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

Abstract

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.

Early events in the mechanism of single-source chemical vapor deposition of zirconium and hafnium diboride: a computational investigation

“…The neutral ligand B 3 H 9 occurring in CpZr-3d has few precedents, despite extensive study of triboron intermediates in the pyrolysis of primary boranes. [62][63][64][65] The optimized structure of Hf-3e appears as Fig. 8(a).…”

The metal-mediated coupling of the diselenaundecaborate anion [Se2B9H10]−

“…Cartesian atomic coordinates for 1, the transition state for 1, and 2 are listed in Tables S5 to S7. 1.941 (4). Angles (°): B(8)-Se(8,9)-B(9) 54.81 (10), B(5)-Se( 6) B( 7) 95.03 (11).…”

“…This can be extended to form larger borane clusters as, in turn, the one electron bridging hydrogen atom in B2H6 or the B-H two-electron vectors can, in principle, be shared with a further BH3 moiety to afford B3H9 and, along the way, stable isolatable species can be formed by a spontaneous loss of dihydrogen to give more stable isolatable molecules such as B4H10 and larger boranes [3]. These can be achieved in practice by pyrolysis [3][4][5][6], or chemically, for example by the reaction of F3B.OEt3 with the [BH4] − anion. The strong {BF3} Lewis acid moiety abstracts a hydride anion, H − , to leave reactive {BH3} intermediate and thence B2H6, which can then sequentially aggregate by hydride abstraction up to [B11H14] − [7].…”

Unexpected cluster formation upon hydroboration of a neutral diborene with 9-BBN

“…Although triborane(7) (B 3 H 7 ) is experimentally unknown, recent theoretical studies have dealt with its role as a catalyst in the pyrolysis of tetraborane(10) (B 4 H 10 ). 27 Another way to describe the clusters 2a,b is as doubly basestabilised B 3 H 5 analogues similar to the B 3 H 6 + analogue presented by Himmel et al 28 The ab initio (MP2) calculations performed by Korkin, Schleyer and McKee provide a range of possible structures for B 3 H 5 consisting of three-membered boron rings. The global minimum geometry of B 3 H 5 possesses a C 2v symmetry and two bridging hydrogen atoms.…”