On the mechanism of reduction of maleate ions by Ni^I complexes with tetraazamacrocyclic ligands in aqueous solutions

Recently, it was shown that Ni^I and Co^I complexes with tetraazamacrocyclic ligands [namely, trans-III-Ni^IL ₃⁺ (L₃ = 1,4,8,11-tetraazacyclotetradecane), Ni^Icyclam and rac-Co^IL₁⁺ (L ₁ = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11- diene, dien)] reduce maleate ions in aqueous solutions. It was decided to expand the study to further Ni^IL_i⁺ complexes. In the present study, the reactions of rac-Ni^IL₁⁺ and meso-Ni^IL₂⁺ (L₂ = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) with maleate ions were investigated. The effect of the addition of maleic acid to aqueous solutions containing these low-valent metal complexes, prepared by the pulse radiolysis technique, was studied. The d→π* complexes formed between monovalent nickel complexes and maleate ions were detected and characterised by UV/Vis spectroscopy. The nature of the final products formed depends strongly on the nature of the metal and the ligand. Detailed mechanisms of the reduction processes are proposed and compared with those previously reported. The results indicate that the electronic configuration of the central cation as well as the reduction potential of the complex and steric hindrance induced by the ligand affect the reduction mechanism of maleate ions by these analogous complexes. The mechanism of reduction of alkenes depends strongly on the electronic configuration of the central cation, the redox potential of the complex and steric factors.