Volume 230, Issue 8

Two uranyl tungstates, (UO 2 )(W 2 O 7 )(H 2 O) 3 ( 1 ) and (UO 2 ) 3 (W 2 O 8 )F 2 (H 2 O) 3 ( 2 ), were synthesized under hydrothermal conditions at 220°C and were structurally, chemically, and spectroscopically characterized. 1 Crystallizes in space group Pbcm , a = 6.673(5) Å, b = 12.601(11) Å, c = 11.552 Å; 2 is in C 2/ m , a = 13.648(1) Å, b = 16.852(1) Å, c = 9.832(1) Å, β = 125.980(1)°. In 1 the U(VI) cations are present as (UO 2 ) 2+ uranyl ions that are coordinated by five oxygen atoms to give pentagonal bipyramids. These share two edges with two tungstate octahedra and single vertices with four additional octahedra, resulting in a sheet with the iriginite-type anion topology. Only water molecules are located in the interlayer. The structural units of 2 consist of (UO 2 ) 2+ uranyl oxy-fluoride pentagonal bipyramids present as either [UO 2 F 2 O 3 ] –6 or [UO 2 FO 4 ] –5 , and strongly distorted tungstate octahedra. The linkage of uranyl pentagonal bipyramids and tungstate octahedra gives a unique sheet anion topology consisting of pentagons, squares and triangles. In 2 , the uranyl tungstates sheets are connected into a novel electro-neutral three-dimensional framework through dimers of uranyl pentagonal bipyramids. These dimers connecting the sheets share an edge defined by F anions. 2 is the first example of a uranyl tungstate oxy-fluoride, and 1 and 2 are rare examples of uranyl compounds containing electro-neutral structural units.

Cu 2 ZnSiSe 4 belong to the adamantine family of quaternary chalcogenides crystallizing in the wurtzstannite structure. Recent ab-initio calculations show, that the lowest energy structure of Cu 2 ZnSiSe 4 is the wurtzkesterite type structure in contrast to wurtzstannite type, usually obtained in experiments. To clarify this issue a structural study on single crystals of Cu 2 ZnSiSe 4 was performed for the first time. The structural characterization of the single crystals was carried out by X-ray diffraction at two different temperatures – room temperature and 150 K. The XRD data analysis shows, that Cu 2 ZnSiSe 4 single crystals adopt the orthorhombic wurtzstannite type structure (space group Pmn 2 1 ) and lattice parameters a = 7.809 Å, b = 6.778 Å, c = 6.447 Å at 150 K, and lattice parameters a = 7.821 Å, b = 6.734 Å, c = 6.453 Å at room temperature were derived. The structural parameters were confirmed for the polycrystalline Cu 2 ZnSiSe 4 bulk sample.

Besides the commonly employed refinement by minimization of the R factor, there is refinement by atomicity. These two refinements lead to two refined structures. Both structures are reasonable and close to each other. The differences between the results of these two refinements are significant. Refinement by atomicity is used as a supplement to refinement by minimization of R factor. For any crystal structure determined by minimization of R factor or other method, with all atoms determined and occupancy of each site equal to 1, in principle, refinement by atomicity should be applied. The purpose is to improve the atomic coordinates. It remains for the X-ray crystallographers to devise a procedure to obtain an improved structure from these two refined structures. The procedure can then be applied to all crystal structures reported in the literatures, or will be published in the future. This means that most articles in X-ray crystallography need to be improved. Many articles and books in other disciplines directly or indirectly contain or use structural informations, such as bond lengths and bond angles, obtained in X-ray crystallography. All these articles and books need to be revised.

Three crystal structures are reported here: ( E,E )-arenecarbaldehyde 7-chloro-1-methyl-4H-quinolinyl-4-ylidene-hydrazones, 3 , [arenecarbaldehyde = 3-MeOC 6 H 4 CHO ( 3a ), 3,4-(MeO) 2 C 6 H 3 CHO ( 3b ) and 2,3-(MeO) 2 C 6 H 3 CHO ( 3c )]. While in each compound π···π stacking and other intermolecular interactions make contributions to the overall supramolecular arrangements, very different arrangements result for the relatively flat molecules of 3a and 3b and the non-planar molecules of 3c . In 3a , all possible π···π interactions [π(quin)···π(quin), π(quin)···π(phen) and π(phen)···π(phen)] are present and together generate a sheet of molecules, one molecule thick) [quin = quinolinyl; phen = methoxyphenyl]. In 3b , π(phen)···π(phen)] interactions are absent: however, π(quin)···π(quin) and π(quin)···π(phen) interactions are both involved in producing a column of molecules, two molecules wide. In 3c , two distinct dimeric π···π stacked structural subsets are formed: ( i ) one dimer is composed of the two independent molecules, A and B, and generated from π(quin)···π(quin) interactions and ( ii ) the other AB dimeric unit is formed from π(phen)···π(phen) interactions. The differences in the π···π intermolecular interactions found in 3a , 3b and 3c are consequences of the steric effects presented by the methoxy substituents in the phenyl ring.

Crystal structure analysis of the isomeric N,N′-bis(pyridin-n-ylmethyl)ethanedithioamides, n = 2 ( 1 ), 3 ( 2 ) and 4 ( 3 ), show a planar conformation for 1 and conformations whereby the pyridyl rings lie orthogonal and to either side of the central residue for each of 2 and 3 . The universal adoption of the all ZZ conformation about the central C–N bonds, which have double bond character, is ascribed to the presence of intramolecular N–H…S hydrogen bonds that close S(5) rings. The gas-phase geometry optimised structure for 1 is the same as the experimental structure which features intramolecular amine-N–H…N(pyridyl) hydrogen bonds. The open structures found for 2 and 3 differ from the somewhat flattened optimised structures. Systematic variations in the geometric parameters characterising the central C 2 N 2 S 2 residue, in particular the double-bond character of the C–N bond and the elongation of the central C–C bond are shown by theory to be due to conjugative n N → π* C=S interactions and n S → σ* C-C hyperconjugation, respectively.

The crystal structures of two closely related compounds, namely, N-(4-fluorobenzoyl)-benzenesulfonamide (I) and N-(4-fluorobenzoyl)-4-methylbenzenesulfonamide (II) are investigated by analysing the packing patterns and intermolecular interactions, and also by Hirshfeld surface analysis. The crystal structure of each of (I) and (II) displays a two-dimensional architecture. Hirshfeld surfaces comprising d norm surface and 2D fingerprint plots were analysed for both molecules in order to understand the relationship between the crystal structures. The analysis shows that the lengths of the observed hydrogen bonds and other intermolecular interactions in (II) are relatively shorter than those observed in (I). Further, the analysis demonstrates the predominant participation of the sulfonyl-O atom and the carbonyl-O atom as the hydrogen bond acceptors in (I) and (II), respectively.

Two polymorphs of [Sc( cpl ) 6 ][Cr(NCS) 6 ] ( cpl =ε- C 6 H 11 NO), trigonal and monoclinic, form purple elongated narrow plates and brownish-purple prisms and are formed concomitantly irrespectively of the crystallization conditions. In the trigonal polymorph both cation and anion possess C 3 i site symmetry while in the monoclinic form cation and anion lie on inversion centre and 2-fold axis respectively. The nature of the polymorphism traces back to a redistribution of inter- and intramolecular hydrogen bonds that causes different conformation of the complex cations, different hydrogen bonding and different molecular packings. The [Sc( cpl ) 6 ] 3+ cations in the structure of the trigonal polymorph form intermolecular N(H)…S, and in the monoclinic form both N(H)…S inter- and N(H)…O intramolecular hydrogen bonds with NCS groups of [Cr(NCS) 6 ] 3− and cpl ligands. This aggregation leads to chains, where the cations and the anions alternate, in the trigonal modification and to layers, in which each ion is surrounded by four counterions, in the monoclinic form. Both polymorphs possess thermochromic properties, and a reversible color change from light purple to dark green takes place at 470–475 K.