Triazolyl Alkoxy Fischer Carbene Complexes in Conjugation with

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Triazolyl Alkoxy Fischer Carbene Complexes in Conjugation with Ferrocene/Pyrene as Sensory Units: Multifunctional Chemosensors for Lead(II), Copper(II), and Zinc(II) Ions Joseph Ponniah S, Subrat Kumar Barik, Arunabha Thakur, R. Ganesamoorthi, and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: The regioselective 1,3-dipolar cycloaddition reaction of alkoxy alkynyl Fischer carbene complex 1 with azidomethyl ferrocene 2 and with azidomethyl pyrene 4 under solvent-free conditions yielded the triazolyl Fischer carbene complexes 3 (C27H21O6N3FeW) and 5 (C33H21O6N3W), respectively. The cation complexation properties of these receptors have been systematically studied using electrochemical and spectroscopic techniques. The exceptional structural feature existing in these receptors is the presence of a Fischer carbene moiety, connected to the ferrocene or pyrene moiety through a 1,2,3-triazole ring. Receptor 3 contains a redox-active ferrocene moiety and is highly selective toward Pb2+ ion, whereas receptor 5, having a fluorescent pyrene unit, selectively recognizes Zn2+ and Cu2+ ions. The binding ability of receptor 3 can be inferred either from the redox shift (the anodic shift ΔE1/2 = 55 mV) or the highly visual output response for Pb2+ ion. Receptor 5 displays considerable chelationenhanced fluorescence (CHEF) upon binding with Zn2+ and Cu2+ ions in an aqueous environment. Further, the proposed binding modes of these receptors and their metal cation complexation properties have been supported by 1H NMR titration and MALDI-MS and a DFT study.

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INTRODUCTION The recognition of toxic heavy- and transition-metal ions such as Hg2+, Pb2+, Zn2+, Cd2+, and Cu2+ has attracted considerable attention from current researchers.1−4 Among the heavy- and transition-metal ions, Fe2+, Zn2+, and Pb2+ are abundant because of their various natural sources and industrial uses. Zn2+ ion is the second most abundant metal ion in the human body after iron and is a fundamental element in natural biological systems.5 It plays critical roles in many biological processes, such as gene expression, apoptosis, metalloenzyme regulation, and neurotransmission.6 Severe neurological diseases, including Alzheimer’s, ischemia, and epilepsy,7 are associated with disorder of Zn2+ metabolism. Failure to maintain zinc homeostasis has been implicated in a number of severe neurological diseases.8 It plays a key role in the synthesis of insulin and the pathological state of diabetes.9 The demand for sensing Zn2+ cations, which are spectroscopically soundless because of their electronic configuration (3d104s0), in competitive media such as Ca2+ and Mg2+ is growing rapidly.10 Most importantly, as cadmium and zinc have similar chemical properties, they usually display similar spectral changes upon interactions with chemosensors. Thus, it is still a great challenge for scientists to develop chemosensors that can differentiate Zn2+ from Cd2+. In this pursuit, a number of fluorescent,11 colorimetric,11k,r−t and redox12 selective Zn2+ chemosensors have been designed and synthesized. In order to provide evidence for the sensing properties of artificial receptors toward particular metal cations, in general traditional analytical techniques such as UV−visible spectrosco© 2014 American Chemical Society

py and cyclic voltametric measurements have been used. However, the recognition and sensing properties of these receptors for metal cations using IR spectroscopy are very rare. Keeping this objective in mind, we focused on the synthesis of a Fischer carbene based ferrocene triazolyl derivative, where [W(CO)5] acts as an active IR fragment. Despite the development of these classical single-signaling approximations, there is a scarcity of use of multichannel receptors as potential guest reporters via multiple signaling patterns. On the other hand, Pb2+, the second most toxic environmental pollutant after Hg2+, is largely used in the automobile industry, batteries, gasolines, and pigments and is responsible for a range of adverse health problems. A variety of symptoms have been attributed to lead poisoning, including anemia, memory loss, muscle paralysis, and particularly mental retardation of children.13 Thus, keeping in view the role of Zn2+ and Pb2+, the detection and monitoring of these metal cations by sensitive and selective chemosensors in various media is of considerable importance. Therefore, the development of suitable chemosensors for detection of these toxic metal cations has become indispensable in this field during the past few years. Current investigations in our laboratory have focused on the development of new and efficient redox-active receptors for heavy-metal ions.14−17 As a part of a comprehensive exploration of new heavy-metal sensing strategies that can detect metal ions Received: March 30, 2014 Published: June 10, 2014 3096

dx.doi.org/10.1021/om5003396 | Organometallics 2014, 33, 3096−3107

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Scheme 1. Synthesis of Triazolyl Fischer Carbene Complexes Based on Ferrocene and Pyrene Units

Figure 1. Molecular structure of receptor 3 with thermal ellipsoids drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): C11−N1 = 1.467(4), C12−N1 = 1.344(4), N2−N3 = 1.304(4), C19−C20 = 1.472(4), C19−N3 = 1.368(4), C20−O1 = 1.310(4), C20−W1 = 2.181(3); O1−C20−W1 = 131.0(2), O1−C20−C19 = 105.0(2), triazole ring N1−C12−C19 = 103.9(3), N3−N2−N1 = 107.6(2), N3−C19−C20 = 121.9(3).

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RESULTS AND DISCUSSION As shown in Scheme 1, the alkoxy alkynyl Fischer carbene complex 1 undergoes a regioselective [3 + 2] cycloaddition reaction with (azidomethyl)ferrocene (2) and (azidomethyl)pyrene (4) to afford the triazolyl Fischer carbene complexes 3 (C27H21O6N3FeW) and 5 (C33H21O6N3W), respectively. Although the procedure has been adopted from recently published work by Sarkar et al.,18 it is superior to earlier known methods for cycloaddition, as it does not require any organic solvent and catalyst. The strongly polarized triple bond of the carbene complex is probably accountable for promoting this reaction without an additive, although the rate may not be competitive with a Cu-catalyzed “click” reaction.18 Compounds 3 and 5 have both been characterized by MALDI-MS, 1H and 13C NMR, and IR analysis. In addition, the solid-state structure of compound 3 has been unambiguously established by X-ray diffraction analysis and is fully in agreement with spectroscopic data. Compounds 3 and 5 are air stable and can be stored at 8−10 °C for months. The metal cation sensing properties of the

in very small quantities, we recently reported triazole appended to various ferrocene-based chemosensors, which behave as very selective redox, chromogenic, and fluorescent chemosensors for various toxic metal cations in an aqueous environment.14−17 We have demonstrated that the selectivity as well as the sensitivity of these receptors highly depends on the nature of the ligands present in the cyclopentadienyl rings.14−17 Traditional analytical techniques such as UV−visible spectroscopy, cyclic voltametric measurements, and fluorescence studies have been used mostly for the detection of toxic metal cations. In this paper, we have explored an efficient and versatile method18 to provide an easy access to triazolyl Fischer carbene complexes via the [3 + 2] cycloaddition reaction of ferrocenyl azide or pyrene azide with an alkynyl Fischer carbene complex. Further, the structural characterization and host−guest complexation properties of these new receptors have been investigated via multiple channels in an aqueous environment. 3097


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Figure 2. (a) UV−vis absorbance spectra of 3 (10−5 M) upon addition of up to 1 equiv of Pb2+ ion in CH3CN/H2O (5/5). (b) Changes in the absorption spectra of 3 (10−5 M) in CH3CN/H2O (5/5) upon addition of several metal cations.

Figure 3. UV−vis absorbance spectra of 5 (10−5 M) upon addition of up to 1 equiv of (a) Cu2+ ion and (b) Zn2+ ion in CH3CN/H2O (5/5).

receptors 3 and 5 have been investigated by optical (UV−vis and fluorescence spectroscopy) and electrochemical methods, 1H NMR titration, and DFT studies. X-ray Structure Analysis of Compound 3. Suitable X-rayquality 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. X-ray single-crystal data were collected using Mo Kα (λ = 0.71073 Å) radiation on a Bruker APEX II diffractometer equipped with a CCD area detector. The data collection, data reduction, and structure solution/refinement were carried out using the SMART APEX software package. The solid-state structure of compound 3 was solved by direct methods and refined in a routine manner.

The single-crystal X-ray analysis revealed that the organometallic compound 3 crystallized in the monoclinic noncentrosymmetric chiral space group P21/c (Figure 1). The asymmetric unit consists of one molecule of 3. The two cyclopentadiene rings in compound 3 are arranged in a eclipsed form. The geometry around the tungsten metal is distorted octahedral with angles ranging from 84.42 to 98.92°. The phenyl and triazole rings are arranged in a perpendicular fashion with an angle of 89.59°. Further, the crystal structure of 3 reveals that the triazole ring attached to the carbene carbon of the complex is oriented nearly orthogonal to the metal carbene σ plane. As shown in Figure 1, all of the C−N bond distances in the fivemembered triazole ring (average ∼1.344 Å) are in the normal range. The N2−N3 bond distance is slightly shorter in 3098


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Figure 4. Evolution of (a) LSV and (b) CV of 3 (1 × 10−4 M) upon addition of up to 1 equiv of Pb2+ ion with [(n-Bu)4N]ClO4 as supporting electrolyte. The scan rate employed was 0.1 V s−1.

Figure 5. Evolution of the color in CH3CN solution of 3 (top) and 5 (below) (10−5 M) after addition of 5 equiv of different cations tested as perchlorate salts.

gradual addition of 1 equiv of Pb2+ ion to 3, the high-energy (HE) absorption band at λ 206 nm (ε = 2770 × 102 M−1 cm−1) is increased and the absorption band centered at 240 nm (ε = 1036 × 102 M−1 cm−1) is decreased. During the titration two isosbestic points at ca. 226 and 255 nm were observed, indicating that only one spectrally distinct [3·Pb2+] complex was formed. The binding assay using the method of continuous variation (Job plot) suggests 1/1 cation/receptor complex formation of 3 with Pb2+ ion (Figure 2b, inset). No changes were observed in the UV−vis spectrum (Figure 2b) upon addition of the aforementioned several metal cations even in large excess. The changes in the UV−vis absorbance spectrum of receptor 5 in CH3CN/H2O (5/5) upon stepwise addition of Zn2+ ion are shown in Figure 3b. The addition of Cu2+ ion to a solution of 5 showed progressive appearance of one new HE band at 225 nm (ε = 3460 × 102 M−1 cm−1) with a concomitant decrease of two lower energy bands at ca. 295 nm (ε = 720 × 102 M−1 cm−1) and

comparison with N1−N2, which may be due to the double-bond character of N2−N3. The triazole ring in 3 is out of the plane with a dihedral angle of 75.14° with respect to the Cp plane. This small distortion may be caused by the repulsion between the neighboring protons of the triazole and the ferrocene proton. All of the terminal carbonyls are almost linear, and the WC bond (2.182 Å) in compound 3 is significantly shorter than that in C18H15O6NW (2.245 Å).19 UV−Visible Absorption Studies. The UV−vis binding interaction studies of receptors 3 and 5 were performed in CH3CN/H2O (5/5, 1 × 10−5 M) against cations of environmental relevance, such as Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Zn2+, Ni2+, Co2+, Fe2+, Tl+, Hg2+, Cd2+, Cu2+, and Pb2+ as perchlorate salts. However, compound 3 shows an absorption response in the presence of Pb2+ ion. The changes in the UV−vis absorbance spectra of receptor 3 in CH3CN/H2O (5/5) due to the stepwise addition of Pb2+ ion are shown in Figure 2a. Upon 3099


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Figure 6. (a) Fluorescence emission (λexc 353 nm) of the free ligand 5 (c = 1.0 × 10−7 M) upon addition of several metal cations in CH3CN/H2O (5/5). (b) Bar diagram of the relative maximum intensity of the ligand 5 upon addition of several cations.

350 nm (ε = 447 × 102 M−1 cm−1) (Figure 3a). Two isosbestic points at 267 and 294 nm were found which indicate the presence of only one distinct complex. Furthermore, the stoichiometry of these complexes have been confirmed by MALDI-MS (Supporting Information, Figures S2−S4) and a Job plot (Supporting Information, Figure S5). The peaks for [3·Pb2+], [5·Cu2+], and [5·Zn2+] were observed at m/z 1129.61, 1029.16, and 1031.12, respectively. The binding constant K (±15%) determined from the increasing absorption intensity for 3 with Pb2+ is 2.72 × 104 M−1 (Supporting Information, Figure S6) and the constants for 5 with Zn2+ and Cu2+ ions are 0.68 × 104 and 2.1 × 104 M−1 (Supporting Information, Figures S7 and S8), respectively. Electrochemical Studies. Chemical receptors having ferrocene nuclei as part of the sensing unit have been broadly studied.20 Earlier, the complexation of ferrocene with different binding sites was studied by cyclic voltammetry, and this study showed a positive shift of the Fe(II)/Fe(III) redox couple as a result of metal−ligand complexation.21 The metal recognition properties of receptor 3 were also evaluated by cyclic (CV), linear sweep (LSV), and differential pulse voltammetry (DPV) analysis in CH3CN solutions containing 0.1 M [(n-Bu)4N]ClO4 as supporting electrolyte. The receptor 3 displays a redox process at E1/2 = 0.44 V due to the ferrocene/ferrocenium redox couple. However, as shown in Figure 4, the original peak gradually decreased upon stepwise addition of Pb2+ ion, while a new peak, associated with the formation of a complexed species, appeared at 0.608 V (ΔE1/2 = 55 mV) for 3. The net anodic shifts are consistent with the formation of a complexed species with Pb2+ ion. The linear sweep voltammetry study (Figure 4a) and the differential pulse voltammetry study (Supporting Information, Figure S9) further revealed results similar to those obtained from CV. No perturbations of the CV, LSV, and DPV voltammograms of 3 were observed in the presence of other metal cations such as Li+, Na+, K+, Ca2+, Mg2+, Cr2+, Zn2+, Ni2+, Fe2+, Co2+, Tl+, Cd2+, and Hg2+ as their appropriate salts. Visual Detection of Pb2+, Cu2+ and Zn2+. When excesses of different metal ions (Na+, Mg2+, Ni2+, Co2+, Fe2+, Pb2+, Mn2+,Cd2+, Cu2+ Hg2+and Zn2+) as their perchlorate salts were separately added to a solution of 3 in CH3CN (10−5 M), no

significant color change was observed except for Pb2+ ion. As shown in Figure 5, Pb2+ shows a distinct color change from yellow to yellowish orange. No significant color change was observed for solutions of 5 except for Zn2+ and Cu2+ ions. The solution color changed moderately from yellow to light orange for Zn2+ ion, and in the case of Cu2+ the solution color drastically changed from yellow to green. More remarkably, the colorimetric responses toward Pb2+, Cu2+, and Zn2+ are preserved in the presence of water as well. Fluorescence Studies. Due to its operational simplicity, high sensitivity, and low cost, fluorescence detection has become the most promising strategy for the detection of Zn2+ ion. The amount to which the fluorescence intensity of receptor 5 was affected in the presence of selected cations was tested by fluorescence spectroscopy. When receptor 5 is excited at λexc 353 nm, it exhibits a weak fluorescence in CH3CN/H2O (5/5) solution (c = 1.0 × 10−7 M). The emission band at 375 nm is attributed to a pyrene monomeric emission. The emission spectrum of receptor 5 shows bands at 375 nm with quantam yield ϕ = 0.002. However, in the presence of Cu2+ and Zn2+ ions, the emission band yielded an important enhancement of the fluorescence emission (CHEF = 21 for Cu2+ and 7 for Zn2+). The quantum yield (ϕ = 0.03) resulted in a 15-fold increase for Cu2+ in comparison to that of free receptor 5 and a 8-fold increase for Zn2+ in comparison to that of 5 (ϕ = 0.016)22a (Figure 6b). The fluorescence enhancement is highly desirable, because many reported fluorescent chemosensors generally undergo fluorescence quenching upon binding with these metal ions via spin− orbit coupling22b or energy or electron transfer.23a Selective fluorescence enhancement by Zn2+ and Cu2+ could be due to the effective coordination of Zn2+ with the nitrogen atom of the triazole ring as well as the oxygen atom of the alkoxy group of 5 over the other metal ions. This restricts the PET23b process and enhances the fluorescence output of 5 via chelation-induced fluorescence enhancement. The mole ratio method was applied to determine the stoichiometries of complexes [5·Cu2+] and [5· Zn2+], which were found to be 1:1 for both cases. Note that, as shown in Figure 6, the fluorescent behavior of receptor 5 did not undergo any considerable changes upon addition of other metal cations. 3100


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Figure 7. Fluorescence emission change of 5 (1.0 × 10−7 M) upon addition of up to 1 equiv of (a) Cu2+ ion and (b) Zn2+ ion in CH3CN/H2O (5/5).

metal ions. Thus, a 5-fold excesses of a number of metal ions (Pb2+, Ni2+, Hg2+, Cd2+, Mg2+, Ca2+, Co2+, Fe2+) were added to solutions of 5 separately, 1 equiv of Zn2+ ion was added into each solution, and then the emission spectra were recorded. In all cases, the addition of Zn2+ resulted in an increase of the fluorescence response of the solution except for Cu2+ ion. In the case of Cu2+, the fluorescence intensity is retained by Zn2+ ion (Figure 8). Furthermore, the addition of 1 equiv of Cu2+ ion to a

To focus on the sensitivity of the receptor 5, we have performed fluorescence titrations of 5 (1 × 10−7 M) in CH3CN/ H2O (5/5) with Cu2+ and Zn2+ ions (1 × 10−7 M) separately (Figure 7). There was no detectable change observed in the fluorescence spectra up to the analyte concentration of 0.15 equiv of Cu2+ ion. An appreciable enhancement of quantum yield by a factor of 5 was observed in the presence of 0.2 equiv of Cu2+ ion, whereas the maximum fluorescence enhancement was observed in the presence of 1 equiv of Cu2+ ion. This experiment shows that the lowest detectable limit of Cu2+ with 5 is 2 × 10−9 M (2 ppb). The enhancement of fluorescence intensity, produced by increasing the concentration of Cu2+ ion from 0 to 2 nM in the solution of receptor 5, is resolved. This shows the increase of the signal to noise ratio of fluorescence emission. The intensity data were normalized between the minimum intensity and the maximum intensity. A linear curve was fitted to the five intermediate values, and the point at which the line crossed the ordinate axis was taken as the detection limit24 and equaled approximately 2 ppb (Supporting Information, Figure S10). Similarly, the detection limit for Zn2+ ion has also been evaluated, and it is equal to 1.7 × 10−7 M (Supporting Information, Figure S11). River water samples were examined by the proposed fluorimetric method under optimized conditions of 0.1 × 10−7 and 1.0 × 10−7 M 5 in an aqueous environment (CH3CN/H2O, 5/5). The binding constant values of 5 with metal ions (Cu2+ and Zn2+) have also been determined from the emission intensity data following the modified Benesi−Hildebrand equation.25a,b From the plot of (Imax − Imin)/(I − Imin) against [C]−1, the value of K (±15%) extracted from the slope is of the same order as that obtained from UV−vis data. The fluorescence response of 5 in the presence of an increasing concentration of Zn2+ ion is shown in Figure 7b. Addition of Zn2+ ion (0−1 equiv) to a solution of 5 induces a remarkable ratiometric change with an increase of monomer and excimer emission intensity. This may be attributed to the flexibility of the triazole moiety of 5 to adopt the appropriate geometry for binding to the Zn2+ ion. The relative intensity ratio of monomer to excimer emission (M375/E494) of the free receptor 5 was 0.66 and increased to 1.16 upon the addition of 1 equiv of Zn2+ (Figure 7b), which is ascribed to formation of a [5·Zn2+] complex. Another important feature for a superior sensor is its ability to retain selectivity and sensitivity in the presence other competitive

Figure 8. Bar plot representation of the fluorescence emission intensity of 5 upon addition of competitive cations in CH3CN/H2O (5/5).

solution of 5 conatining 1 equiv of Zn2+ ion does not display any significance enhancement of CHEF. Therefore, the receptor 5 could be used for the detection of Zn2+ in the presence of other competing metal ions, except for Cu2+ ion. Reversibility Interaction between 3 and Pb2+ and between 5 and Zn2+. For a chemosensor to be widely employed in the detection of specific analytes, reversibility is an important feature. The reversible interaction between 5 and Zn2+ was confirmed by an extraction experiment, as shown in Figure 9. The reversibility was checked by addition of water to a dichloromethane solution of the complex [5·Zn2+]. The fluorescence spectrum of the dichloromethane layer was recorded after the extraction of the metal ion in the aqueous 3101


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Figure 9. (a) Fluorescene spectra showing the reversibility of the interaction between 5 and Zn2+ by the extraction method in CH2Cl2: (yellow) spectrum of the free ligand 5; (red) spectrum after complexation with Zn2+ ion; (pale green) spectrum after extraction with water, showing the reversibility of the complexation process. (b) Stepwise complexation/decomplexation cycles carried out in CH3CN/H2O (5/5) with 5 and Zn2+.

the NMR time scale; therefore, the receptor 3 and complex [3· Pb2+] are observed individually. Moreover, the considerable downfield shift in the 1H NMR signal for protons Ha (Δδ = +0.11 ppm), Hb (Δδ = +0.08 ppm), Hc (Δδ = +0.10 ppm), and HFc (Δδ = +0.10 ppm) indicated that the Pb2+ ion is recognized by the nitrogen atom of the triazole ring and oxygen atom of the OCH2CH3 group (Figure 11). As shown in Figure 12, the most significant 1H NMR spectral changes observed upon the addition of increasing amounts of Zn2+ to a solution of free receptor 5 are as follows: (i) the CH2 proton (Ha) attached to triazole ring is significantly shifted by 0.10 ppm, which probably indicates the coordination of Zn2+ ion with the nitrogen atoms of the triazole rings, and (ii) the Hb and Hc protons are shifted by 0.12 and 0.07 ppm, indicating the coordination of Zn2+ ion with the oxygen atom of the OCH2CH3 group. From the magnitude of the observed 1H chemical shifts, it can be concluded that the plausible binding mode of Zn2+ is the O atom and N atoms of the triazole ring (Figure 13). Computational Details. Computational calculations were carried out to find out the mode of complexation of the Pb2+ ion with L = 3 and of Zn2+ and Cu2+ with L= 5. The formation of a 1:1 [3·Pb2+] complex has already been established by MALDIMS and other techniques. Hence, the mononuclear complexes [L.M]2+ were modeled by DFT calculations. As a result, L and [L· M]2+ were optimized by DFT using the Gaussian 09 package, as explained in the Experimental Section. The starting geometry for receptor 3 was obtained from the crystal structure, and several different starting geometries were used for the geometry optimization for receptor 5 to confirm that the optimized structures correspond to global minima. The corresponding optimized structures are shown in Figures 14 and 15. The geometry optimization for 3 resulted in conformational changes of the arms of the ferrocene unit in order to accommodate the Pb2+ ion. In this process, the nitrogen atom of the triazole moiety and oxygen atom of the OCH2CH3 group enable easy chelation of the metal cation with its two ClO4− counteranions. The minimum-energy structure for the resulting [3·Pb(ClO4)2] complex (Figure 14) features the Pb center in a highly distorted geometry that deviates from a square-planar as well as a tetrahedral binding core. In this binding situation Pb2+ adopts N1O3 coordination by bonding through one nitrogen atom from

layer, and it was found to be the same as that of the free ligand. This process was repeated at least two times without loss of sensitivity of the fluorescence intensity, which clearly demonstrates the high degree of reversibility of the complexation/ decomplexation process between 5 and Zn2+ ion. The interaction between 3 and Pb2+ was reversible, which was confirmed by the introduction of I− into a system containing 3 and Pb2+ in CH3CN. The experiment, detailed in Figure 10,

Figure 10. Reversibility of the interaction between 3 and Pb2+ by the introduction of I− to the system.

showed that after addition of I− (2 equiv to Pb2+) the absorption intensity of 3 was quenched immediately. When Pb2+ was added to the system again, the absorption of 3 was enhanced. This process could be repeated at least two times without loss of sensitivity of the absorbance, which demonstrates the high degree of reversibility of the complexation/decomplexation process between 3 and Pb2+ ion. To seek more detailed information on the mode of binding of receptor 3 with metal ion, a 1H NMR titration experiment was carried out in a CD3CN solvent. As shown in Figure 11, upon gradual addition of Pb2+ salt (up to 1 equiv) to a solution of 3, the resonances corresponding to the protons on receptor 3 were shifted downfield. This result was ascribed to the complexed and decomplexed forms of receptor 3, with Pb2+ being slower than 3102


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Figure 11. 1H NMR titration of 3 upon addition of increasing amount of Pb2+ up to 1 equiv.

Figure 12. 1H NMR titration of 5 upon addition of increasing amounts of Zn2+, up to 1 equiv.

the triazole ring and three oxygen atoms from two ClO4−

(WBI(Pb−OClO4−) = 0.474), and d(Pb−OCH2CH3) = 3.16764 Å (WBI(Pb−O) = 0.101). Similarly, the DFT results predict the optimized geometry for 5 in order to accommodate Zn2+ and Cu2+ ions. In metal complex [5·Zn2+], the Zn2+ center shows a distorted-square-pyramidal geometry binding core in an N1O4 coordination fashion by bonding through one nitrogen atom from the triazole unit,

counteranions. However, there is a weak van der Waals interaction between Pb2+ and the oxygen atom of the OCH2CH3 group. This tetrahedral environment consists of one N donor atom, belonging to the triazole unit: d(Pb−N) = 2.4674 Å (WBI(Pb−N) = 0.395), d(Pb−OClO4−) = 2.2366 Å 3103


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Figure 13. Plausible binding mode for 3 with Pb2+ ion and for 5 with Zn2+ ion.

Figure 14. B3LYP/6-31g*/LANL2DZ optimized structure of free ligand L and mononuclear complex [L·Pb2+].

Figure 15. B3LYP/6-31g*/LANL2DZ optimized structure of free ligand L = 5 and mononuclear complexes [L·Zn2+] and [L·Cu2+].

= 0.440) in [5·Zn2+]. The analogous [5·Cu2+] complex was also computed at the same level of theory, where the Cu2+ prefers a lesser distorted square pyramidal geometry. Detailed bond lengths and WBI values are provided in the Supporting Information (Table S1).

d(Zn−Ntriaz) = 2.0447 Å (WBI(Zn−N) = 0.296) and four oxygen atoms (one from the OCH2CH3 and rest from two ClO4− groups) where d(Zn−OCH2CH3) = 2.4506 Å (WBI(Zn− O) = 0.183) and the average distance of three perchlorate anions with metal is d(Zn−OClO4−) = ca. 2.035 Å (WBI(Zn−OClO4−) 3104


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128.6, 128.3, 80.8, 77.3, 76.8, 76.7, 48.2, 13.8. IR (CH2Cl2, cm−1): 2995, 2911, 2066, 1989, 1955, 1933, 1907, 1532, 1478, 1448, 1400, 1363, 1255, 1228, 1016, 915. MS (MALDI): m/z [M + H] + for C27H21O6N3FeW calculated 724.1609, observed 724.6990. Synthesis of Compound 5. Complex 1 (0.25 g, 0.52 mmol) and 4 (0.20 g, 0.78 mmol) were placed in a two-neck round-bottom flask that was thoroughly degassed with argon for 30 min. The reaction mixture was stirred at 40 °C for 12 h, and the progress of the reaction was monitored by TLC. Upon completion of the reaction the crude product was purified using silica gel column chromatography. Elution with EtOAc/hexane (2/8, v/v) yielded orange-red 5 (0.15g, 40%). Data for 5 are as follows. 1H NMR (CDCl3, 500 MHz): δ 7.85−8.13 (m, 5H, Hphenyl), 7.29−7.33 (m, 9H, Hpyrene), 5.29 (s,2H, Cp-CH2), 4.59−4.63 (q, 2H, −OCH2CH3), 0.87−0.89 (t, 3H, −OCH2CH3). 13C NMR (125 MHz, CDCl3): 299.1, 204.6, 197.9, 157.2, 133.7, 133.4, 130.8, 129.8, 129.7, 129.4, 129.1, 128.9, 128.6, 128.2, 127.0, 126.2, 125.1, 122.8, 78.5, 50.4, 13.8. IR (CH2Cl2, cm−1): 2996, 2913, 2066, 1992, 1955, 1933, 1907, 1532, 1478, 1448, 1412, 1363, 1255, 1228, 1016, 915. MS (MALDI): m/z [M + CH3CN]+ for C33H21N3O6W.CH3CN calculated 780.4325, observed 780.3335. Crystal data for 3: formula C27H21O6N3FeW; crystal system monoclinic, space group P21/c; unit cell dimensions a = 11.7886(5) Å, b = 21.5070(10) Å, c = 11.3200(5) Å, and β = 111.980(2); Z = 4; calcd density 1.805 Mg/m3; μ = 1.805 mm−1; F(000) = 1408; 20634 reflections measured, 6972 of which were unique; goodness of fit F2 = 1.076; θ range for data collection 1.89−31.26°; final R indices (I >2σ(I)) R1 = 0.0280 and wR2 = 0.0562; index ranges −14 ≤ h ≤ 13, −28 ≤ k ≤ 30, −9 ≤ l ≤ 16; Rint = 0.0281; wR2 = 0.0617. Computational Details. Computational calculations were carried out to find the mode of complexation of these metal ions with ligands 3 and 5. As the formation of a 1:1 (L:M2+) complex had already been established by MALDI-MS and was also supported by other techniques, the species were optimized by computational calculations based on DFT28 using the Gaussian0929 package. All of the geometry optimizations were carried out in the gas phase with tight convergence criteria, using the B3LYP30−32 functional together with the 6-31g* basis set for nonmetallic atoms, e.g., C, H, N, O, and LANL2DZ effective core potentials (ECPs) 33 for all transition-metal centers. Bonding interactions were studied by WBI34 values from NBO analysis.35 The nature of the optimized stationary point was confirmed by analytical computation of harmonic force constants at the aforementioned DFT level. The optimized structures were generated by using Gaussview software.36

CONCLUSIONS In summary, we have synthesized the triazole-appended alkoxy Fischer carbene ferrocene and alkoxy Fischer carbene pyrene conjugates 3 and 5, respectively, via regioselective alkyne−azide [3 + 2] cycloaddition reactions. Receptor 3 shows an excellent selectivity and sensitivity toward Pb2+ ion in an aqueous environment. Receptor 5, bearing a pyrene unit, shows significant fluorescence enhancement upon selectively binding with Zn2+ and Cu2+ ions. Although our objective of monitoring the complexation abilities of the receptors 3 and 5 by IR spectroscopy was not very successful, 3 and 5 are unique molecular systems that work well in detecting very low concentrations of Pb2+ and Zn2+, even in the presence of a wide range of competitive non-transition-metal and transitionmetal ions alike in an aqueous environment. Note that Fischer carbene based metal ion sensors which work well in aqueous medium are not known in the literature. Furthermore, their colorimetric responses are preserved in the presence of river water samples, which allows the potential use of receptors 3 and 5 as “naked eye” detectors of these cations.

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EXPERIMENTAL SECTION

Chemicals and General Method. Ferrocene, n-butyllithium, phenylacetylene, tungsten hexacarbonyl, and appropriate perchlorate salts of Na+, K+, Li+, Tl+, Ca2+, Mg2+, Mn2+, Cr2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+, Hg2+, and Pb2+ were purchased from Aldrich and used directly. Sodium azide, sodium borohydride, TMEDA, DMF, acetonitrile (HPLC), and diethyl ether were purchased from local chemical suppliers. Column chromatography was carried out on 3 cm of silica gel in a column of 2.5 cm diameter using 200−400 mesh silica gel. All solvents were dried by conventional methods and distilled under an Ar atmosphere before use. The compounds (azidomethyl)ferrocene (2), alkynyl Fischer carbene complex 1, and 1-(azidomethyl)pyrene (4) were synthesized according to literature procedures.17,26,27 The UV−vis spectra were taken in CH3CN solutions at c = 1 × 10−5 M. Fluorescence spectra were taken in CH3CN solution at c = 1 × 10−7 M. Cyclic voltammetry (CV) was performed with a conventional three-electrode configuration (glassy carbon as working electrode, platinum as auxiliary electrode, and Ag/Ag+ as reference electrode). The experiments were carried out with 10−4 M sample solutions in CH3CN using [(nC4H9)4NClO4] (TBAP) as supporting electrolyte, after deoxygenation of the solution. The working electrode was cleaned at the end of each run. Cyclic voltammograms were recorded at the scan rate 0.1 V s−1. Instrumentation. The 1H and 13C NMR spectra were recorded on 400 and 500 MHz FT-NMR spectrometers, using tetramethylsilane as the internal reference. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) measurements were carried out on a MALDI-TOF instrument; mass spectra were recorded on a Ultraflextreme instrument by using 2,5-dihydroxybenzoic acid as a matrix and a ground steel target plate. The absorption spectra were recorded with a V-650 UV−vis spectrophotometer at 298 K. Fluorescence spectra were recorded with an FP-6300 instrument. The CV measurements were performed on a CH Model CHI630D potentiostat. Caution! Due to the explosive nature of metal perchlorates under certain conditions, precautions should be taken to handle perchlorate salts. Synthesis of Compound 3. Complex 1 (0.25 g, 0.52 mmol) and ferrocenyl monoazide 2 (0.19 g, 0.78 mmol) were palced in a two-neck round-bottom flask that was thoroughly degassed with argon for 30 min. The reaction mixture was stirred at 40 °C for 12 h, and the progress of the reaction was monitored by TLC. Upon completion of the reaction the crude product was purified using silica gel column chromatography. Elution with EtOAc/hexane (1/9, v/v) yielded orange-red 3 (0.17 g, 60%). Data for 3 are as follows. 1H NMR (CDCl3, 500 MHz): δ 7.52−7.16 (m, 5H, Hphenyl), 5.05 (s, 2H, Cp-CH2), 4.64 (s, 2H, −OCH2CH3), 4.12−3.97. (m, 9H, HCp), 0.89−0.86 (t, 3H, −OCH2CH3). 13C NMR (CDCl3, 125 MHz): δ 298.9, 204.49, 197.9, 156.71, 132.3, 129.7, 129.4,

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Figures, a table, and CIF and xyz files giving 1H and 13C NMR and MALDI-MS data of 3 and 5, UV−vis data for 5 upon titration with different metal cations, MALDI-MS spectra of [3·Pb2+], [5· Cu2+], and [5·Zn2+], a Job plot for compounds 5 in the presence of Cu2+ ion, quantitative binding data for 3 and 5 with Pb2+, Cu2+, and Zn2+, fluorescence intensity of 5 for Cu2+and Zn2+, crystallographic data for 3, and all computed molecule Cartesian coordinates in a format for convenient visualization. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*Corresponding Author Tel: (+91) 44 2257 4230. fax: (+91) 44 2257 4202. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support of the Department of Atomic Energy, Board of Research in Nuclear Sciences, BRNS (Project No. 2011/37C/ 3105


Organometallics

Article

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54/BRNS), BARC, Trombay, Mumbai, India, is gratefully acknowledged. J.P.S. and S.K.B. thank the Indian Institute of Technology Madras, India, for research fellowships.

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Triazolyl Alkoxy Fischer Carbene Complexes in Conjugation with

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