Synthesis, characterization and catalytic activity of a Wilkinson's type metal-organic-polyoxometalate hybrid compound

Article abstract of DOI:10.1039/b310236j

A metal-organic-polyoxometalate hybrid compound with two functional centers consisting of a rhodium(I)bis(diphenylphospine) unit connected through two alkylene bridging groups to a lacunary Keggin type polyoxometalate was synthesized and used as an effective, recyclable hydrogenation catalyst in monophasic and aqueous biphasic reaction modes.

Full text of DOI:10.1039/b310236j

Synthesis, characterization and catalytic activity of a Wilkinson’s type
metal-organic–polyoxometalate hybrid compound
Itsik Bar-Nahum and Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel, 76100.
E-mail: Ronny.Neumann@weizmann.ac.il; Fax: +972-8-9344142; Tel: +972-8-9343354
Received (in Cambridge, UK) 27th August 2003, Accepted 10th September 2003
First published as an Advance Article on the web 29th September 2003
A metal-organic–polyoxometalate hybrid compound with
two functional centers consisting of a rhodium( )bis(diphe-
nylphospine) unit connected through two alkylene bridging
groups to a lacunary Keggin type polyoxometalate was
synthesized and used as an effective, recyclable hydro-
genation catalyst in monophasic and aqueous biphasic
reaction modes.
organic solvents by using a lipophilic quaternary ammonium
counter cation to the polyoxometalate unit. After completion of
the reaction, the size and charge of the polyoxometalate moiety
I
6
will potentially allow recovery of the catalyst by nanofiltration.
Alternatively, the catalyst may be precipitated and simply
recovered by addition of ether. In the second case, a water
soluble lithium salt of the RhOM–POM complex was prepared,
leading to effective hydrogenation in an aqueous biphasic
reaction mode that allows catalyst recovery by phase separa-
tion.
Applications of polyoxometalates have become very important
in diverse areas of research.1 Especially, the use of poly-
oxometalates in catalysis has become extensive since the late
The preparation of the Rh( ) metal-organic–polyoxometalate
I
2
1
970’s. Invariably, the catalytic use of polyoxometalates has
hybrid compound was carried out according to the synthetic
method outlined in Scheme 1, and is based on incorporation of
a phosphine containing alkylsilane into the defect position of a
been based on their inherent properties; high acidity in
heteropoly acids, and favorable redox potentials and transition
metal catalyic centers that make them attractive for applications
in acid and oxidation catalysis. Another interesting area of
activity in the chemistry of polyoxometalates has been their
8
2 7,8
lacunary polyoxometalate, [SiW11 In this manner, the
O
39
]
.
organosilane, (OEt) SiCH CH PPh , was reacted with the
3
2
2
2
lacunary polyoxometalate leading to incorporation of the
organosilane moiety into the defect position of the poly-
3
modification by metal-organic moieties. Usually such com-
pounds are based either on an electrostatic interaction of metal-
organic cationic species with polyoxometalate anions or the
attachment of an organic moiety to a metal center incorporated
within the polyoxometalate cluster. Thus, for example, the
possibility of combining noble metal based complexes and
polyoxometalates for catalytic applications would also appear
to be attractive and many noble metal complex–polyox-
ometalate hybrid compounds have been prepared.3 Despite
there being numerous metal-organic–polyoxometalate hybrid
compounds, we know of only one application of such
oxometalate, thereby
yielding
4
K {SiW11O39[O-
(SiCH CH PPh ) ]} in good yields.† The IR spectrum shows
2
2
2 2
2
1
21
peaks at 1048 and 1123 cm (Si–O bonds), 1001 cm (WNO
bonds) and 812, 931, 970 cm (W–O–W bonds), attributable
to the lacunary Keggin structure, that were unchanged by the
2
1
modification with SiCH
of a SiCH CH PPh unit. In the H NMR spectrum one
observes peaks attributable to the CH CH bridging unit and the
2 2 2
CH PPh and absorption bands typical
1
2
2
2
2
2
aromatic rings. After metathetical exchange of the potassium
cation with tetrabutylammonium the integration of the peaks
observable for the tetrabutylammonium moiety can be used as
compounds
(PPh Rh(CO)]
tion–oxidation reaction.
in
catalysis;
the
use
of
[
3
)
2
x
[XW12
O40] for a combined hydroformyla-
internal reference for the incorporation of the SiCH
2
CH
2
PPh
39[O-
2 2 2 2
(SiCH CH PPh ) ]} .‡ The Si NMR spectrum showed the
2
4
units and clearly show the formation of {SiW11
O
42
29
In this paper, we present a new catalytically active noble
metal metal-organic complex–polyoxometalate hybrid com-
pound whereby a Wilkinson’s type catalyst, Rh( )Cl(Ph) , is
3
covalently attached to a Keggin type polyoxometalate through
an alkylene bridging spacer, Scheme 1.
Very recently, a Keggin type polyoxometalate was attached
covalently via an alkylene spacer group to a metallosalen
necessary and expected peaks and intensities at 282.4 and
I
251.4 ppm for the polyoxometalate silicon heteroatom and two
2 2 2
silicon atoms originating from the SiCH CH PPh groups,
31
respectively. The phosphorus atom is observed by P NMR at
1
83
212.43 ppm. Importantly also, the
peaks with relative intensities of 2 : 2 : 1 : 2 : 2 : 2, characteristic
of the C symmetry of the polyoxometalate unit. The present
{SiW11
properties very similar to the previously reported {Si-
W NMR exhibited six
moiety; the polyoxometalate significantly modifies the elec-
s
5
42
tronic properties of the metallosalen unit. The Rh(
I) metal-
O39[O(SiCH
2
CH
2
PPh
2
)
2
]}
anion shows spectral
organic–polyoxometalate compound, RhOM–POM, was used
as an efficient hydrogenation catalyst in two ways. In the first
case, a RhOM–POM complex was prepared that was soluble in
4
2
7
W
11
O39[O(SiR)
2
]} anions (R = (CH
2
)
3
Cl, CHNCH
2
).
The Wilkinson’s type–polyoxometalate hybrid compound,
)Cl]}4
,
2
was ob-
{
SiW11
O
39[O(SiCH
2
CH PPh PPh Rh(
2
2
)
2
3
I
tained by reacting the phosphino functionalized polyox-
4
2
ometalate, {SiW11
rhodium compound, chlorobis(cyclooctene)rhodium(
presence of one equivalent of triphenylphosphine.§ The com-
O
39[O(SiCH
2
CH
2
PPh
2
)
2
]} , with a suitable
I
), in the
3
1
pound is easily identifiable by the characteristic shift in the
NMR spectrum due to complexation by Rh( ). Peaks at 17.33
and 36.46 ppm attributable to the triphenylphosphine and
alkyldiphenylphospine ligands to Rh( ) were clearly observed.
P
I
I
As indicated above, the RhOM–POM hybrid complex was
tested as an alkene hydrogenation catalyst in both (a) a single
liquid organic phase reaction medium, 19 : 1 toluene : ethanol,
8
using the
(
Q
4
{SiW11
O
39[O(SiCH
CH
an aqueous biphasic liquid–liquid reaction with a water soluble
catalyst, Li {SiW11 39[O(SiCH CH PPh PPh Rh( )Cl]},
2
CH
2
PPh
2
)
2
PPh
3
Rh(
I
)Cl]}
8
8
+
Q RhOM–POM; Q = [(C
8
H
17
)
3
3
N ]) as catalyst and (b)
Scheme 1 Synthetic scheme for the preparation of the rhodium based metal-
organic–polyoxometalate hybrid compound.
4
O
2
2
2
)
2
3
I
2
690
CHEM. COMMUN., 2003 2690–2691
This journal is © The Royal Society of Chemistry 2003

Table
SiCH
1
CH
Hydrogenation of alkenes catalyzed by {SiW11
O
39[O-
aqueous solution yielding as
(SiCH CH PPh ]} (3.34 g, 75% yield). The dried powder was stored in a
desiccator. IR (cm ): 507, 535, 690, 710, 812, 931, 970, 1001, 1048, 1123,
179, 1278, 1405, 1440, 1485, 1588, 1619, 2907, 2957, 2996, 3059, 3083.
4
a white precipitate K {SiW11O39[O-
42
(
2
2
2 2 3
PPh ) PPh Rh(I)Cl]} using organic monophasic and aqueous
2
2
2 2
)
21
biphasic reaction protocols
1
Yield/mol%a
1
H-NMR (400 MHz, [D
.05 (m, 4H, PCH ), d = 7.2–7.5 (m, 20H, Ar). { H}-decoupled Si-NMR
(79.495 MHz, [D ]DMSO, 25 °C): d = 282.4 (1Si for the polyoxometalate
heteroatom), d = 251.4 (2Si for the alkylsilyl groups). P-NMR (101.271
6 2
]DMSO, 25 °C): d = 0.52 (m, 4H, SiCH ), d =
1
29
2
2
Organic
monophasic
Aqueous
biphasic
6
31
Substrate
Product
1
83
MHz, [D
6
6
]DMSO, 25 °C): d = 212.43 (bs). W-NMR (16.671 MHz,
1
1
1
-Heptene
-Octene
-Decene
n-Heptane
n-Octane
n-Decane
n-Octane
Ethylbenzene
Bibenzyl
99+
99+
99+
99+
99+
99+
99+
99+
97
98
99+
99+
99+
99
[D ]DMSO, 25 °C): d = 2245.83 (s, 2W), d = 2170.10 (s, 2W), d =
2122.68 (d, 2W), d = 2110.11 (s, 1W), d = 2105.81 (s, 2W), d =
2104.79 (s, 2W).
4
trans-2-Octene
Styrene
cis-Stilbene
Cyclohexene
‡ Q
4
{SiW11
2 2
O39[O(SiCH CH PPh
2 2
) ]} was prepared by suspending
K
4
{SiW11
O
39[O(SiCH CH
2
2
PPh )
2 2
]} (33 mg, 10 mmol) in a few mL of
degassed water; tetrabutylammonium chloride (11.6 mg, 44 mmol)
dissolved in dichloromethane was added and the mixture was vigorously
stirred under argon for three hours. The organic phase was separated and
Cyclohexane
Reaction conditions: organic monophasic - 1 mmol alkene, 4 mmol
{SiW11 39[O(SiCH CH PPh PPh Rh( )Cl]}, 1 mL toluene–EtOH, 2
atm H , 2 h, 60 °C; aqueous biphasic - 1 mmol alkene, 0.25 mL of 1 mM
Li {SiW11 39[O(SiCH CH PPh PPh Rh( )Cl]} in H O, 2 atm H , 2 h, 60
C. The reaction mixtures were analyzed by GC and GC-MS. Yields were
4
4
evaporated to dryness, leaving Q
4
{SiW11
O
39[O(SiCH
2
CH
]DMSO, 25 °C):
CH3, d = 1.31 (m,
CH CH ), d = 2.05
3
), d = 7.2–7.5 (m,
2 2 2
PPh ) ]} Q =
Q
4
O
2
2
)
2 2
3
I
+
1
[(C
4 9 4
H ) N ] as a white powder. H-NMR (400 MHz, [D
6
2
+
d = 0.52 (m, 4H, SiCH
2 2 3
), d = 0.92 (t, 48H, N(CH )
4
O
2
2
)
2 2
3
I
2
2
+
+
a
32H, N(CH
m, 4H, PCH
2 2 2
) CH CH3, d = 1.53 (m, 32H, NCH
2
CH
CH
2
2
3
°
+
(
2
), d = 3.16 (t, 32H, NCH
2
CH
2
CH
2
measured by GC and is mol alkane/mol alkane + alkene + other products.
Occasionally traces of isomerization products were observed.
2
§
0H, Ar).
8
4
Q {SiW11
O
39[O(SiCH
2
CH
4
2
PPh
{SiW11
2
)
2
PPh
39[O(SiCH
{SiW11 39[O(SiCH
47 mg) in toluene (2 mL) and adding chlorobis(cyclooctene)rhodium(
3
Rh(
I
)Cl]} was synthesized
CH PPh ]} (prepara-
CH PPh ]})
8
under argon by dissolving Q
tion the same as described above for Q
O
4
2
2
2 2
)
2
4
O
2
2 2
)
(LiRhOM–POM) and a water immiscible alkene as substrate;
11
I
(
)
no organic solvent was needed. From the results collected in
Table 1, one can observe that the catalytic hydrogenation
reactions proceeded smoothly to give alkanes from alkenes
using both the monophasic and aqueous biphasic reaction
protocols.
The catalytic efficiency of the QRhOM–POM, catalyst in the
monophasic system was compared to that of the classic
(0.5 mmol, 4 mM in toluene) followed by addition of triphenylphosphine (1
mmol) dissolved in toluene (0.5 mL). After few minutes ethanol (2 mL) was
added to prevent dimerization at the Rh(
(SiCH CH PPh )2PPh Rh(
solvents. 3 P-NMR (101.271 MHz, [D8]toluene + 5% EtOH, 25 °C): d =
I
) center. 8
4
Q {SiW11O39[O-
2
2
1
2
3
I)Cl]} was isolated after evaporation of the
1
7.33 (s, Ph
3 2 2
PRh, 1P), d = 36.46 (bs, CH Ph PRh, 2P). The lithium salt of
2
{
SiW11 39[O(SiCH
O
2
CH
2
PPh
2
)
2
PPh
3
Rh(
I
)Cl]}4 was used without isola-
4
tion and was prepared by suspending the desired amount of
39[O(SiCH CH PPh PPh Rh(
4
Q {Si-
(
(
Ph
1 mmol 1-octene, 4 mmol QRhOM–POM, 1 mL toluene–
). The reaction rates were zero order with
respect to both 1-octene and H . The rate constants under the
3
P)
3
Rh( )Cl Wilkinson’s catalyst using 1-octene as substrate
I
W
11
O
2
2
)
2 2
3
I)Cl]} in deionized water followed by
addition of five equivalents of LiCl. After vigorous mixing for two hours the
suspension was filtered and the filtrate obtained was used directly for
catalytic hydrogenation.
EtOH, 2 atm H
2
2
2
3
21
23
21
given conditions 9.0 3 10 M min and 5.8 3 10 M min
for QRhOM–POM and (Ph P) Rh( )Cl, respectively. The
Rh( )metal-organic–polyoxometalate hybrid compound was a
50% more effective catalyst on a molar basis, probably due to
the presence of alkyl chains in the phosphine ligands, which
provide improved stabilization of the intermediate Rh(III
I
1 Chem. Rev., 1998, 98, (1) – thematic issue.
3
3
2
I. V. Kozhevnikov, Catalysis by Polyoxometalates, Wiley, Chichester,
England, 2002; C. L. Hill and C. M. Prosser-McCartha, Coord. Chem.
Rev., 1995, 143, 407; N. Mizuno and M. Misono, Chem. Rev., 1998, 98,
I
~
1
71; R. Neumann, Prog. Inorg. Chem., 1998, 47, 317.
)
3
P. Gouzerh and A. Proust, Chem. Rev., 1998, 98, 77; R. Villanneau, R.
Delmont, A. Proust and P. Gouzerh, Chem. Eur. J., 2000, 6, 1184; C.
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V. Cabuil, Chem. Mater., 2001, 13, 3061; C. Hu, Y. Wang, Y. Li and E.
Wang, Chem. J. Internet, 2001, 3 at http://www.chemistrymag.org/cji/
2001/036022re.htm.
species after oxidative addition of hydrogen. Catalyst recycle
experiments showed that the water soluble LiRhOM–POM
could be recovered by phase separation from the product
(
1-octene was hydrogenated under the conditions described in
Table 1) and reused without loss of activity. Similarly,
QRhOM–POM was recovered from the organic phase by
addition of ether and reused without loss of activity.
4
5
6
A. R. Siedle, C. G. Markell, P. A. Lyon, K. O. Hodgson and A. L. Roe,
Inorg, Chem., 1987, 26, 219.
I. Bar-Nahum, H. Cohen and R. Neumann, Inorg. Chem., 2003, 42,
These catalytic hydrogenation reactions represent the first
example of the use of such metal-organic–polyoxometalate
hybrid complexes as catalysts. Advantages that can be noted in
the use of such hybrid complexes versus the classic Wilkinson’s
catalyst are the potential for dual applications, permitting
3
677.
S. R. Chowdhury, J. E. ten Elshof, N. E. Benes and K. Keizer,
Desalination, 2002, 144, 41.
7 P. Judenstein, C. Deprun and L. Nadjo, J. Chem. Soc., Dalton Trans.,
1991, 1991.
8 W. H. Knoth, J. Am. Chem. Soc., 1979, 101, 2211; M. S. Weeks, C. L.
Hill and R. F. Schinazi, J. Med. Chem., 1992, 35, 1216; A. Mazeaud, N.
Ammari, F. Robert and R. Thouvenot, Angew. Chem., Int. Ed. Eng.,
9
catalyst separation by nanofiltration or by phase separation.
This research was supported by the Minerva Foundation and
the Helen and Martin Kimmel Center for Molecular Design. R.
N. is the Israel and Rebecca Sieff Professor of Organic
Chemistry.
1
996, 35, 1961; F. Zonnevijlle and M. T. Pope, J. Am. Chem. Soc., 1979,
1
01, 2731; P. Judenstein, Chem. Mater., 1992, 4, 4; N. Ammari, G.
Hervé and R. Thouvenot, New J. Chem., 1991, 15, 607; C. R. Mayer and
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I. Fournier and R. Thouvenot, Chem. Eur. J., 2000, 6, 105; C. R. Mayer,
S. Neveu and V. Cabuil, Angew. Chem., Int. Ed., 2002, 41, 501.
9 A highly effective nanofiltration membrane for polyoxometalate
recovery has been developed in cooperation with J. E. ten Elshof and K.
Keizer at the University of Twente. Details will be published
separately.
Notes and references
†
K
4
{SiW11
3
O
39[O(SiCH
2 2 2 2
CH PPh ) ]} was synthesized by adding 6 mL
10
(EtO) SiCH
2
CH PPh to a well stirred solution of K SiW11O39 (4 g, 1.34
2
2
8
mmol) in 100 mL of deionized water and 30 mL of acetonitrile under Ar.
The pH was adjusted to pH = 1 by addition of 1 M HCl and vigorously
stirred for 24 h. A white–yellow sticky precipitate of the undesired
organosilicate oligomers, (CH
and the acetonitrile was evaporated. Degassed isopropanol was added to the
2
CH
2
PPh
2
SiO1.5
)
n
, was removed by filtration
10 A. Tézé and G. Hervé, J. Inorg. Nucl. Chem., 1977, 39, 999.
11 A. van der Ent and A. L. Onderdelinden, Inorg. Synth., 1973, 14, 92.
CHEM. COMMUN., 2003, 2690–2691
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