How molecular oxygen binds to bis[trifluoroacetylacetonato(−1)]cobalt(ii) – ab initio and density functional theory studies

Abstract

Cobalt(II) diketonate complexes, such as bis[trifluoroacetylacetonato(−1)]cobalt(II) [Co(tfa)₂], catalyze the aerobic oxidation of alkenols into functionalized tetrahydrofurans. To gain insight into activation of triplet dioxygen by Co(tfa)₂ in a protic solvent, as used in oxidation catalysis, the electronic structure of aquabis[trifluoroacetylacetonato(−1)]cobalt(II)—Co(tfa)₂(H₂O)—and the derived dioxygen adduct were characterized using ab initio (CASSCF, NEVPT2) and density functional theory (BP86, TPSSh, B3LYP) methods. The ground state of Co(tfa)₂(H₂O) is a high-spin, quartet state. As dioxygen approaches the cobalt atom, the quartet state couples with a triplet dioxygen molecule and forms a sextet, a quartet, and a doublet spin state with the high-spin state being the lowest in energy. At the equilibrium Co–O₂ distance of 1.9 Å, Co(tfa)₂(H₂O)(O₂) has a doublet superoxo Co(III) ground state with the unpaired electron residing on the oxygen moiety, in a nearly unchanged O₂ π* orbital.