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 meso -positions with propyl linkers were synthesized as cis - and trans -isomers. The anion binding properties of cis -1 and cis -3 were screened with ion-mobility mass spectrometry, where cis -1 formed complexes with Cl - , Br - and H 2 PO 4 - , whereas cis -3 formed complexes with most of the investigated anions, including Cl - , Br - , I - , NO 3 - , ClO 4 - , OTf - , SCN - and PF 6 - . The structures of the chloride complexes were further elucidated with density functional theory calculations and a crystal structure obtained for cis -1. In solution, chloride and dihydrogenphosphate anion binding with cis -1 and cis -3 were compared using 1 H NMR titrations. To assess the suitability of two-armed calix[4]pyrroles as anion transporters, chloride transport studies of cis -1, cis -3 and trans -3 were performed using large unilamellar vesicles. The results revealed that cis -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 cis -3 in comparison to cis -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.