Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH(2))(4)NH·BH(3), were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)·BH(3)]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH·BH(3) to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4) BH(2)NMe(2)BH(3)}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe(3))(2)}(2)] (M = Mg, Ca, Sr, Ba), with (CH(3))(2)NH·BH(3) and (i)Pr(2)NH·BH(3) reactivity consistent with successive amidoborane β-hydride elimination and [R(2)N[double bond, length as m-dash]BH(2)] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.     

Computational study of the release of H2 from ammonia borane dimer (BH3NH3)2 and its ion pair isomers

High-level electronic structure calculations have been used to map out the relevant portions of the potential energy surfaces for the release of H2 from dimers of ammonia borane, BH3NH3 (AB). Using the correlation-consistent aug-cc-pVTZ basis set at the second-order perturbation MP2 level, geometries of stationary points were optimized. Relative energies were computed at these points using coupled-cluster CCSD(T) theory with the correlation-consistent basis sets at least up to the aug-cc-pVTZ level and in some cases extrapolated to the complete basis set limit. The results show that there are a number of possible dimers involving different types of hydrogen-bonded interactions. The most stable gaseous phase (AB)2 dimer results from a head-to-tail cyclic conformation and is stabilized by 14.0 kcal/mol with respect to two AB monomers. (AB)2 can generate one or two H2 molecules via several direct pathways with energy barriers ranging from 44 to 50 kcal/mol. The diammoniate of diborane ion pair isomer, [BH4-][NH3BH2NH3+] (DADB), is 10.6 kcal/mol less stable than (AB)2 and can be formed from two AB monomers by overcoming an energy barrier of approximately 26 kcal/mol. DADB can also be generated from successive additions of two NH3 molecules to B2H6 and from condensation of AB with separated BH3 and NH3 molecules. The pathway for H2 elimination from DADB is characterized by a smaller energy barrier of 20.1 kcal/mol. The alternative ion pair [NH4+][BH3NH2BH3-] is calculated to be 16.4 kcal/mol above (AB)2 and undergoes H2 release with an energy barrier of 17.7 kcal/mol. H2 elimination from both ion pair isomers yields the chain BH3NH2BH2NH3 as product. Our results suggest that the neutral dimer will play a minor role in the release of H2 from ammonia borane, with a dominant role from the ion pairs as observed experimentally in ionic liquids and the solid state.     

Formation and hydrogen release of hydrazine bisborane: transfer vs. attachment of a borane

The reactivity of hydrazine in the presence of diborane has been investigated using ab initio quantum chemical computations (MP2 and CCSD(T) methods with the aug-cc-pVTZ basis set). Portions of the relevant potential energy surface were constructed to probe the formation mechanism of the hydrazine diborane (BH(3)BH(3)NH(2)NH(2)) and hydrazine bisborane (BH(3)NH(2)NH(2)BH(3)). The differences between both adducts are established. The release of hydrogen molecules from hydrazine bisborane adducts has also been characterized. Our results suggest that the BH(3)NH(2)NH(2)BH(3) adduct, which has been prepared experimentally, is formed from the starting reactants hydrazine + diborane. The observed adduct is produced by a transfer of a BH(3) group from BH(3)BH(3)NH(2)NH(2) rather than by the direct attachment of a separate BH(3) group, generated by predissociation of diborane, to BH(3)NH(2)NH(2).     

Calcium-amidoborane-ammine complexes: thermal decomposition of model systems

Hydrocarbon-soluble model systems for the calcium-amidoborane-ammine complex Ca(NH(2)BH(3))(2)?(NH(3))(2) were prepared and structurally characterized. The following complexes were obtained by the reaction of RNH(2)BH(3) (R = H, Me, iPr, DIPP; DIPP = 2,6-diisopropylphenyl) with Ca(DIPP-nacnac)(NH(2))?(NH(3))(2) (DIPP-nacnac = DIPP-NC(Me)CHC(Me)N-DIPP): Ca(DIPP-nacnac)(NH(2)BH(3))?(NH(3))(2), Ca(DIPP-nacnac)(NH(2)BH(3))?(NH(3))(3), Ca(DIPP-nacnac)[NH(Me)BH(3)]?(NH(3))(2), Ca(DIPP-nacnac)[NH(iPr)BH(3)]?(NH(3))(2), and Ca(DIPP-nacnac)[NH(DIPP)BH(3)]?NH(3). The crystal structure of Ca(DIPP-nacnac)(NH(2)BH(3))?(NH(3)(3) showed a NH(2)BH(3)(-) unit that was fully embedded in a network of BH???HN interactions (range: 1.97(4)-2.39(4)??) that were mainly found between NH(3) ligands and BH(3) groups. In addition, there were N-H???C interactions between NH(3) ligands and the central carbon atom in the ligand. Solutions of these calcium-amidoborane-ammine complexes in benzene were heated stepwise to 60?°C and thermally decomposed. The following main conclusions can be drawn: 1)?Competing protonation of the DIPP-nacnac anion by NH(3) was observed; 2)?The NH(3) ligands were bound loosely to the Ca(2+) ions and were partially eliminated upon heating. Crystal structures of [Ca(DIPP-nacnac)(NH(2)BH(3))?(NH(3))](∞), Ca(DIPP-nacnac)(NH(2)BH(3))?(NH(3))?(THF), and [Ca(DIPP-nacnac){NH(iPr)BH(3)}](2) were obtained. 3)?Independent of the nature of the substituent R in NH(R)BH(3), the formation of H(2) was observed at around 50?°C. 4)?In all cases, the complex [Ca(DIPP-nacnac)(NH(2))](2) was formed as a major product of thermal decomposition, and its dimeric nature was confirmed by single-crystal analysis. We proposed that thermal decomposition of calcium-amidoborane-ammine complexes goes through an intermediate calcium-hydride-ammine complex which eliminates hydrogen and [Ca(DIPP-nacnac)(NH(2))](2). It is likely that the formation of metal amides is also an important reaction pathway for the decomposition of metal-amidoborane-ammine complexes in the solid state.     

Dehydrogenation mechanisms and thermodynamics of MNH2BH3 (M=Li, Na) metal amidoboranes as predicted from first principles

Electronic structure calculations have been used to determine and compare the thermodynamics of H(2) release from ammonia borane (NH(3)BH(3)), lithium amidoborane (LiNH(2)BH(3)), and sodium amidoborane (NaNH(2)BH(3)). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B-N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H(2) release and the activation energy for dehydrogenation are much lower for NH(3)BH(3) than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH(3)BH(3), 2×LiNH(2)BH(3)) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H(2) release from the 2×NH(3)BH(3) complex involves the formation of a diammoniate of diborane complex ([BH(4)](-)[NH(3)BH(2)NH(3)](+)). A new mechanism is found for dehydrogenation from the 2×LiNH(2)BH(3) dimer that involves the formation of an analogous dibroane complex ([BH(4)](-)[LiNH(2)BH(2)LiNH(2)](+)), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H(2) from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li-H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes.     

Na[Li(NH2BH3)2]--the first mixed-cation amidoborane with unusual crystal structure

We describe the successful synthesis of the first mixed-cation (pseudoternary) amidoborane, Na[Li(NH(2)BH(3))(2)], with theoretical hydrogen capacity of 11.1 wt%. Na[Li(NH(2)BH(3))(2)] crystallizes triclinic (P1) with a = 5.0197(4) ?, b = 7.1203(7) ?, c = 8.9198(9) ?, α = 103.003(6)°, β = 102.200(5)°, γ = 103.575(5)°, and V = 289.98(5) ?(3) (Z = 2), as additionally confirmed by Density Functional Theory calculations. Its crystal structure is topologically different from those of its orthorhombic LiNH(2)BH(3) and NaNH(2)BH(3) constituents, with distinctly different coordination spheres of Li (3 N atoms and 1 hydride anion) and Na (6 hydride anions). Na[Li(NH(2)BH(3))(2)], which may be viewed as a product of a Lewis acid (LiNH(2)BH(3))/Lewis base (NaNH(2)BH(3)) reaction, is an important candidate for a novel lightweight hydrogen storage material. The title material decomposes at low temperature (with onset at 75 °C, 6.0% mass loss up to 110 °C, and an additional 3.0% up to 200 °C) while evolving hydrogen contaminated with ammonia.     

Synthesis and characterization of methylammonium borohydride

A new borohydride, [CH(3)NH(3)](+)[BH(4)](-), has been synthesized through the metathesis of CH(3)NH(3)F and NaBH(4) in methylamine. Room-temperature X-ray diffraction studies have shown that [CH(3)NH(3)](+)[BH(4)](-) adopts a tetragonal unit cell with considerable hydrogen mobility similar to that observed in NH(3)BH(3). The kinetics and thermodynamics of hydrogen release have been investigated and were found to follow a similar pathway to that of [NH(4)](+)[BH(4)](-). Decomposition of [CH(3)NH(3)](+)[BH(4)](-) occurred slowly at room temperature and rapidly at ca. 40 °C to form [BH(2)(CH(3)NH(2))(2)](+)[BH(4)](-), the methylated analogue of the diammoniate of diborane. The decomposition has been investigated by means of in situ X-ray diffraction and solid state (11)B NMR spectroscopy and occurred in the absence of any detectable intermediates to form crystalline [BH(2)(CH(3)NH(2))(2)](+)[BH(4)](-). [(CH(3))(2)NH(2)](+)[BH(4)](-) and [BH(2){(CH(3))(2)NH}(2)](+)[BH(4)](-) have also been synthesized through analogous routes, indicating a more general applicability of the synthetic method.     

Role of NH3 in the dehydrogenation of calcium amidoborane ammoniate and magnesium amidoborane ammoniate: a first-principles study

First-principles calculations show that [NH(3)] molecules play crucial roles as both activator for the break-up of B-H bond and supplier of protic H for the establishment of dihydrogen bonding, which could facilitate the dehydrogenation of Ca(NH(2)BH(3))(2)·2NH(3) or Mg(NH(2)BH(3))(2)·NH(3) occurring at lower temperatures compared to those of Ca(NH(2)BH(3))(2) and Mg(NH(2)BH(3))(2). Moreover, the calculations of Helmholtz Free energy and [NH(3)] molecule removal energy evidence that coordination between [NH(3)] and Mg cation is stronger than that between [NH(3)] and Ca cation; therefore, Mg(NH(2)BH(3))(2)·NH(3) will undergo directly dehydrogenation rather than deammoniation at lower temperatures.     

The diammoniate of diborane: crystal structure and hydrogen release

[(NH(3))(2)BH(2)](+)[BH(4)](-) is formed from the room temperature decomposition of NH(4)(+)BH(4)(-), via a NH(3)BH(3) intermediate. Its crystal structure has been determined and contains disordered BH(4)(-) ions in 2 distinct sites. Hydrogen release is similar to that from NH(3)BH(3) but with faster kinetics.     

Combined formation and decomposition of dual-metal amidoborane NaMg(NH2BH3)3 for high-performance hydrogen storage

As a consequence of the combination of the formation and decomposition reactions of NaMg(NH(2)BH(3))(3), the 3NH(3)BH(3)/NaMgH(3) mixture can rapidly release ca. 10 wt.% of hydrogen at 80 °C within 2 min.     

A new ammine dual-cation (Li, Mg) borohydride: synthesis, structure, and dehydrogenation enhancement

A new ammine dual-cation borohydride, LiMg(BH(4))(3)(NH(3))(2), has been successfully synthesized simply by ball-milling of Mg(BH(4))(2) and LiBH(4)·NH(3). Structure analysis of the synthesized LiMg(BH(4))(3)(NH(3))(2) revealed that it crystallized in the space group P6(3) (no. 173) with lattice parameters of a=b=8.0002(1) ?, c=8.4276(1) ?, α=β=90°, and γ=120° at 50 °C. A three-dimensional architecture is built up through corner-connecting BH(4) units. Strong N-H···H-B dihydrogen bonds exist between the NH(3) and BH(4) units, enabling LiMg(BH(4))(3)(NH(3))(2) to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH(4))(3)(NH(3))(2)/LiBH(4) composite is able to release over 8 wt% hydrogen below 200 °C, which is comparable to that released by Mg(BH(4))(3)(NH(3))(2). More importantly, it was found that release of the byproduct NH(3) in this system can be completely suppressed by adjusting the ratio of Mg(BH(4))(2) and LiBH(4)·NH(3). This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems.     

Li-Na ternary amidoborane for hydrogen storage: experimental and first-principles study

Li-Na ternary amidoborane, Na[Li(NH(2)BH(3))(2)], was recently synthesized by reacting LiH and NaH with NH(3)BH(3). This mixed-cation amidoborane shows improved dehydrogenation performance compared to that of single-cation amidoboranes, i.e., LiNH(2)BH(3) and NaNH(2)BH(3). In this paper, we synthesized the Li-Na ternary amidoborane by blending and re-crystallizing equivalent LiNH(2)BH(3) and NaNH(2)BH(3) in tetrahydrofuran (THF), and employed first-principles calculations and the special quasirandom structure (SQS) method to theoretically explore the likelihood for the existence of Li(1-x)Na(x)(NH(2)BH(3)) for various Li/Na ratios. The thermodynamic, electronic and phononic properties were investigated to understand the possible dehydrogenation mechanisms of Na[Li(NH(2)BH(3))(2)].     

Reactions of diborane with ammonia and ammonia borane: catalytic effects for multiple pathways for hydrogen release

High-level electronic structure calculations have been used to construct portions of the potential energy surfaces related to the reaction of diborane with ammonia and ammonia borane (B2H6 + NH3 and B2H6 + BH3NH3)to probe the molecular mechanism of H2 release. Geometries of stationary points were optimized at the MP2/aug-cc-pVTZ level. Total energies were computed at the coupled-cluster CCSD(T) theory level with the correlation-consistent basis sets. The results show a wide range of reaction pathways for H2 elimination. The initial interaction of B2H6 + NH3 leads to a weak preassociation complex, from which a B-H-B bridge bond is broken giving rise to a more stable H3BHBH2NH3 adduct. This intermediate, which is also formed from BH3NH3 + BH3, is connected with at least six transition states for H2 release with energies 18-93 kal/mol above the separated reactants. The lowest-lying transition state is a six-member cycle, in which BH3exerts a bifunctional catalytic effect accelerating H2 generation within a B-H-H-N framework. Diborane also induces a catalytic effect for H2 elimination from BH3NH3 via a three-step pathway with cyclic transition states. Following conformational changes, the rate-determining transition state for H2 release is approximately 27 kcal/mol above the B2H6 + BH3NH3 reactants, as compared with an energy barrier of approximately 37 kcal/mol for H2 release from BH3NH3. The behavior of two separated BH3 molecules is more complex and involves multiple reaction pathways. Channels from diborane or borane initially converge to a complex comprising the H3BHBH2NH3adduct plus BH3. The interaction of free BH3 with the BH3 moiety of BH3NH3 via a six-member transition state with diborane type of bonding leads to a lower-energy transition state. The corresponding energy barrier is approximately 8 kcal/mol, relative to the reference point H3BHBH2NH3 adduct + BH3. These transition states are 27-36 kcal/mol above BH3NH3 + B2H6, but 1-9 kcal/mol below the separated reactants BH3NH3 + 2 BH3. Upon chemical activation of B2H6 by forming 2 BH3, there should be sufficient internal energy to undergo spontaneous H2 release. Proceeding in the opposite direction, the H2 regeneration of the products of the B2H6 + BH3NH3reaction should be a feasible process under mild thermal conditions.     

Anti and gauche conformers of an inorganic butane analogue, NH3BH2NH2BH3

The crystal structures of an inorganic butane analogue, NH(3)BH(2)NH(2)BH(3) (DDAB), were determined using single crystal X-ray diffraction, revealing both anti and gauche conformations. The anti conformation is stabilized by intermolecular dihydrogen bonds in the crystal whereas two gauche conformations of DDAB observed in its 18-crown-6 adducts are stabilized by an intramolecular dihydrogen bond. The two gauche conformations show rotational isomerization but whether they are a pair of enantiomers is yet to be defined.     

Experimental and computational study of the formation mechanism of the diammoniate of diborane: the role of dihydrogen bonds

The mechanism of formation of ammonia borane (NH(3)BH(3), AB) and the diammoniate of diborane ([H(2)B(NH(3))(2)][BH(4)], DADB) in the reaction between NH(3) and THF·BH(3) was explored experimentally and computationally. Ammonia diborane (NH(3)BH(2)(μ-H)BH(3), AaDB), a long-sought intermediate proposed for the formation of DADB, was directly observed in the reaction using (11)B NMR spectroscopy. The results indicate that dihydrogen bonds between the initially formed AB and AaDB accelerate the formation of DADB in competition with the formation of AB.     

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 Verkade's 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.     

Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties

Alkali- and alkaline-earth metal amidoboranes are a new class of compounds with rarely observed [NH2BH3](-) units. LiNH2BH3 and solvent-containing Ca(NH2BH3)2 x THF have been recently reported to significantly improve the dehydrogenation properties of ammonia borane. Therefore, metal amidoboranes, with accelerated desorption kinetics and suppressed toxic borazine, are of great interest for their potential applications for hydrogen storage. In this work, we successfully determined the structures of LiNH2BH3 and Ca(NH2BH3)2 using a combined X-ray diffraction and first-principles molecular dynamics simulated annealing method. Through detailed structural analysis and first-principles electronic structure calculations the improved dehydrogenation properties are attributed to the different bonding nature and reactivity of the metal amidoboranes compared to NH3BH3.     

Influence of hydrogen bonding on the assembly of six-membered vanadium borophosphate cluster anions: synthesis and structures of (NH4)2(C2H10N2)6[Sr(H2O)5]2[V2P2BO12](6)10H2O, (NH4)2(C3H12N2)6[Sr(H2O)4]2[V2P2BO12](6)17H2O, and (NH4)3(C4H14N2)4 5[Sr(H2O)5]2[Sr(H2O)4][V2P2BO12]6 10H2O

Three new strontium vanadium borophosphate compounds, (NH4)2(C2H10N2)6[Sr(H2O)5]2[V2P2BO12]6 10H2O (Sr-VBPO1) (1), (NH4)2(C3H12N2)6[Sr(H2O)4]2[V2P2BO12]6 17H2O (Sr-VBPO2) (2), and (NH4)3(C4H14N2)4.5[Sr(H2O)5]2[Sr(H2O)4][V2P2BO12]6 10H2O (Sr-VBPO3) (3) have been synthesized by interdiffusion methods in the presence of diprotonated ethylenediamine, 1,3-diaminopropane, and 1,4-diaminobutane. Compound 1 has a chain structure, whereas 2 and 3 have layered structures with different arrangements of [(NH4) [symbol: see text] [V2P2BO12]6] cluster anions within the layers. Crystal data: (NH4)2(C2H10N2)6[Sr(H2O)5]2[V2P2BO12]6 10H2O, monoclinic, space group C2/c (no. 15), a = 21.552(1) A, b = 27.694(2) A, c = 20.552(1) A, beta = 113.650(1) degrees, Z = 4; (NH4)2(C3H12N2)6[Sr(H2O)4]2[V2P2BO12]6 17H2O, monoclinic, space group I2/m (no. 12), a = 15.7618(9) A, b = 16.4821(9) A, c = 21.112(1) A, beta = 107.473(1) degrees, Z = 2; (NH4)3(C4H14N2)4.5[Sr(H2O)5]2[Sr(H2O)4] [V2P2BO12]6 10H2O, monoclinic, space group C2/c (no. 15), a = 39.364(2) A, b = 14.0924(7) A, c = 25.342(1) A, beta = 121.259(1) degrees, Z = 4. The differences in the three structures arise from the different steric requirements of the amines that lead to different amine-cluster hydrogen bonds.     

Thermodynamic properties of molecular borane amines and the [BH4-][NH4+] salt for chemical hydrogen storage systems from ab initio electronic structure theory

The heats of formation for the borane amines BH3NH3, BH2NH2, and HBNH, tetrahedral BH4-, and the BN molecule have been calculated by using ab initio molecular orbital theory. Coupled cluster calculations with single and double excitations and perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation (in kcal/mol) at 0 K in the gas phase are Delta Hf(BH3NH3) = -9.1, Delta Hf(BH2NH2) = -15.9, Delta Hf(BHNH) = 13.6, Delta Hf(BN) = 146.4, and Delta Hf(BH4-) = -11.6. The reported experimental value for Delta Hf(BN) is clearly in error. The heat of formation of the salt [BH4-][NH4+](s) has been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both BH3NH3(g) and [BH4-][NH4+](s) can serve as good hydrogen storage systems which release H2 in a slightly exothermic process. The hydride affinity of BH3 is calculated to be 72.2 kcal/mol, in excellent agreement with the experimental value at 298 K of 74.2 +/- 2.8 kcal/mol.     

(CH3)2NH(CH2)2NH(CH3)2][(UO2)2F2(HPO4)2]: a new organically templated layered uranium phosphate fluoride--synthesis, structure, characterization, and ion-exchange reactions

A new organically templated layered uranium phosphate fluoride, [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)][(UO(2))(2)F(2)(HPO(4))(2)] has been synthesized by hydrothermal reaction of UO(3), H(3)PO(4), HF, and (CH(3))(2)NCH(2)CH(2)N(CH(3))(2) at 140 degrees C. [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)][(UO(2))(2)F(2)(HPO(4))(2)] has a layered crystal structure consisting of seven-coordinated UO(5)F(2) pentagonal bipyramids and four-coordinated HPO(4) tetrahedra. Each anionic layer containing three-, four-, and six-membered rings is separated by [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)](2+) cations. The [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)](2+) cations may be readily exchanged with the M(2+) ions (M = Ba, Sr and Ca) in water to give high crystalline AE(UO(2))(2)(PO(4))(2).6H(2)O (AE = Ca, Sr, Ba).