Mechanistic insights of metal acetylacetonate-aided dehydrocoupling of liquid-state ammonia borane NH3BH3

Abstract

Ammonia borane NH3BH3 solubilized in organic solvent is a potential liquid-state chemical hydrogen storage material. In this study, metal acetylacetonates like Fe(O2C5H7)3, Co(O2C5H7)2, Ni(O2C5H7)2, Pd(O2C5H7)2, Pt(O2C5H7)2 and Ru(O2C5H7)3 are considered for assisting dehydrocoupling of ammonia borane in diglyme (0.135 M) at 50oC. The molar ratio between ammonia borane and metal acetylacetonate is fixed at 100. A protocol for the separation of the soluble and insoluble fractions present in the slurry is proposed; it consists in using acetonitrile to make the precipitation of metal-based compounds easier and to solubilize boron-based intermediates/products. The nature of the metal does not affect the dehydrocoupling mechanisms, the 11B{1H} NMR spectra showing the formation of the same reaction intermediates. The aforementioned metal acetylacetonates do mainly have effect on the kinetics of dehydrocoupling. Dehydrocoupling takes place heterogeneously and dehydrogenation of ammonia borane in these conditions leads to the formation of polyborazylene via intermediates like e.g., B-(cyclodiborazanyl) amine-borane and borazine. Our main results are reported and discussed herein.