Synthesis, theoretical calculations and antimicrobial studies of copper(I) complexes of cysteamine, cysteine and 2-mercaptonicotinic acid

Four new copper(I) complexes, [Cu(Cym-H)Cl] (1), [Cu(Cys-H)Cl] (2), [Cu(Mnt-H)I] (3) and [Cu(Mnt)] (4) (Cym-H = cysteamine, Cys-H = cysteine and Mnt-H = 2-mercaptonicotinic acid) were prepared and characterized by elemental analysis, IR, ¹H & ¹³C NMR spectroscopy and thermal analysis. The absence of S-H vibrations in the IR spectra suggests that the coordination of thiolates to copper(I) occurs through the sulfur atom. In ¹³C NMR spectra of 3 and 4, each resonance splits into three and two resonances respectively, indicating the presence of several species in solution. DFT calculations for 1-4 predicted predominant occurrence of most stable dimers in solution, some of them exhibiting cuprophilic interactions. The electrochemical properties of complexes in DMSO were studied by cyclic voltammetry. Antimicrobial activities were evaluated by minimum inhibitory concentration and complexes 1-4 exhibited significant activities against gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa), while moderate activities were observed (except in case of 3) against molds (Aspergillus niger, Penicillium citrinum) and yeasts (Candida albicans, Saccharomyces cerevisiae).