Syntheses and magnetic properties of stable organic triradicals with quartet ground states consisting of different nitroxide radicals

Article abstract of DOI:10.1021/ja980817g

Bis[p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl] nitroxide (3) was synthesized via desilylation of O-protected diradical followed by autoxidation. By deoxgenation of 3, p-(1-oxyl-4,4,5,5- tetramethylimidazolin-2-yl)phenyl-p-(1-oxyl-3-oxide

Full text of DOI:10.1021/ja980817g

7168
J. Am. Chem. Soc. 1998, 120, 7168-7173
Syntheses and Magnetic Properties of Stable Organic Triradicals with
Quartet Ground States Consisting of Different Nitroxide Radicals
Megumi Tanaka, Kenji Matsuda,† Tetsuji Itoh, and Hiizu Iwamura*,‡
Contribution from the Institute for Fundamental Research in Organic Chemistry, Kyushu UniVersity,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
ReceiVed March 11, 1998
Abstract: Bis[p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl] nitroxide (3) was synthesized via
desilylation of O-protected diradical followed by autoxidation. By deoxgenation of 3, p-(1-oxyl-4,4,5,5-
tetramethylimidazolin-2-yl)phenyl-p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl nitroxide (4) and
bis[p-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)phenyl] nitroxide (5) were prepared. X-ray analysis of a dark
brown needle single crystal for 4 confirmed that 4 consisted of three different kinds of nitroxide radical centers,
which showed three reversible redox waves at +0.42, +0.82, and +1.01 V (vs Ag/Ag+) in CH3CN solution.
The temperature dependence of the molar magnetic susceptibilities for microcrystalline samples of triradicals
3-5 was measured by a SQUID susceptometer, showing the intramolecular ferromagnetic and intermolecular
antiferromagnetic interactions. Experimental data were fitted to the equation derived from an asymmetric
linear three-spin model (H ) -2(J12S1‚S2 + J23S2‚S3)). The best fit parameters were J12/kB ) J23/kB ) 231
( 4 K for 3, J12/kB ) 349 ( 26 K, J23/kB ) 130 ( 3 K for 4, and J12/kB ) J23/kB ) 127 ( 3 K for 5.
Triradicals 3-5 thus have quartet ground states. These exchange coupling parameters are mutually compared
in reference to their molecular structures.
Introduction
the problems of m-phenylenecarbene units, Ishida et al. designed
and synthesized bis[3-tert-butyl-5-(N-oxy-tert-butylamino)phen-
yl] nitroxide 2, and showed that 2 was stable under ambient
temperature and had a large ferromagnetic exchange coupling
of J/kB ) 240 K.3 The oxygen atom of nitroxide radicals is
known to have enough basicity for coordination with transition
metal ions. When three nitroxide radical centers are arranged
trigonally within a triradical molecule, it can serve as bridging
ligands for making high dimensional metal coordination com-
plexes, satisfying the above condition (2).4 From these points
of view, stable polyradicals bearing high-spin ground states such
as 3 were deemed very attractive to us as the coupling units for
highly structured molecular magnets. However, stable quartet
triradicals having multiple ligating sites are still quite limited.5
Due to the valence electrons in organic molecules being
paired, organic compounds are typically insulators and diamag-
netic. The electron spin of the unpaired electrons is needed
for developing strong magnetic properties in organic compounds.
It has been established that (1) a strong inter- and/or intramo-
lecular exchange coupling among the spins and (2) their high
dimensional arrangement in space are required to order them
at finite temperature.1 From the first point of view, intramo-
lecular ferromagnetic interaction among the electron spins in
polyradicals is of great interest in connection with the design
of molecular ferromagnets. Poly(m-phenylenecarbenes) 1 have
been most systematically studied inter alia and are accepted to
represent the highest spin organic molecules.2 The high spin
state of 1 is based on the ferromagnetic exchange coupling
between the one-center π- and σ-unpaired electrons at the
divalent carbons and among the topologically polarized π-spins.
However, carbene species lack the stability for characterization
under ambient conditions and have an inherent drawback for
further extension to usable magnetic materials. To overcome
Results and Discussion
Molecular Design and Synthesis. In place of tert-butyl
nitroxide for the terminal spin sources in 2, Ullman’s nitronyl
nitroxide was selected in this work as in 3 which is more stable
than tert-butyl nitroxide and modifiable to other organic radicals
† Present address: Department of Chemistry and Biochemistry, Graduate
School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan.
(3) Ishida, T.; Iwamura, H. J. Am. Chem. Soc. 1991, 113, 4238.
(4) (a) Caneschi, A.; Gatteschi, D.; Grand, A.; Laugier, J.; Pardi, L.;
Rey, P. Inorg. Chem. 1988, 27, 1031. (b) Caneschi, A.; Gatteschi, D.;
Renard, J. P.; Rey, P. Sessoli, R. J. Am. Chem. Soc. 1989, 111, 785. (c)
Cabello, C. I.; Caneschi, A.; Carlin, R. L.; Gatteschi, D.; Rey, P. Sessoli,
R. Inorg. Chem. 1990, 29, 2582. (d) Caneschi, A.; Fabrizio, F.; Gatteschi,
D.; Rey, P.; Sessoli, R. Inorg. Chem. 1991, 30, 3162. (e) Inoue, K.; Iwamura,
H. J. Am. Chem. Soc. 1994, 116, 3173. (f) Inoue, K. Iwamura, H. J. Chem.
Soc., Chem. Commn. 1994, 2274. (g) Inoue, K.; Hayamizu T.; Iwamura,
H. Chem. Lett. 1995, 745. (h) Inoue, K.; Iwamura, H. Synth. Met. 1995,
71, 1793. (i) Inoue, K.; Hayamizu, T.; Iwamura, H. Mol. Cryst. Liq. Cryst.
1995, 273, 67. (j) Inoue, K.; Iwamura, H. AdV. Mater. 1996, 8, 73. (k)
Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume D.; Ohashi, Y. J. Am.
Chem. Soc. 1996, 118, 1803. (l) Iwamura, H.; Inoue K.; Hayamizu, T. Pure
Appl. Chem. 1996, 68, 243. (m) Inoue K.; Iwamura, H. Mol. Cryst. Liq.
Cryst. 1996, 286, 133. (n) Markosyan, A. S. Hayamizu, T.; Iwamura H.;
Inoue, K. J. Phys., Condens. Matter 1998, 10, 2323.
§ Present address: National Institution for Academic Degrees, 4259
Nagatsuta-cho, Midori-ku, Yokohama 226-0026, Japan.
(1) For reviews see: (a) Iwamura, H. AdV. Phys. Org. Chem. 1990, 26,
179. (b) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88. (c) Iwamura, H.;
Koga, N. Acc. Chem. Res. 1993, 26, 346. (d) Miller, J. S.; Epstein A. J.
Angew. Chem., Int. Ed. Engl. 1994, 33, 385. (e) Rajca, A. Chem. ReV. 1994,
94, 871. (f) Miller, J. S.; Epstein, A. J. Chem. Eng. News 1995, Oct 2, 30.
(g) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (h) Kahn, O.
AdV. Inorg. Chem. 1995, 43, 179. (i) Murray, K. S. AdV. Inorg. Chem.
1995, 43, 261.
(2) (a) Itoh, K. Chem. Phys. Lett. 1967, 1, 235. (b) Mataga, N. Theor.
Chim. Acta 1968, 10, 372. (c) Matsuda, K.; Nakamura, N.; Takahashi, K.;
Inoue, K.; Koga, N.; Iwamura, H. J. Am. Chem. Soc. 1995, 117, 5550. (d)
Matsuda, K.; Nakamura, N.; Inoue, K.; Koga, N.; Iwamura, H. Bull. Chem.
Soc. Jpn. 1996, 69, 1483.
S0002-7863(98)00817-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/10/1998

Syntheses of Stable Organic Triradicals
J. Am. Chem. Soc., Vol. 120, No. 29, 1998 7169
Scheme 1a
such as imino nitroxide as 4 and 5, thus enabling the construc-
tion of organic radicals having three different kinds of nitroxide
radicals. Such organic heterospin systems have a number of
interesting aspects. The singly occupied MOs (SOMOs) have
different spatial distribution and energy levels, and the magni-
tude of the exchange coupling between any of them should be
different. The difference of the ionization potential or shape
of the orbital contributing to the coordination bond may produce
various coordination patterns to transition metals. Thus, for the
construction of a molecular magnet, it is important to investigate
the intramolecular interaction of a polyradical consisting of
different spins and reflect to a later molecular design. Triradi-
cals 3-5 are expected to have five ligating sites for transition
metal ions.
a Reagents and conditions: (a) n-BuLi, THF, isopentyl nitrite, tert-
butyldimethylchlorosilane, triethylamine; (b) n-BuLi, THF, DMF,
27.1%, 2 steps; (c) 2,3-dimethyl-2,3-bis(hydroxyamino)butane, metha-
nol, 73%; (d) NaIO4, H2O, CH2Cl2, 72.4%; (e) tetrabutylammonium
fluoride, THF, 0 °C, 62.5%; (f) p-toluenesulfonyl isocyanate, CH2Cl2,
39%; (g) NaNO2, acetic acid, H2O, CH2Cl2, 73%.
Table 1. Crystallographic Parameters for 4
formula
fw
crystal color,
C26H32N5O4
478.57
dark brown,
c/Å
19.01(1)
2468(3)
V/Å3
Z
4
Dc/(g cm-3
)
1.288
habit
needle
crystal system
space group
a/Å
orthorhombic
Pna21
21.211(4)
6.1216(8)
no. of reflections
R
Rw
2681
0.066
0.096
b/Å
In 1 and 2, atoms carrying the positive spin density are
directly attached to the aromatic ring, and therefore the latter
has to be m-phenylene to have a high-spin ground state as in
m-quinodimethane. The 2-imidazolinyl radical has a negative
spin density at the carbon atom attached to the phenylene ring,
and therefore the topology of the π-system changes by one
carbon atom and the substitution pattern has to be p-phenylene
as in triplet 6 and 7.6
The synthesis of 3 was performed as summarized in Scheme
1. In this synthesis, diphenylhydroxylamine is not stable
sufficiently; it had to be protected by a silyl group without
isolation in situ.
dilithiated and reacted with DMF to give the diformyl compound
(9). Pentakis(hydroxylamine) derivative (10) was obtained by
reaction with 2,3-dimethyl-2,3-bis(hydroxyamino)butane7 and
oxidized with NaIO4. Desilylation followed by autoxidation
of O-protected diradical (11) gave 3 as a purple powder. If
autoxidation was not enough, oxidation by Ag2O was effective.
By deoxygenation of 3 with p-toluenesulfonyl isocyanate,8
4 was prepared as a main product together with a few percents
of 3 and 5. With NaNO2 and acetic acid as a catalyst,9 only 5
was obtained.
p-Dibromobenzene was monolithiated and allowed to react
with isopentyl nitrite. N,N-Bis(p-bromophenyl)hydroxylamine
salt thus formed was treated with tert-butyldimethylsilyl chloride
and triethylamine. The O-protected hydroxylamine (8) was
Crystal Structure. The crystal structure of 4 was determined
by X-ray crystallographic analysis. Crystallographic parameters
are summarized in Table 1. An ORTEP drawing of the essential
part of 4 is shown in Figure 1 with the numbering scheme. The
selected bond distances and angles with their estimated standard
deviations are listed in Table 2.
Figure 1 shows that 4 consists of three different kinds of
nitroxide radicals just by comparing the bond lengths. Judging
from the bond lengths of the benzene ring, to which the nitronyl
nitroxide group is attached, we note a contribution of the
(5) (a) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. J. Phys. Chem. 1993,
97, 13267. (b) Fujita, J.; Tanaka, M.; Suemune, H.; Koga, N.; Matsuda,
K.; Iwamura H. J. Am. Chem. Soc. 1996, 118, 9347. (c) Fujita, J.; Matsuoka,
Y.; Matsuo, K.; Tanaka, M.; Akita T.; Koga, N.; Iwamura H. J. Chem.
Soc., Chem. Commun. 1997, 2393. There are several examples of high-
spin molecules (S g 3/2), but not all species satisfy the requirement of
stability and ligating ability. See: Nau, W. M. Angew. Chem., Int. Ed. Engl.
1997, 36, 2445.
(6) (a) Inoue, K.; Iwamura, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
917. (b) Hosokoshi, Y.; Takizawa, K.; Nakano, H.; Goto, T.; Takahashi,
M.; Inoue, K. J. Magn. Magn. Mater. 1998, 177-181, 634. We thank Prof.
Katsuya Inoue for information prior to publication.
(7) Lamchen, M. Mittag, T. W. J. Chem. Soc. C 1966, 2300.
(8) Ullman, E. F.; Call, L.; Osiecki, J. H. J. Org. Chem. 1970, 35, 3623.
(9) Ullman, E. F.; Osiecki, J. H.; Boocock, D. G. B.; Darcy, R. J. Am.
Chem. Soc. 1972, 94, 7049.

7170 J. Am. Chem. Soc., Vol. 120, No. 29, 1998
Tanaka et al.
Figure 1. An ORTEP drawing of the essential part of 4 with a thermal
ellipsoid plot (50% probability) with the numbering scheme. The
hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 4
Bond Distances (Å)
N(1)-O(1)
N(1)-C(14)
C(2)-C(3)
C(4)-C(5)
C(6)-C(1)
C(15)-C(16)
C(17)-C(18)
C(19)-C(14)
C(7)-N(2)
N(2)-O(2)
N(3)-O(3)
C(17)-C(20)
C(20)-N(4)
N(4)-C(21)
1.289(5) N(1)-C(1)
1.433(8) C(1)-C(2)
1.374(8) C(3)-C(4)
1.431(7) C(5)-C(6)
1.417(7) C(14)-C(15)
1.382(8) C(16)-C(17)
1.392(7) C(18)-C(19)
1.395(7) C(4)-C(7)
1.341(7) C(7)-N(3)
1.270(6) N(2)-C(8)
1.261(6) N(3)-C(9)
1.469(8) C(20)-N(5)
1.384(7) N(4)-O(4)
1.490(7) N(5)-C(22)
1.413(8)
1.400(8)
1.403(7)
1.374(8)
1.398(7)
1.411(7)
1.389(8)
1.472(8)
1.358(7)
1.534(8)
1.513(8)
1.308(8)
1.288(6)
1.522(8)
Figure 2. Cyclic voltammograms of (a) 3, (b) 4, and (c) 5 in acetonitrile
Bond Angles (deg)
(0.1 M TBAP) on a platinum electrode (vs Ag/Ag+).
O(1)-N(1)-C(1)
C(1)-N(1)-C(14)
C(4)-C(7)-N(3)
C(7)-N(3)-O(3)
116.7(5) O(1)-N(1)-C(14)
124.6(4) C(4)-C(7)-N(2)
125.4(5) C(7)-N(2)-O(2)
127.5(5) N(2)-C(7)-N(3)
118.6(5)
125.4(5)
128.6(5)
109.2(5)
Table 3. Electrochemical Data for Triradicals in Acetonitrile (V
vs Ag /Ag+)
triradical
NN•/NN+ E1/2
IM•/IM+ E1/2
Ar2NO•/Ar2NO+ E1/2
C(17)-C(20)-N(4) 123.3(5) C(17)-C(20)-N(5) 124.2(5)
C(20)-N(4)-O(4) 126.0(5) N(4)-C(20)-N(5) 112.5(5)
3
4
5
+0.42
+0.42
+0.97
+1.01
+0.99
+0.82
+0.68
quinonoid resonance structure; bond lengths C(2)-C(3) and
C(5)-C(6) are 1.374 Å, shorter than the other C-C bonds, and
similar to that of p-benzoquinone N-tert-butylimine (1.36 Å)10
{p-benzoquinone (1.344 Å)}. All bond lengths are similar in
the other benzene ring, to which imino nitroxide is attached,
suggesting there is no significant contribution of a quinonoid
resonance. It is also observed that the bond lengths C(7)-N(2)
and C(7)-N(3) and N(2)-C(8) and N(3)-C(9) in the five
membered ring of nitronyl nitroxide differ by only less than
0.02 Å, while C(20)-N(4) and C(20)-N(5) in the imino
nitroxide are different by 0.08 Å.
dipole and exchange interactions among the unpaired electrons
and also probably due to overlap of different resonances of
conformational isomers.
In frozen solution, 3 and 4 showed a large main peak with
weak outside shoulder signals, and 5 showed only a main peak,
suggesting small zero-field splitting parameters; |D| and |E|
(Figure 3).
The temperature dependence of the g ) 2 main peak followed
a Curie law in the range 20-70 K for 3 and 12-100 K for 4.
This result suggests that the observed radicals of 3 and 4 are in
the ground state in this temperature range (inset of Figure 3).
Additionally, the ∆ms ) 2 peak was detected, but ∆ms ) 3
was not observed for all three radicals. Thus, whereas the
quartet was concluded to be either a ground state or fully
degenerate with lower spin states, it was not possible to
determine how close the thermally accessible lower spin states
lie above the ground state.
Magnetic Measurements. Magnetic measurements of three
triradicals were carried out on a SQUID susceptometer from 2
to 300 K. The temperature dependence of the magnetic sus-
ceptibilities (ø) of the microcrystalline samples of three triradi-
cals were measured at 5000 Oe, and are shown in Figure 4.
The øT values of three triradicals at 300 K were 1.37 for 3,
1.35 for 4, and 1.27 emu‚K‚mol-1 for 5, which were close to a
theoretical value of 1.125 for three isolated spins. As the
temperature was increased, the magnetic susceptibilities of 4
increased from 0.54 emu‚K‚mol-1 at 2 K, reached a maximum
of 1.55 emu‚K‚mol-1 at around 65 K, and gradually decreased
Electrochemical Properties. The cyclic voltammograms of
triradicals 3-5 are shown in Figure 2, and the numerical data
are summarized in Table 3.
The cyclic voltammogram of 3 and 5 showed two reversible
waves, and that of 4 showed three. From a comparison of the
oxidation potential of each triradical, the oxidation was assumed
to proceed as follows: first the terminal nitronyl nitroxide (NN),
second the terminal imino nitroxide (IM), and finally diaryl
nitroxide (Ar2NO) at the center. The potential value +0.82 V
of 4 is higher than the corresponding value +0.68 V of 5,
because 4 has a positive charge, and is therefore harder to
oxidize than 5 by Coulombic repulsion.
ESR Measurements. ESR spectra of triradicals 3-5 were
measured in degassed 2-methyltetrahydrofuran fluid solution.
The spectra of 3 and 4 at 210 K and 5 at room temperature
consisted of a multiplet due to nitrogen hyperfine coupling with
broadening. The broadening was due to intramolecular dipole-
(10) Nakazono, S.; Karasawa, S.; Koga, N.; Iwamura, H. Angew. Chem.,
Int. Ed. Engl. in press.

Syntheses of Stable Organic Triradicals
J. Am. Chem. Soc., Vol. 120, No. 29, 1998 7171
Figure 4. øT-T plot for triradicals measured at 5000 Oe. The
solid line indicates a theoretical curve (see text): (a) for 3; (b) for 4;
(c) for 5.
Figure 3. ESR spectra of triradicals in glass (9.31 GHz): (a) for 3 in
MTHF/CH2Cl2 (1:1) glass at 12.0 K; (b) for 4 in MTHF glass at 11.5
K; (c) for 5 in MTHF glass at 7.0 K. The inset shows the temperature
dependence of the ESR signal intensities (Curie plot).
to 1.35 emu‚K‚mol-1 at 300 K. The latter decrease is due to
the thermal population of the spins from a quartet ground state
to exited doublet state, and the initial attenuation of the øT values
at lower temperature is due to intermolecular antiferromagnetic
couplings.
The magnetic interactions in an asymmetric linear triradical
system can be written as in eq 1, where J12 is the exchange
H ) -2(J12S1‚S2 + J23S2‚S3) ) -2J12(S1‚S2 + aS2‚S3) (1)
coupling parameter between S1 and S2, and a is the ratio of J12
and J23, which is the exchange coupling parameter between S2
and S3; for symmetric 3 and 5, a ) 1 (see Figure 5). Equation
1 neglects intramolecular coupling (J′) between two terminal
radicals. For polyradicals like 3-5, J′ is assumed to be 2 orders
of magnitude smaller than J.11
The molar susceptibility is given by eq 2, where all symbols
have their usual meaning. The temperature correction factor
(T - θ) is introduced to represent the Weiss molecular field
approximation. The best fitting parameters are summarized in
Table 4.
Figure 5. Illustrations to understand magnetism: (a) spin alignment
for 3 or 5 and (b) for 4 and (c) energy levels of 3-5.
2
NAg2µB
ø ) f
×
4k(T - θ)
2
x
{10 + exp[(-1 - a - 1 - a + a )(J/kT)] + exp[(-1 -
2
x
a + 1 - a + a )(J/kT)]}/{2 + exp[(-1 - a -
The following explanation is possible for the magnitude of
exchange coupling (Scheme 2). Quinonoid resonance structure
3q contributes to two terminal nitronyl nitroxide radicals to give
J12/kB ) J13/kB ) 231 K. Since the contribution of the
2
x
1 - a + a )(J/kT)] + exp[(-1 - a +
2
x
1 - a + a )(J/kT)]} (2)
quinonoid structure is not conspicuous in 5, the three spins are
(11) Tyutyulkov, N. N.; Karabunarliev, S. C. Int. J. Quantum Chem.
1986. 29, 1325.
more or less localized at two terminal imino nitroxides and the

7172 J. Am. Chem. Soc., Vol. 120, No. 29, 1998
Tanaka et al.
Table 4. Fitting Parameters of the Magnetic Data for Triradical
3-7
magnitude of exchange coupling. Triradicals 3-5 can have five
ligating sites of different basicity and different steric effects.
Our work on isolating metal complexes of interesting magnetic
properties is in progress.
triradical
(J12/kB)/K
(J23/kB)/K
θ/K
f
3
4
231 ( 4
349 ( 26
127 ( 3
319
231 ( 4
130 ( 3
127 ( 3
-3.99 ( 0.03
-4.09 ( 0.03
-3.08 ( 0.03
0.89
0.90
0.92
Experimental Section
5
66
76
A. Materials. 1H and 13C NMR spectra were recorded on a JEOL
EX-270 instrument. IR spectra were obtained on a Hitachi I-5040
spectrometer. UV-vis spectra were recorded on a Hitachi U-3300
spectrophotometer. Mass spectra were obtained by JEOL JMS-
HX110A instruments. Melting points are not corrected.
108
Scheme 2. A Schematic Illustration of the Contribution of
Quinonoid Structures 3q and 4q
Tetrahydrofuran (THF) used in the reactions was distilled, from so-
dium-benzophenone ketyl under a dry nitrogen atmosphere, just before
use. 2-Methyltetrahydrofuran (MTHF) used in the magnetic measure-
ments was purified by distillation from sodium-benzophenone ketyl
under a dry nitrogen atmosphere, just before use. IR spectra were ob-
tained on a Hitachi I-5040 spectrometer. All reactions were performed
under an atmosphere of dry nitrogen unless otherwise specified.
All reactions were monitored by thin-layer chromatography carried
out on 0.2-mm E. Merck silica gel plates (60F-254) using UV light as
detector. Column chromatography was performed on silica gel (E.
Merck, 70-230 mesh) or alumina (Nacalai, Alumina Activated 200,
200 mesh) inactivated with 6% H2O.
N,N-Bis(p-bromophenyl)-N-(tert-butyldimethylsiloxy)amine (8).
p-Dibromobenzene (20 g, 85 mmol) was monolithiated with 1.6 M
n-butyllithium (55 mL) in THF (250 mL) at -78 °C. Into the solution
was dropped isopentyl nitrite (5.7 mL, 43 mmol), and the mixture was
gradually warmed to room temperature. After addition of tert-
butyldimethylsilyl chloride (15 g, 100 mmol) and triethylamine (13
mL, 94 mmol), it was stirred for 3 days. A dilute aqueous solution of
NH4Cl and ether was added, and the organic layer was separated,
washed with water, dried over MgSO4, and concentrated under reduced
pressure. The residue was passed through a column (SiO2/hexane),
and the first fraction was collected. However, it was found to be the
mixture of two products by 1H NMR, the several measurements
confirmed to produce dibromo compound 8. The mixture was used in
the next step without furthermore purification: 1H NMR (CDCl3, 270
MHz) δ 0.05 (s, 6 H, Si-CH3), 0.95 (s, 9 H, t-Bu), 7.05 (d, J ) 8.91
Hz, 4 H, Ar), 7.40 (d, J ) 8.91 Hz, 4 H, Ar); 13C NMR(CDCl3, 67.8
MHz) δ -5.0, 18.0, 25.9, 117.5, 122.4, 131.7, 150.4; FAB HRMS (m
+ 1)/z calcd for C18H24NOBr2Si 455.9994, found 455.9974.
central nitroxide, resulting in relatively small exchange coupling
(J/kB ) 130 K). The magnitude is ca. 60% of that in 6. As
revealed by the X-ray structure analysis the molecular structure
is asymmetric and there are two exchange couplings, J12 and
J23, in 4. The smaller one (J/kB ) 130 K) agrees nicely with
that of 5 for the exchange coupling between imino nitroxide
and central nitroxide. As concerns the larger one, however,
the problem is not so straightforward since J12 is greater than
that of 3 (J/kB ) 231 K). The contribution of quinonoid
structure 4q must be much greater than that of 3 to produce a
larger exchange coupling (J/kB ) 350 K).
N,N-Bis(p-formylphenyl)-N-(tert-butyldimethylsiloxy)amine (9).
To a solution of the mixture of 8 in THF (150 mL) was added 1.6 M
n-butyllithium (33.6 mL), and the mixture was stirred for 1 h at -78
°C. After addition of DMF (30 mL, excess), it was stirred for 1 h and
gradually warmed to room temperature. After aqueous workup as
described above, the residue was chromatographed on silica gel with
hexane and CH2Cl2. By evaporation of the solvent under reduced
pressure, the diformyl compound 9 (4.09 g, 11.5 mmol, 27%, 2 steps)
was obtained: mp 84-86 °C; 1H NMR (CDCl3, 270 MHz) δ 0.06 (s,
6 H, Si-CH3), 0.95(s, 9 H, t-Bu), 7.36(d, J ) 8.75 Hz, 4 H, Ar), 7.86-
(d, J ) 8.75 Hz, 4 H, Ar), 9.94(s, 2 H, -CHO); 13C NMR(CDCl3,
67.8 MHz) δ -5.0, 18.0, 25.8, 120.1, 130.8, 132.6, 154.1, 190.8; FAB
HRMS (m + 1)/z calcd for C20H26NO3Si 356.1682, found 356.1685.
N,N-Bis[p-(1,3-dihydroxy-4,4,5,5-tetramethylimidazolidin-2-yl)-
phenyl]-N-(tert-butyldimethylsiloxy) amine (10). 9 (3 g, 8.45 mmol)
and 2,3-dimethyl-2,3-bis(hydroxyamino)butane7 (5 g, 33.8 mmol) were
dissolved in methanol (50 mL) and stirred for 3 days. Precipitates
were collected by filtration and washed with a small amount of CH2-
Cl2. After drying, pentakis(hydroxyamino) derivative 10 (3.79 g, 6.2
mmol, 73%) was obtained: mp 136 °C; 1H NMR (DMSO-d6, 270 MHz)
δ 0.05 (s, 6 H, Si-CH3), 0.94 (s, 9 H, t-Bu), 1.02, 1.05(ss, 24 H, -CH3),
4.46 (s, 2 H, -CH-), 7.14 (d, J ) 8.25 Hz, 4 H, Ar), 7.40 (d, J ) 8.57
Hz, 4 H, Ar), 7.72(s, 4 H, -OH); 13C NMR (DMSO-d6, 67.8 MHz) δ
-4.9, 17.1, 17.8, 24.5, 26.0, 66.1, 119.9, 128.8, 138.1, 151.3; FAB
HRMS (m + 1)/z calcd for C32H54N5O5Si 616.3894, found 616.3876.
N,N-Bis[p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-
phenyl]-N-(tert-butyldimethylsiloxy)amine (11). To a solution of 10
(600 mg, 0.97 mmol) in CH2Cl2 (500 mL) was added an aqueous
solution of NaIO4 (430 mg, 2 mmol), and the mixture was stirred for
Recently, Hosokoshi et al. reported a very strong exchange
coupling between nitronyl nitroxide or imino nitroxide on one
hand and tert-butyl nitroxide on the other in 6 and 7,6
respectively (see Table 4); J12 of 4 is in good agreement with
that of 6.
Conclusion
In this work we have described a systematic synthesis of three
trinitroxide radicals having quartet ground states. A heterospin
system 4 which contains diphenyl nitroxide, nitronyl nitroxide,
and imino nitroxide was one of them. Synthesis was effected
by the protection of the hydroxyl group of bis(bromophenyl)-
hydroxylamine and the transformation of nitronyl nitroxide to
imino nitroxide by p-toluenesulfonyl isocyanate. The structure
of 4 was determined by X-ray analysis, showing that whereas
the nitronyl nitroxide moiety had a resonance contribution of
quinonoid form 4q, the imino nitroxide moiety appears not to
have such a resonance. Cyclic voltammetry of triradicals
revealed that oxidation took place consecutively and reversibly
in the order the terminal nitronyl nitroxide (NN), the terminal
imino nitroxide (IM), and finally diaryl nitroxide (Ar2NO) at
the center. By the measurement of the temperature dependence
of magnetic susceptibility, these triradicals were confirmed to
have quartet ground states. The exchange interactions between
the heterospins were determined, showing the contribution of
quinonoid resonance forms is important in strengthening the

Syntheses of Stable Organic Triradicals
J. Am. Chem. Soc., Vol. 120, No. 29, 1998 7173
1 h under the open air. The organic layer was separated, washed with
water, dried over MgSO4, and concentrated under reduced pressure.
The residue was chromatographed on silica gel with hexane, CH2Cl2,
and ethyl acetate. By evaporation of the solvent under reduced pressure,
diradical 11 (430 mg, 0.71 mmol, 73%) was obtained: mp 195-196
°C; FAB HRMS (m + 1)/z calcd for C32H48N5O5Si 610.3424, found
610.3431.
The structures were solved by direct methods and expanded using
Fourier techniques. The non-hydrogen atoms were refined anisotro-
pically. Hydrogen atoms were included but not refined. The final cycle
of full matrix least-squares refinement was based on 2226 observed
reflections (I > 2.50σ(I)) and 316 variable parameters and converged
(largest parameter shift was 0.00 times its esd) with unweighted and
weighted agreement factors of
N,N-Bis[p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)-
phenyl] Nitroxide (3). To a solution of 11 (430 mg, 0.71 mmol) in
THF (30 mL) was added tetrabutylammonium fluoride trihydrate (2.2
g, 7.0 mmol) in THF (20 mL), and the mixture was stirred for 30 min.
After aqueous workup followed by autoxidation as described in the
synthesis of 2, the residue was chromatographed on alumina gel with
hexane, CH2Cl2, and ethyl acetate. By evaporation of the solvent under
reduced pressure, 3 (218 mg, 0.44 mmol, 62%) was obtained as a purple
microcrystalline solid: mp 258 °C; IR (KBr, cm-1) 2988, 1584, 1482,
1431, 1420, 1393, 1370, 1296, 1258, 1215, 1167, 833; FAB HRMS
(m + 1)/z calcd for C26H33N5O5 495.2481, found 495.2492.
p-(1-Oxyl-4,4,5,5-tetramethylimidazolin-2-yl)-phenyl-p-(1-oxyl-3-
oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl Nitroxide (4). To
a solution of 3 (100 mg, 0.20 mmol) in CH2Cl2 (10 mL) was added
p-toluenesulfonyl isocyanate (40 mL, 1.30 mmol), and the mixture was
stirred for 2 h. The mixture was chromatographed on alumina with
hexane, CH2Cl2, and ethyl acetate. By evaporation of the solvent under
reduced pressure, 4 (37 mg, 0.08 mmol, 39%) was obtained as a brown-
purple microcrystalline solid: mp 191-192 °C; IR (KBr, cm-1) 2982,
1586, 1483, 1435, 1422, 1412, 1395, 1368, 1296, 1262, 1215, 1169,
1136, 835; FAB HRMS (m + 3)/z calcd. for C26H35N5O4 481.2481,
found 481.2709.
Bis[p-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)phenyl] Nitroxide
(5). To a solution of 3 (100 mg, 0.20 mmol) in CH2Cl2 (20 mL) was
added an aqueous solution of NaNO2 (76 mg, 1.1 mmol) and 10 drops
of acetic acid, and the mixture was stirred for 1 min under the open
air. The organic layer was separated, washed with water, dried over
MgSO4, and concentrated under reduced pressure. The residue was
recrystallized from heptane/CH2Cl2, filtrated, washed with hexane, and
dried in the air. 5 (68 mg, 0.147 mmol, 74%) was obtained as a dark
orange microcrystalline solid: mp 163-165 °C; IR (KBr, cm-1) 2978,
1591, 1545, 1483, 1458, 1439, 1418, 1408, 1389, 1377, 1368, 1308,
1264, 1287, 839; FAB HRMS (m + 1)/z calcd for C26H33N5O3
463.2583, found 463.2559.
R )
||Fo| - |Fc||/ |Fo| ) 0.066
∑
∑
Neutral atom scattering factors were taken from Cromer and Waber.
2
Rw ) { w(|Fo| - |Fc|)2/ wFo }1/2 ) 0.096
∑
∑
Anomalous dispersion effects were included in Fcalc; the values for ∆f ′
and ∆f ′′were those of Creagh and McAuley. The values for the mass
attenuation coefficients are those of Creagh and Hubbel. All calcula-
tions were performed using the teXsan crystallographic software
package of Molecular Structure Corp.
C. Electrochemical Properties. The measurement of electro-
chemical properties of triradicals was performed with a BAS CV-50W
electrochemical analyzer. The electrochemical behavior of triradicals
was examined by means of cyclic voltammetry for oxidation. Mea-
surements were carried out in acetonitrile solution containing tetra(n-
butyl)ammonium perchlorate (TBAP, ca. 1 × 10-1 M) as the supporting
electrolyte. A three-electrode assembly (BAS) was used which was
equipped with a platinum working electrode, a platinum coil as the
counter electrode, and a Ag/Ag+ (TBAP/acetonitrile) electrode as the
reference.
D. ESR Spectroscopy. A Bruker ESP 300E spectrometer was used
to obtain X-band ESR spectra. Temperatures were controlled by an
RMC CT-470-ESR cryogenic temperature controller. The ESR intensi-
ties for Curie plots in the temperature range at 20-70 K for 3 and
12-100 K for 4 were measured at appropriate power attenuation
calibrated to exclude saturation effects.
E. SQUID Measurement. Magnetic susceptibilities of the powder
samples of triradicals were measured by a Quantum Design MPMS-
5S SQUID system. Diamagnetic corrections were made using Pascal’s
constants.
Acknowledgment. This work was supported by a Grant-
in-Aid for COE Research “Design and Control of Advanced
Molecular Assembly Systems” (Grant 08CE2005) from the
Ministry of Education, Science, Sports, and Culture, Japan.
B. Crystallography. 4 was subjected to crystallographic studies.
A single crystal of 4 was mounted on a glass fiber with epoxy resin.
All measurements were made on a Rigaku RAXIS-IV imaging plate
area detector with graphite monochromated Mo KR radiation (λ )
0.710 70 Å). Indexing was performed from three oscillations which
were exposed for 4.0 min. The crystal-to-detector distance was 110.00
mm with the detector at zero swing position. Readout was performed
in the 0.100 mm pixel mode. The data were collected at a temperature
of -150 ( 1 °C to a maximum 2θ value of 55.0°. A total of 215.00°
oscillation images were collected, each being exposed for 30.0 min.
The data were corrected for Lorentz and polarization effects.
Supporting Information Available: X-ray structural infor-
mation on 4 (20 pages, print/PDF). An X-ray crystallographic
file, in CIF format, is available through the Internet only. See
any current masthead page for ordering information and Web
access instructions.
JA980817G