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Nitrite selective anion receptor based on 1-methyl-1H-perimidine
Tetrahedron 71 (2015) 7782e7788
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Nitrite selective anion receptor based on 1-methyl-1H-perimidine Thiravidamani Senthil Pandian a, Venkatesan Srinivasadesikan b, M.C. Lin b, Jongmin Kang a, * a b
Department of Chemistry, Sejong University, Seoul 143-747, South Korea Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2015 Received in revised form 26 February 2015 Accepted 27 February 2015 Available online 5 March 2015
A new anion receptor 1, which utilizes only CeH hydrogen bonding have been designed and synthesized. This receptor utilizes both aromatic CeH (two perimidine C2-H, anthracene 9-H) and aliphatic CeH (two perimidine 1-methyl CeH, two benzylic CeH) as hydrogen bonding moieties. The receptor 1 is found to have considerable binding affinity for weakly basic anions even in polar solvent such as DMSO. In addition, receptor 1 is selective for nitrite. The experimental data from UVevis, fluorescence and 1H NMR titration are consistent with theoretical studies. The order of the interaction of the CeH groups of perimidine containing receptor with NO 2 anion is found to be perimidine (CeH)>methylene (CeH)>methyl (CeH)>anthracene (CeH). Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Anion receptor CeH hydrogen bonds Nitrite Perimidine
1. Introduction Although there have been many examples that CeH hydrogen bonds contribute to anion binding events,1 there appears to be a general consensus among supramolecular chemists that CeH hydrogen bonds are much weaker than traditional OeH and NeH donors. However, when attached to polarizing substituents, CeH groups are moderate-to strong hydrogen bond donors, exhibiting interaction energies comparable to those obtained with OeH and NeH groups. For example, 1,3-disubstituted imidazolium units bind effectively with anionic species through (CeH)þ$$$A hydrogen bonding interactions.2 In quite a number of anion receptors, anion binding is supported by additional weaker interactions of the anion with aromatic CeH bonds. Reports on interactions between anions and nonaromatic CeH groups as hydrogen-bond donors are rare although aliphatic CeH groups form significant interactions with anions in gas phase3 solid state4 and solution.5 In addition, quite a few anion receptors are known, which utilizes only CeH hydrogen bonding for anion binding.6 Therefore, to develop a new anion receptor, which utilizes only CeH hydrogen bonding, we have designed and synthesized a new anion receptor 1, which has anthracene spacer and two 1-methyl1H-perimidine arms. This receptor utilizes both aromatic CeH (two perimidine C2-H, anthracene 9-H) and aliphatic CeH (two perimidine 1-methyl CeH, two benzylic CeH) as hydrogen bonding
* Corresponding author. E-mail addresses: [email protected] (M.C. Lin), [email protected] (J. Kang). http://dx.doi.org/10.1016/j.tet.2015.02.091 0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
moieties. From the experiments, we found that receptor 1 has considerable binding affinity for weakly basic anions even in polar solvent such as DMSO although receptor 1 utilizes only CeH hydrogen bond in the binding event. In addition, receptor 1 is selective for nitrite, which has biological, environmental and synthetic significance.7 The fundamental source of nitrite in our diets is processed (cured) meat or fish, and nitrites are considered a potential reactant precursor for nitrosamines. Nitrosamines are problematic because they become reactive at the cellular level and that means they can alter gene expression and cause DNA damage The U.S. EPA has set an RfD of 0.l mg nitrite nitrogen/kg body weight per day (equivalent to 0.33 mg nitrite ion/kg body weight per day). The most serious health risks caused by high nitrite exposure are blue baby syndrome, occasional intoxications For these reasons, determination of nitrite has received much attention. We report herein the synthesis and binding properties of receptor 1. The binding phenomena of receptor 1 could be monitored by UVevis, fluorescence and 1H NMR spectra. 2. Results and discussion 2.1. Synthesis For the synthesis of receptor 1, 2 equiv of 1-methylperimidine8 was reacted with 1,8-bis(bromomethyl)anthracene.9 Then, anion exchange with ammonium hexafluorophosphate gave receptor 1 bearing two 1-methylperimidine arms in 49% overall yield. Receptor 1 was characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry.
T.S. Pandian et al. / Tetrahedron 71 (2015) 7782e7788
2.2. Interactions with nitrite The ability of receptor 1 to recognize nitrite was studied in DMSO using UVevis titration spectra. The receptor 1 displayed strong absorption band at 261 nm 314 nm, 330 nm, 349 nm, 368 nm and 387 nm in DMSO respectively. Fig. 1a shows the family of UVevis spectra obtained over the course of the titration of solution 1 with tetrabutylammonium nitrite in DMSO. As nitrite ions were added to the 20 mM DMSO solution of 1, the spectra showed small bathochromic shift of lmax along with isosbestic points at 336 nm. This phenomenon indicates complex formation between receptor 1 and nitrite. In addition, the receptor 1 displayed strong fluorescence emission in DMSO as shown in Fig. 1b. The excitation and emission wavelength were 329 and 400 nm, respectively. The intensity of emission spectrum from 20 mM solution of the receptor 1 gradually decreased as the concentration of tetrabutylammonium nitrite salts was increased (1e180 equiv), which also indicates the association between the receptor 1and nitrite.
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The complexation abilities of receptor 1 to nitrite was also measured by standard 1H NMR titration experiments in DMSO-d6 using a constant host concentration (2 mM) and increasing concentrations of nitrite anions. The addition of tetrabutylammonium nitrite salts to the solution of receptor 1 in DMSO-d6 resulted in large downfield shifts of perimidine C2-H (Ha in Fig. 2) and benzylic CeH(Hb). For example, addition of tetrabutylammonium nitrite moved perimidine C2-H from 9.32 to 9.60 ppm and benzylic CeH from 6.04 to 6.12 ppm. The downfield shifts of these protons indicate the presence of a hydrogen bond interaction between these CeH hydrogens and nitrite ion. In addition, perimidine 1-methyl CeH(Hc) moved from 3.65 to 3.67 ppm and anthracene 9-H(Hd) moved from 8.94 to 8.98 ppm. Again, these downfield shifts of CeH protons indicate the presence of a hydrogen bond interaction with nitrite ion although the chemical shifts of these hydrogens are small. The stoichiometry between receptor 1 and nitrite was determined to be 1:1 using a 1H NMR Job plot in DMSO-d6 (Fig. 3).
Fig. 1. Family of UVevis spectra (a) and fluorescence spectra (b) recorded over the course of titration of 20 mM DMSO solutions of the receptor 1 with the standard solution tetrabutylammonium nitrite.
Fig. 2. 1H NMR spectra of 2 mM of receptor 1 containing increasing amounts of tetrabutylammonium nitrite (0e53 equiv) in DMSO-d6.
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2.3. Interactions with halides
Fig. 3. Job plots of receptor 1 with tetrabutylammonium nitrite and bromide obtained by 1H NMR in DMSO-d6.
A BenesieHildebrand plot10 by use of change at 330 nm in UVevis spectrum and 400 nm in fluorescence spectrum gave the association constants. The association constants calculated were 3.3103 M1 from UVevis titration and 3.3103 M1 from fluorescence titration, respectively. In addition, analysis of chemical shift utilizing EQNMR11 gave the association constant of 3.5103 M1 by, which is similar to the values obtained from UVevis and fluorescence titrations.
The abilities of receptor 1 to recognize halides were also studied in DMSO using UVevis titration spectra. When the amount of bromide was increased, small bathochromic shift of lmax along with sharp isosbestic points at 334 nm were observed (Fig. 4a). The existence of isosbestic point for UVevis titrations of receptor 1 with bromide suggests a 1:1 complexation, and this was confirmed by Job’s plot analysis (Fig. 3). In addition, the intensity of emission spectrum from the receptor 1 gradually decreased as the concentration of tetrabutylammonium bromide salts was increased (10e220 equiv), which also indicates the association between the receptor 1 and bromide (Fig. 4b). From these experiments, association constants for bromide were calculated as 1.0103 M1 and 1.2103 M1 from the UVevis and fluorescence titrations, respectively. Hydrogen bond formation was confirmed by 1H NMR titration. When bromide was added, all four hydrogens (HaeHd) moved to downfield (Fig. 5). For example, perimidine C2-H moved from 9.32 to 9.58 ppm and benzylic CeH from 6.04 to 6.15 ppm respectively. In addition, small chemical shifts of perimidine 1-methyl CeH (from 3.65 to 3.67) and anthracene 9-H (from 8.94 to 8.98) were observed. The association constants calculated for bromide were calculated as 1.1103 for 1H NMR titration. Other halides such as chloride and iodide showed similar behaviors to bromide (see Supplementary data); calculated binding constants for other anions are summarized in Table 1. As anions have diverse geometries, complementarity between the receptor
Fig. 4. Family of UVevis spectra (a) and fluorescence spectra (b) recorded over the course of titration of 20 mM DMSO solutions of the receptor 1 with the standard solution tetrabutylammonium bromide.
Fig. 5. 1H NMR spectra of 2 mM of receptor 1 containing increasing amounts of tetrabutylammonium bromide (0e60 equiv) in DMSO-d6.
T.S. Pandian et al. / Tetrahedron 71 (2015) 7782e7788 Table 1 Association constants (M1) of receptors 1 with various anions in DMSO Anion NO 2 Br Cl I
a
H NMRa
UV 3
3.310 0.210 1.01030.510 4.81021.510 1.31022102
1
Fluorescence
3.5103 1.1103 5.0102 1.3102
3.31030.310 1.21031.110 4.81021.710 1.11022.1102
Errors are less than 12%.
and anion is crucial in determining selectivity. The complementarity between the receptor and halides is mostly achieved by the size of the receptor binding site due to the spherical shape of halides. The preference for bromide suggests that the cavity formed by 7 CeH bonds is more complementary to the size of the bromide ion than to the size of other halide ion. 2.4. Interactions with hydroxide, fluoride, acetate and dihydrogen phosphate In order to discriminate between H-bonding and deprotonation, UVevis titration of receptor 1 with tetrabutylammonium hydroxide was carried out (Fig. 6a). Changes in the absorbance spectra in the presence of hydroxide were clearly different from those observed for nitrite and weakly basic halides. In UVevis titration with basic anions such as fluoride, acetate and dihydrogen phosphate, similar phenomenon was observed (Fig. 6bed). Therefore, it could be concluded
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that only deprotonation occurred with these anions. These results are consistent with the work of Gao et al.12 When they tested perimidium receptor with benzene spacer, they observed large changes UVevis and fluorescence spectra with acetate and fluoride. 2.5. Binding energy studies In present study the computational modeling using the B3LYP method has been carried out for perimidine containing receptor with NO 2 , Br , Cl and I anions. All calculations have been computed by using the Gaussian 09 suite of programs.13 The present receptor appears to be formed possibly with seven CeH/O type Hbondings. The NMR results show the shifting of peaks for the formation of H-bondings in hosteanion complex. So, we have decided to investigate the binding poses more accurately using the state-ofthe-art density functional theory. Fig. 7 shows the structure of host 14 and their NO 2 complex optimized at the B3LYP level using the 631g (d) basis set. The binding energies estimated by experiment and theory are presented in Table 2. The results reveal the strong binding energy for Host2þeNO 2 complex, in good agreement with the experimental results, allowing detection of the complex in the lowest concentration compared with the other anions. The strengths of hydrogen bonds and other geometrical parameters are listed in Table 3. The hydrogen bond distance between the H57eO78 is 1.959 A, which is the strong hydrogen bond. The H48/O77 and H55eO77 hydrogen
Fig. 6. Family of UVevis spectra recorded over the course of titrating a 20 mM DMSO solution of receptor 1 with increasing amounts of tetrabutylammonium hydroxide (a), fluoride (b), acetate (c) and dihydrogen phosphate (d).
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Fig. 7. Most stable structure for Host2þ$NO 2 complex in solvent phase (DMSO as a solvent) optimized with B3LYP/6-31g(d) using Gaussian 09. Dashed lines denote the hydrogen bonds (strong and weak). See Table 3 for geometrical parameters and Mulliken charges. Atom numbering included. Atom colors: GraydCarbon; ReddOxygen; BluedNitrogen; WhitedHydrogen.
Table 2 Experimental and computational binding energies (BEs) for Host2þ with X complexes in DMSOa Complex
HosteNO 2 HosteBr HosteCl HosteIb
BE Exp (UV)
Calcd (DFT)
4.74 4.09 3.66 2.88
23.24 15.32 8.17 4.88
a Units are in kcal/mol. Experimental binding energies are derived from UV binding constants. DFT calculations are performed with (B3LYP/6-31G(d)) using a polarizable continuum model in DMSO. b Binding energy of Iodine complex is calculated at the B3LYP/LANL2DZ level of theory.
Table 3 Charges and geometries of Host2þ and their NO 2 complex (cx) Mulliken atomic charges (e)
cx
Host
H72 H58 H55 H48 H54 H57 H74
0.228 0.282 0.219 0.169 0.239 0.275 0.224
0.218 0.266 0.217 0.162 0.226 0.266 0.218
Heavy atom distance ( A) C43eN79 C24eO77 C17eO77 C10eO77 C15eO77 C23eN78 C44eN78
3.460 3.062 3.693 3.329 3.271 3.045 3.511
H-bond distance ( A) H54eO77 H57eO78 H74eO78 H55eO77 H58eO77 H72eO79 H48eO77
2.178 1.959 2.530 2.821 1.987 2.388 2.693
bond distances are 2.693 and 2.821 A, respectively. These two hydrogen bonds observed to be weak in the NO2 complex in solvent phase at B3LYP/6-31g(d) level. The result indicates that the strengths of the hydrogen bonds are in the following order: Permidine CeH/O>benzyllic CeH/O>Methyl CeH/O>anthracene CeH/O in good agreement with the NMR result. The complex with iodine anion is weaker than the rest of the anions studied here. In hosteiodine complex, the two methyl groups are not participating in the complex formation (see Fig. S20), showing more than 4 A distance from one of the H of CH3 to I anion. This result is also in good agreement with the NMR result of the iodine anion complex. Similarly, the bromide and chloride anions form complex with the host and the optimized structures are shown in Supplementary data (Figs. S18 and S19). The hydrogen bonding lengths reveal that the bonding is very weak between the methyl and anions (>3.3 A). The methylene (eCH2e) groups of the host forming weak hydrogen bonds with the Br and Cl anions have been observed to be 2.8 to 2.9 A whereas the iodine anion has been observed to be 3.041 A. Similarly, the perimidine CH protons have been observed to be of weaker for the iodine anion as compared to the Br and Cl anions. 2.6. TD-DFT results To further support the experimental observations of the hosteanion interactions and to provide a better insight into fundamental H-bonded cavity and anions binding interaction, TD-DFT calculations were carried out. The HOMOeLUMO and TD-DFT results are listed in Table 4. The B3LYP optimized structure of Host/NO 2 complex and its electronic delocalization of the frontier molecular orbitals (FMOs) can be seen in Fig. 8. We note that the addition of anions may change the electronic distribution such that the HOMO corresponds to the anthracene part and the LUMO corresponds to the anions interaction with the perimidine parts. Interestingly, we found this to be the case as shown in Fig. 7 for the NO 2 anion complex and the other energy minimized anion complexes are shown in Figs. S21e23 in Supplementary data. These results are in good agreement with the 1H NMR measurements. The predicted absorption wavelengths, oscillator strength (f) and the configuration description of the singlet excited states are shown in Table 4. For Host2þeNO 2 anion complex (see Fig. 8), the lowest
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Table 4 TD-DFT calculated HOMO, LUMO, HOMOeLUMO gap (HLG), wavelength (lmax), and oscillator strength (f) for ANBPM2þeX (X¼Br, Cl, I, NO 2 )complex Complex ANBPMeNO 2
HOMO
LUMO
HLGa (eV)
Excited state
labs,calb (nm)
fc
Configd
Contributione
S1 S2 S1 S2 S1 S2 S1 S2
407.56 395.84 400.26 395.88 399.49 396.58 424.17 423.21
0.0513 0.0185 0.0462 0.0034 0.0284 0.0034 0.0035 0.0021
H/L H/Lþ1 H/L H/Lþ1 H/L H/Lþ1 H/L H/Lþ1
0.63 0.55 0.60 0.53 0.54 0.52 0.49 0.67
5.52
2.04
3.48
ANBPMeBr
5.56
2.06
3.50
ANBPMeCl
5.57
2.06
3.51
ANBPMeIf
5.70
2.29
3.41
a b c d e f
HOMO, LUMO, and HOMOeLUMO gaps are calculated using the B3LYP/6-31G(d) level. Absorption energies are calculated using the TDDFT method at B3LYP6-31G(d) level. Oscillator strength. H and L stands for the predicted HOMO and LUMO, respectively. Only configurations with 5% or greater contributions are included. Iodine complex results are calculated at the B3LYP/LANL2DZ level of theory.
Fig. 8. Molecular orbital distribution plots of HOMO, HOMO-1, LUMO and LUMOþ1 states in the ground state of the ANBPM2þeNO 2 complex.
energy singlet transitions are mainly from HOMO to LUMO, similar to other anion complexes reported here. The HOMOeLUMO gaps (HLG) given in Table 4 indicate that among the anions, NO 2 has the lowest HLG supporting the selective binding and confirming our experimental results. For anthracene and perimidine, both units of FMOs are delocalized over the entire structure. The charge distribution in the receptoreNO2 (in DMSO solvent) complex has been shown in the Fig. 9 through Electrostatic potential surface (EPS). 3. Conclusion In DMSO the NO 2 , Br , Cl and I anions are complexed with perimidine containing receptor 1 via CeH/O type hydrogen bonding. The hydrogen bond strengths vary for the different anions studied here. In the receptor 1, the perimidine eCH/anion type hydrogen bond has been observed to be stronger as compared with the methylene and methyl group hydrogen bonds with the anions. The theoretical binding energies in solvent phase for the Host2þeNO 2 complex has been observed to be the strongest as compared with those of the Br, Cl and I anions. The strong
Fig. 9. Electrostatic potential maps of Host2þeNO 2 complex calculated at B3LYP/631G(d) level in solvent phase (DMSO as a solvent).
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binding energy for the NO 2 anion complex is consistent with the experimental observation such as the lowest saturation point reached during the titration of the NO 2 anion with the perimidine containing receptor as compared with the Br, Cl and I anions studied here. The order of the interaction of the CeH groups of perimidine containing receptor with NO 2 anion is perimidine (CeH)>methylene (CeH)>methyl (CeH)>anthracene (CeH) The hydrogen bond strengths in the solvent phase for the receptoreanion interactions discussed above are also in good agreement with the experimental NMR results. 4. Experimental section 4.1. General 4.1.1. Materials. Tetra-n butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), tetra-n-butylammonium dihydrogen phosphate (TBAH2PO4), tetra-n-butylammonium acetate (TBAA), and tetra-n-butylammonium bromide (TBABr), tetran-butylammonium chloride (TBACl) tetra-n butylammonium iodide (TBAI) and tetra-n butylammonium nitrite were purchased from SigmaeAldrich Chemical Co., Inc., and used as received. 4.1.2. Measurements. Absorption spectra were recorded using a biochrom Libra S70 spectrophotometer (Biochrom Ltd, England). NMR spectra were recorded using a BRUKER spectrometer operated at 500 MHz. ESI MS spectra were obtained using a JMS 700 (Jeol, Japan) double focusing magnetic sector mass spectrometer. All measurements were carried out at room temperature (298 K). 4.1.3. Receptor 1. To a stirred solution of 1-methylperimidine (0.12 g, 0.65 mmol) in toluene (5 mL) was added a solution of 1,8bis(bromomethyl)anthracene (0.12 g, 0.32 mmol) in toluene (5 mL). The resulting mixture was refluxed for 96 h. After the solvent was distilled off under reduced pressure, the crude product was purified by column chromatography (11% MeOH in CH2Cl2) to afford bromide salt. To the solution of bromide salt was added aqueous solution of NH4PF6 (0.537 g, 3.2 mmol). The precipitate was collected by filtration, washed with water and dried in vacuum to give receptor 1 (0.14 g, yellow solid) in 49% yield. 1H NMR (500 MHz, DMSO-d6) d 9.32 (s, 1H), 8.94 (s, 1H), 8.86 (s, 1H), 8.19 (d, 2H, J¼8.6 Hz), 7.79 (d, 2H, J¼7.0 Hz), 7.71 (d, 2H, J¼8.3 Hz), 7.63 (t, 2H, J¼4.0 Hz), 7.64 (d, 2H, J¼2.4 Hz), 7.61 (t, 2H, J¼7.8 Hz), 7.4 (t, 2H, J¼8.1 Hz), 7.2 (d, 2H, J¼7.7 Hz), 6.72 (d, 2H, J¼8.0 Hz), 6.04 (s, 4H) 3.65 (s, 6H). 13C NMR (500 MHz, DMSO-d6): d 154.31, 134.13, 133.02, 131.95, 131.40, 128.77, 128.65, 128.46, 128.30, 128.25, 128.20, 125.36, 124.01, 123.63, 123.53, 120.92, 117.08, 108.44, 108.24, 52.43 mass: calcd for C40H32F12N4P2 m/z¼881.1805, found 881.1808 mp 248e250 C. Acknowledgements This research was supported by the Basic Science Research Program of the Korean National Research Foundation funded by the Korean Ministry of Education, Science and Technology (20100021333). Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2015.02.091.
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R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, M.; Cossi, N.; Rega, J. M.; Millam, M.; Klene, J. E.; Knox, J. B.; Cross, V.; Bakken, C.; Adamo, J.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian: Wallingford CT, 2009. 14. (a) Becke, A. D. Physiol. Rev. 1988, A38, 3098e3100; Becke, A. D. J. Chem. Phys. 1993, 98, 5648e5652; (b) Becke, A. D. J. Chem. Phys. 1997, 107, 8554e8560; (c) Schmider, H. L.; Becke, A. D. J. Chem. Phys. 1998, 108, 9624e9631.
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Nitrite selective anion receptor based on 1-methyl-1H-perimidine Thiravidamani Senthil Pandian a, Venkatesan Srinivasadesikan b, M.C. Lin b, Jongmin Kang a, * a b
Department of Chemistry, Sejong University, Seoul 143-747, South Korea Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2015 Received in revised form 26 February 2015 Accepted 27 February 2015 Available online 5 March 2015
A new anion receptor 1, which utilizes only CeH hydrogen bonding have been designed and synthesized. This receptor utilizes both aromatic CeH (two perimidine C2-H, anthracene 9-H) and aliphatic CeH (two perimidine 1-methyl CeH, two benzylic CeH) as hydrogen bonding moieties. The receptor 1 is found to have considerable binding affinity for weakly basic anions even in polar solvent such as DMSO. In addition, receptor 1 is selective for nitrite. The experimental data from UVevis, fluorescence and 1H NMR titration are consistent with theoretical studies. The order of the interaction of the CeH groups of perimidine containing receptor with NO 2 anion is found to be perimidine (CeH)>methylene (CeH)>methyl (CeH)>anthracene (CeH). Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Anion receptor CeH hydrogen bonds Nitrite Perimidine
1. Introduction Although there have been many examples that CeH hydrogen bonds contribute to anion binding events,1 there appears to be a general consensus among supramolecular chemists that CeH hydrogen bonds are much weaker than traditional OeH and NeH donors. However, when attached to polarizing substituents, CeH groups are moderate-to strong hydrogen bond donors, exhibiting interaction energies comparable to those obtained with OeH and NeH groups. For example, 1,3-disubstituted imidazolium units bind effectively with anionic species through (CeH)þ$$$A hydrogen bonding interactions.2 In quite a number of anion receptors, anion binding is supported by additional weaker interactions of the anion with aromatic CeH bonds. Reports on interactions between anions and nonaromatic CeH groups as hydrogen-bond donors are rare although aliphatic CeH groups form significant interactions with anions in gas phase3 solid state4 and solution.5 In addition, quite a few anion receptors are known, which utilizes only CeH hydrogen bonding for anion binding.6 Therefore, to develop a new anion receptor, which utilizes only CeH hydrogen bonding, we have designed and synthesized a new anion receptor 1, which has anthracene spacer and two 1-methyl1H-perimidine arms. This receptor utilizes both aromatic CeH (two perimidine C2-H, anthracene 9-H) and aliphatic CeH (two perimidine 1-methyl CeH, two benzylic CeH) as hydrogen bonding
* Corresponding author. E-mail addresses: [email protected] (M.C. Lin), [email protected] (J. Kang). http://dx.doi.org/10.1016/j.tet.2015.02.091 0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
moieties. From the experiments, we found that receptor 1 has considerable binding affinity for weakly basic anions even in polar solvent such as DMSO although receptor 1 utilizes only CeH hydrogen bond in the binding event. In addition, receptor 1 is selective for nitrite, which has biological, environmental and synthetic significance.7 The fundamental source of nitrite in our diets is processed (cured) meat or fish, and nitrites are considered a potential reactant precursor for nitrosamines. Nitrosamines are problematic because they become reactive at the cellular level and that means they can alter gene expression and cause DNA damage The U.S. EPA has set an RfD of 0.l mg nitrite nitrogen/kg body weight per day (equivalent to 0.33 mg nitrite ion/kg body weight per day). The most serious health risks caused by high nitrite exposure are blue baby syndrome, occasional intoxications For these reasons, determination of nitrite has received much attention. We report herein the synthesis and binding properties of receptor 1. The binding phenomena of receptor 1 could be monitored by UVevis, fluorescence and 1H NMR spectra. 2. Results and discussion 2.1. Synthesis For the synthesis of receptor 1, 2 equiv of 1-methylperimidine8 was reacted with 1,8-bis(bromomethyl)anthracene.9 Then, anion exchange with ammonium hexafluorophosphate gave receptor 1 bearing two 1-methylperimidine arms in 49% overall yield. Receptor 1 was characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry.
T.S. Pandian et al. / Tetrahedron 71 (2015) 7782e7788
2.2. Interactions with nitrite The ability of receptor 1 to recognize nitrite was studied in DMSO using UVevis titration spectra. The receptor 1 displayed strong absorption band at 261 nm 314 nm, 330 nm, 349 nm, 368 nm and 387 nm in DMSO respectively. Fig. 1a shows the family of UVevis spectra obtained over the course of the titration of solution 1 with tetrabutylammonium nitrite in DMSO. As nitrite ions were added to the 20 mM DMSO solution of 1, the spectra showed small bathochromic shift of lmax along with isosbestic points at 336 nm. This phenomenon indicates complex formation between receptor 1 and nitrite. In addition, the receptor 1 displayed strong fluorescence emission in DMSO as shown in Fig. 1b. The excitation and emission wavelength were 329 and 400 nm, respectively. The intensity of emission spectrum from 20 mM solution of the receptor 1 gradually decreased as the concentration of tetrabutylammonium nitrite salts was increased (1e180 equiv), which also indicates the association between the receptor 1and nitrite.
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The complexation abilities of receptor 1 to nitrite was also measured by standard 1H NMR titration experiments in DMSO-d6 using a constant host concentration (2 mM) and increasing concentrations of nitrite anions. The addition of tetrabutylammonium nitrite salts to the solution of receptor 1 in DMSO-d6 resulted in large downfield shifts of perimidine C2-H (Ha in Fig. 2) and benzylic CeH(Hb). For example, addition of tetrabutylammonium nitrite moved perimidine C2-H from 9.32 to 9.60 ppm and benzylic CeH from 6.04 to 6.12 ppm. The downfield shifts of these protons indicate the presence of a hydrogen bond interaction between these CeH hydrogens and nitrite ion. In addition, perimidine 1-methyl CeH(Hc) moved from 3.65 to 3.67 ppm and anthracene 9-H(Hd) moved from 8.94 to 8.98 ppm. Again, these downfield shifts of CeH protons indicate the presence of a hydrogen bond interaction with nitrite ion although the chemical shifts of these hydrogens are small. The stoichiometry between receptor 1 and nitrite was determined to be 1:1 using a 1H NMR Job plot in DMSO-d6 (Fig. 3).
Fig. 1. Family of UVevis spectra (a) and fluorescence spectra (b) recorded over the course of titration of 20 mM DMSO solutions of the receptor 1 with the standard solution tetrabutylammonium nitrite.
Fig. 2. 1H NMR spectra of 2 mM of receptor 1 containing increasing amounts of tetrabutylammonium nitrite (0e53 equiv) in DMSO-d6.
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2.3. Interactions with halides
Fig. 3. Job plots of receptor 1 with tetrabutylammonium nitrite and bromide obtained by 1H NMR in DMSO-d6.
A BenesieHildebrand plot10 by use of change at 330 nm in UVevis spectrum and 400 nm in fluorescence spectrum gave the association constants. The association constants calculated were 3.3103 M1 from UVevis titration and 3.3103 M1 from fluorescence titration, respectively. In addition, analysis of chemical shift utilizing EQNMR11 gave the association constant of 3.5103 M1 by, which is similar to the values obtained from UVevis and fluorescence titrations.
The abilities of receptor 1 to recognize halides were also studied in DMSO using UVevis titration spectra. When the amount of bromide was increased, small bathochromic shift of lmax along with sharp isosbestic points at 334 nm were observed (Fig. 4a). The existence of isosbestic point for UVevis titrations of receptor 1 with bromide suggests a 1:1 complexation, and this was confirmed by Job’s plot analysis (Fig. 3). In addition, the intensity of emission spectrum from the receptor 1 gradually decreased as the concentration of tetrabutylammonium bromide salts was increased (10e220 equiv), which also indicates the association between the receptor 1 and bromide (Fig. 4b). From these experiments, association constants for bromide were calculated as 1.0103 M1 and 1.2103 M1 from the UVevis and fluorescence titrations, respectively. Hydrogen bond formation was confirmed by 1H NMR titration. When bromide was added, all four hydrogens (HaeHd) moved to downfield (Fig. 5). For example, perimidine C2-H moved from 9.32 to 9.58 ppm and benzylic CeH from 6.04 to 6.15 ppm respectively. In addition, small chemical shifts of perimidine 1-methyl CeH (from 3.65 to 3.67) and anthracene 9-H (from 8.94 to 8.98) were observed. The association constants calculated for bromide were calculated as 1.1103 for 1H NMR titration. Other halides such as chloride and iodide showed similar behaviors to bromide (see Supplementary data); calculated binding constants for other anions are summarized in Table 1. As anions have diverse geometries, complementarity between the receptor
Fig. 4. Family of UVevis spectra (a) and fluorescence spectra (b) recorded over the course of titration of 20 mM DMSO solutions of the receptor 1 with the standard solution tetrabutylammonium bromide.
Fig. 5. 1H NMR spectra of 2 mM of receptor 1 containing increasing amounts of tetrabutylammonium bromide (0e60 equiv) in DMSO-d6.
T.S. Pandian et al. / Tetrahedron 71 (2015) 7782e7788 Table 1 Association constants (M1) of receptors 1 with various anions in DMSO Anion NO 2 Br Cl I
a
H NMRa
UV 3
3.310 0.210 1.01030.510 4.81021.510 1.31022102
1
Fluorescence
3.5103 1.1103 5.0102 1.3102
3.31030.310 1.21031.110 4.81021.710 1.11022.1102
Errors are less than 12%.
and anion is crucial in determining selectivity. The complementarity between the receptor and halides is mostly achieved by the size of the receptor binding site due to the spherical shape of halides. The preference for bromide suggests that the cavity formed by 7 CeH bonds is more complementary to the size of the bromide ion than to the size of other halide ion. 2.4. Interactions with hydroxide, fluoride, acetate and dihydrogen phosphate In order to discriminate between H-bonding and deprotonation, UVevis titration of receptor 1 with tetrabutylammonium hydroxide was carried out (Fig. 6a). Changes in the absorbance spectra in the presence of hydroxide were clearly different from those observed for nitrite and weakly basic halides. In UVevis titration with basic anions such as fluoride, acetate and dihydrogen phosphate, similar phenomenon was observed (Fig. 6bed). Therefore, it could be concluded
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that only deprotonation occurred with these anions. These results are consistent with the work of Gao et al.12 When they tested perimidium receptor with benzene spacer, they observed large changes UVevis and fluorescence spectra with acetate and fluoride. 2.5. Binding energy studies In present study the computational modeling using the B3LYP method has been carried out for perimidine containing receptor with NO 2 , Br , Cl and I anions. All calculations have been computed by using the Gaussian 09 suite of programs.13 The present receptor appears to be formed possibly with seven CeH/O type Hbondings. The NMR results show the shifting of peaks for the formation of H-bondings in hosteanion complex. So, we have decided to investigate the binding poses more accurately using the state-ofthe-art density functional theory. Fig. 7 shows the structure of host 14 and their NO 2 complex optimized at the B3LYP level using the 631g (d) basis set. The binding energies estimated by experiment and theory are presented in Table 2. The results reveal the strong binding energy for Host2þeNO 2 complex, in good agreement with the experimental results, allowing detection of the complex in the lowest concentration compared with the other anions. The strengths of hydrogen bonds and other geometrical parameters are listed in Table 3. The hydrogen bond distance between the H57eO78 is 1.959 A, which is the strong hydrogen bond. The H48/O77 and H55eO77 hydrogen
Fig. 6. Family of UVevis spectra recorded over the course of titrating a 20 mM DMSO solution of receptor 1 with increasing amounts of tetrabutylammonium hydroxide (a), fluoride (b), acetate (c) and dihydrogen phosphate (d).
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Fig. 7. Most stable structure for Host2þ$NO 2 complex in solvent phase (DMSO as a solvent) optimized with B3LYP/6-31g(d) using Gaussian 09. Dashed lines denote the hydrogen bonds (strong and weak). See Table 3 for geometrical parameters and Mulliken charges. Atom numbering included. Atom colors: GraydCarbon; ReddOxygen; BluedNitrogen; WhitedHydrogen.
Table 2 Experimental and computational binding energies (BEs) for Host2þ with X complexes in DMSOa Complex
HosteNO 2 HosteBr HosteCl HosteIb
BE Exp (UV)
Calcd (DFT)
4.74 4.09 3.66 2.88
23.24 15.32 8.17 4.88
a Units are in kcal/mol. Experimental binding energies are derived from UV binding constants. DFT calculations are performed with (B3LYP/6-31G(d)) using a polarizable continuum model in DMSO. b Binding energy of Iodine complex is calculated at the B3LYP/LANL2DZ level of theory.
Table 3 Charges and geometries of Host2þ and their NO 2 complex (cx) Mulliken atomic charges (e)
cx
Host
H72 H58 H55 H48 H54 H57 H74
0.228 0.282 0.219 0.169 0.239 0.275 0.224
0.218 0.266 0.217 0.162 0.226 0.266 0.218
Heavy atom distance ( A) C43eN79 C24eO77 C17eO77 C10eO77 C15eO77 C23eN78 C44eN78
3.460 3.062 3.693 3.329 3.271 3.045 3.511
H-bond distance ( A) H54eO77 H57eO78 H74eO78 H55eO77 H58eO77 H72eO79 H48eO77
2.178 1.959 2.530 2.821 1.987 2.388 2.693
bond distances are 2.693 and 2.821 A, respectively. These two hydrogen bonds observed to be weak in the NO2 complex in solvent phase at B3LYP/6-31g(d) level. The result indicates that the strengths of the hydrogen bonds are in the following order: Permidine CeH/O>benzyllic CeH/O>Methyl CeH/O>anthracene CeH/O in good agreement with the NMR result. The complex with iodine anion is weaker than the rest of the anions studied here. In hosteiodine complex, the two methyl groups are not participating in the complex formation (see Fig. S20), showing more than 4 A distance from one of the H of CH3 to I anion. This result is also in good agreement with the NMR result of the iodine anion complex. Similarly, the bromide and chloride anions form complex with the host and the optimized structures are shown in Supplementary data (Figs. S18 and S19). The hydrogen bonding lengths reveal that the bonding is very weak between the methyl and anions (>3.3 A). The methylene (eCH2e) groups of the host forming weak hydrogen bonds with the Br and Cl anions have been observed to be 2.8 to 2.9 A whereas the iodine anion has been observed to be 3.041 A. Similarly, the perimidine CH protons have been observed to be of weaker for the iodine anion as compared to the Br and Cl anions. 2.6. TD-DFT results To further support the experimental observations of the hosteanion interactions and to provide a better insight into fundamental H-bonded cavity and anions binding interaction, TD-DFT calculations were carried out. The HOMOeLUMO and TD-DFT results are listed in Table 4. The B3LYP optimized structure of Host/NO 2 complex and its electronic delocalization of the frontier molecular orbitals (FMOs) can be seen in Fig. 8. We note that the addition of anions may change the electronic distribution such that the HOMO corresponds to the anthracene part and the LUMO corresponds to the anions interaction with the perimidine parts. Interestingly, we found this to be the case as shown in Fig. 7 for the NO 2 anion complex and the other energy minimized anion complexes are shown in Figs. S21e23 in Supplementary data. These results are in good agreement with the 1H NMR measurements. The predicted absorption wavelengths, oscillator strength (f) and the configuration description of the singlet excited states are shown in Table 4. For Host2þeNO 2 anion complex (see Fig. 8), the lowest
T.S. Pandian et al. / Tetrahedron 71 (2015) 7782e7788
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Table 4 TD-DFT calculated HOMO, LUMO, HOMOeLUMO gap (HLG), wavelength (lmax), and oscillator strength (f) for ANBPM2þeX (X¼Br, Cl, I, NO 2 )complex Complex ANBPMeNO 2
HOMO
LUMO
HLGa (eV)
Excited state
labs,calb (nm)
fc
Configd
Contributione
S1 S2 S1 S2 S1 S2 S1 S2
407.56 395.84 400.26 395.88 399.49 396.58 424.17 423.21
0.0513 0.0185 0.0462 0.0034 0.0284 0.0034 0.0035 0.0021
H/L H/Lþ1 H/L H/Lþ1 H/L H/Lþ1 H/L H/Lþ1
0.63 0.55 0.60 0.53 0.54 0.52 0.49 0.67
5.52
2.04
3.48
ANBPMeBr
5.56
2.06
3.50
ANBPMeCl
5.57
2.06
3.51
ANBPMeIf
5.70
2.29
3.41
a b c d e f
HOMO, LUMO, and HOMOeLUMO gaps are calculated using the B3LYP/6-31G(d) level. Absorption energies are calculated using the TDDFT method at B3LYP6-31G(d) level. Oscillator strength. H and L stands for the predicted HOMO and LUMO, respectively. Only configurations with 5% or greater contributions are included. Iodine complex results are calculated at the B3LYP/LANL2DZ level of theory.
Fig. 8. Molecular orbital distribution plots of HOMO, HOMO-1, LUMO and LUMOþ1 states in the ground state of the ANBPM2þeNO 2 complex.
energy singlet transitions are mainly from HOMO to LUMO, similar to other anion complexes reported here. The HOMOeLUMO gaps (HLG) given in Table 4 indicate that among the anions, NO 2 has the lowest HLG supporting the selective binding and confirming our experimental results. For anthracene and perimidine, both units of FMOs are delocalized over the entire structure. The charge distribution in the receptoreNO2 (in DMSO solvent) complex has been shown in the Fig. 9 through Electrostatic potential surface (EPS). 3. Conclusion In DMSO the NO 2 , Br , Cl and I anions are complexed with perimidine containing receptor 1 via CeH/O type hydrogen bonding. The hydrogen bond strengths vary for the different anions studied here. In the receptor 1, the perimidine eCH/anion type hydrogen bond has been observed to be stronger as compared with the methylene and methyl group hydrogen bonds with the anions. The theoretical binding energies in solvent phase for the Host2þeNO 2 complex has been observed to be the strongest as compared with those of the Br, Cl and I anions. The strong
Fig. 9. Electrostatic potential maps of Host2þeNO 2 complex calculated at B3LYP/631G(d) level in solvent phase (DMSO as a solvent).
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binding energy for the NO 2 anion complex is consistent with the experimental observation such as the lowest saturation point reached during the titration of the NO 2 anion with the perimidine containing receptor as compared with the Br, Cl and I anions studied here. The order of the interaction of the CeH groups of perimidine containing receptor with NO 2 anion is perimidine (CeH)>methylene (CeH)>methyl (CeH)>anthracene (CeH) The hydrogen bond strengths in the solvent phase for the receptoreanion interactions discussed above are also in good agreement with the experimental NMR results. 4. Experimental section 4.1. General 4.1.1. Materials. Tetra-n butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), tetra-n-butylammonium dihydrogen phosphate (TBAH2PO4), tetra-n-butylammonium acetate (TBAA), and tetra-n-butylammonium bromide (TBABr), tetran-butylammonium chloride (TBACl) tetra-n butylammonium iodide (TBAI) and tetra-n butylammonium nitrite were purchased from SigmaeAldrich Chemical Co., Inc., and used as received. 4.1.2. Measurements. Absorption spectra were recorded using a biochrom Libra S70 spectrophotometer (Biochrom Ltd, England). NMR spectra were recorded using a BRUKER spectrometer operated at 500 MHz. ESI MS spectra were obtained using a JMS 700 (Jeol, Japan) double focusing magnetic sector mass spectrometer. All measurements were carried out at room temperature (298 K). 4.1.3. Receptor 1. To a stirred solution of 1-methylperimidine (0.12 g, 0.65 mmol) in toluene (5 mL) was added a solution of 1,8bis(bromomethyl)anthracene (0.12 g, 0.32 mmol) in toluene (5 mL). The resulting mixture was refluxed for 96 h. After the solvent was distilled off under reduced pressure, the crude product was purified by column chromatography (11% MeOH in CH2Cl2) to afford bromide salt. To the solution of bromide salt was added aqueous solution of NH4PF6 (0.537 g, 3.2 mmol). The precipitate was collected by filtration, washed with water and dried in vacuum to give receptor 1 (0.14 g, yellow solid) in 49% yield. 1H NMR (500 MHz, DMSO-d6) d 9.32 (s, 1H), 8.94 (s, 1H), 8.86 (s, 1H), 8.19 (d, 2H, J¼8.6 Hz), 7.79 (d, 2H, J¼7.0 Hz), 7.71 (d, 2H, J¼8.3 Hz), 7.63 (t, 2H, J¼4.0 Hz), 7.64 (d, 2H, J¼2.4 Hz), 7.61 (t, 2H, J¼7.8 Hz), 7.4 (t, 2H, J¼8.1 Hz), 7.2 (d, 2H, J¼7.7 Hz), 6.72 (d, 2H, J¼8.0 Hz), 6.04 (s, 4H) 3.65 (s, 6H). 13C NMR (500 MHz, DMSO-d6): d 154.31, 134.13, 133.02, 131.95, 131.40, 128.77, 128.65, 128.46, 128.30, 128.25, 128.20, 125.36, 124.01, 123.63, 123.53, 120.92, 117.08, 108.44, 108.24, 52.43 mass: calcd for C40H32F12N4P2 m/z¼881.1805, found 881.1808 mp 248e250 C. Acknowledgements This research was supported by the Basic Science Research Program of the Korean National Research Foundation funded by the Korean Ministry of Education, Science and Technology (20100021333). Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2015.02.091.
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