Search

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 S N Ar mechanism to give an inactive salt. In the presence of appropriate substrates, cyclization on the ortho -position of the arene can occur, also by S N Ar. These adducts provide a potential pathway for catalyst decomposition and serve as a caveat to the development of new reactions and catalysts.

Ten silver(I) cyanoximates of AgL composition (L = NC-C(NO)-R, where R is electron withdrawing groups: -CN, -C(O)NR(2), -C(O)R' (alkyl), -C(O)OEt, 2-heteroaryl fragments such as 2-pyridyl, 2-benzimidazolyl, 2-benzoxazolyl, 2-benzthiazolyl) were synthesized and characterized using spectroscopic methods and X-ray analysis. Crystal structures of four complexes were determined and revealed the formation of two-dimensional (2D) coordination polymers of different complexity in which anions exhibit bridging or combined chelate and bridging binding modes. In these compounds, anions are in the nitroso form. All studied AgL complexes are sparingly soluble in water and are thermally stable to 150 °C. Synthesized compounds demonstrated remarkable insensitivity toward visible light and UV-radiation, which was explained based on their polymeric structures with multiple covalent bonds between bridging cyanoxime ligands and Ag(I) centers. All 10 silver(I) cyanoximates were tested in vitro on the subject of their antimicrobial activity against both Gram-positive and Gram-negative microorganisms such as Escherichia coli, Klebsiella pneumoniae, Proteus sp., Pseudomonas aeruginosa, Enterococcus hirae, Streptococcus mutans, Staphylococcus aureus, and Mycobacterium fortuitum as well as against Candida albicans in solutions, and in the solid state as pressed pellets and dried filter paper disks presoaked with solutions of AgL in DMF. Results showed pronounced antimicrobial activity for all investigated complexes. A combination of five factors: (1) light insensitivity, (2) poor water solubility, (3) high thermal stability, (4) lack of toxicity of organic ligands, and (5) in vitro antimicrobial activity allows development of silver(I) cyanoximates for medical applications. These include antimicrobial additives to acrylate glue, cured by UV-radiation, used in introduction of prosthetic joints and dental implants, and prevention of biofilm formation on several types of indwelling medical devices.

Four K, Cs, Ag(I), and Tl(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(I) complexes were obtained for the first time, and crystal structures were determined for Cs{ONC(CN)(2)}, Ag{ONC(CN)(2)}, and Tl{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 H(2), CO, NO, C(2)H(2), and C(2)H(4), the surface 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 Tl[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 6s(2) 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 Tl{ONC(CN)(2)} demonstrated a bathochromic shift of the band's lambda(max) 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.