Location of the Spinel Vacancies in γ‐Al2O3

A non‐spinel model for the structure of γ‐Al₂O₃, with 25 % of the Al³⁺ cations at tetrahedral positions, has been the subject of wide interest. However, ¹⁷O NMR measurements and, more recently, ²⁷Al NMR measurements have shown that there are considerably more Al³⁺ cations at tetrahedral positions. This means that the Al³⁺ vacancies in γ‐Al₂O₃ are not at tetrahedral but at octahedral positions, as in isostructural γ‐Fe₂O₃ and in accordance with density functional theory predictions. This has consequences with regard to the surface structure of γ‐Al₂O₃, and thus, for catalysis.     

Multianvil‐Synthese,Pulver‐Röntgenstrukturanalyse, 31P‐MAS‐NMR‐ und FTIR‐Spektroskopie sowie Materialeigenschaften von γ‐P3N5, einer Hochdruckphase von binärem Phosphor(V)‐nitrid mit verzerrt quadratischen PN5‐Pyramiden und PN4‐Tetraedern

Multianvil Synthesis, X‐ray Powder Diffraction Analysis, ³¹P‐MAS‐NMR, and FTIR Spektroscopy as well as Material Properties of γ‐P₃N₅, a High‐Pressure Polymorph of Binary Phosphorus(V) Nitride, Built up from Distorted PN₅ Square Pyramids and PN₄ Tetrahedra The high‐pressure phase γ‐P₃N₅ was synthesized at a pressure of 11 GPa and a temperature of 1500 °C in a multianvil apparatus. Partially crystalline P₃N₅ has been used as a starting material. The crystal structure was solved by direct methods on the basis of X‐ray powder diffraction data and it was refined by the Rietveld method (Imm2, a = 1287.21(4), b = 261.312(6), c = 440.03(2) pm, Z = 2, R_p = 0.073, wR_p = 0.094, R_F = 0.048). γ‐phosphorus nitride crystallizes in a three‐dimensional network structure built up from corner sharing PN₄ tetrahedra and trans‐edge sharing distorted PN₅ square pyramids. In the ³¹P‐MAS‐NMR spectrum two sharp isotropic resonances with an intensity ratio of 1 : 2.02(5) are observed at —11.95(3) and —101.72(7) ppm, respectively. The IR‐spectroscopic and thermal properties of γ‐P₃N₅ are described. Measurement of the Vickers hardness resulted in a value of 9.7(21) GPa for sintered polycrystalline γ‐P₃N₅, which is significantly higher than that for the partially crystalline normal pressure modification of P₃N₅ (5.1(7) GPa).     

Environmentally Friendly γ‐MnO2 Hexagon‐Based Nanoarchitectures: Structural Understanding and Their Energy‐Saving Applications

Although about 200,000 metric tons of γ‐MnO₂ are used annually worldwide for industrial applications, the γ‐MnO₂ structure is still known to possess a highly ambiguous crystal lattice. To better understand the γ‐MnO₂ atomic structure, hexagon‐based nanoarchitectures were successfully synthesized and used to elucidate its internal structure for the present work. The structural analysis results, obtained from the hexagon‐based nanoarchitectures, clearly show the coexistence of akhtenskite (ε‐MnO₂), pyrolusite (β‐MnO₂), and ramsdellite in the so‐called γ‐MnO₂ phase and verified the heterogeneous phase assembly of the γ‐MnO₂ state, which violates the well‐known “De Wolff” model and derivative models, but partially accords with Heuer's results. Furthermore, heterogeneous γ‐MnO₂ assembly was found to be a metastable structure under hydrothermal conditions, and the individual components of the heterogeneous γ‐MnO₂ system have structural similarities and a high lattice matches with pyrolusite (β‐MnO₂). The as‐obtained γ‐MnO₂ nanoarchitectures are nontoxic and environmentally friendly, and the application of such nanoarchitectures as support matrices successfully mitigates the common problems for phase‐change materials of inorganic salts, such as phase separation and supercooling‐effects, thereby showing prospect in energy‐saving applications in future “smart‐house” systems.     

Two‐step Synthesis of New γ‐Lactones via Cyclization of 7‐Chloro‐2‐(methoxycarbonyl)‐4‐6‐dimethylocta‐(2E,4E,6E)‐trienoic acid

A new rapid synthesis of γ‐lactones, cis fused with a cyclopentenic ring by thermal cyclization of 7‐chloro‐2‐(methoxycarbonyl)‐4‐6‐dimethylocta‐7‐phenyl (or methyl) (2E,4E,6E)‐trienoic acids was reported. The key step implicates an intramolecular cyclization to a cyclopentenyl cation, according to an electrocyclic π_2s + π_2a conrotatory process, published in a recent paper (from the corresponding diacids). We have investigated the thermal behavior of the corresponding half‐esters since; if the cyclization obeys to the proposed mechanism, the diacids, half‐esters must also cyclize in a similar manner. Saponification of these led to γ‐dilactones via intermediary cyclopropanes. Mechanistic pathways were investigated.     

Cu(II) immobilized on guanidinated epibromohydrin‐functionalized γ‐Fe2O3@TiO2 (γ‐Fe2O3@TiO2‐EG‐Cu(II)): A highly efficient magnetically separable heterogeneous nanocatalyst for one‐pot synthesis of highly substituted imidazoles

A simple, efficient and eco‐friendly procedure has been developed using Cu(II) immobilized on guanidinated epibromohydrin‐functionalized γ‐Fe₂O₃@TiO₂ (γ‐Fe₂O₃@TiO₂‐EG‐Cu(II)) for the synthesis of 2,4,5‐trisubstituted and 1,2,4,5‐tetrasubstituted imidazoles, via the condensation reactions of various aldehydes with benzil and ammonium acetate or ammonium acetate and amines, under solvent‐free conditions. High‐resolution transmission electron microscopy analysis of this catalyst clearly affirmed the formation of a γ‐Fe₂O₃ core and a TiO₂ shell, with mean sizes of about 10–20 and 5–10 nm, respectively. These data were in very good agreement with X‐ray crystallographic measurements (13 and 7 nm). Moreover, magnetization measurements revealed that both γ‐Fe₂O₃@TiO₂ and γ‐Fe₂O₃@TiO₂‐EG‐Cu(II) had superparamagnetic behaviour with saturation magnetization of 23.79 and 22.12 emu g^?1, respectively. γ‐Fe₂O₃@TiO₂‐EG‐Cu(II) was found to be a green and highly efficient nanocatalyst, which could be easily handled, recovered and reused several times without significant loss of its activity. The scope of the presented methodology is quite broad; a variety of aldehydes as well as amines have been shown to be viable substrates. A mechanism for the cyclocondensation reaction has also been proposed.     

γ‐Alumina: a single‐crystal X‐ray diffraction study

The structure of γ‐alumina (Al_21+1/3□_2+2/3O₃₂) crystals obtained as a product of a corrosion reaction between β‐sialon and steel was refined in the space group Fdm. The oxygen sublattice is fully occupied. The refined occupancy parameters are 0.83 (3), 0.818 (13), 0.066 (14) and 0.044 (18) for Al ions in 8a, 16d, 16c and 48f positions, respectively. The Al ions are distributed over octa­hedral and tetra­hedral sites in a 63:37 ratio, with 6% of all Al ions occupying non‐spinel positions.     

Copper(II) hypophosphite: the α‐ and β‐forms at 270 and 100 K,and the γ‐form at 270 K

Copper(II) hypophosphite has been shown to exist as several polymorphs. The crystal structures of monoclinic α‐, ortho­rhombic β‐ and ortho­rhombic γ‐Cu(H₂PO₂)₂ have been determined at different temperatures. The geometry of the hypophosphite anion in all three polymorphs is very close to the idealized one, with point symmetry mm2. Despite having different space groups, the structures of the α‐ and β‐polymorphs are very similar. The polymeric layers formed by the Cu atoms and the hypophosphite ions, which are identical in the α‐ and β‐polymorphs, stack in the third dimension in different ways. Each hypophosphite anion is coordinated to three Cu atoms. On cooling, a minimum amount of contraction was observed in the direction normal to the layers. The structure of the polymeric layers in the γ‐­polymorph is quite different. There are two symmetry‐independent hypophosphite anions; the first is coordinated to two Cu atoms, while the second is coordinated to four Cu atoms. In all three polymorphs, the Cu atoms are coordinated by six O atoms of six hypophosphite anions, forming tetragonal bipyramids; in the α‐ and β‐polymorphs, there are four short and two long Cu—O distances, while in the γ‐polymorph, there are four long and two short Cu—O distances.     

2‐Oxo‐2‐phenyl‐N‐[(R)‐1‐phenylethyl]acetamide and N,N‐dimethyl‐2‐(1‐naphthyl)‐2‐oxoacetamide: possibility of Yang photocyclization in a crystal

The crystal structures of 2‐oxo‐2‐phenyl‐N‐[(R)‐1‐phenylethyl]acetamide, C₁₆H₁₅NO₂, (I), and N,N‐dimethyl‐2‐(1‐naphthyl)‐2‐oxoacetamide, C₁₄H₁₃NO₂, (II), were determined in an attempt to understand the reason for the lack of Yang photocyclization in their respective crystals. In the case of (I), the long distance between the O atom of the carbonyl group and the γ‐H atom, and between the C atom of the carbonyl group and the γ‐C atom, preclude Yang photocyclization. For (II), the deviation of the γ‐H atom from the plane of the carbonyl group and interactions between the naphthalene rings are regarded as possible reasons for the chemical inertia. The two independent molecules of (I) differ in their conformation. N—H...O hydrogen bonds link molecules of (I) into chains extended along the b axis.     

PPh3⋅HBr‐DMSO Mediated Expedient Synthesis of γ‐Substituted β,γ‐Unsaturated α‐Ketomethylthioesters and α‐Bromo Enals: Application to the Synthesis of 2‐Methylsulfanyl‐3(2 H)‐furanones

An efficient chemoselective general procedure for the synthesis of γ‐substituted β,γ‐unsaturated α‐ketomethylthioesters from α,β‐unsaturated ketones has been achieved through an unprecedented PPh₃?HBr‐DMSO mediated oxidative bromination and Kornblum oxidation sequence. The newly developed reagent system serves admirably for the synthesis of α‐bromoenals from enals. Furthermore, AuCl₃‐catalyzed efficient access to 3(2H)‐furanones from the above intermediates under extremely mild conditions are described.     

Two‐Step Synthesis of Multifunctional γ‐Lactams from γ‐Lactone

We present herein an efficient and rapid method for the synthesis of N,1‐dialkyl‐4‐(2‐hydroxyethyl)‐5‐oxopyrrolidine‐3‐carboxamides based on the conversion of γ‐lactone to γ‐lactam via the conjugate addition of primary amines to an ethyl α‐functionalized acrylate followed by intramolecular cyclization.     

Spectroscopic and thermal characterization of the host–guest interactions between α‐, β‐ and γ‐cyclodextrins and vanadocene dichloride

Host–guest interactions between α‐, β‐ and γ‐cyclodextrins and vanadocene dichloride (Cp₂VCl₂) have been investigated by a combination of thermogravimetric analysis, differential scanning calorimetry, powder X‐ray diffraction and solid‐state and solution electron paramagnetic resonance (EPR) spectroscopy. The solid‐state results demonstrated that only β‐ and γ‐cyclodextrins form 1:1 inclusion complexes, while α‐cyclodextrin does not form an inclusion complex with Cp₂VCl₂. The β‐ and γ‐CD–Cp₂VCl₂ inclusion complexes exhibited anisotropic electron‐⁵¹V (I = 7/2) hyperfine coupling constants whereas the α‐CD–Cp₂VCl₂ system showed only an asymmetric peak with no anisotropic hyperfine constant. On the other hand, solution EPR spectroscopy showed that α‐cyclodextrin (α‐CD) may be involved in weak host–guest interactions in equilibrium with free vanadocene species. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydroxyl‐Mediated Non‐oxidative Propane Dehydrogenation over VOx/γ‐Al2O3 Catalysts with Improved Stability

Supported vanadium oxides are one of the most promising alternative catalysts for propane dehydrogenation (PDH) and efforts have been made to improve its catalytic performance. However, unlike Pt‐based catalysts, the nature of the active site and surface structure of the supported vanadium catalysts under reductive reaction conditions still remain elusive. This paper describes the surface structure and the important role of surface‐bound hydroxyl groups on VO_x / γ‐Al₂O₃ catalysts under reaction conditions employing in situ DRIFTS experiments and DFT calculations. It is shown that hydroxyl groups on the VO_x /Al₂O₃ catalyst (V?OH) are produced under H₂ pre‐reduction, and the catalytic performance for PDH is closely connected to the concentration of V?OH species on the catalyst. The hydroxyl groups are found to improve the catalyst that leads to better stability by suppressing the coke deposition.     

Diverging Mechanisms: Cytochrome‐P450‐Catalyzed Demethylation and γ‐Lactone Formation in Bacterial Gibberellin Biosynthesis

Biosynthesis of the gibberellin (GA) plant hormones evolved independently in plants and microbes, but the pathways proceed by similar transformations. The combined demethylation and γ‐lactone ring forming transformation is of significant mechanistic interest, yet remains unclear. The relevant CYP112 from bacteria was probed by activity assays and ¹⁸O₂‐labeling experiments. Notably, the ability of tert‐butyl hydroperoxide to drive this transformation indicates use of the ferryl‐oxo (Compound I) from the CYP catalytic cycle for this reaction. Together with the confirmed loss of C20 as CO₂, this necessitates two catalytic cycles for carbon–carbon bond scission and γ‐lactone formation. The ability of CYP112 to hydroxylate the δ‐lactone form of GA₁₅, shown by the labeling studies, is consistent with the implied use of a further oxygenated heterocycle in the final conversion of GA₂₄ into GA₉, with the partial labeling of GA₉, thus demonstrating that CYP112 partitions its reactants between two diverging mechanisms.     

Fe K‐Edge X‐ray Absorption Fine Structure Determination of γ‐Al2O3‐Supported Iron‐Oxide Species

The structure of FeO_x species supported on γ‐Al₂O₃ was investigated by using Fe K‐edge X‐ray absorption fine structure (XAFS) and X‐ray diffraction (XRD) measurements. The samples were prepared through the impregnation of iron nitrate on Al₂O₃ and co‐gelation of aluminum and iron sulfates. The dependence of the XRD patterns on Fe loading revealed the formation of α‐Fe₂O₃ particles at an Fe loading of above 10 wt %, whereas the formation of iron‐oxide crystals was not observed at Fe loadings of less than 9.0 wt %. The Fe K‐edge XAFS was characterized by a clear pre‐edge peak, which indicated that the Fe?O coordination structure deviates from central symmetry and that the degree of Fe?O?Fe bond formation is significantly lower than that in bulk samples at low Fe loading (<9.0 wt %). Fe K‐edge extended XAFS oscillations of the samples with low Fe loadings were explained by assuming an isolated iron‐oxide monomer on the γ‐Al₂O₃ surface.     

Polymorphism in Alkali Metal Uranyl Nitrates: Synthesis and Crystal Structure of γ‐K(UO2)(NO3)3

Single crystals of γ‐K(UO₂)(NO₃)₃ were prepared from aqueous solutions by evaporation. The crystal structure [orthorhombic, Pbca (61), a = 9.2559(3) Å, b = 12.1753(3) Å, c = 15.8076(5) Å, V = 1781.41(9) Å³, Z = 8] was determined by direct methods and refined to R₁ = 0.0267 on the basis of 3657 unique observed reflections. The structure is composed of isolated anionic uranyl trinitrate units, [(UO₂)(NO₃)₃]^–, that are linked through eleven‐coordinated K⁺ cations. Both known polymorphs of K(UO₂)(NO₃)₃ (α‐ and γ‐phases) can be considered as based upon sheets of isolated complex [(UO₂)(NO₃)₃]^– ions separated by K⁺ cations. The existence of polymorphism in the two K[UO₂(NO₃)₃] polymorphs is due to the different packing modes of uranyl trinitrate clusters that adopt the same two‐dimensional but different three‐dimensional arrangements.     

Base‐Catalyzed Highly Stereoselective Conversion of γ‐Hydroxy‐α,β‐acetylenic Esters to γ‐Acetoxy Dienoates

A convenient route with high stereo control to γ‐acetoxy dienoates is provided by the reaction of methyl propiolate with aldehydes in the presence of ZnEt₂ and N‐methylimidazole at room temperature, followed by the catalytic conversion of the resulting γ‐hydroxy‐α,β‐acetylenic esters with p‐N,N‐dimethylaminopyridine (DMAP) in acetic anhydride (see scheme).

     

A structural study of (1RS,2SR,3RS,4SR,5RS)‐2,4‐dibenzoyl‐1,3,5‐triphenylcyclohexan‐1‐ol chloroform hemisolvate and (1RS,2SR,3RS,4SR,5RS)‐2,4‐dibenzoyl‐1‐phenyl‐3,5‐bis(2‐methoxyphenyl)cyclohexan‐1‐ol

(1RS,2SR,3RS,4SR,5RS)‐2,4‐Dibenzoyl‐1,3,5‐triphenylcyclohexan‐1‐ol or (4‐hydroxy‐2,4,6‐triphenylcyclohexane‐1,3‐diyl)bis(phenylmethanone), C₃₈H₃₂O₃, (1), is formed as a by‐product in the NaOH‐catalyzed synthesis of 1,3,5‐triphenylpentane‐1,5‐dione from acetophenone and benzaldehyde. Single crystals of the chloroform hemisolvate, C₃₈H₃₂O₃·0.5CHCl₃, were grown from chloroform. The structure has triclinic (P) symmetry. One diastereomer [as a pair of (1RS,2SR,3RS,4SR,5RS)‐enantiomers] of (1) has been found in the crystal structure and confirmed by NMR studies. The dichoromethane hemisolvate has been reported previously [Zhang et al. (2007). Acta Cryst. E 63 , o4652]. (1RS,2SR,3RS,4SR,5RS)‐2,4‐Dibenzoyl‐3,5‐bis(2‐methoxyphenyl)‐1‐phenylcyclohexan‐1‐ol or [4‐hydroxy‐2,6‐bis(2‐methoxyphenyl)‐4‐phenylcyclohexane‐1,3‐diyl]bis(phenylmethanone), C₄₀H₃₆O₅, (2), is also formed as a by‐product, under the same conditions, from acetophenone and 2‐methoxybenzaldehyde. Crystals of (2) have been grown from chloroform. The structure has orthorhombic (Pca2₁) symmetry. A diastereomer of (2) possesses the same configuration as (1). In both structures, the cyclohexane ring adopts a chair conformation with all bulky groups (benzoyl, phenyl and 2‐methoxyphenyl) in equatorial positions. The molecules of (1) and (2) both display one intramolecular O—H...O hydrogen bond.     

Probing the γ‐Turn in a Short Proline Dipeptide Chain

The small peptide derived from proline, N‐acetyl‐prolinamide (Ac‐Pro‐NH₂), has been investigated using a combination of Fourier transform microwave spectroscopy with laser ablation. Spectral signatures belonging to only one conformer have been detected in the supersonic expansion. Rotational constants and nuclear quadrupole coupling constants of the two ¹⁴N nuclei have been used in the characterization of a γ‐turn structure in the gas phase, which is stabilized by a CO???HN intramolecular hydrogen bond closing a seven‐membered ring. A methyl group internal rotation barrier of 354 cm^?1 has been determined from the analysis of the A–E splittings.     

Crystal chemistry of anion‐excess ReO3‐related phases. III.

The crystal structure of the high‐pressure (4–8 GPa) form of zirconium tetrafluoride, γ‐ZrF₄, is based on the association by corner‐ and edge‐sharing of ZrF₈ triangulated dodecahedra, forming a three‐dimensional framework. It presents some analogies with high‐temperature α‐ZrF₄ but clearly constitutes a new MX₄ structure type. The main MX₄ ionic structure types, and especially those deriving from the `anion‐excess ReO₃‐type', are compared and it is shown that the TeF₄ structure can also be included in this family.     

Nano n‐propylsulfonated γ‐Al2O3: a new,efficient and reusable catalyst for synthesis of spiro[indoline‐3,4‐pyrazolo[3,4‐e][1,4]thiazepine]diones in aqueous media

Nano n‐propylsulfonated γ‐Al₂O₃ is easily prepared by the reaction of nano γ‐Al₂O₃ with 1,3‐propanesultone. This reagent can be used as an efficient catalyst for the synthesis of spiro [indoline‐3,4‐pyrazolo[3,4‐e][1,4]thiazepine]diones in aqueous media. This new method consistently has the advantages of excellent yields and short reaction times. Further, the catalyst can be reused and recovered several times. Copyright © 2013 John Wiley & Sons, Ltd.