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.