Ionic liquids are shown to provide advantageous media for amineborane-based chemical hydrogen storage systems. Both the extent and rate of hydrogen release from ammonia borane dehydrogenation are significantly increased at 85, 90, and 95 degrees C when the reactions are carried out in 1-butyl-3-methylimidazolium chloride compared to analogous solid-state reactions. NMR studies in conjunction with DFT/GIAO chemical shift calculations indicate that both polyaminoborane and the diammoniate of diborane, [(NH3)2BH2+]BH4-, are initial products in the reactions.

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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 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.

A convenient and safe method for the synthesis of ammonia triborane is reported along with studies of its hydrolytic reactions that demonstrate ammonia triborane is both soluble and stable in water but that upon the addition of acid or an appropriate transition metal catalyst it rapidly releases hydrogen. These studies indicate that ammonia triborane is a promising material for chemical hydrogen storage applications.

Significant advantages result from combining the disparate hydrogen release pathways for ammonia-borane (AB) dehydrogenation using ionic liquids (ILs) and transition metal catalysts. With the RuCl(2)(PMe(3))(4) catalyst precursor, AB dehydrogenation selectivity and extent are maximized in an IL with a moderately coordinating ethylsulfate anion.

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