Redox-switching of intramolecular magnetic interaction through π-conjugation mode change of 1,2-bis(4-dianisylamino)-1,2-bis(3-N-oxylamino)- substituted tetraarylethylene

Article abstract of DOI:10.1016/j.poly.2011.02.046

To develop the redox-switching system of intramolecular magnetic interaction, 1,2-bis[3-(N-tert-butyl-N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4- methoxyphenyl)amino}phenyl]ethylene, tetraarylethylene with two nitroxide radical groups at the meta-position,

Full text of DOI:10.1016/j.poly.2011.02.046

Polyhedron 30 (2011) 3106–3111
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Redox-switching of intramolecular magnetic interaction through p-conjugation
mode change of 1,2-bis(4-dianisylamino)-1,2-bis(3-N-oxylamino)-substituted
tetraarylethylene
⇑
Yoshiaki Nakano , Akihiro Ito, Kazuyoshi Tanaka
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 6 March 2011
To develop the redox-switching system of intramolecular magnetic interaction, 1,2-bis[3-(N-tert-butyl-
N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4-methoxyphenyl)amino}phenyl]ethylene,
tetraarylethylene
with two nitroxide radical groups at the meta-position, was synthesized, and characterized by the elec-
trochemical method and ESR spectroscopy. Cyclic voltammetry showed the tetraarylethylene core has
the lower oxidation potential than the substituted nitroxide radical moiety. ESR spectroscopy in frozen
solution revealed that the neutral form shows the fine-structured spectrum characteristic of the spin trip-
let species, while the dicationic form shows the anisotropic hyperfine-structured spectrum characteristic
of the randomly-oriented nitroxide radical, indicating the drastic change of intramolecular magnetic
interaction.
Keywords:
Tetraarylethylene
Nitroxide radical
Diradical
Intramolecular magnetic interaction
Redox
ESR spectroscopy
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Such a drastic structural change including p-conjugation mode
change can be utilized as a redox-switchable spin coupling unit.
For instance, the interconversion of intramolecular magnetic inter-
actions between two radical moieties has been achieved by using
1,2-bis(3-thienyl)ethene and 1,2-bis(2-thienyl)ethene as a photo-
switchable spin coupling unit [3]. To create a new type of redox-
switchable intramolecular magnetic interaction converting organic
molecules, it has beendesigned and synthesized, the novel tetraaryl-
ethylene carrying two nitroxide radical groups, 1,2-bis[4-(N-tert-
butyl-N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4-methoxyphenyl)
amino}phenyl]ethylene (1) [4]. The neutral 1 showed a strong anti-
An understanding of structure–property relationship in conju-
gated molecular materials provides a fundamental basis for molec-
ular design of core units in molecule-based electronic devices.
When the molecular structures are controllable by the external
stimuli, the electronic properties inherent in the molecules can
be efficiently switched on and off, which leads to the application
to molecular switching and memory devices. In this context, it is
well known that tetraarylethylenes exhibit the drastic change in
their molecular structures through the redox reaction; that is to
say, the C@C double bond is elongated like a CAC single bond
and concomitantly rotational motion about the olefinic bond takes
place. Such a free rotation was experimentally confirmed by rever-
sal coulometric experiments. For example, when the cis-form of
1,2-bis(4-dimethylaminophenyl)-1,2-diphenylethylene was sub-
jected to two-electron-oxidation and subsequent two-electron-
reduction, the NMR spectrum of the reduced neutral product gave
the evidence for the mixture of cis- and trans-isomers [1]. More
unequivocal evidence was afforded by the X-ray structural analy-
ses of the isolated mono- and dications of 1,1,2,2-tetrakis(4-
methoxyphenyl)ethylene, in which the rotational angle around
ethylenic CAC bond increases on going from the neutral through
the monocationic to the dicationic states [2].
ferromagnetic interaction through
radical centers. On the other hand, the spin–spin correlation in 12+
disappeared in conjunction with interception of -conjugation
p-conjugated network between
p
between two nitroxide radical groups (Scheme 1). In this work,
1,2-bis[3-(N-tert-butyl-N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4-
methoxyphenyl)amino}phenyl]ethylene (2) as the structural
isomer of 1 was investigated by means of electrochemistry and
ESR spectroscopy.
2. Results and discussion
2.1. Synthesis
The synthesis of bisnitroxide 2 was carried out by the similar
route with that of 1, which is outlined in Scheme 2. Monolithiated
1,3-dibromobenzene was coupled with 2-methyl-2-nitrosopro-
pane to give hydroxylamine 3. The hydroxylamine 3 was protected
⇑
Corresponding author. Present address: Research Center for Low Temperature
and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.:
+81 75 753 4061; fax: +81 75 753 4062.
E-mail address: nakano@kuchem.kyoto-u.ac.jp (Y. Nakano).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.02.046

Y. Nakano et al. / Polyhedron 30 (2011) 3106–3111
3107
OH
N
OSi(CH3)2tBu
N
O
Br
N
H3CO
H3CO
a, b, c
d
OCH3
Br
Br
3 (76%)
Br
4 (96%)
N
N
OSi(CH3)2tBu
H3CO
H3CO
N
1: p,p'-
2: m,m'-
OCH3
N
O
e, f, g
N
O
Strong intramolecular magnetic interaction
5 (75%)
+2e -2e
R
N
H3CO
OCH3
OCH3
O
N
H3CO
H3CO
h
OCH3
OCH3
N
N
N
N
H3CO
N
R
6 (57%): R = OSi(CH3)2tBu
12+: p,p'-
N
i
j
O
22+: m,m'-
7a (52%): R = OH
Weak intramolecular magnetic interaction
2a (24%): R = O
Scheme 1. Redox-switching of through-bond magnetic interaction.
Scheme 2. Synthetic route for diradical 2. Reagents: (a) n-BuLi, THF; (b) t-BuNO,
THF; (c) aqueous NH4Cl; (d) t-Bu(CH3)2SiCl, imidazole, DMF; (e) n-BuLi, THF; (f) 4-
{bis(4-methoxyphenyl)amino}benzonitrile, THF; (g) H2O; (h) TiCl4, Zn, pyridine,
THF; (i) n-Bu4NF, THF; and (j) Ag2O, CH2Cl2.
with tert-butyldimethylsilyl group to afford 4 [5]. Asymmetrically
substituted diarylketone 5 was prepared by addition of 4-{bis(4-
methoxyphenyl)amino}benzonitrile to the lithiated 4 and the sub-
sequent hydrolysis. The reductive coupling of the ketone 5 with a
low-valent titanium reagent [6] gave 6a (trans-isomer) and 6b
(cis-isomer) at the ratio of 6a/6b = 78/22. Here, we regarded 6a
as the trans-isomer because the ESR spectrum for 2a derived from
6a indicates that 2a takes the trans configuration (see Section 2.3).
After desilylation of 6a with tetrabutylammonium fluoride, the de-
sired bisnitroxide 2a was obtained by oxidation of 7a with Ag2O.
Bisnitroxide 2a was unstable as compared with para-nitroxyl-
substituted 1. The instability of this kind of nitroxide seems to be
ascribed to the isomerization to aminoquinone imine N-oxide [7].
However, the ESR spectrum of bisnitroxide 2a in toluene under
an argon atmosphere after 4 days was nearly-unchanged. In this
study, the freshly-prepared sample was used for all measurements
in order to avoid the impurity incorporation.
[8] and the N-tert-butyl-N-phenylnitroxide (9) [9] (Scheme 3), it
is concluded that the first quasi-two-electron transfer process cor-
responds to the oxidation of the tetraarylethylene core, while the
second irreversible two-electron transfer process to the oxidation
of the two nitroxide radical groups (Table 1). This is also supported
by the electronic structure of model compound calculated at the
UB3LYP/6-31G(d,p) level of theory (Table S1 and Fig. S2). The orbi-
tal energy of singly occupied molecular orbitals (SOMOs) localized
on the nitroxide groups is lower than that of HOMO distributed
over the tetraarylethylene core. The same situations are reported
in the literature [10]. The observed separation of the anodic and
cathodic peak potentials for the first redox process (68 mV) is lar-
ger than the theoretical value of 28 mV for the reversible two-elec-
tron transfer process [11]. This indicates that the first oxidation
wave of 2a is the overlap of two closely-spaced consecutive one-
electron-oxidation processes. The similar redox behavior is also
observed for 1,1,2,2-tetrakis(4-methoxyphenyl)ethylene in a polar
solvent like acetonitrile [1]. In 2a, the redox process of the tetraa-
rylethylene core approaches a two-electron transfer, whereas two
resolved one-electron-transfer redox waves were observed in para-
nitroxyl-substituted 1 [4] (Fig. S5). This contrast is closely related
to the fact that the para-substituting nitroxide radical groups affect
the oxidation process of the tetraarylethylene core through
2.2. Electrochemistry
The effective conversion of intramolecular magnetic interaction
before and after the oxidation of 2a postulates the lower oxidation
potential of tetraarylethylene core than the two nitroxide groups.
The cyclic voltammogram of 2a measured in 1 mM benzonitrile
solution is shown in Fig. 1. In comparison with the first oxidation
potentials of the non-nitroxyl-substituted tetraarylethylene (8)

3108
Y. Nakano et al. / Polyhedron 30 (2011) 3106–3111
0.224
5 μΑ
(a)
(b)
160
165
170
(a)
0.504
0.156
(b)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
E / V vs. Fc/Fc+
320
325
330
335
Fig. 1. Cyclic voltammograms of 2a in the cases of (a) lower turn-around potential
and (b) higher turn-around potential in benzonitrile containing 0.1 M n-Bu4NBF4 at
298 K, scan rate 100 mV/s.
H / mT
Fig. 2. (a) ESR spectra of 2a in frozen toluene solution at 123 K. The inset is the
forbidden MS 2 resonance. (b) ESR spectrum of 2a after treatment with two
D
=
equivalents of tris(4-bromophenyl)aminium hexachloroantimonate in n-butyroni-
trile at 123 K.
H3CO
OCH3
coupling constant |aN|/2 of 0.69 mT, indicating that the exchange
coupling between nitroxide radicals is much larger than the hyper-
fine coupling.
In the frozen toluene solution at 123 K, the unresolved fine-
structured spectrum was observed as shown in Fig. 2(a), and the
N
N
O
N
9
H3CO
forbidden
region of the allowed
D
MS = 2 resonance was also detected in the half-field
MS = 1 resonance, indicating the spin trip-
OCH3
8
D
let state. The spectral simulation has been failed so far, which sug-
gests that several conformers with the different parameters have
to be considered. However, given that the zero-field splitting
parameter (D) is 3.04 mT from the separation of the highest and
the lowest field resonances in g = 2 region, the average distance be-
tween the spin centers was estimated to be 9.7 Å on the basis of
the point dipole approximation (Eq. (1)) [12];
Scheme 3. Related compounds.
Table 1
Redox potentialsa of 1, 2a, 8, and 9.
E1
E2
E3
1
2a
8
0.00
0.25
0.49b,c
0.19b
0.15b
0.41c
0.50b,c
D ¼ ð3=2Þðg
l
B=r3Þ;
ð1Þ
9
a
0.1 M n-Bu4NBF4 in PhCN, potential vs. Fc/Fc+, Pt electrode, 298 K, scan rate
where g is the isotropic g-factor, lB the Bohr magneton and r the
average spin–spin distance. On the other hand, the quantum chem-
ical calculations for the model compounds of 2 were carried out at
the UB3LYP/6-31G(d) level of theory [13] (Table S2), and the
through-space distances between two nitroxyl nitrogen atoms were
calculated to be 9.153 or 5.989 Å in the broken-symmetry singlet
state and 9.137 or 5.998 Å in the triplet state for the cis-isomer,
and 10.567 or 9.084 Å in the broken-symmetry singlet state and
10.564 or 9.084 Å in the triplet state for the trans-isomer. Hence,
the average interspin distance estimated from the ESR spectrum is
located midway in the through-space distances between two
nitroxyl nitrogen atoms for cis-isomer and trans-isomer. However,
it is noted that the value estimated from the ESR spectrum should
be shorter than the through-space distance between the two nitrox-
yl nitrogen atoms because the spin density on the nitroxyl moieties
is delocalized over the benzene rings to some extent. Therefore, it is
inferred that the estimated interspin distance of 9.7 Å is slightly
shorter than the calculated through-space distance between the
two nitroxyl nitrogen atoms of 10.567 (broken-symmetry singlet)
100 mV/s.
b
Quasi two-electron transfer.
Anodic peak potential. For the compounds 1, 8, and 9, see also Fig. S5 and Ref.
c
[4].
p
-conjugation as compared with the meta-substituting nitroxide
radical groups. The electrochemical reversibility in the redox pro-
cess of nitroxide group of meta-nitroxyl-substituted 2 was de-
creased as compared with para-nitroxyl-substituted 1. This
difference in electrochemical behavior also originates from the
substitution position of nitroxide radical group.
2.3. ESR spectroscopy
The ESR spectrum of 2a in toluene solution at room tempera-
ture showed the five lines with 1:2:3:2:1 intensity due to two
equivalent nitroxyl nitrogen atoms at g = 2.006 with the hyperfine

Y. Nakano et al. / Polyhedron 30 (2011) 3106–3111
3109
and 10.564 (triplet) Å in trans-isomer, and 2a adopts the structure
4.2. Synthesis
close to the trans-isomer in the frozen solution.
On the other hand, when two equivalents of tris(4-bromophe-
nyl)aminium hexachloroantimonate were added to 2a in n-butyro-
nitrile at ꢀ78 °C, the ESR spectrum1 of 2a2+ completely changed
from that of neutral 2a to the typical anisotropic hyperfine-struc-
tured spectrum for the randomly-oriented N-phenyl-N-tert-butyl-
4.2.1. N-3-Bromophenyl-N-tert-butylhydroxyamine (3)
To a solution of 1,3-dibromobenzene (7.08 g, 30.0 mmol) in THF
(100 mL) was added n-butyllithium (1.55 M hexane solution,
22.0 mL, 34.1 mmol) at ꢀ78 °C under an argon atmosphere. The
solution was stirred for 45 min. A solution of 2-methyl-2-nitroso-
propane (3.14 g, 36.0 mmol) in THF (50 mL) was dropped for
30 min. The mixture was stirred at ꢀ78 °C for 1 h, and then stirred
at room temperature for 4 h. To the mixture was added 5% aqueous
ammonium chloride (100 mL). The organic layer was separated
and the aqueous layer was extracted with Et2O. The organic layer
was combined and dried over MgSO4. After filtration, the solvent
was evaporated. The residue was washed with n-hexane and ethyl
acetate to give 3 as white solid (4.07 g, 56%). The filtrate was evap-
orated to remove solvent and chromatographed on silica gel
(AcOEt:n-hexane = 1:9 as eluent). A fraction (Rf = 0.49) afforded 3
as gray solid (1.51 g, 20%). Total amount of 3 through this reaction
was 5.58 g (76%): 1H NMR (300 MHz, CDCl3) d 7.43–7.42 (m, 1H),
7.24–7.21 (m, 1H), 7.12–7.08 (m, 2H), 6.14 (s, 1H), 1.12 (s, 9H);
13C NMR (75 MHz, CDCl3) d 150.9, 128.7, 128.0, 127.4, 123.2,
121.3, 60.9, 25.9.
nitroxide radical (Fig. 2(b)). The principal value (AZZ
) for the
perpendicular direction to the nitroxide plane of the hyperfine cou-
pling tensor of 2a2+ was estimated to be 2.73 mT [14]. This strongly
indicates that the two nitroxide radical groups in 2a2+ are no longer
coupled magnetically by the
p-conjugation mode change upon the
two-electron oxidation. Indeed, as confirmed by the DFT calculations
on the model compound of 2a2+, the optimized geometry, irrespec-
tive of the spin state, showed the highly distorted conformation
around the olefinic bond [13] (Fig. S3).
3. Conclusion
Tetraarylethylene 2 having nitroxyl groups at the meta-position
was prepared by following the synthetic method of 1, and seems to
be trans-isomer on the basis of the average spin–spin distance esti-
mated from the ESR spectrum. The cyclic voltammetry showed the
tetraarylethylene core has the lower oxidation potential than the
substituted nitroxide radical moiety. However, the first oxidation
process corresponding to the tetraarylethylene core of 2a was
unresolved in contrast to 1, which clearly showed the two-step
one-electron transfer process. It was also found by the ESR spec-
troscopy in frozen solution that the neutral diradical 2a showed
the fine-structured spectrum characteristic of the spin triplet
species, while 2a after treatment with two equivalents of tris(4-
bromophenyl)aminium hexachloroantimonate showed the aniso-
tropic hyperfine-structured spectrum characteristic of the
randomly-oriented nitroxide radical, indicating the drastic change
of intramolecular magnetic interaction, as is the case of para-ni-
troxyl-substituted 1.
4.2.2. N-3-Bromophenyl-N-tert-butyl-N-tert-butyldimethylsiloxyamine
(4)
To
a
solution of tert-butyldimethylchlorosilane (1.36 g,
9.00 mmol) and imidazole (1.28 g, 18.6 mmol) in DMF (5 mL) was
added 3 (1.83 g, 7.50 mmol). The resulting solution was stirred at
40 °C for 16 h under an argon atmosphere. To the mixture was
added CH2Cl2 and water. The organic layer was separated and
the aqueous layer was extracted with CH2Cl2. The organic layer
was combined and dried over MgSO4. After filtration, the solvent
was evaporated. The residue was chromatographed on silica gel
(n-hexane as eluent) to afford 4 as colorless oil (2.58 g, 96%): 1H
NMR (300 MHz, CDCl3) d 7.41 (br-s, 1H), 7.22–7.03 (m, 3H),
7.24–7.21 (m, 1H), 1.07 (s, 9H), 0.89 (s, 9H), ꢀ0.14 (br-s, 6H); 13C
NMR (75 MHz, CDCl3) d 152.7, 128.5, 128.1, 127.7, 123.8, 121.0,
61.2, 26.1, 17.9, ꢀ4.7.
These results strongly suggest that the present diradical 2a can
operate as a redox-switchable intramolecular magnetic interaction
converting system like the para-nitroxyl-substituted 1. However, it
was found that the diradical 2a shows the different electrochemi-
cal behavior from the diradical 1 owing to the difference of substi-
tution patterns of the nitroxide radical groups.
4.2.3. 3-(N-tert-butyl-N-tert-butyldimethylsiloxyamino)-40-
[bis(4-methoxyphenyl)amino]benzophenone (5)
To a solution of 4 (6.18 g, 17.2 mmol) in THF (50 mL) was added
n-butyllithium (1.55 M hexane solution, 13 mL, 20.2 mmol) at
ꢀ78 °C under an argon atmosphere. The solution was stirred for
1 h.
A solution of 4-{bis(4-methoxyphenyl)amino}benzonitrile
4. Experimental
(6.84 g, 20.7 mmol) in THF (60 mL) was dropped for 30 mim. The
mixture was stirred for 1 h at ꢀ78 °C, and then stirred at room
temperature for 1 h. To the mixture was added water (100 mL)
and stirred over night. The organic layer was separated and the
aqueous layer was extracted with Et2O. The organic layer was com-
bined and dried over MgSO4. After filtration, the solvent was evap-
orated. The residue was chromatographed on silica gel (ethyl
acetate:n-hexane = 1:4 as eluent) to afford 5 as yellow solid
(7.93 g, 75%): 1H NMR (400 MHz, CDCl3) d 7.64 (d, J = 8.9 Hz, 2H),
7.57 (br-s, 1H), 7.48 (dt, J = 7.3, 1.3 Hz, 1H), 7.42 (br-s, 1H), 7.31
(t, J = 7.7 Hz, 1H), 7.13 (d, J = 8.9 Hz, 4H), 6.88 (d, J = 8.9 Hz, 4H),
6.82 (d, J = 8.9 Hz, 2H), 3.81 (s, 6H), 1.09 (s, 9H), 0.89 (s, 9H),
ꢀ0.11 (br-s, 6H); 13C NMR (100 MHz, CDCl3) d 195.0, 156.9,
152.5, 150.7, 139.2, 138.0, 131.9, 128.2, 127.9, 127.8, 127.1,
126.1, 125.9, 116.6, 114.9, 61.1, 55.5, 26.2, 26.1, 18.0, ꢀ4.5; 1H
NMR (400 MHz, C6D6) d 7.97 (br-s, 1H), 7.93 (d, J = 8.8 Hz, 2H),
7.65 (d, J = 7.6, 1H), 7.38 (br-s, 1H), 7.06 (dd, J = 7.8, 7.6 Hz, 1H),
7.00 (d, J = 9.0 Hz, 4H), 6.95 (d, J = 8.8 Hz, 2H), 6.69 (d, J = 9.0 Hz,
4H), 3.28 (s, 6H), 1.05 (s, 9H), 0.97 (s, 9H), 0.00 (br-s, 6H); Anal.
Calc. for C37H46N2O4Si: C, 72.75; H, 7.59; N, 4.59. Found: C,
72.59; H, 7.47; N, 4.51%.
4.1. General
Toluene and n-butyronitrile were distilled from CaH2 under an
argon atmosphere, tetrahydrofuran (THF) was distilled from potas-
sium–benzophenone under an argon atmosphere, N,N-dimethyl-
formamide (DMF) was dried over molecular sieves 4A, and
benzonitrile was dried through alumina (ICN, Alumina N, Akt. I)
column with bubbling of argon, just before use. All the other pur-
chased reagents and solvents were used without further purifica-
tion. Column chromatography was performed using silica gel
(Kanto Chemical Co., Inc., Silica gel 60N, spherical neutral). 1H
and 13C NMR spectra were recorded on a JEOL JNM-AL400, JNM-
EX400, or JNM-AL300 spectrometers, and chemical shifts are given
in parts per million (ppm) relative to internal tetramethylsilane (d
0.00 ppm). Elemental analyses were performed by Center for Or-
ganic Elemental Microanalysis, Kyoto University.
1
The spin concentration after oxidation has not been quantitatively-determined
yet. However, no obvious decrease of the spectral intensity was observed, and 2a2+ is
regarded as diradical.

3110
Y. Nakano et al. / Polyhedron 30 (2011) 3106–3111
4.2.4. 1,2-Bis[3-(N-tert-butyl-N-tert-butyldimethylsiloxyamino)
phenyl]-1,2-bis[4-{N,N-bis(4-methoxyphenyl)amino}phenyl]ethylene
(6)
77.47; H, 6.71; N, 5.83; O, 9.99. Found: C, 77.38; H, 6.70; N, 5.75;
O, 9.99%.
Ethylene 6 was prepared by low valent titanium induced reduc-
tive coupling reaction, using the modified method of literature [6].
TiCl4ꢁ2THF complex (4.31 g, 12.5 mmol) was placed in a flask
equipped with a reflux condenser and a dropping funnel under
an argon atmosphere. Dry THF (40 mL) was added at ꢀ10 °C in
ice-acetone bath. Zinc powder (2.27 g, 31.2 mmol) was added
and the resulting mixture was stirred at room temperature for
1 h and refluxed for 3 h. After cooling to ꢀ10 °C in ice-acetone bath,
dry pyridine (1 mL) was added. 7 (3.06 g, 5.01 mmol) in dry THF
(50 mL) was dropped for 20 min. The resulting mixture was
refluxed for 16 h. After cooling to room temperature and diluted
with CH2Cl2, the mixture was poured over cold saturated aqueous
NaHCO3. The suspension was filtered through celite. The organic
layer was separated, and the aqueous layer was extracted with
CH2Cl2. The organic layer was combined and dried over MgSO4.
After filtration, the solvent was evaporated. The residue was chro-
matographed on silica gel (ethyl acetate:CH2Cl2:n-hex-
ane = 1:29:70 as eluent) to afford 6 as yellow solid (fraction A
(6a) 1.33 g, fraction B (6b) 0.37 g; total 1.70 g, 57%, (E)/(Z) = 78/
22): fraction A (6a) (E)-isomer Rf = 0.33 (silica gel, ethyl ace-
tate:n-hexane = 1:9); 1H NMR (400 MHz, CDCl3) d 6.80–7.04 (m,
8H), 6.94 (d, J = 9.1, 8H), 6.75 (d, J = 9.1 Hz, 8H), 6.72 (d,
J = 8.3 Hz, 4H), 6.55 (d, J = 8.3 Hz, 4H), 3.76 (s, 12H), 0.95 (s, 18H),
0.86 (s, 18H), ꢀ0.15 (br-s, 12H); 13C NMR (100 MHz, CDCl3) d
155.4, 150.5, 146.2, 143.9, 140.8, 139.6, 136.1, 131.9, 128.3,
127.8, 126.5, 126.3, 122.9, 119.3, 114.4, 60.6, 55.5, 26.2, 26.1,
18.0, ꢀ4.5; FAB HRMS (m-nitrobenzyl alcohol) m/z calcd for
4.2.6. 1,2-Bis[3-(N-tert-butyl-aminoxyl)phenyl]-1,2-bis[4-{N,N-bis(4-
methoxyphenyl)-amino}phenyl]ethylene (2a)
To a solution of 7a (222 mg, 0.231 mmol) in CH2Cl2 (20 mL) was
added Ag2O (326 mg, 1.39 mmol) and stirred for 2 h at room tem-
perature. The reaction mixture was filtered through celite. The fil-
trate was evaporated to remove solvent and chromatographed on
silica gel (ethyl acetate:CH2Cl2:n-hexane = 20:30:50 as eluent) to
afford 2a as reddish brown solid (52.8 mg, 24%): ESR (toluene) five
lines, g = 2.006, |aN|/2 = 0.69 mT.
4.3. Electrochemistry
The cyclic voltammetry measurements were carried out in ben-
zonitrile solution containing 0.1 M tetrabutylammonium tetrafluo-
roborate 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, the counter, and the reference electrode, respec-
tively. The redox potentials were referenced against ferrocene/fer-
rocenium (Fc/Fc+) couple.
4.4. ESR spectroscopy
ESR spectra were recorded on a JEOL JES-SRE2X spectrometer, in
which temperature was controlled by a JEOL DVT2 variable-tem-
perature unit, and a Mn2+/MnO solid solution was used as a refer-
ence for the determination of g values and hyperfine coupling
constants.
C
74H93N4O6Si2 [M+H]+ 1189.6634, found 1189.6677; Anal. Calc.
for C74H92N4O6Si2: C, 74.71; H, 7.79; N, 4.71. Found: C, 74.41; H,
7.69; N, 4.61%. Fraction B (6b) (Z)-isomer Rf = 0.20 (silica gel, ethyl
acetate:n-hexane = 1:9); 1H NMR (400 MHz, CDCl3) d 7.01 (d,
J = 9.0, 8H), 6.90–6.96 (m, 8H), 6.79 (d, J = 9.0 Hz, 8H), 6.77 (d,
J = 8.5 Hz, 4H), 6.67 (d, J = 8.5 Hz, 4H), 3.77 (s, 12H), 0.90 (s, 18H),
0.85 (s, 18H), ꢀ0.18 (br-s, 12H); 13C NMR (100 MHz, CDCl3) d
155.4, 150.4, 146.4, 143.3, 141.0, 139.7, 136.8, 131.9, 128.4,
127.9, 126.5, 126.2, 122.8, 119.8, 114.5, 60.7, 55.5, 26.2 [two kinds
of carbon concerning tert-butyl group, AC(CH3)3, have the same
chemical shift], 18.0, ꢀ4.5; FAB HRMS (m-nitrobenzyl alcohol) m/
z calcd for C74H93N4O6Si2 [M+H]+ 1189.6634, found 1189.6584;
Anal. Calc. for C74H92N4O6Si2: C, 74.71; H, 7.79; N, 4.71. Found: C,
74.41; H, 7.79; N, 4.61%.
Acknowledgement
The present work was supported by a Grant-in-Aid for Scientific
Research (B) (20350065) from the Japan Society for the Promotion
of Science (JSPS).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.02.046.
References
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