Benzene C−H Bond Activation in Carboxylic Acids Catalyzed by O-Donor Iridium(III) Complexes: An Experimental and Density Functional Study

• Published in: Organometallics 2010-02, Vol.29 (4), p.742-756
• Main Authors: Bischof, Steven M, Ess, Daniel H, Meier, Steven K, Oxgaard, Jonas, Nielsen, Robert J, Bhalla, Gaurav, Goddard, William A, Periana, Roy A
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om900036j

The mechanism of benzene C−H bond activation by [Ir(μ-acac-O,O,C3)(acac-O,O)(OAc)]2 (4) and [Ir(μ-acac-O,O,C3)(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 (ΔH ‡ = 28.3 ± 1.1 kcal/mol, ΔS ‡ = 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 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)(acac-C3)]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.