A mononuclear iron carbonyl complex [Fe(µ-bdt)(CO)2(PTA)2] with bulky phosphine ligands: a model for the [FeFe] hydrogenase enzyme active site with an inverted redox potential.

A mononuclear hexa-coordinated iron carbonyl complex [Fe(µ-bdt)(CO)2(PTA)2] 1 (bdt = 1,2-benzenedithiolate; PTA = 1,3,5-triaza-7-phosphaadamantane) with two bulky phosphine ligands in the trans position was synthesized and characterized by X-ray structural analysis coulometry data, FTIR, electrochemistry and electronic structure calculations. The complex undergoes a facilitated two-electron reduction 1/12- and shows an inverted one-electron reduction for 1/1- at higher potentials. Electrochemical investigations of 1 are compared to the closely related [Fe(bdt)(CO)2(PMe3)2] compound. A mechanistic suggestion for the hydrogen evolution reaction upon proton reduction from acid media is derived. The stability of 1 in both weak and strong acids is monitored by cyclic voltammetry.