A New Approach to Accomplish the Covalent Functionalization of

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A New Approach to Accomplish the Covalent Functionalization of Boron Nitride Nanosheets: Cycloaddition Reactions Pablo A. Denis, and Federico Iribarne J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05907 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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The Journal of Physical Chemistry

A New Approach to Accomplish the Covalent Functionalization of Boron Nitride Nanosheets: Cycloaddition Reactions Pablo A. Denisa,* and Federico Iribarne a- Computational Nanotechnology, DETEMA, Facultad de Química, UDELAR, CC 1157, 11800 Montevideo, Uruguay. * e-mail: [email protected] Tel: 0059899714280, Fax: 00589229241906

Abstract We investigated the occurrence of cycloadditions on dimensional boron nitride. Our results indicated that the [2+2] cycloaddition of benzynes, ethyne and maleic anhydride are allowed both from thermodynamical and kinetic stand points. In fact, our results indicated that the reaction energy associated with the formation of the [2+2] cycloaddition product is more exothermic than that computed for graphene. The activation energy determined for the [2+2] path is 15.9 kcal/mol, suggesting the feasibility of the process. Furthermore, we found that the activation energy for the retro reaction is 30.6 kcal/mol. This value is large enough to guarantee the stability of the functional groups, but not too high to impede their detachment upon heating. In addition to these findings, we found that cycloadditions can be used to reduce the band gap of boron nitride nanosheets. In light of these results, we propose the use of cycloaddition reactions in order to attain the temperature controlled functionalization of boron nitride sheets.

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Introduction Soon after graphene was exfoliated, boron nitride sheets (BN) were obtained using a similar procedure.1-2 Nowadays, thanks to the tremendous progress made in the synthesis of 2D materials, it is possible to produce BN sheets in large scale.3 In line with the results obtained for graphene, it is viable to prepare the BN analogue of graphene oxide,4 even though the method is different because BN is more resistant to oxidation. These oxidized BN sheets react with phenyl isocyanate. Apart from oxidation, other procedures have been reported to functionalize graphene such as: fluorination5 or the addition of long alkyl chain amines on B defects6 and the [2+1] cycloadditions of dibromocarbene7 or nitrenes.8 In the case of graphene, its organic functionalization has been investigated with detail. The addition of

aryl

diazonium

salts,7-9

1,3-dipolar

cycloadditions,10-17

[2+2]

cycloadditions,18-20

[2+1]

cycloadditions21-23 and Diels-Alder reactions24-41 constitute well established procedures. In contrast, for 2D BN there are few examples available, the most relevant ones being the [2+1] cycloaddition of dibromocarbenes,42 nitrenes,43 or the recent amino and silane/functionalization.44 The introduction of these functional groups is very important because they reduce the interlayer interaction. As a consequence, it is possible to obtain better dispersion of BN sheets in solvents and polymers, as recently demonstrated by Liu et al.44 Considering that new methods are needed to functionalize BN sheets and bearing in minds that the BN unit is isoelectronic to CC, we wondered if cycloaddition reactions can occur on 2D BN. For this reason we have undertaken a theoretical investigation comprising both kinetic and thermodynamic aspects of the cycloaddition of benzynes, ethyne, fluorinated maleimides and maleic anhydride to BN sheets. Our results indicate that these processes are allowed from thermodynamical and kinetic stand points. Thus, it would be possible to use these reactions to expand the range of applicability of BN sheets, for example to obtain better dispersion in polymers as observed by Liu at


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al.44 or to anchor metal nanoparticles. It is our hope that this investigation motivates new experimental studies on the covalent functionalization of BN sheets.

2. Theoretical methods Infinite 2D boron nitride was simulated using a 5×5 unit cell, while for BN-circumcoronene was selected as the finite model to perform the investigation of the kinetic aspects of the process. These structures are shown in Figure 1. We performed M06-L45 calculations as implemented in Gaussian 09.46 The basis set used were the 6-31G* and 6-311G*.47 The unit cell was sampled with 1000 k-points and the ultrafine grid was employed. In the case of the calculations carried out using the BN-circumcoronene flake, we calculated vibrational frequencies for all stationary points. For the 5×5 unit cell we undertook periodic VDW-DF48 calculations as implemented in SIESTA.49-50 The double−zeta basis set with polarization functions (DZP) was chosen. The orbital confining cut−off was fixed to 0.01 Ry. The split norm used was 0.15. The DFT implementation in SIESTA can be prone to significant basis set superposition error (BSSE), even with relatively low degree of radial confinement.51 To avoid this problem we used the counterpoise correction suggested by Boys and Bernardi.52 The interaction between ionic cores and valence electrons was described by the Troullier−Martins norm conserving pseudopotentials.53 The Mesh cut−off was fixed to 200 Ry, which gave converged binding energies within 0.02 eV. The lattice parameters were optimized along the a and b directions but the c axis was kept frozen at 20 Å. Geometry optimizations were pursued using the conjugate gradient algorithm until all residual forces were smaller than 0.01 eV/Å. Unit cells were sampled using a Monkhorst−Pack k−point sampling scheme of 40×40×1 k points. In all cases the systems showed zero magnetic moment, despite the fact that we performed spin polarized calculations.

3. Results and discussion


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3.1 Cycloaddition of C6H4: We considered the addition of benzynes to BN sheets along four paths: [2+2], [4+2], 1,3-BB and [1,4]-NN, where BB or NN means that the 1,3 cycloaddition takes place over B or N atoms, respectively. At the M06-L/6-31G* level of theory, we found that the 1,3 cycloadditions are not stable given that the functional groups migrated to the other cycloaddition products during the optimization. This fact is not consistent with the behaviour observed for graphene, since the 1,3 cycloaddition product can be optimized. Yet, it is located 29.6 kcal/mol above the [2+2] adduct. Returning to the discussion about the BN sheet, we found that the reaction energies for the [2+2] and [4+2] paths are -18.1 and -7.9 kcal/mol, respectively. These values can be compared with those computed for the addition of benzynes to graphene: -12.8 and -9.6 kcal/mol for the [2+2] and [4+2] paths, respectively. Interestingly, the formation of the [2+2] cycloaddition product in BN is favoured over graphene, but the situation is reversed when the [4+2] route is considered. When the basis set is extended, the reaction energy becomes -14.6 kcal/mol, at the M06-L/6-311G* for the [2+2] path. This value is only 3.5 kcal/mol weaker than the values computed using the 6-31G* basis set. The cycloaddition products are shown in Figures 2 and 3 for the [2+2] and [4+2] paths, respectively. Taking into account that the cycloadditions take place on different atoms, i.e. B and N, it is not surprising that the CB and CN distances are not equal. In effect, the CN distance is 1.50 Å while the CB one is 1.62 Å, for the [2+2] cycloaddition product. As for the [4+2] path, the distances both 0.04 Å longer. In our recent investigation about Diels-Alder reactions onto graphene we found that cycloadditions are facilitated by the presence of defects and edges.30 Considering that the [2+2] cycloadditions on graphene and BN have similar reaction energies we can expect that the presence of defects and edges is going to promote the reactions studied in this work. The results obtained using periodic conditions and the M06-L functional are supported by periodic VDW-DF/DZP calculations performed with SIESTA. In fact, the reaction energy computed for the [2+2] path is -11.7 kcal/mol, nearly identical to the value previously found for graphene. With


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regard to the deformation experienced by the sheet upon functionalization, we found that this value is 54.9 kcal/mol. Thus we may expect a rather large barrier for the occurrence of these reactions. In Figure 4, we present the band structures calculated for the 5×5 BN sheet before and after functionalization. At the VDW-DF/DZP level, the band gap is 4.8 eV. Conveniently, this value decreases to 4 eV after the [2+2] functionalization with benzynes takes place. This trend is also supported by the M06-L/6-311G* results. Indeed, the band gap for BN sheet is reduced from 5.2 eV to 4.4 eV upon functionalization with this M06-L. Given that most DFT methods are known to underestimate band gaps, we calculated them employing the HSEH1PBE method.54-55 At the HSEH1PBE/6-311G* level of theory the band gap of BN is reduced from 6.1 to 5.3 eV, in agreement with the M06-L and VDW-DF trends. Therefore, in addition to the utility of benzynes to serve as nanoparticle anchors, they may contribute to the band gap tuning of the BN sheet, as observed by Chigo Anota et al.56 for the non covalent functionalization of BN sheets with L guanine. Although we have shown that the cycloadditions of benzynes to the BN sheet are possible from a thermodynamic stand point, this evidence is not enough to guarantee the feasibility of the process. To further analyse this point, we carried out cluster model calculations to assess whether the [2+2] cycloaddition is kinetically allowed or forbidden. In first place, we computed the structure of the cycloaddition product using the BN-circumcoronene model. At the M06-L/6-311G* level of theory, the reaction energy for this path is -14.7 kcal/mol. This value is in excellent agreement with the one obtained with the same functional and the 5×5 unit cell, being just 0.1 kcal/mol lower. The cycloaddition product is shown in Figure 5. Interestingly even the CB and CN distances are similar to the ones computed using periodic conditions, thus confirming that the finite flake selected to model the BN sheet is useful. After checking that all vibrational frequencies of the structure presented in Figure 5a are real, we used it as a starting point to search the transition state for the [2+2] reaction. In Figure 5b, the structure of the transition state is presented. It is located 15.9 kcal/mol above the reactants. We followed


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the reaction path employing the intrinsic reaction coordinate (IRC) algorithm. The results confirmed that the transition state corresponds to the [2+2] cycloaddition of C6H4. The reaction barrier seems to be small enough to ensure the occurrence of the reaction. It is important to mention that this barrier was estimated using the summed energy of the reactants. Another possibility would be to compute the barrier using the structure in which the C6H4 group is non covalently adsorbed on the BN sheet. At the M06L/6-311G* level, the adsorption energy of the benzyne molecule onto BN is -8.6 kcal/mol. Thus, the reaction barrier computed from the adsorbed complex is 24.5 kcal/mol. This is larger than the value mentioned above but still one that can be easily surmounted. With regards to the barrier for the retro reaction, we found that the reverse barrier is almost twice larger, 30.6 kcal/mol, so it would be possible to detach the functional groups upon moderate heating of the cycloaddition product. The whole reaction path is shown in Figure 6. Finally, we have evaluated the effect of solvent on the reaction by means of SMD calculations.57 At the M06-L/6-31G* level of theory we determined that in water the [2+2] cycloaddition of C6H4 onto the BN-circumcoronene model is 2 kcal/mol more exothermic than in the gas phase. Therefore, the reaction is likely to take place. 3.2 Cycloaddition of fluorinated maleimides, ethyne and maleic anhydride: To provide further evidence about the possibility of performing [2+2] cycloaddition reactions onto BN, we considered the addition of a powerful fluorinated maleimide (M3) which was recently added to defect free graphene by Daukiya et al.37 as well as maleic anhydride. The structure of the [2+2] cycloaddition product with the fluorinated maleimide is shown in Figure 7. At the M06-L/6-31G* level, the reaction energy is 33.1 kcal/mol. Although this value is higher than the one computed above for the [2+2] cycloaddition of benzynes, it is 4.6 kcal/mol smaller than the one obtained by us for perfect graphene.24 In the same line, the reaction energy for the [4+2] cycloaddition is 42.1 kcal/mol, again a lower figure than the computed for graphene by 1.3 kcal/mol. In consequence, as observed for benzynes the cycloaddition of fluorinated maleimides is more exothermic than the corresponding reaction with graphene. Finally, the calculations


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for maleimide M3 also supported that the band gap tuning is possible. Indeed, the bad gap computed for the [2+2] cycloaddition product shown in Figure 7 is 3.4 eV. This means that the addition of M3 reduces the gap of BN by 1.8 eV, a value twice as larger than the one computed for BN which was 0.8 eV. Finally we studied the [2+2] cycloaddition of ethyne and maleic anhydride (MA) to 2D boron nitride. As observed for graphene, the reactions are endothermic since the reaction energies are 26.4 and41.3 kcal/mol, for ethyne and MA, respectively, at the M06-L/6-31G* level of theory. With regards to the reaction barrier, we determined that the transition states are located 43.4 and 47.6 kcal/mol, above reactants for C2H2 and MA, respectively. Although these values are significant, the reactions would be possible with moderate heating.

4. Conclusions By means of first principle calculations we showed that cycloadditions of benzynes, ethyne and maleic anhydride over two dimensional boron nitride are feasible. Although the [2+2] cyloaddition is energetically preferred over the [4+2] path, both cycloadditions are, in principle, possible. From a thermodynamic stand point, we found that the reaction energy associated with the formation of the [2+2] cycloaddition product is more exothermic than that calculated for graphene. As for the reaction barrier for the addition of C6H4, the activation energy is 15.9 kcal/mol, small enough to be surmounted by moderate heating. The cycloaddition products are expected to be stable considering that the activation energy for the retro reaction is 30.6 kcal/mol. Nevertheless, it would be possible to detach the functional groups by annealing at higher temperatures. Last but not least, wound that the cycloadditions may be used to reduce the band gap of boron nitride. We expect that this work motivates new investigations targeting the covalent functionalization of boron nitride. Acknowledgments The authors thank PEDECIBA Quimica, CSIC and ANII for financial support.


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electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions. J.Phys.Chem. B 2009, 113, 6378-6396.


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Figure 1. Infinite and finite boron nitride systems employed to study cycloaddition reactions.


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Figure 2. Optimized structure at the M06-L/6-311G* level, for the [2+2] cycloaddition of benzynes to a 5×5 unit cell of 2D boron nitride. Distances in Å.


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Figure 3. Optimized structure at the M06-L/6-311G* level for the [4+2] cycloaddition of benzynes to a 5×5 unit cell of 2D boron nitride. Distances in Å.


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Figure 4. Band structure computed at the VDW-DF/DZP level for perfect and functionalized BN sheet with a benzyne group. Fermi level is at 0 eV.


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Figure 5. a) Optimized structure at the M06-L/6-311G* level for the [2+2] cycloaddition of benzynes to a BN-circumcoronene; b) the corresponding transition state. Distances in Å.


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Figure 6. Reaction path determined for the [2+2] cycloaddition of benzynes to a BN-circumcoronene at the M06-L/6-311G* level.


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Figure 7. Optimized structure at the M06-L/6-311G* level for the [2+2] cycloaddition of maleimide M337 to a 5×5 unit cell of 2D boron nitride. Distances in Å


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TOC Graphic


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TOC Graphic 146x99mm (120 x 120 DPI)


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A New Approach to Accomplish the Covalent Functionalization of

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