The Prototypal Diamondoid Topology Hybrid Ultramicroporous Materials and Their Water Vapor Sorption Properties

We report the green synthesis and sorption properties of [Cu(SnF₅OH)(2,2'-bipyrimidine)], <b>dia-SNIFFOH-1-Cu</b>, the first hybrid ultramicroporous material (HUM) with diamondoid (<b>dia</b>) topology and the first porous material of any class to be comprised of pentafluorohydroxostannate(IV) (SNIFFOH) linker anions. Single crystal and variable temperature powder X-ray diffraction revealed that <b>dia-SNIFFOH-1-Cu</b> exhibits reversible single-crystal-to-single-crystal (SC-SC) transformation between its as-synthesized hydrated form (<b>dia-SNIFFOH-1-Cu-α</b>, Cc, <i>V</i> = 1414.45(6)) and a nonporous anhydrous form (<b>dia-SNIFFOH-1-Cu-β</b>, Cc, <i>V</i> = 1240.8(7)) induced by temperature or humidity swing. Dynamic water vapor sorption experiments revealed S-shaped isotherm profiles with an inflection point at ∼10% relative humidity (RH) and uptake of 12 weight-% (313 cm³/cm³) at 90% RH. Humidity swing cycling (0-30% RH) indicates hydrolytic stability over at least 50 adsorption/desorption cycles. Notably, whereas H₂O induced this phase transformation, CO₂ (195 and 298 K), He (77 and 298 K), N₂ (77 K), and C₂/C₃ hydrocarbons (298 K) registered negligible uptakes. <b>dia-SNIFFOH-1-Cu</b> is to our knowledge the first porous sorbent that can serve as a sieve for water over gases as verified by dynamic breakthrough experiments conducted with wet CO₂. Whereas the pentafluorooxoniobate analog, <b>dia-NBOFFIVE-1-Cu</b>, is isostructural (Cc, <i>V</i> = 1178.52(9)) with <b>dia-SNIFFOH-1-Cu-β</b>, it has different pore chemistry that inhibits H₂O adsorption. <b>dia-SNIFFOH-1-Cu</b> represents a rare example of a flexible desiccant that, thanks to a low RH step and high volumetric uptake, offers high volumetric productivity for atmospheric water harvesting while also serving as a sieve toward CO₂.