Phenoxo-bridged dicopper complexes: Syntheses, characterizations, crystal structures and catecholase activity

Abstract

Two new dinuclear copper(II) complexes are synthesized from two reduced Schiff bases namely 2-(((3-(cyclohexylamino)propyl)amino)methyl)-4-methylphenol (HL1) and 2-(((3-(cyclohexylamino)propyl)amino)methyl)-6-methoxyphenol (HL2): [Cu(L1)(ClO4)]2 (1) and [Cu2(L2)2(μ-ClO4)](ClO4) (2) and characterized by various spectroscopic methods. Structural analyses reveal that in 1, two copper(II) ions are held together by bis(phenoxo) bridges, whereas in 2, the copper centres are connected by bis(phenoxo) and μ1,3-ClO4 bridges. Conductivity measurements in methanol of 1 (180 Ohm−1 cm2 mol−1) and 2 (210 Ohm−1 cm2 mol−1) suggest the formation of dicationic species [Cu2(L1/L2)2]2+ with two perchlorates as counter anions. The cyclic voltammograms show two irreversible cathodic peaks for the one electron reduction process CuIICuII → CuIICuI and CuIICuI → CuICuI. The catecholase activity of complexes 1 and 2 has been investigated in methanol under aerobic condition using 3,5-di-tert-butyl catechol (3,5-DtBC) as a substrate. Both complexes follow the enzymatic Michaelis–Menten kinetics with turnover number (TON) 34.32 h−1 (1) and 26.52 h−1 (2). Both steric and electronic factors favour the higher catalytic activity in 1 compared to 2. The formation of product 3,5-di-tert-butyl-o-benzoquinone (3,5-DtBQ) and catechol–substrate complexes were monitored using ESI-mass spectroscopy. A correlation between observed catalytic activities and electrochemical properties has been attempted.