Crystal structures and polyphenol oxidase activities of dinuclear copper(II) and cobalt(II) complexes with N, N, N′, N′-tetrakis (2′-benzimidazolylmethyl)-1,4-diethylene amino glycol ether (EGTB)

Abstract

Dinuclear complexes [Cu 2(EGTB)(H 2O) 2(ClO 4) 2](ClO 4) 2 · 6H 2O (I) and [Co 2(EGTB)Cl 2](ClO 4) 2 · 5H 2O (II) [EGTB = N, N, N′, N′-tetrakis(2′-benzimidazolylmethyl)-1,4-diethylene amino glycol ether] have been synthesized and characterized. In the crystal structure of complex I, the coordination geometry of each copper(II) atom is distorted octahedral, which consists of three Cu–N bonds and a Cu–O(coordinated water) bond (1.975(2) Å) in the basal plane, as well as axial weak bonds Cu–O(EGTB ether oxygen) (2.4070(19) Å) and Cu–O(perchlorate) (2.610(8) Å). The two Cu–N(benzimidazole, bzim) bond lengths are 1.961(2) and 1.964(2) Å, and the Cu–N(amino) distance is 2.083(2) Å. Each cobalt(II) center of complex II has a trigonal bipyramidal coordination, the average bond lengths of Co–N(bzim), Co–N(amino), Co–O(ether) and Co–Cl are 2.0325, 2.2844, 2.0845 and 2.2937 Å, respectively. The internuclear distances of complex I and II are 8.038(8) and 5.7138(10) Å, respectively. Polyphenol oxidase activities toward pyrogallol and catechol have been studied. Their kinetics obey the Michaelis–Menten equation. The turnover numbers of the complexes toward pyrogallol are 1.92(I) and 2.42(II) min −1, and 0.018(I) and 0.117(II) min −1 for catechol. The Michaelis constants at pH 8.0 are 0.43e − 3(I), 1.34e − 3(II) and 2.15e − 3(I), 2.77e − 3(II) toward pyrogallol and catechol, respectively. By a comparison of the kinetic data, it is found that pyrogallol oxidation is easier than catechol oxidation, and the catalytic activities of complex II are higher than those of complex I, and moreover they increase with increasing pH values. Electrospray ionization mass spectra (ESI-MS) show that pyrogallol is first oxidized to quinone, which is further condensed to give a purpurogallin and its dimer ( m/ z = 436), associating by hydrogen bonds. The catechol oxidation is similar to pyrogallol: first it results in quinone, then quinone is cyclo-opened to produce 2-hydroxymuconic aldehyde, which is followed by condensation with catechol to form a purpurogallinoid, as well as being cleft to give some small molecular ion fragments due to McLafferty rearrangement.