Highly Efficient Ammonia Borane Hydrolytic Dehydrogenation in Neat Water Using Phase-Labeled CAAC-Ru Catalysts

Ammonia borane (AB) has received extensive attention in recent years as an emerging hydrogen storage material due to its high hydrogen density (19.6 wt %), nontoxicity, stability, and water solubility. Although AB itself is stable in water, its catalytic dehydrogenation (2 mol eq) in aqueous media produces borazine whose tandem hydrolytic reaction enables further hydrogen release (1 mol eq). Thus, water serves both as a reaction medium and also a pure hydrogen fuel source (33% of overall released H-2). A high capacity and fast homogeneous AB hydrolytic dehydrogenation system is reported using water-soluble CAAC-Ru carbene catalysts (5 and 6). Applying catalyst 6 at 50 ppm (0.015 mM) loading a high TON of 43,600 can be observed; meanwhile, the yield of the released H-2 remains high (73%, equal to 2.2 released molH(2)/molBH(3)NH(3)). The evolved hydrogen can achieve 2.9 molH(2)/molBH(3)NH(3) ([6] = 3.0 mM) and a TON of 86,100 (equal to 1.70 kg H-2 (released)/g Ru metal (used)) (10 ppm loading, [6] = 0.003 mM). The energy density of 1.70 kg H-2 is equal to that of 6.6 L of gasoline, which is a general consumption for a medium category car/100 km. The reaction yields nonhazardous borates up to 99% yield, which are considered as a recyclable commodity material for hydrogen storage systems. As the metaborate ion (BO2-)-induced catalyst passivation causing a decrease in accessibility of active sites in heterogeneous catalysis does not occur at homogeneous conditions, the reported high TON values can be achieved within significantly shorter reaction times and lower catalyst concentrations.