Theoretical Studies On The Mechanism Of Transition Metal (Fe,Pd) Catalyzed Dehydrogenation Of Ammonia Borane

Ammonia-Borane is considered to be one of the most promising chemical hydrogen storage materials,which has received inordinate attention over the past decade due to the high gravimetric hydrogen storage?19.6 wt%H₂?,low molecular weight and high thermal stability.The pyrolysis temperature of ammonia broane can be lower and the rate of hydrolysis can be faster with transition-metal catalysts.Therefore,theoretical studies on the mechanism of transition metal catalyzed dehydrogenation of ammonia-borane are of great significance.In this thesis,the mechanisms of two kinds of transition metal?Fe and Pd?complexes catalyzed ammonia-borane dehydrogenation reactions have been investigated by the density functional theory?DFT?.The main content of this thesis includes the following two parts:1.A mechanistic investigation on ammonia-borane dehydrogenation catalyzed by the newly developed Iron bis?phosphinite?pincer complexes are reported using density functional theory?DFT?calculations at the M06/6-311+G?d,p?–SDD//M06/6-31G?d?–SDD level of theory.We have shown that AB dehydrogenation proceeds through a ligand-assisted concerted Fe-C_ipso cooperation mechanism,in which the hydrogen atom of B-H moves to metal Fe and proton of N-H transfers to pincer ipso carbon simultaneously.DFT calculations and natural bond orbital?NBO?charge analysis suggest that Fe-POCOP complex with an electron-donating MeO group at the para position to the ipso carbon exhibits the highest catalytic activity with the lowest activation barriers of 17.6 kcal/mol.Moreover,loss of NH₃ from the AB-bound complex leads to the diminished catalytic activity at the late stage of the AB dehydrogenation process.2.The mechanism of tandem dimethylamine-borane?NHMe₂BH₃,DMAB?dehydrogenation and alkene hydrogenation catalyzed by the Pd?0?complex[Pd?NHC??PCy₃?]?NHC=N-heterocycle carbene?has been presented by density functional theory?DFT?calculations at the M06l/6-311+G?d,p?–LANL2DZ//M06l/6-31G?d?–LANL2DZ level of theory.Four possible NHMe₂BH₃ dehydrogenation pathways have been carefully investigated.Theoretical studies show that the N–H proton transfers to ligated carbene carbon and subsequent C–H and B–H activation is the most kinetically favorable with the lowest activation barrier of 26 kcal/mol.It was found that a trans-dihydride Pd?II?species[Pd?H?₂?NHC??PCy₃?],formed in the dehydrogenation process plays a key role in the tandem hydrogenation reaction of alkene.In addition,DFT calculations show that the hydrogenation reaction is through a stepwise H transfer mechanism with a barrier of 19.6 kcal/mol in the RDS,which is kinetically and thermodynamically favored.