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)_nZ 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 T 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.