Easy to make highly efficient FG@ACC stable platform for sp 3 C-CF 3 bond generation and NADH regeneration under sun light

Abstract

The conversion of sun light energy into sustainble greener chemicals are a major obstacle due to the use of expensive and toxic materials. Sun light induced trifluoromethylation emerges as a highly efficient procedure to insert trifluoromethyl groups into the organic compounds. Yet, the expensive and toxic properties of the metal-based photocatalysts creates a major obstacle for the insertion of trifluoromethyl groups. Metal free activated carbon cloth (ACC) emerged as a highly efficient pillar in the area of material science. In this work, we have successfully synthesized self-assembled metal free fast green with activated carbon cloth (FG@ACC) photocatalyst for photocatalytic trifluoromethylation and reduced nicotinamide adenine dinucleotide (NADH) cofactor regeneration (85.89 %, 2 h) under sun light. The sun light induced organic transformation promotes the use of low-cost CF₃SO₂Na as the CF₃ radical source to produce highly selective products with 97% yield.

Keywords

FG@ACC photocatalyst activated carbon cloth NADH regeneration sun light

1 Introduction

The assembly of trifluoromethyl (CF₃) is a most significant building block in medicinal and agrochemical area as it can powerfully improve the lipophilicity, solubility and antioxidizability of the organo fluorine compounds [1 –3]. A novel synthetic photocatalysis technology has been introduced by the researchers for introducing CF₃ group into organic moieties via the generation of the CF₃ radical under the moderate conditions [4, 5]. Usually, metal-based photocatalysts used in such photocatalytic reaction systems, for instance, the iridium, ruthenium copper and iron complexes [6 –10]. Though, metal-based photocatalysts are highly toxic and expensive, so it emerges as a major obstacle for photocatalytic reaction. Nowadays, metal-free photocatalysis technique have been newly establishedfor trifluoromethylation. For instance, some researchers have used organic dye as sun light active catalyst for the organic transformation reaction [11]. In addition, sun light induced trifluoromethylation reactions in the absence of catalysts have also been reported where reactants itself functioned as photocatalyst [12]. But these reactions have some boundaries, for instance, requirement of certain initiators or precise reactants, metal-free sun light active catalytic trifluoromethylation emerge as a field of intense research effort [13, 14]. Currently, activated carbon cloth (ACC) based materials have been considered as a highly efficient metal-free photocatalyst because of their outstanding physical and chemical properties [15]. ACC has received substantial interest as a better adsorbent for water and air treatment applications [16]. Along with this, ACC also contains high storage and separation capacity of gases such as CH₄, H₂, and CO₂ etc [17]. Some researchers have used MoS₂ decorated ACC for enhancing the supercapacitor performances [18]. In addition, many researchers have also reported the adsorption of various metals such as cadmium, palladium, etc over the ACC and utilized it in H₂O₂ synthesis and surface oxidation application [15, 19]. In this article, the synthesis and development of metal-free sun light active photocatalyst that is capable for the insertion of trifluoromethyl groups has been explored. We have used ACC as a template for preparing sun light harvesting photocatalyst. Due to its inexpensive and re-usable nature it emerges as a green precursor in photocatalyst synthesis. But, the ACC backbone suffers with many issues such as poor sun light absorption and band gap problem. Along with these, it also have high recombination rate of photogenerated electron–hole pairs. Higher recombination rate of electron–hole pairs have significantly minimize the photocatalytic efficiency [20]. Self-assembly, functionalization or doping with the excellent sun light harvesting chromophore emerged as an important method to minimize these problems [21, 22]. Nowadays, an organic chromophore fast green (FG) is broadly used in food, medicinal and cosmetics industries [23]. It also possesses excellent sun light absorbing characteristic. In this article, we have designed metal-free fast green with activated carbon cloth (FG@ACC) photocatalyst by self-assembly route. The π–π interaction between the synthesized FG@ACC photocatalyst makes it highly efficient for organic transformation and artificial photocatalysis [24]. The synthesized FG@ACC photocatalyst is highly efficient for sp³C-CF₃ bond generation due to the suitable optical band gap, outstanding molar extinction coeffiecient and low rate of inter system crossing.

Nowadays, the development of artificial photosynthesis system for fine solar chemical synthesis is a highly challenging task. The artificial photosynthesis syetem is similar to natural photosynthetic mechanism. In the natural photosynthetic process, H₂O splits and generate O₂ and converts CO₂ into sugar. The natural photosynthetic mechanism has the excellent redox ability because of the occurence of strong visible light absorbing semiconductors materials. By inspiring the natural photosynthetic system, we have developed the FG@ACC photocatalyst based artificial photosynthetic systems for reduced nicotinamide adenine dinucleotide (NADH) cofactor regeneration. The photocatalytic sp³C-CF₃ bond generation and NADH regeneration is shown in Scheme 1 [25].

Scheme 1

Pictorial Illustration of trifluoromethylation of arenes and NADH regeneration.

2 Results and discussion

2.1 Preparation and characterization of FG@ACC photocatalyst

2.1.1 Materials and methods

Carbon cloth, FG, N, N-dimethyl formamide (DMF), dimeythylsulphoxide (DMSO), toluene, acetonitrile (ACN), distilled water, methanol, dioxane, dicholoromethane (DCM), chloroform, ethylacetate (EA), CF₃SO₂Na, sodium phosphate monobasic dihydrate (NaH₂PO₄.2H₂O), nicotinamide adenine dinucleotide (NAD⁺), 2, 2 bipyridine (pentamethylcyclopentadienyl) rhodium(III) chloride dimer and ascorbic acid were purchased from TCI and Sigma Aldrich and used without further purification.

Synthesis of ACC

A reported method was used to synthesize activated carbon cloth (ACC) [24]. Initially, the carbon cloth (1 cm x 1 cm) was rinsed with acetone and deionized water for multiple times. Then, the carbon cloth was treated with conc. HNO₃ at 100°C for about 4 h. The carbon cloth was properly rinsed with deionized water plus acetone after the acid treatment. The freshly prepared activated carbon cloth was then dried in an oven at 70°C [24].

Synthesis of FG@ACC photocatalyst

Initially, 350 mg of powdered activated carbon cloth and 150 mg of FG was mixed in 20 mL DMF. Then the mixed solution was stirred at room temperature for 2 h. After that, the stirred solution was place in an autoclave at 150°C for 12 h. After this procedure, the excess solvent was evaporated to obtain the desired product. The obtained product was then washed with distilled water for 3 times. Finally, the compound was dried in the oven for overnight (Scheme 2). The yield of the compound obtained was 257 mg [24].

Scheme 2

Chemical synthesis of FG@ACC photocatalyst.

Trifluoromethylation of Toluene

Initially, powdered FG@ACC (20 mg) was dissolved in 1 mL of ACN. Afterward, CF₃SO₂Na (0.5 mmol) and toluene (1 mL) were added to the solution. Then the reaction mixture was stirred for 0.5 h. The resulting slurry was agitated in the dark in presence of air for 0.5 h. Subsequently, the reaction mixture was illuminated under sun light for 24 h. After the completion of reaction, the reacted mixture was filtered and washed with ethyl acetate. Thereafter, the excess solvent was evaporated by rotatory evaporation. The as-obtained crude product was purified by using silica gel column chromatography (eluent: hexane/ethyl acetate: 10 : 1) to afford the pure product [26].

2.1.2 Characterizations

The UV-visible absorption studies of FG@ACC, ACC and FG have been conducted in DMF (Fig. 1). Here, we found that after the self-assembly of FG over the ACC, the resultant FG@ACC photocatalyst shows broad sun light absorbance spectrum in the visible region. The creation of new π–π electonic interaction is responsible for the broad sun light absorbance spectrum [21 –25]. The excellent light harvesting ability and optical band gap suitability makes the FG@ACC photocatalyst more efficient for photocatalytic reaction.

Fig. 1

UV-visible absorption spectra of ACC, FG, and FG@ACC photocatalyst.

The optical band gap of FG@ACC photocatalyst is calculated by using Scherrer equation (1240/λ) [24]. The calculated optical band gap of FG@ACC photocatalyst is 2.86 eV at a wavelength 433 nm, which enhances its sun light harvesting ability in the visible region.

The cyclic voltammetry (CV) experiment backs up the calculated optical band gap (see in Fig. 2) [24, 27]. The redox (oxidation/reduction) potential values of the FG@ACC photocatalyst were determined using a CV experiment. The oxidation and reduction potentials were found to be+1.20 V and -1.58 V, respectively. The calculated band gap from CV supports the optical band gap value of FG@ACC. The bathochromic shift in the absorption spectra of FG@ACC photocatalyst was also observed, which improves its visible light harvesting capabilities.

Fig. 2

a) Cyclic voltammetry and b) Latimer diagram showing photoredox property of FG@ACC.

The FTIR spectra of FG@ACC photocatalyst along with ACC and FG [Fig. 3] gave a clear evidence for the presence of π–π interaction in FG@ACC. The FTIR spectrum of FG@ACC has stretching peak near about at 3450 cm^-1 that confirms the presence of –OH group [28]. The stretching peak of SO₃^- is also observed near about at 1250 cm^-1, which is completely dissappeared in the FTIR spectra of ACC [29, 30].] The outcome confirms the successful creation of π–π interaction in FG@ACC photocatalyst.

Fig. 3

FTIR spectra of ACC, FG and FG@ACC photocatalyst.

2.2 Catalytic Trifluoromethylation of Arene

In Table 1, the photocatalytic efficiency of FG@ACC is compared with previously reported photocatalysts. The result shows the excellent performance of FG@ACC photocatalyst for trifluomethylation and NADH regeneration over the other reported photocatalysts.

Table 1
A comparative literature review on trifluoromethylation of arene and NADH regeneration

Entry Photocatalyst NADH regeneration Trifluoromethylation of arenes Ref.

1. AgF ..................... 60 %, 20 h 31

2. Fmoc-FF/g-CN 62.7%, 3 h ....................... 32

3. N-EGQD 73.31%, 2.5 h ....................... 33

4. FG@ACC 85.89 %, 2 h 97 %, 24 h Our Work

Entry	Photocatalyst	NADH regeneration	Trifluoromethylation of arenes	Ref.
1.	AgF	.....................	60 %, 20 h	31
2.	Fmoc-FF/g-CN	62.7%, 3 h	.......................	32
3.	N-EGQD	73.31%, 2.5 h	.......................	33
4.	FG@ACC	85.89 %, 2 h	97 %, 24 h	Our Work

Reaction condition: Toluene (1 mL), FG@ACC (20 mg), solvent (1 mL) and CF₃SO₂Na (0.5 mmol) in the presence of air under sun light for 24 h.

We have optimized the photocatalytic sp³C-CF₃ bond generation under sun light in Table 2. When the reaction is optimized by choosing toluene (1 mL) as a substrate, FG@ACC (20 mg) as a photocatalyst in EA and dioxane solvent (1 mL) in the presence of air and CF₃SO₂Na under sun light, we have got 61% and 55% yield and 63% and 54% selectivity of the product. Similarly we have also screened the reaction in the presence of other solvents such as DCM, toluene, chloroform, THF, acetone etc, see in Table 2. Furthermore, when the reaction is screened in ACN medium, 97% yield and 98% selectivity of product was acheived. The result suggests that solvent plays an important role in photocatalytic trifluoromethylation reaction. The trifluoromethylation of arenes is favoured in ACN medium, due to the polarity effect [34]. In addition, standard reaction condition using FG photocatalyst gives 36% yield and 38% selectivity of the photocatalytic product. The control experiment is also performed in the abscence of photocatalyst, sun light and solvent. In the abscence of sun light, 4% yield and 6% selectivity of the photocatalytic product was recieved. Furthermore, no product formation was observed in the abscence of photocatalyst, and solvent. The result suggest that FG@ACC, sun light and ACN is essential requirement for photocatalytic trifluoromethylation [26].

Table 2

Optimization of the trifluoromethylation of arenes


Entry	Photocatalyst	Solar light	Solvant	Yield (%)^b	Selectivity (%)^b
1	FG@ACC	Yes	EA	61	63
2	FG@ACC	Yes	Dioxane	55	54
3	FG@ACC	Yes	DCM	41	43
4	FG@ACC	Yes	Toluene	14	12
5	FG@ACC	Yes	Chloroform	37	38
6	FG@ACC	Yes	THF	29	27
7	FG@ACC	Yes	Acetone	56	55
8	FG@ACC	Yes	ACN	97	98
9	FG	Yes	ACN	36	38
10	FG@ACC	No	ACN	4	6
11	Absence	Yes	ACN	0	0
12	FG@ACC	Yes	Absence	0	0

2.2.1 Mechanistic study of the generation of sp³C-CF₃ bond

As shown in Scheme 3, the production of the superoxide radical anion arises via tandem single-electron transfer (SET) from the FG@ACC photocatalyst to oxygen during the reaction. Consequently, a CF₃SO₂Na produced the trifluoromethyl radical and SO₂. The intermediate toluene radical was generated by a radical addition of trifluoromethyl radical to toluene. To complete the aromatization reaction, the superoxide radical abstracted the hydrogen atom from the toluene radical, and the target molecule was obtained [26, 35].

Scheme 3

Mechanistic pathway of sp³C-CF₃ bond generation under sun light.

2.3 An artificial photosynthetic system for photocatalytic NADH regeneration

Scheme 1 depicts a synchronised mechanism of NADH regeneration under sun light by using FG@ACC photocatalyst. Firstly, the photoexcitation of electrons takes place after the absorption of sun light by the FG@ACC photocatalyst [25]. These photo-excited electrons are received by the electron mediator (organometallic rhodium complex). The electron mediator works as a electron transport channel between the FG@ACC photocatalyst and NAD⁺. The electron mediator rhodium complex generally enhances the rate of cofactor regeneration. After recieving the photoexcited electrons, the electron mediator is get reduced [25, 36]. It further absorbs a proton from aqueous solution followed by the transfer of a hydride and electrons to oxidized NAD⁺. During the sun light active catalysis cycle, NAD⁺ accepts a hydride and electrons, itself transforming into NADH cofactor [25, 36].

2.3.1 Generation of various possible isomers during photocatalytic NADH regeneration

During an artificial photocatalysis, enzymatically active 1,4-NADH cofactor is achieved. The oxidized NAD⁺ undergoes a direct electrochemical reduction process, shown in Scheme 4. This process generates numerous NAD isomers, which exist in both enzymatic active/inactive state [25]. Using an electron mediator, the formation of enzymatically inactive isomers are restricted. When the reaction is exposed to sun light, the rhodium complex mediator assist the generation of 1,4 NADH isomer only.

Scheme 4

Diagrammatic illustrations of the generation of various enzymatically active/inactive isomers.

The 1,4-NADH cofactor regeneration was performed in an inert condition at room temperature under sun light irradiation (>420 nm). The reaction medium consist β–NAD⁺ (248μL), AsA (310μL), electron mediator (124μL), and FG@ACC photocatalyst (31μL) in 3. 1 mL of sodium phosphate buffer (100 mM, pH 7.0)]. Firstly, the reaction was first carried out in dark for 30 minutes, no regeneration of 1,4-NADH cofactor was observed. As shown in Fig. 4, the regeneration of the 1,4-NADH cofactor was seen during sun light irradiation. It was noted that when the reaction time increased, the yield of 1,4-NADH increased. The UV-visible spectrum was used to quantify the production of 1,4-NADH by monitoring the absorbance at 340 nm. The molar extinction coefficient of 1,4-NADH at 340 nm is 6.22 mM^-1 cm^-1 [25]. By using a FG@ACC photocatalyst, we have achieved the photocatalytic efficiency of 1,4-NADH is 85.89 % in 2 h. The photocatalytic activity of FG@ACC photocatalyst is higher in both the photocatalytic reactions than that of its precursor ACC, due to the creation of strong π–π interaction between FG and ACC [25, 36].

Fig. 4

The photocatalytic activity of FG@ACC photocatalyst and ACC for NADH regeneration under sun light.

2.3.2 Mechanistic studies of 1,4-NADH Regeneration under sun light

Scheme 5 depicts a potential process for photocatalytic 1,4-NADH regeneration utilising the FG@ACC photocatalyst. The cationic form of electron mediator (A) undergoes hydrolysis during photocatalysis, resulting in the production of a water-coordinated complex (B), designated as [Cp*Rh(bpy)(H₂O)]² ⁺. Through the β-hydride elimination process, the complex (B) reacts with the formate (HCOO^-) ion [25]. This reaction results in the formation of the rhodium hydride complex (C), i.e., [Cp*Rh(bpy)(H)]⁺, along with the loss of CO₂. The reduced intermediate complex (D) is formed, when the FG@ACC photocatalyst donates the electrons to the complex C. Subsequently, NAD⁺ is coordinated with the complex D via its amide group through hydride transfer, which regenerates the regioselective 1,4-NADH cofactors [37].

Scheme 5

Plausible mechanism for photocatalytic 1,4-NADH regeneration.

Scheme 6 demonstrate the carrier generation and migration in photocatalytic system. Firstly, after sun light irradiation, excited electron-hole are generated in the valence band of FG@ACC, which is quenched by ascorbic acid, and photo-excited carriers are then transferred into the conduction band (CB) of FG@ACC. At the same time, excited photo-carriers pass from CB of FG@ACC to NAD⁺ via rhodium complex and take part in regioselective 1,4-NADH regeneration [25].

Scheme 6

Simplified potential energy diagram shows carriers generation and its migration in photocatalytic system.

2.4 Recycling experiments of FG@ACC photocatalyst for multifarious application

We have performed the recycling experiments for checking the stability of the FG@ACC photocatalyst during the photocatalytic reactions, shown in Fig. 5. During the recycling experiment of sp³C-CF₃ bond generation, we have re-used the FG@ACC photocatalyst for five successive runs (i.e., four recycles) under the same reaction conditions [toluene (1 mL), FG@ACC (20 mg), ACN (1 mL) and CF₃SO₂Na (0.5 mmol), air, sun light]. Similarly for the recycling experiment of NADH regeneration, β–NAD⁺ (248μL), AsA (310μL), rhodium complex (124μL), FG@ACC photocatalyst (31μL) in 3. 1 mL of sodium phosphate buffer (100 mM, pH 7.0)] is used. Both the photocatalytic reaction is performed under sun light irridiation. The results demonstrate that the percentage yields of the both photocatalytic reaction is almost constant during each successive runs which show the stable nature of FG@ACC photocatalyst [25]. In photocatalytic NADH regeneration, rhodium complex acts as electron transport channel between FG@ACC photocatalyst and NAD⁺. The constant yield of NADH regeneration after each recycle also suggest the higher stability of rhodium complex, shown in Fig. 5 [25].

Fig. 5

Recycling experiments of FG@ACC photocatalyst for a) NADH regeneration and b) sp³C-CF₃ bond generation.

We have also performed the FTIR analysis of recovered FG@ACC photocatalyst after reactions (trifluoromethylation and NADH regeneration). The FTIR spectrum of recovered FG@ACC also shows the stretching peak at 3443 cm^-1 and 1254 cm^-1 that confirms the existance of –OH and SO₃^- groups, respectively [28 –30]. These stretching peaks are also supported by the FTIR spectrum of FG@ACC photocatalyst (Fig. 6). The result confirms the presence of strong π–π interaction in FG@ACC photocatalyst which make it highly efficient and reuseble for photocatalytic reactions.

Fig. 6

FTIR spectra of recovered FG@ACC photocatalyst after reactions (trifluoromethylation and NADH regeneration).

3 Conclusion

In summary, metal-free photocatalytic trifluoromethylation emerged as a novel and efficient pathway for the introduction of trifluoromethyl groups into the organic moieties. The synthesized metal-free FG@ACC photocatalyst have suitable optical band gap value, broad visible light absorbance which promotes the photocatalytic reaction in a efficient manner. The FG@ACC photocatalyst catalyze the photocatalytic trifluoromethylation of arenes and NADH regeneration via greener route. The FG@ACC photocatalyst provides high yield (97 %) of trifluoromethylation product as well as NADH (85.89 %, 2 h). The current work represents a new yardstick example for highly selective solar chemical synthesis in an eco-friendly manner.

Conflicts of interest

There are no potential conflicts to mention.

Footnotes

Acknowledgments

This work was sustained by Madan Mohan Malaviya University of Technology, Gorakhpur-273010, India.

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