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

The formation of LiNH(2)BH(3) from (LiH)(4) and NH(3)BH(3) 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 NH(3)BH(3) to form LiNH(2)BH(3) plus H(2). The (LiH)(4) tetramer enables dehydrogenation through the exchange of a hydride vertex of (LiH)(4) and NH(2)BH(3)(-) where NH(2)BH(3)(-) is formed when the hydride vertex of (LiH)(4) abstracts a proton from NH(3). The free energy of activation for loss of H(2) is reduced from 37.2 kcal/mol in NH(3)BH(3) to 11.0 kcal/mol in (LiH)(4) + NH(3)BH(3). Further, H(2) elimination from the (LiNH(2)BH(3))(2) dimer is predicted to be much easier than from the monomer which may suggest a cooperative H(2)-loss mechanism is possible in solid LiNH(2)BH(3). While two molecules of H(2) can be lost reversibly from (LiNH(2)BH(3))(2), loss of further H(2) molecules is more difficult but could occur if the lattice energy stabilization accompanying H(2) loss is sufficiently large.