Sequential Phenolate Oxidations in Octahedral Cobalt(III) Complexes with [N₂O₃] Ligands

Abstract

Three six-coordinate cobalt(III) complexes containing electron-rich phenolato pentadentate [N₂O₅] ligands were synthesized and characterized, namely, [Co^III(L¹)(MeOH)] (1), [Co^III(L²)(MeOH)] (2) and [Co^III(L³)(MeOH)] (3), where L¹, L² and L³ are the triply deprotonated, triply negative form of(E)-6,6′-[({2-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]phenyl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol), (E)-6,6′-[({3-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]naphthalen-2-yl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol) and (E)-6,6′-[({2-[(2-hydroxy-3-methoxybenzylidene)amino]phenyl}azanediyl)bis(methylene)]bis(2,4-di-tert-butylphenol), respectively. Crystal structures were obtained for 1–3 and reveal a hexacoordinate cobalt(III) ion bound to the [N₂O₃] donors of each ligand and a methanol molecule occupying the sixth position. The complexes exhibited comparable electronic behavior dominated by phenolate→cobalt charge transfer processes and four redox-accessible states involving three distinct phenolato/phenoxyl radical couples and a fourth process associated with the Co^II/Co^III couple. The redox processes were cycled 30 times without major decomposition at the surface of the electrode for 1 and 2, indicating that the oxidized species should be substitutionally inert and do not degrade significantly upon cycling. Electronic-structure DFT calculations on models 1′ and 2′ favor the generation of localized phenoxyl radicals and suggest distinctive oxidation sequences associated to the nature of the ligands.