Effect of spacer geometry on oxoanion binding by bis- and tetrakis-thiourea hosts

Hosts with thiourea groups bind anions by formation of multiple hydrogen bonds. This contribution discusses how spacers linking two or four thiourea groups affect the host affinity and selectivity. While most of the bis-thioureas bind H₂PO₄⁻ preferentially, the extent of selectivity over chloride, acetate, and H₂AsO₄⁻ is determined by the size of the binding cavity. A tetrakis-thiourea is shown to exhibit a unique H₂AsO₄⁻ selectivity, and the discrimination of chloride is enhanced by specific solvation in dimethyl sulfoxide.