Comparison of the Coordination of B 12 F 12 2- , B 12 Cl 12 2- , and B 12 H 12 2- to Na + in the Solid State: Crystal Structures and Thermal Behavior of Na 2 (B 12 F 12 ), Na 2 (H 2 O) 4 (B 12 F 12 ), Na 2 (B 12 Cl 12 ), and Na 2 (H 2 O) 6 (B 12 Cl 12 ).

The synthesis of high-purity Na2 B12 F12 and the crystal structures of Na2 (B12 F12 ) (5 K neutron powder diffraction (NPD)), Na2 (H2 O)4 (B12 F12 ) (120 K single-crystal X-ray diffraction (SC-XRD)), Na2 (B12 Cl12 ) (5 and 295 K NPD), and Na2 (H2 O)6 (B12 Cl12 ) (100 K SC-XRD) are reported. The compound Na2 (H2 O)4 (B12 F12 ) contains {[(Na(μ-H2 O)2 Na(μ-H2 O)2 )]2+ }∞ infinite chains; the compound Na2 (H2 O)6 (B12 Cl12 ) contains discrete [(H2 O)2 Na(μ-H2 O)2 Na(H2 O)2 ]2+ cations with OH···O hydrogen bonds linking the terminal H2 O ligands. The structures of the two hydrates and the previously published structure of Na2 (H2 O)4 (B12 H12 ) are analyzed with respect to the relative coordinating ability of B12 F12 2- , B12 H12 2- , and B12 Cl12 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 (H2 O)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 B12 Cl12 2- ≪ B12 H12 2- < B12 F12 2- . Differential scanning calorimetry experiments demonstrate that Na2 (B12 F12 ) 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 (B12 H12 ) and Na2 (B12 Cl12 ), respectively. Thermogravimetric analysis demonstrates that Na2 (H2 O)4 (B12 F12 ) and Na2 (H2 O)6 (B12 Cl12 ) undergo partial dehydration at 25 °C to Na2 (H2 O)2 (B12 F12 ) and Na2 (H2 O)2 (B12 Cl12 ) in ca. 30 min and 2 h, respectively, and essentially complete dehydration to Na2 (B12 F12 ) and Na2 (B12 Cl12 ) within minutes at 150 and 75 °C, respectively (the remaining trace amounts of H2 O, if any, were not quantified). The changes in structure upon dehydration and the different vapor pressures of H2 O needed to fully hydrate the respective Na2 (B12 X12 ) compounds provide additional evidence that B12 Cl12 2- is more weakly coordinating than B12 F12 2- to Na+ in the solid state. Taken together, the results suggest that the anhydrous, halogenated closo-borane compounds Na2 (B12 F12 ) and Na2 (B12 Cl12 ), in appropriately modified forms, may be viable component materials for fast-ion-conducting solid electrolytes in future energy-storage devices.