Synthesis and anion binding studies of tris(3-aminopropyl)amine-based tripodal urea and thiourea receptors proton transfer-induced selectivity for hydrogen sulfate over sulfate

摘要

Tris(3-aminopropyl)amine-based tripodal urea and thiourea receptors, tris([(4-cyanophenyl)amino]propyl)urea ( L1 ) and tris([(4-cyanophenyl)amino]propyl)thiourea ( L2 ), have been synthesized and their anion binding properties have been investigated for halides and oxoanions. As investigated by ~(1) H NMR titrations, each receptor binds an anion with a 1?:?1 stoichiometry via hydrogen-bonding interactions (NH?anion), showing the binding trend in the order of F ~(?) > H _(2) PO _(4) ~(?) > HCO _(3) ~(?) > HSO _(4) ~(?) > CH _(3) COO ~(?) > SO _(4) ~(2?) > Cl ~(?) > Br ~(?) > I in DMSO- d _(6) . The interactions of the receptors were further studied by 2D NOESY, showing the loss of NOESY contacts of two NH resonances for the complexes of F ~(?) , H _(2) PO _(4) ~(?) , HCO _(3) ~(?) , HSO _(4) ~(?) or CH _(3) COO ~(?) due to the strong NH?anion interactions. The observed higher binding affinity for HSO _(4) ~(?) than SO _(4) ~(2?) is attributed to the proton transfer from HSO _(4) ~(?) to the central nitrogen of L1 or L2 which was also supported by the DFT calculations, leading to the secondary acid–base interactions. The thiourea receptor L2 has a general trend to show a higher affinity for an anion as compared to the urea receptor L1 for the corresponding anion in DMSO- d _(6) . In addition, the compound L2 has been exploited for its extraction properties for fluoride in water using a liquid–liquid extraction technique, and the results indicate that the receptor effectively extracts fluoride from water showing ca. 99% efficiency (based on L2 ).