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 N-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).