Ammonia borane thermolytic decomposition in the presence of metal (II) chlorides

In the present work, we studied the effect of metal chlorides, MCl2, on the thermal decomposition of ammonia borane NH3BH3 (AB). Some metals from row n = 4 of the periodic table were chosen and used as MCl2: namely, FeCl2, CoCl2, NiCl2, CuCl2, and ZnCl2. In addition, three metals from column VIII of the periodic table were considered: NiCl2, PdCl2 and PtCl2. The AB decomposition was followed by TGA and DSC; the decomposition gases analyzed by μGC/MSD coupling, and the solid by-products identified by XRD, IR and XPS. We observed that the presence of CuCl2 in AB is beneficial, making the decomposition occur in much milder conditions than for pristine AB; for example, the dehydrogenation of CuCl2-doped AB started at 25 °C, with the sample losing about 14 wt% at 85 °C. However, MCl2 does not hinder the evolution of the undesired borazine; it only contributes to a decrease in its content compared to pristine AB. To rationalize the better performance of CuCl2, we propose that Cu offers an optimal doping activity with intermediate binding energies for the intermediates: i.e. with H not too strongly bonded but optimally bonded to the N of AB. The germ Cu⋯NH2–BH2, then formed, acts as a Lewis acid through B and has an optimized reactivity towards a new AB molecule (head-to-tail dehydrocoupling). This is discussed herein.