Volume 227, Issue 6 -

In a systematic approach, the ability of the Maximum Entropy Method (MEM) for localization of missing atoms with low occupancies in the crystal structure from X-ray powder diffraction data and for reconstruction of the most probable electron density distribution of these atoms was evaluated. As a case study, the ambient-temperature crystal structures of Sr- and Ca-apatites with different intercalated metal atoms in the hexagonal channels were investigated in detail. Different combinations of F -constraints (based on the observed structure factors) and G -constraints (based on the structure factors amplitudes of overlapping reflections) were used in the MEM calculations. In particular, a combination of amplitudes of structure factors from a Le Bail fit with phases from Rietveld refinement of the incomplete structure was successfully applied for the localization of missing atoms in centrosymmetric crystal structures.

The crystal structure of manganese(III) dihydrogen triphosphate dihydrate, MnH 2 P 3 O 10 · 2H 2 O, has been studied by means of X-ray powder diffraction data using the Rietveld method. The compound crystallizes in the monoclinic space group P 2/ c , with unit cell dimensions a = 7.8286(2), b = 4.9503(2), c = 12.25415(18) Å, β = 95.7922(11)°, V = 472.47(2) Å 3 , Z = 2. It has a layered structure, with manganese-triphosphate polymeric layers, parallel to the bc plane, and water molecules located in the interlayer space. Powder pattern of the compound exhibits anisotropic peak broadening, which cannot be explained by crystallite size effects. The reflections with k = 2 n remain narrow, whereas those with k = 2 n + 1 and h ≠ 0 are widened substantially. These features have been assigned to stacking faults with a b /2 translation. Simulated diffraction patterns, calculated with the program DIFFaX by recursion method for different values of stacking faults probability, reproduced the experimental X-ray powder pattern. A phenomenological treatment of anisotropic peak broadening was used to complete the Rietveld refinement.

A new tungsten bronze in the Sb—W—O system has been prepared in a solid state reaction from Sb 2 O 3 , WO 3 and W metal powder. The average structure was determined by single crystal X-ray diffraction. Sb x WO 3+ y ( x ∼ 0.11) crystallizes in the orthorhombic space group Pm 2 1 n (no. 31), a = 27.8135(9) Å, b = 7.3659(2) Å and c = 3.8672(1) Å. The structure belongs to the ( n )-ITB class of intergrowth tungsten bronzes. It contains slabs of hexagonal channels formed by six WO 6 octahedra. These slabs are separated by three layers of WO 6 octahedra that are arranged in a WO 3 -type fashion. The WO 6 octahedra share all vertices to build up a three-dimensional framework. The hexagonal channels are filled with Sb atoms to ∼80% and additional O atoms. The atoms are shifted out of the center of the channels. Exit-wave reconstruction of focal series of high resolution-transmission-electron-microscope (HRTEM) images combined with statistical paramäeter estimation techniques allowed to study local ordering in the channels. Sb atoms in neighbouring channels tend to be displaced in the same direction, which is in agreement with total energy calculations on ordered structure models, but the ratio of the occupation of the two possible Sb sites varies from channel to channel. The structure of Sb x WO 3+ y exhibits pronounced local modulations.

A triangular Mn(III) complex with an [Mn III 3 μ 3 -O(sao) 3 ] unit (sao – doubly deprotonated salicyläaldoxime) and unprecedented coordination by a TPO (triphenylphosphine oxide) ligand is the result of a transfer of the TPO ligand from a rhenium(V) complex onto a pre-formed [Mn III 3 O] unit. In the crystal structure, isomeric complex molecules with different patterns of their terminal substituents form hydrogen-bonded dimers.

The tetrachloro and tetrabromo substituted bisimidazoles 1,4-bis(4,5-bis(4-chlorophenyl)-1 H -imidazol-2-yl)benzene ( 1 ) and 1,4-bis(4,5-bis(4-bromophenyl)-1 H -imidazol-2-yl)benzene ( 2 ), respectively, as well as a corresponding aldehydic intermediate imidazole 4-(4,5-bis(4-bromophenyl)-1 H -imidazol-2-yl)benzaldehyde ( 3 ) have been synthesized. Respective solvated complexes [ 1 DMSO (1:2)] ( 1a ), [ 2 (1,4-dioxane) (1:5)] ( 2a ) and [ 3 DMSO · H 2 O (1:1:1)] ( 3a ) were isolated and structurally studied by single crystal X-ray diffraction. Although the central three-membered ring systems in 1 and 2 are almost coplanar, there is no relevant stacking between the molecules. However, 1 and 2 show a distinct difference in conformation with respect to the orientation of the imidazole rings which is cisoid in 1a and transoid in 2a , being attributed to the specific solvent interactions. Hydrogen bonds of N—H···O, C—H···O and C—H···π type are found in all structures while Cl···Cl contacts are characteristic of 1a , Br···O and C—H···Br of 2a , and O—H···O and Br···Br contacts of the crystal structure of 3a . Owing to the formation of solvated crystals, especially the compounds 1 and 2 are represented as potential new inclusion hosts.

Distinct molecular structures and supramolecular association is found in the title crystal structures. When chloroform was diffused into a solution of {Zn[S 2 CN(Me)CH 2 CH 2 OH)] 2 } 2 (N,N‘-bis(pyridin-3-ylmethyl)thioxalamide) in acetonitrile, it was found to have co-crystallised (2:1) with S 8 ( 1 ) as desulphurisation had occurred during crystallisation. By contrast, when dimethylformamide was diffused in an acetonitrile solution of the same compound, a 2:1 dimethylformamide solvate ( 2 ) was isolated. The zinc atom coordination geometry is based on a NS 4 donor set in each case but, this defines an approximate square pyramidal geometry in ( 1 ) and a geometry intermediate between square pyramidal and trigonal bipyramidal in ( 2 ). This difference is ascribed to the hydrogen bonding considerations in ( 2 ). The hydroxyl groups in ( 1 ) self associate leading to a two-dimensional array; the S 8 molecules occupy space between successive layers. The aforementioned hydrogen bonding is disrupted in ( 2 ) where one hydroxyl group interacts with the solvent dimethylformamide molecule. Accordingly, the supramolecular association between molecules is restricted to a one-dimensional chain.

The crystal structures of 4-FCH 2 -1-(4-X-phenyl)-triazole ( 1 : X = Cl, Br and CN) and 4-HOCH 2 -1-(4-X-phenyl)-triazole ( 2 : X = Cl, Br and CN) are reported. Each of the pairs, ( 1 : X = Cl) and ( 1 : X = Br) and ( 2 : X = Cl) and ( 2 : X = Br), are isostructural. There are no strong intermolecular interactions in any of the compounds 1 . Present in ( 1 : X = CN), are C—H···N, C—H···F and C—H···π hydrogen bonds, and π···π stacking interactions, while in ( 2 : X = Cl and Br) present are C—H···N, C—H···F and C—X···π, hydrogen bonds, with resulting different supramolecular arrays produced for the halo, on one hand, and cyano derivatives, on the other. The hydroxyl-triazole nitrogen interactions in 2 are the strongest intermolecular interactions present in 2 . In the case of ( 2 : X = Cl and Br), augmenting this strong O—H···N hydrogen bond with a weaker C—H···O hydrogen bond results in a [ C(6)R 2 2 (10) ] chain of molecules propagated head-to-tail along the b axis. In contrast, this strong O—H···N hydrogen bond in ( 2 : X = CN), now augmented by C—H···N(cyano nitrogen), results in a chain of molecules, [ C(19)R 2 2 (10)R 2 2 (10)] . Further weaker intermolecular interactions of the types, C—H···N and C—H···O hydrogen bonds and π···π stacking interactions for ( 2 : X = CN), and of the types, C—H···O and C—X···π hydrogen bonds, and π···π stacking interactions for ( 2 : X = Cl and Br) generate significantly different supramolecular arrays.

The crystal structure of betulinic acid-DMSO (1:1) solvate has been determined. The betulinic acid, [3 β -hydroxy-lup-20(29)-en-28-oic acid, C 30 H 48 O 3 ] is considered as a promising natural agent for the clinical treatment for various types of cancer. Crystals of betulinic acid for X-ray structural analysis were grown from a DMSO-water (9:1, v/v) solution. Betulinic acid-DMSO (1:1) solvate crystallizes in the orthorhombic symmetry the P 2 1 2 1 2 1 space group, Z = 4. The unit cell parameters are as follows: cell lengths a = 6.9417(2) Å, b = 13.8559(5) Å, c = 31.6232(10) Å, V = 3041.63(17) Å 3 . Betulinic acid molecules are hydrogen bonded to DMSO molecules. The weak intermolecular hydrogen bonds and van der Waals interactions are present in the crystal structure. The crystal and molecular structures of betulinic acid-DMSO solvates are discussed with reference to betulin and betulinic acid derivatives.