Hydrogen Production Catalyzed by Bidirectional, Biomimetic Models of the [FeFe]-Hydrogenase Active Site

Abstract

Active site mimics of [FeFe]-hydrogenase are shown to be bidirectional catalysts, producing H₂ upon treatment with protons and reducing equivalents. This reactivity complements the previously reported oxidation of H₂ by these same catalysts in the presence of oxidants. The complex Fe₂(adt^Bn)(CO)₃(dppv)(PFc*^Et₂ ) ([<b>1</b>]⁰; adt^Bn = (SCH₂)₂NBn, dppv = <i>cis</i>-1,2-bis(diphenylphosphino)ethylene, PFc*^Et₂ = Et₂PCH₂C₅Me₄FeCp*) reacts with excess [H(OEt₂)₂]BAr^F₄ (BAr^F₄^- = B(C₆H₃-3,5-(CF₃)₂)₄^-) to give ∼0.5 equiv of H₂ and [Fe₂(adt^BnH)(CO)₃(dppv)(PFc*^Et₂ )]²⁺ ([<b>1</b>H]²⁺). The species [<b>1</b>H]²⁺ consists of a ferrocenium ligand, an N-protonated amine, and an Fe^IFe^I core. In the presence of additional reducing equivalents in the form of decamethylferrocene (Fc*), hydrogen evolution is catalytic, albeit slow. The related catalyst Fe₂(adt^Bn)(CO)₃(dppv)(PMe₃) (<b>3</b>) behaves similarly in the presence of Fc*, except that in the absence of excess reducing agent it converts to the catalytically <i>in</i>active μ-hydride derivative [μ<b>-</b>H<b>3</b>]⁺. Replacement of the adt in [<b>1</b>]⁰ with propanedithiolate (pdt) results in a catalytically inactive complex. In the course of synthesizing [FeFe]-hydrogenase mimics, new routes to ferrocenylphosphine ligands and nonamethylferrocene were developed.