Rhodium-phosphine complex catalysts tethered on silica-supported heterogeneous metal catalysts: Arene hydrogenation under atmospheric pressure

Article abstract of DOI:10.1016/S1381-1169(99)00182-X

Novel catalysts consisting of a tethered complex on a supported metal (TCSM) are described. The rhodium-phosphine complex, RhCl(CO)[Ph₂P(CH₂)₃Si(OC₂H₅)₃]₂ (Rh-P), was tethered to silica-supported metal heterogeneous catalysts, Pd-SiO₂, Ni-SiO₂ and Au-SiO₂, to produce the tethered complex catalysts Rh-P/Pd-SiO₂, Rh-P/Ni-SiO₂, and Rh-P/Au-SiO₂. The structure of the tethered Rh-P complex in all of these combination catalysts was similar to that of the free Rh-P complex. These TCSM catalysts were used to catalyze the hydrogenation of arenes at 40°C and 1 atm of H₂. Rh-P/Pd-SiO₂ was the most active TCSM catalyst and gave a maximum turnover frequency (TOF, mole H₂/mole Rh min) of 2.9 and a turnover (TO, mole H₂/mole Rh) of 1940 during a 23 hr period in the hydrogenation of toluene. The phosphine ligand was oxidized to the phosphine oxide when the tethered Rh-P complex catalyst stood in air for ≥ 2 mo. This air-aged Rh-P/Pd-SiO₂ was more active than the fresh catalyst for the hydrogenation of toluene, but the air-aged Rh-P/SiO₂ became inactive for this reaction. These two air-aged catalysts were characterized by solid state ³¹P NMR and DRIFTS using CO as a probe.

Full text of DOI:10.1016/S1381-1169(99)00182-X

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Journal of Molecular Catalysis A: Chemical 149 1999 63–74
www.elsevier.comrlocatermolcata
Rhodium–phosphine complex catalysts tethered on
silica-supported heterogeneous metal catalysts: arene
hydrogenation under atmospheric pressure
)
Hanrong Gao, Robert J. Angelici
Department of Chemistry and Ames Laboratory, Iowa State UniÕersity, Ames, IA 50011, USA
Received 11 January 1999; accepted 23 February 1999
Abstract
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.w
Ž
.
Ž
. x Ž
.
The rhodium–phosphine complex, RhCl CO Ph2P CH2 3Si OC2H5
Rh–P , was tethered to silica-supported metal
3 2
heterogeneous catalysts, Pd–SiO2, Ni–SiO2 and Au–SiO2, to give the tethered complex catalysts Rh–PrPd–SiO2,
Ž
.
Rh–PrNi–SiO2 and Rh–PrAu–SiO2. An IR DRIFTS study shows that the structure of the tethered rhodium–phosphine
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.
complex Rh–P in all of these combination catalysts is the same as that of the free Rh–P complex. These heterogenized
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.
complex catalysts consisting of a tethered complex on a supported metal TCSM were used to catalyze the hydrogenation of
arenes under the mild conditions of 408C and 1 atm of H2. The most active TCSM catalyst Rh–PrPd–SiO2 is not only
much more active than the homogeneous Rh–P complex and Pd–SiO2 but also much more active than the rhodium–phos-
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.
phine complex tethered on just SiO2 Rh–PrSiO2 . The Rh–PrPd–SiO2 catalyst gives a maximum turnover frequency
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.
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TOF, mol H2rmol Rh min of 2.9 and a turnover TO, mol H2rmol Rh of 1940 during a 23 h period in the hydrogenation
of toluene. When the tethered rhodium–phosphine complex catalyst stands in air for 2 months or longer, the phosphine
ligand is oxidized to the phosphine oxide. This air-aged Rh–PrPd–SiO2 is much more active than the fresh catalyst for the
hydrogenation of toluene, but the air-aged Rh–PrSiO2 becomes inactive for this reaction. These two air-aged catalysts were
characterized by solid state 31P NMR and DRIFTS using CO as a probe. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Rhodium–phosphine complex; Supported metal catalysts; TCSM; Tethered complex catalysts; Silica; Palladium; Nickel; Gold
1. Introduction
erable work has been devoted to tethering homo-
geneous metal complex catalysts to either or-
w
x
Immobilized homogeneous transition metal
complex catalysts offer the advantage over their
homogeneous counterparts of easy separation
from the reactant and product solutions. Consid-
ganic or inorganic supports 1–11 . In most of
the work reported to date, silica has generally
been used as the support since transition metal
complexes can easily be tethered by reacting the
surface hydroxyl groups of silica with alkoxyl-
or chlorosilane functional groups of ligands in
w
x
)
the complexes 12–21 . Silica-supported hetero-
geneous metal catalysts, such as Pd–SiO2, Ni–
Corresponding author. Tel.: q1-515-294-2603; Fax: q1-515-
294-0105; E-mail: angelici@iastate.edu
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S1381-1169 99 00182-X

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64
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
SiO2 and Au–SiO2, also have surface hydroxyl
groups that could be used to tether homoge-
neous transition metal complex catalysts. This
type of combined catalyst consisting of a teth-
TCSM catalysts in the hydrogenation of arenes,
their characterization by DRIFT spectroscopy,
and their long-term stabilities toward air are
reported.
Ž
.
ered complex on a supported metal TCSM
Ž
.
catalyst Fig. 1 could function by synergistic
action of both catalyst components and poten-
tially have high activity andror selectivity. One
possible mode by which such a TCSM catalyst
could function in hydrogenation reactions in-
volves initial adsorption of H2 on the supported
2. Experimental
2.1. Materials and analyses
w
x
Ž . Ž
Rh2Cl2 CO , PdCl2, HAuCl4 PxH2O Au,
4
metal followed by spillover of hydrogen 22–24
. Ž .
49% and Ni NO3 2 P6H2O were purchased
onto the silica where it is transferred to an
unsaturated hydrocarbon that is activated by
coordination to the metal in the tethered com-
Ž
from Strem. Silica gel 100 B.E.T. surface area,
400 m2 gy1 was obtained from Fluka. Ph2 P
.
w
x
Ž
. Ž .
plex. Recently, we reported 25 the first exam-
ple of such a combined catalyst, one consisting
of a rhodium–isocyanide complex, e.g., RhCl-
CH2 3Si OC2 H5 3 was prepared according to
w
x
the literature method 26 . Toluene and pentane
solvents were dried by distillation over CaH2
under nitrogen prior to use. The arene substrates
were used directly as obtained from commercial
sources without further purification.
w
Ž
.
Ž
.
x
Ž
.
CN CH2 3Si OC2 H5 3 3 Rh–CNR3 tethered
to silica-supported palladium. This Rh–CNR3r
Pd–SiO2 catalyst exhibits high activity for the
hydrogenation of arenes under the mild condi-
tions of 1 atm of H2 and 408C. Its activity
TOFs5.5 and TOs2420 for 8.5 h for
toluene hydrogenation is substantially higher
FTIR and DRIFT spectra were recorded on a
Nicolet 560 spectrophotometer equipped with a
TGS detector in the main compartment and a
MCT detector in the auxiliary experiment mod-
Ž
.
Ž
.
than that of the separate homogeneous RhCl-
ule AEM . The AEM housed a Harrick diffuse
reflectance accessory. The solution IR spectra
were measured in the main compartment using a
solution cell with NaCl plates. The DRIFT spec-
tra were recorded on samples in the Harrick
microsampling cup. A Varian 3400 GC inter-
faced to a Finnigan TSG 700 high-resolution
magnetic sector mass spectrometer with electron
w
Ž
.
Ž
. x
CN CH2 Si OC2 H5 3 3 catalyst, the separate
heterogeneous Pd–SiO2 catalyst, or the rho-
dium–isocyanide complex catalyst tethered on
just silica. In the present study, we describe new
TCSM catalysts that are composed of the
Ž
.w
rhodium–phosphine complex, RhCl CO Ph2-
Ž
.
Ž
. x
P CH2 3Si OC2 H5 3 2, tethered on the silica-
supported metal catalysts M–SiO2 MsPd, Ni
and Au . The catalytic activities of the resulting
Ž
Ž
.
ionization 70 eV was used for all GC-MS
measurements. Gas chromatographic analyses
were performed with a Varian 3400 GC with a
.
FID detector using 25 m HP-1 capillary column.
31
Solution 1H and P H NMR spectra were
measured on Brucker AC 200-MHz and Nicolet
NT 300-MHz spectrometers. All 31P solid-state
NMR spectra were recorded on a Bruker MSL
300 spectrometer. These spectra were measured
at room temperature and referenced with respect
Ä
4
to 80% H3PO4 aq by setting the 31P NMR
peak of solid NH4H2 PO4 to q0.8 ppm. The
Hartmann–Hahn match was set by using solid
Ž
.
Fig. 1. Conceptual illustration of a TCSM catalyst consisting of a
tethered homogeneous complex catalyst on a supported metal
heterogeneous catalyst.

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H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
65
NH4H2 PO4, and a typical 908 pulse width was
4 ms.
The rhodium content of the TCSM catalysts
was determined by atomic emission spectro-
scopy. The samples were prepared for analysis
temperature overnight. After the water was re-
moved in a rotary evaporator at 808C, the result-
ing solid was dried in an oven at 1108C for 5 h.
Then, the solid was reduced by a flow of H2 at
2508C for 4 h to give a red-brown powder of
Ž
.
Ž
.
by treating the catalyst 50 mg successively
with 5 ml of aqua regia and 5 ml of aqueous HF
Au–SiO2 Au, 10 wt.% .
(
)[
(
)
-
2.2.2. Preparation of RhCl CO Ph2 P CH2 3
Ž
.
5% at 908C, and then diluting the resulting
solution with water to 25 ml.
(
) ] ( )
Si OC2 H5 3 Rh–P
2
w
x
Following the procedure 30 for the synthe-
sis of other RhCl CO PR3 2 complexes, a mix-
ture of 0.43 g 1.1 mmol of RhCl CO and
1.71 g 4.4 mmol of C2 H5O Si CH2 3PPh2
in 40 ml of toluene was stirred at room tempera-
ture for 2 h. The solution was evaporated under
vacuum to a small volume, and pentane was
added to give a yellow precipitate of RhCl-
Ž
.Ž
.
2.2. Preparation of the catalysts
Ž
.
w
Ž
.2 x2
.
Ž
.
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.
Ž
3
2.2.1. Preparation of the silica-supported het-
erogeneous metal catalysts M–SiO2 , MsPd,
(
)
Ni, Au
Pd–SiO2: Pd–SiO2 was prepared by a proce-
dure similar to that described in the literature
27 . An aqueous solution of H2 PdCl4 prepared
by dissolving 1.2 g of PdCl2 in 80 ml of
Ž
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.
w
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.
Ž
.
x
Ž
.
CO Ph2 P CH2 3Si OC2 H5 3 2 Rh–P . IR
w
x
Ž
nCO, toluene : 1967 cm 1. Anal. calc. for
y
.
C43H62O7Si2 P2ClRh: C, 54.51; H, 6.59. Found:
C, 55.03; H, 6.14. P H NMR CDCl3, 200
MHz : d 21.92 d, JRh – P s122 Hz .
Ž
..
aqueous HCl 0.2 M was added to a flask
containing 7.0 g of SiO2. After the mixture was
stirred at room temperature overnight, the water
was removed by slow evaporation in a rotary
evaporator at 808C. The resulting solid sample
was dried in an oven at 1108C for 5 h and then
calcined in a tube furnace at 5008C in an air
flow for 4 h. The calcined sample was subse-
quently reduced in the tube furnace in a H2
flow for 4 h at 3808C and then passivated under
a flow of air at room temperature for 1 h to give
31
1
Ä
4
Ž
.
Ž
.
2.2.3. Preparation of the tethered rhodium–
phosphine complex catalysts
A mixture of 0.50 g of M–SiO2 MsPd, Ni,
Au and 0.10 g of Rh–P in 20 ml of toluene
was refluxed under an N2 atmosphere for 12 h.
After filtration, the solid was washed with
Ž
.
Ž
.
Ž
.
toluene 6=15 ml and pentane 15 ml and
dried in vacuum to give the tethered rhodium–
phosphine complex catalysts Rh–PrPd–SiO2
Ž
.
black Pd–SiO2 Pd, 10 wt.% powder.
Ni–SiO2 28 : A solution of Ni NO3 2 P6H2O
2.4 g in 40 ml of water was added dropwise to
w
x
Ž
.
Ž
.
Ž
0.65 wt.% Rh , Rh–PrNi–SiO2 0.60 wt.%
Ž
.
.
Ž
.
Rh and Rh–PrAu–SiO2 0.60 wt.% Rh . IR
a flask containing 5.0 g of SiO2; then about 30
ml of water was added dropwise. The resulting
slurry was stirred vigorously on a hot plate until
all excess liquid was evaporated; the resulting
solid was dried overnight in a tube furnace at
1208C under flowing air. The solid was reduced
under a H2 flow at 4508C for 6 h and then
cooled in a flowing N2 atmosphere. The black
silica-supported nickel catalyst, Ni–SiO2, has a
Ni content of 10 wt.%.
Ž
.
DRIFT, nCO for the three catalysts : 1980
cmy
.
1
The rhodium–phosphine complex catalyst
Ž
.
tethered on SiO2, Rh–PrSiO2, 0.60 wt.% Rh ,
was prepared by the same procedure as that
used for the preparation of Rh–PrM–SiO2 by
using 0.5 g of SiO2 instead of M–SiO2. IR
y
1
Ž
.
DRIFT, nCO : 1980 cm
.
[
2.2.4. Preparation of the adsorbed RhCl-
w
x
(
) ]
(
w
)
Au–SiO2 29 : A mixture of 5.0 g of SiO2
CO 2 catalyst Rh–ClrPd–SiO2
2
Ž
.
Ž
.
2 x2
Ž
and 1.0 g of HAuCl4 PxH2O Au, 49% in 30
A mixture of RhCl CO
30 mg, 0.077
Ž
.
.
ml of aqueous HCl 0.2 M was stirred at room
mmol and 0.50 g of Pd–SiO2 in 10 ml of

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66
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
toluene was refluxed for 4 h. After filtration, the
be tethered to the SiO2 or the silica can be
Ž
.
Ž
.
Ž
.
solid was washed with toluene 2=15 ml and
then dried in vacuum at room temperature. The
rhodium content of the resulting anchored cata-
functionalized with Ph2P CH2 3Si OC2H5 3
first and then reacted with a metal complex
precursor. For the synthesis of TCSM catalysts,
the first approach not only offers control over
the metal–ligand ratio in the complex but also
avoids adsorption or coordination of the phos-
Ž
.
Ž .
lyst was 1.45 wt.%. IR DRIFTS, nCO : 2095 s ,
y
1
Ž .
2028 s cm
.
Ž
.
Ž
.
phine end of C2 H5O Si CH2 3PPh2 on the
3
2.3. Catalytic hydrogenation studies
silica-supported metal. Therefore, this method
was used for the preparation of the TCSM
catalysts described herein. The three TCSM cat-
alysts, Rh–PrPd–SiO2, Rh–PrNi–SiO2 and
Rh–PrAu–SiO2, were prepared by reaction of
The hydrogenation reactions were carried out
in a three-necked, jacketed vessel containing a
stirring bar and closed with a self-sealing silicon
rubber cap; the vessel was connected to a vac-
uumrhydrogen line and to a constant pressure
gas buret. The temperature of the ethylene gly-
col that circulated through the jacket was main-
tained with a constant temperature bath. The
reaction temperature and H2 pressure were 408C
and 1 atm, respectively. After the catalyst was
added and the atmosphere in the vessel was
replaced with hydrogen, the arene substrate was
added and the hydrogen uptake was followed
with the constant-pressure gas buret.
Ž
.
Ž
.
Ž
.
Rh2Cl2 CO with C2 H5O Si CH2 3PPh2 in
a mole ratio of 1:4 to form RhCl CO -
4
3
Ž
.
w
Ž
.
Ž
. x
Ph2 P CH2 3Si OC2 H5 3 2, which was reacted
with hydroxyl groups on the surface of the
silica-supported metal catalysts, Pd–SiO2, Ni–
SiO2 and Au–SiO2 Scheme 1 . DRIFT spectra
of all the TCSM catalysts show a strong CO
band at 1980 cmy1, which is very similar to
Ž
.
Ž
.Ž
Ž
.
those of the analogous trans-RhCl CO PPh3 2
y
1
Ž
.
w
x
Ž
.
.
w
.
30 , RhCl CO Ph2 P CH2 -
3
1980 cm
Ž
y
1
.
x
Ž
Si OC2 H5y3 2 1967 cm , and Rh–PrSiO2
1
Ž
.
1980 cm , which suggests that the trans,
Ž
.Ž
.
square-planar geometry of RhCl CO PPh3 2 is
also present in the tethered complex catalysts.
3. Results
3.1. Synthesis of the TCSM catalysts
3.2. Catalytic actiÕities of the TCSM catalysts
for the hydrogenation of toluene
Since most transition metals form stable
Ž
.
-
3
complexes with phosphines, Ph2 P CH2
Ž
.
Ž
.
Si OC2 H5 3 is often used as the linker to tether
homogeneous transition metal complex catalysts
to silica. Either the preformed complex contain-
The rates of hydrogenation Table 1 of
toluene to methylcyclohexane at 408C and under
1 atm of H2 in the presence of the TCSM
Ž
.
Ž
.
Ž .
Ž
ing the Ph2 P CH2 3Si OC2 H5 3 ligand s can
catalysts Rh–PrPd–SiO2, Rh–PrNi–SiO2 and
Scheme 1.

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)
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
67
Table 1
Hydrogenation of toluene to methylcyclohexanea
Maximum TOFb
TOc mol H2rmol Rh
H2 uptakec mmol
.
Ž
.
Ž
Catalyst
Ž
.
mol H2rmol Rh min
Ž
.
.
Ž
Ž
Ž
Ž
Ž
Ni–SiO2
Au–SiO2
Pd–SiO2
PrPd–SiO2d
Rh–Pe
–
–
–
–
0.07
0.03
0.29
0.25
1.10
2.90
–
–
–
–
0 23
Ž
0 23
1.05 23
0.45 23
0.66 24
0.67 26
1.05 23
Ž
0.79 23
1.74 20
.
.
.
.
.
.
.
.
Ž
Ž
Ž
Ž
Ž
Ž
.
.
.
.
.
.
45 24
33 26
361 23
273 23
665 20
1940 23
RhCl CO PPh3 f2 qPd–SiO2
Rh–PrSiO2
Ž
.Ž
.
Rh–PrNi–SiO2
Rh–PrAu–SiO2
Rh–PrPd–SiO2
Ž
Ž
6.13 23
aReaction conditions: catalyst 50 mg; 5 ml of toluene; 408C, 1 atm.
b TOF is defined as moles of H2 uptake per mole rhodium per min.
c
Ž
.
Ž
.
Turnover TO and H2 uptake correspond to the reaction time in parentheses in h .
d PrPd–SiO2 is Ph2 P CH2 3Si OC2 H5 tethered on Pd–SiO2.
Ž
.
Ž
.
3
e14.5 mmol of Rh–P.
f
Ž
.Ž .
20 mmol of RhCl CO PPh3 .
2
.
Rh–PrNi–SiO2 , the soluble homogeneous
are also more active than the homogeneous
catalyst Rh–P and the heterogeneous supported
metal catalysts Ni–SiO2 and Au–SiO2. The
Ž
.
complex Rh–P catalyst and the heterogeneous
catalysts Pd–SiO2, Ni–SiO2 and Au–SiO2
were determined by following the rate of H2
uptake. Fig. 2 shows plots of turnover frequency
Ž
.
Ž
.
activity TO of Rh–PrAu–SiO2 is about two
times higher than that of Rh–PrSiO2, but Rh–
PrNi–SiO2 is about the same as Rh–PrSiO2.
It can also be seen that the activity of the
non-rhodium-containing catalyst PrPd–SiO2,
Ž
.
TOF vs. time for the hydrogenation of toluene
over the catalysts, Rh–PrPd–SiO2, Rh–PrNi–
SiO2, Rh–PrAu–SiO2 and Rh–PrSiO2. Ex-
cept for Rh–PrNi–SiO2, which shows an in-
duction period of about 1 h after which its
activity increases slowly, all the other catalysts
are active from the beginning. The TOF values
of the four tethered catalysts, Rh–PrM–SiO2
Ž
.
-
3
which was prepared by tethering Ph2 P CH2
Ž
.
MsPd, Ni, Au and Rh–PrSiO2, increase
initially, and then after reaching a maximum,
they slowly decrease with time. From the data
Ž
in Table 1, it is evident that the activity as
measured by the maximum TOF, TO or H2
.
uptake of the TCSM catalyst with Pd as the
supported metal is much greater than those with
Ni or Au. The activity of Rh–PrPd–SiO2 is
about 40 times higher than that of the homoge-
Ž
.
neous catalyst Rh–P , six times higher than the
Ž
heterogeneous silica-supported palladium Pd–
SiO2 , and about six times higher than that of
the complex Rh–P tethered to just SiO2 Rh–
PrSiO2 . Rh–PrNi–SiO2 and Rh–PrAu–SiO2
Ž
.
Fig. 2. Plots of the rates TOF of hydrogenation of toluene over
.
Ž .
Ž .
Ž .
Ž .
a Rh–PrPd–SiO2 , b Rh–PrAu–SiO2 , c Rh-PrSiO2 and d
Rh–PrNi–SiO2. Reaction conditions are the same as those in
Table 1.
Ž
.

(
)
68
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
Ž
.
Ž .
spectra DRIFTS of the used catalysts showed
Si OC2 H5 3 to Pd–SiO2 under the same condi-
tions as those used for making the tethered
y
one Õ CO band very near 1980 cm 1, which is
the same as the fresh catalyst. This suggests that
the tethered rhodium complex is stable under
the hydrogenation conditions.
Ž
.
Ž
.
complex catalyst Rh–PrPd–SiO2 , is much
lower than that of Rh–PrPd–SiO2 and even
lower than that of Pd–SiO2, which suggests that
the tethering conditions are not, in some man-
ner, activating the Pd–SiO2. Furthermore, the
3.3. Effect of air on the hydrogenation actiÕity
Ž
.
activity TO of the soluble, non-tethered rho-
Ž
.Ž
.
dium–phosphine complex RhCl CO PPh3 2 in
combination with Pd–SiO2 is only about 2% of
that of the tethered catalyst Rh–PrPd–SiO2,
which indicates that tethering the complex to
the surface is important for the TCSM catalyst
activity. Although the activity of the Rh–
PrPd–SiO2 TCSM catalyst is lower than that of
the rhodium–isocyanide complexes RhCl CO -
CN CH 2 3Si OC2 H 5
CH2 3Si OC2 H5 3 3 tethered on Pd–SiO2 as
reported in our previous paper 25 , it is, to the
best of our knowledge, higher than that of any
other homogeneous complex or immobilized
complex catalysts reported in the literature for
the hydrogenation of toluene under these mild
conditions. Under similar conditions, Blum et
After the TCSM catalysts and Rh–PrSiO2
were exposed to air by allowing them to stand
in screw-capped vials in air for about 2 months,
their activities in the hydrogenation of toluene
Ž
.
were different Table 2 from those of the fresh
catalysts. The air-aged Rh–PrPd–SiO2 is much
more active than the fresh one. Its maximum
TOF increases from 2.90 to 5.4 and the TO
increases from 1940 during a 23 h time period
to 2464 during a 9 h period. However, the
activities of the other two TCSM catalysts, Rh–
PrNi–SiO2 and Rh–PrAu–SiO2 are about the
same as those of the fresh catalysts. The catalyst
tethered on just SiO2, Rh–PrSiO2, becomes
inactive for the hydrogenation of toluene after it
stands in air for about 2 months. The IR spectra
Ž
Ž
.
w
Ž
Ž
.
Ž
.
3x2
w
,
and RhCl CN-
.
Ž
. x .
w
x
w
x
Ž
.
al. 31 used RhCl3-Aliquat-336 to catalyze the
hydrogenation of toluene in a medium of H2O–
CH2Cl2. After 5 h, only ca. 3 mol of toluene
were converted to methylcyclohexane per mole
of rhodium. The TO value for the immobilized
DRIFTS of all the air-aged TCSM catalysts
and Rh–PrSiO2 show that the intensity of the
CO absorption at 1980 cmy is markedly re-
1
duced compared to that of the fresh catalysts.
After standing in air for about 4 months, the
nCO band of Rh–PrPd–SiO2 at 1980 cmy
1
w
Ž
.x
complex catalyst prepared from RhCl nbd
2
Ž
.
Ž
.
nbdsnorbornadiene and phosphinated silica
disappeared Fig. 3c . It was also observed that
w
x
is 200 at 80 atm of H2 and 308C for 1.5 h 32 .
w
x
Recently, Corma et al. 33 reported that
the rhodium complex Rh COD N–N PF6
COD s 1,5–cyclooctadiene, N–N s 2- 3-tri-
ethoxysilypropyl-aminocarbonyl pyrrolidine
w
Ž
.Ž
.x
Table 2
Hydrogenation of toluene to methylcyclohexane over air-aged
tethered rhodium catalystsa
Catalystb
Ž
Ž
.
.
Maximum TOFb
TOc
anchored on zeolites catalyzes the hydrogena-
tion of arenes under 6 atm of H2 and at 808C.
Hydrogenation of arenes in the presence of
homogeneous catalysts is generally performed
under high G15 atm H2 pressure 34–41 .
Only a few homogeneous catalysts are active
using 1 atm of H2, but their activities are low
42–45 .
After the TCSM, Rh–PrM–SiO2, catalysts
were used for the hydrogenation of toluene, IR
Ž
.
Ž
.
mol H2 rmol Rh min
mol H2 rmol Rh
Ž
.
.
.
Rh–PrSiO2
0
0 15
Ž
250 23
Rh–PrNi–SiO2 0.3
Rh–PrAu–SiO2 1.5
Rh–PrPd–SiO2 5.4
Ž
481 15
Ž .
2464 9
Ž
.
w
x
aReaction conditions are the same as those in Table 1.
bAir-aged for 2 months.
c TOF is defined as moles of H2 uptake per mole rhodium per
min.
w
x
d TO and H2 uptake correspond to the reaction time in parentheses
Ž
.
in h .

(
)
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
69
Table 3
Hydrogenation of arenes over air-aged Rh–PrPd–SiO2a
Substrate
Maximum TOFb
TOc
Ž
.
Ž
.
mol H2 rmol Rh min
mol H2 rmol Rh
Ž .
1780 8
Toluene
Anisole
Benzene
4.5
4.7
5.0
Ž .
2060 8
Ž .
1582 8
Ž .
600 10
Methyl benzoate 1.5
a
Ž
.
Reaction conditions: 25 mg of catalyst air-aged for 2 months ; 5
ml of heptane solvent; 0.5 ml of substrate, 408C, 1 atm.
b TOF is defined as moles of H2 uptake per mole rhodium per
min.
c TO and H2 uptake correspond to the reaction time in parentheses
Ž
.
in h .
Ž .
Ž .
b after Rh–
Fig. 3. IR spectra of
a
fresh Rh–PrPd–SiO2 ,
PrPd–SiO2 was exposed to a mixture of gaseous N2 and H2O for
3 weeks, c air-aged Rh–PrPd–SiO2 after 4 months.
Ž .
after the Rh–PrPd–SiO2 catalyst was exposed
to a mixture of gaseous H2O and N2 for 3
weeks, the nCO absorption at 1980 cmy also
1
Ž
.
decreased Fig. 3b . This suggests that the de-
crease in the nCO intensity of the air-aged cata-
lysts is due to the water in the air, which may
mean that water displaces the CO ligand from
the tethered rhodium complex. A similar effect
of water on IR spectra of catalysts prepared by
w
Ž
.2 x2
the immobilization of RhCl CO
phinated silica was reported by Bartholin et al.
45 .
The air-aged Rh–PrPd–SiO2 catalyst is also
very active for the hydrogenation of other arenes
on phos-
w
x
Ž
.
under the same mild conditions Table 3 . The
higher rate of anisole hydrogenation as com-
pared with that of methyl benzoate suggests that
electron donating substituents in the arene ac-
celerate the rate. A comparison of the hydro-
Table 4
31 P chemical shifts from 31 P MAS NMR studies of the tethered
rhodium–phosphine catalysts and related compounds
Ž31
.
P
Sample
Fresh or air-ageda
d
Ž
.
genation rate maximum TOF and TO of ben-
zene with those of toluene and anisole indicates
that steric effects are relatively unimportant.
PrSiO2b
fresh
–
fresh
fresh
air–aged
air–aged
y17.6
39.5
23.2
23.0
38.6
39.0
P5OrSiO2c
Rh–P
Rh–PrPd–SiO2
Rh–PrPd–SiO2
Rh–PrSiO2
3.4. Characterization of Air-aged Rh–PrPd–
SiO2 and Rh–PrSiO2
aAir-aged for 4 months.
b PrSiO2 is Ph2 P CH2 3Si OC2 H5 tethered on SiO2.
Ž
.
Ž
.
Ž
.Ž
.
Ž 3
.
c P5OrSiO2 is Ph2 P O CH2 3Si OC2 H5 tethered on SiO2.
In order to understand the basis for the effect
of air on the hydrogenation activity of Rh–
3
w
x
See Ref. 12 .

(
)
70
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
TOF of 6.7 and a TO of 3145 during 9 h for the
hydrogenation of toluene, which is higher than
that of the untreated catalyst A. No CO band
was observed in the DRIFT spectrum of the
used catalyst C. Yet when C was treated with
CO, the resulting sample D gives two CO bands
y
1
Ž .
Ž .
at 2093 s and 2023 w cm again. Moreover,
Ž .
after the air-aged Rh–PrPd–SiO2 A was used
for the hydrogenation of toluene followed by
Ž .
treatment with CO, the resulting sample F also
Ž .
gives two CO adsorptions at 2093 s and
y
2023 w cm 1. Although the IR spectra of A, B
Ž .
and F are very similar, F is more active than A
or B in the hydrogenation of toluene. Its maxi-
mum TOF is 10.4 and TO is 3908 during a
period of 8 h. However, the air-aged Rh–Pr
SiO2, even after treatment with CO, is still
inactive for the hydrogenation of toluene.
Ž .
Fig. 4. IR spectra of a air-aged Rh–PrPd–SiO2 after treatment
Ž .
with CO, b air-aged Rh–PrSiO2 after treatment with CO.
and 2023 cmy1. No nCO band was observed at
1980 cmy1. However, the relative intensities
of the two nCO absorptions were different
4. Discussion
Ž
.
for the two catalysts Fig. 4 . For Rh–PrSiO2,
the intensities of the two bands were about the
same, but for Rh–PrPd–SiO2, the intensity of
4.1. TCSM catalysts
the band at 2093 cmy was much stronger than
The simple tethering of RhCl CO Ph2 P-
1
Ž
.w
that of the band at 2023 cmy
.
CH2 3Si OC2 H5 3 2 to the silica surface of
1
Ž . Ž
. x
Ž
.
Ž
Further experiments Scheme 2 showed that
SiO2-supported metal catalysts M–SiO2 Ms
Ž .
A
.
after the air-aged Rh–PrPd–SiO2
treated with CO in toluene at room temperature
overnight, the resulting sample B exhibits a
was
Pd, Ni. Au gives TCSM catalysts Rh–PrM–
SiO2, in which the Rh complex retains its trans-,
square planar geometry as indicated by DRIFT
Scheme 2.

(
)
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
71
w x
adsorption and activation of H2 47 . The induc-
spectra of the catalysts in the nCO region. Since
the nCO value is the same for the Rh–PrM–
SiO2 catalysts and Rh–PrSiO2, there appears
to be no direct interaction between the Rh com-
plex and the supported metal M.
Ž
.
tion period and lower activity Fig. 2d of Rh–
PrNi–SiO2, as compared to Rh–PrPd–SiO2
may be due to a slow and incomplete reduction
of the oxided Ni under the reaction conditions.
In addition, nickel metal is a poorer hydrogen
spillover initiator compared to palladium metal
because the strength of the Ni–H bond is greater
Of the TCSM catalysts, Rh–PrPd–SiO2 is
Ž
.
the most active Table 1 . Its activity is greater
than that of the homogeneous Rh–P complex,
the heterogeneous supported palladium catalyst
w
x
than that of the Pd–H bond 48 .
Ž
.
Pd–SiO2 or Rh–P tethered on just SiO2. Thus,
The activity of Rh–PrPd–SiO2 is also much
Ž
.
both the tethered complex Rh–P and sup-
higher than that of PrPd–SiO2 or the combina-
Ž
.
Ž
.Ž
.
ported metal Pd–SiO2 components are neces-
sary for the high catalytic activity of TCSM
catalysts. The function of the supported metal
tion of RhCl CO PPh3 2 with Pd–SiO2. The
first result excludes the possibility that the high
activity of Rh–PrPd–SiO2 results from an al-
teration of the supported palladium particles
during the tethering process. The second result
shows that tethering of the rhodium complex to
the surface is required for the high activity of
Rh–PrPd–SiO2.
Ž
.
Pd–SiO2 may be to generate spillover hydro-
gen onto the SiO2 surface. It is well known
w
x
22–24 that inorganic oxide supported hetero-
geneous metal catalysts, such as Pd–SiO2, Pt–
SiO2, Pd–Al2O3, etc., can dissociatively adsorb
hydrogen which spills over onto the oxide sur-
face. This behavior of Pd–SiO2 suggests that
the Rh–PrPd–SiO2 catalyst functions by acti-
vating H2 on the Pd–SiO2. The resulting
spillover hydrogen on the silica surface is then
in the vicinity of the tethered Rh–P complex
which coordinates and activates toluene for re-
action with the spillover hydrogen. While this
spillover mechanism logically explains the high
activity of the TCSM catalysts, it does not
exclude the possibility that a mechanism requir-
ing the close proximity of the tethered complex
and supported metal is involved. In such a
mechanism, the tethered Rh may coordinate to
toluene via only two of the toluene carbons.
Then, the remaining four carbons are hydro-
genated on the supported Pd if the tethered
complex is near a Pd island. Supporting this
4.2. Effect of air-aging on Rh–PrPd–SiO2 and
Rh–PrSiO2
Samples of Rh–PrPd–SiO2 that are allowed
to stand in air for 2 months are more active than
the fresh catalyst for the hydrogenation of
toluene. However, air-aged Rh–PrSiO2 is inac-
tive for this hydrogenation. Solid state 31P NMR
and DRIFTS analyses show that the original
Ž
.
rhodium–phosphine complex Rh–P in the
air-aged catalysts has been transformed into a
new species that does not contain a CO ligand.
However, upon treatment of the air-aged Rh–
PrPd–SiO2 and Rh–PrSiO2 with CO, these
samples exhibit DRIFT spectra with nCO bands
y
1
Ž .
Ž .
at 2093 s , 2023 w cm for Rh–PrPd–SiO2
y
1
Ž .
Ž .
and 2093 s , 2023 s cm
These adsorptions are very similar to those
for Rh–PrSiO2.
w
x
type of mechanism is the known 46 hydro-
IŽ
.
2
2
2
q
y1
w
Ž
. Ž
Ž
.
x
Ž
w
Ž .
Ž .
.
genation of Os NH3 5 h -C6H62
to the cy-
2095 s , 2027 s cm
for Rh CO on SiO2
2
q
w
. Ž
.
x
x
clohexene complex Os NH3 5 h -C6H10
in
49,50 . The positions and relative intensities of
the presence of PdrC under 1 atm of H2 and
308C. Under the same conditions, PdrC is inac-
tive for the hydrogenation of benzene.
the nCO bands for CO-treated, air-aged Rh–
IŽ
.
2
PrSiO2 suggest that Rh CO is indeed formed
IŽ
.
on this catalyst. This formation of Rh CO
2
The lower activity of Rh–PrAu–SiO2 as
compared with Rh–PrPd–SiO2 may be related
to the fact that gold is a very poor metal for the
from the tethered complex is accompanied by
the generation of tethered phosphine oxide
Ž .Ž
.
Ž
.
Ph2 P O CH2 3Si OC2 H5 3, which suggests

(
)
72
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
that the air-aging process involves oxidation of
nificant activity of Rh–ClrPd–SiO2 suggests
that there are additional species on the Rh–
ClrPd–SiO2 that give rise to its activity.
the phosphine and formation of RhIrSiO2. To
IŽ
.
further confirm the assignment of Rh CO r
Ž
Ž2.
Ž
SiO2 to the equal intensity bands 2093 s ,
Of the air-aged catalysts Rh–PrSiO2, Rh–
y
1
Ž .
.
.
2023 s cm
of the CO-treated, air-aged Rh–
PrPd–SiO2, Rh–ClrPd–SiO2 , Rh–PrPd–
Ž
.
PrSiO2, we prepared a sample of Rh2Cl2 CO
SiO2 is the most active for the hydrogenation of
toluene. Its IR spectrum, after being treated with
CO, shows two bands of unequal intensity
4
adsorbed on Pd–SiO2 under the same condi-
tions used for preparing the tethered catalysts.
The DRIFT spectrum of this catalyst gives two
y
1
Ž
Ž .
Ž .
.
2093 s , 2023 w cm
that are clearly differ-
.
of the fresh catalyst.
CO bands at 2095 and 2020 cmy with about
ent from that 1980 cm
1
y1
Ž
Ž
.
the same intensities Fig. 5a . Although the
lower energy band is broad, suggesting the pres-
ence of other species, the positions and intensi-
The spectrum also indicates that a species dif-
ferent from those in CO-treated, air-aged Rh–
PrSiO2 and Rh–ClrPd–SiO2 is present. The
31P NMR spectrum of air-aged Rh–PrPd–SiO2
shows that the phosphine ligand is converted to
IŽ
.
ties of these bands indicate that Rh CO is
2
also formed on the Rh–ClrPd–SiO2 catalyst.
Ž
Ž .Ž
.
The activity of Rh–ClrPd–SiO2 for the hydro-
the oxidized phosphine Ph2P O CH2 3Si-
Ž
Ž
. .
genation of toluene maximum TOFs2.4 and
OC2 H5 3 , which may or may not be coordi-
.
TOs521 during 6 h is also lower than that of
nated to the Rh. Whatever Rh species is pro-
duced in air-aged Rh–PrPd–SiO2, this catalyst
is much more active than air-aged Rh–PrSiO2,
Rh–ClrPd–SiO2, Rh–PrNi–SiO2 or Rh–Pr
Au–SiO2. Moreover, the air-aged Rh–Pr
the air-aged Rh–PrPd–SiO2. After Rh–
ClrPd–SiO2 stands in air for about 2 months,
Ž
.
its two Õ CO bands disappear in the DRIFT
Ž
.
Ž
spectrum Fig. 5b . However, its activity maxi-
mum TOFs2.6 and TOs628 during 6 h for
.
Ž
Pd–SiO2 maximum TOFs5.4 mol H2rmol
.
the hydrogenation of toluene is about the same
as that of the fresh sample. Thus, although
Rh–ClrPd–SiO2 and the CO-treated, air-aged
samples of Rh–PrSiO2 have similar IR spectra,
the catalytic inactivity of Rh–PrSiO2 and sig-
Rh min and TOs2464 mol H2rmol Rh in 9 h
is much more active for the hydrogenation of
Ž
toluene than the fresh Rh–PrPd–SiO2 max-
imum TOFs2.9 mol H2rmol Rh min and
.
TOs1940 mol H2rmol Rh in 23 h .
5. Conclusions
The high arene hydrogenation activity of the
fresh Rh–PrPd–SiO2 TCSM catalyst requires
the presence of both components, the tethered
Ž
.
rhodium complex Rh–P and the supported
Ž
.
metal Pd on the SiO2 surface. As compared
with our previously reported TCSM catalysts
w
x
Ž
25 , Rh–CNR3rPd–SiO2 maximum TOFs
5.5 mol H2rmol Rh min and TOs2420 mol
.
H2rmol Rh in 8.5 h and Rh–CNR2rPd–SiO2
Ž
maximum TOFs1.8 mol H2rmol Rh min and
.
TOs1750 mol H2rmol Rh in 8.5 h , prepared
by tethering RhCl CN CH2 3Si OC2 H5 3 3 and
RhCl CO CN CH2 3Si OC2 H5
w
Ž
.
Ž
. x
to Pd–
Ž .
a
Ž .
b air-aged
Fig. 5. IR spectra of
Rh–ClrPd–SiO2.
fresh Rh–ClrPd–SiO2 ,
Ž
.
w
Ž
.
Ž
.
3x2

(
)
H. Gao, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 63–74
73
w
w
w
x
Ž
.
12 J. Blumel, Inorg. Chem. 33 1994 5050.
x
SiO2, Rh–PrPd–SiO2, is somewhat less active
Ž
.
13 B.J. Pugin, J. Mol. Catal. A: Chem. 107 1996 273.
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maximum TOFs2.9 mol H2rmol Rh min,
x
Ž
.
14 A. Kinting, H. Krause, M. Capka, J. Mol. Catal. 33 1985
.
TOs1940 mol H2rmol Rh in 23 h for the
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the Rh–PrPd–SiO2 is allowed to stand in air
215.
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w
w
w
w
w
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x
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´
´
´
Ž
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.
Organomet. Chem. 509 1996 77.
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evident that combinations of tethered complexes
and supported metals offer opportunities for the
creation of highly active catalysts.
x
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