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 O₂. The complexes have the general formula, [Co^II(L)(Tp^R2)] (R = Me, Ph), where L is a bidentate o-aminothiophenolate and Tp^R2 is a hydrotris(pyrazol-1-yl)borate scorpionate with R-substituents at the 3- and 5-positions. Exposure to O₂ at room temperature results in one-electron oxidation and deprotonation of L. 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^II(Tp^Me2)(^X2ITSQ)] complexes, where X = H or tBu, coordinate O₂ reversibly at reduced temperatures to provide Co/O₂ adducts. The O₂ binding reactions closely resemble those previously reported by our group (Kumar et al., J. Am. Chem. Soc. 2019,141, 10984-10987) for the related complexes [Co^II(Tp^Me2)(^tBu2SQ)] and [Co^II(Tp^Me2)(^tBu2ISQ)], where ^tBu2(I)SQ represents 4,6-di-tert-butyl-(2-imino)semiquinonate radicals. In each case, the oxygenation reaction proceeds via the addition of O₂ to both the cobalt ion and the ligand radical, generating metallocyclic cobalt(III)-alkylperoxo structures. Thermodynamic measurements elucidate the relationship between O₂ 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 O₂-absorbing compounds.