Synthesis and Electrochemical Anion-Sensing Properties of a

Article pubs.acs.org/Organometallics

Synthesis and Electrochemical Anion-Sensing Properties of a Biferrocenyl-Functionalized Dendrimer † ́ Carlos Villena,† José Losada,*,† Pilar Garcıa-Armada, Carmen M. Casado,‡ and Beatriz Alonso*,‡ †

Departamento de Quı ́mica Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco 28049-Madrid, Spain, and ‡ Departamento de Ingeniería Quı ́mica Industrial, Escuela Técnica Superior de Ingenieros Industriales, Universidad Politécnica de Madrid, 28006-Madrid, Spain S Supporting Information *

ABSTRACT: The synthesis and electrochemical anionsensing properties of a diaminobutane poly(propyleneimine) dendrimer functionalized with biferrocenyl units 2 are presented. The redox activity of the ferrocenyl centers in 2 has been characterized by cyclic voltammetry. Cyclic and square wave voltammetric investigations demonstrate that tetraferrocenyl compound 2 and the reference compound 1 show electrochemical anion-sensing action: they display a cathodic shift of the ferrocene−ferrocenium redox couple with dihydrogenphosphate and hydrogensulfate anions in solution and immobilized onto electrode surfaces.

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INTRODUCTION Redox-active receptors have great potential in the development of chemical sensors for the detection of nonelectroactive species. A wide variety of metallocene-based endoreceptors (chelates, tripods, porphyrins, calixarenes, etc.) have been designed.1 Receptors containing neutral ferrocenyl units are especially interesting for anion recognition and sensing, since ion-pairing interactions can be switched over by electrochemical oxidation of ferrocene to its cationic ferrocenium form. Additional hydrogen bonding between a guest anion and an appended amide proton is able to enhance anion complexation. Beer et al. have synthesized various endoreceptors attached to amidoferrocenyl groups and found significant variations of the redox potentials of the ferrocene groups in the presence of anions.2 Dendrimers offer an interesting molecular framework for the design and construction of anion recognition sites. Dendrimers may be viewed as exo receptors in which the guest−host interactions occur at the outskirt of the host. The dendritic organization involves channels and microcavities at the surface that provide useful topological conditions for the function as an exo receptor. To take advantage of this dendritic effect, ferrocenyl dendrimers exhibiting anionic electrochemical recognition properties have been reported. The extensive work of Astruc and co-workers3 has made use of ferrocenyl amide terminated dendrimers for the electrochemical recognition of anions. We have also investigated ferrocenylterminated dendrimers as anion receptors.4 An especially interesting approach for the preparation of electrochemical sensory devices is the immobilization of redoxresponsive receptor systems on electrode surfaces.5 Concentration of active recognition and informative sites on solid © 2012 American Chemical Society

surfaces is known to enhance the sensing and binding properties of the resulting material as compared to that of the individual molecules in solution. Dendrimers can adsorb more easily on surfaces than small endoreceptors; this fact allows their use for the modification of electrodes. One objective of our research is to prepare new organometallic derivatives containing ferrocene or cobaltocene units which can act as electrochemical sensors for anions of environmental importance, not only in solution but also confined onto electrode surfaces. Herein, we present the synthesis and anion coordination properties of two poly(propyleneimine) dendrimers bearing in its molecules four ferrocenyl units isolated or joined through a C−C σ bond.

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RESULTS AND DISCUSSION Previously we have synthesized the first-generation organometallic poly(propyleneimine) dendrimer DAB-1-(NHCOFc)4 (1) (Figure 1).6 This compound is accessible by reaction of chlorocarbonylferrocene with the dendritic polyamine DAB-1(NH2)4 via a condensation reaction. Synthesis and Characterization of Dendrimer 2. The key starting step in the preparation of the new compound is the previous synthesis and purification of the biferrocenyl derivative 1′,1‴-bis(chlorocarbonyl)biferrocene.7 This organometallic fragment was prepared by lithiation of dibromobiferrocene8 followed by carbonylation using CO2 and hydrolysis to give the 1′,1‴-biferrocenedicarboxylic acid. This was then converted Received: February 14, 2012 Published: March 30, 2012 3284

dx.doi.org/10.1021/om3001185 | Organometallics 2012, 31, 3284−3291

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different protons, which are in agreement with the expected structures. The ESI-TOF mass spectrum of 2 was recorded in CH3OH/0.1% formic acid and shows dominant peaks at m/z 1161.28 and 581.15 with isotopic patterns matching exactly the calculated values for the molecular singly and doubly protonated ions, respectively. Electrochemical Behavior in Solution. The electrochemical properties of 1 and 2 have been investigated by cyclic voltammetry (CV) in homogeneous solutionand confined to electrode surfaces (i.e., where the dendrimers serve as electrode modifiers). As illustrated in Figure S1 in the Supporting Information, the cyclic voltammogram of dendrimer 2 at low concentrations (about 10−4 M in ferrocene centers) in CH3CN/water (100/1 v/v) solution (0.1 M n-Bu4NPF6) reveals two well-separated and reversible oxidation systems of equal intensity at E1° = 0.58 V and E2° = 0.87 V vs Ag/AgCl, respectively. Both oxidation processes are diffusion-controlled, with the anodic current function i/v1/2 being independent of the scan rate. The ratio of the cathodic to anodic peak current is close to unity for both systems, and the peak potential separations are ΔEp1 = 48 mV and ΔEp2 = 67 mV, respectively. The diffusion coefficient was calculated using the Randles−Sevcik equation, giving D0 = 3.12 × 10−7 cm2/s. This electrochemical stepwise oxidation is consistent with the existence of appreciable interactions between the two iron centers. The initial oxidation occurs at nonadjacent ferrocene sites, which makes the subsequent removal of electrons from the remaining ferrocenyl centers, adjacent to those already oxidized more difficult. From ΔE° values the complex may be considered as belonging to class II of mixed-valence complexes according to the Robin−Day classification.9 As previously reported,6 the CV of 1 in CH2Cl2/0.1 M TBAH is characterized by a reversible wave corresponding to the Fc/Fc+ redox couple. A similar electrochemical response is displayed by 1 in DMSO/0.1 M TBAH solution. The cyclic voltammograms exhibit a reversible Fc/Fc+ wave at E1/2 = 0.19 V (vs Ag/Ag+). The oxidation process is diffusion-controlled, with the anodic current function i/v1/2 independent of the scan rate and the ratio of the cathodic to anodic peak current close to unity. The fact that only a single redox process is observed indicates that the ferrocenyl units are seemingly identical and sufficiently remote from one another to render the electrostatic factor negligible.10 Anion-Sensing Properties in Solution. Measurements could not be performed in acetonitrile containing a significant amount of water. The weakening of the electrostatic interactions and solvation effects are responsible for a significant loss of the electrochemical sensing ability of 2 toward the tested anions. However, quantitative analysis of the anion sensing experiments could be achieved in 0.1 M TBAH/ DMSO employing a clean electrode for each measurement and using Osteryoung square-wave voltammetry (OSWV), because this technique has a lower detection limit than CV. The addition of increasing amounts of anions to solutions of 2 in DMSO, in which it is sufficiently soluble, leads to a progressive increase of the anodic peak current, along with a negative shift of the first ferrocene oxidation wave, which turns less reversible. This behavior is similar to that already found for other ferrocene derivatives, including the unsubstituted ferrocene,11 and constitutes strong evidence for an EC mechanism with adsorption onto the electrode surface of a poorly soluble product that cannot be reduced (see Figure S2

Figure 1. Ferrocenyl dendrimer 1.

into the diacyl chloride by treatment with oxalyl chloride (Scheme 1). Scheme 1. Synthesis of 1′,1‴Bis(chlorocarbonyl)biferrocene

A condensation reaction of 1′,1‴-bis(chlorocarbonyl)biferrocene with the first-generation diaminobutane-based poly(propyleneimine) dendrimer DAB-1-(NH2)4 in dichloromethane at room temperature under conditions of high dilution, followed by the appropriate workup, afforded compound 2 (Scheme 2) as the major product, as an air-stable yellow solid in 86% yield. Scheme 2. Synthesis of Dendrimer 2

The structure of the biferrocenyl dendrimer 2 has been established on the basis of 1H and 13C NMR and was corroborated by electrospray ionization time-of-flight (ESITOF) mass spectrometry. In the 1H NMR spectrum of 2 key signals arising from the organometallic moieties are observed in the range 4.5−4.1 ppm along with signals of the poly(propyleneimine) dendritic framework around 3.1, 2.3, 1.6, and 1.5 ppm. The complete amidation of the peripheral amine groups was supported by the total absence, in the 1H NMR spectra, of the −NH2 signals of the starting dendritic polyamine around 1.3 ppm, the appearance of a new signal at 7.5 ppm due to the amide group, and by the integration ratios of the 3285


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potential shift observed is within a few millivolts of that detected in the presence of H2PO4− alone. On the other hand, HSO4− can be unambiguously detected in an equimolar mixture with the Cl− anion. In addition, as can be seen in Table 1, 2 equiv of HSO4− cannot be determined with lower concentrations of H2PO4− because the cathodic perturbations observed in the OSWV curves are approximately the same as those produced by the corresponding amount of H2PO4− alone. A two-wave behavior was observed in the CV and DPV curves when considering the 1 + H2PO4− systems in DMSO/ 0.1 M TBAH electrolyte (see Figure 3 and Figure S4 in the

in the Supporting Information). The electrochemical response of 2 is strongly affected by the addition of increasing amounts of n-Bu4NH2PO4. The peak corresponding to the first Fc/Fc+ couple measured with OSWV in the absence of the anionic analyte was observed at 0.10 V vs Ag/Ag+. In the presence of increasing concentrations of H2PO4− anion the redox peak current is progressively shifted to less positive potentials (Figure 2).

Figure 2. OSWV curves for dendrimer 2 (5 × 10−5 M) in DMSO/0.1 M TBAH, obtained with a Pt working electrode in the absence and presence of n-Bu4NH2PO4. Figure 3. DPV for dendrimer 1 (5 × 10−5 M) in DMSO/0.1 M TBAH, obtained with a Pt working electrode in the absence (black) and presence of 0.3 equiv (blue), 0.5 equiv (red), 1 equiv (cyan), 1.5 equiv (green), and 2 equiv (blue) of n-Bu4NH2PO4.

This behavior is in accordance with the increase of the effective electron density on the redox center due to the binding of the H2PO4− anion, which makes the oxidation easier. The addition of n-Bu4NHSO4 leads to behavior of the first Fc/Fc+ couple similar to that in the case of n-Bu4NH2PO4, but the potential shifts are smaller. This behavior can be linked to a weaker anion ligand interaction following the oxidation of the ferrocene groups. The addition of Cl− salts to the dendrimer solution gives rise to a small shift of peak potentials (ΔEp ≈ 6 mV) independent of the amount of anion added. Analysis of the OSWV curves as a function of the H2PO4− and HSO4− concentration allows calibration graphs to be established (see Figure S3 in the Supporting Information). Competitive experiments in the presence of H2PO4−, HSO4−, and Cl− anions allowed us to establish the selectivity of the sensing measures (Table 1). When 2 equiv of H2PO4− per ferrocene center is added to a 3.3 × 10−5 M solution of compound 2 in the presence of 4 equiv of HSO4− or Cl−, the

Supporting Information). The addition of increasing amounts of dihydrogenphosphate to solutions of 1 leads to a progressive decrease of the initial Fc/Fc+ wave, along with the growth at less positive potentials of a new peak system corresponding to the Fc/Fc+ couple in the complexed redox probe (ΔEp = 101 mV). No clear anodic new peak can be seen in the curves in the presence of less than 0.3 equiv per ferrocene; nevertheless, amperometric titration curves could be drawn by considering the intensity of the new peaks until the addition of the anion causes precipitation of 1 (Figure S5 in the Supporting Information). The cathodic shifts of the new ferrocene DPV oxidation waves, ΔEpk, increase with the anion concentration (see Figure S6 in the Supporting Information). In the presence of hydrogensulfate anion the DPV or OSWV peak corresponding to the Fc/Fc+ redox couple did not experience significant changes in the potential. In a polar solvent such as DMSO (ε = 46.7) the hydrogen-bonding interactions between the amide functional groups and the anions are usually weakened by competing solvent molecules. In addition, the polarity of the solvent precludes the development of strong electrostatic interactions and leads to a loss of the sensing properties of the receptors. As a consequence, ΔEp is less than 2 mV for the 1 + HSO4− (large excess) system in DMSO. This fact could be evidenced by studies carried out in a less polar solvent such as CH2Cl2 (ε = 8.9). In this media 1 is soluble and shows a two-wave CV behavior in the presence of hydrogensulfate.3c The electrochemical sensing properties of 2 and 1 toward anionic species can be judged from changes in their electrochemical response induced by the progressive addition of a given anion into the electrolyte solutions. For both

Table 1. Cathodic Perturbations of the Ferrocene Redox Couple Observed in the OSWV on Addition of Various Anions as Their TBA+ Saltsa ΔE1/2/mVb,c X− added per ferrocene center/ equiv 0.5 1 2 4

H2PO4− HSO4− Cl− H2PO4− d 14 31 54

5 8 13

6 6

24 35 53

HSO4− e

Cl− e

54 54 54 55

54 54 52 50

Obtained in DMSO containing 0.1 M TBAH. Solutions were 5 × 10−5 M in 2. bΔE1/2 = ΔE1/2(free) − ΔE1/2(X−). ΔE1/2(free) = 104 mV. cThe error in ΔE1/2 values is ±3 mV. dIn the presence of 2 equiv of HSO4−. eIn the presence of 2 equiv of H2PO4−. a

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complexes studied here the shift in potential ΔE1/2 between complexed and free ferrocene is observed toward negative potential. Assuming that no structural effect is involved in the complexation process, this shift is related to the ratio (K+/K0) of the stability constants for the complex in its oxidized (charged) form, K+, and reduced (neutral) state, K0. Two types of voltammetric behavior have been found: (i) a progressive anodic shift of the initial wave with 2 (one-wave behavior) and (ii) the growth at less positive potential of a new redox wave (two-wave behavior) at the expense of the original wave or the free ligand in the case of 1. The different electrochemical behaviora have been rationalized by Miller et al.12 They established that the two-wave behavior can be observed when the binding constant of the neutral ligand K0 is large; therefore, when the strength of the interaction between the anion and the reduced redox form (ferrocenyl groups) is significant, a new wave appears. If the interaction between the ferrocenyl moieties and the anion is weak, only a gradual negative shift is observed in the Fc/Fc+ redox potential. The value of K+, the apparent association constant, between the oxidized form (ferrocenium) and the anion can be estimated by the equation ΔE° = 0.058 log (cK+) at 25 °C, c = [added anion], which gives K+= 6698 ± 200 and 2730 ± 200 for 2 with H2PO4− and HSO4− in DMSO, respectively. The accessibility to the binding sites is a limiting factor for the strength of the anion−receptor interaction. It has been previously found that the strength of the interaction between neutral amidoferrocenyl ligands and the anions is significantly weaker with more rigid ligands.13 Since 1 and 2 have the same number of amide groups, the strength of the interactions between the neutral ligands and the anions depends on the steric constraints. In 2 the amidoferrocenyl moieties are linked by the fulvelenyl bridge, giving a very rigid structure which does not allow a favorable organization of the −CONH groups. Consequently, this ligand does not present significant complexation ability in its reduced form toward all surveyed anions. In contrast, H2PO4− interacts with 1, in which the amidoferrocenyl groups are unhampered at the end of long flexible propyleneimine branches. This fact probably favors an adequate conformation to facilitate the spatial organization of the binding sites and a tighter hydrogen-bonding interaction. Electrochemistry of Modified Electrodes. The coating of Pt or GC electrodes with films of 2 is readily accomplished by repeated cyclic voltammetry scans over the 0.1−0.8 V potential range or by controlled-potential electrolysis at 0.8 V vs Ag/Ag+ in DMSO/0.1 M TBAH electrolyte. The redox behavior of films of compound 2 electrodeposited onto electrode surfaces was studied by CV in fresh CH2Cl2 solutions with different supporting electrolytes containing [ClO4]−, [PF6]− or [B(C6F5)4]− as the anion and tetrabutylammonium ([n-Bu4N]+) as the cation. In all cases, two well-separated and reversible oxidation systems are observed in the first scan corresponding to the interacting ferrocene/ferrocenium couples. This electrochemical response is reminiscent of that observed for the compound in solution. The redox potential of the ferrocene systems depended on the anion and shifted more positively in the order [ClO4]− < [PF6]− < [B(C6F5)4]−. The ΔE1/2 values increase when the supporting electrolyte anion is changed from [ClO4]− to [PF6]− or [B(C6F5)4]−. These facts are in agreement with a decreasing ion-pairing effect.14 The CV curves of films of 2 are deeply modified upon multicyclic scanning in CH2Cl2 solutions containing different

anions. In CH2Cl2/n-Bu4NPF6 the pairs of peaks a/a′ (E1/2 = 0.25 V, ΔEp = 40 mV) and b/b′ (E1/2 = 0.56 V, ΔEp = 43 mV) decrease with a new pair of peaks, c/c′, growing in at E1/2 = 0.37 V (vs Ag/Ag+) and forming isopotential points. The pair of peaks c/c′ continues to grow while the other peaks eventually disappear, as shown in Figure 4. The decrease in the

Figure 4. Consecutive CVs of a Pt electrode modified with a film of dendrimer 2 (Γ = 6.27 × 10−11 mol Fc cm−2), measured in CH2Cl2/ 0.1 M TBAH at 100 mV s−1.

reversibility of the redox system Fc/Fc+ shown by the electrochemical features of the waves c/c′ (ΔEp = 71 mV) could be indicative of the presence of anions in the network of the film. Moreover, a decrease in the electroactivity of the films is observed in the successive CV curves, as already noted for films of other redox polymers.15,16 Charge propagation and, thus, the electroactivity of film appears to be dominated by counterion diffusion and ion-trapping effects. Therefore, the loss of electroactivity could be linked to ion pairing between anions and ferrocenium in the oxidized film, and it can be explained by the irreversible doping of films by anions, which is responsible for a decrease in the rate of charge propagation in the films. The lack of recovery of the initial signal is due to the irreversible trapping of the anions onto the polymer film. During redox cycling an anion incorporation−ejection process takes place; however, the anions are not fully released from the film during the backward reductive scan. Decreasing amounts of counteranions are progressively incorporated into the film until a steady state is reached. The presence of the well-defined isopotential points (common intersection points in current−potential curves that are analogous to isosbestic points in spectrometry) in the cyclic voltammogram (see Figure 4) during the course of the aforementioned process would suggest that there is a transformation between two surface species and an equilibrium is established between these species at a constant coverage.17 We believe that the new peaks and the isopotential points are likely a result of the incorporation of anions into the dendrimer network; this gives rise to a process in which the biferrocene groups become mixed-valence Fe(II)−Fe(III) compounds. It is probable that the closed shape of the biferrocene groups and the very high concentration of amidoferrocene moieties in the film create a favorable topological effect which gives rise to the retention of some of the anions that experience very strong electrostatic interactions with the ferrocenium moieties. The high electron density around the ferrocenium centers, caused 3287


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Figure 5. Shifts of the OSWV oxidation peak potential, ΔEp (a), oxidation peak potential of the generated wave (c), and initial OSWV oxidation peak currents (b, d) for the ferrocenyl groups in dendrimers 1 (black ▲) and 2 (red ■) immobilized on a Pt electrode 1 (Γ = 1.83 × 10−10 mol Fc cm−2) and GC electrode 2 (Γ = 7.6 × 10−11 mol Fc cm−2) vs [H2PO4−] in CH2Cl2/0.1 M TBAH. Inset in (d): OSWV oxidation peak currents of the generated wave.

The shape of the CV wave corresponding to a film stabilized in CH2Cl2/TBAH 0.1 M (see Figure S7 in the Supporting Information) is typical of a surface-confined reversible couple with the expected linear relationship of peak current to potential sweep rate, v, for values up to 500 mV s−1. The peakto-peak separation (ΔEpk) was 54 mV at 5 mV s−1; for higher sweep rates ΔEpk values tended to increase (ΔEpk = 102 mV at 250 mV s−1). Such a peak separation is indicative of slow charge-transfer kinetics. The deposition of 1 can be carried out on Pt or glassy-carbon (GC) electrodes by controlled-potential electrolysis at 0.75 V or by repeated cycling between +0.4 and 0.9 V versus Ag/Ag+ in degassed solutions of the dendrimer in CH2Cl2/0.1 M TBAH. The electrodes modified with films of 1 were rinsed with CH2Cl2 and were characterized by CV in fresh CH2Cl2 with 0.1 M n-Bu4NPF6 or n-Bu4NClO4 as supporting electrolyte. The CVs displayed the electrochemical response of the modified electrode owing to the ferrocene/ferrocenium system in the film, as previously reported.6,18 Initially, a slight decrease in the current response was observed with successive multicyclic scans; however, these decay processes cease after a few cycles. After this stabilizing step, the shape of the features in the cyclic voltammograms is independent of the scan rate and repeated scanning in CH2Cl2 electrolyte solutions did not change the voltammograms, demonstrating that films of the dendrimer remain stable to electrochemical cycling. Anion-Sensing Properties of Modified Electrodes. The sensing properties of dendrimer 2 films toward various anions have been examined by OSWV experiments. Well-behaved OSWV curves are obtained in the absence and in the presence of n-Bu4NH2PO4 in TBAH/CH2Cl2 solutions (Figure S8 in the

by immobilized inserted anions, and the restricted charge propagation may well inhibit the reversible electrochemical response of a fraction of the ferrocene moieties. The binding of counteranions causes progressively larger amount of ferrocenium centers to not be reduced in the reverse scans. The pairs of peaks a/a′ and b/b′ decrease and the new pair of peaks c/c′ increases as the amount of blocked Fc+ centers increases. Thus, the electrostatic effects of trapped counteranions lead to the formation of the mixed-valence state which is reached in the film after several cycles. This postulated behavior is supported by the results obtained from the voltammetric study carried out in various supporting electrolytes. It is known that the order of electrostatic interactions of anions with Fc+ is inverse to the estimated ion size.14b The formation of the ion pairs and the ion-trapping effects of the counteranions are favored in the case of [ClO4]−, which can penetrate into the film and remain inserted easily due to its smaller size and strong ion-pairing ability. When the supporting electrolyte anion is changed from [ClO4]− to [PF6]− or [B(C6F5)4]−, the film doping process and the intensity decay takes place more slowly, a higher number of successive scans being necessary to attain the steady final response. This behavior is in agreement with the lower coordinating ability and/or larger size of these anions. In addition, the loss of electroactivity detected in the films after the cycling period is also dependent on the anion present in the electrolyte solution. When the steady state is reached, the reductions in the size of peak areas are 80%, 52%, and 68% for [ClO4]−, [PF6]−, and [B(C6F5)4]−, respectively. These results are consistent with the effect of size and strong ion-pairing ability. 3288


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Supporting Information). In dihydrogenphosphate-free solution, a peak corresponding to the Fc/Fc+ redox couple is observed at Ep = 0.41 V (vs Ag/Ag+). As in the CV experiments, in the presence of increasing amounts of nBu4NH2PO4, the redox peak current shows a progressive cathodic shift due to the binding of anions, which effectively stabilizes the positive charge of the oxidized film (Figure S9 in the Supporting Information). ΔEp values have been determined in the H2PO4− concentration range 0.1−7.77 μM (Figure 5a). The ΔEp values and the low limit of detection are independent of the apparent surface coverage, Γ, in the range 7.6 × 10−11− 3.65 × 10−10 mol cm−2. The intensity of the OSWV peaks is somewhat depressed in the presence of H2PO4− ions, even at low concentrations. This loss of electroactivity is a consequence of the aforementioned ion-trapping effects. Analysis of the OSWV curves for electrodes modified with films of the dendrimers as a function of the n-Bu4NH2PO4 concentration allows calibration graphs to be established. The intensity of the anodic peak, ipa, of the original wave decreases continuously in the presence of H2PO4−, until it disappears (Figure 5b). The addition of increasing amounts of n-Bu4NHSO4 leads also to a progressive shift of the potential of the Fc/Fc+ wave peak (see Figure S10 in the Supporting Information). In contrast to that observed with H2PO4− the electroactivity of the films diminishes more slowly with the addition of increasing amounts of HSO 4−, owing to the weaker ion-pairing interactions (see Figure S11b in the Supporting Information). The increasing values of ΔEp and decreasing values of ipa can be measured in the HSO4− concentration range from 0.1 to 36.1 × 103 μM (Figure S11a,b). OSWV curves were also recorded after progressive additions of increasing amounts of Cl−. ΔEp values and the intensity values can be detected in the concentration range 0.1−4.86 × 103 μM (Figure S11d,e). Competitive experiments in the presence of H2PO4−, HSO4−, and Cl− anions allowed us to confirm the selectivity of the sensors.5d,16a When the modified electrode is exposed to a solution containing n-Bu4NH2PO4 and n-Bu4NHSO4 or nBu4NCl, all at concentrations of 3 × 10−6 M, the cathodic perturbations observed in the OSWV curves are approximately the same as that induced by the H2PO4− alone. The sensing properties of the electrodes modified with films of 1 toward various anions have been also examined from OSWV experiments. Well-defined OSWV curves are obtained in the absence and in the presence of n-Bu4NH2PO4 in TBAH/ CH2Cl2 solutions (Figure 6). In dihydrogenphosphate-free solution, a peak corresponding to the Fc/Fc+ redox couple is observed at Ep = 0.30 V (vs Ag/Ag+). As in the CV experiments, in the presence of increasing amounts of nBu4NH2PO4, a new peak appears at a less positive potential with a concomitant decrease in the intensity of the initial peak, which disappears after the addition of at least 8 × 10−6 M nBu4NH2PO4. The potential of this second peak that can be detected from 5 × 10−6 M is progressively shifted to less positive potentials with increasing amounts of dihydrogenphosphate. The binding of anions stabilizes the positive charge of the oxidized film, shifting the redox potential (Ep) of the Fc/ Fc+ system to less positive values until the peak potential reaches a constant value. The intensity of the original OSWV peaks is depressed in the presence of H2PO4− ions at low concentrations.

Figure 6. OSWV curves for a Pt electrode modified with dendrimer 1 (Γ = 1.83 × 10−10 mol Fc cm−2) measured in CH2Cl2/0.1 M TBAH, in the absence and presence of n-Bu4NH2PO4.

Analysis of the OSWV curves for electrodes modified with the films of 1 as a function of the n-Bu4NH2PO4 concentration allows calibration graphs to be established (Figure 5a,c,d). The intensity of the anodic peak, ipa, of the original wave decreases continuously in the presence of H2PO4− in the concentration range 0.1−8 μM. The decrease parallels a negative potential shift of this first peak in the same concentration interval. In addition, increasing values of the cathodic shift of the new ferrocene OSWV oxidation wave, ΔEp, can be measured in the n-Bu4NH2PO4 concentration range of 5−30 μM (Figure 5c). Figure 5d (inset) displays the variation of ipa corresponding to the new oxidation wave in the range 5−7.5 μM. Analysis of the intensity of the initial OSWV peak also allows amperometric titration curves to be drawn (Figure 5d) that present low and high limits of detection of 5 and 8 μM in H2PO4−. The sensing properties were further studied by analyzing the OSWV response of dendrimer films (Γ = 2.78 × 10−10 mol cm−2) in the presence of other anions. The addition of increasing amounts of n-Bu4NHSO4 or n-Bu4NCl leads only to a progressive shift of the potential of the Fc/Fc+ peak without the growth of a new redox wave (Figure S12 in the Supporting Information). This behavior can be linked to a weaker interaction of the amido group with HSO4− as compared to that with H2PO4−. This fact is due to a different strength of the NH-oxo anion hydrogen bonding with these oxo anions, since the negative charge on the oxygen atoms is smaller in HSO4− than in H2PO4− (sulfur being more electronegative than phosphorus).3d On the other hand, the electrochemical response to the binding of Cl− with ferrocenyl ligands arises from ion pairing with Fc+ forms. Increasing values of ΔE and decreasing values of ipa can be measured in the HSO4− concentration range from 5 × 10−7 to 1.86 × 10−2 M and in the Cl− concentration range from 10−7 to 3.68 × 10−3 M (see Figure S11a,c,d,f in the Supporting Information). Comparison between results obtained with both compounds in the presence of various anions highlights the main parameters that govern the anion-sensing properties of ferrocene amide receptors in organic electrolytes. From these results, it can be concluded that the strength of the hydrogenbonding and electrostatic interactions between the amidoferrocene groups and the anions depends on the topological characteristics and concentration effects. 3289


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The two-wave behavior shown by the system 1−H2PO4− in DMSO solutions can be facilitated by the topology of 1, which probably favors the encapsulation of the guest.19 When electrodes modified with dendrimer fims are used to sense anions, it can be observed (Figure S11a in Supporting Information) that electrodes modified with films of 2 provide shifts of ferrocene oxidation potential higher than those experienced by electrodes modified with films of dendrimer 1 of nearly the same ferrocene unit coverage. It is noteworthy that even when the association between the neutral receptor and the anion is weak, as in dendrimer 2, the redox ligands are able to recognize the guest anion electrochemically. It can thus be assumed that the oxidation of ferrocene in the vicinity of the amide groups increases the acidity of amide protons and makes hydrogen bonding stronger,20 leading to a synergy between ion pairing and Hbond interactions. Thus, the recognition behavior can be explained in terms of H-bonding complexation, and the magnitude of the electrochemical sensing is essentially dependent on the strength of ion-pairing interactions. Electrodes modified with dendrimer 1 also show a two-wave behavior in the presence of H2PO4− anion. In addition, although the anion−ligand interactions in the reduced state are stronger with 1 than with 2, their recognition properties toward HSO4− are very similar. However, slightly better results are obtained using electrodes modified with 1, since maximum values of ΔEp are reached with these electrodes for analogous anion concentrations. These facts can be explained by the shape of the receptors and the topology of the films, which have an important effect on the complexation and recognition properties. It is likely that, in layers of the exo receptor 1 deposited onto the electrode surface, the open shape of the binding site and the cavities and channels formed by the organometallic units favor the establishment of stronger interactions. In addition, for electrodes prepared with a similar coverage of redox groups, lower ip values were obtained using electrodes modified with 2 dendrimer films. This fact can also be explained by stronger ion−film interactions that cause a greater loss of electroactivity. In conclusion, we have synthesized a new dendrimer (2) containing two biferrocene units, and its anion-sensing ability has been compared to that of the related tetraferrocenyl dendrimer 1. In any instance, the results reported here are remarkable because voltammetric sensing of hydrogenphosphate anions with the dendrimers 1 and 2 and of hydrogensulfate with 2 in solution takes place in a relatively polar solvent (DMSO), where hydrogen-bonding interactions between the amide functional groups and the anion are usually weakened by competing solvent molecules. Furthermore, the data obtained clearly show anion sensing at relatively low (submillimolar) anion concentrations. In addition, electrodes modified with films of both dendrimers are sensitive to the presence of HSO4− anion over a wide range of concentrations, about 10−1−104 μM.

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any further purification. The dendritic polyamine DAB-1-(NH2)4 was purchased from Aldrich and was stripped three times with toluene in order to remove water prior to use. NMR spectra were recorded on a Bruker AMX-300 spectrometer. Chemical shifts are reported in parts per million (δ) with reference to residual solvent resonances. ESI mass spectra were recorded on a “QSTAR Pulsar i” mass spectrometer from Applied Biosystems, equipped with a hybrid analyzer QTOF (quadrupole time of flight). The samples were were directly infused into the ESI source using a syringe pump at a flow rate of 10 μL/min. Electrochemical Measurements. Electrochemical measurements were performed using an Ecochemie BV Autolab PGSTAT 12 instrument. CH2Cl2 (spectrograde) for electrochemical measurements was freshly distilled from calcium hydride under nitrogen. DMSO was obtained from Fluka and used as received. The supporting electrolyte was TBAH (tetra-n-butylammonium hexafluorophosphate) that was purchased from Fluka and was purified by recrystallization from ethanol and dried under vacuum at 60 °C. The supporting electrolyte concentration was typically 0.1 M. A conventional sample cell operating under an atmosphere of prepurified nitrogen was used for cyclic voltammetry. All cyclic voltammetric experiments were performed using either a platinum-disk working electrode (A = 0.070 cm2) or a glassy-carbon-disk working electrode (A = 0.070 cm2), each of which was polished prior to use with either a 0.05 μm alumina/ water slurry or 1 μm diamond paste (Buehler) and rinsed thoroughly with purified water and acetone. Potentials are referenced to a BAS nonaqueous electrode (0.01 M Ag/AgNO3, CH3CN). A coiled platinum wire was used as a counter electrode. Solutions for cyclic voltammetry were typically 1.0 mM in the redox-active species and were deoxygenated by purging with prepurified nitrogen. No iR compensation was used. OSWV was done with a step potential of 1 mV, square wave frequency of 25 Hz, and square wave amplitude of 10 mV. The conditions for DPV were as follows: pulse amplitude, 10 mV; pulse width, 50 ms; sample width, 20 ms; sensitivity, 10 mA V−1; scan rate, 1 mV s−1. n-Bu4NHSO4 (Sigma), n-Bu4NH2PO4 (Sigma), and nBu4NCl (Fluka) were used as received. Synthesis of 1′,1‴-Bis(carboxylic acid)biferrocene. A sample of 1′,1‴-bis(carboxylic acid)biferrocene was prepared according to the following modified procedure.22 Dibromobiferrocene (3.53 g, 6.70 mmol) was placed in a freshly oven-dried three-necked flask (500 mL) and then dried under vacuum for 4 h. Dried THF (50 mL) and then nbutyllithium (5.89 mL; 2.5 M in hexane) were added under N2 at −78 °C. The resulting solution was stirred at −78 °C for 1 h, during which time 1′,1‴-dilithiobiferrocene gradually precipitated. This solution was saturated with anhydrous CO2 gas over a period of 4 h at −78 °C, and the solution was further stirred at room temperature for another 4 h under CO2 bubbling. Water (70 mL) was added, and the resulting mixture was treated with dichloromethane. The aqueous layer was acidified with a small volume of concentrated hydrochloric acid to give 1′,1‴-bis(carboxylic acid)biferrocene as a red-brown solid, which was filtered and dried (2.60 g, 85% yield). Synthesis of 1′,1‴-Bis(chlorocarbonyl)biferrocene. A mixture of 1′,1‴-bis(carboxylic acid)biferrocene (1.00 g, 2.18 mmol), oxalyl chloride (0.93 mL, 10.46 mmol), and 3 drops of pyridine in 30 mL of dry dichloromethane was stirred under Ar in the dark for 4 h at room temperature and then refluxed overnight. The reaction mixture was evaporated to dryness under reduced pressure and the residue extracted repeatedly with pentane. The solvent of the filtrate was removed under reduced pressure to give a deep red solid, which was kept under Ar. Yield: 0.79 g (73%). 1H NMR (300 MHz, CDCl3): δ 4.71 (t, 2H, C5H4), 4.51 (t, 2H, C5H4), 4.45 (t, 2H, C5H4), 4.41 (t, 2H, C5H4). 13C{1H} NMR (75.43 MHz, CDCl3): δ 169.16 (COCl), 84.39, 75.82, 74.99, 73.31, 71.14, 68.61 (C5H4). Synthesis of Dendrimer 2. To a solution of 1′,1‴-bis(chlorocarbonyl)biferrocene (0.12 g, 0.24 mmol) in 100 mL of dry CH2Cl2 was added a solution of DAB-dend-(NH2)4 (0.04 g, 0.12 mmol) and 0.1 mL of triethylamine in 50 mL of dry CH2Cl2 dropwise under an inert atmosphere of Ar. The reaction mixture was stirred for about 12 h under Ar at room temperature. The resulting solution was washed with saturated NaHCO3 aqueous solution and brine, to remove the triethylamine hydrocloride byproduct, and dried over

EXPERIMENTAL SECTION

Materials and Equipment. All reactions were performed under an inert atmosphere (prepurified N2 or Ar) using standard Schlenk techniques. Solvents were dried by standard procedures over the appropriate drying agents and distilled immediately prior to use. The starting materials dendrimer 1,6 1,1′-dibromoferrocene,21 and 1′,1‴dibromobiferrocene8 were prepared as previously described. The starting materials were used as received (Aldrich, Narchem) without 3290


Organometallics

Article

Lobete, F.; Casado, C. M.; Cuadrado, I.; Losada, J. Inorg. Chem. Commun. 2002, 288. (c) Reynes, O.; Gulon, T.; Moutet, J.-C.; Royal, G.; Saint-Aman, E. J. Organomet. Chem. 2002, 656, 116. (d) Reynes, O.; Moutet, J.-C.; Royal, G.; Saint-Aman, E. Electrochim. Acta 2004, 49, 3727. (6) Cuadrado, I.; Morán, M.; Casado, C. M.; Alonso, B.; Lobete, F.; García, B.; Ibisate, M.; Losada, J. Organometallics 1996, 15, 5278. (7) Dong, T.-Y.; Chang, C.-K.; Cheng, C.-H.; Lin, K.-J. Organometallics 1999, 18, 1911. (8) Dong, T.-Y.; Huang, B.-R.; Lin, M.-C.; Chiang, M. Y. Polyhedron 2003, 22, 1199. (9) Robin, M. B.; Day, P. Adv. Inorg. Chem, Radiochem. 1967, 10, 247. (10) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248. (11) Buda, M.; Ion, A.; Moutet, J.-C.; Saint-Aman, E.; Ziessel, R. J. Electroanal. Chem. 1999, 469, 132. (12) Miller, S. R.; Gustowski, D. A.; Chen, Z. H.; Gokel, G. W.; Echegoyen, L.; Kaifer, A. E. Anal. Chem. 1988, 60, 2021. (13) Reynes, O.; Maillard, F.; Moutet, J.-C.; Royal, G.; Saint-Aman, E.; Stanciu, G.; Dutasta, J. P.; Gosse, I.; Mulatier, J. C. J. Organomet. Chem. 2001, 637−639, 356. (14) (a) Camire, N.; Mueller-Westerhoff, U. T.; Geiger, E. G. J. Organomet. Chem. 2001, 637−639, 823. (b) Kondo, T.; Okamura, M.; Uosaki, K. J. Organomet. Chem. 2001, 637−639, 841. (15) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89. (16) (a) Reynes, O.; Royal, G.; Chainet, E.; Moutet, J.-C.; SaintAman, E. Electroanalysis 2003, 15, 65. (b) Collomb-Dunand-Sauthie, M. N.; Deronzier, A.; Moutet, J.-C.; Tingy, S. J. Chem. Soc., Dalton Trans. 1996, 2503. (17) (a) Untereker, D. F.; Bruckenstein, S. Anal. Chem. 1972, 44, 1009. (b) Smith, S. P. E.; Abruña, H. D. J. Phys. Chem. 1998, 102, 3506. (18) Takada, K.; Dı ́az, D. J.; Abruña, H.; Cuadrado, I.; Casado, C. M.; Alonso, B.; Morán, M.; Losada, J. J. Am. Chem. Soc. 1997, 119, 10763. (19) Ornelas, C.; Ruiz, J.; Astruc, D. Organometallics 2009, 28, 4431. (20) Carr, J. D.; Coles, S. J.; Hursthouse, M. B.; Light, M. E.; Tucker, J. H. R.; Westwood, J. Angew. Chem., Int. Ed. 2000, 39, 3296. (21) Dong, T.-Y.; Lay, L.-L. J. Organomet. Chem. 1996, 509, 131. (22) Rausch, M. D.; Ciappenelli, D. J. J. Organomet. Chem. 1967, 509, 131.

MgSO4. The solvent of the organic phase was removed under vacuum to afford 2 as a yellow solid (0.12 g, 86%). 1H NMR (300 MHz, DMSO-d6): δ 7.48 (t, 4H, CONH), 4.53 (t, 8H, C5H4), 4.38 (t, 8H, C5H4), 4.18 (t, 8H, C5H4), 4.08 (t, 8H, C5H4), 3.07 (m, 8H, NHCH2), 2.26 (br, 12H, CH2NCH2), 1.58 (br, 8H, CH2CH2CH2), 1.52 (br, 4H, CH2CH2CH2CH2). 13C{1H} NMR (75.43 MHz, DMSO-d6): δ 168.7 (CONH), 84.22, 78.10, 71.37, 69.43, 67.91 (C5H4), 54.18, 52.45 (CH 2 NCH 2 ), 38.19 (CH 2 NH), 27.59 (CH 2 CH 2 CH 2 ), 25.39 (CH2CH2CH2CH2). MS (ESI+): m/z 1161.28 (M + H+)+, 581.15 (M + 2H+)2+ (M = C60H68O4N6Fe4).

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving additional CVs, OSWVs, and calibration graphs for 1 and 2 and NMR spectra for 1′,1‴-bis(carboxylic acid)biferrocene and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.A.); [email protected] (J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Spanish Ministerio de Ciencia e Innovación (CTQ2009-12332-C02). C.V. acknowledges the Ministerio de Ciencia e Innovación for a predoctoral fellowship.

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REFERENCES

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Synthesis and Electrochemical Anion-Sensing Properties of a

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