Structural and reorientational dynamics of tetrahydroborate (BH4−) and tetrahydrofuran (THF) in a Mg(BH4)2·3THF adduct: neutron-scattering characterization

Abstract

Metal borohydrides are considered promising materials for hydrogen storage applications due to their high volumetric and gravimetric hydrogen density. Recently, different Lewis bases have been complexed with Mg(BH₄)₂ in efforts to improve hydrogenation/dehydrogenation properties. Notably, Mg(BH₄)₂·xTHF adducts involving tetrahydrofuran (THF; C₄H₈O) have proven to be especially interesting. This work focuses on exploring the physicochemical properties of the THF-rich Mg(BH₄)₂·3THF adduct using neutron-scattering methods and molecular DFT calculations. Structural analysis, based on neutron diffraction measurements of Mg(¹¹BH₄)₂·3TDF (D – deuterium), has confirmed a lowering of the symmetry upon cooling, from monoclinic C2/c to P via a triclinic distortion. Vibrational properties are strongly influenced by the THF environment, showing a splitting in spectral features as a result of changes in the bond lengths, force constants, and lowering of the overall symmetry. Interestingly, the orientational mobilities of the BH₄⁻ anions obtained from quasielastic neutron scattering (QENS) are not particularly sensitive to the presence of THF and compare well with the mobilities of BH₄⁻ anions in unsolvated Mg(BH₄)₂. The QENS data point to uniaxial 180° jump reorientations of the BH₄⁻ anions around a preferred C₂ anion symmetry axis. The THF rings are also found to be orientationally mobile, undergoing 180° reorientational jumps around their C₂ molecular symmetry axis with jump frequencies about an order of magnitude lower than those for the BH₄⁻ anions. In contrast, no dynamical behavior of the THF rings is observed with QENS for a more THF-deficient 2Mg(BH₄)₂·THF adduct. This lack of comparable THF mobility may reflect a stronger Mg²⁺–THF bonding interaction for lower THF/Mg(BH₄)₂ stoichiometric ratios, which is consistent with DFT calculations showing a decrease in the binding energy with each additional THF ring in the adduct. Based on the combined experimental and computational results, we propose that combining THF and Mg(BH₄)₂ is beneficial to (i) preventing weakly bound THF from coming free from the Mg²⁺ cation and reducing the concentration of any unwanted impurity in the hydrogen and (ii) disrupting the stability of the crystalline phase, leading to a lower melting point and enhanced kinetics for any potential hydrogen storage applications.