Regioselective near-equatorial chlorination of C_s-C₇₀(CF₃)₈

Abstract

The chlorination of C_s-C₇₀(CF₃)₈ by ICl occurs at near-equatorial [5,6] bond in agreement with quantum chemical predictions; structure of the synthesized C_s-C₇₀(CF₃)₈Cl₂ was proven by ¹⁹F NMR spectroscopy and single crystal X-ray crystallography.

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Trifluoromethylated derivatives of [60] and [70]fullerenes (TFMF) have attracted much attention as prospective materials for construction of photoelectron devices due to their high electron affinity, chemical stability and ability to undergo multiple reversible electrochemical reductions.¹ More than fifty fullerene derivatives of C_60/70(CF₃)_n, n = 2–18 have already been characterized.² Thus, the study of the chemical reactivity of the trifluoromethylated fullerenes as well as ways of their regioselective functionalization have become very important. There are rare examples of functionalization of these compounds by chlorination of S₆-C₆₀(CF₃)₁₂³ and regioselective [2+1]- or [4+2]-cycloadditions to C₁-C₇₀(CF₃)₁₀.⁴ One of the most promising TFMF's which can be obtained in preparative amounts is C_s-C₇₀(CF₃)₈.⁵ Here, we report the first example of regioselective chlorination of C_s-C₇₀(CF₃)₈ resulting in a mixed chloro(trifluoromethyl)fullerene, C_s-C₇₀(CF₃)₈Cl₂. Its structure was predicted by quantum chemical calculations and has been proven by ¹⁹F NMR data and X-ray structure analysis.

A common feature of [70]fullerene derivatives bearing eight or ten addends is their addition pattern in a form of the so-called “equatorial belt”. There are many examples of such compounds, e.g. C₇₀H₈,⁶C₇₀H₁₀,⁶C₇₀Cl₁₀,⁷C₇₀Br₁₀,⁷C₇₀Me₈,⁸C₇₀Me₁₀,⁸C₇₀Ph₈,⁹C₇₀Ph₁₀,⁹C₇₀(CF₃)₈,⁵C₇₀(OO^tBu)₈,¹⁰ possessing such addition patterns with eight or ten addends attached to carbons atoms of edge-sharing equatorial belt of hexagons (indicated by shadowed region on Schlegel diagram in Fig. 1). In all these molecules, the addition of eight groups, regardless of their size, takes place at para-positions of equatorial hexagons giving para⁷ motif, p⁷-C_s-C₇₀X₈ (selected by dotted line on Schlegel diagram in Fig. 1). On the contrary, the attachment positions of an additional two groups to p⁷-C_s-C₇₀X₈ is substantially predetermined by the atom/group size. For example, attachment of two additional small-sized atoms like H, Cl, or Br to the corresponding p⁷-C_s-C₇₀X₈ compound results in C_s-symmetrical C₇₀X₁₀ derivatives where two groups are attached to near-equatorial [5,6] bond (so called para⁹-ortho loop motif).^6,7 Addition of bulkier groups, like CF₃, OO^tBu, occurs in a different way. While C_s-symmetrical C₇₀(CF₃)₈⁵ and C₇₀(OO^tBu)₈¹⁰ demonstrate common para⁷-addition pattern, additional pairs of CF₃ and OO^tBu groups attach either to the polar region or to para-positions of unoccupied equatorial hexagon rather than to near-equatorial [5,6] bond, giving respectively p⁷mp-C₁-C₇₀(CF₃)₁₀¹¹ and p⁹-C₂-C₇₀(OO^tBu)₁₀.¹⁰ Formation of patterns other than para⁹-ortho loop addition pattern in these cases is explained by significant steric repulsion of ortho-positioned bulky groups.

Fig. 1 Schlegel diagram of C₇₀(CF₃)₈Cl₂; sites of CF₃ and Cl addition are denoted by black triangles and white circles, respectively; para⁷ addition pattern is selected by dotted line, para⁹-ortho loop addition pattern is indicated by shadowed region.

Relative DFT energies of C₇₀X₁₀ isomers (X = H, F, Cl, Br, OO^tBu, and CF₃) with p⁹o-loop, p⁹, and p⁷mp-addition patterns are given in Table 1 (values given after slash; see ESI† for calculation details). One can see that the relative stability of the p⁹o-loop pattern decreases along with an increase of van der Waals (vdW) radii of the groups. It should be noted that although the p⁹o-loop-addition pattern in C₇₀(OO^tBu)₁₀ was predicted to be energetically more favorable, only C₂-p⁹-pattern has been found experimentally. Thus, the reaction product is under the control of kinetic factors. Obviously, bulky ^tBu groups surrounding a fullerene cage are the reason for shielding of sp²carbon atom of [5,6] near-equatorial bond and assisting to C₂-p⁹-C₇₀(OO^tBu)₁₀ formation.

Table 1 DFT relative energies (kJ mol⁻¹) of some isomers of C₇₀(CF₃)₈X₂ and C₇₀X₁₀

Addition pattern	ΔE of C70(CF3)8X2/C70X10, X=
	H	F	OO^tBu	Cl	Br	CF3
	1.20^a	1.47^a	1.52^b	1.75^a	1.85^a	2.8^c
a Atomic vdW radius, Å. b Oxygen vdW radius. c Sum of fluorine vdW radii and typical C–F bond length (1.33 Å).
C s -p^9o(loop)	0/0	0/0	0/0	0/0	0/0	12.0
C 2-p^9	55.0/55.1	49.3/43.7	27.6/25.4	24.8/25.6	9.4/15.4	13.0
C 1 -p^7mp	47.2/47.2	51.5/52.1	33.8/33.1	26.0/26.9	9.9/16.0	0.0

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A similar situation was established for C₇₀(CF₃)₈X₂ isomers, when two additional Xgroups are attached to C_s-C₇₀(CF₃)₈. Whereas the addition of bulky CF₃ groups leads to energetically preferable p⁷mp-addition pattern which is experimentally observed for sized C₇₀(CF₃)₁₀, outstanding energetic stability of p⁹o-loop-C₇₀(CF₃)₈X₂ isomers was predicted by quantum chemical calculations for small groupsX like H, F, and Cl. Thus, C_s-p⁹o-loop-C₇₀(CF₃)₈X₂ isomers (X = H, F, and Cl) are at least 25–47 kJ mol⁻¹ more energetically preferable than any other possible isomer.

Site reactivity of the C_s-C₇₀(CF)₈ molecule was evaluated by analysis of frontier molecular orbitals. The highest occupied and lowest unoccupied MOs of the molecule are mostly distributed on near-equatorial [5,6]-bond and contain some percentage of p-orbital component (see ESI†, Fig. S1). Thus, one can expect increased reactivity of this bond toward addition of both small electron donating and electrophilic species. The HOMO−1, which is lower in energy than the HOMO (by ca. 0.36 eV), and the LUMO+1, which is higher in energy than the LUMO (by ca. 0.37 eV), are expected to be less reactive in both types of reactions mentioned above.

These findings prompt us to study the reactivity of near-equatorial [5,6]-bond of C_s-C₇₀(CF₃)₈ towards addends of small size. Radical chlorination by iodine monochloride was chosen as a test reaction. Irrespective of reactive species (atomic chlorine, weak Lewis acid ICl or their complexes), the reactivity of the C_s-C₇₀(CF₃)₈, the predominant choice of a reacting bond will be defined by both the HOMO distribution and relative energy of the radical intermediate. It was found that the lowest energy radical intermediate (among all possible radical intermediates of C₇₀(CF₃)₈Cl˙) is an adduct of chlorine atom at the carbon atoms of near-equatorial [5,6]-bond (see ESI†, Table S4). Both Mulliken and Hirshfeld population analyses of the most energetic preferable radical C₇₀(CF₃)₈Cl˙ intermediate shows maximal spin density at the [5,6]-neighbour carbon atom (see ESI†, Fig. S2). Thus, following radical addition will occur more readily at the second carbon atom of near-equatorial [5,6]-bond under formation of p⁹o-C₇₀(CF₃)₈Cl₂.

The isomer C_s-C₇₀(CF₃)₈ was prepared according to the known two stage method¹² and purified by HPLC. Chlorination of C_s-C₇₀(CF₃)₈ (5 mg, 0.003 mmol) with excess ICl (0.1–0.2 mL, 0.7–1.5 mmol) was carried out at room temperature in o-dichlorobenzene solution for 24 h. After removal of volatile components from the reaction mixture, a yellow colored solid product was obtained.

HPLC analysis (Cosmosil Buckyprep, 4.6 mm i.d. × 25 cm, Nakalai Tesque Inc., toluene as an eluent, 1 ml min⁻¹ flow rate, 290 nm) of the product showed the presence of a major component (ca. 90% of integral HPLC trace area) eluted at 4.72 min, a small amount (ca. 2%) of unreacted C_s-C₇₀(CF₃)₈ eluted at 4.35 min as well as other minor uncharacterized components at 3.27, 3.50, 3.75, and 5.18 min (see Fig. 2a). UV/Vis spectrum of the main chlorination product (toluene : hexane = 9 : 1; λ_max at 310, 397, 421, and 462 nm) differs from that of the starting compound (see inset in Fig. 2). Thus, chlorination resulted in almost quantitative conversion of C_s-C₇₀(CF₃)₈ and the formation of a product with reaction yield ca. 90%. The reaction product is thermally unstable: it substantially degrades under gentle heating at 70 °C under formation of initial C_s-C₇₀(CF₃)₈ and other uncharacterized by-products, according to MALDI mass spectrometry and HPLC data (not shown).

Fig. 2 (a) HPLC traces of the reaction product and starting C_s-C₇₀(CF₃)₈ as well as UV/Vis spectra of C₇₀(CF₃)₈Cl₂, C_s-C₇₀(CF₃)₈, C₇₀Cl₁₀ (shown on insets); (b) The positive ion MALDI mass spectrum of C_s-C₇₀(CF₃)₈Cl₂, experimental and theoretical isotopic abundance of C₇₀(CF₃)₈Cl⁺ are shown on inset.

The negative ion MALDI mass spectrum of the reaction product contains the major peak of C₇₀(CF₃)₈⁻ along with its oxides and hydroxides. The positive ions MALDI mass spectrum also mainly consists of C₇₀(CF₃)₈⁺. However, in the latter, C₇₀(CF₃)₈Cl⁺ was observed as a minor peak along with C₇₀(CF₃)₈(OH)⁺ (Fig. 2b). The chemical detachment of chlorine atoms under the MALDI conditions is quite usual for chlorofullerenes.¹³ For negative ions this detachment may occur in a greater extent than for positive ions. Therefore, the low-chlorinated C₇₀ gives chlorine-containing ions only in positive ion mode of MALDI mass spectra, while the peaks corresponding to parent fullerene are solely present in the negative ion mode.¹⁴ Thus, the presence of chlorinated C₇₀(CF₃)Cl⁺ species in the MALDI mass spectra indicated the formation of low chlorinated products.

Important information about symmetry and structure of the synthesized compound was obtained from comparison of ¹⁹F NMR spectral data of C_s-C₇₀(CF₃)₈ and its chlorinated product (Fig. 3). Both ¹⁹F NMR spectra contain four signals which proves that the molecular symmetry C_s is inherited by chlorinated C_s-C₇₀(CF₃)₈. Chemical shifts of non-terminal trifluoromethyl groups in the ribbon of C₆(CF₃)₂ edge-sharing hexagons are very close to those of C_s-C₇₀(CF₃)₈ while the signal of terminal CF₃ group of chlorinated compound is shifted ca. 3 ppm downfield compared to C_s-C₇₀(CF₃)₈. This is apparently due to proximity of chlorine atoms to the terminal CF₃ groups. This supposition is confirmed by quantum chemical calculations of ¹⁹F NMR chemical shifts in C_s-C₇₀(CF₃)₈Cl₂ and C_s-C₇₀(CF₃)₈ (see ESI†, Table S4).

Fig. 3 ¹⁹F NMR spectra of (a) C_s-C₇₀(CF₃)₈ and (b) C_s-C₇₀(CF₃)₈Cl₂. Zoomed quartet and multiplet regions are shown in insets.

Unambiguous evidence of the structure of chlorinated derivatives of C_s-C₇₀(CF₃)₈ was obtained by means of X-ray single crystal analysis. X-ray single crystal diffraction study of yellow crystals revealed the structure of C_s-C₇₀(CF₃)₈Cl₂ with a para⁹-ortho loop addition pattern (Fig. 1 and 4); IUPAC lowest-locant abbreviation is: 11,29-dichloro-1,4,19,41,49,60,66,69-octa(trifluoromethyl)-[70]fullerene. The C₇₀(CF₃)₈Cl₂ molecule has crystallographically imposed mirror symmetry. The attachment of eight CF₃ groups and two Cl atoms closely resembles the arrangement of ten addends in C₇₀Cl₁₀.^7a (Cl)C–C(Cl) distance is 1.626(10) Å (calculated 1.653 Å), i.e. strongly elongated compared to the same C–C bond in C_s-C₇₀(CF₃)₈ (1.418(6) Å;⁵ DFT calculated 1.414 Å).

Fig. 4 Top and side views of C_s-C₇₀(CF₃)₈Cl₂.

In conclusion, reactivity of near-equatorial [5,6]-bond of trifluoromethylated fullereneC_s-C₇₀(CF₃)₈ to radical addition of a small group was demonstrated by the example of chlorination. It resulted in the high-yield formation of a new mixed compound, C_s-C₇₀(CF₃)₈Cl₂. Its structure was proven by means of ¹⁹F NMR spectroscopy and single crystal X-ray crystallography. Such high regioselectivity is a result of both steric and electronic factors, favoring near-equatorial functionalization of C_s-C₇₀(CF₃)₈ by sterically undemanding groups. It opens a way to near-equatorial conjugation of the molecule with linear electron conducting organic fragments like alkynes and preparation of organic semiconductive materials and molecular switches.

This work was supported in part by the Russian Foundation for Basic Research (grants 09-03-91337 and 09-07-12068).

Experimental

The complete sets of isomers of C₇₀(CF₃)₈X₂ (943 isomers for each X = H, F, Cl, Br, OO^tBu, and CF₃) and radical intermediates C₇₀(CF₃)₈Cl˙ (33 isomers) were constructed by attachment of pair of addends X to unsaturated carbon atoms of C_s-C₇₀(CF₃)₈. Molecular models were initially optimized by molecular mechanics package TINKER 4.2 with MM2 force field set.¹⁵ Preliminary geometry optimization was carried out at the AM1 level of theory using Firefly QC package,¹⁶ which is partially based on the GAMESS (US) source code.¹⁷ Final geometry optimization and NMR chemical shifts calculation were performed at the DFT level of theory using PRIRODA software¹⁸ with the implemented original basis set of triple zeta quality with (11s6p2d)/[6s3p2d] contraction scheme for second row elements and PBE exchange-correlation functional.¹⁹

Mixture of C₇₀(CF₃)_m, m = 12–18 was synthesized by reaction of C₇₀ with CF₃I as described elsewhere.² This product was mixed with C₇₀, evacuated and sealed in a glass ampoule. The ampoule was heated at 440 °C for 50 h. C_s-C₇₀(CF₃)₈ was isolated from prepared mixture of lower trifluoromethylated fullerenes by HPLC (Cosmosil Buckyprep column, 10 mm i.d. × 25 cm, equipped with a guardian precolumn 10 mm i.d. × 20 mm,) using toluene as the eluent, 4.6 ml min⁻¹ flow rate, 290 nm.

Mass spectra (negative and positive ions) of matrix-assisted laser desorption/ionization (MALDI) were acquired with the use of Bruker AutoFlex II time-of-flight reflectron device (N₂ laser, 337 nm, 1 ns pulse). 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB, 99%, Fluka) was chosen as a matrix. Sample preparation was performed with use of the matrix and analyte solutions in toluene and 1,2-dichlorobenzene, respectively, the matrix-to-analyte molar ratio being ≥ 1000.

¹⁹F NMR spectra were recorded on a Bruker Avance 400 spectrometer operating at 376.5 MHz. The solutions of compounds under investigation in CDCl₃ contained C₆F₆ as an internal standard (δ −162.9 ppm).

¹⁹F NMR spectrum of C₇₀(CF₃)₈: δ_F (ppm) −61.40–−61.51 (12F, m, CF₃), −61.68 (6F, m, CF₃), −65.74 (6F, q, J 16.1 Hz, CF₃).

¹⁹F NMR spectrum of C_s-C₇₀(CF₃)₈Cl₂: δ_F (ppm) −61.41–−61.51 (12F, m, CF₃), −61.67 (6F, m, CF₃), −62.36 (6F, q, J 14.9 Hz, CF₃).

Crystal data

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 detector, λ = 0.9050 Å. C₇₀(CF₃)₈Cl₂·2C₆H₄Cl₂: orthorhombic, Pbcm, a = 11.4051(3), b = 19.1550(5), c = 26.914(1) Å, V = 5879.8(3) ų, D_c = 1.986 g cm⁻³, Z = 4. Anisotropic refinement with 6222 reflections and 548 parameters yielded a conventional R₁(F) = 0.094 for 6089 reflections with I > 2σ(I) and wR₂(F²) = 0.213 for all reflections. The C₇₀(CF₃)₈Cl₂ molecule has crystallographically imposed mirror symmetry, whereas o-dichlorobenzene molecule of solvation is situated in a general position in the asymmetric unit. CCDC 768967. See DOI: 10.1039/c0nj00591f for crystallographic data in CIF format.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of ¹⁹F NMR simulation, Schlegel diagrams, DFT relative energies of possible C₇₀(CF₃)₈Cl₂ isomers, selected calculated distances and dihedral angles of C₇₀(CF₃)₈X₂, and HOMO/LUMO calculations and spin density analysis. CCDC reference number 768967. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0nj00591f

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