Latent Porosity in Potassium Dodecafluoro-closo-dodecaborate(2−). Structures and Rapid Room Temperature Interconversions of Crystalline K₂B₁₂F₁₂, K₂(H₂O)₂B₁₂F₁₂, and K₂(H₂O)₄B₁₂F₁₂ in the Presence of Water Vapor

Abstract

Structures of K(2)(H(2)O)(2)B(12)F(12) and K(2)(H(2)O)(4)B(12)F(12) were determined by X-ray diffraction. They contain [K(μ-H(2)O)(2)K](2+) and [(H(2)O)K(μ-H(2)O)(2)K(H(2)O)](2+) dimers, respectively, which interact with superweak B(12)F(12)(2-) anions via multiple K···F(B) interactions and (O)H···F(B) hydrogen bonds (the dimers in K(2)(H(2)O)(4)B(12)F(12) are also linked by (O)H···O hydrogen bonds). DFT calculations show that both dimers are thermodynamically stabilized by the lattice of anions: the predicted ΔE values for the gas-phase dimerization of two K(H(2)O)(+) or K(H(2)O)(2)(+) cations into [K(μ-H(2)O)(2)K](2+) or [(H(2)O)K(μ-H(2)O)(2)K(H(2)O)](2+) are +232 and +205 kJ mol(-1), respectively. The calculations also predict that ΔE for the gas-phase reaction 2 K(+) + 2 H(2)O → [K(μ-H(2)O)(2)K](2+) is +81.0 kJ mol, whereas ΔH for the reversible reaction K(2)B(12)F(12 (s)) + 2 H(2)O((g)) → K(2)(H(2)O)(2)B(12)F(12 (s)) was found to be -111 kJ mol(-1) by differential scanning calorimetry. The K(2)(H(2)O)(0,2,4)B(12)F(12) system is unusual in how rapidly the three crystalline phases (the K(2)B(12)F(12) structure was reported recently) are interconverted, two of them reversibly. Isothermal gravimetric and DSC measurements showed that the reaction K(2)B(12)F(12 (s)) + 2 H(2)O((g)) → K(2)(H(2)O)(2)B(12)F(12 (s)) was complete in as little as 4 min at 25 °C when the sample was exposed to a stream of He or N(2) containing 21 Torr H(2)O((g)). The endothermic reverse reaction required as little as 18 min when K(2)(H(2)O)(2)B(12)F(12) at 25 °C was exposed to a stream of dry He. The products of hydration and dehydration were shown to be crystalline K(2)(H(2)O)(2)B(12)F(12) and K(2)B(12)F(12), respectively, by PXRD, and therefore these reactions are reconstructive solid-state reactions (there is also evidence that they may be single-crystal-to-single-crystal transformations when carried out very slowly). The hydration and dehydration reaction times were both particle-size dependent and carrier-gas flow rate dependent and continued to decrease up to the maximum carrier-gas flow rate of the TGA instrument that was used, demonstrating that the hydration and dehydration reactions were limited by the rate at which H(2)O((g)) was delivered to or swept away from the microcrystal surfaces. Therefore, the rates of absorption and desorption of H(2)O from unit cells at the surface of the microcrystals, and the rate of diffusion of H(2)O across the moving K(2)(H(2)O)(2)B(12)F(12 (s))/K(2)B(12)F(12 (s)) phase boundary, are even faster than the fastest rates of change in sample mass due to hydration and dehydration that were measured. The exchange of 21 Torr H(2)O((g)) with either D(2)O or H(2)(18)O in microcrystalline K(2)(D(2)O)(2)B(12)F(12) or K(2)(H(2)(18)O)(2)B(12)F(12) at 25 °C was also facile and required as little as 45 min to go to completion (H(2)O((g)) replaced both types of isotopically labeled water at the same rate for a given starting sample of K(2)B(12)F(12), demonstrating that water molecules were exchanging, not proton...

Halogenated closo‐Dodecaborate Anions Stabilize Weakly Bound [(Me3NH)3X]2+ (X = Cl, Br) Dications in the Solid State

“…This means that the Coulomb explosion of [Li 2 (SO 2 ) 8 ] 2+ to give two [Li(SO 2 ) 4 ] + cations, which is thermodynamically favorable in the gas phase, is suppressed in the solid state by the high lattice energy of the dication/dianion salt. In line with this, [Cs 2 (NCMe) 2 ] 2+ dications stabilized by [B 12 Y 12 ] (Y = Cl, Br, I) and [K 2 (OH 2 ) n ] 2+ ( n = 2,4) dications stabilized by [B 12 F 12 ] 2– were reported as well . Thus, perhalogenated dodecaborate anions [B 12 Y 12 ] 2– (Y = F, Cl, Br, I) are able to lattice‐stabilize weakly bound dications in the solid state that otherwise would dissociate into two singly charged cations.…”

“…In contrast to other classes of weakly coordinating anions, the closo ‐dodecaborates carry a charge of –2, which causes a higher lattice energy of the respective salts. The higher lattice energy leads to reduced solubility and favors the formation of salts of the type M 2+ [B 12 Y 12 ] 2– , which has been demonstrated before on several occasions . In addition, the spherical [B 12 Y 12 ] 2– anions tend to form distorted close‐packed structures with large holes, which can be filled by suitable cations.…”

Synthesis, structure, and reactivity of AlMe2(1-Me-CB11F11): An AlMe2+ cation-like species bonded to a superweak anion

“…The rhomb has three Á Á Á distances, 6.826, 7.495, and 9.055 Å . The short 6.826 Å distance is among the shortest observed for a 1-R-CB 11 F 11 À or B 12 F 12 2À compound [11][12][13][14], and this distance in [AlMe 2 (1-Me-CB 11 F 11 )] 2 is between anions that are almost perfectly dovetailed (the relevant dihedral angle is 84.58 instead of 908; the four FÁ Á ÁF distances are 2.697, 2.946, 2.978, and 3.083 Å ). Interestingly, the 9.055 Å distance is between two anions that point directly at one another (see the highlighted F and C atoms near the bottom part of Fig.…”

The Structure of (H3O)2B12F12·6H2O – a CCP Lattice of B12F122– Anions Intercalated with a Nonplanar Network of O–H···O Connected O6 Rings

“…Selected interatomic distances and angles are listed in Table 1. The structure of the B 12 F 12 2-anion in this compound is normal, [1,[3][4][5][6] and it is also the most precise structure of any molecular B 12 species, with B-F and B-B esd values of 0.0005-0.0008 Å ( Figure S1). The anions are packed in a cubic close-packed (CCP)-like array, with a network of H 3 O + ions and H 2 O molecules intercalated between the parallel planes of the anions (Figure 1).…”

“…[1][2][3][4][5][6][7] Our hypothesis is that the unique combination of size, shape, high symmetry, thermal stability, and extremely weak Brønsted and Lewis basicity of this anion will lead to unanticipated structures and properties [i.e. unanticipated compared to salts of B 12 H 12 2-, [8][9][10][11] B 12 Cl 12 2-, [12][13][14] B 12 (OH) 12 2-, [15,16] and B 12 (CH 3 ) 12 2- [17] on one hand, and of salts of typical fluoroanions, such as BF 4 -, PF 6 -, Sb 2 F 11 -, SiF 6 2-, and MnF 6 2-, on the other]. For example, K 2 B 12 H 12 exhibits a common antifluorite structure, [10] but K 2 B 12 F 12 exhibits an intermetallic Ni 2 In structure [3] (the B 12 centroids occupy the idealized hexagonal close-packed positions of the In atoms [18] ), which is the first time that this structure has been observed for an ionic compound with a polyatomic anion at ambient temperatures and pressures.…”