Benzene C-H bond activation in carboxylic acids catalyzed by O-donor iridium(III) complexes: An experimental and density functional study

The mechanism of benzene C-H bond activation by [Ir(«-acac-O,O,C ³)(acac-O,O)(OAc)]₂ (4) and [Ir(μ-acac-O,O,C ³)(acac-O,O)(TFA)]₂ (5) complexes (acac = acetylacetonato, OAc = acetate, and TFA = trifluoroacetate) was studied experimentally and theoretically. Hydrogen-deuterium (H/D) exchange between benzene and CD ₃COOD solvent catalyzed by 4 (ΔH = 28.3± 1.1 kcal/mol, ΔS = 3.9±3.0 cal K^-1 mol^-1) results in a monotonie increase of all benzene isotopologues, suggesting that once benzene coordinates to the iridium center, there are multiple H/D exchange events prior to benzene dissociation. B3LYP density functional theory (DFT) calculations reveal that this benzene isotopologue pattern is due to a rate-determining step that involves acetate ligand dissociation and benzene coordination, which is then followed by heterolytic C-H bond cleavage to generate an iridium-phenyl intermediate. A synthesized iridium-phenyl intermediate was also shown to be competent for H/D exchange, giving similar rates to the proposed catalytic systems. This mechanism nicely explains why hydroarylation between benzene and alkenes is suppressed in the presence of acetic acid when catalyzed by [Ir(μ-acac-O,O,C³)(acac-O,O)(acacC³)]₂ (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF₃COOD solvent catalyzed by 5 (ΔH = 15.3 ± 3.5 kcal/mol, ΔS =-30.0 ± 5.1 cal K^-1 mol^-1) results in significantly elevated H/D exchange rates and the formation of only a single benzene isotopologue, (C₆H₅D). DFT calculations show that this is due to a change in the rate-determining step. Now equilibrium between coordinated and uncoordinated benzene precedes a single rate-determining heterolytic C-H bond cleavage step.