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SYNTHESIS, ISOLATION, AND X-RAY STRUCTURAL CHARACTERIZATION OF TRIFLUOROMETHYLATED C90 FULLERENES: C90(30)(CFS)18 AND C90(35)(CFS)14
N. B. Tamm1, S.I. Troyanov1
department of Chemistry, Moscow State University, Moscow, Russia [email protected], [email protected]
PACS 61.48.-c
Two CF3 derivatives of C90, C9o(30)(CF3)i8 and C9o(35)(CF3)i4, have been isolated via HPLC from the products of a high-temperature trifluoromethylation of a C76-C96 fullerene mixture with CF3I. Their molecular structures were determined by single crystal X-ray crystallography using synchrotron radiation. The addition patterns of the new compounds are discussed in comparison with those of the corresponding chlorinated C90.
Keywords: Fullerenes, C90, Trifluoromethylation, HPLC, Structure elucidation. 1. Introduction
Compared to Ceo and C70, the investigation of higher fullerenes has been hampered by their relatively low abundance in fullerene soot and due to the existence of cage isomers [1]. Structural characterization of pristine higher fullerenes is typically accomplished by means of 13C NMR spectroscopy, which provides information on molecular symmetries. However, identification of higher fullerenes by this conventional method is not unambiguous in many cases since several isomers may exhibit the same molecular symmetry [2,3]. An efficient alternative is chemical derivatization of higher fullerenes followed by isolation and structural characterization of the derivatives thus obtained, as exemplified by several examples for C76-C96 [4-12].
Cg0 belongs to the group of higher fullerenes with magic (6n) number of carbon atoms, which usually possess higher abundance and richer isomerism compared to their neighboring members [13]. For Cg0, there are 46 topologically possible isomers obeying the Isolated Pentagon Rule (IPR) [1]. Experimentally, 13C NMR spectra of chromatographic C90 fractions were interpreted as showing the existence of five isomers of C2v, Cs, C2, C1, and C2 symmetry [14,15]. A comparison between the experimental and theoretically predicted 13C NMR shifts allowed the establishment of most probable carbon cages, 28 (C2), 30 (C1), 32 (C1), 35 (Cs), 40 (C2), 45 (C2) and 46 (C2v), which are present in the Cg0 fractions [16]. X-ray crystallographic study of co-crystals of Cg0 from three HPLC fractions (obtained from arc-discharge of Sm2O3-doped graphite rods) with Nin(OEP) (OEP - octaethylporphirin) resulted in the structural confirmation of three isomers of pristine Cg0, 1 (D5h), 30, and 32 [17,18].
A trifluoromethylated derivative of Cg0, Cg0(CF3)12, was suggested to contain C1-Cg0(32) cage on the basis of its 1gF NMR spectrum [19]. X-ray crystallographic investigation of Cg0Clra(n = 22, 24, 28, and 32) unambiguously confirmed the Cg0 cages nos. 28, 30, 32,
34 (Cs), 35, and 46 and significantly contributed to the study of their reactivity towards inorganic chlorides [20,21].
Herein, we report the synthesis, HPLC isolation, and X-ray structure elucidation of trifluoromethylated derivatives of two C90 isomers, C90(30)(CF3)18 and C9o(35)(CF3)14. Addition patterns are discussed and compared with those of the corresponding chlorinated C90.
2. Results
A starting higher fullerene mixture (35-45 mg; MER Corp.) contained C76-C96 fullerenes with the highest abundance of C84 and small admixtures of C60 and C70 [22]. The reaction with CF3I was performed at 420 °C in a glass ampoule for 48 h, whereas the reaction at 560 °C (in a quartz ampoule) lasted only 1 h (see [23] for more details). In both cases, the trifluoromethylation products sublimed in the colder parts of the ampoules contained mixtures of fullerene(CF3)2ra derivatives with 2n in the range of 12 - 20 according to MALDI TOF mass spectrometric analyses (fig. 1).
Fig. 1. Mass spectra of trifluoromethylation products. (a) reaction at 420 °C; (b) reaction at 560 °C. The compositions of C2m(CF3)2ra derivatives are given as 2m/2n.
The product obtained at 420 °C was subjected to a two-step HPLC separation in hexane (Buckyprep column, 10 mm i.d. x 250 mm, flow rate 4.6 mL min"1, monitored at 290 nm). The fraction collected at 13.5 min in the second HPLC step contained mainly a C90(CF3)18 species. Slow concentration of the solution afforded small orange crystals of C90(CF3)18-1.5 Hexane, which have been investigated by singe crystal X-ray diffraction using synchrotron radiation.
The sublimation product from the synthesis at 560 °C was first separated by HPLC in toluene at the same chromatographic conditions; the fraction eluted between 6.3 and 6.6 min was further separated using a toluene/hexane (1/1) mixture as the eluent. The fraction eluted at 19.4 min contained C90(CF3)14 species (fig. 2). Recrystallization from o-dichlorobenzene (o-DCB) afforded small red crystals. The crystal structure of C90(CF3)14-2.5 (o-DCB) was determined by X-ray single crystal crystallography using synchrotron radiation.
Synchrotron X-ray data 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. Crys-tallographic data along with some details of data collection and structure refinements are
I ' I ■ I ■ I—.........—г—'—I—■—I—'-1—■—I—■—1 I-.-1---1-,-1-.-1--I-1-I-1-1-1-•-I-I-1
12 13 14 15 16 17 18 19 20 21 22 23 24 25 15 16 17 18 19 20 21 22 23 24 25
Retention time, milt Retention time, min
Fig. 2. Chromatographic isolation C90(CF3)18 (a) and C90(CF3)14 (b). The fractions collected are indicated by hatching (a) or arrow (b). Insets show mass spectra of the collected fractions. The compositions of C2m(CF3)2ra derivatives are given as 2m/2n.
Table 1. Crystallographic data and some details of data collection and refinement for C90(CF3)18-1.5Hexane and C90(CF3)14-2.5(o-C6H4Cl2)
Compound C90(CF3)18 1.5Hexane C90(CF3)i4-2.5(o-DCB)
Mr 2452.34 2427.41
Crystal system Monoclinic Triclinic
Space group C 2/c P1
a [A] 47.661(4) 14.206(1)
b [A] 22.404(2) 24.343(2)
c [A] 16.352(1) 25.359(2)
a [°] 90 93.312(3)
ß n 97.516(6) 99.786(8)
7 n 90 102.104(3)
V [A3] 17311(2) 8410.0(11)
Z 8 4
Dc [g cm"3] 1.882 1.907
Crystal size [mm] 0.02x0.02x0.01 0.05x0.02x0.01
Detector; A [A] MAR225 / 0.9050 MAR225 / 0.8856
Temperature [K] 100 100
0(max) [deg] 36.94 36.67
Refls collected / R(int) 121922 / 0.051 122394 / 0.191
Data / parameters 17704 / 1712 33354 / 3099
Ri [I ^2a(I)] / wü2 (all) 0.060 / 0.160 0.090 / 0.216
Ap (max / min) [e A"3] 1.107 / -1.265 0.391 / -0.426
presented in Table 1. The structures were solved with SHELXD and anisotropically refined with SHELXL. In the crystal structure of C90(CF3)18-1.5Hexane, seven CF3 groups and sol-vated hexane molecules are strongly disordered. In the crystal structure of C90(CF3)14-2.5(o-C6H4Cl2), there are two crystallographically independent C90(CF3)14 molecules. One CF3 group and three of six independent dichlorobenzene molecules are disordered. Crystallo-graphic data are deposited under CCDC-917604 and -917603, respectively.
3. Discussion
Mass spectrometric MALDI analyses of trifluoromethylation products demonstrate the presence of C90(CF3)2ra species with 2n ranging from 12 to 20, however, without information concerning the C90 cage connectivity and CF3 addition patterns. HPLC separation supported by subsequent MALDI MS analyses of separated fractions indicates the occurrence of several different C90(CF3)2ra isomers of the same composition whereas their assignment to specific C90 cages remained unknown. Growing crystals from separated fractions followed by X-ray crystallographic structure determination using synchrotron radiation was successful in only two cases of C90(CF3)14 and C90(CF3)18, which are the first examples of unambiguous structural characterization of CF3 derivatives of C90 fullerene (fig. 3).
Fig. 3. Projections of the C90(30)(CF3)18 (left) and C90(35)(CF3)14 (right) molecules.
An analysis of the carbon cage connectivities in two molecules reveals that they contain different C90 cages, Cs-C90 ( 35) and C1-C90(30), respectively, i.e., the C90 isomers, which have been already confirmed previously either in a pristine form (disordered C90 (30) cage in co-crystals with Nin(OEP) [18]) or as chlorinated derivatives (both 30 and 35 isomers [21]). The description of molecular structures, their addition patterns, and comparison with corresponding chloro derivatives are convenient to perform using Schlegel diagrams (figs. 4 and 5).
In the C 1-C90(30)(CF3)18 molecule, 18 CF3 groups are attached to a C1-C90 cage exclusively in positions of double hexagon junctions (DHJs), whereas positions of triple hexagon junctions (THJs) remain unoccupied (fig. 4). All cage pentagons contain one or two attached CF3 groups. The stabilization of the addition pattern is achieved due to the formation of five isolated or partially isolated C=C bonds (av. bond length 1.33 A) and one partially isolated benzenoid ring (av. C-C bond length 1.39 A).
The previous X-ray crystallographic characterization of C90(30) fullerene derivative has been performed on the crystal, which contained both C90(30)Cl22 and C90 ( 28)Cl24 molecules in the unit cell [20,21]. A comparison of the addition patterns of C90 ( 30)(CF3)18
Fig. 4. Schlegel diagrams of C90(30)(CF3)18 (left) and C90(30)Cl22 (right). Cage pentagons are highlighted with gray. Black triangles and circles denote attached CF3 groups and Cl atoms, respectively. The isolated or partially isolated C=C bonds and benzenoid rings are also indicated.
and C90 ( 30)Cl22 shows their rather close similarity because of 16 of 18 attachments positions in the former molecule are also occupied in the chlorinated molecule. This results in the same location of three C=C bonds and the benzenoid ring on the C90(30) carbon cage. The differences in the addition patterns concern the attachment of some Cl atoms in adjacent (ortho) positions, which are less favourable for bulkier CF3 groups. For the same reason, the maximum number of CF3 groups attached to fullerene cages is only 20 (structurally confirmed for C70, C84, C88, and C94) [23-26], whereas several fullerene chlorides with 32-34 Cl atoms are known [27,28].
The addition pattern of C90(35)(CF3)14 of both crystallographically independent (but chemically identical) molecules is characterized by CF3 attachment exclusively in positions of DHJs and the occupation of all twelve pentagons with CF3 groups, while two additional CF3 groups contribute to the formation of an isolated C=C bond (av. bond length 1.32 A in two independent molecules) on the fullerene Cs-C90(35) cage (fig. 5). Three partially isolated benzenoid rings are also present on the cage (av. C-C bond length 1.40 A).
Fig. 5. Schlegel diagrams of C90(35)(CF3)14 (left) and C90(35)Cl24 (right). Cage pentagons are highlighted with gray. Black triangles and circles denote attached CF3 groups and Cl atoms, respectively. The isolated or partially isolated C=C bonds and aromatic substructures are also indicated.
The first crystallographic confirmation of the cage connectivity of C90 ( 35) was reported for C90(35)Cl24 and C90 ( 35)Cl28, which possess rather similar chlorination patterns [20,21]. A comparison of the addition patterns in C1-C90 ( 35)(CF3)14 and Cs-C90 ( 35)Cl24 demonstrates their similarity due to 12 common attachment positions. However, due to a large number of attached Cl atoms, several additions in ortho positions are present in the
Cg0(35)Cl24 molecule. Furthermore, all unoccupied carbon atoms on the Cg0 cage are involved into isolated aromatic substructures or isolated C=C bonds. Usually, CF3 and chloro derivatives of fullerenes show rather different addition patterns as can be exemplified by the comparison in pairs of C70(CF3)16 [29] and C70Cl16 [30] or C76(CF3)18 [31] and C76Cl18 [5].
4. Conclusions
Trifluoromethylation of a higher fullerenes mixture followed by HPLC separation, crystallization, and X-ray crystallographic structure determination resulted in the first molecular structures of CF3 derivatives of Cg0 fullerene, Cg0(30)(CF3)18 and Cg0(35)(CF3)14. The comparison of the addition patterns with those of the chlorides of the corresponding Cg0 isomers revealed their close similarities, whereas some differences can be attributed to different sizes of CF3 group and Cl atom as well as to different numbers of attached groups/atoms. It should be noted that Cg0 isomers 30 and 35 belong to the energetically rather stable isomers according to theoretical calculations [16,32,33]. However, the cage connectivity of the most stable C2—Cg0 (45) still remains unconfirmed in any experimental report.
Acknowledgments
This work was supported in part by the Russian Foundation for Basic Research (grants 12-03-00324 and 13-03-91332).
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