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)], dia-SNIFFOH-1-Cu, the first hybrid ultramicroporous material (HUM) with diamondoid (dia) 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 dia-SNIFFOH-1-Cu exhibits reversible single-crystal-to-single-crystal (SC-SC) transformation between its as-synthesized hydrated form (dia-SNIFFOH-1-Cu-α, Cc, V = 1414.45(6)) and a nonporous anhydrous form (dia-SNIFFOH-1-Cu-β, Cc, V = 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. dia-SNIFFOH-1-Cu 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, dia-NBOFFIVE-1-Cu, is isostructural (Cc, V = 1178.52(9)) with dia-SNIFFOH-1-Cu-β, it has different pore chemistry that inhibits H₂O adsorption. dia-SNIFFOH-1-Cu 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.