Molecular and electronic structure, spectroscopic and electrochemical properties of Copper(II) complexes: Experimental and DFT studies

Abstract

Mononuclear copper(II) Schiff base complexes [Cu (L1)2] and [Cu (L2)2] (HL1 = 4-((E)-phenyldiazenyl)-2-((E)-(propylimino) methyl) phenol and HL2 = salicylidenepropylamine) were synthesized and characterized by elemental analysis and spectroscopic techniques. X-ray diffraction analysis showed that, in the solid state, the complex [Cu(L1)2] is centro-symmetric and the copper(II) ion has square-planar coordination geometry. It was found that the crystal packing of the compound is stabilized by π···π interactions between the phenyl and metal chelate rings. Molecular and electronic structures and electronic transitions of [Cu(L1)2], [Cu(L2)2] and free ligands were studied by calculations at DFT/B3LYP level of theory. According to the DFT calculations, the coordination geometry of the copper(II) in [Cu(L1)2] is deformed in the gas phase. The free gaseous [Cu(L2)2] complex was obtained as almost centro-symmetric square planar geometry. DFT calculations indicated that, by reduction of the complexes, the electron is added to the metal dx2-y2 orbital and copper ions in [Cu(L1)2]‾ and [Cu(L1)2]‾ show highly distorted tetrahedral coordination geometry. By one-electron oxidation of the complexes, d9 copper(II)-radical ligand complexes are formed in gaseous state. Moreover, gas phase 3[Cu(L1)2]+ and 3[Cu(L2)2]+ complexes are more stable than the corresponding geometries with singlet spin configuration. The measured UV–Vis spectrum of [Cu(L1)2] is better described by TDDFT calculation results for the optimized compound, rather than the centro-symmetric square-planar complex. Redox peak separations (ΔE = Ea-Ec) for [Cu(L1)2] and [Cu(L2)2] were measured by cyclic voltammetry at the scan rate of 0.1 V-1, to be 340 mV and 240 mV, respectively.