CoIII and CuII complexes of reduced Schiff bases: Generation of phenoxyl radical species

The three complexes [CoIIIL1Cl] (1), [CoIIIL2]+·ClO4− (2+·ClO4−), and [CuIIH2L2]2+·2ClO4− (H232+·2ClO4−) [where H2L1 = N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-di-tert-butylbenzyl)ethylenediamine, H2L2 = N,N′-bis(2-pyridylmethyl)-N,N′-bis(2-hydroxy-3,5-di-tert-butylbenzyl)ethylenediamine] have been prepared. The bis-phenolate and bis-phenol complexes, 2+ and H232+ respectively, have been characterized by X-ray diffraction, showing a metal ion within an elongated octahedral geometry. 1–2 exhibit in their cyclic voltammetry curves two anodic reversible waves attributed to the successive oxidation of the phenolates into phenoxyl radicals. The cobalt radical species (1)+, (2)2+, and (2)3+ have been characterized by combined UV–Vis and EPR spectroscopies. In the presence of one equivalent of base, one phenolic arm of H232+ is deprotonated and coordinates the metal. The resulting complex (H3+) exhibits a single reversible redox wave at ca. 0.3 V. The electrochemically generated oxidized species is EPR silent and exhibits the typical features of a radical compound, with absorption bands at 411 and 650 nm. The fully deprotonated complex 3 is obtained by addition of two equivalents of nBu4N+OH− to H232+. It exhibits a new redox wave at a lower potential (−0.16 V), in addition to the wave at ca. 0.3 V. We assigned the former to the one-electron oxidation of the uncoordinated phenolate into an unstable phenoxyl radical.