Theoretical investigations on the formation and dehydrogenation reaction pathways of H(NH₂BH₂)_nH (n = 1-4) oligomers: Importance of dihydrogen interactions

The H(NH₂BH₂)_nH oligomers are possible products from dehydrogenation of ammonia borane (NH₃BH₃) and ammonium borohydride (NH₄BH₄), which belong to a class of boron-nitrogen-hydrogen (BNH_x) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNH_x-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wave function theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH ₂BH₂)_nH (n = 1-4) oligomers, polyaminoborane (PAB), from NH₃BH₃ monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through computational transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH ₃BH₃ molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH₃BH₃ molecules. We find that the formation of H(NH₂BH₂) _n+1H oligomers occurs first through reactions of the H(NH ₂BH₂)_nH oligomers with BH₃ followed by reactions with NH₃ and the release of H₂, where the BH₃ and NH₃ intermediates are formed through dissociation of NH₃BH₃. We also find that the dimerization of the NH₃BH₃ molecules to form cyclic c-(NH₂BH ₂)₂ is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H₂ molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H₂), with the largest exothermicity for n = 3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (H_N) hydrogens interact with hydridic (H_B) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the R-BH₂(? ²-H₂) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane. © 2010 American Chemical Society.