Redox-switchable π-conjugated systems bearing terminal ruthenium(II) complexes

Article abstract of DOI:10.1016/j.tetlet.2003.08.102

p-Phenylenediamine bearing terminal bipyridyl moieties was synthesized by palladium-catalyzed amination. The corresponding ruthenium(II) complex was formed and characterized, providing a redox-switchable photoinduced electron-transfer system.

Full text of DOI:10.1016/j.tetlet.2003.08.102

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7711–7714
Redox-switchable p-conjugated systems bearing terminal
ruthenium(II) complexes
Xiuliang Shen, Toshiyuki Moriuchi and Toshikazu Hirao*
Department of Materials Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871,
Japan
Received 1 July 2003; revised 27 August 2003; accepted 28 August 2003
Abstract—p-Phenylenediamine bearing terminal bipyridyl moieties was synthesized by palladium-catalyzed amination. The
corresponding ruthenium(II) complex was formed and characterized, providing a redox-switchable photoinduced electron-transfer
system.
© 2003 Elsevier Ltd. All rights reserved.
p-Conjugated polymers and oligomers have received
extensive interest because of the potential application
to electronic materials depending on their electrical
properties.1 The redox-active p-conjugated unit of
polyanilines and oligoanilines exists in three redox
forms, which include the reduced phenylenediamine
dianion, the partially reduced semiquinonediimine rad-
ical anion, and the oxidized neutral quinonediimine.
Coordination of the quinonediimine moiety to transi-
tion metals has been demonstrated to afford a multi-
redox system.2 Although polynuclear bipyridyl
ruthenium(II) complexes linked by a bridging spacer
have been investigated electrochemically and photo-
physically to provide electronic and photo-active
devices,3 only few cases focused on redox-active bridg-
ing spacers.4 A combination with a redox-active p-
phenylenediamine function is expected to afford a
novel redox-active donor–acceptor system. In a previ-
ous paper, two phenylenediamine pendant groups have
been incorporated into the bipyridyl ruthenium com-
plex to give a redox-switching system.5 We herein
report the synthesis of a p-conjugated redox-active p-
phenylenediamine ligand possessing terminal bipyridyl
moieties and its complexation to the corresponding
dinuclear ruthenium(II) complex.
The p-conjugated p-phenylenediamine 1 was prepared
as shown in Scheme 1. 5-Bromo-2,2%-bipyridine was
produced in good yield by modification of the litera-
ture procedure.6 First, 2-bromopyridine was converted
to 2-tributylstannylpyridine. The thus-obtained stan-
nylpyridine underwent Stille-type coupling with 2,5-
dibromopyridine to give 5-bromo-2,2%-bipyridine.7 The
phenylenediamine 1 was conveniently synthesized by
the Pd2(dba)3/S-BINAP-catalyzed amination of 5-
bromo-2,2%-bipyridine with p-phenylenediamine.8 Due
to its insolubility, the BOC derivative was used for the
purification by silica-gel column chromatography, fol-
lowed by thermolysis under an inert atmosphere to
give pure 1 in 95% yield.9a
Scheme 1. Reagents and conditions: (a) BuLi, THF, hexane, Bu3SnCl, −78°C; (b) 2,5-dibromopyridine, cat. Pd(PPh3)4, DMF,
100°C; (c) 4 mol% Pd2(dba)3, 9 mol% S-BINAP, 2.8 equiv. NaOtBu, toluene, 90°C.
Keywords: p-conjugated ligand; redox-active spacer; bipyridyl ruthenium(II) complex; photoinduced electron transfer.
* Corresponding author. Previous department name: Department of Applied Chemistry, Faculty of Engineering. Tel.: +81-6-6879-7413;
fax: +81-6-6879-7415; e-mail: hirao@chem.eng.osaka-u.ac.jp
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.08.102

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X. Shen et al. / Tetrahedron Letters 44 (2003) 7711–7714
The complexation of 1 with cis-Ru(bpy)2Cl2 and suc-
cessive treatment with NH4PF6 led to the formation of
the dinuclear ruthenium(II) complex 2red (Scheme 2).
The redox function of the phenylenediamine moiety
was investigated chemically. On treatment with
Pb(OAc)4, the complex 2red was readily oxidized to 2ox
possessing the quinonediimine moiety. On the other
hand, 2ox could be reduced to 2red with NH2NH2·H2O.
These ruthenium complexes, 2red and 2ox, were fully
characterized by spectral data and elemental analyses.9b
1
The H NMR spectrum of 2red at room temperature is
shown in Figure 1. A combination with the 2D NMR
experiments (HH cosy, HMQC, and HMBC) permitted
assignment of all protons. Two singlet protons of C at
6.87 and 6.86 ppm suggests a syn-conformer as a
possible structure in the solution. Furthermore, the
2D-ROESY of 2red in CD3CN exhibited correlation
between the protons at the different aromatic groups
(Fig. 1), as exemplified by the correlation between the
protons at the 1- or 3-position of C and those of the
pyridyl protons (at the 3-, 4-, 5- and 6-position of D).
The MNDO calculation with ROESY-derived distance
restraints also suggests a syn-conformer of 2red
.
The redox properties were also studied electrochemi-
cally. The ligand 1 in DMF showed two separate
oxidation waves at 0.15 and 0.27 V (E1/2 versus Fc/Fc+)
assignable to the successive one-electron oxidation pro-
cesses of the p-phenylenediamine moiety. In the case of
the complex 2red in CH2Cl2, three separate oxidation
waves were observed at 0.43, 0.71 and 0.81 V. The two
former redox waves are assigned to the successive one-
electron oxidation processes of the p-phenylenediamine
+
’
moiety, giving the oxidized species, 2red and 2ox, as
illustrated in Scheme 3. The complexation resulted in
anodic shift. The redox behavior of the bipyridyl moi-
eties is almost similar to that of (bpy)3Ru(PF6)2 (Table
1).
1
Figure 1. One-dimensional and ROESY H NMR spectra of
2red (CD3CN, 298 K, 400 MHz).
Scheme 3. Redox processes of 2red
.
In the emission spectrum of 2red excited at 450 nm in
CH2Cl2, almost complete quenching was observed (Fig.
2). The quenching of 2red is probably attributed to the
photoinduced electron transfer from the p-phenylenedi-
amine moiety to the bpy-Ru moieties, wherein the
p-phenylenediamine moiety serves as an electron
Scheme 2.

X. Shen et al. / Tetrahedron Letters 44 (2003) 7711–7714
Table 1. Electrochemical dataa of 1, 2red, 2ox and (bpy)3Ru
7713
Compound
Ered3
Ered2
Ered1
Eox1
Eox2
Eox3 (Ru2+/3+
)
1b
2red
0.15
0.43
0.27
0.71
−2.33
−2.39
−2.18
−2.16
−1.98
−1.93
−1.89
−0.84
−1.73
0.81
0.81
0.88
2ox
(bpy)3Ruc
a E1/2/V (versus Fc/Fc+); [Compound]=1.0 mM; Solv. CH2Cl2. Recorded with Bu4NPF6 as an eletrolyte (0.1 M). Potentials were obtained by
cyclic voltammetry with a scan rate of 100 mV s−1 under argon.
b In DMF.
c In MeCN.
References
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and (bpy)3Ru with 1.0 equiv. of 1 (···). Solv. CH2Cl2; under
argon; 2×10−5 M. uex=450 nm.
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donor.5,10 The emission was also quenched in the case
of the oxidized form 2ox. Based on the reported elec-
tron-transfer mechanism of the ruthenium complexes
bearing a viologen or benzoquinone moiety,5,11 this
phenomenon might be explained by the photoinduced
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7. 5-Bromo-2,2%-bipyridine was isolated from unreacted
materials and organotin by-products by acidification of
the reaction mixture and subsequent extraction with
CH2Cl2. 2-Tributylstannylpyridine, 2,5-dibromopyridine,
and tributyltin chloride were extracted into the organic
phase, but 5-bromo-2,2%-bipyridine and another by-
product, terpyridine (tpy), still remained in the aqueous
layer. Addition of NiCl2·6H2O to the aqueous phase
neutralized by 28% ammonia solution, followed by
electron transfer in a direction opposite to that of 2red
.
A much less effective intermolecular quenching of
(bpy)3Ru(PF6)2 with 1 indicates an intramolecular
quenching process for both 2red and 2ox.
In conclusion, the p-phenylenediamine 1 was demon-
strated to serve as a redox-active ligand to afford a
novel redox-active dinuclear ruthenium(II) complex,
which was oxidized to the quinonediimine derivative for
a redox-switchable photoinduced electron-transfer sys-
tem. Further investigation on the photoinduced elec-
tron-transfer mechanism and application to functional
materials is now in progress.
Acknowledgements
This work was partly supported by a Grant-in-Aid for
Scientific Research on Priority Areas (No. 13128206)
from the Ministry of Education, Science, and Culture,
Japan. Thanks are due to the Analytical Center, Fac-
ulty of Engineering, Osaka University, for the use of
their facilities.

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X. Shen et al. / Tetrahedron Letters 44 (2003) 7711–7714
extraction with CH2Cl2 led to the complete separation of
152.0 (A6), 147.2 (B2), 145.6 (B5), 138.7 (G4), 138.7 (E4),
138.7 (F4), 138.5 (A4), 138.5 (D4), 138.1 (B6), 138.0
(B%6), 136.3 (C2), 128.6 (F5), 128.6 (G5), 128.5 (E5),
128.4 (D5), 126.5 (A5), 125.9 (B3), 125.2 (F3), 125.2 (E3),
125.2 (G3), 125.0 (D3), 123.5 (C1 or C3), 123.4 (C3 or
C1), 123.0 (A3), 121.8 (B4), 121.8 (B%4) ppm. MS (FAB):
m/z=1678 (M−PF6)+. Anal. calcd for C66H52N14Ru2-
(PF6)4·0.5H2O: C, 43.27; H, 2.92; N, 10.70. Found: C,
42.91; H, 2.91; N, 10.35. 2ox: mp 257–258°C (decomp).
IR (KBr): 1603, 1585, 1464, 1446, 1314 cm−1. UV–vis
(CH2Cl2): umax (log m)=446 (4.65) nm. 1H NMR
(CD3CN, 400 MHz, syn and anti-isomers): l 8.51–8.43
(m, 24H, A3, B3, D3, E3, F3, and G3), 8.09–8.03 (m,
16H, D4, E4, F4, and G4), 7.98 (td, 4H, J=7.7, 1.5 Hz,
A4), 7.88 (d, 4H, J=5.2 Hz, py6), 7.76 (d, 4H, J=5.5 Hz,
py6), 7.71–7.69 (m, 12H, A6 and py6), 7.54 (dd, 2H,
J=8.4, 1.8 Hz, B4anti), 7.47–7.32 (m, 22H, A5, B4syn, and
py5), 7.27 (d, 2H, J=1.8 Hz, B6syn), 7.21 (d, 2H, J=1.8
Hz, B6anti), 6.98 (d, 2H, J=2.2 Hz, phensyn), 6.89–6.86
(m, 2H, phenanti), 6.61–6.57 (m, 4H, phenanti and
phensyn). 13C NMR (CD3CN, 100 MHz): 161.2 (C: phen),
158.0–157.9 (D2, E2, F2, and G2), 157.7 (A2), 154.4
(B2anti), 154.3 (B2syn), 152.8–152.7 (D6, E6, F6, and G6),
152.6 (A6), 149.6 (B5), 144.6 (B6syn), 144.0 (B6anti),
139.0–138.9 (A4, D4, E4, F4, G4, and phenanti), 137.9
(phensyn), 130.1 (B4anti), 129.7 (B4syn), 128.7–128.6 (D5,
E5, F5, and G5), 128.1 (A5), 126.8 (phensyn), 125.9
(phenanti), 125.5 (B3), 125.3–125.2 (D3, E3, F3, and G3),
124.8 (A3) ppm. MS (FAB): m/z=1676 (M−PF6)+. Anal.
calcd for C66H50N14Ru2(PF6)4·3H2O: C, 42.27; H, 3.01;
N, 10.46. Found: C, 42.12; H, 2.83; N, 10.29.
5-bromo-2,2%-bipyridine from tpy. Finally, the product,
5-bromo-2,2%-bipyridine, was purified as a white solid by
further sublimation.
8. Sadighi, J. P.; Singer, R. A.; Buchwald, S. L. J. Am.
Chem. Soc. 1998, 120, 4960.
9. (a) 1: mp 219–220°C (uncorrected). IR (KBr): 3252, 3035,
1597, 1578, 1566, 1541, 1510, 1460, 1330 cm−1. UV–vis
(CH2Cl2:DMF=10:1): umax (log m)=350 (4.51) nm. 1H
NMR (DMSO-d6, 400 MHz): l 8.58 (d, 2H, J=4.4 Hz,
A6), 8.53 (s, 2H, NH), 8.36 (d, 2H, J=2.6 Hz, B6), 8.23
(d, 2H, J=6.6 Hz, A3), 8.22 (d, 2H, J=8.8 Hz, B3), 7.84
(dd, 2H, J=6.6, 6.6 Hz, A4), 7.49 (dd, 2H, J=8.8, 2.6
Hz, B4), 7.30 (dd, 2H, J=6.6, 4.4 Hz, A5), 7.18 (s, 4H,
C1). 13C NMR (DMSO-d6, 100 MHz): 155.8 (A2), 149.1
(A6), 145.5 (B3), 141.7 (B6), 137.1 (B2), 137.0 (A3), 135.9
(C2), 122.7 (A5), 121.1 (B5), 120.8 (B4), 120.5 (C1), 119.2
(A3) ppm. MS (FAB): m/z=417 (M++1). Anal. calcd for
C26H20N6: C, 74.98; H, 4.84; N, 20.18. Found: C, 74.90;
H, 4.79; N, 20.17; (b) 2red: mp 252–253°C (decomp). IR
(KBr): 3396, 1598, 1512, 1466, 1446, 1324 cm−1. UV–vis
(CH2Cl2): umax (log m)=452 (3.47) nm. 1H NMR
(CD3CN, 400 MHz): l 8.50–8.47 (m, 6H, G3, F3, and
E3), 8.42 (d, 2H, J=8.4 Hz, D3), 8.24 (d, 2H, J=7.3 Hz,
A3), 8.22 (d, 2H, J=9.0 Hz, B3), 8.10–8.01 (m, 6H, G4,
F4, and E4), 7.94 (td, 2H, J=7.9, 1.5 Hz, A4), 7.93 (td,
2H, J=7.9, 1.5 Hz, D4), 7.87 (d, 2H, J=5.8 Hz, G6),
7.77 (d, 2H, J=5.8 Hz, F6), 7.71 (d, 2H, J=5.1 Hz, E6),
7.68 (d, 2H, J=5.1 Hz, D6), 7.58 (d, 2H, J=4.8 Hz, A6),
7.48 (dd, 2H, J=9.2, 2.6 Hz, B4), 7.46–7.41 (m, 4H, G5,
and F5), 7.39–7.36 (m, 2H, E5), 7.31–7.27 (m, 2H, D5),
7.25–7.22 (m, 2H, A5), 7.19 (s, 2H, NH), 7.17 (d, 1H,
J=2.6 Hz, B6), 7.16 (d, 1H, J=2.6 Hz, B%6), 6.87 (s, 2H,
C1 or C3), 6.86 (s, 2H, C3 or C1). 13C NMR (CD3CN,
100 MHz): 158.4 (A2), 158.1 (D2), 158.0 (E2), 158.0 (G2),
157.9 (F2), 152.7 (E6), 152.7 (F6), 152.6 (D6), 152.5 (G6),
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1992, 114, 8081; (b) Goulle, V.; Harriman, A.; Lehn,
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