Topochemical dihydrogen to covalent bonding transformation in LiBH₄·TEA: A mechanistic study

The first mechanistic investigation of a topochemical dihydrogen to covalent bonding conversion is presented. Solid-state decomposition of the LiBH₄·TEA (TEA = triethanolamine) dihydrogen-bonded complex into a covalent material was studied using ¹¹B solid-state MAS NMR, FT-IR, XRD, and optical microscopy. The majority of this solid-state reaction occurs by nucleation and two-dimensional growth of the covalent product nuclei. Variable- temperature kinetics and H/D exchange experiments established that proton transfer between the OH groups of the TEA and the BH₄^- anions, at the reactant/product interface, is the rate-limiting step, with an associated activation barrier of 21.0 ± 2.4 kcal/mol. The activation parameters ΔH(+)and ΔS(+) for the same process were calculated to be 20.1 ± 2.4 kcal/mol and -16.8 ± 6.2 eu, respectively, comparable with the analogous values found for the aqueous hydrolysis of BH₄^- in neutral water, suggesting similar mechanisms for the solid and solution decompositions.