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 fit ting, Fourier self-deconvolution and Partial Least Squares analysis have be en applied to investigate the characteristics of bonding, and the results a re correlated with information obtained from a steric analysis. It is shown that association of the halide guest takes place via hydrogen b onding 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 capabil ity 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 t hat 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.