Influence of a large σ-donor ligand on structural and catalytic properties of di-iron compounds related to the active site of Fe-hydrogenase -: A DFT investigation

The complex (S2C3H6)[Fe-2(CO)(5)P(NC4H8)(3)] (A), a recently synthesized functional model of the active site of Fe-hydrogenases, is able to electrocatalyze proton reduction, leading to molecular hydrogen evolution. Experimental results suggested that the presence of the electron donor P(NC4H8)(3) ligand in A could favor the formation of a mu-CO species similar to that observed in the enzymatic cluster. However, insight into the structural features of key catalytic intermediates deriving from reduction and protonation of A was still lacking. Here we present results obtained using density functional theory to evaluate structures, relative stabilities, and spectroscopic properties of several species relevant for the electrocatalytic H-2 evolving process. The results enabled us to unravel the structure of the mu-CO complex experimentally detected after monoelectronic reduction of A. Moreover, we show that the introduction of the large electron-donor ligand P(NC4H8)(3) in the biomimetic complex does not favour the stabilization of terminal-hydrido adducts, which are expected to be very reactive in terms of H-2 production. The comparison of our findings with previous theoretical and experimental results obtained on similar model complexes suggests that the introduction of an electron donor ligand as good as P(NC4H8)(3), but less sterically demanding, could represent a better choice to facilitate the formation of mu-CO complexes more closely resembling the structure of the enzymatic cluster. ((C) Wiley-VCH Verlaq GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).