Synthesis and magnetic properties of orthogonally linked phenothiazine cation radical dimer and tetramer

Article abstract of DOI:10.1021/ol9025777

[Chemical equaction presented] A 3,3′ -orthogonally linked phenothiazine cation radical dimer with methyl substituents at the 2, 2′, 4, and 4′ positions was synthesized. The dimer was isolated as a stable product which exists in a triplet state due to a torsion angle of ca. 80° between the two phenothiazine units. The tetramer was also synthesized by linking the dimers using a 1,4-phenylene spacer.

Full text of DOI:10.1021/ol9025777

ORGANIC
LETTERS
2010
Vol. 12, No. 3
448-451
Synthesis and Magnetic Properties of
Orthogonally Linked Phenothiazine
Cation Radical Dimer and Tetramer
Hiroyuki Oka*
Department of AdVanced Materials, Institute of Technology and Science, The
UniVersity of Tokushima, 2-1, Minamijosanjima-cho, Tokushima 770-8506, Japan
okah@opt.tokushima-u.ac.jp
Received November 9, 2009
ABSTRACT
A 3,3′-orthogonally linked phenothiazine cation radical dimer with methyl substituents at the 2, 2′, 4, and 4′ positions was synthesized. The
dimer was isolated as a stable product which exists in a triplet state due to a torsion angle of ca. 80° between the two phenothiazine units.
The tetramer was also synthesized by linking the dimers using a 1,4-phenylene spacer.
High-spin organic polymers are important building blocks
for producing organic molecule-based magnets;1 therefore,
a wide variety of organic polyradicals have been investi-
gated.2-10 The π-topological rule is an essential requirement
to achieve high spin states in the ground state, as suggested,
in studies on polycarbenes2,3 and poly(triarylmethine radi-
cal)s4 that have successfully achieved spin states with 10-103
magnitudes. For practical use, polyradical spin sources should
be air-stable organic radicals;5-9 however, most stable
radicals have partial spin density distributions. Therefore,
through-bond spin coupling, which is very important for the
π-topological rule, is often ineffective in stable polyradicals.
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9, 1227. (b) Oka, H.; Tamura, T.; Miura, Y.; Teki, Y. J. Mater. Chem.
2001, 11, 1364. (c) Oka, H.; Kouno, H.; Tanaka, H. J. Mater. Chem. 2007,
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10.1021/ol9025777  2010 American Chemical Society
Published on Web 01/07/2010

Triarylamine cation radicals are an exception and have
successfully led to polymeric systems with high spin
states.5c-e,7 An orthogonal linkage between spin units like
planar π-cation radicals is another requirement for intramo-
lecular spin alignment. However, the requirement has not
been actively pursued, probably due to difficult synthetic
strategies. This requirement is occasionally more predomi-
nant than the π-topological rule when using stable radicals.
Some orthogonally linked dimeric systems have been
synthesized and isolated as stable products.10 For example,
the diradical dication of a spiro-fused triarylamine dimer
formed a stable triplet molecule in the ground state.10b The
diradical dication of a meso-ꢀ linked porphyrin dimer also
existed in a triplet state with a small singlet-triplet energy
gap of 2.1-12 cal mol-1 (1.1-6.0 K).10c Unfortunately, the
development of polymeric systems from these dimers
requires complex synthetic strategies.
Scheme 1. Synthetic Route of 1d(SbCl6)2 and 1t(SbCl6)3
In this work, a novel orthogonally linked dimeric system
1d(SbCl6)2, which consists of a 3,3′-linked diphenothiazine
1d coordinated to two SbCl6 anions, was synthesized.
Phenothiazine cation radicals are well-known stable radicals11
but, suprisingly, have not been used as frequently as
nitroxides, phenoxides, and triarylamine cation radicals.
Complex 1d(SbCl6)2 was substituted by methyl groups at
the 2, 2′, 4, and 4′ positions to generate an orthogonal
conformation between the two phenothiazine units. Some
reports have mentioned that diphenothiazines existed in the
triplet state in sulfuric acid,12 but the triplet species have
not been isolated to date. On the other hand, 1d(SbCl6)2 was
isolated as a stable product. Tetraphenothiazine 1t was also
synthesized using a 1,4-phenyelene spacer to evaluate the
potential development of polymeric systems from 1d(S-
bCl6)2. 10,10′-(1,4-Phenylene)diphenothiazine has been pre-
viously reported as a triplet ground-state molecule in sulfuric
acid.12b Therefore, the complexation of 1t with four SbCl6
anions is expected to generate a quintet species. However,
the complex obtained here, 1t(SbCl6)3, contained three SbCl6
anions instead of four. The synthesis and magnetic properties
of 1d(SbCl6)2 and 1t(SbCl6)3 are reported here.
Phenothiazines 1d and 1t were synthesized from 4,4′-
diamino-2,2′,6,6′-tetramethylbiphenyl (2) which was prepared
by benzidine rearrangement (Scheme 1).13 The reaction of
a diazonium salt prepared from 2 with potassium ethyl
xanthate and the susequent hydrolysis in the presence of
solium hydroxide formed dithiol 3. The reaction of 3 with
2-chloronitrobenzene in the presence of potassium hydroxide
produced 4 in 32% yield from 2. The reaction of 4 with
triethyl phosphite resulted in 3,3′-linked diphenothiazine 5.
This reaction is one of the methods to prepare phenothiazines
and involves a rearrangement through a nitrene intermedi-
ate.14 Because of low solubility except for THF, crude 5 was
directly reacted with 1-bromobutane in the presence of bases
to give 1d and 6 in 5% and 10% yields from 4, respectively.
Pd-catalyzed amination of 2 equiv of 6 with 1,4-dibro-
mobenzene in the presence of tri-tert-butylphoshine and
sodium tert-butoxide produced 1t in 73% yield.
The reaction of 1d with excess pentachloroantimony
formed 1d(SbCl6)2 which was purified by recrystallization
from a dichloromethane-benzene mixture which was evapo-
rated slowly. The isolated yield was 31%, and the experi-
mental values obtained by elemental analysis were in
agreement with the calculated values for 1d(SbCl6)2. In
addition, ESR experiments in dichloromethane indicated that
the spin concentration amounted to 1.90 spins per molecule.
On the other hand, the reaction of 1t with excess pentachlo-
roantimony formed 1t(SbCl6)3 in 56% yield, instead of a
(11) Forrester, A. R.; Hay, J. M.; Thomson, R. H. Organic Chemistry
of Stable Free Radicals; Academic Press: New York, 1968.
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Imakura, T.; Oda, M.; Murai, H. J. Am. Chem. Soc. 1996, 118, 3047.
Org. Lett., Vol. 12, No. 3, 2010
449

complex carrying four SbCl6 anions. Elemental analysis of
1t(SbCl6)3 gave 40.11%, 3.56%, and 2.37% for C, H, and
N, respectively, in agreement with the calculated values (C
) 40.13%, H ) 3.18%, and N ) 2.67%). A spin concentra-
tion of 2.24 spins per molecule was determined by ESR in
dichloromethane for 1t(SbCl6)3.
the lowest spin state. Spin projection was then performed
using eq 2 to eliminate spin contamination effects on the
BS lower spin state from higher spin states.15
ES - ET
J )
(2)
<S2>T - <S2>S
The input geometry for MO calculation was constructed
from the X-ray crystallographic data of the recently synthe-
sized 3,7-diphenyl-10-n-octylphenothiazine cation radical
(Figure 2, left).9e Because J strongly depends on the torsion
angle between the two phenothiazine units, the methyl groups
were not needed. MO calculations were performed at the
UB3LYP/6-31G(d) level using Gaussian 03.16 The dihedral
angle dependence (∠C5-C4-C4′-C3′) of J is shown in
Figure 2 (right). From the J/k value of -38.5 K estimated
above, the torsion angle was found to be ca. 80° in
1d(SbCl6)2, consistent with the corresponding torsion angle
in 2,2′,6,6′-tetramethyl-substituted biphenyls (80-97°).17
Figure 1. Temperature dependence of ESR spectra of 1d(SbCl6)2
in PMMA matrix (3 wt % content). Asterisks denote signals due
to the Mn2+ standard. The IT-T plot was obtained by monitoring
the triplet signal at 318 mT. The solid line in the IT-T plot
represents the least-squares fit using eq 1.
Rigid media ESR spectra of 1d(SbCl6)2 were acquired in
poly(methyl methacrylate) (PMMA) matrix in the temperature
range of 113-333 K (Figure 1). The ESR spectra clearly
showed the triplet signals with the zero-field splitting constants
of |D| ) 7.2 and |E| ≈ 0 mT. The ∆MS ) ( 2 forbidden
transition was also observed at ca. 160 mT. The D value is
larger than the value (3.8 mT) calculated using the point dipole
approximation based on the distance between the centers of two
phenothiazine units (ca. 9.0 Å). This may be due to delocal-
ization of unpaired electrons around the bond linking two
phenothiazine units. The intensity of the triplet signals increased
with decreasing temperature. In the IT-T plot in Figure 1, the
IT value gradually decreased with decreasing temperature,
showing that the triplet state exists in the thermally excited state.
The IT-T behavior was analyzed using the Bleaney-Bowers
singlet-triplet model (eq 1) to estimate the intramolecular
exchange coupling constant (J).
Figure 2. Input geometry for MO calculation (left) and intramo-
lecular exchange coupling constant as a function of dihedral angle
(right). C4-C4′ ) 1.49 Å and ∠C5-C4-C4-C3′ ) 24.2°.
The ESR spectrum of 1t(SbCl6)3 in PMMA matrix showed
a single line but did not exhibit signals characterizing high
spin states. Spins may be far from each other because of the
low spin concentration.
(15) (a) Takano, Y.; Kubo, S.; Onishi, T.; Isobe, H.; Yoshioka, Y.;
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Kitagawa, Y.; Yamanaka, S.; Okumura, M.; Yamaguchi, K. Int. J. Quantum
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(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.
3
IT ) C
(1)
3 + exp(-2J/kT)
The least-squares fit led to a J/k value of -38.5 K (|R| )
0.977).
The torsion angle between the two phenothiazine units in
1d(SbCl6)2 was examined using an MO calculation of the J
value. Energies of singlet and triplet states were calculated
in consideration of the broken-symmetry (BS) problem for
(13) Carlin, R. B. J. Am. Chem. Soc. 1945, 67, 928.
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Soc. C 1970, 2437.
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450
Org. Lett., Vol. 12, No. 3, 2010

The least-squares fit resulted in values of 0.963, 1.99, and
-17.4 K for f, g, and J/k, respectively (|R| ) 0.994).
Figure 4. Potential structure of 1t(SbCl6)3.
The ꢁmT value for 1t(SbCl6)3 at 300 K was 0.867 emu K
mol-1. This value is in good agreement with the value (0.840)
calculated from a spin concentration of 2.24 spins per
molecule and 0.375 emu K mol-1 for a species with S )
1/2. This suggests that there are two isolating S ) 1/2 species
in 1t(SbCl6)3. A potential structure for 1t(SbCl6)3 is shown
in Figure 4. In this structure, one SbCl6 anion is positionned
at the center two phenothiazine units containing the 1,4-
phenylene spacer, while each of the other SbCl6 anions are
at each phenothiazine extremity. Only 0.24 spins may be
generated at the center two phenothiazines circled by the
dotted line in Figure 4. The 3,7-bis(di-4-anisylamino)-10-
n-octylphenothiazine-SbCl5 complex, which was synthe-
sized previously, displayed only a spin concentration of 0.15
spins per molecule.9d This low spin concentration may be
due to unexpected oxidation at the 1,4-phenylenediamino
moieties. A similar unexpected oxidation may occur at the
1,4-phenylenediamino moiety in 1t(SbCl6)3. The ꢁmT value
decreased with decreasing temperature, consistent with
antiferromagnetic interactions.
Figure 3. Temperature dependence of magnetic susceptibility of
(a) 1d(SbCl6)2 and (b) 1t(SbCl6)3 in bulk solid state. Open circles
and triangles correspond to ꢁm and ꢁmT data, respectively. Crosses
are IT data obtained through ESR measurements in PMMA matrix.
The solid line in Figure 3(a) represents the least-squares fit using
eq 3.
The magnetic susceptibilities (ꢁm) of 1d(SbCl6)2 and
1t(SbCl6)3 were measured at various temperatures in bulk
using a SQUID magnetometer (Figure 3). The ꢁm value for
1d(SbCl6)2 increased with decreasing temperature up to 20
K. It decreased gradually from 20 to 6 K and then rapidly
decreased below 6 K. From 300 to 100 K, the ꢁm-T behavior
was in good agreement with the IT-T behavior observed
by ESR measurements in PMMA matrix. ESR signal
intensity is proportional to magnetic susceptibility. The ꢁm-T
behavior up to around 6 K was probably mainly due to an
intramolecular singlet-triplet behavior. The ꢁm-T behavior
below 6 K arose from intermolecular antiferromagnetic
interactions. The ꢁmT-T behavior above 10 K was analyzed
the Bleaney-Bowers singlet-triplet model (eq 3).18
Acknowledgment. I thank the Materials Design and
Characterization Laboratory, Institute for Solid State Physics,
The University of Tokyo, for the use of a SQUID magne-
tometer.
2Ng2ꢀ2
1
ꢁmT ) f
(3)
Supporting Information Available: Synthetic procedure,
ESR spectra, and MO calculation results. This material is
available free of charge via the Internet at http://pubs.acs.org.
k
3 + exp(-2J/kT)
(18) The least-squares fit to the ꢁmT-T plot gave better fitting than that
to the ꢁm-T plot. The solid line to the ꢁm-T plot in Figure 3a is artificially
drawn using parameters estimated in the least-squares fit.
OL9025777
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