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Pentafluorophenyl triazolium carbenes, widely used in NHC catalysis, can decompose by several mechanisms. Under high concentration conditions, the azolium may undergo a pentafluorophenyl exchange by a proposed SNAr mechanism to give an inactive salt. In the presence of appropriate substrates, cyclization on the ortho-position of the arene can occur, also by SNAr. These adducts provide a potential pathway for catalyst decomposition and serve as a caveat to the development of new reactions and catalysts. [ABSTRACT FROM AUTHOR]

Four K, Cs, Ag(I), and TI(I) compounds with a nitrosodicyanomethanide ONC(CN)2- anion have been obtained and characterized using IR spectroscopy; UV-visible spectroscopy; room temperature, solid-state photoluminescence; and solutions electrical conductivity measurements. Cesium and thallium(l) complexes were obtained for the first time, and crystal structures were determined for Cs{ONC(CN)2}, Ag{ONC(CN)2}, and TI{ONC(CN)2}. These structures indicate the formation of completely different 3D polymeric networks in which the anion acts as a bridging ligand of different capacity. The cyanoxime ligand in all complexes studied is in the nitroso form and adopts a planar configuration. The structure of the cesium salt is ionic. The structure of Ag{ONC(CN)2} represents a 3D coordination polymer where the anion acts as a tetradentate ligand with all four bonds between Ag(I) and donor atoms (three N and one O) significantly shorter than the sum of the ionic radii for these elements. The silver(I) atom in this compound has a distorted tetrahedral surrounding. Additionally, Ag{ONC(CN)2} is remarkably UV- and visible-light-insensitive. However, after exposure of the solid complex to gases such as H2, CO, NO, C2H2, and C2H4, the surtace of the compound becomes visible-light-sensitive and changes color with significant darkening, which indicates reduction of the metal. This is reflected in a dramatic decrease of intensity of the photoluminescence of Ag{ONC(CN)2} in the presence of these gases, which might be utilized for nonelectric sensor applications. The TI{ONC(CN)2} complex represents a transitional species between ionic (Cs) and covalent (Ag) compounds. The thallium(I) center has four shorter bonds than the sum of the ionic radii bonds (three with N and one with O atoms) and three longer electrostatic (ionic) contacts with the anion. The 6s2 lone pair is stereoactive, and the coordination polyhedron is best described as a distorted square pyramid. Room temperature diffusion reflectance spectra of solid K{ONC(CN)2}, Ag{ONC(CN)2}, and TI{ONC(CN)2} demonstrated a bathochromic shift of the band's λmax depending on the atomic number of the metal center, indicating a significant role of metal centers in electronic transitions in these compounds. This phenomenon was observed for the first time. The latter complex at 293 K exhibited structured metal-based red photoluminescence in the range of 690-800 nm that depends on the excitation wavelengths. [ABSTRACT FROM AUTHOR]