Nondirected C–H Activation of Arenes with Cp*Ir(III) Acetate Complexes: An Experimental and Computational Study

Combined experimental and computational studies have revealed factors that influence the nondirected C–H activation in Cp*Ir complexes that contain carboxylate ligands. A two-step acetate-assisted pathway was shown to be operational where the first step involves substrate binding and the second step involves cleavage of the C–H bond of the substrate. A nonlinear Hammett plot was obtained to examine substituted arenes where a strong electronic dependence (ρ = 1.67) was observed for electron-donating groups, whereas no electronic dependence was observed for electron-withdrawing groups. Electron-donating substituents in the para position were shown to have a bigger impact on the C–H bond cleavage step, whereas electron-withdrawing substituents influenced the substrate-binding step. Although cleavage of the C–H bond was predicted to be more facile with arenes that contain substituents in the para position by DFT calculations, the cyclometalations of anisole and benzonitrile were observed experimentally. This suggests that these substituents, even though they are weakly directing, still result in cyclometalation because the barriers for activation at the ortho and para positions of arenes are comparable (24.3 and 26.5 kcal/mol, respectively). Incorporation of a weakly bound ligand was found to be necessary for facile reactivity. It is predicted by DFT calculations that the replacement of an oxygen atom with a nitrogen atom in the carboxylate ligand would lead to a dramatic reduction in the barrier for C–H activation, as the incorporation of formimidate and <i>N</i>-methyl­formimidate ligands leads to barriers of 23.4 and 21.7 kcal/mol, respectively. These values are significantly lower than the barrier calculated for the analogous acetate ligand (28.2 kcal/mol)