Mono- and dinuclear manganese(III) complexes showing efficient catechol oxidase activity: syntheses, characterization and spectroscopic studies

Four side-off compartmental ligands L¹–L⁴ [L¹ = N,N′-ethylenebis(3-formyl-5-methyl-salicylaldimine), L² = N,N′-1-methylethylenebis(3-formyl-5-methylsalicylaldimine), L³ = N,N′-1,1-dimethylethylenebis(3-formyl-5-methylsalicylaldimine) and L⁴= N,N′-cyclohexenebis(3-formyl-5-methylsalicylaldimine)] having two binding sites, N₂O₂ and O₄, have been chosen to synthesize mononuclear and dinuclear manganese(III) complexes with the aim to study their catecholase activity using 3,5-di-tert-butylcatechol (3,5-DTBC) as substrate in the presence of molecular oxygen. In all cases only mononuclear manganese complexes (1–4) were obtained, with manganese coordination taking place at the N₂O₂ binding site only, irrespective of the amount of manganese salt used. All these complexes have been characterized by routine physico-chemical techniques. Complex MnL²Cl·4H₂O (2) has further been structurally characterized by X-ray single crystal structure analysis. Four dinuclear manganese complexes, 5–8, were obtained after condensing the two pending formyl groups on each ligand (L¹–L⁴) with aniline followed by reaction with MnCl₂ to put the second Mn atom onto another N₂O₂ site. The catalytic activity of all complexes 1–8 has been investigated following the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) with molecular oxygen in two different solvents, methanol and acetonitrile. The study reveals that the catalytic activity is influenced by the solvent and to a significant extent by the backbone of the diamine and the behavior seems to be related mainly to steric rather than electronic factors. Experimental data suggest that a correlation, the lower the E_1/2 value the higher the catalytic activity, can be drawn between E_1/2 and V_max of the complexes in a particular solvent. The EPR measurements suggest that the catalytic property of the complexes is related to the metal center(s) participation rather than to a radical mechanism.