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.

In situ dehydrogenation of azetidine-BH 3 , 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, whilst the pathway for the unimolecular hydrogen release reaction in the absence and presence of BH 3 as a bifunctional catalyst was predicted at 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 CCSD(T)/CBS level indicates mild reaction conditions would be required for the hydrogen release and that the compound is closer to thermoneutral than the 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 towards hydrogen release; all important factors for hydrogen storage.

The gaseous structures, thermochemical properties and dehydrogenation reaction energy profiles of the borane complexes of pyrrolidine and piperidine have been investigated using gas electron diffraction (GED) and state-of-the-art computational methods. These complexes are of interest because of their potential as hydrogen storage materials for future onboard transport applications. A comparative structural and thermochemical analysis revealed structures with a slight difference in the essential B-N bond length, with the piperidine borane having a longer bond even though it has a stronger B-N bond according to predicted bond dissociation energies, a trend common with amine boranes. To identify the most favourable dehydrogenation pathway, BH3-catalysed and uncatalysed dehydrogenation channels have been explored, where the former has been shown to be the favourable process for both complexes. The energy requirements for the hydrogen release reactions are expected to be minimal as evidenced from the calculated dehydrogenation reaction energies, implying their suitability for onboard chemical hydrogen storage.

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