Selective encapsulation and extraction of hydrogenphosphate by a hydrogen bond donor tripodal receptor

Abstract

Selective encapsulation of an anion by a hydrogen bond donor scaffold demands design and synthesis of suitable receptors which could discriminate between anions of identical size and shape or basicity. Here, we report the anion coordination chemistry of two second generation tripodal receptors (AUL and AAL) based on ¹H-NMR and crystallization experiments. The tripodal urea-based receptor AUL can selectively encapsulate a hydrogenphosphate (HPO₄²⁻) dianion by six strong hydrogen bonds donated from the three urea groups. Theoretical calculations showed that AUL has the highest binding affinity for hydrogenphosphate when compared to other competitive anions (F⁻, CN⁻, CH₃COO⁻ and HSO₄⁻). Because of its HPO₄²⁻ selectivity, AUL has been successfully employed in the extraction of HPO₄²⁻ from water in the presence of competitive anions (F⁻/OH⁻/CH₃COO⁻) by anion exchange between two immiscible phases. On the other hand, the tripodal amide-based receptor AAL when crystallized in the presence of F⁻, CN⁻, CH₃COO⁻, H₂PO₄⁻ and HSO₄⁻ did not yield any hydrogen-bonded receptor–anion complex and instead crystalline AAL was precipitated in each case. ¹H-NMR experiments showed significant broadening and/or downfield shift of –NH signals in AUL and AAL upon additions of F⁻, Cl⁻, CN⁻, CH₃COO⁻ and H₂PO₄⁻ (supplied as tetraalkylammonium salts), indicative of strong hydrogen bonding interactions between –NH donors and anions in the solution-state.