Co and Cu complexes of reduced Schiff bases: Generation of phenoxyl radical species

The three complexes [Co^IIIL¹Cl] (1), [Co^IIIL²]⁺·ClO₄⁻ (2⁺·ClO₄⁻), and [Cu^IIH₂L²]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.