Powerful oxidizing properties of a planar [8]annulene: Tetrakis(perfluorocyclobuta)cyclooctatetraene) and its radical anion

Article abstract of DOI:10.1002/(SICI)1522-2675(19990908)82:9<1434::AID-HLCA1434>3.0.CO;2-7

The central eight-membered ring of tetrakis(perfluorocyclobuta)cyclooctatetraene (1) is flat both in the crystal and in solution, but tetrakis(perfluorocyclopenta)cyclooctatctracne (2) assumes a tub shape similar to that of parent cyclooctatetraene. This sterically induced structural difference strongly affects the physical properties of compounds 1 and 2. The first and second reduction potentials of 1 in solution, + 0.79 and + 0.14 V vs. SCE, are considerably more positive than the corresponding values, + 0.20 and - 0.21 V, of 2. Accordingly, mere contact with Hg metal in N,N-dimethylformamide converts 1 to an unusually stable radical anion 1'- that is persistent in air. The temperature- independent coupling constant, a(F) = + 0.320 mT, of the sixteen equivalent [β¹⁹F-nuclei in 1^- is much smaller than the expected value, and the g factor of 1'^- is markedly lower than those generally found for structurally related radical anions. In contrast, the characteristic data for 2-, generated from 2 with K metal in 1,2-dimethoxyethane, comply with the expected data. The coupling constants a(F) = + 0.976 and +0.935 mT, each of eight β-¹⁹F-nuclei in 2^-, average +0.951 mT upon raising the temperature from 220 to 260 K. The gas-phase electron affinity of 1, A = 3.4 ± 0.2 eV, which was estimated from the reduction potential in solution and from charge- transfer absorption spectra obtained from a series of methylated benzenes and naphthalenes, is among the highest known for neutral organic molecules.

Full text of DOI:10.1002/(SICI)1522-2675(19990908)82:9<1434::AID-HLCA1434>3.0.CO;2-7

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Helvetica Chimica Acta ± Vol. 82 (1999)
Powerful Oxidizing Properties of a Planar [8]Annulene:
1
Tetrakis(perfluorocyclobuta)cyclooctatetraene ) and Its Radical Anion
by Fabian Gerson*, Walter Huber, Pascal Merstetter, Gaby Persy, Robert L. Soulen, Christoph Spöndlin, and
Jakob Wirz*
Institut für Physikalische Chemie der Universität Basel, Klingelbergstrasse 80, CH-4056 Basel
The central eight-membered ring of tetrakis(perfluorocyclobuta)cyclooctatetraene (1) is flat both in the
crystal and in solution, but tetrakis(perfluorocyclopenta)cyclooctatetraene (2) assumes a tub shape similar to
that of parent cyclooctatetraene. This sterically induced structural difference strongly affects the physical
properties of compounds 1 and 2. The first and second reduction potentials of 1 in solution, ‡0.79 and ‡0.14 V
vs. SCE, are considerably more positive than the corresponding values, ‡0.20 and �0.21 V, of 2. Accordingly,
.
�
mere contact with Hg metal in N,N-dimethylformamide converts 1 to an unusually stable radical anion 1 that is
persistent in air. The temperature-independent coupling constant, a
F-nuclei in 1 is much smaller than the expected value, and the g factor of 1 is markedly lower than those
F
ˆ‡0.320 mT, of the sixteen equivalent b-
19
.�
.�
.
�
generally found for structurally related radical anions. In contrast, the characteristic data for 2 , generated from
with K metal in 1,2-dimethoxyethane, comply with the expected data. The coupling constants a
2
F
ˆ‡0.976 and
1
9
.�
‡
0.935 mT, each of eight b- F-nuclei in 2 , average ‡0.951 mT upon raising the temperature from 220 to
2
60 K. The gas-phase electron affinity of 1, A ˆ3.4 Æ0.2 eV, which was estimated from the reduction potential
in solution and from charge-transfer absorption spectra obtained from a series of methylated benzenes and
naphthalenes, is among the highest known for neutral organic molecules.
Introduction. ± When Richard Willstätter first presented his synthesis of cycloocta-
1,3,5,7-tetraene (COT) in a lecture given at the Chemische Gesellschaft Zürich [1],
Alfred Werner immediately pointed out that this molecule might have a nonplanar
structure. Its unexpected reactivity would, therefore, not disprove Thieleꢀs contempo-
rary theory that all cyclic p-systems should be ꢁaromaticꢀ [2]. In fact, COT is by no
means a flat molecule; the dihedral twist angle between adjacent �HCˆCH�
segments is ca. 608 [3]. Nevertheless, even for hypothetically planar COT, one would
nowadays expect an even higher reactivity, comparable to that of ꢁantiaromaticꢀ
cyclobutadiene [4]. The enigmatic special properties of ꢁaromaticꢀ benzene have since
been explained by Hückelꢀs rule of (4n ‡2) [5], but debate on more subtle issues still
continues [6].
The puckered structure of COT should not be attributed to a destabilization energy
of the p-system (ꢁantiaromaticityꢀ), but to the unfavorable C�C�C bond angles of
1
358 and to the nonbonded interactions between the H-atoms in a flat octagonal
structure [7]. Several strategies to force planarity on COT have been explored,
including i) replacement of one or two CˆC bonds by a triple linkage [8], ii) fusion to a
1)
Systematic names according to the IUPAC nomenclature. 1: 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-hexadecafluoro-
1
1
,2,3,4,5,6,7,8-octahydrotetracyclobuta[a,c,e,g]cyclooctene; 2: 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,-
0,11,11,12,12-tetracosafluoro-1,2,3,4,5,6,7,8,9,10,11,12-dodecahydrotetracyclopenta[a,c,e,g]cyclooctene.

Helvetica Chimica Acta ± Vol. 82 (1999)
1435
planar ring system [9], and iii) photoionization of the flat dianion of COT in a rigid
matrix [10]. Work along the lines i and ii has led to the isolation of some novel extended
p-systems incorporating an (essentially) planar eight-membered ring [9], but their
relation to COT is rather remote. Unfortunately, the elegant approach iii was not
successful.
X-Ray crystal-structure analyses [11] showed that the COT core of tetrakis(per-
1
fluorocyclobuta)cyclooctatetraene ) (1) is flat, whereas that of tetrakis(perfluorocy-
1
clopenta)cyclooctatetraene ) (2) assumes a boat conformation closely similar to parent
COT. Hence, the pair of molecules 1 and 2 provides a unique chance to study the effect
of planarization on the physical properties of a cyclic 8p-system.
�
1
The structural difference between 1 and 2 is clearly reflected by the first two p -
ionization bands in the photoelectron (PE) spectra of these compounds [12]: the
energy gap increases by 1 eV, and the lowest ionization energy decreases by the same
�
1
amount on going from 2 to 1. In contrast, the position of the third p band is insensitive
�
1
to the dihedral angle between the p-bonds. Compared to the p band of similar origin
in COT, these ionizations are shifted by ca. 3 eV to higher energies, due to annelation
by the electron-attracting perfluorocycloalkane substituents.
Compound 1 is among the most powerful neutral organic oxidants known. The
ꢀ
1†
cyclic voltammogram of 1 exhibits two reversible reduction waves at E ˆ‡0.79 and
1
=2
ꢀ
2†
E
ˆ‡0.14 V vs. SCE [13]. The first of them is by 2.4 V more positive than the
1
=2
corresponding potential of COT at �1.61 V. This finding can be rationalized by the
cooperative inductive and conformational effects of perfluorocyclobutane annelation:
because neutral 1 already has a planar geometry, little reorganization energy is required
for electron transfer.
The first electronic transition of [4n]annulenes is particularly sensitive to bond-
length alternation and conformational changes [14]. Crystals of 1 are deep red, those of
2
are colorless. By comparing the electronic absorption spectra of 1 in the solid state
and in solution, it is established that the planar structure of 1, as determined by X-ray
crystal-structure analysis, is not due to crystal-packing forces and, thus, persists in

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Helvetica Chimica Acta ± Vol. 82 (1999)
solution. Charge-transfer absorption spectra of 1 in the presence of aromatic donors
are reported and used to estimate the electron affinity of 1. Finally, the electronic
.
�
.�
structure of the radical anions 1 and 2 is probed by ESR spectroscopy.
Results. ± The Electronic Spectrum of 1. Determination of the absorption spectrum
�
1
�1
of 1 was not trivial, because its very weak authentic bands (e ꢁ100 m cm in the
�
1
�1
visible region and e ꢁ1000 m cm down to 300 nm) are easily swamped by strong
absorptions arising from decomposition products. Decay of 1 occurs within seconds in
protic solvents, and more slowly in other solvents such as MeCN or CH Cl ,
2
2
presumably caused by the presence of acid or H O traces. On the other hand, dilute
2
�
3
solutions of 1 in spectrograde air-saturated hexane (up to 10 m) proved to be stable
for years. Solutions prepared from different crystals of 1 gave somewhat variable
results: weak absorption bands in the near UV region (250 ± 400 nm, apparent
2
�1
�1
extinction coefficients of ca. 10 m cm ) were present in some solutions, but not in
others. These variable bands must be attributed to an impurity or a decay product of 1.
Attempts to purify 1 by chromatography on silica gel or Alox failed because of
decomposition; that on cellulose, with benzene as eluent, separated small amounts of a
yellow-colored impurity. An absorption spectrum of 1 in hexane solution, where the
ꢁ
impurity bandsꢀ are absent, is shown in Fig. 1. Also presented in Fig. 1 is the visible-
absorption spectrum of a thin crystal of 1. The apparent distortion of absorbances
exceeding 2 in the spectrum of the crystal (420 ± 500 nm) is most likely an artifact due
to stray light or uneven thickness of the crystal. Thus, apart from an obvious difference
in the resolution of the vibrational fine structure, the two spectra are closely similar.
The frequency shifts, observed by changing the solvent or the temperature, were the
same for all of the vibrational maxima in the visible region, indicating that the entire
absorption band in the range of 360 ± 650 nm arises from a single electronic transition.
Charge-Transfer Absorption Spectra of 1. Addition of aromatic hydrocarbons (ca.
�
2
�4
1
0
m) to dilute solutions of 1 (ca. 2 Â10 m) in hexane led to the appearance of one
�
1
or two broad (width at half-height ca. 6000 cm ), Gaussian-shaped absorption bands in
Fig. 1. Electronic absorption spectrum of 1 in hexane solution (Ð) and in a crystal (±.±)

Helvetica Chimica Acta ± Vol. 82 (1999)
1437
the visible region are attributed to the formation of charge-transfer (CT) complexes
between the acceptor 1 and the aromatic donors. Thereby, p-xylene, which has a narrow
energy gap between the first two ionization potentials, gave two partially overlapping
CT bands. The absorption maxima of the CT bands of 1 with various donors in hexane
solution and the vertical gas-phase ionization energies of the donors, I [15][16], are
v
given in the Table.
Table. Position of the Charge-Transfer Absorption Bands of 1 in Hexane in the Presence of Aromatic Donors
Aromatic donor molecule
l
max/nm
hnmax/eV
I
v
/eV
Ref.
Benzene
p-Xylene (first band)
p-Xylene (second band)
480
620
525
622
850
690
768
728
744
880
881
895
758
756
835
898
2.58
2.00
2.36
1.99
1.46
1.80
1.61
1.70
1.67
1.41
1.41
1.39
1.64
1.64
1.48
1.38
9.23
8.44
9.05
8.42
7.85
8.18
8.01
8.01
7.96
7.82
7.85
7.64
7.89
7.89
7.60
7.76
[15]
[15]
[15]
[15]
[15]
[16]
[16]
[16]
[16]
[16]
[16]
[16]
[16]
[16]
[16]
[16]
1,3,5-Trimethylbenzene
Hexamethylbenzene
Naphthalene
1
2
-Methylnaphthalene
-Methylnaphthalene
Cyclobuta[b]naphthalene
,4-Dimethylnaphthalene
,5-Dimethylnaphthalene
,8-Dimethylnaphthalene
1
1
1
2,3-Dimethylnaphthalene
2,7-Dimethylnaphthalene
2,3,6,7-Tetramethylnaphthalene
Acenaphthene
The intensities of the CT bands with naphthalene as a donor were determined as a
�
3
�2
�4
function of the donor concentration, [D] ˆ2 Â10 to 5 Â10 m, [1] ˆ2 Â10 m. Due
0
to the large excess of naphthalene, its concentration is only marginally reduced by
complex formation, [D] ˆ[D] �[complex] ꢂ[D] (Benesi-Hildebrand conditions
0
0
[
17]). The absorbance of the CT complex, A , is then given by Eqn. 1, where e is
CT CT
the molar extinction coefficient at the absorption maximum of the CT band, d ist the
cell length, and K is the association constant of the complex, Kꢃ[complex]/([D] [1]) ꢂ
[
complex]/{[D] ([1] �[complex])}. Eqn. 1 was not inverted to obtain a linear relation
0
in [D] , because that procedure would give excessive weight to small values of A in a
0
CT
linear regression. Nonlinear fitting of Eqn. 1 to the absorbance data given in
Experimental provided the following values for the fitting parameters: e ˆ(5.4 Æ
CT
3
�1
�1
�1
0
.3) Â10 m cm and K ˆ27 Æ2 m (d ˆ1 cm, fixed).
K[D] ‰1Š
0
0
ACT ˆeCT
d
ꢀ1†
1
‡K[D]0
In Fig. 2 the positions of the CT-band maxima, E , are plotted vs. the vertical
CT
ionization potentials of the donors. The data are reasonably well represented by a linear
relationship ECT ˆaI ‡b. The following values of the parameters a and b were
v
determined by a linear regression: a ˆ0.74 Æ0.04 and b ˆ�4.26 Æ0.34 eV.
Cyclic Voltammetry. The reduction potentials of 2 were measured by cyclic
voltammetry at room temperature. Like 1 [13], compound 2 displays two reversible

1438
Helvetica Chimica Acta ± Vol. 82 (1999)
Fig. 2. Correlation of charge-transfer transition energies of 1 with various aromatic donors in hexane, ECT, with
the vertical ionization energies, I , of the corresponding donors
v
ꢀ
1†
reduction waves in MeCN, albeit at considerably less positive potentials, E ˆ‡
1
=2
ꢀ
2†
0
.20 Æ0.01 and E ˆ�0.21 Æ0.01 V vs. SCE.
1
=2
Radical Anions. Reduction of compounds 1 and 2 to their radical anions was carried
out with K in 1,2-dimethoxyethane (DME), either by a direct contact with the metallic
mirror or indirectly by the method of solvated electrons[18]. For 1, however, mere
shaking of the DME solution of the compound with Hg metal was sufficient to produce
.
�
the highly persistent radical anion 1 which survived even in the presence of air. Its
largely temperature-independent ESR spectrum (g ˆ2.0010 Æ0.0001), shown in Fig. 3,
exhibits 17 equidistant lines, of which the outermost ones could be detected only by
high amplification. Their binomial distribution of intensity and their spacing by the
1
9
coupling constant, ja jˆ0.320 Æ0.001 mT, point to sixteen equivalent b- F-nuclei in
F
2
19
the eight CF groups ). This ja j value was confirmed by the corresponding F-
2
F
ENDOR spectrum. Flanking each hyperfine line, two pairs of satellites with a relative
intensity of ca. 4% indicated an interaction with two sets of eight equivalent 13C-
isotopes. The pertinent coupling constants ja jwere 0.191 Æ0.001 mT and 0.135 Æ
C
0
.001 mT, whereby the lines associated with the smaller ja jvalue were markedly
C
broadened upon lowering the temperature. Based on the arguments presented in the
13
Discussion, this coupling constant was assigned to the C-isotopes in the p-centers of
the cyclooctatetraene ring, leaving the larger ja jvalue for those in the CF groups.
C
2
The temperature-dependent ESR spectra (g ˆ2.0021 Æ0.0001) of the persistent
.
�
radical anion 2 obtained at 205 and 235 K are partly reproduced in Fig. 4. At the lower
temperature, their analysis, corroborated by the corresponding F-ENDOR spectra,
19
yielded coupling constants ja jof 0.976 Æ0.005 and 0.935 Æ0.005 mT, each for a set of
F
eight nuclei, along with ja jˆ0.090 Æ0.001 and 0.059 Æ0.001 mT, each for a set of
F
19
four. The two large ja jvalues were assigned to the b- F-nuclei in the eight CF
F
2
groups bound to the p-centers of the cyclooctatetraene ring and the two small ones to
2
their g-counterparts in the four remaining groups ). According to a TRIPLE-
resonance experiment carried out on the ENDOR signals [19], the signs of the b- and g-
19
F-coupling constants are opposite. On raising the temperature, the two large b values
2)
In ESR spectroscopy, nuclei separated by 0, 1, 2, ... sp -hybridized C-atoms from a p-center are denoted a,
3
b, g, ... .

Helvetica Chimica Acta ± Vol. 82 (1999)
1439
.
�
‡
Fig. 3. ESR Spectrum of the radical anion 1 . Solvent: DME, counterion: K , temperature: 243 K.
19
gradually collapsed into one (0.951 Æ0.005 mT for sixteen F-nuclei, at 260 K), while
the two small g values did not significantly change.
Discussion. ± For a qualitative discussion of the absorption spectrum, the presence
of the perfluorocyclobutane rings will be ignored, i.e., we assume that the spectrum of 1
should correspond to that of hypothetically planar COTwith the same amount of C�C
bond-length alternation [14]. This assumption is justified by the following arguments.
The influence of perfluoroalkyl substituents on the electronic absorption spectra of
conjugated p-systems is generally well described as an inductive effect [20]. In the
present case, all unsaturated C-atoms of 1 are symmetry-equivalent. Thus, there will be
no charge reorganization within the p-system upon electronic excitation, and the
inductive effect on the p-orbital energies will cancel in the transition energies. The
same arguments apply to the absorption spectra of benzene and tris(perfluorocyclo-
buta)benzene, and indeed their spectra are quite similar (benzene, l /nm (log(e/
max
�
1
�1
1
1
1
[
m
cm ]), L : 254 (2.40), L : 204 (3.94); tris(perfluorocyclobuta)benzene, L : 253
b
a
b
1
and 259 (2.38), L : ꢁ200 (ꢄ4.0). In contrast, the first absorption band of 1 (Fig. 1)
a
lies at much longer wavelengths than that of parent COT, which exhibits only an end
absorption in the visible region. This large shift may be attributed to the structural
difference between planar 1 and puckered COT.
The electronic absorption bands of 1 follow the general pattern exhibited by planar
[
4n]hydrocarbons [14], and the assignment of the main bands is straightforward. A
Hückel-orbital scheme for COT is shown in Fig. 5. The degeneracy of the open-shell
orbitals has been removed by assuming some bond-length alternation (b ˆb Ædb)
0
which reduces the molecular symmetry from D to D . The dipole-forbidden HOMO-
8
h
4h
1
LUMO transition (b2u b ) gives rise to the lowest excited A configuration, the
1u
2g
energy of which is equal to 4 db in the Hückel approximation. It is obvious from the
nodal properties of the HOMO and LUMO that the first absorption band of the COT
chromophore will be extremely sensitive to changes in bond lengths and to deviations

1440
Helvetica Chimica Acta ± Vol. 82 (1999)
.
�
‡
Fig. 4. Low-field parts of the ESR spectra of the radical anion 2 . Solvent: DME, counterion: K , temperature
as indicated.
from planarity. The next four configurations (eg
b1u and b2u eg) give rise to two
excited states of E symmetry which are accidentally degenerate in the absence of
u
electronic correlation. They are split by first-order Configuration Interaction (CI).
The results of a standard PPP SCF CI calculation [21] for planar COT, which
included all singly excited configurations and allowed the resonance integrals to reach
self-consistency with the bond orders, b ˆ�2.318 exp(0.335[p �2/3]) eV [22], are in
mn
mn
satisfactory agreement with experiment. The very weak absorption band of 1 observed
1
‡
1
�
in the visible region is predicted to occur at 667 nm ( A2
A , oscillator strength f ˆ
g
1g
0
), while the strong end absorption rising below 210 nm corresponds to the allowed
1
‡
1
�
transition that is calculated to appear at 197 nm ( E
A , f ˆ2.8). The transition to
u
1g
1
�
the lower E state is predicted to lie at 258 nm (f ˆ0, forbidden by the parity rule [21])
u
and is tentatively associated with the weak shoulder observed at ca. 250 nm in the
spectrum of 1. A further forbidden excited state, namely the lowest doubly excited
1
�
state ( A , not included in the CI calculation), may be expected in that region.
1g
Comparison of the first reduction potentials, E , of 1 (‡0.79 V [13]) and 2
1
/2
(
‡0.20 V) with the corresponding value for COT (�1.61 V [13]) indicates that the
shift by ‡1.8 V on going from COT to 2 is due to the annelation by the electron-
attracting perfluoroalkane ring. A further change of ‡0.6 V, on passing from 2 to 1, is
attributed to the structural change, as 1 needs not be flattened on conversion to its
.
�
radical anion 1 . Chen and Wentworth [23] have shown that gas-phase electron

Helvetica Chimica Acta ± Vol. 82 (1999)
1441
Fig. 5. Hückel-orbital energy scheme of a planar cyclooctatetraene with alternating bond lengths
affinities, A, and half-wave reduction potentials, E , obey a direct linear relationship.
1/2
Electron affinities of 23 compounds, for which the reduction potentials were
determined in aprotic solvents vs. SCE, were best reproduced by Eqn. 2. Application
of this relation to 1 gives A(1) ˆ3.28 Æ0.26 eV.
A ˆeE1/2 ‡(2.49 Æ0.26) eV
(2)
Another estimate of A(1) may be obtained from the position of the CT absorption
maxima with various donors. Unfortunately, the parameters of the empirical linear
relationship, ECT ˆaI ‡b (Fig. 2) do not have a simple physical meaning [24].
v
Therefore, we follow the procedure of Chen and Wentworth [23], who have assumed the
validity of Eqn. 3 and have determined the constant C characteristic of various
ꢁ
benzenoidꢀ donor molecules using a series of acceptor molecules of known electron
affinity. They also showed that electron affinities, A, derived from reduction potentials
on the basis of Eqn. 2 and from CT spectra on the basis of Eqn. 3, are in good
agreement. The values for benzene, methylated benzenes, and naphthalene determined
by Chen and Wentworth were in the range of C ˆ�2.8 to �3.2 eV. Based on an average
value of C ˆ�3.0 eV, the data given in the Table provide an electron affinity A(1) ˆ
(
3.38 Æ0.14) eV, in accord with the value estimated above (Eqn. 2). The weighted
average is A(1) ˆ(3.4 Æ0.2) eV, which is among the highest of any neutral organic
compound [25].
Iv ‡C ˆECT ‡A
(3)
The powerful oxidizing properties of 1 are further manifested by the result that its
radical anion can be generated by mere contact with Hg metal. Both the g factor,
1
9
.�
2
.0010, and the F-coupling constant, ja jˆ0.320 mT, for 1 , which are strikingly
F
.
�
lower than the corresponding values, 2.0021 and 0.951 mT, for 2 , must be considered

1442
Helvetica Chimica Acta ± Vol. 82 (1999)
unusual (by chance, ja jˆ0.320 mT is very close to ja jˆ0.323 mT [26] for the
F
H
.
�
eight protons in COT ). It is not easy to find examples of radical anions that could
1
9
.�
.�
serve for predicting the b- F-coupling constants for 1 and 2 . The few pertinent p-
radicals dealt with in the literature are those substituted by CF3 groups [27].
Comparison with structurally related hydrocarbon radicals suggests that, on replace-
ment of a CH by a CF substituent, the ja jvalue should be larger than ja jby a
3
3
F
H
factor of ca. 2, provided that the spin distribution is not markedly altered. For p-radical
anions annelated with one cyclobutane or cyclopentane moiety, hyperfine data of
several derivatives of benzene [28 ± 30], naphthalene [30][31], and COT [30] are
reported. In all cases, the coupling constants of the b-protons in the CH groups lie in
2
the range ja jˆ0.4 ± 0.6 mT, with the values for the cyclobutane being 20 ± 40% larger
H
than those for the cyclopentane rings. We are not aware of data for related radical
anions containing annelated perfluoroalkane rings. According to the findings for CH
3
1
9
.�
.�
and CF substituents, the b- F-coupling constant for 1 and 2 should amount to ja j
3
F
.
�
ꢂ1 mT. This expectation is borne out by ja jˆ0.951 mT for 2 , whereas the
F
.
�
corresponding value, 0.320 mT, for 1 is clearly much smaller. Because of the scarcity
of suitable reference data in the literature and the difficulty of carrying out reliable
1
9
theoretical calculations on F-coupling constants, one must suppose that a 1,3-
.
�
interaction [30] is highly effective in the perfluorocyclobutane rings of 1 , in addition
to the hyperconjugative 1,2-interaction. Whereas hyperconjugation requires, in general,
a positive sign for the coupling constants [32a], the 1,3-interaction could make a
1
9
negative contribution to a , thus decreasing its absolute value. On the whole, b- F-
F
.
�
coupling constants should be positive, i.e., a ˆ‡0.320 mT for 1 and ‡0.951 mT (to
F
.
�
which ‡0.976 and ‡0.935 mT average out) for 2 . In the latter radical anion, an
19
opposite sign is indicated experimentally for the g- F-coupling constants, resulting in
a ˆ�0.90 and �0.59 mT.
F
1
3
.�
Of the two C-coupling constants observed for 1 , the one with the smaller
.
�
absolute value, ja jˆ0.135 mT, is almost identical to that reported for COT
C
(
0.130 mT) [26], a finding which strongly suggests that this coupling constant should
1
3
be assigned to the C-isotopes in the C-atoms of the eight-membered ring. Such an
assignment, with a positive sign of the coupling constant, a ˆ‡0.135 mT, is supported
C
by the broadening of the pertinent satellite lines at high field, diagnostic of a substantial
13
.�
local p-spin population at the C-center [32b]. The C-coupling constant for 1 with the
larger ja jvalues must, therefore, belong to the isotopes in the C-atoms of the eight
C
CF groups and, according to theory [32a], have a negative sign, leading to a ˆ
2
C
�
0.191 mT.
19
.�
It is rather unexpected that the sixteen b- F nuclei in 2 with the large coupling
constants (ja jˆ0.976 and 0.935 mT) become equivalent at higher temperatures,
F
whereas their eight g-counterparts with the smaller ja jvalues (0.090 and 0.059 mT)
F
do not. This finding is, however, not unprecedented, as an analogous observation
was made for the coupling constants of b- and g-protons in the radical anion of
indane [29].
1
Experimental. ± Tetrakis(perfluorocyclobuta)cyclooctatetraene ) (1) and tris(perfluorocyclobuta)benzene
were synthesized by a copper-induced coupling of 1,2-diiodo-3,3,4,4-tetrafluorocyclobutene, and tetrakis(per-
1
fluorocyclopenta)cyclooctatetraene ) (2) was formed by the corresponding treatment of 1,2-diiodo-3,3,4,4,5,5-
hexafluorocyclopentene [33].

Helvetica Chimica Acta ± Vol. 82 (1999)
1443
Synthesis of 1 (improved version of the procedure reported previously in [33]). Dry and freshly prepared
active Cu powder (2.54 g, 0.04 mol [34]; the precipitate was immediately washed with acetone and Et O,
allowing minimum exposure to air) and 1,2-diiodo-3,3,4,4-tetrafluorocyclobutene (1.85 g, 0.0049 mol [35]) were
added to a sublimation apparatus under dry N . The mixture was left overnight, then heated to 90 ± 938 in an oil
2
2
bath for 24 h, and finally to 1358 for another 24 h. The dark red solid (0.65 g) collected from the cold finger was
triturated in 5 ml of hot hexane, and filtered leaving 0.40 g of red crystals. Evaporation of the filtrate left 0.22 g
of a mixture of tris(perfluorocyclobuta)benzene and 2,2'-diiodobis[tetrafluorocyclobutene]. The red crystals
were placed in a glass tube, sealed under vacuum, and heated to 1008 in a thermal gradient sublimation
apparatus. Large, clear-red crystals of 1 (0.38 g, 62.6%, m.p. 201 ± 201.58) were obtained after 48 h. The product
19
gave a single spot by TLC on cellulose; decomposition occurred on Alox, silica gel, or Fluorisil. F-NMR
1
3
(
(
C
6
D
6
): �119.6 (br. s, fwhh ˆ27). C-NMR (C
6
D
6
, F-decoupled): 142.50 (ˆC ); 115.06 ppm (�CF
2
). MS: 496
] ). The electronic absorption
‡
‡
‡
‡
3
100, M ), 477 (48, [M �F] ), 446 (30, [M �CF
2
] ), 427 (61, [M �CF
spectrum is displayed in Fig. 1.
Absorption Spectra. The absorption spectrum of solid 1 was determined by placing a large, thin crystal of 1
on the entrance slit of a polychromator that was equipped with a diode-array detector. Charge-transfer
�
2
absorption spectra were measured with dilute solns. of 1 and a large excess (ca. 10 m) of various aromatic
�
1
�1
donors (Table) in hexane. The following absorbances were determined at lmax ˆ690 nm (eCT/[m cm ]) of the
�
4
�2
CT band with naphthalene as a donor in 2.15 Â10 m solns. of 1 in hexane at r.t., ACT([naphthalene]/10 m):
0
.062 (0.203), 0.158 (0.555), 0.216 (0.766), 0.264 (1.063), 0.310 (1.422), 0.423 (2.227), 0.685 (5.18).
Cyclic Voltammetry. For the measurement of 2 in MeCN, a Pt disk, Pt wire, and Bu
4
NPF
6
were used as
‡
working electrode, counter-electrode, and conducting salt, resp. The ferrocene/ferrocene wave served for
�
1
calibration. The scan was 300 mV s on a Metrohm Polarecord E506 equipped with a VA-stand 663 and a VA-
scanner E612.
ESR Spectra were recorded on a Varian-E9 instrument, while a Bruker-ESP-300 system served for the
ENDOR and TRIPLE-resonance studies.
This work was supported by the Swiss National Science Foundation.
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Received June 16, 1999