Anion binding and transport with meso -alkyl substituted two-armed calix[4]pyrroles bearing urea and hydroxyl groups.

Calix[4]pyrroles bearing hydroxyl (1) or urea (3) groups attached to the -positions with propyl linkers were synthesized as - and -isomers. The anion binding properties of -1 and -3 were screened with ion-mobility mass spectrometry, where -1 formed complexes with Cl, Br and HPO, whereas -3 formed complexes with most of the investigated anions, including Cl, Br, I, NO, ClO, OTf, SCN and PF. The structures of the chloride complexes were further elucidated with density functional theory calculations and a crystal structure obtained for -1. In solution, chloride and dihydrogenphosphate anion binding with -1 and -3 were compared using H NMR titrations. To assess the suitability of two-armed calix[4]pyrroles as anion transporters, chloride transport studies of -1, -3 and -3 were performed using large unilamellar vesicles. The results revealed that -3 had the highest activity among the investigated calix[4]pyrroles, which was related to the improved affinity and isolation of chloride inside the binding cavity of -3 in comparison to -1. The results indicate that appending calix[4]pyrroles with two hydrogen bonding arms is a feasible strategy to obtain anion transporters and receptors with high anion affinity.