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

The three complexes [(CoLCl)-L-III-Cl-1] (1), [(CoL2)-L-III]+center dot ClO4 (2(+)center dot ClO4), and [(CuH2L2)-H-II](2+)center dot 2ClO(4) (H(2)3(2+)center dot 2ClO(4)) [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 H(2)3(2+) 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(center dot))(+), (2(center dot))(2+), and (2(center dot center dot))(3+) have been characterized by combined UV-Vis and EPR spectroscopies. In the presence of one equivalent of base, one phenolic arm of H(2)3(2+) 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 nBu(4)N(+)OH to H(2)3(2+). 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. (C) 2010 Elsevier B.V. All rights reserved.