Hydricity of an Fe–H Species and Catalytic CO_2 Hydrogenation

Abstract

Despite renewed interest in carbon dioxide (CO_2) reduction chemistry, examples of homogeneous iron catalysts that hydrogenate CO_2 are limited compared to their noble-metal counterparts. Knowledge of the thermodynamic properties of iron hydride complexes, including M–H hydricities (ΔG_(H)–, could aid in the development of new iron-based catalysts. Here we present the experimentally determined hydricity of an iron hydride complex: (SiP^(iPr)_3)Fe(H_2)(H), Δ_(G)H– = 54.3 ± 0.9 kcal/mol [SiP^(iPr)_3 = [Si(o-C_6H_4PiPr_2)_3]−]. We also explore the CO_2 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 (SiP^(R)_3)Fe (R = iPr and Ph) and boratoiron (PhBP^(iPr)_3)Fe (PhBP^(iPr)_3 = [PhB(CH_2PiPr_2)_3]^−) systems, as well as the recently reported (CP^(iPr)_3)Fe (CP^(iPr)_3 = [C(o-C_6H_4PiPr_2)_3]^−), are also catalysts for CO_2 hydrogenation in methanol and in the presence of triethylamine, generating methylformate and triethylammonium formate at up to 200 TON using (SiP^(Ph)_3)FeCl as the precatalyst. Under stoichiometric conditions, the iron hydride complexes of this series react with CO_2 to give formate complexes. Finally, the proposed mechanism of the (SiP^(iPr)_3)-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-C_6H_4PPh_2)_3), CO_2 hydrogenation catalyst.

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