Cucurbit[7]urilhost–guest complexes of cholines and phosphonium cholines in aqueous solution

Abstract

The neutral host cucurbit[7]uril forms very stable complexes with a series of cationic cholines (R₃NCH₂CH₂OR′⁺) and their phosphonium analogues (R₃PCH₂CH₂OR′⁺) (R₃ = Me₃, Et₃, or Me₂Bz, or R₃N = quinuclidinium, and R′ = H, COCH₃, CO(CH₂)₂CH₃, or PO₃H), and (±)-carnitine, in aqueous solution. The complexation behaviour has been investigated using ¹H and ³¹P NMR spectroscopies, and ESI mass spectrometry. The complexation-induced chemical shift changes of the guests clearly indicate the effects of replacing the N(CH₃)₃⁺ end group by P(CH₃)₃⁺, and changing the nature of R on the position of the guest with respect to the CB[7] cavity and its polar portal-lining carbonyl groups. This study demonstrates that molecular recognition of cholines in aqueous solution is achievable with a neutral host without the need for aromatic walls for cation–π interactions.