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Timestamp: 2019-04-20 00:15:38+00:00

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The mechanism of the functionalisation of reduced single walled carbon nanotubes with organobromides was monitored by open circuit voltammetry throughout the reaction and further elucidated through a series of comparative reactions. The degree of functionalisation was mapped against the reagent reduction potential, degree of electron donation of substituents (Hammett parameter), and energies calculated, ab initio, for dissociation and heterolytic cleavage of the C–Br bond. In contrast to the previously assumed reduction/homolytic cleavage mechanism, the reaction was shown to consist of a rapid association of carbon–halide bond to the reduced nanotube as a complex, displacing surface-condensed countercations, leading to an initial increase in the net nanotube surface negative charge. The complex subsequently slowly degrades through charge transfer from the reduced single-walled carbon nanotube to the organobromide, utilizing charge, and the carbon–halide bond breaks heterolytically. Electron density on the C–Br bond in the initial reagent is the best predictor for degree of functionalisation, with more electron donating substituents increasing the degree of functionalisation. Both the mechanism and the new application of OCV to study such reactions are potentially relevant to a wide range of related systems.
However, there are some difficulties with the SET mechanism for nanotubide reactions. Most obviously the nanotubide Fermi level, is not thought sufficient to reduce many of the organohalides applied. Mildly charged SWCNTs are known to react with an array of alkyl halides18 despite modest reduction potentials e.g. E°[C20Na] ≈ −0.45 V (all values vs. SHE) as calculated from integrated density of states (more complete description and comparison to literature potentials provided in the ESI†). Even highly charged nanotubide (C5Na ≈ −1.43 V) have reduction potentials lower than some known reagents, notably organochlorides18,35,36 (ca. −2.8 to −3.1 V).37 Whilst absolute Fermi levels for specific (reduced) SWCNTs remain uncertain, even at the absolute limit, the maximum nanotubide Fermi level cannot exceed the reduction potential of the reductant (−2.71 V for sodium) which is still lower than the reactive alkyl chlorides. An additional difficulty is that the degree of functionalisation never exceeds the charging stoichiometry, as might be expected if the mechanism involves radical propagation steps (Fig. 1c and d); further, the proposed CNT-bound radical (Fig. 1c) does not lead to nanotube crosslinking termination, as is seen in other systems involving CNT bound radicals.38 For these reasons, we have studied the mechanism of reductive functionalisation in more detail to provide greater insight into these versatile reactions.
Open circuit voltammetry (OCV) measures the potential of a working electrode versus a reference in the absence of a flowing current, providing a direct measurement of the potential on surface. OVC is used as a characterisation tool for redox rates, commonly for characterising the performance of energy storage devices39 and quantifying corrosion resistance,40 in addition to being a common measurement parameter in chemical sensors.41 Here, OCV is instead applied to monitoring the SWCNT organo-functionalisation in real time. The measured OCV potential must relate to the degree of charging, which shifts the Fermi level of the SWCNTs, modulated by the counterions condensed in the Stern layer (as modelled by Manning-Oosawa theory,42 and applied to SWCNTs15,17,43). By using an assembly of SWCNTs as the working electrode, the effective net charge on the nanotube surface may be monitored, during both the reduction step (addition of sodium naphthalide) and the subsequent functionalisation (addition of alkyl halide).
Fig. 2 OCV (and derivative) versus time of a buckypaper working electrode during addition of sodium naphthalide solution (green), and addition of 1-bromobutane (3 h after NaNp addition, yellow). Dashed line represents dV/dt = 0. Inset shows zoomed region over 2 h around point of organic addition. Derivative before 2 h omitted for clarity with full range supplied in the ESI (Fig. S4†).
To probe the nature of the proposed nanotubide/organohalide complex and the reaction mechanism, several stabilizing factors were considered. Firstly, the mechanism may proceed via either a concerted or stepwise mechanism. A concerted (SN2-like) mechanism would necessitate a carbon-centric trigonal bipyramidal transition state and would be dominated by steric effects due to the large size of the nucleophile (SWCNTn−). Conversely, a stepwise mechanism would involve an electron-poor intermediate, regardless of whether cleavage of the carbon–halogen was a heterolytic SN1-like step, or a homolytic radical step (as is proposed in the SET mechanism, Fig. 1b). The electron-poor species could be stabilized with hyperconjugation from bonds beta to the halide to stabilize the planar intermediate. To distinguish between concerted and stepwise mechanisms of the SWCNT-organohalide complex, different organohalides were used to probe the possible stabilizing factors. The primary, secondary, and tertiary isomers of bromopentane provide a convenient set (Fig. 3): 1-bromopentane (1), 2-bromopentane (2), and 2-bromo-2-methylbutane (3). While alkyl iodides lead to higher degrees of grafting in SWCNT reductive functionalisation than bromides,18 they introduce potentially complicating UV sensitivity.
Fig. 3 (Top) Schematic possible 5-valent and planar intermediates of reductive functionalization of BrC5H11 isomers (1–3) with nanotubide, illustrating stability trends. Most stable state shown with green background, least stable shown with hatched red background. (Bottom) Table of properties of SWCNTs functionalized with various alkyl bromides.
The extent of steric interference is highly dependent on the substituents on the electrophilic carbon (here the brominated carbon), with occlusion small for primary (1, with two small hydrogens and one alkyl substituent) and maximized for tertiary (3, with three alkyls) reagents; thus the grafting trend 1 > 2 > 3 would be expected for a concerted (SN2-like) mechanism. Conversely, the opposite grafting trend 3 > 2 > 1 would be expected for a stepwise, purely electronically-driven mechanism, due to stabilisation from hyperconjugation of both radical and cationic intermediates, as confirmed by ab initio calculations45 (see ESI† for details) of the energies for heterolytic cleavage (ΔEHet) and bond dissociation (ΔEBDE).
The extent of functionalisation was quantified using thermogravimetric analysis (TGA, ESI Fig. S9†) with the grafting density (R/C) calculated as the number of grafted chains per SWCNT carbon atoms. The TGA data show the trend 3 (R/C 0.052) > 1 (0.046) > 2 (0.037). As the sterically hindered (but electronically stable) 3 leads to the highest grafting density, it can be assumed that electronic effects have a significant contribution, and importantly, that the mechanism involves multiple steps and is not concerted. The deviation of 1/2 from the predicted, purely electronic trend implies that steric interactions do play a role in at least one step (in support of previous work18,24) and dominates over the weak electronic contribution of a single methyl group.
The nature of the electronic component of the reaction's rate determining step was investigated in more depth, by comparing the degree of functionalisation with a set of model compounds to four factors: reduction potential (E°), ΔEBDE, ΔEHet, and electron density at the carbon–halide bond. A series of monosubstituted benzyl bromides were selected to facilitate the use of Hammett parameters (vide infra). para and meta substituted methyl, nitro, and trifluoromethyl benzyl bromides were reacted, alongside m-methoxy substituted, p-methylthio substituted, and unsubstituted benzyl bromide (N.B. p-methoxybenzyl bromide was too unstable for use, and m-methylthiobenzyl bromide was not commercially available). These reagents were selected to provide a broad range of electron-withdrawing/donating strengths and stabilities of resonance states of intermediates, with minimal variation in steric bulk of the functionalizing species. ortho-Functionalised benzyl bromides were not tested, as the varying steric influence of the substituents adjacent to the reactive C–Br bond would confound attempts to isolate the electronic effects. In fact, due to this very issue, Hammett parameters are not available for relevant ortho-functionalised species,46 adding a further hurdle to their use in this study.
All benzyl bromides successfully functionalized the SWCNTs, as monitored by TGA (ESI Fig. S10†), and the degree of functionalisation (R/C) was quantified. The R/C was plotted against each of the electronic factors (σ, E°, ΔEBDE, ΔEHet), to allow trends to be elucidated. A linear fit was applied and the coefficient of determination (R2) was used as a crude measure of correlation (Fig. 4). The strongest trends are seen for the Hammett parameter (R2 = 0.871) with more electron rich C–Br bonds being more reactive, and reduction potential (R2 = 0.812) with more easily reduced species functionalizing to a greater extent. These trends imply that the electron density at the C–Br bond is critical, likely in complex formation as seen in the OCV measurements. Further, the organobromide is directly reduced by the nanotubide, akin to the SET mechanism, explaining the residual change posited previously30 and confirmed by OCV in this work. The discrepancy between reduction potential of the alkyl halide and SWCNT Fermi level can be explained by the presence of the intermediate SWCNT–C–Br complex which may lower the energy barrier to reduction, and the more reducing potential of the SWCNT once the counter ions are displaced. However, while heterolytic bond cleavage energies showed a notable correlation (R2 = 0.765, with more easily cleaved C–Br showing greater reactivity), importantly, the homolytic cleavage showed virtually no correlation (R2 = 0.168) with the degree of functionalisation. Even the (weak) trend is in the ‘wrong’ direction with high ΔEBDE (a high enthalpy cost in forming the carboradical) linked to higher functionalisation. At its most simple, the trend implies there is no homolytic cleavage of C–Br, but as ΔEBDE is an indirect probe for the presence of a free R˙ intermediate (ESI Fig. S8†), it implies no such species is present, in contrast to the SET mechanism (Fig. 1b). Given the previous observation of bis-dimers in some reactions, it is possible that a homolytic C–Br cleavage is present as a non-rate-determining step, however, as alternative reactions may explain their presence, and ΔEHet shows a good correlation to functionalisation, the C–Br cleavage during reductive functionalisation is tentatively assigned as heterolytic decomposition of the complexed RBr on the SWCNT surface.
In conclusion, the experiments indicate a development of the previously accepted mechanism reaction between nanotubide and organohalides. The reaction is initiated by rapid complexation of the organohalide to the nanotubide, displacing condensed counterions, before a second slower step involving degradation through halide reduction and cleavage of the carbon–halide bond to liberate the halide anion (Fig. 5). The absence of any nanotube or small molecule radical intermediates in the proposed mechanism is consistent with the lack of observation of either nanotube crosslinking or greater than stoichiometric grafting ratios as would be expected from mechanisms involving radical propagation steps (Fig. 1c and d). In agreement with previous work, steric occlusion of the organohalide is shown to play a significant role in determining the degree of functionalisation, assigned here to the packing of complexes on the SWCNT surface; however, for small molecules at least, steric effects are of lesser importance than electronic stabilisation of the intermediate. Furthermore, the degree of electron-donation from groups adjacent to the carbon–halide bond (i.e. electron density on the C–Br bond) is the strongest predictor for increasing the degree of functionalisation, with more electron donating substituents increasing the degree of functionalisation. Further investigation is required to elucidate a more detailed picture of the functionalisation mechanism, particularly the electronic stability of organohalide/nanotubide complex, the full reaction pathway, and detailed reaction kinetics. The concept of a surface adsorption activated grafting reaction is relevant more generally to other systems that span the molecular-continuum divide, including analogous reactions on graphenides and related materials. The development of OCV, as a method to study the organo-functionalisation, provides a new, simple, and readily accessible tool to investigate a wide range of such systems in real time. To use OCV to monitor surface chemical reactions, a material must be electronically conducting and physically stable in the relevant potential range, either as a cohesive electrode as seen here, or coated on an inert electrode surface. In principle, any reactions involving charge transfer between solvated species and the electrode may be monitored, although high specific surface areas may be needed to amplify the signal; with further refinement, more quantitative interpretation of the charge transfer and surface potentials may be possible. OCV may be useful not only for studying reactions on nanocarbons, but wide range of other systems, including functionalisation of conducting nanomaterials (e.g. transition metal dichalcogonides, nanodots, metallic nanoparticles, etc.), charge transfer to surface-bound ligands, as well as real-time monitoring pathways of heterogeneous catalytic systems.
Fig. 5 Proposed schematic of nanotubide reductive functionalization mechanism. Stern layer (blue), bulk solvent (green), positive charges (yellow), negative charges (red). Alkyl halide rapidly adsorbs onto charged nanotubide surface displacing cations condensed in the Stern layer and forming a more readily reduced complex between SWCNT, and C–Br bond. The complex slowly degrades, using nanotubide charge to break the C–Br bond.
This research was funded by the EPSRC (Doctoral Prize Fellowship, EP/M507878/1). Supporting data can be requested from the corresponding author, but may be subject to confidentiality obligations. P. S. appreciates the financial support from the Ministry of Science and Technology, Royal Thai Government.
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