Article pubs.acs.org/JPCC
Cycloaddition Reactions between Graphene and Fluorinated Maleimides Pablo A. Denis* and Federico Iribarne Computational Nanotechnology, DETEMA, Facultad de Química, UDELAR, CC 1157, 11800 Montevideo, Uruguay ABSTRACT: Herein, we studied [2 + 2] and [4 + 2] cycloaddition reactions between maleimides and free-standing graphene, epitaxial graphene, and graphene deposited over hydrogen terminated SiC. These reactions are strongly endergonic with a ΔGtoluene°298 close to 73 kcal/mol. The inclusion of solvent effects revealed that the reaction became even less favorable in condensed phase as compared with the gas phase process. For epitaxial graphene or graphene deposited on top of hydrogen terminated SiC, we found that the substrate has a small influence over the cycloaddition reaction energies. In effect, they were strongly endergonic much like the case of free-standing graphene. Although the fluorinated maleimides used in recent experiments proved to be more reactive than tetracyanoethylene and maleic anhydride, their cycloaddition reactions with graphene are still strongly endergonic. However, our calculations indicated that these reactions are more likely to occur on the buffer layer of epitaxial graphene.
1. INTRODUCTION Although graphene has some degree of aromatic character,1 it was not until 2011 when a Diels−Alder reaction was performed employing graphene both as diene and dienophile.2 Using strong dienophiles such as tetracyanoethylene (TCNE) and maleic anhydride (MA) or dienes like 2,3-dimethoxy-1,3 butadiene (DMBD) and 9-methylanthracene (9MA), Sarkar et al.2 attained the covalent functionalization of graphene. In the particular case of TCNE, the retro-Diels−Alder reaction was observed that at 100 °C.2 By means of density functional theory calculations we3 and Cao et al.,4 proposed that perfect graphene does not react via the [4 + 2] path with the dienes and dienophiles used by Sarkar et al.,2 namely TCNE, MA, 9MA, and DMBD. For example, the reaction between DMBD and graphene is endergonic by 50 kcal/mol at 298 K.3 In line with these findings, subsequent experimental works showed that Diels−Alder reactions onto graphene are facilitated by defects,3,4 edges,3,4 curvature,5 heating,2 force accelerated processes,6 or the presence of OH groups.7 However, unlike other organic reactions onto graphene such as aryl diazonium salts,8−10 alkylation,11−13 1,3 dipolar cycloadditions,14−20 [2 + 2] cycloadditions,21−24 [2 + 1] cycloadditions,25−27 FriedlCrafts acylation,28 the Grignard reaction with fluorographene,29 and Diels−Alder reactions involving graphene have recently become more controversial. In effect, a recent study by Daukiya et al.30 used fluorinated maleimide molecules to show that cycloaddition reactions can be performed without pre-existing defects onto graphene. Employing XPS analysis, it was shown that functional groups were introduced onto graphene as the signal of the sp3 carbons was increased. The ARPES and Fermi velocity analysis revealed that a band gap was opened. The anisotropic standing wave patterns observed using STM were attributed to the grafted fluorinated maleimides through (1,2) or (1,4) cycloadditions. © XXXX American Chemical Society
Finally, dispersion corrected DFT calculations indicated that the reaction energy between the fluorinated maleimide and graphene is +1.33 eV while the same (1,2) reaction onto epitaxial graphene has a reaction energy of +1.07 eV. For the (1,4) Diels−Alder reaction, the DFT calculations indicated that it is only possible onto free-standing graphene, since no stable cycloaddition product was observed for epitaxial graphene. This result can be considered to be unexpected given that the presence of the SiC substrate improves the reaction energy of the (1,2) path by 0.26 eV, with respect to free-standing graphene. Over the past few years we have been studying the organic chemistry of graphene, with a particular interest in cycloaddition reactions.3,16−18,23,25,31 In fact, in our most recent investigation in this area,31 we proposed that heteroatoms would facilitate Diels−Alder reactions onto graphene. For example, record breaking reaction energy of −112.5 kcal/mol was obtained for the addition of a benzyne group onto Sidoped graphene. Also, we demonstrated that dopants can be used to determine the desired reaction product: [4 + 2] or [2 + 2]. Motivated by the important role that Diels−Alder reations2−7,30−35 are starting to play in the chemistry of graphene, we decided to investigate some aspects reported by Daukiya et al.30 which remain unclear. Although the fluorinated maleimide molecules are more reactive that regular maleimides, it has not been studied in detail how much reactive they are with respect to the reagents used by Sarkar et al.2 Also, as mentioned above, it is unclear why the substrate improves the (1,2) cycloaddition while it has a negative effect for the (1,4) path. Received: April 11, 2017 Revised: May 12, 2017
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DOI: 10.1021/acs.jpcc.7b03413 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
2. METHODS We employed Truhlar’s M06-L36,37 functional to perform periodic density functional calculations. Free standing graphene was modeled using a 5 × 5 unit cell. For graphene deposited onto hydrogen terminated SiC, we used a 4 × 4 unit cell of hydrogen terminated SiC. Silicon carbide was terminated with hydrogen atoms on both sides, and two layers were used. On top of this SiC layer, we deposited a 5 × 5 unit cell of graphene. In the case of epitaxial graphene, we used a 4 × 4 unit cell of SiC. The carbon atoms were saturated with hydrogen atoms but not the Si atoms since a buffer 5 × 5 graphene unit cell was included. Finally, we deposited a 5 × 5 unit cell of graphene on top of the buffer layer. The basis set selected was Pople’s 631G*.38 1000 k-points were used to sample the unit cell and the ultrafine grid was employed. For comparative purposes, calculations using a finite graphene models were also performed. Coronene and circumcoronene, were used to mimic graphene. The M06-L calculations were undertaken with Gaussian 09.39 The methodology is similar to the one previously selected by us to investigate Diels−Alder reactions onto perfect,3 defective,3 and doped graphene.31
Table 1. Adsorption Energies (kcal/mol) and Band Gaps (eV) Calculated for Different Maleimide Molecules Adsorbed onto Graphene, at the M06-L/6-31G* Level of Theory M1 M2 M3 M4 M5 M6
ΔE
gap
−14.4 −9.6 −23.8 −18.8 −25.3 −24.4
0.02 0.04 0.05 0.05 0.04 0.01
3. RESULTS AND DISCUSSION The maleimide molecules involved in the study of the feasibility of cycloadditions onto graphene are shown in Figure 1. Figure 2. Noncovalent adsorption of maleimide M3 onto perfect graphene (C atoms of M3 are shown in different color than those of graphene).
these molecules onto a 5 × 5 graphene sheet opens a small band gap, and charge transfer to Mi is small, 0.02 e- for M3 and 0.21 e- for M5. To gain further insight, we investigated the adsorption of M3 onto coronene and circumcoronene finite models which facilitate the inclusion of solvent effects and thermodynamic corrections. At the M06-L/6-31G* level, ΔE’s are −18.1 and −22.0 kcal/mol. In the gas phase, ΔGgas°298 = −3.9 and −5.1 kcal/mol when M3 is adsorbed onto coronene and circumcoronene, respectively. Finally, when the SMD model is utilized to simulate the presence of toluene, we found that the corresponding ΔEtoluene are −10.9 and kcal/mol −12.7 for coronene and circumcoronene, respectively. Combining the latter values and ΔGgas°298, we obtain that ΔGtoluene°298 = 3.3 and 4.2 kcal/mol, for coronene and circumcoronene, respectively. In the next step, we investigated the [2 + 2] and [4 + 2] cycloaddition reactions of Mi onto perfect graphene. The results are presented in Table 2. The reaction energies obtained for both cycloadditions confirm that M1 is less reactive than M2 and that M3 is more reactive than M2. For the [2 + 2] path, the ΔE computed for M3 is 18.9 and 27.7 kcal/mol, larger than those determined for M1 and M2. These results confirm the increase of reactivity when passing from M1 to M3, proposed by Daukiya et al.30 We tried to modify M3 to improve the ΔE but our attempts proved to be unsuccessful except for the case of M4. In effect, this maleimide molecule and M2 exhibit a curious behavior since the ΔE computed for the [2 + 2] and [4 + 2] paths are nearly identical. This finding is in contrast with the results obtained for M1, M3, M5, and M6 since the [2 + 2] path is always favored for these maleimides. The structure of the [2 + 2] and [4 + 2] cycloaddition products of M3 with graphene are displayed in Figures 3 and 4,
Figure 1. Maleimide molecules employed to investigate [2 + 2] and [4 + 2] cycloaddition reactions onto graphene.
According to Daukiya et al.30 the reactivity is expected to be increased when going from M1 to M3. In fact, M3 was successfully attached to graphene after immersion of the sheets in a toluene solution containing M3. In the first place we analyzed whether the Mi molecules are noncovalently adsorbed onto graphene. The adsorption energies are gathered in Table 1, and the structure of the adduct formed between graphene and M3 is displayed in Figure 2. ΔE were always are quite negative, suggesting that the adsorption is likely to occur. For M3, the ΔE = −23.8 kcal/mol, while M5 is the maleimide more strongly adsorbed onto graphene. In all cases, the adsorption of B
DOI: 10.1021/acs.jpcc.7b03413 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 2. [2 + 2] and [4 + 2] Reaction Energies (kcal/mol) Calculated for Different Maleimide Molecules and Graphene, at the M06-L/6-31G* Level of Theory
Table 3. [2 + 2] and [4 + 2] Reaction Energies (kcal/mol) Calculated for the Addition of M3 to Different Graphene Models, at the M06-L/6-31G* Level of Theory
ΔE
M1 M2 M3 M4 M5 M6
ΔEgas
[2 + 2]
[4 + 2]
[4 + 2] − [2 + 2]
∞ graphene
∞ graphene
∞ graphene
56.6 46.5 37.7 40.5 45.9 39.5
61.3 46.8 43.4 40.7 ADS ADS
4.7 0.3 7.7 0.2 45.9 39.5
∞ graphene coronene circumcoronene graphene over H-SiC-H epitaxial graphene (over SiC(0001)) buffer layer of SiC(0001)
[2 + 2]
[4 + 2]
37.7 43.3 41.9 37.1
43.4 44.5 40.2
41.2
44.2 −33.9
ΔGtoluene°298 [ADS] [2 + 2] [4 + 2] −23.8 −18.1 −22.0
60.6a 68.7 64.8
67.7a 68.8
[ADS] 2.4a 3.3 4.2
−9.8
a
Estimated using the thermodynamic and solvent corrections calculated for circumcoronene.
respectively. The values are close to the ones obtained when using the infinite graphene model, namely 37.7 kcal/mol. When thermodynamic corrections are included, ΔGgas°298 are 61.4 and 58.7 kcal/mol for coronene and circumcoronene, respectively. Clearly, the reaction is strongly endergonic. Given that the experimental investigation of this process was carried out in toluene, we employed the SMD model to estimate the effect of the solvent. Interestingly, we found that ΔGtoluene°298 was 68.7 and 64.8 kcal/mol for coronene and circumcoronene, respectively. In the case of the [4 + 2] path, we found that only the circumcoronene model is useful to study this reaction due to the strong hydrogen bond interactions between the fluorine atoms of M3 and the H atoms of coronene. Using circumcoronene and the same procedure as for the [2 + 2] path, we found that ΔGgas°298 = 61.9 and ΔGtoluene°298 = 68.8 kcal/mol. Consequently, the [4 + 2] path is even more endergonic than the [2 + 2] process. This result is in disagreement with those reported by Daukiya et al.,30 which suggested that the reaction proceeds smoothly in toluene. For the sake of completeness, we also studied the cycloaddition reaction between M3 and graphene supported onto hydrogen terminated SiC, SiC(0001), as well as the buffer layer of epitaxial graphene. For hydrogen terminated SiC, a 5 × 5 graphene sheet was placed on top of a two layer 4 × 4 SiC substrate terminated with H atoms on both sides. The structure of the cycloaddition adduct for the [2 + 2] and [4 + 2] paths is shown in Figure 5. This model is similar to the one built by us to study the reactivity of Li doped graphene when it is over hydrogen terminated SiC.40 At the M06-L/6-31G* level, we found that the ΔE resulted in values of 40.2 and 37.1 kcal/mol, for the [4 + 2] and [2 + 2] cycloaddition products, respectively. Then, for the [4 + 2] path the presence of the substrate improves ΔE by 3.2 kcal/mol. In second place, we considered the reaction between graphene and epitaxial graphene, i.e., a graphene sheet over a graphene buffer layer which is bonded to the SiC substrate via the free Si atoms. We note that only the bottom C atoms are saturated with H atoms. As for graphene deposited over hydrogen terminated SiC, we used 5 × 5 and 4 × 4 unit cells for graphene and SiC. The cycloaddition product is shown in Figure 6. The reaction energy computed for the [2 + 2] and [4 + 2] paths are 44.2 and 47.2 kcal/mol. Again, the reaction is not favorable from a thermodynamic stand point. Finally, we studied the [4 + 2] cycloaddition of M3 onto the buffer layer of epitaxial graphene. In Figure 7, we show the structure of the [4 + 2] cycloaddition product. The C−C bond
Figure 3. [4 + 2] cycloaddition product for the reaction between maleimide M3 and perfect graphene.
Figure 4. [2 + 2] cycloaddition product for the reaction between maleimide M3 and perfect graphene.
respectively. In both cases, the C−C bond lengths between graphene and the adducts are very long: 1.60 and 1.66 Å when M3 reacts with graphene via the [2 + 2] and [4 + 2] paths, respectively. When the reactivity of M3 is compared against the reagents employed by Sarkar et al.,2 we found that it is more reactive than tetracyanoethylene (TCNE) and maleic anhydride (MA). Indeed, the ΔE’s for the [4 + 2] cycloaddition of graphene are 49.7 and 63.2 kcal/mol, for MA and TCNE, respectively.3 These values are 6.3 and 19.8 kcal/mol larger than the ΔE computed for M3. Therefore, we confirm that M3 has been designed with enhanced reactivity. In order to evaluate the endergonic character of the reaction, we studied the cycloaddition of M3 onto coronene and circumcoronene. The results are presented in Table 3. For the [2 + 2] cycloaddition product ΔE = 43.3 and 41.9 kcal/mol, C
DOI: 10.1021/acs.jpcc.7b03413 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. [2 + 2] and [4 + 2] cycloaddition product for the reaction between maleimide M3 and perfect graphene deposited over hydrogen terminated SiC.
reaction onto epitaxial graphene, i.e., 1.67 Å, as shown in Figure 6. The shorter distance is accompanied by a significant change of the reaction energy. Indeed, ΔEgas = −33.9 kcal/mol, while the free energy change is exergonic by −9.8 kcal/mol. Therefore, the substrate plays a key role in facilitating the cycloaddition reaction.
4. CONCLUSIONS We have applied dispersion corrected density functional theory to investigate cycloaddition reactions between maleimide molecules and graphene. In all cases, we concluded that the [2 + 2] and [4 + 2] reactions onto free-standing graphene, epitaxial graphene, and graphene deposited over hydrogen terminated SiC are strongly endergonic. In effect, for the fluorinated maleimide molecule employed in recent experimental works, we found that the free energy change is expected to be close to 60.6 and 67.7 kcal/mol for the [2 + 2] and [4 + 2] cycloadditions. The inclusion of solvent effects revealed that the reaction where even less favorable in the condensed phase. Finally, in order to obtain a better description of the experimental environment, we investigated the reaction between the fluorinated maleimides and graphene deposited on top of hydrogen terminated SiC and also over epitaxial graphene. In both cases it was found that the substrate has a small influence over the cycloaddition reaction energies. Although our calculations indicated that fluorinated maleimides are more reactive than tetracyanoethylene and maleic anhydride, the [2 + 2] and [4 + 2] cycloaddition reactions graphene are strongly endergonic. However, our calculations indicated that these reactions are more likely to occur on the buffer layer of epitaxial graphene.
Figure 6. [4 + 2] cycloaddition product for the reaction between maleimide M3 and perfect graphene deposited over SiC (only C atoms are terminated with H atoms, the Si atoms interact with the buffer layer).
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AUTHOR INFORMATION
Corresponding Author Figure 7. [4 + 2] cycloaddition product for the reaction between maleimide M3 and the buffer layer of epitaxial graphene (only C atoms are terminated with H atoms, and the Si atoms interact with the buffer layer).
*E-mail:
[email protected]. Tel: 0059899714280. Fax: 00589229241906.
distance which connects M3 and the buffer layer is 1.58 Å, much shorter than the distance corresponding to the same
Notes
ORCID
Pablo A. Denis: 0000-0003-3739-5061 The authors declare no competing financial interest. D
DOI: 10.1021/acs.jpcc.7b03413 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS The authors thanks PEDECIBA Quimica, CSIC, and ANII for financial support.
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DOI: 10.1021/acs.jpcc.7b03413 J. Phys. Chem. C XXXX, XXX, XXX−XXX
