Synthesis And Characterization Of Osmium Complexes With Nitrogen-based Ligands And Their Application In Catalytic Dehydrogenation Of Ammonia Borane

In recent years,ammonia-borane（NH₃BH₃,AB）,with the theoretical hydrogen storage capacity of up to 19.6 wt%,has attracted widely attention as a potential hydrogen storage material.However,it is shown that the dehydrogenation of AB requires high temperature,and is accompanied by impurity gas release and volume expansion.In order to solve the problems,catalytic dehydrogenation of AB by metal complexes has become one of most efficient path.Up to now,a number of metal catalysts relative to the first,second and third row transition metals have been applied in the dehydrogenation of AB.However,little attention was paid on the osmium catalysts except only two examples found by Esteruelas’s group.Recently,more and more results have indicated that osmium complexes with N-based ligands demonstrate excellent catalytic performance in the transformation of H₂,which is involved in the catalytic cleavage of O-H and N-H bonds.In consideration that the B-H and N-H bonds in AB can be ruptured by metal catalysts,we propose that the osmium catalyst should also work well in the dehydrogenation of AB if the ligands is chosen appropriately.In this dissertation,N-based ligands such as pyrazole,ethylenediamine,o-phenylenediamine,2,2-dipyridyl and 1,10-phenanthroline were used to react with OsCl₂（PPh₃）₃ to produce the corresponding osmium complexes（2-6²-17）.All the complexes were characterized by multinuclear magnetic resonance,elemental analysis and X-ray single crystal diffraction,and further applied in the catalytic dehydrogenation of AB.It is shown that the synthesized osmium complexes（2-6²-17）exhibits high catalytic activity in the dehydrogenation of AB at 60oC,in the mixed THF/DME solution with the catalyst loading of 5 mol%.In general,the coordination of osmium to the N-based ligands with conjugate or aromatic donor atoms enhances its catalytic activity significantly in comparison with that of the aliphatic donor atoms.Among all the catalysts,compound 2-8 demonstrates the highest activity,which is also the most efficient osmium catalyst in catalytic dehydrogenation of AB until now.