Mechanochemical synthesis, structural characterization, and antimicrobial evaluation of tri-nuclear Co(II), Ni(II), and Cu(II) complexes with vanillin-L-Leucine Schiff base

Abstract

In this paper, a binucleating Schiff base derived from vanillin and L-Leucine and its Co(II), Ni(II), and Cu(II) tri-nuclear metal complexes were synthesized via a mechanochemical route as an alternative to solution-based methods. The compounds were characterized by elemental analysis, ESI-mass spectrometry, infrared spectroscopy, thermogravimetric analysis (TGA), solubility screening, conductivity and magnetic measurements. The FT-IR spectroscopic data provides evidence for the presence of imine linkage between Lleucine and vanillin resulting in the formation of the Schiff base. In addition, the IR spectral data indicate that the C=N stretching band of the Schiff base shifts from 1673 cm-1 to lower frequencies in the range of 1668 to 1629 cm-1 upon coordination with Co(II), Ni(II), and Cu(II) ions. This shift confirms the formation of metalligand bonds. The Schiff base behaves as a tridentate dibasic ligand and binds to metal ions via the phenolic oxygen, imine nitrogen and carboxylic oxygen (O,N,O) resulting in trinuclear complexes. ESI-MS spectra showed m/z = 342.02, 1208.16, 1131.21 and 1412.87 corresponding to singly charged adduct ions, which confirms the molecular weight of the Schiff base and the trinuclear Co(II), Ni(II) and Cu(II) complexes respectively. Data from thermogravimetric analysis (TGA) confirms the presence of both coordinated water and water of crystallization in the metal complexes. The molar conductivity values ranging from 12.04 to 21.68 Ω- 1·cm²·mol⁻1 for the tri-nuclear Co(II), Ni(II), and Cu(II) complexes suggest that the tri-nuclear metal complexes are non-electrolytes. The magnetic moments for the tri-nuclear Co(II), Ni(II), and Cu(II) complexes were found to be 4.51, 3.67, and 1.99 BM, respectively, which suggests paramagnetic metal centers. The percentage yield of the synthesized compounds was found to be in the range of 88.61-94.66%. Anti-bacterial and anti-fungal activities of the Schiff base and its corresponding metal complexes were investigated in vitro using the disc iffusion method against Staphylococcus aureus, Streptococcus pneumoniae, Salmonella typhi, Escherichia coli, Trichophyton tonsurans and Trichophyton rubrum. The results of the anti-microbial assay showed that the metal complexes are more active compared to the Schiff base.