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

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(adtBn)­(CO)3(dppv)­(PFc*Et2) ([1]0; adtBn = (SCH2)2NBn, dppv
= cis-1,2-bis­(diphenylphosphino)­ethylene, PFc*Et2 = Et2PCH2C5Me4FeCp*) reacts with excess [H­(OEt2)2]­BArF4 (BArF4– = B­(C6H3-3,5-(CF3)2)4–) to give ∼0.5 equiv of H2 and [Fe2(adtBnH)­(CO)3(dppv)­(PFc*Et2)]2+ ([1H]2+). The species [1H]2+ consists of a ferrocenium
ligand, an N-protonated amine, and an FeIFeI core. In the presence of additional reducing equivalents in the
form of decamethylferrocene (Fc*), hydrogen evolution is catalytic,
albeit slow. The related catalyst Fe2(adtBn)­(CO)3(dppv)­(PMe3) (3) behaves similarly
in the presence of Fc*, except that in the absence of excess reducing
agent it converts to the catalytically inactive μ-hydride
derivative [μ-H3]+. Replacement
of the adt in [1]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.