Chemical hydrogen storage: ammonia-borane activation by a Pd(II) PCP-pincer complex

Chemical hydrogen storage with small light-weight and hydrogen-rich molecules like ammonia borane (NH3-BH3, AB, 19.3 wt.% H2) or amino boranes of general formula RNH2-BH3 (R = alifatic chain or cycle) is a topic of great interest, as witnessed by the exponential growth of the number of papers published in the recent literature. Transition metal complexes can catalyze H2 release from these species under mild conditions and offer a great potential as storage materials. In this work, the reactivity of the pincer complex [(tBuPCP)Pd(H2O)]PF6 [tBuPCP = 2,6-C6H3(CH2PtBu2)2, Figure] towards AB will be presented and discussed. The reaction was followed through variable-temperature multinuclear (31P, 1H, 11B) NMR spectroscopy in the 190 - 298 K temperature range. At low temperatures, one non-hydridic intermediate was found, whose chemical identity was investigated with the aid of DFT modeling of the reaction mechanism, at the M06//6-31+G(d,p) level. Slow evolution of 1.5 equivalents of H2 in 8 h time and concomitant AB oligomerization also occurs, the final dehydrogenation products ("spent fuels") being polyborazane oligomers of general formula (NH2-BH2)n.