An infrared study of host–guest association in solution by substituted resorcinarene cavitands. Part I. Structural aspects of halide complexation by a tetraurea cavitand

Complexation of chloride, bromide and iodide ion was investigated for a resorcinarene cavitand substituted on its upper rim with four N-(o-nitrophenoxy-n-octyl)ureido groups. Association in chloroform takes place solely via hydrogen bonding by the urea moieties, and is monitored well by infrared spectroscopy. Association constants are high, about 104 M–1, and a small preference for chloride over bromide and iodide is observed.Upon binding of the anion, the array of weak intramolecular bonds is disrupted and replaced by hydrogen bonds towards the halide lone pairs. No significant reorganisation of the urea o-nitrophenoxy-n-octyl substituents was found, which suggests a re-ordering of the urea groups only upon binding. Co-operation of the bonding moieties is observed in the complexation of halide ions as a result of pre-organisation of the urea groups on the upper rim of the resorcinarene cavitand by weak hydrogen bonding interactions.