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(I)(cyclooctene)][PF(6)] at ambient temperature, resulting in a cationic five-coordinate iridium(III) phenyl hydride product. Protonation of the (PONOP)Ir(I) 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.

Three novel pendant acetate complexes, [Rh­(bdmpza)­Cl3]−M+, [Rh­(bdmpza)­Cl2(py)], and [Ir­(bdmpza)­Cl3]−M+ (bdmpza = bis­(3,5-dimethylpyrazol-1-yl) acetate, M+ = Li+, Na+), were synthesized. Abstraction of halide from these complexes with silver salts yielded species capable of C–H activation of arenes. The catalytic H/D exchange reaction between benzene and trifluoroacetic acid-d was optimized, and these conditions were used to evaluate H/D exchange in other arenes. Branched alkyl substituents in alkyl aromatics showed an affinity toward deuterium exchange in the β-alkyl position only. DFT calculations were performed to determine the mechanism of H/D exchange.

Key indicatorsSingle-crystal X-ray study T = 173 K Mean (C-C) = 0.005 Å Disorder in main residue R factor = 0.048 wR factor = 0.126 Data-to-parameter ratio = 10.6For details of how these key indicators were automatically derived from the article, see