Hydride transfer from an octaboranyl Fe(II)—H complex Gabriel Jobin, Marcus Drover Western University, Ontario, Canada
Metal hydrides are important intermediates in the catalytic reduction of protons and CO 2 as well as in the oxidation of H 2. 1 The transfer of hydrides to organic salts is a fundamental step that is found throughout science. 2 The predilection of a transition metal complex ({M}-H) to donate a hydride (H - ) can be quantified using the hydricity value, ΔG H- , where smaller values indicate a higher propensity for hydride release 3 . Given the known hydricity of formate, 44 kcal/mol 4 , we thus require mechanisms to prepare metal complexes with low hydricities. Previous studies found that the presence of a Lewis acid, specifically a trialkylborane can help form a borane- formate intermediate. 5 Consequently, our research team has done extensive research in enhancing reactivity by adding a Lewis acid into the secondary coordination sphere (SCS) of diphosphine complexes having abundant and sustainable transition metals. 6–9 Here we present a new Fe(II)-H complex with 8 pendent boranes in the SCS of the diphosphine backbone. The Lewis acidity in the SCS has been introduced into these systems by hydroboration of the allyl backbone of the phosphine ligand. 6–9 To examine the effect of SCS incorporation, the reactivity of [Fe(tape) 2 (H)(MeCN)]BPh 4 (tape = tetraallylphosphinoethane) is examined with known hydride acceptors (pyridinium analogues); the reactivity of this system is juxtaposed against its hydroborated analogue [Fe(P 2 BCy 4 ) 2 (H)(MeCN)]BPh 4 (P 2 BCy 4 = 1,2-bis[di(3-dicyclohexylboraneyl) propylphosphino]ethane. UV-Vis kinetics data lends credence to an altered rate of hydride transfer with and without the boranes in the SCS. References 1. Bourrez, M., Steinmetz, R., Ott, S., Gloaguen, F. & Hammarström, L. Concerted proton-coupled electron transfer from a metal-hydride complex. Nat Chem 7 , 140–145 (2015). 2. Zhang, F., Jia, J., Dong, S., Wang, W. & Tung, C. H. Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)+ Analogues. Organometallics 35 , 1151–1159 (2016). 3. Schmeier, T. J., Dobereiner, G. E., Crabtree, R. H. & Hazari, N. Secondary coordination sphere interactions facilitate the insertion step in an iridium(III) CO2 reduction catalyst. J Am Chem Soc 133 , 9274–9277 (2011). 4. Waldie, K. M., Brunner, F. M. & Kubiak, C. P. Transition Metal Hydride Catalysts for Sustainable Interconversion of CO2 and Formate: Thermodynamic and Mechanistic Considerations. ACS Sustain Chem Eng 6 , 6841–6848 (2018). 5. Miller, A. J. M., Labinger, J. A. & Bercaw, J. E. Trialkylborane-assisted CO2 reduction by late transition metal hydrides. Organometallics 30 , 4308–4314 (2011). 6. Drover, M. W. et al. Octaboraneyl Complexes of Nickel: Monomers for Redox-Active Coordination Polymers. Chem. -Eur. J. 26 , 11180–11186 (2020). 7. Zurakowski, J. A., Austen, B. J. H. & Drover, M. W. Exterior decorating: Lewis acid secondary coordination spheres for cooperative reactivity. Trends Chem 4 , 331–346 (2022). 8. Zurakowski, J. A., Bhattacharyya, M., Spasyuk, D. M. & Drover, M. W. Octaboraneyl [Ni(H)(diphosphine)2]+Complexes: Exploiting Phosphine Ligand Lability for Hydride Transfer to an [NAD]+Model. Inorg Chem 60 , 37–41 (2021). 9. Zurakowski, J. A., Austen, B. J. H. & Drover, M. W. Wrapping Rhodium in a Borane Canopy: Implications for Hydride Formation and Transfer. Organometallics 40 , 2450–2457 (2021).
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