Heterogenization of [Cu(2,2′-bpy)Cl2] and [Cu(1,10-phen)Cl2] on polyoxometalates: New catalysts for the selective oxidation of tetralin

Article abstract of DOI:10.1016/S1872-2067(10)60239-2

Mononuclear Cu(II) bipyridine (1) and phenantroline complexes (2) were synthesized and immobilized by different procedures on H₃PW ₁₂O₄₀ polyoxometalate (POM). Characterization by XRD and SEM-EDX were performed to assess the preservation of the Keggin structure and stoichiometry of the complex. The immobilized complexes were tested as heterogeneous catalysts for the partial oxidation of tetralin (1,2,3,4-tetrahydronaphthalene) using hydrogen peroxide as oxidant in acetonitrile/water as solvent. [Cu(2,2′-bpy)Cl][H₂PW ₁₂O₄₀] and [Cu(1,10-phen)Cl][H₂PW ₁₂O₄₀] oxidized tetralin at room temperature, with 16 conversion with (2), to 1-tetralone and 2-tetralone with 83 selectivity. However, the selectivity for 1-tetralone was only 56. Different preparation methods for the heterogenization of these complexes on the POM Keggin unit were compared and used to enhance the selectivity to 1-tetralone to 75.

Full text of DOI:10.1016/S1872-2067(10)60239-2

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 5, 2011
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2011, 32: 807–811.
RESEARCH PAPER
Heterogenization of [Cu(2,2'-bpy)Cl2] and
[Cu(1,10-phen)Cl2] on Polyoxometalates: New Catalysts for
the Selective Oxidation of Tetralin
M. BOLTZ, A. BLANC, G. LAUGEL, P. PALE, B. LOUIS*
Laboratoire de Synthèse et Réactivité Organiques, Institut de Chimie, UMR 7177 CNRS, Université de Strasbourg, 4 rue Blaise Pascal, 67000,
Strasbourg Cedex, France
Abstract: Mononuclear Cu(II) bipyridine (1) and phenantroline complexes (2) were synthesized and immobilized by different procedures
on H3PW12O40 polyoxometalate (POM). Characterization by XRD and SEM-EDX were performed to assess the preservation of the Keggin
structure and stoichiometry of the complex. The immobilized complexes were tested as heterogeneous catalysts for the partial oxidation of
tetralin (1,2,3,4-tetrahydronaphthalene) using hydrogen peroxide as oxidant in acetonitrile/water as solvent. [Cu(2,2'-bpy)Cl][H2PW12O40]
and [Cu(1,10-phen)Cl][H2PW12O40] oxidized tetralin at room temperature, with 16% conversion with (2), to 1-tetralone and 2-tetralone with
83% selectivity. However, the selectivity for 1-tetralone was only 56%. Different preparation methods for the heterogenization of these
complexes on the POM Keggin unit were compared and used to enhance the selectivity to 1-tetralone to 75%.
Key words: tetralin; partial oxidation; Cu(II) complexes; 2,2'-bipyridine; 1,10-phenantroline; hydrogen peroxide; polyoxometalate
Tetralone is a keto derivative of tetralin, which is a tetrahy-
drogenated form of naphthalene. Tetralone is used as a solvent
and plastic softener. It is also the reactive intermediate to
pharmaceutical, agrochemical, and dye compounds of indus-
trial importance, such as the insecticide carbaryl, the antide-
pressant sertraline, and the antihypertensive drug benazepril
[1–3]. Tetralone can be produced by different chemical proc-
esses, but the main processes are Friedel-Crafts acylation of
aromatics with acid halide/anhydride or oxidation reactions.
Unfortunately, these processes use, respectively, stoichiometric
amounts of corrosive AlCl3 as catalyst and KMnO4/K2Cr2O7 as
oxidizing agent [4–8]. It is thus worthwhile to develop new
environmentally friendly routes to these industrially important
compounds. Several recent studies have focused on the selec-
tive oxidation of alkylaromatics and cycloalkanes to
higher-added value ketones using green oxidizing agents like
hydrogen peroxide or oxygen [9–13]. Interestingly, copper is a
well known element that interacts with these oxidizing species,
especially in biological systems.
of various proteins and enzymes of numerous organisms,
where it is coordinated to protein residues in mono-, di-, or
trinuclear complexes [13]. Some examples are: copper acts as
the oxygen carrier in hemocyanins [14], and as the redox center
in oxidation reactions in enzymes, such as catechol oxidase
[15], dopamine monoxygenase [16], methane monoxygenase
[17], and tyrosinase [18].
The use of copper complexes for the oxidation of organic
compounds is promising and several approaches have been
developed. Antunes et al. reported the successful oxidation of
benzene, toluene and cyclohexane with hydrogen peroxide
catalyzed by different Cu(II) complexes [19–21]. In our earlier
study, we reported the catalytic activity of 2,2'-bipyridine and
1,10-phenantroline Cu(II) complexes in tetralin oxidation [22].
The aim of the present study is to go one step beyond with the
immobilization of two complexes, [Cu(2,2'-bpy)Cl2] (1) and
[Cu(1,10-phen)Cl2] (2), on polyoxometalates (POM). These
heteropolycompounds are large polyanions that can be used to
immobilize organic parts or organometallic complexes. How-
ever, a change in the redox properties of the grafted metal
cations due to modifications in their electron transfer properties
Copper plays fundamental roles in living systems. These are
mostly in redox processes, and it can be found in the active site
Received 29 March 2011. Accepted 25 April 2011.
*Corresponding author. Tel: +33-3-6885-1488; E-mail: blouis@unistra.fr
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60239-2

M. BOLTZ et al. / Chinese Journal of Catalysis, 2011, 32: 807–811
can be observed. Since these complexes are good homogene-
lents of (1) per equivalent of POM to carry out the grafting of
complex (1) on the POM support. The following [Cu(2,2'-bpy)-
Cl]2[HPW12O40] complex (named as [Cu(bpy)]2POM) was
formed with 69% yield.
ous oxidation catalysts, we have looked at their heterogenei-
zation and hence possible recycling. The catalytic reactions
were carried out using hydrogen peroxide as oxidant in an
acetonitrile/water solvent at room temperature.
Another complex, [Cu(2,2'-bpy)Cl][H2PW12O40], was di-
rectly prepared by the condensation of elementary blocks by
the use of hydrothermal conditions. Na2WO4 (1.058 g, 3.2
mmol) and Cu(2,2'-bpy)Cl2 (72.2 mg, 0.25 mmol) were dis-
solved in distilled water (30 ml). After stirring, a 50 wt%
aqueous solution of H3PO4 was added dropwise to adjust the
pH to 3. The mixture was transferred into a sealed Teflon re-
actor and heated to 160 °C. After 136 h of heating, the reactor
was cooled down to room temperature and the solution was
filtered and washed with water (10 ml). This complex named as
Cu(bpy)POM BB (BB: building blocks) was obtained in 35%
yield.
1 Experimental
1.1 Synthesis of the [Cu(2,2'-bpy)Cl2] and
[Cu(1,10-phen)Cl2] complexes
The detailed preparation procedure for these complexes,
based on original procedures described by Meyer [23], has
been reported in our earlier study [22]. The synthesis of
[Cu(L)]Cl2 was carried out by the addition of one equivalent of
the 2,2'-bipyridyl ligand or 1,10-phenantroline ligand in 50 ml
of ethanol to 1 equivalent of CuCl2·2H2O in 15 ml of ethanol.
The mixture was then stirred for 10 min at room temperature,
and it formed a precipitate after a few minutes. Finally, the
solvent was evaporated slowly at room temperature, and the
complex was collected.
1.3 Characterization
FT-IR spectra were recorded on a Nicolet Spectrometer 760
(CsI pellets). Elemental analyses were obtained using a
Perkin-Elmer 2400 CHN and a microbalance Perkin-Elmer
AD-4 Autobalance. X-ray diffraction (XRD) patterns were
acquired on a D8 Advance Bruker AXS powder diffractometer
using monochromatized Cu-KĮ radiation in the range of 2ș
from 5° to 50°. Scanning electron microscopy (SEM) was
carried out on a JEOL FEG 6700F microscope used at 9 kV
accelerating voltage. Energy-dispersive X-ray (EDX) spectra
were acquired to determine the composition of the material.
The EDX spectra were acquired using 20 kV primary electron
energy. Quantification was done using the standard-less ZAF
correction method with the Genesis software for EDX.
The two complexes were characterized by Fourier transform
infrared (FT-IR) and elemental analysis. The results were as
follows.
[Cu(2,2'-bpy)Cl2] (1). Light blue solid. IR (KBr disc, cmꢀ1):
3068, 3053, 3037, 1602, 1551, 1497, 1473, 1446, 777.
Elemental analysis calculated for C10H8N2Cl2Cu (%): C, 41.27;
H, 2.75; N, 9.63. Found (%): C, 41.10; H, 2.59; N, 9.52.
[Cu(1,10-phen)Cl2] (2). Light green solid. IR (KBr disc,
cmꢀ1): 3080, 3058, 3012, 1607, 1586, 1516, 1495, 1423, 722.
Elemental analysis calculated for C12H8N2Cl2Cu (%): C, 45.78;
H, 2.54; N, 8.9. Found (%): C, 45.59; H, 2.53; N, 8.78.
1.2 Synthesis of Cu(II) complexes-POM
organic-inorganic hybrids
1.4 Catalytic activity evaluation
The different hybrid copper complexes were tested for liquid
phase tetralin oxidation at room temperature. All solvents and
reagents were purchased from Aldrich or Merck. The reactions
were performed in a 20 ml round-bottomed flask with vigorous
stirring. This was sealed with a silicone septum in a nitrogen
atmosphere. The reactions used acetonitrile as solvent, H2O2 as
oxidant, and the Cu(II) hybrid complexes as catalysts. Immo-
bilized Cu complexes on POM (0.014 mmol), tetralin (4.4
mmol), and hydrogen peroxide (131 mol) were dissolved in
acetonitrile (13 ml). The copper catalyst was therefore used
with a 0.3 mol% loading (or 0.6 mol% for the two equivalents
catalyst). The reaction mixture was stirred at room temperature
for up to 6 d. The reactions were followed by a gas chromato-
graph (HP 5890 Series II) equipped with a DB-1 capillary
column (length 30 m, internal diameter 0.53 mm). The reten-
tion times used to characterize most of the reactions products
were first calibrated with standards. Relative yields were ob-
tained by calibration with taking into account the response
Four different hybrid materials were prepared to evaluate the
influence of the ligand, number of Cu2+ cations, and immobi-
lization methodology on oxidative properties. The Cu-complex
(0.25 mmol), either (1) or (2), was dissolved in distilled water
(30 ml). 12-Tungstophosphoric acid (0.25 mmol, H3PW12O40)
was then added. After vigorous stirring for 30 min, the pH was
measured (pH = 3). The reaction mixture was transferred into a
sealed Teflon reactor and heated to 160 °C under autogeneous
pressure. After 65 h of heating, the reactor was cooled down to
room temperature and the mixture was filtered and washed
with distilled water (10 ml). Finally, the hybrid Cu(bpy)POM
or Cu(phen)POM was collected as a colored solid, which was
blue (bipyridine ligand) or green (phenantroline ligand). The
synthesis yields were 42% and 33% for the immobilization of
(1) and (2), respectively. These materials will be considered in
the following sections as standards.
The same procedure was performed starting from 2 equiva-

M. BOLTZ et al. / Chinese Journal of Catalysis, 2011, 32: 807–811
factors of the substrate (tetralin) and major products
(1-tetralone, 2-tetralone). The turnover number (TON) used to
evaluate the catalyst performance was defined as the number of
substrate molecules converted per molecule of catalyst.
2 Results and discussion
2.1 Hybrid complex characterization
Figure 1 presents the XRD patterns of the different
[Cu(2,2'-bpy)Cl2] POMs. It is noteworthy that different
preparation methods for the inorganic-organic hybrid gave
reflections at around 2ș = 9°. This was evidence of the pres-
ervation of the Keggin structure [24]. The XRD pattern of the
[Cu(1,10-phen)Cl2]-POM homolog also showed the same
result (Fig. 2).
5
10
15
20
25
30
35
40
45
2T/( o
)
Fig. 2. XRD pattern of the Cu(phen)POM hybrid material.
The same morphology was probably emerging for the
[Cu(2,2'-bpy)Cl][H2PW12O40] hybrid obtained in situ by using
elementary building blocks to assemble a Keggin-like POM
(Fig. 3(b)). However, the shape remained less defined for these
crystals. Surprisingly, using two equivalents of starting or-
ganometallic complex gave the growth of the cubic crystals for
the [Cu(2,2'-bpy)Cl]2[HPW12O40] hybrid material. These were
also between 1–4 Pm in size (Fig. 3(c)). The EDX analysis
(performed in the SEM chamber) confirmed the
stoichiometries of the first two [Cu(2,2'-bpy)Cl][H2PW12O40]
complexes, which was 1 Cu atom per 12 W atoms. Likewise,
the latter [Cu(2,2'-bpy)Cl]2[HPW12O40] material basically
contained 2 Cu atoms per Keggin unit. The immobilization of
the two starting complexes was possible by simply doubling
the number of equivalents. The influences of the preparation
methodology, nature of starting ligand (bipyridine versus
phenantroline), and amount of complexes grafted were studied
by the catalytic performance of these hybrid materials in the
partial oxidation of tetralin, which has been described else-
where [22].
Since the Keggin structure was maintained after the grafting
of the complexes, the immobilization strategy was successful.
The microstructures of the different hybrid materials, together
with elemental EDX analysis, were observed by SEM. Figure 3
shows the SEM micrographs of the three [Cu(2,2'-bpy)Cl2]
complexes immobilized on H3PW12O40 polyoxometalate. A
dodecahedral shape was observed for the [Cu(2,2'-bpy)Cl]-
[H2PW12O40] hybrid crystals. These were between 1–3 Pm in
size (Fig. 3(a)).
[Cu(bpy)]2POM
Cu(bpy)POM
2.2 Catalyst evaluation
Cu(bpy)POM BB
The catalytic activities of the complexes were investigated
with the partial oxidation of tetralin to 1-tetralone and
2-tetralone as the main evidence. This is depicted in Scheme 1.
The reactions were carried out at room temperature using H2O2
as oxidant and acetonitrile/H2O (5:1) as solvent. Previous
5
10
15
20
25
2T/( o )
30
35
40
45
Fig. 1. XRD patterns of the different [Cu(2,2'-bpy)Cl2] grafted onto
POM hybrid materials.
(a)
(b)
(c)
Fig. 3. SEM images of the Cu(bpy)POM (a), Cu(bpy)POM BB (b), and [Cu(bpy)]2POM (c) hybrid.

M. BOLTZ et al. / Chinese Journal of Catalysis, 2011, 32: 807–811
+
Cl
X
N
Cl
N
Cu
N
N
O
X = H,H or
O
X
+
+
H2O
H2O2
Scheme 1. Tetralin oxidation to 1-tetralone and 2-tetralone.
studies devoted to tetralin or benzene oxidation with peroxides
have shown that the starting [CuLCl2]complexes(L: bipyridine
or phenantroline) were efficient under homogeneous condi-
tions [19–22]. By successfully grafting the copper complexes
onto a Keggin-type POM, it was possible to see the change
from homogeneous to heterogeneous catalysis. This idea gives
some ways to recycle the catalyst.
starting [CuLCl2] complexes were obtained under homogene-
ous conditions, but only after 24 h of reaction. In contrast, the
conversions were below 2% after 24 h for all these hybrid
Cu-POM materials. It is therefore important to point out that
the heterogenization of the [CuLCl2] complexes onto the POM
ligand led to a drastic decrease in their catalytic performances
when compared to the original homogeneous complexes. This
result is in agreement with that the hindered [Cu(2,2'-bpy)3Cl2]
complex gave only 1%–2% tetralin conversion after 24 h in a
homogeneous medium. This indicated that the Cu(II) catalyst,
when sterically hindered by pyridinyl and POM ligands, can
hardly activate hydrogen peroxide to form copper-peroxo
species. These low conversions were probably due to the filled
coordination sphere of the copper center and to ligand steric
hindrance, which limited access to the metal core. On the other
hand, [CuLCl2] complexes having vacant and labile positions
occupied by the chloride ions were more able to perform rapid
electron transfer [27].
Figure 4 presents the tetralin conversion and tetralone se-
lectivity of the Cu-POM hybrid complexes. While each
[Cu(2,2'-bpy)Cl][H2PW12O40] complex approximately yielded
the same tetralin conversion (15%), different selectivities in the
two tetralin isomers were observed. Cu(phen)POM gave 83%
selectivity, while Cu(bpy)POM and Cu(bpy)POM BB prepared
from its elementary blocks gave lower selectivities of, respec-
tively, 65% and 71%. These results agree with the higher se-
lectivity achieved under homogeneous conditions with CuLCl2
complexes [22].
In contrast, an impressive 90% selectivity was observed in
the oxidation over the [Cu(bpy)]2POM hybrid catalyst. Fur-
thermore, it was 1-tetralone that was mainly formed (72%).
This result agreed with earlier studies that showed a different
behaviour of the organic-inorganic POM hybrids of the same
family [25]. Unfortunately, the catalytic activity decreased
dramatically to below 10% over this highly selective catalyst.
Probably, one can recover the activity of this complex using
microwave heating, which was performed by Sithambaram et
al. [26] for tetralin oxidation.
The TON were estimated for each hybrid catalyst. A TON of
25 was achieved with the homogeneous [CuLCl2] complex,
while a TON of 9 was obtained over the hybrid materials but
with the possibility to recycle these catalysts.
The possibility of catalyst recycling was evaluated by al-
lowing the Cu(phen)POM catalyst to react in three consecutive
runs with fresh solutions of the reactants. Unfortunately, the
conversion was seriously diminished after the second run, from
16% after 136 h of reaction, down to 7%. Finally, almost no
catalytic activity was observed for the third catalytic run (yield
in tetralone slightly above 1%). Leaching of the active species
could be responsible for this rapid decrease, especially with the
use of peroxide as reported by Sheldon [28].
For comparison, the same catalytic conversions with the
90
X(tetralin)
S(tetralone)
80
S(1-tetralone)
70
60
50
40
30
20
10
0
The mechanism of the present reaction probably includes
promotion by the generation of peroxo species similar to the
reaction proposed by Mimoun et al. [29] for V-peroxo com-
plexes. Oxygen transfer from the peroxo species to the acti-
vated benzylic position of tetralin ring would occur in a bi-
molecular fashion. The partial oxidation would then take place
by the homolytic addition of oxygen species to the substrate
molecule, similar to that shown for POM catalysts [30].
The present results showed that the inorganic-organic hy-
brids here behave similarly to their homogeneous homologs.
However, they gave a lower activity due to the presence of the
large POM counter anion. In spite of the successful heteroge-
Fig. 4. Conversion of tetralin, selectivity for 1-tetralone and total ke-
tone products (1-tetralone and 2-tetralone) obtained after 136 h.

M. BOLTZ et al. / Chinese Journal of Catalysis, 2011, 32: 807–811
2008, 73: 7365
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some reactions, these positive interactions would guide the
reactants toward the active site, recognize it through specific
interactions, and hence stabilize this couple in an environment
for the desired chemical transformation. Targeting the same
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