Generating Cu-II-Oxyl/Cu-III-Oxo Species from Cu-I-alpha-Ketocarboxylate Complexes and O-2: In Silico Studies on Ligand Effects and C-H-Activation Reactivity

A mechanism for the oxygenation of Cu-I complexes with alpha-keto-carboxylate ligands that is based on a combination of density functional theory and multireference second-order perturbation theory (CASSCF/CASPT2) calculations is elaborated. The reaction proceeds in a manner largely analogous to those of similar Fe-II-alpha-ketocarboxylate systems, that is, by initial attack of a coordinated oxygen molecule on a ketocarboxylate ligand with concomitant decarboxylation. Subsequently, two reactive intermediates may be generated, a Cu-peracid structure and a [CuO](+) species, both of which are capable of oxidizing a phenyl ring component of the supporting ligand. Hydroxylation by the [CuO](+) species is predicted to proceed with a smaller activation free energy. The effects of electronic and steric variations on the oxygenation mechanisms were studied by introducing substituents at several positions of the ligand backbone and by investigating various N-donor ligands. In general, more electron donation by the N-donor ligand leads to increased stabilization of the more Cu-II/Cu-III-like intermediates (oxygen adducts and [CuO](+) species) relative to the more Cu-I-like peracid intermediate. For all ligands investigated the [CuO](+) intermediates are best described as Cu-II-O center dot(-) species with triplet ground states. The reactivity of these compounds in C-H abstraction reactions decreases with more electron-donating N-donor ligands, which also increase the Cu-O bond strength, although the Cu-O bond is generally predicted to be rather weak (with a bond order of about 0.5). A comparison of several methods to obtain singlet energies for the reaction intermediates indicates that multireference second-order perturbation theory is likely more accurate for the initial oxygen adducts, but not necessarily for subsequent reaction intermediates.