Metallaboratrane Facilitated E‒H Bond Activation and Hydrogenation Catalysis Citation Fong, Henry (2015) Metallaboratrane Facilitated E‒H Bond Activation and Hydrogenation Catalysis. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9TX3C9P. https://resolver.caltech.edu/CaltechTHESIS:12152014-181907516 Abstract The E‒H bond activation chemistry of tris -phosophino-iron and -cobalt metallaboratranes is discussed. The ferraboratrane complex ( TPB )Fe(N 2 ) heterolytically activates H‒H and the C‒H bonds of formaldehyde and arylacetylenes across an Fe‒B bond. In particular, H‒H bond cleavage at ( TPB )Fe(N 2 ) is reversible and affords the iron-hydride-borohydride complex ( TPB )(μ‒H)Fe(L)(H) (L = H 2 , N 2 ). ( TPB )(μ‒H)Fe(L)(H) and ( TPB )Fe(N 2 ) are competent olefin and arylacetylene hydrogenation catalysts. Stoichiometric studies indicate that the B‒H unit is capable of acting as a hydride shuttle in the hydrogenation of olefin and arylacetylene substrates. The heterolytic cleavage of H 2 by the ( TPB )Fe system is distinct from the previously reported ( TPB )Co(H 2 ) complex, where H 2 coordinates as a non-classical H 2 adduct based on X-ray, spectroscopic, and reactivity data. The non-classical H 2 ligand in ( TPB )Co(H 2 ) is confirmed in this work by single crystal neutron diffraction, which unequivocally shows an intact H‒H bond of 0.83 Å in the solid state. The neutron structure also shows that the H 2 ligand is localized at two orientations on cobalt trans to the boron. This localization in the solid state contrasts with the results from ENDOR spectroscopy that show that the H 2 ligand freely rotates about the Co‒H 2 axis in frozen solution. Finally, the ( TPB )Fe system, as well as related tris -phosphino-iron complexes that contain a different apical ligand unit (Si, PhB, C, and N) in place of the boron in ( TPB )Fe, were studied for CO 2 hydrogenation chemistry. The ( TPB )Fe system is not catalytically competent, while the silicon, borate, carbon variants, ( SiP R 3 )Fe, ( PhBP i Pr 3 )Fe, and ( CP i Pr 3 )Fe, respectively, are catalysts for the hydrogenation of CO 2 to formate and methylformate. The hydricity of the CO 2 reactive species in the silatrane system ( SiP i Pr 3 )Fe(N 2 )(H) has been experimentally estimated. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Metallaboratrane; hydrogenation catalysis; inorganic chemistry; carbon dioxide Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Peters, Jonas C. Group: JCAP Thesis Committee: Agapie, Theodor (chair) Gray, Harry B. Reisman, Sarah E. Peters, Jonas C. Defense Date: 5 December 2014 Record Number: CaltechTHESIS:12152014-181907516 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:12152014-181907516 DOI: 10.7907/Z9TX3C9P Related URLs: URL URL Type Description http://dx.doi.org/10.1021/om400281v DOI Article adapted for ch. 2 http://dx.doi.org/10.1021/ja508117h DOI Article adapted for ch. 3 http://dx.doi.org/10.1021/ic502508p DOI Article adapted for ch. 4 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 8746 Collection: CaltechTHESIS Deposited By: Henry Fong Deposited On: 19 Dec 2014 19:20 Last Modified: 08 Nov 2023 18:46 Thesis Files Preview PDF - Final Version See Usage Policy. 5MB Repository Staff Only: item control page

Metallaboratrane Facilitated E‒H Bond Activation and Hydrogenation Catalysis Thesis by Henry Fong In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2015 (Defended December 5, 2014) ii  2014 Henry Fong All Rights Reserved iii Acknowledgements First and foremost, I would like to thank my advisor Prof. Jonas Peters. Graduate school is a formative period in a young scientist’s development, and I would not have grown into the scientist that I am today without Jonas’s mentorship. Jonas took a chance on me when I was a graduate student in my first quarter at Northwestern University in 2009. I contacted him about joining his research group, and within a few days he accepted me into his lab at Caltech. I am truly grateful for this. Jonas’s attention to detail, scientific rigor, and approach to research can be summed in a phrase he once told me, “Do it once, and do it right.” Ever since then I have approached my work, both in- and outside of chemistry, by this maxim. This and other lessons from Jonas will remain with me as I embark on my career. I am proud to call him my advisor and scientific mentor. I would also like to acknowledge my committee members: Prof. Theodor Agapie (chair), Prof. Sarah Reisman, and Prof. Harry Gray. Needless to say, the periods leading up to committee meetings and oral exams were invariably stressful, but my committee members always made me feel at ease. I came out of these meetings and exams reinvigorated. They were genuinely interested in my work and ideas, and I appreciated their constructive criticisms and truly enjoyed the discussions. I was also very fortunate to have worked alongside a group of talented individuals in the Peters lab. Each and every member of the group, past and present, has provided me with both professional and personal support throughout my tenure in the group. I would like to mention a few in particular. Prof. Marc-Etienne Moret, with whom I concurrently joined the group, was always generous with his time and effort whenever I needed help and advice. Marc-Etienne, iv Dr. Daniel Suess, and Prof. Hill Harman, who was also my graduate student mentor when I was an undergraduate in the Chris Chang Group at Berkeley, were instrumental in helping me develop a nuanced view of chemical bonding and reactivity. I learned a large number of synthetic inorganic chemistry techniques from Prof. Yunho Lee and Prof. Caroline Saouma. Dr. Charlene Tsay taught me X-ray crystallography. When times were tough, I relied on the calming influence of Dr. Samantha MacMillan and the optimism of Dr. David Lacy. Dr. John Anderson was a blast to be around in the lab. Dr. Ayumi Takaoka, Alicia Chang, and I overlapped as undergraduates in the Chris Chang Group at Berkeley; I will always think fondly back upon the years we worked in the same lab space. Dr. Charles McCrory taught me nearly all I know about electrochemistry, and I will miss the near daily lunches that we had together. I will also miss working with Jonathan “JR” Rittle, Sidney Creutz, and Dr. Tzu-Pin Lin; all of whom I could always turn to for invigorating discussions. I have also had the privilege of working alongside Prof. Chris Uyeda, Dr. Yichen Tan, Dr. Gӓel Ung, Dr. Miles Johnson, Kenny Lotito, Joe Rheinhardt, Ben Matson, Trevor Del Castillo, Tanvi Ratani, Mark Nesbit, Joe Ahn, Niklas Thompson, Kareem Hannoun, Trixia Buscagan, and Gregory Harlow. One of the highlights of my graduate studies has been performing two separate and rare single crystal neutron diffraction experiments at the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee. The experiment and structure determination would not have been possible without the help and support of the instrument team: Dr. Xiaoping Wang, Dr. Christina Hoffmann, and Helen He. In the last two-and-a-half years of my graduate studies, I also worked in the Molecular Catalysis group at the Joint Center for Artificial Photosynthesis (JCAP). The unique, v interdisciplinary research structure at JCAP afforded me a learning experience that complemented my time in a traditional research group setting. I am grateful to have had the chance to interact with Prof. Cliff Kubiak, who is the Project Lead of Molecular Catalysis, as well as Prof. Carl Koval, Prof. Jenny Yang, Dr. Daniel Sieh, Dr. Oana Luca, Dr. David Shaffer, Dr. Suho Jung, Alyssia Lilio, and Michelle Hansen. I would not have been able to perform my research without the assistance of the technical staff at Caltech. Dr. David VanderVelde has on several occasions helped me setup key NMR experiments. Critical parts of my work would not have been possible without X-ray crystallographic assistance from Dr. Mike Takase and Larry Henling, the latter of whom is kind enough to mount crystals at any hour. Dr. Nathan Dalleska of the Environmental Analysis Center at Caltech provided valuable assistance with gas-chromatography analysis. I have also immensely enjoyed the company of my friends at Caltech outside of the lab. I have had a lot of fun playing intramural and recreational league basketball for the “Labinger Panthers,” “3 Turnovers-per-hour” (named after my slow catalysts), and “Buckets.” The teammates with whom I’ve shared this experience include Prof. Tom Teets, Prof. Ian Tonks, Marc-Etienne, Charles McCrory, John Anderson, Josh Buss, Matt Chao, Matt Kovach, Dr. Graham de Ruiter, Dr. Aaron Sattler, Dr. Wes Sattler, Adam Nichols-Nielander, Connie Wang, and last but not least, Guy Edouard, whose easy-going attitude and comic attributes can turn even our most crushing losses into something to laugh about. I have also been very fortunate to attend graduate school close to my extended family and friends. Their support and love throughout this period has helped me maintain a healthy worklife balance. My grandparents, all of my aunts and uncles, as well as my sister Jessica have vi kept me in touch with the real world outside of the graduate school bubble. Alfred Tran, Karyee Chow, Connie Tat, and Melody Wong have taken me out on many tasty and reenergizing coffee and dinner outings throughout L.A. I would also like to thank Julia Ding, who not only didn’t run-away when I uttered the words “nitrogenase” and “CO2 hydrogenation” on our first date, but has been my most enthusiastic supporter since. Her passion for and hard-work in her own profession has helped motivate me in my own work, as well. Finally, I dedicate this thesis to my Dad, Kim Soon Fong, who passed-away two months prior to my graduation from Berkeley, and to my Mom, Fanny Fong, who has bravely gone through so much since then. Both of my parents instilled in me the importance of hard-work and education, and I hope that this thesis makes them proud. vii Abstract The E‒H bond activation chemistry of tris-phosophino-iron and -cobalt metallaboratranes is discussed. The ferraboratrane complex (TPB)Fe(N2) heterolytically activates H‒H and the C‒H bonds of formaldehyde and arylacetylenes across an Fe‒B bond. In particular, H‒H bond cleavage at (TPB)Fe(N2) is reversible and affords the iron-hydride-borohydride complex (TPB)(μ‒H)Fe(L)(H) (L = H2, N2). (TPB)(μ‒H)Fe(L)(H) and (TPB)Fe(N2) are competent olefin and arylacetylene hydrogenation catalysts. Stoichiometric studies indicate that the B‒H unit is capable of acting as a hydride shuttle in the hydrogenation of olefin and arylacetylene substrates. The heterolytic cleavage of H2 by the (TPB)Fe system is distinct from the previously reported (TPB)Co(H2) complex, where H2 coordinates as a non-classical H2 adduct based on X-ray, spectroscopic, and reactivity data. The non-classical H2 ligand in (TPB)Co(H2) is confirmed in this work by single crystal neutron diffraction, which unequivocally shows an intact H‒H bond of 0.83 Å in the solid state. The neutron structure also shows that the H2 ligand is localized at two orientations on cobalt trans to the boron. This localization in the solid state contrasts with the results from ENDOR spectroscopy that show that the H2 ligand freely rotates about the Co‒H2 axis in frozen solution. Finally, the (TPB)Fe system, as well as related tris-phosphino-iron complexes that contain a different apical ligand unit (Si, PhB, C, and N) in place of the boron in (TPB)Fe, were studied for CO2 hydrogenation chemistry. The (TPB)Fe system is not catalytically competent, while the silicon, borate, carbon variants, (SiPR3)Fe, (PhBPiPr3)Fe, and (CPiPr3)Fe, respectively, are catalysts for the hydrogenation of CO2 to formate and methylformate. The hydricity of the CO2 reactive species in the silatrane system (SiPiPr3)Fe(N2)(H) has been experimentally estimated. viii Table of Contents Acknowledgements ................................................................................................................ iii Abstract ...................................................................................................................................vii Table of Contents .................................................................................................................. viii Detailed Table of Contents ...................................................................................................... ix List of Figures .........................................................................................................................xii List of Schemes ......................................................................................................................xiv List of Charts..........................................................................................................................xvi List of Tables ........................................................................................................................xvii List of Abbreviations .............................................................................................................xix Chapter 1. Introduction ....................................................................................................... 1 Chapter 2. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane.............................................................................................. 25 Chapter 3. Neutron Diffraction Structure of an S = ½ Co‒H2 Adduct ............................ 66 Chapter 4. Hydricity of an Fe‒H Species and Catalytic CO2 Hydrogenation ................. 95 Appendix 1. Supplementary Data for Chapter 2 ............................................................... 142 Appendix 2. Supplementary Data for Chapter 3 ............................................................... 159 Appendix 3. Supplementary Data for Chapter 4 ............................................................... 163 ix Detailed Table of Contents Chapter 1. Introduction ..................................................................................................... 1 1.1 Motivation ................................................................................................................... 2 1.2 X‒Y Bond Activation and Hydride Transfer Using Metallaboratranes ................. 10 1.2.1 Precedent for X‒Y Bond Activation ................................................................... 10 1.2.2 Background on (TPB)MX .................................................................................. 12 1.2.3 E‒H Bond Activation by (RDPBAr)MX .............................................................. 13 1.4 Cited References ....................................................................................................... 19 Chapter 2. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane........................................................................................... 25 2.1 Preface ....................................................................................................................... 26 2.2 Introduction ............................................................................................................... 27 2.3 Results and Discussion ............................................................................................. 28 2.3.1 Reversible H2 Addition ....................................................................................... 28 2.3.2 Reaction with Unsaturated Substrates............................................................... 35 2.3.3 Stoichiometric Hydrogenations.......................................................................... 39 2.3.4 Catalytic Hydrogenations .................................................................................. 41 2.4 Conclusions ............................................................................................................... 46 2.5 Experimental ............................................................................................................. 47 2.5.1 General Considerations ..................................................................................... 47 2.5.2 Synthetic Protocols ............................................................................................. 49 2.6 Cited References ....................................................................................................... 62 Chapter 3. 3.1 Neutron Diffraction Structure of an S = ½ Co‒H2 Adduct ...................... 66 Preface ....................................................................................................................... 67 x 3.2 Introduction ............................................................................................................... 69 3.2.1 Collaborative ENDOR Studies .......................................................................... 73 3.3 Results and Discussion ............................................................................................. 80 3.3.1 Neutron Structure of (TPB)Co(H2).................................................................... 80 3.3.2 Comparisons with the ENDOR Data ................................................................. 85 3.4 Conclusions ............................................................................................................... 87 3.5 Experimental Section ................................................................................................ 88 3.5.1 Sample Preparation ............................................................................................ 88 3.5.2 Neutron Diffraction ............................................................................................ 88 3.6 Cited References ....................................................................................................... 91 Chapter 4. Hydricity of an Fe‒H Species and Catalytic CO2 Hydrogenation .......... 95 4.1 Preface ....................................................................................................................... 96 4.2 Introduction ............................................................................................................... 97 4.3 Results and Discussion ........................................................................................... 100 4.3.1 pKa and Hydricity for (SiPiPr3)Fe .................................................................... 100 4.3.2 Stoichiometric Reactivity of Fe‒H Species with CO2...................................... 103 4.3.3 Catalytic Hydrogenation .................................................................................. 109 4.3.4 Influence of Hydricity on the Reaction with CO2 ............................................ 120 4.4 Conclusions ............................................................................................................. 122 4.5 Experimental ........................................................................................................... 123 4.5.1 General Considerations ................................................................................... 123 4.5.2 Synthetic Protocols ........................................................................................... 125 4.5.3 CO2 Hydrogenation Catalysis Protocols. ........................................................ 133 4.5.4 Hydricity Determination .................................................................................. 135 4.5.5 UV-vis Titration of Formate Binding............................................................... 135 4.6 Cited References ..................................................................................................... 137 xi Appendix 1. Supplementary Data for Chapter 2 ........................................................... 142 Appendix 2. Supplementary Data for Chapter 3 ........................................................... 159 Appendix 3. Supplementary Data for Chapter 4 ........................................................... 163 A3.1 Hydricity Determination ......................................................................................... 169 A3.2 UV-vis Titration ...................................................................................................... 172 A3.2 Cited References ..................................................................................................... 173 xii List of Figures Figure 1.1 DFT calculated transition states for borane assisted and unassisted H‒H bond cleavage .................................................................................................. 15 Figure 2.1 XRD structures of (TPB)(μ‒H)Fe(N2)(H) and (TPB)(µ‒H)Fe(CNtBu)(H) ............................................................................. 30 Figure 2.2 XRD and DFT optimized structures of (TPB)(μ‒H)Fe(H2)(H) .................... 31 Figure 2.3 1 Figure 2.4 XRD structures of (TPB)Fe(C2H4) and (TPBH)Fe(C2Tol) .......................... 36 Figure 2.5 Decomposition product of the (TPB)Fe system ............................................ 43 Figure 3.1 Nonclassical W‒H2 adduct ............................................................................. 71 Figure 3.2 Frontier molecular orbital diagram for (SiPiPr3)Fe(H2) and (TPB)Fe(H2) .... 73 Figure 3.3 Q-band pulsed ENDOR spectra and rotational and nuclear spin states ........ 75 Figure 3.4 Potential energy surfaces for H2 rotation ........................................................ 77 Figure 3.5. Single crystal neutron diffraction structure of (TPB)Co(H2) ........................ 81 Figure 3.6 Plot of the isosurface of the promolecule electron density of H VT-NMR spectra of (TPB)(μ‒H)Fe(H2)(H)............................................. 32 (TPB)Co(H2) ................................................................................................... 84 Figure 3.7 Overlay of the core of the (TPB)Co(H2) neutron structure with the potential energy surface. ................................................................................. 86 Figure A1.1 XRD structure of (TPB)Fe(CNtBu) ............................................................. 146 Figure A1.2 XRD structure of (SiPiPr3)Fe(CO)(H) .......................................................... 152 xiii Figure A1.3 Stacked 1H NMR spectra of the reaction of (TPB)Fe(C2H4) with H2 (1 atm) and C2H4 (1 atm) in C6D6 ..................................................................... 153 Figure A1.4 Elimination of H2 from (TPB)(µ-H)Fe(N2)(H)............................................ 154 Figure A1.5 T1 values for (TPB)(µ‒H)Fe(H2)(H) ............................................................ 155 Figure A1.6 T1 values for (SiPiPr3)Fe(H2)(H) ................................................................... 155 Figure A1.7 Variable temperature magnetic susceptibility measurements (method of Evans) of (TPB)Fe(CNtBu) .......................................................................... 156 Figure A1.8 ATR-IR (THF thin film) of A, (TPBH)Fe(Et), (blue) and labeled (TPBD)Fe(Et) (red)....................................................................................... 157 Figure A1.9 Catalytic hydrogenation of styrene with (TPB)Fe(N2) ................................ 158 Figure A1.10 Catalytic hydrogenation of phenylacetylene with (TPB)Fe(N2) .................. 158 Figure A2.1 Solid state neutron diffraction structure of (SiPiPr3)Fe(H2) ......................... 162 Figure A3.1 UV-Vis spectra of the titration of [(SiPiPr3)Fe(N2)](BArF4) with Li(OCHO)...................................................................................................... 172 Figure A3.2 Fit of the UV-vis titration data ...................................................................... 172 xiv List of Schemes Scheme 1.1 Catalytic cycles for olefin hydrogenation ....................................................... 3 Scheme 1.2 Olefin hydrogenation by (PDI)Fe(N2)2........................................................... 5 Scheme 1.3 (PNP)Fe(CO)(Br)(H) for ketone hydrogenation ........................................... 6 Scheme 1.4 Heterolytic H‒H bond cleavage and hydrogenation catalysis by a metallaboratranes ............................................................................................. 9 Scheme 1.5 Reactivity of metallaboratranes toward X‒Y substrates .............................. 11 Scheme 1.6 H‒H and Si‒H bond activation reactions and catalysis by nickelaboratranes ........................................................................................... 14 Scheme 1.7 C‒H bond activation by [(iPrDPBPh)Fe]2(N2) and (iPrDPBPh)Co(N2) ......... 16 Scheme 1.8 Reactivity of [(iPrDPBPh)Fe]2(N2) and (iPrDPBPh)Co(N2) towards PhOH .............................................................................................................. 16 Scheme 1.9 Hydrogenation and hydrosilation of an iron-aminoimide ............................ 17 Scheme 1.10 C‒C coupling and C‒H coupling of an iron-dicarbyne ................................ 18 Scheme 2.1 Related H2 activation across a Ni‒B bond .................................................... 28 Scheme 2.2 H‒H and E‒H bond activations across Fe‒B bonds ..................................... 29 Scheme 2.3 H2 chemistry of the related (SiPiPr3)Fe silatrane system .............................. 35 Scheme 2.4 Ethylene coordination and arylacetylene C‒H bond activation by (TPB)Fe(N2) .................................................................................................. 36 Scheme 2.5 Reversible arylacetylene C‒H bond activation ............................................. 38 xv Scheme 2.6 Stoichiometric hydrogenation reactions........................................................ 39 Scheme 2.7 Mechanism and alkane elimination pathways .............................................. 44 Scheme 4.1 Reactions relevant to the determination of the pKa and hydricity of (SiPiPr3)Fe .................................................................................................... 101 Scheme 4.2 Stoichiometric CO2 hydrogenation to triethylammonium formate ............ 103 Scheme 4.3 Reactivity of (TPB)Fe complexes with CO2 and formic acid.................... 107 Scheme 4.4 Synthetic cycle for CO2 hydrogenation to formate by (SiPiPr3)Fe ............. 108 Scheme 4.5 Proposed catalytic cycle for (SiPiPr3)Fe in MeOH ..................................... 117 Scheme 4.6 Dihydride pathways for catalytic CO2 hydrogenation ................................ 119 Scheme 4.7 Gibbs free energies for the reactions of relevance to CO2 hydrogenation by (SiPiPr3)Fe(H2)(H) .......................................................... 121 Scheme A3.1 Reactions and equations relevant to hydricity determination for (SiPiPr3)Fe(H2)(H)........................................................................................ 169 Scheme A3.2 Equilibrium reaction for competitive THF/H2 coordination by [(SiPiPr3)Fe(L)]+ ........................................................................................... 170 xvi List of Charts Chart 1.1 General forms of (TPB)MX and (RDPBAr)MX complexes......................... 10 Chart 3.1 Open-shell M‒H2 adducts of Fe and Co of interest ...................................... 70 Chart 4.1 Select phosphine-metal complexes of relevance to catalytic CO2 hydrogenation ................................................................................................ 99 xvii List of Tables Table 2.1 Catalytic hydrogenations by (TPB)Fe(N2) with H2...................................... 42 Table 3.1 Selected bond angles and distances for (SiPiPr3)Fe(H2) and (TPB)Co(H2) ................................................................................................. 82 Table 4.1 IR stretching frequencies and solution magnetic moments for ironformate complexes ....................................................................................... 104 Table 4.2 Tris-phosphino-iron catalyzed CO2 hydrogenation .................................... 110 Table 4.3 (SiPiPr3)FeCl catalyzed CO2 hydrogenation under varied conditions ........ 113 Table 4.4 CO2 hydrogenation catalyzed by various (SiPiPr3)Fe species .................... 116 Table A1.5 Crystal data and structure refinement for (TPB)(μ‒H)Fe(N2)(H) ............. 143 Table A1.6 Crystal data and structure refinement for (TPB)(μ‒H)Fe(H2)(H) ............. 144 Table A1.7 Crystal data and structure refinement for (TPB)Fe(CNtBu) ...................... 145 Table A1.8 Crystal data and structure refinement for (TPB)(μ‒H)Fe(CNtBu)(H) ...... 147 Table A1.9 Crystal data and structure refinement for (TPB)Fe(C2H4) ......................... 148 Table A1.10 Crystal data and structure refinement for (TPBH)Fe(C2Tol) .................... 149 Table A1.11 Crystal data and structure refinement for D ................................................ 150 Table A1.12 Crystal data and structure refinement for (SiPiPr3)Fe(CO(H) .................... 151 Table A2.1 Crystal data and neutron structure refinement for (TPB)Co(H2) ............... 160 Table A2.2 Crystal data and neutron structure refinement for (SiPiPr3)Fe(H2)............. 161 Table A3.1 Catalytic hydrogenation results ................................................................... 164 xviii Table A3.2 Experimentally determined ΔGH- for (SiPiPr3)Fe(H2)(H) and pKa values for [(SiPiPr3)Fe(H2)](BArF4) ........................................................... 171 xix List of Abbreviations (BH)+ Generic conjugate acid of base B [M] Generic transition metal complex {1H} Proton decoupled {31P} Phosphorous decoupled ° Degree 11 Boron-11 13 Carbon-13 19 F Fluorine-19 1 H Hydrogen-1 29 Si Silicon-29 2 H Hydrogen-2 or deuterium 2-Me-THF 2-Methyl-tetrahydrofuran 31 P Phosphorous-31 6-311G(d) Basis sets for DFT Å Angstrom A Hyperfine coupling constant Abs Absorbance Anal Analysis Ar Generic aryl group asym Asymmetric B C xx atm Atmosphere ATR Attenuated total reflectance au-3 Inverse cubic atomic units B Generic base B(mimtBu)3 tris(2-mercapto-1-tert-butyl-imidazolyl)-borane B(NIn)3 tris(1H-pyrrolo-2-3-pyridin-1-yl)borane B3LYP DFT functionals BArF4 [B(3,5-(CF3)2-C6H3)4]- br Broad BZQ Benzo[h]quinolin-10-yl C Celsius C(sp) s-p Hybridized carbon atom C(sp2) s-p2 Hybridized carbon atom calc’d Calculated CAr Carbon atom of an aryl group Cipso Ipso carbon atom cm-1 Wave numbers Cn n-Fold rotational symmetry COD Cyclooctadiene CPiPr3 [C(o-C6H4PiPr2)3]- CSiPPh3 [C(Si(CH3)2CH2PPh2)3]- xxi CW Continuous-wave Cy Cyclohexyl D Deuterium d Doublet or deuterium d-d Doublet-of-doublets DFT Density Functional Theory DMF Dimethylformamide E Generic main-group element ENDOR Electron nuclear double resonance EPR Electron paramagnetic resonance Et Ethyl FID Flame ionization detector FpH (C5H5)Fe(CO)2(H) G Gibbs free energy g grams GC Gas chromatography GHz Megahertz h Hour(s) HMDSO Hexamethyldisiloxane I Nuclear spin iPr or iPr iso-Propyl xxii IR Infrared J NMR coupling constant K Kelvin Ka Acid dissociation constant kcal Kilocaloies Keq Equilibrium constant L Generic L-type ligand LF Ligand field m Medium or meta or multiplet M Generic transition metal or molar MeCN Acetonitrile Mes Mesityl mg Milligrams MHz Gigahertz mL Milliliters mm Millimeter mmol Millimoles Mn+ Generic transition metal of oxidation state n mol Moles MS Mass spectrometry n NMR coupling constant between nuclei x and y through n bonds Jx-y xxiii nm Nanometer NMR Nuclear magnetic resonance NPiPr3 N(CH2CH2PiPr2)3 o ortho OTf SO2CF3 p para PDI 2,6-(ArNC(CH3)2)2C5H3N PNP 2,6-(iPr2PCH2)2C5H3N Ph Phenyl PhBPiPr3 [PhB(CH2PiPr2)3]- pKa Negative logarithm of acid dissociation constant Ka PP3 P(CH2CH2PPh2)3 ppm Parts per million quart Quartet quin Quintet R Gas constant or generic functional group R DPBAr Ar(o-C6H4PR2)3B ref Reference RT Room temperature s Singlet or strong S Spin xxiv SiPiPr3 [Si(o-C6H4PiPr2)3]- SiPPh3 [Si(o-C6H4PPh2)3]- sym Symmetric T Temperature t Triplet T1min Minimum spin-lattice relaxation time TBAF Tetrabutylammonium fluoride TBP Trigonal bipyramidal tBu or tBu tert-Butyl TCD Thermal conductivity detector t-d Triplet-of-doublets tetraphos P(o-C6H4PiPr2)3 THF Tetrahydrofuran TOF Time-of-flight or turnover frequency Tol Tolyl TPB B(o-C6H4PiPr2)3 TPBD [DB(o-C6H4PiPr2)3]- TPBH [HB(o-C6H4PiPr2)3]- triphos CH3CH(CH2PPh3)3 UV Ultraviolet vis Visible xxv w Weak wR Weighted R-factor X Generic X-type ligand XRD X-ray diffraction XTmtBu tris(2-mercapto-1-tert-butylimidazolyl)-X-borate δ Chemical shift Δ Difference ΔGH- Hydricity ε Extinction coefficient ηx Heptacity of order X κn Denticity involving n atoms λ Wavelength μB Bohr magneton μeff Effective magnetic moment μL Microliters μM Micromolar μmol Micromoles μ‒X Bridging X ligand π Pi-symmetry orbital σ Sigma symmetry orbital Σ(X‒Y‒Z) Sum of angles defined by atoms X‒Y‒Z 1 Chapter 1. Introduction 2 1.1 Motivation Metal catalyzed bond forming and bond cleavage reactions have played a large role in the growth of organometallic chemistry for decades.1 These bond forming and bond breaking transformations require the formal transfer of multielectron equivalents. Noble metals, such as ruthenium, rhodium, iridium, palladium, and platinum, have proven to be particularly adept at catalyzing these reactions. This is in part due to the predisposition of noble metals to undergo controlled multielectron processes. The hydrogenation of unsaturated E=C bonds is the prototypical two-electron bond forming reaction catalyzed by organometallic complexes. Thus, the principles of hydrogenation chemistry can provide a framework by which to understand related metal-catalyzed bond forming and bond breaking reactions. The discovery of RhCl(PPh3)3 (Wilkinson’s catalyst)2-3 is one of the most important developments in hydrogenation chemistry. The catalyst can operate at ambient temperatures and pressures in the presence of ethanol as a co-solvent. It rapidly hydrogenates unconjugated olefins and acetylenes and is selective for the hydrogenation of olefins in the presence of ester, ketone , and nitroarene functional groups.4 The kinetics for olefin hydrogenation by the Wilkinson system has been extensively studied by Halpern.5-6 The core of the catalytic cycle deduced from Halpern’s studies is shown in Scheme 1.1A, and it illustrates some of the features of a classical (hydrogen-first) hydrogenation mechanism. Notable in the catalytic cycle are the controlled two electron steps that cycle the metal center between formal RhI and RhIII oxidation states. The RhI species B oxidatively adds H2 to afford the RhIII-dihydride C. Subsequent olefin coordination followed by migratory insertion steps afford species D and E, 3 respectively. The product alkane is then released by reductive elimination from the RhIII species E to regenerate the RhI species B and closure of catalytic cycle. Analogous mechanisms involving the transfer of both hydride equivalents from a dihydride intermediate, as in the Wilkinson system, and the similar elementary steps have been discovered for related catalysts. Scheme 1.1 Catalytic cycles for olefin hydrogenation. (A) Core of the catalytic cycle for Wilkinson’s catalyst. (B) Catalytic cycle for Ru(PPh3)3(Cl)(H). L = PPh3, S = co-solvent. Some other catalysts for olefin hydrogenation do not involve oxidative addition and reductive elimination processes.7 For example, the Ru(PPh3)3(H)(Cl) complex (also discovered 4 by Wilkinson) is a catalyst with selectivity for the hydrogenation of terminal over internal olefins.8 The proposed catalytic for hydrogenation by this RuII catalyst is shown in Scheme 1.1B.9-11 The process involves olefin coordination to a metal-monohydride species (G to H) followed by insertion to afford a metal-alkyl species (I). H2 coordination affords J. Release of the alkane product from the Ru-alkyl intermediate J to afford G occurs through hydrogenolysis. By avoiding oxidative addition and reductive elimination steps in the catalytic cycle, the ruthenium center maintains a RuII oxidation state, in contrast to the rhodium system. Other mechanisms for olefin hydrogenation exist,12-14 but like the two mechanisms shown in Scheme 1.1 they rely on the stability of the catalyst to two-electron processes, both with and without formal oxidation state changes. The development of well-defined late first-row transition metal complexes based on iron, cobalt, and nickel for hydrogenation catalysis has lagged behind that of their second and third row counterparts such as ruthenium, rhodium, iridium, palladium, and platinum.15 Part of the reason for this is that these first-row transition metals tend to undergo undesirable one electron reactivity. Reflecting this challenge is the limited number of iron,16-26 cobalt,25,27-34 and nickel32,34-39 catalyzed hydrogenations reported in the literature. Among these example, redoxactive ligand and metal-ligand cooperativity strategies have emerged as two promising methods to engender catalytic reactivity for these first-row transition metal complexes under mild conditions. Chirik reported in 2004 that ligand redox-noninnocent pyridyl-diimine-iron complex, (PDI)Fe(N2)2 (where PDI = 2,6-(ArNC(CH3)2)C5H3N), is a precatalyst for olefin 5 hydrogenation at ambient temperature and 1 atm H2.16 The proposed mechanism for the catalytic hydrogenation of olefins by (PDI)Fe(N2)2 (Scheme 1.2) is similar to the catalytic cycle presented for the Wilkinson catalyst. For (PDI)Fe(N2)2, the bound olefin of the irondihydride intermediate (PDI)Fe(H)2(olefin) (M) inserts into one of the Fe‒H bonds to form the iron-alkyl-hydride intermediate (PDI)Fe(H)(alkyl) (N). Subsequent reductive elimination of the product alkane from N reforms the three-coordinate (PDI)Fe intermediate K and closes the catalytic cycle. The redox-noninnocence of the PDI ligand in this system is significant. Combined experimental and DFT studies suggest that redox changes occur on the PDI ligand rather than the iron center during catalysis. For example, for the series of sequentially one Scheme 1.2 Olefin hydrogenation by (PDI)Fe(N2)2, highlighting ligand redox noninnocence. 6 electron reduced complexes (PDI)FeCl2, (PDI)FeCl, and (PDI)Fe(N2)2, the iron centers are all best formulated as FeII and the ligands as PDI, PDI1-, and PDI2-, respectively.40 By maintaining an FeII oxidation state throughout the catalytic cycle, the iron center avoids an FeIV state or an Fe0 state, which may not be supported by the PDI ligand by virtue of the oxidizing nature of the FeIV oxidation state and the mismatch between the hard nitrogen donors and soft, low valent iron center, respectively. Milstein showed that the (PNP)Fe (PNP = 2,6-(iPr2PCH2)2C5H3N) system heterolytically activates H2 and catalytically hydrogenates ketones through metal-ligand cooperativity41 at ambient temperature and a mild H2 pressure (4 atm).23 The proposed catalytic cycle for this Scheme 1.3 (PNP)Fe(CO)(Br)(H) for ketone hydrogenation, highlighting metal-ligand cooperativity for H‒H bond activation and hydrogenation catalysis. 7 reaction is shown in Scheme 1.3. The precatalyst (PNP)Fe(CO)(Br)(H) enters the catalytic cycle through deprotonation of a benzylic C‒H proton and bromide loss to afford the fivecoordinate FeII species, (PNP*)Fe(CO)(H) (O) (where PNP* is the dearomatized, deprotonated PNP ligand). The ketone subsequently pre-coordinates to the iron center (P) before it inserts into the cis Fe‒H bond to afford the five-coordinate iron-alkoxide intermediate Q. Dihydrogen then adds to intermediate Q to afford R, in which the ligand is re-aromatized. Elimination of the product alcohol from R closes the catalytic cycle. Notable in the catalytic cycle is the metal-ligand cooperativity in the H‒H bond cleavage step from Q to R; a hydride from H2 is formally transferred to the iron center while the proton is transferred to the unsaturated, benzylic carbon atom. In the final elimination step that releases the alcohol product, it is not known experimentally whether it is the hydrogen from the Fe‒H or the benzylic C‒H that is directly transferred to the product. However, given the trans arrangement of the alkoxide to the Fe‒H, transfer of the Fe‒H is unlikely. A previous study on the (PNP)Ir analogue showed that D2 addition to the dearomatized (PNP*)IrX species results in the formation of syn arranged IrD and benzylic C-D bonds, and that E‒H (E = CH2C(O)CH3 or H) elimination involves syn arranged Ir‒E and benzylic C‒D units.42 The data collectively suggest that in the iron system the H atom from the benzylic C‒H position situated syn to the alkoxide ligand is the most likely source of the proton on the alcohol product. 8 Metallaboratranesi are a promising alternative class of complexes for bond activation and catalysis. The Lewis acid BR3 unit in a metallaboratrane can stabilize low valent and/or Lewis basic metal centers through a retrodative MB interaction. These complexes may also activate small molecules (e.g., N2, CO, or H2) and E‒H (e.g., C‒H, H‒H, N‒H) bonds. In a (classical) scenario for the activation of H2, the Lewis basicity of the metal center may react with the πacidic H2 ligand to afford a nonclassical H2 adduct, classical dihydride, or an intermediate M‒H2 species intermediate of the two extremes (Scheme 1.4A top).46 Alternatively, the Lewis basic, low valent metal and the Lewis acidic borane units can in principle work in tandem to heterolytically cleave the H‒H bond to afford [R3B‒H]- and M‒H linkages through the formal transfer of H- to the borane and H+ to the metal (Scheme 1.4A bottom, Scheme 1.4B). This reactivity contrasts with traditional metal-ligand cooperative H‒H bond cleavage reactions, such as in the Milstein example presented above, where the Lewis acidic metal center accepts an H- while a basic site on the ligand accepts H+ from H2 (Scheme 1.4C).23 This type of reaction can be thought of as the “normal polarity” for heterolytic H‒H bond cleavage. Heterolytic H2 cleavage using a metallaboratrane is therefore a formal inversion to the polarity (i.e., “inverse polarity”) of traditional metal-ligand facilitated H‒H bond cleavage reactions (Scheme 1.4B). This strategy is reminiscent of H‒H bond cleavage using frustrated Lewis acid-base pairs.47 Given the success of metal-ligand cooperativity for hydrogenation catalysis, i The metallaboratrane terminology has been used by Hill (see ref 43) to refer to a caged structure in which there exists a transannular dative metal-borane interaction and three supporting heterocylic bridges. Historically, the "atrane" terminology has also referred to transannular interactions supported by three heterocyclic bridges (see ref 44 and 45). 9 the cooperativity of the borane and metal in a metallaboratrane is a promising new strategy for hydrogenation catalysis. In fact, catalytic hydrogenation of alkenes and alkynes using metallaboratranes had not been reported prior to the start of my work on the ferraboratrane system discussed in this thesis. Scheme 1.4 Heterolytic H‒H bond cleavage and hydrogenation catalysis by a metallaboratranes. (A) H‒H bond cleavage to a metallaboratrane results in bond H‒H cleavage (bottom) or H2 addition to the metal center as an H2 adduct, dihydride, or an intermediate M‒H2 species (top). (B) “Inverse polarity” H‒H bond cleavage. (C) “Normal polarity” H‒H bond cleavage. The following chapters of this thesis will discuss my studies on metallaboratrane facilitated E‒H bond cleavage and hydrogenation catalysis. I will present my discoveries from my work on the (TPB)M system (TPB = (TPB = B(o-C6H4PiPr2)3, M = Fe or Co) (Chart 1.1) for H2 activation in Chapters 2 and 3. The olefin and arylacetylene hydrogenation chemistry of the (TPB)Fe system will also be discussed in Chapter 2. Chapter 4 will discuss the CO2 10 hydrogenation chemistry of the (TPB)Fe and related iron-systems. For the remainder of this chapter, I will discuss some relevant aspects metallaboratrane and iron-hydride chemistry. Chart 1.1 General forms of (TPB)MX and (RDPBAr)MX complexes. 1.2 X‒Y Bond Activation and Hydride Transfer Using Metallaboratranes 1.2.1 Precedent for X‒Y Bond Activation Examples of bond activation by metallaboratranes are limited. Parkin reported the first example of X‒Y bond cleavage across a M‒B interaction in a metallaboratrane.48 The ferraboratrane B(mimtBu)3Fe(CO)2 complex (B(mimtBu)3 = tris(2-mercapto-1-tert-butylimidazolyl)-borane) undergoes a formal 1,2-addition of halogen containing X‒Y bonds to afford (XTmtBu)FeY (XTmtBu = tris(2-mercapto-1-tert-butylimidazolyl)-X-borate) (Scheme 1.5A). Notable for this transformation is the breaking of the Fe‒B bond. In contrast, Hill reported that the addition of halogens to the platinum analogue, B(mimtBu)3Pt(PPh3), do not result in Pt‒B bond breakage (Scheme 1.5B).49 Instead, oxidative addition of X2 occurs at the metal center to afford B(mimtBu)3Pt(X)2 with retention of the Pt‒B bond. The first reported 11 example of H2 addition across a M‒B interaction was from Owen for the rhodaboratrane B(NIn)3RhR (B(NIn)3 = tris(1H-pyrrolo-2-3-pyridin-1-yl)borane) (Scheme 1.5C).50 This system is the first example to show that alkyl ligand in a metallaboratranes can be hydrogenated to an alkane. It also shows that the M‒B bond can be broken to form a B‒H bond in metallaboratranes. However, the reaction is stoichiometric, and catalytic reactivity has not been reported. Scheme 1.5 Reactivity of metallaboratranes toward X‒Y substrates. 12 1.2.2 Background on (TPB)MX Our group recently began to study the series of metallaboratranes (TPB)M51-56,57 and (RDPBAr)M39,58,59,60 (RDPBAr = ArB(o-C6H4PR2)3; R = Ph or iPr; Ar = 2,4,6-(Me)3-Ph or Ph; M = Fe, Co, or Ni) (Chart 1.1). At the onset, we were focused on the coordination chemistry of π-acids such as N2 and CO and π-bases such as imides and nitrides trans to the boron in (TPB)Fe.51-52 In light of the limited precedent for bond activation reactivity by metallaboratranes, our group was motivated to study E‒H bond activations. The working hypothesis at the time was that the hemi-labile M‒B bond in the (TPB)MX scaffold may allow the boron and metal center to cleave E‒H bonds. The hemi-lability of the M‒B bond in (TPB)MX complexes was confirmed experimentally. The Fe‒B distance in (TPB)FeX extends from the DFT determined value of 2.2 Å in the irondinitrogen adduct (TPB)Fe(N2) to 2.589 Å in the iron-imide (TPB)Fe(NAr).51-52 The Fe‒B bond in (TPB)FeX thus appears to be highly susceptible to the identity of the X ligand and the formal electron count of the {M-B}n unitii ({Fe‒B}8 and {Fe‒B}6 for (TPB)Fe(N2) and (TPB)Fe(NAr), respectively). Between the isoelectronic (TPB)Ni56 and [(TPB)Cu](BArF4)55 complexes, the M‒B distances are also very different: 2.168 Å in (TPB)Ni and 2.495 Å in [(TPB)Cu](BArF4). This pair of complexes shows that the Lewis basicity of the metal, by substitution of Ni0 for CuI, and/or increasing the formal charge of the complex can disrupt the ii The {M‒E}n notation refers to the number of valence electrons that are formally assigned to the metal (e.g., Fe) and those shared with E (e.g., B). Since the M‒E bond may be covalent and the M‒E interaction is dictated in part by the ligand chelate and M‒E distance, the bonding electrons between M‒E are not reliably assigned to either atom. As such, the {M‒E}n notation tracks the number of valence electrons without assignment of valence or oxidation numbers. See ref 61 and 62. 13 M‒B interaction. The M‒B bond distances in the (TPB)MX complexes are in contrast to the first row metallaboratrane complexes of B(mimtBu)MX (M = Fe(CO),48 Co,63 or Ni;64-65 L = CO, Cl, OAc, NCS, PPh3, N3), where the M‒B distances are consistently short (2.0 – 2.1 Å), regardless of the spin state and identity of M. They also contrast with the nearly static Fe‒Si distances in the (SiPiPr3)MX system,66-67 where the B in TPB is replaced by a Si atom in SiPiPr3, that has been extensively studied in our group. The rigid M‒Si distance in the (SiPR3)MX system reflects the stronger M‒Si bond compared to the M‒B bond. How this difference in M‒E (E = B or Si) bonding affects E‒H bond activation and catalysis for the (TPB)FeX and (SiPiPr3)FeX systems will be discussed in Chapters 2 and 4. 1.2.3 E‒H Bond Activation by (RDPBAr)MX The RDPBAr (Ar = Ph or Mes, R = iPr or Ph) ligand has one less phosphine donor unit compared to the three phosphine donor units on the TPB ligand. (RDPBAr)MX complexes all show η2-arene or Cipso interactions between the metal and arene unit, as shown in Chart 1.1.39,60,68 Compared to the M‒P bond, the M‒arene interaction is more labile. It appears that this lability engenders greater reactivity for (RDPBAr)MX complexes towards E‒H bonds. E‒H bond cleavage and catalysis by (PhDPBMes)Ni were discovered concurrently with the (TPB)Fe results presented in Chapter 2. (PhDPBMes)Ni heterolytically cleaves the E‒H bonds of H‒H39 and R2HSi‒H58 to afford (PhDPBMes)(μ‒H)Ni(E) (E = H or R2HSi). The system is also competent olefin hydrogenation and ketone hydrosilation, respectively (Scheme 1.6A/B). The cleavage of the H‒H bond in H2 by (PhDPBMes)Ni is particularly noteworthy because it is the first well-defined example for the oxidative addition of H2 to a single nickel center. For 14 comparison, the isoelectronic (TPB)Ni does not react with H2 (Scheme 1.6C),56 suggesting that lability of one ligand arm is necessary for E‒H bond activation in these nickelaboratranes. Two independent DFT studies support the cooperativity of the borane and nickel for H‒H bond cleavage.59,69 Calculations at the M06L/6-31+G* level for the model (MeDPBPh)Ni system (the phenyls were replaced with methyls on the phosphines, and mesityl was replaced with phenyl) indicate that the lowest energy transition state (28.1 kcal/mol) for H‒H bond cleavage involves the formation of a B‒H bond in concert with H‒H bond cleavage (Figure 1.1A).59 An alternative mechanism, where B‒H bond formation occurs after oxidative addition of H2 at nickel to afford a nickel-dihydride, is 9 kcal/mol higher in energy (Figure 1.1B). Scheme 1.6 H‒H and Si‒H bond activation reactions and catalysis by nickelaboratranes. 15 Figure 1.1 DFT calculated transition states for borane assisted and unassisted H‒H bond cleavage. Selected bond distances are in Å. (A) Borane assisted H‒H cleavage, ΔG‡ = 28.1 kcal/mol. (B) Oxidative addition of H2 at nickel unassisted by the borane, ΔG‡ = 36.9 kcal/mol. E‒H bond activations by low coordinate iron70 and cobalt71 complexes are known in the literature. Iron and cobalt complexes of the RDPBAr ligand can also activate various E‒H bonds. The C‒H bond activation of BZQ (BZQ = benzo[h]quinolin-10-yl) by the N2-bridged iron-dimer [(iPrDPBPh)Fe]2(N2) affords the six-coordinate iron complex (iPrDPBPh)(μ‒H)Fe(BZQ), which features a hydride ligand bridging the boron and iron atoms, as well as an interaction between the iron and ipso carbon atom of the phenyl arm of the ligand (Scheme 1.7A).72 The cobalt complex (iPrDPBPh)Co(N2) also activates the C‒H bond of BZQ to afford (iPrDPBPh)(μ‒H)Co(BZQ).72 In contrast to the iron-analogue, the cobalt-BZQ complex is pseudo-square pyramidal and does not have a Co‒Cipso interaction (Scheme 1.7B). It is thought that the pyridine group directs the C‒H group towards the metal center to facilitate bond activation. 16 Scheme 1.7 C‒H bond activation by [(iPrDPBPh)Fe]2(N2) and (iPrDPBPh)Co(N2). Scheme 1.8 Reactivity of [(iPrDPBPh)Fe]2(N2) and (iPrDPBPh)Co(N2) towards PhOH. E‒H bond activation by [(iPrDPBPh)Fe]2(N2) and (iPrDPBPh)Co(N2) also extends to benzylic C‒H and N‒H bonds, as well as P‒H, O‒H, and S‒H bonds.72 Differences in their reactivity towards various E‒H bonds exist. One difference between the two systems is their reactivity 17 towards phenol. [(iPrDPBPh)Fe]2(N2) reacts with 2 equiv of phenol to afford the iron-phenolate product (iPrDPBPh)Fe(OPh) and 1 equiv of H2 (Scheme 1.8A). A B‒H linkage is not present in (iPrDPBPh)Fe(OPh). For (iPrDPBPh)Co(N2), O‒H activation does not occur, and phenol ligates through the O-atom as phenol-adduct to cobalt to give (iPrDPBPh)Co(O(H)Ph) (Scheme 1.8B). The activation of E‒H bonds has also been coupled to other bond-forming and bondbreaking transformations. The reaction of PhSiH3 with the aminoimide (PhDPBPh)Fe(NN(SiR3)2) results in the hydrosilation of the Fe‒N bond with delivery of the PhH2Si group to the α-nitrogen of the imide and H to the boron atom, affording the ironhydrazido1- species S (Scheme 1.9).68 The reaction of H2 with (PhDPBPh)Fe(NN(SiR3)2) results in H‒H bond cleavage, formation of B‒H and N‒H linkages, and rupture of the N‒N bond to afford T (Scheme 1.9).68 Scheme 1.9 Hydrogenation and hydrosilation of an iron-aminoimide. The reaction of H2 with the iron-dicarbyne species (iPrDPBPh)Fe(COTMS)2 (TMS = (CH3)3Si) results in the reductive C‒C bond coupling and C‒H bond formation to afford the CO derived disiloxyethylene product (Scheme 1.10).60 The identity of iron containing product from this reaction is not known. Previous examples of reductive CO coupling by iron 18 complexes are rare, and the release of an olefin from hydrogenative CO reductive coupling is not known for iron. Scheme 1.10 C‒C coupling and C‒H coupling of an iron-dicarbyne. In summary, the RDPBAr ligand engenders unusual bond breaking and bond forming transformations, as well as catalytic reactivity, that are otherwise rarely observed or unknown for iron, cobalt, and nickel complexes. The reactivity is facilitated in part by cooperativity between the Lewis acidic borane and Lewis basic metal center, as well as the lability of the aryl unit of ligand. The latter point is notable because differences in hydrogenation catalysis and E‒H bond activation are observed between related (TPB)MX and (RDPBAr)MX complexes. 19 1.4 Cited References (1) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; 1st ed.; University Science Books: Sausalito, California, 2010. (2) Young, J. F.; Osborn, J. A.; Jardine, F. H.; Wilkinson, G. Chemical Communications (London) 1965, 131-132. (3) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem. Soc. A 1966, 1711-1732. (4) James, B. 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(58) MacMillan, S. N.; Hill Harman, W.; Peters, J. C. Chem. Sci. 2014, 5, 590-597. (59) Harman, W. H.; Lin, T.-P.; Peters, J. C. Angew. Chem. Int. Ed. 2014, 53, 1081-1086. 23 (60) Suess, D. L. M.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 12580-12583. (61) Hill, A. F. Organometallics 2006, 25, 4741-4743. (62) Parkin, G. Organometallics 2006, 25, 4744-4747. (63) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2004, 1626-1634. (64) Senda, S.; Ohki, Y.; Hirayama, T.; Toda, D.; Chen, J.-L.; Matsumoto, T.; Kawaguchi, H.; Tatsumi, K. Inorg. Chem. 2006, 45, 9914-9925. (65) Pang, K.; Tanski, J. M.; Parkin, G. Chem. Commun. 2008, 1008-1010. (66) Lee, Y.; Mankad, N. P.; Peters, J. C. Nature Chem. 2010, 2, 558-565. (67) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507-2517. (68) Suess, D. L. M.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 4938-4941. (69) Zeng, G.; Sakaki, S. Inorg. Chem. 2013, 52, 2844-2853. (70) Selected examples of C‒H activation by iron complexes: (a) Klein, H.-F.; Camadanli, S.; Beck, R.; Leukel, D.; Flörke, U. Angew. Chem. Int. Ed. 2005, 44, 975-977; (b) Baker, M. V.; Field, L. D. J. Am. Chem. Soc. 1986, 108, 7433-7434; (c) Field, L. D.; Guest, R. W.; Turner, P. Inorg. Chem. 2010, 49, 9086-9093; (d) Pan, Y. H.; Sohlberg, K.; Ridge, D. P. J. Am. Chem. Soc. 1991, 113, 2406-2411; (e) Bolig, A. D.; Brookhart, M. J. Am. Chem. Soc. 2007, 129, 14544-14545. (f) (e) Wang, D.; Farquhar, E. R.; Stubna, A.; Muenk, E.; Que, L. Nature Chem. 2009, 1, 145-150. 24 (71) Selected examples C‒H activation by cobalt complexes: (a) Klein, H.-F.; Camadanli, S.; Beck, R.; Leukel, D.; Flörke, U. Angew. Chem. Int. Ed. 2005, 44, 975-977; (b) Yoshino, T.; Ikemoto, H.; Matsunaga, S.; Kanai, M. Angew. Chem. Int. Ed. 2013, 52, 2207-2211; (c) Ding, Z.; Yoshikai, N. Beilstein J. Org. Chem. 2012, 8, 1536-1542; (d) Zhang, R.; Zhu, L.; Liu, G.; Dai, H.; Lu, Z.; Zhao, J.; Yan, H. J. Am. Chem. Soc. 2012, 134, 10341-10344; (e) Lee, P.-S.; Fujita, T.; Yoshikai, N. J. Am. Chem. Soc. 2011, 133, 17283-17295; (f) Chen, Q.; Ilies, L.; Nakamura, E. J. Am. Chem. Soc. 2010, 133, 428-429; (g) Hojilla Atienza, C. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 16343-16345; (h) Aubert, C.; Betschmann, P.; Eichberg, M. J.; Gandon, V.; Heckrodt, T. J.; Lehmann, J.; Malacria, M.; Masjost, B.; Paredes, E.; Vollhardt, K. P. C.; Whitener, G. D. Chem. Eur. J. 2007, 13, 74437465. (i) Li, L.; Jones, W. D. J. Am. Chem. Soc. 2007, 129, 10707-10713. (72) Unpublished results. 25 Chapter 2. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane Reproduced in part with permission from: Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Organometallics 2013, 32, 3053-3062. © 2013 American Chemical Society 26 2.1 Preface This chapter describes the heterolytic cleavage of H‒H and C‒H bonds by the ferraboratrane (TPB)Fe(L) (L = N2, CO, tBuNC), as well as catalytic olefin and arylacetylene hydrogenation by (TPB)Fe(N2). The data show that (TPB)Fe(N2) reversibly cleaves H2 to afford (TPB)(μ‒H)Fe(L)(H), which contains B‒H and Fe‒H linkages. The bond cleavage reaction contrasts with the H2 chemistry of the related silatrane (SiPiPr3)Fe scaffold, as well as the cobaltaboratrane (TPB)Co(H2) (see Chapter 3). (TPB)(μ‒H)Fe(L)(H) (L = N2 or H2) and (TPB)Fe(N2) are competent hydrogenation catalysts, and stoichiometric studies indicate that the B‒H unit in (TPB)(μ‒H)Fe(L)(H) is capable of H- transfer to the olefin substrate. These findings are presented in the context of metal-borane cooperative bond activation and catalysis. I am the first author of an article published in Organometallics that describes this work. The other authors of this paper are Marc-Etienne Moret, Yunho Lee, and Jonas Peters. I performed all of the experiments for the (TPB)Fe system described in this chapter. Marc-Etienne, Jonas Peters, and I analyzed the results. Marc-Etienne Moret also assisted me with DFT calculations. Yunho Lee worked on the H2 chemistry of the (SiPiPr3)FeX system. 27 2.2 Introduction Transition metal-catalyzed bond forming reactions often involve formal two-electron redox steps (e.g., oxidative addition and reductive elimination). Noble metal catalysts are commonly used in these reactions due, in part, to their propensity to facilitate multi-electron processes.1 There is growing interest in developing catalysts for bond forming/cleavage reactions based on earth abundant mid-to-late first-row transition metals, a goal that presents a unique set of challenges.2 First-row transition metal catalysts that can circumvent undesirable one electron processes in favor of concerted two electron reaction steps present one plausible design criterion.3 Cooperative catalysis strategies that utilize ligands that operate in tandem with a coordinated metal center to activate substrates have shown promise in addressing this issue. 4-9 First-row metallaboratranes and related compounds that contain a retrodative M→B σ-interaction10 are appealing as catalysts11-12 because of the boron center’s ability to stabilize low valent metals.1319 Akin to frustrated Lewis pairs,20 it has also been recently demonstrated that the metal-boron interaction can cooperatively facilitate the activation of H2.11,21 For instance, our lab reported that the diphosphine-borane nickel (MesDPBPh)Ni complex (MesDPBPh = MesB(o-C6H4PPh2)2) undergoes reversible oxidative addition of H2 to afford a nickel-borohydrido-hydride complex (Scheme 2.1).22 This nickel system is an efficient catalyst for olefin hydrogenation. Kameo and Nakazawa have also reported on the transfer hydrogenation of ketones catalyzed by a related rhodium diphosphine-borane complex.23 28 Scheme 2.1. Related H2 activation across a Ni‒B bond. This chapter describes studies on heterolytic H‒H bond cleavage at a ferraboratrane complex24-25 of a triphosphine-borane (TPB) ligand (TPB = B(o-C6H4PiPr2)3).26 Dihydrogen is shown to add reversibly across the Fe‒B bond of (TPB)Fe(L) complexes to form corresponding iron-borohydrido-hydride complexes of the type (TPB)(µ‒H)Fe(L)(H). Like the nickel system reported previously, olefin hydrogenation catalysis is accessible, albeit much slower, thereby facilitating detailed studies. As discussed below, other E‒H bonds are also activated by the ferraboratanes described, including the terminal C‒H bonds of arylacetylenes and the C‒H bonds of formaldehyde. 2.3 Results and Discussion 2.3.1 Reversible H2 Addition Exposing the previously reported (TPB)Fe(N2) complex24 in d6-benzene to H2 (1.2 equiv) at room temperature results in H2 addition across the Fe‒B bond to give a yellow solution of the six coordinate borohydride-hydride-N2 complex (TPB)(μ‒H)Fe(N2)(H) (Scheme 2.2). The 29 XRD structure of (TPB)(μ‒H)Fe(N2)(H) shows that the Fe‒B distance is significantly elongated relative to (TPB)Fe(N2) (2.604(3) Å in (TPB)(μ‒H)Fe(N2)(H) (Figure 2.1) versus the calculated distance of 2.2 Å in (TPB)Fe(N2)25. A terminal hydride ligand and a bridging hydride ligand located between the B and Fe atoms can be assigned from the electron density Scheme 2.2 H‒H and E‒H bond activations across Fe‒B bonds. 30 Figure 2.1 XRD structures of (TPB)(μ‒H)Fe(N2)(H) and (TPB)(µ‒H)Fe(CNtBu)(H). Ellipsoids shown at 50% probability. Selected bond distances (Å): (left) (TPB)(µ‒H)Fe(N2)(H), Fe1‒H1 = 1.42(2), Fe1‒H2 = 1.49(2), B1‒H2 = 1.17(2), Fe1‒B1 = 2.604(3); (right) (TPB)(µ‒H)Fe(CNtBu)(H), Fe1‒H42 = 1.35(2), Fe1‒H43 = 1.52(2), B1‒H43 = 1.20(2), Fe1‒B1 = 2.673(2). difference map. This structure also prevails in solution. In the 1H NMR spectrum (d6-benzene), the terminal hydride ligand on iron is observed as a triplet-of-doublets at -9.6 ppm, and the bridging hydride is observed as a broad singlet at -30.4 ppm. Replacing H2 with D2 in the reaction gives the corresponding isotopologue (TPB)(µ‒D)Fe(N2)(D). Along with the expected deuteride signals in the 2H NMR spectrum, deuterium signals from the methine and terminal methyl positions of the isopropyl groups of the TPB ligand are observed. This observation establishes that facile scrambling of the hydridic ligands into the TPB isopropyl groups occurs, presumably via a reversible C‒H metalation process. The dinitrogen ligand (νNN = 2070 cm-1) in (TPB)(μ‒H)Fe(N2)(H) is labile, and it can be substituted under excess H2 to give the dihydrogen analogue (TPB)(μ‒H)Fe(H2)(H) (Scheme 2.2). Exposing (TPB)Fe(N2) to excess H2 (1 atm) also generates (TPB)(μ‒H)Fe(H2)(H). Its XRD structure again confirms 31 Figure 2.2 XRD and DFT optimized structures of (TPB)(μ‒H)Fe(H2)(H). Ellipsoids shown at 50% probability. Selected bond distances (Å) and angles (°): (left) XRD, Fe1‒H42 = 1.56(2), B1‒H42 = 1.21(2), Fe1‒B1, 2.63(2), P2‒Fe‒P3 = 136.54(2); (right) DFT predicted, Fe1‒H42 = 1.51, Fe1‒H43 = 1.50, Fe1‒H44 = 1.62, Fe1‒H45 = 1.59, B1‒H42 = 1.24, Fe1‒B1 = 2.63, H44‒H45 = 0.84, P2‒Fe1‒P3 = 140.63. the presence of a bridging hydride (Figure 2.2). Although electron density can be located in the difference map between a widened P‒Fe‒P angle (136.54(2)°) and in the apical position trans to the borohydride unit, the data do not allow us to reliably distinguish between classical and non-classical hydrides. We therefore turned to NMR spectroscopy to aid in the formulation of (TPB)(μ‒H)Fe(H2)(H). The 20 °C 1H NMR spectrum (d8-toluene) of (TPB)(μ‒H)Fe(H2)(H) (Figure 2.3) shows a broad singlet resonance at -15.1 ppm, indicative of hydridic protons. A broad deuteride signal is observed in the 2H NMR spectrum when D2 is used in place of H2, and, like in (TPB)(μ‒H)Fe(N2)(H), deuterium signals are also observed in the methyl and methine positions of the isopropyl groups of the TPB ligand. Cooling a d8-toluene solution of (TPB)(μ‒H)Fe(H2)(H) under an H2 atmosphere to -20 °C leads to sharpening of the resonance at -15.1 ppm, which integrates to three protons (3H). A second, broad hydridic resonance integrating to one proton 32 Figure 2.3 1H VT-NMR spectra of (TPB)(μ‒H)Fe(H2)(H) in d8-toluene under 1 atm of H2. is also observed at -24.9 ppm, and in analogy to (TPB)(μ‒H)Fe(N2)(H), this resonance is assigned to the bridging-borohydride (µ‒H). Cooling the temperature down to -90 °C leads to broadening of the 3H resonance without reaching decoalescence, suggesting that exchange of three hydrogenic ligands is fast on the NMR timescale. Compound (TPB)(μ‒H)Fe(H2)(H) can be heated to 50 °C before significant sample decomposition is observed (vide infra). At 50 °C, the 3H and µ‒H signals both broaden into the baseline, suggesting that exchange of all four hydrogens is facile at this temperature. To further assign the 3H unit in (TPB)(μ‒H)Fe(H2)(H) (i.e., dihydrogen-hydride versus trihydride), we turned to minimal longitudinal relaxation (T1min) measurements.i,27-29 The T1min is 35 ms at -32 °C for the 3H resonance, which suggests that the 3H unit is best described as i The H‒D coupling constant (JH‒D) is also in principle an excellent distinguishing characteristic between the η2dihydrogen and dihydride ligands. Unfortunately, H‒D coupling cannot be resolved in (TPB)(μ‒H)Fe(H2)(H), likely due to fast exchange within the HnD3-n unit and H/D scrambling into the methine and terminal methyl positons on the isopropyl groups of the TPB ligand. 33 dihydrogen-hydride. This interpretation is additionally supported by DFT calculations (RB3LYP/6-31G(d)) that identify a dihydrogen-hydride structure (Figure 2.2 and Appendix 1) as the lowest energy isomer of (TPB)(μ‒H)Fe(H2)(H). A stereoisomer in which the H2 ligand occupies the equatorial position and the hydride ligand is in the axial position trans to boron is calculated to be 6.1 kcal/mol higher in energy. This geometric preference parallels that of the well-characterized iron-dihydrogen-dihydride complex mer-Fe(H2)(H)2(PEt2Ph)3.30 The transition state for conversion of (TPB)(μ‒H)Fe(H2)(H) into this higher energy stereoisomer involving H‒H scission is calculated to be 6.7 kcal/mol above the most stable isomer and is in line with the observed exchange behavior on the NMR timescale. Analogous (TPB)(µ‒H)Fe(L)(H) complexes (L = CNtBu or CO) can be synthesized (Scheme 2.2, Figure 2.1). To explore the effect of the apical ligand L on the H‒H bond activation process, (TPB)Fe(L) complexes were prepared. The previously reported carbonyl complex (TPB)Fe(CO) displays an η3­interaction with a P‒CAr‒CAr unit of the TPB ligand,24 whereas isocyanide adduct (TPB)Fe(CNtBu), whose structure has been determined, does not: its Fe center is rigorously 5-coordinate. Complex (TPB)Fe(CO) is diamagnetic, whereas (TPB)Fe(CNtBu) gives rise to a solution magnetic susceptibility (μeff = 1.7 μB) at room temperature in C6D6. The temperature dependence of the solution susceptibility suggests an S = 0 ground state with a thermally accessible S = 1 state. No reaction occurs between (TPB)Fe(CNtBu) or the previously reported (TPB)Fe(CO) with H2 (1 atm) at room temperature over a period hours. Compound (TPB)Fe(CNtBu) is fully consumed by H2 (1 atm) over the course of 3 days at 40 °C to generate (TPB)(µ‒H)Fe(CNtBu)(H), while compound 34 (TPB)Fe(CO) is fully consumed by H2 (1 atm) over the course of 5 days at 80 °C to give (TPB)(µ‒H)Fe(CO)(H). The increase in temperature and reaction time compared to the facile room temperature reaction between (TPB)Fe(N2) and H2 is consistent with a scenario in which H2 substitution for L occurs prior to H2 addition across the Fe‒B bond. Complex (TPB)(µ‒H)Fe(CO)(H) can be alternatively synthesized from (TPB)Fe(N2) or (TPB)(µ‒H)Fe(N2)(H) and formaldehyde (Scheme 2.2). Dihydrogen addition across the Fe‒B bond of (TPB)Fe(N2) is reversible. Conversion of (TPB)(µ‒H)Fe(H2)(H) to (TPB)(µ‒H)Fe(N2)(H) and subsequently back to (TPB)Fe(N2) can be effected by exposing (TPB)(µ‒H)Fe(H2)(H) to dynamic vacuum and then N2, or by repeated freeze-pump-thaw-N2 cycles. Reformation of the Fe‒B bond can also occur through hydride transfer to unsaturated substrates (vide infra). Dihydrogen elimination from (TPB)(µ‒H)Fe(CNtBu)(H) and (TPB)(µ‒H)Fe(CO)(H) does not occur when treated similarly. Worth underscoring is that cleavage of the Fe‒B bond in the present ferraboratrane system is distinct from the H2 chemistry observed for a structurally related (SiPiPr3)Fe silatrane system (SiPiPr3 = [Si(o-C6H4PiPr2)3]–,) we have introduced elsewhere.31-32 For instance, the reaction between (SiPiPr3)Fe(N2) and H2 affords (SiPiPr3)Fe(H2), and that between [(SiPiPr3)Fe(N2)]+ and H2 affords [(SiPiPr3)Fe(H2)]+. No disruption of the Fe‒Si bond is observed in either case, even if for instance isolated [(SiPiPr3)Fe(N2)]+ or [(SiPiPr3)Fe(H2)]+ is exposed to excess H2. Ligand substitution instead occurs. This sharply contrasts isoelectronic (TPB)Fe(N2), where H2 addition readily affords the cleavage product (TPB)(µ‒H)Fe(N2)(H) or (TPB)(µ‒H)Fe(H2)(H). We also find (this report, Scheme 2.3) that hydrogenolysis of the iron(II) methyl complex 35 (SiPiPr3)Fe(Me)occurs slowly at 60 °C to give an H2/H product, but once again without disruption of the Fe‒Si bond. Addition of exogenous donor ligands such as H2, N2 and CO effects substitution of the coordinated H2 ligand, but the Fe‒Si bond is maintained. One factor contributing to the difference between the two systems is likely the more flexible Fe‒B bond in the (TPB)Fe system, as reflected in the variable Fe‒B bond distances (varying by ca. 0.5 Å)25 versus more rigid Fe‒Si bond distances (varying by ca. 0.2 Å)31-33 in (SiPiPr3)Fe that have been observed over several formal iron oxidation states. The rigidity of the Fe‒Si interaction presumably reflects an appreciably stronger Fe‒Si bond relative to Fe‒B. One can additionally consider the relative Lewis acidity of the Ar3B versus the Ar3Si+ subunit33 in these respective systems, and their propensity to serve as H- acceptors, but one might then predict the Ar3Si+ to be the better acceptor, in contrast to the experimental observations. Indeed, this latter point may be the reason the Fe‒Si interaction is stronger than the Fe‒B interaction. Scheme 2.3 H2 chemistry of the related (SiPiPr3)Fe silatrane system. 36 2.3.2 Reaction with Unsaturated Substrates The ability of the (TPB)Fe scaffold to reversibly cleave H2 prompted us to study if the transfer of hydrogen from (TPB)(µ‒H)Fe(L)(H) to substrates is possible. The reaction of (TPB)Fe(N2) with ethylene, styrene, and arylacetylenes was therefore probed. A degassed d6Scheme 2.4 Ethylene coordination and arylacetylene C‒H bond activation by (TPB)Fe(N2). Figure 2.4 XRD structures of (TPB)Fe(C2H4) and (TPBH)Fe(C2Tol). Ellipsoids shown at 50% probability. Selected bond distances (Å) and angles (°): (right) (TPB)Fe(C2H4) (average for two molecules in the asymmetric unit cell), Fe1‒B1 = 2.491(1), Fe1‒C37 = 2.103(1), Fe1‒C38 = 2.113(1), C37‒C38 = 1.397(2), ∑(P‒Fe‒P) = 338.76(3); (left) (TPBH)Fe(C2Tol) (average for two molecules in the asymmetric unit cell), Fe1‒B1 = 2.761(2), Fe1‒C37 = 1.918(2) C37‒C38 = 1.169(3), ∑(P‒Fe‒P) = 345.07(2). 37 benzene solution of (TPB)Fe(N2) reacts with ethylene to give a light brown solution of the paramagnetic iron-ethylene adduct (TPB)Fe(C2H4) (µeff = 3.2 µB, S = 1) (Scheme 2.4). Brown XRD quality crystals of (TPB)Fe(C2H4) can be grown under an atmosphere of ethylene at 0 °C (Figure 2.4). Two molecules of (TPB)Fe(C2H4) were found in the asymmetric unit cell. The iron center is bound η2 to ethylene ((Fe‒C)avg = 2.108(1) Å) and lies above the plane defined by the phosphine donors by an average distance of 0.641 Å, with a corresponding elongation of the average Fe‒B distance to 2.491(2) Å (compared to 2.2 Å in (TPB)Fe(N2)). The average C‒C bond distance ((C‒C)avg = 1.397(2) Å) of the η2-coordinated ethylene molecule is significantly elongated from that in free ethylene (1.337 Å).34 The data are consistent with π­backbonding from iron to ethylene, which confers significant ferracyclopropane character to (TPB)Fe(C2H4).35 Storing (TPB)Fe(C2H4) for two days under an atmosphere of N2 fully regenerates (TPB)Fe(N2). Compound (TPB)Fe(N2) does not afford a detectable styrene adduct but reacts with both phenyl- and tolylacetylene with formal hydride transfer from the terminal C(sp)‒H of the arylacetylene to the boron, forming S = 2 iron-borohydrido-arylacetylide complexes (TPBH)Fe(C2Ar) (Ar = Ph, µeff = 5.1 µB; Ar = Tol, µeff = 5.2 µB) (Scheme 2.4). Two molecules of (TPBH)Fe(C2Tol) are found in the asymmetric unit, and the XRD structure shows the presence of a tolylacetylide ligand coordinated to a pyramidalized iron center (Figure 2.4). While the hydride on the boron cannot be reliably located by XRD, the IR spectra for both (TPBH)Fe(C2Ph) and (TPBH)Fe(C2Tol) show B‒H stretches at 2490 cm-1 and 2500 cm-1, respectively, most consistent with a non­bridging B‒H unit. The vibrational bands shift 38 to 1826 cm-1 (predicted 1834 cm-1) for (TPBH)Fe(C2Ph) and 1824 cm-1 (predicted 1841 cm-1) for (TPBH)Fe(C2Tol) upon labeling with the monodeuterated arylacetylene (ArC≡CD). The activation of the arylacetylene C(sp)‒H bond by (TPB)Fe(N2) is reversible. Mixing a d6-benzene solution of (TPBH)Fe(C2Ph) with tolylacetylene (4 equiv) and, conversely, mixing a d6-benzene solution of (TPBH)Fe(C2Tol) with phenylacetylene (4 equiv) both result in a mixture of (TPBH)Fe(C2Ph) and (TPBH)Fe(C2Tol) (Scheme 2.5). The corresponding exchange reactions with B‒D labelled isotopologues of (TPBD)Fe(C2Ph) or (TPBD)Fe(C2Ar) and a different all-protio arylacetylene (Ar’C≡CH) result in the exclusive formation of free ArC≡CD, indicating that the arylacetylene unit is reductively eliminated from the ironborohydrido-arylacetylide complexes prior to activation of an incoming acetylene substrate, presumably by reversible hydride transfer from the boron to the arylacetylide to form intermediate π-adducts akin to (TPB)Fe(C2H4) (Scheme 2.5). Scheme 2.5 Reversible arylacetylene C‒H bond activation. See Scheme 2.2 for the detailed ligand representation. 39 2.3.3 Stoichiometric Hydrogenations We also explored whether the transfer of hydrogen from (TPB)(μ‒H)Fe(L)(H) species to unsaturated substrates might be possible. Exposing (TPB)Fe(C2H4) to excess H2 (1 atm) results in complete conversion of ethylene to ethane and (TPB)(µ‒H)Fe(H2)(H) as the ironcontaining product (Scheme 2.6i) in less than 12 h. A paramagnetic intermediate (A) and (TPB)Fe(C2H4) can be observed by in situ 1H NMR spectroscopy. The same paramagnetic intermediate A and (TPB)Fe(C2H4) can also be observed by in situ 1H NMR spectroscopy if the same reaction is run under a mixture ethylene (1 atm) and H2 (1 atm). The IR spectrum of the reaction mixture shows a diagnostic terminal B‒H vibration at 2470 cm-1 that is attributed to A. Akin to the iron-borohydrido-alkynyl complexes (TPBH)Fe(C2Ar), A is assigned to the iron-borohydrido-ethyl complex (Scheme 2.6i).36 Using D2 in place of H2 in the ethylene hydrogenation reaction and monitoring leads to the observation of signals for both B‒D and Scheme 2.6 Stoichiometric hydrogenation reactions. 40 B‒H stretches in the IR spectrum, which is consistent with facile insertion/β-hydride elimination processes prior to ethane elimination from A. We note the similarity of A (and (TPBH)Fe(C2Ar)) to the well-characterized zwitterionic, tris(phosphino)borate-iron-ethyl complex (PhBPiPr3)Fe(Et) (PhBPiPr3 = [PhB(CH2PiPr2)3]-) that was observed as an intermediate in ethylene hydrogenation with the previously reported iron (PhBPiPr3)Fe system.3e Complexes (TPB)Fe(N2), (TPB)(µ‒H)Fe(N2)(H), and (TPB)(µ‒H)Fe(H2)(H) hydrogenate both phenylacetylene (1 equiv) and styrene (1 equiv) to ethylbenzene (1 atm H2) with (TPB)(µ‒H)Fe(H2)(H) as the observable iron-containing product. The in situ 1H NMR spectrum of styrene hydrogenation reactions using (TPB)Fe(N2), (TPB)(µ‒H)Fe(N2)(H), or (TPB)(µ‒H)Fe(H2)(H) show styrene, ethylbenzene, and (TPB)(µ‒H)Fe(H2)(H) in solution during the reaction course. Furthermore, for styrene hydrogenation with D2 the 2H NMR spectrum and GC-MS data of the reaction mixture show incorporation of deuterium onto both olefinic carbon atoms of free styrene, indicating that styrene coordination to the iron-center and insertion/β-hydride elimination processes are reversible. Under stoichiometric conditions, the addition of 1 equiv of styrene to a solution of (TPB)(µ‒H)Fe(N2)(H) in d6-benzene and under N2 (1 atm) cleanly generates 1 equiv of ethylbenzene and (TPB)Fe(N2) (Scheme 2.6ii). In contrast, running the same reaction under a static vacuum yields a mixture of styrene, ethylbenzene, (TPB)Fe(N2), and (TPB)(µ‒H)Fe(N2)(H) (Scheme 2.6iii). These observations suggest that excess H2 or N2 is 41 required for the hydrogenations to proceed to completion. Moreover, the bridging hydride appears competent for transfer to a substrate. Substrates including trans-stilbene, N-benzylideneaniline, acetone, and acetophenone were not hydrogenated under similar conditions. Compounds (TPB)(µ‒H)Fe(CNtBu)(H) and (TPB)(µ‒H)Fe(CO)(H) also do not hydrogenate ethylene, styrene, or phenylacetylene under the same conditions. 2.3.4 Catalytic Hydrogenations Under the catalytic conditions of 0.01 M (TPB)Fe(N2), 1 atm of H2, and 30 equiv of the substrate in d6-benzene at room temperature, ethylene, styrene, and phenylacetylene are hydrogenated to ethane and ethylbenzene, respectively (Table 2.1). Compounds (TPB)(µ‒H)Fe(N2)(H) and (TPB)(µ‒H)Fe(H2)(H) can also be used as precatalysts. Ambient laboratory light does not affect the reaction, and the catalysis is not inhibited by elemental mercury; it appears to be a homogeneous process. Norbornene is hydrogenated to norbornane, and with an atmosphere of D2 in place of H2 the cis-addition product exo,exo-2,3-d2-norbornane37 is exclusively observed, indicating the syn-addition of hydrogen and arguing against radical processes. The hydrogenation catalysis was monitored by 1H NMR spectroscopy with ferrocene as an internal integration standard. As with stoichiometric ethylene hydrogenation, the in situ 1H NMR spectra of the catalytic ethylene hydrogenation reaction indicate the presence of the ethylene adduct (TPB)Fe(C2H4) and the putative ethyl-borohydride intermediate A as the ironcontaining species during the course of the reaction. Complex (TPB)(µ‒H)Fe(H2)(H) is the 42 Table 2.1 Catalytic hydrogenations by (TPB)Fe(N2) with H2. Precatalyst Substrate Product TOF (h-1)c (TPB)Fe(N2) Ethylenea Ethane 15 (TPB)Fe(N2) Styrenea Ethylbenzene 0.27 (TPB)Fe(N2) Phenylacetyleneb Ethylbenzene 0.16 Conditions: Room temperature, 0.01 M (TPB)Fe(N2), 1 atm H2, and 0.01 M ferrocene as an internal integration standard in d6-benzene. a0.3 M substrate, b0.29 M substrate. c As determined by 1H NMR spectroscopy at > 95% product. iron-containing product at the completion of the reaction. For styrene hydrogenation, styrene, ethylbenzene, and (TPB)(µ‒H)Fe(H2)(H) are observed during catalysis, and scrambling of deuterium into the vinylic positions of styrene is observed under D2. In contrast, for phenylacetylene hydrogenation complex (TPBH)Fe(C2Ph) is the only observed iron-species early in the reaction when the phenylacetylene concentration is high. As phenylacetylene is consumed, styrene and (TPB)(µ‒H)Fe(H2)(H) form, and ethylbenzene begins to develop slowly thereafter. Attempts to increase the rate of catalysis by elevating the reaction temperature resulted in catalyst decomposition. The decomposition product (D) can be synthesized independently in near quantitative yields by heating (TPB)(µ‒H)Fe(H2)(H) (80 °C) under H2 (1 atm) for 2 h. The XRD structure of D indicates that a B‒CAr bond is cleaved from the TPB ligand fragment (Figure 2.5). This result offers the cautionary note that B‒CAr bond cleavage to give metal-borohydride products is a viable catalyst decomposition pathway. 43 Figure 2.5 Decomposition product of the (TPB)Fe system. Chemical line representation (left) and XRD structure (right) of D. Ellipsoids shown at 50% probability. Selected bond distances (Å): Fe1‒H1 = 1.59(1), Fe1‒H2 = 1.63(1), Fe1‒B1 = 2.0900(6), Fe1‒P1 = 2.2481(2). Based on the results from the stoichiometric and catalytic experiments, we propose a plausible mechanistic scenario to account for the observed catalytic styrene hydrogenation by (TPB)Fe(N2) (Scheme 2.7A), and in doing so underscore interesting aspects of the mechanism that remain unanswered. Starting from precatalyst (TPB)Fe(N2), addition of H2 generates (TPB)(µ‒H)Fe(H2)(H), a species that can be observed by 1H NMR spectroscopy during catalytic runs (resting state). Subsequent substitution of the apical H2 ligand for styrene forms the unobserved iron-styrene-hydride-borohydride species B, and insertion into the terminal hydride affords the iron-alkyl intermediate A’. Intermediate A’ is analogous to the ethyl species A, and is also related to the structurally characterized acetylide-borohydride complex D. Olefin coordination and insertion appears to be reversible, as labeling studies show deuterium is exchanged into the vinylic positions of free styrene under a D2 atmosphere. Elimination of ethylbenzene in the presence of H2 regenerates the catalyst resting state. 44 Scheme 2.7 Mechanism and alkane elimination pathways. See Scheme 2.2 for the detailed ligand representation. The conversion of (TPB)Fe(N2) to (TPB)(µ‒H)Fe(H2)(H) likely proceeds via the H2-adduct intermediate C depicted in Scheme 2.7B by H2-for-N2 ligand exchange. While we have not detected such a species in the present (TPB)Fe system, the cobalt analogue (TPB)Co(H2) can be isolated and has been thoroughly characterized (see Chapter 3),38 as has the isoelectronic iron complex [(SiPiPr3)Fe(H2)]+.31 N2/H2 exchange is facile in these well-defined Co and Fe systems, and by extension we infer that it would also be facile for (TPB)Fe(N2) to afford 45 (TPB)Fe(H2) C before additional reactions ensue. Since H2 reacts with (TPB)Fe(N2), but not with (TPB)(μ‒H)Fe(CNtBu)(H) and (TPB)(μ‒H)Fe(CO)(H) at room temperature, the facile H2 substitution for N2 most likely occurs prior to H‒H bond cleavage. While this scenario is consistent with the data available, it is not demanded by the available data. For instance, it is alternatively possible that styrene substitution for the N2 ligand in (TPB)Fe(N2) precedes H2 addition to form intermediate B. No direct evidence rules out this possibility. We prefer suggesting that the H2/dihydride species (TPB)(µ‒H)Fe(H2)(H) precedes styrene binding because of the observation that H2 addition/activation by other (TPB)Fe(L) adducts, for example (TPB)Fe(CNtBu) and (TPB)Fe(CO), is very slow, and also because N2, and presumably therefore also H2, displaces ethylene from (TPB)Fe(C2H4) in solution (regenerating (TPB)Fe(N2) or (TPB)Fe(H2)). The final ethylbenzene elimination step can be envisioned to occur through two plausible routes (Scheme 2.7B). One such pathway involves a reductive elimination step where hydride transfer directly from the borohydride subunit generates the alkane product to form (TPB)(µ‒H)Fe(H2)(H), likely via the dihydrogen adduct intermediate C. The other pathway proceeds through alkane elimination by hydrogenolysis of the phenylethyl group without hydride transfer from the borohydride subunit. While the available data do not firmly distinguish between the two product elimination pathways shown in Scheme 2.7B, the stoichiometric hydrogenation studies described above show that 1 equiv of styrene is completely hydrogenated to ethylbenzene by (TPB)(µ‒H)Fe(N2)(H) under an N2 atm (Scheme 2.6ii). This observation implies that 46 (TPB)(µ‒H)Fe(N2)(H) can serve as the source of the two H-atom equivalents delivered to styrene. From complex (TPB)(µ‒H)Fe(N2)(H), substitution of the apical N2 ligand for styrene in (TPB)(µ‒H)Fe(N2)(H) would generate the styrene adduct intermediate B. Subsequent insertion of the bound styrene into the cis Fe‒H would afford intermediate A’, which, in the absence of H2 at least, eliminates ethylbenzene concomitant with N2 binding to reform (TPB)Fe(N2). The presence of exogenous N2 (or alternatively H2) facilitates the generation of ethylbenzene. As noted in Scheme 2.6iii, the stoichiometric reaction under static vacuum between (TPB)(µ‒H)Fe(N2)(H) and styrene is quite slow compared with the same reaction under N2 (Scheme 2.6ii). This observation can be explained by presuming the conversion of intermediate A’ to (TPB)Fe(N2) requires N2 association prior to elimination to generate ethylbenzene. 2.4 Conclusions In summary, the Fe‒B bond in ferraboratranes (TPB)Fe(L) (where L = N2, CNtBu, or CO) can facilitate heterolytic cleavage of H2 and of the C(sp)‒H and C(sp2)‒H bonds of arylacetylenes and formaldehyde, respectively, resulting in Fe‒B bond rupture and formal hydride transfer to the boron of the ligand scaffold. The formal hydride transfer from the C(sp)‒H of arylacetylenes to give iron-acetylide complexes is distinct from traditional syntheses of metal-acetylide complexes in that a hydride equivalent is formally abstracted by the Lewis acidic borane unit in (TPB)Fe(N2) from a C(sp)‒H hydrogen39-41 Dihydrogen addition across the Fe‒B bond is reversible, and the boron is also capable of shuttling the hydride equivalent derived from H2 to unsaturated substrates under stoichiometric 47 hydrogenation conditions. The hydrogen chemistry of this (TPB)Fe(N2) system contrasts with the related nickel42 and cobalt38 complexes of TPB, where the metal-boron bond remains intact under an H2 atmosphere. An understanding of the factors that govern metal-boron bond cleavage will aid in the development of cooperative catalytic reactions in metallaboratranes. The direct role, if any, of the borane ligand in assisting the H2 cleavage step is an interesting question in this context for the present iron and recently reported diphosphine-borane-iron systems,43 and also conceptually related to the nickel system.22 Determining whether there is a cooperative interaction between the coordinated H2 ligand, the iron center, and the borane subunit en route to H‒H cleavage (and its microscopic reverse) in the (TPB)Fe system, akin to H‒H cleavage by frustrated Lewis pairs20 and by (MesDPBPh)Ni,44 calls for detailed theoretical studies. 2.5 Experimental 2.5.1 General Considerations All manipulations were carried out using standard glovebox or Schlenk techniques under an N2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. Deuterated solvents and D2 gas were purchased from Cambridge Isotope Laboratories, INC. The deuterated solvents were degassed and dried over activated 3 Å sieves prior to use. Unless otherwise noted, all compounds were purchased commercially and used without further purification. TPB,26 (SiPiPr3)Fe(Me),32 48 (SiPiPr3)Fe(N2)(H),31 and monodeuterated phenyl- and tolylacetylene (PhC2D and TolC2D) 45 were synthesized by literature procedures. Elemental analyses were performed by Midwest Microlab, LLC., Indianapolis, IN. NMR spectra were recorded on Varian 300 MHz, 400 MHz, and 500MHz spectrometers. 1H and 13C chemical shifts are reported in ppm relative to residual solvent as internal standards. 31 P and 11B chemical shifts are reported in ppm relative to 85% aqueous H3PO4 and BF3∙Et2O, respectively. Multiplicities are indicated by br (broad), s (singlet), d (doublet), t (triplet), quart (quartet), quin (quintet), m (multiplet), d-d (doublet-of-doublets), and t-d (triplet-of-doublets). FT-IR measurements were performed on samples prepared as KBr pellets or in solution using a Bio-Rad Excalibur FTS 300 spectrometer with Varian Resolutions Pro software at 4 cm-1 resolution. The ATR-IR measurements were measured on a thin film of the complex obtained from evaporating a drop of the solution on the probe surface of a Bruker APLHA ATR-IR Spectrometer (Platinum Sampling Module, diamond, OPUS software package) at 2 cm-1 resolution. IR intensities indicated by s (strong), m (medium), and w (weak). X-ray Crystallography. X-ray diffraction was measured on the Bruker Kappa Apex II diffractometer with Mo Kα radiation. Structures were solved using the SHELXS software and refined against F2 on all data sets by full matrix least squares with SHELXL. The crystals were mounted on a glass fiber with Paratone oil. Computational Methods. Geometry optimizations were performed using the Gaussian03 package. B3LYP exchange-correlation functional was employed with a 6-31G(d) basis set. The GDIIS algorithm was used. A full frequency calculation was performed on each structure 49 to ensure that they were the true minima. A single negative vibrational frequency was observed for the transition state between (TPB)(µ‒H)Fe(H2)(H) and its equatorial-H2 isomer, confirming that this structure was the transition state. HD Gas Generation. D2O (1mL) was added to an evacuated, cooled sample (-78 °C) of solid lithium aluminum hydride (316 mg, 8.2 mmol) in a Schlenk flask. An evacuated Schlenk line was filled with the resulting HD gas (ca. 1 atm) as the Schlenk flask was warmed to room temperature. A J-Young NMR tube containing a freeze-pump-thawed solution of the respective complex was exposed to the HD gas. 2.5.2 Synthetic Protocols Synthesis of (TPB)(µ‒H)Fe(N2)(H). A J-Young NMR tube containing a brown-red solution (TPB)Fe(N2) (20.3 mg, 31.1 mmol) in C6H6 (0.8 mL) was subjected to freeze-pumpthaw cycles (3x) and, with the J-Young tube frozen with liquid nitrogen, exposed to H2 (1.2 equiv). The solution was thawed and mixed, giving a yellow solution. An atmosphere of N2 was subsequently introduced and the reaction was mixed for 2 h to yield (TPB)(µ‒H)Fe(N2)(H) (100% yield by 1H NMR spectroscopy with a ferrocene integration standard). Alternatively, (TPB)(µ‒H)Fe(N2)(H) could be synthesized from (TPB)(µ‒H)Fe(H2)(H) by degassing a solution of (TPB)(µ‒H)Fe(H2)(H) of free H2 by freezepump-thaw (3x), exposing it to an N2 atmosphere, and mixing the solution overnight (100 % yield by 1H NMR spectroscopy with a ferrocene integration standard). Yields could not be determined by mass because (TPB)(µ‒H)Fe(N2)(H) was unstable to prolonged exposure to dynamic vacuum. Yellow-orange XRD quality crystals were grown in a concentrated solution 50 of pentane:THF (10:1) at -30 oC. 1H NMR (C6D6, 300 MHz): δ 7.9 (2H, d, 3JH-H = 6 Hz, ArH), δ 7.7 (1H, br s, Ar-H), δ 7.3 (3H, d, 3JH-H = 6 Hz, Ar-H), δ 7.0 (3H, t, 3JH-H = 9 Hz, Ar-H), δ 2.7 (4H, d, 2JP-H = 18 Hz, PCH), δ 2.4 (2H, br s, PCH), δ 1.4 (6H, d, 3JH-H = 6 Hz, CH3), δ 1.3 (12H, br s, CH3), δ 1.1 (6H, d-d, 3JP-H = 15 Hz, 3JH-H = 6 Hz, CH3), δ 0.8 (6H, d, 3JP-H = 9 Hz, CH3), δ -9.6 (1H, d-t, 2JH-Pcis = 81 Hz, 2JH-Ptrans = 36 Hz, Fe-H), δ -30.4 (1H, s, Fe(µ-H)B). 2H NMR (C6H6/C6D6, 76 MHz): δ -9.5 (1D, br s), δ -30.3 (1D, br s). 31P NMR (C6D6, 121 MHz): δ 73.6 (2P, s), δ 64.2 (1P, s). 13C NMR (C6D6, 125 MHz): δ 161.9 (s, CAr), δ 143.5 (s, CAr), δ 141.0 (s, CAr), δ 132.3 (s, CAr), δ 131.8 (s, CAr), δ 131.3 (s, CAr), δ 130.3 (s, CAr), δ 124.6 (s, CAr), δ 124.0 (s, CAr), δ 32.0 (s, PCH), δ 29.4 (s, PCH), δ 28.5 (s, PCH), δ 22.8 (s, CH3), δ 20.1 (s, CH3), δ 19.7 (s, CH3), δ 18.9 (s, CH3). 11B NMR (C6D6, 128 MHz): δ 8.2 (br). IR (KBr, cm1 ): 2071 (s, N≡N), 1960 (w) 1934 (w). UV-vis (THF, nm {M-1 cm-1}): 328 {shoulder, 500}, 280 {shoulder, 11250}. Anal.: Elemental analysis could not be obtained because of the instability of the compound to dynamic vacuum. Synthesis of (TPB)(µ‒H)Fe(H2)(H). A J-Young NMR tube containing a brown-red solution (TPB)Fe(N2) (21 mg, 31.1 mmol) in C6D6 (0.8 mL) was subjected to freeze-pumpthaw cycles (3x). Upon warming to room temperature, the sample was exposed to H2 (1 atm), resulting in a clear yellow solution. The reaction was mixed for 24 h to give (TPB)(µ‒H)Fe(H2)(H) (100 % by 1H NMR spectroscopy with a ferrocene integration standard). The yield could not be determined by weight because (TPB)(µ‒H)Fe(H2)(H) was unstable during prolonged exposure to dynamic vacuum. Yellow-orange XRD quality crystals were grown under 1 atm of H2 in a concentrated solution of pentane:THF (10:1) at -78 oC. 1H 51 NMR (C6D6, 300 MHz): δ 8.0 (3H, d, 3JH-H = 6 Hz, Ar-H), δ 7.3 (3H, t, 3JH-H = 9 Hz, Ar-H), δ 7.2 (3H, t, 3JH-H = 9 Hz, Ar-H), δ 7.0 (3H, d, 3JH-H = 6 Hz, Ar-H), δ 4.47 (s, free H2), δ 2.3 (6H, m, PCH), δ 1.0 (18H, d-d, 3JH-P = 15 Hz, 3JH-H = 6 Hz, CH3), δ 0.8 (18H, d-d, 3JH-P = 15 Hz, 3 JH-H = 6 Hz, CH3), δ -15.1 (br s, 2H). T1min (d8-toluene): 35 ms (δ -15.1, -32 oC). 2H NMR (C6H6/C6D6, 76 MHz): δ –15.4 (1D, br s). 31P NMR (C6D6, 121 MHz): δ 90.0 (3P, s). 13C NMR (C6D6, 125 MHz): δ 163.5 (s, CAr), δ 144.6 (s, CAr), δ 144.1 (s, CAr), δ 130.9 (d, JP-C = 23 Hz, CAr), δ 124.5 (s, CAr), δ 123.9 (s, CAr), δ 28.6 (s, PCH), δ 21.2 (s, CH3), δ 19.9 (s, CH3). 11B NMR (C6D6, 128 MHz): δ 7.5 (br). IR (KBr, cm-1): 2278 (w), 19618 (w), 1845 (w). UV-vis (THF, nm {M-1 cm-1}): 377 {shoulder, 1532}, 275 {14532}. Anal.: Elemental analysis could not be obtained because of the instability of the compound to dynamic vacuum. Synthesis of (TPB)Fe(CNtBu). Tert-butyl isocyanide (20 mg, 0.24 mmol) was added to a brown solution of (TPB)Fe(N2) (40 mg, 59 μmol) in benzene (2 mL), causing an instantaneous darkening upon gentle shaking. The volatiles were removed by lyophilization and the residue was extracted with tetramethylsilane (2 mL). The resulting dark brown solution was slowly concentrated down to ca. 0.2 mL by vapor diffusion into hexamethyldisiloxane. Removal of the mother liquor by decantation, washing with cold tetramethylsilane (2 × 0.1 mL), and drying in vacuo afforded (TPB)Fe(CNtBu) as brown crystals (33 mg, 77 %). 1H NMR (C6D6, 300 MHz): δ 11.2 (3H), δ 9.2 (3H), δ 8.6 (3H), δ 8.5 (9H), δ 6.4 (3H), δ 5.2 (9H), δ 3.7 (12H), δ 2.9 (9H), δ -1.5 (9H), δ -2.3 (3H). IR (KBr, cm-1): 1972 (C≡N). UV-Vis (THF, nm {cm–1M– 1 }): 600 {shoulder, 428}, 910 {70}. μeff (C6D6, method of Evans, 20 °C): 1.7 μB. Anal.: Calc’d for C41H64BFeNP3: C 67.50, H 8.70, N 1.92; found: C 67.20, H 8.54, N 1.72. 52 Synthesis of (TPB)(µ‒H)Fe(CNtBu)(H). A heavy-walled Schlenk tube containing a yellow-brown solution of (TPB) Fe(CNtBu) (16.4 mg, 22.4 mmol) in C6H6 (10 mL) was subjected to freeze-pump-thaw cycles (3x). Upon warming to room temperature, the sample was exposed to H2 (1 atm) for a few minutes. The Schlenk tube was sealed and heated under vigorous mixing at 40 oC for 85 h. Removal of the solvent in vacuo, extraction with C6H6, and lyophilization yielded a solid of (TPB)(μ‒H)Fe(CNtBu)(H) (17.3 mg, 98 %). Room temperature evaporation of a solution of (TPB)(μ‒H)Fe(CNtBu)(H) in a diethyl ether:pentane (2 mL to 1 mL) mixture yielded yellow crystals suitable for XRD analysis. 1H NMR (C6D6, 300 MHz): δ 8.1 (1H, d, 3JH-H = 9 Hz, Ar-H), δ 7.4 (3H, d, 3JH-H = 9 Hz, Ar-H), δ 7.2 (3H, d, 3 JH-H = 6 Hz, Ar-H), δ 7.1 (3H, d, 3JH-H = 9 Hz, Ar-H), δ 2.7 (2H, br s, PCH), δ 2.5 (2H, br s, PCH), δ 2.2 (2H, t, 2JH-P = 9 Hz, PCH), δ 1.3 (16H, m, CH3), δ 1.1 (9H, s, C(CH3)3), δ 1.0 (6H, d, 3JH-H = 6 Hz, CH3), δ 0.8 (6H, d-d, 3JH-P = 15 Hz, 3JH-H = 6 Hz, CH3), δ 0.7 (6H, br s, CH3), δ -11.7 (1H, t-d, 2JH-Pcis = 87 Hz, 2JH-Ptrans = 27 Hz, Fe-H), δ -23.9 (1H, br s, Fe-(µ-H)-B). 13C NMR (C6D6, 125 MHz): δ 176.9 (quart, 2JC-P = 8 Hz, CNtBu), δ 163.5 (br s, CAr), δ 163.0 (br s, CAr), δ 144.9 (m, CAr), δ 143.0 (d, JC-P = 20 Hz, CAr), δ 131.9 (d, 2JC-P = 7.5 Hz, CAr), δ 130.6 (d, JC-P = 3.2 H, CAr), δ 130.5 (d, JC-P = 2.5 H, CAr), δ 128.6 (s, CAr), δ 127.5 (s, CAr), δ 124.2 (s, CAr), δ 123.6 (s, CAr), δ 55.3 (s, C(CH3)3), δ 31.6 (s, PCH), δ 30.9 (s, PCH), δ 29.2 (m, PCH), δ 28.5 (d, 2JC-P = 7.5 Hz, PCH), δ 23.8 (s, CH3), δ 20.4 (s, CH3), δ 20.3 (m, CH3), δ 20.2 (s, CH3), δ 19.9 (s, CH3), δ 19.7 (s, CH3). 31P NMR (C6D6, 121 MHz): δ 81.3 (2P, d, 2JP-P = 63 Hz), δ 72.4 (1P, s). IR (KBr, cm-1): 2027 (s, C≡N), 1942 (w, Fe-H). UV-Vis (THF, nm {cm–1 M–1}): 205 {5530}, 224 {15437}, 245 {17142}, 255 {16635}, 285 {4117}, 335 {shoulder, 2166}, 53 400 {1830}. Anal: Calc’d for C41H66BFeNP3: C 67.32, H 8.96, N 1.91; found: C 66.59, H 8.61, N 1.30. Synthesis of (TPB)(µ‒H)Fe(CO)(H) from (TPB)Fe(CO) and H2. In a J-Young NMR tube, (TPB)Fe(CO) (6.0 mg, 8.9 µmol) was dissolved in C6H6 (0.7 mL) to give a brown-red solution. The solution was subjected to freeze-pump-thaw cycles (3x) and subsequently exposed to H2 (1 atm) for ca. 5 min. The reaction was then heated at 80 oC for 5 days, during which time a clear yellow solution developed. Removal of the solvent in vacuo, extraction with C6H6, and lyophilization yielded a yellow solid of (TPB)(μ‒H)Fe(CO)(H) (5.9 mg, 98 %). Room temperature evaporation of a solution of (TPB)(μ‒H)Fe(CO)(H) in a diethyl ether:pentane (1 mL to 0.5 mL) mixture yielded yellow analytically pure (TPB)(μ‒H)Fe(CO)(H). 1H NMR (C6D6, 300 MHz): δ 8.1 (2H, d, 3JH-H = 6 Hz, Ar-H), δ 7.9 (1H, d, 2JP-H = 9 Hz, Ar-H), δ 7.2 (4H, m, Ar-H), δ 7.0 (4H, m, Ar-H), δ 2.6 (2H, t, 2JH-P = 3 Hz, PCH), δ 2.4 (2H, q, 2JH-P = 6 Hz, PCH), δ 2.2 (2H, t, 2JH-P = 6 Hz, PCH), δ 1.4 (6H, d, 3JH-P = 6 Hz, CH3), δ 1.2 (12H, m, CH3), δ 0.9 (6H, d, 3JH-P = 6 Hz, CH3), δ 0.8 (6H, d-d, 3JH-P = 8 Hz, 3JH-H = 6 Hz, CH3), δ 0.6 (6H, d, 3JH-P = 6 Hz, CH3). δ -11.6 (1H, t-d, 2JH-Pcis = 81 Hz, 2JHPtrans = 21 Hz, Fe-H), δ -20.0 (1H, br s, Fe-(µ-H)-B). 2 H NMR (C6H6, 76 Hz): δ -12.3 (1D, t, 2 JP-D = 10 Hz), δ -20.8 (1D, br s). 13C NMR (THF with 1 drop of C6D6, 125 MHz): δ 222.7 (br s, CO), δ 161.8 (br s, CAr), δ 142.9 (br s, 1JC-P = 19 Hz, CAr), δ 140.8 (br s, 2JC-P = 16 Hz, CAr), δ 131.0 (s, CAr), δ 129.8 (s, CAr), δ 128.5 (s, CAr), δ 128.0 (s, CAr), δ 124.0 (s, CAr), δ 123.4 (s, CAr), δ 30.8 (s, PCH), δ 28.3 (s, PCH), δ 27.7 (s, PCH), δ 22.3 (s, CH3), δ 19.1 (s, CH3), δ 18.8 (s, CH3), δ 18.1 (s, CH3). 31P NMR (C6D6, 121 MHz): δ 83.4 (2P, d, 2JP-H = 21 Hz), δ 72.8 (1P, 54 s). IR (KBr, cm-1): 1898 (s, C≡O), 1967 (w, Fe-H). UV-Vis (THF, nm {cm–1M–1}): 270 {4333}, 280 {4111}, 390 {1400}. Anal.: Calc’d for C37H56BFeOP3: C 65.70, H 8.34; found: C 65.64, H 8.08. Synthesis of (TPB)(µ‒H)Fe(CO)(H) from Formaldehyde. Compound (TPB)Fe(N2) (8 mg, 11.8 µmol) or (TPB)(µ‒H)Fe(N2)(H) (6 mg, 8.9 µmol) was mixed with excess paraformaldehyde in C6H6 for 3 h to give a turbid, light yellow solution. The excess paraformaldehyde was filtered away and the solution was pumped down to give (TPB)(μ‒H)Fe(CO)(H) as a yellow solid (from (TPB)Fe(N2), 8 mg, 100 %; from (TPB)(µ‒H)Fe(N2)(H), 7 mg, 100 %). Spectroscopic data is identical to those listed above. Synthesis of (TPB)Fe(C2H4). A J-Young NMR tube containing an orange solution of (TPB)(µ‒H)Fe(H2)(H) (8.4 mg, 12.5 µmol) in C6H6 (0.8 mL) was subjected to freeze-pumpthaw cycles (3x) and exposed to ethylene gas (1 atm) for ca. 1 minute. Mixing immediately gave a brown solution of (TPB)Fe(C2H4). Removal of solvent in vacuo yielded a brown solid (8.3 mg, 99 %) of (TPB)Fe(C2H4). Dissolution of this solid under N2 atmosphere gave mostly (TPB)Fe(C2H4) and small amounts of (TPB)Fe(N2). Over time, (TPB)Fe(C2H4) in solution converted to (TPB)Fe(N2). Crystals suitable for XRD were grown in a saturated, cold pentane:diethyl ether (2:1) solution under an ethylene atmosphere. 1H NMR (C6D6, 300 MHz): δ 33.1 (1H), δ 28.8 (1H), δ 18.7 (4H), δ 13.3 (1H), δ 5.25 (s, free C2H4), δ 4.9 (3H), δ 4.1 (1H), δ 1.9 (2H), δ -3.0 (10H), δ -6.2 (11H), δ -9.2 (11H), δ -10.0 (3H). UV-Vis (THF, nm {cm–1M– 1 }): 309 {shoulder, 6032}, 553 {1804}, 938 {418}. μeff (C6D6, method of Evans, 20 °C): 3.2 μB 55 (S = 1). Anal.: Elemental analysis could not be obtained because of the instability of the compound to dynamic vacuum. Synthesis of (TPBH)Fe(C2Ph). Phenylacetylene (40.8 mg, 400 µmol) was added to a C6H6 solution (5 mL) of (TPB)(µ‒H)Fe(H2)(H) (9.0 mg, 13 µmol), immediately giving a gray solution. Removal of the solvent in vacuo yielded a black powder of (TPBH)Fe(C2Ph) (10 mg, 100 %). 1H NMR (C6D6, 300 MHz): δ 28.8 (3H), δ 13.1 (4H), δ 4.7 (18H), δ 2.6 (2H), δ 2.3 (4H), δ 1.2 (2H), δ -29.3 (1H), δ -30.9 (5H). μeff (C6D6, method of Evans, 20 °C): 5.1 μB (S = 2). UV-Vis (THF, nm {cm–1M–1}): 325 {20670}, 439 {1051}, 479 {971}, 522 {955}, 601 (br abs extending from 400 to 700 nm, 930}, 883 {1466}. IR (KBr, cm-1): 2040 (s, C≡C), 2490 (m, B-H). Anal.: Calc’d for C44H60FeP3B: C, 70.60; H, 8.08. Found: C, 70.39; H, 7.89. Synthesis of (TPBD)Fe(C2Ph). The B-D labeled isotopologue (TPBD)Fe(C2Ph) was generated by the same method described for (TPBH)Fe(C2Ph), except that PhC2H was replaced with PhC2D. The 1H NMR spectrum was identical to (TPBD)Fe(C2Ar). IR (thin film, cm-1): 1826 (br m, B-D; predicted 1832). Synthesis of (TPBH)Fe(C2Tol). Tolylacetylene (16.1 mg, 138 μmol) was added to a C6H6 solution (5 mL) of 3 (22.9 mg, 33.0 μmol), immediately giving a gray solution. Removal of the solvent in vacuo yielded a black powder of (TPBH)Fe(C2Tol) (24.0 mg 100 %). Black XRD quality crystals of (TPBH)Fe(C2Tol) were grown by layering hexamethyldisiloxane on top of a concentrated THF solution of (TPBH)Fe(C2Tol) and allowing the solution to sit overnight. 1 H NMR (C6D6, 300 MHz): δ 44.1 (1H), δ 29.4 (1H), δ 13.3 (1H), δ 4.6 (2H), δ 3.3 (1H), δ 2.7 (2H), δ 2.3 (4H), δ 1.8 (2H), δ -32.3 (1H). μeff (C6D6, method of Evans, 20 °C): 5.2 μB (S = 2). 56 UV-Vis (THF, nm {cm–1M–1}): 326 {24476}, 444 {1243}, 486 {1138}, 527 {shoulder, 975}, 624 (915}, 887 {1575}. IR (KBr, cm-1): 2039 (s, C≡C), 2500 (m, B-H). Anal.: Calc’d for C45H62FeP3B: C, 70.88; H, 8.20. Found: C, 70.44; H, 7.80. Synthesis of (TPBD)Fe(C2Tol). The B-D labeled isotopologue (TPBD)Fe(C2Tol) was generated by the same method described for (TPBH)Fe(C2Tol), except that TolC2H was replaced with TolC2D. The 1H NMR spectrum was identical to (TPBH)Fe(C2Tol). IR (thin film, cm-1): 1824 (br m, B-D; predicted 1841). Synthesis of D. A yellow, C6H6 solution of (TPB)Fe(N2) (18.2 mg, 27 µmol) was heated in a J-Young NMR tube under H2 (1 atm) at 80 °C for 2 h, giving a turbid red-purple solution. The solvent was removed in vacuo, and the dark crude material was redissolved in hexamethyldisiloxane (3 mL) and filtered through a glass frit to remove a black solid (presumably iron metal). Removal of the solvent in vacuo gave a purple solid that is a mixture i of D (1 equiv) and diisopropyl-phosphino-benzene ( Pr2PPh, 1 equiv). Orange XRD quality crystals of D can be grown from slow evaporation of a concentrated hexamethyldisiloxane solution of 11 at room temperature (5.1 mg, 18 %). Dissolution of these crystals by heating in benzene or THF results in decomposition. Therefore, spectral data is reported on the mixture of i D with Pr2PPh. Compound D appears to be fluxional at RT. 1H NMR (C6D6, 300 MHz): δ 8.4 (4H, s, Ar-H), δ 7.6 (4H, s, Ar-H), δ 7.5 (3H, d, 2JH-P = 6 Hz, Ar-H), δ 2.7 (1H, d, 2JH-P = 6 Hz, PCH), δ 2.5 (1H, s, PCH), δ 1.9 (1H, quart, 3JH-H = 6 Hz, PCH), δ 1.3 (6H, d-d, 3JP-H = 4 Hz, 3 JH-H = 2 Hz, CH3), δ 1.2 (6H, d, 3JH-H = 3 Hz, CH3), δ 1.1 (6H, quart, 3JP-H = 4 Hz, 3JH-H = 2 Hz, CH3), 0.9 (12H, m, CH3), -17.0 (0.25H, t, 2JH-P = 36 Hz, B-H). 13C NMR (C6D6, 125 57 MHz): δ 157.4 (br s, CAr), δ 144.2 (br s, CAr), δ 134.9 (d, 2JC-P = 19 Hz, CAr), δ 130.5 (s, CAr), δ 129.3 (s, CAr), δ 125.7 (d, 2JC-P = 23 Hz, CAr), δ 25.5 (br s, PCH), δ 24.9 (br s, PCH), δ 24.4 (br s, PCH), δ 23.7 (br s, PCH), δ 23.1 (br s, PCH), δ 20.0 (m, CH3), δ 19.4 (m, CH3), δ 18.2 (m, CH3). 31P NMR (C6D6, 121 MHz): δ 99.1 (4P, s), δ 9.5 (2P, s). 11B NMR (C6D6, 128 MHz): δ 41.0 (br). UV-Vis (THF, nm {cm–1M–1}): 264 {shoulder, 31650}, 520 {826}. IR (KBr, cm-1): 2088 (s, B-H), 1838 (m, B-H). Anal.: Calc’d for C51H86FeP4B2Si (i.e., 11 + Me3SiH): C, 65.96; H, 9.33. Found: C, 65.70; H, 9.16. Synthesis of (SiPiPr3)Fe(H2)(H). In a 100 mL Schlenk tube a red solution of (SiPiPr3)Fe(Me) (1.05 g, 1.547 mmol) in C6H6 (50 mL) was degassed by freeze-pump-thaw (3x). H2 gas (1 atm) was charged into the reaction mixture. The reaction was heated at 60 °C for over a week. The reaction solution was then quickly filtered through Celite and volatiles were removed in vacuo to give a light yellow powder. The solid was collected on a glass-frit and washed with pentane (3 mL x 2). The resulting product (SiPiPr3)Fe(H2)(H) (950 mg, 1.425 mmol, 92%) was obtained as a light yellow powder after drying under vacuum. 1H NMR (C6D6, 300 MHz): δ 8.3 (3H, d, 3JH-P = 6.8 Hz, Ar-H), δ 7.3 (3H, m, Ar-H), δ 7.2 (3H, t, 3JH-H = 7.2 Hz, Ar-H), δ 7.1 (3H, t, 3JH-H = 7 Hz, Ar-H), δ 4.5 (s, free H2), δ 2.2 (6H, m, PCH), δ 1.0 (18H, m, CH3), δ 0.8 (18H, br s, CH3), δ 0.16 (s, free CH4), δ –10.0 (3H, quin, 2JP-H = 18.4 Hz, Fe-H). T1min (d8toluene): 32 ms (δ -10.0, -30 oC). 31P NMR (C6D6, 121 MHz): δ 100 (br); (d8-Tol, 121 MHz, 80 °C): δ 117.9 (d, 3JP-P = 43.5 Hz), δ 94.7 (bs), δ 84.7 (d, 3JP-P = 43.5 Hz). 13C NMR (THF with 1 drop of C6D6, 125 MHz): δ 157.3 (d, JC-P = 22.5 Hz, CAr), δ 150.5 (d, JC-P = 21.3 Hz, CAr), δ 130.1 (d, JC-P = 9.3 Hz, CAr), δ 128.6 (s, CAr), δ 127.4 (s CAr), δ 126.0 (d, JC-P = 2.5 Hz, 58 CAr), δ 29.0 (br s, PCH), δ 20.4 (br s, CH3), δ 19.2 (br s, CH3), UV-Vis (THF, nm {cm–1M–1}): 353 {3040}. IR (KBr pellet; cm-1): 1941 (w, Fe-H). Anal.: Elemental analysis could not be obtained because of the instability of the compound under prolonged exposure to N2. Isotopomers of (SiPiPr3)Fe(H2)(H): (SiPiPr3)Fe(H2)(H), (SiPiPr3)Fe(HD)(H), (SiPiPr3)Fe(H2)(D), (SiPiPr3)Fe(D2)(H), (SiPiPr3)Fe(HD)(D), and (SiPiPr3)Fe(D2)(D). HD gas was generated by the method described above and charged into a J-young tube containing a degassed solution of (SiPiPr3)Fe(Me) in C6D6. The solution was heated at 60 oC for over a week. Monitoring the progress of the reaction by 1H NMR revealed the gradual disappearance of (SiPiPr3)Fe(Me) and formation of diamagnetic isotopomers (SiPiPr3)Fe(H2)(H). Spectroscopic features in the 1H NMR spectrum were identical to (SiPiPr3)Fe(H2)(H) except for the hydridic proton resonances, where isotopologues were observed. 1H{31P} NMR (C6D6, 300 MHz): δ -10.0 (3H, quart, 2JP-H = 18.4 Hz, H3), δ -10.0 (t, 1JH-D = 9.5 Hz, H2D), δ -10.2 (quin, 1 JH-D = 9.3 Hz, HD2). Synthesis of (SiPiPr3)Fe(CO)(H). In a 50 mL Schlenk tube a yellow solution of (SiPiPr3)Fe(H2)(H) (330 mg, 0.495 mmol) in C6H6 (20 mL) was subjected to freeze-pump-thaw cycles (3x). The solution was charged with CO (1 atm). The reaction was mixed overnight at RT and then for 1 h at 60 oC, resulting in a light yellow solution. After completion, the solution was degassed by freeze-pump-thaw (3x). The solution was filtered through Celite, and the volatiles were removed in vacuo to give a light yellow powder. The solid was collected on a glass-frit and washed with pentane (5 mL x 3). Removal of the solvent in vacuo yields (SiPiPr3)Fe(CO)(H) (266 mg, 0.384 mmol, 78 %) as a light yellow powder. Crystals suitable 59 for X-ray diffraction were obtained by RT evaporation of a pentane solution of (SiPiPr3)Fe(CO)(H). 1H NMR (d8-toluene, 300 MHz): δ 8.2 (2H, d, 3JH-H = 7.2 Hz), δ 8.1 (1H, d, 3JH-H = 7.2 Hz), δ 7.3 (3H, m), δ 7. 2 (3H, m), δ 7.1 (3H, m), δ 2.7 (2H, m), δ 2.4 (2H, m), δ 2.2 (2H, m), δ 1.5 (6H, d-d, 3JH-P = 15.2 Hz, 3JH-P = 6.8 Hz), δ 1.3 (6H, m), δ 1.1 (6H, d-d, 3JH-P = 12.6 Hz, 3JH-H = 6.6 Hz), δ 0.8 (12H, m), δ 0.5 (6H, s), δ -14.9 (1H, t-d, 3JH-Pcis = 81.0 Hz, 3 JH-Ptrans = 14.1 Hz). 31P NMR (C6D6, 121 MHz, RT): δ 88.0 (br), δ 90.0 (s); (d8-toluene, 121 MHz, –80 °C): δ 109.9 (t, 3JP-P = 65 Hz, 3JP-H = 80 Hz), δ 90.0 (s), δ 77.9 (t, 3JP-P = 65 Hz, 3JP-H = 80 Hz). 13C NMR (THF with 1 drop of C6D6, 125 MHz): δ 223.0 (d, 2JC-P = 6.3 Hz, CO), δ 157.2 (d, JC-P = 8.8 Hz, CAr), δ 155.2 (d, JC-P = 10.6 Hz, CAr), δ 150.5 (d, JC-P = 11.3 Hz, CAr), δ 150.2 (d, JC-P = 8.8 Hz, CAr), δ 148.6 (d, JC-P = 18.1 Hz, CAr), δ 132.4 (d, JC-P = 9.4 Hz, CAr), δ 131.9 (d, JC-P = 8.8 Hz, CAr), δ 130.9 (s, CAr), δ 128.3 (s, CAr), δ 128.5 (s, CAr), δ 127.1 (s, CAr), δ 126.8 (s, CAr), δ 126.4 (s, CAr), δ 125.8 (s, CAr), δ 124.7 (s, CAr), δ 32.4 (s, PCH), δ 30.7 (s, PCH), δ 30.0 (s, PCH), δ 29.6 (s, PCH), δ 28.7 (s, PCH), δ 22.8 (s, CH3), δ 22.1 (s, CH3), δ 20.2 (s, CH3), δ 19.5 (s, CH3), δ 19.2 (s, CH3), δ 19.1 (s, CH3), δ 18.9 (s, CH3), δ 18.0 (s, CH3). UV-Vis (THF, nm {cm–1M–1}): 340 {2,050}, 400 {1,500}. IR (KBr pellet; cm–1): 1882 (s, C≡O), 1944 (m, Fe-H). Anal.: Calc’d for C37H57FeOP3Si: C, 64.16; H, 8.00. Found: C, 64.15; H, 8.13. Synthesis of (SiPiPr3)Fe(13CO)(H). In a J-Young NMR tube an orange C6D6 solution of (SiPiPr3)Fe(H2)(H) was degassed by three freeze-pump-thaw (3x). Subsequently, 13CO (1 atm) was added and the reaction was allowed to mix overnight at RT. The 1H NMR spectrum was identical to (SiPiPr3)Fe(CO)(H). IR (KBr; cm–1): 1836 (s, 13C≡O). 60 Generation of (TPBH)Fe(Et) (A). Compound A was observed as an intermediate of catalytic ethylene hydrogenation under the reaction conditions described below (“Catalytic hydrogen studies”). Complex A can also be generated starting from complex (TPB)Fe(C2H4). The procedure described below is more amendable to observing A spectroscopically. A dark yellow C6D6 solution (0.5 mL) of (TPB)Fe(C2H4) (2.8 mg, 4.2 μmol) under ethylene (1 atm) in a J-Young tube was frozen (-196 °C) and H2 was added (1 atm). The reaction was thawed and quickly mixed only immediately prior to measuring the 1H NMR spectrum, revealing a mixture of (TPB)Fe(C2H4) and A. Further mixing of the solution for ca. 45 min yielded a purple solution of A with a small residual amount of (TPB)Fe(C2H4) by 1H NMR spectroscopy. Free C2H4, C2H6, and H2 were also observed in the 1H NMR spectrum. Compound A could not be isolated as a solid due to its instability. For example, an ATR-IR spectrum of a thin film of the reaction mixture obtained by solvent evaporation under an N2 atmosphere over a period less than 30 sec gave vibrational bands diagnostic of (TPB)Fe(N2), (TPB)(µ‒H)Fe(N2)(H), and A. 1 H NMR (C6D6, 300 MHz): δ 17.3 (1H), δ 6.6 (1H), δ 5.26 (s, free C2H4), δ 4.4 (1H), δ 4.47 (br s, free H2), δ 3.3 (2H), 0.80 (s, free C2H6), δ -1.9 (8H), δ -5.5 (6H). IR (thin film; cm-1): 2470 (br s, B‒H of A), 2069 (m, N≡N of (TPB)(μ‒H)Fe(N2)(H)), 2009 (s, N≡N of (TPB)Fe(N2)). UV-Vis, obtained 45 min after exposing A under 1 atm of ethylene to 1 atm of H2 (THF, nm {cm–1M–1}): 325 {20670}, 439 {1051}, 479 {971}, 522 {955}, 601 (br abs extending from 400 to 600 nm, 930}, 883 {1466}. Magnetic data could not be obtained due to the presence of multiple iron species in the reaction mixture. Anal.: Elemental analysis could not be obtained because of the instability of the compound to dynamic vacuum. 61 Catalytic hydrogenation studies. Compound (TPB)Fe(N2) (0.045 g, 0.07 mmol) and ferrocene (0.012 g, 0.07 mmol) were dissolved in 1.5 mL of C6D6, giving a 0.045 M precatalyst stock solution. Ferrocene was used as an internal 1H NMR integration standard, and it did not affect the rates of hydrogenation. For a catalytic run, 0.1 mL of the stock solution was taken and mixed with 0.35 mL of C6D6, and 30 equiv of substrate in a J-Young NMR tube (3.2 mL capacity). For styrene hydrogenation, this equates to 0.01 M (TPB)Fe(N2) and 0.3 M styrene. For phenylacetylene hydrogenation, this equates to 0.01 M (TPB)Fe(N2) and 0.29 M phenylacetylene. For ethylene hydrogenation, this equates to 0.01 M (TPB)Fe(N2) and 0.30 M ethylene. The sample in the J-Young NMR tube was subsequently degassed by freeze-pumpthaw cycles (3x) and backfilled with 1 atm H2 (0.11 mmol). The J-Young NMR tube was continually rotated (12 min-1) to ensure adequate mass transfer. The tube was periodically refilled with H2 to maintain 1 atm of H2. All reactions were monitored periodically by 1H NMR spectroscopy until >95% completion. All reactions resulted in clean conversion of the substrate to the corresponding product. Catalytic runs in the presence of a drop of mercury or in the absence of ambient laboratory light had no affect on the reactions. Catalytic hydrogenations could also be cleanly effected by pre-generating (TPB)(µ‒H)Fe(N2)(H) or (TPB)(µ‒H)Fe(H2)(H) before the addition of the substrate. 62 2.6 Cited References (1) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; 1st ed.; University Science Books: Sausalito, California, 2010. (2) Bullock, R. M. Catalysis without Precious Metals; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2010. (3) For representative examples of mid-to-late first row transition metal catalysts that perform two-electron transformations see: (a) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104, 6217-6254; (b) Hu, X. Chem. Sci. 2011, 2, 1867-1886; (c) Q. Knijnenburg, Q.; Horton, A. D.; van der Heijden, H.; Kooistra, T. M.; Hetterscheid, D. G. H.; Smits, J. M. M.; de Briun, B.; Budzelaar, P. H. M.; Gal, A. W. J. Mol. Catal. A: Chem 2005, 232, 151-159; (d) Tondreau, A. M. Atienza, C. C. H.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delis, J. G. P.; Chirik, P. J. Science 2012, 335, 567-570; (e) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474-7585; (f) Federsel, C.; Boddien, A.; Jackstell, R.; Jennerjahn, R.; Dyson, P. J.; Scopelliti, R.; Laurenczy, G.; Beller, M. Angew. Chem. Int. Ed. 2010, 49, 9777-9780. (4) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001, 66, 7931-7944. (5) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282-2291. (6) Chakraborty, S.; Guan, H. Dalton Trans. 2010, 39, 7427-7436. (7) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816-5817. (8) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem. Int. Ed. 2008, 47, 940943. 63 (9) Langer, R.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem. Int. Ed. 2011, 50, 21202124. (10) For discussions on the M‒B bonding situation: (a) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583-8586; (b) Amgoune, A.; Bourissou, D. Chem. Comm. 2011, 47, 859-871; (c) Hill, A. F. Organometallics 2006, 25, 4741-4743; (d) Parkin, G. Organometallics 2006, 25, 4744-4747. (11) Owen, G. R. Chem. Soc. Rev. 2012, 41, 3535-3546. (12) Tsoureas, N.; Owen, G. R.; Hamilton, A.; Orpen, A. G. Dalton Trans. 2008, 6039-6044. (13) Crossley, I. R.; Foreman, M. R. S. J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J.; Willis, A. C. Organometallics 2008, 27, 381-386. (14) Crossley, I. R.; Foreman, M. R. S. J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Chem. Commun. 2005, 221-223. (15) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2007, 26, 3891-3895. (16) Crossley, I. R.; Hill, A. F. Dalton Trans. 2008, 201-203. (17) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2008, 27, 312-315. (18) Hill, A. F.; Smith, M. K.; Wagler, J. Organometallics 2008, 27, 2137-2140. (19) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2009, 29, 326-336. (20) Stephan, D. W.; Erker, G. Angew. Chem. Int. Ed. 2010, 49, 46-76. (21) Tsoureas, N.; Kuo, Y.-Y.; Haddow, M. F.; Owen, G. R. Chem. Commun. 2011, 47, 484486. 64 (22) Harman, W. H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080-5082. (23) Kameo, H.; Nakazawa, H. Organometallics 2012, 31, 7476-7484. (24) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063-2067. (25) Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 18118-18121. (26) Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46, 5149-5151. (27) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173-4184. (28) Kubas, G. J. Chem. Rev. 2007, 107, 4152-4205. (29) Morris, R. H. Coord. Chem. Rev. 2008, 252, 2381-2394. (30) Van der Sluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 4831-4841. (31) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 16366-16369. (32) Lee, Y.; Mankad, N. P.; Peters, J. C. Nature Chem. 2010, 2, 558-565. (33) Lee, Y.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 4438-4446. (34) Bartell, L. S.; Roth, E. A.; Hollowell, C. D.; Kuchitsu, K.; Young, J. E. J. Chem. Phys. 1965, 42, 2683-2686. (35) Select iron-ethylene complexes of ferracyclopropane character: (a) Zenneck, U.; Frank, W. Angew. Chem., Int. Ed. 1986, 25, 831-833; (b) Schneider, J. J.; Czap, N.; Blaser, D.; Boese, 65 R. J. Am. Chem. Soc. 1999, 121, 1409-1410; (c) Bennett, M. A.; Ditzel, E. J.; Hunter, A. D.; Khan, K.; Kopp, M. R.; Neumann, H.; Robertson, G. B.; Zeh, H. J. Chem. Soc., Dalton Trans. 2000, 1733-1741; (d) Fürstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.; Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773-8787. (36) A similar paramagnetic (S = 2) iron ethyl-hydro-tris(pyrazolyl)borate complex: Shirasawa, N.; Nguyet, T. T.; Hikichi, S.; Moro-oka, Y.; Akita, M. Organometallics 2001, 20, 3582-3598. (37) Marchand, A. P.; Marchand, N. W. Tetrahedron Lett. 1971, 12, 1365-1368. (38) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 14158-14164. (39) Reinhard, N. Coord. Chem. Rev. 1982, 47, 89-124. (40) Bianchini, C.; Laschi, F.; Masi, D.; Ottaviani, F. M.; Pastor, A.; Peruzzini, M.; Zanello, P.; Zanobini, F. J. Am. Chem. Soc. 1993, 115, 2723-2730. (41) Field, L. D.; George, A. V.; Malouf, E. Y.; Slip, I. H. M.; Hambley, T. W. Organometallics 1991, 10, 3842. (42) Tsay, C.; Peters, J. C. Chem. Sci. 2012, 3, 1313-1318. (43) Suess, D. L. M.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 4938-4941. (44) Harman, W. H.; Lin, T.-P.; Peters, J. C. Angew. Chem. Int. Ed. 2014, 53, 1081-1086. (45) Rubina, M.; Gevorgyan, V. J. Am. Chem. Soc. 2001, 123, 11107-11108. 66 Chapter 3. Neutron Diffraction Structure of an S = ½ Co‒H2 Adduct Reproduced in part with permission from Gunderson, W. A.; Suess, D. L. M.; Fong, H.; Wang, X.; Hoffmann, C. M.; Cutsail, G. E. III, Peters, J. C.; Hoffman, B. M. Journal of the American Chemical Society 2014, 136, 14998-15009 © 2014 American Chemical Society 67 3.1 Preface This chapter describes the single crystal neutron diffraction structure of a cobalt-dihydrogen complex, (TPB)Co(H2). The structure is presented in the context of frozen solution ENDOR studies on (TPB)Co(H2), as well as the previously reported iron-dihydrogen complex (SiPiPr3)Fe(H2).1 The neutron structure clearly shows that (TPB)Co(H2) is a nonclassical dihydrogen adduct, and that the H2 ligand adopts two preferred orientations in the solid state. This is in contrast to the ENDOR data, which suggest that the H2 ligand is freely rotating about the Co‒H2 axis. I am a third author on an article published in Journal of the American Chemical Society that describes this work. The other authors are William A. Gunderson, George E. Cutsail III, and Brian M. Hoffman of Northwestern University, Xiaoping Wang and Christina M. Hoffmann of Oak Ridge National Laboratory (ORNL), and Daniel L. M. Suess and Jonas C. Peters of Caltech. Daniel Suess prepared the first set of (TPB)Co(H2) and (TPB)Co(D2) samples for the ENDOR experiments. I was responsible for writing the proposal to secure instrument time at ORNL for single crystal neutron diffraction, synthesizing and growing single crystals for neutron diffraction, and preparing two sets of samples of (TPB)Co(H2), (TPB)Co(D2), and (TPB)Co(HD) for ENDOR studies. Xiaoping Wang and I, with assistance from Helen He from ORNL, performed the neutron diffraction experiment and solved the structure, and the two of us along with Christina Hoffmann, Daniel Suess, Jonas Peters, and Brian Hoffman analyzed the neutron structure. William Gunderson and George Cutsail performed the ENDOR 68 experiments. William Gunderson, George Cutsail, and Brian Hoffman analyzed the ENDOR data. In this chapter I made a distinction between the ENDOR studies and the neutron structure in order to better reflect my contribution to this topic. In this regard, I have summarized the ENDOR study in the introductory portion of this chapter (section 3.2.1) before presenting the neutron data in the Results and Discussion section 3.3. 69 3.2 Introduction Since Kubas’s discovery2 that transition metals can bind dihydrogen (so-called “nonclassical” behavior in contrast to the formation of a “classical” dihydride), there has been extensive work on the structural and electronic features of a number of closed-shell dihydrogen complexes.3 The binding properties of the dihydrogen ligand, the degree of H‒H bond activation, and its propensity to oxidatively add to a metal center are critical features of intermediate steps in a number of important industrial and biological catalytic cycles. For example, an iron center in the cofactor of the molybdenum-containing nitrogenase enzyme is believed to be the site of N2 binding.4 It is proposed that prior to N2 coordination at this iron center two protons are reduced to H2 as an obligatory component of the catalytic cycle; however, the coordination chemistry and spectroscopic properties of putative intermediate iron-hydride(s) (or -dihydrogen) species remain of interest.5 Furthermore, there has been great interest in using cobalt complexes as electrocatalytic proton reduction catalysts, but the putative CoII‒H2 intermediate proposed in two of the three competing mechanistic hypotheses has never been observed.6 Both these iron- and cobalt-dihydrogen intermediates are open-shell, which are not amenable to the standard NMR techniques used to characterize closed-shell metal-dihydrogen complexes. Whereas a number of diamagnetic closed-shell dihydrogen complexes have been characterized through combined neutron diffraction/scattering, X-ray diffraction (XRD), NMR, and IR studies, examples of open-shell dihydrogen adducts are rare.7 Thus, the properties and reactivity of thoroughly characterized open-shell transition metal- 70 dihydrogen complexes are of significant interest in the context of both synthetic and biological chemistry. Our group recently reported three open-shell metal-dihydrogen complexes. Based on their reactivity, similarities to isoelectronic N2 and/or CO adducts, as well as spectroscopic and XRD data, these complexes were formulated as nonclassical dihydrogen adducts: S = ½ (TPB)Co(H2)8 (TPB = B (o-C6H4PiPr2)3), S = ½ (SiPiPr3)Fe(H2)1 (SiPiPr3 = [Si(o-C6H4PiPr2)3]-), and S = 1 [(SiPiPr3)Fe(H2)][BArF4]1 (BArF4 = [B(3,5-(CF3)2-C6H3)4]-) (Chart 3.1). Although the hydrogenic ligands in the high quality XRD structures of (SiPiPr3)Fe(H2) and (TPB)Co(H2) cannot be reliably assigned, residual electronic density situated trans to the E atom (E = B or Si) was observed in the respective electron density difference map. For the related [(SiPiPr3)Fe(H2)][BArF4], a solid state XRD structure has not been successfully obtained. A method for definitive assignment of the hydrogenic ligands is to use single crystal neutron diffraction. Chart 3.1 Open-shell M‒H2 adducts of Fe and Co of interest. 71 Neutron diffraction, which relies on the scattering cross section of nuclei rather than the scattering by electrons exploited by XRD, is ideal for determining the atomic distances between light atoms such as hydrogen. Single crystal neutron diffraction was critical in confirming the assignment of Kubas’s nonclassical W‒H2 complex, (CO)3(PCy3)2W(H2) (Figure 3.1),9-10 and the technique has been used to characterize many other dihydrogen, dihydride, and polyhydride complexes since.3,11 In the decades since Kubas’s discovery, 1H NMR spectroscopy12-14 has been an important spectroscopic technique for characterizing and definitively assigning M‒H2 adducts, making neutron diffraction largely unnecessary. However, the NMR techniques are only amenable to diamagnetic species and not suitable for paramagnets such as (TPB)Co(H2), (SiPiPr3)Fe(H2), and [(SiPiPr3)Fe(H2)][BArF4]. Other pieces of data such as vibrational spectroscopy and reactivity, while informative, are not diagnostic. This leaves neutron diffraction as one of the few techniques for unequivocal assignment of M‒H2 adducts in paramagnetic complexes. Prior to this study, the neutron diffraction structure of a paramagnetic M‒H2 species had never been reported. Figure 3.1 Nonclassical W‒H2 adduct (CO)3(PCy3)2W(H2). For non-integer spin species, ENDOR (electron nuclear double resonance) spectroscopy can also be used to study M‒H2 species. This is because ENDOR spectroscopy, which is a combined EPR-NMR technique, can resolve coupling between the unpaired electron and the 72 hydrogenic ligands, providing valuable information about the bonding and structure of these complexes.15 For the S = ½ species (SiPiPr3)Fe(H2) and (TPB)Co(H2), this technique can therefore provide informative data on the nature of the hydrogenic ligand (H2 versus dihydride), as well as on the dynamics of the hydrogenic ligand. Indeed, an ENDOR study on (SiPiPr3)Fe(H2) by our collaborators in the Hoffman group at Northwestern University corroborated the assignment of (SiPiPr3)Fe(H2) as a nonclassical H2 adduct in frozen solution at 2 K.1 Furthermore, the 2-D field-frequency ENDOR data suggested that the H2 ligand tunnels/“hops” among localized states that are each parallel to the Fe‒P bond vectors (see section 3.2.1). ENDOR spectroscopy is complementary to single crystal neutron diffraction for the study of non-integer spin M‒H2 species. Whereas neutron diffraction probes static solid state molecular structures, ENDOR spectroscopy probes molecular structure and dynamics, such as the rotation of an H2 ligand. In this study, our collaborators at Northwestern studied (TPB)Co(H2) with ENDOR spectroscopy. The results show that the dihydrogen ligand is a free-rotor, with nearly unhindered rotation about the Co‒H2 axis in solution. This is in contrast to the localized H2 orientations found in the solid state, single crystal neutron diffraction structure, and also differs from the tunneling behavior of the H2 ligand in (SiPiPr3)Fe(H2). This chapter will first summarize the findings and conclusions from the ENDOR studies.i The single crystal neutron diffraction structure for (TPB)Co(H2) will then be presented and discussed in the context of the ENDOR results. Neutron diffraction data for (SiPiPr3)Fe(H2) has i For a thorough discussion of the ENDOR results, including a quantum mechanical description of H2 rotation, see ref 21. 73 also been collected, but due to the presence of a significant amount of impurities, conclusions regarding the H2 ligand in this complex cannot be reliably drawn. 3.2.1 Collaborative ENDOR Studies (TPB)Co(H2) and (SiPiPr3)Fe(H2) have the same orbitally degenerate 2E ground state, both having triply occupied, degenerate dxy/dx2-y2 frontier orbitals (Figure 3.2). Yet the H2 ligands in (TPB)Co(H2) and (SiPiPr3)Fe(H2) exhibit distinct behavior—the H2 ligand undergoes nearly free rotation in (TPB)Co(H2), whereas the H2 ligand in (SiPiPr3)Fe(H2) tunnels among local energy minima.1 As the following discussion will show, these differences are attributed to i) structural distortions of the two complexes arising from the pseudo-Jahn-Teller effect, ii) πbackbonding, and iii) crystal-packing forces and/or the molecular environment. Figure 3.2 Frontier molecular orbital diagram for (SiPiPr3)Fe(H2) and (TPB)Fe(H2). The relative ordering of the fully-occupied orbitals may vary. 74 1 H and 2H ENDOR spectra were collected at 2 K on frozen solution samples of (TPB)Co(H2), (TPB)Co(D2), and (TPB)Co(HD). The Q-band (35 GHz) pulsed 1H ENDOR spectrum of (TPB)Co(H2) is markedly different from the 1H and 2H ENDOR spectra of (TPB)Co(D2) and (TPB)Co(HD) (Figure 3.3C). Whereas (TPB)Co(D2) exhibits a hyperfine feature in the 2H ENDOR spectrum that is due to coupling to the D2 ligand, and (TPB)Co(HD) exhibits the same coupling associated with the HD ligand in both the 1H and 2H ENDOR spectra, the all 1H2 isotopologue (TPB)Co(H2) does not exhibit a hyperfine feature associated with coupling to the H2 ligand. By appropriately adjusting for the different gyromagnetic ratios of 2H and 1H, the 2H ENDOR signal corresponds to a hyperfine feature of A(1H) = 20.8 MHz, which is the same hyperfine coupling frequency observed in the 1H ENDOR spectrum for (TPB)Co(HD) (A(1H) = 20.8 MHz). The presence of 1H ENDOR signals for the HD ligand in (TPB)Co(HD), the presence of 2H ENDOR signals for the respective HD and D2 ligands in (TPB)Co(HD) and (TPB)Co(D2), but the absence of 1H ENDOR signals for the H2 ligand in (TPB)Co(H2), are suggestive of exchange of the two H atoms of the H2 ligand through rotation about the Co‒H2 axis in (TPB)Co(H2). This exchange is subject to constraints on the nuclear wave function imposed by the Pauli exclusion principle.16-17 For a homodiatomic molecule such as H2, the Pauli exclusion principle requires that the total nuclear spin state (Itot), which is a product of spatial rotational (Rot) and spin wave functions, must have definite parity with respect to exchange. For hydrogen (1H), which is a ferimon (I = ½), the total nuclear wave function must be antisymmetric (AS) to exchange. For deuterium (2H) which is a boson (I = 1), the total nuclear 75 wave function must be symmetric (S) to exchange. The Rot ground state for homodiatomic such as H2 is symmetric to exchange (S in Figure 3.3A), and therefore, the spin functions of the 1H in H2 must be antisymmetric with antiparallel nuclear spins (i.e., I = ½ and -½) and Itot = 0. This state does not have allowed ENDOR transitions, which is in agreement with the absence of 1H ENDOR signals for (TPB)Co(H2). For a homodiatomic such as D2, the Rot ground state is also symmetric (S, Figure 3.3B). The total nuclear wave function for D2 (I = 1 for 2H) is Itot = 0,1, or 2. Only Itot = 2 is symmetric to exchange, and it therefore must be associated with the symmetric rotational ground state. This spin state does have allowed A C B Figure 3.3 Q-band pulsed ENDOR spectra and rotational and nuclear spin states. (A) and (B) Spatial rotational (Rot) energy levels and corresponding total nuclear spin (Itot) for (TPB)Co(H2) and (TPB)Co(D2), respectively. (C) 1H ENDOR spectra for (TPB)Co(H2) (red) and (TPB)Co(HD) (black), and 2 H ENDOR spectra for (TPB)Co(D2) (blue) at 2 K . The 1H hyperfine of 20.8 MHz is marked. The frequency scale of the 2H (TPB)Co(D2) spectra has been scaled to match the 1H frequency. 76 ENDOR transitions, which is in agreement with the observation of 2H ENDOR signals for (TPB)Co(D2). Because 2H and 1H in HD are inequivalent, HD rotation in (TPB)Co(HD) is not subject to Pauli principle constraints; both 1H and 2H ENDOR transitions are allowed. The excited Rot state (Figure 3.3A)of (TPB)Co(H2) can exhibit an 1H ENDOR response, because it must correspondingly have a symmetric nuclear spin state with Itot = 1. However, the absence of a 1H ENDOR signal at (TPB)Co(H2) indicates that this state is negligibly occupied at 2 K and that the energy difference (Δ) between the ground and this first excited state must be greater than 7 cm-1. The quantum mechanical model for H2 rotation in a three-fold symmetric complex such as (TPB)Co(H2) predicts that the H2 ligand will rotate about the M‒H2 axis nearly unhindered. This is because for H2 rotation in a C3 symmetric environment, the rotation occurs on a potential energy surface of C6 symmetry (Figure 3.4B). In this symmetry, calculations show that the rotational barriers have little influence on H2 rotation. This is in contrast to H2 rotation about an axis with two-fold symmetry such as (CO)3(PCy3)2W(H2) (Figure 3.4A), wherein the H2 rotation is hindered by potential energy barriers and results in the preferential localization of the H2 ligand. An alternative way to explain the free rotation of the H2 ligand in (TPB)Co(H2) and hindered rotation in (CO)3(PCy3)2W(H2) is to consider the effects of symmetry on πbackbonding to the π-acidic H2 ligand. π-Backbonding can create a rotational barrier, and the H2 ligand can favor a particular orientation(s). In (CO)3(PCy3)2W(H2), the axes of the filled (dπ)4 of dxz and dyz are parallel to the W‒E bond vectors (E = P or C). Because CO is a better 77 π-acceptor than PCy3 (and H2), the π-acidic H2 ligand favors aligning parallel to the W‒P bond vectors where it can better compete for π-backbonding with the slightly π-acidic phosphines. This is illustrated on the potential energy surface in Figure 3.4A, where H2 favors alignment in parallel to the W‒P bond vectors. The H atoms of the H2 ligand tunnel among the two positions along the W‒P bond vectors.18-19 In C3 symmetry, the electron density of the e2 set of degenerate orbitals that participate in π-backbonding are cylindrically symmetric, and therefore π-backbonding cannot generate a rotational barrier. This leads to unhindered rotation of H2 in (TPB)Co(H2). Figure 3.4 Potential energy surfaces for H2 rotation. (A) H2 rotation in C2 symmetric (CO)3(PCy3)2W(H2). (B) Representation of H2 rotation on a C6 potential energy surface, wherein the MP3 plane forms an equilateral triangle, e.g., in (TPB)Co(H2) in frozen solution. (C) Distortion of the MP3 triangle to an acute isosceles triangle results in potential barrier along one M‒P bond vector (red P*) and localization of the H2 ligand along the two remaining M‒P vectors, e.g., in the crystalline state of (TPB)Co(H2) and in (SiPiPr3)Fe(H2). 78 While the discussion above explains the ENDOR response and free rotor behavior of H2 in (TPB)Co(H2), it does not explain why (SiPiPr3)Fe(H2) exhibits a 1H ENDOR response, and why the H2 ligand tunnels among energy minima in (SiPiPr3)Fe(H2) rather that freely rotate. Both complexes have 2E ground states that are subject to symmetry lowering structural distortions through vibronic coupling from the pseudo-Jahn-Teller effect.20 An analysis of the g-tensors from the EPR spectra of (SiPiPr3)Fe(H2) (g = [2.275, 2.064, 2.015]) and (TPB)Co(H2) (g = [2.457, 2.123, 2.029]) through the lens of the pseudo-Jahn-Teller effect shows that (SiPiPr3)Fe(H2) is subject to larger vibronic coupling than (TPB)Co(H2).21 This difference has a large effect on the structural distortions and, therefore, the degree to which H2 can freely rotate. The structural distortion, as shown in Figure 3.4, can be envisioned to occur through distortion of the C3 symmetric MP3 plane. In an idealized C3 symmetry, the MP3 plane can be thought of as an equilateral triangle. In the case of (TPB)Co(H2), it is proposed that a dynamic Jahn-Teller effect would maintain free rotation of the H2 ligand because the distortion of the MP3 plane is “pseudo-rotating” around the symmetry axis.20 The coupling of H2 ligand to this dynamic distortion through π-backbonding would maintain free rotation of the H2 ligand. In contrast, static Jahn-Teller distortion of the MP3 plane in (SiPiPr3)Fe(H2) generates a barrier to rotation. As the neutron structure will illustrate in section 3.3, one possible distortion results in an acute isosceles triangle for the MP3 plane, which generates a potential energy surface that contains a single barrier to rotation along the M‒P* bond vector (Figure 3.4C). In this scenario, free rotation of the H2 ligand is quenched, and the energy surface contains two energy minima 79 for the H2 ligand. Each of the energy minima are parallel to a M‒P bond vector. The hindrance to free rotation of the H2 ligand means that the ENDOR response is no longer subject to Pauli principle selection rules, thereby allowing for both 1H and 2H ENDOR responses, as was observed in (SiPiPr3)Fe(H2).1 The distortion of the MP3 plane in (SiPiPr3)Fe(H2) to an isosceles triangle can be seen in the solid state X-ray structure, which shows that the P1‒Fe‒P3 angle is contracted from the idealized 120 ° to 113 ° (see Table 3.1 in section 3.3.1). This distortion of the MP3 plane away from C3 symmetry also affects the π-backbonding to H2 from the cylindrically symmetric (in C3 symmetry) e2 set of orbitals. The distortion away from C3 symmetry as described above orients one of the two dπ orbitals (either dxz or dyz) towards the “open” M‒P* bond vector, enhancing π-backbonding to this P* donor arm. This outcome of this distortion and enhanced M‒P* π-backbonding, according to calculations, forces the H2 ligand along the two other M‒P bond vectors (Figure 3.4C). The stronger π-backbonding abilities for the iron complex compared to the cobalt complexes can also explain the differences in H2 rotation between (TPB)Co(H2) and (SiPiPr3)Fe(H2). The differences in π-backbonding abilities can be seen in the previously reported (SiPiPr3)Fe(N2)22 and (TPB)Co(N2)8 complexes, which are isoelectronic to (SiPiPr3)Fe(H2) and (TPB)Co(H2), respectively. The substantial differences in the N‒N stretching frequencies (2008 cm-1 for (SiPiPr3)Fe(N2) versus 2089 cm-1 for (TPB)Co(N2)) and N‒N bond lengths (1.125 Å for (SiPiPr3)Fe(N2) versus 1.062 Å for (TPB)Co(N2)) indicate that the M‒N2 π-backbonding is greater, and thus the N‒N bond more activated, in (SiPiPr3)Fe(N2) than (TPB)Co(N2). The 80 similar π-accepting abilities of N2 and H223 mean that these trends should extend to (SiPiPr3)Fe(H2) and (TPB)Co(H2) as well. Thus, the structural distortion and greater π-backbonding abilities for the (SiPiPr3)Fe(H2) complex compared to (TPB)Co(H2) explain why the H2 ligand in (SiPiPr3)Fe(H2) tunnels/hops between potential energy minima, while the H2 ligand in (TPB)Co(H2) freely rotates in frozen solution at 2 K. To probe the structure of (TPB)Co(H2) in the solid state, I collected the single crystal neutron diffraction structure of (TPB)Co(H2). The neutron structure of (TPB)Co(H2) confirms the intact H‒H bond of the H2 ligand and corroborates the conclusion that structural distortions quench free rotor behavior. 3.3 Results and Discussion 3.3.1 Neutron Structure of (TPB)Co(H2) The single crystal neutron diffraction structure of (TPB)Co(H2) has been determined at 100 K, at which temperature any structural changes upon cooling in general have produced a limiting low-temperature structure that is appropriate for comparisons with the ENDOR measurement at still lower temperature. The high-resolution neutron structure of (TPB)Co(H2) clearly resolves the presence of a side-on bound H2 ligand to Co that is positioned trans to boron (Figure 3.5). Thus, the structure confirms the initial assignment,8 and the assignment based on the ENDOR data discussed above, of (TPB)Co(H2) as a cobalt-dihydrogen adduct 81 Figure 3.5. Single crystal neutron diffraction structure of (TPB)Co(H2). (left) The disordered H2 ligand is shown in red and blue for the major and minor components, respectively. (right) View down the Co1‒B1 vector emphasizing the near parallel orientations of the disordered H2 ligand with the B1-C26 (associated with the major component of the disordered H2 ligand) and P1-Co1 bond (associated with the minor component of the disordered H2 ligand). Ellipsoids are shown at the 50 % probability level. Hydrogen atoms on the isopropyl and phenyl groups are omitted for clarity. rather than a cobalt-dihydride complex. The H2 ligand is disordered over two positions—the major component of disorder has a 68.2 % site occupancy. A second component of the disordered H2 was refined to a site occupancy of 25.2 %.ii The two orientations of the H2 ligand can be described as each being associated with a Co‒P bond vector, presumably a reflection of π-backbonding from filled d orbitals of Co to the empty σ* orbital of H2,3,24-25 although being somewhat skewed with respect to them, by ca. 21.2° and ca. 8° for the major ii The remaining 6.8 % nuclear density was modeled as a bromide ligand at 2.383(13) Å away from the Co center, situated trans to B atom, and is attributed to residual Co‒Br starting material. 82 and minor disordered components, respectively. The large degree of skewing of the major component of the H2 ligand (H1 and H2) from the Co1‒P3 places it in near parallel alignment with the B1-C26 bond vector (skewed by 5.7°). The H‒H bond distances for both H2 moieties are identical (0.834(6) and 0.83(2) Å) and elongated from the 0.74 Å bond length3 of free H2. These distances are similar to those found in other metal-dihydrogen complexes characterized by neutron diffraction (0.81 – 0.92 Å)3,11,18 and closely match those for two iron-dihydrogen adducts (0.81 - 0.82 Å).26-27 Table 3.1 Selected bond angles and distances for (SiPiPr3)Fe(H2) and (TPB)Co(H2) determined by X-ray and neutron diffraction. X-ray (SiPiPr3)Fe(H2)a Neutron (TPB)Co(H2)b (TPB)Co(H2) H1‒H2 (Å) 0.834(6) / 0.83(2)c M‒H1 (Å) 1.659(4) / 1.672(7)c M‒H2 (Å) 1.664(4) / 1.671(7)c M‒P1 (Å) 2.2442(9) 2.2412(3) 2.241(3) M‒P2 (Å) 2.260(1) 2.2650(3) 2.280(3) M‒P3 (Å) 2.2631(1) 2.2750(3) 2.262(3) M‒E (Å) 2.254(1) 2.2800(1) 2.287(2) 29.03(11) / 28.91(14)c H1‒M‒H2 (°) P1‒M‒P3 (°) 113.31(3) 119.00(1) 119.40(12) P1‒M‒P2 (°) 118.07(3) 110.97(1) 111.92(11) P2‒M‒P3 (°) 122.36(4) 124.97(1) 123.93(11) E = Si or B aSee ref 1. bSee ref 8. cMetrics for the major and minor components, respectively, of the disordered H2 ligand. 83 The neutron structure shows a deviation of the P‒Co‒P angles away from the 120° of the idealized C3 symmetry of the Co‒P plane (Table 3.1), as was also observed in the X-ray diffraction structures of (TPB)Co(H2) and the closely related complex, (TPB)Co(N2)8: the P1‒Co‒P3 angle is more acute than the other two P‒Co‒P angles, leaving P2 as the unique P*atom at the apex of the isosceles triangle (c.f. Figure 3.4). The same type of structural distortion was seen in the solid state X-ray diffraction structure of (SiPiPr3)Fe(H2). The distortion can be attributed to Jahn-Teller distortion in the solid state as well as crystal packing and other inter- and intramolecular forces, as is illustrated in the electron density isosurface of the solid state structure of Co‒H2 (Figure 3.6).28 The isosurface shows that the disordered H2 ligand is located in the blue-colored triangular cavity and is in close contact with three separate methyl hydrogen atoms from three isopropyl groups on the TPB ligand (Figure 3.6). The H2 orientation with higher occupancy may be favored because it has longer H ∙∙∙ H interactions between the H2 ligand and hydrogen atoms of adjacent methyl groups: the closest H ∙∙∙ H contacts from the neutron structure are 2.174(6) Å and 2.096(14) Å for the major and minor components of the H2 ligand, respectively. These close contacts are likely not present in frozen solutions, where packing forces are not in play. Such crystal-packing influences on H2 rotation have been discussed elsewhere.iii,19 A single crystal neutron diffraction experiment for (SiPiPr3)Fe(H2) was also performed. However, the neutron structure exhibited significant disorder arising from multiple impurities iii The unequal occupation of H2 at the two sites is also predicted by applying an additional, small out-of-registry potential in the modeling of the potential energy surface. 84 Figure 3.6 Plot of the isosurface of the promolecule electron density of (TPB)Co(H2). (left) Isosurface of electron density of (TPB)Co(H2) with the dihydrogen ligand removed. The isovalue of 0.0020e au-3 used in the plot is comparable to the expected van der Waals radii. (right) A translucent view of the same isosurface showing the disordered dihydrogen ligand located in the blue colored triangular cavity. in the crystal: the desired (SiPiPr3)Fe(H2) accounted for ca. 45 % and the previously reported FeII-hydride-dihydrogen complex (SiPiPr3)Fe(H2)(H)29 accounted for another ca. 20 %. The remaining 35 % impurity was assigned to chloride from the (SiPiPr3)FeCl starting material. The significant amounts of impurities, particularly of the (SiPiPr3)Fe(H2)(H), wherein the H2 ligand residues in the same coordination site as the H2 ligand in (SiPiPr3)Fe(H2), rendered this structure inconclusive with respect to the nature of the H2 ligand in (SiPiPr3)Fe(H2). For a figure of the structure, see Figure A2.1 in Appendix 2. 85 3.3.2 Comparisons with the ENDOR Data The localization of the H2 ligand among two preferred orientations in the solid state neutron structure of (TPB)Co(H2) is distinct from the free rotor behavior of the H2 ligand suggested by the frozen solution ENDOR data, but it agrees with the predicted localization for the H2 ligand upon distortion of the MP3 plane. The ENDOR results presented in section 3.2.1 showed that static distortions of the MP3 plane and competitive π-backbonding between the π-acidic phosphines and H2 ligand can lead to preferential alignment of the H2 ligand along two M‒P bond vectors (Figure 3.4C), as was suggested by ENDOR results for the frozen solution samples of (SiPiPr3)Fe(H2). It is evident from the neutron structure for (TPB)Co(H2) that in the solid, crystalline state the MP3 plane is also distorted similarly to the frozen solution structure of (SiPiPr3)Fe(H2) (Table 3.1). Such a distortion in the solid state, as well as additional forces created by the solid, crystalline state molecular environment about the H2 ligand’s coordination sphere, serve to quench free rotation of the H2 ligand. The P1‒Co1‒P2 angle in the solid state structure of (TPB)Co(H2) is contracted from the 120° of an idealized C3 symmetric MP3 core to 111° (Table 3.1), with the MP3 core best described as an acute, isosceles triangle, as shown in Figure 3.4. It was predicted that this geometric distortion quenches free rotor behavior and localizes the H2 ligand among two preferred orientations, where each H2 unit of the disorder lies in parallel with a M‒P bond vector, leaving the third M‒P* bond vector as a non-preferred orientation. Indeed, this is observed in the neutron structure of (TPB)Co(H2). The H2 ligand is localized in two orientations that are each associated with the Co‒P3 and Co‒P1 bond vectors, respectively. 86 The Co‒P2 bond vector is the non-preferred orientation. To illustrate the correspondence between the neutron structure and the potential energy surface, the core of the structure of (TPB)Co(H2) can be overlaid with the potential energy surface, which was originally presented in Figure 3.4C, to give Figure 3.7. The potential energy surface is oriented so as to maximize the overlap of the two H2 orientations with energy minima. The figure illustrates the correspondence between the potential energy surface predicted by quantum mechanics and the neutron diffraction determined localization of the H2 ligand within these potential energy minima. Figure 3.7 Overlay of the core of the (TPB)Co(H2) neutron structure with the potential energy surface for H2 rotation upon distortion of the MP3 core, emphasizing the two preferred orientations of the H2 ligand. The H2 ligand also does not occupy each of the two preferred orientation equally: H1‒H2 has occupancy of 68.2 %, while the H1B‒H2B has occupancy of 25.2 %. H1‒H2 and H1B‒H1B are skewed from the M‒P bond vectors by ca. 21° and ca. 8°, respectively. The deviation from the predicted structure can be explained in part with the isosurface shown in 87 Figure 3.6, which shows that the H2 ligand is in close contact with three separate methyl hydrogen atoms from the three isopropyl group of the TPB ligand. The higher occupancy of H1‒H2 may be favored because it has the longest H ∙∙∙ H close-contact distance of 2.174 Å, compared to the to the significantly shorter H ∙∙∙ H close-contact distance of 2.096 Å for the lower occupancy H1B‒H2B. These close contacts are likely not present in frozen solution, where packing forces are not in play and also contribute to quenching of the free rotation of the H2 ligand. 3.4 Conclusions The neutron diffraction determined H‒H distance of 0.83 Å is evidence that (TPB)Co(H2) is a nonclassical H2 adduct. Comparison of the solid state neutron structure to frozen solution ENDOR data reveals that the rotational dynamics of the H2 ligand are very different in the solid state versus frozen solutions. Whereas the H2 ligand is a free-rotor in frozen solution, arising from H2 residing in an environment of C3 symmetry, the solid state structure reveals that H2 is localized among two preferred orientations. The quenching of free-rotation for the H2 ligand in the solid state is attributed to structural distortions and crystal packing forces that generate barriers to free rotation of the H2 ligand. The neutron structure of (TPB)Co(H2) exhibits exactly what is observed by ENDOR for frozen solution samples of (SiPiPr3)Fe(H2). The contrast between the solid state data and frozen solution data illustrates how single crystal neutron diffraction and ENDOR can be used as complementary tools for the study of molecular dihydrogen complexes in the solid and solution states, respectively. 88 3.5 Experimental Section 3.5.1 Sample Preparation Samples for EPR/ENDOR studies were prepared using a standard glovebox or Schlenk techniques. (TPB)Co(H2), (TPB)Co(D2), and (TPB)Co(HD) were prepared according to literature procedures.8 THF, 2-Me-THF, toluene, HMDSO, and methylcyclohexane were rigorously dried by stirring over Na metal for several days, followed by filtration through a pad of activated alumina. HD gas was generated by the slow addition of D2O into LiAlH4. Q-band ENDOR tubes were charged with the (TPB)Co(H2), (TPB)Co(D2), and (TPB)Co(HD) solutions under an atmosphere of the respective dihydrogen isotopomer. (TPB)Co(H2) crystals of suitable size for single crystal neutron diffraction were grown under 1 atm of H2 in a J-Young NMR tube according to the following procedure. A suspension of (TPB)Co(H2) in 2:1 HMDSO:methylcyclohexane at room temperature (RT) was dissolved by heating to 90 °C in an oil bath. Homogeneity of the solution is critical to growing large crystals suitable for neutron diffraction. The sample was then allowed to cool to RT in the oil bath, which was left undisturbed for three days, yielding large yellow crystals of (TPB)Co(H2). 3.5.2 Neutron Diffraction Single crystal neutron diffraction data were measured on the TOPAZ instrument at the Spallation Neutron Source at Oak Ridge National Laboratory, in the wavelength-resolved time-of-flight Laue diffraction mode using wavelengths in the range 0.4 – 3.5 Å.30 A rodshaped crystal of (TPB)Co(H2) with the dimensions of 0.42  0.60  1.20 mm3 was mounted onto the tip of a polyimide capillary with fluorinated grease in a nitrogen glove box, and 89 transferred onto the TOPAZ goniometer for data collection at 100 K. To ensure good coverage and redundancy, data were collected using 26 crystal orientations optimized with CrystalPlan software.31 Each orientation was measured for approximately 5.9 h. The integrated raw Bragg intensities were obtained using the 3-D reciprocal Q-space integration method in Mantid,32 where Q = 2π/d = 4π(sinθ)/λ. The peaks from (TPB)Co(H2) were found to be triplets in Q-space within less than 0.15 Å-1 radii. Bragg peaks from the major component were used for determination of orientation matrix for the (TPB)Co(H2) crystal. Peak integration was performed accordingly using a radius of 0.15 Å-1 to include contributions from all three components. Data reduction for each sample, including neutron TOF spectrum, detector efficiency, and absorption corrections, was carried out with the ANVRED2 program.33 The reduced data were saved in SHELX HKLF2 format in which the wavelength is recorded separately for each individual reflection, and not merged as a consequence of this saved format. The initial structural model used the unit cell parameters and non-hydrogen atom positions from the single-crystal XRD experiment measured at 100 K. The hydrogen atoms were found from nuclear difference Fourier map of the neutron data, and refined anisotropically using SHELXL-9734 in WinGX.35 The dihydrogen ligand was found to be disordered in two positions with the site occupancy factors refined to 68.2 % and 25.2 %, respectively, for the major and minor components. The remaining 6.6 % nuclear density was modeled as a bromide ligand at 2.383(13) Å away from the Co center (trans to the boron) and is attributed to residual (TPB)CoBr starting material. The neutron structure was validated with Platon and the IUCr online checkcif program. The 90 following is a list of programs used: orientation matrix from live neutron event data, ISAW Event Viewer; data collection strategy, CrystalPlan; Data collection, SNS PyDas; data reductions, Mantid; absorption correction, ANVRED2; structural refinement, SHELXL-97;34 promolecule isosurface plots, CrystalExplore.36 91 3.6 Cited References (1) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 1636616369. (2) Kubas, G. J. J. Chem. Soc., Chem. Commun. 1980, 61-62. (3) Kubas, G. J. Chem. Rev. 2007, 107, 4152-4205. (4) Seefeldt, L. C.; Dance, I. G.; Dean, D. R. Biochemistry 2004, 43, 1401-1409. (5) Hoffman, B. M.; Lukoyanov, D.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C. Chem. Rev. 2014, 114, 4041-4062. (6) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Acc. Chem. Res. 2009, 42, 1995-2004. (7) Proposed nonclassical H2 binding in a paramagnetic metal complex: (a) Hetterscheid, D. G. H.; Hanna, B. S.; Schrock, R. R. Inorg. Chem. 2009, 48, 8569-8577; (b) Kinney, R. A.; Hetterscheid, D. G. H.; Schrock, R. R.; Hoffman, B. M. Inorg. Chem. 2010, 49, 704-713; (c) Bart, S. C.; Lobkovsky, E.; Chirik, P J. J. Am. Chem. Soc. 2004, 126, 13794-13807; (d) Baya, M.; Houghton, J.; Daran, J.-C.; Poli, R.; Male, L.; Albinati, A.; Gutman, M. Chem.-Eur. J. 2007, 13, 5347-5359. (8) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 14158-14164. (9) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451-452. (10) Kubas, G. J.; Unkefer, C. J.; Swanson, B. I.; Fukushima, E. J. Am. Chem. Soc. 1986, 108, 7000-7009. 92 (11) Morris, R. H. Coord. Chem. Rev. 2008, 252, 2381-2394. (12) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173-4184. (13) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc. 1996, 118, 5396-5407. (14) Heinekey, D. M.; van Roon, M. J. Am. Chem. Soc. 1996, 118, 12134-12140. (15) Hoffman, B. M. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 3575-3578. (16) Farkas, A. Orthohydrogen, Parahydrogen, and Heavy Hydrogen; University Press: New York, 1935. (17) McConnell, H. M. J. Chem. Phys. 1958, 29, 1422-1422. (18) Kubas, G. J.; Burns, C. J.; Eckert, J.; Johnson, S. W.; Larson, A. C.; Vergamini, P. J.; Unkefer, C. J.; Khalsa, G. R. K.; Jackson, S. A.; Eisenstein, O. J. Am. Chem. Soc. 1993, 115, 569-581. (19) Eckert, J.; Kubas, G. J. J. Phys. Chem. 1993, 97, 2378-2384. (20) Bersuker, I. B. Chem. Rev. 2013, 113, 1351-1390. (21) Gunderson, W. A.; Suess, D. L. M.; Fong, H.; Wang, X.; Hoffmann, C. M.; Cutsail, G. E. I.; Peters, J. C.; Hoffman, B. M. J. Am. Chem. Soc. 2014, 136, 14998-15009. (22) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507-2517. (23) Morris, R. H.; Earl, K. A.; Luck, R. L.; Lazarowych, N. J.; Sella, A. Inorg. Chem. 1987, 26, 2674-2683. 93 (24) Kubas, G. J. J. Organomet. Chem. 2014, 751, 33-49. (25) Eckert, J. Spectrochim. Acta, Part A 1992, 48, 363-378. (26) Van der Sluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 4831-4841. (27) Ricci, J. S.; Koetzle, T. F.; Bautista, M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer, J. F. J. Am. Chem. Soc. 1989, 111, 8823-8827. (28) Promolecule is the sum of electron density distribution of spherical atoms within the same molecule. see McKinnon, J. J.; Spackman, M. A.; Mitchell, A. S. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2004, 60, 627-668. (29) Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Organometallics 2013, 32, 3053-3062. (30) Jogl, G.; Wang, X.; Mason, S. A.; Kovalevsky, A.; Mustyakimov, M.; Fisher, Z.; Hoffman, C.; Kratky, C.; Langan, P. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2011, 67, 584-591. (31) Zikovsky, J.; Peterson, P. F.; Wang, X. P.; Frost, M.; Hoffmann, C. J. Appl. Crystallogr. 2011, 44, 418-423. (32) Mantid: A high performance framework for reduction and analysis of neutron scattering data, http://www.mantidproject.org (accessed April 2013). (33) Schultz, A. J.; Srinivasan, K.; Teller, R. G.; Williams, J. M.; Lukehart, C. M. J. Am. Chem. Soc. 1984, 106, 999-1003. (34) Sheldrick, G. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112-122. 94 (35) Farrugia, L. J. Appl. Crystallogr. 1997, 30, 565. (36) McKinnon, J. J.; Spackman, M. A.; Mitchell, A. S. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2004, 60, 627-668. 95 Chapter 4. Hydricity of an Fe‒H Species and Catalytic CO2 Hydrogenation Reproduced in part with permission from: Fong, H.; Peters, J. C. Inorganic Chemistry [Online early access]. DOI: 10.1021/ic502508p. © 2014 American Chemical Society 96 4.1 Preface This chapter describes the CO2 hydrogenation chemistry of a series of tris-phosphino-ironhydride complexes. In Chapter 2, I showed that the (TPB)Fe scaffold is a competent for catalytic olefin and arylacetylene hydrogenation and that the hydrogenative reactivity was due in part to iron-borane cooperativity. In this chapter, I will show that the iron-borane cooperativity does not facilitate catalytic CO2 hydrogenation for the (TPB)Fe system. I also studied the CO2 hydrogenation chemistry of a series of structurally related tris-phosphino-iron complexes, and experimentally estimated the hydricity of the iron-hydride species (SiPiPr3)Fe(N2)(H). I am the first author of a paper published in Inorganic Chemistry that describes this work. The other author is Jonas Peters. I performed all of the experiments described in the manuscript, and Jonas Peters and I analyzed the results. Nathan Dalleska of the Environmental Analysis Center at Caltech assisted me in developing a method for the GC-FID measurements. 97 4.2 Introduction The reduction of carbon dioxide into value-added chemicals and liquid fuels has received considerable attention recently due to increasing interest in the development of carbon neutral energy sources.1 The production of liquid fuels such as methanol2 or formic acid3 from CO2 and H2 (or its formal equivalents) is particularly attractive. However, selective production of these products using heterogeneous catalysts remains challenging.4-6 One interesting approach towards CO2 reduction is to use molecular catalysis, where product selectivity may be better controlled than heterogeneous systems.7 The catalytically active species in molecular systems can often be probed either directly or indirectly, thereby offering opportunities to understand the catalytic mechanism and synthetically tune systems in a well-defined manner.8 One of the simplest CO2 reduction reactions is its hydrogenation to formic acid.3 While a number of noble-metal catalysts for the hydrogenation of CO2 to formic acid exist,9-17 there are only a handful of examples using first-row transition metals such as iron18-24 and cobalt,25-28 and information about their thermodynamic properties and elementary reaction steps is needed.29-34 For example, the hydricity (ΔGH-), which is the heterolytic dissociation energy of an [M‒H]n+ into Mn+1 and H- (equation 4.1), has only been experimentally determined for one iron-hydride complex FpH35 (FpH = (C5H5)Fe(CO)2(H)) despite recent reports of ironcatalyzed CO2 hydrogenation.18-24 Knowledge of the hydricities of hydrogenation catalysts can aid the design of new catalysts. This is highlighted by the recent work of Linehan and coworkers on a cobalt-hydride catalyst,26-27 in which the design of this efficient CO2-to- 98 formate hydrogenation system was achieved in part by using a cobalt-hydride that was more hydridic (i.e., < 43 kcal/mol) than the hydricity of formate36 (equation 4.2). [M‒H]n+  Mn+1 + H- ΔGH- (4.1) OCHO-  CO2 + H- ΔGH- = 43 kca/mol (4.2) As part of our exploratory research of phosphine-supported iron complexes in small molecule activation reactions37-42 I were interested in studying the catalytic CO2 hydrogenation chemistry of a series of tris-phosphino-iron species (Chart 4.1): (SiPR3)Fe(L)(H) (L = H2 or N2; SiPR3 = [Si(o-C6H4PR2)3]-, R = iPr or Ph),37,43-44 (PhBPiPr3)Fe(H)3(PMe3) (PhBPiPr3 = PhB(CH2PiPr2)3),45 [(NPiPr3)Fe(N2)(H)](PF6) (NPiPr3 = N(CH2CH2PiPr2)3),46 (TPB)(μ‒H)Fe(L)(H) (L = N2 or H2, TPB = B(o-C6H4PiPr3)3),44 (CPiPr3)FeCl (CPiPr3 = [C(oC6H4PiPr2)3]-),42 and (CSiPPh3)FeCl (CSiPPh3 = [C(Si(CH3)2CH2PPh2)3]-).47 These systems are structurally related to two tetra-phosphino-iron-hydride CO2 hydrogenation catalysts ([(PP3)Fe(H2)(H)](BF4)19,48 and (tetraphos)Fe(H2)(H)](BF4),20 where PP3 = P(CH2CH2PPh2)3 and tetraphos = P(o-C6H4PPh2)3), studied in a similar context by the groups of Beller and Laurenczy (Chart 4.1). A distinguishing feature of the present series of tris-phosphino-iron complexes is that each of the present ligand scaffolds possesses a different apical unit. These include an X-type silyl in SiPR3, an X-type alkyl in CPiPr3 and CSiPPh3, a non-coordinating borate in PhBPiPr3, an L-type amine in NPiPr3, and a Z-type borane in TPB. Each of these apical units can confer different (i) geometries at iron, (ii) formal oxidation states at iron, and (iii) reactivity patterns for otherwise structurally similar species, as we have studied previously with respect to N2 activation chemistry.37-42 99 Chart 4.1 Select phosphine-metal complexes of relevance to catalytic CO2 hydrogenation. This chapter presents the experimentally determined pKa and hydricity values for the (SiPiPr3)Fe system as well as the catalytic and stoichiometric hydrogenation of CO2 in this and related tris-phosphino-iron species shown in Chart 4.1. Under elevated temperatures and pressures of CO2 and H2 and with triethylamine as base, the (SiPiPr3)Fe, (SiPPh3)Fe, (PhBPiPr3)Fe, and (CPiPr3)Fe systems catalytically hydrogenate CO2 to triethylammonium formate and methylformate, while (NPiPr3)Fe, (TPB)Fe, and (CSiPPh3)Fe did not catalyze CO2 100 hydrogenation. We also show that despite a low hydricity (i.e., large ΔGH- value) for the complex (SiPiPr3)Fe(H2)(H) (ΔGH- = 54.3 ± 0.9 kcal/mol) coordination of the formate product to the iron-center following hydride transfer to CO2 provides enough driving force to make the reaction thermally accessible. 4.3 Results and Discussion 4.3.1 pKa and Hydricity for (SiPiPr3)Fe Since Fe‒H species have been invoked as intermediates for CO2 hydrogenation, we were curious if (SiPiPr3)Fe(H2)(H) was sufficiently hydridic to react with CO2. One method for determining hydricities is to use a thermochemical cycle that involves deprotonating the conjugate acid of the metal-hydride of interest. We previously reported the dihydrogen chemistry of the (SiPiPr3)Fe system,44 including the deprotonation of the cationic irondihydrogen complex [(SiPiPr3)Fe(H2)](BArF4) (BArF4 = [B(3,5-CF3-C6H3)4]-) by Hünig’s base under H2 to afford (SiPiPr3)Fe(H2)(H).43 This motivated us to use this deprotonation reaction to experimentally determine the hydricity of (SiPiPr3)Fe(H2)(H) using the series of equations in Scheme 4.1. The equilibrium in equation 4.3 was followed by 1H NMR spectroscopy independently with 1,8-bis(dimethylamino)naphthalene (proton sponge, Keq = 4.3), 2,6-lutidine (Keq = 3.3 x 10-5), and 2,4,6-trimethylpyridine (Keq = 5.1 x 10-5) in d8-THF.i The reverse protonation i of (SiPiPr3)Fe(H2)(H) with the BArF4-salt of 1,8- THF is known to bind competitively with H2 on [(SiPiPr3)Fe(L)](BArF4] (where L = THF or H2), and the THF/H2 binding equilibrium was taken into account in the calculations; see Appendix 3. 101 bis(dimethylammonium)naphthalene (Keq = 2.6) was also followed by 1H NMR spectroscopy in d8-THF. Scheme 4.1 Reactions relevant to the determination of the pKa and hydricity of (SiPiPr3)Fe. See Chart 4.1 for the detailed ligand representation. B = base. Note that the pKa of the dihydrogen ligand in [(SiPiPr3)Fe(H2)](BArF4) can be estimated using equations 4.3 and 4.4. The experimentally determined pKa in THF using this method is pKaTHF = 10.8 ± 0.6 for [(SiPiPr3)Fe(H2)](BArF4). Notably, the pKaTHF agrees very well with the predicted value of 10.2 obtained from the ligand acidity constants method recently developed by Morris.ii,49 It should be cautioned that this is only a rough estimate of the pKa of ii The pKa of [(SiPiPr3)Fe(H2)](BArF4) was estimated using the Morris ligand acidity constants method. These calculations rely on ligand acidity constants for each of the ligands of the conjugate base metal complex, which in this case is the deprotonation product (SiPiPr3)Fe(H2)(H). We note that the ligand acidity constants for H2 and the formally Si- ligands of the conjugate base complex are not known. Therefore, the reported ligand acidity 102 [(SiPiPr3)Fe(H2)](BArF4), because the pKa of [(SiPiPr3)Fe(H2)](BArF4) is a measure of the removal of a proton to afford “(SiPiPr3)Fe(H)”, whereas in the observed deprotonation reaction dihydrogen coordinates to this species to afford the (SiPiPr3)Fe(H2)(H) and contributes to the equilibrium depicted in equation 4.3. With the equilibrium of equation 4.3 in hand, the hydricity of the conjugate base (SiPiPr3)Fe(H2)(H) can be determined by the summation of equations 4.3-4.5 to give equation 4.6.50-51 Most hydricity values have been reported in acetonitrile due in part to the known heterolytic dissociation energy of H2 in acetonitrile (equation 4.5). However, irreversible coordination of acetonitrile to [(SiPiPr3)Fe precluded the use of this solvent. An empirical relationship relates the pKaTHF of a metal complex to the pKa in acetonitrile (pKaMeCN).52 Using this relationship, the pKaMeCN of [(SiPiPr3)Fe(H2)](BArF4) is 15.9 ± 0.7. Combining the pKaMeCN of [(SiPiPr3)Fe(H2)](BArF4) with equation 4.5, the hydricity of (SiPiPr3)Fe(H2)(H) in MeCN is 54.3 ± 0.9 kcal/mol. This is only the second experimentally estimated hydricity value of an iron-hydride complex.35 Formal hydride transfer from phosphine ligated iron-hydride complexes to CO2 to give formate is known in the literature.29-34,53 A comparison of the hydricity of (SiPiPr3)Fe(H2)(H) to that of formate (equation 4.2) indicates that the reaction for hydride transfer from (SiPiPr3)Fe(H2)(H) to CO2 to afford formate is endergonic by over 10 kcal/mol. Yet, as will constants for C2H4 as a model for the H2 ligand and CH3-/H- as a model for the Si- ligand unit were used for these calculations. For the Morris acidity constant method, see ref 49. 103 be shown below, (SiPiPr3)Fe(H2)(H) can still react with CO2 both stoichiometrically and catalytically to afford formate. Scheme 4.2 Stoichiometric CO2 hydrogenation to triethylammonium formate. See Chart 4.1 for the detailed representations of the ligands indicated. 4.3.2 Stoichiometric Reactivity of Fe‒H Species with CO2 In addition to (SiPiPr3)Fe(L)(H)43-44 (where L = N2 or H2), our group has previously reported the synthesis and characterization of three other related tris-phosphino-iron-hydride 104 complexes, (PhBPiPr3)Fe(H)3(PMe3),45 [(NPiPr3)Fe(N2)(H)](PF6),46 and (TPB)(μ‒H)Fe(N2)(H)44 (Chart 4.1), and demonstrated that the two former complexes, (PhBPiPr3)Fe(H)3(PMe3) and (TPB)(μ‒H)Fe(N2)(H), are olefin hydrogenation catalysts. The iron-hydride species of the tris(diphenyl-phosphino)silyl ligand, (SiPPh3)Fe(N2)(H), had not previously been reported, but it has now been synthesized in an analogous manner to the preparation of the isopropyl-analogue (SiPiPr3)Fe(N2)(H) (vide infra). The reactivity of these iron-hydrides to CO2 was probed. Synthesis of iron-formate species. A yellow solution of (SiPPh3)Fe(N2)(H) reacted with CO2 (1 atm) at 50 °C over 1 h to afford the orange insertion product (SiPPh3)Fe(OCHO) (Scheme 4.2a). Consistent with the κ1-bound formate ligand,54 ATR-IR spectroscopy showed two signature vibrational features at 1618 cm-1 and 1316 cm-1 (13CO2: 1587 and 1254 cm-1) with a Δν(O‒C‒O) of 302 cm-1 (Table 4.1). As expected for a five-coordinate (SiPR3)FeII complex,55 Table 4.1 IR stretching frequencies and solution magnetic moments for iron-formate complexes. μeff (μB)a νasym(O‒C‒O) (cm-1)b 13 CO2 CO2 νsym(O‒C‒O) Δνasym (O‒C‒O) (cm-1)b (cm-1)b 13 CO2 CO2 (SiPPh3)Fe(OCHO) 2.7 1618 1587 1316 1254 302 (SiPiPr3)Fe(OCHO) 2.8 1623 1583 --- --- --- (PhBPiPr3)Fe(OCHO) 5.0 1595 1546 1362 1355 233 [(NPiPr3)Fe(OCHO)][PF6] 5.1 1613 1579 --- --- --- (TPB)Fe(OCHO) 4.2 1627 1588 1291 1269 336 a Solution magnetic moments at RT. bATR-IR data of solution thin films. cDifference between νasym(O‒C‒O) and νsym(O-C‒O). 105 (SiPPh3)Fe(OCHO) is S = 1 (2.7 μB in C6D6 at RT). (SiPiPr3)Fe(N2)(H) reacted with CO2 similarly to afford (SiPiPr3)Fe(OCHO). The ATR-IR spectrum showed an asymmetric O‒C‒O stretch at 1623 cm-1 (13CO2: 1583 cm-1). While the symmetric O‒C‒O stretch could not be reliably discerned (Table 4.1), its S = 1 spin state (2.8 μB in C6D6 at RT) and yellow colour indicate a five coordinate (SiPiPr3)Fe(OCHO) complex. Exposing a yellow THF solution of (PhBPiPr3)Fe(H)3(PMe3) to CO2 (1 atm) for 12 h at room temperature afforded the κ2-bound formate adduct (PhBPiPr3)Fe(OCHO) (Scheme 4.2b) as a light yellow solution. The ATR-IR spectrum of this S = 2 species (5.0 μB, C6D6 at RT) exhibited features of a formate ligand at 1595 cm-1 and 1362 cm-1 (13CO2: 1546 and 1355 cm-1) with a Δν(O‒C‒O) = 233 cm-1 that is consistent with the κ2-bound formate assignment.54 The formate coordination mode is in contrast to the κ1-bound formate ligands in (SiPR3)Fe(OCHO), [(NPiPr3)Fe(OCHO)](PF6), and (TPB)Fe(OCHO). Presumably, this arises because of the lower coordination number in (PhBPiPr3)Fe. A yellow THF solution of [(NPiPr3)Fe(N2)(H)](PF6) reacted with CO2 (1 atm) at room temperature over 3 h to afford the formate adduct [(NPiPr3)Fe(OCHO)](PF6) (Scheme 4.2c) as a colorless solution. [(NPiPr3)Fe(OCHO)](PF6) is S = 2 (5.1 μB, C6D6 at RT), analogous to [(NPiPr3)FeCl](PF6).46 Consistent with the iron-formate formulation, ATR-IR spectroscopy showed a diagnostic νasym(O‒C‒O) vibrational feature at 1613 cm-1 that shifts to 1579 cm-1 with 13CO2. However, the accompanying lower energy νsym(O‒C‒O) vibrational feature could not be reliably assigned due to overlapping ligand vibrational modes in the 1200 – 1300 cm-1 106 region. The obscured νsym(O‒C‒O) feature prevented assignment of the formate binding mode, but the νasym(O‒C‒O) most closely matches κ1-bound formate ligands (Table 4.1).54 Mixing a benzene solution of (TPB)(μ‒H)Fe(N2)(H) with CO2 (1 atm) at room temperature afforded the κ1-formate complex (TPB)Fe(OCHO) (Scheme 4.2d) as a yellow solution. The color, 1H NMR spectrum, and solution magnetic moment (4.2 μB, S = 3/2 in C6D6 at RT) are consistent with the formulation of (TPB)Fe(OCHO) as a {Fe‒B}7 species.iii,58 As for (SiPR3)Fe(OCHO), (TPB)Fe(OCHO) exhibited diagnostic κ1-formate ligand vibrational modes at 1627 cm-1 and 1291 cm-1 (13CO2: 1588 and 1269 cm-1) with a Δν(O‒C‒O) of 336 cm-1 (Table 4.1).54 The IR spectrum of (TPB)Fe(OCHO) lacks any feature that is diagnostic for a B‒H unit.59 For comparison, in the related S = 2 (TPBH)Fe(CCAr) (Ar = phenyl or tolyl) complex, where a terminal B‒H is present, the IR spectra exhibits diagnostic B‒H vibrations at 2490 cm-1 for Ar = phenyl and 2500 cm-1 for Ar = tolyl.44 The formation of (TPB)Fe(OCHO) from the reaction of (TPB)(μ‒H)Fe(N2)(H) with CO2 (1 atm) is notable in that there is a formal loss of an H-atom (Scheme 4.3). The loss of 0.5 equiv of H2 (relative to the starting iron complex) was confirmed by gas-chromatography (GC-TCD; 0.44 equiv of H2 quantified). The reaction between the previously reported (TPB)Fe(N2)58 with formic acid also formed (TPB)Fe(OCHO), with 0.42 equiv of H2 detected by GC-TCD as a product (Scheme 4.3). iii The {M‒E}n notation refers to the number of valence electrons that are formally assigned to the metal (e.g., Fe) and those shared with E (e.g., B). Since the M‒E bond may be covalent and the M‒E interaction is dictated in part by the ligand chelate and M‒E distance, the bonding electrons between M‒E are not reliably assigned to either atom. As such, the {M‒E}n notation tracks the number of valence electrons without assignment of valence or oxidation numbers. See ref 56 and 57. 107 Scheme 4.3 Reactivity of (TPB)Fe complexes with CO2 and formic acid. See Chart 4.1 for the detailed ligand representation. Reactivity of Fe‒OCHO species. The formate ligands in all five of the aforementioned ironformate complexes are substitutionally labile. The addition of triethylammonium chloride (10 equiv) into either benzene or methanol solutions of these complexes resulted in the formation of the respective iron-chloride complexes and triethylammonium formate (Scheme 4.2). Furthermore, the iron-chloride products from these metathesis reactions are synthons for the respective iron-hydride complexes. With this metathesis reaction and known reaction chemistry for the (SiPiPr3)Fe scaffold, we can construct a synthetic cycle for CO2 hydrogenation, which may inform the catalytic CO2 hydrogenation reaction (vide infra). Starting from the Fe‒Cl species, (SiPiPr3)FeCl reacts with MeMgCl (1 equiv) to afford the iron-methyl complex (SiPiPr3)FeMe (this work, Scheme 4.4a). Subsequent reaction with H2 affords (SiPiPr3)Fe(N2)(H) (Scheme 4.4b).43 Alternatively, the iron-methyl complex (SiPiPr3)FeMe can be converted into cationic dihydrogen complex [(SiPiPr3)Fe(H2)](BArF4)43 (Scheme 4.4c-d). The dihydrogen ligand in the latter complex can be deprotonated by triethylamine (this work) to generate (SiPiPr3)Fe(L)(H) (where L = N2 or H2) (Scheme 4.4e). As shown above, (SiPiPr3)Fe(N2)(H) reacts with CO2 to afford the 108 (SiPiPr3)Fe(OCHO) (Scheme 4.4f), which can undergo metathesis with (Et3NH)Cl to afford the starting iron-chloride complex (Scheme 4.4g). Reaction of (SiPiPr3)FeCl with H2 is also possible. A CD3OD:d8-THF (10:1) solution of (SiPiPr3)FeCl with excess triethylamine in the presence of H2 and D2 (ca. 1 atm : 1 atm) gives HD (Scheme 4.4h). Related, the cationic dihydrogen adduct [(SiPiPr3)Fe(H2)][BArF4], a model for [(SiPiPr3)Fe(H2)]+, scrambles a mixture of H2 and D2 (ca. 1 atm : 1 atm) to HD in a CD3OD:d8-THF solution (10:1). (SiPiPr3)FeCl is the sole observed iron-containing species by 1 H NMR spectroscopy in the former experiment, indicating that the equilibrium with the putative [(SiPiPr3)Fe(H2)]+ responsible for scrambling H2/D2 heavily favors (SiPiPr3)FeCl. Scheme 4.4 Synthetic cycle for CO2 hydrogenation to formate by (SiPiPr3)Fe. See Chart 4.1 for the detailed ligand representation. Conditions: a) MeMgCl, THF; b) H2, THF (plus N2 workup for L = N2); c) (HBArF4)(Et2O)2, C6H6; d) H2 (forward), N2 (reverse), THF; e) Et3N, THF (plus N2 workup for L = N2); f) CO2, MeOH, THF or C6H6; g) (Et3NH)Cl, C6H6 or MeOH; h) 1:1 atm H2:D2, Et3N, 10:1 CD3OD:d8-THF (HD is produced); i) for L = N2, Et3NHCl, 10:1 CD3OD:d8-THF. 109 4.3.3 Catalytic Hydrogenation Having realized a synthetic cycle for CO2 hydrogenation to formate, the next step was to explore if the process could be made catalytic. Following literature precedent, the trisphosphino-iron-chloride complexes were tested in an initial screen for catalysis,9-16,18-26,28 and triethylamine was added to serve as a base.60 1H NMR spectroscopy with DMF added as an integration standard was used to quantify triethylammonium formate yields. Other known products of CO2 hydrogenation are formate esters such as methylformate obtained from the esterification of formate with methanol.19-20 Due to the volatility and low yields of MeOCHO, GC-FID was used to quantify this product. Under the standardized reaction conditions of 29 atm of CO2 and 29 atm of H2 in methanol solvent with triethylamine, (SiPiPr3)FeCl, (SiPPh3)FeCl, and (PhBPiPr3)FeCl are precatalysts for the hydrogenation of CO2 to triethylammonium formate and methylformate (Table 4.2, entries 1-3). (SiPPh3)FeCl is the most active, having an average turnover number of 200. These three systems are also more selective for (Et3NH)(OCHO) than MeOCHO, with (PhBPiPr3)FeCl being the most selective of the three with a 10:1 (Et3NH)(OCHO) to MeOCHO product ratio. It is also worth noting that the primary coordination sphere of the zwiterionic (PhBPiPr3)Fe system is structurally similar to a known cationic ruthenium system (triphos)Ru (triphos = CH3C(CH2PPh2)3) that hydrogenates CO2 to methanol61 and also dehydrogenates formic acid62 (Chart 4.1). We have also reported the reduction of CO2 to oxalate by (PhBPiPr3)Fe.63 110 Table 4.2 Tris-phosphino-iron catalyzed CO2 hydrogenation. Entry Precatalyst TONa (Et3NH)(OCHO):MeOCHO ratioe 1 (SiPiPr3)FeCl 53 3:1 2 (SiPPh3)FeCl 200 2:1 3 (PhBPiPr3)FeCl 27 10:1 4 [(NPiPr3)FeCl](PF6) 0 0 5 (TPB)FeCl 0 0 6 (CPiPr3)FeCl 27 6:1 7 (CSiPPh3)FeCl 0 0 8 PP3/Fe(BF4)2b,c 486 3:1 9 [(tetraphos)FeF]BF4b,d 1661 1:1 10 FeCl2 0 0 11 FeCl2/4 PPh3 0 0 12 no iron 0 0 Conditions: 0.1 mol % (0.7 mM) iron-precatalyst (relative to Et3N), methanol, 651 mM Et3N, 29 atm CO2 (RT), 29 atm H2 (RT), 100 °C, 20 h. aTurnover number: combined yield (moles) of (Et3NH)(OCHO) and MeOCHO divided by moles of precatalyst. b Previously studied under slightly different conditions. cSee ref 19. dSee ref 20. eRatio of the amount of (Et3NH)(OCHO) product to the amount of MeOCHO product. Under the standard conditions, [(NPiPr3)FeCl](PF6) and (TPB)FeCl are not precatalysts for the reaction (Table 4.2, entries 4 and 5). The recently reported (CPiPr3)FeCl complex (Chart 4.1),42 where the silicon atom in (SiPiPr3)FeCl is substituted by a carbon atom, is also catalytically competent (Table 4.2, entry 6) but is significantly less active than (SiPPh3)FeCl. 111 Another carbon variant of the tris-phosphino-iron series of complexes, (CSiPPh3)FeCl47 (Chart 4.1), is not catalytically competent (Table 4.2, entry 7). For a direct comparison with known iron CO2 hydrogenation catalysts and as a benchmark of the method employed, we subjected the (PP3)Fe19 and (tetraphos)Fe20 systems to our standardized conditions. Beller and Laurenczy reported that a mixture of the PP3 ligand with Fe(BF4)2 is one of the more active conditions for CO2 hydrogenation in the PP3 system. Under the standard conditions of this study, a 1:1 mixture of PP3 and Fe(BF4)2 hydrogenates CO2 to triethylammonium formate and methylformate, as well, at a total TON of 486 (Table 4.2, entry 8). The (tetraphos)Fe(F)(BF4)2 complex also catalyzes CO2 hydrogenation to the same products at a total TON of 1661 (Table 4.2, entry 9). These values are in near agreement with the respective literature reports. The (tetraphos)Fe(F)(BF4)2 complex is the least selective of the series in Table 4.2 for formate production. A series of control experiments were performed to probe the homogeneity of the reaction. The catalytic reaction is uninhibited by the addition of elemental mercury (see Appendix 3). Also, CO2 hydrogenation does not occur with the iron salt FeCl2 (Table 4.2, entry 10) or with a 1:4 mixture of FeCl2 and triphenylphosphine (Table 4.2, entry 11), nor does it proceed in the absence of an iron source (Table 4.2, entry 12). These experiments do not preclude a role for heterogeneous species, but provide evidence consistent with a homogeneous process. To gain insight into the reaction, we chose to study the hydrogenation catalysis by the (SiPiPr3)Fe system further, as it is more active than (PhBPiPr3)FeCl, and because its 112 coordination chemistry has been studied in greater detail than that of its phenyl-analogue (SiPPh3)Fe.43-44 Under standard conditions but in the absence of H2, triethylammonium formate and methylformate were not detected (Table 4.3, entry 1). When the reaction was run in CD3OD instead of CH3OH, (Et3NH)(OCHO) was detected by 1H NMR spectroscopy at the conclusion of the reaction, while (Et3ND)(OCDO) was not detected by 2H NMR spectroscopy (Table 4.3, entry 2). The data collectively indicate that H2 is the source of the H-atom equivalents. High pressures of CO2 and H2 are critical, as the reaction does not proceed at or near atmospheric pressures of H2 and CO2, in agreement with most literature examples (see Appendix 3).3,9-16,18-26,28 Reducing the CO2 pressure to 5.5 atm while keeping the H2 pressure at 29 atm only modestly decreases the overall TON (Table 4.3, entry 13), but significantly increases the selectivity for formate. Reducing the H2 pressure to 5.5 atm while keeping the CO2 pressure at 29 atm stops catalysis (Table 4.3, entry 14). Also critical is methanol, as catalytic activity does not occur in THF under any pressures of CO2 and H2 studied here (see Appendix 3), highlighting the importance of polar, protic solvents in phosphine-iron CO2 hydrogenation catalysis.60 It was determined that 100 °C and 20 h are optimal for the catalytic reaction under the conditions studied here. Running the reaction at 150 °C slightly reduces the turnover relative to the standard conditions (Table 4.3, entry 3), which is likely a result of catalyst decomposition (vide infra). At 20 °C no reaction occurs (Table 4.3, entry 4), and the starting precatalyst, (SiPiPr3)FeCl, is the only iron-containing species at the end of the reaction. Decreasing the 113 Table 4.3 (SiPiPr3)FeCl catalyzed CO2 hydrogenation under varied conditions. Deviation from Standard Conditionsa Entry a TONb (Et3NH)(OCHO):MeOCHO ratiof 0 none 53 3:1 1 0 atm H2 0 0 2c CD3OD 32 2:1 3 150 °C 40 2:1 4 20 °C 0 0 5 2h 16 1:0 6 0.5 equiv (Et3NH)Cld 41 5:1 7 0.5 equiv NaBF4d 93 6:1 8 0.5 equiv NaBArF4d 69 2:1 9 0.5 equiv NaFd 45 8:1 10 0.5 equiv TBAFd,e 33 12:1 11 0.5 equiv CsFd 26 9:1 12 0.5 equiv K2CO3d 57 21:1 13 5.5 atm CO2 47 14:1 14 5.5 atm H2 0 0 Standard conditions: 0.1 mol % (0.7 mM) iron-precatalyst (relative to Et3N), methanol, 651 mM Et3N, 29 atm CO2 (RT), 29 atm H2 (RT), 100 °C, 20 h. bTurnover number: combined yield (moles) of (Et3NH)(OCHO) and MeOCHO divided by moles of precatalyst. c(Et3NH)(OCHO) was detected by 1H NMR spectroscopy, but neither (Et3ND)(OCDO), (Et3NH)(OCDO), nor (Et3ND)(OCHO) was detected by 2H NMR spectroscopy. dRelative to moles of (SiPiPr3)FeCl. eTBAF = tetrabutylammonium fluoride. fRatio of the amount of (Et3NH)(OCHO) product to the amount of MeOCHO product. 114 reaction time to 2 h at 100 °C reduces the TON by a factor of three (Table 4.3, entry 5) compared to the standard conditions. Using the stoichiometric reactions as a guide, the effects of additives and precatalysts on the catalysis was also probed. The stoichiometric metathesis reaction for the transformation of (SiPiPr3)Fe(OCHO) to (SiPiPr3)FeCl suggests that chloride substitution for formate may be a route for formate release. However, the addition of 0.5 equiv of (Et3NH)Cl (relative to iron) into the reaction reduces the TON, although the selectivity for (Et3NH)(OCHO) over MeOCHO slightly increases to 5:1 (Table 4.3, entry 6). It appears that while chloride may substitute for formate, excess chloride may also slow dihydrogen substitution at iron (vide infra) and reduces the overall TON. The addition of a non-coordinating anion in the form of NaBF4 to the catalytic mixture is beneficial, yielding a TON of 93 and 6:1 selectivity for (Et3NH)(OCHO) (Table 4.3, entry 7), while the addition of Na(BArF4) (BArF4 = B(3,5-(CF3)2C6H3)4) only modestly increases the TON to 69 and without significantly affecting selectivity (Table 4.3, entry 8). The origin of the effect from the Na+ and/or borate anion is not understood, but one effect may be that the Na+ facilitates the removal of the inner-sphere chloride as NaCl. Additionally, alkali metals are known to facilitate CO2 coordination to cobalt centres.64 It is also noteworthy that BF4- is the counter anion of the highly active tetraphosphine-iron (PP3)Fe19 and (tetraphos)Fe20 systems and also beneficial for iron catalyzed formic acid dehydrogenation.65 It is unlikely that fluoride, which may be a decomposition product of BF4-, is the source of the positive response, as fluoride-salts decrease the TON but increase the selectivity for (Et3NH)(OCHO) (Table 4.3, entries 9-11). Finally, the 115 addition of K2CO3, which has been reported to enhance CO2 hydrogenation catalysis for some noble- and non-noble metal systems,66 has no effect on the TON but most significantly increases selectivity for (Et3NH)(OCHO) compared to the other additives (Table 4.3, entry 12). The additives containing coordinating anions are more selective for (Et3NH)(OCHO) over MeOCHO, presumably as a result of anion coordination inhibiting iron-catalyzed esterification of formate to methylformate.67 However, it should be cautioned that these results are qualitative. A systematic study of the effects of these and other additives on catalysis would be warranted to draw quantitative conclusions. Other important factors known to affect catalysis are the base identity26 and base concentrations.60 A careful study of the effect of different bases and concentrations on catalysis in the present series is beyond the scope of this report, but note that the pKa of triethylamine is suitably matched to the pKa of [(SiPiPr3)Fe(H2)]+ (vide supra), an intermediate in the catalytic cycle of the (SiPiPr3)Fe system (Scheme 4.5; vide infra). Other (SiPiPr3)Fe species are also competent precatalyst. The iron-formate (SiPiPr3)Fe(OCHO) and iron-hydride complex (SiPiPr3)Fe(N2)(H) are each catalytically competent precatalyst (Scheme 4.4, entries 1 and 2), with TON’s comparable to (SiPiPr)FeCl. The cationic dinitrogen complex [(SiPiPr3)Fe(N2)](BArF4), which is a synthon for (SiPiPr3)Fe(N2)(H) in the presence of H2 and triethylamine (Scheme 4.4), is also a catalytically competent precatalyst (Scheme 4.4, entry 3). Finally, a mixture of the 1:1 free ligand HSiPiPr3 and FeCl2 is significantly less catalytically competent than the synthesized iron complex 116 (SiPiPr3)FeCl (Scheme 4.4, entry 4). All four of these precatalysts are more selective than (SiPiPr3)FeCl for (Et3NH)(OCHO). Table 4.4 CO2 hydrogenation catalyzed by various (SiPiPr3)Fe species. Entry Precatalyst TONa (Et3NH)(OCHO):MeOCHO ratioc 0 (SiPiPr3)FeCl 53 3:1 1 (SiPiPr3)Fe(OCHO) 52 15:1 2 (SiPiPr3)Fe(N2)(H) 47 3:1 3 [(SiPiPr3)Fe(N2)](BArF4) 18 8:1 4 HSiPiPr3/FeCl2 (1:1)b 12 4:1 Conditions: 0.1 mol % (0.7 mM) iron-precatalyst (relative to Et3N), methanol, 651 mM Et3N, 29 atm CO2 (RT), 29 atm H2 (RT), 100 °C, 20 h. aTurnover number: combined yield (moles) of (Et3NH)(OCHO) and MeOCHO divided by moles of precatalyst. b1:1 mixture of HSiPiPr3:FeCl2 (0.7 mM) was used as the precatalyst in place of (SiPiPr3)FeCl. cRatio of the amount of (Et3NH)(OCHO) product to the amount of MeOCHO product. The fate of the iron precatalyst (SiPiPr3)FeCl under the reaction conditions was also probed. At the end of the reaction under standard conditions, the 31P NMR spectrum showed a mixture of phosphorous-containing material, including significant quantities of free ligand (HSiPiPr3). If the reaction was run at room temperature, only the starting precatalyst (SiPiPr3)FeCl was observed by 1H NMR spectroscopy. These observations indicate that while the catalysis requires heating, elevated temperatures lead to eventual catalyst decomposition. 117 Scheme 4.5 Proposed catalytic cycle for (SiPiPr3)Fe in MeOH. See Chart 4.1 for detailed ligand representation. A possible catalytic cycle based in part on the observed stoichiometric reactions discussed in Scheme 4.4 is proposed in Scheme 4.5 for the (SiPiPr3)Fe system. Starting from precatalyst (SiPiPr3)FeCl in Scheme 4.5, dihydrogen substitution forms the cationic dihydrogen adduct [(SiPiPr3)Fe(H2)]+. The viability of this H2 for Cl- substitution step is demonstrated by H/D scrambling experiments discussed above. The cationic dihydrogen adduct [(SiPiPr3)Fe(H2)]+ in the catalytic cycle can be deprotonated by triethylamine to give (SiPiPr3)Fe(H2)(H), as was observed in the stoichiometric reaction (Scheme 4.4e). The reverse of this reaction is also possible: a 1:1 mixture of (Et3NH)Cl and (SiPiPr3)Fe(H2)(H) reacts to afford (SiPiPr3)FeCl (Scheme 4.4i). The iron-hydride intermediate (SiPiPr3)Fe(H2)(H) can then react with CO2 to form the ironformate complex (SiPiPr3)Fe(OCHO), which can subsequently react with (Et3NH)Cl and 118 reform (SiPiPr3)FeCl and release (Et3NH)(OCHO). The direct conversion of (SiPiPr3)Fe(OCHO) to [(SiPiPr3)Fe(H2)]+ may also be a viable pathway, as chloridefree[(SiPiPr3)Fe(N2)](BArF4), (SiPiPr3)Fe(N2)(H), and (SiPiPr3)Fe(OCHO) are catalytically competent. An alternative mechanism involving an iron-dihydride species cannot be ruled out but is unlikely for the (SiPiPr3)Fe system (Scheme 4.6a). A similar mechanism was proposed by Beller et al. for the cationic (tetraphos)Fe catalyst based on in situ NMR data, where the intermediate [(tetraphos)Fe(H2)(H)]+ was deprotonated by Et3N to give (tetraphos)Fe(H)2 (Scheme 4.6b).20 This iron-dihydride intermediate was suggested to react with CO2 to give the iron-hydrido-formate intermediate (tetraphos)Fe(H)(OCHO). However, note that a dihydride intermediate in the (SiPiPr3)Fe system would be unlikely because the analogous deprotonaton of (SiPiPr3)Fe(H2)(H) would form an anionic iron-dihydride species “[(SiPiPr3)Fe(H)2]-”, which is likely to be thermodynamically inaccessible. For example, a solution of (SiPiPr3)Fe(H2)(H) with excess triethylamine is stable for hours at 90 °C. While this does not rule out the possibility of an equilibrium mixture of (SiPiPr3)Fe(H2)(H) and [(SiPiPr3)Fe(H)2]-, heavily favoring the neutral monohydride species, and note that the estimated pKa of the H2 and Hligands in (SiPiPr3)Fe(H2)(H) is greater than 45 in THF,49 vastly higher than for triethylamine ([Et3NH]+ pKa = 12.5 in THF).68 It is of interest to compare the (SiPR3)Fe system to the catalytically incompetent (TPB)Fe system, since (TPB)(μ‒H)Fe(N2)(H) is an olefin hydrogenation catalyst.44 A key step that may be required for catalysis is the substitution of Cl- by H2 in (SiPiPr3)FeCl to give the cationic dihydrogen adduct [(SiPiPr3)Fe(H2)]+. Deprotonation of the dihydrogen ligand in a C6D6:d8- 119 Scheme 4.6 Dihydride pathways for catalytic CO2 hydrogenation. See Chart 4.1 for detailed representation of the ligands indicated. THF mixture by triethylamine leads to the CO2 reactive iron-hydride complex (SiPiPr3)Fe(H2)(H). The initial H2 substitution step, therefore, is critical towards forming (SiPiPr3)Fe(H2)(H). However, the {Fe‒B}7 complexes (TPB)Fe(OCHO) and (TPB)FeCl do not react with H2 (4 atm). Related, the previously reported {Fe‒B}7 [(TPB)Fe](BArF4) complex,41 which has a vacant fifth coordination site, does not react with H2 in the presence of excess triethylamine under 1 atm of H2 at 90 °C for 12 h. Furthermore, [(TPB)Fe](BArF4) does not hydrogenate CO2 under the catalytic conditions (see Appendix 3). [(NPiPr3)FeCl](PF6) is not a hydrogenation precatalyst for possibly the same reason. Qualitatively, it appears that the inability of these latter systems to coordinate H2, presumably a reflection of their weaker 120 ligand-field (LF) strengths by comparison to the SiPR3 system and hence their tendency to populate high spin configurations ([(NPiPr3)FeCl](PF6), S = 2;46 (TPB)FeCl, S = 3/258), limits their efficacy towards CO2 hydrogenation by comparison with the (SiPR3)Fe system ((SiPiPr3)FeCl, S = 155). An additional factor preventing catalysis in the (TPB)Fe system is the unproductive loss of 0.5 equiv of H2 following the reaction of (TPB)(μ‒H)Fe(N2)(H) with CO2, which generates the catalytically incompetent (TPB)Fe(OCHO) (Scheme 4.3). 4.3.4 Influence of Hydricity on the Reaction with CO2 Based only on the hydricity (54.3 ± 0.9 kcal/mol), the reaction of (SiPiPr3)Fe(H2)(H) with CO2 to afford formate (ΔGH- = 43 kcal/mol) is endergonic by over 10 kcal/mol. However, comparisons of only the hydricities of the iron-hydride and formate neglect to take into account the observed formate coordination to iron (Scheme 4.2). To estimate the free energy afforded by formate coordination to iron, the formate binding constant was determined by UVvis titration for the reaction of [(SiPiPr3)Fe(N2)](BArF4) and Li(OCHO) to (SiPiPr3)Fe(OCHO) (Scheme 4.7, equation 4.7). The titration in THF indicates that the binding constant of formate to the iron complex is on the order of 106 M-1. This is equivalent to ΔG < -8 kcal/mol for formate binding. Thus, the added driving force from formate coordination brings the free energy change for the reaction of (SiPiPr3)Fe(H2)(H) and CO2 to form (SiPiPr3)Fe(OCHO) to about 3 kcal/mol (Scheme 4.7, equation 4.10; from the sum of equations 4.7-4.9). This is thermally accessible at the elevated temperatures at which the stoichiometric and catalytic reactions are run. 121 Scheme 4.7 Gibbs free energies for the reactions of relevance to CO2 hydrogenation by (SiPiPr3)Fe(H2)(H). See Chart 4.1 for the detailed ligand representation. It should be cautioned that (SiPiPr3)Fe(H2)(H) may not be the actual iron-hydride intermediate that reacts with CO2, i.e., an intermediate elementary step may occur prior to CO2 reacting with the iron complex. The hydricity of such a species is likely different than (SiPiPr3)Fe(H2)(H) owing to the trans-influencing Si-. Also note that these hydricity values are for acetonitrile, while the catalytic reactions were run in methanol. The magnitude of the difference in hydricities between formate and metal-hydrides is known to decrease upon changing from acetonitrile to water.69 A similar phenomenon may be occurring in methanol, where the difference in hydricity between (SiPiPr3)Fe(H2)(H) and formate may not be as large 122 as the values in acetonitrile. This, combined with formate coordination to iron (Scheme 4.7, equation 4.7), may in fact make this formal CO2 insertion step exergonic in methanol. 4.4 Conclusions In summary, a series of tris-phosphino-iron-hydride complexes, including (SiPR3)Fe(L)(H), (PhBPiPr3)Fe(H)3(PMe3), [(NPiPr3)Fe(N2)(H)](PF6), and (TPB)(μ‒H)Fe(N2)(H), have been studied in the context of CO2 hydrogenation. These iron-hydride complexes react with CO2 to afford iron-formate complexes which can undergo metathesis with triethylammonium chloride to release triethylammonium formate and well-defined iron-chloride complexes, which are themselves synthons for the CO2-reactive iron-hydride complexes (Scheme 4.2). Under the catalytic conditions under elevated pressures of H2 and CO2, (SiPiPr3)FeCl, (SiPPh3)FeCl, and (PhBPiPr3)FeCl are precatalysts for catalytic CO2 hydrogenation to formate and methylformate (Scheme 4.2). (CPiPr3)FeCl, in which carbon replaces the silicon-atom in (SiPiPr3)FeCl, was also a competent catalyst. The catalytic reactions proceeded in methanol but not in THF, highlighting the importance of solvent in the catalytic reaction. 60 As depicted in Scheme 4.5, dihydrogen substitution into (SiPiPr3)FeCl or (SiPiPr3)Fe(OCHO) to form [(SiPiPr3)Fe(H2)]+ followed by deprotonation to form the CO2reactive (SiPiPr3)Fe(H2)(H) are key steps in the catalytic cycle and determine catalytic competency. The proposed mechanism for (SiPiPr3)Fe also differs from the mechanism for the highly active (tetraphos)Fe system, which proceeds through a dihydride intermediate. Finally, the hydricity value of an iron-hydride species has also been experimentally determined. The hydricity of (SiPiPr3)Fe(H2)(H) is 54.3 ± 0.9 kcal/mol in acetonitrile, and the 123 estimated pKaMeCN of the related conjugate acid [(SiPiPr3)Fe(H2)](BArF4) is 15.9 ± 0.7. Despite the low hydricity, (SiPiPr3)Fe(H2)(H) hydrogenates CO2 to formate, and part of the driving force for the reaction is coordination of formate to the iron center. Thus, the free energy change for the reaction between (SiPiPr3)Fe(H2)(H) and CO2 to (SiPiPr3)Fe(OCHO) is only slightly uphill at 3 kcal/mol, and accessible under the reactions conditions. It will be of interest to measure the hydricities of other iron-hydrides, including within the present series of complexes, in the context of CO2 hydrogenation to better understand the factors that may lead to improved catalytic activity. 4.5 Experimental 4.5.1 General Considerations All manipulations were carried out using standard glovebox or Schlenk techniques under an N2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. Deuterated solvents and 13CO2 gas were purchased from Cambridge Isotope Laboratories, INC. The deuterated solvents were degassed and dried over activated 3 Å sieves prior to use. Unless otherwise noted, all compounds were purchased commercially and used without further purification. (SiPiPr3)Fe(N2)(H),43 (SiPiPr3)FeCl,55 [(SiPiPr3)Fe(N2)](BArF4),37 (PhBPiPr3)Fe(H)3(PMe3),45 (SiPPh3)FeCl,55 (PhBPiPr3)FeCl,38 (SiPPh3)FeMe,55 [(NPiPr3)Fe(N2)(H)](PF6),46 [(NPiPr3)FeCl](PF6),46 (TPB)(μ‒H)Fe(N2)(H),44 (CPiPr3)FeCl,42 and (CSiPPh3)FeCl47 were 124 synthesized by literature procedures. Elemental analyses were performed by Robertson Microlit Laboratories, Ledgewood, NJ. NMR spectra were recorded on Varian 300 MHz, 400 MHz, and 500 MHz spectrometers. 1 H and 13C chemical shifts are reported in ppm relative to residual solvent as internal standards. 31 P and 11B chemical shifts are reported in ppm relative to 85 % aqueous H3PO4 and BF3∙Et2O, respectively. Multiplicities are indicated by br (broad), s (singlet), d (doublet), t (triplet), quart (quart), quin (quintet), m (multiplet), d-d (doublet-of-doublets), and t-d (triplet-of-doublets). The ATR-IR measurements were performed in a glovebox on a thin film of the complex obtained from evaporating a drop of the solution on the probe surface of a Bruker APLHA ATR-IR Spectrometer (Platinum Sampling Module, diamond, OPUS software package) at 2 cm-1 resolution. IR intensities indicated by s (strong), m (medium), and w (weak). UV-vis spectra were collected on a Cary 60 UV-Vis Spectrophotometer. The titration experiments were performed in a glovebox using an Ocean Optics HR4000CG Spectrometer. H2 quantification by GC-TCD: H2 was quantified on an Agilent 7890A gas chromotograph (HP-PLOT U, 30 m, 0.32 mm i.d.; 30 °C isothermal; 1 mL/min flow rate; He carrier gas) using a thermal conductivity detector. The total amount of H2 produced was determined as the sum of H2 in the headspace plus dissolved H2 in the solution calculated by Henry’s law with a constant of 328 MPa.70 Methylformate quantification by GC-FID: Methylformate quantification was performed on a 1.2 mL aliquot of the crude reaction mixture by GC-FID against a methylformate calibration curve. GC-FID instrument: Hewlett Packard 5890 with a 57 m Restek RTX-VRX 125 column (0.32 mm inner diameter, 1.8 μm films). Method parameters: He carrier gas, 1 μL injection volume, 200 °C inlet temperature, 250 °C detector temperature, 7:1 split ratio, 2.9 mL/min flow rate, 20 psi pressure, 35 cm/s velocity. Ramp rate: 35 °C initial temperature held for 8 min, followed by 10 °C/min steps up to 100 °C, then immediately followed by 25 °C/min steps up to 230 °C, which was held for 4 min. 4.5.2 Synthetic Protocols Synthesis of (SiPiPr3)FeMe from (SiPiPr3)FeCl. A yellow solution of (SiPiPr3)FeCl (44.4 mg, 73 μmol) in THF (10 mL) was cooled to -78 °C. A solution of MeMgCl (24 μL of a 3 M THF solution, 73 μmol) was diluted with THF (1 mL) and then added dropwise to the stirring reaction, causing a gradual change to a red solution. The stirring solution was allowed to warm to room temperature overnight. The crude mixture was filtered through a glass frit to remove black precipitate, and the volatiles were removed in vacuo to reveal a red solid. The material was taken up in a minimal amount of pentane and allowed to sit at -35 °C overnight, revealing red crystals of (SiPiPr3)FeMe (11.3 mg, 22 %). The 1H and 31P NMR spectra of this material were identical to the reported spectra.55 Synthesis of (SiPiPr3)Fe(OCHO). A yellow THF solution (10 mL) of (SiPiPr3)Fe(N2)(H) (50 mg, 72 μmol) was degassed by freeze-pump-thaw cycles (3x). Subsequently, CO2 (1 atm) was introduced to the thawed solution. The reaction was sealed and then heated for 1 h at 50 °C to give a yellow solution. The volatiles were removed in vacuo to give a yellow solid. The material was extracted with C6H6, and lyophilized to give (SiPiPr3)Fe(OCHO) as a yellow solid (46 mg, 90 %). Analytically pure material was obtained by layering a concentrated solution of 126 (SiPiPr3)Fe(OCHO) in THF (1 mL) under HMDSO (5 mL) and allowing the solution to sit at 35 °C for 3 days. 1H NMR (C6D6, 400 MHz): δ 12.2, δ 12.1, δ 4.7, δ 4.6, δ 1.3, δ 0.2, δ -2.0, δ 2.2, δ -4.9. μeff (C6D6, method of Evans, 20 °C): 2.8 μB (S = 1). IR (thin film, cm-1) 1623 (m, νasym (O‒C‒O)). UV-vis (THF, nm {M-1 cm-1}) 357 {shoulder, 3247}, 426 {2243}, 478 {253}, 963 {br abs starting at 884 nm, 451}. Anal.: Calc’d for C37H55FeO2P3Si: C, 62.71; H, 7.82. Found: C, 61.81; H, 7.24. Synthesis of (SiPiPr3)Fe(O13CHO). The procedures used to synthesize (SiPiPr3)Fe(OCHO) were used here, except that 13CO2 was used in place of CO2. The 1H NMR spectrum was identical to (SiPiPr3)Fe(OCHO). IR (thin film, cm-1): 1583 (m, νasym(O‒13C‒O)). Synthesis of (SiPPh3)Fe(N2)(H). A procedure nearly identical to that used to synthesize (SiPiPr3)Fe(N2)(H) was used to synthesize (SiPPh3)Fe(N2)(H). In a 100 mL Schlenk tube a red solution of (SiPPh3)FeMe (26.3 mg, 30 μmol) in THF (20 mL) was degassed by freeze-pumpthaw cycles (3x). H2 gas (1 atm) was charged into the thawed solution. The reaction was then sealed and heated to 60 °C for over a week. The reaction was then filtered through Celite and volatiles were removed in vacuo to give a light yellow powder. The solid was collected on a glass-frit and washed with pentane (3 mL x 2). The resulting product (SiPPh3)Fe(N2)(H) (24.4 mg, 91 %) was obtained as a light yellow powder after drying under vacuum. Layering a THF solution of (SiPPh3)Fe(N2)(H) under Et2O and letting the solution stand for 2 days yields analytically pure powder of (SiPPh3)Fe(N2)(H). 1H NMR (C6D6, 300 MHz): δ 8.55 (2H, d, 2JHH = 6 Hz, Ar-H), δ 8.32 (2H, d, 2 JH-H = 3 Hz, Ar-H), δ 7.62 (3H, br s, Ar-H), δ 7.45 (2H, br s, Ar-H), δ 7.34 (4H, d, 2JH-H = 6 Hz, Ar-H), δ 6.85 (2H, t, 2JH-H = 6 Hz, Ar-H), δ 6.69 (3H, q, 2JH- 127 H = 3 Hz, Ar-H), δ 6.52 (2H, q, 2 JH-H = 3 Hz, Ar-H), δ -11.88 (1H, t-d, 2JPcis-H = 54 Hz, 2JPtrans-H = 12 Hz, Fe-H). 31P NMR (C6D6, 121 MHz): 85.3 (2P, s), 78.7 (1P, s). 13C NMR (THF with 1 drop of C6D6, 125 MHz): δ 156.3 (d, JC-P = 38 Hz, CAr), δ 155.8 (d, JC-P = 35 Hz, CAr), δ 150.7 (d, JC-P = 5 Hz, CAr), δ 150.4 (s, CAr), δ 150.2 (s, CAr), δ 143.0 (s, CAr), δ 141.6 (d, J = 23 Hz, CAr), δ 141.0 (s, CAr), δ 139.6 (d, J = 28 Hz, CAr), δ 138.6 (d, J = 10, CAr), δ 133.8 (s, CAr) δ 132.6 (s, CAr), δ 132.3 (s, CAr), δ 129.5 (s, CAr), δ 128.4 (s, CAr), δ 128.2 (s, CAr), δ 128.1 (s, CAr), 127.5 (d, J = 5 Hz, CAr). IR (thin film, cm-1): 2073 (s, ν(N≡N)), 1889 (w, ν(Fe‒H)). UVvis (THF, nm {M-1 cm-1}) 335 {shoulder, 8125}, 437 {shoulder, 4500}. Anal.: Calc’d for C54H43FeN2P3Si: C, 72.32; H, 4.83; N, 3.12. Found: C, 72.94; H, 5.22; N, 2.83. Synthesis of (SiPPh3)Fe(OCHO). A yellow THF solution (10 mL) of (SiPPh3)Fe(N2)(H) (51 mg, 57 μmol) was degassed by freeze-pump-thaw cycles (3x). CO2 (1 atm) was introduced to the thawed solution. The reaction was sealed and then heated for 1 h at 50 °C to give a yellow solution. The volatiles were removed in vacuo to give a yellow solid. The material was redissolved in C6H6 and filtered through a pipet filter to remove a small amount of black material. The filtrate was lyophilized in vacuo to give (SiPPh3)Fe(OCHO) as a yellow solid (41 mg, 79 %). Analytically pure material was obtained by layering a concentrated THF solution of (SiPPh3)Fe(OCHO) (3 mL) under pentane (5 mL) and allowing it to stand for 2 days at RT. 1 H NMR (3:2 mixture of C6D6:d8-THF, 300 MHz): δ 12.2, δ 6.5, δ 5.7, δ 4.8, δ -2.1, δ -4.7. μeff (d8-THF, method of Evans, 20 °C): 2.7 μB (S = 1). IR (thin film, cm-1) 1618 (m, νasym(O‒C‒O)), 1316 (m, νsym(O‒C‒O)). UV-vis (THF, nm {M-1 cm-1}) 325 {shoulder, 4775}, 128 415 {4100}, 474 {3700}, 995 {br abs starting at 900 nm, 263}. Anal.: Calc’d for C55H43FeO2P3Si: C, 72.37; H, 4.75. Found: C, 73.21; H, 5.48. Synthesis of (SiPPh3)Fe(O13CHO). The same procedures used to synthesize (SiPPh3)Fe(OCHO) were used here, except that 13CO2 was used in place of CO2. The 1H NMR spectrum of (SiPPh3)Fe(O13CHO) was identical to (SiPPh3)Fe(OCHO). IR (thin film, cm-1): 1587 (m, νasym(O‒13C‒O)), 1254 (m, νsym(O‒13C‒O)). Synthesis of (PhBPiPr3)Fe(OCHO). A yellow THF solution (1 mL) of (PhBPiPr3)Fe(H)3(PMe3) (6.7 mg, 12 μmol) was degassed by freeze-pump-thaw cycles (3x). Subsequently, CO2 (1 atm) was introduced to the thawed solution. The reaction was sealed and then stirred for 12 h at room temperature to give a light yellow solution. The volatiles were removed in vacuo to give an light yellow solid. The material was triterated with pentane, and the solvent was removed in vacuo. The material was then redissolved in C6H6 (3 mL) and filtered through a glass frit to remove a black solid. Removal of the solvent in vacuo gave (PhBPiPr3)Fe(OCHO) (4.7 mg, 70 %) as a light yellow solid. Analytically pure material was obtained by layering HDMSO on top of a THF solution of (PhBPiPr3)Fe(OCHO) and allowing it to stand overnight. 1H NMR (C6D6, 300 MHz): δ 41.1, δ 19.9, δ 18.6, δ 13.5, δ 9.2, δ 4.5, δ 3.6, δ 1.6, δ -1.2, δ -11.2, δ -12.1, δ -32.6, δ -37.7. μeff (C6D6, method of Evans, 20 °C): 5.0 μB (S = 2). IR (thin film, cm-1) 1595 (m, νasym(O‒C‒O)), 1362 (m, νsym(O‒C‒O)). UV-vis (THF, nm {M-1 cm-1}) 298 {1173}, 410 {274}. Anal.: Calc’d for C28H54FeO2P3: C, 57.75; H, 9.35. Found: C, 58.12; H, 9.67. 129 Synthesis of (PhBPiPr3)Fe(O13CHO). The same procedures used to synthesize (PhBPiPr3)Fe(OCHO) were used here, except that 13CO2 was used in place of CO2. The 1H NMR spectrum of (PhBPiPr3)Fe(O13CHO) was identical to (PhBPiPr3)Fe(OCHO). IR (thin film, cm-1): 1546 (m, νasym(O‒13C‒O)), 1355 (m, νsym(O‒13C‒O)). Synthesis of [(NPiPr3)Fe(OCHO)](PF6). A yellow THF solution (10 mL) of [(NPiPr3)Fe(N2)(H)](PF6) (29 mg, 40 μmol) was degassed by freeze-pump-thaw cycles (3x). Subsequently, CO2 (1 atm) was introduced. The reaction was then sealed and stirred for 3 h at room temperature to give a colorless solution. The solvent was removed in vacuo to give a colorless solid. The material was triturated with pentane, and the solvent was removed in vacuo. The solid was washed with diethyl ether (3 x 1 mL) to give [(NPiPr3)Fe(OCHO)](PF6) (29 mg, 97 %) as a white solid. Analytically pure material was obtained by layering Et2O on top of a THF solution of [(NPiPr3)Fe(OCHO)](PF6) and allowing it to stand overnight at -35 °C. 1H NMR (3:2 mixture of C6D6:d8-THF, 300 MHz): δ 27.6, δ 9.4, δ 8.9, δ 6.4, δ 2.1, δ 1.9, δ 0.4, δ -8.3. 31P NMR (3:2 mixture of C6D6:d8-THF, 121 MHz): δ -144.4 (h, 1JP-F = 708 Hz, PF6). 19F NMR (3:2 mixture of C6D6:d8-THF, 282 MHz): -73.4 (d, 1JP-F = 710 Hz, PF6). μeff (C6D6, method of Evans, 20 °C): 5.1 μB (S = 2). IR (thin film, cm-1) 1613 (m, νasym(O‒C‒O)). UV-vis (THF, nm {M-1 cm-1}) 311 {shoulder, 660}, 379 {shoulder, 249}. Anal.: Calc’d for C25H55F6FeNO2P4: C, 43.18; H, 7.97; N, 2.01. Found: C, 44.10; H, 8.25; N, 1.86. Synthesis of [(NPiPr3)Fe(O13CHO)](PF6). The same procedures used to synthesize [(NPiPr3)Fe(OCHO)](PF6) were used here, except that 13CO2 was used in place of CO2. The 1H 130 NMR spectrum of [(NPiPr3)Fe(O13CHO)](PF6) was identical to [(NPiPr3)Fe(OCHO)](PF6). IR (thin film, cm-1): 1579 (m, νasym(O‒13C‒O)). Synthesis of (TPB)FeCl. The procedures used to synthesize (TPB)FeBr58 were used to synthesize (TPB)FeCl, except that FeCl2 was used in place of FeBr2. A Schlenk tube was charged with TPB (117 mg, 172 μmol), FeCl2 (26 mg, 200 μmol), Fe powder (113 mg, 2000 μmol), and THF (20 mL). The reaction was heated to 90 °C for 3 days with vigorous stirring, resulting in a color change of the liquid phase from light yellow to dark green-brown. The remaining iron powder was removed by filtration, and the solvent was removed in vacuo. The residue was taken up in toluene (5 mL), and the solvent was removed in vacuo. Pentane (200 mL) was added, and the mixture was stirred for 3 h and filtered. Removal of the solvent in vacuo yielded a yellow-brown powder of (TPB)FeCl (123 mg, 91 %). 1H NMR (C6D6, 300 MHz): δ 97.6, δ 35.1, δ 23.6, δ 9.6, δ 5.8, δ 3.4, δ 1.9, δ -0.2 δ -2.3, δ -22.5. μeff (C6D6, method of Evans, 20 °C): 4.1 μB (S = 2). UV-vis (THF, nm {M-1 cm-1}) 275 {14086}, 317 {10385}, 556 {sh, 80}, 774 {66}, 897 {91}. Anal.: Calc’d for C36H54BClFeP3: C, 63.41; H, 7.98. Found: C, 64.06; H, 8.89. Synthesis of (TPB)Fe(OCHO). A yellow benzene solution (6 mL) of (TPB)(μ‒H)Fe(N2)(H) (20.7 mg, 31 μmol) was degassed by freeze-pump-thaw cycles (3x). Subsequently, CO2 (1 atm) was introduced. The reaction was then sealed and the yellow solution was mixed for 1 h at room temperature. The solvent was lyophilized in vacuo to give (TPB)Fe(OCHO) as a dark yellow solid (21.0 mg, 99 %). Analytically pure material was obtained by cooling a concentrated pentane solution of (TPB)Fe(OCHO) to -35 °C overnight. 131 1 H NMR (C6D6, 300 MHz): δ 86.1, δ 66.3, δ 38.5, δ 26.3, δ 15.5, δ 4.3, δ 2.7, δ 1.4, δ 1.0, δ - 0.7, δ -2.6, δ -3.5, δ -24.1. μeff (C6D6, method of Evans, 20 °C): 4.2 μB (S = 2). IR (thin film, cm-1): 1627 (m, νasym(O‒C‒O)), 1291 (m, νsym(O‒C‒O)). UV-vis (THF, nm {M-1 cm-1}) 278 {16400}, 317 {12800}, 773 {br abs, 98}, 958 {127}. Anal.: Calc’d for C37H55BFeO2P3: C, 64.27; H, 8.02. Found: C, 63.16; H, 7.75. Synthesis of (TPB)Fe(O13CHO). The same procedures used to synthesize (TPB)Fe(OCHO) were used, except that 13CO2 was used in place of CO2. The 1H NMR spectrum was identical to (TPB)Fe(OCHO). IR (thin film, cm-1): 1588 (m, νasym(O‒13C‒O)), 1269 (m, νsym(O‒13C‒O)). Reaction of (TPB)Fe(N2) with formic acid. (TPB)Fe(N2) (8.2 mg, 12.1 μmol) in 2 mL of THF was charged into a round bottom flask, and the flask was sealed with a rubber septum. Formic acid (3 μL, 80.3 μmol) was added by syringe through the septum, immediately resulting in effervescence of H2 and a yellow-brown solution. The solution was allowed to stir for a few minutes before the volatiles were removed in vacuo to reveal a brown solid. The material was redissolved in benzene and filtered. Removal of the volatiles in vacuo revealed a brown powder of (TPB)Fe(OCHO) (8.1 mg, 96 %). NMR and IR spectral data for this material were identical to (TPB)Fe(OCHO). Deprotonation of [(SiPiPr3)Fe(H2)](BArF4) with Et3N. [(SiPiPr3)Fe(N2)](BArF4) (14.6 mg, 9.4 µmol) and triethylamine (1.7 µL, 9.7 µmol) were charged into an NMR tube with a Jyoung valve with C6D6 and d8-THF (ca. 0.4 mL and 0.1 mL, respectively), yielding a green solution. The solution was degassed by freeze-pump-thaw cycles (3x), revealing an orange 132 solution consistent with [(SiPiPr3)Fe(THF)](BArF4). H2 (1 atm) was charged into the reaction mixture, yielding a transient gray solution (consistent with [(SiPiPr3)Fe(H2)](BArF4) that immediately changed to orange-yellow upon mixing. The reaction was mixed overnight. The NMR data of the iron-species in this reaction mixture were identical to (SiPiPr3)Fe(H2)(H).43 The volatiles were removed in vacuo, and the resulting yellow solid was extracted with pentane. The pentane was removed in vacuo to yield a yellow solid of (SiPiPr3)Fe(N2)(H) (5.1 mg, 88 %). The 1H and 31P NMR spectra of this material are identical to (SiPiPr3)Fe(N2)(H). Quantifying H2 loss from the reaction of (TPB)(μ‒H)Fe(N2)(H) with CO2. Procedures similar to the synthesis of (TPB)Fe(OCHO) were followed. (TPB)(μ‒H)Fe(N2)(H) (20.0 mg, 31 μmol) was dissolved in 6 mL of benzene and charged into a calibrated 200 mL Schlenk tube that had a Teflon valve and 24/40 side joint. The solution was degassed by freeze-pumpthaw cycles (3x), opened to CO2 (1 atm), and agitated for ca. 5 sec to ensure adequate dissolution of CO2 into the solution. The reaction was then sealed at the Teflon valve joint and also with a rubber septum at the 24/40 joint. The reaction was stirred vigorously for 30 min, the Teflon valve was opened, and the headspace was sampled through the rubber septum with a 10 mL gastight syringe, being careful to ensure adequate mixing of the gases from reaction headspace into the 24/40 joint’s headspace by repeated extraction and reinjection (3x) of the headspace gas with the gas-tight syringe before a final aliquot was taken for analysis by GCTCD. 0.44 equiv of H2 (relative to (TPB)(μ‒H)Fe(N2)(H)) found. Quantifying H2 loss from the reaction of (TPB)Fe(N2) with formic acid. Procedures similar to the synthesis of (TPB)Fe(OCHO) from formic acid and (TPB)Fe(N2) were 133 followed. (TPB)Fe(N2) (24.5 mg, 36 μmol) in benzene (3 mL) was charged into a calibrated 100 mL round-bottomed flask and sealed with a rubber septum. Formic acid (1.3 μL, 36 μmol) was added by syringe. Effervescence was immediately visible. The reaction was allowed to stir for a few minutes before the headspace was sampled through the rubber septum with a 10 mL gastight syringe for analysis by GC-TCD. 0.42 equiv of H2 (relative to (TPB)Fe(N2)) found. Metathesis reactions of iron-formate complexes with (Et3NH)Cl. (Et3NH)Cl (10 equiv) was charged into a methanol or benzene solution of (SiPiPr3)Fe(OCHO), (SiPPh3)Fe(OCHO), (PhBPiPr3)Fe(OCHO), [(NPiPr3)Fe(OCHO)](PF6), or (TPB)Fe(OCHO). The resulting suspension was stirred overnight and then filtered through a glass frit. The filtrate was concentrated in vacuo into a solid and then extracted. For (SiPiPr3)Fe(OCHO), (SiPPh3)Fe(OCHO), (PhBPiPr3)Fe(OCHO), and (TPB)Fe(OCHO), pentane (3 x 1 mL) was used for extraction, while a 4:1 C6H6:THF mixture (3 x 1 mL) was used for [(NPiPr3)Fe(OCHO)](PF6)). The respective 1H NMR spectra and IR spectra of the extract showed conversion to (SiPiPr3)FeCl, (SiPPh3)FeCl, (PhBPiPr3)FeCl, [(NPiPr3)FeCl](PF6), or (TPB)FeCl. 4.5.3 CO2 Hydrogenation Catalysis Protocols. High pressure hydrogenation reactions were run in a Parr Instruments Company 4590 Micro Bench Top Reactor, with a 20 mL reaction vessel, controlled by a Parr Instruments Company 4834 Controller. In a typical catalytic run, iron precatalyst in 0.1 mL of THF (to solubilize iron precatalyst), 10 mL methanol, and 1 mL triethylamine was charged into the Parr reactor. The reactor was sealed, stirred with the attached mechanical stirrer (100 rpm), and charged with 134 CO2 until the desired pressure at equilibrium was achieved (ca. 10 min). H2 was subsequently added into the reactor. The gas inlet port was closed, and the reactor was then heated at 100 °C for 20 h. Changes to these conditions were made as described in Tables 4.2, 4.3, and 4.4 and Table A3.1 in Appendix 3. At the conclusion of the reaction, the reactor was cooled to 10 °C with an ice bath over ca. 1.5 h, and the pressure was slowly released through a needle valve. An aliquot of the crude solution was immediately taken for methylformate quantification by GC-FID. The aliquot was then recombined with the crude solution, DMF was added (1 mmol), and 25 μL of this solution was taken into 0.5 mL of CD2Cl2 for triethylammonium formate quantification by 1H NMR spectroscopy. Similar procedures were followed for the low pressure reactions (1 atm CO2, 1-4 atm H2), which were run in a 15 mL Schlenk tube having a Teflon valve. The solution was degassed by freeze-pump-thaw cycles (3x), and CO2 (1 atm) was introduced into the vessel at room temperature. For the reactions requiring 4 atm of H2, the entire body of the Schlenk tube was then cooled in a liquid-N2 bath and 1 atm (RT) of H2 was introduced. For reactions requiring 1 atm of H2, the Schlenk tube was cooled with liquid-N2 up to the solution level, and 1 atm (RT) of H2 was introduced. Analysis of iron-content post catalytic reaction. The reaction was worked up similar to the procedures described above for the hydrogenation runs. After depressurizing the reactor, it was brought into the glovebox for workup. The crude solution was transferred to a scintillation vial, and the volatiles were removed in vacuo. The resulting light yellow solid was dissolved in C6D6 and analyzed by 1H and 31P NMR spectroscopy. 135 4.5.4 Hydricity Determination The hydricity was experimentally determined using the method present by DuBois.50-51 The equilibrium of equation 4.3 (Scheme 4.2) with a given base (proton sponge, 2,6-lutidine, or 2,4,6-trimethylpyridine) was measured in d8-THF. With proton sponge as base, [(SiPiPr3)Fe(N2)](BArF4) (8.0 mg, 5.1 μmol) was mixed with proton sponge (1.1 mg, 5.1 μmol) and the integration standard 1,3,5-trimethoxybenzene (1.2 mg, 7.1 μmol) in d8-THF (0.5 mL). With 2,6-lutidine as base, [(SiPiPr3)Fe(N2)](BArF4) (8.7 mg, 5.6 μmol) was mixed with 2,6lutidine (34 μL, 292 μmol) and the integration standard 1,3,5-trimethoxybenzene (1.1 mg, 6.5 μmol) in d8-THF (0.5 mL). With 2,4,6-trimethylpyridine as base, [(SiPiPr3)Fe(N2)](BArF4) (8.3 mg, 5.3 μmol) was mixed with 2,4,6-trimethylpyridine (1.6 μL, 19.8 μmol) and the integration standard 1,3,5-trimethoxybenzene (1.3 mg, 7.7 μmol) with d8-THF (0.5 mL). The solutions were degassed by freeze-pump-thaw cycles (3x) and H2 (1 atm) was introduced. The solutions were mixed using a mechanical rotator at a rate of ca. 12 min-1, and the reaction was monitored by 1H NMR spectroscopy until equilibration: proton sponge, 6 days; 2,6-lutidine, 5 days; 2,4,6trimethylpyridine, 5 days. Equations 4.3-4.5 were used to calculate ΔGH-. The equilibrium between [(SiPiPr3)Fe(H2)](BArF4) and its THF-adduct [(SiPiPr3)Fe(H2)](BArF4) was also taken into account in the calculations (see Appendix 3). 4.5.5 UV-vis Titration of Formate Binding The UV-vis titration experiments of [(SiPiPr3)Fe(N2)](BArF4) (4.8 mM) with Li(OCHO) (48 mM) in THF was performed by adding aliquots of the formate solution to a solution of the iron complex. The decay of [(SiPiPr3)Fe(N2)](BArF4) was monitored, with the absorbance at 752 nm 136 used for fitting to a quadratic equation against Keq. After the addition of 1 eqv of Li(OCHO), a spectrum corresponding to (SiPiPr3)Fe(OCHO) was observed. 137 4.6 Cited References (1) In Carbon Dioxide as Chemical Feedstock; Wiley-VCH Verlag GmbH & Co. KGaA: 2010. (2) Olah, G. A. Angew. Chem. Int. Ed. 2013, 52, 104-107. (3) Enthaler, S.; von Langermann, J.; Schmidt, T. Energy Environ. Sci. 2010, 3, 1207-1217. (4) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. 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Supplementary Data for Chapter 2 143 Table A1.5 Crystal data and structure refinement for (TPB)(μ‒H)Fe(N2)(H). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Mem123a_cut C36 H56 B Fe N2 P3 676.40 100(2) 0.71073 Crystal system Space group Unit cell dimensions Monoclinic P2(1)/c a = 10.9690(10) Volume Z Density (Mg/m3; calculated) b = 15.8820(14) c = 120.7040(18) 3522.1(5) 4 1.276 Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.592 1444 0.42 x 0.17 x 0.04 Completeness to theta = 28.27° Absorption correction Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 99.6 % Multi-scan 0.7890 to 0.8767 Full-matrix least squares on F2 8699/0/406 1.035 R = 0.0505, wR2 = 0.1111 R = 0.0865, wR2 = 0.1264 Largest diff. peak and hole 0.086 and -0.619 eÅ-3 α = 90.00 β = 102.446(4) γ = 90.00 1.63 to 28.27 -14 ≤ h ≤ 14, -21 ≤ k ≤ 21, -27 ≤ l ≤ 26 64963 8699 144 Table A1.6 Crystal data and structure refinement for (TPB)(μ‒H)Fe(H2)(H). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) hf12 C36 H56 B Fe N2 P3 676.40 100(2) 0.71073 Crystal system Space group Unit cell dimensions Monoclinic P2(1)/n a = 10.6196(6) Volume Z Density (Mg/m3; calculated) b = 21.3418(13) c = 15.6111(9) 3529.4(4) 4 1.218 Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.586 1388 0.28 x 0.16 x 0.15 Completeness to theta = 25.00° Absorption correction Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 95.9 % Multi-scan 0.8530 to 0.9157 Full-matrix least squares on F2 20034/0/286 1.056 R = 0.0548, wR2 = 0.1270 R = 0.1021, wR2 = 0.1511 Largest diff. peak and hole 1.822 and -1.288 eÅ-3 α = 90.00 β = 94.019(4) γ = 90.00 1.91 to 39.27 -17 ≤ h ≤ 18, -37 ≤ k ≤ 37, -26 ≤ l ≤ 27 187522 20034 145 Table A1.7 Crystal data and structure refinement for (TPB)Fe(CNtBu). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Mem129 C43 H69 B Fe N P3 Si0.5 774.61 100(2) 0.71073 Crystal system Space group Unit cell dimensions Rhombohedral R-3 a = 11.2415(3) b = 11.2415(3) c = 59.3261(18) 6492.7(3) 6 1.187 Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.503 2502 0.37 x 0.22 x 0.05 Completeness to theta = 30.04° Absorption correction 99.9 % Multi-scan Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 0.8359 to 0.9753 Full-matrix least squares on F2 4225/88/257 1.082 R = 0.0611, wR2 = 0.1562 R = 0.0771, wR2 = 0.1683 Largest diff. peak and hole 0.079 and -1.970 eÅ-3 α = 90.00 β = 90.00 γ = 120.00 2.06 to 30.04 -15 ≤ h ≤ 15, -14 ≤ k ≤ 15, -83 ≤ l ≤ 83 46175 4245 146 Figure A1.1 XRD structure of (TPB)Fe(CNtBu). Ellipsoids shown at 50 % probability. The CNtBu unit is disordered over three positions, and therefore the Fe‒C13 and C13‒N1 bond distance are reported as an average of the three Fe‒C13 and three C13‒N1 positions, respectively. Selected bond distances (Å) and angles (°): Fe1‒P1 = 2.3193(7), Fe‒P2 = 2.3193(7), Fe‒P3 = 2.3194(7), 2.320(4), Fe‒C13(avg) = 1.861(7), C13‒N1(avg) = 1.179(9), Fe1‒B1 = 2.340(4), ∑(P‒Fe‒P) = 345.03(1). 147 Table A1.8 Crystal data and structure refinement for (TPB)(μ‒H)Fe(CNtBu)(H). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Hf15 C41 H65 B Fe N P3 731.51 100(2) 0.71073 Crystal system Space group Unit cell dimensions Orthorhombic Pna2(1) a = 28.1082(11) Volume Z Density (Mg/m3; calculated) b = 11.7188(5) c = 12.0896(5) 3982.2(3) 4 1.220 Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.528 1576 0.27 x 0.25 x 0.12 Completeness to theta = 25.00° Absorption correction Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 99.9 % Multi-scan 0.8705 to 0.9393 Full-matrix least squares on F2 17351/1/447 1.441 R = 0.0369, wR2 = 0.0807 R = 0.0531, wR2 = 0.0878 Largest diff. peak and hole 0.799 and -0.297 eÅ-3 α = 90.00 β = 90.00 γ = 90.00 2.22 to 35.72 -45 ≤ h ≤ 45, -19 ≤ k ≤ 18, -19 ≤ l ≤ 19 125477 17351 148 Table A1.9 Crystal data and structure refinement for (TPB)Fe(C2H4). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Hf16 C38 H58 B Fe P3 731.51 100(2) 0.71073 Crystal system Space group Unit cell dimensions Triclinic P-1 a = 11.7652(5) b = 18.1293(8) c = 18.8179(8) 3555.0(3) 4 1.260 Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.585 1448 0.29 x 0.24 x 0.22 Completeness to theta = 44.33° Absorption correction 98.0 % Multi-scan Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 0.8487 to 0.8821 Full-matrix least squares on F2 55462/0/831 1.030 R = 0.0428, wR2 = 0.1232 R = 0.0761, wR2 = 0.1475 Largest diff. peak and hole 2.925 and -0.791 eÅ-3 α = 112.296(2) β = 90.626(2) γ = 105.264(3) 1.88 to 44.33 -22 ≤ h ≤ 22, -35 ≤ k ≤ 35, -36 ≤ l ≤ 36 285706 55462 149 Table A1.10 Crystal data and structure refinement for (TPBH)Fe(C2Tol). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Hf36 C45 H62 B Fe N P3 761.51 100(2) 0.71073 Crystal system Space group Unit cell dimensions Monoclinic P2(1)/c a = 25.3343(11) b = 15.1553(6) c = 23.4782(1) 8287.0(6) 8 1.222 Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.510 3264 0.41 x 0.30 x 0.18 Completeness to theta = 25.00° Absorption correction 96.0 % Multi-scan Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 0.8182 to 0.9138 Full-matrix least squares on F2 41264/0/927 1.239 R = 0.0671, wR2 = 0.1565 R = 0.1171, wR2 = 0.1824 Largest diff. peak and hole 4.308 and -0.756 eÅ-3 α = 90.00 β = 113.176(2) γ = 90.00 1.60 to 37.28 -43 ≤ h ≤ 42, -25 ≤ k ≤ 25, -39 ≤ l ≤ 38 256649 41264 150 Table A1.11 Crystal data and structure refinement for D. Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Hf31 C48 H81 B Fe P4 854.44 100(2) 0.71073 Crystal system Space group Unit cell dimensions Monoclinic C2/c a = 14.7183(8) b = 22.2944(13) c = 14.8410(9) 4728.0(5) 8 1.200 Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 0.486 1840 0.33 x 0.30 x 0.24 Completeness to theta = 48.29° Absorption correction 98.9 % Multi-scan Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 0.8561 to 0.8923 Full-matrix least squares on F2 22688/0/265 0.928 R = 0.0373, wR2 = 0.0885 R = 0.0668, wR2 = 0.1027 Largest diff. peak and hole 1.332 and -0.854 eÅ-3 α = 90.00 β = 103.864(3) γ = 90.00 1.69 to 48.29 -30 ≤ h ≤ 30, -46 ≤ k ≤ 46, -30 ≤ l ≤ 30 143163 22688 151 Table A1.12 Crystal data and structure refinement for (SiPiPr3)Fe(CO(H). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) D8_08027_0m C27 H55 Fe P3 Si 692.66 100(2) 1.54178 Crystal system Space group Unit cell dimensions Monoclinic C2/c a = 36.5798(8) b = 11.7777(3) c = 17.1309(4) 7153.6(3) 8 1.286 Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections 5.715 2960 0.25 x 0.20 x 0.15 Completeness to theta = 68.95° Absorption correction 98.8 % Multi-scan Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) 0.3578 to 0.5107 Full-matrix least squares on F2 6580/0/44 1.046 R = 0.0273, wR2 = 0.0693 R = 0.0290, wR2 = 0.0703 Largest diff. peak and hole 0.439 and -0.287 eÅ-3 α = 90.00 β = 104.2430(1) γ = 90.00 2.49 to 68.95 -43 ≤ h ≤ 44, -13 ≤ k ≤ 14, -20 ≤ l ≤ 20 50397 6480 152 Figure A1.2 XRD structure of (SiPiPr3)Fe(CO)(H). Ellipsoids shown at 50 % probability. Selected bond distances (Å) and angles (°): Fe1‒P1 = 2.2559(4), Fe1‒P2 = 2.2495(4), Fe1‒P3 = 2.2214(4), Fe1‒H1 = 1.43(2), Fe1‒C11 = 1.768(2), Fe1‒Si1 = 2.2567(4), P3‒Fe1‒P1 = 106.05(2), P1‒Fe1‒P2 = 110.27(2), P2‒Fe1‒H1 = 66.4(1), P3‒Fe1‒H1 = 73.4(1). 153 Figure A1.3 Stacked 1H NMR spectra of the reaction of (TPB)Fe(C2H4) with H2 (1 atm) and C2H4 (1 atm) in C6D6. (I) Compound (TPB)Fe(C2H4) under a C2H4 atmosphere. (II) Mixture of compounds (TPB)Fe(C2H4) and A immediately following H2 addition. (III) Compound A was formed after mixing the reaction for ca. 45 min; H2 was added to replenish the consumed H2. (IV) Compound (TPB)(μ‒H)Fe(H2)(H) was formed at the conclusion of the reaction. (inset V) Zoom out of spectra. (inset VI) Zoom in to show the H2 resonance. *Residual pentane. +Residual silicon grease. 154 Figure A1.4 Elimination of H2 from (TPB)(µ-H)Fe(N2)(H). Iterative freeze-pumpthaw-N2 cycles were performed to promote H2 release and reformation of (TPB)Fe(N2). Ferrocence (δ 4.0) was used as an integration standard. +Residual (TPB)FeBr (previously reported). *(TPB)FeCl, known decomposition product from (TPB)Fe(N2). 155 T1 (s) T1 vs Temperature 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 -85 -65 -45 -25 -5 15 35 Temp. (oC) Figure A1.5 T1 values for (TPB)(µ‒H)Fe(H2)(H) (d8-toulene). T1min is 35 ms at -32 oC. Figure A1.6 T1 values for (SiPiPr3)Fe(H2)(H) (d8-toluene). T1min is 32 ms at -30 C. 156 Figure A1.7 Variable temperature magnetic susceptibility measurements (method of Evans) of (TPB)Fe(CNtBu). 157 Figure A1.8 ATR-IR (THF thin film) of A, (TPBH)Fe(Et), (blue) and labeled (TPBD)Fe(Et) (red) generated from reaction of (TPB)Fe(C2H4) with H2 or D2, respectively. In the (TPBD)Fe(Et) spectrum, the 2470 cm-1 signal assigned to the B‒H stretch of (TPBH)Fe(Et) is attributed to facile scrambling of B‒D with the hydrogen atoms of the isopropyl groups from the ligand. The 2009 cm-1 signal is assigned to the N‒N stretch of (TPB)Fe(N2) is attributed to the decomposition of the (TPBH)Fe(Et) and (TPBD)Fe(Et) to ethane and (TPB)Fe(N2). 158 Figure A1.9 Catalytic hydrogenation of styrene with (TPB)Fe(N2). Amounts of styrene and ethylbenzene were determined by 1H NMR spectroscopy with ferrocene as an integration standard. Figure A1.10 Catalytic hydrogenation of phenylacetylene with (TPB)Fe(N2). Amounts of phenylacetylene, styrene, and ethylbenzene were determined by 1H NMR spectroscopy with ferrocene as an integration standard. 159 Appendix 2. Supplementary Data for Chapter 3 160 Table A2.1 Crystal data and neutron structure refinement for (TPB)Co(H2). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections Completeness to theta = 78.21° Absorption correction Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Largest diff. peak and hole tpbcoh2 C36 H55.86 B Br0.07 Co P3 656.96 100(2) 0.4 - 3.5 Triclinic P-1 a = 10.8535(2) α = 91.474(2) b = 11.2160(2) β = 101.653(2) c = 16.7367(3) γ = 118.930(2) 1728.21(7) 2 1.264 The linear absorption coefficient is wavelength dependent and is calculated as: μ = 1.543 + 1.718 * λ [cm-1] 108 1.50 x 1.00 x 0.75 7.48 to 79.21 -19 ≤ h ≤ 19, -20 ≤ k ≤ 20, -30 ≤ l ≤ 30 26012 26004 13.60 % sphere 0.4574 to 0.7915 Full-matrix least squares on F2 26012/912/11 1.115 R = 0.0664, wR2 = 0.1288 R = 0.0664, wR2 = 0.1288 0.946 and -0.927 eÅ-3 161 Table A2.2 Crystal data and neutron structure refinement for (SiPiPr3)Fe(H2). Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions Volume Z Density (Mg/m3; calculated) Absorption coefficient F(000) Crystal size (mm) Theta range for data collection (degrees) Index ranges Reflections collected Independent reflections Completeness to theta = 25.00° Absorption correction Max. and min transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Largest diff. peak and hole sip3feh2 C36 H55.52 C10.35 Fe P3 Si 677.44 100(2) 0.4 - 3.5 Triclinic P-1 a = 11.0481(1) α = 92.332(1) b = 11.3328(1) β = 100.440(1) c = 16.8023(2) γ = 119.636(2) 1777.26(3) 2 1.266 The linear absorption coefficient is wavelength dependent and is calculated as: μ = 1.543 + 1.718 * λ [cm-1] 128.0 1.20 x 0.60 x 0.42 7.48 to 79.21 -19 ≤ h ≤ 20, -20 ≤ k ≤ 20, -31 ≤ l ≤ 30 26367 26352 76.8 % sphere 0.7837 to 0.9157 Full-matrix least squares on F2 26367/949/19 1.072 R = 0.0548, wR2 = 0.1148 R = 0.0548, wR2 = 0.1148 0.848 and -0.818 eÅ-3 162 Figure A2.1 Solid state neutron diffraction structure of (SiPiPr3)Fe(H2). The structure is disordered. The hydride and H2 ligands of the disordered components are shown. The majority component (45 %) is the (SiPiPr3)Fe(H2) (blue H atoms). The remaining components are (SiPiPr3)Fe(H2)(H) (red H atom) and (SiPiPr3)FeCl (not shown). 163 Appendix 3. Supplementary Data for Chapter 4 Table A3.1 Catalytic hydrogenation results for (SiPR3)Fe, (PhBPiPr3)Fe, (NPiPr3)Fe, (TPB)Fe, (CPiPr3)Fe, (CSiPPh3)Fe, PP3/Fe(BF4)2, and [(tetraphos)FeF](BF4). Entry Precatalyst (Et3NH)(OCHO) Yield (mmol) MeOCHO Yield (mmol) Total Yield (mmol) (Et3NH)(OCHO) a TON MeOCHO a TON Total a TON Solvent PCO /PH 2 (atm) 2 b Additive Time (h) Temp. (°C) S1 (SiP IPr 3)FeCl 0.00 0.00 0.00 0.00 0.00 0.00 THF 1/1 50 100 S2 (SiP IPr 3)FeCl 0.00 0.00 0.00 0.00 0.00 0.00 THF 1/4 21 100 S3 (SiP IPr 3)FeCl 0.00 0.00 0.00 0.00 0.00 0.00 MeOH 1/4 40 60 S4 (SiP IPr 3)FeCl 0.00 0.00 0.00 0.00 0.00 0.00 THF 29/29 20 100 S5 (SiP IPr 3)FeCl 0.31 0.14 0.45 39.60 18.20 57.80 MeOH 29/29 20 100 S6 (SiP IPr 3)FeCl 0.31 0.13 0.44 37.90 15.30 53.20 MeOH 29/29 20 100 S7 (SiP IPr 3)FeCl 0.31 0.08 0.40 37.90 10.55 48.45 MeOH 29/29 20 100 S8 (SiP IPr 3)FeCl 0.14 0.00 0.14 17.40 0.00 17.40 MeOH 29/29 2 100 S9 (SiP IPr 3)FeCl 0.12 0.00 0.12 15.25 0.00 15.25 MeOH 29/29 2 100 S10 (SiP IPr 3)FeCl 0.26 0.10 0.35 32.54 12.20 44.74 MeOH 29/29 20 150 S11 (SiP IPr 3)FeCl 0.18 0.11 0.29 22.63 13.98 36.61 MeOH 29/29 20 150 S12 (SiP IPr 3)FeCl 0 0 0 0 0 0 MeOH 29/29 20 20 S13 (SiP IPr 3)FeCl 0 0 0 0 0 0 MeOH 29/29 20 20 c (SiP IPr 3)FeCl 0.15 0.12 0.27 18.73 14.98 33.71 CD3OD 29/29 20 100 S14 164 Entry Precatalyst (Et3NH)(OCHO) Yield (mmol) MeOCHO Yield (mmol) Total Yield (mmol) (Et3NH)(OCHO) a TON MeOCHO a TON Total a TON Solvent PCO /PH 2 (atm) S15 c (SiP IPr 3)FeCl 0.16 0.08 0.24 19.40 10.10 29.50 CD3OD 29/29 S16 (SiP IPr 3)FeCl 0.27 0.05 0.32 34.90 6.36 41.26 MeOH 29/29 S17 (SiP IPr 3)FeCl 0.26 0.06 0.32 33.30 7.28 40.58 MeOH 29/29 S18 (SiP IPr 3)FeCl 0.68 0.15 0.83 86.31 18.90 105.21 MeOH 29/29 S19 (SiP IPr 3)FeCl 0.59 0.05 0.64 75.12 6.26 81.38 MeOH 29/29 S20 (SiP IPr 3)FeCl 0.31 0.28 0.59 38.1 34.6 72.7 MeOH 29/29 S21 (SiP IPr 3)FeCl 0.37 0.17 0.54 45.1 20.6 65.7 MeOH 29/29 S22 (SiP IPr 3)FeCl 0.31 0.03 0.34 39.0 4.3 44.3 MeOH 29/29 S23 (SiP IPr 3)FeCl 0.31 0.05 0.35 39.0 6.4 45.4 MeOH 29/29 S24 (SiP IPr 3)FeCl 0.16 0.02 0.18 20.4 2.9 23.3 MeOH 29/29 S25 (SiP IPr 3)FeCl 0.19 0.02 0.21 25.0 2.9 27.9 MeOH 29/29 S26 d (SiP IPr 3)FeCl 0.22 0.01 0.23 28.5 1.4 29.9 MeOH 29/29 S27 d (SiP IPr 3)FeCl 0.24 0.03 0.27 31.1 3.9 35.0 MeOH 29/29 S28 (SiP IPr 3)FeCl 0.39 0.03 0.42 50.2 4.4 54.6 MeOH 29/29 S29 (SiP IPr 3)FeCl 0.45 0.01 0.46 58.3 1.7 60.0 MeOH 29/29 S30 (SiP IPr 3)FeCl 0.00 0.00 0.007 0.00 0.0 0.00 MeOH 29/0 2 b Additive 0.5 equiv (Et3NH)Cl 0.5 equiv (Et3NH)Cl 0.5 equiv NaBF4 0.5 equiv NaBF4 0.5 equiv F NaBAr 4 0.5 equiv F NaBAr 4 0.5 equiv NaF 0.5 equiv NaF 0.5 equiv CsF 0.5 equiv CsF 0.5 equiv TBAF 0.5 equiv TBAF 0.5 equiv K2CO3 0.5 equiv K2CO3 Time (h) Temp. (°C) 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100 165 Entry Precatalyst (Et3NH)(OCHO) Yield (mmol) MeOCHO Yield (mmol) Total Yield (mmol) (Et3NH)(OCHO) a TON MeOCHO a TON Total a TON Solvent PCO /PH 2 (atm) 2 b Additive Time (h) Temp. (°C) S31 (SiP IPr 3)Fe(N2)(H) 0.32 0.08 0.40 41.53 9.74 51.27 MeOH 29/29 20 100 S32 (SiP IPr 3)Fe(N2)(H) 0.25 0.09 0.34 31.79 11.83 43.62 MeOH 29/29 20 100 S33 (SiP IPr 3)Fe(OCHO) 0.39 0.02 0.41 51.66 2.43 54.09 MeOH 29/29 20 100 S34 (SiP IPr 3)Fe(OCHO) 0.36 0.03 0.39 46.98 3.22 50.20 MeOH 29/29 20 100 S35 (SiP 3)FeCl Ph 0.94 0.56 1.51 121.55 72.71 194.26 MeOH 29/29 20 100 S36 (SiP 3)FeCl Ph 1.08 0.52 1.59 139.36 66.44 205.44 MeOH 29/29 20 100 S37 [(SiP iPr F 3)Fe](BAr 4) 0.10 0.03 0.13 12.72 3.82 16.54 MeOH 29/29 20 100 S38 [(SiP iPr F 3)Fe](BAr 4) 0.13 0.00 0.15 16.30 2.80 19.10 MeOH 29/29 20 100 S39 (SiP iPr 3)FeCl 0.39 0.03 0.42 48.69 3.75 52.44 MeOH 29/29 Hg 20 100 S40 (SiP iPr 3)FeCl 0.33 0.00 0.38 41.00 6.24 47.24 MeOH 29/29 Hg 20 100 S41 HSiP iPr 3/FeCl2 (1:1) 0.08 0.03 0.11 9.68 3.10 12.78 MeOH 29/29 20 100 S42 HSiP iPr 3/FeCl2 (1:1) 0.10 0.02 0.12 11.98 1.82 13.80 MeOH 29/29 20 100 S43 (PhBP iPr 3)FeCl 0.24 0.02 0.26 29.00 3.10 32.10 MeOH 29/29 20 100 S44 (PhBP iPr 3)FeCl 0.15 0.02 0.17 19.00 2.40 21.40 MeOH 29/29 20 100 S45 [(NP iPr 3)FeCl](PF6) 0.00 0.00 0.00 0.00 0.00 0.00 MeOH 29/29 20 100 S46 (TPB)FeCl 0.00 0.00 0.00 0.00 0.00 0.00 MeOH 29/29 20 100 166 (Et3NH)(OCHO) Yield (mmol) MeOCHO Yield (mmol) Total Yield (mmol) (Et3NH)(OCHO) a TON MeOCHO a TON Total a TON Solvent [(TPB)Fe](BAr 4) 0.00 0.00 0.00 0.00 0.00 0.00 MeOH S48 (TPB)μ‒H)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 S49 PP3/Fe(BF4)2 3.30 0.91 4.21 395.70 108.50 S50 PP3/Fe(BF4)2 2.69 0.98 3.67 342.24 S51 [(tetraphos)-Fe(F)](BF4) 6.06 7.68 13.74 S52 [(tetraphos)-Fe(F)](BF4) 5.15 7.23 S53 FeCl2 0.00 S54 FeCl2/4 PPh3 S55 no iron S56 (SiP S57 PCO /PH Time (h) Temp. (°C) 29/29 20 100 MeOH 29/29 20 100 504.20 MeOH 29/29 20 100 124.68 466.92 MeOH 29/29 20 100 771.30 976.50 1747.80 MeOH 29/29 20 100 12.38 655.22 919.85 1575.07 MeOH 29/29 20 100 0.00 0.00 0.00 0.00 0.00 MeOH 29/29 20 100 0.00 0.00 0.00 0.00 0.00 0.00 MeOH 29/29 20 100 0.00 0.00 0.00 0.00 0.00 0.00 MeOH 29/29 20 100 IPr 3)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 THF 1/1 20 60 (SiP IPr 3)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 THF 1/4 20 60 S58 (SiP IPr 3)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 C6D6 1/4 20 90 S59 (SiP IPr 3)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 THF 29/29 20 100 S60 (SiP 3)Fe(N2)(H) Ph 0.00 0.00 0.00 0.00 0.00 0.00 THF 29/29 20 100 S61 (PhBP 0.00 0.00 0.00 0.00 0.00 0.00 THF 29/29 20 100 S62 [(NP 0.00 0.00 0.00 0.00 0.00 0.00 THF 29/29 20 100 Entry Precatalyst S47 F iPr 3)FeCl iPr 3)Fe(N2)(H)](PF6) 2 (atm) 2 b Additive 167 Entry Precatalyst (Et3NH)(OCHO) Yield (mmol) MeOCHO Yield (mmol) Total Yield (mmol) (Et3NH)(OCHO) a TON MeOCHO a TON Total a TON Solvent S63 (TPB)(μ‒H)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 THF S64 (TPB)(μ‒H)Fe(N2)(H) 0.00 0.00 0.00 0.00 0.00 0.00 S65 (CP iPr 3)FeCl 0.20 0.03 0.23 24.71 3.43 S66 (CP iPr 3)FeCl 0.18 0.03 0.21 23.15 S67 (C P 3)FeCl 0.00 0.00 0.00 S68 (SiP iPr 3)FeCl 0.36 0.02 S69 (SiP iPr 3)FeCl 0.32 S70 (SiP iPr 3)FeCl 0.00 Si Ph PCO /PH Time (h) Temp. (°C) 29/29 20 100 C6D6 1/4 20 100 28.13 MeOH 29/29 20 100 2.95 26.11 MeOH 29/29 20 100 0.00 0.00 0.00 MeOH 29/29 20 100 0.38 46.68 3.14 49.82 MeOH 5.5/29 20 100 0.02 0.34 41.30 3.15 44.45 MeOH 5.5/29 20 100 0.00 0.00 0.00 0.00 0.00 MeOH 29/5.5 20 100 2 (atm) 2 b Additive Unless otherwise noted, reactions were performed under the standard conditions of 0.7 mM precatalyst, 651 mM of triethylamine, methanol (10 mL), 20 h, 100 °C, 29 atm of CO2, and 29 atm of H2. aTurnover number (TON) is the yield of product divided by the amount of added precatalyst. c(Et3NH)(OCHO) was detected by 1H NMR spectroscopy, but (Et3ND)(OCDO), (Et3NH)(OCDO), and (Et3ND)(OCHO) were not detected by 2H NMR spectroscopy. bBArF4 = [B(3,5-(CF3-C6H3)4)]-; TBAF = tetrabutylammonium fluoride. 168 169 A3.1 Hydricity Determination Reactions relevant to determination of hydricity for (SiPiPr3)Fe(H2)(H) (Fe(H2)(H))with a base (B), equation A3.1-A3.3. The sum of equations A3.1-A3.3 (ΣG, Scheme A3.1) represents the reverse reaction, where hydride is added to Fe(H2)+. Therefore, reversing the reaction and taking negative ΣG (-ΣG, equation A3.4) represents the hydricity (ΔGH-) of Fe(H2)(H). Scheme A3.1 Reactions and equations relevant to hydricity determination for (SiPiPr3)Fe(H2)(H). See Chart 4.1 for detailed ligand representation. 170 Experimentally, the deprotonation reaction of equation A3.1 was run in d8-THF. However, THF is known to coordinate competitively to the cationic FeII complex to give Fe(THF)+.1 This must be taken into account. The equilibrium constant for the competitive coordination of H2 and THF (equation A3.5) has been previously reported (K1 = 1900 M-1, equation A3.5). Scheme A3.2 Equilibrium reaction for competitive THF/H2 coordination by [(SiPiPr3)Fe(L)]+. Therefore, the overall reaction of equations A3.1 and A3.5 is: And the total concentration of iron species in solution is: Experimentally, the equilibrium between the iron-species and base in d8-THF was monitored by 1H NMR spectroscopy with 1,3,5-trimethoxybenzene as an integration standard. The proton resonances from the Fe(H2)(H), base, and conjugate acid of the base were well-resolved and 171 reliably integrated in the 1H NMR spectra, but the paramagnetic Fe(H2)+ and Fe(THF)+ could not be reliably integrated. Therefore, equation A3.7 and K1 were used to determine the respective concentrations of Fe(H2)+ and Fe(THF)+ in order to determine the equilibrium value K2. The activity of hydrogen at 1.0 atm was taken as unity in K2, as this is the reference state of hydrogen for the normal hydrogen electrode.2 Table A3.2 Experimentally determined ΔGH- for (SiPiPr3)Fe(H2)(H) and pKa values for [(SiPiPr3)Fe(H2)](BArF4) using three different bases. Entry Acid Base Proton Sponge THF a (pKa = 11.1) 2,6-Lutidine THF a (pKa = 7.2) 2,4,6-Trimethylpyridine THF a (pKa = 8.1) 1 [(SiP iPr F 3)Fe(H2)](BAr 4) 2 [(SiP iPr F 3)Fe(H2)](BAr 4) 3 [(SiP iPr F 3)Fe(H2)](BAr 4) 4 [Proton Sponge-H](BAr 4) a F (SiP iPr 3)Fe(H2)(H) iPr 3)Fe(H2)(H) [(SiP ΔGH- (kcal/mol) pKa pKa 54.8 10.5 15.5 -5 c 52.9 11.7 16.9 5.1 x 10 -3 d 54.9 10.4 15.4 2.6 54.5 10.7 15.8 -1 (SiP K2 (M ) b 4.31 3.3 x 10 iPr F 3)Fe(H2)](BAr 4) THF MeCN pKa of conjugate acid, see ref 3. b1 equiv of proton sponge used. c292 equiv of 2,6- lutidine used. d20 equiv of 2,4,6-trimethyl-pyridine used. 172 A3.2 UV-vis Titration Figure A3.1 UV-vis spectra of the titration of [(SiPiPr3)Fe(N2)](BArF4) with Li(OCHO) in THF. Figure A3.2 Fit of the UV-vis titration data. Absorbance at 752 nm as a function of the equivalents of Li(OCHO) added (blue diamond). The best fit of the data using Keq = 3.5 x 106 M-1 is in red. 173 A3.3 Cited References (1) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 1636616369. (2) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am. Chem. Soc. 2002, 124, 2984-2992. (3) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019-1028.