Blending materials composed of boron, nitrogen and carbon to transform approaches to liquid hydrogen stores.

Mixtures of hydrogen storage materials containing the elements of boron, nitrogen, carbon, i.e., isomers of BN cyclopentanes are examined to find a 'fuel blend' that remains a liquid phase throughout hydrogen release, maximizes hydrogen storage density, minimizes impurities and remains thermally stable at ambient temperatures. We find that the mixture of ammonia borane dissolved in 3-methyl-1,2-dihydro-1,2-azaborolidine (compound B) provide a balance of these properties and provides ca. 5.6 wt% hydrogen. The two hydrogen storage materials decompose at a faster rate than either individually and products formed are a mixture of molecular trimers. Digestion of the product mixture formed from the decomposition of the AB + B fuel blend with methanol leads to the two corresponding methanol adducts of the starting material and not a complex mixture of adducts. The work shows the utility of using blends of materials to reduce volatile impurities and preserve liquid phase.

Heterolysis of H2 Across a Classical Lewis Pair, 2,6-Lutidine⋠BCl3 : Synthesis, Characterization, and Mechanism.

We report that 2,6-lutidine⋅trichloroborane (Lut⋅BCl3 ) reacts with H2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut⋅BHCl2 . The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut⋅BCl3 was calculated to react with H2 and form the ion pair, [LutH(+) ][HBCl3 (-) ], with a barrier of ΔH(≠) =24.7 kcal mol(-1) (ΔG(≠) =29.8 kcal mol(-1) ). Metathesis with a second molecule of Lut⋅BCl3 produced Lut⋅BHCl2 and [LutH(+) ][BCl4 (-) ]. The overall reaction is exothermic by 6.0 kcal mol(-1) (Δr G°=-1.1). Alternate pathways were explored involving the borenium cation (LutBCl2 (+) ) and the four-membered boracycle [(CH2 {NC5 H3 Me})BCl2 ]. Barriers for addition of H2 across the Lut/LutBCl2 (+) pair and the boracycle BC bond are substantially higher (ΔG(≠) =42.1 and 49.4 kcal mol(-1) , respectively), such that these pathways are excluded. The barrier for addition of H2 to the boracycle BN bond is comparable (ΔH(≠) =28.5 and ΔG(≠) =32 kcal mol(-1) ). Conversion of the intermediate 2-(BHCl2 CH2 )-6-Me(C5 H3 NH) to Lut⋅BHCl2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl3 . Intramolecular protodeboronation, which could form Lut⋅BHCl2 directly, is prohibited by a high barrier (ΔH(≠) =52, ΔG(≠) =51 kcal mol(-1) ).

Experimental and theoretical study of molecular response of amine bases in organic solvents.

Reorientational correlation times of various amine bases (namely, pyridine, 2,6-lutidine, 2,2,6,6-tetramethylpiperidine) and organic solvents (dichloromethane, toluene) were determined by solution-state NMR relaxation time measurements and compared with predictions from molecular dynamics (MD) simulations. The amine bases are reagents in complex reactions catalyzed by frustrated Lewis pairs (FLP), which display remarkable activity in metal-free H2 scission. The comparison of measured and simulated correlation times is a key test of the ability of recent MD and quantum electronic structure calculations to elucidate the mechanism of FLP activity. Correlation times were found to be in the range of 1.4-3.4 (NMR) and 1.23-5.28 ps (MD) for the amines and 0.9-2.3 (NMR) and 0.2-1.7 ps (MD) for the solvent molecules.

A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane-amine Lewis pairs.

Calorimetry is used to measure the reaction enthalpies of hydrogen (H(2)) activation by 2,6-lutidine (Lut), 2,2,6,6-tetramethylpiperidine (TMP), N-methyl-2,2,6,6-tetramethylpiperidine (MeTMP), and tri-tert-butylphosphine (TBP) with tris(pentafluorophenyl)borane (BCF). At 6.6 bar H(2) the conversion of the Lewis acid Lewis base pair to the corresponding ionic pair in bromobenzene at 294 K was quantitative in under 60 min. Integration of the heat release from the reaction of the Frustrated Lewis Pair (FLP) with H(2) as a function of time yields a relative rate of hydrogenation in addition to the enthalpy of hydrogenation. The half-lives of hydrogenation range from 230 s with TMP, ΔH(H2) = -31.5(0.2) kcal mol(-1), to 1400 s with Lut, ΔH(H2) = -23.4(0.4) kcal mol(-1). The (11)B nuclear magnetic resonance (NMR) spectrum of B(C(6)F(5))(3) in bromobenzene exhibits three distinct traits depending on the sterics of the Lewis base; (1) in the presence of pyridine, only the dative bond adduct pyridine-B(C(6)F(5))(3) is observed; (2) in the presence of TMP and MeTMP, only the free B(C(6)F(5))(3) is observed; and (3) in the presence of Lut, both the free B(C(6)F(5))(3) and the Lut-B(C(6)F(5))(3) adduct appear in equilibrium. A measure of the change in K(eq) of Lut + B(C(6)F(5))(3) ⇔ Lut-B(C(6)F(5))(3) as a function of temperature provides thermodynamic properties of the Lewis acid Lewis base adduct, ΔH = -17.9(1.0) kcal mol(-1) and a ΔS = -49.2(2.5) cal mol(-1) K, suggesting the Lut-B(C(6)F(5))(3) adduct is more stable in bromobenzene than in toluene.

3-Methyl-1,2-BN-cyclopentane: a promising H2 storage material?

We provide detailed characterization of properties for 3-methyl-1,2-BN-cyclopentane 1 that are relevant to H(2) storage applications such as viscosity, thermal stability, H(2) gas stream purity, and polarity. The viscosity of 1 at room temperature is 25 ± 5 cP, about one fourth the viscosity of olive oil. TGA/MS analysis indicates that liquid carrier 1 is thermally stable at 30 °C but decomposes slowly at 50 °C. RGA data suggest that the H(2) desorption from 1 is a clean process, producing relatively pure H(2) gas. Compound 1 is a polar zwitterionic-type liquid consistent with theoretical predictions and solvatochromic studies.

Luminescent Diazaborolyl-Functionalized Polystyrene.

We present two different procedures for the synthesis of poly[4-(1',3'-diethyl-1',3',2'-benzodiazaborolyl)styrene] (3a) and poly[4-(1',3'-diphenyl-1',3',2'-benzodiazaborolyl)styrene] (3b). The new polymers were fully characterized by GPC, multinuclear NMR, and elemental analysis. The thermal properties and stability were studied by DSC and TGA, and the optical characteristics were examined by absorption and time-resolved fluorescence spectroscopy. Remarkably high quantum yields of up to 77% were measured. In comparison to molecular species we found significantly shorter lifetimes, likely as a result of incorporation of the chromophores into the polymer structure.

Luminescent triarylborane-functionalized polystyrene: synthesis, photophysical characterization, and anion-binding studies.

A new class of highly fluorescent triarylborane polymers has been prepared from trimethylsilyl-substituted polystyrene via a modular approach that involves selective polymer modification reactions with organometallic reagents. The photophysical properties, environmental stability, and the Lewis acidity of the boron sites have been tailored through modifications in the substitution pattern on boron. The photophysical properties are indicative of electronic communication between the chromophores attached to polystyrene, which has been exploited for the efficient probing of fluoride and cyanide in the micromolar concentration range.