Phosphonate-based hybrid materials for catalysis? Supported rhodium/2,2′-bipyridine complexes as reduction catalysts under hydrogen pressure

Article abstract of DOI:10.1002/1099-0690(200205)2002:10<1685::AID-EJOC1685>3.0.CO;2-I

Supported organometallic catalysts based on covalently immobilized rhodium/2,2′-bipyridine complexes are compared to the parent homogeneous system for the hydrogenation of ketones and carbon-carbon multiple bonds under hydrogen pressure. In alkaline methanol, the supported catalyst is more active for the reduction of C=C and C=C bonds than for the hydrogenation of keto groups; this is in contrast with the analogous system in a homogeneous medium for which the opposite trend is observed. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0690(200205)2002:10<1685::AID-EJOC1685>3.0.CO;2-I

FULL PAPER
Phosphonate-Based Hybrid Materials for Catalysis? Supported Rhodium/
2,2Ј-Bipyridine Complexes as Reduction Catalysts Under Hydrogen Pressure
[a]
[a]
[a]
[a]
´
Celine Maillet, Pascal Janvier, Marie-Jo Bertrand, Thoniyot Praveen, and
Bruno Bujoli*[a]
´
Dedicated to Dr. Jean Villieras on the occasion of his retirement
Keywords: Supported catalysts / Titanium / Phosphonates / Hydrogenation / N ligands / Rhodium
Supported organometallic catalysts based on covalently im-
mobilized rhodium/2,2Ј-bipyridine complexes are compared
to the parent homogeneous system for the hydrogenation of
ketones and carbon-carbon multiple bonds under hydrogen
pressure. In alkaline methanol, the supported catalyst is
more active for the reduction of C=C and CϵC bonds than
for the hydrogenation of keto groups; this is in contrast with
the analogous system in a homogeneous medium for which
the opposite trend is observed.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Introduction
reduction of substrates other than acetophenones, and the
results are described in this paper.
Nowadays, the chemical industry is under increased pres-
sure to develop cleaner production technologies, and homo-
geneous catalysis offers a means to provide more effective
processes and reduction of pollutants. A pitfall, however, is
that the separation of the catalyst from the starting com-
pound and products is difficult and often results in the loss
of the catalytic material. Immobilized catalysts, either en-
trapped or grafted and obtained as insoluble supports, offer
a potential solution that allows an easy separation of the
reaction products by filtration, reducing both waste and
costs.[1Ϫ5] We and others[6] have previously reported that
the immobilization of organometallic catalysts onto solid
supports could be performed using the binding properties
of phosphonic acids [PO3H2]. Recently, we applied this
strategy to the design of new phosphonate-based supported
rhodium/2,2Ј-bipyridine complexes active for the hydro-
genation of a variety of acetophenones under hydrogen
pressure. This system was derived from the early work of
Mestroni et al., who reported a high catalytic activity of
rhodium complexed to 2,2Ј-bipyridine (bpy) for ketone hy-
drogenation in the presence of methanolic NaOH, where
the [Rh(bpy)2]ϩ species was proposed as a catalyst.[7] In a
preliminary work,[8] we have clearly shown how the catalytic
performances could be improved by a careful optimization
of the conditions employed for the preparation of the im-
mobilized catalyst. In a final step, we wanted to know to
what extent this supported catalyst could be used for the
Results and Discussion
In a previous study, a rhodium/2,2Ј-bipyridine complex
functionalized by two phosphonic acid moieties (ligand 1
in Figure 1) was efficiently immobilized onto TiO2 particles
generated in situ.[8] After optimization of the heterogeniz-
ation step, the resulting supported catalyst (labelled catalyst
A) proved to be active for the reduction of aromatic ke-
tones, such as 4Ј-methoxyacetophenone, under hydrogen
pressure in alkaline methanol (Table 1, entry 1), similarly to
the results reported by Mestroni et al.[7] for rhodium/2,2Ј-
bipyridine complexes in a homogeneous medium in the
same solvent. Moreover, the catalyst was recycled four times
with no loss of activity (98% conversion after 21 hours for
successive runs, using a 1.5% Rh/4Ј-methoxyacetophenone
[a]
`
Laboratoire de Synthese Organique, UMR CNRS 6513,
`
2 Rue de la Houssiniere, BP 92208, 44322 Nantes Cedex 03,
France
Figure 1. Schematic representation of the support bipyridine A
Eur. J. Org. Chem. 2002, 1685Ϫ1689  WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0510Ϫ1685 $ 20.00ϩ.50/0
1685

C. Maillet, P. Janvier, M.-J. Bertrand, T. Praveen, B. Bujoli
FULL PAPER
[a]
Table 1. Results from the hydrogenation of various substrates using catalyst A in alkaline methanol under 40 bar H2
Entry[a]
Substrate
Conversion
(%)
Selectivity
(%)
1
2
3
4Ј-methoxyacetophenone
cyclohexanone
4-methylcyclohexanone
Ͼ99
Ͼ99
Ͼ99
4Ј-methoxy-sec-phenethyl alcohol (100)
cyclohexanol (100)
4-methylcyclohexanol (100)
cis/trans: 30:70[b]
4
5
6
2-heptanone
3-pentanone
6-methyl-5-hepten-2-one
90
95
Ͼ99
2-heptanol (100)
3-pentanol (100)
6-methylheptan-2-one (83)
6-methylheptan-2-ol (17)
cyclooctane (100)
octane (100)
1,2-diphenylethane (100)
α-pinane (93)
7
8
9
cyclooctene
1-octene
trans-stilbene
α-pinene
Ͼ99
Ͼ99
Ͼ99
Ͼ99
10
1,2,3,5- and 1,1,3,5-tetramethylcyclohexane (7)
octane (100)
octane (100)
11
12
1-octyne
4-octyne
Ͼ99
Ͼ99
[a]
Catalyst A: 41 mg, 3 mL methanol, 47.5 µL 1  NaOH [NaOH/Rh 5 equiv.], 0.95 mmol substrate [Rh/substrate ϭ 1%], ambient
temperature, 48 h. A 70:30 ratio is obtained in homogeneous medium using 2,2Ј-bipyridine as ligand (ref.[7b]).
[b]
ratio) and the extent of rhodium leaching measured by ICP- formed with an aqueous solution of rhodium(I) complexed
MS analyses of the reaction medium upper phase was very to ligand 1, identical to that used for the preparation of
low (Յ0.065%).[8] It was thus of interest to know if our catalyst A (ligand 1/Rh ϭ 2). With this biphasic catalytic
supported catalyst: (i) was also efficient for the reduction system, after 15 hours, 6-methyl-5-hepten-2-one was quant-
of aliphatic ketones (cyclic or linear), and (ii) was able to itatively reduced to yield 6-methyl-5-hepten-2-ol with a 95%
allow the selective reduction of CϭO groups in the presence selectivity (Rh/substrate ϭ 1%). Similarly, for the reduction
of CϭC bonds.
of a mixture of cyclooctene and acetophenone (1:1), the
As expected, catalyst A was active for the reduction of reduction of the ketone took place first. These results are
aliphatic ketones in good yields (Table 1, entries 2Ϫ5) but, similar to those obtained with the 2,2Ј-bipyridine/rhodium
surprisingly, in the case of 6-methyl-5-hepten-2-one, the re- complex (2:1) in homogeneous conditions, and are therefore
duction of the olefin proceeded faster than the hydrogena- consistent with the presence of similar active species in the
tion of the ketone (Table 1, entry 6). For this reason, vari- two cases. This clearly indicates that the factor responsible
ous alkenes and alkynes were tested in the same conditions, for the change in selectivity (CϭO versus CϭC reduction)
leading to a quantitative conversion into the corresponding occurs during the immobilization process, i.e. when tita-
alkanes (Table 1, entries 7Ϫ12). These results seemed some- nium isopropoxide is added.
what in contradiction with data from the literature,[7] and
we thus decided to compare the catalytic behavior of com-
pound A and a closely related homogeneous system
The activity of compound A seems to be slowed down by
the presence of the carbonyl function on the substrate. This
was confirmed by testing a mixture of cyclooctene and ace-
(Table 2), i.e. rhodium complexed to 2,2Ј-bipyridine (bpy)
tophenone (1:1 molar ratio): after three hours the sup-
with a ligand/metal ratio of two, identical to that used for
ported catalyst A shows only a poor conversion of cyclooc-
the preparation of the supported catalyst A. Obviously, the
tene (5%, entry 9) relative to cyclooctene on its own (95%,
entry 1).
reaction rate for the hydrogenation of olefins is higher for
the supported catalyst (Table 2, entries 1Ϫ4, 13Ϫ14); the
same observation can be made in the case of 4-octyne, with
Our comparison between the homogeneous and sup-
an octane yield (after 8 hours) ten times higher for the sup- ported rhodium/bipyridine systems was extended to the
ported rhodium-bipyridine complex (45%, entry 5) than for case where the bpy/Rh ratio is 1. Mestroni et al. have shown
the homogeneous system (4%, entry 6). On the contrary, that in such conditions the resulting complexes hydrogenate
however, the homogeneous system is far more efficient than both olefins and ketones, as confirmed by the results de-
the supported counterpart for the reduction of CϭO bonds scribed in Table 3 (entries 1/2, 9/10, 13/14). Even though the
(entries 7Ϫ8). Consistently, when the two types of function reaction rate observed for the reduction of CϭC bonds is
are present on the same substrate (e.g. 6-methyl-5-hepten- higher for the 1:1 complex, the reduction of the keto group
2-one) the regioselectivity of the reduction is radically dif- is still favored. On the contrary, CϭO versus CϭC compet-
ferent for the two systems, with the reduction of the CϭC itive reductions (entries 7/8, 11/12), performed with an ana-
bond dominating with the solid catalyst A while the Cϭ logue of catalyst A in which the initial ligand 1/rhodium
O bond is preferably hydrogenated with the Rh/bpy (1:2) ratio used was equal to 1 (catalyst B), showed an increase
complex in solution (entries 11Ϫ12), as reported by Mes- of CϭO reduction products, although the olefin reduction
troni et al. Moreover, some hydrogenation tests were per- was still predominant. It is also worth noting that the hy-
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Eur. J. Org. Chem. 2002, 1685Ϫ1689

Supported Rhodium/2,2Ј-Bipyridine Complexes as Reduction Catalysts Under Hydrogen Pressure
FULL PAPER
Table 2. Comparison of performances of a Rh/bpy (1:2) complex with catalyst A for the reduction of various substrates in alkaline
methanol under 40 bar H2
Entry
Substrate
Reaction
time (h)
Conversion
(%)
Selectivity
(%)
1 (supported)[a]
2 (homogeneous)[b]
3 (supported)[a]
cyclooctene
cyclooctene
1-octene
3
3
4
95
18
97
cyclooctane (100)
cyclooctane (100)
octenes[c] (5)
octane (95)
4 (homogeneous)[b]
1-octene
4
10
2
95
octane (100)
octenes[d] (14)
octane (86)
5 (supported)[a]
6 (homogeneous)[b]
7 (supported)[a]
4-octyne
8
8
90
41
octenes (49)[e]
octane (51)
4-octyne
octenes (90)[f]
octane (10)
4Ј-methoxy-sec-phenethyl alcohol (100)
4Ј-methoxyacetophenone
3
0
30
3
3
89
93
8 (homogeneous)[b]
9 (supported)[a]
4Ј-methoxyacetophenone
acetophenone ϩ cyclooctene
4Ј-methoxy-sec-phenethyl alcohol (100)
phenethyl alcohol (100)
cyclooctane (100)
acetophenone (0)
cyclooctene (5)
acetophenone (20)
cyclooctene (99)
acetophenone (89)
cyclooctene (0)
acetophenone (97)
cyclooctene (66)
70
7
10 (homogeneous)[b]
acetophenone ϩ cyclooctene
3
phenethyl alcohol (100)
cyclooctane (100)
7
11 (supported)[a]
6-methyl-5-hepten-2-one
6-methyl-5-hepten-2-one
15
6-methylheptan-2-one (79)
6-methylheptan-2-ol (12)
6-methyl-5-hepten-2-ol (9)
6-methylheptan-2-one (0.5)
6-methylheptan-2-ol (3.5)
6-methyl-5-hepten-2-ol (96)
3-methylbutan-1-ol (74)
3-methyl-3-buten-1-ol (26)
3-methylbutan-1-ol (63)
3-methyl-3-buten-1-ol (37)
12 (homogeneous)[b]
15
96
13 (supported)[a]
3-methyl-2-buten-1-ol
3-methyl-2-buten-1-ol
24
24
43
13
14 (homogeneous)[b]
[a]
Catalyst A: 41 mg, 3 mL methanol, 47.5 µL 1  NaOH [NaOH/Rh 5 equiv.], 0.95 mmol substrate [Rh/substrate ϭ 1%], ambient
[b]
temperature.
Homogeneous conditions: sample taken from a mother liquor, corresponding to 2.97 mg of 2,2Ј-bipyridine, 2.34 mg of
[Rh(COD)Cl]2, 3 mL methanol, 47.5 µL 1  NaOH [NaOH/Rh 5 equiv.], 0.95 mmol substrate [Rh/substrate ϭ 1%], ambient temperature.
[c]
[f]
[d]
[e]
The major product is trans-2-octene (77%). The major product is trans-2-octene (80%). The major product is cis-4-octene (80%).
The major product is cis-4-octene (90%).
drogenation of alkenes and alkynes is slower for catalyst B
(entries 3/4, 5/6).
In conclusion, it is clear that the immobilization process
is responsible for the catalytic behavior being significantly
Finally, the presence of rhodium metal that might be re- different from that of the parent homogeneous system. This
sponsible for the chemoselectivity in favor of the CϭC hy- phenomenon could arise from: (i) possible strong hydrogen-
drogenation observed with catalyst A was ruled out by tak- bonding of the ketones (compared to olefins) with the TiO2
ing account of the following data: a blank experiment was surface affecting their accessibility to the catalytic sites, and/
run with a solid C obtained using the same deposition or (ii) a mechanism different from that proposed by Mes-
method as for the preparation of catalyst A, but without troni et al. to explain the CϭO selectivity for the hydro-
bipyridine. The resulting pale-yellow solid turned gray irre- genation of unsaturated ketones observed with the Rh/bpy
versibly after one test (due to the formation of rhodium (1:2) homogeneous system. In this mechanism,[7b] various
metal under H2 pressure), while the color of catalyst A was species are present in equilibrium, while the situation is ex-
deep green and switched back to pale-yellow upon pro- pected to be different in the solid state with the supported
longed exposure to air. Moreover, in the case of 4Ј-methox- catalyst; for example, no variation of the rhodium to bipyr-
yacetophenone as the substrate, compound C led to a signi- idine 1 ratio is likely to occur for the immobilized catalyst
ficant amount of by-products corresponding to the hydro- because no mobility of the diamine is allowed due to its
genation of the aromatic ring of the substrate, as expected covalent attachment to the inorganic surface.
for rhodium(0)-based heterogeneous catalysts.[9] No evid-
Work is in progress to understand the exact role of the
ence for reduction of the phenyl ring was observed with inorganic network that might be of importance in the
catalyst A under the same conditions.
course of the reaction.[10] Finally, we would like to point
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Eur. J. Org. Chem. 2002, 1685Ϫ1689

C. Maillet, P. Janvier, M.-J. Bertrand, T. Praveen, B. Bujoli
FULL PAPER
Table 3. Influence of the rhodium/bipyridine ratio using homogeneous and supported catalysts for the reduction of various substrates in
alkaline methanol under 40 bar H2
Entry
Substrate
Reaction
time (h)
Conversion
(%)
Selectivity
(%)
1 (homogeneous)[a]
2 (homogeneous)[a]
3 (catalyst A)[b]
4 (catalyst B)[b]
cyclooctene
(Rh/ligand ϭ 1:2)
cyclooctene
(Rh/ligand ϭ 1)
1-octene
(Rh/ligand ϭ 1:2)
1-octene
(Rh/ligand ϭ 1)
4-octyne
(Rh/ligand ϭ 1:2)
4-octyne
(Rh/ligand ϭ 1)
acetophenone ϩ cyclooctene
(Rh/ligand ϭ 1:2)
acetophenone ϩ cyclooctene
(Rh/ligand ϭ 1)
acetophenone ϩ cyclooctene
(Rh/ligand ϭ 1:2)
acetophenone ϩ cyclooctene
(Rh/ligand ϭ 1)
6-methyl-5-hepten-2-one
(Rh/ligand ϭ 1:2)
3
3
4
4
8
8
7
7
3
3
15
18
100
97
72
90
60
cyclooctane (100)
cyclooctane (100)
octenes[c] (5)
octane (95)
octenes[d] (9)
octane (91)
5 (catalyst A)[b]
6 (catalyst B)[b]
octenes (49)[e]
octane (51)
octenes (78)[f]
octane (22)
phenethyl alcohol (100)
cyclooctane (100)
phenethyl alcohol (100)
cyclooctane (100)
phenethyl alcohol (100)
cyclooctane (100)
7 (catalyst A)[b]
8 (catalyst B)[b]
acetophenone (20)
cyclooctene (99)
acetophenone (63)
cyclooctene (100)
acetophenone (89)
cyclooctene (0)
acetophenone (95)
cyclooctene (40)
70
9 (homogeneous)[a]
10 (homogeneous)[a]
11 (catalyst A)[b]
phenethyl alcohol (100)
cyclooctane (100)
6-methylheptan-2-one (79)
6-methylheptan-2-ol (12)
6-methyl-5-hepten-2-ol (9)
6-methylheptan-2-one(43)
6-methylheptan-2-ol (57)
6-methyl-5-hepten-2-ol (0)
6-methylheptan-2-one (0.5)
6-methylheptan-2-ol (3.5)
6-methyl-5-hepten-2-ol (96)
6-methylheptan-2-one (1)
6-methylheptan-2-ol (76)
6-methyl-5-hepten-2-ol (23)
12 (catalyst B)[b]
6-methyl-5-hepten-2-one
(Rh/ligand ϭ 1)
15
15
15
Ͼ 99
96
13 (homogeneous)[a]
14 (homogeneous)[a]
6-methyl-5-hepten-2-one
(Rh/ligand ϭ 1:2)
6-methyl-5-hepten-2-one
(Rh/ligand ϭ 1)
Ͼ 99
[a]
Homogeneous conditions: sample taken from a mother liquor, corresponding to 2.97 mg (or 1.48 mg) of 2,2Ј-bipyridine, 2.34 mg of
[Rh(COD)Cl]2, 3 mL methanol, 47.5 µL 1  NaOH [NaOH/Rh 5 equiv.], 0.95 mmol substrate [Rh/substrate ϭ 1%], ambient temperature.
[b]
Catalyst A: 41 mg or Catalyst B: 21.3 mg, 3 mL methanol, 47.5 µL 1  NaOH [NaOH/Rh 5 equiv.], 0.95 mmol substrate [Rh/
[c]
[d]
[e]
substrate ϭ 1%], ambient temperature.
product is cis-4-octene (80%). Major product is cis-4-octene (87%).
Major product is trans-2-octene (77%).
Major product is trans-2-octene (68%).
Major
[f]
rhodium (X-ray fluorescence) content[8] in the supernatant liquid
showed that the amount of residual phosphorus was exactly 4 times
as high as the amount of residual rhodium. Thus, no variation of
the Rh/ligand 1 ratio occurred during the heterogenization step.
The solid was washed four times with distilled water in the centrifu-
gation tubes, then three times with acetone and was finally collected
by filtration through a Millipore apparatus. The resulting yellow
powder (990 mg of catalyst A) was dried at room temperature in
air. In agreement with the UV/Visible and X-ray fluorescence meas-
urements mentioned above, chemical analyses of the catalyst A [Rh:
2.21; P: 2.74; N: 2.30] showed Rh/P (calculated 0.25; found 0.24)
and Rh/N (calculated 0.125; found 0.13) values indicative of a rho-
dium/ligand 1 ratio of 1:2 on the supported catalyst. Catalyst A
(41 mg) was then transferred into a 30 mL stainless steel glass-
coated autoclave. Methanol (3 mL), 1  sodium hydroxide aqueous
solution (47.5 µL; NaOH/Rh ϭ 5) and the substrate (with a 1.0
mol % rhodium/substrate ratio) were then added. The autoclave
was purged three times with argon, followed by three times with
hydrogen and the final H2 pressure was adjusted to 40 bar. The
mixture was stirred (200 rpm) at room temperature for the desired
reaction time and then centrifuged. The liquid phase was then sep-
out that different chemoselectivities can be obtained with
ligand 1 depending on the conditions selected for its use:
either biphasic catalysis or supported catalysis.
Experimental Section
Typical Procedure for the Preparation of the Supported 2,2Ј-Bipyrid-
ine Rhodium Complex A and the Evaluation of the Catalytic Activ-
ity: Ligand 1 was prepared as previously described.[11] The follow-
ing procedure used for the synthesis of catalyst A was the result of
an optimization study described previously.[8] Ligand 1 (233.6 mg,
0.48 mmol) was suspended in 40 mL of a distilled water/2-propanol
mixture (1:1) and 2 mL of 1  sodium hydroxide was added. After
complete dissolution, [Rh(COD)Cl]2 (59.2 mg, 0.12 mmol) was ad-
ded and the mixture was stirred for 48 hours at room temperature.
Titanium tetra-iso-propoxide (1.6 mL) was then added in one por-
tion and the mixture stirred for two days. The reaction medium
was then centrifuged. Analysis of the phosphorus (UV/Visible) and
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Eur. J. Org. Chem. 2002, 1685Ϫ1689

Supported Rhodium/2,2Ј-Bipyridine Complexes as Reduction Catalysts Under Hydrogen Pressure
FULL PAPER
[2m]
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arated and analyzed by gas chromatography to determine the con-
Blum, D. Avnir, J. Am. Chem. Soc. 2000, 122, 11999Ϫ12000.
version and reaction yields. Catalyst B was prepared according to
the procedure described for the synthesis of catalyst A, except that
118.4 mg of [Rh(COD)Cl]2 were used, yielding a pale yellow solid
(1.02 g).
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