Theoretical study of elementary reactions of dehydrogenation of magnesium, calcium, zinc, and beryllium amminoborate and amminoalanate complexes

Abstract

The potential energy surfaces along minimal-energy pathways for elementary reactions of hydrogen molecule elimination from the complex molecules of amminoborates and amminoalanates M(LH4)2(NH3) and M(LH4)2(NH3)2 and the [M(LH4)2(NH3)2]2 dimers (M = Be, Mg, Ca, or Zn; L = B or Al) have been calculated by the density functional theory B3LYP method. The equilibrium structures and normal mode frequencies for local minima and transition states corresponding to these molecules and their fragmentation products have been determined. The energies and activation barriers of different decomposition channels have been estimated. It has been demonstrated that the introduction of an ammonia molecule into the coordination sphere of magnesium reduces the barrier to dehydrogenation of ammine complexes as compared with the M(BH4)2 borates and M(AlH4)2 alanates. The barriers for the amminoalanates are lower than those for the amminoborates; however, the former are no lower than ∼30 kcal/mol and hinder the dehydrogenation of ammine complexes at the level of isolated molecules. Going from the M(LH4)2(NH3)2 monomers to the [M(LH4)2(NH3)2]2 dimers has little effect on the barriers. The trends in the behavior of the molecular characteristics of the complexes as we move down Group II of the Periodic table are considered.