Electronic Structures and Reactivity Patterns of Dipalladium(11,11) and Diplatinum(11,11) Complexes Citation Durrell, Alec Charles (2012) Electronic Structures and Reactivity Patterns of Dipalladium(11,11) and Diplatinum(11,11) Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/91HV-2R03. https://resolver.caltech.edu/CaltechTHESIS:05172012-112437443 Abstract This work explores the electronic structures and reactivity patterns of diplatinum(II, II) and dipalladium(II, II) complexes. Complexes containing metal-metal bonds play important roles in both inorganic and organometallic chemistry. Among the many examples of these complexes, dimers of square planar Rh I , Ir I , and Pt II centers comprise a special class that feature attractive d 8 –d 8 interactions. The unique electronic structure characteristic of these complexes gives rise to chemical, photochemical, and photophysical properties that have engaged researchers for the past 35 years. One of the best-known examples of these compounds is tetrakis(µ-pyrophosphito)platinum(II), Pt(pop). Herein, the photophysical properties of Pt(pop) are compared with its fluoroborated analogue, Pt(pop-BF 2 ). This complex possesses eight BF 2 groups that replace the hydrogen atoms located between each “pop” ligand. When compared with Pt(pop), Pt(pop-BF 2 ) has a much greater singlet lifetime (1.56 ns) and singlet quantum yield (0.27). The enhancement is the result of a drastically slower 1 A 2u → 3 A 2u intersystem crossing rate. In particular, the thermal barrier to intersystem crossing is significantly higher in Pt(pop-BF 2 ) (2230 cm -1 vs. 1190 cm -1 ). We believe this is primarily the result of the increased rigidity of the complex afforded by the BF 2 groups. The rigidity increases the energy of symmetry-lowering vibrational modes, which are necessary to promote spin-orbit mixing of the 1 dσ*pσ and 3 dσ*pσ states. Despite the many examples of M-M bonded d 8 –d 8 complexes of Rh I , Ir I , and Pt II in the literature, until recently there were no Pd II complexes fitting this description. Our investigations of clamshell-shaped Pd II dimers [(2-phenylpyridine)Pd(µ-X)] 2 and [(2-p-tolylpyridine)Pd(µ-X)] 2 (X = OAc or TFA) revealed short Pd–Pd distances (~ 2.85 Å). The molecules adopt this unusual geometry in part because of a d 8 –d 8 bonding interaction between the two Pd centers. Density functional theory (DFT) and ab initio (AI) analyses confirm the presence of a Pd–Pd bonding interaction in [(2-phenylpyridine)Pd(µ-X)] 2 and show that the HOMO is a d z 2 σ*Pd–Pd antibonding orbital, while the LUMO and proximal unoccupied orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other Pd II –Pd II dimers that have short Pd–Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(µ-X)] 2 , but quite different from that found for d 8 –d 8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd, and may explain why Pd d 8 –d 8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species. Our work on Pd II –Pd II electronic structures led us to employ these complexes as electrocatalysts in the regioselective chlorination of C–H bonds. Previous work on d 8 –d 8 complexes has established that when treated with halogens (Cl 2 , Br 2 , or I 2 ), the complex undergoes two-center oxidative addition to form an axially coordinated X–d 7 –d 7 –X species. Similar products are observed following electrochemical oxidation in the presence of a halide (Cl – , Br – , or I – ). Recently, related Pd II –Pd II complexes were found to selectively chlorinate benzo[h]quinoline through reductive elimination from a Cl–Pd III –Pd III –Cl species. This led us to probe the viability of the analogous electrochemical route. Cyclic voltammetry, spectroelectrochemistry, and bulk electrolysis measurements confirm that electrochemical oxidation of Pd II –Pd II yields the identical Cl–Pd III –Pd III –Cl intermediate, which is capable of reductive chlorination of C–H bonds. Additional evidence for formation of axially coordinated bromide and acetate species is also presented. Over 10 turnovers of 10-chlorobenzo[h]quinoline were achieved at 80% isolated yield. Further research into the area may lead to a potentially versatile, useful, and green route for C–H bond functionalization reactions. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: d8-d8 complexes; Electronic Structures; Electrocatalysis Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Gray, Harry B. Thesis Committee: Okumura, Mitchio (chair) Bercaw, John E. Lewis, Nathan Saul Gray, Harry B. Defense Date: 11 April 2012 Record Number: CaltechTHESIS:05172012-112437443 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:05172012-112437443 DOI: 10.7907/91HV-2R03 Related URLs: URL URL Type Description http://dx.doi.org/10.1021/ic902189g DOI UNSPECIFIED http://dx.doi.org/10.1021/jp3010053 DOI UNSPECIFIED http://dx.doi.org/10.1021/ja101647t DOI UNSPECIFIED Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 7039 Collection: CaltechTHESIS Deposited By: Alec Durrell Deposited On: 23 May 2012 21:20 Last Modified: 03 Oct 2019 23:55 Thesis Files Preview PDF (Complete Thesis) - Final Version See Usage Policy. 6MB Repository Staff Only: item control page

ELECTRONIC STRUCTURES AND REACTIVITY PATTERNS OF DIPALLADIUM(II,II) AND DIPLATINUM(II,II) COMPLEXES Thesis by Alec Charles Durrell In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2012 (Defended 11 April 2012) ii  2012 Alec Charles Durrell All Rights Reserved ACKNOWLEDGEMENTS iii During my time at Caltech, I have been fortunate enough to work and interact with a number of brilliant people who have had a huge impact on my scientific path and to whom I owe a great deal of thanks. First and foremost, I must thank my advisor, Harry Gray. I am constantly amazed at Harry’s indefatigable enthusiasm and support for me and all his students. He is the model for the chemist I aspire to be. I am a proud member of the Gray Nation. I would also like to thank Jay Winkler. Jay is the prototypical scientist. He is methodical and thorough and goes only where the data takes him. While my enthusiasm sometimes gets the best of me, I have tried to emulate these traits as they can never lead you astray. I am grateful to the other members of my committee, Mitchio Okumura, Nate Lewis, and John Bercaw. While committee meetings were never too common they were always willing to respond to me if I needed them. John, in particular, was always willing to discuss any chemistry concerns I was having. I owe special thanks to Nilay Hazari. I was incredibly fortunate to begin a collaboration with him that ultimately sparked the bulk of my thesis project. More than that, he has been a great friend to me and I look forward to further developing that friendship upon my move to New Haven. Brian Leigh mentored me when I first arrived at Caltech. Beyond showing me the ropes around lab, he welcomed me into his social circle, showed me all the best lunch spots in town, and let me join his flag football team. His generosity made my transition to grad school and the West Coast much easier. Paul Oblad, Charlotte Whited, Josh Palmer, and Heather Williamson were a great class to come through the group with. Paul and Charlotte iv continue to be great friends and I have enjoyed keeping up with them as their lives have taken them to other parts of the country. I am grateful to Josh both for his friendship and for our collaborations, which allow me to make the dubious claim that I am an expert in corrole photophysics. Ian Tonks is another member of the class with whom I was able to work closely on occasion. I enjoyed our collaborations a great deal. Kyle Lancaster was a wonderful office mate, except for those two months when he would not stop quoting from Silence of the Lambs. Despite that, he continues to be a great friend and colleague and I wish him continued success in his scientific pursuits. I thank Bryan Stubbert for many helpful discussions, some of which were even related to science. Jillian Dempsey and I spent a good deal of time together as both TAs for 153 and GLAs in the laser lab. She set a high standard in both and I did my best to maintain that level in my TA and GLA duties after she graduated. As I leave those duties behind, I have complete confidence in Maraia Ener to carry on the standard of excellence. Gretchen Keller has been a great friend and coworker. I am especially grateful to have her as a collaborator on certain difficult projects, where her expertise and demeanor have made things much easier. I have enjoyed working with Yan Choi Lam on his platinum project, and I am happy knowing that he continues to keep d8–d8 chemistry alive. Paul Bracher was an amazing person to have down the hall. I deeply enjoyed our conversations on a wide range of topics. Peter Agbo, if you’re reading this, get back to work. During my tenure in the group I was fortunate enough to overlap with many talented postdocs. Matt Hartings was always available for helpful discussions on a broad range of topics. Hema Karunadasa (along with James McKone and Bryan Stubbert) taught me a v great deal about electrochemistry. Astrid Mueller’s knowledge and experience with the laser systems was truly beneficial. Jeff Warren was always helpful and in good spirits, despite my constant pestering of him. Megan Jackson began working with me as a SURF student her freshman year. From the very beginning, she brought enthusiasm and skill to her work. Had she not joined the lab, the last chapter of my thesis may have been much sparser. I am happy that she will be carrying on d8–d8 chemistry in her final year at Caltech and wish her continued success in whatever she decides to do next. I am deeply grateful for all the love and support my family has shown me while in graduate school. I owe a lot to my mom, who has always been supportive in the pursuit of my goals. I would not be who I am today without her hard work. I would also like to thank my dad, who pushed me to always do my best, and who I deeply miss. Living in Pasadena has given me the opportunity to spend more time with my brother-in-law Eric Notarnicola. Despite him being much cooler than me, he is always eager to come hang out or grab dinner. Finally, I must thank my wife. Cristina, you’ve been there for me since the beginning. You have been supportive, patient, and understanding throughout the entire process. Without your constant encouragement, I probably would not be writing this dissertation. Marrying you was easily the best decision I made while at Caltech. Thank you for putting up with me through almost all these years of grad school. ABSTRACT vi This work explores the electronic structures and reactivity patterns of diplatinum(II, II) and dipalladium(II, II) complexes. Complexes containing metal-metal bonds play important roles in both inorganic and organometallic chemistry. Among the many examples of these complexes, dimers of square planar RhI, IrI, and PtII centers comprise a special class that feature attractive d8–d8 interactions. The unique electronic structure characteristic of these complexes gives rise to chemical, photochemical, and photophysical properties that have engaged researchers for the past 35 years. One of the best-known examples of these compounds is tetrakis(µpyrophosphito)platinum(II), Pt(pop). Herein, the photophysical properties of Pt(pop) are compared with its fluoroborated analogue, Pt(pop-BF2). This complex possesses eight BF2 groups that replace the hydrogen atoms located between each “pop” ligand. When compared with Pt(pop), Pt(pop-BF2) has a much greater singlet lifetime (1.56 ns) and singlet quantum yield (0.27). The enhancement is the result of a drastically slower 1A2u → 3 A2u intersystem crossing rate. In particular, the thermal barrier to intersystem crossing is significantly higher in Pt(pop-BF2) (2230 cm-1 vs. 1190 cm-1). We believe this is primarily the result of the increased rigidity of the complex afforded by the BF2 groups. The rigidity increases the energy of symmetry-lowering vibrational modes, which are necessary to promote spin-orbit mixing of the 1dσ*pσ and 3dσ*pσ states. Despite the many examples of M-M bonded d8–d8 complexes of RhI, IrI, and PtII in the literature, until recently there were no PdII complexes fitting this description. Our investigations of clamshell-shaped PdII dimers [(2-phenylpyridine)Pd(µ-X)]2 and [(2-ptolylpyridine)Pd(µ-X)]2 (X = OAc or TFA) revealed short Pd–Pd distances (~ 2.85 Å). The 8 8 vii molecules adopt this unusual geometry in part because of a d –d bonding interaction between the two Pd centers. Density functional theory (DFT) and ab initio (AI) analyses confirm the presence of a Pd–Pd bonding interaction in [(2-phenylpyridine)Pd(µ-X)]2 and show that the HOMO is a dz2 σ*Pd–Pd antibonding orbital, while the LUMO and proximal unoccupied orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other PdII–PdII dimers that have short Pd–Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(µ-X)]2, but quite different from that found for d8– d8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd, and may explain why Pd d8–d8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species. Our work on PdII–PdII electronic structures led us to employ these complexes as electrocatalysts in the regioselective chlorination of C–H bonds. Previous work on d8–d8 complexes has established that when treated with halogens (Cl2, Br2, or I2), the complex undergoes two-center oxidative addition to form an axially coordinated X–d7–d7–X species. Similar products are observed following electrochemical oxidation in the presence of a halide (Cl–, Br–, or I–). Recently, related PdII–PdII complexes were found to selectively chlorinate benzo[h]quinoline through reductive elimination from a Cl–PdIII–PdIII–Cl species. This led us to probe the viability of the analogous electrochemical route. Cyclic voltammetry, spectroelectrochemistry, and bulk electrolysis measurements confirm that electrochemical oxidation of PdII–PdII yields the identical Cl–PdIII–PdIII–Cl intermediate, which is capable of reductive chlorination of C–H bonds. Additional evidence for formation of axially coordinated bromide and acetate species is also presented. Over 10 viii turnovers of 10-chlorobenzo[h]quinoline were achieved at 80% isolated yield. Further research into the area may lead to a potentially versatile, useful, and green route for C–H bond functionalization reactions. TABLE OF CONTENTS Acknowledgements ............................................................................................ iii Abstract ............................................................................................................... vi Table of Contents ................................................................................................ ix List of Figures ..................................................................................................... xi List of Schemes ................................................................................................. xiv List of Tables ..................................................................................................... xv Chapter 1: Electronic structures, photophysical and photochemical properties, and reactivity of d8–d8 binuclear complexes ..................................... 1 1.1 Introduction ............................................................................................. 2 1.2 Electronic structures ................................................................................ 3 1.3 Photophysical properties ......................................................................... 8 1.4 Chemical properties ............................................................................... 11 1.5 Photochemical properties ...................................................................... 12 1.6 Binuclear palladium(II) complexes....................................................... 15 1.7 References ............................................................................................. 17 Chapter 2: Twist it to mix it! 1A2u-to-3A2u intersystem crossing in diplatinum(II,II) complexes ............................................................................... 20 2.1 Abstract .................................................................................................. 21 2.2 Introduction ........................................................................................... 22 2.3 Photophysical properties ....................................................................... 27 2.4 Concluding remarks .............................................................................. 49 2.5 References ............................................................................................. 50 Chapter 3: Electronic structures of palladium(II) dimers ................................. 53 3.1 Abstract .................................................................................................. 54 3.2 Introduction ........................................................................................... 55 3.3 Results and discussion ........................................................................... 58 3.4 Concluding remarks .............................................................................. 93 3.5 Materials and methods .......................................................................... 93 3.6 References ............................................................................................. 98 Chapter 4: Carbon-chlorine bond formation from a binuclear palladium(II) electrocatalyst ............................................................................ 101 4.1 Abstract ................................................................................................ 102 4.2 Introduction ......................................................................................... 102 4.3 Results and discussion......................................................................... 110 4.4 Concluding remarks ............................................................................ 124 4.5 Materials and methods ........................................................................ 124 ix 4.6 References ........................................................................................... 127 Appendix A: Electron transfer triggered by optical excitation of PTZ-tris(meta-phenylene ethynylene)-(CO)3(bpy)(py)rhenium(I) ................ 129 A.1 Abstract ............................................................................................... 130 A.2 Introduction......................................................................................... 130 A.3 Results and discussion ........................................................................ 131 A.4 Concluding remarks ........................................................................... 150 A.5 Materials and methods........................................................................ 151 A.6 References........................................................................................... 166 Appendix B: Photophysics of Ir(III) corroles ................................................. 170 B.1 Abstract ............................................................................................... 171 B.2 Introduction ......................................................................................... 172 B.3 Results and discussion ........................................................................ 173 B.4 Concluding remarks ............................................................................ 183 B.5 Materials and methods ........................................................................ 183 B.6 References ........................................................................................... 189 Appendix C: Supplemental information: Electronic structures of PdII dimers ............................................................................................................... 191 x xi LIST OF FIGURES Number Page 1.1. Orbital energy level diagram for d8–d8 ........................................................ 5 1.2. Examples of d8–d8 binuclear complexes ...................................................... 6 2.1 Electronic structures of d8–d8 complexes ................................................... 23 2.2 Structure of Pt(pop) and its conversion to Pt(pop-BF2) ............................. 24 2.3 Absorption spectrum of Pt(pop-BF2) .......................................................... 30 2.4 Emission and excitation spectra of Pt(pop-BF2) ......................................... 31 2.5 Temperature dependence of the emission spectrum of Pt(pop-BF2) ......... 33 2.6. Temperature-dependence of the quantum yield of Pt(pop-BF2) ............... 34 2.7. Phosphorescence rise and fluorescence decay of Pt(pop-BF2) ................. 36 2.8. Fluorescence decay of Pt(pop-BF2) measured with a streak camera ........ 37 2.9. Phosphorescence rise of Pt(pop-BF2) measured with a streak camera ..... 38 2.10. Temperature-dependent fluorescence decay of Pt(pop-BF2) .................. 41 2.11. Fitted temp-dependence of Pt(pop-BF2) lifetime and quantum yield ..... 42 3.3. Orbital energy level diagram for d8–d8 ...................................................... 56 3.2. The structures of compounds 1–4 .............................................................. 61 3.3. Selected bond distances in 1–4................................................................... 63 3.4. The structure of [(2-phenylpyridine)Pd(µ-Cl)]2 (5)................................... 64 3.5. Cyclic voltammograms of 1–4 in CH2Cl2 solution ................................... 66 3.6. Differential pulse voltammograms of compounds 1–4 ............................. 67 3.7. Electronic absorption spectra of 1–6 in 2-MeTHF solutions .................... 72 3.8. Emission spectrum of 1 in a 2-MeTHF glass at 77 K ............................... 74 3.9. Absorption and excitation spectra of 2 in 2-MeTHF at 77 K.................... 77 3.10. Selected molecular orbitals of 1 ............................................................... 82 3.11. Molecular orbital diagram for 1 ............................................................... 83 3.12. Complexes with short Pd–Pd interactions for DFT calculations ............ 85 3.13. Complexes with d8–d8 interactions selected for DFT calculations ......... 86 3.14. Plot of charge density and Laplacian against M-M distance .................. 88 4.1. Orbital energy level diagram for d8–d8 .................................................... 109 4.2. Cyclic voltammogram of PdII–PdII in dichloromethane .......................... 112 4.3. CV of [(2-phenylpyridine)PdII(µ-OAc)]2 as with increasing chloride .... 115 4.4. Absorbance spectrum from electrochemical oxidation of PdII–PdII........ 116 4.5. UV–vis spectrum following oxidation of PdII–PdII with chloride .......... 118 4.6. Spectra following oxidation of PdII–PdII with bromide and acetate ....... 118 4.7. Cyclic voltammogram of tris(acetylacetonato)iron(III) .......................... 123 A.1. Absorption (a) and corrected emission (b) spectra of PTZ-bridge-Re... 135 A.2. Quantum yield measurements of PTZ-bridge-Re................................... 136 A.3. Time-resolved emission decay profiles of PTZ-bridge-Re .................... 139 A.4. Concentration-dependent luminescence of PTZ-bridge-Re ................... 140 A.5. Stretched-exponential fit of the rapid decay component ........................ 141 A.6. NNLS fitting of the rapid decay component ........................................... 142 A.7. Structural models of PTZ-bridge-Re....................................................... 143 xii A.8. Absorption spectra of PTZ-bridge-Re at various concentrations ........... 145 A.9. Emission spectra of PTZ-bridge-Re at various concentrations .............. 146 A.10. 1H-NMR spectra of PTZ-bridge-Re at various concentrations. ........... 147 A.11. Calculated m-phenylene-ethynylene fragment from DFT ................... 149 B.1. Iridium(III) corroles ................................................................................. 173 B.2. UV-vis spectra of Ir(III) corroles............................................................. 174 B.3 Emission spectra of Ir(III) corroles a. 298 K; b. 77 K ............................. 175 B.4. Normalized excitation profiles of. Ir(III) corroles .................................. 176 B.5. Shifts of the second Soret peak as a function of solvent polarizability .. 179 B.6. Shifts of other absorption bands as a function of solvent polarizability. 180 B.7. Raman spectra of 1-Ir(tma)2, 1b-Ir(tma)2, and 1-Ir(py)2 ..................... 181 C.1. Cyclic voltammograms of 1–4 in CH2Cl2 ............................................... 191 C.2. Cyclic voltammograms of 1–5 in MeCN ................................................ 192 C.3. Steady-state emission spectra of 1–4, 77 K, λex = 355 nm ..................... 193 C.4. Steady-state emission spectra of 1–5, 77 K, λex = 355 nm ..................... 194 C.5. Excitation spectrum of 1–4, 77 K, λem = 460 nm .................................... 195 C.6. Excitation spectrum of 1–4, 77 K, λem = 790 nm .................................... 196 C.7. Luminescence decay of 1 at 77 K at 460 nm (a) and 720 nm (b)........... 198 C.8. Luminescence decay of 2 at 77 K at 460 nm (a) and 720 nm (b)........... 199 C.9. Luminescence decay of 3 at 77 K at 460 nm (a) and 720 nm (b)........... 200 C.10. Luminescence decay of 4 at 77 K at 460 nm (a) and 720 nm (b)......... 201 C.11. Luminescence decay of 5 at 77 K at 460 nm ........................................ 202 xiii xiv LIST OF SCHEMES Number Page 4.4. Selective chlorination of benzo[h]quinoline by PdII–PdII ....................... 106 4.5. Electrocatalytic chlorination catalyzed by PdII–PdII ................................ 120 A.1. Synthesis of PTZ-bridge-Re .................................................................... 133 xv LIST OF TABLES Number Page 1.6. Metal–metal bond distances for selected selected d8–d8 complexes........... 7 1.7. Absorption properties for selected d8–d8 binuclear complexes ................... 9 1.3 Photophysical parameters for selected d8–d8 binuclear complexes ........... 10 1.4 Ground and excited state ν(M–M) frequencies for selected complexes .... 13 2.1 Absorption and emission properties of Pt(pop-BF2) and Pt(pop) ............. 32 2.2 Temperature dependence of Pt(pop-BF2) quantum yields ......................... 35 2.3 Temperature dependence of Pt(pop-BF2) fluorescence lifetime ................ 43 2.4 Kinetics parameters for 1A2u excited states of Pt(pop-BF2) and Pt(pop) ... 44 3.1 Selected bond distances in 1–4.................................................................... 56 3.2 Reduction potentials for compounds 1–4 in CH2Cl2 .................................. 68 3.3 Reduction potentials for compounds 1–5 in CH3CN ................................. 70 3.4 Absorption and emission data for 1–5 ........................................................ 76 3.5 Energy of the nd → (n+1)p transition ......................................................... 90 3.6 Relative contribution of the d, s, and p orbitals to the M–M orbitals ........ 92 B.1 Photophysical data for Ir(III) corroles in toluene solution....................... 177 B.2 Values for solvatochromic effects iridium(III) corroles .......................... 181 Chapter 1 Electronic structures, photophysical and photochemical properties, and reactivity of d8–d8 bincuclear complexes 1.1 Introduction Binuclear metal complexes are attractive systems of study for the activation of organic substrates due to the potential cooperativity between the two metal sites.1 Many biological processes are driven by metalloenzymes with multiple metal centers and catalyze a diverse array of difficult multi-electron reactions including, nitrogen fixation,2 oxygen reduction,3 methane hydroxylation,4 water oxidation,5 and carbon dioxide reduction. Despite their prominence in Nature, the majority of synthetic work has focused on mononuclear catalysts for performing many reactions, particularly those involved in the functionalization of C–H bonds. Binuclear complexes with square planar RhI, IrI, and PtII centers are members of a special class that feature attractive d8–d8 interactions, as documented by optical and vibrational spectroscopy,6–8 x-ray crystallography,9,10 extended x-ray absorption fine structure,11 and ab initio electronic structure theory.12,13 The first report of these complexes occurred in 1975, when Mann et al. reported the electronic spectroscopic characterization of [Rh(CNPh)4]nn+ (n = 1, 2, or 3) in acetonitrile solution and proposed that aggregation occurred through a direct Rh– Rh interaction. This aggregration was attributable to the unsupported metal–metal interaction between the monomers along the z-axis. Since this initial discovery, many d8–d8 dimers of RhI, IrI, and PtII have been discovered. These complexes possess similar electronic structures and interesting chemical, photophysical, and photochemical properties. Much of the work in this field has focused on complexes of RhI, IrI, and PtII, while binuclear PdII complexes have been largely ignored owing to their lack of 2 8 8 photophysical and photochemical properties when compared to traditional d –d species. However, the recent discovery of productive thermal reactivity of PdII dimers has rejuvenated interest in these complexes.14–17 Electronic structure studies have provided insight into the characteristics that make binuclear PdII complexes chemically unique.18 1.2 Electronic structures The electronic structures of d8–d8 complexes can be interpreted in terms of a simple molecular orbital (MO) model where two d8 square planar fragments combine along the z-axis in a face-to-face orientation (Figure 1.1).7 This leads to substantial interaction between the orbitals on each metal center, particularly those with substantial electron density along the z-axis. This electronic configuration results in a metal–metal antibonding dσ* HOMO and a metal–metal bonding pσ LUMO. Formally, d8–d8 complexes have no metal–metal bond, as the contributions of four filled bonding orbitals are cancelled by four filled antibonding orbitals. However, perturbational mixing between the dz2 and pz orbitals stabilizes the metal– metal dσ and dσ* orbitals (and raises the corresponding pσ and pσ* orbitals), resulting in a net bonding interaction. Such d8–d8 interactions most commonly lead to discrete dimers or trimers, although in some cases higher-order oligomers are formed.7,19 In the excited state, an electron is promoted to the pσ molecular orbital, resulting in formation of a metal–metal single bond. Complexes with d8–d8 interactions exist in two principal geometries. The most common is the face-to-face orientation, where two ML4 square-planar fragments come together along an axis perpendicular to the ML4 plane (limiting D4h 3 8 8 symmetry). This is the preferred geometry of many d –d species, including the original dirhodium(I) isocyanide species as well as many ligand-supported d8–d8 architectures. Alternatively, certain ligands can force d8–d8 complexes to adopt a “clamshell” or “A-frame” orientation (limiting C2v symmetry).20 Examples of these two geometries are shown in Figure 1.2. In this case, the two ML4 fragments combine at an angle to one another. This has the effect of decreasing the extent of electronic interaction between the two fragments due to decreased orbital overlap. The relative ordering of the molecular orbitals remains largely unaffected. Despite the lack of a formal bond, the observed metal–metal bond strength for these complexes can be quite significant.21 Resonance Raman and lowtemperature emission studies of several complexes show ν(M–M) frequencies to be in the range of 50–100 cm-1.22 Spectroscopic analysis of Rh2(bridge)44– (bridge = 1,3-diisocyanopropane) calculated a Rh–Rh ground-state bond energy of 12 ± 6 kcal/mol, consistent with a fractional Rh–Rh bond.23 By contrast, spectroscopic calculations of the excited state show the bond energy to increase to 36 ± 6 kcal/mol, consistent with a metal–metal single bond. 4 5 Figure 1.1. Orbital energy level diagram for interaction along the metal−metal axis of two d8 square-planar units, in both ground (left) and excited (right) states 6 Figure 1.2. Examples of d8–d8 binuclear complexes: (a) Unsupported dimer of tetrakis(phenyl isocyanide) rhodium(I), (b) supported face-to-face d8–d8 binuclear complex, tetrakis(µ-pyrophosphito)diplatinate(II), and (c) supported “clamshell” complex benzo[h]quinolinyl palladium acetate dimer 7 Table 1.1. Metal–metal bond distances for selected selected d8–d8 complexes as determined by x-ray crystallography Complex d(M–M), Å Reference [Rh2(CNPh)8]2+ 3.193 6 [Rh2(bridge)4]2+ a 3.243 24 [Rh2(TMB)4]2+ b 3.262 23 [Ir2(TMB)4]2+ 3.119 25, 26 [Rh2(dimen)4]2+ c 3.861 24 [Ir(µ-pz)COD]2 d 3.216 20 [Pt2(µ-P2O5H2)4]4- 2.925 27, 28 [(bzq)2Pd2(µ-OAc)]2 e 2.842 16 a bridge = 1,3-diisocanyopropane, bTMB = 2,5-dimethyl-2,5-diisocyanohexane, c dimen = 1,8-diisocyano-p-menthane, dpz = pyrazolyl, COD = 1,5-cyclooctadiene, e bzq = benzo[h]quinoline 1.3 Photophysical properties The photophysical properties of these complexes are strongly influenced by electronic structure. Monomeric d8 species often display little color in the visible region as their strong ligand-to-metal-charge-transfer (LMCT) bands are often in the ultraviolet region, particularly amongst 4d and 5d metals.29 By contrast, the optical spectrum of bimetallic d8 complexes are often brightly colored.23,30–33 Their spectrum is dominated by the dσ*→pσ electronic transition, which removes an electron from a metal–metal antibonding orbital and places it in a metal–metal bonding orbital. Direct triplet dσ*→pσ absorption bands are weak and often obscured beneath the allowed singlet transition. The absorption properties for several complexes are presented in Table 1.2. Photophysical studies of numerous d8–d8 complexes have found them to luminesce from both the lowest energy singlet and triplet excited states derived from the (dσ)2(dσ*)1(pσ)1 electronic configuration. 24,26,31–36 Typically, the singlet excited state is short, owing to rapid intersystem crossing rates. The lifetimes for most complexes are well below 1 ns. The triplet state, as expected, is much longer lived, with lifetimes ranging from 30 ns to 10 µs. The emission properties for several complexes are presented in Table 1.3. 8 9 Table 1.2. Absorption properties for selected d8–d8 binuclear complexes 1 3 (dσ*→pσ) (dσ*→pσ) Compound λmax, nm ε, M-1 cm-1 [Rh2(CNPh)4]2+ 568 n/a 7 [Rh2(bridge)4]2+ 565 670 32 [Rh2(TMB)4]2+ 517 13600 32 [Ir2(TMB)4]2+ 625 11200 21 [Ir(µ-pz)COD]2 498 9100 585 260 31 [Pt2(µ-P2O5H2)4]4- 372 452 110 33400 33 λmax, nm ε, M-1 cm-1 Ref. 10 Table 1.3. Photophysical parameters for selected d8–d8 binuclear complexes. All measurements were conducted at room temperature in fluid solution unless otherwise specified. 1 3 (dσ*→pσ) (dσ*→pσ) Compound a Ref. λmax, nm τ, ns Φ λmax, nm τ, ns Φ [Rh2(bridge)4]2+ 656 1.3 0.07 865 8300 0.32 34 [Rh2(TMB)4]2+ 614 0.820 0.055 780 30 0.001 32 [Rh2(Dimen)4]2+ 600 0.23 0.0016 690sha <1 <10-4 24 [Ir2(TMB)4]2+ 735 <2 0.0025 1080 210 [Ir(µ-pz)COD]2 564 <0.1 1×10-4 687 250 7.8×10-3 31 [Pt2(µ-P2O5)4]4- 407 <0.04 1.5×10-4 514 9400 0.61 33, 36 26, 35 Observed in PMMA film at 25oC. No phosphorescence was detected at room temperatures in fluid solution. The excited state can be interpreted as a metal-centered diradical, with one electron localized on the exterior of the metal–metal core (in the dσ* orbital) and the other between the two metal centers (in the pσ orbital). The excited state possesses a formal bond order of one, which results in significantly higher metal– metal bond strengths relative to the ground state. This observation is confirmed by both resonance Raman22,26,32,34,35,37 and absorption/emission studies on several d8–d8 species.24,38 The ground- and excited-state M–M stretching frequencies for several d8–d8 complexes are shown in Table 1.4. A M–M bond length contraction of approximately 0.2–0.3 Å in the excited state has been observed in Pt2(µ-P2O5H2)44– (Pt2) by time-resolved x-ray and electronic spectroscopies.39,40 This value is consistent with the Franck-Condon analysis of Pt2 which predicted a similar contraction in the excited state (Δr = 0.279 Å).41 1.4 Chemical properties Binuclear d8–d8 complexes frequently react with electrophilic reagents (X–Y), undergoing two-center oxidative addition to the axially-ligated d7–d7 (X– M–M–Y) species.16,42–45 Most commonly, reactivity is observed with halogens as well as alkyl halides. For Pt2, chlorine addition was shown to occur through a mixed-valence Pt2X intermediate,46,47 which has been isolated in the solid state.48 In addition, it was shown that chemical or electrochemical oxidation of Pt2 in the presence of certain ligands results in the reversible formation of Pt2X2 species (X = Cl, Br, I, H2O, SCN, MeCN, imidazole).49,50 11 8 8 As expected from the simple MO picture for d –d complexes, oxidation will remove two electrons from the M–M antibonding dσ* orbital and result in the formation of a metal–metal single bond. This is accompanied by a contraction of the metal–metal bond. The degree of contraction is dependent on the extent of electron donation into the dσ* orbital by the axial ligands. For Pt2, Pt–Pt distances range from 2.695 Å (Pt2Cl2) to 2.782 Å (Pt2CH3I). For reference, unmodified Pt2 has a Pt–Pt distance of 2.925 Å.27,28 Similar chemical behavior is displayed by d8– d8 complexes of Rh and Ir.51 1.5 Photochemical properties The relatively long lifetimes of the 3[(dσ*)1(pσ)1] excited state allow for bimolecular reactions to take place. The excited states of metal complexes are simultaneously more powerful reductants and oxidants than the ground-state species. The excited states of d8–d8 complexes are often strong reductants. Electrochemical and spectroscopic measurements allow a modified Latimer diagram to be constructed that shows Eo(M2+/M2*) to range from -0.8 to -2.0 V vs. SSCE.31,33 Bimolecular quenching studies of the A-frame dimer [Ir(µ-pz)COD]2 (pz = pyrazole, COD = 1,4-cycloocatadiene) using a series of pyridinium electron acceptors with varying reduction potentials support these calculations.31 The strongly reducing excited state of [Ir(µ-pz)COD]2, estimated to be ~ -1.8 V vs. SSCE,31 has been exploited in fundamental electron transfer experiments. Photoexcited [Ir(µ-pz)COD]2 has been used to probe the electron transfer “inverted region” in covalent systems as well as to facilitate long-range electron transfer in frozen glasses.52,53 12 13 Table 1.4. Ground and excited state metal–metal stretching frequencies for selected d8–d8 complexes Ground state Excited state ν(M–M), cm-1 ν(M–M), cm-1 [Rh2(CNPh)4]2+ 60 162 22 [Rh2(bridge)4]2+ 79 144 34 [Rh2(TMB)4]2+ 55 151 32 [Ir2(TMB)4]2+ 53 132 26, 35 [Pt2(µ-P2O5)4]2- 118 156 37 Compound Reference 14 Atom-transfer reactions Photoexcited d8–d8 complexes are particularly active in atom-transfer reactions.21,25,33 These reactions are promoted by the diradical nature of the excited state, which places a hole in the dσ* that extends away from the open coordination sites of the metals. Multiple d8–d8 systems are capable of H–atom abstraction from a variety of organic and organometallic substrates. Following excitation into the dσ* → pσ transition, (λmax = 372 nm) Pt2* will react with hydrogen-atom donors such as (CH3)2CHOH, PhCH(OH)CH3, Bu3SnH, Et3SiH, and H3PO3.46,54,55 Rates of reaction of Pt2* with Ph3EH (E = Sn, Ge, Si) decrease as E–H bond strengths increase.33,56 A great deal of mechanistic work on hydrogen–atom in d8–d8 complexes has focused on the light-induced conversion of isopropanol to acetone by Pt2.57 The reaction occurs through initial abstraction of the α-hydrogen to form a monohydride Pt2H species and the alcohol radical. The monohydride complex has been observed through transient absorption spectroscopy before decaying to form the dihydride Pt2H2 complex and acetone. This reaction can be observed with UV–vis through the disappearance of the dσ→dσ* transition at 372 nm and the appearance of the Pt2H2 dσ → dσ* transition at 325 nm. Pt2H2 thermally decomposes into H2 and Pt2; however, this reaction can be accelerated by photoexcitation into the dσ → dσ* transition of the d7–d7 complex, which rapidly liberates H2.58 In addition, Pt2* is readily quenched by halogen atom transfer from alkyl and aryl halides.59 For reactions with aryl halides, reaction rate is again inversely proportional to Ar–X bond strength. Similarly to the ground-state reaction to X2, halogen atom abstraction proceeds by initial formation of mixed-valence Pt2X, followed by disproportionation to Pt2X2 and Pt2. In a similar fashion, illumination of Ir2(TMB)42+ into the dσ* → pσ transition (λmax = 625 nm) in the presence of 9,10-dihydroanthracene (C–H bond strength 77 kcal/mol) results in formation of Ir2(TMB)4H22+ and anthracene. NMR spectroscopy following illumination confirms formation of anthracene as well as Ir2(TMB)4H22+. The dihydride species has also been chemically isolated and structurally characterized.25 Ir2(TMB)44– has also showed photoreactivity with cyclohexadiene, forming benzene. 1.6 Binuclear PdII complexes The great majority of research into d8–d8 complexes has focused on complexes of PtII, RhI, and IrI. However, many examples of PdII dimers with short M–M distances exist in the literature. The lack of reports on the photophysical and photochemical properties of these species are due to the fact that these properties are largely nonexistent. In fact, there are no reports of photochemical reactivity for bimetallic PdII species. Some complexes have been shown to luminesce at cryogenic temperatures,18 and at least one recent report has identified roomtemperature emission from a PdII dimer.60 Experimental and computational work suggests the unique behavior of binuclear PdII species is a result of their altered electronic structure with respect to traditional d8–d8 species. Both systems possess a dσ* M–M-antibonding HOMO. However, rather than a pσ M–M-bonding LUMO, as is the case for most dimeric 15 II I I Pt , Rh , and Ir species, the LUMO of most Pd2 species appear to be ligand-based π* orbitals. The resulting excited state places a large amount of electron density on the ligands. As a result, the Pd2* excited state is often short-lived. In this excited state, the bond order is roughly one-half. The altered electronic structure that leads to a ligand-π* orbital rather than a pσ orbital is a result of the unusually large 4d–5p splitting found in PdII. Gas-phase atomic absorption measurements show this splitting to be nearly 10,000 cm-1 higher than what is observed for PtII, RhI, and IrI.61 As such, the pσ orbital is raised in energy above the ligand-π*. Importantly, this large 4d–5p separation also serves to decrease the extent of d–p mixing. This mixing stabilizes the occupied dσ and dσ* orbitals, resulting in a net bonding interaction in the ground state, resulting in substantially weaker ground-state metal–metal interactions in palladium. Despite their limited photochemical properties, binuclear complexes featuring PdII have demonstrated chemical reactivity that has made them particularly relevant for study. The complex [(benzo[h]quinoline)PdII(µ-OAc)]2 (Pd2) has been shown to facilitate regioselective C–X bond-forming reactions.1516 Moreover, this reaction operates through a cooperative interaction between the two Pd centers. Oxidative addition of X2 (X = Cl, Br) by Pd2II occurs with concomitant formation of a metal–metal bond to yield a [(benzo[h]quinoline)PdIII(µ-OAc)X]2 intermediate which thermally undergoes reductive elimation, releasing a chlorinated benzo[h]quinoline product. This reactivity is analogous to the chemical reactivity of traditional d8–d8 systems, which yield a d7–d7 (X–M–M–X) complex upon treatment with X2 (X = Cl, Br, I). Further, it suggests that Pd2II,II/III,III redox cycles 16 0/II are catalytically relevant, in addition to the more established monometallic Pd and PdII/IV cycles. Since this initial report, additional reactivity of PdII dimers has been discussed.14 1.7 References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) Cotton, F. A. Inorg. Chem. 1998, 37, 5710–5720. Lancaster, K. M.; Roemelt, M.; Ettenhuber, P.; Hu, Y.; Ribbe, M. W.; Neese, F.; Bergmann, U.; DeBeer, S. Science 2011, 334, 974–977. Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. Rev. 1996, 96, 2563–2606. Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Müller, J.; Lippard, S. J. Angew. Chem. Int. Ed. Engl. 2001, 40, 2782–2807. Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Science 2004, 303, 1831–1838. Mann, K. R.; Gray, H. B. In Inorganic Compounds with Unusual Properties - II; Advances in Chemistry; ACS, 1979 vol. 173, pp. 225–235. Mann, K. R.; Gordon, J. G.; Gray, H. B. J. Am. Chem. Soc. 1975, 97, 3553– 3555. Rice, S. F.; Milder, S. J.; Gray, H. B.; Goldbeck, R. A.; Kliger, D. S. Coord. Chem. Rev. 1982, 43, 349–354. Osborn, R. S.; Rogers, D. J. Chem. Soc., Dalton Trans. 1002–1004. Connick, W. B.; Marsh, R. E.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1997, 36, 913–922. Carr, N.; Crossley, J. G.; Dent, A. J.; Gouge, J. R.; Greaves, G. N.; Jarrett, P. S.; Orpen, A. G. J. Chem. Soc., Chem. Commun. 1990,1369–1371. Novoa, J. J.; Aullon, G.; Alemany, P.; Alvarez, S. J. Am. Chem. Soc. 1995, 117, 7169–7171. Aullón, G.; Alvarez, S. Chem. Eur–J 1997, 3, 655–664. Powers, D. C.; Ritter, T. Acc. Chem. Res. 2011. Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 17050–17051. Powers, D. C.; Ritter, T. Nat Chem 2009, 1, 302–309. Lyons, T. W.; Sanford, M. S. Chemical Reviews 2010, 110, 1147–1169. Bercaw, J. E.; Durrell, A. C.; Gray, H. B.; Green, J. C.; Hazari, N.; Labinger, J. A.; Winkler, J. R. Inorg. Chem. 2010, 49, 1801–1810. Wong, K. M.-C.; Yam, V. W.-W. Acc. Chem. Res. 2011, 44, 424–434. Beveridge, K. A.; Bushnell, G. W.; Dixon, K. R.; Eadie, D. T.; Stobart, S. R.; Atwood, J. L.; Zaworotko, M. J. J. Am. Chem. Soc. 1982, 104, 920– 922. Smith, D. C.; Gray, H. B. Coord. Chem. Rev. 1990, 100, 169–181. Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 1595–1596. 17 (23) Rice, S. F.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1988, 27, 4704– 4708. (24) Miskowski, V. M.; Rice, S. F.; Gray, H. B.; Dallinger, R. F.; Milder, S. J.; Hill, M. G.; Exstrom, C. L.; Mann, K. R. Inorg. Chem. 1994, 33, 2799– 2807. (25) Smith, D. C.; Marsh, R. E.; Schaefer, W. P.; Loehr, T. M.; Gray, H. B. Inorg. Chem. 1990, 29, 534–538. (26) Smith, D. C. Ph. D. Thesis. California Institute of Technology, Pasadena, CA, 1989. (27) Dos Remedios Pinto, M. A. F.; Sadler, P. J.; Neidle, S.; Sanderson, M. R.; Subbiah, A.; Kuroda, R. J. Chem. Soc., Chem. Commun. 1980, 13–15. (28) Marsh, R. E.; Herbstein, F. H. Acta Crystallogr. Sect. B–Struct. Sci. 1983, 39, 280–287. (29) Mason, W. R.; Gray, H. B. J. Am. Chem. Soc. 1968, 90, 5721–5729. (30) Marshall, J. L.; Hopkins, M. D.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1992, 31, 5034–5040. (31) Marshall, J. L.; Stobart, S. R.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 3027–3029. (32) Smith, D. C.; Miskowski, V. M.; Mason, W. R.; Gray, H. B. J. Am. Chem. Soc. 1990, 112, 3759–3767. (33) Roundhill, D. M.; Gray, H. B.; Che, C. M. Acc. Chem. Res. 1989, 22, 55– 61. (34) Rice, S. F.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 1593–1595. (35) Harvey, P. D. Coord. Chem. Rev. 2001, 219–221, 17–52. (36) Che, C. M.; Butler, L. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 7796– 7797. (37) Bradley, P. G.; Kress, N.; Hornberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 7441–7446. (38) Miskowski, V. M.; Rice, S. F.; Gray, H. B.; Milder, S. J. J. Phys. Chem. 1993, 97, 4277–4283. (39) Christensen, M.; Haldrup, K.; Bechgaard, K.; Feidenhans’l, R.; Kong, Q.; Cammarata, M.; Russo, M. L.; Wulff, M.; Harrit, N.; Nielsen, M. M. J. Am. Chem. Soc. 2008, 131, 502–508. (40) van der Veen, R. M.; Cannizzo, A.; van Mourik, F.; Vlček, A.; Chergui, M. J. Am. Chem. Soc. 2010, 133, 305–315. (41) Fordyce, W. A.; Brummer, J. G.; Crosby, G. A. J. Am. Chem. Soc. 1981, 103, 7061–7064. (42) Che, C. M.; Schaefer, W. P.; Gray, H. B.; Dickson, M. K.; Stein, P. B.; Roundhill, D. M. J. Am. Chem. Soc. 1982, 104, 4253–4255. (43) Che, C. M.; Mak, T. C. W.; Gray, H. B. Inorg. Chem. 1984, 23, 4386– 4388. (44) Coleman, A. W.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M. J.; Atwood, J. L. J. Am. Chem. Soc. 1982, 104, 922–923. (45) Maverick, A. W.; Smith, T. P.; Maverick, E. F.; Gray, H. B. Inorg. Chem. 1987, 26, 4336–4341. 18 (46) Bryan, S. A.; Dickson, M. K.; Roundhill, D. M. Inorg. Chem. 1987, 26, 3878–3886. (47) Che, C. M.; Lee, W. M.; Cho, K. C. J. Am. Chem. Soc. 1988, 110, 5407– 5411. (48) Che, C. M.; Herbstein, F. H.; Schaefer, W. P.; Marsh, R. E.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 4604–4607. (49) Bryan, S. A.; Dickson, M. K.; Roundhill, D. M. J. Am. Chem. Soc. 1984, 106, 1882–1883. (50) Bryan, S. A.; Schmehl, R. H.; Roundhill, D. M. J. Am. Chem. Soc. 1986, 108, 5408–5412. (51) Cotton, F. A. Multiple bonds between metal atoms. Springer, 2005. (52) Fox, L. S.; Kozik, M.; Winkler, J. R.; Gray, H. B. Science 1990, 247, 1069–1071. (53) Wenger, O. S.; Leigh, B. S.; Villahermosa, R. M.; Gray, H. B.; Winkler, J. R. Science 2005, 307, 99–102. (54) Harvey, E. L. Ph. D. Thesis. California Institute of Technology, Pasadena, CA, 1990. (55) Roundhill, D. M.; Atherton, S. J.; Shen, Z. P. J. Am. Chem. Soc. 1987, 109, 6076–6079. (56) Vlcek, A.; Gray, H. B. Inorg. Chem. 1987, 26, 1997–2001. (57) Roundhill, D. M. J. Am. Chem. Soc. 1985, 107, 4354–4356. (58) Sweeney, R. J.; Harvey, E. L.; Gray, H. B. Coord. Chem. Rev. 1990, 105, 23–34. (59) Che, C. M.; Lee, W. M.; Cho, K. C.; Harvey, P. D.; Gray, H. B. J. Phys. Chem. 1989, 93, 3095–3099. (60) Santana, M. D.; García-Bueno, R.; García, G.; Sánchez, G.; García, J.; Kapdi, A. R.; Naik, M.; Pednekar, S.; Pérez, J.; García, L.; Pérez, E.; Serrano, J. L. Dalton Trans. 2012, 41, 3832–3842. (61) Moore, C. E. Atomic Energy Levels, Vol. III (Molybdenum through Lanthanum and Hafnium through Actinium). Circular of the National Bureau of Standards 467, U.S. Government Printing Office, Washington DC, 1958. 19 20 Chapter 2 Twist it to mix it! 1A2u-to-3A2u intersystem crossing in diplatinum (II,II) complexes This work is a collaborative effort between Alec C. Durrell, Gretchen E. Keller, Yan Choi Lam, Jan Sykora, Harry B. Gray, and Tony Vlcek. 2.1 Abstract The photophysical properties of tetrakis(µ-pyrophosphito)diplatinate(II), abbreviated Pt(pop), are compared with its fluoroborated analogue, Pt(pop-BF2). This complex possesses eight BF2 groups that replace the hydrogen atoms that are located between each “pop” ligand. When compared with Pt(pop), Pt(pop-BF2) has a much greater singlet lifetime (1.5 ns) and singlet quantum yield (0.27). The enhancement is the result of a drastically slower 1A2u → 3A2u intersystem crossing rate. In particular, the thermal barrier to intersystem crossing is significantly higher in Pt(pop-BF2) (2230 cm-1 vs. 1190 cm-1). We believe this is primarily the result of the increased rigidity of the complex afforded by the BF2 groups. The rigidity increases the energy of symmetry-lowering vibrational modes, which are necessary to promote spin-orbit mixing of the 1dσ*pσ and 3dσ*pσ states. 21 22 2.2 Introduction Face-to-face d8–d8 complexes of PtII, RhI, and IrI possess truly remarkable spectroscopic, photophysical, and photochemical properties.1–5 Electronic excitation from a metal-metal antibonding (n-1)dσ* orbital to an npσ bonding orbital creates a net metal–metal bond (Figure 2.1); as a result, the metal–metal distance shrinks in both singlet and triplet (1,3dσ*pσ) excited states. In addition to strong phosphorescence, some of these complexes also show weak fluorescence; and, of the complexes studied to date, fluorescence is most prominent for certain IrI–IrI complexes whose 1dσ*pσ states are able to reduce very low potential substrates.6–9 Tetrakis(µ-pyrophosphito)diplatinate(II), [Pt2(µ-P2O5H2)4]4–, abbreviated Pt(pop), is the prototypal photoactive d8–d8 complex.1 Its molecular structure (Figure 2.2, left) consists of two parallel PtP4 square planar units held in a rigid configuration by four P–O–P bridges. The eight terminal P(O)(OH) groups, which are exposed to solvent, likely play a role in excited-state deactivation.10,11 Pt(pop) shows very weak fluorescence from a 1dσ*pσ state that decays on a tens-of-ps timescale with temperature- and solvent-dependent lifetimes,10,11 and the corresponding triplet state exhibits intense green phosphorescence. Owing to its long lifetime (~ 10 µs)12 and biradical character,13,14 the 3dσ*pσ state reacts with a variety of substrates, including alcohols, hydrocarbons, alkylhalides,2–5 DNA,15 and metal complexes.13,14,16 These reactions, which involve transfer of hydrogen or halogen atoms, can be viewed as photochemical oxidative-addition or inner-sphere electron-transfer processes. 23 Figure 2.1. Electronic structures of d8–d8 complexes in ground (left) and lowest triplet excited states (right) 24 4- 4HO O O O O P O P O P P O HO P HO P O P O O F2B O O Pt OHOH HO Pt P O OH O OH O F3B OEt2 O Room temperature 2 days F2B P O O F2B O P F2B O P O P P O BF2O Pt O O O Pt P O BF2 BF2 P O P O O B F2 O O Figure 2.2. Structure of Pt(pop), left, and its conversion to Pt(pop-BF2), right The structures and dynamics of Pt(pop) 1,3 dσ*pσ excited states have been extensively investigated. Evidence that a Pt–Pt bond is formed upon dσ*→pσ excitation has been extracted from Frank-Condon analysis of the vibronic structure associated with dσ*→pσ absorption bands measured on single crystals at low temperatures (the Pt–Pt stretching frequency increases from 118 cm–1 in the ground state17 to about 150 cm–1 in both excited states,12,18 and 155 cm–1 was found for the triplet by transient Raman spectroscopy).17 A femtosecond time-resolved spectroscopic study11 reported long-lasting (several ps) coherent oscillations of stimulated fluorescence with a 224 fs period corresponding to a 149 cm–1 stretching vibration, a finding that documented the harmonic nature of the 1dσ*pσ potential energy surface with respect to the Pt–Pt coordinate. The structure of the 3dσ*pσ state studied by time-resolved x-ray diffraction from a Pt(pop) single crystal showed a Pt–Pt bond contraction of 0.28 Å.19 The most detailed information on the 3 dσ*pσ structure so far has been obtained by time-resolved x-ray absorption spectroscopy:20 the appearance of a new pre-edge absorption feature upon excitation demonstrated creation of an electron vacancy (“hole”) in the dσ* orbital; EXAFS analysis showed that the Pt–Pt distance contracts by 0.31 Å relative to that in the ground state; and the Pt–P bonds elongate by 0.01 Å, with Pt atoms shifting inward from the planes formed by the four phosphorus donor atoms, a structural picture that accords with excited-state DFT calculations.21,22 Understanding and controlling relaxation pathways of electronically excited d8–d8 complexes remains a major challenge. In particular, several different mechanisms could account for 25 10,11,23 singlet→triplet conversion (i.e., intersystem crossing). It is of special interest that 1dσ*pσ excited-state lifetimes vary from a few ps to almost 1 ns in a d8–d8 Rh2 complex,24 and that substantially different chemical reactivities have been found for singlet and triplet dσ*pσ states of Ir2 derivatives.6,24–26 A major goal of our work is to understand the factors that control the lifetimes of these energy-rich singlet states, as potentially they could be key players in photocatalysis and light-energyharvesting schemes. Although Pt(pop) offers few opportunities for tuning its photobehavior by structural variations, replacing each bridging –O– atom by a –CH2– group, which shortens the 3dσ*pσ lifetime to 55 ns, and accelerates its atom- and electrontransfer reactions.27,28 Since the terminal P(O)(OH) groups are the only other sites that can be derivatized, we set out to study the effects of substituting all eight pop hydrogen atoms by BF2 groups, each linking oxygen atoms of two different pop ligands (Figure 2.2). Such “perfluoroboration" creates a rigid covalent cage around the Pt–Pt central motif, shields it from solvent, and removes potentially energyaccepting –O–H⋅⋅⋅O– vibrations. Moreover, the 3dσ*pσ state is expected to become a stronger oxidant, owing to the presence of electron-withdrawing BF2 groups. We report here that perfluoroboration profoundly changes the properties the 1dσ*pσ state of Pt(pop). Perfluoroboration was found to dramatically lower the rate of intersystem crossing, resulting in a profoundly larger singlet lifetime (~ 100×) and quantum yield (~ 1000×) when compared to the parent Pt(pop) complex. We believe this is primarily the result of the increased rigidity of the complex afforded 26 by the BF2 groups. The rigidity increases the energy of symmetry-lowering vibrational modes, which are necessary to promote spin-orbit mixing of the 1dσ*pσ and 3dσ*pσ states. 2.3 Photophysical Properties The absorption spectrum of Pt(pop-BF2) exhibits an intense band at 365 nm attributable to the dσ*→pσ (1A1g→1A2u) transition, an assignment based on work on Pt(pop), which has a similar absorption feature at 372 nm (Figure 2.3 and Table 2.1).12,18,29 The corresponding spin-forbidden dσ*→pσ transition (1A1g→3A2u) gives rise to a ~ 200× weaker band at 454 nm, virtually identical with that of Pt(pop).12 Both complexes show qualitatively similar spectral patterns at shorter wavelengths [three relatively weak bands at 293 (1/26 main peak intensity), 261 (1/13), and 234 nm (1/4.4) in the spectrum of Pt(pop-BF2), and at 315 (1640), 285 (2550) and 246 (3770) nm for Pt(pop)]. The 2100–3200 cm–1 high-energy shifts of these bands upon perfluoroboration are in line with their LMCT character, as revealed by TD-DFT calculations on Pt(pop).21,22,30 The emission spectra of Pt(pop-BF2) and Pt(pop) are very different in terms of the relative intensities of pσ→dσ* fluorescence and phosphorescence. The spectrum of Pt(pop-BF2) shows intense 3A2u→1A2u phosphorescence at 512 nm and equally strong fluorescence at 393 nm (Figure 2.4 and Table 2.2). Owing to phosphorescence quenching by traces of O2, the intensity ratio of the two bands depends on sample preparation; a limiting phosphorescence:fluorescence peakintensity ratio of 1.15 was obtained from a high-vacuum degassed solution. While 27 Pt(pop) in MeCN also shows strong phosphorescence at 511 nm, the fluorescence at 398 nm is extremely weak,12,31 as documented quantitatively by emission quantum yields: whereas room-temperature phosphorescence yields of the two complexes are comparable, the fluorescence yield is three orders of magnitude larger for Pt(pop-BF2) than for Pt(pop) (2.7×10–1 and 1.5×10–4, respectively)12. The excitation spectra of Pt(pop-BF2) measured at 405 and 512 nm are virtually identical and match perfectly the strong 1A1g→1A2u absorption band at 363 nm (Figure 2.4); and the phosphorescence:fluorescence peak-intensity ratio is independent of excitation wavelength from 350 to 380 nm. Both the fluorescence band intensity and quantum yield decrease with increasing temperature, accompanied by a concomitant increase in phosphorescence intensity and yield (Figures 2.5 and 2.6, and Table 2.2). The total emission quantum yield of ~ 0.75 is temperature independent, behavior that indicates direct conversion of the fluorescent state (1A2u) to the phosphorescent state (3A2u). This conclusion was confirmed by observing identical single-exponential kinetics of fluorescence decay and phosphorescence rise, both occurring with a 1.6 ns lifetime (Figures 2.7, 2.8, and 2.9); the fluorescence and phosphorescence time profiles were measured under exactly the same conditions, except for detection wavelengths, 405 and 512 nm, respectively; and the same results also were obtained using two different techniques, TCSPC (373 nm, ~ 80 ps excitation) and streak camera detection (355 nm, 1 ps excitation). Our findings clearly demonstrate the presence of slow 28 1 3 A2u→ A2u intersystem crossing (ISC) that occurs without any apparent intermediates in Pt(pop-BF2). 29 30 2.5 Y Axis Title 2.0 1.5 x3 x50 1.0 0.5 0.0 200 250 300 350 400 450 500 550 X Axis Title Figure 2.3. Absorption spectrum of Pt(pop-BF2) in MeCN solution Intensity / a.u. 31 350 400 450 500 Wavelength / nm 550 600 Figure 2.4. Red: emission spectrum of Pt(pop-BF2) measured using 373 nm excitation. Blue: normalized excitation spectra of in MeCN obtained at λem = 405 and 512 nm (the two spectra are indistinguishable. Measured in a MeCN solution at 21 °C, freeze-pump-thaw degassed at 4×10–5 mbar 32 Table 2.1. Absorption and emission properties of Pt(pop-BF2) and Pt(pop) in MeCN Pt(pop-BF2) Pt(pop)a Assignment b Absorption, nm (ε, M–1cm–1) 234 (1/4.4) 246 (3770) LMCT c 261 (1/13) 285 (2550) LMCT c 293 (1/26) 315 (1640) LMCT c 365 (1/1) 372 (33400) 1 454 (1/200) 454 (155) 3 (dσ*→pσ) 1A1g→1A2u (dσ*→pσ) 1A1g→3A2u Emission, nm (lifetime at 21 °C) 393 (1.6 ns) 398 (~ 8 ps) 1 (pσ→dσ*) 1A2u→1A1g 512 (8.4 µs) 511 (9.4 µs) 3 (pσ→dσ*) 3A2u→1A1g Emission Stokes shift, cm–1 a 1760 2230 Fluorescence 2460 2500 Phosphorescence Absorption data and emission lifetimes from reference 12, emission wavelengths from reference 31. bBased on references 12, 21, and 22. cThe principal excitations are directed to the pσ LUMO accompanied by smaller contributions from Ptlocalized excitations. 33 Intensity / a.u. 0C 7C 20 C 30 C 40 C 50 C 65 C 400 450 500 550 600 Wavelength / nm Figure 2.5. Temperature dependence of the emission spectrum of Pt(pop-BF2) in MeCN solution. Excitation at 355 nm 34 0.7 Quantum Yield 0.6 0.5 0.4 0.3 0.2 0.1 0.0 260 280 300 320 Temperature / K 340 Figure 2.6. Temperature-dependence of the fluorescence phosphorescence (blue) quantum yield of Pt(pop-BF2) in MeCN (red) and 35 Table 2.2. The temperature dependence of Pt(pop-BF2) fluorescence (φfl) and phosphorescence (φph) quantum yields measured in degassed MeCN solution T [°C] T [K] φfl φph φfl + φph -5 268.15 0.42 - - 0 273.15 0.39 0.34 0.73 7 280.15 - 0.40 - 10 283.15 0.33 - - 20 293.15 0.27 0.49 0.76 30 303.15 0.22 0.54 0.76 40 313.15 0.18 0.58 0.76 50 323.15 0.14 0.61 0.75 60 333.15 0.11 - - 65 338.15 - 0.66 - 70 343.15 0.09 - - Counts / a.u. 36 0 2 4 6 8 Time / ns 10 12 14 Figure 2.7. Phosphorescence rise (red), fluorescence decay (green) of Pt(pop-BF2) in MeCN, shown together with a 373 nm excitation-pulse profile (black). Fluorescence and phosphorescence intensities were measured at 405 and 512 nm, respectively. The blue and magenta curves are single-exponential fits in the 0.3– 17.2 ns interval with lifetimes of 1.56±0.02 and 1.58±0.001 ns, respectively. 37 Fluorescence intensity / counts 2500 2000 1500 1000 500 0 0 2 4 6 Time / ns 8 10 Figure 2.8. Fluorescence decay of Pt(pop-BF2) in MeCN measured with a streak camera, excited at 355 nm, 10 ps. Emission in the 400–415 nm range was selected by a bandpass filter. Red curve: single-exponential fit with τ = 1.58±0.01 ns Phosphorescence intensity / counts 38 120 100 80 60 40 20 0 0 2 4 6 8 10 12 14 16 18 Time / ns Figure 2.9. Phosphorescence rise of Pt(pop-BF2) in MeCN measured with a streak camera, excited at 355 nm, 1 ps pulse. Emission wavelengths > 500 nm were selected by a cut-off filter. Red curve: single-exponential fit with τ = 1.62±0.09 ns The Pt(pop-BF2) room-temperature fluorescence lifetime of 1.6 ns is about 100 times longer than the 8–18 ps values reported for Pt(pop) in MeCN12 and similar solvents.10,11 Figure 2.10 shows that the Pt(pop-BF2) fluorescence decay is strongly dependent on temperature, and the kinetics are predominantly32 singleexponential (lifetimes are collected in Table 2.3). To analyze the temperature dependence of the fluorescence lifetime and quantum yield (Figure 2.11), we express the nonradiative decay rate constant knr as a sum of ISC rate constants, kISC, and the nonradiative decay to the ground state, kd (eq. 1). Owing to the long fluorescence lifetime, the latter cannot be a priori neglected, as in the case of Pt(pop). kISC is assumed to follow Arrhenius-like behavior (eq. 1), as proposed10 for Pt(pop). knr =kd + k ISC =kd + (k0 + A − Ea / kBT ) e k BT (1) The lifetime and quantum yield are then given by equations 2 and 3, respectively. 1 τ = kr + knr = kr + kd + k0 + 1 φ fl = 1+ A − Ea / kBT e k BT knr k + k0 A = + e − Ea / kBT 1+ d kr kr k r k BT (2) (3) Fitting the experimental data to these equations gives: kr + kd + k0 = (2.55±0.07)×108 s–1; A = (3.0±0.2)×1014 cm1/2s–1; Ea = 2229±20 cm–1 for the lifetime, and (kd+k0)/kr = 0.6±0.1; A/kr = (1.7±0.5)×106 cm1/2; Ea = 2233±75 cm–1 for the quantum yield (Figure 2.10). The activation energies from these two fits are 39 virtually identical, in accordance with eqs. 2–3 and the assumption of Tindependent kr and kd (T-dependent kd would require two exponential terms, for which we have no evidence). The radiative rate constant (kr) can be estimated to be 1.7×108 s–1, which is ~ 8 × larger than that of Pt(pop) (2×107 s–1);12 and 8.5×107 s–1 is the rate for T-independent nonradiative decay (kd + k0). Pt(pop-BF2) phosphorescence decay was observed on a longer timescale, occurring with a slightly temperature-dependent lifetime: 8.6, 8.4, and 7.6 µs at 0, 20, and 80 °C, respectively. In absolute terms, the phosphorescence lifetime is comparable to that of Pt(pop), 9.4 µs, which, however, was reported to be independent of temperature.12 The presence of a long-lived strongly emissive singlet excited state together with a strongly phosphorescent triplet are the distinctive photophysical properties of Pt(pop-BF2). The 1A2u→3A2u ISC in Pt(pop) has been studied in detail both experimentally10,11 and theoretically,23 but the mechanism is still open to debate. Herein, we will concentrate on the dramatic ISC slowdown caused by perfluoroboration of the complex. In both Pt(pop) and Pt(pop-BF2), ISC proceeds by a two-channel mechanism, eq. (2): the corresponding kinetics parameters for the two complexes are compared in Table 2.3. 40 41 Fluorescence / counts 10000 Prompt 3.4C 12.0C 20.8C 30.2C 39.4C 49.7C 60.6C 70.9C 8000 6000 4000 2000 0 0 2 4 6 Time / ns 8 10 Figure 2.10. Temperature-dependent fluorescence decay of Pt(pop-BF2) in MeCN solution. Excitation at 373 nm; emission detected at 405 nm 42 2.2 2.0 1/τ [109 s-1] 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.0028 0.0030 0.0032 0.0034 1/T [K-1] 0.0036 0.0038 0.0030 0.0032 0.0034 1/T [K-1] 0.0036 0.0038 12 10 1/φfl 8 6 4 2 0.0028 Figure 2.11. Temperature dependence of Pt(pop-BF2) fluorescence lifetime and quantum yield fitted to equations 2 (red) and 3 (blue), respectively. Data from Tables 2.3 and 2.4. 43 Table 2.3. Temperature dependence of Pt(pop-BF2) fluorescence lifetime. The values were obtained by deconvolution of the TCSPC signal and the actual excitation pulse profile. The nonradiative 1A2u decay rate constants knr were calculated as 1/τ – kr; assuming kr = 1.7×108 s–1. t [°C] T [K] τ1 [ns] τ [ns] A1 [%] A2 [%] knr [108 s–1] 3.4 276.55 0.29 2.20 5 95 2.8 12.0 285.15 0.12 1.86 8 95 3.7 20.8 293.95 0.20 1.56 9 91 4.7 30.2 303.35 0.20 1.28 9 91 6.1 39.4 312.55 0.14 1.03 10 90 8.0 49.7 322.85 0.12 0.81 11 89 10.6 60.6 333.75 0.08 0.63 12 88 14.2 70.9 344.05 0.06 0.50 13 87 18.3 44 Table 2.4. Decay kinetics parameters for the 1A2u excited states of Pt(pop-BF2) and Pt(pop) a Parameter Pt(pop-BF2) Pt(pop)a kr 1.7×108 s–1 2×107 s–1 k0 < 8.5×107 s–1 c 1.5×109 s–1 A 3.0×1014 cm1/2s–1 2.6×1014 cm1/2s–1 Ea 2230 cm–1 1190 cm–1 A/√kB×293 2.1×1013 s–1 1.8×1013 s–1 exp(–Ea/kB×293) 1.749×10–5 2.893×10–3 knr b 4.7×108 s–1 5.4×1010 s–1 kr from reference 12 measured in MeCN. Other parameters from reference 10, measured in a 2-MeTHF/propionitrile mixture. b For Pt(pop-BF2), see Table 2.3. For Pt(pop): calculated from the parameters reported in the Table. c Value corresponds to k0 + kd and thus represents an upper limit of k0. We assume that ISC rates are given by eq. (4), where HSO is the spin-orbit 45 (SO) operator and FC is the Franck-Condon factor that accounts for the thermally weighted overlap between vibrational wavefunctions of the initial and final states. 2 k ISC ∝ 1 A2u H SO 3 A2u FC (4) Temperature-independent 1A2u→3A2u direct tunneling (k0) is more than 18 times slower than that for Pt(pop). Note that direct ISC between states of identical symmetries is allowed only in point groups where one of the rotation components belongs to the totally symmetric representation [it is forbidden in the D4h group of the Pt2(P–O–P)4 core (i.e., 1 A2u H SO 3 A2u = 0 ), but it is allowed in C4h, C2h, or C2]. However, even in D4h, ISC could become partially allowed by SO mixing with higher triplet states33–35 and/or through a spin-vibronic mechanism.23,36–38 In particular, SO coupling between 1A2u and 3Eu LMCT is symmetry allowed, admixing triplet character into 1A2u that then can undergo internal conversion to Eu and/or A1u spin components of the 3A2u state. In both Pt(pop-BF2) and Pt(pop), the higher-lying 3Eu LMCT is the likely coupling state.23 As documented by UV spectra, perfluoroboration raises the energy of LMCT states relative to 1A2u, thereby diminishing the 3A2u–3Eu coupling, which in turn slows ISC (eq. 5). 1 k ISC ∝ A2u H SO 3 Eu 2 E (3 Eu ) − E ( 1 A2u ) FC (5) Irrespective of the details of the indirect spin-orbit interaction, the ISC tunneling pathway is expected to be disfavored in Pt(pop-BF2) because of the smaller FC factor. Since the 1A2u and 3A2u states largely behave as two imbedded 11 Pt–Pt harmonic oscillators, the overlap between their vibrational wavefunctions is small and solvent vibrations gain in importance as energy-accepting modes, as clearly manifested by the strong solvent dependence of the Pt(pop) k0.10 As BF2 groups are much bulkier than –O–H⋅⋅⋅⋅O– units, they will exclude direct interactions between the solvent and terminal P(=O)O groups, as well as with the Pt atoms. Perfluoroboration thus diminishes the ability of the solvent to provide vibrational coupling between the singlet and triplet states, which is required to accept the ~ 5650 cm–1 released during ISC. The thermally activated ISC channel slows 160 times at 293 K on going from Pt(pop) to Pt(pop-BF2) (the activation-energy increases from 1190 to 2230 cm–1, Table 2.3) and is primarily responsible for the lengthened singlet lifetime observed in Pt(pop-BF2). For Pt(pop), the activated pathway may involve thermal population of a higher-lying dσ*→dx2-y2 3B2u state from 1A2u, followed by fast 3B2u decay to 3 A2u.10 However, 3Eu LMCT is a much more likely candidate for the intermediate state, as 3B2u is not coupled to 1A2u by first-order SO (it is at much higher energy than thought previously).21,22,30 Perfluoroboration, which shifts LMCT states to higher energies (Table 2.1), should raise the 1A2u–3Eu barrier, in line with experiment. Although thermal population of an intermediate electronic state is the conventional explanation of T-dependent excited-state nonradiative decay in transition metal complexes, for Pt(pop) and Pt(pop-BF2) it is not likely,33 as we have no evidence for suitable intermediate states. Indeed, a recent ultrafast study of vibrational coherence in the Pt(pop) 1A2u state demonstrated that its potential 46 energy surface has harmonic character, which would suggest the absence of an avoided crossing with any low-lying intermediate state. For the same reason, it would be hard to justify an assumption of strong coupling (i.e., surface crossing) between 1A2u and 3A2u, whose potential energy surfaces appear to be imbedded, owing to the very similar structures of these states. (Note, however, that the Stokes shift is larger for phosphorescence than fluorescence, indicating that 3A2u is more distorted with respect to the ground state than 1A2u.) An alternative way to explain the activated channel is to assume that thermal population of higher levels of certain vibrational modes provide stronger 1 A2u – 3A2u SO coupling than in the ground state. In other words, activated ISC would not occur from the v = 0 level of 1A2u, but from higher levels that are in resonance with target-state (3A2u) vibrational levels. In this scheme, the most efficient ISC activation would be associated with thermal excitation of deformation modes that lower the symmetry to C4h, C2h, or C2, allowing first-order 1A2u–3A2u SO coupling. Since the states involved in ISC are strongly localized along the Pt–Pt bond, it is not expected that symmetry lowering due to changes in the mutual orientation of peripheral BF2 (or, in Pt(pop), –O–H⋅⋅⋅⋅O– groups) would strongly affect the ISC rate. Instead, the distortion would come from excitation of asymmetric vibrations of the Pt2(P–O–P)4 core. Such distortions also could induce mixing between 1,3dσ*pσ and LMCT states, thereby facilitating indirect SO coupling as well. The increase of the apparent activation energy upon perfluoroboration is attributable to the rigidity of covalent –BF2– linkages that 47 increases the energy of activating vibrational modes, so that higher temperatures are required for distortion amplitudes sufficient to induce SO coupling. Long-lived singlet excited states are rare for transition-metal complexes. For example, the 1MLCT states of [MII(bpy)3]2+ (M = Fe, Ru) decay to the corresponding triplets in less than 30 fs,39,40 while 80–150 fs lifetimes have been reported for the singlet excited states of ReI carbonyl-diimine sensitizers.41–43 Besides Pt(pop), ps-lived singlet states were observed at ambient temperatures for d8–d8 complexes of Rh and Ir.6,24,44–46 In particular, [Rh2(1,3-disocyanopropane)4]2+ exhibits room-temperature-solution fluorescence and phosphorescence lifetimes of 1.3 ns and 8.3 µs, respectively, but the fluorescence quantum yield is only 0.07, owing to rapid nonradiative decay to the ground state.45 Other examples of longlived excited singlets include quadruply-bonded metal-metal dimers such as Mo2X4(PR3)4 (X = Cl, Br, I; R = Me, n-Bu) and [Re2X8]2– (X = Cl, Br); these complexes possess 1δδ* states that have 16–140 ns lifetimes, with 3δδ* states that are largely bypassed during 1δδ* decay.47,48 The 1δδ* lifetimes reported for these complexes thus correspond to internal conversion directly to the ground state, the ISC to 3δδ* being much slower, owing to the very large singlet-triplet splitting and vanishing 1δδ*–3δδ* SO coupling. 3δδ* states have been detected only very recently by time-resolved IR spectroscopy of M2(O2CR)4 complexes (M = Mo, W; R=Bu), for which lifetimes of 40 and 50 ps were determined for 1MLCT→3δδ* and 1 δδ*→3δδ* ISC processes.49 And M2(O2CR)4 complexes with electron-accepting substituents R show weakly emissive δ→π*(O2CR) 1MLCT states that decay to the 48 50,51 corresponding nonemissive triplets in 4 to 20 ps. Among mononuclear complexes, picosecond ISC times have been reported for 1MLCT states of flattened-tetrahedral complexes of CuI (13–16 ps)38 and Pt0 (3.2 ps).52 Symmetryforbidden first-order SO coupling between optically excited singlet and lowest triplet states is a common feature for all systems with ps-ns singlet lifetimes. Still, forbidden ISC in metal complexes is many orders of magnitude faster than ISC in organic molecules. Theories of spin-vibronic coupling and ISC rates have been elaborated in detail for organic systems that are characterized by very weak SO and small structural perturbations.23,36–38 Understanding the dynamics of SO-coupled states of metal-containing molecules and developing systems capable of harvesting singlet excitation energy is a major goal of contemporary transition-metal photophysics.33 2.4 Concluding remarks We have shown that modifying all eight P–O–H⋅⋅⋅O–P peripheral sites of Pt(pop) strongly affects its photophysical properties. Pt(pop-BF2), which possesses an unusually long-lived singlet excited state, displays very intense fluorescence and phosphorescence. Excited-state decay of Pt(pop-BF2) is mainly determined by these two radiative pathways along with relatively slow singlet-to-triplet ISC. Nonradiative decay to the ground state is far less important. We can conclude that the dramatic ISC slowing relative to Pt(pop) is attributable to a combination of three effects: (i) the electron-withdrawing power of BF2, which raises pop→pσ LMCT relative to dσ*→pσ state energies and diminishes indirect SO coupling; (ii) 49 a much more rigid structure, owing to covalent –BF2– bridges linking the pop ligands; and (iii) steric protection by BF2 groups, which shield the Pt2(P–O–P)4 core from solvent. The distinctive photophysical features of Pt(pop-BF2) provide a unique opportunity to investigate independently the structures, dynamics, and reactivities of both singlet and triplet dσ*→pσ excited states in the same molecule. We expect that long-lived vibrational coherence will be observed for the singlet state, and that excited-state reactivity will reflect the asymptotic zwitterionic character of the singlet53,54 and the radical-like behavior of the triplet.2,3,5,13,14 2.5 References (1) Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55. (2) Smith, D. C.; Gray, H. B. Coord. Chem. Rev. 1990, 100, 169. (3) Smith, D. C.; Gray, H. B. In ACS Symposium Series 394. The Challenge of d and f Electrons.; Salahub, D. R., Zerner, M. C., Eds. American Chemical Society, Washington, DC, 1989, p. 356. (4) Marshall, J. L.; Stiegman, A. E.; Gray, H. B. In Excited States and Reactive Intermediates. ACS Symposium Series.; Lever, A. B. P., Ed. 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F.; Gray, H. B. J. Phys. Chem. 1993, 97, 4277. (46) Marshall, J. L.; Stobart, S. R.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 3027. (47) Miskowski, V. M.; Goldbeck, R. A.; Kliger, D. S.; Gray, H. B. Inorg. Chem. 1979, 18, 86. (48) Hopkins, M. D.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 2468. (49) Alberding, B. G.; Chisholm, M. H.; Gustafson, T. L. Inorg. Chem. 2012, 51, 491−498. (50) Byrnes, M. J.; Chisholm, M. H.; Gallucci, J. A.; Liu, Y.; Ramnauth, R.; Turro, C. J. Am. Chem. Soc. 2005, 127, 17343. (51) Alberding, B. G.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.; Gustafson, T. L. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 8152. (52) Siddique, Z. A.; Ohno, T.; Nozaki, K. Inorg. Chem. 2004, 43, 663. (53) Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, 705. (54) Benard, M.; Veillard, A. 1982, 90, 160. 52 53 Chapter 3: Electronic structures of palladium(II) dimers The text in this chapter was taken in part from: Bercaw, J. E.; Durrell, A. C.; Gray, H. B.; Green, J. C.; Hazari, N.; Labinger, J. A.; Winkler, J. R. Inorg. Chem. 2010, 49, 1801–1810. 54 3.1 Abstract The PdII dimers [(2-phenylpyridine)Pd(µ-X)]2 and [(2-p-tolylpyridine)Pd(µX)]2 (X = OAc or TFA) do not exhibit the expected planar geometry (of approximate D2h symmetry), but instead resemble an open “clamshell” in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2arylpyridine ligands, with the Pd atoms brought quite close to one another (approximate distance 2.85 Å). The molecules adopt this unusual geometry in part because of a d8-d8 bonding interaction between the two Pd centers. The Pd–Pd dimers exhibit two successive one-electron oxidations: PdII–PdII to PdII–PdIII to PdIII–PdIII. Photophysical measurements reveal clear differences in the UV-visible and low-temperature fluorescence spectra between the clamshell dimers and related planar dimeric [(2-phenylpyridine)Pd(µ-Cl)]2 and monomeric [(2- phenylpyridine)Pd(en)][Cl] (en = ethylenediamine) complexes that do not have any close Pd–Pd contacts. DFT and AIM analyses confirm the presence of a Pd–Pd bonding interaction in [(2-phenylpyridine)Pd(µ-X)]2 and show that the HOMO is a dz2 σ*Pd–Pd antibonding orbital, while the LUMO and close-lying empty orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other PdII–PdII dimers that have short Pd–Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(µ-X)]2, but quite different from that found for d8–d8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd, and may explain why Pd d8-d8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species. 3.2 Introduction Complexes containing metal-metal bonds play an important role in both inorganic and organometallic chemistry.1 Among the many such complexes, dimers of square planar RhI, IrI and PtII centers are members of a special class that feature attractive d8–d8 interactions, as documented by optical and vibrational spectroscopy,2 x-ray crystallography,3 EXAFS4 and ab initio electronic structure theory.5 Overlap in the axial direction between the valence dz2 orbitals of the square planar metal centers results in both filled bonding (dσ) and antibonding (dσ*) orbitals; however, there is a net overall bonding interaction due to symmetryallowed mixing with the (n+1) metal s and pz orbitals. This creates four key orbitals (labeled according to their principal atomic character) in the following order of increasing energy: one strongly bonding (dσ), one weakly antibonding (dσ*), one weakly bonding (pσ), and one strongly antibonding (pσ*); only the first two are filled (Figure 3.1). Such d8–d8 interactions most commonly lead to discrete dimers or trimers, although in some cases higher-order oligomers are formed. 55 Energy 56 Ground State: (dσ*)2(pσ)0 Excited State: (dσ*)1(pσ)1 pσ* pσ* pz pz pz pz pσ pσ dσ* 12 electrons dz2 dz2 dσ* 12 electrons dz2 dxz,dxy,dyz dσ M M-M M-M Bond Order > 0 dz2 dxz,dxy,dyz dσ M M M-M M M-M Bond Order = 1 Figure 3.1: Orbital energy level diagram for interaction along the metal–metal axis of two d8 square-planar units, in both ground and excited states In 1975, Mann et al. reported the electronic spectroscopic characterization of [Rh(CNPh)4]nn+ (n = 1, 2, or 3) in acetonitrile solution and proposed that aggregation occurred through a direct Rh–Rh interaction.2a Since this initial discovery, many d8–d8 dimers of RhI, IrI, and PtII have been found to exhibit a rich diversity of reactions resulting from photoexcitation and the accompanying increase in metal–metal bond order. A particularly widely studied d8–d8 dimer is Pt2(µP2O5H2)44- (Pt-pop),6 which exhibits electronic absorption bands at 367 and 435 nm, assigned to singlet and triplet dσ* → pσ transitions, respectively, as well as 514 nm phosphorescence (τ ~ 9 µs at ambient temperature) from the lowest excited triplet and 407 nm fluorescence from a much shorter-lived (8–40 ps) excited singlet.7 When irradiated at 367 nm, Pt-pop exhibits very rich chemistry, including: electron transfer reactions with various substrates; hydrogen atom abstraction from hydrocarbons, alcohols, stannanes, and silanes; halogen atom abstraction from alkyl and aryl chlorides; duplex DNA cleavage; and catalytic conversion of isopropanol to acetone and hydrogen.6,8 In contrast to well-studied Rh, Ir, and Pt systems, there has been little research into Pd dimers that may experience d8–d8 interactions, and even fewer investigations of their photophysical properties.9 Several complexes have been reported with Pd–Pd distances ranging from 2.55–3.05 Å,9 shorter than the sum of the van der Waals radii (3.26 Å);10 but there has been no agreement on whether a metal–metal bond is present. The limited photophysical investigations have been inconclusive; a weak Pd–Pd bonding interaction in the model compounds [Pd(CN2)(PH3)2]2 (Pd–Pd = 3.107 Å) and [Pd(CN)2(µ-PH2CH2PH2)]2 (Pd–Pd = 57 9b,9c 3.020 Å) was suggested based on MP2 and TDDFT calculations. Although SCF-Xα-SW calculations on the model compound [Pd(HNCHNH)2]2 predicted an even shorter Pd–Pd distance (2.622 Å), it was concluded that there was no bonding interaction.11 As part of an investigation into selective oxidation catalysts,12 we have studied several members of a family of [(2-arylpyridine)PdII(O2CCX3)]2 dimers; notably, there has been considerable recent interest in these and related species as precursors to PdIII-PdIII or PdII-PdIV dimers that are catalytically active for C–C bond forming reactions.13 Here we present spectroscopic data combined with theoretical analyses, using both DFT and Atoms in Molecules (AIM) methods, that strongly support the presence of weakly bonding d8–d8 interactions in these dimers. We extend our results to a number of other PdII species with short Pd–Pd distances, and demonstrate that the ordering of unoccupied molecular orbitals differs from that found for Rh, Ir, and Pt dimers containing d8–d8 interactions, thereby accounting for striking differences in photophysical properties and reactivities. 3.3 Results and discussion Synthesis and solid-state structures The structure of [(2-phenylpyridine)Pd(µ-OAc)]2 (1), which also has been reported elsewhere,14 does not exhibit the expected planar geometry (of approximate D2h symmetry), but instead resembles an open “clamshell” in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2-phenylpyridine ligands, with the Pd atoms brought quite close to one another, 2.86216(11) Å.15 We also synthesized the known compound [(2-p-tolylpyridine)Pd(µ-OAc)]2 (2), and 58 two closely related species, [(2-phenylpyridine)Pd(µ-TFA)]2 (3) and [(2-ptolylpyridine)Pd(µ-TFA)]2 (4) using a modification of a literature procedure.16 The structures of 1–4 are closely related: all four molecules adopt the clamshell geometry (Figure 3.2 and Table 3.1), a consequence, we suggest, of both an attractive d8–d8 interaction between the two Pd centers and a parallel displaced (slipped) π-stacking interaction between the phenylpyridine rings.17 In all structures the distances between the ring centroids vary from 3.70 Å to over 4 Å, and the angles between the ring normal of the plane and the centroid vector are between 20° and 40°, consistent with the approximate upper limits for significant interaction;17 π-stacking between the phenylpyridine rings presumably contributes to the short Pd–Pd distance, but we believe that the d8–d8 interaction plays the main role in determining the overall geometry, because in principle the molecule could π-stack just as effectively if it were planar. The structures of the TFA bridged species 3 and 4 both contain 3 independent molecules, with similar bond distances and angles in the asymmetric unit, while there is disorder associated with the methyl groups of the 2-p-tolylpyridine ligands in 2 and 4. The Pd–Pd distance decreases by approximately 0.01 Å when moving from phenylpyridine to the more electron donating 2-p-tolylpyridine, while moving from OAc to the less-electron-donating TFA increases the distance by 0.01 Å. An interesting feature of the molecules is the relative orientation of the 2arylpyridine rings, which may either be “cis”, with both pyridine rings opposite one other (approximate Cs symmetry), or “trans”, with each pyridine ring opposite a benzene ring (approximate C2 symmetry). In 1 and 3 it is impossible to distinguish 59 between all-cis or a 50%-50% mixture of cis and trans (for which crystallographic averaging would give the observed structure). However, the structure of 2, though disordered, shows a preference for the trans orientation, with only approximately 15% cis. The structure of 4 is even more highly disordered, but both orientations can be observed, again with a preference for trans. In order to assess the presence of Pd–Pd bonding interactions by photophysics, we also examined control samples. The structures of the previously reported18 complexes [(2-phenylpyridine)Pd(µ-Cl)]2 (5) and [(2- phenylpyridine)Pd(en)][Cl] (6, en = ethylenediamine) indicate the absence of any Pd–Pd interaction. 5 has a nearly flat structure, in contrast to 1–4, with the bridging chlorides almost coplanar with the 2-phenylpyridine ligands (Figure 3.3). The dimeric units are stacked, but the closest Pd–Pd contact is 3.691 Å, which is significantly larger than the sum of the van der Waals radii of two Pd atoms. 6 exhibits normal square planar coordination geometry (Figure 3.4); the closest intermolecular Pd–Pd distance is greater than 5 Å. 60 61 (a) (b) Pd–Pd = 2.86216(11) (c) Pd–Pd = 2.8701(4), 2.8963(4), 2.8656(4) Pd–Pd = 2.8567(2) (d) Pd–Pd = 2.8589(12), 2.8801(12), 2.8546(13) Figure 3.2. The structures of [(2-phenylpyridine)Pd(µ-OAc)]2 (1) (a); [(2-ptolylpyridine)Pd(µ-OAc)]2 (2) (only major conformation drawn) (b); [(2phenylpyridine)Pd(µ-TFA)]2 (3) (c); and [(2-p-tolylpyridine)Pd(µ-TFA)]2 (4) (only major conformation drawn) (d). All H atoms were omitted for clarity. 62 Table 3.1. Selected bond distances in 1–4 Pd(1)–C(11) Pd(1)–N(1) Pd(1)–O(3) Pd(1)–O(1) Pd(1)–Pd(2) Pd(2)–C(22) Pd(2)–N(2) Pd(2)–O(2) Pd(2)–O(4) a 1 2 3a 4a 1.9566(8) 1.956(2) 1.969(3), 1.958(3), 1.946(5), 1.925(7), 1.960(4) 1.897(15) 2.008(3), 2.009(3), 1.982(6), 1.974(6), 2.005(3) 2.037(13) 2.093(2), 2.111(3), 2.062(9), 2.081(7), 2.096(2) 2.088(8) 2.171(2), 2.152(2), 2.172(8), 2.120(7), 2.184(3) 2.132(8) 2.8701(4), 2.8589(12), 2.8963(4), 2.8801(12), 2.8656(4) 2.8546(13) 1.960(4), 1.964(4), 1.792(9), 1.974(6), 1.976(4) 1.894(6) 2.000(3), 1.964(4), 2.195(10), 1.999(6), 1.998(4) 2.027(6) 2.077(3), 2.097(3), 2.088(11), 2.106(8), 2.074(3) 2.096(8) 2.163(2), 2.133(3), 2.117(10), 2.157(7), 2.156(2) 2.145(9) 2.0116(6) 2.0473(5) 2.1428(6) 2.86216(11) 1.9616(7) 2.0052(6) 2.0520(6) 2.1562(5) 2.0161(17) 2.0474(14) 2.1378(16) 2.8567(2) 1.937(2) 2.0590(16) 2.0636(17) 2.1407(15) Three independent conformations 63 Figure 3.3. The structure of [(2-phenylpyridine)Pd(µ-Cl)]2 (5). All H atoms were omitted for clarity. Selected bond distances (Å) of 5: Pd(1)–C(11) 1.9865(10), Pd(1)–N(1) 2.0134(9), Pd(1)–Cl(2) 2.3633(3), Pd(1)–Cl(1) 2.4233(3), Pd(2)–C(22) 1.9869(10), Pd(2)–N(2) 2.0146(10), Pd(2)–Cl(2) 2.3725(3), Pd(2)–Cl(1) 2.4271(3). 64 Figure 3.4. The structure of [(2-phenylpyridine)Pd(en)][Cl] (6). All H atoms and the Cl- anion were omitted for clarity. Selected bond distances (Å) of 6: Pd(1)– C(11) 1.9849(12), Pd(1)–N(1) 2.0195(9), Pd(1)–N(2) 2.0652(10), Pd(1)–N(3) 2.1443(11). 65 Electrochemistry Cyclic voltammograms of 1–4 in CH2Cl2 are shown in Figure 3.5; differential pulse voltammetry (Figure 3.6) was used to further resolve the electrochemical features (Table 3.2). Compounds 1 and 2 show one reversible peak (390 and 350 mV, respectively) assigned to PdIII–PdII / PdII–PdII and one irreversible peak (710 and 740 mV) assigned as the PdIII–PdIII / PdII–PdIII couple. The decreased area under the second peak is attributable to disproportionation of the PdII–PdIII mixed-valence state: 2 PdII–PdIII → PdII–PdII + PdIII–PdIII. Such processes also occur in electrochemically generated RhI–RhII systems.19 Irreversible oxidation of the ligand occurs at anodic potentials greater than 1 V (Figure C.1). The PdIII–PdII / PdII–PdII couple is shifted anodically by 300 mV in 3 and 4 and a second Pd–Pd redox event is not observed before the onset of ligand oxidation. We believe that the shift to higher potentials, seen as the bridging ligand is changed from acetate to trifluoroacetate, reflects increased withdrawal of electron density from Pd2, stabilizing the HOMO and rendering oxidation more difficult. The potentials of PdII–PdIII / PdII–PdII as well as PdIII–PdIII / PdII–PdIII couples in 1 and 2 are comparable with those reported recently for a binuclear palladium benzoquinoline acetate complex.13a 66 Figure 3.5. Cyclic voltammograms of 1–4 in CH2Cl2 solution 67 Figure 3.6. Differential pulse voltammograms of compounds 1–4 in CH2Cl2 solution 68 Table 3.2. Reduction potentials for compounds 1–4 (mV vs. Fc+ / Fc) from DPV experiments in CH2Cl2 Compound PdII-PdIII / PdII-PdII PdIIIPdIII / PdII-PdIII 1 390 710 2 350 680 3 660 Not observed 4 660 Not observed A CV for 5 could not be obtained in CH2Cl2 owing to solubility problems; however, measurements made in acetonitrile reveal a single irreversible oxidation near 820 mV, assigned to PdII–PdIII / PdII–PdII (Figure C.2). The CVs for 1–4 in acetonitrile are surprisingly different from those reported in CH2Cl2 (Table 3). The potential associated with the first oxidation depends strongly on the identity of the bridging ligand, ranging from 380 mV (vs. Fc+ / Fc) for acetate-bridged 2 to 1180 mV for TFA-bridged 4, a difference that is much larger than we would have expected. Of greater interest is that the experiments in acetonitrile show clearly that 1 and 2 are more easily oxidized than 5, which is consistent with our electronic structural model: the HOMO is Pd–Pd antibonding in 1 and 2; it is nonbonding in 5. DFT calculations (see below and Supporting Information for details) are consistent with the experimentally determined reduction potentials. Unfortunately, reliable electrochemical data were not obtained for 6, owing to its low solubility in both acetonitrile and CH2Cl2. 69 70 Table 3.3: Reduction potentials for compounds 1–5 (mV vs Fc+/Fc) from DPV experiments in CH3CN Compound PdII–PdIII/PdII–PdII PdIII–PdIII / PdII–PdIII 1 430 1400 2 380 1350 3 1160 1400 4 1180 1370 5 820 1340 Photophysics UV-vis absorption spectra of 1–6 (Figure 3.7) follow Beer’s Law, indicating that aggregration does not occur at concentrations below 200 μM. Intense absorption bands are observed at wavelengths shorter than 350 nm, with molar absorptivities greater than 5000 M-1cm-1 (Table 3.4). Compounds 1 and 2 exhibit weaker absorptions with maxima at 420 nm (ε = 2000 M-1cm-1), whereas absorptions of 3 and 4 in the visible region are broad and featureless. The absorption profiles of 5 and 6 are very similar to those of 1–4 in the UV region, but not at wavelengths greater than 380 nm. The higher energy bands are attributable to [π–π*] intraligand (IL) transitions, and the visible absorption peaks in the spectra of 1 and 2 are assigned to [dσ*(Pd2)–π*(ppy)] metal–metal-to-ligand charge transfer (MMLCT). MMLCT transitions of 3 and 4 are observed as absorptions at higher energy that tail into the visible region. The blue shift is attributable to electron withdrawal from Pd2 to the bridging TFA ligands, which stabilizes the dσ*(Pd2) HOMO. The electronic spectrum of 5, where the Pd–Pd distance is greater than 3.6 Å, is very similar to that of monometallic 6. 71 72 Figure 3.7. Electronic absorption spectra of 1–6 in 2-MeTHF solutions Luminescence was not detected from any of the compounds 1–6 at room temperature; however, all were found to luminesce upon cooling to cryogenic temperatures. Excitation (355 nm) of samples in 2-methyltetrahydrofuran (2MTHF) glasses at 77 K reveals richly structured emission features in the 450–600 nm region. Notably, there is a sharp peak at 461 nm in the emission spectrum of 1 (Figure 3.8), and the spectra of 3, 5, and 6 are similar; the sharp feature is red shifted by 5 nm in 2 and 4 (Table 3.4). The 450–600 nm emission system in each case is likely attributable to an electronic transition that involves a mixture of ligand-centered [π–π*] and metal-to-ligand charge transfer (MLCT) components. Vibronic peaks spaced by 1200 cm-1 indicate that this transition is coupled to aromatic ring-breathing modes. Additional structure on these features with approximately 400 cm-1 spacings suggests that there also is excitation of ground tate Pd–L stretching vibrations. Importantly, the 1–4 450–600 nm emission profiles are similar to those of monomeric metal–phenylpyridine complexes. 73 74 Figure 3.8. Emission spectrum of 1 in a 2-MeTHF glass at 77 K (λex = 355 nm) 75 Table 3.4. Absorption and emission data Absorption Emission a Compound λmax /nm (εmax/M-1cm-1) λem /nm (τo /μs) 1 305 (10300), 315 (8100), 350 (5900), 461 (180 (70%), 70 (30%)), 410 (1800) 740 (150 (50%), 60 (50%)) 309 (10000), 322 (8500), 347 (7200), 466 (240 (70%), 110 (30%)), 373 (5000), 410 (2300) 740 (65 (40%), 12 (60%)) 307 (11800), 317 (11400), 343 (7300), 461 (290 (70%), 80 (30%)), 362 (5400), 400 (2300) 740 (50 (20%), 10 (80%)) 313 (11700), 322 (12200), 347 (8800), 466 (250 (80%), 50 (20%)), 370 (4000), 400 (1900) 740 (60 (30%), 10 (70%)) 5 307 (13800), 317 (14700), 360 (4500) 461 (320) 6 305(4100), 317 (4400), 338 (3300) 461 (350) 2 3 4 a Measurements were made on samples in 2-MTHF glasses at 77 K (λex = 355 nm). Emission spectra of complexes 1–4 (which have short Pd–Pd distances) also exhibit a broad system that peaks at 740 nm. This low-energy band (without any hint of vibronic structure at 77 K) is not observed in the spectra of monomeric phenylpyridine complexes (e.g., 6) or the phenylpyridine chloride dimer 5. We assign this feature to an MMLCT transition, in accord with our calculations of the relative energies of dσ*(Pd2) HOMO and ligand π* LUMO levels (see below). Upon changing the excitation wavelength to 420 nm, the 460 nm emission quantum yields of 1–4 decrease markedly, but the 740 nm emission intensity is largely unaffected. On varying the excitation wavelength from 300 to 420 nm and monitoring luminescence of 1 and 2 at 460 nm, we found that the absorption and excitation spectra are similar between 300 and 380 nm, but not from 380 to 420 nm, a region in which the emission intensity is extremely weak. When a similar experiment was conducted while monitoring emission at 790 nm, the excitation spectrum followed absorbance through the visible region (Figure 3.9). We therefore suggest that there are two competitive pathways for deactivation of the higherenergy IL (π–π*) excited state: one is direct radiative decay to the ground state, and the other is relatively slow internal conversion to the MMLCT excited state. 76 77 Figure 3.9. Absorption and excitation spectra of 2 in 2-MeTHF at 77 K Measurements of radiative decay kinetics have shed light on the nature of the excited state dynamics. With excitation at 355 nm, decays from 1–4 monitored at both 460 and 720 nm are biexponential, with lifetimes in microsecond ranges. In contrast, emission decay from electronic excitation of the chloro-bridged species 5 is monoexponential, with a lifetime consistent with those determined at 77 K for other M(ppy) complexes.20 All emission spectra recorded at 460 nm also exhibited a short-lived fluorescence component.21 We tentatively suggest that the biexponential kinetics associated with 1–4 emissions are attributable to electronically excited stereoisomers (cis- or trans-N-Pd-Pd-N geometry) in the 77 K glasses.22 Density functional theory analysis of bonding in compounds 1–5 DFT studies were performed on 1 (with no symmetry constraints, which also serves as a model for 2), 3 (with no symmetry constraints, which also serves as a model for 4), and 5 (modeled using C2h symmetry). In general there was good agreement between experimental and calculated structures, which suggests that our model is able to reproduce key structural parameters.23 The calculations on 1 and 3 show that the HOMO is a dz2 σ*Pd–Pd antibonding orbital, while the LUMO and close-lying empty orbitals are located on the 2-phenylpyridine rings and possess mainly ligand character. The Pd–Pd σ-bonding character (a mixture of both dz2 and dxy) is spread over two MOs (91 and 93) and mixes with both the bridging acetate and 2-phenylpyridine ligands. As expected, there are a number of occupied nonbonding metal orbitals and ligand-based orbitals between the HOMO and the 78 Pd–Pd bonding orbitals. Selected molecular orbitals of 1, with the percentage contributions from different Pd atomic orbitals, are shown in Figure 3.10. A fragment analysis was performed in which 1 was broken into Pd2, (phenylpyridine)2 and (OAc)2 units; the resulting molecular orbital diagram is shown in Figure 3.11. The analysis confirms that the LUMO and slightly higher energy orbitals are based on the 2-phenylpyridine ligands. Importantly, it shows that although the HOMO is predominantly formed from the Pd dz2 orbitals, there also are small contributions from the 5s and 5pz orbitals due to symmetry-allowed mixing (the HOMO in Figure 3.10 is a linear combination of the 5s and 5pz orbitals; they also make small contributions to the 3σg, which predominantly has dz2 character). Similar mixing occurs in the orbitals with Pd–Pd bonding character (91 and 93), although the contribution from the 5s and 5pz orbitals is smaller due to the greater disparity in orbital energies. The overall effect of this mixing is to increase overlap in the bonding interaction, while decreasing overlap in the antibonding interaction; a Pd–Pd bond order of 0.11 was calculated for 1.24 This is consistent with a weak bonding interaction and suggests the presence of dz2, pz, and s orbital mixing in 1–4, similar to that described earlier for Pt-pop. The AIM approach,25 which uses topological analysis of the electron distribution to characterize bonding interactions, confirms the presence of metal–metal bonding interactions in 1 and 3. In both cases (3, -1), bond-critical points (indicative of the presence of a bonding interaction)26 centered between the two Pd atoms were found. The ellipticity values were 0.004 and 0.003, respectively, indicating a bonding interaction. 79 Frequency calculations on 1 and 3 gave Pd–Pd symmetric and asymmetric stretches around 120–130 cm-1, both heavily coupled to other vibrations such as ring breathing modes. We were unable to obtain direct experimental evidence for a Pd–Pd bonding interaction from polarized single-crystal Raman spectroscopy or low temperature UV-vis spectroscopy, presumably because of this coupling to other bonding modes and the absence of a well-defined uncoupled dz2 σ*-to-pσ transition. In contrast, a highly polarized Pt–Pt stretch has been observed at 118 cm-1 in the Raman spectrum of Pt-pop.27 Calculations on planar [(2-phenylpyridine)Pd(µ-Cl)]2 (5) showed no overlap between the valence dz2 orbitals on the two Pd atoms and hence no Pd–Pd bonding interaction. Instead, there are two essentially degenerate nonbonding linear combinations of the dz2 orbitals, which are of a similar energy to the nonbonding linear combinations of other Pd d orbitals. As in 1 and 3, the LUMO and closelying empty orbitals of 5 are centered on the 2-phenylpyridine ligands. The HOMO–LUMO gap in 5 (2.40 eV) is much larger than in 1 (1.53 eV) or 3 (1.60 eV) because the HOMO in 5 is nonbonding, whereas in 1 and 3 it is an antibonding dz2 Pd–Pd interaction raised in energy relative to the π* system of the 2phenylpyridine ligands. The larger HOMO–LUMO gap in 5 compared with 1 and 3 is consistent with the UV-visible spectroscopy, which showed bands at lower energy in 1–4 than in 5, providing further evidence supporting our assignment of the bands at approximately 420 nm in 1 and 2 as MMLCT transitions (excitation from the Pd–Pd antibonding orbital into the 2-phenylpyridine π* system); these bands are shifted to around 375 nm in 3 and 4 and absent in 5. Furthermore, 80 imposition of a C2 symmetry constraint allowed optimization of the structure 1 in the excited configuration, with one electron promoted from the HOMO to the LUMO; the resulting Pd–Pd distance is 2.6 Å, consistent with an increase in the Pd–Pd bond order and a large geometry change between the ground (calculated Pd– Pd distance = 2.8 Å) and excited states, which may explain the large Stokes shift observed in the emission from 1 on excitation at 420 nm. 81 82 47% 4d 4% 5s 53% 4d 3% 5s Pd-Pd Bonding (93) Pd-Pd Bonding (91) 66% 4d 8% 5s 5% 5p HOMO LUMO LUMO+1 LUMO+2 Figure 3.10. Selected molecular orbitals of 1, with percentage contribution from different Pd atomic orbitals (balance from ligand-based orbitals) 83 Figure 3.11. Molecular orbital diagram for 1 84 DFT-based comparison of bonding interactions in Pd–Pd d8 dimers with those in Rh–Rh, Pt–Pt, and Ir–Ir d8 dimers The results of the spectroscopic and computational studies on compounds 1–4 suggest that the Pd–Pd bonding interaction is different from the d8–d8 interactions in Rh, Pt, and Ir dimers. In the latter systems the LUMO is a metal–metal bonding orbital with s and pz character, whereas in 1–4 the LUMO and other close-lying unoccupied orbitals are ligand based. Indeed, it appears that the energy of the metal–metal bonding pz orbital in 1–4 is much higher than the LUMO, which may explain why the unique photophysical properties arising from a dz2 σ*-to-pσ transition in Pt-pop and other d8–d8 dimers are not observed in the Pd dimers. To explore whether the absence of a low-energy dz2 σ*-to-pσ transition is general to Pd complexes with short Pd–Pd interactions or is unique to 1–4, we performed calculations on the complexes cited earlier,9 shown as (a–h) in Figure 3.12, as well as on Pt-pop (Pt–Pt = 2.925(1)),21,28 [(cod)Ir(µ-pyr)]2 (j, cod = 1,5-cyclooctadiene, pyr = pyrazole, Ir–Ir = 3.216(5) Å),29 and [Rh(CNMe)4]22+ (k) as a model for [Rh(CNPh)4]22+ (Rh–Rh = 3.193(0)Å)30 (Figure 3.13). 85 Me N Pd Me N Pd O N N O O Ph Me O N N PhN NPh Pd Me N Ph N NPh N N H C tolN H C Ntol tolN tol N Ntol Bz S tol N Pd Pd S Cl N Pd O O N Cl S z B S S S N Pd N Pd S S S O Pd NC NC O O Pd O S h N f Pd NC Cy2 P PCy2 Me Me Pd g N Bz S S S S S Ph e S Me N Pd c S S Me Cl Pd S Bz C H d N O O S Pd C H N b Pd tol Cl N N tol Ph Pd PhN a N Pd NC i PCy2 P Cy2 Figure 3.12. Complexes with short Pd–Pd interactions selected for DFT calculations9 86 O O P P P Pt 4- P Pt P P P O N (cod)Ir N (cod)Ir MeNC MeNC MeNC O P = P(OH)O- cod = 1,5-cyclooctadiene MeNC Pt-pop j CNMe CNMe N N P Rh Rh CNMe CNMe k Figure 3.13: Complexes with d8–d8 bonding interactions selected for DFT calculations For all the Pd complexes the calculated HOMO is an antibonding combination of the dz2 orbitals, with a small contribution from the 5s and 5pz orbitals, similar to that described earlier for 1.23 The complementary Pd–Pd σbonding orbital is at lower energy and also has some contribution from the 5s and 5pz orbitals. For all the compounds fragment analysis reveals a favorable bonding interaction, with the bond order varying from 0.1–0.56 (this is a measure of positive orbital overlap rather than bond strength). Additional support for the presence of a Pd–Pd bonding interaction was obtained from AIM analysis: there is an excellent correlation between the Pd distance and both the charge density and the Laplacian at the bond critical points (Figure 3.14), suggesting that the Pd–Pd bonding interaction is related to the separation between the Pd atoms. 87 88 Figure 3.14. Plot of charge density and Laplacian against M-M distance in compounds 1 and (a–h) The calculations also showed that the LUMO and close-lying empty orbitals in the Pd complexes are not based on a bonding linear combination of the s and pz orbitals. Instead, the LUMO consists either of ligand-based orbitals or, when there are no low-lying unoccupied ligand orbitals, an antibonding combination of Pd dx223 y2 and ligand orbitals. In contrast, calculations on Pt-pop and [(cod)Ir(µ-pyr)]2 (j) showed that the LUMO is a metal-based bonding orbital, which contains both s and pz character.31 These results suggest that the larger gap between the (n+1) valence s and p orbitals and the valence d orbitals for Pd compared with Ir or Pt is crucial in determining the relative energy of the unoccupied orbitals in d8–d8 dimers. Even in [Rh(CNMe)4]22+ (k), the LUMO contains some s and p character mixed with a large ligand contribution, suggesting that the gap between Pd 5s and 5p orbitals and the 4d orbitals is large even compared to other elements in the same row. These trends are reflected in the atomic spectra of Rh, Pd, Ir, and Pt (Table 3.5); the lowest Pd dto-p transition is nearly 10,000 cm-1 higher than a corresponding transition in the other metals.32 The ground-state atomic configuration of Pd (4d10) is also consistent with a larger gap between d and s and p orbitals relative to Rh (4d8, 5s1), Ir (5d7, 6s2), and Pt (5d9, 6s1). 89 90 Table 3.5. Energy of the nd → (n+1)p transition as calculated from atomic spectra32 Metal nd  (n+1)p / cm-1 Transition Rh 23766 4d9 (2D)  4d8 (3F) 5p (4Do) Pd 34068 4d10 (1S)  4d9 (2D2 ½) 5p (3P) Ir 23473 5d8 6s (4F)  5d7 6s (5F) 6p (6Do) Pt 26480 5d10 (1S)  5d9 (2D2 ½) 6p0 ½ Since many of the interesting photophysical and reactivity properties of Ptpop, Rh–Rh, and Ir–Ir d8–d8 dimers appear to result from the special nature of the LUMO, we suggest that Pd complexes that display similar behavior may not be attainable, and that the M–M bonding interaction is weaker for Pd because the larger energy difference between the (n+1) valence s and p orbitals and the valence d orbitals disfavors orbital mixing. This view is supported by the relative contributions of the (n+1) valence s and p orbitals to the σ* HOMO and corresponding metal–metal σ-bonding orbital for 1 compared to Pt-pop, [(cod)Ir(µpyr)]2 (j) and [Rh(CNMe)4]22+ (k) (Table 3.6). At present there is no direct experimental data on metal–metal bond strengths that would validate this proposal. 91 92 Table 3.6. Relative contribution of the d orbitals and (n+1) valence s and p orbitals to the metal-metal bonding and antibonding orbitals in 1, Pt-pop, [Rh(CNMe)4]22+ (k), and [(cod)Ir(µ-pyr)]2 (j)a 1 M–M antibonding M–M bonding M–M bonding 66% 4d 47% 4d 53% 4d 8% 5s 4% 5s 3% 5s 48% 4d 79% 4d 70% 4d 29% 5s 6% 5s 2% 5s 33% 5d 56% 5d 52% 5d 50% 6s 23% 6s 7% 6s 3% 6p 1% 6p 69% 5db 26% 5d 67% 5db 10% 6s 20% 6s 7% 6s 1% 6p 1% 6p 5% 5p [Rh(CNMe)4]22+ (k) 2% 5p Pt-pop [(cod)Ir(µ-pyr)]2 (j) a Where numbers to do not add up to 100%, the remainder of the orbital is centered on the ligand. bThe relative d contribution to these orbitals is raised by mixing with a number of d orbitals other than the dz2. 93 3.4 Concluding remarks Crystal structure analyses combined with photophysical, electrochemical and computational investigations have firmly established that there are attractive d8–d8 interactions in PdII dimers of the type [(2-phenylpyridine)Pd(µ -X)]2 and [(2p-tolylpyridine)Pd(µ-X)]2 (X = OAc or TFA). The Pd–Pd HOMO is a weakly metal–metal antibonding dz2 σ* orbital and the LUMO is either a low-lying ligand orbital or a dx2-y2 antibonding orbital. The M–M interaction in Pd d8–d8 dimers is weaker than in Rh, Ir, and Pt systems. The Pd-Pd LUMO also differs from those of d8–d8 complexes of Rh, Ir, and Pt, owing to the unusually large energy gap between Pd 4dz2 and 5s/5p orbitals. Since the increase in Pd–Pd bond order upon HOMO– LUMO excitation is only one-half that of other metal d8–d8 complexes, it is not likely that electronically excited PdII dimers will be able to abstract hydrogen or halogen atoms from substrates as efficiently as PtII analogues. 3.5 Materials and methods General All manipulations were performed in air, unless otherwise stated. Pd(OAc)2, Pd(TFA)2, 2-phenylpyridine (all purchased from Sigma-Aldrich) and 2-p-tolyl-pyridine (purchased from TCI-America) were reagent-grade commercial samples used without further purification. Dichloromethane-d2 and chloroform-d1 were purchased from Cambridge Isotope Laboratories and used as received. [(2phenylpyridine)Pd(OAc)]2 (1), [(2-p-tolylpyridine)Pd(OAc)]2 (2) [(2- phenylpyridine)Pd(Cl)]2 (5), and [(2-phenylpyridine)Pd(en)][Cl] (6) were 16,18,33 1 prepared using literature procedures. H and 13 C NMR spectra were recorded at 298 K using a Varian Mercury 300 MHz spectrometer equipped with the VNMRJ software program, version 2.2d. 1H and 13C NMR spectra were referenced to the residual proton or carbon chemical shifts of the deuterated solvent. The data are reported by chemical shift (ppm) from tetramethylsilane, multiplicity (s, singlet; d, doublet; t, triplet; m, multiplet; dd, double doublet; dt, double triplet), coupling constants (Hz), and integration. Mass spectra were acquired on a Finnigan LCQ ion trap or Agilent 5973 Network mass selective detector and were obtained by peak matching. UV/Vis absorbance spectra were recorded on an Agilent 8453 UV/Vis Spectrometer using a pure sample of the solvent as the background. X-ray crystallographic data were collected on a Bruker KAPPA APEX II instrument, with the crystals mounted on a glass fiber with Paratone-N oil. Structures were determined using direct methods, as implemented in the Bruker AXS software package. Electrochemistry Cyclic voltammetry and differential pulse voltammetry were carried out using a 660 Electrochemical Workstation (CH-Instrument, Austin, TX). Measurements were performed at room temperature in CH2Cl2 or acetonitrile solutions with 0.1 M TBAPF6 as the supporting electrolyte. Sample concentration was kept at approximately 1 mM. A scan rate of 0.1 V/s was used for all cyclic voltammetry measurements. Experiments were conducted using a glassy carbon working electrode, a saturated Ag/AgCl reference electrode, and a platinum wire 94 + counter electrode. Ferrocenium/ferrocene (Fc /Fc) was used as an internal reference. Steady-state and time-resolved luminescence Low-temperature measurements were conducted on samples in 2methyltetrahydrofuran glasses. Samples were placed in quartz EPR tubes and rigorously degassed with five freeze-pump-thaw cycles. For both time-resolved and steady-state measurements at 77 K, samples were submerged in liquid nitrogen within a homemade quartz optical dewar. Steady-state emission spectra were recorded on a Jobin Yvon Spex Fluorolog-3-11. Sample excitation was achieved via a xenon arc lamp with a single monochromator providing wavelength selection. Right-angle light emission was sorted using a single monochromator and fed into a Hammamatsu R928P photomultiplier tube with photon counting. Short- and longpass filters were used where appropriate. Spectra were recorded on Datamax software. Time-resolved measurements were carried out using 10 ns pulses at 355 nm from a Spectra-Physics Quanta-Ray Q-switched Nd:YAG laser operating at 10 Hz. Luminescence decays were detected through an Instruments SA (ISA Edison, NJ) model DH10 (1200 grooves/mm) double monochromator and Hamamatsu R928 PMT with a 5-stage socket made by Products for Research (model R928/17149.00301.0040, Bridgewater, NJ). Signals were amplified with a Phillips Scientific 100 MHz bipolar amplifier (100x) and recorded on a Tektronix model TDS-620A digitizing oscilloscope. 95 Computational details Quantum chemical calculations were performed using density functional methods of the Amsterdam Density Functional (Version ADF2007.01) package.34 TZP basis sets were used with triple-ξ accuracy sets of Slater-type orbitals, with polarization functions added to all atoms. Relativistic corrections were made using the ZORA (zero-order relativistic approximation) formalism35 and the core electrons were frozen up to 1s for C, N, O, and F; 2p for P and Cl; 3d for Rh and Pd; and 4d for Ir and Pt. The local density approximation of Vosko, Wilk, and Nusair36 was utilized. All quoted electronic structure data from optimized structures use an integration grid of 6.0 and were verified as minima using frequency calculations. Fragment analyses use the MOs of the chosen fragments as the basis set for the molecular calculation. Initial spin-restricted calculations were carried out on the fragments with the geometry that they have in the molecule; thus the fragments were in a prepared singlet state. Neutral fragments were chosen, as this assisted in drawing up the MO diagrams. Topological analyses of the electron density were performed using the XAIM program.37 The adf2aim executable, which is provided with the standard source code of the ADF program, was used to convert the tape21 file to a wavefunction (.wfn) file. XAIM was then used to locate and characterize the critical points. 96 Synthesis and characterization of new compounds [(2-phenylpyridine)Pd(TFA)]2 (3). Pd(TFA)2 (0.50 g, 1.50 mmol) was added to a three-neck 250 mL round-bottom flask equipped with a reflux condenser and a stir bar. Chloroform (80 mL, OmnisolvTM) and 2-phenylpyridine (0.233 g, 1.50 mmol) were added, and the solution was heated for five hours at reflux. The yellow solution was allowed to cool, then filtered through Celite to remove any palladium black. The solvent was removed in vacuo to give a brown oil. The crude product was purified using column chromatography (eluting first with CH2Cl2 and then ethyl acetate) to give [(2- phenylpyridine)Pd(TFA)]2 (3) as a yellow powder (0.24 g, 48%). X-ray diffraction quality crystals were grown from saturated solutions of CH2Cl2 or acetone. 1H NMR (300 MHz, chloroform-d1) δ = 7.75 (m, 2H), 7.47 (app td, J = 7.5, 0.9, 2H), 7.15 (d, J = 8.2, 2H), 6.86 (m, 6H), 6.74 (m, 2H), 6.58 (app t, J = 6.4, 2H). 13C{1H} NMR (125 MHz, chloroform-d1) δ = 164.3 (2C), 149.8 (2C), 148.6 (2C), 144.3 (2C), 138.5 (2C), 132.1 (2C), 131.0 (2C), 129.1 (2C), 125.0 (2C), 122.9 (2C), 121.9 (2C), 121.6 (2C), 117.8 (2C). HRMS for [C26H16F6N2O4Pd2]+: calc’d 747.9100 g/mol, found 747.9135 g/mol. [(2-p-tolylpyridine)Pd(TFA)]2 (4). The above procedure was followed, substituting 2-p-tolylpyridine (0.255 g, 1.50 mmol), to give [(2-p-tolylpyridine)Pd(TFA)]2 (4) as a yellow powder (0.21 g, 36%). X-ray diffraction quality crystals were grown from a saturated solution of CH2Cl2. 1H NMR (300 MHz, dichloromethane-d2) δ = 7.69 (m, 2H), 7.47 (app td, J = 7.8, 1.5, 2H), 7.11 (d, J = 8.1, 2H), 6.80 (m, 4H), 6.63 (m, 4H), 2.16 (s, 6H). 13C{1H}NMR (125 MHz, dichloromethane-d2) δ = 164.8 (2C), 97 149.6 (2C), 148.7 (2C) 141.2 (2C), 136.5 (2C), 132.4 (2C), 131.6 (2C), 131.0 (2C), 126.3 (2C), 123.3 (2C), 122.5 (2C), 121.4 (2C), 118.3 (2C), 23.2 (4C). HRMS for [C28H20F6N2O4Pd2]+: calc’d 775.9414 g/mol, found 775.9418 g/mol. 3.6 References (1) (a) Cotton, F. A. Inorg. Chem. 1998, 37, 5710; (b) Cotton, F. A.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons, New York, 1999, p. 647–652. (2) (a) Mann, K. R.; Gordon, J. G.; Gray, H. B. J. Am. Chem. Soc. 1975, 97, 3553; (b) Mann, K. R.; Gray, H. B. Adv. Chem. Ser. 1979, 173, 225; (c) Rice, S. F.; Milder, S. J.; Goldbeck, R. A.; Kliger, D. S.; Gray, H. B. Coord. Chem. Rev. 1982, 43, 349. (3) (a) Osborn, R. S.; Rogers, D. J. Chem. Soc., Dalton Trans. 1974, 1002; (b) Connick, W. B.; Marsh, R. E.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1997, 36, 913. (4) Carr, N.; Crossley, J. G.; Dent, A. J.; Gouge, J. R.; Greaves, G. N.; Jarrett, P. S.; Orpen, A. G. J. Chem. Soc., Chem. Commun. 1990, 11369. (5) (a) Novoa, J. J.; Aullon, G.; Alemany, P.; Alvarez, S. J. Am. Chem. Soc. 1995, 117, 7169; (b) Aullón, G.; Alvarez, S. Chem. Eur. J. 1997, 3, 655. (6) Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55. (7) (a) Che, C.-M.; Butler, L. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 7796; (b) Fordyce, W. A.; Brummer, J. G.; Crosby, G. A. J. Am. Chem. Soc. 1981, 103, 7061. (8) Kalsbeck, W. A.; Grover, N.; Thorp, H. H. Angew. Chem. Int. Ed. 1991, 30, 1517. (9) (a) Yip, H.-K.; Lai, T.-F.; Che, C.-M. J. Chem. Soc. Dalton Trans. 1991, 1639; (b) Xia, B.-H.; Che, C.-M.; Zhou, Z.-Y. Chem. Eur. J. 2003, 9, 3055; (c) Pan, Q.-J.; Zhang, H.-X.; Zhou, X.; Fu, H.-G.; Yu, H.-T. J. Phys. Chem. A 2007, 111, 287; (d) Clement, S.; Aly, S. M.; Bellows, D.; Fortin, D.; Strohmann, C.; Guyard, L.; AbdEl-Aziz, A. S.; Knorr, M.; Harvey, P. D. Inorg. Chem. 2009, 9, 4118. (10) Bondi, A. J. Phys. Chem. 1964, 68, 441. (11) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988, 110, 1144. (12) Bercaw, J. E.; Day, M. W.; Golisz, S. R.; Hazari, N.; Henling, L. M.; Labinger, J. A.; Schofer, S. J.; Virgil, S. Organometallics 2009, 28, 5017. (13) (a) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302; (b) Deprez, N. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 11234. (14) (a) Thu, H.-Y.; Yu, W.-Y.; Che, C.-M. J. Am. Chem. Soc. 2006, 128, 9048; (b) Dinçer, M.; Özdemir, N.; Günay, M. E.; Çetinkaya, B. Acta Crystallogr. Sect. E: Struct. Rep. Online 2008, E64, m381. (15) The two previously reported structures of 1 are polymorphs.14 In this paper we report a low temperature structure (100 K) of 1, which is the same polymorph as 98 14b reported by Dinçer and co-workers. For the sake of accurate comparison with other compounds reported here, we use the geometrical parameters from our structure in further discussions. (16) Aiello, I.; Crispini, A.; Ghedini, M.; La Deda, M.; Barigelletti, F. Inorg. Chim. Acta 2000, 308, 121. (17) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. (18) Craig, C. A.; Watts, R. J. Inorg. Chem. 1989, 28, 309. (19) Mann, K. R.; Hill, M. G. Catal. Lett. 1991, 11, 341. (20) Ivanov, M. A.; Puzyk, M. V. Opt. Spectrosc. 2001, 91, 927. (21) Kim, C. D.; Pillet, S.; Wu, G.; Fullagar, W. K.; Coppens, P. Acta Crystallogr., Sect. A: Found. Crystallogr. 2002, 58, 133. (22) Advances in separation techniques have opened the way for more rigorous investigations of the excited state properties of stereoisomers of organic and inorganic compounds; see Keene, F. R. Coord. Chem. Rev. 1997, 166, 121. Typically, optical isomers have shown minimal differences in excited state behavior; see Rutherford, T. J.; Van Gijte, O.; Kirsch-De Mesmaeker, A.; Keene, F. R. Inorg. Chem. 1997, 36, 4465, or Browne, W. R.; O'Connor, C. M.; Villani, C.; Vos, J. G. Inorg. Chem. 2001, 40, 5461. However, differences are more pronounced for geometric isomers, where excited state lifetimes in some cases vary by more than an order of magnitude, see Spalletti, A.; Bartocci, G.; Galiazzo, G.; Macchioni, A.; Mazzucato, U. J. Phys. Chem. A 1999, 103, 8994, Treadway, J. A.; Chen, P.; Rutherford, T. J.; Keene, F. R.; Meyer, T. J. J. Phys. Chem. A 1997, 101, 6824, or Booth, E. C.; Bachilo, S. M.; Kanai, M.; Dennis, T. J. S.; Weisman, R. B. J. Phys. Chem. C 2007, 111, 17720. (23) See supporting information for more details. (24) Cloke, F. G. N.; Green, J. C.; Jardine, C. N.; Kuchta, M. C. Organometallics 1999, 18, 1087. (25) Bader, R. F. W. Atoms in Molecules: A Quantum Theory. Oxford University Press, New York, USA, 1994. (26) (a) Bader, R. F. W. Chem. Rev. 1991, 91, 893; (b) A (3,-1) critical point is a point at which the electron density is a minimum in the direction of the two bonded atoms and a maximum in the two orthogonal directions. It is taken as indicative of a covalent bond. (27) Che, C.-M.; Butler, L. G.; Gray, H. B.; Crooks, R. M.; Woodruff, W. H. J. Am. Chem. Soc. 1983, 105, 5492. (28) (a) Filomena Dos Remedios Pinto, M. A.; Sadler, P. J.; Neidle, S.; Sanderson, M. R.; Subbiah, A.; Kuroda, R. J. Chem. Soc., Chem. Commun. 1980, 13; (b) Marsh, R. E.; Herbstein, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 1983, 39, 280. (29) Coleman, A. W.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M. J.; Atwood, J. L. J. Am. Chem. Soc. 1982, 104, 922. (30) Mann, K. R.; Lewis, N. S.; Williams, R. M.; Gray, H. B.; Gordon, J. G., II. Inorg. Chem. 1978, 17, 828. (31) (a) Novozhilova, I. V.; Volkov, A. V.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 1079; (b) Pan, Q.-J.; Fu, H.-G.; Yu, H.-T.; Zhang, H.-X. Inorg. Chem. 2006, 45, 8729. 99 (32) Moore C. E, Atomic Energy Levels, Vol. III (Molybdenum through Lanthanum and Hafnium through Actinium), Circular of the National Bureau of Standards 467, U.S. Government Printing Office, Washington DC, (1958). (33) Gutierrez, M. A.; Newkome, G. R.; Selbin, J. J. Organomet. Chem. 1980, 202, 341. (34) (a) Fonseca Guerra, C.; Snijder, J. G.; Te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391; (b) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931; (c) ADF2007.01, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com. (35) (a) Vanlenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597; (b) Vanlenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994, 101, 9783; (c) Vanlenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1996, 105, 6505; (d) Vanlenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943; (e) Vanlenthe, E.; VanLeeuwen, R.; Baerends, E. J.; Snijders, J. G. J. G. Int. J. Quantum Chem. 1996, 57, 281. (36) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (37) Alba, J. C. O.; Jané, C. B. XAim, 1998. (Download free of charge from the web distribution site: http://www.quimica.urv.es/XAIM/.) 100 101 Chapter 4: Carbon–chlorine bond formation from a binuclear palladium(II) electrocatalyst 4.1 Abstract The binuclear PdII complex, [(benzo[h]quinoline)PdII(µ-OAc)]2 (PdII–PdII), has been shown to catalyze the selective chlorination of benzo[h]quinoline. The reaction typically requires the use of the potent oxidant iodosobenzedichloride, PhICl2, and proceeds through reductive elimination of the PdIII–PdIII intermediate [(benzo[h]quinoline)PdIII(µ-OAc)Cl]2. Electrochemical oxidation of PdII–PdII in the presence of chloride at the PdIII–PdII/PdII–PdII potential results in two-electron oxidation with addition of two chloride ions to form the intermediate Cl–PdIII–PdIII– Cl. Selective chlorination of benzo[h]quinoline to 10-chlorobenzo[h]quinoline proceeds to completion with 10% catalyst loading at potentials below the Cl2 / 2Cl couple. 4.2 Introduction Palladium-catalyzed directed C–H functionalization reactions The development of techniques for carbon–hydrogen bond activation and functionalization is a great challenge in modern chemistry. Important to these reactions is that they are able to be performed with high selectivity under relatively mild conditions that are compatible with other functional groups. Traditional methods for the conversion of C–H bonds to carbon–halogen, carbon–oxygen, carbon–nitrogen, or carbon–carbon bonds rely multiple steps or prefunctionalized starting materials that make their implementation in chemical synthesis more costly and time consuming. 102 Direct functionalization of C–H bonds, which overcomes these limitations, is a more attractive option. However, the development of these reactions is made more difficult due to the relative inertness of C–H bonds. Further, most substrates possess many C–H bonds, adding difficulty to enforcing selectivity on these reactions. These difficulties can be overcome through metal-catalyzed reactions and the use of substrates with a coordinating ligand to direct a specific C–H bond to the metal center for activation. This technique, also referred to as cyclometalation, has been employed using Ru, Rh, Pd, and Pt metal centers. However, its use in Pd-catalyzed reactions is particularly interesting. Cyclopalladated PdII complexes are often tolerant of air and moisture, aiding in the ease of their manipulation. Furthermore, the metal is capable of catalyzing a diverse array of C–H functionalizations, including carbon– carbon, carbon–halogen, carbon–oxygen, and carbon–nitrogen bond-forming reactions. Following cyclometalation, substrate functionalization follows either a reductive route, where PdII is reduced to Pd0, or an oxidative route. The mechanism for oxidative functionalization has been a subject of recent discussion as monometallic PdII/IV as well as bimetallic PdII–PdII/PdIII–PdIII cycles have been proposed. Oxidative methods for C–H bond functionalization often require strong oxidants to generate the high-valent Pd-intermediates necessary to drive catalysis. In C–O bond formation, common oxidants include iodine(III) reagents (e.g., PhI(OAc)2) and peroxides. While the use of strong oxidants is the standard, dioxygen has been successfully employed as the terminal oxidant in several 103 1–4 palladium-catalyzed schemes, for example in Wacker-type oxidation, the α- hydroxylation of carbonyl compounds,5 as well as in the ligand-directed acetoxylation of 8-methylquinoline-derived substrates.6 Similarly, palladium-catalyzed carbon–halogen bond formation often requires strong oxidants. The earliest report of ligand-directed carbon–halogen bond-formation occurred in 1970 and demonstrated the ortho-chlorination of azobenzene by Cl2.7 The utility of this reaction was limited by the necessity for chlorine gas; however, it led to the development of chlorination reactions using alternative “X+” sources, in particular PhICl2 and NCS. The mechanism for many Pd-catalyzed C–H bond oxidations was believed to proceed through a monometallic PdIV intermediate. Work by Sanford showed that oxidation of PdII(ppy)2 (ppy = 2-phenylpyridine) by NCS yielded a sixcoordinate [PdIV(ppy)2(Cl)(succinamide)] species. This species was isolated and shown to undergo competing C−Cl, C−C, and C−N bond-forming reductive elimination reactions.8 The results of this and other experiments suggested that monometallic PdII/IV catalytic cycles are viable paths for C–H bond oxidation reactions. Subsequent investigations into the Pd-catalyzed chlorination of arylpyridine substrates suggest some of these reactions may occur through a binuclear PdIII–PdIII intermediate, rather than the monometallic PdIV. Combination of Pd(OAc)2 with benzo[h]quinoline produces the C–H activated dimer [(benzo[h]quinoline)PdII(µOAc)]2. Upon treatment with PhICl2, this species was found to form the PdIII–PdIII species [(benzo[h]quinoline)PdIII(µ-OAc)Cl]2.9,10 This complex is stable and low 104 temperature and has been characterized by x-ray crystallography. Upon warming, the species undergoes reductive elimination, producing 10-chlorobenzo[h]quinoline and PdII products (Scheme 4.1). A binuclear PdII complex was also implicated in the Pd(OAc)2-catalyzed chlorination of benzo[h]quinoline by NCS.11 Prior to this work, there were few examples of PdIII complexes in organometallic chemistry, the first not occurring until 1987.12–15 While the chemistry of Pd0, PdII, and PdIV complexes are well-established, the reactivity of PdIII complexes are largely unexplored. Binuclear d8–d8 complexes The implication of binuclear PdII species in catalysis was particularly interesting to us as we had recently completed a detailed study on the electronic structure of complexes that are nearly identical to those used in the study.16 PdII dimers are part of a subset of binuclear complexes that feature attractive d8–d8 interactions between metal centers. The chemistry of d8–d8 dimers has long been of interest to the group.17–20 Binuclear metal complexes are attractive systems for organic transformations due to the opportunity for cooperativity between metal centers. The benefits of cooperativity include lower oxidation potentials, multielectron redox capability, and multiple metal coordination sites for substrate activation. 105 106 Scheme 4.1. Selective chlorination of benzo[h]quinoline by PdII–PdII proceeds through a PdIII–PdIII intermediate that contains two axial chlorides and a Pd–Pd single bond. Following reductive elimination, treatment with Ag+ returns the catalyst to its resting state. The first example of complexes of this type was discovered in 1975, when square-planar RhI isocyanide complexes were found to form discrete dimers and short oligomers in solution as well as in the solid state.21 Dimerization was attributed to a net bonding interaction between the two metal centers. A simple molecular orbital (MO) diagram would suggest no attractive force between the two centers as there is no net metal–metal bond. However, mixing between the ndz2 and (n+1)pz orbitals stabilizes the metal–metal dσ and dσ* orbitals, resulting in a significant interaction (Figure 4.1). Subsequent work has identified many more examples of complexes of this type featuring PtII, RhI, or IrI. The chemical properties of this family of complexes are fairly consistent. Most participate in two-center oxidative addition when treated with X2 (X = Cl, Br, I). As predicted by the MO diagram, these species possess a metal–metal single bond. In addition to chemical properties, photoexcitation of many of these complexes yields species with relatively long triplet lifetimes and interesting photochemical properties. In particular, they are often capable of hydrogen-atom abstraction from organic and organometallic substrates. Despite over 30 years of work in d8–d8 chemistry, comparatively little attention was paid to binuclear complexes of PdII. This is largely the result of conclusive evidence for Pd–Pd interactions on the scale of what is observed for complexes of PtII, RhI, and IrI.22 Multiple PdII–PdII complexes have been synthesized, including those with metal–metal distances as close as 2.6 Å; however, this was frequently attributed to coordinating ligands forcing a short distance rather 107 23 than an attractive Pd–Pd interaction. Despite this work, conclusive evidence for bonding remained elusive. Interest in PdII–PdII complexes was further hampered by a lack of many of the properties that define traditional d8–d8 chemistry. Prior to our study, few reports existed that had examined explored the properties binuclear PdII complexes following oxidation.12,14,24 The most prominent work was performed by Cotton, who synthesized [Pd2(hpp)4] (hpp = 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine) and treated it with PhICl2 to yield [Pd2(hpp)4Cl2]. As predicted by the molecular orbital description, this complex possesses a Pd–Pd single bond at a distance of 2.391 Å. While the chemical reactivity was more-orless maintained,12 these complexes frequently lack long-lived excited states and therefore also lacked excited-state chemical reactivity. The work by Ritter et al. exploring the role of PdII–PdII in oxidative catalysis has led to rejuvenated interest in the field.9 108 109 Figure 4.1. Symmetry-allowed mixing between the dz2 and pz stabilizes the dσ and dσ* orbitals, resulting in a net metal–metal bonding interaction, despite no formal metal–metal bond. II Our recent investigation of the electronic structures of binuclear Pd –Pd II complexes showed a weak metal–metal interaction in the ground state. The complexes possess a metal–metal dσ* HOMO, reminiscent of the electronic structures of the better-known d8–d8 complexes of PtII, RhI, and IrI. However, unlike these compounds, PdII–PdII species do not possess a pσ LUMO, which resides at higher energy. This large d–p energy gap leads to decreased mixing between the dσ/dσ* and pσ/pσ* orbitals, and a significantly dimished groundstate metal–metal interaction. Further, the altered electronic structure leads to rapid deactivation of the excited state, precluding excited-state reactivity. While our binuclear species are unable to perform photochemistry typical of related d8– d8 complexes, we note the ground-state electronic structure is conserved and oxidative chemistry distinctive of d8–d8 complexes may still be accessed. The newly discovered catalytic properties of [(benzo[h]quinoline)PdII(µOAc)]2, coupled with our recent investigations into the electronic structure of related species,16 led us to explore the electronic structure and properties of these PdII–PdII complexes following oxidation. 4.3 Results and discussion Electrochemical properties Electrochemical oxidation of [(benzo[h]quinoline)PdII(µ-OAc)]2 (PdII– PdII) was conducted in dichloromethane solution. Cyclic voltammograms showed two quasi-reversible waves at 420 mV and 720 mV vs. Fc+/Fc. These waves are attributable to successive one-electron oxidations of PdII–PdII to form PdII–PdIII 110 III III and Pd –Pd , respectively (Figure 4.2). These results are consistent with electrochemical data on the related 2-phenylpyridine and tolyl-pyridine species.16 In addition to participating in two-center oxidative addition with X2 (X = Cl, Br, I), many d8–d8 complexes will also add two equivalents of X when chemically or electrochemically oxidized in the presence of X–.19,25 This reaction occurs at potentials corresponding to oxidation of M2 by a single electron. As we expected the oxidative properties of PdII–PdII to remain intact, we decided to explore the effects of halide ion on electrochemical oxidation. It should be noted that addition of large amounts of chloride and bromide ions (> 20-fold molar excess) led to a noticeable loss of color. This is hypothesized to be the result of displacement of the bridging acetate ligands resulting in a halidebridged dimer. The properties of the chloride-bridged complex [(2- phenylpyridine)PdII(µ-Cl)]2 were examined in our electronic structures study. The crystal structures show that this dimer is in a planar conformation, which separates the two palladium centers and eliminates all metal–metal interaction. Lower concentrations of chloride or bromide did not appear to have a significant effect. Additionally, use of the chelating ligand α,α,α’,α’-tetramethyl-1,3benzenedipropionic acid (H2esp) in place of the two bridging acetates resulted in a complex that was robust to displacement by chloride at very high concentrations. 111 112 Figure 4.2. Cyclic voltammogram of PdII–PdII in dichloromethane The addition of tetraethylammonium chloride to [(2- phenylpyridine)PdII(µ-OAc)]2 results in noticeable changes to the voltammogram (Figure 4.3). The first wave increases anodically to approximately double its previous current with a concomitant decrease in peak current of the second wave. This behavior is consistent with an ECE mechanism for the formation of the complex [(2-phenylpyridine)PdIII(µ-OAc)Cl]2. One-electron oxidation of PdII–PdII is followed by addition of chloride at one or both axial positions. Addition of chloride lowers the potential required for oxidation of the second electron. A related disproportionation mechanism is also possible. Similar electrochemical behavior is observed following oxidation of [(benzo[h]quinoline)PdII(µ-OAc)]2 as well as for [(benzo[h]quinoline)2PdII2(µ-esp)]. Treatment with bromide has similar effects on the electrochemical behavior; however, the bromine/bromide potential is much closer lower and bromide oxidation obscures the second PdII– PdII wave at higher concentrations. The iodine/iodide potential occurs well below the PdII–PdIII/PdII–PdII potential and does not result in any chemical changes to PdII–PdII. Treatment of [(2-phenylpyridine)PdII(µ-OAc)]2 with PhI(OAc)2 was reported to yield a PdIII–PdIII species with axially coordinated acetato groups. When warmed to 40 oC, this complex was found to undergo reductive elimination of acetoxylated 2-phenylpyridine, following a scheme very similar to that presented for the corresponding chlorination reaction. In an effort to test if this reactivity translates to electrochemical reactivity, acetate salts were added to a solution of [(benzo[h]quinoline)PdII(µ-OAc)]2. Oxidation of the palladium 113 complex in the presence of acetate showed similar voltammogram to that of oxidation in the presence of chloride. This suggests that axial ligands beyond simple halogens are possible. Spectroelectrochemical properties Spectroelectrochemical measurements were employed to further explore the effect of anion addition on electrochemical oxidation. One-electron oxidation of [(benzo[h]quinoline)PdII(µ-OAc)]2 was first attempted in the absence of any coordinating ions. The relative reversibility of the first wave, and the ~ 300 mV separation between it and the second suggested that the mixed-valence PdII–PdIII may be stable, at least for brief periods. Previous work by Berry et al. successfully isolated a mixed-valence formamidinate-bridged [Pd2]5+ species. The electronic absorption spectrum of the complex showed a large band centered at approximately 900 nm and was attributable to a transition from the lower-lying metal-based dπ orbitals (dxy, dxz, dyz) to the singly occupied dσ*. An applied potential of +450 mV vs. Fc+/Fc (between the first and second oxidation waves) results in the ingrowth of a broad feature centered at 1010 nm in the UV–vis spectrum (Figure 4.4), which we attribute to a metal-based dπ → dσ* transition. The assignment of this species as [PdII–PdIII]+ is supported by computational results. Theoretical calculations on the [Pd2]5+ complex [(2phenylpyridine)Pd(μ-OAc)]2+ show a dσ* SOMO and predict a metal-based transition at 1022 nm, consistent with experimental results. 114 115 Figure 4.3. Cyclic voltammogram of [(2-phenylpyridine)PdII(µ-OAc)]2 as chloride concentration is increased from 0 mM (blue), to 1 mM (green), to 2 mM (red) 116 Figure 4.4. Absorbance difference spectrum observed during electrochemical oxidation of PdII–PdII at 450 mV vs. Fc+/Fc Oxidation at similar potentials in the presence of chloride yields a strikingly different spectrum. Unlike in the halide-free spectrum, no features in the near-IR appear. Instead, a band centered at ~ 425 nm is observed, giving the solution a red-brown color. The UV–vis spectrum closely follows the spectrum observed for the chemically isolable [(benzo[h]quinoline)PdIII(μ-OAc)Cl]2 and the new band is attributable to the metal–metal based dσ → dσ* transition (Figure 4.5). Isosbestic points observed during electrolysis suggest the reaction occurs directly without build-up of an intermediate species. Similar to other d8–d8 complexes, electrochemical reduction of the oxidized product returns the spectrum to its original pre-electrolysis form, suggesting chloride addition is electrochemically reversible, provided it occurs prior to subsequent chemistry. Isolation of this species was unsuccessful, due to its thermal instability and the presence of large amounts of salt needed as electrolyte. Interestingly, one-electron oxidation of PdII–PdII, followed by deactivation of the electrode and addition of chloride, results in the formation of [(benzo[h]quinoline)PdIII(μ-OAc)Cl]2. While it is impossible to exclude generation of trace amounts of chlorine reacting with PdII–PdII to generate these products, this experiment demonstrates that a scheme proceeding through a [Pd2]5+ intermediate is viable. 117 118 Figure 4.5. UV–vis absorption spectrum of species generated by oxidation of PdII– PdII in the presence of chloride II The spectroelectrochemical properties of Pd –Pd II with other anionic ligands were then examined. In separate experiments, bromide, acetate, and fluoride were added to solutions containing PdII–PdII and electrochemically oxidized. Solutions with bromide result in spectra that closely match the spectrum of [(benzo[h]quinoline)PdIII(μ-OAc)Br]2 that is generated chemically from treatment of PdII–PdII with Br2 (Figure 4.6a). Similarly, oxidation in the presence of acetate yields a complex with an absorbance spectrum reminiscent of [(2phenylpyridine)PdIII(μ-OAc)OAc]2, which is generated by treatment with PhI(OAc)2 (Figure 4.6b). Oxidation in the presence of fluoride did not significantly affect the cyclic voltammogram, nor were large changes observed through spectroelectrochemistry. This is not surprising, as attempts to chemically generate [(benzo[h]quinoline)PdIII(μ-OAc)F]2 with XeF2 were unsuccessful and instead generated linear chains of PdIII.26 Bulk electrolysis It has been previously established that [(benzo[h]quinoline)PdIII(μOAc)X]2 (X= Cl, Br, OAc) will undergo reductive elimination to yield selectively functionalized benzo[h]quinoline products. The electrochemical and spectroelectrochemical results strongly suggest that these solutions were being generated in solution. As such, electrochemical generation of these species should result in product formation (Scheme 4.2). An electrocatalytic route to C–Cl, C– Br, and C–O bond-forming reactions is intriguing as the electrochemical potentials required are low relative to traditional oxidants. 119 120 Scheme 4.2. Electrocatalytic chlorination of benzo[h]quinoline catalyzed by PdII– PdII A bulk electrolysis cell was purchased featuring a reticulated vitreous carbon working electrode with a large surface area and a fritted auxiliary chamber with platinum wire counter electrode. A mixture of 1 mM equivalent PdII–PdII, 10 mM benzo[h]quinoline, and 10 mM of tetraethylammonium chloride were combined in 75 mL dichloromethane solution containing 100 mM TBAPF6. The auxiliary chamber was filled with an excess of tris(acetoacetonato)iron(III) as a sacrificial oxidant. This choice in sacrificial oxidant is advantageous as its potential is insufficient to oxidize PdII–PdII and is electrochemically irreversible, decomposing into bis(acetoacetonato)iron(II) and acetylacetonate (Figure 4.7). Further, it is extremely soluble in methylene chloride (> 20 mM). Electrolysis was conducted at approximately +1.0 V (vs. Fc+ / Fc) and run at room temperature for 48 h. Following electrolysis, > 99% conversion to 10-chlorobenzo[h]quinoline was observed with only trace amounts of benzo[h]quinoline detected by GC–MS. Product purification and isolation Product isolation from electrolyte is easily achieved. Solvent is removed in vacuo and the solid is then washed with ether. The TBAPF6 electrolyte is not soluble and can be recovered by filtration and purified by recrystallization. The supernatant is collected and concentrated. It is then passed through a silica plug eluting with 3:7 ether:hexanes. Removal of solvent 10-chlorobenzo[h]quinoline results in 80% yield as a white crystalline solid. 121 122 (a) (b) Figure 4.6. UV–vis absorbance spectra PdII–PdII following oxidation in the presence of bromide (a) and acetate (b) 123 Figure 4.7. Cyclic voltammogram of tris(acetylacetonato)iron(III) Bulk electrolysis at lower potentials was found to generate 10chlorobenzo[h]quinoline, but at sub-catalytic amounts and with minimal current generation. This effect may be a result of resistance in the bulk electrolysis cell. Indeed, cell design was not optimal for effective ion flow and measurements of the potential difference between the reference and working electrodes suggest the applied potential may have been lower than the programmed 1.0 V. However, particularly at these higher potentials, chloride oxidation to chlorine cannot be excluded. 4.4 Concluding remarks Ongoing studies in our laboratories aim to improve the scope of these electrocatalytic reactions. In summary, we have successfully developed a chemical catalyst for carbon–chlorine bond formation into an efficient electrocatalyst under mild conditions. Capitalizing on the electronic structure and metal–metal interactions between the two palladium(II) centers, we were able to perform two-electron chemistry at the one-electron potential of the complex. Spectroscopic techniques were used to identify reactive intermediates along the proposed catalytic cycle. The properties of these binuclear species may be conducive to more diverse reactivity. Electrocatalytic reactions capable forming C–Br, C–O, C–N, and C–C bonds may be possible from these systems. 4.5 Materials and methods General All manipulations were performed in air, unless otherwise stated. Pd(OAc)2, (purchased from Sigma-Aldrich) and benzo[h]quinoline (purchased 124 from TCI-America) were reagent-grade commercial samples used without further purification. Dichloromethane-d2 and chloroform-d1 were purchased from Cambridge Isotope Laboratories and used as received. [(benzo[h]quinoline)Pd(µOAc)]2 were prepared using literature procedures.9 1H and 13C NMR spectra were recorded at 298 K using a Varian Mercury 300 MHz spectrometer equipped with the VNMRJ software program, version 2.2d. 1H and 13C NMR spectra were referenced to the residual proton or carbon chemical shifts of the deuterated solvent. The data are reported by chemical shift (ppm) from tetramethylsilane, multiplicity (s, singlet; d, doublet; t, triplet; m, multiplet; dd, double doublet; dt, double triplet), coupling constants (Hz), and integration. Mass spectra were acquired on a Finnigan LCQ ion trap or Agilent 5973 Network mass selective detector and were obtained by peak matching. UV/Vis absorbance spectra were recorded on an Agilent 8453 UV/Vis Spectrometer using a pure sample of the solvent as the background. Electrochemistry Cyclic voltammetry and differential pulse voltammetry were carried out using a 660 Electrochemical Workstation (CH-Instruments, Austin, TX). Measurements were performed at room temperature in CH2Cl2 solutions with 0.1 M TBAPF6 as the supporting electrolyte. Concentrations of [(benzo[h]quinoline)Pd(µ-OAc)]2 were kept at approximately 1 mM. Axially coordinating ligands (chloride, bromide, and acetate) were added by titration of a solution containing the corresponding tetraethylammonium salt. A scan rate of 0.1 125 V/s was used for all cyclic voltammetry measurements. Experiments were conducted using a glassy carbon working electrode, a saturated Ag/AgCl reference electrode, and a platinum wire counter electrode. Ferrocenium/ferrocene (Fc+/Fc) was used as an internal reference. Bulk electrolysis experiments were performed on a Pine Instruments WaveNow potentiostat and the data were processed using Igor Pro 5.01 (Wavemetrics). Electrolysis was performed in a bulk electrolysis cell (purchased from BASi MF-1056). The complete bulk cell consists of a 100 mL glass vial, an auxiliary cell with approximately 10 mL volume, a reticulated vitreous carbon working electrode, a 10 cm Pt wire auxiliary electrode, and a Ag/AgCl reference electrode. The auxiliary chamber is separated from the working chamber by a fine-porous frit. Replacement glass parts can be purchased at significantly reduced cost from Ace Glass. Platinum wire can be purchased through Alfa Aesar. The main chamber was filled with 75 mL of 1.0 mM Pd2, 10 mM TEACl, and 10 mM benzo[h]quinoline. The auxiliary chamber was filled with approximately iron(III) tris-(acetylacetonate), which is extremely soluble in dichloromethane and serves as a sacrificial oxidant. Bulk electrolysis was conducted at 100–200 mV beyond the first anodic peak potential (approximately +500–600 mV vs. Fc+/Fc). Working and auxiliary chambers were stirred during electrolysis. Reaction progress was monitored by a combination of coulometry and GCMS measurements of aliquots taken from the reaction mixture. Bulk electrolysis was allowed to proceed 24–48 h at room temperature. 126 127 Product purification and isolation Following electrolysis, the solution was evaporated to dryness and redissolved in diethylether. This solution was then filtered, removing the majority of ammonium salts. Solvent was removed from the filtrate in vacuo and the residue was purified by passing it through a silica plug, eluting with hexanes:diethyl ether (9:1). This afforded the product, 10- chlorobenzo[h]quinoline in 82% isolated yield. 1 H-NMR (300 MHz, CDCl3, 23 ºC, δ): 9.12 (dd, J = 4.4 Hz, 2.0 Hz, 1H), 8.19 (dd, J = 8.3 Hz, J = 2.0 Hz, 1H), 7.84 (td, J = 7.3 Hz, J = 1.0 Hz, 2H), 7.80 (d, J = 8.8 Hz, 1H), 7.72 (d, J = 8.8 Hz, 1H), 7.59–7.55 (m, 2H). 4.6 References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) Smidt, J.; Hafner, W.; Jira, R.; Sieber, R.; Sedlmeier, J.; Sabel, A. Angew. Chem. 1962, 74, 93–102. Ebner, D. C.; Bagdanoff, J. T.; Ferreira, E. M.; McFadden, R. M.; Caspi, D. D.; Trend, R. M.; Stoltz, B. M. Chem. Eur. J. 2009, 15, 12978–12992. Sigman, M. S.; Jensen, D. R. Acc. Chem. Res. 2006, 39, 221–229. Stahl, S. S. Angew. Chem. Int. Ed. 2004, 43, 3400–3420. Chuang, G. J.; Wang, W.; Lee, E.; Ritter, T. J. Am. Chem. Soc. 2011, 133, 1760–1762. Zhang, J.; Khaskin, E.; Anderson, N. P.; Zavalij, P. Y.; Vedernikov, A. N. Chem. Commun. 3625–3627. Fahey, D. R. J. Chem. Soc., Chem. Commun. 1970, 417. Whitfield, S. R.; Sanford, M. S. J. Am. Chem. Soc. 2007, 129, 15142– 15143. Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302–309. Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 17050–17051. Powers, D. C.; Xiao, D. Y.; Geibel, M. A. L.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 14530–14536. Cotton, F. A.; Gu, J.; Murillo, C. A.; Timmons, D. J. J. Am. Chem. Soc. 1998, 120, 13280–13281. Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2010, 132, 7303–7305. (14) Cotton, F. A.; Koshevoy, I. O.; Lahuerta, P.; Murillo, C. A.; Sanaú, M.; Ubeda, M. A.; Zhao, Q. J. Am. Chem. Soc. 2006, 128, 13674–13675. (15) Blake, A. J.; Holder, A. J.; Hyde, T. I.; Schröder, M. J. Chem. Soc., Chem. Commun. 1987, 987–988. (16) Bercaw, J. E.; Durrell, A. C.; Gray, H. B.; Green, J. C.; Hazari, N.; Labinger, J. A.; Winkler, J. R. Inorg. Chem. 2010, 49, 1801–1810. (17) Mann, K. R., Gray, H. B., In Inorganic Compounds with Unusual Properties—II, Advances in Chemistry, ACS, 1979; vol. 173, p. 225–235. (18) Smith, D. C.; Gray, H. B. Coord. Chem. Rev. 1990, 100, 169–181. (19) Roundhill, D. M.; Gray, H. B.; Che, C. M. Acc. Chem. Res. 1989, 22, 55– 61. (20) Marshall, J. L.; Stobart, S. R.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 3027–3029. (21) Mann, K. R.; Gordon, J. G.; Gray, H. B. J. Am. Chem. Soc. 1975, 97, 3553– 3555. (22) Xia, B.; Che, C.; Zhou, Z. Chem. Eur. J. 2003, 9, 3055–3064. (23) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988, 110, 1144–1154. (24) Berry, J. F.; Bill, E.; Bothe, E.; Cotton, F. A.; Dalal, N. S.; Ibragimov, S. A.; Kaur, N.; Liu, C. Y.; Murillo, C. A.; Nellutla, S.; North, J. M.; Villagrán, D. J. Am. Chem. Soc. 2007, 129, 1393–1401. (25) Bryan, S. A.; Schmehl, R. H.; Roundhill, D. M. J. Am. Chem. Soc. 1986, 108, 5408–5412. (26) Campbell, M. G.; Powers, D. C.; Raynaud, J.; Graham, M. J.; Xie, P.; Lee, E.; Ritter, T. Nat. Chem. 2011, 3, 949–953. 128 129 Appendix A: Electron transfer triggered by optical excitation of phenothiazine-tris(metaphenylene-ethynylene)-(tricarbonyl)(bpy)(py)rhenium(I) The text in this chapter was taken in part from: Bingol, B.; Durrell, A. C.; Keller, G. E.; Palmer, J. H.; Grubbs, R. H.; Gray, H. B. J. Phys. Chem. C 2012, (in press). A.1 Abstract We have investigated excited-state electron transfer in a donor-bridgeacceptor complex containing phenothiazine (PTZ) linked via tris(meta-phenyleneethynylene) to a tricarbonyl(bipyridine)(pyridine)Re(I) unit. Time-resolved luminescence experiments reveal two excited-state (*Re) decay regimes: a multiexponential component with a mean lifetime of 2.7 ns, and a longer monoexponential component of 530 ns in dichloromethane solution. The faster decay is attributed to PTZ → *Re electron transfer in a C-shaped PTZ-bridge-Re conformer (PTZ–Re ~ 7.5 Å). We assign the longer lifetime, which is virtually identical with that of free *Re, to an extended conformer (PTZ–Re > 20 Å). The observed biexponential *Re decay requires that interconversion of PTZ-bridge-Re conformers be slower than 106 s-1. A.2 Introduction Photoinduced electron transfer (ET) reactions play key roles in artificial photosynthesis1–5 and optoelectronic devices.6,7 Critical to all of these systems is the precise control of ET rates over long distances. ET has been shown to depend on tunneling distance, donor-acceptor energetics, and chemical structure of the bridge.8,9 Donor-bridge-acceptor complexes allow systematic investigation of the parameters that control ET processes.10–14 Paul Barbara was a leading investigator in this area: his work on strongly coupled mixed-valence complexes helped to fix the speed limit for bridge-mediated ET.15,16 Complexes with highly conjugated bridges such as oligo(phenylene vinylenes),17–19 oligo(fluorenes),20–22 oligo(para- 130 23–29 phenylenes), and oligo(para-phenylene ethynylenes) are of particular interest, as they often exhibit wire-like behavior. Electronic coupling mediated by these bridges promotes rapid forward electron transfer, but in many cases equally fast charge recombination limits their deployment in molecular electronic devices. Donor-acceptor couplings mediated by nonbonded contacts have been investigated in C-shaped complexes containing saturated polycyclic spacers.30–37 The rigid spacer fixes donor-acceptor orientation, providing control of the overall topology and allowing investigations of intramolecular ET processes mediated by noncovalent interactions. Importantly, these investigations have shown that electron tunneling can occur over relatively long distances through organic solvents. Flexible bridges allow for minimal through-space donor-acceptor distances that favor through-space (or through-solvent) over through-bond ET pathways. Here we report the synthesis and photophysical properties of a donorbridge-acceptor (DBA) (tricarbonyl)(bpy)(py)rhenium(I) (Re) linked to phenothiazine (PTZ) by tris(meta-phenylene-ethynylene). We chose PTZ-Re to probe the role of the bridge in ET reactions, as prior work has shed light on the coupling between D and A in this pair.10–13,38–40 A.3 Results and discussion Synthesis In Scheme A.1, a PTZ-bridge ligand (11) covalently coupled to a Re acceptor afforded PTZ-bridge-Re (12). The electron donor is 10-(prop-2-yn-1-yl)10H-phenothiazine (1), and the bridge contains three phenylene-ethynylene units. 131 The PTZ-bridge ligand features a terminal bipyridine, which binds strongly to Re(I). Compound 11 was constructed in a stepwise approach using zinc-mediated, palladium-catalyzed cross-coupling reactions in a microwave reactor (with protecting group strategies).41 More precisely, the synthesis of 11 began with selective coupling of the donor (1) to 3-bromo-5-iodobenzoate (2). Both 1 and 2 were prepared according to published protocols.42,43 Compound 3 was coupled with trimethylsilyl acetylene, followed by removal of the trimethylsilyl group by treatment with tetrabutylammonium fluoride (TBAF) to afford ethynyleneterminated 4. The alkynyl group of 4 was then coupled to 2 to give phenothiazineterminated 5 with a bromo functionality. Assembly of 11 required that an ethynylene-terminated bipyridine connected to a phenylene-ethynylene unit (10) be linked to 5. Synthesis of 10 began with commercial 6-bromo-2,2′-bipyridine, which was acetylated and deprotected before reaction with 2 to yield a bromo-bipyridine derivative attached to a phenylene-ethynylene unit (9). Acetylation of 9 followed by deprotection with TBAF gave 10. Subsequent cross-coupling of 10 to 5 yielded the PTZ-bridge ligand (11). Metalating 11 with rhenium(I) pentacarbonyl chloride,44 followed by reaction with pyridine in the presence of silver perchlorate in the dark, afforded PTZ-bridge-Re (12).44–46 A model of the acceptor, [Re(CO)3(bpy)(py)]+ (bpy = 2,2’-bipyridine, py = pyridine), also was prepared.47,48 132 133 a Scheme A.1. Synthesis of PTZ-bridge-Re (12) COOEt COOEt COOEt H Br N a COOEt H b, c N d N S e, f N h, i g N COOEt COOEt H Br 7 H 10 9 8 COOEt + COOEt COOEt COOEt 5 + 10 Br 2 N N N I N N Br 5 4 3 N Br S S S 1 COOEt N N N S S COOEt COOEt N CO CO Re N N CO N N 12 11 Reagents and conditions: a) propargyl bromide, K2CO3, toluene, reflux. b) 2, bis(triphenylphosphine)palladium(II) dichloride, ZnCl2, THF, Et3N, 100 °C. c) TBAF, THF, room temperature. d) 2, bis(triphenylphosphine)palladium(II) dichloride, ZnCl2, THF, Et3N, 100 °C. e) trimethylsilyl acetytlene, bis(triphenylphosphine)palladium(II) dichloride, ZnCl2, THF, Et3N, 100 °C. f) TBAF, THF, room temperature. g) 2, bis(triphenylphosphine)palladium(II) dichloride, ZnCl2, THF, Et3N, 100 °C. h) trimethylsilylacetylene, bis(triphenylphosphine)palladium(II) dichloride, ZnCl2, THF, Et3N, 100 °C. i) TBAF, THF, room temperature. j) bis(triphenylphosphine)palladium(II) dichloride, ZnCl2, THF, Et3N, 100 °C. k) Re(CO)5Cl, toluene, reflux. l) AgClO4, pyridine, CH3OH, toluene, 50 °C. Physical properties The absorption spectrum of PTZ-bridge-Re in dichloromethane solution displays three main features (Figure A.1a). The intense, high energy bands (260– 320 nm) in the spectrum are attributed to ligand-based π–π* transitions. Similar features are also present in the spectrum of 6-ethynyl-2,2'-bipyridine. A band centered at 255 nm characteristic of phenothiazine is absent in the spectrum of the [Re(CO)3(bpy)(py)]+ model complex, and a broad feature (340–390 nm), attributable to metal-to-ligand charge transfer (MLCT), is absent in the spectrum of PTZ-bridge.49,50 Absorption bands associated with the bridge were not observed; however, broadening of the 255 nm band accompanied cross-coupling of 10-(prop2-yn-1-yl)-10H-phenothiazine to 3-bromo-5-iodobenzoate. The steady-state emission spectrum of PTZ-bridge-Re in dichloromethane solution features a broad band centered at 574 nm (Figure A.1b). Quantum yield measurements were performed in CH2Cl2 using [Re(CO)3(bpy)(py)]+ as a standard. The quantum yield of PTZ-bridge-Re was found to be approximately 10% of the [Re(CO)3(bpy)(py)]+ standard (Figure A.2). Higher concentrations of PTZ-bridgeRe did not exhibit linear behavior as a result of intermolecular self-quenching. 134 135 Figure A.1. Absorption (a) and corrected emission (b) spectra of PTZ-bridge-Re in CH2Cl2 solution at room temperature Integrated Intensity × 10-7 (a.u.) 136 16 Re 14 PTZ-bridge-Re 12 y = 391.1x + 0.6476 R² = 0.9929 10 8 6 4 y = 38.817x + 0.6595 R² = 0.9918 2 0 0 0.05 0.1 0.15 Absorbance at 355 nm Figure A.2. Quantum yield measurements of PTZ-bridge-Re relative to Re 0.2 MLCT emission decay kinetics recorded in degassed dichloromethane solution exhibited two decay regimes. At timescales greater than 50 ns, a 530 ns monoexponential decay lifetime was observed (Figure A.3a). For comparison, the *Re MLCT lifetime of the model complex, [Re(CO)3(bpy)(py)]+, determined under virtually identical experimental conditions, was 560 ns. The excited-state lifetime and steady-state intensity were found to depend on the concentration of PTZbridge-Re in solution. Analysis of the longer component of the excited-state lifetime at a series of concentrations allowed a bimolecular quenching rate constant of 1.1 × 106 M-1 s-1 to be determined. When the lifetime is extrapolated to zero concentration, a natural lifetime of 530 ns is obtained. This value is close to that of the [Re(CO)3(bpy)(py)]+ model compound (560 ns in degassed CH2Cl2, Figure A.4). Emission decay in the 0–50 ns range was highly nonexponential (Figure A.3b). Quality fits required at least three exponential terms, resulting in a 3.2–3.4 ns average lifetime, depending on the fit. Alternatively, a stretched-exponential fit gave τ = 1.73 ns and β = 0.57, indicating a rather broad spread of lifetime values (Figure A.5). A mean value of 2.7–2.8 ns was obtained from a stretchedexponential fit. The presence of several components with different decay kinetics is also confirmed by non-negative least-squares analysis of the numerical inversion of the Laplace transform describing the luminescence intensity decay (Figure A.6). This approach yields a distribution of lifetimes in the 0.2–6 ns range with an average value of τ = 1.9 ns. The decrease in luminescence lifetime and quantum yield is attributed to PTZ→*Re ET. The complex excited-state behavior suggests 137 there are multiple geometries accessible to PTZ-bridge-Re that possess distinct ET rates. Charge recombination must be equally rapid; a PTZ radical cation signal was not observed by transient absorption 10 ns after excitation.13 Unlike rigid oligo(para-phenylene ethynylene) DBA complexes, metalinked species are flexible, allowing short through-space donor-acceptor distances. We applied theoretical calculations using a UFF force field to geneate minimized compact (C-shaped) (Figure A.7a) and extended (Figure A.7b) conformations of PTZ-bridge-Re.51 Intramolecular PTZ → *Re ET in the extended conformation was slow (the lifetime was very near that of the model compound). In the C-shaped conformation, the short through-space distance greatly enhances D:A coupling, which in turn promotes PTZ → *Re ET. The multiexponential character of the decay is consistent with multiple close-contact geometries that exhibit nanosecond electron transfer rates. Interconversion of the extended and C-shaped conformers must be relatively slow (microsecond range or longer) to be consistent with the observed biphasic *Re decay kinetics. Analysis of ET rates for DBA systems linked by oligo(para-phenylene ethynylene) bridges give exponential distance decay (β) constants of 0.3–0.6 Å1 8,28,52 . Meta-linked systems are likely to have higher β values compared to their linear para-linked counterparts; these values, coupled with an estimated ET rate of 1.6 × 1011 s-1 for [Re(CO)3(bpy)(py)]+ and PTZ at contact,13 predict an intramolecular ET rate for extended PTZ-bridge-Re of between 8 × 103 s-1 and 4 × 106 s-1. Molecular modeling of the compact conformation places PTZ and Re within 7.5 Å (C-to-C), consistent with nanosecond ET (Figure A.7).53,54 138 139 Figure A.3. Time-resolved emission decay profiles of PTZ-bridge-Re: (a) monitored to 5 μs; (b) monitored to 40 ns 140 Figure A.4. Concentration-dependent luminescence of PTZ-bridge-Re in CH2Cl2 141 Figure A.5. Stretched-exponential fit of the rapid decay component, τ = 1.72, β = 0.574, <τ> = 2.7 ns 142 Figure A.6. P(k) distributions extracted from non-negative least squares analysis of the rapid component of the luminescence decay. Top panel: residual difference between observed and calculated intensity with minimized χ2. Middle panel: luminescence decay data (circles) fit to a sum of exponentials (line). Bottom panel: probability distribution of rates (log(k)) with an average value of τ = 1.9 ns 143 Figure A.7. Structural models: (a) compact conformation of PTZ-bridge-Re; (b) extended conformation of PTZ-bridge-Re. Structures were modeled using Avogadro molecular editing software.50 Aggregation studies Donor-bridge-acceptor complexes containing meta-phenylene ethynylene linkages have been shown to aggregate under certain conditions.3 Self-association often leads to enhanced quenching, due to the formation of aggregates with short intermolecular donor-acceptor distances. To verify that no aggregation is occuring in PTZ-bridge-Re, a combination of UV-vis, steady-state fluoroescence, and NMR spectroscopies were employed. Absorption and emission spectra of PTZ-bridge-Re at concentrations varying from 32 µM to 1.51 mM did not exhibit significant change in peak position, and increasing concentration did not produce additional spectral features (Figures A.8 and A.9). NMR spectra of PTZ-bridge-Re in dichloromethane solution of concentrations ranging from 0.13 mM to 1.35 mM did not exihibit significant change in the position of resonances, and no additional resonances were observed as concentration was varied (Figure A.10). These results suggest that there is no aggregation under our experimental conditions. 144 145 Figure A.8. UV-vis absorption spectra of PTZ-bridge-Re in CH2Cl2 at various concentrations 146 Figure A.9. Steady-state emission spectra of PTZ-bridge-Re in CH2Cl2 at various concentrations 147 Figure A.10. 1H-NMR spectra of PTZ-bridge-Re complex in CD2Cl2 at various concentrations Computational modeling Density-functional theoretical calculations allowed us to determine the relative energies of two rotational conformers of the bridging tris(meta-phenyleneethynylene) ligand. To decrease computational time, we used a model bridge with no ethyl ester groups (Figure A.11). We wanted to examine the possibility that the donor and acceptor can be both close and remote from one another, which would account for the two sets of observed PTZ-bridge-Re emission lifetimes. We found that we could begin with a model where the bridge is in an extended conformation (and fall into one minimum), or from a model of a fully compact state (in which case we calculated a different minimum energy structure). Both of these geometries were validated as true ground states via numerical frequency calculations; there were no imaginary frequencies or unreasonable bond lengths. According to our calculations, the compact and extended conformers are virtually isoenergetic. 148 149 Figure A.11. Calculated m-phenylene-ethynylene fragment in the compact (a) and extended (b) conformations obtained from DFT calculations A.4 Concluding remarks Electronically excited PTZ-bridge-Re, where a Re electron acceptor is linked by tris(meta-phenylene-ethynylene) bridge to a PTZ donor, exhibits very rapid intramolecular electron transfer. Computational modeling suggests that the reaction occurs in a C-shaped conformer with PTZ-Re at 7.5 Å. The observed biphasic decay shows that the conformational dynamics are substantially shorter than the *Re lifetime, which is on the order of a microsecond. In principle, conformational changes in flexible bridges such as meta-phenylene-ethynylene could retard charge recombination reactions, extending charge-separated lifetimes. In our case, however, back electron transfer is much faster than conversion to the extended (slow electron transfer) structure, disallowing production of long-lived charge-separated extended structures. Variations in electrostatic interactions between D:A pairs could be employed in designs that favor compact conformations prior to electron transfer, but strongly destabilize them following ET. A case in point would be to develop a system consisting of a neutral donor (D) and dicationic acceptor (A2+), which would not experience any charge-charge electrostatic interaction until the formation of ET products D+ and A+. The resulting repulsion would destabilize the compact conformer relative to extended structures, allowing charge separaration to persist for relatively long times. We suggest that tris(meta-phenylene-ethynylene) could be useful as a linker for such D:A pairs, as spectroscopic measurements conducted on PTZ-bridge-Re show that the compact conformation is favored energetically. Repulsive 150 electrostatic forces triggered by ET may be sufficient to drive the conformer equilibrium toward extended structures. We postulate that the pair N,N,N’,N’tetramethyl-p-phenylenediamine (TMPD): ruthenium(II)-tris(bipyridine) could be connected by a phenylene-ethynylene bridge by procedures similar to those used to construct PTZ-bridge-Re. ET-triggered conformational change of this DBA complex would be expected to produce long-lived charge separation that could drive useful multielectron redox chemistry. A.5 Materials and methods General information. Unless otherwise noted, all starting materials were obtained from commercial suppliers and were used without further purification. All air- or moisture-sensitive reactions were performed under an atmosphere of dry nitrogen. Analytical thin-layer chromatography was performed with Kieselgel F-254 precoated TLC plates. Flash column chromatography was carried out with silica gel 60 (230–400 mesh) from EMD Chemicals. Materials. 6-bromo bipyridine, zinc chloride, bis(triphenylphosphine)palladium(II) dichloride, tetrabutyl ammonium tetrafluoroborate, triethyl amine, silver perchlorate, pyridine, pyridine, phenothiazine, and propargyl bromide were purchased from Sigma Aldrich. Rhenium pentacarbonyl chloride was obtained from Strem Chemicals. NMR spectroscopy. NMR spectra were recorded on a INOVA 600 MHz spectrometer equipped with an inverse HCN triple resonance probe with x-, y-, and z-gradients. All spectrometers were running Varian VNMRJ software. Chemical 151 shifts are expressed in parts per million using residual solvent protons as the internal standard (5.32 ppm for CH2Cl2). Coupling constants, J, are reported in Hertz (Hz), and splitting patterns are designated as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad). MestReNova NMR 5.3.2 software was used to analyze all NMR spectra. Mass spectrometry. Mass spectra were obtained through the Mass Spectrometry Facility, Department of Chemistry, California Institute of Technology. Fast atom bombardment (FAB) mass spectra were recorded using a Jeol JMS600H high resolution double-focusing mass spectrometer. Electrospray Ionization (ESI) mass spectra were obtained using a Waters Premier Xe time-of-flight mass spectrometer. Visible spectroscopy. Spectroscopic measurements, including UV-vis, steadystate, and time-resolved luminescence, were carried out at the Beckman Institute Laser Resource Center (BILRC). Prior to any experiments, samples were degassed by three freeze-pump-thaw cycles, unless otherwise noted. UV-vis spectroscopy. UV-vis absorbance spectra were recorded on an Agilent 8453 UV-vis spectrometer using a pure sample of the solvent as the background. Steady-state fluorescence spectroscopy. Steady-state emission spectra were recorded on a Jobin Yvon Spex Fluorolog-3-11. A 450W xenon arc lamp was used as the excitation source with a single monochromator providing wavelength selection. Right-angle light emission was sorted using a single monochromator and fed into a Hamamatsu R928P photomultiplier tube with photon counting. Shortand long-pass filters were used where appropriate. Spectra were recorded on Datamax software and plotted with Origin software. 152 Time-resolved spectroscopy. For experiments focusing on the microsecond timescale, samples were excited at 355 nm with 8 ns pulses from the third harmonic of a Q-switched Nd:YAG laser (Spectra-Physics Quanta-Ray PRO-Series) operating at 10 Hz. Emission wavelengths were selected using a double monochromator (Instruments SA DH-10) with 1 mm slits. Luminescence was detected with a photomultiplier tube (PMT, Hamamatsu R928). The PMT current was amplified and recorded with a transient digitizer (Tektronix DSA 602). Short- and long-pass filters were employed to remove scattered excitation light. Decay traces were averaged over 500 laser pulses. Instruments and electronics in this system were controlled by software written in LabVIEW (National Instruments). Data manipulation was performed and plotted using MATLAB R2008a (Mathworks, Inc.), as well as Microsoft Excel. For experiments focusing on the nanosecond timescale, samples were excited with 1 ps pulses at 355 nm from the third harmonic of a regeneratively amplified mode-locked Nd:YAG laser operating at 10 Hz. Emission at 560 nm was collected on a picosecond streak camera (Hamamatsu C5680 in photon-counting mode) with a 500 nm long-pass filter to exclude scattered light. The measurements were made under magic-angle conditions and data were collected over a 50 ns sweep range with 8,000 exposures. Synthesis and characterization of compounds Synthesis of 10-(prop-2-yn-1-yl)-10H-phenothiazine (1). 10-(prop-2-yn-1-yl)10H-phenothiazine was prepared according to literature procedure.55 153 Synthesis of ethyl 3-bromo-5-iodobenzoate (2). Ethyl 3-bromo-5-iodobenzoate was synthesized according to literature procedure.56 O N O Br S Synthesis of ethyl 3-(3-(10H-phenothiazin-10-yl)prop-1-yn-1-yl)-5- bromobenzoate (3). A microwave flask was charged with 2 (1.55 g, 4.3 mmol), 1 (1.16 g, 4.89 mmol), bis(triphenylphosphine)palladium(II) dichloride (0.102 mg, 0.14 mmol), and zinc chloride (1.108 g, 8.1 mmol), and transferred to the glovebox. Degassed triethylamine (4.86 mL) and tetrahydrofuran (24.36 mL) were added to the mixture in a glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 2 hours. Purification with flash chromatography using hexane/ethyl acetate (9:0.5) afforded 0.89 g product (Yield: 35%). 1H NMR (600 MHz, CD2Cl2) δ 8.12 (t, J = 1.7, 1H), 8.02 (t, J = 1.5, 1H), 7.76 (t, J = 1.7, 1H), 7.28–7.20 (m, 4H), 7.14 (dd, J = 7.6, 1.4, 2H), 6.97 (td, J = 7.5, 1.3, 2H), 4.77 (s, 2H), 4.35 (q, J = 7.1, 2H), 1.37 (t, J = 7.1, 3H). 13C NMR (151 MHz, CD2Cl2) δ 165.10, 144.89, 138.96, 133.37, 133.19, 132.06, 128.33, 127.76, 125.46, 124.09, 123.75, 122.86, 115.60, 87.81, 84.33, 62.48, 39.85, 14.84. HRMS-ES (m/z) [M+] measured 465.0228, calculated: 465.0221. 154 O 155 O N Si S Synthesis of ethyl 3-(3-(10H-phenothiazin-10-yl)prop-1-yn-1-yl)-5- trimethylsilyl)-ethynyl)benzoate. A microwave flask was charged with 3 (1.04 g, 2.2 mmol), bis(triphenylphosphine)palladium(II) dichloride (51 mg, 0.072 mmol), and zinc chloride (0.45 g, 3.3 mmol), and transferred to the glovebox. Degassed triethylamine (2.46 mL), degassed trimethylsilylacetylene (0.35 mL, 2.46 mmol), and tetrahydrofuran (11 mL) were added to the mixture in the glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 2 hours. Purification with flash chromatography using hexane/ethyl acetate (9:0.5) afforded 0.78 g product (Yield: 73%). 1H NMR (600 MHz, CD2Cl2) δ 8.04 (t, J = 1.6, 1H), 8.02 (t, J = 1.6, 1H), 7.68 (t, J = 1.6, 1H), 7.30–7.20 (m, 4H), 7.14 (dd, J = 7.6, 1.5, 2H), 6.98 (td, J = 7.4, 1.2, 2H), 4.76 (s, 2H), 4.35 (q, J = 7.1, 2H), 1.37 (t, J = 7.1, 3H), 0.24 (s, 9H). 13C NMR (151 MHz, CD2Cl2) δ 165.64, 144.94, 139.29, 133.38, 133.10, 132.09, 128.34, 127.75, 124.76, 124.12, 124.04, 123.73, 115.64, 103.61, 87.04, 84.95, 62.27, 47.14, 39.92, 14.90, 0.34. HRMS-ES (m/z) [M+] measured 481.15, calculated: 481.15. O 156 O N H S Synthesis of ethynylbenzoate ethyl (4). Ethyl 3-(3-(10H-phenothiazin-10-yl)prop-1-yn-1-yl)-53-(3-(10H-phenothiazin-10-yl)prop-1-yn-1-yl)-5- trimethylsilyl)-ethynyl)benzoate (0.25 g, 0.53 mmol) was reacted with potassium carbonate (6.25 mg, 0.045 mmol) in a mixture of dichloromethane (1.25 mL) and methanol (3.12 mL). The reaction was allowed to react until the staring material was completely consumed. The reaction was monitored via thin layer chromatography. Purification with flash chromatography using hexane/ethyl acetate (9:0.5) afforded 93.3 mg product (Yield: 43%). 1H NMR (600 MHz, CD2Cl2) δ 8.00 (t, J = 1.6, 1H), 7.97 (t, J = 1.6, 1H), 7.63 (t, J = 1.7, 1H), 7.31–7.08 (m, 6H), 6.96 (td, J = 7.5, 1.2, 2H), ), 4.68 (s, 2H), 4.35 (q, J = 7.1, 2H), 3.20 (s, 1H), 1.37 (t, J = 7.1, 3H). 13C NMR (151 MHz, CD2Cl2) δ 165.42, 144.80, 139.41, 133.46, 133.34, 132.03, 128.19, 127.61, 123.99, 123.95, 123.59, 123.51, 115.50, 87.05, 84.65, 82.23, 79.42, 62.19, 39.77, 14.71. HRMS-ES (m/z) [M+] measured 409.11, calculated: 409.11. O 157 O O O N Br S Synthesis of ethyl 3-(3-(10H-phenothiazin-10-yl)prop-1-yn-1-yl)-5-((3-bromo5-(ethoxycarbonyl)phenyl)ethynyl)benzoate (5). A microwave flask was charged with 2 (0.22 g, 0.62 mmol), 4 (0.26, 0.65 mmol), bis(triphenylphosphine)palladium(II) dichloride (1.36 mg, 0.002 mmol), and zinc chloride (0.126 g, 0.93 mmol), and transferred to a glovebox. Degassed triethylamine (0,65 mL) and tetrahydrofuran (3.1 mL) were added to the mixture in the glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 2 hours. Purification with flash chromatography using hexane/ethyl acetate (18/1) afforded 0.2 g product (Yield: 51%). 1H NMR (600 MHz, CD2Cl2) δ 8.07 (t, J = 1.7, 1H), 8.05 (t, J = 1.6, 1H), 8.04 (t, J = 1.5, 1H), 7.99 (t, J = 1.6, 1H), 7.77 (t, J = 1.7, 1H), 7.69 (t, J = 1.6, 1H), 7.23–7.04 (m,6H), 6.89 (td, J = 7.5, 1.2, 2H), 4.69 (s, 2H), 4.29 (m, 4H), 1.31 (td, J = 7.1, 3.2, 6H). 13C NMR (151 MHz, CD2Cl2) δ 165.38, 164.81, 144.64, 138.73, 138.46, 133.20, 133.08, 133.00, 132.89, 131.98, 131.76, 128.01, 127.42, 125.25, 123.94, 123.77, 123.66, 123.41, 122.67, 115.33, 89.81, 88.71, 86.90, 84.50, 62.19, 62.05, 39.62, 14.51, 14.50. HRMS-ES (m/z) [M+] measured: 637.0718, calculated 637.0745, error: 4.3 ppm. 158 Si N N Synthesis of 6-((trimethylsilyl)ethynyl)-2,2'-bipyridine (7). A microwave flask was charged with 6-bromo-2,2'-bipyridine (0.375 g,1.59 mmol), bis(triphenylphosphine)-palladium(II) dichloride (66 mg, 0.094 mmol), and zinc chloride (0.32 g, 2.34 mmol), and transferred to a glovebox. Degassed triethylamine (3.18 mL), degassed trimethylsilylacetylene (0.45 mL, 3.18 mmol), and tetrahydrofuran (7.95 mL) were added to the mixture in the glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 3 hours. Purification with flash chromatography using CH2Cl2/MeOH (99/1) afforded 0.32 g product (Yield: 79%). 1H NMR (600 MHz, CD2Cl2) δ 8.35 (ddd, J = 4.8, 1.8, 0.9, 1H), 8.12 (dt, J = 8.0, 1.1, 1H), 8.08 (dd, J = 8.0, 1.1, 1H), 7.53 (ddd, J = 8.0, 7.5, 1.8, 1H), 7.48 (t, J = 7.8, 1H), 7.17 (dd, J = 7.7, 1.0, 1H), 7.03 (ddd, J = 7.5, 4.8, 1.2, 1H), 0.00 (s, 9H). 13C NMR (151 MHz, CD2Cl2) δ 156.85, 155.89, 149.76, 142.87, 137.64, 137.48, 128.01, 124.25, 121.60, 121.04, 104.55, 45.62, 0.00. HRMS-ES (m/z) [M+H] observed 252.1160, calculated 253.1161, error 0.4. H N N Synthesis of 6-ethynyl-2,2'-bipyridine (8). To a solution of 7 (0.1 g, 0.40 mmol) in tetrahydrofuran (3.5 mL) was added a solution of tetrabutylammonium flouride in tetrahydrofuran (0.43 mL, 1.0 M). The solution was allowed to react for 5 min then was concentrated in vacuo. The crude product was purified by silica gel column chromatography (CH2Cl2/MeOH, 99/1) to give 70 mg product (Yield: 97%). 1H NMR (600 MHz, CD2Cl2) δ 8.67 (ddd, J = 4.7, 1.7, 0.9, 1H), 8.46–8.43 (m, 2H), 7.87–7.78 (m, 2H), 7.52 (dd, J = 7.6, 1.0, 1H), 7.34 (ddd, J = 7.5, 4.8, 1.2, 1H), 3.26 (s, 1H). 13C NMR (151 MHz, CD2Cl2) δ 157.30, 155.95, 149.94, 142.28, 137.94, 137.68, 128.29, 124.88, 121.78, 121.59, 83.74, 77.25. HRMS-ES (m/z) [M+] M+H measured 181.0772, calculated 181.0766, error: 3.5. N N O O Br Synthesis of ethyl 3-(2,2'-bipyridin-6-ylethynyl)-5-bromobenzoate (9). A microwave flask was charged with 2 (0.114 g, 0.32 mmol), 8 (0.07 g, 0.39 mmol), bis(triphenylphosphine)-palladium(II) dichloride (8.4 mg, 0.012 mmol), and zinc chloride (66 mg, 0.48 mmol), and transferred to a glovebox. Degassed triethylamine (0.38 mL), and degassed tetrahydrofuran (1.61 mL) were added to the mixture in the glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 3 hours and then it was concentrated in vacuo. The crude product was purified by silica gel column chromatography (dichloromethane/methanol, 99/1) to give 70 mg product (Yield: 54%). 1H NMR (600 MHz, CD2Cl2) δ 8.62–8.57 (m, 1H), 8.43–8.29 (m, 2H), 8.16 (t, J = 1.5, 1H), 159 8.13 – 8.07 (m, 1H), 7.89 (t, J = 1.7, 1H), 7.80 – 7.76 (m, 2H), 7.51 (dd, J = 7.6, 1.0, 1H), 7.27 (ddd, J = 7.5, 4.8, 1.1, 1H), 4.31 (q, J = 7.1, 2H), 1.33 (t, J = 7.1, 3H). 13C NMR (151 MHz, CD2Cl2) δ 165.09, 155.96, 149.98, 139.07, 138.00, 137.70, 135.85, 134.08, 133.55, 132.39, 128.26, 125.41, 125.30, 124.90, 123.00, 121.81, 121.51, 91.42, 87.02, 62.52, 14.81. HRMS-ES (m/z) [M+H] measured 409.0358, calculated 409.0275, error: 4.1 N N O O Si Synthesis of ethyl 3-(2,2'-bipyridin-6-ylethynyl)-5- ((trimethylsilyl)ethynyl)benzoate (9a). A microwave flask was charged with 9 (59 mg, 0.144 mmol), bis(triphenylphosphine)-palladium(II) dichloride (5.6 mg, 0.0079 mmol), and zinc chloride (30 mg, 0.22 mmol), and transferred to the glovebox. Degassed triethylamine (0.22 mL), degassed trimethylsilylacetylene (30 μL, 0.217 mmol), and degassed tetrahydrofuran (0.72 mL) were added to the mixture in the glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 3 hours and then it was concentrated in vacuo. The crude product was purified by silica gel column chromatography (CH2Cl2/MeOH, 99/1) to give 20 mg product (Yield: 33%). 1H NMR (600 MHz, CD2Cl2) δ 8.44–8.38 (m, 1H), 8.27–8.07 (m, 2H), 7.96 (t, J = 1.6, 1H), 7.84 (t, J = 1.6, 1H), 7.68–7.50 (m, 160 3H), 7.31 (dd, J = 7.6, 1.0, 1H), 7.17–6.98 (m, 1H), 4.11 (q, J = 7.1, 2H), 1.13 (t, J = 7.1, 3H), 0.00 (s, 9H). HRMS-ES (m/z) [M+H] measured 425.1682, calculated 425.1685, error 0.8 ppm. N N O O H Synthesis of ethyl 3-(2,2'-bipyridin-6-ylethynyl)-5-ethynylbenzoate (10). To a solution of 9a (0.1 g, 0.23 mmol) in tetrahydrofuran (2.12 mL) was added a solution of tetrabutylammonium flouride in tetrahydrofuran (0.26 mL, 1.0 M). The solution was allowed to react for 5 min then was concentrated in vacuo. The crude product was purified by silica gel column chromatography (CH2Cl2/MeOH, 99/1) to give 0.78 g product (Yield: 96%). 1H NMR (600 MHz, CD2Cl2) δ 8.69 (ddd, J = 4.8, 1.8, 0.9, 1H), 8.48 (dt, J = 8.0, 1.1, 1H), 8.47–8.44 (m, 1H), 8.29 (t, J = 1.6, 1H), 8.17 (t, J = 1.6, 1H), 7.93 (t, J = 1.6, 1H), 7.89–7.87 (m, 2H), 7.60 (dd, J = 7.6, 1.1, 1H), 7.37 (ddd, J = 7.5, 4.8, 1.2, 1H), 4.40 (q, J = 7.1, 2H), 3.26 (s, 1H), 1.42 (t, J = 7.1, 3H). 13C NMR (151 MHz, CD2Cl2) δ 164.83, 156.73, 155.35, 149.26, 142.10, 138.92, 137.26, 136.98, 133.24, 133.10, 131.61, 127.53, 124.17, 123.29, 123.12, 121.11, 120.71, 90.28, 86.39, 81.63, 78.92, 61.66, 14.12. HRMS-ES (m/z) [M+H] measured 353.1289, calculated 353.1290, error: 0.3 ppm. 161 162 O O O O N S O O N N Synthesis of ethyl 3-(3-(10H-phenothiazin-10-yl)prop-1-ynyl)-5-((3-((3-(2,2'bipyridin-6-ylethynyl)-5-(ethoxycarbonyl)phenyl)ethynyl)-5ethoxycarbonyl)phenyl)ethynyl)-benzoate (11). A microwave flask was charged with 5 (0.116 g, 0.18 mmol), 10 (0. 89 g, 0.25 mmol) bis(triphenylphosphine)palladium(II) dichloride (5.06 mg, 0.0072 mmol), and zinc chloride (36.57 mg, 0.27 mmol), and transferred to the glovebox. Degassed triethylamine (0.26 mL) and degassed tetrahydrofuran (1.8 mL) were added to the mixture in the glovebox and the flask was sealed. The mixture was stirred at 100 °C in the microwave for 3 hours and then it was concentrated in vacuo. The crude product was purified by silica gel column chromatography (CH2Cl2/MeOH, 99/1) to give 82 mg product 1 (Yield: 50%). H NMR (600 MHz, CD2Cl2) δ 8.69 (ddd, J = 4.8, 1.8, 0.9, 1H), 8.49 (dt, J = 8.0, 1.1, 1H), 8.46 (dd, J = 8.0, 1.1, 1H), 8.29 (t, J = 1.6, 1H), 8.23 (t, J = 1.6, 1H), 8.21 (t, J = 1.6, 1H), 8.19 (t, J = 1.6, 1H), 8.17 (t, J = 1.6, 1H), 8.08 (t, J = 1.6, 1H), 7.99 (t, J = 1.6, 1H), 7.90 (t, J = 1.6, 1H), 7.89–7.84 (m, 2H), 7.80 (t, J = 1.6, 1H), 7.61 (dd, J = 7.6, 1.1, 1H), 7.36 (ddd, J = 7.5, 4.7, 1.2,1 H), 7.33–6.94 (m, 8H), 4.79 (s, 2H), 4.47–4.35 (m, 6H), 1.57–1.30 (m, 9H). 13C NMR (151 MHz, CD2Cl2) δ 164.80, 156.63, 155.41, 149.16, 144.13, 142.02, 138.30, 138.23, 138.02, 137.16, 136.87, 132.85, 132.77, 132.56, 132.54, 132.47, 132.39, 131.60, 131.57, 131.45, 127.51, 127.44, 126.91, 124.06, 123.55, 123.52, 123.51, 123.40, 123.37, 123.30, 123.26, 122.90, 121.01, 120.60, 114.83, 90.23, 88.90, 88.86, 88.84, 88.81, 86.38, 86.36, 84.01, 61.58, 61.57, 61.53, 39.16, 14.04, 14.036, 14.02. HRMS-ES (m/z) [M+H] measured 908.2791, calculated 908.2794, error 0.4. + COOEt COOEt N S COOEt N CO CO Re N N CO 163 Synthesis of PTZ-bridge-Re (12). 13 (40 mg, 0.044 mmol) was reacted with rhenium pentacarbonyl chloride in toluene (1.78 mL). After refluxing the reaction mixtures overnight under argon, the reaction mixture was concentrated in vacuo. The crude product (32 mg, 0.032 mmol) was reacted with silver perchlorate (8.16 mg, 0.0494 mmol) in methanol/toluene (4:1) mixture for 1 day at room temperautre in the dark. Pyridine (76 mg, 0.96 mmol) was added to the reaction mixture and the resulting reaction mixture was stirred at 50 °C for 5 days. Excess pyridine was removed in vacuo. The crude product was purified by silica gel column chromatography (CH2Cl2/MeOH, 95/5) to give 20 mg product (Yield: 36%). 1H NMR (600 MHz, CD2Cl2) δ 9.16 (dd, J = 5.5, 0.8, 1H), 8.46 (d, J = 8.3, 1H), 8.42 (dd, J = 8.3, 1.1, 1H), 8.30–8.27 (m, 1H), 8.27–8.19 (m, 3H), 8.14 (t, J = 1.6, 1H), 8.12–8.09 (m, J = 3H), 8.08 (t, J = 1.6, 1H), 8.02 (dd, J = 7.9, 1.2, 1H), 8.00–7.97 (m, 2H), 7.83 (t, J = 1.6, 1H), 7.81–7.69 (m, 3H), 7.31–7.24 (m, 2H), 7.24–6.85 (m, 8H), 4.70 (s, 2H), 4.49–4.19 (m, 6H), 1.33 (m, 9H). 13C NMR (151 MHz, CD2Cl2) δ 165.42, 165.40, 165.11, 156.96, 156.66, 153.43, 152.38, 147.08, 144.74, 142.09, 141.73, 141.03, 140.48, 139.32, 139.07, 138.83, 138.63, 134.93, 133.79, 133.76, 133.60, 133.33, 133.19, 133.12, 133.00, 132.53, 132.25, 132.09, 131.42, 129.51, 128.11, 127.70, 127.52, 126.47, 125.04, 124.58, 124.20, 123.88, 123.51, 123.08, 122.07, 115.44, 100.35, 90.08, 89.49, 89.30, 89.23, 88.95, 86.99, 84.62, 62.44, 62.23, 62.16, 39.73, 14.64, 14.62. HRMS-FAB (m/z) [M+] measured 1257.2554, calculated 1257.2543. 164 Fitting of the fast component of PTZ-bridge-Re luminescence decay The data collected over the first 50 ns were used to determine the kinetics of the more-rapid decay component. Single and double exponential fitting are incapable of adequately representing the data, even when an additional 530 ns decay component is added. A three-exponential equation (plus a fixed 530 ns component) models the data well, with lifetimes, τ = 10.0 ± 0.8, 2.7 ± 0.2, 0.56 ± 0.02 ns, and an average lifetime <τ> = 3.2 ns. A stretched-exponential function plus a fixed 530 ns exponential was applied and produced values of τ = 1.72, β = 0.574, <τ> = 2.7 ns (Figure S6). These values were relatively unchanged (τ = 1.73, β = 0.571, <τ> = 2.8 ns) when the 530 ns term was removed. y = Ae t β τ −� � 〈𝜏 〉 = + Be −� t � 5.3 × 10−7 𝑡 𝛤 (𝛽 −1 ) 𝛽 In light of the observed non-exponential decay of the faster component (discussed above) we also obtained a rate distribution (P(k)) from the numerical inversion of the Laplace transform that describes the time-resolved luminescence intensity I(t) = Σ P(k)exp(-kt). The data was fit using a MATLAB (The Mathworks) algorithm (LSQNONNEG) that minimizes the sum of the squared deviations (χ2) between the observed and calculated values of I(t), subject to the non-negativity constraint (non-negative least-squares analysis). This produced a P(k) distribution with lifetimes (τ) in the 0.2–6 ns range and an average value of τ = 1.9 ns, in good agreement with the three exponential and stretched-exponential fits. 165 Computational modeling Density functional theoretical calculations were conducted using the ORCA computational chemistry package (F. Neese, version 2.8.0), applying the generalized gradient approximation density functional PW91.57 Initial geometry optimizations were performed using a 2ζ-quality SV-ZORA basis set,58 which employs the zeroth-order relativistic approximation (ZORA) to the Dirac equation to account for relativistic effects. Additionally, the conductor-like screening model (COSMO)59 of a continuum solvent was used, with parameters appropriate for CH2Cl2 solvation, and the empirical van der Waals correction of Grimme was applied.60,61 The initial geometry solutions were then used as input to run more intensive calculations with a 3ζ-quality TZV-ZORA basis set. To test the validity of the calculated geometric parameters, numerical frequency calculations were performed. The lack of any negative (imaginary) frequencies appearing in the list of computed frequencies strongly suggests that we found true minima rather than saddle points on the energy surface. A.6 References (1) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. Chem. Rev. 2010, 110, 7024. (2) Siegbahn, P. E. M.; Blomberg, M. R. A. Chem. Rev. 2010, 110, 7040. (3) Lewis, N. S.; Nocera, D. G. PNAS 2006, 103, 15729. (4) Eisenberg, R.; Gray, H. B. Inorg. Chem. 2008, 47, 1697. (5) Gray, H. B. Nat. Chem. 2009, 1, 7. (6) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515. (7) Saragi, T. P. I.; Spehr, T.; Siebert, A.; Fuhrmann-Lieker, T.; Salbeck, J. Chem. Rev. 2007, 107, 1011. (8) Wenger, O. S. Acc. Chem. Res. 2011, 44, 25. 166 (9) Kilså, K.; Kajanus, J.; Macpherson, A. 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(57) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533. 168 (58) Pantazis, D. A.; Chen, X. Y.; Landis, C. R.; Neese, F. J. Chem. Theory Comput. 2008, 4, 908. (59) Sinnecker, S.; Rajendran, A.; Klamt, A.; Diedenhofen, M.; Neese, F. J. Phys. Chem. A 2006, 110, 2235. (60) Grimme, S. J. J. Comput. Chem. 2004, 25, 1463. (61) Grimme, S. J. J. Comput. Chem. 2006, 27, 1787. 169 170 Appendix B: Photophysics of Ir(III) corroles The text in this chapter was taken in part from: Palmer, J. H.; Durrell, A. C.; Gross, Z.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2010, 132, 9230–9231. B.1 Abstract My interest in corroles began following Joshua Palmer’s synthesis of Ir(III) corroles. He had been interested in exploring their luminescence properties, and I had trained him on the fluorometer. Following his initial round of experiments, he presented data showing luminescence in the near-IR, at the far end of our instrument’s detector limit. I immediately dismissed this evidence as nothing more than an instrument artifact. Fortunately, Josh persisted, and as the evidence accumulated, it became evident that something was there. After purchasing a detector with sensitivity deeper in the near-IR, we were able to more fully characterize the photophysical properties of Ir(III) corroles. The results of our initial collaboration resulted in the publication of our work “Near-IR Phosphorescence of Ir(III) Corroles at Ambient Temperature” in the Journal of the American Chemical Society. This work is presented here. The photophysical properties of Ir(III) corroles differ from those of phosphorescent porphyrin complexes, cyclometalated and polyimine Ir(III) compounds, and other luminescent metallocorroles. Ir(III) corrole phosphorescence is observed at ambient temperature at wavelengths much longer (> 800 nm) than those of most Ir(III) phosphors. The solvatochromic behavior of Ir(III)-corrole Soret and Q absorption bands suggests that the lowest singlet excited states (S2 and S1) are substantially more polar than the ground state. 171 172 B.2 Introduction Porphyrin complexes displaying phosphorescence at ambient temperatures have been employed for photodynamic therapy,1 oxygen detection,2 and organic light-emitting diodes.3 A great deal of research has been done on d8 (mainly PtII, PdII, and AuIII) complexes,4 which emit at relatively long wavelengths (> 600 nm) with lifetimes in the microsecond range. In sharp contrast, d6 metalloporphyrins have scarcely been investigated, although room-temperature phosphorescence of ruthenium(II) porphyrins has been reported.5 Metallocorroles have shown promise as therapeutic agents,6 with biodistribution and bioavailability profiles as well as cellular uptake and intracellular locations7 determined for fluorescent gallium(III) derivatives.8 Although progress has been made, much work remains before we can claim to have developed optimized compounds for optical examination of biological systems.9 It would be beneficial to have agents that emit with microsecond lifetimes beyond 700 nm, as the most common obstacles to efficient biological imaging—tissue absorbance and intrinsic fluorescence—could thus be circumvented. Here we report the photophysical properties of iridium(III) corroles,10 which differ significantly from those of cyclometalated and polyimine Ir(III) compounds,11 other luminescent metallocorroles,12 and free-base corroles.13 Iridium(III) corrole phosphorescence is observed at ambient temperature at wavelengths much longer (> 800 nm) than those of most other luminescent Ir(III) 11 complexes. Our investigations focused on the three corroles shown in Figure B.1. Their absorption spectra are shown in Figure B.2. B.3. Results and discussion. Emission spectra were recorded in toluene at 298 and 77 K (Figure B.3). The spectra display two intense features separated by approximately 1400 cm-1. It is likely that a ring-based vibration is excited in the transition to the lower energy component.14 Excitation spectra conducted at the low- and high-energy bands are identical (Figure B.4). At low temperature, the higher energy emission maximum of each Ir(III) corrole blue shifts by 10 nm (a rigidochromic effect indicating that the transition involves charge transfer)15 (Figure B.3b). The narrow linewidth and ~ 1400 cm-1 spacing suggest that the emission is from a π→π* excited state. Electronic structure calculations place an occupied orbital with metal character close to the HOMO in related Ir(III) corroles,16 indicating that 3MLCT states will be near those of the lowest corrole-localized excited states. Other investigators have reported that 3MLCT is the emissive state in cyclometalated Ir(III) complexes.17 Luminescence quantum yields and lifetimes in degassed and aerated toluene solutions at room temperature (and lifetimes at 77 K) are set out in Table B.1. 1-Ir(tma)2 and 1b-Ir(tma)2 have relatively short lifetimes and low quantum yields. 1-Ir(py)2 exhibits a much higher luminescence quantum yield (1.2%) and a longer lifetime.18 It is apparent even from these initial results that Ir(III)-corrole photophysical properties depend markedly on the nature of the axial ligand. 173 174 Figure B.8. Iridium(III) corroles 175 Figure B.2. UV-vis spectra of Ir(III) corroles in toluene solution at 298 K 176 a b Figure B.3. Emission spectra of Ir(III) corroles in toluene solution (λex = 496.5 nm) (a) 298 K; (b) 77 K. 177 Figure B.4. Normalized excitation profiles of (top to bottom): 1-Ir(tma)2, 1bIr(tma)2, and 1-Ir(py)2 monitored at emission wavelengths of 790 and 890 nm 178 Table B.1. Photophysical data for Ir(III) corroles in toluene solutiona a Compound 1-Ir(tma)2 1b-Ir(tma)2 1-Ir(py)2 Φph 3.3x10-4 3.9x10-3 1.2x10-2 λAr / nm (τAr / µs) 788 (0.220) 795 (1.19) 792 (4.91) λ77K / nm (τ77K / µs) 786 (2.77) 786 (4.72) 793 (7.69) τair / µs 0.170 0.760 0.380 kr / s-1 1.5x103 3.28x103 2.44x103 knr / s-1 4.54x106 8.4x105 2.0x105 At 298 K unless noted otherwise The effect of solvent polarizability on Ir(III)–corrole spectra was investigated to probe the extent of charge transfer in initially formed electronic excited states. UV-vis spectra were obtained in a variety of solvents: band maxima were plotted against polarizability f (Figure B.5), defined as f(n) = (n2 1)/(2n2 + 1),19 where n is the refractive index of the solvent. (The ground states are relatively nonpolar, so inclusion of a solvent dielectric term is not appropriate.) The strong negative correlation (R2 > 0.9) between the polarizability of the solvent and the energy of the Soret transition indicates that in each case the excited state is substantially more polar than the ground state.20 The Q band maxima display a similar trend (Figure B.6, Table B.2). The striking solvatochromic behavior of Ir(III) corroles potentially could be exploited in optical sensors as well as other applications requiring solvent-based tuning of absorption and emission properties. Although the Soret solvatochromic shifts of 1-Ir(tma)2 and 1-Ir(py)2 are similar, 1b-Ir(tma)2 exhibits a somewhat weaker trend, which we suggest is attributable to bromine atom “prepolarization” of the electron density on the corrole, thereby decreasing the change in dipole moment upon excitation. But we cannot rule out a simpler explanation, namely, that the initially formed 1bIr(tma)2 excited state is not as polar as those of the other corroles. 179 180 Figure B.5. Spectral shifts of the absorption maxima as a function of solvent polarizability for second Soret band in various solvents 181 Figure B.6. Spectral shifts of the absorption maxima as a function of solvent polarizability for first Soret band and each Q band in various solvents 182 Table B.2. Slopes and fitting values for solvatochromic effects on each absorption band in the iridium(III) corroles. The correlation values for the Q bands of 1b-Ir(tma)2 are probably poor due to the low intensities of those bands at the concentrations examined. Compound Band Slope of shift (f/cm-1) Correlation (R2) 1-Ir(tma)2 First Soret -3200 0.90 Second Soret -4600 0.90 First Q band -2100 0.77 Second Q band -1800 0.91 First Soret -2500 0.78 Second Soret -2900 0.95 First Q band -1000 0.60 Second Q band -850 0.25 First Soret -4300 0.90 Second Soret -5000 0.94 First Q band -2100 0.75 Second Q band -2000 0.89 1b-Ir(tma)2 1-Ir(py)2 183 B.4 Concluding remarks Our work has established that Ir(III) corroles phosphoresce in the nearinfrared region at ambient temperatures with lifetimes and quantum yields that depend strongly on the nature of the axial ligands. We conclude from emission band shapes and vibronic splittings taken together with results from electronic structure calculations that the phosphorescence in each case is attributable to a transition from a corrole π→π* triplet state that likely has some 3MLCT character. B.5 Materials and methods All reagents were purchased from Sigma-Aldrich and were used as received without further purification. All solvents were purchased through the VWR stockroom at Caltech. Solution-state UV-vis absorption spectra were measured using a Cary 50 scanning spectrophotometer with a pulsed xenon lamp as the excitation source. The error in reported wavelength values is at most 0.5 nm. Extinction coefficients were measured for gravimetrically prepared solutions of iridium corroles in toluene, and should be accurate to ± 10%. Steady-state and time-resolved emission measurements were conducted at the Beckman Institute Laser Resource Center. Emission spectra were recorded on samples dissolved in solution (room temperature) or frozen glass (77 K). Samples were degassed by three freeze-pump-thaw cycles. For steady-state emission spectra, the 496.5 nm line of an argon ion laser (Coherent Inova 70) was used to excite samples. Right-angle emission was collected via with a Melles Griot Fiber Optic Spectrometer (MGSPEC-2048-SPU). Quantum yields were obtained by comparing signal intensity to a tetraphenylporphyrin standard. Absorption values for the samples at 496.5 nm were recorded on a Hewlett-Packard 8451A diode array spectrophotometer. For time-resolved measurements, samples were excited at 440 nm. Pulses of 8 ns duration from the third harmonic of a Q-switched Nd:YAG laser (SpectraPhysics Quanta-Ray PRO-Series) operating at 10 Hz were used to pump an optical parametric oscillator (OPO, Spectra-Physics Quanta-Ray MOPO-700) to provide laser pulses at 440 nm. Emitted light was detected with a photomultiplier tube (PMT, Hamamatsu R928). PMT current was amplified and recorded using a transient digitizer (Tektronix DSA 602). Excitation spectra were recorded on a Jobin Yvon Spex Fluorolog-3-11. Sample excitation was achieved via a xenon arc lamp with a monochromator providing wavelength selection. The excitation wavelength was scanned between 300 nm and 700 nm and recorded at 790 nm and 890 nm. Slits of 2 and 10 nm bandpass were used for excitation and emission, respectively. Right-angle light emission was sorted using a monochromator and fed into a Hamamatsu R928P photomultiplier tube with photon counting. Short- and long-pass filters were used where appropriate. Resonance Raman spectra were recorded using the 488 nm line of an argon ion laser (Coherent Inova 70). Scattered light was sorted by a 0.75 m spectrograph (Spex 750M) and detected with a liquid-nitrogen-cooled CCD (Princeton Instruments). 184 185 Synthesis 5,10,15-tris(pentafluorophenyl)corrolatoiridium(III) bis(trimethylamine), 1- Ir(tma)2. H3tpfc (80 mg), [Ir(cod)Cl]2 (335 mg), and K2CO3 (140 mg) were dissolved/suspended in 150 mL of degassed THF, and the mixture was refluxed under argon for 90 min (until the corrole fluorescence was negligible to the eye upon long-wave irradiation with a hand-held lamp). Tma N-oxide (110 mg) was added, and the solution was allowed to slowly cool to room temperature while open to the laboratory atmosphere. Column chromatography of the black solution (silica, 4:1 hexanes:CH2Cl2) provided purple crystals of 1-Ir(tma)2 (30 mg, 27% yield). 1 H NMR (CD2Cl2): δ 8.93 (d, 2H), 8.54 (d, 2H), 8.42 (d, 2H), 8.12 (d, 2H), -2.96 (s, 18H). 19F NMR (CD2Cl2): δ -139.1 (m, 6H), -156.2 (m, 3H), -164.3 (m, 6H). MS (ESI): 1105.1 ([M+]), 1046.0 ([M+-tma]), 986.5 ([M+-2tma]). UV-vis (toluene, nm, ε in M-1cm-1 x 10-3): 390 (51.9), 413 (64.3), 574 (15.9), 640 (5.64). 2,3,7,8,12,13,17,18-octabromo-5,10,15tris(pentafluorophenyl)corrolatoiridium(III) bis(trimethylamine), 1b-Ir(tma)2. Complex 1-Ir(tma)2 (15 mg) and Br2 (70 μL) were dissolved in 20 mL MeOH and stirred overnight. Column chromatography (silica, 4:1 hexanes:CH2Cl2) of the red solution provided green crystals of 1b-Ir(tma)2 (15 mg, 63% yield). 1 H NMR (CD2Cl2): δ -2.59 (s, 18H). 19F NMR (CD2Cl2): δ -138.4 (q, 2H), -139.0 (q, 4H), -153.9 (t, 3H), -164.4 (m, 4H), -164.7 (m, 2H). UV-vis (toluene, nm, ε in M-1cm-1 x 10-3): 404 (54.0), 424 (62.8), 582 (13.9), 656 (6.15). 186 5,10,15-tris(pentafluorophenylcorrolato)iridium(III) bis-pyridine, 1-Ir(py)2. H3tpfc (40 mg), [Ir(cod)Cl]2 (170 mg), and K2CO3 (70 mg) were dissolved/suspended in 75 mL of degassed THF, and the mixture was heated at reflux under argon for 90 min. Pyridine (1 mL) was added, and the solution was allowed to slowly cool to room temperature while open to the laboratory atmosphere. Column chromatography of the forest green mixture (silica, 4:1 hexanes:CH2Cl2 followed by 3:2 hexanes:CH2Cl2) provided a bright green solution, from which thin, green crystals of 1-Ir(py)2 (26 mg, 50% yield) could be grown by addition of methanol followed by slow evaporation. 1H NMR (CDCl3): δ 8.84 (d, 2H, J = 4.5), 8.53 (d, 2H, J = 4.8), 8.32 (d, 2H, J = 4.8), 8.17 (d, 2H, J = 4.5), 6.21 (t, 2H, J = 7.8), 5.19 (t, 4H, J = 7.0), 1.72 (d, 4H, J = 5.1). 19F NMR (CDCl3): δ -138.68 (m, 6F), -154.84 (t, 2F, J = 22.2), -155.20 (t, 1F, J = 22.2), -163.28 (m, 4F), -163.65 (m, 2F). MS (ESI): 1144.1 ([M+]). UV-vis (toluene, nm, ε x 10-3 M-1cm-1): 392 (46.1), 412 (68.3), 584 (19.4), 621 (10.6). Plotting of solvatochromic effects The refraction of the sodium D line at 20 oC in a given solvent was taken to represent its refractive index, nD20, or simply n. The polarizability of a solvent [f(n)] is related to its refractive index via the following relationship: f(n) = (n2 – 1)/(2n2+1). The extent of the solvatochromic effect exhibited by an absorbing species in a given solvent is then determined by the slope of the line Ef = Ev – (solv)[f(n)], where Ef is the absorption energy in the solvent, Ev is the absorption energy in vacuum, and (solv) is a factor related to the magnitude of the change in the dipole moment of the chromophore upon excitation. In this formalism, the yintercept of the line is equal to the theoretical gas-phase absorption energy of the transition under examination. We have made our solvatochromism plots by setting this value equal to zero and plotting the extent of red-shifting in a variety of solvents. This allows for facile comparison of the three corroles, such that the steepness of the slope scales with the magnitude of the separation between the ground- and excited-state dipole moments. In all cases, the excited state is more polar than the ground state. 187 188 Figure B.7. Raman spectra of 1-Ir(tma)2, 1b-Ir(tma)2, and 1-Ir(py)2. Sample excitation into the Soret was achieved with the 488 nm line of an argon ion laser. 189 Acknowledgment: This work was supported by the NSF Center for Chemical Innovation (CCI Powering the Planet, Grants CHE-0802907 and CHE-0947829), the US-Israel BSF, CCSER (Gordon and Betty Moore Foundation), and the Arnold and Mabel Beckman Foundation. B.6 References (1) Zenkevich, E.; Sagun, E.; Knyukshto, V.; Shulga, A.; Mironov, A.; Efremova, O.; Bonnett, R.; Phinda Songca, S.; Kassem, M. J. Photochem. Photobiol. B: Bio. 1996, 33, 171–180. (2) Papkovsky, D. B.; Ponomarev, G. V.; Trettnak, W.; O'Leary, P. Anal. Chem. 1995, 67,4112–4117. (3) (a) Kwong, R. C.; Sibley, S.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 1999, 11, 3709–3713. (b) Antipas, A.; Dolphin, D.; Gouterman, M.; Johnson, E. C. J. Am. Chem. Soc. 1978, 100, 7705–7709. (4) (a) Sun, Y.; Borek, C.; Hanson, K.; Djurovich, P. I.; Thompson, M. E.; Brooks, J.; Brown, J. J.; Forrest, S. R. App. Phys. Lett. 2007, 90, 213503/1– 213503/3. (b) Wiehe, A.; Stollberg, H.; Runge, S.; Paul, A.; Senge, M. O.; Roder, B. J. Porph. Pthalo. 2001, 5, 853–860. (5) Tait, C. D.; Holten, D.; Barley, M. H.; Dolphin, D.; James, B. R. J. Am. Chem. Soc. 1985, 107, 1930–1934, and references therein. (6) (a) Okun, Z.; Kupershmidt, L.; Amit, T.; Mandel, S.; Bar–Am, O.; Youdim, M. B. H.; Gross Z. ACS Chem. Biol. 2009, 4, 910–914. (b) Agadjanian, H.; Ma, J.; Rentsendorj, A.; Valluripalli, V.; Hwang, J. Y.; Mahammed, A.; Farkas, D. L.; Gray, H. B.; Gross, Z.; Medina–Kauwe, L. K. Proc. Nat. Acad. Sci. 2009, 106, 6100–6105. (7) Agadjanian, H.; Weaver, J. J.; Mahammed, A.; Rentsendorj, A.; Bass, S.; Kim, J.; Domchowski, I. J.; Margalit, R.; Gray, H. B.; Gross, Z.; Medina–Kauwe, L. K. Pharmaceutical Res. 2006, 23, 367–377. (8) Weaver, J. J.; Sorasaenee, K.; Sheikh, M.; Goldschmidt, R.; Tkachenko, E.; Gross, Z.; Gray, H. B. J. Porph. Phthalo. 2004, 8, 76–81. (9) Hwang, J. Y.; Agadjanian, H.; Medina–Kauwe, L. K.; Gross, Z.; Gray, H. B.; Sorasaenee, K.; Farkas, D. L. SPIE 2008, 6859, 68590G. (10) (a) Palmer, J. H.; Day, M. W.; Wilson, A. D.; Henling, L. M.; Gross, Z.; Gray, H. B. J. Am. Chem. Soc. 2008, 130, 7786–7787. (b) Palmer, J. H.; Mahammed, A.; Lancaster, K. M.; Gross, Z.; Gray, H. B. Inorg. Chem. 2009, 48, 9308–9315. (11) (a) Dixon, I. M.; Collin, J.–P.; Sauvage, J.–P.; Flamigni, L.; Encinas, S.; Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385–391. (b) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971–12979. (c) Hung, J.–Y.; Chi, Y.; Pai, I.–H.; Yu, Y.–C.; Lee, G.–H.; Chou, P.–T.; Wong, K.–T.; Chen, C.–C.; Wu, C.–C. Dalton Trans. 2009, 33, 6472– 6475. (d) Shin, C. H.; Huh, J. O.; Lee, M. H.; Do, Y. Dalton Trans. 2009, 33, 6476–6479. (12) (a) Flamigni, L.; Gryko, D. T. Chem. Soc. Rev. 2009, 38, 1635–1646. (b) Nardis, S.; Mandoj, F.; Paolesse, R.; Fronczek, F. R.; Smith, K. M.; Prodi, L.; Montalti, M.; Battistini, G. Eur. J. Inorg. Chem. 2007, 16, 2345–2352. (c) Poulin, J.; Stern, C.; Guilard, R.; Harvey, P. D. Photochem. Photobiol. 2006, 82, 171– 176. (d) Liu, X.; Mahammed, A.; Tripathy, U.; Gross, Z.; Steer, R. P. Chem. Phys. Lett. 2008, 459, 113–118. (13) Ding, T.; Alemán, E. A.; Modarelli, D. A.; Ziegler, C. J. J. Phys. Chem. A. 2005, 109, 7411–7417. (14) Steene, E.; Wondimagegn, T.; Ghosh, A. Inorg. Biochem. 2002, 88, 113– 118. (15) Chen, P.; Meyer, T. J. Chem. Rev. 1998, 98, 1439–1478. (16) Dong, S. S.; Nielsen, R. J.; Palmer, J. H.; Gray, H. B.; Gross, Z.; Dasgupta, S.; Goddard, W. A. Inorg. Chem. 2011, 50, 764–770. (17) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem. A. 2009, 113, 5927–5932. (18) Ir(III) complexes with 3% quantum yields have been used successfully to stain living cells. See: Yu, M.; Zhao, Q.; Shi, L.; Li, F.; Zhou, Z.; Yang, H.; Yi, T.; Huang, C. Chem. Commun. 2008, 2115–2117. (19) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd. Ed. Klewer Academic/Plenum Publishers, New York, 1999. (20) (a) Marcus, R.A. J. Chem Phys. 1965, 43, 1261–1274. (b) Mody, V. V.; Fitzpatrick, M. B.; Zabaneh, S. S.; Czernuszewicz, R. S.; Galezowski, M.; Gryko, D. T. J. Porph. Pthalo. 2009, 13, 1040–1052. 190 191 Appendix C: Supporting information: Electronic structures of PdII dimers The data presented here represents much of the raw data used to fully characterize the electronic properties of the dipalladium(II) species. It is presented largely without comment. Electrochemistry Figure C.1. Cyclic voltammograms of 1–4 in CH2Cl2 192 193 Figure C.2. Cyclic voltammograms of 1–5 in MeCN Photophysics Figure C.3. Steady-state emission spectra of 1–4, 77 K, λex = 355 nm 194 195 . Figure C.4. Steady-state emission spectra of 1–5, 77 K, λex = 355 nm 196 Figure C.5. Excitation spectrum of 1–4, 77 K, λem = 460 nm 197 Figure C.6. Excitation spectrum of 1–4, 77 K, λem = 790 nm Time-resolved luminescence decay Time-resolved data were collected in MTHF glasses at 77 K. All lifetimes are reported in microseconds and the prefactors reflect the intensity of the signal in mV. The short-lived fluorescence components observed at 460 nm were not fit, as their lifetimes exceeded the limits of the instrument (τ < 20 ns). 198 a) b) Figure C.7. Luminescence decay of 1 at 77 K at 460 nm (a) and 720 nm (b) 199 a) b) Figure C.8. Luminescence decay of 2 at 77 K at 460 nm (a) and 720 nm (b) 200 201 a) b) Figure C.9. Luminescence decay of 3 at 77 K at 460 nm (a) and 720 nm (b) a) b) Figure C.10. Luminescence decay of 4 at 77 K at 460 nm (a) and 720 nm (b) 202 203 Figure C.11. Luminescence decay of 5 at 77 K at 460 nm. No luminescence observed at higher wavelengths Density Functional Theory XYZ coordinates of optimized structures Compound 1 Atom X Y Z (Angstrom) 1.Pd -0.015348 -1.411688 0.087827 2.Pd 0.015348 1.411688 0.087827 3.O -1.500074 1.073325 1.480057 4.O -1.434283 -1.178652 1.484098 5.C -1.864868 -0.059158 1.884683 6.C -2.948716 -0.104711 2.911512 7.H -3.922853 -0.087146 2.394642 8.H -2.891036 -1.032063 3.495669 9.H -2.895562 0.778455 3.561457 10.O 1.500074 -1.073325 1.480057 11.O 1.434283 1.178652 1.484098 12.C 2.948716 0.104711 2.911512 13.C 1.864868 0.059158 1.884683 14.H 2.891036 1.032063 3.495669 15.H 2.895562 -0.778455 3.561457 16.H 3.922853 0.087146 2.394642 17.N -1.250971 1.847612 -1.353349 18.C -2.571072 1.888701 -1.187181 19.C -3.427210 2.155687 -2.233800 20.C -2.891637 2.373012 -3.494219 21.C -1.523280 2.335998 -3.659037 22.C -0.702720 2.082700 -2.569541 23.H -2.912908 1.703127 -0.164559 24.H -4.503654 2.190641 -2.056975 25.H -3.544563 2.577341 -4.346602 26.H -1.067027 2.513086 -4.636009 27.C 0.735972 2.075293 -2.555547 28.C 0.702720 -2.082700 -2.569541 29.C -0.735972 -2.075293 -2.555547 30.C 1.538420 2.341708 -3.659102 31.C 1.523280 -2.335998 -3.659037 32.C -1.538420 -2.341708 -3.659102 33.C 2.911745 2.377383 -3.517244 34.C 2.891637 -2.373012 -3.494219 35.C -2.911745 -2.377383 -3.517244 36.C 3.478860 2.154550 -2.268587 37.C 3.427210 -2.155687 -2.233800 38.C -3.478860 -2.154550 -2.268587 39.C 2.683763 1.877334 -1.166354 204 40.C 41.C 42.C 43.N 44.C 45.H 46.H 47.H 48.H 49.H 50.H 51.H 52.H 53.H 54.H 55.H 56.H 2.571072 -1.888701 -1.187181 -2.683763 -1.877334 -1.166354 1.305437 1.824233 -1.294263 1.250971 -1.847612 -1.353349 -1.305437 -1.824233 -1.294263 1.082697 2.533435 -4.636259 1.067027 -2.513086 -4.636009 -1.082697 -2.533435 -4.636259 3.547700 2.592693 -4.379214 3.544563 -2.577341 -4.346602 -3.547700 -2.592693 -4.379214 4.565591 2.207922 -2.151998 4.503654 -2.190641 -2.056975 -4.565591 -2.207922 -2.151998 3.136506 1.720651 -0.181717 2.912908 -1.703127 -0.164559 -3.136506 -1.720651 -0.181717 Compound 3 Atom X Y Z (Angstrom) 1.Pd 8.405940 11.170688 6.682973 205 2.Pd 3.F 4.F 5.F 6.F 7.F 8.F 9.O 10.O 11.O 12.O 13.N 14.N 15.C 16.C 17.C 18.C 19.C 20.C 21.C 22.C 23.C 24.C 25.C 26.C 27.C 28.C 29.C 30.C 31.C 32.C 33.C 34.C 35.C 36.C 37.C 38.C 39.C 40.C 41.H 42.H 43.H 44.H 45.H 46.H 9.001517 13.190823 4.756391 7.079164 15.977153 8.120844 5.603517 14.470104 8.594779 5.470082 15.558200 6.735815 4.597094 10.168097 4.230325 5.853791 10.236662 2.468456 4.717854 11.983236 3.061309 7.405606 12.881381 7.530108 7.696986 14.350726 5.819304 6.788729 10.851505 5.461157 7.340858 12.278520 3.779528 10.061647 11.326680 7.788633 10.394366 12.100990 3.824211 10.269199 12.267519 8.712638 11.471881 12.390683 9.380600 12.501210 11.512787 9.079994 12.284293 10.537067 8.127796 11.054715 10.449032 7.486170 10.685090 9.477863 6.482796 11.517303 8.467413 5.997072 11.048383 7.584143 5.042243 9.745747 7.703454 4.575671 8.911619 8.709194 5.046605 9.371637 9.614885 5.992632 10.162554 11.184600 2.881609 11.158434 10.345477 2.421651 12.435232 10.456151 2.948014 12.696740 11.467241 3.847759 11.659127 12.276938 4.293235 11.778833 13.334581 5.271905 12.979175 13.743021 5.852454 12.992056 14.798155 6.747314 11.804097 15.452756 7.051938 10.598204 15.036233 6.492729 10.571473 13.970067 5.606973 6.640393 11.420865 4.352452 5.418459 10.948421 3.529983 7.223331 13.948384 6.913065 6.314490 14.995154 7.600201 9.435697 12.945347 8.923437 11.584648 13.173704 10.134477 13.462235 11.596472 9.598608 13.058431 9.810716 7.863235 12.550079 8.359411 6.349619 11.694875 6.792211 4.651382 206 47.H 48.H 49.H 50.H 51.H 52.H 53.H 54.H 55.H 56.H 9.381634 6.999025 3.819931 7.888276 8.778246 4.662483 9.142531 11.112712 2.485189 10.938711 9.593410 1.659871 13.192187 9.725314 2.643267 13.704748 11.647066 4.232876 13.910029 13.234055 5.581135 13.929122 15.129747 7.205167 11.820920 16.310846 7.732551 9.665135 15.558403 6.733072 207 Compound a Atom X Y Z (Angstrom) 1.Pd 1.264011 0.000000 0.000000 2.Pd -1.264011 0.000000 0.000000 3.O -1.225263 1.487607 -1.323780 4.O 1.225263 -1.487607 -1.323780 5.O -1.225263 -1.487607 1.323780 6.O 1.225263 1.487607 1.323780 7.N 1.106177 1.332913 -1.518155 8.N -1.106177 -1.332913 -1.518155 9.N 1.106177 -1.332913 1.518155 10.N -1.106177 1.332913 1.518155 11.C -0.108147 1.839090 -1.874483 12.C -0.162808 2.805087 -2.903456 13.C 0.979195 3.194345 -3.545370 14.C 2.201988 2.632104 -3.182307 15.C 2.230322 1.704645 -2.166369 16.C 0.108147 -1.839090 -1.874483 17.C 0.162808 -2.805087 -2.903456 18.C -0.979195 -3.194345 -3.545370 19.C -2.201988 -2.632104 -3.182307 20.C -2.230322 -1.704645 -2.166369 21.C -0.108147 -1.839090 1.874483 22.C -0.162808 -2.805087 2.903456 23.C 0.979195 -3.194345 3.545370 24.C 2.201988 -2.632104 3.182307 25.C 2.230322 -1.704645 2.166369 26.C 0.108147 1.839090 1.874483 27.C 0.162808 2.805087 2.903456 28.C -0.979195 3.194345 3.545370 29.C -2.201988 2.632104 3.182307 30.C -2.230322 1.704645 2.166369 31.H 3.133162 -2.908137 3.682893 32.H 0.929753 -3.938703 4.346450 33.H -1.149053 -3.202883 3.155006 34.H -1.149053 3.202883 -3.155006 35.H 0.929753 3.938703 -4.346450 36.H 3.133162 2.908137 -3.682893 37.H 1.149053 -3.202883 -3.155006 38.H -0.929753 -3.938703 -4.346450 39.H -3.133162 -2.908137 -3.682893 40.H 1.149053 3.202883 3.155006 41.H -0.929753 3.938703 4.346450 42.H -3.133162 2.908137 3.682893 43.C -3.485168 1.051892 1.729359 208 44.H 45.H 46.H 47.C 48.H 49.H 50.H 51.C 52.H 53.H 54.H 55.C 56.H 57.H 58.H -4.341820 1.412962 2.315728 -3.666891 1.248293 0.658826 -3.395675 -0.042091 1.844057 -3.485168 -1.051892 -1.729359 -3.666891 -1.248293 -0.658826 -3.395675 0.042091 -1.844057 -4.341820 -1.412962 -2.315728 3.485168 1.051892 -1.729359 3.666891 1.248293 -0.658826 3.395675 -0.042091 -1.844057 4.341820 1.412962 -2.315728 3.485168 -1.051892 1.729359 4.341820 -1.412962 2.315728 3.666891 -1.248293 0.658826 3.395675 0.042091 1.844057 Compound b Atom X Y Z (Angstrom) 1.N -0.901383 -0.843200 -1.928842 2.N -1.455852 1.646117 -0.660848 3.N -0.510441 2.512108 -0.669410 4.N 0.679138 2.158806 -0.348112 5.N 0.195731 -0.641199 -2.561591 6.N 1.200875 -0.134247 -1.947277 7.N 1.474688 -1.708610 0.417026 8.N 0.509360 -2.507722 0.687152 209 9.N 10.N 11.N 12.N 13.Pd 14.Pd 15.C 16.H 17.H 18.H 19.C 20.H 21.H 22.H 23.C 24.H 25.H 26.H 27.C 28.H 29.H 30.H 31.C 32.H 33.H 34.H 35.C 36.H 37.H 38.H 39.C 40.H 41.H 42.H 43.C 44.H 45.H 46.H -0.699977 -2.087168 0.632586 0.954379 0.574095 2.003745 -0.197052 0.645049 2.564231 -1.253273 0.406904 1.877040 1.241168 0.256988 0.031680 -1.241557 -0.256750 -0.027799 -1.889962 -1.556902 -2.678778 -2.011866 -2.581180 -2.282413 -2.866018 -1.051186 -2.599374 -1.585329 -1.620613 -3.737029 2.305836 0.194556 -2.795405 3.244122 -0.195407 -2.366277 2.148584 -0.231371 -3.800533 2.411009 1.292743 -2.885792 2.767593 -2.321853 0.407883 3.454829 -1.788540 1.087488 2.684791 -3.377011 0.715919 3.202011 -2.277836 -0.607207 -1.680177 -3.064692 1.003160 -1.219475 -3.855073 1.619643 -2.480614 -2.572719 1.581295 -2.145574 -3.528620 0.111765 -2.478700 0.692710 2.557952 -2.273903 0.940316 3.612598 -2.988452 1.551230 2.082988 -3.161142 -0.172430 2.503048 1.628492 3.229736 -0.324931 2.157985 3.239338 0.644683 2.388421 3.099952 -1.119259 1.112219 4.194357 -0.466376 2.059040 0.674846 2.907387 2.778564 1.432080 2.552648 1.698342 0.943521 3.914174 2.593259 -0.291741 2.967177 -2.709074 2.131916 -1.151566 -2.644863 3.217154 -1.336995 -2.974248 1.622070 -2.096114 -3.513208 1.925636 -0.423961 210 211 Compound c Atom X Y Z (Angstrom) 1.Pd 0.000000 0.000000 1.278900 2.Pd 0.000000 0.000000 -1.279033 3.Cl 1.745299 -0.789015 -3.719094 4.Cl -1.745299 0.789015 -3.719094 5.Cl -1.212059 -1.481652 3.719031 6.Cl 1.212059 1.481652 3.719031 7.O 0.995361 1.725601 -1.217104 8.O 1.656566 -1.106672 1.217015 9.O -0.995361 -1.725601 -1.217104 10.O -1.656566 1.106672 1.217015 11.N 1.124957 1.674332 1.120125 12.N 1.750835 -1.002252 -1.120230 13.N -1.124957 -1.674332 1.120125 14.N -1.750835 1.002252 -1.120230 15.C 1.403170 2.224100 -0.103992 16.C 2.179126 3.404169 -0.151221 17.C 2.649068 3.973573 0.998825 18.C 2.366170 3.393866 2.236028 19.C 1.604779 2.255095 2.225920 20.C 2.243420 -1.372135 0.103947 21.C 22.C 23.C 24.C 25.C 26.C 27.C 28.C 29.C 30.C 31.C 32.C 33.C 34.C 35.H 36.H 37.H 38.H 39.H 40.H 41.H 42.H 43.H 44.H 45.H 46.H 3.461917 -2.086177 0.151417 4.136120 -2.387733 -0.998449 3.626341 -1.992603 -2.235692 2.439062 -1.309376 -2.225889 -1.403170 -2.224100 -0.103992 -2.179126 -3.404169 -0.151221 -2.649068 -3.973573 0.998825 -2.366170 -3.393866 2.236028 -1.604779 -2.255095 2.225920 -2.243420 1.372135 0.103947 -3.461917 2.086177 0.151417 -4.136120 2.387733 -0.998449 -3.626341 1.992603 -2.235692 -2.439062 1.309376 -2.225889 -2.720832 -3.805724 3.181278 -3.252363 -4.885209 0.951427 -2.379322 -3.818696 -1.141799 2.379322 3.818696 -1.141799 3.252363 4.885209 0.951427 2.720832 3.805724 3.181278 3.824347 -2.369582 1.142113 5.081035 -2.937446 -0.950891 4.127527 -2.203768 -3.180743 -3.824347 2.369582 1.142113 -5.081035 2.937446 -0.950891 -4.127527 2.203768 -3.180743 212 213 Compound d Atom X Y Z (Angstrom) 1.Pd -0.001348 -0.003657 -1.270687 2.Pd 0.000573 -0.003941 1.267476 3.N -1.036355 1.715600 -1.134027 4.N -0.626264 1.907213 1.145346 5.N 1.729089 1.019246 -1.141064 6.N 1.904947 0.633055 1.145825 7.N 1.018038 -1.735443 -1.148418 8.N 0.636992 -1.909764 1.140109 9.N -1.728470 -1.027821 -1.147703 10.N -1.903757 -0.648618 1.140689 11.C -1.099463 2.391461 0.005070 12.C 2.384293 1.116877 0.008590 13.C 1.118346 -2.389397 0.002026 14.C -2.401491 -1.089311 -0.007645 15.C 1.704187 -2.223450 -2.306451 16.H 1.410764 -3.256783 -2.582933 17.H 1.464704 -1.572046 -3.159078 18.H 2.809447 -2.216129 -2.191355 19.C 1.818500 -3.711965 -0.010995 20.H 1.235762 -4.467916 -0.566924 21.H 2.787672 -3.617539 -0.528904 22.H 2.005401 -4.090145 1.002388 23.C 0.546227 -2.723920 2.314232 24.H 0.269494 -3.773456 2.098339 25.H 1.484341 -2.737378 2.909236 26.H -0.241227 -2.311187 2.964758 27.C -2.724236 -0.618904 2.314371 28.H -3.117937 -1.616621 2.597417 29.H -2.119453 -0.251741 3.156002 30.H -3.595795 0.063494 2.219062 31.C -3.779544 -1.673733 0.000794 32.H -3.776527 -2.695258 0.422266 33.H -4.447102 -1.069237 0.635314 34.H -4.209184 -1.719422 -1.008998 35.C -2.191347 -1.709850 -2.318172 36.H -2.656796 -2.689370 -2.097606 37.H -2.921035 -1.115038 -2.908431 38.H -1.327863 -1.902844 -2.974243 39.C -0.544972 2.713629 2.326786 40.H -1.527532 2.873557 2.817199 41.H 0.098536 2.196591 3.054660 42.H -0.092417 3.709238 2.147246 43.C 44.H 45.H 46.H 47.C 48.H 49.H 50.H 51.C 52.H 53.H 54.H 55.C 56.H 57.H 58.H 59.C 60.H 61.H 62.H -1.751860 3.738121 -0.016676 -2.847862 3.628600 -0.101418 -1.545186 4.309000 0.898093 -1.419337 4.326366 -0.887118 -1.710420 2.196124 -2.300527 -1.903418 1.345600 -2.973272 -2.685810 2.669795 -2.076799 -1.107806 2.933302 -2.873039 2.714127 0.553681 2.324620 2.283575 -0.210112 2.991321 3.758945 0.251975 2.118672 2.743525 1.504458 2.898379 3.706379 1.816912 -0.005057 4.452137 1.251626 -0.591686 3.603517 2.800989 -0.492869 4.101877 1.973982 1.006675 2.219944 1.704758 -2.298195 1.556753 1.484470 -3.146638 2.237104 2.808789 -2.173346 3.244734 1.392603 -2.586579 214 215 Compound e Atom X Y Z (Angstrom) 1.Pd 0.000000 0.000000 1.573542 2.Pd 0.000000 0.000000 -1.105541 3.S -2.162553 -0.809085 1.644804 4.S -0.841421 2.148563 1.659921 5.S -1.579099 -1.677986 -1.201531 6.S -1.693879 1.568707 -1.195281 7.C -2.493975 -1.615868 0.207416 8.C -1.707856 2.429906 0.241937 9.C -3.786900 -2.376081 0.182077 10.C -2.662375 3.586102 0.300922 11.C -4.764056 -1.869729 -0.836420 12.C -5.192383 -0.546668 -0.808590 13.C -6.109088 -0.091250 -1.740781 14.C -6.609120 -0.948584 -2.710077 15.C -6.185605 -2.267471 -2.742936 16.C -5.265331 -2.722210 -1.810980 17.C -4.058525 3.137460 0.627641 18.C -5.088255 3.344885 -0.280773 19.C -6.381825 2.945644 0.016216 20.C -6.656415 2.321156 1.222014 21.C -5.631133 2.105989 2.131902 22.C -4.341119 2.512797 1.837120 23.H -3.557635 -3.434896 -0.030362 24.H -4.230135 -2.340810 1.192296 25.H -2.664438 4.110441 -0.667476 26.H -2.304141 4.292873 1.069425 27.H -4.798229 0.143775 -0.048081 28.H -6.432661 0.954413 -1.699338 29.H -7.332460 -0.585134 -3.447041 30.H -6.572619 -2.951182 -3.504819 31.H -4.923378 -3.763304 -1.838848 32.H -4.868152 3.834574 -1.236231 33.H -7.186132 3.130412 -0.702139 34.H -7.675583 2.000133 1.455671 35.H -5.839986 1.615827 3.087262 36.H -3.525758 2.336220 2.549996 37.S 2.162553 0.809085 1.644804 38.S 0.841421 -2.148563 1.659921 39.S 1.579099 1.677986 -1.201531 40.S 1.693879 -1.568707 -1.195281 41.C 2.493975 1.615868 0.207416 42.C 1.707856 -2.429906 0.241937 43.C 44.C 45.C 46.H 47.H 48.C 49.H 50.H 51.C 52.C 53.C 54.C 55.C 56.H 57.C 58.H 59.C 60.H 61.C 62.H 63.C 64.H 65.H 66.C 67.H 68.H 69.H 70.H 3.786900 2.662375 4.764056 3.557635 4.230135 4.058525 2.664438 2.304141 5.192383 5.265331 5.088255 4.341119 6.109088 4.798229 6.185605 4.923378 6.381825 4.868152 5.631133 3.525758 6.609120 6.432661 6.572619 6.656415 7.186132 5.839986 7.332460 7.675583 2.376081 0.182077 -3.586102 0.300922 1.869729 -0.836420 3.434896 -0.030362 2.340810 1.192296 -3.137460 0.627641 -4.110441 -0.667476 -4.292873 1.069425 0.546668 -0.808590 2.722210 -1.810980 -3.344885 -0.280773 -2.512797 1.837120 0.091250 -1.740781 -0.143775 -0.048081 2.267471 -2.742936 3.763304 -1.838848 -2.945644 0.016216 -3.834574 -1.236231 -2.105989 2.131902 -2.336220 2.549996 0.948584 -2.710077 -0.954413 -1.699338 2.951182 -3.504819 -2.321156 1.222014 -3.130412 -0.702139 -1.615827 3.087262 0.585134 -3.447041 -2.000133 1.455671 Compound f Atom X Y Z (Angstrom) 1.Pd 1.318882 0.018442 -0.004188 216 2.Pd 3.S 4.S 5.S 6.S 7.N 8.N 9.N 10.N 11.C 12.C 13.C 14.C 15.C 16.C 17.C 18.C 19.C 20.C 21.C 22.C 23.C 24.C 25.C 26.C 27.C 28.C 29.C 30.C 31.H 32.H 33.H 34.H 35.H 36.H 37.H 38.H 39.H 40.H 41.H 42.H 43.H 44.H 45.H 46.H -1.318882 -0.018442 -0.004188 -1.359885 1.734423 -1.513117 1.359885 -1.734423 -1.513117 -1.312218 -1.775636 1.500981 1.312218 1.775636 1.500981 1.349786 1.356298 -1.514685 -1.349786 -1.356298 -1.514685 1.383676 -1.312507 1.510915 -1.383676 1.312507 1.510915 0.266431 1.990334 -2.020918 0.457512 2.954747 -3.028915 1.710500 3.256418 -3.488127 2.809111 2.582263 -2.955973 2.575375 1.646549 -1.982469 -0.266431 -1.990334 -2.020918 -0.457512 -2.954747 -3.028915 -1.710500 -3.256418 -3.488127 -2.809111 -2.582263 -2.955973 -2.575375 -1.646549 -1.982469 0.319000 -1.976144 2.018508 0.536194 -2.924453 3.036253 1.795971 -3.179781 3.505095 2.874574 -2.474562 2.972396 2.615533 -1.556958 1.988063 -0.319000 1.976144 2.018508 -0.536194 2.924453 3.036253 -1.795971 3.179781 3.505095 -2.874574 2.474562 2.972396 -2.615533 1.556958 1.988063 3.899374 -2.633963 3.313289 1.947456 -3.924603 4.292372 -0.338399 -3.446839 3.434077 -0.431679 3.452821 -3.425540 1.841334 4.014289 -4.266581 3.829992 2.779742 -3.288599 0.431679 -3.452821 -3.425540 -1.841334 -4.014289 -4.266581 -3.829992 -2.779742 -3.288599 0.338399 3.446839 3.434077 -1.947456 3.924603 4.292372 -3.899374 2.633963 3.313289 -3.413624 0.966630 1.530045 -3.390549 -1.079226 -1.525323 3.390549 1.079226 -1.525323 3.413624 -0.966630 1.530045 217 218 Compound g Atom X Y Z (Angstrom) 1.Pd -0.501075 -1.249450 0.002655 2.Pd 0.501075 1.249450 0.002655 3.S -2.336510 -0.617680 1.250175 4.S -0.617859 1.788151 1.941511 5.C -1.953354 0.779441 2.105559 6.S -1.690386 -0.874610 -1.939425 7.S 1.270245 -2.048232 -1.233328 8.S 0.617859 -1.788151 1.941511 9.S -1.270245 2.048232 -1.233328 10.S 1.690386 0.874610 -1.939425 11.S 2.336510 0.617680 1.250175 12.C -1.952973 0.779450 -2.100579 13.C 1.952973 -0.779450 -2.100579 14.C 1.953354 -0.779441 2.105559 15.C 2.903145 -1.182914 -3.175525 16.H 2.418575 -1.056669 -4.158388 17.H 3.805047 -0.552925 -3.163580 18.H 3.186309 -2.240852 -3.071942 19.C 2.943805 -1.199873 3.136463 20.H 3.829434 -1.629164 2.638391 21.H 3.279694 -0.341014 3.736306 22.H 23.C 24.H 25.H 26.H 27.C 28.H 29.H 30.H 2.519580 -1.971618 3.795634 -2.943805 1.199873 3.136463 -3.829434 1.629164 2.638391 -3.279694 0.341014 3.736306 -2.519580 1.971618 3.795634 -2.903145 1.182914 -3.175525 -2.418575 1.056669 -4.158388 -3.805047 0.552925 -3.163580 -3.186309 2.240852 -3.071942 Compound h Atom X Y Z (Angstrom) 1.Pd -0.241391 1.209384 0.782951 2.Pd -0.180446 -1.198104 -0.766736 3.O 0.987196 0.105674 1.995307 4.O 1.411503 -1.527232 0.490925 5.O 1.389817 1.659366 -0.379055 6.O 1.000408 0.039115 -1.909852 7.C -1.590072 2.611297 0.006913 219 8.C 9.C 10.C 11.H 12.H 13.H 14.C 15.C 16.H 17.H 18.H 19.C 20.C 21.H 22.H 23.H 24.H 25.H 26.C 27.C 28.H 29.H 30.H 31.H 32.H -1.374273 -2.790937 -0.133494 1.614170 -0.906405 1.572347 2.706166 -1.438956 2.442581 2.903691 -0.760680 3.282034 3.615498 -1.583954 1.840214 2.411055 -2.428889 2.825827 1.610773 1.054260 -1.466915 2.686262 1.613075 -2.340582 3.271743 0.797196 -2.788016 3.330038 2.298147 -1.773780 2.212823 2.170837 -3.165559 -2.159795 -1.822451 -0.787097 -1.779031 -1.444710 -2.090426 -2.226945 -0.557683 -2.548589 -1.387468 -2.195895 -2.787402 -0.949793 -3.629190 -0.699328 -1.506939 -2.955357 0.939936 -2.870605 -1.215713 -0.212913 -2.279467 1.511804 0.544911 -2.016340 1.169982 1.887765 -2.784389 0.804647 -0.125698 -2.360777 0.207760 2.277836 -1.865467 1.960678 2.633139 -1.385313 3.490840 0.630958 -1.602146 2.780485 -1.074079 220 221 Compound Pt-pop Atom X Y Z (Angstrom) 1.Pt 1.344705 -0.432943 0.152621 2.P 2.076525 1.006043 -1.522572 3.P 1.040940 -2.165667 -1.386568 4.P 0.730669 -1.932427 1.827031 5.P 1.798215 1.244792 1.695919 6.O 2.471138 0.367008 -2.873936 7.O 3.264581 1.944377 -1.073902 8.H 3.180497 2.084155 0.048679 9.O -0.553120 -2.321185 -1.805023 10.O 1.768566 -1.962412 -2.768313 11.H 2.098618 -0.879022 -2.859224 12.O 1.391535 -3.584411 -0.873656 13.O 1.191276 -3.420350 1.561339 14.H 1.289588 -3.552040 0.445090 15.O 1.163006 -1.559840 3.263294 16.O 0.914242 2.132703 -1.868633 17.O 3.001194 2.158205 1.351288 18.O 1.989615 0.739122 3.178420 19.H 1.601369 -0.315675 3.256278 20.Pt -1.344385 0.433054 -0.152563 21.P -2.076768 -1.006024 1.522922 22.O -2.478207 -0.364435 2.871381 23.O -3.261869 -1.947922 1.074362 24.H -3.180811 -2.084046 -0.048905 25.O -0.911656 -2.130550 1.870640 26.P -0.728137 1.931821 -1.827290 27.P -1.042821 2.165489 1.386401 28.O 0.550050 2.318460 1.809400 29.P -1.798675 -1.244105 -1.695830 30.O -3.004497 -2.153694 -1.351301 31.O -1.987144 -0.740706 -3.179419 32.H -1.598504 0.314117 -3.257327 33.O -1.773823 1.964150 2.766826 34.H -2.104387 0.881534 2.857351 35.O -1.391255 3.584552 0.872584 36.O -1.190202 3.419401 -1.562055 37.H -1.289245 3.550941 -0.445396 38.O -1.157122 1.556327 -3.263876 222 Compound j Atom X Y Z (Angstrom) 1.Ir -0.185905 0.010207 1.546019 2.N -1.205522 1.541593 0.673423 3.C -1.737286 2.695238 1.099630 4.C -2.104251 3.458103 0.000436 5.C 1.278157 -1.443125 2.007400 6.C 0.244605 -1.466921 2.972004 7.C 2.646655 -0.877714 2.299802 8.C 0.357627 -0.865337 4.341560 9.H -1.848501 2.895689 2.163139 10.N -1.474864 -1.303588 0.671060 11.C 1.436414 1.300995 1.930383 12.C 0.566978 1.284614 3.044616 13.N -1.206374 1.541125 -0.673551 14.C -1.738332 2.695088 -1.099114 15.C 2.758634 0.594608 1.908373 16.C 0.925115 0.552358 4.315661 17.N -1.475077 -1.304415 -0.671751 18.C -2.300660 -2.268939 1.099246 19.Ir -0.187042 0.008264 -1.546657 20.H -1.850312 2.896097 -2.162382 21.C 22.C 23.C 24.C 25.C 26.C 27.H 28.C 29.C 30.C 31.C 32.H 33.H 34.H 35.H 36.H 37.H 38.H 39.H 40.H 41.H 42.H 43.H 44.H 45.H 46.H 47.H 48.H 49.H 50.H 51.H 52.H 53.H 54.H 55.H 56.H 57.H 58.H -2.300692 -2.269965 -1.099252 -2.858690 -2.905620 0.000274 1.278607 -1.443257 -2.008040 1.435184 1.300729 -1.927739 0.245548 -1.466929 -2.973289 0.566496 1.285406 -3.042429 -3.596367 -3.704974 0.000453 2.646987 -0.876538 -2.298706 2.757797 0.595312 -1.905280 0.358614 -0.862885 -4.341891 0.925645 0.555076 -4.313984 -2.602812 4.425050 0.000869 1.237425 -2.215027 1.224827 -0.484668 -2.286127 2.891674 3.410328 -1.463202 1.760565 2.867607 -1.012323 3.374724 0.964325 -1.515967 5.004562 -0.655044 -0.842558 4.779609 1.357314 2.161486 1.248879 -0.110924 2.142058 3.162340 3.156433 0.666532 0.882168 3.491502 1.119163 2.555259 2.025933 0.530493 4.414910 0.553294 1.115332 5.187879 -2.468595 -2.425093 2.162313 -2.468009 -2.426791 -2.162321 1.238183 -2.215783 -1.226083 1.355232 2.160090 -1.244839 -0.483063 -2.286836 -2.894327 -0.111583 2.142906 -3.158741 2.868775 -1.009406 -3.373603 3.410720 -1.462165 -1.759778 3.154743 0.666488 -0.878653 3.490791 1.121138 -2.550960 -0.654272 -0.839365 -4.779751 0.965464 -1.512396 -5.006139 2.026458 0.534254 -4.412853 0.553602 1.119029 -5.185366 223 224 Compound k Atom X Y Z (Angstrom) 1.Rh -0.125563 1.476608 1.322813 2.C -0.829828 1.898338 3.079839 3.N -1.286016 2.243905 4.095857 4.C 1.696703 1.520388 1.978132 5.N 2.808727 1.471494 2.326121 6.C 0.521718 1.475244 -0.497882 7.N 0.872227 1.482688 -1.611447 8.C -1.937067 1.346562 0.651927 9.N -3.018646 1.182672 0.246129 10.Rh 0.126003 -1.479468 1.324489 11.C 0.829623 -1.902567 3.080140 12.C -1.694131 -1.525083 1.982130 13.C -0.523199 -1.476125 -0.494704 14.C 1.935389 -1.347648 0.651304 15.N 1.284096 -2.248915 4.096695 16.N -2.805772 -1.477078 2.331614 17.N -0.873925 -1.482747 -1.608259 18.N 3.016105 -1.181750 0.243517 19.C 4.140058 1.376043 2.735411 20.H 4.576982 2.379106 2.860074 21.H 4.207026 0.838348 3.693805 22.H 4.729351 0.826485 1.982729 23.C -1.831550 2.730365 5.287219 24.H -1.828904 3.832268 5.282576 25.H -2.869314 2.380404 5.399111 26.H -1.243063 2.376452 6.147389 27.C 1.831455 -2.726294 5.291002 28.H 2.129055 -1.885596 5.937021 29.H 30.H 31.C 32.H 33.H 34.H 35.C 36.H 37.H 38.H 39.C 40.H 41.H 42.H 43.C 44.H 45.H 46.H 47.C 48.H 49.H 50.H 1.092879 -3.341755 5.827711 2.717456 -3.345103 5.078534 -4.138857 -1.364518 2.730298 -4.416343 -0.300734 2.821681 -4.798528 -1.850654 1.993716 -4.292555 -1.850911 3.705794 -4.303069 0.940182 -0.245330 -4.825323 1.888848 -0.444825 -4.247963 0.362404 -1.181166 -4.889104 0.361863 0.487703 4.299405 -0.926562 -0.244358 4.465624 0.161456 -0.320303 5.058093 -1.363165 0.424168 4.425830 -1.371398 -1.243653 -1.266030 -1.490808 -2.948515 -1.388894 -0.458072 -3.315852 -0.509704 -2.003281 -3.563878 -2.224078 -2.020624 -3.066214 1.269501 1.498406 -2.950301 1.376145 2.534297 -3.308923 2.234051 0.980824 -3.069805 0.519400 0.987108 -3.575398 225 i i 5