Synthesis, structural analysis, and thermal decomposition studies of [(NH3)2BH2]B3H8

Abstract

Boron–nitrogen containing compounds with high hydrogen contents as represented by ammonia borane (NH₃BH₃) have recently attracted intense interest for potential hydrogen storage applications. One such compound is [(NH₃)₂BH₂]B₃H₈ with a capacity of 18.2 wt% H. Two safe and efficient synthetic routes to [(NH₃)₂BH₂]B₃H₈ have been developed for the first time since it was discovered 50 years ago. The new synthetic routes avoid a dangerous starting chemical, tetraborane (B₄H₁₀), and afford a high yield. Single crystal X-ray diffraction analysis reveals N–H^δ+⋯H^δ−–B dihydrogen interactions in the [(NH₃)₂BH₂]B₃H₈·18-crown-6 adduct. Extended strong dihydrogen bonds were observed in pure [(NH₃)₂BH₂]B₃H₈ through crystal structure solution based upon powder X-ray analysis. Pyrolysis of [(NH₃)₂BH₂]B₃H₈ leads to the formation of hydrogen gas together with appreciable amounts of volatile boranes below 160 °C.