Synthesis, characterization, density functional study and antimicrobial evaluation of a series of bischelated complexes with a dithiocarbazate Schiff base ligand

A nitrogen-sulfur Schiff base HL (1) derived from S-hexyldithiocarbazate and 4-methylbenzaldehyde has been reacted with different divalent metal ions in 2:1 molar ratio, producing neutral complexes (2–7) of general formula M^IIL₂ (where M = Ni, Cu, Zn, Cd, Pd and Pb). All compounds were characterized using established physico-chemical and spectroscopic methods. The single crystal structures of Cu^II and Zn^II complexes are compared and discussed with those of Ni^II and Pd^II already reported by us, underlining the geometrical variations occurring in the HL ligand upon coordination. The metal complexes, as revealed by the X-ray diffraction analyses, show a square planar or tetrahedral coordination geometry, and in the former case either a cisoid or transoid configuration of chelating ligands. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations have been performed on the isolated cis/trans complexes of Ni and Pd complexes in order to evaluate the stability of the isomer isolated in solid state. The thermodynamic parameters for trans to cis isomerization of NiL₂ complex [ΔH = −29.12 kJ/mol and ΔG = −43.97 kJ/mol] indicated that the trans isomer (observed in solid state) is more stable than the cis one. On the other hand, relative enthalpy [ΔH = −4.37 kJ/mol] and Gibbs free energy [ΔG = −5.50 kJ/mol] of PdL₂ complex disclosed a small difference between the energies of the two isomers. Experimental UV–vis and TD-DFT calculation confirmed that these complexes have distinctive LMCT bands with a broad shoulder at 400–550 nm. With the purpose of providing insight into the properties and behavior of the complexes in solution, photoluminescence and electrochemical experiments have been also performed. Finally, the anti-bacterial activity of these compounds was evaluated against three pathogenic Gram-negative organisms such as Escherichia coli, Salmonella typhi and Shigella flexneri, but only the free ligand 1 showed anti-bacterial property against all three tested organisms.