Eco-friendly Synthesis, Characterization, and Antimicrobial Evaluation of Transition Metal (II) Complexes with Thiophene-Derived Tridentate (S N N Donor) Heterocyclic Schiff Base Ligand

A new unsymmetric tridentate (SNN donor) Schiff base ligand was synthesized from the 1:1 M condensation of thiophene-2-carboxaldehyde with 5-amino-1,3,4-thiodiazol-2-thiol, under microwave irradiation in the presence of a green catalyst PPA-SiO2. The bivalent 3d transition metal (M = Co2+, Ni2+ and Cu2+) complexes were obtained by the reaction of M(II) chloride with tridentate Schiff base ligand (L) in a 1:2 M ratio under microwave irradiation in aqueous ethanol medium. The Schiff base and its complexes have been characterised by FTIR, 1H NMR, UN/Vis, elemental analyses, conductometry and magnetic measurements. The IR results showed that the Schiff base ligand acts as a neutral unsymmetrical tridentate with SNN donor sequence in E isomeric form towards the metal ion through azomethine-N, thiadiazole-N and thiophene-S. The electronic spectral results and magnetic measurement data revealed six coordinated octahedral geometries having the formula [M(L)2]Cl2 for the synthesised metal complexes. The melting point supported all the compounds' thermal stability and non-hygroscopic nature. The molar conductance data revealed the 1:1 electrolytic nature of the metal complexes. The in vitro biological activities of the free ligand and its metal complexes against a few bacteria and fungi were screened by the disc diffusion technique. The relative order of potency of antimicrobial activity against pathogens is Cu(II) > Co(II) > Ni(II) > Ligand.