Photochemical oxidation of a manganese(III) complex with oxygen and toluene derivatives to form a manganese(V)-Oxo complex

Article abstract of DOI:10.1021/ic402121j

Visible light photoirradiation of an oxygen-saturated benzonitrile solution of a manganese(III) corrolazine complex [(TBP₈Cz)Mn^III] (1): [TBP₈Cz = octakis(p-tert-butylphenyl)corrolazinato^3-] in the presence of toluene derivatives resulted in formation of the manganese(V)-oxo complex [(TBP₈Cz)Mn^V(O)]. The photochemical oxidation of (TBP₈Cz)Mn^III with O ₂ and hexamethylbenzene (HMB) led to the isosbestic conversion of 1 to (TBP₈Cz)Mn^V(O), accompanied by the selective oxidation of HMB to pentamethylbenzyl alcohol (87%). The formation rate of (TBP ₈Cz)Mn^V(O) increased with methyl group substitution, from toluene, p-xylene, mesitylene, durene, pentamethylbenzene, up to hexamethylbenzene. Deuterium kinetic isotope effects (KIEs) were observed for toluene (KIE = 5.4) and mesitylene (KIE = 5.3). Femtosecond laser flash photolysis of (TBP₈Cz)Mn^III revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state was shown to be the key, activated state that reacts with O₂ via a diffusion-limited rate constant. The data allow for a mechanism to be proposed in which the tripseptet excited state reacts with O₂ to give the putative (TBP₈Cz)Mn ^IV(O₂^?-), which then abstracts a hydrogen atom from the toluene derivatives in the rate-determining step. The mechanism of hydrogen abstraction is discussed by comparison of the reactivity with the hydrogen abstraction from the same toluene derivatives by cumylperoxyl radical. Taken together, the data suggest a new catalytic method is accessible for the selective oxidation of C-H bonds with O₂ and light, and the first evidence for catalytic oxidation of C-H bonds was obtained with 10-methyl-9,10-dihydroacridine as a substrate.

Full text of DOI:10.1021/ic402121j

Article
pubs.acs.org/IC
Photochemical Oxidation of a Manganese(III) Complex with Oxygen
and Toluene Derivatives to Form a Manganese(V)-Oxo Complex
Jieun Jung,† Kei Ohkubo,† Katharine A. Prokop-Prigge,‡ Heather M. Neu,‡ David P. Goldberg,*,‡
and Shunichi Fukuzumi*,†,§
†Department of Material and Life Science, Graduate School of Engineering, ALCA, Japan Science and Technology Agency,
Osaka University, Suita, Osaka 565-0871, Japan
‡Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States
§Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: Visible light photoirradiation of an oxygen-
saturated benzonitrile solution of a manganese(III)
corrolazine complex [(TBP8Cz)MnIII] (1): [TBP8Cz =
octakis(p-tert-butylphenyl)corrolazinato3−] in the presence of
toluene derivatives resulted in formation of the manganese(V)-
oxo complex [(TBP8Cz)MnV(O)]. The photochemical oxidation
of (TBP8Cz)MnIII with O2 and hexamethylbenzene (HMB) led
to the isosbestic conversion of 1 to (TBP8Cz)MnV(O), accompanied by the selective oxidation of HMB to pentamethylbenzyl alcohol
(87%). The formation rate of (TBP8Cz)MnV(O) increased with methyl group substitution, from toluene, p-xylene, mesitylene, durene,
pentamethylbenzene, up to hexamethylbenzene. Deuterium kinetic isotope effects (KIEs) were observed for toluene (KIE = 5.4) and
mesitylene (KIE = 5.3). Femtosecond laser flash photolysis of (TBP8Cz)MnIII revealed the formation of a tripquintet excited state,
which was rapidly converted to a tripseptet excited state. The tripseptet excited state was shown to be the key, activated state that
reacts with O2 via a diffusion-limited rate constant. The data allow for a mechanism to be proposed in which the tripseptet excited state
reacts with O2 to give the putative (TBP8Cz)MnIV(O2•−), which then abstracts a hydrogen atom from the toluene derivatives in the
rate-determining step. The mechanism of hydrogen abstraction is discussed by comparison of the reactivity with the hydrogen
abstraction from the same toluene derivatives by cumylperoxyl radical. Taken together, the data suggest a new catalytic method is
accessible for the selective oxidation of C−H bonds with O2 and light, and the first evidence for catalytic oxidation of C−H bonds was
obtained with 10-methyl-9,10-dihydroacridine as a substrate.
electron oxidation of metal complexes with H2O as the
oxygen source [eq 3].17−23
INTRODUCTION
■
High-valent metal-oxo species are key oxidizing intermediates
in a variety of biological oxidation reactions mediated by heme
and nonheme metalloenzymes involved in respiration, metabolism,
and photosynthesis.1−4 Synthetic high-valent metal-oxo
complexes (M(n+2)+(O)) are usually formed by reactions of
metal complexes (Mn+) with two-electron oxidants such as
iodosylarenes, peroxy acids, and H2O2 [eq 1].5−16 However,
M
n+ + H2O − 2H+ − 2e− → M(n+2)+(O)
(3)
There have been comparatively few reports on the formation
of discrete, high-valent metal-oxo complexes using O2.24−26
The activation of O2 and stabilization of high-valent metal-oxo
species is challenging to carry out in a single ligand environ-
ment. Porphyrinoid ligands, and in particular corroles and
corrolazines (Czs), are designed to stabilize high-valent species
including high-valent metal-oxo complexes, and there are
examples of CrV(O) corroles generated from O2.27 It has
recently been reported that the oxidation of a MnIII corrolazine
complex with O2 in cyclohexane or toluene as solvent under
visible light irradiation leads to the quantitative production of a
well-characterized, isolable MnV(O) complex.28 Relatively short-
lived metal-oxo species have been generated by photoinitiated
homolytic axial ligand cleavage reactions with metalloporphyrins,
as well as photochemical splitting of metalloporphyrin μ-oxo
M
n+ + H2O2 → M(n+2)+(O) + H2O
(1)
enzymes such as cytochrome P450 use dioxygen (O2)
together with the addition of two electrons and two protons,
which is equivalent to H2O2, to generate a high-valent metal-
oxo species [eq 2].1,2 On the other hand,
M
n+ + O2 + 2H+ + 2e− → M(n+2)+(O) + H2O
(2)
high-valent manganese-oxo species are implicated in the
water oxidation mechanism of Photosystem II, and may
involve the formal two-electron oxidation of a H2O molecule
by a tetranuclear manganese complex to give high-valent
Mn(O) intermediates.4,5 Synthetic high-valent metal-oxo
complexes have also been reported to be formed by two-
Received: August 20, 2013
© XXXX American Chemical Society
A
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dimers.29,30 However, to our knowledge the former photo-
initiated reaction involving MnIII(Cz) is the first example of the
production of a well-defined MnV(O) species from O2.
authentic samples. Compounds were quantified by comparison against
a known amount of detected products using a calibration curve con-
sisting of a plot of mole versus area. Calibration curves were prepared
by using concentrations in the same range as that observed in the
actual reaction mixtures. Mass spectra were recorded with a JEOL
JMS-700T Tandem MS station, and the GC-MS analyses were carried
out by using a Shimadzu GCMS-QP2000 gas chromatograph mass
spectrometer. GC-MS conditions in these experiments were performed
as follows: an initial oven temperature of 60 °C was held for 1 min and
then raised 30 °C min−1 for 7.3 min until a temperature of 280 °C was
reached, which was then held for further 10 min. This experiment was
repeated three times, exhibiting error in yields within 10%.
In the former study, the combined data suggested a
mechanism for the generation of MnV(O)(Cz) that involved
solvent-assisted autoxidation, in which the solvent (cyclohexane
or toluene) served as a sacrificial reductant for the activation
and cleavage of a putative photactivated Mn−O2 intermediate.
The nature of the photoexcited state and the product(s) of sol-
vent oxidation were not identified in this study. The proposed
mechanism suggested that, perhaps under conditions involving
an inert solvent, the controlled oxidation of substrates such as
toluene derivatives could be mediated by MnIII(Cz)/O2/light.
We report herein the use of benzonitrile (PhCN) as an inert
solvent for the reaction of the MnIII complex [(TBP8Cz)MnIII:
TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3−] with O2
under visible light irradiation. The conversion of (TBP8Cz)-
MnIII to (TBP8Cz)MnV(O) does not occur in PhCN in the
absence of a proton/electron source, but with the addition of a
series of toluene derivatives as substrates the smooth production
of the MnV(O) complex is observed. The concomitant oxidation
of the toluene derivatives occurs with high selectivity and effi-
ciency, leading to monohydroxylation of a benzylic position.
Femtosecond laser flash photolysis (LFP) measurements
resulted in the spectroscopic observation of a short-lived,
O2-reactive (TBP8Cz)MnIII excited state. The LFP experi-
ments, together with product analyses and kinetic measure-
ments, including kinetic deuterium isotope effects, provide
valuable insights into the mechanism of generation of the
MnV(O) complex by the photochemical oxidation of the MnIII
complex using O2 as an oxygen source and toluene derivatives
as the source of protons and electrons.
Spectral and Kinetic Measurements. The photochemical
oxidations of (TBP8Cz)MnIII (7.6 × 10−6 M) with excess O2 were
examined by monitoring UV−vis spectral changes in the presence
of a large excess of toluene derivatives (2.0 × 10−2 to 9.4 M) at 298 K
using a Hewlett-Packard HP8453 diode array spectrophotometer
under continuous irradiation of white light from a Shimadzu RF-
5300PC fluorescence spectrophotometer at the same time. Typically,
an aerated PhCN solution of (TBP8Cz)MnIII was added with a micro-
syringe to an aerated PhCN solution containing the toluene derivatives
in a quartz cell (total vol. 2.0 mL). Rates of oxidation reaction of
(TBP8Cz)MnIII to produce (TBP8Cz)MnV(O) were monitored by
the decrease in the absorption band due to (TBP8Cz)MnIII (λmax
=
695 nm, εmax = 3.51 × 104 M−1 cm−1). The concentration of toluene
derivatives were maintained in large excess as compared to
[(TBP8Cz)MnIII] for all kinetic measurements. The limiting
concentration of O2 in PhCN solution was prepared by a mixed gas
flow of O2 and N2. The mixed gas was controlled by using a gas mixer
(Kofloc GB-3C, KOJIMA Instrument Inc.), which can mix two or
more gases at a certain pressure and flow rate.
Quantum Yield Determination. A standard actinometer
(potassium ferrioxalate)34 was used for the quantum yield determination
of the photochemical oxidation of (TBP8Cz)MnIII with O2 and toluene
derivatives in O2-saturated PhCN. Typically, a square quartz cuvette
(10 mm i.d.), which contained an O2-saturated PhCN solution (2.0 mL)
of (TBP8Cz)MnIII (7.6 × 10−6 M) and a toluene derivative was
irradiated with monochromatic light of λ = 450 nm from a Shimadzu
RF-5300PC fluorescence spectrophotometer. Under the conditions of
actinometry experiments, the actinometer and (TBP8Cz)MnIII absorbed
essentially all of the incident light at λ = 450 nm. The light intensity of
monochromatized light at λ = 450 nm was determined to be 3.7 × 10−8
einstein s−1. The quantum yields were determined by monitoring the
disappearance of absorbance at 695 nm due to (TBP8Cz)MnIII.
Photocatalytic Activity Measurements. The photocatalytic
reactivity of (TBP8Cz)MnIII (1.7 × 10−4 M) with excess of O2 was
examined by monitoring the UV−vis spectral changes in the presence
of a large excess of AcrH2 (0.2 M) in a quartz cell (optical path length
1 mm) using a Xe lamp (500 W) for irradiation through a transmitting
glass filter (λ > 480 nm) at room temperature. In a typical experiment,
(TBP8Cz)MnIII (1.7 × 10−4 M) was dissolved in PhCN (0.5 mL)
containing excess AcrH2 (0.2 M). The solution was purged with O2
gas for 10 min in the quartz cell, and then the reaction was initiated
by irradiating the solution with a Xe lamp (500 W) transmitting through a
glass filter (λ > 480 nm). The UV−vis spectra of the solution were mea-
sured using a Hewlett-Packard HP8453 diode array spectrophotometer
every 1 h. The amount of 10-methyl-(9,10H)-acridinone (Acr=O)
produced was quantified by an increase in the absorption band due to
Acr=O (λ = 402 nm, εmax = 8.6 × 103 M−1 cm−1 in PhCN).
EXPERIMENTAL SECTION
■
Materials. The starting material (TBP8Cz)MnIII was synthesized
according to published procedures.31 The commercially available
reagents (p-xylene, mesitylene, durene, pentamethylbenzene, and hexa-
methylbenzene) were purchased with the best available purity and
used without further purification. Toluene and benzonitrile (PhCN)
were dried according to literature procedures32 and distilled under Ar
prior to use. The reagents 2,3,4,5,6-pentamethylbenzyl alcohol and
pentamethylbenzaldehyde were also purchased with the best available
purity and used without further purification from Wako Pure Chemical
Industries, Ltd. and Tokyo Kasei Co., Ltd., respectively. Di-tert-
butylperoxide was purchased from Nacalai Tesque Co., Ltd. and
purified by chromatography through alumina, which removes traces
of the hydroperoxide. 10-Methyl-9,10-dihydroacridine (AcrH2−) was
prepared from 10-methylacridinium perchlorate (AcrH+ClO4 ) by
reduction with NaBH4 in methanol and purified by recrystallization
from ethanol.33 Cumene was purchased from Tokyo Kasei Co., Ltd.
and also purified by chromatography through alumina. Deuterated
toluene and mesitylene were purchased from Cambridge Isotopes in
the highest purity and used as received.
Product Analysis. A PhCN solution of (TBP8Cz)MnIII (5.0 ×
10−4 M) was added with a microsyringe into an O2-saturated PhCN
solution containing hexamethylbenzene (2.0 × 10−2 M) as a substrate
in a quartz cell. A rather large concentration of (TBP8Cz)MnIII
was employed to obtain detectable amounts of products by GC-MS.
The mixture of the reaction was filled with O2 and then irradiated
for 24 h at room temperature, after which the solution changed from
the green-brown color indicative of (TBP8Cz)MnIII to the bright-green
color of (TBP8Cz)MnV(O). Complete conversion of (TBP8Cz)MnIII
to (TBP8Cz)MnV(O) was confirmed by UV−vis spectroscopy by
sampling an aliquot of the reaction mixture. An aliquot was removed
and injected directly into the GC-MS for analysis. All peaks of interest
were identified by comparison of retention times and coinjection with
Femtosecond Laser Flash Photolysis Measurements. Meas-
urements of transient absorption spectra in the oxidation reaction of
(TBP8Cz)MnIII were performed according to the following procedures.
An O2- or N2-saturated PhCN solution containing (TBP8Cz)MnIII (6.8 ×
10−5 M) was excited using an ultrafast source, Integra-C (Quantronix
Corp.), an optical parametric amplifier, TOPAS (Light Conversion Ltd.),
and a commercially available optical detection system, Helios provided
by Ultrafast Systems LLC. The source for the pump and probe pulses
were derived from the fundamental output of Integra-C (λ = 786 nm,
2 mJ/pulse and fwhm = 130 fs) at a repetition rate of 1 kHz. 75% of the
fundamental output of the laser was introduced into a second harmonic
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Scheme 1. Formation of (TBP8Cz)MnV(O) from (TBP8Cz)MnIII with O2 and Toluene Derivatives under Photoirradiation
(White Light)
generation (SHG) unit: Apollo (Ultrafast Systems) for excitation light
generation at λ = 393 nm, while the rest of the output was used for white
light generation. The laser pulse was focused on a sapphire plate of 3 mm
RESULTS AND DISCUSSION
Photochemical Oxidation of (TBP8Cz)MnIII with O2 and
■
Toluene Derivatives. The photochemical generation of a
MnV(O) complex using O2 as an oxygen source was performed
by photoirradiation of an aerated PhCN solution containing a
MnIII complex [(TBP8Cz)MnIII] and toluene derivatives
(Scheme 1). Monitoring this reaction by UV−vis spectroscopy
revealed the transformation of (TBP8Cz)MnIII (λmax = 695 nm,
εmax = 3.51 × 104 M−1 cm−1) to the MnV(O) complex
[(TBP8Cz)MnV(O)] (λmax = 634 nm, εmax = 2.0 × 104 M−1
cm−1)28 with isosbestic points as shown in Figure 1. No further
thickness and then white light continuum covering the visible region from
λ = 410 to 800 nm was generated via self-phase modulation. A variable
neutral density filter, an optical aperture, and a pair of polarizers were
inserted in the path to generate stable white light continuum. Prior to
generating the probe continuum, the laser pulse was fed to a delay line
that provides an experimental time window of 3.2 ns with a maximum
step resolution of 7 fs. In our experiments, a wavelength at λ = 393 nm of
SHG output was irradiated at the sample cell with a spot size of 1 mm
diameter where it was merged with the white probe pulse in a close angle
(<10°). The probe beam after passing through the 2 mm sample cell
was focused on a fiber optic cable that was connected to a CMOS
spectrograph for recording the time-resolved spectra (λ = 410−800 nm).
Typically, 1500 excitation pulses were averaged for 3 s to obtain the tran-
sient spectrum at a set delay time. Kinetic traces at appropriate wave-
lengths were assembled from the time-resolved spectral data. The decay
rate of the tripquintet (5T1) obeyed the first-order kinetics given by eq 4
where A1 is the pre-exponential factor for the absorbance changes and A2
is the final absorbance at 530 nm
ΔAbs = A1exp(−k1t) + A2
(4)
and k1 is the rate constant of the decay of the tripquintet (5T1) after
irradiation. The slower decay rate of the tripseptet (7T1) also obeyed the
first-order kinetics given by eq 5, where A1 and A2 are pre-exponential
factors for the absorbance changes, A3 is the final absorbance at 774 nm
and k2 is the rate
Figure 1. (a) UV−vis spectral changes and (b) time profiles of
absorption changes at λ = 695 and 634 nm for the photochemical
oxidation reaction of (TBP8Cz)MnIII (7.6 × 10−6 M) under irradiation
(white light) in an aerobic solution of PhCN containing hexamethyl-
benzene (0.08 M) as a substrate at room temperature. The dotted line is
the best fit of the initial rate.
ΔAbs = A1exp(−k1t) + A2exp(−k2t) + A3
(5)
constant of the decay of 7T1. All measurements were conducted at room
temperature, 298 K.
EPR Measurements. Photoirradiation of an oxygen-saturated
propionitrile solution containing di-tert-butylperoxide (1.0 M) and
cumene (1.0 M) with a 1000 W mercury lamp (Ushio-USH1005D)
through an aqueous filter resulted in formation of cumylperoxyl radical
(g = 2.0156).35 The electron paramagnetic resonance (EPR) spectra
were measured with a JEOL X-band spectrometer (JES-RE1XE) and
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 value
was calibrated by using a Mn2+ marker. Upon cutting off the light
irradiated through a window directly, the decay of the EPR intensity
was recorded with time. The decay rates were accelerated by the
presence of toluene derivatives, indicating hydrogen atom transfer
occurred. Rates of hydrogen atom transfer from toluene derivatives to
cumylperoxyl radical were monitored by measuring the decay of the
EPR signal in the presence of various concentrations of toluene
derivatives in propionitrile at −80 °C. Pseudo-first-order rate constants
were determined by a least-squares curve fitting procedure. The first-
order plots of ln(I − I∞) vs time (I and I∞ are the EPR intensity at
time t and the final intensity, respectively) were linear for three or
more half-lives with the correlation coefficient, ρ > 0.99. Plot of the
pseudo-first-order rate constants against the substrate concentrations
gave linear lines.
reaction of (TBP8Cz)MnV(O) with toluene occurred for another
1 h (Supporting Information, Figure S1), as expected from pre-
vious work.28,36 No formation of (TBP8Cz)MnV(O) was observed
in the absence of O2 or toluene derivatives, indicating both toluene
derivatives and O2 are certainly required to generate (TBP8Cz)-
MnV(O) (Supporting Information, Figures S2 and S3).
The photochemical oxidation of (TBP8Cz)MnIII with O2 and
hexamethylbenzene to produce (TBP8Cz)MnV(O) is accom-
panied by the oxidation of hexamethylbenzene. Analysis of
the reaction mixture by GC-MS reveals the major product
of oxidation to be the pentamethylbenzyl alcohol in 87% yield
(based on total Mn content), along with a small amount of
pentamethylbenzaldehyde (8%) (see Supporting Information,
Figures S4−S5, and Table S1). Thus the oxidation of hexa-
methylbenzene occurs with high selectivity, and the stoichiometry
of the main reaction is given by eq 6.
(TBPCz)MnIII + O2 + C6Me6
8
hv
→ (TBPCz)MnV(O) + C6Me5CH2OH
(6)
8
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Kinetics. Rates of formation of (TBP8Cz)MnV(O) in the
photochemical oxidation of (TBP8Cz)MnIII with O2 in the air
and toluene derivatives were monitored from the decay of
absorbance at 695 nm due to (TBP8Cz)MnIII in aerated PhCN
at 298 K. The zeroth-order rate constant (kobs) was determined
from the initial rate in Figure 1b to avoid the decrease in the
light intensity absorbed by (TBP8Cz)MnIII in the course of
the photochemical reaction. The kobs values were derived from the
initial slopes of the plots of [(TBP8Cz)MnIII] vs time for the
different substrates, as shown in Supporting Information, Figures
S6−S11. The observed rate constants are proportional to
concentrations of substrates, as shown in Figure 2 (see Supporting
Table 1. Second-Order Rate Constants for the Oxidation
of (TBP8Cz)MnIII (kox) by O2 with Toluene Derivatives
in Aerated PhCN at 298 K and for the Hydrogen-Atom
Transfer from Toluene Derivatives to Cumylperoxyl
Radicals (k′H) in Aerated Propionitrile at 193 K
substrate
toluene
kox, M−1 s−1
k′H, M−1 s−1
(4.0 0.4) × 10−7
(1.1 0.3) × 10−7
(2.7 0.1) × 10−6
(1.2 0.4) × 10−6
(3.6 0.1) × 10−5
(8.0 0.2) × 10−5
(6.5 0.2) × 10−3
(8.3 0.3) × 10−3
(9.2 0.4) × 10−3
(2.3 0.1) × 10−2
(4.7 0.2) × 10−2
(1.3 0.1) × 10−1
p-xylene
mesitylene
durene
pentamethylbenzene
hexamethylbenzene
groups in the substrate. A dramatic 200-fold rate enhancement
is observed in the presence of hexamethylbenzene over toluene.
When toluene (2.0 M) was replaced with its deuterated
analogue (C6D5CD3), the reaction rate of formation of
(TBP8Cz)MnV(O) with O2 became significantly slower as
shown in Figure 4a (blue for C6H5CH3 and red for C6D5CD3).
Figure 2. Plots of the observed zeroth-order rate constants (kobs) of the
oxidation reaction of (TBP8Cz)MnIII (7.6 × 10−6 M) under irradiation
(white light) of an aerated PhCN solution containing toluene (red, 1.0 to
8.2 M), p-xylene (orange, 1.0 to 4.0 M), mesitylene (green, 0.5 to 2.0 M),
durene (blue, 0.1 to 0.4 M), pentamethylbenzene (purple, 2.0 × 10−2 to
1.0 × 10−1 M) or hexamethylbenzene (black, 2.0 × 10−2 to 1.0 × 10−1 M)
as a substrate at 298 K.
Figure 4. Time dependence of photochemical oxidation of (TBP8Cz)-
MnIII (8.0 × 10−6 M) with (a) C6H5CH3 (blue, 2.0 M), C6D5CD3 (red,
2.0 M), (b) mesitylene (blue, 2.0 M), or mesitylene-d12 (red, 2.0 M)
under irradiation (white light) in O2-saturated PhCN at 298 K.
Information, Figure S12 for separate figures). The rates are also
proportional to concentration of O2 (Figure 3). Thus, the rate law
is given by eq 7, where kox is a second-order
The deuterium kinetic isotope effect (KIE) for toluene was
determined to be 5.4. A similar KIE was obtained for the case
of mesitylene (KIE = 5.3) as shown in Figure 4b (blue for
mesitylene and red for mesitylene-d12). The bond-dissociation
energy of the aryl C−H for benzene (109.8 kcal mol−1) is over
20 kcal mol−1 higher than the methyl C−H for toluene
(87.2 kcal mol−1).38 Thus H-atom abstraction from the methyl
C−H is dramatically favored, and it is reasonable to expect that
H-atom transfer from the methyl C−H occurs rather than from
the aryl C−H. In fact, the photochemical reaction of benzene
showed negligible formation of (TBP8Cz)MnV(O) (Supporting
Information, Figure S13). The fact that the methyl group of
hexamethylbenzene was oxygenated to yield pentamethylbenzyl
alcohol provides additional evidence for this mechanism. Thus
the KIE value corresponds to that of H-atom transfer from
the methyl C−H rather than the aryl C−H. Taken together, the
findings of increasing reaction rate with increasing the number
of abstractable hydrogen atoms along with the observed large
KIEs suggest that hydrogen-atom abstraction from the toluene
derivative is directly involved in the rate-determining step for
the photochemical oxidation of (TBP8Cz)MnIII with O2 and
toluene derivatives.
−d[(TBPCz)MnIII]/dt = kox[S][O2]
(7)
8
Figure 3. Plot of the observed zeroth-order rate constants (kobs) for
the oxidation of (TBP8Cz)MnIII (7.6 × 10−6 M) with O2 and hexa-
methylbenzene (0.10 M) under photoirradiation (white light) in
PhCN vs the oxygen concentration (0−8.5 × 10−3 M)37 at 298 K.
constant and [S] is substrate concentration. The slope of
the best-fit lines for the kobs values for the different substrates in
Figure 2 yield first-order rate constants with respect to substrate
concentration. These values can then be divided by the excess O2
concentration (1.7 × 10−3 M) to give the second-order rate
constants (kox) listed in Table 1. Interestingly, the second-order
rate constants increase with an increasing number of methyl
Femtosecond Transient Absorption Measurements.
To detect the photoexcited state involved in the photochemical
oxidation of (TBP8Cz)MnIII with O2, the femtosecond laser
flash photolysis measurements of (TBP8Cz)MnIII were per-
formed in the absence and presence of O2 in PhCN. The MnIII
metal ion has a high spin d4 ground state electronic configuration
D
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Figure 5. (a) Transient absorbance spectral changes (purple after 1 ps, blue 10 ps, green 100 ps, orange 1000 ps, and red 3000 ps) after
photoexcitation of (TBP8Cz)MnIII in PhCN. Time profile of the generation and decay of [(TBP8Cz)MnIII]* (5T1) at λ = 530 nm under (b) N2 and
(c) O2. The red lines are exponential fits given in eq 4 (see Experimental Section). A1 = 3.3 × 10−3, A2 = 1.5 × 10−4, and k1 = 1.8 × 1010 s−1 for (b)
and A1 = 3.1 × 10−3, A2 = 1.5 × 10−4, and k1 = 1.7 × 1010 s−1 for (c).
2
2
(S = 2) with only the high energy dx −y orbital unoccupied in the
ground state.39 Because of the coupling between unpaired elec-
trons of the metal with the π electrons of the corrolazine ring,
the ground state of (TBP8Cz)MnIII is a singquintet (5S0). Upon
femtosecond laser excitation, instantaneously formed minima
at λmin = 450 and 695 nm can be observed as shown in Figure 5a
which closely mirror the ground-state absorption spectrum
(Figure 1a). A new absorption maximum at λmax = 530 nm is also
formed and can be assigned to the tripquintet (5T1) excited state.
It is known that first row paramagnetic complexes, such as MnIII
complexes, undergo an extremely rapid intersystem crossing
process from the singquintet (5S1) excited state to the tripquintet
(5T1) excited state because of the presence of unpaired elec-
trons.40 In MnIII porphyrins, for example, the existence of two tri-
pmultiplet levels was suggested where a tripquintet (5T1) relaxes
to a long-lived tripseptet (7T1), which requires a spin conversion
to go back to the quintet ground state.40,41 In the Mn corrolazine
complex, we attribute the decay of the absorbance band at
efficiently to produce the Mn(IV)-superoxo complex, (TBP8Cz)-
MnIV(O2•−).47
An alternative mechanism could involve direct energy transfer
from [(TBP8Cz)MnIII]* (7T1) to O2 to produce singlet oxygen
(1O2*) and the ground state (TBP8Cz)MnIII (S = 2), which is a
spin-allowed process. In the initial report on the photochemical
oxidation of (TBP8Cz)MnIII to (TBP8Cz)MnV(O), a significant
1
role for singlet oxygen was ruled out by use of the O2* trap,
9,10-dimethylanthracene.28 However, to eliminate the possibility
1
of O2* as the major oxidant under inert solvent conditions as
employed in the present study, we looked for the presence of
1O2* by its phosphorescence spectrum (λmax = 1270 nm).37,48
The photoexcitation of (TBP8Cz)MnIII in O2-saturated C6D6
1
(PhCN could not be used because of the short lifetime of O2)
results in a much smaller phosphorescence signal at 1270 nm
than that obtained by photoexcitation of C60 under the same con-
ditions.49 (see Supporting Information, Figure S14) Thus, the con-
1
tribution of O2* for the photochemical oxidation of (TBP8Cz)-
MnIII with O2 may be negligible as compared with an electron-
transfer pathway from [(TBP8Cz)MnIII]* (7T1) to O2.
5
7
530 nm to intersystem crossing from the T1 state to the T1
state, which has a small absorption band at 774 nm. The decay
rate of the tripquintet (5T1) obeyed first-order kinetics with a rate
constant of 1.8 × 1010 s−1 in deaerated PhCN (Figure 5b).
The similar decay rate constant was observed in O2-saturated
PhCN (Figure 5c) showing no oxygen dependence on the rate of
intersystem crossing.
Reaction Mechanism. Based on the photodynamics of
(TBP8Cz)MnIII, together with the effect of added substrate and
the large observed KIEs, the mechanism of photochemical oxi-
dation of (TBP8Cz)MnIII with O2 and toluene as the substrate
to produce (TBP8Cz)MnV(O) is proposed as shown in Scheme 2.
Upon photoexcitation of (TBP8Cz)MnIII, the produced tripquin-
tet excited state ([(TBP8Cz)MnIII]* (5T1)) is converted rapidly
by intersystem crossing to the triplet excited state ([(TBP8Cz)-
MnIII]* (7T1)). Electron transfer from [(TBP8Cz)MnIII]* (7T1)
to O2 occurs to produce the superoxo complex [(TBP8Cz)-
MnIV(O2•−)]. Hydrogen-atom transfer from toluene to
(TBP8Cz)MnIV(O2•−) then follows as the rate-determining step,
generating the hydroperoxo complex (TBP8Cz)MnIV(OOH) and
benzyl radical. This reaction is most likely in competition with the
back-reaction of electron transfer to regenerate the ground state
(TBP8Cz)MnIII and O2. The subsequent homolytic O−O bond
cleavage by benzyl radical may occur rapidly inside the reaction
7
The absorbance at 774 nm due to T1 remained unchanged
up to 3000 ps in deaerated PhCN (Figure 6b), whereas the
absorbance decays in O2-saturated PhCN (Figure 6c), signaling
a direct reaction between the excited state and O2. The second-
order rate constant of the decay of [(TBP8Cz)MnIII]* (7T1) in
the presence of O2 was determined to be 4.9 × 109 M−1 s−1,
which is comparable to the diffusion-limited rate constant
in PhCN.42 Because the one-electron oxidation potential of
[(TBP8Cz)MnIII]* (7T1) (−0.90 V vs SCE)43−45 is more negative
than the one-electron reduction potential of O2 (−0.87 V vs SCE),46
electron transfer from [(TBP8Cz)MnIII]* (7T1) to O2 may occur
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Figure 6. (a) Transient absorbance spectral changes (purple after 1 ps, blue 10 ps, green 100 ps, orange 1000 ps, and red 3000 ps) after
photoexcitation of (TBP8Cz)MnIII in PhCN. Decay time profiles of absorbance at 774 nm due to [(TBP8Cz)MnIII]* (7T1) (b) in N2-saturated
PhCN and (c) in O2-saturated PhCN. Best-fit lines (red) obtained from a double-exponential kinetic model given in eq 5 (see Experimental
Section). A1 = 1.4 × 10−3, A2 = 6.7 × 10−4, A3 = 3.6 × 10−5, k1 = 4.8 × 109 s−1 and k2 = 6.3 × 107 s−1 for (b) and A1 = 1.0 × 10−3, A2 = 1.1 × 10−3, A3 =
−4.5 × 10−4, k1 = 5.0 × 109 s−1 and k2 = 1.1 × 108 s−1 for (c).
Scheme 2. Mechanism of Photochemical Oxidation of
(TBP8Cz)MnIII with O2 and Toluene for Generation of
(TBP8Cz)MnV(O)
(S) to (TBP8Cz)MnIV(O2•−), ket is the rate constant of electron
transfer from [(TBP8Cz)MnIII]* (7T1) to O2 to produce
(TBP8Cz)MnIV(O2•−), and k−et is the back electron transfer
from the O2 moiety to the (TBP8Cz)MnIV moiety to
•−
regenerate (TBP8Cz)MnIII and O2. Equation 8 agrees with the
empirical rate law [eq 7], in which kox = Φ0(kHket/(kH + k−et))
In (In = light intensity absorbed by (TBP8Cz)MnIII).50 Because
significant KIEs were observed, as shown in Figure 4, the kH
value may be much smaller than the k−et value: kH ≪ k−et
,
suggesting the back electron transfer pathway is much more
favored.
The mechanism in Scheme 2 is similar to O2 activation by
[(TMC)FeII]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-
cyclotetradecane) with a hydrogen donor (RH) to produce
[(TMC)FeIV(O)]2+. In this case, electron transfer from [(TMC)-
FeII]2+ to O2 occurs thermally to produce [(TMC)FeIII(O2•−)]2+.
This is followed by hydrogen transfer from RH to [(TMC)-
FeIII(O2•−)]2+ to give R• and [(TMC)FeIII-(OOH)]2+, which
undergoes an oxygen rebound reaction via the homolytic
O−O bond cleavage by R• to yield [(TMC)FeIV(O)]2+ and
ROH.51
If a free radical (e.g., pentamethylbenzyl radical) were produced
following hydrogen-atom transfer from a toluene derivative to
(TBP8Cz)MnIV(O2•−), the benzyl radical should react with O2
to produce the peroxyl radical, which may act as a chain carrier for
the autoxidation of hexamethylbenzene to produce pentamethyl-
benzyl hydroperoxide. The disproportionation of the peroxyl
radical may yield equimolar pentamethylbenzyl alcohol and
pentamethylbenzaldehyde. The fact that neither pentamethylben-
zyl hydroperoxide nor pentamethylbenzaldehyde were produced
and that pentamethylbenzyl alcohol was the major product,
provides strong evidence for the conclusion that the rebound
pathway to produce pentamethylbenzyl alcohol is faster than the
rate of the reaction of pentamethylbenzyl radical with O2. Judging
from the reported rate constant of benzyl radical with O2 (2.6 ×
109 M−1 s−1)52 to produce benzyl peroxyl radical, and the O2
cage before the reaction of benzyl radical with O2 to yield
(TBP8Cz)MnV(O) and benzyl alcohol (an oxygen rebound
pathway). Alternatively, electron transfer may occur from benzyl
radical derivatives to (TBP8Cz)MnIV(OOH) to produce an ion
pair {(TBP8Cz)MnIII(OOH)}− and benzyl cation (PhCH2)+,
followed by heterolytic O−O bond cleavage to yield (TBP8Cz)-
MnV(O) and benzyl alcohol derivatives. At present the homolysis
vs heterolysis pathway has yet to be distinguished.
According to Scheme 2, the quantum yield of formation of
(TBP8Cz)MnV(O) is given by eq 8, where Φ0 is the quantum
Φ = Φ0(kHket/(kH + k−et))[S][O2]
(8)
yield of photoinduced formation of (TBP8Cz)MnIV(O2•−), kH
is the rate of hydrogen-atom transfer from toluene derivatives
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concentration (8.5 mM) of the reaction mixture, the lifetime of
the radical is expected to be shorter than 45 ns = (2.6 × 109 ×
8.5 × 10−3)−1. The lifetime of (TBP8Cz)MnIV(OOH) is also
expected to be shorter than 45 ns. In such a case, it is very difficult
to detect (TBP8Cz)MnIV(OOH), because this is a high energy
species, which goes back to (TBP8Cz)MnIII, O2, and H+.53,54
Comparison of Hydrogen Transfer Reactivity. To gain
deeper insight into the rate-determining hydrogen transfer from
toluene derivatives to (TBP8Cz)MnIV(O2•−), the reactivity was
compared with that of hydrogen transfer from the same toluene
derivatives employed in this study to cumylperoxyl radical, which is
regarded as an authentic hydrogen-transfer reaction.35,55,56 The
rates of hydrogen transfer from toluene derivatives to cumylperoxyl
radical were determined by monitoring the reactions by EPR spec-
troscopy. Cumylperoxyl radicals were generated by photoirradi-
ation of an aerated propionitrile solution containing di-tert-
butylperoxide (tBuOOtBu) and cumene [PhCH(CH3)2] with
a 1000 W Mercury lamp via a radical chain process as shown in
Scheme 3.35 The UV light irradiation of tBuOOtBu results in the
Figure 7. Time profiles of the EPR intensity change of cumylperoxyl
radical in the absence (black line) and presence of toluene as a
substrate (3 M, blue line) in aerated propionitrile at 193 K.
Scheme 3. Generation of Cumylperoxyl Radical and
Hydrogen Transfer from Toluene to PhCMe2OO•
t
O−O bond cleavage to produce BuO•. This radical can then
Figure 8. Plots of k′obs vs concentrations of toluene derivatives for
hydrogen-atom transfer from toluene derivatives to cumylperoxyl
radical in aerated propionitrile at 193 K.
readily abstract a hydrogen from cumene to generate cumyl
radical [PhC•(CH3)2]. Rapid O2 addition to the cumyl radical
affords the cumylperoxyl radical (PhC(CH3)2OO•). Once
generated, this autoxidation process continues until cumylper-
oxyl radicals decay via ß-scission producing acetophenone and
CH3O•.57
After cutting off the light, the decay rates of cumylperoxyl
radical were monitored by the decrease of EPR signal inten-
sity at g = 2.0156 in the presence of a toluene derivative in
propionitrile at 193 K, as shown in Figure 7. The decay rates
of cumylperoxyl radical obeyed first-order kinetics under the
conditions with large excess toluene derivatives.58 The pseudo-
first-order rate constants (k′obs) were linearly proportional to
concentrations of toluene derivatives as shown in Figure 8. The
second-order rate constants (k′H) of the hydrogen atom transfer
from toluene derivatives to cumylperoxyl radicals were determined
from the slopes of linear plots in Figure 8. The k′H values are also
listed in Table 1. The decay rates of cumylperoxyl radical in the
presence of a toluene derivative become faster than in the absence
of the toluene derivative.
These results suggest that hydrogen-atom transfer from the
toluene derivative to cumylperoxyl radical readily occurs. Figure 9
shows plots of the logarithm of the kox values of the photochemical
oxidation of (TBP8Cz)MnIII with O2 in the presence of toluene
derivatives and the k′H values of hydrogen-atom transfer from
toluene derivatives to cumylperoxyl radicals vs the one-electron
oxidation peak potentials of the toluene derivatives (Epox), which
were reported previously.59 There is a linear correlation between
log kox and Epox with a larger slope (3.2) than that between log k′H
and Epox (1.7) in Figure 9. The slopes of 3.2 and 1.7 correspond to
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Figure 10. (a) UV−vis spectral changes and (b) time course of formed
Acr=O under photoirradiation (λ > 480 nm) of an aerobic solution
(0.5 mL) containing (TBP8Cz)MnIII (1.7 × 10−4 M) and AcrH2 (blue,
0.2 M) or AcrD2 (red, 0.2 M).
Figure 9. Plots of log kox and log k′H against the oxidation potential
(Epox) of toluene derivatives for hydrogen-atom transfer with (a)
(TBP8Cz)MnIII and (b) cumylperoxyl radical.
triplet excited state of acetophenone derivatives and the ionization
potentials of hydrogen donors.62
19% (3.2/16.9) and 7% (1.7/26.0) of the difference in the free
energy change of electron transfer, because the difference in 1 V of
Eox corresponds to the difference in terms of logarithm of the rate
constant to be 2.3RT = 26.0 and 16.9 at 193 and 298 K,
respectively. The k′H value of hexamethylbenzene increases by a
factor of 20 as compared with that of toluene, while the number
of hydrogen atoms increases by a factor of 6. Thus, a small
3-fold “net” increase was observed in the rates of cumylperoxyl
radical decay rate in the presence of hexamethylbenzene compared
to its reaction with toluene. In general, the k′H value is expected to
increase with decreasing the C−H bond dissociation energies of
toluene derivatives.60 However, the C−H bond dissociation
energies of toluene derivatives have been reported to be only
slightly affected by the electron-donating or -withdrawing
substituents.61 The small “net” difference in the hydrogen-transfer
reactivity of cumylperoxyl radical with hexamethylbenzene vs
toluene results from the small change in the C−H bond
dissociation energies of toluene derivatives. The larger slope
(3.2) in the plot of log kox vs Epox than the slope (1.7) in the plot
Photocatalytic Reactivity with AcrH2. The photocatalytic
reactivity of (TBP8Cz)MnIII was examined under irradiation
(white light) of a reaction solution (0.5 mL) containing
(TBP8Cz)MnIII (1.7 × 10−4 M) and AcrH2 (0.2 M) which has a
weaker C−H bond than toluene derivatives. The time course
for formation of Acr=O as the product quantified by
absorbance at 402 nm (λ = 402 nm, εmax = 8.6 × 103 M−1
cm−1)63 is shown in Figure 10a. The turnover number was
determined to be 11 in 5 h based on the initial amount of
(TBP8Cz)MnIII. A negligible amount of Acr=O was produced
from a reaction solution without (TBP8Cz)MnIII as a control
experiment.
When AcrH2 was replaced by the dideuterated compound
AcrD2, the photocatalytic reactivity dramatically decreased with
a large kinetic deuterium isotope (KIE) value of 16 as shown in
Figure 10b (blue for AcrH2 and red for AcrD2).64 This KIE
suggests that hydrogen-atom transfer from AcrH2 to
(TBP8Cz)MnIV(O2•−) is involved in the rate-determining
step as seen in the case of toluene derivatives in Scheme 2.
This hydrogen transfer may also proceed via electron transfer
from AcrH2 to (TBP8Cz)MnIV(O2•−), followed by rate-limiting
of log k′H and Ep in Figure 9 indicates the higher degree of
ox
charge transfer in the transition state of the hydrogen-transfer
reactions of (TBP8Cz)MnIV(O2•−) as compared with those of
cumylperoxyl radical, as reported for the linear relations between
logarithm of the rate constants of hydrogen-transfer reactions of
proton transfer from AcrH2 to (TBP8Cz)MnIV(O22−) to
•+
produce AcrH• and (TBP8Cz)MnIV(OOH−). As seen for the
toluene derivatives, oxygen rebound via homolytic O−O bond
Scheme 4. Mechanism of Photocatalytic Reaction of (TBP8Cz)MnIII with O2 and AcrH2
H
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cleavage by AcrH• yields (TBP8Cz)MnV(O) and AcrHOH.
The electron-donating OH group in AcrHOH may make it
easier to be oxidized by (TBP8Cz)MnV(O) as compared to
AcrH2 via electron and proton transfer, yielding Acr=O and
regenerating (TBP8Cz)MnIII (Scheme 4). The stoichiometry of
the photocatalytic reaction is given by eq 9. The slow catalytic
oxidation of AcrH2 may result from the much faster back
electron transfer in (TBP8Cz)MnIV(O2•−) as compared with
the hydrogen-transfer reaction with AcrH2 in Scheme 4.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp (S.F.).
*E-mail: dpg@jhu.edu (D.P.G.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid (No. 20108010 to
S.F. and 23750014 to K.O.), the NSF (CHE0909587 and
CHE121386 to D.P.G.), the NIH (GM101153 to D.P.G) by a
Grant-in-Aid (20108010) and a Global COE Program (The
Global Education and Research Center for Bio-Environmental
Chemistry) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan, and by KOSEF/MEST
through the WCU Project (R31-2008-000-10010-0). K.A.P. is
grateful for a Harry and Cleio Greer Fellowship.
CONCLUSIONS
■
Photoirradiation of (TBP8Cz)MnIII with O2 and toluene deri-
vatives in the inert solvent PhCN results in the rare formation
of a high-valent manganese(V)-oxo complex with O2 as the
oxidant. At the same time, the substrate hexamethylbenzene is
selectively oxidized in good yield to a single, monohydroxylated
benzyl alcohol product. The photochemical mechanism was
interrogated by femtosecond laser flash photolysis measure-
ments, leading to the first direct spectroscopic observation of a
novel photoexcited state ([(TBP8Cz)MnIII]* (7T1)), which was
found to be responsible for the initial reaction with O2. The
LFP experiments, together with kinetic studies involving
substrate dependence and KIEs, indicate that the mechanism
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7
of photochemical oxidation involves the short-lived T1 excited
state reacting with O2 via binding and electron-transfer to
produce the proposed superoxo complex (TBP8Cz)MnIV(O2•−).
The superoxo complex then abstracts a hydrogen atom from the
toluene derivatives in the rate-determining step to produce
(TBP8Cz)MnIV(O2H), followed by oxygen rebound of the
resulting benzyl radical derivatives with (TBP8Cz)MnIV(OOH)
to yield the corresponding benzyl alcohol derivatives and
(TBP8Cz)MnV(O) (Scheme 2). The hydrogen-atom abstracting
reactivity of cumylperoxyl radical was examined with the same
set of toluene derivatives, and comparison of the kinetic data
confirmed that H-atom abstraction was the rate-determining step
for the Mn complex, and indicated that the putative (TBP8Cz)-
MnIV(O2•−) species exhibited more electrophilic character
toward H-atom transfer than the cumylperoxyl radical. Our
mechanistic findings suggested that catalytic oxidations should
be possible with the appropriate substrate, and indeed, it was
shown that carrying out the reaction with a stronger hydrogen
donor such as AcrH2 leads to the photocatalytic oxidation of this
substrate by O2 and (TBP8Cz)MnIII as catalyst. We have thus
revealed a new method for the selective oxidation of benzylic C−H
bonds involving only O2, light, and a discrete MnIII complex. The
mechanistic findings and accompanying model (Scheme 2)
provide valuable insights into the generation of high-valent
metal-oxo intermediates via O2 activation in the presence of a
hydrogen-atom (electron and proton) source.
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ASSOCIATED CONTENT
* Supporting Information
■
S
UV−vis spectra (Figure S1, S2, S3 and S13), GC charts (Figure S4),
calibration plots for GC measurements (Figure S5), kinetic plots
(Figures S6, S7, S8, S9, S10, S11, and S12), phosphorescence
spectra (Figures S14), and yields of the products (Table S1).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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•+
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•−
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