Regioselective arene hydroxylation mediated by a (μ-peroxo)diiron(III) complex: A functional model for toluene monooxygenase

Article abstract of DOI:10.1021/ja063987z

Integration of mononuclear [Cr(CN)₆]³⁺ and preorganized trinuclear [Co2Ln(L)₂]³⁺ complexes provides novel trimetallic magnets having a 3-D pillared-layer framework with an alternate array of 2-D layer extended b

Full text of DOI:10.1021/ja063987z

Published on Web 12/15/2006
Regioselective Arene Hydroxylation Mediated by a (µ-Peroxo)diiron(III)
Complex: A Functional Model for Toluene Monooxygenase
Mai Yamashita,† Hideki Furutachi,† Takehiko Tosha,‡ Shuhei Fujinami,† Wataru Saito,†
Yonezo Maeda,# Kenji Takahashi,† Koji Tanaka,§ Teizo Kitagawa,‡ and Masatatsu Suzuki*,†
DiVision of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa UniVersity,
Kakuma-machi, Kanazawa 920-1192, Japan, Okazaki Institute for IntegratiVe Bioscience and Okazaki Institute for
Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, 444-8585, Japan, and Department of
Chemistry, Graduate School of Sciences, Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Received June 6, 2006; E-mail: suzuki@cacheibm.s.kanazawa-u.ac.jp
The diiron center of toluene monooxygenase hydroxylase
(TMOH) is capable of oxidizing arenes, alkenes, and haloalkanes.1
Recently, the diiron center of toluene/o-xylene monooxygenase hy-
droxylase (ToMOH) has been shown to have a carboxylate-rich
environment similar to that of soluble methane monooxygenase
(MMOH).1a,2 Extensive studies of MMOH have provided a fun-
damental basis for the understanding of the structural and spectro-
scopic properties and the reaction mechanisms. In the catalytic
process of MMOH, peroxodiiron(III) and bis(µ-oxo)diiron(IV)
intermediates have been detected, the latter of which is responsible
for hydroxylation of methane.2,3 In addition, a peroxo-intermediate
has been shown to be capable of the epoxidation of alkenes.4 Very
recently, two intermediates, a peroxodiiron(III) form and a diiron-
(III, IV) form having a W‚ radical, have been discovered in I100W
ToMOH mutant.5 Another type of diiron center having a histidine-
rich environment has also been proposed for alkane ω-hydroxylase
(AlkB),6 suggesting the diversity of the coordination environments
of the diiron centers in the dioxygen activating enzymes.
Some synthetic TMOH model complexes have been reported,
which hydroxylate phenyl group of the supporting ligand in the reac-
tions with oxidants such as H2O2, m-CPBA, and O2.7 However, no
intermediate has been observed in the hydroxylation process. Herein,
we report two types of peroxodiiron(III) complexes, [Fe2(LPh4)-
(RCO2)(O2)]2+ (R ) Ph3C (oxy-1) and Ph (oxy-2)) generated in
the reaction of diiron(II) complexes [Fe2(LPh4)(RCO2)]2+ (R ) Ph3C
(1) and Ph (2)) with O2; the former leads to regioselective hydrox-
ylation of a phenyl group of LPh4 and the latter exhibits reversible
deoxygenation (LPh4 ) N,N,N′,N′-tetrakis[(1-methyl-2-phenyl-4-
imidazolyl)methyl]-1,3-diamino-2-propanolate). The reactions mimic
TMOH and hemerythrin (Hr) reactivity, respectively. They provide
further insights into the reactivity of the peroxodiiron(III) complexes
having a nitrogen-rich coordination environment like AlkB and Hr.
nals corresponding to 2 (m/z 501.2)) in CH2Cl2 at -40 °C. The
electronic spectra showed absorption bands at 665 nm (∼2300 M-1
cm-1) for oxy-1 and ∼710 nm (∼1500 M-1 cm-1) for oxy-2 (Figure
2-
S4) assignable to the CT transitions from O2 to Fe(III) center
(LMCT). The absorption band of oxy-2 is very broad compared to
that of oxy-1, suggesting some electronic and/or structural change-
(s) in those complexes. Mo¨ssbauer spectra of powdery samples of
oxy-1‚BPh4 and oxy-2‚BPh4 at 80 K showed a quadrupole doublet
with δ (∆EQ) ) 0.57 (1.44) and 0.58 (1.17) mm s-1, respectively
(Figures S6 and S7), which are in the range of those of the
peroxodiiron(III) complexes.9,10
Both oxy-species are stable at -40 °C for several days. Oxy-2
is reversibly deoxygenated by bubbling N2 and warming a solution
up to room temperature (Figure S4B). In contrast, oxy-1 did not
show reversible deoxygenation; warming a solution up to 25 °C
with bubbling of N2 generated a dark-blue solution (vide infra).
Resonance Raman spectra of oxy-1 and oxy-2 showed isotope
sensitive bands at 873 cm-1
(
16-18∆ ) 50 cm-1) and 480 cm-1 (16
16-18∆ ) 45 cm-1) and 459 cm-1 (20 cm-1),
cm-1) and at 840 cm-1
(
respectively (Figure 1). The bands at 873 and 840 cm-1 can be
assigned to the ν(O-O) vibrations, and the bands at 480 and 459
cm-1 can be assigned to the νs(Fe-OO-O) vibrations. It is noted
that there is a significant difference in both ν(O-O) and νs(Fe-
OO-O) frequencies between these two complexes. The ν(O-O) and
νs(Fe-OO-O) have been shown to depend on the Fe-O-O angle;
as the Fe-O-O angle becomes larger, the ν(O-O) becomes higher
and the νs(Fe-OO-O) lower by mechanical coupling.11 This is not
the case for those of the present complexes. It has also been shown
that the LMCT transition energy and the ν(O-O) and νs(Fe-OO-O
)
frequencies of the peroxodiiron(III) complexes can be good probes
for elucidation of the bonding nature of the Fe-OO-O and Lewis
acidity of Fe(III) center.2d,10,12 The high ν(O-O) and νs(Fe-OO-O
)
frequencies of oxy-1 relative to those of oxy-2 suggest a stronger
Fe-OO-O bonding owing to stronger σ and π donation from the
peroxo ligand, which decreases the electron densities of the π*
orbitals of the peroxo ligand. This is in line with a blue shift of the
LMCT transition energy of oxy-1 relative to that of oxy-2.
Furthermore, this effect seems to be responsible for the high
dioxygen affinity of oxy-1 relative to that of oxy-2 (vide infra).
However, the origin of such a significant difference between these
two complexes remains unclear.
Complex 2 did not produce measurable oxy-2 in CH2Cl2 at 25 °C
under O2, whereas irreversible oxidation occurred for several hours
to give a brown species. In contrast, 1 is oxygenated under O2 at
25 °C to give oxy-1, which is not stable and decomposes to generate
a dark-blue species (a red line in Figure 2). The crystal structure
of this species ([Fe2(LPh4-O)(Ph3CCO2)(OH)](ClO4)2‚3CH2Cl2‚2H2O
(1-O‚ClO4)) revealed that one of the phenyl groups of LPh4 is regio-
Crystal structures of 1 and 2 revealed that they are pentacoor-
dinate, and two irons are bridged by alkoxide of LPh4 and an exo-
genous carboxylate (Figure S1, Supporting Information).8 They
reacted with O2 to generate dark-green oxy-1 (ESI-MS: m/z 600.2
and 602.2 for 18O2 sample) and oxy-2 (ESI-MS exhibited only sig-
† Kanazawa University.
‡ Okazaki Institute for Integrative Bioscience.
§ Okazaki Institute for Molecular Science.
# Kyushu University.
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10.1021/ja063987z CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have succeeded in the synthesis of peroxodiiron-
(III) complexes which have two faces modulated by the stereo-
chemistry of bridging carboxylates: oxy-1 having Ph3CCO2- causes
arene hydroxylation of the supporting ligand LPh4 which mimics
-
the function of TMOH, whereas oxy-2 having PhCO2 exhibits
reversible deoxygenation like Hr. For a fuller understanding of such
a remarkable change in reactivity, structural information of the oxy-
species is needed.
Acknowledgment. Financial support by Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan (H.F., T.K., and M.S.) is acknowledged.
Figure 1. Resonance Raman spectra of oxy-1 prepared from (A) 16O2 and
(B) 18O2 in CH2Cl2/CH3CN (1:1) at -80 °C and oxy-2 prepared from (C)
16O2 and (D) 18O2 in CH2Cl2 at -80 °C (607 nm laser excitation).
Supporting Information Available: Details of syntheses, charac-
terization of the complexes, X-ray crystallography, kinetics, and ligand
recovery experiment. This material is available free of charge via the
Internet at http://pubs.acs.org.
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latter of which was identified as {Fe2(LPh4-O)(Ph3CCO2)(OMe)}2+ (m/z
607.2) by the addition of MeOH into the reaction solution. Mo¨ssbauer
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0.48 (1.63) mm s-1) (see Figures S5 and S6).
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