A new azaferrocenophane with an azobenzene-containing ligand. Remote control of photoisomerization of the azobenzene group by redox of the iron center

Article abstract of DOI:10.1021/om034163i

Azobenzene-introduced azaferrocenophanes have been newly synthesized by coupling reactions catalyzed by transition-metal complexes. The photochemical response of the azobenzene group is controlled by the oxidation state of the molecule.

Full text of DOI:10.1021/om034163i

18
Organometallics 2004, 23, 18-20
A New Aza fer r ocen op h a n e w ith a n
Azoben zen e-Con ta in in g Liga n d . Rem ote Con tr ol of
P h otoisom er iza tion of th e Azoben zen e Gr ou p by Red ox
of th e Ir on Cen ter
Masaki Horie, Tatsuaki Sakano, Kohtaro Osakada,* and Hidenobu Nakao
Chemical Resources Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, J apan
Received September 9, 2003
Sch em e 1. Syn th esis of tr a n s-1a
Summary: Azobenzene-introduced azaferrocenophanes
have been newly synthesized by coupling reactions
catalyzed by transition-metal complexes. The photo-
chemical response of the azobenzene group is controlled
by the oxidation state of the molecule.
Organotransition-metal complexes that have azo-
benzene-containing ligands are of recent interest be-
cause they can exhibit photochemical properties which
are not achieved by azobenzene and its organic deriva-
tives.1 Connecting the metal center and azobenzene with
an organic linker having suitable length and/or a
π-conjugated system enables communication of the
electronic states between the metal and the azobenzene
group. Nishihara reported the unique properties of
m-ferrocenylazobenzene, in which azobenzene is directly
bonded to a Cp ligand of ferrocene.2 One-electron
oxidation of the compound decreased the thermody-
namic stability of the cis-azobenzene group bonded to
ferrocene. This oxidation led to the formation of a trans-
rich mixture in the Fe(III) state under irradiation with
a single green light and its conversion to a cis-rich
mixture upon electrochemical reduction to Fe(II). This
control of photoinduced cis-trans isomerization of the
azobenzene by changing the electronic state of the metal
center formed a smart on-off switching system.3 Alter-
natively, the introduction of a functional group to the
organic azobenzene can accelerate or retard the cis-
trans isomerization, depending on the kind of group.4
Thus,the electronic character of the substituent bonded
to azobenzene changes the photochemical properties.
a
Conditions: (a) 3-bromoaniline, RuCl2(PPh3)3 (5 mol %),
180 °C, NMP; (b) (4-phenylamino)azobenzene, Pd2(dba)3 (dba
) dibenzylidenacetone, 1.3 mol %), NaO-t-Bu (1.5 equiv), P(t-
Bu)3 (7.5 mol %), 100 °C, toluene.
We recently found that azaferrocenophanes undergo
reversible electron transfer between the Fe and N atoms
in the electrochemically oxidized state.5 Electronic com-
munication between the azaferrocenophane, showing
reversible redox behavior, and the substituent bonded
to azobenzene would provide a new approach for the
control of photochemical output of the molecule by
electrochemical stimuli. In this paper, we report the
preparation of a new azaferrocenophane containing an
azobenzene group and its electrochemical control over
the photochemical response via intramolecular electron
transfer.
Scheme 1 summarizes the preparation of the aza-
ferrocenophane trans-1, which contains an azobenzene
group, via coupling reactions catalyzed by Ru6 and Pd
complexes.7,8 The N atom, bonded to a phenyl carbon of
* To whom correspondence should be addressed. E-mail: kosakada@
res.titech.ac.jp.
(1) (a) Coe, B. J .; J ones, C. J .; McCleverty, J . A.; Bloor, D.; Kolinsky,
P. V.; J ones, R. J . J . Chem. Soc., Chem. Commun. 1989, 1485. (b) Lee,
S.-M.; Marcaccio, M.; McCleverty, J . A.; Ward, M. D. Chem. Mater.
1998, 10, 3272. (c) Barlow, S.; Bunting, H. E.; Ringham, C.; Green, J .
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Chem. Soc. 1999, 121, 3715. (d) Walter, D. G.; Compbell, D. J .; Mirkin,
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T. Macromolecules 1998, 31, 355. (b) Nishimura, N.; Sueyoshi, T.;
Yamanaka, H.; Imai, E.; Yamamoto, S.; Hasegawa, S. Bull. Chem. Soc.
J pn. 1976, 49, 1381. (c) Nishimura, N.; Kosako, S.; Sueishi, Y. Bull.
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Chem. Soc. 1996, 118, 3626. (b) Hartwig, J . F. Synlett 1997, 329. (c)
Driver, M. S.; Hartwig, J . F. J . Am. Chem. Soc. 1997, 119, 8232. (d)
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L. M.; Guzei, I. A. J . Am. Chem. Soc. 2000, 122, 4618.
10.1021/om034163i CCC: $27.50 © 2004 American Chemical Society
Publication on Web 12/09/2003

Communications
Organometallics, Vol. 23, No. 1, 2004 19
F igu r e 1. UV-vis spectra of trans-1 in toluene (a) before
irradiation and (b) after irradiation at 420 nm from a cutoff-
filtered xenon lamp for 1 h at 20 °C (photostationary state).
F igu r e 2. Cyclic voltammogram of trans-1 (1.0 mM) in
CH2Cl2 containing 0.10 M n-Bu4NPF6 and 25 °C. Sweep
rate: 0.10 V s-1. E1/2 ) 0.22 V and Epa ) 0.73 V.
the azobenzene group, is connected to the N atom of the
ferrocenophane group by a m-phenylene group.
X-ray crystallography of the azaferrocenophane re-
vealed a structure with a trans-azobenzene group.9
Figure 1 shows the absorption spectra of trans-1, which
exhibits a π-π* transition at 420 nm (ꢀ ) 31 500 M-1
cm-1). Irradiation of the solution at 420 nm decreases
the absorption because of photoinduced isomerization
of the trans-azobenzene group to its cis form. An n-π*
transition of the cis-azobenzene group much weaker
than the π-π* transition of the trans-azobenzene group
was reported in many organic azobenzene derivatives.10
This isomerization generates a photostationary state,
which contains mainly the compound with a cis-azo-
benzene unit. The azaferrocenophane generated by
irradiation undergoes thermal isomerization to regener-
ate trans-1. This isomerization is completed in 24 h at
10 °C (0.5 h at 70 °C).
F igu r e 3. First-order plots of thermal isomerization (283
K) of cis-1 in the photostationary state (a) without oxidation
and (b) after oxidation at 0.4 V. At denotes the absorption
at t.
The cyclic voltammogram of trans-1 shows one re-
versible redox and one irreversible oxidation, as shown
in Figure 2. The first wave is assigned to electrochemical
oxidation and reduction of the Fe center, while the
second wave at a higher potential is due to the electro-
chemical oxidation of the organic moiety.11,12 Electro-
chemical oxidation of 1 in the photostationary state at
0.4 V (vs Ag+/Ag) led to a solution whose spectrum is
similar to that of trans-1. This result is rationalized by
assuming a significant acceleration of the thermal
isomerization of the cis-azobenzene to the trans form.
Electrochemical oxidation of a toluene solution of trans-1
at 0.4 V (vs Ag+/Ag) by a flow electrolysis method and
subsequent irradiation of this solution at 420 nm caused
a slight decrease of the absorption due to photoisomer-
ization of the trans-azobenzene to the cis isomer,13 and
the spectrum rapidly returned to that of trans-1, as a
result of the rapid thermal isomerization of the cis-
azobenzene group. The thermal isomerization was moni-
tored by the spectroscopic change at 10 °C. Completion
of the isomerization required a time shorter than 5 min
at that temperature. Figure 3 shows a comparison of
first-order plots of the reaction to those of the thermal
isomerization of the isomer mixture of 1 in the photo-
stationary state to pure trans-1. The first-order rate
constant of the thermal reaction of 1 after the electro-
chemical oxidation (1.0 × 10-2 s-1; 283 K) is much faster
than that without electrochemical oxidation (1.4 × 10-4
s-1). Thus, the cis-azobenzene group of the oxidized form
of 1 undergoes much more rapid isomerization to the
trans form than does compound 1 without electrochemi-
cal oxidation.
(8) (a) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1348. (b) Wolfe, J . P.; Wagaw, S.; Buchwald,
S. L. J . Am. Chem. Soc. 1996, 118, 7215. (c) Wagaw, S.; Rennels, R.
A.; Buchwald, S. L. J . Am. Chem. Soc. 1997, 119, 8451. (d) Singer, R.
A.; Sadighi, J . P.; Buchwald, S. L. J . Am. Chem. Soc. 1998, 120, 213.
(e) Old, D. W.; Harris, M. C.; Buchwald, S. L. Org. Lett. 2000, 2, 1403.
(9) Crystal data for trans-1: C36H30FeN4, Mr ) 574.51, red prisms
(0.70 × 0.50 × 0.40 mm), triclinic, P1h, a ) 10.362(2) Å, b ) 18.639(4)
Å, c ) 7.399(2) Å, R ) 97.97(2)°, â ) 102.67(2)°, γ ) 86.58(2)°, V )
1380.1(5) Å3, Z ) 2, Fcalcd ) 1.382 g/cm3, µ ) 5.79 cm-1, minimum/
maximum transmission 0.9169/1.0000, 2θmax ) 55.0°, λ(Mo KR) )
0.710 69 Å, T ) 293 K, 5457 total reflections (R(int) ) 0.051), R1 )
0.047, wR2 ) 0.033 (I > 3.0σ(I)), GOF (F2) ) 1.65.
Scheme 2 summarizes a mechanism that accounts for
rapid isomerization of the oxidized species. A one-
electron oxidation of cis-1 would form the Fe(III) com-
plex, which could easily be converted into cis-A and cis-
B, having an Fe(II) center and a cation radical at the
N atom,11 via electron transfer among the Fe and two
N atoms.5 cis-B is responsible for the rapid isomeriza-
(10) (a) Delaire, J . A.; Nakatani, K. Chem. Rev. 2000, 100, 1817. (b)
Ichimura, K. Chem. Rev. 2000, 100, 1847.
(11) (a) Somoano, R.; Hadek, V.; Yen, S. P. S.; Rembaum, A. J . Chem.
Phys. 1975, 62, 1061. (b) Yamaguchi, J .; Fujita, H. Bull. Chem. Soc.
J pn. 1990, 63, 2928.
(12) (a) McDiarmid, A. G.; Chiang, J .-C.; Halpern, M.; Huang, W.-
S.; Mu, S.-L.; Somasiri, N. L. D.; Wu, W.; Yaniger, S. I. Mol. Cryst.
Liq. Cryst. 1985, 121, 173. (b) Awano, H.; Murakami, H.; Yamashita,
T.; Ohigashi, H.; Ogata, T. Synth. Met. 1991, 39, 327. (c) Kang, E. T.;
Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277.
(13) The degree of change of the absorption peak by irradiation is
17% of the difference between (a) and (b) in Figure 1. It can be ascribed
to rapid thermal isomerization of the azobenzene and shift of the trans/
cis equilibrium in the photostationary state of the oxidized azaferro-
cenophane.

20 Organometallics, Vol. 23, No. 1, 2004
Communications
Sch em e 2
tion of cis- to trans-azobenzene. The positive charge at
the N atom attached to the azobenzene unit enhances
the isomerization of the NdN bond significantly, due
to the N-N bond rotation of the oxidized form of cis-B
being more rapid than that of the unoxidized form of
cis-1. In summary, the azaferrocenophane in this study
undergoes photoinduced isomerization of the trans-
azobenzene group to the cis form, and the UV-vis
spectrum of a photostationary state has been estab-
lished. Upon electrochemical oxidation, the spectro-
scopic signature of the photostationary state becomes
smaller, due to acceleration of the thermal isomeriza-
tion. The control of the photochemical output of this
molecule by the oxidation state results from an elec-
tronic communication between the Fe atom and the
organic ligand.
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research from the Ministry
of Education, Culture, Sports, Science and Technology
of J apan.
Su p p or tin g In for m a tion Ava ila ble: Text, tables, and
figures giving experimental procedures for synthesis of the
complexes, results of X-ray crystallography for trans-1, and
kinetics data; X-ray data are also available in electronic form
as CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM034163I