Reactions of Diborane with Ammonia and Ammonia Borane: Catalytic Effects for Multiple Pathways for Hydrogen Release

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Abstract

High-level electronic structure calculations have been used to construct portions of the potential energy surfaces related to the reaction of diborane with ammonia and ammonia borane (B2H6 + NH3 and B2H6 + BH3NH3)to probe the molecular mechanism of H2 release. Geometries of stationary points were optimized at the MP2/aug-cc-pVTZ level. Total energies were computed at the coupled-cluster CCSD(T) theory level with the correlation-consistent basis sets. The results show a wide range of reaction pathways for H2 elimination. The initial interaction of B2H6 + NH3 leads to a weak preassociation complex, from which a B-H-B bridge bond is broken giving rise to a more stable H3BHBH2NH3 adduct. This intermediate, which is also formed from BH3NH3 + BH3, is connected with at least six transition states for H2 release with energies 18-93 kal/mol above the separated reactants. The lowest-lying transition state is a six-member cycle, in which BH3exerts a bifunctional catalytic effect accelerating H2 generation within a B-H-H-N framework. Diborane also induces a catalytic effect for H2 elimination from BH3NH3 via a three-step pathway with cyclic transition states. Following conformational changes, the rate-determining transition state for H2 release is approximately 27 kcal/mol above the B2H6 + BH3NH3 reactants, as compared with an energy barrier of approximately 37 kcal/mol for H2 release from BH3NH3. The behavior of two separated BH3 molecules is more complex and involves multiple reaction pathways. Channels from diborane or borane initially converge to a complex comprising the H3BHBH2NH3adduct plus BH3. The interaction of free BH3 with the BH3 moiety of BH3NH3 via a six-member transition state with diborane type of bonding leads to a lower-energy transition state. The corresponding energy barrier is approximately 8 kcal/mol, relative to the reference point H3BHBH2NH3 adduct + BH3. These transition states are 27-36 kcal/mol above BH3NH3 + B2H6, but 1-9 kcal/mol below the separated reactants BH3NH3 + 2 BH3. Upon chemical activation of B2H6 by forming 2 BH3, there should be sufficient internal energy to undergo spontaneous H2 release. Proceeding in the opposite direction, the H2 regeneration of the products of the B2H6 + BH3NH3reaction should be a feasible process under mild thermal conditions.

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