The three dinuclear silver(I) complexes of N-heterocyclic carbene (NHC) ligands, [Ag2(L-1a)](PF6)2·2CH3CN (Ag-1a), [Ag2(L-2)2](PF6)2·H2O (Ag-2), and [Ag2(L-4)2](PF6)2·2CH3CN (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-1b(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-1a 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 (1H and 13C) NMR spectroscopy. Organometallic silver(I) complexes Ag-1a, 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.
Keywords: Antimicrobial activities; Dinuclear silver(I) complexes; Molecular structures; N-heterocyclic carbene (NHC) ligands; Structure-activity relationship.
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