Catalytic role of borane and alane in hydrogen release from cyclic amine adducts CnH2n+1N•XH3 [X = B, Al; n=2-5]: a theoretical interpretation

The reaction pathways for H-2 release from cyclic amine-alane adducts in the presence and absence of alane (AlH3) are investigated and the associated potential energy surfaces are constructed by employing the composite G4MP2 method. The computed Gibbs free energy change values are potential proof of the thermodynamic feasibility of the unimolecular dehydrogenation reactions. Unlike cyclic amine-borane adducts, the unimolecular dehydrogenation barrier in cyclic amine-alane adducts is found to be lower than the Al-N bond dissociation energy in all cases but one. The calculated results clearly demonstrate that alane plays a significant catalytic role in H-2 release from cyclic amine-alane adducts. The catalytic effect of molecular borane (BH3) on the dehydrogenation of cyclic amine-borane adducts is also investigated and it is observed that AlH3 exhibits a larger catalytic effect than BH3. A systematic comparison of the unimolecular as well as catalyzed H-2 loss from cyclic amine-alane and amine-borane systems reveals that the former is a better candidate for releasing molecular hydrogen than the latter.