Multisite Anion Receptors: Interplay of OH and CH Binding Sites in Sulfate-Bridged Resorcinarenes

We report sulfate-bridged resorcin[4]arenes as hetero-multisite anion receptors combining OH and CH binding sites within a single macrocyclic framework. DFT calculations show that ─SO₂─ bridges strongly increase the electrostatic surface potential (ESP) at lower rim CH sites. Receptors bearing two ─SO₂─ bridges at opposite corners and various upper rim substituents (H, Br, NO₂), and free OH groups were synthesized; NMR spectroscopy and single-crystal X-ray diffraction confirm cone conformations with inward-oriented ─SO₂─ bridges. Anion binding studies with chloride in THF, THF:water, and chloroform show that replacing two ─CH₂─ bridges with ─SO₂─ bridges enhances CH binding by up to two orders of magnitude. Both CH and OH sites act as effective binding motifs, with their relative contributions governed by substituents and solvent. The solvent dependence arises from the distinct characterisation of the two sites: OH sites display highly localized positive ESPs, while CH sites possess more diffuse positive ESPs. THF selectively solvates OH sites whereas chloroform solvates CH sites while leaving OH binding intact. NMR, UV-Vis, and ITC confirm distinct thermodynamic signatures for CH- and OH-dominated complexes, establishing a solvent-controlled hierarchy between the two motifs and enabling multisite receptors to remain effective across diverse environments.