Experimental and quantum computational study of two new bridged copper(II) coordination complexes as possible models for antioxidant superoxide dismutase: Molecular structures, X-band electron paramagnetic spectra and cryogenic magnetic properties

Abstract

In the present study, we use a dual approach comprising experimental and quantum computational studies of two new bridged copper(II) coordination complexes with NNO donor ligands, viz., [Cu2(μ-sulfato)(L)2(2H2O)]·1.5H2O (1) and [Cu2(μ-succinato)(L)(HL)(H2O)]ClO4 (2), where HL/L = N′-[(E)-pyridin-2-ylmethylidene]benzohydrazide, have been synthesized and characterized using various physico-chemical techniques. Both complexes are structurally characterized using single crystal X-ray diffraction studies. The distances between two copper centers are 3.270(2) Å and 3.178(1) Å, for 1 and 2, respectively. On the basis of quantum computational DFT study, electronic excitations involve transitions mainly from metal-ligand bonding MO’s to the β-LUMO within the dominant Cu atom exhibiting dxy character and to the β-LUMO+1. EPR spectra for these polycrystalline samples were determined for the copper(II) hyperfine structures as well their zero-field splitting which are appropriate for the triplet state of such dimers. Cryomagnetic behavior is consistent with weak antiferromagnetic interactions between the Cu(1)⋯Cu(2) centers in both complexes. The magnetic exchange coupling constant (J) between the Cu(1)⋯Cu(2) centers for 1 and 2 were determined to be J = −1.50(1) and J = −7.7(1)cm−1, respectively. CH⋯π, π⋯π, and lone pair⋯π interactions which have gained attention and their role in bimolecular structure analysis has been recognized. In addition, antioxidant superoxide dismutase activity measurements have showed that homodinuclear complexes give significant scavenging effects against superoxide free radicals. Complex 2 is more antioxidant superoxide dismutase active than 1.