The synthesis and characterisation of phenolate complexes of Cu(II) and Ni(II) that are capable of supporting a phenoxyl radical ligand

Abstract: New CuII and NiII complexes of potentially tridentate N2O Schiff base ligands 1 and 2 have been synthesised and characterised. [Cu(2)(OH2)]+ possesses a square planar geometry in the solid state whereas [Ni(1)2] possesses a distorted octahedral geometry in which the amine donors of 1 coordinate weakly to the NiII centre. EPR spectroscopy demonstrates that the N2O2 coordination sphere of [Cu(2)(OH2)]+ is retained in CH2Cl2 solution. [Cu(2)(OH2)]+ exhibits a reversible one electron oxidation at E 1/2 =0.54V versus [Fc]+/[Fc], the product of which has been characterised by UV–Vis absorption and EPR spectroscopies. The spectroscopic signature of the oxidised product is consistent with the formation of a stable phenoxyl radical ligand bound to a CuII centre. [Ni(1)2] possesses a reversible metal-based oxidation process at E 1/2 =0.03V versus [Fc]+/[Fc] and a further oxidation, attributed to the generation of a phenoxyl radical centre, at =0.44V versus [Fc]+/[Fc]. UV–Vis absorption and EPR spectroscopic studies indicate that the lower potential process is a formal NiIII/II couple. In contrast, the pro-ligands 1H and 2H exhibit chemically irreversible oxidation processes at =0.42 and 0.40V versus Fc+/Fc, respectively, and do not support the formation of stable phenoxyl radical species. [Copyright &y& Elsevier]