Single-component CT crystals based on 1,4-benzoquinone derivatives and TEMPO radical

Article abstract of DOI:10.1246/cl.2008.788

Four kinds of TEMPO-substituted 1,4-benzoquinone derivatives were found to afford single-component CT crystals formed by the interactions between a benzoquinone ring and one or two TEMPO groups of the neighboring molecules. Copyright

Full text of DOI:10.1246/cl.2008.788

788
Chemistry Letters Vol.37, No.7 (2008)
Single-component CT Crystals Based on 1,4-Benzoquinone Derivatives and TEMPO Radical
Mitsunori Nobusawa, Hiroki Akutsu, Jun-ichi Yamada, and Shin’ichi Nakatsujiꢀ
Graduate School of Material Science, University of Hyogo,
3-2-1 Kouto, Kamigori, Akou-gun, Hyogo 678-1297
(Received May 9, 2008; CL-080476; E-mail: nakatuji@sci.u-hyogo.ac.jp)
Table 1. CV data of benzoquinone derivativesa
Four kinds of TEMPO-substituted 1,4-benzoquinone
derivatives were found to afford single-component CT crystals
formed by the interactions between a benzoquinone ring and
one or two TEMPO groups of the neighboring molecules.
RED
RED
OX
Compound
E1
E2
E1
1a
1b
2a
2b
3a
3b
ꢁ0:26
ꢁ0:27
ꢁ0:40
ꢁ0:39
ꢁ0:37
ꢁ0:39
ꢁ0:13
ꢁ0:12
ꢁ0:92
ꢁ0:92
ꢁ0:91
ꢁ0:96
ꢁ0:96
ꢁ0:92
ꢁ0:84
ꢁ0:79
0.72
0.72
0.75
0.74
0.73
0.75
CT complexes are generally formed by two component sys-
tems normally composed of a donor and an acceptor by an inter-
molecular interaction.1 Although a number of such molecules
are known that have both donor and acceptor unit in a single
molecule, examples of single-component CT complexes exhibit-
ing remarkable functionality are still limited2 probably because
of the difficulty of controlling the donating and accepting ability
and/or steric effects detrimental to forming a CT complex with
sufficient CT interaction. To our knowledge, there is so far no
example of a single-component CT complex based on stable rad-
icals, even though several CT complexes involving stable radi-
cals have been reported, consisting of two or more components.3
During our studies toward the development of organic mul-
tifunctional spin systems,4 we prepared several benzoquinone
derivatives carrying 4-amino-TEMPO radical to form CT com-
plexes with TTF and its derivative.5 Furthermore, it was clarified
that 4-amino-TEMPO derivatives act themselves as donors to
afford intermolecular CT complexes with some acceptors.6
We wish to report in this paper the preparation of several
1,4-benzoquinone derivatives with 4-oxy-TEMPO radical 1–3
(Chart 1) and serendipitous formations of the first examples of
single-component CT crystals based on a stable radical together
with their structures and magnetic properties.
The reaction of chloranil or bromanil with 4-hydroxy-
TEMPO in the presence of potassium carbonate in DMF gave
after separation by column chromatography on SiO2 and recrys-
tallization 2-(4-oxy-TEMPO)-3,5,6-trihalo-1,4-bezoquinone 1a
or 1b as the major products (1a: 48%, 1b: 65%). Noteworthy
was the black color of the recrystallized crystals of 1a and 1b
from the pale yellow pentane–n-hexane (1:1) solution, indicating
occurrence of charge transfer within the molecules. Further
examination of other eluants revealed the formation of bis-
TEMPO-substituted benzoquinones 2 and 3 as the minor prod-
ucts (each <10%), which could be purified by recrystallization
and elucidated by X-ray analyses. Again black crystals were
obtained in the cases of 2a and 2b, while orange crystals were
grown in the cases of 3a and 3b, suggesting occurrence of CT
Chloranil
Bromanil
TEMPO
0.70
aV vs. SCE, 0.1 M n-Bu4NClO4 in CH3CN.
in the former crystals but not in the latter.
The reduction and oxidation potentials of each benzoqui-
none derivative were estimated by cyclic voltammetry and the
data are summarized in Table 1.
It is apparent from the data that 1a and 1b are weaker
acceptors compared to choranil or bromanil and still weaker
acceptors are 2 and 3. There is no significant difference between
the redox data of chloro-series and bromo-series and between 2
and 3. On the contrary, the colors of the crystals of 2 and 3
are different in spite of the similarity of their redox data and that
implies the difference of their crystal structures.
The X-ray analyses of 1a and 1b indicate that they have
similar molecular/crystal structures.7 The TEMPO groups of
1a and 1b are largely distorted from the corresponding benzo-
quinone ꢀ-planes and a couple of benzoquinone rings face each
˚
˚
other with the plane-to-plane distance of 3.61 A in 1a and 3.64 A
in 1b, respectively (Figure 1). The most remarkable features of
their crystal structures are the very close proximities of oxygen
atoms of the radical moieties to the benzoquinone rings in 1a and
˚
˚
1b (2.79 A in 1a and 2.83 A in 1b), suggesting the occurrence of
charge transfer between them and reflecting the black colors of
their crystals (vide supra).
Antiferromagenetic interactions obeying the Curie–Weiss
law are observed in the spins of 1a and 1b with the Weiss tem-
perature of ꢁ0:41 K for 1a and ꢁ1:87 K for 1b, respectively.8
As no close contact is observed in their crystal structures other
than those described above, the antiferromagnetic interactions
observed are probably due to those between the spin centers
mediated by a couple of benzoquinone rings.
Again, similar molecular/crystal structures are apparent
between 2,5-disubstituted derivatives 2a and 2b as shown in
Figure 2. In this case, the benzoquinone ring of a molecule is
being put between two TEMPO groups of two neighboring
O
O
O
N
O
N
O
O
N
N
O
O
X
O
X
O
X
O
X
X
X
O
X
O
˚
˚
molecules with a short distance of 2.90 A for 2a and 2.89 A
for 2b, respectively, resulting in the occurrence of charge trans-
fer between the donor and acceptor groups.
O
N
O
O
1a: X = Cl
1b: X = Br
2a: X = Cl
2b: X = Br
3a: X = Cl
3b: X = Br
Chart 1.
The magnetic data of these compounds obey a singlet–triplet
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.7 (2008)
789
b
b
b
3.646 Å
4.371(12) Å
4.276(8) Å
2.831 Å
a
a
c
3.315(8) Å
3.351(13) Å
o
c
c
o
o
a
Figure 1. Crystal structures of 1a (left) and 1b (right). Four
molecules are depicted in each structure. The arrows indicate
short intermolecular contacts.
Figure 3. Crystal structures of 3a (left) and 3b (right). Five
molecules are depicted in each structure. The broken lines
indicate short intermolecular contacts.
In summary, several single-component CT crystals based on
TEMPO-substituted 1,4-benzoquinone derivatives were found to
form by the interactions between a benzoquinone ring and one or
two TEMPO groups of the neighboring molecules, giving the
first examples of single-component CT complexes involving a
stable radical.
References and Notes
1
For reviews on CT complexes, see for example: a) H. A.
Bent, Chem. Rev. 1968, 68, 587. b) Organic Charge-transfer
Complexes, ed. by R. Foster, Academic Press, London, 1969.
c) The Donor-Acceptor Approach to Molecular Interactions,
ed. by V. Gutmann, Plenum Press, New York, 1978.
a) M. Iwamatsu, T. Kominami, K. Ueda, T. Sugimoto, T.
Adachi, H. Fujita, H. Yoshino, Y. Mizuno, K. Murata, M.
Shiro, Inorg. Chem. 2000, 39, 3810. b) H. Tanaka, Y. Okano,
H. Kobayashi, W. Suzuki, A. Kobayashi, Science 2001, 291,
285. c) D. Belo, H. Alves, E. B. Lopes, M. T. Duarte, V.
Figure 2. Crystal structures of 2a (left) and 2b (right). Three
molecules are depicted in each structure. The arrows indicate
short intermolecular contacts.
model and almost the same weak antiferromagnetic interactions
with J ¼ ꢁ2:9 K for 2a and J ¼ ꢁ3:0 K for 2b are estimated
from the data possibly by intermolecular spin–spin interactions.9
Although the magnetic data cannot be well understood yet by
considering the structural motifs, singlet formation is apparently
operating through a benzoquinone ring at cryogenic temperature
for each crystal.
Contrary to the previous crystals, no close contact is seen
between TEMPO and benzoquinone groups in the crystals of
3a and 3b, while somewhat short distances are observed between
the oxygen atoms of a TEMPO group in a molecule and that of
2
´
´
Gama, R. T. Henriques, M. Almeida, A. Perez-Benıtez, C.
Rovira, J. Veciana, Chem.—Eur. J. 2001, 7, 511.
S. Nakatsuji, in Nitroxides: Applications in Chemistry,
Biomedicine, and Materials Science, ed. by G. I.
Likhtenstein, J. Yamauchi, S. Nakatsuji, A. I. Smirnov, R.
Tamura, Wiley-VCH, Weinheim, 2008, p. 168.
3
˚
˚
a neighboring molecule (4.28 A for 3a and 4.37 A for 3b) and
between the oxygen atom of another TEMPO group of the
molecule and the carbon atom of a methyl group of another
4
5
S. Nakatsuji, J. Synth. Org. Chem. Jpn. 2003, 61, 670.
S. Nakatsuji, N. Akashi, K. Suzuki, T. Enoki, H. Anzai,
J. Chem. Soc., Perkin Trans. 2 1996, 2555.
S. Nakatsuji, A. Takai, K. Nishikawa, Y. Morimoto, N.
Yasuoka, K. Suzuki, T. Enoki, H. Anzai, J. Mater. Chem.
1999, 9, 1747.
Crystal data of 1–3 reported in this paper have been deposit-
ed with Cambridge Crystallographic Data Centre. Copies
of the data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html.
Magnetic data of 1–3 are electronically available as Support-
ing Information which is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/.
The J-values were estimated by using Bleany–Bowers
equation: B. Bleaney, K. D. Bowers, Proc. R. Soc. London,
Ser. A. 1952, 14, 451.
˚
˚
neighboring molecule (3.31 A for 3a and 3.35 A for 3b) as
shown in Figure 3. The orange colors of the crystals suggest
no occurrence of charge transfer interactions in the crystals in
these cases.
6
7
Whereas the intermolecular magnetic interactions of 3a are
antiferromagnetic, ferromagnetic interactions are observed in
the spins of 3b. The difference of the magnetic interactions is
supposed to reflect the difference of the distances between the
spins described above. The shorter O–O distance found in 3a
compared to that of 3b may result in the predominance of
antiferromagnetic interactions in 3a, while ferromagnetic inter-
actions due to spin polarization through the hydrogen bonds
may comparatively contribute to the ferromagnetic interactions
observed between the spins of 3b.
8
9
Published on the web (Advance View) June 21, 2008; doi:10.1246/cl.2008.788