Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate

Article abstract of DOI:10.1021/ja512584r

Mononuclear copper complexes, [(tmpa)Cu^II(CH₃CN)](ClO₄)₂ (1, tmpa = tris(2-pyridylmethyl)amine) and [(BzQ)Cu^II(H₂O)₂](ClO₄)₂ (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduction of O₂ by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)₃) in acetone, whereas 1 catalyzes the four-electron reduction of O₂ by the same reductant in the presence of Bronsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II) complex [(tmpa)Cu^II(O₂)Cu^II(tmpa)]²⁺, the two-electron reduced product of O₂ with Sc³⁺ is observed to be scandium peroxide ([Sc^III(O₂^2-)]⁺). In the presence of 3 equiv of hexamethylphosphoric triamide (HMPA), [Sc^III(O₂^2-)]⁺ was oxidized by [Fe(bpy)₃]³⁺ (bpy = 2,2-bipyridine) to the known superoxide species [(HMPA)₃Sc^III(O₂^-)]²⁺ as detected by EPR spectroscopy. A kinetic study revealed that the rate-determining step of the catalytic cycle for the two-electron reduction of O₂ with 1 is electron transfer from Fc to 1 to give a cuprous complex which is highly reactive toward O₂, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O₂ following electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O₂-reaction stoichiometry from four-electron with Bronsted acids to two-electron reduction in the presence of Sc³⁺ and also for the change in the rate-determining step is clarified based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc³⁺ on copper-oxygen intermediates.

Full text of DOI:10.1021/ja512584r

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Article
Lewis Acid-Induced Change from Four- to Two-Electron Reduction of
Dioxygen Catalyzed by Copper Complexes Using Scandium Triflate
Saya Kakuda, Clarence Rolle, Kei Ohkubo, Maxime A. Siegler, Kenneth D. Karlin, and Shunichi Fukuzumi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja512584r • Publication Date (Web): 07 Feb 2015
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Lewis Acid-Induced Change from Four- to Two-Electron Reduction of
Dioxygen Catalyzed by Copper Complexes Using Scandium Triflate
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‡,
†
‡
,‡
Saya Kakuda,†, Clarence Rolle, Kei Ohkubo, Maxime A. Siegler, Kenneth D. Karlin,* and Shunichi Fuku-
#
#
zumi*,†
†Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, ALCA (JST),
Osaka University, Suita, Osaka 565-0871, Japan
‡Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA
#These authors contributed equally to this work
ABSTRACT: Mononuclear copper complexes, [(tmpa)CuII(CH3CN)](ClO4)2 (1, tmpa
= tris(2-pyridylmethyl)amine) and
[(BzQ)CuII(H2O)2](ClO4)2 (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduc-
tion of O2 by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)3), in acetone, whereas 1 catalyzes the four-electron reduc-
tion of O2 by the same reductant in the presence of Brønsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II)
complex [(tmpa)CuII(O2)CuII(tmpa)]2+, the two-electron reduced product of O2 with Sc3+ is observed to be scandium peroxide ([Sc3+(O2
2–
)]+). In the presence of three equiv of hexamethylphosphoric triamide (HMPA), [Sc3+(O22–)]+ was oxidized by [Fe(bpy)3]3+ (bpy = 2,2’-
bipyridine) to the known superoxide species [(HMPA)3Sc3+(O2•–)]2+ as detected by EPR spectroscopy. A kinetic study revealed that the rate-
determining step of the catalytic cycle for the two-electron reduction of O2 with 1 is electron transfer from Fc* to 1 to give a cuprous complex
which is highly reactive toward O2, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O2 fol-
lowing electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O2-reaction stoichiometry from four-
electron with Brønsted acids to two-electron reduction in the presence of Sc3+ and also for the change in the rate-determining step is clarified
based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc3+
on copper-oxygen intermediates.
variety of factors, including the ligand type and resulting nature of
copper-oxygen intermediates formed as reactive species in the O2
reduction catalysis.24,25 The counter anions of proton sources
employed also affects the O2-reduction catalytic reactivity with
copper complexes, with-respect-to the observed stoichiometry
and/or mechanism of reaction.26,27 However, there has been no
report on the change in the number of electrons to reduce O2
(two-electron vs four-electron) in O2 reduction catalysis induced
by metal ions acting as Lewis acids.
We report herein the drastic change to a two-electron from a
four-electron reduction of O2 with [(tmpa)CuII(CH3CN)]-
(ClO4)2 (1) induced by the Lewis acid [Sc(OTf)3]. In contrast to
the case of 1, the selective two-electron reduction of O2 occurred
with [(BzQ)CuII(H2O)2](ClO4)2 (2) in the presence of triflic
acid (HOTf) as well as Sc(OTf)3. The mechanism of the selective
two-electron reduction of O2 with 1 and 2 is examined by a
kinetics study of the overall catalytic cycle as well as each step of
the catalytic cycle with study of the observed effects of Sc3+ on
copper-oxygen intermediates.
Introduction
Copper proteins play important roles in oxidation of substrates
accompanied by two-electron reduction of dioxygen (O2) to
hydrogen peroxide (H2O2) or four-electron reduction of O2 to
water (H2O) depending on the type of enzymes.1 For example, in
the oxidation of their substrates, galactose oxidases2 and amine
oxidases3 effect the two-electron reduction of O2 to hydrogen
peroxide4 whereas multicopper oxidases (MCO’s)5 and heme-
copper oxidases (HCO’s)6 facilitate the four-electron reduction of
O2 to H2O. The catalytic four-electron reduction of O2 with
synthetic copper complexes as well as other metal complexes has
merited special attention because of not only the mechanistic
interest in relation to MCO’s but also in development of a fuel cell
technology and their application using earth-abundant metals
such as iron, cobalt and copper.7-15 On the other hand, the
catalytic two-electron reduction of O2 to H2O2 has also attracted
increasing interest, because H2O2 is regarded as a promising
candidate as a high-density energy carrier as compared with
gaseous hydrogen and also H2O2 can be used as a liquied fuel in
simple one-compartment fuel cells.16-18 There have been many
reports on the electrocatalytic and homogeneous four-electron
reduction of O2 with copper complexes.16-24 In contrast, there has
been only few examples for the catalytic two-electron reduction of
O2 using a copper complex.25 Whether copper complexes are
effective for two-or four-electron reduction of O2 depends on a
Experimental Section
Materials. The following reagents were obtained commer-
ceally and used as received: Scandium triflate [Sc(OTf)3], de-
camethylferrocene (Fc*), 1,1’-dimethylferrocene (Me2Fc), fer-
rocene (Fc), perchloric acid (70%), trifluoroacetic acid, hydrogen
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peroxide (30%) and NaI (Wako Pure Chemical Industries). Fol-
M) or [(BzQ)CuII(H2O)2](ClO4)2 (2) (1.0 × 10–4 M) and Fc*
lowing literature procedures,28 acetone was dried and distilled
under Ar. The compounds [(tmpa)CuII(CH3CN)](ClO4)2 (1)29
and bis(2-quinolinylmethyl)benzylamine (BzQ)30 were prepared
as described.
(2.0 × 10–3 M). Fc*+ and Me2Fc+ concentrations being produced
during the reaction were determined from known absorptivity
data, λmax = 780 nm (ε = 500 M–1 cm–1 at 298 K and 600 M–1 cm–1
at 213 K) for Me2Fc*+ and λmax 650 nm, εmax = 360 M–1 cm–1 for
Me2Fc+. For Fc*+ the extinction coefficient was estimated by car-
rying out a Fc* electron-transfer oxidation using [RuIII(bpy)3]-
(PF6)3. The limiting concentration of O2 in an acetone solution
was prepared by a mixed gas flow of O2 and N2, using a gas mixer
(Kofloc GB-3C, KOJIMA Instrument Inc.), that able to effect
controlled pressure and flow rate mixing of two gases. Hydrogen
peroxide determination was carried out by standard iodide titra-
tion where the O2 reduction product solution in acetone was di-
1
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[(BzQ)CuII(H2O)2](ClO4)2; (2) A 50 mL flask with BzQ
(389 mg, 1.0 mmol) and Cu(ClO4)2 ·6H2O (370 mg, 1.0 mmol),
as prepared and 20 mL MeOH was then added; the solution be-
came blue and with stirring over 1 h, it yielded a precipitate con-
sisting of blue microcrystals. The solid product 2 was collected
employing a vacuum filtration procedure and then and washed
with 15mL MeOH, and dried under vacuum.. (599 mg, 0.86
mmol, 86% yield) Anal. Calcd (C28H29Cl2CuO10N3): C, 47.14; H,
3.96; N, 6.11. Found: C, 47.15; H, 3.87; N, 6.06. X-ray quality
crystals were obtained by allowing pentane to slowly diffuse into a
saturated acetone solution of 2.
Single Crystal X-ray Crystallography. All reflection intensi-
ties were measured at 110(2) K using a SuperNova diffractometer
(with Atlas detector) and Cu Kα radiation (λ = 1.54178 Å) using
the CrysAlisPro program (Version 1.171.36.32 Agilent Tech-
nologies, 2013); the latter was also employed to refine cell dimen-
sions and for data reduction. The structure was solved and refined
on F2 using SHELXS-2013 (Sheldrick, 2013). Analytical numeric
absorption corrections based on a multifaceted crystal model were
applied using CrysAlisPro. The data collection temperature was
controlled using a Cryojet (Oxford Instruments) system. Unless
otherwise specified, the H atoms were placed at calculated posi-
tions using AFIX 23, AFIX 43 or AFIX 137 instructions with isot-
ropic displacement parameters with values 1.2 or 1.5 times Ueq
those for the C atoms attached. The H atoms attached to O1Wn,
O2Wn and O3Wn (n = 1, 2) (coordinated water molecules) were
found from difference Fourier maps, and their coordinates were
refined freely (DFIX instructions were used to restrain the O−H
and H…H distances within acceptable ranges).
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–
luted and reacted with NaI in excess. Quantitation of the I3
formed was then calculated using its visible spectrum (λmax = 361
nm, ε = 2.5 × 104 M–1 cm–1).31 All low-temperature UV–vis ab-
sorption spectra and spectral changes were recorded using a Hew-
lett Packard 8453A diode array spectrophotometer with attached
liquid nitrogen cooled cryostat (Unisoku USP-203-A).
Kinetic Measurements. Fast reaction with short half-lives (≤
10 s) at 298 K were performed with a UNISOKU RSP-601
stopped flow spectrophotometer possessing a MOS-type high
selective photodiode array and attached Unisoku thermostated
cell holder. The kinetics of electron-transfer from Fc* to 1 were
analyzed by monitoring absorption band changes due to Fc*+
formation. Pseudo-first-order conditions were used throughout,
with Fc* concentrations kept at more than a 10-fold excess com-
pared to that of 1.
Electrochemistry. Copper(II) complex cyclic voltammetry
using an ALS 630B electrochemical analyzer was utilized for
measurements at 1 atm and under nitrogen or argon, both in the
presence and absence of Sc(OTf)3; conditions included use of
deaerated acetone solutions with 0.1
M [(n–butyl)4N]PF6
(TBAPF6) all at RT.. A platinum working electrode (surface area
of 0.3 mm2) was employed within a conventional three-electrode
cell using a Pt counter electrode. The BAS platinum working elec-
trode was often polished with an alumina suspension (BAS); prior
to use this, acetone was used to wash the electrode. The reference
electrode used was Ag/AgNO3 (0.01 M) and to convert potentials
to values vs the SCE, 0.29 V was added.32
EPR Measurements. A JEOL JES-RE1XE spectrometer was
used to record EPR spectra of Cu(II) and scandium superoxide
complexes. The modulation amplitude employed was selected to
optimize the signal-to-noise (S/N) ratio and resolution under
conditions of non-saturating microwave power. A Mn2+ marker
inserted into the EPR cavity was used to determine g values and
hyperfine coupling constants.
Theoretical Calculations. Using a 32-processor Quantum-
CubeTM with Gaussian 09 (revision A.02), DFT calculations on
copper complexes were performed. A UCAM-B3LYP/6-311G(d)
level of theory was employed for geometry optimization.33–36
Computational results graphical output were generated using
GaussView (ver. 3.09; Semichem, Inc.).37
There are three crystallography independent Cu(II) com-
−
plexes, six ClO4 perchlorate anions, plus seven lattice acetone
solvate molecules per asymmetric unit. The structure is mostly
ordered. Five of the six counterions are disordered over two
orientations. The occupancy factors of the major components of
the disorder refine to 0.871(6), 0.54(2), 0.661(6), 0.591(4),
0.560(16).
C102H123Cl6Cu3N9O37
: moiety formula: 3(C27H27CuN3O2),
6(ClO4), 7(C3H6O), Fw = 2470.41, blue block, 0.34 × 0.33 × 0.18
mm3, monoclinic, P21/n (no. 14), a
12.18146(19), c = 39.6106(6) Å, β = 92.6704(13)°, V
=
22.8236(3), b
=
=
-
11000.8(3) Å3, Z = 4, Dx = 1.492 g cm−3, µ = 2.760 mm−1, Tmin
T
max: 0.474−0.684. 73408 Reflections were measured up to a
resolution of (sin θ/λ)max = 0.62 Å−1. 21523 Reflections were
unique (Rint = 0.0243), of which 19284 were observed [I > 2σ(I)].
1682 Parameters were refined using 746 restraints. R1/wR2 [I >
2σ(I)]: 0.0341/0.0914. R1/wR2 [all refl.]: 0.0388/0.0948. S =
1.028. Residual electron density found between −0.55 and 0.66 e
Å−3. The molecular weight is given for the moiety formula.
Reaction Procedure. Spectral changes (Hewlett Packard
8453 photodiode-array spectrophotometer with a quartz cuvette
(path length = 10 mm)) at 298 K were observed as a function of
varying Sc(OTf)3 concentrations during the dioxygen catalytic
reduction experiments. Employing a microsrying, an acetone
solution of Sc(OTf)3 was added to an O2-saturated acetone solu-
tion containing [(tmpa)CuII(CH3CN)](ClO4)2 (1) (1.0 × 10–5
Results and Discussion
Catalytic Two-Electron Reduction of O2 by Fc* with 1 in
the Presence of Sc(OTf)3. We have previously reported that
[(tmpa)CuII(CH3CN)](ClO4)2
(1)
(tmpa
=
tris(2-
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4.0
pyridylmethyl)amine) catalyzed the four-electron reduction of O2
1
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8
by decamethylferrocene (Fc*) to H2O in the presence of
perchloric acid or trifluoroacetic acid in acetone as shown in eq 1,
3.0
2.0
1.0
0
4Fc*+ + 2H2O
4Fc* + O2 + 4H+
(1)
2+
N
N
N
CuII
N
[(tmpa)CuII]2+ (1)
0
0.5
1.0
1.5
2.0
9
[Sc(OTf)3], mM
where four-equiv of Fc*+ (decamethylferrocinium ion) relative to
O2 were consumed.24a,26 When Sc(OTf)3 is employed as a Lewis
acid instead of HClO4 or CF3COOH, 1 also efficiently catalyzes
the reduction of O2 where Fc*+ is also produced (Figure 1). In this
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Figure 2. Plot of absorbance at 780 nm due to Fc*+ vs concentration of
Sc(OTf)3 in the two-electron reduction of O2 (2.5 mM) by Fc* (2.0 mM) with
Sc(OTf)3 (0-2.0 mM) catalyzed by 1 (40 µM) in acetone at 298 K.
(UHF) SCF optimization using the 6-311++G** basis set.39 In
contrast to the case for this superoxo complex, DFT calculations
suggest that the side on structure of the peroxo complex
([Sc3+(O22–)]+) is more stable than an end-on coordinated
peroxo-scandium species (Figure S2 in SI).
Figure 1. (a) UV-vis spectral changes observed in the two-electron and four-
electron reduction of O2 (0.5 mM) by Fc* (2.0 mM) with Sc(OTf)3 (2.0 mM)
catalyzed by 1 (40 µM) in acetone at 298 K. (b) Time courses of absorbance
at 780 nm due to Fc*+ in the two-electron and four-electron reduction of O2
(0.5 mM) by Fc* (2.0 mM) catalyzed by 1 (40 µM) in the presence of
Sc(OTf)3 (2.0 mM) and HOTf (40 mM), respectively.
Figure 3. EPR spectrum observed after addition of [FeIII(bpy)3]3+ and HMPA
(30 mM) to an N2-saturated acetone solution of [Sc3+(O22–)]+, which was
produced by the two-electron reduction of O2 (11 mM) by Fc* in the presence
of [(tmpa)CuII](ClO4)2 (1) (10 µM) and Sc(OTf)3 (10 mM) in acetone at
298 K. The g value is 2.011, confirming the production of the known HMPA-
Sc-superoxide complex.
case, however, the stoichiometry of O2 reduction is different and
follows eq 2,
2–
2Fc* + O2 + Sc3+ → 2Fc*+ + Sc3+(O2
)
(2)
1
where two-equivalents of Fc*+ (λmax = 780 nm) relative to O2 are
produced and one equiv of Sc3+ is consumed instead of four
protons (eq 1), as shown in the spectral titration in Figure 2. The
reduced product of O2 is [Sc3+(O22–)]+, based on the
stoichiometry determined, Figure 2. The yield of [Sc3+(O22–)]+
was determined to be 100% based on an iodometric titration
(Figure S1 in Supporting Information (SI)).29
To further support the formation of scandium peroxide
([Sc3+(O22–)]+), the reaction mixture was oxidized using the one-
electron oxidant ([Fe(bpy)3]3+), which was stabilized in the
presence of three equiv of the hexamethylphosphoric triamide
(HMPA) ligand to produce ([(HMPA)3Sc3+(O2•–)]2+), as shown
by eq 3.37 The formation of [(HMPA)3Sc3+(O2•–)]2+ was detected
Kinetics and Mechanism of Catalytic Two-Electron
Reduction of O2 by Fc* with 1. The rate of formation of Fc*+ in
the two-electron reduction of O2 by Fc* with 1 was monitored by
an increase in absorbance at 780 nm due to Fc*+. The rate obeyed
pseudo-first-order kinetics in the presence of excess Sc(OTf)3 and
O2 relative to Fc* (Figure 4a). The observed pseudo-first-order
rate constant (kobs
)
increases linearly with increasing
concentration of 1 (Figure 4b), whereas the kobs value remained
constant with increasing concentration of O2 (Figure 4c). The kobs
value was also constant at the Sc(OTf)3 concentration above 5
mM (Figure 4d). Thus, the kinetic formulation of the two-
electron reduction of O2 by Fc* with 1 in the presence of large
excess Sc(OTf)3 is given by eq 4,
[Sc3+(O22–)]+ + 3HMPA + [Fe(bpy)3]3+
]
d[Fc*+]/dt = kcat[1][Fc*]
(4)
[(HMPA)3Sc3+(O2•–)]2+ + [Fe(bpy)3]2+
(3)
where kcat is the second-order catalytic rate constant of 1. The kcat
value was determined to be (9.4 ± 0.5) × 104 M–1 s–1 at 298 K. This
value is twice of the rate constant (ket) of electron transfer from
by EPR spectroscopy as shown in Figure 3. The g value (2.0112)
and and superhyperfine coupling constant due to scandium (I
=7/2; aSc = 3.82 G) are the same as those reported previously.38
Fc* to 1 (5.0 ± 0.1) × 105 M–1 s–1 in acetone at 298 K,24a i.e., kcat
=
2ket, because the electron transfer should occur twice to reduce O2
in the catalytic cycle when two equiv of Fc*+ are formed. This
inidicates that electron transfer from Fc* to 1 is the rate-
•–
The end-on coordination of O2 to Sc3+ is indicated by
inequivalent a(17O) values (14 and 17 G)38 and supported by
optimized geometry calculations by an unrestricted Hartree-Fock
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Figure 6. (a,c) UV-vis absorption spectral change in the reaction of
[(tmpa)CuI]+ (2.0 mM) with O2 in O2-saturated acetone (black), followed by
addition of Sc(OTf)3 (2.0 mM) to the resulting solution at 213 K (red) at (a)
0-4 s and (c) 4-10 s time delays. (b) Absorption time profiles at 394 nm due to
the Sc3+-bound peroxo complex, [(tmpa)CuII(O2)Sc(OTf)3]2+ and 520 nm
due to [(tmpa)CuII(O2)CuII(tmpa)]2+. Sc(OTf)3 was added at 2 s time delay.
Figure 4. (a) Time profiles of formation of Fc*+ monitored by absorbance at
780 nm (ε = 500 M–1 cm–1) in the two-electron reduction of O2 by Fc* (2.0
mM) with Sc(OTf)3 (10 mM) catalyzed by 1 (2-10 µM) in saturated ([O2] =
11 mM) acetone at 298 K. Inset: First-order plots. (b) Plot of kobs vs [1] for
the two-electron reduction of O2 by Fc* (2.0 mM) in the presence of
Sc(OTf)3 (10 mM) in acetone at 298 K. (c) Plot of kobs vs [O2] for the two-
electron reduction of O2 by Fc* (2.0 mM) catalyzed by 1 (2.0 µM) in
saturated ([O2] = 11 mM) acetone at 298 K. (d) Plot of kobs vs [Sc(OTf)3] for
the two-electron reduction of O2 by Fc* (2.0 mM) with Sc(OTf)3 (5-25 mM)
catalyzed by 1 (2 µM) in saturated ([O2] = 11 mM) acetone at 298 K.
([(tmpa)CuII(O2)CuII(tmpa)]2+) (Figure 6), where the
absorption band at 520 nm due to the peroxo compelex was
observed by the reaction of [(tmpa)CuI]+ with O2 at 213 K.24a,26
The addition of one equiv of Sc(OTf)3 (2 mM) to an acetone
solution of the peroxo complex resulted in disappearance of the
absorption band due to the peroxo complex, accompanied by
appearance of the absorption band at 394 nm, which is tentatively
assigned to the Sc3+-bound peroxo complex, [(tmpa)CuII(O2)-
Sc(OTf)3]2+; the conversion exhibits an isosbestic point (Figures
6a and 6b). At prolonged reaction times, absorption bands at both
394 and 520 nm decayed to yield [(tmpa)CuII]2+ (blue line in
Figure 6c) and [Sc3+(O22–)]+.
Based on the results described above, the catalytic cycle of the
two-electron reduction of O2 by Fc* with 1 in the presence of
Sc(OTf)3 is proposed as shown in Scheme 1. Electron transfer
from Fc* to 1 is the rate-determining step to produce Fc*+ and
[(tmpa)CuI]+, which rapidly reacts with O2 to produce the
superoxo complex ([(tmpa)CuII(O2)]+). There are two pathways
of the further reaction of [(tmpa)CuII(O2)]+. One is the reaction
of [(tmpa)CuII(O2)]+ with [(tmpa)CuI]+ to produce the peroxo
complex ([(tmpa)CuII(O2)CuII(tmpa)]2+) which reacts with Sc3+
to yield [(tmpa)CuII]2+ and [Sc3+(O22–)]+ (Figure 6). In such a
case, the catalytic rate constant would be the same as the rate
constant of electron transfer from Fc* to [(tmpa)CuII]2+ (kcat = ket).
determining step in the catalytic two-electron reduction of O2 by
Fc* with 1. In such a case, 1 should remain as the cupric complex
[(tmpa)CuII]2+ during the catalytic two-electron reduction
reaction. This was confirmed by the observation of the EPR
spectrum of 1 as measured during catalysis as shown in Figure 5,
where the EPR spectrum (red line) during the reaction is the
same as [(tmpa)CuII]2+ before the reaction (black line).
Because kcat
= 2ket (vide supra), the superoxo complex
Figure 5. EPR spectra of [(tmpa)CuII]2+ 1 (0.10 mM) (black line) measured
at 77 K during the catalytic two-electron reduction of O2 (11 mM) by Fc* (2
mM) with Sc(OTf)3 (10 mM) at 298 K (red line). EPR parameters of
[(tmpa)CuII]2+: g⊥ = 2.21, A⊥= 100 G, g// = 2.00, A// = 64 G.
([(tmpa)CuII(O2)]+) may be rapidly reduced by Fc* with Sc3+ to
produce Fc*+ and [Sc3+(O22–)]+, accompanied by regeneration of
[(tmpa)CuII]2+ (Scheme 1).
When Sc(OTf)3 was replaced by trivalent metal trifates such as
Yb(OTf)3, Y(OTf)3 and Lu(OTf)3, which are weaker Lewis acid
than Sc(OTf)3,38-40 the catalytic reactivity of the two-electon
reduction of O2 by Fc* with 1 becomes lower than that the case of
Sc(OTf)3 as shown in Figure 7. Di-valent metal triflates such as
Mg(OTf)2 and Ca(OTf)2, which are still weaker Lewis acid than
trivalent metal triflates,39-42 exhibited lower reactivity and the
Electron transfer from Fc* to 1 is followed by the well-
established nearly diffusion controlled binding of O2 to
[(tmpa)CuI]+ producing superoxo complex ([(tmpa)CuII-
(O2)]+) that further rapidly reacts with [(tmpa)CuI]+ to afford the
peroxo complex (trans–µ–1,2–peroxo-dicopper complex
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2Me2Fc + O2 + Sc3+ → 2Me2Fc+ + Sc3+(O2
2–
Scheme 1
)
(5)
1
2
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
reaction was stopped before completion (Figure 7). Thus, strong
Lewis acidity of metal ions is required for the efficient catalytic
two-electron reduction of O2 by Fc* with 1.
Figure 8. Displacement ellispoid plot (50% probability level)of one
crystallographically
independent
cation
of
[CuII(BzQ)(H2O)2]-
(ClO4)2•2.33(C3H6O) BzQCuII; the two remaining cations, the ClO4
counteranions, and the lattice acetone solvent molecules have been omitted
for clarity. Selected bond distances: Cu1-N1A,1.9950(14) Å; Cu1-N2A,
2.0494(14) Å; Cu1-N3, 1.9958(14) Å; Cu1-O1W1, 2.1934(12) Å Cu1-
O1W2 2.0002 (13). Selected bond angles: N1A-Cu1-N3A, 165.60(6)°;N1A-
Cu1-N2A, 83.21(6)°;N2A-Cu1-N3A, 83. 82.39(6)°; N1A-Cu1-O1W1,
90.69(5)°; O1W1-Cu1-O1W2, 108.97(5)°; N2A-Cu1-O1W2, 141.95(6)°.
–
The stoichiomety is the same as in eq 2, where one equiv of Sc3+
was consumed for formation of two equiv of Me2Fc+ (Figure S3).
Ferrocene itself can also be used to reduce O2 to [Sc3+(O22–)]+
with 2 and Sc(OTf)3 although the rate is slower than that of
Me2Fc because of the higher Eox value of Fc (0.37 V vs SCE) than
that of Me2Fc (Eox = 0.28 V vs SCE).
Figure 7. Time courses of absorbance at 780 nm due to Fc*+ in the two-
electron reduction of O2 (11 mM) by Fc* (2.0 mM) with metal triflates
[Sc(OTf)3, Yb(OTf)3, Y(OTf)3, Lu(OTf)3, Mg(OTf)2, and Ca(OTf)2] (2.0
mM) catalyzed by 1 (40 µM) in acetone at 298 K.
The rate of formation of Me2Fc+ obeyed pseudo-zero-order
kinetics as shown in Figure 9a, where the inital rate of formation
of Me2Fc+ (Ri) is independent of concentration of Me2Fc. Ri is
also independent of concentration of Sc3+ (Figure 9b), whereas Ri
is proportional to concentrations of O2 and 2 as shown in Figure
9c and 9d, respectively. Thus, the rate of formation of the two-
electron reduction of O2 by Me2Fc with 2 in the presence of large
excess Sc(OTf)3 is given by eq 6,
When Fc* was replaced by a weaker one-electron reductant
such as 1,1’-dimethylferrocene (Me2Fc), no electron tranfser from
Me2Fc (Eox = 0.28 V vs SCE) to 1 (Ered = -0.05 V vs SCE)
occurred, leading to no catalytic reduction of O2 in the presence of
Sc(OTf)3. Thus, we examined the catalytic reduction of O2 by
Me2Fc in the presence of Sc(OTf)3 using a copper(II) complex
which has a more positive Ered value than 1 (vide infra).
Catalytic Two-Electron Reduction of O2 by Me2Fc with 2 in
the Presence of Sc(OTf)3. Pyridyl ligand alterations which
introduce steric effects are known to result in a decrease in the
donor ability to a Cu(II) center, which causes a positive shift in
the redox potential of Cu(II) complexes.29 In order to use milder
reductants for the reduction of dioxygen, we synthesized
[(BzQ)CuII](ClO4)2 (2) as a potential catalyst. Complex 2 was
generated by the addition of BzQ to Cu(ClO4)2·6H2O in MeOH
and characterized by elemental analysis. Recrystallization of 2
from acetone/pentane afforded crystals suitable for X-ray
structure determination; the structure of 2 is shown in Figure 8.43
The steric effect of quinoline ligand is recognized as the elongated
Cu-N bonds as compared with those of [(tmpa)CuII]2+.
d[Me2Fc+]/dt = k’cat[O2][2]
(6)
where k’cat is the the second-order catalytic rate constant.
Because the catalytic rate is proportional to concentrations of
O2 and 2, but independent of concentrations of Me2Fc or Sc3+, the
the rate-determining step in the catalytic cycle is the reaction of
[(BzQ)CuI]+ with O2. In such
a case, 2 is converted to
[(BzQ)CuI]+ during the catalytic two-electron reduction of O2 by
Me2Fc with 2. This was confirmed by disappearace of the EPR
spectrum of 2 measured during the catalysis as shown in Figure 10,
where the EPR signal due to 2 (black line) is converted to the CuI
complex, which is EPR silent (red line).
The reaction of the CuI complex of 2 with O2 was previously
reported to afford copper(II)-oxygen intermediates different than
that known for the case of 1, that are
peroxo)dicopper(II) complex (λmax = 362 and 535 nm) plus a
The Ered value of 2 was determined to be 0.44 V vs SCE, which
is much more positive than that of 1 (-0.05 V vs SCE), thus the
two-electon reduction of O2 by Me2Fc became possible using 2 as
a catalyst in the presence of Sc(OTf)3 in acetone at 298 K (eq 5).
a
(η2:η2-
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(a)
(b)
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0
1
2
3
4
5
6
1.0
0.8
0.6
0.4
0.2
0
7
8
0
1000 2000 3000 4000 5000
Time, s
300
400 500 600 700
Wavelength, nm
800 900
9
(c)
1.2
1.0
0.8
0.6
0.4
0.2
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
300
400 500 600 700
Wavelength, nm
800 900
Figure 11. (a) UV-vis spectral changes observered upon the addition of 2 eq
of HOTf (4.0 mM) to the mixture of the µ-η2-η2-(side-on) peroxo dinuclear
copper(II) complex and the bis-µ-oxo dinuclear copper(III) complex. (b)
Absorption time profiles at 394 nm due to the addition of HOTf (4.0 mM) to
to the mixture of the µ-η2-η2-(side-on) peroxo dinuclear copper(II) complex
and the bis-µ-oxo dinuclear copper(III) complex.. (c) UV-vis spectral changes
observered upon the addition of Sc(OTf)3 (4.0 mM) to the mixture of the µ-
η2-η2-(side-on) peroxo dinuclear copper(II) complex and the bis-µ-oxo
dinuclear copper(III) complex.
Figure 9. (a) Plot of the initial rate of formation of Me2Fc+ vs [Me2Fc] in the
two-electron reduction of O2 by Me2Fc with Sc(OTf)3 (10 mM) catalyzed by
2 (0.2 mM) in saturated ([O2] = 11 mM) acetone at 298 K. (b) Plot of the
initial rate of formation of Me2Fc+ vs [Sc(OTf)3] in the two-electron reduction
of O2 by Me2Fc (10 mM) with Sc(OTf)3 catalyzed by 2 (0.20 mM) in
saturated ([O2] = 11 mM) acetone at 298 K. (c) Plot of kobs vs [O2] for the
two-electron reduction of O2 by Me2Fc (10 mM) with Sc(OTf)3 (10 mM)
catalyzed by 2 (0.2 mM) in acetone at 298 K. (d) Plot of kobs vs [2] for the
two-electron reduction of O2 by Me2Fc (10 mM) with Sc(OTf)3 (10 mM)
catalyzed by 2 in saturated ([O2] = 11 mM) acetone at 298 K.
Scheme 2
[LCuIIO2]+
[LCuI]+
O2
+
[LCuI]+
O
O
CuIIL]2+
+ Sc3+
[LCuII
no reaction
O
O
CuIIIL]2+
[LCuIII
Thus, once the µ-η2-η2-(side-on) peroxo dinuclear copper(II)
complex and the bis-µ-oxo dinuclear copper(III) complex are
formed via the superoxo complex, no catalytic reduction of O2 by
Me2Fc would occur with 2 in the presence of Sc(OTf)3. Under the
catalytic conditions, the superoxo complex is reduced by Me2Fc in
the presence of Sc3+ to yield Me2Fc+ and Sc(O2)+, accompanied by
regeneration of 2 without formation of the µ-η2-η2-(side-on)
peroxo dinuclear copper(II) complex or the bis-µ-oxo dinuclear
copper(III) complex as shown in Scheme 3. The rate-determining
step in Scheme 3 is the reaction of [(BzQ)CuI]+ with O2 to
produce the superoxo complex, when the catalytic rate is
proportional to concentrations of O2 and 2, but independent of
concentrations of Me2Fc or Sc3+ as observed in Figure 9.
Figure 10. EPR spectra of [[(BzQ)CuII]2+] (2) (0.10 mM) (black line)
observed at 77 K, [[(BzQ)CuI]+] (0.05 mM) produced during the catalytic
reduction of oxygen (2.2 mM) in the presence of Me2Fc (10 mM) and
Sc(OTf)3 (10 mM) (red line).
bis(µ-oxo)dicopper(III) species (λmax = 394 nm) (see Figure 11a,
black line); the mixture had been characterized by resonance
Raman spectroscopy.29 The addition of 2 equiv HOTf to an
acetone solution of the peroxo and bis-µ-oxo complexes resulted
in the decomposition of the Cu-O2 species to release H2O2
(Figure 11b). In contrast, the addition of Sc(OTf)3 to an acetone
solution of the peroxo and bis-µ-oxo complexes resulted in little
change in absorption bands of the Cu-O2 intermediates (Figure
11c), indicating that these complexes are stable against Sc3+
(Scheme 2).
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Scheme 3
(3) Mukherjee, A.; Smirnov, V. V.; Lanci, M. P.; Brown, D. E.; Shepard, E.
1
2
3
4
5
6
7
8
M.; Dooley, D. M.; Roth, J. P. J. Am. Chem. Soc. 2008, 130, 9459−9473.
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(5) (a) Blanford, C. F.; Heath, R. S.; Armstrong, F. A. Chem. Commun.
2007, 1710. (b) Mano, N.; Soukharev, V.; Heller, A. J. Phys. Chem. B
2006, 110, 11180−11187.
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2001, 1505, 185. (b) Winter, M.; Brodd, R. J. Chem. Rev. 2004, 104,
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O2
[LCuI]+
Me2Fc+
+
O
LCuII
O
Me2Fc
[LCuII]2+
Me2Fc + Sc3+
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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43
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Me2Fc+ + ScO2
+
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Conclusion
The four-electron reduction of O2 by Fc* with a mononuclear
complex [(tmpa)CuII](ClO4)2 (1) in the presence of a proton
source (HOTf) was changed to the two-electron reduction of O2
by replacing Brønsted acids by Sc(OTf)3 that acts as a strong Lew-
is acid. The rate-determining step of the catalytic cycle is found to
be electron transfer from Fc* to O2. When 1 was replaced by a
copper(II) complex [(BzQ)CuII](ClO4)2 (2), which has a more
positive reduction potential as compared with 1, the catalytic two-
electron reduction of O2 is made possible by using a weaker one-
electron reductant than Fc* such as Me2Fc and Fc. In this case, the
rate-determining step is the reaction of [(BzQ)CuI]+ with O2 to
produce the superoxo complex. The Lewis acid-induced change in
the stoichiometry of the catalytic O2 reduction provides a new
way to control this important biological or chemical “fuel-cell”
reaction which can produce either hydrogen peroxide or water.
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ASSOCIATED CONTENT
Supporting Information. Spectral and kinetic analytical data, theo-
retical calculation data (Figures S1-S6), and X-ray crystallographic
data (pdf); crystallographic file (cif). This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp; karlin@jhu.edu
ACKNOWLEDGMENT
This work was supported by an Advanced Low Carbon Technology
Research and Development (ALCA) program from Japan Science
Technology Agency (JST) to S.F., the Japan Society for the
Promotion of Science (JSPS: Grants 20108010 to S.F. and 26620154
and 26288037 to K.O.) sponsored by the MEXT (Japan), and by
KOSEF/MEST through the WCU Project (R31-2008-000-10010-0).
K.D.K. also acknowledges support from the USA National Institutes
of Health.
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