The synthesis of a Cu(ii) Schiff base complex using a bidentate N2O2 donor ligand: crystal structure, photophysical properties, and antibacterial activities

In this paper, a new mononuclear Cu(ii) complex was synthesized by using a bidentate N2O2 Schiff base ligand, (E)-2-bromo-4-chloro-6-[(2,6-dimethylphenylimino)methyl]phenol (HL), with a copper(ii) salt in a methanol solvent. The structures of the HL ligand and the complex were characterized by Fourier transform infrared (FTIR) spectroscopy, single crystal X-ray diffraction (SCXD), and elemental analysis (EA). The Cu(ii) center in the complex is four-coordinated by a bidentate N2O2 donor ligand, forming slightly distorted square planar geometry. The detailed studies of their photophysical properties such as UV-Vis and fluorescence were done and the X-ray diffraction (XRD) patterns were investigated in the powder forms. Density functional theory calculations were carried out on both the HL ligand and the Cu(ii) complex to investigate the changes in the structural parameters and 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 the donor unit and the benzene ring of 3-bromo-5-chlorosalicylaldehyde as well as the chelate ring as the acceptor unit. The effective HOMO-LUMO separation helps 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) ions, which was most likely due to the Cu(ii) perturbation effect. These theoretical calculations supported the experimentally observed results. The antibacterial activities of the HL ligand and the Cu(ii) complex were studied on Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. The obtained data confirmed their potent antimicrobial activity and the Cu(ii) complex had the MIC value of 1.25 mmol L-1 against Escherichia coli.