Activation and Oxidation of Mesitylene C–H Bonds by (Phebox)Iridium(III) Complexes

A pincer iridium­(III) complex, (Phebox)­Ir­(OAc)₂OH₂ (<b>1</b>) (Phebox = 3,5-dimethylphenyl-2,6-bis­(oxazolinyl)), selectively cleaves the benzylic C–H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)­Ir­(mesityl)­(OAc) (<b>3</b>), in >90% yield. The trifluoroacetate analogue, (Phebox)­Ir­(OCOCF₃)₂OH₂ (<b>2</b>), was synthesized to compare with complex <b>1</b> for C–H activation, and (Phebox)­Ir­(mesityl)­(OCOCF₃) (<b>4</b>) was synthesized by ligand exchange of complex <b>3</b>. Both complexes <b>1</b> and <b>2</b> catalyze H/D exchange between mesitylene and D₂O at 180 °C, exclusively at the benzylic position; <b>2</b> gave a higher turnover number (11 TO) than <b>1</b> (6 TO) in 12 h. Using <i>d</i>₄-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex <b>1</b>. (Phebox)­Ir­(OCOCF₃)₂OH₂ catalyzed the benzylic C–H oxidation of mesitylene using Ag₂O as a terminal oxidant at 130 °C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C–H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C–H hydroxylation. Results of calculations of the C–H activation pathway appear to be the more consistent with the experimental observations