Reversible Dioxygen Binding to Co(II) Complexes with Noninnocent Ligands.

A series of mononuclear Co(II) complexes with noninnocent (redox-active) ligands are prepared that exhibit metal-ligand cooperativity during the reversible binding of O. The complexes have the general formula, [Co()(Tp)] (R = Me, Ph), where is a bidentate -aminothiophenolate and Tp is a hydrotris(pyrazol-1-yl)borate scorpionate with R-substituents at the 3- and 5-positions. Exposure to O at room temperature results in one-electron oxidation and deprotonation of . The oxidized derivatives possess substantial "singlet diradical" character arising from antiferromagnetic coupling between an iminothiosemiquinonate (ITSQ) ligand radical and a low-spin Co(II) ion. The [Co(Tp)(ITSQ)] complexes, where X = H or Bu, coordinate O reversibly at reduced temperatures to provide Co/O adducts. The O binding reactions closely resemble those previously reported by our group (Kumar et al., . 10984-10987) for the related complexes [Co(Tp)(SQ)] and [Co(Tp)(ISQ)], where (I)SQ represents 4,6-di--butyl-(2-imino)semiquinonate radicals. In each case, the oxygenation reaction proceeds via the addition of O to both the cobalt ion and the ligand radical, generating metallocyclic cobalt(III)-alkylperoxo structures. Thermodynamic measurements elucidate the relationship between O affinity and redox potentials of the (imino)(thio)semiquinonate radicals, as well as energetic differences between these reactions and conventional metal-based oxygenations. The results highlight the utility and versatility of noninnocent ligands in the design of O-absorbing compounds.