Syntheses, crystal structure, and nuclease activity of oxalato-bridged dicopper(II) complexes with planar N-donor heterocyclic bases

Abstract

Oxalato-bridged dicopper(II) complexes [{CuL(H₂O)}₂(μ-ox)](ClO₄)₂ (1-4), where L is a bidentate chelating heterocyclic base such as 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyridoquinoxaline (dpq, 3) and dipyridophenazine (dppz, 4), were prepared by treatment of copper(II) perchlorate hexahydrate with sodium oxalate and L in aqueous ethanol. Complex 3 has been structurally characterised. The coordination geometry of the copper centres is essentially square-pyramidal. The {CuL(H₂O)} units are bridged by an oxalate moiety to form the discrete dimeric core. Magnetic studies in the 20-300 K temperature range show the presence of an antiferromagnetically coupled dicopper(II) unit, giving -2J values in the 346-384 cm^-1 range. The redox-active complexes retain the dimeric core in solution, and two quasireversible Cu^II/Cu^I couples are observed in the potential range from 0.2 to -0.4 V vs. SCE in a DMF/tris-HCl/0.1 M KCl buffer system (1:1, v/v, pH = 7.2) with a glassy carbon working electrode. The binding and cleavage of DNA by the complexes have been studied. The intercalating ability of the planar bases, determined from the decrease of the fluorescence intensity of ethidium bromide bound calf thymus DNA on addition of the complex, gave the order: 3 > 4 > 2 >> 1. The nuclease activity of the complexes with supercoiled pUC19 DNA in DMF/tris-HCl/NaCl buffer (pH = 7.2) in the presence of ascorbic acid follows the order: 3 > 2 >> 4 ≈ 1. The dpq complex exhibits the most efficient DNA binding and cleavage activity. The dppz complex shows good binding ability but poor nuclease activity.