Spectroelectrochemical, photochemical and theoretical study of octaazamacrocyclic nickel(II) complexes exhibiting unusual solvent-dependent deprotonation of methylene group

Abstract

The reduction potentials and the reversibility of corresponding redox processes in a series of nickel(II) complexes, featuring 15-membered (1–5) and 14-membered (6) octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide, strongly depend on the number of phenyl and methyl substituents on the macrocycle. The reaction mechanism for the ligand-based reduction of 1–5 is proposed. One-electron reduction generates unstable ligand-centred radical anions that lose a hydrogen atom from the macrocycle, and transform to stable monoanionic EPR silent species. After the re-oxidation at the strongly shifted anodic potential, a tautomerised neutral Ni(II) complex can be reduced again to the nickel(II) ligand-based radical. This was confirmed by NMR, UV–vis–NIR and EPR spectroelectrochemical measurements in different media. Density functional theory (DFT) calculations were performed to support the assignment of species generated upon reduction. Additionally, we showed that photochemical stability and photochemically induced reactions of 1–5 strongly depend on the form of the complex in the solution, where the deprotonated form is more reactive.