In situ multinuclear NMR spectroscopic studies of the thermal decomposition of ammonia borane in solution

Selective dehydrogenation of ammonia borane to borazine and derivatives by rhodium olefin complexes

This report presents a selective synthetic approach towards borazine from ammonia borane using a dinuclear rhodium olefin homogeneous catalyst. The synthesis and spectroscopic characterization of a dirhodium ammonia borane complex as an intermediate provides insight into a possible mode of activation.

Study of the first step of hydrogen release in ammonia borane using high-resolution Raman spectroscopy and different heating ramps

Ammonia borane, as a source of hydrogen, has attracted much attention due to its high hydrogen content, low molecular weight, and high stability in solution. However, the process and enhancement of hydrogen release must be done practically under ambient conditions. For this work, Raman spectroscopy, principal component analysis (PCA), and molecular simulation were applied to study the hydrogen release process of ammonia borane. Three stages of release were observed from room temperature to 1300 °C. The shift, the appearance, and the disappearance of the Raman bands were evident in the whole process. In-situ monitoring of Raman and PCA, with four different heating rates between 70 and 130 °C, was done; ammonia borane showed visible variations in its first release step, in which a fast rate helped reduce distortion in the release process. Finally, molecular simulation of ammonia borane using the Density Functional Theory (DFT) in calculations showed that dihedral rotation and stretching of the hydrogen bonds can occur before the first release step.

Borophenes made easy

To date, the scalable synthesis of elemental two-dimensional materials beyond graphene still remains elusive. Here, we introduce a versatile chemical vapor deposition (CVD) method to grow borophenes, as well as borophene heterostructures, by selectively using diborane originating from traceable byproducts of borazine. Specifically, metallic borophene polymorphs were successfully synthesized on Ir(111) and Cu(111) single-crystal substrates and conjointly with insulating hexagonal boron nitride (hBN) to form atomically precise lateral borophene-hBN interfaces or vertical van der Waals heterostructures. Thereby, borophene is protected from immediate oxidation by a single hBN overlayer. The ability to synthesize high-quality borophenes with large single-crystalline domains in the micrometer scale by a straight-forward CVD approach opens up opportunities for the study of their fundamental properties and for device incorporation.

A comparison of hydrogen release kinetics from 5- and 6-membered 1,2-BN-cycloalkanes

The reaction order and Arrhenius activation parameters for spontaneous hydrogen release from cyclic amine boranes, i.e., BN-cycloalkanes, were determined for 1,2-BN-cyclohexane (1) and 3-methyl-1,2-BN-cyclopentane (2) in tetraglyme. Computational analysis identified a mechanism involving catalytic substrate activation by a ring-opened form of 1 or 2 as being consistent with experimental observations.

The reaction order and Arrhenius activation parameters for spontaneous hydrogen release from cyclic amine boranes, i.e., BN-cycloalkanes, were determined for 1,2-BN-cyclohexane (1) and 3-methyl-1,2-BN-cyclopentane (2) in tetraglyme.

Mechanistic insights into the thermal decomposition of ammonia borane, a material studied for chemical hydrogen storage

We have now a better understanding of the mechanisms of thermal decomposition of ammonia borane, a widely studied hydrogen storage material.

Catalytic Dehydrogenation of Ammonia Borane Mediated by a Pt(0)/Borane Frustrated Lewis Pair: Theoretical Design

A new efficient metal-based frustrated Lewis pair constructed by (P^t Bu₃ )₂ Pt and B(C₆ F₅ )₃ was designed through density functional theory calculations for the catalytic dehydrogenation of ammonia borane (AB). The reaction was composed by the successive dehydrogenation of AB and H₂ liberation, which occurs through the cooperative functions of the Pt(0) center and the B(C₆ F₅ )₃ moiety. Two equivalents of H₂ were predicted to be liberated from each AB molecule. The generation of the second H2 is the rate-determining step, with a Gibbs energy barrier and reaction energy of 27.4 and 12.8 kcal/mol, respectively.

An experimental approach for controlling confinement effects at catalyst interfaces

Catalysts are conventionally designed with a focus on enthalpic effects, manipulating the Arrhenius activation energy. This approach ignores the possibility of designing materials to control the entropic factors that determine the pre-exponential factor. Here we investigate a new method of designing supported Pt catalysts with varying degrees of molecular confinement at the active site. Combining these with fast and precise online measurements, we analyse the kinetics of a model reaction, the platinum-catalysed hydrolysis of ammonia borane. We control the environment around the Pt particles by erecting organophosphonic acid barriers of different heights and at different distances. This is done by first coating the particles with organothiols, then coating the surface with organophosphonic acids, and finally removing the thiols. The result is a set of catalysts with well-defined "empty areas" surrounding the active sites. Generating Arrhenius plots with >300 points each, we then compare the effects of each confinement scenario. We show experimentally that confining the reaction influences mainly the entropy part of the enthalpy/entropy trade-off, leaving the enthalpy unchanged. Furthermore, we find this entropy contribution is only relevant at very small distances (<3 Å for ammonia borane), where the "empty space" is of a similar size to the reactant molecule. This suggests that confinement effects observed over larger distances must be enthalpic in nature.

Ammonia Borane: An Extensively Studied, Though Not Yet Implemented, Hydrogen Carrier

Ammonia borane H3N−BH3 (AB) was re-discovered, in the 2000s, to play an important role in the developing hydrogen economy, but it has seemingly failed; at best it has lagged behind. The present review aims at analyzing, in the context of more than 300 articles, the reasons why AB gives a sense that it has failed as an anodic fuel, a liquid-state hydrogen carrier and a solid hydrogen carrier. The key issues AB faces and the key challenges ahead it has to address (i.e., those hindering its technological deployment) have been identified and itemized. The reality is that preventable errors have been made. First, some critical issues have been underestimated and thereby understudied, whereas others have been disproportionally considered. Second, the potential of AB has been overestimated, and there has been an undoubted lack of realistic and practical vision of it. Third, the competition in the field is severe, with more promising and cheaper hydrides in front of AB. Fourth, AB has been confined to lab benches, and consequently its technological readiness level has remained low. This is discussed in detail herein.

Boron: Its Role in Energy-Related Processes and Applications

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications.

Changes in microstructure of two ammonium-based protic ionic liquids proved by in situ variable-temperature 1 H NMR spectroscopy: influence of anion

In this work, changes in microstructure of two protic ionic liquids (PILs), namely n-butylammonium acetate (N4Ac) and n-butylammonium nitrate (N4NO₃ ), are proved by in situ variable-temperature ¹ H NMR spectroscopy at the temperature range from 25 to 115 °C, and the influence of the nature of anion is discussed accordingly. The results demonstrate that ¹ H NMR chemical shifts of alkyl protons of both N4Ac and N4NO₃ are almost not changed with the increasing of temperature, due to the absence of hydrogen bond interaction between alkyl protons with anions. Whereas those of ⁺ N-H of cation decrease linearly with the temperature increasing, indicating that the hydrogen bond interaction between ⁺ N-H and anion weakens gradually. In addition, the strength of hydrogen bond interaction between ⁺ N-H and NO₃^- is stronger than that between ⁺ N-H and Ac^- , suggesting that anions have a significant influence on microstructure due to the acidity of a Brønsted acid. Consequently, the proton transfer from cation to anion is much easier in N4Ac compared to N4NO₃ . Further analyses of ¹ H NMR chemical shifts of ⁺ N-H in N4Ac at the temperature range from 100 to 115 °C suggest that the splitting of ⁺ N-H peak may be attributed to obvious evidence of the existence of the proton transfer from ⁺ N-H to Ac^- , which leads to dissociate the contact ion-pair in N4Ac to form the neutral ion-pair 'molecule'. The results will help us to extensively understand the behavior of proton transfer and offer us some valuable information for the design of PILs. Copyright © 2017 John Wiley & Sons, Ltd.

Cleavage of Dinitrogen from Forming Gas by a Titanium Molecular System under Ambient Conditions

Simple exposure of a hexane solution of [TiCp*Me₃ ] (Cp*=η⁵ -C₅ Me₅ ) to an atmosphere of commercially available and inexpensive forming gas (H₂ /N₂ mixture, 13.5-16.5 % of H₂ ) at room temperature leads to the methylidene-methylidyne-nitrido cube-type complex [(TiCp*)₄ (μ₃ -CH)(μ₃ -CH₂ )(μ₃ -N)₂ ] via dinitrogen cleavage. This paramagnetic compound reacts with [D₁ ]chloroform to give the titanium(IV) methylidyne-nitrido species [(TiCp*)₄ (μ₃ -CH)₂ (μ₃ -N)₂ ], whereas its one-electron oxidation with AgOTf or [Fe(η⁵ -C₅ H₅ )₂ ](OTf) (OTf=O₃ SCF₃ ) yields the diamagnetic ionic derivative [(TiCp*)₄ (μ₃ -CH)(μ₃ -CH₂ )(μ₃ -N)₂ ](OTf). The μ₃ -nitrido ligands of the methylidyne-nitrido cubane complex can be protonated with [LutH](OTf) (Lut=2,6-lutidine) or hydrogenated with NH₃ ⋅BH₃ to afford μ₃ -NH imido moieties.

A non-dissociative open-flask hydroboration with ammonia borane: ready synthesis of ammonia-trialkylboranes and aminodialkylboranes

Under open-flask conditions, ammonia borane hydroborates olefins in refluxing tetrahydrofuran. Unlike conventional hydroboration, the Lewis base (ammonia) is not dissociated from the boron center. Terminal alkenes selectively provide ammonia-trialkylborane complexes. On the other hand, internal alkenes afford aminodialkylboranes via a metal-free hydroboration-dehydrogenation sequence. Alkaline hydrogen peroxide oxidation of the products provides the corresponding alcohols in high yields.

Synthesis of Ammonia Borane Nanoparticles and the Diammoniate of Diborane by Direct Combination of Diborane and Ammonia

Pure nanoparticle ammonia borane (NH3 BH3 , AB) was first prepared through a solvent-free, ambient-temperature gas-phase combination of B2 H6 with NH3 . The prepared AB nanoparticle exhibits improved dehydrogenation behavior giving 13.6 wt. % H2 at the temperature range of 80-175 °C without severe foaming. Ammonia diborane (NH3 BH2 (μ-H)BH3 , AaDB) is proposed as the intermediate in the reaction of B2 H6 with NH3 based on theoretical studies. This method can also be used to prepare pure diammoniate of diborane ([H2 B(NH3 )2 ][BH4 ], DADB) by adjusting the ratio and concentration of B2 H6 to NH3 .

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.

Study of the competitive mechanisms of cyclohexane dehydrogenation by gas-phase Ni2(+) cationic dimer: one-face dehydrogenation versus flip dehydrogenation

The mechanism of cyclohexane dehydrogenation catalyzed by the cationic dimer Ni2 (+) has been investigated at the B3LYP level of density functional theory. The first dehydrogenation occurs readily (it is exothermic by 30 kcal/mol), whereas the second and third dehydrogenations show weaker exothermicity than the first (23 and 21 kcal/mol, respectively). These three hydrogenations corresponding to the total dehydrogenation of one face of cyclohexane mainly proceed in the doublet state due to the presence of significant minimum-energy crossing points (MECPs). In addition, because the elimination of non-negligible amounts of [H2,2D2] and [2H2,D2] in this reaction was also observed in a previous experiment, we calculated a flip mechanism which would yield results that agree with those experimental results. This flip process includes two MECPs, meaning that the reaction mainly proceeds along the doublet potential energy surface but finishes in the quartet state. The rate-limiting step ((2)IM9 → (2)TS9/10 → (2)IM10) of the flip process is endothermic by 3 kcal/mol and the barrier to this step is 33 kcal/mol. Our calculations indicate that one-face dehydrogenation is a more favorable channel than the flip one. We excluded the possibility that eliminations of [H2,2D2] or [D2,2H2] could proceed through a mechanism involving Ni2 (+) dissociation, or that [H-D] scrambling could occur through (2)TS11/13 ((4)TS12/15), due to the large amounts of energy required. In the dissociation of (2)IM19, (2)[(H2)Ni2(C6H6)](+), a molecule of hydrogen first dissociates, leaving a final product of (2)[Ni2(C6H6)](+). Neither C6H6 nor (H2)Ni2 (+) can easily dissociate from (2)IM19 due to π backdonation.

Dehydrogenation of ammonia-borane by cationic Pd(ii) and Ni(ii) complexes in a nitromethane medium: hydrogen release and spent fuel characterization

This work provides a comprehensive experimental study on the mechanism of AB dehydrogenation with the [Pd(MeCN)₄][BF₄]₂ (1) catalyst.

Cobalt-catalyzed ammonia borane dehydrocoupling and transfer hydrogenation under aerobic conditions

Simple cobalt compounds engage in ammonia borane dehydrocoupling and transfer hydrogenation.

Probing the second dehydrogenation step in ammonia-borane dehydrocoupling: characterization and reactivity of the key intermediate, B-(cyclotriborazanyl)amine-borane

While thermolysis of ammonia-borane (AB) affords a mixture of aminoborane- and iminoborane oligomers, the most selective metal-based catalysts afford exclusively cyclic iminoborane trimer (borazine) and its B-N cross-linked oligomers (polyborazylene). This catalysed dehydrogenation sequence proceeds through a branched cyclic aminoborane oligomer assigned previously as trimeric B-(cyclodiborazanyl)amine-borane (BCDB). Herein we utilize multinuclear NMR spectroscopy and X-ray crystallography to show instead that this key intermediate is actually tetrameric B-(cyclotriborazanyl)amine-borane (BCTB) and a method is presented for its selective synthesis from AB. The reactivity of BCTB upon thermal treatment as well as catalytic dehydrogenation is studied and discussed with regard to facilitating the second dehydrogenation step in AB dehydrocoupling.

Molybdenum catalyzed ammonia borane dehydrogenation: oxidation state specific mechanisms

Though numerous catalysts for the dehydrogenation of ammonia borane (AB) are known, those that release >2 equiv of H2 are uncommon. Herein, we report the synthesis of Mo complexes supported by a para-terphenyl diphosphine ligand, 1, displaying metal-arene interactions. Both a Mo(0) N2 complex, 5, and a Mo(II) bis(acetonitrile) complex, 4, exhibit high levels of AB dehydrogenation, releasing over 2.0 equiv of H2. The reaction rate, extent of dehydrogenation, and reaction mechanism vary as a function of the precatalyst oxidation state. Several Mo hydrides (Mo(II)(H)2, [Mo(II)(H)](+), and [Mo(IV)(H)3](+)) relevant to AB chemistry were characterized.