Generating CuII-Oxyl/CuIII-Oxo species from CuI-a-Ketocarboxylate complexes and O2: In silico studies on ligand effects and C-H-Activation reactivity

A mechanism for the oxygenation of CuI complexes with a-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 FeII-a-ketocarboxylate systems, that is, by initial attack of a coordinated oxygen molecule on a ketocarboxylate ligand with concomitant decarboxyla-tion. Subsequently, two reactive intermediates may be generated, a Cu-pera-cid 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 substitu-ents 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 CuII/CuIII-like intermediates (oxygen adducts and [CuO]+ species) relative to the more CuI-like peracid intermediate. For all ligands investigated, the [CuO] + intermediates are best described as Cu11-O-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.