Synthesis, crystal studies, antimicrobial activity, and BSA binding studies of metal complexes derived from pyridyl-based hydrazone: Multi-spectroscopic and DFT approach

Abstract

We describe herein the synthesis of a pyridyl-benzothiazole hydrazone ligand (BP) and its metal complexes (BP1–BP4) incorporating Co(II), Ni(II), Cu(II), and Zn(II) cations. The molecular structures of BP and two of its complexes, BP1 and BP3, have been determined by single-crystal X-ray diffraction analysis. Octahedral complexation of the central metal ion by the NNN donor ligand in a 1:2 stoichiometry has been confirmed by various spectroscopic techniques, that is, IR spectroscopy, NMR spectrometry, and UV/Vis spectrophotometry, and corroborated by electrospray-ionization mass spectrometry and cyclic voltammetry. Diffuse-reflectance spectra revealed the presence of d-d transitions in all of the complexes, except in the case of BP4, which has no unpaired electron. The thermal stabilities of the complexes have been assessed by thermogravimetry and differential thermogravimetry, which revealed stepwise decomposition. Significant differences in the morphologies of the complexes have been discerned by FE-SEM analysis. Hirshfeld surface analysis has revealed the establishment of N⋅⋅⋅H, H⋅⋅⋅H, and C⋅⋅⋅H noncovalent interactions. Density functional theory (DFT)-based calculations have been applied to investigate the chemical reactivities and optimized structures of BP and BP1–BP4. A 1,3-proton shift in BP was confirmed and the HOMO-LUMO energy gap was assessed through DFT-based methods. Photophysical studies have revealed a large Stokes shift for BP, which is reduced upon complexation with metal ions. In vitro anti-microbial screening of the prepared compounds has been performed against four bacterial strains (Bacillus cereus, Staphylococcus aureus, Escherichia coli, and Xanthomonas campestris) and two fungal strains (Candida albicans and Fusarium oxysporum), which revealed most potent inhibition of bacterial and fungal growth, with minimum inhibitory concentrations of 128 and 256 μg/mL, by BP1 and BP3, respectively. BP1 exhibits broad-spectrum anti-bacterial activity exceeding that of the standard drug chloramphenicol. Spectroscopic methods have been utilized to explore the interaction between BP1 and bovine serum albumin (BSA). A hypsochromic shift in fluorescence intensity and a hyperchromic shift in the absorption band of BSA with different concentrations of BP1 have yielded significant binding information. BSA binding studies have revealed a negative value of Gibbs free energy, confirming the spontaneity of BSA–BP1 complex formation.