An infrared study of host-guest association in solution by substituted resorcinarene cavitands: Part II. Comparison of halide complexation by tetrathiourea cavitands and a simple thiourea

Binding of halide has been studied for two thioureido-substituted resorcin[4]arenes and a model compound, with special attention to the NH stretching region and the bonding behaviour. Mathematical techniques such as curve fitting, Fourier self-deconvolution and Partial Least Squares analysis have been applied to investigate the characteristics of bonding, and the results are correlated with information obtained from a steric analysis.It is shown that association of the halide guest takes place via hydrogen bonding between halide ions and thioureido moieties solely for all ligands. Compared to a simple thiourea compound, pre-organisation of the thioureide side-chains on the upper rim by intramolecular bonding enhances the capability of association by a factor of 10 to 100.An analogy is found between complexation of a halide anion in the cavitands and complexation of an iodide anion by the model compound. This suggests that within the cavitand, chloride, bromide and iodide are all complexed by at least two thioureido moieties, whereas a simple thiourea binds chloride and bromide in a 1-to-1 fashion, and iodide in a 2-to-1 fashion.