Syntheses, Structures, and Antimicrobial Activities of Gold(I) - and Copper(I)-N-Heterocyclic Carbene (NHC) Complexes Derived from Basket-Shaped Dinuclear Ag(I)-NHC Complex

Novel dinuclear gold(I)- and copper(I)-N-heterocyclic carbene (NHC, L-1a) complexes [M-2(L-1a)](PF6)(2) (M = Au-1 (Au-1 center dot PF6) and M = Cu-I (Cu-1 center dot PF6)) were synthesized by transmetalation of the dinuclear silver(I)-NHC complex [Ag-2(L-1a)](PF6)(2) (Ag-1 center dot PF6) with [AuCl(Me2S)] or CuI in over 70% yield. These NHC complexes were characterized by CHN elemental analysis, Fourier transform infrared spectroscopy, thermogravimetry/differential thermal analysis, and solution (H-1 and C-13) NMR spectroscopy. X-ray crystallography revealed that Au-1 center dot PF6 and Cu-1 center dot PF6 are dinuclear molecules consisting of two linear intramolecular C-M-C bonds (M = Au-I and Cu-I), one M center dot center dot center dot M interaction, and two metal atoms arranged in a T-shaped geometry; their molecular structures are very similar to the basket-shaped cage structure of the parent complex Ag-1 center dot PF6. Because of the smaller ionic radius of copper(I), Cu-1 center dot PF6 has the smallest upper space of the basket among the three complexes. Au-1 center dot PF6 and Cu-1 center dot PF6 were very stable in the solid state and in solution. They did not undergo nucleophilic reaction with thiols 2-mercaptoethanol and 2-benzimidazolethiol (Hbmt) at least for several hours, whereas Ag-1 center dot PF6 did react, forming precipitates of silver(I) thiolate and the free ligand L-1a center dot PF6. The minimum inhibitory concentration values toward a panel of bacteria, yeasts, and molds were examined in a water suspension system and a solution system containing dimethyl sulfoxide. The antimicrobial spectra of the three compounds were metal-dependent, and Au-1 center dot PF6 showed the greatest activity toward Gram-positive bacteria.