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FLUORINATED FULLERENE DERIVATIVES: SYNTHESIS,

STRUCTURE, AND ELECTRONIC PROPERTIES

Alexey A. Goryunkov, Marina G. Apenova, Eugenia V. Borkovskaya, Victor A. Brotsman,

Nikita M. Belov, Ilya N. Ioffe, Vitaliy A. Ioutsi, Natalia S. Lukonina, Vitaliy Yu. Markov,

Alexey V. Rybalchenko, Olesya O. Semivrazhskaya

Chemistry Department, Lomonosov Moscow State University, Leninskie Gory, 1, 119991,

Moscow (Russia)

Abstract

The structural features and synthetic procedures for three main classes of the fluorinated

fullerene derivatives, C60Rn, R=F, CF2, and CF3 are discussed. The electronic properties of these

compounds are largely determined by the number of addends and their attachment pattern. New

approach with low-computational cost for predictions of the isomeric composition of fullerene

polyadducts was developed and one was verified on the example of perfluoroalkylated

fullerenes.

Introduction

Fullerenes and their derivatives have been proposed to be employed in the design of

materials for a considerable variety of applications including nano- and molecular electronics

(photovoltaic cells, optical limiters, organic light-emission diodes and field-effect transistors)(1-

3) and medicine (antioxidants, neuroprotective agents, antimicrobial agents, agents for

photodynamic therapy and magnetic resonance imaging)(4). Their value in electronics stems

from their spherically conjugated π-electron system that gives rise to n-type conductivity, low

reduction potential and low reorganization energy in electron-transfer reactions. In addition,

there is a broad selection of fullerene derivatives suitable for solution-based technology.

However, there is still a need for improvement and fine tuning of their electronic properties,

which requires development of the regioselective techniques of exohedral functionalization as

well as skeletal transformation.

Contrary to the anticipated enormous compositional and isomeric complexity to arise from

concurrence of a multitude of reaction sites, these fullerene derivatives can be prepared as

relatively simple mixtures of products or even as individual isomers. According to our extensive

quantum chemical DFT surveys, the observed selectivity can be explained by both

thermodynamic and kinetics factors. We found that the addition motifs of the initially attached

groups controls the regioselectivity of further derivatization. Efficient and reliable methods to

predict the structure of the diversely functionalized fullerene compounds are presented below.

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Synthetic pathways, structural features and electronic properties of fluorinated fullerenes

derivatives

Enhancement of electron withdrawing

properties of a fullerene cage is readily and

efficiently achievable via derivatization with

fluorinated moieties. To date, three rather broad

and extensively explored classes of the fluorine-

containing fullerene derivatives are available.

These are fluorofullerenes (e.g. C60Fn, n=2–

48),(5-9) trifluoromethylfullerenes (C60(CF3)2n

and C70(CF3)2m,(10-12) n=1–9 and m=1–10),

and difluoromethylenofullerenes (C60(CF2)n,

C70(CF2)n, n=1–3).(13-15) Most of these

compounds exhibit enhancement the electron

withdrawing properties and processability due

to high solubilities in the common organic

solvents and capability to sublime without

decomposition (except for

difluoromethylenofullerenes).

The methods for the synthesis of polyfluorinated derivatives of fullerenes can be divided

into three groups: (i) fluorination of fullerene by molecular fluorine, (ii) fluorination by fluorides

of transitional metals in high oxidation states and (iii) fluorination of chloro- or bromofullerenes.

(16, 17) Fluorination of C60 with molecular fluorine yields two D3- and S6-symmetrical isomers

C60F48,(7) whereas heating of the C60 with MnF3 under the dynamic vacuum conditions leads to

mixture of the three T-, C3-, and C1-symmetrical isomers C60F36.(9) Low fluorinated fullerenes

C60Fn, n=2, 4, 6, 18, and 20 were prepared in the reaction of the C60 with complex fluorides like

K2PtF6 and KMnF4 during heating under the dynamic vacuum conditions.(5, 6, 8, 18, 19)

According to X-Ray single crystal and 19

F NMR data the addition of the fluorine atoms occurs at

C-C double bonds with formation of the continues patterns of ortho-attached atoms (see Fig 1).

High fluorination degree of the fullerenes increases the electron affinities (EA) of the

compounds. So, measured EA values for C60F2 and C60F48 are 2.74(7)(20) and 4.06(3)(21) eV,

whereas EA of C60 is 2.683(8) eV.(22) The same trend was demonstrated by electrochemical

studies of fluorofullerenes.(23-25) The cathodic peaks (Epc) of the first reduction potentials of

Fig. 1. Schlegel diagrams of fluorofullerenes

C60Fn, n=2, 4, 6, 18, 36, and 48 (a–f).

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C60F2, C60F4, C60F18, C60F36, and C60F48 are anodic shifted relatively Epc of C600/–

couple for

values of 0.04, 0.08, 0.26, 0.63, and 1.38 V, respectively. The electron transfer occurs

electrochemically and chemically irreversible: defluorination of the compounds is observed

during reduction. Therefore applicability of the fluorofullerenes for doping of semiconducting

polymers is restricted by their degradability during electron transfer.

There are about 70 structurally characterized trifluoromethylfullernes, C60(CF3)2n and

C70(CF3)2m, n=1–9 and m=1–10.(10-12) The available synthetic techniques make it possible to

control the average number of addends. Thus, when reacting C60 or C70 with silver(I)

trifluoroacetate under the dynamic vacuum conditions, the number of the CF3 addends may be

varied from a minimum of two up to maximum of 20–22.(26, 27) Trifluoromethylation under the

atmosphere of CF3I allows formation of molecules with intermediate to high degrees of addition.

In particular, the reaction with CF3I in a sealed ampoule shifts the composition toward higher

functionalized derivatives C60/70(CF3)n, n = 12–20,(28, 29) while in a flow of CF3I, which

conditions enable rapid removal of the products from the reaction zone, one generally obtains

C60/70(CF3)n compounds within the range of n = 6–14.(30, 31) Further, lower functionalized

trifluoromethylated fullerenes can be prepared via thermal treatment of the higher functionalized

ones. For example, sublimation of products of the fullerene/CF3COOAg reaction (n = 2–20)(10,

27, 32) or heating the C60/70(CF3)n, n = 2–20, in a mixture with pristine fullerene(33-35)

facilitates conversion into lower CF3-derivatives.

Attachment moieties for several trifluoromethylfullerenes are shown on Fig 2. One can

see, attachment moieties form a ribbon of edge-sharing para-C6(CF3)2 and meta-C6(CF3)2

hexagons. Compact arrangement of the groups is sterically prohibited because of bulkiness of

CF3 groups, that leads to a greater isomeric variety of trifluoromethylfullerenes in comparison

with fluorofullerenes. It is important, that attachment patterns dictates the electronic withdraw

properties of the trifluoromethylfullerenes. For example, family of C70(CF3)n demonstrates

variability of the first reduction potentials within range from –0.11 to 0.34 V vs C700/–

.(36) At

contrary to fluorofullerenes, the reduction of vast majority of trifluoromethylfullerenes is

electrochemical and chemical reversible. However, variation of the reduction potentials is

counterintuitive. So, compound with 10 CF3 groups, C70(CF3)10 (Fig 2b, 1070-I), is weaker

oxidizing agent comparatively to pristine C70 (Epc –0.11 V vs C700/–

couple), whereas dramatic

anodic shift of the reduction potential (0.34 V vs C700/–

couple) is observed for C2-symmetrical

C70(CF3)8 (870-II). Isomeric Cs-symmetrical C70(CF3)8 (870-I) differs position of only one CF3

group. But it results in cathodic shift of the reduction potential for 0.3 V (0.04 V vs C700/–

couple). This behavior is correlated with DFT estimated LUMO energies.(36)

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Fig. 2. Schlegel diagrams of trifluoromethylfullerenes (a) C60(CF3)n, n=2–8, and (b) C70(CF3)m,

m=2–10.

Thus, a broad family of trifluoromethylated fullerenes bearing from 2 to 20 CF3 groups

per fullerene cage and manifold addition patterns enlarges the library of the fullerene-based

compounds with “tuneable” electronic properties and attracts attention as prospective building

blocks for organic electronics useful as electron acceptors due to chemical stability during

electron transfer.

Third class of the fluorinated fullerenes

derivatives is difluoromethylene fullerenes. These

compounds are prepared in the reaction of the

fullerene with :CF2 carbene generated in situ by

thermolysis of alkaline difluorochloroacetates.(13-15)

Insertion of difluoromethylene fragment into a [6,6]-

bond leads to cleavage of the said bond with

associated redistribution of the π-electron bonding in

the cage (see Fig 3a). The resulting so-called “[6,6]-

open” adducts contain 1,6-

difluoromethano[10]annulene moeity where the sp2

hybridization of the bridgehead carbon atoms is

retained and, accordingly, all carbon atoms of the

parent fullerene remain within the spherical π-

system, though its connectivity slightly decreases.

Fig. 3. Molecular structures of C60(CF2) (a),

cis-2-C60(CF2)2 (b), and isomers C70(CF2)-I

and -II (c, d).

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These compounds with open [6,6] C–C bond (homofullerenes) are quite unusual among fullerene

exohedral derivatives. Though still very little is known about the reactivity of homofullerenes, it

is clear that the preservation of the spherical π-system of fullerenes with all carbon atoms

remaining sp2 hybridized makes homofullerenes more sensitive than methanofullerenes to any

sort of further functionalization. Our recent electrochemical investigation of the [6,6]-open

C60(CF2) and C70(CF2) demonstrated that the electron affinity of this compound is well above the

values obtained for the other known C60 adducts with one bivalent or two monovalent

addends.(14, 15)

Genetic protocol for identification of favorable isomers of trifluoromethylfullerenes

The feature of the fullerenes is a great

number of reaction sites with similar reactivity

leading to different isomers. Therefore one can

await a exponential growth of the isomeric

complexity of fullerene polyadducts (see Fig 4).

Nevertheless trifluoromethylation lead to

formation of the mixture of the products with

narrow isomeric compositions. The general

observations regarding the trifluoromethylation of

fullerenes (some energetically favorable isomers

remain missing, trifluoromethylation of pre-

isolated individual isomers mostly affords

products of direct addition without

rearrangements, many structural relations can be

found that connect various isolated structures)

indicate that the key role in this process is being

played by direct consecutive addition of CF3

groups. The relative formation energies of vast majority of experimentally isolated

trifluoromethylated fullerenes fall within the range of 0–30 kJ∙mol–1

. Importantly, many of these

compounds are apparent precursors for certain isolated higher trifluoromethylated molecules.

This led us to a general assumption, which we exploit below, that each consecutive step of CF3

addition is energetically controlled.

In an effort to understand whether the composition of trifluoromethylation products is,

notwithstanding the rearrangements, really determined by the energetic factors we have

Fig. 4. Comparison of numbers of total

possible isomers C60/70(CF3)n (n = 1–18/20)

and the isomers within the framework of

our kinetic model.

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performed massive quantum chemical calculations on the formation energies of the possible

molecular and radical products of the consecutive CF3 addition to fullerene. In order to decrease

the number considered structures, the following genetic algorithm was used. On the first step, we

considered the complete sets of the isomeric C60(CF3)2 bisadducts and C70(CF3)• radicals at the

semi-empirical AM1 level. Then, the isomers that fell within a predetermined range of relative

AM1 energy 0–ΔE (50–80 kJ∙mol–1

to ensure completeness of our survey) were used as

precursors, and all possible closed-shell C60/70(CF3)2 products of second addition to any of them

were generated. The same procedure of energy-based selection and subsequent addition was then

repeated for each consecutive passage from C60/70(CF3)k to C60/70(CF3)k+1 up to 18 and 20 CF3

groups added per C60 and C70 cages, respectively. Finally, the molecular geometry and the

relative energies of the above obtained sets of C60/70(CF3)n isomers with relative energy within

20–90 kJ∙mol–1

AM1 energy gap were refined at the DFT level of theory. It turns out that the

number of structures under consideration rapidly becomes dramatically lower than the total

number of possible isomers, as demonstrated in Fig. 4. The number of structures considered at

the AM1 level did not exceed 3·104 for any particular degree of addition, which made it possible

to accomplish these preliminary semiempirical calculations with the use of several conventional

PCs.

The above approach appears to be highly predictive for both C60 and C70 as it captured all

the experimentally isolated C60/70(CF3)n isomers with n up to 18/20, even though it ignores the

possibility of isomerization. On the other hand, it does not provide any means to filter off those

stable isomers that were not found to form (or to accumulate). A useful hint comes from a

consideration that our procedure missed none of the synthesized compounds despite there might

exist some stable structures that have no sufficiently stable precursors. This suggests that some

energy-based criteria act on each consecutive stage and, accordingly, they must be carefully

considered stepwise rather than globally.

Kinetic model based on the BEP relations

The selectivity in formation of particular stable isomers of C60/70(CF3)n can be

rationalized using the Bell-Evans-Polanyi (BEP) principle. The BEP model states that the

activation barriers (and, consequently, the rates) for a given type of processes must correlate with

their enthalpies. When applied to our case of sequential trifluoromethylation, this means that (i)

CF3 addition to a given structure will follow the most exothermic routes and (ii) accumulation of

each particular isomer will depend on exothermicity of its formation and of its further

trifluoromethylation. Perhaps, more important should be the slower stages of addition to closed-

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shell molecules yielding radicals with odd number of addends while their subsequent

recombination with yet another CF3 group will proceed much faster.

Fig. 5. Most energetically favorable pathways from C70 to C70(CF3)2. Hereinafter we indicate the

corresponding enthalpies of CF3 addition (kJ∙mol–1

, DFT), numberation is given in the order of

relative DFT energy (shown in parentheses, kJ·mol–1

). Roman numerals indicate the

experimentally obtained compounds. The dotted arrows indicate lower-rate processes.

As an introductory example, we consider consecutive addition of two CF3 groups to

fullerene C70 (see Fig. 5). There are five nonequivalent carbon atoms in the D5h-C70 cage, so one

can obtain five different radical species ·C70CF3. Four addition enthalpies that correspond to the

structures 1170

–1470

are almost indistinguishable while the fifth alternative turns out to be

significantly less favorable. Thus, according to the BEP principle, radical intermediate 1570

can

be discarded. The addition of further CF3 group to the intermediates 1170

–1470

gives 136 possible

structures C70(CF3)2; however, only those designated 2170

–2570

fall within a range of 30 kJ·mol–1

.

Furthermore, clearly preferable is formation of 2170

, 2270

, 2370

. Indeed, synthetically available are

only 2170

and 2270

also known in the literature as 270

-II and 270

-I, respectively. The third one,

2370

, shows higher reactivity towards further addition and its motif can be found in many higher

trifluoromethylated derivatives of C70. Such considerations make it possible to track the whole

sequence of structures up to C70(CF3)10 consisting of isomers with relative energy less then 30

kJ·mol–1

.

Consecutive trifluoromethylation of C60 can be rationalized on the same basis as above.

In fig. 6 we exemplify this with the most likely kinetic pathways between C60 and C60(CF3)4.

While addition of the first CF3 to C60 produces only one possible radical intermediate, C60(CF3)2

already has 23 possible isomers. However, the para- or 1,7-isomer finds no competition: the next

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stable 1,9-C60(CF3)2 (with ortho-addition) is founds 33 kJ·mol–1

higher, and 1,7-C60(CF3)2 is the

only experimentally available product with two CF3 groups.

Fig. 6. Major pathways of the consecutive trifluoromethylation of fullerene C60. The

corresponding enthalpies of ·CF3 addition as well as relative energies of isomers (values in

parentheses) are indicated (kJ∙mol–1

, DFT). Roman numerals indicate the experimentally

obtained compounds. The dotted arrows indicate lower-rate processes.

The course of further addition is illustrated on Fig. 6. The most energetically preferable

addition of the third CF3 group occurs in the para position, resulting in C60(CF3)3· 31 radical.

Formation of the next by energy isomers 32–36 are less preferable for 13–20 kJ·mol–1

.

Nevertheless attachment of the forth CF3 group to these isomers lead to experimental found

isomers of C60(CF3)4 4-II, 4-III, and 4-I (latter one was observed as an oxide, C60(CF3)4O,

because of high reactivity of fulvene moiety). According to BEP principle, the rate limiting stage

is formation of C60(CF3)3· radical species again, whereas addition of the fourth CF3 group will be

easily occurs.

In discussing the highest degrees of addition, 18 CF3 groups, the key role direct

consecutive addition of CF3 groups should be used with caution and the isomerization processes

cannot be neglected at these stages. First of all there are the direct experimental data of the

rearrangement of C70(CF3)12 isomers at trifluoromethylation.(37) Possible pathways of the

formation of C3v-C60(CF3)18 provided direct addition CF3 groups are given on fig. 7. In the

presented cases the isomers with high relative energy (up to 52 kJ·mol–1

) can not be excluded

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from the sequence of structures up to abovementioned compounds. Such isomers never were

found in experiments and their absents give additional proof in favor of rearrangements.

Theoretical consideration states that the activation energy of the CF3 rearrangements became less

when the energy difference of isomers increases and can be considered as a reason of elimination

isomers with high relative energies.(38, 39)

Fig. 7. Possible pathways of the formation of C3v-C60(CF3)18 not involving rearrangements of

CF3 groups. The reaction enthalpies and the relative energies (kJ·mol-1

, DFT) are shown.

Conclusions

The richly branched process of fullerene trifluoromethylation requires careful analysis in

order to achieve good quantitative predictability. However, the energetic correlations in kinetic

behavior simplify the task. We found that the purely thermodynamic model affords at least

satisfactory prediction of the results even for evidently non-equilibrium syntheses. Still, such

simplified description is imperfect, and the most problematic aspect is hindered formation of

certain kinetically inaccessible isomers or, on the contrary, fast conversion of certain structures

that are kinetically labile towards further trifluoromethylation. These special cases can be

identified by means of detailed kinetic modeling based on the BEP approach that includes

stepwise energetic analysis of the possible trifluoromethylation sequences. The BEP survey

makes it possible to explain the discrepancies of thermodynamic modeling and the experimental

results. To eliminate such discrepancies in practice, one can employ additional post-annealing of

the trifluoromethylated products, which brings the trifluoromethylated fullerene mixtures closer

to equilibrium and promotes formation of otherwise inaccessible stable isomers, possibly

through rearrangements in the shells of CF3 addends.

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The modeling approaches developed in this work are quite general and may be thus

applied to other processes of stepwise functionalization of fullerenes. However, necessary

alterations are to be introduced in accordance with the reaction mechanisms (e.g. nucleophilic

rather than radical) and the nature of the intermediates.

Experimental

Initially, molecular geometry of the isomers was optimized by TINKER 4.2 molecular

mechanic package with MM2 parameter sets.(40) Further, preliminary geometry optimization for

these molecules was carried out at the AM1 level of theoryPreliminary geometry optimizations

were carried out sequentelly by molecular mechanics with MM2 force field with at the AM1

level of theory with the use of Firefly QC 8.0 package(41) partially based on the GAMESS (US)

software.(42) Single point DFT (spDFT) calculations of the isomers within the AM1 energy

range of 20–90 kJ∙mol–1

, and final optimization of those of them that fell within the spDFT

energy range of 20–50 kJ∙mol–1

were performed with the use of PRIRODA package.(43) The

PBE exchange-correlation functional(44) and the built-in TZ2P-quality basis set were used.

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