Synthesis, structure and the dehydrogenation mechanism of calcium amidoborane hydrazinates

Abstract

The calcium amidoborane hydrazinates, Ca(NH₂BH₃)₂·nN₂H₄, were firstly synthesized by reacting different molar ratios of Ca(NH₂BH₃)₂ and N₂H₄. In particular, Ca(NH₂BH₃)₂ and N₂H₄ with a molar ratio of 1 : 2 crystallizes into the orthorhombic symmetry P2₁2₁2₁ space group with the lattice parameters of a = 6.6239(4) Å, b = 13.7932(6) Å, c = 4.7909(2) Å. The dehydrogenations of calcium amidoborane hydrazinates are two-step reactions, exhibiting superior dehydrogenation properties compared with those of pristine Ca(NH₂BH₃)₂. For Ca(NH₂BH₃)₂–1/2N₂H₄, approximately 4.6 equiv. hydrogen (or 7.9 wt% hydrogen) can be released at 150 °C. Kinetic analysis shows that the activation energies for the two steps of hydrogen desorption from Ca(NH₂BH₃)₂·2N₂H₄ are much lower than those of pristine Ca(NH₂BH₃)₂, suggesting an improvement in the dehydrogenation kinetics of Ca(NH₂BH₃)₂ after coordinating with N₂H₄. Isotopic labeling results show that the driving force for the dehydrogenation of calcium amidoborane hydrazinates is the combination mechanism of protonic hydrogen and hydridic hydrogen (H^δ+ and H^δ−). In addition, initial H₂ release from calcium amidoborane hydrazinates originates from the interaction of [–BH₃] and N₂H₄, rather than [–BH₃] and [–NH₂] (in [–NH₂BH₃]).