Concerted dehydrogenation reactions of magnesium borohydride with oxygen and nitrogen heterocyclic compounds

Abstract

This dissertation focuses on work resulting from the progress in developing a two-way hy-drogen carrier system consisting of solutions of furans and pyrroles containing Mg(BH4)2 and an irridum pincer complex catalyst. Previous studies have focused on magnesium borohy- dride (Mg(BH4)2) due to its high hydrogen content and favorable thermodynamics. Further studies have shown the kinetic enhancement of the addition of sub-stoichiometric amounts of Lewis bases on cycling. While the kinetic enhancement of the addition of O-heterocycles has been explored the effects of N-heterocycles have not been. Since the discovery in 1997 of the first reported homogeneous catalyst for the dehydrogenation cycloalkanes to arenes, the “pincer” complex IrH2(2,6 -C6H3-(CH2PtBu2)2) and related pincer complexes, are known to be highly active catalysts for the dehydrogenation of heterocyclic liquid or- ganic hydrogen carriers (LOHC). While attempting the tandem dehydrogenation of magne- sium borohydride and pyrrolidine, we found Mg(BH4)2 reacts with pyrrolidine to produce tris(pyrrolidino)borane and bis(pyrrolidino)borane, irreversibly releasing up to 6 wt% H2. The proportions of the pyrrolidino boranes are highly dependent on reaction times and conditions. We have also observed that the iridium pincer complex catalyzes the direct de- hydrogenation of tris(pyrrolidino)borane to tris(1H-pyrrole-1-yl)borane, a significant finding in view of its theoretical, 5.2 wt% gravimetric and >80 g/L volumetric hydrogen capacities. Full characterization of the products along with their hydrogen cycling behavior is discussed.