Mixed Valent, Fe(II)Fe(I), Diiron Complexes Reproduce the Unique Rotated State of the [FeFe]Hydrogenase Active Site

ABSTRACT

The reversible Fe^IFe^I / Fe^IFe^II couple of an N-heterocyclic carbene dinuclear Fe^IFe^I complex, (μ-pdt)[Fe^I(CO)₂(PMe₃)][Fe^I(CO)₂(IMes)] (IMes= 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), 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.  This is the rare example of Fe^IFe^II paramagnetic H₂ase model complex studied by X-ray diffraction.  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, 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 a 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 bulky NHC carbene ligand. Other asymmetric disubstituted diiron complexes, (μ-pdt)[Fe^I(CO)₂(P)][Fe^I(CO)₂L)] with a selection of P-donor and NHC ligands designed to illustrate the principles that govern stability and function of the Fe(II)Fe(I) are being studied. Reactivity of Fe(II)Fe(I) species with H₂, CO and CN^- are described.