Synthesis and dehydrogenation of novel N-substituted ammonia borane derivatives

Hydrogen is considered one of the most promising alternatives to fossil fuels as an energy carrier due to its high energy density and environmental friendliness and sustainability. However, a significant obstacle to the widespread application of hydrogen is the lack of a compact, safe, and cost-effective storage method. Solid hydrogen storage is regarded as a long-term solution among various hydrogen storage methods. Solid materials based on chemical sorption include interstitial hydrides, metal hydrides, complex hydrides, chemical hydrides, and reactive hydride composites, all of them have garnered significant attention due to their higher hydrogen content. Nevertheless, these solid materials possess both advantages and limitations for hydrogen storage. Regarding this thesis, the primary focus was on developing solid materials based on ammonia borane derivatives for hydrogen storage. This work is divided into two different but complementary approaches: 1) synthesis of ammonia borane derivatives solid materials for optimizing hydrogen storage properties by introducing functional groups on the nitrogen atoms, 2) understanding the relationship between material structure and hydrogen storage properties to establish new design principles of hydrogen storage materials. In this thesis, the properties of ammonia borane derivatives were fine-tuned by introducing -CH3 and -CH2CH2- groups. A series of corresponding metallic complexes were synthesized through the optimized mechanochemical or solution syntheses, as discussed in Chapters 3-6. Chapter 3 and 5 focus on synthesis and hydrogen storage properties of compounds containing -CH3 and -CH2CH2- substituent on nitrogen atoms of the parent compound, Na[BH3NH2BH2NH2BH3]. These substitutions effectively suppress the release of by-products such as NH3, B2H6, and other large fragments. Consequently, these modifications significantly enhance the production of pure hydrogen. Notably, the derivative with -CH2CH2- releases approximately 7.4 wt.% of pure hydrogen below 260°C. However, the impact of substitution with -CH3 and -CH2CH2- on the nitrogen atoms in Na[Al(NH2BH3)4] on hydrogen storage properties is slightly different, as explored in Chapters 4 and 6. Na[Al(CH3NHBH3)4] exhibits significant mass loss upon heating. These results indicate the substituents on ammonia borane derivatives and the structure of metallic complexes, including all the specific network of intermolecular interactions, define together the hydrogen storage properties. This cooperative effect is also illustrated in Chapter 6, where the derivatives Li[Al(BH3NHCH2CH2NHBH3)2] and Na[Al(BH3NHCH2CH2NHBH3)2] featuring -CH2CH2- substitutions, exhibit high ability to suppress the release of ammonia, diborane, and ethylenediamine. Specifically, Li[Al(BH3NHCH2CH2NHBH3)2] releases about 6.6 wt.% pure hydrogen below 280°C. As with any complex materials, the optimization of ammonia borane derivatives for hydrogen storage inherently demands systematic control over multiple factors, encompassing the substitution patterns on ammonia borane, the choice of metal, and the intermolecular interactions. Our findings, however, unequivocally demonstrate that the introduction of -CH3 and -CH2CH2- groups onto nitrogen atoms of ammonia borane derivatives positively impacts hydrogen storage performance. This substitution helps to suppress the release of unwanted by-products such as NH3 and B2H6. Therefore, the incorporation of alkyl-groups and the electron donating groups onto nitrogen atoms of NH3BH3 emerges as a promising strategy for optimizing the hydrogen storage properties of amidoboranes and their metal complexes.