New Isolated-Pentagon-Rule Isomer of C92 Isolated as

Nov 4, 2015 - Chemistry Department, Moscow State University, Leninskie Gory, 119991 Moscow, Russia. Inorg. Chem. , 2015, 54 (22), pp 10527–10529.Mis...
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New Isolated-Pentagon-Rule Isomer of C92 Isolated as Trifluoromethyl and Chlorido Derivatives: C92(38)(CF3)14/16 and C92(38)Cl20/22 Nadezhda B. Tamm and Sergey I. Troyanov* Chemistry Department, Moscow State University, Leninskie Gory, 119991 Moscow, Russia S Supporting Information *

section with CF3I should be held at room temperature, thus providing a pressure of ca. 6 bar.7) The trifluoromethylation product containing C92(CF3)10−18 was dissolved in n-hexane and subjected to HPLC separation using a Buckyprep column (4.6 × 250 mm, Nacalai Tesque Inc.) and n-hexane as the eluent at 1.0 mL min−1 flow rate [see the Supporting Information (SI) for details]. A total of 6 from 29 fractions, with retention times of 4.1, 39.7, 46.8, 50.6, 73.0, and 96.7 min, gave small crystals upon the slow removal of hexane or recrystallization from p-xylene or odichlorobenzene. An X-ray diffraction study with the use of synchrotron radiation revealed the molecular structures of C92(38)(CF3)14 (as a solvent-free compound and a solvate with p-xylene) and five isomers of C92(38)(CF3)16-I−V, which are numbered consecutively according to retention times of the corresponding HPLC fractions (see the selected crystallographic data in Table S1 in the SI). In another series of studies, the same C92 HPLC fraction was chlorinated with excess VCl4 (or VCl4 containing a drop of SbCl5) in thick-walled glass ampules at 350 °C for 2−3 weeks. (Caution! At 350 °C, the saturation pressure of VCl4 is 20−25 bar.) All of these experiments gave small crystals suitable for X-ray diffraction study with the use of synchrotron radiation that revealed the molecular structure of C92(38)Cl20/22 (see Table S1 in the SI). The C92(38)(CF3)14/16 molecules are shown in Figure 1 as projections in the same direction relative to the carbon cage. Because of the asymmetric structure of isomer C1-C92(38), all of its trifluoromethyl derivatives also possess only trivial C1 symmetry. An addition pattern of 14 CF3 groups in the C92(38)(CF3)14 molecule (see the Schlegel diagram in Figure 2) is characterized by the attachments to all 12 pentagons plus two CF3 groups, which isolate a double CC bond on the carbon cage [the bond length is 1.310(5) Å]. Most elongated on the cage are sp2−sp3 C−C bonds at the attachment position of the CF3 groups with an average bond length of 1.534 Å. C−CF3 bonds are, on average, 1.547 Å in length. Five isomers of C92(38)(CF3)16 possess structures with similar addition patterns all containing two isolated double CC bonds on the carbon cage (Figure 2). The differences in the addition patterns of C92(38)(CF3)16 are mostly due to the positions of the CC bonds. Isomers C92(38)(CF3)16-I, -II, and -IV also contain isolated or nearly isolated benzenoid rings [with six and five adjacent C(sp3) atoms, respectively] on the carbon cages, which can be considered as additional stabilizing factors. Remarkably,

ABSTRACT: High-temperature trifluoromethylation and chlorination of the C92 fraction followed by single-crystal X-ray diffraction with the use of synchrotron radiation resulted in the structure determination of C92(38)(CF3)14, five isomers of C92(38)(CF3)16, and C92(38)Cl20/22. Their addition patterns are stabilized by the formation of isolated CC bonds and aromatic substructures. According to quantum-chemical calculations, the newly detected C1− C92(38) belongs to the most stable isomers of C92.

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ow abundance and the existence of many isomers hamper the investigation and even identification of higher fullerenes. Structural studies of some empty higher fullerenes such as C86, C90, and C96 were possible through their cocrystallization with metal porphyrins,1 which restricts the rotational/librational mobility of fullerenes in crystals. Derivatization (mostly trifluoromethylation and chlorination) of a fullerene or fullerene mixtures followed by separation and structural characterization of individual derivatives was proven to be an effective tool for the identification of many pristine fullerene isomers from C84 up to C104.2 Fullerene C92 has 86 topologically possible isolated-pentagonrule (IPR) isomers.3 Theoretical studies of the formation energies pointed out several C92 isomers, D2-C92(82), D2C92(81), D2-C92(84), C1-C92(38), and D3-C92(28), as the most stable ones (the numerals in parentheses indicate the number of isomers according to the spiral algorithm3), however, in different orders of stability depending on the calculation method.4 Earlier, the 13C NMR spectroscopic data on a C92 isomeric mixture was interpreted as the coexistence of four isomers with D2, D2, C2, and C2 symmetry.5a A multistep high-performance liquid chromatography (HPLC) separation of a C92 fraction resulted in the detection of a C2-C92 isomer by 13C NMR spectroscopy, however, without its further identification.5b The only crystallographic study has been performed for a trifluoromethyl derivative, C92(CF3)16, and revealed the presence of either or both C92(82) and C92(81) cages.6 Herein we report the isolation and structural characterization of CF3 and chlorido derivatives of a new C92 isomer, C92(38). The addition patterns of CF3 groups and Cl atoms on the C92(38) carbon cage are discussed in terms of the formation of isolated double CC bonds and aromatic substructures. Trifluoromethylation of a C92 HPLC fraction (ca. 4 mg) with gaseous CF3I was carried out in a quartz ampule at 450 °C for 2.5 h following a procedure described previously. (Caution! The © 2015 American Chemical Society

Received: July 31, 2015 Published: November 4, 2015 10527

DOI: 10.1021/acs.inorgchem.5b01740 Inorg. Chem. 2015, 54, 10527−10529

Communication

Inorganic Chemistry

formation energies. It can be suggested that the lowest relative energy of isomer I, i.e., its higher energetic stability, is due to the presence of two stabilizing nearly isolated benzenoid rings on the carbon cage (the outer hexagon on the Schlegel diagram is the second one). Isomers II and IV with slightly higher formation energies have only one benzenoid ring on the cage, whereas isomers III and V have none. At the same time, the differences in the trifluoromethylation energies are not large, so that all of these isomers possess close stability and, therefore, they all are present in the trifluoromethylation products. The addition patterns of the known C2-C92(82/81)(CF3)166 also contain two isolated double CC bonds. A more detailed comparison with C1-C92(38)(CF3)16 isomers is hardly possible because of the higher symmetry of the pristine D2-C92(82/81), resulting in a more uniform distribution of triple-hexagonjunction (THJ) positions, which substantially determines the trifluoromethylation pattern. Several experiments on chlorination of the C92 fraction afforded crystals of the average composition C92(38)Cl20.84. In the crystal structure, very similar C92(38)Cl20 and C92(38)Cl22 molecules occupy the same crystallographic position. The other cases of overlaps of two (or more) molecules with similar composition and shapes have also been found for fullerene chlorides such as C76(1)Cl24/28, C76(1)Cl32/34,9a C86(17)Cl18/20/22,9b and C90(34,46)Cl32.9c Two projections of the C92(38)Cl20 molecule and its Schlegel diagram are shown in Figure 3. The chlorination pattern is characterized by the

Figure 1. Projections of C92(38)(CF3)14 (a) and five isomers of C92(38)(CF3)16-I−V (b−f, respectively). The orientations of the carbon cages are the same in all projections.

Figure 2. Schlegel diagrams of C92(38)(CF3)14 and five isomers of C92(38)(CF3)16 abbreviated as 92(38)/14 and 92(38)/16-I−V, respectively. The relative formation energies (kJ mol−1) of C92(38)(CF3)16 isomers are given in parentheses. Cage pentagons are highlighted with gray. Black triangles denote the positions of attached CF3 groups. Isolated CC bonds and benzenoid rings are also indicated.

Figure 3. Two projections of the C92(38)Cl20 molecule and its Schlegel diagram. Cage pentagons are highlighted with gray. Black circles denote the positions of the attached Cl atoms, whereas two gray circles show the location of two additional Cl atoms in C92(38)Cl22. Isolated CC bonds and benzenoid rings are also indicated.

presence of three isolated CC bonds and three isolated or nearly isolated benzenoid rings on the carbon cage. In contrast to the trifluoromethylation patterns of C92(38)(CF3)14/16, the chlorination pattern of C92(38)Cl20 contains a pair of attachments in the adjacent positions and one Cl attachment in the THJ position. Probably, the generally unfavorable attachment in the THJ is compensated for by the formation of a stabilizing, fully isolated benzenoid ring (the outer hexagon of the Schlegel diagram). In the carbon cage of the C92(38)Cl20 molecule, the isolated CC bonds are 1.31−1.33 Å long, the C−C bonds in the benzenoid rings are 1.40 Å, on average, whereas the sp2−sp3 C− C bonds at the CF3 attachments are, on average, 1.514 Å in length. The longest C−C bond is of the sp3-sp3 type on the place of the 1,2-addition, 1.582(4) Å. Most C−Cl bonds are in the typical range of 1.801−1.829 Å (average 1.816 Å),2b,d,10 whereas C−Cl at the addition to THJ is slightly longer, 1.840(3) Å. The chlorination pattern of the C92(38)Cl22 molecule contains two additional Cl atoms, which isolate an additional double CC bond, and two more attachments in the ortho positions on the cage, as shown on the Schlegel diagram in Figure 3. A comparison

the addition pattern of C92(38)(CF3)14 is present in isomers C92(38)(CF3)16-I, -III, and -V as a substructure. Therefore, C92(38)(CF3)14 can be regarded as a possible precursor of these three isomers with 16 CF 3 groups in the course of trifluoromethylation. There are several types of C−C bonds in the C92(38)(CF3)16 molecules. In the most accurately determined structure of C92(38)(CF3)16-IV with an estimated standard deviation of C−C bonds of 0.004 Å, isolated CC bonds are the shortest, with typical lengths of 1.316(4) and 1.331(4) Å. The average C−C bond length of the benzenoid ring is 1.394 Å, whereas the longest C−C bonds are of the sp2−sp3 type (radiating from the attachment positions), with an average length of 1.529 Å. C−CF3 bonds are, on average, 1.553 Å in length. Five C92(38)(CF3)16 isomers can be compared with each other by their relative formation energy. Density functional theory (DFT) calculation with PRIRODA software and the PBE exchange-correlation functional8 revealed that isomer C92(38)(CF3)16-I possesses the lowest energy, whereas isomers II−V have respectively 15.1, 19.5, 8.9, and 21.0 kJ mol−1 higher 10528

DOI: 10.1021/acs.inorgchem.5b01740 Inorg. Chem. 2015, 54, 10527−10529

Inorganic Chemistry

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of the addition patterns of C92(38)Cl20 and the isomers of C92(38)(CF3)16 revealed the closest similarity with isomer C92(38)Cl22-I, which possesses 14 common addition positions with the chloride. DFT calculation of an average chlorination enthalpy per one Cl atom in C92(38)Cl20 and C92(38)Cl22 gave respectively values of 4.9 and 3.1 kJ mol−1 higher than the chlorination enthalpy of D3d-C60Cl30 taken as a standard.11 These relations can be considered as expected because, as a rule, the lower chlorination degrees correspond to the higher chlorination enthalpies, as was found earlier on the examples of many chlorides of IPR fullerenes regardless of the cage size.2d,9a,12 Theoretical calculations by different methods predict a rather high stability of C92(38) among 86 possible IPR isomers of C92 fullerene. While the earlier report on the calculations by HF/431G indicated the highest stability of C92(38),4a the subsequent calculations by SAM1 revealed the highest stability of C92(82) and C92(81), followed by C92(38) and C92(84). Further calculations at the B2LYP/6-31G* level changed the order of the stability to C92(84) > C92(38) ≈ C92(28) > C92(82).4b,c In our structural studies, both D2-C92(82) [and, possibly, D2C92(81)] and C1-C92(38) have been unambiguously confirmed as CF3 derivatives, with the latter also being isolated as a chloride. It is worth noting that the earlier experimental data of 13C NMR spectroscopy for the C92 mixtures were interpreted as the presence of two D2 and two C2 isomers5a or a C2 isomer plus an inseparable mixture of C92 isomers,5b but no C1 or D3 isomers were detected. In summary, the presence of a new isomer of C92, C92(38), in the fullerene soot, was confirmed for the first time by the isolation of several CF3 derivatives and one chloride and their subsequent investigation by X-ray crystallography. The addition patterns of C92(38)(CF3)14/16 and C92(38)Cl20/22 are characterized by the formation of isolated double CC bonds and isolated or partially isolated benzenoid rings on the fullerene cage. The addition patterns of C92(38)(CF3)16-I and C92(38)Cl20 demonstrate very similar features. Theoretical calculations suggest that, besides so far known D2-C92(82), D2-C92(81), and C1-C92(38), other isomers such as D2-C92(84) and D3C92(28) can be present in the fullerene soot. The most reliable method of detection of new fullerene isomers seems to be their derivatization, followed by the separation and structure determination of the derivatives, as has been shown by numerous previous cases of the investigation of higher fullerenes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +007 495 9395396. Fax: +007 495 9391240. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research (Grant 15-03-04464). REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01740. Data on the isolation of C92(CF3) n and selected crystallographic data (PDF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) Extended crystallographic data in CIF format (CIF) 10529

DOI: 10.1021/acs.inorgchem.5b01740 Inorg. Chem. 2015, 54, 10527−10529