The recognition of 1,5-naphthalenedisulfonate by a protonated azamacrocyclic ligand

Abstract

A protonated azamacrocyclic chemosensor containing two anthracene fragments can recognize 1,5-naphthalenedisulfonate (1,5-NDS) over halides and other oxoanions, ¹H NMR spectra and crystal structure analysis give further insight into the binding interactions.

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Anion recognition and sensing continue to attract much attention due to their wide applications in chemistry,¹ biology,² medicine,³ and environment science.⁴ In general, to encapsulate an anion, the receptor should provide a cavity which contains functional groups capable of interacting with the guest, and the selectivity depends both on the energy terms (related to the intensity of the receptor–substrate interaction) and on geometrical factors (size and shape matching between the receptor and substrate).^1e Custelcean and Moyer⁵ also suggested that the selectivity would be affected by the organizational rigidity of the receptor, including both the rigidity and structural uniqueness. In the past few decades, many polyaza ligands (including cryptands) have been designed and synthesized for the recognition and sensing of anions.⁶ Among them, fluorescent chemosensors draw particular attention due to the simple instrumentation required and usually low detection limit reached.⁷

In the present paper, we focused our attention on the polyaza macrocyclic ligand L¹ (Scheme 1), which was previously shown as a fluorescent molecular switch driven by the addition sequence of metal cations (Zn²⁺ and Cd²⁺).⁸ We noted that L¹ contains two polyamine binding units and two fluorescent anthracene fragments. Upon protonation, L¹ can recognize anions through both energy terms (electrostatic, hydrogen-bonding and anion–π/π–π interactions) and geometrical factors (size matching effect). So the protonated L¹ should be a good receptor and fluorescent chemosensor for anions which contain aromatic groups. Indeed, the 1,5-naphthalenedisulfonate (1,5-NDS) anion, which is a pollutant in industrial dying wastewater,⁹ was found to be recognized selectively by the protonated L¹. In acidic conditions, the fluorescence of the protonated L¹ is quenched significantly upon the addition of 50 equiv of 1,5-NDS, and crystal structure analysis provided further insight into the binding interactions between the host and guest. For comparison, a similar azamacrocyclic ligand L² was also synthesized, and its recognizing property for 1,5-NDS has also been investigated.

Scheme 1 The structures of the receptors and substrates.

Protonated receptors H₈L¹Cl₈ and H₆L²Cl₆ (Scheme 1) were synthesized according to literature methods.^8,10 Both receptors display strong fluorescence at low pH values, as the electron transfer process is prohibited by the protonation of amines, as observed in previous studies.^8,11 Fluorescent spectra of [H₈L¹]⁸⁺ with various anions in aqueous solution (Britton–Robinson buffer, pH 1.81) were investigated. As shown in Fig. 1, the fluorescent intensities of [H₈L¹]⁸⁺ do not change even when 50 equiv of F⁻, Cl⁻, Br⁻, HCO₃⁻, NO₃⁻, ClO₄⁻ were added, and only slightly decrease in the presence of 50 equiv of I⁻, HSO₄⁻, and C₂O₄²⁻. However, it is very interesting to note that the fluorescence of [H₈L¹]⁸⁺ was significantly quenched when 50 equiv of 1,5-NDS was added, indicating [H₈L¹]⁸⁺ can recognize 1,5-NDS over other anions. The Job’s plot of [H₈L¹]⁸⁺ with 1,5-NDS in ¹H NMR measurements displays a maximum chemical shift of H_f in [H₈L¹]⁸⁺ at [H₈L¹]/{[H₈L¹] + [1,5-NDS]} = 0.5 ([H₈L¹] is the concentration of [H₈L¹]⁸⁺), indicating a 1 : 1 inclusion compound was formed (see Fig. S1, ESI†).¹² In addition, the fluorescent titration of [H₈L¹]⁸⁺ with 1,5-NDS (Fig. 2) gave an association constant (K_a) of 4.0 ± 0.3 × 10⁴ M⁻¹ (R² = 0.9963).¹³ We postulate that the fluorescence quenching effect by 1,5-NDS is caused by π–π stacking interactions between the two anthracene fragments of [H₈L¹]⁸⁺ and the naphthalene fragment of 1,5-NDS, through an energy transfer process,¹⁴ and this was confirmed by the results of the UV–vis titration of [H₈L¹]⁸⁺ with 1,5-NDS. As shown in Fig. S2, ESI,† the absorption bands at 356, 375 and 395 nm for [H₈L¹]⁸⁺ are red shifted upon addition of 1,5-NDS, and the red shift is attributed to the π–π stacking interactions between the two anthracene fragments of [H₈L¹]⁸⁺ and the naphthalene fragment of 1,5-NDS.^14b To reveal the binding mechanism, fluorescent spectra of [H₈L¹]⁸⁺ with naphthalene (NA), 1-naphthalenesulfonate (1-NS), and 1,4-naphthalenedicarboxylic acid (1,4-NDC) were also investigated (see Fig. S3–S5, ESI†). However, the fluorescent intensities of [H₈L¹]⁸⁺ do not change or slightly decrease upon adding NA, 1-NS and 1,4-NDC, indicating NA, 1-NS and 1,4-NDC can not be encapsulated into the cavity of [H₈L¹]⁸⁺. This implies that electrostatic and hydrogen-bonding interactions between [H₈L¹]⁸⁺ and 1,5-NDS play a pivotal role during the association.

Fig. 1 Fluorescent spectra of [H₈L¹]⁸⁺ (5 μM) upon additions of 50 equiv of different anions at pH 1.81 (Britton–Robinson buffer) (excitation at 368 nm).

Fig. 2 Fluorescent titration of [H₈L¹]⁸⁺ (5 μM) upon additions of 1,5-NDS at pH 1.81 (Britton–Robinson buffer) (excitation at 368 nm). Inset: curve-fitting analysis of the fluorescence emission change at 425 nm.

To confirm that 1,5-NDS was encapsulated by [H₈L¹]⁸⁺, ¹H NMR experiments were carried out in DMSO-d₆–D₂O (1 : 1 v/v, pH 1.81). As shown in Fig. 3, the addition of 2 equiv of 1,5-NDS resulted in significant upfield shifts of the anthracene protons H_a and H_b, demonstrating the presence of π–π stacking interactions between the two anthracene fragments of [H₈L¹]⁸⁺ and the naphthalene fragment of 1,5-NDS. On the other hand, the signals of the methylene protons (H_d, H_e, H_f) in [H₈L¹]⁸⁺ moved downfield, implying strong interactions between the protonated polyamine binding units in [H₈L¹]⁸⁺ and the sulfonate groups of 1,5-NDS.¹⁵ In contrast to 1,5-NDS, the addition of NA or 1-NS caused no change or a very small change (Δδ < 0.05 ppm) in the chemical shifts of the protons in [H₈L¹]⁸⁺ (Fig. 3), implying neither NA nor 1-NS was encapsulated by [H₈L¹]⁸⁺. The K_a value of [H₈L¹]⁸⁺ with 1,5-NDS was calculated by monitoring the chemical shifts of the proton H_f in [H₈L¹]⁸⁺ with 1 : 1 binding stoichiometry, giving the association constant (K_a) 1.6 ± 0.3 × 10⁴ M⁻¹ (Fig. S6 and S7, ESI†).¹⁶ The different values of the binding constants (K_a) obtained by the two different methods are attributed to the different measuring solvents.¹⁷

Fig. 3 Partial ¹H NMR spectra for (a) [H₈L¹]⁸⁺ (2 mM), (b) [H₈L¹]⁸⁺ + NA (2 equiv), (c) [H₈L¹]⁸⁺ + 1-NS (2 equiv), (d) [H₈L¹]⁸⁺ + 1,5-NDS (2 equiv) in DMSO-d₆–D₂O (1 : 1 v/v, pH 1.81, adjusted by DCl).

To further understand the encapsulating nature of [H₈L¹]⁸⁺ towards 1,5-NDS, the crystal structure of [(H₈L¹)(1,5-NDS)][1,5-NDS]₂[HPO₄](H₂O)₁₆ (1·16H₂O) was investigated. Single crystals of 1·16H₂O were obtained by diffusing an aqueous solution of H₈L¹Cl₈ with a methanol solution of 1,5-NDS. As shown in Fig. 4 and S8, ESI,† one divalent 1,5-NDS anion is encapsulated into the cavity of [H₈L¹]⁸⁺, forming a sandwich like structure. The naphthalene ring of the guest and the two anthracene rings of the host are subparallel, with dihedral angles of 7.863° and 10.504°, respectively. The shortest C⋯C contacts between the naphthalene ring of 1,5-NDS and the two anthracene rings of [H₈L¹]⁸⁺ are 3.331(7) Å (C(30)⋯C(46)) and 3.454(8) Å (C(23)⋯C(49)), respectively, demonstrating there are π–π stacking interactions between the host and the guest.¹⁸ In addition, the encapsulated 1,5-NDS anion forms four hydrogen bonds with four protonated nitrogen atoms of [H₈L¹]⁸⁺, with O⋯N hydrogen bonding distances of 2.721(5)–2.885(5) Å (Table S1†). The other two 1,5-NDS and one HPO₄²⁻ counter anions locate outside the cavity of [H₈L¹]⁸⁺ for charge balance (see Fig. S9, ESI†). No complex crystals could be obtained when 1,5-NDS was replaced by NA or 1-NS under the same conditions, due to the weaker interactions between the receptor and the two substrates.

Fig. 4 The structure of the [(H₈L¹)(1,5-NDS)]⁶⁺ cation in 1.

To see if the similar [H₆L²]⁶⁺ receptor can also encapsulate 1,5-NDS or not, the fluorescent titration of [H₆L²]⁶⁺ with 1,5-NDS was performed (see Fig. S10, ESI†), which showed less fluorescence decrease in comparison with the fluorescent titration of [H₈L¹]⁸⁺ with 1,5-NDS, indicating 1,5-NDS may not be encapsulated into the cavity of [H₆L²]⁶⁺, and this was confirmed by the result of the crystal structure analysis. The crystal structure of [H₆L²][1,5-NDS]₃(H₂O)₆ (2·6H₂O) demonstrates that [H₆L²]⁶⁺ does not encapsulate 1,5-NDS due to the size mismatch between the host and guest. As shown in Fig. 5, 1,5-NDS anions locate outside the cavity of [H₆L²]⁶⁺, and interact with [H₆L²]⁶⁺ through intermolecular hydrogen bonds. Two oxygen atoms O(8) and O(9) in one 1,5-NDS anion form two hydrogen bonds with two hydrogen atoms of protonated N(2) and N(2A), resulting in the N(2)⋯N(2A) distance (5.22 Å) being much shorter than the distance between two anthracene rings (10.13 Å). In addition, each [H₆L²]⁶⁺ forms twelve hydrogen bonds with six 1,5-NDS, and each 1,5-NDS forms four hydrogen bonds with two [H₆L²]⁶⁺, with O⋯N hydrogen bonding distances of 2.679(4)–3.003(4) Å (see Fig. S11 and Table S1, ESI†).

Fig. 5 The structure of the [(H₆L²)(1,5-NDS)₂]²⁺ cation in 2.

In summary, we have shown here the highly selective recognition of 1,5-NDS over halide anions and other oxoanions by [H₈L¹]⁸⁺ in water. The 1,5-NDS anion is encapsulated in the cavity of [H₈L¹]⁸⁺, causing the fluorescence quenching of [H₈L¹]⁸⁺ through π–π stacking interactions. The high affinity of [H₈L¹]⁸⁺ toward 1,5-NDS is attributed to electrostatic, hydrogen-bonding, and π–π stacking cooperative interactions, as well as the size matching effect between the host and guest.

Acknowledgements

This work was supported by the 973 Program of China (2012CB821705) and NSFC (Grant Nos. 20831005, 21121061 and 91127002).

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Footnotes

† Electronic Supplementary Information (ESI) available: experimental methods, NMR spectra and fluorescence data, calculation details, supplementary figures and crystallographic data in CIF (PDF). See DOI: 10.1039/c2ra22423b

‡ Crystal data for 1·16H₂O: C₇₄H₁₁₅N₈O₃₈PS₆, M = 1948.07, triclinic, P1̄, a = 12.9514(4), b = 14.2719(5), c = 25.9721(9) Å, α = 87.967(3)°, β = 86.878(3)°, γ = 67.322(3)°, V = 4422.4(3) ų, Z = 2, μ = 2.413 mm⁻¹, D_c = 1.463 Mg m⁻³, F(000) = 2060. 14678 unique (R_int = 0.0367), R₁ = 0.0641, wR₂ = 0.1802 [I > 2σ(I)], GOF = 1.032. For 2·6H₂O: C₇₀H₈₂N₆O₂₄S₆, M = 1583.78, monoclinic, P2₁/c, a = 13.3839(7), b = 22.0863(13), c = 11.7496(5) Å, β = 95.793(4)°, V = 3455.5(3) ų, Z = 2, μ = 2.576 mm⁻¹, D_c = 1.522 Mg m⁻³, F(000) = 1664. 5792 unique (R_int = 0.0493), R₁ = 0.0616, wR₂ = 0.1648 [I > 2σ(I)], GOF = 1.036. CCDC 874373(1) and 874374(2).

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