Icosahedral B12, macropolyhedral boranes, β-rhombohedral boron and boron-rich solids

The preference for icosahedral B₁₂ amongst polyhedral boranes and elemental boron is explained based on an optimization of overlap model. The ingenious ways in which elemental boron and boron-rich solids achieve icosahedron-related structures are explained by a fragment approach. The Jemmis mno rules are used to get the electron requirements. The extra occupancies and vacancies in β-rhombohedral structures are shown to be inevitable results of electron requirements. The detailed understanding of the structure suggests ways of doping β-rhombohedral boron with metals for desired properties. Theoretical studies of model β-rhombohedral solids with metal dopings provide support for the analysis.     

Synthesis, crystal growth and structure of Mg containing β-rhombohedral boron: MgB17.4

For the first time, single crystals of Mg containing β-rhombohedral boron MgB_17.4 were synthesised from the elements in a Mg/Cu melt at 1600 °C. The crystal structure determined by the refinement of single crystal data (space group R-3m, , , 890 reflections, 123 variables, R₁(F)=0.049, wR₂(I)=0.122) improves and modifies the former structure model derived from earlier investigations on powder samples. Mg is located on four different positions with partial occupation. While the occupation of the sites D (53.3%), E (91%) and F (7.2%) is already known from other boron-rich borides related to β-rhombohedral boron, the occupation of the fourth position (18h, 6.7%) is observed for the first time. Two boron positions show partial occupation. The summation reveals the composition MgB_17.4 and Mg_5.85B_101.9, respectively, confirmed by WDX measurements. The single crystals of MgB_17.4 show the highest Mg content ever found. Preliminary measurements indicate no superconductivity.     

On the diffusion of free carriers in β-rhombohedral boron

To determine the diffusion of untrapped carriers in β-rhombohedral boron, we constructed a feedback pico-ammeter based on pulse integration technique. This enabled measuring deviations from the bias in a 10⁹ Ω sample in the order of 1 nA with 0.7 ms time resolution. For the first time, we obtained the drift velocity of optically generated untrapped electron-hole pairs 106(20) cm s⁻¹ yielding for the band-determined diffusion coefficient and for the carrier mobility . Fitting Fick's second law to the measured trap-determined dispersion of carriers yields the ambipolar diffusion coefficient D^*=0.043(14) and 0.28(10) cm² s⁻¹ at 260 and 340 K, respectively. The thermal activation energy of 0.18 eV agrees with the well-known trapping levels in β-rhombohedral boron.     

Physical-mechanical properties of Zr-(Re)-doped β-rhombohedral boron

Effect of doping with Zr(Re) on the structure and physical-mechanical properties of β-rhombohedral boron has been studied. In all specimens p-type conductivity was found. Internal friction and dynamic shear modulus of the specimens were investigated at frequencies of torsion oscillations (0.5-5 Hz) in the temperature range 80<T<1000 K. The increase of Zr(Re) concentration in the samples results in increase of their hole concentration, this increasing and shifting the observed IF maxima to lower temperatures; activation energy of the maxima and frequency factor of the relaxation processes decrease by 10-15%. Effects of change of the structure-sensitive properties observed in Zr-(Re)-doped boron are analyzed in view of changes of activation energy necessary for the motion of twinning boundaries and stacking faults.     

Synthesis of dipyrrolo[2,3-a:1′,2′,3′-fg]acridin-12(1H)-ones

Acylation reactions of 4,6-dimethoxyindoles with glyoxyloyl chlorides were achieved by the use of graphite powder in 1,2-dichloroethane at reflux. The products were monoketones as a result of decarbonylation, rather than the expected 1,2-diketones. Treatment of these monoketones with base led to their cyclisation and elimination of methanol to afford the novel dipyrrolo[2.3-a:1′,2′,3′-fg]acridin-12(1H)-ones.     

High-energy-resolution electron energy-loss spectroscopy study of the electronic structure of Cu- and Mg-Si-doped β-rhombohedral boron crystals

Electronic structures of Cu and Mg-Si-doped β-rhombohedral boron (β-r-B) crystals were studied by using a high-energy-resolution electron energy-loss spectroscopy microscope. Boron 1s electron excitation spectra, which show the density of states of the conduction bands, of the crystals were obtained from single crystalline areas of 100 nm in diameter. The spectrum of Cu-doped β-r-B showed a chemical shift to a lower binding energy side. It means an electron transfers from the doped Cu atoms to B atoms. The intensity distributions of the spectrum was almost the same as that of the non-doped β-r-B, which suggests that all of the doped electrons occupy the intrinsic acceptor level just above the valence bands. The spectrum of Mg-Si-doped β-r-B showed not only a chemical shift to a lower binding energy side but also a sharp intensity increase at the onset with a width of an energy resolution of the experiment. The sharp onset may be assigned to a Fermi edge. It indicates that the doped electrons fill up the acceptor level and occupy the conduction bands forming the Fermi edge, a metallization of β-r-B by the Mg-Si-doping.     

Three-dimensional grids based on graphene oxide and 3,3’,4,4’-tetraaminodiphenyl oxide for supercapacitor electrodes

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Phase transition induced by lithium insertion in αI- and αII-VOPO4

Lithium insertion in α_I-VOPO₄ and α_II-VOPO₄, either by chemical or electrochemical route, leads to the same new compound: α_I-LiVOPO₄ (space group P4/nmm). The structure, resolved by neutron and synchrotron diffraction, is made up of planes of corner-connected PO₄ and VO₅ polyhedra, whereas lithium atoms are located between the layers. The reversal of the short vanadyl bond that corresponds to the insertion-induced α_II-α_I transition finds an explanation in terms of lattice energy. It favors the migration of lithium ions in the (0 0 1) interlayer planes, a key parameter for the electrochemical performance as electrode material in Li-ion batteries.     

Synthesis of a phosphotyrosyl analogue having χ1, χ2 and φ angles constrained to values observed for an SH2 domain-bound phosphotyrosyl residue

Src homology 2 (SH2) domains provide connectivity in protein-tyrosine kinase (PTK)-dependent signaling through their high affinity association with phosphotyrosyl (pTyr)-containing peptide sequences. Because recognition of pTyr residues is central to SH2 domain-binding affinity, design of pTyr-mimicking residues has been one component of SH2 domain signaling antagonist development. Reported herein is the synthesis of (±)-(rel-1R,2R,5S)-3-acetyl-1,2,3,4,5,6-hexahydro-8-O-phosphoryl-1,5-methano-3-benzazocine-2-carboxylic acid methyl ester (3c) as a monomeric pTyr-mimicking analogue that constrains three torsion angles (χ₁=168°; χ₂=−85°; φ₁=−113°) to values approximating those observed for a pTyr residue bound to the Grb2 SH2 domain (χ₁=182°; χ₂=−89°; φ₁=−132°). Compound 3c differs from our previously reported analogue, (±)-(rel-1R,2R,5S)-3-acetyl-1,2,3,4,5,6-hexahydro-1-methyl-1,5-methano-3-benzazocin-8-ol, in lacking a methyl substituent at the bridgehead 1-position. Molecular modeling studies had indicated that this methyl group could potentially hinder SH2 domain binding. Synthesis of the desmethyl derivative was achieved by formation of the methanobenzazocine ring system using an intramolecular electrophilic cyclization that proceeds through an activated acyliminium intermediate. Importantly, the correct relative (2R) stereochemistry at the ‘α-carboxyl’-bearing carbon is obtained through base-catalyzed equilibration of a (2S/2R) diastereomeric mixture that results from intramolecular ring closure. Comparison of Grb2 SH2 domain-binding affinity of 3c (IC₅₀=1167 μM) with conformationally flexible phosphorylated (±)-N-acetyl-tyrosine methyl ester (15; IC₅₀=1469 μM) revealed no apparent enhancement in affinity. This apparent ineffectiveness of ‘local conformational constraint’ on SH2 domain-binding affinity of the monomeric pTyr mimetic is consistent with previous reports obtained by conformationally constraining pTyr-mimicking residues that were contained within peptide platforms. Although not providing high binding affinity in its current form, the novel 1,5-methano-3-benzazocine ring system may afford a novel platform for further elaboration and development of small molecule SH2 domain signaling antagonists.     

Enantioselective α-chlorination of β-keto esters and amides catalyzed by chiral imidodiphosphoric acids

Chiral imidodiphosphoric acids were employed as catalysts for the enantioselective α-chlorination of β-keto esters and amides using NCS as the chlorine source, providing a series of optically active products with good to high enantioselectivities (74–95% ee) and excellent yields (92–99%). This represents the first report of the Brønsted acid catalyzed enantioselective α-chlorination of cyclic β-keto derivatives.     

An access to α, β-unsaturated ketones via dual cooperative catalysis

A dual cooperative organocatalytic approach for the synthesis of α, β-unsaturated ketones is described. This one pot transformation is realized via a domino Knoevenagel-Michael-retro Michael reaction sequence. Various aliphatic ketones reacted smoothly with aromatic as well as aliphatic aldehydes in presence catalytic amount of Meldrum’s acid and bifunctional amine. The highlights of this protocol are the easy availability of catalysts, high selectivity, and functional group tolerance. The reaction proved to highly E-selective with no side products emanating from self-condensation, unlike the base-mediated reactions.     

Raman characterization of α- and β-LiFe5O8 prepared through a solid-state reaction pathway

Lithium ferrite, a mixed-inverse spinel of type A_xB_y[A_1−xB_1−y]O₄ was produced through solid state synthesis by calcining a Li₂CO₃/Fe₂O₃ mixture at 900 °C. The presence of both the ordered α-phase and disordered β-phase of LiFe₅O₈ was confirmed by XRD analysis, while formation of the latter was achieved by air quenching from high temperature. Laser Raman analysis was performed on both the α-LiFe₅O₈ and β-LiFe₅O₈ powders in order to achieve a reference set of Raman shifts for the spinel. The strongest, characteristic Raman peaks were determined to be 493, 382, 358, 300, and 263 cm⁻¹ for both phases while smaller peaks at 202 and 236 cm⁻¹ present in the α-phase were diminishing in intensity when the β-phase was present, thus providing unique identifiers for the presence of the disordered ferrite structure. SEM images taken of the synthesized LiFe₅O₈ powders showed particle sizes of less than 300 nm and an even particle size distribution.     

SiCl4-catalyzed/PR3-mediated β-C(sp3)−H functionalization of nitrones to α,β-unsaturated imines and aromatic heterocycles

A novel method of SiCl₄-catalyzed/PR₃-mediated β-C(sp³)?H functionalization of nitrones with aldehydes/ketones to α,β-unsaturated imines was developed. The synthesis of α,β-unsaturated imines mainly invovles deoxygenation and aldol condensation, each proceeding under a cooperation effect between Lewis acid and Lewis base. In addition, both the acidity and hydrolytic stability of the weak SiCl₄ were supposed to be enhanced by coordination with phosphine oxide (R?=?Et) or phosphoric triamide (R?=?NMe₂) that originated from deoxygenation of nitrones by PR₃. In the case of 6-membered nitrone, a [1,3]-hydride shift within the resulted α,β-unsaturated imines renders the aromatization leading to 3,5-dialkylpyridines.     

Local crystal chemistry, induced strain and short range order in the cubic pyrochlore (Bi1.5−αZn0.5−β)(Zn0.5−γNb1.5−δ)O(7−1.5α−β−γ−2.5δ) (BZN)

Clear evidence (in the form of structured diffuse scattering) is found for short range ordering of metal ions and associated induced structural relaxation in two members of the cubic BZN pyrochlore (Bi_1.5−αZn_0.5−β)(Zn_0.5−γNb_1.5−δ)O_{(7−1.5α−β−γ−2.5δ)} solid solution. An average neutron powder diffraction structure refinement is carried out for one of these. Electron probe micro-analysis suggests that the primary mechanism for non-stoichiometry in cubic BZN is the removal of ZnO from the nominally fully occupied (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O₇ end-member. A detailed bond valence sum analysis of a recently reported average crystal structure is used to suggest possible local cation ordering schemes along with the induced displacive relaxation that is likely to accompany such local cation ordering. The observed diffuse distribution is qualitatively interpreted via Monte Carlo modelling.     

Pressure-induced transformations in α- and β-Ge3N4: in situ studies by synchrotron X-ray diffraction

Metastable high-pressure transformations in germanium nitride (α- and β-Ge₃N₄ polymorphs) have been studied by energy- and angle-dispersive synchrotron X-ray diffraction at high pressures in a diamond anvil cell. Between P=22 and 25 GPa, the phenacite-structured β-Ge₃N₄ phase (P6₃/m) undergoes a 7% reduction in unit-cell volume. The densification is primarily concerned with the a-axis parameter, in a plane normal to the hexagonal c-axis. Based on results of previous LDA calculations and Raman spectroscopic studies, we propose that the structural collapse is due to transformation into a new metastable polymorph (δ-Ge₃N₄) that has a unit-cell symmetry based upon P3, that is related to the low-pressure β-Ge₃N₄ phase by concerted displacements of N atoms away from special symmetry sites in the plane normal to the c-axis. No such transformation occurs for α-Ge₃N₄, due to the different stacking of linked GeN₄ layers. All three polymorphs (α-, β- and δ-Ge₃N₄) are based on tetrahedrally coordinated Ge atoms, unlike the spinel-structured γ-Ge₃N₄ phase, that contains octahedrally coordinated Ge⁴⁺. Experimentally determined bulk modulus values for α-Ge₃N₄ (K₀=165(10) GPa, K₀′=3.7(4)) and β-Ge₃N₄ (K₀=185(7) GPa, K₀′=4.4(5)) are in excellent agreement with theoretical predictions. The bulk modulus for the new δ-Ge₃N₄ polymorph is only determined above the β-δ transition pressure (P=24 GPa); K=161(20) GPa, assuming K′=4. Above 45 GPa, both α- and δ-Ge₃N₄ polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering.     

Polymorphism in PbBiOXO4 compounds (X=V, P, As). Part I: Crystal structures of α- and β-PbBiOVO4

Reinvestigation of PbBiOVO₄ thermal behaviour revealed a phase transition. As shown by single-crystal X-ray diffraction and high-resolution neutron powder diffraction, α-PbBiOVO₄ transforms to β-PbBiOVO₄ at 550 °C. At 25 °C, α-PbBiOVO₄ is triclinic, S.G. P-1, Z=2, with a=5.6088(3), b=7.1109(3), c=7.2978(3) Å, α=108.957(2), β=111.889(2), and γ=94.833(2)°. Above 550 °C, β-PbBiOVO₄ is monoclinic, S.G. C2/m, Z=4, with a=13.61(1), b=5.64(1), c=7.18(1) Å, and β=113.75(1)°. Both structures are built upon (O₂Bi₂Pb₂)_∞ chains parallel to the [100] direction in the α polymorph and [001] in the β-polymorph. These chains are undulated in α and linear in β. In both structures, VO₄ tetrahedra are organized in two sets of rows parallel to (O₂Bi₂Pb₂)_∞ chains, thus building layers of (OBiPb) sandwiched by two layers of VO₄ oriented head to tail; VO₄ displays different orientations in α- and β-PbBiOVO₄.     

Preparation and Double-Helix Icosahedra Structure of δ-Co2Zn15

The first known example of a regular face-sharing icosahedra helix is presented in the novel crystal structure of δ-Co₂Zn₁₅, solved and refined from single-crystal X-ray and neutron powder diffraction data. The compound δ-Co₂Zn₁₅ is also the first example of an intermetallic compound crystallizing in the acentric hexagonal space group P6₂. The parameters from the single crystal refinement are a=11.292(2) Å, c=7.750(1) Å, Z=4, and R_w=0.025.     

Crystal Structure, Electric and Magnetic Properties of Layered Cobaltite β-NaxCoO2

The layered oxide thermoelectric material β-Na_0.67CoO₂ has been studied by powder neutron diffraction, electric and magnetic measurements. This compound includes an edge-sharing CoO₆ slab and a highly vacant Na⁺ sheet in a unit cell (space group symmetry C2/m, a=4.9023(4) Å, b=2.8280(2) Å, c=5.7198(6) Å and β=105.964(6)° at 300 K). The evaluated formal valence of cobalt ion, +3.33(1), is ascribed to the coexistence of Co³⁺ and Co⁴⁺ in the ratio 2:1. Polycrystalline β-Na_0.67CoO₂, a p-type thermoelectric material, exhibits metallic behavior of the electric resistivity below 300 K. The Curie-Weiss-type magnetic susceptibility indicates antiferromagnetic interactions between magnetic cobalt ions in the edge-sharing CoO₆ slab.     

Structure, bonding, and magnetic response in two complex borides: Zr2Fe1−δRu5+δB2 and Zr2Fe1−δ(Ru1−xRhx)5+δB2

Polycrystalline samples of two complex intermetallic borides Zr₂Fe_1−δRu_5+δB₂ and Zr₂Fe_1−δ(Ru_1−xRh_x)_5+δB₂ (δ=ca. 0.10; x=0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc₂Fe(Ru_1−xRh_x)₅B₂ and crystallize in the space group P4/mbm (no. 127) with the Ti₃Co₅B₂-type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c-axis with an interatomic metal-metal distance of 3.078(1) Å, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr₂FeRu₅B₂” using LDA yielded a narrow peak at the Fermi level assigned to Fe-Fe antibonding interactions along the c-axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains.