Preparation and Electrochemical Investigation of 4-Amino-TEMPO Derivatives

Nitroxides as stable free radicals are widely used as owing to their interesting redox behavior.¹ They are applied to radical batteries,² spin probes,³ antioxidants,⁴ magnetro-active materials,⁵ and radiation protective agents.⁶ Recently these nitoxides are interesting to organic electrolyte in redox flow batteries(RFBs) for offering the possibility of wide potential windows.⁷

In this study, we have investigated the relationship between electrochemical properties of 4-amino-TEMPO derivatives which are different electronic effects. To do this we prepared 4-amino-TEMPO derivatives 1-6 (Fig. 1) and investigated cyclic voltammetry (CV) results depending on their different functional group. Ultimately these materials will be applied for Fiber-Shaped Dye-Sensitized Solar Cells (FDSSCs) to improve efficiency and stability.⁸

Figure 1. 4-Amino-TEMPO and 4-Amino-TEMPO Derivatives.

Preparation of 4-Amino-TEMPO Derivatives 2-6⁹

4-Acetamide-TEMPO 2 in 98% yield was prepared by treatment of acetyl chloride and 4-amino-TEMPO 1 with N,N'-diisopropylethylamine (DIPEA) at 0 ℃ for 2 hours. 4-Acetamide-TEMPO 2 was characterized by IR at 3293 cm^-1 (NH stretching), 1697 cm^-1 (C=O stretching) (Scheme 1).

Scheme 1. Preparation of 4-Acetamide-TEMPO 2.

2-Benzamide-TEMPO 3 in 96% yield was prepared by treatment of benzoyl chloride and 4-amino-TEMPO 1 with DIPEA at 0 ℃ to room temperature. 2-Benzamide-TEMPO 3 was characterized by IR at 3308 cm^-1 (NH stretching), 1634 cm^-1 (C=O stretching) (Scheme 2).

Scheme 2. Preparation of 4-Benzamide-TEMPO 3.

4-Methanesulfonamide-TEMPO 4 in 94% yield was prepared by treatment of methansulfonyl chloride and 4-amino-TEMPO 1 with N,N’-diisopropylethylamine (DIPEA) at 0 ℃ to room temperature for 3 hours. 4-Methanesulfonamide-TEMPO 4 was characterized by IR at 3249 cm^-1 (NH stretching), 1326 cm^-1 (S=O asymmetric stretching), 1150 cm^-1 (S=O symmetric stretching) (Scheme 3).

Scheme 3. Preparation of 4-Methanesulfoneamide-TEMPO 4.

4-p-Methoxybenzenesulfonamide-TEMPO 5 in 58% yield was prepared by treatment of benzenesulfonyl chloride and 4-amino-TEMPO 1 with N,N'-diisopropylethyl-amine(DIPEA) at 0 ℃ to room temperature for 3 hours. 4-Benzenesulfonamide-TEMPO 5 was characterized by IR at 3293 cm^-1 (NH stretching), 1330 cm^-1 (S=O asymmetric stretching), 1162 cm^-1 (S=O symmetric stretching) (Scheme 4).

Scheme 4. Preparation of 4-Benzenesulfonamide-TEMPO 5.

4-p-Methoxybenzenesulfonamide-TEMPO 6 in 80% yield was prepared by treatment of p-methoxybenzenesulfonyl chloride and 4-amino-TEMPO 1 with N,N'-diisopropylethylamine (DIPEA) at 0 ℃ to room temperature for 3 hours. 4-p-Methoxybenzenesulfonamide-TEMPO 6 was characterized by IR at 3293 cm^-1 (NH stretching), 1331 cm^-1 (S=O asymmetric stretching), 1153 cm^-1 (S=O symmetric stretching) (Scheme 5).

Scheme 5. Preparation of 4-p-Methoxybenzenesulfonamide-TEMPO 6

Electrochemical Properties of 4-Amino-TEMPO Derivatives 1-6

Electrochemical Measurement

All electrochemical measurements were conducted on WPG100e potentiosta/Galvanostat (WonTech). The glassy carbon rotating disk electrode, Pt foil and saturated calomel electrode were used as the working electrode, counter electrode and reference electrode, respectively. The potential was reported relative to the standard hydrogen electrode. The 4-amino-TEMPO derivatives were dissolved in 0.1 M TEABF₄ with acetonitrile solution, served as electrolyte solution.

Electrochemical Properties

To compare the CV results of functionalized 4-amino-TEMPO derivatives 2-6, commercially available 4-amino-TEMPO 1 is determined by CV in 0.1 M TEABF₄ with acetonitrile solution using glassy carbon electrode(GC). Cyclic voltammogram of 4-amino-TEMPO 1 shows the half-wave potential (E_1/2) in potential range -0.05 V and I_pa/I_pc: 0.98 vs Ag/Ag+ reference electrode at 50 mV/s (Fig. 2).

Figure 2. Cyclic Voltammogram of 4-amino-TEMPO 1 in 0.1 M TEA·BF₄ in acetonitrile solution.

4-Acetamide-TEMPO 2 was determined by CV in 0.1 M TEABF₄ with acetonitrile solution using glassy carbon electrode (GC). Cyclic voltammogram of 4-acetamide - TEMPO 2 shows the half-wave potential (E_1/2) in the potential range 0.94 V and I_pa/I_pc: 0.96 vs Ag/Ag+ reference electrode at 50 mV/s (Fig. 3).

Figure 3. The CV of 4-Acetamide-TEMPO 2.

4-Benzamide-TEMPO 3 was determined by CV in 0.1 M TEABF₄ with acetonitrile solution using glassy carbon electrode (GC). Cyclic voltammogram of 4-benzamide-TEMPO 3 shows the half-wave potential (E_1/2) in the potential range 0.97 V and I_pa/I_pc: 0.97 vs Ag/Ag+ reference electrode at 50 mV/s (Fig. 4).

Figure 4. The CV of 4-Benzamide-TEMPO 3.

4-Methanesulfonamide-TEMPO 4 was determined by CV in 0.1 M TEABF₄ with acetonitrile solution using glassy carbon electrode (GC). Cyclic voltammogram of 4-Methanesulfonamide-TEMPO 4 shows the half-wave potential (E_1/2) in the potential range 0.98 V and I_pa/I_pc: 0.76 vs Ag/Ag+ reference electrode at 50 mV/s (Fig. 5).

Figure 5. The CV of 4-Methanesulfonamide-TEMPO 4.

4-Benzenesulfonamide-TEMPO 5 was determined by CV in 0.1 M TEABF₄ with acetonitrile solution using glassy carbon electrode (GC). Cyclic voltammogram of 4-benzenesulfonamide-TEMPO 5 shows the half-wave potential (E_1/2) in potential range 1.01 V and I_pa/I_pc: 0.85 vs Ag/Ag+ reference electrode at 50 mV/s (Fig. 6).

Figure 6. The CV of Benzenesulfonamide-TEMPO 5.

4-p-Methoxybenzenesulfonamide-TEMPO 6 was determined by CV in 0.1 M TEABF₄ with acetonitrile solution using glassy carbon electrode (GC). Cyclic voltammogram of 4-p-Methoxybenzenesulfonamide-TEMPO 6 shows the half-wave potential (E_1/2) in the potential range 1.04 V and I_pa/I_pc: 0.77 vs Ag/Ag+ reference electrode at 50 mV/s (Fig. 7).

Figure 7. The CV of 4-p-Methoxybenzenesulfonamide-TEMPO 6.

Comparing to CV results of 4-amino-TEMPO derivatives 1-6, 4-amino-TEMPO derivatives 2-6 are better than 4-amino-TEMPO 1 in half-wave potential (E_1/2), but the value of I_pa/I_pc is not improved. Also comparing to CV results of 4-amino-TEMPO derivatives 2-6, 4-p-methoxybenzenesulfonamide-TEMPO 6 is best in half-wave potential (E_1/2) in 4-amino-TEMPO derivatives 2-6.

In these results, we investigated the sulfonyl functional group is better than the carbonyl functional group in half-wave potential (E_1/2) by some different electronic effects.

In 4-amino-TEMPO derivatives 4-6 which has the sulfonyl functional group, p-methoxybenzenesulfonyl functional group is better than methyl and benzenesulfonyl functional group. In our laboratories, we are continuing study the reason of different half-wave potential (E_1/2) and I_pa/I_pc depending on functional group (Table 1).

Table 1. 4-Amino-TEMPO 1 and 4-Amino-TEMPO Derivatives 2-6

0.025 M 4-Amino-TEMPO derivatives and 1 M TEABF₄ in ACN at scan rate 50 mV/s