An Efficient Ferrocene Derivative as a Chromogenic, Optical, and

Article pubs.acs.org/Organometallics

An Efficient Ferrocene Derivative as a Chromogenic, Optical, and Electrochemical Receptor for Selective Recognition of Mercury(II) in an Aqueous Environment Arunabha Thakur and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: The synthesis, electrochemical, optical, and cation-sensing properties of two triazole-tethered ferrocenyl benzylacetate derivatives, C36H36O6N6Fe (2) and C23H23O3N3Fe (3), are presented. The binding event of both the receptors can be inferred either from a redox shift (2, ΔE1/2 = 106 mV for Hg2+ and ΔE1/2 = 187 mV for Ni2+; 3, ΔE1/2 = 167 mV for Hg2+ and ΔE1/2 = 136 mV for Ni2+) or a highly visual output response (colorimetric) for Hg2+, Ni2+, and Cu2+ cations. Remarkably, the redox and colorimetric responses toward Hg2+ are preserved in the presence of water (CH3CN/H2O, 2/8), which can be used for the selective colorimetric detection of Hg2+ in an aqueous environment over Ni2+ and Cu2+ cations. The changes in the absorption spectra are accompanied by the appearance of a new low-energy (LE) peak at 625 nm for both compounds 2 and 3 (2, ε = 2500 M−1 cm−1; 3, ε = 1370 M−1 cm−1), due to a change in color from yellow to purple for Hg2+ cations in CH3CN/H2O (2/8).

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molecular recognition.15−24 Thus, from a synthetic standpoint, ferrocene is a very attractive building block for redox-active ligands because of the robustness of ferrocene under aerobic conditions and the ease of functionalization. These facts, coupled with its electrochemical and UV−vis spectroscopic properties, which can be perturbed by the proximity of bound guests, reveal that ferrocene is an important functional tentacle in this research area. In this article, the host−guest complexation properties of two triazoletethered ferrocenyl benzyl acetate derivatives toward Hg2+ and Ni2+ metal cations have been investigated.

INTRODUCTION The development of selective and sensitive imaging tools capable of monitoring heavy- and transition-metal ions has attracted interest because of their wide use in various fields of science and the subsequent effects of these metals in the environment and nature.1 The Hg2+ ion is considered a highly toxic element, and its detection is currently a task of key importance for environmental and biological concerns.2−5 However, the design and the advancement of new and practical chemosensors, which offer a promising advance for mercuric ion detection, is still a great challenge in supramolecular chemistry.6−11 On the other hand, the Ni2+ ion is also considered as a highly toxic trace element which causes chronic bronchitis and lung and kidney function problems. The progress of multichannel (chromogenic/optical/electrochemical) Ni2+ selective chemosensors is, as far as we know, an unexplored subject and only a very few receptors have been described,12 where the binding event of the Ni2+ ion can be inferred only on the basis of the redox shift of ferrocene unit. As a result, the development of good sensors for the detection of these toxic metal cations through multiple channels is of key importance for the environment and human health.13 To develop sensitive sensors, various receptors consisting of a mercuric ion recognition unit and a probe exhibiting physical responses upon coordination of Hg2+ have been reported.14 One of the most elegant ways of achieving sensor design is to functionalize a receptor capable of both selective substrate binding with a metal cation and reporting on the recognition event through a variety of physical responses. In this context, the design of redox-active receptors in which a change in electrochemical behavior can be used to monitor complexations of guest species is an increasingly important area of © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Materials and Methods. Perchlorate salts of Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Co2+, Cu2+, Fe2+, Zn2+, Cd2+, Ni2+, Pb2+, and Hg2+, propargyl bromide, butyllithium, and tetramethylethylenediamine (TMEDA) purchased from Aldrich were used directly without further purification. Ferrocene, sodium ascorbate, benzyl-2-bromo acetate, sodium azide, and acetonitrile were of analytical grade and were used without 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 2.5 cm diameter column. Column chromatography was carried out using 60−120 mesh silica gels. All the solvents were dried by conventional methods and distilled under a N2 atmosphere before use. Benzyl 2-azidoacetate25 and compounds 1a,b [Fc(CH2OCH2CCH)n]26 (1a, n = 2; 1b, n = 1; Fc = ferrocene) were synthesized as per literature procedures. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed with a conventional three-electrode configuration consisting of glassy carbon as the working electrode, platinum Received: January 1, 2011 Published: January 26, 2012 819

dx.doi.org/10.1021/om201073g | Organometallics 2012, 31, 819−826

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3: 1H NMR (CDCl3, 400 MHz) δ 7.62 (s, 1H, Htriazole), 7.32−7.39 (m, 5H, Ar H), 5.21 (s, 2H, OCH2Ph), 5.16 (s, 2H, NCH2CO), 4.64 (s, 2H, OCH2), 4.35 (S, 2H, OCH2), 4.17 (s, 4H, HCp), 4.08−4.14 (t, 5H, HCp); 13C NMR (CDCl3, 100 MHz) δ 166.17 (OCO), 146.06 (Ctriazole), 134.52 (Ar C), 128.90 (Ar C), 128.80 (Ar C), 128.64 (Ar C), 123.95 (Ctriazole), 82.87 (OCH2Ph), 69.65 (NCH2CO), 68.74 (Ccp), 68.70 (Ccp), 68.53 (Ccp), 68.06 (Ccp), 63.14 (OCH2), 50.84 (OCH2); electrospray MS m/z (relative intensity) 446 (M+ + 1), 469 (M+ + 23). Anal. Calcd for C23H23FeO3N3: C, 62.04; H, 5.21; N, 9.44. Found: C, 62.08; H, 4.95; N, 9.30. X-ray Crystallographic Analysis. Suitable X-ray-quality crystals of 3 were grown by slow diffusion of a hexane/EtOAc (6/4 v/v) solution, and a single-crystal X-ray diffraction study was undertaken. Xray single-crystal data were collected using Mo Kα (λ = 0.710 73 Å) radiation on a Bruker APEX II diffractometer equipped with a CCD area detector. Data collection, data reduction, and structure solution/ refinement were carried out using the software package of SMART APEX. All structures were solved by direct methods 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 3: formula C23H23FeO3N3; crystal system, space group monoclinic, C2/c; unit cell dimensions a = 49.128(3) Å, b = 5.7090(3) Å, c = 36.0273(17) Å, β = 125.257(3); Z = 16. calcd density 1.434 Mg/m3; final R indices (I > 2σ(I)) R1 = 0.0355, wR2 = 0.0474 (all data); index ranges −13 ≤ h ≤ 13, −10 ≤ k ≤ 10, −13 ≤ l ≤ 12, 29 688 reflections collected, 1826 independent reflections, Rint = 0.0705; wR2 = 0.0496 (I > 2σ(I)), R1 = 0.0772, goodness of fit on F2 0.954.

as an auxiliary electrode, and Ag/Ag+ as a reference electrode. The experiments were carried out with a 10−3 M solution of sample in CH3CN 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 s−1. The UV−vis spectra were carried out in CH3CN solutions at c = 1 × 10−4 M. Instrumentation. The 1H and 13C NMR spectra were recorded on Bruker 400 and 500 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 and DPV measurements were performed on a CH Model 660B potentiostat. Caution! Metal perchlorate salts are potentially explosive under certain conditions. All due precautions should be taken while handling perchlorate salts. Synthesis of 2 and 3. To a well-stirred solution of 1a (0.5 g, 1.55 mmol) and benzyl 2-azidoacetate (0.592 g, 3.1 mmol) in 15 mL of acetone/H2O (2/1) was added an aqueous solution of CuSO4·5H2O (0.077 g, 0.31 mmol). To this resultant mixture was added freshly prepared sodium ascorbate solution (0.122 g, 0.62 mmol), and the mixture was stirred at room temperature for 12 h. A 30 mL portion of ethyl acetate was added to the reaction mixture, and the organic layer was washed several times with water and finally with brine (15 mL) and dried over anhydrous sodium sulfate. 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) yielded yellow 2 (0.97 g, 89%). The compound 3 was prepared by following the procedure adopted for 2, where the amounts used were alkyne 1b used 0.5 g (1.96 mmol), benzyl 2-azidoacetate 0.374 g (1.96 mmol), aqueous CuSO4·5H2O 0.097 g (0. 392 mmol), and sodium ascorbate 0.15 g (0.784 mmol). The crude product was purified by silica gel column chromatography and elution with EtOAc/ hexane (7/2 v/v) to yield pure yellow 3 (0.74 g, 84%). 2: 1H NMR (CDCl3, 400 MHz) δ 7.59 (s, 2H, Htriazole), 7.26−7.29 (m, 10H, Ar H), 5.13 (s, 4H, OCH2Ph), 5.11 (s, 4H, NCH2CO), 4.55 (s, 4H, OCH2), 4.25 (s, 4H, OCH2), 4.05−4.12 (d, 8H, HCp); 13C NMR (CDCl3, 100 MHz) δ 165.19 (OCO), 144.77 (Ctriazole), 133.48, (Ar C), 127.81 (Ar C), 127.72 (Ar C), 127.56 (Ar C), 123.06 (Ctriazole), 82.54 (OCH2Ph), 68.96 (NCH2CO), 68.13 (Ccp), 67.33 (Ccp), 66.96 (Ccp), 62.10 (OCH2), 49.77 (OCH2); electrospray MS m/z (relative intensity) 727 (M++23, 100). Anal. Calcd for C36H36FeO6N6: C, 61.37; H, 5.15; N, 11.93. Found: C, 60.96; H, 4.89; N, 11.12.

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RESULT AND DISCUSSION The precursors 1a,b for ferrocenyl benzyl acetate derivatives 2 and 3 were obtained from following literature procedures.26 As shown in Scheme 1, the precursors 1a,b both undergo “click reactions” with benzyl 2-azidoacetate to generate compounds 2 and 3 in 89% and 84% yields, respectively. Compounds 2 and 3 have been characterized by ESI-MS spectrometry, IR and 1H and 13C NMR spectroscopy, and analytical techniques. The solid-state structure of 3 was unambiguously established by Xray analysis and is in full agreement with spectroscopic data. Compounds 2 and 3 are both moderately stable and could be stored in the freezer for months. The complexation properties of the receptors 2 and 3 have been investigated by electrochemistry and UV−vis spectroscopic measurements.

Scheme 1. Synthesis of Mono- and Diferrocene Benzyl Acetate Derivatives 2 and 3

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Figure 1. Molecular structure of 3 with thermal ellipsoids drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): C(Cp)− Fe = 2.016(3)−2.046(3), C11−O1 = 1.434(3)−1.440(3); triazole ring NN = 1.308(3)−1.311(2), N−N = 1.332(3)−1.336(2), N−C = 1.333(4)−1.337(2), N−C = 1.334(3)−1.353(2), C−C = 1.348(3)−1.351(4); ester C−O = 1.310(3)−1.326(3), CO = 1.167(3)−1.184(3), C−O = 1.453(3)−1.467(3), C−C = 1.493(3)−1.510(4); Ar C−C = 1.345(5)−1.392(5), C−C−Fe = 69.47(14)−70.64(17); triazole ring C−C−N = 105.8(2)−108.8(3), NN−N = 106.92(18)−106.98(12); Ar C−C−C = 119.5(4)−120.7(4); ester O−C−O = 125.6(3)−126.3(3).

X-ray Structure Analysis. Single-crystal X-ray analysis revealed that the organometallic compound 3 crystallized in the monoclinic centrosymmetric space group C2/c (Figure 1). The asymmetric unit consists of two molecules of 3, which are crystallographically independent. From the crystal data, it is clear that the C−N bond distances of the five-membered rings for both crystallographically independent molecules are in the ideal range (C−N = 1.307(3)−1.370(4) Å). Moreover, in both molecules one of the N−N bond distances is slightly shorter (NN = 1.311(3)−1.313(3) Å) than the other (N−N = 1.336(2)−1.354(4) Å), which is in fact due to the double-bond character of the triazole ring. The carbonyl CO oxygen of the ester group is involved in hydrogen bonding with CH2 via a C− H···O interaction (C···O = 3.405(5) Å; ∠C−H···O = 150(2)°), that leads to the formation of a one-dimensional (1D) hydrogen-bonded network (Figure 2b). Such 1D networks

sheet (Figure 2c). Interestingly, 2D sheets are packed in a slightly offset manner sustained by a C−H···N interaction (C···N = 3.483(3) Å; ∠C−H···N = 135(4)°) (Figure 2d). Electrochemical Studies. Chemical sensors bearing ferrocene nuclei as part of the sensing unit have been broadly studied.29 Earlier, the complexation of ferrocene with a variety of binding ligands have been studied by cyclic voltammetry and it has shown a positive shift of the Fe(II)/Fe(III) redox couple as a result of metal−ligand complexation.30 The metalrecognition properties of receptors 2 and 3 were evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) analysis. The reversibility and relative oxidation potential of the redox process were determined by CV and DPV in CH3CN solutions containing 0.1 M [(n-Bu)4N]ClO4 as the supporting electrolyte. Compounds 2 and 3 both display a reversible one-electron-oxidation process at E1/2 = 0.494 and 0.482 V, respectively, due to the ferrocene/ferrocenium redox couple. No perturbation of the CV and DPV voltammograms of 2 and 3 were observed in the presence of several metal cations such as Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Zn2+, Co2+, Fe2+, Cd2+, and Pb2+ as their appropriate salts, even in large excess. However, as shown in Figures 3 and 4, the original peak gradually decreased upon stepwise addition of Hg2+ and Ni2+ ions, while a new peak, associated with the formation of a complexed species, appeared at 0.60 and 0.681 V, respectively, for 2. In the case of 3 the peak appears at 0.649 and 0.618 V for Hg2+ and Ni2+ ions, respectively (Supporting Information, Figure S3). DPV experiments show that the peak corresponding to uncomplexed species completely disappeared upon addition of 1 equiv of these metal cations. Interestingly, the redox responses toward Hg2+ cations for compounds 2 and 3 are preserved in the presence of CH3CN/H2O (2/8). Interestingly, as shown in Figure 5, no additional peak appeared upon addition of Cu2+ ion for the compounds 2 and 3 (Supporting Information, Figure S4); only the voltammetric wave shifted toward more cathodic current. In addition, linear sweep voltammetry (LSV) studies were carried out upon addition of Hg2+, Cu2+, and Ni2+ to CH3CN solutions of receptors 2 and 3, which are shown in Figure 6 and in the Supporting Information (Figure S5), respectively. The shift of the voltammetric wave toward cathodic current in the case of the Cu2+ cation indicates that this metal cation promotes the oxidation of the free receptors 2 and 3 with concomitant reduction to Cu+. This is in agreement with the CV as shown in Figure 5 (for 2) and Figure S4 (for 3,

Figure 2. Crystal structure description of 3: (a) molecular structure of 3 with thermal ellipsoids drawn at the 50% probability level (two crystallographically independent molecules were present in the asymmetric unit); (b) 1D hydrogen-bonded network formed by the C−H···O hydrogen-bonding interaction; (c) 2D hydrogen-bonded corrugated sheets via C−H···π; (d) offset packing of 2D sheets via C− H···N interactions.

are further self-assembled via C−H···π (C···π = 3.822(2) Å) interactions27,28 and form a 2D hydrogen-bonded corrugated 821


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Figure 3. Evolution of CV (a) and DPV (b) of 2 (10−3 M) in CH3CN/H2O (2/8) using [(n-Bu)4N]ClO4 as supporting electrolyte when Hg(ClO4)2 is added up to 0−1 equiv.

Figure 4. Evolution of CV (a) and DPV (b) of 2 (10−3 M) in CH3CN using [(n-Bu)4N]ClO4 as supporting electrolyte when Ni(ClO4)2 is added up to 0−1 equiv.

method of continuous variations (Job plot) strongly suggest 1:1 (cation/receptor) complex formation with Hg2+ ion for compound 2 (Figure 7, inset). Further, the stoichiometry of the complex has also been confirmed by ESI-MS, where peaks at m/z 905 for [2·Hg2+] and m/z 646 for [3·Hg2+] are observed (Supporting Information, Figure S8). The binding constant determined from the increasing absorption intensity at 630 nm for 2 and 3 is K (±15%) = 2.35 × 103 and 1.9 × 103 M−1, respectively. Likewise, the addition of increasing amount of Cu2+ ions to a solution of 2 and 3 showed progressive appearance of one new LE band at 637 nm for 2 and 625 nm for 3. Well-defined isosbestic points at 350 nm for 2 and 442 nm for 3 were found, indicating that only one spectrally distinct complex was present. The new LE bands at 637 and 625 nm are accountable for the change of color from yellow to bluish green. The UV−vis spectral change, shown in Figure 8, 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.31 Note that there was no significant spectral

shown in the Supporting Information). In contrast, the same experiment was carried out upon addition of Hg2+ and Ni2+ cations, which revealed a shift of the LSV toward positive potential for the receptor 2 (Figure 6b,c), which is in agreement with the complexation process previously observed by CV (Figures 3a and 4a). UV−Visible Absorption Studies. The UV−vis binding interaction studies of receptors 2 and 3 in CH3CN (1 × 10−4 M) against cations of environmental relevance, such as Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Zn2+, Co2+, Fe2+, Cd2+, and Pb2+ as perchlorate salts, show selective response to Hg2+, Ni2+, and Cu2+. The change in the UV−vis absorbance spectra of receptors 2 and 3 in CH3CN due to the stepwise addition of Hg2+ ion are shown in Figure 7 and in the Supporting Information (Figure S7), respectively. Upon addition of 1 equiv of Hg2+ to 2 the high-energy (HE) absorption band at λ 432 nm disappeared and a new peak at ca. 294 nm appeared. Further, as shown in Figure 7, a new and weak lower-energy (LE) absorption band appeared at λ 625 nm (ε = 2500 M−1 cm−1) for 2, due to the change in color. Binding assays using the 822


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change observed upon addition of Cu+ and Ag+ cations, which indicates that these cations do not promote the

oxidation of the ferrocene moiety (Supporting Information, Figure S9). The change in the UV−vis absorbance spectra of receptors 2 and 3 in CH3CN due to the stepwise addition of Ni2+ ion are shown in Figure 9. Upon addition of 1 equiv of Ni2+, the high-energy (HE) absorption band at λ 361 nm was reduced with the concomitant increase of the peak at λ 435 nm (ε = 1380 M−1 cm−1) for the receptors 2 and 3. Well-defined isosbestic points at ca. 395 and 289 nm indicated the presence of a unique complex with a neutral ligand. Binding assays using the method of continuous variations (Job plot) clearly indicate 1:1 (cation/receptor) complex formation with Ni2+ ion for both compounds 2 and 3 (Supporting Information, Figure S10). The binding constants determined32 from the increasing absorption intensity at 630 nm for 2 and 3 are K (±15%) = 2.6 × 103 and 2.36 × 103 M−1, respectively. Visual Detection of Hg2+, Ni2+, and Cu2+. When an excess of different metal cations (Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Zn2+, Ni2+, Fe2+, Co2+, Cd2+, Hg2+, and Pb2+) as their perchlorate salts are separately added to solutions of 2 and 3 in CH3CN (10−3 M) no significant color change was observed, except for Hg2+, Ni2+, and Cu2+. As shown in Figure 10, Hg2+ shows a drastic color change from yellow to purple and Ni2+ from yellow to pale blue, whereas Cu2+ shows a color change

Figure 5. Evolution of the CV of 2 (10−3 M) in CH3CN using [(nBu)4]ClO4 as supporting electrolyte upon addition of increasing amounts of Cu2+ metal cation up to 1 equiv. Arrows indicate the movement of the wave during the experiments.

Figure 6. Evolution of LSV of 2 (10−3 M) in CH3CN during addition of (a) Cu2+, (b) Ni2+, and (c) Hg2+ using [(n-Bu)4N]ClO4 as supporting electrolyte and a scan rate of 0.1 V s−1. 823


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absorption of the 2−Cu2+ system is influenced by the Ni2+ and Hg2+ ions, but Cu2+ did not interfere with the absorption of 2− Ni2+ and 2−Hg2+. Except for Hg2+ ion, the absorption intensity of 2 in the presence of 10 equiv of the Ni2+ ion was almost unaffected by the addition of 10 equiv of competing metal ions (for example, Li+, Na+, K+, Ca2+, Mg2+, Cr2+, Co2+, Cu2+, Fe2+, Zn2+, Cd2+, and Pb2+). On addition of an equal amount of Hg2+ ion into a solution of 2·Ni2+ 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+ and Ni2+ ion. In addition, 2 can be utilized for the detection of Ni2+ ion in the presence of other competing ions, except Hg2+ ion. Reversibility 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 (10−4 M) and Hg2+ (2 equiv). The experiment, shown in Figure 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 was 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. On the other hand, the reversibility of the complexation process of Ni2+ ions was performed by an extraction experiment,33 which specifies the high level of reversibility of the complexation/decomplexation process (Supporting Information, Figure S11). 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 CD3CN solution. 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.34 The most significant spectral changes (1H NMR; Supporting Information, Figure S12) 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 broadening of the −CH2COO and −OCH2Ph protons in the 1H NMR spectrum; (ii) the hydrogen atom within the triazole ring showed

from yellow to bluish green (due to the oxidation of ferrocene to ferrocenium ion). The sensing potential of 3 toward Hg2+, Ni2+, and Cu2+ in solution is very similar to that of 2. Most remarkably, as shown in the Figure 11, the colorimetric response toward Hg2+ is preserved in the presence of water. Thus, addition of Hg2+ cations to a solution of receptor 2 or 3 in CH3CN/H2O (2/8) induced a red shift of the absorption band from 312 to 326 nm (Δλ = 14 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 an aqueous environment, because no changes were observed in the absorption spectrum after addition of either Ni2+ or Cu2+ cations. The competition experiments were carried out for the interaction of 2 between Hg2+, Ni2+, and Cu2+ in pure CH3CN solution in order to ascertain the selectivity. It shows that the

Figure 7. Changes in the absorption spectra of 2 (10−4 M) in CH3CN/H2O (2/8) upon addition of increasing amounts of Hg2+ up to 1 equiv. Inset: Job plot of 2 with Hg2+ cations, indicating the formation of a 1:1 complex.

Figure 8. Changes in the absorption spectra of (a) 2 and (b) 3 (10−4 M) in CH3CN upon addition of increasing amounts of Cu2+ up to 1 equiv. 824


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Figure 9. Changes in the absorption spectra of (a) 2 and (b) 3 (10−4 M) in CH3CN upon addition of increasing amounts of Ni2+ up to 1 equiv.

Figure 10. Visual features observed in CH3CN solution of 2 (10−3 M) after addition of 10 equiv of different metal cations tested.

a significant downfield shift by ca. 0.10 ppm; (iii) the observed downfield shifts for the cyclopentadienyl ring hydrogen atoms in ferrocene were not prominent. Similar types of chemical shifts were also observed for the Ni2+ metal ion. The above metal-ioninduced chemical shift changes support that metal cations (Hg2+ and Ni2+) are bound to the triazole (N1) ring and to the −OCH2 units of the ester group.

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CONCLUSION In conclusion, we have described two simple triazole-based, easy-to-synthesize, multisignaling chemosensors 2 and 3, which selectively bind with Hg2+ and Ni2+ through multiple channels. Interestingly, the redox and colorimetric response toward Hg2+ are preserved in the presence of water, which can be used for the selective colorimetric (naked-eye) detection of Hg2+ in an aqueous environment over other cations, including the strong competitors Ni2+, Pb2+, and Cd2+. To the best of our knowledge, the ferrocene-based ligand 2 is the rare example of a dual electrochemical−optical Ni2+ ion sensor in which the Fc/Fc+ redox couple is significantly shifted (ΔE1/2 = 187 mV) on complexation, with a concomitant change in color from yellow to blue, that allows its potential use for “naked eye” detection.

Figure 11. Visual color changes observed in a CH3CN/H2O (2/8) solution of 2 (10−3 M) after addition of Hg2+, Ni2+, and Cu2+ cations.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving 1H and 13C NMR and ESI-MS data of 2 and 3, electrochemical data for 2 and 3 upon titration with Cu2+ and different metal ions, UV−vis spectra upon titration with different metal ions, ESI-MS spectrum of [2·Hg2+] and [3·Hg2+], evolution of 1H NMR spectra of 2 in CD3CN upon addition of increasing amounts of Hg2+, reversibility

Figure 12. 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+. 825


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experiments of 2 with Ni2+ ions, and quantitative binding data for 2 and 3 with Hg2+ and Ni2+ and a CIF file giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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(24) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. Engl. 2001, 40, 485. (25) The benzyl 2-azidoacetate was prepared from the corresponding bromide using NaN3. (26) Thakur, A.; Adarsh, N. N.; Chakraborty, A.; Devi, M.; Ghosh, S. J. Organomet. Chem. 2010, 695, 1059. (27) Nishio, M. Cryst. Eng. Commun. 2004, 6, 130. (28) Nishio, M.; Umezawa, Y.; Honda, K.; Tsuboyamad, S.; Suezawa, H. Cryst. Eng. Commun. 2009, 11, 1757. (29) (a) Zhu, X. J.; Fu, S. T.; Wong, W. K.; Guo, J. P.; Wong, W. Y. Angew. Chem. 2006, 45, 3222. (b) Molina, P; Tárraga, A.; Caballero, A. Eur. J. Inorg. Chem. 2008, 22, 3401. (30) (a) López, J. L.; Tárraga, A.; Espinosa, A.; Velasco, M. D.; Molina, P.; Lloveras, V.; Vidal-Gancedo, J.; Rovira, C.; Veciana, J.; Evans, D. J.; Wurst, K. Chem. Eur. J. 2004, 10, 1815. (b) Martínez, R.; Espinosa, A.; Tárraga, A.; Molina, P. Org. Lett. 2005, 7, 5869. (c) Zapata, F.; Caballero, A.; Espinosa, A.; Tárraga, A.; Molina, P. Org. Lett. 2007, 9, 2385. (31) Lloveras, V.; Caballero, A.; Tárraga, A.; Velasco, M. D.; Espinosa, A.; Wurst, K.; Evans, D. J.; Vidal-Gancedo, J.; Rovira, C.; Molina, P.; Veciana, J. Eur. J. Inorg. Chem. 2005, 2436. (32) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703. (33) One equivalent of [Ni(ClO4)2] was added to a CH2Cl2 solution of receptor 2 to yield the complex [2·Ni2+], and the UV−vis spectrum was recorded. The CH2Cl2 solution of the complex was washed with water several times to obtain pure compound 2 in the organic layer. The organic layer was dried, and the optical spectrum was recorded; it was found to be identical with that of the free receptor 2. On further addition of 1 equiv of [Ni(ClO4)2] to this solution the initial UV−vis spectrum of [2·Ni2+] was completely recovered. This experiment was carried out over several times, and the UV−vis spectrum was recorded after each step. (34) Live, D.; Chan, S. I. J. Am. Chem. Soc. 1976, 98, 3769.

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*Tel: (+91) 44 2257 4230. Fax: (+91) 44 2257 4202. E-mail: [email protected].

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ACKNOWLEDGMENTS Generous support of the Indo-French Centre for the Promotion of Advanced Research (IFCPAR-CEFIPRA), No. 4405-1, New Delhi, India, is gratefully acknowledged. A.T. is grateful to the Council of Scientific and Industrial Research (CSIR) of India for research fellowships.

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