Combination catalysts consisting of a homogeneous catalyst tethered to a silica-supported palladium heterogeneous catalyst: Arene hydrogenation [10]

Full text of DOI:10.1021/ja9710058

J. Am. Chem. Soc. 1997, 119, 6937-6938
6937
Combination Catalysts Consisting of a
Homogeneous Catalyst Tethered to a
Silica-Supported Palladium Heterogeneous
Catalyst: Arene Hydrogenation
Hanrong Gao and Robert J. Angelici*
Department of Chemistry and Ames Laboratory
Figure 1. Conceptual illustration of a TCSM catalyst consisting of a
tethered homogeneous complex catalyst on a supported metal hetero-
geneous catalyst.
Iowa State UniVersity, Ames, Iowa 50011
ReceiVed March 31, 1997
6
Transition metal complex catalysts tethered to organic or
inorganic supports1 have received much attention in the past
few decades because they can, in principle, combine the
advantages of homogeneous and heterogeneous catalysts. Such
complexes can be easily tethered on silica surfaces through a
ligand in the complex which has alkoxy- or chlorosilane
functional groups that react with surface hydroxyl groups on
the SiO2.2 Silica-supported heterogeneous metal catalysts such
as Pd-SiO2, Rh-SiO2, and Pt-SiO2 also have surface hydroxyl
groups that could be used to tether transition metal complex
homogeneous catalysts. These combination catalysts consisting
of a tethered complex on a supported metal (TCSM) catalyst
(Figure 1) could function by synergistic action of both catalyst
components. For hydrogenation reactions of unsaturated organic
substrates, one might imagine that these TCSM catalysts could
function in a way that H2 is activated on the supported metal
(e.g., Pd, Rh, or Pt) with the resulting hydrogen atoms spilling
over onto the silica where they could react with the unsaturated
organic substrate that is simultaneously coordinated and acti-
vated by the tethered complex. This mechanism for the
functioning of a TCSM catalyst depends on the well-known
phenomenon of hydrogen spillover on supported metal cata-
lysts.3 In other mechanisms, the tethered complex may interact
more directly with molecules that are activated on the supported
metal. In this paper, we report an example, the first to our
knowledge, of a tethered complex on a supported metal (TCSM)
catalyst, whose activity for the hydrogenation of arenes is
substantially higher than that of the tethered complex or the
supported metal separately. In fact, its activity is higher than
that of any reported homogeneous or immobilized metal
complex catalyst under the mild conditions of 1 atm of H2 and
40 °C.
CN(CH2)3Si(OC2H5)3 in toluene, in a reaction similar to that
described for the synthesis of RhCl(CO)[CNBut]2.7 The
complex RhCl[CN(CH2)3Si(OC2H5)3]3 (Rh-CNR3)8 was pre-
pared in the reaction of [Rh(COD)Cl]29 (COD ) cyclooctadiene)
with 6 equiv of CN(CH2)3Si(OC2H5)3 according to a procedure
used for the preparation of RhCl[CN(2,6-xylyl)]3.10 The toluene
solution containing RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 or RhCl-
[CN(CH2)3Si(OC2H5)3]3 was refluxed with the silica-supported
11
palladium catalyst Pd-SiO2 (Pd, 10 wt %) for 4 h. After
filtration, the solid was washed with toluene and then dried in
vacuum at room temperature. The resulting tethered catalysts,
Rh-CNR2/Pd-SiO2 (Rh content, 1.10 wt %) and Rh-CNR3/Pd-
SiO2 (Rh content, 1.35 wt %), gave IR spectra (DRIFTS) with
ν(CN-) and ν(CO) bands (2197 (s) and 2017 (s) cm-1 for Rh-
CNR2/Pd-SiO2; 2176 (s) and 2124 (w) cm-1 for Rh-CNR3/Pd-
SiO2) that are very similar in position and relative intensity to
those of the untethered Rh-CNR2 and Rh-CNR3 complexes,4,8
which indicates that the complexes retain their structures after
being tethered to the Pd-SiO2 surface.
The rates of hydrogenation (Table 1) of toluene to methyl-
cyclohexane at 40 °C while being stirred under 1 atm of H2 in
the presence of the TCSM catalysts or the separate homogeneous
and heterogeneous catalysts were determined by following the
rate of H2 uptake. The catalysts are active from the outset but
the TOF (turnover frequency) values increase to a maximum
value of 4.8 for Rh-CNR2/Pd-SiO2 after 1 h and to 5.5 for Rh-
CNR3/Pd-SiO2 after 6.5 h. After several hours at the maximum
TOF levels, the activities decrease slightly. From the data in
Table 1, it can be seen that the Rh-CNR2/Pd-SiO2 catalyst
activity (as measured by the maximum TOF, turnover number
(TO), or H2 uptake) is at least 7 times greater than that of the
simple heterogeneous SiO2-supported Pd (Pd-SiO2), the Rh-
CNR2 complex tethered to just SiO2(Rh-CNR2/SiO2), just the
ligand (CN(CH2)3Si(OC2H5)3) tethered to Pd-SiO2(CNR/Pd-
SiO2), or the homogeneous catalyst (Rh-CNR2) even with
relatively large amounts of Rh (20 µmol) as compared with 6.3
µmol in Rh-CNR2/Pd-SiO2. Similarly, Rh-CNR3/Pd-SiO2 is at
least 9 times more active than Pd-SiO2, homogeneous Rh-CNR3,
tethered Rh-CNR3/SiO2, or CNR/Pd-SiO2. The most active
TCSM catalyst, Rh-CNR3/Pd-SiO2, has a maximum turnover
frequency of 5.5 mol H2/(mol of Rh min) and a turnover number
Two TCSM catalysts were prepared by tethering either of
the rhodium isocyanide complexes, RhCl[CN(CH2)3Si(OC2H5)3]3
or RhCl(CO)[CN(CH2)3Si(OC2H5)3]2, to a silica-supported
palladium metal catalyst (Pd-SiO2). The rhodium isocyanide
complex RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 (Rh-CNR2)4 was
5
prepared by the reaction of [Rh(CO)2Cl]2 with 4 equiv of
(1) (a) Hartley, F. R. Supported Metal Catalysts; Reidel: Dordrecht, The
Netherlands, 1985. (b) Iwasaka, Y. Tailored Metal Catalysts; Reidel: Tokyo,
1986. (c) Cornils, B.; Hermann, W. A. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: Weinheim, 1996; p 351.
(2) (a) Blumel, J. Inorg. Chem. 1994, 33, 5050. (b) Capka, M.; Czakova´,
M.; Wlodzimierz, U.; Schubert, U. J. Mol. Catal. 1992, 74, 335. (c) Allum,
K. G.; Hancock, R. D.; Howell, I. V.; McKenzie, S.; Pitkethly, R. C.;
Robinson, P. J. J. Organomet. Chem. 1975, 87, 203. (d) Capka, M.; Hetflejs,
J. Collect. Czech. Chem. Commun. 1974, 39, 154. (e) Pugin, B. J. Mol.
Catal. A: Chem. 1996, 107, 273. (f) Czakova´, M.; Capka, M. J. Mol. Catal.
1981, 11, 313.
(6) (CH3CH2O)3SiCH2CH2CH2NC was prepared from (CH3CH2O)3-
SiCH2CH2CH2NHCHO and Cl3COC(dO)Cl following a procedure devel-
oped for the synthesis of other alkyl isocyanides (Skorna, G.; Ugi, I. Angew.
Chem., Int. Ed. Engl. 1977, 16, 259); IR (in CH2Cl2), ν(CN-) 2150 cm-1
;
1H NMR (CDCl3) δ 3.81 (q, 6H, OCH2CH3), 3.38 (m, 2H, CNCH2), 1.78
(m, 2H, CH2CH2CH2), 1.20 (t, 9H, OCH2CH3), 0.72 (t, 2H, SiCH2).
(7) Deeming, A. J. J. Organomet. Chem. 1979, 175, 105.
(3) (a) Pajonk, G. M.; Teichner, S. J.; Germain, J. E. SpilloVer of
Adsorbed Species; Elsevier: Amsterdam, 1983. (b) Conner, W. C., Jr.;
Pajonk, G. M.; Teichner, S. J. AdV. Catal. 1986, 34, 1. (c) Conner, W. C.,
Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (d) Inui, T.; Fujimoto, K.;
Uchijima, T.; Masai, M. New Aspects of SpilloVer Effects in Catalysis;
Elsevier: Amsterdam, 1993.
(8) Selected data for RhCl[CN(CH2)3Si(OC2H5)3]3: 1H NMR (CDCl3)
δ 3.82 (q, 18H, OCH2CH3), 3.58 (t, 4H, CNCH2), 3.46 (t, 2H, CNCH2),
1.85 (m, 6H, CH2CH2CH2), 1.23 (t, 27H, OCH2CH3), 0.73 (t, 6H, SiCH2);
IR (in toluene) ν(CN-) 2158 (s), 2119 (m) cm-1. Anal. Calcd for
C30H63O9N3Si3ClRh: C, 43.28; H, 7.63; N, 5.05. Found: C, 42.70; H, 7.37;
N, 4.57.
(4) Selected data for RhCl(CO)[CN(CH2)3Si(OC2H5)3]2: 1H NMR
(CDCl3) δ 3.82 (q, 12H, OCH2CH3), 3.67 (t, 4H, CNCH2), 1.90 (m, 4H,
CH2CH2CH2), 1.21 (t, 18H, OCH2CH3), 0.75 (t, 4H, SiCH2); IR (in toluene)
(9) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88.
(10) Yamamoto, Y.; Yamazaki, H. J. Organomet. Chem. 1977, 140, C33.
(11) Pd-SiO2 was prepared by the incipient wetness method by impreg-
nation of SiO2 using an aqueous solution of H2PdCl4, calcining at 500 °C
for 4 h and reducing with H2 at 380 °C for 4 h.
ν(CN-) 2192 (s) cm-1, ν(CO) 1996 (s) cm-1
.
(5) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 84.
S0002-7863(97)01005-6 CCC: $14.00 © 1997 American Chemical Society

6938 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Table 1. Hydrogenation of Toluene to Methylcyclohexanea
Communications to the Editor
maximum TOFf
catalyst
Pd-SiO2
Rh contente (wt %)
reaction time (h)
(mol of H2/mol of Rh min)
TOg (mol of H2/mol of Rh)
H2 uptakeg (mmol)
0
0
23
15
18
8.5
8.5
6.0
8.5
8.5
1.0
0.62
1.1
0
8.4
0
b
CNR/Pd-SiO2
c
Rh-CNR2
0.08
0
4.8
0
0.7
5.5
57
0
1750
0
143
2420
d
Rh-CNR2/SiO2
1.30
1.10
Rh-CNR2/Pd-SiO2
c
Rh-CNR3
d
Rh-CNR3/SiO2
Rh-CNR3/Pd-SiO2
1.00
1.35
0.94
9.5
a Reaction conditions: 50 mg of solid catalyst, 5 mL of toluene, 40 °C, 1 atm of H2. Methylcyclohexane was identified by GC-MS. b This
catalyst was prepared in the same way as described for the TCSM catalysts except CN(CH2)3Si(OC2H5)3 was used instead of Rh-CNR2 or Rh-
CNR3. c Using 20 µmol of the homogeneous catalysts, Rh-CNR2 or Rh-CNR3. d These catalysts were prepared in the same way as that used for the
corresponding catalysts tethered on Pd-SiO2, except SiO2 was used instead of Pd-SiO2. e Rhodium content determined by atomic emission analysis.
f Turnover frequency defined as moles of H2 per mole of rhodium per minute. g Turnover and H2 uptake during the entire reaction time; they also
coincide with the amounts of product formed as determined by GC analyses.
Table 2. Hydrogenation of Arenes over Rh-CNR3/Pd-SiO2 and
Rh-CNR2/Pd-SiO2
of 2420 during an 8.5 h period. To our knowledge, this catalyst
is more active for the hydrogenation of arenes under these mild
a
substrate (mmol)
TOFb
TOc
product
conditions than any other reported homogeneous or immobilized
complex catalyst.12a-14 Hydrogenation of arenes in the presence
of homogeneous catalysts is generally performed under high
H2 pressures (g10 atm).15 Only a few homogeneous catalysts
are active using 1 atm of H2, but their activities are low.12
The two TCSM catalysts are also very active for the
hydrogenation of other arenes under the same mild conditions
(Table 2). The higher rate of hydrogenation of anisole as
compared with methyl benzoate in the presence of either catalyst
suggests that electron-donating substituents in the arene ac-
celerate the rate. The lower rates for diphenyl methane and
phenyl ether compared to those for toluene and anisole indicate
that steric effects may be involved. No hydrogenolysis products
were detected in any of the hydrogenation reactions.
Rh-CNR3/Pd-SiO2 catalyst
methyl benzoate (4.0)
diphenylmethane (3.0)
1.5
1.5
1760 (26) C6H11CO2Me
1910 (24) (C6H11)2CH2 (7.6%)
C6H11(Ph)CH2 (47.9%)
2760 (22) (C6H11)2O (24.3%)
C6H11OPh (42.3%)
3700 (19) C10H12 (88%)
C10H18 (12%)
phenyl ether (3.1)
naphthalene (5.0)
2.8
4.7
toluene (4.7)
anisole (4.6)
5.6
6.6
1120 (9)
2810 (9)
C6H11CH3
C6H11OCH3
Rh-CNR2/Pd-SiO2 Catalyst
methyl benzoate (8.0)
anisole (9.2)
1.6
5.7
6.3
6.5
1050 (12) C6H11CO2Me
1390 (7)
1560 (6)
1750 (8)
C6H11OCH3
C10H12
naphthalene (5.0)
toluene (9.4)
C6H11CH3
The durability of the Rh-CNR3/Pd-SiO2 catalyst was tested
by using it for three successive hydrogenations of toluene using
the conditions in Table 1. In the first cycle, the maximum TOF
was 5.5 and TO was 6920 after 24.5 h. Then, the catalyst was
filtered from the mixture, washed two times with toluene in
air, and dried under vacuum at room temperature. In the second
hydrogenation cycle, this catalyst gave a maximum TOF of 4.6
and TO values of 5710 after 24.5 h and 6950 after 31 h. After
the catalyst was treated as described after the first cycle, it was
used in a third cycle for which the maximum TOF was 4.0 and
TO was 6770 after 24.5 h and 7160 after 27 h. Thus, after
three cycles and a total use period of 82.5 h, the catalyst has
essentially the same TO activity as in the first cycle, which
indicates that the Rh has not been leached from the tether.
Even after three catalytic cycles, the Rh-CNR3/Pd-SiO2
catalyst still exhibits ν(CN-) absorptions in the DRIFT spectrum
at 2177(s) and 2124(w) cm-1, which are the same as those in
the freshly prepared catalyst. Also, there neither is any evidence
for a ν(CN-) band in the region of 2150 cm-1 corresponding to
a Reaction conditions: 25 mg of Rh-CNR3/Pd-SiO2 (Rh, 1.35 wt
%) or 50 mg of Rh-CNR2/Pd-SiO2 (Rh, 1.10 wt %); 5 mL of heptane
solvent; 40 °C; 1 atm of H2; mmole of arene substrate is given in the
table. Products were identified by GC-MS. b Turnover frequency is
the maximum TOF defined as moles of H2 uptake per mole rhodium
per minute. c Turnover number corresponds to the reaction time in
parentheses (in hours); it also coincides with the amount of product
formed as determined by GC analysis.
uncoordinated CN-R groups nor a band in the ν(N-H) region
(3300 ∼ 3500 cm-1) that would indicate hydrogenation of the
isocyanide groups to CH3NH-. Thus, the tethered Rh-CNR3
complex on the Rh-CNR3/Pd-SiO2 catalyst appears to retain its
structure even after extended use. In order to determine whether
the tether is essential to the activities of these catalysts, the
soluble, nontethered RhCl(CNBun)3 together with Pd-SiO2 was
used to catalyze the hydrogenation of toluene, but the activity
was only one-fifth that of Rh-CNR3/Pd-SiO2, which indicates
that the tether is required for the high TCSM catalyst activity.
Although it has not been established whether these arene
hydrogenation reactions proceed by a hydrogen spillover
mechanism or by some other process, it is clear that the tethered
homogeneous complex and the supported metal cooperate in a
manner that gives these TCSM catalysts activities higher than
either of the catalysts separately. It is expected that this type
of catalyst could incorporate a wide range of tethered homo-
geneous complexes and supported metals, which would permit
the design of TCSM catalysts for a variety of reactions. Studies
are in progress.
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Acknowledgment. This work was supported by the U.S. Depart-
ment of Energy, Office of Basic Energy Sciences, Chemical Sciences
Division, under contract W-7405-Eng-82.
JA9710058