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

New Cu^II and Ni^II 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 Ni^II 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 = 0.54 V 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 Cu^II centre. [Ni(1)₂] possesses a reversible metal-based oxidation process at E_1/2 = 0.03 V versus [Fc]⁺/[Fc] and a further oxidation, attributed to the generation of a phenoxyl radical centre, at  = 0.44 V versus [Fc]⁺/[Fc]. UV-Vis absorption and EPR spectroscopic studies indicate that the lower potential process is a formal Ni^III/II couple. In contrast, the pro-ligands 1H and 2H exhibit chemically irreversible oxidation processes at  = 0.42 and 0.40 V versus Fc⁺/Fc, respectively, and do not support the formation of stable phenoxyl radical species.