Latent Porosity in Potassium Dodecafluoro-closo-dodecaborate(2−). Structures and Rapid Room Temperature Interconversions of Crystalline K2B12F12, K2<

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• 作者：Dmitry V. Peryshkov ; Alexey A. Popov ; Steven H. Strauss
• 刊名：Journal of the American Chemical Society
• 出版年：2010
• 出版时间：October 6, 2010
• 年：2010
• 卷：132
• 期：39
• 页码：13902-13913
• 全文大小：483K
• 年卷期：v.132,no.39(October 6, 2010)
• ISSN：1520-5126

文摘

Structures of K₂(H₂O)₂B₁₂F₁₂ and K₂(H₂O)₄B₁₂F₁₂ were determined by X-ray diffraction. They contain [K(μ-H₂O)₂K]²⁺ and [(H₂O)K(μ-H₂O)₂K(H₂O)]²⁺ dimers, respectively, which interact with superweak B₁₂F₁₂²⁻ anions via multiple K···F(B) interactions and (O)H···F(B) hydrogen bonds (the dimers in K₂(H₂O)₄B₁₂F₁₂ 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(H₂O)⁺ or K(H₂O)₂⁺ cations into [K(μ-H₂O)₂K]²⁺ or [(H₂O)K(μ-H₂O)₂K(H₂O)]²⁺ are +232 and +205 kJ mol⁻¹, respectively. The calculations also predict that ΔE for the gas-phase reaction 2 K⁺ + 2 H₂O → [K(μ-H₂O)₂K]²⁺ is +81.0 kJ mol, whereas ΔH for the reversible reaction K₂B₁₂F_12(s) + 2 H₂O_(g) → K₂(H₂O)₂B₁₂F_12(s) was found to be −111 kJ mol⁻¹ by differential scanning calorimetry. The K₂(H₂O)_0,2,4B₁₂F₁₂ system is unusual in how rapidly the three crystalline phases (the K₂B₁₂F₁₂ structure was reported recently) are interconverted, two of them reversibly. Isothermal gravimetric and DSC measurements showed that the reaction K₂B₁₂F_12(s) + 2 H₂O_(g) → K₂(H₂O)₂B₁₂F_12(s) was complete in as little as 4 min at 25 °C when the sample was exposed to a stream of He or N₂ containing 21 Torr H₂O_(g). The endothermic reverse reaction required as little as 18 min when K₂(H₂O)₂B₁₂F₁₂ at 25 °C was exposed to a stream of dry He. The products of hydration and dehydration were shown to be crystalline K₂(H₂O)₂B₁₂F₁₂ and K₂B₁₂F₁₂, 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 H₂O_(g) was delivered to or swept away from the microcrystal surfaces. Therefore, the rates of absorption and desorption of H₂O from unit cells at the surface of the microcrystals, and the rate of diffusion of H₂O across the moving K₂(H₂O)₂B₁₂F_12(s)/K₂B₁₂F_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 H₂O_(g) with either D₂O or H₂¹⁸O in microcrystalline K₂(D₂O)₂B₁₂F₁₂ or K₂(H₂¹⁸O)₂B₁₂F₁₂ at 25 °C was also facile and required as little as 45 min to go to completion (H₂O_(g) replaced both types of isotopically labeled water at the same rate for a given starting sample of K₂B₁₂F₁₂, demonstrating that water molecules were exchanging, not protons. Significant portions of mass (m) vs time (t) plots for the ^1,2H₂O_(g)/K₂(^2,1H₂O)₂B₁₂F_12(s) exchange reactions fit the equation m e^−kt, with 10³k = 1.9 s⁻¹ for one particle size distribution and 10³k = 0.50 s⁻¹ for another. Finally, K₂(H₂O)₂B₁₂F₁₂ was not transformed into K₂(H₂O)₄B₁₂F₁₂ after prolonged exposure to 21 Torr H₂O_(g) at 25 °C, 37 Torr H₂O_(g) at 35 °C, or 55 Torr H₂O_(g) at 45 °C.