Unprecedented tetranuclear complexes with “weighing balance shaped” topology: single crystal structures, unusual EPR spectra, magnetic properties and antioxidant activity

Abstract

Using the 2-hydroxy-N'-[(E)-phenyl(pyridin-2-yl)methylidene]benzohydrazide ligand and dicarboxylate anions (succinate and malonate), two new copper(ii) complexes of formula [Cu₄(μ-L)₄(μ-CH₂(COO)₂)(H₂O)₃](NO₃)₂·4H₂O (1) and [Cu₄(μ-L)₄(μ-CH₂-CH₂(COO)₂)(H₂O)](NO₃)₂] (2) have been synthesized and characterized by single crystal X-ray diffraction. Complexes 1 and 2 contain a dicarboxylate bridged tetranuclear Cu(ii) cationic unit with "weighing balance shaped" topology. These compounds have also been studied by variable temperature magnetic susceptibility measurements, X-band EPR spectroscopy and electrochemistry. The unprecedented tetranuclear copper complexes 1 and 2 exhibit antiferromagnetic interactions between adjacent copper(ii) centers with magnetic exchange coupling constants (J) of J₁ = -6.8 cm^-1, J₂ = -20.6 cm^-1 and g = 2.03 for 1 and J₁ = -3.1 cm^-1, J₂ = -13.9 cm^-1 and g = 2.02 for 2. The goal for our research work is to understand the factors that govern the stability of the Cu-O(-O-C-O-) bond towards reactive oxygen species (ROS) in Cu-containing mimics utilizing a systematic approach. The antioxidant superoxide dismutase measurements indicate that these complexes behave as superoxide dismutase mimics.