Multielectron oxidation of anthracenes with one-electron oxidant via water-accelerated electron-transfer disproportionation of the radical cations as the rate-determining step

Article abstract of DOI:10.1021/jp990541e

The six-electron oxidation of anthracene and the four-electron oxidation of 9-alkylanthracene occurred with [Ru(bpy)₃]³⁺ (bpy = 2,2-bipyridine) in acetonitrile (MeCN) containing H₂O to yield anthraquinone and 10-alkyl-10-h

Full text of DOI:10.1021/jp990541e

11212
J. Phys. Chem. A 1999, 103, 11212-11220
Multielectron Oxidation of Anthracenes with a One-Electron Oxidant via
Water-Accelerated Electron-Transfer Disproportionation of the Radical Cations as the
Rate-Determining Step
Shunichi Fukuzumi,* Ikuo Nakanishi, and Keiko Tanaka
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
Suita, Osaka 565-0871, Japan
ReceiVed: February 15, 1999
The six-electron oxidation of anthracene and the four-electron oxidation of 9-alkylanthracene occur with
[Ru(bpy)3]3+ (bpy ) 2,2-bipyridine) in acetonitrile (MeCN) containing H2O to yield anthraquinone and 10-
alkyl-10-hydroxy-9(10H)anthracenone, respectively. The direct detection of radical cations of anthracene and
its derivatives formed in the multielectron oxidation with [Ru(bpy)3]3+ and the extensive kinetic analysis are
performed with the use of a stopped-flow technique. Both the rates of decay of anthracene radical cations
and the formation of [Ru(bpy)3]2+ obey the second-order kinetics. The kinetic deuterium isotope effects and
the dependence of the rates on the concentrations of [Ru(bpy)3]3+, anthracenes, and H2O have revealed that
the six-electron oxidation of anthracene and the four-electron oxidation of alkylanthracene proceed via the
rate-determining electron-transfer disproportionation of radical cations of anthracene and alkylanthracene,
which is accelerated by H2O due to the complex formation between the corresponding dications and H2O.
The electron-transfer disproportionation of anthracene radical cations is followed by the facile nucleophilic
attack of H2O on the resulting dication leading to six-electron oxidized product, i.e., anthraquinone associated
with rapid electron transfer from [Ru(bpy)3]3+ and anthracene radical cation in the presence of more than 6
equiv of [Ru(bpy)3]3+ and less than 1 equiv of [Ru(bpy)3]3+, respectively. The reorganization energy for the
self-exchange between 9,10-dimethylanthracene and the radical cation in MeCN is also determined by analyzing
line width variations of the ESR spectra at different concentrations of 9,10-dimethylanthracene. The
reorganization energy is used to evaluate the rate constant of electron-transfer disproportionation of 9,10-
dimethylanthracene radical cations in light of the Marcus theory of electron transfer, which agrees with the
experimental value determined from the second-order decay of the radical cations.
Introduction
the energy gap of both an electron transfer and a singlet-triplet
excitation is small.15 To gain more comprehensive and confir-
mative understanding of the reactions of radical cations with
nucleophiles, it is certainly desired to obtain more kinetic data
by detecting the transient radical cations directly to follow the
reactions. In this regard, nanosecond laser flash photolysis or
pulse radiolysis techniques have frequently been applied to
obtain a time-resolved spectroscopic evaluation of the kinetics
of the reactions of radical cations with nucleophiles.16-21
However, there have so far been no kinetic studies of radical
cation intermediates detected directly in multielectron oxidation
of aromatic compounds such as the six-electron oxidation of
anthracene to anthraquinone, although anodic six-electron
oxidation of anthracene to anthraquinone has been long known.22
The electrochemical method for the detection of radical cation
intermediates is limited to studies involving stable radical cations
such as the radical cation derived from 9,10-diphenylan-
thracene.23
Electron-transfer oxidation of aromatic hydrocarbons has long
attracted considerable interest in view of a drastic change in
the reactivity of the radical cations which become much stronger
acids, electrophiles, or oxidants than the parent molecules.1-5
For example, the radical cation of toluene has a tremendous
acidity (pKa ) -13)4 and thereby the deprotonation from the
alkylbenzene radical cations is the predominant decay process
of the radical cations.6-8 In the case of alkylanthracene radical
cations which are much weaker acids (e.g., pKa ) -6 for 9,-
10-dimethylanthracene radical cation)4 as compared with toluene
radical cation, the deprotonation leading to the side chain
oxidation is known to be sluggish relative to nucleophilic attack
by nucleophiles leading to the ring oxidation.9-11 The low
reactivity of nucleophiles toward radical cations has earlier been
noted by Eberson.12 These reactions are classified by Pross as
“forbidden” because of the double excitation in the curve
crossing diagram with respect to the ground state of the
nucleophile/radical cation pair in contrast with “allowed”
reactions of regular cations.13 The “allowed-forbidden” clas-
sification was caught in a dilemma, since the rapid rates (107-
109 M-1 s-1) of radical cations of anthracene derivatives with
strong base nucleophiles were reported and compared with the
rates of regular cations.14 Shaik and Pross then re-evaluated the
“allowed-forbidden” classification based on a semiquantitative
analysis to show that double excitation can often be small when
This study reports the first direct detection of radical cations
of anthracene and its derivatives formed in the multielectron
oxidation with [Ru(bpy)3]3+ (bpy ) 2,2′-bipyridine) in aceto-
nitrile (MeCN) containing H2O and the extensive kinetic analysis
for the decay of the radical cations with use of a stopped-flow
technique combined with the characterization of the oxygenated
products.24 The self-exchange rate constant between 9,10-
dimethylanthracene and the radical cation in MeCN is also
10.1021/jp990541e CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/25/1999

Multielectron Oxidation of Anthracenes
J. Phys. Chem. A, Vol. 103, No. 50, 1999 11213
determined by analyzing line width variations of the ESR spectra
at different concentrations of 9,10-dimethylanthracene in order
to estimate the rate of electron transfer between the radical
cations. Simultaneous determination of the decay rates of
anthracene radical cation and the rates of reduction of [Ru-
(bpy)3]3+ to [Ru(bpy)3]2+, the kinetic deuterium isotope effects
and the dependence of the rates on the concentrations of [Ru-
(bpy)3]3+ and H2O provide valuable and comprehensive insight
into the mechanistic aspects in the multielectron oxidation.
various concentrations of H2O were carried out using a Union
RA-103 stopped-flow spectrophotometer under deaerated condi-
tions. Typically, deaerated MeCN solutions of anthracene (1.2
× 10-3 M) and [Ru(bpy)3](PF6)3 (7.0 × 10-3 M) were
transferred to the spectrophotometric cell by means of a glass
syringe which had earlier been purged with a stream of argon.
Both the decay of anthracene radical cation produced in electron
transfer from anthracene to [Ru(bpy)3]3+ and the formation of
[Ru(bpy)3]2+ associated with the six-electron oxidation of
anthracene in deaerated MeCN at 298 K were monitored by
following a decrease in absorbance at 720 nm (ꢀ ) 8.60 × 103
M-1 cm-1) due to anthracene radical cation and an increase in
Experimental Section
Materials. Anthracene and 9,10-dimethylanthracene were
obtained from Tokyo Chemical Industry Co., Ltd., Japan and
recrystallized from ethanol prior to use. Anthraquinone was
purchased from Aldrich and recrystallized from chloroform.
9-Ethylanthracene and 9-benzylanthracene were prepared from
Grignard reaction of anthrone with the corresponding alkyl-
magnesium bromide and purified through an alumina column.
Tris(2,2′-bipyridine)ruthenium dichloride hexahydrate, [Ru-
(bpy)3]Cl2‚H2O, was obtained commercially from Aldrich. The
oxidation of [Ru(bpy)3]Cl2 with lead dioxide in aqueous H2-
SO4 gives [Ru(bpy)3]3+ which was isolated as the PF6 salt, [Ru-
(bpy)3](PF6)3.25 1-Benzyl-1,4-dihydronicotinamide was prepared
by the literature method.26 Tris(1,10-phenanthroline)iron(III)
perchlorate, [Fe(phen)3](ClO4)3, was prepared by oxidizing the
iron(II) complex with ceric sulfate in aqueous H2SO4.27 Anhy-
drous magnesium perchlorate was obtained from Nakalai
Tesque, Inc. Tetra-n-butylammonium perchlorate (TBAP) was
purchased from Sigma Chemical Co., recrystallized from
ethanol, and dried under vacuum at 40 °C prior to use.
Acetonitrile was purchased from Wako Pure Chemical Ind. Ltd.
and purified by successive distillation over CaH2 according to
standard procedures.28
absorbance at 450 nm (ꢀ ) 1.38 × 104 M-1 cm-1 25
)
due to
[Ru(bpy)3]2+. The ꢀ values of the radical cations of anthracene
and 9-alkylanthracene were determined from the initial absor-
bances due to the radical cations at time zero by taking account
of the mixing time. The second-order plots of (A∞ - A)-1 vs
time (A∞ and A are the final absorbance and the absorbance at
the reaction time, respectively) were linear with the correlation
coefficient F > 0.999. Second-order rate constants (kobs) were
determined by a least-squares curve fit using a Macintosh
personal computer. In each case, it was confirmed that the kobs
values derived from at least five independent measurements
agreed within an experimental error of (5%.
Cyclic Voltammetry. The fast cyclic voltammetry measure-
ments of anthracene and 9-alkylanthracene in deaerated MeCN
containing 0.1 M TBAP as supporting electrolyte were per-
formed on a Fuso model HECS 972 potentiostat-galvanostat
by using the platinum microelectrode (10 µm i.d.) at the sweep
rate of 2000 V s-1. The counter electrodes were platinum, while
Ag/AgNO3 (0.01 M) was used as the reference electrode. All
potentials are reported as V vs SCE. The one-electron oxidation
potential (E0ox) value of ferrocene used as a standard is 0.37 V
vs SCE in MeCN under our solution conditions.30
ESR Measurements. The ESR measurements were per-
formed on a JEOL X-band spectrometer (JES-RE1XE). Deaer-
ated MeCN solutions of various concentrations of 9,10-
dimethylanthracene and [Fe(phen)3](ClO4)3 under atmospheric
pressure of nitrogen were mixed in the capillary cell by using
a JEOL JES-SM-1 rapid mixing flow apparatus. The ESR
spectra were recorded under nonsaturating microwave power
conditions. The magnitude of modulation was chosen to
optimize the resolution and the signal-to-noise (S/N) ratio of
the observed spectra. The g values were calibrated with a Mn2+
marker and hyperfine splitting values were determined by
computer simulation using an Calleo ESR version 1.2 program
coded by Calleo Scientific Software Publishers on a Macintosh
personal computer.
Reaction Procedure. The isolation of the oxidation products
was obtained by ether extraction. The isolated yield of an-
thraquinone was 91%. MS m/z 208 (M+). The 1H NMR
measurements were performed using a JNM-GSX-400 NMR
1
spectrometer. H NMR (CDCl3) δ 7.86-7.89 (m, 4H), 8.25-
8.28 (m, 4H). The isolated yield of 10-benzyl-10-hydroxy-
1
9(10H)anthracenone was 80%. MS m/z 300 (M+). H NMR
(CDCl3) δ 2.61 (s, 1H), 3.22 (s, 2H), 6.13 (d, 2H, J ) 7.3 Hz),
6.89 (t, 2H, J ) 7.5 Hz), 7.04 (t, 1H, J ) 7.5 Hz), 7.45 (t, 2H,
J ) 7.8 Hz), 7.66 (t, 2H, J ) 7.8 Hz), 7.90 (d, 2H, J ) 7.8
Hz), 8.03 (d, 2H, J ) 7.8 Hz). 10-Ethyl-10-hydroxyanthrone:
1H NMR (CD3CN) δ 0.29 (t, 3H, J ) 7.3 Hz), 1.98 (q, 2H, J
) 7.3 Hz), 7.4-7.6 (m, 2H), 7.73 (t, 2H, J ) 7.3 Hz), 7.92 (d,
2H, J ) 7.8 Hz), 8.15 (d, 2H, J ) 7.8 Hz).
Spectral and Kinetic Measurements. Typically, a 10 µL
aliquot of [Ru(bpy)3](PF6)3 (6.3 × 10-3 M) in MeCN containing
H2O (7.0 × 10-3 M) was added to a quartz cuvette (10 mm
i.d.) which contained anthracene (2.1 × 10-5 M) in deaerated
MeCN (3.0 mL). The concentration of H2O in MeCN was
determined by the complex formation of 1-benzyl-1,4-dihy-
dronicotinamide with anhydrous magnesium perchlorate ac-
cording to the literature method.29 UV-visible spectral changes
associated with the oxidation with different concentrations of
[Ru(bpy)3](PF6)3 were monitored using a Hewlett-Packard 8453
diode array spectrophotometer. The same procedure was used
for spectral measurements for the oxidation of alkylanthracenes
(RAn) with [Ru(bpy)3](PF6)3. All measurements were carried
out in a dark cell compartment using deaerated solutions to avoid
the photochemical reactions of anthracenes.
Results and Discussion
Multielectron Oxidation of Anthracenes. The one-electron
reduction potential of [Ru(bpy)3](PF6)3 (E0 ) 1.24 V)31 in
red
MeCN is more positive than the one-electron oxidation potential
of anthracene in the same solvent (E0ox ) 1.19 V). Thus, electron
transfer from anthracene to [Ru(bpy)3](PF6)3 is energetically
feasible. UV-visible spectra observed after the oxidation of
anthracene with different concentrations of [Ru(bpy)3]3+ are
shown in Figure 1 where the absorption band at 450 nm due to
[Ru(bpy)3]2+ increases upon successive addition of [Ru(bpy)3]3+
to a deaerated MeCN solution containing anthracene (2.1 × 10-5
M) and H2O (7.0 × 10-3 M). The spectral titration (inset of
Figure 1) reveals that 6 equiv of [Ru(bpy)3]3+ are consumed to
oxidize 1 equiv of anthracene. In fact, anthraquinone, which is
the six-electron oxidized product of anthracene, is isolated (91%
Kinetic measurements of the oxidation of anthracene and
9-alkylanthracene with [Ru(bpy)3](PF6)3 in MeCN containing

11214 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Fukuzumi et al.
Figure 1. Spectral changes observed upon addition of [Ru(bpy)3]3+
(2.1 × 10-5, 4.3 × 10-5, 6.4 × 10-5, 8.5 × 10-5, 1.1 × 10-4, 1.3 ×
10-4, 1.5 × 10-4, 1.7 × 10-4, 1.9 × 10-4, and 2.1 × 10-4 M) to an
MeCN solution of An (2.1 × 10-5 M). Inset: plot of the absorbance
at 450 nm vs [Ru(bpy)3]3+/[An].
Figure 2. Transient absorption spectrum of An•+ formed in the
electron-transfer oxidation of An (6.0 × 10-4 M) with [Ru(bpy)3]3+
(5.0 × 10-3 M) in deaerated MeCN.
yield, see Experimental Section) after the oxidation of an-
thracene with 6 equiv of [Ru(bpy)3]3+. The product was
1
identified by the H NMR spectrum and mass spectroscopy.
Thus, the stoichiometry of the oxidation of anthracene with [Ru-
(bpy)3]3+ is given by eq 1.
When anthracene is replaced by 9-benzylanthracene, the ring
oxidation occurs to yield the four-electron oxidized species, 10-
benzyl-10-hydroxy-9(10H)-anthracenone as shown in eq 2. The
product was also identified by the 1H NMR spectrum and mass
spectroscopy. The oxidation of 9-ethylanthracene (EtAn) also
yields the 10-ethyl-10-hydroxy-9(10H)-anthracenone (see Ex-
perimental Section).
Figure 3. Time course of the absorption changes (a) at 720 nm due to
decay of An•+ and (b) at 450 nm due to formation of [Ru(bpy)3]2+ in
the electron-transfer oxidation of An (6.0 × 10-4 M) with [Ru(bpy)3]3+
(3.6 × 10-3 M) in MeCN at 298 K. Inset: the second-order plot.
Decay Kinetics of Anthracene Radical Cations. Mixing an
MeCN solution ([H2O] ) 7.0 × 10-3 M) of anthracene (An:
6.0 × 10-4 M) with 6 equiv of [Ru(bpy)3](PF6)3 (3.6 × 10-3
M) in a stopped-flow spectrometer results in an instant appear-
ance of a new absorption band at λmax ) 720 nm due to
anthracene radical cation (An•+)32 as shown in Figure 2. The
decay of absorbance at 720 nm due to An•+ coincides with an
appearance of absorbance at 450 nm due to 5 equiv of [Ru-
(bpy)3]2+ following the initial rapid increase due to formation
of 1 equiv of [Ru(bpy)3]2+, accompanied by the formation of
An•+.33 Both the rates of decay of An•+ and the formation of 5
equiv of [Ru(bpy)3]2+ obeys the second-order kinetics as shown
in the linear plots of (A∞ - A)-1 vs time (see insets in Figure
3; A is absorbance at time t and A∞ is the final absorbance after
the reaction). After the complete decay of anthracene radical
cation, there is no further increase in absorbance due to [Ru-
(bpy)3]2+. It should be emphasized that the consumption of 5
equiv of [Ru(bpy)3]3+ coincides exactly with the decay of
anthracene radical cation derived from 1 equiv of anthracene.
Thus, the kinetics of decay of An•+ and that of formation of
[Ru(bpy)3]2+ are given by eqs 3 and 4, where [Ru(bpy)3]3+/2+
is represented as
-d[An•+]/dt ) kobs[An•+]2
d[Ru2+]/dt ) (kobs/5){[Ru2+]∞ - [Ru2+]}2 ) 5kobs[An•+]2
(3)
(4)
Ru3+/2+ and [Ru2+]∞ is the final concentration of [Ru(bpy)3]2+
which is equal to 5[An]0 (the subscript 0 denotes the initial

Multielectron Oxidation of Anthracenes
J. Phys. Chem. A, Vol. 103, No. 50, 1999 11215
TABLE 1: Absorption Maxima (λmax) and Extinction
Coefficients (Emax) of 9-Alkylanthracene Radical Cations
(RAn•+), the One-Electron Oxidation Potentials (E0ox) of
RAn, the Observed Second-Order Rate Constants (kobs) for
the Decay of RAn•+ Generated by Electron-Transfer
Oxidation of RAn with [Ru(bpy)3](PF6)3 (2.0 × 10-2 M), and
Rate Constants (k1) for Disproportionation Reaction of
RAn•+ in Deaerated MeCN Containing 7.0 × 10-3 M H2O at
298 K
a
b
b
b
λmax 10-3 ꢀmax E0ox (vs SCE)
kobs
M-1 s-1
k1
M-1 s-1
RAn
An
PhCH2An 720
nm M-1 cm-1
V
720
8.6
7.0
8.0
8.6
1.19
1.20
1.14
1.14
1.1 × 106 1.1 × 106
1.3 × 105
EtAn
Me2An
700
680
1.4 × 105 1.6 × 105
1.5 × 103 2.4 × 103
a The experimental error is (10 nm. b The experimental error is
(10%.
concentration). The observed second-order rate constant (kobs
)
derived from eq 3 (1.2 × 10-6 M-1 s-1) agrees well with that
derived from eq 4 (1.3 × 10-6 M-1 s-1). Such an agreement
indicates strongly that the rate-determining step for the six-
electron oxidation of anthracene is the bimolecular reaction of
anthracene radical cation.
Figure 4. Plots of (a) kobs vs [Ru3+] (O) for the decay of An•+ formed
in the electron-transfer oxidation of An (6.0 × 10-4 M) with
[Ru(bpy)3]3+ and (b) kobs vs [An] (b) for the decay of An•+ formed in
the electron-transfer oxidation of An with [Ru(bpy)3]3+ (5.6 × 10-4
)
in deaerated MeCN at 298 K.
Similar transient absorption spectra of radical cations derived
alkylanthracenes are detected in the reactions of alkylanthracenes
with [Ru(bpy)3](PF6)3. The absorption maxima (λmax) and the
extinction coefficients (ꢀmax) of radical cations of anthracenes
are listed in Table 1, together with the one-electron oxidation
potentials of anthracites determined by the fast cyclic voltam-
mograms (see Experimental Section). In each case, both the
rates of decay of alkylanthracene radical cation and the
formation of [Ru(bpy)3]2+ obeys the second-order kinetics and
the kobs values for the multielectron oxidation of anthracene (An)
and alkylanthracenes (RAn) with [Ru(bpy)3]3+ (2.0 × 10-2 M)
in MeCN containing 7.0 × 10-3 M H2O at 298 K are listed in
Table 1. The kobs value of RAn•+ decreases in the order: An >
PhCH2An = EtAn > Me2An.
The second-order kinetics for the decay of RAn•+ holds at
various concentrations of [Ru(bpy)3]3+, [RAn], and H2O. The
kobs values for the decay of anthracene radical cation in the
oxidation of anthracene with more than 6 equiv of [Ru(bpy)3]3+
and with less than 1 equiv of [Ru(bpy)3]3+ are independent of
changes in the concentrations of [Ru(bpy)3]3+ and anthracene
as shown in parts a and b of Figure 4, respectively. The kobs
value with excess anthracene is 6 times larger than that with
excess [Ru(bpy)3]3+ (eq 5).
Figure 5. Plot of kobs vs [H2O] for the decay of An•+ formed in the
electron-transfer oxidation of An (6.0 × 10-4 M) with [Ru(bpy)3]3+
(1.0 × 10-2 M) in deaerated MeCN at 298 K.
sp2 to sp,3,9d,34 that occurs during the decay of the radical cation
as discussed later.
kobs([An] > [Ru3+]) ) 6kobs([Ru3+] > 6[An])
(5)
The kobs value for the decay of 9-ethylanthracene radical
cation (EtAn•+) as well as 9,10-dimethylanthracene radical
cation (Me2An•+) increases with an increase in the [Ru(bpy)3]3+
concentration to reach a constant value as shown in Figure 6
(part a and b, respectively). In the case of 9-benzylanthracene
(PhCH2An), however, the kobs value increases linearly with an
increase in the [Ru(bpy)3]3+ concentration exhibiting no satura-
tion up to 2.0 × 10-2 M as shown in Figure 7. Since the kobs
value of PhCH2An is relatively small as compared with the
corresponding value of An (see Table 1), the acceleration effect
of H2O on kobs can be examined in the presence of much higher
concentrations of H2O than the case of An (Figure 5). The kobs
value increases with an increase in [H2O] to exhibit first-order
dependence on [H2O] at low concentrations, changing to second-
order dependence at high concentrations, as shown in Figure 8.
When H2O is replaced by D2O, the same kobs values are obtained
at the same water concentrations (kobs(H2O) ) 2.1 × 104 M-1
s-1; kobs(D2O) ) 2.0 × 104 M-1 s-1 at [PhCH2An] ) 7.0 ×
The decay of An•+ is accelerated by the presence of H2O.
The kobs value increases linearly with an increase in [H2O] as
shown in Figure 5. When H2O is replaced by D2O, the same
kobs values are obtained at the same water concentrations (kobs
-
(H2O) ) 3.20 × 106 M-1 s-1; kobs(D2O) ) 3.17 × 106 M-1 s-1
at [An] ) 6.0 × 10-5 M; [Ru(bpy)3]3+ ) 6.0 × 10-5 M; [H2O
or D2O] ) 2.5 × 10-2 M). Thus, there is no kinetic deuterium
isotope effect of water for the decay of anthracene radical cation.
When anthracene is replaced by the deuterated compound,
anthracene-d10, the larger kobs value (1.39 × 106 M-1 s-1) is
obtained for the decay of the radical cation in the oxidation of
anthracene-d10 (6.0 × 10-4 M-1 s-1) with [Ru(bpy)3]3+ (5.0 ×
10-3 M) as compared to the corresponding kobs value of
anthracene (9.85 × 105 M-1 s-1). Such inverse kinetic isotope
effects (kH/kD ) 0.71) may be due to the hybridization change,

11216 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Fukuzumi et al.
Figure 6. Plots of kobs vs [Ru3+] for the decay of EtAn•+ (b) and
Me2An•+ (O) formed in the electron-transfer oxidation of EtAn (6.0 ×
10-4 M) and Me2An (6.0 × 10-4 M), respectively, with [Ru(bpy)3]3+
in deaerated MeCN at 298 K.
Figure 8. Plots of kobs vs [H2O] for the decay of PhCH2An•+ formed
in the electron-transfer oxidation of PhCH2An (7.0 × 10-5 M) with
[Ru(bpy)3]3+ (7.0 × 10-4 M) in deaerated MeCN at 298 K.
SCHEME 1
Figure 7. Plot of kobs vs [Ru3+] for the decay of PhCH2An•+ formed
in the electron-transfer oxidation of PhCH2An (6.0 × 10-4 M) with
[Ru(bpy)3]3+ in deaerated MeCN at 298 K.
(2) the four-electron oxidation of 9-alkylanthracene to 10-
alkyl-10-hydroxy-9(10H)-anthracenone (eq 2);
10-4 M; [Ru(bpy)3]3+ ) 5.6 × 10-4 M; [H2O or D2O] ) 0.05
M). Thus, there is no kinetic deuterium isotope effect of water
for the decay of PhCH2An•+ as the case of An•+.
(3) the second-order kinetics for both the decay of anthracene
radical cation (eq 3) and formation of [Ru(bpy)]2+ (eq 4) and
the relation, d[Ru2+]/dt ) -5d[An•+]/dt (Figure 3);
(4) the kobs value with excess anthracene being 6 times larger
than that with excess [Ru(bpy)]3+ (Figure 4, eq 5);
(5) the dependence of kobs on [Ru3+] (Figures 4, 6, 7);
(6) the accelerating effects of H2O on the kobs values (Figures
5 and 8);
Mechanisms of Multielectron Oxidation of Anthracenes.
Since the rate-determining step for the multielectron oxidation
of anthracenes with [Ru(bpy)3]3+ is found to be the bimolecular
reactions of anthracene radical cations produced in the initial
electron transfer from anthracenes to [Ru(bpy)3]3+, the decay
kinetics of anthracene radical cations by itself provides no direct
information on the mechanisms of the product-forming steps.
Nonetheless, such finding on the rate-determining step for the
multielectron oxidation of anthracenes indicates that the decay
of anthracene radical cations results in formation of products
which are rapidly oxidized to lead to the final multielectron
oxidized products. The detailed kinetic results combined with
the product analyses provide valuable information on the overall
mechanism for the multielectron oxidation of anthracenes, since
any mechanism should explain all the following experimental
observations:
(7) the absence of kinetic deuterium isotope effects of D2O
on kobs
;
(8) the observation of inverse kinetic isotope effects of
anthracene-d10 on kobs
For the sake of clarifying and better understanding of this
study, we first present the reaction mechanism of the six-electron
oxidation of anthracene with [Ru(bpy)3]3+ as shown in Scheme
1 and then discuss how this mechanism can explain all the
experimental observations summarized above.
The six-electron oxidation of anthracene (An) is started by a
rapid electron transfer from An to [Ru(bpy)3]3+ to produce the
radical cation (An•+). The electron-transfer disproportionation
between An•+ occurs to give An and An2+. The free energy
(1) the six-electron oxidation of anthracene to anthraquinone
(Figure 1, eq 1);

Multielectron Oxidation of Anthracenes
J. Phys. Chem. A, Vol. 103, No. 50, 1999 11217
SCHEME 2
change of electron transfer between An•+ (∆G0 ) in the absence
et
of H2O is given by eq 6,
∆G0et ) F(E0ox - E0
)
(6)
red
where E0ox is the one-electron oxidation potential of An•+, E0
red
is the one-electron reduction potential of An•+ (E0red) which is
equivalent to the one-electron oxidation potential of An, and F
is the Faraday constant. Since the one-electron oxidation
potential of An•+ is more positive than the one-electron oxidation
potential of An,35 the electron transfer between An•+ is
endergonic (∆G0 > 0). In such a case, the electron transfer
et
(k1) should be followed by rapid back electron transfer (k-1) to
regenerate the product pair (Scheme 1). In the present case,
however, electron transfer from An to [Ru(bpy)3]3+ is energeti-
cally feasible (vide infra), and thereby the electron-transfer
disproportionation of An•+ to give An and An2+ should be
followed by a fast electron transfer from An to [Ru(bpy)3]3+
(k2) to reproduce An•+. Since An2+ is a strong electrophile, An2+
may react rapidly with H2O (k3) to produce the dihydroxy
adduct. This can be rapidly oxidized to anthraquinone by 4 equiv
of [Ru(bpy)3]3+ when more than 6 equiv of [Ru(bpy)3]3+ are
used for the oxidation of An (Scheme 1).
When less than 1 equiv of [Ru(bpy)3]3+ is used to start the
oxidation of An, [Ru(bpy)3]3+ is consumed completely in the
initial electron transfer from An to [Ru(bpy)3]3+ to generate
An•+. In such a case, the dihydroxy adduct formed by the
reaction of An2+ with H2O may be oxidized to anthraquinone
by 4 equiv of An•+ (Scheme 1).
According to Scheme 1, the decay rate of An•+ in the presence
of more than 6 equiv of [Ru(bpy)3]3+ is expressed by eq 7 as a
function of [Ru3+]. The rate of formation of [Ru(bpy)3]2+ is
given by eq 8.
-d[An•+]/dt ) k1(k2[Ru3+] + 2k3)/(k-1 + k3 +
k2[Ru3+])[An•+]2 (7)
d[Ru2+]/dt ) k1(5k2[Ru3+] + 4k3)/(k-1 + k3 +
k2[Ru3+])[An•+]2 (8)
Figure 9. Plots of 1/kobs vs 1/[Ru3+] for the decay of EtAn•+ (b) and
Me2An•+ (O) formed in the electron-transfer oxidation of EtAn (6.0 ×
10-4 M) and Me2An (6.0 × 10-4 M), respectively, with [Ru(bpy)3]3+
in deaerated MeCN at 298 K.
Under the conditions that [Ru3+] . k3 . k-1, eqs 7 and 8 are
reduced to eqs 9 and 10, respectively. Equations 9 and 10 agree
with the experimental observations in eqs 3 and 4, respectively,
where kobs corresponds to the rate constant of electron-transfer
disproportionation (k1).
The four-electron oxidation of RAn (R ) Et and PhCH2) with
more than 4 equiv of [Ru(bpy)3]3+ may also proceed via the
electron-transfer disproportionation of RAn•+ as shown in
Scheme 2 for the case of PhCH2An. In this case, the dihydroxy
adduct of PhCH2An2+ is further oxidized to 10-alkyl-10-
hydorxy-9(10H)anthracenone by 2 equiv of [Ru(bpy)3]3+. The
involvement of two H2O molecules in the electron-transfer
disproportionation of PhCH2An•+ in Scheme 2 will be discussed
in relation with the catalytic effect of H2O in the next section.
According to Scheme 2, the decay rate of RAn•+ is also
expressed as a function of [Ru3+] by eq 13, and the observed
second-order rate constant kobs is given by eq 8 which is
rewritten by eq 14.
-d[An•+]/dt ) k1[An•+]2
(9)
d[Ru2+]/dt ) 5k1[An•+]2 ) -5d[An•+]/dt
(10)
When less than 1 equiv of [Ru(bpy)3]3+ is used to start the
six-electron oxidation of An, no [Ru(bpy)3]3+ is left after the
electron transfer from An to [Ru(bpy)3]3+ to produce An•+. In
such a case, 4 equiv of An•+ is rapidly consumed following the
formation of the dihydroxy adduct (Scheme 1). Then, the decay
rate of An•+ in the presence of less than 1 equiv of [Ru(bpy)3]3+
is expressed by eq 11. Since k3 . k-1 (vide supra), the observed
second-order rate constant kobs is expressed by eq 12 which
agrees with the experimental observation in Figure 4 (eq 5),
where kobs ([Ru3+] > 6[An]) ) k1 (vide supra).
kobs ) k1k2[Ru3+]/(k-1 + k2[Ru3+])
(13)
(14)
kobs-1 ) k1-1 + (k1k2[Ru3+]/k-1)-1
A linear correlation between kobs and [Ru3+ -1
]
predicted by
-1
-d[An•+]/dt ) 6k1k3/(k-1 + k3)[An•+]2
(kobs([An] > [Ru3+]) ) 6k1
(11)
(12)
eq 14 is shown in Figure 9 for the kobs values of EtAn•+ and
Me2An•+. From the intercepts of the linear plots are obtained
the k1 values, which are listed in Table 1. The k1 value of An•+,

11218 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Fukuzumi et al.
electron acceptor ability. In such a case, the reaction between
PhCH2An•+-H2O and PhCH2An•+ (the first-order dependence
of kobs on [H2O]) would be faster than the reaction between the
PhCH2An•+-H2O complexes (the second-order dependence of
kobs on [H2O]). Thus, the second-order acceleration of the rate
with an increase in [H2O] can only be explained by the complex
formation of PhCH2An2+ with 2 equiv of H2O. Although the
complex formation occurs after the electron transfer between
PhCH2An•+, the 1:1 and 1:2 complex formation of PhCH2An2+
with H2O causes a decrease in the free energy change of electron
transfer because of the negative shift of the oxidation potential
of PhCH2An•+ (eq 15). This results in the first-order and second-
order acceleration of the rate of electron transfer (eq 16). Such
a first-order and second-order catalysis on an electron-transfer
reaction has been well documented for a Mg2+-catalyzed
electron-transfer reaction from a one-electron reductant to
p-benzoquinone, which is ascribed to the 1:1 and 1:2 complex
formation of semiquinone radical cation and Mg2+ 36
Such
catalysis of metal ions on both thermal and photoinduced
electron-transfer reactions has recently been reviewed in com-
parison with catalysis on polar reactions.37,38
.
Figure 10. Plot of kobs/[H2O] vs [H2O] for the decay of PhCH2An•+
formed in the electron-transfer oxidation of PhCH2An (7.0 × 10-5 M)
with [Ru(bpy)3]3+ (7.0 × 10-4 M) in deaerated MeCN at 298 K.
The electron-transfer disproportionation of An•+ may also be
accelerated by the 1:1 and 1:2 complex formation of An2+ with
H2O although the observed kobs values of An•+ in the presence
of large concentrations of H2O, where the kobs value of An•+
may exhibit the second-order dependence on [H2O], were so
rapid as to fall outside the stopped-flow range.
which is obtained as a constant value in Figure 4, is also listed
in Table 1. In the case of PhCH2An•+, the k-1 value may be
much larger than the k2[Ru3+] values under the experimental
conditions in Figure 7, where the kobs value increases linearly
with increase in [Ru3+].
Catalysis of Water on Electron-Transfer Disproportion-
ation. The first-order and second-order dependence of kobs of
PhCH2An•+ on [H2O] in Figure 8 may be explained by the
complex formation of PhCH2An2+ with one and two H2O
molecules as shown in Scheme 2. The complex formation of
PhCH2An2+ and H2O should result in the negative shift of the
one-electron oxidation potential of PhCH2An•+ (Eox). The Nernst
equation may be given by eq 15, where E0ox is the one-electron
oxidation potential of PhCH2An•+ in the absence of H2O,
The absence of kinetic deuterium isotope effect of D2O on
kobs for the decay of An•+ (vide supra) indicates that no oxygen-
hydrogen bond cleavage is involved in the water-accelerated
electron-transfer disproportionation of An•+. On the other hand,
the observation of the inverse secondary kinetic isotope effect
(kH/kD ) 0.71, vide supra) suggests that the interaction of An2+
with H2O results in the hybridization change from sp2 to sp3 as
shown in Scheme 1.9d,34 As suggested by Shaik and Pross,15
such a strong interaction between a radical cation and a
nucleophile may be “forbidden” but the “allowed” interaction
of the dication with H2O may be strong enough to cause the
hybridization change in the transition state of electron transfer.
Evaluation of Rate Constants of Electron-Transfer Dis-
proportionation. Since the multielectron oxidation of An and
RAn proceeds via the rate-determining electron-transfer dis-
proportionation of the corresponding radical cations as shown
in Schemes 1 and 2, respectively, it is required to evaluate the
free energy change of electron transfer between the radical
cations (∆G0et) in order to understand the difference in the
Eox ) E0ox - (2.3RT/F) log K1[H2O](1 + K2[H2O]) (15)
and K1[H2O] . 1. From eq 15 is derived the dependence of
the rate constant of electron-transfer disproportionation (k1) on
[H2O] as given by eq 16, where k0 is the rate constant in the
absence of H2O. Since kobs is proportional to k1 (eq 13), the
validity to eq 16 is confirmed by
(k1 - k0)/[H2O] ) k0K1(1 + K2[H2O])
(16)
the linear plot of kobs/[H2O] vs [H2O] for the data in Figure 8
as shown in Figure 10. The absence of the kinetic deuterium
isotope effect of water (Figure 8) is also consistent with Scheme
2 where no cleavage of O-H bond is involved in the water-
accelerated electron-transfer reaction between PhCH2An•+.
The second-order dependence of kobs of PhCH2An•+ on [H2O]
may be alternatively explained by the bimolecular reactions of
PhCH2An•+ which forms a complex with H2O (eq 17). In such
a case, the bimolecular reaction between PhCH2An•+-H2O
complexes (eq 18) would be responsible for the second-order
dependence on [H2O].
reactivity of An and RAn. The ∆G0 values in the absence of
et
H2O can be obtained from the E0 and E0 values of An•+
ox
red
and RAn•+ (eq 6). The E0 values of An•+ and RAn•+ are
red
determined as the first one-electron oxidation potentials of An
and RAn as listed in Table 1. However, it is extremely difficult
to determine the E0 values of An•+ and RAn•+ because of
ox
instability of the corresponding dications (An2+ and RAn2+).
When the strongly reactive 9- and 10-positions of anthracene
are protected by CH3 substitution, a reversible cyclic voltam-
mogram of Me2An is obtained for the first and second oxidation
step.35 Thus, the ∆G0 /F value for the electron-transfer dispro-
et
portionation of Me2An•+ is obtained from the difference in the
PhCH2An•+ + H O f PhCH An•+-H O
(17)
2(PhCH2An•+-H2O) f PhCH2An2+-2H2O + PhCH2An
(18)
first and second oxidation potentials as 0.44 V.35
r
2
2
2
The dependence of the activation free energy ∆Gq of electron
transfer on ∆G0 has been well established by Marcus and
et
follows the relationship shown in eq 19,
However, the complex formation of PhCH2An•+ with H2O may
result in an increase in the donor ability but a decrease in the
∆Gq ) (λ/4)(1 + ∆G0 /λ)2
(19)
et

Multielectron Oxidation of Anthracenes
J. Phys. Chem. A, Vol. 103, No. 50, 1999 11219
minimum value of the rate constant of electron-transfer dispro-
portionation of anthracene radical cations, since the much
stronger interaction of other anthracene dications with H2O
causes the negative shift in the E0ox value, resulting in the much
larger k1 values. The observed reactivity order of anthracene
radical cations (An > EtAn > PhCH2An > Me2An) may be
determined by the thermodynamic stability of the complex
formed between corresponding dication and H2O. Thus, intro-
duction of substituents at 9- and 10-positions of anthracene
decreases the reactivity of electron-transfer disproportionation
of the radical cation which is the rate-determining step for the
multielectron oxidation of anthracenes.
Acknowledgment. We are grateful to Dr. M. Fujita for his
valuable contribution in the early stage of the present work.
This work was partially supported by a Grant-in-Aid for
Scientific Research Priority Area (10149230 and 1025220) from
the Ministry of Education, Science, Culture and Sports, Japan.
Figure 11. Plot of [Me2An] vs ∆Hmsl of ESR spectra of Me2An•+ in
MeCN at 298 K. Inset: ESR spectrum of Me2An•+ generated in the
electron-transfer oxidation of Me2An (1.0 × 10-2 M) with [Fe(phen)3]3+
(1.0 × 10-2 M) in MeCN at 298 K.
References and Notes
(1) (a) Parker, V. D. Acc. Chem. Res. 1984, 17, 243. (b) Hammerich,
O.; Parker, V. D. AdV. Phys. Org. Chem. 1984, 20, 55.
where λ is the reorganization energy of the electron-transfer
reaction.39 The ∆Gq values are obtained from the rate constant
(2) Bard, A. J.; Ledwith, A.; Shine, H. J. AdV. Phys. Org. Chem. 1976,
12, 115.
(3) (a) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456. (b) Bordwell,
F. G.; Bausch, M. J. J. Am. Chem. Soc. 1986, 108, 2473. (c) Bordwell, F.
G.; Cheng, J.-P.; Bausch, M. J. J. Am. Chem. Soc. 1988, 110, 2867. (d)
Bordwell, F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1989, 111, 1792. (e) Zhang,
X.; Bordwell, F. G. J. Org. Chem. 1992, 57, 4163. (f) Zhang, X.-M.;
Bordwell, F. G.; Bares, J. E.; Cheng, J.-P.; Petrie, B. C. J. Org. Chem.
1993, 58, 3051.
(4) (a) Nicholas, A. M. P.; Arnold, D. R. Can. J. Chem. 1982, 60,
2165. (b) Nicholas, A. M. P.; Arnold, D. R. Can. J. Chem. 1984, 62, 1850.
(c) Nihcolas, A. M. P.; Arnold, D. R. Can. J. Chem. 1984, 62, 1860.
(5) (a) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J.
J. Am. Chem. Soc. 1993, 115, 8960. (b) Fukuzumi, S.; Koumitsu, S.;
Hironaka, K.; Tanaka, T. J. Am. Chem. Soc. 1987, 109, 305.
(6) (a) Baciocchi, E.; Bietti, M.; Steenken, S. J. Am. Chem. Soc. 1997,
119, 4078. (b) Baciocchi, E.; Delgiacco, T.; Elisei, F. J. Am. Chem. Soc.
1993, 115, 12290. (c) Baciocchi, E.; Cort, A. D.; Eberson, L.; Mandolini,
L.; Rol, C. J. Org. Chem. 1986, 51, 4544. (d) Baciocchi, E.; D’Acunzo, F.;
Galli, C.; Lanzalunga, O. J. Chem. Soc., Perkin Trans. 2 1996, 133. (d)
Fujita, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S. J. Am. Chem. Soc. 1996,
118, 8566.
of electron transfer (ket) and the diffusion rate constant (kdiff
)
using eq 20, where Z is the collision frequency taken as 1 ×
1011 M-1 s-1, F is the Faraday constant, the kdiff value in MeCN
is 2.0 × 1010 M-1 s-1 and the other notations are conventional.
∆Gq ) (2.3RT/F) log[Z(ket-1 - kdiff-1)]
(20)
To determine the self-exchange rate constant between Me2An•+
and Me2An (eq 21),
kex
{\
Me2An + Me2An•+
} Me2An•+ + Me2An
(21)
the ESR spectra of Me2An•+ produced by electron transfer from
Me2An to [Ru(bpy)3]3+ were measured in the presence of
various concentrations of Me2An in MeCN (inset in Figure 11).40
The maximum slope line width (∆Hmsl) of the ESR spectrum
of Me2An•+ increases linearly with an increase in the concentra-
tion of Me2An as shown in Figure 11. The rate constants (kex)
of the electron exchange reactions between Me2An•+ and Me2-
An (eq 19) were determined as 5.0 × 108 M-1 s-1 using eq 22,
(7) (a) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc.
1984, 106, 3567. (b) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Am.
Chem. Soc. 1984, 106, 7472.
(8) (a) Albini, A.; Sulpizio, A. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 88. (b)
Albini, A.; Fasani, E.; Mella, M. Topp. Curr. Chem. 1993, 168, 143.
(9) (a) Parker, V. D.; Chao, E. T.; Zheng, G. J. Am. Chem. Soc. 1997,
119, 11390. (b) Reitsto¨en, B.; Norrsell, F.; Parker, V. D. J. Am. Chem.
Soc. 1989, 111, 8463. (c) Parker, V. D.; Chao, Y. T.; Reitsto¨en, B. J. Am.
Chem. Soc. 1991, 113, 2336. (d) Reitsto¨en, B.; Parker, V. D. J. Am. Chem.
Soc. 1991, 113, 6954. (e) Reitsto¨en, B.; Parker, V. D. J. Am. Chem. Soc.
1990, 112, 4968. (f) Reitsto¨en, B.; Parker, V. D. Acta Chem. Scand. 1992,
46, 464.
(10) (a) Tolbert, L. M.; Khanna, R. K.; Popp, A. E.; Gelbaum, L.;
Bottomley, L. A. J. Am. Chem. Soc. 1990, 112, 2373. (b) Sirimanne, S. R.;
Li, Z. Z.; VanderVeer, D. R.; Tolbert, L. M. J. Am. Chem. Soc. 1991, 113,
1766. (c) Tolbert, L. M.; Li, Z. Z.; Sirimanne, S. R.; VanDerveer, D. G. J.
Org. Chem. 1997, 62, 3927.
kex ) 1.52 × 107(∆Hmsl - ∆H0msl)/{(1 - Pi)[Me2An]}
(22)
where ∆Hmsl and ∆H0msl are the maximum slope line widths of
the ESR spectra in the presence and absence of Me2An,
respectively, and Pi is a statistical factor.41 The reorganization
energies (λ) of the electron-transfer reaction is obtained as 12.5
kcal mol-1 from the kex values using eq 23 (Z ) 1011 M-1 s-1).
(11) (a) Tanko, J. M.; Wang, Y. Chem. Commun. 1997, 2387. (b) Wang,
Y.; Tanko, J. M. J. Am. Chem. Soc. 1997, 119, 8201.
kex ) Z exp(-λ/4RT)
(23)
(12) Eberson, L.; Blum, Z.; Helge´e, B.; Nyberg, K. Tetrahedron 1978,
34, 731.
(13) Pross, A. J. Am. Chem. Soc. 1986, 108, 3537.
Assuming that the λ value for the electron-transfer dispro-
portionation of Me2An•+ is the same as that for the electron
exchange between Me2An•+ and Me2An, the k1 value is
(14) (a) Parker, V. D.; Tilset, M. J. Am. Chem. Soc. 1987, 109, 2521.
(b) Parker, V. D.; Tilset, M. J. Am. Chem. Soc. 1988, 110, 1649.
(15) (a) Shaik, S. S.; Pross, A. J. Am. Chem. Soc. 1989, 111, 4306. (b)
Shaik, S.; Reddy, A. C.; Ioffe, A.; Dinnocenzo, J. P.; Danovich, D.; Cho,
J. K. J. Am. Chem. Soc. 1995, 117, 3205.
(16) (a) Workentin, M. S.; Parker, V. D.; Morkin, T. L.; Wayner, D. D.
M. J. Phys. Chem. A 1998, 102, 6503. (b) Workentin, M. S.; Schepp, N.
P.; Johnston, L. J.; Wayner, D. D. M. J. Am. Chem. Soc. 1994, 116, 1141.
(c) Workentin, M. S.; Johnston, L. J.; Wayner, D. D. M.; Parker, V. D. J.
Am. Chem. Soc. 1994, 116, 8279.
estimated from the λ and ∆G0 values as 3.0 × 103 M-1 s-1
.
et
This value agrees well with the k1 value (2.4 × 103 M-1 s-1) in
Table 1. Such an agreement between the k1 value and the
calculated value of the electron-transfer rate constant strongly
supports that the observed second-order decay of Me2An•+ is
ascribed to electron-transfer disproportionation between Me2-
An•+. The k1 value of Me2An•+ may be considered as the

11220 J. Phys. Chem. A, Vol. 103, No. 50, 1999
Fukuzumi et al.
(17) (a) Johnston, L. J.; Schepp, N. P. J. Am. Chem. Soc. 1993, 115,
6564. (b) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1994, 116, 6895.
(c) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1994, 116, 10330.
(18) (a) Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.;
Gould, I. R.; Todd, W. P.; J. Am. Chem. Soc. 1997, 119, 1876. (b)
Dinnocenzo, J. P.; Zuilhof, H.; Lieberman, D. R.; Simpson, T. R.;
McKechney, M. W. J. Am. Chem. Soc. 1997, 119, 994. (c) Dinnocenzo, J.
P.; Todd, W. P.; Simpson, T. R.; Gould, I. R. J. Am. Chem. Soc. 1990,
112, 2462. (d) Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R.;
Todd, W. P.; Mattes, S. L. J. Am. Chem. Soc. 1989, 111, 8973.
(19) (a) Brede, O.; David, F.; Steenken, S. J. Chem. Soc., Perkin Trans.
2 1995, 23. (b) Koike, K.; Thomas, J. K. J. Chem. Soc., Faraday Trans.
1992, 88, 195.
(20) (a) Baciocchi, E.; Bietti, M.; Putignani, L.; Steenken, S. J. Am.
Chem. Soc. 1996, 118, 5952. (b) Baciocchi, E.; Bietti, M.; Lanzalunga, O.:
Steenken, S. J. Am. Chem. Soc. 1998, 120, 11516. (b) Steenken, S.;
McClelland, R. A. J. Am. Chem. Soc. 1989, 111, 4967.
(21) (a) Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc.
1998, 120, 2826. (b) Hubig, S. M.; Bockman, T. M.; Kochi, J. K. J. Am.
Chem. Soc. 1997, 119, 2926. (c) Hubig, S. M.; Bockman, T. M.; Kochi, J.
K. J. Am. Chem. Soc. 1996, 118, 3842.
(22) (a) Majeski, E. J.; Stuart, J. D.; Ohnesorge, E. J. Am. Chem. Soc.
1968, 90, 633. (b) Parker, V. D. Acta Chem. Scand. 1970, 24, 2757.
(23) Evans, J. F.; Blount, H. N. J. Phys. Chem. 1979, 83, 1970.
(24) A preliminary report for the four-electron oxidation of 9-alkylan-
thracene with Fe(ClO4)3‚9H2O has appeared. Fujita, M.; Fukuzumi, S. Chem.
Lett. 1993, 1911.
(25) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
(26) Mauzerall, D.; Westheimer, F. H. J. Am. Chem. Soc. 1955, 77, 2261.
(27) Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5593.
(28) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: Elmsford, 1966.
(31) Fukuzumi, S.; Nakanishi, I.; Tanaka, K.; Suenobu, T.; Tabard, A.;
Guilard, R.; Van Caemelbecke, E.; Kadish, K. M. J. Am. Chem. Soc. 1999,
2, 785.
(32) The absorption spectrum of RAn•+ in Figure 1 is essentially the
same as that of anthracene radical cation generated by the electrochemical
oxidation, photoinduced electron transfer, and γ-irradiation. (a) Masnovi,
J. M.; Kochi, J. K.; Hilinski, E. F.; Rentzepis, P. M. J. Am. Chem. Soc.
1986, 108, 1126. (b) Rodgers, M. A. Trans. Faraday Soc. 1971, 67, 1029.
(c) Shida, T.; Iwata, S. J. Am. Chem. Soc. 1973, 95, 3473.
(33) The appearance of absorbance at 720 nm due to An•+ and the
concomitant increase in absorbance at 450 nm due to [Ru(bpy)3]2+ were
too fast to be followed by the stopped-flow technique.
(34) Streitwieser, A., Jr.; Jagow, R. H.; Fahey, R. C.; Suzuki, S. J. Am.
Chem. Soc. 1958, 80, 2326.
(35) Kubota, T.; Kano, K.; Uno, B.; Konse, T. Bull. Chem. Soc. Jpn.
1987, 60, 3865.
(36) Fukuzumi, S.; Okamoto, T. J. Am. Chem. Soc. 1993, 115, 11600.
(37) Fukuzumi, S. Bull. Chem. Soc. Jpn. 1997, 70, 1.
(38) Fukuzumi, S.; Itoh, S. In AdVances in Photochemistry; Neckers,
D. C., Volman, D. H., Bu¨nau, G., Eds.; Wiley: New York, 1999; Vol. 25,
Chapter 3.
(39) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b)
Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (c) Eberson,
L. AdV. Phys. Org. Chem. 1982, 18, 79.
(40) The hyperfine coupling constants for Me2An•+ in Figure 11 (inset)
agree with the reported values; see: Buchanan, A. C., III; Livingston, R.;
Dworkin, A. S.; Smith, G. P. J. Phys. Chem. 1980, 84, 423.
(41) (a) Ward, R. L.; Weissman, S. I. J. Am. Chem. Soc. 1957, 79, 2086.
(b) Chang, R. J. Chem. Educ. 1970, 47, 563. (c) Watts, M. T.; Lu, M. L.;
Chen, R. C.; Eastman, M. P. J. Phys. Chem. 1973, 77, 2959. (d) Cheng, K.
S.; Hirota, N. In InVestigation of Rates and Mechanisms of Reactions;
Hammes, G. G., Ed.; Wiley-Interscience: New York, 1974; Vol. VI, p 565.
(e) Nakanishi, I.; Itoh, S.; Suenobu, T.; Fukuzumi, S. Chem. Commun. 1997,
1927.
(29) Fukuzumi, S.; Kondo, Y.; Tanaka, T. Chem. Lett. 1983, 485.
(30) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Non-
aqueous Systems; Marcel Dekker: New York, 1990.