- Photolysis of argon matrices containing tribromoboron and dihydrogen: Synthesis of hydroboranes via dibromoboron
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Ultraviolet irradiation (λ = 254 nm) of argon matrices containing BBr3 and high concentrations of H2 produces an intermediate, BBr2, which reacts with H2 to form HBBr2, H2BBr, and HBr. The intermediate is completely destroyed by irradiation at 680 nm, the position of a broad absorption band, producing the same hydrogenation products that form by using ultraviolet radiation. The reaction of BBr2 with HD gives HBr and DBBr2 1.6 times more readily than DBr and HBBr2. The spectrum of BBr2 has been reported earlier; herein we reassign the totally symmetric B-Br stretching mode to a weak band at 551.0 cm-1. There is no evidence that either BBr2 or BBr3 forms a ground-state complex with hydrogen in spite of the fact that BBr2 forms more readily in the presence of H2 and that it is readily converted into HBBr2 and H2BBr by H2. However, a complex of hydrogen and BBr2 in its first excited state may lead to the hydrogenation products. The yield of H2BBr is greatest when the concentration of hydrogen in the matrix is greater than about 10 mol %. Both hydrogen atoms that make up the H2BBr come from the same molecule of hydrogen. Mixtures of H2 and D2 give H2BBr and D2BBr with only a trace of HDBBr whereas matrices containing HD produce only HDBBr.
- Moroz, Antoni,Sweany, Ray L.
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p. 5236 - 5242
(2008/10/08)
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- Reactions of the diboron tetrahalides B2Cl4 and B2Br4 with B5H9: Preparation and properties of the (dihaloboryl)pentaborane derivatives 1-BX2B5H8, (X = Br, Cl, F, OCH3, t-Bu, H) and synthesis of (BCl2)3B5H6
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The reactions of B2Cl4 with excess B5H9 yield 1-BCl2B5H8 (73%) while those of B2Br4 generate 1-BBr2B5H8 (80%). Ligand exchange of 1-BCl2B5H8 with excess BBr3 forms 1-BBr2B5H8 (86%), that with Hg(CF3)2 results in 1-BF2B5H8 (96%), that with CH3OH generates 1-B(OCH3)2B5H8 (46%), and that with Li(t-Bu) prepares B(t-Bu)(Cl)B5H8 (23%) and B(t-Bu)2B5H8 (20%). The relative thermal stabilities of these products are BF2B5H8 > BCl2B5H8 > BBr2B5H8 > B(OCH3)2B5H8 > B(t-Bu)2B5H8. All of these BX2B5H8 compounds (X = F, Cl, OCH3, t-Bu) decompose to form BX3 and B5H9 as the volatile products. Reactions of BCl2B5H8 with excess B2Cl4 yield (BCl2)3B5H6, a compound of limited thermal stability, but no evidence for further BCl2 substitution on the pentaborane cage was obtained. Reductions of BCl2B5H8 with LiBH4 in C6H5Cl or C6H4Cl2 form apparent equilibrium mixtures of 1:1′,2′-[B5H8][B2H5] and 1:1′-[B5H8][B2H5]. One or both of these compounds decompose with the evolution of B2H6, B5H9, and coupled pentaborane cages (B5H7)n, where n can be at least as large as 8. The 11B NMR and mass spectrometric evidence from the last reaction is consistent with the initial dimerization of the hexaborane 1-BH2B5H8, which is rapidly followed by the formation of 1:1′-[B5H8][B2H5], the cross product arising from the interaction of B2H6 with (BH2B5H8)2, and then isomerization of this heptaborane to 1:1′,2′-[B5H8][B2H5].
- Saulys,Morrison
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p. 4174 - 4179
(2008/10/08)
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- Gaseous boroxine: Mechanism of reaction with boron trihalides
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Low-pressure reactions of gaseous boroxine (H3B3O3) with boron trihalides to produce dihaloborane and boric oxide have been studied by infrared absorption techniques. Kinetic data indicate that the reaction is close to first order in boroxine pressure. The pressure dependence of BF3 in the BF3-H3B3O3 reaction is close to zero order for BF3 pressures above 10 mm but tends toward a higher order at lower pressures. Reactions are accelerated in cells coated with B2O3(s). Reaction rates increase in the order BF3 3 3. Spectra of products of the reaction of 10B-labeled compounds show that the boron atoms in H3B3O3 appear in HBX2 and the boron atom in BX3 appears in B2O3(s). A surface mechanism which accounts for the 10B distribution in the products is proposed.
- Nadler,Porter, Richard F.
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p. 1192 - 1196
(2008/10/08)
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