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.

Utilizing the Combined Power of Theory and Experiment to Understand Molecular Structure – Solid-State and Gas-Phase Investigation of Morpholine Borane

The molecular structure of morpholine borane complex has been studied in the solid state and gas phase using single-crystal X-ray diffraction, gas electron diffraction, and computational methods. Despite both the solid-state and gas-phase structures adopting the same conformation, a definite decrease in the B–N bond length of the solid-state structure was observed. Other structural variations in the different phases are presented and discussed. To explore the hydrogen storage potential of morpholine borane, the potential energy surface for the uncatalyzed and BH3-catalyzed pathways, as well as the thermochemistry for the hydrogen release reaction, were investigated using accurate quantum chemical methods. It was observed that both the catalyzed and uncatalyzed dehydrogenation pathways are favourable, with a barrier lower than the B–N bond dissociation energy, thus indicating a strong propensity for the complex to release a hydrogen molecule rather than dissociate along the B–N bond axis. A minimal energy requirement for the dehydrogenation reaction has been shown. The reaction is close to thermoneutral as demonstrated by the calculated dehydrogenation reaction energies, thus implying that this complex could demonstrate potential for future on-board hydrogen generation.

Direct Experimental Observation of in situ Dehydrogenation of an Amine-Borane System Using Gas Electron Diffraction

In situ dehydrogenation of azetidine-BH₃, which is a candidate for hydrogen storage, was observed with the parent and dehydrogenated analogue subjected to rigorous structural and thermochemical investigations. The structural analyses utilized gas electron diffraction supported by high-level quantum calculations, while the pathway for the unimolecular hydrogen release reaction in the absence and presence of BH₃ as a bifunctional catalyst was predicted at the CBS-QB3 level. The catalyzed dehydrogenation pathway has a barrier lower than the predicted B-N bond dissociation energy, hence favoring the dehydrogenation process over the dissociation of the complex. The predicted enthalpy of dehydrogenation at the CCSD(T)/CBS level indicates that mild reaction conditions would be required for hydrogen release and that the compound is closer to thermoneutral than linear amine boranes. The entropy and free energy change for the dehydrogenation process show that the reaction is exergonic, energetically feasible, and will proceed spontaneously toward hydrogen release, all of which are important factors for hydrogen storage.

Dehydrogenation of Amine-Boranes Using p-Block Compounds

Amine-boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine-boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine-boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine-borane dehydrogenation and discuss the underlying mechanisms.

A Computational Study of the Reactivity of 3,5-(Oxo/Thioxo) Derivatives of 2,7-Dimethyl-1,2,4-Triazepines. Keto-Enol Tautomerization and Potential for Hydrogen Storage

The G4 level of theory was used to evaluate the acidity of a series of triazepines, that is, 3-thioxo-5-oxo-, 5-thioxo-3-oxo-, 3,5-dioxo-, and 3,5-dithioxo- derivatives of 2,7-dimethyl-[1,2,4]-triazepine. The ability of their available nitrogen lone pair to form a dative bond with BH₃ was also studied to highlight the resulting changes in acidity and to understand the behavior of the complexes formed. The effect of the substitution of sulfur by oxygen on the stability of the complex and the activation barrier of dehydrogenation was also evaluated. The formation of these triazepine:BH₃ complexes, accompanied by the loss of H₂ molecular hydrogen, is a strongly exothermic process. With one triazepine the pathway for H₂ elimination from [triazepine]-BH₃ is characterized by a small energy barrier ranging from 11 to 23 kJ/mol. The second H₂ elimination is relatively more energetic than the first one (∼27 kJ/mol). Because of the steric hindrance associated with the addition of two molecules of triazepine (triazepine)₂-BH₂, the third dehydrogenation step is relatively less favorable than the two preceding steps, particularly in the case of the 3,5-dithio- derivative. The potential energy surface associated with the dehydrogenation reaction of all triazepine derivatives was explored. The thermodynamic favorability reported in this study could allow triazepine-borane to be used as a material for H₂ storage applications.

Mono- and di-cationic hydrido boron compounds

Stable mono- and di-cationic hydrido boron compounds featuring CH₂BH₂(μ-H)BH₂CH₂ and CH₂BH(μ-H)₂BHCH₂ cores are readily accessible by dehydrogenative hydride abstractions. NBO calculations revealed the occurrence of two B–H–B 3c–2e bonds in the HB(μ-H)₂BH moiety of di-cations (7 and 8), where 43% is located at the H bridges and ∼28% at each boron atom.