Rhodium amine complexes tethered on silica-supported gold-palladium bimetal catalysts. Arene hydrogenation

Article abstract of DOI:10.1016/S1381-1169(99)00209-5

The effects of various loadings of Pd and Au supported on silica to which rhodium complexes of the type Rh(COD)[H₂NCH₂CH₂NH(CH₂)₃Si(OCH₃)₃](X) were tethered were investigated at 40°C and 1 atm H₂. The influence of the X^- anion in the complex on the activities of the tethered complex on supported metal catalysts was also studied. The arene hydrogenation activities of the catalysts were examined. PF₆-Rh(N-N)/Au:Pd-SiO₂, with the weakly coordinating PF₆^- anion, was the most active catalyst, and remained highly active for toluene hydrogenation through at least catalytic cycles involving 2900 mole H₂/mole Rh turnovers over 19 hr. The catalysts with added Au were less active by a factor of two, but the activity was dependent on the Pd and Rh complex loadings. A mechanism which involved dissociative adsorption of H₂ on the supported Pd was proposed. The hydrogen spilled over onto the silica surface where it was used by the Rh complex to hydrogenate the arene substrate.

Full text of DOI:10.1016/S1381-1169(99)00209-5

Ž
.
Journal of Molecular Catalysis A: Chemical 149 1999 99–111
www.elsevier.comrlocatermolcata
Rhodium amine complexes tethered on silica-supported
gold–palladium bimetal catalysts. Arene hydrogenation
)
M. Ann D.N. Perera, Robert J. Angelici
Department of Chemistry and Ames Laboratory, Iowa State UniÕersity, Gilman Hall, Ames, IA, 50011, USA
Received 11 January 1999; accepted 23 February 1999
Abstract
Arene hydrogenation activities of catalysts prepared by tethering Rh COD H 2 NCH 2CH 2 NHCH 2-
Ž
.w
Ž
. xŽ .
CH2CH2Si OCH3 X on the silica support of the bimetal catalyst Au:Pd–SiO2 were studied under the mild conditions of
3
y
Ž
.
408C and 1 atm H2 pressure. The most active catalyst PF6–Rh N–N rAu:Pd–SiO2, with the weakly coordinating PF6
anion, remains highly active for toluene hydrogenation through at least three catalytic cycles involving 2900 mol H2rmol
Ž
.
Rh turnovers over a 19-h period. As compared with the activity of the Pd-supported catalyst PF6–Rh N–N rPd–SiO2, the
catalysts with added Au are less active only by a factor of two. But the activity depends strongly on the Pd and Rh complex
Ž
.
loadings. Although the detailed functioning of these TCSM catalysts tethered complex on a supported metal is not known,
a mechanism that is consistent with the results involves dissociative adsorption of H2 on the supported Pd. The hydrogen
spills over onto the silica surface where the Rh complex uses it to hydrogenate the arene substrate. q 1999 Elsevier Science
B.V. All rights reserved.
Keywords: TCSM; Tethered complex catalysts; Bimetallic catalysts; Toluene hydrogenation; Palladium; Gold
w x
1. Introduction
4 . Among these, silica is widely used to tether
homogeneous transition metal complexes that
The heterogenization of metal complex cata-
lysts on solid supports generates catalysts that
have the advantage over their homogeneous
counterparts that they may be easily separated
contain siloxy groups that can bind to the silica
w
x
surface 5,6 . Supported bimetallic heteroge-
neous catalysts have been studied extensively
w x
7 and show improved characteristics such as
high activity and stability over those of mono-
w
x
from the reaction products 1–3 . Several mate-
rials including organic polymers as well as inor-
ganic solids such as silica, zeolites, glass, clay
and metal oxides have been used as supports
w x
metallic supported catalysts 8 . In this research
group, a new type of heterogeneous catalyst has
been developed by tethering homogeneous com-
plexes to the silica of supported metal catalysts
w x
such as Pd–SiO2 9 . These TCSM catalysts
Ž
.
)
tethered complex on supported metal function
by the synergistic effects of both the tethered
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.
Ž
.
PII: S1381-1169 99 00209-5

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)
100
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
complex and supported metal. TCSM catalysts
consisting of tethered rhodium amine complexes
on Pd–SiO2, Pt–SiO2 and Au–SiO2 have been
catalyst for arene hydrogenation under the mild
w
x
conditions of 408C and 1 atm H2 pressure 10 .
Only a few homogeneous catalysts are active
under the low pressure of 1 atm H2, but their
w
x
used for the hydrogenation of arenes 10 . Their
activities are much higher than those of the
tethered complex or the supported metal sepa-
rately. A possible explanation for the high activ-
ities of these catalysts is that the supported
metal, e.g., Pd, is the site at which H2 is
activated to produce spillover hydrogen which
moves onto the SiO2 surface where the tethered
complex uses it for the hydrogenation of the
w
x
activities are low 17–19 . Otherwise, such cata-
lysts require high pressures for the hydrogena-
w
x
tion of arenes 20–22 . On the other hand,
TCSM catalysts are active for the hydrogenation
of arenes under mild conditions such as 408C
w
x
and 1 atm H2 pressure 9,10 .
In the present study of toluene hydrogenation
reactions, we examine the effects of various
loadings of Pd and Au supported on silica to
w x
arene substrates 9 . In order to explore the
Ž
.
effect of two supported metals on the catalytic
activities of TCSM catalysts consisting of a
rhodium amine complex tethered on silica-sup-
ported Au and Pd, we performed the studies
described in this paper.
which rhodium complexes of the type Rh COD
w
Ž
.
Ž
. xŽ .
H2 NCH2CH2 NH CH2 3Si OCH3 3 X
are
tethered. We also describe the influence of the
X
y anion in the complex on the activities of the
TCSM catalysts.
Although several rhodium diamine com-
plexes have been prepared, few have been used
w
x
w x
as catalysts 11,12 . Gokak et al. 13 studied
2. Experimental
Ž
.
polymer-supported Rh N;N Cl3, where N;
Ns1,2-diaminoethane, for the hydrogenation
of 1-octene at atmospheric pressure and in the
temperature range 30–558C; the polymer was
2.1. General procedures
styrene-divinylbenzene. The rhodium complex
Syntheses of the rhodium complex catalysts
were performed using standard Schlenk tech-
niques under a nitrogen atmosphere. Rhodium
q
Ž
.Ž
.
Ž
Rh COD N;N
N;Nspyridyl-imine lig-
.
and was used by Brunner et al. to catalyze the
w
x
w
x
Ž
.
hydrosilylation of ketones 14 . Capka et al. 15
trichloride RhCl3 P3H2O was purchased from
Pressure Chemicals. PdCl2 and HAuCl4 PxH2O
ˆ
studied the silica-anchored rhodium carbonyl
Ž
.
Ž
.
Ž
.
complex prepared from CH3O Si CH2 2-py
Au, 49% were purchased from D.F. Goldsmith
Chemical and Stevens Metallurgical, respec-
3
Ž
.
Ž
.
pys2-pyridyl and Rh2Cl2 CO in the hy-
4
w
Ž
.
x
drogenation of 1-octene under the conditions of
658C and 1 atm H2 pressure. They observed
that the immobilized rhodium complex was three
times more active than the analogous homoge-
tively. The rhodium dimer Rh COD Cl was
prepared according to the literature method 23 .
The Rh2 m-S CH2 3Si OCH3 3 2 CO
plex Rh–S was prepared by the published
2 w
x
w
Ž
Ž
Ž
.
Ž
. . Ž
.
4x
com-
.
w
x
w x Ž
neous catalyst. Corma et al. 16 recently re-
ported a zeolite-supported rhodium complex
procedure 24 . Silica gel 100 B.E.T. surface
2
.
Ž
.
area, 400 m rg and 3 2-amino ethylamino -
Ž
.Ž
.
w
propyltrimethoxysilane H 2 NCH 2CH 2 NH-
Rh COD N ; N PF6, where COD s 1,5-
Ž
Ž
. Ž
. x
cyclooctadiene, N ; N s 2- 3-triethoxysilyl-
CH2 3Si OCH3 3 were obtained from Fluka.
.
Ž
.
propyl-aminocarbonyl pyrrolidine, that cata-
The 1,5-cyclooctadiene COD was purchased
from Aldrich. Solvents were dried by refluxing
over CaH2 under nitrogen before use. All other
reagents and chemicals were commercial sam-
ples and were used as received, unless other-
wise mentioned.
lyzes the hydrogenation of arenes under 6 atm
of H2 pressure and 808C. As noted above, a
rhodium complex with the diamine ligand
H2 NCH2CH2 NHCH2CH2CH2Si OCH3 3 teth-
ered on Pd–SiO2 is a highly active TCSM
Ž
.

(
)
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
101
1H NMR spectra were obtained on samples
in CDCl3 solvent using a Varian VXR 300-MHz
NMR spectrometer with TMS as the internal
rotary evaporator at 808C. The resulting sample
was dried at 1108C for 5 h in an oven and then
calcined at 5008C in an air flow for 4 h in a tube
furnace. The calcined material was subsequently
reduced in a H2 flow for 4 h at 3808C and then
passivated under a flow of air at room tempera-
Ž
.
reference ds0.00 ppm . FTIR and DRIFT
spectra were recorded on a Nicolet 560 spectro-
photometer equipped with TGS and MCT detec-
tors in the main compartment and in the auxil-
Ž
ture for 1 h to give black PdrSiO2 Pd 9.9
Ž
.
.
iary experiment module AEM , respectively.
The AEM housed a Harrick diffuse reflectance
accessory. The solution IR spectra were mea-
sured in the main compartment in a cell with
NaCl salt plates. The DRIFT spectra were
recorded with the samples in the Harrick mi-
crosampling cup. A Varian 3400 GC interfaced
to a Finnigan TSG 700 high-resolution magnetic
sector mass spectrometer with electron ioniza-
wt.% powder.
2.2.1.2. Au–SiO2. Following the procedure in
w
x
Ref. 26 , a mixture of 5.00 g of SiO2 and 1.0 g
Ž
.
of HAuCl4 PxH2O Au, 49% in an aqueous
Ž
.
HCl solution 0.200 M, 75.0 ml was stirred at
room temperature overnight. After the water
was removed by rotary evaporation at ca. 808C,
the 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
Ž
.
tion 70 eV was used for all GC–MS measure-
ments. Gas chromatographic analyses were per-
formed with a Hewlett Packard HP 6890 GC
system with an FID detector using a 25 m HP-1
capillary column. Elemental microanalyses were
performed on a Perkin-Elmer 2400 Series 11
CHNSrO analyzer. The rhodium contents of
the TCSM catalysts were determined using
atomic emission spectroscopy. The analytical
samples were prepared by first treating the cata-
Ž
.
AurSiO2 Au 10 wt.% .
2.2.1.3. Au:Pd–SiO2. A series of Au:PdrSiO2
catalysts with different wt.% of Pd and Au were
prepared by incipient wetness coimpregnation
w
x
27 of silica with appropriate amounts of the
Ž
metal chlorides. Both HAuCl4 PxH2O 0.085–
.
Ž
.
0.861 g and H2 PdCl4 0.201–0.833 g
Ž
0.200 M HCl in solution were stirred with
silica overnight at room temperature in order to
achieve the desired Au and Pd wt.% mentioned
at the end of this paragraph. Since about 2.20
Ž
.
lysts 50.0 mg with 5.00 ml of aqua regia at
room temperature for 5–10 min with shaking.
H2 PdCl4 was prepared by dissolving PdCl2 in
.
Ž
.
Then 5.00 ml of aqueous HF 5% was added
and the mixture was heated until all of the solid
dissolved. The resulting solution was diluted to
25.0 ml with distilled water.
ml of impregnation solution per gram of SiO2
2
Ž
.
)400 m rg was sufficient to bring about
w
x
2.2. Preparation of the catalysts
incipient wetness 28 , the amount of water
added was determined by the amount of SiO2.
The slurries obtained after impregnation were
dried on a hot plate at 808C. Then they were
calcined at 5008C in a stream of air for 5 h and
finally reduced in a H2 flow for 18 h at 3808C
to give black Au:Pd–SiO2 powder. Two series
of bimetallic catalysts were prepared by this
procedure. Series 1: The Pd wt.% was a con-
stant 9.9%. Au wt.% increased as follows: 0.80,
2.1, 4.2%. Series 2: The Au wt.% was a con-
stant 4.2%. Pd wt.% increased as follows: 1.0,
4.1, 9.9%.
2.2.1. Preparation of the silica-supported het-
erogeneous metal and bimetal catalysts
2.2.1.1. Pd–SiO2. This was prepared according
w
x
to the general procedure described in Ref. 25 .
Ž
An aqueous solution of H2 PdCl4 PxH2O pre-
pared by dissolving 1.20 g of PdCl2 in 80.0 ml
Ž
..
of aqueous HCl 0.200 M was added to a flask
containing 7.00 g of SiO2. After the mixture
was stirred at room temperature overnight, the
water was removed by slow evaporation in a

(
)
102
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
Ž
.
Ž
.
2.2.2. Preparation of the rhodium complexes
s, 9H, OC H3 , 2.97 m, 2H, H2 NC H2CH2 ,
Ž
.
Ž
2.72 m, 2H, H2 NCH2C H2 NH , 2.49 m, 2H,
CH2 HNC H2CH2 , 2.33 m, 4H, C H2CH5CH ,
1.70–1.80 m, 6H, C H2CH2Si and C H2-
CH5CH , 0.56 t, 2H, C H2Si . C16H34F6-
N2O3 PRhSi : Found calc. : % C 32.91 33.22 ,
% H 5.59 5.55 , % N 5.12 4.84 .
(
)[ )
2.2.2.1. Rh COD NH2CH2CH2 NH CH2 -
3
(
.
Ž
.
(
) ]( ) (
(
))
Ž
Si OCH3 3 Cl Cl–Rh N–N . A mixture of
w
Ž
.
x
Ž
.
.
Ž
.
Ž
Rh COD Cl
Ž
0.090 g, 0.181 mmol and
2
Ž
.
Ž
. .
Ž
0.120
.
Ž
.
Ž
.
NH2CH2CH2 NH CH2 3Si OCH3 3
.
Ž
.
Ž
.
Ž
.
ml, 0.501 mmol in CH2Cl2 10.0 ml was
stirred under nitrogen at room temperature for 1
h. The solution became yellow, and the solvent
was removed under vacuum at room tempera-
ture. The yellow solid was washed with hexanes
(
)[ )
(
2.2.2.3. Rh COD NH2CH2CH2 NH CH2 -
Si OCH3
Rh COD NH2CH2CH2 NH CH2 3Si OCH3 3
(
) ](
)
CF3 SO3
(
(
)3)
CF3 SO3–Rh N–N .
3
Ž
.Ž
.
w
Ž
.
.
Ž
.
x
-
Ž
.
Ž
5.00 ml=2 and dried under vacuum to give a
Cl 0.171 g, 0.360 mmol was dissolved in
Ž
.
yellow powder of Cl–Rh N–N in 91% yield.
20.0 ml of MeOH. To this solution, NaO3SCF3
The 1H-NMR assignments are based on the
Ž
.
0.080 g, 0.460 mmol dissolved in 4.00 ml of
w
x
reported values 29 for the free ligands COD
MeOH was added. After the solution was stirred
for 30 min at room temperature, MeOH was
removed under vacuum, and the resulting solid
1H
ppm : 5.00 br, 1H, CH2-
w
Ž
.
Ž
.
x
and NH2CH2CH2 NH CH2 3Si OCH3 3
.
Ž
.
Ž
NMRrCDCl3
d
.
Ž
.
Ž
Ž
.
NHCH2 , 4.39 br, 2H, H2 NCH2 4.22 s, br,
was dissolved in CH2Cl2 10.0 ml . This solu-
tion was filtered to remove the NaCl; then the
CH2Cl2 was removed under vacuum. The re-
sulting pale yellow solid was recrystallized from
.
Ž
.
Ž
4H, C H5C H , 3.61 s, 9H, OC H3 , 2.97 m,
2H, H2 NC H2CH2 , 2.73 m, 2H, H2 NCH2-
C H2 NH , 2.51 m, 2H, CH 2 HNC H2-
CH2 , 2.36 m, 4H, C H2CH5CH , 1.76–1.82
m, 6H, C H2CH2Si and C H2CH5CH , 0.57 t,
2H , C H 2 Si .
Found calc. : % C, 40.98 40.53 , % H 7.31
7.02 , % N 5.97 6.03 .
.
Ž
.
Ž
.
Ž
.
Ž
CH2Cl2rdiethyl ether to give CF3SO3–Rh N–
1
Ž
.
Ž
.
Ž
.
N in 61% yield. H NMRrCDCl3 d ppm :
.
Ž
.
Ž
.
Ž
C 16
H 34C lN 2 O 3 R hSi :
5.00 br, 1H, CH2 NHCH2 , 4.39 br, 2H, H2 N-
Ž
.
Ž
.
.
Ž
.
Ž
CH2 ,4.02 s, br, 4H, HC5C H , 3.56 s, 9H,
OC H3 ,2.83 m, 2H, NH2C H2CH2 , 2.63
Ž
C H2CH5CH , 2.36 m, 2H, CH2 NHC H2-
CH2 , 1.82 m, 4H, C H2CH5CH , 1.66 m,2H,
C H2CH2Si , 0.57 t, 2H, C H2Si . C17H34-
F3N2O6RhSSi : Found calc. : % C35.44-
Ž
.
Ž
.
.
Ž
.
.
Ž
m, 2H, H2 NCH2C H2 NH , 2.49 m,4H,
(
)[ )
2.2.2.2. Rh COD NH2CH2CH2 NH CH2 -
3
(
.
Ž
(
) ](
) (
(
))
.
Ž
.
.
Ž
Si OCH3 3 PF6 PF6–Rh N–N . Following a
w
x
Ž
. Ž
previously reported method 30 , a solution of
Ž
.
w
Ž
.
Ž
. x
.
Ž
.
Rh COD NH2CH2CH2 NH CH2 3Si OCH3 3
Ž
.
Ž
.
Ž
.
Ž
.
Cl 0.171 g, 0.360 mmol dissolved in 20.0 ml
35.17 , % H 5.76 5.55 , % N 5.06 4.82 , % S
Ž
.
of MeOH was treated with the slow addition of
5.80 5.52 .
Ž
.
0.121 g 0.650 mmol of KPF6 dissolved in 5.00
ml of distilled water. This mixture was stirred at
room temperature under nitrogen for 30 min.
After most of the MeOH was removed under
vacuum, the solid was filtered off and dissolved
in CH2Cl2. After the KCl was separated by
filtration, the CH2Cl2 was removed from the
filtrate under vacuum to leave a dark yellow
(
)[ )
2.2.2.4. Rh COD NH2CH2CH2 NH CH2 -
3
Si OCH3 3 I–Rh N–N . To a solution of
Rh COD NH2CH2CH2 NH CH2 3Si OCH3 3
Ž
acetone, NaI 0.060 g, 0.401 mmol in 5.00 ml
of toluene was added. The solution was stirred
under nitrogen for 20 min at room temperature
as it became brown in color. The solution was
filtered and the acetone was evaporated under
vacuum. The resulting brown solid was washed
with diethyl ether and dried under vacuum to
(
(
) ]]( ) (
(
))
I
Ž
. Ž
.
w
Ž
.
.
Ž
. x
Cl 0.172 g, 0.360 m mol in 20.0 ml of dry
Ž
.
Ž
solid which was washed with hexanes 5.00
.
ml=2 and dried under vacuum to give PF6–
Rh N–N in 85% yield. 1H NMRrCDCl3
d
Ž
.
Ž
.
Ž
.
.
Ž
ppm : 5.00 br, 1H, CH2 NHCH2 , 4.39 br,
Ž
.
Ž
.
2H, H2 NCH2 4.21 s, br, 4H, C H5C H , 3.56
obtain the complex I–Rh N–N in 70% yield.

(
)
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
103
1
Ž
.
Ž
Ž
.
H NMRrCDCl3 d ppm : 5.00 br, 1H, CH2-
10.0 ml=3 . The tethered rhodium-amine
.
Ž
.
Ž
Ž
.
NHCH2 , 4.39 br, 2H, H2 NCH2 , 4.14 s, br,
4H, HC5C H , 3.53 s, 9H, OC H3 , 2.94 m,
2H, NH2C H2CH2 , 2.75 m, 2H, H2 NCH2-
C H2 NH , 2.40 m, 2H, CH2 NHC H2CH2 , 2.50
complex catalyst X–Rh N–N rPd–SiO2 was
obtained after drying in vacuum.
.
Ž
.
Ž
.
Ž
y
y
.
Ž
.
(
)
(
2.3.2. X–Rh N–N rAu:Pd–SiO2 XsCl , I ,
y
PF6 , BPhy4 , CF3SO3y
Ž
.
Ž
)
m, 4H, C H2CH5CH , 1.60–1.79 m, 6H,
.
Ž
C H2CH2Si and C H2CH5CH , 0.57 t, 2H,
C H2Si . C16H34IN2O3RhSi : Found calc. : %
C 34.46 34.30 , % H 5.84 5.73 , % N 5.36
These bimetallic catalysts were prepared in
. Ž
.
Ž
.
Ž
.
the same manner as X–Rh N–N rPd–SiO2 by
using Au:Pd–SiO2 prepared in two series as
mentioned in Section 2.2.1.3 instead of Pd–
Ž
.
Ž
.
Ž
Ž
.
.
5.10 .
Ž
.
Ž
SiO2. The X–Rh N–N rAu:Pd–SiO2 X s
PF6y and Cly catalysts were characterized by
(
)[ )
2.2.2.5. Rh COD NH2CH2CH2 NH CH2 -
3
Si OCH3
Rh COD NH2CH2CH2 NH CH2 3Si OCH3 3
(
.
Ž
(
Ž
) ](
)
BPh4
(
(
.
))
.
BPh4–Rh N–N . To
stirring them 0.102 g in 4.0 ml of toluene
under a CO atmosphere for 5 h at room temper-
ature. After filtration, the solid catalyst was
washed with toluene 10 ml=2 and dried un-
der vacuum. The resulting X –Rh N –
N rAu:PdrSiO2 catalysts Au:Pd wt.% 4.2:1.0
gave IR spectra DRIFT with two n CO ab-
sorptions: XsCl , 2085 s , 2037 vs cm and
XsPF6 , 2093 s , 2047 vs cm
.
w3
Ž
Ž
.
x
-
Ž
.Ž
.
Cl 0.172 g, 0.360 mmol dissolved in 20.0 ml
Ž
.
Ž
.
of MeOH, NaBPh4 0.121 g, 0.420 mmol dis-
solved in 5.00 ml of distilled water was added.
This mixture was stirred under nitrogen for 30
min at room temperature. A yellow solid was
filtered off and washed with a mixture of MeOH
Ž
.
Ž
.
Ž
.
Ž
.
y
y1
Ž .
.
Ž
.
.
y
y1
Ž
.
Ž .
Ž
and H2O 1:1 and dried under vacuum. The
resulting pale yellow solid was washed with
Ž
.
Ž
.
[
(
(
(
)
(
) ) ( ) ]
)
hexanes 5.00 ml=2 to give BPh4–Rh N–N
2.3.3. Rh2 m-S CH2 3 Si OCH3 3 2 CO
r
4
1
Ž
.
in 73% yield. H NMRrCDCl3 d ppm : 6.75–
7.15 m, 20H, Ph , 5.00 br, 1H, CH2 NHCH2 ,
4.39 br, 2H, H2 NCH2 , 3.75 s, br, 4H,
HC5C H , 3.47 s, 9H, OC H3 , 2.38 m, 2H,
NH2C H2CH2 , 2.09 m, 2H, H2 NCH2C H2-
NH , 1.50 m, 2H, CH2 NHC H2CH2 , 1.95 m,
4H, C H2CH5CH , 1.34 m, 4H, C H2CH5CH ,
1.22 m, 2H, C H2CH2Si , 0.89 m, 2H, C H2Si .
Au:Pd–SiO2 Rh–SrAu:Pd–SiO2
Ž
.
Ž
.
w
Ž
.
Ž
.
x
Ž
.
4
The Rh2 m-S CH2 3Si OCH3 3 2 CO com-
Ž
.
Ž
Ž
.
plex 0.049 g, 0.065 mmol in 15.0 ml of
toluene was added to 1.00 g of previously de-
gassed Au:Pd–SiO2 Au:Pd wt.% 4.2:1.0 and
refluxed for 4 h under nitrogen. The solid cata-
lyst was separated by filtration and washed with
.
Ž
.
Ž
.
Ž
Ž
.
.
Ž
.
Ž
.
Ž
.
.
.
Ž
Ž
Ž
.
toluene 15 ml=3 . After drying under vac-
uum, the Rh–SrAu:Pd–SiO2 catalyst analyzed
Ž
Ž
.
Ž
.
C40H54BN2O3RhSi : Found calc. : % C 64.27
64.00 , % H 7.08 6.93 , % N 3.97 3.73 .
.
Ž
.
Ž
.
Ž
.
Ž .
for 1.53 wt.% Rh. The IR spectrum DRIFT of
Ž
.
this catalyst gave n CO values of 2072 s ,
2051 s , 2002 s cm 1. These wavenumbers and
y
Ž .
Ž .
2.3. Preparation of the tethered rhodium com-
plex TCSM catalysts
Ž
Ž
Ž
.
Ž .
intensities are similar to those 2070 m , 2050 s
.
and 2002 s of the free Rh2 m-S CH2 -
3
Si OCH3 3 2 CO complex in toluene and to
those 2081 m , 2064 s and 2020 s cm
previously reported for this complex tethered on
just SiO2 24 .
Ž ..
. . Ž
w
Ž
Ž
.
4x
y
y
y
1
(
)
(
Ž
Ž
.
Ž .
Ž .
.
2.3.1. X–Rh N–N rPd–SiO2 XsCl , I ,
PF6y, BPhy4 , CF3SO3y
)
y
y
Ž
. Ž
w
x
A solution of X–Rh N–N XsCl , I ,
PF6y, BPhy4 , CF3SOy3 dissolved in 15 ml of
.
CH2Cl2 was added to previously degassed Pd–
2.4. Hydrogenation reactions
Ž
.
SiO2 1.00 g under vacuum. This mixture was
Ž
.
refluxed while stirring for 4 h under nitrogen.
The hydrogenation reactions were carried out
Ž
The solid was filtered and washed with CH2Cl2
in a three necked, jacketed vessel about 100

(
)
104
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
.
ml closed with a self-sealing silicon rubber cap
and containing a stirring bar; the vessel was
connected to a vacuumrH2 line and a constant
pressure 1 atm gas burette. The temperature of
the circulating ethylene glycol passing through
the jacket was maintained by a thermostated
tected during the reactions. Table 1 gives the
hydrogenation activities of the Au:Pd–SiO2 cat-
Ž
.
alyst, the PF6–Rh N–N complex tethered on
w
x
just SiO2 PF6–RhrSiO2 and three TCSM cata-
lysts; these activities are expressed in terms of
Ž
.
Ž
TOF turnover frequency , TO turnover num-
Ž
.
.
bath Haake B3-DC1 . After the catalyst was
added and the air in the vessel was replaced
with H2, the substrate was injected into the
vessel. While being constantly stirred at a speed
of 550 rpm, H2 uptake was followed with the
constant pressure gas burette. The rates of the
reactions measured by the rate of H2 uptake,
were reproducible within "3% for typical reac-
tions. The products were identified by GC and
GC–MS.
ber or mmol of H2 uptake as defined in the
table.
Ž
The first two TCSM catalysts, Cl–Rh N–
N rAu:Pd–SiO2 and PF6–Rh N–N rAu:Pd–
.
Ž
.
SiO2 are much more active than either the
Ž
tethered rhodium complex on just silica PF6–
Ž
.
.
Rh N–N rSiO2 or just the supported bimetal-
lic Au:Pd–SiO2 Au:Pd wt.% 4.2:1.0 catalyst.
Of these two TCSM catalysts, PF6–Rh N–
Ž
.
Ž
.
N rAu:Pd–SiO2 shows the higher activity.
However, the TCSM catalyst Rh–SrAu:Pd–
SiO2, containing the rhodium carbonyl thiolate
Ž
.
complex Rh–S , had no activity and is there-
fore less active than just Au:Pd–SiO2. Di-
rhodium complexes of the general formula
3. Results and discussion
Ž
. Ž
.
Rh2 m-SR CO
4
were previously demon-
strated to be active olefin hydroformylation cat-
2
3.1. Hydrogenation of toluene with the TCSM
catalysts
w
x
alysts 31 . When tethered on phosphine-mod-
ified silica, P– Pd–SiO2 , where P s
Ph2 P CH2 3Si OC2 H5 3, it was an active
Ž
.
Ž
.
w
Ž
Ž
.
Ž
.
The TCSM catalysts Rh COD NH2CH2-
Ž
.
Ž
. x Ž .
CH2 NH CH2 3Si OCH3 3 X r Pd–SiO2,Rh-
TCSM catalyst for the hydrogenation of toluene
Ž
.
w
Ž
.
Ž
.
x
w x
32 .
COD NH2CH2CH2 NH CH2 y3Si OCH3 3
-
y
y
y
Ž .
Ž
Ž .
Since PF6–Rh N–N rAu:Pd–SiO2 showed
X rAu:Pd–SiO2, XsCl , I , PF6 , BPh4 ,
CF3SOy3 and Rh2 m-S CH2 3Si OCH3 3 2
good activity for the hydrogenation of toluene,
it was studied in other arene hydrogenations
under the same mild conditions to give the
.
w
Ž
.
Ž
. x
-
Ž
.
CO rAu:Pd–SiO2, were used to catalyze the
4
hydrogenation of toluene to methylcyclohexane
under the conditions of 408C and 1 atm of H2
pressure. No hydrogenolysis products were de-
Ž
.
corresponding cyclohexane products Table 2 .
The percent conversions increase in the follow-
Table 1
Hydrogenation of toluene with TCSM catalystsa
TOb mol H2r
H2 uptakeb
Ž
.
Ž
Ž
Catalyst
Rh %
TOF mol H2r
.
.
Ž
.
mol Rh min
mol Rh
mmol
c
Ž
.
Ž
.
Ž
.
Cl–Rh N–N rAu:Pd–SiO2 c
3.6
0.61
1.5
1.4
0.00
0.239
3.73
0.00
0.00
–
155 10 h
2.69 10 h
Ž
.
Ž
.
.
.
Ž
.
.
.
PF6–Rh N–N rAu:Pd–SiO2
1655 7 h
4.49 7 h
Rh–SrAu:Pd–SiO2c
0.00 5 h
0.00 5 h
Ž
Ž
Ž
.
Ž
Ž
PF6–Rh N–N rSiO2
0.00 5 h
0.00 5 h
Au:Pd–SiO2c
–
1.56 19 h
Ž
.
aReaction conditions: 50 mg solid catalyst, 5.00 ml toluene, 408C, 1 atm H2.
b TO and H2 uptake values correspond to the reaction times in hours in parentheses.
cAu:Pd wt.% is 4.2:1.0%.
Ž
.

(
)
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
105
Table 2
Arene hydrogenation using the PF6–Rh N–N rAu:Pd–SiO2 catalysta
Ž
.
Substrate
TOF mol H2r
TOb mol H2r
H2 uptakeb
Ž
Ž
.
.
Ž
.
mol Rh min
mol Rh
mmol
Ž
.
.
.
.
Ž
.
.
.
.
anisole
toluene
benzene
ethyl benzene
0.294
0.124
0.176
0.055
488 26 h
4.74 26 h
Ž
Ž
1.91 23 h
198 23 h
Ž
Ž
165 26 h
1.56 26 h
Ž
Ž
1.33 23 h
138 23 h
a
Ž
.
Ž
.
Reaction conditions: 1 ml substrate, 4 ml heptane spectroscopic grade , 50 mg solid catalyst Au:Pd 4.2:1.0 wt.% and Rh 2.0 wt.% , 408C,
1 atm H2 pressure.
b Footnote is the same as in Table 1.
Ž
.
ing order: ethylbenzene 4.6%, 23 h -benzene
genation reactions. When the untethered PF6–
Ž
Ž
.
Ž
.
Ž
.
Ž
.
4.8%, 26 h -toluene 6.6%, 23 h -anisole
13.3%, 26 h . Anisole reacts fastest of the
Rh N–N complex was treated with CO 1 atm
.
Ž Ž .
in toluene, it gave a similar spectrum 2094 s ,
y
1
Ž .
Ž
.
studied arenes, indicating that electron donating
substituents on the arene ring accelerate the
rate. A comparison of the rate of hydrogenation
of toluene with that of ethylbenzene indicates
the size of the alkyl substituent affects the rate
little. Similar substituent effects on arene hydro-
genation rates were observed with other TCSM
catalysts such as Rh–CNR2rPd–SiO2 and Rh–
2025 s cm
also indicating the formation of
q
.Ž
.
a Rh diamine CO complex. These results are
2
consistent with the formation of
a
q
Ž
.Ž
.
Rh diamine CO
2
complex even after three
hydrogenation cycles.
In order to test for the possibility that the Rh
complexes leach from the catalyst and homoge-
neously catalyze the toluene hydrogenation, the
solution after each of the three runs was treated
w
x
CNR3rPd–SiO2 9,33 .
Ž
.
Ž
.
The durability of the PF6–Rh N–N rAu:Pd–
SiO2 catalyst Rh 1.2%, Au:Pd wt.% 4.2: 1.0
with more toluene 5 ml and tested for H2
uptake, but there was none after 7 h. Atomic
emission spectroscopic analyses of the used so-
lutions gave no detectable Rh, which suggests
that Rh did not leach from the catalyst.
Ž
.
was tested by running three successive hydro-
genation reactions of toluene using the condi-
tions in Table 1. In the first cycle, the maximum
TOF was 1.52 and TO was 696 for a 7-h run.
Then, the catalyst was filtered from the reaction
y
(
)
on toluene
3.2. Effect of counter anion X
Ž
.
mixture, washed with fresh toluene 5 ml=2
hydrogenation
in air, dried under vacuum at room temperature
and then used for the second run. The second
hydrogenation cycle gave a maximum TOF of
3.20 and TO of 1227 after 6 h. After the catalyst
was treated as described after the first cycle, the
third run gave a maximum TOF of 2.47 and a
TO of 971 for 6 h. Thus, the catalyst was more
active in the second and third cycles than in the
Ž
.
Since the Cl–Rh N–N rAu:Pd–SiO2 and
Ž
.
PF6–Rh N–N rAu:Pd–SiO2 TCSM catalysts
had significantly different toluene hydrogena-
tion activities Table 1 , we decided to examine
Ž
.
Ž
.
the X–Rh N–N rAu:Pd–SiO2 TCSM catalysts
with other counter anions, PF6y, BPhy4 , CF3SOy3 ,
Iy and Cl , in the tethered complex Table 3 .
y
Ž
.
Ž
.
first, and after the three runs 19 h , the total TO
was 2900 mol H2rmol Rh. The DRIFT spec-
trum of the catalyst after three cycles, followed
For the series of catalysts in which the X–
Rh N–N complexes were tethered on Au:Pd–
SiO2 Au:Pd wt.%, 4.2: 1.0% , the rates of
Ž
.
Ž
.
by treatment with 1 atm CO, contained bands at
y
y
2092 s and 2049 vs cm 1, which are the same
Ž .
Ž
.
y
1
Ž
Ž .
Ž
.
.
as those 2093 s , 2047 vs cm
observed for
the CO-treated catalyst before use in the hydro-

(
)
106
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
Table 3
Ž
.
Ž
.
Comparison of rates of toluene hydrogenation with different counter anions in X–Rh N–N rAu:Pd–SiO2 and X–Rh N–N rPd–SiO2
catalystsa
Xy
Rh %
TOF mol H2r
TOb mol H2r
H2 uptakeb
Ž
.
Ž
Ž
.
.
Ž
.
mol Rh min
mol Rh
mmol
X–Rh N–N rAu:Pd–SiO2 catalystsc
(
)
PF6y
0.47
0.94
1.3
9.3
3.3
1.2
7.4
0.18
3160 6 h
6.95 6 h
Ž
.
.
.
Ž
.
.
.
BPhy4
CF3SOy3
Iy
1211 6 h
Ž
Ž
5.45 6 h
Ž
Ž
504 6 h
3.17 6 h
Ž
.
Ž
.
0.26
3.6
786 23 h
0.91 23 h
Cly
91 6 h
1.58 6 h
Ž
.
Ž
.
X–Rh N–N rPd–SiO2 catalystsd
(
)
BPhy4
PF6y
0.70
0.82
1.1
12
5738 7 h
Ž
.
Ž
19.4 7 h
.
Ž
.
Ž
.
6.9
3.2
2.6
1.4
2931 6.5 h
11.7 6.5 h
Iy
1389 7 h
7.6 7 h
Ž
.
Ž
.
CF3SOy3
Cly
1.7
2.3
1336 7.25 h
10.9 7.25 h
Ž
.
Ž
.
Ž
.
Ž
.
576 7 h
6.34 7 h
aReaction conditions: 5 ml toluene, 50 mg solid catalyst, 408C, 1 atm H2 pressure.
b Footnote is the same as in Table 1.
cAu:Pd 4.2:1.0 wt%.
d Pd 9.8 wt%.
BPhy4 ) CF3SOy3 ) Cly. The I–Rh N –
.
Studies of the effects of anions on catalyst
activity have been performed in relatively few
Ž
N rAu:Pd SiO2 catalyst showed a compara-
tively high rate at the beginning of the reaction
but became inactive after about 2 h as indicated
in Fig. 1. The rates of toluene hydrogenation as
w
x
other systems. Burckhardt et al. 34 investi-
gated anion effects on the enantioselectivity of
asymmetric allylic amination of 1,3-diphenyl-
prop-2-enylethylcarbonate with benzyl amine
using a homogeneous Pd-catalyst with ferro-
cenyl phosphine ligands. They observed en-
Ž
a function of the anion for the X–Rh N–
.
N rPd–SiO2 catalysts, in which Pd–SiO2 is
the supported metal, follow a similar, but not
y
Ž
.
Ž
.
identical, trend Table 3 and Fig. 2 : BPh4 )
PF6y)IyfCF3SOy3 )Cly. In general, the most
weakly coordinating anions, PF6y and BPhy4 ,
gave the most active catalysts.
hancement of ee to )99.5% with small hard
anions such as Fy and BHy4 while very low ee
Fig. 1. Kinetic plots for the hydrogenation of toluene over the
Fig. 2. Kinetic plots for the hydrogenation of toluene over the
X–Rh N–N rAu:Pd–SiO2ycatalysts: Xys a PF6y,
Iy,
X–Rh N–N rPd–SiO2 catalysts: Xys a BPhy4 ,
PF6y,
c
Ž .
Ž
.
Ž .
Ž .
b
Ž .
c
Ž
Ž .
d
.
Ž .
Ž .
b
y
y
BPh4 , d CF3SO3 , e Cl . Reaction conditions are the same as
those in Table 3.
Iy,
CF3SOy3 , Cly. Reaction conditions are the same as
e
Ž .
Ž .
Ž .
those in Table 3.

(
)
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
107
Ž
.
-10% ee were obtained with non-coordinat-
increase from 24 to 408C then decrease at 708C.
For the Pd-supported TCSM catalysts, the activ-
ity of the XysBPhy4 catalyst increases from 24
to 408C but decreases upon further heating to
708C. However, for XysPF6y, the rates in-
crease from 248 through to 708C. The increase
in rate with increasing temperature is expected
for most reactions and is observed for the
ing anions such as PF6y. Studies of the hydro-
w
x
w
. Ž
genation of imines 35 such as ArMeC5
NCH2 Ph using Rh y -dbpp NBD ClO4 as
x
wŽ
.
x
Ž
.
y
Ž
.
a catalyst and a sulfonate RSO3 co-catalyst
showed high enantioselectivity. On the other
hand, the Rh complex together with halide co-
catalysts gave an almost complete reversal of
enantioselectivity. The authors proposed that the
hydrogenations proceed via a monohydride
pathway in the presence of RSOy3 in non-polar
solvents, but in the presence of halides it occurs
Ž
.
Ž
BPh4–Rh N–N rAu:Pd–SiO2 and PF6–Rh N–
.
N rPd–SiO2 catalysts. On the other hand, the
Ž
.
rates for the PF6–Rh N–N rAu:Pd–SiO2 and
Ž
.
BPh4–Rh N–N rPd–SiO2 catalysts go through
a maximum at 408C. Although rate maxima for
the latter two catalysts are surprising, such be-
havior has been observed previously with sim-
ple heterogeneous catalysts. For example, Joice
w
x
by a dihydride pathway 35 . Thus, anions in
other catalytic systems also have a marked ef-
fect on the course of reactions.
w
x
3.3. Effect of temperature on toluene hydro-
genation rates
and Rooney 36 observed that the rates of
hydrogenation of 1-butene on catalysts com-
Ž
.
posed of Au:Pd Au mol% 0–65 supported on
pumice increase up to 908C but then decrease at
higher temperatures. On the other hand, the rate
of neopentane conversion to methane, ethane,
propane, isobutane, isopentane, and n-pentane
The effects of temperature on the catalytic
Ž
.
Ž
activities of the X–Rh N–N complexes Xs
PF6y and BPhy4 , when tethered on Au:Pd–SiO2
.
Ž
.
Au:Pd wt.%, 4.2:1.0 and Pd–SiO2, were stud-
Ž
.
Ž
.
ied. The results Table 4 show that for the
Au:Pd–SiO2-supported catalysts, the rates in-
crease as the temperature increases from 24 to
708C for XysBPhy4 , but the rates for XysPF6y
on 2 wt.% total metal Au:Pd–SiO2 increases
with an increase in temperature: 240-2608C
27 . For the TCSM catalysts, PF6–Rh N–N r
Au:Pd–SiO2 and BPh4–Rh N–N rPd–SiO2,
w
x
Ž
.
Ž
.
Table 4
Study of the temperature effect on toluene hydrogenationa
TOb mol H2r
H2 uptakeb
Ž
Ž
Catalyst
Temperature
TOF mol H2r
Ž .
8C
.
.
Ž
.
mol Rh min
mol Rh
mmol
c
Ž
.
Ž
.
.
.
.
.
.
.
.
.
.
Ž
.
.
.
.
.
.
.
BPh4–Rh N–N rAu:Pd–SiO2
70
40
24
70
40
24
70
40
24
70
40
24
3.8
3.3
1.0
5.2
3.7
0.55
6.3
13.1
5.3
29
1500 6 h
6.75 6 h
Ž
.
Ž
Ž
5.45 6 h
Rh wt.% 0.94
1211 6 h
Ž
Ž
516 6 h
2.32 6 h
c
Ž
.
Ž
Ž
4.19 6 h
PF6–Rh N–N rAu:Pd–SiO2
Rh wt.% 0.61
1414 6 h
Ž
.
Ž
Ž
1655 7 h
4.49 7 h
Ž
Ž
0.36 6 h
124.0 6 h
d
Ž
.
Ž
Ž
BPh4–Rh N–N rPd–SiO2
2432 6 h
8.27 6 h
Ž
.
Ž
Ž
.
Rh wt.% 0.70
4980 6 h
16.9 6 h
Ž
Ž
.
2679 6 h
9.11 6 h
d
Ž
.
Ž
Ž
.
PF6–Rh N–N rPd–SiO2
Rh wt.% 0.82
10 572 6 h
42.2 6 h
11.7 6.5 h
10.0 6 h
Ž
.
Ž
.
Ž
Ž
.
6.9
6.3
2931 6.5 h
Ž
.
.
2522 6 h
aReaction conditions: 5 ml toluene, 50 mg catalyst, 1 atm H2 pressure.
b Footnote is the same as in Table 1.
cAu:Pd wt.% is 4.2:1.0%.
d Pd wt.% is 9.8.

(
)
108
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
Table 5
Ž
.
amounts of H2 mmol consumed in 19–24 h
for Au:Pd–SiO2 catalysts with Pd metal mole
percentages ranging from 10.1 to 100%. Sur-
prisingly, these rates are all very similar. As
another measure of catalytic activity, we deter-
mined the time that it took for 1.0 ml of H2 to
be consumed at the beginning of the reaction.
Hydrogenation of toluene using Au:Pd–SiO2 catalystsa
Mol%
H2 uptakeb
Initial timec
Ž .
Ž
.
Ž
.
wt.%
mmol
period h
Au
Pd
Ž
.
.
.
.
.
Ž
.
Ž
.
.
.
.
.
0.0 0.0
100 9.9
1.77 24.0 h
3.0
2.5
4.0
2.0
2.5
Ž
Ž
Ž
Ž
Ž
.
.
Ž
18.6 5.0
81.3 11.6
61.5 4.07
30.6 1.0
10.1 0.41
1.59 22.5 h
Ž
Ž
1.57 19.0 h
38.4 4.2
Ž
.
Ž
69.3 4.2
1.39 22.5 h
Ž
.
Again, the times are all very similar 2.0–4.0 h
Ž
.
Ž
1.56 22.5 h
89.8 6.6
for the different Au and Pd loadings. It should
be emphasized that these rates are all slow as
indicated by the observation that it takes only
35 min to consume 1.0 ml of H2 when using
PF6–Rh N–N rAu:Pd–SiO2 0.47% Rh, Au:Pd
wt.% 4.2:1.0 catalyst.
aReaction conditions: 50 mg of Au:Pd–SiO2 , 5.0 ml of toluene,
408C, 1 atm pressure.
b Total reaction period in parentheses.
c Time period required to consume 1 ml of H2 at the beginning of
the reaction.
Ž
.
Ž
.
the maxima in rates are presumably related to
the behavior of the Xy anion in the complex.
3.4.2. Effect of Õarious gold loadings with con-
stant palladium loading on Cl–Rh N–N r
Au:Pd–SiO2 catalytic actiÕity
The catalyst Cl–RhrAu:Pd–SiO2 that con-
tains no Au is more active than any of the
(
)
3.4. Effects of Au and Pd loadings on the
(
)
catalytic actiÕity of Cl–Rh N–N rAu:Pd–SiO2
catalysts
Ž
.
catalysts that contain some Au Table 6 . For
the series of catalysts in which the Pd loading is
The purpose of these investigations was to
understand the effects of the Au and Pd load-
ings on the TCSM catalyst activities.
Ž
.
kept constant at 9.8%, the rate in parentheses
as measured by the TOrh for the total reaction
time changes with increasing Au content %
Ž
. Ž . Ž
3.4.1. ActiÕity of Au:Pd–SiO2 supported cata-
lysts
Au in the order: 0.8% Au 35 TOrh , 2.1% 48
.
Ž
.
TOrh , 4.2% 43 TOrh . The variation in rate
is rather small with increasing Au loading, and
the irregular trend in rates may be due to changes
in the morphology of the supported bimetal
catalysts. Studies of Au:Pd–SiO2 performed by
Prior to examining the TCSM catalysts, we
studied the Au:Pd–SiO2 catalysts in toluene
hydrogenation under the conditions of 408C and
1 atm pressure. The activities of all of these
catalysts are much lower than those of the TCSM
w
x
Juszczyk et al. 27 showed that in all catalysts
that they prepared with different Au:Pd mole
Ž
.
catalysts Table 1 . In Table 5 are shown the
Table 6
Effect of Au and Pd loadings on the activities of Cl–Rh N–N rAu:Pd–SiO2 catalysts in toluene hydrogenationa
Ž
.
TOb
TOrhc mol H2r
Ž
.
Ž
Ž
Au:Pd
Rh %
TOF mol H2r
.
Ž
.
.
mol Rh h
mol Rh min
mol H2rmol Rh
Ž
.
.
.
.
.
0.0:9.8
0.8:9.8
2.1:9.8
4.2:9.8
4.2:4.1
4.2:1.0
2.3
1.2
0.82
1.5
2.5
3.6
1.3
707 9.5 h
74
35
48
43
25
16
Ž
0.48
0.74
0.79
0.63
0.23
266 7.5 h
Ž
481 10 h
Ž
304 7.0 h
Ž
455 18 h
Ž
.
155 10 h
aReaction conditions: toluene 5 ml, catalyst 50 mg, 1 atm H2 pressure.
b Footnote is the same as in Table 1.
c TOrh for the total reaction time.

(
)
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
109
ratios, there are two types of particles on the
surface: Pd-rich and Au-rich particles. At low
morphology of the Au:Pd–SiO2 part of the
catalysts undoubtedly changes as the amount of
Ž
w x
Au loadings e.g., 4 mol% Au and 96 mol%
Pd increases 27 . Another explanation for the
increasing rate with increasing % Pd involves
considering the % Rh in the catalyst. Although
the tethering process was the same for all of the
Au:Pd–SiO2 catalysts, the amount of complex
tethered in these catalysts decreases as the % Pd
increases. Since TO increases as the Rh loading
decreases, as noted above, the trend in increas-
ing TOrh values may be reasonably understood
in terms of the decreasing Rh loading. Thus, the
observed increase in TOrh values may be due
to either the increase in % Pd loading or to the
decrease in % Rh loading.
.
Pd , the Pd-rich islands dominate and the metal
dispersion is 0.80. At higher Au loadings e.g.,
30 mol% Au and 70 mol% Pd , both Pd-rich
Ž
.
and Au-rich particles are present; and the Au-
rich islands are composed of 91% Au. The
metal dispersion of the 30:70 Au:Pd catalyst is
only 0.53. These results suggest that there are
also probably substantial structural changes in
Ž
the Au:Pd–SiO2 portion of the Cl–Rh N–
.
N rAu:Pd–SiO2 catalyst. These changes may
account for the irregularly changing activities as
the Au loading is increased.
On the other hand the % Rh is also changing
in these catalysts even though the tethering
process is the same for all of the catalysts. If
TO were to change with the Rh complex load-
ing, this would also affect the activities of these
3.5. Comments on the form of the rhodium on
the catalyst
Ž
.
catalysts. In fact, for Cl–Rh N–N rAu:Pd–
The hydrogenation of toluene to methylcy-
clohexane is catalyzed by the series of TCSM
Ž
.
Ž
SiO2 Au:Pd 4.2:1.0% , TO 128 mol H2rmol
Rh for 6 h is significantly higher when the %
Rh is 1.5 than it 91 mole H2rmol Rh for 6 h
is when the % Rh is 3.6. Thus, TO decreases as
the %Rh increases, which is similar to the trend
in rates observed for the three catalysts with
.
Ž
catalysts of the composition X–Rh N–
.
Ž
.
N rAu:Pd–SiO2. Reactions of the catalysts with
CO help to characterize the form of the cata-
Ž
.
lysts. When PF6–Rh N–N rAu:Pd–SiO2
Ž
.
Au:Pd wt.% 4.2:1.0 with Rh 1.2% reacts with
CO 1 atm in toluene before use in a hydro-
genation reaction, a Rh complex with n CO
bands at 2093 s and 2047 vs cm is formed;
Ž
.
Ž .
increasing Au content Table 6 . Thus, the ob-
Ž
.
Ž
.
served changes in Cl–Rh N–N rAu:Pd–SiO2
catalyst activity with Au loading may be due to
changes in either the morphology of the Au:Pd–
SiO2 portion of the catalyst or the changing
amounts of Rh complex tethered to the surface.
y
1
Ž .
Ž
.
the positions of these bands suggest that teth-
q
Ž
.Ž
.
ered Rh diamine CO
is formed. This result
2
indicates that the COD ligand is displaced by
Ž
CO in this reaction. The PF6–Rh N–
.
N rAu:Pd–SiO2 catalyst, even after being used
in three toluene hydrogenation cycles, exhibits
3.4.3. Effect of Õarious palladium loadings with
(
)
Ž
.
constant gold loading on Cl–Rh N–N r
the same n CO bands after treatment with CO.
This suggests that either the COD ligand is still
present after the hydrogenation and is displaced
by CO or the COD ligand is no longer present
as a result of having been hydrogenated and
Au:Pd–SiO2 catalytic actiÕity
Ž
.
For the series of Cl–Rh N–N rAu:Pd–SiO2
Ž
.
catalysts Table 6 in which the % Au is held
constant at 4.2%, the TOrh values increase as
Ž
Ž
the % Pd increases in the order: 1.0% Pd 16
some other donor groups stabilize the Rh di-
q
.
Ž
.
Ž
.
Ž
.Ž
.
TOrh -4.1% Pd 25 TOrh -9.8% Pd 43
amine
fragment. In Rh COD PR3 q2 -cata-
.
TOrh . Thus, the rate of toluene hydrogenation
lyzed olefin hydrogenation reactions, it is ob-
served 37 that the COD ligand is hydrogenated
at 258C. It is also presumably hydrogenated off
w
x
appears to increase linearly with the amount of
Pd, which is somewhat surprising because the

(
)
110
M.A.D.N. Perera, R.J. AngelicirJournal of Molecular Catalysis A: Chemical 149 1999 99–111
Ž
.
from the Rh complex in the PF6–Rh N–N
4. Summary
The TCSM catalysts consisting of Rh COD
rAu:Pd–SiO2 catalysts. Since PF6y is a very
weakly coordinating ligand, it seems likely that
surface oxygen atoms on the SiO2 will occupy
Ž
.
w
Ž
.
Ž
.
x
Ž .
NH2CH2CH2 NH CH2 3Si OCH3 3
X
teth-
ered on a SiO2-supported Au–Pd bimetal cata-
lyst are highly active for the hydrogenation of
toluene under the mild conditions of 408C and 1
the sites formerly occupied by COD, thereby
q
Ž
.
stabilizing the Rh diamine
unit. This SiO2-
q
Ž
.
stabilized Rh diamine
unit could also react
Ž
.
atm H2 pressure. These X–Rh N–N rAu:Pd–
SiO2 catalysts are most active with the weakly
coordinating Xy anions PF6y and BPhy4 . The
q
Ž
.Ž
.
with CO to give the Rh diamine CO
2
com-
plex. It is also conceivable that under the condi-
tions of the reaction RhI is removed from the
diamino ligand and deposits on the Au:Pd–SiO2
surface. Such a deposition has been observed
when Rh2Cl2 CO complexed to phosphinated
silicas is treated with H2 for several days at
Ž
.
Cl–Rh N–N rAu:Pd–SiO2 catalyst retains its
activity during three cycles over a period of 19
h and 2900 mol H2rmol Rh turnovers. This and
other evidence indicate that the Rh does not
leach from the catalyst. The activity of the
Ž
.
4
w
x
room temperature 38 . These catalysts showed
Ž
.
PF6–Rh N–N rAu:Pd–SiO2 catalyst changes
y
1
Ž .
Ž .
bands at 2048 s and 1983 s cm which were
attributed to CO adsorbed on metallic rhodium.
Ž
.
little when the Pd loading is kept constant 9.8%
while the Au loading increases from 0.8 to
4.2%. In a series of catalysts in which the Au
loading is held constant at 4.2% and the Pd%
increases from 1.0 to 9.8% and Rh decreases
In our studies, we see no band in the 1983 cmy
1
region which suggests that metallic Rh is not
present. However, Rh CO has been identified
IŽ
x
.
2
w
in several studies 39 of Rh on silica. This
Ž
.
from 3.6 to 1.5%, the rate TOrh of toluene
hydrogenation increases by a factor of three
which may be due either to the high loading of
Pd or the lower loading of the Rh complex.
Although other interpretations are possible, a
mechanism involving H2 adsorption on Pd fol-
lowed by hydrogen spillover onto the SiO2
where the tethered rhodium complex uses it to
hydrogenate the arene substrate is consistent
with the observed results.
species has bands at 2093, 2038 cmy1, which
y
1
Ž
.
are quite similar to those 2093, 2047 cm
observed in our studies of the PF6–Rh N–
Ž
.
N rAu:Pd–SiO2 catalyst both before and after
toluene hydrogenation. Thus, these bands are
IŽ
.
consistent with the presence of a Rh CO
2
q
Ž
.Ž
.
species or a tethered Rh diamine CO
com-
2
plex. From the spectroscopic studies, it is not
possible to distinguish these alternatives. If it
IŽ
.
were the Rh CO species, one would expect
2
the RhI on the surface to be an effective arene
w
x
hydrogenation catalyst 40–42 , which is ob-
served. On the other hand, the catalyst prepared
Acknowledgements
This work was supported by the U.S. Depart-
ment of Energy, Office of Science, Office of
Basic Sciences, Chemical Sciences Division,
under contract W-7405-Eng-82 with Iowa State
University.
Ž
.
by tethering PF6–Rh N–N to only SiO2 is
totally inactive as an arene hydrogenation cata-
Ž
Ž
.
.
lyst see PF6–Rh N–N rSiO2 in Table 1 . If Rh
were to be removed under the hydrogenation
conditions to give RhI, it should be an active
catalyst. Perhaps Pd must be present in order to
remove the Rh from the complex andror acti-
vate the RhI on the catalyst surface. Thus, it is
not possible to definitely conclude whether the
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