Novel conjugated 5-pyridin-2-ylmethylidene 2-thio-4H-imidazol-4-ones and their complexes with copper(II) chloride

Three novel 2-thio-4H-imidazol-4-ones containing conjugated five, six- or seven-membered S,N-heterocycle and pyridin-2-ylmethylidene moiety (L) were synthesized by two-step reaction sequence starting from 2-thiohydantion. The ligand structures were confirmed by ¹H NMR, ¹³C NMR and HRMS. The reactions of the ligands L with copper chloride dihydrate give the complexes of LCuCl₂ composition. Copper coordination compounds with 6-(pyridin-2-ylmethylidene)-2,3-dihydroimidazo[2,1-b][1,3]thiazol-5(6H)-one (L1) and 2-(pyridin-2-ylmethylidene)-6,7-dihydro-5H-imidazo[2,1-b][1,3]thiazin-3(2H)-one (L2) were characterized by the X-ray data, demonstrating the distorted tetrahedral copper coordination environment. It was shown that the presence in the ligand L1 of a five-membered thiazolidine ring fused with imidazolone leads to a significant change in the geometric characteristics of the complex: an increase in the Cu-S distance and the flattening of copper coordination polyhedron. Both synthesized complexes were evaluated using cyclic voltammetry and demonstrated the quasi-reversible reduction of Cu(II) at ∼ −0.25 V. Testing of the antibacterial activity on the reporter strains E. coli ΔtolC pDualrep2 (AmpR) and E. coli lptDmut pDualrep2 (KanR) showed a lower activity of the synthesized copper-containing complexes compared to the complexes of similar ligand that did not have an additional condensed ring in its structure.