Mechanistic Study of LiNH₂BH₃ Formation from (LiH)₄ + NH₃BH₃ and Subsequent Dehydrogenation

Abstract

The formation of LiNH₂BH₃ from (LiH)₄ and NH₃BH₃ and the subsequent dehydrogenation have been studied computationally at the CCSD(T)/6-311++G(3d,2p)//MP2/6-311++G(2d,p) level. A cubic unit of (LiH)₄ is predicted to react readily with NH₃BH₃ to form LiNH₂BH₃ plus H₂. The (LiH)₄ tetramer enables dehydrogenation through the exchange of a hydride vertex of (LiH)₄ and NH₂BH₃⁻ where NH₂BH₃⁻ is formed when the hydride vertex of (LiH)₄ abstracts a proton from NH₃. The free energy of activation for loss of H₂ is reduced from 37.2 kcal/mol in NH₃BH₃ to 11.0 kcal/mol in (LiH)₄ + NH₃BH₃. Further, H₂ elimination from the (LiNH₂BH₃)₂ dimer is predicted to be much easier than from the monomer which may suggest a cooperative H₂-loss mechanism is possible in solid LiNH₂BH₃. While two molecules of H₂ can be lost reversibly from (LiNH₂BH₃)₂, loss of further H₂ molecules is more difficult but could occur if the lattice energy stabilization accompanying H₂ loss is sufficiently large.