O2 activation and external substrate oxidation capability of a Co(ii)-semiquinonato complex

Article abstract of DOI:10.1039/c3nj00215b

The synthesis, characterization and catalytic oxidizing activity of a cobalt(ii)-semiquinonato complex with hydrotris(3,5-dimethyl-1-pyrazolyl)borate (=Tp^Me2) have been investigated. The cobalt(ii)-semiquinonato complex can be synthesized by two alternative routes; (i) dehydrative condensation of a cobalt(ii)-hydroxo complex with 3,5-di-tert-butylcatechol and following O ₂ oxidation, or (ii) one-electron oxidation of catechol by a dinuclear cobalt(iii)-bis(μ-oxo) complex. The synthesized cobalt(ii)-semiquinonato complex can bind O₂ at low temperatures and catalyzes the oxidation of ortho- and para-hydroquinones to the corresponding benzoquinones concomitant with the generation of hydrogen peroxide. The resulting H₂O₂ is utilized as an oxidant for the epoxidation of allyl alcohol catalyzed by peroxotungstate. Aerobic oxidation of benzyl alcohol to the corresponding aldehyde is also promoted by the cobalt(ii)-semiquinonato complex.

Full text of DOI:10.1039/c3nj00215b

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Short text and Figure for TOC
A cobalt(II)-semiquinonato complex can bind O2 at low temperature and catalyzes
aerobic oxidations of hydroquinones and alcohols concomitant with the generation of
hydrogen peroxide.
+O2
–O2
O
III
Co
Aromatic
cleavage
or
O
O
O
N
N
N
N
H B
Co =
Co
+ H2O2
N
N

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O2 activation and external substrate oxidation capability of
Co(II)-semiquinonato complex
Atsushi Ikeda,a Kazuya Hoshino,a Hidehito Komatsuzaki,b Minoru Satoh,b
Jun Nakazawaa and Shiro Hikichi*a
a Department of Material and Life Chemistry
Faculty of Engineering
Kanagawa University
3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686 (Japan)
Fax: +81-45-413-9770
E-mail: hikichi@kanagawa-u.ac.jp
b Department of Chemistry and Material Engineering
Ibaraki National College of Technology
866 Nakane, Hitachinaka, Ibaraki 312-8508, Japan

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Abstract
Synthesis, characterization and catalytic oxidizing activity of a cobalt(II)-semiquinonato
complex with hydrotris(3,5-dimethyl-1-pyrazolyl)borate (= TpMe2
)
have been
investigated. The cobalt(II)-semiquinonato complex can be synthesized by two
alternative routes that dehydrative condensation of a cobalt(II)-hydroxo complex with
3,5-di-tert-butylcatechol and following O2 oxidation, or one-electron oxidation of
catechol by
a
dinuclear cobalt(III)-bis(ꢀ-oxo) complex.
The synthesized
cobalt(II)-semiquinonato complex can bind O2 at low temperature and catalyzes the
oxidation of ortho- and para-hydroquinones to the corresponding benzoquinones
concomitant with the generation of hydrogen peroxide. The resulting H2O2 is utilized
as oxidant for the epoxidation of allyl alcohol catalyzed by peroxotungstate. Aerobic
oxidation of benzyl alcohol to the corresponding aldehyde is also promoted by the
cobalt(II)-semiquinonato complex.

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Introduction
Oxidation of organic compounds with molecular oxygen is still important subject for
modern synthetic chemistry in order to develop the environmentally benign chemical
transformation processes.1
Some metalloenzymes catalyze organic compounds
oxidation with O2 under physiological mild condition. Therefore, design and synthesis
of O2 activating complexes of transition metals are attractive approach to green
oxidation catalysts based on bio-inspired concept.2
In the active sites of some non-heme metalloenzymes such as galactose
oxidase3 and catechol dioxygenases,4 redox-active ligand complexes are formed during
the catalytic cycles. The ligand-based radical might work as an accepter of another
radicals such as H atom derived from substrate and metal-bound superoxide anion
–
radical (= O2 •).
As well as the radical trapping, intramolecular electronic
communication between the metal center and the redox-active ligands are indispensable
for the emergence of catalysis. Therefore, combination of redox-active ligands and
transition metal ions is interesting approach for designing the bio-inspired oxidation
catalyst.5
To date, a large number of redox-active transition metal complexes with
dioxolene ligands have been investigated. The chemistry of iron-dioxolene complexes
have been extensively attracted much attention because of the relevance to catechol
dioxygenases, which catalyze oxidative aromatic ring cleavage to carbonyl compounds.6
Copper-dioxolene complexes have also been investigated from an interest in
catecholases, which catalyze oxidation of catechols to quinones.7 Cobalt complexes
with dioxolene ligands, including cobalt(II)-semiquinonato and cobalt(III)-catecholato
complexes, have been investigated mainly from the viewpoint of their unique electronic

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configuration and magnetism.8 However, O2 reduction capability as well as organic
substrate oxidation potential of cobalt(II)-semiquinonato complexes have not been
turned attention so far, although O2 binding ability and oxidizing catalyses of the same
group 9 metal complexes (Rh and Ir) have been investigated.9 As similar to the active
site of the non-heme metalloenzymes mentioned above, the cobalt(II)-semiquinonato
complexes are composed of the radical ligand and the redox-active transition metal
center. Therefore, such complexes are expected to show multi-electronic oxidaizing
activity.
In this study, potentials of a cobalt(II)-semiquinonato complex,
[CoII(DTBSQ)(TpMe2)] (1; TpMe2 = hydrotris(3,5-dimethylpyrazolyl)borate, DTBSQ =
3,5-di-tert-butylsemiquinonate) to promote biomimetic oxidation of OH-containing
substrates have been explored.
Results and discussion
Synthesis and characterization of cobalt(II)-semiquinonato complex 1
Compound 1 could be synthesized according to the similar procedures for the
previously reported cobalt(II)-semiquinonato complexes.10
Oxidation of the
stoichiometric reaction mixture of DTBCH2 and the cobalt(II)-hydroxo complex,
[(CoIITpMe2)2(ꢀ-OH)2] (2),11 yielded a dark green compound 1 (Scheme 1). UV-vis
spectral pattern of 1 was similar to that of the bulky TpR ligand analogue,
[CoII(DTBSQ)(TpCum,Me)] (TpCum,Me = hydrotris(3-cumyl-5-methylpyrazolyl)borate)
reported by Pierpont and coworkers10; both complexes exhibited the strong LMCT
band around 380 nm as well as the semiqunonate ligand-based absorption bands
around 300 nm and 720 nm (Fig. 1(a)).
In addition,
1
showed
1
paramagnetically-shifted H NMR signals from –62 to +61 ppm region at room

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temperature (Fig. S1). These spectral characteristics support the assignement of 1
as the cobalt(II)-semiquinonato complex. These spectral features are very similar
to those of an an analogous cobalt(II)-DTBSQ complex with TpMe3 (=
hydrotris(3,4,5-trimethylpyrazolyl)borate), of which the molecular structure has
been confirmed by X-ray crystallography (See Figs. S2 and S3 and Tables S1 and S2
in supporting information). The shortening of the O–C lengths and the lost of
aromaticity of the six-membered ring imply the dioxolene ligand is semiquinonate as
found in the Pierponts’ compound.10
[Scheme 1]
[Fig. 1]
Compound 1 could be synthesized alternatively by reaction of a dinuclear
cobalt(III)-bis(ꢀ-oxo) complex, [(CoIIITpMe2)2(ꢀ-O)2] (3),11 with two equiv. of
DTBCH2 under argon (Scheme 1). When one equiv. of DTBCH2 was added, 50 %
of 3 was consumed to yield 1. This fact suggests that the formation of the
cobalt(II)-SQ species 1 proceeds through 1e– oxidation of DTBCH2 by 3, or reaction
of a basic oxo ligand of 3 with DTBCH2 yielding a putative cobalt(III)-catecholato
species and following intramolecular electron transfer from the catecholate ligand to
cobalt. It should be noted that reaction of dinuclear copper(III)-bis(ꢀ-oxo) species
with DTBCH2 lead to 1e– or 2e– oxidation of catechol depending on the metal
supporting ligands: a 1,4,7-tribenzyl-1,4,7-triazacyclononane ligand complex plays
as
a
1e– oxidant to yield
a
copper(II)-SQ species, whereas an
N,N,N’,N’-tetramethyl-1,3-propanediamine ligand complex works as a 2e– oxidizing
reagent to give o-benzoquinone.12
Compound 1 exhibited an affinity toward O2 as shown in Fig. 1.

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Introduction of O2 to the CH2Cl2 solution of 1 at low temperature (203 K) resulted in
the color change from dark green to pale brown, and the initial dark green color was
recovered by evacuation even at 203 K (Fig. S4). Such color change were not
observed under argon atmosphere at 195 K and under O2 atmosphere at ambient
temperature. Therefore, an O2 adduct of 1 (= 1•O2) forms in the pale brown
solution, but we have not met with success to isolate 1•O2 due to low O2 binding
affinity of the cobalt center of 1. To date, the molecular structure of the
alkylperoxo complexes of rhodium(III) and iridium(III), which are derived from the
reaction of O2 with the corresponding catecholato precursors have been
charactrized.9c,d In line with an analogy of the catecholase activity of 1•O2 and the
iridium(III)-alkylperoxo complex with tridentate phosphine ligand9c (see below), it is
plausible that an assignment of 1•O2 is a cobalt(III)-alkylperoxo species (Compound
P in Scheme 2). Disappearance of the absorption band of 1 around visible to
near-IR region (~ 720 nm) upon contacts with O2 at low temperature seems to
suggest the lost of radical character of the dioxolene ligand,10 and that is consistent
with the structure of the postulated alkylperoxo complex. At this moment, however,
we cannot exclude the possibility that 1•O2 is a cobalt(III)-superoxo species with
DTBSQ ligand (Compound S in Scheme 2).13 Attempt to detect an O–O vibration
by IR spectroscopy of the solution of the in situ generated 1•O2 at 193 K was failed
(Fig. S5).
[Scheme 2]
Previously, we have demonstrated that a related iron(III)-catecholato
complex,
[FeIII(DTBC)(TpiPr2)]
(4;
TpiPr2
=
hydrotris(3,5-di-iso-propylpyrazolyl)borate, DTBC = 3,5-di-tert-butylcatecholate),

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exhibits catechol dioxygenase activity: The iron-bound DTBC ligand is converted
to the corresponding oxygenated compounds (intra- and extradiol cleavage products)
and no DTBQ (= 3,5-di-tert-butylquinone) is formed upon stoichiometric reaction
(i.e., reaction of 4 with O2 in the absence of external DTBCH2).14 In an analogous
manganese
chemistry,
reaction
of
DTBCH
and
a
dinuclear
manganese(III)-bis(ꢀ-oxo) complex with TpiPr2 yields an O2 sensitive
manganese(II)-semiquinonato complex, [MnII(DTBSQ)(TpiPr2)] (5), and reaction of
5 and O2 yields the both intra- and extradiol cleavage products as well as DTBQ.15
In contrast to these iron and manganese analogues 4 and 5, further oxidation of the
coordinated semiquinonate ligand giving oxidative ring-cleavage products did not
occur from 1•O2. Notably, the cobalt(II)-substituted extradiol enzyme retains
primary catalytic activity.16 The postulated intermediates in the enzymatic system
–
are cobalt(II)-SQ-O2 • and cobalt(II)-alkylperoxo species and following O–O
activation will lead to the oxygenation of the substrate. No dioxygenase activity of
1•O2 might arise from the difference of the oxidation state of the cobalt centre. In
our synthetic compound system, the oxidation state of the cobalt center of 1•O2 is +3
and further O–O activation is difficult due to high oxidation potential of cobalt(III)
to cobalt(IV).
Hydroquinone oxidation and concomitant H2O2 generation catalyzed by 1
Compound 1 exhibited catecholase activity, although the structure of 1 is
different from the dicopper active site of the enzyme. O2 oxidation of ortho- and
para-hydroquinones to the corresponding benzoquinones were catalyzed by 1 as
summarized in Table 1.17 Not only the non-substituted and alkyl-substituted

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para-hydroquinones but also a halogenated derivative were oxidized although the
reaction rate for the halogenated substrate was relatively low (Fig. S6). Oxidation
rates of hydroquinone were depended on the reaction conditions as shown in Fig. S7.
[Table 1]
As similar to the previously reported iridium(III)-catecholato complex,9c
hydrogen peroxide was formed concurrent with the consumption of the
hydroquinone substrates.18,19 When less water-soluble DTBCH2 was used as a
substrate, the catalytically generated H2O2 could be extracted to an aqueous phase
under organic (toluene) – water (acetate buffer; pH 4.4) biphasic condition. In
toluene phase, DTBCH2 was oxidaized to DTBQ by action of 1 (1 mol %) under O2
(1 atm) at 313 K. At the same time, generated H2O2 was extracted to 2.0 mL of the
aqueous phase, and the concentration of H2O2 was 0.13 mol/L (34 % of the
generated DTBQ) determined by iodometry (entry 1 of Table 2).
[Table 2]
Also, we examined trapping of H2O2 by easy-oxidizable substrate Ph3P in
homogeneous reaction media. Co-existence of para-hydroquinone and Ph3P with 1
in the homegeneous solution resulted in the catalytic oxidation of the both substrates
(Fig. 2 and entry 2 of Table2). In the absence of hydroquinone, 1 could not
catalyze Ph3P oxygenation. Therefore, the catalytically generated H2O2 during
hydroquinone oxidation was trapped by Ph3P. In contrast to the sole hydroquinone
substrate system, the presence of Ph3P affected the oxidation rate of hydroquinone
(Fig. S8): Ph3P seems to work as a competitive inhibitor for the O2 activation by
the cobalt center of 1, whereas the consumption of H2O2 by Ph3P leads to accelate
catalytic turnover. Notably, the rhodium-catecholato complex catalyzes Ph3P

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oxygenation with O2.9a Lack of the oxygen atom-transfer ability of 1•O2 implies
hardness of further O–O bond activation, and that may be consistent with no
dioxygenase activity toward the DTBSQ ligand. In general, stoichiometry of the
substrate to oxidant in the O2 oxidation of hydroquinones by metal catalysts is
represented as hydroquinones:O2 = 2:1. One plausible explanation for this
stoichiometry is that the redox active catalyst mediate the disproportionation of the
intermediary generated H2O2 to water and O2.20
In contrast, compound 1 was
inactive for H2O2 degradation and that resulted in the catalytic generation of H2O2.
[Figure 2]
The biphasic reaction system could be applied to oxidation of another organic
compound.
Combination with a H2O2-activating peroxotungstate catalyst,
[{W(=O)(η2-O2)2(H2O)}2(ꢀ-O)]2– (6),21 resulted in the catalytic epoxidation of allyl
alcohol concomitant with the O2 oxidation of DTBCH2 catalyzed by 1 (entry 3 of
Table 2). Catalytic DTBCH2 oxidation by 1 with O2 proceeded in the organic
solvent (toluene) phase, and the resulting H2O2 was extracted in water phase and
consumed for the epoxidation of allyl alcohol catalyzed by 6 (Scheme 3). In this
catalytic epoxidation reaction, O2 and DTBCH2 (hydroquinone) are formally oxidant
and reductant, respectively.22
From biomimetic viewpoints, the redox of
quinone/hydroquinone is often utilized as the electron transfer mediator for the
turnover of the catalysts of the oxidation reactions.23 Taking the place of NADH in
biological system, hydroquinones work as proton and electron source in our system,
and the combination with the reductive O2 activation and H2O2 activation systems
leads to mimic the reaction catalyzed by monooxygenases.
[Scheme 3]

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It should be noted that the cobalt(II)-hydroxo complex 2, which is the
precursor of 1, also catalyzed oxidation of para-hydroquinones to the corresponding
benzoquinones. When hydroquinone was a sole substrate, TOF of 2 (> 20 min–1)
was higher than that of 1 (ca. 1.5 min–1). When both hydroquinone and Ph3P were
used as co-substrate, however, only a trace amount of Ph3P=O was yielded upon the
reaction by 2. These facts suggest that the catalytic reaction mechanism of 1 is
different from that of 2, and the DTBSQ ligand of 1 might remain to coordinate
during the oxidation of para-hydroquinones. A candidate of the active oxidant for
2e– oxidation of the hydroquinones coupled with H2O2 generation is the alkylperoxo
species (P in Scheme 2) as well as the iridium complex.9c
Another candidate is
–
the cobalt(III)-DTBSQ-O2 • species (S in Scheme 2), which may be a transient
intermediate from 1 to the alkylperoxo species: The superoxide ligand captures H
atom and H+ from substrate to give H2O2. Electron transfer from the resulting
semiquinonate radical (derived from the substrate) to the cobalt(III) center leads to
turnover to 1.
Alcohol oxidation capability
We also examined aerobic alcohol oxidation ability of 1, because 1 is
composed of the radical ligand with the redox-active transition metal center and this
composition is similar to that of the active species of the galactose oxidase.3,24,25
Galactose oxidase catalyzes oxidation of primary alcoholic moiety of substrate to the
corresponding aldehyde product concomitant with reduction of O2 to H2O2. An
active site of the enzyme is a mononuclear copper complex with a cysteine-cross
linked tyrosine radical ligand. This unique phenoxy-radical ligand is proposed as a

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hydrogen atom acceptor from the C–H moiety adjacent to OH of the alcoholic
substrate. Two electron oxidation of alcohol to aldehyde is achieved due to
combination of the H atom capturing by the radical ligand and an electron transfer
from the substrate to the copper(II) center giving a copper(I) species.3, 24
An aerobic oxidation of benzyl alcohol to benzaldehyde was mediated by 1
(eq. 1). A Michaelis-Menten type kinetics, as was evidenced by dependence of the
initial reaction rate on the catalyst condensation and a liner Lineweaver-Burk plot
(Fig. S9), suggest that this reaction proceeds through the catalyst-alcohol complex
intermediate.
The yield of acetophenone derived from oxidation of
1-phenylethanol (secondary alcohol) was lower than that of benzaldehyde from
benzylalcohol (Fig. 3), and this result also supports the mechanism through the the
catalyst-alcohol complex because of the steric hindrance of benzylacohol is lower
than that of 1-phenylethanol.26 Oxidation of Ph3P occurred on the co-substrates
system (Fig. S10) and that suggests the concomitant H2O2 generation as found in the
hydroquinone/Ph3P system. However, the reaction rate was not so high and the
reaction was terminated before the turn over numbers of catalyst 1 reached to 2
under any conditions. Moreover, the alcohol oxidation did not occur in the absence
of O2. Thus, 1 itself could not work as 2e– oxidant, and the reaction mechanism of
the alcohol oxidation mediated by 1 might be diffent from that mediated by the
enzyme. In the enzymatic process, both copper(II) center and the phenoxy radical
of the cysteine-cross linked tyrosine ligand play as a 1e– acceptor, and that leads to
complete 2e– oxidation of alcohol to aldehyde. In contrast, the cobalt(II) center of
1 cannot work as a 1e– acceptor, and therefore, O2 is essential to oxidize cobalt(II) to
cobalt(III).
Several examples of alcohol oxidation by radical ligand-containing

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copper complexes have been reported so far.25 To our knowledge, the present
result is the first example of Co(II)-semiquinonato complex mediated reaction,
although the reaction mechanism is not clear.
[Figure 3]
Conclusion
The cobalt(II)-semiquinonate complex 1 can be synthesized by oxidation of the
cobalt(II)-catecholato species generated by reaction of the cobalt(II)-OH precursor with
catechol, or anaerobic reaction of the cobalt(III)-oxo species with catechol.
Compound 1 shows affinity toward O2. Moreover, compound 1 exhibits catecholase
rather catechol dioxygenase activity. During the catalytic oxidation of hydroquinones
to benzoquinones mediated by 1, concomitant H2O2 generation occurs. Such O2
activating and catecholase like behaviours of 1 are similar to those of the previously
reported iridium-catecholato complex. Biomimetic monooxygenase-type oxygenation
process can be composed by combination of the hydroquinone oxidation catalyzed by 1
with another appropriate H2O2 activating system. The cobalt-based radical ligand
complex 1 also promoted the O2 oxidation of alcohols to the corresponding carbonyl
compounds, although catalytic performance is low. Our findings demonstrate the
redox active dioxolene ligand plays important roles for the emergence of oxidation
catalyses.

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Experimental Section
General
All manipulations were performed under argon by standard Schlenk techniques.
Solvents used in this study (CH2Cl2, toluene, MeCN) were purified by Glass Counter
System by Grubbs’ method. The other reagents of the highest grade commercially
available were used without further purification. The starting materials of 1, i.e.
cobalt(II)-hydroxo complex 2 and dinuclear cobalt(III)-bis(ꢀ-oxo) complex 3,11 and the
olefin epoxidation catalyst, K2[{W(=O)(η2-O2)2(H2O)}2(ꢀ-O)]·2H2O (6),21 were
prepared by the methods described previously. IR measurements were carried out as
KBr pellets using JASCO FT/IR-4200 spectrometer. IR spectra of the solution
samples were recorded using a Mettler Toledo ReactIR iC10. 1H NMR spectra were
recorded on JEOL JNM EX-270 spectrometer. Chemical shifts (δ) were reported in
ppm downfield from internal SiMe4. UV-vis spectra were recorded on JASCO V-670
spectrometer. Gas chromatographic (GC) analyses were carried out on Shimadzu
GC-2010 instrument with a flame ionization detector equipped with RESTEK Rtx-1 or
Rtx-1701 (30 m, 0.25 mmID, 0.25 ꢀmdf) capillary columns. GC-MS analyses were
carried out on Shimadzu PARVUM2 system equipped with RESTEK Rxi-5MS (30 m,
0.25 mmID, 0.25 ꢀmdf) capillary column.
Synthesis of the Co(II)-semiquinonato complex [CoII(DTBSQ)(TpMe2)] (1)
The hydroxo complex 2 (82 mg; 0.11 mmol) was dissolved in 30 mL of CH2Cl2 under
argon. To this solution, toluene solution (10 mL) of DTBCH2 (51 mg; 0.23 mmol) was
added. The reaction mixture was exposed to O2 (1 atm) and stirred for 2 h. The
voletiles were evaporated under vacuum, then recrystallization of the resulting solid
from MeCN at 243 K afforded the dark green powder of 1 (72 mg; 0.12 mmol; 55%
yield based on 2). Anal. Calcd for C29H42N6O2BCo (1): C, 60.43; H, 7.34; N, 14.58.

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Found: C, 59.95; H, 6.83; N, 14.08. IR (KBr, ν/cm–1): 2508 (BH). UV-vis
(CH2Cl2, r.t., nm, ε/M–1cm–1): 305 (2990), 329 (sh, 2220), 379 (2100), 596 (180). 1H
NMR (CDCl3, r.t.): δH –61.8 (9H), –2.9 (9H), 9.8 (9H), 37.5 (3H), 39.2 (9H), 61.2 (1H).
Catalytic oxidation
Catalyst 1 (10 ꢀmol), substrate (hydroquinones and alcohols; 1.0 mmol) and decane (an
internal standard for quantitative analysis by GC; 0.10 mmol) were dissolved in MeCN
(4 mL) under argon atmosphere and then degassed by freeze-pump-thaw cycles (twice).
This degassed reaction solution was exposed to O2 (1 atm) and stirred at 305 K, and
analyzed by gas chromatography.
H2O2 detection experiment
Generated H2O2 (derived from DTBCH2 oxidation) was directly detected by extraction
to water. DTBCH2 (1.0 mmol) and 1 mol % of 1 (10 ꢀmol) were dissolved in toluene
(2.0 mL) under argon atmosphere. Acetate buffer solution (pH 4.4; 2.0 mL) was added
to this toluene solution and then degassed by freeze-pump-thaw cycles (twice). This
degassed reaction solution was exposed to O2 (1 atm) and stirred at 313 K for 24 h.
The organic phase was analyzed by gas chromatography with internal standard, and the
yield of DTBQ was 77 % (0.77 mmol; based on initial DTBCH2). The concentration
of H2O2 in aqueous phase was 0.13 mol/L determined by iodometry.
Co-oxidation of para-hydroquinone and Ph3P with O2 catalyzed by 1
Catalysts 1 (10 ꢀmol), para-hydroquinone (100 ꢀmol), and Ph3P (100 ꢀmol) were
dissolved in the mixture of MeCN (2.0 mL) and CH2Cl2 (2.0 mL) under argon and then
degassed by freeze-pump-thaw cycles (twice). This degassed reaction solution was
exposed to O2 (1 atm) and stirred at 305 K for 90 min. The organic products were
analyzed by GC. The final yields of products: benzoquinone, 87 ꢀmol; Ph3P=O, 64
ꢀmol.

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Co-oxidation of DTBCH2 and allylalcohol with O2
Catalysts
1
(10
ꢀmol)
and
the
potassium
salt
of
6
(=K2[{W(=O)(η2-O2)2(H2O)}2(ꢀ-O)]·2H2O; 5.0 ꢀmol), DTBCH2 (1.0 mmol), and
allylalcohol (2.5 mmol) were dissolved in the mixture of toluene (2.0 mL) and acetate
buffer solution (pH 4.4; 2.0 mL) under argon and then degassed by freeze-pump-thaw
cycles (twice). This degassed reaction solution was exposed to O2 (1 atm) and stirred
at 313 K for 24 h. The organic products were analyzed by GC. The yields of
products: DTBQ, 0.17 mmol; glysidol, 0.13 mmol.
Acknowledgements
This work was supported in part by a Grant in-Aid for Scientific Research (Nos.
20037063 and 20360367), a Scientific Frontier Research Project, and a Strategic
Development of Research Infrastructure for Private Universities from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT), Japan. S. H. and J. N.
also thank Kanagawa University for financial support.

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Catalysis, Wiley-VCH, Weinheim, 2007.
2
Reviews for example: (a) Biomimetic Oxidations Catalyzed by Transition Metal
Complexes (Ed.: B. Meunier), Imperial College Press, London, 2000; (b)
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Eldik and J. Reedijk), Adv. Inorg. Chem., 2006, 58, 1 – 290; (c) Metal-Oxo and
Metal-Peroxo Species in Catalytic Oxidations (Ed.: B. Meunier), Struct. Bonding,
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Examples of the cobalt complexes catalyzed aerobic benzoquinone formation:

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24
An iridium-aminyl radical complex, which is designed based on similar
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Table 1. Oxidation of hydroquinones with O2 catalyzed by 1
Reaction time
[min]
Entry
Substrate
Product
Yield [%]
1
15
60
>98
98
2
3
4
5
120
180
1440
88
O
O
14
52
Reaction conditions: Catalyst (1), 10 ꢀmol; substrate, 1.0 mmol; O2, 1 atm; solvent
(MeCN), 4.0 mL; reaction temperature, 315 K. The yields were determined by gas
chromatography.

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Table 2. Trapping of H2O2 generating through the catalytic O2 oxidation of hydroquinones by 1. a
Hydroquinone
Substrate
Benzoquinone
Product
Entry
Solvents (condition)
(amount / mmol) (amount / mmol) (amount / mmol) (amount / mmol; yield based on benzoquinone)
Toluene/acetate buffer
(biphase)
H2O2
1
none
(0.26 mmol; 34 %)
(1.0 mmol)
(0.77 mmol)
(0.087 mmol)
MeCN / CH2Cl2
(homogeneous)
Ph3P
Ph3P=O
2
3
(0.10 mmol)
(0.064 mmol; 74 %)
(0.10 mmol)
OH
Toluene/acetate buffer
(biphase)
(2.5 mmol)
(0.13 mmol; 76 %)
(1.0 mmol)
(0.17 mmol)
a Detail reaction conditions are stated in experimental section.

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List of the Legends of Schemes and Figures
Scheme 1 Synthesis of 1.
Scheme 2 Reaction of 1 with O2.
Scheme 3 Biphasic system composed of 1 and peroxotungstate catalyst (5) for O2
epoxidation of allyl alcohol.
Fig. 1
Fig. 2
Fig. 3
UV-vis spectra of a CH2Cl2 solution of 1 and in situ-generated dioxygen
adduct of 1 (= 1•O2) recorded at 203 K. (a: solid line) spectrum of 1
under argon atmosphere (b: gray line) spectrum of 1•O2 under O2
atmosphere.
Time course for the co-oxidation of para-hydroquinone and
triphenylphosphine with O2 catalyzed by 1. Reaction conditions: Catalyst
(1), 10 ꢀmol; substrate, 100 ꢀmol; O2, 1 atm; solvent (1:1 mixture of
CH2Cl2 and MeCN), 4.0 mL, Reaction temperature, 305 K.
Time course for the aerobic oxidation of primary and secondary alcohols
mediated by 1. Reaction conditions: Catalyst (1), 10 ꢀmol; substrate, 100
ꢀmol; O2, 1 atm; solvent (1:1 mixture of CH2Cl2 and MeCN), 4.0 mL,
Reaction temperature, 305 K. The yields were determined by gas
chromatography.

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Scheme 1 Synthesis of 1.

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Scheme 2 Reaction of 1 with O2.

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O2
1
1 O2
H2O2 generation
tBu
tBu
O
O
HO
HO
tBu
tBu
Organic phase
Aqueous phase
H2O2
VI
W
(6)
VI
W
O
O
O
Oxygen transf er
O
OH
OH
2-
O
O
VI
VI
VI
O
O O
W
W
=
W
O
O
O
O
O
H2O
O
O
OH2
Scheme 3 Biphasic system composed of 1 and peroxotungstate catalyst (6) for O2
epoxidation of allyl alcohol.

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3
2
1
(a) 1 (under Ar)
(b) 1•O2 (under O2)
0
400
600
800
1000
λ / nm
Fig. 1
UV-vis spectra of a CH2Cl2 solution of 1 and in situ generated dioxygen
adduct of 1 (= 1•O2) recorded at 203 K. (a: solid line) spectrum of 1 under
argon atmosphere (b: gray line) in situ generated 1•O2 under O2
atmosphere.

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90
90
60
60
30
30
Ph3P=O
0
0
0
0
30
30
60
60
90
90
Reaction time / min
Fig. 2
Time course for the co-oxidation of para-hydroquinone and
triphenylphosphine with O2 catalyzed by 1. Reaction conditions: Catalyst
(1), 10 ꢀmol; substrate, 100 ꢀmol; O2, 1 atm; solvent (1:1 mixture of CH2Cl2
and MeCN), 4.0 mL, Reaction temperature, 305 K.