Advances in Chemistry

Volume 2014 (2014), Article ID 987481, 7 pages

http://dx.doi.org/10.1155/2014/987481

Research Article

Synthesis and Characterization of Ferrocene Derivatives and Preliminarily Electrocatalytic Oxidation of L-Cysteine at Nafion-Ferrocene Derivatives Modified Glassy Carbon Electrode

Jianping Yong,¹ Xiaoyu Jiang,² Xiaoyuan Wu,¹ Shuijin Huang,¹ Qikai Zhang,¹ and Canzhong Lu¹

¹State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

²Fujian University of Technology, Fuzhou 350108, China

Received 2 April 2014; Revised 23 April 2014; Accepted 7 May 2014; Published 25 May 2014

Academic Editor: Fazlul Haq

Copyright © 2014 Jianping Yong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Five new structural ferrocene derivatives (2a~2e) were firstly synthesized and characterized by ¹H NMR, ¹³C NMR, ESI-MS, and XRD. Subsequently, the preliminarily electrocatalytic oxidation of L-cysteine (L-Cys) at nafion-ferrocene derivatives modified glassy carbon electrode (GCE) has also been investigated by cyclic voltammetry. The results showed that 2e can dramatically electrocatalyze the oxidation of L-cysteine at its modified GCE in 0.1 mol L⁻¹ NaNO₃ aqueous solution with a quasireversible process with  mV.

1. Introduction

Ferrocene derivatives have attracted considerable attention for their potential applications as nonlinear optical devices, functional materials in electrochemical sensor [1–4], and chiral catalysts [5–7]. In addition, ferrocene derivatives have also attracted wide interest for their considerably biological activities [8–15]. For example, the ferroquine (FQ, SSR97193, Figure 1) is about to complete phase II clinical trials as a treatment for uncomplicated malaria [16].

Figure 1

L-Cysteine (L-Cys) plays a crucial role in both bio- and environmental chemistry and can be applied in many biochemical processes and diagnosis of disease states. Especially, L-Cys provides a modality for the intramolecular crosslinking of proteins through disulfide bonds to support their secondary structures and functions. Therefore, it is very important to develop simple and effective methods to trace L-Cys detection. Many methods, such as spectrometric method [17], chromatography [18], and electrochemical method [19–23], have been used to detect the trace content of L-Cys. However, electrochemical method has attracted considerable attention for simple operation, fast response, and sensitive in situ detection.

The oxidation of L-Cys at Hg, Au, Ag, Pt, and diamond electrodes has been reported [24–27]. However, the direct oxidation of L-Cys at GCE is very sluggish. Ferrocene derivatives have been used as the selective probes for detecting the trace ions, biomacromolecules [28–32]. Nafion is a special material, which possesses widespread applications in the field of electrochemistry analysis, chemical sensor, and nanomaterials [33, 34]. According to our survey, no electrocatalytic oxidation of L-Cys at the nafion-ferrocene derivatives modified GCE has been reported. In this work, five new structural ferrocene derivatives (2a~2e) (Scheme 1) were firstly synthesized and then confirmed by IR, ¹H NMR, ¹³C NMR, MS, and XRD. Subsequently, their preliminarily electrocatalytic oxidation of L-Cys at the nafion-ferrocene derivatives modified GCE has also been investigated.

Scheme 1: Synthetic routes of ferrocene derivatives.

2. Experimental Details

2.1. Materials and Apparatus

2-Chloro-4,6-dimethoxy-[1,3,5]triazine (CDMT), N,N ′-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt), 6-Chloro-1-hydroxybenzotriazole (6-Cl-HOBt), 1-hydroxyl-7-azabenzotriazole (HOAt), 4-dimethylaminopyridine (DMAP), and N-methylmorpholine (NMM) were purchased from Aladdin Reagent company. 5 wt% Nafion solution (DuPont, USA) and other chemicals were commercially available and used without further purification. All melting points were determined on X-4 micro-melting points apparatus and values are uncorrected. ¹H (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded on a 400 MHz Bruker AVANCE III spectrometer in CDCl₃. The chemical shifts are expressed in ppm relative to tetramethylsilant (TMS) as the internal standard; ESI-MS was performed on a DECAX-30000 LCQ Deca XP (70 Ev). FTIR spectra were recorded on Spectrum One with KBr disks in the 600–3600 cm⁻¹ region. All electrochemical experiments were carried out using an electrochemistry workstation CHI760 (CH Instrument Inc., USA). The working electrode used in cyclic voltammetry experiments was a CHI104 glassy carbon electrode with a geometric area of ca. 0.15 cm²; a twisted platinum wire auxiliary electrode and Ag/AgCl served as the reference electrode. Tetrahydrofuran (THF) was distilled over sodium and benzophenone before being used; all electrolyte solutions were prepared with doubly distilled water.

2.2. Synthesis Parts

2.2.1. Synthesis of 2-Ferrocene Carboxylic Acid-1,3-dicyclohexyl-isourea (2a) [35]

Ferrocene carboxylic acid 1 (0.46 g, 2 mmol) and DCC (0.453 g, 2.2 mmol) were added into a 100 mL one necked round bottom flask with 20 mL dry THF. This mixture was stirred under cold bath and a THF solution (10 mL) containing DMAP (0.268 g, 2.2 mmol) was added dropwise by syringe, the mixture was stirred under cold bath for 30 min, and then temperature rose naturally to room temperature. The completion of reaction was judged from the simple TLC analysis. The mixture was evaporated under reduced pressure and the residual directly purified by column chromatography (EtOAc/petroleum ether: 5 : 1 to 2 : 1) to give the desired compound 2a.

Light yellow solid (0.75 g, 86% yield). M.p. 173°C, IR (cm⁻¹): 3235 (NH), 3039 (=C–H), 2931 (C–H), 1701 (O=C–O–), 1598 (C=C), 1542 (C=N), 1182 (C–O); ¹H NMR (CDCl₃) (m, 8H), 1.35–1.66 (m, 5H), 1.79–1.85 (m, 6H), 1.99–2.09 (m, 2H), 3.48–3.53 (m, 1H), 4.19 (s, 5H, C₅H₅), 4.28 (t,  Hz, 2H of C₅H₄), 4.67 (t,  Hz, 2H of C₅H₄), 6.04 (d,  Hz, 1H, O=C–NH); ¹³C NMR (CDCl₃) , 25.4, 25.5, 26.3, 30.9, 31.1, 31.2, 31.3, 32.6, 47.7, 49.8, 56.8, 70.4, 77.9 (C₅H₄), 70.3 (C₅H₅), 154.9 (C=N), 171.9 (–C=O); MS (ESI): m/z 436 [M]⁺.

2.2.2. Synthesis of Ferrocene Carboxylic Acid Benzotriazol-1-yl Ester (2b)

Ferrocene carboxylic acid 1 (0.46 g, 2 mmol), DCC (0.453 g, 2.2 mmol), and HOBt (0.337 g, 2.5 mmol) were added into a 100 mL one necked round bottom flask with 20 mL dry THF. This mixture was stirred under cold bath and a THF solution (10 mL) containing DMAP (0.268 g, 2.2 mmol) was added dropwise by syringe, the mixture was stirred under cold bath for 30 min, and then temperature rose naturally to room temperature. The completion of reaction was judged from the simple TLC analysis. The mixture was evaporated under reduced pressure and the residual purified directly by column chromatography (EtOAc/petroleum ether: 5 : 1 to 2 : 1) to give the desired compound 2b.

Light yellow solid (0.638 g, 92% yield). M.p. 141-142°C, IR (cm⁻¹): 3035 (=C–H), 1776 (O=C–O–), 1441 (C=C), 1261 (C=N), 1054 (C–O), 768 (Ph); ¹H NMR (CDCl₃) (s, 5H, C₅H₅), 4.67 (t,  Hz, 2H of C₅H₄), 5.07 (t,  Hz, 2H of C₅H₄), 7.39–7.45 (m, 2H), 7.52 (d,  Hz, 1H), 8.08 (d,  Hz, 1H); ¹³C NMR (CDCl₃) , 70.8, 73.5 (C₅H₄), 71.0 (C₅H₅), 108.4, 120.5, 124.7, 128.6, 128.9, 143.6 (benzotriazol-ring), 168.3 (–O–C=O); MS (ESI): m/z 347[M]⁺.

2.2.3. Synthesis of Ferrocene Carboxylic Acid 6-Chloro-benzotriazol-1-yl Ester (2c)

The detailed process for synthesis of 2c is the same as of 2b. Red crystal (0.723 g, 75% yield). M.p. 112-113°C, IR (cm⁻¹): 3100 (=C–H), 1792 (O=C–O–), 1445 (C=C), 1286 (C=N), 1029 (C–O), 881 (C–Cl), 827 (Ph); ¹H NMR (CDCl₃) (s, 5H, C₅H₅), 4.72 (t,  Hz, 2H of C₅H₄), 5.10 (t,  Hz, 2H of C₅H₄), 7.41 (dd, , 1.6 Hz, 1H), 7.49 (d,  Hz, 1H), 8.03 (d,  Hz, 1H); ¹³C NMR (CDCl₃) , 71.3, 73.9 (C₅H₄), 71.4 (C₅H₅), 108.6, 121.8, 126.4, 129.9, 135.5, 142.4 (benzotriazol-ring), 168.5; MS (ESI): m/z 482[M]⁺.

2.2.4. Synthesis Ferrocene Carboxylic Acid [1,2,3]triazolo[4,5-b]pyridin-3-yl Ester (2d)

The detailed process for synthesis of 2d is the same as synthesis of 2b. Light yellow solid (0.557 g, 80% yield). M.p. 136-137°C, IR (cm⁻¹): 3108 (=C–H), 1775 (O=C–O), 1448 (C=C), 1265 (C=N), 1058 (C–O), 887; ¹H NMR (400 MHz, CDCl₃) (s, 5H, C₅H₅), 4.61 (s, 2H of C₅H₄), 5.03 (s, 2H of C₅H₄), 7.38 (dd, , 4.8 Hz, 1H), 8.38 (d,  Hz, 1H), 8.68 (d,  Hz, 1H); ¹³C NMR (CDCl₃) , 71.1, 73.5 (C₅H₄), 71.3 (C₅H₅), 129.5, 135.1, 140.9, 151.7 ([1,2,3]triazolo[4,5-b]pyridin-ring), 168.1 (–O–C=O); MS (ESI): m/z 348[M]⁺.

2.2.5. Synthesis of Ferrocene Carboxylic Acid 4,6-Dimethoxy-[1,3,5]triazin-2-yl Ester (2e)

Ferrocene carboxylic acid 1 (0.46 g, 2 mmol) and CDMT (0.438 g, 2.5 mmol) were added into a 100 mL one necked round bottom flask with 20 mL dry THF. This mixture was stirred under cold bath and a THF solution (10 mL) containing NMM (0.253 g, 2.5 mmol) was added dropwise by syringe, the mixture was stirred under cold bath for 30 min, and then temperature rose naturally to room temperature. The completion of reaction was judged from the simple TLC analysis. The mixture was evaporated under reduced pressure and the residual purified directly by column chromatography (EtOAc/etroleum ether: 5 : 1 to 2 : 1) to give the desired compound 2e.

Light yellow solid (0.863 g, 92% yield). M.p. 90–92°C, FTIR (cm⁻¹): 3104 (=C–H), 2951 (C–H), 1747 (O=C–O), 1577 (C=C), 1469 (C=N), 1366 (C–O); ¹H NMR (400 MHz, CDCl₃) δ (ppm): 4.08 (s, 6H, 2OCH₃), 4.39 (s, 5H, C₅H₄), 4.56 (s, 2H of C₅H₄), 4.95 (s, 2H of C₅H₄); ¹³C NMR (CDCl₃) (OCH₃), 70.6, 71.3, 73.1 (C₅H₄), 71.4 (C₅H₅), 167.9 (–C=O), 171.2, 174.5 ([1,3,5]triazin-ring); MS (ESI): m/z 369[M]⁺.

2.3. Electrochemistry

2.3.1. The Process for Preparation of the Solutions of Nafion-Ferrocene Derivatives

Compound 2a (6.6 mg, 0.015 mmol) and 50 μL 5% nafion solution were added in a 2 mL centrifugal tube, then 1 mL of absolute alcohol was added, and the mixture was sonicated for 10 min to obtain the solution of  mol L⁻¹ 2a.

The detailed process for preparing the solutions of 1.5 ×10−2 mol L−1 2b, 2c, 2d, and 2e is the same as 2a.

2.3.2. General Process for Preparing the Modified Glassy Carbon Electrode and Preliminarily Electrocatalytic Oxidation of L-Cysteine

A GCE was polished with 0.1 μm α-Al₂O₃, rinsed with doubly distilled water, sonicated in 1 mol L⁻¹ H₂SO₄ for 5 min, and rinsed with doubly distilled water again prior to each experiment. Then, it was dried naturally.

A modified GCE was prepared by placing 5 μL of solution prepared in Section 2.3.1 onto the dried GCE surface by microsyringe. The electrode was dried naturally to obtain the modified GCE.

The modified GCE, a twisted platinum wire electrode, and Ag/AgCl electrode were dipped into 20 mL (V/V, 1 : 1) water solution of 6.0 × 10−3 mol L⁻¹ L-Cys and 0.1 mol L⁻¹ NaNO₃. The Ag/AgCl electrode was connected to the main body of the cell through a luggin capillary whose end was connected on the modified GCE and positioned close to the electrode surface. The solution was thoroughly flushed with high purity nitrogen for 5 min before each run to remove the oxygen from the solution in the electrochemical cell. All experiments were carried out at room temperature.

3. Results and Discussion

3.1. Chemistry

Ferrocene derivatives (2a~2e) were firstly synthesized in higher yield and their structures were confirmed by IR, ¹H and ¹³C NMR, and mass spectrometry.

There showed three kinds of proton signals for ferrocene core of the ferrocene derivatives (2a~2e) in ¹H NMR spectra (Figure 2), the protons of C₅H₅ showed a singlet, the C₅H₄ showed two triplets of 2a~2c, but the C₅H4 showed two singlets of 2d. Three signals of the carbon atoms of C₅H₄ and one signal of carbon atom of C₅H₅ appear in 13C NMR spectra, while two carbon signals of C₅H₄ appear upfield and one carbon signal of C₅H₄ appears downfield compared to the carbon signals of C₅H₅.

Figure 2: Characteristic fragments of ¹H spectra of ferrocene ring in ferrocene derivatives 2a~2d.

3.2. X-Ray Crystallographic Study

X-ray diffraction data for 2c was collected on a Siemens Smart CCD diffractometer equipped with a graphite-monochromated CuKa radiation ( Ǻ). The structure was solved by direct method using the program SHELXL-97 and refined by full-matrix least-squares techniques on with SHELXL-97 program package [36, 37]. The molecular structure (Figure 3) and crystal data of 2c are listed in Tables 1 and 2.

Table 1: Crystallographic data and structure refinement for 2c.

Crystal Data
2c

Experical formula
C₁₇H₁₂ClFeN₃O₂

MW
381.60

Crystal system
monoclinic

Space group
P2(1)/c

9.6361(8)

13.7876(7)

13.1082(8)

(°)
90.00

(°)
108.881(8)

(°)
90.00

1647.83(19)

4

(g cm⁻¹)
1.538

(mm⁻¹)
8.958

(000)
776

range (°)
4.80 to 74.56

Reflections collected/unique
6006/3248

0.0230

,
, = 0.0911

, (all data)
, = 0.1027

GOF on
0.999

Table 2: Selected bond lengths (Ǻ) and angles (°) for 2c.

Bond lengths
Bond angle (°)

Fe(1)–C(9)
2.007(3)
C(9)–Fe(1)–C(8)
41.50(12)

Fe(1)–C(1)
2.025(3)
C(9)–Fe(1)–C(2)
153.96(14)

C(9)–C(11)
1.445(4)
C(9)–Fe(1)–C(5)
127.51(17)

C(11)–O(12)
1.185(4)
C(8)–Fe(1)–C(10)
69.73(14)

O(13)–N(14)
1.362(3)
O(12)–C(11)–C(9)
130.8(3)

N(14)–C(22)
1.361(3)
O(13)–N(14)–N(15)
118.6(3)

C(20)–Cl(23)
1.730(3)
N(16)–C(17)–C(22)
109.1(3)

N(15)–N(16)
1.295(4)
N(14)–C(22)–C(21)
133.8(2)

C(19)–C(20)
1.408(4)
C(19)–C(20)–Cl(23)
117.6(2)

Figure 3: Crystal structure of 2c.

3.3. Cyclic Voltammetric Behavior of Compounds 2a~2e and Nafion Modified GCE Electrocatalyze the Oxidation of L-Cys

The preliminarily electrochemical oxidation of L-Cys with the nafion-ferrocene derivatives modified GCE in 0.1 mol L⁻¹ different supporting electrolyte solutions, such as NaCl, NaNO₃, NaAc, and Na₂SO₄, has been investigated by cyclic voltammetry. The results (Figure 4) showed that 2e can promote the electrochemical oxidation of L-Cys dramatically in 0.1 mol L⁻¹ NaNO₃ solution with a quasireversible process with  mV (curve (d) in Figure 4), while direct oxidation of L-Cys at a GCE is very sluggish (cruve (b) in Figure 4).

Figure 4: Cyclic voltammograms of (a) nafion modified GCE in 0.1 mol L⁻¹ NaNO₃; (b) nafion modified GCE in solution of 0.1 mol L⁻¹ NaNO₃ and 6.0 × 10−3 mol L⁻¹ L-Cys; (c) 2e-nafion modified GCE in 0.1 mol L⁻¹ NaNO₃ solution; (d) 2e-nafion modified GCE in solution of 0.1 mol L⁻¹ NaNO₃ and 6.0 × 10−3 mol L⁻¹ L-Cys.

4. Conclusion

In this study, five ferrocene derivatives (2a~2e) were firstly synthesized and structurally characterized using spectroscopic methods. The preliminarily electrochemical oxidation of L-Cys with the nafion-ferrocene derivatives modified GCE in 0.1 mol L⁻¹ NaNO₃ solution has also been investigated by cyclic voltammetry. The results showed that 2e can promote the electrochemical oxidation of L-Cys dramatically at its modified GCE with a quasireversible process with  mV, while L-Cys itself showed a sluggish electrochemical response at GCE. Thus, 2e modified GCE can be used to detect the trace L-Cys content. (The quantitative analysis of trace L-Cys and electrochemical kinetics of 2e modified GCE are undergoing.) Based on this research, more ferrocene derivatives will be synthesized and the oxidation of L-Cys and other biological molecules at the modified GCE will be studied soon.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was financially supported by the 973 Key Program of the MOST (2010CB933501, 2012CB821705), the Chinese Academy of Sciences (KJCX2-YW-319, KJCX2-EW-H01), the National Natural Science Foundation of China and the Natural Science Foundation of Fujian Province (2007HZ0001-1, 2009HZ0005-1, 2009HZ0006-1, and 2006L2005), and Postdoctoral Foundation of Fujian Province.

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