Reaction of C70 with ten equivalents of silver(I) trifluoroacetate at 320-340 degrees C followed by fractional sublimation at 420-540 degrees C and HPLC processing led to the isolation of a single abundant isomer of C70(CF3)n for n = 2, 4, 6, and 10, and two abundant isomers of C70(CF3)8. These six compounds were characterized by using matrix-assisted laser desorption ionization (MALDI) mass spectrometry, 2D-COSY and/or 1D 19F NMR spectroscopy, and quantum-chemical calculations at the density functional theory (DFT) level. Some were also characterized by Raman spectroscopy. The addition patterns for the isolated compounds were unambiguously found to be C1-7,24-C70(CF3)2, C1-7,24,44,47-C70(CF3)4, C2-1,4,11,19,31,41-C70(CF3)6, Cs-1,4,11,19,31,41,51,64-C70(CF3)8, C2-1,4,11,19,31,41,51,60-C70(CF3)8, and C1-1,4,10,19,25,41,49,60,66,69-C70(CF3)10 (IUPAC numbering). Except for the last compound, which is identical to the recently reported, crystallographically characterized C70(CF3)10 derivative prepared by a different synthetic route, these compounds have not previously been shown to have the indicated addition patterns. The largest relative yield under an optimized set of reaction conditions was for the Cs isomer of C70(CF3)8 (ca. 30 mol % of the sublimed mixture of products based on HPLC integration). The results demonstrate that thermally stable C70(CF3)n isomers tend to have their CF3 groups arranged on isolated para-C6(CF3)2 hexagons and/or on a ribbon of edge-sharing meta- and/or para-C6(CF3)2 hexagons. For Cs- and C2-C70(CF3)8 and for C2-C70(CF3)6, the ribbons straddle the C70 equatorial belt; for C1-C70(CF3)4, the para-meta-para ribbon includes three polar hexagons; for C1-7,24-C70(CF3)2, the para-C6(CF3)2 hexagon includes one of the carbon atoms on a C70 polar pentagon. The 10.3-16.2 Hz 7JF,F NMR coupling constants for the end-of-ribbon CF3 groups, which are always para to their nearest-neighbor CF3 group, are consistent with through-space Fermi-contact interactions between the fluorine atoms of proximate, rapidly rotating CF3 groups.
A number of C60(CF3)n compounds with n = 2–10 have been synthesized by the reaction of C60 with silver trifluoroacetate and successfully isolated by means of HPLC. This resulted in the first crystal structure determination of six lower trifluoromethyl derivatives with n = 2 (single isomer), 4 (two isomers), and 6 (three isomers). A kinetic model of sequential trifluoromethylation based on the Bell–Evans–Polanyi principle has been used to explain the experimentally observed isomeric distribution in the mixtures of C60(CF3)n compounds up ton = 6. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
In recent years, many higher fullerenes that obey the isolated pentagon rule (IPR) were found capable of rearranging into molecules with adjacent pentagons and even with heptagons via chlorination-promoted skeletal transformations. However, the key fullerene, buckminsterfullerene I -C, long seemed insusceptible to such rearrangements. Now we demonstrate that buckminsterfullerene yet can be transformed by chlorination with SbCl at 420-440 °C and report X-ray structures for the thus-obtained library of non-IPR derivatives. The most remarkable of them are non-IPR CCl and CCl with fundamentally rearranged carbon skeletons featuring, respectively, four and five fused pentagon pairs (FPPs). Further high-temperature trifluoromethylation of the chlorinated mixture afforded additional non-IPR derivatives C(CF) and C(CF), both with two FPPs, and a nonclassical C(CF)F with a heptagon, two FPPs, and a fully fused pentagon triple. We discuss the general features of the addition patterns in the new non-IPR compounds and probable pathways of their formation via successive Stone-Wales rearrangements.
Investigation of higher fullerenes and their derivatives is hampered not only by the small quantities of materials available but also because for each fullerene two or more cage isomers can exist that obey the Isolated Pentagon Rule (IPR).[1] Typically higher fullerenes and their mixtures are characterized by 13 C NMR spectroscopy of HPLC fractions which yields information on molecular symmetry but not always on definitive isomer cages. [2,3] Theoretical calculations provide information concerning the relative stability or the expected line distribution in the NMR spectra, thus assisting the cage assignment. [4][5][6] Derivatization can help in characterizing higher fullerenes because their derivatives are easier to separate. In this way, metalation and chlorination of higher fullerenes contributed to the determination of cage connectivities in C 76 -C 80 , C 84 , and C 90 . Perfluoroalkylation followed by HPLC separation and X-ray structure determination of perfluoroalkyl (R F ) derivatives confirmed the cage connectivities in isomers of C 76 -C 78 , [7] C 84 -C 88 , [8,9] and C 92 . [9] Experimental observations of even higher fullerenes, C 94 and C 96 , have been limited to 13 C NMR and UV/Vis spectra of the HPLC fractions containing isomer mixtures. [2,3,10] Here we report the synthesis, separation, and X-ray structure determination of R F derivatives C 94 (CF 3 ) 20 and C 96 (C 2 F 5 ) 12 . The results provide direct proof of the cage connectivities of the highest fullerenes investigated so far, C 94 and C 96 , for which respectively 134 and 187 IPR isomers are possible.[1]A mixture of higher fullerenes C 76 -C 96 (30 mg) also containing small amounts of C 60 and C 70 was allowed to react with excess CF 3 I (98 %) in a glass ampoule at 400-420 8C and a pressure of roughly 6 bar for 3 days. The orange-colored sublimate that deposited in the colder part of the ampoule contained a complex mixture of CF 3 derivatives of C 60 , C 70 , and C 76 -C 96 according to MALDI MS analysis. The number of attached CF 3 groups ranged from 12 to 20. The mixture was partially dissolved in hexane and subjected to HPLC separation using a Cosmosil Buckyprep column (10 mm i.d., 25 cm length), hexane as the eluent (4.6 mL min À1 ), monitored at the wavelength of 290 nm. Several fractions containing C 94 (CF 3 ) n compounds were eluted between 4 and 38 min. The fraction eluting at 4.64 min contained C 94 (CF 3 ) 20 as the main component. After an additional HPLC separation and recrystallization from a toluene/hexane mixture, small orange crystals were obtained which were investigated by X-ray diffraction with the use of synchrotron radiation.[11] The same reaction batch provided some other CF 3 compounds which were isolated and characterized by X-ray diffraction: C 84 -(CF 3 ) 12 [9] and C 92 (CF 3 ) 16 .[9]The same mixture of higher fullerenes (15 mg) was allowed to react with excess C 2 F 5 I (98 %, Apollo) in a glass ampoule at 250 8C under a pressure of about 20 bar for 5 days. After the ampoule had been opened, the excess C...
Five C70(CF3)14 isomers have been isolated chromatographically from the mixture produced in the ampoule reaction between C70 and CF3I at 390 °C. Molecular structures of four isomers have been determined in a single‐crystal X‐ray diffraction study. A quantum chemical survey of the theoretically possible isomers demonstrated that the structures obtained are energetically favorable but that there is probably no full thermodynamic control in the trifluoromethylation process.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
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