Comparison of the Coordination of B12F122-, B12Cl122-, and B12H122- 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 B12F122-, B12H122-, and B12Cl122- 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 B12Cl122- ≪ B12H122- < B12F122-. 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 B12Cl122- is more weakly coordinating than B12F122- 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.