Coordination and conformational isomers in mononuclear iron complexes with pertinence to the [FeFe] hydrogenase active site

A series of six mononuclear iron complexes of the type [Fe(X-bdt)((P2N2Ph)-N-R)(CO)] ((P2N2Ph)-N-R = 1,5-diaza-3,7-diphosphaoctane, bdt = benzenedithiolate with X = H, Cl-2 or Me and R = Ph, Bn, Cyc or tert-Bu) was prepared. This new class of penta-coordinate iron complexes contains a free coordination site and a pendant base as essential structural features of the [FeFe]-hydrogenase active site. The bidentate nature of the (P2N2Ph)-N-R ligands was found to be crucial for the preferential formation of coordinatively unsaturated penta-coordinate complexes, which is supported by first principle calculations. IR-spectroscopic data suggest the presence of coordination isomers around the metal center, as well as multiple possible conformers of the (P2N2Ph)-N-R ligand. This finding is further corroborated by X-ray crystallographic and computational studies. P-31{H-1}-NMR- and IR-spectroscopic as well as electrochemical measurements show that the electronic properties of the complexes are strongly, and independently, influenced by the P-substituents at the (P2N2Ph)-N-R ligand as well as by modifications of the bdt bridge. These results illustrate the advantages of this modular platform, which allows independent and selective tuning through site specific modifications. Potential catalytic intermediates, namely singly reduced and protonated complexes, have been further investigated by spectroscopic methods and exhibit remarkable stability. Finally, their general capacity for electro-catalytic reduction of protons to molecular hydrogen was verified.