The effect of the NH2 substituent on NH3: hydrazine as an alternative for ammonia in hydrogen release in the presence of boranes and alanes.

Potential energy surfaces for H(2) release from hydrazine interacting with borane, alane, diborane, dialane and borane-alane were constructed from MP2/aVTZ geometries and zero point energies with single point energies at the CCSD(T)/aug-cc-pVTZ level. With one borane or alane molecule, the energy barrier for H(2)-loss of approximately 38 or 30 kcal mol(-1) does not compete with the B-N or Al-N bond cleavage ( approximately 30 or approximately 28 kcal mol(-1)). The second borane or alane molecule can play the role of a bifunctional catalyst. The barrier energy for H(2)-elimination is reduced from 38 to 23 kcal mol(-1), or 30 to 20 kcal mol(-1) in the presence of diborane or dialane, respectively. The mixed borane-alane dimer reduces the barrier energy for H(2) release from hydrazine to approximately 17 kcal mol(-1). A systematic comparison with the reaction pathways from ammonia borane shows that hydrazine could be an alternative for ammonia in producing borane amine derivatives. The results show a significant effect of the NH(2) substituent on the relevant thermodynamics. The B-N dative bond energy of 31 kcal mol(-1) in NH(2)NH(2)BH(3) is approximately 5 kcal mol(-1) larger than that of the parent BH(3)NH(3). The higher thermodynamic stability could allow hydrazine-borane to be used as a material for certain energetic H(2) storage applications.