Synthesis, spectral characterization, crystal structures, biological activities, theoretical calculations and substitution effect of salicylidene ligand on the nature of mono and dinuclear Zn(II) Schiff base complexes

Abstract

Two new zinc(II) Schiff base complexes (Z1 and Z2) are prepared by treating ONNO tetradentate Schiff base ligands (H2L1 = [6,6′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol)] and H2L2 = [2,2′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(4-methoxyphenol)]) with acetylacetonate salt of zinc. The synthesized Schiff base ligands and their respective zinc(II) complexes were characterized by elemental analysis (CHN) and various spectroscopic techniques like FT-IR and 1H NMR. The single crystal X-ray structures of Z1 and Z2 show the effect of salicylidene ligand on the nature of mono and dinuclear Zn(II) Schiff base complexes. In Z1 complex, a square pyramidal geometry is adapted by the complex, supported by the ONNO donor atoms of the tetradentate Schiff base and a coordinated water molecule. Structural analysis of Z2 complex revealed that the compound is a centrosymmetric dimer in which the five coordinated Zn(II) atoms are linked to the opposite metal center by making μ-phenoxo bridges through one of the phenolic oxygen atoms of Schiff base ligands. The nature of various kinds of non-covalent interactions present among the sample molecules was also investigated by using QTAIM and NCI calculations. The theoretical calculations were performed by DFT using the B3LYP/Def2-TZVP level of theory, which indicated that the intended outcomes are in compliance with the actual consequences. Furthermore, the antibacterial potential of the Schiff base ligands and their corresponding zinc complexes was measured by testing them against two Gram-positive (Staphylococcus aureus and Bacillus cereus) and two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacterial strains. The agar well diffusion method and poisoned media technique were used for screening of bacterial and fungal species, respectively. According to the results of the zone of inhibition assessments, metal complexes have comparatively more inhibition potential relative to free ligands.