Synthesis, Crystal Structure, Photophysical Properties, and Antibacterial Activities of the Copper(II) Complex Derived from 4-Chloro-2-{[(2,6-Dimethylphenyl)Imino]Methyl}Phenol

Abstract

A new asymmetrical Schiff base ligand, (E)-4-chloro-2-{[(2,6-dimethylphenyl)amino] methyl}phenol (HL), and its mononuclear Cu(II) complex are synthesized. The molecular structures and spectroscopic properties of the ligand and its complex are experimentally characterized by single crystal X-ray diffraction, elemental analysis, FTIR, NMR and UV-Vis spectroscopic techniques. The analysis confirms that the Cu atom is generally four-coordinated in the complex by one imino N and one phenolic O atoms of the Schiff-base ligand, adopting a trans-configuration of N₂O₂ donor atoms around the metal ion, forming a square planar geometry. The density functional calculation is carried out for both HL and the copper(II) complex to investigate changes in the structural parameters and HOMO and LUMO energies. The results demonstrate that the HOMO and the LUMO are effectively separated by the benzene ring of 2,6-dimethylbenzenamine as a donor unit and the benzene ring of 5-chlorosalicylaldehyde as an acceptor unit. The effective HOMO–LUMO separation helps to induce intramolecular charge transfer from the HOMO to the LUMO. The HOMO–LUMO energy gap becomes smaller when the Schiff base ligand is coordinated with the Cu(II) ion, which is most likely to be due to the interaction of copper(II) d orbitals with the HOMO and/or LUMO of the ligand. These theoretical calculations support the experimentally observed results. The biological assay reveals that both the Schiff base and the complex have different antimicrobial activities. The complex has the MIC value of 0.0781 mmol/L against Escherichia coli.