102 research outputs found
Structural links beetween zeolite-type and clathrate hydrate-type materials synthesis and crystal structure of [NMe4]16[Si8O20][OH]8.116H2O
A novel crystalline tetramethylammonium silicate hydroxide polyhydrate has been obtained by fractional crystallization at room temperature from a highly alkaline aqueous NMe4OH-SiO2 solution. X-Ray single-crystal studies at 200 K revealed the trigonal crystal system, the unit-cell constants (hexagonal setting) a = 18.126(2) and c = 57.529(9) A°, the space group R3 and Z = 3 formula units of [NMe4]16[Si8O20][OH] 8·116H2O per unit cell. The crystal structure has been determined from 1416 unique Mo-Ka¯ data and refined to R = 0.104. The host-guest compound is of polyhedral clathrate type with a mixed three-dimensional, (mainly) four-connected tetrahedral network composed of oligomeric silicate anions [Si8O20]8-, OH- ions and H2O molecules linked via hydrogen bonds O-H?O. The silicate anions are each built up of eight SiO4 tetrahedra sharing corners to form a cube (double four-ring structure). Cationic guest species NMe4+ are enclosed in large polyhedral cavities [4668], [425865], [51264] and [425867] of the host structure; small [46] cages (i.e. the double-ring anions) and [4356] cages are unoccupied. Each silicate anion is the centre of a specific and probably very stable finite cluster [(NMe4)6(Si8O20)·24H 2O]2-. It is suggested that the polyhydrate may be taken as a crystalline model system for studies on the structures of aqueous tetramethylammonium silicate solutions
Metal-organic framework nanofibers via electrospinning
A hierarchical system of highly porous nanofibers has been prepared by electrospinning MOF (metal-organic framework) nanoparticles with suitable carrier polymers. Nitrogen adsorption proved the MOF nanoparticles to be fully accessible inside the polymeric fibers. © 2011 The Royal Society of Chemistry
In situ static and dynamic light scattering and scanning electron microscopy study on the crystallization of the dense zinc imidazolate framework ZIF-zni
The kinetics and mechanism of crystallization of the dense zinc imidazolate framework with zni topology, from comparatively dilute methanol solutions containing Zn(NO3)·6H2O and imidazole with variation of the zinc-to-imidazole ratio, were followed in situ by time-resolved static and dynamic light scattering. The light scattering data revealed that metastable primary particles of about 100 nm in diameter form rapidly upon mixing the component solutions. After a lag time that is dependent on the imidazole concentration, the primary particles aggregate into secondary particles by a monomer addition mechanism with the primary particles as the monomers. Complementary scanning electron microscopy revealed that further evolution of the secondary particles is a complex process involving polycrystalline intermediates, the non-spherical morphologies of which depend on the initial zinc-to-imidazole ratio. Time and location of the first appearance of crystalline order could so far not be established. The pure-phase ZIF-zni crystals obtained after 240 min are twins. The aspect ratio of the tetragonal crystals can be controlled via the zinc-to-imidazole ratio. © 2011 The Owner Societies
Benzamidinium tetraÂhydroÂpentaÂborate sesquihydrate
The asymmetric unit of the title compound [systematic name: benzamidinium 3,3′,5,5′-tetraÂhydrÂoxy-1,1′-spirobi[2,4,6-trioxa-1,3,5-triboracycloÂhexaÂne](1−) sesquihydrate], C7H9N2
+·B5H4O10
−·1.5H2O, is composed of two protonated benzamidinium cations, two tetraÂhydroÂpentaÂborate anions and three water molÂecules of crystallization. The ions and water molecules are associated in the crystal structure by an extensive three-dimensional hydrogen-bonding network, which consists mainly of cation-to-anion N—H⋯O and anion-to-anion O—H⋯O hydrogen bonds
A zeolitic imidazolate framework with conformational variety: Conformational polymorphs versus frameworks with static conformational disorder
We show via structural considerations and DFT calculations that for a zeolitic imidazolate framework (ZIF) with sodalite (SOD) topology, [Zn(dcim)2]-SOD (dcim = 4,5-dichloroimidazolate), structural models of an infinite number of hypothetical conformational polymorphs with distinct linker orientations can be generated, which can be interconverted most likely only via reconstructive structural transitions. The relative total energies suggest that some of those polymorphs might be synthetically accessible. Efforts in that direction led to the synthesis of new trigonal 1 and previously known cubic 2 with improved crystallinity. According to structural analyses based on powder X-ray diffraction (PXRD) methods supported by NMR spectroscopy, 1 is the most stable of the theoretically predicted SOD-type framework conformers (isostructural to ZIF-7), whereas 2, at variance to a recent proposal, is a SOD-type material with a high degree of orientational disorder of the dcim linker units. The statistics of the linker orientations in 2 is close to that in 1, indicating that the disorder in 2 is not random. Rather crystals of 2 are likely twins consisting of nanoscopic domains of trigonal 1 that are deformed to a cubic metric, with linker disorder being located in the domain interfaces. As structural differences appear to be more related to characteristics of the real as opposed to the ideal crystal structures, we propose to not consider 1 and 2 as true conformational polymorphs. Systematic investigations of solvent mixtures led to the discovery of intermediate materials of 1 and 2. The PXRD patterns and SEM images indicate that they belong to a complete series of structural intermediates. Differences in the Ar adsorption/desorption behaviours reveal that 1, in contrast to 2, is a flexible ZIF framework
A zeolitic imidazolate framework with conformational variety
We show via structural considerations and DFT calculations that for a zeolitic imidazolate framework (ZIF) with sodalite (SOD) topology, [Zn(dcim)2]-SOD (dcim = 4,5-dichloroimidazolate), structural models of an infinite number of hypothetical conformational polymorphs with distinct linker orientations can be generated, which can be interconverted most likely only via reconstructive structural transitions. The relative total energies suggest that some of those polymorphs might be synthetically accessible. Efforts in that direction led to the synthesis of new trigonal 1 and previously known cubic 2 with improved crystallinity. According to structural analyses based on powder X-ray diffraction (PXRD) methods supported by NMR spectroscopy, 1 is the most stable of the theoretically predicted SOD-type framework conformers (isostructural to ZIF-7), whereas 2, at variance with a recent proposal, is a SOD-type material with a high degree of orientational disorder of the dcim linker units. The statistics of the linker orientations in 2 is close to that in 1, indicating that the disorder in 2 is not random. Rather, crystals of 2 are likely twins consisting of nanoscopic domains of trigonal 1 that are deformed to a cubic metric, with linker disorder located in the domain interfaces. As structural differences appear to be more related to characteristics of real as opposed to ideal crystal structures, we propose to not consider 1 and 2 as true conformational polymorphs. Systematic investigations of solvent mixtures led to the discovery of intermediate materials of 1 and 2. The PXRD patterns and SEM images indicate that they belong to a complete series of structural intermediates. Differences in the Ar adsorption/desorption behaviours reveal that 1, in contrast to 2, is a flexible ZIF framework.DFG/Priority Program/141
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A new polyborate anion, [B 7 O 9 (OH) 6 ] 3− : Self assembly, XRD and thermal properties of s-fac- [Co(dien) 2 ][B 7 O 9 (OH) 6 ]·9H 2 O
ChemInform Abstract: Orientational Disorder of the Hydrogen Dihydroxide Anion, O2H- 3, in Sodium Hydroxosodalite Dihydrate, Na8(Al6Si6O24)(OH)2×2 H2O: Single-Crystal X-Ray and Powder Neutron Diffraction and MAS NMR and FT IR Spectroscopy.
ChemInform Abstract: Sodium Aluminogermanate Hydroxosodalite Hydrate Na6+x(Al6Ge6O24)(OH)x. times.nH2O (x ≈ 1.6, n ≈ 3.0): Synthesis, Phase Transitions and Dynamical Disorder of the Hydrogen Dihydroxide Anion, H3O- 2, in the Cubic High-Temperature Form.
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