Halogenated benzene cation radicals

Article abstract of DOI:10.1002/chem.201102960

The halogenated benzenes C₆HF₅, 2,4,6-C ₆H₃F₃, 2,3,5,6-C₆H₂F ₄, C₆F₆, C₆Cl₆, C ₆Br₆, and C₆I₆ were converted into their corresponding cation radicals by using various strong oxidants. The cation-radical salts were isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy and by single-crystal X-ray diffraction. The thermal stability of the cation radicals increased with decreasing hydrogen content. As expected, the cation radicals [C₆HF₅] ⁺ and 2,3,5,6-[C₆H₂F₄]⁺ had structures with the same geometry as C₆HF₅ and 2,3,5,6-[C₆H₂F₄]. In contrast, the cation radicals [C₆F₆]⁺, [C₆Cl ₆]⁺, and possibly also [C₆Br₆] ⁺ exhibited Jahn-Teller-distorted geometries in the crystalline state. In the case of C₆F₆⁺Sb₂F ₁₁^-, two low-symmetry geometries were observed in the same crystal. Interestingly, the structures of the cation radicals 2,4,6-[C ₆H₃F₃]⁺ and C₆I ₆⁺ did not exhibit Jahn-Teller distortions. DFT calculations showed that the explanation for the lack of distortion of these cations from the D_3h or D_6h symmetry of the neutral benzene precursor was different for 2,4,6-[C₆H₃F ₃]⁺ than for [C₆I₆]⁺. Copyright

Full text of DOI:10.1002/chem.201102960

FULL PAPER
DOI: 10.1002/chem.201102960
Halogenated Benzene Cation Radicals
Matthias J. Molski,[a] Doreen Mollenhauer,[a] Sebastian Gohr,[a] Beate Paulus,[a]
Monther A. Khanfar,[b] Hashem Shorafa,[a] Steven H. Strauss,[c] and Konrad Seppelt*[a]
+
Abstract: The halogenated benzenes
C6HF5, 2,4,6-C6H3F3, 2,3,5,6-C6H2F4,
C6F6, C6Cl6, C6Br6, and C6I6 were con-
verted into their corresponding cation
radicals by using various strong oxi-
dants. The cation-radical salts were iso-
lated and characterized by electron
paramagnetic resonance (EPR) spec-
troscopy and by single-crystal X-ray
diffraction. The thermal stability of the
cation radicals increased with decreas-
ing hydrogen content. As expected, the
cation radicals [C6HF5]+ and 2,3,5,6-
[C6H2F4]+ had structures with the same
geometry as C6HF5 and 2,3,5,6-
[C6H2F4]. In contrast, the cation radi-
cals [C6F6]+, [C6Cl6]+, and possibly also
[C6Br6]+ exhibited Jahn–Teller-distort-
ed geometries in the crystalline state.
In the case of C6F6 Sb2F11À, two low-
symmetry geometries were observed in
the same crystal. Interestingly, the
structures of the cation radicals 2,4,6-
+
[C6H3F3]+ and C6I6 did not exhibit
Jahn–Teller distortions. DFT calcula-
tions showed that the explanation for
the lack of distortion of these cations
from the D3h or D6h symmetry of the
neutral benzene precursor was differ-
ent for 2,4,6-[C6H3F3]+ than for [C6I6]+.
Keywords: benzene · cations · crys-
tal structures · Jahn–Teller distor-
tion · X-ray diffraction
Introduction
zenes, see the Supporting Information, Table SI-1). By anal-
ogy to other high-IP molecules, such as O2 (12.07 eV), Cl2
(11.51 eV), Br2 (10.51 eV), and I2 (9.22 eV), all of which
have been successfully oxidized to produce cation radicals
that were isolable without further reaction (i.e., [O2]+,[4]
[Cl4]+,[5] [Br2]+,[6] and [I2]+,[7] respectively), we reasoned that
some benzene cation radicals should be obtainable.
Benzene, C6H6, is one of the archetypal organic molecules
owing to its D6h symmetry, aromaticity, ring current, and sta-
bility. Because its exceptional stability is related to the de-
localization of six p electrons, it is expected that variation of
the electron count, such as in [C6H6]+, [C6H6]2+, and
[C6H6]À, would result in highly reactive species. Herein, we
The cation radical [C6H6]+ has been observed spectroscop-
ically in the gas phase.[8,9] However, in a condensed phase, it
may be the most-difficult benzene cation radical to study.
Although C6H6 has a lower IP than the other benzenes stud-
ied herein, side-reactions following oxidation, such as the
loss of a proton, may be very rapid and therefore unavoida-
ble. Nevertheless, EPR spectra have been observed. The ox-
idation of benzene by persulfate in concentrated sulfuric
acid gave a poorly resolved seven-line spectrum, and a reac-
tion mixture containing C6H6 and NbF5 has been interpreted
+
focused on the cation radicals C6X6 , with X=F, Cl, Br, and
I (hereafter, to avoid confusion, the generic term “benzene”
is used to refer to C6H6 and all of its possible monocyclic de-
rivatives, the formula “C6H6” is used to refer specifically to
the parent molecule). In the gas phase, C6H6 has a first ioni-
zation potential (IP) of 9.25(2) eV,[1] and the substitution of
hydrogen by halogen atoms causes this to increase in a sys-
tematic manner[1–3] (for a list of IP values for relevant ben-
as that of the ion-pair [C6H6 Nb2F11À].[10] On the other hand,
+
C6F6, with an IP of 9.88(5) eV,[1] should be the most-difficult
benzene to oxidize, but, once obtained, its cation radical
might be more stable than all other benzene cation radicals.
As we will show below, the stability of [C6H6-nFn]+ cation
radicals increases as the value of n increases.
[a] M. J. Molski, D. Mollenhauer, S. Gohr, Prof. Dr. B. Paulus,
Dr. H. Shorafa, Prof. Dr. K. Seppelt
Freie Universitꢀt Berlin
Institut fꢁr Chemie und Biochemie
Fabeckstrasse 34-36, 14195 Berlin (Germany)
Fax : (+49)30-838-54289
In addition to demonstrating that benzene cation radicals
can be isolated in a condensed phase, the central focus of
this work was the determination of their structures. For neu-
tral precursors with D6h and D3h symmetry, Jahn–Teller dis-
tortions are expected following oxidation into the cation
radicals. But what particular lower-symmetry structure
should be present for a given cation radical in a given con-
densed-phase environment is an open question. It has been
shown theoretically that gas-phase [C6H6]+, [C6F6]+, and
2,4,6-[C6H3F3]+ should distort into D2h ([C6H6]+, [C6F6]+) or
E-mail: seppelt@zedat.fu-berlin.de
[b] Prof. M. A. Khanfar
Chemistry Department, The University of Jordan
Amman (Jordan)
[c] Prof. S. H. Strauss
Department of Chemistry, Colorado State University
Fort Collins, CO 80523 (USA)
Supporting information for this article, including additional informa-
tion on selected structures, calculated orbital energies, and EPR spec-
tra, is available on the WWW under http://dx.doi.org/10.1002/
chem.201102960.
Chem. Eur. J. 2012, 00, 0 – 0
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C2v structures (2,4,6-[C6H3F3]+).[11–21] However, in each case,
two alternative structures were predicted that were very
close in energy: a quinoid structure and a bis-allyl-like struc-
ture (hereinafter referred to as the bis-allyl structure or ge-
ometry). The quinoid cations were predicted to have two
Table 2. Figures 1–6 show the structures of most of the ben-
zene cation radicals and, only where relevant, details of
cation–anion interactions.
DFT-predicted relative energies for three limiting geome-
tries for the isolated [C6X6]+ cation radicals are listed in
À
À
short C C bonds and four longer ones, whereas the bis-allyl
Table 3 (X=F, Cl, Br, I). DFT-predicted C C bond lengths
À
cations were predicted to have four short and two longer C
were compared with their corresponding lengths as deter-
mined by X-ray crystallography for neutral molecules C6X6
and for the [C6X6]+ cation radicals in Table 4. Similar com-
parisons were made between the [C6HF5]+, 2,4,6-[C6H3F3]+,
and 2,3,5,6-[C6H2F4]+ cation radicals in Table 5, which also
C bonds.
All benzene cation radicals can be unstable at room tem-
perature and hence susceptible to secondary reactions.
Therefore, our plan was to first detect these species qualita-
tively by their often intense color, then to study them in so-
lution by EPR spectroscopy, and finally to isolate them as
their crystalline salts and determine their structures by X-
ray diffraction. The latter part of the plan was deemed nec-
essary because the Jahn–Teller distortion of benzene cation
radicals, which were made from a D6h-symmetry- or D3h-
symmetry-neutral benzene precursor, should not be observa-
ble in isotropic EPR spectra owing to the rapid interconver-
sion of equivalent Jahn–Teller-distorted isomers in solution.
However, the observation of Jahn–Teller distortions by crys-
tallography could be hampered by positional disorder of the
pseudo-hexagonal cations. In an attempt to avoid this prob-
lem, special emphasis was placed on growing single crystals
very slowly at low temperatures.
À
includes the DFT-predicted C C bond lengths for the
2,4,5,6- and 3,4,5,6-[C6H2F4]+ cation radicals.
Discussion
AHCTUNGTRENNUNG
[C6F6]+: This cation radical may have been first observed by
EPR spectroscopy in a mixture of C6F6 and SbF5, which ex-
hibited hyperfine splittings that were attributable to six
equivalent F atoms.[22] Later, the yellow compound [C6F6]+-
ACHUTNGRENNUG CHTUNGTRENNUNG
[AsF6]À was obtained by oxidizing C6F6 with [O2]+A[AsF6]À,
but information about the structure of the cation radical
could not be obtained.[23,24] It has also been reported that
CrF5 can oxidize C6F6 into [C6F6]+.[25]
In 2009, we reported that OsF6/SbF5 or [O2] CTHNUGTRNENUG
+A[Sb2F11]À oxi-
dized C6F6 into [C6F6]+ in anhydrous HF solution. Our EPR
spectrum,[26] the parameters of which are listed in Table 1,
was identical to the previously published spectrum.[22] Two
Results
EPR g values and hyperfine coupling constants (aF and aH)
salts, [C6F6] CTHUGNERTNNUNG CHTNUGTRENNUNG
+A[Os2F11]À and [C6F6]+A[Sb2F11]À, were isolated
for solutions of [C6HF5]
[SbF6]À in SO2, [C6F6]+A[Sb2F11]À and [C6Cl6]
SO2ClF, and microcrystalline [C6I6]
+A[AsF6]À and [C6I6]+-
[SbF6]À are listed in Table 1. The EPR spectra of [C6F6]+-
[Sb2F11]À, [C6HF5]+, and [C6H2F4]+ are shown in the Sup-
C
and structurally characterized (they were found to be iso-
structural).[26] The X-ray structures revealed two pleasant
surprises: 1) the two independent, Jahn–Teller-distorted
[C6F6]+ cation radicals in the asymmetric unit were not dis-
ordered, and 2) they had different distorted geometries
(Figure 1). In addition, both structures were determined
very precisely. The R1 value and the standard error for indi-
À
A
U
+A[Sb2F11]À in
CHTUNGTRENNUNG
CTHUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
porting Information, and matched those reported in the lit-
erature.[22]
vidual C C bond lengths for [C6F6] CTHNUGTRNENUG
+A[Sb2F11]À were 0.015 and
either 0.3 or 0.4 pm, respectively. The two “bond-stretch iso-
mers”[27] discovered in the same unit cell were indeed the
quinoid and bis-allyl isomers predicted by DFT and ab initio
calculations.[21,26]
Table 1. EPR spectroscopic parameters.[a]
Compound
Solvent
T
[8C]
g
Hyperfine coupling
constants [G]
A
[Sb2F11]À[b]
CHTUNGTRENNUNG
SO2ClF
SO2
À20 2.0022
À70 2.004
aF =13.65
aF(1,5)–aF(2,4) =26.9
The isomeric [C6F6]+ cations were in distinctly different
lattice environments, and that electrostatic cation···anion in-
teractions caused the relative stabilization of one geometry
E
a
F(3) =4.8,
aH =1.4
aF =25.6,
aH =1.0
2,3,5,6-[C6H2F4]+-
[SbF6]À[d]
[C6Cl6]
+A[Sb2F11]À
[C6I6]
+A[AsF6]À,
[C6I6]
+A[SbF6]À
SO2
À70 2.004
À
or the other. In our analysis, only C F atomic distances
ACHTUNGTRENNUNG
shorter than about 326 pm were considered because the van
der Waals radii of F and C atoms are 147–150 and 176–
177 pm, respectively. The bis-allyl structure, with four short
A
N
SO2ClF
none
25 2.010
25,
broad envelope
very broad signal
C
N
A
E
(solid)
À70,
À196
À
and two longer C C bonds, should have more positive
[a] All data are from this work unless otherwise noted. [b] Data taken
from reference [26]. [c] Literature values from reference [22]: g=2.0036,
aF(1,5)–aF(2,4) =25.8
reference [22]: g=2.0039, aF =(25.8Æ0.3) G.
charge localized on the C2 and C3 atoms than on the C1
and C4 atoms. In harmony with this prediction, the closest
C1···F and C4···F contacts were 325.5(3) (ꢃ2) and
318.2(3) pm (ꢃ2), respectively, whilst the closest C2···F con-
tacts were 272.4(3) and 275.7(3) pm, respectively, and the
closest C3···F contacts were 272.2(3) and 281.9(3) pm, re-
spectively, about 50 pm shorter. The situation for the qui-
A
Selected crystal data and structure-refinement parameters
for the X-ray structures reported herein are listed in
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Halogenated Benzene Cation Radicals
FULL PAPER
Table 2. Crystal data and structure-refinement parameters.
À
Compound
[C6HF5]
E
[C6H2F4]
E
[C6H3F3]
G
[C6H3F3]
[SbF6
]
ACHUTGTNREN[NUG C6Cl6] CHTUNGTRENNUNG
+A[Sb2F11]À
formula
Mr
C6HF11As
356.99
C12F21H5Sb2
791.66
C6F9H3As
321.00
C6F9H3Sb
367.83
C6Cl6F11Sb2
737.27
crystal system
space group
a [pm]
b [pm]
c [pm]
a [8]
monoclinic
P21/n
790.74(8)
2054.7(2)
1170.91(1)
90
100.461(3)
90
1870.9(3)
8
triclinic
P1
hexagonal
R3
973.6(1)
973.6(1)
1579.3(2)
90
90
120
1296.5(3)
6
2.467
4.051
hexagonal
R3c
991.66(7)
991.66(7)
1605.5(2)
90
90
120
1367.3(2)
6
2.680
monoclinic
P21/m
789.9(2)
1530.2(2)
830.3(2)
90
116.80(1)
90
895.9(3)
2
¯
¯
¯
805.3(2)
1020.2(3)
1273.3(4)
99.33(1)
98.28(1)
93.27(1)
1018.0(5)
2
ß [8]
g [8]
V, ꢃ106 [pm3]
Z
1calcd [MgmÀ3
]
2.535
3.787
2.583
2.841
2.733
4.006
m [mmÀ1
]
3.139
crystal size [mm]
color
independent data
Rint
parameters
R1, wR2
0.3ꢃ 0.1ꢃ 0.1
yellow
3290
0.045
336
0.15ꢃ 0.14ꢃ 0.05
green
10086
0.0251
336
0.5ꢃ 0.01ꢃ 0.01
red
443
0.0230
0.2ꢃ 0.05ꢃ 0.05
red brown
468
0.0181
30
0.4ꢃ 0.2ꢃ 0.01
blue
2752
0.0277
115
29
0.0328, 0.0901
0.026, 0.0543
0.0188, 0.0477
0.0111, 0.0304
0.0237, 0.0513
A
E
[C6I6]
E
[C6I6]+[H
G
2,3,5,6-C6H2F4
formula
Mr
C6F6I6As
1022.38
monoclinic
C2/c
1820.2(14)
669.6(6)
1456.2(13)
90
109.36(2)
90
1674(3)
4
C6F6I6Sb
1069.21
monoclinic
C2/c
1838.9(2)
673.54(7)
1484.2(2)
90
109.243(5)
90
1735.5(3)
4
C8F6HI6O6S2
1132.61
C6F4H2
150.08
monoclinic
P21/c
448.0(1)
1025.1(2)
630.1(2)
90
107.99(3)
90
275.3(1)
2
crystal system
space group
a [pm]
b [pm]
c [pm]
a [8]
triclinic
¯
P1
945.2(7)
986.2(5)
1274.2(9)
75.39(2)
89.76(2)
81.92(3)
1137.3(13)
2
ß [8]
g [8]
V, ꢃ106 [pm3]
Z
1calcd [MgmÀ3
]
4.056
13.133
4.092
12.303
3.307
8.448
1.811
0.201
m [mmÀ1
]
crystal size [mm]
color
independent data
Rint
parameters
R1, wR2
0.15ꢃ 0.04ꢃ 0.01
black
2559
0.0291
87
0.3ꢃ 0.05ꢃ 0.01
black
2627
0.0283
87
0.3ꢃ 0.05ꢃ 0.01
black
4935
0.0387
256
0.2ꢃ 0.2ꢃ 0.2
colorless
1948
0.0177
50
0.0248, 0.0496
0.0292, 0.0475
0.0321, 0.0772
0.0537, 0.1310
Table 3. B3LYP-predicted relative energies [kJmolÀ1] for the [C6X6]+
cation radicals.
partially fluorinated benzenes. The impurities, mostly
C6HF5, were as high as 2 mol%. If a large excess of C6F6
was used relative to the oxidant, the resulting product would
contain [C6HF5]+ as well as [C6F6]+, because hydrogen-con-
taining fluorobenzenes have lower IPs than C6F6. This result
could only be avoided by using C6F6 with a purity of 99.9%
or above, and the molar excess used in oxidation reactions
should be as low as practically possible (preferably less than
a two-fold excess of C6F6).
Cation radical
Quinoid
structure
Bis-allyl
structure
D6h symmetry
point
A
0
0
0
0.4
0.3
0.3
12[a]
6.7[a]
4.2[a]
0
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] Jahn–Teller stabilization energy at the intrinsic linearly-interpolated
coordinates.
AHCTUNGTRENNUNG
[C6HF5]+: When one of the six F atoms in C6F6 was replaced
noid [C6F6]+ cation in [C6F6]
[Sb2F11]À was the complete op-
by a H atom, calculations predicted that the separation of
the quinoid and bis-allyl isomer energies increased to about
34 kJmolÀ1,[20] with the bis-allyl structure becoming the
posite, although the differences were only about 10 pm in-
stead of 50 pm. A more-detailed analysis of the cationic···a-
nionic interactions is given in the Supporting Information.
One experimental problem that should be mentioned
before discussing the other benzene cation radicals is that
commercially available C6F6 contained various amounts of
ground state. The compound [C6HF5] CTHGUNTRNENUG
+A[AsF6]À prepared
herein was a yellow crystalline solid; its EPR spectrum had
the expected hyperfine structure and resembled the spec-
trum reported more than 40 years ago.[22] In crystals of this
Chem. Eur. J. 2012, 00, 0 – 0
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K. Seppelt et al.
À
Table 4. X-ray and B3LYP-predicted average C C bond lengths for sets of real or idealized symmetry-related
compound, there were two
unique [C6F5H]+ cations and
two unique AsF6 anions in the
asymmetric unit, and the struc-
tures of the two cation radicals
were very similar: both cations
had the C2v bis-allyl geometry
(Figure 2). As was the case with
the [C6F6]+ ions, the long and
bonds in neutral C6X6 and [C6X6]+ cation radicals [pm].[a]
À
X
Neutral C6X6,
D6h geometry
G
N
ACHTUNGTRENNUNG
X-ray
DFT
X-ray
DFT
X-ray
DFT
F
137.8(2)
138.9
136.8(4)ꢃ2,
137.0ꢃ2,
137.7(4)ꢃ4’
138.8ꢃ4,
141.0(3)ꢃ4[b,c] 142.7ꢃ4
138.2ꢃ2,
143.4(3)ꢃ2[b,c] 144.8ꢃ2
Cl 139.5(5)[d] 139.8
Br 140.1(5)[d] 140.0
140.2(3)ꢃ4,
139.9ꢃ4,
144.8ꢃ4
138.8ꢃ2,
142.8ꢃ4
144.1(5)ꢃ2[b] 144.7ꢃ2
140.0ꢃ4,
144.3ꢃ2
À
short C C atomic distances
I
139.9(7)[e] 140.8
139.6(5)ꢃ6[f] 139.7ꢃ6
were clearly statistically differ-
ent, even though the standard
errors were higher: the differ-
ence between the average of
[a] All data are from this work unless otherwise noted (T=À1408C). For the X-ray structures, the standard
À
error shown is the standard error for an individual C C distance, not the standard deviation of the average.
[b] Reference [26] (T=À1408C). [c] [Sb2F11]À salt. [d] Reference [29] (T=À1738C). [e] Reference [37] (T=
À1738C). [f] [SbF6]À salt.
À
one set of short C C bond
lengths, 136.8(7) pm, and the
average of the long bond
lengths, 143.7(7) pm, was about
10 times the standard error for
À
Table 5. X-ray and B3LYP-predicted average C C bond lengths for sets of real or idealized symmetry-related
bonds in [C6HnFm]+ cation radicals [pm].[a]
Compound
[C6HnFm]+ quinoid
N
[C6HnFm]+ D3h
ACHTUNGTRENNUNG
geometry
DFT
geometry
X-ray
À
X-ray
X-ray
DFT
an individual C C bond length.
Furthermore,
as
observed
A
E
137.1(7)ꢃ2,
143.7(7)ꢃ2,
136.8(7)ꢃ2
137.9ꢃ2,
144.8ꢃ2,
138.9ꢃ2
À
above, the DFT-predicted C C
bond lengths closely matched
the X-ray-determined bond
lengths (Table 5).
2,3,5,6-[C6H2F4]
N
137.2(3)ꢃ4,
137.9ꢃ4
143.9(3)ꢃ2[b] 144.9ꢃ2
144.2ꢃ2
2,4,6-[C6H3F3]
142.6ꢃ2
140.7(2)ꢃ6[c]
141.6ꢃ2
136.3ꢃ2
138.0ꢃ2
138.4ꢃ2
2,3,5,6-[C6H2F4]+: Tetrafluoro-
benzene 2,3,5,6-C6H2F4 was oxi-
dized to produce crystals of
3,4,5,6-[C6H2F4]+
2,4,5,6-[C6H2F4]+
142.0ꢃ2
136.2ꢃ2,
142.1ꢃ2
both
[SbF6]À·0.25HF and 2,3,5,6-
[C6H2F4]
+A[Sb2F11]À. The EPR
spectrum of the 2,3,5,6-
2,3,5,6-[C6H2F4]+-
141.4ꢃ2
136.2ꢃ2,
143.5ꢃ2
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
[a] All data are from this work (T=À1408C). For the X-ray structures, the standard error shown is the stan-
À
[C6H2F4]+ cation radical was
a straightforward quintet (aF =
25.6 G) of triplets (aH =1.0 G).
dard error for an individual C C distance, not the standard deviation of the average. [b] The three correspond-
ing distances for neutral 2,3,5,6-C6H2F4 (T=À1408C) are 138.2(1), 138.3(1), and 138.3(1) pm. [c] The B3LYP-
predicted difference in energies between the quinoid and bis-allyl (ground state) structures of 2,4,6-[C6H3F3]+
cation is 1.2 kJmolÀ1
.
A
single crystal of 2,3,5,6-
[C6H2F4]
CTHUNGTRENNUNG
precise X-ray structure, so only these data will be discussed
Figure 2. The [C6HF5]+ ion in [C6HF5]+ [AsF6]À; ellipsoids set at 50%
probability, except for the H atom, which is shown as a sphere of arbitra-
Figure 1. The two [C6F6]+ cation radicals in the crystal of
[C6F6]+ [Sb2F11]À, ellipsoids set at 50% probability (see reference [26]).
À
À
ry size. The two distances [pm] given for each C C bond refer to the two
Left: the quinoid cation. Right: the bis-allylic cation. Only the unique C
À
crystallographically independent cations in the asymmetric unit. The
C bond lengths [pm] are shown. The average C C distance in neutral
À
numbers in italics are the DFT-predicted C C bond lengths for the gas-
phase cation radical.
C6F6 is 137.8(2) pm (average of nine values; see the Supporting Informa-
tion).
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Halogenated Benzene Cation Radicals
FULL PAPER
herein (although the cations in both structures had very sim-
ilar geometries).
There were four independent 2,3,5,6-[C6H2F4]+ cations,
four [SbF6]À anions, and two HF molecules in the asymmet-
ric unit. All of the cations possessed the bis-allyl geometry
(Figure 3), which agreed with the predicted geometry
À
(Table 5, also see the Supporting Information). The C C
bond lengths in the cations were compared with neutral
2,3,5,6-C6H2F4, which was readily obtained as single crystals
À
owing to its relatively high melting point. The C C bond
lengths were all statistically the same in the structure of this
neutral tetrafluorobenzene (138.23(9)–138.33(9) pm; see the
Supporting Information, Figure SI-4).
Figure 4. Left: The formula unit in the structure of 2,4,6-
[C6H3F3]+ [SbF6]À; ellipsoids set at 50% probability, except for the H
atoms, which are shown as spheres of arbitrary size. The rigorously
À
planar cation is in the plane of the page and the SbF6 anion is below the
À
À
À
plane of the page. Only the unique C C, C H, and C F distances are
shown. The corresponding distances in the structure of 2,4,6-[C6H3F3]+-
AHCTUNGTRENNUNG
[AsF6]À are 140.5(2), 84(4), and 131.3(3) pm, respectively. Right: The cat-
ionbanion packing along the threefold crystallographic axis in 2,4,6-
[C6H3F3]
CTHUNGTRENNUNG
lected interatomic distance [pm] and angles [8]:Sb–F 187.90(8);
F2···C1···F2’ 173.88(8), F-Sb-F 90.41(4), 89.59(4), and 180.00(4).
ally symmetric and efficient cation···anion packing in the
crystal. Figure 4 shows the columnar arrangement of alter-
nating cations and anions along the hexagonal axis of the
rhombohedral crystal. There were relatively short contacts
(292.7(1) pm) between the three benzene C(F) atoms and
the anion F atoms; this was a sensible result because the
C(F) atoms were expected to carry most of the positive
charge of the cation. The substantial charge-neutralization
and concomitant large lattice energy caused by the efficient
ion packing sterically enforced the three-fold symmetry on
the 2,4,6-[C6H3F3]+ ion, despite the electronically-preferred
distorted geometry. This result also explains why only the
Figure 3. The
2,3,5,6-[C6H2F4]+
cation
radical
in
[C6H2F4]+ [SbF6]À·0.25HF; ellipsoids set at 50% probability, except for
the H atoms, which are shown as spheres of arbitrary size. The first four
numbers are the C C bond lengths [pm] for the four crystallographically
independent cations in the lattice. Only the unique C C bond lengths are
shown. The first number in italics is the DFT-predicted C C distance for
À
À
À
the gas-phase cation radical; the second number in italics is the DFT-pre-
À
dicted C C distance for the neutral precursor 2,3,5,6-C6H2F4 (also see the
Supporting Information).
2,4,6-[C6H3F3]+: Owing to the D3h symmetry of the parent
benzene, 2,4,6-C6H3F3, the corresponding cation should ex-
hibit a Jahn–Teller distortion. Again, the two possible iso-
mers, quinoid and bis-allyl, should be very close in energy,
with the quinoid form possibly being the minimum
À
SbF6 compound could be isolated: the low solubility of
2,4,6-[C6H3F3]
[SbF6]À+SbF5 to the right, even when the oxidation with
[O2] CHUTNGTRENNUGN
+A[Sb2F11]À was carried out with a large excess of SbF5.
+A[SbF6]À drove the equilibrium [Sb2F11]ÀQ
CHTUNGTRENNUNG
(Table 3). Oxidation of 2,4,6-C6H3F3 with [O2]
[O2]
+A[AsF6]À in liquid HF produced 2,4,6-[C6H3F3]+SbF6
and 2,4,6-[C6H3F3]
+A[AsF6]À, respectively, as ruby-red,
needle-shaped crystals in almost quantitative yield. Interest-
+A
[Sb2F11]À and
We were not able to isolate a salt of the 2,4,6-[C6H3F3]+
cation with a lower-symmetric anion, either by starting with
an alternative oxidant or through an anion-exchange reac-
tion following the oxidation.
À
CHTUNGTRENNUNG
CTHUNGTRENNUNG
ingly, the use of [O2]
C
Our current interpretation of the structure of this com-
pound is that the electrostatic interaction provides enough
energy to hold the cation in the observed symmetric geome-
try. Our calculations predicted that the energy needed to
overcome the Jahn–Teller distortion was only about
12 kJmolÀ1.
[SbF6]À salt, even in the presence of a large excess of SbF5.
Both salts resulted in very precise crystal-structure deter-
minations. The crystals were isomorphous, and the space
¯
group R3c resulted in the cations and anions being in special
positions. This structure required rigorous D3d symmetry for
the anions and, more importantly, rigorous D3h symmetry
for the 2,4,6-[C6H3F3]+ cation radical. Although distorted
cations that were three-fold disordered could not be exclud-
ed, the small and relatively symmetric thermal displacement
parameters (at À1408C) did not give any indication of disor-
der. We believe that the lack of an observable Jahn–Teller
distortion in the cation, despite the prediction that the
cation should be distorted, could be explained by the unusu-
AHCTUNGTRENNUNG
[C6Cl6]+ and [C6Br6]+: As expected, C6Cl6 and C6Br6 were
easier to oxidize than the fluorinated benzenes (IP: 9.13(3)
and 8.80 eV, respectively; see the Supporting Information,
Table SI-1). Neat SbF5 alone could also act as the oxidant
for these hexahalobenzenes. Recrystallization from SO2FCl
afforded crystals of [C6Cl6]
The crystal structure of [C6Cl6] CTHGNUTRENNUNG
G
+A[Sb2F11]À.
CHTUNGTRENNUNG
+A[Sb2F11]À was precise enough
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K. Seppelt et al.
to draw definite conclusions about the geometry of the
[C6Cl6]+ cation radical. However, the crystallographic char-
Hypothetically, if the [C6Cl6]+ cation radical had the quinoid
geometry, we would expect the C2 atoms to have closer
and/or a greater number of contacts with the F atoms of the
anions relative to the C1 and C3 atoms.
acteristics of [C6Br6] CTHUNGTRNEUNG
+A[Sb2F11]À were poor. The [C6Br6]+
cation seemed to have a quinoid structure, as predicted by
computation, but better structural data are needed.
However, the [C6Cl6]+ cation had a bis-allyl geometry
[C6I6]+: C6I6 was the benzene with the lowest ionization po-
ACHTUNGTRENNUNG
À
(Figure 5), with two long C C bonds (144.5(5) and
tential studied herein (IP=7.9 eV; see the Supporting Infor-
mation, Table SI-1). Several attempts to oxidize C6I6 have
been reported, and black products were always observed in
H2O2/CF3SO3H solutions. These products have been de-
scribed as [C6I6]2+ salts.[31,32] The crystal structure of a com-
2+
À
pound with the formula [C6I6
]ACHTNGUTERNU[NG (CF3SO3 )2] was described
in a PhD dissertation,[33] but, to the best of our knowledge,
such a structure has not been published elsewhere. Herein,
we oxidized C6I6 by using SbF5 or AsF5 in HF, which indeed
resulted in such black precipitates. However, single crystals
were obtained with the compositions [C6I6]
+A[SbF6]À and
CHTUNGTRENNUNG
[C6I6]
CTHUNGTRENNUNG
The [C6I6]+ cation radicals in both structures had undis-
torted hexagonal geometries (Figure 6). Because the packing
arrangements of the ions were quite irregular (unlike the sit-
uation in the 2,4,6-[C6H3F3]+ salts), the symmetric geome-
tries observed for the [C6I6]+ cation radicals in [C6I6]+
[SbF6]À and [C6I6]+ [AsF6]À were quite clearly not a result of
differential cation···anion interactions. In fact, a symmetric
geometry was predicted by our calculations. The HOMO of
C6I6 was a non-bonding A1g iodine-centered orbital that was
represented by a combination of the six iodine p-orbitals
lying in the molecular plane in an antibonding manner.
Therefore, one-electron oxidation of C6I6 removed an elec-
tron largely from the I atoms, not from the C atoms (i.e.,
Figure 5. The structure of [C6Cl6]+ [Sb2F11]À, which shows two symmetry-
related anions and one cation radical; ellipsoids set at 50% probability.
Selected interatomic distances [pm]: C1–C2 139.9(3), C1–C1A 144.5(5),
C2–C3 140.5(3), C3–C3A 144.1(5), C1···F4 317.3(3), C1···F5B 300.4(3),
C2···F5B 312.5(3), C3···F3 326.6(3), C3···F2B 319.1(3).
À
not from a degenerate C C p orbital), and should not result
À
144.1(5) pm) and four shorter C C bonds (in the range
in a Jahn–Teller distortion of the molecule (unlike the other
halogens, the ionization potential of an I atom, 10.45 eV, is
lower, not higher, than that of a C atom, 11.26 eV). Further-
more, oxidation of the [C6I6]+ cation radical should result in
a [C6I6]2+ dication that also has a hexagonal geometry. It has
been suggested that removal of the two A1g electrons from
C6I6 would result in a s-aromatic ring-current in the [C6I6]2+
dication,[31,32,35,36] in contrast to the situation for [C6Cl6]2+.[37]
À
139.9(3)–140.5(3) pm). The three unique C C distances in
neutral C6Cl6 were statistically the same and ranged from
138.6(5)–140.0(5) pm at À1738C[29] and from 139.1(3)–
139.5(3) pm at 24(1)8C.[30] Although this result contradicted
the theoretical prediction that the ground state of the
[C6Cl6]+ cation should adopt the quinoid geometry, the cal-
culated difference in energy was only 0.24 kJmolÀ1 between
the quinoid ground state and
the bis-allyl transition state.
There were two C···F(Sb) inter-
actions for the C1 atom and
two for the C3 atom, which
ranged
326.6(3) pm, but only one
C2···F(Sb) interaction
from
300.4(3)
to
(312.5(3) pm), as expected for
a bis-allyl structure. In fact, it
can be argued that the C2···F5B
contact was really
a conse-
quence of the proximity of the
F5B atom to the C1 atom
(C1···F5b=300.4(3) pm), which
required that the F5B atom
Figure 6. Left: The [C6I6]+ cation radical in [C6I6]+ [AsF6]À and [C6I6]+ [SbF6]À, which are isostructural; ellip-
soids set at 50% probability. The top number refers to the [AsF6]À salt and the bottom number to the [SbF6]À
+
salt. Only the unique C C distances [pm] are shown. Right: The crystal structure of [C6I6] [H
(SO3CF3)2]À.
À
must be close to the C2 atom. The anion has a short O(H)···O distance of 243.9 pm.
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Halogenated Benzene Cation Radicals
FULL PAPER
The average intramolecular I···I distance for the [C6I6]+
cation radical (347.8 pm) was only marginally shorter than
the corresponding average distance in neutral C6I6
(351.3 pm).[37] This was a sensible result, because the as-
sumed half-bond between the I atoms in [C6I6]+, spread as
over six intramolecular I···I interactions, would not be ex-
pected to result in a significant amount of shortening. Fur-
thermore, the shortening was not due to differences in the
displacements of the I atoms from the C6 planes of C6I6 and
[C6I6]+: in both cases, the mean displacement was only about
4 pm.
zenes occasionally resulted in the formation of salts of the
perhalogenated benzenium cations [C6Cl7]+, [C6Cl5F2]+, and
[C6Br5F2]+. These compounds will be reported in a future
report.
Computational results: Perhalogenated benzenes: Our DFT-
predicted and X-ray structural parameters for the neutral
benzenes C6X6 (D6h symmetry; X=F, Cl, Br, I) and the
cation radicals [C6X6]+ are listed in Table 4. At the level of
À
theory used in this work, the C X bond lengths increased
from 1.331 ꢄ in C6F6 to 2.123 ꢄ in C6I6, as did the intramo-
lecular X···X distance, from 2.270 ꢄ to 3.531 ꢄ. Interesting-
We believed that the compound previously reported to be
À
[C6I6]2+
G
E
ly, the aromatic-ring C C bond lengths also increased as the
À
Dissolution of [C6I6]
formation of small amounts of crystalline [C6I6 ][H-
(CF3SO3)2]À, which we analyzed by X-ray crystallography.
[AsF6]À in CF3SO3H resulted in the
size of the halogen atoms increased, from 1.389 ꢄ in C6F6 to
1.408 ꢄ in C6I6. The two-fold degenerate HOMO in C6F6
À
A
was a pair of C C p orbitals, labeled as B (a bis-allyl-like or-
Our compound and the compound previously reported as
bital) and Q (a quinoid-like orbital) in Figure 7. The bis-
À
[C6I6]2+
G
were
crystallographically identical in
all respects. As in the structures
of [C6I6]
[AsF6]À, the monovalent cation
CHTUNGTRENNUNG
+A[SbF6]À and [C6I6]+-
ACHTUNGTRENNUNG
+
radical [C6I6]+ in [C6I6 ][H-
ACHTUNGTRENNUNG
hexagonal geometry. The over-
looked hydrogen atom in the
previously reported crystal
structure was not only detected
in this work by a difference
Fourier analysis, its presence
was also obvious because of the
short O···O distance (243.9 pm),
which is the hallmark of
À
a
strong O H···O hydrogen
bond.[38] Furthermore the two
À
S O bonds that participated in
the hydrogen bond were elon-
Figure 7. The HOMO energies for C6X6 (X=F, Cl, Br, and I) are shown in a partial MO diagram. B=bis-allyl
orbital; Q=quinoid orbital; XX3*=totally symmetric antibonding in-plane halogen-atom p-orbital with three
nodal planes. Orbital plots for C6F6 are shown as an example (B3LYP/tzvpp).
gated by 5–8 pm relative to the
À
other S O bonds.
The absence of an observable
EPR spectrum was previously cited as evidence for the exis-
allyl orbital had a perpendicular nodal plane that bisected
tence of the diamagnetic [C6I6]2+ dication.[33] However, in
opposite C C bonds, whilst the quinoid orbital had a perpen-
À
+
our hands, all of the C6I6 salts exhibited very broad EPR
dicular nodal plane that included two para-C atoms. The
À
signals at À1968C. Taken together, our crystallographic and
spectroscopic results led us to conclude that the [C6I6]2+ di-
cation remains an unknown species. Nevertheless, if it is
ever isolated, it will probably be found to have a symmetric
hexagonal geometry like C6I6 and [C6I6]+, as predicted by
calculations.
third C C p orbital was HOMO-1. Below the energy of
HOMO-1 in C6F6 were six orbitals that were composed of
linear combinations of in-plane X-atom p orbitals (X=F).
One of these six orbitals (XX3*, Figure 7) was important be-
cause its energy was especially sensitive to the nature of the
halogen atom. The XX3* energy was 0.175 below the B/Q
energy for C6F6, but this difference decreased to 0.039 for
C6Cl6 and to 0.003 for C6Br6. For C6I6, XX3* was the
HOMO and the degenerate B/Q orbital was HOMO-1.
The molecular orbital diagrams indicated that Jahn–
Teller-distorted geometries should be found for [C6X6]+ with
X=F, Cl, and Br but not for X=I. Two distorted cations,
a compressed one with the quinoid orbital as a singly occu-
pied orbital and an elongated one with the bis-allyl orbital
Halogenated benzenium ion salts: Salts of benzenium cat-
ions are intermediates in electrophilic aromatic substitution
reactions and therefore have been studied extensively. In
many cases, they can be isolated, particularly if electron-do-
nating substituents are present.[39] The parent cation [C6H7]+
was recently isolated by using a weakly-coordinating carbor-
ane anion.[39] Our oxidation reactions of halogenated ben-
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K. Seppelt et al.
as a singly occupied orbital, were found for [C6X6]+, with
X=F, Cl, and Br. The compressed (quinoid) cation geome-
tries were classified as minima, with the elongated (bis-allyl)
geometries as transition states. The [C6I6]+ cation retained
the D6h symmetry of neutral C6I6. The behavior of the orbi-
tals was as expected from the molecular orbital diagram of
the neutral molecule. The difference between the quinoid
and bis-allylic orbitals was about 0.13–0.18 for the cations
(X=F, Cl, Br), and the XX3* orbitals became closer in
energy to the quinoid and bis-allylic orbitals. Significantly,
the barrier between the distorted ground-state geometries
and the hexagonal geometry decreased in the order
[C6F6]+ >[C6Cl6]+ >[C6Br6]+.
Linearly interpolated intrinsic coordinates (LIICs) for the
cation radicals [C6F6]+, [C6Cl6]+, and [C6Br6]+ were generat-
ed to calculate the Jahn–Teller stabilization energy, eJT, as
Figure 8. The quinoid- and bis-allyl-distorted cation-radical structures are
connected by two paths. The path through the D6h symmetry point can
be described by calculating the LIICs; the other path, which leads from
the bis-allyl cation as a transition state to the minimum-energy quinoid
cation, can be calculated by using IRCs. The combined paths can be used
to construct a potential energy surface for the system, which has the ap-
pearance of a “Mexican hat” (see reference [26]).
the difference E
(D6h)ÀE
ACHUTNGTRENN(UG D2h), where ECAHTUNGTRENN(GUN D2h) was the energy
+
of the bis-allyl cation radical. For C6F6 this difference was
about 12 kJmolÀ1, which decreased to 6.7 kJmolÀ1 for
[C6Cl6]+ and to 5.4 kJmolÀ1 for [C6Br6]+. The bis-allyl and
quinoid structures were nearly isoenergetic. In [C6F6]+, their
energy difference was only 0.4 kJmolÀ1, and the difference
was even smaller for [C6Cl6]+ and [C6Br6]+ (Table 3). These
values were certainly within the uncertainty of the computa-
tional methods used, but the calculated trend was independ-
ent of the basis set and the method used. A calculation
using intrinsic reaction coordinates (IRCs) offered a possible
reaction path from the bis-allylic cation to the quinoid
cation. In both paths, the LIICs and the IRCs connected the
two distorted structures. These are the two typical paths for
the interconversion of geometries in such systems,[19] and
they can be combined to calculate a potential energy surface
(Figure 8).
À
because the maximum difference in the C C bond lengths
was smaller than 1 pm. Inspection of the molecular orbital
diagrams for these molecules showed that the bis-allyl and
quinoid p orbitals were no longer degenerate. In C6HF5 and
2,3,5,6-C6H2F4, the quinoid orbital was the HOMO, and so
removing an electron would lead to a bis-allyl-distorted
cation radical. In 2,4,5,6-C6H2F4 and 3,4,5,6-C6H2F4, the bis-
allyl orbital was the HOMO and a quinoid-distorted cation
radical would be expected (Figure 9).
The Jahn–Teller distortions in the [C6X6]+ cation radicals
were quantified geometrically by comparing the difference
À
between the elongated and compressed C C bond lengths;
we found that this difference was predicted to decrease
from 6.0 pm for both types of [C6F6]+ cations to 4.8 and
4.6 pm for the bis-allyl and quinoid [C6Cl6]+ cations, respec-
tively, to 4.3 and 4.0 pm for the bis-allyl and quinoid
[C6Br6]+ cations, respectively. Experimentally, the observed
differences were 6.0(5) and 5.4(4) pm for the bis-allyl
[C6F6]+ cation in [C6F6]
[Sb2F11]À and 4.6(6) and 3.6(6) pm
+A[Sb2F11]À.
for the bis-allyl [C6Cl6]+ cation in [C6Cl6]
CTHUNGTRENNUNG
The average Mulliken charge on the F atoms in [C6F6]+
was predicted to be slightly negative (À0.1 e). In contrast, it
was slightly positive (+0.1 e) on the Cl and Br atoms in
[C6Cl6]+ and [C6Br6]+ and +0.25 e on the I atoms in [C6I6]+.
Both the Mulliken charge and the occupancy of the orbitals
indicated a significant positive charge on the I atoms in the
cation radical.
Figure 9. A partial molecular orbital diagram for fluorinated benzenes
showing the high-energy occupied orbitals, including the bis-allyl (B) and
quionid (Q) p orbitals. Orbital plots for C6F6 are shown as an example
(B3LYP/tzvpp).
Partially fluorinated benzene derivatives: The molecular
symmetry of the partially-fluorinated neutral molecules
C6HF5, 2,3,5,6-C6H2F4, and 2,4,5,6-C6H2F4 was D2h, and for
The explanation of these differences is as follows: In C6F6,
the bis-allyl and quinoid p orbitals contained an antibonding
contribution from the in-plane F atom p-orbitals, thus rais-
ing their energy. For the bis-allyl p orbital, two large and
four small out-of-plane F atom p-orbital contributions were
À
3,4,5,6-C6H2F4 it was C2v. Nevertheless, the hexagonal C C
framework in all four molecules had effective D6h symmetry
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Halogenated Benzene Cation Radicals
FULL PAPER
[C6F6]+ [Os2F11]À and [C6F6]+ [Sb2F11]À were prepared according to litera-
ture procedures.[26]
present, whilst for the quinoid p orbital, four large out-of-
plane contributions were present. In C6HF5 and 2,3,5,6-
C6H2F4, one and two of the large out-of-plane contributions
in C6F6 were missing due to the presence of the H atoms, re-
spectively. This result lowered the energy of the bis-allyl or-
bital, thereby making the quinoid orbital the HOMO. In
3,4,5,6-C6H2F4 and 2,4,5,6-C6H2F4, both orbitals were stabi-
lized as follows: for the bis-allylic orbital, two small anti-
bonding F atom p-orbital contributions were missing, and
for the quinoid orbital two large antibonding F atom p-orbi-
tal contributions were missing. Thus, the quinoid p orbital
was stabilized to a greater extent, and the bis-allyl orbital
became the HOMO.
After removing an electron from the neutral molecules,
structural optimizations were performed, starting with the
same symmetry. In each case, a distorted cation was found.
As expected, [C6HF5]+ and 2,3,5,6-[C6H2F4]+ had bis-allyl
geometries and 2,4,5,6-[C6H2F4]+ and 3,4,5,6-[C6H2F4]+ had
quinoid geometries. The difference for the bis-allylic and
quinoid orbitals increased from the neutral compounds
(average: 0.016) to the cation radicals (average: 0.197), but
the order was the same. These results were in full agreement
with previous calculations reported for such systems.[20] Fol-
lowing ionization, the molecules were predicted to exhibit
distorted bis-allyl geometries, with maximum differences in
Physical methods: X-band EPR spectra were recorded on a Bruker ER
200 D-SRC spectrometer equipped with an ER 4111 variable-tempera-
ture unit that allowed for low-temperature measurements. Except for the
[C6I6]+ salts, all samples were freshly prepared in either SO2 or SO2ClF
(Table 1) and sealed in 4 mm O.D. PFA tubes. Samples of [C6I6]+ [AsF6]À
and [C6I6]+ [SbF6]À were weighed into PFA tubes, then anhydrous
CF3SO3H was added, and the tubes were sealed under an Ar atmosphere
(PFA=poly(perfluorether-tetrafluorethylene)). Single crystals suitable
for X-ray diffraction were mounted on a Bruker Smart 2000 diffractome-
ter (MoKa radiation) under oxygen- and moisture-free conditions at tem-
peratures below À1008C by using a special device of local design. All
data collections were performed at À1408C. After semiempirical absorp-
tion corrections, the structures were solved and refined by using the pro-
gram SHELXL.[44] Non-hydrogen atoms were refined anisotropically; H
atoms were refined isotropically. Relevant data collection and refinement
parameters are listed in Table 2.
Synthesis of new compounds: General procedures: Moisture-sensitive
nonvolatile materials were handled in a glovebox in an inert atmosphere
of less than 0.1 ppm H2O and O2. Volatile materials were handled in
glass or stainless steel vacuum lines as appropriate. The oxidation reac-
tions were generally carried out in 8 mm PFA tubes that were connected
to a vacuum line. Weighed portions of the nonvolatile oxidants were
transferred into the tube in the glovebox, after which the solvent and the
benzene reagent were added by using the vacuum line. The oxidation re-
actions frequently occurred rapidly even at low temperatures, and were
signaled by the formation of an intense color. The cation-radical-salt
products often decomposed at 258C, with the concomitant loss of color.
AHCTUNGTRENNUNG
[C6HF5]+ [AsF6]À: A sample of C6HF5 (600 mg, 3.57 mmol) was cooled to
À
C C distance of 6.4–7.3 pm. The differences in experimental
À
À488C, the melting point of this compound. Small portions of [O2]+-
+
C C bond lengths for the [C6HF5] and 2,3,5,6-[C6H2F4]+
AHCTUNGTRENNUNG
[AsF6]À (total 100 mg, 0.453 mmol) were added under an Ar atmosphere;
cations agreed with the calculated values to within (Æ2) and
(Æ1) pm, respectively (these two cation radicals did have
bis-allyl geometries in the solid state).
the mixture was shaken carefully after each portion (about 5 mg) was
added. The resulting green suspension was warmed to À358C and all of
the volatile compounds were removed under vacuum. A yellow powder
was obtained in 70 mg (43% based on [O2] CTHNUGTRNENUG
+A[AsF6]À). The dry product
Finally, all of our attempts to find other relatively stable
distorted geometries for the cation radicals were unsuccess-
ful, including LIICs between other assumed geometries and
for geometries distorted along symmetry-reducing coordi-
nates.
decomposed within minutes at 258C. Recrystallization from anhydrous
HF (4 mL) at À788C afforded large yellow needles after 24 h.
2,3,5,6-[C6H2F4]
2,3,5,6-C6H2F4 (60 mg, 0.4 mmol) was cooled to À788C and small por-
tions of [O2]
+A[Sb2F12]À (total 200 mg, 0.413 mmol) were added under an
+ A[SbF6]À:
CHTUNGTERNNNUG A mixture of anhydrous C6F14 (4 mL) and
CTHUNGTRENNUNG
Ar atmosphere; the mixture was shaken carefully after each portion
(about 5 mg) was added. The resulting yellow–green suspension was
warmed to À458C and all of the volatile compounds were removed
under vacuum. The product, as a yellow powder, decomposed immediate-
ly at 258C and was recrystallized from a green solution in anhydrous HF
(2 mL) as green crystals after 1 week at À788C.
Experimental Section
Caution: Handling anhydrous HF, dioxygenyl salts, SbF5, and AsF5 re-
quires eye and skin protection.
2,4,6-[C6H3F3] CTHNGURTENNUNG CHUTNGTRENNGUN
+ A[AsF6]À: [O2]+A[AsF6]À (100 mg, 0.453 mmol) and AsF5
(about 2 mL, 4.3 g, 25 mmol) were mixed in a PFA tube. The volatile
compound 2,4,6-C6H3F3 (54 mg, 0.41 mmol) was added to this mixture at
À1968C. Warming the mixture to 258C afforded a clear, wine-red solu-
tion (note that the compound did not decompose rapidly at 258C).
Slowly cooling this solution to À788C yielded red–brown crystals. Anhy-
drous HF and SO2ClF were also used as solvents with similar results.
Reagents and solvents: The following reagents were purchased from the
indicated vendor: C6HF5, 2,4,6-C6H3F3 and 2,3,5,6-C6H2F4 (ABCR
GmbH), C6Cl6 and C6Br6 (Sigma–Aldrich Chemie GmbH), elemental As
(ABCR GmbH), SO2Cl2 (Merck Schuchardt), SbF5 (Fluorochem Ltd.),
NH4F (E. Merck), KI (Acros Organics), HIO4 (H5IO6; E. Merck),
CF3COOH (Merck Schuchardt), CF3SO3H (Merck Schuchardt; distilled
from P2O5 (Sigma Aldrich GmbH)). The five benzenes were checked for
purity by NMR spectroscopy and were redistilled or resublimed as neces-
sary. C6I6 was prepared by treating benzene with periodic acid in the
presence of potassium iodide.[40] Arsenic pentafluoride was prepared by
passing F2 gas over elemental arsenic.[41] [O2]+ [AsF6]À and [O2]+ [Sb2F11]À
were prepared from O2, F2, and either AsF5 or SbF5 by irradiation with
UV light.[42] Solvent SO2ClF was prepared by treating a mixture of
SO2Cl2 and NH4F with CF3COOH.[43] Perfluorohexane (ABCR GmbH)
was dried and purified by treatment with KF followed by
[XeF]+ [Sb2F11]À. Anhydrous HF was vacuum-distilled into a stainless
steel cylinder containing BiF5 to remove any residual H2O. SbF5 was
vacuum-distilled twice by using a glass vacuum line with a À308C trap.
The resulting liquid was clear, colorless, and highly viscous.
2,4,6-[C6H3F3] CTHNGURTENNUNG CHUTNGTRENNGUN
+ A[SbF6]À: [O2]+A[Sb2F11]À (100 mg, 0.206 mmol) was dis-
solved in anhydrous HF (2 mL) at 258C in a PFA tube. The mixture was
cooled to À1968C and 2,4,6-C6H3F3 (26 mg, 0.20 mmol) was added by
vacuum transfer. The mixture was slowly warmed to 258C, and produced
the same wine-red solution as described above. Slow cooling to À788C
yielded dark-brown needle-shaped crystals.
+ A[Sb2F11]À: C6Cl6 (50 mg, 0.18 mmol) was dissolved in SbF5
ACHUTNGREN[NUG C6Cl6] CHTUNGTRENNGUN
(100 mg, 0.461 mmol) at 258C in a PFA tube, thereby forming a dark-
blue solution after 15–30 min. The solution was frozen at À1968C and an-
hydrous HF (2 mL) was added by vacuum transfer. The mixture was
slowly warmed to 08C, and afforded a clear, dark-blue solution. Slow
cooling of this solution to À788C resulted in the formation of dark-blue
crystals.
Chem. Eur. J. 2012, 00, 0 – 0
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K. Seppelt et al.
ACHTUNGTRENNUNG[C6Br6] CHTUNGTRENNUNG
+ A[Sb2F11]À: C6Br6 (50 mg, 0.091 mmol) was dissolved in SbF5
Acknowledgements
(100 mg, 0.461 mmol) at 258C in a PFA tube, thereby forming a dark-
green solution after 2–5 min. The solution was frozen at À1968C and
SO2ClF (2 mL) was added by vacuum transfer. The mixture was slowly
warmed to 08C and afforded a clear, dark-green solution. Slow cooling
of this solution to À788C resulted in the formation of black crystals. a=
802.9(5), b=855.3(5), c=874.0(5) pm; a=63.12(1), b=76.13(1), g=
63.088; V=476.9(5)ꢃ106 pm3; Z=1; disordered [Sb2F11]À anion; R1 =
0.10; wR2 =0.3202.
This work was made possible by the Deutsche Forschungsgemeinschaft
(Graduiertenkolleg “Fluorine as a Key Element”), the Fonds der Chemi-
schen Industrie, and a DFG grant (to M.K.). Research internship stu-
dents C. Noufele and K. Schulz performed preliminary calculations.
ACHTUNGTRENNUNG[C6I6] CHTUNGTRENNUNG
+ A[SbF6]À: SbF5 (0.42 g, 1.94 mmol), C6I6 (0.5 g, 0.60 mmol), and an-
[1] I. D. Clark, D. C. Frost, J. Am. Chem. Soc. 1967, 89, 244–247.
[2] R. Bralsford, P. V. Harris, W. C. Price, Proc. R. Soc. London Ser. A
1960, 258, 459–469.
[3] N. Sato, K. Seki, H. Inokuchi, J. Chem. Soc. Faraday Trans. 1981, 77,
1621–1633.
[4] N. Bartlett, D. H. Lohmann, Proc. Chem. Soc. 1962, 115–116.
[5] S. Seidel, K. Seppelt, Angew. Chem. 2000, 112, 4072–4074; Angew.
Chem. Int. Ed. 2000, 39, 3923–3925.
[6] A. J. Edwards, G. R. Jones, R. J. C. Sills, J. Chem. Soc. Chem.
Commun. 1968, 1527–1528.
[7] C. G. Davies, R. J. Gillespie, R. J. Ireland, J. M. Sowa, Can. J. Chem.
1974, 52, 2048–2052.
[8] R. Lindner, H. Sekiya, B. Beyl, K. Mꢁller-Dethlefs, Angew. Chem.
1993, 105, 631–634; Angew. Chem. Int. Ed. Engl. 1993, 32, 603–606.
[9] M. Iwasaki, K. Toriyama, K. Nunone, J. Chem. Soc. Chem.
Commun. 1983, 320–322.
[10] a) J. R. Bolton, A. Carrington, Proc. Chem. Soc. 1961, 174–175;
b) F. Marchetti, C. Pinzino, S. Zachini, G. Pampaloni, Angew. Chem.
2010, 122, 5396–5400; Angew. Chem. Int. Ed. 2010, 49, 5268–5272.
[11] a) T. J. Sears, T. A. Miller, V. E. Bondybey, J. Chem. Phys. 1981, 74,
3240–3248; b) W. D. Hobey, A. D. McLachlau, J. Chem. Phys. 1960,
33, 1695–1703.
[12] C. A. Coulson, A. Golebiewski, Mol. Phys. 1962, 5, 71–84.
[13] L. C. Snyder, J. Chem. Phys. 1960, 33, 619–621.
[14] A. D. Liehr, Z. Phys. Chem. (Muenchen Ger.) 1956, 9, 338–354.
[15] K. Raghavachari, R. C. Haddon, T. A. Miller, V. E. Bondybey, J.
Chem. Phys. 1983, 79, 1387–1395.
hydrous HF (2.0 mL) were mixed in a PFA tube at 258C until a deep-
blue precipitate had formed. All volatile compounds were removed
under vacuum to afford a deep-blue powder in quantitative yield. A por-
tion (20 mg) of the powder was dissolved in anhydrous CF3SO3H (2 mL)
and heated to 2008C for 5 min. Slow cooling of the hot solution to 258C
resulted in the formation of deep-blue needle-shaped crystals. The com-
pound was stable at 258C under an inert atmosphere, but it decomposed
within 10 min when exposed to air.
ACHTUNGTRENNUNG[C6I6] CHTUNGTRENNUNG
+ A[AsF6]À: AsF5 (4.00 g, 23.5 mmol), C6I6 (1.00 g, 1.20 mmol), and
anhydrous HF (1.3 mL) were mixed in a PFA tube at À788C until
a deep-blue precipitate had formed. All volatile compounds were re-
moved under vacuum to afford a deep-blue powder in quantitative yield.
A portion (20 mg) of the powder was dissolved in anhydrous CF3SO3H
(2 mL) and heated to 2008C for 5 min. Slow cooling of the hot solution
to 258C resulted in the formation of deep-blue needle-shaped crystals.
The compound was stable at 258C in an inert atmosphere, but it decom-
posed within 10 min when exposed to air.
ACHTUNGTRENNUNG[C6I6] CHTUNGTRENNUNG CTHNUGTRENNUGN
+ A[CF3SO3]À: When [C6I6]+A[AsF6]À (120 mg) was heated to 1458C
under high vacuum (in an attempt to sublime the salt intact), the sample
started to slowly decompose into orange C6I6, purple I2, and other un-
identified products without any indication of the sublimation of [C6I6]+-
ACHTUNGTRENNUNG
[AsF6]À. The apparatus was cooled to 258C after approximately half of
the sample had been converted into C6I6. To recover the remaining unde-
composed sample of [C6I6] small amount of anhydrous
+A[AsF6]À,
CF3SO3H was added under an Ar atmosphere. After 2 days at 258C,
C
a
deep-blue crystals of [C6I6]
tion.
+A[CF3SO3]À had crystallized out of this solu-
CHTUNGTRENNUNG
[16] J. Eiding, R. Schneider, W. Domke, H. Koppel, W. von Niessen,
Chem. Phys. Lett. 1991, 177, 345–351.
[17] R. Lindner, K. Mueller-Dethlefs, E. Wedum, K. Haber, E. R. Grant,
Science 1996, 271, 1698–1702.
[18] K. Mueller-Dethlefs, J. B. Peel, J. Chem. Phys. 1999, 111, 10550–
10551.
[19] B. E. Applegate, T. A. Miller, J. Chem. Phys. 2002, 117, 10654–
10674.
[20] V. P. Vysotsky, G. E. Salmikov, L. N. Shchgoleva, Int. J. Quantum
Chem. 2004, 100, 469–476.
[21] K. F. Hall, A. M. Tokmachev, M. J. Bearpark, M. Boggia-Pasqua, M.
Robb, J. Chem. Phys. 2007, 127, 134111–1–134111–11.
[22] N. M. Bazhin, Yu. V. Pozdnyakovich, V. D. Shteingarts, G. G. Yakob-
son, Izv. Akad. Nauk SSSR Ser. Khim. 1969, 10, 2300–2302.
[23] T. J. Richardson, N. Bartlett, J. Chem. Soc. Chem. Commun. 1974,
427–428.
[24] T. J. Richardson, F. L. Tanznella, N. Bartlett, Adv. Chem. Ser 1987,
217, 169–176.
[25] S. D. Brown, T. M. Loehr, G. L. Gard, J. Fluorine Chem. 1976, 7,
19–32.
[26] H. Shorafa, D. Mollenhauer, B. Paulus, K. Seppelt, Angew. Chem.
2009, 121, 5959–5961; Angew. Chem. Int. Ed. 2009, 48, 5845–5847.
[27] W. D. Stohrer, R. Hoffmann, J. Am. Chem. Soc. 1972, 94, 779–786.
[28] A similar “mexican hat” energy surface has been observed in C5H5
(see: H. J. Wçrner, F. Markt, J. Chem. Phys. 2007, 127, 034303) and
in AuX3 (X=F, Cl, Br; see: A. Schulz, M. Hargittai, Chem. Eur. J.
2001, 7, 3657–3669).
[29] C. M. Reddy, M. T. Kirchner, R. C. Gundakaram, K. A. Padmanab-
han, G. R. Desiraju, Chem. Eur. J. 2006, 12, 2222–2234.
[30] G. M. Brown, O. W. Strydom, Acta Crystallogr Sect. B. 1974, 30,
801–804.
Quantum-chemical calculations: Halogenated benzenes C6X6 (X=F, Cl,
Br, I) and partially-fluorinated benzenes C6HF5 and 2,3,5,6-, 2,4,5,6-, and
3,4,5,6-C6H4F2 as well as their corresponding single-positively charged
cation radicals were investigated by quantum-chemical calculations by
using the B3LYP DFT-functional[45] and the TZVPP basis set.[46] For I
atoms, the Def2-ECP28 pseudo-potential[47] was applied. Test calculations
for C6F6 and [C6F6]+ with different basis sets and different functionals re-
sulted in similar relative energies, as did the B3LYP/TZVPP and
CCSD(T) levels of theory.[26] The structures of the neutral molecules and
the cation radicals were optimized. The two energetically similar isomers
with the same symmetry were calculated by starting with models that had
À
À
4 C C bond lengths longer than 2 C C bond lengths and vice versa. Fre-
quency calculations were performed to determine whether the optimized
structures were a true minimum or a transition state. The program pack-
ages used were GAUSSIAN 03[48] and TURBOMOLE5v10.[49]
One reviewer brought to our attention a distantly related Jahn–Teller
isomer problem: the [CuACHTNUTGRNEUNG
(NO2)6]4À anion has been found to exist both as
an elongated and a compressed octahedron, depending on the type of
cation. Furthermore there even exists a regular octahedral anion that is
generated by a dynamic Jahn–Teller effect.[50, 51]
CCDC-703472 ([C6F6]+ [Os2F11]À), CCDC-703473 ([C6F6]+ [Sb2F11]À),
CCDC-703623 ([C6F6]), CCDC-844888 ([C6HF5]+ [AsF6]À), CCDC-844885
([C6H2F4]+ [SbF6]À·0.5HF), CCDC-844884 ([C6H2F4]), CCDC-844886
([C6H3F3]+ [AsF6]À), CCDC-844887 ([C6H3F3]+ [SbF6]À), CCDC-844883
([C6Cl6]+ [Sb2F11]À), CCDC-844882 ([C6Br6]+ [Sb2F11]À), CCDC-844889
([C6I6]+ [AsF6]À), CCDC-844890 ([C6I6]+ [SbF6]À), and CCDC-844891
([C6I6]+ [HS2O6C2F6]À) contain the supplementary, crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[31] D. J. Sagl, J. C. Martin, J. Am. Chem. Soc. 1988, 110, 5827–5833.
[32] J. C. Martin, L. J. Schaad, Pure Appl. Chem. 1990, 62, 547–550.
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Halogenated Benzene Cation Radicals
FULL PAPER
[33] K. M. Patel, Diss. Abstr. Int. B 1994, 54, 4679, Vanderbilt Univ.,
Nashville, TN, USA.
[48] Gaussian 03, Revision C.02, M. J.Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Fores-
man, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Na-
nayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wallingford
CT, 2004.
[34] A crystal structure of [C6I6]+ [AsF6]+ was also presented in 1997 at
the 15th International Symposium on Fluorine Chemistry (Vancou-
ver, CA) by J. B. Chestnut, R. D. Rogers, J. S. Thrasher, A. Water-
feld, (personal communication from A. Waterfeld and J. S. Thrash-
er).
[35] R. W. A. Havenith, P. W. Fowler, S. Fias, B. Bultwick, Tetrahedron
Lett. 2008, 49, 1421–1424.
[36] I. Ciofini, P. P. Laine, C. Adamo, Chem. Phys. Lett. 2007, 435, 171–
175.
[37] S. Ghosh, C. M. Reddy, G. R. Desiraju, Acta Crystallogr. Sect. E
2007, 63, o910-o911.
[38] S. H. Strauss, K. D. Abney, O. P. Anderson, Inorg. Chem. 1986, 25,
2806–2812.
[39] C. A. Reed, K.-C. Kim, E. S. Stoyanov, D. Stasko, F. S. Tann, L. J.
Mueller, P. D. W. Boyd, J. Am. Chem. Soc. 2003, 125, 1796–1804.
[40] D. L. Mattern, J. Org. Chem. 1983, 48, 4772–4773.
[41] O. Ruff, A. Braida, O. Breitschneider, W. Menzel, H. Plaut, Z.
Anorg. Allg. Chem. 1932, 206, 59–64.
[42] J. Shamir, J. Binenboym, Inorg. Chim. Acta 1968, 2, 37–38; D. E.
McKee, N. Bartlett, Inorg. Chem. 1973, 12, 2738–2740.
[43] V. P. Reddy, D. R. Bellew, G. K. S. Prakash, J. Fluorine Chem. 1992,
56, 195–197.
[44] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112.
[45] A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652.
[46] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297–
3305.
[47] K. A. Peterson, D. Figgen, E. Goll, H. Stoll, M. Dolg, J. Chem.
Phys. 2003, 119, 11113–11123.
[49] TURBOMOLE V5.10 2008, a development of University of Karls-
ruhe and Forschungszentrum Karlsruhe GmbH, 1989–2007, TUR-
BOMOLE GmbH, since 2007; available from http://www.turbomole.
com.
[50] D. L. Cullen, E. C. Lingafelter, Inorg. Chem. 1971, 10, 1264–1268.
[51] M. D. Joesten, S. Tagagi, P. G. Lenhart, Inorg. Chem. 1977, 16, 2680–
2685.
Received: September 21, 2011
Revised: January 18, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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K. Seppelt et al.
The new radicals: The [C6I6]+ radical
cation is, in contrast to all other
[C6X6]+ radical cations presented
herein, not Jahn–Teller distorted.
Jahn–Teller distortion
M. J. Molski, D. Mollenhauer, S. Gohr,
B. Paulus, M. A. Khanfar, H. Shorafa,
S. H. Strauss, K. Seppelt* . . . &&&& — &&&&
Halogenated Benzene Cation Radicals
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