Cobalt(II) and Silver(I) Complexes with 2-Amino-4-Thiazolinone: Synthesis, Structural Characterization, and Antibacterial Potential

Selective control of amino- (L1) and imino- (L2) tautomeric forms of 2-amino-4-thiazolinone was achieved via precise pH regulation (pH 11 and 6.5, respectively) during hydrochloride precursor neutralization, as confirmed by ¹H and ¹³C NMR spectroscopy. Reaction of protonated ligand with cobalt(II) chloride and silver(I) nitrate yielded [Co(L1)2Cl2] and [Ag(L1)3NO3] complexes. X-ray diffraction, IR, and UV–Vis spectral analyses revealed a distorted tetrahedral geometry for [Co(L1)2Cl2] with monodentate endocyclic nitrogen coordination. In contrast, [Ag(L1)3NO3] adopted an irregular coordination environment via endocyclic nitrogen and carbonyl oxygen atoms, assembling into chains. In vitro antimicrobial assays against Gram-positive (S. aureus, B. subtilis), Gram-negative (E. coli, P. aeruginosa) bacteria, and Candida albicans demonstrated a clear structure–activity relationship: free L1 < [Co(L1)2Cl2] < [Ag(L1)3NO3]. Uncoordinated L1 showed low-to-moderate activity (MIC = 64–128 μg/mL), silver complexation significantly enhanced potency, displaying broad-spectrum efficacy (MIC = 4–8 μg/mL) comparable to reference drugs. Molecular docking simulations against key microbial targets (DNA gyrase, penicillin-binding proteins, and CYP51) corroborated experimental findings, revealing the strongest binding affinity for [Ag(L1)3NO3] (up to –8.8 kcal/mol against S. aureus DNA gyrase) stabilized by extensive hydrogen bonding, electrostatic, and hydrophobic interactions. These results highlight [Ag(L1)3NO3] as a promising lead candidate for novel antimicrobial agents.