C O M M U N I C A T I O N S
The unequilibrated (i-rich) divalent state was prepared by
oxidation of [Cu(FcMpmpy)(LAnth)]+ at 213 K. Then, the sample
was warmed to 223 K to monitor the i- to o-inversion. Upon
warming, the absorption spectrum gradually evolved with an
isosbestic point (Figure 2b). The spectral change could be described
as breaching at 450 nm (metal-to-ligand CT (MLCT) on Cu(I))
and 700 nm (LMCT on ferrocenium), with a rise at 533 nm (CT
from Cu(II) to ferrocene) signifying valence electron rearrangement.
EPR also confirmed that warming the sample to room temperature
after 1e- oxidation at 193 K increased the spin density from 0.36
to 0.86, with a significant change in the sample color (Figure 2c).
At present, we propose that the difference in stability of the Cu(II)
coordination structure is the major factor for the reversed ET
behavior of isomers. The Cu(II) state is likely to exist in a
pentacoordinated form, and weak coordination of another species
(anion or solvent) seems to be strongly hindered by the methyl
substituent in the i-isomer form, destabilizing the Cu(II) state.
In conclusion, we have constructed a novel intramolecular
electron gating system that converts the rotative motion of a
coordinative pyrimidine ring into directional electron transfer
between two redox centers. The rotational equilibrium in the
monovalent state appears to be controllable by application of a weak
interaction. We are currently developing molecular structures in
which rotational motion can be driven by light-induced structural
rearrangement.
Figure 2. (a) Conceptual illustration of the rotation-triggered intramolecular
ET process. (b) Time-course UV-vis absorption spectral changes of
[Cu(FcMpmpy)(LAnth)]2+ in acetone formed by the addition of
(NH4)2[Ce(NO3)6] to a solution of [Cu(FcMpmpy)(LAnth)]BF4 at 213 K and
upon warming to 223 K. (c) EPR spectra of [Cu(FcMpmpy)(LAnth)]2+
measured in acetone after addition of (NH4)2[Ce(NO3)6] to a solution of
[Cu(FcMpmpy)(LAnth)]BF4 at 193 K, followed by cooling to 100 K (blue),
and after the subsequent warming to 298 K followed by cooling to 100 K
via 193 K (red). The appearance of the sample at 193 K in each step is
shown in the inset photographs.
determined, because the o-isomer is thermally dominant in both
divalent and trivalent states. Upon 1e- oxidation at 293 K, an
anisotropic EPR signal characteristic to Cu(II) with a spin density
of 1.0 was observed (Figure S6).
Acknowledgment. This work was supported by Grants-in-Aid
from MEXT of Japan (20750044, 20245013, and 21108002) and
the Global COE Program for Chemistry Innovation.
Thus, it is concluded that an electron is extracted first from
copper, then by ferrocene in the o-isomer form. The UV-vis spectra
of each oxidized form could subsequently be assigned. A charge-
transfer (CT) band from ferrocene to Cu(II) appears at 530 nm in
the divalent state, and a typical ligand-to-metal charge transfer
(LMCT) band of ferrocenium is found at ∼700 nm in the trivalent
state (Figure S7).
Supporting Information Available: Materials and methods, crystal
structure data (CIF), and electrochemical and spectral data. This material
References
(1) (a) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (b) Park, J.;
Pasupathy, A. N.; Goldsmith, J. I.; C. Chang, C.; Yaish, Y.; Petta, J. R.;
Rinkoski, M.; Sthena, J. P.; Abrun˜a, H. D.; McEuen, P. L.; Ralph, D. C.
Nature 2002, 417, 722–725. (c) Perepichka, D. F.; Bryce, M. R. Angew.
Chem., Int. Ed. 2005, 44, 5370–5373. (d) Lacroix, J. C.; Chane-Ching, K. I.;
Maque´re, F.; Maurel, F. J. Am. Chem. Soc. 2006, 128, 7264–7276. (e)
Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Bredas, J.-L.; Stuhr-Hansen,
N.; Hedegard, P.; Bjornholm, T. Nature 2003, 425, 698–701.
(2) (a) Yee, G. T.; Miller, J. S. In Magnetism: Molecules to Materials V; Miller,
J. S., Drillon, M., Eds.; Wiley-VCH: Weinheim, Germany, 2005; pp 223-
260. (b) Chang, H.-C; Kitagawa, S. Angew. Chem., Int. Ed. 2002, 41, 130–
132. (c) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K Science 1996,
272, 704–5. (d) Matsushita, M. M.; Kawakami, H.; Sugawara, T.; Ogata,
M. Phys. ReV. B 2008, 77, 195208.
The 1e- oxidation at 213 K, when ring inversion was thermally
prohibited, afforded different spectral changes of two successive
processes (Figure S8). The first spectral change (0-0.4 equiv) was
characterized by a new absorption peak at 530 nm, which strongly
suggests the formation of Cu(II) in the o-isomer form. In the second
change (0.4-1.0 equiv), a new broad absorption peak at ∼700 nm
appeared, whose energy and molar extinction coefficient are
characteristic of ferrocenium LMCT. Thus, the oxidation can be
attributed to the ferrocene moiety in the i-isomer form. The
possibility of a second oxidation of the o-isomer can be excluded,
because the disappearance of the CT band at 530 nm was not
accompanied with ferrocenium LMCT emergence (Figure S7).
These assignments are supported by the EPR spectrum of the sample
oxidized with 1e- at 193 K, which shows a Cu(II) pattern with a
smaller spin density (0.38) than the spectrum at 293 K.
(3) (a) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Yu.; Luo, C.; Sakata,
Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617–6628. (b) Herrero,
C.; Lassale-Kaiser, B.; Leibl, W.; Rutherford, A. W.; Aukauloo, A. Coord.
Chem. ReV. 2008, 252, 456–468.
(4) (a) Pierpont, C. G. Coord. Chem. ReV. 2001, 216, 99. (b) Gu¨tlich, P.; Dei,
A. Angew. Chem., Int. Ed. 1997, 36, 2734.
(5) (a) Nishida, S.; Morita, Y.; Fukui, K.; Sato, K.; Shiomi, D.; Takui, T.;
Nakasuji, K. Angew. Chem., Int. Ed. 2005, 44, 7277–7280.
The above results suggest that the 1e--oxidation center is
reversed with the ring inversion, and the structural isomers (i- and
o-) directly relate to the valence isomer (Cu(I)-ferrocenium and
Cu(II)-ferrocene) in the divalent state. This means that the motions
of pyrimidine inversion can induce directional electron transfer from
copper to the ferrocene moiety (Figure 2a). This was experimentally
demonstrated by monitoring the i- to o- isomerization by elevating
the temperature to unlock the pyrimidine inversion.
(6) Wu, H.; Zhang, D.; Su, L.; Ohkubo, K.; Zhang, C.; Yin, S.; Mao, L.; Shuai,
Z.; Fukuzumi, S.; Zhu, D. J. Am. Chem. Soc. 2007, 129, 6839–6846.
(7) (a) Balzani, V.; Credi, A.; Venturi, M. Molecular DeVices and Machines,
2nd ed.; Wiley-VCH: Weinheim, 2008. (b) Stoddart, J. F. Chem. Soc. ReV.
2009, 38, 1802–1820. (c) Bonnet, S.; Collin, J.-P.; Koizumi, M.; Pierre,
M.; Sauvage, J.-P. AdV. Mater. 2006, 18, 1239–1250. (d) Vives, G.; Carella,
A.; Launay, J.-P.; Rapenne, G. Coord. Chem. ReV. 2008, 252, 1451–1459.
(8) Nomoto, K.; Kume, S.; Nishihara, H. J. Am. Chem. Soc. 2009, 131,
3830–3831.
JA906684G
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14199