Synthesis, spectroscopic, theoretical and biological evaluation of novel Schiff base complexes of divalent transition metals

In the present manuscript, a new series of triazole-based Schiff base, (E)-4-((3-fluoro-4-methoxybenzylidene)amino)-5-propyl-4H-1,2,4-triazole-3-thiol (FMBT) and its metal complexes with divalent cobalt, nickel, copper, zinc and palladium are prepared and investigated for biological activities. The structural assignments of all the synthesized ligand and its metal complexes were done using different spectral methods like mass, infrared (IR), H-1-NMR, elemental analysis, electron spin resonance (ESR) and ultraviolet (UV)-visible. The magnetic properties, fluorescence profile and redox (cyclic voltammetry) behaviour of all the complexes were also investigated. Based on the spectral studies, the octahedral (Co[II], Ni[II] and Zn[II]) and square planar (Cu[II] and Pd[II]) structural environment have been proposed to the metal complexes. The results obtained from thermal and IR data confirmed the presence of coordinated water in metal complexes. The conductance measurements confirmed the non-electrolytic nature of metal complexes. Density functional theory (DFT)-based methods were used to ascertain the stability and chemical inertness of the compounds. Various electrostatic properties were also investigated by utilising the value of energy associated with different frontier molecular orbitals. The decomposition behaviour and stability of all the metal complexes was established from different thermodynamical functions calculated using Coats-Redfern method. The bio-efficacy of bidentate ligand and its complexes was evaluated against a wide spectrum of microbial cultures using Broths dilution method. Among the novel metal complexes Ni (1:2) (minimum inhibitory concentration [MIC] = 62.5 mu g ml(-1)), Cu (1:1) (MIC = 62.5 mu g ml(-1)) and Zn (1:2) (MIC = 12.5 mu g ml(-1)) complexes were found to be more potent against Escherichia coli than the standard drugs.