Synthesis, structural characterization, and luminescence properties of multinuclear silver complexes of pyrazole-functionalized NHC ligands containing Ag–Ag and Ag–π interactions

A few pyrazole-functionalized imidazolium salts have been prepared via the reactions of N-alkylimidazole and 3,5-bis(chloromethyl)pyrazole or 2-(1-(2-chloroethyl)-5-methyl-1H-pyrazol-3-yl)-6-(5-methyl-1-vinyl-1H-pyrazol-3-yl) pyridine. Reactions of these imidazolium salts with Ag2O led to the successful isolation of tetranuclear [Ag4(L)2](X)2 (X = PF6− or BF4−; H3L1 = 3,5-bis(N-benzylimidazoliumyl)pyrazole, H3L2 = 3,5-bis(N-(2,4,6-trimethylphenyl)imidazoliumyl)pyrazole, H3L3 = imidazolium cyclophane from the condensation of 3,5-bis(chloromethyl)pyrazole and 1,4-bis(imidazolyl)butane) and trinuclear silver clusters supported by N-heterocyclic carbene ligands in high yields. The molecular structures of these silver complexes have been confirmed by 1H, 13C NMR, ESI-MS spectroscopy, and X-ray diffraction analyses. The tetranuclear complexes [Ag4(L1)2](PF6)2 (1) and [Ag4(L2)2](BF4)2 (2) consist of a pair of Ag–Ag contacts (ca. 3.11 Å) showing weak silver–silver interaction. [Ag4(L3)2](PF6)2 (3) has a square planar Ag4 core sandwiched by two NHC cyclophanes with Ag–Ag distances of 3.22 Å. All the silver atoms in 1–3 are located in the same linear C–Ag–N coordination environment. [Ag3(L4)2] (PF6)3 (HL4 = 2-(1-(2-methylimidazoliumylethyl)-5-methyl-1H-pyrazol-3-yl)-6-(5-methyl-1-vinyl-1H-pyrazol-3-yl) pyridine) (4) is a trinuclear complex in which the three silver are bridged by two L4 molecules, and the Ag3 units form one-dimensional chain via Ag–π interaction. The luminescence properties of the imidazolium salts and their silver complexes were also studied.