Larry G. Sneddon

Base-Promoted Ammonia Borane Hydrogen-Release

The strong non-nucleophilic base bis(dimethylamino)naphthalene (Proton Sponge, PS) has been found to promote the rate and extent of H(2)-release from ammonia borane (AB) either in the solid state or in ionic-liquid and tetraglyme solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) containing 5.3 mol % PS released 2 equiv of H(2) in 171 min at 85 degrees C and only 9 min at 110 degrees C, whereas comparable reactions without PS required 316 min at 85 degrees C and 20 min at 110 degrees C. Ionic-liquid solvents proved more favorable than tetraglyme since they reduced the formation of undesirable products such as borazine. Solid-state and solution (11)B NMR studies of PS-promoted reactions in progress support a reaction pathway involving initial AB deprotonation to form the H(3)BNH(2)(-) anion. This anion can then initiate AB dehydropolymerization to form branched-chain polyaminoborane polymers. Subsequent chain-branching and dehydrogenation reactions lead ultimately to a cross-linked polyborazylene-type product. AB dehydrogenation by lithium and potassium triethylborohydride was found to produce the stabilized Et(3)BNH(2)BH(3)(-) anion, with the crystallographically determined structure of the [Et(3)BNH(2)BH(3)](-)K(+).18-crown-6 complex showing that, following AB nitrogen-deprotonation by the triethylborohydride, the Lewis-acidic triethylborane group coordinated at the nitrogen. Model studies of the reactions of [Et(3)BNH(2)BH(3)](-)Li(+) with AB show evidence of chain-growth, providing additional support for a PS-promoted AB anionic dehydropolymerization H(2)-release process.

Ammonia Borane Hydrogen Release in Ionic Liquids

The rate and extent of H(2)-release from ammonia borane (AB), a promising, high-capacity hydrogen storage material, was found to be enhanced in ionic-liquid solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) (50:50-wt %) exhibited no induction period and released 1.0 H(2)-equiv in 67 min and 2.2 H(2)-equiv in 330 min at 85 degrees C, whereas comparable solid-state AB reactions at 85 degrees C had a 180 min induction period and required 360 min to release approximately 0.8 H(2)-equiv, with the release of only another approximately 0.1 H(2)-equiv at longer times. Significant rate enhancements for the ionic-liquid mixtures were obtained with only moderate increases in temperature, with, for example, a 50:50-wt % AB/bmimCl mixture releasing 1.0 H(2)-equiv in 5 min and 2.2 H(2)-equiv in only 20 min at 110 degrees C. Increasing the AB/bmimCl ratio to 80:20 still gave enhanced H(2)-release rates compared to the solid-state, and produced a system that achieved 11.4 materials-weight percent H(2)-release. Solid-state and solution (11)B NMR studies of AB H(2)-release reactions in progress support a mechanistic pathway involving: (1) ionic-liquid promoted conversion of AB into its more reactive ionic diammoniate of diborane (DADB) form, (2) further intermolecular dehydrocoupling reactions between hydridic B-H hydrogens and protonic N-H hydrogens on DADB and/or AB to form neutral polyaminoborane polymers, and (3) polyaminoborane dehydrogenation to unsaturated cross-linked polyborazylene materials.

The characterization of surface acetylene and ethylene species on Pt(111) by angle resolved photoemission using synchrotron radiation

Abstract Angle resolved photoemission using synchrotron radiation was employed to elucidate the molecular structure of the species present in the low and high temperature phases of ethylene and acetylene on Pt(111). The plane polarized nature of synchrotron radiation allows the use of simple symmetry arguments to determine the orientation of an adsorbed species relative to the surface. In the low temperature phases of acetylene and ethylene the data are consistent with the carbon-carbon bond axis being parallel to the surface in agreement with earlier work. The high temperature phases of both molecules were found to consist of identical surface complexes where the carbon-carbon bond axis is normal or nearly normal to the surface. The orbital symmetries determined from this study favor the ethylidyne structure originally proposed by Kesmodel et al.

Double-Walled Boron Nitride Nanotubes Grown by Floating Catalyst Chemical Vapor Deposition

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as the active elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralities are semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction with a floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotube production, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructures with high production volumes.

Syntheses and structural characterizations of anionic borane-capped ammonia borane oligomers: evidence for ammonia borane H2 release via a base-promoted anionic dehydropolymerization mechanism.

Studies of the activating effect of Verkades base, 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (VB), on the rate and extent of H(2) release from ammonia borane (AB) have led to the syntheses and structural characterizations of three anionic aminoborane chain-growth products that provide direct support for anionic dehydropolymerization mechanistic steps in the initial stages of base-promoted AB H(2) release reactions. The salt VBH(+)[H(3)BNH(2)BH(2)NH(2)BH(3)](-) (1) containing a linear five-membered anionic aminoborane chain was produced in 74% yield via the room-temperature reaction of a 3:1 AB/VB mixture in fluorobenzene solvent, while the branched and linear-chain seven-membered anionic aminoborane oligomers VBH(+)[HB(NH(2)BH(3))(3)](-) (2a) and VBH(+)[H(3)BNH(2)BH(2)NH(2)BH(2)NH(2)BH(3)](-) (2b) were obtained from VB/AB reactions carried out at 50 °C for 5 days when the AB/VB ratio was increased to 4:1. X-ray crystal structure determinations confirmed that these compounds are the isoelectronic and isostructural analogues of the hydrocarbons n-pentane, 3-ethylpentane, and n-heptane, respectively. The structural determinations also revealed significant interionic B-H···H-N dihydrogen-bonding interactions in these anions that could enhance dehydrocoupling chain-growth reactions. Such mechanistic pathways for AB H(2) release, involving the initial formation of the previously known [H(3)BNH(2)BH(3)](-) anion followed by sequential dehydrocoupling of B-H and H-N groups of growing borane-capped aminoborane anions with AB, are supported by the fact that 1 was observed to react with an additional AB equivalent to form 2a and 2b.

Ammonia Triborane: A New Synthesis, Structural Determinations, and Hydrolytic Hydrogen-Release Properties

Iodine oxidation of B(3)H(8)(-) in glyme solution to produce (glyme)B(3)H(7), followed by displacement of the coordinated glyme by reaction with anhydrous ammonia provides a safe and convenient preparation of ammonia triborane, NH(3)B(3)H(7) (1). X-ray crystallographic determinations and DFT computational studies of both NH(3)B(3)H(7) and the NH(3)B(3)H(7) x 18-crown-6 adduct demonstrate that while computations predict a symmetric single bridging-hydrogen conformation, NH(3)B(3)H(7) has a highly asymmetric structure in the solid-state that results from intermolecular N-H(+)...H(-)-B dihydrogen bonding interactions. Studies of its hydrolytic reactions have shown that upon the addition of acid or an appropriate transition metal catalyst, aqueous solutions of 1 rapidly release hydrogen, with 6.1 materials wt % H(2)-release being achieved from a 22.7 wt % aqueous solution of 1 at room temperature in the presence of 5 wt % Rh/Al(2)O(3) (1.1 mol% Rh). The rate of H(2)-release was controlled by both the catalyst loadings and temperature.

Second‐generation polymeric precursors for BN and SiNCB ceramic materials

Our recent work directed at the design, synthesis, characterization and applications of new types of polyborazylene and polyborosilazane polymers is reviewed with a focus on the use of these polymers as processable precursors to BN and SiNCB composites. A design strategy based on the controlled functionalization of preformed polymers with pendant groups of suitable compositions and crosslinking properties has been employed to yield second-generation dipentylamine‐polyborazylene (DPA) and pinacolborane‐hydridopolysilazane (PIN‐HPZ) polymers, which, unlike the parent polyborazylene (PB) and the borazine‐hydridopolysilazane (B‐HPZ) polymers, are stable as melts and can be easily melt-spun into polymer fibers. Subsequent pyrolyses of these polymer fibers then provide excellent routes to BN and SiNCB ceramic fibers.# 1998 John Wiley & Sons, Ltd.

Oxygen chemisorption on copper (110)

High resolution electron energy loss spectroscopy (EELS) and angle-resolved ultra-violet photoelectron spectroscopy (UPS) have been used: (1) to study a surface phonon of Cu(110) as a function of oxygen coverage, (2) to identify oxygen adsorption site(s) in the p(2×1)O, c(6×2)O, and disordered oxygen overlayer (formed by O2 exposure at 100 K), and (3) to determine whether molecular adsorption or dissociation of O2 followed by atomic adsorption occurs after oxygen exposure at 100 K. With EELS, a continuous shift in energy of the surface phonon as a function of oxygen exposure at 300 K is observed. Our EELS data for the p(2×1)O overlayer support previous reports of a single long-bridge adsorption site, while indicating two sites are populated in the c(6×2)O overlayer: a long-bridge site and a four-coordinated site. The long-bridge site is populated at all coverages while the four-coordinated sites is occupied only after high exposures (≥2×104 L) at room temperature, or after exposures >2 L at low temperature (100 K). For both conditions the oxygen coverages are greater than 0.5 monolayer. Also, EELS and complementary UPS data clearly show that oxygen adsorbs dissociatively on Cu(110) after O2 exposure at 100 K. At this temperature, LEED results indicate that the oxygen atoms are adsorbed without long-range order; however, local adsorption sites, which are similar to those in the c(6×2)O surface, are observed.

Polymeric precursors to boron based ceramics

Our recent work is described that h as resulted in the production of several new boron-polymer systems, including poly(viny1- borazine), poly(viny1pentaborane) and poly( borazylene) , that have proven to be high yield precursors to boron ceramics such as, boron nitride and boron carbide.

Direct insertion of transition metals into polyhedral carboranes. Structurally novel mono-, di-, and trimetallic small cage systems

Abstract : Metallocarboranes of iron, cobalt and nickel have been prepared by the direct reaction of the small polyhedral carboranes 1,5-C2B3H5, 1,6-C2B4H6, or 2,4-C2B5H7 with organometallic and metal carbonyl reagents in the gas phase or in solution, without the use of a prior cage-opening step. Novel 6-vertex cages as well as 7-vertex species were obtained, including mono-, di-, and trimetallocarborane species. Molecular structures were assigned to the new compounds on the basis of boron-11 and proton nmr spectra. Fine structure in the carborane CH proton nmr signals exhibited by several of the cobaltacarborane species was interpreted on the basis of H-C-B-H proton-proton coupling. (Modified author abstract)