New mechanistic insight into intramolecular arene hydroxylation initiated by (μ-1,2-peroxo)diiron(III) complexes with dinucleating ligands

Article abstract of DOI:10.1039/c5dt04088d

(μ-1,2-Peroxo)diiron(iii) complexes (2-R) with dinucleating ligands (R-L) generated from the reaction of bis(μ-hydroxo)diiron(ii) complexes [Fe₂(R-L)(OH)₂]²⁺ (1-R) with dioxygen in acetone at -20°C provide a diiron-centred electrophilic oxidant, presumably diiron(iv)-oxo species, which is involved in aromatic ligand hydroxylation.

Full text of DOI:10.1039/c5dt04088d

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New mechanistic insight into intramolecular arene
hydroxylation initiated by (μ-1,2-peroxo)diiron(III)
complexes with dinucleating ligands†
Cite this: DOI: 10.1039/c5dt04088d
Received 19th October 2015,
Accepted 23rd November 2015
a
a
a
a
a
Mio Sekino, Hideki Furutachi,* Kyosuke Tasaki, Takanao Ishikawa, Shigeki Mori,
DOI: 10.1039/c5dt04088d
a
a
a
b
b
Shuhei Fujinami, Shigehisa Akine, Yoko Sakata, Takashi Nomura, Takashi Ogura,
Teizo Kitagawa and Masatatsu Suzuki
b
c
www.rsc.org/dalton
(μ-1,2-Peroxo)diiron(III) complexes (2-R) with dinucleating ligands ligand hydroxylation was not investigated. To gain further
(
R-L) generated from the reaction of bis(μ-hydroxo)diiron(II) com- insights into the arene hydroxylation performed by (μ-peroxo)-
2
+
plexes [Fe
2
(R-L)(OH)
2
]
(1-R) with dioxygen in acetone at −20 °C diiron(III) species, further (μ-peroxo)diiron(III) model complexes
provide a diiron-centred electrophilic oxidant, presumably diiron(IV)- are demanded, which exhibit the monooxygenase activity like
oxo species, which is involved in aromatic ligand hydroxylation.
ToMO.
In this study, we have applied the dinucleating ligands
R-L = 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]-5-R-
benzene; R = tBu, H, and NO ) to the diiron complex system in
The Fe/O - and Fe /O -mediated arene hydroxylation is of
2
2
2
(
current interest for understanding the reaction mechanism of
dioxygen activating non-heme mononuclear and dinuclear
iron enzymes such as tetrahydropterin-dependent aromatic
2
order to examine the Fe
2 2
/O -mediated arene hydroxylation
(
Scheme 1), since R-L provides crucial active oxygen species
1
amino acid hydroxylase and toluene/o-xylene monooxygenase
that are capable of hydroxylating the xylyl linker of R-L via an
2
(
ToMO). In these enzymes, an iron(IV)-oxo and a (μ-peroxo)-
electrophilic aromatic substitution mechanism, as found for
1
,2
diiron(III) species have been spectroscopically identified,
11
12
dicopper and dinickel complexes with R-L. Herein, we
report intramolecular arene hydroxylation together with inter-
molecular acetone oxidation initiated by (μ-1,2-peroxo)diiron(III)
complexes (2-R) with dinucleating ligands R-L (Scheme 1),
which are responsible for arene hydroxylation. To date, intra-
3
–6
molecular aromatic ligand hydroxylation and intermolecular
7
hydroxylation of aromatic compounds such as benzoic acid,
8
9
benzene, and anthracene by synthetic mononuclear iron
model complexes with various oxidants such as O , H O ,
generated from the reaction of bis(μ-hydroxo)diiron(II)
2+
2
2 2
complexes [Fe
2
(R-L)(OH)
2
]
(1-R) with dioxygen in acetone
m-CPBA, t-BuOOH, and PhIO have been extensively studied. In
some of these, the iron(IV)-oxo species have been identified as
the active oxidant in intramolecular aromatic ligand hydroxyla-
at −20 °C. We found decisive mechanistic evidence that a
diiron-centred electrophilic oxidant, presumably diiron(IV)-
oxo species, is involved in the aromatic ligand hydroxylation,
whose species is capable of exchanging with exogenous water.
5
b,6b,9
tion.
Some synthetic ToMO model complexes have also
1
0
been reported so far; yet only for one, a (μ-peroxo)diiron(III)
2+
The diiron(II) complex [Fe (H-L)(OH) ] (1-H) was obtained
2
2
Ph4
2+
complex [Fe (L )(O )(Ph CCO )] with a dinucleating ligand
2
2
3
2
by treatment of [Fe(CH
3
CN)
4
(OTf)
2
] with H-L in the presence
Ph4
(L
=
N,N,N′,N′-tetrakis(1-methyl-2-phenyl-4-imidazolyl)-
methyl-1,3-amino-2-propanolate) has been shown to be directly
1
0d
involved in intramolecular aromatic ligand hydroxylation,
although the electronic effect of substituents on the aromatic
a
Department of Chemistry, Division of Material Sciences, Graduate School of Natural
Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192,
Japan. E-mail: h-furutachi@se.kanazawa-u.ac.jp
b
Picobiology Institute, Graduate School of Life Science, University of Hyogo, Ako-gun,
Hyogo 678-1297, Japan
c
Department of Chemistry and Biochemistry, Graduate Engineering,
Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
†
Electronic supplementary information (ESI) available: Experimental details of
synthesis and structural, spectroscopic, and kinetic data. CCDC 1430633 and
430634. For ESI and crystallographic data in CIF or other electronic format see
Scheme 1 Intramolecular arene hydroxylation and intermolecular
acetone oxidation initiated by (μ-1,2-peroxo)diiron(III) complexes (2-R)
with dinucleating ligands (R-L).
1
DOI: 10.1039/c5dt04088d
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of triethylamine and H
2
O in dry-THF under an N
2
atmosphere.
2-H is unstable at −20 °C and decomposes to give a red
1
-H has a bis(μ-hydroxo)diiron(II) core bridged by exogenous solution that showed an absorption band at 550 nm (Fig. 2(A),
hydroxo groups (Fig. 1 and S1†) as found for a closely related (b)). The red solution slowly converted into a purple solution
2
+ 11
dicopper(II) complex [Cu
2
(H-L)(OH)
2
] .
at ambient temperature that afforded purple crystals of
Reaction of a pale green acetone solution of 1-H with O at [Fe (OH) {H(H-L-O)} ](OTf) (3-H) suitable for X-ray crystallo-
2
2
2
2
4
−20 °C generated a dark green peroxo species, 2-H, that exhi- graphy. The crystal structure of 3-H clearly shows that the xylyl
bits a broad absorption band at 500–800 nm (Fig. 2(A), (a): linker is hydroxylated and the resulting phenolates coordinate
1
−1
−1
−1
5
30 nm, ε = ∼710 M− cm ; 700 nm, ε = ∼645 M cm ) that to the iron(III) centers (Fig. 3 and S2†). Decomposition of 2-tBu
2
−
can be assigned to the CT transition from O
2
to the Fe(III) and 2-NO
2
also afforded the corresponding hydroxylated
ligands tBu-L-OH and NO -L-OH (Fig. S5†). Unlike O , no
Similar UV-vis spectral features were also observed for 2-tBu hydroxylation occurred when H O was used as an oxidant.
1
3,14
center, as found in related (μ-peroxo)diiron(III) complexes.
2
2
2
2
and 2-NO
2
(Fig. S3†). Unlike in acetone, no peroxo complexes The yields of hydroxylated ligands (R-L-OH) were determined
1
2
-R were generated in acetonitrile and dichloromethane. The by H NMR spectroscopy (Fig. S6–8, details in the ESI†). The
1
6
resonance Raman (rR) spectrum of 2-H prepared by O2 hydroxylation yields of the xylyl linker depend on the electron-
−
1
−1
showed a band at 845 cm , which shifted to 800 cm when donating ability of the substituent (R-) of R-L and decreased as
1
8
was used (Fig. 2(B) and S4†). The band at 845 cm−1 follows: tBu-L-OH (∼32%) > H-L-OH (∼26%) > NO
O
2
2
-L-OH
can be assigned to ν(O–O) vibration and is similar to those (∼8%) (Fig. S9(a)†). A similar observation was also made for a
2
2
of well characterized (μ-oxo)(μ-peroxo)diiron(III) complexes series of (μ–η ;η -peroxo)dicopper(II) complexes [Cu
2
2
(R-L)-
(R-L)-
835–874 cm− ), suggesting that 2-H has a similar (μ-oxo)- (O
1
14
)] (Cu-R) and bis(μ-oxo)dinickel(III) complexes [Ni
μ-peroxo)diiron(III) core. In contrast to ν(O–O) vibration, (O)2] (Ni-R), suggesting that aromatic hydroxylation of R-L
2+
11
(
2
2
+
12
(
ν(Fe–O) and ν(Fe–O–Fe) vibrations of 2-H were not observed in the present diiron system also proceeds via an electrophilic
(Fig. S4†), which might be due to their poor signal-to-noise aromatic substitution mechanism. Thus, the substituent-
ratios.
dependent intramolecular arene hydroxylation observed upon
decay of (μ-peroxo)diiron(III) species 2-R is closely relevant to
arene hydroxylation catalyzed by ToMO, where a (μ-peroxo)-
diiron(III) intermediate is directly involved in the arene
2
hydroxylation. However, the hydroxylation yield of 2-R is lower
Ph4
2
than those of Cu-R (98–72%), Ni-R (88–30%), and [Fe (L )-
2
+
(
O )(Ph CCO )] (∼90%), suggesting that some other side
2
3
2
reaction(s) takes place simultaneously in the present diiron
system.
The ESI-TOF/MS of a red solution (Fig. 2(A) and (b))
obtained by decomposition of 2-H at −20 °C showed a signal
2
at m/z 379.1 (Fig. S10(a)†), which can be assigned as [Fe (H-L-
2
+
−
O)(O)(CH CO )] having a hydroxylated ligand (H-L-O ) and
3
2
an acetate. The signal at m/z 379.1 in acetone shifted to m/z
80.6 when 2-H decomposed in d -acetone (Fig. S10(b)†), indi-
cating that acetone solvent is indeed the source of acetate in
this species. The acetic acid was also identified by GC-MS ana-
lysis (Fig. S11†). Thus, these results clearly demonstrate intra-
3
6
2+
2 2
Fig. 1 An ORTEP view of [Fe (OH) (H-L)] (1-H). Hydrogen atoms are
omitted for clarity.
Fig. 2 (A) Electronic spectra of 2-H (a) and its decomposition species
(
b) in acetone at −20 °C. (B) Resonance Raman spectra of 2-H generated
16 18
from the reaction of 1-H with
O
2
(c) and
2 6
O (d) in d -acetone at
4+
−
40 °C with a 647.1 nm laser excitation. The asterisk denotes the solvent
Fig. 3 An ORTEP view of [Fe
2
(OH)
2
{H(H-L-O)}
2
]
(3-H). Hydrogen
band.
atoms are omitted for clarity.
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molecular arene hydroxylation and intermolecular acetone oxi- tially involved in the rate determining step of the decay of 2-R
dation initiated by 2-H. Unlike the hydroxylation yields of R-L- as well as O–O bond cleavage of 2-R (Scheme 2, step a), since a
H, the electron-donating ability of the substituent (R-) of R-L large negative activation entropy has been observed for the
has only a small influence on the oxidation yields of acetone, reaction system in which bimolecular reaction is dominantly
1
7
where ∼40% yield of acetic acid was obtained in each ligand included in the rate-limiting step.
system (Fig. S9(b), details in the ESI†). Moreover, the yields of
R-L-OH and acetic acid observed upon decay of 2-R are not using
It should be noted that isotope labeling experiments by
1
8
18
2 2
O and H O strongly support involvement of the
influenced by the reaction conditions under both O and N , diiron(IV)-oxo species in the aromatic ligand hydroxylation.
2
2
1
8
indicating that there is no participation of radical species in Surprisingly, decomposition of 2-H generated by
the present reaction system. It has been reported that acetic acetone resulted in only ∼33% incorporation of O into
2
O in
1
8
acid is formed by decomposition of Fe- and Cu-HPP (HPP = H-L-OH, which is confirmed by ESI-TOF/MS (Fig. S14(b)†).
Ph4
2
2 2
-hydroxy-2-peroxy-propane) species generated by the reaction This observation is in contrast to that of [Fe (L )(O )-
2
+ 10d
18
of mononuclear Fe and Cu complexes with H
2
O
2
in acetone (Ph
3
CCO
2
)] ,
where incorporation of the O label mainly
1
5,16
18
solvent.
lation by a mononuclear Cu-HPP complex has also been generated by
reported by Itoh et al., where an acetate species was not borates this result (Fig. S14(c)†). Thus, incorporation of the
Recently, an electrophilic aromatic ligand hydroxy- originates from O . The complementary experiment with 2-H
2
1
6
18
2 2
O in the presence of H O (1000 equiv.) corro-
1
8
18
formed and only acetone is regenerated during the course of
O label from H2 O into H-L-OH requires the participation of
1
5
reaction. Thus, a diiron-HPP species, probably produced by a an oxidant capable of exchanging with exogenous water, such
nucleophilic attack of 2-R to acetone, seems to be a candidate as a diiron(IV)-oxo species, that carries out the ligand hydroxy-
of the precursor to give acetic acid, but not to give rise to lation (Scheme 2, step c). A similar labeling result has
ligand hydroxylation in the present system (Scheme 2).
been reported for the formation of the DOPA208 residue
To gain further insight into the oxidation mechanism, from the F208Y mutant of the ribonucleotide reductase
1
8
kinetic studies of 2-R were carried out by monitoring the (RNR) R2 protein as well as a few synthetic iron model
3a,4c,10c
absorbance change at 700 nm in acetone at −20 °C. Decompo- complexes.
An electrophilic character of this species is
sition of 2-R under the conditions obeys first-order kinetics supported by the observed substituent-dependent hydroxy-
1
8
(
2
Fig. S12†). It should be noted that the decomposition rates of lation yields of the xylyl linker. Thus, observation of O incor-
1
8
2
-R are independent of the electron-donating ability of the sub- poration from H O as well as substituent-dependent
stituent (R-) of R-L (Fig. S12(d)†). In contrast to those of substi- hydroxylation yields of the xylyl linker in this system are
2
2
tuent-dependent decay observed for the (μ–η ;η -peroxo)- indirect but compelling evidence for high-valent diiron(IV)-oxo
2
+
11
dicopper(II) complex [Cu
μ-oxo)dinickel(III) complex [Ni
vation indicates that the rate determining step in the decay of
-R does not involve an electrophilic attack by the peroxo results mentioned above, we thus propose a plausible mechan-
2
(R-L)(O
2
)] (Cu-R) and the bis- species bearing an electrophilic character, which is well-known
2
+
12
3–9
(
2
(R-L)(O)
2
]
(Ni-R), this obser- for synthetic mononuclear iron(IV)-oxo model complexes.
Based on the kinetics data together with the experimental
2
species 2-R to the xylyl linker, but involves unimolecular reac- ism for intramolecular arene hydroxylation and intermolecular
tion such as conversion of 2-R into a diiron(IV)-oxo species. acetone oxidation initiated by 2-R (Scheme 2), which involves
Activation parameters obtained from the temperature depen- rate determining O–O bond cleavage of 2-R into diiron(IV)-oxo
‡
−1
‡
dence of decay of 2-R are ΔH = 76–80 kJ mol and ΔS = species (step a) and nucleophilic attack of 2-R to acetone into
−
8 to −26 J K− mol (Fig. S13, Tables S3 and S4†). The diiron-HPP species (step b) prior to rapid decay. Conversion of
1
−1
observed small negative activation entropy implies that a bimo- (μ-peroxo)diiron(III) species to high-valent diiron(IV)-oxo species
lecular reaction such as a nucleophilic attack of 2-R to acetone has also been reported for a few synthetic diiron model
1
4a,d,f,19
affording a diiron-HPP species (Scheme 2, step b) is also par- complexes.
The generated diiron(IV)-oxo species is the
Scheme 2 Proposed mechanism for intramolecular arene hydroxylation and intermolecular acetone oxidation initiated by 2-R.
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electrophilic active oxidant capable of exchanging with exogen-
ous water prior to an electrophilic attack on the aromatic ring,
(c) S. Chatterjee and T. K. Paine, Angew. Chem., Int. Ed.,
2015, 54, 9338.
1
8
to provide the partially O labeled phenol (R-L-OH), whose
hydroxylation yields are substituent-dependent (step c). It has
been reported that a well-defined mononuclear non-heme
iron(IV)-oxo complex shows a poor nucleophilic oxidative reac-
tion toward 2-phenylpropionaldehyde compared with (hydro)-
5 (a) S. Sahu, L. R. Widger, M. G. Quesne, S. P. de Visser,
H. Matsumura, P. Moënne-Loccoz, M. A. Siegle and
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1
7a,20
peroxo-iron(III) complexes.
Thus, the diiron(IV)-oxo species
derived from 2-H seems not to give rise to acetone oxidation
via the formation of diiron-HPP species (step e), although the
formation of diiron-HPP species derived from the diiron(IV)-
oxo species may not be ruled out. Decay of diiron-HPP species
is independent of the electron-donating ability of the substitu-
ent (R-) of R-L to give acetic acid and the yield of acetic acid is
expected to be nearly constant in each ligand system (step d),
which is consistent with the observed yield of acetic acid
(
∼40%).
In summary, we have succeeded in intramolecular arene
hydroxylation and intermolecular acetone oxidation initiated
by the (μ-1,2-peroxo)diiron(III) complexes (2-R) with the di-
nucleating ligands (R-L) for the first time. The former reaction
2
mimics the function of ToMO and involves the diiron(IV)-oxo
species as the electrophilic active oxidant capable of exchanging
with exogenous water, whereas the latter one is closely relevant
to a nucleophilic attack of peroxo species to the CvO group pro-
8 (a) V. Balland, D. Mathieu, Y. M. N. Pons, J. F. Bartoli,
F. Banse, P. Battioni, J. J. Girerd and D. Mansuy, J. Mol.
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17a,21
posed for aldehyde deformylating oxygenase (ADO).
The
observed oxidation reactions initiated by 2-R provide a chemical
insight into the nature of (μ-1,2-peroxo)diiron(III) species for oxi-
dation reactivity and O–O bond activation, which are found for
dioxygen-activating non-heme diiron proteins, although further
comprehensive functional model studies are needed.
9 S. P. de Visser, K. Oh, A.-R. Han and W. Nam, Inorg. Chem.,
2007, 46, 4632.
This work was supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, 10 (a) S. Ménage, J.-B. Galey, J. Dumats, G. Hussler, M. Seité,
Science and Technology, Japan and Kanazawa University SAKI-
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