Low-Temperature Rotational Tunneling of Tetrahydroborate Anions in Lithium Benzimidazolate-Borohydride Li2(bIm)BH4

To investigate the dynamical properties of the novel hybrid compound, lithium benzimidazolate-borohydride Li2(bIm)BH4 (where bIm denotes a benzimidazolate anion, C7N2H5–), we have used a set of complementary techniques: neutron powder diffraction, ab initio density functional theory calculations, neutron vibrational spectroscopy, nuclear magnetic resonance, neutron spin echo, and quasi-elastic neutron scattering. Our measurements performed over the temperature range from 1.5 to 385 K have revealed the exceptionally fast low-temperature reorientational motion of BH4– anions. This motion is facilitated by the unusual coordination of tetrahedral BH4– anions in Li2(bIm)BH4: each anion has one of its H atoms anchored within a nearly square hollow formed by four coplanar Li+ cations, while the remaining −BH3 fragment extends into a relatively open space, being only loosely coordinated to other atoms. As a result, the energy barriers for reorientations of this fragment around the anchored B–H bond axis are very ...