Synthesis and structural characterization of highly chlorinated C₇₀, C₇₀Cl₂₈

Abstract

Chlorination of [70]fullerene with SbCl₅, VCl₄ or PCl₅ yielded C₇₀Cl₂₈ comprising three isomers, all containing four isolated benzenoid rings in the fullerene cage. This demonstrates, for the first time for C₇₀ derivatives, a stabilization effect due to planar aromaticity.

---

To date, halogenated derivatives of C₇₀ remain much less studied than those of C₆₀. Fluorination of C₇₀ yielded complex mixtures of highly fluorinated products; HPLC separation and subsequent characterization by means of ¹⁹F NMR, mass spectrometry and IR spectroscopy revealed the presence of several C₇₀F_x fluorides with x ranging from 34 to 44.¹ C₇₀ bromination in organic solvents was previously supposed to yield C₇₀Br₁₄;² however, a more recent study demonstrated that the only product of bromination of C₇₀ in o-C₆H₄Cl₂ or liquid bromine is C₇₀Br₁₀.³ The only known C₇₀ chloride, C₇₀Cl₁₀, was synthesized by the reaction between C₇₀ and ICl in benzene. ¹³C NMR data⁴ suggested that its molecular structure comprises a belt of chlorine atoms which splits the π-system into two isolated halves, and this arrangement was later confirmed in the structural study of C₇₀Ph₁₀ obtained via substitution of chlorine atoms with phenyl or methyl groups.⁵

Recently, we suggested a novel class of reagents for deep chlorination of fullerenes: higher chlorides of the variable valency elements.⁶ The most promising results were obtained using SbCl₅ and VCl₄, which allow the selective synthesis of T_h-C₆₀Cl₂₄ with high yield and isomeric purity, as proven on the basis of a comparison between the experimental and calculated IR spectra. Having been applied to the chlorination of C₇₀, these chlorinating agents were found to lead to the formation of a compound with the molecular formula C₇₀Cl₂₈, as preliminarily suggested on the basis of the elemental analysis data.⁶ In the work presented here we describe the preparation of C₇₀Cl₂₈ and its characterization by means of X-ray single crystal diffraction studies, IR spectroscopy and quantum-chemical calculations.

C₇₀Cl₂₈ samples with reproducible IR spectra (KBr pellet) and elemental composition as determined by chemical analysis (Cl : C = 27.8–28.1), were prepared in sealed glass ampoules by reacting C₇₀ with excess liquid VCl₄ (140–160 °C, 7 days), SbCl₅ (200 °C, 1 day) or PCl₅ (180–200 °C, 2 days). Alternatively, a similar result can be achieved by chlorination of C₇₀Br₁₀ with SbCl₅ at 120–140 °C for 7 days. Due to the high internal pressure when heated (ca. 6–10 atm), ampoules with an inner diameter of no more than 5–7 mm were used, each of which was placed into a metallic tube for safety reasons. Removal of the inorganic components of the reaction mixture was carried out by sublimation in vacuo (SbCl₅, SbCl₃, PCl₅, PCl₃) or by washing off with 15% HCl followed by drying the product in vacuo over P₂O₅. This procedure yielded a yellowish-brown polycrystalline powder of C₇₀Cl₂₈ with low solubility in organic solvents. The synthesized compound was found to be stable when exposed to air at room temperature. According to the thermal analysis data, its decomposition into C₇₀ and gaseous Cl₂ in an inert atmosphere occurs in the temperature range 320–380 °C. A typical IR spectrum of C₇₀Cl₂₈ is presented in Fig. 1. The most prominent absorption bands are observed in the region of deformational carbon cage vibrations and C–Cl stretching vibrations at 413, 431, 453, 470, 592, 724, 782, 814, 839, 847, 886, 915, 948, 1105, and 1161 cm⁻¹.

Fig. 1 Experimental (above) and calculated (below) IR spectra of C₇₀Cl₂₈.

Previously, it had been theoretically predicted that the attachment of 6–12 bulky X atoms to the C₇₀ cage should occur around the equator, leading to the predominant formation of 1,4-pairs.⁷ The known C₇₀X₁₀ structures,^3,4 which have nine 1,4-contacts and one 1,2-contact between the X groups in the equatorial X₁₀ belt, constitute a good example of such trends. However, our DFT calculations of the relative stability of various possible C₇₀Cl_x isomers (x ≥ 24) have demonstrated that the principles governing the addition patterns for highly chlorinated molecules are quite different.† It appeared that the main factor affecting stability was the number of isolated benzenoid cycles in the molecule. The stabilizing effects of formation of such aromatic fragments were found to surpass the negative steric effects of the 1,2-contacts of chlorine atoms except for the cases where some chlorine atoms have three such contacts. The maximal number of benzenoid rings of a closed shell derivative of C₇₀, provided all the addends have no more than two 1,2-contacts with the others, is four, and their location is unique. As a result, among the numerous isomers of C₇₀Cl₂₈ considered in the calculations, those containing four benzenoid rings were found to be 80–100 kJ mol⁻¹ more stable than the structures having only three rings, and around 200 kJ mol⁻¹ more stable than the isomers with a minimal number of 1,2-contacts and no aromatic fragments. Here we see an obvious analogy with the addition patterns observed in some highly fluorinated fullerene molecules such as C₆₀F₃₆ (T isomer)⁸ and C₇₄F₃₈,⁹ though our calculations predict the benzenoid cycles in C₇₀Cl₂₈ to be less planar, i.e. less aromatic.

An X-ray single crystal diffraction study was carried out for the C₇₀Cl₂₈·1.76Br₂ crystals grown from liquid bromine.‡ The experimentally determined structure of the C₇₀Cl₂₈ molecule has crystallographically imposed C₂ symmetry which most likely originates from the superposition of two orientations of one isomer having C₁ symmetry (Fig. 2a) and two isomers possessing C₂ symmetry. (Fig. 2b and 2c). All three isomers contain four benzenoid rings with their addition patterns exhibiting minor differences, which relate to positions labeled as “1”, “2”, and “3”. The fractional experimental occupancies of these positions, namely 0.50 for 1, 0.17 for 2, and 0.33 for 3, can be accounted for by the statistically disordered distribution of isomers a (0.33), a′ (0.33), b (0.17), and c (0.17) in the crystal structure. The experimentally determined molecular structure with the location of the Cl atoms corresponding to isomer b is presented in Fig. 3. DFT calculations demonstrate that isomer a is approximately 9 kJ mol⁻¹ more stable than isomer c and about 40 kJ mol⁻¹ more stable than isomer b; at the same time, among more than 30 isomers of C₇₀Cl₂₈ containing four benzenoid rings, many possess comparable stability, lying in the range between isomers a and b. This implies that the observed isomers are partially kinetic products, therefore, formation of other isomers is, in principle, possible.

Fig. 2 Schlegel diagrams of experimentally observed isomers of C₇₀Cl₂₈ (a–c) and the possible structurally related isomer of C₇₀Cl₃₀ (d). The numbers 1–3 denote the positions differently occupied in the C₇₀Cl₂₈ isomers.

Fig. 3 Top (left) and side (right) views of the b isomer of C₇₀Cl₂₈. The numbers 1–3 denote the same positions as in Fig. 2.

The aromatic rings (marked with circles in Fig. 2 and with dashes in Fig. 3) reveal observable deviations from planarity in contrast to the analogous cycles in C₆₀F₃₆ or C₆₀F₁₈.^8,10 This is most probably due to a lower number of addends and, consequently, sp³ carbons in the adjacent cycles, because the degree of planarity of a conjugated fragment is directly connected with buckling of its surroundings, which originates from the elongation of C–C bonds upon changing hybridization from sp² to sp³. Generally, C–C bonds can be separated into four major groups according to their nature. These are isolated double or conjugated bonds connecting non-aromatic sp² carbons (average length 1.38 Å observed/1.38 Å calculated for the isomer b), aromatic bonds (1.39 Å/1.40 Å), sp²–sp³ bonds (1.50 Å/1.51 Å) and finally, considerably elongated sp³–sp³ bonds (1.61 Å/1.62 Å). For comparison, the average length of sp³–sp³ bonds in C₆₀F₁₈ is 1.60 Å.¹⁰ The C–Cl bonds (1.82 Å/1.83 Å ) are also elongated compared to the typical value for conventional chlorocarbons (1.77 Å), which suggests that they represent relatively weak bonds. DFT-simulated IR spectra of the isomers a–c were found to be rather similar, especially in the most prominent region near 800 cm⁻¹, and revealed good qualitative agreement with the experimental data (Fig. 1).

The highest degree of chlorination satisfying the “four rings and no triple Cl–Cl contacts” condition, is represented by the single isomer of C₇₀Cl₃₀ (see Fig. 2d). According to our DFT results, the enthalpy of consecutive chlorination with Cl₂ remains negative only up to C₇₀Cl₂₈. However, C₇₀Cl₃₀ is still rather stable and use of strong chlorinating agents can make its formation thermodynamically favorable. Further chlorination of C₇₀Cl₃₀, which would result in the destruction of the benzenoid rings and an increase in the Cl–Cl adjacency, is unlikely due to high endothermicity.

INI is thankful to Russian Presidential grant MK-2734.2004.3.

Notes and references

1. R. Taylor, A. K. Abdul-Sada, O. V. Boltalina and J. K. Street, J. Chem. Soc., Perkin Trans. 2, 2000, 1013.
2. G. Waidmann and M. Jansen, Z. Anorg. Allg. Chem., 1997, 623, 623.
3. S. I. Troyanov, A. A. Popov, N. I. Denisenko, O. V. Boltalina and E. Kemnitz, Angew. Chem., Int. Ed., 2003, 42, 2395.
4. P. R. Birkett, A. G. Avent, A. D. Darwish, H. W. Kroto, R. Taylor and D. R. M. Walton, J. Chem. Soc., Chem. Commun., 1995, 683.
5. A. G. Avent, P. R. Birkett, A. D. Darwish, H. W. Kroto, R. Taylor and D. R. M. Walton, Tetrahedron, 1996, 52, 5235; H. Al-Matar, A. K. Abdul-Sada, A. G. Avent, R. Taylor and X.-W. Wei, J. Chem. Soc., Perkin Trans. 2, 2002, 1251.
6. S. I. Troyanov, N. B. Shustova, A. A. Popov, M. Feist and E. Kemnitz, Zh. Neorg. Khim., 2004, 49, 1413.
7. S. J. Austin, P. W. Fowler, J. P. B. Sandall, P. R. Birkett, A. G. Avent, A. D. Darwish, H. W. Kroto, R. Taylor and D. R. M. Walton, J. Chem. Soc., Perkin Trans. 2, 1995, 1027; B. W. Clare and D. L. Kepert, J. Mol. Struct.: THEOCHEM, 1999, 491, 249.
8. P. B. Hitchcock and R. Taylor, Chem. Commun., 2002, 2078.
9. A. A. Goryunkov, V. Yu. Markov, I. N. Ioffe, R. D. Bolskar, M. D. Diener, I. V. Kuvychko, S. H. Strauss and O. V. Boltalina, Angew. Chem., Int. Ed., 2004, 43, 997.
10. I. S. Neretin, K. A. Lyssenko, M. Yu. Antipin, Yu. L. Slovokhotov, O. V. Boltalina, P. A. Troshin, A. Yu. Lukonin, L. N. Sidorov and R. Taylor, Angew. Chem., Int. Ed., 2000, 39, 3273.

---

Footnotes

† The program PRIRODA with the implemented original basis set of TZ2P quality (D. N. Laikov, Chem. Phys. Lett., 1997, 281, 151) and the PBE exchange–correlation functional (J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865) were used.

‡ Crystal data: C₇₀Cl₂₈·1.76Br₂, monoclinic, P2/n, a = 13.003(1) Å, b = 17.362(1) Å, c = 15.020(1) Å, β = 102.458(3)°, V = 3311.0(4) ų, D_c = 2.12 g cm⁻³, Z = 2, T = 100 K. Data collection on a MAR345 image plate using synchrotron radiation at the BESSY storage ring (λ = 0.9184 Å), PSF BL 14.2 of the Free University of Berlin, Germany. One Cl atom was found to be disordered over three positions with site occupancies of 0.50, 0.33, and 0.17. Some small peaks assigned as additional Cl atoms with partial (0.12) occupancies originated from a complex overlap of three molecules with different shapes. The positions of two solvate Br₂ molecules are also partially occupied. Anisotropic refinement with 5425 reflections and 527 parameters yielded a conventional R₁ (F) = 0.086 for 3960 reflections with I > 2σ(I) and wR2 (F²) = 0.259 for all reflections. CCDC 247934. See http://www.rsc.org/suppdata/cc/b4/b412448k/ for crystallographic data in .cif or other electronic format.

---