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

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Three six‐coordinate cobalt(III) complexes containing electron‐rich phenolato pentadentate [N2O5] ligands were synthesized and characterized, namely, [CoIII(L1)(MeOH)] (1), [CoIII(L2)(MeOH)] (2) and [CoIII(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 CoII/CoIII 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.