Synthesis, crystal structure, redox behavior and DNA-binding properties of a series of copper(II) complexes with two new carboxamide derivatives

Four mononuclear copper(II) complexes of two new carboxamide derivatives formulated as [Cu(L¹)₂](ClO₄)₂ (<b>1a</b>), [Cu(L¹)₂](NO₃)₂ (<b>1b</b>), [Cu(L²)₂(H₂O)₂](ClO₄)₂ (<b>2a</b>) and [Cu(L²)₂(H₂O)](NO₃)₂ (<b>2b</b>) have been isolated in pure form from the reaction of <b>L</b>^<b>1</b> and <b>L</b>^<b>2</b> [where <b>L</b>^<b>1</b> = <i>N</i>-(furan-2-ylmethyl)-2-pyridinecarboxamide and <b>L</b>^<b>2</b> = <i>N</i>-(thiophen-2-ylmethyl)-2-pyridinecarboxamide] with copper(II) salts of perchlorate and nitrate. All the complexes were characterized by physico-chemical and spectroscopic tools along with single crystal X–ray diffraction studies. The structural analyses showed that <b>1</b> is monomeric of square planar geometry with copper(II) chelated by two <b>L</b>^<b>1</b> ligands. Complexes <b>2</b> differs in coordination geometry, being octahedral and distorted square pyramidal. Two <b>L</b>^<b>2</b> ligands occupy the equatorial positions of the octahedral <b>2a</b> and the basal sites of the pyramidal <b>2b</b>, with water molecules that complete the coordination sphere in each case. Electrochemical studies using cyclic voltammetry showed a reversible redox behavior of the copper(II) in <b>1</b> and <b>2</b>. The electronic spectroscopic behavior and the trend of one electron equivalent redox potential corresponding to a Cu^II/Cu^I couple have also been confirmed by DFT calculations. The spectroscopic and viscosity measurement study in tris-HCl buffer suggested an intercalative interaction of <b>1a</b> and <b>2</b> with calf thymus-DNA (CT-DNA) likely due to the stacking between the non-coordinated furan or thiophene chromophore with the base pairs of DNA.