Xiqu Wang

Tossing and turning: guests in the flexible frameworks of metal(III) dicarboxylates.

Single crystals of Ga(OH)(C(8)H(4)O(4)).0.74C(8)H(6)O(4) (2) and Ga(OH,F)(C(8)H(4)O(4)).0.74C(8)H(6)O(4) (3) were obtained under hydrothermal conditions. The structures of 2 and 3 have the same topological framework as the previously reported aluminum 1,4-benzenedicarboxylate (BDC), Al(OH)(C(8)H(4)O(4)).0.7C(8)H(6)O(4) (1). The frameworks are built by interconnecting M-OH-M chains (M = Al, Ga) with BDC anions to form large diamond-shaped one-dimensional channels filled with additional H(2)BDC guest molecules occupying disordered positions in the channels. Upon removal of H(2)BDC, other guest molecules such as H(2)O and pyridine can be inserted. In this work, we present a study of the intercalation of aromatic guests (BDC and pyridine) into frameworks of 1-3 by liquid and vapor diffusion into the empty channels of 1 and by single-crystal-to-single-crystal solvothermal guest exchange for 2 and 3. In the case of Al(OH)BDC and Ga(OH,F)BDC, two interconvertible, guest-concentration-dependent phases with different orientations of the pyridine guests have been observed, while only one pyridine orientation is found in Ga(OH)BDC.

Studies on bond and atomic valences. I: Correlation between bond valence and bond angles in SbIII chalcogen compounds : the influence of lone-electron pairs

In the present bond-valence concept the bond-valence parameter ro is treated as constant for a given pair of atoms, and it is assumed that the bond valence sij is a function of the corresponding bond length Dij, and that the atomic valence is an integer equal to the formal oxidation number forVi derived from stoichiometry. However, from a statistical analysis of 76 [SbIIISn] and 14 [SbIIISen] polyhedra in experimentally determined structures, it is shown that for SbIII—X bonds (X = S, Se), ro is correlated with {\bar \alpha}i, the average of the X—Sb—X angles between the three shortest Sb—X bonds. This is interpreted as a consequence of a progressive retraction of the 5s lone-electron pair from the SbIII nucleus, which can be considered as continuous change of the actual atomic valence actVi of Sb from +3 towards +5. A procedure is derived to calculate an effective atomic valence effVi of SbIII from the geometry, {\bar \alpha}i and Dij, of the [SbIIIXn] polyhedra, which approximates actVi and is a better description of the actual valence state of SbIII than the formal valence forVi. Calculated effVSbIII are found to vary between +2.88 and +3.80 v.u. for [SbIIISn] and between +2.98 and +3.88 v.u. for [SbIIISen] polyhedra. It is suggested that a corresponding modification of the present bond-valence concept is also required for other cations with lone-electron pairs.

Hydrothermal synthesis and structures of the new open-framework uranyl silicates Rb4(UO2)2(Si8O20)(USH-2Rb), Rb2(UO2)(Si2O6)·H2O (USH-4Rb) and A2(UO2)(Si2O6)·0.5H2O (USH-5A; A = Rb, Cs)

Four new open-framework alkali metal uranyl silicates, Rb4(UO2)2(Si8O20) (USH-2Rb), Rb2(UO2)(Si2O6)·H2O (USH-4Rb), and A2(UO2)(Si2O6)·0.5H2O (USH-5A; A = Rb, Cs), have been prepared hydrothermally at 245 °C. The structures of USH-2, -4, and -5 contain UO6 tetragonal bipyramidal units connected by double chains, 4-membered rings, and single chains of SiO4 tetrahedra, respectively. The compounds have been characterized by single crystal X-ray diffraction, scanning electron microscopy, UV-vis and infrared spectroscopy, and thermogravimetric analysis.

The novel open-framework uranium silicates Na2(UO2)(Si4O10).2.1H2O (USH-1) and RbNa(UO2)(Si2O6).H2O (USH-3)

The new open-framework uranium silicates Na2(UO2)(Si4O10)·2.1H2O (USH-1) and RbNa(UO2)(Si2O6)·H2O (USH-3) have been synthesized by hydrothermal reactions at 230 °C. USH-1 has a framework structure formed from silicate single layers cross-linked by interlayer UO6 tetragonal bipyramids. The silicate layers consist of interconnected tetrahedral 4-rings and 8-ring voids. The 8-ring voids are lined up to form 1-dimensional channel systems in the framework. In contrast to the silicate layers of USH-1, individual 4-rings of silicate tetrahedra are found in USH-3 which are cross-linked by UO6 tetragonal bipyramids to form an open framework.

Breathing and twisting: an investigation of framework deformation and guest packing in single crystals of a microporous vanadium benzenedicarboxylate.

The structural details of the compounds vanadium benzenedicarboxylate VO(bdc)·Guest, where the Guests are the absorbed six-ring molecules: benzene, 1,4-cyclohexadiene, 1,3-cyclohexadiene, cyclohexene and cyclohexane, have been determined from single crystal X-ray data. All of the six-ring guest molecules show a high degree of ordering inside the channels of VO(bdc). The interactions between the guests and the host framework are dominated by van der Waals bonding. The six-ring molecules are all packed in two columns in the channels, either in herringbone or close to parallel patterns. The packing changes the space group symmetry of VO(bdc) from Pnma to the noncentrosymmetric space group P2(1)2(1)2(1). The VO(bdc) framework deforms to closely adapt to the shape and thickness changes of the double columns of the guest molecules. In addition to the well studied breathing deformation, a twisting deformation mechanism that involves a cooperative rotation of the octahedral chains accompanied by bending of the bdc ligand is apparent in the detailed structural data. More quantitative information on the remarkable flexibility of the VO(bdc) framework was obtained from ab initio calculations.

A Three-Dimensional Coordination Polymer with an Expanded NbO Structure

The connection of square-planar [Ni(CN)4]2− ions with Ph3Sn+ ions results in 1, a noninterpenetrated three-dimensional coordination polymer with an expanded NbO structure. The (Ph3Sn)2Ni(CN)4 framework (shown schematically without phenyl groups and solvent molecules) displays a large central cavity that is occupied by Ph3SnOH, acetonitrile, and water molecules.

Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situ neutron diffraction

The layered perovskite NdBaCo2O5+δ (NBCO) was characterized using neutron powder diffraction under in situ conditions from 577–852 °C and in 10−1 to 10−4 atm oxygen. The best fit to the data was obtained in the tetragonal (P4/mmm) space group. No oxygen atom vacancy ordering was observed that warranted a lowering of the symmetry to orthorhombic (Pmmm). Two P4/mmm structural models were investigated: Model 1 (no split sites) and Model 2 (split Nd and O2 sites). Transport of oxygen through the material via the vacancy hopping mechanism likely involves the nearest-neighbor oxygen atom sites in the Nd layer. Total oxygen stoichiometry values were in the range 5.51 ≤ δ ≤ 5.11. The tetragonal lattice parameters increased with temperature as expected. However, the a-axis expands while the c-axis contracts with decreasing pO2 at a given temperature due to increasing vacancy concentration in the Nd layer.

Homochiral frameworks formed by reactions of lanthanide ions with a chiral antimony tartrate secondary building unit.

A chiral cluster compound, dipotassium bis(μ-tartrato)diantimony(III), K(2)Sb(2)L(2) (H(4)L = L-tartaric acid), was used as a secondary building unit to react with lanthanide ions. Three series of homochiral coordination compounds were obtained: 0D [La(H(2)L)(H(2)O)(4)](2)[Sb(2)L(2)]·7H(2)O (0D-La), 1D Ln(Sb(2)L(2))(H(2)O)(5)(NO(3))·H(2)O (1D-Ln) (Ln = La-Lu or Y, expect Pm), 2D(I) [(Ln(H(2)O)(5))(2)(Sb(2)L(2))(3)]·5H(2)O (2D(I)-Ln) (Ln = La, Ce, Pr), and 2D(II) [(La(H(2)O)(5))(2)(Sb(2)L(2))(3)]·6H(2)O (2D(II)-La). Single-crystal X-ray diffraction studies indicated that 0D-La crystallizes in space group P1, and the structure contains isolated Sb(2)L(2)(2-) units located between chains of composition La(H(2)L)(H(2)O)(4). The series of 1D-Ln compounds is isostructural and crystallizes in space group P2(1)2(1)2(1). In the structure, Sb(2)L(2)(2-) units are coordinated to two Ln ions by two out of the four free tartrate oxygen atoms to form a linear chain. To the best of our knowledge, this is the first example of a homochiral structure that can be formed for the whole lanthanide series. In the 2D(I)-Ln structure series, which crystallizes in space group P2(1), the Sb(2)L(2)(2-) units have two distinct coordination modes: one is the same as that found in the 1D structure, while in the other all four free tartrate oxygen atoms are coordinated to four Ln ions in a very distorted tetrahedral arrangement. The connectivity between Sb(2)L(2)(2-) secondary units and LnO(9) polyhedra gives rise to infinite layers. 2D(II) [(La(H(2)O)(5))(2)(Sb(2)L(2))(3)]·6H(2)O, which crystallizes in space group C2, has a similar network to the 2D(I)-Ln compounds. The trends in lattice parameters, bond lengths, and ionic radii in the 1D-Ln series were analyzed to show the effect of the lanthanide contraction.

Influence of polyhedron distortions on calculated bond-valence sums for cations with one lone electron pair

In the present bond-valence model (BVM), the bond-valence parameters r(0) and b are, in general, supposed to be constant for each A-X pair and equal to 0.37 A for all A-X pairs, respectively. For [A(i)(X(j))(n)] coordination polyhedra that do not deviate strongly from regularity, these suppositions are well fulfilled and calculated values for the bond-valence sums (BVS)(i) are nearly equal to the whole-number values of the stoichiometric valence. However, application of the BVM to 2591 [L(i)(X(j))(n)] polyhedra, where L are p-block cations, i.e. cations of the 13th to 17th group of the periodic system of elements, with one lone electron pair and X = O(-II), S(-II) and Se(-II), shows that r(0i) values of individual [LX(n)] polyhedra are correlated with the absolute value /Phi(i)/ of an eccentricity parameter, Phi(i), which is higher for more distorted [LX(n)] polyhedra. As a consequence, calculated (BVS)(i) values for these polyhedra are also correlated with /Phi(i)/, rather than being numerically equal to the stoichiometric valence of L. This is interpreted as the stereochemical influence of the lone electron pair of L. It is shown that the values of the correlation parameters and the R(2) values of the correlation equations depend on the position of the L cation in the periodic system of elements, if the correlations are assumed to be linear. This observation suggests that (BVS)(L) describes a chemical quantity that is different from the stoichiometric valence of L.

Crystal structure of the microporous titanosilicate ETS-10 refined from single crystal X-ray diffraction data

ETS-10 crystals up to 45 µm in size have been synthesized at 240 °C under a pressure of 80 MPa and the structure refined from single crystal X-ray diffraction data.