Synthesis, structural characterization, and luminescence of new silver aggregates containing short Ag-Ag contacts stabilized by functionalized bis(N-heterocyclic carbene) ligands

The imidazolium salts bis(N-(pyridylmethyl) imidazoliumyl) methane hexafluorophosphate (H2L1,(PF6)(2)), 2,2'-bis(N-(pyridylmethyl) imidazoliumyl) ethyl ether (H2L2 center dot(PF6)(2)), 2,6-bis(N-(pyridylmethyl) imidazoliumyl) pyridine (H2L3 center dot(PF6)(2)), and 3,6-bis(N-(pyridylmethyl) imidazoliumyl) pyridazine (H2L4 center dot(PF6)(2)) have been prepared and characterized. They reacted with Ag2O in acetonitrile to produce the silver complexes [Ag-4(L-1)(2)(CH3CN)(2)](PF6)(4)center dot 2CH(3)CN ( 1), [Ag-4(L-2)(2)](PF6)(4)center dot 2CH(3)CN ( 2), [Ag-4(L-3)(2)](PF6)(4)center dot Et2O center dot CH3CN ( 4), and [Ag-6(L-4)(4)](PF6)(6) ( 5), respectively. The reaction of 2 with 4 equiv of Au(Et2S) Cl afforded the dimetallic gold complex [Au-2(L-2)](PF6)(2)center dot 2CH(3)CN ( 3). Complexes 1-4 were characterized by H-1 NMR, C-13 NMR, luminescence spectroscopy, elemental analyses, and X-ray crystallography. Complex 1 is a tetrasilver complex consisting of a zigzag silver chain with silver-silver separations of 2.911( 1) and 3.288( 1) angstrom. Complexes 2 and 4 have orthogonal tetrasilver cores in which four silver atoms are held together by two L-2 or L-3 ligands with relatively short Ag-Ag contacts ranging from 2.790( 1) to 3.293( 2) angstrom. The digold complex consists of a macrometallocycle formed by two gold atoms and one L-2 ligand, and the intramolecular Au-Au bond distance is 3.267( 1) angstrom. Complex 5 is a hexamer with six silver atoms bridged by four L-4 ligands forming three silver-silver bonds of distances 3.249( 2), 3.014( 2), and 3.089( 2) angstrom. ESI-MS spectra show that these multinuclear silver cores are retained in acetonitrile solution. All of these silver complexes are intensely luminescent in their solid state.