Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H2N=BH2) Molecule

Aminoborane (H2NBH2) is an elusive entity and is thought to be produced during dehydropolymerization of ammonia borane, a molecule of prime interest in the field of chemical hydrogen storage. The entrapment of H2NBH2 through hydroboration of exogenous cyclohexene has emerged as a routine technique to infer if free H2NBH2 is produced or not during metal-catalyzed ammonia borane dehydrogenation reactions. But to date, the underlying mechanism of this trapping reaction remains unexplored. Herein, by using DFT calculations, we have investigated the mechanism of trapping of H2NBH2 by cyclohexene. Contrary to conventional wisdom, our study revealed that the trapping of H2NBH2 does not occur through direct hydroboration of H2NBH2 on the double bond of cyclohexene. We found that autocatalysis by H2NBH2 is crucial for the entrapment of another H2NBH2 molecule by cyclohexene. Additionally, nucleophilic assistance from the solvent is also implicated for the entrapment reaction carried out in nucleophilic solvents. In THF, the rate-determining barrier for formation of the trapping product was predicted to be 16.7kcalmol(-1) at M06L(CPCM) level of theory.