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Addition of Carbene to the Equator of C 70 To Produce the Most Stable C 71 H 2 Isomer: 2 a H ‐2(12)a‐Homo(C 70 ‐ D 5 h (6) )[5,6]fullerene

2009; Wiley; Volume: 49; Issue: 5 Linguagem: Inglês

10.1002/anie.200905263

1521-3773

Bao Li, Chunying Shu, Xin Lü, Lothar Dunsch, Zhongfang Chen, T. John S. Dennis, Zhiqiang Shi, Li Jiang, Taishan Wang, Wei Xu, Chunru Wang,

Carbon Nanotubes in Composites

At long last, the elusive thermodynamically most stable isomer of C71H2 (see structure) has been synthesized through the addition of CH2 across an equatorial bond of C70: a site at which such a reaction is not kinetically favorable. Theoretical studies revealed that the pyrogenic synthetic method used enables reactants to overcome the higher activation barrier to equatorial addition. Unlike C60, in which all carbon-atom environments are identical, C70 has five distinct carbon-atom environments, which give rise to eight distinct CC bond types. Hence, the addition chemistry of C70 involves both chemo- and regioselectivity. The synthetic chemistry of C70 is centered on the areas near the poles,1, 2 as these areas have the highest curvature and hence high bond strain.3, 4 This relatively high bond strain in turn makes the polar regions the most reactive sites of the molecule. The equatorial region of C70, on the other hand, has little curvature and hence lower bond strain. Thus, the carbon atoms at the equator are much less reactive, as there is a much higher activation barrier to be overcome before reactions can occur. For example, carbene (CH2) has been added to the polar region of C70, and several isomers of C71H2 have been synthesized and fully characterized.5–8 However, the addition of carbene to the equatorial bond of C70 (to form C2v-C71H2) has not been detected. We were prompted to synthesize this elusive C2v C71H2 isomer for several reasons: 1) The hydrogen-atom chemical shifts of the carbene adducts are useful for probing the local ring currents of C70 and its hexaanion.7, 9–11 Computations show that the equatorial six-membered ring of C70 has the largest diamagnetic ring current,11, 12 but no solid experimental evidence exists. 2) Theoretical studies have indicated that the sidewall of nanotubes can be opened by chemical modifications of divalent groups, such as carbene,13 as was also confirmed indirectly by Umeyama et al.14 C70 can be considered as the shortest (5,5) nanotube; fully characterized C2v C71H2 would provide us with direct hard evidence of the structure of nanotube carbene adducts. 3) The density functional computations in this study show that the C2v structure has the lowest energy of all possible isomers of C71H2 and is an open [6,6] homofullerene. Addition at the [6,6] junction of fullerenes mostly results in [6,6] closed adducts;15 [6,6] open homofullerenes are stable only in special cases.16, 17 Herein we report the synthesis, characterization, and theoretical studies of the missing C71H2 isomer—a homofullerene with a CH2 group attached to the C70 equator. Access to this elusive C2v C71H2 isomer not only enabled satisfactory clarification of the local electron delocalization of the C70 equatorial rings and provided unambiguous support for nanotube-sidewall opening, but also provided a new member of the homofullerene family. Moreover, the pyrogenic synthesis proved to be a highly efficient approach to overcome the high activation barriers to the formation of the thermodynamically most stable isomers, as also demonstrated, for example, by the synthesis of C60Cl8 and C60Cl12, in which the C60 cage violates the isolated pentagon rule (IPR),18 and by the synthesis of the stable unconventional fulleride C64H4.19 The structures of all chemically possible C70-fullerene-based isomers of C71H2 were optimized by density functional calculations at the B3LYP/6-31G* level. There are eight different types of bonds in C70: aa, ab, bc, cc, cd, dd, de, and ee; among them, the ab and dd bonds are the shortest and exhibit chemical reactivity like that of a CC double bond, for example, they may undergo [2+1] cycloaddition with an incoming carbene, whereas the bonds in the equatorial pentaphenyl belt are benzene-like (not quinoid-like) and far less reactive.5–8, 11, 12 Topologically, there are potentially 16 isomers of C71H2: eight methanofullerenes (CH2 adds across one of each of the eight CC bonds) and eight homofullerenes (CH2 replaces one of each of the eight CC bonds).6, 20 Depending on which of the eight bonds undergoes reaction, and irrespective of whether the CC bond is crossed or replaced, these isomers have Cs, Cs, C1, Cs, C1, Cs, C1, and C2v symmetry, respectively (according to the list of bond types above). Our computations show that homofullerenes are formed when the CH2 addend is attached to an aa, bc, cd, dd, or ee bond, whereas methanofullerenes are obtained when the CH2 group is attached to an ab, cc, or de bond (Figure 1).21 Similar to the case of carbon nanotubes,13 the enhanced stability of homofullerenes is due to homoaromatic stabilization and the avoidance of strain energy; the addition of CH2 would otherwise result in the formation of a three-membered ring (as in methanofullerenes) and loss of the homoaromatic stabilization (as in bridged 1,6-X-[10]annulenes).22 C70 and the eight computationally identified isomers of C71H2. The five different types of carbon atom in C70 are assigned as a, b, c, d, and e. The e–e isomer has two mirror planes marked with dashed lines. Isomers a–a, b–c, c–d, d–d, and e–e are homofullerenes; isomers a–b, c–c, and d–e are methanofullerenes. Of the eight calculated C71H2 isomers, the e–e isomer has by far the lowest energy (Table 1). Thus, reactions at the poles are kinetically rather than thermodynamically controlled. Our calculations agree well with those of Smith et al.,7 who obtained the kinetically favorable a–a/b–c and a–b/c–c isomers by thermolysis and irradiation of the precursor, respectively; these isomers were among those low in energy. The C71Cl2 isomer synthesized by Kiely et al. with CCl2 bridging d,d carbon atoms8 is the isomer of third-lowest energy. Considering the rather high relative energies, it is understandable that the d–e isomer of C70O23 and the c–d and d–e isomers of C71H2 have not been detected so far. Bonds bridged by CH2 C–C separation [Å] Relative energy [kcal mol−1] e–e 2.315 0 a–b 1.647 9.9 d–d 2.154 11.5 c–c 1.611 11.9 a–a 2.184 12.6 b–c 2.181 15.3 c–d 2.179 22.8 d–e 1.709 24.8 There are several possible methods for the synthesis of C71H2, such as solution chemical reactions,6–8 the Krätschmer–Huffman method,24 and a photochemical reaction.20 The method7, 8 for the synthesis of a–a, a–b, b–c, c–c, and d–d isomers involves kinetically favorable [2+1] cycloaddition reactions in solution at relatively low temperature. To overcome the possible higher activation barrier and obtain the product of equatorial addition to C70, pyrogenic synthetic methods were adopted in this study. The C2v C71H2 homofullerene was synthesized by a modified Krätschmer–Huffman method in the presence of methane (see the Experimental Section) and separated from other hydrogenated fullerene derivatives as well as C70 and C70O by recycling HPLC. The purity of the final sample was approximately 99 %, as estimated from the HPLC profile (Figure 2) and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. The mass spectrum (see the Supporting Information) exhibited only one molecular-ion peak, at m/z 854, which corresponds to C71H2. Recycling-HPLC profiles (three recycles) of a) the C70/C71H2 mixture, b) pure C71H2 (solid line) together with pure C70 (dashed line), and c) pure C71H2 produced by the Krätschmer–Huffman method (solid line) and by the CVD method (dashed line). Conditions: Buckyprep-M column (20×250 mm2); flow rate: 12 mL min−1; eluent: toluene. NMR spectroscopy is an effective tool for the structural characterization of fullerenes and their derivatives,25 and symmetry considerations are critical in the correlation of NMR spectroscopic data with possible structures.7 The 13C NMR spectrum of the product obtained by this method exhibits 22 lines (14×4, 7×2, 1×1; Figure 3). This pattern can only be consistent with the C2v homo or methano isomer since the symmetry of the other isomers is far too low. Although the C2v homo and methano isomers have patterns of the same intensity, 13C NMR spectroscopy can readily distinguish between the two isomers, as in general sp3- and sp2-hybridized carbon atoms have vastly different chemical shifts. The measured spectrum with 21 lines (14×4, 7×2) in the range δ=115–155 ppm (corresponding to sp2-hybridized carbon atoms) and a single line (1×1) at δ=30.2 ppm (corresponding to the sp3-hybridized methylene carbon atom) is only consistent with the e–e homofullerene isomer with C2v symmetry, since the methano isomer should have 20 lines (14×4, 6×2) for the sp2-hybridized carbon atoms and two lines for a twofold degenerate and a methylene sp3-hybridized carbon atom. The 13C NMR spectrum simulated at the B3LYP/6-31G* level for the e–e isomer agrees reasonably well with the experimental result (Figure 3). Notably, the e,e bridgehead carbon atoms have a chemical shift of 118.64 ppm, which is consistent with the computationally optimized structure, homofullerene, owing to homoaromaticity.26 a) 13C NMR spectrum (100 MHz) of C71H2 (CS2/[D6]acetone) in the range δ=115–155 ppm. b) Computed (B3LYP/6-31G* level) 13C NMR spectrum of C71H2 with C2v symmetry. Signals marked with * are single-intensity resonances (all others are double intensity). The heteronuclear multiple quantum coherence (HMQC) NMR spectrum (see the Supporting Information) shows a 13C–1H correlation at the intersection of δC=30.2 ppm with δH=1.27 ppm; this correlation is consistent with a methylene functional group. Of all eight computationally predicted isomers of C71H2, only isomers a–b and e–e contain two equivalent hydrogen atoms. The resonances for the methylene hydrogen atoms of the a–b isomer were reported to occur at δ=2.88 ppm.7 The singlet that we observed at δ=1.27 ppm for the C2v C71H2 isomer indicated unambiguously that our spectrum was that of the e–e isomer. The high-field chemical shift of this signal indicates that the methylene hydrogen atoms are more shielded than those in the other experimentally accessible C71H2 isomers (δ=2.91/6.52 for a–a, 2.88 for a–b, 2.78/5.23 for b–c, and 2.56 ppm for c–c)7 and provides strong evidence that the equatorial hexagonal rings of C70 are the most aromatic.11, 12 UV/Vis spectra exhibit different absorption characteristics for different isomers as a result of changes in the molecular-orbital levels. According to Smith et al.,7 the absorptions of the a–a isomer (homofullerene) are much more similar than those of the a–b isomer (methanofullerene) to those of C70, as the homofullerene retains the π-electron conjugation system of the C70 skeleton to the maximum extent. The spectrum of our arc-produced e–e isomer exhibits similar absorptions to those of pristine C70 (Figure 4). This result is again consistent with a homofullerene structure. UV/Vis spectra (in toluene) of C70, arc-produced C71H2 (dashed line), and CVD-produced C71H2 (solid line). Three basic formation mechanisms can be envisioned for C71H2 in the direct-current (DC) arc. First, C70 and C71H2 form simultaneously under the conditions of the arc. Second, C70 forms initially in the arc and then reacts with CHx to form several isomers of C71H2, which isomerize into the e–e isomer. The third mechanism is similar to the second except that the e–e isomer is the only survivor of the several isomers of C71H2 formed during the subsequent cooling collisions. To confirm that high temperatures are required to overcome an activation barrier to produce thermodynamically stable e–e-bonded adduct, and to probe the formation mechanism, we produced C71H2 by another high-temperature technique. This technique was similar to chemical vapor deposition (CVD)27 but involved the direct reaction of C70 with CH4 in the gas phase at approximately 1100 °C. The C71H2 product obtained by the "CVD" method was essentially identical to that synthesized by the modified Krätschmer–Huffman method (Figures 2 and 4). As the temperature of the "CVD" method is about 3000 K lower than that of the arc method, it is likely that C2v-C71H2 is produced by the second or third mechanism; that is, C70 forms in the arc and subsequently reacts whilst hot with CH4. In conclusion, we synthesized the elusive but thermodynamically most stable carbene adduct of C70: an isomer of C71H2 in which CH2 has added across an e–e bond to form a C2v homofullerene. The product was characterized by NMR and UV/Vis spectroscopy, as well as mass spectrometry. Theoretical studies (B3LYP/6-31G*) confirmed that this isomer is the most stable, although the direct addition of carbene at the equatorial sites is not kinetically favorable. This DC-arc production of the C71H2 e–e isomer appears to be thermodynamically controlled, as confirmed by a CVD-based method for the direct reaction of C70 with CH4 in the gas phase. The production and characterization of the thermodynamically most stable but kinetically unfavorable C71H2 isomer, together with the systematic theoretical studies, not only offer us direct experimental evidence in answer to some important chemical questions, such as the local aromaticity of C70 and the structure of divalent-group adducts of normal-diameter single-walled nanotubes, but also introduce a new member into the homofullerene family and provide a new route for the synthesis of novel stable fullerene derivatives that can not be obtained under routine synthetic conditions. More studies on this homofullerene and related systems are in progress. In the modified Krätschmer–Huffman method, helium and methane (ca. 100:1) were introduced into the DC-arc oven at a total pressure of 26.7 kPa. A spectroscopically pure graphite rod (length 30 mm, ø 8 mm) was used as the anode, a graphite disk as the cathode. The voltage and intensity of the current were maintained at 40 V and 160 A, respectively. The as-produced soot was extracted by Soxhlet extraction with toluene for 48 h, and the extracts were separated by HPLC (see the Supporting Information). The total amount of C71H2 obtained was approximately 3 mg (ca. 0.05 % in the produced soot). The retention time for C71H2 was approximately 19.5 and 12.2 min on Buckyprep and Buckprepy-M columns (20×250 mm2, Cosmosil; detector: 310 nm), respectively, at a flow rate of 12 mL min−1 in toluene. In the CVD-based method, C70 vapor, sublimed from the purified C70 powder at 800 °C, was brought into the reaction area (1100 °C) by a flow of CH4. The products were deposited at the collection zone (room temperature), then extracted with toluene from a sootlike residue (amorphous carbon resulting from the decomposition of CH4 and C70), and then separated by recycling HPLC (Buckyprep-M column). 13C NMR spectroscopy of C71H2 was carried out on a 100 MHz NMR spectrometer (Bruker AV400) with a BBO probe (5 mm). The sample was dissolved in CS2, with [D6]acetone in a capillary as an internal lock. The 1H–13C HMQC spectrum of C71H2 was acquired on a Bruker AV400 instrument (600 MHz) with a BBI probe (5 mm), with CS2 as the solvent and [D6]acetone as an internal lock. 13C NMR (100 MHz, [D6]acetone, 25 °C): δ=30.2 (1 C), 118.6 (2 C), 129.3 (2 C), 129.7 (2 C), 130.7 (2 C), 131.1 (2 C), 139.1 (4 C), 139.3 (4 C), 141.4 (4 C), 144.6 (4 C),144.9 (4 C), 145.2 (4 C), 145.4 (2 C), 145.9 (4 C), 146.0 (4 C), 146.7 (4 C), 147.3 (4 C), 148.0 (4 C),149.7 (4 C), 150.0 (4 C), 151.2 (2 C), 152.7 ppm (4 C). 1H NMR (400 MHz, [D6] acetone, 25 °C, Si(CH3)4): δ=1.27 ppm. UV/Vis spectra of C71H2 and C70 were recorded on a UV spectrometer (Unico UV4802) in toluene. C71H2: λmax=333, 380, 471, 550, 600, 618, 640, 660 nm (ε380=32 970 L mol−1 cm−1); C70: λmax=316, 335, 365, 383, 472, 550, 600, 620, 640, 660 nm (ε383=36 067 L mol−1 cm−1). Full geometry optimization and 13C NMR chemical-shielding computations were carried out for all possible C71H2 isomers at the B3LYP/6-31G* level of theory. 13C NMR chemical-shielding values were evaluated by employing the gauge-independent atomic orbital (GIAO) method. They were calculated relative to C60 and converted to the tetramethylsilane scale. All calculations were carried out with the Gaussian 03 program.28 Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.