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N,N’-Diphenyldecanediamide: A Fluoride Ion Sensitive and Selective Amide

Meenakshi Thakran, Anek pal Gupta, Neeru Dabas*

Amity School of Applied Sciences, Amity University Haryana, Panchgaon, Manesar, Gurgaon, Haryana, India

Abstract: A fluoride ion (F⁻) sensitive organic ligand N, N’-Diphenyldecanediamide (L1), has been synthesized by the reaction of sebacoyl chloride with aniline in presence of triethylamine at room temperature. Spectroscopic investigation revealed F⁻ interacts strongly with L1 in comparison to other competitive anions (Cl⁻, Br⁻ and NO₃⁻) and as a consequence induces deprotonation in the NH fragment of L1. Different spectroscopic techniques such as ¹H NMR, UV-Vis and fluorescence emission spectroscopy supports the fast and distinct response behavior of N, N’-substituted polymethylene diamide towards F⁻.

Keywords: fluoride, responsive, selective, flexible, polymethylene diamide

Citation: Meenakshi Thakran, Anek pal Gupta, Neeru Dabas, N,N’-Diphenyldecanediamide: A Fluoride Ion Sensitive and Selective Amide. American Journal of Sensor Technology, 4 (1). 1-9. January 2017. doi:10.12691/ajst-4-1-1

1. Introduction

Recognition of inorganic and organic anions has becoming increasingly important because of their participation in chemical, biological and environmental processes [1,2,3,4]. Among various inorganic anions, F- is of prime concern as the critical presence of this ion in environment is associated with several health hazards such as neurotoxicity, fluorosis, and urolithiasis [5,6,7,8,9]. Therefore, the search of a molecular species that give quick response towards F- remains of utmost importance at all times. In this direction, different types of sensor molecules have been synthesized [10-23]. Among these, the amide based organic molecules appear to be simplest and effective synthetic target because they have polarized NH bond, which can be easily approached by anions through H-bonding. Undoubtedly, the hydrogen bonding ability of different anions vary with an amide based receptor, among them F- will have foremost and strongest H-bonding due to its small size and high electronegativity. The varying hydrogen bonding ability of different anions forms the basis of design of amide based receptors for selective recognition of a particular anion. Although at many instances, the central theme in design of a receptor molecule for anions has been the use of rigid or semi-rigid scaffolds to hold polarised NH fragment, however, a key aspect that has been missing is the use of flexible scaffold, polymethylene [24,25]. Flexibility is an important aspect of ligand design and is warranted in creating the quick response as required for different sensing applications [26]. To the best of our knowledge, no amide based sensor on flexible polymethylene backbone has been reported so far to evaluate the response toward fluoride ion. In the present work, we report synthesis of a flexible, acyclic polymethylene amide based species, N, N’-diphenyldecanediamide (L1) and its response toward different uninegative ions. L1 was found suitable for providing distinct response towards F- in DMSO and CH3CN and the response was monitored by ¹H NMR, UV-Vis and fluorescence emission spectroscopy. The uniqueness of the present system, L1, is that it can be easily synthesized at room temperature, in one step only and is highly selective and sensitive towards fluoride ion.

2. Experimental

2.1. Materials and Methods

Commercially available reagents such as aniline, triethylamine were purchased from Spectrochem Pvt. Ltd., India and dried and distilled following standard procedures. Sebacoyl chloride and tetrabutylammonium salts were procured from Alfa Aesar and Sigma-Aldrich respectively and used as such without any further purification. Solvents, acetonitrile and DMSO were used after drying with suitable drying agent followed by the distillation. ¹H and ¹³C NMR spectra were recorded on a Bruker spectrospin DPX 300/Avance 300 FT-NMR Spectrometer at 300 MHz and 75 MHz (in DMSO-d₆). IR spectra in the range of 4000-400 cm⁻¹ were recorded on a Nicolet Protégé 460 FTIR spectrometer. The samples were prepared with KBr as pellets. UV-visible spectra were recorded at 25°C on Perkin Elmer Bio Lambda 20 spectrophotometer. Fluorescence measurements were carried out on a Horiba-Jobin Yvon Scientific Fluoromax-4 at 25 °C.

2.2. Synthesis and Characterization

To a stirred solution of aniline (3.72g, 40 mmol) in CH₃CN (30 ml), sebacoyl chloride (4.78g, 20 mmol) was added dropwise followed by an addition of triethylamine (4.04 g, 40 mmol) at room temperature. The reaction mixture was allowed to stir for 5 hours and reaction completion was monitored by thin layer chromatography using silica coated plate. The reaction mixture was evaporated to complete dryness and residue was washed with cold water (3×50 ml) that resulted into crude solid product. Crystallization of crude mass from DMSO/CHCl₃ (1:2) afforded analytically pure species. Yield: 4.83 g (68.6 %); ¹H NMR (300 MHz, DMSO-d₆) δ in ppm: 9.84 (s, 2H, NH), 7.59 (d, 4H, Ar), 7.28 (t,4H, Ar), 7.02 (t, 2H, Ar), 2.29 (t, 4H, -CH₂-), 1.59 (s, 4H, -CH₂-), 1.30 (s, 8H, -CH₂-); ¹³C NMR (75 MHz, DMSO-d₆) δ in ppm: 171, 139, 129, 123, 119, 36, 29, 25; IR (KBr pellet, ν/cm⁻¹): 3307 cm⁻¹ (amide NH stretch), 3049 cm⁻¹ (aromatic CH stretch), 2935 cm⁻¹, 2854 cm⁻¹ (aliphatic CH stretch), 1659 cm⁻¹ (CO stretch), 1597 cm⁻¹ (aromatic C=C stretch), 1533 cm⁻¹ (NH bend), 756 cm⁻¹, 690 cm⁻¹ (out of plane bending for monosubstituted aromatic ring); ESI-MS: 352.2151, found: 375.2057 [M+ Na]⁺.

3. Results and Discussion

Species L1 was prepared by the reaction of one equivalent of sebacoyl chloride with two equivalents of aniline in acetonitrile (CH₃CN) using triethyl amine as a base (Scheme 1). It was crystallized further in DMSO/CHCl₃ (1:2) and the crystallized species was characterized by various physicochemical and spectroscopic techniques. Charcaterization data (¹H NMR, ¹³C NMR, mass and FT-IR spectrum) for L1 have been given in supporting Figure S1 to Figure S4. Species L1 has poor to moderate solubility in organic solvents such as chloroform, dichloromethane, ethanol, methanol, THF, and acetonitrile and very good solubility in solvents such as DMF and DMSO.

Scheme 1. Recation scheme for synthesis of L1

3.1. Anion Influenced Studies

Response of L1 toward different uninegative ions such as, F⁻, Cl⁻, Br⁻ and NO₃⁻ was examined using tetrabutylammonium salts, Bu₄N⁺X⁻ ( X= F, Cl, Br, NO₃ ) and monitored with the help of ¹H NMR, UV-Vis and Fluorescence emission spectroscopic techniques. The anion influenced studies were carried out in polar aprotic solvents, DMSO and CH₃CN since the polar protic solvents may compete with L1 for the anion.

3.2. ¹H NMR Studies

Anion influenced changes in L1 were studied with the help of ¹H NMR spectroscopy, carried out in DMSO- d₆. A solution of concentration 10⁻² mol dm⁻³ of L1 was prepared to evaluate the response towards different anions carrying uninegative charge and of competitively similar nature. Changes in ¹H NMR spectrum of L1 were recorded after addition of 30 equivalents of tetra butyl ammonium salts, Bu₄N⁺X⁻ (X= F, Cl, Br, NO₃) to solution of the species L1 in DMSO-d₆ and have been demonstarted in supporting Figure S5 to Figure S8 respectively. The ¹H NMR spectrum of species L1 in DMSO- d₆ shows that it has three types of protons namely, aliphatic, aromatic and amide (-CO-NH-) (Supporting Figure S1). The aliphatic protons appear between 1.30 ppm to 2.29 ppm and aromatic protons appear between 6.99 ppm to 7.60 ppm. There are two chemically and magnetically equivalent NH protons in L1 that exhibited a singlet at 9.84 ppm (Figure 1a). The change in ¹H NMR spectrum of L1 upon addition of 30 equivalents of Bu₄N⁺F⁻ has been shown in Figure 1c. It is clearly observable from the ¹H NMR spectrum that the two equivalent amide protons (-CO-NH-) of L1 at 9.84 ppm disappeared upon addition of Bu₄N⁺F⁻. The disappearance of NH protons signal in ¹H NMR could be understood by considering the nature of fluoride ion in DMSO. In polar aprotic solvent such as DMSO, anion is not solvated as effectively as the cation and hence anion is free to demonstrate its intrinsic activity. F⁻ being small in size has high charge density and exhibits high basicity in polar aprotic solvents and because of high basicity usually results in deprotonation of polarized H-A bond, where A is any electronegative substitutent such as nitrogen [27]. In addition to this, F⁻ has been reported as a strong proton acceptor that interacts with amide derivatives containing only a single proton transfer donor group [28]. However in present case, deprotonation of two equivaent amide protons (-NH-CO-) of L1 has been observed after addition of 30 equivalents of fluoride source, Bu₄N⁺F⁻ leading to disappearance of NH protons. The immediate disappearance of NH protons signal in ¹H NMR spectrum of L1 upon addition of Bu₄N⁺F⁻ indicates the species L1 is senstive to F⁻. It is also evident from the ¹H NMR spectrum that species L1 interacts to F⁻ through amide protons (-NH-CO-) as the addition of Bu₄N⁺F⁻ to L1 results in disappearence of protons signal at 9.84 ppm that corresponds to amide protons. To evaluate the response of L1 towards other anions, ¹H NMR spectroscopic investigation was extended to other anions, Cl⁻, Br⁻ and NO₃⁻. It should be noted that during ¹H NMR spectroscopic investigation all the parameters such as concentration of L1, concentartion of anion source and solvent were kept constant and similar to ¹H NMR spectroscopic investigation carried out for fluoride ion. The ¹H NMR spectral response in L1 as measured upon addition of 30 equivalents of Bu₄N⁺Cl⁻ has been demonstrated in Figure 1b.

Figure 1. ¹H NMR spectrum of (a) L1 (1×10⁻² mol dm⁻³) (b) L1(1×10⁻² mol dm⁻³) upon addition of 30 equivalents of Bu₄N⁺Cl⁻ (c) L1(1×10⁻² mol dm⁻³) upon addition of 30 equivalents of Bu₄N⁺F⁻

Changes in ¹H NMR spectrum of L1 with chloride ion were found unlike fluoride ion. The deprotonation of amide proton of L1 as observed with fluoride ion was not seen with chloride ion, however it was found that two equivalent amide protons (-CO-NH-) of L1 undergoes a slight downfield shift of 0.4 ppm immediately upon addition of tetrabutyl ammonium chloride (Bu₄N⁺Cl⁻ ) into the solution of L1 in DMSO - d₆ (Figure 1b). The shift in ¹H NMR spectrum of L1 upon addition of Bu₄N⁺Cl⁻ indicates that L1 is also sensitive to Cl⁻ but not as sensitive as F⁻. The shift in the two chemically and magnetically equivalent NH protons of LI upon addition of Cl⁻ source indicates that Cl⁻ weakly interact with both amide NH proton equally through intermeolecular H-bonding (-CO-NH….Cl⁻) as representated in the Figure 2a.

The involvement of amide NH protons in hydrogen bonding with chloride ion, decreases the elctron density around proton resulting in deshielding of amide protons and so the NH signals appeared in downfield region (at 10.26 ppm) compared to L1. 1H NMR spectroscopic investigation of L1 with Bu₄N⁺Cl⁻ and Bu₄N⁺F⁻ reveals that in both the cases it is the the amide NH proton of L1 that interacts with the anion. Though the two competitive halide ions, Cl⁻ and F⁻ interact with L1 through the amide protons NH but the degree to which they do so is quite different. Upon addition of equal amounts of Cl⁻ and F⁻ into L1, earlier results in deshielding of NH protons while the later results in deprotonation. The interaction and reaction outcomes of Cl⁻ and F⁻ with L1 have been demonstrated in Figure 2a and Figure 2b respectively. Deprotonation of NH protons of L1 is observed only with F⁻ not with Cl⁻ because later being larger in size is not a strong base like F- and hence unable to deprotonate L1. It has been previously reported by Elmes et al. that chloride ion interacts to amide NH protons through classical H-bonding at lower concentartion but can also results in deprotonation phenomenon if present in higher concentration upto 30 equivalents [29]. However, for present sytem, L1, deprotonation of NH protons was not observed even when chloride ion is present at high concentration (30 equivalent) so, L1 can be used for selective recognition of fluoride ion in presence of chloride ion at higher concentrations. Further, the response of L1 was monitered toward Br⁻ and NO₃⁻ using similar ¹H NMR spectroscopic technique keeping the identical concentrations of L1 and anion source (Bu₄N⁺Br⁻ and Bu₄N⁺NO₃⁻). The addition of 30 equivalents of Bu₄N⁺Br⁻ and 30 equivalents of Bu₄N⁺NO₃⁻ into L1 in two separate sets of experiments could not reveal any appreciable chemical shift in ¹H NMR spectrum of L1 (Figure S7 and S8 in supplementary data).

Since, addition of Br⁻ and NO₃⁻ did not induce change in ¹H NMR spectrum of L1, so the species L1 is not suitable for their recognition or sensing. Summarizing the ¹H NMR spectroscopic results it was found that a sharp and distinct change in ¹H NMR spectrum of N, N’-Diphenyldecanediamide (L1) was observed upon addition of fluoride ion only ⁻ and not with Cl⁻, Br⁻ and NO₃⁻ anions, used in present study. The deprotonation of amide protons of L1 upon addition of F⁻ eventually explains enhanced selectivity and sensitivity of L1 toward F⁻ over other uninegative ions. Highly sensitive nature of N, N’-Diphenyldecanediamide towards F⁻ is further supported and ascertained by UV-Vis absorption and Fluorescence emission studies.

Figure 2. Pictorial representation of interaction of L1 with (a) Cl⁻ (b) F⁻

3.3. UV-Vis Study

Uv-Vis spectroscopic studies were carried out in acetonitrile. Species L1, in CH₃CN, exhibited two absorption bands at wavelength, 206 nm and 241 nm that arises due to π → π* transition of aromatic ring. The changes in UV-Vis absorption spectrum of L1 were evaluated after succesive addition of fluoride source. A decrease in absorbance of L1 was continued upto addition of 7 equivalents of F- (Figure 3a). However no noticeable change in absorption wavelength was observed. A distinct red shift of 5 nm in UV-Vis spectrum of L1 was observed only upon the addition of 30 equivalents of Bu₄N⁺F⁻ into L1 (10⁻⁴ M , in CH₃CN) as illustrated in Figure 3b. It should be noted that such type of red shift in absorption spectrum of a receptor were observed in few cases only upon addition of more than 30 equivalents of fluoride salt and has been rationalized in term of deprotonation of the receptor [30]. The UV-Vis spectroscopic study corroborate well with the result of ¹H NMR spectroscopy, where deprotonation of L1 was observed upon addition of 30 equivalent of fluoride source. The red shift in UV-Vis spectrum of species L1 is ascribed due to its interaction with F⁻ that concomitantly lead to deprotonation of NH fragment.

Figure 3. Changes in UV-Vis spectrum of L1 (10⁻⁴M, CH₃CN) (a) with an increasing amount of Bu₄N⁺F⁻ (b) upon addition of 30 equivalents of Bu₄N⁺F⁻

3.4. Fluorescence Emission Spectroscopy

The resposive nature of L1 towards F⁻ was further monitered by emission spectroscopy. Similar to the changes observed in UV-Vis absorption spectrum the changes in emission spectrum of L1 were also seen upon the addition of F⁻ into L1 as shown in Figure 4. Receptor 1 itself gives emission at 311 nm in CH₃CN upon excitation at 270 nm. A significant quenching in fluorescence intensity of L1 (10⁻⁴ M, CH₃CN) was observed with appearance of a low intensity band at 422 nm upon the addition of 30 equivalents of Bu₄N⁺F⁻ salt.

Figure 4. Fluorescence emission change of L1 in presence of 30 equivalents of Bu₄N⁺F⁻ in CH₃CN (λexc 270 nm)

The quenching in fluorescence intensity of L1 upon addition of Bu₄N⁺F⁻ is indicative of enhanced intermolecular charge transfer (ICT) that arises due to change in electron density of the amide and is also indicative of strong interaction of receptor L1 with F⁻ [31].

4. Conclusion

Response of a flexible, acyclic, artificial organic ligand, N, N’diphenyldecanediamide (L1) has been studied toward different anions using equal concentration of anion source, tetrabutylammonium salts, Bu₄N⁺X⁻ (X =F, Cl, Br, NO₃) with the help of ¹H NMR spectroscopy. ¹H NMR spectrum of L1 exhibits complete disappearance of NH protons signal, upon addition of Bu₄N⁺F⁻. However a shift of 0.4 ppm in NH protons was observed in ¹H NMR spectrum of L1 with Cl⁻. No change in ¹H NMR spectrum of L1 was observed for Br⁻ and NO₃⁻. ¹H NMR spectroscopic investigation indicates that L1 is selective and more sensitive for F⁻, compared to Cl⁻ and not at all sensitive for Br⁻ and NO₃⁻. Since the deprotonation process of L1 is achieved only in presence of F⁻ and not with any other anion so the utility of current system is quite appealing in terms of selective fluoride ion sensor in presence of Cl⁻, Br⁻ and NO₃⁻.

Acknowledgements

Authors are thankful to Prof. P. B. Sharma, vice chancellor, Prof. Padmakali Banerjee, pro vice chancellor, Prof. R. K. Thakur, Director Amity School of Applied Sciences, Amity University Haryana, Gurgaon 122413, India for providing research facilities and encouragement. We also thank Indian Institute of Technology Delhi-110016, India to facilitate instrumental analysis to carry forward this research work.

References

Supporting Figures

Figure S1. ¹H NMR spectrum of L1 at 300 MHz in DMSO-d₆

Figure S2. ¹³C NMR spectrum of L1 at 75 MHz in DMSO-d₆

Figure S3. ESI-MS spectrum of L1

Figure S4. FT-IR spectrum of L1

Figure S5. ¹H NMR spectrum of (a) L1 (1×10⁻² mol dm⁻³) at 300 MHz in DMSO-d6 upon addition of 30 equivalents of Bu₄N⁺F⁻

Figure S6. ¹H NMR spectrum of (a) L1 (1×10⁻² mol dm⁻³) at 300 MHz in DMSO-d6 upon addition of 30 equivalents of Bu₄N⁺Cl⁻

Figure S7. ¹H NMR spectrum of (a) L1 (1×10⁻² mol dm⁻³) at 300 MHz in DMSO-d6 upon addition of 30 equivalents of Bu₄N⁺Br⁻

Figure S8. ¹H NMR spectrum of (a) L1 (1×10⁻² mol dm⁻³) at 300 MHz in DMSO-d6 upon addition of 30 equivalents of Bu₄N⁺NO₃⁻

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