Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

Lanthanide hexaborides (LnB₆) have disparate and often anomalous properties, from structurally homogeneous mixed valency, to superconductivity, spectral anomalies, and unexplained phase transitions. It is unclear how such a diversity of properties may arise in the solids of identical crystal structures and seemingly very similar electronic structures. Building on our previous model for SmB₆ (mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB₆ that explains simultaneously EuB₆ (possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB₆ (mixed valent topological insulator), and rather ordinary LaB₆. We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB₆ family, also easily access alternative bonding states through the electron–phonon coupling. The accessibility, relative energetics, and bonding nature of the states involved dictate the properties.     

Direct Chemical Fine‐Tuning of Electronic Properties in Sc2Ir6−xPdxB

Crystal orbital Hamilton population (COHP) bonding analysis has predicted that ScPd₃B_0.5 is the least stable compound of the entire series Sc₂Ir_6?xPd_xB. Here, we report a systematic study of Sc₂Ir_6?xPd_xB (x=3, 5 and 6) by means of ¹¹B nuclear magnetic resonance (NMR), Knight shift (K) and nuclear spin‐lattice relaxation rate (1/T₁). NMR results combined with theoretical band structure calculations provide a measure of s‐ and non‐s‐character Fermi‐level density of states. We present direct evidence that the enhanced s‐state character of the Fermi level density of states (DOS) in ScPd₃B_0.5 reduces the strength of the B 2p and Pd 4d hybridized states across the entire Sc₂Ir_6?xPd_xB series. This hybridization strength relates to the opening of a deep pseudogap in the density of states of Sc₂IrPd₅B and the chemical bonding instability of ScPd₃B_0.5. This study is an experimental realization of a chemical fine‐tuning of the electronic properties in intermetallic perovskites.     

Rare Earth Ruthenium Gallides with the Ideal Composition Ln2Ru3Ga5 (Ln = La–Nd,Sm) crystallizing with U2Mn3Si5 (Sc2Fe3Si5) Type Structure

The rare earth ruthenium gallides Ln₂Ru₃Ga₅ (Ln = La, Ce, Pr, Nd, Sm) were prepared by arc‐melting of cold‐pressed pellets of the elemental components. They crystallize with a tetragonal structure (P4/mnc, Z = 4) first reported for U₂Mn₃Si₅. The crystal structures of the cerium and samarium compounds were refined from single‐crystal X‐ray data, resulting in significant deviations from the ideal compositions: Ce₂Ru_2.31(1)Ga_5.69(1), a = 1135.10(8) pm, c = 580.58(6) pm, R_F = 0.022 for 742 structure factors; Sm₂Ru_2.73(2)Ga_5.27(2), a = 1132.95(9) pm, c = 562.71(6) pm, R_F = 0.026 for 566 structure factors and 32 variable parameters each. The deviations from the ideal compositions 2:3:5 are discussed. A mixed Ru/Ga occupancy occurs only for one atomic site. The displacement parameters are relatively large for atoms with mixed occupancy within their coordination shell and small for atoms with no neighboring sites of mixed occupancy. Chemical bonding is analyzed on the basis of interatomic distances. Ln–Ga bonding is stronger than Ln–Ru bonding. Ru–Ga bonding is strong and Ru–Ru bonding is weak. The Ga–Ga interactions are of similar strength as in elemental gallium.     

Volumetric,dielectric, calorimetric and X-ray studies of smectogenic 10PBO8 at atmospheric and elevated pressures

The synthesis and pressure–volume–temperature (PVT), differential thermal analysis (DTA), dielectric and X-ray diffraction data of 2-(4-octylcarbonyloxyphenyl)-5-decylpyrimidine (10PBO8) are presented. The substance exhibits two crystalline and smectic C (SmC) phases on heating and a SmC–monotropic crystalline smectic B (SmB_cr) SmB_cr–crystal sequence of phase transitions on cooling. Above ca. 15 MPa, the SmB_cr phase becomes enantiotropic (reversible polymorphism). The phase behaviour and molecular dynamics in the liquid crystalline phases are analysed and discussed, with the conformational component of the total entropy for the SmC–isotropic liquid transition estimated. We also calculate from the PVT results the potential parameter characterising the steepness of the interaction potential.     

Identification of the Structural Boundary between [Ln(18‐crown‐6)(NO3)3] and [Ln(NO3)3(H2O)3] · 18‐crown‐6 Motifs in the Lanthanide Series

Recrystallization of Ln(NO₃)₃ (Ln = Sm, Eu, Yb) in the presence of 18‐crown‐6 under aqueous conditions yielded [Ln(NO₃)₃(H₂O)₃] · 18‐crown‐6. X‐ray crystallography revealed isomorphous structures for each of the lanthanide complexes where [Ln(NO₃)₃(H₂O)₃] is involved in hydrogen bonding interactions with 18‐crown‐6. The transition point where the structural motif changes from [Ln(18‐crown‐6)(NO₃)₃] (with the metal residing in the crown cavity) to [Ln(NO₃)₃(H₂O)₃] · 18‐crown‐6 has been identified as at the Nd/Sm interface. A similar investigation involving [Ln(tos)₃(H₂O)₆] (tos = p‐toluenesulfonate) and 18‐crown‐6 were resistant to crown incorporation. X‐ray studies show extensive intra‐ and intermolecular hydrogen bonding is present.     

Structural and energetic exploration of a boron‐rich sulfide cluster B6S

Boron and mixed‐boron clusters have received considerable attention because of their wide applications and their essential roles in advancing chemical bonding models. Bearing the bright prospects as building blocks to form novel polymeric materials, the sulfur‐rich boron sulfides have been greatly studied. However, the knowledge of the boron‐rich boron sulfides is much rare. In this article, we report an extensive theoretical study on the structural, energetic, and stability features of a hitherto unknown septa‐atomic cluster B₆S at the CCSD(T)/6‐311+G(2df)//B3LYP/6‐311+G(d) level. The local minimum isomers were obtained through our recently developed program “grid‐based comprehensive isomeric search algorithm.” The results show that the planar knife‐like isomer B₅(?BS) 01 (0.0 kcal/mol) containing the ?BS moiety is the lowest energy, followed by the quasi‐planar belt‐like isomer B₆(>S) 02 (6.7 kcal/mol) and the pyramid‐like isomer B₆(>S) 03 (8.4 kcal/mol). Notably, the three singlet isomers all have good kinetic stability on the basis of the potential energy surface analysis. The B/S‐centered wheel‐like isomers are unfavorable in thermodynamics and kinetics. The triplet state structures generally can not compete with the singlet ones. The results are compared to the analogous and isoelectronic cluster B₆O. Our work is expected to provide useful information for understanding the structures and stability of boron sulfides. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Confined Raman Scattering – Easy Access to the Surface Phonons of Specific Crystalline Solids

An easily feasible method to measure surface phonons of specific crystalline solids is described. Special experimental conditions allow confining Raman scattering to the surface region of crystals, while the contribution of bulk scattering to the spectra is largely excluded. Such conditions exist for example in the case of hexaborides, using FT Raman spectroscopy with a commercial spectrometer with ND:YAG laser excitation. We show results obtained on LaB₆, SmB₆ and boron carbide. The surface phonon spectra of LaB₆ are analyzed by comparing them with bulk spectra and discussed in comparison with results of other experimental methods and theoretical calculations.     

New Architecture of Liquid‐Crystalline Polymers from Fluorinated Vinylcyclopropanes

A new class of fluorinated polymers was prepared by radical ring‐opening homopolymerization of vinylcyclopropane monomers with a perfluorinated (CF₂)_nF chain (n = 6, 8, or 10). The polymers were in fact copolymers composed of 1,5‐linear and cyclobutane isomer units, the relative content of which depended on n. Surprisingly, they formed liquid‐crystalline mesophases (SmB_d and/or SmA_d), which was attributed to phase separation of the incompatible fluorocarbon and hydrocarbon components of the repeat unit.     

Nb3Ru5B2 – The First Fully Characterized Ternary Phase in the Nb‐Ru‐B System: Synthesis,Crystal Structure,and Bonding Analysis

The first fully characterized phase in the Nb‐Ru‐B system, Nb₃Ru₅B₂, was successfully synthesized as polycrystalline powders as well as single crystals and characterized by EDX analysis and X‐ray diffraction methods. It is the first ternary phase of the type A₃T₅B₂ adopting the Ti₃Co₅B₂ structure type and containing a group eight transition metal at the T sites. According to COHP bonding analysis the Nb–Nb interactions between two pentagonal prisms are strongly bonding and thus weaken the Nb–Ru interactions, which become significantly weaker than those found in the tetragonal prisms. Furthermore a deep pseudogap is found around the Fermi Level of the calculated DOS and the phase is predicted to be a metallic conductor as expected for this metal‐rich boride.     

Hot π‐Electron Tunneling of Metal–Insulator–COF Nanostructures for Efficient Hydrogen Production

A metal–insulator–semiconductor (MIS) photosystem based on covalent organic framework (COF) semiconductors was designed for robust and efficient hydrogen evolution under visible‐light irradiation. A maximal H₂ evolution rate of 8.42 mmol h^?1 g^?1 and a turnover frequency of 789.5 h^?1 were achieved by using a MIS photosystem prepared by electrostatic self‐assembly of polyvinylpyrrolidone (PVP) insulator‐capped Pt nanoparticles (NPs) with the hydrophilic imine‐linked TP‐COFs having =C=O?H?N= hydrogen‐bonding groups. The hot π‐electrons in the photoexcited n‐type TP‐COF semiconductors can be efficiently extracted and tunneled to Pt NPs across an ultrathin PVP insulating layer to reduce protons to H₂. Compared to the Schottky‐type counterparts, the COF‐based MIS photosystems give a 32‐fold‐enhanced carrier efficiency, attributed to the combined enhancement of photoexcitation rate, charge separation, and oxidation rate of holes accumulated in the valence band of the TP‐COF semiconductor.     

Thermally Stable Porous Hydrogen‐Bonded Coordination Networks Displaying Dual Properties of Robustness and Dynamics upon Guest Uptake

Two series of microporous lanthanide coordination networks of the general formula, {[Ln(ntb)Cl₃] ? x H₂O}_n (series 1 : monoclinic C2/c, Ln=Sm and Tb; series 2 : hexagonal P3₁/c, Ln=Sm and Eu; ntb=tris(benzimidazol‐2‐ylmethyl)amine, x=0–4) have been synthesized and characterized by IR, elemental analyses, thermal gravimetry, and single‐crystal and powder X‐ray diffraction methods. In both series, the monomeric [Ln(ntb)Cl₃] coordination units are consolidated by N? H???Cl or C? H???Cl hydrogen bonds to sustain three‐dimensional (3D) networks. However, the different modes of hydrogen bonding in the two series lead to crystallization of the same [Ln(ntb)Cl₃] monomers in different forms (monoclinic vs. hexagonal), consequently giving rise to distinct porous structures. The resulting hydrogen‐bonded coordination networks display high thermal stability and robustness in water removal/inclusion processes, which was confirmed by temperature‐dependent single‐crystal‐to‐single‐crystal transformation measurements. Adsorption studies with H₂, CO₂, and MeOH have been carried out, and reveal distinct differences in adsorption behavior between the two forms. In the case of MeOH uptake, the monoclinic network shows a normal type I isotherm, whereas the hexagonal network displays dynamic porous properties.     

Tuning the Electronic Properties of the Dative N−B Bond with Associated O−B Interaction: Electron Localizability Indicator from X‐Ray Wavefunction Refinement

Despite the immense growth in interest in difluoroborate dyes, the nature of the interactions of the boron atom within the N‐BF₂‐O kernel is not yet fully understood. Herein, a set of real‐space bonding indicators is used to quantify the electronic characteristics of the dative N?B bond in difluoroborate derivatives. The atoms‐in‐molecules (AIM) partitioning scheme is complemented by the electron localizability indicator (ELI‐D) approach, and both were applied to experimental and theoretical electron‐density distributions (X‐ray constrained wavefunction fitting vs. DFT calculations). Additionally, Fermi orbital analysis was introduced for small DFT models to support and extend the findings for structures that contain BF₂.     

Preparation and Characterization of Mesoporous Ce0.5Zr0.5O2 Mixed Oxide

Mesoporous （MSU） Ce0.5Zr0.5O2 mixed oxide with a high specific surface area has been synthesized under weak acidic condition in the presence of an anionic surfactant, sodium dodecylbenzenesulfonate. The effect of the pH value on the formation of mesostructure and the thermal stability of the material has been evaluated. The products were characterized by transmission electron microscopy, powder X-ray diffraction and nitrogen adsorption-desorption measurements. The results showed that the as-prepared Ce0.5Zr0.5O2 mixed oxide possessed a specific surface area of 163.3 m^2·g^-1, which had a cubic fluorite-type structure and possessed specific surface areas of 148.4 and 62.4 m^2·g^-1 after calcination at 500 and 800 ℃ for 2 h, respectively. The material showed excellent thermal stability.     

Synthesis of ceria–zirconia mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

Ceria–zirconia mixed oxide was successfully synthesized via the sol–gel process at ambient temperature, followed by calcination at 500, 700 and 900 °C. The synthesis parameters, such as alkoxide concentration, aging time and heating temperature, were studied to obtain the most uniform and remarkably high‐surface‐area cubic‐phase mixed oxides. The thermal stability of both oxides was enhanced by mutual substitution. Surface areas of the Ce_xZr_1?xO₂ powders were improved by increasing ceria content, and their thermal stability was increased by the incorporation of ZrO₂. The most stable cubic‐phase solid solutions were obtained in the Ce range above 50 mol%. The highest surface area was obtained from the mixed catalyst containing a ceria content of 90 mol% (200 m²/g). Temperature programmed reduction results show that increasing the amount of Zr in the mixed oxides results in a decrease in the reduction temperature, and that the splitting of the support reduction process into two peaks depends on CeO₂ content. The CO oxidation activity of samples was found to be related to its composition. The activity of catalysts for this reaction decreased with a decrease in Zr amount in cubic phase catalysts. Ce₆Zr₄O₂ exhibited the highest activity for CO oxidation. Copyright © 2006 John Wiley & Sons, Ltd.     

Bonding in Diborane–Metal Complexes: A Quantum‐Chemical and Experimental Study of Complexes Featuring Early and Late Transition Metals

The coordination chemistry of the doubly base‐stabilised diborane(4), [HB(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido‐[1,2‐a]pyrimidinate), was extended by the synthesis of new late transition‐metal complexes containing Cu^I and Rh^I fragments. A detailed experimental study was conducted and quantum‐chemical calculations on the metal–ligand bonding interactions for [HB(hpp)]₂ complexes of Group 6, 9, 11 and 12 metals revealed the dominant B? H? M interactions in the case of early transition‐metal fragments, whereas the B? B? M bonding prevails in the case of the late d‐block compounds. These findings support the experimental results as reflected by the IR and NMR spectroscopic parameters of the investigated compounds. DFT calculations on [MeB(hpp)]₂ and model reactions between [B₂H₄ ? 2NMe₃] and [Rh(μ‐Cl)(C₂H₄)₂] showed that the bicyclic guanidinate allows in principle for an oxidative addition of the B? B bond. However, the formation of σ‐complexes is thermodynamically favoured. The results point to the selective B? H or B? B bond‐activation of diborane compounds by complexation, depending on the chosen transition‐metal fragment.     

Bismuth–Boron Multiple Bonding in BiB2O− and Bi2B−

Despite its electron deficiency, boron is versatile in forming multiple bonds. Transition‐metal–boron double bonding is known, but boron–metal triple bonds have been elusive. Two bismuth boron cluster anions, BiB₂O⁻ and Bi₂B⁻, containing triple and double B−Bi bonds are presented. The BiB₂O⁻ and Bi₂B⁻ clusters are produced by laser vaporization of a mixed B/Bi target and characterized by photoelectron spectroscopy and ab initio calculations. Well‐resolved photoelectron spectra are obtained and interpreted with the help of ab initio calculations, which show that both species are linear. Chemical bonding analyses reveal that Bi forms triple and double bonds with boron in BiB₂O⁻ ([Bi≡B−B≡O]⁻) and Bi₂B⁻ ([Bi=B=Bi]⁻), respectively. The Bi−B double and triple bond strengths are calculated to be 3.21 and 4.70 eV, respectively. This is the first experimental observation of Bi−B double and triple bonds, opening the door to design main‐group metal–boron complexes with multiple bonding.     

新的杯[4]芳烃衍生物通过缔合乙腈进行自插入的X-射线衍射和密度泛函理论研究

合成和表征了一个新的杯[4]芳烃衍生物，11,23-二羟亚胺甲基-25,27-二羟基-26,28-二丙氧基杯[4]芳烃 (B)及其与乙腈生成的组成为B·2CH₃CN的化合物。¹H NMR显示，在B·2CH₃CN中B采取锥型构象，X-射线衍射分析确证在溶液中所发现的构象。在晶格网络中存在着B·2CH₃CN以二聚体形式的自插入现象。在B3LYP/6-311G(d)水平上计算了该自插入二聚体中的非共价相互作用能，并对基集叠加误差进行了校正。在二聚体中的B·2CH₃CN，一个CH₃CN通过与羟亚胺基形成氢键使之稳定，结合能为–5.02 kJ·mol^-1，另一个CH₃CN则通过与另一个羟亚胺基形成氢键以及与另一B·2CH₃CN中B苯环空腔间的C–H···π相互作用使之稳定，结合能分别为–14.23 kJ·mol^-1和–3.77 kJ·mol^-1。自插入的驱动能为–7.54 kJ·mol^-1。     

Molecular dynamics simulations of the effects of carbon dioxide on the interfacial bonding of polystyrene thin films

Fabrication of nanoscale polymer‐based devices, especially in biomedical applications, is a challenging process due to requirements of precise dimensions. Methods that involve elevated temperature or chemical adhesives are not practicable due to the fragility of the device components and associated deformation. To effectively fabricate devices for lab‐on‐a‐chip or drug delivery applications, a process is required that permits bonding at low temperatures. The use of carbon dioxide (CO₂) to assist the bonding process shows promise in reaching this goal. It is now well established that CO₂ can be used to depress the glass transition temperature (T_g) of a polymer, allowing bonding to occur at lower temperatures. Furthermore, it has been shown that CO₂ can preferentially soften a polymer surface, which should allow for effective bonding at temperatures even below the bulk T_g. However, the impact of this effect on bonding has not been quantified, and the optimal bonding temperature and CO₂ pressure conditions are unknown. In this study, a molecular dynamics model is used to examine the atomic scale behavior of polystyrene in an effort to develop understanding of the physical mechanisms of bonding and to quantify how the process is impacted by CO₂. The final result is the identification of a range of CO₂ pressure conditions which produce the strongest bonding between PS thin films at room temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

B(C6F5)3/Chiral Phosphoric Acid Catalyzed Ketimine–Ene Reaction of 2‐Aryl‐3H‐indol‐3‐ones and α‐Methylstyrenes

The enantioselective ketimine–ene reaction is one of the most challenging stereocontrolled reaction types in organic synthesis. In this work, catalytic enantioselective ketimine–ene reactions of 2‐aryl‐3H‐indol‐3‐ones with α‐methylstyrenes were achieved by utilizing a B(C₆F₅)₃/chiral phosphoric acid (CPA) catalyst. These ketimine–ene reactions proceed well with low catalyst loading (B(C₆F₅)₃/CPA=2 mol %/2 mol %) under mild conditions, providing rapid and facile access to a series of functionalized 2‐allyl‐indolin‐3‐ones with very good reactivity (up to 99 % yield) and excellent enantioselectivity (up to 99 % ee). Theoretical calculations reveal that enhancement of the acidity of the chiral phosphoric acid by B(C₆F₅)₃ significantly reduces the activation free energy barrier. Furthermore, collective favorable hydrogen‐bonding interactions, especially the enhanced N?H???O hydrogen‐bonding interaction, differentiates the free energy of the transition states of CPA and B(C₆F₅)₃/CPA, thereby inducing the improvement of stereoselectivity.