Triradical cation of p-phenylenediamine having two nitroxide radical groups: Spin alignment mediated by delocalized spin

Article abstract of DOI:10.1021/ja056318e

The cationic state of a p-phenylenediamine (PDA) molecule having two nitroxide radical groups was prepared and characterized using electrochemical, electron spin resonance (ESR) spectroscopic, and absorption spectroscopic methods. The delocalized interval

Full text of DOI:10.1021/ja056318e

Published on Web 02/11/2006
Triradical Cation of p-Phenylenediamine Having Two Nitroxide
Radical Groups: Spin Alignment Mediated by Delocalized
Spin
Akihiro Ito,*,† Yoshiaki Nakano,† Masashi Urabe,† Tatsuhisa Kato,*,‡ and
Kazuyoshi Tanaka*,†,§
Contribution from the Department of Molecular Engineering, Graduate School of Engineering,
Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Department of Chemistry,
Josai UniVersity, 1-1 Keyakidai, Sakado, Saitama 350-0295, Japan, and CREST,
Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
Received September 13, 2005; E-mail: aito@scl.kyoto-u.ac.jp; rik@josai.ac.jp; a51053@sakura.kudpc.kyoto-u.ac.jp
Abstract: The cationic state of a p-phenylenediamine (PDA) molecule having two nitroxide radical groups
was prepared and characterized using electrochemical, electron spin resonance (ESR) spectroscopic, and
absorption spectroscopic methods. The delocalized intervalence state of the p-phenylendiamine (PDA)
moiety was detected in the cationic state. From the pulsed ESR measurements, it was confirmed that the
delocalized spin induces parallel spin alignment between the localized two nitroxide groups which are
magnetically weakly coupled in the neutral state. It was found that the resulting high-spin alignment does
not seriously affect the delocalized intervalence state of the PDA radical cation.
Introduction
Organic intervalence compounds are the simple models for
understanding of the intramolecular electron-transfer process or
To control spin preference in multispin molecular systems,
it should be established that a lot of stable organic radical centers
are assembled to have desirable intramolecular magnetic
interaction. In the past few decades, significant progress has
been made in rational design and synthesis of high-spin organic
molecules containing several stable radical centers.1 In many
cases, the stable radical centers are linked by the so-called
ferromagnetic coupling units such as 1,3-benzenediyl to generate
the (quasi-)degenerate singly occupied nonbonding molecular
orbitals (NBMOs) according to Hund’s rule.2
In addition, the control of charge distribution in the above-
mentioned multispin organic molecular systems is also regarded
as an important issue in conjunction with the potential applica-
tion to spin electronics devices.3 When a few electrons are added
to or removed from the multispin organic system, important
problems of the relationship between the spin preference and
the extra charge distribution emerge from alterations to oc-
cupancy number of the NBMOs: whether the high-spin prefer-
ence among the surviving unpaired electrons remains unchanged
or not; whether the extra electrons (or “holes”) are localized or
delocalized.2c With these questions in mind, several polyradical
anions4 and cations5 have exhibited intriguing aspects of the
charge distribution and its influence on the spin preference.
for examining to what degree the extra charge is delocalized
over a molecule, and nowadays a lot of delocalized intervalence
compounds have been reported.6-8 However, the delocalized
spin associated with the delocalized intervalence state has never
been used to mediate an intramolecular magnetic interaction,
in particular, high-spin alignment. As has been described in our
previous theoretical study,3 the ferromagnetic coupling between
the localized multispin system and the delocalized electron spin
is closely related to the realization of spin-polarized molecular
wire. In the present study, we report on the observation of spin
alignment mediated by a delocalized spin in a multispin organic
system. We selected para-phenylenediamine (PDA) derivative
1 carrying two persistent nitroxide groups (Figure 1) since
radical cations of PDA molecules are widely accepted as
important organic delocalized intervalence radical cations.6,8
Furthermore, the relationship between spin alignment and
intervalence state in 1+ is also discussed in comparison with
(4) (a) Matsushita, M.; Nakamura, T.; Momose, T.; Shida, T.; Teki, Y.; Takui,
T.; Kinoshita, T.; Itoh, K. J. Am. Chem. Soc. 1992, 114, 7470. (b) Rajca,
S.; Rajca, A. J. Am. Chem. Soc. 1995, 117, 9172. (c) Nakamura, T.;
Momose, T.; Shida, T.; Kinoshita, T.; Takui, T.; Teki, Y.; Itoh, K. J. Am.
Chem. Soc. 1995, 117, 11292. (d) 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. (e) Sedo´, J.;
Ruiz, D.; Vidal-Gancedo, J.; Rovira, C.; Bonvoisin, J.; Launay, J.-P.;
Veciana, J. AdV. Mater. 1996, 8, 748.
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T.; Kinoshita, T.; Itoh, K. Bull. Chem. Soc. Jpn. 1993, 66, 1333. (b) Izuoka,
A.; Hiraishi, M.; Abe, T.; Sugawara, T.; Sato, K.; Takui, T. J. Am. Chem.
Soc. 2000, 122, 3234. (c) Nakazaki, J.; Chung, I.; Matsushita, M. M.;
Sugawara, T.; Watanabe, R.; Izuoka, A.; Kawada, Y. J. Mater. Chem. 2003,
13, 1011. (d) Harada, G.; Jin, T.; Izuoka, A.; Matsushita, M. M.; Sugawara,
T. Tetrahedron Lett. 2003, 44, 4415.
† Kyoto University.
‡ Josai University.
§ Japan Science and Technology Agency.
(1) Crayston, J. A.; Devine, J. N.; Walton, J. C. Tetrahedron 2000, 56, 7829.
(2) (a) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88. (b) Iwamura, H.; Koga,
N. Acc. Chem. Res. 1993, 26, 346. (c) Rajca, A. Chem. ReV. 1994, 94,
871.
(3) Ito, A.; Urabe, M.; Tanaka, K. Polyhedron 2003, 22, 1829 and references
therin.
(6) (a) Nelsen, S. F. Chem.sEur. J. 2000, 6, 581. (b) Launay, L.-P. Chem.
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9
2948
J. AM. CHEM. SOC. 2006, 128, 2948-2953
10.1021/ja056318e CCC: $33.50 © 2006 American Chemical Society

Spin Alignment Mediated by Delocalized Spin
A R T I C L E S
2-(di-tert-butylphosphino)biphenyl (150 mg, 0.50 mmol) in toluene was
refluxed for 1 day. The resulting mixture was concentrated by
evaporation under reduced pressure. The residue was diluted with Et2O
and water, and the resulting organic layer was separated and dried over
MgSO4. After evaporation of the solvent, the crude product was
chromatographed on silica gel (CH2Cl2/n-hexane ) 1:4 as eluent) to
afford 7 (4.80 g, 54%) as a white solid: 1H NMR (400 MHz, C6D6) δ
6.96-6.99 (m, 8H), 6.91 (br-s, 2H), 3.07 (s, 6H), 1.30 (s, 18H), 1.21
(s, 18H), 1.03 (s, 18H), 0.93 (br-s, 12H); 13C NMR (100 MHz, C6D6)
δ 151.9, 151.0, 148.8, 144.3, 122.7, 115.6, 114.5, 113.6, 61.0, 40.6,
35.0, 31.6, 26.6, 26.5, 18.4, -4.2. Anal. Calcd for C48H82N4O2Si2: C,
71.76; H, 10.29; N, 6.97. Found: C, 71.58; H, 10.44; N, 6.81.
N,N′-Bis[3-tert-butyl-5-(N-tert-butyl-N-hydroxyamino)phenyl]-
N,N′-dimethyl-p-phenylenediamine (8). To an ice-cooled solution of
7 (564 mg, 0.70 mmol) in 5 mL of THF were added slowly 7 mL (7
mmol) of a 1.0 M solution of tetrabutylammonium fluoride in THF.
Stirring was continued for 45 min at 0 °C and for 2 h at room
temperature. After addition of water and Et2O, the organic layer was
separated, and the aqueous layer was extracted with Et2O. The organic
layers were combined and dried over MgSO4. By evaporation under
reduced pressure, a slightly reddish brown solid was obtained and
washed with n-hexane containing a small amount of Et2O to give 8
(355.4 mg, 88%) as a white solid: 1H NMR (400 MHz, acetone-d6) δ
7.36 (br-s, 2H), 6.96 (s, 4H), 6.84 (t, J ) 1.9 Hz, 2H), 6.79 (t, J ) 1.9
Hz, 2H), 6.73 (t, J ) 1.9 Hz, 2H), 3.27 (s, 6H), 1.26 (s, 18H), 1.11 (s,
18H); 13C NMR (100 MHz, acetone-d6) δ 151.8, 151.0, 149.2, 144.4,
122.6, 115.4, 114.0, 113.3, 60.3, 40.8, 35.2, 31.6, 26.6.
N,N′-Bis[3-tert-butyl-5-(N-tert-butyl-N-oxylamino)phenyl]-N,N′-
dimethyl-p-phenylenediamine (1). To a solution of 8 (76 mg, 0.13
mmol) in CH2Cl2 (10 mL) was added an excess of Ag2O (308 mg, 1.3
mmol), and the mixture was stirred for 2 h. After filtration through
Celite, the solvent was evaporated under reduced pressure. The residue
was chromatographed on silica gel (Et2O/CH2Cl2/n-hexane ) 1:4:5 as
eluent) to afford 1 (63 mg, 83%) as a red solid: ESR (toluene at room
temperature) g ) 2.006, |aN| ) 6.3 G; FAB HRMS (m-nitrobenzyl
alcohol) m/z (relative intensity %) calcd for C36H52N4O2 [M]+ 572.4090,
found 572.4085 (7.4); C36H53N4O2 [M + H]+ 573.4169, found 573.4162
(6.2); C36H54N4O2 [M + 2H]+ 574.4247, found 574.4208 (7.4). Anal.
Calcd for C36H52N4O2: C, 75.48; H, 9.15; N, 9.78; O, 5.59. Found: C,
75.36; H, 9.10; N, 9.60; O, 5.58.
Figure 1. Spin alignment mediated by intervalence state in 1+.
the intervalence compound, N,N′-dimethyl-N,N′-diphenyl-p-
phenylenediamine (3) radical cation.
Experimental Section
1H and 13C spectra were recorded on a JEOL JNM-AL400 or a JNM-
EX400 spectrometer, and chemical shifts are given in parts per million
(ppm) relative to internal tetramethylsilane (δ 0.00 ppm). Elemental
analyses were performed by the Center for Organic Elemental Mi-
croanalysis, Kyoto University.
Toluene and n-butyronitrile were distilled from CaH2 under an argon
atmosphere, tetrahydrofuran (THF) was distilled from sodium ben-
zophenone or potassium benzophenone under an argon atmosphere,
N,N-dimethylformamide (DMF) was dried over molecular sieves 4A,
and benzonitrile was purified through an alumina (ICN, Alumina N,
Akt. I) column with bubbling of argon, just before use. All the other
purchased reagents and solvents were used without further purification.
Column chromatography was performed using silica gel (Kanto
Chemical Co., Inc., Silica gel 60N, spherical neutral) or alumina (Kanto
Chemical Co., Inc., Aluminum Oxide, Activated for column chroma-
tography).
N,N′-Bis[3-tert-butyl-5-(N-(tert-butyldimethylsiloxy)-N-tert-bu-
tylamino)phenyl]-N,N′-dimethyl-p-phenylenediamine (7). A mixture
of 5 (1.50 g, 11.0 mmol), N-(3-bromo-5-tert-butylphenyl)-N-(tert-
butyldimethylsiloxy)-N-tert-butylamine (6)9 (10.0 g, 24.1 mmol),
NaOtBu (2.50 g, 25.5 mmol), Pd(OAc)2 (150 mg, 0.65 mmol), and
Electrochemical Measurements. The cyclic voltammetry (CV)
measurements were carried out in benzonitrile solution containing
0.1 M n-tetrabutylammonium tetrafluoroborate (n-Bu4NBF4) as a
supporting electrolyte (25 °C, scan rate 100 mV/s) using an ALS/chi
Electrochemical Analyzer model 612A. A three-electrode assembly was
used, which was equipped with a platinum disk (2 mm2), a platinum
wire, and Ag/0.01 M AgNO3 (acetonitrile) as the working electrode,
the counter electrode, and the reference electrode, respectively. The
redox potentials were referenced against a ferrocene/ferrocenium (Fc/
Fc+) couple.
UV/vis/NIR Spectrum Measurements. UV/vis/NIR spectra were
obtained with a Perkin-Elmer Lambda 19 spectrometer. Spectroelec-
trochemical measurements were carried out with a custom-made
optically transparent thin-layer electrochemical (OTTLE) cell (light pass
length ) 1 mm) equipped with a platinum mesh, a platinum coil, and
a silver wire as the working electrode, the counter electrode, and the
pseudo-reference electrode, respectively. The potential was applied with
an ALS/chi Electrochemical Analyzer model 612A.
ESR Measurements. ESR spectra were recorded on a JEOL JES-
SRE2X or a JEOL JES-TE200 X-band spectrometer, in which the
temperature was controlled by a JEOL DVT2 variable-temperature unit
or an Oxford ITC503 temperature controller combined with an ESR
910 continuous flow cryostat, respectively. A Mn2+/MnO solid solution
was used as a reference for the determination of g-values and hyperfine
coupling constants. Pulsed ESR measurements were carried out on a
Bruker ELEXES E580 X-band FT ESR spectrometer.
(7) (a) Mazur, S.; Schroeder, A. H. J. Am. Chem. Soc. 1978, 100, 7339. (b)
Nelsen, S. F.; Thomson-Colon, J. A.; Katfory, M. J. Am. Chem. Soc. 1989,
111, 2809. (c) Almlo¨f, J. E.; Feyereisen, M. W.; Josefiak, T. H.; Miller, L.
L. J. Am. Chem. Soc. 1990, 112, 1206. (d) Maslak, P.; Augustine, M. P.;
Burkey, J. D. J. Am. Chem. Soc. 1990, 112, 5359. (e) Utamapanya, S.;
Rajca, A. J. Am. Chem. Soc. 1991, 113, 9242. (f) Telo, J. P.; Shohoji, M.
C. B. L.; Herold, B. J.; Grampp, G. J. Chem. Soc., Faraday Trans. 1992,
88, 47. (g) Bonvoisin, J.; Launay, J.-P.; Van der Auweraer, M.; De Schryver,
F. C. J. Phys. Chem. 1994, 98, 5052. (h) Lahil, K.; Moradpour, A.; Bowlas,
C.; Menou, F.; Cassoux, P.; Bonvoisin, J.; Launay, J.-P.; Dive, G.;
Dehareng, D. J. Am. Chem. Soc. 1995, 117, 9995. (i) Bonboisin, J.; Launay,
J.-P.; Verbouwe, W.; Van der Auweraer, M.; De Schryver, F. C. J. Phys.
Chem. 1996, 100, 17079. (j) Nelsen, S. F.; Tran, H. Q.; Nagy, M. A. J.
Am. Chem. Soc. 1998, 120, 298. (k) Lambert, C.; Gaschler, W.; Schma¨lzlin,
E.; Meerholz, K.; Bra¨uchle, C. J. Chem. Soc., Perkin Trans 2 1999, 577.
(l) Rovira, C.; Ruiz-Molina, D.; Elsner, O.; Vidal-Gancedo, J.; Bonvoisin,
J.; Launay, J.-P.; Veciana, J. Chem.sEur. J. 2001, 7, 240. (m) Mayor, M.;
Bu¨schel, M.; Fromm, K. M.; Lehn, J.-M.; Daub, J. Chem.sEur. J. 2001,
7, 1266. (n) Lambert, C.; No¨ll, G.; Hampel, F. J. Phys. Chem. A 2001,
105, 7751. (o) Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi, J. K. J.
Am. Chem. Soc. 2002, 124, 842. (p) Lambert, C.; No¨ll, G. Chem.sEur. J.
2002, 8, 3467. (q) Dumur, F.; Gautier, N.; Gallego-Planas, N.; Sahin, Y.;
Levillain, E.; Masino, M.; Girlando, A.; Lloveras, V.; Vidal-Gancedo, J.;
Veciana, J.; Rovira, C. J. Org. Chem. 2004, 69, 2164.
(8) (a) Lambert, C.; No¨ll, G. J. Am. Chem. Soc. 1999, 121, 8434. (b) Lambert,
C.; No¨ll, G.; Schelter, J. Nat. Mater. 2002, 1, 69. (c) Lambert, C.; No¨ll, G.
J. Chem. Soc., Perkin Trans. 2 2002, 2039.
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9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 2949

A R T I C L E S
Scheme 1
Ito et al.
Figure 2. Cyclic voltammograms of 1 in PhCN containing 0.1 M n-Bu4-
NBF4 at 298 K (scan rate 0.1 V s-1): (a) the scan at lower turnaround
potential and (b) the scan at higher turnaround potential.
Scheme 2
Table 1. Redox Potentials (V) of 1 and Related Compoundsa
compd
E1
E2
E3
1
3
4
0.00
-0.02
0.41b
0.52b
0.48
0.55
a 0.1 M n-Bu4NBF4 in PhCN, potential versus Fc/Fc+, Pt electrode, 298
K, scan rate 100 mV/s. b Oxidation peak potential.
Magnetic Susceptibility Measurements. Magnetic susceptibilities
of the powder samples were measured by a Quantum Design MPMS-
5S system. The raw data were corrected for both the magnetization of
the sample holder alone and the diamagnetic contribution of the sample
itself. The estimation of the diamagnetic contribution was done by using
Pascal’s constants.
the oxidation process of the nitroxide groups and/or the oxidation
process from a semiquinone to quinone of the central PDA
moiety. It is interesting to note that the difference between the
first and second oxidation potentials of PDA derivatives (∆E)
is closely related to the twist angle between the amino group
and 1,4-phenylenediyl, which determines the electronic coupling
between charge-bearing units.10 Although the rigorous value of
∆E for 1 is not obtained owing to the overlap of the second
and third oxidation processes, the ∆E can be roughly estimated
to be 0.52-0.55 V, similar to the value 0.5 V for 3. This
suggests that the substituting nitroxide groups do not seriously
affect the twist angle between the amino group and 1,4-
phenylenediyl.
The observed continuous wave ESR (cw-ESR) spectrum of
1 in a frozen n-butyronitrile solution consisted of a single signal
with unresolved hyperfine and/or fine structure at 123 K. In
addition, the forbidden ∆MS ) (2 resonance was also observed,
suggesting the presence of the triplet state between the two
nitroxide groups in 1. On the other hand, the temperature
dependence of the magnetic susceptibility measured by a SQUID
magnetometer exhibited a very weak ferromagnetic interaction
between the two nitroxide groups in 1 (J/kB ) 0.6 K).11 This
Results and Discussion
As shown in Scheme 1, the bisnitroxide 1 was prepared in
three steps. The coupling reaction of N,N′-dimethyl-p-phe-
nylenediamine 5 with the protected hydroxylamine 6 in the
presence of palladium catalyst affords the precursor 7. After
desilylation of 7 with tetrabutylammonium fluoride, the desired
bisnitroxide 1 was obtained as a reddish brown solid by
oxidation of the bishydroxylamine 8 with Ag2O.
A prerequisite for generating a triradical cation of 1 is that
the central PDA moiety has a lower oxidation potential than
those of the two nitroxide groups. The oxidation potentials of
1 were evaluated by cyclic voltammetry in a benzonitrile
solution at 298 K. The observed voltammogram showed three
oxidation waves (Figure 2b). The first oxidation process was
found to be reversible by lowering the turnaround potential
(Figure 2a). From the comparison of the oxidation potentials
for 3 with that for the tert-butylphenyl nitroxide (4)(Scheme
2), the first reversible oxidation wave of 1 can be ascribed to
the removal of one electron from the central PDA moiety (Table
1). The remaining two oxidation peaks probably correspond to
(10) Nishiumi, T.; Nomura, Y.; Chimoto, Y.; Higuchi, M.; Yamamoto, K. J.
Phys. Chem. B 2004, 108, 7992.
(11) The temperature dependence of magnetic susceptibility for 1 was analyzed
by the modified Bleaney-Bowers equation for singlet-triplet model.
Bleaney, B.; Bowers, K. D. Proc. R. Soc., London Ser. A 1952, 214, 451.
9
2950 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Spin Alignment Mediated by Delocalized Spin
A R T I C L E S
Figure 4. ESTN spectrum of 1+ in frozen n-butyronitrile solution at 5 K
[1 was oxidized with tris(4-bromophenyl)aminium hexachloroantimonate].
Figure 3. cw-ESR spectrum of 1+ in n-butyronitrile at 123 K. Inset: The
forbidden ∆MS ) (2 resonance at 123 K.
indicates that the two nitroxide groups are virtually magnetically
uncoupled. On the other hand, in the cw-ESR spectrum of the
oxidized species of 1 treated with up to 1 molar equiv of tris-
(4-bromophenyl)aminium hexachloroantimonate at 195 K, no
definite fine-structured spectrum characteristic of the spin quartet
state was observed at 123 K, although broad shoulders were
seen beside the intense central line (Figure 3). However, the
half-field resonance corresponding to the ∆MS ) (2 transition
was detected, and furthermore, the signal intensity of this
forbidden transition was inversely proportional to temperature,
suggesting the possibility of a high-spin ground state of the
triradical cation 1+. In addition, the DFT calculations (UB3LYP/
6-31G*) on the cation of the simplified model compound for 1
gave the energy difference between the doublet and quartet states
(∆ED-Q) of 0.81 kcal/mol, supporting the spin quartet ground
state for the triradical cation 1+.12 To identify unequivocally
spin multiplicity of the triradical cation 1+ at low temperature,
we measured the electron spin transient nutation (ESTN)
spectrum based on the pulsed ESR method (Figure 4).13 Judging
from the ratio13 between the nutation frequency in question and
1
1
that for the |S, MS ) | /2, +1/2 T | /2, -1/2 transition (23
MHz), the main nutation signals observed at 39 and 45 MHz
3
3
Figure 5. Spectroelectrochemical study of (a) 2 to 2+ and (b) 3 to 3+ in
CH2Cl2/0.1 M n-Bu4NBF4 at room temperature.
were found to be assignable to |S, MS ) | /2, (3/2 T | /2,
(1/2 and | /2, /2 T | /2, -1/2 transitions, respectively. The
weak nutation signal at 33 MHz corresponds to the |1, 0 T |1,
(1 transition of neutral 1. This also supports that the spin
quartet state of 1+ is dominant at 5 K.
3
1
3
energy absorption band. According to Robin and Day,14 the
intervalence compounds are classified into three categories on
the basis of the size of the electronic interaction (H) between
the charge-bearing units: Class I compounds have complete
charge localization (H ) 0), Class II compounds have inter-
mediate electronic interaction that allows partial charge delo-
calization between two charge-bearing units, and in Class III
compounds, the electronic interaction is so strong that the charge
is fully delocalized over two charge-bearing units. In Class II
compounds, the lowest-energy absorption band (intervalence
band) has a very broad Gaussian-shaped peak that is explainable
by charge transfer between two charge-bearing units, as pointed
out by Hush.15 On the other hand, delocalized Class III
compounds exhibit rather narrow intervalence bands with a
partially resolved vibrational fine structure.16,17 Therefore, we
To confirm the presence of the intervalence state in triradical
cation 1+, the best way is to observe the characteristic lowest-
(12) For the neutral and cationic state of 1 and 3, the hybrid HF/DF (B3LYP)
calculations were performed by using 6-31G* basis sets. For 1, all tert-
butyl groups were replaced by the methyl groups. Full geometry optimiza-
tions were carried out under Ci symmetrical constraint. All the calculations
were done with the Gaussian 98 program package.
(13) The magnetic moments with distinct spin quantum numbers (S) precess
with their specific nutation frequency (ωn) in the presence of a microwave
irradiation field and a static magnetic field. The nutation frequency for a
transition from |S, MS to |S, MS + 1 can be expressed as ωn ) [S(S + 1)
- MS(MS + 1)]1/2ω0 under certain conditions. This indicates that ωn can
be scaled with the total spin quantum number S and the spin magnetic
quantum number MS in the unit ωn ()ω0) for the doublet species; x2 for
S ) 1, x3 and 2 for S ) 3/2. For determination of spin multiplicity for
high-spin molecules by using the pulsed ESR technique, see: (a) Isoya, J.;
Kanda, H.; Norris, J. R.; Tang, J.; Brown, M. K. Phys. ReV. B 1990, 41,
3905. (b) Astashkin, A. V.; Schweiger, A. Chem. Phys. Lett. 1990, 174,
595. (c) 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. (d) Ito, A.; Ino, H.; Matsui, Y.; Hirao, Y.; Tanaka, K.; Kanemoto,
K.; Kato, T. J. Phys. Chem. A 2004, 108, 5715.
(14) Robin, M.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(15) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Hush, N. S.
Electrochem. Acta 1968, 13, 1005. (c) Creutz, C. Prog. Inorg. Chem. 1983,
30, 1. (d) Hush, N. S. Coord. Chem. ReV. 1985, 64, 135.
(16) Bailey, S. E.; Zink, J. I.; Nelsen, S. F. J. Am. Chem. Soc. 2003, 125, 5939.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 2951

A R T I C L E S
Ito et al.
Table 2. Observed and Koopmans-Based Transition Energies for
p-Phenylenediamine Intervalence Radical Cations in CH2Cl2
E
A (obsd)
A1
(cm-
E
B (obsd)
B
1 (and B2)
1
1
1
1
compd
(cm-
)
)
(cm-
)
(cm-
)
1
14 430
16 260
15 950
15 210a
18 330b
15 790c
25 510
29 810a
(33 810a)
29 710b
2
3
26 390 (sh)
30 400
26 530 (sh)
28 250
(33 310b)
30 900c
(31 220c)
a UB3LYP/6-31G* calcd. b B3LYP/6-31G* calcd from ref 20. c B3LYP/
6-31G* calcd.
Figure 6. Lower-energy NCG transitions (Hoijtink type A and B
transitions) and the selected frontier MO levels for 3+ at the B3LYP/6-
31G* level. The E1 to E4 MO levels are derived from partial orbital
interaction between the diabatic orbitals of two charge-bearing units and
the benzene e1g HOMOs and e2u LUMOs. HOMO-1 (and LUMO+4) stems
from the remaining benzene e1g HOMO (and e2u LUMO).
Figure 7. Spectroelectrochemical study of 1 in CH2Cl2/0.1 M n-Bu4NBF4
at room temperature: (a) 1 to 1+ and (b) 1+ to 12+
.
have measured the UV/vis/NIR absorption spectral change
during the course of the oxidation of 1 by using an optically
transparent thin-layer electrochemical cell. For a reference, the
spectral changes of N,N,N′,N′-tetramethyl-p-phenylenediamine
(2) and N,N′-dimethyl-N,N′-diphenyl-p-phenylenediamine (3)
were also measured in the same experimental condition.
As shown in Figure 5a, the two absorption bands EA and EB,
which correspond to the lowest- and next-lowest-energy bands,
respectively, grew upon oxidation of 2 to the monoradical cation
2+, and the lowest-energy band (EA) showed a typical vibrational
fine structure, as examined by Nelsen et al.16 On the other hand,
the EA band in 3+ was a broad one with an unresolved
vibrational fine structure (Figure 5b). This broad EA band in
3+ is also seen in some N,N,N′,N′-tetraaryl-p-phenylenediamine
radical cations. Recently, the detailed study on the N,N,N′,N′-
tetraphenyl-p-phenylenediamine radical cation concluded that
the radical cation adopts a delocalized Class III structure both
in the solid state and in solution, despite the broad EA band
shape which was different from that of the delocalized Class
III compound 1+.18 Moreover, Nelsen et al. pointed out that
the Marcus-Hush two-state model should not be applicable to
the delocalized Class III system and that the transition energy
obtained from the EA absorption band does not directly relate
to the electronic interaction between two charge-bearing units.19,20
Delocalized intervalence compounds have symmetrical charge
distributions and, hence, have a charge on the bridging p-
phenylene moiety. As is apparent from Figure 6, the partial
molecular orbital interaction between the diabatic orbitals
originating from two charge-bearing units and the benzene e1g
(18) Szeghalmi, A. V.; Erdmann, M.; Engel, V.; Schmitt, M.; Amthor, S.;
Kriegisch, V.; No¨ll, G.; Stahl, R.; Lambert, C.; Leusser, D.; Stalke, D.;
Zabel, M.; Popp, J. J. Am. Chem. Soc. 2004, 126, 7834.
(19) Nelsen, S. F.; Weaver, M. N.; Zink, J. I.; Telo, J. P. J. Am. Chem. Soc.
2005, 127, 10611.
(20) Nelsen, S. F.; Weaver, M. N.; Telo, J. P.; Lucht, B. L.; Barlow, S. J. Org.
Chem. 2005, 70, 9326.
(17) Nelsen, S. F.; Konradsson, A. E.; Weaver, M. N.; Telo, J. P. J. Am. Chem.
Soc. 2003, 125, 12493.
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2952 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Spin Alignment Mediated by Delocalized Spin
A R T I C L E S
Figure 8. Lower NCG transition energies (Hoijtink type A and B transitions) and the selected frontier MO levels for model compounds of 1+ at the
UB3LYP/6-31G* level. The transition from R(HOMO) to R(LUMO+1) is forbidden.
HOMOs and e2u LUMOs leads to four adiabatic MOs, shown
as E1 to E4 (neighboring orbital model19).12 In this situation,
the lowest transition energy in the optical spectra corresponds
to the promotion of an electron from the HOMO to the singly
occupied MO (SOMO) (Hoijtink type A transition) or that from
the SOMO to LUMO (Hoijtink type B transition).21 These
transition energies in delocalized intervalence compounds can
be well estimated by single-point MO calculations with a neutral
charge at optimized radical cation geometries (neutral in cation
geometry or NCG)19,20 on the basis of Koopmans’ theorem. The
observed transition energies and the Koopman-based band
assignments by using the NCG method are summarized in Table
2. The calculated transition energies are in good agreement with
the experimental values, and the EA and EB bands can be
assigned to the Hoijtink type A and B transitions, respectively.
Note that the EB band of 3+ has a shoulder. This is because the
LUMO and LUMO+1 are quasi-degenerate (the calculated
energy difference is 320 cm-1) and have an au orbital symmetry
(B1 and B2 transition in Figure 6). Such a shoulder peak is also
observed in 2+ (Figure 5a).
In the case of 1, the EA and EB bands grew at 14 430 cm-1
and 25 510 cm-1, respectively, upon oxidation to the mono-
radical cation 1+, and further oxidation to the dication 12+
resulted in the disappearance of these two band, and a new band
at 18 730 cm-1 was slightly apparent as shown in Figure 7.
These spectral changes indicate that the delocalized electron
spin was generated over the central PDA moiety after one-
electron oxidation of 1. Note that the same spectral changes
were also seen by the chemical oxidation with SbCl5 in CH2-
Cl2. For 1+, the wave function obtained by the NCG calculation
is of the unrestricted type (UB3LYP/6-31G*), and therefore,
the MOs are actually split into R and â spin sets.12 The lowest
Hoijtink type A transition corresponds to one from â (HOMO)
to â (LUMO), while the lowest Hoijtink type B transition
corresponds to one from R (HOMO) to R (LUMO), as shown
in Figure 8. From the comparison between Figures 6 and 8, the
MOs associated with the type A and B transitions in 1+ are
similar to ones in 3+, and furthermore, the MOs (R(HOMO-1)
and R(HOMO-2)) originating from the nitroxide groups are well
localized and are lower in energy than the MO (R(HOMO))
from the PDA radical cation. Therefore, it can be concluded
that the two nitroxide groups do not greatly affect the delocalized
intervalence band of the PDA moiety in 1+.
In summary, we have demonstrated the first example of
parallel spin alignment mediated by the delocalized intervalence
state in a multispin organic molecule 1+. The present triradical
cation is also considered to be intriguing in that the localized
and delocalized spins coexist in a single molecular system.
Acknowledgment. This work was supported by CREST
(Core Research for Evolutional Science and Technology) of the
Japan Science and Technology Agency (JST) and by a Grant-
in-Aid for Scientific Research from the Japan Society for the
Promotion of Science (JSPS). Thanks are due to the Research
Center for Molecular-Scale Nanoscience, the Institute for
Molecular Science for assistance in obtaining the pulsed ESR
spectra.
(21) (a) Hoijtink, G. J.; Weijland, W. P. Recl. TraV. Chim. Pays-Bas 1957, 76,
836. (b) Buschow, K. J. J.; Dieleman, J.; Hoijtink, G. J. Mol. Phys. 1963-
1964, 7, 1.
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