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The international journal for inorganic, organometallic and bioinorganic chemistry



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Dalton Trans., 2009, 8755 - 8764, DOI: 10.1039/b902498k

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

Kazi Sabnam Banu, Tanmay Chattopadhyay, Arpita Banerjee, Madhuparna Mukherjee, Santanu Bhattacharya, Goutam Kumar Patra, Ennio Zangrando and Debasis Das

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 E1/2 value the higher the catalytic activity, can be drawn between E1/2 and Vmax 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.

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