Investigations of Iridium-Mediated Reversible C-H Bond Cleavage: Characterization of a 16-Electron Iridium(III) Methyl Hydride Complex

New iridium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP), have been prepared and used in the study of hydrocarbon C-H bond activation. Intermolecular oxidative addition of a benzene C-H bond was directly observed with [(PONOP)Ir-1(cyclooctene)][PF6] at ambient temperature, resulting in a cationic five-coordinate iridium(III) phenyl hydride product. Protonation of the (PONOP)Ir-1 methyl complex yielded the corresponding iridium(III) methyl hydride cation, a rare five-coordinate, 16-valence electron transition metal alkyl hydride species which was characterized by X-ray diffraction. Kinetic studies of C-H bond coupling and reductive elimination reactions from the five-coordinate complexes have been carried out. Exchange NMR spectroscopy measurements established a barrier of 17.8(4) kcal/mol (22 degrees C) for H-C-aryl bond coupling in the iridium(III) phenyl hydride cation and of 9.3(4) kcal/mol (-105 degrees C) for the analogous H-C-alkyl Coupling in the iridium(III) methyl hydride cation. The origin of the higher barrier of H-C-aryl relative to H-C-alkyl bond coupling is proposed to be influenced by a hindered rotation about the Ir-C-aryl bond, a result of the sterically demanding PONOP ligand.