[Fefe]-hydrogenase models: Overpotential control for electrocatalytic H_2 production by tuning of the ligand π-acceptor ability

摘要

In the search for synthetic competitive catalysts that function with hydrogenase-like capability, a series of (Pyrrol-1-yl)-phosphane-substituted diiron complexes [(μ-pdt)Fe_2(CO)_5L] [pdt = propanedithiolate, L = Ph_2PPyr (2), PPyr_3 (4); Pyr = pyrrolyl] and [(μ-pdt)Fe_2(CO)_4L_2] [L = Ph_2PPyr (3), PPyr_3 (5)] were prepared as functional models for the active site of Feonly hydrogenase. The structures of these complexes were fully characterized by spectroscopy and X-ray crystallography. In the IR spectra the CO bands for complexes 2-5 are shifted to higher energy relative to those of complexes with "traditional" phosphane ligands, such as PPh_3, PMe_3, and PTA (1,3,5-triaza-7-phosphaadamantane), indicating that (pyrrol-1-yl)phosphanes are poor σ-donors and better π-acceptors. The electrochemical properties of complexes 2-5 were studied by cyclic voltammetry in CH_3CN in the absence and presence of the the weak acid HOAc. The reduction potentials of these complexes show an anodic shift relative to other phosphane-substituted derivatives. All of the complexes can catalyze proton reduction from HOAc to H_2 in CH_3CN at their respective Fe~IFe~0level. Complex 4 is the most effective electrocatalyst, which catalytically generates H2 from HOAc at-1.66 V vs. Fc+/Fc with only ca. 0.2 V overpotential in CH_3CN.