Journal of Structural Chemistry最新文献

Ni_xCe_1–xAlO_3–δ catalysts (x = 0-0.5) are prepared by the solution combustion synthesis method. According to the powder XRD data, thermal treatment of the products at 700 °C in air leads mainly to the formation of the cerium aluminate phase and to small amounts of highly dispersed cerium dioxide phase. An increase in the nickel content in the sample significantly decreases the content of the cerium dioxide phase. The prepared Ni²⁺-modified cerium aluminate samples are precursors of catalysts for the dry reforming of methane reaction (DRM) and are characterized by high specific surface area and a developed porous structure. The study of the phase composition of these samples after their preliminary reduction (“activation”) in a hydrogen-containing gas mixture and subsequent performance in the reaction medium conditions shows that the Ni²⁺ ions are reduced, come out from the structure to the surface, and form metal Ni particles with a size of 5-8 nm. The influence of catalysts′ chemical composition on their properties in the DRM reaction is studied. The representative Ni_0.5Ce_0.5AlO_3–δ catalyst shows stable performance in the DRM reaction for 30 h at T = 700 °C with a CH₄ conversion of 77%, an H₂ yield of 43%, and an extremely short contact time τ of 30 ms. High catalytic activity of the obtained catalysts in the DRM reaction is due to the formation of the active phase represented by highly-dispersed coking-resistant metal Ni nanoparticles on the inherently large specific surface area of the Ni²⁺-modified cerium aluminate.

Silver malate [Ag₂(mal)] (1) (mal = O₂C–CH₂–CH(OH)–CO₂) with a polymeric structure is prepared and characterized by elemental analysis and IR spectroscopy. This complex can be dissolved upon heating in pyridine (py) or its derivatives. Crystals of the [Ag₂(py)₄(mal)]·H₂O (2a), [Ag₂(3-Me-py)₄(mal)] (3a) and [Ag₂(4-Me-py)₄(mal)]·0.6(4-Me-py) (4a) coordination polymers are prepared from solutions. The crystals can be characterized by XRD but undergo rapid degradation and pyridine loss when extracted from the mother liquor or slightly heated. The desolvation and decoordination of 2a, 3a, 4a yields [Ag₂(py)₂(mal)] (2), [Ag₂(3-Me-py)₃(mal)] (3), [Ag₂(4-Me-py)₃(mal)] (4), respectively, as stable products.

A seven-coordinate bisligand complex of vanadium(IV) (1) is formed as a result of the interaction between glyoxal-bis(2-hydroxy-3,5-di-tert-butylphenyl)imine LH₂ and vanadyl sulfate in the presence of Et₃N base. Its structure is determined by the single crystal X-ray diffraction analysis. The reaction is accompanied by redox processes involving LH₂ and its isomerization. The redox state of the ligands and the metal center in complex 1 is confirmed by quantum chemical computations. One of the organic ligands is the radical trianion and is tetradentate coordinated by the metal atom. The second being the monoanion undergoes intramolecular oxidative cyclization and is characterized by tridentate coordination with vanadium(IV). Compound 1 is diamagnetic due to the strong antiferromagnetic interaction between the paramagnetic metal cation and the organic radical. The complex under study is additionally characterized by NMR, IR, UV spectroscopic techniques and cyclic voltammetry.

A procedure is proposed for the quantitative powder X-ray diffraction (XRD) analysis of powder mixtures of nanocrystalline low-temperature alumina forms. It is based on the presentation of the XRD pattern of the mixture being analyzed by a linear combination of the XRD profiles of reference oxides. The purpose of the quantitative analysis is the least-squares refinement of weight coefficients of the profiles until agreement is achieved between the experimental and generated XRD patterns. The pre-normalization procedure of the experimental XRD profiles with conversion to electronic units enables the analysis of data obtained on instruments with different measurement geometries and instrumental parameters. The procedure is verified using γ-Al₂O₃ and χ-Al₂O₃ mixtures of set compositions, and the accuracy of this procedure is compared with that of the calibration plot method. The absolute error of the χ-Al₂O₃ weight fraction determined by the procedure proposed is two times lower than that of the calibration plot method: 5 wt.% against 10 wt.%. By simulating the noise in the model XRD profiles it is shown that the random error contribution is about 1 wt.%.

A new crystal form (Z′ = 3) of 15-ene-steviol, 13-hydroxy-ent-kaur-15-ene-19-oic acid, is prepared. A homochiral “head-to-head, tail-to-tail” cyclic dimer (previously unknown associate for compounds with a kaurane skeleton) is determined by XRD. The conducted quantum chemical calculations indicate that the dimer has a strained geometry, both in crystal and gas phases, which is manifested as a significant deviation from the carboxyl synthon plane. The topological analysis and the electron density Laplacian analysis show that the lone pairs of carbonyl groups deviate from bond paths.

Solid solutions of the nominal composition La_1–y□_yMn_1–xFe_xO_3±δ (y = 0, 0.1, 0.2, x = 0.5, 0.55, 0.58) with the non-stoichiometric concentration of La cations are synthesized by the polymer-salt composition method and characterized by a set of physicochemical techniques. An increase in the degree of Fe cation doping is shown to decrease the particle size of solid solutions with retaining the perovskite structure. Particle enrichment with Fe cations and the presence of disordered crystal structure layers on the perovskite phase surface is detected by high-resolution transmission electron microscopy. According to the thermogravimetric data, the composition with x = 0.58 is characterized by a smaller weight loss, which seems to be due to the partial blocking of the perovskite phase surface by disordered layers enriched with Fe.

The crystal structure of 4,10-dibromo-6,12-bis((triisopropylsilyl)ethynyl)dibenzo-[def,mno]chrysene and its UV-Vis and photoluminescence spectra are described. The structure of the compound is a packing of planar molecules arranged in stacks due to the π–π interaction of aromatic nuclei, which is identical to the known structure of the unbrominated analogue. The spectra of the brominated compound are slightly shifted to the long-wave region relative to the unbrominated analogue.

Unusual condensation of phenylpropargyl aldehyde with acetylacetone in acetonitrile in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene as a catalyst is described. It proceeds at room temperature with the formation of a racemic mixture of (2S,3S,4S,5S,6S)- and (2R,3R,4R,5R,6R)-3,5-diacetyl-2-methyl-4,6-bis(phenylethynyl)tetrahydro-2H-pyran-2-ylacetate instead of expected 3-(3-phenylprop-2-in-1-ylidene)pentane-2,4-dione. The X-ray diffraction and NMR studies of the molecular and crystal structures of this compound reveals an anometric effect that manifests itself at the axial position of the acetoxy group relative to the six-membered heterocycle.

Photocatalytic processes utilizing the energy of light quanta to synthesize valuable compounds are studied quite widely, but particular attention is paid to the development of effective and stable photocatalysts. The present study considers the transformation of photocatalysts based on TiO₂ modified with copper and its oxide compounds (CuO_x) in the conditions of CO₂ reduction under the LED irradiation (397 nm). The influence of TiO₂ precalcination on the state of the copper cocatalyst is demonstrated. It is shown by in situ X-ray absorption (XANES) that modifying the calcined TiO₂ photocatalyst with copper results in higher content of (0) and (1+) copper sates than in the case of the non-calcined photocatalyst. It is established that the fraction of metal copper in both photocatalysts increases during the reaction due to the accumulation of photogenerated electrons on the copper cocatalyst.

Structure of the non-nucleoside HIV transcriptase inhibitor elsulfavirine and that of its active form VM-1500A are studied by powder XRD. Their molecules in crystals are stabilized by hydrogen bonds of amide and amino groups and by stacking interactions between phenolic groups. The corresponding complex is prepared by the co-crystallization of the elsulfavirine sodium salt and 15-crown-5. The structure of this compound is determined by single-crystal XRD. It is shown that the coordination of the sodium atom with 15-crown-5 prevents the formation of a polymer structure due to weak Na–O bonds and that the molecules of the complex are connected into layers by stacking interactions. The conformational analysis of VM-1500A shows that the most energy profitable conformations are stabilized due to the conjugation between phenolic fragments and the sulfo group.