Early stages of LiX, BeH₂X, and BH₃X pyrolysis (X stands for NH₃ or OH₂): A theoretical study of weak dative complex stability

Abstract

In this work, we study theoretically the thermal decomposition of weak dative complexes (namely, the complexes between BH3, BeH2, or LiH and ammonia or water). The most accurate calculations have been carried out at the ~~4sdtq/6-311+ +G(3df,2p) level on MP2fu11/6-31c** fully optimized geometries. The transition structures for hydrogen evolution are described. The rate constants were obtained using conventional transition-state theory. Stability conditions for the complexes were considered. Water complexes are less stable than are the corresponding ammonia derivatives. Lithium complexes seem to be very unstable. It seems that beryllium compounds could be synthesized. Borane derivatives are known experimentally. We find, as observed, that the first hydrogen evolution occurs only above room temperature. Iminoborane seems not to be easily accessible under pyrolysis conditions. Other reactions require less activation energy. Finally, the early stages of the ammonia-borane pyrolysis and paths to open-chain polymers as well as to cyclic structures are described. 0 1993 John Wiley & Sons, Inc.

Combining Protons and Hydrides by Homogeneous Catalysis for Controlling the Release of Hydrogen from Ammonia–Borane: Present Status and Challenges

“…40.0 kcal/mol. 5 Incidentally, the N−B bond of AB cleaves at a cost of 23.0 kcal/mol. 6 Experimentally it is known that the onset of AB decomposition in solid state occurs at 120 °C under atmospheric pressure, whereas it was shown by Geanangel et al and other experimental groups that the decomposition of AB initiates at much lower temperatures in ether solvents.…”

“…Sana and co-workers used gas-phase computations to predict that the first equivalent of H 2 loss occurs through a very high activation energy of 40.0 kcal/mol. 5 Later, Dixon et al suggested that BH 3 produced from the dissociation of the N−B bond in AB, can act as a Lewis acidic catalyst and brings down the activation barrier (ca. 20.9 kcal/mol) for AB dehydrogenation (see Scheme 1).…”

“…However, it turns out that in a single molecule, the theoretically estimated barrier for the combination of proton and hydride to release H 2 is prohibitively high, ca. 40.0 kcal/mol . Incidentally, the N–B bond of AB cleaves at a cost of 23.0 kcal/mol .…”

“…The complex processes of solid- and solution-state decomposition of AB elicited considerable interest in the computational community, and a large body of information has accumulated over the past few years. ,, AB as a molecule is endowed with the presence of neighboring protons and hydrides located on N and B atoms, respectively . Intuitively, the combination of proton and a hydride would produce H 2 from AB yielding AOB as a side product.…”

“…Intuitively, the combination of proton and a hydride would produce H 2 from AB yielding AOB as a side product. Sana and co-workers used gas-phase computations to predict that the first equivalent of H 2 loss occurs through a very high activation energy of 40.0 kcal/mol . Later, Dixon et al suggested that BH 3 produced from the dissociation of the N–B bond in AB , can act as a Lewis acidic catalyst and brings down the activation barrier (ca.…”