A chiral macrocyclic receptor for sulfate anions with CD signals

Abstract

A novel macrocyclic binaphthalene derivative (L) containing amide and triazole units has been synthesized by “click” reaction. The binding behaviors of this receptor toward anions have been studied by ¹H NMR titration, circular dichroism (CD) spectroscopy. The tetrahedral sulfate anion predominantly interacts with L through hydrogen bonds which could tune the dihedral angle between the two naphthalene rings and supply tunable CD output signals to form chiral receptor.

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Introduction

During the past decade, scientists have developed many techniques in supramolecular chemistry leading to assemblies possessing various architectures and properties.^1–4 Anions are playing important roles to understand self-assembly strategies of structures based on intermolecular interactions.^5–7 As a result, there is much attention paid to tetrahedral anion receptors,^8–10 which have wide applications in chemical sensors, molecular medicines and catalysis. Tetrahedral oxyanions expand the functionality of the supramolecular system and provide a means to control molecular interactions for improving physical and chemical properties for application in chemical, material and biological fields. Therefore, design and application of efficient receptors based on tetrahedral oxyanions have been actively exploited during the last decades to develop new self-assembly structures and devices by weak interactions in supramolecular chemistry.^11–15 Hydrogen bonding is an important approach to induce supramolecular aggregations, which is widely used in the design and synthesis of anion receptors. Meanwhile, typical anion recognition moieties such as amide,^16–20 pyrrole,²¹ urea,^22–25 ammonium,^26,27 thiourea,²⁸ and imidazolium^29–32 have been widely studied as hydrogen donors to bind the anion species (N–H⋯X⁻) to realize supramolecular architectures. 1,2,3-Triazoles can serve as a surrogate for amide bonds,^32–35 which give us the possibility to replace one of the amide NH groups in the urea moiety with 1,2,3-triazole to generate a versatile anion-receptor building block that combines the characteristics of urea moiety and triazole.^36,37

NMR, UV/vis or fluorescence spectroscopies were used to monitor anion binding processes.^38,39 CD spectroscopy is a convenient method to monitor conformational changes of the supramolecular systems, which is a powerful tool for sensing anions when the supramolecular response detected from other spectroscopies is particularly weak.⁴⁰

Molecular sensors for sulfate anions mainly based on triazolium, imidazolium, pyrrole, amide, urea and heterotopic combination of these binding motifs. Amide and urea NH groups have been employed to produce a wide range of sulfate receptors.⁴¹ For example, Das, Ganguly and co-workers have reported a tren based tris(urea) receptor which demonstrated selective binding with sulfate both in solution and solid states.⁴² Amide-based macrocyclic receptors are also found to be particularly well-suited for encapsulating sulfate anions.⁴³ All these receptors prefer to bind sulfate over dihydrogenphosphate, acetate, nitrate and perchlorate. However, the construction of chiral receptors for sulfate anions is relatively rare.⁴⁴ Several examples of chiroptical sensors that provide significantly differentiated CD signals upon binding of metal ions, neutral guests, or chiral polymers have been recently reported.^45,46

Herein, we designed a molecule L (see Scheme 1) based on the chiral framework of binaphthalene,⁴⁷ with 1,2,3-triazoles and amide units as the binding motifs. Anions can form strong intermolecular hydrogen bonds with the amide groups and triazole units which may result in the changes of the dihedral angle between the two naphthalene rings, thus the variation of the CD signals from the binaphthalene framework can be observed. This chiral molecular receptor shows an efficient sensor property with obvious CD signal changes in the presence of sulfate anion, however, it only exhibits weak CD signal changes when binding to other anions, such as H₂PO₄⁻, HSO₄⁻, AcO⁻ and Br⁻.

Scheme 1 The synthetic scheme for the preparation of the compound L.

Results and discussion

1,1-Binaphthyl derivatives containing chiral units have been found extensive applications in molecular recognition, asymmetric synthesis, and applied materials.^48,49 binaphthol (BINOL) moiety also has unique features, such as stable chiral configuration ((R)- and (S)-BINOL), easy modification at 2-, 3-, 4-, 5- and 6-positions. We chose 2,2-dihydroxy-1,1-binaphthyl and 1,2,3-triazole as the functional linking groups in our molecule. The synthesis of the binaphthyl derivatives were performed following a stepwise strategy as shown in Scheme 1. The terminal azide 2 was afforded at 45% yield under gentle basic conditions (potassium carbonate) by previous methods.⁵⁰ Compound 3 was synthesized by the amide-formation coupling reaction.⁵¹ Compound 3 was coupled with 2via click reaction in toluene, giving compound L at 40% yield. The COSY-NMR spectrum (Fig. 1) was used to identify protons of the receptor L.

Fig. 1 COSY-NMR spectra of L in CD₃CN at 298 K.

Sulfate is a tetrahedral anion which can form NH⋯X⁻ hydrogen bonds through two oxygen atoms with amide groups, the remaining oxygen atoms can form hydrogen bonds with CH of the 1,2,3-triazole. ¹H NMR titration was carried out to understand the binding mode of L with sulfate anion. Fig. 2b showed the ¹H NMR spectra of L with different amounts of bis(tetrabutylammonium) sulfate (TBA₂SO₄) in [D₆] acetonitrile. In general, π–π stacking results in upfield shift of protons, hydrogen-bonding interaction results in downfield shift of protons. Upon the addition of 1.0 equivalent of TBA₂SO₄, the signals from amide NH (Δδ_Hj = 3.35 ppm), triazole CH (Δδ_Hd = 2.13 ppm) and benzene CH (Δδ_Ha = 1.25 ppm) protons showed downfield shifts, due to the multiple hydrogen bonds between the sulfate and L. With the addition of 1.5 equivalents to 10.0 equivalents of TBA₂SO₄, the signals from the protons of triazole CH (H_d), amide NH (H_j), benzene CH (H_a) showed upfield shifts, and those from the protons H_m, H_h of the binaphthalene also showed slight upfield shifts.

Fig. 2 (a) Illustration of the sulfate-binding process of receptor L, (b) ¹H NMR spectra of L in CD₃CN at 298 K upon titrational addition of SO₄²⁻, (c) NOESY spectra of L with 1.0 equiv. of SO₄²⁻ in CD₃CN at 298 K. (d) NOESY spectra of L with 1.5 equiv. of SO₄²⁻ in CD₃CN at 298 K.

As shown in Fig. 2c, there were strong NOE connections between the H_j and H_a (H_d–H_j, H_d–H_e, H_j–H_e) in the presence of 1.0 equivalent of sulfate. Ascribed to the unique configuration of the L, one amide NH and two triazoles CH of L could bind with one molecule of sulfate by multiple hydrogen bonds as 1 : 1 host–guest complex. NOESY spectroscopy in the presence of 1.5 equivalents of sulfate (Fig. 2d) showed some new NOE connections between the benzene protons (H_a), triazole protons (H_d), and binaphthalene protons (H_f) (H_f–H_d, H_a–H_f, H_a–H_c). Due to the long distances between protons H_f and H_d, H_a and H_f, H_a and H_c in one molecule, intramolecular cross peaks in NOESY NMR could not be observed, these signals (H_f–H_d, H_a–H_f, H_a–H_c) must correspond to intermolecular contacts between different chains, indicating that two macrocyle L arranged in head to tail conformation in the final 2 : 3 complex which was stabilized by π–π stacking between the binaphthalene unit and benzene unit.

These results clearly indicated that the addition of sulfate to a solution of host L first led to the formation of a 1 : 1 host–guest complex, two 1 : 1 complexes then sandwiched another sulfate with intermolecular hydrogen bonds, which resulted in the final 2 : 3 binding stoichiometry between L and sulfate (as shown on Fig. 2a). This configuration can explain the small upfield shift of the signals from triazole proton H_d, amide H_j, and even the up-field shift of those from the aromatic protons (H_m, H_a, H_h), which is due to the interaction of π–π stacking (upfield shift).

UV-vis spectral studies were carried out in acetonitrile to evaluate the binding properties of compound L (Fig. 3a). The absorption spectra of L exhibit an intense absorption band (λ_max = 227 nm), assigned to the π–π* transition of the naphthalene chromophores. Upon addition of sulfate, the absorption at λ_max = 227 nm decreased until one equivalent of sulfate was added. This absorption change was due to the destruction of the π–π conjugation within the binaphthaene rings through the binding of sulfate anions. The binding stoichiometry of the sulfate–L interactions was calculated to be 2 : 3 from the Job's plot (Fig. S11†). The fluorescence intensity was quenched upon the addition of SO₄²⁻ (Fig. S18†), due to the electron transfer from electron-rich sulfate anions to binaphthalene chromophore.

Fig. 3 (a) UV-vis absorption spectra of L (1 × 10⁻⁵ mol L⁻¹ in acetronitrile) upon addition of SO₄²⁻ in acetronitrile, (b) CD spectra of L (1 × 10⁻⁵ mol L⁻¹) upon addition of SO₄²⁻ in acetronitrile.

The multi-hydrogen bonding interaction can modulate the dihedral angle of naphthalene rings which leads to the changes of CD signals. As shown in Fig. 3b, the CD spectra of compound L (1.0 × 10⁻⁵ M) were changed upon the addition of SO₄²⁻. Compound L exhibits CD signals at 220 nm and 239 nm (Fig. 3b) due to the chiral binaphthane-rings. The intensity of CD band at 239 nm was increased upon the addition of SO₄²⁻, and it was increased about 200% (Fig. 3b) with 5.0 equiv. of SO₄²⁻, further addition of SO₄²⁻ just increased CD signals slightly (Fig. S7†).

On the basis of previous studies,^53,54 the enhancement of CD band intensity reflects that the dihedral angle of binaphthalene is reduced. As the energy-minimized structure of compound L and sulfate–L shown in Fig. S31,† the dihedral angle of binaphthalene ring in L was calculated to be 93°, for the sulfate–L complex, the dihedral angle of binaphthalene ring was calculated to be 76°.⁵⁵ The dihedral angles of compound L and sulfate–L complex were respectively about 92° and 73° based on CD data according to the method described by Salvadori et al.⁵⁶

The binding behaviors of L with other anions were also investigated with ¹HNMR titration. As shown in Fig. S19–S24,† with the addition of I⁻ (Fig. S20†), the signals from protons H_j (Δδ = 1.0 ppm), H_a (Δδ = 0.6 ppm), H_d (Δδ = 0.5 ppm) showed downfield shifts, which was due to the weak multiple hydrogen-bonding between compound L and I⁻. While in the presence of Cl⁻ (as shown in Fig. S19†), only the signals from protons H_j (Δδ = 1.7 ppm) and H_a (Δδ = 2.0 ppm) showed downfield shifts, those from triazole H_d and other protons showed no chemical shifts. Similar phenomenon was observed for Br⁻ titration (Fig. S19†). Upon the addition of H₂PO₄⁻ (Fig. S20†), the signal of amide proton H_j (Δδ_Hj = 2.4 ppm) showed downfield shifts. Those from other protons such as triazole CH (H_d), aromatic proton (H_a) showed weak chemical-shifts. While in the presence of HSO₄⁻, (as shown in Fig. S24†), the signals from protons H_j (Δδ = 2.2 ppm), H_a (Δδ = 0.5 ppm), H_d (Δδ = 0.4 ppm) showed downfield shifts. The spectra of receptor L with various amounts of AcO⁻ anions were shown in Fig. S21,† the signal of amide (NH) shifted downfield more largely (Δδ_Hj = 2.8 ppm) than those of the triazole CH (Δδ_Hd = 0.5 ppm) and aromatic proton CH (Δδ_Ha = 1.0 ppm).

The binding stoichiometry of the L–anion (I⁻, Cl⁻, Br⁻, AcO⁻, H₂PO₄⁻) interaction was confirmed to be 1 : 1 from the Job's plot (as shown in Fig. S12 and S13†). The stability constant of I⁻ and AcO⁻ with L were determined by the analysis of the signal shifts of benzene proton H_a, because of line broadening of the binding-site proton signals in the ¹H NMR spectra. With the addition of H₂PO₄⁻, the signal from proton H_j shifted downfield (as shown on Fig. S20†). The stability constant of the H₂PO₄⁻ was obtained by the analysis of the amide NH (H_j) signals (as shown in Fig. 4). The stability constant of the SO₄²⁻ was calculated by fitting the chemical shifts of proton H_j (as shown on Fig. S27†). While the stability constant of the HSO₄⁻ was calculated by fitting the chemical shifts of proton Ha (as shown on Fig. S24†). The data were fitted with the WinEQNMR software to give K (Table 1).⁵² While the stability constant (K) of Br⁻, Cl⁻ anions could not be determined by analysis of signal shifts of protons due to the nonlinear line broadening. Benesi–Hildebrand curve method⁴⁷ was employed to calculate the stability constant K from spectropolarimetric titration data (Fig. S29 and 30†). The stability constant of SO₄²⁻ is undoubtedly the biggest one among the anions studied.

Fig. 4 Changes in the chemical shift of proton H_j (L–H₂PO₄⁻) upon addition of H₂PO₄⁻ to a solution of L in CD₃CN at 298 K. Square symbols represent experimental data points; continuous lines represent fitted curves.

Table 1 Binding constants (K) determined by ¹H NMR or UV titration

Anions	Br^−^b	I^−^a	HSO4^−^a	Cl^−^b	SO4^2−^a	H2PO4^−	AcO^−
a Anions as nBu4N^+ salts in [D3] CH3CN, triazole protons Ha fitted with WinEQNMR,^20 errors less than 10%. b Benesi–Hildebrand curve methods. c r = I0 − Ix/I0 (I0 is the CD intensity of L at 238 nm. Ix is the CD intensity of the L after addition of anions. r showed ratio of CD signals changes).
K	113 ± 1	108 ± 3	360 ± 4	120 ± 1.5	12970 ± 146	300 ± 5	140 ± 2
r ^c (%)	28%	30%	60%	28%	28%	28%	28%

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As shown in Fig. 2a, compound L has three typical anion recognition sites: amide, triazole and benzene ring. Amide (NH) group is a typical strong hydrogen bond donor which can easily form hydrogen bonds as NH⋯X⁻ complex. The strength of the hydrogen-bonding is influenced by the radius size and the basicity of X⁻.^57,58 The smaller radius size and stronger basicity of X⁻ can result in high strength hydrogen-bonding. From this theory, the anion binding constant with amide (NH) should be in the order of AcO⁻ > H₂PO₄⁻ ≫ Br⁻, but from the binding constants (K) (as shown in Table 1), L showed the binding order of H₂PO₄⁻ > AcO⁻ >Br⁻. This strange binding order is due to the specific structural compensation of the receptor and anions. For example, AcO⁻ is a planar structure, in which two oxygen atoms preferentially bind with one amide and one triazole units or one benzene protons on the same side of the L through hydrogen-bonding, while SO₄²⁻ with four binding sites could cooperatively bind with L. Due to the specific structure of compound L, L can bind with one sulfate as 1 : 1 complex, then the complex binds with another sulfate as 2 : 3 host–guest complex, which shows large binding constants (K). We figure out its 1 : 1 complex binding constants (K₁) by WinEQNMR software (as shown on Table 1). For tetrahedral H₂PO₄⁻ and SO₄²⁻, H₂PO₄⁻ should have bigger binding constants than SO₄²⁻ based on basicity, while the size of SO₄²⁻ is more suitable for the cavity of the compound L than that of H₂PO₄⁻, and the cooperative effect in the sulfate–L complex further increased the binding strength. On the other hand, the charge difference also contributed to binding affinity. Halogen anions are weak base anions with small size, which bind with compound L weakly.

The structural compensation between the receptor and anions also influenced the CD signals of this system. The trend of CD signal changes (Table 1) is consistent with the binding constants. Sulfate induced the largest CD signal increment (about 200%), while other anions caused weak or moderate CD signal changes (from 28% to 60%). In compound L, there are two pairs of amide and triazole units, anions which could form multiple hydrogen-bonds with both pairs of binding units of L could rotate the naphthane rings and tune the dihedral angle. Thus tetrahetral anions (SO₄²⁻, H₂PO₄⁻) induced more CD signal changes than the planar AcO⁻ and sphere halides (Cl⁻, Br⁻, I⁻), and the smaller sulphate with two charges dragged both sides more strongly than H₂PO₄⁻, which showed the largest CD signal changes. Tetrahedral sulfate anions which could form multiple hydrogen bonds with both pairs of binding units of L could tune the CD signals of the binaphthalene framework.^53,59

Conclusion

In summary, we have successfully synthesized a new macrocyclic chiral compound L based on the axially chiral binaphthalene containing two pairs of amide and triazole units. The anion binding studies of L indicated that the structural compensation between the receptor and anions greatly influenced the binding strength and selectivity. The results showed that we could apply molecular recognition principles to fabricate chiral molecular receptor based on the binaphthalenes with amide and 1,2,3-triazoles units. This study provides an ideal structure model for building anion-active receptor.

Experimental section

General methods and materials

Synthesis used chemicals and solvents that were reagent grade purchased from Aldrich, Acros Chemical Co., and used without further purification. The silica gel for column chromatography was purchased from JIYIDA Silica Gel Corp. in Qing Dao (200–300 mesh). Melting points were measured with an XT4-100X apparatus and uncorrected. ¹H NMR and ¹³C NMR spectra were recorded with Bruker 400 MHz spectrometers. Infrared spectra were obtained on a Perkin-Elmer System 2000 FT-IR spectrometer. MS spectra were determined with BEFLEX III for TOF-MS and AEI-MS 50 for EI-MS. HRMS was determined with FTICR-APEX. CD spectra were recorded in a Jasco J-810spectrophotometer, the scan rate was 1000 nm min⁻¹, and all of the spectra were accumulated two times.

Compound (L)

8-Diaza [5.4.0] bicycloundec-7-ene (DBU) (4.0 mmol, 0.7 mL) was added to toluene (400 mL), degassed (argon) for 30 minutes and heated to 75 °C while flushing with argon. At 75 °C, CuI (0.10 mmol, 20 mg) was added to the mixture. A solution of 2 (85.0 mg, 0.2 mmol) and 3 (50.0 mg, 0.2 mmol) in THF (5 mL) and toluene (50 mL) was added to the solution slowly over 10 h and stirred for another 12 h under argon. The reaction was quenched with water and washed with H₂O (100 mL × 3), dried over anhydrous Na₂SO₄ and concentrated under reduced pressure. The product was purified via chromatography (SiO₂, CH₂Cl₂ : MeOH 25 : 1) to afford L (20 mg, 30%) as a white solid. ¹H NMR (CDCl₃,400 MHz, δ): 8.13 (s, 1H), 8.08 (t, J = 8.4 Hz, 2H), 7.78 (d, J = 8.6 Hz, 2H), 7.68 (d, J = 8.6 Hz, 2H)7.64 (t, J = 8.6 Hz, 1H),7.59 (s, 2H),7.31 (d, J = 6.3 Hz, 2H), 7.20 (d, J = 6.3 Hz, 2H)7.17 (s, 2H), 7.02 (d, 2H), 4.50–4.48 (t, J = 7.3 Hz, 2H), 4.45–4.41 (t, J = 8.3 Hz, 2H), 4.38–4.21 (t, J = 8.3 Hz, 4H), 4.19–4.15 (t, J = 8.3 Hz, 4H). ¹³CNMR (CDCl₃, 100 MHz, δ): 166.9, 153.7, 134.6, 133.7, 131.4, 130.2, 129.9, 129.7, 128.2, 127.1, 125.1, 124.9, 121.1, 117.1, 69.5, 50.5, 35.4, MS (MALDI-TOF): m/z 687.3 [M + Na], HRMS (ESI): found, 664.2555 (M), Calcd for C₃₈H₃₂N₈O₄, 664.2547.

Acknowledgements

This study was supported by the National Basic Research 973 Program of China (2011CB932302 and 2012CB932900) and the National Nature Science Foundation of China (201031006, 21290190, 21322301).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR and ¹³C NMR of compound L. UV-vis Spectra and CD Signals of compound L with various anions, See DOI: 10.1039/c3ra46049e

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