Design, preparation, and electronic structure of high-spin cation diradicals derived from amine-based spin-polarized donors

Article abstract of DOI:10.1021/ja994547t

Amine-based donor radicals, such as dimethylamino-, diethylamino-and morpholinonitronyl nitroxides, were synthesized, and their donor abilities were examined by cyclic voltammetry. ESR spectra of the singly oxidized donor radicals showed ground state triplet signals derived from the cation radicals of these donor radicals. The result was interpreted by the presence of the ferromagnetic coupling between the generated π-spin and the localized unpaired electron on the radical unit. According to the molecular orbital calculations, the ferromagnetic coupling between these two spins is originated from the space-sharing nature between SOMO which is localized on the radical unit and SOMO' which is derived from HOMO of the donor radical upon one-electron oxidation. Therefore, the cation-radicals of these donor radicals can be regarded as a heteroanalogue of trimethylenemethane. p-and m-dimethylaminophenyl nitronyl nitroxides were synthesized in order to expand the π-electronic structure of the donor unit, and the singly oxidized species were also found to afford triplet ESR signals. The origin of the ferromagnetic coupling of these aminophenyl nitronyl nitroxides was discussed on the same basis. Charge transfer complexes of some of the donor radicals with chloranil or 2,3-dichloro5,6-dicyano-1,4-benzoquinone were prepared, and the conducting and magnetic properties were examined.

Full text of DOI:10.1021/ja994547t

J. Am. Chem. Soc. 2000, 122, 9723-9734
9723
Design, Preparation, and Electronic Structure of High-Spin Cation
Diradicals Derived from Amine-Based Spin-Polarized Donors
Hiromi Sakurai,† Akira Izuoka,‡ and Tadashi Sugawara*
Contribution from the Department of Basic Science, Graduate School of Arts and Sciences,
The UniVersity of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
ReceiVed December 30, 1999
Abstract: Amine-based donor radicals, such as dimethylamino-, diethylamino- and morpholinonitronyl
nitroxides, were synthesized, and their donor abilities were examined by cyclic voltammetry. ESR spectra of
the singly oxidized donor radicals showed ground state triplet signals derived from the cation radicals of these
donor radicals. The result was interpreted by the presence of the ferromagnetic coupling between the generated
π-spin and the localized unpaired electron on the radical unit. According to the molecular orbital calculations,
the ferromagnetic coupling between these two spins is originated from the space-sharing nature between SOMO
which is localized on the radical unit and SOMO′ which is derived from HOMO of the donor radical upon
one-electron oxidation. Therefore, the cation-radicals of these donor radicals can be regarded as a heteroanalogue
of trimethylenemethane. p- and m-dimethylaminophenyl nitronyl nitroxides were synthesized in order to expand
the π-electronic structure of the donor unit, and the singly oxidized species were also found to afford triplet
ESR signals. The origin of the ferromagnetic coupling of these aminophenyl nitronyl nitroxides was discussed
on the same basis. Charge transfer complexes of some of the donor radicals with chloranil or 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone were prepared, and the conducting and magnetic properties were examined.
Introduction
have chosen TMM as the fundamental skeleton and intend to
modify the basic TMM spin system in accord with the advanced
purpose.
Molecular magnetism has originated from the discovery of
the ground state quartet m-dicarbene, m-phenylenebis(phenyl-
methylene),1 and has been developed rapidly during these two
decades.2 The high-spin state of m-dicarbene is derived from
two types of strong ferromagnetic couplings: one is a one-
centered exchange interaction at the divalent carbon atoms, and
the other is a multicentered exchange interaction in the
m-phenylene unit having two degenerated nonbonding molecular
orbitals (NBMOs). The latter interaction aligns a pair of triplet
spins at the carbenic carbon atoms ferromagnetically, giving
rise to a ground state quintet species. Until now, a number of
high-spin polycarbenes and polyradicals with a m-phenylene
skeleton have been reported as the fruitful result of skillful
synthetic efforts.3 While a m-phenylene unit is frequently used
as a ferromagnetic coupler in the preparation of high-spin
molecules, there is a more basic diradical, that is, trimethylene-
methane (TMM), of which the ground state is determined to
be triplet, both experimentally4 and theoretically.5 For innovating
a spin system which can be modulated by external stimuli, we
Since the intrinsic character of an organic spin system is that
unpaired electrons occupy π-orbitals, modulation of the π-sys-
tem is expected to control the magnetic interaction between
unpaired electrons in π-orbitals.6 The reason we are interested
in a heteroanalogue of TMM for the above purpose is 2-fold.
First, by introducing an oxidizable core (e.g., a dimethylamino
group) into one of the peripheral carbon atoms, one can generate
a diradical species by one-electron oxidation (Scheme 1, Step
1). Second, the remaining allyl radical unit can be stabilized
substantially by replacing a heteroatom-centered radical source
(e.g., nitronyl nitroxide) (Scheme 1, Step 2). The resultant
heteroanalogue of TMM is DMANN+• which can to be
generated by one-electron oxidation of a nitronyl nitroxide
derivative with a dimethylamino group (DMANN).
In this paper, we describe the molecular design and prepara-
(3) (a) Rajca, A.; Rajca, S.; Wongsriratanakul, J. J. Am. Chem. Soc. 1999,
121, 6308-6309. (b) Matsuda, K.; Nakamura, N.; Inoue, K.; Koga, N.;
Iwamura, H. Bull. Chem. Soc. Jpn. 1996, 69, 1483-1494. (c) Wienk, M.
M.; Janssen, R. A. J. J. Am. Chem. Soc. 1997, 119, 4492-4501. (d) Selby,
T. D.; Blackstock, S. C. J. Am. Chem. Soc. 1998, 120, 12155-12156. (e)
Nishide, H.; Miyasaka, M.; Tsuchida, E. Angew. Chem., Int. Ed. 1998, 37,
2400-2402. (f) Bushby, R. J.; Gooding, D. J. Chem. Soc., Perkin Trans.
2 1998, 1069-1075. (g) Lahti, P. M.; Esat, B.; Walton, R. J. Am. Chem.
Soc. 1998, 120, 5122-5123. (h) Sato, K.; Yano, M.; Furuichi, M.; Shiomi,
D.; Takui, T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. J.
Am. Chem. Soc. 1997, 119, 6607-6613.
(4) (a) Baseman, R. J.; Pratt, D. W.; Chow, M.; Dowd, P. J. Am. Chem.
Soc. 1976, 98, 5726-2727. (b) Wenthold, P. G.; Hu, J.; Squires, R. R.;
Lineberger, W. C. J. Am. Chem. Soc. 1996, 118, 475-476.
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74(4), 2256-2259.
† Present address: Japan Association for International Chemical Informa-
tion (JACI).
‡ Present address: Department of Chemistry, Faculty of Science, Ibaraki
University.
(1) (a) Itoh, K. Chem. Phys. Lett. 1967, 1, 235-238. (b) Wasserman,
E.; Murray, R. W.; Yager, W. A.; Trozzolo, A. M.; Smolinsky, G. J. Am.
Chem. Soc. 1967, 89, 5076-5078.
(2) (a) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker: New York, 1999. (b) Rajca, A. Chem. ReV. 1994, 94, 871-
893. (c) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994, 33,
385-415. (d) Gatteschi, D. AdV. Mater. 1994, 6, 635-645. (e) Kollmar,
C.; Kahn, O. Acc. Chem. Res. 1993, 26, 259-265. (f) Dougherty, D. A.
Acc. Chem. Res. 1991, 24, 88-94. (g) Iwamura, H. AdV. Phys. Org. Chem.
1990, 26, 179-253.
(6) (a) Ishiguro, K.; Ozaki, M.; Sekine, N.; Sawaki, Y. J. Am. Chem.
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16-17. (c) Hamachi, K.; Matsuda, K.; Iwamura, H. Bull. Chem. Soc. Jpn.
1998, 71, 2937-2943.
10.1021/ja994547t CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/21/2000

9724 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Sakurai et al.
Scheme 1. Molecular Design for a Heteroanalogue of TMM
Scheme 2. Preparation of (a) Amine-Based Donor Radicals
and (b) Aminophenyl-Based Donor Radicals
Figure 1. ORTEP drawing of the molecular structure of DMANN at
30% probability.
Table 1. Selected Bond Lengths, Bond Angles, and Torsion
Angles for DMANN and pAPNN in pAPNN‚CA
DMANN pAPNN in pAPNN‚CA
Bond Lengths (Å)
O(1)-N(2)
O(2)-N(3)
C(3)-N(3)
C(3)-N(2)
1.282(3) O(1)-N(2)
1.289(3) O(2)-N(3)
1.342(3) C(9)-N(3)
1.341(4) C(9)-N(2)
1.268(10)
1.296(10)
1.34(1)
1.39(1)
Bond Angles (deg)
C(1)-N(1)-C(3)
C(1)-N(1)-C(2)
C(2)-N(1)-C(3)
119.0(3)
118.1(3)
121.4(3)
C(1)-N(1)-C(3)
C(1)-N(1)-C(2)
C(2)-N(1)-C(3)
123.5(9)
117.2(8)
119.3(9)
Torsion Angles (deg)
C(1)-N(1)-C(3)-N(2) 47.1(4)
C(2)-N(1)-C(3)-N(3) 32.3(4)
C(2)-N(1)-C(3)-C(5)
0(1)
C(1)-N(1)-C(3)-C(4) -3(1)
C(6)-C(8)-C(9)-N(2) 2(1)
C(6)-C(8)-C(9)-N(3) -1(1)
tion of amine-based donor radicals in which a radical unit is
connected with a donor unit in a cross-conjugating manner. It
turned out that one-electron oxidation of the prepared donor
radicals gives rise to ground state triplet cation diradicals
reflecting their unique electronic structure.7 We also point out
that the ground state spin multiplicity of heteroanalogues of
π-conjugated spin systems is not necessarily predicted by
Ovchinnikov’s rule.8 The spin correlation in singly oxidized
donor radicals, however, is found to be rationalized by Borden’s
criterion on the basis of their space-sharing nature of two types
of SOMOs (singly occupied molecular orbitals).9 Finally,
application of the novel spin system derived from spin-polarized
donors is documented.
Dihedral Angle (deg)
between plane 1-2a
1.23
a Plane 1 is a least-squares plane consisting of C(3)-C(4)-C(5)-
C(6)-C(7)-C(8), and plane 2 is of C(9)-N(2)-N(3)-O(1)-O(2).
2. Molecular Structures of Donor Radicals. The molecular
structure of neutral DMANN was determined by X-ray crystal-
lographic analysis. Crystallographic parameters are given in the
Experimental Section. An ORTEP drawing of the DMANN
molecule is shown in Figure 1 with the numbering scheme. The
selected bond lengths, bond angles, and torsion angles with their
estimated standard deviations are listed in Table 1. The bond
lengths, O(1)-N(2) and O(2)-N(3), C(3)-N(3) and C(3)-N(2)
are equal within the standard deviations. The twist angle (cf. θ
in Figure 13) between the dimethylamino group with a shallow
pyramidal form and the nitronyl nitroxide group is ca. 40°.
A charge transfer complex composed of pAPNN and chlor-
anil (CA) was prepared by mixing benzene solutions of pAPNN
and CA in a 1:1 molar ratio, followed by slow evaporation of
the solvent at room temperature. Single crystals of the CT
complex (pAPNN‚CA) were obtained as small thin plates. This
CT complex can be regarded as a model to discuss the molecular
structure of pAPNN in the singly oxidized state. Crystal-
lographic parameters of pAPNN‚CA are summarized in the
Experimental Section, although the quality of the collected
crystallographic data is poor due to the kinetic instability of
the complex. An ORTEP drawing of pAPNN in the CT complex
is shown in Figure 2a with a numbering scheme. The selected
bond lengths, bond angles, and torsion angles with their
estimated standard deviations are listed in Table 1. The dihedral
angle between the phenyl ring and the nitronyl nitroxide ring
is 0.78°. While pAPNN exists as a twist form in the neutral
crystal,10 it turns out to be almost planar in pAPNN‚CA. The
crystal structure of pAPNN‚CA is displayed in Figures 2b and
2c. Planar pAPNNs and acceptor molecules (CAs) are stacked
alternately along the c axis (Figure 2b), and the distance between
Results
1. Preparations of Donor Radicals. Amine-based donor
radicals (DMANN, DEANN, and MORNN) were prepared by
treating bromo nitronyl nitroxide 2 with the corresponding
secondary amines (Scheme 2a). Aminophenyl-based donor
radicals with a different substitution pattern (p&mAPNN) were
synthesized from the corresponding formyl derivatives 3 and 7
(Scheme 2b). As for the m-formyl derivative 7, the correspond-
ing ester 5 was reduced to m-dimethylaminobenzyl alcohol 6,
which was oxidized by pyridinium chlorochromate to afford
the desired compound (see Experimental Section). The ring-
closure reaction of formyl compounds 3 and 7 with bishydrox-
ylamine was catalyzed by bishydroxylamine sulfate to afford
cyclic hydroxylamines 4 and 8, respectively, in high yields.
Cyclic hydroxylamines 4 and 8 were oxidized by PbO2 to yield
p- and mAPNN, respectively.
(7) (a) Kumai, R.; Matsushita, M. M.; Izuoka, A.; Sugawara, T. J. Am.
Chem. Soc. 1994, 116, 4523-4524. (b) Kumai, R.; Sakurai, H.; Izuoka,
A.; Sugawara, T. Mol. Cryst. Liq. Cryst. 1996, 279, 133-138. (c) Sakurai,
H.; Kumai, R.; Izuoka, A.; Sugawara, T. Chem. Lett. 1996, 879-880. (d)
Sakurai, H.; Izuoka, A.; Sugawara, T. Mol. Cryst. Liq. Cryst. 1997, 306,
415-421. (e) Sugawara, T.; Izuoka, A. Mol. Cryst. Liq. Cryst. 1997, 305,
41-54.
(8) Ovchinnikov, A. A. Theor. Chim. Acta (Berl.). 1978, 47, 297-304.
(9) (a) Borden, W. T. Mol. Cryst. Liq. Cryst. 1993, 232, 195-218. (b)
Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel
Dekker: New York, 1999; pp 61-102.
(10) Caneschi, A.; Gatteschi, D.; le Lirzin, A. J. Mater. Chem. 1994,
4(2), 319-326.

High-Spin Cation Diradicals
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9725
Table 3. g Values and hfc Constants of Donor Radicals in THF
Measured at 30 K
gx
gy
gz
azN (2 N)/mT
DMANN
DEANN
MORNN
pAPNN
mAPNN
2.0105
2.0105
2.0102
2.0117
2.0114
2.0062
2.0069
2.0069
2.0055
2.0060
2.0016
2.0012
2.0011
2.0013
2.0012
1.88
1.87
1.84
1.81
1.86
of the reference donor compounds are between 0.82 and 1.49
V and 0.82 V for 2-hydro-4,4,5,5-tetramethylimidazoline-3-
oxide-1-oxyl (NNH). It is to be noted that the oxidation
potentials of all the donor radicals composed of a donor unit
and a radical unit are lower than those of the parent compounds.
Among them, DMANN turned out to have the lowest oxidation
potential of 0.53 V.
4. Triplet ESR Spectra of Cation Diradicals Derived from
Donor Radicals. The ESR spectra of neutral donor radicals in
THF matrices were measured at 30 K. The spectrum of
DMANN in a THF matrix showed the anisotropic g values gx
) 2.0136, gy ) 2.0062, and gz ) 2.0016 and the hyperfine
structure of azN ) 1.88 mT (2 N) on the z-line.11 The g values
and the hyperfine coupling (hfc) constants of other donor
radicals are listed in Table 3.
The cation diradical of DMANN was generated by addition
of the excess iodine to a THF solution of DMANN at room
temperature. The ESR spectrum of DMANN+• in a frozen
matrix is shown in Figure 3a. The resonance lines assignable
to a triplet species were observed at 292.9, 305.8, 310.5, 335.0,
340.3, and 353.5 mT. The triplet signals were tentatively
assigned to a pair of X, Y, and Z lines as shown in the figure.
The zero-field splitting parameters calculated from these
resonance fields were |D| ) 0.0276 cm-1 and |E| ) 0.0016
cm-1. These parameters are summarized in Table 4 together
with the gav values which were calculated as (gx + gy + gz)/3.
Compared to other lines, the Z-line was rather broad presumably
because of the presence of hfc of the nitrogen nuclei. Final
chemical consequence of the singly oxidized DMANN was
examined based on ESR spectroscopic data.12 The temperature
dependence of the low-field Y-line signal intensity followed the
Curie law in the temperature range of 8-100 K (Figure 3b).
One-electron oxidation of the other donor radicals, DEANN,
MORNN, pAPNN and mAPNN, afforded similar triplet ESR
spectra as in the case of DMANN. The temperature dependence
of the triplet ESR signal intensities of these donor radicals also
obeyed Curie law. Figures 4a and 4b show the triplet ESR
spectrum of an oxidized species of mAPNN and the Curie plot
of the triplet signal, respectively. Calculated zero-field splitting
parameters and anisotropic g values for all the observed triplet
species are shown in Table 4. All the values were about the
same as those of DMANN+•.
Figure 2. (a) ORTEP drawing of the molecular structure of pAPNN
in pAPNN‚CA at 30% probability and (b) crystal structure of pAPNN‚
CA: view along the a axis, (c) in the ab plane.
Table 2. Oxidation Potentials of Donor Radicals and Reference
Compounds vs Ag/AgCl
compounds
NNH
dimethylamine
diethylamine
morpholine
E1/2a/V
compounds
DMANN
DEANN
MORNN
pAPNN
mAPNN
E1/2a/V
0.82
1.27 (irr.)
1.49 (irr.)
1.25 (irr.)
0.82 (irr.)
0.53
0.60
0.58
0.64
0.75
dimethylaniline
1.31 (irr.)
1.17 (irr.)
a Experimental condition: acetonitrile solution in the presence of
0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte;
5. Physical Properties of a Charge Transfer Complex
Composed of a Donor Radical. The UV-NIR spectrum of a
CT complex, pAPNN‚CA, in a KBr pellet showed a broad peak
at around 11000 cm-1 (1.4 eV) which was assignable to a charge
scanning rate was 200 mV‚s-1
.
the aromatic ring and the quinone ring is ca. 4.3 Å. Viewed
along the c-axis, the benzene ring of pAPNN and the quinone
ring of CA overlap each other almost completely (Figure 2c).
A benzene molecule is incorporated within a cavity created by
the methyl groups of the nitronyl nitroxide moieties of the facing
pAPNNs (Figures 2b and 2c).
3. Electrochemical Properties of Donor Radicals. Oxidation
potentials of the five donor radicals prepared here were
determined by cyclic voltammetry. The half-wave potentials are
listed in Table 2, together with those of the reference com-
pounds. The first oxidation potentials of amine- or aminophenyl-
based donor radicals are between 0.53 and 0.75 V, while those
(11) The anisotropic g values are tentatively assigned to gx, gy and gz
according to the decreasing order.
(12) After warming the sample to room temperature, neither neutral
DMANN or the singly oxidized DMANN was detected by the ESR
measurement. The strong signal at the central magnetic field in the frozen
sample, therefore, was ascribed to the decomposed compounds derived from
the oxidized species. The ESR spectrum of the decomposed sample
measured at room temperature was assigned to a mixture of two doublet
species. The species at g ) 2.0064 with the hyperfine structure of a )
0.73 mT coupled with two equivalent I ) 1 nucleus. The other at g )
2.0066 of a ) 0.16 mT of I ) 1 with a ) 0.49 mT with six equivalent I
1
) /2 nuclei.

9726 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Sakurai et al.
Figure 3. (a) ESR spectrum of DMANN+• in THF matrix at 8 K. Central signals are due to the decomposed radicals.12 (b) Temperature dependence
of the signal intensity of DMANN+•
.
Table 4. Zero-Field Splitting Parameters and g Values for Cation
Diradicals in a THF Frozen Matrix at 8 K
Temperature dependence of the magnetic susceptibility of the
complex was measured by the Faraday method in a temperature
range of 3-270 K. The magnetic property of the complex was
basically paramagnetic. The Curie constant was 0.31 emu‚K‚
mol-1 at higher temperatures. This value was about 18% less
|D|/cm-1 |E|/cm-1
gx
gy
gz
gav
DMANN+• 0.0276
0.0016 2.0097 2.0077 2.0067 2.0080
0.0017 2.0069 2.0080 2.0073 2.0074
0.0018 2.0087 2.0067 2.0087 2.0080
0.0018 2.0075 2.0066 2.0091 2.0077
0.0020 2.0106 2.0088 2.0081 2.0092
DEANN+•
0.0273
1
MORNN+• 0.0272
than that of the S ) /2 radical. Although the øp‚T plot could
pAPNN+•
mAPNN+•
0.0272
0.0263
not be reproduced by the Curie-Weiss equation throughout the
temperature region measured, a weak antiferromagnetic interac-
tion (θ ) -1 K) was at least detected from the plot at lower
temperatures. The field dependence of the magnetization of the
complex was measured at 3 K. The curve almost fitted the
Brillouin function of S ) 1/2. Conductivity of the freshly
prepared complex in a compressed pellet was measured by
means of a two-probe method to showed a low conductivity of
ca. 10-6 S‚cm-1) at room temperature.
transfer band. The IR spectrum of pAPNN‚CA in the solid state
showed the carbonyl stretching mode of CA at 1682 cm-1. The
position of the absorption band of the complex is close to those
of the uni-component sample (1692 and 1681 cm-1). The other
peaks of the spectrum were also interpreted by the superposition
of those of individual components of the complex.
The magnetic susceptibility of pAPNN‚CA was measured
by the Faraday method in a temperature range of 3-270 K.
The magnetic property of the complex was basically paramag-
netic with a weak antiferromagnetic interaction (θ ) -0.8 K).
The Curie constant of 0.35 emu‚K‚mol-1 at higher temperatures
Discussion
1. Molecular Structures of Donor Radicals in Uni-
Component Crystals or in Charge Transfer Complexes. To
discuss the molecular structure of a donor radical, an X-ray
crystallographic analysis was performed on a single crystal of
DMANN. From the fact that the bond lengths of O(1)-N(2)
and O(2)-N(3), C(3)-N(3) and C(3)-N(2) are equal within
the standard deviations, the nitrone and the nitroxide moieties
are considered to be equivalent due to the resonance effect
(Table 1). The twist angle between the direction of a lone pair
of electrons of the dimethylamino group and the normal axis
to the best plane of the nitronyl nitroxide group is ca. 40° (Figure
1). The twist may be caused not only by the steric repulsion
between the methyl groups of the dimethylamino group and
the oxygen atoms of the nitronyl nitroxide but also by the
repulsive electronic interaction between a lone pair of electrons
of the former group and a π-orbital of the latter. This means
that if the twist angle becomes zero, the orbital energies, in
particular that of HOMO of the eclipse conformer, must be
raised because HOMO (highest occupied molecular orbital) has
large coefficients at the heteroatoms involved in DMANN (see
Appendix).
1
was slightly less (7%) than 1 mol unit of S ) /2 spins. The
electrical conductivity of pAPNN‚CA was lower than 10-6
S‚cm-1 at room temperature when measured by a two-probe
method along the crystal c axis: The axis corresponds to the
direction of the donor-acceptor stacking.
A charge-transfer complex (DMANN‚DDQ) was obtained
as a black powder by grinding DMANN (Eox ) 0.53 V) and
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (Ered ) 0.59
V).13 All attempts to obtain single crystals have been unsuc-
cessful. The VIS-NIR spectrum of the DMANN‚DDQ complex
in a KBr pellet showed a broad band from 10000 cm-1 to an
IR region with a maximum absorption at 5300 cm-1 (0.65 eV),
which was assignable to a CT band (Figure 5). The IR spectrum
of DMANN‚DDQ was totally different from those of the neutral
compounds. The tail of the CT absorption band extended over
to ca. 1700 cm-1 (0.21 eV), and most of the peaks in the range
of 1700-500 cm-1 were broadened. The stretching of the cyano
group of DDQ in the complex was observed at 2219 cm-1, while
those of neutral DDQ and K+DDQ-• were at 2234 and 2217
cm-1, respectively.14 The absorption position of the cyano
stretching corresponds to that of the anion radical salt.15
(14) Miller, J. S.; Krusic, P. J.; Dixon, D. A.; Reiff, W. M.; Zhang, J.
H.; Anderson, E. C.; Epstein, A. J. J. Am. Chem. Soc. 1986, 108, 4459-
4466.
(15) When the ratio of DDQ was increased, only signals of excess DDQ
appeared in the spectrum of DMANN‚DDQ, suggesting that DMANN and
DDQ form a stable 1:1 complex.
(13) Toda, F.; Miyamoto, H. Chem. Lett. 1995, 861.

High-Spin Cation Diradicals
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9727
Figure 4. (a) ESR spectrum of mAPNN+• in THF matrix at 8 K. Central signals are due to neutral mAPNN. (b) Temperature dependence of the
signal intensity of mAPNN+•
.
difference between the redox potentials of the donor (Eox
)
0.64 V) and the acceptor (Ered ) 0.09 V) and also from the IR
spectral data as described in the Results section 5. The above
results suggest that the molecular structure of the cation diradical
derived from one-electron oxidation of a donor radical could
also be planar.
2. PMO Analysis of Electronic Structure for Donor
Radicals. The characteristic electronic structure of the cross-
conjugated donor radical may be best understood by a pertur-
bational molecular orbital (PMO) method.18 Namely, a donor
radical (e.g., DMANN) is composed by union of a donor
molecule (e.g., dimethylamine) and a radical molecule (e.g.,
2-hydro-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl, NNH).
The electronic structure of such a donor radical may be
represented by either (a) or (b) as shown in Figure 6: The
highest occupied molecular orbital of the donor, the singly
occupied molecular orbital of the radical, and the next highest
occupied molecular orbital of the radical are defined as homo,
somo, and homo, respectively. In case (a), the homo of the donor
and somo of the radical interact mutually, and the resulting
singly occupied molecular orbital (SOMO) of the radical donor
locates itself above the highest fully occupied molecular orbital
(HOMO). On the other hand, in case (b), the homo of the donor
and the homo of the radical interact mutually, not interacting
with somo of the radical, because of the mismatch of the
symmetry of the relevant partial molecular orbitals. Since this
electronic interaction raises the energy level of the resultant
HOMO of the donor radical, it may be possible to locate HOMO
above SOMO. Such an exotic structure can be maintained, if
the on-site Coulombic repulsion of SOMO is larger than the
orbital energy difference (∆E) between HOMO and SOMO.
Whereas a donor radical of type (a) in Figure 6 should afford
a closed shell cation species upon one-electron oxidation, one-
Figure 5. UV-NIR spectrum of DMANN‚DDQ complex.
The crystal structure of neutral pAPNN has been reported
by Gatteschi et al.10 The unit cell of pAPNN contains two
crystallographically independent molecules. The dihedral angles
between the best plane of the O-N-C-N-O moiety and the
phenyl ring of each pAPNN are 25° and 23° in a conrotatory
manner. The result is consistent with the tendency that the
dihedral angles of other para-substituted aromatic nitronyl
nitroxides are usually larger than 20°; for example, 36° for
p-fluorophenyl, 32° for p-iodophenyl, and 25° for p-chloro-
phenyl nitronyl nitroxide.16 The reason for the twisting may be
interpreted in the same manner as in the case of DMANN.
Contrary to the twisted molecular structure of the uni-
component pAPNN, the molecular structure of pAPNN in the
charge transfer complex (pAPNN‚CA) is almost planar (Figure
2b). The molecular packing shows that HOMO of the donor
radical and LUMO (lowest unoccupied molecular orbital) of
the acceptor molecule overlapped each other exactly in phase
(Figure 2c) in a mixed-stack columnar structure (Figure 2b).
This geometrical feature is consistent with the observation of a
charge transfer band (11000 cm-1) in the absorption spectrum
of pAPNN‚CA. Although several CT complexes composed of
an organic radical and an acceptor are reported,17 this may be
the first example in which a π-donor radical and a π-acceptor
are stacked alternately in a columnar structure. The complex
is, however, considered to be almost neutral, judging from the
(17) (a) Nakatsuji, S.; Takai, A.; Nishikawa, K.; Morimoto, Y.; Yasuoka,
N.; Suzuki, K.; Enoki, T.; Anzai, H. J. Mater. Chem. 1999, 9, 1747-1754.
(b)Sugimoto, T.; Tsujii, M.; Suga, T.; Hosoito, N.; Ishikawa, M.; Takeda,
N.; Shiro, M. Mol. Cryst. Liq. Cryst. 1995, 272, 183-194. (c) Ishida, T.;
Tomioka, K.; Nogami, T.; Iwamura, H.; Yamaguchi, K.; Mori, W.; Shirota,
Y. Mol. Cryst. Liq. Cryst. 1993, 232, 99-102. (d) Sugano, T.; Fukasawa,
T.; Kinoshita, M. Synth. Met. 1991, 41-43, 3281-3284.
(16) (a) Hosokoshi, Y.; Tamura, M.; Kinoshita, M.; Sawa, H.; Kato, R.;
Fujiwara, Y.; Ueda, Y. J. Mater. Chem. 1994, 4, 1219-1226. (b) Hosokoshi,
Y. Ph.D. Thesis, The University of Tokyo, 1995.
(18) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587-
4594.

9728 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Sakurai et al.
Figure 6. Schematic drawing of the electronic configuration of radicals carrying a donor unit. (a) homo of the donor site interacting with somo of
the radical site. (b) homo of the donor site interacting with homo of the radical site.
electron oxidation of a donor radical of type (b) occurs from
HOMO to afford an open-shell cation diradical (Figure 6b).
Next, we will discuss how DMANN satisfies the aforemen-
tioned requirements. As seen in the previous section, the
oxidation potential of DMANN (Eox ) 0.53 V) is lower than
those of dimethylamine (Eox ) 1.27 V) and NNH (Eox ) 0.82
V): The latter two are regarded as parent compounds of
DMANN. The lowering of the oxidation potential reflects the
electronic interaction between the two units. Moreover, the
detection of a triplet signal in the ESR spectrum of the singly
oxidized DMANN strongly suggests that the electronic structure
of DMANN can be represented as case (b) in Figure 6. The
proof for a large on-site Coulombic repulsion of SOMO of a
nitronyl nitroxide derivative may be obtained from the fact that
the nitronyl nitroxide group is reluctant to be reduced.19 This
may be because SOMO is localized only on the nitronyl
nitroxide group.
The fact that triplet ESR spectra were observed in all the
oxidized species in Table 4 indicates that their electronic
structure can also be classified as the case shown by Figure 6b.
It is to be stressed that close examination of the electronic feature
of DMANN and other amine-based donor radicals gives a clue
for the design of a donor radical which affords a ground state
triplet species upon one-electron oxidation.
3. Origin of Ferromagnetic Coupling in Cation Diradicals
Derived from Spin-Polarized Donors. The Curie plot of the
triplet signal of DMANN+• afforded a straight line in the
temperature range of 8-100 K (Figure 3b). The experimental
result strongly suggests that the oxidized species exists as a
cation diradical with a triplet ground state. To rationalize the
ground state of the cation diradical derived from a donor radical,
a molecular orbital calculation was performed on the cation
diradical of DMANN using a PM3/RHF method. Two singly
occupied molecular orbitals of DMANN+• correspond to
nonbonding molecular orbitals (NBMOs) of alternate hydro-
carbons (trimethylmethane, TMM for example), although the
degeneracy was removed by lower symmetry.20 Coefficients of
the singly occupied molecular orbitals of DMANN+• are
displayed in Figure 7 together with those of NBMOs of TMM.
As can be seen from Figure 7, coefficients of SOMO are
restricted on the nitronyl nitroxide group, while those of SOMO′,
which are derived from one-electron oxidation of HOMO, spread
over the entire molecule. Since the coefficients of SOMO are
shared by those of SOMO′ especially at the nitronyl nitroxide
group, they can be designated as space-sharing type molecular
orbitals. The cation diradicals which have such space-sharing
SOMOs, therefore, should have a positive exchange interaction
Figure 7. Coefficients of singly occupied molecular orbitals for
DMANN+• and TMM calculated by the PM3/RHF method.
between two unpaired electrons. As far as the distribution of
the coefficients of the singly occupied molecular orbitals for
DMANN+• is concerned, SOMO and SOMO′ closely resemble
two degenerated NBMOs of TMM (Figure 7). Exchange
interactions in DEANN+• and MORNN+• can be discussed in
the same manner.
The zero-field splitting parameters of DMANN+• were |D|
) 0.0276 cm-1 and |E| ) 0.0016 cm-1 as summarized in Table
4. The calculated D value for the presumed average distance
(2.1 Å) between two unpaired electrons is 0.28 cm-1 21
This
.
value is substantially larger than the observed one (0.0276
cm-1). The discrepancy can be rationalized by considering the
negative spin density on the R-carbon of the nitronyl nitroxide
group.22 In fact, the experimentally determined D value of TMM
in a hexafluorobenzene matrix23 is also much smaller than the
geometrically estimated one, and it is only 0.024 cm-1, which
is close to that of DMANN+•. Thus, the theoretical consideration
described above and the results from the ESR measurement
provide evidence for the close resemblance of the electronic
structure between DMANN+• and TMM of which the ground
state triplet spin multiplicity was proved both theoretically24 and
experimentally.4 Thus, the cation diradical of DMANN can be
regarded as a heteroanalogue of TMM, although the degeneracy
of SOMO and SOMO′ is removed in the former.25
It is worthwhile discussing the remarkable difference in
(21) The distance between the “unpaired” electrons of DMANN+• was
defined as the distance between the center of two N-O groups and the
aminium nitrogen atom and was evaluated from the molecular structure
which was optimized by a molecular mechanics calculation.
(22) Zheludev, A.; Barone, V.; Bonnet, M.; Delley, B.; Grand, A.;
Ressouche, E.; Rey, P.; Subra, R.; Schweizer, J. J. Am. Chem. Soc. 1994,
116, 2019-2027.
(23) Dowd, P. J. Am. Chem. Soc. 1966, 88, 2587-2589.
(24) Cramer, C. J.; Smith, B. A. J. Phys. Chem. 1996, 100, 9664-9670.
(25) The ground-state spin multiplicity of the hetero analogues of TMM
has been calculated recently. (Li, J.; Worthington, S. E.; Cramer, C. J. J.
Chem. Soc., Perkin Trans. 2 1998, 1045-1051.) The ground state of
iminoallyl which can be obtained by replacing the terminal carbon atoms
of TMM with an imino group is calculated to be triplet. However, the energy
difference between the triplet ground state and the singlet state becomes
closer by 7 kcal/mol, compared to that of TMM. Moreover, the ground
(19) The reduction potentials of NNH and phenyl nitronyl nitroxide are
-0.78 and -0.82 V, respectively.
(20) A qualitative method for predicting the ground state of non-Kekule´
hydrocarbons is documented by Borden.9 According to this method, when
the connectivity of two NBMOs are nondisjoint, i.e., whose NBMOs cannot
be confined to separate atoms by any linear combination, the triplet state
will be the ground state for this diradical.
state of imium dimethylene diradical, which is isoelectronic with DMANN+•
,
was calculated to be singlet. Therefore, even if the ground state of
DMANN+• is triplet, the triplet-singlet energy difference is likely to be
smaller than that of TMM.

High-Spin Cation Diradicals
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9729
Figure 8. Electronic structures of (a) ordinary radical and (b, c) spin-polarized donor radical. The curved arrow represents an electron to be
removed by one-electron oxidation.
Figure 9. Distribution of coefficients of SOMOs of diradicals composed of (a) dimethylanilinium and allylic radicals and (b) benzyl radical allylic
radicals.
kinetic stability between DMANN+• and TMM. In the case of
TMM, this diradical is photogenerated from a diazo precursor
at liquid nitrogen temperature and it exists only at cryogenic
temperatures. On the contrary, DMANN+• can be generated at
room temperature and it survives in a certain time period. Such
a significant stabilization of the triplet diradical cation can be
achieved by replacing the allyl radical unit with a nitronyl
nitroxide group. However, our attempts to isolate cation radical
salts derived from amine- or aminophenyl-based donor radicals
have not been successful so far. To increase the intensity of
triplet ESR signals, the oxidation conditions of spin-polarized
donors by iodine have been thorously examined. The signal
intensity turned out not to increase appreciably even when the
oxidation was carried out at lower temperatures e.g. 0, -20, or
-40 °C. On the contrary, the triplet signal was found to be
increased by leaving the oxidized sample at room temperature
for 30-60 s before freezing the sample. The result suggests
that the concentration of the triplet cation diradical is not
governed by the rate of its decomposition but by the rate of its
generation from a CT complex between the donor-radical and
iodine. When the temperature of the sample is lowered, the
equilibrium between the ion radical salt and the charge transfer
complex inclines to the complex side.
4. Spin Polarization in Donor Radicals Described by a
UHF Method. The electronic structure of donor radicals was
calculated by a UHF method in order to elucidate the charac-
teristic of donor radicals more precisely. First, phenyl nitronyl
nitroxide was selected as an ordinary radical for the comparison
with amine-based donor radicals. The orbital energy of SOMO-
(R) of phenyl nitronyl nitroxide is higher than those of HOMO-
(R) and HOMO(â), although HOMO(â) is located slightly above
HOMO(R) (Figure 8a). The energy difference between the
HOMO(â) and HOMO(R) can be regarded as an energetic spin-
polarization. Since the spin-polarization in phenyl nitronyl
nitroxide is not large enough to locate HOMO(â) above SOMO-
(R), one-electron oxidation of this radical will afford a closed-
shell cationic species, removing an electron from SOMO(R).
On the other hand, the energy level of HOMO(â) is located
above SOMO(R), in the case of case (b) in Figure 8. As a result,
one-electron oxidation will remove the electron of HOMO(â)
to afford a ground state triplet species. The electronic structure
of DMANN falls into this category especially in an eclipsed
conformation. Here we define a donor radical in which HOMO-
(â) is located above SOMO(R) as a spin-polarized donor. If
the donor character is even more stronger, the orbital energies
of both HOMO(R) and -(â) becomes higher than that of SOMO-
(R) as represented in Figure 8c (see the next section for
pAPNN).
5. Extension of Spin-Polarized Donors to an Aromatic
System. To expand the π-electronic structure of a spin-polarized
donor, aromatic amines are chosen as a donor unit. The
electronic structure of a cation diradical of the aminophenyl-
based donor radical, which was composed of N,N-dimethyl-
anilinium and allyl radicals, was examined in terms of a PMO
method.18 Contrasting to the case of a benzyl radical, the
electronic structure of which is characterized as an alternate
hydrocarbon (Figure 9b), the coefficients of a dimethylanilinium
radical distribute not only at the ortho- and para-positions but
also at the meta, although the coefficient at the latter position
is much smaller (Figure 9a). Consequently, the diradicals
consisting of anilinium and allyl radicals can be classified as a
nondisjoint type, regardless of the substitution positions. The
ferromagnetic coupling between these two radical units, there-
fore, can be expected both in para- and meta-isomers.
The ground state of cation diradicals was examined more
precisely in terms of a semiempirical molecular orbital calcula-
tion as in the case of amine-based donor radicals. According to
the molecular orbital calculation using a PM3/UHF method, the
energy level of HOMO(â) of pAPNN turns out to be the highest
(Figure 8c).26 Thus, the one-electron oxidation removes the
â-spin of HOMO to afford a ground state triplet diradical. When
coefficients of the SOMO and SOMO′ of pAPNN+• are
examined, it is found they share the atomic orbitals on the
nitronyl nitroxide group (Figure 10a). According to Borden’s
criterion9 such a species is thought to have a triplet ground state.
Spin correlation in the meta-substituted derivative is also
discussed on the basis of Borden’s criterion. Although the
coefficients of SOMO and SOMO′ are shared by the atomic
orbitals on the nitronyl nitroxide group in mAPNN+• (Figure
10b), the overlap of the coefficients of SOMO and SOMO′ of
mAPNN+• is smaller than that in the para-substituted system.
The ferromagnetic exchange interaction, therefore, is considered
to be much smaller.
(26) See also: Yamaguchi, K.; Okamura, M.; Nakano, M. Chem. Phys.
Lett. 1992, 191, 237-244.

9730 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Sakurai et al.
coefficients at the meta positions of the aminium radical as stated
above.
6. Charge Transfer Complexes Composed of Spin-Polar-
ized Donors. It has been reported that some donor radicals form
charge transfer complexes with acceptors.17 The magnetic
properties of those complexes were examined, and a part of
them exhibited ferromagnetic intermolecular interaction. The
electronic feature of those donor radicals, however, has not been
fully elucidated, that is, whether an unpaired electron on the
radical site has a ferromagnetic coupling with a π-electron spin
generated by one-electron oxidation of the donor part. In this
respect, it is interesting to study the magnetic properties of a
CT complex of the spin-polarized donor, because it is proved
to afford a ground state triplet species upon one-electron
oxidation.
On the basis of the temperature dependence of the magnetic
susceptibility, pAPNN‚CA turned out to be a paramagnet. This
may be because the ground state of the complex is basically
neutral (see Results section 5). If the degree of the charge
transfer is large enough to afford a ionic ground state, the
complex is expected to provide a ferrimagnetic property (Figure
12a).28 Although the CT complex with 2,3,5,6-tetrafluoro-
7,7,8,8-tetracyanoquinodimethane (TCNQ-F4) was prepared,17d
the paramagnetic spins were found to have almost disappeared,
presumably due to the decomposition of the cation diradical.
To obtain a CT complex of an ionic type,29 DMANN of lower
oxidation potential (Eox ) 0.53 V) and DDQ (Ered ) 0.59 V)
were used (cf., the oxidation potential of pAPNN is 0.64 V
and the reduction potential of CA is 0.11 V) The spectral data
were consistent with the ionic character of the complex (see
Results. 5). The temperature dependence of the magnetic
susceptibility of the DMANN‚DDQ complex revealed that the
complex, however, is paramagnetic. The Curie constant of the
complex was almost constant at 0.31 emu‚K‚mol-1 in the
temperature range of 20-250 K. Incidentally, the øp‚T value at
higher temperatures was ca. 18% less than one paramagnetic
spin per complex. The lack of the expected value is caused by
the decomposition of the complex, since the DMANN‚DDQ
complex turned out to be kinetically unstable. The result of the
magnetic measurement may be interpreted as follows: In this
complex, there are three kinds of spins; D+•, a cation radical
on the donor site of DMANN; R•, a localized spin on the radical
site of DMANN; A-•, and an anion radical of DDQ. Judging
from the Curie constant, electron spins of the cation radical (D+•)
and the anion radical (A-•) are supposed to be coupled strongly
in an antiferromagnetic manner, leaving the spin (R•) on the
radical site as a paramagnetic spin. Such an undesirable
interaction may be derived from a tight overlap between the
donor site of DMANN and DDQ, although no structural
information is available at the present stage.
Figure 10. Coefficients of SOMO and SOMO′ of (a) pAPNN+• and
(b) mAPNN+• calculated by a PM3/RHF method.
Oxidation of pAPNN and mAPNN by iodine afforded ESR
spectra of the triplet species. Zero-field splitting (ZFS) param-
eters of these triplet species were not significantly different from
those of the amine-based nitronyl nitroxides shown in Table 4.
This may be because the ZFS parameter is governed by the
spin distribution close to the nitronyl nitroxide group. The spin
densities of DMANN and pAPNN calculated by a DFT method
are depicted in Figure 11. The geminal dimethyl groups of NN
were replaced by hydrogen atoms. As seen from the figure, the
spin densities at the N-O groups, the C(2) carbon of the NN
group, and the amino-nitrogen or the aromatic carbon atom at
the foot of the NN group of DMANN or pAPNN are similar to
each other. The tendency of the spin densities of these
compounds can be understood by the canonical structures in
the valence bond description (Scheme 3).
In the point-dipole approximation, the D value is estimated
by the sum of the products of spin densities (Fa ‚ Fb) divided
by r3 , where r is the average distance between the partial
electron spins (spin densities). Therefore, the D values of
DMANN and pAPNN are mainly governed by spin densities
of the closer sites to the N-O groups aforementioned. Contribu-
tion from other sites should be much smaller because the average
distances between the N-O groups and the other sites are
substantially longer. Thus, it is not strange that the D values of
DMANN and pAPNN are not significantly different from each
other.
Since the temperature dependence of the triplet signal obeys
the Curie law in the temperature range of 8-100 K, the ground
state of pAPNN+• is considered to be triplet, taking the result
of molecular orbital calculation into account. On the other hand,
the ground state of mAPNN+• cannot be determined strictly. A
triplet state may be the ground state or the energy difference
between the triplet and the low-lying singlet is less than 10 cal/
mol. The observation of the distinct triplet signal, however, is
in contrast to a diradical of an odd alternate structure, because
the substitution pattern of mAPNN is predicted to be sin-
glet according to Ovchinikov’s rule.8 If the ground state of
mAPNN+• is triplet, introduction of heteroatoms may produce
a novel spin system which cannot be achieved by hydrocarbon
analogues.27 The difference in the spin-polarization between
mAPNN+• and the corresponding hydrocarbon diradical (Figure
9b) may be derived from the significant contribution of
Yamaguchi has claimed that ferrimagnets may be constructed
when a triplet cation diradical and a doublet anion radical are
stacked alternately, with an antiferromagnetic interaction be-
tween them (Figure 12a).28 If triplet cation radicals and anion
radicals stack separately in a mixed valent state, even a
conducting ferromagnet may be realized (Figure 12b). Although
it is possible for the DMANN‚DDQ complex to satisfy either
of the requirements, the complex, in fact, exhibited neither ferro-
nor ferrimagnetic intermolecular interactions. The discrepancy
(28) Yamaguchi, K.; Namimoto, H.; Fueno, T.; Nogami, T.; Shirota, Y.
Chem. Phys. Lett. 1990, 166, 408-414.
(29) (a) Sugimoto, T.; Tsujii, M.; Murahashi, E.; Nakatsuji, H.; Yamagu-
chi, K.; Fujita, H.; Kai, Y.; Hosoito, N. Mol. Cryst. Liq. Cryst. 1993, 232,
117-134. (b) Nakamura, Y.; Koga, N.; Iwamura, H. Chem. Lett. 1991,
69-71.
(27) Okada, K.; Imakura, T.; Oda, M.; Kajiwara, A.; Kamachi, M.;
Yamaguchi, M. J. Am. Chem. Soc. 1997, 119, 5740-5741.

High-Spin Cation Diradicals
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9731
Figure 11. Spin densities of DMANN+• calculated by a DFT method. Spin densities of pAPNN+• are also depicted.
Scheme 3. Structures of DMANN+• and pAPNN+•
have been synthesized. Such a donor radical turned out to afford
a ground state triplet cation diradical upon one-electron oxidation
even at room temperature. Since two parallel spins in DMANN+•
reside on nondegenerated molecular orbitals, its electronic
structure resembles that of a photoexcited triplet state of an
organic π-molecule. In other words, the electronic structure of
the photoexcited triplet state (T0) was stabilized into a ground
state according to a sophisticated molecular design. Such an
exotic electronic structure can be maintained by virtue of a large
Coulombic repulsion of SOMO and of a relatively small
difference in the orbital energies of SOMO and SOMO′. Such
type of donor radicals was designated as spin-polarized donors.
Figure 12. Schematic drawing of charge transfer complexes composed
of spin-polarized donor. (a) Ferrimagnetic spin system and (b) conduct-
ing ferromagnetic spin system.
The concept of a spin-polarized donor was successfully
extended to aminophenyl-based donor radicals. A high-spin
cation diradical species was observed even from meta-substituted
dimethylaminophenyl nitronyl nitroxide (mAPNN+•). The ob-
served tendency apparently violates the prediction by Ovchin-
nikov’s rule,8 when the rule is formally applied to the
heteroanalogue of an odd alternate hydrocarbon. The apparent
violation, however, can be rationalized by applying Borden’s
concept on the space-sharing molecular orbitals to the hetero-
atom-containing extended π-system.
The outcome of these spin-polarized donors will provide a
novel spin system which is characteristic of organic materials.
The difference in the electronic structure between TMM and
the cation diradicals reported here (e.g., DMANN+•) is that the
unpaired electrons of the diradical reside in two singly occupied
molecular orbitals (SOMO) of different character. Accordingly,
each unpaired electron in the nondegenerated SOMO plays an
individual role, constructing a unique spin system. For example,
if spin-polarized donors are stacked and partially doped, the
conduction electron, which itinerates along the columnar stack
of the donor units, may align local spins on the radical units
ferromagnetically on the basis of the strong ferromagnetic
intramolecular coupling.31 The ferromagnetic transition tem-
peratures of organic ferromagnetic metals composed of spin-
polarized donors are, therefore, expected to be raised substan-
tially higher than those of organic ferromagnets discovered so
far.32 Moreover, even an organic spin device may be constructed
may be derived from the fine structure of the complex. If
DMANN+• and DDQ-• are strongly dimerized, the antiferro-
magnetic interaction between the cation diradical and the anion
radical may become even larger than the ferromagnetic in-
tramolecular interaction between the π-spin on the donor site
of DMANN and the localized spin on the radical site. In such
a case, a ferrimagnetic spin ordering is not achieved. Conse-
quently, the structural requirement should be examined more
precisely in order to a realize ferrimagnetic spin system as shown
in Figure 12a. Isolation of the ionic type complexes of the
amine-based spin-polarized donors has so far not been success-
ful. Recently, we have prepared TTF-based spin-polarized
donors and have examined the conditions to isolate a charge
transfer complex and/or ion-radical salts derived from the new
type of donor radicals.30
At any rate, the CT complex composed of a spin-polarized
donor and an acceptor has been prepared, and the ionic complex
exhibits a conducting property. If the ion radical salt of a
segregated columnar stacking type with mixed valency is formed
using a spin-polarized donor and an acceptor with a reasonably
high reduction potential, the salt may exhibit both metallic
conductivity and ferromagnetism. It is to be stressed that the
spin-polarized donors which we presented here are promising
building blocks to construct a novel spin system.
Summary
To construct a novel high-spin system which can be generated
(31) (a) Dormann, E.; Nowak, M. J.; Williams, K. A.; Angus, R. O., Jr.;
Wudl, F. J. Am. Chem. Soc. 1987, 109, 2594-2599. (b) Yamaguchi, K.;
Okamura, M.; Fueno, T. Synth. Met. 1991, 41-43, 3631-3634.
(32) Nogami, T.; Ishida, T.; Tsuboi, H.; Yoshikawa, H.; Yamamoto, H.;
Yasui, M.; Iwasaki, F.; Iwamura, H.; Takeda, N.; Ishikawa, M. Chem. Lett.
1995, 635-636 and references therein.
by redox processes, amine-based donor radicals (e.g., DMANN)
(30) (a) Nakazaki, J.; Matsushita, M. M.; Izuoka, A.; Sugawara, T.
Tetrahedron Lett. 1999, 40, 5027-5030. (b) Nakazaki, J.; Ishikawa, Y.;
Izuoka, A.; Sugawara, T.; Kawada, Y. Chem. Phys. Lett. 2000, 319, 385-
390.

9732 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Sakurai et al.
The molecular and crystal structures in this paper were drawn by
Moldraw.37
using a spin-polarized polyradical donors the spin multiplicity
of which can be converted reversibly through redox processes.33
Electrochemical Measurements. Cyclic voltammograms were
recorded in dry acetonitrile in the presence of 0.1 M tetra-n-
butylammonium perchlorate (Nacalai Tesque, Inc.) as an electrolyte
with a platinum working electrode using a potentiostat/galvanostat HAB
151 (Hokuto Denko Ltd.) at room temperature. An Ag/AgCl electrode
Experimental Section
General Methods. Gel permeation chromatography (GPC) was
performed by LC908 or LC908-C60 (Japan Analytical Industry Co.,
Ltd.) equipped with JAIGEL-1H and -2H columns. Chloroform was
used as an eluent. 1H NMR spectra were measured by an R-500 (JEOL)
spectrometer in chloroform-d1 or dimethylsulfoxide-d6 solutions. Chemi-
cal shifts were reported in δ relative to TMS, 0.00 ppm or dimethyl-
sulfoxide, 2.49 ppm as an internal standard. Infrared spectra were
recorded on a Perkin-Elmer 1400 series infrared spectrometer using
KBr pellets. Elemental analyses were performed by YANACO CHN
corder MT-5 at the Center of Organic Elemental Analyses in the
University of Tokyo. Mass spectra were measured by a GCMS-QP1000
(Shimadzu) quadrupole mass-spectrometer (EI) in a direct ionization
mode (70 V). Melting points were obtained with a MEL-TEMP II
(Laboratory Devices) heating block and were uncorrected. Reflection
spectra were recorded by a V-570 UV/VIS/NIR spectrophotometer
(JASCO) using KBr pellets. Observed reflection spectra were converted
to absorption spectra using a Kubelka-Munk transform.
was used for a reference electrode. Scanning rate was 200 mV‚s-1
.
ESR Measurements. ESR spectra were measured on a JEOL JES-
RE2X spectrometer (X-band microwave unit, 100 kHz field modulation)
equipped with a liquid helium transfer system (Air Products and
Chemicals, Inc., LTR-3). Temperature was controlled by a digital
temperature indicator/controller (Scientific Instruments Inc., 3700).
Temperature close to the sample was measured separately with a digital
multithermometer (Advantest, TR2114H) with a gold-iron thermo-
couple (of an estimated precision of 0.5 K). Temperature calibration
was performed by monitoring Ge resistance thermometer (Lake Shore,
GR-200B-1500, in the temperature range of 6 to 100 K). Iodine was
sublimed under a reduced pressure (ca. 5 mmHg). In a side arm portion
made of Pyrex, was placed a THF solution (ca. 0.5 cm3) of a large
excess of iodine, and the solution was degassed by three freeze-thaw
cycles through a high-vacuum stopcock (Flon Ind., F-6001 92-03). The
solution was transferred to crystals of a donor radical (ca. 1 mg) which
was placed at the bottom of a main-arm portion made of quartz. After
shaking the mixed sample vigorously at room temperature, the sample
tube was rapidly cooled in a precooled ESR cavity. The microwave
frequency was measured with a TR5212 meter (Advantest), and the
resonance magnetic fields of the signals were measured with the aid
of an ES-FC5 NMR field meter (Echo Electronics). Dependence of
the signal intensities on temperatures was measured by the use of a
temperature-controlling accessory. Microwave power dependence of
the signal intensities was checked by several power settings to avoid a
power saturation at the low-temperature region.
Measurements of Magnetic Susceptibility. Magnetic susceptibilities
of charge transfer complexes (pAPNN‚CA and DMANN‚DDQ) were
measured by Faraday-type magnetic balance system (Oxford, 5 T).38
The force loaded on the sample was measured by a Cahn 1000 electric
microbalance. The quartz-basket containing the sample (ca. 10-15 mg)
was hung from an electrobalance with a tungsten wire (0.08 mm φ) in
a sample chamber of the cryostat. The outputs from the electricbalance
system were processed by an NEC PC9801vm microcomputer, which
was also used to control the temperature of the cryostat and the power
supplied for the superconducting magnets.
Electrical Conductivity Measurement. A compaction pellet with
a diameter of 3 mm of DMANN‚DDQ was prepared by a compressor.
The conductivity of DMANN‚DDQ was measured by a two-probe
method. For measuring the electric conductivity of the single crystal
of pAPNN‚CA, a two-probe method was used. Gold wires (25 µm φ)
were attached to the sample with gold paste (Tokuriki Kagaku) as a
contact.
Molecular Orbital Calculations. For semiempirical molecular
orbital calculations, CAChe (including Editor, molecular mechanics,
MOPAC, Tabulator, and Visualizer+), a software provided by CAChe
Scientific, Inc., was used. The geometrical structure was first optimized
by molecular mechanics, and molecular orbital calculations using PM3
were conducted on a Power Macintosh 8500/120 computer.
Materials. Most of the chemicals were purchased from Tokyo Kasei
Kogyo Co. Ltd., Wako Pure Chemicals Industries Ltd., and Aldrich
Chemical Co. Inc. (Japan). Preparation of 2,3-bis(hydroxylamino)-2,3-
dimethylbutane and 2,3-bis(hydroxylamino)-2,3-dimethylbutane sulfate
were performed according to the published procedures.39 Diethyl ether
was distilled under nitrogen from sodium benzophenone ketyl before
use. Dichloromethane was dried over molecular sieves (3 Å). Deacti-
vated silica gel (Fuji Silysia Chemical Co., Ltd., BW-820MH) was
prepared by the treatment with methanol before use. Florisil (100-
200 mesh) was purchased from Katayama Chem. Co. Ltd., and Celite
(No. 545) was purchased from Wako Pure Chemicals Industries Ltd.
X-ray Crystal Structure Analyses. Blue rod crystals of DMANN
were obtained by slow evaporation of the solvent from a n-hexane
solution. A single crystal with dimensions of 0.60 × 0.20 × 0.16 mm3
was used for the X-ray analysis. The intensity data collection was
performed at an ambient temperature on a Rigaku AFC-5 four-circle
diffractometer using graphite-monochromatized Mo KR radiation. Three
standard reflections were measured for every 100 reflections and showed
no significant variations throughout the data collection; 2616 reflections
(4° e 2θ e 55°) measured, 1123 independent reflections (I > 3σ(I))
were used for analysis. Structure was solved by direct methods (SAPI-
91)34 and Fourier techniques (DIRDIF-94).35 The non-hydrogen atoms
were refined anisotropically. The hydrogen atom coordinates were
refined but their isotropic B’s were held fixed. Final refinement was
performed with anisotropic thermal factors for all non-hydrogen atoms;
181 parameters, R ) 0.053, Rw ) 0.063, S ) 1.67; maximum and
minimum heights in final difference Fourier map 0.16 and -0.19 e-
/
Å3. Crystal data: C9H18N3O2, FW ) 200.26, monoclinic, space group
P21/n, a ) 11.083(4) Å, b ) 11.497(4) Å, c ) 9.263(3) Å, â )
106.03(3)°, V ) 1134.5(7) Å3, Z ) 4, Dc ) 1.172 g/cm3.
Bluish plate crystals of pAPNN‚CA were prepared as described at
a later part of the Experimental Section. A single-crystal having
approximate dimensions of 0.80 × 0.28 × 0.01 mm3 was used for the
X-ray analysis. All measurements were performed at an ambient
temperature on a Rigaku AFC7R diffractometer with graphite-mono-
chromatized Mo KR radiation. Three standard reflections were measured
for every 150 reflections. The intensities were decreased by ca. 5%
during the measurement. Of the 5591 reflections (6° e 2θ e 55°)
collected, 1419 reflections (I > 3σ(I)) were used for analysis. The
structure was solved by direct methods (SHELX-86)36 and Fourier
techniques (DIRDIF-94).35 Final refinement was performed with
anisotropic thermal factors for all non-hydrogen atoms; 316 parameters,
R ) 0.071, Rw ) 0.053, S ) 3.26; maximum and minimum heights in
final difference Fourier map 0.31 and -0.30 e-/Å3. Crystal data:
C24H25N3O4Cl4, FW ) 561.29, triclinic, space group P1h, a ) 8.690(5)
Å, b ) 22.16(2) Å, c ) 6.725(6) Å, R ) 92.07(8)°, â ) 90.51(8)°, γ
) 91.41(5)°, V ) 1293(1) Å3, Z ) 2, Dc ) 1.441 g/cm3.
(33) (a) Nakamura, T.; Momose, T.; Shida, T.; Sato, K.; Nakazawa, S.;
Kinoshita, T.; Takui, T.; Itoh, K.; Okuno, T.; Izuoka, A.; Sugawara, T. J.
Am. Chem. Soc. 1996, 118, 8684-8687. (b) Rajca, S.; Rajca, A. J. Am.
Chem. Soc. 1995, 117, 9172-9179.
(34) Fan, H.-F. Structure Analysis Programs with Intelligent Control;
Rigaku Corporation: Tokyo, Japan, 1991.
(35) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-94 program system;
Technical Report of the Cryatallography Laboratory: University of
Nijmegen, The Netherlands, 1994.
(36) Sheldrick, G. M. Crystallographic Computing 3; Sheldrick, G. M.,
Kruger, C., Goddard, R.. Eds.; Oxford University Press: 1985; pp 175-
189.
(37) Molecular graphics for Machintosh: Cense, J.-M. Tetrahedron
Comput. Mothodol. 1989, 2, 65-71.
(38) Awaga, K.; Maruyama, Y. Chem. Mat. 1990, 2, 535-539.
(39) Lamchen, M.; Mittag, T. W. J. Chem. Soc. C 1966, 2300-2303.

High-Spin Cation Diradicals
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9733
Dry benzene, which was distilled over sodium metal, was used
especially for the preparation of charge-transfer complexes of donor
radicals. Chloranil (CA) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) were recrystallized from dichloromethane and chloroform
solution, respectively. All the other reagents were used as purchased
without further purification.
Preparation. Amine- or aminophenyl-based donor radicals were
synthesized from the corresponding compounds as shown in Scheme
1.
2-Bromo-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (2). In
reference 40 BrNN was prepared from NNH by treating with Br2 in
carbon tetrachloride/pyridine or by treating bicarbonate solution of NNH
with cyanogen bromide. The method described below was found to
yield BrNN more conveniently even in a better yield.
1260s, 1217, 1200, 1135, 1114s, 1004, 873, 857, 538; mp 151-152
°C; EI-MS m/e 242 (M+). Anal. (percent) calcd for C11H20N3O3, C
54.53, H 8.32, N 17.35, found C 54.36, H 8.25, N 17.35.
2-(4′-Dimethylaminophenyl)-1,3-dihydroxy-4,4,5,5-tetrameth-
ylimidazolidine (4). Preparation of the cyclic hydroxylamine 4 was
reported in reference 42. The addition of the hydroxylamine sulfate
was found to afford the cyclic hydroxylamine 4 in higher yield by the
procedure described below.
In a 50 mL round-bottomed flask, 600 mg (4.02 mmol) of
4-(dimethylamino)benzaldehyde 3, 900 mg (6.08 mmol) of 2,3-bis-
(hydroxylamino)-2,3-dimethylbutane, and 300 mg of 2,3-bis(hydrox-
ylamino)-2,3-dimethylbutane sulfate as a catalyst were suspended in
30 mL of methanol. Between the neck of a reaction flask and a reflux
condenser, a dropping funnel filled with molecular sieves (4 Å) was
placed for dehydration. The reaction mixture was refluxed under
nitrogen for overnight. After the solvent was concentrated to about half
of the starting amount with a rotary evaporator, precipitates were filtered
by suction and was washed with cold methanol. The obtained product
was dried under a reduced pressure to give 1.12 g (94%) of colorless
powders: 1H NMR (DMSO-d6, ppm) 7.58 (s, 2H), 7.41 (d, J ) 9 Hz,
2H), 6.67 (d, J ) 9 Hz, 2H), 4.39 (s, 1H), 2.85 (s, 6H), 1.04 (s, 6H),
1.02 (s, 6H); IR (KBr pellet, cm-1) 3241 br, 2974, 2894, 1614, 1522,
1480, 1445, 1378, 1314, 1192, 1163, 1140, 804.
To an ice-cooled solution of 2.10 g (52.5 mmol) of NaOH in 12
mL of water was added 2.8 g (17.5 mmol) of Br2 dropwise. A mixture
of 0.21 mg (1.3 mmol) of 2-hydro-4,4,5,5-tetramethylimidazoline-3-
oxide-1-oxyl (1), 50 mL of ice-water, and 1.0 mL of NaOBr aq
obtained by the above procedure were shaken for 30 s in a 500 mL
separating funnel. The solution was extracted with 40 mL of dichlo-
romethane twice, washed with 50 mL of water twice, dried over MgSO4,
and filtered off, and then the solvent was concentrated down to ca. 3
mL by a rotary evaporator. Concentration of the solution by a slow
evaporation of the solvent at 0 °C under N2 yielded 0.26 g of red-
purple crystals of the desired product (yield 84%). The product was
purified by GPC: ESR in benzene solution aN ) 0.782 mT (2 N) at g
) 2.0060; IR (KBr pellet, cm-1) 2991, 1448, 1413, 1374, 1360, 1171,
1132; mp 134-138 °C (decomp).
2-Dimethylamino-4,4,5,5-tetramethylimidazoline-3-oxide-1-ox-
yl (DMANN).41 To an ice-cooled solution of 0.30 g (1.3 mmol) of 2
dissolved in 10 mL of water was added 4 mL of a 50% aqueous solution
of dimethylamine. After stirring for 4 h at room temperature, the
solution was extracted with two 30 mL portions of dichloromethane,
washed with 30 mL of water, and dried over MgSO4. After filtered off
the desiccant, the solvent was evaporated under a reduced pressure.
The obtained product was recrystallized from hot n-hexane to give 0.14
g of blue needles (yield 55%): ESR in benzene solution aN ) 0.741
mT (2 N) at g ) 2.0059; IR (KBr pellet, cm-1) 2986, 2932, 1595,
1329, 1207, 1127, 1062, 874, 539; mp 112-113 °C. Anal. (percent)
calcd for C9H18N3O2, C 53.98, H 9.06, N 20.98, found C 53.70, H
8.80, N 20.80.
2-Diethylamino-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl
(DEANN). To an ice-cooled solution of 0.26 g (1.1 mmol) of 2
dissolved in 8 mL of water was added 3 mL of diethylamine. After
stirring for 24 h at room temperature, the solution was extracted with
50 mL of dichloromethane, washed with water, dried over MgSO4,
and filtered off, and then the solvent was evaporated under a reduced
pressure. The obtained product was dissolved in hot n-hexane and
insoluble byproducts were separated. After a removal of the solvent,
the residue was purified on deactivated silica gel using 3% methanol-
diethyl ether as an eluent. Evaporation of the solvent gave 25 mg of
blue needles (yield 10%): ESR in benzene solution aN ) 0.747 mT (2
N) at g ) 2.0060; IR (KBr pellet, cm-1) 2980, 2932, 1598, 1451, 1387,
1369, 1330, 1296, 1208, 1145, 1127, 1102, 1088, 871, 541; mp 88.5-
91.0 °C; EI-MS m/e 228 (M+).
2-Morpholino-4,4,5,5-tetramethylimidazoline-3-oxide-1-
oxyl (MORNN). To 10 mL of morpholine in a 30 mL round-bottomed
flask cooled by an ice bath was added 210 mg (0.890 mmol) of 2.
After stirring the mixture for an hour, the ice bath was removed and
the solution was stirred for overnight. Excess morpholine was removed
under a reduced pressure at room temperature. The obtained product
was dissolved in diethyl ether, and insoluble byproducts were separated.
After a removal of the solvent, the residue was column chromatographed
on silica gel using ethyl acetate as an eluent. The obtained product
was recrystallized from n-hexane to give 100 mg of blue needles (yield
46%): ESR in benzene solution aN ) 0.734 mT (2 N) at g ) 2.0056;
IR (KBr pellet, cm-1) 2985, 2900, 2856, 1595, 1368, 1340s, 1319s,
2-(4′-Dimethylaminophenyl)-4,4,5,5-tetramethylimidazoline-3-
oxide-1-oxyl (pAPNN).42 To a mixture of 500 mg (1.79 mmol) of cyclic
hydroxylamine 4 and 30 mL of benzene were aded 500 mg of powdered
anhydrous potassium carbonate and 2.0 g of PbO2, and the suspension
was stirred vigorously for 1.5 h. After filtering the reaction mixture
through Celite, the solvent was removed by a rotary evaporator. The
blue residue was chromatographed on a deactivated silica gel using
chloroform as an eluent. The obtained product was recrystallized from
hot ethanol to give 350 mg (yield 71%) of blue plates: ESR in benzene
solution aN ) 0.760 mT (2 N) at g ) 2.0059; IR (KBr pellet, cm-1
)
2985, 1608, 1498, 1387, 1369, 1358, 1298, 1207, 1132, 946, 815, 541;
mp 153-154 °C. Anal. (percent) calcd for C15H22N3O2, C 65.19, H
8.02, N 15.21, found C 64.90, H 8.10, N 15.38.
3-(Dimethylamino)benzyl Alcohol (6).43 In a 100 mL three-necked
round-bottomed flask fitted with a 50 mL dropping funnel with a
pressure-equalization arm and a reflux condenser equipped with a
nitrogen balloon were placed with 1.30 g (34.0 mmol) of lithium
aluminum hydride and 80 mL of anhydrous diethyl ether. After
refluxing the suspended mixture for an hour, the dropping funnel was
charged with 10.0 g (55.8 mmol) of 3-(dimethylamino)benzoic acid
methyl ester 5 dissolved in 30 mL of anhydrous diethyl ether. While
the suspension was gently stirred under nitrogen, the solution of ester
5 was added dropwise at a rate maintaining a gentle reflux. When the
addition was complete, the reaction mixture was refluxed for 3 h. Then
20 mL of ethyl acetate was added and the solution was refluxed for
other 2 h. The gray reaction mixture was hydrolyzed by an addition of
1 mL of 1 N HCl and 1 mL of water. The reaction mixture was filtered
through Celite, washed with diethyl ether, dried over MgSO4, and
filtered off, and the solvent was evaporated under a reduced pressure.
The obtained yellow oil (4:3 mixture of benzyl alcohol and a chlorinated
compound) was dissolved in 40 mL of ethanol and was added to NaOH
aq (80 g, 0.20 mol in 40 mL of water). After refluxing the mixture for
1.5 h, 100 mL of water was added. The obtained product was extracted
by 400 mL of diethyl ether, washed with water, and dried over MgSO4.
After the evaporation of the solvent, 6.87 g (yield 81%) of yellow oil
was obtained. No further purification was performed before the next
step: 1H NMR (CDCl3, ppm) 7.23 (t, J ) 8 Hz, 1H), 6.75 (d, J ) 2
Hz, 1H), 6.71 (d, J ) 7 Hz, 1H), 6.68 (dd, J ) 8 and 2 Hz, 1H), 4.65
(s, 2H), 2.96 (s, 6H); IR (KBr pellet, cm-1) 3364 br, 2873, 2803, 1606,
1582, 1499, 1439, 1352, 997, 776, 694.
3-(Dimethylamino)benzaldehyde (7). In a 200 mL round-bottomed
flask, 1.34 g (8.86 mmol) of benzyl alcohol 6 was dissolved in 50 mL
(42) Ullman, E. F.; Osiecki, J. H.; Boocock, D. G. B.; Darcy, R. J. Am.
Chem. Soc. 1972, 94, 7049-7059.
(40) Boocock, D. G. B.; Ullman, E. J. Am. Chem. Soc. 1968, 90, 6873-
6874.
(41) Ullman, E. F.; Call, L.; Leute, R. K.; Osiecki, J. H. U.S. Patent
3,740,412 (Cl.260-309.6; C07d), 1973.
(43) Tsuchiya, M.; Yoshida, H.; Ogata, T.; Inokawa, S. Bull. Chem. Soc.
Jpn. 1969, 42, 1756-1757.
(44) Cocker, W.; Harris, J. O.; Loach, J. V. J. Chem. Soc. 1938, 751-
753.

9734 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Sakurai et al.
of dry dichloromethane. To this flask were added 2 mL of dry pyridine
and 3.83 g (17.7 mmol) of powdered pyridinium chlorochromate. After
an addition of 1.0 g (11 mmol) of powdered anhydrous sodium
carbonate to the reaction mixture, the suspension was refluxed for
overnight under nitrogen. When the mixture was cooled to room
temperature, dry diethyl ether was added and the precipitates were
filtered off through Florisil. The crude product was purified by GPC
and then by silica gel column chromatography with CHCl3 as eluent,
giving 67 mg (yield 5%) of a yellow oil: 1H NMR (CDCl3, ppm) 9.96
(s, 1H), 7.39 (t, J ) 8 Hz, 1H), 7.20 (d, J ) 8 Hz, 1H), 7.19 (s, 1H),
6.97-6.99 (m, 1H), 3.02 (s, 6H); IR (KBr pellet, cm-1) 2809, 1698s,
1602s, 1576, 1498, 1357, 1206, 998, 779, 683.
2-(3′-Dimethylaminophenyl)-1,3-dihydroxy-4,4,5,5-tetrameth-
ylimidazolidine (8). In a 50 mL round-bottomed flask, 217 mg (1.45
mmol) of 7, 434 mg (2.93 mmol) of 2,3-bis(hydroxylamino)-2,3-
dimethylbutane, and 100 mg of 2,3-bis(hydroxylamino)-2,3-dimeth-
ylbutane sulfate as a catalyst were dissolved in 20 mL of methanol.
Between the neck of a reaction flask and a reflux condenser, a dropping
funnel filled with molecular sieves (4 Å) was placed for dehydration.
The reaction mixture was refluxed under nitrogen overnight. After the
solvent was concentrated to about half of the starting amount with a
rotary evaporator, precipitates were filtered by suction and was washed
with cold methanol. The obtained product was dried under a reduced
pressure to give 345 mg (yield 85%) of colorless powders: 1H NMR
(CDCl3, ppm) 7.24 (d, J ) 8 Hz, 1H), 6.91 (s, 1H), 6.90 (d, J ) 9 Hz,
1H), 6.70-6.72 (m, 1H), 4.92 (s, 2H), 4.73 (s, 1H), 2.95 (s, 6H), 1.13
(s, 12H); IR (KBr pellet, cm-1) 3265 br, 2981, 2912, 1608, 1504, 1378,
1218, 1145, 917, 776, 697.
Figure 13. Angular dependence of the orbital energy for DMANN.
(Dotted line, HOMO(â); solid line, HOMO(R); gray line, SOMO(R)).
pAPNN‚CA complex. The authors are indebted to Prof. Kunio
Awaga (The University of Tokyo) for his generous provision
of the Faraday-type magnetic balance. This work was supported
by CREST (the Core Research for Evolutional Science and
Technology) of JST (the Japan Science and Technology Corp.)
and by a Grant-in-Aid for Scientific Research on Priority Area
“Molecular Magnetism” (Area No. 228/04 242 104) from the
Ministry of Education, Science and Culture, Japan.
Appendix
2-(3′-Dimethylaminophenyl)-4,4,5,5-tetramethylimidazoline-3-
oxide-1-oxyl (mAPNN). To a mixture of 193 mg (0.691 mmol) of
cyclic hydroxylamine 8 and 10 mL of benzene were added 170 mg of
powdered anhydrous potassium carbonate and 660 mg of PbO2, and
the mixture was stirred vigorously. The resulting colored mixture was
stirred for 1 h. After filtering through Celite, the solvent was removed
under a reduced pressure. The residue was column chromatographed
on deactivated silica gel with 3% ethyl acetate-chloroform as an eluent.
The obtained product was recrystallized from hot n-hexane to give 145
mg (yield 76%) of blue block crystals: ESR in benzene solution aN )
0.748 mT (2 N) at g ) 2.0061; IR (KBr pellet, cm-1) 2982, 1580,
1575, 1493, 1454, 1415, 1395, 1360, 1216, 1190, 1165, 1137, 999,
856, 785, 688, 542; mp 105.0-105.5 °C; EI-MS m/e 276 (M+). Anal.
(percent) calcd for C15H22N3O2, C 65.19, H 8.02, N 15.21, found C
65.28, H 8.07, N 15.04.
Preparation of CT Complexes. pAPNN‚CA:pAPNN (19 mg, 0.067
mmol) and chloranil (CA) (17 mg, 0.067 mmol) were dissolved
separately in 0.6 and 1.4 mL of dry benzene, respectively. Each solution
was filtrated and transferred into a 10 mL Erlenmeyer flask. The flask
was kept standing in a desiccator in a dark cool place for 4 days. Small
thin blue plates of pAPNN‚CA were obtained after a slow evaporation
of the solvent: IR (KBr pellet, cm-1) 1682, 1603, 1563, 1540, 1416,
1366, 1299, 1215, 1132, 1112, 945, 835, 809, 741, 707. DMANN‚
DDQ: Grinding DDQ (2.0 mg, 0.0089 mmol) with crystals of DMANN
(1.8 mg, 0.0089 mmol) by an agate mortar and pestle in an equal molar
amount under a nitrogen atmosphere gave a black powder of DMANN‚
DDQ: IR (KBr pellet, cm-1) 2219, 1675 br, 1500 br, 1412, 1225 br,
1188 br, 1038, 782, 509.
The heat of formation of DMANN was calculated using a
PM3/UHF method, changing the angle (θ in Figure 13) between
the direction of the lone pair of the amino group and the
π-orbital of the nitronyl nitroxide group by every 5° in the range
of 0-90°. The hybridization of the nitrogen atom of the amino
group is fixed to sp2 during the process. The most stable
conformation was obtained with the twist angle of θ ) 45°,
while the perpendicular conformation (θ ) 90°) turned out to
be the least stable. This twist angle (θ ) 45°) was close to the
value (θ ≈ 40°) that was obtained from the X-ray analysis.
Preference of the twist conformation may be rationalized by
the steric and electronic repulsion. Figure 13 shows the angular
dependence of the orbital energy of HOMO(R) and HOMO(â)
and SOMO(R). Judging from the calculated result, one-electron
oxidation will remove a â-spin of HOMO to afford an open-
shell cation diradical. Since the electronic interaction is largest
at θ ) 0°, the largest spin-polarization of HOMO is obtained
at this angle. Because the electrochemical oxidation of a donor
radical in solution is considered to occur from a conformation
with the lowest oxidation potential, the one-electron oxidation
will occur when θ becomes close to 0°. In this conformation,
oxidation must occur from HOMO(â) to afford the ground state
triplet cation diradical as in Figure 8b.
Supporting Information Available: Crystal structure in-
formation for DMANN and pAPPN‚CA (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. The authors gratefully acknowledge Dr.
Reiji Kumai (Joint Research Center for Atom Technology) for
helpful discussion and for the X-ray measurement of the
JA994547T