Study on Synthesis and Electrochemical Properties of a Novel

Qiaohua Tan, Li Wang,* Haojie Yu, and Libo Deng ... are recorded, and the major influential factors, namely, potential scan rate, concentration of ele...
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J. Phys. Chem. B 2007, 111, 3904-3909

Study on Synthesis and Electrochemical Properties of a Novel Ferrocene-Based Compound and Its Application in Anion Recognition Qiaohua Tan, Li Wang,* Haojie Yu, and Libo Deng State Key Laboratory of Polymer Reaction Engineering, College of Materials Science and Chemical Engineering, Zhejiang UniVersity, Hangzhou, China, 310027 ReceiVed: December 2, 2006; In Final Form: January 28, 2007

The novel ferrocene-based compound 2-Fc is synthesized through the reaction of ferrocene carbonate acid and epoxy rosin E-51, and characterized by 1H NMR. The CV behaviors of 2-Fc in different organic solvents are recorded, and the major influential factors, namely, potential scan rate, concentration of electrolyte, and concentration of 2-Fc, and the mechanism of the electrode process are investigated. The CV behaviors of 2-Fc film on electrode in LiClO4 aqueous and organic solutions and the application of 2-Fc in H2PO4recognition are also studied.

1. Introduction Anion guests are found to play a key role in medicine,1,2 the environment,3,4 and especially biology, because most enzyme substrates and cofactors are negatively charged and the genetic information carrier DNA is also a polyanion.5,6 Thus, recognition of anions will be meaningful for the study of biological systems. Researchers have been trying to design artificial anion receptors that can bind anions effectively and give macroscopic physical phenomena allowing the detection of complexation events.7-12 This requires that there must be at least a binding site in the anion receptor molecule structure to effectively bind the target anions and also a signaling unit to transport the binding information that can be detected by some methods.13 Ferrocene-based compounds have been proven to be good anion receptors, because the ferrocene group designed to directly connect to the “binding site” is not only a high-effective electrochemical signaling unit, it can also participate in the binding event through electrostatic interaction.13 Electrochemical methods such as cyclic voltammetry (CV) are usually and maturely used to study ferrocene derivatives for anion recognition.14-19 Amideferrocene,14-16 urea-ferrocene,17,18 and quaternized-nitrogen ferrocene groups13,19 have been used as effective parts of ferrocene-based anion receptors. Also, special molecular structures in these anion receptors are required to effectively and electively bind target anions.12 In this paper, a novel ester-ferrocene compound with a special structure is designed and synthesized through the ringopening-addition reaction of ferrocene carbonate acid and epoxy rosin E-51. In this novel ester-ferrocene compound molecule, a ferrocene group and a hydroxyl group are directly connected to a carbonyl group. Thus this unique structure satisfies the essential requirement of anion receptors. This novel esterferrocene compound as an anion receptor and its electrochemical behaviors in solutions and films are studied. 2. Experimental Section 2.1. Instruments and Reagents. The CV measurements of solution were carried out with a CHI-600A electrochemical analyzer (CH Instruments, Inc., Austin, TX) in an undivided * Corresponding author. Telephone: +86-571-8795-3200. Fax: +86571-8795-1612. E-mail: [email protected].

three electrode cell. The working electrode was a Teflonshrouded Pt disk electrode (2 mm, geometric area ) 0.0314 cm2), which was polished to a mirror finish with 0.05-µm Al2Cl3 paste on felt, cleaned by ultrasonication successively in 0.1 M NaOH, 1:1 HNO3, anhydrous ethyl alcohol, and doubly distilled water, and then dried and used for electrochemical measurements. Pt wire counter electrode and Ag wire reference electrode were used. The CV measurements of film were carried out with the same instrument as the solution CV measurements. The difference was that the working electrode was a Teflon-shrouded glassy carbon disk electrode (Φ ) 3 mm, geometric area ) 0.071 cm2), and the reference electrode was Ag/AgCl (3 M KCl). The solution ohmic potential (iR) drop of the investigated solution was measured and autocompensated with the CHI-600A electrochemical analyzer. Every CV experiment was done three times to obtain reproducible results. All the reagents in use were from commercial markets. Before use, ferrocene carbonate acid (AR) was dried under vacuum condition, and solvents such as dichloromethane (AR), tetrahydrofuran (AR), dimethylformamide (AR), and dimethyl sulfoxide (AR) were distilled twice in the presence of a molecular sieve. Other solid reagents such as LiClO4 (AR) and Bu4NBF4 (AR) were directly used. 2.2. Synthesis and Characterization of Ferrocene-Based Compound 2-Fc. Epoxy E-51 (0.4950 g, 1.607 mmol), ferrocene carbonate acid (1.4808 g, 6.438 mmol), and tetrabutylammonium bromide (AR, 0.0120 g, 0.0037 mmol) reacted in 10 mL of dimethyl sulfoxide (DMSO) with agitation at the temperature 120 °C for 2.5 h. Then the solution was sedimented in 220 mL of 0.27% NaHCO3 and filtered under reduced pressure. The solid products were washed in 220 mL of 0.27% NaHCO3 until the solution was clear, and then washed with 200 mL of distilled water five times. The solid products were then solved in 30 mL of CHCl3. After filtration and evaporation, 1.092 g of a deep brown solid product (2-Fc) was obtained. The 1H NMR spectrum of 2-Fc was recorded with a 500 MHz AVANCE NMR spectrometer (Model DMX500) in CDCl3, using TMS as the standard.

10.1021/jp068289f CCC: $37.00 © 2007 American Chemical Society Published on Web 03/23/2007

Electrochemistry of a Ferrocene-Based Compound

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Figure 1. 1H NMR of the ferrocenyl compound 2-Fc.

Figure 2. Cyclic voltammogram of 2-Fc in four different organic solvents. Electrolyte, 0.01 M Bu4NBF4; [Fe], 0.25 mM; scan rate, 0.10 V/s.

2.3. 2-Fc Film-Coated Electrodes. 2-Fc film-coated electrodes were prepared by covering the glassy carbon disk electrodes with a solution of 2-Fc in CH2Cl2 (0.033 mol/L) and then allowing the solvent to evaporate at room temperature. 3. Results and Discussion The structure of 2-Fc is characterized by 1H NMR (Figure 1). The 1H NMR spectrum of 2-Fc shows peaks with the following shifts: 7.13, 6.83 (-C6H4-), 4.82, 4.42, 4.20 (protons from ferrocene rings), 4.47(-CH-OH), 4.26 (-CO-O-CH2), 4.12 (-OH), 4.09 (-C6H4-O-CH2-), 1.63(-CH3); these correspond to the structure of expected product. Before studying the application of 2-Fc in anion recognition through an electrochemical method, it is helpful to first study its electrochemical behaviors in solutions and films and the major factors that affect its behaviors, such as the polarity of the solvent, scan rate in CV experiment, electrolyte concentration, and concentration of the redox substance, so that proper conditions can be found.

3.1. CV Behaviors of 2-Fc in Different Organic Solvents. The CV behaviors of 2-Fc in CH2Cl2, tetrahydrofuran (THF), dimethylformamide (DMF), and DMSO were investigated (Figure 2). The CV results indicated that the polarity of the solvent has a great effect on the CV behavior of 2-Fc. The increase of the polarity of the solvent leads to the deformation of the peak shapes and the decrease of the peak currents. Also, the reduction peak potential increases with the increase of the solvent polarity, which indicates that the larger the solvent polarity is, the harder 2-Fc is to reduce.20 These phenomena may be explained as that the polarity of solvent has great influence on charge transport and the stability of the positively charged oxidation state of 2-Fc. The larger the solvent polarity is, the more stable the oxide 2-Fc is, and thus the harder 2-Fc is to reduce. 3.2. Influence of Scan Rate on CV Behaviors of 2-Fc. In CH2Cl2 with 0.01 M Bu4NBF4, the CV behaviors of 2-Fc at different scan rates were investigated as shown in Figure 3. It is found that the CV peak current values ip of 2-Fc are

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Tan et al.

Figure 3. CV of 2-Fc in CH2Cl2 at scan rates of 20, 40, 60, 80, 100, 120, and 140 mV/s from inner to outer. Electrolyte, 0.01 M Bu4NBF4; concentration of 2-Fc, 0.25 mM (in Fe units).

Figure 5. CV of 2-Fc in THF with different electrolyte concentrations: (a) 0.01 M; (b) 0.05 M; (c) 0.10 M. [Fe], 0.25 mM; scan rate, 0.10 V/s.

Figure 4. Peak potential of 2-Fc vs potential scan rate. Electrolyte, 0.01 M Bu4NBF4; concentration of 2-Fc, 0.25 mM (in Fe units).

Figure 6. CV of 2-Fc in CH2Cl2 with different 2-Fc concentrations of 0.25, 0.50, 0.75, and 1.00 mM from inner to outer. Electrolyte, 0.01 M Bu4NBF4; scan rate, 0.10 V/s.

TABLE 1: Relationship between Peak Currents and Potential Scan Ratesa for Sample 2-Fc rate range (V/s)

linear equation

correln coeff r

0.02-0.12 0.02-0.12 0.14-0.20 0.14-0.20

ipc ) 1.21522 + 21.76994V1/2 ipa ) -4.80249 + 5.49512V1/2 ipc ) 0.33878 - 14.17674V1/2 ipa ) -3.94609 - 1.79963V1/2

0.999 69 0.998 23 -0.989 35 -0.995 31

a

Peak search potential range is 0.8.

proportional to the square root of the scan rate (V1/2) over a wide scan rate range. Furthermore, the increase of the reduction peak currents ipc with increasing scan rate is faster than that of oxidation currents ipa. The results of the linear correlation between ip and V1/2 are listed in Table 1. These relationships indicate that at room temperature the charge transport obeys Fick’s law.21 The electrode processes in solution are diffusion controlled. Wave shapes were sensitive to the scan rate and became more distorted at higher scan rates. The potential of the reduction peak shifted toward more negative potentials, and the potentials of the oxidation peaks shifted to more positive potentials; thus, the peak-to-peak separation ∆Ep increased with increasing scan rate. Compared with the oxidation peaks, the reduction peaks were widened and shifted negatively much more, so the apparent formal potential E° ′, where E° ′ ) (Epc + Epa)/2, shifted to slightly more negative potentials with increasing scan rate. The relationship between the peak potential (Ep - E° ′) and the logarithm of potential scan rate ln V of 2-Fc is shown in Figure 4. The peak potential of 2-Fc shifted with increasing potential scan rate, but was nonlinear. At slow scan rates, the

variability of peak potential with potential scan rate changed slightly. Along with the potential scan rate increase, the variability of peak potential with scan rate increased. According to the theory of CV,21 the peak potential Ep is independent of scan rate and ∆Ep < 60 mV in a reversible electrode process. In a totally irreversible process, there is a linear relationship between the peak potential and the logarithm of the scan rate and ∆Ep > 60 mV. The experimental results indicate that the electrochemical processes of 2-Fc in CH2Cl2 are neither totally irreversible nor simply reversible. It is obvious that the electrontransport rate in the process is slow. Although the electrode processes are diffusion controlled, the rate of the electrode reaction is rather slow. Thus, the larger the potential scan rate is, the less possibility the electrode reaction has to reach completion in time. Therefore, the CV process is controlled by both mass diffusion and electrode reaction. 3.3. Influence of Concentration of Electrolytes on CV Behavior of 2-Fc. The influence of electrolyte concentration on CV behaviors of 2-Fc is also investigated. Because in CH2Cl2 the electrolyte concentration has much influence on the dissolvability of 2-Fc (in the solution of 0.01 M Bu4NBF4 and 0.50 mM 2-Fc, 2-Fc will slowly precipitate), THF is used as the solvent. The results are shown in Figure 5. With the increase of Bu4NBF4 concentration, the oxidation peak shifted toward negative potential and the reduction peak shifted to positive potential, and thus the peak-to-peak separation ∆Ep decreased. However, when the concentration of Bu4NBF4 was low, the influence was apparent. When the concentration was higher, the influence reached a much lower degree. Similarly, the

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Figure 7. CV of 2-Fc film in 0.1 M LiClO4 aqueous solution. (a) Peak current increases with sweep time. (b) Peak current reaches dynamic balance with sweep time. (c) Peak current decreases with sweep time. (d) Peak current reaches steady state. Coverage, 2.8 × 10-6 mol cm-2 Fe units; scan rate, 0.10 V/s.

Figure 8. CVs of 2-Fc film in organic solutions and comparison with the one in aqueous solution: (a) acetone; (b) acetonitrile; (c) ethanol; (d) water. Coverage, 2.8 × 10-6 mol cm-2 Fe units; electrolyte, 0.1 M LiClO4; scan rate, 0.10 V/s.

increase of electrolyte concentration led to the increase of oxidation peak current and the decrease of reduction peak current. These may be explained as that the increasing electrolyte concentration makes charge transport easier and the precipitation of 2-Fc on the electrodes easier. However, when the concentration of electrolyte increases, the effect on charge transport and the precipitation gradually reaches saturation. 3.4. Influence of Concentration of 2-Fc on its CV Behavior. The influence of the concentration of 2-Fc on its CV behavior in CH2Cl2 is investigated as well. The results are shown in Figure 6. With the increase of the 2-Fc concentration, there are phenomena similar to those with the increase of potential scan rate. The oxidation peak shifted toward positive potential

Figure 9. CV changes of 2-Fc in CH2Cl2 with the addition of H2PO4-. Curve 1, free; curve 2, H2PO4-/2-Fc ) 0.1; curve 3, H2PO4-/2-Fc ) 0.3; curve 4, H2PO4-/2-Fc ) 0.5; curve 5, H2PO4-/2-Fc ) 0.6; curve 6, H2PO4-/2-Fc ) 0.7.

SCHEME 1: Scheme of Kaifer-Echegoyen Model23

and the reduction peak shifted to negative potential, and thus the peak-to-peak separation ∆Ep increased. The peak currents also increased. These can be explained as that the increase of the concentration of redox substances makes the velocity of mass

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Figure 10. Changes in UV-vis spectrum of 2-Fc after the addition of H2PO4-. H2PO4-/2-Fc is 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, and 2.4 from outer to inner.

Figure 11. Decrease of intensity of the characteristic peak at 304 nm with the addition of H2PO4-.

diffusion become quicker and thus the electrode processes become more controlled by the electrode reaction, which is characterized by the superpotential ∆Ep. 3.5. CV Behavior of 2-Fc Film in LiClO4 Aqueous Solution and Organic Solutions. The CV behaviors of 2-Fc films on electrodes in LiClO4 aqueous solution were investigated. The results are shown in Figure 7. The CV behaviors appeared to be four successive phases according to continuous changes of the peak currents. Figure 7a shows the first phase, in which the peak currents increased with successive scans. Figure 7b shows the second phase, in which the peak currents achieved balance and did not change with successive scans. Figure 7c shows the third phase, in which the peak currents decreased with successive scans. Figure 7d shows the last phase, in which the peak currents reached steady state. These are explained as that CV behaviors

Figure 12. Probable models of the binding of 2-Fc and H2PO4-.

Tan et al. of 2-Fc film are controlled by two opposite factors, one of which is the gradual swelling of the film and the permeation of supporting electrolytes that cause the increase of the peak currents. The other factor is the activity decrease of 2-Fc film, which may be caused by the decomposition of the positively charged oxidative 2-Fc and leads to the decrease of the peak currents. From the results, the swelling of the film and the permeation of supporting electrolytes have great influence on CV behavior of 2-Fc film. However, in aqueous solution, water has a low ability to swell the 2-Fc films. Thus CV behaviors of 2-Fc film in organic solvents are also investigated. Figure 8 shows the CV behaviors of 2-Fc film in LiClO4-acetone, -acetonitrile, and -ethanol solutions and the comparison of them with aqueous solution. It is found that in acetone and acetonitrile solutions, the CV oxidation peak potential was more negative and the reduction peak potential was more positive than in aqueous solution, and the peak currents were also larger, which indicates that acetone and acetonitrile are better solvents to swell 2-Fc film and the supporting electrolytes are easier to permeate into the films.22 However, in ethanol, the peak potentials and peak currents had nearly no difference from the aqueous solution, indicating that the film-swelling abilities of ethanol and water are more or less the same. 3.6. Application of 2-Fc in Recognition of H2PO4-. Application of 2-Fc in anion recognition was studied. CV was used to study the recognition of H2PO4- with 2-Fc in 0.01 M Bu4NBF4-CH2Cl2 solution. Figure 9 shows the CV changes after the addition of H2PO4-. With the addition of H2PO4-, the reduction peak (0.74 V) gradually disappeared and a new reduction peak appeared at the potential less positive (0.35 V), which indicates that H2PO4- has been bound by 2-Fc.14-17 According to the Kaifer-Echegoyen model (Scheme 1),23 the peaks shifted to less positive potential because the ability of positively charged ferrocenium 2-Fc to bind to H2PO4-, characterized by the apparent association constant K(+), is much larger than that of neutral ferrocene 2-Fc, characterized by K(0). From eq 1, which comes from the Kaifer-Echegoyen model, the ratio of the apparent association constants of the ferrocenium 2-Fc and the ferrocene 2-Fc K(+)/K(0) can be given by the shift value of the peak potential.

E°free - E°bound ) ∆E° ) 0.058 ln(K(+)/K(0))

(1)

The UV-vis spectrum is also used to help investigate the recognition of H2PO4- by 2-Fc. The results are shown in Figure 10. After the addition of H2PO4- to the 2-Fc-CH2Cl2 solution (0.25 mM Fe units), the intensity of the peak at 304 nm, which is the characteristic peak of the carbonate group, gradually

Electrochemistry of a Ferrocene-Based Compound decreased, indicating that H2PO4- has been bound and the carbonate group was involved in the binding event. From the intensity decrease with the addition of H2PO4-, the equivalent point at about 0.6 equiv per Fe is found (Figure 11), which indicates that the model of the binding of 2-Fc and H2PO4involves two 2-Fc molecules with a H2PO4-. This conclusion agrees with the one from CV. Thus, several possible models of the binding of 2-Fc and H2PO4- can be inferred and are given in Figure 12. However, it needs more experiments to tell which model is the dominant one. 4. Conclusions A novel ester-ferrocene compound 2-Fc was synthesized, its CVs in different organic solutions were recorded, and the influences of solvent were discussed. The major influential factors, namely, the scan rate, the concentration of electrolyte, the concentration of 2-Fc, and the mechanism of the electrode process, were investigated. The CV behaviors of 2-Fc film on electrodes in LiClO4 aqueous and organic solutions were also investigated. Finally, the application of 2-Fc in H2PO4- recognition was studied. The results indicate that solvents with different polarities have different influences on charge transport and the stability of the positively charged ferrocenium 2-Fc, thus affecting the CV behaviors of 2-Fc. The increasing scan rate leads to the increase of the CV peak currents, and over a wide potential range, the peak currents ip have linear relations with the square root of the potential scan rate (V1/2), indicating that the charge transport obeys Fick’s law and the electrode processes in solution are diffusion controlled. The peak potential shifts with the increasing scan rate, but does not have a linear relationship, indicating that the electrode process is controlled by both the electrode reaction and mass diffusion. In low electrolyte concentration, the concentration of electrolyte has a large influence on CV behavior. However, in high electrolyte concentration, the electrolyte concentration has low influence. The increase of the concentration of 2-Fc makes the mass diffusion easier and thus the electrode process becomes controlled by electrode reaction, which is characterized by the increase of superpotential ∆Ep. The CV behavior of 2-Fc film in LiClO4 aqueous solution appears to be four successive phases according to the continuous changes of peak currents with successive sweeps. These can be explained as the process of the swelling of the film, the permeation of the supporting electrolyte, and the activity decrease of the film. Acetone and acetonitrile have better

J. Phys. Chem. B, Vol. 111, No. 15, 2007 3909 abilities for swelling the film; thus, the CV behaviors appear to be more reversible than in aqueous solution. 2-Fc has an apparent response to the addition of H2PO4-. The addition of H2PO4- makes the induction peak of CV gradually disappear, and a new reduction peak appears at less positive potential. The addition of H2PO4- also leads to the decrease of the intensity of the characteristic peak at 304 nm of the UV-vis spectrum of 2-Fc. From both CV and UV-vis, equivalent points can be found, and the possible models of the binding of H2PO4- by 2-Fc are inferred. Acknowledgment. This work is supported by the National Nature Science Foundation of China (No. 20572097 and No. 20672097). References and Notes (1) Quinton, P. M. FASEB J. 1990, 4, 2709. (2) Renkawek, K.; Bosman, M. Neuroreport 1995, 6, 929. (3) Holm, E. Radiochim. Acta 1993, 63, 57. (4) Moss, B. Chem. Ind. 1996, 407. (5) Stry, L. Biochemistry, 4th ed.; W. H. Freeman & Co.: New York, 1988. (6) Lang, L. G.; Riordan, J. F.; Vallee, B. L. Biochemistry 1974, 13, 4361. (7) Beer, P. D.; Elizabeth, J. H. Coord. Chem. ReV. 2003, 240, 67. (8) Beer, P. D.; Philip, A. G. Angew. Chem., Int. Ed. 2001, 40, 486. (9) Beer, P. D.; James, C. Coord. Chem. ReV. 2000, 205, 131. (10) Beer, P. D.; Philip, A. G.; George, Z. C. Coord. Chem. ReV. 1999, 185-186, 3. (11) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S. Chem. ReV. 1995, 95, 2529. (12) Beer, P. D.; Graydon, A. R.; Johnson, A. O. M.; Smith, D. K. Inorg. Chem. 1997, 36, 2112. (13) Olivier, R.; Jean, C. M.; Jacques, P.; Guy, R.; Eric, S. A. J. Electroanal. Chem. 2005, 580, 291. (14) Marie, C. D.; Jaime, R.; Jean, C. B.; Nathalie, D.; Didier, A. Chem.sEur. J. 2003, 9, 4371. (15) Jaime, R.; Maria, J. R.; Marie, C. D.; Jean, C. B.; Didier, A. Chem. Commun. 2003, 464. (16) Labande, A.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2002, 124, 1782. (17) Alonso, B.; Casado, C. M.; Cuadrado, I.; Mora´n, M.; Kaifer, A. E. Chem. Commun. 2002, 1778. (18) Oton, F.; Tarraga, A.; Espinosa, A.; Velasco, M. D.; Bautista, D.; Molina, P. J. Org. Chem. 2005, 70, 6603. (19) Olivier, R.; Guy, R.; Eric, C.; Jean, C. M.; Eric, S. A. J. Electroanal. Chem. 2003, 15, 65. (20) Tao, C.; Li, W.; Guohua, J.; Jianjun, W.; Xuejie, W.; Junfeng, Z.; Jianfeng, W.; Chang, C.; Wei, W.; Haoqi, G. J. Electroanal. Chem. 2006, 586, 122. (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons Inc.: New York, 1980; Chapter 6. (22) Wang, X.; Wang, L.; Wang, J.; Chen, T. J. Phys. Chem. B 2004, 108, 5627. (23) Carmen, M. C.; Isabel, C.; Beatriz, A.; Moises, M.; Jose, L. J. Electroanal. Chem. 1999, 463, 87.