DOI: 10.1002/chem.201302946ACHTUNGTRENNUNG[6,6]-Open and [6,6]-Closed Isomers of C70 ACHTUNGTRENNUNG(CF2): Synthesis, Electrochemicaland Quantum Chemical Investigation

Nataliya A. Samoylova,[a] Nikita M. Belov,[a] Victor A. Brotsman,[a] Ilya N. Ioffe,[a]

Natalia S. Lukonina,[a] Vitaliy Yu. Markov,[a] Adrian Ruff,[b, c] Alexey V. Rybalchenko,[a]

Paul Schuler,[b] Olesya O. Semivrazhskaya,[a] Bernd Speiser,[b] Sergey I. Troyanov,[a]

Tatiana V. Magdesieva,*[a] and Alexey A. Goryunkov*[a]

Introduction

Fullerenes and their derivatives are of particular interest forthe design and synthesis of molecules with specific proper-ties required for applications in areas such as material sci-ence (photovoltaic cells, optical limiters)[1] and medicine(antioxidants, neuroprotective agents, antimicrobial agents,agents for photodynamic therapy and magnetic resonance

imaging).[2] Typically, fullerenes are functionalised by vari-ous addition reactions to the double bonds, whereas trans-formations of the fullerene cage itself are much harder tocarry out due to high stability of the network of carbon–carbon fullerene bonds.

Recently it has been demonstrated[3] that sites of en-hanced reactivity can emerge in the fullerene cage uponopening of the [6,6]-bonds. Synthesis of the so-called homo-fullerene compounds by insertion of a bivalent addend (e.g.the difluoromethylene fragment) into a [6,6]-bond leads tocleavage and associated redistribution of the p-electronbonding in the cage. The resulting [6,6]-open adducts con-tain the 1,6-difluoromethano[10]annulene moiety in whichthe sp2 hybridisation of the bridgehead carbon atoms is re-tained and, accordingly, all carbon atoms of the parent ful-ACHTUNGTRENNUNGlerene remain within the spherical p system, although itsconnectivity decreases slightly. These compounds with anopen [6,6] C�C bond (homofullerenes) are quite unusualamongst fullerene exohedral derivatives. Until recently, onlya few examples of [6,6]-open structures were known, all ofthem with nitrogen-containing bridges.[4–6] Preparation ofthe first [6,6]-open carbon-containing derivatives C60 ACHTUNGTRENNUNG(CF2)[predicted theoretically in 1998][7] and cis-2-C60ACHTUNGTRENNUNG(CF2)2 wasreported in our pioneering works in 2006.[3,8, 9] Although stillvery little is known about the reactivity of homofullerenes,it is clear that preservation of the spherical p system of ful-lerenes with all carbon atoms remaining sp2 hybridised

[a] N. A. Samoylova, N. M. Belov, V. A. Brotsman, Dr. I. N. Ioffe,Dr. N. S. Lukonina, Dr. V. Y. Markov, A. V. Rybalchenko,O. O. Semivrazhskaya, Prof. Dr. S. I. Troyanov,Prof. Dr. T. V. Magdesieva, Dr. A. A. GoryunkovChemistry DepartmentLomonosov Moscow State UniversityLeninskie Gory, 1, 119991, Moscow (Russia)Fax: (+7) 495-939-1240E-mail : tvm@org.chem.msu.ru

aag@thermo.chem.msu.ru

[b] Dr. A. Ruff, P. Schuler, Prof. Dr. B. SpeiserInstitut f�r Organische ChemieUniversit�t T�bingen, Auf der Morgenstelle 1872076 T�bingen (Germany)

[c] Dr. A. RuffInstitut f�r PolymerchemieUniversit�t Stuttgart, Pfaffenwaldring 5570569 Stuttgart (Germany)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201302946.

Abstract: Novel difluoromethylenated[70]fullerene derivatives, C70ACHTUNGTRENNUNG(CF2)n (n=

1–3), were obtained by the reaction ofC70 with sodium difluorochloroacetate.Two major products, isomeric C70 ACHTUNGTRENNUNG(CF2)mono-adducts with [6,6]-open and[6,6]-closed configurations, were isolat-ed and their homofullerene and metha-nofullerene structures were reliably de-termined by a variety of methods thatincluded X-ray analysis and high-levelspectroscopic techniques. The [6,6]-open isomer of C70ACHTUNGTRENNUNG(CF2) constitutes thefirst homofullerene example of a non-

hetero [70]fullerene derivative in whichfunctionalisation involves the most re-active bond in the polar region of thecage. Voltammetric estimation of theelectron affinity of the C70 ACHTUNGTRENNUNG(CF2) iso-mers showed that it is substantiallyhigher for the [6,6]-open isomer (the70-electron p-conjugated system is re-

tained) than the [6,6]-closed form, thelatter being similar to the electron af-finity of pristine C70. In situ ESR spec-troelectrochemical investigation of theC70 ACHTUNGTRENNUNG(CF2) radical anions and DFT calcu-lations of the hyperfine coupling con-stants provide evidence for the first ex-ample of an inter-conversion betweenthe [6,6]-closed and [6,6]-open forms ofa cage-modified fullerene driven by anelectrochemical one-electron transfer.Thus, [6,6]-closed C70 ACHTUNGTRENNUNG(CF2) constitutesan interesting example of a redox-switchable fullerene derivative.

Keywords: density functional calcu-lations · fullerenes · molecularswitches · spectroelectrochemistry ·structure elucidation

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makes homofullerenes more sensitive than methanofuller-enes to further functionalisation. Our recent electrochemicalinvestigation of [6,6]-open C60ACHTUNGTRENNUNG(CF2)

[3] demonstrated that theelectron affinity of this compound is well above the valuesobtained for other known C60 adducts with one bivalent ortwo mono-valent addends. As an illustration, a plot of thefirst reduction potentials for various methano[60]fullerenesversus the sum of Hammett constants, sm, of the substitu-ents[10] based on the literature data[11] and our previous find-ings[3] is provided in Figure 1. Only the C60ACHTUNGTRENNUNG(CF2) homofuller-

ene shows a marked departure from the conventional lineartrend, which reveals a key influence of the bond opening onthe redox behaviour.

The replacement of C60 with C70 offers broader possibili-ties for investigation of the influence of the local geometryof the fullerene p system on the electronic properties of theadducts. Compared to C60, in which all [6,6]-bonds areequivalent, the skeleton of C70 contains four distinct types of[6,6]-bonds with somewhat different local curvature of thecarbon cage. This introduces more diversity in the function-alisation reactions and makes it possible to obtain both[6,6]-open and [6,6]-closed isomers. A comparative investi-gation of the electronic structure of such compounds byusing high-level experimental methods and quantum chemi-cal calculations will provide deeper insight into the geomet-ric and electronic aspects of the reactivity of homofuller-enes.

However, targeted synthesis of the [6,6]-open and [6,6]-closed isomers is not a trivial task. Previously, we have dem-onstrated that the bivalent CF2 addend facilitates selectiveformation of the [6,6]-open form of C60 ACHTUNGTRENNUNG(CF2).[8,9] For C70, thesituation is more complex because theoretically eight differ-ent C70 ACHTUNGTRENNUNG(CF2) regioisomers can be expected upon difluorocar-bene addition to the C70 cage. The present paper describesthe development of a new chemical approach to derivatisa-tion of the [70]fullerene cage with the CF2 addend to yieldeither [6,6]-open homofullerene or [6,6]-closed methanoful-lerene isomers. We also elucidate the connections betweenthe structural and electronic properties of these compoundsby using a variety of electrochemical and spectroscopic tech-

niques and quantum chemical calculations. Notably, one ofthe reported compounds constitutes the first example ofa non-hetero [70]fullerene derivative that has the most reac-tive bond at the pole of the cage cleaved to give a [6,6]-open adduct.

Results and Discussion

Synthesis and structures of the C70ACHTUNGTRENNUNG(CF2) isomers : The syn-thesis of the difluoromethylenated derivatives of C70 wasperformed by using a protocol similar to that used for C60:[9]

C70 was heated at reflux with CF2ClCOONa in ortho-di-chlorobenzene (oDCB, 180 8C) in the presence of a catalyticamount of [18]crown-6. MALDI mass spectrometric andHPLC analysis showed products of mono-, bis- and tris-ad-dition of CF2 groups to be dominant, whereas tetra-adductswere only minor components (Figure 2). Additional treat-

ment with sodium difluorochloroacetate resulted in a de-crease in the relative abundance of C70 ACHTUNGTRENNUNG(CF2), whereas thecontent of the C70 ACHTUNGTRENNUNG(CF2)n poly-adducts (n=2–4) was en-hanced. Though the reaction was performed under air, onlysmall amounts of the oxygen-containing derivativesC70 ACHTUNGTRENNUNG(CF2)2O and C70ACHTUNGTRENNUNG(CF2)3O were detected.

The reaction mixture was separated by HPLC with tolu-ene/hexane (8:2 v/v) as the eluent (Figure 2 b). Eleven frac-tions were isolated and identified by MALDI mass spec-trometry (Table 1). Three major fractions (p2, p3 and p5)correspond to unreacted C70 and to the two isomers ofC70 ACHTUNGTRENNUNG(CF2), respectively. The first eluted fraction (p1) con-tained the C70 ACHTUNGTRENNUNG(CF2)2 bis-adduct, comparable to the C60ACHTUNGTRENNUNG(CF2)n

mixture from which the early eluted fraction wascis-2-C60 ACHTUNGTRENNUNG(CF2)2.

[8] According to MALDI mass spectrometry,fraction p6 contained a third (minor), non-characterised,

Figure 1. First reduction potential values of some C60 ACHTUNGTRENNUNG(CR1R2) compoundsversus the sum of sm for substituents R1 and R2.

Figure 2. a) Negative ion MALDI mass spectrum of the products of[70]fullerene difluoromethylenation; b) HPLC trace (Cosmosil Bucky-prep 10 mm I.D.� 25 cm, toluene/hexane= 8:2, 4.6 mL min�1) of the reac-tion mixture.

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C70 ACHTUNGTRENNUNG(CF2) isomer, along with the bis- and tris-adducts. Theother fractions comprised complex mixtures of bis- to tetra-CF2 adducts and their oxygenated derivatives.

It was found that the optimal method for isolation of theC70 ACHTUNGTRENNUNG(CF2) isomers is a two-step procedure: initial coarse sep-

aration of the desired fractions by elution with toluene, fol-lowed by fine chromatographic purification with a less polareluent (6:4 toluene/hexane; a further increase in hexanecontent does not improve the separation and considerablyincreases the retention time, see the Supporting Informa-tion, Figure S1, for details). Thus, isomerically pure samplesof two C70 ACHTUNGTRENNUNG(CF2) isomers (early eluted C70 ACHTUNGTRENNUNG(CF2)-I and subse-quently eluted C70 ACHTUNGTRENNUNG(CF2)-II) were obtained after HPLC sepa-ration in amounts sufficient for spectral, X-ray and electro-chemical analysis.

The C70ACHTUNGTRENNUNG(CF2) isomers were characterised by 19F and13C NMR spectroscopic analysis (Figure 3). The 19F NMRspectra show signals typical for CF2 groups: an AB spinsystem at d=�126.37 ppm (AB q, DdAB =2.55 ppm, J=

169.3 Hz, 2 F) for C70ACHTUNGTRENNUNG(CF2)-I and a more downfield singletsignal (d=�111.9 ppm) for C70 ACHTUNGTRENNUNG(CF2)-II. The presence of anAB spin system in the spectrum of the C70ACHTUNGTRENNUNG(CF2)-I isomer isan indication that the chemical environment of the fluorineatoms is non-equivalent, contrary to C70ACHTUNGTRENNUNG(CF2)-II for whichthe singlet peak suggests equivalence of the fluorine atoms.

We found that the fluorine chemical shift is characteristicfor distinction of methano- and homofullerene structures,

Table 1. HPLC separation and mass spectrometric characterisation ofthe reaction products.

Fraction tR [min][a] Composition[b]

p1 18.9–20.1 C70ACHTUNGTRENNUNG(CF2)2

p2 22.0–22.8 C70

p3 22.8–23.8 C70ACHTUNGTRENNUNG(CF2) [isomer I]p4 25.1–26.5 C70ACHTUNGTRENNUNG(CF2)n, n=2, 3p5 27.1–28.5 C70ACHTUNGTRENNUNG(CF2) [isomer II]p6 29.0–31.2 C70ACHTUNGTRENNUNG(CF2)n, n=1–3p7 31.3–32.6 C70ACHTUNGTRENNUNG(CF2)nOm, n=2–4, m=0–2p8 32.9–34.5 C70ACHTUNGTRENNUNG(CF2)n, n=2–3p9 34.5–36.2 C70ACHTUNGTRENNUNG(CF2)nOm, n=2–3, m=0–1p10 36.3–38.0 C70ACHTUNGTRENNUNG(CF2)nOm, n=2–4, m=0–2p11 38.2–38.9 C70ACHTUNGTRENNUNG(CF2)nOm, n=2–3, m=0–2

[a] Retention time. Conditions: Cosmosil Buckyprep (10 mm I.D.�25 cm), toluene/hexane= 8:2, 4.6 mL min�1. [b] Determined by MALDIMS.

Figure 3. a), d) 19F NMR and b), c), e), f) 13C NMR spectra of C70 ACHTUNGTRENNUNG(CF2)-I (top) and C70 ACHTUNGTRENNUNG(CF2)-II (bottom); [D4]oDCB 13C NMR signals are marked with as-terisks. DFT-optimised structures of C70ACHTUNGTRENNUNG(CF2)-I and C70ACHTUNGTRENNUNG(CF2)-II, as well as Schlegel diagrams with the numeration of bonds (position of CF2 groups aremarked with dumbells), are shown.

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which differ in hybridisation of the bridgehead carbonatoms. The signal due to C70 ACHTUNGTRENNUNG(CF2)-II is shifted downfield (byabout 7 ppm) with respect to that observed for [6,6]-openC60 ACHTUNGTRENNUNG(CF2) (d=�119 ppm), which supports the homofullerenestructure of C70ACHTUNGTRENNUNG(CF2)-II. On the contrary, the AB spinsystem of C70ACHTUNGTRENNUNG(CF2)-I is shifted upfield, rather typical for 1,1-difluorocyclopropane derivatives with sp3-hybridised bridge-head carbon atoms. For example, CF2 chemical shift valuesfor saturated 7,7-difluoronorcarane[12] are dF = �129 and159 ppm; for 11,11-difluoro-1,6-methano-[10]annulene[13]

and difluorocyclopropabenzene,[14] which contain sp2-bridge-head carbon atoms, d= �126.2 and �80.4 ppm, respectively.

The 13C NMR spectrum of C70 ACHTUNGTRENNUNG(CF2)-I features 34 sp2

carbon atom signals in the range d= 124–153 ppm plus twotriplet signals at d=69.4 (3JACHTUNGTRENNUNG(C,F)=23 Hz) and 102.8 ppm(1J ACHTUNGTRENNUNG(C,F)=285 Hz) attributed to the bridgehead sp3 carbonatoms and the sp3 carbon of the CF2 group, respectively. Thesignals observed within the range d=68–80 ppm are typicalfor bridgehead carbon atoms of methanofullerenes, as fol-lows from the literature data obtained for [5,6]-closed C60-ACHTUNGTRENNUNG(GeR2),[15] [6,6]-closed C60ACHTUNGTRENNUNG(CR2), R=H, Cl, Br, I,[16–19] andC70 ACHTUNGTRENNUNG(CCl2).[20] The 13C NMR spectrum of C70 ACHTUNGTRENNUNG(CF2)-II contains35 sp2 carbon atom signals in the range d=125–155 ppm,a triplet signal at d= 111.6 ppm (1J ACHTUNGTRENNUNG(C,F)= 257 Hz; CF2) andtwo triplet signals at d=99.2 and 98.2 ppm (3J ACHTUNGTRENNUNG(C,F)= 37.4and 38.5 Hz, respectively). The last two triplet signals can beascribed to the two inequivalent bridgehead carbon atoms.The signals of the bridgehead carbon atoms of C70ACHTUNGTRENNUNG(CF2)-IIare dramatically shifted downfield (by about 30 ppm) rela-tive to those in C70 ACHTUNGTRENNUNG(CF2)-I. The apparent reason is pro-nounced sp2 hybridisation of the bridgehead atoms inC70 ACHTUNGTRENNUNG(CF2)-II. The same effect has been observed for bridge-head cage carbon atoms of the homofullerenes[6,6]-C60 ACHTUNGTRENNUNG(CF2),[9] [5,6]-C70ACHTUNGTRENNUNG(CCl2)

[20] and [6,6]-equatorial C2v-C70(CH2)

[21] within the range d=107–119 ppm. Thus thestructural predictions from the 19F and the 13C NMR spectraare in good qualitative agreement.

The detection of only 34 and 35 sp2 carbon atom signalsin the 13C NMR spectra of C70 ACHTUNGTRENNUNG(CF2)-I and C70 ACHTUNGTRENNUNG(CF2)-II, re-spectively, points to Cs symmetry of both molecules. The ex-istence of C2 symmetry can be ruled out because witha single CF2 addend only a more symmetric C2v structurewith far fewer NMR spectral lines can possibly form and in-volve bond 1 (see Figure 3 for bond enumeration). Accord-ing to 19F NMR spectroscopic data, both fluorine atoms inthe C70ACHTUNGTRENNUNG(CF2)-I isomer should lie in the mirror plane whereasin the C70 ACHTUNGTRENNUNG(CF2)-II case they are symmetric to it. This is onlyconsistent with addition to bond 7 for C70ACHTUNGTRENNUNG(CF2)-II and tobonds 4, 5, or 8 for C70ACHTUNGTRENNUNG(CF2)-I. However, addition to thesingle [5,6]-bonds 4 and 8 is most unlikely because it wouldresult in their inevitable cleavage (the DFT-estimated C···Cdistances are 2.19 and 2.30 �, respectively, see Table 3below) whereas C70ACHTUNGTRENNUNG(CF2)-I belongs to the methanofullerenetype with the respective C�C bond retained according tothe 19F and 13C NMR spectroscopic data. Thus, 19F and13C NMR spectroscopy unambiguously characterise the

C70 ACHTUNGTRENNUNG(CF2)-I and C70 ACHTUNGTRENNUNG(CF2)-II isomers to be the products of CF2

addition to bonds 5 and 7, respectively (Figure 3).Ultimately, the structure of the C70ACHTUNGTRENNUNG(CF2)-I isomer was

proven by means of single-crystal X-ray diffraction(Figure 4). Crystals of sufficient quality were prepared by

co-crystallisation of C70 ACHTUNGTRENNUNG(CF2)-I and Ni(II) octaethylporphy-ACHTUNGTRENNUNGrin. The experimental C�C bond length between the bridge-head carbon atoms (1.707(8) �) is in good agreement withthe DFT estimated value (1.72 �). Typical bond lengths ofsuch C�C bonds in methanofullerenes are about 1.65 �.[22]

The longest C�C bond of approximately 1.70 � was detect-ed in D3d-C60Cl30.

[23] Thus, in spite of significant elongationof C�C bond 5 after CF2 addition, one can assume that thebonding between the bridgehead carbon atoms in C70 ACHTUNGTRENNUNG(CF2)-Iis preserved. It should be also mentioned that, in addition tothe elongation of the aforementioned C�C bond 5, notice-ACHTUNGTRENNUNGable elongation of all neighbouring C�C cage bonds in C70-ACHTUNGTRENNUNG(CF2)-I (�0.015–0.025 � relative to the same bonds in pris-tine fullerene) is observed. This is due to the change in hy-bridisation of the bridgehead carbon atoms from sp2 to sp3.The CF2-Cbridgehead-Cbridgehead angle in the cyclopropane ring is54.2(3)8, closer to the perfect 608 than in C60 ACHTUNGTRENNUNG(CF2). Theshortest Ni···Ccage and N···Ccage distances are 2.816(4) and3.026(6) �, respectively, which is typical for co-crystals offullerene derivatives with Ni(II) octaethylporphyrin and iscommonly attributed to p–p interactions.[24]

The [6,6]-closed and [6,6]-open structures of C70 ACHTUNGTRENNUNG(CF2)-Iand C70ACHTUNGTRENNUNG(CF2)-II are also supported by characteristic featuresof their UV/Vis spectra (Figure 5). The spectra of [6,6]-openC70 ACHTUNGTRENNUNG(CF2)-II and C70 (Figure 5 b, c) are similar, a fact that is

Figure 4. X-ray structure of the C70ACHTUNGTRENNUNG(CF2)-I adduct with Ni(II) octaethyl-porphyrin (C70 ACHTUNGTRENNUNG(CF2)·2NiII ACHTUNGTRENNUNG(OEP)·C6H5 ACHTUNGTRENNUNG(CH3), 50% probability thermal el-lipsoids are shown; toluene molecule and hydrogen atoms are omittedfor clarity.

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T. V. Magdesieva, A. A. Goryunkov et al.

typical for homofullerenes with weakly perturbed p-electronsystems.[25] It has been shown that the UV/Vis spectrum ofthe C60 ACHTUNGTRENNUNG(CF2) homofullerene is almost identical to that ofC60.

[9] On the contrary, the UV/Vis spectrum of C70 ACHTUNGTRENNUNG(CF2)-I issignificantly different from that of C70 (Figure 5 a versus c).In the region of p–p* transitions (l=300–500 nm), insteadof four bands observed for C70, only three bandsare detected, and they are blueshifted by Dl�10–20 nm. This shift indicates a slight increase in thep–p* transfer energy, typical for all methanofuller-enes.[9] This blueshift reflects partial destruction ofthe fullerene p-electron system due to rehybridisa-tion of two cage carbon atoms to the sp3 state afterfunctionalisation.

Voltammetric measurements : With sufficient quan-tities of the compounds in hand, we next investigat-ed the influence of local perturbation of the fuller-ene p system on the electronic properties of themolecules. The C70 framework enables the firstdirect comparison of [6,6]-open and [6,6]-closed de-rivatives of the same carbon cage. The cyclic vol-tammograms for the [6,6]-open and [6,6]-closedC70 ACHTUNGTRENNUNG(CF2) isomers are shown in Figure 6. In bothcases, at least three reversible one-electron reduc-

tions can be observed, their peak potential values and peakcurrent ratios (Ia/Ic) are summarised in Table 2. In all cases,the Ia/Ic values were close to unity, evidence of the stabilityof the mono-, di- and tri-anions towards any follow-upchemical reactions, at least within the cyclic voltammetry(CV) timescale. The peak currents for both isomers werediffusion controlled, as follows from the linear dependenceof the current on the square root of the potential scan rate.The potential separation between the forward and reversepeaks is about 60 mV for all the redox couples observed,which conforms to the reversibility criterion. In the case ofthe [6,6]-open isomer, after the second reduction a small re-versible redox couple (marked with asterisks in Figure 6) ata potential close to the second C70 reduction (C70

�/2�) can beobserved in the CV curve. This was possibly the result ofa small impurity or an indication that the C70ACHTUNGTRENNUNG(CF2)

2� di-anion may slowly decompose. In the latter case, the pres-ence of the corresponding set of peaks for C70 in the reverse

Figure 5. UV/Vis spectra for C70ACHTUNGTRENNUNG(CF2) isomers a) I , c =4.49 � 10�2 mm ; b)II, c=7.91 � 10�2 mm and c) C70, c =7.54 � 10�2 mm, measured in solutionin CH2Cl2.

Table 2. Electrochemical data (Pt, oDCB, 0.1m Bu4NBF4, E vs. Fc0/+, scan rate=

100 mV s�1) for C70, C70ACHTUNGTRENNUNG(CF2)-I, C70ACHTUNGTRENNUNG(CF2)-II and C60ACHTUNGTRENNUNG(CF2).

Compound Process Epc[a] Epa

[b] DE Ia/Ic ACHTUNGTRENNUNG(Epc+Epa)/2 [V][V] [V] [V] vs. Fc0/+ vs. C70

�n/�(n+1)

C70 0/1� �1.09 �1.02 0.07 0.92 �1.06 –1�/2� �1.47 �1.41 0.06 0.97 �1.44 –2�/3� �1.89 �1.83 0.06 1.00 �1.86 –

C70ACHTUNGTRENNUNG(CF2)-IACHTUNGTRENNUNG[6,6]-closed0/1� �1.08 �1.02 0.06 0.98 �1.05 0.011�/2� �1.37 �1.31 0.06 1.00 �1.34 0.102�/3� �1.83 �1.78 0.05 0.97 �1.81 0.06

C70ACHTUNGTRENNUNG(CF2)-IIACHTUNGTRENNUNG[6,6]-open0/1� �0.94 �0.88 0.06 1.00 �0.91 0.151�/2� �1.27 �1.21 0.06 0.95 �1.24 0.202�/3� �1.85 �1.79 0.06 1.00 �1.82 0.04

C60ACHTUNGTRENNUNG(CF2)ACHTUNGTRENNUNG[6,6]-open0/1� �0.91 �0.86 0.05 1.00 �0.89 0.171�/2� �1.25 �1.19 0.06 1.00 �1.22 0.222�/3� �1.85 �1.80 0.05 0.95 �1.83 0.04

[a] Reduction peak potential. [b] Oxidation peak potential.

Figure 6. CV curves obtained for the [6,6]-closed and [6,6]-open C70ACHTUNGTRENNUNG(CF2)isomers (Pt, 0.15 m Bu4NBF4 , oDCB, scan rate=100 mV s�1, E vs. Fc0/+).

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anodic scan would also be expected. However, the concen-tration of hypothetically formed C70 is too small to ensureall peaks are detected.

The calculated formal potential values that correspond tothe formation of mono-, di- and trianions of both isomersare summarised in Table 2. The potential of the C70 ACHTUNGTRENNUNG(CF2)

0/�

redox couple is approximately 140 mV more anodic in thecase of the [6,6]-open isomer, which means that the electronaffinity (EA) of the [6,6]-open isomer is higher than that ofthe [6,6]-closed form or pristine C70. These results are in linewith our previous data; the electrochemical study of [6,6]-open C60 ACHTUNGTRENNUNG(CF2) revealed a similar anodic shift (150 mV) ofthe first reduction potential relative to C60.

[3] Furthermore,the experimental results obtained are in agreement with thetheoretical estimations; the observed anodic shift correlateswith DFT-predicted EA values (2.77, 2.80 and 2.93 eV forC70, C70ACHTUNGTRENNUNG(CF2)-I and C70ACHTUNGTRENNUNG(CF2)-II, respectively).

The EA of [6,6]-closed C70ACHTUNGTRENNUNG(CF2) almost coincides withthat of C70 (the peak potential values for one-electron re-duction are almost identical too). Probably, a decrease inEA due to impaired p conjugation (68 electrons versus 70)is compensated for by the addition of an electron-withdraw-ing CF2 group.

The distances between the corresponding redox peaks ofthe two isomers become attenuated with an increase in thenegative charge (Table 2). The formal redox potentials ofthe [6,6]-closed and [6,6]-open isomers for the consecutiveC70 ACHTUNGTRENNUNG(CF2)

0/�, C70ACHTUNGTRENNUNG(CF2)�/2� and C70 ACHTUNGTRENNUNG(CF2)

2�/3� couples are sepa-rated by 140 mV, 100 mV, and �20 mV, respectively. Thismay be due to gradual smoothing of the structural differen-ces (most importantly, the geometry of the CF2 bridge) inthe highly charged states. One can expect, by analogy to theincreased bond opening in the mono-anions of C70ACHTUNGTRENNUNG(CF2) dis-cussed below, that further charging may facilitate evenstronger C···C elongation between the bridgehead carbonatoms in both isomers. Notably, the successive redox poten-tials of the two compounds that already have an open con-figuration in their neutral state, C70 ACHTUNGTRENNUNG(CF2)-II and C60 ACHTUNGTRENNUNG(CF2),remain constantly spaced (ca. 20 mV, see Table 2).

Relative energies and structural and electronic features ofC70 ACHTUNGTRENNUNG(CF2) isomers : Table 3 summarises our DFT (PBE/TZ2P) results for the relative energies and geometry of theC�CF2�C fragment of the eight theoretically possible iso-mers of C70 ACHTUNGTRENNUNG(CF2). For comparison, experimental bondlengths in C70

[26] are given. The most energetically preferableC70 ACHTUNGTRENNUNG(CF2) isomer with C2v-symmetry, the formal product ofCF2 addition to the equatorial [6,6]-bond 1, is the least ki-netically favourable (see Figures S2–S9 in the Supporting In-formation for the calculated reaction profiles). Although therespective activation energy of 58 kJ mol�1 is quite accessiblein itself, it is considerably higher than the barrier heights foraddition to the other bonds. Despite being [6,6]-bonds, theequatorial bonds 1 in C70 are 1.48 � long, thus exhibitsingle-bond character.[26] Moreover, they are the longestC�C bonds in the C70 molecule, with the lowest bond order(1.23) among the [6,6] bonds. Also, they are formed betweenthe two chemically less-active carbon atoms at the triplehexagon junctions (THJs). Therefore, it is not unexpectedthat the insertion of the CH2 groups into the equatorialbonds to yield the [6,6]-open C2v-C70ACHTUNGTRENNUNG(CH2) derivative is ob-served only at elevated temperatures (approximately1100 8C).[21] Similarly, other elongated C�C bonds (3, 6 and8 ; 1.44–1.46 �, bond orders: 1.21–1.23) are unlikely to be in-volved in carbene addition under mild conditions due tocompetition from shorter bonds.

The shortest bonds, 5 (1.37 �) and 7 (1.38 �) are charac-terised by the highest bond orders (1.41 and 1.45, respective-ly). As a result, they easily undergo [2+1] cycloadditionwith DCCl2 and [3+2] cycloaddition with diazomethane toresult in the corresponding cyclopropane and pyrazoline de-rivatives, respectively.[20,25] Analogously, CF2 addition tothese bonds is characterised by EA =18 and 19 kJ mol�1, re-spectively, the lowest amongst the considered set of isomericadditions.

Bonds 2 and 4 constitute an intermediate case; bond 2 isslightly shorter than bond 4 (1.42 versus 1.45 �) and theirbond orders (1.34 and 1.30, respectively) are in between theextreme cases discussed above. Thus, reactions that involvethese bonds are relatively less probable. Addition to bond 2should be also impeded by the low reactivity of the THJ

Table 3. Schlegel diagram of C70 (bond indices and bond lengths [�] are given) and DFT relative energies, activation energy (EA) values for DCF2 addi-tion and distances between the bridgehead carbon atoms for the various theoretically possible isomers of C70 ACHTUNGTRENNUNG(CF2) [in order of increasing EA, numberedaccording to the bond being functionalised].

Isomer C�C bond in C70 Corresponding C�C bonds in C70 ACHTUNGTRENNUNG(CF2) isomerType Length[a]

[�]Mulliken bondorder[b]

EA[c]ACHTUNGTRENNUNG[kJmol�1]

DEACHTUNGTRENNUNG[kJmol�1]C···C distance[�]

7 II [6,6] 1.38 (1.40) 1.45 18 0 2.095 I [6,6] 1.37 (1.39) 1.41 19 17.6 1.724 ACHTUNGTRENNUNG[5,6] 1.45 (1.44) 1.30 25 �2 2.192 ACHTUNGTRENNUNG[6,6] 1.42 (1.42) 1.34 26 40.8 2.196 ACHTUNGTRENNUNG[5,6] 1.46 (1.45) 1.21 28 8.9 2.218 ACHTUNGTRENNUNG[5,6] 1.46 (1.45) 1.23 28 0.3 2.303 ACHTUNGTRENNUNG[5,6] 1.44 (1.45) 1.23 29 36.9 2.201 ACHTUNGTRENNUNG[6,6] 1.48 (1.47) 1.23 58 �44 2.33

[a] X-ray structural data from ref. [26]; the DFT estimated values are given in parentheses. [b] DFT (PBE/TZ2P) data. [c] When alternative inequivalentreaction paths exist, the lowest of the EA values is given, for details see the Supporting Information (Figures S2–S9).

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T. V. Magdesieva, A. A. Goryunkov et al.

carbon atom involved in this bond. However, it is knownthat treatment of C70 with the Seyferth reagent,PhHgCCl2Br (which releases DCCl2 upon heating), results inaddition to bond 4 to yield a [5,6]-open C70ACHTUNGTRENNUNG(CCl2) homoful-lerene, along with formation of the two isomeric C70ACHTUNGTRENNUNG(CCl2)methanofullerenes (adducts at bonds 5 and 7). The ratio ofthe products was 1:1.2:1.3 according to HPLC data.[20, 27]

Functionalisation of the fullerene cage with DCF2 differsfrom the reactions with its bis-chlorinated analogue becausedifluorocarbene more readily forms open adducts, as followsfrom the experimental findings reported for C60.

[3,8,9] In caseof C70, C70ACHTUNGTRENNUNG(CF2)-I and C70 ACHTUNGTRENNUNG(CF2)-II show quite pronounceddifferences in the distance between the bridgehead carbonatoms (1.72 and 2.09 �, respectively; see Table 3). TheseDFT results are consistent with our 19F and 13C NMR spec-troscopic data and confirm that C70ACHTUNGTRENNUNG(CF2)-I has a [6,6]-closedmethanofullerene structure, whereas C70ACHTUNGTRENNUNG(CF2)-II is of the[6,6]-open homofullerene type. Lack of bonding betweenthe bridgehead carbon in C70ACHTUNGTRENNUNG(CF2)-II is also conveniently il-lustrated by the electron density distribution given inFigure 7. An appreciable (though still lower than for con-ventional C�C s-bonds) electron density can be observedalong the line that connects the bridgehead carbon atomsonly in the case of C70 ACHTUNGTRENNUNG(CF2)-I. It is noteworthy that cleavageof bond 7, the most reactive and one of the two shortestbonds in C70, was not previously observed for non-hetero de-rivatives.

After the discovery of the structural difference we decid-ed to study how other CX2 substituents alter the bond towhich they are attached. Figure 7 shows the relaxed scans ofthe potential energy surfaces (PES) for the neutral isomersof C70 ACHTUNGTRENNUNG(CX2) [X=F, H, Cl] of types 5 and 7 along the stretch-ing coordinate coupling the bridgehead carbon atoms.

Notably, in the CF2 adducts only very shallow potentialwells are observed at the equilibrium inter-atomic distances;the energy variation within a broad range of inter-atomicdistances are of the order of 10 kJ mol�1. This suggestsa high amplitude and a largely anharmonic CF2 opening–clo-sure mode in the C70 ACHTUNGTRENNUNG(CF2) isomers. At the same time, theC70 ACHTUNGTRENNUNG(CCl2) and C70 ACHTUNGTRENNUNG(CH2) compounds of type 5 are character-ised by markedly steeper PES profiles. In the isomers oftype 7, the steepness is comparable to that in C70ACHTUNGTRENNUNG(CF2)-IIbut the equilibrium inter-atomic distance is shifted to1.65 �, which may be interpreted as s bonding. In general,we see a clear trend towards the formation of closed metha-nofullerene compounds in the cases of C70ACHTUNGTRENNUNG(CCl2) andC70 ACHTUNGTRENNUNG(CH2).

The flatness of the PES in the C70 ACHTUNGTRENNUNG(CF2) isomers, as well asfor C60 ACHTUNGTRENNUNG(CF2),[8] can be rationalised by using a simple modelbased on electrostatic and steric factors. One can considerthe C70ACHTUNGTRENNUNG(CF2) molecule as a combination of a polarisablesphere and a CF2 dipole, the strongest among other possibleCX2 groups. Attraction between the dipole and the polarisa-ble sphere brings the CF2 group closer and thus forces bondopening. Also, the CF2 carbon atom bears high effectivepositive charge, which induces negative charges at thebridgehead carbons and additionally contributes to their re-

pulsion. The resulting s-bond cleavage is energetically un-favourable in itself but this is partly compensated by the re-duction of steric strain in the cyclopropane ring. The degreeof opening, however, should be limited by another stericeffect: the bridgehead atoms in the sp2 state will prefermore-or-less planar coordination, which can be observed forseparations of about 2.1 �. The interplay of the above fac-tors results in structural flexibility of the carbon cage. Theminor differences that favour a closed configurationC70 ACHTUNGTRENNUNG(CF2)-I and open configuration in C70 ACHTUNGTRENNUNG(CF2)-II are likelyassociated with local curvatures of the respective sections ofthe carbon cage and with the ensuing differences in the en-ergetics of the steric effects.

In situ ESR spectroelectrochemistry of C70ACHTUNGTRENNUNG(CF2) radicalanions—the structure of C70ACHTUNGTRENNUNG(CF2) anionic forms : In situESR spectroelectrochemical investigation of the products of

Figure 7. a), b) PES scans for C70 ACHTUNGTRENNUNG(CX2), X =F, H, Cl and c) DFT-calculat-ed electron density distribution in C70ACHTUNGTRENNUNG(CF2)-I and C70ACHTUNGTRENNUNG(CF2)-II (contourlevels are given for values of 0.01, 0.05, 0.1, 0.2 and 0.3 e ��3).

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FULL PAPERIsomers of C70(CF)2

one-electron reduction of the C70 ACHTUNGTRENNUNG(CF2) isomers provides fur-ther evidence of their remarkable structural flexibility. Thestructural information for the radical anions can be extract-ed from the g-factor and hyperfine coupling (hfc) values(aF), and the influence of the negative charging on the bond-ing between the bridgehead carbon atom is of particular in-terest. As above, we complemented the experiment withDFT calculations of the PES and the hfc.

As follows from Figure 8, negative charging alters theshape of the PES for both isomers. In C70ACHTUNGTRENNUNG(CF2)-II, this re-sults in additional elongation of the distance between thebridgehead carbon atoms up to 2.23 � (Figure 8, bottom).DFT analysis of the net charge distribution shows that thebridgehead carbon atoms bear the highest effective netcharge (calculated relative to the effective charges in theneutral molecules): 1.6–2.5 times higher than on the othercarbon atoms. Apparently, this results in additional Cou-lomb repulsion and, consequently, elongation of the C···Cdistance and emergence of a deeper potential well.

The behaviour of the C70 ACHTUNGTRENNUNG(CF2)-IC� radical anion (Figure 8,top) is much trickier. Unlike the closed neutral molecule,here we see two almost isoenergetic minima separated by

a low barrier (DE�3 kJ mol�1). Thus, the radical anion canswitch between the two comparably stable configurations.The somewhat deeper minimum corresponds to the [6,6]-open configuration of C70 ACHTUNGTRENNUNG(CF2)-IC� (2.17 �) and the secondminimum, only 1 kJmol�1 above, is closed (1.73 �). Thehighest effective net charges in the [6,6]-open configurationare, as in the previous case, on the bridgehead carbonatoms, although the overall net charge distribution is moreuniform. In the case of the [6,6]-closed configuration, how-ever, the highest negative charge density is observed for thecarbon atoms in the polar regions of the molecules and netcharges on the bridgehead carbon atoms are much lower.

The spin-density distribution is also substantially differentfor the [6,6]-closed and [6,6]-open configurations of theC70 ACHTUNGTRENNUNG(CF2)-IC� isomer. This difference enables ESR-basedstructural assignment of the C70ACHTUNGTRENNUNG(CF2)-IC� radical anion.

Spectroelectrochemical measurements were performed ina two-electrode cell in oDCB. The potentials were tuned tothe onset of the generation of the paramagnetic radicalanions C70ACHTUNGTRENNUNG(CF2)C

�. ESR spectra for both isomers exhibit anisotropic three-line signal due to the hyperfine interactionwith the pair of fluorine nuclei (Figure 9, Table 4).

The g-factor for C70 ACHTUNGTRENNUNG(CF2)-IC� is the same as was measuredin C60ACHTUNGTRENNUNG(CF2)C

� (2.0010).[3] The g-factor for C70ACHTUNGTRENNUNG(CF2)-IIC� ishigher (2.0026) and quite similar to those observed formono-functionalised C70RC radicals (2.0024–2.0028).[28] Onthe contrary, the g-factor for C70C

� is markedly lower(2.002).[29] Peak-to-peak line widths are significantly nar-rowed (�0.5 and 0.6 G) relative to the Ih-C60C

� and D5h-C70C�

radical anions,[29] in which small Jahn–Teller distortions andthe associated dynamic effects play a role.

Figure 8. PES cross-sections along C···C stretching coordinates for neutraland radical anion forms of the C70ACHTUNGTRENNUNG(CF2) isomers and their DFT optimisedgeometries.

Figure 9. Experimental (c) and simulated (g) ESR spectra of elec-trochemically generated radical anions of a) C70 ACHTUNGTRENNUNG(CF2)-I and b) C70ACHTUNGTRENNUNG(CF2)-II in Bu4NPF6 (0.1 m) in oDCB.

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T. V. Magdesieva, A. A. Goryunkov et al.

Simulation of the experimental ESR spectra based onLorentz functions (Figure 9) was performed with the as-sumption of symmetry equivalence of the fluorine nuclei inC70 ACHTUNGTRENNUNG(CF2)-II and by application of no constraints in C70ACHTUNGTRENNUNG(CF2)-I. The parameters obtained from the fitting procedure areaF ACHTUNGTRENNUNG(1F)= 1.62 G, aF’ ACHTUNGTRENNUNG(1 F)=1.6 G, Lorentzian linewidth =

0.37 G (isomer I) and aF ACHTUNGTRENNUNG(2 F)=1.01 G, Lorentzian line-width= 0.81 G (isomer II).

To interpret the ESR data, we considered the DFT-calcu-lated spin distributions and the spin-density values at thefluorine nuclei. For a better description of the spin polarisa-tion of the core s orbitals, we employed a larger built-inquadruple-zeta core-valence basis set with a (14s8p3d2f)/ ACHTUNG-TRENNUNG[8s4p3d2f] contraction scheme for the first-row atoms. Theresults shown in Table 4 demonstrate good agreement withthe experimental values and clearly support our conclusionthat C70 ACHTUNGTRENNUNG(CF2)-IC� exists in the open configuration. Thus, one-electron reduction of C70ACHTUNGTRENNUNG(CF2)-I triggers opening of the C�Cbond between the bridgehead carbon atoms.

The aF value appears to be a convenient test for distin-guishing between open and closed configurations of the rad-ical anions of difluoromethylenated fullerenes. Indeed, theaF value correlates with the distance between the fluorineatom and the centres of spin localisation. In the closed con-figurations, the sp3-hybridised bridgehead carbon atoms loseany appreciable spin population, which yields considerableweakening of the hyperfine coupling with the fluorinenuclei. This is the case for [6,6]-closed C70ACHTUNGTRENNUNG(CF2)-IC�. In theopen configuration the unpaired electron is distributed overthe whole p system of the fullerene cage, including thebridgehead carbon atoms. As a result, the aF value is greaterthan that for the closed form. Besides, in the closed configu-ration the distance between the fluorine atoms and the full-erene cage is increased.

The half-life (t1/2) of the radical anions was also measuredin the ESR experiments. The estimated values for theC70 ACHTUNGTRENNUNG(CF2)-IC� and C70ACHTUNGTRENNUNG(CF2)-IIC� radical anions are approxi-mately 7 and 9 min, respectively (Table 4). This is an orderof magnitude longer than the lifetime of C60 ACHTUNGTRENNUNG(CF2)C

� (t1/2 =

58.1�0.2 s, in good agreement with the literature value oft1/2 =51.7�0.5 s, measured with a different apparatus).[3]

The reason for such a difference is not yet understood andneeds further investigation.

Conclusion

Novel isomeric difluoromethylenated derivatives ofC70 with [6,6]-open and [6,6]-closed configurationsmake an important addition to the narrow family ofstructurally characterised CF2 derivatives of ful-lerenes. Notably, [6,6]-open C70 ACHTUNGTRENNUNG(CF2) constitutes thefirst example of a non-hetero [70]fullerene deriva-tive in which bond opening occurs at the most reac-tive bond in the polar region of the cage.

The synthesis of the two isomeric C70ACHTUNGTRENNUNG(CF2) com-pounds made it possible to compare open and

closed adducts of the same fullerene cage. The electron-withdrawing properties of the C70 ACHTUNGTRENNUNG(CF2) isomers were investi-gated by CV. We observed that open structures are betteracceptors, whereas in closed structures the impairment of p

conjugation upon formation of a cyclopropane moietynearly cancels out the acceptor effect of the CF2 group. Fur-ther charging attenuates the differences.

In situ ESR spectroelectrochemical investigation of theC70 ACHTUNGTRENNUNG(CF2) mono-anions supplemented with DFT calculationsproved to be an efficient tool for structural characterisationof the charged species. On the basis of these studies, wehave obtained experimental and theoretical evidence for thefirst example of transitions between the [6,6]-closed and[6,6]-open forms in C70ACHTUNGTRENNUNG(CF2)-I, driven by electrochemicalelectron transfer. Thus, the CF2 derivatives of fullerenesmay have interesting prospects for the design of molecularswitches.

Experimental Section

Negative-ion MALDI mass spectra were recorded with a Bruker Auto-Flex II reflector time-of-flight mass spectrometer equipped with a N2

laser (l =337 nm, 3 ns pulse) and 2-[(2E)-3-(4-tert-butylphenyl)-2-methyl-prop-2-enylidene]malononitrile (DCTB, �98%, Sigma Aldrich) asa matrix. High-resolution MALDI mass spectra were recorded witha Thermo Scientific LTQ Orbitrap XL Hybrid Fourier Transform MassSpectrometer. HPLC analysis was performed on an Agilent 1100 seriesliquid chromatograph fitted with a Cosmosil Buckyprep column (4.6 mmI.D. � 25 cm) at 25 8C. Isolation of individual isomers was carried out ona Waters chromatograph equipped with a Cosmosil Buckyprep column(10 mm I.D.� 25 cm). Toluene (99.8 %, Khimmed, Russia) and hexane(99.7 %, Khimmed, Russia) were purified by distillation. The UV/Visspectra of the toluene solutions were obtained by using a diode array de-tector (DAD) [l=290–950 nm with 2 nm resolution]. 19F and 13C NMRspectra were recorded with a Bruker Avance III spectrometer operatedat 564.7 and 150.9 MHz, respectively. [D4]oDCB was used as the solventwith a small amount of TMS and hexafluorobenzene (dF =�162.9 ppm)as internal standards.

Synthesis of C70 ACHTUNGTRENNUNG(CF2): Sodium difluorochloroacetate (10 equiv) wasadded in two portions, with a 1 h interval, to a solution of C70 (1 equiv,123.6 mg, 0.172 mmol) and [18]crown-6 (cat.) in oDCB (0.5 mg mL�1) andthe reaction mixture was heated at reflux (180 8C). The reaction coursewas monitored by HPLC; the maximum yield of C70(CF2) mono-adductswas achieved after 3 h. Inorganic precipitate (NaCl, residualCF2ClCOONa) was filtered from the reaction mixture and the solventwas evaporated at 90 8C under reduced pressure. The solid precipitate ob-tained was dissolved in toluene and the solution was passed througha column packed with silica gel. MALDI MS analysis indicated a mixtureof C70 ACHTUNGTRENNUNG(CF2)n (n=1–4) was formed. Two dominant mono-adducts

Table 4. Experimental and predicted[a] ESR parameters for C70 ACHTUNGTRENNUNG(CF2) and C60 ACHTUNGTRENNUNG(CF2).

Anion t1/2 [s] g-factor Hyperfine coupling constant (aF) [G]Exp.[b] DFT (PBE/QZ3P)ACHTUNGTRENNUNG[6,6]-open ACHTUNGTRENNUNG[6,6]-closed

C70ACHTUNGTRENNUNG(CF2)-IC� 453�6 2.0010(2)1.62 1.66 0.271.61 1.64 0.17

C70ACHTUNGTRENNUNG(CF2)-IIC� 565�1 2.0026(2) 1.01 0.99 –

C60ACHTUNGTRENNUNG(CF2)C� 58.1�0.2 2.0010(2) 1.65 1.74 –

[a] Predicted by DFT calculations. [b] Experimental data.

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FULL PAPERIsomers of C70(CF)2

C70(CF2)-I (21.9 mg, 17.7 %) and C70ACHTUNGTRENNUNG(CF2)-II (28.3 mg, 22.9 %) were iso-lated by HPLC (elution with toluene, then toluene/hexane =3:2, v/v).

Compound C70 ACHTUNGTRENNUNG(CF2)-I : HPLC (Cosmosil Buckyprep 4.6 mm I.D. �25 cm): tR =5.75 min at a toluene flow rate of 2 mL min�1 and tR =

35.8 min at a toluene/hexane (3:2) flow rate of 1 mL min�1; 19F NMR(564.7 MHz, C6D4Cl2): d =�126.37 ppm (AB q, DdAB =2.55 ppm, JAB =

169.3 Hz, 2 F); 13C NMR (150.9 MHz, C6D4Cl2): d=69.4 (t, 2J ACHTUNGTRENNUNG(C,F)=

23 Hz, 2 C; Ccage-CF2), 102.8 (t, 1J ACHTUNGTRENNUNG(C,F)=285 Hz, 1C; CF2), 126.1, 130.7,131.3, 131.9, 132.0, 133.0, 135.2, 141.37, 141.44, 142.8, 143.6, 144.3, 144.4,144.5, 144.9, 145.4, 145.7, 145.9, 146.3, 146.7, 146.8, 146.9, 147.3, 148.0,148.1, 148.15, 148.2, 148.3, 149.3, 149.4, 149.6, 150.0, 151.0, 152.9 ppm (34signals of expected 32� 2C+4�1 C=36 signals of sp2 carbon cage atoms;two sp2-carbon signals are masked by signals of [D4]oDCB);IR, (KBr,4 cm�1 spectral resolution), n=578, 888, 969, 998, 1082, 1162, 1188, 1209,1239, 1252, 1277, 1315, 1378, 1435, 1460 cm�1; UV/Vis (toluene, l= 290–950 nm): lmax =322 (sh), 376, 450 nm (br); UV/Vis (CH2Cl2): lmax (e): 321(24320), 372 (24160), 445 nm (15662 mol�1 m3 cm�1); MS (MALDI): m/z(%): 890.0 [C70ACHTUNGTRENNUNG(CF2)]� (100), 940.0 [C70 ACHTUNGTRENNUNG(CF2)2]

�(2), 1140.1 [C70-ACHTUNGTRENNUNG(CF2)·DCTB]� (3); HRMS (MALDI): m/z calcd for C71F2: 889.9968;found: 889.9960.

The crystallisation of C70ACHTUNGTRENNUNG(CF2)-I was performed by slow mutual diffusionof saturated toluene solutions of C70ACHTUNGTRENNUNG(CF2)-I and Ni(II) octaethylporphyr-in [NiII ACHTUNGTRENNUNG(OEP)] in a glass capillary to afford orange plate crystals (0.03 �0.03 � 0.01 mm3). Data collection for a single crystal at 100 K was per-formed with a MAR-225 CCD detector by using synchrotron radiation(l=0.83774 �) at the BESSY storage ring (BL14.2, PSF of the Free Uni-versity Berlin, Germany). The structure was solved by usingSHELXS97[30] and anisotropically refined with SHELXL97.[31] Crystaldata for C70CF2·2NiII ACHTUNGTRENNUNG(OEP)·C7H8: Mr =2165.77; monoclinic; space groupP21/c ; a =27.734(2), b=14.7911(5), c =25.084(2) �; b=106.600(4)8 ; V=

9861.0(11) �3; Z=4; Anisotropic refinement with 24041 reflections and1476 parameters yielded a conventional R1(F)=0.094 for 22244 reflec-tions with I>2s(I) and wR2(F2)=0.240 for all reflections. CCDC-950915contains the supplementary crystallographic data for this paper. Thesedata can be obtained free of charge from The Cambridge Crystallograph-ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Compound C70ACHTUNGTRENNUNG(CF2)-II : HPLC (Cosmosil Buckyprep 4.6 mm I.D.�25 cm) tR =6.7 min at a flow rate of 2 mL min�1 in toluene and tR =

43.9 min at a flow rate of 1 mL min�1 in toluene/hexane =3:2; 19F NMR(564.7 MHz, C6D4Cl2): d =�111.9 ppm (s, satellite signals: 1J ACHTUNGTRENNUNG(C,F)=

257.0 Hz, 3J ACHTUNGTRENNUNG(C,F)=37.7 Hz, 2 F); 13C NMR (150.9 MHz, C6D4Cl2): d=

98.2 (t, 3J ACHTUNGTRENNUNG(C,F) =37.4 Hz, 1C; Cacage-CF2), 99.2 (t, 3J ACHTUNGTRENNUNG(C,F)= 38.5 Hz, 1C;

Cbcage-CF2), 111.6 (t, 1J ACHTUNGTRENNUNG(C,F)=257 Hz, 1 C; CF2), 128.3, 129.1, 129.7, 130.9,

132.0, 135.5, 136.9, 137.0, 140.0, 140.9, 141.4, 144.70, 144.72, 144.8, 144.93,144.94, 145.0, 145.1, 145.7, 146.2, 146.78, 146.85, 146.9, 147.2, 147.27,147.3, 147.6, 148.4, 148.6, 149.1, 149.6, 150.1, 150.5, 151.4, 153.4 (35 signalsof expected 33� 2 C+2 � 1C= 35 signals of sp2 carbon cage atoms);IR(KBr, 4 cm�1 spectral resolution): n=531 (w), 576 (w), 637 (vw), 671 (w),726 (w), 801 (s), 869 (w), 991 (m), 1022 (s), 1095 (s), 1143 (m), 1187 (m),1261 (s), 1379 (w), 1415 (m), 1430 (m), 1454 cm�1 (m); UV/Vis (toluene,l= 290–950 nm): lmax =338, 384, 474 nm (br); UV/Vis (CH2Cl2): lmax

(e)=334 (16324), 364 (13153), 382 (16033), 466 nm(10259 mol�1 m3 cm�1); MS (MALDI): m/z (%): 890.0 [C70ACHTUNGTRENNUNG(CF2)]� (100),940.0 [C70 ACHTUNGTRENNUNG(CF2)2]

� (1.3), 1140.1 [C70 ACHTUNGTRENNUNG(CF2)·DCTB]� (7); HRMS (MALDI):m/z calcd for C71F2: 889.9968; found: 889.9959.

Voltammetric experiments : oDCB was stirred over CaH2 for 48 h underargon and then distilled under reduced pressure. The experiments wereperformed in a one-compartment 10 mL cell with a platinum-wire coun-ter electrode (CE) and a Ag/AgCl/KCl (aq) reference electrode (RE)with the use of an IPC-Win potentiostat. The working electrode (WE)was either a Pt or glassy carbon (GC) disk electrode with an active sur-face area of 0.049 or 0.070 cm2, respectively. The concentrations ofC70(CF2)-I and C70 ACHTUNGTRENNUNG(CF2)-II were 2� 10�3 and 1� 10�3

m, respectively;Bu4NBF4 (0.15 m) was used in all experiments as the supporting electro-lyte. All solutions were thoroughly deoxygenated by bubbling Ar gasthrough the solution prior to the experiments and above the solutionduring the measurements. The formal potential of the ferrocene couple

(Fc0/+) versus our RE is about 0.56 V in oDCB/Bu4NBF4. No backgroundcorrection of registered voltammograms was performed.

ESR experiments : oDCB was distilled (3 �) under an argon atmosphereover CaH2. ESR spectroelectrochemical experiments were performedwith a Bruker ESR 300 E spectrometer at rt under an argon atmospherein a thin-layer quartz cell (path length�1 mm). The working electrodewas a Pt net. The counter electrode consisted of a Pt wire (diameter=

0.5 mm). For the reduction of C70 ACHTUNGTRENNUNG(CF2)-I and C70ACHTUNGTRENNUNG(CF2)-II (c=6 � 10�5m

for both isomers) terminal voltages of 3.7 and 3.5 V were applied, respec-tively. Bu4NPF6 (0.1 m) was used as the supporting electrolyte (synthes-ised according to ref. [32]. For reliability, the experiments were repeatedthree times. Simulations were performed with the P.E.S.T. WinSim v.1.02002 software by using the implemented custom LBM1 algorithm.[33] Forcoupling constants see main text. Electrolytes were deareated by argonbubbling.

Quantum chemical calculations : Initial geometry optimisation of theC70(CF2) isomers was carried out at the AM1 level of theory with the useof the Firefly QC package,[34] which is partially based on the GAMESS(US) source code.[35] Final optimisation of molecular geometry (r.m.s.gradient 10�5–10�6 a.e./�), relative energies, adiabatic electron affinities(EA), as well as spin distributions and spin-density values of the radicalanions, were calculated at the DFT level with the use of the PRIRODAsoftware[36] and employment of original TZ2P and QZ3P (for spin-densi-ty values) basis sets and a PBE exchange-correlation functional.[37] TheEA values were corrected (multiplied by 0.94) according to the ratio be-tween the experimental EA value of C70 obtained by photoelectron spec-troscopy measurements[38] and that predicted by DFT (2.765�0.010 and2.95 eV, respectively). Atomic partitions of the charge and spin densitiesare given according to the Hirschfeld method.[39]

Acknowledgements

This work was partially supported by the Russian Foundation for BasicResearch (projects No 12–03–31513, 12–03–31524, 12–03–00615, 12–03–00858), the Ministry of Education and Science of Russia (project NoMD-5540.2013.3) and the Deutsche Forschungsgemeinschaft (project NoDFG Sp 265/25–1). The reported study was supported by the Supercom-puting Center of Lomonosov Moscow State University.[40] We thankMarina V. Polyakova and Andreas Schank for registration of UV/Visspectra and technical help with solvent preparation.

[1] a) J. L. Delgado, P. Bouit, S. Filippone, M. A. Herranz, N. Martin,Chem. Commun. 2010, 46, 4853 –4865; b) A. J. Ferguson, J. L. Black-burn, N. Kopidakis, Mater. Lett. 2013, 90, 115 –125; c) Y. Liang, Z.Xu, J. Xia, S. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010,22, E135 – E138.

[2] a) Medicinal Chemistry and Pharmacological Potential of Fullerenesand Carbon Nanotubes (Eds.: F. Cataldo, T.Da Ros), Springer 2008 ;b) P. Anilkumar, F. Lu, L. Cao, P. G. Luo, J. Liu, S. Sahu, K. N. Tack-ett II, Y. Wang, Y. Sun, Curr. Med. Chem. 2011, 18, 2045 –2059.

[3] A. A. Goryunkov, E. S. Kornienko, T. V. Magdesieva, A. A. Kozlov,V. A. Vorobiev, S. M. Avdoshenko, I. N. Ioffe, O. M. Nikitin, V. Y.Markov, P. A. Khavrel, A. K. Vorobiev, L. N. Sidorov, Dalton Trans.2008, 6886 – 6893.

[4] M. R. Banks, J. I. G. Cadogan, I. Gosney, A. J. Henderson, P. K. G.Hodgson, W. G. Kerr, A. Kerth, P. R. R. Langridge-Smith, J. R. A.Millar, A. R. Mount, J. A. Parkinson, A. T. Taylor, P. Thornburn,Chem. Commun. 1996, 507 – 508.

[5] G. Schick, A. Hirsch, H. Mauser, T. Clark, Chem. Eur. J. 1996, 2,935 – 943.

[6] G. Schick, T. Jarrosson, Y. Rubin, Angew. Chem. 1999, 111, 2508 –2512; Angew. Chem. Int. Ed. 1999, 38, 2360 – 2363.

[7] C. H. Choi, M. Kertesz, J. Phys. Chem. A 1998, 102, 3429 –3437.

www.chemeurj.org � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 17969 – 1797917978

T. V. Magdesieva, A. A. Goryunkov et al.

[8] A. S. Pimenova, A. A. Kozlov, A. A. Goryunkov, V. Y. Markov, P. A.Khavrel, S. M. Avdoshenko, V. A. Vorobiev, I. N. Ioffe, S. G. Sakhar-ov, S. I. Troyanov, L. N. Sidorov, Dalton Trans. 2007, 5322 – 5328.

[9] A. S. Pimenova, A. A. Kozlov, A. A. Goryunkov, V. Y. Markov, P. A.Khavrel, S. M. Avdoshenko, I. N. Ioffe, S. G. Sakharov, S. I. Troya-nov, L. N. Sidorov, Chem. Commun. 2007, 374 –376.

[10] A. J. Gordon, R. A. Ford, The Chemist�s Companion: A Handbookof Practical Data, Techniques, and References, John Wiley & Sons,1973.

[11] M. Keshavarz-K, B. Knight, R. C. Haddon, F. Wudl, Tetrahedron1996, 52, 5149 –5159.

[12] G. A. Wheaton, D. J. Burton, J. Fluorine Chem. 1977, 9, 25 –44.[13] V. Rautenstrauch, H. J. Scholl, E. Vogel, Angew. Chem. 1968, 80,

278 – 279; Angew. Chem. Int. Ed. Engl. 1968, 7, 288 – 289.[14] E. Vogel, S. Korte, W. Grimme, H. Gunther, Angew. Chem. 1968,

80, 279 –280; Angew. Chem. Int. Ed. Engl. 1968, 7, 289 –290.[15] Y. Kabe, H. Ohgaki, T. Yamagaki, H. Nakanishi, W. Ando, J. Orga-

nomet. Chem. 2001, 636, 82– 90.[16] A. M. Benito, A. D. Darwish, H. W. Kroto, M. F. Meidine, R. Taylor,

D. R. M. Walton, Tetrahedron Lett. 1996, 37, 1085 – 1086.[17] A. B. Smith III, R. M. Strongin, L. Brard, G. T. Furst, W. J. Roma-

now, K. G. Owens, R. C. King, J. Am. Chem. Soc. 1993, 115, 5829 –5830.

[18] M. Tsuda, T. Ishida, T. Nogami, S. Kurono, M. Ohashi, TetrahedronLett. 1993, 34, 6911 –6912.

[19] Z. Yinghuai, J. Phys. Chem. Solids 2004, 65, 349 –353.[20] A. F. Kiely, R. C. Haddon, M. S. Meier, J. P. Selegue, C. P. Brock,

B. O. Patrick, G. Wang, Y. Chen, J. Am. Chem. Soc. 1999, 121,7971 – 7972.

[21] B. Li, C. Shu, X. Lu, L. Dunsch, Z. Chen, T. J. S. Dennis, Z. Shi, L.Jiang, T. Wang, W. Xu, C. Wang, Angew. Chem. 2010, 122, 974 –978;Angew. Chem. Int. Ed. 2010, 49, 962 –966.

[22] a) V. P. Gubskaya, L. S. Berezhnaya, A. T. Gubaidullin, I. I. Fain-gold, R. A. Kotelnikova, N. P. Konovalova, V. I. Morozov, I. A. Lit-vinov, I. A. Nuretdinov, Org. Biomol. Chem. 2007, 5, 976 – 981; b) J.Osterodt, M. Nieger, F. Voegtle, J. Chem. Soc. Chem. Commun.1994, 1607 – 1608.

[23] a) C. B. H�bschle, S. Scheins, M. Weber, P. Luger, A. Wagner, T.Korits�nszky, S. I. Troyanov, O. V. Boltalina, I. V. Goldt, Chem. Eur.J. 2007, 13, 1910 –1920; b) P. A. Troshin, R. N. Lyubovskaya, I. N.

Ioffe, N. B. Shustova, E. Kemnitz, S. I. Troyanov, Angew. Chem.2005, 117, 238 –241; Angew. Chem. Int. Ed. , 2005, 44, 234 – 237.

[24] a) S. Stevenson, C. B. Rose, J. S. Maslenikova, J. R. Villarreal, M. A.Mackey, B. Q. Mercado, K. Chen, M. M. Olmstead, A. L. Balch,Inorg. Chem. 2012, 51, 13096 –13102; b) M. Suzuki, Z. Slanina, N.Mizorogi, X. Lu, S. Nagase, M. M. Olmstead, A. L. Balch, T. Akasa-ka, J. Am. Chem. Soc. 2012, 134, 18772 – 18778.

[25] A. B. I. Smith, R. Strongin, L. Brard, G. Furst, W. Romanow, K. G.Owens, R. Goldschmidt, R. C. King, J. Am. Chem. Soc. 1995, 117,5492 – 5502.

[26] S. I. Troyanov, Russ. J. Inorg. Chem. 2001, 46, 1612 – 1616.[27] A. F. Kiely, M. S. Meier, B. O. Patrick, J. P. Selegue, C. P. Brock,

Helv. Chim. Acta 2003, 86, 1140 –1151.[28] R. Borghi, L. Lunazzi, G. Placucci, P. J. Krusic, D. A. Dixon, N. Mat-

suzawa, M. Ata, J. Am. Chem. Soc. 1996, 118, 7608 –7617.[29] C. A. Reed, R. D. Bolskar, Chem. Rev. 2000, 100, 1075 –1120.[30] G. M. Sheldrick, SHELXS-97, Program for Solution of Crystal

Structures from Diffraction Data, Universit�t Gçttingen, Germany1997.

[31] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Re-finement, Universit�t Gçttingen, Germany 1997.

[32] S. D�mmling, E. Eichhorn, S. Schneider, B. Speiser, M. W�rde,Curr. Sep. 1996, 15, 53 –56.

[33] D. R. Duling, J. Magnetic Res. B 1994, 104, 105 –110.[34] A. Granovsky, http://classic.chem.msu.su/gran/gamess/index.html.[35] M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S.

Gordon, J. J. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. Su,T. L. Windus, M. Dupuis, J. A. Montgomery, J. Comput. Chem. 1993,14, 1347 –1363.

[36] D. N. Laikov, Chem. Phys. Lett. 1997, 281, 151 – 156.[37] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77,

3865 – 3868.[38] X.-B. Wang, H. K. Woo, X. Huang, M. M. Kappes, L. S. Wang, Phys.

Rev. Lett. 2006, 96, 143002.[39] F. L. Hirshfeld, Theor. Chim. Acta 1977, 44, 129 –138.[40] V. Sadovnichy, A. Tikhonravov, Vl. Voevodin, V. Opanasenko in

Contemporary High Performance Computing: From Petascaletoward Exascale (Ed.: J. S. Vetter), CRC Press, Boca Raton, USA,2013, pp. 283 –307.

Received: July 26, 2013Published online: November 18, 2013

Chem. Eur. J. 2013, 19, 17969 – 17979 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 17979

FULL PAPERIsomers of C70(CF)2