Pentaspiranes and hexaspiranes with 1,3-dioxane or 1,3-oxathiane rings: synthesis and stereochemistry

The synthesis and stereochemistry of the first reported pentaspiro- and hexaspiro-1,3-dioxane and polyspiro-1,3-oxathiane (from dispiro to hexaspiro) derivatives are described. The crystal structures of a dispiro- and tetraspiro-1,3-oxathiane were determined by X-ray diffraction methods. NMR and chiral column HPLC investigations in solution revealed flexible and semiflexible structures for which syn-anti, cis-trans and d,l isomers were observed.     

Eu10Mn6Sb13: a new ternary rare-Earth transition-metal Zintl phase

A new transition-metal-containing Zintl compound, Eu(10)Mn(6)Sb(13), was prepared by a high-temperature Sn-flux synthesis. The structure was determined by single-crystal X-ray diffraction. Eu(10)Mn(6)Sb(13) crystallizes in the monoclinic space group C2/m with a = 15.1791(6) A, b = 19.1919(7) A, c = 12.2679(4) A, beta = 108.078(1)*, Z = 4 (R1 = 0.0410, wR2 = 0.0920), and T = 90(2) K. The structure of Eu(10)Mn(6)Sb(13) is composed of double layers of Mn-centered tetrahedra separated by Eu(2+) cations. The double layers are composed of edge- and corner-sharing Mn-centered tetrahedra which form cavities occupied by Eu(2+) cations and [Sb(2)](4-) dumbbells. Linear [Sb(3)](5-) trimers bridging two tetrahedra across the cavity are also present. Bulk susceptibility data indicate paramagnetic behavior with a ferromagnetic component present below 60 K. Temperature-dependent electrical resistivity measurements show semiconducting behavior above 60 K (E(a)() = 0.115(2) eV), a large and unusually sharp maximum in the resistivity at approximately 40 K, and metallic behavior below 40 K. (151)Eu M?ssbauer spectra confirm that the europium is divalent with an average isomer shift of -11.2(1) mm/s at 100 K; the spectra obtained below 40 K reveal magnetic ordering of six of the seven europium sublattices and, at 4.2 K, complete ordering of the seven europium sublattices.     

A europium-151 Mössbauer spectral study of Eu(14)MnP(11), Eu(14)MnAs(11), and Eu(14)MnSb(11)

The europium-151 Mossbauer spectra of the Eu(14)MnP(11), Eu(14)MnAs(11), and Eu(14)MnSb(11) Zintl compounds, measured between 4.2 and 100 K, reveal europium(II) for all four crystallographically inequivalent europium sites in Eu(14)MnAs(11) and Eu(14)MnSb(11) and europium(II) and europium(III) for the three 32g and the 16f europium sites in Eu(14)MnP(11), respectively. Below the ordering temperatures of 52, 74, and 92 K, only very small hyperfine fields of 2-4 T are observed at the europium sites as a result of the polarization by the manganese magnetic moments. At 4.2 K, the europium(II) magnetic moments are ordered, and hyperfine fields of 24.4, 24.8, and 19.3 T are observed in Eu(14)MnP(11), Eu(14)MnAs(11), and Eu(14)MnSb(11), respectively, fields that are typical for magnetically ordered europium(II) ions. At 4.2 K the 16f europium(III) sites in Eu(14)MnP(11) experience a transferred hyperfine field of 33 T from the neighboring ordered europium(II) moments. Between its Curie temperature and 4.2 K, the europium-151 Mossbauer spectra of Eu(14)MnSb(11) reveal that the europium(II) moments order below ca. 13 K, i.e., below the second magnetic transition observed in magnetic measurements. Between their Curie temperatures and 4.2 K, the europium-151 Mossbauer spectra of Eu(14)MnP(11) and Eu(14)MnAs(11) are complex and have been analyzed with two models, models that give equivalently good fits. However, the second model in which the spectra are fit with a three-dimensional relaxation of the europium(II) and europium(III) hyperfine fields is preferred for its physical meaning and its reduced number of fitted parameters.     

Preparation of 2-alkenylperhydro-1,3-benzoxazines and application for 2-isoxazolines synthesis

Various 2-substituted-N-benzyl-4,4,7-trimethyl-trans-octahydro-1,3-benzoxazines 2 were prepared from the condensation of (?)-8-benzylaminomenthol 1 derived from (+)-pulegone, with acrolein, crotonaldehyde, cinnamaldehyde, 2(E)-N,N-diisopropyl-4-oxobut-2-enamide, ethyl (2E)-4-oxobut-2-enoate, and 2-furaldehyde in 71–96% yield. The 1,3-dipolar cycloaddition with aceto- and benzonitrile oxide gave the corresponding 2-isoxazoline cycloadducts. The origin of the stereoselectivity (4′S, 5′S-cycloadducts up to 64% de) arises from the cycloaddition of the dipole to the top of the Re,Re-alkene face of dipolarophile 2.     

Polyethylene Aerogels with Combined Physical and Chemical Crosslinking: Improved Mechanical Resilience and Shape‐Memory Properties

While the introduction of polymers into aerogels strongly enhances their toughness, truly elastic monolithic aerogels which restore their dimensions upon extensive compression are still challenging to synthesize. In this context hydrophobic semi‐crystalline polymers with low glass transition temperatures, and combined stiffness and flexibility, have only recently attracted attention. Shown here is that polyethylene aerogels with a low density, and combined chemical crosslinking and high crystallinity, display high moduli and excellent mechanical resilience. To maximize the crystallinity of these aerogels while maintaining a high crosslinking density, polyethylene networks with well‐defined segments were synthesized by hydrosilylation crosslinking of telechelic, vinyl‐functionalized oligomers obtained from catalyzed chain‐growth polymerization. Recoverable deformations both above and below the melting temperature of polyethylene affords remarkable shape‐memory properties.     

Complex magnetic ordering in Eu3InP3: a new rare earth metal zintl compound

Eu3InP3 has been prepared as large single crystals with an indium flux reaction. The structure of the new compound is isotypic to Sr3InP3 and crystallizes in the orthorhombic space group Pnma with unit cell dimensions of a = 12.6517(15) A, b = 4.2683(5) A, and c = 13.5643(14) A (Z = 4, T = 140 K, R1 = 0.0404, wR2 = 0.0971 for all data). The structure consists of one-dimensional chains of corner-shared distorted [InP2P2/2]6- tetrahedra separated by rows of Eu2+ ions. Two of the three crystallographically distinct europium sites have a short Eu(1)-Eu(2) distance of 3.5954(7) A, which yields Eu-Eu dimers. The Eu-P bond distances range from 2.974(2) to 3.166(2) A. The temperature dependence of the conductivity indicates that Eu3InP3 is a small band gap semiconductor. Both magnetization and Eu-151 Mossbauer spectral measurements indicate that the europium in Eu3InP3 is divalent and that at least two magnetic transitions occur. Magnetization studies reveal magnetic transitions at 14, 10.4, and approximately 5 K. These transitions are also observed in heat capacity studies of Eu3InP3. The Mossbauer spectra indicate that the two europium sites are ordered at 12 K and that all three europium sites are ordered at 8 K.     

Binding of the oxo-rhenium(V) core to methionine and to N-terminal histidine dipeptides

The ReOX(2)(met) compounds (X = Cl, Br) adopt a distorted octahedral structure in which a carboxylato oxygen lies trans to the Re=O bond, whereas the equatorial plane is occupied by two cis halides, an NH(2), and an SCH(3) group. Coordination of the SCH(3) unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr(2)(d,l-met).1/2H(2)O and ReOBr(2)(d,l-met).1/2CH(3)OH contain only the syn isomer (S-CH(3) bond on the side of the Re=O bond), whereas ReOCl(2)(d-met) and ReOCl(2)(d,l-met) consist of the pure anti isomer. (1)H NMR spectroscopy shows that both isomers coexist in equilibrium in acetone (anti/syn ratio = 1:1 for X = Br, 3:1 for X = Cl). Exchange between these two isomers is fast above room temperature, but it slows down below 0 degrees C, and the sharp second-order spectra of both isomers at -20 degrees C were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX(2)(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr(2)(His-Ala)]Br reveals the presence of distorted octahedral cations containing the Re=O(3+) core and a dipeptide coordinated through the histidine residue via the imidazole nitrogen, the terminal amino group, and the amide oxygen, the site trans to the Re=O bond being occupied by the oxygen. The alanine residue is ended by a protonated carboxylic group that does not participate in the coordination. The constant pattern of the(1)H NMR signals for the protons in the histidine residue confirms that the various dipeptides adopt a similar binding mode, consistent with the solid-state structure being retained in CD(3)OD solution.