Dual chemosensing properties of new ferrocene-based receptors towards fluoride and copper(II) ions

Inorganic Chemistry Communications 14 (2011) 1596–1601

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Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e

Dual chemosensing properties of new ferrocene-based receptors towards fluoride and copper(II) ions Soosai Devaraj ⁎, Veeramuthu Stalin Elanchezhian, Muthusamy Kandaswamy Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai-600 025, India

a r t i c l e

i n f o

Article history: Received 12 May 2011 Accepted 14 June 2011 Available online 21 June 2011 Keywords: Chemosensor Ferrocene Cationic sensor Anionic sensor Electrochemical studies

a b s t r a c t The new ferrocene based receptors N-[4-ferrocenyl-2-methyl-4-oxobut-1-enyl]-N′-phenylthiourea (1), N-[4ferrocenyl-2-methyl-4-oxobut-1-enyl]-N′-[4-nitrophenyl]thiourea (2) were synthesized and characterized. Fluorescence titrations of receptors 1 and 2 with various transition metal ions showed selective response to Cu 2+ ions and the emission intensities quenched significantly. Electrochemical titrations with anions revealed that receptors 1 and 2 sensed the F− anion in high selectivity with a cathodic shift of 100 mV. © 2011 Elsevier B.V. All rights reserved.

The development of receptors for recognizing cation, anion and neutral species has attracted much dynamic in molecular recognition study and supramolecular chemistry [1]. The sensing function is generally achieved by the coupling of two-well defined parts: a) selective binding sites and b) signaling subunits e.g. redox shifts, colour changes and fluorescence quenching or enhancement [2]. The binding events have been converted into an electrochemical [3] or fluorescent [4] change or, more directly, a colorimetric change detectable by the naked eye [5]. The incorporation of luminescent chromophores into the receptor has recently gained considerable attention due to their high sensitivity and low detection limit [6–8]. Many fluorescent sensors have been developed on the basis of a variety of signaling mechanisms such as competitive binding, photoinduced electron transfer, metal–ligand charge transfer, and intramolecular charge transfer [9–12]. Commonly, electrochemical anion sensing has been achieved potentiometrically by ferrocenyl species or other organometallic derivatives [13,14]. It has been postulated by Beer et al. that amide spacer units gave good coupling between the binding and electrochemical readout sites, and the –NH fragments were excellent hydrogen bond donors for anions [15]. In addition, recently some papers reported a few compounds, which display simultaneously both electrochemical and fluorescent signaling for cation sensing [16,17]. Sensing of a fluoride anion, the smallest anion, has attracted growing attention due to its beneficial effects (e.g., prevention of dental caries) and detrimental (e.g., fluorosis) ⁎ Corresponding author at: Department of Applied Chemistry, Providence University, 200 Chungchi Road, Sha-Lu, Taichung Hsien 433, Taiwan. Tel.: +886 4 26328001x15218; fax: +886 4 26327554. E-mail address: [email protected] (S. Devaraj). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.06.016

roles [18]. In order to make beneficial use of fluorescent and electrochemical dual sensing molecules for the detection of the fluoride and copper (II) ion, here we report N-[4-ferrocenyl-2methyl-4-oxobut-1-enyl]-N′-phenylthiourea (1), N-[4-ferrocenyl-2methyl-4-oxobut-1-enyl]-N′-[4-nitrophenyl]thiourea (2), which consist of a redox active ferrocene group and nitro phenyl rings as well as anion binding thiourea groups as a biresponsive fluorescent and electrochemical chemosensor. The host–guest complexation for sensing fluoride and copper(II) ions through electrochemical and optical responses were investigated. The receptors 1 and 2 were synthesized by Schiff base condensation (Scheme 1) of ferrocenoylacetone [19] with 1-(2-aminophenyl)3-phenylthiourea and 1-(2-amino phenyl)-3-(4-nitrophenyl)thiourea respectively [20]. The receptors 1 and 2 were characterized by elemental analysis, IR, 1H and 13C-NMR [Figs. S1–S4], and ESI Mass [Figs. S5 and S6] spectroscopic methods [21]. The anion binding interactions of receptors 1 and 2 were investigated using UV–visible, fluorescence and electrochemical studies. The UV–visible titrations were carried out in CH3CN by adding aliquots of solutions of different halide anions as its tetrabutylammonium salts. Fig. 1 displayed absorption spectral changes of receptor 1 (5 × 10 −5 M in CH3CN) in the presence of F − (5 × 10 −3 M) ions. As depicted in Fig. 1, three absorption peaks were observed at 222 or 243, 308 and 364 nm in the absence of F − ions, might attribute to ICT (intramolecular charge transfer) absorption bands. A prominent absorption band with λmax 222, 243 and 308 nm, can ascribed to a high energy ligand-centered π–π* electronic transition. The band at 364 nm was assigned to n–π* transition. In addition, another absorption band in the visible region at 480 nm (ε = 200 M −1 cm −1) was assigned to localized excitation with a low

S. Devaraj et al. / Inorganic Chemistry Communications 14 (2011) 1596–1601

1597

H2N O

O HN

Fe

+

S

HN

O

N

S

HN

EtOH reflux

HN R Fe

R 1=H 2 = NO2

R R = H, NO 2 Scheme 1. Synthesis of receptors 1 and 2.

energy (LE) which was produced by two nearly degenerated transitions and Fe(II) d–d transition [Fig. 1]. Upon the addition of F − ions [0 to 2 equiv.], the absorption peak at 480 nm decreased with a blue shift of 15 nm. A typical titration curve of fluoride ions by receptor 2 was shown in Fig. S7. The longer wavelength absorption at λmax = 485 nm decreased with a blue shift of 15 nm in the presence of 0 to 2 equivalents of F − ions. Simultaneously the peak at 222 nm

increased and the peaks at 310 and 360 nm decreased gradually along with the increased concentration of F − ion. Exposure to chloride, bromide and iodide anions did not result in any spectral changes in the above receptors [22,23]. The binding constants of F − complexation for the receptors 1 and 2 were estimated and tabulated (see Table 1, Fig. S9 and Fig. S10 respectively). Receptor 2 showed a higher binding constant, attributing the presence of nitro group.

1.80 1.6 1.4

Absorbance

1.2 1.0

1 equiv 0.66 equiv 0.33 equiv 0 equiv

0.8 0.6 0.4 0.2 0.00 200.0

250

300

350

400

Wavelength (nm) 1.60 1.4

1 equiv 0.9 equiv 0.8 equiv 0.7 equiv 0.6 equiv 0.5 equiv 0.4 equiv 0.3 equiv 0.2 equiv 0.1 equiv 0 equiv

Absorbance

1.2 1.0 0.8 0.6 0.4 0.2 0.03 400.0

450

500

550

600

650

700

750

800.0

Wavelength (nm) Fig. 1. Absorption spectra of receptor 1 recorded in CH3CN (5.0 × 10−5 M) after the addition of 0 to 1 equiv. of TBAF.

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S. Devaraj et al. / Inorganic Chemistry Communications 14 (2011) 1596–1601

Table 1 Data obtained from the UV–visible spectra upon titration of receptors 1 and 2 with anionsa and cations in CH3CN. Receptor

1 2

Binding constants F−

Cu2+

Cl−, Br−, I−

(1.2 ± 0.21)b × 103 (24.0 ± 0.12)b × 103

(1.5 ± 0.12)b × 103 (9.2 ± 0.20)b × 103

NDc NDc

Correlation coefficient (R) 0.9959 0.9940

*ND-not determined. a Countercation was tetrabutylammonium salts for anions. All errors are ± 5%. b The error value was obtained by the result of linear fitting. All errors are ± 5%. c Changes in the UV–visible spectra were not enough to calculate the binding constant.

The binding ability of 1 and 2 in CH3CN (5 × 10 −5 M) against cations of environmental relevance, such as Mg 2+, Ca 2+, Cd 2+, Ni 2+, Co 2+, Zn 2+, Mn 2+ and Cu 2+ as its perchlorate salts showed unique selective response towards Cu 2+ ions alone. This observation was consistent with Irving–William series. Copper(II) has a specific high thermodynamic affinity for typical N-donor ligands and fast metal-toligand binding kinetics. Figs. 2 and S11 displayed absorption spectral changes of the receptors 1 and 2 respectively in the presence of Cu 2+ ions. The colorimetric sensing ability of 1 and 2 in CH3CN (1 × 10 −3 M) was monitored by the naked eye and visible spectroscopy. The

sequential addition of Cu 2+ ions from 0 to 2 equivalents to the receptor 1 [Fig. 2] showed a gradual decrease in absorbance at 481 nm and intensity enhancement of absorption at 651 nm with colorimetric change from pale yellow to green [Fig. 3]. As shown in fig. S10, the intensity of 2 at 640 nm increased, while that of 490 nm decreased with the colour changes from orange to dark green [Fig. 3]. These new peaks and intensity of the colours for the receptors 1 and 2 reached their limiting value after the addition of two equivalents of Cu 2+ ions. The binding constants of copper(II) complexation for the receptors 1 (Fig. S11) and 2 (Fig. S12) were shown in Table 1. No significant spectral changes are observed for receptors 1 and 2 in the presence of other metal ions viz Mg 2+, Ca 2+, Cd 2+, Ni 2+, Co 2+, Zn 2+ and Mn 2+. Fluorescence monitoring of F - and Cu 2+ ions were also performed for the receptors by using 5 × 10 −5 M solution of 1 and 2 in CH3CN. The emission spectra of 1 and 2 were characterized by the emission maximum observed at 438 nm and 445 nm respectively upon excited at 325 nm. Upon increasing the addition amounts of F - ion (0 to 3 equiv.) to the solution of receptor 1, a significant increase in fluorescence emission at 438 nm was observed [Fig. 4]. The increase in fluorescence emission induced by F - ion was attributed to 1.F − complex. Similarly, upon addition of F − ion, the emission band at 445 nm of 2 was also gradually increased [Fig. S13]. Fluorescence monitoring of other halides such as Cl −, Br − and I − was also carried

2.00 1.8 1.6

Absorbance

1.4

2 equiv 1.66 equiv 1.33 equiv 1 equiv 0.66 equiv 0.33 equiv 0 equiv

1.2 1.0 0.8 0.6 0.4 0.2 0.00 200.0

250

300

350

400

450

Wavelength (nm)

2 equiv 1.8 equiv 1.6 equiv 1.4 equiv 1.2 equiv 1 equiv 0.8 equiv 0.6 equiv 0.4 equiv 0.2 equiv 0 equiv

1.50 1.4

Absorbance

1.2 1.0 0.8 0.6 0.4 0.2 0.07

440.0 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800.0

Wavelength (nm) Fig. 2. Absorption spectra of receptor 1 recorded in CH3CN (5.0 × 10−5 M) after the addition of 0 to 2 equiv. of Cu2+ ions.

S. Devaraj et al. / Inorganic Chemistry Communications 14 (2011) 1596–1601

1

1+Cu2+

2+Cu2+

2

Fig. 3. Colour changes of 1 and 2 in CH3CN (5.0 × 10−5 M) before and after the addition of 2 equiv. of Cu2+ ions.

out under the same conditions as the F - ion. Beside F − ion; no significant fluorescence change of receptors 1 and 2 was observed even in the presence of 10 equiv.

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On increasing the concentrations of Cu 2+ ions [0 to 3 equiv., Fig. 5] to the receptor 1, the emission band was quenched at 438 nm with the blue shift of 10 nm. A selective quenching with blue shift (15 nm) was also observed for the receptor 2 at 455 nm [Fig. S14]. The emission spectrum of 1 and 2 were unaffected upon the addition of other metal ions. The ferrocene conjugated system could allow the modulation of optical or photochemical properties of the molecule via change in the electronic states of the ferrocene moiety by the involvement of Cu(II) ion binding with donor atoms. The quenching effect could be attributed to an electron or energy transfer quenching of the π* emissive state through low-lying metal-centered unfilled d-orbitals of paramagnetic Cu(II)[24]. Figs. 6 and S15 show electrochemical changes of 1 and 2 in CH3CN solution upon addition of F − ions at 100 mV scan rate, using TBAP as a supporting electrolyte. The cyclic voltammetric (CV) response of 1 showed a reversible oxidation process at E1/2 = 645 mV which was attributed to the Fc/Fc + couple. On stepwise addition of F − ion (1 equiv.) to a solution of receptor 1 in CH3CN, a decrease in current intensity of the reduction wave was observed, which might be attributed to the formation of receptor–anion complex [Fig. 5] [25].

900.0 800

3 equiv 2.66 equiv 2.33 equiv 2 equiv 1.66 equiv 1.33 equiv 1 equiv 0.66 equiv 0.33 equiv 0 equiv

700

Intensity

600 500 400 300 200 100 2.4 400.0

420

440

460

480

500

520

540

560

Wavelength (nm) Fig. 4. The changes in the fluorescence emission spectra of receptor 1 (5.0 × 10−5 M) upon titration with solutions of F− ions (0–3 equiv.) in CH3CN.

310.0

250

Intensity

200

3 equiv 2.5 equiv 2 equiv 1.5 equiv 1 equiv 0.5 equiv 0 equiv

150 100 50 1.3 400.0

420

440

460

480

500

520

540

560

580

600

620.0

Wavelength (nm) Fig. 5. The changes in the fluorescence emission spectra of receptor 1 (5.0 × 10−5 M) upon titration with solutions of Cu2+ ions (0–3 equiv.) in CH3CN.

1600

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2 equiv 1 equiv 0 equiv

Fig. 6. Cyclic voltammetric response of receptor 1 in CH3CN (with 0.1 M TBAClO4) upon the addition of F− ion (0–2 equiv.) at the scan rate of 100 mV.

Further addition of F − anion had an effect on the position of Eox, which shifted cathodically from 690 mV to 600 mV. Cathodic shifts were often seen in 1-F − anion binding because the oxidation process became easier in the presence of the negatively charged ion as a consequence of electrostatic stabilization [26]. Receptor 2 showed a similar response to TBAF, the potential of Eox cathodically shifted from 790 mV to 690 mV with E1/2 = 685 mV [Fig. S15]. There was no significant CV response of receptors 1 and 2 upon addition of other halide and metal ions. In this present study, we have developed selective and sensitive chromo as well as fluorogenic receptors 1 and 2 using ferrocene derivatives for the determination of F − and Cu 2+. These receptors selectively recognize F − ions even in the presence of other halide ions and shows higher selectivity towards Cu 2+ ions than other metal ions studies. Acknowledgements SD gratefully acknowledges SRF (CSIR) for financial assistance. MK also acknowledges the Department of Science and Technology (Government of India) and UGC, Delhi for the project grant. Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10. 1016/j.inoche.2011.06.016. References [1] J.M. Lehn (Ed.), Comprehensive Supramolecular Chemistry, Elsevier, Amsterdam, The Netherlands, 1996. [2] (a) R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476; (b) P.A. Gale, Coord. Chem. Rev. 240 (2003) 191–221. [3] (a) C. Dusemund, K.R.A.S. Sandayanake, S. Shinkai, J. Chem. Soc. Chem. Commun. (1995) 333–334; (b) H. Yamamoto, A. Ori, K. Ueda, C. Dusemund, S. Shinkai, Chem. Commun. (1996) 407–408; (c) D. Jimenez, R. Martínez-Manez, F. Sancenon, J. Soto, Tetrahedron Lett 43 (2002) 2823–2825. [4] (a) C.R. Cooper, N. Spencer, T.D. James, Chem. Commun. (1998) 1365–1366; (b) M. Nicolas, B. Fabre, J. Simonet, Chem. Commun. (1999) 1881–1882; (c) S. Yamaguchi, S. Akiyama, K. Tamao, J. Am. Chem. Soc. 122 (2000) 6793–6794; (d) P. Anzenbacher Jr., K. Jursikova, J.L. Sessler, J. Am. Chem. Soc. 122 (2000) 9350–9351; (e) S.K. Kim, J. Yoon, Chem. Commun. (2002) 770–771; (f) T.-H. Kimand, T.M. Swager, Angew. Chem. Int. Ed. 42 (2003) 4803–4806; (g) D.H. Lee, J.H. Im, J.H. Lee, J.I. Hong, Tetrahedron Lett. 43 (2002) 9637–9640;

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[21] To the solution of 1-(2-amino phenyl)-3-phenylthiourea (0.3 g, 1.2 mmol) in ethanol (25 ml), ferrocenoylacetone (0.33 g, 1.2 mmol) in ethanol (25 ml) was added under stirring. The resulting mixture was refluxed for 3 h and cooled to room temperature. The solid product was collected by filtration and washed with cold ethanol. The solid was recrystallized using hot chloroform. The receptor 1 was obtained as red powder. (a) Receptor 1: Yield : 1.2 g (72%) m.p.: 110 °C. Analytical data for C27H25N3OSFe Calculated (%) C, 65.46; H, 5.09; N, 8.48 Found (%) :C, 65.44; H, 5.07; N, 8.45. IR data (KBr, ν/cm-1): 3250 (NH stretching), 1672 (C = O stretching), 1625 (C = N stretching), 1592 (aromatic C-H), 1322 (C = S stretching), 3080, 1100, 998 & 813 (Ferrocene) ESI Mass (m/z):495.1 (M)+ . 1H NMR (400 MHz, (CD3)2SO, ppm): δ 11.44 (bs, 2H), δ 7.26-6.88 (m, 4H), δ 4.81 (s, 1H), δ 4.72 (s, 1H), δ 4.53 (s, 1H), δ 4.28 (s, 1H), δ 2.41 (s, 3H), δ 1.25 (s, 2H). 13 C NMR (100 MHz, (CD3)2SO, ppm): δ 22.4, 29.7, 69.5, 69.7, 70.3, 72.8, 73.0, 80.3, 109.4-132.8, 141.5, 166.8, 190.1. (b) Receptor 2: Yield : 1.2 g (72%) m.p.: 176 °C. Analytical data for C27H24N4O3SFe Calculated (%):C, 60.01; H, 4.48; N, 10.37 Found (%):C, 60.00; H, 4.46; N, 10.35. IR data (KBr, ν/cm-1): 3312 (NH stretching), 1671 (C = O stretching), 1631 (C = N stretching), 1588 (aromatic C-H), 328 (C = S stretching), 485 & 1345 (NO2 stretching), 3082, 1102, 997 & 815 (Ferrocene). ESI Mass (m/z) 540.1 (M)+. 1H NMR (400 MHz, (CD3)2SO, ppm): δ 10.58 (bs, 2H), δ 7.26-6.60 (m, 4H), δ 4.79 (s, 1H), δ 4.72 (s, 1H), δ 4.54 (s, 1H), δ 4.28 (s, 1H), δ 2.41 (s, 3H), δ 1.25 (s, 2H). 13 C NMR (100 MHz, (CD3)2SO, ppm): δ 20.5, 29.7, 69.5, 69.7, 70.3, 73.0, 73.1, 80.3, 109.4-138.9, 141.6, 166.7, 190.2

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