Evaluating the Role of Triborane(7) As Catalyst in the Pyrolysis of Tetraborane(10)

The initial steps in the B₄H₁₀ pyrolysis mechanism have been elucidated. The mechanism can be divided into three stages: initial formation of B₄H₈, production of volatile boranes with B₃H₇ acting as a catalyst, and formation of nonvolatile products. The first step is B₄H₁₀ decomposition to either B₄H₈/H₂ or B₃H₇/BH₃ where the free energy barrier for the first pathway is 5.6 kcal/mol higher (G4, 333 K) than the second pathway when transition state theory (TST) is used. When variation transition state theory (VTST) is used for formation of B₃H₇/BH₃, the two pathways become very similar in free energy with the B₄H₈/H₂ pathway becoming favored at G4 by 1.0 kcal/mol at 333 K (33.1 versus 34.1 kcal/mol). The experimental activation energy for B₄H₁₀ pyrolysis is about 10 kcal/mol lower than the calculated barrier for B₄H₁₀ → B₄H₈ + H₂, which indicates that this reaction is not the rate-determining step. We suggest that the rate-determining step is B₄H₁₀ + B₃H₇ → B₄H₈ + H₂ + B₃H₇ where B₃H₇ acts as a catalyst. The role of reactive boron hydrides such as B₃H₇ and B₄H₈ as catalysts in the build-up of larger boron hydrides may be more common than that previously considered.