Theoretical Studies On The Metal Complexes Catalyzed Hydrolysis Of Ammonia Borane

Ammonia borane is a promising candidate for a H2 storage material.The hydrolysis of ammonia borane catalyzed by metal complex is one of the highly potential processes for which 3 equivalents of hydrogen may be released.In recent years,considerable efforts were devoted to the development of efficient metal catalysts for the hydrolysis of ammonia borane.Because of the short existence time of some key intermediates,the feasibility of experimental methods to study the hydrolysis mechanism were partially restricted.Thanks to the rapid development of sophisticated computational routines and modern computers,theoretical and computational chemistry has reached a level that allows it to be easily studied the mechanism of metal complex catalyzed hydrolysis of ammonia borane.In this thesis,theoretical calculations are carried out to examine 3 metal complexes catalyzed hydrolysis of ammonia borane for releasing three equivalents of hydrogen.We hope that the insight provided might be helpful for designing the new industrial catalyst for hydrolysis of ammonia borane.This thesis mainly includes the following three parts:1.The reaction mechanism of Ru(II)catalyzed ammonia borane hydrolysis.(1)Ru(II)-catalyzed hydrolysis of ammonia borane to release of the first equivalent of H2 includes four major stages:(a)ammonia borane coordination to Ru(II)-catalyst,(b)SN2-type nucleophilic attack of H2 O on the activated boron center,(c)the attack of the protic O-H of [NH3BH2(H2O)]+,(d)releasing the first equivalent of H2 and regenerating the Ru(II)catalyst.(2)The feasibility of the out of cycle hydrolysis of amine-hydroxyl boranes accounting for the second and third equivalents of hydrogen is examined and at the same time an alternative catalytic mechanism is proposed.Our calculations show that the second and third equivalents of H2 are generated through the catalytic process which is similar to the calculation of the first equivalent of H2.(3)To further compared the Ru(II)-catalyzed hydrolysis and dehydrogenation of ammonia borane,we investigate the reaction mechanism of Ru(II)-catalyzed dehydrogenation of ammonia borane.The concerted activation of N-H and B-H pathway,the stepwise B-H and N-H activation pathway and the direct N-H activation pathway are calculated.The results show that the reaction barriers of Ru(II)-catalyzed dehydrogenation of ammonia borane lies higher than the Ru(II)-catalyzed hydrolysis of ammonia borane.2.The comparison studies on the mononuclear and binuclear Ru(II)complex catalyzed hydrolysis of ammonia borane.Following the same reaction mechanism shown in the previous chapter,the energy profiles of mononuclear and binuclear Ru(II)complex catalyzed hydrolysis of ammonia borane are calculated.Our calculations show that the overall reaction barrier of hydrolysis of ammonia borane with binuclear Ru(II)complex is lower than that with mononuclear Ru(II)complex.This result is consisted with the experimental observation that the binuclear Ru(II)complex is more activity than mononuclear Ru(II)complex.3.The reaction mechanism of hydrolysis of ammonia borane catalyzed by the functionalized Ir complexes.(1)Under basic conditions,the pendent hydroxyl group in Ir complexes was deprotonated to yield O-,a stronger electron donor and favors enhancement of catalytic activities.In the present of the pendent O-,the reaction mechanism includes(a)the B-H in ammonia borane which formed a hydrogen bond with water molecule coordinates to the Ir center and generates a stable intermediate,(b)the oxidative addition of coordinated NH3BH3 to Ir center cleaves a B-H single bond,the water-assisted proton transfer from nitrogen atom of NH3BH3 to pendent O-of releases NH3BH2 OH complex,(c)release of H2 with assistance of pendent OH to complete catalytic cycle.(2)Our calculations show that the release of the other two equivalents of H2 is very similar to that calculated for the generation of first equivalent of H2.