Synthesis, Isolation and Structures of Trifluoromethylated Fullerenes D2-C76, C76(1)(CF3)10–18

Article abstract of DOI:10.1002/asia.201800774

High-temperature trifluoromethylation of fullerene C₇₆ chlorination products followed by HPLC separation of C₇₆(CF₃)_n derivatives resulted in the isolation and X-ray structural characterization of thirteen C

Full text of DOI:10.1002/asia.201800774

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Synthesis, Isolation and Structure of Trifluoromethylated
Fullerene D2-C76, C76(1)(CF3)10-18
Authors: Nadezhda B. Tamm, Daria V. Ignat'eva, Leonid A. Aslanov,
and Sergey Troyanov
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To be cited as: Chem. Asian J. 10.1002/asia.201800774
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Chemistry - An Asian Journal
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DOI: 10.1002/asia.200((will be filled in by the editorial staff))
Synthesis, Isolation and Structure of Trifluoromethylated Fullerene D2-C76,
C76(1)(CF3)10-18
Nadezhda B. Tamm, Daria V. Ignat’eva, Leonid A. Aslanov, and Sergey I. Troyanov*[a]
Abstract: High-temperature trifluoromethylation of fullerene C76
chlorination products followed by HPLC separation of C76(CF3)n
derivatives resulted in the isolation and X-ray structural
characterization of thirteen C76(1)(CF3)n compounds including
nine new isomers such as one isomer of C76(1)(CF3)10, two
C76(1)(CF3)12, three C76(1)(CF3)14, one C76(1)(CF3)16, and two
isomers of C76(1)(CF3)18. Dependent on their addition patterns,
C76(1)(CF3)n isomers are divided into three subgroups and
discussed in terms of trifluoromethylation pathways and relative
formation energies.
process was used which included high-temperature chlorination
with SbCl5 followed by high-temperature trifluoromethylation of
the chlorination products with gaseous CF3I. It was suggested that
the formation of the C76(NC2) cage occurred upon chlorination
while trifluoromethylation transforms fullerene chlorides into CF3
derivatives without any change of the cage connectivity.
The formation of mixed CF3/F derivatives was primarily
ascribed to the catalytic influence of antimony compounds which
can be present in the system even after the removal of
SbCl5/SbCl3 by sublimation at elevated temperatures. To check
this
assumption,
a
separate
experiment
on
chlorination/trifluoromethylation of C76 was carried out with the
only difference that the chlorination was performed at lower
temperature, 140 °C instead of 340 °C in ref. [7c], while the
following operations were performed at the same experimental
conditions. The following HPLC separation of CF3 derivatives,
crystallization of the isolated fractions, and structure
determination by single crystal X-ray diffraction revealed the
formation of numerous C76(CF3)n compounds with the carbon
cage connectivity of the starting IPR C76(1) fullerene. No CF3 or
CF3/F derivatives of non-IPR C76 were found among the
trifluoromethylation products thus excluding the catalytic
influence of Sb compounds on the formation of mixed CF3/F
derivatives. At the same time, the isolation and structure
determination of many new CF3 derivatives prompted us to revisit
the structural chemistry of C76(1)(CF3)n compounds.
Trifluoromethylated fullerenes represent the most studied
derivatives of fullerenes including C60, C70, and individual
isomers of higher fullerenes.[1a] These easily accessible and
thermally stable compounds possess high electron affinities which
allows one to consider them as potential components of energy
storage systems. Synthesis, isolation and structural investigation
of trifluoromethylated derivatives allowed identification and
reactivity studies of several IPR isomers of higher fullerenes C78–
C96.[2]
Fullerene C76 may exist in form of two isolated-pentagon-
rule (IPR) isomers, D2-C76(1) and Td-C76(2),[3] but only the former
can be extracted from the fullerene soot. The chemistry of D2-
C76(1) includes several compounds obtained by Bingel or Diels-
[4]
[5]
Alder reactions
as well as many chloro
and CF3
derivartives.[6] Besides, isomer C76(1) provided the first example
In this communication, we report the synthesis and isolation
of C76(1)(CF3)10-18 compounds whose molecular structures have
been reliably determined by X-ray diffraction. The comparative
topological analysis of the addition patterns and formation energy
calculations allowed us to suggest the structural classification of
C76(1)(CF3)n and possible pathways of their formation in the
course of trifluoromethylation.
Ca. 10 mg of fullerene C76 was placed into a quartz ampoule
and 1 mL of SbCl5 was added. The mixture was heated at 140 °C
under reflux of SbCl5 for 20 h. The reaction mixture was then
heated at 150 °C in vacuum to remove SbCl5 and SbCl3. Ca. 1 mL
of CF3I was condensed into the ampoule under cooling and the
ampoule was evacuated and sealed off. Trifluoromethylation was
carried out at 450 °C for 3 h under the pressure of gaseous CF3I of
ca. 6 atm affording a mixture of CF3 derivatives as a brown-
colored residue in the hot zone and an orange colored sublimate in
the colder zone of the ampoule. According to MALDI TOF mass
of chlorination-promoted cage transformations by Stone-Wales
[3]
rearrangements (SWRs, i.e., CC bond rotations by ca. 90
)
which resulted in a non-IPR derivative.[7a] At present, three
18917
18387
different non-IPR C76 cages,
C ,
76
C , and non-classical
76
(NC) C76(NC2) containing two heptagons, are known to form as a
result of high-temperature chlorination. Whereas two former
cages were identified as chlorides,[7a,b] the non-IPR C76(NC2) was
obtained in the form of CF3 and CF3/F derivatives.[7c] Two-step
[a]
Dr. N. B. Tamm, Dr. D. V. Ignat’eva, Prof.Dr. L. A. Aslanov, Prof. Dr. S.
I. Troyanov
Chemistry Department, Moscow State University
Leninskie Gory, 119991 Moscow (Russia)
Fax: (+007) 495-9391240
E-mail: stroyano@thermo.chem.msu.ru
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.2018.....
1
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10.1002/asia.201800774
Chemistry - An Asian Journal
spectrometric analysis, both products possessed similar
composition, C76(CF3)n,, with n ranging from 10 to 18 for the
residue and from 12 to 18 for the sublimate (see the Supporting
Information for more details).
isomer notation used for six isomers in refs. 6b,c. For a sake of
brevity, C76(1)(CF3)n isomers are denoted as N-M with N (an
Arabic number) indicating the number of the attached CF3 groups
and M (a Roman number) corresponding to the consecutive
number of isomer of a given composition. Thus, the isomers
reported in the previous X-ray diffraction studies are abbreviated
as 14-I, 16-I, 16-II, 16-III, 18-I and 18-II. The present
investigation allowed structural confirmation for isomers 14-I
(two crystal structures), 16-II, 16-III, and 18-I. The newly isolated
and structurally characterized isomers have notations 12-I (76-12-
2 in ref. 6a) and 12-II, 14-II, 14-III, 16-IV, 16-IV), 18-III, and 18-
IV, with two latter being present in the same crystal structure.
Isomer 16-IV is present in the crystal structure as the major
component (72%) in the mixture with its epoxide which, most
probably, formed as an oxidation product during crystals growing
on air. In addition, isomer C76(1)(CF3)10 (10-II) was isolated from
trifluoromethylation products of a higher fullerene mixture (MER
Corp.) analogously to the process described in ref. 2c. The
notation 10-I is used for the compound isolated and characterized
by 19F NMR spectroscopy in ref. 6a (76-10-1).
Selected molecular structures of the new C76(1)(CF3)n are
presented in Figure 1. For comparison purposes, the projections
are given along the two-fold axes of D2-C76(1) carbon skeleton,
though the most molecular structures are asymmetric. The
comparison of the three molecular structures, 10-II, 12-II, and 14-
III, indicates that simple addition of two CF3 groups occurs as the
number of attached groups increases from 10 to 12 and from 12 to
14. Similar relations can be seen when structures 16-IV and 18-III
are compared with one another.
Both trifluoromethylation products were dissolved in n-
hexane and were subjected to HPLC separation in n-hexane using
a semi-preparative Buckyprep column (10 × 250 mm, Nacalai
Tesque Inc.) and a flow rate of 4.6 mL min−1 (see the SI for more
details). Several fractions of both separation routes gave small
crystals which were investigated by X-ray diffraction with the use
of synchrotron radiation (selected crystallographic data are given
in the SI). Most of the structural determinations revealed new
C76(1)(CF3)10-18 isomers whereas some others identified the
isomers known from previous studies crystallized with different
solvated molecules or possessing different crystal packing.
Detailed characterization of the chlorination products was
not performed because the two-step synthesis was aimed to
disclose the influence of antimony compounds on the composition
of trifluoromethylation products. It was concluded that
chlorination proceeded to a considerable extent because a large
amount of SbCl3 was observed as crystalline deposition on the
walls during gentle heating of the chlorination product in vacuum.
It is known that chlorination of C76(1) performed with the use of
weaker chlorination agents results in the formation of C76(1)Cln. It
was reported that chlorination with a (TiCl4 + Br2) mixture at
150 °C produced C76(1)Cl18,[5a] whereas the reaction with ICl at
180 °C afforded C76(1)Cl34.[5b] In our former experiments on
chlorination of C76(1) with VCl4 at 200-250 °C or PCl5 at 220-
240 °C C76(1)Cln compounds were isolated with n ranging from
24 to 32.[5c] Therefore, it was assumed that the chlorination of
C76(1) with SbCl5 at comparatively low temperature of 140 °C
produces only C76(1)Cln compounds which was proven by the
following trifluoromethylation and structure determination of CF3
derivatives. However, no structural similarity between the
starting C76(1)Clm and final C76(CF3)n can be expected because of
different sterical demands of Cl atoms and CF3 groups.
A more detailed comparison of the relations between the
isomeric structures and the structure-substructure relations
between the compounds of different composition is presented in
Figure 2 by means of Schlegel diagrams. Isomeric structures of
each composition are compared by the number of common
position of attached CF3 groups on the C76(1) carbon cage (the
numbers above thin black two-end arrows) and also by relative
[8]
The addition patterns of several C76(1)(CF3)n compounds
were reported previously as a result of single crystal diffraction or
19F NMR spectroscopic studies.[6] The latter method allowed a
reliable identification of two compounds with ten and twelve
attached CF3 groups (76-10-1 and 76-12-2) and a hypothetical
identification of some isomers with six and eight CF3 groups.[6a]
Previous X-ray diffraction studies resulted in unambiguous
identification of six C76(1)(CF3)n compounds with n = 14 – 18
which were divided into two structural groups on the basis of their
addition patterns.[6b,c] In the discussion below, we follow the
formation energy calculated at the DFT level of theory
and
given in the parentheses. Thick green arrows indicate that two
isomers are connected by a structure/substructure relation, i.e. by
the simple addition of two CF3 groups (and one additional
position interchange in 18-II as compared with 16-III). Such kind
of relations can reflect the formation pathway of CF3 isomers in
the course of trifluoromethylation. In Figure 2, the whole set of 16
isomers with known addition patterns is divided by black lines
into three subsets which contain isomers with similar structural
features.
Figure 1. Top and side views of the selected C76(1)(CF3)n molecules given along orthogonal C2 axes of the D2-C76(1) carbon cages. The abbreviations for individual isomers
include the number of the attached CF3 groups (Arabic numerals) and a consecutive number of an isomer of this composition (Roman numerals). In the case of 16-IV, the minor
component.( 28%), C76(CF3)16O epoxide, is shown whereas the main component (72%) is represented by the non-oxidized structure.
2
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10.1002/asia.201800774
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Figure 2. Schlegel diagrams of fifteen C76(1)(CF3)10-18 isomers structurally investigated in this work and isomer C76(CF3)10 reported in ref. [6a]. Only the number of attached CF3
groups and the sequential number of an isomer of a given composition are indicated. Relative formation energy for each composition is given in parentheses. Cage pentagons are
highlighted with red. Black circles denote the positions of CF3 attachments. Isolated and nearly isolated C=C bonds and benzenoid substructures are indicated. Green arrows
connect isomers with n and n+2 attached CF3 groups which differ by the positions of only two added groups (and the change of one position in for a pair 16-III / 18-II). The
figures above two-end arrows indicate the number of common attachment positions. The crosses on the diagram of 10-II indicate the positions of three two-fold axes of D2-C76(1).
[6a]
Two C76(1)(CF3)10 isomers, 10-I
and 10-II, differ by the
Simple additions of two CF3 groups to isomers 12-I and 12-II
result respectively in the formation of isomers 14-II and 14-III,
the latter retaining one cage pentagon free from CF3 attachment.
Whereas the most stable isomer 14-IV also contains one non-
occupied cage pentagon, isomer 14-I possesses one CF3
attachment in the position of triple-hexagon junction (THJ).
Attachments at THJs are generally not favorable due to small
pyramidalization angles but they occur in some cases when
stabilization effects are prevailing as, in isomer 14-I, due to
formation of isolated butadiene-like substructure on the cage.
Remarkably, this feature is also present in the addition patterns of
16-II and 16-III of the same subset of closely related structures.
Furthermore, the trifluoromethylation pattern of isomer 18-II
contains the second attachment at THJ which is most probably
stabilized by the formation of benzenoid substructure though the
formation energy of 18-II is rather high when compared with
other known C76(1)(CF3)18 isomers. Additional stabilizing factors
in isomers 16-II, 16-III, and 18-II is the presence of several
isolated or nearly isolated C=C bonds. Surprisingly, both
structurally related isomers 16-III and 18-II contain one non-
occupied cage pentagon in their addition patterns.
positions of only two CF3 groups. Isomers 10-I is C2 symmetric
and is 11 kJ mol1 more stable than asymmetric isomer 10-
II .Note that both isomers can be converted into isomer 12-II by
the simple addition of two CF3 groups. A remarkable feature of
the addition pattern of 12-II is the presence of two cage pentagons
which are not occupied by CF3 groups (highlighted by dark red).
In most known fullerene(X)12 structures, all 12 pentagons are
occupied by X groups/atoms,[2,6a,9] whereas few exceptions can be
accounted for by the formation of stabilizing aromatic
substructures or isolated double C=C bonds (the case of 12-II) on
the carbon cage. The addition pattern of the C2 symmetric isomer
12-I is characterized by an uniform distribution of CF3 groups on
12 pentagons. Its addition pattern corresponds exactly to that of
isomer 76-12-2 which was proposed on the basis of 19F NMR
spectroscopy.[6a] Although nearly isoenergetic, isomers 12-I and
12-II belong to different structural subsets of C76(1)(CF3)n isomers
due to their considerably different addition patterns. In contrast to
12-II, isomer 12-I contains some CF3 additions in the positions of
ICCBs (Interpentagonal C-C Bonds) which are avoided in most
CF3 derivatives of other higher fullerenes.[9c-h] Apparently, this
feature is due to the specific distribution of pentagons on the D2-
C76(1) carbon cage, and it retains in all isomers C76(1)(CF3)14-18
presented in this work.
A separate subset of closely related C76(1)(CF3)n isomers
includes structures 16-I and 16-IV as well as 18-I, 18-III, and 18-
IV, which contain 13 CF3 attachments in common and no
3
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10.1002/asia.201800774
Chemistry - An Asian Journal
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of two CF3 groups to isomer 16-IV. These relations can reflect
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fifteen C76(1)(CF3)10-18 revealed a rich isomerism of this group of
compounds making it one of the most studied among
trifluoromethylated derivatives of higher fullerenes. Sixteen
C76(CF3)10-18 structures can be divided into three subgroups each
containing isomers with closely related addition patterns which
possess many attachment positions in common. Moreover, some
C76(1)(CF3)14-18 structures of the largest subgroup contain the
additions to triple-hexagon junctions which are rarely observed in
other fullerene(CF3)n compounds. Some precursor relations in the
course of successive addition of CF3 groups were found thus
indicating possible pathways of trifluoromethylation. However,
the processes of CF3 rearrangements on the carbon cage at high-
temperature reaction conditions cannot be excluded.
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Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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4
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