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Anion response of dimeric hydrazide derivatives: Dependence on the nature of terminal substituents
Journal of Molecular Liquids 204 (2015) 100–105
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Anion response of dimeric hydrazide derivatives: Dependence on the nature of terminal substituents Zhenhua Wei a,b, Jue Wei a, Binglian Bai a,⁎, Haitao Wang b, Min Li b,⁎ a b
College of Physics, Jilin University, Changchun 130012, PR China Key Laboratory for Automobile Materials (JLU), Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 1 September 2014 Received in revised form 23 January 2015 Accepted 28 January 2015 Available online 2 February 2015 Keywords: Anion sensing Hydrazide derivatives Highly selective detection Terminal substituent
a b s t r a c t Three dimeric hydrazide derivatives with nitro, phenyl, and methyl terminal substituents were synthesized and their anion responsive behaviors were studied. The UV–vis spectra showed that the compounds with methyl groups and phenyl groups can allow highly selective fluoride detection, whereas the compound with terminal nitro substituent can respond to F−, AcO− and H2PO− 4 , due to the distinction on acidity of compounds which is caused by electronic effect and field effect of terminal substituents. The 1H NMR spectra revealed that the − anion responsive mechanism for F−, AcO− and H2PO− 4 was different due to the much lower basicity of AcO − and H2PO− 4 compared to F . Whereas the number of hydrazide groups and the nature of the terminal substitute do not affect the fluoride anion responsive mechanism. It demonstrates that, by altering terminal substituent groups, the anionic recognition ability of compounds could be adjusted and controlled. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The research for the recognition and sensing of biologically important ions has emerged as a research field which has considerable attention [1–4]. The majority of these synthetic chemosensors generally contains an optical-signaling chromophoric fragment linking to a neutral anion receptor with urea [1,5–7], thiourea [6,8], calix [4]pyrrole [9], indole [10] and amide [11] subunits which can provide one or more hydrogen bond (H-bond) donor sites for selective binding and recognizing of special anions. Among numerous anions, the fluoride anion is significantly important for health and environmental issues. For example, fluoride can be applied on dental care and treatment of osteoporosis, whereas superfluous intake of F− would cause fluorosis [12]. So the recognizing and sensing of F− have received considerable attention. The selectivity mainly relate to the structure of the synthetic chemosensors and the basicity of the anions [13,14]. As the most electronegative atom, F− usually forms strong H-bond interaction with – NH or –OH fragment [12] of the receptor, and then the processes of charge displacement and deprotonation take place [13,15,16], which mainly depend on the inherent acidity of the H-bond donor fragment of the artificial chemosensors. Recently, we reported the anion responsive behavior of the 4nitrobenzohydrazide derivative C8 (Scheme 1) (containing one hydrazide per molecule) and proposed the possible mechanism for the F− responsive process [17]. In order to study the influence of the number of ⁎ Corresponding authors. E-mail addresses: [email protected] (B. Bai), [email protected] (M. Li).
http://dx.doi.org/10.1016/j.molliq.2015.01.049 0167-7322/© 2015 Elsevier B.V. All rights reserved.
hydrazide groups, we synthesized the dimeric 4-nitrobenzohydrazide derivative (containing two hydrazide groups per molecule) with more target sites for forming H-bond with anions, namely, 1,6-Bis[N-(4nitrobenzoyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (N6). In addition, in order to study the properties of the terminal substituent groups in the anion responsive process, the dimeric hydrazide derivatives with methyl groups and phenyl groups, namely, 1,6-bis[N-(4nitrobenzoyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (N6), 1,6-bis[N(4-biphenylcarbonyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (B6) and 1,6-bis[N-(4-methylbenzoyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (M6) (as shown in Scheme 1) were also synthesized. Compound N6 with terminal nitro substituents can respond to F−, AcO− and H2PO− 4 , which is similar to that of the reported hydrazide derivatives C8, whereas M6 with methyl groups and B6 with benzene groups can allow highly selective fluoride detection. 2. Experimental and characterization The compounds N6, B6 and M6 were synthesized in our laboratory, and their structures confirmed by FTIR, 1H NMR spectra and elemental analysis. The synthetic details were reported elsewhere [18]. Spectrophotometric titrations were performed on solutions of receptors in DMSO at room temperature. Anions (F−, Cl−, Br−, I−, AcO− and H2PO− 4 ) were used in solution by adding relative alkylammonium salts [1,6,19]. 1 H NMR spectra were recorded with a Bruker Avance 500 MHz spectrometer, using dimethyl sulfoxide-d as solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00). UV–vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer.
Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
Ha
Hc
Hb O
R
C
H N
H N
Hd
O
O
C
O(CH2)6O
N6: R= NO2 , B6: R=
O H O2N
101
C
N
, M6: R=
H O N
C
C
H N
H N
O C
R
CH3
O(CH2)7CH3 O(CH2)7CH3 O(CH2)7CH3
C8 Scheme 1. Molecular structures of compounds N6, B6, M6, and C8 [17].
3. Results and discussion 3.1. Anion-binding studies and color change The interactions of receptors (N6, B6 and M6) with multiple anions (F−, Cl−, Br−, I−, AcO− and H2PO− 4 ) were investigated in DMSO solutions through UV–vis spectra experiments. The anions were added as the state of TBA salts, stepwise into the solution of receptor at room temperature. As shown in Fig. 1a, N6 exhibited various responses for anions. In addition of 16 equiv. F−, the intrinsic maximum absorption of N6 at 257 nm, had an evident decrease and a bathochromic shift to 290 nm, and a new absorption band appeared at 456 nm. In contrast, in the presence of 16 equiv. AcO− and H2PO− 4 , the absorbance band at 257 nm decreased with no red shift, and a new absorbance appear at 456 nm which is the same with the situation when F− added, but the absorbance strength
Fig. 1. UV–vis absorption spectra of a) N6, b) B6 and c) M6 in the presence of various anions (16 equiv.) in DMSO (5 × 10−5 mol/L).
Fig. 2. The corresponding color changes of three dimeric hydrazide derivatives (1 × 10−2 mol/L) upon addition of 10 equiv. various anions in DMSO.
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Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
Fig. 3. UV–vis absorption spectrum of a) N6 in the presence of F− (0–12 equiv., 1 equiv. interval), b) M6 (0–12 equiv., 2 equiv. interval) and c) B6 (0–12 equiv., 2 equiv. interval) in the presence of F− in DMSO (5 × 10−5 mol/L) at room temperature.
was lower than that of the F− response process. Due to the appearance of a new band in the visible region, macroscopic phenomenon was a vivid color change caused by the addition of F−, AcO− and H2PO− 4 . Upon the addition of F−, AcO− and H2PO− 4 , the solution of N6 turned from colorless to dark red (Fig. 2a), whereas no color changes were observed when the other anions were added. However, B6 and M6 can allow highly selective fluoride detection. As can be observed in Fig. 1b and c, in the presence of 16 equiv. of F−, the absorption band of B6 at 270 nm (257 nm for M6) decreased, and a new absorption band for B6 appeared at 362 nm (347 nm for M6). However, − H2PO− 4 , AcO and other halide anions did not induce any significant spectral changes, suggesting no bonding or very weak interactions with the
receptor B6 and M6. Due to the appearance of a new band near 350 nm, the addition of fluoride led to a noticeable color change from colorless to brilliant yellow, whereas no color changes were observed when the other anions were added (Fig. 2b and c). 3.2. Fluoride anion binding behavior studies by UV–vis spectra The interaction between receptors and F− was investigated in detail through UV–vis spectra titration experiment, which was taken in DMSO with TBAF at room temperature. Fig. 3a shows the complete family of spectra during the titration of a 5 × 10− 5 mol/L solution of N6 in DMSO with TBAF. With the increase of F −, the maximum
Fig. 4. 1H NMR spectra of N6 in addition of F− in DMSO-d6.
Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
103
3.3. 1H NMR spectra of fluoride anion sensing process
Fig. 5. Partial 1H NMR spectrum of a) M6 and b) B6 in addition of F− in DMSO-d6.
absorption band of N6 at 257 nm gradually decreased, and a new absorption band came out (abs. maxima at 456 nm). Addition of high equiv. F− caused a red shift from 257 nm to 290 nm, the band at 457 nm increased notably. Fig. 3b and c shows the whole family of spectra obtained during the titrations of solutions of M6 and B6 in DMSO with TBAF. As is shown in Fig. 3b, in the M6 titration experiment, with the increase of F −, the band at 257 nm gradually decreased and gone through a very slightly hypochromatic shift, at the same time a new absorption band came out (abs. maxima at 347 nm). Similar change occurred on the UV–vis spectra of B6 titration experiment (Fig. 3c), the inherent maximum absorption of B6 at 270 nm had a diminution and a new absorption rose at 362 nm as a sign of F− responding process [5,11].
The interactions between three dimeric hydrazide derivatives and F− were investigated by 1H NMR titration experiments in DMSO-d6 (1 × 10−2 mol/L). Fig. 4 shows the 1H NMR spectra of N6 and illustrates the spectral shifts and intensity changes after F− recognizing process occurred. In addition F−, two hydrazide NH proton signals at 10.81 ppm and 10.49 ppm vanished and a single weak peak appeared. Along with the amount of F− increasing, spectral shifts of protons of phenyl ring were observed. In this process, the chemical shifts of Ha, Hb, Hc and Hd all get conspicuous high field shifts, and the signals of Ha and Hb have an obvious trend of combining. After the F− exceed 6 equiv., signal of Ha incorporated with Hb. In the meantime, a series of faint signals arise near 16 ppm, they were assigned to the [HF2]− dimer [20] (the new weak triplet signal appeared after 10 equiv. of F− was added), which demonstrate that the hydrazide NH groups undergo the deprotonation process [7]. Similar [HF2]− dimer formation processes were observed in 1H NMR titration spectra of M6 and B6 with F− (Fig. 5). These results of the 1H NMR upon the addition of F− are the same as that of hydrazide derivative C8 [17], it can be considered that the F− response mechanism of dimeric hydrazide derivatives is the same as that of the reported hydrazide derivatives, although the number of hydrazide groups is different. Upon the addition of the lower equiv. F−, the proton was abstracted slightly from the hydrazide subunit and resulted in the formation of a supramolecular complex. On the addition of the higher equiv. F−, the base becomes very strong, and F− exhibits a large affinity toward H+, which induce the deprotonation of the complex to form the very stable [HF2]− dimer. Meanwhile the chargetransfer [21] might result in a five-membered ring based on intramolecular hydrogen bonding between the oxygen atom nearby deprotonation nitrogen atom and the other NH could thus be formed [17]. 3.4. Other anion sensing process When receptor N6 was titrated with AcO− and H2PO− 4 (Fig. 6), the UV–vis spectroscopic patterns were similar to that of titrated with lower equiv. F−. The absorbance band at 257 nm cannot shift no matter how much AcO− and H2PO− 4 were added. In addition, when re1 ceptor N6 was titrated with AcO− and H2PO− 4 (Fig. 6), the H NMR patterns were also similar to that of titrated with lower equiv. F−. Even on excess addition of AcO− and H2PO− 4 , no new triplet peak at about 16 ppm appeared and only a very little upfield shift of CH protons of benzene ring was observed (Fig. 7). So we can conclude that only a stable hydrogen-bond complex is produced in response to
Fig. 6. UV–vis absorption spectra of N6 (5 × 10−5 mol/L) in the presence of a) AcO− and b) H2PO− 4 in DMSO (0–18 equiv., 2 equiv. interval) at room temperature.
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Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
Fig. 7. 1H NMR spectra of N6 in addition of a) AcO− and b) H2PO− 4 in DMSO-d6.
the interaction between N6 and AcO− or H2PO− 4 maybe due to the − much lower basicity of AcO− and H 2PO − [12]. 4 compared to F These results are also consistent with those of hydrazide derivatives C8 [17]. 3.5. Effect of terminal substitute on the anion responsive properties It should be emphasized that the nature of the terminal substitute has a profound influence on the anion responsive properties of these symmetric dimeric hydrazide derivatives. Compounds N6 with terminal nitro substituents can respond to F−, AcO− and H2PO− 4 , whereas M6 with methyl groups and B6 with phenyl groups can allow highly selective fluoride detection. The difference in anion responsive behavior of these compounds was mainly due to the acidity of compound induced by the terminal substitute. The different substituent groups can exert different influences on acidities of compounds mainly through electronic effect [22,25] and field effect. The nitro substituent, a strong electronwithdrawing group [23,24], has a negative conjugative effect, which could be delivered from benzene ring to hydrazide group through alternant polarization. As a consequence, the electron density of hydrazide reduced, and the N–H bond of hydrazide group is more likely to fracture and deprotonate [21]. Furthermore, the –NO2 has a field effect on the deprotonated –NH and the field effect diffuses the electron density of the deprotonated –NH, and as a result the deprotonated N6 is more stable. In general, the acidity of N6 is enhanced by –NO2. For M6 and B6, the methyl and phenyl terminal substituent groups, the electron-pushing group, act in contrary way, so the acidity of M6 and B6 is lower than that of N6, thus M6 and N6 only can interact with the most alkaline anion (F−). So we can conclude that the hydrazide derivatives with highly selective fluoride detection can be obtained through regulating the terminal substituent groups, whereas the number of hydrazide groups does not affect the response mechanism of anions. In addition, the red shift from 257 nm to 290 nm during the titration
of a 5 × 10−5 mol/L solution of N6 in DMSO with TBAF may be due to the combined effect of nitro group and F−. 4. Conclusions The anion responsive behaviors of three dimeric hydrazide derivatives with nitro (N6), phenyl (B6), and methyl (M6) terminal substituents were detected in DMSO solution. Compound N6 with terminal nitro substituent can respond to F−, AcO− and H2PO− 4 , whereas M6 with methyl groups and B6 with phenyl groups can allow highly selective fluoride detection. This is due to the nature of the terminal substitute that has a profound influence on the acidity of these symmetric dimeric hydrazide derivatives. The F− response mechanism of dimeric hydrazide derivatives is the same as that of the reported hydrazide derivatives C8, although the number of hydrazide groups is different. It can be concluded that the hydrazide derivatives with highly selective fluoride detection can be obtained through regulating the terminal substituent groups, whereas the number of hydrazide groups and the nature of the terminal substitute do not affect the fluoride anion responsive mechanism. The results might offer guidance and provide the basis for designing colorimetric anion sensors with excellent selectivity. Acknowledgments This work was supported by the National Natural Science Foundation of China (21072076, 51103057, 51073071), the project of Science and Technology Development Plan of Jilin Province (20140414020GH), and Project 985-Automotive Engineering of Jilin University. References [1] E.J. Cho, J.W. Moon, S.W. Ko, J.Y. Lee, S.K. Kim, J. Yoon, K.C. Nam, J. Am. Chem. Soc. 125 (2003) 12376–12377.
Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105 [2] S. Xu, B. Liu, H. Tian, Prog. Chem. 18 (2006) 687–697. [3] M. Teng, G. Kuang, X.R. Jia, M. Gao, Y. Li, Y. Wei, J. Mater. Chem. 19 (2009) 5648–5654. [4] Y. Zhou, Prog. Chem. 23 (2011) 125–135. [5] E.J. Cho, B.J. Ryu, Y.J. Lee, K.C. Nam, Org. Lett. 7 (2005) 2607–2609. [6] D.A. Jose, D.K. Kumar, B. Ganguly, A. Das, Org. Lett. 6 (2004) 3445–3448. [7] T.H. Kim, M.S. Choi, B.H. Sohn, S.Y. Park, W.S. Lyood, T.S. Lee, Chem. Commun. (2008) 2364–2366. [8] M. Vázquez, L. Fabbrizzi, A. Taglietti, R.M. Pedrido, A.M. González-Noya, M.R. Bermejo, Angew. Chem. Int. Ed. 43 (2004) 1962–1965. [9] C.J. Woods, S. Camiolo, M.E. Light, S.J. Coles, M.B. Hursthouse, M.A. King, P.A. Gale, J.W. Essex, J. Am. Chem. Soc. 124 (2002) 8644–8652. [10] J.L. Sessler, D.G. Cho, V. Lynch, J. Am. Chem. Soc. 128 (2006) 16518–16519. [11] P. Xue, Y. Zhang, J. Jia, D. Xu, X. Zhang, X. Liu, H. Zhou, P. Zhang, R. Lu, M. Takafujic, H. Ihara, Soft Matter 7 (2011) 8296–8304. [12] Y. Zhou, J. Zhang, J. Yoon, Chem. Rev. 114 (2014) 5511–5571. [13] X. He, S. Hu, K. Liu, Y. Guo, J. Xu, S. Shao, Org. Lett. 8 (2006) 333–336.
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[14] D. Esteban-Gómez, L. Fabbrizzi, M. Licchelli, J. Org. Chem. 70 (2005) 5717–5720. [15] M. Boiocchi, L.D. Boca, D.E. Gómez, L. Fabbrizzi, M. Licchelli, E. Monzani, J. Am. Chem. Soc. 126 (2004) 16507–16514. [16] C. Wang, D. Zhang, D. Zhu, Langmuir 23 (2007) 1478–1482. [17] B. Bai, J. Ma, J. Wei, J. Song, H. Wang, M. Li, Org. Biomol. Chem. 12 (2014) 3478–3483. [18] H. Wang, B. Bai, P. Zhang, B. Long, W. Tian, M. LI, Liq. Cryst. 33 (2006) 445–450. [19] Y. Peng, Y. Dong, M. Dong, Y. Wang, J. Org. Chem. 77 (2012) 9072–9080. [20] Y. Zhang, S. Jiang, Org. Biomol. Chem. 10 (2012) 6973–6979. [21] C. Ravikumar, I. Hubert Joe, V. Jayakumar, Chem. Phys. Lett. 460 (2008) 552–558. [22] Y. Zhu, W. Yan, Y. Zhang, J. Organomet. Chem. 696 (2011) 1850–1855. [23] L. Wang, W. Wei, Y. Guo, J. Xu, S. Shao, Spectrochim. Acta 78 (2011) 726–731. [24] S. Ivanov, N. Giricheva, M. Fedorov, I. Men'shikova, T. Nurkevich, E. Tarasova, Russ. J. Phys. Chem. A 87 (2013) 608–614. [25] G. Scholes, K. Ghiggino, A. Oliver, M. Paddon-Row, J. Am. Chem. Soc. 115 (1993) 4345–4349.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Anion response of dimeric hydrazide derivatives: Dependence on the nature of terminal substituents Zhenhua Wei a,b, Jue Wei a, Binglian Bai a,⁎, Haitao Wang b, Min Li b,⁎ a b
College of Physics, Jilin University, Changchun 130012, PR China Key Laboratory for Automobile Materials (JLU), Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 1 September 2014 Received in revised form 23 January 2015 Accepted 28 January 2015 Available online 2 February 2015 Keywords: Anion sensing Hydrazide derivatives Highly selective detection Terminal substituent
a b s t r a c t Three dimeric hydrazide derivatives with nitro, phenyl, and methyl terminal substituents were synthesized and their anion responsive behaviors were studied. The UV–vis spectra showed that the compounds with methyl groups and phenyl groups can allow highly selective fluoride detection, whereas the compound with terminal nitro substituent can respond to F−, AcO− and H2PO− 4 , due to the distinction on acidity of compounds which is caused by electronic effect and field effect of terminal substituents. The 1H NMR spectra revealed that the − anion responsive mechanism for F−, AcO− and H2PO− 4 was different due to the much lower basicity of AcO − and H2PO− 4 compared to F . Whereas the number of hydrazide groups and the nature of the terminal substitute do not affect the fluoride anion responsive mechanism. It demonstrates that, by altering terminal substituent groups, the anionic recognition ability of compounds could be adjusted and controlled. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The research for the recognition and sensing of biologically important ions has emerged as a research field which has considerable attention [1–4]. The majority of these synthetic chemosensors generally contains an optical-signaling chromophoric fragment linking to a neutral anion receptor with urea [1,5–7], thiourea [6,8], calix [4]pyrrole [9], indole [10] and amide [11] subunits which can provide one or more hydrogen bond (H-bond) donor sites for selective binding and recognizing of special anions. Among numerous anions, the fluoride anion is significantly important for health and environmental issues. For example, fluoride can be applied on dental care and treatment of osteoporosis, whereas superfluous intake of F− would cause fluorosis [12]. So the recognizing and sensing of F− have received considerable attention. The selectivity mainly relate to the structure of the synthetic chemosensors and the basicity of the anions [13,14]. As the most electronegative atom, F− usually forms strong H-bond interaction with – NH or –OH fragment [12] of the receptor, and then the processes of charge displacement and deprotonation take place [13,15,16], which mainly depend on the inherent acidity of the H-bond donor fragment of the artificial chemosensors. Recently, we reported the anion responsive behavior of the 4nitrobenzohydrazide derivative C8 (Scheme 1) (containing one hydrazide per molecule) and proposed the possible mechanism for the F− responsive process [17]. In order to study the influence of the number of ⁎ Corresponding authors. E-mail addresses: [email protected] (B. Bai), [email protected] (M. Li).
http://dx.doi.org/10.1016/j.molliq.2015.01.049 0167-7322/© 2015 Elsevier B.V. All rights reserved.
hydrazide groups, we synthesized the dimeric 4-nitrobenzohydrazide derivative (containing two hydrazide groups per molecule) with more target sites for forming H-bond with anions, namely, 1,6-Bis[N-(4nitrobenzoyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (N6). In addition, in order to study the properties of the terminal substituent groups in the anion responsive process, the dimeric hydrazide derivatives with methyl groups and phenyl groups, namely, 1,6-bis[N-(4nitrobenzoyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (N6), 1,6-bis[N(4-biphenylcarbonyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (B6) and 1,6-bis[N-(4-methylbenzoyl)-N′-(benzoyl-4′-oxy)hydrazine]hexane (M6) (as shown in Scheme 1) were also synthesized. Compound N6 with terminal nitro substituents can respond to F−, AcO− and H2PO− 4 , which is similar to that of the reported hydrazide derivatives C8, whereas M6 with methyl groups and B6 with benzene groups can allow highly selective fluoride detection. 2. Experimental and characterization The compounds N6, B6 and M6 were synthesized in our laboratory, and their structures confirmed by FTIR, 1H NMR spectra and elemental analysis. The synthetic details were reported elsewhere [18]. Spectrophotometric titrations were performed on solutions of receptors in DMSO at room temperature. Anions (F−, Cl−, Br−, I−, AcO− and H2PO− 4 ) were used in solution by adding relative alkylammonium salts [1,6,19]. 1 H NMR spectra were recorded with a Bruker Avance 500 MHz spectrometer, using dimethyl sulfoxide-d as solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00). UV–vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer.
Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
Ha
Hc
Hb O
R
C
H N
H N
Hd
O
O
C
O(CH2)6O
N6: R= NO2 , B6: R=
O H O2N
101
C
N
, M6: R=
H O N
C
C
H N
H N
O C
R
CH3
O(CH2)7CH3 O(CH2)7CH3 O(CH2)7CH3
C8 Scheme 1. Molecular structures of compounds N6, B6, M6, and C8 [17].
3. Results and discussion 3.1. Anion-binding studies and color change The interactions of receptors (N6, B6 and M6) with multiple anions (F−, Cl−, Br−, I−, AcO− and H2PO− 4 ) were investigated in DMSO solutions through UV–vis spectra experiments. The anions were added as the state of TBA salts, stepwise into the solution of receptor at room temperature. As shown in Fig. 1a, N6 exhibited various responses for anions. In addition of 16 equiv. F−, the intrinsic maximum absorption of N6 at 257 nm, had an evident decrease and a bathochromic shift to 290 nm, and a new absorption band appeared at 456 nm. In contrast, in the presence of 16 equiv. AcO− and H2PO− 4 , the absorbance band at 257 nm decreased with no red shift, and a new absorbance appear at 456 nm which is the same with the situation when F− added, but the absorbance strength
Fig. 1. UV–vis absorption spectra of a) N6, b) B6 and c) M6 in the presence of various anions (16 equiv.) in DMSO (5 × 10−5 mol/L).
Fig. 2. The corresponding color changes of three dimeric hydrazide derivatives (1 × 10−2 mol/L) upon addition of 10 equiv. various anions in DMSO.
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Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
Fig. 3. UV–vis absorption spectrum of a) N6 in the presence of F− (0–12 equiv., 1 equiv. interval), b) M6 (0–12 equiv., 2 equiv. interval) and c) B6 (0–12 equiv., 2 equiv. interval) in the presence of F− in DMSO (5 × 10−5 mol/L) at room temperature.
was lower than that of the F− response process. Due to the appearance of a new band in the visible region, macroscopic phenomenon was a vivid color change caused by the addition of F−, AcO− and H2PO− 4 . Upon the addition of F−, AcO− and H2PO− 4 , the solution of N6 turned from colorless to dark red (Fig. 2a), whereas no color changes were observed when the other anions were added. However, B6 and M6 can allow highly selective fluoride detection. As can be observed in Fig. 1b and c, in the presence of 16 equiv. of F−, the absorption band of B6 at 270 nm (257 nm for M6) decreased, and a new absorption band for B6 appeared at 362 nm (347 nm for M6). However, − H2PO− 4 , AcO and other halide anions did not induce any significant spectral changes, suggesting no bonding or very weak interactions with the
receptor B6 and M6. Due to the appearance of a new band near 350 nm, the addition of fluoride led to a noticeable color change from colorless to brilliant yellow, whereas no color changes were observed when the other anions were added (Fig. 2b and c). 3.2. Fluoride anion binding behavior studies by UV–vis spectra The interaction between receptors and F− was investigated in detail through UV–vis spectra titration experiment, which was taken in DMSO with TBAF at room temperature. Fig. 3a shows the complete family of spectra during the titration of a 5 × 10− 5 mol/L solution of N6 in DMSO with TBAF. With the increase of F −, the maximum
Fig. 4. 1H NMR spectra of N6 in addition of F− in DMSO-d6.
Z. Wei et al. / Journal of Molecular Liquids 204 (2015) 100–105
103
3.3. 1H NMR spectra of fluoride anion sensing process
Fig. 5. Partial 1H NMR spectrum of a) M6 and b) B6 in addition of F− in DMSO-d6.
absorption band of N6 at 257 nm gradually decreased, and a new absorption band came out (abs. maxima at 456 nm). Addition of high equiv. F− caused a red shift from 257 nm to 290 nm, the band at 457 nm increased notably. Fig. 3b and c shows the whole family of spectra obtained during the titrations of solutions of M6 and B6 in DMSO with TBAF. As is shown in Fig. 3b, in the M6 titration experiment, with the increase of F −, the band at 257 nm gradually decreased and gone through a very slightly hypochromatic shift, at the same time a new absorption band came out (abs. maxima at 347 nm). Similar change occurred on the UV–vis spectra of B6 titration experiment (Fig. 3c), the inherent maximum absorption of B6 at 270 nm had a diminution and a new absorption rose at 362 nm as a sign of F− responding process [5,11].
The interactions between three dimeric hydrazide derivatives and F− were investigated by 1H NMR titration experiments in DMSO-d6 (1 × 10−2 mol/L). Fig. 4 shows the 1H NMR spectra of N6 and illustrates the spectral shifts and intensity changes after F− recognizing process occurred. In addition F−, two hydrazide NH proton signals at 10.81 ppm and 10.49 ppm vanished and a single weak peak appeared. Along with the amount of F− increasing, spectral shifts of protons of phenyl ring were observed. In this process, the chemical shifts of Ha, Hb, Hc and Hd all get conspicuous high field shifts, and the signals of Ha and Hb have an obvious trend of combining. After the F− exceed 6 equiv., signal of Ha incorporated with Hb. In the meantime, a series of faint signals arise near 16 ppm, they were assigned to the [HF2]− dimer [20] (the new weak triplet signal appeared after 10 equiv. of F− was added), which demonstrate that the hydrazide NH groups undergo the deprotonation process [7]. Similar [HF2]− dimer formation processes were observed in 1H NMR titration spectra of M6 and B6 with F− (Fig. 5). These results of the 1H NMR upon the addition of F− are the same as that of hydrazide derivative C8 [17], it can be considered that the F− response mechanism of dimeric hydrazide derivatives is the same as that of the reported hydrazide derivatives, although the number of hydrazide groups is different. Upon the addition of the lower equiv. F−, the proton was abstracted slightly from the hydrazide subunit and resulted in the formation of a supramolecular complex. On the addition of the higher equiv. F−, the base becomes very strong, and F− exhibits a large affinity toward H+, which induce the deprotonation of the complex to form the very stable [HF2]− dimer. Meanwhile the chargetransfer [21] might result in a five-membered ring based on intramolecular hydrogen bonding between the oxygen atom nearby deprotonation nitrogen atom and the other NH could thus be formed [17]. 3.4. Other anion sensing process When receptor N6 was titrated with AcO− and H2PO− 4 (Fig. 6), the UV–vis spectroscopic patterns were similar to that of titrated with lower equiv. F−. The absorbance band at 257 nm cannot shift no matter how much AcO− and H2PO− 4 were added. In addition, when re1 ceptor N6 was titrated with AcO− and H2PO− 4 (Fig. 6), the H NMR patterns were also similar to that of titrated with lower equiv. F−. Even on excess addition of AcO− and H2PO− 4 , no new triplet peak at about 16 ppm appeared and only a very little upfield shift of CH protons of benzene ring was observed (Fig. 7). So we can conclude that only a stable hydrogen-bond complex is produced in response to
Fig. 6. UV–vis absorption spectra of N6 (5 × 10−5 mol/L) in the presence of a) AcO− and b) H2PO− 4 in DMSO (0–18 equiv., 2 equiv. interval) at room temperature.
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Fig. 7. 1H NMR spectra of N6 in addition of a) AcO− and b) H2PO− 4 in DMSO-d6.
the interaction between N6 and AcO− or H2PO− 4 maybe due to the − much lower basicity of AcO− and H 2PO − [12]. 4 compared to F These results are also consistent with those of hydrazide derivatives C8 [17]. 3.5. Effect of terminal substitute on the anion responsive properties It should be emphasized that the nature of the terminal substitute has a profound influence on the anion responsive properties of these symmetric dimeric hydrazide derivatives. Compounds N6 with terminal nitro substituents can respond to F−, AcO− and H2PO− 4 , whereas M6 with methyl groups and B6 with phenyl groups can allow highly selective fluoride detection. The difference in anion responsive behavior of these compounds was mainly due to the acidity of compound induced by the terminal substitute. The different substituent groups can exert different influences on acidities of compounds mainly through electronic effect [22,25] and field effect. The nitro substituent, a strong electronwithdrawing group [23,24], has a negative conjugative effect, which could be delivered from benzene ring to hydrazide group through alternant polarization. As a consequence, the electron density of hydrazide reduced, and the N–H bond of hydrazide group is more likely to fracture and deprotonate [21]. Furthermore, the –NO2 has a field effect on the deprotonated –NH and the field effect diffuses the electron density of the deprotonated –NH, and as a result the deprotonated N6 is more stable. In general, the acidity of N6 is enhanced by –NO2. For M6 and B6, the methyl and phenyl terminal substituent groups, the electron-pushing group, act in contrary way, so the acidity of M6 and B6 is lower than that of N6, thus M6 and N6 only can interact with the most alkaline anion (F−). So we can conclude that the hydrazide derivatives with highly selective fluoride detection can be obtained through regulating the terminal substituent groups, whereas the number of hydrazide groups does not affect the response mechanism of anions. In addition, the red shift from 257 nm to 290 nm during the titration
of a 5 × 10−5 mol/L solution of N6 in DMSO with TBAF may be due to the combined effect of nitro group and F−. 4. Conclusions The anion responsive behaviors of three dimeric hydrazide derivatives with nitro (N6), phenyl (B6), and methyl (M6) terminal substituents were detected in DMSO solution. Compound N6 with terminal nitro substituent can respond to F−, AcO− and H2PO− 4 , whereas M6 with methyl groups and B6 with phenyl groups can allow highly selective fluoride detection. This is due to the nature of the terminal substitute that has a profound influence on the acidity of these symmetric dimeric hydrazide derivatives. The F− response mechanism of dimeric hydrazide derivatives is the same as that of the reported hydrazide derivatives C8, although the number of hydrazide groups is different. It can be concluded that the hydrazide derivatives with highly selective fluoride detection can be obtained through regulating the terminal substituent groups, whereas the number of hydrazide groups and the nature of the terminal substitute do not affect the fluoride anion responsive mechanism. The results might offer guidance and provide the basis for designing colorimetric anion sensors with excellent selectivity. Acknowledgments This work was supported by the National Natural Science Foundation of China (21072076, 51103057, 51073071), the project of Science and Technology Development Plan of Jilin Province (20140414020GH), and Project 985-Automotive Engineering of Jilin University. References [1] E.J. Cho, J.W. Moon, S.W. Ko, J.Y. Lee, S.K. Kim, J. Yoon, K.C. Nam, J. Am. Chem. Soc. 125 (2003) 12376–12377.
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