The synthesis, characterization and catecholase activity of dinuclear cobalt(ii/iii) complexes of an O-donor rich Schiff base ligand?

New Journal of Chemistry Pub Date: 2014-08-26 DOI: 10.1039/C4NJ00715H

Abstract

Three dinuclear CoIII/II complexes [(CoIII2(H2L)2(OAc)2)]·CH3OH (1), [CoIICoIII(H2L)2(OAc)]·2CH3OH·H2O (2), and [CoIICoIII(H2L)2(CH3CN)(H2O)]Cl·CH3CN·4H2O (3) [H4L is (3,5-di-tert-butyl-2-hydroxybenzylideneamino)-2-(hydroxymethyl)propane-1,3-diol] were synthesized and characterized using single crystal X-ray diffraction and other analytical methods. Complex 1 having two CoIII centres was the only one found to show catecholase activity, first order with respect to the substrate in the oxidation of 3,5-di-tert-butyl catechol (DTBC) to 3,5-di-tert-butyl benzoquinone (DTBQ). In contrast complexes 2 and 3, which have a CoII and CoIII per molecule, show no catechol oxidase activity although the CoII ion has labile sites in both 2 and 3. The cyclic voltammetry studies show that only 1 exhibits a CoIII/II redox couple whereas the metal centers in 2 and 3 do not show any redox activity. The kinetic studies confirm that the turnover number (kcat) is 79.8 h?1. Unlike the enzyme which coordinates to one molecule of catechol during a catalytic cycle, the mass spectral studies support the coordination of two molecules of DTBC simultaneously during a catalytic cycle to the two CoIII centres in 1 rather than one DTBC bridging the two CoIII centres, which renders this complex unique among the mimics of catechol oxidase. The mechanistic studies show no involvement of singlet oxygen, superoxide or hydroxyl radical as ROS. However the results support the production of hydrogen peroxide during oxidation of DTBC to DTBQ. We found that esters of amino acids completely inhibit the oxidation of DTBC through competitive coordination to 1.

Graphical abstract: The synthesis, characterization and catecholase activity of dinuclear cobalt(ii/iii) complexes of an O-donor rich Schiff base ligand
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