Crystal Structures of C₉₄(CF₃)₂₀ and C₉₆(C₂F₅)₁₂ Reveal the Cage Connectivities in C₉₄ (61) and C₉₆ (145) Fullerenes

Abstract

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...

Capturing C90 Isomers as CF3 Derivatives: C90(30)(CF3)14, C90(35)(CF3)16/18, and C90(45)(CF3)16/18

“…Thus, similar shapes of both molecules are responsible for identical (or very close) retention times during HPLC separation and fort heir capability for mutual substitution in the crystal packing. Similar co-crystallization phenomena of the derivatives of higher fullerenes with the same addition patterns but different cage isomers have been also found in the crystal structures of C 78 (2,3)Br 18 , [13a] C 84 (11,14,16) [13d]Ac omparison of the molecular structures of C 90 (CF 3 ) 14À18 with carbon cages of C 90 (30), C 90 (35), and C 90 (45) demonstrates some common features of their addition patterns. Most of additions of CF 3 groupso ccur in para positions with the formation of chains of adjacent C 6 (CF 3 ) 2 hexagons.…”

“…The stabilizing factor is the formation of one, two or three isolated C=Cb onds in the molecules with 14, 16, or 18 attached CF 3 groups, respectively.Further stabilization occurs due to formation of aromatic substructures, isolated or nearly isolated benzenoid rings, on the carbon cages. The addition patterns of the C 90 (CF 3 ) 14,16,18 molecules exhibit similar features as the most extensively studied CF 3 derivatives of C 84 (isomers1 6, 18, 22, and 23), in which the addition to THJ has not been observed.[11] However,t he addition of two CF 3 groups in THJs was established in the overcrowdedm olecule of C 94 (61)(CF 3 ) 20 .[14a]SeveralC 76 (1)(CF 3 ) 14À18 molecules contain one or two CF 3 groups in THJs.[14b] In the latter cases, the presence of additions in THJs can be explained by the simultaneous formation of isolated benzenoid rings or/and isolatedb utadiene fragments on the C 76 carbon cage.An important result of the presentw ork is the first confirmation of the presence of isomer C 90 (45) in the fullerenes oot, which was highlye xpectedt ob ei solated experimentally as the most stable isomer of C 90 according to quantum chemical calculations by different methods.[10] Now the set of seven experimentally confirmed isomers of C 90 includes all isomers of high (nos. 45, 46, 1) or middle relative stability (nos.…”

“…[3a] Recently,t wo other CF 3 derivatives, C 90 (30)(CF 3 ) 18 and C 90 (35)(CF 3 ) 14 ,h ave been isolated and crystallographically characterized.…”

“…Single crystal X-ray diffraction with the use of synchrotronr adiation resulted in the structure determination of C 90 (30)(CF 3 ) 14 ,C 90 (35)(CF 3 ) 16/18 , and C 90 (45)(CF 3 ) 16/18 .T heir addition patterns are discussed and compared with the known isomers C 90 (30)(CF 3 ) 18 and C 90 (35)(CF 3 ) 14 ,r espectively.T he presence of the most stable C 90 isomer,C 90 (45), in the fullerene soot has been confirmed for the first time.…”

Higher fullerenes: Compositional analysis by EDXD and molecular dynamics

“…Therefore, although the existence of fullerene isomers up to C540 [12] has been hypothesized, only isomers up to C116 have been, so far, isolated [13] and only the structure of isomers up to C96 has been determined [14]. Derivatization is also an effective tool for isolating a specific isomer, since it enhances the variation of physical properties [15]. A way of circumventing the difficulties of lengthy and expensive fullerene purification steps is using the powder as it is obtained from the synthesis, provided it is properly characterized.…”