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A colorimetric sensor for Fe2 + ion
Inorganic Chemistry Communications 41 (2014) 88–91
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A colorimetric sensor for Fe2 + ion Chang-Hung Chen a, Chien Cho a, Chin-Feng Wan b, An-Tai Wu a,⁎ a b
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan School of Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
a r t i c l e
i n f o
Article history: Received 20 November 2013 Accepted 20 December 2013 Available online 30 December 2013
a b s t r a c t A simple Schiff-based receptor 1 was synthesized and investigated its binding properties toward various metal ions in polar solvent (MeOH). Receptor 1 showed a dramatic color change from colorless to black which could easily be detected by the naked-eye upon binding with Fe2+. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Schiff-base Colorimetric Chemosensor Reversibility
Fe2+ or Fe3+ is the most abundant transition-metal ion in humans and other mammals, and it plays important roles in various biological systems [1]. Fe2+ or Fe3+ deficiency leads to anemia, liver and kidney damages, diabetes, and heart diseases [2]. Therefore, detection of Fe2+ or Fe3 + is crucial in controlling its concentration levels in the biosphere and its direct impact on human health. In recent years, many chemosensors specific for Hg2+, Cu2+, Zn2+ or other transition metals have been developed [3–16]. Compared to these transition metal ions, only a few chemosensors have been reported for detection of Fe2+ or Fe3+ [17–27]. However, the majorities of the reported Fe2+ or Fe3+ have limitations such as requiring complicated synthesis and are insoluble in polar solutions. In addition, most of these possess selective and sensitive signaling mechanisms that were detected by fluorescence spectrophotometer. Obviously, expensive instruments and complicated synthesis cannot be avoided. For practical applications, it is necessary to develop Fe2+ or Fe3+ sensors that are easily prepared and easily detected without the help of instruments. Hence, colorimetric sensors are useful to develop simple-to-use, naked eye diagnostic tools. Herein, our target is to design a highly selective colorimetric sensor in polar solvent, which can discriminate between Fe2+ and Fe3+. Schiff bases (imines) are known to be good ligands for metal ions [6b,7,28–37]. Most of the Schiff bases sensors are selective toward Zn2+ or Al3+, but to the best of our knowledge, a Schiff base sensor specific to Fe2+ is still unexplored. Therefore, we reported a highly sensitive Schiff base colorimetric receptor 1 toward Fe2+ by UV/vis spectrophotometric detection. Receptor 1 can be readily prepared by a simple coupling reaction of pyridine-2,6-dicarbaldehyde with 2-aminoethanol, as shown in
⁎ Corresponding author. E-mail address: [email protected] (A.-T. Wu).
Scheme 1. The structure of receptor 1 was confirmed by NMR spectra (Fig. S1–S2). The recognition behavior of receptor 1 on various metal ions was investigated by UV/vis measurements. As shown in Fig. 1, receptor 1 does not give rise to any absorption spectra except upon coordination to Fe2 +, while no meaningful changes in absorption intensity were observed upon addition of the following 15 metal ions (as perchlorate salts): Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pb2 +, Cd2 +, Zn2 + and Al3 +. In the presence of Fe2 +, the absorption spectra of receptor 1 in MeOH show two major bands at 475 and 600 nm. Meanwhile, the solution of receptor 1-Fe2+ showed a dramatic color change from colorless to black which could easily be detected by the naked-eye (Fig. 2). The absorption enhancement efficiency observed at 480 nm was 105-fold greater than the control in the absence of Fe2+ (Fig. S3). The observed absorption enhancement may be attributed to the interaction of Fe2+ ion with the imino nitrogen leading to the intramolecular charge transfer from the pyridine moiety to the imino groups. In addition, we also try to confirm the effect of water on the detection. The chemosensor behavior of receptor 1 with cations was performed in MeOH–H2O (1:9 v/v). From the UV/vis spectra of receptor 1 (Fig. S4), Fe2+ ion showed weak absorption with a red shift, in addition Fe3 + also showed a broad absorption with a red shift upon addition of the following 15 metal ions. The result indicated that the content of the water would affect the selectivity of receptor 1 toward cations. To further investigate the sensing properties of receptor 1, UV/vis titration of receptor 1 with Fe2 + was performed. As shown in Fig. 3, the addition of increasing amounts of Fe2 + to a solution of receptor 1 in MeOH, the intensity of the two absorption bands at 475 and 600 nm increased gradually. As shown in Fig. S5, a Job plot indicated a 1:1 stoichiometric complexation of receptor 1 with Fe2+. In addition, the formation of a 1:1 complex between 1 and Fe2 + was further
1387-7003/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.12.027
C.-H. Chen et al. / Inorganic Chemistry Communications 41 (2014) 88–91
N O
H2N
N
dry MeOH 60%
O
1.0 eq
N
OH
N
OH
Absorbance (AU)
H
HO 1
Scheme 1. Synthesis of receptor 1.
1.5
0.6
Absorbance (AU)
2.0
H
89
0.5 0.4 0.3 0.2 0.1 0.0 0.00000
1.0
0.00005
0 eq
0.00010 2+
0.00015
[Fe ] M
0.5
Absorbance (AU)
0.6
+
+
+
2+
host and K , Li , Na , Mg , 2+ 3+ 3+ 2+ 2+ Mn , Al , Fe , Co , Ni , 2+ 2+ 2+ 2+ 2+ 2+ Cu , Ca , Cd , Pb , Hg , Zn
0.0 250
400
450
500
550
450
500
550
600
650
700
Fig. 3. UV/vis spectra of receptor 1 (160 μM) in MeOH upon addition of increasing concentrations Fe2+; Inset is a plot of absorbance change vs. equiv. of Fe2+ added.
0.0 350
400
2+
0.2
300
350
Wavelength (nm) Fe
0.4
300
600
650
Wavelength (nm) Fig. 1. UV/vis spectra of 1 (80 μM) recorded in MeOH after addition of 1.0 equiv of various metal ions.
confirmed by the appearance of a peak at m/z 376, assignable to [receptor 1 + Fe2+ + ClO− 4 ] in the ESI/MS (Fig. 4). From the UV/vis titration profiles (Fig. 3), the association constant for 1-Fe2+ in MeOH was determined as 1.31 × 107 M−1 by a Hill plot (Fig. S6). The selectivity toward Fe2 + was ascertained by the competition experiment. As shown in Fig. S7, receptor 1 was treated with 1.0 equiv. of Fe2 + in the presence of the same concentration of other metal ions. Relatively low interference was observed for the detection of Fe2 + in the presence of other metal ions. Thus, receptor 1 can be used as a selective sensor for Fe2+ in the presence of most competing metal ions. Reversibility is a prerequisite in developing a novel sensor for practical application. The reversibility of the recognition process of receptor 1 was performed by adding a Fe2+ bonding agent, Na2EDTA (Fig. 5). The addition of Na2EDTA to a mixture of receptor 1 and Fe2+ resulted in diminution of the absorption intensity at 475 nm and 600 nm. Meanwhile, the color of the solution changed back to the original colorless instantly, which indicated the regeneration of the free receptor 1. The absorption band was recovered by the addition of Fe2 + again. Such reversibility and regeneration are important for the fabrication of devices to sense the Fe2+. In addition, we also performed the IR experiment. The IR spectrum of receptor 1 was compared with the spectrum of Fe2+ complex. The typical IR spectra were shown in Fig. S8. The IR spectrum of receptor 1 indicates a sharp peak at 1647 cm−1, which is
assigned to C_N stretching of imine group. The IR spectrum of Fe2+ complex exhibits a broad band at 1655 cm− 1. In addition, the Fe2 + complex displays a symmetrical stretching band near 1100 cm−1. The result showed the presence of Fe2+ complex. To better understand the complexation behavior of receptor 1 with Fe2 +, 1H NMR titration experiments were carried out in CD3OD. The spectral differences were depicted in Fig. 6. The imine proton (H3) of receptor 1 at around 8.31 ppm was shifted down-field toward 8.42 ppm upon the addition of Fe2 +. In addition, the protons of ethylene (H1 and H2) were obvious shifted high-field from 3.74 and 3.68 ppm to 2.63 and 2.91 ppm, respectively. Obvious changes of the chemical shifts indicated that receptor 1 could form a stable complex with Fe2+. These results suggest that the oxygen atom of the ethylene moiety and the nitrogen atom of the imine moiety might be involved in Fe2+ coordination. Based on the 1HNMR titration, Job plot, IR and ESI-mass spectrometry analysis, we propose the structure of a 1:1 complex of receptor 1 and Fe2+, as shown in Scheme 2. In summary, we prepared a novel receptor 1 for the detection of selected metal ions. The receptor 1 displayed significant absorption and color changes after addition of Fe2 +. It means that receptor 1 could serve as a selective and efficient colorimetric sensor for the detection of Fe2+ by the UV/vis spectra or naked-eye method. In addition, the addition of EDTA quenches the absorption of receptor 1·Fe2+ complex indicated that receptor 1 was a reversible chemosensor. Acknowledgment We thank the National Science Council of Taiwan for the financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2013.12.027.
Fig. 2. The color changes observed by naked eye of receptor 1 (80 μM) upon addition of 1.0 equiv of various metal ions in MeOH.
90
C.-H. Chen et al. / Inorganic Chemistry Communications 41 (2014) 88–91
Fig. 4. ESI Mass spectrum for [receptor 1-Fe2+ + ClO− 4 ] complex.
Ar
1.0 Host Host+Fe2+ Host+Fe2+ +EDTA Host+Fe2+ +EDTA+Fe2+
Absorbance (AU)
0.8
Fe2+
N N
N
N
HO
O H
0.6
OH 0.4
3
N Fe2+
N
2
O H
1
Scheme 2. Proposed binding mode of receptor 1 with Fe2+.
0.2
References 0.0 300
400
500
600
700
800
Wavelength (nm) Fig. 5. UV/vis spectra of receptor 1 (80 μM) in the presence of Fe2+ (1.0 equiv) or EDTA disodium (2.0 equiv) in MeOH solution.
Fig. 6. 1H NMR titration plot of receptor 1 with Fe2+ in CD3OD.
[1] J.J.R. Fausto da Silva, R.J.P. Williams, The Biological Chemistry of the Elements, Oxford University, New York, 1992. [2] C. Brugnara, Clin. Chem. 49 (2003) 1573–1578. [3] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515–1566. [4] B. Valeur, I. Leray, Coord. Chem. Rev. 205 (2000) 3–40. [5] S.H. Kim, H.S. Choi, J. Kim, S.J. Lee, D.T. Quang, J.S. Kim, Org. Lett. 12 (2010) 560–563. [6] (a) Y.C. Hsieh, J.L. Chir, H.H. Wu, C.Q. Guo, A.-T. Wu, Tetrahedron Lett. 51 (2009) 109–111; (b) H.-H. Wu, Y.-L. Sun, C.-F. Wan, S.-T. Yang, S.-J. Chen, C.-H. Hu, A.-T. Wu, Tetrahedron Lett. 53 (2012) 1169–1172. [7] (a) W.H. Hsieh, C.-F. Wan, D.-J. Liao, A.-T. Wu, Tetrahedron Lett. 53 (2012) 5848–5851; (b) H.-Y. Lin, P.-Y. Cheng, C.-F. Wan, A.-T. Wu, Analyst 137 (2012) 4415–4417; (c) Y.-W. Liu, C.-H. Chen, A.-T. Wu, Analyst 137 (2012) 5201–5203. [8] H.N. Lee, H.N. Kim, K.M.K. Swamy, M.S. Park, J. Kim, H. Lee, K.-H. Lee, S. Park, J. Yoon, Tetrahedron Lett. 49 (2008) 1261–1265. [9] J. Kim, T. Morozumi, H. Nakamura, Org. Lett. 9 (2007) 4419–4422. [10] M.E. Huston, K.W. Haider, A.W. Czarnik, J. Am. Chem. Soc. 110 (1988) 4460–4462. [11] M. Formica, C. Fusi, L. GIorgi, M. Micheloni, Coord. Chem. Rev. 256 (2012) 170–192. [12] T.-H. Ma, A.-J. Zhang, M. Dong, Y.-M. Dong, Y. Peng, Y.-W. Wang, J. Lumin. 130 (2010) 888–892. [13] M. Dong, Y.-W. Wang, Y. Peng, Org. Lett. 12 (2010) 5310–5313. [14] M. Dong, T.-H. Ma, A.-J. Zhang, Y.-M. Dong, Y.-W. Wang, Y. Peng, Dyes Pigm. 87 (2010) 164–172.
C.-H. Chen et al. / Inorganic Chemistry Communications 41 (2014) 88–91 [15] Y. Zhou, F. Wang, Y. Kim, S.-J. Kim, J. Yoon, Org. Lett. 11 (2009) 4442–4445. [16] V. Bhalla, Roopa, M. Kumar, Org. Lett. 14 (2012) 2802–2805. [17] S.-L. Hu, N.-F. She, G.-D. Yin, H.-Z. Guo, A.-X. Wua, C.-L. Yang, Tetrahedron Lett. 48 (2007) 1591–1594. [18] J.L. Bricks, A. Kovalchuk, C. Trieflinger, M. Nofz, M. Buschel, A.I. Tolmachev, J. Daub, K. Rurack, J. Am. Chem. Soc. 127 (2005) 13522–13529. [19] H. Ouchetto, M. Dias, R. Mornet, E. Lesuisse, J.M. Camadro, Bioorg. Med. Chem. 13 (2005) 1799–1803. [20] G.E. Tumambac, C.M. Rosencrance, C. Wolf, Tetrahedron 60 (2004) 11293–11297. [21] Y. Ma, W. Luo, P.J. Quinn, Z. Liu, R.C. Hider, J. Med. Chem. 47 (2004) 6349–6362. [22] R. Nudelman, O. Ardon, Y. Hadar, Y. Chen, J. Libman, A. Shanzer, J. Med. Chem. 41 (1998) 1671–1678. [23] L. Praveen, M.L.P. Reddy, R. Luxmi Varma, Tetrahedron Lett. 51 (2010) 6626–6629. [24] C. Bhaumik, S. Das, D. Maity, S. Baitalik, Dalton Trans. 40 (2011) 11795–11808. [25] S. Sen, S. Sarkar, B. Chattopadhyay, A. Moirangthem, A. Basu, K. Dhara, P. Chattopadhyay, Analyst 137 (2012) 3335–3342.
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[26] Z.-Q. Liang, C.-X. Wang, J.-X. Yang, H.-W. Gao, Y.-P. Tian, X.-T. Tao, M.-H. Jiang, New J. Chem. 31 (2007) 906–910. [27] I. Grabchev, J.-M. Chevelon, X. Qian, New J. Chem. 27 (2003) 337–340. [28] L. Salmon, P. Thuéry, E. Rivière, M. Ephritikhine, Inorg. Chem. 45 (2006) 83–93. [29] D.M. Epstein, S. Choudhary, M.R. Churchill, K.M. Keil, A.V. Eliseev, J.R. Morrow, Inorg. Chem. 40 (2001) 1591–1596. [30] Y. Xu, J. Meng, L. Meng, Y. Dong, Y. Cheng, C. Zhu, Chem. Eur. J. 16 (2010) 12898–12903. [31] R. Joseph, J.P. Chinta, C.P. Rao, J. Org. Chem. 75 (2010) 3387–3395. [32] B. Pedras, E. Oliveira, H. Santos, L. Rodriguez, R. Grehuet, T. Avilés, J.L. Capelo, C. Lodeiro, Inorg. Chim. Acta 362 (2009) 2627–2635. [33] L. Wang, W. Qin, W. Liu, Inorg. Chem. Commun. 13 (2010) 1122–1125. [34] D. Udhayakumari, S. Saravanamoorthy, M. Ashok, S. Velmathi, Tetrahedron Lett. 52 (2011) 4631–4635. [35] Y. Dong, J. Li, X. Jiang, F. Song, Y. Cheng, C. Zhu, Org. Lett. 13 (2011) 2252–2255. [36] P.G. Cozzi, Chem. Soc. Rev. 33 (2004) 410–421. [37] D. Pucci, I. Aiello, A. Bellusci, A. Crispini, M. Ghedini, M.L. Deda, Eur. J. Inorg. Chem. (2009) 4274–4281.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A colorimetric sensor for Fe2 + ion Chang-Hung Chen a, Chien Cho a, Chin-Feng Wan b, An-Tai Wu a,⁎ a b
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan School of Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
a r t i c l e
i n f o
Article history: Received 20 November 2013 Accepted 20 December 2013 Available online 30 December 2013
a b s t r a c t A simple Schiff-based receptor 1 was synthesized and investigated its binding properties toward various metal ions in polar solvent (MeOH). Receptor 1 showed a dramatic color change from colorless to black which could easily be detected by the naked-eye upon binding with Fe2+. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Schiff-base Colorimetric Chemosensor Reversibility
Fe2+ or Fe3+ is the most abundant transition-metal ion in humans and other mammals, and it plays important roles in various biological systems [1]. Fe2+ or Fe3+ deficiency leads to anemia, liver and kidney damages, diabetes, and heart diseases [2]. Therefore, detection of Fe2+ or Fe3 + is crucial in controlling its concentration levels in the biosphere and its direct impact on human health. In recent years, many chemosensors specific for Hg2+, Cu2+, Zn2+ or other transition metals have been developed [3–16]. Compared to these transition metal ions, only a few chemosensors have been reported for detection of Fe2+ or Fe3+ [17–27]. However, the majorities of the reported Fe2+ or Fe3+ have limitations such as requiring complicated synthesis and are insoluble in polar solutions. In addition, most of these possess selective and sensitive signaling mechanisms that were detected by fluorescence spectrophotometer. Obviously, expensive instruments and complicated synthesis cannot be avoided. For practical applications, it is necessary to develop Fe2+ or Fe3+ sensors that are easily prepared and easily detected without the help of instruments. Hence, colorimetric sensors are useful to develop simple-to-use, naked eye diagnostic tools. Herein, our target is to design a highly selective colorimetric sensor in polar solvent, which can discriminate between Fe2+ and Fe3+. Schiff bases (imines) are known to be good ligands for metal ions [6b,7,28–37]. Most of the Schiff bases sensors are selective toward Zn2+ or Al3+, but to the best of our knowledge, a Schiff base sensor specific to Fe2+ is still unexplored. Therefore, we reported a highly sensitive Schiff base colorimetric receptor 1 toward Fe2+ by UV/vis spectrophotometric detection. Receptor 1 can be readily prepared by a simple coupling reaction of pyridine-2,6-dicarbaldehyde with 2-aminoethanol, as shown in
⁎ Corresponding author. E-mail address: [email protected] (A.-T. Wu).
Scheme 1. The structure of receptor 1 was confirmed by NMR spectra (Fig. S1–S2). The recognition behavior of receptor 1 on various metal ions was investigated by UV/vis measurements. As shown in Fig. 1, receptor 1 does not give rise to any absorption spectra except upon coordination to Fe2 +, while no meaningful changes in absorption intensity were observed upon addition of the following 15 metal ions (as perchlorate salts): Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pb2 +, Cd2 +, Zn2 + and Al3 +. In the presence of Fe2 +, the absorption spectra of receptor 1 in MeOH show two major bands at 475 and 600 nm. Meanwhile, the solution of receptor 1-Fe2+ showed a dramatic color change from colorless to black which could easily be detected by the naked-eye (Fig. 2). The absorption enhancement efficiency observed at 480 nm was 105-fold greater than the control in the absence of Fe2+ (Fig. S3). The observed absorption enhancement may be attributed to the interaction of Fe2+ ion with the imino nitrogen leading to the intramolecular charge transfer from the pyridine moiety to the imino groups. In addition, we also try to confirm the effect of water on the detection. The chemosensor behavior of receptor 1 with cations was performed in MeOH–H2O (1:9 v/v). From the UV/vis spectra of receptor 1 (Fig. S4), Fe2+ ion showed weak absorption with a red shift, in addition Fe3 + also showed a broad absorption with a red shift upon addition of the following 15 metal ions. The result indicated that the content of the water would affect the selectivity of receptor 1 toward cations. To further investigate the sensing properties of receptor 1, UV/vis titration of receptor 1 with Fe2 + was performed. As shown in Fig. 3, the addition of increasing amounts of Fe2 + to a solution of receptor 1 in MeOH, the intensity of the two absorption bands at 475 and 600 nm increased gradually. As shown in Fig. S5, a Job plot indicated a 1:1 stoichiometric complexation of receptor 1 with Fe2+. In addition, the formation of a 1:1 complex between 1 and Fe2 + was further
1387-7003/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.12.027
C.-H. Chen et al. / Inorganic Chemistry Communications 41 (2014) 88–91
N O
H2N
N
dry MeOH 60%
O
1.0 eq
N
OH
N
OH
Absorbance (AU)
H
HO 1
Scheme 1. Synthesis of receptor 1.
1.5
0.6
Absorbance (AU)
2.0
H
89
0.5 0.4 0.3 0.2 0.1 0.0 0.00000
1.0
0.00005
0 eq
0.00010 2+
0.00015
[Fe ] M
0.5
Absorbance (AU)
0.6
+
+
+
2+
host and K , Li , Na , Mg , 2+ 3+ 3+ 2+ 2+ Mn , Al , Fe , Co , Ni , 2+ 2+ 2+ 2+ 2+ 2+ Cu , Ca , Cd , Pb , Hg , Zn
0.0 250
400
450
500
550
450
500
550
600
650
700
Fig. 3. UV/vis spectra of receptor 1 (160 μM) in MeOH upon addition of increasing concentrations Fe2+; Inset is a plot of absorbance change vs. equiv. of Fe2+ added.
0.0 350
400
2+
0.2
300
350
Wavelength (nm) Fe
0.4
300
600
650
Wavelength (nm) Fig. 1. UV/vis spectra of 1 (80 μM) recorded in MeOH after addition of 1.0 equiv of various metal ions.
confirmed by the appearance of a peak at m/z 376, assignable to [receptor 1 + Fe2+ + ClO− 4 ] in the ESI/MS (Fig. 4). From the UV/vis titration profiles (Fig. 3), the association constant for 1-Fe2+ in MeOH was determined as 1.31 × 107 M−1 by a Hill plot (Fig. S6). The selectivity toward Fe2 + was ascertained by the competition experiment. As shown in Fig. S7, receptor 1 was treated with 1.0 equiv. of Fe2 + in the presence of the same concentration of other metal ions. Relatively low interference was observed for the detection of Fe2 + in the presence of other metal ions. Thus, receptor 1 can be used as a selective sensor for Fe2+ in the presence of most competing metal ions. Reversibility is a prerequisite in developing a novel sensor for practical application. The reversibility of the recognition process of receptor 1 was performed by adding a Fe2+ bonding agent, Na2EDTA (Fig. 5). The addition of Na2EDTA to a mixture of receptor 1 and Fe2+ resulted in diminution of the absorption intensity at 475 nm and 600 nm. Meanwhile, the color of the solution changed back to the original colorless instantly, which indicated the regeneration of the free receptor 1. The absorption band was recovered by the addition of Fe2 + again. Such reversibility and regeneration are important for the fabrication of devices to sense the Fe2+. In addition, we also performed the IR experiment. The IR spectrum of receptor 1 was compared with the spectrum of Fe2+ complex. The typical IR spectra were shown in Fig. S8. The IR spectrum of receptor 1 indicates a sharp peak at 1647 cm−1, which is
assigned to C_N stretching of imine group. The IR spectrum of Fe2+ complex exhibits a broad band at 1655 cm− 1. In addition, the Fe2 + complex displays a symmetrical stretching band near 1100 cm−1. The result showed the presence of Fe2+ complex. To better understand the complexation behavior of receptor 1 with Fe2 +, 1H NMR titration experiments were carried out in CD3OD. The spectral differences were depicted in Fig. 6. The imine proton (H3) of receptor 1 at around 8.31 ppm was shifted down-field toward 8.42 ppm upon the addition of Fe2 +. In addition, the protons of ethylene (H1 and H2) were obvious shifted high-field from 3.74 and 3.68 ppm to 2.63 and 2.91 ppm, respectively. Obvious changes of the chemical shifts indicated that receptor 1 could form a stable complex with Fe2+. These results suggest that the oxygen atom of the ethylene moiety and the nitrogen atom of the imine moiety might be involved in Fe2+ coordination. Based on the 1HNMR titration, Job plot, IR and ESI-mass spectrometry analysis, we propose the structure of a 1:1 complex of receptor 1 and Fe2+, as shown in Scheme 2. In summary, we prepared a novel receptor 1 for the detection of selected metal ions. The receptor 1 displayed significant absorption and color changes after addition of Fe2 +. It means that receptor 1 could serve as a selective and efficient colorimetric sensor for the detection of Fe2+ by the UV/vis spectra or naked-eye method. In addition, the addition of EDTA quenches the absorption of receptor 1·Fe2+ complex indicated that receptor 1 was a reversible chemosensor. Acknowledgment We thank the National Science Council of Taiwan for the financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2013.12.027.
Fig. 2. The color changes observed by naked eye of receptor 1 (80 μM) upon addition of 1.0 equiv of various metal ions in MeOH.
90
C.-H. Chen et al. / Inorganic Chemistry Communications 41 (2014) 88–91
Fig. 4. ESI Mass spectrum for [receptor 1-Fe2+ + ClO− 4 ] complex.
Ar
1.0 Host Host+Fe2+ Host+Fe2+ +EDTA Host+Fe2+ +EDTA+Fe2+
Absorbance (AU)
0.8
Fe2+
N N
N
N
HO
O H
0.6
OH 0.4
3
N Fe2+
N
2
O H
1
Scheme 2. Proposed binding mode of receptor 1 with Fe2+.
0.2
References 0.0 300
400
500
600
700
800
Wavelength (nm) Fig. 5. UV/vis spectra of receptor 1 (80 μM) in the presence of Fe2+ (1.0 equiv) or EDTA disodium (2.0 equiv) in MeOH solution.
Fig. 6. 1H NMR titration plot of receptor 1 with Fe2+ in CD3OD.
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