The complexation of halide ions by a calix[6]pyrrole

The X-ray crystal structures of the 1∶1 complexes formed by calix[6]pyrrole 1b with Bun4NCl and Bun4NBr show the macrocycle to adopt D3d symmetry and to encapsulate the halide ions within the macroring cavity via six N–H···X– hydrogen bonds; the macrocycle subtly adjusts its conformation to accommodate the differently sized anions; preliminary extraction experiments indicate that 1b is a dramatically stronger chloride ion complexing agent than its smaller analogue calix[4]pyrrole 1a.