Hydricity of an Fe–H Species and Catalytic CO_2 Hydrogenation
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.
© 2014 American Chemical Society. Received: October 18, 2014; Published: December 31, 2014. This material is based upon work performed by the Joint Center of Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award DE-SC0004993. The Bercaw Group at the California Institute of Technology is acknowledged for their assistance with the high-pressure reactions. GC-FID instrumentation at the Environmental Analysis Center (EAC) at the California Institute of Technology was used in this work. We acknowledge Dr. Nathan Dalleska of the EAC for his assistance with the GC-FID measurements.
Supplemental Material - ic502508p_si_001.pdf