Synthesis, crystal structures, photophysical properties
and antibacterial activities of copper(II) complex derived
from 4-chloro-2-{[(2,6-dimethylphenyl)imino]methyl}phenol

A new asymmetrical Schiff base ligand, (E)-4-chloro-2-{[(2,6-dimethylphenyl)amino]methyl}phenol(HL), and its mononuclear Cu(II) complex have been synthesized. The molecular structures and spectroscopic properties of the ligand and its complex were experimentally characterized by single crystal X-ray diffraction, elemental analysis, FTIR, NMR and UV-Vis spectroscopic techniques. The analysis confirmed that the Cu atom in complex is generally four-coordinated by one imino N and one phenolic O atoms of a Schiff-base ligand adopting a trans-configuration of N₂O₂ donor atoms around the metal ion, forming a square planar geometry. Density functional calculation is being carried out on both HL and copper(II) complex to investigate changes in structural parameters, energies of the HOMO and LUMO. The results demonstrated that the HOMO and LUMO were effectively separated with 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 become smaller when the Schiff base ligand coordinated with Cu(II) ion, most likely due to the interaction of the d-orbitals of copper(II) with the HOMO and/or LUMO of the ligand. These theoretical calculations supported the experimentally observed results. The biological assay revealed that both Schiff base and the complex have different antimicrobial activity. The complex has the MIC value of 0.0781 mmol/L against Escherichia coli.