Mechanochemical Synthesis and Thermal Dehydrogenation of Novel Calcium-Containing Bimetallic Amidoboranes

Metallic amidoboranes are widely investigated candidates for solid-state hydrogen storage, and much focus shifted recently toward bimetallic amidoboranes. Bimetallic amidoboranes are expected to introduce novel and enhanced physicochemical properties regarding storage and stability. However, these materials are still scarce and mostly grouped around magnesium- or aluminum-containing compounds. We present here a rapid and green mechanochemical solvent-free synthesis of two novel calcium-containing bimetallic amidoboranes, Li2Ca­(NH2BH3)4 and Na2Ca­(NH2BH3)4, from metal hydrides and ammonia borane. The insight into mechanochemical syntheses is provided by the in situ tandem synchrotron X-ray diffraction and thermal monitoring. The in situ data reveal how the choice of alkali metal hydride governs the course of reactions and their thermal profiles. In situ monitoring of thermal dehydrogenation of these materials is conducted by mass spectrometry and infrared spectroscopy, showing how the course of thermal decomposition varies depending on the structure of the amidoborane, resulting however in the same final products. These new hydrogen-rich bimetallic amidoboranes are structurally characterized by high-resolution powder X-ray diffraction, and they both show potential for hydrogen storage applications: high theoretical gravimetric capacities and low desorption temperatures of hydrogen without the significant presence of harmful gases. We also show how the choice of the milling reactor material can be decisive for the efficiency and overall success of the mechanochemical synthetic procedure, which may impact the design of milling syntheses for other thermally labile chemical systems.

金属氨基硼烷（metallic amidoboranes）是固态储氢领域广受研究的候选材料，近年来研究焦点逐渐转向双金属氨基硼烷（bimetallic amidoboranes）。双金属氨基硼烷有望在储氢性能与稳定性方面展现出新颖且优化的物理化学特性，但目前这类材料仍较为稀缺，且大多集中于含镁或铝的化合物体系。本研究报道了一种快速绿色的无溶剂机械化学合成方法，可通过金属氢化物与氨硼烷制备两种新型含钙双金属氨基硼烷：Li₂Ca(NH₂BH₃)₄与Na₂Ca(NH₂BH₃)₄。研究借助原位同步辐射X射线衍射与热监测联用技术，阐明了该机械化学合成的反应机理：原位数据揭示了碱金属氢化物的选择如何调控反应进程与热分解行为。本研究还通过质谱与红外光谱对材料的热脱氢过程进行原位监测，结果表明尽管不同氨基硼烷的热分解路径因结构差异而有所不同，但最终产物一致。通过高分辨粉末X射线衍射对这两种新型富氢双金属氨基硼烷完成结构表征后发现，二者均具备良好的储氢应用潜力：理论重量储氢容量较高，氢气脱附温度较低，且不会产生大量有害副产物气体。此外，本研究还证实球磨反应器材质的选择对机械化学合成的效率与整体成败具有决定性影响，该结论可为其他热不稳定化学体系的球磨合成工艺设计提供借鉴。