A Mixed-Valent, Fe(II)Fe(I), Diiron Complex Reproduces the Unique Rotated State of the [FeFe]Hydrogenase Active Site

The fully reversible FeIFeI ⇆ FeIFeII couple of an N-heterocyclic carbene dinuclear FeIFeI complex, (μ-pdt)[FeI(CO)2(PMe3)][FeI(CO)2(IMes)], complex D, has led to the isolation of the mixed-valent cationic complex Dox 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 (μ-pdt)[Fe(CO)2(PMe3)][Fe(CO)2(IMes)]+ cation, Dox, to exist as a “rotated” structure, with structural and spectroscopic similarities to the diiron unit of Has isolated or Hox. 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.