Structure Elucidation of a Melam–Melem Adduct by a Combined Approach of Synchrotron X-ray Diffraction and DFT Calculations

Melam-melem (1:1), an adduct compound that can be obtained from dicyandiamide in autoclave reactions at 450 °C and elevated ammonia pressure, had previously been described based on mass spectrometry and NMR spectroscopy, but only incompletely characterized. The crystal structure of this compound has now been elucidated by means of synchrotron microfocus diffraction and subsequent quantum-chemical structure optimization applying DFT methods. The structure was refined in triclinic space group P based on X-ray data. Cell parameters of a=4.56(2), b=19.34(8), c=21.58(11) Å, α=73.34(11)°, β=89.1(2)°, and γ=88.4(2)° were experimentally obtained. The resulting cell volumes agree with the DFT optimized value to within 7 %. Molecular units in the structure form stacks that are interconnected by a vast array of hydrogen bridge interactions. Remarkably large melam dihedral angles of 48.4° were found that allow melam to interact with melem molecules from different stack layers, thus forming a 3D network. π-stacking interactions appear to play no major role in this structure.     

Polymorphism of N,N'-diacetylbiuret studied by solid-state 13C and 15N NMR spectroscopy, DFT calculations, and X-ray diffraction

The molecular configuration and crystal structure of solid polycrystalline N,N′′‐diacetylbiuret (DAB), a potential nitrogen‐rich fertilizer, have been analyzed by a combination of solid‐ and liquid‐state NMR spectroscopy, X‐ray diffraction, and DFT calculations. Initially a pure NMR study (“NMR crystallography”) was performed as available single crystals of DAB were not suitable for X‐ray diffraction. Solid‐state ¹³C NMR spectra revealed the unexpected existence of two polymorphic modifications (α‐ and β‐DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X‐ray diffraction. The X‐ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two‐fold screw in the case of α‐DAB and a c‐glide plane in the case of β‐DAB, lead to a more symmetric (α‐DAB) or asymmetric (β‐DAB) intermolecular hydrogen‐bonding pattern for each molecule.     

Single Crystals of CaNa[ReO4]3: Serendipitous Formation and Systematic Characterization

In an attempt to crystallize Ce[ReO₄]₄ · xH₂O from aqueous solutions of equimolar amounts of Ce[SO₄]₂ and Ba[ReO₄]₂ via salt‐metathesis the serendipitous formation of colorless, transparent, rod‐shaped single crystals of CaNa[ReO₄]₃ was observed as a result of calcium and sodium impurities within the improperly deionized water used. Structure analysis by X‐ray diffraction lead to the conclusion that the title compound crystallizes in the ThCd[MoO₄]₃ structure type with the hexagonal space group P6₃/m and the lattice parameters a = 991.74(6) pm, c = 636.53(4) pm, c/a = 0.642 for Z = 2. The crystal structure contains purely oxygen surrounded and crystallographically unique cations, namely Ca²⁺ in tricapped trigonal prismatic (d(Ca–O) = 6 × 249 pm + 3 × 254 pm), Na⁺ in octahedral (d(Na–O) = 6 × 241 pm), and Re⁷⁺ in tetrahedral coordination (d(Re–O) = 171–173 pm). Furthermore, it was possible to yield an almost phase‐pure microcrystalline powder of the title compound from a melt of equimolar amounts of Na[ReO₄] and Ca[ReO₄]₂ stemming from aquatically obtained precursors.     

Melamium Thiocyanate Melam,a Melamium Salt with Disordered Anion Sites

Melamium salts are a group of ionic carbon nitride type compounds that has been investigated only scarcely. We herein present a novel representative of this group. A melamium thiocyanate melam (1:1) adduct was synthesized from dicyandiamide and ammonium thiocyanate in sealed glass ampoules. The structure of the adduct was determined from single‐crystal X‐ray diffraction. Melamium thiocyanate melam crystallizes in monoclinic space group P2₁/c (no. 14) with lattice parameters of a = 3.6041(11), b = 28.532(7), c = 10.937(4) Å, β = 99.051(14)°, and Z = 4. While the melamium ions form 2D extended hydrogen bridged networks, the thiocyanate ions are disordered and no distinct structural sites could be assigned to the respective atoms. Instead, continuous columns of electron density located in channels in the porous structure were identified as potential space for anion locations. The compound was further characterized by elemental analysis, FTIR spectroscopy and solid‐state MAS‐NMR spectroscopy of the nuclei ¹H, ¹³C and ¹⁵N. Rietveld refinement of powder samples was performed for phase analysis. Furthermore, DSC‐TG was used to investigate the thermal behavior of the compound.     

Solid-state molecular organization and solution behavior of methane-1,1-diphosphonic acid derivatives of heterocyclic amines: the role of the topochemical ring modification and the intramolecular hydrogen bonds in monosubstituted piperid-1-ylmethane-1,1-diphosphonic acids

The crystal structures of 3-methylpiperid-1-ylmethane-1,1-diphosphonic (2), 4-methylpiperid-1-ylmethane-1,1-diphosphonic (3), 2-ethylpiperid-1-ylmethane-1,1-diphosphonic (4), and 2-methylpiperid-1-ylmethane-1,1-diphosphonic (5) acids have been determined and are discussed with respect to their molecular organization and crystal-packing preferences. The chair conformation, predominant also in solution, favors equatorial positioning of the bulky substituents of the heterocyclic N and C atoms. The molecular geometry also provides access to intramolecular hydrogen-bond formation between the axial protons located on the nitrogen atoms, as well as the carbon atoms closest to it, and phosphonic/phosphonate oxygen atoms. The molecules preferably arrange in monolayers, observed in all crystals with an exception of 3. The layers are held in place in the third direction through van der Waals interactions. The analysis of two-dimensional hydrogen-bonded networks is concentrated on revealing how the substituent's topology of the molecule affects the solid-state organization in well-defined structures and is aimed at unraveling the consequences and the possible conformational changes by stepwise network disruption upon crystal dissolution in water. The solution NMR studies are focused on revealing the role that the topochemistry of the substituent plays for the stereodynamics in 2-5. It is demonstrated that in contrast to piperid-1-ylmethane-1,1-diphosphonic acid (1), in which the ring inversion/rotation around the C-N bond concerted with the N-H...O hydrogen-bond breaking/formation process leads to a mixture of two interconverting conformers, the concerted N-H...O breaking/rotation/N-H...O formation process in 2 and 3 allows for a predominance of one conformer in solution. However, placement of a substituent at 2-position in the ring hampers the rotation around the C-N bond; this makes 4 and 5 significantly less flexible relative to compounds 1-3. In addition, both compounds 4 and 5 are proved to exist as a mixture of two conformers, the equilibrium of which in acidic solution is shifted towards the conformer found in solid state. In alkaline solutions of 4 and 5, the equilibrium is shifted towards the conformer that is forced by the flipping of the heterocyclic ring. These results correlate well with recently documented differences in the biological potency of this group of compounds.     

Creation of Ternary Multicomponent Crystals by Exploitation of Charge‐Transfer Interactions

Four new ternary crystalline molecular complexes have been synthesised from a common 3,5‐dinitrobenzoic acid (3,5‐dnda) and 4,4′‐bipyridine (bipy) pairing with a series of amino‐substituted aromatic compounds (4‐aminobenzoic acid (4‐aba), 4‐(N,N‐dimethylamino)benzoic acid (4‐dmaba), 4‐aminosalicylic acid (4‐asa) and sulfanilamide (saa)). The ternary crystals were created through the application of complementary charge transfer and hydrogen‐bonding interactions. For these systems a dimer was created through a charge‐transfer interaction between two of the components, while hydrogen bonding between the third molecule and this dimer completed the construction of the ternary co‐crystal. All resulting structures display the same acid ??? pyridine interaction between 3,5‐dnba and bipy. However, changing the third component causes the proton of this bond to shift from neutral OH ??? N to a salt form, O^? ??? HN⁺, as the nature of the group hydrogen bonding to the carboxylic acid was changed. This highlights the role of the crystal environment on the level of proton transfer and the utility of ternary systems for the study of this process.     

New Phases in the Lithiumnitridovanadate System — The Solid Solution Li7‐2xMgx[VN4] with 0 ≤ x ≤ 1

Solid solution phases Li_7‐2xMg_x[VN₄] (0 < x ≤ 1) with varying Mg‐content are obtained as yellow microcrystalline powders from heat treatment of mixtures of VN, Li₃N and Mg₃N₂ or from mixtures of Li₇[VN₄] and Mg₃N₂ at 1370 K in N₂ atmosphere at ambient pressure. At substitution parameter values of x > 0.5 a subsequent distortion from the ideal cubic unit cell to an orthorhombic unit cell is observed. The crystal structure of Li_7‐2xMg_x[VN₄] with x ≈ 1 was refined from neutron and X‐ray powder diffraction data (space group Pbca, No. 61, a = 963.03(3) pm, b = 958.44(3) pm, c = 951.93(2) pm, neutron pattern 14° — 156° 2θ, step non‐linear ≈ 0.0782° 2θ, No. of measured points 1816, R_p = 0.089, R_wp = 0.115, R_Bragg = 0.155, R_F = 0.114; X‐ray pattern 10° — 98° 2θ, step 0.005° 2θ, No. of measured points 17600, R_p = 0.028, R_wp = 0.045, R_Bragg = 0.113, R_F = 0.133, structure variables: 45). The crystal structure resembles a Li₂O type superstructure with the atomic arrangement of β‐Li₇[VN₄] and with two crystallographic Li‐sites each substituted by Mg with statistical occupation factors of 0.5. Chemical analyses prove the composition and XAS spectroscopy at the V K‐edge support the +5 oxidation state assignment for vanadium. XAS data also support the tetrahedral coordination of vanadium by N as indicated by the structure refinements.     

Synthesis and Structure of Melamium Bromide C6N11H10Br and Melamium Iodide C6N11H10I

Melamium bromide and melamium iodide were synthesized from dicyandiamide in the presence of ammonium halides in evacuated Duran glass ampoules at temperatures of 450 °C. The crystal structures of both compounds were obtained from single‐crystal X‐ray diffraction. Melamium bromide C₆N₁₁H₁₀Br crystallizes in space group P2₁/n [no. 14, a = 7.0500(5), b = 28.7096(18), c = 10.8783(8) Å, β = 96.060(2)°, Z = 8, wR₂ = 0.2231] and exhibits a layer‐like arrangement of melamium ions, wherein both planar as well as twisted molecular structures of the cations occur. Melamium iodide C₆N₁₁H₁₀I crystallizes in space group P2₁/c [no. 14, a = 6.8569(3), b = 11.9949(6), c = 14.0932(6) Å, β = 97.613(2)°, Z = 4, wR₂ = 0.0654], however in a structure completely different from the one of melamium bromide. The melamium iodide structure is comprised of stacks of planar melamium ions that form complex, hydrogen‐bonded network layers with iodide ions within the layers. Both compounds were further characterized by FTIR spectroscopy, mass spectrometry, and elemental analyses. Melamium bromide and melamium iodide could be obtained as air stable and colorless crystals. Samples are crystallographically phase pure as shown by Rietveld refinement.     

Crystallography Aided by Atomic Core-Level Binding Energies: Proton Transfer versus Hydrogen Bonding in Organic Crystal Structures

Ionic bond or hydrogen bridge? Br?nsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter.     

In situ Untersuchungen der Reaktion von Ammoniummonomolybdat, (NH4)2MoO4, mit Ammoniak: Die Struktur von (NH4)2[Mo3O10]

In situ Investigation of the Reaction of Ammonium Monomolybdate (NH₄)₂MoO₄ with Ammonia: The Structure of (NH₄)₂[Mo₃O₁₀] The reactivity of both polymorphs of (NH₄)₂MoO₄ with ammonia was investigated in a temperature range between 20 and 180 °C. Time and temperature controlled X‐ray powder diffraction as well as thermogravimetrical and differential thermal analysis were used to investigate this reaction.The formation of (NH₄)₂[Mo₃O₁₀] from (NH₄)₂MoO₄ is reversible in a humid and irreversible in a dry NH₃ gas flow. Heating (NH₄)₂MoO₄(mP60) under an atmosphere of humid NH₃ at about 170 °C forms (NH₄)₂[Mo₃O₁₀] and succesively cooling yields the (NH₄)₂MoO₄(mS60) polymorph. (NH₄)₂[Mo₃O₁₀] crystallises isostructural to the potassium compound with space group C2/c (No. 15) and lattice constants a = 1398.2(4), b = 804.1(2), b = 921.0(3) pm and β = 98.833(4)°.     

tert‐Butoxysilanols as model compounds for labile key intermediates of the sol–gel process: crystal and molecular structures of (t‐BuO)3SiOH and HO[(t‐BuO)2SiO]2H

The tert‐butoxychlorosilanes (t‐BuO)₃SiCl ( 1 ), (t‐BuO)₂SiCl₂ ( 2 ), and [(t‐BuO)₂SiCl]₂O ( 3 ) were prepared by the reaction of SiCl₄ or (Cl₃Si)₂O with t‐BuOK. Subsequent hydrolysis afforded the tert‐butoxysilanols (t‐BuO)₃SiOH ( 4 ), (t‐BuO)₂Si(OH)₂ ( 5 ), HO[(t‐BuO)₂SiO]₂H ( 6 ) in high yields. The controlled condensation of 2 and 5 provided HO[(t‐BuO)₂SiO]₃H ( 7 ) in reasonable yields. The tendency of 4 – 7 to undergo self‐condensation is small, thus enabling their characterization in solution and in the solid state by ²⁹Si NMR spectroscopy, IR spectroscopy and electrospray mass spectrometry, and in the case of 4 and 6 also by X‐ray diffraction. The key feature of the crystal structures is the incorporation of tert‐butoxy groups into the hydrogen bonding. The results obtained are discussed in relation to the sol–gel process. Copyright © 2002 John Wiley & Sons, Ltd.     

Octahedral adducts of dichlorosilane with substituted pyridines: synthesis, reactivity and a comparison of their structures and (29)si NMR chemical shifts

H(2)SiCl(2) and substituted pyridines (Rpy) form adducts of the type all-trans-SiH(2*)Cl(2)2 Rpy. Pyridines with substituents in the 4- (CH(3), C(2)H(5), H(2)C=CH, (CH(3))(3)C, (CH(3))(2)N) and 3-positions (Br) give the colourless solids 1 a-f. The reaction with pyrazine results in the first 1:2 adduct (2) of H(2)SiCl(2) with an electron-deficient heteroaromatic compound. Treatment of 1 d and 1 e with CHCl(3) yields the ionic complexes [SiH(2)(Rpy)(4)]Cl(2*)6 CHCl(3) (Rpy=4-methylpyridine (3 d) and 4-ethylpyridine (3 e)). All products are investigated by single-crystal X-ray diffraction and (29)Si CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the Si-N distances vary between 193.3 pm for 1 c (4-(dimethylamino)pyridine complex) and 197.3 pm for 2. Interestingly, the pyridine moieties are coplanar and nearly in an eclipsed position with respect to the SiH(2) units, except for the ethyl-substituted derivative 1 e, which shows a more staggered conformation in the solid state. Calculation of the energy profile for the rotation of one pyridine ring indicates two minima that are separated by only 1.2 kJ mol(-1) and a maximum barrier of 12.5 kJ mol(-1). The (29)Si NMR chemical shifts (delta(iso)) range from -145.2 to -152.2 ppm and correlate with the electron density at the Si atoms, in other words with the +I and +M effects of the substituents. Again, compound 1 e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the (29)Si NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent Si-H bonds and very polar Si-Cl bonds, with the highest bond polarisation being observed for the Si-N interaction, which must be considered a donor-acceptor interaction. An analysis of the topological properties of the electron distribution (AIM) suggests a Lewis structure, thereby supporting this bonding situation.