Binuclear Copper(II) Chelates with Heptadentate Ligands: Synthesis, Structure, Magnetic Properties, DFT Studies, and Catecholase and Hydrolytic DNA Cleavage Activity

Abstract

Two binucleating heptadentate ligands 2,6-bis[{[(2-hydroxybenzyl)(N,N-(dimethylamino)ethyl]amino}methyl]-4-methylphenol (H₃L¹) and 2,6-bis[{(2-hydroxybenzyl)(N-(2-pyridylmethyl)amino}methyl]-4-methylphenol (H₃L²) were used to synthesize the two new copper(II) complexes [Cu₂(L¹)(N₃)]·2H₂O (1·2H₂O) and [Cu₂(HL²)(O₂CPh)(H₂O)]PhCO₂·H₂O (2·H₂O). X-ray diffraction studies disclose that 1 is made up from bridging phenoxido and azido group in an equatorial fashion, whereas 2 is bridged axially–equatorially through a central cresolato and syn–syn benzoate moiety. The geometry around the copper(II) centers is distorted square pyramid in both cases. Variable-temperature magnetic susceptibility data reveals that 1 is moderately antiferromagnetically coupled (J = –119 cm^–1) and 2 is very weakly antiferromagnetic (J = –1.0 cm^–1). The structural features, as well as the presence of orbital countercomplementary effects, are associated with the magnetic behavior. Theoretical calculations with the use of broken symmetry density functional theory also establish the experimental values of the exchange coupling constants (J). In our case, only 2 exhibits catalytic activity in the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) at pH 9.5 and hydrolytic cleavage of plasmid DNA in the absence of any added cofactor, whereas complex 1 cannot display catecholase activity or DNA interaction as a result of strong Cu^II–azido binding.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)