Jahn–Teller Effect in the B₁₂F₁₂ Radical Anion and Energetic Preference of an Octahedral B₆(BF₂)₆ Cluster Structure over an Icosahedral Structure for the Elusive Neutral B₁₂F₁₂

The B₁₂F₁₂^– radical anion was generated by oxidation of [CoCp₂⁺]₂B₁₂F₁₂^2– with AsF₅ in SO₂. In the crystal structure of [CoCp₂⁺]­B₁₂F₁₂^–, the anion displays a lowered symmetry (<i>D</i>_2<i>h</i>) instead of an <i>I_h</i>-symmetric dianion as a result of Jahn–Teller distortion. Moreover, shortening of the B–F bonds and subtle changes of the B–B bonds are observed. DFT calculations show that, for the unknown neutral B₁₂F₁₂, unprecedented structural isomers [e.g., octahedral B₆(BF₂)₆] are energetically favored instead of an icosahedral structure. The structures and energetics are compared with those of the analogous chlorine compounds

Jahn–Teller Effect in the B₁₂F₁₂ Radical Anion and Energetic Preference of an Octahedral B₆(BF₂)₆ Cluster Structure over an Icosahedral Structure for the Elusive Neutral B₁₂F₁₂

The B₁₂F₁₂^– radical anion was generated by oxidation of [CoCp₂⁺]₂B₁₂F₁₂^2– with AsF₅ in SO₂. In the crystal structure of [CoCp₂⁺]­B₁₂F₁₂^–, the anion displays a lowered symmetry (<i>D</i>_2<i>h</i>) instead of an <i>I_h</i>-symmetric dianion as a result of Jahn–Teller distortion. Moreover, shortening of the B–F bonds and subtle changes of the B–B bonds are observed. DFT calculations show that, for the unknown neutral B₁₂F₁₂, unprecedented structural isomers [e.g., octahedral B₆(BF₂)₆] are energetically favored instead of an icosahedral structure. The structures and energetics are compared with those of the analogous chlorine compounds

Structures of M₂(SO₂)₆B₁₂F₁₂ (M = Ag or K) and Ag₂(H₂O)₄B₁₂F₁₂: Comparison of the Coordination of SO₂ versus H₂O and of B₁₂F₁₂^2– versus Other Weakly Coordinating Anions to Metal Ions in the Solid State

The structures of three solvated monovalent cation salts of the superweak anion B₁₂F₁₂^2– (Y^2–), K₂(SO₂)₆Y, Ag₂­(SO₂)₆Y, and Ag₂­(H₂O)₄Y, are reported and discussed with respect to previously reported structures of Ag⁺ and K⁺ with other weakly coordinating anions. The structures of K₂(SO₂)₆Y and Ag₂­(SO₂)₆Y are isomorphous and are based on expanded cubic close-packed arrays of Y^2– anions with M­(OSO)₆⁺ complexes centered in the trigonal holes of one expanded close-packed layer of B₁₂ centroids (⊙). The K⁺ and Ag⁺ ions have virtually identical bicapped trigonal prism MO₆F₂ coordination spheres, with M–O distances of 2.735(1)–3.032(2) Å for the potassium salt and 2.526(5)–2.790(5) Å for the silver salt. Each M­(OSO)₆⁺ complex is connected to three other cationic complexes through their six μ-SO₂-κ¹<i>O</i>,κ²<i>O</i>′ ligands. The structure of Ag₂­(H₂O)₄Y is unique [different from that of K₂­(H₂O)₄Y]. Planes of close-packed arrays of anions are offset from neighboring planes along only one of the linear ⊙···⊙···⊙ directions of the close-packed arrays, with [Ag­(μ-H₂O)₂­Ag­(μ-H₂O)₂)]_∞ infinite chains between the planes of anions. There are two nearly identical AgO₄F₂ coordination spheres, with Ag–O distances of 2.371(5)–2.524(5) Å and Ag–F distances of 2.734(4)–2.751(4) Å. This is only the second structurally characterized compound with four H₂O molecules coordinated to a Ag⁺ ion in the solid state. Comparisons with crystalline H₂O and SO₂ solvates of other Ag⁺ and K⁺ salts of weakly coordinating anions show that (i) N­[(SO₂)₂­(1,2-C₆H₄)]⁻, BF₄^–, SbF₆^–, and Al­(OC­(CF₃)₃)₄^– coordinate much more strongly to Ag⁺ than does Y^2–, (ii) SnF₆^2– coordinates somewhat more strongly to K⁺ than does Y^2–, and (iii) B₁₂Cl₁₂^2– coordinates to K⁺ about the same as, if not slightly weaker than, Y^2–

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 <i>closo</i>-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

Latent Porosity in Alkali-Metal M₂B₁₂F₁₂ Salts: Structures and Rapid Room-Temperature Hydration/Dehydration Cycles

Structures of the alkali-metal hydrates Li₂(H₂O)₄Z, LiK­(H₂O)₄Z, Na₂(H₂O)₃Z, and Rb₂(H₂O)₂Z, unit cell parameters for Rb₂Z and Rb₂(H₂O)₂Z, and the density functional theory (DFT)-optimized structures of K₂Z, K₂(H₂O)₂Z, Rb₂Z, Rb₂(H₂O)₂Z, Cs₂Z, and Cs₂(H₂O)­Z are reported (Z^2– = B₁₂F₁₂^2–) and compared with previously reported X-ray structures of Na₂(H₂O)_0,4Z, K₂(H₂O)_0,2,4Z, and Cs₂(H₂O)­Z. Unusually rapid room-temperature hydration/dehydration cycles of several M₂Z/M₂(H₂O)_<i>n</i>Z salt hydrate pairs, which were studied by isothermal gravimetry, are also reported. Finely ground samples of K₂Z, Rb₂Z, and Cs₂Z, which are not microporous, exhibited latent porosity by undergoing hydration at 24–25 °C in the presence of 18 Torr of H₂O­(g) to K₂(H₂O)₂Z, Rb₂(H₂O)₂Z, and Cs₂(H₂O)­Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24–25 °C in dry N₂ to the original anhydrous M₂Z compounds in 61, 25, and 76 min, respectively (the exact times varied from sample to sample depending on the particle size). The hydrate Na₂(H₂O)₂Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na₂(H₂O)₃Z and dehydration back to Na₂(H₂O)₂Z at 23 °C. For the K₂Z, Rb₂Z, and Cs₂Z transformations, the maximum rate of hydration (rh_max) decreased, and the absolute value of the maximum rate of dehydration (rd_max) increased, as <i>T</i> increased. For K₂Z ↔ K₂(H₂O)₂Z hydration/dehydration cycles with the same sample, the ratio rh_max/rd_max decreased 26 times over 8.6 °C, from 3.7 at 23.4 °C to 0.14 at 32.0 °C. For Rb₂Z ↔ Rb₂(H₂O)₂Z cycles, rh_max/rd_max decreased from 0.88 at 23 °C to 0.23 at 27 °C. For Cs₂Z ↔ Cs₂(H₂O)­Z cycles, rh_max/rd_max decreased 20 times over 8 °C, from 6.7 at 24 °C to 0.34 at 32 °C. In addition, the reversible substitution of D₂O for H₂O in fully hydrated Rb₂(H₂O)₂Z in the presence of N₂/16 Torr of D₂O­(g) was complete in only 60 min at 23 °C