A Recyclable Nanoparticle-Supported Rhodium Catalyst for Hydrogenation Reactions

Abstract

Catalytic hydrogenation under mild conditions of olefins, unsaturated aldeydes and ketones, nitriles and nitroarenes was investigated, using a supported rhodium complex obtained by copolymerization of Rh(cod)(aaema) [cod: 1,5-cyclooctadiene, aaema-: deprotonated form of 2-(acetoacetoxy)ethyl methacrylate] with acrylamides. In particular, the hydrogenation reaction of halonitroarenes was carried out under 20 bar hydrogen pressure with ethanol as solvent at room temperature, in order to minimize hydro-dehalogenation. The yields in haloanilines ranged from 85% (bromoaniline) to 98% (chloroaniline).

Full text

Molecules 2010, 15, 3311-3318; doi:10.3390/molecules15053311
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
A Recyclable Nanoparticle-Supported Rhodium Catalyst for
Hydrogenation Reactions
Maria Michela Dell’Anna, Vito Gallo, Piero Mastrorilli * and Giuseppe Romanazzi
Dipartimento di Ingegneria delle Acque e di Chimica del Politecnico di Bari, via E.Orabona, 4 I-70125
Bari, Italy; E-Mail: mm.dellanna@poliba.it (M.M.D.)
* Author to whom correspondence should be addressed; E-Mail: p.mastrorilli@poliba.it;
Tel.: +39-080-596-3605; Fax: +39-080-596-3611.
Received: 31 March 2010; in revised form: 27 April 2010 / Accepted: 29 April 2010 /
Published: 5 May 2010
Abstract: Catalytic hydrogenation under mild conditions of olefins, unsaturated aldeydes
and ketones, nitriles and nitroarenes was investigated, using a supported rhodium complex
obtained by copolymerization of Rh(cod)(aaema) [cod: 1,5-cyclooctadiene, aaema–:
deprotonated form of 2-(acetoacetoxy)ethyl methacrylate] with acrylamides. In particular,
the hydrogenation reaction of halonitroarenes was carried out under 20 bar hydrogen
pressure with ethanol as solvent at room temperature, in order to minimize hydro-
dehalogenation. The yields in haloanilines ranged from 85% (bromoaniline) to 98%
(chloroaniline).
Keywords: hydrogenation; supported rhodium complex; nitroaromatics; halonitroanilines
1. Introduction
Heterogeneous transition metal catalysts leading to their efficient recyclability without a significant
loss in activity as well as to facile separation of products from reaction mixtures without contamination
of metal residues play an important role in economically and environmentally benign chemical
processes. The immobilization of homogeneous metal coordination complexes onto an insoluble
support is a very useful methodology for the synthesis of heterogeneous catalysts. Several supports
have been employed for the immobilization of various homogeneous complexes including polymeric
organic and inorganic supports [1–7].
OPEN ACCESS

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Among heterogeneous catalysts, supported Rh complexes have been widely employed for
promoting hydrogenation reactions [8–11]. In the field of the hydrogenation of unsaturated compounds
the catalytic hydrogenation of aromatic halonitro compounds to yield anilines is very important, since
haloanilines are a class of industrially interesting compounds used as starting materials or
intermediates of many fine chemicals, such as dyes, drugs, herbicides, cosmetic products, pesticides
and polymers [12,13]. The catalysts usually employed in the hydrogenation of halonitroarenes are
based on transition metals [14], such as noble metals and Raney nickel, which is very sensitive toward
the moisture in air and may burn.
Moreover, the hydrogenation of halo-substituted nitroaromatic compounds poses a serious problem
due to the tendency towards hydro-dehalogenation, which is enhanced by amino substitution in the
aromatic ring [15]. Various hydrogenation methods for nitroarenes with palladium [16–18], platinum
[19–24], and ruthenium [25,26] heterogeneous catalysts have been recently proposed, all aimed at
improving the selectivity towards the corresponding haloanilines and at minimizing the dehalogenation
process.
Furthermore, the selective transfer hydrogenation of halonitroarenes promoted by transition metal
catalysts in the presence of different hydrogen donors such as alcohols or formic acid and its salts has
been reported [27,28]. Of remarkable interest is the novel transfer hydrogenation system developed by
Yan and coworkers [29]. In this protocol, twelve “active hydrogens” can be transferred from
water/ethanol system as the efficient hydrogen donor and used directly for the hydrogenation of
halogenated nitrobenzene over Ru-Fe/C catalyst, obtaining o-chloroaniline with 98.0% selectivity at
99.8% conversion without dehalogenation.
In the framework of our research on hybrid catalysis, we have synthesised a polymerizable
heteroleptic complex of Rh(I) bearing 1,5-cyclooctadiene (cod) and the anion of 2-(acetoacetoxy)ethyl
methacrylate (aaema-) as ligands [30]. Rh(cod)(aaema) was copolymerized with N,N’-methylene
bisacrilamide and N,N-dimethylacrilamide in dimethylformamide to yield a supported complex in
which the catalytically active centres are dispersed onto an organic polymer matrix (Scheme 1).
Scheme 1. Synthesis of the supported rhodium catalyst.
Rh O
OO
OO
DMF
100°C
N,N-dimethylacrylamide
N,N'-methylene bisacrylamide
Rh-pol
This nano-structured catalyst has been also employed for the synthesis of poly(phenylacetylene)s
with nanospherical morphology, reproducible and stable over time [31]. We report here on the
catalytic activity of the above mentioned supported rhodium complex in heterogeneous hydrogenation

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reactions under mild conditions of several organic substrates such as olefins, unsaturated aldehydes
and ketones, nitrobenzenes and nitriles. Moreover, we gained insight into the hydrogenation of halo-
substituted nitroaromatic compounds at room temperature under 20 bar hydrogen pressure.
2. Results and Discussion
The hydrogenation of cyclohexene at room temperature and pressure yielded cyclohexane in 2 h
(entry 1, Table 1). The recyclability of the catalyst was verified by submitting the same recovered Rh
supported catalyst to five subsequent cycles of this reaction and no appreciable loss in activity was
observed. The results obtained using supported rhodium complex (Rh pol) in the hydrogenation of
several organic substrates are summarized in Table 1.
Table 1. Catalytic hydrogenations promoted by Rh-pol. Reaction conditions:
substrate/supported rhodium = 160 mol/mol; T = 298 K.
Entry Substrate Time (h)
PH2
(bar) Conv.
(%) Products Selectivity
(%)
1 a
2 1 100 100
2 a 4 1 100 100
3 a
O
(−)-carvone
6.5
1
100
O
O
77
23
4 b
O
(−)-carvone
23
1
100
O
O
92
8
5 b
H
O
8
20
95 H
O
OH
72
24
6 a CN
1 20 83 CH2=N-CH2 100
7 a,c NO2
18 1 100 NH2100
a: in CH3OH;
b
: in CH2Cl2; c: at 323 K.
In all cases good selectivities were achieved using mild reaction conditions and acceptable reaction
times. It is worth noting that when the hydrogenation of (R)-(-)-carvone (entry 3) was carried out in
methanol the selectivity for carvotanacetone was 77% after 6.5 h reaction. By performing the same
reaction in CH2Cl2 (entry 4) the reaction rate decreased, but the selectivity improved.
The encouraging result obtained in the hydrogenation of nitrobenzene (entry 7) prompted us to
study the hydrogenation of halonitroarenes promoted by Rh-pol. A paradigmatic substrate such as

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1-chloro-4-nitrobenzene (1a) was submitted to hydrogenation at room temperature affording the
corresponding p-chloroaniline (2a, Scheme 2), an important intermediate in the chemistry of dyes,
drugs and pesticides.
Scheme 2. Hydrogenation of halonitroarenes promoted by Rh-pol.
NO2
X
NH2
X
+ 3 H2Rh-pol
298 K + 3 H2O
1a: X=Cl
1b: X=F
1c: X=Br
2a: X=Cl
2b: X=F
2c:X=Br
No hydrogenation took place if the reaction was carried out at atmospheric pressure of H2, but high
yields of 2a (65-98%) were obtained raising the hydrogen pressure in the range 10-50 bar, as shown in
Table 2.
Table 2. Effect of hydrogen pressure on the hydrogenation reaction of 1a (2.12 mmol;
solvent: EtOH, 7 mL; T= 298 K; substrate/Rh = 160 mol/mol).
Entry Pressure
(bar) Time
(h) Conversion
(%) Selectivity for 2a
(%)
1 10 72 65 100
2 20 72 100 98
3 50 72 100 65
It is apparent from Table 2 that the best yield in 2a was obtained when the reaction was carried out
under 20 bar H2 (98% 2a after 72 h, entry 2). Under 50 bar H2, there was a significant decrease in the
selectivity in 2a (65%, entry 3) due to the reductive dechlorination of 1a – in fact 35% of aniline was
detected in the reaction mixture after 72 h reaction.
All the described reactions were carried out in ethanol, a typical solvent for the hydrogenation of
unsaturated substrates. When the same reactions were carried out in methylene chloride, the formation
of several hydrogenation by-products was detected regardless of the H2 pressure used. For example,
under 20 bar H2 the yield in 2a dramatically decreased down to 20% (at 100% conversion) after 72 h.
Subsequently, using Rh-pol as catalyst and the proper choice of process conditions, p-
halonitrobenzenes 1a-c were selectively hydrogenated to p-haloanilines 2a-c (Scheme 2) giving high
yields of the desired products (Table 3).
As summarized in Table 3, a long reaction time was required to convert 1b into the corresponding
fluoroaniline 2b (95% in 7 days, entry 1). However, no dehalogenation product was observed.
p-Chloronitrobenzene (1a, entry 2) was selectively hydrogenated into chloroaniline 2a, as shown
before. Only 2% of aniline, the product of reductive debromination, was found in the hydrogenation of
bromonitrobenzene 1c, affording, after 24 h, bromoaniline 1c in 90% yield (entry 4).

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Table 3: Hydrogenation of halonitrobenzenes under 20 bar of H2 at 298 K in the presence
of Rh-pol (substrate: 2.12 mmol; solvent: EtOH 7 mL; substrate/Rh = 160 mol/mol).
Entry Substrate Time
(days) Conversion
(%) Product Yield
(%)
1 1b 7 100 2b 95
2
3 a
1a
” 3
2
100
100
2a
” 98
94
4
5 a
1c
„ 1
0.5
98
100
2c
” 90
90
a: Recycle of the previous run.
The recyclability of the supported catalyst was tested by submitting the resin recovered from entries
2 and 4 to recycles. A higher hydrogenation rate was observed in both recycles (average t.o.f.'s pass
from 2.2 h-1 to 6.3 h-1 in the case of 1a, entries 2 and 3 and from 7.6 h-1 to 12.0 h-1 in the case of 1c,
entries 4 and 5), but the observed selectivity was slightly lower. In fact, at the end of the reaction, 6%
(entry 3) or 8% (entry 5) of by-products other than aniline was detected by GCMS, including p-halo-
nitrosobenzene and 4,4’-dihaloazobenzene.
The reason of the major activity of the resin in the recycle reactions may be the by-passing of the
induction time necessary to the catalyst for its activation under 20 bar hydrogen pressure. Atomic
absorption analyses of the recycled catalyst showed that the metal load in the resin was fully retained
also after two reaction cycles.
3. Experimental
3.1. General
Rh(cod)(aaema) and its copolymer with N,N’-methylenebisacrylamide and N,N-dimethylacrylamide
were synthesised according to literature methods [30]. The rhodium content of the resin was 5.59%.
Atomic absorption analyses for the determination of the Rh content in the catalyst were performed
with a Perkin Elmer 3110 instrument using a hollow cathode lamp. GLC analysis of the products was
performed using a HP 6890 instrument equipped with a FID detector and a Supelcowax.-10 capillary
column (30.0m x 0.25mm x 0.25 μm). GCMS data were acquired on a HP 6890 instrument using a HP
5MS 5% phenyl-methylsiloxane (30 m x 0.25 mm x 0.25 μm) coupled with a HP 5973 (EI, 70 eV)
mass spectrometer. Conversions and yields were calculated by GLC analysis as moles of hydrogenated
product per mole of starting nitro-compound by using dodecane as internal standard.
3.2. Hydrogenation reactions
A Schlenk tube charged with the unsaturated substrate (3.5 mmol) and the supported complex
(containing 0.022 mmol of rhodium) in methanol or methylene chloride (3 mL) as reported in the
caption of Table 1 was stirred under dihydrogen at 298 K monitoring the reaction course via GLC and
GCMS. At the end of the reaction the heterogeneous catalyst was recovered by filtration, washed with
methanol and diethyl ether, dried under vacuum and opportunely recycled. After duty the resin was
analysed to determine the residual metal content. Negligible loss of rhodium was observed in all cases.

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In the case of reactions carried out at pressures other than ambient, a 50 mL steel autoclave equipped
with an on-line spilling device was used. The hydrogenation of p-halonitrobenzenes was carried out in
a 100 mL stainless steel autoclave equipped with a sampling device.
Typically, substrate (2.12 mmol), Rh-pol (substrate/Rh = 160 mol/mol) and dodecane (54 mg, as an
internal standard for GLC) in ethanol (7 mL) were stirred vigorously under 20 bar hydrogen at room
temperature and the reaction course was monitored via GLC and GCMS. At the end of the reaction,
the supported catalyst was recovered by filtration, washed with ethanol and diethyl ether, dried under
vacuum and opportunely recycled.
4. Conclusions
The nanoparticle supported Rh catalyst synthesised by copolymerization of Rh(cod)(aaema) with
suitable co-monomers and cross-linkers showed a high activity and selectivity towards the
hydrogenation reaction of several unsaturated substrates under mild conditions.
In particular, the hydrogenation reaction of p-halonitrobenzenes to p-haloanilines (halogen = F, Cl,
Br) catalysed by Rh-pol under 20 bar hydrogen pressure at 298 K proceeded selectively without
affecting the halo-groups on the aromatic ring. The recycled catalyst showed higher activity compared
with the first cycle and did not leach out metal in solution.
Acknowledgements
We thank the Italian Ministero dell'Istruzione, dell'Università e della Ricerca, MIUR (PRIN 2007,
project 2007X2RLL2) for financial support.
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Sample Availability: Samples of the rhodium supported catalyst are available from the authors.
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