Zinc(II) thione and selone complexes: The effect of metal redox activity on ligand-based oxidation

Upon redox-inactive Zn(II) coordination, thione and selone ligands are harder to oxidize than similar complexes with redox-active metals, demonstrating the importance of metal coordination on ligand-based antioxidant activity. [Display omitted] •Zn complexes with imidazole thione and selone ligands were synthesized.•Electronic spectra and DFT studies of Zn(II) compared with Fe(II) complexes.•Zn(dmit/dmise)2Cl2 complexes oxidize upon hydrogen peroxide addition.•Zn(II) binding decreases thione/selone ligand oxidation relative to Fe(II) binding.•Antioxidant properties of thione/selone ligands depend upon metal redox activity. Thione and selone ligands preferentially oxidize to protect redox active-metals from oxidation, but the effects of thione or selone coordination to a redox-inactive metal have not been examined. Therefore, Zn(II)-containing thione and selone complexes of the formulae Zn(L)2Cl2 and [Zn(L)4]2+ (L = N,N′-dimethylimidazole thione, dmit, and selone, dmise) have been synthesized and their properties, including electronic spectra and DFT calculations, compared with analogous Fe(II) complexes. The Zn(dmit/dmise)2Cl2 complexes were also oxidized with H2O2, resulting in oxidation and CS/Se bond cleavage of the dmit and dmise ligands. Electrochemical studies show that upon redox-inactive Zn(II) coordination, dmit- and dmise-based oxidation potentials decrease relative to those of analogous redox-active Fe(II) complexes, demonstrating that the redox activity of the metal ion significantly effects the redox properties of the ligands.