Wenchuan
Lai
,
Yuehui
Yuan
,
Xu
Wang
,
Yang
Liu
,
Yulong
Li
and
Xiangyang
Liu
*
College of Polymer Science and Engineering, State Key Laboratory of Polymer Material and Engineering, Sichuan University, Chengdu 610065, People's Republic of China. E-mail: lxy6912@sina.com; Fax: +86 28 85405138; Tel: +86 28 85403948
First published on 24th November 2017
The mechanism of nucleophilic substitution deserves more investigation to include more reaction systems such as two-dimensional (2D) materials. In this study, we used fluorinated graphene (FG) as a representative 2D material to reveal the in-depth mechanism of its defluorination and nucleophilic substitution reaction under attack of common nucleophiles to explore the chemistry of 2D materials and enrich the research on the nucleophilic substitution reaction. DFT calculations and electron paramagnetic resonance spectroscopy (EPR) demonstrated that defluorination of FG occurred via a radical mechanism after a single electron transfer (SET) reaction between the nucleophile and C–F bond, and a spin center was generated on the nanosheet and fluorine anion. Moreover, neither the SN1 nor SN2 mechanism was suggested to be appropriate for the substitution reaction of FG with a 2D structure due to the corresponding kinetics or thermodynamics disadvantage; hence, its nucleophilic substitution was proved to occur via a radical mechanism initiated by the defluorination step. The proposed substitution mechanism of FG demonstrates that nucleophilic substitution via a radical mechanism can also be applied to the attacking process of common nucleophiles without any particular conditions. Furthermore, it has been discovered that triethylamine without active hydrogen can be covalently attached to graphene nanosheets via a nucleophilic substitution reaction with FG; this further indicates a radical process for the nucleophilic substitution of FG rather than an SN1 or SN2 mechanism. The detailed process of the nucleophilic substitution reaction of FG was revealed to occur via a radical mechanism depending on the 2D structure of FG, which could also represent the typical characteristic of 2D chemistry.
Fluorinated graphene (FG), a representative 2D material and graphene derivative, has also received significant interest since its first report.17–19 It should be noted that its relatively simple chemical structure,17 high functionalization density,20 high-performance characteristics,21–23 and subsequent reactivity24,25 have made FG stand out from a variety of graphene derivatives. Moreover, these characteristics can enable FG to be an appropriate representative material for investigating the chemistry of 2D materials; accordingly, in recent years, some studies have been reported that shed light on the derivative chemistry, mainly including reductive defluorination26,27 and nucleophilic substitution, of FG.24,28–33 In these studies, the graphene functional material has been successfully prepared with desired structures based on the reactivity of FG;28,31–33 however, the detailed mechanism of the derivatization reactions is not revealed. For instance, the mechanism for defluorination of FG caused by reductive or alkaline reagents are still unknown, and the destination of fluorine atoms removed from FG is also unknown.24,31,34 Furthermore, during the displacement of the fluorine atoms, the nucleophilic substitution of the C–F bonds in FG has been designed to excessively depend on the substitution of the carbon–halogen bond of halohydrocarbon micromolecules,30,35 whereas the substitution process can be special for FG with 2D structures due to its special C–F bonds.36–38 A new thought should be introduced to the study of the derivatization reactions of FG, especially its nucleophilic substitution reactions, as well as to the research of the 2D chemistry.
Herein, we take FG as a representative 2D material to investigate the in-depth mechanism of its derivatization reactions, the chemistry of 2D materials, as well as enrich the research on nucleophilic substitution in the field of organic chemistry. We have studied the detailed mechanism of derivatization reactions of FG under attack of usual nucleophiles such as amines, phosphines, and potassium hydroxide. DFT calculations and electron paramagnetic resonance spectroscopy (EPR) demonstrated that the defluorination of FG occurred via a radical mechanism after a single-electron transfer (SET) reaction39–41 between the nucleophile and C–F bond; this resulted in a spin center on the nanosheet and fluorine anion. More importantly, it has been indicated that both the SN1 and SN2 mechanism are inappropriate for describing the nucleophilic substitution reaction of FG with a rigid 2D structure due to relevant thermodynamics or kinetics disadvantage. In contrast, the nucleophilic substitution of the C–F bonds in FG is also thought to occur via a radical mechanism initiated by the defluorination step, which is defined as a DR mechanism and is analogous to the SRN1 mechanism. The DR mechanism was then proven using some experimental results including the covalent attachment of a triethylamine fragment to graphene nanosheets by the nucleophilic substitution of FG. The mechanism of the nucleophilic substitution reaction of FG was first revealed and also used to explore the chemistry of 2D materials.
The detection of intermediate radicals during the nucleophilic reaction between FG and nucleophilic reagents, including EDA, N2H4, DPA, DPP, KOH, and TEA, was performed using PBN or MNP as a radical trapper. Herein, 0.1 mmol of radical trapper and 0.3 mmol of the nucleophilic reagent were first dissolved in 10 mL of ethanol and mixed adequately by 10 min of sonication. Then, 10 mg of FG was added to the resulting solution followed by 30 min of sonication to accomplish the reaction. The reaction mixture was then centrifuged for 20 min, and the liquid supernatant was used for EPR measurements to detect the radical intermediates.
The main derivatization reactions of FG with various nucleophilic reagents were performed using a solution method. Herein, 60 mg of FG was adequately dispersed in 50 mL of ethanol by 30 min of sonication in a round bottom flask. Then, 10 mmol of the nucleophilic reagent was gradually added. After 4 h of reaction, the mixture was centrifuged for 20 min, whereas the sediment was filtered. The filter cake was washed using a Soxhlet extractor for 24 h to remove any adsorbed nucleophilic reagent and then dried for 4 h at 60 °C under vacuum conditions. The derivatization reaction of FG with FM (formamide) in the blank group was performed using a similar method, whereas that in the control group was performed in the same way under ultra-violet irradiation for 30 min at the beginning of the reaction.
Fig. 1 The reaction energy (dissociation of the C–F bond) for various FG defluorination pathways based on DFT calculations. |
Scheme 1 The proposed defluorination mechanism for FG under nucleophilic attack. GF means the chemical structure of FG. |
SET reaction system | EDA (Nu–H) | OH− (Nu−) | TEA (N–R3) |
---|---|---|---|
Energy of reaction (kcal mol−1) | −2.10 | −5.21 | −8.78 |
On the other hand, the single electron transferred to the C–F bond during the SET process was donated by the electron donor. This SET process between fluorinated carbon materials (FCMs) and electron donors has been proposed in previous reports, whereas the related donors are mainly restricted in active metals41 and do not include usual nucleophilic reagents such as amines or anionic nucleophiles. However, as the following content suggests, the SET process between FCMs, such as FG and conventional nucleophiles, can also occur. The detailed R3 reaction in the presence of nucleophiles is shown as step 1 in Scheme 1, which would result in a radical intermediate related to the nucleophile. However, due to its very short life, it is usually difficult to demonstrate the existence of this radical intermediate to prove the rationality of the defluorination process step 1 in Scheme 1. A radical trapping technique was then introduced into the defluorination study of FG using N-tert-butyl-alpha-phenylnitrone (PBN) and 2-methyl-2-nitroso-propane (MNP) as radical trappers.48,49 The EPR spectra of the various radical intermediates captured during the defluorination step by nucleophiles, including amines, phosphines, and sodium hydroxide, are exhibited in Fig. 2. For the PBN-FG blank group without the nucleophilic reagent, no radical intermediate micromolecule was discovered as no evident EPR signal was observed. In contrast, the defluorination groups, including MNP-FG-EDA (ethanediamine), MNP-FG-N2H4 (hydrazine), PBN-FG-DPA (diphenylamine), PBN-FG-DPP (diphenylphosphine), and PBN-FG-KOH (potassium hydroxide), have evident EPR spin signals, which are likely attributed to the corresponding captured radical intermediates depicted in Fig. 2. The corresponding possible radical intermediates may be neutral in charge, as depicted, or in their electropositive form (perhaps [˙Nu–H]+) originating from their relevant nucleophilic reagent. It should be noted that even defluorination caused by potassium hydroxide may be involved with the hydroxyl radical resulting from the SET reaction between FG and the hydroxyl anion; this is quite interesting and beyond common thought. However, the successful detection of these radical intermediates has first proved the mechanism step 1 in Scheme 1 proposed for the defluorination of FG caused by conventional nucleophiles, including the defluorination reaction under nucleophilic attack from potassium hydroxide, which is usually thought to be non-reductive. Moreover, the deciduous fluorine atoms after defluorination of FG caused by nucleophiles, such as EDA, were confirmed to be in the form of fluorine anions, as suggested by the fluorine ion chromatography curve shown in Fig. S5 (ESI†); this further demonstrated the reasonability of step 2 in Scheme 1.
Fig. 2 The EPR spectra of the various radical intermediates captured by the radical trappers PBN and MNP during the derivatization reactions of FG. |
The EPR spectra of FG and FG treated with various nucleophilic reagents (T–FG) are shown in Fig. 3. Compared to FG, the T–FG samples have a relatively more intense EPR signal, implying that a number of new spin centers are generated on the graphene nanosheets during the defluorination step caused by the nucleophilic reagents; this is consistent with step 2 in Scheme 1. The existence of these spin centers on the nanosheets depends on the stabilizing effect of the π-conjugation system and rigid structure of the FG derivatives.1,50–53 After step 2, the coupled reaction of two closer spin centers on a nanosheet may occur and thus result in a CC bond, as shown in step 3 of Scheme 1, whereas some of the spin centers can survive and remain in the final graphene products. In addition, it appeared that two spin centers on two different nanosheets can also couple; this leads to the formation of a C–C bond and cross-linking of two nanosheets (step 4 Scheme 1). Herein, it has been demonstrated that the defluorination of FG caused by nucleophilic reagents occurs via a radical mechanism generating spin centers on the nanosheets, and the subsequent coupled reaction of the spin centers will generate CC bonds; this means that the defluorination step has been accomplished. It is found that these spin centers play an important role in the defluorination process of FG. In addition to this, it must be emphasized that these spin centers will also affect the nucleophilic substitution of FG to a great extent, just as we has discussed in the following section.
Substrate molecule | Energy of reaction (kcal mol−1) | Energy of barrier (kcal mol−1) |
---|---|---|
Fluoromethane | −0.01 | 48.21 |
FG model molecule | 17.36 | 104.72 |
As abovementioned, a classical nucleophilic substitution mechanism was not appropriate for the substitution process of C–F bonds in FG; therefore, a new thought should be introduced into this field. Since configuration transformation is difficult for the C–F bonds located on the rigid 2D structure, a substitution process without excessive configuration transformation will be kinetically favourable. Moreover, as Fig. 3 shows, the EPR spectra of the FG derivatives display much more intense spin signals than that of the original FG sample. This implied that a number of spin centers were formed after the reaction of FG with the nucleophiles, which would be involved in both the defluorination as well as the nucleophilic substitution mechanism. Moreover, it was demonstrated that almost all the nucleophilic substitution reactions of FG in previous studies were accompanied by its defluorination reaction; this suggested the extraordinary correlation between these two kinds of derivatization reactions of FG.
Based on these discussions, we first proposed the whole derivatization reaction process of FG via a radical mechanism including its defluorination and substitution reactions, which was named as the DR mechanism and depicted in Scheme 2. The proposed mechanism is analogous to the radical nucleophilic substitution (SRN1) mechanism of micromolecules, whose application is limited by the required rigorous reaction conditions including peculiar substrate molecules, attacking reagents or initiation conditions. However, it is demonstrated by the DR mechanism that nucleophilic substitution via a radical mechanism can also be applied to attacking process of common nucleophiles, such as amines or hydroxyl ions, without special conditions. The whole derivatisation reaction via a radical mechanism shown in Scheme 2 clearly reveals the reaction process of FG under attack by a neutral nucleophilic reagent (with active hydrogen). The DR mechanism contains three types of reaction: the initiation, defluorination, and substitution reactions. The initiation and defluorination process have been discussed above, including the SET process between the nucleophile and C–F bonds in FG (step 1) followed by dissociation of the C–F− structure that results in a spin center on the graphene nanosheet and fluorine anion (or HF, step 2), as well as the following formation of the CC bond (step 3).
Scheme 2 A schematic of the proposed radical derivatization mechanism of FG under attack of neutral nucleophilic reagents. |
The substitution reaction of FG also subsequently occurs via a radical mechanism. As step 4 shows, on the one hand, the coupled reaction between the spin center on the nanosheet and radical center of the micromole ˙Nu from the SET process in step 1 may happen, which brings about the formation of the C–Nu bond or the accomplishment of nucleophilic substitution of the C–F bond. On the other hand, the nucleophile can directly attack the spin center on the nanosheet, generating the [GF–C–Nu]˙− structure. The [GF–C–Nu]˙− structure can be regarded as an intermediate, allowing the existence of an additional unpaired electron, which is delocalized and distributed in the –C–Nu– structure regions on the graphene nanosheets. Furthermore, as step 5 suggests, the intramolecular SET reaction of the intermediate with a neighbouring C–F bond will result in another new spin center adjacent to the original spin as well as the deciduous fluorine anion, which is similar to the intramolecular transfer of spin centers. The new spin center formed in step 5 can be involved with another substitution reaction in step 4. It was discovered from the analysis of step 4 and step 5 that the radical chain reaction was started after the initiation and defluorination process. Moreover, as step 6 suggests, the interaction of GF–C˙ with Nu–H results in hydrogen abstraction to provide GF–C–H, and ˙Nu can also happen, which brings about the formation of another substitution product of FG.
The abovementioned explanations explicitly demonstrate the detailed process of the nucleophilic substitution reaction of FG via a radical mechanism instead of the classical SN2 mechanism, which is initiated by the SET reaction between the C–F bonds and nucleophilic reagent. It should be noted that due to the pyramidal configuration of the spin centers on the nanosheets, the nucleophilic substitution process in the proposed mechanism was not accompanied by the evident configuration transformation; this demonstrated the thermodynamic and kinetic advantages of this kind of substitution process in comparison to the case of other substitution reactions with an inversion of configuration. The 2D structure where the C–F bonds in FG are located leads to the unfavourable kinetics of nucleophilic substitution via an SN2 mechanism as well as the stable existence of spin centers on the nanosheets due to their rigid structure and conjugation effects;50–53 this brings about a greater possibility of nucleophilic substitution via a radical process. It can be deduced that the radical process can also represent the typical characteristics of the chemistry of 2D materials.
Similar to the reaction process shown in Scheme 2, the derivatization reaction of FG under attack of an electronegative nucleophile also occurs via a radical mechanism, as shown in Scheme 3. The difference between the two processes is that the SET reaction between a neutral nucleophile and the C–F bond will result in a cation radical [˙Nu–H]+, which will dissociate into a Nu˙ radical and a proton. It can be discovered from Schemes 1 and 2 that in the DR mechanism, the defluorination process of FG, generating spin centers on the graphene nanosheets, becomes the requirement of the subsequent nucleophilic substitution. This is consistent with the previous reports stating that the substitution of FG is always accompanied by a defluorination reaction. It was also demonstrated in a previous study that the grafting ratio of a nucleophilic amino group on the FG nanosheets was improved as the nucleophilicity or reducibility of amine increased; this was in agreement with the increasing degree of defluorination. The relationship between these two kinds of reactions is not a total competitive relation, which is another feature of the DR mechanism, distinguished from that of the typical SN2 process.
Scheme 3 A schematic of the proposed radical derivatization mechanism of FG under attack of an electronegative nucleophilic reagent. GF represents the chemical structure of FG. |
Scheme 4 A schematic of the proposed nucleophilic attack of triethylamine (TEA) to FG. R represents the ethyl group. |
Furthermore, by comparing the XPS spectra obtained for the FG and FG-TEA samples in Fig. 5, the different signature of FG-TEA whereby a significantly less intense peak for the C–F group and an enhanced peak for the CC bond in the C 1s spectrum, as well as the new emerging peak in the N 1s spectrum were observed. Therefore, it can be tentatively concluded that the TEA fragment has been attached to the graphene nanosheets; this is also supported by the FTIR spectrum of FG-TEA shown in Fig. S6 (ESI†). The elemental analysis given in Table S1 (ESI†) further demonstrates that the new introduced species into the graphene nanosheet is a covalently attached group in the form of –N(CH2CH3)2 instead of a physically absorbed species N(CH2CH3)3 because of the increased molar ratio for H/N of around 10 rather than 15 in the FG-TEA sample. The negligible nitrogen element in the blank group G-TEA (graphene treated by TEA) further confirmed that the increased nitrogen-related group in FG-TEA was indeed a covalently attached group. However, for nucleophilic substitution via an SN2 mechanism, the TEA fragment cannot replace fluorine atom at all. These discussions effectively demonstrate the rationality of Scheme 4 as well as the proposed DR mechanism shown in Scheme 2, in which the nucleophilic substitution of FG occurs via a radical mechanism after the SET and defluorination reactions. In addition, it should be noted that the oxygen element content of the derivative sample FG-TEA is slightly increased as compared to that of the FG sample (O/C ratio 0.24 vs. 0.18); this has occurred in almost all the previously reported derivatization reactions of FG.29,33,34 Considering the spin centers on the graphene nanosheets, it is suggested by Scheme S1 (ESI†) that the increasing oxygen element originates from the coupled reaction between the spin centers and oxygen molecules.54,55 The reaction mechanism for the introduction of oxygen has again showed the significance of the spin centers in the derivatization reactions of FG.
Fig. 5 The XPS C 1s spectra of the FG and FG-TEA samples. The insets correspond to the F 1s spectrum of FG and N 1s spectrum of FG-TEA. |
As we have emphasized above, the DR mechanism shows a special characteristic different from the SN1 or SN2 mechanisms in which the defluorination process is the requirement for the subsequent nucleophilic substitution reaction of FG. In the case of the nucleophilic reaction of FG with a relatively inert nucleophile, the DR process cannot be adequately initiated; this results in a low degree of defluorination and low grafting ratio of nucleophile. In contrast, the introduction of a reductive reagent, such as pyridine, into the reaction system will promote the defluorination step as well as the degree of substitution; this may be the reason of the catalytic effect of pyridine during the derivatisation of FCMs.33,34,56 In addition, it has been suggested that FG can be defluorinated using ultra-violet radiation; this leads to the formation of new spin centers on the graphene nanosheets (Fig. S7, ESI†), which can be the attaching position of relatively inert nucleophiles on the nanosheets. A grafting experiment using formamide (FM) under ultra-violet irradiation was then performed, and the grafting amount is given in Table S2 (ESI†). It was demonstrated that ultra-violet irradiation indeed promoted the defluorination of FG under attack of FM as well as the substitution reaction; this resulted in a higher degree of defluorination and a higher grafting ratio of formamide on the graphene nanosheets. These results further indicate the rationality of our proposal that the nucleophilic substitution of FG occurs via a radical mechanism, which is dependent on the preceding defluorination process generating spin centers on the nanosheets. It will be easier to understand the derivative chemistry of FG with 2D structures based on the discussions proposed above.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cp06708a |
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