A novel TTF-based donor carrying four nitronyl nitroxides

Article abstract of DOI:10.1016/S0040-4039(03)00840-2

A novel TTF-based tetraradical donor was synthesized and its electronic structure was fully characterized by spectroscopic measurements, magnetic susceptibility measurements, and electrochemical analyses.

Full text of DOI:10.1016/S0040-4039(03)00840-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4415–4418
A novel TTF-based donor carrying four nitronyl nitroxides
Genta Harada,a Toshio Jin,a Akira Izuoka,a,b Michio M. Matsushitaa and Tadashi Sugawaraa,*
aDepartment of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro,
Tokyo 153-8902, Japan
bDepartment of Materials and Biological Sciences, Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito,
Ibaraki 310-8512, Japan
Received 17 February 2003; revised 20 March 2003; accepted 25 March 2003
Abstract—A novel TTF-based tetraradical donor was synthesized and its electronic structure was fully characterized by
spectroscopic measurements, magnetic susceptibility measurements, and electrochemical analyses. © 2003 Elsevier Science Ltd.
All rights reserved.
The chemical properties and electronic structures of
polyradicals, where stable radical units are connected
with a p-conjugating core, have been explored for a
long time and many types of polyradicals have been
prepared so far.1 Although the exchange coupling
between unpaired electrons on radical units was
observed, the intramolecular spin–spin interaction was
not large in most of cases due to the ‘disjoint character’
of the spin system.2 Accompanied by the progress of
molecular magnetism, high-spin polyradicals have been
synthesized on the basis of the spin-correlation in odd-
alternate hydrocarbons of a non-disjoint type.2 More-
over, dynamic molecular spin systems, in which the
magnetic property can be altered by modulating a
coordination-active,3 photo-active,4 or redox-active5 p-
conjugating core bearing radial units, have recently
been reported.
Meanwhile,
several
tetrathiafulvalene(TTF)-based
donor radicals have also been prepared.6 When a
nitronyl nitroxide group was introduced to the TTF
core through a p-phenylenethio unit as in the case of
donor radical (1), the donor radical turned out to
afford a ground state triplet cation diradical upon
one-electron oxidation due to the ‘non-disjoint’ charac-
ter of the resulting cation diradical species. This result
indicates the validity of the p-phenylenethio group as a
ferromagnetic coupler.
Here we report preparation of TTF-based tetraradical
donor (2), in which four vinylic protons of TTF are all
substituted with p-thiophenyl nitronyl nitroxides. One
of the methyl groups of the nitronyl nitroxide of 2 is
substituted by a hexyl group in order to increase the
solubility of the precursor of nitronyl nitroxide, that is
a cyclic bishydroxylamine. A preparative method for
the modified bishydroxylamine is also described. In
addition, the electronic structure and magnetic property
of this novel TTF-based tetraradical have been fully
Keywords: polyradical; p-ionradical; tetrathiafulvalene; nitronyl
nitroxide; high-spin molecule; spin multiplicity conversion.
* Corresponding author. Tel.: +81-3-5454-6742; fax: +81-3-5454-6997;
e-mail: suga@pentacle.c.u-tokyo.ac.jp
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00840-2

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G. Harada et al. / Tetrahedron Letters 44 (2003) 4415–4418
studied by UV–vis absorption spectroscopy, ESR spec-
troscopy, magnetic measurements, and electrochemical
analyses.
ture of two enantiomers, the mixture was used for later
reactions without enantiomeric separation. It should be
noted that this bishydroxylamine 5b is suitable for the
preparation of a cyclic bishydroxylamine, that is a
precursor of a nitronyl nitroxide derivative, especially
when the solubility of the corresponding tetramethyl
derivative is poor. Introduction of a long alkyl chain
also contributes to the fabrication of nitronyl nitrox-
ides, in particular, for the formation of a thin film.
Tetraradical donor 2 was prepared by the following
reaction scheme (Scheme 1).7 Vinyl protons of TTF
were lithiated by adding 5 equiv. of lithium di-iso-
propylamide in diethylether and the lithiated species
was reacted with acetal-disulfide to afford tetraacetal
(3), which was then hydrolyzed to tetraformyl deriva-
tive (4). The reaction between tetraformyl derivative 4
and 2,3-dimethyl-2,3-di(hydroxyamino)butane (5a) was
found not to proceed because of the poor solubility of
the condensation products. In order to increase the
Tetraformyl compound 4 was reacted with bishydroxyl-
amine 5b to afford tetrakis(cyclic bishydroxylamine)
(7),11 which was oxidized by excess lead dioxide in THF
to give the desired tetraradical 2.12 The crude tetra-
radical 2 was purified by column chromatography
(silica-gel) with THF as an eluant and was isolated as
green powders. It was found that tetraradical 2 is very
stable and can be stored for a year in a refrigerator.
solubility,
2,3-dimethyl-2,3-di(hydroxyamino)nonane
(5b), in which one of the methyl groups of 5a is
substituted by a hexyl group, was prepared according
to the following reaction scheme (Scheme 1, middle).
Although the preparation of an alkylated bishydroxyl-
amine has been reported by Tamura et al.8 the prepar-
The UV–vis absorption spectrum of 2 in dichloro-
methane showed broad absorption bands at 378 and
605 nm: these bands are characteristic to a nitronyl
nitroxide group directly attached to a p-conjugating
system. They are red-shifted in comparison with those
of phenyl nitronyl nitroxide at 362 and 597 nm, similar
to those of 1,6a suggesting the presence of the electronic
interaction between the TTF unit and the NN-
phenylthio groups even in the neutral state.
ative condition has
a
room for improvement
considerably. We have optimized the preparation route
as follows. Dinitro compound (6) was obtained accord-
ing to the literature based on Kornblum’s method.9
Reduction of dinitro compound 6 was successfully car-
ried out with aluminum-amalgam10 in aqueous tetra-
hydrofuran (THF) to produce 2,3-dimethyl-2,3-di-
(hydroxyamino)nonane 5b in a reasonably good yield.
Although bishydroxylamine 5b was obtained as a mix-
Scheme 1. Synthetic route to tetraradical donor 2.

G. Harada et al. / Tetrahedron Letters 44 (2003) 4415–4418
4417
Figure 2. Temperature dependence of the magnetic suscepti-
bility of 2. The open circle shows the product of the magnetic
susceptibility and temperature (pT) of 2, whereas the solid
line shows the calculated value with q=−0.88 K.
Figure 1. (a) ESR spectrum of TTF-based tetraradical 2 in
benzene at room temperature. (b) Simulated spectrum with
g=2.0062, aN=0.181 mT, and half line-width of 0.18 mT for
w=9.14548 GHz.
smaller than that of the intermolecular one in the
neutral state.
Figure 3a shows the cyclic voltammogram of tetra-
radical 2. Three reversible redox waves indicates that
each oxidation state is reasonably stable. Since the
separation of the redox waves was not sufficient, the
differential pulse voltammogram of 2 was measured for
the determination of the accurate redox potentials. As
shown in Figure 3b, three redox peaks were observed at
720, 850, and 1070 mV versus Ag/AgCl with the ratio
of 1:4:1. Judging from the first redox potential, tetra-
radical 2 has a reasonable donor ability. The first,
The ESR spectrum of a benzene solution of tetraradical
2 showed multiple lines with g=2.0062 and aN=0.181
mT as shown in Figure 1a. The g value of 2 is close to
that of monoradical 1, and the aN value is 1/4 of that of
the monoradical,13 strongly suggesting that the
unpaired electron of 2 is exchange coupled with four
equivalent nitronyl nitroxides. Although the spectrum
of 2 was poorly resolved, the spectral shape was repro-
duced reasonably well using the g value, the aN of eight
equivalent nitrogen atoms, and the half line-width of
0.18 mT as shown in Figure 1b.14 These data provide a
good proof for the tetraradical structure of 2. Although
the line width of 2 is broader than that of monoradical
1, the broadened line shape of 2 was not sharpened
even when the temperature was raised up to 346 K.
This result suggests that the line broadening is derived
from neither the restricted rotation of the congested
substituents nor the slow tumbling motions of the
whole molecule in solution. The line-broadening was
presumably due to the presence of five diastereomers
derived from the chirality in four radical moieties.
The temperature dependence of the magnetic suscepti-
bility of a powdered sample of 2 was measured by a
SQUID magnetometer and the pT plot of 2 was shown
in Figure 2, together with the calculated line according
to Curie–Weiss law, p=C/(T−q). The constant pT
values at higher temperatures suggest the paramagnetic
behavior of tetraradical 2. The Curie constant (C) of
1.51 was four times larger than the value (0.375) for
paramagnetic spins of S=1/2, indicating the presence
of four radical centers in one molecule. The negative
Weiss constant of q=−0.88 K was reproduced reason-
ably well by the mean-field approximation. Therefore,
the negative Weiss constant may be ascribed to the
intermolecular antiferromagnetic interaction. The
intramolecular spin–spin coupling is, if any, to be
Figure 3. Redox potentials of TTF-based tetraradical donor 2
measured in dichloromethane in the presence of 0.1 mol l−1
tetra-n-butylammonium perchlorate as an electrolyte, using a
BAS CV-50 W electrochemical analyzer. A platinum electrode
(BAS PTE, 1.6 mm diameter) was employed as the working
electrode, an Ag/AgCl electrode (BAS RE-1B) as the refer-
ence electrode, and the counter electrode was a platinum wire
(0.5 mm diameter). (a) Cyclic voltammogram of 2 with a
scanning rate of 200 mV s−1. (b) Differential pulse voltam-
mogram of 2.

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G. Harada et al. / Tetrahedron Letters 44 (2003) 4415–4418
second, and third redox peaks are assigned to the
oxidation process of the TTF moiety, four nitronyl
nitroxides, and the cation radical of TTF moiety,
respectively, on the basis of the relative intensity of
redox peaks and the redox potentials of reference com-
pounds. It is important that the first oxidation does not
occur at the radical moiety, because if the first oxida-
tion process removed the unpaired electrons from the
radical sites of tetraradical 2, it would afford an unde-
sired closed-shell tetracation species.
6. (a) Nakazaki, J.; Matsushita, M. M.; Izuoka, A.; Sug-
awara, T. Tetrahedron Lett. 1999, 40, 5027; (b) Nakazaki,
J.; Ishikawa, Y.; Izuoka, A.; Sugawara, T.; Kawada, Y.
Chem. Phys. Lett. 2000, 319, 385–390.
7. An ordinary method for the preparation of nitronyl
nitroxides, see: (a) Osiecki, J. H.; Ullman, E. F. J. Am.
Chem. Soc. 1968, 90, 1078; (b) Ullman, E. F.; Osiecki, J.
H.; Boocock, D. G. B.; Darcy, R. J. Am. Chem. Soc.
1972, 94, 7049.
8. Matsuura, H.; Tamura, R.; Yamauchi, J. Proceedings of
the 14th Symposium on Fundamental Organic Chemistry
in Japan, 1998, 359.
Based on above experimental data, the singly oxidized
species of the tetraradical 2 is considered to exist as a
pentaradical of a ground state sextet spin-multiplicity,15
9. (a) Kornblum, N.; Singh, H. K.; Kelly, W. J. J. Org.
Chem. 1983, 48, 332; (b) Kerber, R. C.; Urry, G. W.;
Kormblum, N. J. Am. Chem. Soc. 1965, 87, 4520.
10. Weis, C. D.; Newkome, G. R. Synthesis 1995, 1053.
11. The radical precursor 7 is also a mixture of optical and
diastereomeric isomers. Spectrum data for 7: 1H NMR
(DMSO-d6): l=0.87 (t, 12H), 0.98, 1.02, 1.05, 1.08, 1.09
(s, 36H), 1.18–1.77 (m, 40H), 4.51, 4.54 (s, 4H), 7.40 (d,
8H, J=7.69 Hz), 7.51 (d, 8H, J=7.69 Hz), 7.48 (s, 2H),
7.69 (s, 2H), 7.74 (s, 2H), 7.96 ppm (s, 2H). FAB-MS;
calcd for C78H116N8O8S8 (M+): 1550.33. Found: 1549.9.
12. Elemental analysis data of 2; calcd for C78H104N8O8S8: C,
60.90; H, 6.81; N, 7.28; O, 8.32; S, 16.68%. Found: C,
61.01; H, 6.85; N, 7.31; S, 16.50%. IR (KBr, cm−1); 2927
(s), 2855 (m), 1466 (w), 1424 (s), 1363 (s), 1300 (s), 826
(w).
because the p-phenylenethio unit act as a ferromagnetic
6a
coupler between TTF+ and nitronyl nitroxide groups.
’
Moreover, if the tetraradical 2 forms charge transfer
complexes with acceptors, it may provide a ferrimag-
netic spin system.16 In summary, the newly synthesized
TTF-based tetraradical 2 would be an important para-
magnetic building block to construct novel organic spin
systems.
Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research (13304056) from Ministry of Education, Sci-
ence, Technology, Sports and Culture, Japan.
13. The ESR spectrum of a benzene solution of monoradical
1 shows five lines of the 1:2:3:2:1 relative intensity with
g=2.0072, aN=0.75 mT for two equivalent nitrogen
atoms. See Ref. 6a.
References
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36: 112: 266: 504: 784: 1016: 1107: 1016: 784: 504: 266:
112: 36: 8: 1.
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15. Although the fine structures were observed in the ESR
spectrum of the singly oxidized species of 2 in frozen
matrix of 2-methyltetrahydrofuran and they were pre-
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be discussed elsewhere.
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