Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant

Article abstract of DOI:10.1016/j.ica.2011.08.004

Fe(II)-tris(2-pyridylmethyl)amine complexes, Fe(II)-tpa, having different co-existing anions, [Fe(tpa)(MeCN)₂](ClO₄)₂ (1), [Fe(tpa)(MeCN)₂](CF₃SO₃)₂ (2) and [Fe(tpa)Cl₂] (3), were prepared. Effective magnetic moments (evaluated by the Evans method) revealed that while 1-3 in acetone and 3 in acetonitrile (MeCN) have a high-spin Fe(II) ion at 298 K, the Fe(II) ions of 1 and 2 are in the low-spin state in MeCN. The aerobic oxidation of 1-3 was monitored by UV-Vis spectral changes in acetone or MeCN under air at 298 K. Only the high-spin Fe(II)-tpa complexes were oxidized with rate constants of k _obs = 0.1-1.3 h^-1, while 1 and 2 were stable in MeCN. The aerobic oxidation of 1 or 2 in acetone was greatly accelerated in the presence of pure, peroxide-free cyclohexene (1000 equiv.) and yielded a large amount of oxidized products; 2-cyclohexe-1-ol (A) and 2-cyclohexene-1-one (K) (A + K: 23 940% yield based on Fe; A/K = 0.3), and cyclohexene oxide (810%). Besides cyclohexene, aerobic oxidation of norbornene, cyclooctene, ethylbenzene, and cumene proceeded in the presence of 1 in acetone at 348 K without any reductant. Essential factors in the reaction are high-spin Fe(II) ion and labile coordination sites, both of which are required to generate Fe(II)-superoxo species as active species for the H-atom abstraction of hydrocarbons.

Full text of DOI:10.1016/j.ica.2011.08.004

Inorganica Chimica Acta 378 (2011) 19–23
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Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme
Fe(II) complexes without any reductant
a
a
Shinya Furukawa a, Yutaka Hitomi b, , Tetsuya Shishido , Tsunehiro Tanaka
⇑
a Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 606-8501, Japan
b Department of Molecular Chemistry and Biochemistry, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2011
Received in revised form 28 July 2011
Accepted 2 August 2011
Available online 11 August 2011
Fe(II)–tris(2-pyridylmethyl)amine complexes, Fe(II)–tpa, having different co-existing anions, [Fe(tpa)
(MeCN)2](ClO4)2 (1), [Fe(tpa)(MeCN)2](CF3SO3)2 (2) and [Fe(tpa)Cl2] (3), were prepared. Effective mag-
netic moments (evaluated by the Evans method) revealed that while 1–3 in acetone and 3 in acetonitrile
(MeCN) have a high-spin Fe(II) ion at 298 K, the Fe(II) ions of 1 and 2 are in the low-spin state in MeCN.
The aerobic oxidation of 1–3 was monitored by UV–Vis spectral changes in acetone or MeCN under air at
298 K. Only the high-spin Fe(II)–tpa complexes were oxidized with rate constants of kobs = 0.1–1.3 hÀ1
,
Keywords:
while 1 and 2 were stable in MeCN. The aerobic oxidation of 1 or 2 in acetone was greatly accelerated
in the presence of pure, peroxide-free cyclohexene (1000 equiv.) and yielded a large amount of oxidized
products; 2-cyclohexe-1-ol (A) and 2-cyclohexene-1-one (K) (A + K: 23 940% yield based on Fe; A/K =
0.3), and cyclohexene oxide (810%). Besides cyclohexene, aerobic oxidation of norbornene, cyclooctene,
ethylbenzene, and cumene proceeded in the presence of 1 in acetone at 348 K without any reductant.
Essential factors in the reaction are high-spin Fe(II) ion and labile coordination sites, both of which are
required to generate Fe(II)-superoxo species as active species for the H-atom abstraction of hydrocarbons.
Ó 2011 Elsevier B.V. All rights reserved.
Aerobic oxidation
Hydrocarbon oxidation
High-spin Fe(II) complex
Nonheme Fe complex
1. Introduction
dioxygen activation ability of simple Fe(II)–tpa complexes. Diebold
and Hagen reported that the spin state of the Fe(II)–tpa complex is
The catalytic oxidation of hydrocarbons such as alkanes, alkenes
and aromatic compounds to oxygen-containing materials is one of
the most important chemical transformations in industrial chemis-
try [1]. For economic and environmental reasons, the use of diox-
ygen as sole oxidant is highly desirable [2]. Since it reacts slowly
with hydrocarbons owing to its triplet ground state, appropriate
activation of dioxygen is required for efficient oxidation of
hydrocarbons using dioxygen. In nature, such dioxygen activation
is performed by iron- and/or copper-dependent enzymes such as
cytochrome P450 [3], methane monooxygenase [4], tyrosinase
[5], dopamine b-monooxygenase [6] and various types of mononu-
clear nonheme iron dioxygenases [7], where dioxygen is reduc-
tively activated via the Cu(I) or Fe(II) ion(s). It is noteworthy that
a high-spin Fe(II) center activates dioxygen in heme and nonheme
iron-dependent enzymes [8] and model complexes [9–11].
different depending on its counter anions [13]. Therefore, we pre-
pared three Fe(II)–tpa complexes; [Fe(tpa)(MeCN)2](ClO4)2 (1),
[Fe(tpa)(MeCN)2](CF3SO3)2 (2) and [Fe(tpa)Cl2] (3), and examined
their spin states and dioxygen activation ability. We also examined
the aerobic oxidation of hydrocarbons in the presence of the Fe(II)–
tpa complexes.
2. Results and discussion
2.1. Characterizations of Fe(II)–tpa complexes
The effective magnetic moments of complexes 1–3 were mea-
sured in acetonitrile (MeCN) and acetone by the Evans NMR method
[14]. The leff values, listed in Table 1, indicate that 1 and 2 possess
mainly a low spin Fe(II) ion in MeCN, whereas 3 is in the high-spin
state in MeCN. The spin states of Fe(II)–tpa type complexes in MeCN
have been widely investigated [13,15–17]. The present results in
MeCN are well consistent with the reported data [13,15,16]. In
contrast, all the Fe(II) ions of 1–3 were in the high-spin states in
acetone. The complexes 1 and 2 are known to be in the low spin
state in the solid state [13,15]. Therefore, the high spin state in ace-
Previously, we reported that low spin Fe(III)–tpa catecholate
complexes (tpa: tris(2-pyridylmethyl)amine, Fig. 1) have a lower
dioxygen reactivity than the high-spin species, even though the
former has a much higher Fe(II)-semiquinoate character [12]. This
result motivated us to examine the spin-state dependency on the
⇑
tone suggests the dissociation of coordinated
p-accepting ligand
Corresponding author. Tel.: +81 077 465 7437.
E-mail address: yhitomi@mail.doshisha.ac.jp (Y. Hitomi).
MeCN. Then, we conducted 19F and 1H NMR study to clarify the
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.004

20
S. Furukawa et al. / Inorganica Chimica Acta 378 (2011) 19–23
Fig. 1. Structure of the tpa ligand.
coordination environment of the Fe(II) complex in acetone (see Figs.
S1 and S2 in Supplementary material). The chemical shift of the
fluorine atom of triflate anion with paramagnetic Fe(II) complex
widely varies based on the chemical environment; ca.+60 (bridging
triflate), ca. À10 (terminal) and ca. À80 ppm (free triflate) [18,19].
The 19F NMR spectrum of 2 in acetone-d6 exhibited a single broad
Fig. 2. UV–Vis spectral changes of 3 in MeCN upon the reaction with O2. Inset:
change in absorbance with time at 430 nm and fitted curve.
peak at À41.4 ppm (m1/2 = 1036 Hz), which suggest that the triflate
anions weakly coordinated to Fe(II) and they are equivalent [16,20].
The observed line broadening indicates a fast exchange between
coordinated and free triflate anions [16]. In addition, the 1H NMR
spectrum of 2 in acetone-d6 shows that the all three pyridylmethyl
moieties are equivalent, suggesting a fast exchange between a six-
coordinate and a five-coordinate conformation as proposed with
the similar Fe(II) complexes by Britovsek et al. [16], which is shown
in Scheme 1a. The electronic states of 1–3 in solution were also
investigated by UV–Vis absorption spectroscopy (Table 1). Intense
MLCT bands were observed for 1 and 2 in MeCN, but the MLCT
bands for 1 and 2 in acetone were weaker in intensity. The MLCT
bands of 3 were weak in intensity in both MeCN and acetone, sim-
ilar to those observed for 1 and 2 in acetone. Thus, the intensity of
the MLCT band was higher with low-spin Fe(II)–tpa complexes than
with the high-spin. The similar correlation between intensity of the
MLCT band and spin states of Fe(II) complexes has been reported by
Britovsek et al. without rationale [16]. The correlation suggests that
The preferential high spin state of 1 and 2 in acetone can be
rationalized by coordination of the counter anions to the Fe(II)
ion in acetone. On the other hand, the solvent-independent high
spin state of 3 suggests that the two coordinated chloride ions of
3 are not exchanged by MeCN or acetone and result in the Fe(II)
ion being in a high spin state [21].
2.2. Reaction with dioxygen
The reactions of 1–3 with dioxygen were examined by UV–Vis
spectroscopy. In MeCN, only 3 showed a spectral change upon reac-
tion with O2 (Fig. 2), while 1 and 2 did not show any spectral
changes for at least one day. The MLCT band of 3 disappeared in a
first-order decay fashion with an isosbestic point at 413 nm over
several hours. The observed first-order rate constant kobs is
comparable with the reported value (0.68 hÀ1) by Thallaj et al.
the low spin complex has the stronger interactions between dp and
pyridine p⁄ orbitals compared with the high-spin complexes.
Table 1
UV–Vis absorption properties, effective magnetic moments, rate constant of oxygen activation (kobs) and catalytic activity of cyclohexene oxidation (TON) of Fe(II)–tpa complexes
1–3 in solution states.
Entry
Solvent
Complex
kmax
(
ea
)
leff
(lB
)
kobs(hÀ1
)
TONb
1
2
3
4
5
6
acetonitrile
1
2
3
1
2
3
368 sh, 398 (6379), 506 (150)
368 sh, 398 (6608), 521 (141)
426 (1468)
362 (1268)
364 (1476)
0.740
0.834
5.29
5.07
5.33
5.46
0c
8.7
8.8
1.3
239
253
0.1
0c
0.529
1.28
0.118
0.311
acetone
470 (1331)
a
b
c
Units of MÀ1 cmÀ1
.
TON was defined as mol of products (2-cyclohexene-1-ol and 2-cyclohexene-1-one)/mol of complex (5
No spectral change was observed after 24 h. In the absence of the catalyst, only a small amount of product was evolved (15
lmol) after 24 h.
l
mol in acetone after 24 h).
Scheme 1. Reaction of dioxygen with the Fe(II) center via dissociative mechanism: (a) fast ligand exchange; (b) coordination of dioxygen; and (c) release of superoxide.

S. Furukawa et al. / Inorganica Chimica Acta 378 (2011) 19–23
21
[17] (Table 1). In acetone, in contrast to MeCN, all complexes
reacted with dioxygen (Table 1). These results clearly show the
higher dioxygen reactivity of high spin Fe(II)–tpa complexes rather
than low spin. Mandon and his co-workers has proposed that
Fe(II)–tpa chloride complex (here in 3) reacts with dioxygen in a
dissociative fashion [17,22,23]. Our NMR study also supports the
possible coordination of dioxygen to Fe(II) in five-coordinate
(Scheme 1b). Interestingly, the kobs value of 2 in acetone is smaller
than that of 3 in acetone or MeCN, which is contrary to the order of
binding strength of the counter anion. This may be due to the differ-
ence in steric hindrance between bulky triflate and chloride. The
fact that 1, which has the most weakly coordinating counter anions,
showed the fastest oxidation reaction rate indicates that 1 was oxi-
dized preferentially through the formation of a dioxygen adduct of
the Fe(II) complex, that is, an Fe(III)-superoxo species (Scheme 1b)
[9,11,24,25]. The Fe(III)-superoxo might convert to Fe(III) and
superoxide in the presence of a nucleophile such as chloride
(Scheme 1c), as found for the autooxidation of oxymyoglobin and
oxyhemoglobin [26].
2.3. Aerobic oxidation of alkenes in the presence of the Fe(II) complexes
To examine the effect of the Fe(III)-superoxo species on the aer-
obic oxidation of alkenes, we first chose cyclohexene as a substrate
with weak allylic C–H bonds; bond dissociation energy
(BDE) = 81 kJ/mol [27]. The aerobic oxidation of cyclohexene was
carried out in the presence of 1–3 (0.1 mol%) both in MeCN and
acetone. Gas chromatography analysis of the products revealed
that 1 and 2 are capable of promoting aerobic oxidation of cyclo-
hexene in acetone but not in MeCN (Table 1). Thus, these results
clearly indicate that the high-spin Fe(II)–tpa complex can be a good
promoter in the aerobic oxidation of cyclohexene. Complex 1 gave
allylic oxidation products of cyclohexene, 2-cyclohexen-1-ol and
2-cyclohexene-1-one, (y. 23940% based on Fe), and cyclohexene
oxide (y. 810%) in acetone after 24 h. The alcohol/ketone (A/K) ratio
was ca. 1.0 at the beginning and decreased to 0.3 as the reaction
proceeded (Fig. 3). The decrease in A/K ratio from 1.0 indicates that
the allylic oxidation occurs via a radical auto-oxidation process,
followed by over-oxidation of the alcohol to ketone by radicals or
other oxidants. Production of cyclohexene oxide, however, implies
that metal-based oxidants are also formed during the oxidation
reaction. Interestingly, 3 is virtually ineffective in initiating the aer-
obic oxidation of cyclohexene in both MeCN and acetone (Table 1).
Thus, these results suggest that labile coordination sites on the
high-spin Fe(II) center are a requisite for efficient aerobic oxidation
of cyclohexene. In the presence of 1, oxidation of other alkenes and
alkanes with weak or moderate C–H BDE (81–97 kJ/mol) were also
examined, whose results are listed in Table 2. At 298 K, cumene
(BDE: 83 kJ/mol) [28] was oxidized; however, hydrocarbons with
C–H BDE of more than 85 kJ/mol such as cyclooctene (85 kJ/mol)
[27], ethylbenzene (86 kJ/mol) [27] and norbornene (97 kJ/mol)
[29] were intact under these conditions. The latter hydrocarbons
were oxidized when the temperature was elevated to 348 K in 2-
butanone. The product yield in the oxidation of cumene was effec-
tively enhanced with increase in temperature. These results clearly
show that the C–H activation of hydrocarbon is the key step in the
catalytic system.
Fig. 3. Time course of (a) product yield based on Fe and (b) A/K ratios in the
oxidation of cyclohexene with 1 (filled symbols) and 2 (open symbols) in acetone.
absence of cyclohexene (ca. 2 min). These results indicate that
cyclohexene-derived radicals induce the oxidation of Fe(II) species.
We propose that H-atom abstraction from cyclohexene by an
Fe(III)-superoxo species is the key step in producing cyclohex-
ene-derived radicals, which quickly oxidize Fe(II) species (Scheme
2). The radicals generated would propagate the radical chain oxida-
tion of cyclohexene. Thus, the observed induction period corre-
sponds to the initial dioxygen activation step. The fact that 3 was
aerobically oxidized but did not initiate the aerobic oxidation of
cyclohexene implies that Fe(III)-superoxo species immediately
converts to Fe(III) and superoxide by nucleophilic attack of the
chloride anion as discussed above. Superoxide has been reported
to perform H-atom abstraction only from weak N–H [30] and
O–H [31] bonds. The Fe(III)-superoxo species is also likely to gen-
erate a
l-peroxo dimer, followed by formation of a l-oxo dimer
as widely invoked in aerobic oxidation of heme and non-heme
Fe(II) complexes [32–34]. The formation of stable
can finally lead to deactivation of the catalysis.
l-oxo dimer
Que and his co-workers have been studying hydrocarbon oxida-
tion catalyzed by the Fe(II)–tpa complex and its derivatives with
H2O2 or tBuOOH as oxidant [21,35,36]. Recently, they reported that
Fe(III)-hydroperoxo and -alkylperoxo species supported by the tpa
ligand were transformed to Fe(IV)-oxo species in acetone [37]. Pro-
duction of cyclohexene oxide supports the generation of Fe(IV)-oxo
species in our system, although the aerobic oxidation mainly pro-
ceeds via radical chain oxidation, as shown by the A/K ratio.
The reactivity of metal-superoxo species involved in hydrocar-
bon oxidation reactions has attracted much attention recently in
the field of bioinorganic chemistry, since Cu(II)-superoxide species
2.4. Reaction mechanism
Next, we monitored UV–Vis absorption spectral changes of 1 in
the presence of cyclohexene in acetone. The abundance fraction of
Fe(II) species estimated by intensity of the MLCT band is plotted
against reaction time in Fig. 4. The Fe(II) species disappeared with-
in 5 s with a small induction period in the presence of cyclohexene,
which is significantly faster than the oxidation rate of 1 in the

22
S. Furukawa et al. / Inorganica Chimica Acta 378 (2011) 19–23
Table 2
Product yields in the aerobic oxidation of hydrocarbons in the presence of 1.
Substrate
Solvent
T (K)
Product
Yield (%)a
Sel. (%)
bNorbornene
Cyclooctene
2-butanone
2-butanone
348
348
norbornene oxide
cyclooctene oxide
1,2-cyclooctanediol
cyclooctanone
cyclohexene oxide
2-cyclohexene-1-ol
2-cyclohexene-1-one
acetophenone
1-phenylethanol
acetophenone
cumylalcohol
acetophenone
cumylalcohol
18 340
12 300
13 250
14 830
810
6070
17 870
2000
1700
2200
100
51c
21
23
26
2.9
22
64
54
32
93
4.0
72
28
50
50
Cyclohexene
acetone
298
Ethylbenzene
Cumene
2-butanone
2-butanone
acetone
348
348
313
298
940
360
130
130
acetone
acetophenone
cumylalcohol
a
b
c
Based on Fe (5
2100 equiv. of norbornene used.
Several byproducts also detected.
l
mol) after 24 h of reaction.
have been postulated to initiate H-atom abstraction from sub-
strates in the catalytic cycles of copper-dependent enzymes such
as dopamine b-monooxygenase and peptidylglycine a-hydroxylat-
ing monooxygenase (PHM) [38–40], and the Fe(III)-superoxo spe-
cies have been proposed as active species in a di-iron-dependent
enzyme, myo-inositol oxygenase [41,42]. It has been reported by
using model complexes that Cu(II)-superoxo species can perform
H-atom abstraction from weak O–H and N–H bonds [43–45].
Recently, Itoh and co-workers synthesized a series of Cu(II) end-
on superoxo species which mimic a tetrahedral geometry in the
X-ray structure of oxy-PHM reported by Amzel and co-workers
[46,47], and demonstrated that this species also mimics the H-atom
abstraction from aliphatic C–H bonds [47]. H-atom abstraction by
Fe(III)-superoxo species had been reported rarely, but recently,
Fukuzumi and Nam et al. reported that Fe(III)-superoxo species
generated in the reaction of the mononuclear nonheme Fe(II) com-
plex with dioxygen performs H-atom abstraction from cycloalkenes
[48,49]. We believe that the same mechanism should be operative
in the aerobic oxidation reaction promoted by 1 in acetone.
Fig. 4. Change in fraction of Fe(II) species with time during oxidation of 1 in acetone
in the presence (filled circle) or absence (open circle) of cyclohexene upon the
reaction with O2. The fraction of Fe(II) species was calculated from the MLCT band
intensity. Inset shows the initial phase.
3. Conclusions
We found that aerobic oxidation of alkenes was efficiently pro-
moted by the Fe(II)–tpa complex, when the Fe center was in the
high-spin state. It is noteworthy that the aerobic oxidation pro-
ceeded without any reductants, since most of the aerobic oxidation
systems catalyzed by heme or nonheme iron complexes require
stoichiometric co-reductants such as aldehydes and Zn/AcOH
[50]. We propose that an Fe(III)-superoxo species is a key interme-
diate in the efficient aerobic oxidation of alkenes. The essential fac-
tor in the reaction is the presence of labile coordination sites that
provides stable coordination of dioxygen, giving Fe(III)-superoxo
species active for the H-atom abstraction; the strong interaction
of MeCN with the low-spin iron(II) center would prevent the for-
mation of the Fe(III)-superoxo species, and chloride ion may short-
en the life-time of the Fe(III)-superoxo species probably through
nucleophilic attack. Thus, the ligand-exchange chemistry including
dioxygen should be of important in the performance of the efficient
aerobic oxidation1. Consequently, high-spin Fe(II)–tpa complexes
except for the chloride complex effectively initiated aerobic oxida-
tion of hydrocarbons. We will further study the relationship
between spin sate and dioxygen activation ability.
1
Scheme 2. Proposed reaction paths in the initial phase of oxidation of 1 in the
We appreciate an anonymous referee for valuable and insightful comments on
presence of cyclohexene.
our manuscript.

S. Furukawa et al. / Inorganica Chimica Acta 378 (2011) 19–23
23
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Oxygen was bubbled for 1 min into 5 ml of 1.0 mM Fe(II)–tpa
complex solution containing 1000 equiv. of hydrocarbon as sub-
strate, followed by vigorous stirring under 1 atm O2 in a batch
reactor tube equipped with an O2 balloon. Organic products were
analyzed by FID-GC (Shimadzu GC-14B) and GC–MS (Shimadzu
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sieves 4 Å and deoxygenated by N2 bubbling. 2-Butanone was used
instead of acetone for the reaction at 348 K to avoid boiling.
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This work was supported by a Grant-in-Aid for Scientific Re-
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and Technology, Japan. S.F. thanks the JSPS Research Fellowships
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.08.004.
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