Comparison of the Coordination of B₁₂F₁₂^2–, B₁₂Cl₁₂^2–, and B₁₂H₁₂^2– to Na⁺ in the Solid State: Crystal Structures and Thermal Behavior of Na₂(B₁₂F₁₂), Na₂(H₂O)₄(B₁₂F₁₂), Na₂(B₁₂Cl₁₂), and Na₂(H₂O)₆(B₁₂Cl₁₂)

The synthesis of high-purity Na₂B₁₂F₁₂ and the crystal structures of Na₂(B₁₂F₁₂) (5 K neutron powder diffraction (NPD)), Na₂(H₂O)₄(B₁₂F₁₂) (120 K single-crystal X-ray diffraction (SC-XRD)), Na₂(B₁₂Cl₁₂) (5 and 295 K NPD), and Na₂(H₂O)₆­(B₁₂Cl₁₂) (100 K SC-XRD) are reported. The compound Na₂(H₂O)₄(B₁₂F₁₂) contains {[(Na­(μ-H₂O)₂Na­(μ-H₂O)₂)]²⁺}_∞ infinite chains; the compound Na₂(H₂O)₆­(B₁₂Cl₁₂) contains discrete [(H₂O)₂Na­(μ-H₂O)₂Na­(H₂O)₂]²⁺ cations with OH···O hydrogen bonds linking the terminal H₂O ligands. The structures of the two hydrates and the previously published structure of Na₂(H₂O)₄(B₁₂H₁₂) are analyzed with respect to the relative coordinating ability of B₁₂F₁₂^2–, B₁₂H₁₂^2–, and B₁₂Cl₁₂^2– toward Na⁺ ions in the solid state (i.e., the relative ability of these anions to satisfy the valence of Na⁺). All three hydrated structures have distorted octahedral NaX₂(H₂O)₄ 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 B₁₂Cl₁₂^2– ≪ B₁₂H₁₂^2– < B₁₂F₁₂^2–. Differential scanning calorimetry experiments demonstrate that Na₂(B₁₂F₁₂) 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 Na₂(B₁₂H₁₂) and Na₂(B₁₂Cl₁₂), respectively. Thermogravimetric analysis demonstrates that Na₂(H₂O)₄­(B₁₂F₁₂) and Na₂(H₂O)₆­(B₁₂Cl₁₂) undergo partial dehydration at 25 °C to Na₂(H₂O)₂­(B₁₂F₁₂) and Na₂(H₂O)₂­(B₁₂Cl₁₂) in ca. 30 min and 2 h, respectively, and essentially complete dehydration to Na₂(B₁₂F₁₂) and Na₂(B₁₂Cl₁₂) within minutes at 150 and 75 °C, respectively (the remaining trace amounts of H₂O, if any, were not quantified). The changes in structure upon dehydration and the different vapor pressures of H₂O needed to fully hydrate the respective Na₂(B₁₂X₁₂) compounds provide additional evidence that B₁₂Cl₁₂^2– is more weakly coordinating than B₁₂F₁₂^2– to Na⁺ in the solid state. Taken together, the results suggest that the anhydrous, halogenated closo-borane compounds Na₂(B₁₂F₁₂) and Na₂(B₁₂Cl₁₂), in appropriately modified forms, may be viable component materials for fast-ion-conducting solid electrolytes in future energy-storage devices.