Monovalent K, Cs, Tl, and Ag Nitrosodicyanomethanides: Completely Different 3D Networks with Useful Properties of Luminescent Materials and Nonelectric Sensors for Gases

Abstract

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.

An Excursion into the Intriguing World of Polymeric Tl(I) and Ag(I) Cyanoximates

“…With the exception of only two complexes-Tl(BIHCO) [12] and Tl(CCO) [19]-all other investigated TlL complexes do not exhibit appreciable emission properties.…”

“…An extensive bridging exclusively via oxygen atoms was detected in only one complex-Tl(ACO) [35] (Scheme 2(D)). The only one example of reliably assigned monodentate coordination of the cyano-group to the metal center was observed in another monomeric Tl(I) macrocyclic complex with H(BTCO) 2 − anion [36], while formation of the coordination polymer with coordinated CN-group was recently reported in the structure of Tl(CCO) [19] as sketched in Scheme 2(D). Central metal ions in Tl(I) coordination polymers that have been synthesized and studied so far exhibit a variety of coordination numbers, and shapes of the coordination polyhedra are presented in detail in the ESI 5-10, but are out of the scope of this presentation.…”

“…Practically all TlL can be solubilized in water, forming dilute solutions upon application of the ultrasound and moderate heating to ~65 °C, but the attainable concentration of complexes depends on the hydrophilicity/hydrophobicity of the cyanoxime ligand. All TlL complexes that are soluble in water lose their solid state structures in solutions and form 1:1 type electrolyte which evidences their ionic character [19]. Synthesized TlL are much better soluble in strong donor solvents such as DMSO, DMF, pyridine and 2-picoline, and also form electrolytic solutions.…”

“…The deprotonation of cyanoximes with the base (typically NaOH, or K 2 CO 3 ) leads to yellow solutions of respective conjugated anions [19,20,50], which rapidly, within seconds react with aqueous AgNO 3 to form silver(I) cyanoximates of AgL composition. In all cases resulting complexes are thick, sparingly soluble in aqueous solutions precipitates of various colors that and can be easily separated from the reaction mixture by filtration, washed with water and cold methanol and dried in vacuum desiccator over H 2 SO 4 (c).…”

“…These compounds turned out to be coordination polymers of different complexity in which anionic cyanoximes act as bridging ligands sometimes showing unusual binding modes. Silver(I) polymeric cyanoximates showed quite remarkable stability towards visible light, which can be utilized in several practical applications [19][20][21], while polymeric thallium(I) cyanoximates are attractive building blocks and templates [22] for the preparation of mixed valence Tl(I)/Tl(III) systems that may exhibit electrical conductivity. This is an important property in design of molecular wires and in other molecular electronics applications [23].…”