Comparative study of inclusion complexation of tetraalkylphosphonium and ammonium salts with cucurbit[7]uril

Inclusion complexation of tetraalkylphosphonium salts (PSs) with cucurbit[7]uril (CB[7]) was studied spectrophotometrically using methylene blue as a chemical indicator. Differences in the inclusion behaviour caused by the central ions in PSs and tetraalkylammonium salts (ASs) are described herein. The inclusion complexation constant (K) of PS3 with a C3-alkyl chain is remarkably smaller than those of the other PSs, indicating that PS3 is most suitable for clathrate-hydrate formation in bulk solution. In the AS inclusions, AS4 with a C4-alkyl chain showed the small K value. Furthermore, the K values of PSs with CB[7] were measured under high pressure. The K values of CB[7] increased with increasing external pressure and decreasing solvent polarity. From the high-pressure results, the volume change (ΔVrepel) caused by water molecules released from the CB[7] cavity was evaluated. A volumetric study for the inclusion of PSs with CB[7] indicated that in PS6 and PS8 with long C6 and C8 chains, respectively, one alkyl chain was encapsulated in the CB[7] cavity. In the other PSs with short chains, two alkyl chains could be accommodated in the cavity. Based on the effects of temperature, substituents, and external pressure, differences in the inclusion mechanisms of PSs and ASs for CB[7] are discussed.