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A phenylhydrazone-based indole receptor for sensing acetate
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Talanta 74 (2008) 1122–1125
A phenylhydrazone-based indole receptor for sensing acetate Yuehong Wang a , Hai Lin b , Jie Shao a , Zun-Sheng Cai a , Hua-Kuan Lin a,∗ a
b
Department of Chemistry, Nankai University, Tianjin 300071, China State Key Laboratory of Functional Polymer Materials of Ministry of Education Nankai University, Tianjin 300071, China Received 8 June 2007; received in revised form 8 August 2007; accepted 13 August 2007 Available online 22 August 2007
Abstract A novel phenylhydrazone-based indole anion receptor is synthesized by a simple method, and the receptor binds acetate anions with good selectivity in DMSO solution, which is related to the structure of the receptor matching with the anion. The binding properties of the host are examined by UV–vis changes. The color changes of the host upon the addition of a variety of structurally different anions were also utilized as nake-eyed recognization (from orange to purple, first figure), The hydrogen bonds between recognization sites (phenylhydrazone and indole N–H) and anions were determined on the basis of 1 H NMR experiments. © 2007 Elsevier B.V. All rights reserved. Keywords: Indole; Phenylhydrazone; Receptor; Nake-eyed recognization
1. Introduction Anion recognition has attracted considerable attention due to their medicinal and environmental potential application [1–3]. Receptors [4] and sensors [5–7] having strong affinity and selectivity for specific anions arouses of great interest in supramolecular chemistry [8]. So, the design of anion receptors with high selectivity is challenging [9–11]. Synthetic receptors for anions are usually based on urea/thiourea [12,13], amides [14,15,25], macrocyclic ammonium/guanidinium [16,17], functionalized calixarenes [18,19], phenylhydrazone [20], indole [21] and –OH [26]. Take the phenylhydrazone for example, first, this kind of the receptor is neutral and the synthetic method is simple. Secondly, the reactant indoline-2,3-dione and 2,4nitrophenylhydrazine could recognize anion by itself and had a nake-eyed color change. Of particular interest in this regard is the color of phenylhydrazone, which changed so obviously (from orange to purple) upon addition of AcO− anion that we could use the nake-eyed detection without resorting to any spectroscopic instrumentation. In the present work, we designed and synthesized the 3-(2 ,4 dinitrophenylhydrazone)indolin-2-one. It is provided with the
∗
Corresponding author. Fax: +86 22 23502458. E-mail address: [email protected] (H.-K. Lin).
0039-9140/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2007.08.015
merit, which we referred to. We believe that the receptor will have a better-applied foreground. 2. Experimental 2.1. Reagents All anions, in the form of tetrabutylammonium salts, were purchased from Sigma–Aldrich Chemical Co., stored in a desiccator under vacuum containing self-indicating silica, and used without any further purification. Solvents were purified prior to use and stored under nitrogen. Dimethyl sulfoxide was dried with calcium hydride and distilled at reduced pressure. Unless stated otherwise, commercial grade chemicals were used without further purification. 2.2. Synthesis 3-(2 ,4 -Dinitrophenylhydrazone)indolin-2-one. A solution of 2,3-dikedoine (isatin) (4 mmol) in ethanol (20 ml) was added dropwise to a solution of 2,4-nitrophenylhydrazine (4 mmol) in ethanol (60 ml) with stirring at reflux. After being stirred for 5 h, the solvent was removed by evaporation. Recrystallization (ethanol) yielded yellow crystals. δH (400 MHz, DMSO-d6 , Me4 Si): 11.73 (s, 1H, N–H), 10.91 (s, 1H, N–H), 8.92 (d, 1H, Ar–H, J = 4 Hz), 8.59 (d, 1H, Ar–H, J = 8 Hz), 8.16 (d, 1H,
Y. Wang et al. / Talanta 74 (2008) 1122–1125
1123
Ar–H, J = 8 Hz), 7.89 (d, 1H, Ar–H, J = 8 Hz), 7.48 (t, 1H, Ar–H, J = 12 Hz), 7.21 (t, 1H, Ar–H, J = 12 Hz), 6.97 (d, 1H, Ar–H, J = 8 Hz). Anal. Calcd. for C14 H9 N5 O5 : C 51.38, H 2.77, N 21.40, Found C 51.54, H 2.38, N 21.55. 2.3. General methods Unless otherwise specified, all of the experiments were carried out at 298 ± 1 K. The 1 H NMR spectra were recorded on a Varian UNITY-plus 400 MHz spectrometer using tetramethylsilane (TMS) as an internal standard. UV–vis spectra were recorded on a Shimadzu UV-2450 PC spectrophotometer.
Fig. 1. Color changes from receptor (3 × 10−5 M) to receptor + AcO− (3 × 10−4 M) in DMSO.
2.4. Absorption titration studies The binding ability of receptor for CH3 CO2 − , H2 PO4 − , and halide anions (as tetrabutylammonium salts) was investigated by UV–vis spectroscopy in DMSO solution using a constant host concentration (3.0 × 10−5 M) and increasing concentrations of anions. The change in absorbance at 561 nm for receptor was plotted against anion concentration and fitted by the equation as described by Connors [22]. 2.5. 1 H NMR titrations Receptor 1 (5.0 × 10−3 M in DMSO-d6 ) was titrated against acetate anions (tetrabutylammonium salts) by addition of excess anion in DMSO-d6 . 3. Results and discussion Here, we reported a remarkable selective indolehydrazonebased acetate chromo-chemosensor of a Schiff-base compound, 3-(2 ,4 -dinitrophenylhydrazone)indolin-2-one. It contained an indolehydrazone group that could be transformed to phenol form in the presence of anions (Scheme 1) [23,24]. 3.1. UV–vis experiments
Fig. 2. The UV–vis spectra of receptor (3 × 10−5 M) in DMSO solution during the titration with tertrabutylammonium (TBA) acetate (0, 0.067, 0.133, 0.266, 0.5, 0.633, 0.67, 0.766, 1.2, 1.6, 1.8, 2.0, 4.2, 5.6 equiv.). Inset: titration plots observed binding profiles and corresponding nonlinear fit plots monitored by the absorption increase at 561 nm.
of the absorption peak at 383 nm was remarkably reduced with a simultaneous growth of the peak at 561 nm (Fig. 2) and meanwhile the solution color changed from orange to purple (Fig. 1). The presence of isobestic points during titration with acetate revealed the formation of 1:1 complexes (Fig. 3 ). No significant changes in absorption or a noticeable color changes were observed for other anions such as Cl− , Br− , I− .
In DMSO solution, the receptor could form hydrogen bond with AcO− , F− and H2 PO4 − anions, respectively, and the color of solution changed significantly (Fig. 1 ). The anion-binding properties were investegated by UV–vis titration of the recepror in DMSO solution using standard tetrabutylammonium salts of AcO− , F− and H2 PO4 − . As shown in Fig. 2 , the titration of receptor (c = 3.0 × 10−5 M), exhibited a broad strong absorption band at 383 nm due to the nitrophenylhydrazone. The intensity
Scheme 1. Proposed mode of transformatin of the receptor upon binding anion.
Fig. 3. A job plot of receptor with tetrabutylammonium acetate.
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Y. Wang et al. / Talanta 74 (2008) 1122–1125
Table 1 Association constants Kass of receptors 1 with anions in DMSO Anion
AcO−
F−
H2 PO4 −
Cl−
Br−
I−
Kass (M−1 )
2.32 × 105
6.22 × 104
2.25 × 104
ND
ND
ND
The anions were added as their tetrabutylammonium salts. All errors are ±10%; ND: the spectra have too small change with adding anion so we cannot determine the affinity constant by the spectra.
Detail data was given in Table 1. The association constant Kass was calculated by non-linear least square regression method [22]. For the receptor, the association constant for AcO− was larger than that of F− and H2 PO4 − . The reason was probably that the selective recognition for acetate anion is related to the configuration of the acetate matching with the receptor (Fig. 1) and the alkalescence of the anion, as well as the basicity of receptor. Because acetate anion was a plane and triangular and the angle of O–C–O was about 120◦ , the distance of two oxygen atoms might be fit to the two hydrogen atoms on recognition sites of the receptor in the triangular configuration, Furthermore, the alkalescence of acetate anion was also stronger than the other anions (Cl− , Br− , I− ). So, the association constant Kass for acetate was maximal. The angle of O–P–O is about 108◦ for H2 PO4 − (tetrahedral configuration) and the distance of two oxygen atoms of AcO− was longer than that of H2 PO4 − ; so two oxygen atoms of H2 PO4 − could not match well with two hydrogen atoms in the receptor. As for F− , it was global and had the smallest atom radius in halide atom, but the better alkalescence made itself have the higher association constant Kass than H2 PO4 − . Table 1 illustrated that the receptor can bind anions in the order AcO− > F− > H2 PO4 − > Cl− ∼ Br− ∼ I− .
N–H proton, as well as the hydrazone N–H signal shift downfield over the course of the titration. However, the fact the N–H signals were observable throughout the titration was consistent with a hydrogen-binding process involving the receptor. In the specific case of receptor with acetate anion, the singlet peak of the indole N–H protons broadened and meanwhile shifted from 11.73 to 11.94 ppm upon addition of 0.8 equiv. of tetrabutylammonium acetate (Fig. 4). Addition of the second equivalent of acetate caused substantial broadening and downfield shift of all indole N–H protons, as well as all hydrazone N–H protons. Similarly, much broadening of the aromatic protons was observed in NMR titrations of all anions. These results clearly indicated that, in addition to the formation of hydrogen bonds between acetate and N–H groups. But some shifted slightly upfield, the others appeared slightly downfield for the aromatic protons, and two effects were responsible for the spectral changes of the aromatic protons upon NH-anion hydrogen bond formation (1) throughbond effects, which increased the electron density of the phenyl ring and promoted upfield shifts, and (2) through-space effects, which polarized the C–H bond in proximity to hydrogen bonds, created partial positive charge on the proton, and caused downfield shifts. As mentioned above, the proposed binding mode was shown in Scheme 1.
3.2. 1 H NMR titrations 4. Analytical application Further supports for the notion that the formation of hydrogen bonds between N–H groups and anions came from 1 H NMR spectroscopic analyses. These were carried out in DMSOd under normal conditions of so-called NMR titration, with the spectra of the receptors being recorded in the presence of increasing concentrations of anions (Fig. 4 ). A notable feature of these titrations was that the resonances corresponding to the indole
Fig. 4. 1H NMR titration of a DMSO-d6 solution of receptor with AcO− .
The phenylhydrazone-based indole receptor would have potential application in analytical chemistry, because the receptor was applied in the monitoring of binding ability with various anions (AcO− , H2 PO4 − , F− , Cl− , Br− , and I− ), which existed widely in biological, industrial, and environmental processes; e.g., carboxylate anions (CH3 COO− ) played an important role in organic, environmental, biological processes, etc. The carboxylate functions of enzymes and antibodies ascribed to their specific biochemical behaviors. Moreover, they were critical components of numerous metabolic processes. So, the development of sensitive detection systems for their binding-ability was very important. In the paper, the selective recognization of the receptor for acetate anion was designed; synthesized and studied. In the first place, color changes of the receptor solution was tested in DMSO (3 × 10−5 M) and the solution color changed from orange to purple upon addition of 10 equiv. AcO− , which was shown in the Fig. 1. In the next place; the binding ability of the host with different anions was investigated through UV–vis titration in DMSO. Obviously; AcO− anion; whose association constant was maximal; could be recognized selectively from the other anions tested according to the affinity constants in the Table 1. Especially, the interactions between the receptor and anions accompanied the naked-eyed color changes (from orange
Y. Wang et al. / Talanta 74 (2008) 1122–1125
to purple), which were expected to be convenient and feasible application in the practical detection of anions. So; the receptor could be considered as a potential AcO− anion sensor in many biological and analytical applications.
[5] [6] [7] [8] [9]
5. Conclusion Examination of the binding patterns of chemosensors by UV–vis titrations with a series of structurally different anions (in solvents with strong polarity) provided very important information that the chemsensor preferred structure matching anion such as acetate. Such behavior is speculated to reflect the sizes of the binding sites of these chemosensors. The solution color of the receptor changed obviously (from orange to purple) with the addition of AcO− , F− and H2 PO4 − , thus usefullness in distinguishing these anions. 1 H NMR Titrations indicated that the pattern of the receptor interaction with anion was hydrogen bond. Acknowledgements
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
This work was supported by a project 20371028, 20671052 from the National Natural Science Foundation of China and a project 023605811 from the Natural Science Foundation of Tianjin. References [1] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486. [2] P.A. Gale, Coord. Chem. Rev. 199 (2000) 181. [3] A. Bianchi, K. Bowman-James, E. Garcia-Espana, Supramolecular Chemistry of Anions, Wiley–VCH, New York, 1997. [4] F.P. Schmidtchen, M. Berger, Chem. Rev. 97 (1997) 1609–1646.
[21]
[22] [23] [24] [25] [26]
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R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476. C. Suksai, T. Tuntulani, Chem. Soc. Rev. 32 (2003) 192–202. P.D. Beer, Acc. Chem. Res. 31 (1998) 71–80. J.L. Atwood, J.E.D. Davies, D.D. MacNicol, F. Vogtle, K.S. Suslick (Eds.), Comprehensive Supramolecular Chemistry, Pergamon, Oxford, 1996. Y. Kubo, M. Tsukahara, S. Ishihara, S. Tokita, Chem. Commun. (2000) 653. M.J. Deetz, M. Shang, B.D. Smith, J. Am. Chem. Soc. 122 (2000) 6201. K. Choi, A.D. Hamilton, J. Am. Chem. Soc. 123 (2001) 2456. B.H.M. Snellink-Ruel, M.G.M. Antonisse, J.F.J. Engbersen, P. Timmerman, D.N. Reinhoudt, Eur. J. Org. Chem. (2000) 165. E. Fan, S.A. Van Arman, S. Kincaid, A.D. Hamilton, J. Am. Chem. Soc. 115 (1993) 369. P.B. Beer, Chem. Commun. (1996) 689. S. Valiyaveettil, J.F.J. Engbersen, W. Verboom, D.D. Reinhoudt, Angew. Chem. Int. Ed. 32 (1993) 900. E. Kimura, S. Aoki, T. Koike, M. Shiro, J. Am. Chem. Soc. 119 (1997) 3068. K. Niikura, A. Metzger, E.V. Anslyn, J. Am. Chem. Soc. 120 (1998) 8533. Y. Morzherin, D.M. Rudkevich, W. Verboom, D.N. Reinhoudt, J. Org. Chem. 58 (1993) 7602. J. Schreeder, J.F.J. Engbersen, A. Casnati, R. Ungaro, D.N. Reinhoudt, J. Org. Chem. 60 (1995) 6448. Z.-H. Lin, S.-J. Ou, C.-Y. Duan, B.-G. Zhang, Z.-P. Bai, Chem. Commun. (2006) 624–626. V.M. Sharma, P. Prasanna, K.V.A. Seshu, B. Renuka, C.V.L. Rao, G.S. Kumar, C.P. Narasimhulu, P.A. Babu, R.C. Parank, D. Subramanyam, A. Venkateswarlu, S. Rajagopal, K.B.S. Kumar, C. Seshagiri Rao, N.V.S. Rao Mamidi, D.S. Deevi, R. Ajaykumar, R. Rajagopalan, Bioorg. Med. Chem. Lett. 12 (2002) 2303. K.A. Connors, Binding Constants, 1st ed., John Wiley & Sons, New York, 1987. B. Chiswell, F. Lions, M.L. Tomlinson, Inorg. Chem. 3 (1964) 492–499. H. Tong, G. Zhou, L. Wang, X. Jing, F. Wang, J. Zhang, Tetrahedron Lett. 44 (2003) 131–134. X.-F. Shang, H. Lin, Z.-Sh. Cai, H.-K. Lin, Talanta 73 (2007) 296–303. A.K. Jain, V.K. Gupta, L.P. Singh, P. Srivastava, J.R. Raisoni, Talanta 65 (2005) 716–721.
Talanta 74 (2008) 1122–1125
A phenylhydrazone-based indole receptor for sensing acetate Yuehong Wang a , Hai Lin b , Jie Shao a , Zun-Sheng Cai a , Hua-Kuan Lin a,∗ a
b
Department of Chemistry, Nankai University, Tianjin 300071, China State Key Laboratory of Functional Polymer Materials of Ministry of Education Nankai University, Tianjin 300071, China Received 8 June 2007; received in revised form 8 August 2007; accepted 13 August 2007 Available online 22 August 2007
Abstract A novel phenylhydrazone-based indole anion receptor is synthesized by a simple method, and the receptor binds acetate anions with good selectivity in DMSO solution, which is related to the structure of the receptor matching with the anion. The binding properties of the host are examined by UV–vis changes. The color changes of the host upon the addition of a variety of structurally different anions were also utilized as nake-eyed recognization (from orange to purple, first figure), The hydrogen bonds between recognization sites (phenylhydrazone and indole N–H) and anions were determined on the basis of 1 H NMR experiments. © 2007 Elsevier B.V. All rights reserved. Keywords: Indole; Phenylhydrazone; Receptor; Nake-eyed recognization
1. Introduction Anion recognition has attracted considerable attention due to their medicinal and environmental potential application [1–3]. Receptors [4] and sensors [5–7] having strong affinity and selectivity for specific anions arouses of great interest in supramolecular chemistry [8]. So, the design of anion receptors with high selectivity is challenging [9–11]. Synthetic receptors for anions are usually based on urea/thiourea [12,13], amides [14,15,25], macrocyclic ammonium/guanidinium [16,17], functionalized calixarenes [18,19], phenylhydrazone [20], indole [21] and –OH [26]. Take the phenylhydrazone for example, first, this kind of the receptor is neutral and the synthetic method is simple. Secondly, the reactant indoline-2,3-dione and 2,4nitrophenylhydrazine could recognize anion by itself and had a nake-eyed color change. Of particular interest in this regard is the color of phenylhydrazone, which changed so obviously (from orange to purple) upon addition of AcO− anion that we could use the nake-eyed detection without resorting to any spectroscopic instrumentation. In the present work, we designed and synthesized the 3-(2 ,4 dinitrophenylhydrazone)indolin-2-one. It is provided with the
∗
Corresponding author. Fax: +86 22 23502458. E-mail address: [email protected] (H.-K. Lin).
0039-9140/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2007.08.015
merit, which we referred to. We believe that the receptor will have a better-applied foreground. 2. Experimental 2.1. Reagents All anions, in the form of tetrabutylammonium salts, were purchased from Sigma–Aldrich Chemical Co., stored in a desiccator under vacuum containing self-indicating silica, and used without any further purification. Solvents were purified prior to use and stored under nitrogen. Dimethyl sulfoxide was dried with calcium hydride and distilled at reduced pressure. Unless stated otherwise, commercial grade chemicals were used without further purification. 2.2. Synthesis 3-(2 ,4 -Dinitrophenylhydrazone)indolin-2-one. A solution of 2,3-dikedoine (isatin) (4 mmol) in ethanol (20 ml) was added dropwise to a solution of 2,4-nitrophenylhydrazine (4 mmol) in ethanol (60 ml) with stirring at reflux. After being stirred for 5 h, the solvent was removed by evaporation. Recrystallization (ethanol) yielded yellow crystals. δH (400 MHz, DMSO-d6 , Me4 Si): 11.73 (s, 1H, N–H), 10.91 (s, 1H, N–H), 8.92 (d, 1H, Ar–H, J = 4 Hz), 8.59 (d, 1H, Ar–H, J = 8 Hz), 8.16 (d, 1H,
Y. Wang et al. / Talanta 74 (2008) 1122–1125
1123
Ar–H, J = 8 Hz), 7.89 (d, 1H, Ar–H, J = 8 Hz), 7.48 (t, 1H, Ar–H, J = 12 Hz), 7.21 (t, 1H, Ar–H, J = 12 Hz), 6.97 (d, 1H, Ar–H, J = 8 Hz). Anal. Calcd. for C14 H9 N5 O5 : C 51.38, H 2.77, N 21.40, Found C 51.54, H 2.38, N 21.55. 2.3. General methods Unless otherwise specified, all of the experiments were carried out at 298 ± 1 K. The 1 H NMR spectra were recorded on a Varian UNITY-plus 400 MHz spectrometer using tetramethylsilane (TMS) as an internal standard. UV–vis spectra were recorded on a Shimadzu UV-2450 PC spectrophotometer.
Fig. 1. Color changes from receptor (3 × 10−5 M) to receptor + AcO− (3 × 10−4 M) in DMSO.
2.4. Absorption titration studies The binding ability of receptor for CH3 CO2 − , H2 PO4 − , and halide anions (as tetrabutylammonium salts) was investigated by UV–vis spectroscopy in DMSO solution using a constant host concentration (3.0 × 10−5 M) and increasing concentrations of anions. The change in absorbance at 561 nm for receptor was plotted against anion concentration and fitted by the equation as described by Connors [22]. 2.5. 1 H NMR titrations Receptor 1 (5.0 × 10−3 M in DMSO-d6 ) was titrated against acetate anions (tetrabutylammonium salts) by addition of excess anion in DMSO-d6 . 3. Results and discussion Here, we reported a remarkable selective indolehydrazonebased acetate chromo-chemosensor of a Schiff-base compound, 3-(2 ,4 -dinitrophenylhydrazone)indolin-2-one. It contained an indolehydrazone group that could be transformed to phenol form in the presence of anions (Scheme 1) [23,24]. 3.1. UV–vis experiments
Fig. 2. The UV–vis spectra of receptor (3 × 10−5 M) in DMSO solution during the titration with tertrabutylammonium (TBA) acetate (0, 0.067, 0.133, 0.266, 0.5, 0.633, 0.67, 0.766, 1.2, 1.6, 1.8, 2.0, 4.2, 5.6 equiv.). Inset: titration plots observed binding profiles and corresponding nonlinear fit plots monitored by the absorption increase at 561 nm.
of the absorption peak at 383 nm was remarkably reduced with a simultaneous growth of the peak at 561 nm (Fig. 2) and meanwhile the solution color changed from orange to purple (Fig. 1). The presence of isobestic points during titration with acetate revealed the formation of 1:1 complexes (Fig. 3 ). No significant changes in absorption or a noticeable color changes were observed for other anions such as Cl− , Br− , I− .
In DMSO solution, the receptor could form hydrogen bond with AcO− , F− and H2 PO4 − anions, respectively, and the color of solution changed significantly (Fig. 1 ). The anion-binding properties were investegated by UV–vis titration of the recepror in DMSO solution using standard tetrabutylammonium salts of AcO− , F− and H2 PO4 − . As shown in Fig. 2 , the titration of receptor (c = 3.0 × 10−5 M), exhibited a broad strong absorption band at 383 nm due to the nitrophenylhydrazone. The intensity
Scheme 1. Proposed mode of transformatin of the receptor upon binding anion.
Fig. 3. A job plot of receptor with tetrabutylammonium acetate.
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Y. Wang et al. / Talanta 74 (2008) 1122–1125
Table 1 Association constants Kass of receptors 1 with anions in DMSO Anion
AcO−
F−
H2 PO4 −
Cl−
Br−
I−
Kass (M−1 )
2.32 × 105
6.22 × 104
2.25 × 104
ND
ND
ND
The anions were added as their tetrabutylammonium salts. All errors are ±10%; ND: the spectra have too small change with adding anion so we cannot determine the affinity constant by the spectra.
Detail data was given in Table 1. The association constant Kass was calculated by non-linear least square regression method [22]. For the receptor, the association constant for AcO− was larger than that of F− and H2 PO4 − . The reason was probably that the selective recognition for acetate anion is related to the configuration of the acetate matching with the receptor (Fig. 1) and the alkalescence of the anion, as well as the basicity of receptor. Because acetate anion was a plane and triangular and the angle of O–C–O was about 120◦ , the distance of two oxygen atoms might be fit to the two hydrogen atoms on recognition sites of the receptor in the triangular configuration, Furthermore, the alkalescence of acetate anion was also stronger than the other anions (Cl− , Br− , I− ). So, the association constant Kass for acetate was maximal. The angle of O–P–O is about 108◦ for H2 PO4 − (tetrahedral configuration) and the distance of two oxygen atoms of AcO− was longer than that of H2 PO4 − ; so two oxygen atoms of H2 PO4 − could not match well with two hydrogen atoms in the receptor. As for F− , it was global and had the smallest atom radius in halide atom, but the better alkalescence made itself have the higher association constant Kass than H2 PO4 − . Table 1 illustrated that the receptor can bind anions in the order AcO− > F− > H2 PO4 − > Cl− ∼ Br− ∼ I− .
N–H proton, as well as the hydrazone N–H signal shift downfield over the course of the titration. However, the fact the N–H signals were observable throughout the titration was consistent with a hydrogen-binding process involving the receptor. In the specific case of receptor with acetate anion, the singlet peak of the indole N–H protons broadened and meanwhile shifted from 11.73 to 11.94 ppm upon addition of 0.8 equiv. of tetrabutylammonium acetate (Fig. 4). Addition of the second equivalent of acetate caused substantial broadening and downfield shift of all indole N–H protons, as well as all hydrazone N–H protons. Similarly, much broadening of the aromatic protons was observed in NMR titrations of all anions. These results clearly indicated that, in addition to the formation of hydrogen bonds between acetate and N–H groups. But some shifted slightly upfield, the others appeared slightly downfield for the aromatic protons, and two effects were responsible for the spectral changes of the aromatic protons upon NH-anion hydrogen bond formation (1) throughbond effects, which increased the electron density of the phenyl ring and promoted upfield shifts, and (2) through-space effects, which polarized the C–H bond in proximity to hydrogen bonds, created partial positive charge on the proton, and caused downfield shifts. As mentioned above, the proposed binding mode was shown in Scheme 1.
3.2. 1 H NMR titrations 4. Analytical application Further supports for the notion that the formation of hydrogen bonds between N–H groups and anions came from 1 H NMR spectroscopic analyses. These were carried out in DMSOd under normal conditions of so-called NMR titration, with the spectra of the receptors being recorded in the presence of increasing concentrations of anions (Fig. 4 ). A notable feature of these titrations was that the resonances corresponding to the indole
Fig. 4. 1H NMR titration of a DMSO-d6 solution of receptor with AcO− .
The phenylhydrazone-based indole receptor would have potential application in analytical chemistry, because the receptor was applied in the monitoring of binding ability with various anions (AcO− , H2 PO4 − , F− , Cl− , Br− , and I− ), which existed widely in biological, industrial, and environmental processes; e.g., carboxylate anions (CH3 COO− ) played an important role in organic, environmental, biological processes, etc. The carboxylate functions of enzymes and antibodies ascribed to their specific biochemical behaviors. Moreover, they were critical components of numerous metabolic processes. So, the development of sensitive detection systems for their binding-ability was very important. In the paper, the selective recognization of the receptor for acetate anion was designed; synthesized and studied. In the first place, color changes of the receptor solution was tested in DMSO (3 × 10−5 M) and the solution color changed from orange to purple upon addition of 10 equiv. AcO− , which was shown in the Fig. 1. In the next place; the binding ability of the host with different anions was investigated through UV–vis titration in DMSO. Obviously; AcO− anion; whose association constant was maximal; could be recognized selectively from the other anions tested according to the affinity constants in the Table 1. Especially, the interactions between the receptor and anions accompanied the naked-eyed color changes (from orange
Y. Wang et al. / Talanta 74 (2008) 1122–1125
to purple), which were expected to be convenient and feasible application in the practical detection of anions. So; the receptor could be considered as a potential AcO− anion sensor in many biological and analytical applications.
[5] [6] [7] [8] [9]
5. Conclusion Examination of the binding patterns of chemosensors by UV–vis titrations with a series of structurally different anions (in solvents with strong polarity) provided very important information that the chemsensor preferred structure matching anion such as acetate. Such behavior is speculated to reflect the sizes of the binding sites of these chemosensors. The solution color of the receptor changed obviously (from orange to purple) with the addition of AcO− , F− and H2 PO4 − , thus usefullness in distinguishing these anions. 1 H NMR Titrations indicated that the pattern of the receptor interaction with anion was hydrogen bond. Acknowledgements
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
This work was supported by a project 20371028, 20671052 from the National Natural Science Foundation of China and a project 023605811 from the Natural Science Foundation of Tianjin. References [1] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486. [2] P.A. Gale, Coord. Chem. Rev. 199 (2000) 181. [3] A. Bianchi, K. Bowman-James, E. Garcia-Espana, Supramolecular Chemistry of Anions, Wiley–VCH, New York, 1997. [4] F.P. Schmidtchen, M. Berger, Chem. Rev. 97 (1997) 1609–1646.
[21]
[22] [23] [24] [25] [26]
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