C-H bond activation by cationic iridium (III) NHC complexes: A combined experimental and computational study

The cationic complexes [Cp*Ir(NHC)Me(solv)]+[MeB- (C6F5)3]− were prepared and studied as models for methane oxyfunctionalization catalysts (Cp* = ƒÅ5-C5Me5; NHC = 1,3,4,5-tetramethylimidazol- 2-ylidene (MeIMe, 3a), 1,3-dimethylimidazol-2-ylidene (IMe, 3b), 1,3-dimethylbenzimidazol-2-ylidene (BIMe, 3c); solv = solvent or open site). These complexes were targeted on the basis of the C−H bond activation reactions of the previously reported complexes [Cp*Ir(PMe3)R]+ (R = Me, H) and the general robustness of Ir−NHC complexes under oxidizing conditions. The syntheses of the new iridium(III)complexes Cp*Ir(NHC)Me2 are described (NHC = MeIMe (4a), IMe (4b), BIMe (4c)). When 4a−c were allowed to react with B(C6F5)3 in CH2Cl2, the methyl abstraction products [Cp*Ir(NHC)Me(solv)]+[MeB(C6F5)3]ñY (3a−c) were produced. Complexes 3a−c reacted with arenes to form the aryl complexes [Cp*Ir(NHC)Ar(solv)]+[MeB(C6F5)3]ñY and methane (Ar = C6H5 (7), C6H4F (8)). Complexes 3a−c reacted very slowly with alkanes; the slow reaction rate is attributed to steric congestion due to the NHC ligand. DFT calculations support this hypothesis: the barriers to C−H activation are in qualitative agreement with the empirical reaction rates, and the C−H activation transition state structures show significant steric crowding. Several of these complexes have been analyzed by X-ray diffraction.