Exploring the Photophysics and Reactivity of Nickel–Bipyridine Cross-Coupling Catalysts Citation Cagan, David Abraham (2024) Exploring the Photophysics and Reactivity of Nickel–Bipyridine Cross-Coupling Catalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n3xz-6v34. https://resolver.caltech.edu/CaltechTHESIS:05092024-225449838 Abstract Ni(II)–bipyridine (bpy) aryl halide complexes have been prized for nearly a decade for their catalytic potency to facilitate cross-coupling reactions. To achieve these transformations, the energy from light is leveraged to drive the key catalytic processes. Thus, Ni-mediated photoredox catalysis provides an attractive and sustainable means to replace precious metal catalysts. However, precise mechanistic information regarding how these transformations occur is limited. This thesis thus focuses on a dual experimental and computational analysis of Ni(II)–bpy aryl halide complexes and their photoproducts to provide insight into the specific photophysical and chemical pathways that these catalysts undertake for cross-coupling reactions. The first chapter is a review of the proposed mechanisms presented for Ni-mediated photoredox catalysis. Therein, certain portions of this work are also summarized. The second chapter provides a computational description of the Ni(II) excited states. The third chapter expands on this analysis with experiment, elucidating the photophysical pathway that grants entry into dark Ni(I)/Ni(III) catalytic cycles. Together, chapters two and three show that Ni(II)–bpy aryl halide complexes form low-valent Ni(I)–bpy halide species by an aryl-to-Ni ligand-to-metal charge transfer. Chapter four outlines a method to generate and study these reactive Ni(I)–bpy halide intermediates, identifying their mechanism of C(sp 2 )–Cl bond activation as nucleophilic aromatic substitution, tunable via the energies of the 3 d -orbitals and the effective nuclear charge of Ni. The final chapter finds that these low-valent Ni species are competitive light-absorbers, and it presents a study into their ultrafast photophysics, marking the first of its kind on any Ni(I) complex. The excited-state relaxation dynamics of Ni(I)–bpy halide complexes are well described by vibronic Marcus theory, spanning the normal and inverted regions as a result of simple changes to the bpy substituents. Altogether, these studies have provided a framework to gain electronic structural control over Ni-meditated photoredox catalysis and, thus, guides the use of photonic energy as a sustainable alternative to precious metal catalysis. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Catalysis; Nickel; Mechanism; Photoredox; Spectroscopy; Kinetics; Cross-coupling Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Awards: Graduate Deans’ Award, 2024. Thesis Availability: Public (worldwide access) Research Advisor(s): Hadt, Ryan G. Thesis Committee: Peters, Jonas C. (chair) Stoltz, Brian M. Reisman, Sarah E. Hadt, Ryan G. Defense Date: 29 May 2024 Funders: Funding Agency Grant Number Ford Foundation Pre-Doctoral Research Fellowship UNSPECIFIED NSF Graduate Research Fellowship DGE-1144469 National Institute of General Medical Sciences (NIGMS) R35-GM142595 Record Number: CaltechTHESIS:05092024-225449838 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:05092024-225449838 DOI: 10.7907/n3xz-6v34 Related URLs: URL URL Type Description https://doi.org/10.1021/acscatal.4c02036 DOI Article adapted for Chapter 1 https://doi.org/10.1021/acs.jpca.0c08646 DOI Article adapted for Chapter 2 https://doi.org/10.1021/jacs.2c01356 DOI Article adapted for Chapter 3 https://doi.org/10.1021/acs.inorgchem.3c00917 DOI Article adapted for Chapter 4 https://doi.org/10.1021/jacs.4c04091 DOI Article adapted for Chapter 5 ORCID: Author ORCID Cagan, David Abraham 0000-0002-4719-2789 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16378 Collection: CaltechTHESIS Deposited By: David Cagan Deposited On: 06 Jun 2024 21:46 Last Modified: 08 Jul 2024 19:07 Thesis Files PDF - Final Version See Usage Policy. 20MB Repository Staff Only: item control page

Exploring the Photophysics and Reactivity of Nickel–Bipyridine Cross-Coupling Catalysts Thesis by David A. Cagan In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2024 (Defended May 29, 2024) ii © 2024 David A. Cagan ORCID: 0000-0002-4719-2789 iii ACKNOWLEDGEMENTS The journey to earn my doctorate in chemistry has been transformative. I have found chemistry to be an empowering mechanism to express my creativity and individuality while trying to understand the world. This scientific method was given to me by my professors, my mentors, my friends, and my family. I am deeply indebted to each of them, and I would like to take this time to thank them. Thank you to my Pasadena City College professors. Professor Castro, you taught me the history of chemical discovery from alchemy to Bohr. Your lectures contextualized and humanized science. Thank you for your passion and your representation. Professor Harman, you showed me that chemistry and art are not so separate. Your discussions allowed me to see science as a creative process. Thank you for encouraging my individuality. Professor Ansari, you taught me to join math and chemistry together for the first time. Your notes on reaction kinetics carried me into my subdiscipline. Thank you for showing me the value of organization and structure in my learning. Professor Sweimeh, you not only uncovered my love for organic chemistry, but you also demonstrated the value of mentorship. Your handson approach to teaching and mentoring has made a tremendous impact in my own goals as an educator. Thank you for believing in me before I did. Professor Ganapathi, you presented an enthusiastic and logical approach to organic mechanisms. Your lectures gave me an intuition for reactivity that has been vital for my research. Thank you for showing me the joy in a mechanistic puzzle. Professor Jaramillo, you hosted the STEM study center. Your willingness to maintain our study space gave me a second home. Thank you for supporting my all-hours education. Professor Socrates, you taught me the artistry of linear algebra. Your teaching became the seed for my love of inorganic chemistry. Thank you for showing me the value of an interdisciplinary understanding. Professor Riley, you expanded my appreciation of reaction kinetics by teaching me differential equations. Your support of my project modeling the Belousov–Zhabotinsky reaction solidified physical chemistry as a subdiscipline of mine. Thank you for always having time for your students. Professor Underwood, you expanded my horizon of understanding to the intersection of hermeneutics and science. Your lectures fostered my love for academic writing and challenged my critical iv thinking skills. Thank you for teaching me that a methodological scientific undertaking is still a human endeavor, and as such, should be conducted with openness and an awareness of my own biases. Thank you to my Cal State LA professors. Professor Foster, you furthered my understanding of physical chemistry. Your discussion of statistical thermodynamics was very helpful; I have used your notes several times throughout my research. Thank you for making these complicated subjects intelligible to me. Professor Liu, you taught me the fundamentals of inorganic chemistry and group theory. Your lectures have carried me throughout my advanced studies. Thank you for teaching me the importance of a rigorous scientific foundation. Professor Yap, you demonstrated advanced synthetic methods with an infectious casual demeanor. Your laboratory tutelage encouraged my appreciation for inorganic synthesis. Thank you for making inorganic chemistry colorful. Professor Tunstad, you ran the research programs while I was at Cal State LA. Your grant writing gave me funding for my research, allowing me to not have to hold another job. Thank you for letting research be a priority for me and for showing me what opportunities could lie ahead. Professor Selke, you taught me how to conduct research. Your mentorship, friendship, and instruction have been invaluable to me. Thank you for teaching me how to begin, lead, and finish a project. Thank you for your willingness to discuss with me until well into the evening. Thank you for teaching me the value of a well-thought-out failure. Thank you for showing me how to recover detailed information from simple experiments. Thank you for exemplifying an excellent PI. Thank you to my Caltech professors (who I will call by first name as is customary at Caltech). Theo, you expanded my understanding of inorganic chemistry. Your lecture notes have been a valuable resource; I keep them printed out in my desk drawer for reference. Thank you for your enthusiastic instruction and thank you for your leadership of FUTURE Ignited. It was a pleasure to work with you on it for two years. Greg, you took an interest in my research and supported me in collaboration. Your knowledge of catalysis is seemingly endless, and you took the time to share your thoughts with me to guide my research plans. Thank you for v hosting me at your subgroups. Jay, thank you for your many discussions on photophysics and spectroscopy. I have the utmost respect for your understanding of kinetics. Thank you for sharing your thoughts with me and for allowing me to bounce ideas off of you. Doug, thank you for leading DICI and for being a mentor to me in DEI work. I am always happy to see you on campus and catch up. Harry, you took a chance on me as a summer WAVE student. Your mentorship allowed me to grow in understanding and in confidence. Thank you for teaching me the value of humility in research. Thank you for championing me on into graduate school. Thank you for teaching me advanced ligand field theory and for having me as a TA for the same class. You will find that I have used your lectures in all of my papers. Thank you to my dissertation committee. Jonas, you have always asked the tough questions. Thank you for maintaining a strict academic standard. Thank you for teaching me the importance of each statement or claim I make. Thank you for serving as the chair of my committee. Brian, you also took a chance on me as a summer WAVE student, for which I am very grateful. Thank you for being a mentor to me, both academically and professionally. Your leadership of the CCE DEI Committee is outstanding; thank you for making Caltech a welcoming place. Thank you for serving on my committee. Sarah, you have inspired me as a leader in your field. Your ability to tackle complex organic chemistry and present it with clarity and enthusiasm is a goal of mine. Thank you for challenging me to think beyond my field. Thank you for serving on my committee. Thank you to my thesis advisor. Ryan, you have been a mentor, counselor, and friend. Thank you for your guidance as I navigated my research. Thank you for your openness to my ideas. It has been a privilege to work with you and under your leadership. Joining your group at its early stages has been hugely educational for me. Thank you for your transparency in discussing some of the challenges associated with being a new PI. Thank you for trusting me to lead Team Nickel. Thank you for your willingness to try a risky new project and for being “all about the science”. Thank you for our many one-on-one meetings and countless hours vi of discussion. Thank you for supporting me in my DEI work and for being vital to the start of Rising Tide. Thank you for believing in me. Thank you to the many administrators and staff who have supported me. Candace, Maria, and Carol, the three queens of the Student-Faculty Programs Office, thank you for supporting my time as a WAVE student, for making Rising Tide more than just a dream of mine, and for hosting key programs like the GSRI. You three are core to what makes Caltech a great place for research. Paul, your expertise in EPR has made my detailed mechanistic inquiries possible. Thank you for being open to collaboration, for running my many (sadly) EPR silent Ni samples, and for being a friend to me. Thank you to my coworkers. Ruben, thank you for being the first one to welcome me into the lab. I have enjoyed chatting science with you and playing basketball together. Gautam, thank you for teaching me my first bit of computational chemistry. I wish you the best at U Chicago. Daniel, Brendon, and Erica, thank you for being incredibly supportive postdocs and research teammates. It has been a pleasure working with you and learning from you. Katie, thank you for being a bright presence in the group. Your expertise in physical chemistry has been very valuable to me. Thank you for entertaining my random questions and for proofreading my emails. Thank you for teaching me your synthetic inorganic techniques and for being co-lab managers with me. Nathanael, isn’t it time you start your own lab? Thank you for your patience in teaching me MATLAB, collaborating with me on many projects, your openness to discussing science and philosophy, and your general supportiveness of my research. Chris (Goose), thank you for answering all my chemical biology questions and for being such a good sport. Stephen, thank you for teaching me about your approach to quantum yields and rate constants. It has been fun working with you. Kay, Thais, Edwin, and Stefan, although we have spent less time together in the group, I am grateful for all our discussions. Maria and Keon Ha, you are doing fantastic. Thank you for bringing such good energy and research enthusiasm to the group. I am excited to read your future published projects. vii Thank you to my research friends. Nick, Robert, and Brian, thank you for being such good friends to me. Even though our cohort suffered due to the pandemic, I have enjoyed sharing meals together and discussing science/general research frustrations. I look forward to our friendship beyond Caltech. Robert, I am also thankful for all your helpful discussions about nickel with myself and Maria. The DICI team, including Trixia, Javier, Mary, Joel, Karen, Kim, Levi, Melinda, Marva, Cami, Jacob, and so many others. Thank you for making DICI a successful program. Thank you for working to make Caltech a more equitable place. You have each played a key role in my time at Caltech. Julian, thank you for mentoring me when I was a WAVE student. I am confident that your review of my graduate application was a significant contributor to my entrance into doctoral programs. Arman, thank you for being a friend for these many years. I am thrilled to hear of your successes at U of O, and I look forward to your independent career. You have made my time at Caltech more enjoyable. Thank you to my family. John, you are first-class. Thank you for carrying me through all these years. I could not have done it without you. I respect you deeply. Thank you for being such an outstanding brother to me. It was all not for nothing. To my parents, Mama and Papa, thank you for supporting me all these years. Thank you for understanding my busyness, for being patient as I figured out my plans, and for being excited to learn of my research. I also could not have gotten here without your support. Thank you to my partner. Lizette, you have taught me a great deal about myself. I owe you so much of my mindset. Thank you for being understanding of me and for listening to me. You are my best friend. Thank you for giving me the stability I needed to pursue my research. Thank you for supporting me, for keeping me grounded, and for inspiring me to be my best self. You were there when I started this journey, and you have carried me through. You are my home. viii ABSTRACT Ni(II)–bipyridine (bpy) aryl halide complexes have been prized for nearly a decade for their catalytic potency to facilitate cross-coupling reactions. To achieve these transformations, the energy from light is leveraged to drive the key catalytic processes. Thus, Ni-mediated photoredox catalysis provides an attractive and sustainable means to replace precious metal catalysts. However, precise mechanistic information regarding how these transformations occur is limited. This thesis thus focuses on a dual experimental and computational analysis of Ni(II)–bpy aryl halide complexes and their photoproducts to provide insight into the specific photophysical and chemical pathways that these catalysts undertake for crosscoupling reactions. The first chapter is a review of the proposed mechanisms presented for Ni-mediated photoredox catalysis and serves as an introduction to this work. Therein, certain portions of this work are also summarized. The second chapter provides a computational description of the Ni(II) excited states. The third chapter expands on this analysis with experiment, elucidating the photophysical pathway that grants entry into dark Ni(I)/Ni(III) catalytic cycles. Together, chapters two and three show that Ni(II)–bpy aryl halide complexes form low-valent Ni(I)–bpy halide species by an aryl-to-Ni ligand-to-metal charge transfer. Chapter four outlines a method to generate and study these reactive Ni(I)–bpy halide intermediates, identifying their mechanism of C(sp2)–Cl bond activation as nucleophilic aromatic substitution, tunable via the energies of the 3d-orbitals and the effective nuclear charge of Ni. The final chapter finds that these low-valent Ni species are competitive lightabsorbers, and it presents a study into their ultrafast photophysics, marking the first of its kind on any Ni(I) complex. The excited-state relaxation dynamics of Ni(I)–bpy halide complexes are well described by vibronic Marcus theory, spanning the normal and inverted regions as a result of simple changes to the bpy substituents. Altogether, these studies have provided a framework to gain electronic structural control over Ni-mediated photoredox catalysis and, thus, guides the use of photonic energy as a sustainable alternative to precious metal catalysis. ix PUBLISHED CONTENT AND CONTRIBUTIONS 1. Multireference Description of Nickel–Aryl Bond Dissociation Processes in Photoredox Catalysis. Cagan, D.A.;† Stroscio, G.D.;† Cusumano, A.Q.; Hadt, R.G. J. Phys. Chem. A. 2020, 124(48), 9915-9922. DOI: 10.1021/acs.jpca.0c08646. ◊ Contribution: Co-first authorship with Gautam Stroscio. We both performed the computational work, conducted the data analysis, and contributed to the writing of the publication. 2. Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts. Cagan, D. A.; Bím, D.; Silva, B.; Kazmierczak, N. P.; McNicholas, B. J.; Hadt, R. G. J. Am. Chem. Soc. 2022, 144(14), 6516-6531. DOI: 10.1021/jacs.2c01356. ◊ Contribution: As first author, I performed the experimental work and some of the initial computations. I also conducted the data analysis and contributed to the writing of the publication. 3. Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)–Cl Oxidative Addition and Dimerization Reactivity Pathways. Cagan, D. A.;† Bím, D.;† McNicholas, B. J.; Kazmierczak, N. P.; Oyala, P. H.; Hadt, R. G. Inorg. Chem. 2023, 62(24), 9538–9551. DOI: 10.1021/acs.inorgchem.3c00917. ◊ Contribution: Co-first authorship with Daniel Bím. I conceptualized the project, performed experimental work, and conducted the data analysis. D.B. assisted with the experimental work and performed the computational analysis. We both contributed to the writing of the publication. 4. Mechanisms of Photoredox Catalysis Featuring Nickel–Bipyridine Complexes. Cagan, D. A.; Bím, D.; Kazmierczak, N. P.; Hadt, R. G. ACS Catalysis, 2024, Accepted. ◊ Contribution: As first author, I performed the bulk of the literature review, organized its structure, and contributed to the writing of the manuscript. 5. Ultrafast Photophysics of Ni(I)–Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes. Sutcliffe, E.;† Cagan, D. A.;† Hadt, R. G. J. Am. Chem. Soc. 2024, Accepted. ◊ Contribution: Co-first authorship with Erica Sutcliffe. I conceptualized the project, performed the synthetic work, and conducted the computational analysis. E.S. performed the ultrafast spectroscopy and data analysis. We both contributed to the writing of the manuscript. x TABLE OF CONTENTS Acknowledgements ................................................................................................................................iii Abstract ....................................................................................................................................................viii Published Content and Contributions ................................................................................................ix Table of Contents ...................................................................................................................................x List of Figures, Schemes, and Tables..................................................................................................xii Nomenclature and Abbreviations .......................................................................................................xvi Chapter 1. Mechanisms of Photoredox Catalysis Featuring Nickel–Bipyridine Complexes ..1 §1-1. Introduction ...........................................................................................................................2 §1-2. Nickel Electronic Structure Primer ....................................................................................4 §1-3. Summary and Comparisons of Proposed Photoredox Mechanisms ..........................10 §1-3.1. Photosensitization ..............................................................................................................10 §1-3.1.1. Reductive SET..........................................................................................................10 §1-3.1.2. Key Considerations for the Reductive SET Mechanism ..........................12 §1-3.1.3. Oxidative SET ..........................................................................................................14 §1-3.1.4. Key Considerations for the Oxidative SET Mechanism...........................15 §1-3.1.5. Photosensitization for Homolysis (SET vs. 3EnT)............................................17 §1-3.1.6. Key points of the Photosensitization for Homolysis Mechanisms .........19 §1-3.1.7. Triplet Energy Transfer (3EnT).............................................................................22 §1-3.1.8. Key Components of the 3EnT Mechanism .................................................23 §1-3.1.9. SET for Active Ni(I) ...............................................................................................25 §1-3.1.10. Key Components of the SET for Active Ni(I) Mechanism ...................29 §1-3.2. Direct Excitation ................................................................................................................32 §1-3.2.1. Direct Excitation for Dark Cycle Initiation ........................................................32 §1-3.2.2. Direct Excitation for Reductive Elimination......................................................39 §1-3.2.3. Key Components of Direct Excitation ........................................................40 §1-4. Conclusions and Outlook ....................................................................................................45 §1-5. References ...............................................................................................................................47 Chapter 2. Multireference Description of Nickel–Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis 63 §2-1. Introduction ...........................................................................................................................64 §2-2. Computational Methods ......................................................................................................66 §2-3. Results and Discussion .........................................................................................................68 §2-4. Conclusions ............................................................................................................................75 §2-5. References ...............................................................................................................................76 Chapter 3. Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel– Bipyridine Photoredox Catalysts .............................................................................................................................82 §3-1. Introduction ...........................................................................................................................83 §3-1.1 Ni(II)–bpy Photoredox Catalysis ..............................................................................83 §3-1.2. Mechanistic Hypotheses for Excited-State Ni(II)–C Bond Homolysis ...........85 §3-2. Results and Analysis ..............................................................................................................87 §3-2.1. Experimental Studies..................................................................................................87 §3-2.1.2. Steady-State UV–Vis Spectroscopy ......................................................................88 §3-2.1.3. Photochemical Investigations ................................................................................90 xi §3-2.1.4. Preliminary Investigations of the Photochemically Generated Species....... 95 §3-2.1.5. Further Examination of the Mechanism of Excited-State Bond Homolysis . 98 §3-2.2. Computational Studies ......................................................................................................101 §3-2.2.1. DFT versus CASSCF/QD-NEVPT2 Ground and Excited States ...............101 §3-2.2.3. Investigating the Mechanism of Excited-State Ni(II)–C Bond Homolysis ..104 §3-3. Discussion ...............................................................................................................................110 §3-4. Conclusions ............................................................................................................................116 §3-5. References ...............................................................................................................................118 Chapter 4. Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)–Cl Oxidative Addition and Dimerization Reactivity Pathways ...................................................................................................................................................127 §4-1. Introduction ...........................................................................................................................128 §4-2. Results and Analysis ..............................................................................................................131 §4-2.1 Photochemical Synthesis and Spectroscopic Characterization of Ni(I)–bpy Halide Complexes .................................................................................................................................131 §4-2.2. Oxidative Addition Kinetics with 2-Chloro-toluene ............................................135 §4-2.3. Oxidative Addition Mechanistic Investigations ....................................................139 §4-2.4. Thermodynamics of the Dimerization of Ni(I)–bpy Halides.............................144 §4-2.5. Dimerization Mechanistic Investigations ...............................................................147 §4-3. Discussion ...............................................................................................................................149 §4-4. Conclusions ............................................................................................................................154 §4-5. References ...............................................................................................................................156 Chapter 5. Ultrafast Photophysics of Ni(I)–Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes. ............................................................................................................164 §5-1. Introduction ...........................................................................................................................165 §5-2. Results......................................................................................................................................168 2.1. Steady State and Ultrafast UV-vis-NIR Spectroscopy...............................................168 §5-2.2. Isolation and Characterization of Ni(I)(MeO2Cbpy)Cl, 5′........................................172 §5-3. Discussion ...............................................................................................................................174 §5-4. Conclusions ............................................................................................................................179 §5-5. References ...............................................................................................................................180 Appendix 1. Supporting Information for Chapter 2: Multireference Description Of Nickel– Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis ......................................187 Appendix 2. Supporting Information for Chapter 3: Elucidating The Mechanism of ExcitedState Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts .............................................188 Appendix 3. Supporting Information for Chapter 4: Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)– Cl Oxidative Addition and Dimerization Reactivity Pathways ...............................................................................................189 Appendix 4. Supporting Information for Chapter 5: Ultrafast Photophysics of Ni(I)– Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes ..190 xii LIST OF FIGURES, SCHEMES, AND TABLES Item Page Figure 1.1. Qualitative molecular orbital correlation diagram of four Ni(II)–bpy species of potential relevance in photocatalytic pathways........................................... 6 Figure 1.2. UV-vis absorption spectra of a common Ir(III) photosensitizer and various Ni complexes.................................................................................................... 8 Figure 1.3. Proposed Reductive SET mechanism....................................................... 11 Figure 1.4. Proposed Oxidative SET mechanism....................................................... 15 Figure 1.5. Proposed Photosensitization for Homolysis Mechanism (Doyle, Shields).......................................................................................................................... 18 Figure 1.6. Proposed Photosensitization for Homolysis Mechanism (Molander)...... 18 Figure 1.7. Proposed 3EnT Mechanism....................................................................... 23 Figure 1.8. Proposed SET for Active Ni(I) Mechanism for C(sp2)–S coupling......... 27 Figure 1.9. Proposed SET for Active Ni(I) Mechanism............................................. 28 Figure 1.10. Proposed direct excitation for photohalogen elimination from fivecoordinate Ni complexes.............................................................................................. 34 Figure 1.11. Proposed direct excitation for Ni(II)–C(aryl) bond homolysis and C(sp2)–O product formation mechanism...................................................................... 37 Figure 2.1. Mechanistic hypotheses related to Ni–bpy photoredox catalysis............. 65 Figure 2.2. Simplified MO diagram of 1 calculated by CASSCF/QD-NEVPT2......... 69 Figure 2.3. Ni(II)–C bond dissociation from the lowest energy singlet and triplet states in 1....................................................................................................................... 71 Figure 2.4. DFT vs. CASSCF/QD-NEVPT2 description of the formal Ni(I) species formed upon homolytic Ni–C bond cleavage.............................................................. 72 xiii Figure 2.5. CASSCF/QD-NEVPT2 relaxed ground- and excited-state PESs along the Ni–C coordinate of 1................................................................................................ 74 Figure 3.1. Plausible photocatalytic approaches to Ni(II)–bpy-mediated C–O bond formation, direct excitation energy diagram, and summary of work Chapter 3......... 85 Figure 3.2. Matrix of Ni(Rbpy)(R′Ph)Cl (R = MeO, t-Bu, H, and MeOOC; R′ = ortho-CH3, H, OMe, F, and CF3) complexes examined in Chapter 3...................... 88 Figure 3.3. UV–vis spectra in THF of complexes studied and correlation between λmax and the Hammett parameter............................................................................... 89 Figure 3.4. Photolysis profiles of 1A–1D in THF for 390 nm excitation.................. 92 Table 3.1. Summary of UV-vis λmax and first-order rate constants............................... 93 Figure 3.5. Correlation between normalized rate constants (390 nm excitation) of excited-state bond homolysis and specific Hammett σ parameters of the bpy and aryl ligands........................................................................................................................... 95 Figure 3.6. Photolysis profile of 1B–Br in THF, comparison between long-time UV–vis spectra of five compounds, and comparison between UV–vis spectra for 1B–Br in THF and DMF before and after irradiation.............................................. 96 Figure 3.7. Eyring analysis of the temperature-dependent photolysis rates................. 99 Figure 3.8. Wavelength-dependent photolysis for 1B, 1D, and 5D............................. 99 Figure 3.9. Correlation between the calculated 1MLCT transition energies and the specific Hammett σ parameters..................................................................................... 103 Figure 3.10. Two plausible excited-state mechanisms of Ni(II)–C bond homolysis... 106 Figure 3.11. Computed excited-state activation energies plotted against experimental ln(kobs,1)................................................................................................ 109 Figure 3.12. Comparison between the change in energy of the MLCT energy and experimental ln(kobs,1) and energetic barrier for photolysis; PES diagram illustrating how the barrier for photolysis is dependent on both the 1MLCT and repulsive 3(MLCT+LMCT) surfaces........................................................................ 113 xiv Figure 4.1. Catalytic cycle displaying the photochemical activation of Ni(II)–bpy aryl halide complexes and two competing pathways for the Ni(I) complexes.............. 131 Figure 4.2. Spectroscopic characterization of the Ni(I)–bpy halides examined in Chapter 4....................................................................................................................... 133 Table 4.1. Experimental g values for the Ni(I)–bpy halide complexes in 2-MeTHF at 5 K.............................................................................................................................. 134 Scheme 4.1. Oxidative addition reaction conducted with photogenerated Ni(I)–bpy halide............................................................................................................................. 136 Figure 4.3. Experimental kinetic analysis of the oxidative addition reaction for 1Cl, 2-Cl, and 3-Cl.......................................................................................................... 138 Table 4.2. Rate constants for room temperature oxidative addition for the reaction between 2-chloro-toluene and various Ni(I)(Rbpy)X species....................................... 139 Figure 4.4. Hammett analysis for the reaction of 1-Cl with 4-substituted aryl chlorides........................................................................................................................ 140 Figure 4.5. Computational analysis of the oxidative addition process of Ni(I)(Rbpy)X with 2-chloro-toluene............................................................................. 142 Figure 4.6. Experimental thermodynamic analysis of the dimerization reaction for 1-Cl, 2-Cl, and 3-Cl...................................................................................................... 146 Table 4.3. Thermodynamic parameters for dimerization of Ni(I)–bpy halide complexes...................................................................................................................... 147 Figure 4.7. Computational analysis of the dimerization of Ni(I)‒bpy halides............. 148 Figure 4.8. Reaction coordinate and state energy diagrams for the Ni(I)–bpy halide complexes...................................................................................................................... 152 Figure 5.1. Photogeneration and cross-coupling reactivity of Ni(I)–bipyridine halides, the overlapping absorption of Ni(I)–bpy with Ni(II)–bpy, and schematic of pump-probe transient absorption spectroscopy............................................................. 167 Figure 5.2. Structures and UV-vis-NIR absorption spectra of the photochemically generated Ni(I)–bpy halides in THF.............................................................................. 169 xv Figure 5.3. Cascaded difference spectra of 1 and 5....................................................... 170 Table 5.1. Lowest-energy MLCT absorption positions and relaxation time constants for all compounds studied in Chapter 5......................................................................... 172 Figure 5.4. Photochemical generation and isolation of 5′............................................. 173 Figure 5.5. Modeling excited state relaxation dynamics of Ni(I) compounds.............. 177 xvi NOMENCLATURE AND ABBREVIATIONS 2-MeTHF 2-methyltetrahydrofuran 3EnT Triplet energy transfer Å Angstrom Ac Acetate Ar Aryl BDE Bond dissociation energy BDFE Bond dissociation free energy bpy 2,2'-bipyridine BSS Broken-symmetry singlet CASSCF Complete active space self-consistent field theory CI Configuration interaction COD Cyclooctadiene CPCM Conductor-like polarizable continuum model CSS Closed-shell singlet CW Continuous wave DABCO 1,4-diazabicyclo[2.2. 2]octane d–d Refers to a ligand field electronic transition DFT Density functional theory DMF Dimethylformamide E° Reduction potential EnT Energy transfer EPR Electron paramagnetic resonance Es Taft’s steric parameter ESA Excited-state absorption Et Ethyl Fc Ferrocene GC-MS Gas chromatography with mass spectrometry GSB Ground-state bleach xvii IR Infrared k Rate constant LMCT Ligand-to-metal charge transfer LUMO Lowest unoccupied molecular orbital MCD Magnetic circular dichroism Me Methyl MLCT Metal-to-ligand charge transfer NIR Near infrared NMR Nuclear magnetic resonance OAc Acetate PES Potential energy surface Ph Phenyl ppy 2-phenylpyridine Q Nuclear coordinate QD-NEVPT2 Quasi-degenerate N-electron valence second-order perturbation theory RAMO Redox-active molecular orbital RI Resolution of identities approximation RI Resolution of identities RIXS Resonant inelastic X-ray scattering RKS Restricted Kohn-Sham formalism RKS Restricted Kohn-Sham formalism S Spin quantum number SCE Saturated calomel electrode SCF Self-consistent field SET Single electron transfer SNAr Nucleophilic aromatic substitution SOMO Singly occupied molecular orbital TA Transient absorption TBA Tetrabutyl ammonium t-Bu Tert-butyl xviii TDDFT Time-dependent density functional theory THF Tetrahydrofuran TMEDA N,N,N′,N′-tetramethylethylenediamine UKS Unrestricted Kohn-Sham formalism UV-Vis Ultraviolet-visible V Volt VT Variable temperature X Halide XRD X-ray diffraction Zeff Effective nuclear charge Δ Contextual: ligand field splitting energy or "change in" ε Molar extinction coefficient ⟨ℏ𝜔⟩ Average vibrational energy λ Contextual: wavelength in nanometers or total reorganization energy λmax Wavelength at which the absorbance is maximized λs Solvent contribution to λ λv Vibrational contribution to λ μeff Effective magnetic moment ν Frequency (energy) in inverse centimeters νmax Wavenumber at which the absorbance is maximized ρ Hammett reaction rate constant σF Taft’s field parameter σm Meta-Hammett parameter σp Para-Hammett parameter τ Excited-state lifetime Φ Quantum yield 1 Chapter 1 Mechanisms of Photoredox Catalysis Featuring Nickel–Bipyridine Complexes Adapted with permission from: Cagan, D. A.; Bím, D.; Kazmierczak, N. P.; Hadt, R. G. Mechanisms of Photoredox Catalysis Featuring Nickel–Bipyridine Complexes. ACS Catalysis, 2024, Accepted. Copyright 2024 American Chemical Society. 2 §1-1. Introduction Pd-catalyzed cross-coupling reactions have transformed organic chemistry with their synthetic contributions to drug discovery and development.1–3 Although subtle differences emerge between reactions, the majority of Pd-catalyzed couplings leverage a mechanism featuring dominantly two-electron processes: oxidative addition, transmetalation, and reductive elimination.4 Going beyond Pd, a precious metal and limited resource, significant strides have been made toward more sustainable approaches to catalysis. These advances feature critical contributions from methodology-driven research into homogenous crosscoupling catalysis by first-row transition metal complexes, which are becoming more widely adopted for enabling the construction of new C–X (X = C, O, N, F, etc.) bonds.5–7 Mechanistic studies highlight the complexities of these ground-state cross-coupling reactions, but also bring to light new possibilities stemming from one-electron redox processes and the variety of intermediates involved in the underlying bond-formation and bond-rupture processes.8 Ni-mediated catalysis has emerged as a key alternative to Pd, as it can access a range of formal oxidation and/or spin states (Figure 1.1) and facilitate numerous complex substrate transformations.9–11 In addition to metal redox, ligand-based redox (i.e., ligand non-innocence and potential multireference character)12–15 further increases reaction complexity by providing important, yet poorly understood, electronic structure contributions. These can result in noble-metal-like reactivity in base-metal catalysts and provide a basis for transformative structure/function relationships.7 Metallaphotoredox catalysis has had a profound influence on many areas of organic chemistry, including cross-coupling reactions. This approach uses photosensitizers to generate metal-based intermediates that can be active in dark cycles.16–22 These intermediates often form due to their propensity for single electron transfer (SET).23 Photosensitizers can additionally transfer energy to metal complexes to form reactive excited states.24,25 The merger of photoredox catalysis with Ni–bipyridine (bpy) complexes has claimed a prominent place in the organic, inorganic, and physical chemistry communities owing to its wide synthetic utility and rich photophysical aspects.20,26–32 In addition to light absorption by the 3 photosensitizers present in reaction mixtures (often cyclometalated Ir(III) heteroleptic complexes33–35), these Ni–bpy co-catalysts also absorb strongly across the UV–vis region and can directly harvest light to access key excited states.22,36–40 In principle, ultrafast spectroscopic methods should be critical to studying the photophysical processes that undergird the overall chemical bond transformations.41 However, as discussed below, there are often strongly competing intramolecular excited-state relaxation pathways, and care needs to be taken to account for low quantum yield processes that can be difficult to probe directly using time-resolved spectral methods. Overall, the elucidation of mechanistic routes requires the knowledge of both light- and thermally-driven components and the interplay between them. As discussed below, this has proven to be a difficult task for light-driven, Nimediated catalytic cycles, and our overall understanding of how photon energy drives organic transformations is still superficial. The aforementioned progress motivates further efforts to elucidate the geometric and electronic structures of critical inorganic species and photoinduced states that are involved in metallaphotoredox cross-coupling reactions. We believe these knowledge gaps can be addressed by a synergistic combination of synthesis, spectroscopy, and computation to define electronic structure contributions to reactivity, and we hold that there is significant general potential linked to leveraging these complexities for cross-coupling catalysis. To do so, however, significant strides need to be made toward detailed and fundamental studies of discrete light and dark reaction steps that constitute photoredox catalytic cycles. Ultimately, in concert with additional methodological studies, this understanding will help inform chemists how to leverage the inherent properties of first-row transition metals and, thus, guide academic and industrial research toward sustainable approaches for bond constructions in organic synthesis. While previous reviews have highlighted the tremendous advancements made in the development of new photoredox-enabled transformations,19,20,31 this review seeks to compare and evaluate mechanisms that have been proposed in the literature, with a focus on Ni–bpy complexes. We note that additives can influence the catalytic pathway. However, mechanistic analysis of their contributions is quite limited. Thus, while potentially important 4 to consider, this review does not provide a complete picture of their potential mechanistic roles. Given the growing importance of ground- and excited-state processes in metallaphotoredox catalysis, the review first features a brief electronic structure primer, which discusses key aspects of different electronic states of Ni at a broadly accessible level. We subsequently provide a summary and comparison of proposed photoredox mechanisms. Divided into two main sections, we firstly summarize mechanisms featuring key photosensitization steps. Secondly, we discuss mechanisms that feature direct excitation of Ni-based species for bond-homolysis-driven dark cycle initiation or excited-state bond-formation reactions. The mechanistic summaries are further bolstered by “Key Consideration” sections designed to highlight the importance of Ni-based intermediates and their electronic structures. By doing so, we hope to 1) demonstrate the importance and need for further mechanistic studies of metallaphotoredox reactions, even beyond Ni, and 2) highlight the interdisciplinary nature of this growing area, hopefully motivating future synergistic contributions that will span the physical, organic, and inorganic chemistry communities. §1-2. Nickel Electronic Structure Primer Prior to embarking on our review of light-activated catalytic cycles featuring Ni complexes, it is valuable to consider the distinct electronic structures of the commonly invoked Ni intermediates. Even within a given oxidation state, such as Ni(II), disparate geometries, spin states, and ligand field strengths can lead to unique properties for different species.22 These changes have direct implications for evaluating the plausibility of ground- and excited-state reactivity, including mechanistic steps such as light harvesting, energy/electron transfer, and electrophile activation. Nickel is most stable in the 2+ oxidation state with a d8 electron configuration.42 Many stable four-coordinate Ni(II) species are known, and several feature prominent roles in the mechanisms outlined below. Given the importance of these species and their reactivity, we consider their geometric and electronic structures at length. As this review focuses on Ni– 5 bpy complexes, we assume that two of the four coordination sites are occupied by the bpy ligand. Charge balance requires the remaining two ligands be anionic. Common options in the context of cross-coupling include aryl and halide ligands, which could potentially be arranged in either a square planar (D4h) or a pseudo-tetrahedral (Td) geometry (Figure 1.1). These options are not independent of the ligand character, as described below. In the square planar geometry, the vast majority of the σ* character is concentrated in the 3d(x2–y2) orbital, resulting in a large ligand field splitting energy, Δ (Figure 1.1).43 This splitting is greater than the electron-electron repulsion (i.e., spin pairing energy) incurred by having the seventh and eighth electrons occupying the same orbital (the 3d(z2) orbital here). Thus, it is more energetically favorable to adopt a low-spin, S = 0 configuration with a doubly unoccupied 3d(x2–y2) orbital. If the ligands are rotated into a pseudo-Td geometry, multiple nearly degenerate orbitals share the σ* character, leading to a small Δ relative to the square planar case and a high-spin, S = 1 d8 configuration. (Note that calculations of molecular orbital energies for related pseudo-Td Ni(II) complexes suggest two main σ* orbitals, as opposed to the three σ*-orbitals found in a perfect tetrahedron.)44,45 Accordingly, population of the strongly antibonding 3d(x2–y2) orbital in the D4h geometry (such as through metalcentered photoexcitation) induces a geometric rotation to the pseudo-Td geometry to minimize the σ* overlap. The choice between a square planar and pseudo-Td geometry can thus be understood as a competition between electron repulsion (spin pairing energy) and the ligand field splitting energy.46 The D4h S = 0 state pays the energetic penalty for pairing electrons, but it avoids populating the high-lying 3d(x2–y2) orbital and is therefore unaffected by larger values of Δ (Figure 1.1, A and D). On the other hand, the pseudo-Td S = 1 state avoids the energetic penalty for pairing electrons in the same orbital, yet it populates both the 3d(z2) and 3d(x2–y2) orbitals, which each experience an energetic disadvantage according to the magnitude of Δ (Figure 1.1, B and C). Accordingly, strong-field ligands favor the square planar geometry, while weak-field ligands favor the pseudo-Td geometry.47 6 Figure 1.1. Qualitative molecular orbital correlation diagram of four Ni(II)–bpy species of potential relevance in photocatalytic pathways; each feature distinct geometric and electronic structures, ligand field splitting energies (Δ), and σ* effects. The Ni(II)–bpy aryl halide (A) adopts a square planar (D4h) geometry, leading to a diamagnetic S = 0 ground state. The high spin S = 1 geometry (B) is observed as a relaxed excited-state intermediate; population of the 3d(x2–y2) orbital induces a rotation into a pseudo-Td geometry. Ni(II)–bpy dihalide (C) is stable as a Td triplet ground state. For completeness, we also show this complex in a square planar geometry (D). This singlet state is energetically disfavored and yet-to-be identified to date. Select molecular orbitals (computed with density functional theory at the B3LYP/def2-TZVP48–50 level) are depicted at the top of the figure for illustration of σ* effects. Herein arises the essential difference between aryl and halide ligands. From the perspective of ligand field theory, the aryl is considered a strong-field ligand, while halides are weakfield ligands.51 As such, the gap between the σ* orbital(s) and the remaining, lower-lying 3d- 7 orbitals will be large for a Ni(II)–bpy aryl halide species, but comparatively small for Ni(II)–bpy dihalides. For this reason, Ni(II)–bpy aryl halides feature singlet, square planar ground states, while Ni(II)–bpy dihalides feature pseudo-Td triplet ground states. Note that pseudo-halide ligands (such as alcohols or acetates) result in similar electronic structures as halides; alkyl ligands behave as aryls, but with larger values for Δ, as they are stronger σdonors. For Ni(II)–bpy aryl halides vs. Ni(II)–bpy dihalides, their distinct geometries and spin states have significant implications for electron transfer in catalysis owing to the divergent energies of the redox-active molecular orbital (RAMO). For the ground-state Ni(II)–bpy aryl halide, the lowest unoccupied molecular orbital (LUMO) is not metal-based. The strong σ* overlap of 3d(x2–y2) orbital raises its energy above the bpy π* orbital manifold. As such, the first reduction event for Ni(II)–bpy aryl halide is observed on the bpy ligand rather than the metal.52,54 Reduction of the complex results in an anionic [Ni(II)–bpy•– aryl halide]–, which slowly decomposes to a three-coordinate Ni(I)–bpy aryl species.52,53 When the aryl ligand is replaced by a halide, the reduction in σ-donation strength and associated antibonding character leads to a significant decrease in the 3d(x2–y2) orbital energy. Furthermore, as the ground-state Ni(II)–bpy dihalide adopts a pseudo-Td geometry, an additional stabilization in the Ni-based RAMO is expected. One-electron reduction of this complex affords a doubly occupied σ* 3d(z2) orbital, resulting in ejection of a halide to give a Ni(I)–bpy halide complex.55,56 The reduction potential of the complex trends with the energy of the LUMO. In Ni(II)(t-Bubpy)(o-tolyl)Cl, the first reduction event is found to be –1.6 V vs. SCE, corresponding to electrochemically reversible bpy reduction. Irreversible Ni-based reduction appears at ~–1.8 V vs. SCE57. By contrast, the first reduction event for Ni(II)(t-Bubpy)Cl2 is at –1.3 V vs. SCE (Ni-based and irreversible).55,56,58 The activity of a Ni(II) complex toward reductive steps in a catalytic cycle is dramatically influenced by ligand field strength and coordination geometry;54,59,60 similar considerations were also demonstrated for S = 1 chiral enantioselective Ni(II)–diimine dihalide cross-coupling catalysts.44,61,62 Ligand field analysis 8 of molecular orbital energies indicates the relative plausibility of various catalytic reduction events. Figure 1.2. UV–vis absorption spectra of a common Ir(III) photosensitizer and various Ni complexes. (A) Strongly absorbing complexes with charge transfer bands, [Ir(III)[Rppy]2(t-Bubpy)]PF6 (green line, R = 2-(2,4-difluorophenyl)-5-trifluoromethyl), and Ni(II)(t-Bubpy)(o-tolyl)Cl (S = 0, blue line), are highlighted. (B) An expanded view of complexes with only ligand field transitions in the visible region, Ni(II)(TMEDA)(o-tolyl)Cl (S = 0, orange line), Ni(II)(TMEDA)Cl2 (S = 1, red line), and Ni(II)(t-Bubpy)Cl2 (S = 1, black line). Solvent = THF. Spectra were digitized and scaled with permission from references 36, (Copyright 2018 American Chemical Society) 38 (Copyright 2022 American Chemical Society), 65 (available under a CC-BY NC 3.0 Deed license, copyright 2024 Bryden and Zysman-Colman), 66 (Copyright 2016 John Wiley and Sons) and 67 (Copyright 2020 American Chemical Society). 9 In addition to redox potentials, the geometric and electronic structures of Ni–bpy complexes determine their light-harvesting ability through the molar absorption coefficients of the UV–vis transitions. As a ligand with a significant π-conjugation, the bpy possesses low-lying π*-orbitals capable of backbonding with the metal center. These bpy orbitals serve as acceptors for metal-to-ligand charge transfer (MLCT) transitions in the visible absorption spectrum and possess significant electron delocalization, leading to a large transition dipole moment. Replacement of the bpy ligand for aliphatic N,N,N′,N′-tetramethylethylenediamine (TMEDA) exemplifies this point, where only ligand field bands become possible, leading to reduced values of ε (Figure 2).38,63,64,66 Ni(II)–bpy aryl halide complexes exhibit MLCT transitions (350 nm–550 nm) that possess molar extinction coefficients of comparable magnitude as iridium photosensitizers (ε = 103–104 M-1 cm-1),36,38,65,67 rendering these Ni(II) species competitive for photocatalytic light harvesting. Ni(II)–bpy dihalide complexes show orbitally-forbidden ligand field transitions in the visible to near-infrared region with ε = 101–102 M-1 cm-1 (Figure 1.2).36 Similar analyses may be conducted for other oxidation states. Three-coordinate Ni(I) complexes adopt an approximately planar geometry; while the 3d(x2–y2) σ* interaction is somewhat lessened due to the loss of fourfold symmetry and consequent orbital overlap, there nonetheless remains a large energetic separation between the 3d(x2–y2) orbital and the remainder of the 3d-manifold due to σ* interactions with the bpy and π* interactions with the halide. The d9 Ni(I) configuration implies single occupation of the high-energy σ*orbital; however, this is tolerated, and such Ni(I) compounds have been characterized.54,56,68 However, further reduction of Ni(I) to Ni(0) requires the introduction of an additional electron into the destabilized σ* 3d(x2–y2) orbital. The reduction potentials for such an event are thought to be high, and it is unclear whether Ni(0) is catalytically accessible69,70 (see Reductive SET mechanism below). Indeed, Ni(0)–bpy cyclooctadiene (COD) exhibits a large degree of bpy ligand redox non-innocence and is proposed to exist as Ni(I)(bpy•–)(COD).71 Interestingly, Ni(I)–bpy halide complexes exhibit MLCT transitions across a wide wavelength range (350 nm–1400 nm) and have molar extinction coefficients 10 of equal or greater magnitude than Ni(II)–bpy aryl halides, marking yet another competitive light-harvesting species in photocatalytic cycles.56 §1-3. Summary and Comparisons of Proposed Photoredox Mechanisms Key consideration sections are provided for each of the mechanisms summarized herein, with the goal of connecting these considerations to experimental observations that are emphasized across all Ni–bpy-based photoredox mechanisms, both in terms of direct excitation and photosensitization. §1-3.1. Photosensitization §1-3.1.1. Reductive SET The first metallaphotoredox reactions using light-activated nickel were reported independently in 2014 by the groups of Molander29 and Doyle and MacMillan,30 where C(sp2)–C(sp3) cross-couplings were discovered in reactions combining Ni(0)–bpy, an Ir(III) photosensitizer, and organic coupling partners. The reaction scope was further extended to C(sp2)–C(sp2) and C(sp3)–C(sp3) couplings in 2015 and 2016, respectively,72–74 then for the activation of aliphatic C–H bonds in 2018,75 and to alkyl chloride substrates in 201976 and 2020;77 enantioselective cross-coupling was seen a year later.78 Based on a thermodynamic redox potential argument, it was speculated that the iridium excited state, *Ir(III), carried out two separate SET events. This mechanism is termed “Reductive SET” herein, as the first (and only) proposed interaction between iridium and nickel is a reduction of Ni(I) to Ni(0) (Figure 1.3). 11 Figure 1.3. Proposed Reductive SET mechanism. C(sp2)–C(sp3) coupling is presented as a representative example. LG = leaving group. In the Reductive SET mechanism, the Ir(III) photosensitizer is the sole excited-state active species. In one SET, *Ir(III) oxidizes the alkyl coupling partner, affording C(alkyl)• and Ir(II). In another SET, Ir(II) reduces a Ni(I)–bpy halide complex (top box, Figure 1.3) to Ni(0)–bpy, which can undergo oxidative addition with an aryl halide to generate a squareplanar (S = 0) Ni(II)–bpy aryl halide complex (bottom box, Figure 1.3). This Ni(II) complex captures the *Ir(III)-generated alkyl radical, and the resultant pentacoordinate Ni(III) species undergoes reductive elimination to form a Ni(I)–bpy halide and the C(sp2)–C(sp3) crosscoupled product. The cycle continues upon further reduction of Ni(I)–bpy halide by Ir(II) to Ni(0)–bpy and Ir(III). 12 §1-3.1.2. Key Considerations for the Reductive SET Mechanism 1. Ir(III) acts as the sole light-harvesting species. This is a critical point for any photoredox cycle featuring multiple intermediates that could absorb photons with energies matching those of the irradiation source. For example, Ni(II)–bpy aryl halide complexes (bottom box, Figure 1.3) are now known to be photoactive in C(sp2)–C(sp3) cross-coupling upon direct excitation via a Ni(II)–C(aryl) to Ni(I) + C(aryl)• bond homolysis step.36–38,79 Even a small amount of photogenerated Ni(I) through this alternative step may be sufficient to catalyze the reaction. These examples are discussed in Section §1-3.1.9. Importantly, both the Ir photosensitizer and the Ni(II)–bpy aryl halide complexes absorb light in the visible region with molar extinction coefficients of 103 M-1 cm-1 (Figure 1.2). The molar absorptivities of the various Ni intermediates possible in the reaction cycle are largely unknown. 2. *Ir(III) is sufficiently oxidizing to react with alkyl substrates, doing so preferentially. Redox interactions between *Ir(III) and substrate can be probed through electrochemical measurements and the oxidation state of the Ir complex tracked by absorption spectroscopy. Interactions between *Ir(III) and species in solution other than the organic substate, including any Ni complexes in the putative cycle, are possible and should be evaluated. For example, the alkyl substrates used in the abovementioned work have accessible oxidation potentials of ~ 1 V versus SCE,29,30,80 but these neighbor the oxidation potential of Ni(II)–bpy aryl halide (~ 0.8–0.9 V versus SCE). As will be seen below, related interactions between *Ir(III) and Ni complexes are invoked in the Oxidative SET mechanism (Section §1-3.1.3). Furthermore, both SET and triplet energy transfer (3EnT) are possible from *Ir(III) to Ni(II),81 further complicating analyses (see Sections §1-3.1.3 and §1-3.1.7). 3. Ni(0)–bpy undergoes oxidative addition, while Ni(I)–bpy halide does not. Both Ni(0) and Ni(I) can undergo oxidative addition with aryl halides. However, Ni(I)-Ni(III) oxidative addition would divert the proposed Reductive SET mechanism from Ni(0)-Ni(II) oxidative addition. The reactivity of Ni(0) and Ni(II) vs. Ni(I) and Ni(III) are distinct. Furthermore, the presence of Ni(I) and Ni(III) can lead to facile comproportionation to S = 0 Ni(II)–bpy aryl halide and S = 1 Ni(II)–bpy dihalide,82 another chemically distinct species that is not 13 considered in this mechanism but is important for others (Section §1-3.1.9). Additionally, Oderinde, Johannes, and coworkers noted the reduction potential of Ir(II) is scarcely able to reduce various Ni(I) complexes, finding their potentials to be similar ([IrIII/IrII] = −1.37 V vs. SCE, [NiI/Ni0] = −1.41 V vs. SCE), and that Ni(0) is ineffective to turn over the cycle.69 Further disfavoring Ni(0), Gutierrez, Martin and coworkers found that Ni(II)–bpy dihalide complexes engage in rapid, facile comproportionation with Ni(0)–bpy species in solution, affording Ni(I)–bpy halide species.83 However, Plasson, Fensterbank, Grimaud and coworkers argued that Ni(0) is indeed a vital source of Ni(II)–bpy aryl halide,84 and Bahamonde and coworkers argued that oxidative addition to Ni(0) outcompeted the comproportionation reaction, supporting an Oxidative SET mechanism, though 3EnT pathways were not discarded85 (see Section §1-3.1.7). Altogether, the requirement of Ni(0) for catalytic cycle turnover is still debated. 4. Alkyl radicals are preferentially captured by Ni(II), not Ni(0). Given that Ni(0) and Ni(II) complexes are present in the proposed mechanism, a comparison between the relative rates of radical capture by both of these species would help confirm the Ni(II) to Ni(III)– alkyl hypothesis. Computations by Molander, Kozlowski, and coworkers suggest both oxidation states should be productive toward radical capture.86 While kinetic analysis for radical capture at Ni(II) was recently reported (k = 106–107 M-1 s-1)87, we are unaware of studies for C(alkyl)• capture by Ni(0). 5. Ir(II) is sufficiently reducing to regenerate Ni(0) and Ir(III). The presence of Ir(II) presupposes that Reductive SET is indeed operative (see point 2 above). Given the highly reducing nature of Ir(II), one must also consider its potential interaction with Ni(II) and Ni(III). Reduction of Ni(II) to Ni(I) would present an alternative mechanistic route, potentially favoring a Ni(I/III) catalytic cycle (see point 3). Additionally, Neurock, Minteer, Baran, and coworkers reported that pentacoordinate Ni(III) complexes are readily reduced to Ni(II) via Ni–X heterolysis.55 It is possible the Ni(III) species could be intercepted by Ir(II) prior to reductive elimination and thereby be diverted from the cycle making C(sp2)–C(sp3) bonds. Again, relative reactivity rates between Ir(II) and the relevant Ni species would prove invaluable for mechanistic insight. 14 There have been limited experimental mechanistic studies conducted on this reaction, but one notable example is the work by Lloyd-Jones and coworkers in 2022.88 Careful kinetic analysis using radiolabeled substrates and 13C NMR identified the Ni(II)–bpy aryl halide as a genuine intermediate. From the kinetic modeling, three plausible mechanisms were proposed for the reaction, including one which is akin to the Reductive SET mechanism illustrated above. Interestingly, this mechanistic possibility was the only one of the three the researchers were able to rule out. The remaining two mechanisms proposed by Lloyd-Jones and coworkers centered around *Ir(III) promoting a photoinduced Ni–halide bond homolysis step, referred to here as “Photosensitization for Homolysis” (see Section §1-3.1.5). However, the three mechanisms considered therein are not an exhaustive list, as noted by the authors.88 Nonetheless, based on these considerations and the recent kinetics study, the initially proposed Reductive SET mechanism is unlikely operative. Additional detailed experimental studies are necessary, however, particularly addressing the five points outlined above. §1-3.1.3. Oxidative SET The expansion of dual Ni/Ir metallaphotoredox reactions to C(sp2)–X coupling led to an additional mechanistic hypothesis, Oxidative SET, as proposed for C(sp2)–N coupling by Jamison and coworkers in 201589 and C(sp2)–O/N coupling by MacMillan and Buchwald and coworkers28,90 in 2015 and 2016, respectively. In the Reductive SET mechanism for C– C bond coupling, Ir(II) interacted with a Ni(I)–bpy halide complex, reducing it by one electron in a dark reaction. Keeping with the naming convention adopted herein, the Oxidative SET mechanism features a SET wherein *Ir(III) oxidizes a Ni(II)–bpy aryl alkoxide complex (right box, Figure 1.4), leading to a Ni(III) species and Ir(II). As in Reductive SET, the Ir(III) complex acts as the sole excited-state active species in Oxidative SET. Ir(II) reduces a Ni(I)–bpy halide species to generate Ni(0)–bpy, which undergoes oxidative addition of an aryl halide coupling partner to form a Ni(II)–bpy aryl halide species (bottom box, Figure 1.4). Ligand substitution of the alcohol (or amine) via the assistance of exogenous base generates the aforementioned four-coordinate, square-planar Ni(II)–bpy aryl 15 alkoxide (right box, Figure 1.4). The critical chemical impetus behind this mechanism is the Ni(III)-promoted reductive elimination of the C–X product, akin to the one-electron oxidation chemistry developed by Hillhouse and coworkers.91,92 Initial reports founded this reaction scheme on the basis of redox potentials and reductive elimination thermodynamics for Ni(II) vs. Ni(III). Figure 1.4. Proposed Oxidative SET mechanism. C(sp2)–O coupling (alcohols) is shown as a representative example. §1-3.1.4. Key Considerations for the Oxidative SET Mechanism 1. Ni(II)–bpy aryl alkoxide is the SET partner with *Ir(III). While oxidation of the Ni(II)–bpy aryl alkoxide species to formal Ni(III) may be necessary to drive reductive elimination, there are additional Ni species present, including the Ni(II)–bpy aryl halide complex. It is currently unclear why *Ir(III) would preferentially oxidize one and not the other. Additionally, if Ir(II) is competent for the reduction of Ni(I) to Ni(0), why either of 16 these Ni(II) species is not also reduced presents an open question. As demonstrated by Diao and coworkers, electrochemical reduction of Ni(II)–bpy aryl halide to Ni(I)–bpy aryl represents an important step in alternative cross-coupling mechanisms.52 Indeed, through electrochemical and computational mechanistic analysis, Oderinde and coworkers presented an alternative mechanism wherein Ni(II)–bpy aryl halide is reduced by Ir(II) to form Ni(I)– bpy aryl.53 This reduction was also suggested to be important through computations by Molander, Gutierrez, and coworkers.93 Thus, there may be additional, alternative routes aiding in or solely responsible for the production of cross-coupled product. Mechanistic analyses of these discrete steps, particularly those involving key interactions between Ir and Ni, are needed. 2. The proposed cycle rests on Ni(0)–bpy/Ni(II)–bpy aryl halide as the starting source of nickel. While Oxidative SET features Ni(0)–bpy to Ni(II)–bpy aryl halide oxidative addition, Buchwald, MacMillan, and co-workers also find that beginning with high-spin (S = 1) Ni(II)–bpy dichloride is suitable for the transformation.90 Indeed, the substrate scope and product yields are all achieved using this NiCl2 starting species, not Ni(0). This switch in Ni precursor presents a dilemma, namely that the electronic structure, redox potential, and behavior of high-spin Ni(II)–bpy dihalide vary considerably compared to the low-spin Ni(II)–bpy aryl halide that arises from Ni(0). Little-to-no experimental mechanistic analysis on this reaction beginning with Ni(0) has been reported.94 Detailed follow-up work was done on this reaction by Nocera and coworkers95 to interrogate the cycle beginning with Ni(II)–bpy dihalide, and their analysis argued against the Oxidative SET mechanism (see SET for Active Ni(I)). Furthermore, in the closely related C(sp2)–O cross-coupling of aryl–acetate substrates,96 a 3EnT mechanism was favored over Oxidative SET by experimental mechanistic work.97 We therefore find it plausible that either the Oxidative SET mechanism is not operative for C(sp2)–X coupling, or it is only operative when beginning with a Ni(0)–bpy/Ni(II)–bpy aryl halide precursor combination—a pathway still underexplored mechanistically. The electronic structure of the Ni precursor is non-trivial for dictating the mechanistic pathway for catalysis (see Section §1-2). 17 §1-3.1.5. Photosensitization for Homolysis (SET vs. 3EnT) Two versions of the Photosensitization for Homolysis mechanism were invoked by concurrent works in 2016, one by Doyle, Shields and coworkers57 and another by Molander and coworkers.98 The two studies identified a simplified version of a C(sp2)–C(sp3) coupling reaction that no longer required an easily oxidized alkyl coupling partner for C• generation. Instead, the groups found ethereal solvent (THF) to be a suitable C(sp3) source. The two versions of the Photosensitization for Homolysis mechanism are shown below, one involving SET (Figure 1.5), another 3EnT (Figure 1.6). The original SET mechanism of Doyle, Shields, and coworkers57 involves oxidative addition of an aryl halide coupling partner to Ni(0)–bpy, affording Ni(II)–bpy aryl halide. Rather than engaging an organic substrate, *Ir(III) oxidizes this Ni(II) complex to a four-coordinate, cationic [Ni(III)–bpy aryl halide]+ species and Ir(II). Ir(III) is no longer the primary lightabsorber in this mechanism. Here the Ni(III) intermediate must undergo photon absorption as well, which promotes halide-to-Ni(III) ligand to metal charge transfer (LMCT). This electron excitation populates a Ni(III)–X antibonding orbital, resulting in an excited-state bond homolysis, ejection of an in-cage X•, and formation of a three-coordinate Ni(II)–bpy aryl species. Note this step is analogous to the excited-state bond homolysis for isolable Ni(III) trihalide species,99,100 wherein the apical Ni(III)–X bond cleaves due to a dissociative LMCT excited-state potential energy surface (PES). The X• abstracts a hydrogen atom from neighboring ethereal solvent (THF in this case), generating an in-cage C• and HCl. The C• is captured by the three-coordinate Ni(II)–bpy aryl species, resulting in the formation of the cationic [Ni(III)–bpy aryl alkyl]+ complex (upper left box, Figure 1.5). Rapid reductive elimination follows, affording C(sp2)–C(sp3) coupled product and Ni(I)–bpy. Ir(II) reduces this Ni(I) species to Ni(0)–bpy, returning Ir(III). 18 Figure 1.5. Proposed Photosensitization for Homolysis Mechanism (Doyle, Shields).57 Figure 1.6. Proposed Photosensitization for Homolysis Mechanism (Molander).98 19 98 Molander and coworkers proposed a related catalytic cycle featuring the same Ni(0) to Ni(II) oxidative addition (bottom of Figure 1.6). However, instead of undergoing subsequent SET, the Ni(II)–bpy aryl halide acts as a 3EnT acceptor from *Ir(III). Thus, in this mechanism, Ir(III) is again the sole light-harvesting species. Upon photosensitization, excited *[Ni(II)–bpy aryl halide] can follow either stepwise out-of-cage or concerted in-cage Ni–X bond homolysis and alkyl solvent capture. The product of either process is a Ni(II)– bpy aryl alkyl species (left box, Figure 1.6), which undergoes thermal reductive elimination to yield the aryl–alkyl coupled product and Ni(0). §1-3.1.6. Key points of the Photosensitization for Homolysis Mechanisms 1. In addition to Ir(III), a Ni–bpy aryl halide species also absorbs light. While the cycle in Figure 1.6 requires photon absorption by Ir(III), additional mechanistic analysis has found that direct irradiation of the reaction mixture with high-energy light (290–315 nm) without the Ir(III) complex also yields the desired cross-coupled product.98 Relatedly, the cycle in Figure 5 necessitates additional photon absorption by a Ni(III) complex in the cycle. As such, identifying 1) the relative absorption cross-sections and 2) the resulting quantum yields of ensuing processes for the Ir(III), Ni(II)–bpy aryl halide, and cationic [Ni(III)–bpy aryl halide]+ species is imperative for evaluating these potential mechanistic pathways. Furthermore, oxidation of Ni(II)–bpy aryl halide has been demonstrated to rapidly afford the aryl halide substrate.36 The proposed intermediacy of a [Ni(III)–bpy aryl halide]+ species is therefore critical. In order for this species to avoid reduction by iridium or aryl halide reductive elimination, it must 1) remain stable in room-temperature solution long enough to outcompete Ir(III) as a light-harvesting species and 2) have an LMCT within the energy range of the excitation source. The Ir(III) complex has a near unity quantum yield (Φ = 1) for *Ir(III) formation,34,67,101 which can react through near-diffusion-controlled quenching with Ni(II)–bpy aryl halide (kq = 109 M-1 s-1)57,95 to regenerate ground-state Ir(III); both *Ir(III) and Ir(II) are thermodynamically suitable reductants for Ni(III). 2. *Ir(III) can undergo SET or 3EnT with Ni(II)–bpy aryl halides. Determination of whether *Ir(III) facilitates SET, 3EnT, or both when combined with Ni(II)–bpy aryl halide is 20 at the core of the Photosensitization for Homolysis mechanisms shown in Figures 1.5-1.6. It is evident from Stern–Volmer analysis that Ni(II)–bpy aryl halide complexes do quench 3 Ir(III) excited states (vide supra), but the mechanism of quenching is still undetermined. Reports by MacMillan, Scholes, and coworkers on Ni(II)–bpy aryl acetate complexes favor 3 EnT over reductive SET,97 but it is possible that halide to acetate ligand exchange sufficiently alters the electronic structure of the compound to favor one mechanism vs. another. Mechanism switches have been reported for Ni(II)–polypyridyl complexes upon halide to acetate ligand substitution.83 Moreover, reduced reactivity was observed by Molander and coworkers when employing strongly oxidizing photocatalysts that were unproductive for 3EnT,98 a result that contrasts the good product yields observed by MacMillan and coworkers when using an external chemical oxidant in place of the photosensitizer.97 Beginning with Ni(0) and aryl halide, Rueping and coworkers102 proposed a similar mechanism to that proposed by Molander and coworkers (Figure 1.6). In this case, strongly oxidizing photocatalysts also did not provide good product yields, but neither did direct excitation of the independently synthesized Ni(II)–bpy alkyl bromide complex. Notably, no direct evidence for Ni–X homolysis was provided in this work. In related reports, Ni(II)–bpy acyl chlorides were examined by Shibasaki and coworkers103 in 2017; they found photosensitizers with high triplet energies and low oxidizing power alone gave good product yields (favoring 3EnT Photosensitization for Homolysis). However, Paixão, König and coworkers104 surmised that beginning with a high-spin Ni(II)–bpy dihalide precursor gave entry via SET into the Photosensitization for Homolysis mechanism (Figure 1.5) and not the 3EnT pathway (Figure 1.6). Notably, alternative routes have been demonstrated for the combination of Ni(II)–bpy dihalide and photocatalyst. Therefore, the electronic structure of the receiving low-spin Ni(II)–bpy complex is susceptible to changes by both ligands, the aryl and the halide, and it is possible that high-spin Ni(II)–bpy dihalide precursors can enter into a variety of pathways upon photosensitization. One should take care when extending mechanistic analysis of one Ni complex to even seemingly similar ones. Careful experimental electronic structure-centered analysis on the mechanism of excitedstate quenching between Ni(II)–bpy aryl halide and *Ir(III) is still needed. 21 3. Halide radicals are photogenerated. Mechanisms outlined in Figure 1.5-1.6 both rest on a critical Ni–X bond homolysis induced via the photosensitizer, either through SET or 3 EnT. Kinetic isotope effect measurements supported the generation of radicals, with halide radicals being favored over aryl radicals by Doyle, Shields, and coworkers.57,105 Evidence of aryl radical generation upon direct excitation of Ni(II)–bpy aryl halide complexes has also been provided on numerous accounts (pathway discussed in Section §1-3.2.1). Evans– Polanyi analysis conducted on the Photosensitization for Homolysis pathway by Doyle and coworkers in 2018 determined an α-value of 0.44,106 near that for the proposed Cl• (αCl = 0.45), but also near CH3• (αCH3 = 0.45), and H• (αH = 0.43)107; the αC6H5 value is unknown. Discrete experiments using Ni(II)–bpy aryl halide in conjunction with chemical oxidant and light source found that 1) the dehalogenated arene, Ar–H, is produced in 40% yield, 2) the direct reductive elimination product, Ar–X, is produced in 12% yield, 3) the solvent-aryl cross-coupled product is produced in 7% yield, and 4) the bis-aryl is produced 2% yield.36 These values, in particular the large amounts of Ar–H, are suggestive of aryl radical generation, not halide radicals. These disparate conclusions call for more detailed work to evaluate these proposed mechanisms. 4. Reductive elimination proceeds from a Ni–bpy aryl alkyl species. In Figure 1.5, crosscoupled product is the result of reductive elimination from an oxidized [Ni(III)–bpy aryl alkyl]+ complex. Indeed, reductive elimination from more highly oxidized Ni complexes is well established.91,92 However, the direct Ni(II)-Ni(0) reductive elimination in Figure 1.6 is less common. Stable Ni(II)–bpy aryl alkyl complexes have been synthesized and isolated by Park and coworkers; reductive elimination does not proceed under irradiation or at elevated temperatures (75 °C).108,109 Thus, it remains unclear whether Ni(II)-Ni(0) C(sp2)–C(sp3) reductive elimination is thermodynamically feasible near room temperature. We note that, shortly before finalizing this Review, a mechanistic study by Doyle and coworkers110 was deposited to the ChemRxiv preprint server. In this detailed work, the authors revisit the abovementioned 2016 proposals, finding that 1) 3EnT from *Ir(III) to Ni(II)–bpy aryl halide promotes reductive elimination of the aryl halide. 2) Direct light absorption by a Ni(II)–bpy aryl halide complex affords an aryl radical and Ni(I)–bpy halide; 22 photohalogen elimination from the Ni(II) complex is not favored. 3) In addition to aryl radical generation, excitation of the Ni(II) complex likely also facilitates excited-state reductive elimination of the aryl halide to afford Ni(0)–bpy. 4) In situ oxidation to [Ni(III)– bpy aryl halide]+ immediately releases the aryl halide at room temperature. However, at cryogenic temperatures, the Ni(III) species persists long enough to absorb an additional photon, again ejecting an aryl radical and not the halogen, in agreement with previous computational work.14 5) C(sp2)–C(sp3) reductive elimination from a Ni(II)–bpy aryl alkyl species is faced with a substantial room temperature barrier of ~25 kcal mol-1. However, absorption of high energy light (390–470 nm) by this complex promotes the formation of aryl–alkyl product. This mechanistic work illustrates the importance of thorough experimental consideration of each step in proposed mechanistic cycles, such as those presented in the Key Points highlighted above. §1-3.1.7. Triplet Energy Transfer (3EnT) In 2017, McCusker, MacMillan and coworkers demonstrated C(sp2)–O cross-coupling of aryls and carboxylic acids;96 product yields correlated with the 3EnT ability of the photocatalyst, not its oxidizing potential, a result that argued against the Oxidative SET mechanism outlined in Figure 1.4. Rather, a 3EnT mechanism was proposed (Figure 1.7). The 3EnT mechanism also features oxidative addition of the aryl halide to Ni(0)–bpy to form a Ni(II)–bpy aryl halide species (left box, Figure 1.7). Base assisted ligand substitution generates a Ni(II)–bpy aryl acetate species (bottom box, Figure 1.7). The Ir(III) complex acts as the primary light-absorbing species to form *Ir(III). This excited state is quenched by the Ni(II)–bpy aryl acetate complex through Dexter EnT (kq = 109 M-1 s-1 by Stern–Volmer analysis)95,98 affording ground-state Ir(III) and an excited-state Ni(II) complex, *[Ni(II)–bpy aryl acetate]. Reductive elimination was proposed to occur from a Ni(II)-based ligand field excited state, resulting in C(sp2)–O cross-coupled product and Ni(0)–bpy. 23 Figure 1.7. Proposed 3EnT Mechanism. C(sp2)–O coupling (carboxylic acids) coupling is shown as a representative example. §1-3.1.8. Key Components of the 3EnT Mechanism 1. Ir(III) is the primary light-harvesting species, but potentially not the only one. While it is clear from Stern–Volmer analysis that the Ni(II)–bpy aryl acetate quenches *Ir(III), excitation without the photosensitizer also results in cross-coupled product (albeit with slower kinetics under the given conditions).96 Furthermore, the precursor Ni(II)–bpy aryl halide species also absorbs light;37 direct excitation of Ni(II)–bpy aryl halide in the presence of cross-coupling partners is productive for C–O bond formation (discussed in Section §1-3.2.2),36,79 marking at least three distinct light-harvesting species present in the reaction mixture (Figure 1.2). It is unclear if one or more mechanisms are operative. However, given the photosensitizer’s high absorption cross-section and quantum yield for 3 Ir(III) formation,101 its excitation may be dominant. 2. 3EnT, not SET, is the dominant mechanism promoting photosensitized crosscoupling reactions. The proposed divergence away from SET to favor 3EnT for C(sp2)–O 24 111 coupling seems contingent upon the presence of the Ni(II)–bpy aryl acetate species. Previous reports favored SET to the Ni(II)–bpy aryl halide precursor (vide supra, Section §1-3.1.3). However, several observations support 3EnT from *Ir(III) to the Ni(II)–bpy aryl acetate species: 1) the lack of reactivity with a chemical reductant, 2) a 3EnT threshold for product yield of ~40 kcal mol-1, 3) product generation upon direct excitation of the Ni complex alone, and 4) an inverse correlation between product and oxidizing power of the photocatalyst.96 Furthermore, computational analysis by Chen and coworkers13 and followup ns transient absorption studies on C(sp2)–O coupling reactivity by MacMillan, Scholes, and coworkers97 demonstrated additional support for the 3EnT mechanism, although in the latter, chemical oxidant was still effective for product yield. Recent work by Oderinde, Hudson, and coworkers finds that a series of organic 3EnT photosensitizers are also competent photocatalysts for C(sp2)–O esterification reactions when used in combination with Ni(II)–bpy aryl acetate species.112 It may be the case that both the aryl halide and aryl acetate complexes undergo 3EnT with *Ir(III), but only in the case of the aryl acetate is it irreversible and productive for catalysis. The aryl halide coupling partner has also been implicated as redox non-innocent. Pieber, Seeberger, and coworkers113 conducted a kinetic analysis of the aryl–acetate C(sp2)–O coupling presented by McCusker, MacMillan and coworkers in 2017, but they began with Ar–I instead of the Ar–Br substrates. Evidence supported rapid SET from *Ir(III) to Ar–I; this off-cycle electron transfer, which resulted in a dehalogenated Ar–H product, was said to be involved in turnover-limiting oxidative addition of the substrate to Ni(0). However, the Ni precursor used was Ni(II)–bpy dihalide, not Ni(0), making the presence of Ni(0) somewhat speculative (see Point 3 below). Nonetheless, the authors cited previous work to propose that 3EnT between *Ir(III) and Ni(II)–bpy aryl acetate was the active mechanism for C–O coupled product formation.113 3. The proposed cycle rests on the Ni(0)–bpy/Ni(II)–bpy aryl halide combination as the starting source of nickel. As in the Oxidative SET Mechanism, the reaction was initiated with a Ni(0) source to give the Ni(II)–bpy aryl halide precatalyst. However, optimized reaction conditions utilized high-spin Ni(II)–bpy dihalide as the precursor nickel source.96 25 3 The same complication is then introduced in this EnT proposal. It is unclear if the experimental mechanistic analysis conducted between the Ir photosensitizer and the Ni(II)– bpy aryl acetate complex holds true for the Ir and Ni(II)–bpy dihalide combination. Important and complementary analysis was conducted by Li, Huang, Zhang and coworkers114 in 2018 on the aryl–acetate coupling reaction but using organic photosensitizers in place of the Ir(III) and beginning with Ni(II)–bpy dihalide. Under these conditions, strongly oxidizing photocatalysts gave an undesired dehalogenation of the aryl halide and no cross-coupled product, consistent with the 2017 study.96 Transitioning to 3EnT-active photosensitizers with lowered oxidation potentials gave some product yield, but still saw ~20% dehalogenation product. Thus, it was reasoned that when using Ni(II)–bpy dihalide as the precursor species, SET is favored over 3EnT and proceeds prior to energy transfer.114 Only careful and deliberate suppression of oxidation allows for 3EnT to become the major pathway. Under standard conditions (i.e., with Ir(III)), SET is therefore likely the sole or dominant mechanism, but it is unclear from these studies if the oxidation occurs with Ni, with the substrate coupling partners, or with the exogenous amine base. This electron transfer event is explicitly considered in the SET for Active Ni(I) Mechanism (Section §1-3.1.9), a proposal that appears to be the principal pathway when combining Ir(III) and Ni(II)–bpy dihalide, thereby marking a critical mechanistic switch when using high- vs. low-spin Ni(II) precursors. It is clear from these studies that the highly potent and versatile *Ir(III) may engage in multiple pathways at once, including 3EnT and SET. Diversion from one route to another depends on the Ni catalyst precursor, substrate coupling partners, exogenous base, and even photon intensity.58,115 §1-3.1.9. SET for Active Ni(I) Dual Ir/Ni cross-coupling reactions beginning with Ni(II)–bpy dihalide precursors have seen wide use, largely due to their incredible substrate scope potential. In 2016, Oderinde, Johannes, and coworkers uncovered C(sp2)–S coupling reactivity by combining an aryl 26 69 halide and thiol with Ni(II)–bpy dichloride and Ir(III). Interestingly, it was found through Stern–Volmer analysis and radical traps that the thiol appreciably quenches *Ir(III) (k = 105 M-1 s-1) to generate thiyl radicals and Ir(II). Ir(II) reduces the high-spin Ni(II)–bpy dihalide complex, affording Ni(I)–bpy halide and Ir(III) (Figure 1.8). The Ni(I)–bpy halide is at the core of the catalytic cycle turnover. First, it is intercepted by the thiyl radical to make a Ni(II)–bpy halide sulfide complex, which is reduced by a second equivalent of Ir(II) to Ni(I)–bpy sulfide. This Ni(I) undergoes oxidative addition with aryl halide to form a Ni(III)– bpy aryl halide sulfide species. This complex undergoes rapid reductive elimination to return Ni(I)–bpy halide and the C(sp2)–S coupled product (Figure 1.8).116 The generalizability of the reaction became evident by the extension to C(sp2)–N coupling of aryls and amines.117 Inspired by the 2018 work of Miyake and coworkers,118 which demonstrated the formation of arylamines upon direct excitation of a nickel–amine complex formed in situ form Ni(II)Br2, Neurock, Minteer, Baran, and coworkers55 eliminated the photocatalyst, demonstrating that the Ni(II)-Ni(I) initiation step could be achieved through applied electrochemical potential. As discussed in the Ni Electronic Structure Primer (Section 2), the high-spin Ni(II)–bpy dihalide has a less-negative (more accessible) reduction potential than a Ni(II)–bpy aryl halide species, an additional species formed in the catalytic cycle. By controlling the applied potential, the researchers ruled out Oxidative SET; electrochemical oxidation of Ni(II)–bpy aryl halide did not occur within the solvent window. Ni(I)–bpy halide intermediates were observed spectroelectrochemically and were demonstrated to be active toward oxidative addition, thereby facilitating reaction turnover.55 27 Figure 1.8. Proposed SET for Active Ni(I) Mechanism for C(sp2)–S coupling.69 While examining the C(sp2)–O coupling of aryl halide and alcohol coupling partners pioneered by MacMillan and coworkers28 in 2015 (see Oxidative SET, Figure 1.4, above), Nocera and coworkers95 instead found support for the SET for Active Ni(I) mechanism (Figure 1.9). Again, Ir(III) is the sole light-harvesting species, being promoted to *Ir(III). Like in the case of aryl thiolate formation, the exogenous base used for the ligand substitution step (i.e., quinuclidine), was found to quench *Ir(III) with ease, generating Ir(II) and amine cation radicals. Ir(II) reduces the Ni(II)–bpy dihalide to Ni(I)–bpy halide and Ir(III). This Ni(I) species undergoes oxidative addition to form a Ni(III)–bpy aryl dihalide species, followed by ligand substitution of the halide by the alcohol/alkoxide and subsequent reductive elimination of the C(sp2)–O product. However, it was also proposed that the Ni(I)– bpy halide species can be diverted via comproportionation with the Ni(III)–bpy aryl dihalide species to regenerate the S = 1 Ni(II)–bpy dihalide complex and the S = 0 Ni(II)–bpy aryl 28 halide species. The former acts to regenerate the cycle, while the latter presents an offpathway sink for diminished catalysis. Indeed, the Ni(II)–bpy aryl halide itself can aggregate with the Ni(I)–bpy halide, as observed by Nocera and coworkers95 (using X-ray crystallography and electron paramagnetic resonance), as well as Hadt and coworkers56 (using temperature-dependent spectroscopic methods). Figure 1.9. Proposed SET for Active Ni(I) Mechanism. C(sp2)–O coupling (alcohols) is shown as a representative example. Following these three studies, researchers began to revisit previous mechanistic proposals. In 2020, MacMillan and coworkers67 conducted a detailed study on the mechanism proposed in 2016 for C(sp2)–N coupling90 (see Oxidative SET, Figure 1.4) and found the SET for Active Ni(I) Mechanism was better supported by their data, despite using the amine base DABCO (1,4-diazabicyclo[2.2. 2]octane) instead of quinuclidine. DABCO quenches *Ir(III) 29 with a quantum yield near unity (Φ > 0.99) and near-diffusion-controlled kinetics (k = 10 9 M-1 s-1), affording amine•+ and Ir(II). This initial quenching step was confirmed using spectroelectrochemistry and transient absorption spectroscopy and was not found to be sensitive to the addition of either aryl halide or high-spin Ni(II)–bpy dihalide. When varying the photosensitizer, the data further indicated that SET from Ir(II) to Ni(II) was involved in the rate-determining step of the overall catalytic cycle.67 §1-3.1.10. Key Components of the SET for Active Ni(I) Mechanism 1. Ir(III) is the sole light-harvesting species, and its excited state is quenched by exogenous organic base (e.g., amine or thiol in solution) to generate Ir(II). *Ir(III) is quenched by the amine or thiol in solution (Figures 8-9); this fact has been verified by numerous sources via Stern–Volmer analysis.67,95,118–120 In the case of C(sp2)–S coupling, twelve discrete rate constants have been elucidated, altogether pointing to a self-sustained Ni(I)/(III) cycle with product Φ > 1.119 Nocera and coworkers also demonstrated that Ni(II)– bpy dihalide is an effective quencher of *Ir(III), but with a quenching rate constant ~six times smaller than that of quinuclidine.95 Interestingly, the Ni(II)–bpy aryl alkoxide complex generated by comproportionation of Ni(I) and Ni(III) also quenches *Ir(III), but with a rate constant twice as large as that of quinuclidine.95 It is therefore possible that the crosscoupling reaction begins with the SET for Active Ni(I) Mechanism, but once sufficient concentration of the Ni(II)–bpy aryl alkoxide species is generated, the mechanism diverts to one in which the S = 0 Ni(II) species is the dominant quencher (e.g., the 3EnT mechanism discussed above). Computational evidence by Liu, Tlili, and by Zhu and Guan and their coworkers lends preliminary support to this hypothesis.121,122 A switch in mechanism, or multiple, simultaneous kinetically competing mechanisms occurring in dual Ir/Ni(II)–bpy dihalide catalysis is likely. MacMillan found that C(sp2)–N coupling for arylamines proceeded rapidly with DABCO present, but it was not switched off with DABCO absent; the reaction rate decreased by ~nine times as a function of decreasing DABCO concentration, but 14% product yield was still obtained without DABCO.67 Thus, the more kinetically active mechanism involves the quenching of the amine, but *Ir(III) is 30 also quenched by other species present in solution (including high-spin Ni(II)–bpy dihalide). Discrete reactivity pathways with *Ir(III) and Ni(II)–bpy dihalide in the absence of other quenchers were also examined, finding that no Ir(II) spectral features were observed without the organic quencher and, thus, invoking another cycle that does not involve the reduced Ir(II) species as an intermediate.67 This analysis was computationally extended by Su, Guan, and coworkers,123 then experimentally by Escobar, Thordarson, Johannes, Miyake, and coworkers.124 It was proposed through ns transient absorption spectroscopy and oxidative spectroelectrochemistry that *Ir(III) is oxidatively quenched by high-spin Ni(II)– bpy dihalide to give Ni(I)–bpy halide and Ir(IV);124 computations by Su in 2018 also favor this pathway.125 The Ir(IV) is reduced downstream to return the starting Ir(III) by SET from a redox non-innocent aryl halide substrate.124 2. Ni(II)–bpy dihalide is the precursor source of Ni and is reduced by Ir(II) to Ni(I)– bpy halide. The reduction of Ni(II) to Ni(I) by Ir(II) has been demonstrated multiple times (vide supra). Additionally, the importance of Ni(I) for product yields was established through a photosensitizer screen by MacMillan and coworkers.67 Analysis of a library of Ir(III) photocatalysts demonstrated that product yields and reaction rates trend with Ir(II) reduction potential. However, the Ir(II) reduction potential also trends well with 3EnT capability. The first non-functioning photosensitizer has an emission energy of ~45 kcal mol-1 and a reduction potential of –0.77 V vs. SCE. This energy transfer threshold is similar to that used to support 3EnT,96 which makes these trends alone a poor distinction between mechanisms. However, the first functioning photosensitizer (albeit with low product yields of ~3%) makes a notable exception to this trend. It has an emission energy of ~62 kcal mol-1 and reduction potential of –1.23 V vs. SCE. The best performing Ir photocatalyst (100% yield) has a 3EnT potential of 61 kcal mol-1 but reduction potential of –1.91 V vs. SCE, confirming that reduction potential, not 3EnT energy, is a good predictor of productive catalysis.67 In accord, Rovis and coworkers utilized a red light-absorbing Os(II) photocatalyst with a highly reducing potential to successfully achieve C(sp2)–N coupling.126 Therefore, Ni(I) formation is on-pathway and required for product formation. This result was further supported by Nocera and coworkers, who found an induction period when organic quenchers were not 31 present to transform *Ir(III) to Ir(II), the preferred species for Ni(II) to Ni(I) reduction, 119 and by Liu, Tlili, and coworkers who leveraged active Ni(I) for C(sp2)–N coupling with CO2 as electrophile.121 3. Ni(I)–bpy halide supports a dark Ni(I)/(III) cycle. The reactivity of Ni(I)–bpy halide toward oxidative addition and subsequent reductive elimination is well established.70,127 Recent mechanistic studies have identified rate constants for the reaction between Ni(I)–bpy halide and aryl iodides, bromides, and chlorides.56,60,82,105 The immediate product of this reaction has been confirmed as Ni(III) by comparison to model complexes.82,128 Importantly, Ni(I)–bpy halide species undergo dimerization or oligomerization to binuclear or polynuclear Ni species, respectively, representing significant off-cycle deactivation pathways.56,82,95,129 Because of the exponential rate law dependence on the Ni(I) concentration in these reactions, maintaining lower Ni(I) concentration leads to improved cross-coupling yields. Indeed, by modulating the flux of the incident light to minimize the rate of *Ir(III) formation (and therefore the downstream concentration of Ni(I) at a given time), the quantum yield for product formation could be increased ~15 times (from Φ = 1.6 to Φ = 25).95 The observation of a quantum yield greater than one at even high flux levels supported a dark Ni(I)/(III) cycle. A similar observation was made for aryl thiolate formation.119 The prevalence of the SET for Active Ni(I) Mechanism cannot be overstated. König reports a general reaction beginning with high-spin Ni(II)–bpy dihalide that is competent for C(sp2)– X (X = C(sp, sp2, sp3), S, Se, N, O, P, B, Si, Cl) coupling.130 Mechanistic work is needed on a case-by-case basis, but the authors propose that this dramatic substrate scope is largely, if not fully, dominated by a Ni(I)/(III) self-sustaining cycle, even when alternative mechanisms are possible. 32 §1-3.2. Direct Excitation This section, divided into two parts, describes research invoking direct photon absorption by specific Ni-based species involved in catalytic cycles. The first part considers cases where direct photoexcitation generates Ni-based intermediates for dark reactions that mediate crosscoupling. The second considers cases in which the cross-coupling events occur directly from transient excited states. §1-3.2.1. Direct Excitation for Dark Cycle Initiation 1. Photoinduced Ni–X Bond Homolysis Photoinduced Ni–X bond homolysis has been proposed as an initiation step that generates reactive species involved in key dark reactions. Often drawing comparisons to photohalogen elimination from five-coordinate Ni complexes, researchers have proposed pathways for metal–halide bond homolysis from four-coordinate Ni (Figure 1.10). These include the generation of triplet excited states of Ni(II)–bpy aryl halide species, photohalogen elimination from a Ni(III)–bpy aryl halide intermediate, the generation of intraligand charge transfer excited states that relax to dissociative metal-based excited states, and the direct generation of photoactive triplet metal-centered excited states of Ni(II) complexes with triplet ground states. In 2016, Molander and coworkers reported C(sp3)–H arylation using a Ni(II)–bpy aryl bromide catalyst98 (Section §1-3.1.5, Figure 1.4; Figure 1.10C). Arylated product was observed when the reaction was carried out with an Ir(III) photosensitizer, as discussed above. However, it was noted that arylated product could be detected when the reaction mixture was irradiated at specific wavelengths without the Ir(III) photosensitizer. Visible-light excitation (~400–600 nm) did not lead to product, while UV-B irradiation (290– 315 nm) did. Based on these results and additional control photosensitization experiments, a catalytic mechanism was proposed that included a triplet excited state responsible for Ni reactivity. Mechanistic scenarios were presented for the generation of reactive Ni intermediates, all of which involved Ni(II)–Br bond homolysis. In the case of direct UV-B 33 excitation, a high-energy singlet excited state was proposed to relax via nonradiative intersystem crossing to a triplet excited state from which the Ni(II)–Br homolysis could occur.98 As discussed above, the resultant bromine radicals were suggested to activate THF through hydrogen atom abstraction, and coupling to the aryl ligand could occur via reductive elimination from Ni(II) in an overall Ni(0)/Ni(II) cycle (see Figure 1.4). Also in 2016, Doyle and coworkers reported a C(sp3)–H cross-coupling platform with Ni(II)– bpy aryl chloride and an Ir(III) photocatalyst57 (Section §1-3.1.5, Figure 1.4; Figure 1.10D). As discussed above, the Ir(III) photocatalyst is proposed to carry out reductive SET to generate Ni(0) species for oxidative addition with the aryl chloride, as well as oxidative SET to oxidize the Ni(II)–bpy aryl chloride. The resulting cationic [Ni(III)–bpy aryl chloride]+ intermediate was proposed to undergo direct photon absorption to drive excited-state Ni(III)– Cl bond homolysis via an LMCT state in a manner analogous to an isolable pentacoordinate Ni(III) species99,100 (Figure 1.10A). Ni(II)–X bond homolysis was further proposed in a Ni(II)–bpy dihalide S = 1 system that featured a dicarbazolyl functionalized bpy ligand (Figure 1.10E).131 The extended ligand alters the bpy orbital energies levels such that intra-ligand charge transfer (ILCT) states are present in the visible region. Combining transient absorption with computational analysis, the mechanism of Ni(II)–Cl bond homolysis was proposed to involve an initial excitation into a 3ILCT state, followed by relaxation into an optically dark square-planar metal-centered state. This state was proposed to feature antibonding character along the Ni–halide bond, thereby facilitating Ni(II)–X bond homolysis and formation of catalytically relevant Ni(I) species. 34 Figure 1.10. Proposed direct excitation for photohalogen elimination from five-coordinate Ni complexes studied by (A) Nocera95,96 and (B) Mirica,122 and from four-coordinate Ni complexes studied by (C) Molander,94 (D) Doyle, Shields,56 and (E) van der Veen, Thomas, and Pieber.125 2. Photoinduced Ni–C Bond Homolysis. In addition to excited-state Ni–X bond homolysis, recent studies have invoked analogous Ni–C bond homolysis.36–38 For Ni(II)–bpy complexes, after photoexcitation and carbon radical formation, the resultant Ni(I)–bpy halides mediate dark chemistry leading to the cross-coupled products (including C(sp2)–C(sp3), O, N, S coupling)36,79,109,132,133 via the proposed mechanism outlined in Figure 1.11. In 2018, Doyle and coworkers utilized transient absorption spectroscopy to study the excitedstate dynamics of isolable Ni(II)–bpy aryl halide compounds.36 It was proposed that the 35 3 excitation of the complex resulted in MLCT excited states that could undergo bimolecular electron transfer with a ground-state Ni(II)-bpy aryl halide species. The downstream result of this photochemical process was aryl ligand loss and the generation of a three-coordinate Ni(I)–bpy halide that would engage in Ni(I)/Ni(III) oxidative addition/reductive elimination cycles for C(sp2)–O cross-coupled product formation. However, later studies conducted by MacMillan, Scholes, and coworkers97 indicated that this bimolecular photoinduced disproportionation pathway is not operative, as Stern–Volmer studies did not find appreciable Ni(II) excited-state quenching by ground-state Ni(II)–bpy aryl halide. In a subsequent 2020 study, Doyle and coworkers expanded on their earlier transient absorption analysis and proposed an alternative excited-state relaxation pathway that could lead to Ni(II)–C(aryl) bond homolysis via a triplet ligand field (3d-d) excited state of Ni(II)– bpy aryl halides.37 Transient spectroscopic measurements carried out with either a 530–590 nm (for transient absorption) or 610 nm (time-resolved IR) laser pump demonstrated that initial excitation dominantly populates a 1MLCT excited-state manifold, which can relax through additional MLCT states to ultimately form the 3d-d state. By correlating density functional theory (DFT) calculations to transient absorption and 2D exchange NMR experiments, it was proposed that the 3d-d state features a pseudo-Td geometry (see Figure 1.1B) that can be accessed photochemically or thermally at room temperature. This pseudoTd geometry featured electron population of a σ* orbital, reducing the Ni–aryl bond order to one half, thereby activating it for thermally driven homolysis. In support of that, DFT predicted a significantly weaker Ni(II)–Cl bond in the 3d-d excited state (~24 kcal mol-1) vs. the ground state (~35 kcal mol-1). Both 1H NMR and electron paramagnetic resonance (EPR) spectroscopy confirmed the generation of aryl radicals; no chlorine radicals were trapped, arguing against photoinduced Ni(II)–Cl homolysis.37 Also in 2020, Hadt and coworkers explored mechanistic aspects of excited-state Ni(II)– C(aryl) bond homolysis from Ni(II)–bpy aryl halides using quantum chemical calculations of both ground- and excited-state PESs.12 Multireference/multiconfigurational calculations suggested intractable energies for thermal bond dissociation from the lowest-energy 3d-d state, with calculated bond strengths differing significantly from those predicted by DFT. 36 This study also suggested an alternative mechanism of excited-state Ni(II)–C(aryl) bond homolysis that featured 1) initial 1MLCT formation and 2) intersystem crossing and aryl-toNi LMCT to form repulsive triplet excited-state PESs.12 These MLCT/LMCT surfaces featured a Ni(II)–C(aryl) σ → σ* electron excitation, which reduces the bond order to zero, hence the repulsive excited-state PESs. Notably, such description of dissociative excitedstate bond homolysis conceptually resembles the isolable pentacoordinate Ni(III) photochemistry,99 as well as the mixed MLCT/σπ* (σ bond to ligand charge transfer) photoinduced radical formation in Re(I) and Ru(II) complexes.136–138 In 2021, Park and coworkers proposed an analogous mechanism, utilizing Ni(II) complexes with cyclic ligands inherently predisposed to facile photochemical reductive elimination.109 Regardless of the bidentate backbone ligand (diimine, diamine, or diphosphine), all studied complexes exhibited photoactivity under irradiation. The 3LMCT excited-state PES was suggested to initiate carbon radical generation. This could occur through pathways from a preferred 1MLCT state in aromatic diimine complexes or from a 1d-d state in aliphatic diamine or diphosphine complexes lacking low-lying unoccupied ligand-based orbitals. Additionally, both 1d-d and 1MLCT excited states may operate simultaneously, with their ratio depending on the ground-state complexes' electronic structure and molar absorptivities. Importantly, charge transfer excitations exhibit orders of magnitude higher molar absorptivity than ligand field transitions, but vibronic coupling with a weakly absorbing, dissociative triplet state can potentially mediate intersystem crossing and Ni–C bond homolysis. 37 Figure 1.11. Proposed direct excitation for Ni(II)–C(aryl) bond homolysis and C(sp2)–O product formation mechanism (as a representative example). Following light-initiation, Ni(I)–bpy halide participates in “dark” substrate turnover but can be deactivated via off-cycle dimerization. Following their earlier work, Hadt and coworkers provided further exploration of the excitedstate bond homolysis mechanism.38 In 2022, experimental analysis of a library of Ni(II)–bpy aryl halides found that 1) Ni(II)–C(aryl) bond homolysis was dependent on the bpy (MLCT acceptor) and aryl (LMCT donor) ligand substituents. A linear relationship was found for bpy/aryl Hammett parameters139 and the rate constants and quantum yields for photochemical aryl radical generation; switching the halide from Cl to Br to I increased the rate of Ni–C bond homolysis. Notably, the quantum yields were very low (Φ = 10–3–10–4 at 390 nm for Ni(II)–bpy aryl halides). 2) Temperature-dependent rate analysis revealed that there existed a modest barrier for excited-state Ni(II)–C(aryl) bond homolysis (~4 kcal mol1 ); this barrier was well below the predicted values for thermal dissociation from the 3d-d 38 excited state. 3) Quantum yields and rate constants for excited-state bond homolysis were highly wavelength-dependent; excitation into the lowest-energy MLCT (which relaxes to the 3 d-d state) was unproductive. Only high-energy light (>525 nm, ~55 kcal mol-1) afforded aryl radicals. These experiments, alongside an expanded computational analysis, supported a dissociative MLCT/LMCT excited state for C(aryl)• generation.38 Experiments further supported that Ni(I)–bpy halide species were the products of the unimolecular excited-state bond homolysis.56 In 2024, Hadt and coworkers expanded their photochemical analysis of Ni(II)–bpy aryl halides to a Ni(II)–Phbpy chloride complex that features a covalent bond between the aryl ligand and the bpy backbone.140 This geometrically constrained complex demonstrated apparent photochemical stability over a broad wavelength range. However, evidence for Ni(I) generation upon irradiation was demonstrated through a reaction with an introduced aryl bromide, wherein substrate activation outcompeted radical recombination of the tethered aryl group. From transient absorption experiments, the structural constraint of the ligand prevented access to a 3d-d state by prohibiting the formation of a pseudo-Td geometry (see Section §1-2 for electronic structure implications). The retention of light-promoted activation of an electrophile in this tethered complex again suggested that the triplet charge transfer dissociative pathway (MLCT/LMCT) likely facilitates excited-state Ni–C bond homolysis. As noted above, due to the small quantum yields for Ni(II)–C(aryl) homolysis, transient spectroscopies largely probe unproductive background excited-state relaxation processes that do not lead to the formation of Ni(I) intermediates and organic radicals, making the assignment of the photochemical pathway challenging and inconclusive.41 We briefly note that in addition to Ni(II)–C(aryl) bond homolysis, Ni(II)–C(alkyl) homolysis has been observed. For example, Park prepared Ni(II)–bpy methyl thiolate complexes to corroborate the possibility of carbon radical formation (see above).109 These complexes were designed to prefer irreversible Ni(II)–C(alkyl) bond homolysis and yielded ethane as the dominant photoproduct, thereby confirming methyl radical generation. Furthermore, aliphatic nickellacycles generated β-hydride elimination products under 390 nm irradiation. Similarly, Oderinde and coworkers questioned if Ni(II)–bpy dimethyl catalysts could 39 2 3 141 mediate C(sp )–C(sp ) cross-coupling reactions under visible-light irradiation. Indeed, stoichiometric cross-coupled products were observed upon irradiation of the Ni(II) complex alongside 5-bromophthalide substrate using blue or violet light. Methyl radicals were confirmed via EPR radical-trap experiments followed by GC-MS analysis. These results reveal that alkyl radical formation through a Ni(II)–C(alkyl) bond homolysis pathway can be photoinduced with a variety of ligand backbones, including sulfur-ligated systems, an aromatic ligand (such as bpy), an aliphatic ligand (such as TMEDA), and occurs in both cyclic and acyclic compounds. §1-3.2.2. Direct Excitation for Reductive Elimination Excited states that serve to either 1) oxidize Ni via Ni-to-ligand change transfer transitions or 2) populate Ni–ligand antibonding orbitals via ligand field or ligand-to-Ni transitions have been suggested to promote direct intramolecular reductive elimination of organic substrates. In either case, experimental mechanistic analysis is lacking, marking an opportunity for interdisciplinary follow-up analysis. Absorption of a photon by Ni(II)–bpy complexes has been suggested to drive excited-state reductive elimination. Originally conceptualized by the McCusker and MacMillan groups in 2017,96 it was proposed that direct excitation of a Ni(II)–bpy aryl acetate complex ultimately resulted in the population of a low-energy triplet ligand field state, from which intramolecular reductive elimination could afford an aryl-acetate product with new C(sp2)–O bond and a reduced Ni(0)–bpy complex. Follow-up ns transient absorption on a mixture of Ir(III) photosensitizer and Ni(II)–bpy aryl acetate was conducted in 2020, wherein it was surmised that *Ir(III) underwent Dexter EnT to a ground-state Ni(II) complex (see Section §1-3.1.7 above), populating a long-lived triplet excited state. The nature of this state, i.e., charge transfer vs. metal centered, was not described. It was proposed, however, that this excited state was active for reductive elimination. Notably, ultrafast transient absorption on the independent excited-state dynamics of the Ni(II)–bpy aryl acetate was not presented. Computational assessment of the excited-state relaxation pathways of a Ni(II)–bpy aryl acetate was undertaken by Ma and coworkers that same year.13 Therein, it was proposed that 40 142,143 direct excitation of the Ni(II) complex into a high-energy, anti-Kasha •– Ni(III)–bpy MLCT state was responsible for intramolecular reductive elimination, where the oxidation of the Ni(II) center serves as the driving force for substrate formation.91,92,144 Excited-state-driven reductive elimination was also seen by Lloyd-Jones and coworkers on Ni(II)–bpy aryl halides.88 Energy transfer from *Ir(III) to generate a triplet excited-state Ni(II) complex resulted in the formation of an aryl halide substrate and Ni(0)—a reversible process as oxidative addition from Ni(0) is readily accessible at room temperature. Following these results, the Lin and Doyle groups noted that direct excitation of the Ni complex is also productive for the same reductive elimination/oxidative addition equilibrium.110,145 Indeed, this reversible light-driven chemistry was utilized for ligand exchange, promoting a (retro)Finkelstein reaction. The long-lived excited state of Ni(II)–bpy aryl halides is a 3d-d state, which is populated after relaxation from higher-energy MLCT states.37 It is unclear if a charge transfer or metal-centered excited state is productive for reductive elimination; further experimental mechanistic work is still needed to elucidate the photophysics of this process. Direct aryl–alkyl C(sp2)–C(sp3) reductive elimination from excited-state, high-valent Ni(III/IV)–bpy complexes was demonstrated by Park in 2020.108 In this case, a LMCT promoted the cross-coupled product via the population of a Ni–C σ*-orbital, increasing the rate of substrate formation by up to a factor 105 when compared to thermal, dark reactivity. Interestingly, these complexes were penta-coordinate, suggesting similarities to the lightdriven Ni–X homolysis reactivity seen by Nocera, Mirica, and coworkers.99,100,128 A recent report by the Doyle group finds evidence for excited-state intramolecular C(sp2)–C(sp3) reductive elimination from Ni(II)–bpy aryl alkyl complexes;110 the mechanism for this process is presently unknown. §1-3.2.3. Key Components of Direct Excitation 1. Ni(II)–bpy aryl halides are light-harvesting species. The absorptive nature of Ni(II)– bpy aryl halide complexes is well described. The primary absorption features in the visible-light region are Ni(II)-to-bpy MLCT in nature, with molar absorptivities in the range of 103–104 M-1 cm-1.37,38 Hadt and coworkers found that these transitions can be further 41 separated into low- and high-energy MLCTs, with the various bpy π* acceptor orbitals marking the difference between the two.12,38 The low-energy bands are typically ~ 415–580 nm, while the high-energy bands are found between ~ 340–400 nm, with additional transitions extending into the higher-energy region. If present, d-d bands are likely obscured by MLCT transitions due to their relatively low molar absorptivities (101–102 M-1 cm-1) (Figure 1.2). While low-temperature magnetic circular dichroism (MCD) can enhance ligand field transitions relative to charge transfer due to different selection rules relative to UV–vis, the greatest utility involves C-term intensity, which requires a paramagnetic ground state.146 The ground states of Ni(II)–bpy aryl halide compounds are diamagnetic, however, which would lead to an absence of C-term intensity. Thus, additional spectroscopic methods may be required to locate and assign ligand field excited states in these compounds. 2p3d resonant inelastic X-ray scattering (RIXS) may be a viable technique due to its ability to resonantly excite metal-based states.147–149 Higher-energy incident wavelengths (< 330 nm) populate ILCT (bpy π → π*) transitions. These have been found to relax into the MLCT manifold via transient absorption spectroscopy.36,131 Similarly, the MLCT transitions relax into d-d excited states before ultimately returning to the ground state.37,150 2. Ni(I)–bpy halide is produced via direct excitation, not Ni(I)–bpy aryl. Although the very low quantum yields for photoinduced Ni–ligand homolysis from Ni(II)–bpy aryl halides have made them challenging to study by transient spectroscopies, steady-state methods including UV–vis, NMR, EPR, and GC-MS have verified the formation of aryl radicals, not halogen radicals, upon light absorption (vide supra). Indeed, to the best of our knowledge, no direct experimental evidence of halogen radical production upon irradiation of Ni(II)–bpy aryl halide complexes has been provided. The exclusive observation of aryl radicals is in contrast to the early mechanistic proposals by Molander, Doyle, and Shields.57,98 These 2016 reports featuring key Ni(II/III)–X bond homolysis refer to the work by Nocera and coworkers developed in the context of HX splitting for solar energy storage.99,100 It is important to highlight the distinctions in the photohalogen elimination chemistry between these systems. In HX splitting, the Ni(III) species is an isolable, penta-coordinate Ni(III)– dppe trichloride complex (dppe = bis(diphenylphosphino)ethane). Here, a common 42 dissociative excited-state surface is accessed upon 370 and 434 nm irradiation and was proposed to be responsible for the photoelimination of the apical chlorine ligand (Figure 1.10A). Based on time-dependent DFT calculations (TDDFT), the Ni(III)–Cl bond cleavage was attributed to a LMCT excitation into the unoccupied p(z)/d(z2) antibonding Ni-based hole, reducing the bond order between Ni(III) and the apical Cl to zero. The resulting photoproduct, Ni(II)(dppe)Cl2, exhibited a square-planar structure with a singlet ground state; no further halogen photoelimination occurred. In 2022, Mirica and Na reported a study128 with a similar isolable, five-coordinate Ni(II) complex that featured a tridentate pyridinophane ligand (Figure 1.10B). Their proposed mechanism also featured chlorine photoelimination, both Ir(III)-facilitated via SET and under direct light excitation of the S = 1 Ni(II) complex. The latter displayed accessible triplet Ni-to-ligand charge transfer states. Excitation into one of these triplet states promoted an electron from the Ni 3d-orbital manifold, thus generating a transient Ni(III) complex. It was proposed that subsequent relaxation gave rise to a 3d-d state with significant σ* character along the Ni–Cl bond, triggering homolysis. Although chlorine radical trapping experiments were not presented, the lability of the Ni–Cl bonds was demonstrated in the chemically oxidized cationic Ni(III) complex. Thus, photohalogen elimination is possible with penta-coordinate Ni(II/III) di- or trihalide complexes, but it is disfavored when using four-coordinate Ni(II/III) aryl halide complexes. The MLCT/LMCT process seen in Ni(II)–bpy aryl halide complexes is akin to that considered using time-dependent density functional theory (TDDFT) in the Ni(III)–Cl photohalogen elimination chemistry, but with the distinct difference that the LMCT originates from an aryl donor, not the halide. This is unsurprising, as the DFT predicted bond dissociation energy for Ni(II)–X homolysis is roughly twice that of Ni(II)–C(aryl).37 Interestingly, the Ni(II)–C(aryl) homolysis pathway is promiscuous with respect to the backbone ligand, as demonstrated by Park and coworkers and by Hadt and coworkers for aliphatic TMEDA ligands.38,109 It is therefore the presence of the aryl group that governs the selectivity for radical generation. 43 This notion has been corroborated experimentally. Xue and coworkers 133 demonstrated that even when replacing the halide with a stronger field ligand, as in the Ni(II)–bpy aryl cyanide complex, aryl radicals are still preferentially generated upon light absorption. Here, the starting Ni(II) complex was initially presented as a potential reactive intermediate in C(sp2)–N coupling reactions, with the authors suggesting that reductive elimination from a Ni(III) state would yield C(sp2)–N coupled products. Reductive elimination was indeed observed after single-electron oxidation via an excited photosensitizer. However, in the absence of photosensitizer, no reductive elimination occurred; the reaction instead resulted in biphenyl product formation, suggesting photochemical Ni(II)–C(aryl) bond homolysis (which was later confirmed by EPR). The formation of the d9 Ni(I)–bpy cyanide intermediate upon irradiation of the parent Ni(II) structure was also confirmed by EPR.133 Furthermore, recent work suggests that high-spin (S = 1) Ni(II)–bpy dihalide (and Ni(II)–TMEDA dihalide) engage in photohalogen elimination upon direct excitation with high-energy light, thereby recovering the photohalogen elimination pathway by removing the aryl group from the parent Ni(II) complex.110,151–153 Similarly, photo-pseudo-halogen elimination from Ni(II)–bpy diacetate has been proposed by Xue and coworkers when using purple light.154– 156 3. Ni(I)–bpy halide is the active species for oxidative addition, but also suffers from offcycle dimerization. The potency of Ni(I)–bpy halide for the oxidative addition of aryl halide substrates has been demonstrated experimentally. Hammett analysis by the groups of Bird and MacMillan, Sigman, and Doyle suggested that aryl iodides and bromides are activated via a concerted, two-electron oxidative addition mechanism.60,82,157 When studying reactivity of Ni(I)–bpy halides with aryl chlorides, Hadt and coworkers found a relatively higher ρ value from Hammett analysis, which suggested the activation step is characterized by a concerted, two-electron nucleophilic substitution mechanism (SNAr).56 For this, the nucleophilic site is the doubly occupied 3d(z2) orbital, which carries out a two-electron transfer into the C(sp2)–Cl σ* orbital. It was found that the reactivity of the Ni(I)–bpy halide species can be tuned via the energy of this orbital; bpy ligand modifications alter the effective nuclear charge (Zeff) of the Ni(I) and serve to increase (via electron-donating groups) or 44 decrease (via electron-withdrawing groups) the reactivity of the complex toward substrate activation. Although one might speculate that cross-coupling reactions would be accelerated by increased Ni(I) concentration in solution, Nocera and coworkers found that doing so (by increasing the photon flux of the irradiation source) was actually detrimental to productive catalysis.95 At a high concentration of Ni(I), the low-valent species is prone to either direct dimerization to yield formal [Ni(I)–bpy halide]2 complexes (Figure 1.11) or to aggregation with the parent Ni(II)–bpy aryl halide species—both of which are off-cycle products. On the basis of X-ray photoelectron spectroscopy on a synthesized [Ni(I)–bpy halide]2 complexes, Hazari and coworkers described this species as a dimeric Ni(II)–bpy•– halide.129 It was also found to be inert toward aryl halide substrates.129,157 Importantly, the same electronic structure effects that govern the reactivity of Ni(I)–bpy halide species also dictate their tendency toward this dimerization pathway; electron-rich ligands promote dimerization, while electron-deficient ligands slow or fully inhibit room temperature dimerization (ΔG‡ ~25 kcal mol-1).56 Therefore, the relative rates of Ni(I)–bpy halide formation, substrate activation, and off-cycle dimerization should be considered when optimizing catalytic cycles. 4. Penta-coordinate Ni(III) undergoes reductive elimination and/or comproportionation to close the cycle. As has been discussed above, Ni(III) is prone to rapid reductive elimination. Indeed, mechanistic work by Mirica and Doyle and coworkers found that reductive elimination is facile, even at low temperatures.110,128 In good agreement with these experimental observations, DFT calculations by Hadt and coworkers suggest that reductive elimination of an aryl halide from a Ni(III)(bpy)(aryl)X2 species is effectively barrierless.56 Nonetheless, under continuous irradiation and formation of both Ni(I) and Ni(III) species, there exists a non-negligible resting state concentration of Ni(III) in solution that can undergo comproportionation with Ni(I) to afford S = 0 Ni(II)–bpy aryl halide and S = 1 Ni(II)–bpy dihalide.158,159 This has been experimentally demonstrated by Doyle and coworkers in 202284 and 2024110 and by Hadt and coworkers in 202356 by NMR analysis. The low-spin Ni(II)–bpy aryl halide is thereby returned to the catalytic cycle, where it can absorb a photon and be transformed anew to Ni(I)–bpy halide (Figure 1.11). However, S = 45 1 Ni(II)–bpy dihalide accumulates following comproportionation over multiple turnovers, potentially diverting the cycle to one beginning at the high spin species. Detailed mechanistic work on the photophysical pathway upon direct excitation of Ni(II)–bpy dihalide complexes is still needed. §1-4. Conclusions and Outlook In this Review, we have provided a summary of the various mechanisms presented for metallaphotoredox reactions featuring Ni(II)–bpy catalysts. A few common mechanistic observations have arisen: 1) Commonly employed photosensitizers such as cyclometalated Ir(III) heteroleptic complexes can engage in numerous excited-state quenching pathways, including 3EnT and reductive/oxidative SET, complicating mechanistic analysis. Furthermore, in metallaphotoredox cycles there are often numerous species capable of quenching the photosensitizer excited state, organic and inorganic alike. The precise mechanism of quenching, and the quenching species, are still largely debated. Detailed experimental mechanistic work (including Stern–Volmer analysis) is needed for individual steps in Ir/Ni dual photocatalytic cycles. 2) The electronic structure of the Ni(II)–bpy species present in the reaction governs the specific mechanistic pathway it follows; this electronic structure is highly sensitive to both ligands and the surrounding environment. To highlight a few key structures, Ni(II)–bpy aryl halides and Ni(II)–bpy aryl acetates are both photosensitized species and direct lightharvesters. Ni(II)–bpy aryl halides undergo photochemical Ni(II)–C(aryl) bond homolysis, a key step for initiation into Ni(I)/Ni(III) dark cycles, while Ni(II)–bpy aryl acetates have been proposed to follow an excited-state intramolecular reductive elimination path. Ni(II)–bpy dihalide species are primarily photosensitized complexes, typically resulting in a reduced Ni(I)–bpy halide. However, with high-energy light, these complexes have been proposed to also engage in direct *Ni(II)–X bond homolysis to form the same Ni(I) intermediate. 46 3) Regardless of the Ni(I)–bpy halide photochemical generation mechanism, a consistent Ni(I)/Ni(III) cycle has been proposed for most cross-coupling reactions. The key catalytic steps consist of an oxidative addition of electrophile (such as aryl halide) to Ni(I) to form a reactive Ni(III)(bpy)(Ar)X2 intermediate. This Ni(III) can undergo X ligand exchange with a nucleophile (presumably facilitated by a base), followed by reductive elimination to form the cross-coupled product and regenerate Ni(I)–bpy halide. Continuous irradiation is often necessary, likely due to the formation of off-cycle, catalytically inactive Ni(II)–bpy dihalide via Ni(I)/Ni(III) comproportionation. However, continuous high photon flux leads to an accumulation of Ni(I)–bpy halide, resulting in aggregation with other Ni species in solution or dimerization to [Ni(I)/Ni(I)] off-cycle products. Viewing these light-driven mechanisms from an electronic structure perspective has granted key considerations for the steps in each cycle, particularly pertaining to Ni-based intermediates. Although tremendous advancements have been made in reaction development in this field, unified, experimentally supported mechanisms are still lacking for many of the reactions. Due to the inherent complexity in these cycles, care should be taken to evaluate individual steps independently. We hope that presenting this Review from the standpoint of the outlined key considerations has provided a model logic from which to conduct such analyses. Finally, thermally driven enantioselective cross-coupling catalysis, often utilizing metal-based reductants instead of photochemical processes, feature similar complementary mechanistic considerations.6,10,61,62,160,161 As such, mechanistic studies in this field are of direct relevance for designing future studies. We hope research groups from the physical, organic, and inorganic fields will take on interdisciplinary research to elucidate the precise mechanisms of Ni-mediated cross-coupling catalysis, providing rationale to improve product scope and reaction efficiency and laying a foundation to extend catalyst activity to other transition metals. 47 §1-5. References (1) Chen, X.; Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Palladium(II)-Catalyzed C–H Activation/C–C Cross-Coupling Reactions: Versatility and Practicality. Angew. Chem. Int. Ed. 2009, 48 (28), 5094–5115. https://doi.org/10.1002/anie.200806273. (2) Torborg, C.; Beller, M. 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ACS Catal. 2020, 10 (15), 8237–8246. https://doi.org/10.1021/acscatal.0c01842. 63 Chapter 2 Multireference Description of Nickel–Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis Adapted with permission from: Cagan, D.A.;† Stroscio, G.D.;† Cusumano, A.Q.; Hadt, R.G. Multireference Description of Nickel–Aryl Bond Dissociation Processes in Photoredox Catalysis. J. Phys. Chem. A. 2020, 124(48), 9915-9922. DOI: 10.1021/acs.jpca.0c08646. †Co-first author. Copyright 2020 American Chemical Society. 64 §2-1. Introduction Merging thermal catalysis with photochemistry (i.e., photoredox catalysis) has provided new, more sustainable routes to bond activations and coupling reactions in organic synthesis.1–10 An extension of solar energy conversion, photoredox catalysis utilizes photosensitizers to harvest photon energy and transform it into chemical potential to drive single electron transfer (SET) processes to generate reactive high- and/or low-valent species and important organic radicals. However, photoredox reactions feature complex mechanisms that are challenging to elucidate, and our understanding of how photon energy drives organic transformations is therefore still growing. Beyond SET, photosensitizer energy transfer can form photocatalyst excited states that can be uniquely reactive relative to ground states.11–16 The photocatalyst can also potentially act as both the light-absorbing and catalytic unit through direct excitation.17,18 In direct excitation and energy-transfer-mediated catalysis, the ultrafast photophysical processes of transition metal excited-state relaxation can also contribute to reactivity.9 Notably, Ni(II) complexes of 2,2′-bipyridine (bpy) exhibit photocatalytic activity for coupling reactions using either energy transfer11,19 or direct excitation.20 Several mechanistic hypotheses have been discussed and are summarized in Figure 1. In one scenario, energy transfer to a Ni(II)–bpy aryl acetate complex induces reductive elimination from a triplet excited state of Ni(II) (Figure 2.1A, bottom), originally proposed to be ligand field in nature.11 Ni(II)-to-bpy metalto-ligand charge transfer (MLCT) excited states have also been suggested to: 1) mediate bimolecular electron transfer to generate Ni(I) and Ni(III) species for catalysis,17 or 2) directly mediate reductive elimination.21 The latter consideration encompasses the oneelectron oxidatively induced ground state formal Ni(III) reactivity discovered by Hillhouse and co-workers.22,23 65 Figure 2.1. (A) Two previous mechanistic hypotheses related to Ni–bpy photoredox catalysis and (B) findings in this study. Complex 1 = Ni(II)(t-Bubpy)(o-tolyl)Cl; Complex 2 = Ni(II)(bpy)(ph)(ac) (ph = phenyl, ac = acetate). Thermally assisted homolytic Ni(II)–C bond dissociation from photochemically formed triplet ligand field excited states in Ni(II)–bpy aryl halide complexes has also been proposed. This process results in the formation of formal Ni(I) and aryl radicals (Figure 2.1A, top).18 While it is unclear whether these species initiate a subsequent Ni(I)/Ni(III) catalytic cycle, this represents an intriguing means to photochemically generate reduced Ni species and organic radicals for ground-state thermal catalysis.18,20 Overall, more detailed experimental and theoretical descriptions of the ground-state bonding and excited-state relaxation processes in Ni–bpy complexes (and other Ni heteroaromatic complexes24) are critical for developing synthetic applications. 66 Here we describe a new electronic structural framework to interpret experimental data on Ni(II)–bpy complexes of relevance to photoredox catalysis. Of particular importance is the multireference description (relative to DFT), which manifests in mixed ground and excitedstate wave functions and potential energy surfaces (PESs) in Ni(II)–C homolytic bond dissociation. Intractable barriers are found for thermal bond dissociation from the lowestenergy triplet ligand field excited state within the multireference framework. However, higher-energy repulsive triplet excited states are found here and are proposed to be responsible for homolytic bond dissociation. These triplet excited states feature a high-spin Ni(II) coupled to anionic bpy and neutral aryl radicals and can be generated from initial 1 MLCT excitation (Ni(II)-to-bpy) followed by intersystem crossing and intramolecular charge transfer (aryl-to-Ni(III)) (Figure 2.1B). §2-2. Computational Methods Calculations were performed using ORCA25,26 version 4.2.1. The BP8627-29 functional was used for geometry optimizations and frequency calculations, including both full geometry optimizations and constrained optimizations where the Ni–C bond length was systematically varied. The 6-311G(d)30 basis set was used on all atoms, and AutoAux31 was used as the auxiliary basis set. Split-RI-J, the default and recommended version of resolution of identities32–35 (RI) approximation was used. The finest available DFT grids were used (GRID7 NOFINALGRID). Very tight SCF convergence criteria, which has a convergence tolerance of 10-9 Hartrees, was applied for all DFT calculations. The restricted Kohn–Sham formalism (RKS) was used for the singlet ground-state optimizations; the unrestricted Kohn– Sham formalism (UKS) was used for the triplet optimizations. Additional single point calculations using the B3LYP28,36 functional, the def2-TZVP37 basis, and implicit solvation by tetrahydrofuran (THF) modeled by the conductor-like polarizable continuum model38 (CPCM) were performed on optimized structures. Here the RIJCOX39 approximation was used with fine DFT grids (GRID7 NOFINALGRID GRIDX9). At this level of theory, broken-symmetry singlet (BSS) and unrestricted triplet single point calculations were performed on the S = 0 and S = 1 optimized geometries, respectively. Likewise, TDDFT calculations were performed using these same settings. Applying a Yamaguchi spin 67 40,41 correction did not significantly affect the BSS dissociations energies of 1 and 2. It lowered the dissociation energy of 1 from 43.3 kcal mol-1 to 41.7 kcal mol-1 (Table S1P); it barely changed the dissociation energy of 2 from 44.9 kcal mol-1 to 45.1 kcal mol-1 (Table S2L). Sample input DFT and TDDFT parameters are given on page S3. Quasidegenerate N-electron valence second-order perturbation theory42 (QD-NEVPT2) corrected complete active space self-consistent field (CASSCF) single point calculations were performed on DFT-optimized geometries. As suggested in the documentation for ORCA CASSCF-based calculations, tight SCF convergence criteria with an energy tolerance of 10-7 Hartrees were applied. The def2-TZVP basis set was used on all atoms, and the RIJCOSX approximation was employed. Note that the number of states averaged was varied (see Table S1G-1–S1G-3), and it was found that a state-averaging with fifteen singlets and twenty-five triplets yielded a thorough description of the ground and excited states of interest while maintaining reasonable computational costs. Therefore, state-averaged CASSCF/QDNEVPT2 single point calculations utilized fifteen singlets and twenty-five triplets throughout. The recommended Nakano formalism was used, and the corresponding CI vectors are tabulated below. A comparison between gas phase and solvent corrected CASSCF/QD-NEVPT2 single point calculations yielded qualitatively similar results at both the singlet equilibrium geometry and at longer Ni–C distances (3.2 Å for 1 and 3.1 Å for 2); therefore, gas phase calculations were conducted on all structures (comparisons are tabulated below). Sample input files for CASSCF/QD-NEVPT2 calculations are given on page S3. The size of the active space was varied until a thorough description of 1 and 2 was reached (comparisons between active space sizes are tabulated below). Active spaces are shown in Figures S1C and S1E for 1 (S = 0 and S = 1) and Figure S2C and S2E for 2 (S = 0 and S = 1). The first 10-20 lowest-energy roots, CI vectors, transitions, and oscillator strengths are tabulated below. An active space consisting of nine orbitals filled with ten electrons (9o/10e): d(xy), d(z2), d(xz), d(yz), a pair of bonding and antibonding orbitals from the d(x2–y2) and the C(sp2) orbital on the dissociating phenyl group, and three π* orbitals on the bipyridine ligand, were found to be thorough descriptors of the S=0 equilibrium geometry of 1, while 68 an additional orbital was added for 2. The additional orbital in 2 is a bonding d(xy)/C(π) orbital (see Figure S2C), which was kept in the active space due to its partially unfilled occupancy of 1.93 (for compound 1, this orbital has occupancy of ~2 (1.99), and thus was not needed to generate a complete active space). However, as can be seen in Table S2C1-3, the additional orbital in 2 was not involved in any critical transitions. At all other Ni–C bond lengths, the third bpy π* orbital exhibited very low occupancy and was removed to aid convergence. For example, the ten electrons in nine orbitals CASSCF calculation using the 3.6 Å geometry of 1, the third π* orbital had an extremely low active space occupancy value of 0.00004. As the Ni–C bond was elongated and eventually cleaved, the molecular geometry along the singlet surface approached that of the optimized triplet surface. This observation is particularly clear for 2. Geometry coordinates along the Ni–C scan are listed in the Appendix portion of the Supporting Information. The active space for the triplet scan of 1 again consisted of the ten electrons in eight orbitals (active space with the third π* removed, Figure S1E). Here the third bpy π* again had very low active space occupancy values (~0.0001). This was true in geometries ranging from 2.0 Å to 3.6 Å. For triplet structures of 1 with a short Ni–C bond (between 1.6-2.0 Å), the second π* orbital was similarly removed to aid convergence. For the triplet scan of 2, it was possible to use an active space of ten electrons in nine orbitals active space for the entire scan (Figure S2E). §2-3. Results and Discussion Homolytic bond dissociation is an inherently multireference process that can pose difficulties for DFT.43,44 Analyses therefore began by comparing the ground-state wave functions of Ni(II)(t-Bubpy)(o-tolyl)Cl (1) and Ni(II)(bpy)(ph)(ac) (ph = phenyl, ac = acetate) (2), as well as their lowest-energy singlet and triplet bond dissociation energies (BDEs) using both DFT and multireference ab initio calculations (i.e., CASSCF and quasidegenerate N-electron valence state second-order perturbation theory (QD-NEVPT242)) within ORCA25,26 (see Supporting Information for computational details). 69 Figure 2.2. Simplified MO diagram of 1 as calculated by CASSCF/QD-NEVPT2 using a nine orbital, ten electron active space. Natural orbitals energies are plotted; orbital occupancies are labeled. CASSCF/QD-NEVPT2 calculations on 1 and 2 exhibit appreciable ground-state multiconfigurational character (S1C/S2C and Tables S1C/S2C-1). Using a nine orbital, ten electron active space (Figure 2.2, S1C), the dominant contributions to the configuration interaction (CI) vector of the singlet ground state of 1 are ~58 % low-spin d8 (closed-shell singlet, CSS) and ~22 % 1MLCT (Table S1E). Similar values are obtained for 2 (~57% CSS and ~23 % 1MLCT) (Table S2D-2). With only an eight electron, five 3d-orbital active space, the low-spin d8 character increases to ~95 % in both 1 and 2 (Table S1A-2/S2B-2). Thus, the unoccupied bpy π* orbitals, which have high active space occupancies, play a critical role in the degree of multiconfigurational ground-state bonding. 70 It is interesting to consider the multireference data in the context of the DFT bonding description. The low-spin d(x2–y2) ground states of 1 and 2 are highly covalent (~56/57 % Ni(II) and ~11/13 % bpy character), with some back-bonding (~7-8 % occupied Ni(II) character in the bpy-based unoccupied π* orbitals of both 1 and 2) (Figure S1B/S2B). This highly covalent bonding framework is not particularly amenable to formal redox state assignment and is more consistent with a multiconfigurational bonding description.45,46 PESs for Ni(II)–C bond dissociations from 1 and 2 are given in Figures 2.3A and S2G, respectively. The DFT BDEs are ~43 kcal mol-1 and ~31 kcal mol-1 starting from the relaxed, lowest-energy singlet and triplet structures of 1, respectively, consistent with the ~32 kcal mol-1 from a study invoking thermal homolysis on the triplet PES.18 Values for 2 are similar (~45 kcal mol-1 and ~38 kcal mol-1). The multireference bond dissociation is fundamentally different than DFT, with significantly higher BDEs (~87/65 kcal mol-1 and 73/70 kcal mol-1 from the lowest-energy singlet and triplet states of 1/2), suggesting the Ni(II)–C bonds are stronger than in DFT and will not be thermally cleaved, even upon formation of the relaxed lowest-energy triplet ligand field excited state. This difference is important, as the ~30 kcal mol-1 barrier was used to rationalize photochemical formation of radicals and reduced Ni species from 1.18 71 Figure 2.3. Ni(II)–C bond dissociation from the lowest-energy singlet and triplet states in 1. (A) Relaxed DFT vs. CASSCF/QD-NEVPT2 PESs and (B) DFT Löwdin spin densities for both the singlet (BSS) and triplet states and (C) the CASSCF/QD-NEVPT2 lowest-energy singlet CI vector. From Löwdin spin density plots in Figures 2.3B and S2H, the DFT-based homolytic bond dissociation results in the formation of Ni(I) and neutral aryl radicals for 1 and 2. The 72 compositions of the multiconfigurational ground-state CI vectors of 1 and 2 upon bond dissociation from the singlet ground state are given in Figures 2.3C and S2I, respectively, and describe the nature of bond homolysis. Upon initial elongation of the Ni–C bond, the amount of low-spin d8 character (CSS) decreases significantly, with a concomitant increase in the weighting of 1MLCT character at 2.4 Å, beyond which the CI vector becomes dominantly d(xz)/d(x2–y2) → C(sp2)*/π*, formally corresponding to a high-spin Ni(II) coupled to anionic bpy and neutral aryl radicals. Some additional formal Ni(I) character is also present (~7 %). Independent DFT vs. multireference calculations on the formal Ni(I) species after homolytic bond dissociation (Figure 2.4) further support this description. Notably, similar ligand redox has been observed for reduced formal Ni(I) species in groundstate cross-coupling reactions.24,47 Figure 2.4. DFT (left) vs. CASSCF/QD-NEVPT2 (right) description of the formal Ni(I) species formed upon homolytic Ni–C bond cleavage. Given the intractability of thermally assisted Ni(II)–C homolysis and radical formation from the lowest-energy triplet ligand field excited states, we now further describe the excited-state PESs/manifolds of 1 and 2 to develop new understanding of the mechanism of homolytic bond dissociation. TDDFT and CASSCF/QD-NEVPT2 calculated excited-state manifolds at the ground-state singlet relaxed structures of 1 and 2 are given in Figure S1G and Tables S1IK, S2A, and S2C. Both methods predict a set of lower and higher-energy 1,3MLCTs. 73 However, their relative oscillator strengths differ somewhat from one another and, for 1, the experimental spectrum. The CASSCF/QD-NEVPT2 ground and excited-state PESs along the Ni(II)–C coordinate of 1 and 2 are given in Figure 2.5 and S2J, respectively, while the analogous TDDFT PESs are given in Figure S1F and S2K, respectively. From Figure 2.5, repulsive excited states are present (left panel: black, red, and yellow lines). The higher-energy MLCT excited states (A in Figure 2.5, left) cross the repulsive surfaces at Ni–C bond distances of ~2.3–2.4 Å (circled in Figure 2.5) in both 1 and 2 with an activation energy from the Franck-Condon point of ~25 kcal mol-1. Thus, the multireference approach predicts homolytic bond dissociation occurs via population of a 1MLCT excited state (Ni(II)-to-bpy) followed by an intersystem crossing and intramolecular charge transfer (aryl-to-Ni(III)) (Figure 2.1B). The resulting multiconfigurational species can be described as a high-spin Ni(II) with antiferromagnetically coupled electrons on the bpy and phenyl ligands (see Supporting Information Tables S1Q–R and S2M–N for more details). Interestingly, this description is conceptually similar to that given for the mixed MLCT/σπ* (sigma bond to ligand charge transfer) photoinduced radical formation in Re(I) and Ru(II) complexes.48-57 Here, the intersystem crossing could occur between the 1,3MLCT states or the 1MLCT and dissociative triplet state. Overall, this represents a novel homolytic bond dissociation mechanism in nickel catalysis, which we propose derives from the redox noninnocent and multiconfigurational ground and excited-state bonding in Ni(II)–bpy complexes. Experimentally, the lowest-energy triplet ligand field excited states of Ni(II)–bpy aryl halide complexes are populated in ~5-10 ps.18 Given an estimated Ni(II)–C frequency of ~250 cm-1, ~40-80 vibrational periods could occur to drive intersystem and surface crossings that could compete with population of the lowest-energy triplet ligand field state. A higher-energy aryl vibration (~650 cm-1; Figures S1I/S2L for 1 and 2, respectively) exhibiting significant changes in Ni–C bond distance may also provide ~100-200 vibrational periods to drive these processes. 74 Figure 2.5. CASSCF/QD-NEVPT2 relaxed ground- and excited-state PESs along the Ni–C coordinate of 1. Left: Vertical excitation (black vertical arrow), the higher (A) and lowerenergy (B) manifolds of MLCTs, and the crossings between the higher-energy MLCTs and repulsive triplets (circled) are depicted. Singlet states, circles; triplets, squares. Right: Simplified depiction of the UV light photoinduced Ni–C bond homolysis process. The yield of cross-coupled product obtained from direct excitation of 1 is incident light dependent; high yields are only observed with UV light (390-395 nm or ~70 kcal mol-1),20 corresponding to excitation into the higher-energy manifold of MLCT states (Figure 2.5). Of particular relevance to compound 2, variations in the energy of the photosensitizer triplet state demonstrated C–O coupling occurs when ~40-45 kcal mol-1 is transferred to the Ni catalyst.11 This energy would excite complexes to the lower-energy manifold of MLCT states (Figure 2.5, S2J), resulting in thermodynamically unfavorable radical formation (~45 kcal mol-1) for both 1 and 2. Thus, an alternative relaxation pathway and mechanism may exist for photosensitized cross-coupling. In fact, triplet ligand field excited-state formation, reductive elimination, and homolytic bond dissociation may all be possible for a given Ni(II)–bpy complex. We believe the ligands in addition to bpy will be of particular importance in determining the relative propensity for specific relaxation pathways. For example, reductive elimination is disfavored for the aryl halide (1) relative to the aryl carboxylate (2). This may preferentially lead to excited-state processes that favor the 75 formation of radicals and ligand field excited states over an intractable photosensitized or direct excitation induced reductive elimination. A future study will aim to elucidate and analyze these potentially competitive pathways. §2-4. Conclusions In summary, we have provided a new electronic structural framework to interpret UV lightinduced homolytic bond dissociation in Ni(II)–bpy complexes of relevance for photoredox catalysis. Compared to DFT, multireference ab initio calculations predict: 1) thermal homolysis from the lowest-energy triplet ligand field excited state is not energetically favorable (barriers: DFT ~30 kcal mol-1, CASSCF/QD-NEVPT2 ~70 kcal mol-1), 2) initial population of a Ni(II)-to-bpy 1MLCT excited state can be followed by intersystem crossing and aryl-to-Ni(III) intramolecular charge transfer, resulting in the formation of repulsive triplet excited states described as a high-spin Ni(II) coupled to anionic bpy and neutral aryl radicals. Formally, this represents an overall two-electron transfer process driven by a single photon. 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(42) Angeli, C.; Borini, S.; Cestari, M.; Cimiraglia, R. A Quasidegenerate Formulation of the Second Order N-Electron Valence State Perturbation Theory Approach. J. Chem. Phys. 2004, 121, 4043–4049. (43) Bao, J. L.; Odoh, S. O.; Gagliardi, L.; Truhlar, D. G. Predicting Bond Dissociation Energies of Transition-Metal Compounds by Multiconfiguration Pair-Density Functional Theory and Second-Order Perturbation Theory Based on Correlated Participating Orbitals and Separated Pairs. J. Chem. Theory Comput. 2017, 13, 616–626. (44) Cramer, C. J.; Truhlar, D. G. Density Functional Theory for Transition Metals and Transition Metal Chemistry. Phys. Chem. Chem. Phys. 2009, 11, 10757–10816. (45) Wilson, S. A.; Kroll, T.; Decréau, R. A.; Hocking, R. K.; Lundberg, M.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. Iron L-Edge X-Ray Absorption Spectroscopy of Oxy- 80 Picket Fence Porphyrin: Experimental Insight into Fe–O2 Bonding. J. Am. Chem. Soc. 2013, 135, 1124–1136. (46) Yan, J. J.; Kroll, T.; Baker, M. L.; Wilson, S. A.; Decréau, R.; Lundberg, M.; Sokaras, D.; Glatzel, P.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. Resonant Inelastic X-Ray Scattering Determination of the Electronic Structure of Oxyhemoglobin and Its Model Complex. Proc. Natl. Acad. Sci. 2019, 116, 2854–2859. (47) Diccianni, J. B.; Diao, T. Mechanisms of Nickel-Catalyzed Cross-Coupling Reactions. Trends Chem. 2019, 1, 830–844. (48) Rossenaar, B. D.; Kleverlaan, C. J.; Stufkens, D. J.; Oskam, A. Photochemistry of ReR(CO)3(Pri-dab)(R=Me, Et, Bn; dab=1,4-diazabuta-1,3-diene): Homolysis of the Re–R bond, its Dependence on R and Evidence for the Reactive σbπ* State from Transient Absorption Spectra. J. Chem. Soc., Chem. Commun. 1994, 63-64. (49) Nieuwenhuis, H. A.; van de Ven, M. C. E.; Stufkens, D. J.; Oskam, A.; Goubitz, K. Photochemistry of [Ru(I)(iPr)(CO)2(iPr-DAB)] (iPr-DAB = N,N'-diisopropyl-1,4-diaza-1,3butadiene): Homolysis of the Metal-Alkyl Bond from the σb(Ru-iPr)π* State. Crystal Structure of the Photoproduct [Ru(I)2(CO)2(iPr-DAB)]. Organometallics 1995, 14, 780-788. (50) Rossenaar, B. D.; George, M. W.; Johnson, F. P. A.; Stufkens, D. J.; Turner, J. J.; Vlček, A., Jr. First Direct Structural Information on a Reactive σπ* Excited State: Time-Resolved UV–Vis and IR Spectroscopic Study of Re(benzyl)(CO)3(iPr-DAB). J. Am. Chem. Soc. 1995, 117, 11582-11583. (51) Rossenaar, B. D.; Lindsay, E.; Stufkens, D. J.; Vlček, A., Jr. Properties and Dynamics of the σ(M'–Re)π* Excited State of Photoreactive Dinuclear LnM'–Re(CO)3(α-diimine) (LnM' = Ph3Sn, (CO)5Mn, (CO)5Re; α-diimine = bpy', iPr-PyCa, iPr-DAB) Complexes Studied by Time-Resolved Emission and Absorption Spectroscopies. Inorg. Chim. Acta 1996, 250, 5-14. (52) Aarnts, M. P.; Wilms, M. P.; Stufkens, D. J.; Baerends, E. J.; Vlček, A., Jr. σ–π* Electronic Transition of the Di- and Trinuclear Complexes Ru(E)(E')(CO)2(iPr-DAB): Resonance Raman, Electronic Absorption, Emission, and Density Functional Study (E = Me, SnPh3, M(CO)5; E' = M(CO)5; M = Mn, Re; iPr-DAB = N,N'-Diisopropyl-1,4-diaza-1,3butadiene). Organometallics 1997, 16, 2055-2062. (53) Stufkens, D. J.; Aarnts, M. P.; Rossenaar, B. D.; Vlček, A., Jr. A New Series of Re- and Ru-Complexes Having a Lowest σπ* Excited State That Varies from Reactive to Stable and Long Lived. Pure Appl. Chem. 1997, 69, 831-835. 81 (54) Kleverlaan, C. J.; Martino, D. M.; van Slageren, J.; van Willigen, H.; Stufkens, D. J.; Oskam, A. FT-EPR Study of Alkyl Radicals Formed in the Photochemical Reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) Complexes. Appl. Magn. Reson. 1998, 15, 203-214. (55) Kleverlaan, C. J.; Stufkens, D. J.; Clark, I. P.; George, M. W.; Turner, J. J.; Martino, D. M.; van Willigen, H.; Vlček, A., Jr. Photoinduced Radical Formation from the Complexes [Re(R)(CO)3(4,4'-Me2-bpy)] (R = CH3, CD3, Et, iPr, Bz): A Nanosecond Time-Resolved Emission, UV–Vis and IR Absorption, and FT-EPR Study. J. Am. Chem. Soc. 1998, 120, 10871-10879. (56) Weinstein, J. A.; van Slageren, J.; Stufkens, D. J.; Záliš, S.; George, M. W. A TimeResolved Infrared Spectroscopic Study of [M(SnR3)2(CO)2(α-diimine)] (M = Ru, Os; R = Ph, Me): Evidence of Charge Redistribution in the Lowest-Excited State. J. Chem. Soc., Dalton Trans. 2001, 2587-2592. (57) Stufkens, D. J.; Vlček, A., Jr., Ligand-dependent Excited State Behaviour of Re(I) and Ru(II) Carbonyl–Diimine Complexes. Coord. Chem. Rev. 1998, 177, 127-179. 82 Chapter 3 Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts Adapted with permission from: Cagan, D. A.; Bím, D.; Silva, B.; Kazmierczak, N. P.; McNicholas, B. J.; Hadt, R. G. Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts. J. Am. Chem. Soc. 10.1021/jacs.2c01356. Copyright 2022 American Chemical Society. 2022, 144(14), 6516-6531. DOI: 83 §3-1. Introduction §3-1.1 Ni(II)–bpy Photoredox Catalysis Merging thermal catalysis with photochemistry (i.e., photoredox catalysis) has had a profound influence within organic chemistry, including coupling reactions forging sp2– sp3 and sp3–sp3 C–C or C–X bonds and their applications to medicinal chemistry.1–9 By leveraging photonic energy to drive key catalytic processes and utilizing earth-abundant transition metals, photoredox catalysis provides an attractive and sustainable means to replace precious metal catalysts.10–14 The disparate electron-transfer properties of first-row transition-metal catalysts can also provide pathways to new reactive intermediates and/or excited-state avenues that can unlock synthetic possibilities for drug development and discovery. However, while methodological studies have demonstrated the power of photoredox approaches in achieving bond-forming reactivity, the mechanisms that underlie these processes are largely unknown. In response, recent research has taken key steps toward a deeper mechanistic understanding, utilizing a combination of experiment and theory.4,5,15−19 Mechanistic survey of photoredox catalysts requires thorough exploration due to the numerous possible photophysical pathways present. For example, reactive molecular excited states can be generated photochemically through photosensitized energy transfer17,20−22 or direct excitation.16,23,24 In either case, the ensuing transition-metal photophysics will strongly influence the overall catalytic efficacy by directing the photonic energy to specific pathways, only some of which may be productive to the target reaction. This complexity motivates highly detailed studies of the excited-state PESs that govern the important photophysics underlying photoredox catalysis. Being catalytically active via photosensitization or direct excitation, Ni(II) complexes featuring the bidentate 2,2′–bipyridine (bpy) ligand have received a great deal of attention due to their many applications in photoredox catalysis. For example, MacMillan et al. demonstrated a photosensitized, energy-transfer-mediated approach to enable Ni(II)–bpycatalyzed coupling of aryl halides with carboxylic acids.20 In particular, an Ir(III) 84 photosensitizer enabled triplet energy transfer to a ground-state Ni(II)–bpy aryl acetate complex (formed in situ from a Ni(II)–bpy aryl halide). Energy transfer from the Ir(III) complex generates a long-lived triplet excited state of the Ni(II)–bpy complex, which can subsequently undergo reductive elimination of the aryl and acetate ligands, forming a new C–O bond (Figure 3.1A, top).25 The mechanism of this photosensitized, energy-transfermediated reaction is still being investigated. However, ab initio calculations have suggested that a triplet metal-to-ligand charge-transfer (3MLCT) state may be active for excited-state C–O bond formation.26 Furthermore, the chemical oxidation of the groundstate Ni(II)–bpy complex also facilitates reductive elimination.25 These ground- and excited-state pathways are consistent with earlier research from the Hillhouse group demonstrating that the ground-state chemical oxidation of Ni(II) complexes to Ni(III) can trigger reductive elimination and the formation of new C–X bonds.27,28 In addition to energy-transfer pathways, photocatalytic cross-couplings can also be driven by direct excitation and can circumvent the need for external photosensitizers, which often contain precious metals. For example, irradiation of the Ni(II)–bpy aryl halide complex in the presence of ancillary ligands enables the downstream formation of new C–O bonds (Figure 3.1A, bottom).23,24 Previous research has noted that the direct excitation of the Ni(II)–bpy complex homolytically cleaves the Ni(II)–C(aryl) bond, generating aryl radicals and a formal Ni(I) species. This reduced Ni species may allow access to catalytically active Ni(I)/Ni(III) cycles.16,23,24 While the use of light-induced homolysis to generate reactive Ni species has broad implications for photoredox catalysis, the precise mechanism of this critical bondrupture step is not yet well understood and is the main subject of this study. 85 Figure 3.1. (A) Two possible photocatalytic approaches to Ni(II)–bpy-mediated C–O bond formation. (B) Left: energy diagram showing the direct excitation mechanistic pathway proposed in ref (16). The structure of the tetrahedral triplet ligand field excited state 3(d–d) is shown. Right: PESs as described in ref (29) with ab initio calculations showing the 3LMCT-based repulsive surface (in red) responsible for Ni–C bond homolysis. Note that T1 is the 3(d–d) state. The antibonding d(x2–y2)/C(sp2)* orbital is depicted. (C) Summary of this research. §3-1.2. Mechanistic Hypotheses for Excited-State Ni(II)–C Bond Homolysis There are two proposed excited-state Ni(II)–C bond homolysis mechanisms in Ni(II)–bpy aryl halide complexes. Using a combination of transient optical and IR spectroscopies, Doyle et al. demonstrated that the excitation of Ni(II)–bpy singlet metal-to-ligand charge 86 transfer ( MLCT) (λpump = 530 nm) resulted in the formation of triplet Ni(II) ligand field 1 excited states (3(d–d)).16 Intersystem crossing occurs in ∼5 to 10 ps, and the 3(d–d) state has a lifetime of ∼4 ns. Subsequent correlation to DFT calculations led to the proposal that Ni(II)–C homolysis occurs thermally from this photochemically formed Ni(II) 3(d–d) state (Figure 3.1B), which features a tetrahedral coordination geometry and a weakened Ni(II)– C bond. With DFT, the calculated homolytic BDE is ∼25 kcal mol–1. However, no direct experimental evidence was provided to demonstrate homolysis from the 3(d–d) state. Ab initio multiconfigurational/multireference calculations suggested an alternative mechanism of excited-state Ni(II)–C bond homolysis that is also consistent with the experimental data provided by Doyle et al. (Figure 3.1B).29 This approach yielded larger homolytic BDEs (∼90 kcal mol–1 from the S = 0 geometry, ∼70 kcal mol–1 from S = 1 geometry) than DFT and highlighted a putative one-photon, two-electron process leading to Ni(II)–C bond homolysis. In this mechanism, the initial excitation of the S = 0 complex forms a 1MLCT state (Ni(II)-to-bpy). From this PES (blue curve in Figure 3.1B), a ligandto-metal (aryl-to-Ni(III)) charge-transfer (LMCT) PES can be accessed. Critically, this LMCT results in the population of the antibonding d(x2–y2)/C(sp2)* orbital (Figure 3.1B, right), which reduces the bond order and results in a repulsive triplet PES, leading to homolytic bond rupture (red curve in Figure 1B, right).29 Notably, the energy difference between the MLCT/repulsive triplet crossing point (purple circle in Figure 3.1B, right) and the Frank–Condon point of the MLCT state constitutes the energy barrier (Ea) for bond rupture. Thus, it was reasoned that the structural and electronic control over the key MLCT/LMCT PESs, and, consequently, the barrier for photolysis, will result in variable rates of excited-state Ni(II)–C bond homolysis (Figure 3.1C), but new experimental data are required to further elucidate the overall mechanism. Indeed, we demonstrate a direct correlation between experimental rate constants at given excitation wavelengths and the energies of both of these excited-state PESs. Furthermore, we provide an experimental measure of the excited-state energetic barrier for homolysis in Ni(II)–bpy aryl halide complexes utilized as photoredox catalysts. The homolysis rate 87 constants are wavelength-dependent, and we have demonstrated a minimum energy threshold for photochemical activation. Coupled to extensive computational analyses, these data provide experimental evidence implicating high-energy, repulsive aryl-to-Ni LMCT PESs as being vital to homolytically cleaving the Ni(II)–C bond, a critical process in the photocatalytic C–X cross-coupling catalysis. The dynamics of the excited states of these Ni(II) complexes resemble those previously associated with third-row transitionmetal catalysts (e.g., Re complexes),30−39 unveiling underexplored reactivity pathways in these earth-abundant transition-metal catalysts. Beyond fundamental interest, demonstrating structural and electronic control over the key PESs in photoredox catalysis will, for example, allow chemists to tune the rates of formation of novel reactive intermediates and guide the discovery of new photon-driven organic methodological approaches to coupling reactions. §3-2. Results and Analysis §3-2.1. Experimental Studies In Sections §3-2.1.1–§3-2.1.5, we detail the syntheses and spectroscopic/photochemical characterizations of a matrix of Ni(II)–bpy aryl halide complexes (Figure 3.2). We demonstrate direct correlations between ligand-based electronic perturbations, observable MLCT transition energies, and rate constants of excited-state bond homolysis. Temperature- and wavelength-dependent studies provide experimental barriers and energetic thresholds for excited-state Ni(II)–C bond homolysis, respectively. §3-2.1.1. Synthetic Approach To probe the mechanism of excited-state Ni(II)–C bond homolysis, we targeted the matrix of Ni(II)(Rbpy)(R′Ph)Cl complexes (R = CH3O, t-Bu, H, CH3OOC; R′ = ortho-CH3, H, CH3O, F, CF3), 1A–5D, shown in Figure 3.2. Two primary synthetic approaches were utilized: (1) oxidative addition and (2) ligand substitution (Scheme S1). In the former, bis(1,5-cyclooctadiene) nickel(0) was prestirred with a given bpy ligand; subsequent reaction with the specific aryl halide resulted in the target complex. 88 Figure 3.2. Matrix of Ni(Rbpy)(R′Ph)Cl (R = MeO, t-Bu, H, and MeOOC; R′ = ortho-CH3, H, OMe, F, and CF3) complexes examined in this study. The bpy ligand varies down a column, while the aryl ligand varies across a row. Complexes shaded in blue were synthesized and examined both experimentally and computationally, while those in orange were examined only computationally. The latter method called for either bis(triphenylphosphino)(2– methylphenyl)chloronickel(II) or the independent preparation of a precatalyst complex, Ni(TMEDA)(R′Ph)Cl, R′ = CH3 or CF3, TMEDA = N,N,N′,N′–tetramethyl ethylenediamine.40 These TMEDA compounds afforded a more labile ligand that could be substituted by bpy.41 The precatalyst complexes themselves were prepared by oxidative addition. Ligand substitution was used in cases where oxidative addition proved slow, yielded inconsistent results, or would not produce the desired product. Full synthetic details for both previously prepared16 and novel compounds are available in Supporting Information Section S1.3. §3-2.1.2. Steady-State UV–Vis Spectroscopy The steady-state UV–vis spectra of the Ni(II)–bpy aryl halide series in tetrahydrofuran (THF) are given in Figure 3.3A. Molar absorptivity plots in both THF and toluene are 89 given in Figures S5 and S6 and are consistent with previous spectral assignments of dominantly MLCT intensity across the UV–vis range (Table S1). Figure 3.3. (A) Normalized UV–vis spectra in THF of complexes studied here (overall Δνmax = 4350 cm–1). (B) Correlation between λmax (dashed lines in panel A given for the extrema) in THF and the Hammett parameter (σp) for each bpy substituent or (σm) for each aryl substituent. Analogous plots are given in Figure S6 for toluene; molar extinction coefficient data are summarized in Table S1. The legend code used hereafter references the matrix of complexes in Figure 3.2: red, squares = column 1; blue, triangles = column 5, orange, circles = row B. The spectral assignments are discussed further in Section §3-2.2.1 and given explicitly in Table S10. It is also noted that the MLCT transition energies are generally solvatochromic, with transition energies being lower in toluene relative to THF (Figures S5 and S6). 90 Increasing the electron-withdrawing effect of the bpy substituents (proceeding down the columns in Figure 3.2) generally decreases the energies of the 1MLCT transitions. The MLCT λmax for spectra in Figure 3.3A (extrema denoted by dashed lines) correlates linearly with the Hammett parameter42 (σp) for each bpy substituent (blue and red curves in Figure 3.3B; Δνmax = 3000 cm–1). Similar Hammett relationships have been shown for Cu and Re bipyridine complexes.43,44 Variation in the aryl ligand (rows in Figure 3.2) also modulates λmax (Figure 3.3A) (Δνmax = 1500 cm–1), with increases in the electron-withdrawing group strength leading to increases in the energy of the 1MLCT transitions. While the aryl ligands are all modified at the ortho-position with respect to the Ni(II)–C bond, the MLCT λmax correlates with the corresponding meta-Hammett parameter (σm) (orange line in Figure 3.3B). This demonstrates a larger contribution of electrostatic and inductive effects over resonance effects upon variation of the aryl substituent relative to bpy.45−50 Accordingly, this series (1B–5B) also trends with Taft’s field parameter, σF (Figure S7);42 for consistency, we use σm in the main text of this manuscript. No linear trend was observed when using Taft’s steric (Es) parameter,42 as the aryl ligand and its substituent are rotated orthogonal to the plane of the molecule (Figures S7 and S55).45,50 §3-2.1.3. Photochemical Investigations We first sought to confirm the formation of aryl radicals upon irradiation (see Supporting Information S1.2 for experimental setup). Irradiating well-characterized 1B and analogous 5B at 390 nm results in distinctive 1H NMR peaks assigned to aryl radical products, 2-(o-tolyl)tetrahydrofuran, and 2,2′-dimethyl-1,1′–biphenyl (Figures S11 and S12).23 Using 19F NMR, 5B revealed new peaks associated with the free aryl ligand, concomitant with a loss of aryl peaks in the 1H NMR after 390 nm irradiation (Figure S14). As demonstrated previously, homolysis does not occur in the absence of light; only minor degradation is observed when the complex is heated to 55 °C for 60 min (Figure S13), but no radical products are observed.23,24 We also noted the formation of radical products upon extended irradiation of the analogous precatalyst complexes, Ni(TMEDA)(R′Ph)Cl (R′ = 91 CH3, CF3), implicating ligand field excited states as operative for photolysis in the diamine complexes.51 More detailed discussion regarding this result relative to the photochemistry of Ni(II)–bpy complexes is available in Supporting Information Section S1.6. Time-dependent absorption spectra were obtained during photolyses (390 nm) in THF of all complexes 1A–5D. Photolysis kinetics were monitored at two wavelengths (arrows in Figure 3.4 and Figures S19 and S20). From these kinetics, the observed rate constant (kobs,1) of the excited-state Ni(II)–C bond homolysis can be obtained. For a representative compound, 1B, concentration dependence studies found a negligible change in kobs,1 across the absorbance window tested (Figure S44). The data for series 1A–1D, which varies only the bpy substituent, are provided in Figure 3.4. Note that this series of complexes (1A– 1D) has previously been investigated using transient absorption spectroscopy (the only difference being EtOOC versus MeOOC in 1D here).16 Clear rate constant changes are observed upon variation of the bpy ligand (Figure 3.4 and Table 3.1). The largest rate constant was found for 1D, which underwent photolysis with an observed rate constant of kobs,1 = (17.0 ± 0.7) × 10–2 min–1. Compounds 1A–1C presented smaller, but noticeable, differences in their decay rate constants (Table 3.1). For 1A, background scattering from precipitation precluded a clear observation of the decay of the starting material. Excited-state homolysis of the Ni(II)–C bond yielded a new product with absorbance in the visible region for each compound. Isosbestic points are observed in the photolysis data for all compounds studied here except 1A, where light scattering contributes to the timedependent spectra. While the general absorption profiles of these new species are similar, the primary low-energy features shift from ∼650 nm in 1A to 805 nm for 1D (Figure 3.4). The spectral shift over this series suggests that the bpy ligand is present in the new species. 92 Figure 3.4. Photolysis profiles of 1A–1D in THF for 390 nm excitation. Photolysis kinetics were monitored at two wavelengths indicated by the blue and orange arrows in each panel. The insets correspond to the fitted kinetic data (blue curve for the decay of the starting material, orange curve for the formation of the new species). Data were fit using a single exponential; error bars are 1 standard deviation. For 1A, background scattering from precipitation precluded a clear observation of the decay of the starting material. This scattering also contributes to the kinetics measured at longer wavelengths for 1A. Photolysis profiles of 5A–5D and 2B–4B are given in Figures S19 and S20. The analogous time-dependent UV–vis spectra for compounds 5A–5D are given in Figure S19. There are significant changes in the rate constants of excited-state Ni(II)–C bond homolysis across these compounds, with kobs,1 varying over an order of magnitude (Figure S19 and Table 3.1). However, these compounds generally exhibit much smaller rate constants than the complementary 1A–1D series. Thus, the electron-withdrawing effect of the aryl ligand also impacts the rate. The growth of a new species was also observed for 93 these complexes (∼650 nm in 5A to 805 nm in 5D), albeit at significantly lower quantities. To further investigate the dependence of kobs,1 on the variation of the aryl ligand, analogous time-dependent UV–vis data were obtained for complexes 2B–4B (Figure S20). From these data, the full trend is revealed: increasing the electron-withdrawing nature of the aryl ligand (left to right in the row of Figure 3.2) again resulted in smaller rate constants across the series. Table 3.1. Summary of UV–vis λmax and first-order rate constants.c λMLCT νMLCT kobs,1 kp kobs,2 Compound (nm) (cm-1) (x 10-2 min-1) (x 10-2 min-1) (x 10-2 min-1) 1A 462 21645 n.d.a 0.9 ± 0.1 1.4 ± 0.4 1B 475 21053 2.5 ± 0.2 4.1 ± 0.4 7.6 ± 0.7 1C 483 20704 3.6 ± 0.6 3.3 ± 0.5 2.9 ± 0.3 1D 532 18797 17.0 ± 0.7 15.0 ± 0.8 10.1 ± 3.9 2B 471 21231 2.6 ± 0.1 4.4 ± 0.1 6.5 ± 0.4 3B 464 21552 0.55 ± 0.02 1.6 ± 0.1 2.3 ± 0.2 4B 447 22371 0.219 ± 0.002 0.14 ± 0.18 10.4 ± 5.2 5A 432 23148 0.13 ± 0.01 0.08 ± 0.03 0.40 ± 0.15 b 5B 443 22573 0.16 ± 0.03 n.d. 2.3 ± 1.1 5C 453 22075 0.19 ± 0.05 0.16 ± 0.01 1.4 ± 0.4 5D 497 20121 1.39 ± 0.02 3.0 ± 0.5 5.1 ± 1.0 1B–Br 479 20877 6.9 ± 0.4 8.1 ± 0.8 9.2 ± 0.7 a Because the new species formed upon Ni(II)–C bond homolysis has absorption underneath that of the starting material, we were unable to obtain k1,obs for this compound. The rate is approximated by k2,obs. bComplex 5B was omitted owing to poor convergence of the global kinetics model. cSolvent, THF. Errors are listed as one standard deviation over three trials. This behavior is opposite to that observed for variations in the electron-withdrawing effect of the bpy ligand, hence the opposite slopes in Figure 3.3B. Note also that the primary low-energy absorption feature of the new species is not dependent on the aryl ligand (λ = 660 nm for 1B–5B; Figures S41–S43). 94 We note that in certain regions, the absorption spectrum of the photolysis product overlaps with that of the starting material, including where decay kinetics are measured (blue arrows in Figures 3.4 and S19 and S20). Furthermore, kobs,1 is in most cases less than kobs,2. To deconvolute the spectral overlap and rationalize these differences, global kinetics modeling was carried out (full discussion and details of the kinetic modeling are available in Supporting Information Section S1.9). Good agreement is seen between the observed rate constants and those obtained from the global fits, and the kinetic trends across the matrix of compounds are preserved (Figure S40). Comparison between kobs,1 and rate constants from global fitting (kp) is also given in Table 1. Additionally, using the method developed by Gescheidt et al.,52 we calculated the quantum yields for each complex to account for differential absorbance at the 390 nm excitation wavelength (see Supporting Information Section S1.11 for complete details). We found good linear agreement between the observed photolysis rates and the calculated quantum yields (R2 = 0.9730; Figure S53), further supporting our kinetic analysis. Furthermore, the rate constants of excited-state Ni(II)–C bond homolysis correlate linearly with specific Hammett parameters of the bpy and aryl ligands. As shown in Figure 3.5, linear relationships are observed upon plotting log(kobs,1/kobs,1(H)) versus σp or σm (for Rbpy or R′Ph, respectively) (R2 ≥ 0.95) (ρ = ∼1.4 for Rbpy and ρ = ∼−2.6 for R′Ph). Thus, the rate of excited-state Ni(II)–C homolysis is sensitive to electronic structure perturbations from both the bpy and aryl ligands. Electronically stabilizing the Ni(II)-to-bpy MLCT transition energies by increasing the bpybased electron-withdrawing effect accelerated the rate of photolysis. Conversely, increasing the aryl-based electron-withdrawing effect resulted in increased MLCT transition energies and slower rates of photolysis (see the oppositely signed slopes in Figure 3.3B). These data reflect competing effects on the excited-state PESs involved in homolysis and are further described in Section 2.2. 95 Figure 3.5. Correlation between normalized rate constants (390 nm excitation) of excitedstate bond homolysis and specific Hammett σ parameters of the bpy and aryl ligands. Note that in 1A, the rate is approximated by k2,obs. Analogous plot of the relative quantum yields versus specific Hammett σ parameters is given in Figure S54. §3-2.1.4. Preliminary Investigations of the Photochemically Generated Species The immediate product of Ni(II)–C bond homolysis has been proposed to be a threecoordinate Ni(I)(Rbpy)Cl complex. A recent study by Bird et al. on the related Ni(I)(t-Bubpy)Br reported its UV–vis spectrum as generated by pulse radiolysis or electrolysis.53 For direct comparison, we synthesized Ni(t-Bubpy)(CH3Ph)Br, 1B–Br, and subjected it to the same photolysis conditions as above. We found a roughly threefold enhancement in the rate constant of photolysis for 1B– Br (kobs,1 = (6.9 ± 0.4) x 10–2 min–1) relative to 1B (kobs,1 = (2.5 ± 0.2) x 10–2 min–1) and a change in the absorption spectrum of the product species (Figure 3.6A). The primary low-energy absorption feature of the product appears at higher energy when produced from the bromo-complex (653 nm) versus the chloro-complex (660 nm). Thus, there is a halide dependence on the absorption spectrum of the product compound. A comparison between the long-time spectra of the photoproducts from compounds 1B–Br, 1B, 3B, 1C, and 1D is given in Figure 3.6B, illustrating a change in peak maxima when changing the bpy or halide ligands, but not the aryl ligand. 96 Figure 3.6. (A) Photolysis profile of 1B–Br in THF (390 nm excitation) monitored at two wavelengths indicated by the blue and orange arrows in each panel. The inset corresponds to the first-order kinetics data (blue curve for the decay of the starting material, and orange curve for the formation of the new species). Error bars are 1 standard deviation. (B) Comparison between long-time UV–vis spectra for 1B–Br (λmax = 653 nm), 1B (λmax = 660 nm), 3B (λmax = 660 nm), 1C (λmax = 673 nm), and 1D (λmax = 805 nm). (C) Comparison between UV–vis spectra for 1B–Br in THF and DMF before and after irradiation with 390 nm light. 97 We also followed the photolysis of 1B–Br in dimethylformamide (DMF), the same solvent used by Bird et al. (Figure S51).53 We first note that the steady-state UV–vis data are solvatochromic, with the main MLCT bands being lower in energy in THF relative to DMF (Figure 3.6C, blue versus orange lines, respectively). The homolysis product UV–vis spectra are also solvatochromic (Figure 3.6C, dashed lines). In particular, DMF solutions exhibit the same characteristic UV–vis features for the three-coordinate monomeric species (430, 620, and 860 nm), as observed by Bird et al. (Figure 3.6C, orange dashed line). We further note that the rate constant of excited-state Ni(II)–C bond homolysis is smaller in DMF relative to THF. We tentatively ascribe these differences in rate constants to changes in the MLCT energies and overall excited-state PESs (vide infra). These interesting solvent effects on the excited-state PESs and, thus, rates of homolysis are currently under more detailed investigation. Monomeric Ni(I)(t-Bubpy)X, X = Cl, Br has been shown to be active toward the oxidative addition of aryl iodides, while the dimeric form, [Ni(t-Bubpy)X]2, is unreactive with the same.53,54 We irradiated a sample of 1B–Br in THF, generating the photoproduct. Addition of 2-iodotoluene to this solution revealed rapid reactivity and complete removal of the characteristic absorption feature at 653 nm (Figure S52). Furthermore, the absorption spectrum of the dimeric species shows peaks only in the UV region, further implicating the monomeric form as the photoproduct.54 Therefore, we postulate that the new species formed here upon excited-state Ni(II)–C homolysis are three-coordinate Ni(I)(Rbpy)X complexes (R = MeO, t-Bu, H, and MeOOC, X = Cl or Br), as they have been shown by steady-state UV–vis spectroscopy to (1) contain the bpy ligand, (2) not contain the aryl ligand, (3) contain the halide and, in the case of 1B– Br, (4) exhibit the same absorption profile as Ni(I)(t-Bubpy)Br, and (5) exhibit oxidative addition reactivity with iodotoluene. A detailed comparative study of the reactivities and further spectroscopic characterizations of these species is currently underway. 98 §3-2.1.5. Further Examination of the Mechanism of Excited-State Bond Homolysis To further investigate the mechanism of excited-state Ni(II)–C bond homolysis, we carried out temperature-dependent photolyses of 1B and 1B–Br. Among the matrix of complexes studied here, these two are most often utilized for synthetic applications, giving their analyses direct implications for photoredox catalysis.24 Eyring plots of temperature-dependent rate constants for these complexes are given in Figure 3.7. From these data, the enthalpy and entropy of activation for the excited-state Ni(II)–C bond homolysis in 1B are ΔH‡ = 4.4 ± 0.6 kcal mol–1 and ΔS‡ = −45.3 ± 1.8 cal mol–1 K–1, with ΔG‡(298 K) = 17.9 ± 0.8 kcal mol–1. Similar analysis of 1B–Br gives ΔH‡ = 2.1 ± 0.1 kcal mol–1 and ΔS‡ = −49.3 ± 0.4 cal mol–1 K–1, with ΔG‡(298 K) = 16.8 ± 0.2 kcal mol–1. At high temperatures (328 K), thermal decay of the starting material occurs for 1B– Br, resulting in a downturn in the temperature-dependent rate constants (dashed yellow line; Figure 3.7). Because of this, the linear fit utilized a room temperature point. As expected, the barrier for excited-state Ni(II)–C bond homolysis is lower in 1B–Br than in 1B, consistent with its larger rate constant. In addition to being dependent on temperature, the rate constant of excited-state bond homolysis in 1B is also highly dependent on the excitation wavelength (Figure 3.8A). Varying incident wavelengths (390, 427, 456, and 525 nm; Figure S21) revealed a minimum energy threshold for the excited-state Ni(II)–C bond homolysis of ∼55 kcal mol–1 (525 nm, 19,050 cm–1) in 1B; below this incident energy, no homolysis is observed (Figure S22). 99 Figure 3.7. Eyring analysis of the temperature-dependent photolysis rates of 1B (blue, squares) and 1B–Br (orange, circles); error bars are one standard deviation over three trials. Previous optical transient absorption measurements on 1B were carried out using λpump = 530 nm.16 Laser excitation at this wavelength results in the formation of a Ni(II)-based triplet ligand field excited state, from which homolysis was proposed on the basis of DFT calculations. Notably, however, very limited photolysis occurs here using a 525 nm excitation light source. These results demonstrate that the lower-energy ligand field state is not responsible for excited-state Ni(II)–C bond homolysis but rather indicate the involvement of higher-energy excited states. Figure 3.8. Wavelength-dependent photolysis for 1B, 1D, and 5D. The absorption spectra are shown in blue; the observed photolysis rate constants (squares) and incident wavelengths are given in orange. Analogous plot of wavelength dependency of quantum yields is given in Figure S25. To search for general trends across compounds considered here, we also conducted wavelength-dependent studies on 1D and 5D (Figure 3.8B,C). Altogether, these complexes 100 span a wide range of photolysis rate constants, have varying MLCT transition energies, and feature electronic structure differences provided by the bpy and aryl ligands. In each case, a clear wavelength dependence was observed, and high-energy incident light was required for homolysis. No appreciable decay was observed using low-energy light (Figures S23 and S24), again implicating high-energy excited states in the mechanism of lightinduced homolysis. We also evaluated the wavelength dependency of quantum yields for each complex, accounting for variable LED power and complex absorbance at each wavelength, and found that their behavior mirrors the photolysis kinetics (Figure S25). In summary, through experimental analyses of a matrix of Ni(II)–bpy aryl halide complexes, we have demonstrated the following: (1) a dependence between the MLCT λmax and the Hammett parameters of the bpy and aryl substituents over the 1A–5D series (Figure 3.3B), (2) linear correlations between the Hammett parameters of the bpy and aryl substituents and the rate constants of excited-state Ni(II)–C bond homolysis over the 1A–5D series, interestingly with oppositely signed slopes (Figure 3.5), (3) the barrier for excited-state bond homolysis is moderate (e.g., ΔH‡ = 4.4 ± 0.6 kcal mol–1 in 1B using 390 nm excitation; Figure 3.7), and (4) excited-state bond homolysis is distinctly wavelength-dependent (Figure 3.8); e.g., in 1B, requiring a minimum of ∼55 kcal mol–1 (525 nm, 19,050 cm–1). These experimental observations are discussed below in the context of computational studies, which further aid in the elucidation of the mechanism of excited-state Ni(II)–C bond homolysis. 101 §3-2.2. Computational Studies In the following computational Sections §3-2.2.1–§3-2.2.3, we first compare the groundand excited-state properties of 1A–5D computed at different levels of theory. We discuss the possible photoactivation pathways that are accessible in the energy range of the external light sources used in the photolysis experiments, as well as those pathways that are consistent with the experimental barrier. Notably, the incident light energy required for photolysis (as determined from wavelength-dependent kinetic experiments) is substantially greater than the energy of the 3(d–d) bands in 1A–5D, and the calculated barriers for homolysis from these states are significantly larger than the experiment. These points indicate that the thermally driven excited-state Ni(II)–C bond homolysis from a spin-forbidden ligand field state is not the operative mechanism. Instead, we focus on the possible photolysis pathways that exploit triplet excited-state LMCT-based repulsive PESs. We propose a working mechanism that can ultimately be described as 1MLCT [Ni d → bpy π*(2)] excitation followed by surface hopping to a repulsive 3LMCT (aryl-to-Ni) PES (3MLCT + LMCT). This mechanism is in agreement with the experimentally derived reaction rates and thermodynamic barriers determined herein. §3-2.2.1. DFT versus CASSCF/QD-NEVPT2 Ground and Excited States To evaluate the geometric and electronic structures of 1A–5D, we compared their groundand excited-state properties calculated with either DFT/TD-DFT or ab initio complete active space self-consistent field theory with the quasidegenerate N-electron valence state perturbation theory correction (CASSCF/QD-NEVPT2);55−58 full computational details are available in Supporting Information Section S2.1. With DFT (B3LYP),59−61 all Ni(II) complexes are predicted to have low-spin, singlet (S = 0) ground states with square-planar geometries (note that the x-axis is directed along the Ni– halide bond and the y-axis is along the Ni–aryl bond). The fully optimized triplet (S = 1) ligand field excited states are in all cases ∼10 kcal mol–1 higher in energy with pseudotetrahedral geometries. The valence electronic configuration of the d8 ground state is [d(xy)]2[d(yz)]2[d(xz)]2[d(z2)]2, with three unoccupied bpy-based π* orbitals and a highly 102 covalent antibonding [d(x –y )/C(sp )*] orbital (see Figure S59 for an example 2 2 2 0 molecular orbital diagram for 1D). The orbital energies are modulated by the bpy substituents; increasing the electron-withdrawing effect of the bpy ligand (columns in Figure 3.2) decreases the energies of the bpy π* orbitals, reducing the Ni(II)-to-bpy MLCT energy (Figure S60), consistent with the red-shifted experimental λmax features in Figure 3.3. On the other hand, the HOMO and the bpy π* orbital energies remain essentially unchanged when modulating the aryl substituent (rows in Figure 3.2), contrasting with the blue shift in Figure 3.3. The correct behavior can be recovered at the TD-DFT level, which accounts for orbital mixing in the excited states (Figure S62). Interestingly, the changes in orbital energies are not translated into changes in the covalencies of the ground states, which remain ∼51 to 54% Ni d and ∼11 to 13% bpy characters for 1A–5D (Table S9). The calculated TD-DFT absorption spectra agree well with the experimental UV–vis data (see overlaid spectra in Figure S62) and also demonstrate a similar linear relationship with the substituent-specific Hammett σ parameters (Figure 3.9, top). The broad feature at longer wavelengths (∼400 to 600 nm, ∼25,000 to 16,500 cm–1) encompasses all of the “lowenergy” 1MLCT transitions [Ni d → bpy π*(1)], with [d(yz) → bpy π*(1)] having the highest calculated oscillator strength. The shoulder at ∼350 to 370 nm (∼28,500 to 27,000 cm–1) apparent in most of the experimental UV–vis spectra of 1A–5D can be similarly assigned to a [d(yz) → bpy π*(2)] transition; other “high-energy” 1MLCT [Ni d → bpy π*(2)] transitions are predicted to fall in the ∼300 to 450 nm (∼33,000 to 22,000 cm–1) range. The 1(d–d) transitions are calculated to be comparable in energy to the “low-energy” 1MLCT bands (∼400 to 500 nm, 25,000 to 20,000 cm–1) and are not visible in the experimental UV–vis spectra of 1A–5D. This assignment is also consistent with the energy of the observable 1(d–d) band [d(yz) → d(x2–y2)/C(sp2)*] in the Ni(II)(TMEDA)(CH3Ph)Cl complex that is detected in the visible range (found at ∼470 nm, 21,280 cm–1; calculated at 533 nm, 18,760 cm–1); the less-intense bands observed near ∼635 nm (15,750 cm–1) can be assigned to the spin-forbidden triplet transitions. For example, the [d(xy) → d(x2–y2)/C(sp2)*] triplet transition is calculated at 642 nm (15,580 cm–1). Others, including [d(xz) → d(x2–y2)/C(sp2)*] and [d(yz) → d(x2–y2)/C(sp2)*], are calculated at 847 nm (11,810 –1 –1 103 cm ) and 945 nm (10,580 cm ), respectively. All relevant TD-DFT electronic transition energies are given in Table S10. Figure 3.9. Correlation between the calculated 1MLCT [d(yz) → π*(1)] transition energies (CPCM solvation model, THF) and the specific Hammett σ parameters using both TD-DFT (top) and CASSCF/QD-NEVPT2 (bottom) methods. Therefore, the light sources exhibiting most appreciable excited-state Ni(II)–C bond homolysis (i.e., 390 nm, 25,640 cm–1 and 427 nm, 23,420 cm–1) in 1A–5D favor initial excitation into high-energy 1MLCT bands, [Ni d → bpy π*(2)], while longer-wavelength light sources from the wavelength dependence study in Figure 3.8 (456 nm, 21,930 cm–1 and 525 nm, 19,050 cm–1) are in the range of the low-energy 1MLCT bands, [Ni d → bpy π*(1)]. The minimum energy threshold of ∼525 nm, below which no photolysis is observed, is not consistent with the previous DFT-based assignment of bond rupture from a low-lying triplet 104 16 ligand field state (Figure 3.1B, left). The lack of bond homolysis from the low-lying MLCTs is likely the result of an increased barrier for homolysis (discussed further in Section §3-2.2.3). Multiconfigurational/multireference CASSCF/QD-NEVPT2 calculations were conducted using the DFT-optimized singlet geometries. State-averaged CASSCF calculations (15 singlets, 25 triplets) were performed with an active space of ten electrons in nine orbitals (10e, 9o): d(yz), d(z2), d(xy), d(xz), a pair of bonding and antibonding Ni d(x2–y2)/aryl C(sp2) orbitals, and three bpy π* orbitals (Figure S56). The method and active space follow our previous study on 1B.29 For 1A–5D, the ground-state wave functions exhibit a substantial multiconfigurational character. While the CI vectors (Figure S57 and Table S9) are primarily comprised of the CSS configuration (i.e., the configuration also acquired with DFT), both the MLCT and LMCT configurations contribute significantly to the character of the groundstate wave function. The calculated CASSCF/QD-NEVPT2 absorption spectra generally agree with the observed bands in the experimental UV–vis data (Figure S63; tabulated in Table S11). However, the λmax is notably red-shifted (∼5000 cm–1) as compared with TD-DFT (Figure S62 and Table S10). The composition of the ground-state CI vector, particularly the total 1MLCT contribution, also closely correlates with the transition energy in the calculated UV–vis spectra. As the 1MLCT [Ni d(xz) → bpy π*(1)] weight increases, the calculated energy of λmax in the absorption spectrum decreases (Figure S65). The multiconfigurational/multireference calculations also demonstrate a similar linear relationship with the substituent-specific Hammett σ parameters (Figure 3.9, bottom). §3-2.2.3. Investigating the Mechanism of Excited-State Ni(II)–C Bond Homolysis DFT-relaxed PES scans along the Ni(II)–C bond coordinates of 1A–5D revealed a singlet/triplet degenerate homolytic dissociation product of ∼40 kcal mol–1 above the singlet equilibrium geometry. Thermal dissociation barriers of ∼30 kcal mol–1 were calculated from the triplet equilibrium geometries, consistent with a previous study.16 Given the large 105 differences in BDEs between DFT and CASSCF/QD-NEVPT2 observed previously for 1B,27 we have also evaluated the BDEs using different levels of theory. Referencing to the experimental energy of the 3(d–d) state found by Doyle et al. (12 kcal mol–1),16 we find BDEs of ∼40 kcal mol–1 (SCS-MP2/QZ), ∼41 kcal mol–1 (DLPNO-CCSD(T)/QZ), and ∼76 kcal mol–1 (CASSCF/QD-NEVPT2; 10e, 8o) (Tables S6 and S7), suggesting that the DFT BDE of ∼30 kcal mol–1 represents the lower limit for thermally driven, excited-state Ni(II)–C bond homolysis from the triplet ligand field equilibrium geometry. These calculated barriers are all significantly higher than the experimental value of ∼5 kcal mol–1 demonstrated above. Therefore, we again find that thermal dissociation from the lowest-energy triplet ligand field state is not consistent with the experimental data provided here and, thus, is not a viable mechanism for excited-state Ni(II)–C bond homolysis. Instead of thermally driven excited-state Ni(II)–C bond homolysis from ligand field states, we propose that the mechanism of bond homolysis exploits excited-state triplet repulsive aryl-to-Ni LMCT PESs (outlined in Figure 3.10). Critically, in these scenarios, the transfer of an electron between the bonding and antibonding d(x2–y2)/C(sp2) orbitals significantly lowers the overall bond order, facilitating bond rupture. We have found two pathways utilizing either 1(d–d) (blue panel in Figure 3.10) or 1MLCT (orange panel in Figure 10) transitions as the initial entrance to the excited-state mechanism. In both cases, these initial excitations are followed by intersystem crossing (ISC) and formation of an aryl-to-Ni 3LMCT state. Accordingly, two principally different triplet repulsive surfaces are viable for homolysis: a “one-photon, one-electron” excited state 3[d(x2–y2)/C(sp2) → d(x2–y2)/C(sp2)*] and a “one-photon, two-electron” excited state 3[d(x2–y2)/C(sp2) + d(yz) → d(x2–y2)/C(sp2)* + π*(1)] (green and red surfaces in Figure 10 (right), respectively). This former pathway expands our initially delineated mechanism in reference,29 wherein the probability for population of the 1(d–d) excited states versus the 1MLCT upon irradiation would contribute to determining the excited-state surfacemediating homolysis. Indeed, it is likely that this “one-photon, one-electron” repulsive surface facilitates Ni(II)–C bond homolysis in the Ni(TMEDA) aryl halide complexes, as 106 described in Section §3-2.1.3; a recent report by Park et al. implicated the corresponding surface in the homolysis of related Ni(II) complexes.51 Figure 3.10. Two plausible excited-state mechanisms of Ni(II)–C bond homolysis. Initial 1(d–d) or 1MLCT excitation (blue or orange boxes, respectively) is followed by ISC and 3LMCT formation. On the right, this surface hopping mechanism is exemplified with the shaded purple circles connecting the 1(d–d) excited-state surface (purple line) to the oneelectron triplet repulsive 3LMCT surface (green line) or 1MLCT excited-state surface (blue line) to the two-electron triplet repulsive 3(MLCT + LMCT) surface (red line). Corresponding plots computed at the TD-DFT and CASSCF/QD-NEVPT2 levels are provided in Figures S68 and S69. For clarity, exchange splitting between α and β orbitals is neglected. Illustrative orbital energies and ordering on the left are provided from the ground state and are held constant through steps 1–4; the right panel gives a more accurate depiction of relative state energies, reflecting a small thermal barrier relative to the excitation energy. From the singlet ground state, the standard TD-DFT approach (i.e., single-electron excitations) cannot access the two-electron nature of the red surface in Figure 3.10 (right). 107 Also, the spin-flipped α-to-β excitations (needed for description of the green surface in Figure 3.10, right) are only available from the singlet ground state using the restricted Kohn–Sham orbitals, which are inadequate for producing the accurate excited-state chargetransfer states, especially for out-of-equilibrium geometries with increased charge separation. Nevertheless, we were able to identify both of these triplet repulsive PESs using TD-DFT from the high-spin (S = 1) triplet reference state (i.e., the electronic configuration of the middle structure in the blue panel of Figure 3.10). From this configuration, the oneelectron or two-electron triplet repulsive surfaces can be obtained via β-to-β [d(x2–y2)/C(sp2) → d(z2)] or [d(x2–y2)/C(sp2) → bpy π*(1)] excitations. However, we note that, due to the triplet reference state containing the singly occupied Ni d(z2) orbital configuration, our description of the 3MLCT surfaces and two-electron triplet repulsive surface is restricted to those with [Ni d(z2) → bpy π*] MLCT transitions. Detailed discussion on the limitations of this approach is given in Supporting Information Section S2.2. Despite these limitations, we were able to obtain an excellent energetic correlation between the “one-photon, two-electron” triplet repulsive surface at the equilibrium geometry and both the experimental ln(kobs,1) and Hammett σ parameters (Figures S66 and S67), suggesting that this surface (or an analogous surface with another singly occupied Ni d-orbital) is involved in the mechanism of excited-state bond homolysis. In contrast, a significantly poorer correlation was found for the energy of the “one-photon, one-electron” 3[d(x2–y2)/C(sp2) → d(x2–y2)/C(sp2)*] repulsive surface (Figure S66). Furthermore, when considering that the calculated 1/3(d–d) transitions are near or below the minimum excitation energy threshold obtained from the wavelength-dependent experiments in Figure 3.8, it appears that the oneelectron triplet repulsive surface is not likely operative for the excited-state Ni(II)–C bond homolysis in 1A–5D. The activation energies derived from the high-energy 1MLCT excited states (as defined by the crossing of the MLCT/triplet repulsive surface and the Frank–Condon point of the MLCT ‡ 108 state) agree qualitatively well with those of ΔH = 4.4 ± 0.6 and 2.1 ± 0.3 kcal mol–1 measured for 1B and 1B–Br (Figure 3.11). The best fit between the calculated activation energies and the experimental ln(kobs,1) was observed for the 1MLCT [d(yz) → π*(2)] excited surface hopping to the two-electron repulsive surface, 3[d(x2–y2)/C(sp2) + d(z2) → d(x2–y2)/C(sp2)* + π*(1)] (Figure 3.11). It should also be noted that the MLCT transitions from the donor d(yz) orbitals correspond to the absorption bands with greatest oscillator strengths, making the [d(yz) → π*(2)] surface most likely to be populated from the initial photon absorption. However, due to the aforementioned limitations of the TD-DFT approach (Section S2.2), we cannot exclude an equal or better correlation for the same pathway utilizing another 3(MLCT + LMCT) excited state instead. The CASSCF/QD-NEVPT2 computations provide the same photolysis mechanism for 1A– 5D as that obtained from TD-DFT (Figure 3.10, left). However, the predicted activation energy is increased by ∼20 to 25 kcal mol–1 (Figure 3.11). This increase is likely the result of the higher Ni(II)–C BDEs found by CASSCF/QD-NEVPT2, as increasing the overall BDE also increases the energies of the crossing point between the initial 1MLCT and the repulsive two-electron excited-state surfaces. As discussed in detail in Supporting Information Section S2.2, we believe that the higher BDEs for these calculations originate from a potentially incomplete description of the CASSCF reference wave function. From the initial 1MLCT [d(yz) → π*(2)] excitation, we have evaluated the activation energies for this surface crossing from all of the accessible 3(MLCT + LMCT) repulsive states. Although the correlation is satisfactory with the same 3[d(x2–y2)/C(sp2) + d(z2) → d(x2–y2)/C(sp2)* + π*(1)] surface as explored by TD-DFT (with R2 = 0.79), an even better correlation was observed for surface hopping to the 3[d(x2–y2)/C(sp2) + d(yz) → d(x2– y2)/C(sp2)* + π*(1)], characterized by R2 = 0.91 (Figure 3.11, bottom). 109 Figure 3.11. Computed excited-state activation energies (TD-DFT, top; CASSCF/QDNEVPT2, bottom) for 1A–5D plotted against experimental ln(kobs,1) obtained using 390 nm excitation. Note that for 1A, k1,obs is approximated by k2,obs. The activation energies are estimated from the surface crossing between the high-energy 1MLCT [d(yz) → π*(2)] PES and the “one-photon, two-electron” triplet repulsive surface, 3(MLCT + LMCT) (more details can be found in Supporting Information Section S2.2; see Figure S66 for an analogous plot with the “one-photon, one-electron” triplet repulsive surface). Additionally, we have found an activation energy of ∼2.2 kcal mol–1 for the 1B–Br complex, which is in excellent agreement with the experimental ΔH‡. Overall, both TD-DFT and CASSCF/QD-NEVPT2 methods predict analogous photolysis mechanisms that exploit triplet repulsive “one-photon, two-electron” excited-state PESs, and the calculations further capture the critical aspects that lead to geometric and electronic structural control over the excited-state PES manifold that ultimately governs the photochemical behavior. However, each method is associated with its own limitations, precluding a definitive assignment of which 1/3MLCT is initially operative or at which 110 stage ISC occurs (i.e., before or after the surface hopping between MLCT and MLCT + LMCT surfaces). We also speculate that this mechanism may not be identical for all Ni complexes used in photoredox catalysis. Additional possibilities, including the “onephoton, one-electron” 1/3(d–d) → 3LMCT pathway, might be operative depending on the energetics of the individual excited states, the probability of the initial light-induced transitions, and/or the probability of the surface hopping to the triplet repulsive PESs. §3-3. Discussion Light-induced homolysis provides a powerful means to activate ligand–metal bonds for the generation of reactive radical species involved in targeted catalytic bond transformations.5,62,63 Defining the photophysical processes underlying the mechanisms of light-induced homolysis will therefore aid chemists in elucidating the role(s) of photogenerated intermediates in currently established photoredox catalytic cycles, as well as further guide the development of novel bond-formation reactions in organic chemistry. Here, we have provided new combined experimental and computational insights that have aided in the elucidation of the mechanism of excited-state Ni(II)–C bond homolysis in commonly employed Ni(II)–bpy aryl halide photoredox catalysts. In particular, rate constants of excited-state homolysis depend on the temperature (Figure 3.7) and the wavelength of light excitation (Figure 3.8). From these observations, we conclude that there exists an energy barrier between the light-absorbing and homolytically dissociative excited states. For 1B, ΔH‡ = 4.4 ± 0.6 kcal mol–1 using 390 nm excitation. Note that ΔH‡ is the energy parameter that is most relevant for comparisons to calculated excited-state PESs. From the wavelength dependence on the rate constants, it is also clear that the lower-energy 1MLCTs near ∼525 nm (19,050 cm–1, 55 kcal/mol) are not productive for excited-state homolysis. Rather, higher-energy excitation is required. Overall, these observations are not consistent with thermally driven Ni(II)–C bond homolysis from excited ligand field states. We further note that similar wavelength dependence trends have been demonstrated for Ni(II)–bpy-mediated C–O cross-coupling product yields, directly implicating these high-energy 1MLCT states in photoredox 111 24 catalysis. Substituent-driven modulation of the critical PESs and bond homolysis rates may provide a synthetic handle to promote (or discourage) radical formation during catalysis. In addition to the temperature and wavelength dependence, we have found that the rate constants of homolysis in Ni(II)–bpy aryl halide complexes are also sensitive to variations in both the solvent and the nature of all three ligands. For example, the rate constants of excited-state Ni(II)–C bond homolysis for 1B and 1B–Br are slower in toluene and DMF (Figures S48–S51) relative to THF, suggesting a solvent influence on the excited-state PESs. Solvent dependence has been observed for other light-induced homolysis reactions that feature activation energies, and similar arguments regarding perturbations to the barrier have been made.30,31 Furthermore, exchanging chloride with bromide (1B versus 1B–Br) increases the observed rate of homolysis from 2.5 ± 0.2 to 6.9 ± 0.4 × 10–2 min–1. The calculated barriers and excited-state PESs for these two compounds are similar (Figure S70). However, changing Cl to Br increases the ligand-based spin-orbit coupling constant.64 This increased spin-orbit coupling may increase the rate of surface crossing and suggests that spin-vibronic effects may be important to consider (vide infra).65−67 Furthermore, light-induced homolysis results in the production of an additional visiblelight absorbing species (Section §3-2.1.4). Based on the correlation between the UV–vis spectrum of the intermediate formed from 1B–Br via photolysis to that identified recently by Bird et al.,53 we tentatively assign this species as a three-coordinate, formal Ni(I)(t-Bubpy)(Br) complex. While further study of this intermediate is outside the scope of this study, it is worth noting that, under controlled air- and moisture-free conditions, excited-state homolysis provides a means to generate and isolate putative reactive intermediates for detailed studies of their spectroscopic properties and reactivity patterns. Thus, the rate of formation of these intermediates is tunable via structural and electronic control over key PESs, affording chemists new photon-driven synthetic possibilities in photoredox catalysis.68 112 Variation of the bpy ligand (columns in Figure 3.2) modifies the MLCT λmax (Figure 3.3B) and the rate constant of homolysis at a given excitation wavelength; there is in fact a linear correlation between the normalized logarithm of the rate constant and the σp of the bpy substituent (Figure 3.5). Increased electron-withdrawing strength of the bpy lowers the energies of the MLCT excited-state PES manifold (e.g., the MLCT of 1D is 8.1 kcal mol–1 lower relative to 1A; see Figure 3.12A). If the MLCT surface was the only excitedstate PES involved in the bond-rupture mechanism, one would expect an increased barrier and an accordingly slower rate for homolysis in 1D than for 1A. However, when considering the barrier as governed by the crossing point between the MLCT and 3(MLCT + LMCT) states, the correct rate trend is recovered. Increasing the electron-withdrawing strength of the bpy enables more facile aryl-to-Ni(III) charge transfer, lowering the LMCT energies. Furthermore, from the Hammett trends/slopes (Figure 3.5), we see a greater impact on the rate constant when modulating the aryl substituent (i.e., the LMCT component) versus the bpy substituent. Thus, while the energetic shifts of the 1MLCT and 3(MLCT+LMCT) PESs are of the same sign, they shift at different rates as a function of ligand perturbation. The ligand-based effects on the 3(MLCT + LMCT) repulsive excited state outweigh the drop in the initial MLCT energy. Ultimately, these effects result in a lower barrier for homolysis (e.g., 1D has a barrier of 1.4 kcal mol–1 lower than that for 1A; Figure 3.12B) and, thus, a larger rate constant. The ligand-based perturbations to the excited-state PESs described in Figure 3.12C demonstrate that the coupled changes to the overall excited-state PES manifold, including surface intersections and barriers, govern the barriers and rates of Ni(II)–C bond homolysis. 113 Figure 3.12. Comparison between the change in energy of the MLCT λmax and (A) the experimental ln(kobs,1) obtained with 390 nm excitation and (B) the energetic barrier for photolysis for 1A–1D and 1B–5B. Barriers are estimated using the Arrhenius equation (assuming a uniform pre-exponential factor) and are normalized to the experimental value obtained for 1B. (C) PES diagram illustrating how the barrier for photolysis is dependent on both the 1MLCT and repulsive 3(MLCT+LMCT) surfaces. Full detailed PESs for 1A– 5D are given in Figures S68 and S69. Electronic structure calculations have provided further insights into these experimental observations, successfully correlating the calculated PES features to the experimental rate constants by capturing the critical aspects that lead to geometric and electronic structure control over the excited-state PES manifolds (Figure 3.9). Also, both TD-DFT and CASSCF/QD-NEVPT2 methods predict analogous mechanisms that exploit “one-photon, two-electron” repulsive 3(MLCT + LMCT) excited-state PESs. In particular, Figure 3.11 demonstrates a strong correlation between the computed excited-state activation energies and the experimental ln(kobs,1) obtained using 390 nm excitation. The activation 114 energies are predicted to arise from the surface crossing between the highenergy 1MLCT [d(yz) → π*(2)] PES and the two-electron triplet repulsive surface, 3(MLCT + LMCT). Thus, the calculations are consistent with the overall experimental data and mechanism presented in Figure 3.12C. Further comment on the specific mode of entry into the repulsive 3(MLCT + LMCT) state is warranted. Given that the UV–vis electronic absorption spectra of Ni(II)–bpy compounds exhibit dominantly MLCT intensity, there is a minimum energy threshold for homolysis, and the calculated ligand field transitions are lower than photochemically active higher-energy MLCT excited states, we posit that MLCT surfaces provide an entry into the 3(MLCT + LMCT) state. However, we do not know if ISC occurs between 1,3MLCT states or the 3(MLCT + LMCT) states. It may also be the case that vibronic coupling between charge-transfer and ligand field states may be an important component of the mechanism (vide infra).67 Therefore, experimental determination of the energies of the ligand field transitions across these Ni(II)–bpy aryl halide complexes would be informative. If the 1(d–d) transitions are indeed experimentally obscured by the dominant, photochemically active MLCT intensity, electronic absorption will not be appropriate. Due to the C-term intensity mechanism, low-temperature optical MCD would be ideal for this. However, the diamagnetic ground states of these complexes will also likely complicate ligand field assignments. As an alternative, 2p3d RIXS is a less commonly employed, but powerful, methodology to obtain spectroscopic insights into the spin-allowed and forbidden ligand field excited-state manifolds.69−74 This technique represents a potential approach to observe the ligand field excited-state manifold in the presence of chargetransfer bands, which will provide important insight into potential 1(d–d) states as a mode of entry to a dissociative 3LMCT state.51 Ni(II)–bpy aryl halide complexes feature a high density of excited states (Figure S68 and Tables S10 and S11), many vibrational degrees of freedom, and large spin-orbit coupling. The combination of these factors can complicate mechanistic analyses that feature discrete processes of vibrational relaxation, internal conversion, and ISC, especially when they 115 occur on the time scale of molecular vibrations (i.e., sub-picosecond time scale).39,67 Thus, the ultrafast spectroscopic characterization of the homolytically active state in Ni(II)–bpy aryl halide complexes may be hampered by the nature of the excitedstate homolysis mechanism. That is, the spectral dynamics will be dominated by MLCT and ligand field states, and there may be little-to-no dynamics detectable for the repulsive 3(MLCT + LMCT) surfaces proposed here, unless they can be accessed in sufficient amounts, perhaps with high-energy excitation. Additionally, it may be that the TD-DFT and multiconfigurational/multireference mechanistic pictures provided by the calculated PESs need to be further supplemented to account for more complex aspects of spin-vibronic coupling.67 Further experimental characterization of aspects such as the specific state that provides entry to the repulsive 3(MLCT + LMCT) states, as well as more detailed experimental characterization of potential spin-vibronic coupling effects, will be useful for better understanding how they manifest in the mechanism of excited-state bond homolysis. Finally, it is instructive to note that findings reported here resemble results from detailed photochemical investigations of d6 Re-complexes, Re(R)(CO)3(α–diimine), where R = Me, Et, Bz.30−38 For these complexes, it was found that the excited-state Re–R bond homolysis is a result of the formation of a repulsive σπ* excited state. In the case of the Re systems, the σπ* excited state refers to an electronic configuration in which one electron is added to a π* orbital of the α-diimine ligand (1MLCT) and one electron is depleted from the Re–R bond (LMCT). The final dissociative state is not accessible via direct excitation but is instead accessed from excitation of the 1MLCT state followed by the LMCT. Moreover, excited-state homolysis in the Re(Me)(CO)3(α–diimine) complex occurs on the subpicosecond time scale with a small quantum yield, is dependent on temperature and excitation wavelengths, and has activation energies between ∼1.8 and 5.5 kcal mol–1 using 458–502 nm excitation (∼21,830 to 19,920 cm–1).31 These photophysical properties and excited-state barriers are in the range of those determined for the Ni(II)–bpy aryl halide complexes studied here and are indicative of a dissociative excited state that is higher in energy than the 1MLCT excited state, creating a barrier and surface crossing (Figure 116 3.12A,B). It was further noted in the Re complex that enhanced quantum yields correlated with increasing energy of excitation; this behavior mirrors the rate accelerations we observed herein upon increased excitation energy (Figure S25). This observation was interpreted as arising from excitation into higher-energy vibronic levels of the 1MLCT state, which promotes the 1MLCT → σπ* surface hopping.30,36 Based on these similarities, in particular, the increased homolysis rate constants with increasing energy of excitation into the higher-energy MLCT transitions (Figure 3.8), we hypothesize that the mechanism of excited-state homolysis in Ni(II)–bpy aryl halide complexes may indeed feature a similar vibronically mediated component, wherein the optically excited 1MLCT is vibronically coupled to dissociative 3(MLCT + LMCT) states or even potentially to weakly absorbing ligand field excited states, as observed by Park et al.51 Overall, we highlight Relike reactivity in Ni-catalysts, adapting the excited-state dynamics of third-row transitionmetal complexes to first-row, earth-abundant metal-based systems. §3-4. Conclusions In summary, we have demonstrated that rate constants of excited-state Ni(II)–C bond homolysis are temperature- and wavelength-dependent. Both of these experimental observations point to a thermal barrier involved in the photophysical mechanism. The barrier is moderate and inconsistent with thermally driven homolysis from a low-energy ligand field excited state. Additionally, we have demonstrated a linear correlation between bpy and aryl perturbations and the observed rate constants of homolysis using a consistent energy of irradiation. In this way, pinning the excitation wavelength reveals the ligandinduced electronic perturbations to the energetic barrier for Ni(II)–C bond homolysis by controlling the key excited-state MLCT/LMCT surfaces. In accordance with this, electronic structure calculations predict a mechanism that exploits “one-photon, twoelectron” repulsive 3(MLCT + LMCT) excited-state PESs and reveal a strong correlation between the computed excited-state activation energies and the experimental ln(kobs,1) obtained using 390 nm excitation. The activation energies are predicted to arise from the surface crossing between the high-energy 1MLCT [d(yz) → π*(2)] and the two-electron triplet repulsive surfaces, highlighting the specific excited-state PESs that contribute to 117 Ni(II)–bpy-mediated photoredox catalysis. This study provides insights into the electronic structural control over light-induced homolysis and, thus, guides the use of photonic energy as a sustainable alternative to coupling catalysis as carried out by precious metals. 118 §3-5. References (1) Nicewicz, D. A.; MacMillan, D. W. C. Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes. 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Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis. Angew. Chem. Int. Ed. 2021, 60 (39), 21100–21115. https://doi.org/10.1002/anie.202100270. (63) Hwang, S. J.; Powers, D. C.; Maher, A. G.; Anderson, B. L.; Hadt, R. G.; Zheng, S.L.; Chen, Y.-S.; Nocera, D. G. Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes. J. Am. Chem. Soc. 2015, 137 (20), 6472–6475. https://doi.org/10.1021/jacs.5b03192. (64) Richards, W. G.; Trivedi, H. P.; Cooper, D. L. Spin-Orbit Coupling in Molecules. In The International series of monographs on chemistry, Clarendon Press, 1981, pp 17. (65) Harvey, J. N. Understanding the Kinetics of Spin-Forbidden Chemical Reactions. Phys. Chem. Chem. Phys. 2007, 9 (3), 331–343. https://doi.org/10.1039/B614390C. 125 (66) Harvey, J. N. Spin-Forbidden Reactions: Computational Insight into Mechanisms and Kinetics. WIREs Comput. Mol. Sci. 2014, 4 (1), 1–14. https://doi.org/10.1002/wcms.1154. (67) Penfold, T. J.; Gindensperger, E.; Daniel, C.; Marian, C. M. Spin-Vibronic Mechanism for Intersystem Crossing. Chem. Rev. 2018, 118 (15), 6975–7025. https://doi.org/10.1021/acs.chemrev.7b00617. (68) Gisbertz, S.; Reischauer, S.; Pieber, B. Overcoming Limitations in Dual Photoredox/Nickel-Catalysed C–N Cross-Couplings Due to Catalyst Deactivation. Nat. Catal. 2020, 3 (8), 611–620. https://doi.org/10.1038/s41929-020-0473-6. (69) Wang, R.-P.; Liu, B.; Green, R. J.; Delgado-Jaime, M. U.; Ghiasi, M.; Schmitt, T.; van Schooneveld, M. M.; de Groot, F. M. F. Charge-Transfer Analysis of 2p3d Resonant Inelastic X-Ray Scattering of Cobalt Sulfide and Halides. J. Phys. Chem. C 2017, 121 (45), 24919–24928. https://doi.org/10.1021/acs.jpcc.7b06882. (70) Al Samarai, M.; Hahn, A. W.; Beheshti Askari, A.; Cui, Y.-T.; Yamazoe, K.; Miyawaki, J.; Harada, Y.; Rüdiger, O.; DeBeer, S. Elucidation of Structure-Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L-Edge X-Ray Absorption Spectroscopy and 2p3d Resonant Inelastic X-Ray Scattering. ACS Appl. Mater. Interfaces 2019, 11 (42), 38595–38605. https://doi.org/10.1021/acsami.9b06752. (71) de Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. 2p X-Ray Absorption of 3d Transition-Metal Compounds: An Atomic Multiplet Description Including the Crystal Field. Phys. Rev. B 1990, 42 (9), 5459–5468. https://doi.org/10.1103/PhysRevB.42.5459. (72) Van Kuiken, B. E.; Hahn, A. W.; Maganas, D.; DeBeer, S. Measuring Spin-Allowed and Spin-Forbidden d–d Excitations in Vanadium Complexes with 2p3d Resonant Inelastic X-Ray Scattering. Inorg. Chem. 2016, 55 (21), 11497–11501. https://doi.org/10.1021/acs.inorgchem.6b02053. (73) Hahn, A. W.; Van Kuiken, B. E.; Chilkuri, V. G.; Levin, N.; Bill, E.; Weyhermüller, T.; Nicolaou, A.; Miyawaki, J.; Harada, Y.; DeBeer, S. Probing the Valence Electronic Structure of Low-Spin Ferrous and Ferric Complexes Using 2p3d Resonant Inelastic XRay Scattering (RIXS). Inorg. Chem. 2018, 57 (15), 9515–9530. https://doi.org/10.1021/acs.inorgchem.8b01550. 126 (74) Hahn, A. W.; Van Kuiken, B. E.; al Samarai, M.; Atanasov, M.; Weyhermüller, T.; Cui, Y.-T.; Miyawaki, J.; Harada, Y.; Nicolaou, A.; DeBeer, S. Measurement of the Ligand Field Spectra of Ferrous and Ferric Iron Chlorides Using 2p3d RIXS. Inorg. Chem. 2017, 56 (14), 8203–8211. https://doi.org/10.1021/acs.inorgchem.7b00940. 127 Chapter 4 Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)–Cl Oxidative Addition and Dimerization Reactivity Pathways Adapted with permission from: Cagan, D. A.;† Bím, D.;† McNicholas, B. J.; Kazmierczak, N. P.; Oyala, P. H.; Hadt, R. G. Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)–Cl Oxidative Addition and Dimerization Reactivity Pathways. Inorg. Chem. 2023, 62(24), 9538–9551. DOI: 10.1021/acs.inorgchem.3c00917. †Co-first author. Copyright 2023 American Chemical Society. 128 §4-1. Introduction Photoredox catalysis has captivated the fields of organic and inorganic chemistry, with nickel(II)–bipyridine (bpy) aryl halide complexes retaining a prominent place as the metalligand scaffold of choice for numerous cross-coupling reactivity pathways.1–11 Among these, C–C and C–heteroatom couplings have been facilitated by Ni(II)–bpy complexes through either the use of an external photosensitizer (e.g., Ir(ppy)3) or via direct excitation of the Ni(II)–bpy aryl halide complex.12–18 Due to the diverse reactivity and intriguing photophysics of these complexes, much interest has been placed on understanding the underlying photophysical and thermal processes involved in photoredox mediated cross-coupling reactivity.19–28 While originally thought to proceed through a Ni(0)/Ni(II) cycle,6 recent work has instead supported a Ni(I)/Ni(III) cycle in the direct excitation pathway (Figure 4.1A).23,29,30 Through analysis of a library of Ni(II)–bpy aryl halide complexes, we have revealed that excited-state Ni(II)–Caryl bond homolysis from the S = 0 square planar Ni(II) ground state features a key ligand-to-metal charge transfer (LMCT) process. This LMCT results in electron excitation between the Ni–aryl σ and σ* orbitals, which lowers the bond order from one to zero, resulting in repulsive homolytic bond cleavage and the generation of an aryl radical and a three-coordinate Ni(I)–bpy halide species.27 Related Ni(I)–bpy halide complexes have been prepared by alternate methods, including pulse radiolysis, electrolysis, and independent synthesis.30–33 Importantly, these compounds have demonstrated potency for the activation of aryl halide substrates. Following the work by Vicic et al. on Ni(I)–terpyridine complexes,34–36 Bird and MacMillan et al. reported a nickel(I)(4,4′-di-tert-butyl bipyridine)bromide complex (Ni(I)(t-Bubpy)Br) that exhibited rapid reactivity toward aryl iodides with second-order rate constants of ~104 M-1 s-1.31 The activation of C(sp2)–Br substrates was demonstrated by Doyle et al. using a nickel(I)(diethyl2,2′-bipyridine-4,4′-dicarboxylate)chloride complex, Ni(I)(EtOOCbpy)Cl. Activation of the stronger aryl bromide bond proceeded with slower, but still catalytically relevant, rate 129 constants of ~10 –10 M s . Notably, C(sp )–Cl bonds could not be activated by this -2 1 -1 -1 30 2 Ni(I) complex. Careful kinetic analysis has been employed to understand the mechanism of this reactivity;30,37 the Ni(I) species has been suggested to undergo two-electron oxidative addition to form a short-lived five-coordinate Ni(III)–bpy aryl dihalide complex. This Ni(III) intermediate rapidly decays by comproportionation with another equivalent of Ni(I), forming Ni(II)–bpy aryl halide and Ni(II)–bpy dihalide complexes (Figure 4.1A).30 Using threecoordinate pyridinophane Ni(I) model complexes, Mirica et al. have also provided evidence for this Ni(I)/Ni(III) oxidative addition pathway.29 Therefore, these and related studies have argued for the importance of three-coordinate Ni(I)–bpy halide complexes to facilitate the oxidative addition step, without which the cross-coupling catalytic cycle would not close.21,23,38,39 Figure 4.1. (A) Catalytic cycle displaying the photochemical activation of Ni(II)–bpy aryl halide complexes proposed in ref. 27 for the generation of Ni(I) and Ni(III) intermediates. (B) Two competing pathways for the Ni(I) complexes investigated here: oxidative addition and dimerization. (C) Photogenerated Ni(I)(Rbpy)X structures examined in this work (R = t-Bu, H, MeOOC; X = Cl, Br, I). 130 While the reactivity of Ni(I)–bpy complexes has proven desirable for organic synthesis, their solution-phase stabilities vary widely, limiting their scope. Numerous Ni(I) dimerization products have been characterized, and the resultant polypyridyl species are no longer reactive toward oxidative addition with aryl halides.30,32,40,41 Dimerization thereby acts as an off-cycle sink with diminished catalytic activity (Figure 4.1A). Hazari et al. independently synthesized and characterized the dimeric [Ni(I)(t-Bubpy)Cl]2 complex. Once formed, the compound was stable in solution and exhibited no oxidative addition reactivity even with weak C(sp2)–I bonds.32 The related [Ni(I)(t-Bubpy)Br]2 complex was studied by Bird and MacMillan et al. and similarly showed no oxidative addition reactivity with aryl iodides.31 Additionally, tetrameric [Ni(I)(EtOOCbpy)Cl]4 and dimeric [Ni(I)(EtOOCbpy)Cl]2 complexes were studied by Doyle et al.30 In solution, both species can dissociate to form the monomeric Ni(I)(EtOOCbpy)Cl complex mentioned above. This three-coordinate species was reported to exist in solution-phase equilibria with Ni(II)Cl2 and Ni(0)(EtOOCbpy)2. However, the oxidative addition reactivity was favored from the Ni(I) complex, not the Ni(0) species, and no reactivity was found for the dimeric or tetrameric species. Understanding the interplay between the oxidative addition and dimerization/oligomerization pathways available to three-coordinate Ni(I)–bpy halide complexes is critical for optimized ground-state and metallaphotoredox catalysis and reaction development (Figure 4.1B). There is also a fundamental knowledge gap related to specific structure-function relationships underlying the elementary oxidative addition step in Ni(I)–bpy complexes, a key component of the scope of bond activations that can be achieved. To the best of our knowledge, no systematic study has either compared the oxidative addition reactivity and solution-phase stability of Ni(I)–bpy complexes or rationalized the relative rates of oxidative addition upon structural perturbation. Here we accomplish both of these aims through the facile photogeneration of a series of Ni(I)(Rbpy)X species (R = t-Bu, H, MeOOC; X = Cl, Br, I; Figure 4.1C). We provide direct evidence for C(sp2)–Cl bond activation with Ni(I)–bpy complexes, which thus far has only been implied as part of catalytic cycles.33,42 Furthermore, we broadly detail the kinetics and 131 2 mechanisms of their oxidative addition reactivity toward C(sp )–X (X = Cl, Br, I) bonds, and their thermal barriers for dimerization. Results reported herein implicate key ligand effects on the electronic structure of the Ni(I)–bpy halide complexes that result in tunable reactivity and variable solution-phase lifetimes. Ultimately, the ability of Ni(I)–bpy complexes to activate challenging C(sp2)–Cl bonds stems from bpy-induced modulation of the effective nuclear charge (Zeff) of the Ni(I) center. Related analyses reported here have afforded a molecular orbital-based picture of oxidative addition reactivity, which provides specific electronic structure benchmarks that need to be achieved to activate strong C(sp2)– X bonds and opens the door for the targeted synthesis of next-generation cross-coupling catalysts. §4-2. Results and Analysis §4-2.1 Photochemical Synthesis and Spectroscopic Characterization of Ni(I)–bpy Halide Complexes Parent four-coordinate Ni(II)(Rbpy)(o-tolyl)X (R = tert-butyl, H, MeOOC; X = Cl, Br, I) complexes were synthesized according to a previous report.27 Three-coordinate Ni(I)(Rbpy)X species, which are important reaction intermediates in photoredox cross-coupling catalysis, can be accessed directly from these precursors by air- and moisture-free irradiation at 370 nm using purple LEDs. Light excitation drives a key LMCT process that results in Ni(II)– Caryl excited-state bond homolysis; this shorter wavelength enhances the quantum yield of Ni(I)(Rbpy)X generation relative to more commonly used, longer wavelength blue light sources (Table S2). Typical irradiation times were ~30–60 minutes; irradiation time variations among complexes depended on the rate of photo-driven decay of the starting Ni(II) complex and the rate of Ni(I) decomposition. Global kinetic analysis provides strong support for a first-order process in which the Ni(II)–bpy aryl halide parent complex is cleanly photolyzed to the Ni(I)–bpy halide (for more details, see Supporting Information Section S1.5, including Figures S6-S10). To systematically probe the reactivity and stability of the photochemically generated Ni(I) complexes, the bpy ligand was varied with electron donating tert-butyl groups 132 t-Bu (Ni(I)( H bpy)Cl, 1-Cl), electronically neutral H-atoms (Ni(I)( bpy)Cl, 2-Cl), and electron-withdrawing methyl ester groups (Ni(I)(MeOOCbpy)Cl, 3-Cl). The halide was also varied by first synthesizing Ni(II)(t-Bubpy)(o-tolyl)Br and Ni(II)(t-Bubpy)(o-tolyl)I parent complexes, followed by irradiation to form Ni(I)(t-Bubpy)Br (1-Br) and Ni(I)(t-Bubpy)I (1-I), respectively. Note that 1-Br is the same as that examined by Bird and MacMillan et al. by pulse radiolysis,31 while 3-Cl is similar to that studied by Doyle et al.,30 the only difference being the appended methyl ester vs. ethyl ester substituents, following our previous study.27 Having direct access to the Ni(I)–bpy halide complexes via photolysis allows for their characterization using UV–vis electronic absorption and EPR spectroscopies (Figure 4.2A and Figure 4.2C, respectively). Irradiation of Ni(II) parent compounds and spectral analysis were done in tetrahydrofuran (THF). The primary UV–vis absorption band in the ~550 nm– 900 nm region of the three-coordinate Ni(I)(Rbpy)X species shifts strongly with changes to the bpy ligand from 660 nm (1-Cl) to 805 nm (3-Cl). Moderate shifts are also observed when changing the halide (640 nm for 1-I to 660 nm for 1-Cl, Figure 4.2A). To further corroborate the spectral assignment of UV–vis peak to the Ni(I)–bpy halides, we conducted reductive spectroelectrochemistry on Ni(t-Bubpy)Cl2 to form Ni(t-Bubpy)Cl; the resultant spectrum is identical to that of photogenerated 1-Cl (Figure 4.2B). From TD-DFT calculated spectra (bottom of Figure 4.2A; see Computational Details in Supporting Information Section S2.1), the intensity of the primary absorption band is attributed to Ni(I)-to-bpy MLCT transitions (3d(xz/yz) → π*(1); note these orbitals are labeled according to their parallel (||) or perpendicular (⊥) orientation to Ni‒halide bond— see details in Supporting Information Section S2.4). Consistent with the experimental spectra, smaller shifts are observed for variations in the halide; the MLCT energies are more sensitive to bpy variation, with an energy trend of 1-Cl > 2-Cl > 3-Cl. Through analyses of the molecular orbital energy diagrams for these complexes (Figures S40-S44 and Table S7), the MLCT energy shift arises mainly from the stabilization of the bpy π* acceptor orbitals as a function of electron-withdrawing substituents. 133 Figure 4.2. Spectroscopic characterization of the Ni(I)–bpy halides examined in this study. (A) UV–vis absorption spectra (THF) of the three-coordinate Ni(I)–bpy halide complexes, highlighting the Ni(I)-to-bpy MLCT transitions and the excellent spectral agreement between experiment and TDDFT calculations (dashed lines). (B) Top: UV–vis spectrum for the photochemical preparation of 1-Cl (solid line) from its Ni(II)–bpy aryl halide parent (dashed lines) using 370 nm LEDs. Bottom: Spectroelectrochemistry of Ni(II)(t-Bubpy)Cl2 (dashed lines) forming Ni(I)(t-Bubpy)Cl (solid line) in THF with TBAPF6 electrolyte. For more details, see Figure S13. (C) Frozen-glass X-band CW-EPR spectra for the S = 1/2 Ni(I) region (T = 5 K; solvent = 2-MeTHF; frequency for 1-Cl, 1-Br, 3-Cl = 9.637 GHz, for 1-I = 9.646 GHz; power = 2.2 mW; modulation amplitude = 8 G). Corresponding g values are given in Table 4.1. Starred peaks may correspond to THF coordination. 134 Table 4.1. Experimental g values for the Ni(I)–bpy halide complexes in 2-MeTHF at 5 K. DFT computed g values are given in parentheses. Compound gz,exp. (gz,calc.) gx,exp. (gx,calc.) gy,exp. (gy,calc.) giso,exp. (giso,calc.) 1-Cl 2.248 (2.238) 2.050 (2.070) 2.070 (2.174) 2.123 (2.161) 1-Br 2.255 (2.246) 2.042 (2.080) 2.079 (2.166) 2.125 (2.164) 1-I 2.370 (2.262) 2.044 (2.094) 2.075 (2.161) 2.163 (2.172) 2-Cla — (2.238) — (2.066) — (2.190) — (2.165) 3-Cl 2.217 (2.252) 2.171 (2.057) 2.195 (2.251) 2.194 (2.187) a Sufficiently high concentration samples of 2-Cl were precluded due to precipitation. As such, no EPR signal could be resolved experimentally. 14N hyperfine values of Ax = 146 MHz, Ay = 91 MHz, and Az = 64 MHz were fit for 3-Cl and required to model the line shape. Computational details are given in Supporting Information Section S2.1. While room temperature EPR analysis provided no resolvable signals, likely due to rapid spin relaxation times (Figure S31), spectra taken at 5 K in 2-methyl tetrahydrofuran (2MeTHF) after irradiation provided signals characteristic of S = 1/2 Ni(I) species (Figure 4.2C), with g values in the range of gz = ~2.22–2.37, gx = ~2.04–2.17, gy = ~2.07–2.20, and giso = ~2.12–2.19 (Figure 4.2C; Table 4.1). The axial g tensor values are overall consistent with a single unpaired electron in the Ni(I) 3d(x2–y2) orbital. The EPR spectra and g values of compounds 1-Cl/Br and 3-Cl are congruent with previous reports for Ni(I) halide species with aromatic ligand backbones (e.g., phenanthroline, bipyridine, and bis(pyrazolyl)pyridines).30,39,42–45 The 14N hyperfine values employed in the 3-Cl simulation are of comparable magnitude to those previously reported for Ni(I)-neocuproine complexes (0, 50, and 170 MHz).32 Sufficiently high concentration samples of 2-Cl were precluded due to a precipitation (dimerization) pathway (vide infra, Section §4-2.4); as such, no EPR signal could be detected. The intermediate peak at ~310 mT in 1-Cl/Br/I is likely attributable to THF solvent coordination (see Supporting Information Section 2.7), as it does not appear in previous spectra recorded in toluene but does appear in toluene:THF solvent mixtures and in neat 2-MeTHF.42,43 These published data share excellent agreement with the spectra collected and presented in Figure 4.2C. Importantly, the relative intensities of the gz and 135 intermediate peaks remain approximately constant with variations in photolysis time, indicating there is not a second Ni(I) species with a distinct kinetic profile (Figures S33– S35). The DFT calculated g values are in general agreement with the experimental data, with the exception of the gz value for 1-I and significantly more predicted rhombicity throughout (computed gx and gy in Table 4.1). We also note the presence of additional complex signals in the half-field region of the concentrated samples of 1-Cl in the low-temperature (5 K) EPR (Figures S33-S35). Variabletemperature (VT) UV–vis experiments indicate that these are attributable to a reversible concentration- and temperature-dependent speciation (see Supporting Information Section S1.9). That is, these additional species form rapidly upon freezing the EPR samples. Future studies should consider speciation changes that can occur upon freezing samples for lowtemperature spectroscopic characterization. That said, under standard catalysis conditions (~0.2 mM, room temperature) studied herein, the Ni(I)–bpy halide complexes corresponding to the S = 1/2 signals are the dominant species after irradiation is terminated (~95% Ni(I) by VT UV–vis analysis), with the additional species (~5%) being the starting Ni(II)–bpy aryl halide complex. §4-2.2. Oxidative Addition Kinetics with 2-Chloro-toluene Having demonstrated near quantitative conversion of parent Ni(II) complexes to the threecoordinate Ni(I) complexes, we sought to gauge their relative reactivity toward oxidative addition. As expected, room temperature addition of excess 2-bromo-toluene or 2-iodotoluene (0.2 mL) to all photogenerated Ni(I) complexes studied herein resulted in immediate color changes, with loss of the Ni(I)-bpy MLCT features observed in the UV–vis spectra (Figures S14-S15). The diagnostic S = 1/2 Ni(I) EPR signal is also quenched upon addition of aryl halide (Figure S36). Again, reactivity with C(sp2)–I and C(sp2)–Br bonds is consistent with previous reports.30,31 However, interestingly, many of the Ni(I) intermediates studied herein also react with 2-chloro-toluene—the exception being 3-Cl. As a control, addition of 0.2 mL of toluene did not result in UV–vis spectral changes, further implicating the reactivity 136 of the Ni(I) species with the C(sp )–Cl bond. The lack of reactivity of 3-Cl is also 2 significant and is directly related to the bpy ligand, as discussed further in Section §4-2.3. Taking 1-Cl as a representative compound, we sought to confirm the C(sp2)–Cl reactivity by stoichiometric 1H NMR studies in d8-THF. However, to facilitate reaction turnover at a rate greater than the decay of the Ni(I) intermediate, a large excess of 2-chloro-toluene is necessary, which overwhelms the 1H NMR analysis. Using fewer equivalents of aryl halide, the reaction between 2-chloro-toluene and the photogenerated Ni(I) species is too slow and precludes definitive product speciation assignments (Supporting information Section S1.7). This can be circumvented using a fluorinated aryl halide and 19F NMR analysis. As depicted in Scheme 4.1, addition of 20 μL of 2-chloro-α,α,α-trifluorotoluene (~200-fold excess, 19F NMR peak at –63 ppm) to a solution of 1-Cl afforded a new peak in the 19F NMR spectrum at –58 ppm. Independent synthesis27 confirms this peak originates from the four-coordinate complex, Ni(II)(t-Bubpy)(CF3Ph)Cl (Figure S18). Thus, this experiment provides direct evidence of a Ni(I)–bpy halide complex activating an aryl chloride substrate. We now describe the relative reactivity of the Ni(I)–bpy complexes toward C(sp2)–Cl oxidative addition. Scheme 4.1. Oxidative addition reaction conducted with the photogenerated Ni(I)–bpy halide, 1-Cl. Incident wavelength = 370 nm; solvent = d8-THF. Formation of Ni(II)(t-Bubpy)(CF3Ph)Cl was verified by 19F NMR; the high spin Ni(II)–bpy dihalide is paramagnetic and not observed, but is invoked to account for the mass balance on the basis of previous work.30 Pseudo-first-order kinetics investigations with quantitative addition of excess 2-chlorotoluene to the photogenerated Ni(I) complexes were carried out by monitoring the fast decay 137 of the primary MLCT absorption feature via UV–vis spectroscopy (Figure 4.3A and Figure S20A). The natural logarithm of the normalized peak absorbance varies linearly with time for ~3-5 half-lives, depending on the magnitude of starting Ni(I) absorption (Figure 4.3B and Figure S20B). The slope of this correlation provides kobs values that vary linearly with 2-chloro-toluene concentration (Figure 4.3C and Figure S20C), yielding second-order rate constants for oxidative addition, kOA (M-1 s-1) (Table 4.2). The bimolecular rate constants are on the order of 10-2 M-1 s-1, six orders of magnitude slower than the reaction of 1-Br with aryl iodides and two orders of magnitude slower than the reaction of Ni(I)(EtOOCbpy)Cl with aryl bromides.30,31 This reduction in rate constant for the activation of 2-chloro-toluene is attributable to the increased bond dissociation free energy (BDFE) of the carbon–halogen bond (BDFE of C(sp2)–Cl > C(sp2)–Br > C(sp2)–I, Table S15) and a steric effect of the orthomethyl group (Figure 4.4). Analysis of ligand effects reveals that changes to the halide manifest in minor changes in kOA (Figure S20). These results agree well with the electronic effects of the halide as predicted by the Hammett parameters (σp = 0.23 for Cl and Br, and 0.18 for I).46 More pronounced effects are observed upon variation of the bpy substituent. Substitution of the electrondonating tert-butyl groups (σp = –0.20) for hydrogens (σp = 0.0) in going from 1-Cl to 2-Cl results in a twofold decrease in kOA from 7.2 ± 0.2 x 10-2 M-1 s-1 to 3.2 ± 0.2 x 10-2 M-1 s-1. Furthermore, introduction of the electron-withdrawing methyl ester groups (σp = 0.45) eliminates C(sp2)–Cl oxidative addition reactivity altogether. As described further below in Section §4-2.3, these differences in oxidative addition reactivity can be traced directly to ligand-induced differences in Zeff on the metal, as changes in Zeff tune the energy of the RAMO involved in the oxidative addition reaction. Follow-up work is currently underway to push the limits of Ni(I)-facilitated oxidative addition reactivity by substituent-based modulation of Zeff. 138 Figure 4.3. Experimental kinetic analysis of the oxidative addition reaction for 1-Cl, 2-Cl, and 3-Cl. (A1, B1, C1) UV–vis absorption plots in THF showing the addition of 2-chlorotoluene to photochemically generated Ni(I)–bpy halides (initial spectrum of Ni(I)–bpy halide, blue line (~10-4 M); after 2-chloro-toluene addition, orange line). Note the orange spectra depict unreacted and newly generated parent four-coordinate Ni(II)–bpy aryl halide (Figure S1-S2). (A2, B2, C2) Linear plots of ln([Ni(I)]/[Ni(I)]0) vs. time showing the pseudofirst-order nature of the oxidative addition reaction over several half-lives; [2-Cl-Tol] = 0.14 M (red circles), 0.20 M (orange triangles), 0.27 M (blue squares), and 0.41 M (purple diamonds). (A3, B3, C3) Plots of kobs vs. [2-Cl-Tol]. Error bars are one standard deviation of three trials. Slope of the fitted line gives second-order rate constants, kOA (M-1 s-1). Complex 3-Cl showed no reaction with 2-chloro-toluene. Analogous data for 1-Cl, 1-Br, and 1-I are given in Figure S20. 139 Table 4.2. Rate constants for room temperature oxidative addition for the reaction between 2-chloro-toluene and various Ni(I)(Rbpy)X species (R = t-Bu, H, MeOOC; X = Cl, Br, I).a Compound σp (R) σp (X) kOA (x10-2 M-1 s-1) 1-Cl -0.20 0.23 7.2 ± 0.2 1-Br -0.20 0.23 7.0 ± 0.4 1-I -0.20 0.18 6.5 ± 0.6 2-Cl 0.00 0.23 3.2 ± 0.2 3-Cl 0.45 0.23 no reaction a Reactions were done under inert atmosphere in anhydrous THF and followed by UV–vis spectroscopy. Reported errors are one standard deviation of three trials. Hammett σp values taken from ref. 46. §4-2.3. Oxidative Addition Mechanistic Investigations Having examined the effect of the substituents on the Ni(I)–bpy halides toward the C(sp2)– Cl oxidative addition reactivity, we next turned to the kinetic dependence of the substituents on the aryl chloride substrates. We performed a Hammett analysis on the reactivity of series of para-substituted aryl chlorides with 1-Cl. The rate constants (kOA) of the formal Ni(I)/Ni(III) oxidative addition process increased by two orders of magnitude when switching from electron-donating to electron-withdrawing substituents (σp = – 0.27 to 0.50, Figure 4.4). We further found the slope (ρ) of the plot of ln(kx/kH) vs. σp was ~5, indicative of an SNAr-type activation47 of the C(sp2)– Cl bond, wherein the Ni(I) metal center acts as a nucleophile, attacking the polarized C(sp2)– Cl bond at the electron-deficient carbon. While an activation pathway that proceeds by single electron transfer (SET) also has a fairly large ρ value (~4),48 the reduction potentials of the aryl chlorides are too negative (experimentally found to be less than –3.2 V vs. Fc+/Fc, calculated as –3.5 V vs. Fc+/Fc; for more details see Supporting Information Sections S1.6 and S2.6) to be accessed by the Ni(I)–bpy halides (calculated Ni(II)/Ni(I) potential of 0.2 V vs. Fc+/Fc).49 140 Interestingly, we note that ρ ~5 represents a change in the specific mechanism of C(sp2)– X bond activation by Ni(I) halides. Aryl bromides and aryl iodides exhibit Hammett slope values of ρ ~2-3, typical of a concerted oxidative addition pathway,30,31,50,51 but the activation of the stronger, more polarized C(sp2)–Cl bond is better described as nucleophilic aromatic substitution. Figure 4.4. Hammett analysis for the reaction of 1-Cl with 4-substituted aryl chlorides. Recall that the reaction of 1-Cl with sterically hindered 2-chloro-toluene has kOA = 0.072 M-1 s-1. We note that Hammett analysis is commonly presented with base 10 log instead of the natural log, but we present the latter here for direct comparison to literature values for ρ.30,31,50,51 Hammett σp values taken from ref. 46. Reported standard errors are propagated from the linear least-squares analysis (Figures S21-S25). DFT calculations were used to further detail the mechanism of oxidative addition (i.e., stepwise vs. concerted two-electron nucleophilic attack by Ni) and to identify the key frontier molecular orbitals involved in this elementary reaction step. Oxidative addition of 2-chlorotoluene to a three-coordinate Ni(I)(Rbpy)X species yields a five-coordinate Ni(III)(Rbpy)(otolyl)(Cl)(X) intermediate. Following the initial oxidative addition step, comproportionation of the Ni(III) complex with an additional Ni(I)(Rbpy)X species can take place with a calculated Gibbs free energy of ~–23 kcal mol-1, which produces a considerable driving force for the overall reaction. Comproportionation can occur via transfer of a halide from the five- 141 coordinate intermediate, which is readily accessible through a barrierless transition state (Figure 4.5A). This step therefore results in the regeneration of a parent Ni(II)‒bpy aryl halide complex and the formation of a Ni(II) dihalide (i.e., the speciation determined experimentally by Diao et al. and Doyle et al. and 19F NMR and UV–vis studies presented herein (vide supra, Section 4.2.2)).30,39 The rate-determining step of the overall reaction is found to be oxidative addition, with calculated Gibbs free-energy barriers of ~21–23 kcal mol-1, consistent with the slow reaction rates at room temperature and the requirement of a large excess of 2-chloro-toluene (Figure 4.5A). The computed free energy barriers vary by only ~1.3 kcal mol-1 for 1-Cl, 1-Br, 1-I, and 2-Cl (i.e., within the general accuracy obtainable with DFT calculations).55,56 However, this observation is still in accord with the differences in kOA for the compounds observed experimentally (Table 4.2). Furthermore, the highest computed activation free energy is observed for 3-Cl (~23 kcal mol-1, red in Figure 4.5A), consistent with its lack of reactivity with 2-chloro-toluene. We have also computationally reproduced the Hammett analysis for the reaction of 1-Cl with 4-substituted aryl chlorides and have found excellent agreement with experiment (calculated ρ ~5.8; see Supporting Information Section S2.6). Furthermore, the calculated bond lengths in the transition state show that the new Ni–Cl bond is ~0.3 Å longer than the new Ni–C bond (potentially implying the Ni– C bond is formed prior to the Ni–Cl bond and corroborating an SNAr-type mechanism, Figure 4.5B). Additionally, we observe that the experimental rate constants trend linearly and positively with the Löwdin atomic charges, the Mulliken atomic charges, and the natural population analysis (NPA) charges at the carbon in the Ar–Cl bond, but they do not trend well with the energy of the LUMO of the aryl chlorides (Figure S66). This observation is also indicative of an SNAr mechanism as the electrophilicity of this carbon is related to the rate and not the reduction potential of the aryl chlorides (as would be expected for SET).57 142 Figure 4.5. (A) DFT energetics of the oxidative addition reactions of Ni(I)(Rbpy)X with 2chloro-toluene and (B) the transition state structure of 1-Cl with 2-chloro-toluene. Relative Gibbs free energy values were computed at the B3LYP-D3/def2-TZVP(CPCM)//BP86D3/def2-TZVP(CPCM) level.52–54 Computed free energies are provided in Table S13. A related plot showing the DFT energetics of the oxidative addition with 2-bromo-toluene and 2-iodo-toluene is given in Figure S65. (C) Computational analysis of the oxidative addition process. The doubly occupied Ni 3d(z2) orbital transforms into a new Ni–Caryl σ-bond (blue), transferring electrons into the Caryl‒Cl σ* orbital (orange). This breaks the Caryl‒Cl σ bond, affording a new Ni‒Cl σ-bond. Tabulated are the energies of the β Ni 3d(z2) orbital for several steps along the intrinsic reaction coordinate (IRC), the Ni 3d character in the Caryl‒Cl σ* orbital at the transition state, and the Löwdin spin density at the Ni center. Note energies of Ni 3d(z2) orbitals trend with the Ni(I) reactivity (see Table 4.2), while the other tabulated properties uncover the two-electron nature of the nucleophilic attack by the Ni(I) at the Caryl site. See Supporting Information Section S2.3 for more detailed intrinsic bond orbital (IBO) analysis. 143 To further elucidate the origin of the different reactivity profiles for the different Ni(I) species, we analyzed the initial oxidative addition step in the context of intrinsic bond orbital (IBO) progression.58,59 IBO analysis allowed for the identification of orbital changes (δ-orb) along the intrinsic reaction coordinate (IRC), wherein the separated Ni(I) and aryl halide reactants come together to form the five-coordinate Ni(III) adduct. Although the unpaired electron is located in the Ni 3d(x2–y2) orbital (cf. EPR analysis in Section §4-2.1), the largest δ-orb effects are observed in the doubly occupied Ni 3d(z2) orbital and Caryl–Cl σ orbital. These are transformed into new Ni‒Caryl and Ni‒Cl σ bonds, respectively (Figure S39). This change in bonding is the outcome of two-electron transfer from the occupied Ni 3d(z2) orbital into the virtual Caryl– Cl σ* orbital (Figure 4.5C). Direct observation of Ni-to-(Caryl–Cl) σ* backbonding at the transition state is evidenced by significant mixing of filled 3d orbital character into the unoccupied Caryl–Cl σ* orbital along the IRC (Figure 4.5C, middle). For example, from 1-Cl to 2-Cl to 3-Cl, the Löwdin Ni 3d character varies from 24.0 to 29.4 to 39.4. Previous analysis on Pd-catalyzed aryl chloride activation found that the increase in Pd backbonding to the Caryl–Cl σ* orbital was related to an increase in the barrier for the reaction.60 This same trend is seen in our Ni-based C(sp2)–Cl bond activation. Thus, backbonding in the transition state may play an important role in activating stronger C(sp2)– Cl bonds by Ni(I) species. As discussed further below, this backbonding will increase with less-negative electron binding energies on the metal. At the transition state, the α- and β-type orbitals are transformed in conjunction, suggesting a two-electron nucleophilic attack by the Ni(I) 3d(z2) orbital on the Caryl atom, yielding a Ni(III) intermediate. This concerted two-electron transfer is supported by a small change of Löwdin spin density on the Ni center throughout the reaction (~1 unpaired electron throughout; see inset table in Figure 4.5C); one-electron transfer or stepwise two-electron transfer would reveal more substantial metal-based spin density changes along the IRC. The small increase in the spin density at the transition state arises from the slightly misaligned attack initiated by β Ni 3d(z2) orbital, which is ~0.8 eV higher in energy than the α Ni 3d(z2) orbital due to spin polarization arising from the presence of five vs. four 3d electrons in α and β manifolds, respectively (Table S7). That Caryl–Cl bond-breaking is rate determining is 144 consistent with the more facile oxidative addition activation of 2-bromo-toluene and 2iodo-toluene substrates (e.g., their reactivity is observed even with 3-Cl due to their weaker Caryl–halogen σ bonds and lower-energy Caryl–Cl σ* orbitals, which facilitates backbonding from the metal). From the preceding analysis, the reactivity of individual Ni(I) complexes should correlate with the energies of the Ni 3d(z2) orbitals. Indeed, the calculated Ni(I)–bpy 3d(z2) orbital energies trend with the kOA rate constants (Figure 4.5C and Table 4.2). For example, the most- and least-reactive complexes, 1-Cl and 3-Cl, have β 3d(z2) orbital energies of –4.92 eV and –5.44 eV, respectively (Δ = –0.52 eV). There is also a linear trend between the energy of the 3d(z2) orbital and the bpy σp Hammett parameters for 1-Cl, 2-Cl, and 3-Cl (Figure S45). The less-negative electron binding energies of 1-Cl will increase nucleophilicity and propensity for a two-electron reduction. As discussed below, this effect on the 3d orbital energies is not necessarily specific to 3d(z2) alone, specifically because the bpy-based σ interaction only involves the torus of the 3d(z2) orbital. Rather, the bpy ligand ultimately tunes the entire 3d orbital manifold via changes in Zeff of the metal, making the ligand substitution effects a key predictor of reactivity. In further support of these changes, the calculated Ni(I) 1s and 2p(x,y,z) orbital energies of 1-Cl, 2-Cl, and 3-Cl all trend linearly with bpy-based Hammett parameters and oxidative addition rate constants, with slopes similar to the 3d orbitals (Figure S45-S46). Moving forward, the calculated 3d orbital energies of three-coordinate Ni(I) intermediates will be a useful predictor for oxidative addition reactivity and will bracket relative reactivity for specific C(sp3)–X and C(sp2)–X bonds. §4-2.4. Thermodynamics of the Dimerization of Ni(I)–bpy Halides Having established the reactivity of the Ni(I) complexes toward oxidative addition, we sought to better understand their general stability in solution. Their stability is of particular interest, as unreactive halide-bridged dimers have been proposed to form in bpy and related systems.30–32,40 Photogeneration of the Ni(I) compounds was again achieved using 370 nm LEDs in THF. Even in the absence of aryl halide, the characteristic MLCT band of the Ni(I)– 145 bpy halide complexes decayed over time; this decay was accelerated by increased temperature. In all cases, this decomposition results in the formation of a precipitate. However, we found that the precipitate of complexes 1-Cl/Br exhibited slight solubility in THF, likely owing to the bulky, non-polar tert-butyl substituents on the bipyridine. Taking 1-Cl as a representative complex, we thus collected the precipitation product and analyzed it by UV–vis, 1H NMR, and EPR (in THF, d8-THF, and 2-MeTHF, respectively; Figures S28– S30). We found that its spectra matched that of independently synthesized [Ni(I)(t-Bubpy)Cl]2, suggesting that the primary thermal decomposition product of the Ni(I)– bpy halides is their halide-bridged dimers (see Supporting Information Section S1.8). Furthermore, this thermal decay pathway of the Ni(I)–bpy halides can be monitored by the decrease of their characteristic UV–vis spectra over time (Figure 4.6, top). Linear fits were obtained when plotting the reciprocal of the absorbance change vs. time (Figure 4.6, middle), consistent with a decay process that is second-order in nickel concentration. Together with the comparison to independent synthesis, this pathway is therefore assigned to Ni(I)–bpy halide dimerization. The slope of the linear fit yielded second-order dimerization rate constants, kD, which were observed to be temperature-dependent (Figure 4.6, bottom and Table 4.3). Eyring analysis of the temperature-dependent rate constants afforded enthalpic and entropic thermodynamic parameters, ΔH‡ and ΔS‡ (Table 4.3).61–63 Rather large enthalpic barriers are observed for all compounds, ranging from ~11 kcal mol-1 to 19 kcal mol-1. The most significant trend follows ΔH‡ of 3-Cl > 2-Cl > 1-Cl. ΔS‡ values ranged from ~ –15 cal mol-1 K-1 to –38 cal mol-1 K-1. The fairly large, negative values of ΔS‡ are consistent with an associative (i.e., bimolecular) transition state. ΔG‡(298 K) values yielded small overall differences, however, falling in the range of ~22 kcal mol-1 to 25 kcal mol-1. The largest ΔG‡(298 K) is observed for 3-Cl; at ~25 kcal mol-1, this barrier is consistent with very slow decomposition at room temperature and is in agreement with the overall stability of the related Ni(I)(EtOOCbpy)Cl complex.30 We note, however, that dimer precipitation can offset the chemical equilibrium, acting as a thermodynamic sink and driving the Ni(I) monomer to dimer conversion. 146 Figure 4.6. Experimental thermodynamic analysis of the dimerization reaction for 1-Cl, 2Cl, and 3-Cl. (A1, B1, C1) UV–vis absorption plots in THF showing the starting spectra of the Ni(I)–bpy halide generated by irradiation with 370 nm light (blue line) and the final spectra after thermally induced dimerization (orange line). Note that in the orange spectra, there remains some unreacted parent four-coordinate Ni(II)–bpy aryl halide (Figure S1-S2). (A2, B2, C2) Linear plots of 1/([Ni(I)]-[Ni(I)]0) vs. time showing the second-order nature of the dimerization; temperatures between 20 °C and 55 °C were chosen. (A3, B3, C3) Eyring plots of ln(kD/T) vs. 1/T giving thermodynamics data. Prohibitively slow dimerization made low-temperature data collection challenging for 3-Cl, so an intermediate temperature point was added. Analogous data for 1-Cl, 1-Br, and 1-I are given in Figure S27. We again turn to substituent-based electronic effects to rationalize the changes in dimerization rate constants and thermodynamics. The ΔH‡ for dimerization is lowest for 1-Cl/Br/I and higher for 2-Cl/3-Cl, a trend similar to that observed for oxidative addition (vide supra, Section §4-2.2). Electron-withdrawing effects weaken the nucleophilicity of the Ni(I) center, lowering the propensity for dimerization. Conversely, electron donation from the ligands enhances the reactivity of the Ni(I) center for both oxidative addition and dimerization. The halide appears to have a lesser effect on the dimerization thermodynamics 147 than the bpy, with 1-Br exhibiting the lowest enthalpic barrier of the three halide variants (Figure S27). Table 4.3. Thermodynamic parameters for dimerization of Ni(I)–bpy halide complexes.a Compound kD (x10-5 M-1 s-1)b ΔH‡ (kcal mol-1) ΔS‡ ΔG‡(298 K) (cal mol-1 K-1) (kcal mol-1) 1-Cl 4.0 16.6 ± 1.0 -19.3 ± 1.1 22.3 ± 0.5 1-Br 3.6 11.1 ± 1.2 -38.1 ± 4.1 22.4 ± 2.4 1-I 1.8 15.8 ± 1.6 -25.1 ± 2.5 23.3 ± 2.3 2-Cl 3.8 18.1 ± 1.1 -14.5 ± 0.9 22.4 ± 1.3 3-Cl 0.1 19.2 ± 2.9 -18.3 ± 5.8 24.6 ± 3.7 a Dimerization is monitored under nitrogen atmosphere in anhydrous THF and followed by UV–vis spectroscopy. Reported standard errors are propagated from the linear least-squares analysis. bSecond-order rate constants for the dimerization reaction (kD) are given at 30 °C (303 K), as no rate constant for 3-Cl was measurable at room temperature. §4-2.5. Dimerization Mechanistic Investigations DFT calculations also support the decay mechanism of Ni(I)‒bpy halides through dimerization. The experimental second-order decay in Ni(I) allows for a direct 2 Ni(I)(Rbpy)X → [Ni(I)(Rbpy)X]2 pathway (Figure 4.7A). We note that the experimental kinetic analysis does not rule out the inclusion of an intermediate step, 2 Ni(I)(Rbpy)X → Ni(0)(Rbpy) + Ni(II)(Rbpy)X2 → [Ni(I)(Rbpy)X]2. In either case, the starting reactants are two Ni(I)(Rbpy)X species, and the ultimate decomposition product is the [Ni(I)(Rbpy)X]2 dimer, making the overall reaction free energy (ΔG) the same for both pathways. We have elected to computationally examine the simpler case which does not invoke a Ni(0) species. 148 Figure 4.7. (A) Calculated energetics of the dimerization of Ni(I)‒bpy halides and (B) the transition state structure for the dimerization of 1-Cl. Relative Gibbs free energy values were computed at the B3LYP/def2-TZVP(CPCM)//BP86/def2-TZVP(CPCM) level. Computed free energies are tabulated in Table S16. Like the oxidative addition mechanism discussed in Section §4-2.3, DFT finds that dimer formation is initiated by nucleophilic attack by the doubly occupied Ni 3d(z2) orbital. Rather than interacting with the C(sp2)–X σ* orbital of an aryl halide, the acceptor orbital for dimerization is an unoccupied halide p-orbital of another three-coordinate Ni(I) complex. This attack results in a transition state with a pseudo-tetrahedral geometry (i.e., a formal Ni(I)–bpy dihalide complex bridged via a halide to a second Ni(I)–bpy (Figure 4.7B)). The predicted barrier for this transformation is ~17–22 kcal mol-1 across the complexes. Two distinct dimerization product geometries were obtained by following the downhill reaction coordinate from the transition state, both featuring two bridging halides but with different orientations (i.e., “peaked” vs. “flat” geometry in Figure 4.7A). At the DFT level, both dimer geometries feature two nickel sites in the Ni(I) oxidation state (Figure S68). Although both dimers are rather high in energy—alone not providing enough driving force for the dimerization—they show distinct similarities to the dimer structures obtained previously in the solid phase.30,32,64 Given the low solubility of the [Ni(I)(Rbpy)X]2 dimers in THF, we argue that precipitation drives the dimerization process in the forward direction and makes the dissociation of the dimer (i.e., the back-reaction) less favorable. 149 The free energies of activation are comparable to the experiment for 1-Cl/Br/I; however, the DFT transition states are predicted too low in energy for 2-Cl and 3-Cl (orange and red data in Figure 4.7A). The discrepancy between experimental vs. DFT values may again be attributed to the better solubility of 1-Cl/Br in THF, influencing the reversibility of the speciation. The higher multiconfigurational character for Ni(I)‒bpy halides26,27 and likely for the dimeric structures (not captured by DFT) could also contribute to the differences between computed and experimental data. These should be present to a greater degree in complexes with electron-withdrawing substituents on the bpy (i.e., the electron-withdrawing trend of 3-Cl > 2-Cl > 1-Cl). Therefore, it may be the case that the starting three-coordinate Ni(I)‒ bpy halides and their corresponding dimers are predicted to be too high in energy for 2-Cl and 3-Cl due to insufficient account of ligand non-innocent radical character on the bpy ligand, which may result in smaller effective barriers for dimerization. Nevertheless, computational analysis provides a plausible mechanism for Ni(I)–bpy halide dimerization. §4-3. Discussion Photoredox catalysis mediated by electronically excited nickel complexes, coupled to thermal reactivity pathways, can accomplish bond transformations with broad substrate versatility. Ni(I) intermediates are thought to be critical for specific bond activation steps facilitating reaction turnover (Figure 4.1A). Here we have demonstrated the facile photochemical generation of a library of Ni(I)(Rbpy)X complexes (R = t-Bu, H, MeOOC; X = Cl, Br, I). Through experimental and computational analyses, we have evaluated their competitive reactivity with challenging C(sp2)–Cl bonds and tendency toward dimerization and have developed structure-function relationships between ligand set and reactivity. UV–vis electronic absorption and EPR spectroscopies have provided an initial characterization of the electronic structures of the photochemically generated Ni(I)–bpy complexes. By UV–vis spectroscopy, all of the Ni(I) species exhibit relatively low-energy MLCT transitions (Figure 4.2A), the energies of which trend with the electronic effects of the bpy substituents. More electron-withdrawing functionalities lead to the shift of the most intense low-energy MLCT bands from ~660 nm to ~805 nm going from 1-Cl to 3-Cl. This 150 shift is attributed to the stabilization of the bpy π* acceptor orbitals. In contrast, the variation of the halides shows a rather small influence on the MLCT energies. EPR spectra reflect S = 1/2 Ni(I) ground-state electronic configurations with giso values of ~2.12 – ~2.19 (Table 4.1). The giso observed here deviate somewhat from the free electron g value (2.0023) due to the presence of ground-state orbital angular momentum. They are larger than those observed for methyl Ni(II)–terpyridine•– (giso = 2.02) and mesityl Ni(II)– phenanthroline•– (giso = 2.01) but smaller than observed for Ni(I)–bisoxazoline bromide (giso = 2.24).34,39,45 These giso values are consistent with greater ligand character for the terpyridine and phenanthroline alkyl complexes and greater Ni(I) character for the bisoxazoline bromide complex. While the giso of Ni(I)-bipyridine halide compounds would be consistent with Ni(I) character intermediate to these complexes, a full analysis of the aggregate giso values will require the additional assignment of the ligand field transitions that spin-orbit couple with the ground state, giving rise to orbital angular momentum. Current efforts are underway to assign the UV–vis transitions across various isolable Ni(I) complexes to elucidate the amount of ligand vs. metal character, as well as ligand field vs. MLCT contributions to the observed g values. These may be useful to evaluate the degree of electron delocalization onto the bpy ligand. Indeed, multireference/multiconfigurational calculations predict significant multiconfigurational character (i.e., Ni(I)(bpy)X vs. Ni(II)(bpy•–)X).26,27 A full analysis of the Ni(I) g values will require experimental characterization of the Ni-based ligand field transitions. The g values report on the spin-orbit coupling in the ground state of Ni 3d(x2-y2) SOMO parentage. However, importantly, the electronic effects of the bpy ligand and its substituents extend beyond the Ni 3d(x2-y2) SOMO and allow for energetic tuning of the lower-energy, occupied Ni 3d orbitals that are actually involved in the elementary steps of oxidative addition and dimerization (as further discussed below). When examining the Ni(I)–based reactivity toward C(sp2)–Cl oxidative addition, we find that complexes 1-Cl/Br/I, which feature the most electron-donating tert-butyl substituent on 151 bpy, exhibit the largest rate constants for the oxidative addition of 2-chloro-toluene (kOA ~7.0 x10-2 M-1 s-1, Figure 4.3 and Table 4.2). Changing this bpy substituent to hydrogen atoms (2-Cl) reduces the rate constant by twofold, while changing to the electronwithdrawing methyl ester (3-Cl) completely eliminates oxidative addition reactivity. Furthermore, 1-Cl is competent towards the activation of a variety of aryl chlorides, with rate constants spanning two orders of magnitude (from kOA ~ 8.0 x10-2 M-1 s-1 to kOA ~ 6.4 M-1 s-1). Hammett relationships indicate that the mechanism of the C(sp2)–Cl bond activation proceeds by a SNAr-type pathway, marking a discrete change in mechanism for the oxidative addition of aryl chlorides vs. aryl bromides or aryl iodides. These reactivity trends have been rationalized herein through analysis of the associated Ni-based 3d(z2) orbital involved in the nucleophilic two-electron transfer. In particular, we find linear trends between the 3d(z2) RAMO and the Hammett parameter of the bpy ligand (Figure S45), with electron donation and electron-withdrawing substituents destabilizing or stabilizing the RAMO, respectively. Interestingly, the energetic (de)stabilization is observed to occur for the entire 3d orbital manifold (Figure 4.8B). This observation is consistent with bpy-induced modifications of the Zeff of the metal center; the calculated Ni(I) 1s and 2p(x,y,z) orbital energies also trend linearly with the observed rate constants (Figure S46). Increased Zeff on the metal stabilizes the 3d orbital manifold and perturbs the overall reactivity of the complex (Figure 4.8A) through the kinetic barrier (ΔG‡). The bpy and halide substituents therefore present themselves as synthetic handles by which the reactivity of Ni(I)–bpy halide complexes can be adjusted and evaluated. For example, compound 3-Cl is selective toward the oxidative addition of C(sp2)–Br/I bonds, while 1-Cl demonstrates robust reactivity toward even the challenging C(sp2)–Cl functionality. The relative reactivity of 1-Cl/Br/I, 2-Cl, and 3-Cl allows for an estimate of the minimum energy of the Ni 3d(z2) orbital required to activate the challenging C(sp2)–Cl bond (Figure 4.8B). 152 Figure 4.8. Reaction coordinate and state energy diagrams for the Ni(I)–bpy halide complexes. (A) Blue (1-Cl) and orange (3-Cl) lines depict the photoexcitation mechanism (the MLCT+LMCT ‘one-photon, two-electron’ excitation previously discussed in ref. 27) for the formation of Ni(I) complexes. Subsequent reaction coordinate diagrams for the Ni(I)based oxidative addition (if aryl halide is present) or dimerization pathways are given for 1-Cl (blue) and 3-Cl (orange), highlighting the difference in kinetic barriers, ΔG‡, for the two complexes. (B) Molecular orbital diagram for the Ni(I) species, demonstrating the (de)stabilization of the β 3d orbital manifolds due to changes in Zeff; the chemically active, nucleophilic 3d(z2) orbital is highlighted in orange. (C) Plot of the oxidative addition rate constants, kOA, vs. the average change in Ni 3d orbital energy relative to 1-Cl. Furthermore, as the Ni 3d orbitals are stabilized in energy, the rate constant decreases linearly (R2 = 0.97, Figure 4.8C). Extrapolating this correlation to the x-axis provides an approximation of the minimum orbital energy required for oxidative addition reactivity. Notably, 3-Cl, which is not reactive toward C(sp2)–Cl bonds, falls well off the correlation 153 for the other complexes. Thus, from this picture, in order to activate reactivity, one would need to move the 3-Cl point along the x-axis until it intersects with the slope for the other compounds (i.e., increase its 3d orbital energies). Correlations of this type over other Ni(I) species reacting with a variety of C–X bonds will be useful for generating robust reactivity predictions. It further indicates that tuning the energy of the Ni 3d orbitals is a key aspect for reactivity. X-ray absorption spectroscopies will prove useful to quantify ligand contributions to Zeff and the 3d(z2) RAMO.65 Outside of ligand contributions, which thus far appear to be non-specific and affect all 3d orbitals, it may be possible to utilize other contributions such as solvent or additive effects as another handle by which to specifically target the axially accessible 3d(z2) orbital and modulate the energy and reactivity (and thereby the selectivity) of these complexes toward oxidative addition. Current work is underway to further define the reactivity capabilities of these Ni(I)–bpy halide complexes. Not only is the oxidative addition reactivity of these complexes tunable via the Ni 3d(z2) orbital, but their general solution-phase stability is as well. Analysis of the temperaturedependent kinetics indicates that 3-Cl has a markedly decreased tendency toward dimerization, with a measured room temperature free energy of activation, ΔG‡, of ~25 kcal mol-1. Conversely, 1-Cl readily dimerizes at room temperature suggesting that this decomposition pathway may be of concern during catalysis. These dimerization pathways also feature a nucleophilic attack by the Ni 3d(z2) orbital, but in this case, the acceptor is the unoccupied p orbital of the halide on a second complex. Thus, the competitive oxidative addition and dimerization reactivity pathways are both correlated with the Ni 3d(z2) orbital energy, suggesting it as a sensitive target for researchers to tune to discourage Ni(I) reactant decomposition and increase the catalytic turnover number. One can also envision similar concepts from a multiconfigurational bonding perspective. That is, increasing a ligand noninnocent Ni(II)(bpy•–)X configuration over the metal-centered Ni(I)(bpy)X configuration will deactivate the complex toward oxidative addition and dimerization. Future studies examining additional factors (e.g., steric effects, solvent coordination, etc.) that could suppress Ni(I) dimerization without affecting its performance toward oxidative addition would be of interest. 154 §4-4. Conclusions In summary, we report the straightforward photochemical generation of a series of Ni(I)–bpy halide complexes, which has allowed for detailed studies of their relative reactivity toward competitive pathways of oxidative addition and dimerization/oligomerization. Interestingly, many of the Ni(I)–bpy halide complexes studied herein are active toward oxidative addition of high-energy C(sp2)–Cl bonds. Through time-resolved UV–vis kinetic analysis, we have determined rate constants for these oxidative addition reactions. A Hammett analysis on substituted aryl chlorides elucidated the specific mechanism for the C(sp2)–Cl bond activation step as proceeding through an SNAr-type pathway, wherein the Ni(I) complex acts as a nucleophile. Relatedly, computational studies have also supported this mechanism, well reproducing the experimental ρ value, and reveal a nucleophilic two-electron transfer from the 3d(z2) RAMO into the Caryl–X σ* orbital. While the Ni(I)-mediated reactivity is not strongly dependent on the identity of the halide ligand, we find that it is strongly influenced by the identity of the bpy substituents. Electronwithdrawing substituents increase the energetic barrier for oxidative addition and can even completely abolish reactivity with C(sp2)–Cl bonds. Electron-donating substituents, however, promote reactivity. The role of the bpy ligand in reactivity has been traced directly to substituent-induced changes in the Ni(I)-based Zeff. For example, the most reactive complex, 1-Cl, has the Zeff decreased by electron donation from the tert-butyl substituents on the bpy. The enhanced electron density on the metal more effectively screens the nuclear charge through electron-electron Coulomb repulsion; this decrease in Zeff destabilizes the entire 3d-orbital manifold. Thus, the energy of the nucleophilic 3d(z2) orbital is modulated through changes in Zeff, and greater energetic destabilization results in greater reactivity due to decreases in electron binding affinities. While not evaluated here, the low coordination number and planarity of the bpy ligand also likely reduce steric contributions allowing for an ideal orbital overlap between the 3d(z2) and Caryl–X σ* orbitals to be achieved. We have additionally utilized temperature-dependent reaction kinetics to determine the thermodynamics associated with Ni(I) complex dimerization. We again find that stabilization 155 of the complexes is made possible via ligand effects, reporting room temperature dimerization barriers that range from moderate to untenable. Dimerization reactivity considerations parallel those of oxidative addition. Altogether, geometric changes to the ligand scaffold about the Ni(I) center in these and other catalytically relevant complexes have important implications for their relative electronic structures and reactivities. Modulation of the 3d(z2) orbital (by ligand substitution or through yet-to-be explored solvent coordination) represents a tunable target by which the reactivity of Ni(I)–bpy halide complexes can be altered, potentially unveiling new ways to drive Ni-mediated photocatalytic cycles and a direct route toward stimulating reactivity with even stronger C–X bonds. 156 §4-5. References (1) Nicewicz, D. A.; MacMillan, D. W. C. Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes. Science 2008, 322 (5898), 77–80. https://doi.org/10.1126/science.1161976. (2) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81 (16), 6898–6926. https://doi.org/10.1021/acs.joc.6b01449. 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Electronic Structures of Metal Sites in Proteins and Models: Contributions to Function in Blue Copper Proteins. Chem. Rev. 2004, 104 (2), 419–458. https://doi.org/10.1021/cr0206317. 164 Chapter 5 Ultrafast Photophysics of Ni(I)–Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes. Adapted with permission from: Sutcliffe, E.;† Cagan, D. A.;† Hadt, R. G. Ultrafast Photophysics of Ni(I)–Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes. J. Am. Chem. Soc. 2024, Accepted. †Co-first author. Copyright 2024 American Chemical Society. 165 §5-1. Introduction Ni(II)–bipyridine (bpy) aryl halide complexes are widely used for their ability to facilitate light-driven cross-coupling reactions.1–12 Excited-state Ni(II)–C(aryl) bond homolysis results in the formation of low-valent Ni(I)–bpy halide species13–15 that can activate aryl halides through a subsequent dark catalytic cycle. While aryl iodides and bromides can be activated with relative ease, increasing the electron-donating ability of the bpy substituents allows for activation of stronger C(sp2)–Cl bonds.16–24 Coupling light reactions for in situ Ni(I) generation to dark reactions for C–X bond activation enables Ni(I)/(III) catalytic cycles in the presence of nucleophilic coupling partners (e.g., amines, alcohols, thiols, etc., Figure 1A).25–30 Indeed, Ni(I)–bpy halide species have been shown to facilitate cross-coupling with remarkable scope, forging new C(sp2)–X bonds in good-to-excellent yields.31 As such, recent efforts have been made to understand the properties and reactivity profiles of Ni(I)–bpy halide complexes. However, they are prone to dimerization at elevated temperatures or high concentrations and exhibit rapid decomposition in the presence of oxygen or water, making their direct structural and spectroscopic characterization challenging.24,32–34 Ni(I)–bpy species can be stabilized through steric protection or backbonding to coordinating olefins.35,36 While these routes provided some of the first X-ray crystal structures of Ni(I)–bpy complexes, the stabilization slows or fully inhibits aryl halide oxidative addition. Alternatively, in situ formation of Ni(I)–bpy complexes has allowed for detailed mechanistic analysis. Pulsed radiolysis and electrochemical methods identified a Ni(I)(t-Bubpy)Br species and have provided rate constants for aryl iodide oxidative addition.21,37 Solid-state polynuclear Ni species (e.g., [Ni(I)(EtO2Cbpy)Cl]n, n = 2 or 4) are precursors to monomeric Ni(I) and did likewise for aryl bromides.19 From Hammett analysis, both of these aryl halide classes are thought to be activated by a concerted two-electron oxidative addition. Additionally, air- and moisture-free irradiation of parent Ni(II) complexes leads to stoichiometric conversion to Ni(I)–bpy halide photoproducts, many of which activate aryl chlorides.24 Analogous Hammett analysis demonstrated a nearly twofold increase in ρ-value relative to aryl bromides or iodides, suggesting a two-electron 166 2 nucleophilic aromatic substitution (SNAr) mechanism for the activation of the C(sp )–Cl bond.24 Interestingly, all the Ni(I)–bpy species observed thus far absorb light across a wide wavelength range with similar or greater extinction coefficients than their parent Ni(II) complexes.24 More generally, many of the previously proposed metallaphotoredox reactions invoke photon absorption by specific Ni species and/or photosensitizers, indicating that in situ generated intermediates also absorb a significant fraction of photons during LED irradiation (Figure 1B). To date, there have been no direct studies of the photophysical properties of Ni(I) intermediates that form as part of metallaphotoredox catalytic cycles. Given the importance of light activation for Ni(II), Ni(I) excited states may also participate in processes that could influence catalysis. Furthermore, there has been significant interest in understanding geometric and electronic structural factors that contribute to charge transfer excited-state lifetimes in first-row transition metal complexes, including recent work on Ni(II).14,18,38–40 167 Figure 5.1. (A) Photogeneration and cross-coupling reactivity of Ni(I)–bipyridine halides. (B) Comparison of Ni(II)(t-Bubpy)(o-tolyl)Cl and photogenerated Ni(I)(t-Bubpy)Cl, illustrating their overlapping absorbance profiles. (C) Schematic of pump-probe transient absorption spectroscopy. Herein, we have utilized ultrafast transient absorption (TA) spectroscopy (Figure 1C) to study a library of photogenerated Ni(I)(Rbpy)X species (R = t-Bu, Me, H, Ph, MeO2C; X = Cl, Br, I) and an independently isolated Ni(I)(MeO2Cbpy)Cl complex, the first such photophysical study for any Ni(I) species. The TA data are consistent with the formation and decay of 2Ni(I)-to-bpy metal-to-ligand charge transfer (2MLCT) excited states with lifetimes ranging from ~10–30 ps. We also find the bpy substituent strongly influences the mechanism of 2 MLCT excited-state deactivation. Tuning from electron-donating to electron- withdrawing substituents has provided a rare example of a series of related complexes whose excited-state deactivation processes span the Marcus normal and inverted regimes. 168 §5-2. Results 2.1. Steady-State and Ultrafast UV–vis–NIR Spectroscopy Following our previous work,24 Ni(I)(Rbpy)X complexes were photogenerated from Ni(II)(Rbpy)(o-tolyl)X parents (R = t-Bu, Me, Ph, H, MeO2C; X = Cl, Br, I) (1–5, Figure 2 and Figures S7-S8). Complete synthetic details are provided in Supporting Information Section 1.2. In general, Ni(II) complexes were dissolved in tetrahydrofuran (THF) and irradiated under air- and moisture-free conditions using purple or blue PR160L Kessil LEDs (370 or 390 nm incident light) to afford >95% Ni(II)(Rbpy)(o-tolyl)X to Ni(I)(Rbpy)X conversion. UV–vis–NIR absorption spectra of 1–5 feature transitions with molar absorptivity on the order of 103 M-1 cm-1 (Figure 2), with minor energy shifts upon variation of the halide; more dramatic shifts are observed upon modifications of the bpy substituents. As discussed in Supporting Information Section 1.4, the absorption bands can be assigned as Ni(I)-to-bpy MLCTs, consistent with our previous work.24 DFT calculated molecular orbital diagrams are given in Figures S34-S41, and TDDFT predicted absorption transitions are tabulated in Supporting Information Section S2.4. We have used ultrafast TA spectroscopy to elucidate the photophysical properties of the Ni(I) intermediates. TA spectra for all compounds were measured in THF using either 700 or 800 nm pump pulses. While some residual Ni(II) precursor can be present in solution due to the in situ generation of 1–5, the precursors do not absorb light near the 700 or 800 nm excitation.14,15 Nevertheless, additional pump wavelengths were also used to determine the effect of pumping alternative, higher-energy transitions. Full details of the experimental setup and subsequent analysis are given in Supporting Information Section 1.1 and 1.5. 169 Figure 5.2. Structures and UV–vis–NIR absorption spectra of the photochemically generated Ni(I)–bpy halides in THF. Boxed section is expanded on the right. Analogous figure with wavenumber axis is given as Figure S9. Similar to their Ni(II) precursors, all Ni(I) compounds showed a sizeable ultrafast TA response. While the Ni(II) precursors exhibit 3d-d lifetimes of several nanoseconds,14,18 Ni(I) excited states decay more rapidly, with no measurable signal after a few hundred ps. Difference spectra for the longest-lived state of each compound (Figure S32) exhibit groundstate bleach (GSB) signals that clearly align with the MLCT bands in the Ni(I) steady-state absorption, along with significant excited-state absorption (ESA) in the UV-visible region; complexes 4 and 5 also show sizeable signal in the NIR region (Figures S21 and S31). 170 Figure 5.3. Cascaded difference spectra of (A) 1 and (B) 5, across two time regions with TA prior to photoexcitation plotted in black. Kinetic traces at representative GSB and ESA wavelengths plotted in bottom panel alongside the fit to the data. Insets are an enlarged view of the shaded area showing the evolution of the ESA. All TA spectra, alongside their corresponding global fits, are presented in Figures S11-S32. Changing the Cl ligand in 1 to Br (1-Br) or to I (1-I) slightly blue-shifts the MLCT bands in the ground-state absorption spectra (~160 cm-1 and 310 cm-1, respectively). TA spectra for these halide variants were similar to 1, but with a small increase in lifetime of the long-lived state going down the group. Substituting Cl in 5 for Br (5-Br) also gave a small change in 171 the TA spectra, but with the reverse trend; the long-lived state showed a slight decrease in lifetime for Br versus Cl. Thus, the halide has a minor but measurable impact on the relaxation kinetics. Five bpy variants were also investigated (1–5). Compounds 1–3 have comparable absorption spectra; these similarities are reflected in their ultrafast behavior, with all three compounds exhibiting decay pathways with similar time constants. In contrast, 4 and 5 showed a longer-lived three-component decay pathway. Notably, the lowest-energy MLCT bands of 4 and 5 extend into the NIR, while the lowest-energy MLCTs of complexes 1–3 are located in the visible. Thus, the bpy substituents significantly perturb the steady-state electronic structure and the excited-state relaxation kinetics. The ultrafast dynamics of the eight compounds fall into two groups (Table 1); analysis of 1 and 5 are presented as representative compounds. Difference spectra across two ranges of time delays following photoexcitation of 1 (700 nm, 1 μJ/pulse) into its lowest-energy MLCT are plotted in Figure 3A. Within the first picosecond, GSB and ESA peaks are observed at 390 and 540 nm, respectively, with the ESA red-shifting by ~300 cm-1. Subsequently, a simultaneous decay of the whole difference spectrum occurs from 1.5 to 40 ps, as evidenced by isosbestic points at 479 and 583 nm. By 100 ps, the ground state of 1 has fully recovered. We note that there remains a small, flat, negative feature in the TA; this minor signal has no spectral features in common with the Ni(I) TA signal and is assigned to fine particulate scatter following the pump pulse (Supporting Information Section 1.5). Using global multiexponential fitting to quantify the relaxation dynamics, the TA spectrum of 1 is found to be well described by two exponentials with time constants of 0.3 and 10.9 ps, plus a small constant offset. Difference spectra for 5 following photoexcitation (800 nm, 1 μJ/pulse) (Figure 3B) exhibit a GSB ~25-fold larger than 1, despite the comparable concentrations and pump power. The magnitude of the GSB signal of 5 is contrasted by its smaller ESA at 640 nm (Figure 3B, inset), which reveals an initial growth and blue shift (~380 cm-1) in the first picosecond, followed by a significant broadening and loss of intensity. Fifteen picoseconds after the excitation pulse, isosbestic points appear at 605 and 662 nm, signifying the recovery of the whole difference spectrum to the ground state. Three exponentials are required to fit to the data: the shortest time constant (τ1 = 0.6 ps) corresponds to the initial 172 growth of the spectrum, while the intermediate and longer time constants correspond to the blue-shift/spectral broadening (τ2 = 5.6 ps) and the relaxation of the difference spectrum to the ground state (τ3 = 24.2 ps), respectively. Table 5.1. Lowest-energy MLCT absorption positions and relaxation time constants for all compounds in THF. MLCT λmax τ1 (ps)a τ2 (ps)a τ3 (ps) (nm / cm-1) 1 t-Bu / Cl 660 / 15 150 0.3 10.9c 1-Br t-Bu / Br 653 / 15 310 0.5 13.9 1-I t-Bu / I 640 / 15 625 0.4 1.2 15.4 2 Me / Cl 660 / 15 150 12 3 H / Cl 673 / 14 860 0.4 10 b 4 Ph / Cl 1175 / 8510 0.5 5.2 22.3 5 MeO2C / Cl 1178 / 8490 0.6 5.6 24.2c 5′ MeO2C / Cl 1178 / 8490 0.2 5.0 23.9 5-Br MeO2C / Br 1167 / 8570 0.4 5.6 23.8 a In the case of the quickly-relaxing compounds (1–3), we are only able to resolve one fast Compound Rbpy / halide component (see Discussion Section below). bIdentified by using both the UV–vis–NIR absorption peak and the GSB feature in the TA spectrum (Figure S31). cLifetimes of 12.4 and 29.2 ps for 1 and 5 in toluene, respectively. As discussed in Supporting Information Section 1.5, the difference in behavior between 1 and 5 is not the result of pumping different MLCT bands, solvent coordination, pump saturation, or multi-photon effects. §5-2.2. Isolation and Characterization of Ni(I)(MeO2Cbpy)Cl, 5′ Given the disparate excited-state lifetimes for 1 and 5, we sought their isolation and independent study. Unfortunately, this was not possible for 1, as it was prone to dimerization, consistent with previous works.24,32 Compound 5, however, does not dimerize at room temperature. Thus, we approached the synthesis of 5 photochemically. The parent Ni(II)(MeO2Cbpy)(o-tolyl)Cl complex is only sparingly soluble in diethyl ether; we reasoned the more polar, three-coordinate Ni(I)(MeO2Cbpy)Cl complex would be even less so. Indeed, 173 irradiation of the deep purple precursor rapidly afforded a blue precipitate, (5′), which was collected and analyzed. Figure 5.4. Photochemical generation and isolation of 5′. Top: Synthetic route for the isolation of solid sample, 5′. Bottom left: Powder X-band CW-EPR spectra of 5′ (T = 5 K; powder sample; frequency = 9.638 GHz; power = 2.2 mW; modulation amplitude = 8 G). Simulation parameters: gx = 2.053, gy = 2.123, gz = 2.262, g(strain)x = 0.025, g(strain)y = 0.035, g(strain)z = 0.033. The sharp signal denoted with the triangle at g = 2.003 likely corresponds to a trace amount of an organic radical impurity (present in a ~1:10 000 ratio relative to 5′ by spin standard measurements with TEMPO). Bottom middle: UV–vis–NIR spectra of 5 and 5′ in THF. Bottom right: 1H NMR spectra (d8-THF) of a partially photolyzed sample of parent Ni(II)(MeO2Cbpy)(o-tolyl)Cl complex to generate 5 (starred peaks) compared to the isolated complex 5′. While small and darkly colored crystals were grown by slow evaporation of a concentrated solution of 5′ in either 2-methyl THF or toluene, they were highly air sensitive and too small for XRD analysis. We instead turned to EPR. Solid-state EPR at 5 K showed a broadened rhombic signal (gavg = 2.146, gx = 2.053, gy = 2.123, gz = 2.262; Figure 4). The g values agree well with previously reported frozen-glass solutions of Ni(I)–bpy complexes (gavg = 2.12–2.24) and the predicted gavg of the photogenerated species 5 (calculated gavg = 174 24,35,36 2.187). Interestingly, IR spectra of solid samples exhibited a lowered carbonyl stretching frequency (νC=O = 1716 cm-1) in 5′ relative to its precursor Ni(II) complex (νC=O = 1730 cm-1),15 indicative of greater electron density in the bpy ligand π*-orbital via increased back-bonding from Ni(I). Redissolving 5′ in THF gave an identical UV–vis–NIR spectrum to 5 (Figure 4). Evans Method in deuterated benzene gave μeff = 1.9 (ne- = 1.2), and EPR spectra of frozen solutions were consistent with a Ni(I) species (Figures S1–S2). Furthermore, comparison between the paramagnetic 1H NMR spectra of in situ photogenerated 5 and isolated 5′ showed excellent agreement (Figure 4). Notably, the NMR does not match the related tetrameric sample, [Ni(I)(EtO2Cbpy)Cl]4, and we see no evidence of a redox equilibrium with Ni(0), as was seen previously for the tetramer (Figure S55).19 Addition of 2-bromobenzotrifluoride to a solution of 5′ gave rapid conversion to the oxidative addition product, Ni(II)(MeO2Cbpy)(o-CF3Ph)Br, as confirmed by UV–vis–NIR and 19F NMR spectra of the independently synthesized Ni(II) complex (Figures S3–S4). Finally, TA measurements on 5′ gave signals identical to 5 (Figures S28-S29). Altogether, 5′ is most likely monomeric Ni(I)(MeO2Cbpy)Cl. §5-3. Discussion Given their broad absorption cross-sections and relevance for photoredox reactions, Ni(I)– bpy halide complexes are important targets for fundamental and applied photophysical investigations. Herein, we have studied a library of Ni(I)–bpy complexes, 1–5, using ultrafast TA spectroscopy. Following excitation, relaxation proceeds through a rapid, multistep process accompanied by red- or blue-shifting of peaks and spectral broadening, characteristic of internal conversion through intermediate MLCT manifolds, vibrational cooling, and/or solvation effects.14,38,39,41–43 Components t1 and t2 are therefore assigned to relaxation into the lowest-energy MLCT manifold (Figure 5A). For the fastest-relaxing compounds (1–3), we are unable to separately resolve these components. Subsequently, ESA and GSB features recover on a longer time scale with clearly defined isosbestic points; the presence of significant ESA in the visible at all times suggests persistent reduction of the bpy ligand in an MLCT state.14 The kinetics for ground-state reformation are independent of pump 175 wavelength, further indicating the rate-limiting step involves nonradiative relaxation out of the lowest-energy MLCT (τ3 in Table 1, Figure 5A). Overall charge transfer lifetimes span ~10–30 ps, comparable to some of the longest reported Ni(II) MLCT lifetimes only obtained through the synthetic incorporation of significant steric constraints.38,40 The simple three-coordinate structures of these Ni(I) intermediates may lead to limited vibrational degrees of freedom to relax to the ground state, possibly representing an alternative approach to prolonging excited-state lifetimes. Interestingly, the values of t3 for the eight complexes also fall into two clear classes (exemplified by 1 and 5). Nonradiative decay can be examined by the approach of Englman and Jortner,44 which relates the decay rate constant (k = τ3–1) and the energy gap between fully relaxed excited and ground states, ΔG°. This model considers two limiting cases of the coupling between molecular vibrations and the decay rate, namely weak or strong coupling. The weak coupling limit applies to excited states with a small displacement from the ground state along the vibrational coordinate. This regime is found to approximately result in the energy-gap law through the interaction of the excited state with the highest vibrational mode(s) of the ground state; the decay rate constant is linearly governed by the energy gap between the relaxed ground and excited state, i.e., ln(k) ∝ –ΔG°. A full discussion of the weak coupling limit is given in Supporting Information Section 1.6. In the strong coupling limit, the displacement between the ground- and excited-state potential energy surfaces is large, making their intersection thermally accessible (Figure 5A). This activation energy gives rise to a Gaussian dependence on the energy gap, as in Marcus theory for intermolecular electron transfer.45–48 For small energy gaps, an increase in ΔG° leads to a decrease in the kinetic barrier, ΔG‡, for back-electron transfer (e.g., the relaxation from the MLCT to the ground state). Accordingly, the decay rate constant increases. For large energy gaps, ΔG° and ΔG‡ scale together; a larger energy gap corresponds to a reduced decay rate constant, akin to the Marcus inverted region. These two regimes of strong coupling behavior can be described by equation 1, 176 2 ln(𝑘) = ln(𝐴) − (Δ𝐺° + 𝜆) , 2𝜆⟨ℏ𝜔⟩ (5.1) where λ is the reorganization energy, ⟨ℏ𝜔⟩ the average vibrational energy, and 𝐴 contains various transition-dependent parameters. Substituting ⟨ℏ𝜔⟩ = 2𝑘𝐵 𝑇, where 𝑘𝐵 is the Boltzmann constant and 𝑇 is the temperature, recovers the well-known Marcus equation. This difference in denominator follows from their derivations in the classical (Marcus) and quantum limits (strong coupling). Notably, a plot of ln(k) versus –ΔG° results in a parabolic shape, which captures both the Marcus normal and inverted regimes.49,50 The value of the energy gap is often not easily accessible experimentally. However, 𝜆 is defined as the difference between the vertical transition energy (EMLCT) and ΔG°, enabling the substitution of the energy gap for the more experimentally accessible vertical transition energy, ΔG° = 𝜆 − 𝐸MLCT44 ln(𝑘) = ln(𝐴) − (𝐸MLCT − 2𝜆)2 . 2𝜆⟨ℏ𝜔⟩ (5.2) By considering the coupling of vibrations to intermolecular electron transfer, Jortner and Ulstrup developed an extended version of Marcus theory that has also seen success in modeling intramolecular electron transfer.51–57 Aside from vibrational considerations, the major difference of this model is the splitting of the reorganization energy into contributions from the solvent (𝜆𝑆 ) and vibrational modes (𝜆𝑉 ), or 𝜆 = 𝜆𝑆 + 𝜆𝑉 . Making the approximation that one dominant vibronic mode (or a representative average mode) with energy ℏ𝜔 couples to the transfer, this model simplifies to ∞ 𝑘 = 𝐴′ ∑ 𝑛=0 (EMLCT − 2λS − 𝜆𝑉 − 𝑛ℏ𝜔)2 𝑒 𝑆𝑆𝑛 exp (− ). 𝑛! 4𝜆𝑆 𝑘𝐵 𝑇 (5.3) 177 Figure 5.5. Modeling excited state relaxation dynamics of Ni(I) compounds. (A) PESs and Jablonski diagram (in red) demonstrating the relaxation pathways within the strong coupling limit, which qualitatively correspond to the Marcus normal (left) and inverted (right) regimes along the nuclear coordinate, Q. (B) A plot of the experimentally determined rate constants for relaxation from the lowest-energy MLCT back to the ground state as a function of the MLCT vertical transition energy (values taken from Table 1). The data reveal a clear parabolic shape and are fit to the four models discussed in the text, with vibronic Marcus giving the best fit. Error bars were extrapolated from the standard deviation of replicate measurements (see Supporting Information Section 1.1). Data were collected in THF; complexes 1 and 5 were also measured in toluene; 1 eV = 8065 cm-1. 178 Here 𝐴 is a constant defined in equation S6 and S = 𝜆𝑉 /ℏ𝜔. We term this form of Jortner ′ and Ulstrup’s model the vibronic Marcus model to distinguish it from the simpler, classically derived Marcus theory. To evaluate which models are most representative of the relaxation kinetics of 1–5, we plot ln(k) against EMLCT (Figure 5B). Two clear clusters are visible; while 1–3 appear to follow the energy gap law, 4–5 exhibit the opposite behavior. The division of the excited-state dynamics into two clusters cannot be explained solely within the weak coupling regime, which predicts a single monotonic relationship over all complexes. This is highlighted by the fit to the weak coupling model (equation S3) shown in Figure 5B. On the other hand, the strong coupling and Marcus models present a simple alternative to describe the two classes of relaxation kinetics. Using equation 2 with 𝐴, 𝜆, and ⟨ℏ𝜔⟩ as variables, the strong coupling fit shown in Figure 5B is achieved. The model shows excellent agreement with the data; 1–3 and 4–5 lie in the inverted and normal regions, respectively. We find ⟨ℏ𝜔⟩ = 0.13(7) eV and 𝜆 = 0.77(3) eV. Perhaps coincidentally, ⟨ℏ𝜔⟩ is comparable to the value predicted by DFT (0.14 eV, Table S4). The reorganization energy is of comparable magnitude to similar metal-to-bpy charge transfer processes.50,58,59 Alternatively, fitting to the classical Marcus model also gives a reasonable, but marginally worse, fit to the data with 𝜆 varying no more than the error of the original fit. The best fit to the data is achieved by the vibronic Marcus model (equation 3), giving 𝜆 = 𝜆𝑆 + 𝜆𝑉 = 0.76 eV. This can also be roughly approximated from Gaussian fits to the widths of MLCT transitions in the absorption spectrum, which results in 𝜆 ~ 0.5 eV, in fair agreement (Figure S10).53,60 The dominant vibrational mode is fit as ℏ𝜔 = 0.21 eV. Previous studies have maintained that intramolecular charge transfer typically couples to vibrations of the ligand backbone, such as bpy breathing modes in polypyridyl–Fe complexes.52–54,61 These modes are predicted by DFT at ~0.2 eV for 1–5, agreeing with the vibronic Marcus fit and these previous studies. The solvent and vibrational contributions to the reorganization energy are found by the fit to be 𝜆𝑆 = 0.54 eV and 𝜆𝑉 = 0.22 eV, 179 respectively. These individual contributions are hard to measure experimentally, but 𝜆𝑉 can be estimated from TD-DFT (Figure S44). In 1 and 5, 𝜆𝑉 is ~0.18 and ~0.33 eV, respectively (average value of 0.26 eV), which is close to the value obtained from the vibronic Marcus model. We note, however, that DFT is found to have limited applicability in these systems (Supporting Information Section 2.5) and reported values of 𝜆𝑉 vary greatly.52,54,55,61 Nonetheless, the total reorganization energy found by the vibronic Marcus fit is consistent with both the classical Marcus and strong coupling models. §5-4. Conclusions Herein, we have examined a library of Ni(I)–bpy halide complexes, including an isolable species accessible through a simple photochemical route. To the best of our knowledge, this research represents the first ultrafast TA spectroscopic characterization of any Ni(I) complex. As such, it is also the first ultrafast characterization of a Ni(I) intermediate in metallaphotoredox catalysis. Ni(I)–bpy halide 2MLCT lifetimes are < 50 ps, prohibiting diffusion-controlled bimolecular chemistry. Varying the bpy substituents significantly influences the excited-state relaxation dynamics, with kinetics that span the Marcus normal and inverted regimes. 180 §5-5. 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Chem. 1996, 100 (46), 18269–18274. https://doi.org/10.1021/jp960423g. 187 Appendix A Supporting Information for Chapter 2: Multireference Description Of Nickel– Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.0c08646 and includes the tabulation of TDDFT and CASSCF/QD-NEVPT2 energetics; tabulation of CASSCF/QD-NEVPT2 CI vectors; plotted data for 2 analogous to data for 1 presented in the manuscript; and DFT-optimized structures. 188 Appendix B Supporting Information for Chapter 3: Elucidating The Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts The Supporting Information is https://pubs.acs.org/doi/10.1021/jacs.2c01356 available and free includes of the charge at experimental and computational methods, UV–vis/photochemical data, X-ray crystallography, NMR spectra, calculated spectra/properties, global analysis modeling, and additional comments. 189 Appendix C Supporting Information for Chapter 4: Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)–Cl Oxidative Addition and Dimerization Reactivity Pathways The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c00917 and includes the experimental and computational methods; UV–vis/photochemical data; NMR and EPR spectra; calculated spectra/properties; additional comments; and DFT-optimized XYZ files of all structures. 190 Appendix D Supporting Information for Chapter 5: Ultrafast Photophysics of Ni(I)–Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes 191 S1. Experimental Section. S1.1. General Considerations. All purchased compounds were used as received unless otherwise noted. Bis-(1,5-cyclooctadiene) nickel(0) was purchased from Strem Chemicals. Ligands N,N,N′,N′-tetramethyl ethylenediamine (TMEDA), 4,4′-di-tert-butyl-2,2′-bipyridine t-Bu Me ( bpy), 4,4′-dimethyl-2,2′-bipyridine ( bpy), 2,2′-bipyridine (bpy), 4,4'-diphenyl-2,2'bipyridine (Phbpy), and dimethyl-2,2′-bipyridine-4,4′-dicarboxylate (MeO2Cbpy) were purchased from Sigma-Aldrich. Aryl halide compounds, 2-chloro-toluene, 2-bromo-toluene, 2-iodo-toluene, and 2-bromo-α,α,α-trifluorotoluene were also obtained from Sigma-Aldrich. Solids were dried under vacuum and brought into a nitrogen-atmosphere glove box; liquids (including aryl halides) were sparged (N2) and degassed via freeze-pump-thaw techniques, brought into the glove box, and stored over 3 Å molecular sieves. All solvents were air-free and collected from a solvent purification system (SPS), then stored in the glove box over 3 Å molecular sieves in amber jars. Tetrahydrofuran (THF), 2-methyl tetrahydrofuran (2-MeTHF), and d8-tetrahydrofuran (d8-THF) were inhibitor-free. All deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. and also dried and stored over activated 3 Å molecular sieves in a nitrogen-filled glove box for at least three days before use. All synthesized compounds were made using air-free Schlenk techniques or made in the glove box. All synthesized complexes are considered air and moisture sensitive. Light sensitivity was also seen even in the solid state if left exposed for extended time. UV–vis spectra of the complexes were obtained on a Varian Cary 500 spectrophotometer or a StellarNet, Inc., Black Comet UV–vis spectrophotometer. Starna Cells 6-Q 2- or 10-mm path length cuvettes fitted with air-tight seals were used. Proton nuclear magnetic resonance (1H NMR) and fluorine nuclear magnetic resonance (19F NMR) spectra were recorded on a 400 MHz Varian Spectrometer with broadband auto-tune OneProbe. 19F NMR were externally referenced to neat fluorobenzene (δ = -113.15 ppm). 13C NMR spectra were collected on a Bruker AV-III HD 400 MHz spectrometer and were 1H decoupled. Chemical shifts are reported in parts per million (δ in ppm, s: singlet, d: doublet, t: triplet, m: multiplet) and are referenced to residual solvent signal (THF-d8 = 3.58 ppm). NMR samples were prepared in the glove box into Norell J-Young tubes. IR measurements were performed on a Bruker Alpha Platinum ATR spectrometer. Samples were analyzed with high resolution mass spectrometry (HRMS) by Field Desorption ionization using a JEOL AccuTOF GCAlpha (JMS-T2000GC) mass spectrometer interfaced with an Agilent 8890 GC system. Electron paramagnetic resonance (EPR) spectroscopy was collected on a Bruker EMX Xband CW-EPR Spectrometer using either an Oxford ESR 900 liquid helium/nitrogen flowthrough cryostat or a liquid nitrogen immersion dewar for experiments at a fixed temperature of 77 K. The recorded spectra were simulated in EasySpin for MATLAB.1 EPR samples were prepared in the glove box into Wilmad quartz low pressure/vacuum EPR tubes fitted with a with air-tight PTFE piston. 192 Ultrafast laser pulses used for the transient absorption (TA) measurements originate from a Coherent Astrella Ti:Sapphire amplifier system, which generates 5 mJ, 40 fs pulses centered on 800 nm at a 1 kHz repetition rate. These pulses were passed into an Ultrafast Systems Helios spectrometer system to carry out the transient absorption spectroscopy measurements. Pulses from the Astrella were attenuated and then delayed by the Helios’ built-in 7 ns delay stage before undergoing supercontinuum generation to generate a white-light probe. Three different non-linear optical media were used to generate white light across the UV to the NIR as needed: CaF2 (330–650 nm), Sapphire (470–750 nm), YAG (850–1600 nm). The majority of the beam was focused onto the sample and then subsequently into a fiber spectrometer, while the remaining portion of the beam bypassed the sample and was focused into a second fiber spectrometer to act as a reference. Pump pulses were generated through various means using a stronger portion of the Astrella output than that of the probe. To generate 560, 700 and 1200 nm pump pulses, a Coherent OPerA optical parametric amplifier was used in various configurations. For pumping at 800nm, the fundamental output of the Astrella was used and for 400 nm the fundamental was frequency doubled in a β-barium borate (BBO) crystal (EKSMA Optics, 10 mm x 10 mm x 0.2 mm, θ = 29.2º, ɸ = 90º, P/P@400–800 nm) crystal. After wavelength manipulation, the pump was chopped, attenuated with a variable neutral density filter, focused onto the sample, and subsequently blocked. TA samples were prepared in a nitrogen-filled glovebox using 2 mm quartz cuvettes fitted with air-tight PTFE piston seals (Schlenk cuvettes). Concentration varied between measurements but was chosen to maximize absorbance while minimizing the dimerization occurring at high concentrations. Thus, typical absorbances at the pumped wavelength were around 0.1–0.3 OD. Some compounds were especially prone to dimerization and the resultant precipitate caused significant scattered pump light in the results. This was minimized through the use of spectral filters (ThorLabs FELH0450, FESH0650, FESH0750) to block the pump wavelengths, but these also blocked wavelengths shorter than 400 nm so were only used when necessary. A magnetic stirrer was also used to stir the sample over the course of the measurement. Data were acquired using the Helios control software and subsequently exported to a custom MATLAB script for processing. Each datapoint was averaged for 2 s (1000 total pump-probe cycles) and an exponentially spaced time array was used to capture the decay of the signal. Each scan was repeated five times. Background points collected before time-zero where no signal was present were subtracted from the remaining data to remove pump scatter and other unwanted effects. Following this, the chirp in the data was corrected by tracking the position of the center of the cross-phase modulation (XPM) feature across the first picosecond of the spectrum and resampling the data with the original time array to remove the chirp. The data were then globally fit through nonlinear least-squares to a series of exponentials convoluted 193 with a Gaussian instrument response function. Since the true pulse length at the sample is unknown, the width of the response function was set to 70 fs to best fit the data. In most cases, the XPM was many times larger than the actual signal and so greatly biased the fit. In this work our interest lies with the dynamics after the first picosecond so the signal between -0.3 and 0.3 ps (0.5 ps for toluene due to a wider XPM) was excluded from the fit. The fitting procedure yields several fitted time constants alongside the decay-associated spectrum (DAS) corresponding to each exponential decay. For some cases, the least-squared algorithm would not converge so coarse manual tuning of the parameters was necessary to provide a good fit. Further processing can convert the DAS into evolution- or species-associated spectra but the presence of significant vibrational cooling violates the assumption of bilinearity required by the global fitting procedure making subsequent postprocessing of the spectrum questionable. However, the time constants are still valid and the DAS can still be used to understand the origin of each component. Errors on the fitted parameters can be estimated through the residuals and numerical Jacobian matrix outputted by the fitting algorithm. The errors on the time constants calculated this way ranged from around 0.1 to 1%. However, errors calculated this way notoriously underestimate the true uncertainty and more accurate errors can only be garnered from more complex statistical methods such as bootstrapping.2,3 Therefore, we adopt a compromise here. Across all compounds, 1 and 5 were studied most thoroughly so the standard deviation of 𝜏3 across many different samples and measurements was taken as the error and found to be 5 and 4%, respectively. Such a large number of individual measurements were not possible for all compounds, so instead several permutations of repeats of the noisiest, 4, were fitted and their standard deviation found to also be around 5%. Therefore, we approximate the error on 𝜏3 to be 5% for all compounds. The shorter time constants typically correspond to a much smaller change in the signal and so have larger uncertainty. However, these components are of less relevance to the study as a whole and thus a detail consideration of their corresponding errors is considered beyond the scope of this work. 194 S1.2. Synthetic Details. The parent four-coordinate complexes, Ni(II)(t-Bubpy)(o-tolyl)Cl, Ni(II)(t-Bubpy)(o-tolyl)Br, Ni(II)(t-Bubpy)(o-tolyl)I, Ni(II)(bpy)(o-tolyl)Cl, and Ni(II)(MeO2Cbpy)(o-tolyl)Cl, were synthesized according to previous reports.4–6 Their spectroscopic properties were identical to those described prior. The precatalyst, Ni(II)TMEDA(o-tolyl)Br, and the parent complexes, Ni(II)(Mebpy)(o-tolyl)Cl, Ni(II)(Phbpy)(o-tolyl)Cl, and Ni(II)(MeO2Cbpy)(o-tolyl)Br were prepared as given below. Preparation of Parent Ni(II)–bpy Aryl Halide Complexes. Ni(Mebpy)(o-tolyl)Cl. In a nitrogen-filled glove box, a 20 mL scintillation vial was charged with a Teflon coated stir bar, bis-(1,5-cyclooctadiene) nickel(0) (0.240 g, 0.870 mmol, 1.00 eq.), and 4,4′-dimethyl-2-2′-bipyridine (0.165 g, 0.896 mmol, 1.03 eq.). To this vial, 5.0 mL THF was added, and the mixture was stirred for 90 minutes affording a deep purple solution. Subsequently, 1.5 mL of 2-chloro-toluene (excess) was added dropwise, while stirring. An orange solid precipitated after 3.5 hours alongside a gray-black solid. Pentane (10 mL) were added to the mixture to complete precipitation. The crude solid mixture was then collected by filtration, washed with pentane and heptane (3x5 mL each), and the filtrate discarded. Into a second, clean filter flask, the solid mixture was rinsed with diethyl ether, affording a red/orange filtrate; the insoluble solids were discarded. To this filtrate, pentane was added to precipitate an orange solid. This solid was collected by filtration, washed again with pentane and heptane (3x5 mL each) then dried under vacuum (0.075 g, 23% yield). Note: The solid product is prone to decomposition over the course of days/weeks, even in the glove box, becoming an orange/brown solid. Sample should be stored at low temperature, if possible. Solutions decompose at room temperature over the course of several hours to days. Solutions should be used immediately to avoid insoluble decomposition products. UV–vis (THF): λMLCT = 477 nm / 20,964 cm-1 (εMLCT = 4530 cm-1 M-1). 1H NMR (400 MHz, CD2Cl2): δ 8.97 (d, J = 6.0 Hz, 1H), 7.71 (d, J = 1.6 Hz, 1H), 7.67 (d, J = 1.5 Hz, 1H), 7.54 – 7.48 (m, 1H), 7.35 (dd, J = 5.2, 1.6 Hz, 1H), 7.10 (d, J = 5.7 Hz, 1H), 6.91 (dd, J = 5.5, 1.9 Hz, 1H), 6.83–6.74 (m, 3H), 3.04 (s, 3H), 2.49 (s, 3H), 2.37 (s, 3H). 13C{1H}NMR (100 MHz, CD2Cl2) δ 156.1, 151.3, 150.5, 149.2, 142.7, 135.8, 127.5, 127.2, 123.3, 123.2, 122.8, 121.8, 121.0, 25.2, 21.7, 21.5. FT-IR (ATR, cm-1): 3036, 2977, 1615, 1556, 1478, 1445, 1418, 1024, 1018, 921, 846, 827, 733, 650, 556, 515. HRMS (FD-MS): calculated for [C19H19N2NiCl]+: 368.0590 found: 368.0584. 195 Ni(Phbpy)(o-tolyl)Cl. Synthetic procedure was adapted from a literature method.5 In a nitrogen-filled glove box, a 4 mL vial with air-tight septa cap was charged with a Teflon coated stir bar and Ni(TMEDA)(o-tolyl)Cl (0.024 g, 0.080 mmol, 1.00 eq.). To this vial, 4,4′diphenyl-2,2′-bipyridine (0.026 g, 0.085 mmol, 1.06 eq.) was added along with 1.6 mL of benzene. The vial was capped and sealed with three turns of electrical tape, removed from the glove box, and stirred at 45 ⁰C for 4 hours affording a dark red solution with precipitate. After allowing the vial to cool, it was brought back into the glove box where the red solid was collected by vacuum filtration, washed with benzene (2x2 mL), diethyl ether (2x2 mL), and pentane (4x2 mL), then was dried under vacuum (0.028 g, 73% yield). Spectroscopic properties were identical to those reported previously. UV–vis (THF): λMLCT = 500 nm / 20,000 cm-1 (εMLCT = 5300). 1H NMR (400 MHz, CD2Cl2): δ 9.25 (d, J = 5.7 Hz, 1H), 8.20 (s, 1H), 8.15 (s, 1H), 7.83–7.76 (m, 3H), 7.73–7.68 (m, 2H), 7.63–7.49 (m, 7H), 7.36 (d, J = 3.0 Hz, 2H), 6.89–6.78 (m, 3H), 3.09 (s, 3H). HRMS (FDMS): calculated for [C29H23N2NiCl]+: 492.0903 found: 492.0923. Ni(TMEDA)(o-tolyl)Br. In a nitrogen-filled glove box, a 20 mL scintillation vial was charged with a Teflon coated stir bar and bis-(1,5-cyclooctadiene) nickel(0) (0.250 g, 0.909 mmol, 1 eq.). Via micro syringe, 0.175 mL (1.182 mmol, 1.3 eq.) of N,N,N′,N′-tetramethyl ethylenediamine was added along with 3.25 mL of 2-bromo-toluene (excess). A red/orange solid began precipitating in the vial. After 5 hours stirring at room temperature, hexanes was added (10 mL) to further precipitate the solid; the mixture was left overnight. The red/orange solid was collected by vacuum filtration where it was rinsed thoroughly with hexane and pentane, and dried (290 mg, 92% yield). H NMR (400 MHz, CD2Cl2): δ 7.38 (dd, J = 7.3, 1.4 Hz, 1H), 6.70–6.62 (m, 2H), 6.61–6.53 (m, 1H), 3.41 (s, 4H), 2.79–2.31 (m, 12H), 2.17 (d, J = 10.4 Hz, 2H), 1.78 (s, 2H). 13 C{1H}NMR (100 MHz, CD2Cl2) δ 144.8, 143.7, 136.2, 126.5, 122.5, 121.9, 61.4, 57.3, 50.1, 49.1, 47.7, 47.0, 26.7. FT-IR (ATR, cm-1): 3037, 2971, 2893, 2840, 2784, 1558, 1456, 1277, 1123, 1047, 1018, 1010, 953, 806, 772, 749, 648, 605. HRMS (FD-MS): calculated for [C13H23N2NiBr]+: 344.0398 found: 344.0396. 1 196 Ni(MeO2Cbpy)(o-tolyl)Br. In a nitrogen-filled glove box, a 100 mL Schlenk flask was charged with a Teflon coated stir bar and Ni(TMEDA)(o-tolyl)Br (0.250 g, 0.723 mmol, 1.00 eq.). To this vial, dimethyl-2,2′-bipyridine-4,4′-dicarboxylate (0.240 g, 0.881 mmol, 1.22 eq.) was added along with 24 mL of heptane and 4 mL toluene (7:1 heptane/toluene). Upon removal from the glove box, the flask was sonicated to promote solubilization of the reagents. The flask was attached to the nitrogen Schlenk line, covered in aluminum foil, and stirred at 60 ⁰C for 24 hours affording a deep purple solution with purple precipitate. Orange starting material was still seen in the flask after inspection, so the temperature was then increased to 65 ⁰C and the reaction continued for an additional 48 hours. After allowing the Schlenk flask to cool, it was brought back into the glove box. A purple solid had precipitated; it was collected by vacuum filtration, washed copiously with heptane, diethyl ether (3x2 mL), and excess pentane, then was dried under vacuum (0.255 g, 70% yield). UV–vis (THF): λMLCT = 538 nm / 18,587 cm-1 (εMLCT = 5100 M-1 cm-1). 1H NMR (400 MHz, CD2Cl2): δ 9.73 (d, J = 5.0 Hz, 1H), 8.58 (d, J = 2.5 Hz, 1H), 8.51 (d, J = 1.4 Hz, 1H), 8.11 (dd, J = 5.7, 1.7 Hz, 1H), 7.71 (dd, J = 6.0, 1.8 Hz, 1H), 7.48 (dd, J = 6.9, 1.8 Hz, 1H), 7.43 (d, J = 6.1 Hz, 1H), 6.88 – 6.76 (m, 3H), 4.03 (s, 3H), 3.97 (s, 3H), 2.96 (s, 3H). 13C{1H}NMR (100 MHz, CD2Cl2) δ 164.1, 163.9, 155.8, 153.0, 152.2, 151.6, 147.4, 142.1, 139.7, 138.3, 135.6, 127.8, 126.3, 125.8, 123.5, 122.9, 120.8, 120.1, 25.1. FT-IR (ATR, cm-1): 3028, 2950, 1725, 1556, 1433, 1398, 1322, 1250, 1232, 1121, 1012, 962, 884, 841, 840, 766, 737, 715, 649. HRMS (FD-MS): calculated for [C21H19N2O4NiBr]+: 499.9882 found: 499.9899. 197 Ni(MeO2Cbpy)(o-CF3Ph)Br. In a nitrogen-filled glove box, a 20 mL scintillation vial was charged with a Teflon coated stir bar, bis-(1,5-cyclooctadiene) nickel(0) (0.220 g, 0.800 mmol, 1.00 eq.), and dimethyl-2,2′-bipyridine-4,4′-dicarboxylate (0.225 g, 0.826 mmol, 1.03 eq.). To this vial, 5.0 mL THF was added, and the mixture was stirred for 90 minutes affording a deep purple solution. Subsequently, 2.0 mL of 2-bromobenzotrifluoride (excess) was added while stirring. The product solid precipitated after 4 hours. Hexane (10 mL) was added to the mixture to complete precipitation, and the mixture was placed in the glovebox freezer (-35 °C) for 60 hours. The solid was finally collected by filtration, washed thoroughly with hexane, diethyl ether (3x2 mL), and excess pentane, then dried under vacuum (0.405 g, 91% yield). UV–vis (THF): λMLCT = 499 nm / 20,040 cm-1 (εMLCT = 4370 M-1 cm-1). 1H NMR (400 MHz, d8-toluene): δ 9.74 (d, J = 5.7 Hz, 1H), 7.96–7.92 (m, 1H), 7.91 (s, 1H), 7.74 (s, 1H), 7.65 (s, 1H), 7.37 (d, J = 8.3 Hz, 2H), 7.22 (t, J = 5.2 Hz, 2H), 6.55 (d, J = 5.4 Hz, 1H), 3.40 (s, 3H), 3.36 (s, 3H). 19F NMR (400 MHz, CD2Cl2) δ –58.6 ppm; (400 MHz, d8-THF) δ –58.4 ppm. 13 C{1H}NMR (100 MHz, CD2Cl2) δ 163.8, 155.8, 152.8, 152.1, 146.1, 137.9, 137.1, 128.3, 126.0, 125.7, 123.0, 120.8, 120.3, 120.2. Low signal to noise precluded the resolution of the JC–F coupling values for the trifluorotoluene peak. FT-IR (ATR, cm-1): 3061, 2960, 1723, 1558, 1435, 1398, 1311, 1232, 1148, 1092, 1020, 967, 883, 842, 764, 735, 702, 675, 638. HRMS (FD-MS): calculated for [C21H16N2O4F3NiBr]+: 553.9599 found: 553.9594. 198 Photochemical Preparation of Ni(I)–bpy Halide Complexes. Following our previous report,6 the Ni(I)(Rbpy)X (R = t-Bu, Me, Ph, H, MeO2C; X = Cl, Br, I) compounds studied herein were accessed directly from their parent Ni(II)–bpy aryl halide precursors by air- and moisture-free irradiation (370 nm or 390 nm). Stock solutions of parent Ni(II)–bpy aryl halide complexes (0.5–1 mM) were prepared in a nitrogen-filled glove box and distributed into separate spectroscopic cuvettes (Starna Cells, 2- or 10-mm path length) fitted with air-tight PTFE piston seals (Schlenk cuvettes). Solutions were prepared fresh daily for analysis. Each cuvette was placed 5 cm away from either a Gen 2 Kessil PR160L 370 nm LED or Kessil PR160L 390 nm LED on highest setting. A cooling fan was used to maintain room-temperature irradiation during the experiment. Note: Kessil LEDs may auto-shut off if left on for extended periods without the external fan due to overheating. Typical irradiation times, t, were ~60 minutes, but these varied with each complex and are listed below. Ni(I)(t-Bubpy)Cl, 1. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(t-Bubpy)(o-tolyl)Cl, for 60 minutes using a Gen 2 Kessil PR160L 370 nm LED afforded the title compound. UV–vis (THF): λ1 = 660 nm (15,152 cm-1), λ2 = 422 nm, (23,700 cm-1). Spectroscopic properties were identical to those reported previously. Ni(I)(t-Bubpy)Br, 1-Br. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(t-Bubpy)(o-tolyl)Br, for 45 minutes using a Kessil PR160L 390 nm LED afforded the title compound. UV–vis (THF): λ1 = 653 nm (15,314 cm-1), λ2 = 386 nm, (25,906 cm-1). Spectroscopic properties were identical to those reported previously. Ni(I)(t-Bubpy)I, 1-I. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(t-Bubpy)(o-tolyl)I, for 60 minutes using a Kessil PR160L 390 nm LED afforded the title compound. UV-vis (THF): λ1 = 640 nm (15,625 cm-1), λ2 = 382 nm, (26,178 cm-1). Spectroscopic properties were identical to those reported previously. 199 Ni(I)(Mebpy)Cl, 2. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(Mebpy)(o-tolyl)Cl, for 60 minutes using a Kessil PR160L 390 nm LED afforded the title compound. UV–vis (THF): λ1 = 660 nm (15,152 cm-1), λ2 = 440 nm, (22,727 cm-1). Ni(I)(Hbpy)Cl, 3. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(Hbpy)(o-tolyl)Cl, for 75 minutes using a Kessil PR160L 390 nm LED afforded the title compound. Precipitation can occur after extended irradiation; it can be filtered off in a glove box to yield a homogenous filtrate solution of the Ni(I) complex. UV–vis (THF): λ1 = 673 nm (15,625 cm-1), λ2 = 431 nm, (23,200 cm-1). Spectroscopic properties were identical to those reported previously. Ni(I)(Phbpy)Cl, 4. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(Phbpy)(o-tolyl)Cl, for 75 minutes using a Gen 2 Kessil PR160L 370 nm LED afforded the title compound. UV–vis (THF): λ1 = 1175 nm (8,510 cm-1), λ2 = 915 nm, (10,929 cm-1), λ3 = 690 nm, (14,493 cm-1), λ4 = 485 nm, (20,619 cm-1). Ni(I)(MeO2Cbpy)Cl, 5. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(MeO2Cbpy)(o-tolyl)Cl, for 45 minutes using a Gen 2 Kessil PR160L 370 nm LED afforded the title compound. UV–vis (THF): λ1 = 1178 nm (8,490 cm-1), λ2 = 805 nm (12,422 cm-1), λ3 = 523 nm (19,120 cm-1). Spectroscopic properties were identical to those reported previously. 200 Ni(I)(MeO2Cbpy)Br, 5-Br. Air- and moisture-free irradiation of the parent Ni(II) compound, Ni(II)(MeO2Cbpy)(o-tolyl)Br, for 45 minutes using a Gen 2 Kessil PR160L 370 nm LED afforded the title compound. UV–vis (THF): λ1 = 1167 nm (8,570 cm-1), λ2 = 785 nm (12,739 cm-1), λ3 = 520 nm (19,231 cm-1). Table S1. Summary of the photochemical parameters used to generate the Ni(I)–bpy halide complexes from their Ni(II)–bpy aryl halide parents (Ar = o-tolyl) alongside their photochemical properties. MLCT peak positions and molar extinction coefficients are given for the lowest-energy MLCT transition seen in the UV–vis–NIR data. Solvent = THF; t = irradiation time. Ni(II) MLCT Ni(II) εMLCT LED t (nm / cm-1) (M-1 cm-1) (nm) (min) 475 / 21 053 4970 Ni(II)(t-Bubpy)(Ar)Cl 370 60 (II) t-Bu 479 / 20 877 3100 Ni ( bpy)(Ar)Br 390 45 488 / 20 492 2200 Ni(II)(t-Bubpy)(Ar)I 390 60 465 / 21 505 4540 Ni(II)(Mebpy)(Ar)Cl 390 60 483 / 20 704 4070 Ni(II)(Hbpy)(Ar)Cl 390 75 501 / 19 960 5400b Ni(II)(Phbpy)(Ar)Cl 370 75 532 / 18 797 6100 Ni(II)(MeO2Cbpy)(Ar)Cl 370 45 (II) MeO2C 538 / 18 587 5100 Ni ( bpy)(Ar)Br 370 45 Ni(I) Ni(I) MLCT Ni(I) εMLCT Parent Ni(II) Complex Complex (nm / cm-1) (M-1 cm-1)a (II) t-Bu 660 / 15 150 2000 Ni ( bpy)(Ar)Cl 1 653 / 15 310 2100 Ni(II)(t-Bubpy)(Ar)Br 1-Br 640 / 15 625 1000 Ni(II)(t-Bubpy)(Ar)I 1-I (II) Me 660 / 15 150 1900 Ni ( bpy)(Ar)Cl 2 (II) H 673 / 14 860 2100 Ni ( bpy)(Ar)Cl 3 1175c / 8510 800 Ni(II)(Phbpy)(Ar)Cl 4 1178 / 8490 5500 Ni(II)(MeO2Cbpy)(Ar)Cl 5 1167 / 8570 2500 Ni(II)(MeO2Cbpy)(Ar)Br 5-Br a Values obtained following complete photolysis of parent Ni(II) complexes and may be underestimated. b Reference value5. cIdentified by using both the UV–vis–NIR absorption peak and the GSB feature in the TA spectrum (Figure S31). Parent Ni(II) Complex 201 Isolation of a Ni(I)–bpy Halide Complex. Ni(I)(MeO2Cbpy)Cl, 5′. In a nitrogen-filled glove box Ni(II)(MeO2Cbpy)(o-tolyl)Cl6 (0.160 g, 0.350 mmol) was dissolved in 175.0 mL of diethyl ether. This purple solution was filtered to ensure homogeneity and transferred to a 350 mL Schlenk flask with a Teflon coated stir bar. The flask was sealed and removed from the glovebox. The flask was attached to a nitrogen Schlenk line and allowed to stir. Two Gen 2 Kessil PR160L 370 nm LEDs were pointed at the flask (one on either side); a fan was pointed at the entire setup to ensure room-temperature irradiation. The LEDs were allowed to irradiate the solution on their maximum setting for 48 hours. During the course of the irradiation, a dark precipitate could be seen evolving from the solution. The flask was then returned to the glovebox, and the solid collected by vacuum filtration (fraction 1). The purple filtrate was transferred back into the Schlenk flask, removed from the glovebox, and reattached to the nitrogen line. Irradiation of the solution was continued in the same manner as before for an additional 24 hours, affording more precipitated solid. The flask was again returned to the glovebox, and the new solid collected by vacuum filtration (fraction 2); the filtrate was discarded. The collected navy blue solid was washed with excess diethyl ether, 2-methyl tetrahydrofuran (5x1 mL), hexane (3x2 mL), and pentane (3x2 mL), then dried under reduced pressure (fraction 1: 0.072 g, fraction 2: 0.029 g, combined fractions: 0.101 g, 81% yield). Note: The title compound is highly air and moisture sensitive. It is insoluble in pentane, hexane, heptane, and ether. It is sparingly soluble in 2-MeTHF and is soluble in benzene, toluene, and THF. The compound reacts readily with dichloromethane (affording a red solution) and decomposes in acetonitrile (becoming a black solution). The compound readily reacts with aryl bromides. Powder sample X-band CW-EPR (T = 5 K; frequency = 9.638 GHz; power = 2.2 mW; modulation amplitude = 8 G): gavg = 2.146 (gx = 2.053, gy = 2.123, gz = 2.262; g(strain)x = 0.025, g(strain)y = 0.035, g(strain)z = 0.033. FT-IR solid sample (ATR, cm-1): 3073, 2957, 1716, 1567, 1511, 1435, 1398, 1316, 1277, 1224, 1102, 1014, 992, 885, 836, 751, 726, 698, 542. Frozen solution (toluene) X-band CW-EPR (T = 5 K; frequency = 9.639 GHz; power = 2.2 mW; modulation amplitude = 8 G): giso = 2.2011; g(strain) = 0.5. Frozen solution (THF) Xband CW-EPR (T = 5 K; frequency = 9.639 GHz; power = 2.2 mW; modulation amplitude = 8 G): g1,iso = 2.2011; g1(strain) = 0.5; g2,iso = 2.1870; g2(strain) = 0.0902. UV–vis (THF): 202 λ1 = 1178 nm (8,490 cm-1), λ2 = 805 nm (12,422 cm-1), λ3 = 523 nm (19,120 cm-1). UV–vis (Benzene): λ2 = 1181 nm (8,490 cm-1), λ2 = 820 nm (12,422 cm-1), λ3 = 526 nm (19,120 cm-1). Paramagnetic 1H NMR (400 MHz, d8THF): δ 10.51 (br s, 1H), 8.31 (br s, 2H), 3.81 (br s, 3H). Effective magnetic moment (Evans method, 298 K, C6D6): 1.9 µB. Figure S1. Frozen solution (toluene) X-band CW-EPR spectrum and fit (blue and orange lines, respectively) of 5′ (T = 5 K; frequency = 9.639 GHz; power = 2.2 mW; modulation amplitude = 8 G). Anisotropy in the signal could not be resolved due to line broadening, likely arriving from suspended particles of the complex which precipitated upon freezing or were not fully solvated. Simulation values: giso = 2.201; g(strain) = 0.5. Figure S2. Frozen solution (THF) X-band CW-EPR spectrum and fit (blue and orange lines, respectively) of 5′ (T = 5 K; frequency = 9.639 GHz; power = 2.2 mW; modulation amplitude = 8 G). (Left) Full spectrum plotted showing two species; anisotropy in the signal could not be resolved due to line broadening, likely arising from suspended particles of the complex which precipitated upon freezing or were not fully solvated. The second feature in the spectrum (labeled as g2) is attributed to a small fraction of the species coordinating to THF, which may occur during freezing of the sample. Dual spin simulation values: g1,iso = 2.201; g1(strain) = 0.5; g2,iso = 2.187; g2(strain) = 0.090. (Right) Spectrum after subtraction of the broad signal corresponding to g1. Simulation values: gz = 2.230, gx = 2.180, gy = 2.146, gavg = 2.185; gz(strain) = 0.043, gx(strain) = 0.043, gy(strain) = 0.065. 203 Figure S3. UV–vis–NIR spectra of 5′ pre- and post-addition (blue and orange lines, respectively) of 100 μL of 2-bromobenzotrifluoride. Figure S4. Aryl bromide reactivity analysis of 5′ by 19F NMR (d8-THF). (A) Reaction scheme for the oxidative addition of the aryl bromide by Ni(I), forming Ni(II)(MeO2Cbpy)(oCF3Ph)Br and paramagnetic Ni(II)(MeO2Cbpy)Cl2.6,7 (B) 19F NMR spectrum of independently synthesized Ni(MeO2Cbpy)(o-CF3Ph)Br. (C) 19F NMR spectrum of 5′ post-addition of 100 μL of 2-bromobenzotrifluoride. 204 S1.3. X-ray Crystallography. Collection and Refinement Details for Ni(II)(MeO2Cbpy)(o-tolyl)Cl. Ni(II)(MeO2Cbpy)(o-tolyl)Cl was crystallized by slow evaporation in diethyl ether. Lowtemperature diffraction data (-and -scans) were collected on a Bruker AXS D8 VENTURE KAPPA diffractometer coupled to a PHOTON II CPAD detector with Mo K radiation ( = 0.71073 Å) from an IμS micro-source for the structure of compound V23337. The structure was solved by direct methods using SHELXS8 and refined against F2 on all data by full-matrix least-squares with SHELXL-20199 using established refinement techniques.10 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included into the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups). All disordered atoms were refined with the help of similarity restraints on the 1,2- and 1,3-distances and displacement parameters as well as enhanced rigid bond restraints for anisotropic displacement parameters. Ni(II)(MeO2Cbpy)(o-tolyl)Cl crystallizes in the monoclinic space group P21/n with one molecule in the asymmetric unit. The o-tolyl group was modeled as a two-component disorder. These data are provided free of charge from The Cambridge Crystallographic Data Centre by The Cambridge Crystallographic Data Centre. top view side view Figure S5. Top down and side views of the refined crystal structure of Ni(II)(MeO2Cbpy)(otolyl)Cl. The o-tolyl group was modeled as a two-component disorder, with one conformer pointing up and the other pointing down. 205 Figure S6. The refined crystal structure of Ni(II)(MeO2Cbpy)(o-tolyl)Cl showing only one aryl ligand conformer for clarity. Compound Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 67.679° Absorption correction Max. and min. transmission Refinement method Data / restraints / parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole Ni(II)(MeO2Cbpy)(o-tolyl)Cl C21H19ClN2NiO4 457.54 g mol-1 100(2) K 1.54178 Å Monoclinic P21/n a = 7.1614(11) Å a= 90° b = 22.156(2) Å b= 102.715(10)° c = 12.972(3) Å g = 90° 3 2007.8(6) Å 4 1.514 Mg/m3 2.881 mm-1 944 3 0.150 x 0.100 x 0.050 mm 3.990 to 74.733°. –8 ≤ h ≤ 8, –27 ≤ k ≤ 27, –16 ≤ l ≤ 12 23344 4086 [R(int) = 0.1627] 99.9 % Semi-empirical from equivalents 0.7538 and 0.5432 Full-matrix least-squares on F2 4086 / 384 / 330 1.073 R1 = 0.0699, wR2 = 0.1544 R1 = 0.1140, wR2 = 0.1754 n/a -3 0.674 and –0.450 e.Å 206 S1.4. Steady-State UV–vis–NIR Spectroscopy. Steady-State Spectra Figure S7. Time-course UV–vis–NIR spectra in THF of the parent Ni(II)–bpy aryl halide compounds (blue line) being photogenerated to the Ni(I)–bpy halide complexes (orange line) examined in this work. (A) Photochemical conversion of Ni(II)(t-Bubpy)(o-tolyl)Cl to Ni(I)(tBu bpy)Cl, 1. (B) Photochemical conversion of Ni(II)(t-Bubpy)(o-tolyl)Br to Ni(I)(t-Bubpy)Br, 1-Br. (C) Photochemical conversion of Ni(II)(t-Bubpy)(o-tolyl)I to Ni(I)(t-Bubpy)I, 1-I. (D) Photochemical conversion of Ni(II)(Mebpy)(o-tolyl)Cl to Ni(I)(Mebpy)Cl, 2. 207 Figure S8. Time-course UV–vis–NIR spectra in THF of the parent Ni(II)–bpy aryl halide compounds (blue line) being photogenerated to the Ni(I)–bpy halide complexes (orange line) examined in this work. (A) Photochemical conversion of Ni(II)(Hbpy)(o-tolyl)Cl to Ni(I)(Hbpy)Cl, 3. Starred peak corresponds to an aggregation between Ni(II) and Ni(I) which is seen at low Ni(II) conversion.6 (B) Photochemical conversion of Ni(II)(Phbpy)(o-tolyl)Cl to Ni(I)(Phbpy)Cl, 4. (C) Photochemical conversion of Ni(II)(MeO2Cbpy)(o-tolyl)Cl to Ni(I)(MeO2Cbpy)Cl, 5. (D) Photochemical conversion of Ni(II)(MeO2Cbpy)(o-tolyl)Br to Ni(I)(MeO2Cbpy)Br, 5-Br. 208 Figure S9. UV–vis–NIR absorption spectra of the photochemically generated Ni(I)–bpy halides in THF. Boxed section is expanded on the right. Analogous figure with wavelength axis is given as Figure 2. The electronic structure of 1-5 and related compounds have been assigned by EPR and quantum chemical calculations as having a [d(xy)]2, [d(yz)]2, [d(xz)]2, [d(z2)]2, [d(x2–y2)]1 ground state.11–14 The four β-β ligand field transitions are predicted by TDDFT to require only ~0.5–1.1 eV (~4050–8900 cm-1) of photonic energy, placing them all in the IR to NIR region (~2500-1100 nm, see Supporting Information Section 2.4). Given the formal electric dipole forbidden nature of these transitions, their intensities would be much lower than those seen here. Contrastingly, the bpy π* orbitals lie energetically between the metal-based HOMO and LUMO (β-3d(x2–y2)), allowing for numerous spin- and orbitally-allowed d-π* transitions across the full wavelength range shown. From these observations, the absorption bands can be assigned as Ni(I)-to-bpy MLCTs, consistent with our previous work.11 209 Fitting of Steady-State Spectra Figure S10. Fits to absorption spectra (red) alongside the raw spectrum (solid black) for compound 1 (left) and 5 (right) and the components to the fit as individual Gaussians underneath. The data were fit only in the wavelength region shown; components required to satisfy the boundaries are shown as dotted lines and not included in the fit. We focus here on compounds 1 and 5 as representative examples. Given the number of transitions expected in these compounds and the inaccuracy of the TD-DFT, we cannot fit the data to a series of vibronic progressions. This would provide the best comparison to the values obtained from equation 3, but the number of unknowns would severely limit the reliability of the fit. Therefore, we instead approximate the lineshape of each transition as a simple Gaussian and the absorption spectrum can be fitted to estimate the contributions of each transition to the absorption spectrum, 𝐴(𝐸). Mathematically, we fit to 𝑁 (𝐸 − 𝑏𝑖 )2 𝐴(𝐸) = ∑ 𝑎𝑖 exp (− ), 2𝑐𝑖2 (S1) 𝑖=1 where 𝐸 is photon energy, and 𝑎𝑖 , 𝑏𝑖 and 𝑐𝑖 are fitting parameters which describe the amplitude, center, and width of each transition. Excellent fits to the data with only minor deviations are achieved using 𝑁 = 9 for 1 and 𝑁 = 8 for 5 with all parameters shown in Table S2; these are plotted in Figure S10. The dashed Gaussians are used to account for the boundaries of the fit and are not included the subsequent analysis. The total reorganization energy, 𝜆, can be approximated from the peak widths as 𝜆~ 𝑐2 2𝑘𝐵 𝑇 (S2) Averaging over 𝜆 and taking the error on the mean gives 𝜆 = 0.47(5) eV and 𝜆 = 0.6(1) eV for 1 and 5, respectively. Given the number of fitting parameters used, the overlapping nature of the peaks, and the assumed lineshape as a perfect Gaussian (no vibronic considerations) this fit is unlikely to be unique and constitutes an underestimate of the true 210 𝜆. Despite this, all fitted peaks have similar widths suggesting that this is a reasonable approximation. Table S2. Parameters for the fits shown in Figure S10 for complexes 1 and 5. Components not included in the average are underneath the dotted line. 1 5 𝒂 (OD) 𝒃 (eV) 𝒄 (eV) 𝒂 (OD) 𝒃 (eV) 𝒄 (eV) 0.040 1.75 0.15 0.25 1.04 0.06 0.14 1.87 0.14 0.35 1.23 0.20 0.14 2.15 0.16 0.48 1.53 0.14 0.13 2.47 0.17 0.33 1.80 0.16 0.050 2.98 0.10 0.26 2.08 0.13 0.16 3.2 0.15 0.51 2.35 0.18 0.21 2.80 0.18 0.40 2.75 0.25 0.075 1.46 0.31 0.38 3.25 0.17 0.22 3.81 0.64 - 211 S1.5 Time-Resolved Spectroscopy Transient Absorption Spectra Figure S11. TA data for 1 in THF following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 212 Figure S12. TA data for 1 in THF following 700 nm, 0.3 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 213 Figure S13. TA data for 1 in THF following 700 nm, 1.5 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 214 Figure S14. TA data for 1 in toluene following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 215 Figure S15. TA data for 1-Br in THF following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 216 Figure S16. TA data for 1-I in THF following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 217 Figure S17. TA data for 2 in THF following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. Peak at 510 nm is a result of pump scatter. 218 Figure S18. TA data for 3 in THF following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 219 Figure S19. TA data for 4 in THF following 700 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 220 Figure S20. TA data for 5 in THF following 800 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 221 Figure S21. TA data for 5 in THF following 800 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 222 Figure S22. TA data for 5 in THF following 1200 nm photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 223 Figure S23. TA data for 5 in THF following 560 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. The peak at ~1120 nm is due to scattered pump light. 224 Figure S24. TA data for 5 in toluene following 800 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 225 Figure S25. TA data for 5 in THF following 800 nm, 0.15 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 226 Figure S26. TA data for 5 in THF following 800 nm, 0.3 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 227 Figure S27. TA data for 5 in THF following 800 nm, 2 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 228 Figure S28. TA data for 5ʹ in THF following 1200 nm photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 229 Figure S29. TA data for 5ʹ in THF following 800 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 230 Figure S30. TA data for 5-Br in THF following 800 nm, 1 μJ photoexcitation. (A,B) Cascaded difference spectra across the two time regions indicated. (C,D) and (E,F) show results of three and four component global fits, respectively. (C,E) Kinetic traces at indicative wavelengths across two time regions. Inset is an enlarged view of the traces at long times. (D,F) DAS corresponding to the time constants indicated in the legend. 231 Figure S31. NIR difference spectra for (A) 4 and (B) 5-Br at selected time delays following 700 nm and 800nm, 1 μJ photoexcitation, respectively. The GSB is partially obscured by the overlapping ESA in 4. Noise precluded reliable fitting, but the features are shown for comparison to other measurements. Figure S32. Difference spectra for all compounds overlaid following 1 μJ photoexcitation at either 700 or 800 nm. Time delays chosen to represent difference spectrum of rate-limiting process: compounds 1, 1-Br, 2 and 3 are shown at 2 ps, 1-I at 5 ps, and 4, 5 and 5-Br at 15 ps. The spectrum for 5 has been scaled by 0.1 to fit on the same y-axis as the other compounds. In this section, we have presented TA spectra measured for each compound (Figures S11S31) using the procedure described in Section S1. Where possible, each dataset is shown 232 with fits to two kinetic models. The parameters obtained from the better of these two fits are given in Table S3 below. TA spectra were also measured for 5 following photoexcitation into its MLCT bands at 1200 and 560 nm (Figures S22-S23). The resulting spectra and time constants agree well with the results for 800 nm photoexcitation, indicating the difference in behavior between 1 and 5 is not the result of pumping different MLCT bands; the lowest-energy MLCT dictates the overall ultrafast behavior. We additionally questioned if solvent coordination to Ni obfuscated the comparison between 1 and 5. Thus, TA spectra were also measured in toluene. The spectral and kinetic profiles of 1 and 5 were largely unchanged (still requiring two- and three-component fits, respectively) and both exhibited slightly slower relaxation to the ground state (12.4 ps in 1 and 29.2 ps in 5). The difference in kinetic behavior between 1 and 5 is therefore not due to solvent coordination; we do not observe any evidence of a difference in relaxation pathway between solvents. Finally, we repeated the above TA measurements on 1 and 5 at a range of pump powers to ensure the observed differences in dynamics were not due to saturation or multi-photon effects. For both 1 and 5, the kinetics were independent of pump power from 0.15 to 2 mW (Table S3). 233 Table S3. Table of relaxation time constants for all compounds in THF. Pump Pump 𝜏1 𝜏2 𝜏3 𝜏4 𝜏5 Compound Wavelength Power Solvent a (ps) (ps) (ps) (ps) (ps)a (nm) (μJ) 1 700 1.0 THF 0.3 10.9 ∞ 700 THF ∞ 1 0.3 0.4 11.2 1 700 1.5 THF 0.3 10.7 ∞ 1 700 1.0 Toluene 0.5 12.4 ∞ 1-Br 700 1.0 THF 0.4 13.9 ∞ 1-I 700 1.0 THF 0.4 1.2 15.4 2 700 1.0 THF 12 80 ∞ 3 700 1.0 THF 0.4 10 300 ∞ 4 800 1.0 THF 0.5 5.2 22.3 ∞ 5 800 1.0 THF 0.6 5.6 24.2 b 5 800 1.0 THF 0.1 4.2 21.8 c 5 1200 n.d THF 0.2 4.8 25.1 5b 560 1.0 THF 0.5 6.9 25.6 5 800 1.0 Toluene 0.6 6.5 29.2 5 800 0.15 THF 0.2 5.5 24.7 5 800 0.3 THF 0.2 4.8 23.2 5 800 2.0 THF 0.2 4.8 25.1 c 5′ 1200 n.d THF 0.4 6.0 24.3 ∞ 5′ 800 1.0 THF 0.2 5.0 23.9 5-Br 800 1.0 THF 0.4 5.6 23.8 ∞ a Time constants correspond to growth of long-lived feature discussed below, with those much longer than the time window considered infinite. bSpectra recorded with NIR probe. c Not determined due to lack of a suitable NIR power meter. 234 Discussion of Anomalous Long-lived Feature As noted in the main text, a long-lived feature is observed in some spectra. This feature is negative and flat with little-to-no shape and appears to increase in magnitude as time-delay increases. The DAS corresponding to the shortest time constants show signals that clearly correspond to the growth or decay of the GSB and ESA of interest. Given the overlap of the positions of the GSB with the peaks in the static absorbance spectrum, these features must correspond to the Ni(I) species. In contrast, the final time constant corresponds to a constant offset which does not decay on the timescale of the experiment. The lack of any GSB features in the DAS corresponding to the static Ni(I) absorbance show that this signal does not arise from the Ni(I) species. For most compounds, this long-lived signal is very small and has minimal impact on the results other than necessitating an extra constant-offset component in the fitting procedure. However, in the samples containing 3 (Figure S18), this long-lived feature is non-negligible, and four components are needed for a reasonable fit to the data. To better understand the kinetics of 3, we turn again to the DAS. The shortest two components clearly correspond to the Ni(I) complex, due to GSB peaks with the same positions as peaks in the steady-state absorbance spectrum. The near-UV band (~350 nm) in the 10 ps DAS is often diagnostic of the presence of reduced bpy (an MLCT state). In contrast, the other two DAS are largely flat, and as such are not included in the analysis in the rest of this work. These latter two time components correspond to the slow growth of the flat feature and persist beyond the experimental timescale. Notably, consecutive repeats of this TA (Figure S33A) overlap exactly, showing that the signal is not due to sample degradation over the course of the experiment. Thus, we find a strong Ni(I) signal which decays alongside a slowly growing, broad, negative feature. Given that the long-lived signal is largest in 3, we investigated it in more depth using 3 as a model system. Photogeneration of 3 from its parent Ni(II) complex resulted in the formation of some precipitate. We brought the sample into the glove box, filtered the cuvette solution through a Kimwipe, and returned the filtrate for TA analysis. TA on this solution did not exhibit a noticeable change to the unfiltered sample. We further questioned if this signal was from a Ni(I) complex. Ni(I) species are known to react quickly with aryl halides. In the glove box, aryl bromide was added to the solution. Doing so removed the TA signal originating from the Ni(I) species, but the long-lived signal was still present in the sample (Figure S33B). Therefore, we find that the short-lived signals assigned to 3 do indeed arise from the Ni(I)– bpy halide complex, but that the long-lived signal does not. 235 Figure S33. TA spectra for sample containing 3 under various conditions following 700 nm, 1 μJ photoexcitation. (A) Consecutive scans of the TA measurement showing the reproducibility of the long-lived feature. (B) Difference spectra before and after addition of aryl bromide showing disappearance of Ni(I) signal. (C) Difference spectra at 1 ns (solid lines) recorded at different irradiation times alongside the corresponding absorption spectra (dotted lines). We next turned to a photogeneration time-course experiment to try to identify the origin of the signal. A solution of the Ni(II) parent complex for 3 was irradiated for different lengths of time and the TA signal at 1 ns was plotted alongside the UV–vis data (Figure S33C). Pumping the compound at 700 nm avoids exciting any parent Ni(II) left in solution. Furthermore, by looking at only the TA signal at a long time delay (1 ns) wherein all 236 photoexcited 3 will be back in the ground state, we can select for only the long-lived feature. From the absorbance data, the Ni(II) peak (~480 nm) decreases monotonically with time, while the peak around 670 nm grows in and then decreases. This lower-energy peak is indicative of the formation and then subsequent thermal degradation of 3. In the corresponding TA, a monotonic growth of the signal magnitude with increasing irradiation time is observed. The signal presents as a broad feature that slowly grows towards the blue with two small peaks around 350 and 450 nm. From comparison to Figure S18, it is clear that the shape of the long-lived feature varies between samples but is reproducible for multiple measurements of the same solution. The dependence of the long-lived TA signal on irradiation time is markedly different than that of the steady-state Ni(I) peak at 670 nm, further supporting the idea that this long-lived feature does not originate from 3. While we can conclude that the long-lived feature originates from neither photoexcited 3 nor its Ni(II) parent, its true identity is unknown. Previous studies into the speciation which occurs when Ni(II)–bpy aryl halide complexes are irradiated present three potential other candidates: Ni(II)/Ni(I) bimetallic aggregates,6,16 Ni(I)/Ni(I) dimers,6,17,18 or high-spin Ni(II)–bpy dihalide.6,7 The first of these require there be significant Ni(II) parent in solution, which does not fit with the above kinetic observations. Most likely for our case is the insoluble (or sparingly soluble) Ni(I)/Ni(I) dimer which forms irreversibly and depends only on the Ni(I) concentration (formation of the dimer is expected to be the main thermal degradation pathway for 3)6 or further downstream decomposition resulting in the high-spin Ni(II)–bpy dihalide. Therefore, we tentatively postulate that the pump pulse deposits a significant amount of thermal energy into the system causing dissolved dimers or high-spin Ni species to precipitate. The newly formed precipitate at the focus of the light would then slowly diffuse outwards, scattering a greater proportion of light as the delay time increases. This would be evidenced by a growing GSB in the TA signal, such as what we observe. Then, over the subsequent microseconds, the precipitate would diffuse far enough away from the focused light as to not affect the transmitted signal (or the precipitate could redissolve), thereby making the TA repeatable. Whatever the origin of the signal, it arises from neither photoexcited parent Ni(II) nor the Ni(I) complexes studied herein. 237 S1.6 Fits to Alternative Relaxation Models In an attempt to rationalize the differences between 1-3 and 4-5, we considered the possibility of alternative decay pathways, including relaxation through the intermediacy of an optically dark metal-centered state (2d-d) or a 4MLCT state accessed by intersystem crossing (ISC). Rate-limiting decay of a dark 2d-d state can be excluded by considering that the relaxation proceeds through simultaneous recovery of the ESA and GSB with isosbestic points for all compounds and the presence of strong ESA in the visible. Therefore, if a dark 2d-d state is formed, it must relax much faster than the 2MLCT. The 2d-d → 2GS transition has an energy gap similar to that of the MLCT state and is formally orbitally forbidden, making it unlikely that it is significantly faster than a rate-limiting 2MLCT → 2d-d step (Figure S44). Conversely, optical TA is not sensitive to state multiplicity, allowing for the possibility of relaxation from 2MLCT → 4MLCT by fast ISC followed by spin-forbidden relaxation to the ground state (4MLCT → 2GS, Figure S42). Albeit not completely unprecedented, the observed values for 3 would constitute very short time constants for reverse ISC to the ground state.5 We further find that the relaxation kinetics of this step show no dependence on pump wavelength (Table S3). As such, we propose that all compounds relax from the same MLCT excited state (Figure 5A). Several models have been presented for intramolecular electron transfer (ET) over the past decades; in this section we discuss four of these to understand the data presented in main text Figure 5B. These models all rely on the same four approximations: 1. 2. 3. 4. The series of compounds relax from the same MLCT excited state. EMLCT is an accurate reflection of the lowest MLCT state. The electronic coupling is roughly the same along the series. The series of compounds show roughly the same reorganization energies between ground and excited states. That all compounds in the series relax from the same MLCT state is discussed above, and the association of this state to EMLCT is supported by the TD-DFT assignments (Tables S5S12) of the absorption spectra presented in Section S1.4. The lowest MLCT transition energy is only slightly lower than that of the lowest high-absorbance MLCT, justifying this approximation. The latter two are assumed for almost all literature examples of Marcus theory discussed below, and we see them as equally valid in this case. Only the bpy substituents or the halide change between compounds; the bpy coordination sites that have most influence on the MLCT character are mostly unchanged by these modifications. Therefore, we expect that the electronic coupling and reorganization energies are similar across all compounds. Any additional model-specific assumptions are discussed where relevant in the following paragraphs. 238 Weak Coupling Model Jortner and co-workers19,20 developed one such model to describe the non-radiative relaxation of excited states. Their framework relates the rate of relaxation to the deformation of the excited state relative to the ground state. If the deformation is relatively small—the socalled weak coupling limit—then the intersection between the ground- and excited-state PESs lies at very high energy. Therefore, relaxation proceeds through coupling of the excited state to high-energy vibronic modes of the ground state. At room temperature, this leads to Eq. (S3). 𝑘= 𝑉2 2𝜋 Δ𝐺° Δ𝐺° √ ) − 1]) exp (− [log ( ℏ ℏ𝜔𝑀 Δ𝐺° ℏ𝜔𝑀 𝑑𝜆 (S3) Here ℏ𝜔𝑀 is the energy of the highest vibrational mode with degeneracy, 𝑑. 𝑉 is the matrix element for the transition, and Δ𝐺° is the energy gap between the relaxed excited and ground states. Given the explicit dependence on the maximum vibrational frequency, the rate is expected to show a strong dependence on the character of this transition. To derive Eq. (S4), Δ𝐺° it is assumed that the reorganizational energy is small such that [log ( 𝑑𝜆 ) − 1] > 0. This results in a slightly stronger than exponential dependence on Δ𝐺° and thus recovers the empirical energy gap law. Our main interest here lies with the dependence on Δ𝐺° so we can simplify to 𝑘= 𝐴 √EMLCT − 𝜆 exp (− EMLCT − 𝜆 EMLCT − 𝜆 ) − 1]), [log ( ℏ𝜔𝑀 𝑑𝜆 (S4) where the pre-factors have been condensed into 𝐴. The same assumption as in the main text that Δ𝐺 𝑜 = 𝜆 − 𝐸𝑀𝐿𝐶𝑇 is also made to relate the model back to the experimentally accessible vertical transition energies. For all compounds, the highest vibrational modes are C–H stretches on the bpy at around 0.39 eV (Table S4); six of these are in common to all 1–5. While not strictly degenerate, these are similar in energy so we follow literature precedent19,21 in setting 𝑑 = 6 and fit for the remaining parameters. This results in the fit shown in Figure 5B, with 𝜆 = 0.09 eV, ℏ𝜔𝑀 = 0.41 eV and 𝐴 = 0.26 eV1/2 ps–1. While it can describe the high-energy results well, the monotonically increasing function cannot account for the slower relaxation of the lower-energy compounds. Despite this, it does seem to recover the energy of the C–H stretches, suggesting that there may be some merit to the fit in the high-energy region. Observation of a large change in rate upon deuteration of the bpy could interrogate this experimentally, as the energy change from C–H to C–D vibrational modes (ℏ𝜔𝑀 (C–D) ~ 0.28 eV)19 are expected to result in sizable changes to the coupling between the ground and excited states. 239 Importantly, a globally poor fit to our data is to be expected from the weak coupling model given the assumptions taken in its derivation. Here we observe MLCT transitions where there will, by definition, be a significant redistribution of charge. Therefore, unlike a π→π* transition, for example, the reorganization energy here is relatively large, as is observed in the absorption spectra in Figure S10. Thus, again we find that our system most likely does not lie within the weak coupling limit. Strong Coupling and Classical Marcus Models When the excited-state deformation is large enough, the intersection between the surfaces becomes thermally accessible. The system will instead move along the vibrational coordinate with a small activation energy into the ground-state PES through the intersection. This behavior is shown schematically in the main text Figure 5A and corresponds to the strong coupling limit presented by Jortner et al.19 and given mathematically by main text Eq. (1). As Δ𝐺° decreases, excited- and ground-state PESs transition from nested to displaced, and the resulting change in activation energy yields a parabolic dependence on Δ𝐺°. In deriving Eq. (1), it is approximated that probability of surmounting the energy barrier is determined by the mean vibrational energy. Here we again relate back to 𝐸𝑀𝐿𝐶𝑇 to get Eq. (2) and then fit this to the observed rate constants. A good fit is achieved (Figure 5B) with fitting parameters: 𝜆 = 0.77(3) eV, ln(𝐴) = 25.7(6), ⟨ℏ𝜔⟩ = 0.13(7) eV. As discussed in the main text, ⟨ℏ𝜔⟩ is indeed close to the calculated average vibrational energy; the reorganizational energy is larger than that obtained from fitting the steady-state spectra, but of similar magnitude. The most successful theory of electron transfer (ET) is Marcus theory.22–24 However, this was developed for ET between two relatively independent species rather than the nonradiative relaxation we observe here.25 These could be different molecules in the case of intermolecular ET or even a large molecule with well-separated donor and acceptor. This places limits on the applicability of the model to intramolecular systems. Despite this fact, we can still utilize Marcus theory to approximate the dynamics of charge transfer transitions. Making the same assumption that Δ𝐺 𝑜 = 𝜆 − 𝐸𝑀𝐿𝐶𝑇 leads to Eq. (S5) (𝐸MLCT − 2𝜆)2 (S5) ln(𝑘) = ln(𝐴′′) − . 4𝜆𝑘𝐵 𝑇 Fitting this to the data in Figure 5B and fixing 𝑇 to 298 K, yields 𝜆 = 0.76(1) eV, ln(𝐴) = 27.1(2). This fit is plotted alongside the other three models in Figure 5B, where it is clearly worse than that of Eq. (2) yet still captures the main features of the data; the reorganization energy is consistent between the two models. The worse fit is to be expected given that the ET in question is short-range and intramolecular, and there is one fewer fitted parameter. 240 Likewise, the consistency in 𝜆 likely only reflects the very similar functional forms of the models. Nonetheless, the loose applicability of classical Marcus theory here is borne out by the similarities between the Marcus equation and the strong coupling model discussed in the main text and Eq. (2). Both the classical Marcus theory and strong coupling models have the same form, save for the substitution ⟨ℏ𝜔⟩ = 2𝑘𝐵 𝑇, which physically corresponds to the charge transfer being driven by vibrational modes rather than thermal energy. This difference follows from their derivation in classical and quantum regimes, respectively. Accordingly, the semiclassical derivation of Marcus theory leads to the same dependence on ⟨ℏ𝜔⟩.26 Vibronic Marcus Model Classical Marcus theory can be extended into a quantum regime by considering coupling of the electron transfer with vibronic modes.27 Summation over all vibronic modes is not practical for fitting experimental data, so this can be simplified by assuming quantum coupling to just one mode—either one dominant mode, or a representative average mode— while low frequency modes are treated classically.28 This semiclassical vibronic Marcus model was first applied in the context of long-distance intramolecular electron transfer29 and splits the reorganization energy into contributions from the solvent and vibrations, 𝜆𝑆 and 𝜆𝑉 , respectively. Mathematically, this gives rate constants of ∞ (Δ𝐺° + λS + 𝑛ℏ𝜔)2 𝜋 𝑒 𝑆𝑆𝑛 𝑘= 𝑉 √ 2 ∑ exp (− ), ℏ 𝜆𝑆 𝑘𝐵 𝑇 𝑛! 4𝜆𝑆 𝑘𝐵 𝑇 2 (S6) 𝑤=0 where 𝑆 = 𝜆𝑉 /ℏ𝜔 and ℏ𝜔 is the energy of the mode to which the ET couples. The sum over 𝑤 accounts for transitions into each of the ground-state vibronic modes and the weighting of the exponential corresponds to the Franck-Condon factor for each transition. Substituting Δ𝐺 𝑜 = 𝜆 − 𝐸𝑀𝐿𝐶𝑇 = (𝜆𝑆 + 𝜆𝑉 ) − 𝐸𝑀𝐿𝐶𝑇 results in ∞ 𝑘 = 𝐴′ ∑ 𝑛=0 (EMLCT − 2λS − 𝜆𝑉 − 𝑛ℏ𝜔)2 𝑒 𝑆𝑆𝑛 exp (− ), 𝑛! 4𝜆𝑆 𝑘𝐵 𝑇 (S7) where the pre-factors are again collated into A. This constitutes the fourth model we fit to our data, resulting in parameters 𝐴 = 0.044 ps−1 , ℏ𝜔 = 0.21 eV, 𝜆𝑆 = 0.54 eV, 𝜆𝑉 = 0.22 eV; the fit to this vibronic Marcus model is plotted in Figure 5B. Accurate estimates of errors on these values could not be obtained due to the infinite sum requiring a more involved fit. Given the explicit dependence on solvent, we exclude the toluene datapoints from the fit here, but their inclusion has a negligible impact on the fitted parameters. The Franck-Condon weighted sum results in an asymmetric, near-parabolic dependence that gives the best fit to 241 the data out of all models considered here (albeit with the greatest number of fitted parameters). Previous studies on Fe–polypyridyl complexes15 supposed that bpy breathing modes couple to MLCT transitions. Vibronic coupling to a particular mode is typically accompanied by corresponding vibronic progressions into the absorption spectrum. However, given the linewidth and number of transitions expected in the spectra for 1–5, we are not able to say whether significant vibronic transitions are present. Despite this, the 0.21 eV value obtained from the fit does lie within the range of bpy breathing modes of 1–5, which are predicted by DFT to be ~0.2 eV. It is also often assumed that ET along organic spacers is coupled to general skeletal modes in the region of 0.19 eV,29–31 further corroborating the fitted value. Only the solvent contribution to the reorganization energy affects the linewidths of the peaks in the absorption spectrum. Indeed, 𝜆 matches the widths obtained from Section S1.4. This value is also identical to that observed for Zn porphyrin complexes in THF.30 The vibrational reorganization energy is much harder to determine experimentally, but it can be approximated through calculations of the PESs in Figure S44. These calculations (with implicit solvation model, CPCM) do not account for reorganization of the solvent, reducing the predicted contribution of 𝜆𝑆 . Consequently, they should roughly correspond to 𝜆𝑉 and are found to be 0.18 eV and 0.33 in 1 and 5, respectively, which is close to the 0.22 eV given by the fit. However, as discussed in Section S2.5, there are several limitations to DFT in these systems. Literature values for 𝜆𝑉 are found in the range 0.15–0.6 eV,29–32 reflecting the dependence on excited-state distortion, which can vary greatly between compounds. The total reorganization energy 𝜆 = 𝜆𝑆 + 𝜆𝑉 = 0.76 eV found by the vibronic Marcus fit is consistent with the classical Marcus and strong coupling models. Comparing Eqs. S6 and S7, 𝑉 = 1.4 meV ≪ 𝑘𝐵 𝑇, indicating that the relaxation is nonadiabatic, which is assumed by the Marcus model and implies that crossing the PES is the rate-determining step.29 Despite being developed for longer-range ET, the vibronic Marcus model still appears to provide a self-consistent description of the data here. The model has also been applied with success to short-range intramolecular charge transfer transitions in both inorganic and organic systems, suggesting that its semiclassical nature is not a significant limitation.15,30–35 Both strong coupling and vibronic Marcus models appear to give good descriptions of the data, and both of these arise from considering the intersection of displaced PESs. Therefore, while Marcus theory was initially developed for long-distance charge transfer, it is functionally the same as strong coupling in this case. In compounds 1–3 with their larger 𝐸𝑀𝐿𝐶𝑇 , the ground-state and MLCT PESs are nested (known as “inverted” in the language 242 of Marcus theory) and so behave in a manner reminiscent of the energy gap law. For 4– 5, which have smaller 𝐸𝑀𝐿𝐶𝑇 , the PES are displaced (normal) so the energy gap law behavior is effectively reversed. 243 S2. Computational Section. S2.1. General Computational Details. All the computations were performed using ORCA 5.0.3 software.36,37 Molecular structures were optimized with DFT using the BP86 functional38,39 with the def2-TZVP basis set40 on all atoms except Ni which received def2-TZVPP. The Weigend auxiliary basis set, def2/J was used.41 The calculations were expedited by employing the resolution-of-identity (RI) approximation. The D3BJ dispersion correction42,43 was applied; the conductor-like polarizable continuum (CPCM) solvation model44,45 was used for implicit solvation. Single point calculations with the hybrid B3LYP functional46,47 and def2-TZVP+def2TZVPP(Ni) basis set were used to refine the electronic energies and molecular properties. Again, def2/J was used as auxiliary basis, alongside D3BJ and CPCM for dispersion correction and solvation modeling, respectively. The chain-of-spheres approximation, RIJCOSX,48 was utilized as is default for hybrid-DFT in ORCA 5. For equilibrium geometries, the terms contributing to Gibbs free energy were calculated as follows: G = Eel + Gsolv + [EZPVE + RT – RT ln (Q)], (S8) where, i) Eel is the in vacuo electronic energy; calculated using RI-B3LYP-D3 method as above, ii) Gsolv is the free energy of solvation; calculated using CPCM, iii) [EZPVE + RT – RT ln(Q)] corresponds to the thermal enthalpic and entropic contributions to the solute energy with EZPVE and Q being the zero-point vibrational energy and the molecular partition function, respectively; obtained from frequency calculations with the rigid rotor/harmonic oscillator approximation (for p = 1 bar, T = 298 K). The spin states considered in our computational analysis of 1-5 were all doublets (S = 1/2), taken following experimental data. For 1 and 5, ISC to a quartet (S = 3/2) was also considered. On top of the DFT-optimized geometries, TDDFT was used to predict the excited states and compare the computational absorption patterns with experimental UV–vis–NIR spectra. For each TDDFT calculation, 100 roots were considered. Relaxed excited-state geometries were also found using TDDFT via the iRoot keyword in ORCA. The XYZ coordinate system used for labeling orbitals throughout the manuscript was selected according to the parent Ni(II) complexes to maintain consistency with the previous studies,4,6,49 i.e., the x and y axes are oriented along the Ni‒N(bpy) axes, making the singly occupied orbital to be the 3d(x2-y2). By this, the orbitals parallel and perpendicular to the Ni‒ 244 halide axis are the mixtures of d(xz) and d(yz) orbitals. To distinguish them, we thus label the orbitals as d(xz/yz,‖) and d(xz/yz,⊥) according to their parallel and perpendicular orientation to the Ni‒halide axis. 245 S2.2. Sample ORCA Inputs. Example DFT Geometry Optimization ! UKS BP86 def2-TZVP def2/J RI D3BJ ! TightSCF CPCM(THF) SlowConv ! OPT FREQ Example DFT Single Point Calculation ! UKS B3LYP def2-TZVP def2/J RIJCOSX ! D3BJ CPCM(THF) SlowConv ! MOREAD ! SP %basis newgto Ni "def2-TZVPP" end end %moinp "optimization.gbw" *xyzfile 0 2 structure.xyz %basis newgto Ni "def2-TZVPP" end end *xyzfile 0 2 optimized-structure.xyz Example TDDFT Calculation ! UKS B3LYP def2-TZVP RIJCOSX ! D3BJ CPCM(THF) SlowConv Example TDDFT Excited State Optimization def2/J ! UKS B3LYP def2-TZVP def2/J RIJCOSX ! D3BJ CPCM(THF) SlowConv ! OPT Keepdens %basis newgto Ni "def2-TZVPP" end end %basis newgto Ni "def2-TZVPP" end end %tddft nroots 100 maxdim 5 end %tddft nroots 5 maxdim 5 IRoot 1 end *xyzfile 0 2 optimized-structure.xyz * xyzfile 0 2 optimized-structure.xyz 246 S2.3. DFT Molecular Orbital Diagrams and Vibrational Energies Figure S34. Molecular orbital diagram for 1 at the DFT(B3LYP) level. 247 Figure S35. Molecular orbital diagram for 1-Br at the DFT(B3LYP) level. 248 Figure S36. Molecular orbital diagram for 1-I at the DFT(B3LYP) level. 249 Figure S37. Molecular orbital diagram for 2 at the DFT(B3LYP) level. 250 Figure S38. Molecular orbital diagram for 3 at the DFT(B3LYP) level. 251 Figure S39. Molecular orbital diagram for 4 at the DFT(B3LYP) level. 252 Figure S40. Molecular orbital diagram for 5 at the DFT(B3LYP) level. 253 Figure S41. Molecular orbital diagram for 5-Br at the DFT(B3LYP) level. 254 Table S4. Vibrational energies for complexes 1-5 computed at the DFT (BP86) level. The highest vibrational level for all the complexes is a C–H stretching frequency on the bipyridine ligand. Compound Average Vibrational Energy, Highest Vibrational Energy ⟨ℏ𝝎⟩ (cm-1 / eV) (cm-1 / eV) 1 1241.1 / 0.154 3145.3 / 0.390 1-Br 1240.0 / 0.154 3145.1 / 0.390 1-I 1239.9 / 0.154 3148.1 / 0.390 2 1175.9 / 0.146 3124.6 / 0.387 3 1142.8 / 0.142 3141.4 / 0.389 4 1145.1 / 0.142 3135.7 / 0.389 5 1087.4 / 0.135 3155.3 / 0.391 5-Br 1086.3 / 0.135 3155.4 / 0.391 Average 1169.8 / 0.145 3143.9 / 0.390 255 Figure S42. Computed PESs for 1 (left) and 5 (right) showing the positions of the calculated TD-DFT transitions (see below) relative to the TA pump (800 or 1200 nm). Relaxed structures computed at the DFT BP86 level for the doublet ground state (blue surface) and relaxed quartet excited state (orange surface) are shown. Open circles indicate computed values and correspond to the energy markers on the y-axis. Calculations were performed to investigate the possibility of ISC to a quartet state (see Supporting Information Section 1.6; Fits to Alternative Relaxation Models for more details). Since both quartets are nested in the doublet surface (energy gap law behavior/Marcus inverted region), one would expect 5 to relax more quickly than 1 given its lower energy (a result opposite of that to the experiment). Thus, DFT suggests that a rate-limiting 4MLCT → 2GS is unlikely. We note, however, that accurate computed displacements are precluded without the use of an explicit solvation model to account for outer sphere reorganization energy. Vertical and relaxed excited-state energies of 5 are likely to be overestimated (see Supporting Information Section 2.5; Limitations of DFT/TDDFT for more details). 256 S2.4. TDDFT Spectra and Tabulated Transitions Figure S43. Experimental UV–vis–NIR spectra of complexes 1-5 (blue lines) and their predicted excited-state transitions (orange sticks) using TDDFT (B3LYP) with the CPCM(THF) solvation model. MLCT Manifolds are labeled from lowest to highest energy and are denoted by the highlighted regions (Manifold 1 = magenta, 2 = yellow, 3 = cyan, 4 = green). Complexes 4, 5, and 5-Br are not well modeled by TDDFT (note the absence of the low-energy MLCT Manifold 1). 257 Table S5. Absorption transitions for the equilibrium structure of 1 at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. E State E (cm-1) fosc Transition Assignment (nm) 1 9305 1075 0.0004300 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 9758 1025 0.0000523 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 3 10781 928 0.0000023 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 4 11462 873 0.0014185 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 12973 771 0.0008758 β-Ni 3d(z²) → β-π*(1) 6 14931 670 0.0148499 β-Ni 3d(xz/yz,‖) → β-π*(1) 7 15269 655 0.0105068 β-Ni 3d(xz/yz,⊥) → β-π*(1) 8 18675 536 0.0000537 α-Ni 3d(x²-y²) → α-π*(1) 9 19490 513 0.0000902 β-Ni 3d(xy) → β-π*(1) 10 20225 494 0.0010854 α-Ni 3d(z²) → α-π*(1) 11 21293 470 0.0035294 α-Ni 3d(xz/yz,⊥) → α-π*(1) 12 21575 464 0.0276969 α-Ni 3d(xz/yz,‖) → α-π*(1) 13 22119 452 0.0002490 β-Ni 3d(z²) → β-π*(2) 14 23376 428 0.0321762 β-Ni 3d(xz/yz,‖) → β-π*(2) 15 23483 426 0.0000779 α-Ni 3d(xy) → α-π*(1) 16 24201 413 0.0217040 β-Ni 3d(xz/yz,⊥) → β-π*(2) 17 24448 409 0.0000061 β-Ni 3d(z²) → β-π*(3) 18 25159 398 0.0253318 β-Ni 3d(xz/yz,‖) → β-π*(3) 19 26720 374 0.0069969 α/β-π → α/β-π*(1) 20 26879 372 0.0148811 β-Ni 3d(xz/yz,⊥) → β-π*(2) 21 26899 372 0.0060159 α-Ni 3d(x²-y²) → α-π*(2) 22 28373 352 0.0000002 β-Ni 3d(xy) → β-π*(2) 2 23 28664 349 0.0002781 α-Ni 3d(z ) → α-π*(2) 24 29219 342 0.0000696 α-Ni 3d(x²-y²) → α-π*(3) 25 29386 340 0.0208177 α-Ni 3d(xz/yz,‖) → α-π*(2) 26 29945 334 0.0160776 α-Ni 3d(xz/yz,⊥) → α-π*(2) 27 30712 326 0.0000008 β-Ni 3d(xy) → β-π*(3) 2 28 31189 321 0.0000156 α-Ni 3d(z ) → α-π*(3) 29 31378 319 0.0105837 α-Ni 3d(xz/yz,‖) → α-π*(3) 30 31867 314 0.0000012 α-Ni 3d(xy) → α-π*(2) 31 32464 308 0.0356698 α-Ni 3d(xz/yz,⊥) → α-π*(3) 32 33280 301 0.0052250 α/β-π → α/β-π*(2) 258 Table S6. Absorption transitions for the equilibrium structure of 1-Br at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. E State E (cm-1) fosc Transition Assignment (nm) 1 8910 1122 0.0003991 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 9364 1068 0.0000572 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 3 10612 942 0.0000099 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 4 11396 878 0.0012884 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 13312 751 0.0008388 β-Ni 3d(z²) → β-π*(1) 6 15204 658 0.0098469 β-Ni 3d(xz/yz,⊥) → β-π*(1) 7 15677 638 0.0169887 β-Ni 3d(xz/yz,‖) → β-π*(1) 8 18794 532 0.0001154 α-Ni 3d(x²-y²) → α-π*(1) 9 20078 498 0.0002039 β-Ni 3d(xy) → β-π*(1) 10 20432 489 0.0009542 α-Ni 3d(z²) → α-π*(1) 11 21288 470 0.0082070 α-Ni 3d(xz/yz,⊥) → α-π*(1) 12 21841 458 0.0316036 α-Ni 3d(xz/yz,‖) → α-π*(1) 13 22335 448 0.0002508 β-Ni 3d(z²) → β-π*(2) 14 23755 421 0.0457485 β-Ni 3d(xz/yz,‖) → β-π*(2) 15 23832 420 0.0000714 α-Ni 3d(xy) → α-π*(1) 16 24292 412 0.0079633 β-Ni 3d(xz/yz,⊥) → β-π*(2) 17 24722 405 0.0000136 β-Ni 3d(z²) → β-π*(3) 18 25572 391 0.0187942 β-Ni 3d(xz/yz,‖) → β-π*(3) 19 26754 374 0.0000144 α/β-π → α/β-π*(1) 20 26952 371 0.0017301 α-Ni 3d(x²-y²) → α-π*(2) 21 27024 370 0.0279220 β-Ni 3d(xz/yz,⊥) → β-π*(2) 22 28843 347 0.0000016 β-Ni 3d(xy) → β-π*(2) 2 23 28893 346 0.0002863 α-Ni 3d(z ) → α-π*(2) 24 29363 341 0.0002532 α-Ni 3d(x²-y²) → α-π*(3) 25 29497 339 0.0386636 α-Ni 3d(xz/yz,‖) → α-π*(2) 26 29991 333 0.0036054 α-Ni 3d(xz/yz,⊥) → α-π*(2) 27 31259 320 0.0000037 β-Ni 3d(xy) → β-π*(3) 2 28 31360 319 0.0000669 α-Ni 3d(z ) → α-π*(3) 29 31468 318 0.0047503 α-Ni 3d(xz/yz,‖) → α-π*(3) 30 32167 311 0.0000001 α-Ni 3d(xy) → α-π*(2) 31 32539 307 0.0407090 α-Ni 3d(xz/yz,⊥) → α-π*(3) 32 33281 301 0.0040320 α/β-π → α/β-π*(2) 259 Table S7. Absorption transitions for the equilibrium structure of 1-I at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. E State E (cm-1) fosc Transition Assignment (nm) 1 8308 1204 0.0003008 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 8829 1133 0.0000591 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 3 10281 973 0.0000098 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 4 11238 890 0.0011694 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 13731 728 0.0007837 β-Ni 3d(z²) → β-π*(1) 6 15603 641 0.0071086 β-Ni 3d(xz/yz,⊥) → β-π*(1) 7 15983 626 0.0229720 β-Ni 3d(xz/yz,‖) → β-π*(1) 8 18840 531 0.0001058 α-Ni 3d(x²-y²) → α-π*(1) 9 20550 487 0.0008454 β-Ni 3d(xy) → β-π*(1) 10 20661 484 0.0003566 α-Ni 3d(z²) → α-π*(1) 11 21430 467 0.0212783 α-Ni 3d(xz/yz,⊥) → α-π*(1) 12 21779 459 0.0404195 α-Ni 3d(xz/yz,‖) → α-π*(1) 13 22648 442 0.0002552 β-Ni 3d(z²) → β-π*(2) 14 23905 418 0.0376620 β-Ni 3d(xz/yz,‖) → β-π*(2) 15 23985 417 0.0008700 α-Ni 3d(xy) → α-π*(1) 16 24509 408 0.0085618 β-Ni 3d(xz/yz,⊥) → β-π*(2) 17 25028 400 0.0000163 β-Ni 3d(z²) → β-π*(3) 18 25953 385 0.0167115 β-Ni 3d(xz/yz,‖) → β-π*(3) 19 26754 374 0.0001335 α/β-π → α/β-π*(1) 20 26919 372 0.0001459 α-Ni 3d(x²-y²) → α-π*(2) 21 27132 369 0.0250047 β-Ni 3d(xz/yz,⊥) → β-π*(2) 22 29026 345 0.0008326 α-Ni 3d(z2) → α-π*(2) 23 29179 343 0.0007470 β-Ni 3d(xy) → β-π*(2) 24 29219 342 0.0420723 α-Ni 3d(xz/yz,‖) → α-π*(2) 25 29398 340 0.0003774 α-Ni 3d(x²-y²) → α-π*(3) 26 29821 335 0.0000016 → β-π*(1) β-Br p(x/y,⊥) 27 30087 332 0.0053483 α-Ni 3d(xz/yz,⊥) → α-π*(2) 28 31007 323 0.0038798 → β-π*(1) β-Br 3p(z) 29 31270 320 0.0049428 β-Ni 3d(xz/yz,‖) → β-π*(3) 30 31478 318 0.0000444 α-Ni 3d(z2) → α-π*(3) 31 31624 316 0.0000028 β-Ni 3d(xy) → α-π*(3) 32 31796 315 0.0000026 → α-π*(1) α-Br 3p(x/y,⊥) 33 32222 310 0.0000639 α-Br 3p(x/y,‖) → α-π*(2) 34 32239 310 0.0005966 α-Br 3p(z) → α-π*(1) 35 32707 306 0.0410170 α-Ni 3d(xz/yz,⊥) → α-π*(3) 36 33242 301 0.0026747 α/β-π → α/β-π*(2) 260 Table S8. Absorption transitions for the equilibrium structure of 2 at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. E State E (cm-1) fosc Transition Assignment (nm) 1 9316 1073 0.0004425 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 9715 1029 0.0000437 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 3 10799 926 0.0000010 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 4 11474 872 0.0013918 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 12493 801 0.0008308 β-Ni 3d(z²) → β-π*(1) 6 14396 695 0.0141462 β-Ni 3d(xz/yz,‖) → β-π*(1) 7 14840 674 0.0096912 β-Ni 3d(xz/yz,⊥) → β-π*(1) 8 18193 550 0.0000616 α-Ni 3d(x²-y²) → α-π*(1) 9 19026 526 0.0000801 β-Ni 3d(xy) → β-π*(1) 10 19810 505 0.0010316 α-Ni 3d(z²) → α-π*(1) 11 20808 481 0.0047675 α-Ni 3d(xz/yz,⊥) → α-π*(1) 12 21373 468 0.0333436 α-Ni 3d(xz/yz,‖) → α-π*(1) 13 22397 447 0.0002836 β-Ni 3d(z²) → β-π*(2) 14 23062 434 0.0000109 α-Ni 3d(xy) → α-π*(1) 15 23560 425 0.0351858 β-Ni 3d(xz/yz,‖) → β-π*(2) 16 24395 410 0.0103361 β-Ni 3d(xz/yz,⊥) → β-π*(2) 17 24653 406 0.0000010 β-Ni 3d(z²) → β-π*(3) 18 25222 397 0.0217931 β-Ni 3d(xz/yz,‖) → β-π*(3) 19 26536 377 0.0002506 α/β-π → α/β-π*(1) 20 27022 370 0.0248685 β-Ni 3d(xz/yz,⊥) → β-π*(3) 21 27136 369 0.0000233 α-Ni 3d(x²-y²) → α-π*(2) 22 28672 349 0.0000001 β-Ni 3d(xy) → β-π*(2) 2 23 28956 345 0.0003385 α-Ni 3d(z ) → α-π*(2) 24 29420 340 0.0000264 α-Ni 3d(x²-y²) → α-π*(3) 25 29714 337 0.0254983 α-Ni 3d(xz/yz,‖) → α-π*(2) 26 30185 331 0.0074432 α-Ni 3d(xz/yz,⊥) → α-π*(2) 27 30928 323 0.0000002 β-Ni 3d(xy) → β-π*(3) 2 28 31403 318 0.0000074 α-Ni 3d(z ) → α-π*(3) 29 31574 317 0.0046334 α-Ni 3d(xz/yz,‖) → α-π*(3) 30 32170 311 0.0000011 α-Ni 3d(xy) → α-π*(2) 31 32618 307 0.0323619 α-Ni 3d(xz/yz,⊥) → α-π*(3) 261 Table S9. Absorption transitions for the equilibrium structure of 3 at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. E State E (cm-1) fosc Transition Assignment (nm) 1 9360 1068 0.0005099 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 9822 1018 0.0000331 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 3 11017 908 0.0000012 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 4 11518 868 0.0014098 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 12214 819 0.0008277 β-Ni 3d(z²) → β-π*(1) 6 14304 699 0.0152190 β-Ni 3d(xz/yz, ‖) → β-π*(1) 7 14507 689 0.0068829 β-Ni 3d(xz/yz,⊥) → β-π*(1) 8 17984 556 0.0000108 α-Ni 3d(x²-y²) → α-π*(1) 9 18676 536 0.0000766 β-Ni 3d(xy) → β-π*(1) 10 19629 510 0.0011378 α-Ni 3d(xz/yz,‖) → α-π*(1) 11 20849 480 0.0012266 α-Ni 3d(xz/yz,⊥) → α-π*(1) 12 21240 471 0.0256761 α-Ni 3d(z²) → α-π*(1) 13 22078 453 0.0001954 β-Ni 3d(z²) → β-π*(2) 14 22873 437 0.0000108 α-Ni 3d(xy) → α-π*(1) 15 23254 430 0.0342562 β-Ni 3d(xz/yz,‖) → β-π*(2) 16 23809 420 0.0000003 β-Ni 3d(z²) → β-π*(3) 17 24018 416 0.0120753 β-Ni 3d(xz/yz,⊥) → β-π*(2) 18 24352 411 0.0183904 β-Ni 3d(xz/yz,‖) → β-π*(3) 19 26411 379 0.0336622 β-Ni 3d(xz/yz,⊥) → β-π*(3) 20 26464 378 0.0008086 α/β-π → α/β-π*(1) 21 26658 375 0.0000053 α-Ni 3d(x²-y²) → α-π*(2) 22 28207 355 0.0000234 β-Ni 3d(xy) → β-π*(2) 23 28322 353 0.0002583 α-Ni 3d(x²-y²) → α-π*(3) 24 28756 348 0.0000260 α-Ni 3d(xz/yz,‖) → α-π*(2) 25 29489 339 0.0208373 α-Ni 3d(z2) → α-π*(2) 26 29918 334 0.0036422 α-Ni 3d(xz/yz,⊥) → α-π*(2) 27 29965 334 0.0000001 β-Ni 3d(xy) → β-π*(3) 28 30568 327 0.0000013 α-Ni 3d(xz/yz,‖) → α-π*(3) 29 30928 323 0.0049812 α-Ni 3d(z2) → α-π*(3) 30 31742 315 0.0000018 α-Ni 3d(xy) → α-π*(2) 31 32255 310 0.0443949 α-Ni 3d(xz/yz,⊥) → α-π*(3) 32 33020 303 0.0055649 α/β-π → α/β-π*(2) 262 Table S10. Absorption transitions for the equilibrium structure of 4 at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. State E (cm-1) E (nm) fosc Transition Assignment 1 9315 1074 0.0012938 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 9757 1025 0.0000222 → β-Ni 3d(x²-y²) β-Ni 3d(xz/yz,⊥) 3 10904 917 0.0000046 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 4 11368 880 0.0023779 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 11712 854 0.0007462 β-Ni 3d(z²) → β-π*(1) 6 13592 736 0.0236330 β-Ni 3d(xz/yz, ‖) → β-π*(1) 7 13959 716 0.0110160 β-Ni 3d(xz/yz,⊥) → β-π*(1) 8 17543 570 0.0000432 α-Ni 3d(x²-y²) → α-π*(1) 9 18154 551 0.0000570 β-Ni 3d(xy) → β-π*(1) 10 19220 520 0.0010219 α-Ni 3d(z²) → α-π*(1) 11 20014 500 0.0036455 → α-π*(1) α-Ni 3d(xz/yz,⊥) 12 20455 489 0.0250978 α-Ni 3d(xz/yz,‖) → α-π*(1) 13 20888 479 0.0000001 β-Ni 3d(z²) → β-π*(2) 14 21296 470 0.0728130 β-Ni 3d(xz/yz,‖) → β-π*(2) 15 21315 469 0.0001460 β-Ni 3d(z²) → β-π*(3) 16 21742 460 0.0233042 β-Ni 3d(xz/yz,‖) → β-π*(3) 17 22418 446 0.0000110 α-Ni 3d(xy) → α-π*(1) 18 23073 433 0.0091206 → β-π*(3) β-Ni 3d(xz/yz,⊥) 19 23927 418 0.1216611 → β-π*(2) β-Ni 3d(xz/yz,⊥) 20 25614 390 0.0002358 α-Ni 3d(x²-y²) → α-π*(2) 21 25808 388 0.0000905 α-Ni 3d(x²-y²) → α-π*(3) 22 26063 384 0.0019861 α/β-π(1) → α/β-π*(1) 23 27039 370 0.0000042 β-Ni 3d(xy) → β-π*(2) 24 27418 365 0.0000243 β-Ni 3d(xy) → β-π*(3) 25 27492 364 0.0585519 α-Ni 3d(xz/yz,‖) → α-π*(2) 26 27685 361 0.0000517 α-Ni 3d(z²) → α-π*(2) 27 27815 360 0.0398972 α/β-π(2) → α/β-π*(1) 28 27900 358 0.0000528 α-Ni 3d(z²) → α-π*(3) 29 28080 356 0.0130280 α-Ni 3d(xz/yz,‖) → α-π*(3) 30 28518 351 0.0088201 α-Ni 3d(xz/yz,‖) → α-π*(2) 31 28828 347 0.0041067 → α-π*(3) α-Ni 3d(xz/yz,⊥) 32 29706 337 0.1162010 → α-π*(2) α-Ni 3d(xz/yz,⊥) 33 30679 326 0.0000027 α-Ni 3d(xy) → α-π*(2) 34 30909 324 0.0000251 α-Ni 3d(xy) → α-π*(3) 35 31051 322 0.0330242 β-Ni 3d(xz/yz,‖) → β-Ph π*(1) 36 31471 318 0.0000043 β-Ni 3d(z²) → β-Ph π*(1) 37 31940 313 0.0028909 α/β-π(1) → α/β-π*(3) 38 32016 312 0.0042637 β-Ni 3d(xz/yz,‖) → β-Ph π*(2) 39 32078 312 0.4313805 α-π(3) → α-π*(1) 40 32318 309 0.0000060 α-Ph π*(1) → α-π*(1) 263 Table S11. Absorption transitions for the equilibrium structure of 5 at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. State E (cm-1) E (nm) fosc Transition Assignment 1 9091 1100 0.0002290 β-Ni 3d(z²) → β-π*(1) 2 9151 1093 0.0033827 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 3 10389 963 0.0004404 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 4 11264 888 0.0030372 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 5 11392 878 0.0000812 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 6 11860 843 0.0284356 β-Ni 3d(xz/yz,‖) → β-π*(1) 7 12658 790 0.0009997 β-Ni 3d(xz/yz,⊥) → β-π*(1) 8 15983 626 0.0000290 β-Ni 3d(xy) → β-π*(1) 9 16030 624 0.0000045 α-Ni 3d(x²-y²) → α-π*(1) 10 17987 556 0.0463427 β-Ni 3d(xz/yz,‖) → β-π*(2) 11 18260 548 0.0009906 α-Ni 3d(z²) → α-π*(1) 12 18809 532 0.0000022 β-Ni 3d(z²) → β-π*(2) 13 19552 512 0.0106762 β-Ni 3d(xz/yz,⊥) → β-π*(2) 14 19828 504 0.0246655 α-Ni 3d(xz/yz,⊥) → α-π*(1) 15 20886 479 0.0003944 β-Ni 3d(xz/yz,‖) → β-π*(3) 16 21168 472 0.0000018 α-Ni 3d(xy) → α-π*(1) 17 22193 451 0.0000033 β-Ni 3d(z²) → β-π*(3) 18 23219 431 0.0054451 α-Ni 3d(x²-y²) → α-π*(2) 19 23248 430 0.1213548 α-Ni 3d(xz/yz,‖) → α-π*(1) 20 23662 423 0.0050371 β-Ni 3d(xz/yz,⊥) → β-π*(3) 21 24509 408 0.0000006 β-Ni 3d(xy) → β-π*(2) 22 25113 398 0.0032781 α/β-π → α/β-π*(1) 23 25853 387 0.0000009 α-Ni 3d(z²) → α-π*(2) 24 26043 384 0.0000004 α-Ni 3d(x²-y²) → α-π*(3) 25 26468 378 0.0399679 α-Ni 3d(xz/yz,‖) → α-π*(2) 26 27775 360 0.0000103 β-Ni 3d(xy) → β-π*(3) 27 28457 351 0.1106965 α-Ni 3d(xz/yz,⊥) → α-π*(2) 28 28589 350 0.0000012 α-Ni 3d(xy) → α-π*(2) 29 28780 348 0.0000007 α-Ni 3d(z²) → α-π*(3) 30 29229 342 0.0634764 α-Ni 3d(xz/yz,‖) → α-π*(3) 31 29675 337 0.0070271 α-Ni 3d(xz/yz,⊥) → α-π*(3) 32 30412 329 0.0012561 α/β-π → α/β-π*(2) 33 30628 327 0.0023853 α/β-π → α/β-π*(3) 34 31416 318 0.0000132 α-Ni 3d(xy) → α-π*(3) 35 31968 313 0.3709734 β-Ni 3p(z) → β-π*(1) 36 32798 305 0.0000048 β-Ni 3p(x/y,⊥) → β-π*(1) 37 32920 304 0.1317423 β-Ni 3d(xz/yz,‖) → β-π*(3) 264 Table S12. Absorption transitions for the equilibrium structure of 5-Br at the TDDFT(B3LYP) level with the CPCM(THF) solvation model. State E (cm-1) E (nm) fosc Transition Assignment 1 8849 1130 0.0024990 β-Ni 3d(z²) → β-Ni 3d(x²-y²) 2 9135 1095 0.0000423 β-Ni 3d(xz/yz,⊥) → β-Ni 3d(x²-y²) 3 10326 969 0.0005262 β-Ni 3d(z²) → β-π*(1) 4 11131 898 0.0000184 β-Ni 3d(xz/yz,‖) → β-Ni 3d(x²-y²) 5 11266 888 0.0033695 β-Ni 3d(xy) → β-Ni 3d(x²-y²) 6 12189 820 0.0281527 β-Ni 3d(xz/yz,‖) → β-π*(1) 7 12595 794 0.0020666 β-Ni 3d(xz/yz,⊥) → β-π*(1) 8 16002 625 0.0000073 α-Ni 3d(x²-y²) → α-π*(1) 9 16481 607 0.0000518 β-Ni 3d(xy) → β-π*(1) 10 18208 549 0.0009511 α-Ni 3d(z²) → α-π*(1) 11 18375 544 0.0328136 β-Ni 3d(xz/yz,‖) → β-π*(2) 12 18871 530 0.0000601 β-Ni 3d(z²) → β-π*(2) 13 19437 515 0.0172895 β-Ni 3d(xz/yz,⊥) → β-π*(2) 14 19764 506 0.0326760 α-Ni 3d(xz/yz,⊥) → α-π*(1) 15 21205 472 0.0000214 α-Ni 3d(xy) → α-π*(1) 16 21301 470 0.0014812 β-Ni 3d(xz/yz,‖) → β-π*(3) 17 22197 451 0.0001503 β-Ni 3d(z²) → β-π*(3) 18 22899 437 0.1336416 α-Ni 3d(xz/yz,‖) → α-π*(1) 19 23179 431 0.0001572 α-Ni 3d(x²-y²) → α-π*(2) 20 23632 423 0.0038845 β-Ni 3d(xz/yz,⊥) → β-π*(3) 21 24849 402 0.0000008 β-Ni 3d(xy) → β-π*(2) 22 25045 399 0.0022873 α/β-π → α/β-π*(1) 23 25798 388 0.0001838 α-Ni 3d(z²) → α-π*(2) 24 25982 385 0.0010291 α-Ni 3d(x²-y²) → α-π*(3) 25 26096 383 0.0375426 α-Ni 3d(xz/yz,‖) → α-π*(2) 26 28025 357 0.0010304 β-Ni 3d(xy) → β-π*(3) 27 28171 355 0.1092840 α-Ni 3d(xz/yz,⊥) → α-π*(2) 28 28615 350 0.0000006 α-Ni 3d(xy) → α-π*(2) 29 28719 348 0.0095568 α-Ni 3d(z²) → α-π*(3) 30 28794 347 0.0453802 α-Ni 3d(xz/yz,‖) → α-π*(3) 31 29330 341 0.0000008 β-Ni 3p(x/y,⊥) → β-π*(1) 32 29561 338 0.0029779 α-Ni 3d(xz/yz,⊥) → α-π*(3) 33 30407 329 0.0010950 α/β-π → α/β-π*(2) 34 30567 327 0.0000111 β-Ni 3p(z) → β-π*(1) 35 30605 327 0.0015624 α/β-π → α/β-π*(3) 36 31202 321 0.0000006 α-Ni 3p(x/y,⊥) → α-π*(1) 37 31406 318 0.0000256 α-Ni 3d(xy) → α-π*(3) 38 32009 312 0.4327586 β-π → β-π*(1) 39 32588 307 0.0130867 α-Ni 3p(z) → α-π*(1) 40 33226 301 0.0321215 β-Ni 3d(xz/yz,‖) → β-π*(3) 265 Figure S44. Computed PESs for 1 (left) and 5 (right) showing the positions of the calculated transitions relative to the TA pump (800 or 1200 nm). Relaxed structures computed at the DFT BP86 level for the ground state. Lowest-energy excited states corresponding to β-Ni 3d(z²)→β-Ni 3d(x²–y²) for the metal-centered state (red surface) and β-Ni 3d(z²)→bpy π*(1) for the MLCT state (orange surface) were optimized using TDDFT at the B3LYP level; structures are shown. Open circles indicate computed values and correspond to the energy markers on the y-axis. Calculations were performed to investigate the possibility surface crossing into a ligand field excited state and to interrogate the relative geometries of the 2 MLCT excited states vs. the ground-state structures (see Supporting Information Section 1.6; Fits to Alternative Relaxation Models for more details). We find that only in 1 is there a 2 d-d state lower than the MLCT. Given the similar energies of the 2d-d in 1 and the 2MLCT in 5 (the lowest-energy surfaces are both ~19 kcal mol-1 above the ground state by TD-DFT), one would expect their relaxation times to be similar under energy gap law behavior (a result opposite of that to the experiment). Thus, DFT/TD-DFT suggests that a rate-limiting 2d-d → 2 GS is unlikely; relaxation is from the 2MLCT (a point better emphasized by the experimental TA spectra). The 2MLCT geometry of 5 is more distorted than for the 2MLCT of 1, indicative of an excited-state surface that is nested with the ground state in 1 (Marcus inverted) but a displaced excited-state surface in 5 (Marcus normal). We note, however, that the relative widths and displacements along Q of the excited-state surfaces are qualitative; accurate computed displacements are precluded without the use of an explicit solvation model to account for outer sphere reorganization energy. Vertical and relaxed excited-state energies of 5 are likely to be overestimated (see Supporting Information Section 2.5; Limitations of DFT/TDDFT). 266 S2.5 Limitations of DFT/TDDFT As can be noted in the experimental versus computed absorption spectra (Figure S43 and Tables S5-S12), TDDFT well simulates the Ni(I) complexes with electron-rich bpy ligands, particularly those with electron-rich substituents (i.e., 1-3). However, it quickly fails to adequately stabilize the charge transfer excited states of those with an electron poor bpy ligand or with an extended π-system (i.e., 4-5). The lowest-energy MLCTs of these complexes (labeled Manifold 1 in Figure S43) are either substantially blue-shifted in energy or not accounted for at all. Taking complex 5 as an example, we attempted to better model the experiment by adjusting the functional used in the TDDFT calculations50 and thereby tuning the amount of exact exchange (which can favor open-shell electronic configurations)51,52 from 20% in B3LYP46,47 to 10% in meta-hybrid TPSSh53–55 and 0% in the standard generalized gradient approximation (GGA) functional, BP86.38,39 However, in none of these cases were we able to reproduce the experimental spectrum of 5 well (Figure S45). Figure S45. Comparison of the experimental absorption spectrum of 5 in THF (top left) and the computed spectra of the same using TDDFT at the B3LYP (top right), TPSSh (bottom left), and BP86 (bottom right) levels. Although the electronic transitions red-shift, their positions and intensities are still poor models of the experiment. Excited-state optimizations using TDDFT, particularly those of the MLCT transitions, would similarly suffer. Relaxed MLCT geometries of 4-5 are likely computed too high in energy. Additionally, an implicit solvation model CPCM is used to simulate THF, but it cannot adequately describe outer sphere reorganization energies and related effects that contributing to the experimental value of λ presented in the manuscript. 267 A detailed computational analysis of these electron-deficient bpy-ligated Ni(I) complexes is needed, likely employing multireference calculations (which have been previously demonstrated to reveal substantial bpy ligand non-innocence effects)4,49 and/or excited-state molecular dynamics, both of which are beyond the scope of this work. As such, the analysis of 1-5 given in the manuscript main text rests on the experimental data; computational analysis of 4-5 is provided here for completeness. 268 S3. NMR and IR Spectra. Proton nuclear magnetic resonance (1H NMR) and fluorine nuclear magnetic resonance (19F NMR) spectra were recorded on a 400 MHz Varian Spectrometer with broadband auto-tune OneProbe. Fluorine NMR were externally referenced to neat fluorobenzene (δ = -113.15 ppm). All 13C NMR spectra were collected on a Bruker AV-III HD 400 MHz spectrometer and were 1H decoupled. Chemical shifts are reported in parts per million and are referenced to residual solvent signal. Figure S46. 1H NMR (400 MHz, CD2Cl2) spectra of Ni(Mebpy)(o-tolyl)Cl, parent compound for 2. 269 Figure S47. 13C NMR (100 MHz, CD2Cl2) spectra of Ni(Mebpy)(o-tolyl)Cl, parent compound for 2. Figure S48. 1H NMR (400 MHz, CD2Cl2) spectra of Ni(Phbpy)(o-tolyl)Cl, parent compound for 4. Proton NMR agrees with previous report.5 270 Figure S49. 1H NMR (400 MHz, CD2Cl2) spectra of Ni(TMEDA)(o-tolyl)Br. Figure S50. 13C NMR (100 MHz, CD2Cl2) spectra of Ni(TMEDA)(o-tolyl)Br. 271 Figure S51. 1H NMR (400 MHz, CD2Cl2) spectra of Ni(MeO2Cbpy)(o-tolyl)Br, parent compound for 5-Br. Figure S52. 13C NMR (100 MHz, CD2Cl2) spectra of Ni(MeO2Cbpy)(o-tolyl)Br, parent compound for 5-Br. 272 Figure S53. (Left) 1H NMR (400 MHz, d8-toluene) and (right) 19F NMR (400 MHz, CD2Cl2) spectra of Ni(MeO2Cbpy)(o-CF3Ph)Br. Residual hexanes from the workup/washing steps are seen at ~1.26 ppm and ~0.89 ppm in the proton spectrum. Figure S54. 13C NMR (100 MHz, CD2Cl2) spectra of Ni(MeO2Cbpy)(o-CF3Ph)Br. 273 Figure S55. 1H NMR (400 MHz, d8-THF) spectra of (A) Ni(MeO2Cbpy)(o-tolyl)Cl, (B) the photochemical conversion of Ni(MeO2Cbpy)(o-tolyl)Cl to 5, and (C) paramagnetic, isolated 5′. Boxed insets in (B) and (C) are shown in panels (B1) and (C1), respectively. Assignments for Ni(MeO2Cbpy)(o-tolyl)Cl are as described previously.4 274 Figure S56. Solid-state IR spectra of the Ni(II) complexes newly synthesized in this work and the isolated complex, 5′. From top to bottom: Ni(II)(Mebpy)(o-tolyl)Cl, Ni(II)(TMEDA)(o-tolyl)Cl, Ni(II)(MeO2Cbpy)(o-tolyl)Br, Ni(II)(MeO2Cbpy)(o-CF3Ph)Br, Ni(I)(MeO2Cbpy)Cl. Structures shown on each spectrum. 275 S4. Appendix. 1 C N C C C C C C C C C N C C Ni Cl C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H -2.029767481 -2.993320211 -4.295276591 -4.630774991 -3.635290044 -2.306945286 -5.261779440 -6.648934787 -7.493591358 -6.868329156 -5.485454487 -4.677959590 -9.016340381 -9.601934114 -2.752347324 -1.059195981 -4.022766266 -4.891752963 -9.489648427 -9.532749178 -2.793524766 -4.838551620 -7.065085163 -7.454021382 -4.984029087 -5.681886650 -1.472701894 -1.003355313 -3.125386762 -2.189664978 -2.149507033 -10.587640101 -9.170794673 -9.111098939 -9.123131571 -9.255802893 -10.629749085 -4.243248926 -5.753897280 -5.131020694 -4.332046802 -5.190577441 -5.804367177 -9.250452573 -10.700034884 -9.316120791 -5.482378378 -4.541935687 -4.956603212 -6.307361731 -7.296086104 -6.846340471 -3.849662986 -4.006354925 -2.892159547 -1.631051876 -1.534413023 -2.617770489 -3.002867842 -2.328966391 -2.623508518 -1.305289447 -8.773587920 -9.069960068 -4.463198015 -2.273372247 -9.693511954 -9.067196224 -5.010934268 -0.712160239 -0.566301298 -6.592145825 -7.544815970 -5.113796749 -10.741100459 -9.541107276 -9.535050064 -4.487448094 -5.023994300 -4.982162212 -2.731811081 -1.210271219 -2.339348662 -8.857561688 -8.461049022 -10.127257327 -8.868430762 -10.128344393 -8.458291521 -2.833680152 -2.384748993 -1.269916189 1-Br 0.242513014 0.229603853 0.200439005 0.186192007 0.202279473 0.230069863 0.190029655 0.151789945 0.143655115 0.179230886 0.215901240 0.220269810 0.098655540 1.359270503 0.259827385 0.317631596 0.191453470 1.434425899 0.051075312 -1.161574862 0.216089041 -1.087289670 0.127776804 0.178081633 0.242288659 0.165020124 0.242795152 0.264629404 0.204986446 1.122413974 -0.661128316 0.014460344 0.941803493 -0.841603077 -2.073275273 -1.156192434 -1.203057528 -1.987679478 -1.131336205 -1.104148959 2.359199251 1.433356604 1.443432679 2.270759038 1.333256842 1.419775304 C N C C C C C C C C C N C C Ni Br C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H 0.659321000 -0.397352000 -1.649222000 -1.840237000 -0.746495000 0.526474000 -2.727948000 -4.091702000 -5.047239000 -4.557068000 -3.191298000 -2.278114000 -6.550778000 -7.183911000 -0.358827000 1.053673000 -0.974933000 -1.869149000 -6.873605000 -7.156813000 0.342297000 -1.689293000 -4.402007000 -5.235811000 -2.791397000 -2.854691000 1.429217000 1.641470000 0.124197000 0.884848000 1.004616000 -7.964180000 -6.474049000 -6.471514000 -6.718811000 -6.984190000 -8.242662000 -1.074874000 -2.660069000 -1.866678000 -1.388861000 -2.040007000 -2.847755000 -6.762950000 -8.269275000 -7.016833000 -2.259714000 -1.428101000 -1.968507000 -3.333722000 -4.209229000 -3.632789000 -0.975037000 -1.272852000 -0.257758000 1.053054000 1.290266000 0.301427000 -0.522399000 -0.000721000 0.480878000 2.255198000 -5.704285000 -6.245857000 -2.016991000 0.234846000 -6.493501000 -5.906205000 -2.308624000 1.900709000 2.297545000 -3.720427000 -4.238110000 -1.794106000 -7.560941000 -6.384022000 -6.172965000 -2.150816000 -2.599403000 -2.435361000 -0.118922000 1.317268000 0.064168000 -5.521211000 -5.391982000 -6.978306000 -6.100021000 -7.322462000 -5.747024000 -0.522027000 -0.177178000 1.077236000 0.041788000 0.119515000 0.012015000 -0.178781000 -0.260353000 -0.142727000 0.116787000 0.071256000 0.181630000 0.333112000 0.376550000 0.274279000 0.143140000 1.452255000 0.358578000 0.638329000 -0.474611000 0.662745000 0.001797000 -1.059583000 -0.483851000 -1.829868000 -0.046145000 0.421825000 0.498717000 -0.267245000 -0.193174000 0.132408000 -0.627315000 0.466347000 -1.301145000 -0.012640000 0.844773000 -0.932513000 -2.004097000 -0.983942000 -1.095596000 -2.656492000 -1.853258000 -1.999478000 1.641047000 0.517547000 0.681522000 2.324143000 1.433738000 1.581444000 276 1-I C N C C C C C C C C C N C C Ni I C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H -0.751175000 2.863174000 0.216287000 1.927032000 1.517527000 2.346358000 1.846986000 3.698260000 0.847183000 4.683179000 -0.479159000 4.228310000 2.491693000 1.244221000 3.877410000 1.409261000 4.728096000 0.299192000 4.112406000 -0.965072000 2.729961000 -1.070136000 1.918915000 0.008728000 6.249761000 0.417804000 6.778836000 -0.367947000 -0.009358000 0.014083000 -1.757424000 -1.706865000 1.227408000 6.162348000 2.055040000 6.462742000 6.713712000 1.877312000 6.831188000 -0.191122000 -0.008786000 7.073668000 2.077925000 6.462862000 4.287757000 2.415278000 4.705457000 -1.878916000 2.232167000 -2.039731000 2.896881000 3.988782000 -1.316660000 4.922644000 -1.776107000 2.490317000 0.314596000 8.123904000 -0.639190000 6.912120000 -0.622112000 6.912334000 7.811562000 1.907254000 6.331195000 2.353487000 6.392446000 2.476063000 6.470584000 0.355448000 6.550799000 -1.247781000 7.929022000 -0.129330000 1.507143000 6.256388000 2.995528000 5.858824000 2.368469000 7.523581000 1.467437000 6.256957000 2.346227000 7.523249000 2.971510000 5.857978000 6.371029000 0.041487000 7.875457000 -0.295007000 6.511232000 -1.431864000 2 0.039848000 0.046683000 0.066315000 0.074519000 0.064787000 0.048675000 0.070050000 0.118877000 0.113278000 0.055949000 0.016149000 0.024492000 0.159594000 1.379607000 0.017072000 0.005504000 0.068929000 1.338405000 0.273631000 -1.136123000 0.057297000 -1.185246000 0.163858000 0.043982000 -0.022912000 0.084580000 0.041484000 0.025775000 0.060367000 0.943558000 -0.840898000 0.312162000 1.188210000 -0.591113000 -2.019487000 -1.246572000 -1.114004000 -2.102118000 -1.208574000 -1.189549000 2.244712000 1.347555000 1.378164000 2.315115000 1.418533000 1.323651000 C N C C C C C C C C C N Ni Cl H H H H H H C H H H C H H H -0.710483000 0.262518000 1.559679000 1.890582000 0.888165000 -0.441423000 2.527817000 3.913083000 4.750709000 4.133307000 2.750849000 1.947017000 0.023437000 -1.691356000 4.348734000 4.730038000 2.247241000 2.935772000 -1.268961000 -1.731723000 6.243486000 6.724932000 6.630314000 6.548672000 1.213724000 0.817025000 2.296930000 0.750804000 2.863308000 1.931301000 2.351214000 3.706989000 4.678474000 4.224544000 1.250223000 1.408225000 0.291287000 -0.971932000 -1.068540000 0.014365000 0.013952000 -1.276324000 2.407504000 -1.882751000 -2.035856000 4.014392000 4.931670000 2.482348000 0.429869000 -0.087009000 -0.031285000 1.483447000 6.143338000 6.607006000 6.312533000 6.665870000 0.180761000 0.097130000 0.045922000 0.080218000 0.166719000 0.216764000 -0.044628000 -0.114486000 -0.196705000 -0.205470000 -0.134968000 -0.055631000 0.046003000 0.095598000 -0.105601000 -0.267441000 -0.140650000 0.040845000 0.284708000 0.219264000 -0.272213000 0.571548000 -1.193411000 -0.255454000 0.203972000 1.119609000 0.168362000 -0.646746000 277 3 C N C C C C C C C C C N Ni Cl H H H H H H H H 0.473042000 -0.621213000 -1.820768000 -1.933067000 -0.798133000 0.426311000 -2.929113000 -4.254095000 -5.214232000 -4.822396000 -3.487035000 -2.551375000 -0.679100000 0.725756000 -4.532733000 -6.252626000 -5.539336000 -3.133022000 -2.898610000 -0.868067000 1.337867000 1.403530000 -2.474101000 -1.686728000 -2.247454000 -3.610491000 -4.417499000 -3.837966000 -1.285073000 -1.595831000 -0.585292000 0.717024000 0.963078000 -0.010033000 0.210618000 1.738839000 -2.618318000 -0.810518000 1.537014000 1.961473000 -4.037241000 -5.482974000 -4.431333000 -1.976099000 4 0.301340000 0.241593000 -0.095944000 -0.379539000 -0.316330000 0.030900000 -0.124235000 -0.438432000 -0.427954000 -0.101945000 0.200276000 0.191995000 0.584668000 1.122454000 -0.689887000 -0.670461000 -0.081022000 0.459119000 -0.647427000 -0.534779000 0.092479000 0.575931000 C N C C C C C C C C C N Ni Cl H H H H H H C C C C C C H H H H H C C C C C C H H H H H -0.303121000 0.370314000 1.340992000 1.643905000 0.948392000 -0.049372000 2.002125000 3.017789000 3.583328000 3.070632000 2.057866000 1.520684000 0.120011000 -1.159087000 3.359026000 3.475143000 1.652308000 2.409633000 -0.614348000 -1.064154000 1.252568000 0.244798000 2.556277000 0.531524000 2.842500000 1.831232000 -0.776753000 3.358085000 -0.264922000 3.861054000 2.055069000 4.663419000 4.790750000 5.590001000 5.809841000 6.611721000 6.725020000 4.070040000 5.526701000 5.886107000 7.327196000 7.523016000 2.357166000 1.197550000 0.888638000 1.733652000 2.941798000 3.239870000 -0.397495000 -0.943208000 -2.184070000 -2.829197000 -2.233660000 -1.038404000 -0.109760000 -0.274252000 -0.413421000 -3.784416000 -2.709981000 1.443726000 4.169553000 2.569899000 3.851686000 4.652755000 3.937364000 5.508562000 4.797980000 5.585590000 4.584838000 3.345468000 6.113522000 4.859297000 6.256808000 -2.778875000 -4.176191000 -1.962084000 -4.738031000 -2.525698000 -3.915699000 -4.826883000 -0.877222000 -5.822953000 -1.877112000 -4.355894000 -1.786324000 -1.632083000 -2.542159000 -3.605965000 -3.774420000 -2.825965000 -2.277626000 -3.057500000 -2.722633000 -1.580223000 -0.843102000 -1.170501000 -0.235981000 1.478289000 -3.945518000 -1.247827000 0.050026000 -4.323622000 -2.880582000 -1.034839000 -4.894780000 -5.464168000 -5.419065000 -6.527592000 -6.478636000 -7.038813000 -5.087682000 -4.975335000 -6.963318000 -6.864606000 -7.869049000 -3.532174000 -3.645886000 -4.207824000 -4.414914000 -4.971884000 -5.080299000 -3.148912000 -4.111536000 -4.500356000 -5.479460000 -5.679758000 278 5 C N C C C C C C C C C N C O C Ni Cl C O C O O H H H H H H H H H H H H 0.568503000 -0.519621000 -1.753839000 -1.908807000 -0.779763000 0.483694000 -2.850198000 -4.199573000 -5.149743000 -4.707806000 -3.349024000 -2.427774000 -6.612290000 -6.882910000 -8.286502000 -0.529882000 0.960350000 -0.977021000 0.190825000 0.088721000 -7.463500000 -2.069176000 -4.515737000 -5.423238000 -2.962519000 -2.892830000 1.388707000 1.527777000 -8.839144000 -8.706483000 -8.311075000 -0.519047000 -0.363635000 1.117044000 -2.394588000 -1.595903000 -2.162453000 -3.537154000 -4.362047000 -3.773251000 -1.186377000 -1.503469000 -0.476662000 0.844794000 1.092138000 0.104205000 -0.734410000 -2.048339000 -2.386728000 0.317468000 1.824056000 -5.831976000 -6.504183000 -7.943740000 0.142614000 -6.361714000 -2.536785000 1.665127000 2.103126000 -3.985669000 -4.376553000 -1.892921000 -2.065172000 -1.902810000 -3.475297000 -8.361820000 -8.193319000 -8.311511000 5-Br 0.316547000 0.306684000 0.148607000 -0.001582000 0.010679000 0.173273000 0.156934000 0.021577000 0.047900000 0.209841000 0.339974000 0.316439000 -0.088046000 -0.229848000 -0.365605000 0.498309000 0.786525000 -0.149263000 -0.112213000 -0.257699000 -0.071618000 -0.295546000 -0.102283000 0.233770000 0.468761000 -0.128370000 0.188369000 0.444695000 0.525972000 -1.256203000 -0.466662000 0.554406000 -1.225558000 -0.202772000 C N C C C C C C C C C N C O C Ni C O C O O H H H H H H H H H H H H Br 0.764158000 -0.323481000 -1.558559000 -1.714520000 -0.586011000 0.677975000 -2.655609000 -4.004795000 -4.955627000 -4.515313000 -3.156501000 -2.235630000 -6.418504000 -6.687937000 -8.091449000 -0.337623000 -0.785026000 0.381935000 0.278198000 -7.269897000 -1.878068000 -4.320438000 -5.231610000 -2.770426000 -2.698855000 1.582467000 1.723792000 -8.643445000 -8.512503000 -8.114985000 -0.328688000 -0.175988000 1.306252000 1.211822000 -2.191930000 -1.393036000 -1.957846000 -3.332451000 -4.158180000 -3.570739000 -0.981640000 -1.300035000 -0.273731000 1.048328000 1.297154000 0.309625000 -0.533285000 -1.847857000 -2.188373000 0.517928000 -5.628340000 -6.301605000 -7.741378000 0.343453000 -6.156553000 -2.333754000 1.867955000 2.308504000 -3.780523000 -4.174833000 -1.690908000 -1.862308000 -1.709724000 -3.277492000 -8.157626000 -7.991643000 -8.110018000 2.149558000 0.350402000 0.341233000 0.184071000 0.034530000 0.046305000 0.207383000 0.191745000 0.057076000 0.081623000 0.240264000 0.370217000 0.348946000 -0.052459000 -0.187666000 -0.320409000 0.540196000 -0.112243000 -0.075498000 -0.218972000 -0.040063000 -0.256796000 -0.064544000 0.262336000 0.497451000 -0.091230000 0.221837000 0.478182000 0.569907000 -1.213329000 -0.415395000 0.594718000 -1.185777000 -0.165190000 0.887246000 279 S5. 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