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 small, and at low temperatures, this motion can be described as rotational tunneling. The tunnel splitting derived from the neutron spin echo measurements at 3.6 K is 0.43(2) μeV. With increasing temperature, we have observed a gradual transition from the regime of low-temperature quantum dynamics to the regime of classical thermally activated jump reorientations. The jump rate of the uniaxial 3-fold reorientations reaches 5 × 1011 s–1 at 80 K. Nearer room temperature and above, both nuclear magnetic resonance and quasi-elastic neutron scattering measurements have revealed the second process of BH4– reorientations characterized by the activation energy of 261 meV. This process is several orders of magnitude slower than the uniaxial 3-fold reorientations; the corresponding jump rate reaches ∼7 × 108 s–1 at 300 K.