Catalytic electron-transfer oxygenation of substrates with water as an oxygen source using manganese porphyrins

Article abstract of DOI:10.1002/chem.201202041

Manganese(V)-oxo-porphyrins are produced by the electron-transfer oxidation of manganese-porphyrins with tris(2,2'-bipyridine)ruthenium(III) ([Ru(bpy) ₃]³⁺; 2 equiv) in acetonitrile (CH₃CN) containing water. The rate const

Full text of DOI:10.1002/chem.201202041

DOI: 10.1002/chem.201202041
Catalytic Electron-Transfer Oxygenation of Substrates with Water as an
Oxygen Source Using Manganese Porphyrins
Shunichi Fukuzumi,*[a, b] Takuya Mizuno,[a] and Tetsuya Ojiri[a]
Abstract: Manganese(V)–oxo–porphyr-
ins are produced by the electron-trans-
fer oxidation of manganese–porphyrins
ides were converted to the correspond-
ing diols by hydrolysis, and were fur-
ther oxidized to the corresponding al-
dehydes. The turnover numbers vary
significantly depending on the type of
manganese–porphyrin used owing to
the difference in their oxidation poten-
tials and the steric bulkiness of the
ligand. Ethylbenzene was also oxidized
to 1-phenylethanol using manganese–
porphyrins as electron-transfer cata-
lysts. The oxygen source in the sub-
strate oxygenation was confirmed to be
water by using 18O-labeled water. The
rate constant of the reaction of the
manganese(V)–oxo species with cyclo-
hexene was determined directly under
single-turnover conditions by monitor-
ing the increase in absorbance attribut-
with tris(2,2’-bipyridine)ruthenium
ACHTUNGTNER(NUNG III)
([Ru
ACHTUNGTRENNUNG
(CH3CN) containing water. The rate
constants of the electron-transfer oxi-
dation of manganese–porphyrins have
been determined and evaluated in light
of the Marcus theory of electron trans-
able to the manganeseACTHNUGRTNEUNG(III) species pro-
duced in the reaction with cyclohexene.
It has been shown that the rate-deter-
mining step in the catalytic electron-
transfer oxygenation of cyclohexene is
fer. Addition of [RuACHTUNGTRENUNG
(bpy)3]3+ to a solu-
tion of olefins (styrene and cyclohex-
ene) in CH3CN containing water in the
presence of a catalytic amount of man-
ganese–porphyrins afforded epoxides,
diols, and aldehydes efficiently. Epox-
Keywords: electron
transfer
·
electron transfer from [Ru
the manganese–porphyrins.
(bpy)3]3+ to
ACHTUNGTRENNUNG
homogeneous catalysis
·
manga-
nese · oxygenation · porphyrinoids
Introduction
were the first to report cytochrome P450 type activity in a
model system using iron–tetraphenylporphyrin–chloride
[(tpp)FeIII(Cl)] (tpp2ꢀ =tetraphenylporphyrin dianion) and
iodosylbenzene (PhIO) as an oxidant, which can oxidize the
Cytochrome P450 enzymes catalyze a number of important
metabolic oxidation reactions by dioxygen.[1,2] The remarka-
ble catalytic reactivity results from an oxoiron(IV)–porphyr-
in p-radical cation (compound I), which has been suggested
to be the ultimate oxidant in these enzymatic systems.[1,2]
The reaction mechanism to produce compound I in the cyto-
chrome P450 enzymes involves two distinct electron-transfer
processes to activate dioxygen (Scheme 1).[1–3] An oxoiro-
n(IV)–porphyrin p-radical cation (compound I) derived
from horseradish peroxidase (HRP) has been produced by
reaction with H2O2.[4] In addition, the use of synthetic model
systems has provided valuable mechanistic insights into the
molecular catalytic mechanism of P450.[5–11] Groves et al.[9,10]
III
IV
+
C
Fe –porphyrin directly to produce [(tpp)Fe (O)] in a so-
called “shunt” pathway. Groves et al.[11] also prepared and
characterized synthetic oxoiron(IV)–porphyrin p-radical
cation complexes and the oxygen-transfer reactions with
these reactive intermediates was monitored successfully.
High-valent metal–oxo–porphyrins are now generally pro-
duced by oxygen-atom transfer from active oxygen species
[a] Prof. Dr. S. Fukuzumi, T. Mizuno, T. Ojiri
Department of Material and Life Science
Graduate School of Engineering
Osaka University, ALCA
Japan Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7368
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[b] Prof. Dr. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University, Seoul 120-750 (Korea)
E-mail: wwnam@ewha.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202041.
Scheme 1.
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FULL PAPER
such as iodosylbenzene, hydroperoxide, m-chloroperoxyben-
over conditions) as well as the kinetics of the overall catalyt-
ic reactions, which provides valuable insight into the catalyt-
ic mechanism for the electron-transfer oxygenation of sub-
strates using water as an oxygen source. Manganese–por-
phyrins employed as catalysts in this study are [(tmp)MnIII]+
(tmp2ꢀ =5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrin
dianion), [(tdcpp)MnIII]+ (tdcpp2ꢀ =5,10,15,20-tetrakis(2,6-
ꢀ
zoic acid (mCPBA), and oxone (HSO5 ).[5–8]
Among high-valent metal–oxo species, MnV(O) species
have merited special attention because they are postulated
as important intermediates in the conversion of water to di-
oxygen during water oxidation in photosynthesis.[12,13] The
active species in the oxygenation of substrates with manga-
nese–porphyrins (P) are generally believed to be
[(P)MnV(O)].[14,15] The [(P)MnV(O)] intermediate has been
isolated and characterized and the oxygenation of substrates
by [(P)MnV(O)] has been studied extensively.[16–21] The
[(P)MnV(O)] and [(P)MnIV(O)] species have also been pro-
duced by the photoexcitation of the (P)MnIII–perchlorate
and –chlorate complexes, respectively.[22] The high reactivity
of the [(P)MnV(O)] species has been demonstrated by deter-
mining the rate constant of the oxo transfer from
[(P)MnV(O)] to a substrate with use of laser flash photolysis
measurements; the representative rate constant for the reac-
tion of [(tpfpp)MnV(O)] (tpfpp: 5,10,15,20-tetrakis(penta-
dichlorophenyl)porphyrin
dianion),
[(tmopp)MnIII]+
(tmopp2ꢀ =5,10,15,20-tetrakis(2,4,6-trimethoxyphenyl)por-
phyrin dianion), and [(dtmpd)MnIII
]
2
(dtmpd4ꢀ =di(trime-
2+
sitylporphyrin)dibenzofuran tetraanion).
fluorophenyl)porphyrin)
with
cis-stilbene
is
6.1ꢁ
105 mꢀ1 sꢀ1.[22] Because [(P)MnIV(O)] and [(P)MnV(O)] are
the one-electron and two-electron oxidized species of
(P)MnIII, respectively, they can be formed by the electron-
transfer oxidation of (P)MnIII with appropriate one-electron
oxidants.[23] However, the kinetics and mechanism of forma-
tion of [(P)MnIV(O)] and [(P)MnV(O)] by the electron-
transfer oxidation of (P)MnIII or the catalytic electron-trans-
fer oxygenation of substrates, in which water rather than
oxygen is used as an oxygen source, have yet to be reported.
We report herein the catalytic electron-transfer oxygena-
tion of alkanes and olefins using a one-electron oxidant,
[RuACHTUNGTRENNUNG
(bpy)3]3+ (bpy=2,2’-bipyridine), rather than active
Experimental Section
oxygen species and manganese–porphyrins as the catalyst in
the presence of water, which acts as an oxygen source as
shown in Scheme 2. The electron-transfer oxidation of
(P)MnIII to [(P)MnIV(O)] and [(P)MnV(O)] has also been ex-
amined and the electron-transfer kinetics were analyzed to
determine the reorganization energy of the electron transfer
in light of the Marcus theory of electron transfer.[24,25] Fol-
lowing this, a detailed kinetic study was performed on the
oxygenation of cyclohexene with [(P)MnV(O)] (single-turn-
General: 1H NMR spectra were measured on a JEOL JNM-AL300 NMR
spectrometer. GC analyses were performed on a Shimadzu GC-17A
equipped with a DB-5MS column (Agilent Technologies, 30 m) and a
mass spectrograph (Shimadzu QP-5050) as a detector. Fast-atom-bom-
bardment mass spectra (FAB-MS) were obtained on a JEOL JMS-
DX300 mass spectrometer. Elemental analyses were performed on a
Perkin–Elmer 240C elemental analyzer.
Materials: Acetonitrile (CH3CN) used as solvent was purified and dried
by a standard procedure.[26] A small amount of H2O (6.5 mm) was added
to CH3CN to study oxidation of manganese–porphyrins in the presence
of H2O. [Ru
ACHUTGTNRNENUG(bpy)3]ACHTUNGTRENNUNG
erature method.[27] MnCl2·4H2O and Mn
AHCTUNGTRENNUNG
from Wako Pure Chemical Ind. Ltd., Japan. Porphyrins tmp and tdcpp
were purchased from Tokyo Kasei Organic Chemicals. Porphyrins tmopp
and dtmp were prepared according to the literature method.[28–31]
[(tmp)MnIII
N
ACHTNUGTERN(NUGN PF6) was prepared by adding MnCl2·4H2O to the
AHCTUNGTRENNUNG
CH2Cl2, and AgPF6 (1 equiv) was added. The reaction mixture was fil-
tered to remove AgCl (1 equiv) according to the literature method.[32]
[(tmp)MnIII(OH)] was prepared by adding Mn
ACTHNUTRGNEUNG(OAc)2 to the porphyrin,
followed by heating under reflux conditions for 24 h in DMSO according
to the literature method.[33] [(dtmpd)Mn2]Cl2 has been synthesized and
characterized according to the literature (see the Supporting Information,
Figures S1–S3).[31a,34] All ethylbenzene and olefins (styrene, cyclohexene)
used in this study were purchased from Tokyo Kasei Organic Chemicals.
CH3CN and toluene used as a solvent were purified and dried by using
Scheme 2.
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15795

S. Fukuzumi et al.
the standard procedure.[33] Purification of water (18.2 MWcm) was per-
formed with a Milli-Q system (Millipore; Milli-Q Jr). [D3]Acetonitrile,
deuterium oxide, and [D]chloroform were obtained from EURI SO-TOP,
CEA, France. H218O (95%) was purchased from Wako Pure Chemical
Ind. Ltd., Japan. TLC Alufolien Kieselgel and flash column chromatogra-
phy were performed with Art. 5554 DC-60 F254 (Merck) and Fujisilica
BW300, respectively.
Spectral measurements: The formation of [(tmp)MnIV(O)] and
[(tmp)MnV(O)]+ was examined from the change in the UV/Vis spectrum
+
of a solution of [(tmp)MnIII
]
in CH3CN in the presence of [RuACTHNGUTRENU(NG bpy)3]-
ACHTUNGTRENNUNG(PF6)3 and H2O by using a Hewlett Packard 8453 diode array spectropho-
tometer with a quartz cuvette (path length=10 mm) at various tempera-
tures (233–298 K).
Kinetic measurements: All kinetic measurements were performed on a
UNISOKU RSP-601 stopped-flow spectrophotometer with an MOS-type
high-selective photodiode array at various temperatures (233–298 K)
using a Unisoku thermostated cell holder designed for low-temperature
experiments. In a typical reaction,
a solution of each porphyrin in
CH3CN containing a substrate was transferred to the spectrophotometric
cell with a glass syringe. Rates of the oxidation reactions catalyzed by
+
[(tmp)MnIII
]
in CH3CN/H2O (9:1 v/v) at 298 K were monitored by fol-
lowing an increase in absorbance at l=450 nm attributable to
[Ru(bpy)3]2+. The rate constants of oxidation reactions of substrates cata-
lyzed by the porphyrins were determined by pseudo-first-order plots in
which concentrations of substrates were maintained at more than a ten-
fold excess amount of the porphyrins.
Electrochemical measurements: Electrochemical measurements were
performed on a BAS 100 W electrochemical analyzer in CH3CN contain-
ing H2O (6.5 mm) and Bu4NPF6 (TBAPF6: 0.10m) as a supporting electro-
lyte at 298 K. Differential pulse voltammetry measurements were also
performed on a BAS 50 W electrochemical analyzer in CH3CN contain-
ing H2O (6.5 mm) and TBAPF6 (0.10m) as a supporting electrolyte at
298 K (10 mVsꢀ1). A conventional three-electrode cell was used with a
platinum working electrode (surface area of 0.3 mm2) and a platinum
wire as the counter electrode. The Pt working electrode (BAS) was rou-
tinely polished with a BAS polishing alumina suspension and was rinsed
with acetone before use. The measured potentials were recorded with re-
spect to the Ag/AgNO3 (0.01m) reference electrode. All potentials (vs.
Ag/Ag+) were converted to values versus SCE by adding 0.29 V.[35] All
electrochemical measurements were carried out under an atmospheric
pressure of Ar.
EPR measurements: The EPR spectra of high-valent manganese–oxo–
porphyrins produced by adding [RuACTHNUTRGENNUG(bpy)3]ACHTUNGTREN(NUNG PF6)3 (2 equiv) to a deaerated
solution of each porphyrin in CH3CN were recorded on a JEOL X-band
spectrometer (JES-RE1XE) with a quartz EPR tube (4.5 mm i.d.). The
EPR spectra were measured under nonsaturating microwave-power con-
ditions. The magnitude of modulation was chosen to optimize the resolu-
tion and the signal-to-noise (S/N) ratio of the observed spectra. The g
values were calibrated with an Mn2+ marker.
Figure 1. Cyclic voltammograms and differential pulse voltammograms
(DPV) of a) [(tmp)MnIII(Cl)] (5.0ꢁ10ꢀ4 m) and b) [(tmp)MnIII
ACHTUNGTRENNUNG
.
[(tmp)MnIII(Cl)] is replaced by [(tmp)MnIII
two reversible waves merge into a single wave, which corre-
sponds to the two-electron oxidation of [(tmp)MnIII
(H2O)2]-
(PF6) as shown in Figure 1b.[37] Because the one-electron re-
duction potential of [Ru
(bpy)3]3+ (Ered versus SCE=1.24 V)
is more positive than the second oxidation potential of
[(tmp)MnIII(Cl)] (Eox versus SCE=1.22 V), not only the
one-electron oxidation but also the two-electron oxidation
of [(tmp)MnIII(Cl)] is thermodynamically feasible.
ACHTUTGNRENGU(N H2O)2]ACHTUNGTRENNUNG(PF6),
General procedure for catalytic oxygenation reactions: In a typical reac-
tion, a solution of 4.0ꢁ10ꢀ3 m [Ru
ACTHUNRGTENNGU(bpy)3]ACHTUGNTREN(NUGN PF6)3 in CD3CN (0.50 mL) was
delivered by syringe pump to a solution of manganese–porphyrin (2.0ꢁ
10ꢀ5 m) and substrate (2.0ꢁ10ꢀ3 m) in CH3CN/H2O (9:1 v/v; 0.50 mL) .
The solution was stirred for 15 min. The products were identified by com-
parison of their GC retention times, GC/MS spectra, and 1H NMR spec-
tra with those of authentic compounds.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Results and Discussion
Figure 2 shows the spectral change upon the electron-
Formation of high-valent manganese–oxo porphyrins: The
cyclic voltammogram and differential pulse voltammogram
of [(tmp)MnIII(Cl)] in CH3CN exhibits two reversible waves
consisting of the first one-electron oxidation at Eox1 versus
SCE at 1.02 V and the second one-electron oxidation at Eox2
versus SCE at 1.22 V as shown in Figure 1a.[36] When
transfer oxidation of [(tmp)MnIII(Cl)] with one equivalent of
[RuACHTNUTGRNEUNG
(bpy)3]3+ in CH3CN at 233 K. A new absorption band
appears at 411 nm accompanied by the disappearance of the
absorption band at 477 nm attributed to [(tmp)MnIII(Cl)].
The appearance of the new absorption band supports that
reported for [(tmp)MnIV(O)] as shown in Figure S1 in the
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Catalytic Electron-Transfer Oxygenation
FULL PAPER
may be formed by the one-electron oxidation of
[(tmp)MnIV(O)].[39]
When [(tmp)MnIII(Cl)] is replaced by [(tmp)MnIII
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
a
AHCTUNGTRENNUNG
Figure 2. UV/Vis spectra of a) [(tmp)MnIII(Cl)] and b) [(tmp)MnIV(O)]
formed in the electron-transfer oxidation of [(tmp)MnIII(Cl)] (1.6ꢁ
ACHTUNGTRENNUNG
10ꢀ5 m) by [Ru(bpy)3]3+ (1.6ꢁ10ꢀ5 m) in CH3CN containing H2O (6.5 mm)
at 233 K.
Supporting Information, which is prepared by the reaction
of [(tmp)MnIII(Cl)] with mCPBA. The EPR spectrum of the
resulting solution of the one-electron oxidation of
Figure 4. UV/Vis spectra of a) [(tmp)MnIII
b), c) of [(tmp)MnV(O)]+ formed in the electron-transfer oxidation of
[(tmp)MnIII (PF6) in the presence of b) 1.2ꢁ10ꢀ5 and c) 2.4ꢁ10ꢀ5
(H2O)2]
[Ru
(bpy)3]3+ in CH3CN containing H2O (6.5 mm) at 233 K.
(PF6) (1.2ꢁ10ꢀ5 m) and
ACHUTGTNREN(NUG H2O)2]ACHTUNGTRENNGUN
ACTHNUTRGNEUNG
[(tmp)MnIII(Cl)] in CH3CN with [Ru(bpy)3]3+ (1 equiv) is
shown in Figure S2 (see the Supporting Information). The
observed broad EPR signal at g=4 agrees with that of
[(tmp)MnIV(O)].[38] Thus, the electron-transfer oxidation of
[(tmp)MnIII(Cl)] occurs at the metal center rather than at
the porphyrin ligand to produce [(tmp)MnIV(O)]. The addi-
G
E
m
AHCTUNGTRENNUNG
tion band appears at 443 nm, but the Soret band at 472 nm
attributable to [(tmp)MnIII(H2O)2]+ still remains with half
intensity (Figure 4b). The addition of a further equivalent of
[Ru
(bpy)3]3+ (total two equivalents) results in complete dis-
tion of two equivalents of [Ru
N
ACHTUNGTRENNUNG
[(tmp)MnIII(Cl)] in CH3CN results in the disappearance of
the Soret band at 477 nm attributable to [(tmp)MnIII(Cl)]
and the appearance of the absorption bands at 389 and
650 nm in Figure 3. In Figure S2b (see the Supporting Infor-
mation), no EPR signal attributable to the further oxidation
of [(tmp)MnIV(O)] was observed; the only bands appearing
AHCTUNGTRENNUNG
appearance of the Soret band at 472 nm accompanied by a
further increase in absorbance at 443 nm (Figure 4c). This
indicates that
a
two-electron transfer of [(tmp)MnIII-
AHCTUNGTRENNUNG
in the EPR spectrum are those attributable to [Ru
in the two-electron oxidation of [(tmp)MnIII(Cl)] in CH3CN
with two equivalents of [Ru
(bpy)3]3+. Thus, [(tmp)MnV(O)]+
A
tron oxidized species.
ACTHNUTRGNEUNG
Such two-electron oxidation of [(tmp)MnIII(H2O)2]+ with
A
[RuACTHUNGETRNNUG
(bpy)3]3+ is consistent with the observation of the two-
electron oxidation wave in the cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) of [(tmp)MnIII-
AHCTUNGTRENNUNG
(H2O)2]+ shown in Figure 1b. In contrast with the case of
[(tmp)MnIII(Cl)], no EPR signal attributable to
[(tmp)MnIV(O)] was observed in the solution of the elec-
tron-transfer oxidation of [(tmp)MnIII(Cl)] with one equiva-
lent of [Ru
porting Information). Thus, the two-electron oxidation of
[(tmp)MnIII(H2O)2]+ occurs even with one equivalent of
[Ru to produce the EPR-silent species,
(bpy)3]3+
[(tmp)MnV(O)]+, when a half equivalent of [(tmp)MnIII-
(bpy)3]3+ in CH3CN (see Figure S3 in the Sup-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(H2O)2]+ remains unreacted. The absorption band at 445 nm
attributable to [(tmp)MnV(O)]+ agrees with the Soret band
at 450 nm due to [(tmpyp)MnV(O)]+ produced by the laser
flash photolysis of [(tmpyp)MnIII]+ perchlorate (tmpyp2ꢀ
=
tetra-N-(methylpyridyl)porphyrinato dianion).[22]
Figure 3. UV/Vis spectra of a) [(tmp)MnIII(Cl)] and b) [(tmp)MnV(O)]
formed in the electron-transfer oxidation of [(tmp)MnIII(Cl)] (1.6ꢁ
ACHTUNGTRENNUNG
10ꢀ5 m) by [Ru(bpy)3]3+ (3.2ꢁ10ꢀ5 m) in CH3CN containing H2O (6.5 mm)
Electron-transfer dynamics of the formation of high-valent
at 233 K.
manganese–oxo–porphyrins: The electron-transfer dynamics
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15797

S. Fukuzumi et al.
of formation of high-valent manganese–oxo–porphyrins was
examined by stopped-flow measurements at 233 K. When
more than two equivalents of [RuACHTUNGTRNEUNG
(bpy)3]3+ are employed for
the oxidation of [(tmp)MnIII(Cl)], the rapid increase in ab-
sorbance at 450 nm attributable to the formation of
[Ru
[Ru
ACHTUNGTRENNUNG
(bpy)3]2+ is followed by a slower further increase in
ACHTUNGTRENNUNG
(bpy)3]3+. Figure 5 shows the time profile for the initial
Figure 6. a) Time profile of formation of [Ru
dation step monitored by absorbance at l=450 nm (e=13800mꢀ1 cmꢀ1
in the electron-transfer oxidation of [(tmp)MnIII(Cl)] (5.0ꢁ10ꢀ6 m) by
[Ru
(bpy)3]3+ (2.0ꢁ10ꢀ4 m) in CH3CN containing H2O (6.5 mm) at 233 K.
(bpy)3]2+ for the second oxi-
ACHTUNGTRENNUNG
)
AHCTUNGTRENNUNG
Inset: First-order plots. b) Plot of kobs versus [RuACTHNUTRGNEUNG
(bpy)33+] for the second
oxidation step of [(tmp)MnIII(Cl)] (5.0ꢁ10ꢀ6 m) with [Ru
CH3CN containing H2O (6.5 mm) at 233 K.
(bpy)3]3+ in
ACHTUNGTRENNUNG
Table 1. Rate constants (ket) and free-energy changes (DGet) of the elec-
tron transfer in the oxidation of [(tmp)MnIII(OH)] in CH3CN containing
H2O (6.5 mm) at 233 K.
Figure 5. a) Time profile of the formation of [Ru
idation step monitored by absorbance at l=450 nm (e=13800mꢀ1 cmꢀ1
in the electron-transfer oxidation of [(tmp)MnIII(Cl)] (5.0ꢁ10ꢀ6 m) with
[Ru
(bpy)3]3+ (2.0ꢁ10ꢀ4 m) in CH3CN containing H2O (6.5 mm) at 233 K.
(bpy)3]2+ for the first ox-
ACHTUNGTRENNUNG
)
Porphyrin
[(tmp)MnIII(Cl)] (1st)
[(tmp)MnIII(Cl)] (2nd)
[(tmp)MnIII
(H2O)2](PF6)
Oxidant
DGet [eV] ket [mꢀ1 sꢀ1
]
(bpy)3]3+
(bpy)3]3+
(bpy)3]3+
(bpy)3]3+
(bpy)3]3+
ꢀ0.22
ꢀ0.02
ꢀ0.09
ꢀ0.31
ꢀ0.13
4.2ꢁ105
4.8ꢁ102
3.0ꢁ105
2.0ꢁ105
2.3ꢁ102
1.3ꢁ107
1.0ꢁ107
1.9ꢁ106
1.3ꢁ105
8.5ꢁ104
ACHTUNGTRENNUNG
N
[Ru
[Ru
[Ru
Inset: First-order plots. b) Plot of kobs versus [RuACTHNUTRGNEUNG
(bpy)33+] for the first ox-
R
idation step of [(tmp)MnIII(Cl)] (5.0ꢁ10ꢀ6 m) with [Ru
CH3CN containing H2O (6.5 mm) at 233 K.
(bpy)3]3+ in
ACHTUNGTRENNUNG
A
E
E
E
N
ACHUTNGRENNUG CAHTUNGTRENNUNG
[(dtmpd)MnIII2(Cl)2] (2nd) [Ru
A
[Fe(5-NO2phen)3]3+ ꢀ0.19
one-electron oxidation of [(tmp)MnIII(Cl)] to produce
[(tmp)MnIV(O)]. The rate of increase in absorbance at
450 nm obeys pseudo-first-order kinetics. The observed
pseudo-first-order rate constant (kobs) increases linearly with
E
[Ru
(bpy)3]3+
ꢀ0.16
ꢀ0.09
0.02
N
[Fe(5-Cl-phen)3]3+
[Fe
(bpy)3]3+
[Ru
(bpy)3]3+
T
ACHTUNGTRENNUNG
A
G
ꢀ0.02
increasing concentration of [RuACHTUNGTRNEUNG
(bpy)3]3+ (Figure 5b). The
second slower step in Figure 6a corresponds to the further
oxidation of [(tmp)MnIV(O)] to [(tmp)MnV(O)]+. The rate
constant (ket) of the first and second electron-transfer steps
are determined from the slopes of the linear plots of kobs
first-order conditions using large excess amounts of oxi-
dants.
The four-electron oxidation of a manganese–porphyrin
ACTHNUTRGNEUNG
dimer [(dtmp)MnIII2(Cl)2] with [Ru(bpy)3]3+ also proceeds
versus [Ru
N
by means of two-step electron-transfer processes (Figures
S4a and S5a in the Supporting Information). In each step, a
two-electron oxidation of the dimer occurs simultaneously
at the two MnIII centers. The ket values are determined from
the slopes of the linear plots of kobs versus concentration of
ket values thus determined are listed in Table 1. Those of the
first electron-transfer step from [(tmp)MnIII(OH)] to
[Fe(bpy)3]3+
,
[Fe
N
,
[Ru
(bpy)3]3+
,
and
[Fe(5-NO2phen)] were also determined under the pseudo-
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Catalytic Electron-Transfer Oxygenation
FULL PAPER
[Ru
G
on the free-energy change of electron transfer (DG0et) is
shown in Figure 8, in which the logket value increases with
increasing driving force of electron transfer (ꢀDGet). Such
mation, respectively). The ket values thus determined are
also listed in Table 1.
In contrast with the cases of [(tmp)MnIII(Cl)] and
[(dtmp)MnIII2(Cl)2],
the
(H2O)2]+ with
[Ru(bpy)3]3+ occurs by a single step as shown in Figure 7a,
[(tmp)MnIII
Figure 8. Dependence of log(ket, mꢀ1 sꢀ1) on DGe0t for the electron-transfer
oxidation of [(tmp)MnIII(OH)] with various oxidants:
~
~
^
[Fe(5-NO2phen)3]
G
(
), [Ru
G
G
(
), [Fe(5-Cl-phen)3]-
A
(
), [Fe
N
(PF6)3 (&) and of [(tmp)MnIII
(H2O)2]+
(
!
*
),
ACTHNUTRGNEUNG
[(tmp)MnIII(Cl)] ( ), and [(dtmpd)Mn 2(Cl)2] ( ) with [Ru(bpy)3]3+ in
&
CH3CN containing H2O (6.5 mm). The solid line represents the fits to
Equation (1) with (1) l=24 kcalmolꢀ1, (2) l=35 kcalmolꢀ1, and (3) l=
40 kcalmolꢀ1
.
driving-force dependence of logket can be fitted well using
the Marcus theory of adiabatic outer-sphere electron trans-
fer [Eq. (1)], in which Z is the collision frequency taken as
1ꢁ1011 mꢀ1 sꢀ1 and l is the reorganization energy of the elec-
tron transfer.[24]
2
ð1Þ
ket ¼ Z exp½ꢀðl=4Þð1 þ DGet=lÞ =RTꢁ
The best-fit value of l in Figure 8a, which includes both
Figure 7. a) Time profile of the formation of [Ru
[(tmp)MnIII
(H2O)2](PF6) for one-step oxidation monitored by absorbance
at l=450 and 472 nm in the electron-transfer oxidation of [(tmp)MnIII
(H2O)2]
(PF6) (8.0ꢁ10ꢀ6 m) by [Ru(bpy)3]3+ (1.0ꢁ10ꢀ4 m) in CH3CN con-
taining H2O (6.5 mm) at 233 K. b) Plot of kobs versus [Ru
(bpy)3]3+ for the
(bpy)3]2+ and
ACHTUNGTRENNUNG
electron
transfer
from
[(tmp)MnIII(OH)]
and
G
ACHTUNGTRENNUNG
-
[(tmp)MnIV(O)] to one-electron oxidants, is determined to
be 24 kcalmolꢀ1. The larger l value (35 kcalmolꢀ1) is ob-
tained for the first and second electron transfer from
N
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
one-step oxidation of [(tmp)MnIII (PF6) (5.0ꢁ10ꢀ6 m) with
ACTHNUTRGENN(UG H2O)2]ACHTUNGTRENNGUN
ACTHNUTRGNEUNG
[(tmp)MnIII(Cl)] to [Ru(bpy)3]3+ (Figure 8b). The larger l
[Ru(bpy)3]3+ in CH3CN containing H2O (6.5 mm) at 233 K. The kobs
value may result from the reorganization of the Cl ligand as-
*
values were determined from the decay of absorbance at 472 nm ( ) and
sociated with the electron transfer.[25] Thus, the two-electron
*
the rise of absorbance at 450 nm ( ).
oxidation
of
[(tmp)MnIII(Cl)]
which is
may
different
afford
from
[(tmp)MnV(O)(Cl)],
in which the disappearance of absorbance at 472 nm attrib-
[(tmp)MnV(O)]+ produced by the two-electron oxidation of
utable to [(tmp)MnIII
(H2O)2]+ coincides with the appearance
ACTHNGUTERNNUG
[(tmp)MnIII(H2O)2]+.[41,42] The different absorption spectra
of absorbance at 450 nm attributable to two equivalents of
[Ru
(bpy)3]2+. Both the decay of [(tmp)MnIII(H2O)2]+ and
the formation of [Ru
(bpy)3]2+ obey pseudo-first-order kinet-
between [(tmp)MnV(O)(Cl)] (Figure 3) and [(tmp)-
MnV(O)]+ (Figure 4) result from the binding of Clꢀ in the
former case.
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ics with the same pseudo-first-order rate constant (kobs). The
ket value was determined from the slope of the linear plot of
The ket value of electron transfer from [(tmp)MnIII-
ACHUTNGRENNUG CAHTUNGTRENNUGN
(H2O)2]+ to [Ru(bpy)3]3+ (filled circle in Figure 8), deter-
kobs versus the concentration of [Ru
(bpy)3]3+ (Figure 7b).
mined as one half of the observed rate constant of the two-
3+
electron oxidation of [(tmp)MnIII
is significantly smaller than the ket value of electron transfer
from [(tmp)MnIV(O)] to [Ru(bpy)3]3+ (diamond in Figure 8)
determined as the second electron transfer from
(H2O)2]+ with [Ru
ACHTUNGTRENNUGN ACHTUNGTRENNUNG(bpy)3 ,
Driving-force dependence of the electron transfer: The de-
pendence of the logket of the electron-transfer reactions of
manganese–porphyrins with one-electron oxidants (Table 1)
AHCTUNGTRENNUNG
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15799

S. Fukuzumi et al.
[(tmp)MnIII(OH)] to [Ru
the fact that the two-electron oxidation of [(tmp)MnIII-
(H2O)2]+ with [Ru(bpy)3]3+ occurs by means of a single
step: the rate-determining electron transfer from
[(tmp)MnIII(H2O)2]+ (bpy)3]3+
to [Ru to produce
[(tmp)MnIV(O)], followed by rapid electron transfer from
[(tmp)MnIV(O)] to [Ru(bpy)3]3+
(bpy)3]3+. This is consistent with
ACHTUNGTRENNUNG
Table 2. Turnover numbers (TONs) in the oxygenation of cyclohexene
ACTHNUTRGNEUNG
(5.0ꢁ10ꢀ3 m) with [Ru(bpy)3]3+ (1.0ꢁ10ꢀ2 m) catalyzed by solutions of
various manganese–porphyrins in CD3CN/D2O (9:1 v/v) at 298 K.
G
ACHTUNGTRENNUNG
Catalyst
Ar
TON
Cyclohexene Styrene
G
ACHTUNGTRENNUNG
A
130
170
20
65
E
G
ACHTUNGRTEN(NUNG PF6)
N
.
A
160
210
70
85
Catalytic oxygenation by electron transfer: Because a high-
valent metal–oxo species ([(tmp)MnV(O)]+) is produced by
E
R
ACHTUNGRTEN(NUNG PF6)
the electron-transfer oxidation with [Ru
lytic oxygenation of substrates by electron transfer with
[Ru
(bpy)3]3+ in the presence of water was examined using
manganese–porphyrins as catalysts (see below). Addition of
[Ru
(bpy)3]3+ to a solution of olefins (styrene, cyclohexene)
(bpy)3]3+, the cata-
ACHTUNGTRENNUNG
N
80
110
trace
35
ACHTUNGTRENNUNG
N
E
A
ACHTUNGTRENNUNG
G
20
80
trace
20
in CH3CN containing water in the presence of a catalytic
amount of manganese–porphyrins afforded epoxides, diols,
and aldehydes. Epoxides were converted to the correspond-
ing diols by hydrolysis, and the diols were further oxidized
to the corresponding aldehydes, which are four-electron oxi-
dized products of olefins (Scheme 3). These products were
detected by 1H NMR spectroscopy (see the Experimental
Section).
G
E
ACHTUGNTERN(NUNG PF6)2
Table 3. Redox potentials of various manganese–chloride–porphyrins
[(P)Mn(Cl)] in CH3CN containing H2O (6.5 mm) at 298 K.
tmopp
tmp
tdcpp
Eox1 vs. SCE [V][a]
Eox2 vs. SCE [V][a]
0.95
1.11
1.02
1.20
1.20
1.48
[a] TBAPF6: 0.10m.
cause the tdcpp ligand is less sterically hindered than the
tmp ligand and exhibits higher redox potentials (Table 3).[44]
In the case of [(dtmpd)Mn2(Cl)2], the TON values are sig-
nificantly smaller than those of the corresponding monomer,
[(tmp)Mn(Cl)], because the MnV(O) species derived from
[(dtmpd)Mn2(Cl)2] may be produced inside the diporphyrin
cavity to prevent the interaction with substrates.
Scheme 3.
Alkane hydroxylation by catalytic electron-transfer reac-
tions: Hydroxylation of alkane (ethylbenzene) also occurs
by the manganese–porphyrin-catalyzed electron-transfer ox-
It was confirmed that no oxygenated product was ob-
tained in the absence of a manganese–porphyrin under
otherwise the same experimental conditions. When
idation with [RuACTHNUTRGNENGU ACHTUGNTRENNUGN
(bpy)3]3+. Addition of [Ru(bpy)3]3+ to a sol-
[Ru
N
U
ution of alkane (ethylbenzene) in CH3CN containing water
in the presence of a catalytic amount of manganese–por-
phyrins resulted in the formation of alcohols (Scheme 4).[45]
product was obtained in the presence of [(tmp)MnIII(OH)].
This indicates that the formation of [(tmp)MnV(O)]+ is es-
sential for the oxygenation of olefins. The turnover numbers
(TONs) were calculated on the basis of the amount of cata-
lysts and the results are summarized in Table 2. TONs vary
significantly depending on the type of manganese–porphyr-
ins owing to the difference in their oxidation potentials
(Table 3) and the steric bulkiness of the porphyrin ligand.
Scheme 4.
When [(tmp)MnIII
ACHTUNGTRENNUNG(H2O)2]ACHTUNGTERN(NUGN PF6) was used as the catalyst,
oxygenation of cyclohexene occurred more efficiently than
that of styrene probably because of the stronger steric repul-
sion of styrene against the bulky tmp ligand of
[(tmp)MnV(O)]+ compared with that of cyclohexene. Such
preference of cyclohexene for styrene in the catalytic oxy-
genation has also been observed under photocatalytic oxy-
genation of olefins using porphyrins.[43] In the case of the
tdcpp ligand, the oxygenation of cyclohexene and styrene is
more reactive than that with the tmp ligand (Table 2) be-
1
The oxygenated products were detected by H NMR spec-
troscopy (see the Experimental Section). TONs were deter-
mined and the results are given in Table 4. Ethylbenzene is
also oxidized to 1-phenylethanol using [(tmp)MnIII
ACHTUNGTRENNUNG
A
ACHUTGTNREN(NUG H2O)2]ACHTNUGTRENNNUG
A
ACHTUNGTRENNUNG
on the type of manganese–porphyrins as a result of their dif-
ferent oxidation potentials (Table 3).
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Catalytic Electron-Transfer Oxygenation
FULL PAPER
Table 4. Turnover numbers (TONs) in the oxygenation of ethylbenzene
ACHTUNGTRENNUNG
(5.0ꢁ10ꢀ3 m) with [Ru(bpy)3]3+ (1.0ꢁ10ꢀ2 m), catalyzed by solutions of
various manganese–porphyrins in CD3CN/D2O (9:1 v/v) at 298 K.
Catalyst Ar
TON
15
ACHTUNGTRENNUNG ACHTUNGTRENNUNG(H2O)2]ACHTUNGTRENNUNG(PF6)
[(tmp)MnIII
A
70
30
R
N
ACHTUNGTRENNUNG(PF6)
Figure 9. Mass spectra of styrene oxide produced by styrene (1.0ꢁ10ꢀ2 m)
with [Ru
(bpy)3]3+ (4.0ꢁ10ꢀ2 m) catalyzed by
a
solution of
[(tmp)MnIII(OH)] (5.0ꢁ10ꢀ7 m) in CH3CN/H2O (9:1 v/v) at 298 K. Using
A
N
(PF6)
0
a) H218O and b) H216O as an oxygen source.
18O-labeling experiments: The
oxygen source in the manga-
nese–porphyrin-catalyzed elec-
tron-transfer oxygenation of
substrates with [Ru
was examined by 18O-labeling
(bpy)3]3+
ACHTUNGTRENNUNG
experiments
using
[(tmp)MnIII(OH)] as a catalyst.
The epoxidation of styrene
and the hydroxylation of ethyl-
benzene with [RuACHTUNGTRENNUNG
(bpy)3]3+ in
the presence of 95% 18O-la-
beled water containing a cata-
lytic amount of manganese–
porphyrin afforded the corre-
sponding epoxide and alcohol
with 90% incorporation of 18O
in the oxygenated products as
shown in Figure 9 and Fig-
ure S6 (see the Supporting In-
formation), respectively. Thus,
the oxygen source in the oxy-
genated products in the man-
ganese–porphyrin-catalyzed
electron-transfer oxygenation
of substrates with [RuACTHNUTRGNEUNG
(bpy)3]3+
has been confirmed to be
water in the mixed solvent
(CH3CN/H2O).
Figure 10. a) Time profile of the formation of [Ru
13800 mꢀ1 cmꢀ1) in the electron-transfer oxidation of [Ru
(H2O)2]+ (1.0ꢁ10ꢀ5 m), with substrate (cyclohexene; 5ꢁ10ꢀ3 m) in CH3CN/H2O (9:1 v/v) at 298 K. Inset: First-
order plot. b) Plot of kobs versus [Ru
(bpy)33+] in the presence of 1.0ꢁ10ꢀ5 m [(tmp)MnIII(H2O)2]+ and 5.0ꢁ
10ꢀ3 m cyclohexene in CH3CN:H2O (9:1 v/v) at 298 K. c) Plot of kobs versus [cyclohexene] in the presence of
1.0ꢁ10ꢀ5 m [(tmp)MnIII(H2O)2]+ and 1.0ꢁ10ꢀ4 m [Ru(bpy)3]3+ in CH3CN/H2O (9:1 v/v) at 298 K. d) Plot of kobs
versus [(tmp)MnIII(H2O)2]+ in the presence of 1.0ꢁ10ꢀ4 m [Ru(bpy)3]3+ and 5.0ꢁ10ꢀ3 m cyclohexene in
CH3CN/H2O (9:1 v/v) at 298 K.
ACHTUNGTRENNUNG
G
-
AHCTUNGTRENNUNG
Catalytic
mechanism: The rate of forma-
tion of [Ru
(bpy)3]2+ in the
[(tmp)MnIII
(H2O)2](PF6)-cata-
lyzed oxygenation of cyclohex-
ene with [Ru
(bpy)3]3+ in the
electron-transfer
G
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
U
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
presence of 5.5m H2O in CH3CN was monitored by an in-
[RuACTHUNGETRNNUG
(bpy)3]3+ (Figure 10b) and cyclohexene (Figure 10c). In
crease in the absorption band at 450 nm attributable to
contrast, the observed first-order rate constant increased lin-
early with an increase in concentration of the catalyst,
[RuACHTUNGTRENNUNG
(bpy)3]2+ (Figure 10a). The rate obeyed first-order kinet-
ics (inset of Figure 10a). The observed first-order rate con-
stant remained constant with the change in concentration of
[(tmp)MnIII
mation of [Ru
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Chem. Eur. J. 2012, 18, 15794 – 15804
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www.chemeurj.org
15801

S. Fukuzumi et al.
oxygenation of cyclohexene with [Ru
Equation (2).
(bpy)3]3+ is given by
The rate constant of the electron transfer from [(tmp)Mn-
ACHUTNGRENNUG CAHTUNGTRENNUGN
(H2O)2]+ to [Ru(bpy)3]3+ under the same conditions as
those employed for the catalytic oxygenation of cyclohexene
d½½RuðbpyÞ3ꢁ2þꢁ=dt ¼ ket½½ðtmpÞMnIIIꢁþꢁ½½RuðbpyÞ3ꢁ3þꢁ
ð2Þ
with [Ru
AHCTUNGTRENNUNG
time profile of the electron transfer under stoichiometric
conditions (Figure 12). The rate obeyed second-order kinet-
Because the rate is independent of the cyclohexene con-
centration, the reaction of [(tmp)MnV(O)]+ with cyclohex-
ene is faster than the electron-transfer step to generate [Ru-
ACHTUNGTRENNUNG
(bpy)3]2+, which is the rate-determining step.
To confirm this point, the rate constant of the reactions of
[(tmp)MnV(O)]+ with cyclohexene was determined directly
under stoichiometric (noncatalytic) reaction conditions by
monitoring an increase in the absorption band at 470 nm at-
tributable to [(tmp)MnIII]+, which was produced by the oxy-
genation of cyclohexene with [(tmp)MnV(O)]+, in the pres-
ence of various concentrations of cyclohexene in CH3CN/
H2O (9:1 v/v) at 298 K. The rate obeyed pseudo-first-order
kinetics (Figure 11a) and the observed first-order rate con-
stant (kobs) increased linearly with increasing cyclohexene
concentration (Figure 11b). From the slope of the linear plot
of kobs versus cyclohexene concentration in Figure 11b the
second-order rate constant (kox) of the reaction of
[(tmp)MnV(O)]+ with cyclohexene in CH3CN/H2O (9:1 v/v)
at 298 K was determined to be 1.6ꢁ105 mꢀ1 sꢀ1.
Figure 12. Time profile of a) the formation of [Ru
decay of [(tmp)MnIII(H2O)2]+ for oxidation monitored by absorbance l=
450 and 465 nm in the electron-transfer oxidation of [(tmp)MnIII(H2O)2]+
(5.0ꢁ10ꢀ6 m) by [Ru(bpy)3]3+ (1.0ꢁ10ꢀ5 m) in CH3CN/H2O (9:1 v/v).
Inset: Second-order plot.
(bpy)3]2+ and b) the
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ics and the second-order rate constant of the electron trans-
fer (ket) was determined to be 2.4ꢁ106 mꢀ1 sꢀ1 in CH3CN/
H2O (9:1 v/v) at 298 K. This ket value is much larger than
the value (3.0ꢁ105 mꢀ1 sꢀ1) determined in CH3CN at 233 K.
More importantly, the ket value determined directly under
stoichiometric conditions agrees well with the value (2.6ꢁ
106 mꢀ1 sꢀ1) determined under catalytic conditions (Fig-
ure 10d). Thus, it was confirmed that electron transfer from
[(tmp)Mn
termining step in the catalytic oxygenation of cyclohexene
(H2O)2]+ to [Ru(bpy)3]3+ was indeed the rate-de-
ACHUTGTNRENNUG CAHTUNGTRENNUGN
with [RuACHTUNGTRNEUNG
(bpy)3]3+, when the first-order rate constant of the
reaction of [(tmp)MnV(O)]+ with 1.0ꢁ10ꢀ3 m cyclohexene
(1300 sꢀ1) is significantly larger that the value of formation
of [(tmp)MnV(O)] with 1.0ꢁ10ꢀ4 m (260 sꢀ1).
Based on the above results, the catalytic mechanism of
the manganese–porphyrin-catalyzed oxygenation of sub-
strates (S: alkanes and olefins) with [Ru
ized as shown in Scheme 5. The electron transfer from [Ru-
(bpy)3]3+ to [(P)MnIII(H2O)2] generates [(P)MnV(O)]+,
(bpy)3]3+ is summar-
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
which can oxygenate a substrate (S) to yield SO in the pres-
ence of H2O to regenerate [(P)MnIII(OH)]. The presence of
H2O is essential to accelerate the oxygenation of S with
[(P)MnV(O)]+. In this context, it is interesting to note that
the high reactivity of [(P)MnV(O)]+ with a large rate con-
stant (6.5ꢁ105 mꢀ1 sꢀ1) has been reported using a water-solu-
+
Figure 11. a) Time profile of the formation of [(tmp)MnIII
]
monitored
by absorbance at l=465 nm in the oxidation of cyclohexene (5.0ꢁ
10ꢀ3 m), in the presence of [(tmp)MnV(O)]+ in CH3CN/H2O (9:1 v/v) at
298 K. Inset: First-order plot. b) Plot of kox versus [cyclohexene] in the
ble
porphyrin
of [(tmpyp)MnV(O)]+ with a water-soluble olefin (carbama-
manganese–porphyrin,
tetra-N-(methylACHTUNGTRENNUNGpyridyl)-
A
R
ACHTUNGTRENNUNG
presence of 5.0ꢁ10ꢀ6 m [(tmp)MnIII(H2O)2]+ and 1.0ꢁ10ꢀ5 m [Ru(bpy)3]3+
ACTHNUTRGENNUG CAHTUNGTRENNUNG .
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Catalytic Electron-Transfer Oxygenation
FULL PAPER
Acknowledgements
This work was supported by a Grant-in-Aid (no. 20108010) and a Global
COE program, “the Global Education and Research Center for Bio-En-
vironmental Chemistry” from the Ministry of Education, Culture, Sports,
Science and Technology, Japan and also by NRF/MEST through WCU
(R31-2008-000-10010-0), Korea.
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the strong solvation and hydrogen bonding to the substrate
radical cation.[46] In the case of HRP compound I, the reac-
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reported to proceed through initial electron transfer from
the substrates to the oxoiron(IV)–porphyrin p-radical
cation.[47] The N-demethylation of N,N-dimethylaniline by a
synthetic oxoiron(IV)–porphyrin p-radical cation also pro-
ceeds by electron transfer.[47] Such electron-transfer process-
es are enhanced in polar solvents, whereas hydrogen- or
oxygen-atom transfer reactions are known to be rather in-
sensitive to the polarity of solvents.[48] However, the role of
water to accelerate the oxygenation of S with [(P)MnV(O)]+
has yet to be further clarified.
The direct determination of the rate constants of manga-
nese(V)–oxo species with substrates has also been made by
Zhang and Newcomb,[49] who reported the rate constant of
manganese(V)–oxo species with substrates by using laser
flash photolysis. A large rate constant (6.1ꢁ105 mꢀ1 sꢀ1)[49]
was determined for the reaction of [(F20tpp)MnV(O)]+
(F20tpp=tetrakis(pentafluorophenyl)porphyrinato dianion)
with cis-stilbene in the absence of water in CH3CN. This
value is larger than the kox values in Figure 11d.
Conclusion
The present study has demonstrated that the electron-trans-
ACTHNUTRGNEUNG
fer oxidation of (P)MnIII with [Ru(bpy)3]3+ in the presence
of H2O affords [(P)MnIV(O)] and [(P)MnV(O)] and the elec-
tron-transfer rate constants were thoroughly analyzed in
light of the Marcus theory of electron transfer. The
[(P)MnV(O)] oxygenates substrates to reproduce (P)MnIII
when catalytic electron-transfer oxygenation of substrates
occurs efficiently with [RuACHTNUGRTNEUNG
(bpy)3]3+ as a one-electron oxi-
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Chem. Eur. J. 2012, 18, 15794 – 15804
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: June 8, 2012
Published online: November 5, 2012
15804
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15794 – 15804