Theoretical exploration of H2X (X = O, S, Se) and HY (Y = F, Cl, Br) assisted H2-release from ammonia-borane and related compounds: mechanistic insights from theoretical viewpoint

Reaction pathways for H-2 release mediated by H2X (X = O, S, Se) and HY (Y = F, Cl, Br) set of reagents from ammonia-borane (NH3BH3) have been explored in gas phase using density functional theory (DFT). The optimized geometries of reactant complexes, transition states and product complexes have been obtained using M06-2X functional in conjunction with 6-311+G(d, p) basis set. The study of catalytic effect of these cheap and easily available, yet highly effective, assisting reagents has farther been extended to analyze the thermochemistry involved in H-2 departure from two other ammonia-borane-related compounds: aziridine-borane [C2H5N center dot BH3] and 3-methyl-1,2-BN-cyclopentane. The mechanism of H-2 release channel is identical for all three compounds considered. The free energy barrier falls significantly from H2O to H2S to H2Se for the H2X set of reagents and from HF to HCl to HBr for the HY analogs. Our study endorses the fact that the HY set of reagents serves as better assisting reagents kinetically compared to H2X, with HBr being the best of the lot. HBr reduces the free energy barrier to 41.9 kJ mol(-1) for ammonia-borane [NH3BH3], 42.7 kJ mol(-1) for aziridine-borane [C2H5N center dot BH3] and 30.2 kJ mol(-1) for 3-methyl-1,2-BN-cyclopentane in H-2 release at 1 atm. pressure and 298 K temperature.