Many Simple Molecular Cavitands Are Intrinsically Porous (Zero-Dimensional Pore) Materials

The guest-free crystal forms of eight related small molecule cavitands (simplified nomenclature: R, R ′, Y ) are investigated as candidate discrete molecule microcavity materials (DMMMs). Due to their rigid bowl-like molecular structures, many cavitands are incapable of efficient crystal packing in pure form, yielding zero-dimensional porous apohost phases. By molecular modifications that eschew self-inclusion, emphasis is placed on engineering structures that exhibit uniform microcavities that are large enough to accommodate small molecules of interest (e.g., gases or volatile organic compounds). The most thermodynamically stable guest-free crystal forms of several cavitands—namely, H, H, CH 2, H, Me, CH 2, α- Me, H, CH 2, Me, Me, CH 2, Br, Me, CH 2, Me, Et, CH 2, Me, Et, SiMe 2, and Me, i- Bu, CH 2 —appear to be as close packed as possible, yet they exhibit relatively large microcavities (or, zero-dimensional pores) in the range of 27–115 Å 3 . Where self-inclusion is ineffective, the microcavities predictably assimilate the intrinsic cavitand molecular cavity, yet the ultimate size and shape of cavities are also strongly influenced by crystal packing. It is demonstrated that some cavitand solvates, CH 2 Cl 2 @ H, Me, CH 2, x H 2 O@ Me, Et, SiMe 2, and CH 2 Cl 2 @ Me, i -Bu, CH 2 (84:16 rccc / rcct ), maintain host crystal packings that are equivalent to their empty, intrinsically porous phases, and it is argued that the intrinsic pores of DMMMs are particularly suited to selective gas enclathration and/or storage. As a proof-of-concept demonstration, the porous phase of Me, Et, SiMe 2 is shown to capture and temporarily hold Freon-41 (fluoromethane, bp = −78 °C) at room temperature. A single crystal of empty Me, Et, SiMe 2 is shown to uptake CO 2 gas at room temperature, allowing structure determination of x CO 2 @ Me, Et, SiMe 2, and single crystal-to-single crystal dehydration of x H 2 O@ Me, Et, SiMe 2 demonstrates its permeability to water.