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

A pincer iridium(III) complex, (Phebox)Ir(OAc)₂OH₂ (1) (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) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF₃)₂OH₂ (2), was synthesized to compare with complex 1 for C-H activation, and (Phebox)Ir(mesityl)(OCOCF₃) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D₂O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d₄-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (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. (Chemical Equation Presented).