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Hydricity of an Fe–H Species and Catalytic CO2 Hydrogenation

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journal contribution
posted on 01.06.2015, 00:00 by Henry Fong, Jonas C. Peters
Despite renewed interest in carbon dioxide (CO2) reduction chemistry, examples of homogeneous iron catalysts that hydrogenate CO2 are limited compared to their noble-metal counterparts. Knowledge of the thermodynamic properties of iron hydride complexes, including M–H hydricities (ΔGH), could aid in the development of new iron-based catalysts. Here we present the experimentally determined hydricity of an iron hydride complex: (SiPiPr3)­Fe­(H2)­(H), ΔGH = 54.3 ± 0.9 kcal/mol [SiPiPr3 = [Si­(o-C6H4PiPr2)3]]. We also explore the CO2 hydrogenation chemistry of a series of triphosphinoiron complexes, each with a distinct apical unit on the ligand chelate (Si, C, PhB, N, B). The silyliron (SiPR3)Fe (R = iPr and Ph) and boratoiron (PhBPiPr3)­Fe (PhBPiPr3 = [PhB­(CH2PiPr2)3]) systems, as well as the recently reported (CPiPr3)Fe (CPiPr3 = [C­(o-C6H4PiPr2)3]), are also catalysts for CO2 hydrogenation in methanol and in the presence of triethylamine, generating methylformate and triethylammonium formate at up to 200 TON using (SiPPh3)­FeCl as the precatalyst. Under stoichiometric conditions, the iron hydride complexes of this series react with CO2 to give formate complexes. Finally, the proposed mechanism of the (SiPiPr3)-Fe system proceeds through a monohydride intermediate (SiPiPr3)­Fe­(H2)­(H), in contrast to that of the known and highly active tetraphosphinoiron, (tetraphos)­Fe (tetraphos = P­(o-C6H4PPh2)3), CO2 hydrogenation catalyst.