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An Iridium(IV) Species, [Cp*Ir(NHC)Cl]+, Related to a Water-Oxidation Catalyst

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posted on 14.03.2011 by Timothy P. Brewster, James D. Blakemore, Nathan D. Schley, Christopher D. Incarvito, Nilay Hazari, Gary W. Brudvig, Robert H. Crabtree
The Ir precatalyst (3) contains both a Cp* and a κ2C2,C2-1,3-diphenylimidazol-2-ylidene ligand, a C−C chelate, where one C donor is the NHC and the other is a cyclometalated N-phenyl wingtip group. The structure of 3 was confirmed by X-ray crystallography. Like our other recently described Cp*Ir catalysts, this compound is a precursor to a catalyst that can oxidize water to dioxygen. Electrochemical characterization of the new compound shows that it has a stable iridium(IV) oxidation state, [Cp*IrIV(NHC)Cl]+, in contrast with the unstable Ir(IV) state seen in our previous cyclometalated [Cp*IrIII(2-pyridyl-2′-phenyl)Cl] catalyst. The new iridium(IV) species has been characterized by EPR spectroscopy and has a rhombic symmetry, a consequence of the ligand environment. These results both support previous studies which suggest that Cp*Ir catalysts can be advanced through the relevant catalytic cycle(s) in one-electron steps and help clarify the electrochemical behavior of this class of water-oxidation catalysts.

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