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

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journal contribution
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.