Hydrolysis Of Ammonia Borane Catalyzed By Proton Responsive Iridium Complexes

With drastic consumption of fossil fuel and increasing demand of energy, exploring alternative energy is necessary. Hydrogen attracts extensive attention as a new efficient and green energy source. Safe and effective hydrogen storage becomes the key to promote the wide application of hydrogen and development of a hydrogen economy. Ammonia borane (AB) owning a high hydrogen storage capacity (19.6 wt%) is one of the most promising hydrogen storage materials. Among the diverse types of hydrogen generation, hydrogen evolution from the hydrolysis of ammonia borane possesses numerous advantages including high rate, evolving large amount of H2, operating at low temperature, and producing no toxic by-products. Although considerable studies on heterogeneous catalysis of hydrogen evolution form AB have been reported, most present heterogeneous catalysts require large amounts of catalysts loading, showed poor selectivity and recyclability and low rate of hydrogen generation. In contrast, homogeneously catalytic hydrolysis of AB showed higher activity and selectivity due to the reactive centers dispersed well. However, transition-metal catalyzed homogeneously catalytic hydrolysis of AB has been barely reported because development of stable and water soluble catalysts is highly challenging.In this thesis, a series of water-soluble complexes were synthesized and applied in the homogeneously catalytic hydrolysis of ammonia borane. Meanwhile, the catalytic activity and catalytic mechanism were explored.1) A series of bipyridyl and bipyrimidine bidentate ligands bearing hydroxyl groups were synthesized and reacted with [Cp*Ir(OH2)3]SO4 or [Cp*IrCl]2 to gave four types of corresponding proton-responsive iridium complexes. Because of the existence of hydroxyl groups, the complexes showed superior water solubility and stability.2) The synthesized proton-responsive iridium complexes were utilized in hydrogen evolution from the catalytic hydrolysis of AB. The effect of temperature, catalyst loading, solution pH, and concentration of AB was studied. With comprehensive comparison, reaction conditions of 65 ℃,1 μmol of catalyst, pH=9,0.1 mol/L of AB were obtained as optimal conditions.3) In addition, the effect of pressure was also studied. The hydrolysis of AB was performed in a sealed autoclave. It was found that the catalysts still retained catalytic activity and released 3 equivalents of hydrogen. All the AB substrates (3.6 mmol) were consumed within 3 h and a TON of 3600 was obtained.4) Under the optimal conditions, [Cp*Ir(6,6’-(OH)2-bpy)(OH2)]SO4 exhibited the best catalytic performance releasing 3 equivalents of hydrogen within 0.7 h and gave a high initial turnover frequency (TOF) of 5496 h-1 (initial 5 min).According to the experiment results and previous reports, we proposed a catalytic mechanism of the catalytic hydrolysis of AB with our proton-responsive catalysts. With a water molecule participated, the AB dehydrogenation is assisted by the synergistic effect between hydroxyl group on the ligand and metal center.