2,6,10-Tris(dianisylaminium)-3,7,11-tris(hexyloxy)triphenylene: A robust quartet molecule at room temperature

Article abstract of DOI:10.1021/ol060379f

A new triradical molecule, 2,6,10-tris(dianisylaminium)-3,7,11- tris(hexyloxy)triphenylene 1³⁺, was synthesized by oxidative trimerization, palladium-catalyzed amination, and subsequent oxidation. It was chemically stable with a half-life > 1 m

Full text of DOI:10.1021/ol060379f

ORGANIC
LETTERS
2006
Vol. 8, No. 9
1835-1838
2,6,10-Tris(dianisylaminium)-3,7,11-
tris(hexyloxy)triphenylene: A Robust
Quartet Molecule at Room Temperature
Eiji Fukuzaki and Hiroyuki Nishide*
Department of Applied Chemistry, Waseda UniVersity, Tokyo 169-8555, Japan
nishide@waseda.jp
Received February 14, 2006
ABSTRACT
+
A new triradical molecule, 2,6,10-tris(dianisylaminium)-3,7,11-tris(hexyloxy)triphenylene 13 , was synthesized by oxidative trimerization, palladium-
catalyzed amination, and subsequent oxidation. It was chemically stable with a half-life > 1 month and displayed the magnetic parameter of
S
) 3/2 even at room temperature.
There has been a continuous interest in the synthesis of high-
spin organic molecules1 because they are expected to be the
spin module for forming a magnetically active bulk material.
The high-spin behavior of molecules is derived from the spin
alignment among the radicals’ unpaired electrons conjugated
with an aromatic pathway in a non-Kekule´ and nondisjoint
fashion (or a ferromagnetic connectivity).2 The 1,3,5-
substituted benzene core3,4 and π-conjugated cyclic mol-
ecules, such as 2,8,14-benzenedehydro[12]annulene-5 and
calix[4]arene-based polyradicals,6 have been proposed and
synthesized as a ferromagnetic coupler for the multiradicals.
However, these molecules have been obtained via tedious
reaction steps and/or were difficult to utilize as a high-spin
module for extending them to a high-spin polyradical or
magnetically active and organic-based material.
Triphenylene7 is known to have a cyclic and planar
π-conjugated skeleton, and its derivatives have been exten-
sively synthesized and studied as discotic liquid crystal
molecules with functionalities, such as a photoconductive
property.8,9 For example, the columnar mesophase stack of
hexapentyloxytriphenylene showed a charge migration with
a significantly high mobility along the column.10 In this study,
we expected the triphenylene core to function as a new
ferromagnetic coupler: through the planar and the cyclic
(1) (a) Lahti, P. M. Magnetic Properties of Organic Materials; Marcel
Dekker: New York, 1999. (b) Itoh, K.; Kinoshita, M. Molecular Magnetism;
Kodansha: Tokyo and Gordon and Breach: Amsterdam, 2000. (c) Rajca,
A. Chem.-Eur. J. 2002, 8, 4834. (d) Baumgarten, M. In Progress in
Theoretical Chemistry and Physics; Lund, A., Shiotani, M., Ed.; Kluwer
Academic Publishers: Dordrecht, 2003; Vol. 10, Chapter 12. (e) Makarova,
T. L.; Palacio, F. Carbon Based Magnetism: An OVerView of the Magnetism
of Metal Free Carbon-Based Compounds and Materials; Elsevier: Am-
sterdam, 2006. (f) Fukuzaki, E.; Nishide, H. J. Am. Chem. Soc. 2006, 128,
996.
(2) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587.
(3) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994, 116, 11576.
(4) Nishide, H.; Doi, R.; Oyaizu, K.; Tsuchida, E. J. Org. Chem. 2001,
66, 1680.
(6) Rajca, A.; Rajca, S.; Desai, S. R. J. Am. Chem. Soc. 1995, 117, 806.
(7) (a) Mannich, C. Ber. 1907, 40, 153. (b) Mannich, C. Ber. 1907, 40,
159.
(8) Adam, D.; Schuhmacher, P.; Siemmerer, J.; Ha¨ussling, L.; Siemens-
meyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371,
141.
(9) Maliszewskyj, N. C.; Heiney, P. A.; Josefowicz, J. Y.; McCauley,
J., Jr.; Smith, A. B., III. Science 1994, 264, 77.
(10) Adam, D.; Schuhmacher, P.; Siemmerer, J.; Ha¨ussling, L.; Paulas,
W.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. AdV.
Mater. 1995, 7, 276.
(5) Nishide, H.; Takahashi, M.; Takashima, J.; Pu, Y.; Tsuchida, E. J.
Org. Chem. 1999, 64, 7375.
10.1021/ol060379f CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/05/2006

π-conjugated skeleton of triphenylene, which has two rigid
pathways for the interaction, the substituted three radicals’
unpaired electrons could strongly interact and a spin defect
which interrupts the interaction would not be vital, leading
to a triplet molecule at room temperature.
Several groups reported the synthesis of symmetrical,
unsymmetrical, and functionalized triphenylene derivatives.
Some of the triphenylene derivatives are prepared by the
trimerization of halogenated aryls using Pd coupling11 and
lithiation.12 The unsymmetrical triphenylenes13 are also
synthesized by the coupling of the biphenyl precursor and
the aryl substituted with different substituents. Among these
synthetic routes of the triphenylene derivatives, the oxidative
trimerization of phenyl ether14 is one of the efficient routes
to obtain a triphenylene with substituent groups on the 2, 6,
and 10 positions, which possibly produce the ferromagnetic
coupling.
This paper describes a room-temperature robust quartet
molecule, 2,6,10-tris(dianisylaminium)-3,7,11-tris(hexyloxy)-
triphenylene 13+. We selected, for the first time, the 2,6,-
10-substituted triphenylene as a planar and π-conjugated
ferromagnetic coupling core for the high-spin alignment. The
2,6,10-position of the triphenylene was very appropriate for
significantly reducing both the steric hindrance of the radical
moieties and electrostatic repulsion between the cationic
radicals, which lead to the chemical stable molecule 13+ with
the spin alignment of S (spin quantum number) ) 3/2.
The triphenylene core 3 was prepared by the oxidative
trimerization of the 2-hexyloxyanisole 2 with FeCl3 (Scheme
1).19 The methoxy groups of 3 were converted to triflate
Scheme 1
For the molecular design of high-spin molecules, the
second requisite is the selection of a stable organic radical
species as the spin source. Molecules substituted with the
triarylmethane radical and the phenoxyl radical have been
studied;15,16 however, their high-spin states were observed
only in a low-temperature range because of the chemical
instability of these radical species. Chemically stable radical
species are desired in such high-spin molecules to raise the
temperature range of the spin alignment and possibly use
them as a module for a molecular-based material. From
among the list of radical species, the triarylaminium radical
is a favorable candidate for the spin source which could fulfill
the criteria of having both a substantial chemical stability
even at room temperature and a strong spin polarization for
spin alignment.1f,17 From such a viewpoint, a series of high-
spin aminium radical modules have been synthesized;18
however, they often lacked stability at room temperature and
the quantitative radical (spin) generation because of their high
oxidation potential and/or side reactions.
(11) Pena, D.; Perez, D.; Guitian, E.; Castedo, L. Org. Lett. 1999, 1,
1555.
(12) Heaney, H.; Mann, F. G.; Millar, T. J. Chem. Soc. 1957, 3930.
(13) Borden, N.; Bushby, R. J.; Cammidge, A. N. J. Chem. Soc., Chem.
Commun. 1994, 465.
(14) Naarmann, H.; Hanack, M.; Mattmer, R. Synthesis 1994, 477.
(15) (a) Rajca, A.; Wongsriratanakul, J.; Rajca, S. Science 2001, 294,
1503. (b) Rajca, A.; Wongsriratanakul, J.; Rajca, S. J. Am. Chem. Soc. 2004,
126, 6608.
(16) (a) Nishide, H.; Nambo, M.; Miyasaka, M. J. Mater. Chem. 2002,
12, 3578. (b) Kaneko, T.; Makino, T.; Miyaji, H.; Teraguchi, M.; Aoki, T.;
Miyasaka, M.; Nishide, H. J. Am. Chem. Soc. 2003, 125, 3554.
(17) (a) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem.
1997, 62, 448. (b) Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc.
1997, 119, 4492. (c) Bushby, R. J.; Gooding, D. J. Chem. Soc., Perkin
Trans. 2 1998, 1069. (d) Selby, T. D.; Blackstock, S. C. J. Am. Chem. Soc.
1999, 121, 7152. (e) Goodson, F. E.; Hauck, S. I.; Hartwig, J. F. J. Am.
Chem. Soc. 1999, 121, 7527. (f) van Meurs, P. J.; Janssen, R. A. J. J. Org.
Chem. 2000, 65, 5712. (g) Nelsen, S. F.; Trieber, D. A., II.; Ismagilov, R.
F.; Teki, Y. J. Am. Chem. Soc. 2001, 123, 5684. (h) Yamamoto, K.; Higuchi,
M.; Shiki, S.; Tsuruta, M.; Chiba, H. Nature 2002, 415, 509. (i) Michinobu,
T.; Inui, J.; Nishide, H. Org. Lett. 2003, 5, 2165. (j) Murata, H.; Takahashi,
M.; Namba, K.; Takahashi, N.; Nishide, H. J. Org. Chem. 2004, 69, 631.
(k) Murata, H.; Yonekuta, Y.; Nishide, H. Org. Lett. 2004, 6, 4889. (l)
Wu, J.; Baumgarten, M.; Debije, M. G.; Warman, J. M.; Mu¨llen, K. M.
Angew. Chem., Int. Ed. 2004, 43, 5331.
groups in compound 5 via selective ether cleavage and
esterification. The 2,6,10-tris(p-dianisylamino)-3,7,11-tris-
(hexyloxy)triphenylene 1 was obtained by Pd coupling of
the triflates 5 with dianisylamine in the presence of Cs2CO3.20
The yields of the trimerization and ether cleavage were 10
and 75%, respectively, which were comparable to those
previously reported19 for the corresponding reactions and
fairly good for obtaining 1. For the process from 5 to 1, the
triflate is a good leaving group to be substituted with the
dianisylamine by the Pd coupling. 5 may be an effective
intermediate to yield other radical (phenoxyl, galvinoxyl,
nitroxide, and nitronyl nitroxide) precursor-substituted tri-
phenylenes. The NMR, IR, and mass spectra of 1 supported
its structure. For example, two singlet signals of the protons
and triplet signals of the hexyloxy groups on the triphenylene
(18) (a) Selby, T. D.; Blackstock, S. C. Org. Lett. 1999, 1, 2053. (b)
Hauck, S. I.; Lakshmi, K. V.; Hartwig, J. F. Org. Lett. 1999, 1, 2057. (c)
Selby, T. D.; Stickley, K. R.; Blackstock, S. C. Org. Lett. 2000, 2, 171. (d)
Ito, A.; Ono, Y.; Tanaka, K. Angew. Chem., Int. Ed. 2000, 39, 1072.
(19) Boden, N.; Bushby, R. J.; Martin, P. S.; Evans, S. D.; Owens, R.
W.; Smith, D. A. Langmuir 1999, 15, 3790.
(20) (a) A° hman, J.; Buchwald, S. L. Tetrahedron Lett. 1997, 38, 6363.
(b) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 1997, 62, 1264.
1836
Org. Lett., Vol. 8, No. 9, 2006

core were ascribed to the symmetrical structure of 1 and
significantly shifted to a lower magnetic field (8.11, 7.61,
and 3.89 ppm) due to the strong ring current effect of the
triphenylene core in comparison with the protons on the
diphenylamine groups.
Cyclic voltammetry of the triaminotriphenylene 1 showed
characteristic redox waves. The first and second oxidation
waves were chemically reversible (inset a of Figure 1). The
Figure 2. Electrolytic UV/vis spectrum of 1 (0.03 mM) in CH2-
Cl2 with 0.1 M (C4H9)4NBF4 at room temperature, at the given
potential of 0.9 V (a) and oxidized at 1.5 V (b) vs Ag/AgCl.
with g ) 2.0031, indicating the generation of a nitrogen-
centered radical. The ESR spectrum changed to a strong
unimodal signal attributed to a high concentration with the
applied time. These results indicated that the triaminotri-
phenylene 1 was oxidized to the aminium radical at 0.9 V,
and the radical was chemically stable in CH2Cl2 at room
temperature.
On the other hand, the absorption spectrum of the aminium
radical disappeared at the applied potential of 1.5 V, and a
new absorption appeared at 540 nm with isoberic points at
329, 457, and 643 nm (Figure 2b). This spectrum resembled
that of a dicationic phenylenediamine.21 The ESR absorption
completely disappeared at the applied potential of 1.5 V.
These results suggested the overoxidation of 13+ to an ESR-
silent cationic state. These electrochemical measurements
indicated that the triaminium triphenylene 13+ was generated
at the applied potential of 0.9 V vs Ag/AgCl.
On the basis of the electrochemical oxidation result, AgBF4
(E°′ ) 0.65 vs ferrocene in CH2Cl222) was selected as the
oxidizing agent of 1 to yield 13+. AgBF4 is also characterized
by its counteranion that possibly stabilizes the formed
aminium cationic radical. The triaminotriphenylene 1 was
treated with an equimolar amount of AgBF4 solubilized in
the acidic mixture of trifluoroacetic acid, trifluoro anhydride,
and CH2Cl2 (vol 0.25/0.25/99.5). Upon oxidation, the solution
turned a deep green, and the ESR spectrum gave a strong
unimodal signal (g ) 2.003). The half-life of 13+ was
estimated by the ESR signal intensity to be 1.2 months at
room temperature. The ESR at 100 K gave a quartet signal
with a D value of 0.0022 cm-1. The D value was compatible
with that (0.0026 cm-1) reported for the 1,3,5-benzene-based
cationic triradical 1,3,5-tris(triphenylenediamino)benzene,17a
which supported the formation of a N-centered and sym-
metrical triradical.
Figure 1. Cyclic and differential pulse voltammogram of 1 (0.33
mM) in CH2Cl2 with 0.1 M (C4H9)4NBF4 at room temperature and
a 50 mV s-1 scan rate. Inset (a): repeatedly recorded voltammogram
of 1 in the potential range of 0-1.0 V. Inset (b): controlled potential
coulometry at 0.9 V for 1 (1.67 × 10-6 mol) in CH2Cl2 with 0.1
M (C4H9)4NBF4 at room temperature. Potential vs Ag/AgCl.
voltammogram was repeatedly recorded 100 times for the
potential (on 0-1.0 V) sweeps at room temperature. The
redox potential was E°′(1) ) 0.60 and E°′(2) ) 0.73 V vs
Ag/AgCl and ascribed to the one- and two-electron processes,
respectively, also based on the differential pulse voltammetry
of 1 (0.59 and 0.68 V, respectively). However, the third
oxidation wave at E°′(3) ) 1.30 V (1.25 V in the differential
pulse voltammetry) was irreversible, which was probably
ascribed to overoxidation of the aminium cation or oxidation
of the triphenylene core. Controlled potential coulometry was
carried out on the 1 solution at 0.90 V (inset b of Figure 1),
which quantitatively supported the triequivalent (three elec-
trons) oxidation of 1 at the given potential.
The CH2Cl2 solution of the triaminotriphenylene 1 was
electrochemically oxidized at the applied potential of 0.9 or
1.5 V, and the reaction was monitored by in situ UV/vis
and ESR spectroscopies. The UV/vis absorption (λmax ) 270,
343, nm) of the starting ESR-silent 1 decreased under the
applied potential of 0.9 V, and new absorptions appeared at
442 and 782 nm with isoberic points at 276 and 365 nm
(Figure 2a). These absorptions fairly agreed with those of a
triarylaminium radical previously reported.21 Upon the initial
oxidation, the ESR showed a three-line absorption spectrum
(21) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D.
W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498.
(22) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
Org. Lett., Vol. 8, No. 9, 2006
1837

The magnetization and static magnetic susceptibility of
the triaminium triphenylene 13+ in the frozen acidic CH2Cl2
were measured by SQUID. The magnetization normalized
with the saturated magnetization (M/Ms) vs the ratio of
magnetic field and temperature (H/(T - θ)) plots for 13+
with a spin concentration (per the amine residue) of 0.923
were very close to the theoretical Brillouin curve for S )
3/2, indicating the quartet state of 13+. The ratio of the
effective magnetic moment (µeff) and the Bohr magneton
(µB), µeff/µB, vs temperature plots are shown in the inset of
Figure 3. The µeff/µB values of 13+ were almost constant in
with a very strong spin-exchange interaction between the
unpaired electrons for 13+.
The magnetic susceptibility of 13+ was also measured by
the NMR shift method and the Evans equation25 in the acidic
CD2Cl2 solution at high temperature. The susceptibility was
estimated from the peak shift for a concentration series of
the standard peak of cyclohexane in the deutrated C6D6
solution. The µeff/µB value at 297 K was 3.78 (3.87 for the
theoretical value of S ) 3/2) and was constant up to 333 K
(60 °C).26 These results revealed that a quartet state is stable
even at room temperature.
In summary, three aminium radicals substituted on the 2,6,-
10-triphenylene in 13+ were strongly coupled through the
π-conjugated triphenylene core to be a quartet molecule even
at room temperature. This quartet molecule is expected to
be an effective module forming a high-spin polyradical and
a high-spin liquid crystal.
Acknowledgment. This work was partially supported by
Grant-in-Aids for Scientific Research on the Priority Area
Super-Hierarchical Structures and for the COE Research
Programs Practical Nano-Chemistry and Molecular Nano-
Engineering from MEXT, Japan.
Supporting Information Available: Synthesis and char-
acterization of the compounds, ESR spectra, and data of the
NMR shift method. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 3. Normalized plots of magnetization (M/Ms) vs the ratio
of the magnetic field and temperature (H/(T - θ)) for the triaminium
triphenylene radical 13+ with a spin concentration of 0.90 in CF3-
COOH/(CF3CO)2O/CH2Cl2 (0.25/0.25/99.5 vol %) at T ) 2.5 (b),
3 (O), and 5 (0) K and theoretical Brillouin curves corresponding
to S ) 2/2, 3/2, and 4/2. Inset: µeff/µB vs T plots for 13+ with a
spin concentration of 0.90.
OL060379F
(23) The spin concentration was determined by the saturated magnetiza-
tion of the 13+ sample.
(24) Above 180 K, the frozen medium was melted and the µeff/µB could
not be determined because of the difficulty in the diamagnetic susceptibility
correction.
(25) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Cotton, F. A.; Murillo,
C. A.; Wang, X. Inorg. Chem. 1999, 38, 6294.
(26) Above 333 K, the C6D6, whose boiling point is about 353 K, could
not be locked by the NMR spectrometer.
the range of 20-150 K24 and agreed with the theoretical
value of 3.87 for S ) 3/2. These SQUID data indicated that
the aminium triphenylene 13+ is in a quartet ground state
1838
Org. Lett., Vol. 8, No. 9, 2006