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

Abstract

Three dinuclear Co^III/II complexes [(Co^III₂(H₂L)₂(OAc)₂)]·CH₃OH (1), [Co^IICo^III(H₂L)₂(OAc)]·2CH₃OH·H₂O (2), and [Co^IICo^III(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 Co^III 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 Co^II and Co^III per molecule, show no catechol oxidase activity although the Co^II ion has labile sites in both 2 and 3. The cyclic voltammetry studies show that only 1 exhibits a Co^III/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 (k_cat) 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 Co^III centres in 1 rather than one DTBC bridging the two Co^III 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.