Introduction

Anions are ubiquitous. Water-soluble anions (fluoride, chloride, bromide, phosphate, etc.) play crucial roles in a wide range of biological phenomena, and chemical and environmental processes [1–3]. Anions are present in c. 70 % of all enzymatic sites, which play essential roles in many proteins and are critical for the manipulation and storage of genetic information (DNA and RNA are poly-anions) [4]. They are also essential in the formation of the majority of enzyme–substrate and enzyme–cofactor complexes as well as in the interactions of proteins with DNA or RNA. Anions are involved in the regulation of osmotic pressure, activating signal transduction pathways, maintaining cell volumes, and in the production of electrical signals [4]. Recently, several X-ray crystal structures have been solved that have allowed the direct visualizations of enzyme–anionic substrate complexes that are stabilized via multiple hydrogen-bonding interactions [4, 5]. Among the different bio-active anions, there is interest in the detection and recognition of the fluoride ion because of its significant role in dental caries, clinical treatment for osteoporosis, toxicity resulting from its over-accumulation in the bone [6, 7], and association with hydrolysis of the nerve gas sarin [8]. The fluoride levels in blood have been reported to be in the range 20–60 μg L−1 due to its easy absorption by the body, but it is excreted slowly from the body. Therefore, the presence of excess fluoride ions can result in chronic poisoning [9]. Hence, noble methods for the detection of fluoride have become a hot topic.

Because of the advancement in the supramolecular concepts on host–guest chemistry, a number of suitable receptors have been developed for the selective encapsulation and sensing of the biologically important anions for qualitative and quantitative determination. Particularly, the sensing of anions by the naked-eye (colorimetric), fluorescent and/or electrochemistry responses have attracted considerable attention [10–29]. In designing such sensing materials, Schiff bases containing phenolic and nitro groups are a recent choice for the development of sensing material for anions because of the ability of the phenolic-OH group to interact with anions through hydrogen bonding even in aqueous solution and of the nitro group to act as a chromogenic unit. As part of our ongoing research on anion recognition by Schiff bases [28, 29], we have investigated the anion recognition ability of an old but novel Schiff base, bis(5-nitrosalycilaldehyde)ethylenediamine (5-NO2-Salen, L), with different anions such as F−, I−, Cl−, and Br− using spectrophotometric and computational approaches.

Experimental

Materials and methods

All the starting materials were obtained commercially in pure form and were used without further purification. The solvents used for the experiments were purchased commercially in the purest form and were used without further purification. All the anions were used in the form of tetrabutylammonium (TBA) salts [(n-C4H9)4NF (n-C4H9)4NCl (n-C4H9)4NBr (n-C4H9)4NI and (n-C4H9)4NOH] and were purchased from Spectrochem, India. The synthesis of the receptor 5-NO2-Salen (L) was done by following the available literature [30]. The absorption spectra were measured on a Cary 50 Varian UV–Vis spectrophotometer at room temperature. Stock solutions of the receptor (1 × 10−3 M) and anions (1 × 10−3 M) were prepared in DMSO. These solutions were used for all spectroscopic studies after appropriate dilution. Then, the required amounts of the receptor and anions were taken directly into a cuvette by using a micropipette for spectroscopic titrations.

Computational methods

In order to investigate the anion binding behavior of the receptor, theoretical calculations were carried out with the Gaussian 09W computer program [31]. Optimizations of the receptor and anions have been carried out without symmetry constraints by applying the semi-empirical PM6 method. The harmonic vibrational frequency calculations using the same methods as for the geometry optimizations were used to ascertain the presence of a local minimum. Further, the interaction energy (∆E int) for the anion–receptor complex has been determined from the following complexation equilibrium:

$$ {\bf L} \, + \, m{\text{X}}^{n - } \rightleftarrows \left[ {{\text{X}}_{m} {\bf L}} \right]^{mn - } $$
(1)
$$ \Updelta E_{\text{int}} = \, E\left( {\left[ {{\text{X}}m{\bf L}} \right]^{mn - } } \right) - E\left( {\bf L} \right) - mE\left( {{\text{X}}^{n - } } \right). $$
(2)

Results and discussion

The qualitative anion binding abilities of the 5-NO2-Salen (L) with halide anions (F−, Cl−, Br−, and I−) were investigated by colorimetric and spectrophotometric experiments in DMSO. The colorimetric responses of the receptor L (5.0 × 10−5 M) were investigated before and after the addition of equivalent amount of each anion. As shown in Fig. 1, we observed a visually detectable color change from light yellow to intense yellow in the presence of F− anions. However, no obvious color change of L was observed in the presence of Cl−, Br−, and I−; even the anions were excessive presumably due to weaker interactions with the receptors which failed to alter any structural changes. However, the intensity of color decreases reversibly on addition of a small amount of water because of the competitive interactions of water molecules with anions for binding sites, and disturbance of the H-bond interactions between the host and the anionic guest.

Fig. 1
figure 1

Color change of receptor L (5.0 × 10−5 M) on addition of equivalent amounts of different anions

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The UV–Vis spectrum of the free receptor exhibited two absorption bands with maxima at 373 and 425 nm. The absorption above 400 nm indicates the presence of a ketoenamine form of receptor L in DMSO. The nitro being an electron-withdrawing group enhances the hydrogen bonding ability with anions and increases the tautomerization processes in solution. On the addition of various anions, an obvious hyperchromic spectral shift of the receptor bands was observed in the presence of fluoride but not appreciably with other halides. As shown in Fig. 2, the successive addition of anions to a fixed concentration of the receptor resulted in a hyperchromic shift at 425 nm, with the formation of an isosbectic point at ~362 nm indicating the formation of a single complex species between the receptor L and the added anions. At the same time, the absorption peaks became sharper, which was related to the decrease of the molecular conjugacy. The spectral results were used to calculate the binding constant (K) for F− by applying the Benesi–Hildebrand equation. The calculated K value for F− was 3.06 × 103 M−1. The stoichiometry analysis of the receptor with the fluoride anion by Job’s plot method indicates the formation of an anion–receptor complex in the ratio 2:1 (Fig. 2). The proposed binding mode of the receptor L with F− is shown in Fig. 3. The small-sized fluoride anion fails to bring the two binding moieties close together, and interacts in the ratio 1:2 to form a receptor L (F−)2 complex.

Fig. 2
figure 2

UV–Vis spectral titration of the receptor L (1.0 × 10−5 M) upon addition of different equivalents of F−. Inset shows Job’s plots with F−

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Fig. 3
figure 3

Proposed binding of the receptor L with F−

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The geometries of the receptor L and its tautomer were optimized (Fig. 4) using the semi-empirical method by applying PM6 Hamiltonian. The optimized global minimum structure of receptor L shows that the two nitro-salicylaldimine moieties are placed in the opposite direction, and therefore the receptor facilitates the incoming fluoride anion to interact with the receptor L through opposite directions with respect to the molecular baseline. As a result, the receptor L can form possible 1:2 stable complexes with the fluoride ion. The calculated results on total energy and the energy gap of E LUMO, E HOMO of L and its tautomer are listed in Table 1. From Table 1, one can observe that (1) the total energy of compound L is higher than that of its tautomer by 12.46 kJ mol−1; and (2) the energy gap ∆E (∆E = E LUMO − E HOMO) of compound L is larger than that of its tautomer. The above results imply that the tautomer of L (ketoenamine) is more stable than compound L. Hence, receptor L (ketoenamine) converts to the tautomer (phenolimine), which participates in the anion recognition process in the solution that corroborates well with our experimental findings.

Fig. 4
figure 4

Optimized geometry of receptor L, and the tautomer of L

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Table 1 The total energy (E T) and energy gap ∆E(∆E = E LUMO − E HOMO) of L and its tautomer obtained by the semi-empirical PM6 method
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Further, considering the higher affinity for the fluoride anion and to obtain insight into the anion-sensing mechanism, the interactions between receptor L and halides (X− = F−, Cl−, Br−, and I−) have been investigated by applying the semi-empirical PM6 method. The main geometrical parameters for the receptor L and complexes L (X−)2 (X = F, Cl, Br, and I), and the interaction energy (∆E int) are given in Table 2. The optimized structural parameters for the anion complexes L (X−)2 inferred that the O–H bond distance increases as the hydrogen bonding ability of the anion increases in the order F− > Cl− > Br− > I−, as compared to the free receptor L. Simultaneously, for the same reason, a decrease in the C–O bond distance was observed in the same manner (Table 2), which clearly indicates that the hydrogen bonding interaction between L and anions promotes the receptor L to undergo a facile charge transfer state mainly from phenolic-OH to the nitro group. Moreover, in the case of L (F−)2, the O–H bond distance is stretched significantly in comparison with the enolimine form of L (Table 2), and the H+ moves close to F− to more precisely form a deprotonated complex L 2−(HF)2. However, such effects were decreased with the increase in the size of the halides. Therefore, the receptor prefers to bind selectively with the F− anion. Furthermore, the interaction energy of L 2−(HF)2 clearly demonstrated that the receptor L is much more favorable for the distinct selectivity of F− than other anions. In addition, the frontier molecular orbitals (FMOs) for receptor L, tautomer of L, and deprotonated L 2− and L 2−(HF)2 (Fig. 5) are qualitatively analyzed in order to explain the electronic excitation and electron density redistribution that affects the molecular geometry during the host–guest complexation process. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that largely acts as the electron acceptor [32]. The highest HOMO density and highest LUMO density in receptor L is distributed uniformly between the two salicylaldimine rings, whereas in the tautomer of L it is found to be located in only one ring responsible for the absorbance at the higher wavelength. Similar to the tautomer of L, the HOMO and LUMO of deprotonated L 2− and L 2−(HF)2 are located in one salicylaldimine ring. The FMOs analyses indicate that the tautomer of L interacts with the fluoride anion through the phenolic-OH group by hydrogen bonding and causes deprotonation of the phenolic-OH group of L. It is obvious that deprotonation strengthens the electron-donating ability of the phenol group. As a consequence, the electrons can flow easily from the phenol moiety to the electron-withdrawing nitro group resulting in the hyperchromic and red shift in the absorption spectra of L. Thus, the interaction of receptor L with the F− anion resulted in the deprotonation from the phenol moiety to form the deprotonated complex L 2−(HF)2. Further to confirm the deprotonation of the receptor L during the fluoride recognition process, we performed the absorption titration of L with a strong base, tetrabutylammonium hydroxide (TBAOH) (Fig. 6). The similar spectral changes observed for both F− and OH− on interaction with the receptor L indicate that the anions are functioning here as a base and forming the same species in solution by the deprotonation process [33].

Table 2 Some useful optimized parameters and the interaction energy (∆E int, kJ mol−1) of the complexes of receptor L and the anions
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Fig. 5
figure 5

FMOs for the neutral receptor L, tautomer of L, complex L (F−)2, and deprotonated L 2− obtained at the semi-empirical PM6 method

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Fig. 6
figure 6

UV–Vis spectral titration of the receptor L (2.0 × 10−5 M) upon addition of different equivalents of TBAOH

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The anion recognition ability of L was also investigated in CH2Cl2. The receptor L showed both naked-eye detectable color change from colorless to yellow and spectral responses in the presence of F− (Fig. 7). The free receptor exhibited a strong absorption band at 318 nm along with a weak band at 405 nm, which indicates the presence of the enolimine form of the receptor L predominantly in the low polar solvent CH2Cl2. In the presence of F−, the absorption band of the receptor at 318 nm was decreased with the concomitant hyperchromic and red shift at 405–421 nm, due to the formation of an anion–receptor complex species followed by the deprotonation of the phenolic-OH groups of the receptor L. Considering the affinity of the receptor L towards fluoride in CH2Cl2, the possible analytical applicability of L was investigated by performing the extraction of fluoride from aqueous solution using a liquid–liquid extraction strategy in the presence of TBAI as a phase transfer agent. The result shown in Fig. 7c clearly indicates the fluoride extraction ability of the receptor L from aqueous solution.

Fig. 7
figure 7

a UV–Vis spectra and b color change of the receptor L (2.5 × 10−5 M) in the absence (a) and presence of TBAF (b: 2.5 × 10−5 M; c: 5.0 × 10−5 M) in CH2Cl2. c Color change of the receptor L (d) before and (e) after the extraction of fluoride from aqueous solution to the CH2Cl2 solution containing the receptor L and TBAI as phase transfer agent

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Conclusions

The anion binding abilities of the Schiff base 5-NO2-Salen with different halide anions such as F−, I−, Cl−, and Br− were investigated using spectrophotometric and computational methods. The Schiff base receptor showed equilibrium between the ketoenamine and enolimine in DMSO. Due to the higher basicity, deprotonating ability, and smaller size as compared to other tested halides, the addition of F− intensified the color of the receptor giving a hyperchromic and red shift in the UV–Vis spectrophotometer, thereby increasing the tautomerization rate of the receptor. Other anions showed slight changes. Importantly, the extraction of fluoride from aqueous solution by L was achieved by solubilizing L in CH2Cl2 in the presence of TBAI (which act as a phase transfer agent) followed by a liquid–liquid extraction technique. The promising preliminary results of this work inferred that the Schiff base 5-NO2-Salen and its related derivatives can be used to develop colorimetric sensing devices for fluoride detection and fluoride extraction from aqueous solution.