Homoleptic and heteroleptic silver(I) complexes bearing diphosphane and thioamide ligands: Synthesis, structures, DNA interactions and antibacterial activity studies

Three silver(I) complexes bearing different combinations of diphosphanes and N-heterocyclic thioamides or thioamidates as ligands have been synthesized and structurally characterized: the ionic, homoleptic compound [Ag(xantphos)(2)][BF4] (1), where xantphos = 4,5-bis(diphenylphosphano)-9,9-dimethyl-xanthene, and the neutral, heteroleptic compounds [Ag(xantphos)(kappa-S-pymt)] (2), where pymt = pyrimidine-2-thiolate, and [AgCl (dppbz)(kappa-S-mtdztH)] (3), where dppbz = bis(diphenylphosphano)benzene and mtdztH = 5-methyl-1,3,4-thiadiazole-2-thione. X-ray crystallography studies reveal tetrahedral coordination environments around the silver (I) ions in compounds 1 and 3, while a trigonal planar arrangement of the P2S donor set has been found around the metal center in compound 2. The interaction of the three compounds with calf-thymus DNA was monitored by UV-vis spectroscopy, DNA-viscosity measurements and indirectly by testing their ability to compete with ethidium bromide for DNA intercalation sites studied by fluorescence emission spectroscopy. Intercalation was revealed as the most possible binding mode for the neutral compounds 2 and 3 and electrostatic interactions for the cationic complex [Ag(xantphos)(2)](+) in 1. Complexes 1-3 have also been found to display moderate in vitro antibacterial activity against the Gram-positive B. cereus, S. aureus and the Gram-negative E. coli bacterial strains, with the homoleptic bis-phosphane silver(I) compound 1 exhibiting a lower activity than the other two neutral compounds.