Synthesis, characterization, and structure-activity relationship of the antimicrobial activities of dinuclear N-heterocyclic carbene (NHC)-silver(I) complexes

The three dinuclear silver(I) complexes of N-heterocyclic carbene (NHC) ligands, [Ag-2(L-1a)](PF6)(2)center dot 2CH(3)CN (Ag-1a), [Ag-2(L-2)(2)](PF6)(2)center dot H2O (Ag-2), and [Ag-2(L-4)(2)](PF6)(2)center dot 2CH(3)CN (Ag-4), were synthesized by reactions of Ag2O with the corresponding PF6- salt of the NHC precursors indicated as H4L-1(PF6)(4). H2L-2(PF6)(2). and H2L-4(PF6)(2). H4L-1(PF6)(4), which is the precursor of ligand L-1, was formed as mixed crystals of two geometric isomers, i.e., H4L-1a(PF6)(4) (major) and H4L-lb(PF6)(4) (minor), each of which was not isolated as single species. Ag2O reacted with the mixed isomers of H4L-1(PF6)(4) to give a single, pure crystalline silver(I) complex Ag-la with one isomer (L-1a) as the major product. The molecular structures of the precursors and their silver(I) complexes were determined by X-ray crystallography. A mononuclear NHC-silver(I) complex (Ag-3) was prepared by the reaction of Ag2O with the precursor HL-3Cl. The silver(I) complexes and NHC precursors prepared here were characterized by CHN elemental analysis, FTIR, Thermogravimetry/Differential thermal analysis, X-ray crystallography and solution (H-1 and C-13) NMR spectroscopy. Organometallic silver(I) complexes Ag-la, Ag-2, and Ag-4 were dinuclear C-Ag-C bonding complexes, whereas Ag-3 was a mononuclear C-Ag-Cl bonding complex. These complexes are highly soluble in organic solvents such as acetone, acetonitrile, and dimethyl sulfoxide, and light-stable in the solid-state and in solution over one year. The antimicrobial activities of four silver(I) complexes and their NHC precursors against selected bacteria, yeasts, and molds in water-suspension systems were evaluated via the minimum inhibitory concentration; the activities were strongly dependent on the molecular structures of the dinuclear silver(I) complexes, which suggests a structure-activity relationship. (C) 2016 Elsevier Inc. All rights reserved.