Base-Promoted Ammonia Borane Hydrogen-Release

Abstract

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.

Accelerating the Understanding and Development of Hydrogen Storage Materials: A Review of the Five-Year Efforts of the Three DOE Hydrogen Storage Materials Centers of Excellence

“…However, both require that the substrate, ammonia borane in this case, be in a liquid state. A number of approaches to developing liquid storage materials were examined ranging from solutions of ammonia borane in water and in conventional organic solvents to more concentrated solutions of ammonia borane in IL (low melting organic salts) as demonstrated by Sneddon and his colleagues, [88][89][90][91][92] to various organic amine derivatives of ammonia borane that had melting points at or below room temperature in a few cases. The CHSCoE's research began by exploring the homogeneous catalyst approach owing to its experimental simplicity, and with the hope that this could guide future heterogeneous catalyst selection.…”

Amineborane Based Chemical Hydrogen Storage - Final Report

“…For example, the H 3 BNH 2 -Li + salt has previously been prepared in situ from AB and n-BuLi at 0 o C. 56 More recently, ball milling 55,58,59 and solution reactions 60,61 of AB with metal hydrides have been used to generate H 3 BNH 2 -M + (M = Li and Na) and (H 3 BNH 2 -) 2 Ca 2+ . We found 29 that AB deprotonation could also be easily achieved by its reaction with either lithium or potassium triethylborohydride (Equation 4.7), but that in these cases, a Et 3 B-stabilized anion was formed, with the observed resonances in the 11 B NMR spectra of the Li + (-7.5 (s) and -23.8 (q) ppm) and K + (-7.8 (s) and -23.6 (q) ppm) compounds being assigned based on the DFT/GIAO calculations to the BEt 3 -(calc -10.6) and terminal -BH 3 The structures of H 3 BNH 2 -M + (M = Li and Na) and (H 3 BNH 2 -) 2 Ca 2+ have recently been established from the analyses of their high resolution powder diffraction data, 59,61 and that of (H 3 BNH 2 -) 2 Ca 2+ #2THF by a single-crystal X-ray determination. 60 The crystallographic determination of the [Et 3 BNH 2 BH 3 ] -K + #18-crown-6 complex in Figure 4.17, 29 confirmed that following AB nitrogen-deprotonation by the triethylborohydride, the Lewis-acidic triethylborane group coordinated at the nitrogen.…”

“…We found 29 that AB deprotonation could also be easily achieved by its reaction with either lithium or potassium triethylborohydride (Equation 4.7), but that in these cases, a Et 3 B-stabilized anion was formed, with the observed resonances in the 11 B NMR spectra of the Li + (-7.5 (s) and -23.8 (q) ppm) and K + (-7.8 (s) and -23.6 (q) ppm) compounds being assigned based on the DFT/GIAO calculations to the BEt 3 -(calc -10.6) and terminal -BH 3 The structures of H 3 BNH 2 -M + (M = Li and Na) and (H 3 BNH 2 -) 2 Ca 2+ have recently been established from the analyses of their high resolution powder diffraction data, 59,61 and that of (H 3 BNH 2 -) 2 Ca 2+ #2THF by a single-crystal X-ray determination. 60 The crystallographic determination of the [Et 3 BNH 2 BH 3 ] -K + #18-crown-6 complex in Figure 4.17, 29 confirmed that following AB nitrogen-deprotonation by the triethylborohydride, the Lewis-acidic triethylborane group coordinated at the nitrogen. Electronic structure calculations on [H 2 NBH 3 ] -M + have predicted 59,61 that the B-H hydrogens in these complexes will be much more negatively charged and, as a result, have a higher bascity than in AB.…”

“…of H 2 can be achieved, rather than the 3 equiv. predicted by Equation 3.1, even with prolonged heating at 85 o C. 1,7 As a result, a number of approaches are now being explored to induce efficient AB H 2 -release, including, for example, activation by transition metal catalysts, [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] acid catalysts, 28 base catalysts, 29 and nano and meso-porous scaffolds. [30][31][32] Most of these additives use organic solvents either as the reaction medium or as the AB transport and loading method.…”

Moderate Temperature Dense Phase Hydrogen Storage Materials within the US Department of Energy (DOE) H2 Storage Program: Trends toward Future Development

“…Numerous examples of catalytically amended AB were demonstrated over the course of the CHSCoE. Homogenous catalysis efforts included Lewis acids [83], non-nucleophilic bases (i.e., Proton Sponge) [84], and various transition metals [83,[85][86][87][88]. One of the most effective homogenous catalysts was demonstrated by Baker et al [89] using nickel N-heterocyclic carbene (Ni-NHC) complex generated through an in situ reaction with bis(cyclooctadiene)Ni(0).…”