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.

A few trinuclear and tetranuclear silver clusters supported by pyrazole-functionalized N-heterocyclic carbene ligands have been prepared and fully characterized by NMR spectroscopy and X-ray diffraction analyses. Weak Ag–Ag and Ag–π interactions were observed. The luminescence properties of the imidazolium salts and the silver complexes were studied in their solid states.Figure optionsDownload as PowerPoint slide