Ruthenium complexes bearing π-conjugated pendant moieties for a redox-switching system

Article abstract of DOI:10.1039/b010039k

A ruthenium(II) complex bearing N,N'-bis(4-aminophenyl)-1,4-phenylenediamine moieties, which can be chemically oxidized to the corresponding N,N'-bis(4-aminophenyl)-1,4-benzoquinonediimine moieties, was synthesized, characterized electrochemically and pho

Full text of DOI:10.1039/b010039k

Ruthenium complexes bearing p-conjugated pendant moieties for a
redox-switching system
Toshikazu Hirao* and Koichiro Iida
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871,
Japan. E-mail: hirao@ap.chem.eng.osaka-u.ac.jp
Received (in Cambridge, UK) 15th December 2000, Accepted 4th January 2001
First published as an Advance Article on the web 14th February 2001
A ruthenium(II) complex bearing N,NA-bis(4-aminophenyl)-
1,4-phenylenediamine moieties, which can be chemically
oxidized to the corresponding N,NA-bis(4-aminophenyl)-
1,4-benzoquinonediimine moieties, was synthesized, charac-
terized electrochemically and photochemically, and found to
afford a redox-switching system.
The construction of efficient systems for electron transfer is a
key factor in the development of versatile catalysts and
functionalized materials. Photoinduced electron-transfer sys-
tems have been investigated with a variety of ruthenium
bipyridyl complexes. A ruthenium complex bearing a redox-
active moiety provides an electro-photoswitching device, as
exemplified by complexes bearing a benzoquinone1 or viologen
function.2 Ruthenium(II) complex systems have also been
recognized as receptors for anion recognition3 and photo-
synthetic models.4 In a previous paper, quinonediimine deriva-
tives were revealed to afford redox-active catalysts and d,p-
conjugated complexes.5 Furthermore, four p-conjugated
pendant groups have been incorporated into a porphyrin
scaffold to give atropisomeric three-dimensionally oriented
redox-active systems.6 We herein report redox-switchable
ruthenium(II) complexes bearing p-conjugated pendant moie-
ties.
A bipyridyl ligand 1 bearing aniline trimer pendant groups
Scheme 1
was prepared by amidation of the acid chloride derivative of
2,2A-bipyridine-4,4A-dicarboxylic acid with 2 mol equiv. of N-
(4-acetylaminophenyl)-NA-(4-aminophenyl)-1,4-phenylenedia-
mine.† The ruthenium(II) complex 2 was synthesized by
complexation with cis-Ru(bpy)2Cl2 and subsequent treatment
with NH4PF6. Chemical oxidation of complex 2 was achieved
with 2 mol equiv. of Ag2O to give the corresponding oxidized
complex 3 (Scheme 1). Furthermore, 3 could be reduced to 2 on
treatment with N2H4·H2O at room temperature for 3 h under
argon. These complexes were identified by spectral data and
cyclic voltammetry.‡ For example, 3 exhibited a CT band at ca.
400–600 nm together with an MLCT band, in contrast to the
observation of only the MLCT band for 2.
formation of 6 (g = 2.004, AN = 6.7 G, AH(CH) = 1.6 G, AH(NH)
= 3.4 G). These findings are consistent with the reported redox
behavior of N,NA-diphenyl-1,4-benzoquinonediimine and N,NA-
diphenyl-1,4-phenylenediamine.7 The above-mentioned sol-
vent effect might be due to the difference in equilibrium
between the diprotonated quionediimine and corresponding
reduced species, and the radical cationic species.
In the emission spectrum of complex 2 excited at 477 nm,
almost complete quenching was observed in acetonitrile (Fig.
Table 1 Electrochemical data for 2, 4 and 5a
The redox properties were studied by cyclic voltammetry.
The redox waves were assigned by comparison with those of the
corresponding complexes 4 and 5 bearing anilino and anilino-
anilino groups, respectively (Table 1). The electrochemical
behavior of complex 2 in acetonitrile is explained by the redox
processes D0?D+?D2+?D4+ of the quinonediimine moieties
as shown in Scheme 2. The redox behavior of the ruthenium
bipyridyl moiety was similar to that for complexes 4 or 5. A
multiredox system has thus been achieved with complex 2. It
should be noted that the redox processes were found to depend
on the solvent. The redox waves in DMF were different from
those observed in acetonitrile, suggesting the redox processes
D0?D2+?D3+?D4+ in the former. Two quinonediimine
moieties exhibited the same redox potential at each step in both
solvents.
The radical cationic species 6 could be generated by
treatment of 2 with 3 in the presence of a proton source (HClO4)
with the appearance of a broad absorption (600–1100 nm) in the
UV–VIS–NIR spectrum.§ EPR spectroscopy also supported the
DOI: 10.1039/b010039k
Chem. Commun., 2001, 431–432
This journal is © The Royal Society of Chemistry 2001
431

intramolecularly. Such intramolecular quenching processes are
considered to be thermodynamically feasible.
In conclusion, we have described a versatile and efficient
redox-switching system with variable oxidation state of the p-
conjugated pendant moieties. Further investigation on the
electron-transfer mechanism and molecular recognition is now
in progress.
Thanks are due to the Analytical Center, Faculty of
Engineering, Osaka University, for the use of their facilities.
Partial financial support by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan is also acknowledged.
Notes and references
† The acid chloride derivative prepared by treatment of 2,2A-bipyridine-
4,4A-dicarboxylic acid (122 mg, 0.50 mmol) with SOCl2 (238 mg, 2.0 mmol)
was dissolved in DMF (40 mL) and added dropwise over 2 h to a solution
of N-(4-acetylaminophenyl)-NA-(4-aminophenyl)-1,4-phenylenediamine5
(332 mg 1.0 mmol), 4-dimethylaminopyridine (12 mg, 0.10 mmol) and
Et3N (0.50 mL) in DMF (20 mL) at 0 °C. After stirring at room temp. for
24 h, work-up gave the pure bipyridyl ligand 1 (342 mg, 0.39 mmol, 78%).
1: IR (KBr) 3408, 3305, 3265, 1652 cm21; dH(300 MHz, DMSO-d6) 10.51
(s, 2H), 9.69 (s, 2H), 8.94 (d, 2H, J 5.2 Hz), 8.91 (d, 2H, J 1.6 Hz), 7.97 (dd,
2H, J 5.2, 1.6 Hz), 7.87 (s, 2H), 7.76 (s, 2H), 7.62 (d, 4H, J 9.0 Hz), 7.39 (d,
4H, J 8.8 Hz), 7.04–6.90 (m, 16H), 1.99 (s, 6H); MS (FAB) m/z 872 M+.
‡ 2: IR (KBr) 3405, 3291, 1654 cm21; dH(600 MHz, DMSO-d6) 10.55 (s,
2H), 9.67 (s, 2H), 9.34 (s, 2H), 8.88–8.87 (m, 4H), 8.23–8.20 (m, 4H),
7.96–7.95 (m, 4H), 7.89 (s, 2H), 7.81–7.75 (m, 4H), 7.76 (s, 2H), 7.59–7.54
(m, 4H), 7.54 (d, 4H, J 8.9 Hz), 7.38 (d, 4H, J 8.9 Hz), 7.01–6.96 (m, 12H),
6.91 (d, 4H, J 8.9 Hz), 1.99 (s, 6H); MS (FAB) m/z 1431 (M 2 PF6)+; UV–
VIS (MeCN) labs/nm (log e) 289 (5.08) 304 (5.04) 477 (4.42). 3: IR (KBr)
3404, 1669, 1529 cm21; UV–VIS (MeCN) labs/nm (log e) 288 (5.00) 482
(4.64).
Scheme 2
1). Such quenching was not observed with 4, indicating that the
p-conjugated chain of 2 contributes to the quenching. An
efficient photoinduced electron transfer is likely to operate in
complex 2, where the reduced form of the p-conjugated pendant
groups serves as an electron donor. Use of the oxidized form 3
also resulted in a quenched spectrum upon excitation at 477 nm.
Taking the reported electron-transfer mechanism of complexes
bearing viologen or benzoquinone moiety into account,1,2 this
result might be explained by electron transfer in a direction
opposite to that of 2. A much less effective quenching of
ruthenium(II) complex 4 with N,NA-bis(4-acetylaminophenyl)-
1,4-phenylenediamine or N,NA-bis(4-acetylaminophenyl)-
1,4-benzoquinonediimine was observed intermolecularly, in-
dicating that the quenching process for both 2 and 3 occurs
§ UV–VIS–NIR (MeCN) labs/nm (log e) 288 (4.96) 405 (4.72) 847
(4.44).
1 V. Goulle, A. Harriman and J.-M. Lehn, J. Chem. Soc., Chem. Commun.,
1993, 1034.
2 E. H. Yonemoto, R. L. Riley, Y. I. Kim, S. J. Atherton, R. H. Schmehl and
T. E. Mallouk, J. Am. Chem. Soc., 1992, 114, 8081.
3 P. D. Beer, Acc. Chem. Res., 1998, 31, 71 and references therein; S.
Watanabe, O. Onogawa, Y. Komatsu and Y. Katsuhira, J. Am. Chem.
Soc., 1998, 120, 229; M. J. Deetz and B. D. Smith, Tetrahedron Lett.,
1998, 39, 6841; S.Watanabe, N. Higashi, M. Kobayashi, K. Hamanaka,
T. Tanaka and Y. Katsuhira, Tetrahedron Lett., 2000, 41, 4583.
4 M. Seiler, H. Dürr, I. Willner, E. Joselevich, A. Doron and J. F. Stoddart,
J. Am. Chem. Soc., 1994, 116, 3399; S. Yamada, Y. Koide and T. Matsuo,
J. Electroanal. Chem., 1997, 426, 23; A. Magnuson, H. Berglund, P.
Korall, L. Hammarström, B. Åkermark, S. Styring and L. Sun, J. Am.
Chem. Soc., 1997, 119, 10720; S. H. Bossmann, M. F. Ottaviani, D.
Loyen, H. Dürr and C. Turro, Chem. Commun., 1999, 2487; K. A.
Maxwell, M. Sykora, J. M. DeSimone and T. J. Meyer, Inorg.
Chem.,2000, 39, 71; D. Burdiniski, E. Bothe and K. Wieghardt, Inorg.
Chem., 2000, 39, 105; Y.-Z. Hu, S. Tsukiji, S. Shinkai, S. Oishi, H. Dürr
and I. Hamachi, Chem. Lett., 2000, 442.
5 T. Hirao and S. Fukuhara, J. Org. Chem., 1998, 63, 7534; T. Moriuchi, S.
Bandoh, M. Miyaishi and T. Hirao, Eur. J. Inorg. Chem., 2001, 651.
6 T. Hirao and K. Saito, Tetrahedron Lett., 2000, 41, 1413.
7 J. F. Wolf, C. E. Forbes, S. Gould and L. W. Shacklette, J. Electrochem.
Soc., 1989, 2887.
Fig. 1 Emission spectra of 2 or 3 (·-·-), 4 (—), 4 with 2.0 mol equiv. of N,NA-
bis(4-acetylaminophenyl)-1,4-phenylenediamine (·····), and 4 with 2.0 mol
equiv. of N,NA-bis(4-acetylaminophenyl)-1,4-benzoquinonediimine (- -).
[Complex] = 2.0 3 1025 M; solv, MeCN, lexc = 477 nm.
432
Chem. Commun., 2001, 431–432