Volume 230, Issue 6

Quenching cubic high-temperature polymorphs of (GeTe) n Sb 2 Te 3 (n ≥ 3) yields metastable phases whose average structures can be approximated by the rocksalt type with 1/(n + 3) cation vacancies per anion. Corresponding diffraction patterns are a superposition of intensities from individual twin domains with trigonal average structure but pseudo-cubic metrics. Their four orientations are mirrored in structured diffuse streaks that interconnect Bragg reflections along the [001] directions of individual disordered trigonal domains. These streaks exhibit a “comet-like” shape with a maximum located at the low-angle side of Bragg positions (“comet head”) accompanied by a diffuse “comet tail”. 2D extended cation defect ordering leads to parallel but not equidistantly spaced planar faults. Based on a stacking fault approach, the diffuse scattering was simulated with parameters that describe the overall metrics, the concentration and distribution of cation defect layers, atom displacements in their vicinity and the stacking sequence of Te atom layers around the planar defects. These parameters were varied in order to derive simple rules for the interpretation of the diffuse scattering. The distance between Bragg positions and “comet heads” increases with the frequency of planar faults. A sharp distance distribution of the planar faults leads to an intensity modulation along the “comet tail” which for low values of n approximates superstructure reflections. The displacement of atom layers towards the planar defects yields “comets” on the low-angle side of Bragg positions. A rocksalt-type average structure is only present if the planar defects correspond to missing cation layers in the “cubic” ABC stacking sequence of the Te atom layers. An increasing amount of hexagonal ABA transitions around the defect layers leads to increasing broadening and splitting of the Bragg reflections which then overlap with the diffuse scattering. Based on these rules, the diffuse scattering of (GeTe) n Sb 2 Te 3 (n = 2, 4, 5, 12) crystals was analyzed by comparing simulated and experimental reciprocal space sections as well as selected streaks extracted from synchrotron data. With decreasing n, both the average distance between faults and thus the slab thickness decrease, whereas the probability of hexagonal ABA transitions increases. The quenched metastable phases can be understood as intermediates between the stable high-temperature phases, which exhibit a rocksalt-type structure with randomly disordered cations and vacancies on the cation position, and the trigonal layered structures, which are stable at room temperature and consist of distorted rocksalt-type slabs separated by equidistant defect layers.

The crystal and molecular structures of three ammonium salts derived from sulfathiazole are described. In each case, the anion is in the azanide form, features an intramolecular S←O interaction, and adopts a U-shape. The structures of two cations, [R(HOCH 2 CH 2 )NH 2 ] + , namely for R = Me ( 1 ) and iPr ( 2 ), are unprecedented in the crystallographic literature. Extensive hydrogen bonding is observed in all crystal structures and leads to a two-dimensional array for 1 , and three-dimensional architectures for each of 2 and 3 (R = CH 2 CH 2 OH). The salts exhibited anti-microbial activity against a range of Gram-positive and Gram-negative bacteria, and proved bactericidal toward Vibrio parahaemolyticus , but had no advantage over sulfathiazole itself.

The crystal and molecular structure of a second polymorph ( P 2 1 / n ) of [1,4-ROC(=S)N(H)C(=O)C 6 H 4 C(=O)N(H)C(=S)OR], R = Et, is shown to have inversion symmetry and to have a more twisted conformation than the previously reported P 4 3 2 1 2 form which has 2-fold symmetry. Despite utilising equivalent atoms in forming intermolecular interactions, very distinct crystal packing patterns are observed. Crystal characteristics and theory (DFT) are consistent with the P 4 3 2 1 2 form being more stable, a conclusion correlated with the observation that this form is the overwhelming majority of the sample (PXRD). An analysis of a binuclear copper(I) complex containing the R = iPr analogue, reveals the neutral ligand to bind via the thione-S atom resulting in a distorted ClP 2 S tetrahedral geometry.

The dinuclear complex tris(2-aminoethyl)aminezinc(II)-μ-cyanothreecyanozincate(II) hemihydrate, [Zn(tren)Zn(μ-CN)(CN) 3 ]·0.5H 2 O ( 1 ) (tren = tris(2-aminoethyl)amine), has been synthesized and characterized by spectral (FT-IR and Raman), elemental, thermal analysis (TG, DTG and DTA) as well as single crystal X-ray diffraction techniques. The asymmetric unit is composed of two Zn(II) ions, one tren ligand, four cyanide ligands and a half crystal water molecule which is situated at the special position. Zn1 ion exhibits tetrahedral coordination geometry with four carbon atoms of four cyanide ligands. Zn2 ion is five-coordinated by five nitrogen atoms from one tren and one cyanide ligands in a trigonal bipyramid coordination geometry. The cyanide nitrogen is in the axial position. Adjacent dinuclear units are connected by hydrogen bonding interactions to form three dimensional network. The decomposition reaction takes place in the temperature range 30–700 °C in the static air atmosphere.

A two-dimensional coordination polymer [Zn(L) 2 ] n ( 1 ) (L = 3-aminopyrazine-2-carboxylate) synthesized from the solvothermal reaction of zinc(II) salt with 3-aminopyrazine-2-carboxylic acid (HL) is described. Compound 1 has been characterized by single X-ray diffraction, IR spectra and thermogravimetric analyses. Crystal structure analysis reveals that each hexacoordinated zinc(II) center adopts a distorted octahedral geometry occupied by three O carboxylate and three N pyrazine atoms. The L – ligand binds the metal cation by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. A three-dimensional supramolecular associate in the crystal lattice of 1 has been stabilized by a number of non-covalent interactions. The IR spectroscopic and TGA properties are investigated in this work. Topological analysis of the two-dimensional network has been also discussed.

A new one-dimensional Zn(II)–cyanide complex with 1-methylimidazole ligand, [Zn(μ-CN)(CN)(1-meim)] n (1-meim: 1-methylimidazole) has been synthesized and characterized by spectral (FT-IR and Raman) methods, elemental analysis, thermal (TG, DTG and DTA) analysis and single crystal X-ray diffraction techniques. The complex crystallizes in the monoclinic system, P2 1 space group. The asymmetric unit contains one Zn(II), one 1-meim and two cyanide ligands. The coordination sphere of the Zn(II) ion exhibits a distorted tetrahedral geometry. In the crystal structure, the symmetry related zinc(II) atoms are bridged by the cyanide anions to form one dimensional chains running along the b-axis. Intermolecular C–H···N hydrogen bonds link the adjacent polynuclear chains forming two dimensional layers through the R1818(2)$R_{18}^{18}(2)$ ring motifs, where the layers formed are oriented in parallel to (020).

The X-ray molecular structure of two benzimidazoles unsubstituted on the nitrogen atoms (2-ethyl-1 H -benzimidazole and 2-propyl-1 H -benzimidazole) has been determined and analysed in the context of crystallographic and solid-state NMR data of seven substituted benzimidazoles. The first compound presents polymorphism, and two polymorphs [forms 3a and 3c ] were obtained from different solutions of dichloromethane/hexane. Polymorph ( 3a ) contains two independent molecules differing in the conformation of the ethyl group. Polymorph ( 3c ) contains only one molecule in the asymmetric unit, with similar configuration as the ordered molecule of polymorph ( 3a ). Another polymorphic form ( 3b ) is obtained by cooling form ( 3a ) below 179 K. The transition is related to an order-disorder process and involves a change in the hydrogen-bonding scheme. The second compound ( 4 ) presents enantiomorphism and its asymmetric unit contains four independent molecules that differ in the conformation of the 2-propyl group. For form ( 3a ), the splitting of NMR signals is almost perfectly consistent with the features of the crystal structure, whereas not splitting is observed for compound ( 4 ). Agreements between NMR and X-ray crystallography techniques are also observed in other simple 2-R-1 H -benzimidazoles (R = C 4 H 9 , CH 2 C 6 H 5 and C 6 H 5 ), but disagreements are found for R = H, CH 3 .