Reactive ZrIV and HfIV butterfly peroxides on polyoxometalate surfaces: Bridging the gap between homogeneous and heterogeneous catalysis

Article abstract of DOI:10.1002/chem.201003103

At variance with previously known coordination compounds, the polyoxometalate (POM)-embedded Zr^IV and Hf^IV peroxides with formula: [M₂(O₂)₂(α-XW ₁₁O₃₉)₂]^12- (M=Zr^IV, X=Si (1), Ge (2); M=Hf^IV, X=Si (3)) and [M₆(O ₂)₆(OH)₆(γ-SiW₁₀O ₃₆)₃]^18- (M=Zr^IV (4) or Hf ^IV (5)) are capable of oxygen transfer to suitable acceptors including sulfides and sulfoxides in water. Combined ¹HNMR and electrochemical studies allow monitoring of the reaction under both stoichiometric and catalytic conditions. The reactivity of peroxo-POMs 1-5 is compared on the basis of substrate conversion and kinetic. The results show that the reactivity of POMs 1-3 outperforms that of the trimeric derivatives 4 and 5 by two orders of magnitude. Reversible peroxidation of 1-3 occurs by H ₂O₂ addition to the spent catalysts, restoring oxidation rates and performance of the pristine system. The stability of 1-3 under catalytic regime has been confirmed by FT-IR, UV/Vis, and resonance Raman spectroscopy. The reaction scope has been extended to alcohols, leading to the corresponding carbonyl compounds with yields up to 99 % under microwave (MW) irradiation. DFT calculations revealed that polyanions 1-3 have high-energy peroxo HOMOs, and a remarkable electron density localized on the peroxo sites as indicated by the calculated map of the electrostatic potential (MEP). This evidence suggests that the overall description of the oxygen-transfer mechanism should include possible protonation equilibria in water, favored for peroxo-POMs 1-3. Copyright

Full text of DOI:10.1002/chem.201003103

FULL PAPER
DOI: 10.1002/chem.201003103
IV
IV
Reactive Zr and Hf Butterfly Peroxides on Polyoxometalate Surfaces:
Bridging the Gap between Homogeneous and Heterogeneous Catalysis
[
a]
[b]
[c]
[a]
Mauro Carraro,* Nadeen Nsouli, Holger Oelrich, Andrea Sartorel,
[
a]
[b]
[a]
[c]
Antonio Sorarꢀ, Sib Sankar Mal, Gianfranco Scorrano, Lorenz Walder,*
[
b]
[a]
Ulrich Kortz,* and Marcella Bonchio*
Abstract: At variance with previously
known coordination compounds, the
kinetic. The results show that the reac-
tivity of POMs 1–3 outperforms that of
the trimeric derivatives 4 and 5 by two
orders of magnitude. Reversible perox-
idation of 1–3 occurs by H O addition
to the spent catalysts, restoring oxida-
tion rates and performance of the pris-
tine system. The stability of 1–3 under
catalytic regime has been confirmed by
FT-IR, UV/Vis, and resonance Raman
spectroscopy. The reaction scope has
been extended to alcohols, leading to
the corresponding carbonyl compounds
with yields up to 99% under micro-
wave (MW) irradiation. DFT calcula-
tions revealed that polyanions 1–3 have
high-energy peroxo HOMOs, and a re-
markable electron density localized on
the peroxo sites as indicated by the cal-
culated map of the electrostatic poten-
tial (MEP). This evidence suggests that
the overall description of the oxygen-
transfer mechanism should include pos-
sible protonation equilibria in water,
favored for peroxo-POMs 1–3.
polyoxometalate
Zr and Hf peroxides with formula:
(POM)-embedded
IV
IV
1
2À
IV
[
M (O ) (a-XW O ) ]
(M=Zr ,
2
2
2
11 39
2
2
2
IV
X=Si (1), Ge (2); M=Hf , X=Si (3))
1
8À
and
[M (O ) (OH) (g-SiW O ) ]
6 2 6 6 10 36 3
IV
IV
(
M=Zr (4) or Hf (5)) are capable
of oxygen transfer to suitable acceptors
including sulfides and sulfoxides in
1
water. Combined H NMR and electro-
chemical studies allow monitoring of
the reaction under both stoichiometric
and catalytic conditions. The reactivity
of peroxo-POMs 1–5 is compared on
the basis of substrate conversion and
Keywords: hafnium · homogeneous
catalysis · oxidation · peroxides ·
polyoxometalates · zirconium
Introduction
tion with H O , in the presence of nitrogen–oxygen/sulphur
2
2
[4,5]
IV
donor ligands.
Of major importance is the access to Zr
Though solid metal oxides, including zirconia-based cata-
peroxides by ligand-mediated O activation, as demonstrat-
ed by the work of Abu-Omar et al., in which an h -bisper-
2
[1]
2
lysts, are considered interesting in oxidation catalysis, lim-
ited knowledge is available on the structural and electronic
properties of surface-anchored peroxo sites that have been
[
6]
oxo complex containing bidentate diimines was obtained.
IV
However, the so-far characterized Zr –peroxo complexes,
featuring multidentate organic ligands, are catalytically inert
towards the oxidation of organic substrates, such as phos-
[
2,3]
postulated as competent oxidants.
In a few cases, stable
IV
monoperoxo complexes of Zr have been isolated by reac-
[4,5]
IV
phines, alkenes, and alcohols.
Hf analogues are even
less documented. An interesting discovery is the structural
IV
[
a] Dr. M. Carraro, Dr. A. Sartorel, A. Sorarꢀ, Prof. G. Scorrano,
Dr. M. Bonchio
Department of Chemical Sciences
ITM-CNR and University of Padova
Via Marzolo 1, 35131 Padova (Italy)
E-mail: mauro.carraro@unipd.it
characterization of peroxo-bridged Hf tetraarylporphyrina-
2À
to dimers, displaying two O ligands with a rather unique,
2
2
2
[7]
side-on bis-m-h :h arrangement.
IV
IV
The introduction of Zr and Hf ions within the frame-
work of polyoxometalates (POMs) of W and Mo has
also been reported, in which M-(m-O)-M (M=Zr , Hf )
VI
VI
IV
IV
marcella.bonchio@unipd.it
[8]
[
b] Dr. N. Nsouli, Dr. S. S. Mal, Prof. U. Kortz
School of Engineering and Science
Jacobs University
junctions promote the formation of oligomeric structures.
IV
IV
Due to the large size of Zr and Hf ions, coordination
numbers up to seven can be accessed, including one or more
terminal water molecules. This structural feature is expected
to boost catalysis by providing accessible and adjacent sites
for ligand exchange, becoming available for simultaneous
P.O. Box 750 561, 28725, Bremen (Germany)
E-mail: u.kortz@jacobs-university.de
[
c] Dr. H. Oelrich, Prof. L. Walder
Institute of Chemistry, University of Osnabrꢁck
Barbarastrasse 7, 49069 Osnabrꢁck (Germany)
E-mail: LoWalder@uos.de
[9]
substrate and peroxide activation at the metal center. In
this chemistry, the POM framework can offer some key im-
provements being a totally inorganic, highly robust, and
electron-withdrawing environment, with structural and coor-
dination properties modeling a discrete fragment of solid
Supporting Information for this article (kinetic data obtained from
1
H NMR and electrochemical data; Cartesian coordinates corre-
sponding to the optimized geometries of 1–5) is available on the
WWW under http://dx.doi.org/10.1002/chem.201003103.
Chem. Eur. J. 2011, 17, 8371 – 8378
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8371

[
10]
18À
3
metal oxides. In this respect, Zr and Ti-based POMs have
been suggested as molecular models for the well-known het-
erogeneous titanium silicalite catalyst, TS-1, with major in-
dustrial appeal in the field of H O activation and selective
[M (O ) (OH) (g-SiW O ) ]
(M=Zr (4) or Hf (5)) dis-
6
2
6
6
10 36
playing a cyclic 6-peroxo-6-metal core stabilized by three
decatungstosilicate units (Figure 1).
[16]
We present herein a combined kinetic, electrochemical,
and computational study to investigate the potential of Zr/
Hf butterfly peroxides for oxygen transfer in water, under
stoichiometric and catalytic regimes. Our results highlight
the role of the POM ligand powering the reactivity of the
Zr/Hf peroxides, in contrast with the generally found inert-
ness of related classical coordination compounds. The results
presented herein point to structure–reactivity correlations in
the realm of functional metal oxides. These are instrumental
for the rational design and synthesis of innovative ligands/
catalysts taken from the POM pool and their further up-
2
2
[
8i,11]
oxidation.
ed POMs can mimic the active site geometry of natural oxy-
On the other hand, transition-metal-substitut-
[
9,12–14]
genases.
Indeed, natureꢃs strategy to face some formi-
dable catalytic challenges is the stabilization of adjacent
transition-metal centers through multiple-m-hydroxo/oxo
bridging units, that is, the basic structural motif of the POM
molecular connectivity.
We have recently reported the structural characterization
and preliminary catalytic studies of novel heteropolytung-
states incorporating Zr and Hf peroxides. These are
unique examples of bimetallic peroxides, having so-called
butterfly-type cores with a mono- or bis-m-h :h arrangement
IV
IV
grade to molecular materials. ACHUTNGRENNUG[ 17,18]
2
2
of the peroxo ligand for each Zr/Hf center embedded within
[
15,16]
the POM scaffold (Figure 1).
Such a structural feature is shared by three isostructural
Results and Discussion
1
2À
dimers, [{M(O )(a-XW O )} ]
(M=Zr, X=Si (1), Ge
The peroxo functions of complexes 1–5 are readily reduced
by iodide in water, and they can be thus quantitatively titrat-
ed by a standard thiosulfate solution. Reductive expulsion
of the peroxo functions is also observed by cyclic voltamme-
2
11 39
15]
2
[
(
2); M=Hf, X=Si (3)),
and by the trimeric derivatives
À
try (two e per peroxo function). This fact allows to deter-
mine the ratio of peroxo POM to peroxo-free POM in a
simple double scan on the reductive wave appearing at ꢀ
À0.8 V (vs. Ag/AgCl, see Figures S1–S5 in the Supporting
[15,19]
Information).
The reactivity of POMs 1–5 has been as-
sessed for oxygen transfer to l-methionine (S) in aqueous
solution. This benchmark transformation occurs through the
mono- and bis-oxygenation of the sulfur atom, yielding the
sulfoxide (SO) and sulfone (SO ) products, respectively
2
(
Figure 1). Such a consecutive two-step process has been
1
monitored with complementary H NMR and fast electro-
chemical scanning techniques, by following the substrate/
product time-evolution or the fading of the peroxo-POM
concentration, respectively, upon oxygen transfer. In partic-
1
ular, H NMR analysis provides direct evidence of the oxi-
dation yield and product distribution as diagnostic signals
are found at 2.1, 2.6, and 3.2 ppm attributed to the S, SO
and SO terminal methyl groups, respectively (Figure S6 in
2
the Supporting Information). On the other hand, voltam-
metric analysis, also implemented with rotating disk setup at
a fixed potential (E=À0.85 V), adds a convergent scenario
indicating the progressive quenching of the peroxo function-
ality upon stepwise addition of l-methionine (Figure 2 and
[20]
Figures S7–S12 in the Supporting Information).
This combined screening strategy allows us to match the
diverse kinetic domains of the oxidation experiments, span-
ning from a few seconds to hundreds of minutes, depending
on the reactivity behavior of the complexes under examina-
tion. In particular, fast kinetics that could not be successfully
1
accomplished by means of conventional H NMR methods
Figure 1. Combined
{M(O )(a-XW11
3)), and of [M (O
reaction scheme represents the oxygen transfer to l-methionine (S) to
give dl-methionine sulfoxide (SO) and methionine sulfone (SO ).
polyhedral/ball-and-stick
representation
of
have been addressed by rotating disk voltammetry (vide
infra and Table 1). The reactivity of peroxo-POMs 1–5 is
compared in Table 1 on the basis of substrate conversion
and kinetic data. In such a two-step process, the selectivity
1
2À
[
(
2
O
39)}
2
]
(M=Zr, X=Si (1), Ge (2); M=Hf, X=Si
1
8À
6
2
)
6
(OH)
6
(g-SiW10
O
36
)
3
]
(M=Zr (4) or Hf (5)). The
2
8372
www.chemeurj.org
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8371 – 8378

IV
IV
Zr and Hf Butterfly Peroxides on Polyoxometalate Surfaces
FULL PAPER
complex 1 (see Experimental Section). Concerning the
oxygen-transfer rate, POMs 1–3 turn out to outmatch the
trimeric derivatives 4 and 5 by two orders of magnitude (en-
tries 1–3 vs. 4 and 5 in Table 1). While the nature of both
the Hf or Zr center and POM ligand plays a minor role, a
major impact on reactivity stems from the coordination
mode and structural environment of the Zr/Hf peroxides. In
2
2
this regard, the bis-m-h :h peroxides 1–3, in which each Zr/
Hf center is connected by four M-O-W bonds to the POM
scaffold, are powerful electrophiles, and hence more reactive
sulfoxidation reagents than the related trimers. These trim-
ers have a looser connectivity to the Group IV heterometals
with just two M-O-W units per each Hf/Zr center.
2
2
The superior reactivity of the bis-m-h :h -peroxides 1–3 is
also apparent in the second oxidation, converting SO to
SO . Indeed, overoxidation to SO can be achieved with
À4
Figure 2. Rotating disk voltammetry of 1 (n=3.86 mmol, 6.43ꢄ10 m)
2
2
during substoichiometric additions of l-methionine (1 mmol portions) in
POMs 1–3 (entries 6, 8, 10 in Table 1), while it is negligible
with complexes 4 and 5. Polyanions 1–3 behave as electro-
philic oxidants, displaying an enhanced reactivity towards
the electron-rich thioether with respect to the sulfoxide
0
0
.5m HAc/KAc buffer (pH 4.7) at a glassy carbon electrode (A=
.07 cm ) and a fixed potential (E=À0.85 V). Inset: kinetic trace of per-
2
oxide consumption derived from the first step of the current/time plot.
[21]
moiety. In this respect, a reactivity ratio of S/SO>100 can
be calculated on the basis of the oxidation rates determined
for all complexes (entries 1–3 and 6, 8, 10 in Table 1). This is
also consistent with the sharp selectivity favoring the first
step of the oxidation, that is, the sulfoxide production (en-
Table 1. Oxidation of l-methionine (S) and/or dl-methionine oxide (SO)
by peroxo-POMs 1–5 in aqueous solution.
[
a]
[
b]
[c]
Sub
POM
Time [min]
Conv. [%]
Initial rate, R
0
À1 À1 [e]
[molL
s
]
[
[
[
[
[
[
d]
d]
d]
d]
d]
g]
À3[f]
À3[f]
À3[f]
À5
1
2
3
4
5
6
7
8
9
0
1
S
S
S
S
1 (Zr
2 (Zr
3 (Hf
4 (Zr
5 (Hf
1 (Zr
1 (Zr
2 (Zr
2 (Zr
2
2
Si)
Ge)
<1
<1
<1
240
50
90
90
120
120
60
>99
>99
>99
97
99
94
94
83
87
88
2.7ꢄ10
1.8ꢄ10
3.1ꢄ10
1.0ꢄ10
3.9ꢄ10
1.1ꢄ10
1.0ꢄ10
6.8ꢄ10
6.6ꢄ10
1.4ꢄ10
1.3ꢄ10
[21b]
tries 1–3 in Table 1).
Oxygen transfer to SO by POMs 1–
2
Si)
Si)
3 has been further investigated monitoring the time profile
of simultaneous SO conversion and peroxide consumption
6
À5
S
6
Si)
1
by H NMR and voltammetric analysis, respectively (Fig-
À5[i]
À5
SO
SO
SO
SO
SO
SO
2
Si)
Si)
Ge)
Ge)
[
[
[
h]
h]
h]
[g]
ure S13–S26 in the Supporting Information). Upon compari-
son of the oxidation rates observed for 1–3, the tungstosili-
cate ligand performs slightly better than the germanate ana-
logue, while the Hf and Zr species behave similarly (en-
tries 6, 8, 10 in Table 1). To get further mechanistic insights,
a set of reactions with variable concentration of 3 has been
2
2
2
[
g]
À6[i]
À6
[g]
[
g]
À5[i]
À5
1
1
3 (Hf
3 (Hf
2
Si)
Si)
[g]
2
60
96
[
a] In all reactions stoichiometric substrate (Sub) and peroxide content
act) have been established vis-ꢅ-vis the dimeric or trimeric structure of
POMs 1–5 ([Sub]=[Oact]=7.27 mm), in D O (500 mL), at 288C. [b] See
(
O
1
carried out and monitored by H NMR spectroscopy
2
Figure 1 for POM abbreviations. [c] % Substrate conversion. [d] Selective
(Figure 3).
1
SO formation as determined by H NMR spectroscopy. [e] Initial oxida-
1
tion rate, determined for <15% conversion by H NMR spectroscopy.
[
f] Initial oxidation rate, determined by rotating disk voltammetry (see
Experimental Section). [g] Selective SO formation as determined by
H NMR spectroscopy. [h] Recharge experiments, with further addition
of SO and H to restore their intial amounts (3.63 mmol). [i] Fitting of
CV data provide R
–3, respectively, (see Experimental Section).
2
1
2
O
2
À5
À5
À5
À1 À1
0
=1.8ꢄ10 , 1.0ꢄ10 , 1.2ꢄ10 moll
s
for POMs
1
of the oxidation depends on the relative reactivity of the Zr/
Hf peroxides towards the sulfide and the sulfoxide function-
[21,22]
al groups.
Under stoichiometric conditions, complexes
1–5 all display a remarkable activity towards S oxidation,
yielding quantitative oxygen transfer to the electron-rich
sulfur atom (entries 1–5 in Table 1), to give SO as the only
product (see Experimental Section for the detailed proce-
dure). In particular, sulfoxidation by polyanions 1–3 is com-
plete within seconds. In this time-frame, rotating disk vol-
tammetry was used to gain further evidence by kinetic fit-
ting of the reaction transients, as depicted in Figure 2 for
Figure 3. Kinetic traces obtained during oxidation of SO (7.27 mm) with
3
, in the concentration range (0.958–15.3 mm; top to bottom). Inset: de-
pendence of the initial oxidation rates (R ) on the concentration of 3.
0
Chem. Eur. J. 2011, 17, 8371 – 8378
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8373

M. Bonchio, U. Kortz, L. Walder et al.
Increasing the concentration of the POM leads to rate sat-
uration (inset in Figure 3), which is generally indicative of
association pre-equilibria. Indeed, as the reaction pH is
close to the isoelectric point of methionine (pI=5.74), an
electrostatic interaction between the anionic POM and the
protonated amino group of the substrate can be envisaged.
Complementary charge interactions are known to affect the
reactivity and selectivity of POM reagents both in aqueous
performed on recharging the spent reaction mixtures with a
supplementary and stoichiometric amount of both H O and
substrate (entries 7, 9, and 11 in Table 1). The oxidation ac-
tivity was immediately restored, with analogous conversion,
selectivity, and kinetics behavior for all the three systems
under investigation. Superimposable time-evolution profiles
2
2
for SO conversion and parallel SO formation can indeed be
2
observed (Figure 5 and Table 1 entries 7, 9, and 11). This
finding points to the selective addition of H O to the Zr/Hf
[23]
solution and in organic solvents.
2
2
After oxygen transfer, the stability of the spent peroxo-
POMs has been confirmed by FT-IR, UV/Vis, and reso-
nance Raman analysis. Superimposable spectra are shown
centers within the POM framework.
[24]
in Figure 4 for the representative case of POM 3 (see also
Figure 5. Oxidation of dl-methionine sulfoxide (SO) by 1 and recharged
experiment with additional H and substrate. In the first run, SO=
.27 mm, 1=3.63 mm; in the recharge, H =7.27 mm (entries 6 and 7 in
Table 1), in D O (500 mL).
2
O
2
7
2 2
O
2
Catalytic oxidation of S and SO has thus been successfully
achieved in the presence of an excess of H O , yielding up
2
2
to 45 turnovers, 70–99% yield, and a reactivity order 3>1>
2
, in line with the stoichiometric behavior (Table 2 and Fig-
[27,28]
ures S37–S42 in the Supporting Information).
Vice-
versa, spectroscopic analysis of spent 4 and 5 indicates a
substantial modification of the POM features, and H O re-
2
2
charge under catalytic conditions cannot be attributed to a
well-identified reactive system (Figures S35–S36 in the Sup-
[29]
porting Information).
The reaction scope in aqueous solution has been extended
Figure 4. FT-IR (top), UV/Vis (middle), and resonance Raman spectra
(
bottom) of 3 a) before and b) after the reaction with l-methionine. Re-
to alcohol oxidation catalyzed by POMs 1–3, through a mi-
actions performed under the experimental conditions described for
entry 10 in Table 1.
Table 2. Catalytic oxidation of l-methionine (S) or dl-methionine oxide
[
a]
(
SO) by peroxo-POMs 1–3 and H
2
O
2
in aqueous solution.
[
b]
[c]
Sub
POM
Conv [%]
Time [h]
Initial rate, R
0
Figures S27–S29 and S32–S34 in the Supporting Informa-
tion). The structural integrity of POM 1–3 scaffold is gener-
ally confirmed by multiple techniques. The resonance
À1 À1 [d]
[
molL s ]
[
[
e]
e]
À5
1
2
3
4
5
6
S
S
S
SO
SO
SO
1 (Zr
2 (Zr
3 (Hf
1 (Zr Si)
2 (Zr
3 (Hf
2
2
Si)
Ge)
>99
>99
>99
70
80
93
0.25
0.5
0.08
48
48
20
2.9ꢄ10
1.0ꢄ10
5.5ꢄ10
5.8ꢄ10
6.9ꢄ10
1.1ꢄ10
À5
À5
À8
À8
À7
À1
Raman spectrum shows very weak signals at 550–650 cm ,
[e]
f]
[f]
2
Si)
[
the expected region for M(O ) absorption, while the sharp-
2
2
À1
ening of the 980 cm feature, in the region of W=O bands,
2
Ge)
Si)
[
f]
is likely induced by the reversible rearrangement of the
2
[8h,25]
POM scaffold upon peroxide reduction.
[a] In all reactions [Sub]=7.27 mm, [POM]=0.16 mm, [H
in D O (500 mL), at 288C. [b] See Figure 1 for POM abbreviations. [c] %
Substrate conversion. [d] Initial oxidation rate determined for <15%
conversion by H NMR spectroscopy. [e] Selective SO formation as deter-
2 2
O ]=7.27 mm,
2
The maintenance of the POM framework is pivotal for
the upgrade of the system to catalytic conditions, thus gener-
ating the competent peroxo-POMs in situ by addition of
1
1
mined by H NMR spectroscopy. [f] Selective SO
2
formation as deter-
[26]
1
excess H O .
Exploratory experiments have thus been
mined by H NMR spectroscopy.
2
2
8374
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Chem. Eur. J. 2011, 17, 8371 – 8378

IV
IV
Zr and Hf Butterfly Peroxides on Polyoxometalate Surfaces
FULL PAPER
[30]
crowave (MW)-assisted protocol.
The best performance,
in terms of efficiency and catalyst stability has been ach-
ieved with 2, for the transformation of representative cyclic
[31]
and benzylic secondary alcohols (Table 3).
Employing a
Table 3. Catalytic oxidation of alcohols by 2 and H
tion.
2
O
2
in aqueous solu-
Conv
[
a]
Substrate
Product
[
b]
[
%]
1
2
3
4
5
cyclopentanol
cyclohexanol
cycloheptanol
indanol
cyclopentanone
cyclohaxanone
cycloheptanone
indanone
99
99
99
99
80
phenylethanol
phenyl methyl ketone
[
(
a] Alcohol 0.8–1.0 mmol, POM 1.6 mmol, H
500 mL). Irradiation power 70 W, time 50 min, Tbulk 908C, with applied
compressed air. [b] % Conversion calculated by GLC analysis, on the
limiting reagent H
2 2 2
O 0.08–0.1 mmol in H O
2
2
O .
monomodal MW lab-station, with 70 W power irradiation,
selective and complete conversion of H O is obtained in ap-
2
2
[32]
proximately 50 min. Target substrates include the aromat-
ic indanol, which is quantitatively oxidized to the carbonyl
product, a well-known intermediate for pharmaceutical and
agrochemical applications. The present protocol represents a
further advancement with respect to on-water catalysis and
[33,34]
environmentally sustainable oxidations.
Moreover, by virtue of the POM robustness and fast ki-
netics, the catalytic set-up can be readily implemented in a
flow-mode within microfluidic devices in which the H O
2
2
Figure 6. a) Calculated electrostatic potential surface of 2. b) Graphical
representation of the HOMO for 2.
feeding can be properly tuned to match the addition/con-
sumption rates and optimize the steady-state concentration
[30b]
of the competent peroxidic intermediate.
The impact of the geometry/structure of the peroxide
bridge on the oxygen-transfer catalysis has been further ad-
dressed by DFT computations, including relativistic and sol-
vent effects. The POM structures were optimized using the
ZORA-BP86 functional, with TZP basis sets; a single-point
calculation using a TZ2P basis set was then performed on
In general, the calculated MEP for the isostructural 1–3
clearly shows that a high electron density is localized on the
peroxidic moieties and on W-O-M bridging oxygen atoms,
[35,36]
while terminal WÀO sites are relatively electron-poor.
0
The molecular orbitals distribution of fully-d POMs is
generally described by two identifiable sets of molecular or-
bitals: the doubly occupied ones, delocalized on oxygen
atoms (oxo band), and a set of virtual ones corresponding to
the d orbitals of the transition metals (metallic band). In
this scenario, the POM-embedded peroxo moiety gives rise
to one occupied orbital, which is found half way in energy
[35]
the final geometry. Due to the large size of the molecules
and to the large number of heavy metals, the internal or
core electrons were kept frozen (see Experimental Section).
The calculated geometries are in very good agreement with
[15,16]
the experimental ones obtained by XRD analysis,
con-
firming the adherence of the calculation method to the ex-
perimental data for the title compounds. A detailed compar-
ison of relevant experimental and calculated geometric pa-
rameters is provided in the Supporting Information
[37]
between the oxo and the metallic bands. A graphical rep-
resentation of the peroxo-HOMO is given in Figure 6b for
2; a similar orbital shape is also observed for the isostructur-
al complexes 1 and 3. The high energy of such peroxo-
HOMO orbitals and the resulting HOMO–LUMO low-
energy gap stand in contrast with the strong electrophilic be-
havior displayed by the peroxo-POMs 1–3, with regard to
the sulfide and sulfoxide oxidation. On the other hand, the
proton affinity of the peroxide moiety can be controlled by
its HOMO energy. Moreover, protonation equilibria are
known to trigger the electrophilic reactivity of organic and
(
Table S1).
As a first tool to investigate the electronic character of
the competent peroxo moieties, the calculated map of the
electrostatic potential (MEP) provides a qualitative descrip-
tion of the electron distribution over the polyanion. This is a
direct hint on the relative nucleophilicity/basicity of the
oxygen sites, which can undergo protonation in aqueous
[25,35]
media under the catalytic regime.
MEP of 2 is represented in Figure 6a.
As an example, the
[25]
inorganic peroxides as oxygen-transfer agents.
Chem. Eur. J. 2011, 17, 8371 – 8378
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8375

M. Bonchio, U. Kortz, L. Walder et al.
It is therefore of interest to extend this type of analysis to
the trimeric peroxo-POMs 4 and 5, which are remarkably
less reactive than 1–3 (see previous discussion). While no
significant changes are observed for the energies of the oxo
and metallic bands, a strong difference is noticed for the
peroxo-HOMOs (Figure 7), which are found at a lower
As frontier molecular orbital analysis addresses the differ-
ence in reactivity between polyanions 1–3 and the related
complexes 4 and 5 in terms of protonation affinity of the
peroxide moiety, further attention will be devolved to the
effect of acidic additives in terms of catalysis rates and se-
lectivity. To date, very few reports exist on the functional
properties of Zr- or Hf-based peroxo complexes. In this
regard, the unique reactivity exhibited by the totally inor-
ganic POM-based molecular species sheds further light on
the nature of transient oxidants generated on the surface of
[39]
extended metal oxides applied as heterogeneous catalysts.
Experimental Section
General procedures: Compounds 1–5 were prepared as described else-
[
15,16]
where.
Commercially available l-methionine (98%), dl-methionine
(35%), and all other reagents were used as purchased
oxide (99%), H
2
O
2
without further purification. Infrared spectra were recorded on a Nicolet
1
Avatar spectrophotometer. H NMR spectra were recorded on a Bruker
AV 300 instrument, equipped with a multinuclear probe. UV/Vis spectra
were recorded with a Varian UV-NIR Cary 5000 instrument, using quartz
cells with 1 cm path length. Resonance Raman spectra were acquired
with a Renishaw instrument, model Invia reflex, by using the laser at
5
32 nm.
Figure 7. Frontier orbital energies calculated for 1–5. EHOMO =À3.934,
À3.933, À3.827, À4.607 and À4.563 eV for 1–5, respectively.
Electrochemistry: All cyclic voltammetry measurements and rotating
disk experiments were performed in three-electrode systems under
argon, using a potentiostat PGSTAT 20 from Autolab interfaced with a
personal computer running under GPES for Windows, version 4.9 (ECO
Chemie). In addition to the glassy carbon working electrode, an Ag/AgCl
reference electrode (Metrohm, 6.0724.140), separated by a salt bridge
[38]
energy in 4 and 5 with respect to 1–3, by 0.629–0.782 eV.
Such a result points to an enhanced proton affinity of perox-
À1
containing the electrolyte was used. The scan rate was 100 mVs . All
ides 1–3 with respect to 4 and 5, likely ascribed to the di-
voltammograms were recorded after 5 s of equilibration at the starting
potential. The rotating disk experiments were performed with an elec-
trode rotation speed of 3000 rpm.
verse arrangement of the MO moieties in the two families
2
of complexes. Based on such evidence, the overall descrip-
tion of the oxygen-transfer mechanism should include possi-
ble protonation equilibria in water, which are favorable for
Oxidation of l-methionine and dl-methionine oxide: In all reactions, a
stock aqueous solution (D
2
O) of the substrate (37.3 mm) was diluted to
match the stoichiometric content of the active peroxide (7.27 mm) in
1
–3.
5
00 mL total volume. After total dissolution of the catalyst, (3.64 mm for
1
–3 or 1.61 mm for 4 and 5), the oxidation of S and SO was monitored by
1
H NMR spectroscopy (T=301 K), taking into account the time required
to prepare the sample before spectra acquisition. Recharged experiments
were performed after the reaction, by adding a new aliquot of the sub-
Conclusion
strate and a stoichiometric amount of H
trations.
2 2
O , to restore the initial concen-
Water-soluble Zr/Hf butterfly peroxo-polyoxotungstates are
reactive oxidants under stoichiometric and catalytic condi-
tions. The active core of these molecular catalysts is a
Catalytic oxidation of l-methionine and dl-methionine oxide: A stock
solution of substrate or SO in (0.189m) was prepared.
3.64 mmol sample was transferred to a 5 mm NMR tube and diluted with
O (500 mL) to obtain a final concentration of 7.27 mm. POMs 1–3 were
S
D
2
O
A
2
2
mono- or bis-m-h :h -peroxo ligand for each Zr/Hf center
D
2
embedded within the POM scaffold. The isostructural
added in the tube, using a stock solution 0.86 mm, to provide 0.081 mmol.
After the total dissolution of the catalyst, a stoichiometric amount of
8
À
dimers 1–3 feature two [a-XW O ] ions (X=Si, Ge)
1
4+
1
39
4
+
4+
linked by a {M (O ) } (M=Zr , Hf ) fragment, while
the derivatives 4 and 5 display a cyclic {M (O ) }
2 2
H O was added (7.27 mm), and the oxidation of the substrate was moni-
2
2 2
1
2+
1
tored by H NMR spectroscopy (T=301 K).
hexame-
6
2 6
Oxidation of methionine monitored by rotating disk voltammetry: A so-
ric core stabilized by three decatungstosilicate units [g-
8À
lution of the POMs (1–3, c
0
=0.30–0.64 mm) in 0.5m HAc/KAc buffer
SiW O ] . Analysis of oxygen-transfer to l-methionine
1
0
36
(
pH 4.7) was prepared and transferred into an electrochemical cell. l-Me-
and its sulfoxide rank the POM performance in the range
thionine was added in portions of 1 mmol (1, 2) or 0.5 mmol (3), thus pro-
viding 7.1–14 mm (11.8–28 equiv). After total dissolution of the substrate,
the reduction of the POM was monitored by CV. Initial rates, expected
for the conditions employed during NMR experiments, were calculated
by using bimolecular rate constants.
3
ꢀ1>2@4ꢀ5. In this chemistry, polyanions 1–3 behave as
strong electrophiles with a reactivity ratio S/SO>100, thus
showing a major impact of the POM structure on the reac-
tivity of the peroxide moiety. Recharging experiments
proved that these POMs can be reactivated after the deple-
tion of the peroxidic content and used for multi-turnover
processes.
Oxidation of methionine oxide monitored by CV: A solution of the
À4
À4
0
POMs (1–3, c =5.0ꢄ10 –6.0ꢄ10 m) in 0.5m HAc/KAc buffer (pH 4.7)
was prepared and transferred into an electrochemical cell. dl-Methionine
oxide was added, thus providing 7.1–14 mm (11.8–28 equiv). After total
8376
www.chemeurj.org
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8371 – 8378

IV
IV
Zr and Hf Butterfly Peroxides on Polyoxometalate Surfaces
FULL PAPER
dissolution of the substrate, the reduction of the POM was monitored by
CV. Initial rates, expected for the conditions employed during NMR ex-
periments, were calculated by using bimolecular rate constants.
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Catalytic oxidation of alcohols: A 500 mL sample of an aqueous solution
containing the alcohol (0.8–1.0 mmol), H O (0.08–0.1 mmol), and cata-
2 2
lyst (1.6 mmol) was poured into a MW test tube. The reaction was carried
out in a monomodal MW oven, upon irradiation at 70 W and application
of compressed air to avoid overheating of the sample. The reaction mix-
ture, after checking for the presence of H O with iodinated paper, was
2 2
extracted with ethyl acetate (4ꢄ1.2 mL) and analyzed by GLC with the
internal standard method.
Computational details: Computational resources and assistance were pro-
vided by the Laboratorio Interdipartimentale di Chimica Computazio-
nale (LICC) at the Department of Chemical Sciences of the University
of Padova. DFT calculations were carried out using the Amsterdam den-
[
40]
sity functional (ADF) code; scalar relativistic effects were taken into
account by means of the two-component zero-order regular approxima-
[
41]
tion (ZORA) method,
adopting the Becke 88 exchange plus the
[
42]
Perdew 86 correlation (BP) functional. The basis functions for describ-
ing the valence electrons are triple-zeta quality, singly or doubly polar-
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ternal or core electrons (O: 1s; Si: 1s to 2sp; Ge: 1s to 3spd; Zr: 1s to
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[
[
43]
solvent effect was modeled by means of the ADF implementation of
[
44]
the COSMO method.
This method required a prior definition of
atomic radii, which were set at their following recommended values (in
ꢆ): H: 1.3500; O: 1.5167; Si: 1.9083; Ge: 2.033; Zr: 2.1167; Hf: 2.1083;
W: 1.9917. In addition to the dielectric permittivity e, the solvent was also
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[
45]
terms derived from the solvation of alkanes.
Since these parameters
[
[
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Financial support from CNR, MIUR, University of Padova (PRAT
CPDA084893/08) and the ESF COST D40 action are gratefully acknowl-
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spectra. U.K. thanks the German Research Council (Grant DFG-KO-
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À1
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8
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[
31] FT-IR of the recovered catalyst confirms its stability even in such
conditions (Figure S43). POMs 1 and 3 showed a decreased efficien-
cy under identical conditions.
Received: October 27, 2010
Published online: June 8, 2011
8378
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