Modulation of the Selectivity in Anions Recognition Processes by Combining Hydrogen- and Halogen-Bonding Interactions

Abstract

Most of the halogen bonding receptors for anions described use halogen bonding binding sites solely in the anion recognition process; only a few examples report the study of anion receptors in which the halogen bonding interaction has been used in combination with any other non-covalent interaction. With the aims to extend the knowledge in the behaviour of this kind of mixed receptors, we report here the synthesis and the anion recognition and sensing properties of a new halogen- and hydrogen- bonding receptor which binds anions by the cooperation of both non-covalent interactions. Fluorescence studies showed that the behaviour observed in the anion recognition sensing is similar to the one previously described for the halogen analogue and is quite different to the hydrogen one. On the other hand, the association constants obtained by ¹H-NMR data demonstrate that the mixed halogen- and hydrogen-bonding receptor is more selective for SO₄²⁻ anion than the halogen or hydrogen analogues.

1. Introduction

The field of anion recognition chemistry has become one of the most important areas in supramolecular chemistry due to the important role that anions play in numerous biological and environmental processes [1,2,3,4].

The hydrogen bond (HB) has been probably the most non-covalent interaction used in the design of anion receptors in the last two decades in which amides [5], ureas [6], pyrroles [7] or imidazole [8,9] among others have been uses as hydrogen donors as anion binding sites.

The use of the halogen bonding (XB) interaction as an alternative to the hydrogen bonding in the design of new anion receptors has emerged strongly in the last years. Halogen bonding is a noncovalent bonding interaction between halogen atoms that function as electrophilic centers (Lewis acids) and neutral or anionic Lewis bases [10]. Theoretical calculations indicate that the origin of this attraction arises from the positive electrostatic potential located at the terminus of the R–X axis (σ hole), thus resulting in a strongly linear geometry that maximizes the interface of opposite charges [11]. Although a number of solid state examples and theoretical studies revealed the potential of halogen bonding interactions for anion recognition, only very recently has it been exploited in solution phase. Practically all the halogen bonding receptors for anion described in the literature use halogen bonding binding sites solely to bind anions. Despite that the combination of two or more different non-covalent interactions in the same receptors is becoming an emerging strategy in the design of new anion receptors [12], only a few examples use the halogen bonding interaction in combination with others in the anion recognition process [13,14,15,16,17,18,19,20,21,22,23].

Recently, we have reported the synthesis and the anion sensing properties of two-armed charge-assisted ditopic imidazolium and haloimidazolium bidentade receptors which recognize anions by hydrogen or halogen bonding interaction, respectively [24]. Taking into account the important differences found in the sensing behavior between these two receptors, we decided to perform the synthesis and study of a new charge-assisted bidentade receptor containing both halogen and hydrogen bonding sites by incorporation in its structure of one haloimidazolium, halogen bonding binding site, and one imidazolium, hydrogen bonding binding site, which could act through the cooperative and simultaneous action of halogen bonds and hydrogen bonds during the anion recognition event. The receptor also incorporates two end-caped photoactive anthracene rings as fluorescent signaling units into this host framework (Figure 1).

2. Results and Discussion

2.1. Synthesis

The novel bis-imidazolium receptor 5²⁺·2PF₆⁻ was prepared with a 31% overall yield by a stepwise procedure that involved the alkylation of 1-((anthracen-9-yl)methyl)-4,5-dimethyl-1H-imidazole 2 with 2,7-bis(bromomethyl)naphthalene 1, providing the intermediate 3⁺·Br⁻ (63% yield), which was reacted with 1-((anthracen-9-yl)methyl)-2-bromo-4,5-dimethyl-1H-imidazole 4, yielding the receptor as bromide salt in moderate yield (25%). Anion exchange with the corresponding hexafluorophosphate salt was achieved on addition of aqueous NH₄PF₆. The compounds 2 and 4 were synthesized by alkylation of 4,5-dimethyl-1H-imidazole [25] and 2-bromo-4,5-dimetil-1H-imidazol [26] with 9-(bromomethyl)anthracene, respectively (Scheme 1).

2.2. Anion Binding Studies

The binding and sensing ability of the receptor 5²⁺·2PF₆⁻ toward several anions (F⁻, Cl⁻, Br⁻, I⁻, SO₄²⁻, HSO₄⁻, PhCOO⁻, ClO₄⁻, HP₂O₇³⁻, AcO⁻, NO₃⁻ and H₂PO₄⁻) as tetrabutylammonium salts were investigated by spectroscopic measurements and ¹H-NMR spectroscopy.

The emission spectrum of the receptor 5²⁺·2PF₆⁻ (Φ = 0.036) in CH₃CN showed the characteristic anthracene monomeric bands at λ = 396, 418 and 442 nm, when excited at λ = 370 nm. The addition of the above-mentioned set of anions to a solution of the receptor 5²⁺·2PF₆⁻ in CH₃CN (c = 1 × 10⁻⁵ M) showed that Cl⁻, Br⁻, I⁻, HSO₄⁻, PhCOO⁻, ClO₄⁻ and NO₃⁻ anions had no effect on the emission spectrum, whereas the addition of F⁻, HP₂O₇³⁻, SO₄²⁻, AcO⁻ and H₂PO₄⁻ anions promoted significant changes in the emission spectrum of receptor 5²⁺·2PF₆⁻. These changes were strongly dependent on the anion added and different responses have been observed (Figure 2).

The presence of AcO⁻ (Figure S3, Supplementary Material) and SO₄²⁻ anions (Figure 3a) induced a decrease in the monomer emission band at λ = 419 nm of the receptor 5²⁺·2PF₆⁻ I_receptor/I_complex = 2.55 and 2.13, respectively, and also a decrease of the quantum yield from Φ = 0.036 to Φ = 0.025 for AcO⁻ and Φ = 0.019 for SO₄²⁻ anions. A different fluorescent response was observed for H₂PO₄⁻ anions (Figure 3b) in which the presence of this anion promoted a remarkable decrease of the monomer emission bands at λ = 396, 418 and 412 nm and a progressive increase in a new broad and structureless emission band at λ = 465 nm, attributed to the anthracene excimer emission band (Φ = 0.058) and finally, when HP₂O₇³⁻ (Figure 3c) and F⁻ (Figures S1 and S2) anions were added, two different effects were observed: first the addition up to 1.8 equiv. of HP₂O₇³⁻ anions or 1 equiv. F⁻ anion induced an increase in the monomer emission band of the receptor, subsequent addition of more than 1.8 equiv. of HP₂O₇³⁻ anions and 1 equiv. in the case of F⁻, promoted a continuous decrease in the monomer emission band.

A comparative analysis of the response observed in the fluorescence study indicates that the mixed hydrogen- and halogen- bonding receptor 5²⁺·2PF₆⁻ shows changes similar to the halogen bonding receptor 7²⁺·2PF₆⁻ Both receptors 5²⁺·2PF₆⁻ (HB and XB) and 7²⁺·2PF₆⁻ (XB) act through a photoinduced electron transfer (PET) mechanism in the presence of SO₄²⁻ anions, and therefore a decrease in the intensity of the emission bands was observed [27]. In addition, a broad emission band at λ = 465 nm due to a π-stacking interaction between the two anthracene units [28] appears in the spectrum of the receptor 5²⁺·2PF₆⁻ during the addition of H₂PO₄⁻ anions as it was also observed in the halogen bonding receptor 7²⁺·2PF₆⁻. Interestingly, the remarkable changes observed in the emission spectrum of the receptor 5²⁺·2PF₆⁻ after addition of HP₂O₇³⁻ and F⁻ anions are consistent with a chemodosimeter behavior, which was also observed in the halogen bonding receptor 7²⁺·2PF₆⁻. Interestingly, the emission spectrum of the receptor 5²⁺·2PF₆⁻ undergoes important changes in the presence of AcO⁻ anions to difference with the described for the halogen bonding receptor 7²⁺·2PF₆⁻, whereas the behavior towards H₂PO₄⁻ and SO₄²⁻ anions is similar to that found for 7²⁺·2PF₆⁻. On the other hand, the fluorescent response of the mixed XB and HB receptor 5²⁺·2PF₆⁻ is quite different than for the hydrogen bonding receptor 8²⁺·2PF₆⁻, in which the receptor acted on by a photoinduced electron transfer mechanism with all the tested anions.

The stoichiometry of the complexes was determinate by Job-plot experiments using the changes in the fluorogenic response of the receptor 5²⁺·2PF₆⁻ in the presence of varying concentrations of the tested anions. The results clearly indicated the formation of 1:2 complexes in the case of H₂PO₄⁻ (Figure S7) and AcO⁻ anions (Figure S9), and 1:1 for SO₄²⁻ anions (Figure S8). The association constant values were calculated by fitting the fluorescence data in CH₃CN with the host-guest binding model obtained using the Dynafit program [29] and are summarized in

Table 1. Association constants for receptor 5²⁺·PF₆⁻, 7²⁺·2PF₆⁻ and 8²⁺·2PF₆⁻ with H₂PO₄⁻, SO₄²⁻ and AcO⁻ anions in CH₃CN measured by fluorescence technique. Errors (in percent) are given in parentheses.

Receptor	H2PO4−	SO42−	AcO−
52+·PF6−	β = 5.9 × 1010 M−2 (15)	K = 5.7 × 104 M−1 (14)	β = 2.16 × 109 M−2 (10)
72+·2PF6−	β = 5.0 × 1011 M−2 (16)	K = 8.0 × 104 M−1 (14)	-
82+·2PF6−	β = 9.8 × 1011 M−2 (13)	K = 1.4 × 106 M−1 (6)	-

together with the values previously reported for the halogen bonding receptor 7²⁺·2PF₆⁻ and the hydrogen bonding receptor 8²⁺·2PF₆⁻. The association constants obtained indicate that the hydrogen bonding receptor 8²⁺·2PF₆⁻ binds the recognized anions stronger than the analogous halogen 7²⁺·2PF₆⁻ or the mixed halogen and hydrogen bonding receptor 5²⁺·2PF₆⁻ in the pure organic solvent CH₃CN, while the monohalogenated receptor 5²⁺·2PF₆⁻ and the dihalogenated receptor 7²⁺·2PF₆⁻ bind the anions with similar strength.

The calculated detection limits of the receptor 5²⁺·2PF₆⁻ for H₂PO₄⁻, SO₄²⁻, and AcO⁻ anions were 3.90 × 10⁻⁶ M, 3.57 × 10⁻⁵ M, and 4.59 × 10⁻⁵ M respectively (Figures S4–S6).

The recognition properties of the receptor towards the above-mentioned set of anions were also evaluated by UV-vis spectroscopy. Absorption spectrum of the receptor 5²⁺·2PF₆⁻ in CH₃CN (c = 1 × 10⁻⁵ M) displays two intense absorption bands at λ = 230 nm (ε = 1.31 × 10⁵ M⁻¹ cm⁻¹) and λ = 255 nm (ε = 2.39 × 10⁵ M⁻¹ cm⁻¹) along with the characteristic absorption bands attributed to the anthracene moieties at λ = 334 (ε = 6 × 10³ M⁻¹ cm⁻¹), λ = 351 (ε = 11.14 × 10⁴ M⁻¹ cm⁻¹), λ = 370 (ε = 1.5 × 10⁴ M⁻¹ cm⁻¹) and λ = 390 nm (ε = 1.35 × 10⁴ M⁻¹ cm⁻¹). The addition of increase amounts of SO₄²⁻, AcO⁻, HP₂O₇³⁻ and F⁻ anions to a solution of the receptor in CH₃CN (c = 1 × 10⁻⁵ M) caused small perturbations in the absorption spectrum of the receptor; by contrast, the addition of H₂PO₄⁻ anions induced a decrease in the absorption bands at λ = 250 and λ = 255 nm while a decrease and red-shifted ∆λ = 2 nm in the anthracene absorption bands was observed. The well-defined isosbestic point observed at λ = 260, λ = 343, λ = 373, λ = 382 and λ = 390 nm indicates that a neat interconversion between the uncomplexed and complexed species occurs (Figure 4). Unfortunately, these changes do not allow an accurate determination of the association constant.

In order to obtain additional information about the binding mode of the receptor 5²⁺·2PF₆⁻ with the previously tested anions, ¹H-NMR titration experiments were performed. The ¹H-NMR spectrum of the receptor in CD₃CN/CD₃OD (8/2) exhibit the following characteristic signals: (a) the protons of the methyl groups of the Br-imidazolium unit appears at δ = 1.86 and 2.15 ppm and the ones corresponding to the H-imidazolium moiety at δ = 2.15 and 2.59 ppm; (b) the protons of the methylenes naphthalene–CH₂–imidazolium and imidazolium–CH₂–anthracene appear as four different singlets around δ = 5.12 and 5.50 ppm and δ = 6.18 and 6.45 ppm, respectively; and (c) the protons of the naphthalene and the anthracene groups are embeded in the aromatic region in the range δ = 6.94–8.79 ppm.

The addition of increase amounts of H₂PO₄⁻ (Figure 5) anions to a solution of the receptor 5²⁺·2PF₆⁻ in CD₃CN/CD₃OD (8/2) induced the splitting and a significant downfield shifts of the inner naphthalene protons H_a,a’ (∆δ ~ 0.23 ppm). The methylene protons H_b, H_b’, H_c and H_c’ were also downfield shifted; ∆δ = 0.09, 0.11, 0.05 and 0.07 ppm, respectively. Unfortunately the observation of the downfield shift of the H-imidazolium proton was not detected due to the signal of this proton within the aromatic region; nevertheless, the simultaneous participation in the anion binding process of the H and the Br sited at C-2 of the H-imidazolium and Br-imidazolium rings, respectively, was supported by the upfield shift observed in the methyl protons of the Br-imidazolium ring H_d (∆δ = −0.16 ppm) and in the methyl protons H_e’ of the H-imidazolium ring (∆δ = −0.10 ppm) (Figure S13).

Similar changes were observed with the presence of SO₄²⁻ anions but to a lesser extent than those produced with H₂PO₄⁻ anions (Figure S10).

The addition of increasing amounts of HP₂O₇³⁻ (Figure S11) and F⁻ anions (Figure S12) to a solution of the hydrogen and halogen bonding receptor 5²⁺·2PF₆⁻ produce different changes in the ¹H-NMR than those previously described for H₂PO₄⁻ and SO₄²⁻ anions. In these cases, the addition of HP₂O₇³⁻ and F⁻ anions up to 1 equiv. promote the downfield shift of the internal naphthalene protons H_a,a’ (∆δ ~ 0.09 ppm) as well as the methylene protons H_b, H_b’, H_c and H_c’ (∆δ ~ 0.03 ppm), suggesting that a recognition processes is taking place. However, the addition of more than 1 equiv. of HP₂O₇^3- induced the appearance of new signals and the progressive disappearance of the signals assigned to the complexes formed between the receptor 5²⁺·2PF₆⁻ with HP₂O₇³⁻ or F⁻ anions. After addition of 2 equiv. of anion, the ¹H-NMR spectrum showed only one set of signals. The titration was also followed by mass spectrometry. The results indicate that a debromination process is taking place in the bromo-imidazolium ring by the formation of an imidazolone ring to generate the new compound 6²⁺·PF₆⁻ (Scheme 2). Interestingly the H-imidazolium ring remains unperturbed in the presence of HP₂O₇³⁻ and F⁻ anions. These results are consistent with those reported previously for the halogen and hydrogen bonding receptor 7²⁺·2PF₆⁻ and 8²⁺·2PF₆⁻, respectively [10] and dramatically reveal the lability of the C2–X bond of the haloimidazolium ring that undergoes cleavage in the presence of basic anions, which constitutes a clear and important limitation for the use of such kind of receptors against basic anions.

The association constants calculated from the ¹H-NMR titration data using the Dynafit program [15] are shown in

Table 2. Association constants for receptors 5²⁺·2PF₆⁻, 7²⁺·2PF₆⁻ and 8²⁺·2PF₆⁻ with H₂PO₄⁻ and SO₄²⁻ anions in CD₃CN/CD₃OD 8:2 v/v measured using the ¹H-NMR technique. Errors (in percent) are given in parentheses.

Receptor	H2PO4−	SO42−	K(SO42−)/β(H2PO4−)
52+·PF6−	β = 1.37 × 102 M−2 (4)	K = 2.6 × 103 M−1 (4)	18
72+·2PF6−	β = 9.9 × 103 M−2 (3)	K = 5.5 × 103 M−1 (4)	0.55
82+·2PF6−	β = 5.6 × 103 M−2 (4)	K = 2.2 × 103 M−1 (4)	0.39

.

A comparative study of the association constants obtained for the receptor 5²⁺·PF₆⁻ with those reported for the halogen bonding receptor 7²⁺·2PF₆⁻ and the hydrogen bonding receptor 8²⁺·2PF₆⁻ in the competitive mixture CD₃CN/CD₃OD 8:2 v/v indicates that the receptors bind the SO₄²⁻ anions following the trend 7²⁺·2PF₆⁻ (XB receptor) > 5²⁺·PF₆⁻ (XB and HB) > 8²⁺·2PF₆⁻ (HB). Thus, the presence of halogen bonding interactions increases the strength of the receptors for SO₄²⁻ anion. Interestingly, the association constant between the receptor 5²⁺·PF₆⁻ and SO₄²⁻ anions is 18 times higher than with the H₂PO₄⁻ anion, while higher association constants for the receptors 7²⁺·2PF₆⁻ and 8²⁺·2PF₆⁻ were found for H₂PO₄⁻ anion, and therefore the halogen and hydrogen bonding receptor 5²⁺·PF₆⁻ is the receptor which shows the highest selectivity for SO₄²⁻ anions.

3. Experimental Section

3.1. General Comments

All reactions were carried out using solvents that were dried by routine procedures. All melting points were determined by means of a Kofler hot-plate melting-point apparatus (Wagner & Munz, München, Germany) and are uncorrected. Solution ¹H- and ¹³C-spectra were recorded with Bruker 200, 300, 400, or 600 MHz spectrometers (Bruker Corporation, Billerica, MA, USA). The following abbreviations have been used to state the multiplicity of the signals: s (singlet), m (multiplet), and q (quaternary carbon atom). Chemical shifts (δ) in the ¹H- and ¹³C-NMR spectra are referenced to tetramethylsilane (TMS). UV−vis and fluorescence spectra were carried out in the solvents and concentrations stated in the text and in the corresponding figure captions, using a dissolution cell with 10 mm path length, and they were recorded with the spectra background corrected before and after sequential additions of different aliquots of anions. Quantum yield values were measured with respect to anthracene as the standard (Φ = 0.27 ± 0.01) using the equation:

Φ_x/Φ_s = (S_x/S_s)((1 − 10^−As)/(1 − 10^−Ax))(n_s²/n_x²)

(1)

where x and s indicate the unknown and standard solution, respectively, Φ is the quantum yield, S is the area under the emission curve, A is the absorbance at the excitation wavelength and n is the refractive index. Mass spectra were recorded with a MLC-MS TOF 6220 (Agilent Technologies Germany, Waldbronn, Germany).

3.2. General Procedure for Synthesis of Compounds 2 and 4

To a solution of 4,5-dimethyl-1H-imidazol (0.27 g, 2.81 mmol) or 2-bromo-4,5-dimethyl-1H-imidazol (0.3 g, 1.71 mmol) in CH₃CN (120 mL) was added dropwise an aqueous solution of 1 M NaOH (3.27 mmol). The reaction medium was stirred for 10 min and after this time, 9-(bromomethyl)anthracene (0.5 g, 1.75 mmol) was added in one portion; the resulting mixture was stirred at 0 °C for 30 min in the case of compound 2 and 2h at room temperature for compound 4. The resulting precipitate was separated by filtration and purified by silica gel column chromatography (CH₂Cl₂/CH₃OH 95:5) to yield compound 2. In the case of compound 4, the resulting precipitate was filtered and dissolved in CH₂Cl₂ (50 mL) and then washed with water (2 × 50 mL). The organic solvent was collected and dried with anhydrous Na₂SO₄. After filtration, the solvent was removed under reduced pressure to yield the desired compound.

1-((anthracen-9-yl)methyl)-4,5-dimethyl-1H-imidazole (2).Yellow solid, yield 19%, ¹H-NMR (CDCl₃, 400 MHz) δ = 8.55 (s, 1H); 8.06–8.01 (m, 4H), 7.53–7.46 (m, 4H); 6.56 (s, 1H); 5.76 (s, 2H); 2.41 (s, 3H); 2.18 (s, 3H); ¹³C-NMR (CDCl₃, 100 MHz) δ = 134.3; 133.6; 131.4; 130.9; 129.3; 127.3; 125.3; 124.1; 123.1; 122.2; 41.7; 12.7; 9.0; MS (ESI, m/z): 287.10 [M + H]⁺.

1-((anthracen-9-yl)methyl)-2-bromo-4,5-dimethyl-1H-imidazole (4). Yield 48%, ¹H-NMR (CDCl₃, 400 MHz) δ = 8.48 (s, 1H); 8.10–8.05 (m, 4H), 7.57–7.48 (m, 4H); 6.06 (s, 2H); 1.97 (s, 3H); 1.22 (s, 3H); ¹³C-NMR (CDCl₃, 100 MHz) δ = 135.6; 132.1; 131.6; 130.4; 130.3; 128.1; 127.5; 126.1; 125.1; 124.1; 117.1; 45.4; 12.9; 10.9; MS (ESI, m/z): 364.08 [M + H]⁺.

3.3. Synthesis of Compound 3⁺·Br⁻

To a solution of compound 2 (0.25 g, 0.87 mmol) in CH₃CN (180 mL), 2,7-bis(bromomethyl)naphthalene (0.65 g, 2 mmol) was added in one portion. The mixture was stirred for 90 min at 60 °C; the volatile compounds were removed under vacuum and the resulting residue was purified by silica gel column chromatography (CH₂Cl₂/CH₃OH 9:1) to yield compound 3 as bromide salt. Yield 63%, ¹H-NMR (CDCl₃, 400 MHz) δ = 9.54 (s, 1H); 8.57 (s, 1H), 8.35 (d, 2H, J = 9 Hz), 8.06 (d, 2 H, J = 9 Hz), 7.73–7.66 (m, 5H); 7.40-7.46 (m, 4H), 6.45 (s, 2H); 5.48 (s, 2H); 4.71 (s, 1H), 4.61 (s, 1H), 2.03 (s, 3H); 1.66 (s, 3H); ¹³C-NMR (CDCl₃, 75 MHz) δ = 136.0; 135.4; 132.8; 132.5; 131.2; 131.0; 130.7; 130.6; 129.6; 128.9; 128.4; 128.2; 127.9; 127.6; 127.4; 126.4; 125.5; 125.0; 122.9; 121.3; 50.9; 45.1; 33.6; 9.7; 8.7; MS (ESI, m/z): 519.20 [M]⁺.

3.4. Synthesis of Receptor 5²⁺·2Br⁻

To a solution of compound 3⁺·Br⁻ (0.3 g, 0.5 mmol) in acetonitrile (200 mL), compound 4 (0.36 g, 0.98 mmol) was added. The reaction mixture was stirred at 60 °C for 48 h. The solvent was then removed under reduced pressure and the crude was purified by silica gel column chromatography (CH₂Cl₂/CH₃OH 9:1) to yield the desired compound as bromide salt. Yield 25%, ¹H-NMR (d₆-DMSO, 300 MHz) δ = 8.90 (s, 1H); 8.83 (s, 1H), 8.38 (d, 2H, J = 9 Hz), 8.30 (s, 1H), 8.26–8.17 (m, 6H), 7.95 (d, 1H, J = 9 Hz), 7.88 ( d, 1H, J = 8 Hz), 7.70–7.55 (m, 10H), 7.23 (dd, 1H, J = 9 Hz, J = 0.5 Hz), 7.00 (dd, 1H, J = 9 Hz, J = 0.5 Hz), 6.62 (s, 2H), 6.37 (s, 2H), 5.66 (s, 2H), 5.34 (s, 2H); 2.59 (s, 3H), 2.17 (s, 3H); 2.13 (s, 3H), 1.82 (s, 3H); ¹³C-NMR (d₆-DMSO, 75 MHz) δ = 135.6; 134.4; 133.8; 133.3; 132.6; 132.3; 132.1; 131.8; 131.5; 131.3; 131.1; 130.9; 130.2; 130.1; 129.1; 129.0; 128.3; 127.2; 127.1; 126.9; 126.4; 126.2; 125.8; 125.0; 124.7; 124.3; 123.4; 122.9; 51.6; 50.8; 48.5; 45.1; 11.4; 10.4; 9.5; MS (ESI, m/z): 885.2 [M²⁺ + Br⁻]⁺.

3.5. Synthesis of Receptor 5²⁺·2PF₆⁻

A solution of the bis-imidazolium receptor as the bromide salt 5²⁺·2Br⁻ (0.1 g, 0.1 mmol) in CH₂Cl₂ (20 mL) was washed with a saturated solution of NH₄PF₆ (4 × 20 mL) for 30 min. The organic layer was separated, dried over anhydrous Na₂SO₄, filtered and evaporated under reduced pressure, yielding the desired receptor. Yield 62%, ¹H-NMR (d₆-DMSO, 400 MHz) δ = 8.90 (s, 1H); 8.83 (s, 1H), 8.33 (d, 2H, J = 8 Hz), 8.30 (s, 1H), 8.26 (d, 2H, J = 8 Hz), 8.22 (d, 2H, J = 8 Hz), 8.14 (d, 2H, J = 8 Hz), 7.94 (d, 1H, J = 8 Hz), 7.87 (d, 1H, J = 8 Hz), 7.70-7.58 (m, 8H), 7.53 (s, 1H), 7.49 (s, 1H), 7.19 (dd, 1H, J = 8 Hz, J = 0.5 Hz), 6.95 (dd, 1H, J = 8 Hz, J = 0.5 Hz), 6.57 (s, 2H), 6.33 (s, 2H), 5.65 (s, 2H), 5.32 (s, 2H); 2.58 (s, 3H), 2.17 (s, 3H); 2.13 (s, 3H), 1.40 (s, 3H); ¹³C-NMR (d₆-DMSO, 100 MHz) δ = 134.0; 132.9; 132.3; 132.0; 131.9; 131.2; 130.9; 130.8; 130.6; 130.4; 130.1; 129.7; 129.5; 128.9; 128.7; 128.6; 126.9; 125.9; 125.6; 125.5; 125.0; 124.8; 124.4; 123.4; 123.1; 122.8; 121.8; 121.3; 50.2; 48.4; 46.8; 43.5; 9.8; 8.9; 8.4; 7.9; MS (ESI, m/z): 951.2 [M²⁺ + PF₆⁻]⁺.

4. Conclusions

We have reported the synthesis of a novel anion sensor 5²⁺·2PF₆⁻ which binds anions by the cooperative action of halogen- and hydrogen-bonding interactions. Several anion-binding experiments have been carried out in order to make a comparative study of the sensing capabilities of the novel XB and HB receptor 5²⁺·2PF₆⁻ regarding the XB and HB analogues 7²⁺·2PF₆⁻ and 8²⁺·2PF₆⁻, respectively. Evaluation of the sensing properties by fluorescence in CH₃CN reveals important similarities in the sensing behaviour between the halogenated receptor 7²⁺·2PF₆⁻ and the mixed XB and HB receptor 5²⁺·2PF₆⁻. The XB and HB receptor 5²⁺·2PF₆⁻ acts as a selective fluorescent molecular sensor for H₂PO₄⁻ anion, because it is the unique anion which promotes the appearance of the anthracene excimer emission band. In addition, the presence of the HP₂O₇³⁻ anion produces the selective transformation in the corresponding mono-imidazolone 6²⁺·2PF₆⁻ after debromination of the Br-imidazolium ring while the H-imidazolium ring remained unperturbed. The analysis of the obtained association constant values obtained from the ¹H-NMR titration experiments in CD₃CN/MeOD (8/2) reveals that the selectivity showed for the mixed XB and HB receptor 5²⁺·2PF₆⁻ for SO₄²⁻ anions is higher than the ones observed for the halogen or hydrogen bonding analogues. The magnitude of the association constant with SO₄²⁻ anions of the mixed XB and HB receptor 5²⁺·2PF₆⁻ is intermediate between the halogen bonding receptor 7²⁺·2PF₆⁻ and the hydrogen bonding receptor 8²⁺·2PF₆⁻ in this competitive methanolic medium. These results highlight the importance of the combinations of different non-covalent interactions in selective anion recognition and sensing.