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.