Water as an oxygen source in the generation of mononuclear nonheme iron(IV) oxo complexes

Article abstract of DOI:10.1002/anie.200805670

Give me an "O"! Mononuclear nonheme iron(IV) oxo complexes have been generated using water as an oxygen source and cerium(IV) as an oxidant. The high-yield oxygenation of organic substrates in this system (see picture, Fe green, O red, N blue, C gray) is catalyzed by iron(II) complexes. The source of oxygen in the iron(IV) oxo complexes and the oxygenated products has been assigned unambiguously by isotopic labeling experiments.

Full text of DOI:10.1002/anie.200805670

Angewandte
Chemie
DOI: 10.1002/anie.200805670
Enzyme Models
Water as an Oxygen Source in the Generation of Mononuclear
Nonheme Iron(IV) Oxo Complexes**
Yong-Min Lee, Sunder N. Dhuri, Sarvesh C. Sawant, Jaeheung Cho, Minoru Kubo,
Takashi Ogura, Shunichi Fukuzumi,* and Wonwoo Nam*
High-valent iron(IV) oxo species have been implicated as the
active oxidizing species in metabolically important oxidative
transformations performed by mononuclear nonheme iron
enzymes.[1] Such nonheme iron(IV) oxo species were
observed and characterized recently in enzymatic and bio-
mimetic reactions.[2,3] For example, intermediate high-spin
iron(IV) oxo species were identified and proposed as active
oxidants in the catalytic cycles of E. coli taurine:a-ketoglu-
tarate dioxygenase (TauD), prolyl-4-hydroxylase, and halo-
genase CytC3.[2] In biomimetic studies, a number of mono-
nuclear nonheme iron(IV) oxo complexes bearing tetraden-
tate N4 and pentadentate N5 and N4S ligands were synthe-
sized and characterized with various spectroscopic techni-
ques.[3] A notable example is the first crystal structure of a
nonheme iron(IV) oxo complex, [FeIV(O)(tmc)(NCCH3)]2+
(tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetrade-
cane).[4] The mononuclear iron(IV) oxo species were synthe-
sized using artificial oxidants, such as iodosylbenzene, per-
acids, oxone, N2O, XOÀ (X = Cl, Br), ozone, H2O2, and alkyl
hydroperoxides,[4,5] and more recently using molecular
oxygen.[6] The synthetic iron(IV) oxo intermediates have
shown reactivities in a variety of oxidation reactions, includ-
ing alkane hydroxylation,[7a] olefin epoxidation,[5d] alcohol
oxidation,[7b] aromatic hydroxylation,[7c] nitrogen dealkyla-
tion,[7d] and the oxidation of sulfides[7e] and PPh3.[4,7f]
High-valent metal oxo species have also been invoked as
key intermediates in the oxygen-evolving complex (OEC) in
photosystem II.[8] At the OEC, manganese(V) oxo species
have been proposed to be formed by the oxidation of water in
a proton-coupled electron transfer (PCET) mechanism.[8]
Thus, the oxygen atom in the MnV oxo species derives from
water. In biomimetic studies, the formation of high-valent
ruthenium oxo complexes has been well established for water
oxidation in the presence of a strong oxidant, such as
cerium(IV) or [Ru(bpy)3]3+ [9–11]
Since water is the most
.
abundant, readily available oxygen source on earth, we
attempted to generate mononuclear nonheme iron(IV) oxo
complexes with water as an oxygen source.[12] We now report
for the first time the generation of mononuclear nonheme
iron(IV) oxo complexes and the catalytic oxygenation of
organic substrates using water as an oxygen source and
cerium(IV) as a one-electron oxidant (Scheme 1).
[*] Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering, Osaka University
SORST (Japan) Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Scheme 1.
Addition of [CeIV(NO3)6](NH4)2 (cerium(IV) ammonium
nitrate, CAN; 4 mm) to a reaction solution containing non-
heme iron(II) complexes (1 mm) [FeII(N4Py)](ClO4)2 or
[FeII(Bn-tpen)](OTf)2 (N4Py = N,N-bis(2-pyridylmethyl)-N-
bis(2-pyridyl)methylamine,
tris(2-pyridylmethyl)ethane-1,2-diamine,
see Figure S1 in the Supporting Information) afforded the
corresponding iron(IV) oxo complexes [FeIV(O)(N4Py)]2+ (1)
or [FeIV(O)(Bn-tpen)]2+ (2)[7a] in H2O, H2O/CH3CN, or
buffered H2O/CH3CN at 258C (Figure 1a and Figure S2a in
the Supporting Information).[13] The electrospray ionization
mass spectra (ESI MS) of 1 and 2 exhibit prominent ion peaks
at m/z 219.7 and 247.6, respectively (Figure 1b and Figur-
e S2 b in the Supporting Information), the mass and isotope
distribution patterns of which correspond to [FeIV(O)-
(N4Py)]2+ (calcd m/z 219.6) and [FeIV(O)(Bn-tpen)]2+ (calcd
m/z 247.6). When the reactions were carried out in isotopi-
cally labeled water (H218O), mass peaks corresponding to 1
and 2 appeared at m/z 220.7 and 248.6, respectively, thus
indicating that the oxygen atom in the iron(IV) oxo com-
Dr. Y.-M. Lee, Dr. S. N. Dhuri, Dr. S. C. Sawant, Dr. J. Cho,
Prof. Dr. W. Nam
Department of Chemistry and Nano Science and
Department of Bioinspired Science, Ewha Womans University
Seoul 120–750 (Korea)
Bn-tpen = N-benzyl-N,N’,N’-
À
OTf = CF3SO3
;
Fax: (+82)2-3277-4441
E-mail: wwnam@ewha.ac.kr
Dr. M. Kubo, Prof. Dr. T. Ogura
Picobiology Institute, Graduate School of Life Science
University of Hyogo
Koto 3-2-1, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan)
[**] The research at EWU was supported by KOSEF/MOST through CRI
Program and the WCU Project (to W.N.), a Grant-in-Aid (No.
19205019 to S.F.), and a Global COE program, “the Global
Education and Research Centre for Bio-Environmental Chemistry”
from the Ministry of Education, Culture, Sports, Science and
Technology (Japan) (to S.F.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805670.
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The stability of 1 and 2 generated in the present study (t1/2
ꢀ 5 and 1 day at 258C for 1 and 2, respectively) was found to
be greater than that of the compounds generated in the
reaction with PhIO in CH3CN (t1/2 ꢀ 60 and 6 h at 258C for 1
and 2, respectively).[7a] We therefore attempted to generate
other nonheme iron(IV) oxo complexes with water as an
oxygen source and CAN as an oxidant at 258C; these were
synthesized only at low temperature (e.g. at À408C) owing to
their thermal unstability. Such complexes include [FeIV(O)-
(tpa)]2+ (3, tpa = tris-(2-pyridylmethyl)amine),[5d] [FeIV(O)-
(bpmen)]2+ (4, bpmen = N,N’-dimethyl-N,N’-bis(2-pyridylme-
thyl)ethane-1,2-diamine),[18a] and [FeIV(O)(bqen)]2+ (5,
bqen = N,N’-dimethyl-N,N’-bis(8-quinolyl)ethane-1,2-diami-
ne;[18b] see Figure S1 in the Supporting Information). Inter-
estingly, we observed the formation of the iron(IV) oxo
species in the reactions of the corresponding iron(II) com-
plexes and CAN in the presence of H2O and the slow decay of
the intermediates at 258C (Figure S3 in the Supporting
Information), thus indicating that the intermediates gener-
ated by H2O and CAN are stabilized under the present
reaction conditions.[19] As the enhanced stability of the iron
oxo intermediates might be due to the high acidity of CAN,[10a]
we compared the stability of 3 in the presence and absence of
HClO4 in H2O/CH3CN at 258C (Figure S4 in the Supporting
Information).[5b,20] The stability of 3 increased by a factor of
approximately four when it was prepared with CH3CO3H in
the presence of HClO4, thus indicating that the high stability
of nonheme iron(IV) oxo intermediates prepared with H2O
and cerium(IV) can be ascribed to the low pH value of the
reaction solution.[5b] Furthermore, and in contrast to the
reactions of H2O and CAN (see above), the formation of 4
and 5 was not observed when [FeII(bpmen)]2+ and [FeII-
(bqen)]2+ were treated with CH3CO3H in the presence and
absence of HClO4 at 258C. It is also worth noting that other
nonheme iron(IV) oxo complexes reported in the literature,
including [FeIV(O)(tmc)]2+,[3,4] were easily prepared using
water as an oxygen source and CAN as an oxidant.
In light of the observation that nonheme iron(IV) oxo
complexes are formed using water as an oxygen source, we
investigated the catalytic oxygenation of organic substrates by
the nonheme iron(II) catalyst [FeII(Bn-tpen)](OTf)2 and
CAN in H2O/CH3CN (1:3) at 258C. As the results in
Table 1 show, high product yields were obtained in all of the
reactions. In the oxidation of thioanisole (Table 1, entry 1),
methyl phenyl sulfoxide was the major product, and no
formation of sulfone was observed. Benzyl alcohol was
oxidized to benzaldehyde with the formation of a small
amount of benzoic acid (Table 1, entry 3). In the oxidation of
cyclohexene (Table 1, entry 4), hexanedioic acid, which is the
product of eight-electron oxidation, was formed, as reported
in the ruthenium-catalyzed oxidation of cyclohexene by
CAN.[10a] The hydroxylation of ethylbenzene (Table 1,
entry 5) yielded acetophenone as the major product with
the formation of a small amount of sec-phenethyl alcohol, as
observed in the hydroxylation of ethylbenzene by 2.[7a] The
turnover number increased by reducing the catalyst amount
(Table 1, entry 2). In the absence of the iron catalyst, only
negligible amounts of products were formed by H2O and
CAN under the reaction conditions.
Figure 1. a) UV/vis spectral changes showing the formation of
[(N4Py)FeIV(O)]2+ (red line) in the reaction of [(N4Py)FeII](ClO4)2
(1 mm, blue line) and [CeIV(NO3)6](NH4)2 (4 mm) in H2O/CH3CN (1:3)
at 258C. b) ESI MS of [(N4Py)FeIV(O)]2+ formed in the reaction of
[(N4Py)FeII](ClO4)2 and CAN (4 equiv) in H2O/CH3CN (1:3) at 258C. A
peak at m/z 537.9 corresponds to [FeIV(O)(N4Py)(ClO4)]+. Insets show
the observed isotope distribution patterns for [(N4Py)FeIV(16O)]2+ (left
panel, prepared with H216O) and [(N4Py)FeIV(18O)]2+ (right panel,
prepared with H218O). c) Resonance Raman spectra (08C, 407 nm
excitation) of [(N4Py)FeIV(16O)]2+ (blue line) formed by the reaction of
[(N4Py)FeII](ClO4)2 (16 mm) and four equivalents CAN in H216O/
CH3CN (1:3) and of [(N4Py)FeIV(18O)]2+ (red line) formed in H218O/
CH3CN (1:3). The peaks marked with * are from solvent.
plexes derives from water [Eq. (1)].[14] The resonance Raman
(rR) spectrum of 1 exhibits a vibration at 840 cmÀ1 that shifts
to 806 cmÀ1 upon introduction of 18O (Figure 1c).[15,16] By
comparison with the reported rR data of [FeIV(O)-
(tmc)(X)]n+ [17]
,
the observed Fe O stretching frequency and
À
the isotope shift of À34 cmÀ1 with 18O substitution demon-
2+
strate unambiguously that [(N4Py)FeIV O] was formed in
=
the oxidation of [FeII(N4Py)]2+ by water and CAN and that
the oxygen atom in the iron oxo group derives from water
[Eq. (1)].
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Chemie
Table 1: Catalytic oxidation reactions using [FeII(Bn-tpen)]2+ as a catalyst,
[3] a) W. Nam, Acc. Chem. Res. 2007, 40, 522 – 531; b) L. Que, Jr.,
water as an oxygen source, and CAN as an oxidant.[a]
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2003, 299, 1037 – 1039.
Entry
Substrate
Products
TON[b]
1
thioanisole
thioanisole
benzyl alcohol
cyclohexene
ethylbenzene
methyl phenyl sulfoxide
methyl phenyl sulfoxide
benzaldehyde
hexanedioic acid
sec-phenethyl alcohol
acetophenone
10Æ1
95Æ5
10Æ1
5.6Æ0.5
1Æ0.2
9Æ1
2[c]
3
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4
5
[a] Reaction conditions: CAN (10 mm, 50 equiv relative to iron catalyst)
was added to a reaction solution containing [FeII(Bn-tpen)](OTf)2
(0.2 mm) and substrate (40 mm) in H2O/CH3CN (1:3) at 258C. After
30 min stirring, the reaction solution was analyzed by HPLC for the
reactions of thioanisole, benzyl alcohol, and ethylbenzene and by
1H NMR spectroscopy for the reaction of cyclohexene.[10a] [b] Turnover
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The catalytic oxidation of thioanisole was also carried out
in H218O/CH3CN (1:3) to confirm the source of oxygen in the
oxygenated products. The methyl phenyl sulfoxide produced
contained 93% 18O derived from 95% 18O-enriched H218O.[21]
The 18O percentage in the sulfoxide product was sensitive
neither to the ratio of H218O to CH3CN in the solvent
mixture[14] nor to the presence of O2. The labeled-water
experiments demonstrate unambiguously that water was the
oxygen source and no autooxidation reaction was involved in
the oxidation reactions under aerobic conditions, as reported
in [Ru(tpa)]-catalyzed oxygenation reactions with CAN.[10a]
In conclusion, we have reported the first example of using
water as an oxygen source in the generation of mononuclear
nonheme iron(IV) oxo complexes and the catalytic oxygen-
ation of organic substrates. The source of oxygen in the iron
oxo complexes and in the oxygenated products was assigned
unambiguously by carrying out experiments with isotopically
labeled water. We also demonstrated that nonheme iron(IV)
oxo complexes are facilely synthesized under mild conditions
(e.g. at ambient temperature) using water as an oxygen source
and cerium(IV) as an oxidant. In light of the precedent
established by water oxidation in manganese and ruthenium
chemistry,[9,22] design of ligands for dinuclear iron complexes
(e.g., [{Fe(O)}2L] for FeIV or FeV) and control of the redox
potential of the iron catalysts might allow the present results
to be expanded to a system in which nonheme iron complexes
can oxidize water to O2.
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[12] Electrochemical generation of a nonheme iron(IV) oxo complex
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[13] The generation and stability of iron(IV) oxo intermediates do
not depend on solvent systems (e.g. organic vs. aqueous
solutions).
Received: November 20, 2008
Published online: January 13, 2009
Keywords: bioinorganic chemistry · enzyme models · iron ·
.
oxygenation · reaction mechanisms
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2420. Thus, the full 18O incorporation from H218O into the iron
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À
[15] Very recently, Solomon and co-workers reported the Fe O
stretching vibration of 1 to be 820 cmÀ1 (788 cmÀ1 with 18O) by
nuclear resonance vibrational spectroscopy (NRVS; see
Ref. [16]). We find that 1 prepared with PhIO in CH3CN
exhibits a vibration at 840 cmÀ1 that shifts to 807 cmÀ1 upon
[19] The relative stability of iron(IV) oxo complexes under the
present conditions is in the order 1 > 2 > 3 > 5 ꢁ 4.
18
À
introduction of O. The latter result indicates that the Fe O
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