Methane activation by silica-supported Zr(IV) hydrides: the dihydride [(triple bond)SiO)2ZrH2] is much faster than the monohydride [(triple bond)SiO)3ZrH

The silica-supported Zr(iv) dihydride [(triple bond)SiO)2ZrH2] reacts quickly and completely with methane to yield [(triple bond)SiO)2ZrMe2] through the intermediate [(triple bond)SiO)2ZrHMe], while its monohydride analogue [(triple bond)SiO)3ZrH] yields the monomethylated product [(triple bond)SiO)3ZrMe] slowly and incompletely.     

Fast characterization of functionalized silica materials by silicon-29 surface-enhanced NMR spectroscopy using dynamic nuclear polarization

We demonstrate fast characterization of the distribution of surface bonding modes and interactions in a series of functionalized materials via surface-enhanced nuclear magnetic resonance spectroscopy using dynamic nuclear polarization (DNP). Surface-enhanced silicon-29 DNP NMR spectra were obtained by using incipient wetness impregnation of the sample with a solution containing a polarizing radical (TOTAPOL). We identify and compare the bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions. Furthermore, the remarkable gain in time provided by surface-enhanced silicon-29 DNP NMR spectroscopy (typically on the order of a factor 400) allows the facile acquisition of two-dimensional correlation spectra.     

On the use of phosphoramidite ligands in copper-catalyzed asymmetric transformations with trialkylaluminum reagents

[reaction: see text] Phosphoramidites based on BINOL readily react with trimethylaluminum in "noncoordinating" solvents, leading to the corresponding aminophosphine which is the real ligand in copper-catalyzed asymmetric transformations. This artifact explains the experimental differences in the asymmetric ring opening of meso bicyclic hydrazines using dialkylzinc or trialkylaluminum reagents as nucleophiles.     

Diminishing blocking interval in particle-conveying channel bundles

We consider a channel bundle consisting of N_c parallel channels conveying a particulate flux. Particles enter these channels according to a homogeneous Poisson process and exit after a fixed transit time, τ. An individual channel blocks if N particles are simultaneously present. When a channel is blocked the flux previously entering it is redistributed evenly over the remaining open channels. We perform event driven simulations to examine the behaviour of an initially empty channel bundle with a total entering flux of intensity Λ. The mean blockage time of the kth channel is denoted by ? t_k ? ,k = 1,...,N_c. For N = 1, as shown previously, the interval between successive blockages is constant, while for N> 1 an accelerating cascade, i.e. one in which the interval between successive blockages decreases, is observed. After an initial transient regime we observe a well-defined universal regime that is characterized by\hbox{$\Delta_k^{(N)} = (-1)^{N-1}\frac{[(N-1)!]^2}{(\Lambda\tau)^N}$}Δk(N)=(?1)N?1[(N?1)!]2(Λτ)Nwhere \hbox{$\Delta_k^{(1)}=\langle t_k \rangle-\langle t_{k-1}\rangle$}Δk(1)=?tk???tk?1? and \hbox{$\Delta_k^{(j)}=\Delta_k^{(j-1)}-\Delta_{k-1}^{(j-1)}$}Δk(j)=Δk(j?1)?Δk?1(j?1) denotes the jth order difference.     

Modifying the reactivity in the homologation of propane by introducing aryloxide ligands on a silica supported zirconium alkyl system

Grafting the well-defined molecular complexes [(ArO)Zr(CH2tBu)3], , and [(ArO)2Zr(CH2tBu)2], , on SiO2-(700) (ArO=2,6-Ph2C6H3O) gives the corresponding monosiloxy surface complexes [([TRIPLE BOND]SiO)Zr(CH2tBu)2(OAr)] and [([TRIPLE BOND]SiO)Zr(CH2tBu)(OAr)2] as major surface species as evidenced by mass balance analysis, IR and NMR spectroscopies. In both cases, minor cyclometallated species (ca. 20%) are also probably formed during the grafting process. While /SiO2-(700) catalytically transforms propane into its lower and higher homologues, /SiO2-(700) remains inactive. Moreover, the formation of butane as the major higher homologues is consistent with the formation of metallocarbene intermediates in this system in contrast to what was observed for the corresponding homologation reaction on silica supported zirconium hydrides.     

A slowly relaxing rigid biradical for efficient dynamic nuclear polarization surface-enhanced NMR spectroscopy: expeditious characterization of functional group manipulation in hybrid materials

A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T(1e)) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors (ε) of over 100 at a magnetic field of 9.4 T and sample temperatures of ~100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements (Σ(?)) are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T(1e), which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12,700, making the acquisition of (13)C and (15)N one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material.     

Structural Expansion of Chalcogenido Tetrelates in Ionic Liquids by Incorporation of Sulfido Antimonate Units

Multinary chalcogenido (semi)metalate salts exhibit finely tunable optical properties based on the combination of metal and chalcogenide ions in their polyanionic substructure. Here, we present the structural expansion of chalcogenido germanate(IV) or stannate(IV) architectures with Sb^III, which clearly affects the vibrational and optical absorption properties of the solid compounds. For the synthesis of the title compounds, [K₄(H₂O)₄][Ge₄S₁₀] or [K₄(H₂O)₄][SnS₄] were reacted with SbCl₃ under ionothermal conditions in imidazolium-based ionic liquids. Salt metathesis at relatively low temperatures (120 °C or 150 °C) enabled the incorporation of (formally) Sb³⁺ ions into the anionic substructure of the precursors, and their modification to form (Cat)₁₆[Ge₂Sb₂S₇]₆[GeS₄] ( 1 ) and (Cat)₆[Sn₁₀O₄S₂₀][Sb₃S₄]₂ ( 2 a and 2 b ), wherein Cat=(C₄C₁C₁Im)⁺ ( 1 and 2 a ) or (C₄C₁C₂Im)⁺ ( 2 b ). In 1 , germanium and antimony atoms are combined to form a rare noradamantane-type ternary molecular anion, six of which surround an {GeS₄} unit in a highly symmetric secondary structure, and finally crystallize in a diamond-like superstructure. In 2 , supertetrahedral oxo-sulfido stannate clusters are generated, as known from the ionothermal treatment of the stannate precursor alone, yet, linked here into unprecedented one-dimensional strands with {Sb₃S₄} units as linkers. We discuss the single-crystal structures of these uncommon salts of ternary and quaternary chalcogenido (semi)metalate anions, as well as their Raman and UV-visible spectra.     

Clay-fulleropyrrolidine nanocomposites

In this work, we describe the insertion of a water-soluble bisadduct fulleropyrrolidine derivative into the interlayer space of three layered smectite clays. The composites were characterized by a combination of powder X-ray diffraction, transmission electron microscopy, X-ray photoemission and FTIR spectroscopies, and laser flash photolysis measurements. The experiments, complemented by computer simulations, give insight into the formation process, structural details, and properties of the fullerene/clay nanocomposites. The reported composite materials constitute a new hybrid system, where C(60) differs from its crystals or its solutions, and open new perspectives for the design and construction of novel C(60)-based organic/clay hybrid materials.     

Skeletal rearrangements in the 2,3-diazanorbornene series. A fast access to highly functionalized cyclopentanes

[reaction: see text] Acid-catalyzed nucleophilic substitution of bicyclic hydrazine-epoxide involves nitrogen participation, leading to a skeletal rearrangement. This transformation enables the fast preparation of disubstituted bicyclic hydrazines in a regio- and stereoselective manner, leading to several polyfunctional diaminocyclopentanes after hydrogenolysis.