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Molecules 2017, 22(12), 2273; doi:10.3390/molecules22122273

Article
Modulation of the Selectivity in Anions Recognition Processes by Combining Hydrogen- and Halogen-Bonding Interactions
Dto de Química Orgánica, Universidad de Murcia, Campus de Espinardo, E-30100 Murcia, Spain
*
Authors to whom correspondence should be addressed.
Received: 17 November 2017 / Accepted: 18 December 2017 / Published: 20 December 2017

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.
Keywords:
halogen-bonding; hydrogen-bonding; anion; fluorescence; imidazolium; recognition; sensing

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 Ireceptor/Icomplex = 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 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 Ha,a’ (∆δ ~ 0.23 ppm). The methylene protons Hb, Hb’, Hc and Hc’ 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 Hd (∆δ = −0.16 ppm) and in the methyl protons He’ 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 Ha,a’ (∆δ ~ 0.09 ppm) as well as the methylene protons Hb, Hb’, Hc and Hc’ (∆δ ~ 0.03 ppm), suggesting that a recognition processes is taking place. However, the addition of more than 1 equiv. of HP2O73- 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.
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:
Φxs = (Sx/Ss)((1 − 10−As)/(1 − 10−Ax))(ns2/nx2)
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.

Supplementary Materials

The supplementary materials are available online. 1H- and 13C-NMR spectra of the compound, Figures S1–S3: Changes in the emission spectrum of 52+·2PF6 upon addition of F and AcO anions, Figures S4–S6: Calculation of the detection limits, Figures S7–S9: Job´s Plot experiments, Figures S10–S13: 1H-NMR spectral changes of the receptor 52+·2PF6 during the addition of SO42−, HP2O73− and F anions.

Acknowledgments

This work was funded by the Ministerio de Economía y Competitividad Government of Spain and European FEDER CTQ2013-46096-P as well as by the Fundación Séneca Región de Murcia (CARM) Projects 18948/JLI/13 and 19337/PI/14. P.S. acknowledges the University of Murcia for a FPU predoctoral grant. F.Z. and A.C. acknowledge the Government of Spain for a Juan de la Cierva and Ramon y Cajal contract, respectively.

Author Contributions

F.Z., A.C. and P.M. conceived and designed the experiments; F.Z., P.S. and S.J.B.-B. performed the experiments; F.Z., P.S., S.B., A.C. and P.M. analyzed the data; F.Z., A.C. and P.M. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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  • Sample Availability: Samples of the compounds are not available from the authors.
Figure 1. Structure of the halogen and hydrogen bonding receptors 72+·2PF6 and 82+·2PF6 reported in [24] and the structure of the mixed halogen and hydrogen bonding receptor 52+·2PF6 proposed in this work.
Figure 1. Structure of the halogen and hydrogen bonding receptors 72+·2PF6 and 82+·2PF6 reported in [24] and the structure of the mixed halogen and hydrogen bonding receptor 52+·2PF6 proposed in this work.
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Scheme 1. Synthesis of the halogen and hydrogen bonding receptor 52+·2PF6.
Scheme 1. Synthesis of the halogen and hydrogen bonding receptor 52+·2PF6.
Molecules 22 02273 sch001
Figure 2. Emission spectrum of the receptor 52+·2PF6 (c = 1 × 10−5 M) in CH3CN (red line) and after the addition of 5 equiv. of H2PO4 (blue line), SO42− (pink line), AcO (orange line), HP2O73− (green line) and F (yellow line).
Figure 2. Emission spectrum of the receptor 52+·2PF6 (c = 1 × 10−5 M) in CH3CN (red line) and after the addition of 5 equiv. of H2PO4 (blue line), SO42− (pink line), AcO (orange line), HP2O73− (green line) and F (yellow line).
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Figure 3. Changes in the fluorescence spectra of receptor 52+·2PF6 (c = 1 × 10−5 M) in CH3CN upon addition of (a)SO42−, (b) H2PO4, (c) HP2O73− from 0 to 1.8 equiv., and (d) HP2O73− from 1.8 to 100 equiv. Arrows indicate the emission bands that increase or decrease during the titration.
Figure 3. Changes in the fluorescence spectra of receptor 52+·2PF6 (c = 1 × 10−5 M) in CH3CN upon addition of (a)SO42−, (b) H2PO4, (c) HP2O73− from 0 to 1.8 equiv., and (d) HP2O73− from 1.8 to 100 equiv. Arrows indicate the emission bands that increase or decrease during the titration.
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Figure 4. Changes in the absorption spectra of the receptor 52+·2PF6 (c = 1 × 10−5 M in CH3CN) upon addition of increasing amounts of H2PO4 anions. Arrows indicate the absorption bands that increase or decrease during the titration.
Figure 4. Changes in the absorption spectra of the receptor 52+·2PF6 (c = 1 × 10−5 M in CH3CN) upon addition of increasing amounts of H2PO4 anions. Arrows indicate the absorption bands that increase or decrease during the titration.
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Figure 5. 1H-NMR spectral changes observed in the receptor 52+·2PF6 in CD3CN/CD3OD (8/2, v/v) during the addition from 0 (top) to 50 equiv. (bottom) of H2PO4 anions.
Figure 5. 1H-NMR spectral changes observed in the receptor 52+·2PF6 in CD3CN/CD3OD (8/2, v/v) during the addition from 0 (top) to 50 equiv. (bottom) of H2PO4 anions.
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Scheme 2. Synthesis of imidazolone 62+·PF6 from the receptor 52+·2PF6 by the HP2O73− anion.
Scheme 2. Synthesis of imidazolone 62+·PF6 from the receptor 52+·2PF6 by the HP2O73− anion.
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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.
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.
ReceptorH2PO4SO42−AcO
52+·PF6β = 5.9 × 1010 M−2 (15)K = 5.7 × 104 M−1 (14)β = 2.16 × 109 M−2 (10)
72+·2PF6β = 5.0 × 1011 M−2 (16)K = 8.0 × 104 M−1 (14)-
82+·2PF6β = 9.8 × 1011 M−2 (13)K = 1.4 × 106 M−1 (6)-
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.
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.
ReceptorH2PO4SO42−K(SO42−)/β(H2PO4)
52+·PF6β = 1.37 × 102 M−2 (4)K = 2.6 × 103 M−1 (4)18
72+·2PF6β = 9.9 × 103 M−2 (3)K = 5.5 × 103 M−1 (4)0.55
82+·2PF6β = 5.6 × 103 M−2 (4)K = 2.2 × 103 M−1 (4)0.39

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