title: Mono- and dinuclear manganese(III) complexes showing efficient catechol oxidase activity: syntheses, characterization and spectroscopic studies.
abstract: 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 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.
authors.affiliation: Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata-700 009, India.
authors.forename: Kazi Sabnam
authors.initials: KS
authors.lastname: Banu
authors.forename: Tanmay
authors.initials: T
authors.lastname: Chattopadhyay
authors.forename: Arpita
authors.initials: A
authors.lastname: Banerjee
authors.forename: Madhuparna
authors.initials: M
authors.lastname: Mukherjee
authors.forename: Santanu
authors.initials: S
authors.lastname: Bhattacharya
authors.forename: Goutam Kumar
authors.initials: GK
authors.lastname: Patra
authors.forename: Ennio
authors.initials: E
authors.lastname: Zangrando
authors.forename: Debasis
authors.initials: D
authors.lastname: Das
mesh_terms: D004156:Catechol Oxidase / Q000737:chemistry / Q000378:metabolism; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D008024:Ligands; D008345:Manganese / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D008345:Manganese; D004156:Catechol Oxidase