Comparison of the Coordination of B<sub>12</sub>F<sub>12</sub><sup>2–</sup>, B<sub>12</sub>Cl<sub>12</sub><sup>2–</sup>, and B<sub>12</sub>H<sub>12</sub><sup>2–</sup> to Na<sup>+</sup> in the Solid State: Crystal Structures and Thermal Behavior of Na<sub>2</sub>(B<sub>12</sub>F<sub>12</sub>), Na<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>(B<sub>12</sub>F<sub>12</sub>), Na<sub>2</sub>(B<sub>12</sub>Cl<sub>12</sub>), and Na<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>(B<sub>12</sub>Cl<sub>12</sub>)
journal contribution
posted on 2017-04-06, 15:18 authored by Eric V. Bukovsky, Dmitry V. Peryshkov, Hui Wu, Wei Zhou, Wan Si Tang, W. Matthew Jones, Vitalie Stavila, Terrence J. Udovic, Steven H. StraussThe synthesis of
high-purity Na<sub>2</sub>B<sub>12</sub>F<sub>12</sub> and the crystal
structures of Na<sub>2</sub>(B<sub>12</sub>F<sub>12</sub>) (5 K neutron
powder diffraction (NPD)), Na<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>(B<sub>12</sub>F<sub>12</sub>) (120 K single-crystal X-ray diffraction
(SC-XRD)), Na<sub>2</sub>(B<sub>12</sub>Cl<sub>12</sub>) (5 and 295
K NPD), and Na<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>(B<sub>12</sub>Cl<sub>12</sub>) (100 K SC-XRD) are reported. The compound
Na<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>(B<sub>12</sub>F<sub>12</sub>) contains {[(Na(μ-H<sub>2</sub>O)<sub>2</sub>Na(μ-H<sub>2</sub>O)<sub>2</sub>)]<sup>2+</sup>}<sub>∞</sub> infinite
chains; the compound Na<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>(B<sub>12</sub>Cl<sub>12</sub>) contains discrete [(H<sub>2</sub>O)<sub>2</sub>Na(μ-H<sub>2</sub>O)<sub>2</sub>Na(H<sub>2</sub>O)<sub>2</sub>]<sup>2+</sup> cations with OH···O hydrogen
bonds linking the terminal H<sub>2</sub>O ligands. The structures
of the two hydrates and the previously published structure of Na<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>(B<sub>12</sub>H<sub>12</sub>)
are analyzed with respect to the relative coordinating ability of
B<sub>12</sub>F<sub>12</sub><sup>2–</sup>, B<sub>12</sub>H<sub>12</sub><sup>2–</sup>, and B<sub>12</sub>Cl<sub>12</sub><sup>2–</sup> toward Na<sup>+</sup> ions in the solid state (i.e.,
the relative ability of these anions to satisfy the valence of Na<sup>+</sup>). All three hydrated structures have distorted octahedral
NaX<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> coordination spheres (X
= F, H, Cl). The sums of the four Na–O bond valence contributions
are 71, 75, and 89% of the total bond valences for the X = F, H, and
Cl hydrated compounds, respectively, demonstrating that the relative
coordinating ability by this criterion is B<sub>12</sub>Cl<sub>12</sub><sup>2–</sup> ≪ B<sub>12</sub>H<sub>12</sub><sup>2–</sup> < B<sub>12</sub>F<sub>12</sub><sup>2–</sup>. Differential
scanning calorimetry experiments demonstrate that Na<sub>2</sub>(B<sub>12</sub>F<sub>12</sub>) undergoes a reversible, presumably order–disorder,
phase transition at ca. 560 K (287 °C), between the 529 and 730
K transition temperatures previously reported for Na<sub>2</sub>(B<sub>12</sub>H<sub>12</sub>) and Na<sub>2</sub>(B<sub>12</sub>Cl<sub>12</sub>), respectively. Thermogravimetric analysis demonstrates that Na<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>(B<sub>12</sub>F<sub>12</sub>) and Na<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>(B<sub>12</sub>Cl<sub>12</sub>) undergo partial dehydration at 25 °C to Na<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>(B<sub>12</sub>F<sub>12</sub>) and Na<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>(B<sub>12</sub>Cl<sub>12</sub>) in ca. 30 min and 2 h, respectively, and essentially
complete dehydration to Na<sub>2</sub>(B<sub>12</sub>F<sub>12</sub>) and Na<sub>2</sub>(B<sub>12</sub>Cl<sub>12</sub>) within minutes
at 150 and 75 °C, respectively (the remaining trace amounts of
H<sub>2</sub>O, if any, were not quantified). The changes in structure
upon dehydration and the different vapor pressures of H<sub>2</sub>O needed to fully hydrate the respective Na<sub>2</sub>(B<sub>12</sub>X<sub>12</sub>) compounds provide additional evidence that B<sub>12</sub>Cl<sub>12</sub><sup>2–</sup> is more weakly coordinating
than B<sub>12</sub>F<sub>12</sub><sup>2–</sup> to Na<sup>+</sup> in the solid state. Taken together, the results suggest that the
anhydrous, halogenated <i>closo</i>-borane compounds Na<sub>2</sub>(B<sub>12</sub>F<sub>12</sub>) and Na<sub>2</sub>(B<sub>12</sub>Cl<sub>12</sub>), in appropriately modified forms, may be viable
component materials for fast-ion-conducting solid electrolytes in
future energy-storage devices.
