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Synthesis, isolation, and X-ray structural characterization of trifluoromethylated C78 fullerenes:
C78(2)(CFs)i0/i2 and C78(3)(CFs)i2/14
N.B. Tamm, M.P. Kosaya, M.A. Fritz, S.I. Troyanov
Department of Chemistry, Moscow State University, Moscow, Russia tamm@thermo.chem.msu.ru, stroyano@thermo.chem.msu.ru
PACS 61.48.-c DOI 10.17586/2220-8054-2016-7-1-111-117
Four CF3 derivatives of C78, C78(2)(CF3)10/i2 and C78(3)(CF3)12/i4, have been isolated via HPLC from the products of high-temperature trifluoromethylation of a C76-C96 fullerene mixture or a C78 fraction. Their molecular structures were determined by single crystal X-ray crystallography using synchrotron radiation. The addition patterns of the new compounds are compared with each other and with the previously known C78(2)(CF3)10 and C78(3)(CF3)i2.
Keywords: higher fullerenes, C78, trifluoromethylation, HPLC, structure elucidation. Received: 20 November 2015
1. Introduction
Higher fullerenes are characterized by the existence of multiple cage isomers and relatively lower abundance compared to those of C6o and C70, and thus their isolation and further study of their structure and properties are much more difficult. Among the family of higher fullerenes, C78 is present in moderate abundance in fullerene mixtures because it, along with C84, C90, and C96, belongs to the group of C6n fullerenes, which have richer isomeric distribution and have been more comprehensively investigated [1]. The C78 fullerene possesses five topologically-distinct isolated pentagon rule (IPR) isomers, D3-C78(1), C2v-C78(2), C2v-C78(3), D3h-C78(4), and D3h-C78(5) [2]. The C78(2) - C78(5) isomers can be converted into each other by Stone - Wales rearrangements (SWR) of the pyracylene type, whereas C78(1) cannot be transformed into other isomers of C78 by SWRs. The abundances of C78(1) - C78(3) isomers in the fullerene soot are comparable, being dependent on the method of fullerene synthesis [3,4]. The D3-C78(1) and C2v-C78(2) isomers were isolated chromatographically and their cage structures were confirmed by 13C NMR spectroscopy [3]. At the same time, isomer D3h-C78(5) is elusive because of a small band gap resulting in a very low solubility; its presence in the fullerene soot could be unambiguously confirmed by the structural study of a CF3 derivative, C78(5)(CF3)12 [5]. The D3h-C78(4) isomer possesses the lowest stability; it has never been found in the fullerene soot. These differences are satisfactorily explained by the differences in the relative formation energies of the C78(1) - C78(5) isomers [3].
The chemical reactivity of C78 fullerene was studied in cyclopropanation, halogena-tions, trifluoromethylation, and other reactions [6-14]. Structural characterization of the derivatives was carried out by 1H, 13C, and 19F spectroscopy as well as by X-ray crystallography. Among the most structurally investigated compounds are the bromides, C78(2,3)Br18 [8], chlorides, C78(2,3,5)Cl18 [9-11] and C78(1,2)Cl30 [12,13], and several trifluoromethylated derivatives, C78(CF3)2n [14]. For the latter group, structural data obtained by 19F NMR spectroscopy and/or X-ray studies are available for compositions C78(3)(CF3)8, C78(1,2)(CF3)10,
and C78(3,5)(CF3)12. The only structural X-ray study of a pentafluoroethyl derivative concerns the compound C78(2)(C2F5)i0 [15]. Investigations of the structural chemistry of CF3 derivatives have been restricted to only several examples, which hampers a comprehensive comparison, even within the derivatives of a distinct isomer of C78. The aim of the present study is to expand the chemistry of trifluoromethylated C78 derivatives for different isomers. The synthesis, HPLC isolation, and X-ray crystallographic study was performed for several CF3 derivatives of C78(2) and C78(3) with 10-14 CF3 groups, which enables comparison with previously known C78(2,3)(CF3)n isomers. Addition patterns are discussed in terms of the partial of full occupation of 12 pentagons and formation of aromatic substructures and isolated double C=C bonds on the fullerene cage.
2. Results
Two different starting higher fullerene mixtures were used for trifluoromethylation with gaseous CF3I in quartz ampules following a previously described procedure [16-18]. The mixture of higher fullerenes C76 - C96 (45 mg; MER Corp.) was trifluoromethylated at 560 °C for 1 h, whereas a C78 fraction (60 mg) obtained by HPLC from the C76 - C90 mixture (Suzhou Dade Carbon Nanotechnology Co.) was reacted at 450 °C for 1.5 h. In both cases, the trifluoromethylation products were sublimed into the colder parts of the ampoules and were collected from there. The products obtained from the C76 - C96 mixture contained a complex mixture of fullerene(CF3)2n compounds with 2n in the range of 12 - 20 according to MALDI TOF mass spectrometric analyses, whereas CF3 derivatives of C78 were represented by C78(CF3)i0-i6 with the maximum abundance of C78(CF3)i2 species. Thifluoromethylation of the C78 fraction gave a mixture of C78(CF3)12-18 with the highest abundance of C78(CF3)14 and C78(CF3)16. It can be concluded that the lower reaction temperature (450 vs. 560 °C) somewhat shifts the composition in the mixture to the compounds with a large number of CF3 groups. This effect can be explained by the increase of the compound volatility with the increasing number of CF3 groups which influences the kinetics of sublimation in the reaction ampoule [19].
Both products were dissolved in toluene and subjected to HPLC separation in toluene (Buckyprep column, 10 mm i.d. x 250 mm, Nacalai Tesque Corp.) with a flow rate of 4.6 mL-min-1 monitored at 290 nm. A typical HPLC trace for the C76_g6(CF3)n product is shown in Fig. 1a. The fractions obtained were further separated by HPLC using toluene/n-hexane mixtures or pure n-hexane. The second-step separation of the toluene fraction, which eluted at 6.4 min, was carried out in a toluene/hexane mixture with v/v = 1/1 and a subtraction eluted at 20.2 min gave a compositionally pure C78(CF3)10 compound. Recrystallization from o-dichlorobenzene and p-xylene afforded small crystals which were then investigated by X-ray diffraction with the use of synchrotron radiation, thus revealing molecular structures of C78(2)(CF3)10 in the form of solvates with o-dichlorobenzene and ^-xylene, respectively. HPLC separation of the toluene fraction eluted at 6.8 min was performed in a toluene/hexane (v/v = 1/1) eluent and a subfraction eluted at 23.1 min afforded a compositionally pure C78(CF3)12 compound. Slow evaporation of solvent gave small crystals which were studied by X-ray crystallography revealing the molecular structure of C78(2)(CF3)12.
Trifluoromethylation products of the C78 fraction were also separated by two-step HPLC using the same Buckyprep column. The second-step HPLC of the first toluene fraction in a toluene/hexane 15/85 v/v mixture (not shown) gave a compositionally pure C78(CF3)12 fraction according to MALDI TOF mass-spectrometry. Recrystallization from toluene afforded small crystals of C78(3)(CF3)12 ■ 1.5 (toluene). Finally, the second-step HPLC separation of the first toluene fraction in hexane at flow rate of 1.5 mL-min-1 allowed the isolation of several C78(CF3)12-18 compounds (Fig. 1b). A fraction eluted at 17.7 min (indicated by the arrow),
a) L 7E<3>m
b)
78(3)/14
,, 78(3)/12
J
10 11
10 12 14 16 18 20 22 24 28 2fi retention time, miri
retention time, min
Fig. 1. HPLC trace of a fullerene(CF3)2n mixture in toluene (a) and of a C78(CF3)2n mixture in hexane (b). The collected C78(CF3)2n fractions are indicated by arrows. The compositions of C78(CF3)2n derivatives are given as 78(N)/2n, where N denotes the number of a C78 isomer according to the spiral algorithm
which contained predominantly C78(CF3)14 admixed by small amount of C78(CF3)16/18, afforded crystals after recrystallization from p-xylene. X-ray diffraction revealed the structure of a solvate, C78(3)(CF3)14 ■ 2.5 (p-xylene). Most other peaks of this separation also gave crystals which were shown to be CF3 derivatives of C78(1) with 12-18 attached groups. These data will be presented in a separate publication elsewhere later.
Synchrotron X-ray data for the obtained crystals were collected at 100 K at the BL14.2 at the BESSY storage ring (PSF at the Free University of Berlin, Germany) using a MAR225 CCD detector. Crystallographic data, along with some details of data collection and structure refinements, are presented in Table 1. The structures were solved with SHELXD and anisotropically refined with SHELXL [20]. All crystal structures except C78(2)(CF3)12 show disordering phenomena, most of which concern disorder of solvent molecules and CF3 groups. The latter was caused by the librational movement of CF3 groups around the C-CF3 bonds or due to statistical overlap of similar molecules in the same crystallographic site. In the crystal structure of C78(2)(CF3)10 ■ 0.5 (o-dichlorobenzene), one CF3 group and the solvated molecule of o-dichlorobenzene are disordered over two positions each. In the crystal structure of C78(2)(CF3)10- p-xylene, the molecule of solvation is strongly disordered over several positions. In the crystal structure of C78(3)(CF3)12 ■ 1.5 (toluene), there is a disorder of two CF3 groups and one toluene molecule. In the crystal structure of C78(3)(CF3)14- p-xylene, there is an overlap of the main molecule with its enantiomer (ca. 16 %) and the molecule of an epoxide, C78(3)(CF3)14O (ca. 33 %), in the same crystallographic site, which is accompanied by a disorder of seven CF3 groups. Crystallographic data are deposited under CCDC 1408116 - 1408120.
3. Discussion
Mass spectrometric MALDI TOF analyses of the raw trifluoromethylation products demonstrate the presence of C78(CF3)2n species with 2n ranging from 10 to 18, however, without information concerning C78 cage connectivity and CF3 addition patterns. HPLC separation supported by subsequent MALDI MS analyses of separated fractions indicated the presence of several different C78(CF3)2n isomers of the same composition, whereas their assignment to specific C78 frameworks remained unknown. Crystal growth from separated fractions, followed by
Table 1. Crystallographic data and details of data collection and refinement for CzsCCFsV compounds
Compound C78(2)(CF3)IO C78(2)(CF3)l0 C78(2)(CF3)l2 C78 (3)(CF3)12 C78 (3)(CF3)141
Solvate 0.5 o-CeH4Cl2 ^-CeH4(CH3)2 — 1.5 Co H5(CH3) 2.5 p- C6 H4(CH3)2
Mr 1700.38 1733.04 1764.90 1903.10 2174.44
Crystal system triclinic orthorhombic monoclinic triclinic monoclinic
Space group P 1 P212121 P21/n P 1 C2/c
a [A] 11.537(1) 11.218(1) 11.490(1) 11.878(1) 21.280(2)
b [A] 14.421(1) 19.022(1) 26.243(1) 12.374(1) 17.637(1)
c [A] 18.038(1) 28.842(2) 19.154(1) 24.267(2) 42.567(3)
a [°] 85.60(1) 90 90 89.485(9) 90
P [° ] 84.155(9) 90 91.409(10) 86.869(8) 90.02(1)
Y [°] 73.83 90 90 66.236(8) 90
V [A3] 2863.7(4) 6174.6(8) 5773.8(6) 3259.1(5) 15976(2)
Z 2 4 4 2 8
Dc [g-cm-3] 1.972 1.870 2.030 1.939 1.807
Crystal size [mm] 0.02x0.02 x0.01 0.03x0.03 x0.01 0.03x0.02 x0.01 0.03x0.03 x0.01 0.03x0.03 x0.01
A [A] 0.9050 0.8434 0.8434 0.8434 0.8551
Temperature [K] 100 100 100 100 100
0(max)[deg] 36.66 33.70 34.21 34.74 34.75
Refls col-lected/R(int) 37547 / 0.064 87675 / 0.025 91568 / 0.028 51625 / 0.026 84029 / 0.073
Data / parameters 10391 / 11148 14259 / 1144 13460 / 1135 13634 / 1252 18585 / 1463
R1[1 > 2a(/)]/ wR2 (all) 0.087 / 0.233 0.048 / 0.114 0.054 / 0.135 0.077 / 0.174 0.116 / 0.290
Ap(max / min)[e A-3] 0.90 / -0.49 0.57 / -0.44 0.56 /-0.39 0.55 /-0.55 0.67 / -0.50
:The crystal structure contains ca. 30 % admixture of an epoxide, C78(3)(CF3)14O, localized in the same crystallographic site.
single crystal X-ray structure determination using synchrotron radiation was successful for only some cases of C78(CF3)10-14, which are additional examples of unambiguous structural characterization of CF3 derivatives for the most abundant isomers 1 - 3 of C78 fullerene (Fig. 2). CF3 derivatives of the elusive C78(5) have not been detected, most probably, due to its very low content (or perhaps very low solubility), whereas the isolation of CF3 derivatives of C78(4) was not even expected because of its obvious absence in the fullerene soot.
Two of the three CF3 derivatives of C78(2) isolated and structurally characterized in this work contain the same Cs-C78(2)(CF3)10 molecule, which is known from Ref. [14] and designated there as 78-10-2. The differences concern only the solvate molecules, o-dichlorobenzene,
C.-C^XCF,),, C2v-Cis(2)(CF3),, C|-Cjb(3)(CFj)I3 C,-C7S(3)(CF,)!4
Fig. 2. Projection of the Cs-C78(2)(CF3)10 molecule parallel to the mirror plane; the C2v-C78(2)(CF3)12 molecule is shown along the C2 axis, whereas the C1 -C78(3)(CF3)12 and C1-C78(3)(CF3)14 molecules are presented along the C2 axes of the C78(3) carbon cages
p-xylene, and toluene in [14]. The formation of a toluene solvate with similar unit cell parameters and the space group P21 /m was also observed in one of our crystallization experiments. The addition pattern of 10 CF3 groups contains a single ribbon of para attachments in nine edge-sharing p-C6(CF3)2 hexagons, which is designated as p9 (Fig. 3). This addition pattern is additionally stabilized by the formation of two nearly isolated benzenoid substructures on the fullerene cage. An isomeric p4, p4 structure of C2 symmetry (78-10-3), which differs by the position of only one CF3 group (in the same pentagon near the C2 axis), has been proposed in [14] based on 19F NMR spectroscopy data. Note that the proposed 78-10-3 also contains two stabilizing nearly isolated benzenoid substructures. According to DFT calculations, Cs-C78(2)(CF3)10 is 21.3 kJ-mol-1 more stable than the isomer with C2 symmetry.
C,-C7S(2)(CFj)|0 C2v-C,s(2)(CF3)I2 C,-G,s(3)(CF3)i2 CrCls(3)(CF3),
Fig. 3. Schlegel diagrams of CS-C78(2)(CF3)10, C2v-C78(2)№)12, C-C78(3)(CF3)12, and C1-C78(3)(CF3)14 molecules. Cage pentagons are highlighted with gray. Black triangles denote the positions of attached CF3 groups. The isolated C=C bonds are denoted by double lines. Aromatic nearly isolated and isolated benzenoid as well as triphenylene substructures are also indicated
The addition pattern of 12 CF3 groups in the structure of C2v-C78(2)(CF3)12 is interesting in several aspects. Importantly, the structure of C2v-C78(2)(CF3)12 retains the symmetry of the pristine C2v-C78(2) cage (Fig. 3). Its addition pattern contains the addition patterns of both Cs-C78(2)(CF3)10 and C2-C78(2)(CF3)10 as substructures. Most probably, both molecules serve as precursors of C2v-C78(2)(CF3)12 in the course of trifluoromehylation. The attachment of two CF3 groups transforms the p9 ribbon into a p12 loop. At the same time, two nearly isolated benzenoid rings become fully isolated and one carbon-carbon bond becomes an isolated double C=C bond, both acting as additional stabilizing factors. It should be noted that the addition pattern of C2v-C78(2)(CF3)12 exhibits an extremely rare example of a fullerene(CF3)12 structure
where not all pentagons are occupied by CF3 groups. In fact, two pentagons remain unsubsti-tuted in the experimentally determined structure. If the addition of two more groups occurs in these pentagons (for example, in the left-most and right-most positions of free pentagons), the hypothetical Cs-C78(2)(CF3)12 molecule with all 12 pentagons occupied by CF3 groups would contains two groups attached in isolated positions, which can be considered as a destabilizing feature. Indeed, DFT calculations of the formation energies of the hypothetical and the experimentally determined C78(2)(CF3)12 molecules revealed that the former is 10.1 kJ-mol-1 less stable than the latter.
Structural relations between C78(2)(CF3)10 and C78(2)(CF3)12 molecules are very similar to those reported for C84(18)(CF3)10 and C84(18)(CF3)12 [18]. The addition pattern of Cs-C84(18)(CF3)10 contains a single p9 ribbon which is very similar to that in Cs-C78(2)(CF3)10. Though the experimentally determined structure of Cs-C84(18)(CF3)12 is characterized by the attachment of twelve CF3 groups in all pentagons (with two additional groups in a separate p-C6(CF3)2 hexagon), a theoretically predicted structure with a p12 loop (84(18)/12 — 3 in [18]), i.e., an analog of C78(2)(CF3)12, is only 6 kJ-mol-1 less stable so that its presence in the trifluoromethylation products of C84(18) cannot be excluded. In fact, the only example of addition-free pentagons in the molecule with more than 12 addends on a fullerene cage has been reported for C88(17)Cl16 containing two such pentagons [21]. The stabilizing factors of this structure include the formation of one isolated benzenoid ring and three isolated C=C bonds on the fullerene C88(17) cage.
The crystal structure of C78(3)(CF3)12 ■ 1.5(toluene) contains the C1-C78(3)(CF3)12 molecule, which is known from the previous structure determination for a solvate with bromobenzene (78-12-2) [14]. Its asymmetric addition pattern of 12 CF3 groups occupying all 12 pentagons is characterized by the presence of a short p3 ribbon and a longer p5mp ribbon (m for a meta-C6(CF3)2 hexagon) on the fullerene cage (Fig. 3). Thus, it differs considerably from the additions pattern of C78(2)(CF3)12 which comprises a loop of exclusively para additions in C6(CF3)2 hexagons (p12) and containing two unoccupied pentagons. For comparison, the addition pattern of C2-C78(5)(CF3)12 [5] contains a single p11 ribbon of 12 CF3 groups on the C78(5) cage which differs from the C78(3) cage by the position of only one C-C bond.
The molecular structure of C78(3)(CF3)14 was determined for the first time. Its addition pattern consists of a short pmp ribbon and a long p10 ribbon of edge-sharing C6(CF3)2 hexagons (Fig. 3). There is an isolated double C=C bond and an isolated triphenylene-like substructure on the fullerene cage. 13 of 14 CF3 groups are arranged mirror-symmetrically on the cage. This feature explains the existence of packing errors with a number of enantiomeric molecules located in the same crystallographic sites as the main molecules in the crystal structure. C1-C78(3)(CF3)14 and C1-C78(3)(CF3)12 possess only seven CF3 groups attached in common positions. Therefore, the latter cannot be regarded as a precursor of the former.
4. Conclusions
Trifluoromethylation of a higher fullerenes mixture and a C78 fraction followed by HPLC separation, crystallization, and X-ray diffraction studies resulted in structure determination of several CF3 derivatives, C78(2,3)(CF3)10-14. Although the carbon cages of C2v-C78(2) and C2v-C78(3) differ by the position of only one C-C bond [2], the addition patterns of the derivatives with equal numbers of CF3 groups, C78(2,3)(CF3)12, differ significantly. A similar phenomenon has also been found for (CF3)12 derivatives of isomers C84(22) and C84(23), which also differ by the position of one bond on the carbon cages [16,17]. In contrast, the addition patterns of C78X18 (X = Cl, Br) are the same for isomers C78(2), C78(3), and even C78(5) [8-11] which give rise to co-crystallization phenomena for these halides [8,11] thus demonstrating the
levelling effect for a large number of addends on the addition patterns of fullerene derivatives with similar carbon cages. The existence of similar effects for CF3 derivatives of isomers of C78 could be clarified by structural study of compounds with 16-18 attached groups.
Acknowledgements
This work was supported in part by the Russian Foundation for Basic Research (grant 1503-04464).
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