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

Abstract

Potential energy surfaces for H₂ 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 H₂-loss of ∼38 or 30 kcal mol⁻¹ does not compete with the B–N or Al–N bond cleavage (∼30 or ∼28 kcal mol⁻¹). The second borane or alane molecule can play the role of a bifunctional catalyst. The barrier energy for H₂-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 H₂ release from hydrazine to ∼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 NH₂ substituent on the relevant thermodynamics. The B–N dative bond energy of 31 kcal mol⁻¹ in NH₂NH₂BH₃ is ∼5 kcal mol⁻¹ larger than that of the parent BH₃NH₃. The higher thermodynamic stability could allow hydrazine–borane to be used as a material for certain energetic H₂ storage applications.