Redox properties of zinc complexes of verdazyl radicals and diradicals

Reactions of ZnCl2 with tridentate ligands based either on a 2,2′-bipyridine-substituted verdazyl radical (1) or a 2,6-pyridine-linked verdazyl diradical (2) give trigonal bipyramidal complexes 1·ZnCl2 and 2·ZnCl2, respectively in which the Zn–N bonds to the verdazyl nitrogen atoms are considerably longer than the Zn–N (pyridine) bonds. Electronic and EPR spectroscopy and magnetic susceptibility studies indicate that the perturbation of the verdazyl chromophore in the two Zn complexes is small. However, the redox properties of the Zn complexes are substantially different from those of the free ligand. The oxidation and reduction potentials of the verdazyl radical 1 both shift to more positive potentials in 1·ZnCl2. Voltammetry studies of diradical complex 2·ZnCl2 reveal substantial changes: reduction of the two verdazyl moieties in the complex occurs in a stepwise manner, whereas in the free ligand the two verdazyls have nearly identical reduction potentials. Oxidation of 2·ZnCl2 appears to lead to hemilabile behavior, i.e. the verdazyl–Zn bonds are broken reversibly upon oxidation, based on the voltammetric profile and also based on spectroscopic studies of the neutral and oxidized form of this complex.