A Systematic Study of the Solid-State Pathway from Melamine via Melaminate to Carbodiimide under Evolution of Hydrogen.

Thermal analysis techniques, such as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), can provide valuable insights into thermal properties, intermediate phases, and phase transitions; sometimes even a whole series of compounds appears in a given system. The solid-state reaction pathway from melamine to carbodiimide, monitored by DSC, involves a sequence of chemical reactions and intermediate phases departing from the reaction of potassium hydride and melamine. The fully analyzed reaction cascade begins with the formation of potassium melaminate, K(C3N6H5), and progresses through several intermediate phases, each with distinct structures and properties, before ultimately yielding ß-K2(CN2). All crystalline compounds appearing in this reaction sequence are identified using X-ray diffraction analyses. With a 6:1 ratio of potassium hydride and melamine, equal numbers of protic and hydridic hydrogen atoms in the starting materials favor the release of H2 until the formation of the final product K2(CN2), which appears with two modifications.

High-Yield Synthesis Route, Post-Synthesis Treatment, and Insights into the Photoluminescence and Magnetic Properties of Tricopper(I) Melaminate Cu3(C3N6H3).

A novel and more efficient synthesis of Cu3(C3N6H3) is presented through a salt-metathesis reaction using copper(I) chloride and sodium hydrogen cyanamide. This synthesis yields a melaminate trianion through the cyclotrimerization of (HNCN)- ions, offering an alternative route to the deprotonation of melamine for synthesizing melaminate. The reaction is analyzed via differential thermal analysis. After the unconventional formation of this MOF-like structure by solid-state reaction, this material still requires treatment via solvent exchange and vacuum drying due to the presence of a host molecule inside the channels of the structure. The process and the impact of the treatment of Cu3(C3N6H3) on its XRD pattern are thoroughly discussed. Additionally, results from stability, NMR and infrared spectroscopy, and thermogravimetric analysis. Finally, photoluminescence and magnetic studies are conducted to evaluate their potential as a functional material.

Ta4SBr11: A Cluster Mott Insulator with a Corrugated, Van der Waals Layered Structure.

The compound Ta4SBr11 was prepared by a comproportionation reaction of tantalum bromide with tantalum and elemental sulfur. The crystal structure, as refined by single-crystal X-ray diffraction, is composed of clusters with Ta4S cores, arranged in corrugated van der Waals layers. Individual layers appear to be displaced relative to each other along one direction. Successful crystal growth in a melt of CsBr yielded black platelets of Ta4SBr11, which were used to investigate the electrical properties of the compound. The electronic structure was studied by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and by density functional theory (DFT) band structure calculations, revealing this material to be a small-gap semiconductor. DFT results, in combination with magnetic susceptibility measurements, suggest that metallicity originating from the one unpaired Ta d electron per cluster is most likely suppressed by electronic correlations, forming a cluster Mott insulator.

Pnictide-Capped Butterfly Cluster in the Crystal Structure of Nb4PnX11 (Pn = N, P; X = Cl, Br, I).

Niobium pnictide halides Nb4PnX11 (Pn = N, P; X = Cl, Br, I) are reported with their average crystal structures. Individual pnictide-capped butterfly cluster cores [Nb4P] in the structure are interconnected into two-dimensional layers, with their electronic and magnetic properties being analyzed.

Formation of a Polar Structure in the Metallic Niobium Sulfide Nb4S3.

The synthesis of Nb4S3, a previously undiscovered binary sulfide, was achieved using Nb3Br7S as a precursor. Its structure is composed of Nb6S triangular prisms arranged in a polar (Imm2) configuration, with sulfur atoms lying in channels along the a axis. Electrical resistivity measurements and density functional theory calculations were used to determine that Nb4S3 is metallic and therefore a polar metal, with metallic bands occupied by electrons with primarily niobium character. The electrons near the Fermi level are so closely associated with the niobium sublattice that the sulfur atoms have positive Born effective charges, indicating that the electrostatic interactions between sulfur atoms are unscreened. Calculations of the dependence of the electron density on the sulfur atomic positions confirm that the metallic electrons do not screen the dipole-dipole interactions between sulfur atoms, which allows polarity and metallicity to coexist in Nb4S3. These findings suggest that applied electric fields might be able to reverse the polarity of thin films of Nb4S3.

Tricopper Melaminate, a Metal-Organic Framework Containing Dehydrogenated Melamine and Cu-Cu Bonding.

Crystals of Cu3(C3N6H3) are formed by a solid-state reaction of CuCl with melamine to form a layered framework structure with open pores running along the hexagonal axis direction of the P6/mcc structure. The compound contains the hitherto unknown (C3N6H3)3- ion, assigned as melaminate. Bonding interactions within and between Cu-Cu dumbbells, which connect melaminate ions into layers, are analyzed by density functional theory calculations of the electron localization function and directional Young's modulus. Band structure calculations reveal the material to be a semiconductor with a band gap on the order of 2 eV.

Reversible Iodine Intercalation into Tungsten Ditelluride.

The new compound WTe2I was prepared by a reaction of WTe2 with iodine in a fused silica ampule at temperatures between 40 and 200 °C. Iodine atoms are intercalated into the van der Waals gap between tungsten ditelluride layers. As a result, the WTe2 layer separation is significantly increased. Iodine atoms form planar layers between each tungsten ditelluride layer. Due to oxidation by iodine the semimetallic nature of WTe2 is changed, as shown by comparative band structure calculations for WTe2 and WTe2I based on density functional theory. The calculated phonon band structure of WTe2I indicates the presence of phonon instabilities related to charge density waves, leading to an observed incommensurate modulation of the iodine position within the layers.

Origins of Iodine-Rich W6I12 Cluster Compounds and the Soluble Compound W6I22.

In search of iodine-rich compounds with an octahedral tungsten cluster, we explored the treatment of ß-W6I12, the most stable tungsten iodide cluster compound, with liquid iodine. The most iodine-rich compound obtained from these reactions was W6I22, whose crystal structure adopts two closely related modifications. The remarkable connectivity of [W6I8]4+ clusters in the structure of W6I22 makes this compound the first example of a soluble binary octahedral tungsten iodide cluster, as demonstrated by dissolution experiments in several solvents. Differential scanning calorimetry showed that the thermolysis of triclinic α-W6I22 proceeds via a phase transformation into monoclinic ß-W6I22, followed by the formation of W6I18 and W6I16 with release of iodine. A corresponding ambient-pressure study by combined differential thermal analysis and thermal gravimetry revealed the transformation of ß-W6I22 into W6I14 and ß-W6I12, which finally decomposes into the elements. On the basis of this simple example, we demonstrate how a complete reaction sequence, including preparation and subsequent phase transformations, can be monitored and analyzed by thermal scanning methods. Moreover, a reaction cycle is reported that relates a whole series of binary tungsten iodides. Syntheses of the new compounds α- and ß-W6I22, and W6I14 are reported, and their crystal structures, as determined by X-ray diffraction techniques, are presented.

Synthesis, Structure, and Electronic Properties of Sn(CN2) and Sn4Cl2(CN2)3.

A solid-state metathesis reaction between equimolar amounts of Li2(CN2) and SnCl2 revealed the formation of two new compounds, Sn4Cl2(CN2)3 and Sn(CN2). Thermal analysis of this reaction indicated that Sn4Cl2(CN2)3 forms exothermically near 200 °C and subsequently transforms into Sn(CN2) at higher temperatures. The crystal structures of both compounds are presented. According to optical measurements and band structure calculations, Sn(CN2) can be considered as a semiconductor with a band gap on the order of 2 eV. The presence of Sn2+ ions in the structure of Sn(CN2) with a toroidally shaped lone pair is indicated by electron localization function calculations. The structure of Sn(CN2) is shown to be related to the structures of FeS2 and CaC2.

Synthesis, Structure, and Electronic Properties of Sn9O5Cl4(CN2)2.

The formation of the new compound Sn9O5Cl4(CN2)2 is reported and placed in the context of several other recently discovered tin carbodiimide compounds (Sn(CN2), Sn2O(CN2), and Sn4Cl2(CN2)3), all of which contain divalent tin. The crystal structure of Sn9O5Cl4(CN2)2, as determined by X-ray powder diffraction, includes an unusual [Sn8O3] cluster, in which tin atoms form the motif of a hexagonal bipyramid. An additional tin atom and two oxygen atoms connect these clusters into chains. Mössbauer spectroscopy shows tin to predominantly adopt the +2 oxidation state, and electronic structure calculations predict Sn9O5Cl4(CN2)2 to be a semiconductor.

Lithium and Sodium Ion Distributions in A2- x[W6I14] Structures.

Ag2[W6I14] and A2- x[W6I14] compounds with A = Na, Li were prepared from binary tungsten iodides (W3I12) and corresponding metal iodides. Their crystal structures are analyzed on the basis of X-ray diffraction data. 7Li and 23Na solid-state NMR measurements reveal that Li+ and Na+ ions are distributed over two sites in the respective structures. These results shed some new light on A x[M6I14] with A = alkali and M = Mo, W compounds being reported with x = 1 and 2, which exhibit photophysical properties. The lithium compound is an exception in the series of A2- x[W6I14] compounds, because it is the only compound which is soluble in water.

A Reaction Cycle for Octahedral Tungsten Iodide Clusters.

A reaction cycle is shown for octahedral tungsten iodide compounds. The thermal transformation of W6I16 (W6I12·2I2) via release of iodine proceeds via the new W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) to yield a new modification of W6I12, denoted as α-W6I12. When heated, α-W6I12 is converted into the known (ß-)W6I12. The reaction of (ß-)W6I12 with I2 leads to the formation of the starting compound W6I16. The new compounds W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) and α-W6I12 are structurally characterized by powder and single-crystal X-ray diffraction techniques. The thermal decomposition of W6I16 and the monotropic phase transition of α-W6I12 into ß-W6I12 are examined by differential scanning calorimetry measurements.

Thermal Iodine Loss Cascade of W5I16.

Tungsten iodide compounds feature a surprising diversity of binary compounds. Their formation conditions and phase equilibria depend very much on the surrounding iodine partial pressure and temperature. Herein we focus on square-pyramidal tungsten iodide cluster compounds with their iodine loss, structural rearrangements, cross-linking behavior, and final transformation into the octahedral cluster compound ß-W6I12 at elevated temperatures. Reactions depart from W5I16 at different iodine pressures and temperatures. The thermal decomposition of W5I16 at low iodine pressure passes through a number of cluster compounds (W5I15, W5I13·xI2, ß-W5I12, and W5I11) under release of iodine. At higher iodine pressures, only one intermediate compound (α-W5I12) was found. Thermal decomposition of W5I16 was examined by differential thermal/thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction (XRD) measurements. New compounds (W5I15, W5I13·xI2, and W5I11) were structurally characterized by means of XRD techniques. The crystal structure of W5I11 reveals a nice relationship to its transformation product W6I12.

Formation, Structure, and Frequency-Doubling Effect of a Modification of Strontium Cyanurate (α-SCY).

The low-temperature modification of Sr3(O3C3N3)2 was prepared and assigned as α-SCY after the high-temperature phase (now called ß-SCY) and its frequency-doubling properties were reported recently. The crystal structure of α-SCY was solved and refined by single-crystal X-ray diffraction. Both modifications of SCY crystallize in noncentrosymmetric space groups, with the low-temperature phase (α-SCY) adopting the lower symmetry structure (Cc). Atomic positions in α-SCY (Cc) reveal only small deviations in comparison to those in the structure of ß-SCY (R3c). The reversible phase transition between both modifications of SCY was studied by means of temperature-dependent powder X-ray diffraction. NLO measurements of both SCY modifications are reported in comparison to the commercial frequency-doubling material KTiOPO4 (KTP).

Cluster harvesting in the WBr6-P system.

A combined thermal scanning-X-ray diffraction (XRD) approach was performed for the WBr(6)-P system to detect and analyze phases in this system, including metal-rich phases generated with increasing amounts of elemental (red) phosphorus under partial PBr(3) release. Phases were characterized by powder XRD. A black crystalline powder of W(4)(PBr)Br(10) was obtained after reduction of WBr(6) with elemental phosphorus at 450 °C. The crystal structure of the new compound was found to be isotypic with the structure of W(4)(PCl)Cl(10) on the basis of powder XRD data. The structure of W(4)(PBr)Br(10) is represented by a cyclobutadiene-like tetranuclear tungsten cluster interconnected into a layered (W(4)(µ(4)-PBr)Br(6)(i))Br(8)/(2)(a-a) arrangement via outer bromide ligands. The µ(4)-capping bromophosphinidene ligand was verified by solid-state magic-angle spinning (31)P NMR spectroscopy.

From WCl6 to WCl2: Properties of Intermediate Fe-W-Cl Phases.

Phenomenological studies of WCl6 reduction with transition metal powders M = Mn, Fe, and Co have been recently reported. These reactions involve a series of reductive intercalation steps of M atoms into layered tungsten chloride arrangements, followed by exsolution of MCl2. In the series M = Fe, the presence of divalent iron is evidenced for Fe(x)WCl6, FeW2Cl10, Fe2W2Cl10, and (Fe,W)Cl2 by Mössbauer spectroscopy. Magnetic properties are reported. Bonding characteristics between tungsten atoms in edge-sharing [W2Cl10](n-) bioctahedra reveal that a double bond can be addressed to FeW2Cl10. A similar situation appears for Fe2W2Cl10, due to the localized and thus nonbonding character of the two electrons in the δ orbitals of this compound.

Cluster harvesting by successive reduction of a metal halide with a nonconventional reduction agent: a benefit for the exploration of metal-rich halide systems.

The preparation of thermally labile compounds is a great temptation in chemistry which requires a careful selection of reaction media and reaction conditions. With a new scanning technique denoted here as Cluster Harvesting, a whole series of metal halide compounds is detected by differential thermal analysis (DTA) in fused silica tubes and structurally characterized by X-ray powder diffraction. Experiments of the reduction of tungsten hexahalides with elemental antimony and iron are presented. A cascade of six compounds is identified during the reduction with antimony, and five compounds or phases are monitored following the reduction with iron. The crystal structure of Fe2W2Cl10 is reported, and two other phases in the Fe-W-Cl system are discussed.

Preparation, photoluminescence and excited state properties of the homoleptic cluster cation [(W6I8)(CH3CN)6]4.

Solvated tungsten iodide cluster compounds are presented with the homoleptic cluster cation [(W6I8)(CH3CN)6]4+ and the heteroleptic [(W6I8)I(CH3CN)5]3+, synthesized from W6I22 in acetonitrile. Crystal structures were solved and refined on deep red single-crystals of [(W6I8)(CH3CN)6](I3)(BF4)3·H2O, [(W6I8)I(CH3CN)5](I3)2(BF4), and on a yellow single-crystal of [W6I8(CH3CN)6](BF4)4·2(CH3CN) on the basis of X-ray diffraction data. The structure of the homoleptic [(W6I8)(CH3CN)6]4+ cluster is based on the octahedral [W6I8]4+ tungsten iodide cluster core, coordinated by six apical acetonitrile ligands. The electron localisation function of [(W6I8)(CH3CN)6]4+ is calculated and solid-state photoluminescence and its temperature depedence are reported. Additionally, photoluminescence and transient absorption measurements in acetonitrile are shown. Results of the obtained data are compared to compounds containing [(M6I8)I6]2- and [(M6I8)L6]2- (M = Mo or W; L = ligand) clusters.

W2O3I4 and WO2I2: metallic phases in the chemical transport reaction of tungsten.

The crystal structures of the hitherto unknown phase W2O3I4 and of WO2I2, a compound that is known to play an important role in the chemical vapor transport of elemental tungsten are reported. We demonstrate that WO2I2 transforms into W2O3I4, and then into WO2 with increasing temperature. Crystals of WO2I2 appear as thin platelets, showing metallic luster; crystals of W2O3I4 appear as black colored needle-shaped platelets. Both compounds adopt layered structures; electronic band structure calculations reveal metallic conductivity for W2O3I4 and surprisingly also for WO2I2.

Heterogeneous photoactive antimicrobial coatings based on a fluoroplastic doped with an octahedral molybdenum cluster compound.

Despite the wide variety of strategies developed to combat pathogenic microorganisms, the infectious diseases they cause remain a worldwide health issue. Hence, the search for new disinfectants, which prevent infection spread, constitutes an extremely urgent task. One of the most promising methods is the use of photoactive compounds - photosensitizers, capable of generating reactive oxygen species, in particular, singlet oxygen (O2(1Δg)), which causes rapid and effective death of microorganisms of all types. In this work, we propose the utilization of the powdered cluster complex (Bu4N)2[{Mo6I8}(OTs)6] as a photoactive additive to commercially available fluoroplastic lacquer F-32L to create heterogeneous self-sterilizing coatings. We show that soaking of the prepared films in water for 60 days did not lead to a decrease in their photosensitization properties indicating their excellent stability. Moreover, the use of the cluster complex in the solid state allowed significant expansion of the operating wavelength range, which covers the UV region and a large part of the visible region (250-650 nm). The films displayed high photoantimicrobial activity against five common pathogens (bacteria and fungi) under white-light irradiation. Overall, the properties demonstrated make these materials promising for practical use in everyday outdoor and indoor disinfection since they are active under both sunlight and artificial lighting.