Luminescent closed shell nickel(II) pyridyl-azo-oximates and the open shell anion radical congener: molecular and electronic structure, ligand redox behaviour and biological activity

Luminescent nickel(II) complexes of type [Ni^II(L^−I)₂], 2, have been synthesized using photosensitizer redox-active oxime ligands HL, 1, incorporating the π-acidic azo and pyridyl functions. The redox non-innocent behaviour of the ligands has been exploited to isolate the open shell Ni(II)-bound azo-oxime anion radical complexes of type Et₄N[Ni^II{(L^−I)₂}^•−], 3, via reduction with NaBH₄. The superior stabilization of the unpaired spin over the ligand framework has been established by EPR and DFT studies. The anti-bacterial activity of 2 has been scrutinized and it has been found to exhibit potential radical scavenging activity.