Anion binding and transport with -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. The cis isomer of a calix[4]pyrrole bearing two urea-functionalized arms wraps around a chloride anion and shows 10 times higher transport activity into liposomes than the corresponding trans isomer, which can only use one arm for anion binding.