A mixed-valent, Fe(II)Fe(I), diiron complex reproduces the unique rotated state of the [FeFe]hydrogenase active site

The fully reversible (FeFeI)-Fe-I -> (FeFeII)-Fe-I couple of an N-heterocyclic carbene dinuclear (FeFeI)-Fe-I complex, (mu-pdt)[Fe-I(CO)(2)(PMe3)][Fe-I(CO)(2)(IMes)], complex D, has led to the isolation of the mixed-valent cationic complex D-ox as a biomimetic of the 2Fe2S subsite of the oxidized H cluster in [FeFe]hydrogenase. During the review of this submission a second complex was reported. As compared to complex D, a remarkable reorientation of the IMes NHC ligand enables the (mu-pdt)[Fe(CO)(2)(PMe3)][Fe(CO)(2)(IMes)](+) cation, D-ox, to exist as a rotated structure, with structural and spectroscopic similarities to the diiron unit of H-as isolated or H-ox. The structural makeup of the model includes an Fe-Fe distance that matches that of the enzyme, a semi-bridging CO group, and a pseudo-octahedral iron with open site blocked by a strategically positioned arene group from the IMes-NHC carbene ligand.