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.

Electronic structure and transport in the potential Luttinger liquids CsNb3Br7S and RbNb3Br7S.

The crystal structures of ANb3Br7S (A = Rb and Cs) have been refined by single crystal X-ray diffraction, and are found to form highly anisotropic materials based on chains of the triangular Nb3 cluster core. The Nb3 cluster core contains seven valence electrons, six of them being assigned to Nb-Nb bonds within the Nb3 triangle and one unpaired d electron. The presence of this surplus electron gives rise to the formation of correlated electronic states. The connectivity in the structures is represented by one-dimensional [Nb3Br7S]- chains, containing a sulphur atom capping one face (µ3) of the triangular niobium cluster, which is believed to induce an important electronic feature. Several types of studies are undertaken to obtain deeper insight into the understanding of this unusual material: the crystal structure, morphology and elastic properties are analysed, as well the (photo-)electrical properties and NMR relaxation. Electronic structure (DFT) calculations are performed in order to understand the electronic structure and transport in these compounds, and, based on the experimental and theoretical results, we propose that the electronic interactions along the Nb chains are sufficiently one-dimensional to give rise to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic electrons.

Facile Energy Release from Substituted Dewar Isomers of 1,2-Dihydro-1,2-Azaborinines Catalyzed by Coinage Metal Lewis Acids.

Molecular solar thermal systems (MOST) represent an auspicious solution for the storage of solar energy. We report silver salts as a unique class of catalysts, capable of releasing the stored energy from the promising 1,2-dihydro-1,2-azaborinine based MOST system. Mechanistic investigations provided insights into the silver catalyzed thermal backreaction, concurrently unveiling the first crystal structure of a 2-aza-3-borabicyclo[2.2.0]hex-5-ene, the Dewar isomer of 1,2-dihydro-1,2-azaborinine. Quantification of activation energies by kinetic experiments has elucidated the advantageous energy change associated with Lewis acid catalysts, a phenomenon corroborated through computational analysis. By means of low temperature NMR spectroscopy, mechanistic insights into the coordination of Ag+ to the 1,2-dihydro-1,2-azaborinine were gained.

Synthesis of Sterically Encumbered Thiourea S-Oxides through Direct Thiourea Oxidation.

Thiourea S-oxides can be viewed as formal analogs of the currently unknown diamino-substituted Criegee intermediates (urea O-oxides). However, the preparation of such S-oxides is rather challenging, and the direct oxidation of thioureas typically only leads to formation of desulfurized products. Employing the accurate revDSD-PBEP86-D4 double hybrid density functional, it was found that the peracid mediated oxidation of thiourea S-oxides exhibits a lower reaction barrier than the oxidation of the corresponding thiourea itself in contrast to most other ordinary thioketones. The undesired overoxidation reactivity, which is associated with strong π-donation from the thiourea's nitrogen atoms, can be partially suppressed by introduction of bulky substituents and the utilization of protic solvents. In this regard, we managed to prepare two sterically encumbered thiourea S-oxides in isolated yields of 35-40 %. The S-oxides are stable in the solid state and in alcoholic solutions at room temperature for extended periods of time, but swiftly decompose in aprotic solvents by disproportionation. A dimesityl-substituted thiourea S-oxide complexed with residual mCBA could be characterized by means of X-ray crystallography, confirming the importance of hydrogen bonding in the stabilization of the amino-substituted C=S+ -O- moiety.

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.

Modulating the Electronic and Solid-State Structure of Organic Semiconductors by Site-Specific Substitution: The Case of Tetrafluoropentacenes.

The properties as well as solid-state structures, singlet fission, and organic field-effect transistor (OFET) performance of three tetrafluoropentacenes (1,4,8,11: 10, 1,4,9,10: 11, 2,3,9,10: 12) are compared herein. The novel compounds 10 and 11 were synthesized in high purity from the corresponding 6,13-etheno-bridged precursors by reaction with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate at elevated temperatures. Although most of the molecular properties of the compounds are similar, their chemical reactivity and crystal structures differ considerably. Isomer 10 undergoes the orbital symmetry forbidden thermal [4+4] dimerization, whereas 11 and 12 are much less reactive. The isomers 11 and 12 crystallize in a herringbone motif, but 10 prefers π-π stacking. Although the energy of the first electric dipole-allowed optical transition varies only within 370 cm-1 (0.05 eV) for the neutral compounds, this amounts to roughly 1600 cm-1 (0.20 eV) for radical cations and 1300 cm-1 (0.16 eV) for dications. Transient spectroscopy of films of 11 and 12 reveals singlet-fission time constants (91±11, 73±3 fs, respectively) that are shorter than for pentacene (112±9 fs). OFET devices constructed from 11 and 12 show close to ideal thin-film transistor (TFT) characteristics with electron mobilities of 2×10-3 and 6×10-2  cm2 V-1 s-1 , respectively.

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 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.

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.

Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers: Exploring the Domain of Resonant Energy Transfer at 1 eV.

The photophysics of selected tungsten iodide clusters was examined with respect to their role as a photosensitizer for the production of singlet oxygen, O2(a1Δg). We examined all-iodo octahedral clusters, [W6I8(I6)]2-, and ligand-substituted octahedral clusters, [W6I8(L6)]2-, in which the ligand, L, occupies the outer apical positions surrounding the cluster core. We also examined a square-pyramidal cluster, [W5I8(I5)]-, in which the tungsten core was presumably more accessible to diffusional encounter with ground state oxygen, O2(X3Σg-). For the compounds examined, we find pronounced cluster-dependent changes in the yield of photosensitized O2(a1Δg) production. In particular, although the iodine-encased octahedral cluster, [W6I8(I6)]2-, is an efficient O2(a1Δg) sensitizer, the pyramidal cluster, [W5I8(I5)]-, does not make O2(a1Δg) at all. The latter provides fundamental insight into the important case where the sensitizer triplet state is nearly degenerate with the O2(X3Σg-)-O2(a1Δg) transition energy at 1 eV. Our data indicate that even with near resonance, energy transfer to form O2(a1Δg) will not occur within the 3sensitizer-O2(X3Σg-) encounter pair if other more efficient channels for energy dissipation are available.

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).

Facile Way of Synthesis for Molybdenum Iodides.

Molybdenum iodides are prepared based on a new way of synthesis, namely, via reductive metathesis reaction of MoCl5 with SiI4. MoI3 was formed at 150 °C. Mo6I12 and the new molybdenum iodides Mo6I16 and Mo6I18 with the well-known [Mo6I8]4+ cluster core were obtained in the temperature range 550-600 °C. Compounds were structurally characterized by powder and single-crystal X-ray diffraction techniques.

A Facile Method for the Synthesis of Binary Tungsten Iodides.

The preparation of tungsten iodides in large quantities is a challenge because these compounds are not accessible using an easy synthesis method. A new, remarkably efficient route is based on a halide exchange reaction between WCl6 and SiI4. The reaction proceeds at moderate temperatures in a closed glass vessel. The new compounds W3I12 (W3I8 ⋅2 I2) and W3I9 (W3I8 ⋅½I2) containing the novel [W3I8] cluster are formed at 120 and 150 °C, and remain stable in air. W3I12 is an excellent starting material for the synthesis of other metal-rich tungsten iodides. At increasing temperature these trinuclear clusters undergo self-reduction until an octahedral tungsten cluster is formed in W6I12 . The synthesis, structure, and an analysis of the bonding of compounds containing this new trinuclear tungsten cluster are presented.

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.