Syntheses, electronic structures, and EPR/UV-vis-NIR spectroelectrochemistry of nickel(II), copper(II), and zinc(II) complexes with a tetradentate ligand based on S-methylisothiosemicarbazide

Template condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione and triethyl orthoformate at elevated temperature resulted in metal complexes of the type MIIL, where M = Ni, Cu and H2L = novel tetradentate organic ligand. These complexes are relevant to the active site of the copper enzymes galactose oxidase and glyoxal oxidase. Demetallation of NiIIL with gaseous hydrogen chloride in chloroform afforded the metal-free ligand H2L. Then by the reaction of H2L with Zn(CH3COO)2·2H2O in 1:1 molar ratio in chloroform/methanol 1:2 the complex ZnIIL(CH3OH) was prepared. The three metal complexes and the prepared ligand were characterized by spectroscopic methods (IR, UV–vis and NMR spectroscopy), X-ray crystallography and DFT calculations. Electrochemically generated one-electron oxidized metal complexes [NiL]+, [CuL]+ and [ZnL(CH3OH)]+ and the metal-free ligand cation radical [H2L]+• were studied by EPR/UV–vis–NIR and DFT calculations. These studies demonstrated the interaction between the metal ion and the phenoxyl radical.