[FeFe]-Hydrogenase Models: Overpotential Control for Electrocatalytic H2 Production by Tuning of the Ligand π-Acceptor Ability

In the search for synthetic competitive catalysts that function with hydrogenase-like capability, a series of (Pyrrol-1-yl)-phosphane-substituted diiron complexes [(mu-pdt)Fe-2(CO)(5)L] [pdt = propanedithiolate, L = Ph(2)PPyr (2), PPyr(3) (4); Pyr = pyrrolyl] and [(mu-pdt)Fe-2(CO)(4)L-2] [L = Ph(2)PPyr (3), PPyr(3) (5)] were prepared as functional models for the active site of Fe- only 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 PPh3, PMe3, and PTA (1,3,5-triaza-7-phosphaadamantane), indicating that (pyrrol-1-yl)phosphanes are poor sigma-donors and better pi-acceptors. The electrochemical properties of complexes 2-5 were studied by cyclic voltammetry in CH3CN 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 CH3CN at their respective (FeFe0)-Fe-I level. Complex 4 is the most effective electrocatalyst, which catalytically generates H-2 from HOAc at -1.66 V vs. Fc(+)/Fc with only ca. 0.2 V over-potential in CH3CN.