Synthesis of cyclic carbonates from epoxides and carbon dioxide catalyzed by talc and other phyllosilicates.

Naturally occurring phyllosilicate minerals such as talc and vermiculite in conjunction with n-tetra butyl ammonium bromide (TBAB) co-catalyst were found to be efficient in the coupling of CO2 with epoxides to form cyclic carbonates. The reaction was carried out in a pressurized autoclave reactor at moderate pressures of 10-35 bars and temperatures of 100-150 °C. The optimized catalyst system exhibited > 90% conversion of the epoxides and > 90% selectivity for the desired cyclic carbonates, in the presence or absence of a solvent. The selectivity of the catalytic system could be improved with heat pre-treatment of the phyllosilicates albeit this resulted in slightly lower epoxide conversion. The results obtained using the heat treated phyllosilicates strongly support the hydrogen bond assisted mechanism for the cycloaddition of epoxides and CO2. The cycloaddition reaction could also be carried out in the absence of TBAB, although lower cyclic carbonate yields were observed. The phyllosilicate part of the catalyst system is heterogeneous, easy to separate after completion of reactions and reusable a number of runs without loss of activity.

ß-Li2Zn5: A Low Symmetric Polar Intermetallic Compound.

The crystal structure of the high-temperature modification of the compound Li2Zn5 was determined using single-crystal X-ray diffraction data. It is the first representative of a new binary structure type with triclinic space group P1̅, where the parameters of the unit cell are a = 8.0073(3)Å, b = 11.5956(6)Å, c = 15.2956(5)Å, α = 96.39°, ß = 101.92°, and γ = 108.13°, the formula sum is Li11.748Zn31.113, Z = 2, and CCDC deposit number is 1861780. The title compound has a pseudohexagonal motif, made of 7- and 17-atom Li-zigzag chains and continuous Zn-chains, with the the relative placement of these large aggregates violating hexagonal symmetry. The structure may be decomposed into fragments, related to the AlB2 structure type, and could be obtained from multiplication of the unit cell, multiple substitution of Li atoms by triangles of Zn, insertion of Zn atoms, and deformation. Most structures of Li-Zn compounds, except LiZn, are highly symmetric but disordered. The possible causes of lower symmetry of ß-Li2Zn5 were analyzed using the results of DFT calculations. ß-Li2Zn5 shows high polarity of bonds, and Li atoms donate part of the electron density to Zn atoms.

Penta-lanthanum zinc diplumbide, La5Zn1-x Pb2+x (x ≃ 0.6).

The title non-stoichiometric penta-lanthanum zinc diplumbide, La5Zn1-x Pb2+x (x ≃ 0.6), was prepared from the elements in an evacuated silica ampoule. It adopts the Nb5Sn2Si-type structure (space group I4/mcm, Pearson symbol tI32), a ternary ordered superstructure of the W5Si3 type. Among the four independent crystallographic positions, three are fully occupied by La (Wyckoff 16k), La (4b), and Pb (8h) and one is occupied by a statistical mixture [occupancy ratio 0.394 (12):0.606 (12)] of Zn and Pb (4a). The structure is constructed by face-sharing 10-vertex polyhedra around the unmixed Pb sites. These fragments enclose channels of trans-face-sharing tetra-gonal anti-prisms occupied by the disordered Zn and Pb sites.

Tb3Sn7: polymorphism and crystal structure of high-temperature modification.

During the systematic investigation of the tin-rich side of the Tb-Sn binary system temperatures of non-variant transformations were established. The crystal structure of ßTb3Sn7 was determined for the first time from single-crystal diffraction data. Parameters of the unit cell have been determined previously by Manfrinetti & Palenzona [(1993), J. Alloys Compd. 201, 43-47], but the structure was not solved until now. The compound belongs to a new structure type with space group Pmmm. The unit cell contains two split positions: Sn7 (Wyckoff 1a), Sn7A (Wyckoff 2q) and Sn8 (Wyckoff 1d), Sn8A (Wyckoff 2s). Structures of both α- and ß-modifications of Tb3Sn7 can be described as close packing of the coordination polyhedra of Tb atoms. In turn these polyhedra can be constructed of fragments of simple AuCu3 and CsCl types.

Lanthanum tetrazinc, LaZn4.

The structure of lanthanum tetrazinc, LaZn(4), has been determined from single-crystal X-ray diffraction data for the first time, approximately 70 years after its discovery. The compound exhibits a new structure type in the space group Cmcm, with one La atom and two Zn atoms occupying sites with m2m symmetry, and one Zn atom occupying a site with 2.. symmetry. The structure is closely related to the BaAl(4), La(3)Al(11), BaNi(2)Si(2) and CaCu(5) structure types, which can be presented as close-packed arrangements of 18-vertex clusters, in this case LaZn(18). The kindred structure types contain related 18-vertex clusters around atoms of the rare earth or alkaline earth metal.

Terbium (lithium zinc) distannide, TbLi(1-x)Zn(x)Sn(2) (x = 0.2).

The new terbium (lithium zinc) distannide, TbLi(1-x)Zn(x)Sn(2) (x = 0.2) crystallizes in the ortho-rhom-bic CeNiSi(2) structure type with space group Cmcm and Pearson symbol oS16. Of the four independent 4c atom positions (m2m site symmetry), three are fully occupied by individual atoms (two by Sn and one by Tb atoms) and the fourth is occupied by Li and Zn atoms with a statistical distribution. The Tb coordination polyhedron is a 21-vertex pseudo-Frank-Kasper polyhedron. One Sn atom is enclosed in a tricapped trigonal prism, the second Sn atom is in a cubocta-hedron and the statistically distributed (Li,Zn) site is in a tetra-gonal anti-prism with one added atom. Electronic structure calculations were used for the elucidation of reasons for and the ability of mutual substitution of lithium and transition metals. Positive charge density was observed around the rare earth atom and the Li and Zn atoms, the negative charge density in the proximity of the Sn atoms.

Redetermination of LaZn(5) based on single crystal X-ray diffraction data.

The crystal structure of the already known binary title compound LaZn(5) (lanthanum penta-zinc) (space group P6/mmm, Pearson symbol hP6, CaCu(5) structure type) has been redetermined from single-crystal X-ray diffraction data. In contrast to previous determinations based on X-ray powder data [Nowotny (1942). Z. Metallkd.34, 247-253; de Negri et al. (2008). Inter-metallics, 16, 168-178], where unit-cell parameters and assignment of the structure type were reported, the present study reveals anisotropic displacement parameters for all atoms. The crystal structure consists of three crytallographically distinct atoms. The La atom (Wyckoff site 1a, site symmetry 6/mmm) is surrounded by 18 Zn atoms and two La atoms. The coordination polyhedron around one of the Zn atoms (Wyckoff site 2c, site symmetry -6m2) is an icosa-hedron made up from three La and nine Zn atoms. The other Zn atom (Wyckoff site 3g, site symmetry mmm) is surrounded by four La and eight Zn atoms. Bonding between atoms is explored by means of the TB-LMTO-ASA (tight-binding linear muffin-tin orbital atomic spheres approximation) program package. The positive charge density is localized around La atoms, and the negative charge density is around Zn atoms, with weak covalent bonding between the latter.

LaZn(12.37†(1)), a zinc-deficient variant of the NaZn(13) structure type.

The title compound (lanthanum dodecazinc), LaZn(12.37 (1)), is confirmed to be a nonstoichiometric (zinc-deficient) modification of the NaZn(13) structure type, in which one Zn atom (Wyckoff site 8b, site symmetry m[Formula: see text]) has a fractional site occupancy of 0.372 (11). The other Zn atom (96i, m) and the La atom (8a, 432) are fully occupied. The coordination polyhedra of the Zn atoms are distorted icosa-hedra, whereas the La atoms are surrounded by 24 Zn atoms, forming pseudo-Frank-Kasper polyhedra. Electronic structure calculations indicate that Zn-Zn bonding is much stronger than La-Zn bonding.

La(5)Zn(2)Sn.

A single crystal of penta-lanthanum dizinc stannide, La(5)Zn(2)Sn, was obtained from the elements in a resistance furnace. It belongs to the Mo(5)SiB(2) structure type, which is a ternary ordered variant of the Cr(5)B(3) structure type. The space is filled by bicapped tetra-gonal anti-prisms from lanthanum atoms around tin atoms sharing their vertices. Zinc atoms fill voids between these bicapped tetra-gonal anti-prisms. All four atoms in the asymmetric unit reside on special positions with the following site symmetries: La1 (..m); La2 (4/m..); Zn (m.2m); Sn (422).

TbNb(6)Sn(6): the first ternary compound from the rare earth-niobium-tin system.

The title compound, terbium hexa-niobium hexastannide, TbNb(6)Sn(6), is the first ternary compound from the rare earth-niobium-tin system. It has the HfFe(6)Ge(6) structure type, which can be analysed as an inter-growth of the Zr(4)Al(3) and CaCu(5) structures. All the atoms lie on special positions; their coordination geometries and site symmetries are: Tb (dodeca-hedron) 6/mmm; Nb (distorted icosa-hedron) 2mm; Sn (Frank-Caspar polyhedron, CN = 14-15) 6mm and m2; Sn (distorted icosa-hedron) m2. The structure contains a graphite-type Sn network, Kagome nets of Nb atoms, and Tb atoms alternating with Sn2 dumbbells in the channels.