A triazole tethered triferrocene derivative as a selective chemosensor for mercury(II) in aqueous environment

Polyhedron 52 (2013) 1109–1117

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A triazole tethered triferrocene derivative as a selective chemosensor for mercury(II) in aqueous environment Dipendu Mandal, Arunabha Thakur, Sundargopal Ghosh ⇑ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

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Article history: Available online 3 July 2012 Dedicated to Professor Alfred Werner on the occasion of the 100th anniversary of his Nobel Prize in Chemistry Keywords: Hg2+ chemosensor Triferrocene Aqueous environment 1,2,3 Triazole ring

a b s t r a c t The synthesis, electrochemical, optical and cation-sensing properties of two triazole tethered ferrocene derivatives, C25H25ON3Fe2, 2 and C40H40O2N6Fe3, 3 are presented. The solid state structure of compound 2 has been established by X-ray diffraction analysis which reveals that the unit cell of molecule 2 consists of 3-D helical chain formed via CH  N interaction and p  p stacking. The complexation properties of the receptors can be inferred either from redox shift or visual output response (colorimetric) for Hg2+ and Cu2+ cations. The common structural feature of these ligands is the presence of other ferrocene moiety as redox unit. Interestingly, the redox and colorimetric responses, towards Hg2+ are preserved in the presence of water (CH3CN/H2O, 2/8), which can be used for the selective colorimetric detection of Hg2+ in aqueous environment over other competitor cations. The changes in the absorption spectra are accompanied by the appearance of a new low energy (LE) peak at ca. 626 nm for 2 and 632 nm for 3 (2: e = 669 M1 cm1 and 3: e = 1150 M1 cm1), due to the change in color from yellow to purple for Hg2+ cations in CH3CN/H2O (2:8). Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The heavy metal contamination, in particular mercury, has severe harmful effects on human health and the environment. The contamination of mercury is a global problem and it arises from a variety of natural sources, such as combustion of fossil fuel, gold mining, oceanic and volcanic eruptions [1–3]. Therefore, to increase the understanding of mercury pollution, efforts are being made to develop new mercury-sensing strategies that can detect mercury ions even in very small quantities in the environment [4]. Although efforts have been made to develop several chemosensors for the detection of Hg2+, the design and advancement of new and practical chemosensors that offer a promising advance for mercuric ion detection is still a great challenge in supramolecular chemistry [5–10]. Such mercury sensors should display high solubility in water and a high selectivity for mercury ions against a background of challenging analytes. However, most of these molecules have limitations due to the interference from other contending metal ions as well as low water solubility. As a result, not many molecules are known that selectively sense mercury ion in aqueous environment. With a vision to advancing chemical sensor technology, considerable interest is being shown in the synthesis of redox-active receptors that contain a redox centre in close proximity to a cation ⇑ Corresponding author. Tel.: +91 44 2257 4230; fax: +91 44 2257 4202. E-mail address: [email protected] (S. Ghosh). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.06.060

binding site [11–17]. In this context, ferrocene and their derivatives are found to be very convenient building block for redoxactive ligands. Ferrocene derivatives in which the cyclopentadienyl rings bear same substituents have attracted substantial interest, owing to their usefulness in the synthetic application of large ferrocene-based assemblies [18–26]. Current investigations in our laboratory focus on the development of new synthetic methods to make redox-active receptors for mercury ion [27,28]. As a part of comprehensive exploration of new mercury-sensing strategies that can detect mercury ions in very small quantities, we recently reported two ferrocene–glycine conjugates that behave as very selective redox, chromogenic and fluorescent chemosensor for Hg2+ in aqueous environment [27,28]. Here in this article, we demonstrate the synthesis, characterization and metal cation complexation properties of two new triferrocene derivatives 2 and 3 through multiple channels.

2. Experimental 2.1. Materials, methods and instrumentation Perchlorate salts of Li+, Na+, K+, Ca2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Cu , Zn2+, Cd2+, Ni2+, Pb2+, and Hg2+, propargyl bromide, butyl-lithium, tetramethylethylenediamine (TMEDA) were purchased from Aldrich and used directly without further purification. Ferrocene, allyl bromide, NaH, copper iodide, 1,8-Diazabicyclo[5.4.0]undec7-ene (DBU), acetonitrile (HPLC) were purchased and used without 2+

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further purification. DMF was purchased from Aldrich and freshly distilled prior to use. Chromatography was carried out on 3 cm of silica gel in a column of 2.5 cm diameter. Column chromatography was carried out using 100–200 mesh silica gel. All the solvents were dried by conventional methods and distilled under Ar atmosphere before use. Compounds [Fc(CH2OCH2CCH)n], [29] 1a–b (1a: n = 1 and 1b: n = 2; Fc = ferrocene) and ferrocene methyl azide [30,31] were synthesized as per literature procedures. The cyclic voltammetry (CV) was performed with a conventional three-electrode configuration consisting of glassy carbon as working electrode, platinum as an auxiliary electrode and Ag/Ag+ as a reference electrode. The experiments were carried out with a 104 M solution of sample in CH3CN or CH3CN/H2O (2:8) containing 0.1 M [(n-C4H9)4NClO4] (TBAP) as supporting electrolyte. Deoxygenation of the solutions was achieved by bubbling nitrogen for at least 10 min, and the working electrode was cleaned after each run. The cyclic voltammograms were recorded at a scan rate 0.1 V s1. The UV–Vis spectra were carried out in CH3CN/H2O (2:8) solutions at c = 1  104 M. The 1H and 13C NMR spectra were recorded on Bruker 400 MHz FT-NMR spectrometers, using tetramethylsilane as the internal reference. Electrospray ionization mass spectrometry (ESI-MS) measurements were carried out on a Qtof Micro YA263 HRMS instrument. The absorption spectra were recorded with a JASCO V-650 UV–Vis spectrophotometer at 298 K. The CV measurements were performed on a CH potentiostat model 668.

Caution! Metal perchlorate salts are potentially explosive in certain conditions. All due precautions should be taken while handling perchlorate salts!

2.2. Synthesis of 2 A solution of the (azidomethyl) ferrocene (0.4 g, 1.66 mmol) and the mono alkynyl ferrocene derivative, 1a (0.4 g, 1.66 mmol) in dry DMF was degassed three times with Ar. The degassed and diluted catalyst solution (Catalyst solution: CuI:DBU (1:1 M)) was added to it and the resulting solution was heated at 60 °C for 4 h. The reaction mixture was diluted with dichloromethane and the organic layer was separated and washed several times with excess of water/methanol (1:1 v/v). The solvent was removed under reduced pressure and the crude product was purified by silica gel column chromatography. Elution with EtOAc/hexane (8:2 v/v) to yield pure yellow 2 (0.65 g, 83%). 2: 1H NMR (400 MHz, CDCl3, 22 °C): d = 7.36 (s, 1H, Htriazole), 5.25 (s, 2H, OCH2), 4.54 (s, 2H, OCH2), 4.26 (s, 2H, NCH2), 4.17– 4.20 (m, 18H, HCp); 13C NMR (CDCl3, 100 MHz, 22 °C): d = 145.4, 122.0, 80.8, 70.3, 69.7, 69.2, 69.1, 69.0, 68.9, 68.6, 67.5, 63.4, 50.2; UV–Vis (nm): 305, 434; ESI-MS (relative intensity): m/z 496 (M++1); Anal. Calc. for C25H25Fe2ON3: C, 60.64; H, 5.09; N, 8.49. Found: C, 61.43; H, 4.90; N, 8.43.

Scheme 1. Synthesis of compounds 2 and 3.

Fig. 1. Molecular structure of 2 with thermal ellipsoid drawn at the 30% probability level. Selected bond length (Å) and angles (°): C(Cp)–Fe 2.010(4)–2.038(4), C12–O1 1.448(5), C13–O1 1.397(5), [triazole ring N@N = 1.321(6), N–N 1.328(5), N–C 1.319(6), N–C 1.350(6), C–C 1.332(5), C16–N3 1.470(5); Fc–C–O 111.8(3), C–C–Fe 70.64(17), [triazole ring C–C–N 109.4(4), N@N–N 106.3(4), N2–N3–C15 118.9(4)].

D. Mandal et al. / Polyhedron 52 (2013) 1109–1117

1111

Fig. 2. Crystal structure description of 2: left handed helical chain in ball and stick model displaying C–H  p interactions (black dotted line) (a) and spacefill model (b) along crystallographic axis ‘c’; triple helical motif [shown in red, orange and black] (c); interaction of triple helices displaying the 3D network structure along crystallographic axis ‘c’ (d) and ‘a’ (e) [triple helices are shown in red-orange-black and blue-green-purple]. (Color online.)

2.3. Synthesis of compound 3 Compound 3 was prepared in good yield following the procedure adopted for 2 from (azidomethyl) ferrocene (1.44 g, 6.03 mmol) and the di-alkynyl ferrocene derivative, 1b (0.4 g, 1.24 mmol). The crude product was purified by silica gel column chromatography and elution with EtOAc/hexane (9:1, v/v) to yield pure yellow 3 (0.84 g, 85%). 3: 1H NMR (CDCl3, 400 MHz, 22 C): d = 7.37 (s, 2H, Htriazole), 5.26 (s, 4H, OCH2), 4.51 (s, 4H, OCH2), 4.30 (s, 4H, NCH2), 4.11–4.26 (m, 26H, HCp); 13C NMR (CDCl3, 100 MHz, 22 °C): d = 145.1, 121.9, 83.3, 80.7, 69.9, 69.08, 69.0, 68.9, 68.4, 68.1, 63.2, 50.0; UV–Vis (nm):

309, 434; ESI-MS, m/z (relative intensity): 805 (M++1); Anal. Calc. for C40H40Fe3O2N6: C, 59.73; H, 5.01; N, 10.45. Found: C, 59.51; H, 4.98; N, 10.36. 2.4. X-ray crystallographic analysis Suitable X-ray quality crystals of 2 were grown by slow diffusion of a hexane: EtOAc (6:4 v/v) solution and single crystal Xray diffraction study was undertaken. X-ray single crystal data were collected using Mo Ka (k = 0.71073 Å) radiation on a BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/refinement were carried

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0.000035

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Fig. 3. Evolution of CV (a) and DPV (b) of 2 (104 M) in CH3CN using [(n-Bu)4N]ClO4 as the supporting electrolyte when 0–1 equiv of Hg(ClO4)2 is added.

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Fig. 4. Evolution of (a) CV and (b) LSV of 3 (104 M) in CH3CN using [(n-Bu)4N]ClO4 as the supporting electrolyte when 0–1 equiv of Hg(ClO4)2 is added.

(b) 0.00004

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Fig. 5. Evolution of CV of 2 (a) and 3 (b) (104 M) in CH3CN using [(n-Bu)4N]ClO4 as the supporting electrolyte when 0–1 equiv of Hg(ClO4)2 is added.

out using the software package of SMART APEX. All structures were solved by direct method and refined in a routine manner. In most of the cases, non-hydrogen atoms were treated anisotropically. Whenever possible, the hydrogen atoms were located on a difference Fourier map and refined. In other cases, the hydrogen atoms were geometrically fixed. Crystal data for 2: Formula, C25H25Fe2ON3; Crystal system, space group: monoclinic, C2/c. Unit cell dimensions, a = 8.7962(2) Å, b = 9.4869(2) Å, c = 13.1578(4) Å, b = 96.6590(10); Z = 2. Dcalc 1.508 mg/m3. Final R indices [I > 2r(I)] R1 = 0.0358, wR2 = 0.0965 (all data). Index ranges 13 6 h 6 13, 10 6 k 6 10, 13 6 l 6 12.

Reflections collected 4254, independent reflections 3884, Rint = 0.0410; wR2 = 0.0925 (I > 2r(I)), Goodness-of-fit on F2 1.062.

3. Results and discussion 3.1. Synthesis As shown in Scheme 1, compounds 1a and 1b undergo the ‘‘click reaction’’ with mono (azidomethyl) ferrocene to produce compounds 2 and 3 in 83% and 85% yields respectively. Both the

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Potential (v) Fig. 6. Evolution of LSV of 2 and 3 (104 M) in CH3CN during the addition of (a–b) Hg2+ and (c) Cu2+ with [(n-Bu)4N]ClO4 as the supporting electrolyte scanned at 0.1 V s1.

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Fig. 7. Changes in the absorption spectra of 2 (104 M) in CH3CN upon the addition of increasing amounts of Hg2+ up to 1 equiv.

compounds 2 and 3 have been characterized by ESI-mass spectrometry, 1H and 13C NMR spectroscopy and elemental analysis. In addition, the solid state structure of compound 2 has been unambiguously established by X-ray diffraction analysis. Both the compounds 2 and 3 are highly stable and could be stored at 25 °C for months. The complexation properties of both the receptors have been investigated by electrochemistry, UV–Vis spectroscopic measurements and 1H NMR titration.

3.2. X-ray structural analysis Single crystal X-ray analysis revealed that the organometallic compound 2 crystallized in monoclinic non-centrosymmetric chiral P21 space group (Fig. 1). The asymmetric unit consists of one molecule of 2. Interestingly, the C–H  p interactions involving the ferrocene rings leads to the formation of left handed helical chain (Fig. 2a and b). One of such helix is further interact with

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Fig. 8. Changes in the absorption spectra of 3 (10

M) in CH3CN upon the addition of increasing amounts of Hg2+ up to 1 equiv.

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Fig. 9. Changes in the absorption spectra of 2 (a) and 3 (b) (104 M) in CH3CN upon the addition of increasing amounts of Cu2+ up to 1 equiv.

Fig. 10. Visual features observed in CH3CN solution of 2 (103 M) after addition of 10 equiv of different cation tested as perchlorate salt.

other two helices via C–H  N, C–H  p and p  p stacking and form a triple helix (Fig. 2c). Such triple helices are further interacts with each other via weak interactions to form a 3D network (Fig. 2d and e). 3.3. Electrochemical studies Earlier, the complexation of ferrocene with a variety of binding ligands, particularly 1,2,3 triazole has been studied by cyclic vol-

tammetry (CV), and it showed a positive shift of the FeII/FeIII redox couple as a result of metal ligand complexation [32–37]. Furthermore, few ferrocene based triazole ligands have been reported as a metal cation sensor. The metal-recognition properties of receptors 2 and 3 were evaluated by CV and DPV analysis. The reversibility and relative oxidation potential of the redox process were determined by CV and DPV in CH3CN (104 M) solutions containing 0.1 M [(n-Bu)4N]ClO4 as the supporting electrolyte. Under these conditions, receptor 2 exhibited two reversible one-electron redox

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IET

½L  Cu2þ   ½Lþ  Cuþ   Lþ þ Cuþ

Fig. 11. Visual color changes observed in CH3CN/H2O (2:8) solution of 2 (103 M) after addition of Hg2+ and Cu2+ cation.

0.55 0.50

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0.45 Abs. (319 nm)

3.5 3.0 Absorbance

waves at the half-wave potential values of E11/2 = 0.421 V and E21/ 2 = 0.424 V whereas the receptors 3 exhibited one reversible redox waves at E1/2 = 0.437 V. For 2, the first oxidation wave corresponds to the monosubstituted ferrocene moiety close proximity to the triazole ring and the second oxidation wave to another ferrocene moiety which is attached with more electronegative oxygen atom [38,39]. No perturbation of the cyclic and differential pulse voltammograms of 2 and 3 were observed in the presence of other metal cations such as Li+, Na+, K+, Ca2+, Mg2+, Cr2+, Zn2+, Ni2+, Fe2+, Co2+, Cd2+, and Pb2+ as their appropriate salts, even in large excess. However, as shown in Figs. 3 and 4, the original peak gradually decreased upon the stepwise addition of Hg2+ ions, while a new peak, associated with the formation of a complexed species, appeared at 0.444 V (DE11/2 = 0.02 V) and 0.461 V (DE21/2 = 0.04 V) for 2. Receptor 3 also showed perturbation of the oxidation peaks in the presence of Hg2+ ions and a new peak at 0.513 V (DE1/ 2 = 0.076 V) appeared. Likewise, the DPV voltammogram also exhibits two oxidation peaks at the same potential values as CV for the receptor 2. Interestingly, as shown in the Fig. 5, no additional peak appeared upon addition of Cu2+ ion for the compounds 2 and 3, only the voltammetric wave shifted towards more cathodic current. In addition, linear sweep voltammetry (LSV) studies, shown in Fig. 6, carried out upon the addition of Cu2+ to a CH3CN solution of receptor 2 showed a significant shift of the voltammetric wave toward the cathodic current, indicating that this metal cation promotes oxidation of the free receptor with its incidental reduction to Cu+. Furthermore, studies about the formation of copper(II) complexes with nitrogen donor ligands bearing ferrocenyl pendants revealed that upon addition of Cu2+ ion to the free ferrocenyl receptor, L a [LCu2+] complex have been formed rapidly at the initial stage. However, in a second step, a slower intramolecular electron–electron transfer process, between Fe2+–Cu2+ and Fe3+–Cu+ valence tautomers occurs [40–43] which leads to the formation of a [L+Cu+] complex. Further, [L+Cu+] complex undergoes rapid decomposition to give the oxidized free ligand L+ together with Cu+ (Eq. (1)). On contrary, the same experiment carried out upon the addition of Hg2+ cations revealed a shift of the LSV toward a more positive potential (Fig. 6a–b), which is in agreement with the complexation process previously observed by CV and DPV (Figs. 3 and 4).

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3.4. UV–Vis absorption study The UV–Vis binding interaction studies of receptors 2 and 3 in CH3CN (104 M) against cations of environmental relevance, such as of Li+, Na+, K+, Ca2+, Mg2+, Cr2+, Zn2+, Fe2+, Ni2+, Co2+, Cd2+ and Pb2+ as perchlorate salts, show selective response to Hg2+ and Cu2+. The changes in the UV–Vis absorbance spectra of both the receptors in CH3CN due to the stepwise addition of Hg2+ ion are shown in the Figs. 7 and 8. Upon the addition of 1 equiv of Hg2+ to 2, the high-energy (HE) absorption band at k = 434 nm (e = 3010 M1 cm1) got disappeared and a new peak at ca. 305 (e = 640 M1 cm1) nm appeared. The HE band at k = 330 nm got disappeared with concomitant diminution of one other HE band at 436 nm (e = 1250 M1 cm1) in the case of 3. In addition, as shown in Figs. 7 and 8, a new and weak lower-energy (LE) absorption band appeared at k = 632 nm for both 2 (e = 669 M1 cm1) and 3 (e = 1150 M1 cm1). These facts are responsible for the change of color from yellow to purple, which is visible to the naked eye. Binding assays using the method of continuous variations (Job’s plot) strongly suggest 1:1 (cation/receptor) complex formation with Hg2+ ion both for compounds 2 (Fig. 7b) and 3. The

Fig. 12. The reversibility of the interaction between 2 and Hg2+ by the introduction of I to the system. (Inset) Stepwise complexation/decomplexation cycles carried out in CH3CN with 2 and Hg2+.

stoichiometries of the complexes have also been confirmed by ESI-MS, where peaks at m/z 718 for the [2-Hg2+] complex (Supporting information, Fig. S4). The binding constant determined [44] from the increasing absorption intensity at 626 and 632 nm is K = 1.46 ± 0.26  103 M1 and 2.21 ± 0.32  103 M1 for 2 and 3 respectively (Supporting information, Fig. S5). Similarly, the addition of increasing amount of Cu2+ ions to a solution of 2 and 3 showed progressive appearance of one new LE band at 626 nm for 2 and 632 nm for 3. Two well-defined isosbestic points at 415 and 462 nm for 2 were found indicating that only one spectral distinct complex was present. The new LE bands at 626 and 632 nm are accountable for the change of color from yellow to bluish green. The UV–Vis spectral change, shown in Fig. 9, suggests that the ferrocene moiety is oxidized upon interaction with Cu2+ ion and the change of color to green is characteristic of the formation of ferrocenium ion [45]. Note that: there was no significant spectral change observed upon addition of Cu+ and Ag+ cations, which indicates that these cations do not promote the oxidation of ferrocene moiety.

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3.5. Visual detection of Hg2+ and Cu2+ When an excess of different metal ions (Li+, Na+, K+, Ca2+, Mg2+, Cr , Zn2+, Ni2+, Co2+, Fe2+, Cd2+, Hg2+, and Pb2+) is separately added to the solution of 2 and 3 in CH3CN (103 M), no significant color change is observed, except for Hg2+ and Cu2+. As shown in Fig. 10, Hg2+ shows a drastic color change from yellow to purple, whereas Cu2+ shows a color change from yellow to bluish green. The sensing potential of 3 toward Hg2+ and Cu2+ in solution is very similar to 2. Most remarkably, as shown in the Fig. 11, the colorimetric response toward Hg2+ is preserved in the aqueous environment. Thus, addition of Hg2+ cations to a solution of receptor 2 or 3 in CH3CN/H2O (2:8) induced the appearance of a new peak at 305 and 626 nm along with reduction of one HE band at k = 434 nm. These changes are responsible for the change of color from yellow to purple that can be used for the selective colorimetric detection of Hg2+ in aqueous environment, because no changes were observed in the absorption spectrum after addition of either Cu2+ or other competitor cations. The competition experiments were carried out for interaction of 2 between Hg2+ and Cu2+ in pure CH3CN solution in order to ascertain the selectivity. It shows that the absorption of the 2-Cu2+ system is influenced by the Hg2+ ion, but Cu2+ did not interfere with the absorption of 2-Hg2+ system. Except for Hg2+ ion, the absorption intensity of 2 in the presence of 10 equiv of the Cu2+ ion was almost unaffected by the addition of 10 equiv of competing metal ions for example, Li+, Na+, K+, Ni2+, Ca2+, Mg2+, Cr2+, Co2+, Cu2+, Fe2+, Zn2+, Cd2+ and Pb2+). On addition of an equal amount of Hg2+ ion into a solution of [2Cu2+] complex, the absorption intensity was shown to be dominated by the Hg2+ ion. Therefore 2 could be used for the detection of Hg2+ in the presence of other competing metal ions, including Cu2+ ion. In addition, 2 can be utilized for the detection of Cu2+ ion in the presence of other competing ions except Hg2+ ion. This experiment could be repeated several

times and it clearly shows the strong affinity of receptor 2 towards Hg2+ ion.

2+

3.6. Reversibility of the interaction of 2 and Hg2+ The reversible interaction between 2 and 3 with Hg2+ was confirmed by the introduction of I into the system containing 2 (104 M) and Hg2+ (2 equiv). The experiment, shown in Fig. 12, showed that the introduction of 5 equiv of I into Hg2+ immediately quenched the absorption of 2. When Hg2+ was further added to the system, the absorption of 2 enhanced again. This process could be repeated several times without loss of sensitivity of the absorbance, which clearly demonstrates the high degree of reversibility of the complexation/decomplexation process between 2 and Hg2+ ions. To support the results obtained by electrochemical and spectroscopic experiments, and to get an insight about the coordination mode of these metal cations by receptor 2, we performed the 1H NMR spectroscopic analysis in d6-DMSO solution. The binding of metal ion by the molecule containing 1,2,3 triazole ring is well documented in the literature [46–51]. The binding of metal ion by a host molecule is accompanied by conformational and electronic changes, which in turn are reflected by variations in the 1H chemical shifts of those centers close to the site of complexation [52]. The most significant spectral changes, shown in Fig. 13, observed upon addition of increasing amounts of Hg2+ ions to a solution of the free receptor 2 are the following: (i) Hg2+ metal ions caused considerable amounts of chemical shifts of the –CH2OCH2-triazole and –CH2OCH2-triazole protons in the 1H NMR spectrum by 0.32 and 0.25 ppm respectively; (ii) the hydrogen atom within the triazole ring showed a significant downfield shift by ca. 0.85 ppm and (iii) the observed downfield shifts for the cyclopentadienyl ring hydrogen atoms in ferrocene were not prominent. The above metal ion-induced chemical shift changes support that metal cations,

Fig. 13. 1H NMR titration of 2 upon addition of increasing amount of Hg2+ up to 1 equiv.

D. Mandal et al. / Polyhedron 52 (2013) 1109–1117

Hg2+ are bound to the triazole (N1) ring and to the oxygen atom of ether group. 4. Conclusion In conclusion, we have described the synthesis and structural characterization of one simple helical fashioned diferrocene and one symmetrically substituted tri-ferrocenes derivative and studied their properties as highly selective Hg2+ chemosensors through electrochemical and optical probes. The unit cell packing of 2 is remarkable as it form triple helix superstructure via CH  N and p  p stacking interaction. These chemosensors not only exhibit the capability of highly selective detection of Hg2+ cation through electrochemical or optical probe but also comply with the facile colorimetric sensing of Hg2+ cation, thus allowing the perceptibility to ‘‘naked-eye’’ detection over some other cations. Acknowledgements Generous support of the Board of research in nuclear science (BRNS), No. 2011/37C/54/BRNS, Government of India is gratefully acknowledged. A.T. is grateful to the Council of Scientific and Industrial Research (CSIR), India for research fellowships.

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

[31] [32] [33]

Appendix A. Supplementary data CCDC 878827 contains the supplementary crystallographic data for compound 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.poly.2012.06.060. References [1] F.Z. Renzoni, E. Franchi, Environ. Res. 77A (1998) 68. [2] Update: Impact on Fish Advisories Mercury. EPA Fact Sheet EPA-823-F-01-011; EPA. Office of Water, Washington DC, 2001. [3] O. Malm, Environ. Res. 77 (1998) 73. [4] 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. [5] A. Caballero, R. Martinez, V. Lloveras, I. Ratera, J. Vidal-Gancedo, K. Wurst, A. Tarraga, P. Molina, J. Veciana, J. Am. Chem. Soc. 127 (2005) 15666. [6] C. Diez-Gil, A. Caballero, I. Ratera, A. Tarraga, P. Molina, J. Veciana, Sensors 7 (2007) 3481. [7] J. Huang, Y. Xu, X. Qian, J. Org. Chem. 74 (2009) 2167. [8] H. Lu, L. Xiong, H. Liu, M. Yu, Z. Shen, F. Li, X. You, Org. Biomol. Chem. 7 (2009) 2554. [9] J. Gong, T. Zhou, D. Song, L. Zhang, X. Hu, Anal. Chem. 82 (2010) 567. [10] F. Loe-Mie, G. Marchand, J. Berthier, N. Sarrut, M. Pucheault, M. BlanchardDesce, F. Vinet, M. Vaultier, Angew. Chem., Int. Ed. 49 (2010) 424. [11] J.M. Lehn, Angew. Chem., Int. Ed. Engl. 27 (1988) 89.

[34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]

[46] [47] [48] [49] [50] [51] [52]

1117

T. Saji, J. Kinoshita, J. Chem. Soc., Chem. Commun. (1986) 716. G.W. Gokel, Chem. Soc. Rev. 21 (1992) 39. P.D. Beer, Adv. Inorg. Chem. 39 (1992) 79. A.E Kaifer, S. Mendoza, in: G. Gokel (Ed.), Comprehensive Supramolecular Chemistry, Vol. 1, Pergamon, Oxford, 1996, p. 701. P.L. Boulas, M. Gómez-Kaifer, L. Echegoyen, Angew. Chem., Int. Ed. Engl. 37 (1998) 216. P.D. Beer, P.A. Gale, G.Z. Chen, Coord. Chem. Rev. 186 (1999) 3. D. Seyferth, H.P. Withers Jr., Organometallics 1 (1982) 1275. M.E. Wright, Organometallics 9 (1990) 853. L.-L. Lai, T.-Y. Dong, J. Chem. Soc., Chem. Commum. (1994) 2347. G. Iftime, C. Moreau-Bossuet, E. Manoury, G.G.A. Balavoine, J. Chem. Soc., Chem. Commun. (1996) 527. T.-Y. Dong, L.-L. Lai, J. Organomet. Chem. 509 (1996) 131. M.C. Grossel, D.G. Hamilton, T.A. Vine, Tetrahedron Lett. 38 (1997) 4639. Y. Xu, H.B. Kraatz, Tetrahedron Lett. 42 (2001) 2601. J.G. Rodríguez, S. Pleite, J. Organomet. Chem. 230 (2001) 637. J. Chiffre, F. Averseng, G.G.A. Balavoine, J.C. Daran, G. Iftime, P.G. Lacroix, E. Manoury, K. Nakatani, Eur. J. Inorg. Chem. (2001) 2221. A. Thakur, S. Sardar, S. Ghosh, Inorg. Chem. 50 (2011) 7066. A. Thakur, S. Ghosh, Organometallics 31 (2012) 819. A. Thakur, N.N. Adarsh, A. Chakraborty, M. Devi, S. Ghosh, J. Organomet. Chem. 695 (2010) 1059. F. Pérez-Balderas, M. Ortega-Munoz, J. Morales-Sanfrutos, F. HernándezMateo, F.G. Calvo-Flores, J.A. Calvo-Asín, J. Isac-García, F. Santoyo-González, Org. Lett. 5 (2003) 1951. M.J. Casas-Solvas, A. Vargas-Berenguel, F.L. Capitán-Vallvey, F. SantoyoGonzález, Org. Lett. 6 (2004) 3687. T. Romero, R.A. Orenes, A. Espinosa, A. Tárraga, P. Molina, Inorg. Chem. 50 (2011) 8214. J.L. Lopez, A. Tarraga, A. Espinosa, M.D. Velasco, P. Molina, V. Lloveras, J. VidalGancedo, C. Rovira, J. Veciana, D.J. Evans, K. Wurst, Chem. Eur. J. 10 (2004) 1815. R. Martinez, A. Espinosa, A. Tarraga, P. Molina, Org. Lett. 7 (2005) 5869. F. Zapata, A. Caballero, A. Espinosa, A. Tarraga, P. Molina, Org. Lett. 9 (2007) 2385. A.J. Evans, S.E. Watkins, C. Craig, S.B. Colbran, Dalton Trans. (2002) 983. K.T. Potts, M. Keshavarz-K, F.S. Tham, H.D. Abruna, C.R. Arana, Inorg. Chem. 32 (1993) 4422. A. Thakur, N.N. Adarsh, A. Chakraborty, M. Devi, S. Ghosh, J. Organomet. Chem. 695 (2009) 1059. D.W. Hall, C.D. Russel, J. Am. Chem. Soc. 89 (1967) 2316. X. Cui, H.M. Carapuca, R. Delgado, M.G.B. Drew, V. Felix, Dalton Trans. (2004) 1743. F. Oton, A. Tarraga, A. Espinosa, M.D. Velasco, P. Molina, Dalton Trans. (2006) 3685. M. Sato, M. Katada, S. Nakashima, H. Sano, J. Chem. Soc., Dalton Trans. (1990) 1979. H. Plenio, C. Aberle, Y.A. Shihadeh, J.M. Lloris, R. Martnez-Manez, T. Pardo, J. Soto, Chem. Eur. J. 7 (2001) 2861. H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703. V. Lloveras, A. Caballero, A. Tarraga, M.D. Velasco, A. Espinosa, K. Wurst, D.J. Evans, J. Vidal-Gancedo, C. Rovira, P. Molina, J. Veciana, Eur. J. Inorg. Chem. (2005) 2436. H. Struthers, T.L. Mindt, R. Schibli, Dalton Trans. 39 (2010) 675. Y.H. Lau, P.J. Rutledge, M. Watkinson, M.H. Todd, Chem. Soc. Rev. 40 (2011) 2848. K.J. Kilpin, E.L. Gavey, C.J. McAdam, C.B. Anderson, S.J. Lind, C.C. Keep, K.C. Gordon, J.D. Crowley, Inorg. Chem. 50 (2011) 6334. T. Romero, R.A. Orenes, A. Espinosa, A. Tarraga, P. Molina, Inorg. Chem. 50 (2011) 8214. F. Otón, M.del.C. González, A. Espinosa, A. Tárraga, P. Molina, Organometallics 31 (2012) 2085. S. Badèche, J.-C. Daran, J. Ruiz, D. Astruc, Inorg. Chem. 47 (2008) 4903. D. Live, S. Chan, J. Am. Chem. Soc. 98 (1976) 3769.