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

Abstract

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 ¹H 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^- > H₂PO₄^- > HCO₃^- > HSO₄^- > CH₃COO^- > SO₄^2- > Cl^- > Br^- > I in DMSO-d₆ . 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^-, H₂PO₄^-, HCO₃^-, HSO₄^- or CH₃COO^- due to the strong NH⋯anion interactions. The observed higher binding affinity for HSO₄^- than SO₄^2- is attributed to the proton transfer from HSO₄^- 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-d₆ . 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).

Fig. 1

Partial ¹H NMR spectra of L1 (2mM) in the presence of one equivalent of different anions in DMSO-d₆ (H1 = CONHAr, H2 = CH₂NHCO).

Fig. 2

Partial ¹H NMR spectra of L2 (2mM) in the presence of one equivalent of different anions in DMSO-d₆ (H1 = CSNHAr, H2 = CH₂NHCS).

Fig. 3

Partial ¹H NMR titration of L2 showing changes in the NH chemical shifts of the receptor with an increasing amount of HSO₄⁻ in DMSO-d₆. (H1 = CSNHAr and H2 = CH₂NHCS).

Fig. 4

¹H NMR titration plot of changes in the NH (CH₂NHCO) chemical shifts of L1 with an increasing amount of different anions in DMSO-d₆.

Fig. 5

¹H NMR titration plot of changes in the NH (CH₂NHCS) chemical shifts of L2 with an increasing amount of different anions in DMSO-d₆.

Fig. 6

¹H NMR titration plot of changes in the aromatic CH chemical shifts (CHCNH) of L1 and L2 with an increasing amount of F⁻ in DMSO-d₆.

Fig. 7

2D NOESY NMR of (a) Free L1, (b) Free L2, (c) L1 + HSO₄⁻ (1 eq.) and (d) L2 + HSO₄⁻ (1 eq.) (H1 = ArNH and H2 = CH₂NH).

Fig. 8

Optimized structures of (a) L1 and (b) L2 calculated at the M06-2X/6-31G(d,p) level of theory.

Fig. 9

Optimized structures of (a) [L1(SO₄)]²⁻ and (b) [L2(SO₄)]²⁻ calculated at the M06-2X/6-31G(d,p) level of theory.

Fig. 10

Optimized structures of (a) [L1(HSO₄)]⁻ and (b) [L2(HSO₄)]⁻ calculated at the M06-2X/6-31G(d,p) level of theory.

Fig. 11

Comparative ¹H NMR spectra of (a) L2, (b) Extracted fluoride-L2 complex, (c) L2 in the presence of one equivalent of [n-Bu₄N]⁺F⁻ in DMSO-d₆. (H1 = CSNHAr and H2 = CH₂NHCS).

Fig. 12

Comparative FT-IR spectra of L2 (black) and extracted fluoride-L2 complex (red).

Scheme 1

Schematic representation of chemical structures of L1 and L2 (a), and electrostatic potential map for L1 (b) and L2 (c) calculated at M06-2X/6-31G(d,p) level theory (red is negative potential and blue is positive potential).

Scheme 2

Synthetic pathway of L1 and L2.