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(mu-acac-O,O,C-3) (acac-O,O)(OAc)](2) (4) and [Ir(mu-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 CD3COOD solvent catalyzed by 4 (Delta H-double dagger =228.3+/-1.1 kcal/mol, AS = 3.9+/-3.0 cal K-1 mol(-1)) results in a monotonic 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 in iridium-phenyl in termed late. 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(mu-acac-O,O,C-3)(acac-O,O)(acac- C-3)](2) (3) (Matsumoto ct al..J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF3COOD solvent catalyzed by 5(Delta H-double dagger = 15.3+/-3.5 kcal/mol, Delta S-double dagger = -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.