PNNL

Abstract

Ammonia borane [NH3BH3, AB] is a prototypical Lewis acid (BH3)"Lewis base (NH3) adduct that is a stable molecular crystal at ambient temperature and pressure.[1] Upon heating, AB releases H2 in an exothermic reaction to form polyaminoborane [(NH2BH2)x, PAB] at temperatures below 1208C and decomposes further to polyiminoborane [(NHBH)x, PIB] above 1208C (Scheme S1, Supporting Information).[2-4] The structure and dynamics of AB, PAB and PIB is influenced by dihydrogen bonding (DHB)[5, 6] arising from the electrostatic interactions between protonic (Hd+) and hydridic (Hd*) H atoms and dative bonding due to donation of non-bonding lone pair on nitrogen to the empty p-orbital on boron.[7] Recently, it has been discovered that the presence of organic ligands on Lewis bases and/or donors creates steric hindrance resulting in a frustration of the dative bonding and termed as "frustrated Lewis pairs" (FLPs).[8] Early reports of phosphine-borane FLPs revealed a striking reactivity towards H2 activation[9, 10] and now seen in amine-borane FLPs as well.[11] We have recently reported on pressureinduced H2 interactions with AB resulting in the formation of AB*H2 complexes.[12] Herein, we demonstrate reactions of H2 and D2 with PAB and PIB in the 2-4 GPa pressure range and temperatures up to 2208C, in situ in a diamond anvil cell (DAC).[13] Isotopic scrambling of H2 and D2, when thermodynamics prevent reversible reactions,[4, 14] is proposed to occur by a mechanism involving "polymeric" FLPs. We thank the USDOE, Office of BES, for funding and NNSA (CDAC) and NSF-DMR for instrumentation support. PNNL is operated by Battelle.