Latent Porosity in Potassium Dodecafluoro-closo-dodecaborate(2-). Structures and Rapid Room Temperature Interconversions of Crystalline K_(2)B_(12)F_12, K_(2)(H_(2)O)_(2)B_(12)F_(12), and K_2(H_(2)O)_(4)B_(12)F_(12) in the Presence of Water Vapor

摘要

Structures of K_2(H_2O)_2B_(12)F_(12) and K_2(H_2O)_4B_(12)F_(12) were determined by X-ray diffraction. They contain [K(μ-H_2O)_2K]~(2+) and [(H_2O)K(μ-H_2O)_2K(H_2O)]~(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_2O)_4B_(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 △E values for the gas-phase dimerization of two K(H_2O)~(+) or K(H_2O)_2~(+) cations into [K(μ-H_2O)_2K]~(2+) or [(H_2O)K(μ-H_2O)_2K(H_2O)]~(2+) are +232 and +205 kJ mol~(-1), respectively. The calculations also predict that △E for the gas-phase reaction 2 K~(+) + 2 H_(2)O → [K(μ-H_2O)_2K]~(2+) is +81.0 kJ mol, whereas AH for the reversible reaction K_2B_(12)F_(12)_(s) + 2 H_(2)O_(g) → K_2(H_2O)_2B_(12)F_(12(S)) was found to be -111 kJ mor- 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_2B_12F_12 structure was reported recently) are interconverted, two of them reversibly. Isothermal gravimetric and DSC measurements showed that the reaction K_2B_(12)F_(12(S)) + 2 H_2O_(g) → K_2(H_2O)_2B_(12)F_(12(S)) was complete in as little as 4 min at 25 ℃ when the sample was exposed to a stream of He or N_2 containing 21 Torr H_2O_(g). The endothermic reverse reaction required as little as 18 min when K_2(H_2O)_2B_(12)F_(12) at 25 ℃ was exposed to a stream of dry He. The products of hydration and dehydration were shown to be crystalline K_2(H_2O)_2B_(12)F_(12) and K_2B_(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_2O_(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_2O_(a) with either D_2O or H_2~(18)O in microcrystalline K_2(D_2O)_2B_(12)F_(12) or K_2(H_2~(18)O)_2B_(12)F_(12) at 25 ℃ was also facile and required as little as 45 min to go to completion (H_2O_(g) replaced both types of isotopically labeled water at the same rate for a given starting sample of K_2B_(12)F_(12), demonstrating that water molecules were exchanging, not protons. Significant portions of mass (m) vs time (t) plots for the ~(12)H_2O_(g)/K_(2)~(2,1)H_2O)_2B_(12)F_(12 (s)) exchange reactions fit the equation m ∝ e~(-kt), with 10~3k= 1.9 s-1 for one particle size distribution and 10~3k = 0.50 s- for another. Finally, K_2(H_2O)_4B_(12)F_(12) was not transformed into K_2(H_2O)_4B_(12)F_(12) after prolonged exposure to 21 Torr H_2O_(g) at 25 ℃, 37 Torr H_2O_(g) at 35 ℃, or 55 Torr H_2O_(g) at 45 ℃.