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

Authors: Jianping Yong; Xiaoyu Jiang; Xiaoyuan Wu; Shuijin Huang; Qikai Zhang; Canzhong Lu
Journal: Advances in Chemistry
 (2014)

---

## 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 Δ
E
p
≈
55 mV.


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## Body

## 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 1L-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), andN-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 acid1 (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 compound2a.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) δ
=
0.92
–
1.33 (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, J
=
2.0 Hz, 2H of C5H4), 4.67 (t, J
=
1.6 Hz, 2H of C5H4), 6.04 (d, J
=
6.4 Hz, 1H, O=C–NH); 13C NMR (CDCl3) δ
=
24.6, 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 acid1 (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 compound2b.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) δ
=
4.42 (s, 5H, C5H5), 4.67 (t, J
=
2.0 Hz, 2H of C5H4), 5.07 (t, J
=
1.0 Hz, 2H of C5H4), 7.39–7.45 (m, 2H), 7.52 (d, J
=
7.2 Hz, 1H), 8.08 (d, J
=
8.4 Hz, 1H); 13C NMR (CDCl3) δ
=
63.7, 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 of2c is the same as of2b. 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) δ
=
4.47 (s, 5H, C5H5), 4.72 (t, J
=
2.1 Hz, 2H of C5H4), 5.10 (t, J
=
1.3 Hz, 2H of C5H4), 7.41 (dd, J
=
1.6, 1.6 Hz, 1H), 7.49 (d, J
=
1.6 Hz, 1H), 8.03 (d, J
=
8.8 Hz, 1H); 13C NMR (CDCl3) δ
=
63.7, 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 of2d is the same as synthesis of2b. 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) δ
=
4.44 (s, 5H, C5H5), 4.61 (s, 2H of C5H4), 5.03 (s, 2H of C5H4), 7.38 (dd, J
=
4.8, 4.8 Hz, 1H), 8.38 (d, J
=
8.4 Hz, 1H), 8.68 (d, J
=
4.4 Hz, 1H); 13C NMR (CDCl3) δ
=
63.5, 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/z348[M]+.


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

Ferrocene carboxylic acid1 (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 compound2e.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) δ
=
56.4 (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

Compound2a (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 1.5
×
10
-
2 mol L−1
2a.The detailed process for preparing the solutions of 1.5 ×10−2mol L−1
2b,2c,2d, and2e is the same as2a.


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

A GCE was polished with 0.1μm 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 5μL 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 × 10−3mol 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.


## 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), andN-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 acid1 (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 compound2a.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) δ
=
0.92
–
1.33 (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, J
=
2.0 Hz, 2H of C5H4), 4.67 (t, J
=
1.6 Hz, 2H of C5H4), 6.04 (d, J
=
6.4 Hz, 1H, O=C–NH); 13C NMR (CDCl3) δ
=
24.6, 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 acid1 (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 compound2b.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) δ
=
4.42 (s, 5H, C5H5), 4.67 (t, J
=
2.0 Hz, 2H of C5H4), 5.07 (t, J
=
1.0 Hz, 2H of C5H4), 7.39–7.45 (m, 2H), 7.52 (d, J
=
7.2 Hz, 1H), 8.08 (d, J
=
8.4 Hz, 1H); 13C NMR (CDCl3) δ
=
63.7, 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 of2c is the same as of2b. 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) δ
=
4.47 (s, 5H, C5H5), 4.72 (t, J
=
2.1 Hz, 2H of C5H4), 5.10 (t, J
=
1.3 Hz, 2H of C5H4), 7.41 (dd, J
=
1.6, 1.6 Hz, 1H), 7.49 (d, J
=
1.6 Hz, 1H), 8.03 (d, J
=
8.8 Hz, 1H); 13C NMR (CDCl3) δ
=
63.7, 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 of2d is the same as synthesis of2b. 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) δ
=
4.44 (s, 5H, C5H5), 4.61 (s, 2H of C5H4), 5.03 (s, 2H of C5H4), 7.38 (dd, J
=
4.8, 4.8 Hz, 1H), 8.38 (d, J
=
8.4 Hz, 1H), 8.68 (d, J
=
4.4 Hz, 1H); 13C NMR (CDCl3) δ
=
63.5, 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/z348[M]+.


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

Ferrocene carboxylic acid1 (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 compound2e.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) δ
=
56.4 (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]+.


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

Ferrocene carboxylic acid1 (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 compound2a.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) δ
=
0.92
–
1.33 (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, J
=
2.0 Hz, 2H of C5H4), 4.67 (t, J
=
1.6 Hz, 2H of C5H4), 6.04 (d, J
=
6.4 Hz, 1H, O=C–NH); 13C NMR (CDCl3) δ
=
24.6, 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 acid1 (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 compound2b.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) δ
=
4.42 (s, 5H, C5H5), 4.67 (t, J
=
2.0 Hz, 2H of C5H4), 5.07 (t, J
=
1.0 Hz, 2H of C5H4), 7.39–7.45 (m, 2H), 7.52 (d, J
=
7.2 Hz, 1H), 8.08 (d, J
=
8.4 Hz, 1H); 13C NMR (CDCl3) δ
=
63.7, 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 of2c is the same as of2b. 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) δ
=
4.47 (s, 5H, C5H5), 4.72 (t, J
=
2.1 Hz, 2H of C5H4), 5.10 (t, J
=
1.3 Hz, 2H of C5H4), 7.41 (dd, J
=
1.6, 1.6 Hz, 1H), 7.49 (d, J
=
1.6 Hz, 1H), 8.03 (d, J
=
8.8 Hz, 1H); 13C NMR (CDCl3) δ
=
63.7, 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 of2d is the same as synthesis of2b. 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) δ
=
4.44 (s, 5H, C5H5), 4.61 (s, 2H of C5H4), 5.03 (s, 2H of C5H4), 7.38 (dd, J
=
4.8, 4.8 Hz, 1H), 8.38 (d, J
=
8.4 Hz, 1H), 8.68 (d, J
=
4.4 Hz, 1H); 13C NMR (CDCl3) δ
=
63.5, 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/z348[M]+.


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

Ferrocene carboxylic acid1 (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 compound2e.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) δ
=
56.4 (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

Compound2a (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 1.5
×
10
-
2 mol L−1
2a.The detailed process for preparing the solutions of 1.5 ×10−2mol L−1
2b,2c,2d, and2e is the same as2a.


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

A GCE was polished with 0.1μm 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 5μL 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 × 10−3mol 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.


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

Compound2a (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 1.5
×
10
-
2 mol L−1
2a.The detailed process for preparing the solutions of 1.5 ×10−2mol L−1
2b,2c,2d, and2e is the same as2a.


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

A GCE was polished with 0.1μm 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 5μL 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 × 10−3mol 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 of2a~2c, but the C5H4showed two singlets of2d. Three signals of the carbon atoms of C5H4 and one signal of carbon atom of C5H5 appear in13C 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 of1H spectra of ferrocene ring in ferrocene derivatives2a~2d.


### 3.2. X-Ray Crystallographic Study

X-ray diffraction data for2c was collected on a Siemens Smart CCD diffractometer equipped with a graphite-monochromated CuKa radiation (λ
=
1.54180 Ǻ). The structure was solved by direct method using the program SHELXL-97 and refined by full-matrix least-squares techniques on F
2 with SHELXL-97 program package [36, 37]. The molecular structure (Figure 3) and crystal data of2c are listed in Tables 1 and 2.Table 1
Crystallographic data and structure refinement for2c.
Crystal Data
2c
Experical formula
C17H12ClFeN3O2
MW
381.60
Crystal system
monoclinic
Space group
P2(1)/c
a  (
Ǻ
)
9.6361(8)
b   (
Ǻ
)
13.7876(7)
c   (
Ǻ
)
13.1082(8)
α (°)
90.00
β (°)
108.881(8)
γ (°)
90.00
V  (
Ǻ
3
)
1647.83(19)
Z
4
D
c (g cm−1)
1.538
μ (mm−1)
8.958
F (000)
776
θ range (°)
4.80 to 74.56
Reflections collected/unique
6006/3248
R
int
⁡
0.0230
R
1, w
R
2
(
I
>
2
σ
(
I
)
)
R
1
=
0.0361, w
R
2 = 0.0911
R
1, w
R
2 (all data)
R
1
=
0.0566, w
R
2 = 0.1027
GOF onF
2
0.999Table 2
Selected bond lengths
(Ǻ) and angles (°) for2c.
Bond lengths(
Ǻ
)
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 of2c.


### 3.3. Cyclic Voltammetric Behavior of Compounds2a~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 that2e can promote the electrochemical oxidation of L-Cys dramatically in 0.1 mol L−1 NaNO3 solution with a quasireversible process with Δ
E
p
≈
55 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 × 10−3mol 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 × 10−3mol L−1 L-Cys.


## 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 of2a~2c, but the C5H4showed two singlets of2d. Three signals of the carbon atoms of C5H4 and one signal of carbon atom of C5H5 appear in13C 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 of1H spectra of ferrocene ring in ferrocene derivatives2a~2d.


## 3.2. X-Ray Crystallographic Study

X-ray diffraction data for2c was collected on a Siemens Smart CCD diffractometer equipped with a graphite-monochromated CuKa radiation (λ
=
1.54180 Ǻ). The structure was solved by direct method using the program SHELXL-97 and refined by full-matrix least-squares techniques on F
2 with SHELXL-97 program package [36, 37]. The molecular structure (Figure 3) and crystal data of2c are listed in Tables 1 and 2.Table 1
Crystallographic data and structure refinement for2c.
Crystal Data
2c
Experical formula
C17H12ClFeN3O2
MW
381.60
Crystal system
monoclinic
Space group
P2(1)/c
a  (
Ǻ
)
9.6361(8)
b   (
Ǻ
)
13.7876(7)
c   (
Ǻ
)
13.1082(8)
α (°)
90.00
β (°)
108.881(8)
γ (°)
90.00
V  (
Ǻ
3
)
1647.83(19)
Z
4
D
c (g cm−1)
1.538
μ (mm−1)
8.958
F (000)
776
θ range (°)
4.80 to 74.56
Reflections collected/unique
6006/3248
R
int
⁡
0.0230
R
1, w
R
2
(
I
>
2
σ
(
I
)
)
R
1
=
0.0361, w
R
2 = 0.0911
R
1, w
R
2 (all data)
R
1
=
0.0566, w
R
2 = 0.1027
GOF onF
2
0.999Table 2
Selected bond lengths
(Ǻ) and angles (°) for2c.
Bond lengths(
Ǻ
)
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 of2c.


## 3.3. Cyclic Voltammetric Behavior of Compounds2a~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 that2e can promote the electrochemical oxidation of L-Cys dramatically in 0.1 mol L−1 NaNO3 solution with a quasireversible process with Δ
E
p
≈
55 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 × 10−3mol 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 × 10−3mol 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 that2ecan promote the electrochemical oxidation of L-Cys dramatically at its modified GCE with a quasireversible process with Δ
E
p
≈
55 mV, while L-Cys itself showed a sluggish electrochemical response at GCE. Thus,2emodified GCE can be used to detect the trace L-Cys content. (The quantitative analysis of trace L-Cys and electrochemical kinetics of2emodified 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.



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*Source: 987481-2014-05-25.xml*