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Receptor with an Active Methylene Group as Binding Site for Extraction of Inorganic Fluoride Ions from Seawater [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Two new receptors R1 and R2 based on triphenylphosphonium salts with an active methylene group as binding site have been designed and synthesised for the detection of F- ions. The detection limit of these receptors in organic media was found to be 0.2 ppm. Upon adding F- ions, a ??max of 188 nm and 256 nm was observed for receptors R1 and R2, respectively. The detection process followed deprotonation of the methylene proton, which has been confirmed by 1 H NMR titration. The receptors were evaluated for real-life applicability by extracting F- ions from aqueous media and seawater into organic media. Receptor R2 was able to extract F- ions from seawater with 99 % efficiency. The level of F- ions present in seawater has been quantified and found to be 1.4 ppm, which is comparable to the reported literature value. Keywords: colorimetry · extraction · fluoride · receptors · water chemistry Introduction The design and synthesis of organic receptors for the detec- tion of anions through their optical, electrochemical and mag- netic resonance response have received considerable attention owing to the vital applications of anions in various industrial, chemical, biological and environmental processes. In this regard, organic receptors with neutral binding sites for various anions have been synthesised for decades.[1] Recently, cationic receptors with ammonium, quinolinium, imidazolium and gua- nidinium salts were used for anion detection.[2] These cationic receptors utilise electrostatic interaction between anion and cation for anion recognition. Along with the electrostatic inter- actions, other weaker noncovalent interactions, such as anion- p interaction, were used for anion detection.[3] Among the wide range of anions, the design and synthesis of an effective receptor for F^ ion detection have received con- siderable attention because of its vital role in industrial usage, environmental pollution and significance in clinical applica- tions. In contrast, excess fluoride consumption is always a health concern as it can cause dental fluorosis[4] and skeletal fluorosis.[5] Acute fluoride exposure may cause collagen break- down, depression in thyroid activity, bone cancer, immune system disturbance and anaemia.[6] Thus, the real-time moni- toring of the F^ ion is most significant over other anion detec- tion. Receptors with well-known functionalities, such as urea/thio- urea, amide, imide, pyrrole and imidazolium, as binding sites for the F^ ion have been reported in which conventional hy- drogen-bond formation has been used for detection.[7] Un- fortunately, the majority of them are capable of working only in absolute non-aqueous conditions for the detection of organ- ic fluoride sources such as tetrabutylammonium fluoride (TBAF). On the other hand, only a few organic receptors have been reported for the detection of F^ ions in aqueous media.[8] To the best of our knowledge, only one report has been pub- lished so far on the selective extraction of the F^ ion from aqueous solution with measurable visual detection.[9] Anion-binding receptors with alkyl triphenylphosphonium salts with an active/acidic methylene group as a binding site were not studied to the extent of receptors with ^NH or ^OH binding sites. Vicens et al. synthesised alkyl triphenylphospho- nium salts attached to calix[4]arene and reported the forma- tion of ion-pair-type complexes with a range of anions such as halides, AcO^ ,HPO42^ and ClO4^ .[10] Das et al. synthesised alkyl triphenylphosphonium salts attached to an anthraquinone skeleton, which displayed high selectivity towards F^ ions.[9] In addition, this receptor displayed a unique anion extraction from aqueous solution to organic solvent. With this background, herein we report new receptors based on triphenylphosphonium salts that contain an active methylene (^CH2^) group as a binding site for anion detection. The receptors were designed on the binding site-spacer-sig- nalling unit approach, in which the binding site and signalling unit were separated by a "spacer" diazo group (Scheme 1). Re- ceptor R1 contains an ethoxyphenyl unit as signalling unit and receptor R2 contains a coumarin as signalling unit. These re- ceptors are used for the detection of F^ ions in organic media and to extract inorganic F^ ions from aqueous media into or- ganic media. The extraction process has been visualised by an instantaneous optical change in organic media. The recep- tors R3 and R4 were synthesised to evaluate the role of the active methylene group in the detection process. Results and Discussion A single crystal of receptor R3 suitable for X-ray diffraction analysis was grown by slow evaporation of an ethanol/di- chloromethane (1:1) solution at room temperature. The ORTEP diagram (50 % probability) of receptor R3 is given in Figure 1. Receptor R3 was crystallised in a monoclinic lattice. Detailed crystallographic data of receptor R3 are given in Table 1. A selective anion detection study for receptors R1 - R4 was performed with the help of UV/Vis spectroscopy. Solutions (1.0 ^ 10^5m) of receptors R1 and R2 in dry dichloromethane solutions were treated with two equivalents of different anions, such as fluoride, chloride, bromide, iodide, nitrate, hy- drogen sulfate, dihydrogen phosphate and acetate in the form of tetrabutylammonium (TBA) salts. In the case of receptor R1, a significant shift in the absorbance was observed with the ad- dition of F^ and AcO^ ions (Figure 2). The intensity of the ab- sorption band in UV/Vis spectra for AcO^ ions was much less than that for F^ ions. This signifies that the interaction be- tween receptor R1 and the F^ ion is stronger and the interac- tion between receptor R1 and the AcO^ ion is much weaker. All other anions did not show any change in UV/Vis spectra, which indicates that these anions either did not interact with receptor R1 or the interaction was not enough to produce any changes in the spectra. Receptor R2 showed similar changes with the addition of different anions (Figure S7 in the Support- ing Information). In addition, receptor R1 was further evaluated for the colori- metric detection of anions. The receptor R1 solution (1.0 ^ 10^5 m) in dry dichloromethane showed significant colour change from pale yellow to pink instantaneously with the ad- dition of F^ ions. However, no colour change was noticed upon addition of other anions (Figure 3). The addition of AcO^showed slight change in the absorbance of the UV/Vis spectra ; however, it failed to induce any significant colour change to receptor R1. To understand the nature of the receptor-anion interactions, a UV/Vis titration experiment was performed between recep- tor R1 and TBAF (Figure 4). With the incremental addition of TBAF to receptor R1 (1.0 ^10^5m), the absorbance at 356 nm, corresponding to the phenyldiazene group in receptor R1, de- creased constantly and a new absorption band at 544 nm ap- peared and gradually increased. This observation is owing to the formation of charge-transfer (CT) transitions involving the electron-rich methylene functionality as donor group and the phenylenediazene group as the acceptor unit. The intensity of the absorption band attained saturation after adding two equi- valents of F^ ions. The bathochromic shift of 188 nm with the formation of an isosbestic point at 401 nm was attributed to the formation of a CT complex between the receptor and F^ ion. The binding stoichiometry between receptor R1 and F^ ions was determined by the Benesi-Hildebrand method using UV spectrometric titration data at 544 nm. The linearity of the graph confirms the formation of a stable 1:2 receptor/F^ ion stoichiometric complex (Figure 4, inset). Further, the colorimetric investigation was extended to re- ceptor R2. The receptor R2 (1.0 ^ 10^5 m) was treated with two equivalents of different anions in dry dichloromethane (Figure 5). The addition of F^ ions resulted in a significant colour change from pale yellow to dark blue, whereas AcO^ ions showed slight variation in colour from pale yellow to light blue. This difference in the colour intensity indicated a stronger binding interaction between receptor R2 and F^ ions and a sig- nificantly weaker interaction between receptor R2 and AcO^ ions. On the other hand, other anions either did not interact or interacted feebly with receptor R2 and did not show any visual colour change. Receptor R2 was further analysed quantitatively with UV/Vis spectroscopic titration by adding TBAF to dry dichloromethane solution (Figure 6). The addition of TBAF resulted in the gener- ation of a new absorption band at 573 nm. With the incremen- tal addition of F^ ions to the receptor R2 solution, the absorb- ance band at 317 nm, corresponding to the phenyldiazene chromenone unit, decreased gradually and simultaneously the absorption band at 573 nm increased progressively owing to the development of CT transitions between the electron-rich methylene functionality, which it acts as an electron donor, and the phenylenediazene chromenone group, which acts as an electron acceptor unit. The intensity of this new absorption band attained saturation after the addition of two equivalents of TBAF. The stoichiometric complexation ratio was determined with the Benesi-Hildebrand method. The Benesi-Hildebrand plot was obtained by using UV spectrometric titration data at 573 nm. The plot showed linearity only at the square of the concentration of F^ ions. This clearly indicates the formation of a 1:2 stoichiometric complex between receptor R2 and F ^ ions (Figure 6, inset). Correspondingly, the UV/Vis spectroscopic titration was per- formed by adding tetrabutylammonium acetate to the recep- tors R1 and R2 in dichloromethane. The spectral pattern dis- played similar changes to that of TBAF (Figure S12 for recep- tor R1 and Figure S13 for receptor R2). However, the intensity of the new absorption band was much less than that of TBAF. This is perhaps a result of weaker interactions between recep- tors and AcO^ ions. The coumarin moiety is a well-known fluorophore, so it was interesting to study fluorescence changes upon F^ ion binding to receptor R2. Therefore, a fluorescence titration was per- formed by adding TBAF to receptor R2 in dichloromethane (Figure S16). As expected, upon increasing the concentration of F^ ions the quenching of fluorescence was observed owing to the deprotonation of receptor R2. The binding constant for both receptors (R1 and R2)was calculated using the Benesi-Hildebrand equation and found to be (4.58^ 0.02)^ 107 m^2 for receptor R1 and (7.5 ^ 0.03) ^ 107 m^2 for receptor R2. This result shows that the F^ ion binds more strongly to receptor R2 than receptor R1. This sensitivity is perhaps because of the presence of the coumarin unit in re- ceptor R2, which is a strong signalling unit when compared with the ethoxyphenyl group in receptor R1. Further, the binding mechanism of receptor R2 to F^ ions was proposed by evaluating the results obtained from UV/Vis titration and the Benesi-Hildebrand method. On compiling the results of UV/Vis experiments, it is evident that the colorimetric detection of the F^ ion with receptor R2 is a two-step process. Initially, the F^ ion binds to the active methylene (^CH2^) group of receptor R2 through hydrogen bonding, which re- sults in a 1:1 adduct to form the R1···F^ complex. A second F^ ion leads to deprotonation of the active methylene group to form R2^ (Scheme 2).[11] As a result, the electron density of re- ceptor R2 increases, which leads to intramolecular CT interac- tion between the electron-rich phenyldiazene chromenone functionality and the electron-deficient [PPh3] + group. Thus, receptor R2 shows intense colour change instantaneously upon addition of F^ ions. The binding mechanism was further confirmed by 1H NMR ti- tration (Figure 7) of receptor R2 performed in deuterated di- methyl sulfoxide ([D6]DMSO) solution. The 1H NMR signal at d=5.32ppm correspondsto twoprotonsofthemethylene(^ CH2^) group of receptor R2. The splitting pattern of this signal appeared as a doublet because of coupling with the adjacent phosphorus.[12] Upon adding 0.5 equivalents of F^ ions, a slight downfield shift from d = 5.32 to 5.33 ppm and d = 5.35 to 5.36 ppm was observed. This downfield shift resulted from the formation of a hydrogen bond between the methylene proton and F^ ion. Upon increasing the concentration of F^ ions to one equivalent, the signal corre- sponding to this methylene group disappeared completely, which indicated a fast proton ex- change between the methylene proton and the F^ ion. Further, the addition two equivalents of F^ ions resulted in the deproto- nation of receptor R2 to form R2^ and simultaneously the elec- tron density over the receptor molecule increased. Therefore, the signal corresponding to the protons of the ^CH2^ group shifted downfield from d = 5.32 to d =5.76 ppm. Concurrently, the splitting pattern disappeared because of the fast proton exchange within the receptor. The multiplet signals for aromat- ic protons experienced a downfield shift (from d= 7.24-7.63 to d = 7.54-7.64 ppm) and merged together to form two major signals with multiple splitting. This result further confirms the increase in electron density over receptor R2. In addition, upon adding two equivalents of F^ ions to the receptor R2 solution, the active methylene group undergoes deprotonation. This de- protonation process was confirmed from the appearance of the 1H NMR signal at d = 16.1 ppm (Figure 7, inset), which cor- responds to HF2^ .[13] As evidence for the deprotonation mechanism, UV/Vis spec- troscopic titration was performed by adding tetrabutylammo- nium hydroxide (TBAOH) to dry dichloromethane solutions of receptors R1 and R2 (Figure S14 for receptor R1 and Fig- ure S15 for receptor R2). The UV/Vis spectra of TBAOH for both the receptors showed similar changes to that of TBAF, which clearly indicates the detection process follows the deprotona- tion mechanism. Receptors R3 and R4, which do not contain an active meth- ylene group, were studied for their anion detection ability in dichloromethane. As expected, these receptors neither showed any colour changes (Figures S8 and S9) nor displayed any UV/ Vis spectral changes (Figures S10 and S11) even with the addi- tion of 20 equivalents of anions. This finding clearly suggests that the active methylene group is responsible for the F^ ion detection. Inorganic fluoride such as NaF is an essential nutrient for living organisms ; however, at higher concentrations it is haz- ardous to health. Therefore, the World Health Organization has restricted the F^ ion concentration level to 1 ppm in drinking water.[14] Keeping this fact in mind, the real-life applicability of extraction of F^ ions from aqueous media with receptors R1 and R2 has been evaluated. Standard solutions of NaF at differ- ent concentrations were prepared in water and used as inor- ganic F^ ion source for the extraction study. These standard solutions were treated with receptor R1 and R2 solutions in di- chloromethane. Upon vigorous shaking of these organo-aque- ous mixtures, the receptor solutions were able to extract F^ ions from aqueous media. As a result, the colour of recep- tors R1 and R2 changed from yellow to pink and yellow to deep blue, respectively. Further, these receptors were tested for the extraction of F^ ions from seawater collected from the Arabian Sea (latitude 138 0' 33.99", longitude 74847' 17.23") to examine the practical applicability. The extraction experiment was performed by treating stan- dard solutions of NaF (1-4 ppm) and seawater (1.5 mL each) with dichloromethane solutions of receptor R2, in glass vials. The organo-aqueous solutions were shaken well to extract the F^ ions. The vials containing above 1 ppm NaF solution showed significant colour change from yellow to blue, which clearly indicates that the receptor is capable of extracting F^ ions from water (Figure 8). The instantaneous colour change (blue) observed in the organic layer during extraction experi- ments suggests that receptor R2 can extract F^ ions efficiently from seawater, and thus confirms that other competing anions, such as chloride, bromide, iodide, phosphate and so forth, present in seawater did not interfere in the detection/ex- traction process. Further, the experimental studies revealed that receptor R2 was able to extract F^ ions from aqueous NaF solution and seawater with 99 % efficiency. The extraction study was further quantified by UV/Vis spec- troscopy. Standard solutions of NaF with varying concentra- tions from 1 to 4 ppm were prepared in water. Portions (1 mL) of these standard solutions were subjected to extraction (three times for each NaF solution) using receptor R2 dissolved in di- chloromethane. The organic phases of each solution were sep- arated, diluted four times and the UV/Vis spectrum was mea- sured. The same procedure was repeated for seawater. A cali- bration curve of absorbance versus concentration of F^ ions (in ppm) was plotted (Figure 9), from which the concentration of F^ ions in seawater was found to be 1.4 ppm. This result was comparable with the previously reported literature value.[14b] Similar extraction experiments were performed for recep- tor R1, which showed a significant colour change in the organ- ic medium only with organo-aqueous solutions that contained a minimum 1.5 ppm of NaF. Therefore, receptor R1 was unable to extract the F^ ions from seawater (Figure 10). This is per- haps owing to the less extended conjugation as the ethoxy- phenyl group is not participating in the detection process. Conclusion Two new receptors with an active methylene group as binding site have been designed and synthesised for the selective de- tection of F^ ions. On adding F^ ions, the receptors R1 and R2 displayed a colour change from pale yellow to pink and pale yellow to dark blue along with a significant bathochromic shift of 188 nm and 256 nm, respectively. This colour change and bathochromic shift resulted from the charge-transfer transi- tions in the receptors on adding F^ ions. These receptors were able to detect F^ ions even at concentrations as low as 0.2 ppm in organic media, which is much less than the World Health Organization permissible level. In addition, these recep- tors were able to extract F^ ions from aqueous media into or- ganic solutions with a substantial colour change. The real-life applicability of the receptors was evaluated by extracting F^ ions from seawater. Receptor R2 was able to extract F^ ions from seawater with 99 % efficiency. In addition, the level of F^ ions present in seawater was determined by using receptor R2 and was found to be 1.4 ppm, which is in good agreement with literature reports. Currently, we are concentrating on the design and synthesis of new and more efficient receptors for the extraction of F^ ions from industrial wastes. Experimental Section General information All chemicals were purchased from Sigma-Aldrich, Alfa Aesar or Spectrochem and used without further purification. All the solvents were procured from SD Fine, India, were of HPLC grade and used without further distillation. The 1H NMR spectra were recorded on a Bruker Avance II (500 MHz) instrument using TMS as internal reference and [D6]DMSO as solvent. Resonance multiplicities are described as s (singlet), d (doublet), t (triplet) and m (multiplet). Melting points were measured on a Stuart SMP3 melting-point apparatus in open capillaries. Infrared spectra were recorded on a Thermo Nicolet Avatar-330 FTIR spectrometer ; signal designations: s (strong), m (medium) and w (weak). UV/Vis spectroscopy was performed with an Analytikjena Specord S600 spectrometer in standard 3.5 mL quartz cells (two optical windows) with 10 mm path length. Fluorescence spectroscopy was performed with a JASCO FP-6200 spectrofluorometer in standard 3.5 mL quartz cells (four optical windows) with 10 mm path length. Elemental analyses were done using a Flash EA1112 CHNS analyser (Thermo Electron Corpora- tion). Synthesis of (E)-4-(p-tolyldiazenyl)phenol (1) and (E)-2-hy- droxy-5-[(p-tolylimino)methyl] benzaldehyde (2 ; Scheme 3) The diazonium salt of p-toluidine was prepared by adding sodium nitrate (3.5 mmol) to a stirred solution of p-toluidine (2.33 mmol) in concd HCl (2 mL) at 0 8C. A mixture of R (phenol, 2.33 mmol or 2- hydroxybenzaldehyde, 2.33 mmol) and NaOH (4.66 mmol) was added slowly to the diazonium salt at 0 8C and stirred for 10 min. The reaction mixture was then stirred at room temperature for 30 min and the pH was adjusted to 6 to obtain the solid product. The solid was isolated by filtration, washed with water and dried to obtain the desired products 1 (0.446 g) or 2 (0.5 g). Synthesis of (E)-1-(4-ethoxyphenyl)-2-p-tolyldiazene (R3 ; Scheme 4) Anhydrous K2CO3 (0.488 g, 3.53 mmol) and ethyl iodide (0.231 g, 1.48 mmol) were added to a solution of 1 (0.25 g, 1.18 mmol) in dry acetonitrile (ACN). The reaction mixture was stirred at 50 8C for 8 h. The completion of reaction was checked by thin-layer chroma- tography (TLC). Excess of K2CO3 was removed by filtration and the filtrate was evaporated under reduced pressure to yield pure brown product R3 (0.226 g). Synthesis of (E)-3-acetyl-6-(p-tolyldiazenyl)-2H-chromen-2- one (R4 ; Scheme 5) Ethyl acetoacetate (0.135 g, 1.04 mmol) was added to a mixture of 2 (0.25 g, 1.04 mmol) and piperidine (0.018 g, 0.21 mmol) in etha- nol, and stirred for 5 h at room temperature. The completion of re- action was confirmed by TLC. The solid obtained was isolated by filtration, washed with ethanol and dried to obtain a yellow pure product R4 (0.316 g). Synthesis of (E)-1-[4-(bromomethyl)phenyl]-2-(4-ethoxyphe- nyl)diazene (3) and (E)-3-acetyl-6-{[4-(bromomethyl)phenyl]- diazenyl}-2H-chromen-2-one (4 ; Scheme 6) N-Bromosuccinimide (NBS ; 0.162 g, 0.91 mmol for R3 and 0.183 g, 1.03 mmol for R4) and a catalytic amount of dibenzoyl peroxide (Bz2O2) were added to a solution of R3 or R4 (0.2 g, 0.83 mmol or 0.288 g, 0.94 mmol) in CCl4 (20 mL). The resulting mixture was heated at reflux for 8 h. The decomposed product of NBS was then separated by filtration and the filtrate was evaporated to dryness to afford a yellow solid. The solid was triturated with diethyl ether to yield pure product 3 (0.24 g) or 4 (0.343 g). Synthesis of (E)-1-(4-ethoxyphenyl)-2-{4-[(triphenylphosphi- no)methyl]phenyl}diazene bromide (R1) and (E)-3-acetyl-6- ({4-[(triphenylphosphino)methyl]phenyl}diazenyl)-2H-chro- men-2-one bromide (R2 ; Scheme 7) A solution of 3 or 4 (0.2 g, 0.63 mmol or 0.3 g, 0.78 mmol) and tri- phenylphosphine (0.182 g, 0.693 mmol for 3 and 0.225 g, 0.858 mmol for 4) in dry chloroform (20 mL) was heated at reflux for 4 h and then stirred at room temperature for 8 h. The reaction mixture was evaporated under reduced pressure to afford a hygro- scopic residue, which was stirred with diethyl ether until it formed a solid residue. The solid was isolated by filtration and dried to give the pure product R1 (0.34 g) or R2 (0.47 g). Characterisation data (E)-4-(p-Tolyldiazenyl)phenol (1): Yield: 90 %; m.p. 135-136 8C; FTIR: ñ= 3356.9 (br m), 3040.3 (m), 2936.4 (m), 1601.2 cm^1 (s); elemental analysis calcd (%) for C13H12N2O: C 73.56, H 5.70, N 13.20 ; found : C 73.59, H 5.66, N 13.19. (E)-2-Hydroxy-5-[(p-tolylimino)methyl] benzaldehyde (2): Yield : 89%; m.p. 119-1218C; FTIR: ñ=3364.9 (br m), 3072.3 (w), 2835.7 (w), 1698.9 (s), 1602.3 cm^1 (s) ; elemental analysis calcd (%) for C14H12N2O2: C 69.99, H 5.03, N 11.66 ; found: C 70.02, H 5.06, N 11.69. (E)-1-(4-Ethoxyphenyl)-2-p-tolyldiazene (R3): Yield: 80 %; m.p. 126- 1278C; 1H NMR (500 MHz, [D6]DMSO, 258C): d=7.88 (d, 3J(H,H)= 7Hz, 2H; Ar-H), 7.77 (d, 3J(H,H)=8Hz, 2H; Ar-H), 7.39 (d, 3J(H,H)= 8Hz, 2H; Ar-H), 7.13 (d, 3J(H,H)=7Hz, 2H; Ar-H), 4.15 (q, 3J(H,H)= 7 Hz, 2H; -CH2-), 1.38 ppm (t, 3J(H,H)=7 Hz, 3H; -CH3); FTIR: ñ= 3041.4 (m), 2940.3 (m), 1602.4 cm^1 (s) ; elemental analysis calcd (%) for C15H16N2O: C 74.97, H 6.71, N 11.66; found: C 74.95, H 6.69, N 11.69. (E)-3-Acetyl-6-(p-tolyldiazenyl)-2H-chromen-2-one (R4): Yield: 95 %; m.p. 177-1788C; 1HNMR (500 MHz, [D6]DMSO, 258C): d=8.82 (s, 1H; Ar-H), 8.50 (s, 1H; Ar-H), 8.22 (d, 3J(H,H)=9Hz, 1H; Ar-H), 7.85 (d, 3J(H,H)=8.5Hz, 2H; Ar-H), 7.66 (d, 3J(H,H)=8.5 Hz, 1H; Ar-H), 7.44 (d, 3J(H,H)=8Hz, 2H; Ar-H), 2.61 (s, 3H; -COCH3), 2.43ppm (s, 3H; -CH3); FTIR: ñ=3042.4 (m), 2885.9 (m), 1699.4 (s), 1684.4 (s), 1601.5 cm^1 (m) ; elemental analysis calcd (%) for C18H14N2O3 :C 70.58, H 4.61, N 9.15; found: C 70.54, H 4.58, N 9.18. (E)-1-[4-(Bromomethyl)phenyl]-2-(4-ethoxyphenyl)diazene (3): Yield : 90%; m.p. 156-1588C; FTIR: ñ=3056.9 (m), 2986.4 (m), 1601.4 (s), 1247.3 cm^1 (m) ; elemental analysis calcd (%) for C15H15BrN2O: C 56.44, H 4.74, N 8.78 ; found : C 56.49, H 4.76, N 8.80. (E)-3-Acetyl-6-{[4-(bromomethyl)phenyl]diazenyl}-2H-chromen-2- one (4): Yield: 91%; m.p. 188-1908C; FTIR: ñ=3040.2 (w), 2889.3 (m), 1700.3 (s), 1687.4 (s), 1602.5 cm^1 (m) ; elemental analysis calcd (%) for C18H13BrN2O3: C 56.12, H 3.40, N 7.27; found: C 56.17, H 3.46, N 7.29. (E)-1-(4-Ethoxyphenyl)-2-{4-[(triphenylphosphino)methyl]phenyl}dia- zene bromide (R1): Yield: 92%; m.p. 146-1478C;1HNMR (500MHz, [D6]DMSO, 258C): d=7.62-7.94 (m, 23H; Ar-H); 5.30 (d, J=16 Hz, 2H; P-CH), 4.16 (q, J=7Hz, 2H; -CH2-), 1.38ppm (t, J=7Hz, 3H; -CH3); FTIR: ñ=3462.3 (m), 3045.6 (m), 2985.7 (m), 1604.3cm^1 (s); ESI-MS: m/z : calcd : 581.5 ; found : 581.1; elemental analysis calcd (%) for C33H30BrN2OP: C 68.16, H 5.20, N 4.82; found: C 67.53, H 5.58, N 4.86. (E)-3-Acetyl-6-({4-[(triphenylphosphino)methyl]phenyl}diazenyl)-2H- chromen-2-one bromide (R2): Yield: 92%; m.p. 159-1618C;1H NMR (500 MHz, [D6]DMSO, 25 8C): d = 7.24-7.64 (m, 23 H; Ar-H); 5.36 (d, J=16 Hz, 2H; P-CH), 2.42 ppm (s, 3H; -CH3); FTIR: ñ=3462.3 (m), 3045.6 (m), 2985.7 (m), 1604.3 cm^1 (s); ESI-MS: m/z: calcd : 647.5; found : 648.5 ; elemental analysis calcd (%) for C36H28BrN2O3P: C 66.78, H 4.36, N 4.33; found: C 66.23, H 4.68, N 4.73. CCDC 985440 (R3) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Extraction efficiency measurement of receptor R2 A solution of TBAF (1.0 ^ 10^ 4 m , 1 mL) was treated with recep- tor R2 solution (1.0 ^ 10^5 m, 19 mL) in absolute dry CH2Cl2 (20 mL total volume). The absorbance was recorded by a UV/Vis spectrom- eter. An aqueous NaF solution (1.0 ^ 10^4 m) was prepared and ex- tracted three times with receptor R2 solution in CH2Cl2. The ex- tracted non-aqueous solutions were combined, diluted to 20 mL and the absorbance recorded. The absorbance of both solutions at 573 nm was compared to obtain the extraction efficiency and the receptor was found to be 99 % efficient. Acknowledgements We acknowledge The Director and The HOD (Department of Chemistry), NITK, for providing the research infrastructure. Mad- huprasad is grateful to NITK for the research fellowship. We thank the Department of Science and Technology, Government of India for providing the SCXRD facility under the FIST program, CSMCRI-Bhavnagar and Manipal Institute of Technology-Manipal for the spectral analysis. We thank the reviewers for their insight- ful suggestions and comments to improve the manuscript scien- tifically. [1] a)S. K. Menon, N. R. Modi, A. Pandya, A. Lodha, RSC Adv. 2013, 3, 10623-10627; b) J. Zhao, G. Li, C. Wang, W. 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