Cation binding resorcinarene bis-crowns: the effect of lower rim alkyl chain length on crystal packing and solid lipid nanoparticles

Abstract

A group of seven resorcinarene bis-crown ethers (CNBC5) with two polyether bridges at the upper rim and either propyl, butyl, pentyl, heptyl, nonyl, decyl or undecyl groups at the lower rim were synthesized and their binding properties with Cs⁺ were investigated by NMR titration. The bis-crowns form 1 : 2 complexes with Cs⁺ with binding constants of log K 4–5. Crystal structures of bis-crowns and their Cs⁺ and K⁺ complexes were studied and different packing motifs were found depending on the alkyl chain length. Short ethyl, propyl and butyl alkyl chains gave a layer or pillar packing where the polar and non-polar regions cannot be distinguished, whereas longer pentyl and decyl chains formed bilayers. Amphiphilic properties and self-assembly in water were studied by preparing solid lipid nanoparticles (SLNs) from the bis-crowns. All investigated compounds formed stable SLNs showing amphiphilic character, which in the case of the short chain bis-crowns probably arises from their locked boat conformation separating the polar face of the molecule from the non-polar face.