Receptor design and extraction of inorganic fluoride

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Downloaded by Central Salt & Marine Chemicals Research Institute (CSMCRI) on 07 March 2012 Published on 31 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CC11458A

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Receptor design and extraction of inorganic fluoride ion from aqueous mediumw Priyadip Das, Amal K. Mandal, Manoj K. Kesharwani, E. Suresh, Bishwajit Ganguly* and Amitava Das* Received 14th March 2011, Accepted 28th April 2011 DOI: 10.1039/c1cc11458a A receptor with acidic methylene hydrogens is found to act as an efficient binding mode for F. This reagent could as well be used for selective and quantitative extraction of F from the aqueous solution of NaF and sea water. Ion specific interactions and recognitions play a vital role in various bio-inspired processes and thus, enormous efforts are put forward by researchers from different areas for developing a better insight in understanding the ion-specific interaction of certain functionalities in designer receptors.1 This has led to an exponential growth in studies in the area of molecular recognition with varying binding motifs.2 Fluoride ion, owing to its duplicitous nature and significance in biology and environmental pollution, has been one of most popular target for recognition and sensing studies.3 A variety of systems having hydrogen bond donor functionalities, like HNUrea/Thiourea, HNImidazole/Pyrrole/Indole, HNAmine, HNAmide and HOPhenol/Catechol have been used as the key binding motifs in designing receptors and such examples have been discussed in numerous reviews.4 More recently, F–p interaction is used for designing a colorimetric sensor for F.5 In some of our recent articles, we have shown that apart from the charge density of the anionic analyte, spatial arrangements of the binding motif(s) in the receptor and the geometry of the anions are also crucial in influencing receptor-analyte binding efficiency and specificity.6 Even though some chemodosimeters are able to detect F in water,7a–d the challenge for selective extraction or detection of F in aqueous solution through a measurable and visually detectable output still remains.7e In pursuit of these, we are reporting herein a new class of receptor with a novel binding motif for selective extraction and colorimetric sensing of the F, present either as NaF or TBAF in aqueous solution. A recent report reveals that a calix[4]arene bearing alkyl triphenylphosphonium salts were found to form ion-pair type complexes with a range of anions like halides, CH3COO, HPO42 and ClO4.8 The proposed mode of binding was the strong

Central Salt & Marine Chemicals Research Institute, Bhavnagar, 364002, Gujarat, India. E-mail: [email protected], [email protected]; Fax: +91 2782 567562; Tel: +91 2782 567760 w Electronic supplementary information (ESI) available: Synthesis & characterization data, results of various spectral studies for unraveling the binding process. See DOI: 10.1039/c1cc11458a

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electrostatic interaction between cationic PPh3+ fragment and the anionic analyte, along with an anion-p interaction. Thus, till date there is no definite example which has conclusively demonstrated the design of an anion receptor based on the hydrogen bonded interaction between the acidic methylene (–CH2) hydrogen and any anionic analyte. In order to examine such a possibility, we have synthesized a new receptor (L), having an anthraquinone skeleton as a building block, as well as the signaling unit (Scheme 1). This reagent was characterized using standard analytical and spectroscopic techniques.w This reagent allowed visual detection through significant change in colour on binding to F, while a pale blue colour developed on binding to H2PO4. The role of the spatial orientation of two methylene groups (in L) in influencing the specificity and efficiency was also analyzed using density functional theory (DFT) studies (vide infra). Electronic spectra for L (2.12 105 M) were recorded in a CH3CN solution in the absence and presence of the tetrabutyl ammonium (TBA) salt of all common anionic analytes like F, Cl, Br, I, HSO4, NO2, NO3, N3, CH3COO, ClO4, IO4 and H2PO4. Spectra for, L showed a p–p* based transition at 328 nm and a weak shoulder at B400 nm (charge transfer transition). A distinct change in spectral pattern, as well as in colour, was observed in the presence of either F or H2PO4 among all other above mentioned anions used (Fig. 1A); the extent of changes for H2PO4 was much less. Two new absorption bands appeared with lmax at 439 and 606 nm, along with a simultaneous growth in the absorbance with a blue shifted maxima at 318 nm (Fig. 1A) when F or H2PO4 was added to the acetonitrile solution of L. Systematic spectral studies with varying [F] or [H2PO4]wenabled us to evaluate the formation constant for two respective adducts, L.2F and L.2H2PO4 (KfF : (2.24 0.1) 106 M2; KfH2PO4 : (8.98 0.8) 104 M2). Binding stoichiometry of 1 : 2 for L:X was evaluated using a B–H plot.w The difference in binding affinities of these anions were also examined using 1H NMR studies, where the extent of spectral

Scheme 1 Molecular structure of L(PF6)2.

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Fig. 1 (A) Electronic spectra of L (2.12 105M) in the absence and presence of the TBA salt of different anions in acetonitrile medium (X is Cl, Br, I, HSO4, NO2, NO3, N3, CH3COO, ClO4, IO4); Insert: Colour of the solution of L in acetonitrile with different anions: (a) L, (b) Cl or Br, (c) F, (d) HSO4, (e) N3, (f) CH3COO, (g) ClO4, (h) IO4. (B) Benesi–Hildebrand plot for UV-vis titration of L with varying [F] in CH3CN with lmon = 440 nm for evaluation of the binding stoichiometry.

change varied depending upon the extent of interaction with respective anions and this followed the order F > H2PO4 c Cl/Br > HSO4. The extent of the shifts for Cl/Br/HSO4 were nominal and suggested a weak or negligible interaction between the –[CH2]– proton and these anions in acetonitrile solution. No detectable change was observed for other anions.w For L, –[CH2]– hydrogens appear as doublet at 3.97 ppm with J = 15.5 HZ, which is typical for coupling with the adjacent phosphorous atom. This agreed well with the results of the proton decoupled 13C NMR spectra for L;w a doublet at 26.72 ppm with J = 78 Hz for –[CH2]– functionality confirmed coupling with adjacent P[PPh3]+. In 1H NMR spectra, all other aromatic protons for L appeared within 8.174 to 7.495 ppm and noticeable changes were observed when recorded in the presence of F; while extent of change was much smaller for H2PO4. The methylene peak at d 3.97 ppm for L was found to be very broad in the presence of H2PO4, while this disappeared in the presence of F.w Further studies with TBAF and t-BuOK revealed that deprotonation of L took place only in the presence of 50 mole equivalence of t-BuOK or TBAF.w Difference in the electronic spectra of the deprotonated form (LH+) (for [F] Z 50 mole equivalent) and L.2F (for F o 10 mole equivalent) revealed that in the 1 H NMR spectra, the disappearance of the methylene proton signal in L.2F at 25 1C was not due to any deprotonation phenomena and this was confirmed from the results of 1 H NMR spectra recorded at 20 1C.w More importantly, the characteristic signal for H2F appeared at 16 ppm when [F] Z 50 mole equiv. was used, which was absent when [F] o 10 mole equivalent.w The 31P NMR spectra of receptor L in the absence and presence of F/H2PO4 were recorded (Fig. 2). A downfield shift of 4.873 ppm for P[PPh3]+ was observed in the presence of F, while the extent of shift was much smaller for H2PO4

Fig. 2 Partial 31P NMR spectra for L in the absence and presence of (A) F and (B) H2PO4 in CD3CN at room temperature.

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(Dd = 3.137 ppm). This confirms a stronger binding of F to the –[CH2]– protons, as compared to H2PO4. This shift was insignificant for Cl (DdCl = 0.26 ppm) and other anions studied.w Binding of the electron rich F to the –[CH2]– protons favours the charge delocalization and thus imparts certain double bond character to the H2C–P[PPh3]+ bond and possible anisotropy could account for this observed downfield shift.9 An earlier report on a phosphonium ion-based receptor for anions suggests an upfield shift of B0.02 to 0.16 ppm on binding to anions due to a tight ion-pair formation between the anions and P[PPh3]+ 8. However, our results validate a H-bonded adduct formation between –[CH2]– and the F or H2PO4, and thereby confirms a different binding mode. To examine and rationalize the binding affinity and stoichiometry of receptor L towards anions F and H2PO4, DFT calculations have been performed. All geometries were optimized with GGA/BLYP/DNP methods10 using DMol3 density functional program (version 4.1.2) of Accelrys Inc.11 The optimized geometry of receptor L is shown in Fig. 3A. Though experimentally obtained stoichiometry was 1 : 2 (i.e. L:2F and L:2H2PO4) for binding of F or H2PO4 to L, different binding stoichiometries like 1 : 1 and 1 : 2 were examined computationally. The calculated binding affinity for 1 : 1 complexes of L with F and H2PO4 ions are 184.2 kcal mol1 and 131.4 kcal mol1, respectively.w The 1 : 2 stoichiometry showed higher binding energies for L with F (328.6 kcal mol1) and H2PO4 (215.9 kcal mol1) compared to 1 : 1 stoichiometric ratio (Fig. 3). The receptor L can accommodate two F or H2PO4 ions using the active methylene hydrogens. Further, the strong binding affinity of F with L is also borne out in the computational study. This observation is in accord with the experimentally obtained stoichiometry that was evaluated from the results of UV-vis spectral studies. NaF is an essential nutrient for mammalians. However, when present in higher concentration, apart from causing commonly known orthopedic disorders, it acts as a potent G-protein activator, Ser/Thr phosphate inhibitor and affects essential cell signalling processes. Higher levels of NaF is also known to induce cell apoptosis.12 Thus, we explored the possibility of using L as an reagent for real-time quantitative extraction and detection of F from aqueous solution of NaF, fresh sea water (Arabian Sea) collected at Bhavnagar (high tide water, latitude 21147 0 20.33 0 0 N, longitude 7217 0 24.49 0 0 E), Okha (latitude 22128 0 9 0 0 N, longitude 6913 0 38 0 0 E) and water from Sambhar lake in Jaipur, India (latitude 26158 0 N, longitude 7515 0 E). [F] is known to be higher than the permissible level at Sambhar lake.13 Experimental studies reveal that [F], as

Fig. 3 GGA/BLYP/DNP optimized geometries and important distances (A˚) of (A) L, (B) L.2F and (C) L.2H2PO4. (yellow: C; red: O; cyan: F; orange: P; white: H).

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Fig. 4 (i) A plot of absorbance of L.2F, extracted in the CH2Cl2 layer from an aqueous solution of NaF vs. [F] that was used in preparing the standard aqueous solution of NaF; points in colour represents absorbances for L.2F, extracted from 2 ml of sea water collected from ( ) Bhavnagar, ( ) Okha, and ( ) Sambhar lake;w (ii) Photograph of the solvent extracted L.2F in CH2Cl2 layer from (A) 2 ppm aq. solution of NaF, (B) 2 ml sea water from Okha and (C) 3 ml water from Sambhar lake; (iii) Photograph of L.2F, extracted in the CH2Cl2 layer from an aqueous solution having varying [NaF]: A (0.1 ppm), B (10 ppm), C (15 ppm), D (25 ppm).

low as 0.06 ppm, could be selectively extracted into the organic layer (e.g. CH2Cl2 or CH3Cl) from neutral aqueous solution of NaF with a 99.3% extraction efficiency (Fig. 4); while according to the WHO norms, the permissible [F] in drinking water is 1 ppm.14 An UNICEF report states that 65% of India’s villages are exposed to F risk.13 The presence of the L.2F in the organic (CH2Cl2) layer after extraction from the aqueous phase was ascertained from electronic spectral studies; the presence of the hydrated fluoride ion i.e. L.2(FH2O) was identified from the mass spectra data and this agreed well with independent reports from Rissanen et al. and Ghosh. et al.4a,15 Our control studies also ascertained that phosphate did not contribute to the colour of the extracted fluoride from sea or lake water.w Thus, the measurable [F] level in water using reagent L is much lower than the WHO permissible [F] in drinking water. Fig. 4 further reveals that this reagent could be used successfully for the visual detection and quantitative extraction of the [F] in drinking or other saline water, where other competing ions are present in large excess. According to the prescribed WHO methodology,14 sulfophenyl azo dihydroxy naphthalene disulfonic acid is being used as the colorimetric reagent for detection of F with a lower detection limit of 0.1 ppm. Considering this, the new reagent, L is more efficient in detecting lower concentration of F in water. In this communication, we have demonstrated a new reagent for selective and quantitative extraction, as well as visual detection of an ultratrace quantity of inorganic F present in water. Thus, this reagent has implication towards a large cross-section of the village population in India. Receptor L also provides a new binding motif for further exploration of variety of anion recognition. DST and NWP-53(CSIR) have supported this work. PD and AKM acknowledge CSIR for SRF fellowship. We thank reviewers for insightful comments and suggestions.

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