Synthesis, characterization, and catalytic oxygenation activity of dinuclear iron(III) complex supported by binaphthol-containing chiral ligand

Article abstract of DOI:10.1016/j.molcata.2004.08.032

A diiron(III) complex supported by a new dinucleating ligand containing a chiral binaphthol spacer has been synthesized and characterized by elemental analysis and spectroscopic methods (ESI-MS, CD, and UV-vis). The complex has a (μ-oxo)(μ-carboxylato) doubly bridged diiron(III) core, the structural motif of which resembles that of the active site of sMMO and related diiron enzymes. Catalytic oxidation of alkanes with m-CPBA proceeded very efficiently to give the corresponding alcohols as the major products together with ketones as minor products. The alcohol-selectivity (alcohol/ketone = 3.8-8.8) as well as the kinetic deuterium isotope effect (k_H/k_D = 3.0 for the oxidation of cyclohexane) and significantly high regioselectivity (tertiary carbon/secondary carbon = 17.1) in the oxidation of adamantane strongly suggested a reaction mechanism involving a highly reactive metal-based oxidant rather than an autooxidation mechanism involving free radical species. A (μ-oxo)(μ-carboxylato) doubly bridged diiron(III) complex supported by a new dinucleating ligand containing a chiral binaphthol spacer has been synthesized and characterized. Catalytic oxidation of alkanes with m-CPBA proceeded very efficiently to give the corresponding alcohols as the major products together with ketones as minor products. Mechanistic studies have suggested that the reaction involves a highly reactive metal-based oxidant.

Full text of DOI:10.1016/j.molcata.2004.08.032

Journal of Molecular Catalysis A: Chemical 225 (2005) 103–109
Synthesis, characterization, and catalytic oxygenation activity of dinuclear
iron(III) complex supported by binaphthol-containing chiral ligand
Takayuki Nagataki, Yoshimitsu Tachi, Shinobu Itoh∗
a
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
Received 12 July 2004; received in revised form 31 August 2004; accepted 31 August 2004
Available online 7 October 2004
Abstract
A diiron(III) complex supported by a new dinucleating ligand containing a chiral binaphthol spacer has been synthesized and characterized
by elemental analysis and spectroscopic methods (ESI-MS, CD, and UV–vis). The complex has a (␮-oxo)(␮-carboxylato) doubly bridged
diiron(III) core, the structural motif of which resembles that of the active site of sMMO and related diiron enzymes. Catalytic oxidation of
alkanes with m-CPBA proceeded very efficiently to give the corresponding alcohols as the major products together with ketones as minor
products. The alcohol-selectivity (alcohol/ketone = 3.8–8.8) as well as the kinetic deuterium isotope effect (kH/kD = 3.0 for the oxidation
of cyclohexane) and significantly high regioselectivity (tertiary carbon/secondary carbon = 17.1) in the oxidation of adamantane strongly
suggested a reaction mechanism involving a highly reactive metal-based oxidant rather than an autooxidation mechanism involving free radical
species.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Oxygenation; Alkane hydroxylation; Diiron complex; m-CPBA; Binaphthol
1. Introduction
Recently, remarkable advances have been made in the
chemistry of non-heme iron enzymes, especially, those with
two proximal metal ions that yield novel active oxygen in-
termediates capable of performing diverse organic function-
alizations [2,3,5,15]. A notable example is soluble methane
monooxygenase (sMMO), in which a high-valent diiron-oxo
species has been suggested to act as the key reactive inter-
mediate for the alkane hydroxylation [2]. Significant insights
into the dinuclear iron site not only of sMMO but also the
related diiron enzymes have been attained through the syn-
thetic modeling approach, whereby the detailed characteriza-
tion of O2 adducts or derived species provides fundamental
spectroscopic, structural, and reactivity benchmarks for un-
derstanding the diiron active sites and, eventually, developing
new oxidation catalysts [15,19].
Developmentofanefficientcatalystforselectiveoxygena-
tion is an important objective in synthetic organic chemistry.
Metalloenzymes accomplish highly efficient and selective
oxygenation of organic molecules under very mild condi-
tions, where molecular oxygen is mainly employed as the
source of reactive intermediates [1–9]. Biomimetic studies
have greatly contributed to characterization of the active oxy-
gen intermediates involved in the enzymatic reactions and
to understanding of their chemical reactivities and functions
[10–17]. Recent advances in structural biology have also pro-
vided a great deal of information about mechanistic details
of the enzymatic reactions [18], stimulating many synthetic
chemists to design highly efficient catalysts for practical ap-
plications [10–13].
In this study, we have designed a new dinucleating ligand
LH2 in order to replicate the structure and reactivity of the
active sites of diiron enzymes. The ligand involves a binaph-
thol spacer to which N,N-bis(2-pyridylmethyl)aminomethyl
group (Pym2) is attached at the 3- and 3ꢀ-positions (Fig. 1).
∗
Corresponding author. Tel.: +81 6 6605 2564; fax: +81 6 6605 2564.
E-mail address: shinobu@sci.osaka-cu.ac.jp (S. Itoh).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.08.032

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T. Nagataki et al. / Journal of Molecular Catalysis A: Chemical 225(2005) 103–109
a Perkin-Elmer or a Fisons instruments EA 1108 Elemental
analyzer.
2.2. Synthesis of ligand
2.2.1. (S)-3,3ꢀ-Bis[N,N-bis(2-pyridylmethyl)
aminomethyl]-2,2ꢀ-bis(methoxymethoxy)-1,1ꢀ-binaphthyl
[(S)-Lmom
]
Bis(2-pyridylmethyl)amine (Pym2) (0.86 g, 4.3 mmol),
K2CO3 (0.96 g, 6.9 mmol), and dry CH3CN (5 ml) were
placed in a round bottom flask under N2 atmosphere at room
temperature. To this mixture, a CH3CN solution (40 ml) of
(S)-1 (0.81 g, 1.7 mmol) was added dropwise under N2 atmo-
sphere over 30 min. After stirred for 44 h, insoluble materials
were removed by filtration, and washed with CH2Cl2 sev-
eral times until the washing solvent becomes colorless. The
combined filtrate was concentrated, and the resulting residue
was purified by alumina column chromatography by using
ethyl acetate as an eluent to give a yellow oil (0.86 g, 62%).
1H NMR (CDCl3, 300 MHz): δ 2.68 (6 H, s, MOM CH3),
3.94 (4 H, d, J = 14.6 Hz, N CH2 Py), 4.00 (4 H, d, J =
14.6 Hz, N CH2 Py), 4.04 (2 H, d, J = 9.3 Hz, Ar CH2 N),
4.08 (2 H, d, J = 15.6 Hz, Ar CH2 N), 4.35 (2 H, d, J =
5.7 Hz, MOM CH2), 4.46 (2 H, d, J = 5.9 Hz, MOM CH2),
7.13–7.23 (8 H, m, Ar and Py), 7.38 (2 H, t, J = 7.3 Hz, Ar),
7.65–7.72 (8 H, m, Py), 7.89 (2 H, d, J = 8.0 Hz, Ar), 8.33
(2 H, s, Ar), 8.54 (4 H, dt, J = 4.8 Hz, 1.3 Hz, Py); IR (KBr):
1064(C N), 1147, 968(MOMgroup), 1589, 1506, 754 cm−1
(aromatic); HR-MS (FAB+): m/z 797.3810 [M + H]+, calcd
for C50H49N6O4797.3815. The (R)-isomer was prepared in
the same way by using (R)-1 instead of (S)-1.
Fig. 1. Dinucleating ligand LH2.
The Pym2 groups collaborate with the binaphthol group to
provide an appropriate dinuclear metal binding site, where
the binaphthol moiety is expected to work not only as a
spacer to hold the Pym2 groups at a desirable distance but
also as a chiral source for enantioselective oxidation. So
far, a great deal of efforts have been made to develop non-
heme iron catalysts for alkane hydroxylation, some of which
have been demonstrated to produce a high-valent iron-oxo
species as a reactive intermediate [20–22]. However, most
of the catalysts so far been reported are those generated
from mononuclear iron-complex precursors, whereas din-
uclear iron catalysts supported by well-designed dinucle-
ating ligands are very rare [23–25]. Moreover, there has
been no report of a chiral dinuclear iron complex aim-
ing for developing a catalyst for asymmetric hydroxylation
[21].
2.2.2. (S)-3,3ꢀ-Bis[N,N-bis(2-pyridylmethyl)
2. Experimental
aminomethyl]-1,1ꢀ-bi-2-naphthol [(S)-LH2]
To a THF solution (10 ml) of (S)-Lmom (591 mg,
0.74 mmol) was added 6N HCl (10 ml). Then, the mixture
was stirred for 15 h at room temperature. After removal of
the solvent under reduced pressure, the resulting residue
was dissolved in water (30 ml) and the aqueous solution
was treated with a saturated Na2CO3 aq. The mixture was
extracted with CH2Cl2 (20 ml × 3), and the combined or-
ganic layer was washed with water and brine, and then dried
over Na2SO4. After removal of Na2SO4 by filtration, evap-
oration of the solvent gave the title compound as a yellow
2.1. General
The reagents and the solvents used in this study ex-
cept for the ligand and the complex were commercial prod-
ucts of the highest available purity and were further pu-
rified by the standard methods, if necessary [26]. Bis(2-
pyridylmethyl)amine was prepared according to the reported
procedure [27]. (S)- and (R)-3,3ꢀ-Bis(chloromethyl)-2,2ꢀ-
bis(methoxymethoxy)-1,1ꢀ-binaphthyl(1)waspreparedfrom
(S)- and (R)-1,1ꢀ-bi-2-naphthol according to the reported pro-
cedures with a little modification [28]. FT-IR spectra were
recorded with a Shimadzu FTIR-8200PC. Mass spectra were
recorded with a JEOL JMS-700T Tandem MS station or
1
oil (489 mg, 93%). H NMR (CDCl3, 400 MHz): δ 3.93 (4
H, d, J = 14.6 Hz, N CH2 Py), 3.97 (4 H, d, J = 14.6 Hz,
N CH2 Py), 4.06 (2 H, d, J = 13.4 Hz, Ar CH2 N), 4.18
(2 H, d, J = 13.4 Hz, Ar CH2 N), 7.00–7.07 (6 H, m, Ar,
Py), 7.15 (2 H, d, J = 8.1 Hz, Ar), 7.24 (2 H, t, J = 7.4 Hz,
Ar), 7.36–7.44 (8 H, m, Py), 7.76 (2 H, s, Ar), 7.80 (2 H,
d, J = 8.1 Hz, Ar), 8.41 (4 H, d, J = 4.9 Hz, Py), 10.74 (2
H, s br, ArOH); IR (KBr): 3062 (O H), 1110 (C N), 1635,
1436, 750 cm−1 (aromatic); HR-MS (FAB+): m/z 709.3300
[M + H]+, calcd for C46H41N6O2709.3291. The (R)-isomer
was prepared in the same way by using (R)-Lmom instead of
1
a PE SCIEX API 150EX (for ESI-MS). H NMR spectra
were recorded on a JEOL LMN-ECP300WB or a LMX-
GX400. ESR measurements were performed on a JEOL
JES-ME spectrometer at −150 ◦C equipped with a vari-
able temperature cell holder. UV–vis spectra were measured
using a Hewlett-Packard HP8453 diode array spectropho-
tometer. CD spectra were recorded using a Jasco J-720W
spectropolarimeter. Elemental analyses were performed on
(S)-Lmom
.

T. Nagataki et al. / Journal of Molecular Catalysis A: Chemical 225(2005) 103–109
105
2.3. Preparation of diferric complex
The kH/kD value for cyclohexanol was calculated by
[FeI2II((S)-L)(O)(OBz)]ClO4
(moles of the products from cyclohexane)
/(moles of the products from cyclooctane)
A water solution (2 ml) of sodium benzoate (91 mg,
0.63 mmol) was added into an ethanol solution (3 ml) of
FeIIICl3·6H2O (66 mg, 0.24 mmol) with stirring at room
temperature. The mixture was turned to an orange sus-
pension. After the mixture was stirred for 30 min, lig-
and (S)-LH2 (50.0 mg, 0.07 mmol) was added into the
mixture. The resulting mixture was further stirring for
2 h, during which the solution was turned to black. Af-
ter adding NaClO4 (87 mg, 0.71 mmol), the mixture was
further stirred at room temperature for 24 h. The result-
ing black precipitates were collected by filtration, washed
with water and ethanol, and dried (67.6 mg, 91%). IR
(KBr): 1095 cm−1 (ClO4); MS (ESI, positive): m/z 955.2
[M − ClO4]+, calcd for C53H43N6O5Fe2955.2; Anal. Calcd
for C53H47N6O11Fe2Cl·2H2O: C, 58.34; H, 4.34; N, 7.70.
Found: C, 57.97; H, 4.04; N, 7.31. The (R)-isomer was
prepared in the same way by using (R)-LH2 instead of
(S)-LH2.
.
(moles of the products from cyclohexane-d12)
/(moles of the products from cyclooctane)
2.6. Determination of oxidant efficiency
Afterthecatalyticreactionfor15 min, thereactionmixture
was diluted with methanol to a total volume of 20 ml. The
mole of remaining oxidant was determined by iodometric
titration as follows. A portion (1.0 ml) of the diluted methanol
solution was added to 1 M H2SO4 aqueous solution (20 ml)
containing 1 mmol potassium iodide and a small amount of
starch. The mixture turned brown due to generation of I2.
Then, the mixture was titrated with 0.02 M Na2S2O3 aqueous
solution until the color disappeared.
3. Results and discussion
2.4. Catalytic oxidation of hydrocarbons
3.1. Synthesis and characterization
Typically, the diiron(III) complex [FeI2II((S)-L)(O)
(OBz)]ClO4 (2 ␮mol) in CH3CN CH2Cl2 (1:1 v/v) (2 ml)
was added to a CH2Cl2 (2.4 ml) solution containing cy-
clohexane (15 mmol) and m-CPBA (2 mmol) under N2
atmosphere with vigorous stirring at room temperature. The
reaction mixture were analyzed with a Shimadzu GC–14A
gas chromatograph equipped with a Restek Rtx–1701
capillary column (30 m × 0.25 mm). All peaks of interest
were identified by comparison of retention times and
co-injection with authentic samples. The products were
quantified by comparison against a known amount of internal
standard using a calibration curve consisting of a plot of
mole ratio (moles of organic compound/moles of internal
standard) versus area ratio (area of organic compound/area
of standard). In the case of tetralin as the substrate, the
optical purity of the hydroxylated product was determined
by using an HPLC system consisting of a Shimadzu LC-6A
chromatographic pump and a Shimadzu UV–vis spectropho-
tometric detector SPD-6AV equipped with a chiral column
(CHIRALCEL OB 250 mm × 4.6 mm, Daicel Chemical
Industries).
Dinucleating ligand LH2 was synthesized by the SN2 reac-
tion between bis(chloromethyl) binaphthol derivative 1 and
bis(2-pyridylmethyl)amine followed by deprotection of the
methoxymethyl (mom) groups as indicated in Scheme 1. The
structure of LH2 was well confirmed by H NMR, IR, and
HR-MS (see Section 2).
In Fig. 2 is shown the CD spectra of (S)- and (R)-LH2
in CH2Cl2 at 25 ◦C. The CD spectrum of the (S)-isomer is
the mirror image to that of the (R)-isomer with respect to
the X-axis, confirming that the enantiomeric purity of the
binaphthyl group is completely retained during the synthetic
processes.
The optically pure diferric complexes [(S)- and (R)-
derivatives] were synthesized by treating the ligands, (S)-
and (R)-LH2, and three-fold excess of ferric chloride hex-
ahydrate in the presence of 9 equiv. of sodium benzoate in
ethanol. Black powder was obtained after addition of sodium
perchlorate into the final reaction mixture. In spite of our
great efforts, any single crystals of the complex suitable
for X-ray crystallographic analysis could not be obtained.
However, the elemental analysis and the ESI-MS analy-
sis shown in Fig. 3 unambiguously demonstrated that the
complex was a diiron(III) complex containing (␮-oxo)(␮-
carboxylato) bridges. Thus, an estimated structure of the di-
iron(III) complex is as illustrated in Fig. 4. The proposed
structure of the diiron(III) complex was further supported
by its UV–vis spectrum, which exhibits intense absorption
bands at 340 nm (ε = 18400 M−1 cm−1; 9200 M−1 cm−1/Fe)
and 590 nm (6800 M−1 cm−1; 3400 M−1 cm−1/Fe) as shown
in Fig. 5. The spectral shape and the peak positions
of the present complex are very close to those of the
1
2.5. Determination of kinetic deuterium isotope effect
The diiron(III) complex [FeIII(L)(O)(OBz)]ClO4
(2 ␮mol) in CH3CN CH2Cl2 (1:1 v/v) 2(2 ml) was added to a
CH2Cl2 (2.4 ml) solution containing cyclohexane (0.81 ml,
7.5 mmol), cyclooctane (0.81 ml, 6.0 mmol), and m-CPBA
(2 mmol) under N2 atmosphere with vigorous stirring at
room temperature. The oxidation products were analyzed
by the GC method described above. The reaction was also
carried out using cyclohexane-d12 instead of cyclohexane.

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T. Nagataki et al. / Journal of Molecular Catalysis A: Chemical 225(2005) 103–109
Scheme 1. Synthesis of ligand LH2.
Fig. 4. Estimated structure of the diiron(III) complex.
was also similar to that of the Que’s complex [29]. In Fig. 6
are shown the CD spectra of the (S)- and (R)-isomers of the
diiron(III) complex in CH3CN. The spectra of (S)- and (R)-
isomers are also completely mirror image each other with
respect to the X-axis, demonstrating that the optical purity
was maintained during the complex formation process.
Fig. 2. CD spectra of (R)-LH2 (solid line) and (S)-LH2 (dotted line) (1.0 ×
10−4 M) in CH2Cl2 at 25 ◦C.
diiron(III)–diphenolate complex containing (␮-oxo) and (␮-
carboxylato) bridges reported by Que and coworkers [336 nm
(ε = 8900 M−1 cm−1/Fe) and 522 nm (3300 M−1 cm−1/Fe)
[29]. The lower energy band was assigned to a phenolate-
to-Fe(III) charge transfer transition and the higher energy
bandhasbeenattributedtobothphenolate-andoxo-to-Fe(III)
charge transfer transitions [29]. The complex was ESR-silent,
presumably due to a strong antiferromagnetic interaction be-
tween the two ferric ions, the magnetic property of which
3.2. Catalytic oxygenation of alkanes
As demonstrated above, the complex has a (␮-oxo)(␮-
carboxylato) doubly bridged diiron(III) core, the structural
motif of which resembles that of the active sites of sMMO
and related enzymes [2,3]. Thus, catalytic activity of the di-
iron(III) complex was examined in the oxygenation reaction
Fig. 3. ESI-MS spectrum of [Fe2(L)(O)(OBz)]ClO4. The solid line shows
the observed spectrum. The bars represent the isotope distribution calculated
for [C53H43N6O5Fe2]+.
Fig. 5. UV–vis spectrum of [Fe2(L)(O)(OBz)]ClO4 (1.0 × 10−4 M) in
CH3CN at 25 ◦C.

T. Nagataki et al. / Journal of Molecular Catalysis A: Chemical 225(2005) 103–109
107
tion mechanism of the former is different from that of the
laters.
Oxidation of adamantane with m-CPBA also proceeded
efficiently with a high TON (172.8) to give the corresponding
alcohols together with a small amount of ketone. In this case
as well, TBHP and H2O2 were poorer oxidants. It should
be noted that the regioselectivity between secondary (2◦) and
tertiary (3◦) carbons was quite high as 17.1 (the value derived
from the amount of 1-adamantanol divided by the amounts
of 2-adamantanol and 2-adamantanone and multiplied by 3
to correct for the higher number of secondary C H bonds).
The catalytic activity of the diferric complex was further
examined using cyclooctane, ethylbenzene, and tetralin as
the substrates and m-CPBA as the oxidant (Table 1). In all
the cases, the corresponding alcohols were obtained as the
major products together with the corresponding ketones as
the minor products with A/K ratio = 3.8–8.8. The difference
in apparent TON among the substrates can be attributed in
part to the difference in the number of C H bonds to be
oxidized. Thus, the TONs normalized just for one C H bond
of the substrates were rather constant (22.5 for cyclohexane,
24.3 for cyclooctane, 19.0 for ethylbenzene, and 24.5 for
tetralin) except for adamantane (36.8 for 3◦-carbon and 10.8
for 2◦-carbon).
Fig. 6. CD spectra of (R)-[Fe2(L)(O)(OBz)]ClO4 (solid line) and (S)-
[Fe2(L)(O)(OBz)]ClO4 (dotted line) (1.0 × 10−4 M) in CH3CN at 25 ◦C.
of alkanes as summarized in Table 1. In the reaction of cy-
clohexane, oxidation with m-CPBA (m-chloroperoxybenzoic
acid) proceeded catalytically to give cyclohexanol as the ma-
jor product together with cyclohexanone as a minor prod-
uct, where the product ratio of alcohol/ketone (A/K) was 6.5.
The total turnover number (TON = moles of products/moles
of catalyst) of the catalyst reached 270 within 15 min, and
the oxidant efficiency (moles of product/moles of m-CPBA
consumed after the reaction) was 61% (see Section 2). Fur-
thermore, an appreciable amount of kinetic deuterium iso-
tope effect (kH/kD = 3.0) was obtained when cyclohexane-
d12 was employed as the substrate (see Section 2). On the
other hand, the reaction with other oxidants such as TBHP
(t-BuOOH) and H2O2 gave much lower TON and lower
A/K values (1–1.7) (see Table 1), suggesting that the reac-
The enantioselectivity in the m-CPBA-oxidation was
tested by using tetralin as the substrate and the (S)-isomer
complex as the catalyst under otherwise the same experi-
mental conditions. Enantiomeric excess (ee) of the product
was determined by HPLC with a chiral column (see, Section
2). Unfortunately, however, the ee value was unexpectedly
low (∼10%; (R)-isomer was the major product of alcohol).
Table 1
a
Catalytic oxidation of alkanes by [FeIII(L)(O)(OBz)]ClO4
Substrate
Oxidant
Time
Products and TONb
234
m-CPBA
TBHP
H2O2
15 min
24 h
24 h
36c
5.4
3.4
9.1
3.2
d
m-CPBA
15 min
147
20
5.8
TBHP
H2O2
24 h
24 h
6.0
5.2
1.1
1.6
0.6
0.7
m-CPBA
15 min
341
48
m-CPBA
m-CPBA
15 min
15 min
30
8.0
10
88 (9.9% ee)
a
Reaction conditions: [catalyst] = 0.33 mM; [oxidant] = 0.24 M; [substrate] = 2.5 M in CH2Cl2/CH3CN at room temperature.
Turnover number (moles of product/moles of catalyst) determined by GC using calibration curves of the products.
A trace amount of ␧-caprolactone was formed.
b
c
d
The reaction was carried out under diluted conditions because of the low solubility of adamantane in the solvents: [catalyst] = 0.17 mM; [oxidant] = 61 mM;
[substrate] = 0.33 M.

108
T. Nagataki et al. / Journal of Molecular Catalysis A: Chemical 225(2005) 103–109
Scheme 2. Proposed mechanism for the alkane hydroxylation.
3.3. Mechanistic consideration
case, sodium benzoate may act as a base to enhance deproto-
nation of m-CPBA, accelerating the ligand exchange reaction
(Scheme 2).
Attempts to detect any intermediates derived from the re-
action of the diiron(III) complex and m-CPBA have been
unsuccessful. Namely, no spectral change was observed at a
lowtemperature(−40 ◦C), whereasraisingthetemperatureto
room temperature only resulted in decomposition of the cata-
lyst in the absence of the substrate. Nonetheless, the observed
KIE (kH/kD = 3.0) and the significantly large regioselectivity
(tertiary carbon/secondary carbon = 17.1) in the adamantane
oxidation, as well as the large A/K ratio strongly suggested a
reaction mechanism involving a highly reactive metal-based
oxidant rather than an autooxidation mechanism involving
free radical species [21,22].
Although a high turnover number of diferric complex was
attained in the initial stage of the m-CPBA-oxidation of alka-
nes, the catalytic activity was almost lost within 15 min as
shown in Fig. 7. In order to get information about the degra-
dation product of catalyst, the reaction mixture was analyzed
by ESI-MS (Fig. 8). There was a prominent peak at m/z = 656,
the peak position and the isotope distribution pattern of which
were consistent with the chemical structure of the decompo-
sition product IV (inset in Fig. 8). Thus, the self-oxidation
One of the possible reaction pathways of the catalytic
oxygenation reaction is shown in Scheme 2. Ligand ex-
change between the benzoate bridging ligand and m-CPBA
may take place to give a peracid-diiron(III) intermediate I,
from which a Fe(III) O Fe(V) O type intermediate II may
be formed via heterolytic cleavage of the O O bond. The
high valent iron oxo species may oxidize the substrate via
hydrogen atom abstraction and subsequent oxygen rebound
mechanism or its concerted variant as proposed in the re-
lated model reactions and the sMMO reaction [3,22]. The
resulting Fe(III) O Fe(III) species III may react with an-
other molecule of m-CPBA to regenerate the intermediate I,
completing the catalytic cycle. In support of this mechanism,
addition of sodium benzoate (50 equiv. of the catalyst) into
the reaction mixture enhanced the oxygenation reaction of
cyclohexane (TON = 369, oxidant efficiency = 85%). In this
Fig. 7. Time courses for the formation of products in the oxidation
of cyclohexane with m-CPBA catalyzed by [Fe2(L)(O)(OBz)]ClO4 in
CH2Cl2/CH3CN at 25 ◦C. Products: cyclohexanol (᭹), cyclohexanone (ꢀ),
and ␧-caprolactone (ꢁ).

T. Nagataki et al. / Journal of Molecular Catalysis A: Chemical 225(2005) 103–109
109
Isamu Kinoshita of Osaka City University for his help in ob-
taining the CD spectra.
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at the benzylic position of the ligand sidearm might occur to
induce the catalyst-degradation.
In summary, a dinuclear iron(III) complex has been syn-
thesized using a new chiral dinucleating ligands LH2. As we
expected, the ligand supports two ferric ions in an appropri-
ate position to give the dinuclear iron(III) complex with (␮-
oxo)(␮-carboxylato) bridges, the structural motif of which is
close to that of non-heme diiron metalloenzymes. The axial
chirality of the binaphthyl moiety was maintained through-
out the ligand synthesis and the complex formation. A sig-
nificantly high catalytic activity of the diferric complex was
obtained in the oxidation of a series of alkanes with m-CPBA,
where formation of a high valent iron-oxo species has been
suggested by the high alcohol-selectivity as well as the KIE
(kinetic deuterium isotope effect) and significantly high re-
gioselectivity (tertiary carbon vs. secondary carbon) in the
adamantane oxidation. However, the enantioselectivity and
the stability of catalyst were not satisfactory. Thus, rational
ligand design will be further needed to enhance the stereos-
electivity and stability of the catalysts.
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This work was financially supported in part by Grant-in-
Aid for Scientific Research on Priority Area (No. 11228206),
Grant-in-Aid for Scientific Research (Nos. 13480189 and
15350105), and Grant-in-Aid for Young Scientists (B) (No.
14740368) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. The authors also thank Prof.
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