Oxidation reactivity of a structurally and spectroscopically well-defined mononuclear peroxocarbonatoiron(III) complex

Article abstract of DOI:10.1246/cl.141058

Mononuclear peroxocarbonatoiron(III) complex [Fe-(6Me-pic)₂(O₂C(O)O)]^- (1-O₂C(O)O) with bidentate ligands (6Me-pic), prepared by the reaction of a carbonatoiron(III) complex [Fe(6Me-pic)₂(CO₃)]^- (1-CO₃) with H₂O₂, was fully characterized. 1-O₂C(O)O showed reversible OO bond cleavage and reformation of the peroxo group under CO₂ at 25 °C. 1-O₂C(O)O is capable of not only oxidizing the C=C bond of cyclooctene but also the CH bond of toluene. As for cyclooctene, epoxidation is favorable under CO₂ in the presence of H₂O, while cis-dihydroxylation precedes under N₂, indicating that the oxidation reactivity of 1-O₂C(O)O toward cyclooctene can be tuned by changing the concentration of CO₂ and H₂O.

Full text of DOI:10.1246/cl.141058

CL-141058
Received: November 18, 2014 | Accepted: December 3, 2014 | Web Released: December 12, 2014
Oxidation Reactivity of a Structurally and Spectroscopically Well-defined
Mononuclear Peroxocarbonato-Iron(III) Complex
Tomohiro Tsugawa,1 Hideki Furutachi,*1 Megumi Marunaka,1 Taichi Endo,1 Koji Hashimoto,1 Shuhei Fujinami,1 Shigehisa Akine,1
Yoko Sakata,1 Shigenori Nagatomo,2 Takehiko Tosha,3 Takashi Nomura,4 Teizo Kitagawa,4 Takashi Ogura,4 and Masatatsu Suzuki*5
1Department of Chemistry, Division of Material Sciences, Graduate School of Natural Science and Technology,
Kanazawa University, Kakuma-machi, Kanazawa 920-1192
2Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571
3Biometal Science Laboratory, RIKEN SPring-8 Center, Kouto, Sayo, Hyogo 679-5148
4Picobiology Institute, Graduate School of Life Science, University of Hyogo, Ako-gun, Hyogo 678-1297
5Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Moto-oka, Nishi-ku, Fukuoka 819-0395
(E-mail: h-furutachi@se.kanazawa-u.ac.jp)
A mononuclear peroxocarbonato-iron(III) complex [Fe-
(6Me-pic)2(O2C(O)O)] (1-O2C(O)O) with bidentate ligands
(A)
(B)
¹
O
O
N
N
N
CH3
O
(6Me-pic), prepared by the reaction of a carbonato-iron(III)
O
O
O
O
O
O
O
O
¹
C
O
C
O
O
O–
O
O–
complex [Fe(6Me-pic)2(CO3)] (1-CO3) with H2O2, was fully
Fe
Fe
H3C
N
N
O
characterized. 1-O2C(O)O showed reversible O-O bond cleav-
age and reformation of the peroxo group under CO2 at 25 °C.
1-O2C(O)O is capable of not only oxidizing the C=C bond
of cyclooctene but also the C-H bond of toluene. As for
cyclooctene, epoxidation is favorable under CO2 in the presence
of H2O, while cis-dihydroxylation precedes under N2, indicating
that the oxidation reactivity of 1-O2C(O)O toward cyclooctene
can be tuned by changing the concentration of CO2 and H2O.
CH3
6Me-pic
qn
N
O
O
1-O2C(O)O
2-O2C(O)O
(C)
CO2
O
O
O
O
O
O
C
O
O
C
O
C
O-
(L)2Fe3+
(L)2Fe3+
(L)2Fe4+
(L)2Fe5+
or
O•
O
O
O
CO2
1- or 2-oxo
1- or 2-O2C(O)O
L = 6Me-pic (1), L = qn (2)
1- or 2-O2
¹
Scheme 1. (A) Bidentate ligand 6Me-pic and its peroxocarbonato-
iron(III) complex 1-O2C(O)O. (B) Bidentate ligand qn and its peroxo-
carbonato-iron(III) complex 2-O2C(O)O. (C) Reversible cleavage and
reformation of the peroxo O-O bond of 1- or 2-O2C(O)O (left) and an
equilibrium with 1- or 2-O2C(O)O and 1- or 2-O2 (right) under CO2.
Peroxocarbonate (or peroxomonocarbonate, HCO4 ) is a
moderately reactive oxidant that can be classified as a peroxy
acid, such as peracetic acid and m-chloroperbenzoic acid that are
extensively utilized for the oxidation/oxygenation of organic
¹
compounds. HCO4 generated from the reaction of H2O2 with
¹
HCO3 in H2O1-5 has demonstrated its competence in the
epoxidation of various olefins,2 N-oxidation of tertiary amines,3
the poor solubility of 2-O2C(O)O in conventional organic
solvents prevented the investigating of its oxidation reactivity.
We have improved its solubility by altering the bidentate ligand
(6Me-pic instead of qn) in order to examine the oxidation
reactivity of the peroxocarbonato-iron(III) species, whose per-
oxo moiety shows reversible O-O bond cleavage and reforma-
tion. We report the oxidation reactivity of a structurally and
spectroscopically well-defined peroxocarbonato-iron(III) com-
and S-oxidation of organic sulfides4 and thiols.5 It has been
¹
shown that the epoxidation reaction with HCO4 is significantly
accelerated by the addition of some transition-metal (Mn, Fe,
etc.) salts, whose activity is further enhanced by additives such
as salicylic acid.6 A putative peroxocarbonato-metal species has
been proposed as a key intermediate in the epoxidation of
olefins;6 the active species is yet to be identified. Some peroxo-
carbonato-metal complexes have been prepared to date;7-9
however, their oxidation abilities toward external substrates
have not been studied in detail.9a,9c-9f
¹
plex [Fe(6Me-pic)2(O2C(O)O)] (1-O2C(O)O)13 (Scheme 1A)
toward substrates including PPh3, cis-cyclooctene, naphthalene,
toluene, and cyclohexanecarboxaldehyde (CCA). We found
interesting selectivity control of cyclooctene oxidation by 1-
O2C(O)O due to the reversible release and uptake of CO2
(Scheme 1C).
Previously, we reported the crystal structure of a mononu-
clear peroxocarbonato-iron(III) complex [Fe(qn)2(O2C(O)O)]
¹
(2-O2C(O)O)8a with a high-spin iron(III) center (Scheme 1B).
Furthermore, 2-O2C(O)O showed reversible cleavage and
reformation of the peroxo O-O bond of 2-O2C(O)O via the
formation of a high-valent iron-oxo species (FeIV=O or FeV=O:
2-oxo), in which an iron-peroxo species, (FeIII(O2): 2-O2), and
CO2 are under equilibrium conditions (Scheme 1C).8b Such
iron(III)-peroxo and high-valent iron-oxo species are crucial
intermediates that are responsible for alkane hydroxylation
and arene dihydroxylation found in non-heme mononuclear
iron enzymes10 such as taurine/α-ketoglutarate dioxygenase
(TauD)10,11 and naphthalene dioxygenase (NDO).10,12 However,
We have previously shown that 2-O2C(O)O was prepared by
the reaction of a bis(¯-hydroxo)diiron(III) complex [Fe2(qn)4(¯-
OH)2] with ca. 10 equiv of H2O2 in the presence of DBU or (n-
Bu4N)(OAc) in DMF at ¹60 °C under CO2.8a In contrast, it
was found that the peroxocarbonato-iron(III) complex [Fe(6Me-
¹
pic)2(O2C(O)O)] (1-O2C(O)O) can be prepared by the reaction
¹
of a carbonato-iron(III) complex [Fe(6Me-pic)2(CO3)] (1-CO3)
with only 1 equiv of H2O2 in acetonitrile at ¹40 °C under CO2,
which was confirmed by UV-vis and ESI-TOF/MS measure-
ments (Figure S2, Supporting Information).14
330 | Chem. Lett. 2015, 44, 330–332 | doi:10.1246/cl.141058
© 2015 The Chemical Society of Japan

O
O
O
O
O
O
O
O
C
O
C
O
C
O
C
O
Fe3+
Fe3+
Fe3+
Fe3+
+
O
O
O
O
1-18/18O2C(O)O
1-18/16O2C(O)O 1-16/18O2C(O)O
1-16/16O2C(O)O
CO2
CO2
O
O
O
O
C
O
O
Fe3+
Fe3+
O
O
C
O•
Fe4+
O
O
1-18/16O2
O
or
CO2
O
Fe5+
O
C
O-
O
O
O
Fe3+
O
O
CO2
O = 18
O = 16
O
O
C
O
C
O
Fe3+
Fe3+
O
O
1-oxo
1-18/18O2
Figure 1. ORTEP views (50% probability) of complex anions of
¹
(A) [Fe(6Me-pic)2(CO3)] (1-CO3) and (B) [Fe(6Me-pic)2(O2C(O)-
O)] (1-O2C(O)O).
Scheme 2. Possible conversion pathways from 1-18/18O2C(O)O
through 1-18/16O2C(O)O and 1-16/18O2C(O)O to 1-16/16O2C(O)O.
The following reactions to 1-16/16O2C(O)O proceed in the same way.
¹
The crystal structures of 1-CO3 and 1-O2C(O)O showed that
the complex anions have a distorted octahedral geometry around
the iron atom with a trans-N2O4 donor set composed of a
bidentate carbonate (for 1-CO3) or peroxocarbonate (for 1-
O2C(O)O) and two bidentate 6Me-pic ligands (Figures 1 and
S1).15 The metric parameters of 1-O2C(O)O (Table S2), includ-
ing the peroxo O-O bond length (1.457(4) ¡), were comparable
to those of 2-O2C(O)O (O-O, 1.455(5) ¡).8a
and cis-1,2-cyclooctanediol (ca. 10%) together with a trace
amount of 2-cycloocten-1-one (ca. 1%) under CO2,18 whereas
1-O2C(O)O under N2 afforded the epoxide and cis-diol in
respective yields of ca. 3% and ca. 28% with a trace amount
of 2-cycloocten-1-one.19 The results are in contrast to those of
¹
the selective epoxidation observed for HCO4 /transition-metal
The physicochemical properties (Figures S3-S9) of [Fe-
salts6 and the related peracetato-iron(III) complex [Fe(6Me2-
BPP)(CH3C(O)O2)]+.20 It should be noted that the cis-diol (D)/
epoxide (E) product ratio (D/E ratio, hereafter) dramatically
changes from 1/1 under CO2 to 10/1 under N2, resulting in a
10-fold change in the selectivity of cyclooctene oxidation from
epoxidation to cis-dihydroxylation. This result indicates that the
D/E selectivity by 1-O2C(O)O toward cyclooctene is dominated
by the concentration of CO2, although the oxidation ability of
1-O2C(O)O is modest. The observed cis-dihydroxylation is
important and closely relevant to naphthalene cis-dihydroxyla-
tion catalyzed by NDO;10,12 however, only a limited number of
examples of cis-dihydroxylation by iron complexes are known
so far.21,22 Similar high D/E selectivity has also been achieved
for catalytic cyclooctene oxidation by iron(II) complexes with
tetradentate N4 ligands having 6-methyl-2-pyridylmethyl side
arm(s), where the D/E ratios are 7.0/1 ([Fe(6Me3-TPA)(CH3-
CN)2]2+),21 5.0/1 ([Fe(6Me2-BPMEN)(OTf)2]),21a and 5.5/1
([Fe(Me,MePy-tacn)(OTf)2]).22 Que et al. have proposed that a
putative high-spin η2-FeIII-OOH species or the corresponding
O-O bond cleaved HO-FeV=O species is responsible for the
cis-dihydroxylation of olefin catalyzed by [Fe(6Me3-TPA)-
¹
(6Me-pic)2(O2C(O)O)] (1-O2C(O)O) were quite similar to
¹
those of [Fe(qn)2(O2C(O)O)] (2-O2C(O)O),8 except for a
higher solubility in acetonitrile. Furthermore, 1-18/18O2C(O)O
also showed reversible O-O bond cleavage and reformation of
the peroxo group under CO2 at 25 °C, as found for 2-18/18O2C-
(O)O.8b The successive conversion from 1-18/18O2-C(O)O (m/z
408) through 1-18/16O2C(O)O and 1-16/18O2-C(O)O (m/z 406) to
1-16/16O2C(O)O (m/z 404) was confirmed by ESI-TOF/MS and
rR measurements (Figures S7 and S8), where the oxygen atoms
of the peroxo moiety are derived from CO2.8b It should be noted
that the ESI-TOF mass spectrum of 1-18/18O2C(O)O showed a
¹
mononuclear peroxo-iron(III) complex [Fe(6Me-pic)2(18O2)]
(1-18/18O2: m/z 364)16 together with
a signal of 1-
18/18O2C(O)O (Figure S8), whereas the corresponding analogue
[Fe(qn)2(18O2)] (2-18/18O2) was not detected in the previous
¹
study.8b The successive conversion from 1-18/18O2 (m/z 364)
through 1-18/16O and 1-16/18O2 (m/z 362) to 1-16/16O2 (m/z 360)
was also observed, where the intensity ratios of these isotopom-
ers are the same as those of the corresponding isotopomers of 1-
O2C(O)O (Figure S8). Thus, these observations strongly support
the successive O-O bond conversion pathways of 1-O2C(O)O,
involving a rapid exchange of the CO2 moiety of 1-O2C(O)O
(Scheme 2),8b which was previously proposed for 2-O2C(O)O.
As mentioned above, there is a rapid equilibrium between 1-
O2C(O)O and 1-O2 + CO2.17 Thus, the reactivity of 1-O2C(O)O
both under CO2 and N2 was investigated in electrophilic,
nucleophilic, and C-H bond oxidation reactions with several
substrates such as PPh3, cis-cyclooctene, naphthalene, toluene,
and CCA in acetonitrile at 25 °C (details are given in the
Supporting Information). The oxidation products determined by
GC-MS are listed in Table S3. In the reaction of 1-O2C(O)O
with an excess amount (50 equiv) of PPh3, 1-O2C(O)O acted
as an oxo-transfer reagent to produce O=PPh3 (ca. 95%)
either under CO2 or N2, as found for the peroxocarbonato-metal
complexes.9a,9c-9f The reaction of 1-O2C(O)O with a 1000-fold
excess of cis-cyclooctene afforded cyclooctene oxide (ca. 10%)
(CH3CN)2]2+ 21 Thus, in the present system, a similar η2-FeIII-
.
OOH species formed by the protonation of 1-O2, derived from
1-O2C(O)O, appears to be dominant for cis-dihydroxylation,
whereas a putative high-valent iron-oxo species (FeIV=O or
FeV=O: 1-oxo) or 1-O2C(O)O seems to give rise to the epoxide.
Although 1-O2C(O)O under N2 strongly favors cis-dihydrox-
ylation, no oxidation products of naphthalene were observed.
Furthermore, unlike other well-defined non-heme η2-peroxo-
iron(III) complexes,23 1-O2C(O)O under N2 showed no nucle-
ophilic oxidative reaction toward CCA.
A noteworthy fact is that the addition of 1000 equiv of water
into the reaction mixtures significantly improved the epoxidation
selectivity of 1-O2C(O)O under CO2 (D/E ratio from 1/1 to
1/8.5) and N2 (D/E ratio from 10/1 to 1/1.5) (Table S3). These
results then indicate that the oxidation reactivity of 1-O2C(O)O
toward cyclooctene can be tuned by regulating the concentration
Chem. Lett. 2015, 44, 330–332 | doi:10.1246/cl.141058
© 2015 The Chemical Society of Japan | 331

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of not only CO2 but also H2O, where a 83-fold change in the
D/E ratio is observed on going from the condition under CO2 +
H2O (D/E = 1/8.3) to under N2 (D/E = 10/1). Although the
influence of water on the epoxidation is uncertain at the present
time, the presence of water may affect the nature of the active
oxidizing species.
We have also investigated the reactivity of 1-O2C(O)O
toward toluene (1000 equiv). 1-O2C(O)O showed C-H bond
oxidation ability toward toluene either under CO2 or N2 to give
benzyl alcohol (ca. 2%) and benzaldehyde (ca. 10%) together
with 1,2-diphenylethane (ca. 2%), although their yields were
poor (Table S3). However, unlike cyclooctene, the reaction
conditions have only a small influence on the product ratio of 1-
O2C(O)O toward toluene. The formation of 1,2-diphenylethane
indicates the involvement of a benzyl radical that exist long
enough to form the coupling dimer.24
In summary, we have succeeded in the structural and
spectroscopic characterization of a new peroxocarbonato-
iron(III) complex 1-O2C(O)O, whose peroxo moiety shows a
reversible O-O bond cleavage and reformation. The oxidation
abilities of 1-O2C(O)O toward external substrates have also been
explored and 1-O2C(O)O is capable of not only oxidizing the
C=C bond of cyclooctene but also the C-H bond of toluene. As
for cyclooctene, selective epoxidation and cis-dihydroxylation
by 1-O2C(O)O were achieved by controlling the reaction
conditions. Studies on the oxidation reactivity of 1-O2C(O)O
toward other olefins are in progress.
10 M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem. Rev.
2004, 104, 939.
11 a) V. Purpero, G. R. Moran, J. Biol. Inorg. Chem. 2007, 12, 587.
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12 A. Karlsson, J. V. Parales, R. E. Parales, D. T. Gibson, H. Eklund,
S. Ramaswamy, Science 2003, 299, 1039.
13 Abbreviations used: qn, quinaldinate; 6Me-pic, 6-methylpicoli-
nate; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; 6Me2-BPP, N,N-
bis(6-methyl-2-pyridylmethyl)-3-aminopropionate; 6Me3-TPA,
tris(6-methyl-2-pyridylmethyl)amine; Me,MePytacn, N,N¤-dimeth-
yl-N¤¤-(6-methyl-2-pyridylmethyl)triazacyclononane; 6Me2-
BPMEN, N,N¤-bis(6-methyl-2-pyridylmethyl)-N,N¤-dimethyl-1,2-
diaminoethane.
¹
14 We also attempted to prepare [Mn(6Me-pic)2(O2C(O)O)] (3-
O2C(O)O) because manganese salts show the highest activity
¹
among epoxidation of olefins by combination of HCO4 with
transition-metal salts.6b ESI-TOF/MS study revealed that, unlike
¹
1-CO3, the reaction of [Mn(6Me-pic)2(CO3)] (3-CO3)15 with
1 equiv of H2O2 in acetonitrile at ¹40 °C under CO2 did not
generate the peroxocarbonate species 3-O2C(O)O, whereas a
¹
peroxo species [Mn(6Me-pic)2(O2)] (3-O2) was detected upon
addition of 20 equiv of H2O2 to the solution of 3-CO3 (Figure S10).
This reactivity is in contrast to that of 1-CO3. Thus, it is obvious
that the metal ion plays a critical role in the formation of
peroxocarbonate species in this system, although the origin of such
a difference remains unclear. Unfortunately, 3-O2 was not isolated
at ¹40 °C in this study.
Financial support of this research by the Ministry of
Education, Culture, Sports, Science and Technology (MEXT)
Grant-in-Aid for Scientific Research to H.F. and M.S. is
gratefully acknowledged.
15 The crystal structure of 3-CO3 was also determined by X-ray
crystallography (Figure S1).
¹
16 Peroxo-iron(III) species [Fe(6Me-pic)2(O2)] (1-O2) has been
Supporting Information is available electronically on J-STAGE.
detected only under ESI-TOF/MS spectrometry conditions.
17 Thermal stability of 1-O2C(O)O in acetonitrile at 25 °C is highly
dependent on the reaction conditions as found for 2-O2C(O)O in
DMF; 1-O2C(O)O decayed for 55 h under CO2 and for 3 h under
N2 in acetonitrile at 25 °C (Figure S9).
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5
6
7
8
9
332 | Chem. Lett. 2015, 44, 330–332 | doi:10.1246/cl.141058
© 2015 The Chemical Society of Japan