Iron-substituted polyoxotungstates as inorganic synzymes: Evidence for a biomimetic pathway in the catalytic oxygenation of catechols

Article abstract of DOI:10.1002/chem.200901392

Combined and convergent structural, spectroscopic, and mechanistic evidence on the bio-inspired catalysis by Krebs-type Fe₄X₂W ₁₈ polytungstates in promoting the cleavage of catechols was reported. The biomimetic function of the polyoxometalates (POM)-based catalyst was assessed by spectroscopic and structure-reactivity relationships. The results show that the cleavage products are generated from the rearrangement and hydrolysis of a transient alkylperoxo intermediate. The biometric Fe-catecholate intermediates show two catecholate-to-Fe charge transfer bands in UV/Vis-NIR spectra. The energies of the catecholate-to-Fe charge-transfer transitions can be tuned by the POM composition and these bands provide a sensitive probe for the Lewis acidity of the iron center.

Full text of DOI:10.1002/chem.200901392

DOI: 10.1002/chem.200901392
Iron-Substituted Polyoxotungstates as Inorganic Synzymes: Evidence for a
Biomimetic Pathway in the Catalytic Oxygenation of Catechols
[
b]
[b]
[b]
[a]
Andrea Sartorel, Mauro Carraro, Gianfranco Scorrano, Bassem S. Bassil,
[
a]
[c]
[c]
[a]
Michael H. Dickman, Bineta Keita, Louis Nadjo, Ulrich Kortz,* and
[
b]
Marcella Bonchio*
Dedicated to the Centenary of the Italian Chemical Society
III
III
IV
IV
The oxygenase activity of natural metallo-enzymes offers
a unique paradigm of what is the Holy Grail of oxidation
X=As , Sb ; n=6; X=Se , Te ; n=4), display two of
the four iron sites with three terminal, substitution-labile co-
ordination positions, typical of non-heme dioxygenase en-
[1–4]
chemistry.
An innovative approach to the design of oxy-
[18]
genase functional mimics is the adoption of a totally inor-
ganic ligand system derived from polyoxometalates
zymes (Figure 1).
tabolism/biodegradation of dihydroxylated aromatic com-
pounds. While several functional models have been designed
with coordination complexes using polydentate organic lig-
The latter are responsible for the ca-
AHCTUNGTRENNUNG
[
5–7]
(
POMs),
coordination complexes.
tion-metal-substituted POMs (TMSP, with TM being Fe,
as an alternative to organic or organometallic
[8–9]
Bio-inspired activity of transi-
[8]
AHCTUNGTRENNUGNa nds, very few examples deal with POM-based systems,
[10–14]
III
Mn, Ru) has been proposed in the recent literature.
and Fe framework-incorporated POMs as multi-turnover
catalysts for the aerobic cleavage of catechols are unprece-
However, due to the severe mechanistic complexity, which
is frequently associated with metal-mediated aerobic oxida-
tions, it is a major challenge to unravel the catalystꢀs role
along oxygen-transfer pathways. Therefore, in the realm of
POM-based oxygenase mimics, the adherence to bio-in-
spired mechanistic features is often a matter of debate and
[19]
dented.
[15,16]
retains a fundamental interest.
Among the class of Fe-
substituted polyoxotungstates some structural types are
known, where the coordination geometry of the iron center
[17]
exhibits a striking oxygenase synzyme motif (Figure S1).
In particular, the tetrasubstituted Krebs-type polyanions
nÀ
with general formula [Fe4
A
H
U
G
R
N
U
G
A
H
N
T
E
N
N
2
10
9
33
2
4
2
18
Figure 1. Structure of a Krebs-type Fe-substituted polyoxotungstate (left)
and of the active site coordination geometry of protocatechuate-3,4-dioxy-
genase (right).
[
a] Dr. B. S. Bassil, Dr. M. H. Dickman, Prof. U. Kortz
School of Engineering and Science
Jacobs University P.O. Box 750 561, 28725 Bremen (Germany)
Fax : (+49)421-200-3229
E-mail: u.kortz@jacobs-university.de
Herein we report combined and convergent structural,
spectroscopic and mechanistic evidence on the bio-inspired
catalysis by Krebs-type Fe X W polytungstates in promot-
[
b] Dr. A. Sartorel, Dr. M. Carraro, Prof. G. Scorrano, Dr. M. Bonchio
ITM-CNR and Department of Chemical Science
University of Padova, via Marzolo 1, 35131 Padova (Italy)
Fax : (+39)0498275239
4
2
18
ing the cleavage of catechols. The biomimetic function of
the POM-based catalyst is assessed by spectroscopic and
structure–reactivity relationships. According to the generally
accepted hypothesis (Scheme 1), the initial steps of the en-
zymatic mechanism involve catechol deprotonation and its
E-mail: marcella.bonchio@unipd.it
[
c] Dr. B. Keita, Prof. L. Nadjo
CNRS, Equipe d’Electrochimie et Photoꢁlectrochimie
Universitꢁ Paris-Sud, Bꢂtiment 350, 91405 Orsay Cedex (France)
[
8,20]
III
chelation to the Fe center (a–b in Scheme 1).
The Fe –
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901392.
II
catecholate intermediate displays a Fe –semiquinonate char-
7854
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 7854 – 7858

COMMUNICATION
acter vis-ꢄ-vis ligand-to-metal charge transfer interactions (b
and c in Scheme 1). This is the turning point of catalysis,
In all cases, the formation of a POM–catecholate complex
can be monitored by visible spectroscopy, through an in-
crease of the broad absorption bands in the region 570–
750 nm (Table 1). Titration profiles with binding saturation
are generally observed. Noteworthy, the solvent effect (sol-
vatochromism) on such visible lmax follows the expected
trend observed for LMCT transitions, whereby less polar,
aprotic solvents give rise to more pronounced red-shifted
being responsible for substrate activation towards O elec-
2
trophilic attack (d in Scheme 1). Then, cleavage products
are generated from the rearrangement and hydrolysis of a
transient alkylperoxo intermediate (e–g in Scheme 1). Mile-
stone results supporting this scenario are provided by mech-
III
anistic studies involving mononuclear Fe complexes and
[23]
polydentate ligands mimicking the coordination geometry,
bands (Table 1). Moreover, the inorganic POM ligand re-
[8,21]
III
spectroscopy and function of the natural enzyme.
3,5-Di-
markably influences the Fe –catechol interaction, resulting
[24,25]
tert-butylcatechol (DTBC) is generally used as the substrate
analogue, due to its favorable electron-donating character
in diverse lmax values (Table 1).
The energies of the cat-
III
echolate-to-Fe charge-transfer transitions can therefore be
tuned by the POM composition. As pointed out above,
these bands provide a sensitive probe for the Lewis acidity
of the iron center, depending on the donor properties of the
ligand set within its coordination sphere. A key aspect is to
address the electronic character of the POM moiety by com-
paring the new observations with the well established data-
base available for classical polydentate organic ligands. In
particular, a most informative study has been realized with
and stability of the related cleavage products. The biomi-
III
ACHTUNGTRENNUNGm etic Fe –catecholate intermediates have shown some key
spectroscopic features, specifically: i) two catecholate-to-
III
Fe charge transfer bands in the UV/Vis-NIR spectra; ii)
the dependence of the complexꢀs reactivity on the energy of
such LMCT transitions, as a function of the Lewis acidity of
the iron center.
tetradentate tripod ligands, of general formula N
ACHTUNGTRENNUNG( CH X) ,
2 3
where X is a phenolate, carboxylate, pyridine or benzimida-
[22,26,27]
zole functionality (Scheme 2).
Table 1 sheds light on
the ligand effect induced by the POM framework. The
Lewis acidity of the Fe substituent is not substantially
quenched by the overall negative charge of the polyanion.
The latter (À4 for Fe Se W and Fe Te W , and À6 for
4
2
18
4
2
18
Fe As W and Fe Sb W ) is largely associated with the
4
2
18
4
2
18
XO hetero groups with some delocalization over the tung-
3
sten–oxo framework. Indeed, for Fe X W , in EtOH as sol-
4
2
18
vent, the LMCT band falls at lmax > 700, that is, at lower
energy compared to a tricarboxylate-based ligand while ap-
proaching a mixed ligand set, incorporating two carboxy-
Scheme 1. Biomimetic mechanism for intra-diol cleavage of catechol by
Fe complexes. Spectator ligands on the iron centre are omitted for clari-
ty.
III
[22,26]
lates and one pendant pyridine (Scheme 2).
Indeed, red-shifted LMCT transition maxima are associat-
ed with a favorable donor–acceptor interaction, thus en-
hancing the semiquinone character of the bound catecho-
[22]
late, and its reactivity with dioxygen. Following these ar-
guments, the biomimetic potential of the Fe X W com-
4
2
18
plexes has been initially screened for DTBC binding, in a
weakly coordinating solvent such as 1,2-dichloroethane
(
(
DCE), and in protic media such as wet THF and ethanol
Table 1).
4 2
Table 1. UV/Vis characterization of Fe X W18/DBTC complexes at 258C.
Scheme 2. Comparison of the inorganic POM ligand (middle) with organ-
ic tetradentate, tripod-type N(CH X) ligands (left and right), influencing
the spectroscopic properties of Fe–catecholate complexes.
À1
À1
FeÀPOM
l
max [nm] (e [m cm ])
A
H
U
G
R
N
U
G
2
3
A( THF/H O)
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
A
H
U
G
R
N
N
(DCE)
ACHTUNGTRNENUNG( EtOH)
Fe
Fe
Fe
Fe
4
4
4
4
Te
2
W
18 632, (sh, 450), 698
520)
18 594 (490), 703 (440)
618 (sh, 1600) 743
(2700)
624 (sh, 2600) 729
715
(>2000)
715
(>2000)
726
(>3000)
714
(
Sb
2
W
It is noteworthy that the LMCT band red-shifts signifi-
(
3400)
623 (sh, 2100) 714
2400)
626 (sh 1800) 706
1900)
As
2
W
18 665 (550)
cantly in the sequence Fe Se W
4
2
18
<
Fe As W
18
4
2
<
(
Fe Sb W < Fe Te W .
4 2 18 4 2 18
Direct evidence of the substrate-binding mode is provided
by the solid-state structures of discrete tetra-oxalato deriva-
2
Se W18 574 (176)
(
(>1000)
Chem. Eur. J. 2009, 15, 7854 – 7858
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7855

M. Bonchio, U. Kortz et al.
tives of Fe X W (see Figure 2). We have prepared three
Table 2. Aerobic oxidation of DTBC catalyzed by Fe
4 2 18
X W .
4
2
18
[
a]
oxalato
XW O ) ]
derivatives,
namely
[Fe
A
T
N
T
E
N
G
A
H
U
T
E
N
N
Entry Catalyst [mm] Solvent Conv. [%] Cleavage [%]
2/3 ratio
4
III
2
4
III
4
2
1
4À
III
(Ox -Fe X W ; X=As , Sb , Bi ) by reac-
[b]
[b]
[b]
9
33
2
4
4
III
2
18
1
2
3
4
5
6
7
8
9
Fe As W
DCE
DCE
DCE
DCE
THF/H
THF/H O 91
THF/H O 93
THF/H
THF/H
93
91
75
88
95
<2
<2
<2
<2
19
20
9
31
43
–
–
–
–
0.2
0.28
0.12
2.1
6.5
4
4
4
4
4
2
18
III
III
tion of Fe X W (As , Sb , Bi ) with Na C O in aqueous
Fe
Fe
Fe
Fe
Sb
2
W
18
4
2
18
2
[28]
Se
Te
2
W
W
18
acidic medium at pH 4. Recently Dolbecq et al. reported
on the structure of the oxalate–antimony derivative Ox -
[
b]
2
18
4
[c]
[c]
As
2
W
18
18
18
18
18
2
O
[
29]
Fe Sb W .
Oxalate can be considered as an unreactive
4
2
18
Fe Sb W
4
2
2
III
[
c]
[c]
c,d]
analogue of DTBC, coordinating the embedded Fe centers
Fe Se W
4
4
4
2
2
2
2
[29]
in a bidentate fashion via two oxygens. Interestingly, the
Fe–Ooxalate distances resemble those found for some repre-
sentative Fe–catecholate complexes, in both abiotic and en-
zymatic systems (Table S4).
Fe
Fe
Te
Te
W
W
2
O
O
91
35
[
2
[a] % cleavage selectivity determined by total cleavage products (2+3)/
substrate conversion; [b] reaction performed with catalyst (8.0 mm), 3,5-
DTBC (200 mm) at 608C for 3 h, P(O
action performed with catalyst (8.0 mm), 3,5-DTBC (60 mm) at 258C in
THF/H O (98:2) in the presence of BHT (0.2m) for 68 h, P(O )=1 atm,
turnovers; [d] reaction performed with 2.5 mm catalyst in the presence
2
)=1 atm, 19–23 turnovers; [c] re-
2
2
7
of 1m BHT, 8 turnovers.
Figure 2. Left: Combined ball-and-stick/polyhedral view representative
1
4À
III
III
for [Fe
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(C
2
O
4
)
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(H
2
O)
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(b-XW
9
O
33
)
2
]
(Ox
4
ÀFe
4 2 18
X W ; X=As , Sb ,
III
Bi ). Right: Inner coordination sphere of the two “external” Fe centers.
Of the three terminal, labile water ligands two have been replaced by a
bidentate oxalate ligand. The three remaining oxygen atoms link to the
tungsten-oxo fragment of the POM.
Scheme 3. Competing autoxidation and cleavage pathways in the aerobic
oxidation of DTBC catalyzed by Fe X W .
4 2 18
III
The voltammetric analysis of Ox -Fe X W (X=As ,
4
4
2
18
III
III
Sb ) in aqueous media shows a reduction of the Fe cen-
ters with a 100 mV negative peak potential shift compared
to the parent Fe X W . This is expected, considering the
In an attempt to improve the cleavage selectivity, catalysis
[30]
has also been explored in other media. The autoxidation
[18]
process turns out to be faster in CH CN and DCE, while it
4
2
18
3
significant increase in negative charge upon oxalate binding.
Importantly, the reduction of Ox -Fe Sb W (two closely
is sensibly slowed down in acetone or in wet THF, in agree-
ment with previous evidence (Figure S21).
[31]
4
4
2
18
spaced waves at À0.280 V and À0.424 V vs SCE, pH 5)
occurs at a less negative potential with respect to the As
analogous Ox -Fe As W (a single composite four-electron
Adding to these observations, we also investigated the ox-
idative screening in wet THF and in the presence of 2,6-di-
tert-butylcresol (BHT), as radical scavenger to inhibit free-
radical oxidation and polymerization pathways (entries 5–9
in Table 2). Indeed, a favorable quinone abatement (<6%)
and a parallel enhancement of the cleavage selectivity up
to>40% is obtained (entry 9 in Table 2). In all cases, cata-
lytic cleavage occurs yielding both products 2 and 3, deriv-
ing from an intra- and extra-diol cleavage mechanism, re-
4
4
2
18
wave at À0.456 V vs SCE, pH 5), in line with the spectro-
scopic results discussed above (Figure S18).
The ensemble of structural/spectroscopic/redox evidence
allows us to establish a reactivity scenario with a valuable
potential in the aerobic oxidation of catechols. Oxidation of
III
III
IV
IV
DTBC by Fe X W (X=As , Sb , Se , Te ) has been ex-
4
2
18
[32]
amined in DCE at 1 atm O pressure and 608C. Under the
spectively. The Krebs-type structures Fe X W appear to
4 2 18
2
condition explored, 80–90% substrate conversion occurs in
be ideally suited for catechol cleavage catalysis. Other types
of Fe-containing POMs such as the Keggin derivatives [Fe-
3
h (entries 1–5 in Table 2). In all cases, the two-electron ox-
5À
6À
idation product 3,5-di-tert-butyl-1,2-benzoquinone (1) is
formed in 50–60% yield, along with a mixture of polymeric
tars.
Such product distribution indicates that the autoxidation
pathway dominates over an almost silent cleavage routine
ACHTUNGTNERNUNG( H O)(a-SiW O )] and [g-Fe ACHTUNTGERNUN(G H O) SiW O ] are inac-
2 11 39 2 2 2
10 38
tive, although they coordinate DTBC (Figures S2, S3, S8,
S9). This result is consistent with the key requirements of
iron-mediated cleavage of catechols (Scheme 1). Only the
Krebs-type POMs feature external Fe centers easily accessi-
ble for both catechol and dioxygen binding, thus allowing a
possible mechanistic pathway along bio-inspired guide-
(
Scheme 3). Indeed, of the typical intra- or extra-diol cleav-
age products: 3,5-di-tert-butyl-muconic acid anhydride (2) or
,6-di-tert-butyl-1-oxacyclohepta-4,6-diene-2,3-dione (3) , re-
spectively, the former is detected only in traces (<2%).
[33]
4
lines.
The isostructural Krebs-type catalysts Fe X W
4 2 18
offer the unique opportunity to correlate the exhibited
7856
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ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 7854 – 7858

Inorganic Synzymes
COMMUNICATION
cleavage selectivity and the Fe-catechol LMCT interaction
Keywords: catechol cleavage · homogeneous catalysis ·
iron · polyoxometalates · synzymes
(
see above). Inspection of entries 7–10 in Table 2 shows that
the cleavage performance increases in the order Fe Se W
4
2
18
<
Fe As W ꢀ Fe Sb W < Fe Te W indicating that the
4
2
18
4
2
18
4
2
18
hetero atom type is crucial. The reactivity trend for DTBC
cleavage selectivity of the Fe X W catalysts is inversely
4
2
18
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18
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products (Scheme 1), in agreement with the enzymatic para-
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[
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4 2
X W18 catalysts as a function
of their LMCT energies in THF/H O 98:2 (see also Table 2).
2
9
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Acknowledgements
We thank Ciprian Padurariu for assistance in polyanion synthesis and
Francesco Vivan for preliminary catalytic experiments. Financial support
from CNR, MIUR (FIRB CAMERE-RBNE03 JCR5), and the ESF
COST D40 action are gratefully acknowledged. UK thanks Jacobs Uni-
versity and the Fonds der Chemischen Industrie for research support.
[22] H. G. Jang, D. D. Cox, L. Que, Jr., J. Am. Chem. Soc. 1991, 113,
9200–9204.
[23] D. D. Cox, S. J. Benkovic, L. M. Bloom, F. C. Bradley, M. J. Nelson,
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Chem. Eur. J. 2009, 15, 7854 – 7858
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7857

M. Bonchio, U. Kortz et al.
[
[
24] The stability of the polyanions upon DTBC addition or adventitious
was confirmed by FT-IR (see the Supporting Information).
cal CO
2
. These experiments have been performed with Fe
4 2 18
As W ,
H
2
O
2
isolated as a THA, TBA or PPh
offers an optimal synthetic strategy for tuning the solubility of the
POM catalyst (Table S5 in the Supporting Information).
4
salt by cation metathesis, which
The same complexes have been found highly robust under harsher
autoxidation conditions with microwave assisted protocols see M.
Bonchio, M. Carraro, G. Scorrano, U. Kortz, Adv. Synth. Catal.
2
[31] THF/H O 98:2 was reported to be an effective solvent for this reac-
2
005, 347, 1909–1912.
tion, see: T. Funabiki, I. Yoneda, M. Ishikawa, M. Ujiie, Y. Nagai, S.
Yoshida, J. Chem. Soc. Chem. Commun. 1994, 1453.
[32] GC-MS and H NMR exclude the formation of five-member ring 2-
25] A blue-shift of the lmax values is observed upon coordination of less
1
electron donating catechols due to less favorable LMCT transitions:
indeed lmax values of Fe
and 4-chlorocatechol in DCE are at 729 nm (shoulder at 652) and
71 nm (shoulder at 622 nm), respectively.
4
Te
2
W
18 upon addition of 4-tert-butylcatechol
pyrones. A non-regioselective cleavage is generally observed in
abiotic systems. Computational arguments point at a subtle interplay
of factors that might modulate the oxidative cleavage preference.
See reference [21].
6
[
[
26] D. D. Cox, L. Que, Jr., J. Am. Chem. Soc. 1988, 110, 8085–8092.
27] a) M. Pascaly, M. Duda, F. Schweppe, K. Zurlinden, F. K. Muller, B.
Krebs, J. Chem. Soc. Dalton Trans. 2001, 828–837; b) P. Mialane, L.
Tchertanov, F. Banse, J. Sainton, J.-J. Girerd, Inorg. Chem. 2000, 39,
[33] Control experiments with iron-free derivatives allow to establish
that the observed reactivity is ascribable to the POM embedded
4
iron centers Specifically, isostructural complexes of the type [b-M -
nÀ
III
II
II
II
II
II
2
440–2444.
A
H
U
G
R
N
U
G
2
O)10
A
H
U
G
E
N
N
(XW
9
O
33
)
2
]
where M is Cr , Mn , Co , Ni , Cu , and Cd
[
28] Synthesis and crystal data of Ox -Fe X W18 are reported in the Sup-
4 4 2
porting Information. CCDC 724754, 724755, and 724756 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[34] L. Que, Jr., R. C. Kolanczyk, L. S. White, J. Am. Chem. Soc. 1987,
109, 5373–5380.
[35] The pioneering work on transition-metal supported POMs is inspir-
ing as this POM structural type displays cis-coordination sites, coor-
dinative unsaturation, interesting reactivity and selectivity, see refer-
ence [19a] and H. Weiner, Y. Hayashi, R. G. Finke, Inorg. Chem.
1999, 38, 2579–2591.
[
[
29] A. Dolbecq, J.-D. Compain, P. Mialane, J. Marrot, E. Riviꢆre, F. Sꢁ-
cheresse, Inorg. Chem. 2008, 47, 3371–3378.
30] Reactivity screening has also been extended in the following sol-
vents: DMF, CH
3
CN, acetone, chlorobenzene, toluene, and in mixed
CN 1:2, DCE/tert-butanol 1:1,
media: CH CN/pyridine 1:8, THF/CH
3
3
Received: May 25, 2009
CH
3
CN/tert-butanol 1:1, as well as the non-conventional supercriti-
Published online: July 8, 2009
7858
www.chemeurj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 7854 – 7858