Iridium(I) and Iridium(III) Complexes Supported by a Diphenolate Imidazolyl-Carbene Ligand

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Abstract

Deprotonation of 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride (1a) followed by reaction with chloro-1,5-cyclooctadiene Ir(I) dimer affords the anionic Ir(I) complex [K][{OCO}Ir(cod)] (2: OCO = 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl; cod = 1,5-cyclooctadiene), the first Ir complex stabilized by a diphenolate imidazolyl-carbene ligand. In the solid state 2 exhibits square-planar geometry, with only one of the phenoxides bound to the metal center. Oxidation of 2 with 2 equiv of [FeCp2][PF6] generates the Ir(III) complex [{OCO}Ir(cod)(MeCN)][PF6] (3). Reaction of 3 with H2 results in the liberation of cyclooctane and a species capable of catalyzing the hydrogenation of cyclohexene to cyclohexane. Displacement of cyclooctadiene from 3 can be achieved by heating in acetonitrile to form [{OCO}Ir(MeCN)3][PF6] (4) or by reaction with either PMe3 or PCy3 to generate [{OCO}Ir(PMe3)3][PF6] (5) or [{OCO}Ir(PCy3)2(MeCN)][PF6] (6), respectively. 6 reacts with CO in acetonitrile to give an equilibrium mixture of 6 and [{OCO}Ir(PCy3)2(CO)][PF6] (7) and with chloride to generate [{OCO}Ir(PCy3)2Cl] (8). The solid-state structure of 8 shows that the diphenolate imidazolyl-carbene ligand is distorted from planarity; DFT calculations suggest this is due to an antibonding interaction between the phenolates and the metal center in the highest occupied molecular orbital (HOMO) of the complex. 8 undergoes two successive reversible one-electron oxidations in CH2Cl2 at −0.22 and at 0.58 V (vs ferrocene/ferrocenium); EPR spectra, mass spectroscopy, and DFT calculations suggest that the product of the first oxidation is [{OCO}Ir(PCy3)2Cl]+ (8+), with the unpaired electron occupying a molecular orbital that is delocalized over both the metal center and the diphenolate imidazolyl-carbene ligand.

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