Metal ion-promoted intramolecular electron transfer in a ferrocene-naphthoquinone linked dyad. Continuous change in driving force and reorganization energy with metal ion concentration

Article abstract of DOI:10.1021/ja034831r

Thermal intramolecular electron transfer from the ferrocene (Fc) to naphthoquinone (NQ) moiety occurs efficiently by the addition of metal triflates (Mⁿ⁺: Sc(OTf)₃, Y(OTf)₃, Eu(OTf)₃) to an acetonitrile solution of a ferrocene-naphthoquinone (Fc-NQ) linked dyad with a flexible methylene and an amide spacer, although no electron transfer takes place in the absence of Mⁿ⁺. The resulting semiquinone radical anion (NQ^.-) is stabilized by the strong binding of Mⁿ⁺ with one carbonyl oxygen of NQ^.- as well as hydrogen bonding between the amide proton and the other carbonyl oxygen of NQ^.-. The high stability of the Fc⁺-NQ^.-/Mⁿ⁺ complex allows us to determine the driving force of electron transfer by the conventional electrochemical method. The one-electron reduction potential of the NQ moiety of Fc-NQ is shifted to a positive direction with increasing concentration of Mⁿ⁺, obeying the Nernst equation, whereas the one-electron oxidation potential of the Fc moiety remains the same. The driving force dependence of the observed rate constant (k_ET) of Mⁿ⁺-promoted intramolecular electron transfer is well evaluated in light of the Marcus theory of electron transfer. The driving force of electron transfer increases with increasing concentration of Mⁿ⁺ [Mⁿ⁺], whereas the reorganization energy of electron transfer decreases with increasing [Mⁿ⁺] from a large value which results from the strong binding between NQ^.- and Mⁿ⁺.

Full text of DOI:10.1021/ja034831r

Published on Web 05/15/2003
Metal Ion-Promoted Intramolecular Electron Transfer in a
Ferrocene-Naphthoquinone Linked Dyad. Continuous Change
in Driving Force and Reorganization Energy with Metal Ion
Concentration
Ken Okamoto,† Hiroshi Imahori,*,‡ and Shunichi Fukuzumi*,†
Contribution from the Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation, Suita,
Osaka 565-0871, Japan, Department of Molecular Engineering, Graduate School of
Engineering, Kyoto UniVersity, PRESTO, Japan Science and Technology Corporation (JST),
Sakyo-ku, Kyoto 606-8501, Japan, and Fukui Institute for Fundamental Chemistry,
Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan
Received February 24, 2003; E-mail: fukuzumi@ap.chem.eng.osaka-u.ac.jp; imahori@sci.kyoto-u.ac.jp
Abstract: Thermal intramolecular electron transfer from the ferrocene (Fc) to naphthoquinone (NQ) moiety
occurs efficiently by the addition of metal triflates (Mn+: Sc(OTf)3, Y(OTf)3, Eu(OTf)3) to an acetonitrile
solution of a ferrocene-naphthoquinone (Fc-NQ) linked dyad with a flexible methylene and an amide spacer,
although no electron transfer takes place in the absence of Mn+. The resulting semiquinone radical anion
(NQ•-) is stabilized by the strong binding of Mn+ with one carbonyl oxygen of NQ•- as well as hydrogen
bonding between the amide proton and the other carbonyl oxygen of NQ•-. The high stability of the Fc+-
NQ•-/Mn+ complex allows us to determine the driving force of electron transfer by the conventional
electrochemical method. The one-electron reduction potential of the NQ moiety of Fc-NQ is shifted to a
positive direction with increasing concentration of Mn+, obeying the Nernst equation, whereas the one-
electron oxidation potential of the Fc moiety remains the same. The driving force dependence of the observed
rate constant (kET) of Mn+-promoted intramolecular electron transfer is well evaluated in light of the Marcus
theory of electron transfer. The driving force of electron transfer increases with increasing concentration of
Mn+ [Mn+], whereas the reorganization energy of electron transfer decreases with increasing [Mn+] from a
large value which results from the strong binding between NQ•- and Mn+
.
Introduction
donor-acceptor linked systems with inert rigid spacers have
also been developed to study the intramolecular electron-transfer
reactions between the donor (D) and acceptor (A) molecules at
a fixed distance.7-15 The electron-transfer dynamics between
D and A at a fixed distance are now well understood in light of
Electron transfer plays a pivotal role not only in chemical
processes but also in biological redox processes that have
tremendous relevance to our life such as photosynthesis and
respiration.1-4 To understand factors to control important
electron-transfer processes in biological systems, electron-
transfer dynamics between donor and acceptor molecules bound
to proteins have been studied extensively.5,6 A number of
(6) (a) Moser, C. C.; Keske, J. M.; Warncke, K.; Farid, R. S.; Dutton, P. L.
Nature 1992, 355, 796. (b) Kirmaier, C.; Holton, D. In The Photosynthetic
Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San
Diego, 1993; Vol. II, pp 49-70. (c) Langen, R.; Chang, I.-J.; Germanas,
J. P.; Richards, J. H.; Winkler, J. R.; Gray, H. B. Science 1995, 268, 1733.
(d) Page, C. C.; Moser, C. C.; Chen, X.; Dutton, P. L. Nature 1999, 402,
47.
(7) (a) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000;
Vol. 8, pp 153-190. (b) Gust, D.; Moore, T. A.; Moore, A. L. In Electron
Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001;
Vol. 3, pp 272-336. (c) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed.
Engl. 1995, 34, 849. (d) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem.
Res. 2001, 34, 40.
† Osaka University.
‡ Kyoto University.
(1) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
2001; Vols. 1-5.
(2) (a) Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M.
T., Bauer, C. E., Eds.; Kluwer Academic Publishing: Dordrecht, 1995.
(b) Armstrong, F. A.; Kaim, W.; Schwederski, B. Bioinorganic Chemis-
try: Inorganic Chemistry in the Chemistry of Life; Oxford University: U.K.,
1995.
(8) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Imahori, H.; Sakata,
Y. Eur. J. Org. Chem. 1999, 2445, 5. (c) Fukuzumi, S.; Imahori, H. In
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
2001; Vol. 2, pp 927-975.
(3) (a) Kochi, J. K. Acc. Chem. Res. 1992, 25, 39. (b) Rathore, R.; Kochi, J.
K. AdV. Phys. Org. Chem. 2000, 35, 193.
(4) (a) Fukuzumi, S. In AdVances in Electron-Transfer Chemistry; Mariano,
P. S., Ed.; JAI Press: Greenwich, CT, 1992; pp 67-175. (b) Fukuzumi,
S.; Tanaka, T. In Photoinduced Electron Transfer; Fox, M. A., Chanon,
M., Eds.; Elsevier: Amsterdam, 1988; Part C, Chapter 10.
(5) (a) Winkler, J. R.; Gray, H. B. Chem. ReV. 1992, 92, 369. (b) McLendon,
G.; Hake, R. Chem. ReV. 1992, 92, 481. (c) Isied, S. S.; Ogawa, M. Y.;
Wishart, J. F. Chem. ReV. 1992, 92, 381. (d) Tollin, G. In Electron Transfer
in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 4, pp
202-231.
(9) (a) Wasielewski, M. R. In Photoinduced Electron Transfer; Fox, M. A.,
Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, pp 161-206. (b)
Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
(10) (a) Paddon-Row, M. N. In Electron Transfer in Chemistry; Balzani, V.,
Ed.; Wiley-VCH: Weinheim, 2001; Vol. 3, pp 179-271. (b) Jordan, K.
D.; Paddon-Row, M. N. Chem. ReV. 1992, 92, 395. (c) Paddon-Row, M.
N. Acc. Chem. Res. 1994, 27, 18.
(11) Verhoeven, J. W. AdV. Chem. Phys. 1999, 106, 603.
9
7014
J. AM. CHEM. SOC. 2003, 125, 7014-7021
10.1021/ja034831r CCC: $25.00 © 2003 American Chemical Society

Metal Ion-Promoted Intramolecular Electron Transfer
A R T I C L E S
the Marcus theory of electron transfer.16 Once the driving force
(-∆GET0) and reorganization energy (λDA) of electron transfer
between D and A are determined, the activation free energy of
electron transfer (∆GqET) is well predicted using the Marcus
equation (eq 1).16
Scheme 1
2
∆GqET ) (λDA/4)(1 + ∆GET0/λDA
)
(1)
The driving force of electron transfer is obtained from the
one-electron oxidation potential of electron donor (Eox0) and
systems, a variety of metal ion enzymes are also involved to
finely control electron-transfer processes.1 In the case of metal
ion-promoted electron-transfer reactions between D and A, the
driving force of electron transfer is altered by the addition of
metal ions which can interact with A•-. However, the instability
of the metal ion complexes with A•- has precluded the
determination of the one-electron reduction potentials of A in
the presence of metal ions. As such, the driving force depen-
dence of rates of metal ion-promoted electron-transfer reactions
has yet to be examined. Moreover, the reorganization energy
of electron transfer is also expected to be altered in the presence
of metal ions, because the binding of metal ions associated with
electron transfer certainly requires a much larger reorganization
energy than the value without metal ion. Under such circum-
stances, the applicability of the Marcus equation (eq 1) to metal
ion-promoted electron-transfer reactions has remained unex-
plored as compared to well-established electron-transfer pro-
cesses involving only D and A.
We report herein the driving force dependence of metal ion-
promoted intramolecular electron transfer. The results are
evaluated in light of the Marcus theory of electron transfer. The
D-A system employed in this study is a ferrocene-naphtho-
quinone dyad (Fc-NQ) in which the metal ion-promoted thermal
intramolecular electron-transfer reaction proceeds efficiently
(Scheme 1). The metal ion complexes with NQ•- moiety of
Fc-NQ are stable enough to determine the one-electron reduction
potentials of Fc-NQ in the presence of metal ion. This enables
us to determine the driving force of metal ion-promoted electron
transfer and hence to examine the driving force dependence of
the electron-transfer dynamics in light of the Marcus theory of
electron transfer.
0
the one-electron reduction potential of electron acceptor (Ered
)
using eq 2, where F is the Faraday constant. The reorganization
energy
0
∆GET0 ) F(Eox0 - Ered
)
(2)
of electron transfer between D and A (λDA) is obtained as the
average of the reorganization energy for the electron self-
exchange between D and D•+ (λD) and that between A and A•-
(λA): eq 3.16 Thus, knowledge of the fundamental redox
properties of D (Eox0 and λD) and A (Ered0 and λA) is sufficient
λDA ) (λD + λA)/2
(3)
to predict the ∆GqET values using eq 1.16,17 In other words, the
electron-transfer reactivity is automatically determined once the
combination of D and A is fixed. However, it has recently been
demonstrated that the scope of electron-transfer reactivity can
be expanded much further by introducing a third component
such as a metal ion to the D-A system provided that the metal
ion can interact with one of the products of electron transfer,
for example, the radical anion (A•-).18,19 Although photoexci-
tation of the D or A moiety or radiolysis is normally required
to start the electron transfer of the D-A linked system, thermal
intramolecular electron-transfer reactions of D-A linked sys-
tems, which would otherwise never occur, can be started by
the addition of the appropriate metal ions.20,21 There have also
been a number of examples of metal ion-promoted intermo-
lecular electron-transfer reactions.22-26 In biological redox
(12) (a) Chambron, J. C.; Chardon-Noblat, S.; Harriman, A.; Heitz, V.; Sauvage,
J.-P. Pure Appl. Chem. 1993, 65, 2343. (b) Harriman, A.; Sauvage, J.-P.
Chem. Soc. ReV. 1996, 25, 41. (c) Blanco, M.-J.; Consuelo Jime´nez, M.;
Chambron, J.-C.; Heitz, V.; Linke, M.; Sauvage, J.-P. Chem. Soc. ReV.
1999, 28, 293.
Experimental Section
General. 1H NMR spectra were measured on a JEOL JNM-AL300
NMR spectrometer. Fast atom bombardment mass spectra (FAB-MS)
were obtained on a JEOL JMS-DX300 mass spectrometer. Melting
points were recorded on a Yanagimoto micro-melting point apparatus
and not corrected. IR spectra were measured on a Shimadzu FT-IR
8200 PC as KBr disks. Elemental analyses were performed on a Perkin-
Elmer model 240C elemental analyzer.
(13) (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759. (b) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi,
M. A. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, 2001; Vol. 3, pp 337-408.
(14) (a) Maruyama, K.; Osuka, A.; Mataga, N. Pure Appl. Chem. 1994, 66,
867. (b) Osuka, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69,
797. (c) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143.
(15) (a) Guldi, D. M. Chem. Commun. 2000, 321. (b) Guldi, D. M.; Prato, M.
Acc. Chem. Res. 2000, 33, 695.
Materials. All solvents and chemicals were of reagent grade quality,
obtained commercially, and used without further purification unless
otherwise noted. Tris(2,2′-bipyridyl)ruthenium(III) hexafluorophosphate
[Ru(bpy)3(PF6)3] was prepared according to the literature.27 Scandium
triflate [Sc(OTf)3 (99%, FW ) 492.16)] was purchased from Pacific
(16) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus, R.
A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (c) Marcus, R. A.; Sutin,
N. Biochim. Biophys. Acta 1985, 811, 265.
(17) (a) Eberson, L. AdV. Phys. Org. Chem. 1982, 18, 79. (b) Eberson, L.
Electron-Transfer Reactions in Organic Chemistry; ReactiVity and Struc-
ture; Springler: Heidelberg, 1987; Vol. 25.
(18) Fukuzumi, S. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 4, pp 3-67.
(19) (a) Fukuzumi, S. Org. Biomol. 2003, 1, 609. (b) Fukuzumi, S.; Itoh, S. In
AdVances in Photochemistry; Neckers, D. C., Volman, D. H., von Bu¨nau,
G., Eds.; Wiley: New York, 1998; Vol. 25, pp 107-172.
(20) Fukuzumi, S.; Okamoto, K.; Imahori, H. Angew. Chem., Int. Ed. 2002, 41,
620.
(24) (a) Fukuzumi, S.; Okamoto, T.; Otera, J. J. Am. Chem. Soc. 1994, 116,
5503. (b) Fukuzumi, S.; Satoh, N.; Okamoto, T.; Yasui, K.; Suenobu, T.;
Seko, Y.; Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 7756.
(25) (a) Fukuzumi, S.; Mori, H.; Imahori, H.; Suenobu, T.; Araki, Y.; Ito, O.;
Kadish, K. M. J. Am. Chem. Soc. 2001, 123, 12458. (b) Fukuzumi, S.;
Fujii, Y.; Suenobu, T. J. Am. Chem. Soc. 2001, 123, 10191.
(21) Fukuzumi, S.; Okamoto, K.; Yoshida, Y.; Imahori, H.; Araki, Y.; Ito, O.
J. Am. Chem. Soc. 2003, 125, 1007.
(26) (a) Fukuzumi, S.; Ohkubo, K.; Okamoto, T. J. Am. Chem. Soc. 2002, 124,
14147. (b) Fukuzumi, S.; Inada, O.; Satoh, N.; Suenobu, T.; Imahori, H. J.
Am. Chem. Soc. 2002, 124, 9181. (c) Fukuzumi, S.; Yuasa, J.; Suenobu,
T. J. Am. Chem. Soc. 2002, 124, 12566.
(22) (a) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305. (b) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Chem.
Soc., Perkin Trans. 2 1985, 371.
(23) Fukuzumi, S.; Ohkubo, K. Chem.-Eur. J. 2000, 6, 4532.
(27) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7015

A R T I C L E S
Okamoto et al.
1
Scheme 2
Fc-NQ as a brown solid (55 mg, 0.10 mmol, 32%). H NMR (300
MHz, CDCl3): δ 8.11 (ddd, 2H, J ) 2, 2, 7 Hz), 7.74 (ddd, 2H, J )
2, 7, 7 Hz), 7.45 (m, 4H), 7.42 (s, 1H), 6.16 (s, 1H), 4.60 (t, 2H, J )
2 Hz), 4.29 (t, 2H, J ) 2 Hz), 4.06 (t, 2H, J ) 7 Hz), 4.03 (s, 5H),
2.44 (t, 2H, J ) 7 Hz), 1.97 (q, 2H, J ) 7 Hz), 1.90 (q, 2H, J ) 7 Hz),
1.64 (q, 2H, J ) 7 Hz). 13C NMR (300 MHz, CDCl3): δ 184.9 (NQ,
CdO), 171.1 (NQ, CdO), 159.7 (amide, CdO), 136.0 (Ph), 135.2
(NQ), 134.4 (NQ), 133.3 (NQ), 132.1 (NQ), 131.3 (NQ), 126.7 (Fc),
126.5 (Fc), 126.2 (Fc), 119.8 (Ph), 110.3 (NQ), 66.2 (-CH2-O), 37.5
(OdC-CH2), 27.6 (-CH2-), 26.0 (-CH2-), 25.2 (-CH2-). FAB-
MS m/z 547. Anal. Calcd for C32H29FeNO4: C, 70.21; H, 5.34; N,
2.56. Found: C, 69.81; H, 5.41; N, 2.44. FT-IR (KBr): 1700, 1680,
1650, 1610, 1580 cm-1. mp 130.0-132.0 °C.
Spectral Measurements. The formation of the Fc+-NQ•-/Sc3+
complex was examined from the change in the UV-visible spectrum
of an MeCN solution of Fc-NQ in the presence of Mn+ by using a
Hewlett-Packard 8453 diode array spectrophotometer with a quartz
cuvette (path length ) 10 mm) at 298 K.
Metals Co., Ltd. (Taiheiyo Kinzoku). Yttrium triflate [Y(OTf)3] and
europium triflate [Eu(OTf)3] were prepared by the literature method.28
Metal triflates were dried under vacuum evacuation at 403 K for 40 h
prior to use. Magnesium perchlorate [Mg(ClO4)2] and barium perchlo-
rate [Ba(ClO4)2] were obtained from Wako Pure Chemical Ind. Ltd.,
Japan. Acetonitrile (MeCN) used as a solvent was purified and dried
by the standard procedure.29 Chloroform-d was obtained from EURI
SO-TOP, CEA, France. Thin-layer chromatography (TLC) and flash
column chromatography were performed with Art. 5554 DC-Alufolien
Kieselgel 60 F254 (Merck), and Fujisilica BW300, respectively.
Synthesis. The synthetic route to Fc-NQ is summarized in Scheme
2. Ferrocene derivatives 1 and 2 were prepared according to the
literature.30
Kinetic Measurements. Kinetics measurements of intramolecular
ET reactions from the ferrocene moiety to naphthoquinone moiety in
donor-acceptor systems in the presence of Mn+ were performed on a
Hewlett-Packard 8453 diode array spectrophotometer when the rates
were slow enough to be determined accurately. When the rates were
too fast (<10 s) to be followed by the photodiode array spectropho-
tometer, kinetics measurements were performed on a UNISOKU RSP-
601 stopped-flow spectrophotometer with the MOS-type high selective
photodiode array at various temperature (243-298 K) using a Unisoku
thermostated cell holder designed for low-temperature experiments.
Electrochemical Measurements. Electrochemical measurements
were performed on a BAS 100 W electrochemical analyzer in deaerated
MeCN containing 0.1 M Bu4NPF6 (TBAPF6) as supporting electrolyte
at 298 K. A conventional three-electrode cell was used with a platinum
working electrode (surface area of 0.3 mm2), and a platinum wire was
used as the counter electrode. The Pt working electrode (BAS) was
routinely polished with a BAS polishing alumina suspension and rinsed
with acetone before use. The measured potentials were recorded with
respect to the Ag/AgNO3 (0.01 M) reference electrode. The second-
harmonic alternating current voltammetry (SHACV) measurements of
Fc-NQ in the presence of Y(OTf)3 were carried out with a BAS 100B
electrochemical analyzer in deaerated MeCN containing 0.1 M Bu4-
NPF6 (TBAPF6) as supporting electrolyte.31 All potentials (vs Ag/Ag+)
were converted to values versus SCE by adding 0.29 V.32 All
electrochemical measurements were carried out under an atmospheric
pressure of Ar.
ESR Measurements. The ESR spectra of Fc-NQ•- and 2-methoxy-
1,4-naphthoquinone radical anion (2-MeONQ•-) were produced by the
chemical reduction with the naphthalene radical anion generated by
naphthalene (0.5 g) in THF with sodium (0.075 g). The concentration
of the naphthalene radical anion was determined by the appearance of
the absorption band at the characteristic peak of the semiquinone radical
anion (ꢀ (422 nm) ) 6.0 × 103 M-1 cm-1 in MeCN) produced by the
titration of the naphthalene radical anion to p-benzoquinone. The
solution containing the radical anion was transferred to an ESR tube
under an atmospheric pressure of Ar. The ESR spectra were recorded
on a JEOL X-band spectrometer (JES-RE1XE) with a quartz ESR tube
(1.2 mm i.d.). The ESR spectra were measured under nonsaturating
microwave power conditions. The magnitude of modulation was chosen
Fc-Br. A solution of THF containing 4-(dimethylamino)pyridine
(DMAP: 200 mg, 1.06 mmol), 1-hydroxybenzotriazole (HOBt: 160
mg, 1.19 mmol), and 6-bromohexanoic acid (800 mg, 4.10 mmol) was
treated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydro-
chloride (EDC: 400 mg, 2.08 mmol) under nitrogen at 0 °C. The
mixture was stirred for 1 h at 0 °C. 4-Aminophenylferrocene (2) (150
mg, 0.54 mmol) was added to the mixture and stirred at 0 °C for 30
min. The mixture was then stirred at room temperature overnight. The
mixture was concentrated in vacuo and poured into a 5% HCl aqueous
solution and extracted with chloroform (50 mL × 3). The crude product
was purified by flash chromatography (SiO2, EtOAc:CHCl3 ) 1:10, Rf
) 0.67). Subsequent recrystallization from chloroform and hexane gave
1
Fc-Br as an orange solid (200 mg, 0.44 mmol, 81%). H NMR (300
MHz, CDCl3): δ 7.43 (s, 4H), 7.08 (s, 1H), 4.60 (t, 2H, J ) 2 Hz),
4.29 (t, 2H, J ) 2 Hz), 4.03 (s, 5H), 3.43 (t, 2H, J ) 7 Hz), 2.39 (t,
2H, J ) 7 Hz), 1.92 (q, 2H, J ) 7 Hz), 1.79 (q, 2H, J ) 7 Hz), 1.64
(q, 2H, J ) 7 Hz). 13C NMR (300 MHz, CDCl3): δ 159.5 (amide,
CdO), 136.1 (Ph), 126.6 (Fc), 119.8 (Ph), 37.5 (OdC-CH2-), 33.7
(-CH2-Br), 32.4 (-CH2-), 27.8 (-CH2-), 24.8 (-CH2-). FAB-
MS m/z 455, 453. Anal. Calcd for C22H24BrFeNO: C, 58.18; H, 5.33;
N, 3.08; Br, 17.59. Found: C, 58.10; H, 5.21; N, 3.17; Br, 17.47. FT-
IR (KBr): 1700, 1650, 1620, 1580 cm-1. mp 135.5-137.5 °C.
Fc-NQ. A solution of DMF containing 2-hydroxy-1,4-naphtho-
quinone (900 mg, 5.17 mmol), Fc-Br (150 mg, 0.33 mmol), and
dehydrated potassium carbonate (600 mg, 4.35 mmol) was stirred under
nitrogen at room temperature for 10 min. The reaction mixture was
then refluxed for 30 min. The mixture was evacuated in vacuo. The
crude product was purified by flash chromatography (SiO2, EtOAc:
CHCl3 ) 1:10, Rf ) 0.27). The brown fraction was concentrated in
vacuo. Subsequent recrystallization from chloroform and hexane gave
(31) The SHACV method provides a superior approach to directly evaluating
the one-electron redox potentials in the presence of a follow-up chemical
and reaction, relative to the better-known dc and fundamental harmonic ac
methods. See: (a) McCord, T. G.; Smith, D. E. Anal. Chem. 1969, 41,
1423. (b) Bond, A. M.; Smith, D. E. Anal. Chem. 1974, 46, 1946. (c)
Wasielewski, M. R.; Breslow, R. J. Am. Chem. Soc. 1976, 98, 4222. (d)
Arnett, E. M.; Amarnath, K.; Harvey, N. G.; Cheng, J. J. Am. Chem. Soc.
1990, 112, 344. (e) Patz, M.; Mayr, H.; Maruta, J.; Fukuzumi, S. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1225.
(28) (a) Forsberg, J. H.; Spaziano, V. T.; Balasubramanian, T. M.; Liu, G. K.;
Kinsley, S. A.; Duckworth, C. A.; Poteruca, J. J.; Brown, P. S.; Miller, J.
L. J. Org. Chem. 1987, 52, 1017. (b) Kobayashi, S.; Hachiya, I. J. Org.
Chem. 1994, 59, 3590.
(29) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
(30) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607.
(32) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous
Systems; Marcel Dekker: New York, 1990.
9
7016 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

Metal Ion-Promoted Intramolecular Electron Transfer
A R T I C L E S
Figure 1. UV-vis spectral change after the addition of Sc3+ (1.0 × 10-3
M) into a deaerated MeCN containing Fc-NQ (2.0 × 10-4 M) at 298 K.
Inset: Time course of the absorption change at 420 and 800 nm due to
formation of the Fc+-NQ•-/Sc3+ complex.
Figure 2. Plots of the rate constant (kET) determined at 420 nm versus
concentration of Mn+ for Mn+-promoted intramolecular ET of Fc-NQ (2.0
× 10-4 M) in deaerated MeCN at 298 K.
to optimize the resolution and the signal-to-noise (S/N) ratio of the
observed spectra. The g values were calibrated with an Mn2+ marker,
and the hyperfine coupling (hfc) constants were determined by computer
simulation using the Calleo ESR version 1.2 program coded by Calleo
Scientific on an Apple Macintosh personal computer.
Table 1. First-Order Rate Constants (kET), Driving Forces
(-∆GET), Reorganization Energies (λ), and Activation Free
Energies (∆GqET) of Metal Ion-Promoted Intramolecular Electron
Transfer in Fc-NQ with Various Metal Ion Concentrations [Mn+
]
in Deaerated MeCN at 298 K
Theoretical Calculations. Density functional (DFT) calculations
were performed with a basis function: double-ú Slater-type orbital set
(frozen core: C (1s), N (1s), O (1s); ADF basis set II (large)) on a
COMPAQ DS20E computer using the Amsterdam density functional
(ADF) program version 1999.02 developed by Baerends et al.33 The
gradient corrections of Becke88 (exchange) and Perdew86 (correlation)
were included in the exchange-correlation functional. The hfc values
are derived from multiplying 506.82 and the spin density values of
each hydrogen.
q
ET
n+
n+
a
b
M
[M ], M
kET, s-1
−∆G , eV
λ, eV
∆G , eV
ET
Sc3+
5.0 × 10-4
1.0 × 10-3
2.0 × 10-3
3.0 × 10-3
5.0 × 10-3
1.0 × 10-2
1.0 × 10-2
2.0 × 10-2
0.76
1.7
3.7
6.2
9.3
16
24
31
0.810
0.828
0.846
0.856
0.869
0.887
0.897
0.905
4.53
4.46
4.42
4.38
4.36
4.33
4.31
4.29
0.764
0.739
0.722
0.709
0.699
0.684
0.676
0.668
Y3+
5.0 × 10-3
1.0 × 10-2
2.0 × 10-2
0.025
0.043
0.060
0.506
0.523
0.541
4.36
4.34
4.33
0.852
0.839
0.829
Results and Discussion
Metal Ion-Promoted Intramolecular Electron Transfer.
No electron transfer from the ferrocene (Fc) to naphthoquinone
(NQ) moiety occurs in Fc-NQ with a flexible methylene spacer
including an amide linkage thermally in MeCN at 298 K, as
expected from the highly positive value of free energy change
Eu3+
3.0 × 10-3
2.5 × 10-2
5.0 × 10-2
0.0010
0.0080
0.015
0.403
0.457
0.475
4.51
4.40
4.36
0.935
0.883
0.865
a Determined from the equation ∆GET (Mn+) ) 1.19-0.059 log(Kred
-
[Mn+]). b Determined from eq 8.
0
of electron transfer (∆GET ) 1.19 eV). However, addition of
scandium triflate (Sc(OTf)3: 1.0 × 10-3 M) to an MeCN
solution of Fc-NQ (2.0 × 10-3 M) results in the formation of
Fc+ as indicated by the appearance of the absorption band due
to Fc+ at 860 nm34 with the absorption band at λmax ) 420 nm
(Scheme 1, Figure 1). This indicates that an intramolecular
electron transfer occurs in the presence of Sc3+ to produce Fc+-
NQ•-/Sc3+ in which Sc3+ is bound to NQ•- and that the
absorption band at 420 nm is due to the NQ•-/Sc3+ complex.
The rates of appearance of the absorption band at 420 and 800
nm obeyed first-order kinetics, and the same first-order rate
constants (kET) were obtained from the first-order plots at 420
and 800 nm. The kET values determined at 420 nm increase
linearly with increasing [Sc3+], [Y3+], and [Eu3+], respectively
(Figure 2). The first-order dependence of kET on [Mn+] is
consistent with the complexation of one Sc3+ with the NQ•-
moiety in Fc+-NQ•-. The binding strength of NQ•- with Mn+
results in a slower electron-transfer rate, and by much weaker
Lewis acids such as Ba2+, Ca2+, and Mg2+ results in no
occurrence of electron transfer from Fc to NQ in Fc-NQ. The
k
ET values determined in the presence of various concentrations
of Mn+ are summarized in Table 1.
The ESR spectra were measured upon addition of Y(OTf)3
or Sc(OTf)3 (2.0 × 10-2 M) to an MeCN solution of Fc-NQ
(9.1 × 10-3 M) to confirm the formation of Fc+-NQ•-/Y3+ and
Fc+-NQ•-/Sc3+ complexes at 298 K. The observed ESR spectra
are shown in Figure 3a (Fc+-NQ•-/Y3+) and Figure 3b (Fc+-
NQ•-/Sc3+). The g values are determined as 2.0037 (Fc+-NQ•-/
Y
3+) and 2.0038 (Fc+-NQ•-/Sc3+). The hyperfine structures are
quite different depending on the type of metal ion. This indicates
the binding of metal ions with the NQ•- moiety of Fc+-NQ•-.
The hyperfine coupling (hfc) constants are determined by the
computer simulation spectra (see Supporting Information S1).
In contrast to the instability of the metal ion complexes of radical
anions of p-benzoquinone derivatives,26a the Fc+-NQ•-/Mn+
complex particularly in the case of the Sc3+ complex is very
stable even at 298 K.36 Such a remarkable stability of the Fc+-
NQ•-/Mn+ complex may be ascribed to the hydrogen bonding
of the amide proton with one carbonyl oxygen of the semi-
is expected to vary depending on the Lewis acidity of Mn+ 23,35
.
The replacement of Sc3+ by weaker acids such as Y3+ and Eu3+
(33) (a) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41. (b) te
Valde, B.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.
(34) The absorption band at 860 nm due to Fc+ in Fc-NQ was confirmed by
electron-transfer oxidation by Fc-NQ with Ru(bpy)3(PF6)3.
(35) Fukuzumi, S.; Ohkubo, K. J. Am. Chem. Soc. 2002, 124, 10270.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7017

A R T I C L E S
Okamoto et al.
Figure 3. (a) ESR spectrum of Fc-NQ (9.1 × 10-3 M) in the presence of
Y3+ (2.0 × 10-2 M) in deaerated MeCN at 298 K. (b) ESR spectrum of
Fc-NQ (9.1 × 10-3 M) in the presence of Sc3+ (2.0 × 10-2 M) in deaerated
MeCN at 298 K.
Figure 5. Optimized structures of (a) Ph-NQ•- and (b) 2-MeONQ•-
obtained by ADF calculation with the II (large) basis set.
Ph-NQ•- in which the Fc moiety is omitted as indicated in
Figure 4 (the values in parentheses).37 The optimized structures
of Ph-NQ•- and 2-MeONQ•- obtained by the Amsterdam
density function (ADF) calculation with the II (large) basis set
are shown in Figure 5. The O-H distance between the carbonyl
oxygen of NQ•- and the amide proton in Ph-NQ•- is 1.62 Å.
This value is even shorter than the hydrogen bonding distance
between the semiquinone radical anion and water (1.78 Å).38
The radical anion (2-MeONQ•-) without hydrogen bonding
is also produced by the electron-transfer reduction of 2-MeONQ
by the naphthalene radical anion as shown in Figure 4c. The
simulated hfc values can also be well reproduced by the ADF
calculation as indicated in Figure 4d (the values in parentheses),
and they are quite different from those observed in Fc-NQ•-
with hydrogen bonding.
Figure 4. (a) ESR spectrum of Fc-NQ•- (4.0 × 10-3 M) in deaerated
MeCN at 298 K and (b) the computer simulation spectrum (the values in
parentheses are provided by ADF calculation). (c) ESR spectrum of
2-MeONQ•- (5.0 × 10-5 M) in deaerated MeCN at 298 K and (d) the
computer simulation spectrum (the values in parentheses are provided by
ADF calculation).
Change in the One-Electron Reduction Potential of Fc-
NQ in the Presence of Metal Ions. The remarkable stability
of Fc+-NQ•-/Mn+ complexes mentioned above allows us to
determine the one-electron reduction potentials of Fc-NQ (Ered
)
quinone radical anion. The presence of the hydrogen bond in
the radical anion of Fc-NQ was confirmed by examining the
spin density distribution of Fc-NQ•- and the radical anion of
2-methoxy-1,4-naphthoquinone (2-MeONQ•-) as follows.
Fc-NQ•- is produced by the electron-transfer reduction of
Fc-NQ by naphthalene radical anion (Scheme 3). The ESR
spectrum of Fc-NQ•- thus produced is shown in Figure 4a
together with the computer simulation spectrum (Figure 4b).
The g value of Fc-NQ•- in Figure 4a is determined as 2.0060,
which is significantly larger than those of the metal ion
complexes in Figure 3. This indicates that more spin is localized
on the oxygen atom which has a large spin-orbit coupling
constant in the case of Fc-NQ•- as compared to those of the
metal ion complexes. The simulated hyperfine coupling con-
stants (hfc) can be well reproduced by the ADF calculation of
in the presence of various concentrations of metal ions by cyclic
voltammetry measurements as shown in Figure 6. The one-
electron reduction potential of the NQ moiety is observed as a
well-defined reversible wave at -0.81 V (Figure 6a).39 In the
presence of 7.0 × 10-3 M Sc3+, the Ered value exhibits a
remarkable positive shift from -0.81 to 1.26 V,40 whereas the
one-electron oxidation potential of the Fc moiety remains the
same irrespective of the absence or presence of Sc3+ (Figure
6b). The cyclic voltammograms of Fc-NQ in the presence of
various concentrations of Sc3+ are shown in the Supporting
(37) The hfc values were also calculated using the B3LYP method. The ADF
calculation predicts the hfc values in closer agreement with the experimental
results.
(38) O’Malley, P. J. Phys. Chem. A 1997, 101, 6334.
(39) The one-electron oxidation potential for the Fc+/Fc couple in Fc-NQ agrees
0
with that of ferrocene (Eox vs SCE ) 0.37 V), see: Fukuzumi, S.;
Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28, 2459.
(40) The anodic peak current for the oxidation of the Fc-NQ•-/Sc3+ complex is
smaller than the cathodic peak current for the reduction of Fc-NQ in the
presence of Sc3+ because of the instability of the Fc-NQ•-/Sc3+ complex.
A similar trend is observed for cyclic voltammograms of Fc-NQ in the
presence of other metal ions.
(36) The semiquinone radical anion complexes with metal ions have been only
observed only at low temperature by ESR.26a In contrast, the Fc+-NQ•-
/
Sc3+ complex has a long lifetime (>3 h) at 298 K. The Fc+-Q•-/Sc3+
complex reported in ref 21 is also too unstable to detect the ESR spectrum
at 298 K because of the fast disproportionation of the Q•-/Sc3+ complex.
9
7018 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

Metal Ion-Promoted Intramolecular Electron Transfer
A R T I C L E S
Scheme 3
Figure 7. Plots of ∆E1/2 versus log[Mn+] for the one-electron reduction of
Fc-NQ in the presence of Sc3+ (b), Y3+ (9), and Eu3+ (2) in MeCN at
198 K. The plot of each [Mn+] affords the same slope of 0.059.
The plots of ∆Ered versus log[Mn+] (Mn+ ) Sc3+, Y3+, and
Eu3+) are shown in Figure 7.
The slope of each plot is determined as 0.059, which agrees
with the expected slope ()2.3RT/F at 298 K) by the Nernst
equation (eq 6). The intercepts of linear plots in Figure 7 thus
Figure 6. (a) Cyclic voltammogram of Fc-NQ (5.0 × 10-4 M) in deaerated
MeCN containing 0.1 M Bu4NPF6 at 298 K with sweep rate of 50 mV s-1
.
(b) Cyclic voltammogram of Fc-NQ (5.0 × 10-4 M) in the presence of
Sc3+ (7.0 × 10-3 M) in deaerated MeCN containing 0.1 M Bu4NPF6 at
298 K with sweep rate of 50 mV s-1. (c) Cyclic voltammogram of Fc-NQ
(5.0 × 10-4 M) in the presence of Eu3+ (7.0 × 10-3 M) in deaerated MeCN
afford the binding constants Kred(Sc3+) ) 1.6 × 1037 M-1, Kred
-
.
(Y3+) ) 1.1 × 1031 M-1, and Kred(Eu3+) ) 3.3 × 1029 M-1
The Kred values correspond to the free energy changes of the
metal ion binding: 2.20 eV (Sc3+), 1.83 eV (Y3+), and 1.74
eV (Eu3+).42 Such a large binding energy results in a remarkable
change in the driving force of electron transfer in Fc-NQ from
a highly negative value to a positive value in the presence of
n+. The driving force of electron transfer in the presence of
Mn+ (-∆GET) is given by eq 7
containing 0.1 M Bu4NPF6 at 298 K with sweep rate of 100 mV s-1
.
Information (S2). Similar positive shifts of Ered are observed in
the presence of Eu3+ (Figure 6c) and Y3+ (S3).
The positive shift of Ered in the presence of metal ion is
ascribed to the binding of metal ion with NQ•- (eq 4) as
indicated in Figure 3. In such a case, Ered is given as a function
M
Kred
{
-∆GET ) -∆GET0 + RT ln(Kred[Mn+])
(7)
Fc-NQ•- + Mn+
} Fc-NQ•-/Mn+
(4)
of the concentration of Mn+, in accordance with the Nernst
equation (eq 5), where Ered is the one-electron reduction
potential in the absence of metal ion, Kred is the formation
constant of the Fc-NQ•-/Mn+ complex, and Kox is the formation
constant of the Fc-NQ/Mn+ complex.41
where -∆GET0 is the driving force in the absence of Mn+. The
-∆GET values determined in the presence of various concentra-
tions of Mn+ are summarized in Table 1.
0
Driving Force Dependence of Metal Ion-Promoted Elec-
tron Transfer. Figure 8 shows the driving force dependence
of log kET of Mn+-promoted electron transfer, where the kET
Ered ) Ered0 + (2.3RT/F) ×
0
and -∆GET values are taken from Table 1. Three sepa-
rate linear correlations are obtained for the case of Sc3+, Y3+
,
log{(1 + Kred[Mn+])/(1 + Kox[Mn+])} (5)
and Eu3+ (Figure 8). The slope of each linear plot is deter-
mined as 16.9 (eV)-1, which corresponds to 1/2.3kBT at 298
K, where kB is the Boltzmann constant. This indicates that the
change in driving force with concentration of Mn+ is directly
reflected on the change in the activation free energy, that is,
Because Kred[Mn+] . 1, and Kox[Mn+] , 1, eq 5 is written by
eq 6, where ∆Ered is the potential shift in the presence of Mn+
∆Ered ) (2.3RT/F) log Kred[Mn+]
from the value in its absence.
(6)
∂(∆Gq )/∂(∆GET) ) 1. This is quite different from the slope
ET
expected from the Marcus equation (eq 1): ∂(∆Gq )/∂(∆GET
)
ET
(41) Meites, L. Polarographic Techniques, 2nd ed.; Wiley: New York, 1965;
pp 203-301.
(42) Binding free energy changes are calculated as the follow equation: (binding
free energy changes) ) -RT ln Kred
.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7019

A R T I C L E S
Okamoto et al.
Figure 8. Plots of log kET versus -∆GET in Mn+-promoted intramolecular
electron transfer in Fc-NQ in the addition of Sc3+ (b), Y3+ (9), and Eu3+
(2) in deaerated MeCN at 298 K. The plot of kET on each [Mn+] gives a
straight line with a slope of 16.9. The broken lines (a), (b), and (c) represent
the fit to eq 9 with (a) λ ) 4.3 eV, (b) λ ) 4.4 eV, and (c) λ ) 4.5 eV.
Figure 9. Plots of -RT ln(kET[Mn+
]
-1/2) versus ∆GET in Mn+-promoted
intramolecular electron transfer in Fc-NQ in the addition of Sc3+ (b), Y3+
(9), and Eu3+ (2) in deaerated MeCN at 298 K. The plot of -RT
ln(kET[Mn+
]
-1/2) versus ∆GET on each [Mn+] gives a straight line with a
slope of 0.5.
Scheme 4
0
energy λDA is given by eq 11, where λDA ) (λD + λA0)/2.
Equation 11 indicates that the λDA value decreases with
increasing [Mn+] as observed in Figure 8.
λA ) λA0 - 4RT ln(Kox[Mn+])
λDA ) λDA0 - 2RT ln(Kox[Mn+])
(10)
(11)
Because λDA . -∆GET in Figure 8, eq 1 is simplified to eq
12 under the present experimental conditions. The driving force
dependence of kET is derived from eqs 8, 11, and 12 as eq 13
) 0.5. The driving force dependence of kET expected from the
Marcus equation is shown as broken lines in Figure 8, where
three different λDA values (4.3, 4.4, and 4.5 eV) are used for
the calculation using eq 1. The ∆GqET value is converted to the
∆GqET ) (λDA/4) + (∆GET/2)
(12)
(13)
corresponding kET value using eq 8, by assuming that the Mn+
-
-RT ln(kET/[Mn+]1/2) ) C + (∆GET/2)
kET ) (kBT/h) exp(-∆GqET/kBT)
(8)
where C is a constant which is independent of the concentration
of Mn+, given by eq 14.
promoted intramolecular electron transfer in Fc-NQ is adia-
batic.43 Hereby, h is the Planck constant. From the comparison
of the calculated Marcus lines with the observed lines, it is
apparent that the λDA value changes with the concentration of
C ) (λDA0/4) - RT ln[kBT(Kox)1/2/h]
(14)
A plot of -RT ln(kET/[Mn+ 1/2
]
) versus ∆GET including all of
M
n+. Such a change of λDA with [Mn+] can be evaluated within
the data in Table 1 is shown in Figure 9, where a single linear
correlation is obtained. The slope is determined as 0.5, which
agrees with expectation from eq 13.
the context of the Marcus theory of electron transfer (vide infra).
The λDA value consists of λD for the electron self-exchange
between D and D•+ and λA for that between A and A•- (eq 3).
Because the metal ion is involved only in the acceptor part, the
dependence of λA on [Mn+] should be considered. The electron
self-exchange between Fc-NQ and the Fc-NQ•-/Mn+ complex
occurs via formation of the Fc-NQ/Mn+ complex as shown in
Scheme 4. According to Scheme 4, the electron self-exchange
rate constant (kex) is given by eq 9, where Z is the frequency
factor for an intermolecular reaction, and λA0 is the reorganiza-
The single linear correlation in Figure 9 indicates that the C
value is constant irrespective of the type of metal ions. The
larger the binding between NQ•- and Mn+, the larger is the λDA
0
value and also the larger is the Kox value for the binding between
NQ and Mn+. In such a case, the effects of different metal ions
may be largely canceled in the C value in eq 14.
The large reorganization energy involved in the Mn+
-
promoted electron transfer was also confirmed by the temper-
ature dependence of kET. Eyring plots of Sc3+-promoted electron
transfer in Fc-NQ are shown in Figure 10. The activation
parameters obtained from the Eyring plots are listed in Table
2. The same activation enthalpy (∆Hqobs ) 52 kJ mol-1 ) 0.54
eV) is obtained irrespective of the difference in Sc3+ concentra-
kex ) ZKox[Mn+] exp(-λA /4kBT)
(9)
0
tion energy for the electron self-exchange between Fc-NQ/Mn+
and Fc-NQ•-/Mn+. The reorganization energy between Fc-NQ
and Fc-NQ•-/Mn+ (λA) is then given by eq 10, by comparing
eq 9 with kex ) Z exp(-λA/4kBT). From eq 3, the reorganization
tion (Table 2). The observed activation enthalpy (∆Hoqbs
)
consists of the activation enthalpies of the electron self-exchange
between the donor Fc and Fc+ (∆HDq ) and between the
acceptor NQ and NQ•- (∆HAq ): ∆Hoqbs ) (∆HDq + ∆HqA)/2.
(43) Because Fc-NQ has a long flexible methylene spacer, there may be
sufficient interaction between the Fc and NQ moieties to make the electron
transfer adiabatic.
9
7020 J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003

Metal Ion-Promoted Intramolecular Electron Transfer
A R T I C L E S
as compared to the NQ•-/Sc3+ complex with the same nuclear
configuration as the neutral NQ/Sc3+ complex.45 Because the
stabilization free energy is obtained as 2.20 eV, which is derived
from the Kred value for the NQ•-/Sc3+ complex (vide supra),
0
the λA value is estimated as 4.40 eV. This value is consistent
with the estimation from the ∆Hq value: λA > 3.44 eV.
0
A
In conclusion, metal ion-promoted intramolecular electron
transfer in Fc-NQ proceeds via strong binding of the NQ•-
moiety with metal ion, which results in a drastic change of the
free energy change of electron transfer from a highly positive
value to a highly negative value. Such a drastic change of driving
force of electron transfer is accompanied by a large reorganiza-
tion energy required for the metal ion-promoted electron transfer.
The driving force dependence of kET of the metal ion-promoted
intramolecular electron transfer can be well evaluated within
the context of the Marcus theory of electron transfer in which
the driving force increases with increasing concentrations of
metal ions, whereas the reorganization energy of electron
transfer decreases with concentrations of metal ions.
Figure 10. Eyring plots of ln(kETT-1) versus T-1 in Mn+-promoted
intramolecular electron transfer in Fc-NQ (2.0 × 10-4 M) in the presence
of Sc3+ [2.0 × 10-2 M (b), 1.0 × 10-2 M (9), and 5.0 × 10-3 M (2)] in
deaerated MeCN.
Table 2. Activation Parameters for Sc3+-Promoted Intramolecular
Electron Transfer in Fc-NQ with Various Concentrations of
Sc(OTf)3 in Deaerated MeCN at 298 K
Acknowledgment. This work was supported by the Academy
of Finland, the National Technology Agency of Finland, and
Grant-in-Aids (No. 11740352 to H.I., Nos. 11555230 and
11694079 to S.F.) and the Development of Innovative Technol-
ogy (No. 12310) from the Ministry of Education, Science,
Sports, and Culture, Japan. H.I. thanks the Nagase Foundation
for financial support.
q
q
q
a
[Sc3+], M
∆Hobs, kJmol-1
∆Sobs, JK-1mol-1
∆Gobs, eV
5.0 × 10-3
1.0 × 10-2
2.0 × 10-2
52
52
52
-53
-49
-43
0.701
0.686
0.667
a T ) 298 K.
Supporting Information Available: ESR simulation of Fc+-
NQ•-/Mn+ (S1), CVs of Fc-NQ in the presence of Sc3+ (S2),
and CV and SHACV of Fc-NQ in the presence of Y3+ (S3)
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Using this relation, we determined the ∆HqA value as 0.86 eV
from the ∆Hqobs value (0.54 eV) and the reported value of ∆HDq
(0.22 eV).44 Thus, the λA0 value for the electron self-exchange
0
between NQ/Sc3+ and NQ•-/Sc3+ is estimated as λA > 3.44
q
eV, because λA ) 4(∆HqA -T∆Sq) and ∆SA < 0. The
0
0
reorganization energy λA consists of twice the stabilization
JA034831R
energy of the NQ•-/Sc3+ complex with the optimized structure
(45) The stabilization energy of the NQ•-/Sc3+ complex is equal to the
destabilization of the NQ/Sc3+ complex with the same nuclear configuration
as the NQ•-/Sc3+ complex as compared to the NQ/Sc3+ complex with the
optimized structure.
(44) (a) Yang, E. S.; Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1980, 84, 3094.
(b) Fukuzumi, S.; Nakanishi, I.; Suenobu, T.; Kadish, K. M. J. Am. Chem.
Soc. 1999, 121, 3468.
9
J. AM. CHEM. SOC. VOL. 125, NO. 23, 2003 7021