Synthesis of dendritic polyoxometalate complexes assembled by ionic bonding and their function as recoverable and reusable oxidation catalysts

Article abstract of DOI:10.1002/anie.200453870

Regenerable and reusable dendritic catalysts have been synthesized by the ionic assembly of polyammonium dendrimers and polyoxometalate (POM) trianionic units. These dendrimers (see example depicted, in which the POM is [PO ₄{WO(O₂)<

Full text of DOI:10.1002/anie.200453870

Communications
Dendrimers
Synthesis of Dendritic Polyoxometalate
Complexes Assembled by Ionic Bonding and
Their Function as Recoverable and Reusable
Oxidation Catalysts**
Lauriane Plault, Andreas Hauseler, Sylvain Nlate,
Didier Astruc,* Jaime Ruiz, Sylvain Gatard, and
Ronny Neumann
Dedicated to Professor David Carrillo
on the occasion of his 65th birthday
Dendrimers[1] are known for a number of potential applica-
tions in supramolecular chemistry,[2] nanosciences,[3] biology,
and medicine,[4] and many of their transition- metal com-
plexes[5] are especially promising in catalysis.[6] Whereas
covalent-based metallodendrimers have been shown to act
as catalysts in numerous cases, the ionic-bonding approach
has only recently been mentioned.[7] We now wish to report a
[*] L. Plault, Dr. A. Hauseler, Dr. S. Nlate, Prof. D. Astruc, Dr. J. Ruiz
Nanosciences and Catalysis Group
LCOO, UMR CNRS N8 5802
UniversitØ Bordeaux I
351 Cours de la LibØration, 33405 Talence Cedex (France)
Fax: (+33)5-4000-6646
E-mail: d.astruc@lcoo.u-bordeaux1.fr
Dr. S. Gatard, Prof. R. Neumann
Department of Organic Chemistry
Weizmann Institute of Science
76100 Rehovot (Israel)
[**] We are grateful to the Institut Universitaire de France (DA), the
CNRS (AH), the Alexander von Humboldt foundation (AH), the
Aron Zandman Postdoctoral FellowshipProgram in Organic
Chemistry (SG), the University Bordeaux I, the Weizmann Institute,
the MRT (LP) and the French–Israel AFIRST grant for financial
support. RN is the Rebecca and Israel Sieff Professor of Organic
Chemistry.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200453870
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new series of five metallodendrimers based on ionic bonding
between dendritic polyammonium polycations and trianionic
polyoxometalates (POMs) and their use as recoverable and
reusable catalysts in oxidation reactions. POMs are the source
of fascinating architectures[8] and very rich redox chemistry[9]
that provides the basis for their catalytic activity in oxidation
reactions.[10] There is only one report of catalysis by using a
dendritic-type framework that contains POMs: the oxidation
of tetrahydrothiophene to its sulfoxide by tBuOOH and H2O2
was catalyzed by a compound consisting of four POM units
covalently bonded to a tetradirectional core.[11]
Among the POM oxidation catalysts, we have selected the
peroxophosphotunsgtate [PO4{WO(O2)2}4]3À, which is known
for its catalytic properties in olefin epoxidation and alcohol
oxidation.[11,12] To build the supramolecular assembly of the
dendritic oxidation catalysts in which each POM trianion is
connected to a trication, we have designed tricationic tripods
that consist of triammonium groups. For this purpose, our
synthetic strategy involves polymethyl arenes in which each
methyl group can be functionalized by three allyl groups by
using the temporary complexation/activation by the 12-
electron fragment [CpFe]+ (see Scheme 1, the example is
mesitylene).[13] Regioselective hydroboration of the nona-
alcene 2, which was produced from 1, and oxidation of the
nona-borane to give the primary nona-ol 3 followed by
treatment with SiMe3Cl/NaI provided the nona-iodo com-
pound 4.[14] Alkylation of tri-hexylamine by using 4 gave the
nona-ammonium salt 5 as an iodide that cannot be used as
such for oxidation chemistry, because of facile oxidation of
the iodide anion.[15] Treatment of 5 with AgBF4 in ethanol
À
gave the BF4 ammonium salt. The latter was transformed
into the desired nona-ammonium salt 6 of [PO4[WO(O2)2}4]3À
in dichloromethane by treatment with an aqueous solution of
the commercial heteropolyacid H3PW12O40 and H2O2. A
peroxotungstate product, [{WO(O2)2(H2O)}2O]2À, was easily
removed, as it remained in the aqueous phase.[12d] This
procedure was also applied to toluene and p-xylene providing
the mono and bis(POM)-triammonium salts 7 and 8. Standard
1
characterization techniques, such as H and 31P NMR spec-
troscopies and elemental analysis, were applied to the
dendrimers, the results of which are reported in the Support-
ing Information. AFM characterization of the tris(POM)
dendrimer is shown in Figure 1, and the height of the complex
recorded by this technique, 2.9 nm, indicates the presence of
Figure 1. AFM images of the tris(POM) dendrimer 6 on graphite
HOPG support shown in two dimensions (left) and three dimensions
(right). The height is reproducibly 2.9 nm (bottom) indicating a mono-
layer of monomeric tris(POM) dendrimer 6. AFM was operated in the
tapping-mode with a resonance frequency of around 200 MHz. The tip
has a radius of 10 nm.
=
Scheme 1. Synthesis of the tris(POM) complex 6. a) KOH, CH2 CH-
CH2Br, DME, À508C to RT. b) PPh3, CH3CN, RT, 12 h hn (Hg lamp).
c) BH3, isobutene, THF, RT, 12 h. d) aq NaOH, 3m, H2O, 30% H2O2,
508C, 12 h. e) NaI, CH3CN, Me3SiCl, 508C, 12 h. f) NHex3, CH3CN,
808C, 12 h. g) AgBF4, EtOH, RT. h) H3PW12O24, H2O2, H2O, CH3Cl.
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monomeric species, a feature that is evidenced by the good
solubility properties.
Epoxidation of cyclooctene by 35% aq H2O2 in a biphasic
water/CDCl3 system was carried out at room temperature,
and was catalyzed by 0.3% equivalents of POM embedded in
the ionic dendrimer. Under these conditions, monitoring of
the kinetics by plotting the ratio between the intensity of the
Since the trifunctionalization of polymethyl arenes used
here requires that each methyl group be free of its methyl
neighbor in the arene structure, thus it cannot be extended to
more than three methyl groups on a single arene ring. To
extend the arene system to four methyl groups, we used the
dinuclear tetramethylbiphenyl complex 9, which was recently
shown to provide the dodeca-allyl derivative 10 by treatment
with KOH/DME (DME = 1,2-dimethoxy ethane) and allyl
bromide at room temperature.[16] This synthetic strategy was
successfully employed and also provides the tetra(POM)
polyammonium compound 11 according to Scheme 2.
1
disappearing H NMR signal of cyclooctene at d = 5.6 ppm
versus TMS and that of the rising peak of the epoxide at d =
2.9 ppm versus TMS showed that the reaction was quantita-
tive after 5h. A comparison of the homogeneous mono-, bis-,
tris-, and tetra(POM) catalysts indicated that there was no
measurable dendritic effect on the reaction kinetics within
this series.
A dendritic effect, however, was noted in the recovery of
the catalysts. The catalysts are precipitated from the organic
phase by the addition of pentane leaving a clear solution in
the case of the tris and tetra(POM) catalysts 6 and 11, whereas
precipitation of the mono(POM) catalyst 7 is tedious and
leaves a white colloidal solution. Therefore, the recovery of
the tri and tetra(POM) catalysts is easier than that of the
mono(POM) catalyst. Out of a catalytic amount as low as
20 mg, the recovery of 6 by precipitation and filtration was
between 80 and 85% after each catalytic cycle, and the
efficiency of this catalyst was reproducible. With 11, the
catalyst recovery reached 96%. After catalysis, the 31P NMR
spectra of 6 and 11 were unchanged.
It was noticed however that, if the dendritic POM
catalysts were left for several days in air, they became less
soluble in CDCl3, and their 31P NMR spectrum changed (peak
shift from d = 3.45to 1.11 ppm). Then, their catalytic activity
was suppressed. Preliminary experiments indicate that the
mono(POM) catalyst is more sensitive to this deactivation
than the tris and tetra(POM) catalysts. If the POM catalysts
are kept under an inert atmosphere, they are not the subjects
of deactivation. Although this deactivation procedure was
very slow with 6, since two catalytic cycles with recovery in air
could be performed without storing this catalyst under an
inert atmosphere, it is best to store the POM catalyst under
argon for prolonged periods of time. Indeed, storage of 6 for
several months under an inert atmosphere allows further
catalytic use without problem.
Scheme 2. Synthesis of the tetra(POM) complex 11. a) tBuOK,
=
CH2 CHCH2Br, THF, 3 d, b) PPh3, CH3CN, RT, 12 h hn (Hg lamp).
c) BH3, isobutene, THF, RT, 12 h. d) aq. NaOH, 3m, H2O, 30% H2O2,
508C, 12 h. e) NaI, CH3CN, Me3SiCl, 508C, 12 h. f) NHex3, CH3CN,
808C, 12 h. g) AgBF4, EtOH, RT, h) H3PW12O24, H2O2, H2O, CH3Cl.
A successful catalysis experiment was also carried out by
using the heterogeneous hexa(POM) catalyst 17, which led
quantitatively to epoxycyclooctane and to direct recovery of
the catalyst by filtration.
The oxidation of thioanisole to the corresponding sulfone
by H2O2 in the biphasic water/CDCl3 system could also be
catalyzed by 0.1% of the tris(POM) complex 6 and was
selective and quantitative at room temperature after two
hours. This reaction does not work in the absence of catalyst.
The catalyst 6 was recovered analogously, as in the epoxida-
tion experiments, and it was checked by 31P NMR spectros-
copy that it was unchanged after recovery.
In conclusion, we have designed and assembled nanosized
ionic dendritic polyammonium POM oxidation catalysts 6, 7,
8, 11, and 17, in which a peroxophosphotungstate trianion is
bound to a tripod terminated by three trihexylammonium
groups. A comparison of the homogeneous POM dendritic
epoxidation catalysts showed that their efficiency (kinetics)
were the same, but recovery of the soluble tris and tetra-
We also wished to compare the stability, solubility,
recovery, and reuse properties within a series of ionic
dendrimers that contain between 1 and 6 POM units. Thus,
we designed a new pathway to synthesize a hexa(POM)
ammonium compound. Therefore, we used the new [CpFe]+-
induced hexa-alkenylation of the C6Me6 ligand of 12 to obtain
13 (Scheme 3),[17] followed by the analogous synthesis of the
hexa-ol 14 and hexamesylate 15. Then, the latter was allowed
to react with the phenolate dendron p-OÀ-C6H4C(allyl)3.[14,18]
to give the 18-allyl dendrimer 16, which was further converted
to the octadecyl ammonium iodide, BF4À, and hexa(POM)
salt 17 in a way similar to that described above for the mono-,
bis-, and tris(POM). An important difference between this
hexa(POM) salt 17 and the other salts (6, 8, and 11) however,
is that it is insoluble in all common solvents, which indicates a
polymeric form. Nevertheless, 17 has been characterized by
its correct elemental analysis.
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Experimental Section
General procedure for the synthesis of den-
dritic POM complexes: The dendritic poly-
allyl, polyol, and polyiodo derivatives were
synthesized according to procedures analo-
gous to those described in references [14] and
[16]. For the synthesis of the dendritic poly-
ammonium polyiodide salts, a mixture of
iodo compound and trihexylamine in CH3CN
was stirred for 12 h at 808C. After removal of
the solvent under vacuum, the residue was
washed with pentane and dissolved in about
2 mL of CH3CN. Then, 30 mL pentane was
added to precipitate the polyammonium
À
polyiodide salt. The polyammonium BF4
salt was prepared by adding AgBF4 to an
ethanol solution of ammonium iodide salt.
AgI precipitated immediately and was
removed by filtration, the ethanol was
removed by evaporation, and the residue
was extracted with CH2Cl2. The solvent was
then removed under vacuum, yielding the
À
ammonium BF4 salt. The dendritic polyam-
monium POM salts were synthesized by
adding H2O2 (35% in water) to a water
solution of commercial heteropolyacid
H3PW12O40. The mixture was stirred at
room temperature for 30 min. Then,
a
À
CH2CH2 solution of ammonium BF4 salt
was added, and the mixture was stirred for an
additional hour. The CH2Cl2 layer was dried
over sodium sulphate and evaporated under
vacuum, thus providing the desired dendritic
POM complex as a white solid.
General procedure for catalytic reactions
and catalyst recycling: The POM catalyst 6, 7,
8
or 11 (4.68 mmol) and cyclooctene
(284 equiv per POM unit) were dissolved in
3 mL CDCl3, then H2O2 (35% in water,
773 equiv per POM unit) was added. The
mixture was stirred at room temperature, and
the reaction was followed by 1H NMR
spectroscopy. After completion of the reac-
tion, the CDCl3 layer was concentrated under
reduced pressure to about 1 mL, and 5mL of
pentane was added producing a white pre-
cipitate. The solid was filtered over celite
then redissolved with CH2Cl2, and the solu-
tion was dried over sodium sulfate. The
solvent was then removed under vacuum,
yielding the POM complex as a white solid
that was analyzed by 1H and 31P NMR
spectroscopies, stored under nitrogen, and
reused for a subsequent catalytic experiment.
Supporting Information available: Complete synthetic proce-
dures and standard characterizations for all the dendrimers.
Scheme 3. Synthesis of the hexa(POM) complex 17. a) KOH, 1-iodo-11-undecene, DME, 608C,
3 d. b) PPh3, CH3CN, hn,16 h (Hg lamp), RT. c) BH3, isobutene, THF, 1 d, RT. d) NaOH, 30%
H2O2, H2O. e) CH3SO3Cl, C5H5N, HCl, 4 h, À58C-08C. f) p-OÀ-C6H4C(allyl)3, KCO3, Me2CO,
reflux 2.5 days.[14] g) BH3, isobutene, THF, RT, 12 h, then aq NaOH, 3m, H2O, 30% H2O2, 508C,
12 h, then NaI, CH3CN, Me3SiCl, 508C, 12 h, then NHex3, CH3CN, 808C, 12 h, then AgBF4,
EtOH, RT, then H3PW12O24, H2O2, H2O, CH3Cl. The locations of the counteranions are arbitrary.
(POM) catalysts 6 and 11 was complete, which was in contrast
to that of the mono(POM) 7, whereas the insoluble hexa-
(POM) 17 worked heterogeneously. Characterization of 6 by
AFM confirms its monomeric structure, and it can be stored
under an inert atmosphere for months without change for
efficient reuse as a catalyst. Finally, this synthetic and catalytic
study shows the usefulness of the ionic-dendrimer approach
for the design and efficient use of fully recyclable oxidation
catalysts. The large dendritic POM catalysts may also prove
interesting in the near future for continuous epoxidation by
using a membrane reactor.[19]
Received: January 27, 2004 [Z53870]
Keywords: dendrimers · epoxidation · homogeneous catalysis ·
.
polyoxometalates · tungsten
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