Rh2Cl2(CO)4 adsorbed and tethered on gold powder: IR spectroscopic characterization and olefin hydrogenation activity

Article abstract of DOI:10.1139/v00-190

Catalysts were prepared by adsorbing Rh₂Cl₂(CO)₄ directly on gold powder or on gold that contained the tethered ligands 2-(diphenylphosphino)ethane-1-thiol (DPET) or methyl 2-mercaptonicotinate (MMNT). Infrared (IR) studies (diffuse reflectance infrared Fourier transform (DRIFT)) of the catalyst Rh-Au prepared by adsorbing Rh₂Cl₂(CO)₄ directly on Au indicate that a Rh^I(CO)₂ species is present. IR studies of Rh-DPET-Au suggest that tethered cis-Rh(DPET)(CO)₂Cl is the major species at relatively high Rh₂Cl₂(CO)₄ loadings, but trans-Rh(DPET)₂(CO)Cl is observable at low Rh₂Cl₂(CO)₄ loadings. Spectral investigations of the catalyst Rh-MMNT-Au prepared by adsorbing Rh₂Cl₂(CO)₄ on MMNT-Au suggest that tethered [cis-Rh(MMNT)₂(CO)₂]⁺Cl^- and (or) Rh(MMNT)(CO)₂Cl are the major species at low Rh₂Cl₂(CO)₄ loadings, while a new unidentified species predominates at high Rh₂Cl₂(CO)₄ loadings. All three catalysts are active 1-hexene hydrogenation catalysts under the mild conditions of 40°C and 1 atm of H₂; they are much more active than Au powder or Rh₂Cl₂(CO)₄ in solution. Of the three catalysts, Rh-Au is the most active with a maximum turnover frequency (TOF) of 800 mol H₂ per mol Rh per min while its turnover (TO) is 29 600 mol H₂ per mol Rh during a 2-hour run. Under the conditions of 1-hexene hydrogenation, the catalysts lose their CO ligands. Thus, it appears that a form of Rh metal on Au is the catalytically active species.

Full text of DOI:10.1139/v00-190

5
78
Rh Cl (CO) adsorbed and tethered on gold
2
2
4
powder: IR spectroscopic characterization and
olefin hydrogenation activity
Hanrong Gao and Robert J. Angelici
Abstract: Catalysts were prepared by adsorbing Rh Cl (CO) directly on gold powder or on gold that contained the
2
2
4
tethered ligands 2-(diphenylphosphino)ethane-1-thiol (DPET) or methyl 2-mercaptonicotinate (MMNT). Infrared (IR)
studies (diffuse reflectance infrared Fourier transform (DRIFT)) of the catalyst Rh–Au prepared by adsorbing
I
Rh Cl (CO) directly on Au indicate that a Rh (CO) species is present. IR studies of Rh–DPET-Au suggest that teth-
2
2
4
2
ered cis-Rh(DPET)(CO) Cl is the major species at relatively high Rh Cl (CO) loadings, but trans-Rh(DPET) (CO)Cl is
2
2
2
4
2
observable at low Rh Cl (CO) loadings. Spectral investigations of the catalyst Rh–MMNT-Au prepared by adsorbing
2
2
4
+
–
Rh Cl (CO) on MMNT-Au suggest that tethered [cis-Rh(MMNT) (CO) ] Cl and (or) Rh(MMNT)(CO) Cl are the ma-
2
2
4
2
2
2
jor species at low Rh Cl (CO) loadings, while a new unidentified species predominates at high Rh Cl (CO) loadings.
2
2
4
2
2
4
All three catalysts are active 1-hexene hydrogenation catalysts under the mild conditions of 40°C and 1 atm of H2;
they are much more active than Au powder or Rh Cl (CO) in solution. Of the three catalysts, Rh–Au is the most ac-
2
2
4
tive with a maximum turnover frequency (TOF) of 800 mol H per mol Rh per min while its turnover (TO) is 29 600
2
mol H per mol Rh during a 2-hour run. Under the conditions of 1-hexene hydrogenation, the catalysts lose their CO
2
ligands. Thus, it appears that a form of Rh metal on Au is the catalytically active species.
Key words: catalysis, olefin hydrogenation, gold powder, tethered rhodium complexes, infrared studies, adsorption, rho-
dium complexes.
Résumé : On a préparé les catalyseurs mentionnés dans le titre en faisant absorber du Rh Cl (CO) directement sur de
2
2
4
l’or en poudre ou sur de l’or qui contient les ligands fixés 2-(diphénylphosphino)éthane-1-thiol (DPET) ou 2-
mercaptonicotinate de méthyle (MMNT). Des études infrarouges («DRIFT») du catalyseur (Rh–Au) préparé par adsorp-
I
tion de Rh Cl (CO) directement sur de l’or indiquent la présence d’une espèce Rh (CO) . Les études IR du Rh–DPET-
2
2
4
2
Au suggèrent que, pour des chargements relativement élevés de Rh Cl (CO) , le cis-Rh(DPET)(CO) Cl est l’espèce
2
2
4
2
fixée principale; on peut toutefois observer la formation de trans-Rh(DPET) (CO)Cl avec des chargements faibles de
2
Rh Cl (CO) . Des études spectrales du catalyseur (Rh–MMNT-Au) préparé par adsorption de Rh Cl (CO) sur du
2
2
4
2
2
4
+
–
MMNT-Au suggèrent que, pour des chargements faibles de Rh Cl (CO) , le [cis-Rh(MMNT) (CO) ] Cl et (ou) le
2
2
4
2
2
Rh(MMNT)(CO) Cl sont les espèces fixées principales; toutefois, pour des chargements élevés de Rh Cl (CO) ,
2
2
2
4
l’espèce principale est une nouvelle espèce qui n’a pas été identifiée. Les trois catalyseurs sont des catalyseurs actifs
pour l’hydrogénation de l’hex-1-ène dans des conditions douces, soit 40°C et 1 atm. de H ; en solution, ils sont beau-
2
coup plus actifs que l’or en poudre ou le Rh Cl (CO) . Des trois catalyseurs, le Rh–Au est le plus actif avec une va-
2
2
4
leur maximale de “TOF” de 800 mol H /mol Rh·min alors que son “TO” est de 29,600 mol H /mol Rh pour une
2
2
expérience de deux heures. Dans les conditions d’hydrogénation de l’hex-1-ène, les catalyseurs perdent leurs ligands
CO. Il semble donc que l’espèce catalytiquement active soit une forme métallique de Rh sur de l’or.
Mots clés : catalyseur, hydrogénation d’oléfines, poudre d’or, complexes de rhodium fixés, études infrarouges, com-
plexes de rhodium.
[
Traduit par la Rédaction] 586
Introduction
Gao and Angelici organometallic complexes to insoluble organic polymers or
inorganic oxides (2). Little information, however, has been
Heterogenized transition-metal complex catalysts, which
offer distinct advantages over their homogeneous counter-
parts, have been extensively studied in the past few decades
reported on transition-metal complexes tethered to metallic
surfaces (3).
The adsorption of organic molecules on solid metal sur-
faces has been extensively studied in recent years (4). Gold
(
1). Much work in this area has been dedicated to tethering
Received July 12, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on May 30, 2001.
Dedicated with best wishes to Professor Brian James on the occasion of his 65th birthday.
1
H. Gao and R.J. Angelici. Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011, U.S.A.
1Corresponding author (telephone: (515) 294-2603; fax: (515) 294-0105; e-mail: angelici@iastate.edu).
Can. J. Chem. 79: 578–586 (2001)
DOI: 10.1139/cjc-79-5/6-578
© 2001 NRC Canada

Gao and Angelici
579
metal has been investigated extensively because it does not
form a surface oxide under atmospheric conditions at room
temperature (5, 6). Among the organic compounds (7, 8)
that adsorb on gold are thiols (RSH) and disulfides (RSSR),
which produce RS groups on the surface (9). For example,
terminal alkyl thiols, X(CH ) SH (n ≥ 1 and X = COOH,
Scheme 1.
2
n
CONH , CN, CH NH , etc.) adsorb on gold films as X(CH ) S
2
2
2
2 n
groups (10).
Various transition-metal complexes have been tethered on
polymer or inorganic oxide supports using a ligand that co-
ordinates to the transition-metal complex through a donor
ligand at one end and to the support through a surface-
reactive group at the other end (11). Some of these tethered
complexes have been used as catalysts (2). In our previous
studies, we formed catalysts by tethering complexes on sil-
ica that also contained a supported metal, e.g., Pd (12).
These tethered complex on a supported metal (TCSM) cata-
lysts are unusually active for the hydrogenation of arene
substrates. In the present report, we describe catalysts that
are prepared by tethering a Rh complex directly to the sur-
face of a metal (Au). The tethering ligand is attached to the
Au through a thiol group and to the Rh complex by a phos-
phine or pyridine donor group. It was hoped that an unsatu-
rated organic substrate would be activated by coordination to
the Rh complex and hydrogen would be activated by Au to
give a catalyst that was more active than either the complex
or the Au metal separately. Although gold is generally con-
sidered to be a poor catalyst, small-particle gold has recently
been reported (13) to be active for alkene hydrogenation. Al-
though there are a few examples (14) of metal complexes
tethered on gold, none have been used in catalytic hydroge-
nation reactions.
SOCl (20 mL) in a 250 mL flask connected to a mineral oil
bubbler in order to prevent moisture from contaminating the
2
reaction mixture. Evolution of SO and HCl was observed
2
while the suspension was heated at reflux, and a red precipi-
tate of anhyd gold(III) chloride was visible in the reaction
flask. When the evolution of SO and HCl ceased, the re-
2
maining SOCl was removed by syringe and the red solid
2
was dried in vacuum (1 mm Hg, 1 mm Hg = 133.322 Pa) at
6
5°C for 4 h. Then THF (100 mL) was added to the reaction
flask, and 20 mL (1.0 M) of a THF solution of
Li[B(C H ) H] was added dropwise with stirring at room
2
5 3
temperature. After stirring for 12 h at room temperature, the
mixture was filtered and the solid was washed successively
with 80 mL aliquots of THF, methanol, THF, pentane, and
water, and then dried under vacuum (0.02 mm Hg) at 80°C.
The resulting Au powder was cleaned with freshly prepared
“
piranha” solution (20 mL) composed of a 50:50 mixture of
concd H SO and 30% aq H O (caution: carefully add the
2
4
2
2
H SO to the H O ) in a 250 mL beaker with slow stirring.
2
4
2
2
Nitrogen was blown over the mixture to reduce gold loss
due to foaming. The mixture was stirred for 10 min, filtered
on a coarse frit, washed with water (10 × 50 mL) and etha-
nol (2 × 20 mL), and dried under a nitrogen stream. The Au
powder was then baked in an oven at 120°C overnight. Rou-
tine BET surface area measurements were performed on a
Micromeritics AccuSorb 2100 E instrument. With Kr as the
adsorbate gas at 77 K, the surface area of the resulting Au
In this study, the tethering ligands, methyl 2-
mercaptonicotinate (MMNT) or 2-(diphenylphosphino)ethane-
1
-thiol (DPET), were adsorbed on gold powder to give the
ligand-modified gold powders MMNT-Au and DPET-Au
Scheme 1). These modified gold powders were reacted with
Rh Cl (CO) to give Au powders with tethered rhodium-
(
2
–1
powder was determined to be 0.96 ± 0.07 m g . An XPS
analysis (on an AEI 200B spectrometer) of the Au powder
showed it to be pure Au(0). Neither Au(I) nor other impuri-
ties such as lithium and boron were detected.
2
2
4
carbonyl complexes. We also observed that Rh Cl (CO)
2
2
4
could be adsorbed directly onto Au powder without a tether.
All three of these rhodium-complex – Au-metal materials
were characterized by diffuse reflectance infrared Fourier
transform (DRIFT) spectroscopy and evaluated for their cat-
alytic activities in the hydrogenation of 1-hexene under the
mild conditions of 40°C and 1 atm of H2.
Infrared spectroscopy
All FT-IR and DRIFT spectra were recorded on a Nicolet
10 spectrophotometer equipped with a TGS detector in the
7
main compartment and a MCT detector in the auxiliary
experimental module (AEM). The AEM housed a Harrick
diffuse reflectance accessory. All solution IR spectra were
recorded using a NaCl cell (0.50 mm thickness) in the main
Experimental
–
1
Materials
compartment with the instrument set for 4 cm resolution
and 128 scans. All DRIFT spectra were recorded with sam-
ples in the Harrick microsampling cup using 600 scans and
Rh Cl (CO) and HAuCl ·xH O (Au, 49%) were purchased
2
2
4
4
2
from Strem. The 2-(diphenylphosphino)ethane-1-thiol (DPET)
15) and methyl 2-mercaptonicotinate (MMNT) (16) tethers
–
1
(
4 cm resolution. The background spectrum used for the
DRIFT spectra was clean gold powder. Spectra were routinely
baseline and purge corrected. The abscissa of the spectra
were recorded and are reported in absorbance units even
though the radiation collected is actually reflectance. Kubelka–
Munk transformations of the spectra were not possible be-
cause of the low intensities of all bands; the bands were
eliminated by the transformation. The pseudo-absorbance units
used do appear to obey Beer’s law, at least approximately.
were prepared by the literature methods. Toluene was dried
over CaH before use, while THF, ethanol, and olefin sub-
2
strates were used directly as commercial products without
further purification.
The gold powder was prepared by reducing anhyd AuCl3
(
17) with Li[(C H ) BH] in THF using the following modi-
2 5 3
fied literature procedures (18). Hydrogen tetrachloroaurate
HAuCl ·3H O) (2.00 g) was heated under reflux with
(
4
2
©
2001 NRC Canada

5
80
Can. J. Chem. Vol. 79, 2001
Preparation of ligand-modified Au powders
Fig. 1. DRIFT spectrum of Rh–Au.
MMNT-Au
The Au powder (300 mg) was mixed with a THF solution
1 mL) of methyl 2-mercaptonicotinate (MMNT) (3 mM) in
(
a test tube and shaken for 2 min on a Genie Vortex mixer.
After allowing the mixture to stand overnight at room tem-
perature, the solution was separated by centrifugation and
analyzed for the amount of MMNT by measuring the intensity
–
1
of the ester ν(C=O) absorption at 1720 cm and converting
it to MMNT concentration using Beer’s law. Subtracting this
concentration from the original concentration of the MMNT
solution gave the amount of MMNT (0.30 µmol) adsorbed
on the 300 mg sample of Au powder. Increasing the original
concentration of MMNT in the THF solution did not change
the amount of MMNT adsorbed on the Au powder. The Au
solid was washed with THF (1 mL) and dried in vacuum at
room temperature to give the MMNT-modified Au powder
(
MMNT-Au). The DRIFT spectrum of MMNT-Au shows a
–
1
–1
carbonyl band at 1728 cm . This 8 cm increase from that
of free MMNT (1720 cm ) is similar to that observed in the
–
1
adsorption of MeO C(CH ) SH on a Au film (10a, 19).
Rh Cl (CO) tethered on MMNT-Au (Rh–MMNT-Au)
2 2 4
2
2 n
A mixture of MMNT-Au (300 mg) and a toluene solution
1 mL) of Rh (CO) Cl of the desired concentration was
(
2
4
2
DPET-Au
The Au powder (300 mg) was mixed with a 1,2-
dichloroethane (DCE) solution (1 mL) of 2-
diphenylphosphino)ethane-1-thiol (DPET) (5 mM). After be-
ing shaken for 2 min on a Genie Vortex mixer, the mixture
was allowed to stand at room temperature overnight. The so-
lution was separated by centrifugation and analyzed for the
amount of DPET by measuring the intensity of the C-H
stretching absorption at 3073 cm–1 and converting it to DPET
concentration using Beer’s law. Subtracting this concentra-
tion from the original concentration of the DPET solution
gave the amount (0.63 µmol) of DPET adsorbed on the
stirred at room temperature overnight under an N atmo-
2
sphere. The mixture was centrifuged for 5 min to separate
the toluene solution. The separated toluene solution was
analyzed by the same IR method that was used in the prepa-
ration of the Rh–Au catalyst to determine the amount of
Rh Cl (CO) adsorbed on the 300 mg of MMNT-Au powder.
(
2
2
4
The Au solid was washed (4 × 1 mL) with toluene and dried in
vacuum at room temperature to give the tethered Rh Cl (CO)
2
2
4
catalyst Rh–MMNT-Au. The mol of tethered Rh Cl (CO) ,
2
2
4
and the DRIFT spectra of the Rh–MMNT-Au catalysts pre-
pared by using different concentrations of Rh Cl (CO) solu-
2
2
4
tions are shown in Table 1 and Fig. 2.
3
00 mg sample of Au powder. Increasing the original con-
centration of DPET in the DCE solution did not change the
amount of DPET adsorbed on the Au powder. The Au solid
was washed with DCE (1 mL) and dried in vacuum at room
temperature to give the modified Au powder, DPET-Au.
Rh Cl (CO) tethered on DPET-Au (Rh–DPET-Au)
2
2
4
Rh–DPET-Au was prepared by the same procedure as that
used for the preparation of Rh–MMNT-Au using DPET-Au
instead of MMNT-Au. The mol of tethered Rh Cl (CO) ,
2
2
4
and DRIFT spectra of the Rh–DPET-Au catalysts prepared
by using different concentrations of Rh Cl (CO) solutions
are shown in Table 2 and Fig. 3.
2
2
4
Preparation of Rh Cl (CO) -modified Au powder
catalysts
2
2
4
Hydrogenation reactions
Rh Cl (CO) adsorbed on Au powder (Rh–Au)
2
2
4
The hydrogenation reactions were carried out in a 50 mL
three-necked jacketed glass-vessel closed with a self-sealing
silicon rubber cap and connected to a vacuum–hydrogen line
and a constant pressure (1 atm) gas buret. The temperature
of the ethylene glycol circulating through the jacket was
maintained at 40.0°C by a constant temperature bath. After
the catalyst was added and the atmosphere in the vessel was
replaced with hydrogen, solvent was added and the mixture
was stirred for about 5 min. Then, the substrate was added
and the hydrogen volume uptake was followed by the con-
stant-pressure gas buret. The products were analyzed by gas
chromatography on a Varian 3400 GC using a 25 m HP-1
capillary column with a FID detector. When the catalyst was
used in several successive 1-hexene hydrogenations, the re-
action mixture after the first cycle was filtered, and the solid
The Au powder (300 mg) was mixed with a toluene solu-
tion (1 mL) of Rh Cl (CO) (0.6 mM), and the mixture was
2
2
4
stirred at room temperature overnight under an N atmo-
2
sphere. The solution was separated by centrifugation and an-
alyzed for the amount of Rh Cl (CO) by measuring the
2
2
4
–
1
intensity of the ν(CO) band at 2088 cm and converting it to
Rh Cl (CO) concentration using Beer’s law. Subtracting
2
2
4
this concentration from the original concentration of the
Rh Cl (CO) solution gave the amount of Rh Cl (CO)
4
2
2
4
2
2
(
0.72 µmol) adsorbed on the 300 mg sample of Au powder.
The gold powder was washed with toluene (4 × 1 mL) and
dried at room temperature under vacuum to give the catalyst
–
1
Rh–Au. IR (DRIFT) (cm ): 2105 (s) (CO), 2045 (s) (CO)
Fig. 1).
(
©
2001 NRC Canada

Gao and Angelici
581
Table 1. Adsorption of Rh Cl (CO) from a 1 mL toluene solution on 300 mg of MMNT-
2
2
4
Au powder.
Concentration of
Rh adsorbed
µmol)b
Rh Cl (CO) before
Rh:MMNT mol
ratioa
2
2
4
Entry
adsorption (mM)
MMNT:Rhc
(
a
b
c
d
e
a
1.00
0.60
0.30
0.15
0.075
9
4
2
1
0.56
0.56
0.42
0.30
0.15
0.53
0.53
0.71
1.0
0.5
2.0
Mol ratio of rhodium in the initial toluene solution to the MMNT ligand adsorbed on 300 mg of
MMNT-Au.
b
On 300 mg of MMNT-Au.
c
Ratio on Rh–MMNT-Au.
–
1
Fig. 2. DRIFT spectra of the Rh–MMNT-Au samples containing
various amounts of rhodium (µmol) per 300 mg catalyst: (a)
Rh Cl (CO) (2115 (w), 2095 (s), and 2045 (s) cm in
2 2 4
I
Nujol™ mull (20)), they are also similar to those of Rh (CO)
2
–
1
0
.56; (b) 0.56; (c) 0.42; (d) 0.30; (e) 0.15.
(approximately 2095 (s) and 2032 (s) cm ) on SiO or
2
Al O (21). Thus, the Rh carbonyl group on Rh–Au appears
2
3
I
to be some type of Rh (CO) species, perhaps associated
with Cl ligands in some way. The broadness of the 2045 cm
2
–
–1
absorption suggests that other species, perhaps monocarbonyl
I
Rh (CO) species, are also present. It is unlikely that any of
these species is associated with CO adsorbed on Au as CO
does not adsorb on pure Au powder under these conditions.
Rh–MMNT-Au
The tethered Rh Cl (CO) catalyst Rh–MMNT-Au was
2
2
4
obtained by stirring a mixture of Rh Cl (CO) and MMNT-
2
2
4
Au in toluene at room temperature overnight. Table 1 gives
the number of mol of rhodium anchored on 300 mg of
MMNT-Au when 1 mL toluene solutions of Rh Cl (CO) in
2
2
4
concentrations ranging from 0.075 to 1.0 mM were used.
The MMNT:Rh ratio on the Au surface ranges from 0.53 to
2
.0. In Table 1, it is evident that the number of mol of rho-
dium anchored on 300 mg of MMNT-Au increases as the
initial concentration of Rh Cl (CO) in the toluene solution
2
2
4
increases. When the initial concentration of Rh Cl (CO) in
2
2
4
catalyst was washed with ethanol, dried under vacuum, and
used for the hydrogenation of the second batch of 1-hexene
by following the same procedure as that used in the first cy-
cle. After the second cycle, the catalyst was treated as after
the first cycle; the isolated catalyst was then used for the
third cycle.
the toluene solution reaches 0.60 mM (Table 1, entry b), the
amount of Rh anchored on 300 mg of MMNT-Au is 0.56 µmol
and does not increase further when the initial concentration
of Rh Cl (CO) in the toluene solution is increased to
2
2
4
1
.0 mM. To gain some insight into the structure of the rho-
dium species, DRIFT spectra (Fig. 2) were recorded on the
five Rh–MMNT-Au samples. When the amount of Rh on the
MMNT-Au is relatively low (Table 1, entries d and e), the
DRIFT spectra (Fig. 2d and e) show one relatively sharp
Results and discussion
IR characterization of catalysts consisting of
–1
ν(CO) band at 2120 (s) cm and one broad band in the region
Rh Cl (CO) adsorbed or tethered on Au powder
2
2
4
–1
2
060–2000 cm ; these absorptions are somewhat similar to
–
1
Rh–Au
After stirring a mixture of Rh Cl (CO) and Au powder in
those of the Rh–Au catalyst (2105 (s) and 2045 (s) cm ).
Although this suggests that the structure of the major rho-
dium-carbonyl species formed on these two Rh–MMNT-Au
samples might be similar to that on Rh–Au, the two ν(CO)
bands at 2120 (s) and 2060–2000 (vs, br) could also be as-
signed to the square planar [(cis-Rh(MMNT) (CO) ] Cl ;
the related cis-Rh(py) (CO) complex is known to have
2
2
4
toluene at room temperature overnight, IR analyses of both
the solution and the Au solid showed that rhodium-carbonyl
species were present on the Au powder surface. A DRIFT
spectrum (Fig. 1) of the resulting Au powder (Rh–Au) shows
+
–
2
2
–
1
+
two ν(CO) bands at 2105 (s) and 2045 (s) cm . The DRIFT
spectrum of Rh–Au remained the same after it was washed
with toluene several times, which indicates that the rhodium-
carbonyl species is strongly adsorbed on the Au surface. Al-
though the ν(CO) bands of Rh–Au are similar to those of
2 2
–
1
ν(CO) bands at 2095 (vs) and 2020 (vs) cm in CHCl solu-
3
tion (22a). In pyridine solution under a CO atmosphere, cis-
+
–1
Rh(py) (CO) has bands at 2100 and 2038 cm (22b). Still
2
2
another possible assignment for the species with bands at
©
2001 NRC Canada

5
82
Can. J. Chem. Vol. 79, 2001
Table 2. Adsorption of Rh Cl (CO) from a 1 mL toluene solution on 300 mg of DPET-
2
2
4
Au powder.
Concentration of
Rh adsorbed
µmol)b
Rh Cl (CO) before
Rh:DPET mol
ratioa
2
2
4
Entry
adsorption (mM)
DPET:Rhc
(
a
b
c
d
a
3.00
1.00
0.30
0.15
9
3
1
0.5
0.52
0.48
0.45
0.28
1.2
1.3
1.4
2.3
Mol ratio of rhodium in the initial toluene solution to the DPET ligand adsorbed on 300 mg of
DPET-Au.
b
On 300 mg of DPET-Au.
c
Ratio on Rh–DPET-Au.
2
120 (s) and 2060–2000 (vs, br) cm–1 is Rh(MMNT)(CO) Cl;
Fig. 3. DRIFT spectra of the Rh–DPET-Au samples containing
various amounts of rhodium (µmol) per 300 mg catalyst: (a)
0.52; (b) 0.48; (c) 0.45; (d) 0.28.
2
this assignment is based on the ν(CO) spectrum of
Rh(py)(CO) Cl which has bands at 2090 (s) and 2020
2
–
1
(
s) cm in CHCl solution (23). In fact, there may be an
3
+
–
equilibrium (eq. [1]) between [Rh(MMNT) (CO) ] Cl and
2
2
Rh(MMNT)(CO) Cl as the MMNT:Rh ratio on the surface
2
decreases from 2.0 to 1.0. This equilibrium may account for
the change in band shapes between spectra in Fig. 2d and e.
–
1
The shoulder on the low-energy side of the 2060–2000 cm
absorption in Rh–MMNT-Au might be attributed to a
+
–
–1
[
Rh(MMNT) (CO)] Cl species based on a 1993 cm band
3
+
reported for Rh(py) (CO) , which was observed in pyridine
3
(
22b).
+
–
[
1] [Rh(MMNT) (CO) ] Cl + 1/2Rh Cl (CO)
4
2
2
2
2
W
2Rh(MMNT)(CO) Cl
2
When the initial mol ratio of Rh–MMNT increases to 2, the
Rh–MMNT-Au sample (Table 1, entry d) has ν(CO) absorp-
–
1
tions at 2120 (s), 2095 (m), and 2030 (s) cm . As the initial
mol ratio of Rh–MMNT increases further, the relative inten-
–
1
sity of the ν(CO) band at 2120 cm decreases, but the rela-
–
1
tive intensity of the ν(CO) band at 2095 cm increases. The
Rh–DPET-Au
–
1
growth of the 2095 cm band (Fig. 2a–c) is in samples in
which the MMNT:Rh ratio is less than 1.0, which means that
there is less than one MMNT ligand per Rh. A species with
only one MMNT per two Rh atoms is Rh Cl (CO) (MMNT);
The tethered Rh Cl (CO) catalyst Rh–DPET-Au was pre-
2
2
4
pared by reacting DPET-Au with toluene solutions (1 mL) of
Rh Cl (CO) ranging in concentration from 3.0 to 0.15 mM
2
2
4
to give Au powders (Rh–DPET-Au) containing different
amounts of tethered Rh Cl (CO) . The data in Table 2 show
that the amount of rhodium anchored on 300 mg of DPET-
Au increases with an increase in the amount of Rh Cl (CO)
4
in the initial toluene solution; the DPET:Rh ratio on the Au
surface increases from 1.2 to 2.3 in the samples. Figure 3
shows that the DRIFT spectra of the four Rh–DPET-Au
samples (Table 2) change as the DPET:Rh ratio increases.
All of the samples exhibit one relatively sharp ν(CO) band at
about 2070 cm and one broad ν(CO) band in the region
2025–1975 cm , and as expected, the intensities of the
bands increase as the loading of Rh Cl (CO) increases. The
reaction of Rh Cl (CO) with PPh in solution is known (24)
to give a complicated distribution of various mononuclear
2
2
3
however, the model complex Rh Cl (CO) (py) for this spe-
2
2
3
2
2
4
–
1
cies is unknown. Another possible species for the 2095 cm
–
1
band (and probably a companion band at 2030 cm ) is one
in which Rh is bonded to the sulfur atom tethered to the sur-
face. At this point, it is not possible to make an assignment
to these new bands. It was also observed that when the Rh–
MMNT-Au sample (Table 1, entry d) was stirred in a toluene
solution (1 mL) of Rh Cl (CO) (0.60 mM), an additional
2
2
2
2
4
–
1
0
.24 µmol of Rh Cl (CO) was tethered on the Au powder,
2
2
4
–
1
giving a sample with 0.56 µmol of Rh, which is the same as
that of the Rh–MMNT-Au (0.56 µmol) sample used in Ta-
ble 1, entry b. The DRIFT spectra of these two Rh–MMNT-
Au samples are similar as well. Because of the broadness of
2
2
4
2
2
4
3
the ν(CO) bands, these assignments are tentative, and it is
[RhCl(CO)3–n(PPh ) ] and dinuclear [Rh (µ-Cl) (CO)4–m-
3 n
2
2
I
not possible to exclude the presence of the Rh (CO) species
(PPh ) ] complexes depending on the ratio of reactants. The
2
3 m
resulting from the adsorption of Rh Cl (CO) directly on the
Au.
complex cis-Rh(PPh )(CO) Cl is assigned to the two ν(CO)
absorptions at 2090 (s) and 2005 (s) cm , which are in a
2
2
4
3
2
–
1
©
2001 NRC Canada

Gao and Angelici
583
Scheme 2.
Fig. 4. DRIFT spectra of Rh–Au as a function of treatment un-
der vacuum (0.1 mm Hg) at different temperatures: (a) original;
b) 50–60°C for 1.5 h; (c) 70–80°C for 1.5 h; (d) 100–110°C for
.5 h; (e) after stirring sample (d) in toluene under CO (1 atm)
(
1
at room temperature overnight.
–
1
similar position (2070 and 2025–1975 cm ) to those
Fig. 3a–c) of the samples in which the DPET:Rh ratio is
(
near unity (1.2–1.4). In the spectrum (Fig. 3d), in which the
DPET:Rh ratio is higher (2.3), there is evidence of another
band at lower energy, which is similar in position to that re-
–
1
ported for trans-Rh(PPh ) (CO)Cl (1959 cm ) (24). Thus,
3
2
the spectra in Fig. 3 are consistent with cis-Rh(DPET)(CO) Cl
2
(
Scheme 2a) as the predominant species at high Rh Cl (CO)
2 2 4
loadings where the DPET:Rh ratio is 1.2–1.4, whereas trans-
Rh(DPET) (CO)Cl (Scheme 2b) becomes a contributing spe-
2
cies when the DPET:Rh ratio is higher (24). It should be
noted that none of the four Rh–DPET-Au samples exhibit
I
ν(CO) bands that are characteristic of Rh (CO) resulting
2
from the adsorption of Rh Cl (CO) directly on Au.
2
2
4
Decarbonylation of Rh Cl (CO) adsorbed and tethered
2
2
4
on Au powder
Fig. 5. DRIFT spectra of Rh–DPET-Au with heat and CO treat-
ment: (a) original; (b) heating at 50–60°C under vacuum for
The decarbonylation of the Rh–Au and Rh–DPET-Au cat-
alysts was examined by heating under vacuum. Figure 4
shows changes in the ν(CO) bands for the Rh–Au catalyst as
a function of treatment under vacuum (0.1 mm Hg) and at
1
.5 h; (c) stirring sample (b) in toluene at room temperature
overnight under CO (1 atm).
–1
different temperatures (no absorptions in the 1700–1900 cm
range were observed in the DRIFT spectra of any of the
heat-treated samples). After treatment at 50–60°C under vac-
–
1
uum for 1.5 h, the ν(CO) band at 2105 cm disappeared but
–
1
the band at 2045 cm remained, although its intensity de-
I
creased (Fig. 4b); this suggests that the original Rh (CO)
2
species has been lost and small amounts of perhaps a
I
monocarbonyl Rh (CO) species remain. After further treat-
–
1
ment at 70–80°C for 1.5 h, the ν(CO) band at 2045 cm
–
1
shifted to about 2025 cm and broadened (Fig. 4c). After
this sample was further treated at 100–110°C for 1.5 h, only
–
1
a weak, broad band at 2005 cm remained (Fig. 4d). How-
ever, when this final sample was stirred in toluene under a
CO atmosphere at room temperature overnight, the DRIFT
spectrum (Fig. 4e) of the resulting sample was similar to that
of the original Rh–Au catalyst, although the ν(CO) bands are
at slightly lower values, which suggests the formation of a
I
I
Rh (CO) species that is not identical to the original Rh (CO)
2
2
species.
After a sample of Rh–DPET-Au containing 0.45 µmol Rh
per 300 mg of DPET-Au was heated at 60°C under vacuum
2045 cm–1 would overlap with that of cis-RhCl(CO) (DPET)
2
(
0.1 mm Hg) for 1.5 h, the DRIFT spectrum showed only
(Scheme 2). Thus, it appears that during the heat treatment,
three weak ν(CO) bands at 2105 (sh), 2076 (s), and 2007 (s,
some of the rhodium is deposited on the Au surface and is
–
1
I
br) cm (Fig. 5b). When the heat-treated sample was stirred
in toluene under a CO atmosphere at room temperature over-
night, the sample exhibited ν(CO) bands at 2105 (sh), 2076
carbonylated to Rh (CO) under CO.
2
Hydrogenation of 1-hexene with catalysts consisting of
Rh Cl (CO) adsorbed and tethered on Au powder
–
1
(
2
s), and 2007 (s, br) cm . The absorptions at 2076 (s) and
007 (vs, br) indicate that at least some of the original Rh–
2
2
4
The Rh–Au, Rh–MMNT-Au, and Rh–DPET-Au materials
were used to catalyze the hydrogenation of 1-hexene under
–1
DPET-Au (2076 (s) and 2008 (s) cm ) has been regenerated.
The ν(CO) band at 2105 (sh) corresponds to a Rh (CO) spe-
cies with ν(CO) bands at 2105 and 2045 cm (the band at
I
the conditions of 40°C and 1 atm of H . Table 3 gives hydro-
2
2
–
1
genation activities for the homogeneous Rh Cl (CO) and
2
2
4
©
2001 NRC Canada

5
84
Can. J. Chem. Vol. 79, 2001
a
Table 3. Hydrogenation of 1-hexene catalyzed by Rh Cl (CO) adsorbed and tethered on gold powder.
2
2
4
Yields of products (%)e
Reaction
time (h)
Conversion
(%)b
Max
trans-2-
Hexene
cis-2-
Hexene
TOFc
TOd
Hexane
Rh (µmol)
Catalyst
Au
Rh–Au
—
7.5
2
6
6
5
9.2
100
89.7
56.3
0
—
—
0.7
100
49
23.3
0
5.9
0
24
17.4
0
2.6
0
16.7
15.6
0
f
0.12
0.08
0.08
10
800
116
23.2
0
29 600
14 500
6 900
0
g
Rh–DPET-Au
h
Rh–MMNT-Au
Rh Cl (CO)
2
2
4
Rh Cl (CO) + PPh (1:1)
10
6
0
0
0
0
0
0
2
2
4
3
a
b
c
Reaction conditions: catalyst (50–55 mg), 1-hexene (0.5 mL), ethanol (5 mL), 40°C, 1 atm of H2.
Conversion of 1-hexene determined by GC at the indicated reaction time.
Maximum TOF defined as mol of H uptake per mol rhodium per min.
2
d
Turnover (mol of H uptake per mol Rh) at the indicated reaction time.
2
e
f
Yields of products determined by GC.
0
.72 µmol Rh per 300 mg catalyst.
g
h
0
0
.48 µmol Rh per 300 mg catalyst.
.42 µmol Rh per 300 mg catalyst.
Table 4. Hydrogenation of 1-hexene using Rh–
Table 5. Durability of the Rh–Au and Rh–DPET-Au catalysts in
MMNT-Au catalysts with different amounts of
the hydrogenation of 1-hexene.a
a
rhodium.
Catalyst
Rh–Au
Cycle
1st
Max TOFb
TOb
Rh on 300 mg of
Rh–MMNT-Au (µmol)
800
528
350
116
35
29 600(2)
29 500(2.5)
28 500(3)
18 400(6)
9 640(6)
Max TOFb
TOc
2
nd
0
0
0
0
.15
.30
.42
.56
a
48.0
23.2
16.0
7.0
10 800(5)
6 900(6)
3 280(5)
1 740(5)
3rd
1st
2nd
Rh–DPET-Au
a
Reaction conditions are the same as those in Table 3.
Footnotes are the same as those in Table 4.
b
Reaction conditions are the same as those in Table 3.
b
TOF defined as mol of H uptake per mol Rh per min.
2
c
TO (mol of H uptake per mol rhodium) at the indicated
2
ings. Table 5 shows the durability of the Rh–Au and Rh–
DPET-Au catalysts during repeated use in the hydrogenation
of 1-hexene. The activities of both catalysts in the second
cycle are lower than in the first cycle. The DRIFT spectra of
the used Rh–Au and Rh–DPET-Au catalysts, which were
isolated from the reaction mixtures after Rh–Au was used
for three cycles and Rh–DPET-Au was used for two cycles,
show no ν(CO) absorptions. Even after these two used cata-
lysts were stirred in toluene under CO (1 atm) overnight, no
ν(CO) bands were observed in the DRIFT spectra of the
samples. Thus, the nature of the Rh species on the Au has
changed dramatically during the hydrogenation. It seems un-
likely that all of the rhodium has leached from the surface
since the catalysts still have appreciable activity after two or
three runs even though they exhibit no ν(CO) absorptions.
reaction time (h) shown in parentheses.
Rh Cl (CO) -PPh (1:1) catalysts, Au powder, Rh–Au, Rh–
2
2
4
3
DPET-Au (0.48 µmol Rh per 300 mg of DPET-Au), and
Rh–MMNT-Au (0.42 µmol Rh per 300 mg of MMNT-Au).
From the data in Table 3, it can be seen that the homogeneous
rhodium complex catalysts Rh Cl (CO) and Rh Cl (CO) -
2
2
4
2
2
4
PPh are inactive for 1-hexene hydrogenation under these
3
mild conditions. The Au powder exhibits a very low activity
for the hydrogenation. However, Rh–Au, Rh–MMNT-Au,
and Rh–DPET-Au are all active for the hydrogenation of 1-
hexene under these reaction conditions. Their activities de-
crease in the following order: Rh–Au > Rh–DPET-Au > Rh–
MMNT-Au. The maximum turnover frequency (TOF) and
turnover (TO) values for the Rh–Au catalyst are 800 mol H2
per mol Rh per min and 29600 mol H per mol Rh in 2 h.
2
Conclusions
To the best of our knowledge, this activity is greater than
that of any homogeneous or immobilized rhodium complex
catalyst for the hydrogenation of 1-hexene reported in the
literature (11f, 25–27). For the Rh–DPET-Au and Rh–
MMNT-Au catalysts, substantial amounts of isomerization
to the 2-hexenes also occur.
Table 4 gives the results of hydrogenation of 1-hexene
over Rh–MMNT-Au catalysts with different amounts of rho-
dium, prepared as described in Table 1. It can be seen that
the activities (per mol of Rh) of the Rh–MMNT-Au catalysts
decrease as the amount of rhodium on the catalyst increases.
This decrease may be due to changes in the catalyst precur-
sor complexes present on the surface at different Rh load-
Although the original goal of this investigation was to
probe the hydrogenation activities of catalysts that consist of
metal complexes tethered on a gold surface, we find that the
most active catalyst is Rh–Au in which Rh Cl (CO) is sim-
2
2
4
ply adsorbed on gold. This catalyst is far more active than
Au metal or Rh Cl (CO) separately. While the initial rho-
2
2
4
I
dium species on the Au appears to be some type of Rh (CO)
unit, it loses CO under 1-hexene hydrogenation conditions
2
(40°C, 1 atm H ). Thus, the active catalyst is a noncarbonyl
2
Rh species, which also does not adsorb CO. Although the
active form of the Rh is not known, it is possibly small Rh
metal aggregates. The formation of this active Rh species is
©
2001 NRC Canada

Gao and Angelici
585
apparently facilitated in some manner by the Au. This is
supported by the observation that Rh–Au and Rh–DPET-Au
undergo decarbonylation even at 55°C under vacuum
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2
4
not undergo decarbonylation even when heated at 120°C un-
–
3
der vacuum (1 × 10 mm Hg) for 24 h. It has also been re-
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for a material prepared by adsorbing Rh Cl (CO) from tolu-
3
4
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ene solution onto Al O do not change when it is heated at
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8
0°C under 1 atm of H for 1 h (2l). This suggests that the
2
Au powder facilitates the decarbonylation of the rhodium
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an unusually active form of Rh for 1-hexene hydrogenation.
While less active, the tethered Rh complex catalysts, Rh–
DPET-Au and Rh–MMNT-Au, also catalyze the hydrogena-
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Rh complexes that are present in the unused catalysts lose
their CO ligands to generate a noncarbonyl form of Rh, per-
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alytically active species. The activities of the Rh–Au and
Rh–DPET-Au catalysts decrease upon repeated use. This de-
crease may be due to migration of the Rh(0) into the Au to
form an inactive alloy.
These studies demonstrate that it is possible to form active
hydrogenation catalysts by adsorbing Rh complexes on Au
metal. Although Au is too expensive to be used as a catalyst
support for most studies, the results do suggest that the ad-
sorption of metal complexes on less expensive metal sup-
ports may lead to unusually active hydrogenation catalysts.
(
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7
Acknowledgement
(
This work was supported by the U.S. Department of En-
ergy, Office of Science, Office of Basic Energy Sciences,
Chemical Sciences Division, under contract W-7405-Eng-82
with Iowa State University.
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