Sequential phenolate oxidations in octahedral cobalt(III) complexes with [N _2O _3] ligands (Conference Paper)

摘要

Three six-coordinate cobalt(III) complexes containing electron-rich phenolato pentadentate [N _2O _5] ligands were synthesized and characterized, namely, [Co ~(III)(L ~1)(MeOH)] (1), [Co ~(III)(L ~2)(MeOH)] (2) and [Co ~(III)(L ~3)(MeOH)] (3), where L ~1, L ~2 and L ~3 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 [N _2O _3] 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.