Pyridine-Stabilized Rhodium Nanoparticles in Ionic Liquids as Selective Hydrogenation and Transfer Hydrogenation Catalysts

Article abstract of DOI:10.1002/ejic.201900223

Rhodium nanoparticles (RhNPs) stabilized with pyridine-based ligands in the ionic liquid [BMIM][BF₄] (RhNPs-I to III) were synthesized from the organometallic precursor [Rh(μ-OMe)COD]₂ under dihydrogen pressure. The pyridine-stabilized RhNPs showed smaller size compared to the ligand free RhNPs-V and presented higher activity and selectivity in the hydrogenation of acetophenone to 1-phenylethanol. In the case of pyridine-capped RhNPs-I, the system was reused for several runs without loss of activity and selectivity. Nitrobenzene was reduced to aniline with dihydrogen in the presence of RhNPs-I with moderate activity. When the hydrogen source was formic acid-Et₃N azeotrope (transfer hydrogenation) the reaction was completed within minutes with high selectivity. Under transfer hydrogenation conditions, it was possible to apply the catalytic system RhNPs-I in multistep processes for the generation of substituted arylic amines through the reductive N-alkylation of nitrobenzene and benzaldehyde; and the synthesis of substituted pyrroles through the nitroarene reduction/Paal-Knorr condensation.

Full text of DOI:10.1002/ejic.201900223

A Journal of
Accepted Article
Title: Pyridine-stabilized rhodium nanoparticles in ionic liquid as
selective catalysts in hydrogenation and transfer hydrogenation
processes
Authors: Alejandro Serrano-Maldonado, Erika Martin, and Itzel
Guerrero-Ríos
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
Pyridine-stabilized rhodium nanoparticles in ionic liquid as
selective catalysts in hydrogenation and transfer hydrogenation
processes
Alejandro Serrano-Maldonado,[a] Erika Martinᵻ[a] and Itzel Guerrero-Ríos*[a]
ᵻIn memory of Prof. Erika Martin (1963-2017)
Rhodium nanoparticles (RhNPs) stabilized with pyridine-based
ligands in the ionic liquid [BMIM][BF4] (RhNPs-I to III) were
synthesized from the organometallic precursor [Rh(-OMe)COD]2
under dihydrogen pressure. The pyridine-stabilized RhNPs showed
smaller size compared to the ligand free RhNPs-V and presented
higher activity and selectivity in the hydrogenation of acetophenone to
1-phenylethanol. In the case of pyridine-capped RhNPs-I, the system
was reused for several runs without loss of activity and selectivity.
Nitrobenzene was reduced to aniline with dihydrogen in the presence
of RhNPs-I with moderate activity. When the hydrogen source was
formic acid-Et3N azeotrope (transfer hydrogenation) the reaction was
completed within minutes with high selectivity. Under transfer
hydrogenation conditions, it was possible to apply the catalytic system
RhNPs-I in multistep processes for the generation of substituted arylic
amines through the reductive N-alkylation of nitrobenzene and
benzaldehyde; and the synthesis of substituted pyrroles through the
nitroarene reduction/Paal-Knorr condensation.
RhNPs catalysts in ionic liquids control the selectivity towards
phenylethanol in the reduction of acetophenone. Although high
selectivity of phenylethanol was achieved at low conversion for
RhNPs in [BMIM][PF6], when the reaction was carried out to
completion, the selectivity diminished.[23] It has been reported that
bipyridine (bipy) ligands enhance the stability and the catalytic
performance of RhNPs in imidazolium-based ILs, obtaining low
selectivity to 1-phenylethanol in the reduction of acetophenone
with 2,2’-bipyridine as ligand.[24] Rh/bipy ratio was important to
control the selectivity of RhNPs in this process. It has been
proposed that the rate-determining step in hydrogenation of
arenes is the decoordination of bipy from the nanoparticle
surface.[25] The rational design of ancillary stabilizers that contain
bipyridine and imidazolium groups had remarkable impact on the
activity and selectivity of RhNPs. The ancillary ligand affords the
proper stabilization of RhNPs and at the same time the formation
of ligand-free RhNPs surface.[25] In our recent work, thiolate-
stabilized RhNPs in the ionic liquid [BMIM][BF4], showed
selectivity towards the reduction of alkene, nitro and imine groups
of aromatic substrates, and in the N-reductive alkylation of
nitrobenzene and benzaldehyde.[26]
Introduction
As an alternative to the hydrogenation with molecular hydrogen,
the catalytic transfer hydrogenation (TH) has received special
attention due to the simplicity of the equipment and the selectivity
control.[27,28] The use of formic acid as hydrogen source is
environmentally preferable, since it can be produced from
biomass in large scale and CO2 is formed as byproduct during the
TH reaction.[29,30] Many MNPs supported catalysts have been
developed for the TH of nitroarenes; however, to attain high yields,
long reaction times and complex supports are required.[31–37]
Herein, RhNPs were synthesized in the ionic liquid [BMIM][BF4]
and stabilized with pyridines to enhance its catalytic performance
in the reduction of arylketones and nitroarenes. The catalytic
systems RhNPs/Py/[BMIM][BF4] were applied on the
hydrogenation of arylketones with H2, as well as in the hydrogen
transfer reactions for the selective reduction of nitroarenes and in
multistep reactions using formic acid-Et3N azeotrope as hydrogen
source.
Today, many industrial processes rely on rhodium catalysts due
to its extraordinary and often unique catalytic properties. Rhodium
nanoparticles (RhNPs) as catalysts have emerged as a strategy
to improve the catalytic performance.[1,2] RhNPs combine the
advantages of homogeneous and heterogenous catalysts,
performing chemical transformations under mild conditions due to
their high surface area. The possibility of catalyst reutilization is
especially favored when ionic liquids (ILs) are employed as
stabilizing agents and reaction media.[3–10]
Metal nanoparticles (MNPs) are especially attractive in the
chemoselective reductions of aromatic carbonyl compounds and
nitroarenes, using environmentally benign hydrogen sources.[11–
13] Ligand-stabilized MNPs (amines, phosphines, thiolates, NHCs)
improve the catalytic activity and chemoselectivity as well.[14–17]
RhNPs have shown that surface modification using P-donor
ligands as stabilizers enhance the chemoselectivity of the
reduction of arenes with H2; however, bulky P-donor ligands
reduce conversion rates.[18–22]
Results and Discussion
Synthesis of Pyridine-stabilized RhNPs in [BMIM][BF4]
[a]
M. Sc. A. Serrano-Maldonado, Dr. Erika Martin, Dr. I. Guerrero-Ríos
Departamento de Química Inorgánica, Facultad de Química,
Universidad Nacional Autónoma de México,
Av. Universidad 3000, 04510 CDMX, Mexico
E-mail: itzelgr@unam.mx
Pyridine-stabilized RhNPs (RhNPs-I) were synthetized by
decomposition of the organometallic precursor [Rh(-
OMe)(COD)]2 under reductive H2 atmosphere, using one
equivalent of pyridine as nanoparticles stabilizer and the ionic
liquid [BMIM][BF4] as solvent. A pressure of 10 bar and 80°C
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
during 18 h was required to carry out the complete reduction of
the Rh precursor (figure 1) to the desired RhNPs together with
cyclooctane and methanol as organic byproducts. When RhNPs
were synthesized with 0.2 equivalents of Py, only rhodium
agglomerates were observed (Supporting Information, figure S1).
Figure 1. Synthesis of pyridine-stabilized RhNPs in [BMIM][BF4].
The system RhNPs-I was characterized by HAADF-STEM (figure
2), revealing pseudo-spherical nanoparticles with a narrow size
distribution of 2.8±0.4 nm for 400 nanoparticles (Supporting
Information, figures S2). High-resolution images of a single
nanoparticle showed interplanar distances of 2.2 Å,
corresponding to the crystallographic planes (111) of Rh with fcc
structure (figure 2). EDX analysis evidenced the presence of Rh
(Supporting Information, figures S2).
Figure 3. HR-XPS spectra for Rh 3d region (A) and N 1s region (B)
Further evidence of the presence of pyridine stabilizer was
confirmed by TGA, where mass loss around 205°C,
a
corresponding to the total pyridine added during the RhNPs
synthesis (Supporting Information, figure S6), points at a strong
interaction of pyridine not only with the nanoparticle surface, but
also with the ionic liquid.
RhNPs stabilized by 4-trifluoromethylpyridine (RhNPs-II), 4-tert-
butylpyridine (RhNPs-III),
and
4-(3-phenylpropyl)pyridine
(RhNPs-IV) were synthesized in the ionic liquid [BMIM][BF4]. TEM
analyses (figure 4) showed spherical nanoparticles and narrow
size distributions with similar size (3.0 – 3.4 nm, figures S7-S9)
for all the systems (RhNPs-II, III, IV) and in comparison with
RhNPs-I, larger nanoparticles were obtained with the substituted
pyridine stabilizers. RhNPs-III with 4-tert-butylpyridine as
stabilizer showed a tendency to agglomerate probably due to the
presence of bulky groups in the stabilizer that hamper a good NPs
dispersion. The absence of pyridine as previously shown for
RhNPs in [BMIM][BF4][26] generated nanoparticles with a mean
diameter of 5.2±0.8 nm, clearly showing an effect of the stabilizer
on the size control.
Figure 2. HR HAADF-STEM image of RhNPs-I (A); magnification of circled
zone (B); HR HAADF-STEM image of a single Rh nanoparticle with atom-
resolved structure, interplanar distances of 2.2 Å (111) (C); crystallographic
plane spots observed by FFT of C (0.455 Å-1) (D).
XPS characterization of RhNPs-I was performed to identify
rhodium species at the nanoparticle surface, revealing the
presence of Rh together with N and F (Supporting Information,
figure S3), indicative of the presence of pyridine stabilizer and
ionic liquid as well. Deconvolution analysis of HR-XPS for Rh 3d
showed only the presence of Rh(0) at the surface (figure 3A),
neglecting the formation of oxides on the surface, due to the
stabilization of ionic liquid. In the case of HR-XPS of N 1s, only
one species at 398.43 eV was observed, that corresponded to the
coordinated pyridine (figure 3B).
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
Figure 4. TEM images of RhNPs-II (A), RhNPs-III (B) and RhNPs-IV (C).
Table 1. Hydrogenation of acetophenone (1) catalyzed by RhNPs in [BMIM][BF4].
% Selectivitya
1c
Substrate
(mmol)
Temperature
(0C)
Entry
Catalytic system
Time (h)
% Conversiona
1a
97
90
89
78
84
89
95
94
62
92
95
99
38
-
1b
2
1d
<1
1
1
RhNPs-I
1
1
80
66
81
87
86
85
89
52
58
93
91
89
55
70
0
1
2
RhNPs-I
1
1.5
1
80
2
7
3
RhNPs-I
1
100
100
100
100
100
100
100
100
100
100
80
3
7
1
4
RhNPs-II
RhNPs-III
RhNPs-IV
RhNPs-V
RhNPs-V
RhNPs-I
1
1
5
10
9
7
5
1
1
5
2
6
1
1
3
7
1
7b
8b
9
1
1
2
2
1
1
2
3
2
1
0.5
2
1
7
23
4
8
10
11
12
13c
14d
RhNPs-I
1
2
2
RhNPs-I
5
1
2
2
1
RhNPs-I
10
1
1
<1
22
-
<1
40
-
<1
0
RhNPs-bipy[24]
RhNPs-(SR)[26]
15
1
1
80
-
Results from duplicated experiments. Reaction conditions: 1 mL of the catalytic solution of RhNPs (10-2 mol L-1, 0.01 mmol of total Rh), H2 (5
bar), 1 h. a Determined by GC using n-decane as internal standard. b agglomeration of nanoparticles was observed at the end of reaction. c From
ref. [24], using 40 bar H2 pressure. d From ref. [26], using 50 bar H2 pressure
to cyclohexylethanol (1c), and the hydrogenolysis reaction of 1a
Hydrogenation of acetophenones
to ethyl benzene (1d) as side reaction.
When RhNPs-I were evaluated in the hydrogenation of 1 at 80°C
and 1 h reaction time (table 1, entry 1), 97% selectivity to
phenylethanol (1a) was observed with 66% of conversion.
Increasing the reaction time further half hour (entry 2, table 1),
increased the conversion to 81% but decreased the selectivity to
1a (90%), due to the formation of 1c (7%) at expenses of 1a.
Increasing the temperature to 100°C at 1 h reaction time (entry 3,
The synthesized RhNPs, were evaluated in the reduction of
acetophenone (1) with dihydrogen and the results are
summarized in Table 1. Among the possible products expected
for the reduction of 1 (scheme in Table 1), the selective
hydrogenation of ketone could lead to phenylethanol (1a), of
aromatic ring to acetylcyclohexane (1b), complete hydrogenation
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
table 1), increased the conversion to 87% with an 89% selectivity
to 1a and 7% to 1c.
reports where the coordination of substrate to the surface of
nanoparticle forming di--bonded species is necessary to carry
out the hydrogenation of substrates.[22,38] Furthermore, evidence
of heterogenous catalytic behavior by RhNPs-I was confirmed by
addition of Hg(0) after a 15 min catalytic run, when the reaction
turnover was inhibited (entries 5-6, table S1).[39] Considering
these results, two possible reaction pathways were observed:
route 1, wherein the ketone is reduced to form phenylethanol (1a);
and route 2, wherein the aromatic ring was reduced to obtain
cyclohexenone (1b) (figure 6). The products cyclohexylethanol
(1c) and ethylbenzene (1d) can be obtained only from 1a.
The RhNPs stabilized with different pyridines were also evaluated
in the hydrogenation of 1. Electron withdrawing substituents on
the stabilizer 4-trifluoromethylpyridine in RhNPs-II reduce the
selectivity to product phenylethanol 1a (78%) in comparison with
RhNPs-I (entry 4 vs entry 3, table 1). The minor binding strength
of
4-trifluoromethylpyridine
in
RhNPs-II,
favors
ring
hydrogenation as observed by Dyson et al. in the hydrogenation
of toluene with different substituted bipiridynes used as stabilizers
for RhNPs.[25] When pyridines with sterically demanding groups
were used as stabilizers in RhNPs-III (4-tertbutylpyridine) and in
RhNPs-IV (4-(3-phenylpropyl)pyridine) (entries 5 and 6, table 1),
similar activities and selectivities compared to RhNPs-I were
observed. The absence of pyridine stabilizers on RhNPs affected
the catalytic performances as shown by RhNPs-V (entries 7 and
8 in table 1) that reached 58% conversion with high selectivity to
1a. Formation of agglomerates was observed at the end of the
catalytic reactions catalyzed with RhNPs-V, accounting for the
low activity attained. The results suggested the beneficial effect of
pyridine as stabilizer of RhNPs to avoid nanoparticles
agglomeration and to obtain high conversion and selectivity.
The concentration of substrate affected the selectivity of products
in the reactions catalyzed by RhNPs-I (entries 9-12 in table 1). At
low concentration of substrate (entry 9, table 1), we observed
higher conversion (93%) and lower selectivity towards
phenylethanol (1a) (62%) with higher concentration of
cyclohexylethanol (1c) (23%). Increasing the amount of
acetophenone (1) to 2 mmol (entry 10, table 1), increased the
selectivity (92%) to 1a with a good conversion (91%). Further
increment of substrate 1 to 5 and 10 mmol (entries 13 and 12,
table 1) increased the selectivity to product 1a to 98% but at
higher concentration of substrate the conversion was diminished
to 55%. The above results indicated that the reduction of carbonyl
is much faster than that of the aromatic ring (product 1a vs 1c)
and formation of 1c is competitive when the substrate 1 is
consumed. The pyridine stabilized RhNPs-I of this work (entries
1-2, table 1), showed a remarkable activity and selectivity
compared to bipyridine stabilized RhNPs[24] (entry 13, table 1) and
thiolate-capped RhNPs[26] (entry 14, table 1). The higher activity
observed by RhNPs-I at similar rhodium concentration and
rhodium:donor atom ratio (nitrogen or sulphur), could be
accounted for the stronger binding strength of bipyridine and
thiolate to rhodium.
Figure 5. Monitoring of acetophenone hydrogenation catalyzed by RhNPs-I.
Reaction conditions: 1 mmol of acetophenone (1) and 1 mL of the catalytic
solution of RhNPs (10-2 mol L-1, 0.01 mmol of total Rh), H2 (5 bar), 100°C.
Conversion and selectivity determined by GC using n-decane as internal
standard.
Figure 6. Hydrogenation pathways of acetophenone (1) catalyzed by RhNPs.
To further study the influence of substrate and product
concentration on the kinetics of the reaction, the hydrogenation
reaction of acetophenone catalyzed by RhNPs-I was monitored
during 2 h (figure 5, entries 1-4, table S1). At high concentrations
of acetophenone (1), the reduction proceeds fast obtaining
phenylethanol (1a) selectively. With a low concentration of 1, the
reaction is slower and the concentration of products,
cyclohexylethanol (1c) and ethyl benzene (1d), increased due to
the reduction of 1a. The latter is a slow process as shown by the
direct reduction of 1a (Supporting Information, figure S12), where
only 31% of conversion was observed with 77% selectivity of 1c
in 1 h. Regarding the path in which product 1c is generated, it is
possible to discard the direct reduction of acetylcyclohexane (1b),
as shown by the low concentration observed of 1b during the
monitoring experiment, and by an independent experiment of the
hydrogenation of 1b (Supporting Information, figure S13), where
no products were detected. This behavior agrees with previous
Recycling studies were carried out employing the catalytic system
RhNPs-I and 2 mmol of acetophenone (1). After separation of
products, the catalytic system was reused for up to ten runs
without important loss of activity and selectivity, demonstrating
that the catalytic system is robust and stable (figure 7). After the
tenth catalytic run, the conversion amounted to 85% with 85%
selectivity to phenylethanol (1a). Loss of activity and selectivity
due to loss of rhodium was excluded by ICP-MS analysis of
organic extracts. TEM analysis of RhNPs-I after the tenth catalytic
run, evidenced agglomeration of nanoparticles (figure S11) that
could account for the reduced activity and selectivity observed in
the recycling experiments.
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
Figure 7. Recycling experiments of RhNPs-I in the hydrogenation of
acetophenone (1). Reaction conditions: 2 mmol of acetophenone (1) and 1 mL
of the catalytic solution of RhNPs-I (10-2 mol L-1, 0.01 mmol of total Rh), H2 (5
bar), 100°C, 1 h. Conversion and selectivity determined by GC using n-decane
as internal standard.
The catalytic system RhNPs-I was applied in the reduction of
substituted aryl ketones (substrates 2-8, figure 8), achieving lower
conversion compared to acetophenone 1, with high selectivity to
the corresponding aryl alcohol. The presence of a substituent in
the aromatic ring limits the planar coordination of the substrate to
the nanoparticle surface, minimizing ring hydrogenation and
decreasing
conversion.
Electron-withdrawing
group
trifluoromethyl on aryl ketones (substrates 2 and 3, figure 8)
promoted the hydrogenation of the aromatic ring (10%) with
conversions of 63% when para-substituted substrates were used,
and 73% with meta-substituted substrates. Electron donor
substituents on substrate 4 (figure 8) showed complete selectivity
to the corresponding arylic alcohol with low conversion (34%) due
to the inductive effect of methoxy groups. Substrates with methyl
substituent in para and ortho positions (5 and 6, figure 8) that
could affect the activity due to a possible steric effect, showed
similar activity and selectivity; however, when the aromatic ring
was para-substituted with a bulky group (7, figure 8), the
conversion decreased to 11%. The substrate 4-iso-
butylacetophenone (8, figure 8), an intermediate in the ibuprofen
synthesis, was hydrogenated with high selectivity and moderate
conversion (62%) to the corresponding alcohol. Varying the steric
effect over the ketone group with substrates propiophenone (9,
figure 8) and benzophenone (10, figure 8), affected moderately
the activity and decreased the selectivity to alcohol. The reduction
of non-conjugated aryl ketones (11 and 12, figure 8) were also
tested, observing an important decrease in conversion and
selectivity. Benzaldehyde (13, figure 8) was also employed as
substrate working under the same reaction conditions, obtaining
only 9% of conversion. An increase in the pressure of H2 to 30 bar
and 6 h reaction time were necessary to obtain 95% of conversion
with 97% of selectivity to benzylic alcohol (Supporting Information,
table S3). Interestingly, when the reaction was carried out adding
0.5 mL of acetone at 5 bar of H2 pressure during 1 h, the
conversion increased to 53%, suggesting that the low conversion
of 13 is limited by low diffusion in the ionic liquid.
Figure 8. Scope of reaction. Reaction conditions: 1 mmol of substrate and 1 mL
of the catalytic solution of RhNPs-I (10-2 mol L-1, 0.01 mmol of total Rh), H2
5
bar), 100°C, 1 h. Conversion and selectivity determined by GC using n-decane
as internal standard.
Transfer hydrogenation of nitrobenzene and imine
derivatives
The catalytic reduction of nitrobenzene to selectively obtain
aniline, an important building block of many compounds, was
tested under the reaction conditions used for the reduction of aryl
ketones with the catalytic system RhNPs-I (figure 9). Working
under these conditions, 62% conversion and 90% selectivity to
aniline (14a) were obtained.
Figure 9. RhNPs-I catalyzed nitrobenzene reduction with dihydrogen. Reaction
conditions: 1 mmol of substrate and 1 mL of the catalytic solution of RhNPs-I
10-2 mol L-1, 0.01 mmol of total Rh). Conversion and selectivity determined by
GC using n-decane as internal standard.
To improve the selectivity and activity in the reduction of
nitroarenes, another source of hydrogen was studied. The use of
formic acid-Et3N azeotrope for transfer hydrogenation reactions
(TH) have been used for the reduction of nitroarenes to anilines
with several supported nanoparticle catalysts (metal or metal
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
oxides), with high activity and selectivity.[35,37,40] The reduction of
nitrobenzene (14) catalyzed by the system RhNPs-I, under TH
conditions was completed in only 5 min, with total selectivity to
aniline (14a) (entry 1, table 2) in simple reflux reaction conditions.
The reaction did not proceed when a less reactive substrate as
nitrodecane (15) was used.
reaction using benzaldehyde 13, was probably due to the
interaction of chloride and methoxy substituents with the
nanoparticle surface, hampering the hydrogenation of
nitrobenzene (14).
Table 3. RhNPs-I catalyzed N-reductive alkylation by transfer hydrogenation
with HCOOH
Table 2. RhNPs-I catalyzed transfer hydrogenation with HCOOH
Entry
Substrate
Product
Conversion[a]
Time (min)
5
1
>99
[Rh]
(mol %)
Conversion
(%)[a]
imine/amine
(%)[a]
Entry
Aldehyde
2
3
0
0
60
60
1
2
3
4
5
6
13
13
18
18
19
19
1
99
99
99
30
99
48
9/91
0.1
1
5/95
10/90
52/48
7/93
0.1
1
4
0
60
5
0.1
41/59
Results from duplicated experiments. Reaction conditions:
1 mmol of
nitrobenzene (14) and 1 mmol of selected benzaldehyde; 1 mL of the catalytic
solution of RhNPs-I (10-2 mol L-1, 0.01 mmol of total Rh), 10 eq. of
HCOOH/NEt3 (5:2), 120°C. 2h. [a] Determined by GC using n-decane as
internal standard.
5[b]
>99
Results from duplicated experiments. Reaction conditions: 1 mmol of substrate
and 1 mL of the catalytic solution of RhNPs-I (10-2 mol L-1, 0.01 mmol of total
Rh), 5 eq. of HCOOH/NEt3 (5:2), 100°C. [a] Determined by GC using n-decane
as internal standard. [b] Conversion determined by 1H NMR.
Pyrrole synthesis through a nitroarene reduction and Paal-
Knorr condensation
Under similar conditions, the reduction of acetophenone (1) or
benzaldehyde (9) (entries 3 and 4, table 2), was not possible, and
when 4-nitroacetophenone (15) was used as substrate (entry 5,
table 2), complete conversion to 4-acetylaniline (15a) was
obtained. The results indicate that transfer hydrogenation with
formic acid-Et3N catalyzed by RhNPs-I is highly active and
Pyrrole ring is a heterocycle present in a large variety of biological
active alkaloids, with a wide range of applications in the synthesis
of
drugs,
pigments,
conducting
materials
and
electroluminescence devices.[41] The Paal-Knorr reaction is a
route to obtain pyrroles and consists in the condensation between
a 1,4-dicarbonyl compound and a primary amine under acidic
conditions. Considering the activity of RhNPs-I in the
nitrobenzene reduction by transfer hydrogenation with formic
acid-Et3N azeotrope (see above), we carried out a tandem
process to prepare N-substituted aryl pyrroles starting from
nitroarene and 2,5-hexanodione with this catalytic system. The
reaction proceeds in two steps: a) nitroarene (14 or 16) reduction
to the corresponding aniline; b) condensation between aniline
(14a or 16a) and 2,5-hexanedione (22) to form the corresponding
pyrrole under acidic conditions provided by the presence of formic
acid. When nitrobenzene (14) was used as substrate, the reaction
proceeded with 92% yield of the corresponding pyrrole (23) (figure
10). When 4-nitroacetophenone (16) was used as substrate, the
yield is reduced to 69%, probably due to low diffusion of
intermediate products. This strategy showed good yields at short
reaction times and is completely selective to the pyrrole since the
presence of pyrrolidine was not observed.
selective to nitrobenzene reduction. The latter suggests
a
different mechanism for transfer hydrogenation reaction for
nitroarenes compared to the one observed in the hydrogenation
of acetophenone (see above).
Considering the selectivity to nitroarenes of RhNPs-I in the
transfer hydrogenation reaction, the one pot synthesis of N-
benzylamine (16a) through the reductive N-alkylation of
nitrobenzene (14) and benzaldehyde (13) was studied under
these reaction conditions. The reaction proceeds through three
steps: a) reduction of 14 to aniline (14a); b) condensation of 14a
with 13 to form N-benzylidenaniline (17); and c) hydrogenation of
17 to N-benzylaniline (17a). To attain high selectivity to amine the
amount of formic acid-Et3N azeotrope was increased to 10
equivalents at 120°C and 2 h reaction time (entry 1, table 3).
Under these reaction conditions RhNPs-I afforded complete
conversion and 91% selectivity to 17a. Furthermore, reducing Rh
concentration to 0.1 mol % (entry 2, table 3) resulted in complete
conversion and 95% selectivity to 17a. To our knowledge, this
nanoparticle-based catalyst is one of the most active systems
reported.
The N-reductive alkylation was also studied with substituted
benzaldehydes p-anisaldehyde (18) and 4-chlorobenzaldehyde
(19). When the reactions were carried out with a Rh concentration
of 1 mol %, complete conversion was observed with high
selectivity to amines 20a and 21a (entries 3 and 5, table 3).
However, when the Rh concentration was decreased to 0.1 mol %,
the conversion was only 30% of substrate 18 and 48% of 19. The
reduction of imine to amine was also lower, with 48% of amine
with 18 and 59% of amine with 19 (entries 4 and 6, table 3). The
decay in activity for aldehydes 18 and 19 in comparison with the
Figure 10. RhNPs-I catalyzed tandem nitroarene reduction/Paal Knorr
condensation. Reaction conditions: 1 mmol of nitroarene, 1 mmol of 2,5-
hexanedione (22), 100 l of the catalytic solution of RhNPs-I (0.001 mmol of
total Rh), 10 eq. of HCOOH/NEt3 (5:2). Yield determined by GC and 1H NMR.
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
Reaction Station RR91023 with 5 mL glass tubes. ICP-MS experiments
were performed on a Bruker Aurora M90 equipment, digesting the sample
with nitric acid. XPS measurements were performed using a PHI 5000
VersaProbe II (from Physical Electronics), with an Al-K X-ray source
(h = 1486.6 eV), a beam size of 100 m, power of 25 W and X-ray voltage
of 25 KV. XPS spectra were obtained at 45° from the normal surface in the
constant pass energy mode (CAE), E0 = 117.3 eV and 23.5 eV for survey
and high-resolution narrow scan with a step size of 1 eV and 0.2 eV
respectively. Peak positions were referenced to the Shirley background Ag
3d5/2 core level at 368.20 eV, Au 4f7/2 at 84.00 eV and C 1s hydrocarbon
groups at 284.00 eV central peaks. Analyses were performed in ultra-high
vacuum (10-8 mbar). XPS spectra were fitted with the program
CasaXPS.[45] The assignment of chemical components of core level N 1s
and Rh 3d were made using the referenced value reported in the
literature.[46,47]
Conclusions
In summary, we found that the use of pyridine derivatives as
stabilizers during the synthesis of rhodium nanoparticles in ionic
liquid [BMIM][BF4] generated well-dispersed systems of RhNPs
with small diameter (2.8-3.4 nm).
The systems catalyze the hydrogenation of acetophenone, where
the pyridine stabilized nanoparticles RhNPs-I showed the best
activity and selectivity to phenylethanol. RhNPs-V without
pyridine stabilizer showed low activity due to the formation of
inactive agglomerates on the reaction conditions used. For
pyridine-stabilized RhNPs, the reduction of aromatic ring is slower
than ketone reduction thus favoring the selectivity to arylic alcohol.
The catalytic system was reused for up to ten runs without loss of
activity and selectivity.
RhNPs-I showed good selectivity for the reduction of substituted
arylketones to the corresponding alcohol, where bulky
substituents in the aromatic ring hampered reduction. In addition,
substrates with non-conjugate ketone groups hinder the formation
of di--bonded species on the nanoparticle surface, diminishing
ketone reduction.
The reduction of nitroarenes to the corresponding aniline by
RhNPs-I with dihydrogen showed moderate activity and
selectivity, whereas the transfer hydrogenation using formic acid-
Et3N azeotrope showed remarkable activity in the reduction of
nitroarenes. Under transfer hydrogenation conditions, the
General synthesis of RhNPs in [BMIM][BF4]. The precursor [Rh(-
OMe)(COD)]2 (0.025 mmol, 0.05 mmol of total Rh), the selected pyridine
(0.05 mmol) and ionic liquid [BMIM][BF4] (5 mL) were placed in a 25 mL
stainless steel reactor vessel. The mixture was magnetically stirred and
heated for 15 min at 70°C, followed by pressurization with H2 (10 bar) and
stirring for 18 h at 80°C. The reactor vessel was cooled to room
temperature and depressurized. The resulting black homogeneous
solution of dispersed nanoparticles was washed with hexanes (3 x 5 mL)
to remove organic byproducts and dried under vacuum (0.05 mmHg) for
18 hours.
General procedure for catalytic hydrogenation with H2. Dispersed
RhNPs in ionic liquid (1 mL, 0.01 mmol of Rh) were placed in a 25 mL
stainless steel reactor vessel together with the set amount of selected
substrate and n-decane as internal standard (97 l, 0.5 mmol). The reactor
was pressurized with H2 at the desired pressure, temperature and time.
After the reaction was completed, the reactor vessel was cooled to room
temperature and depressurized. Products were extracted with a solvent
mixture of hexanes/toluene 10:1 (5 x 3 mL) and analyzed by gas
chromatography by comparison with the commercial product.
Hydrogenation of substituted acetophenones were also monitored by 1H
NMR.
acetophenone was not transformed suggesting
reduction mechanism.
a different
The versatility of the catalytic system allowed the application in
multistep processes, such as the N-reductive alkylation to form
substituted anilines, and the sequential nitroarene reduction
followed by Paal-Knorr condensation to generate substituted
pyrroles. An advantage of these reactions is that no specialized
equipment is needed (autoclaves, reactors), since the multistep
reactions can be performed in a simple reflux system.
Recycling experiments of catalytic hydrogenation. After products
extraction, the catalytic phase (1 mL of RhNPs-I) was dried under vacuum
at 80°C to remove volatiles for 3 h. Additional substrate and standard were
added and the same procedure for hydrogenation reaction was followed.
The presence of Rh in extracted products was discarded by ICP-MS.
Experimental Section
General
General procedure for catalytic transfer hydrogenation and multistep
processes. The set amount of dispersed RhNPs in ionic liquid were
placed in a 5 mL reaction glass tube together with the substrate(s) (1
mmol) and n-decane as internal standard (97 l, 0.5 mmol). The tube was
heated to the set temperature and amount of the azeotrope HCOOH/NEt3
(5:2) was added to start the reaction. After the reaction was completed, the
reaction tube was cooled down with an ice bath. Products were extracted
with a solvent mixture of hexanes/toluene 10:1 (5 x 3 mL) and analyzed by
gas chromatography by comparison with the commercial product. N-
reductive alkylation reactions with substituted aldehydes were monitored
by 1H-NMR.
All manipulations were carried out in
a dry, oxygen-free nitrogen
atmosphere using standard Schlenk techniques. Ionic liquid was dried
under vacuum (0.05 mmHg) at 60°C for 18 h before use. All reagents were
acquired from commercial suppliers; benzaldehyde were purified
according to reported methods prior to use,[42] and the rest of reagents
were used as received. Ionic liquid 1-butyl-3-methylimidazolium
tetrafluoroborate ([BMIM][BF4]) (BASF grade) was purchased from Sigma-
Aldrich. [Rh(-OMe)(COD)]2 was synthesized following reported
methods.[43] NMR were obtained on a Varian VNMRS spectrometer at 400
MHz for 1H. Chemical shifts () are given in ppm relative to residual solvent
peak. Thermogravimetric analysis was performed on a Perkin Elmer
TGA4000 system, employing
a sample of RhNPs as prepared.
Transmission electron microscopy for high resolution images were
acquired on a JEOL ARM200F microscope with HAADF-STEM detector,
and the rest of images were acquired on a JEOL JEM-2010 microscope
with bright field detector. Nanoparticle diameter was determined counting
at least 400 individual nanoparticles with the software package Digimizer
4.6.1.[44] Catalytic conversions were determined on a Varian 3800 gas
chromatograph with a capillary column DB-Wax (30 m x 0.32 mm x 0.25
mm) coupled to a FID detector, using n-decane as internal standard.
Synthesis of nanoparticles and catalytic hydrogenations were carried out
on a Parr Multi Reactor 500 system with 25 mL reactor vessels. Catalytic
transfer hydrogenations were carried out on a Radleys Carousel 12 Plus
Acknowledgements
This work was financially supported by the project DGAPA-
PAPIIT IN226616 and IA205816. A.S.-M. thanks CONACyT for a
PhD grant. Authors thank Dr. Salas-Martin for technical support.
Special thanks for Dr. Zuñiga for helpful discussions.
Keywords: Rhodium nanoparticles • ionic liquids • selective
reduction • transfer hydrogenation • multistep processes
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
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10.1002/ejic.201900223
European Journal of Inorganic Chemistry
FULL PAPER
FULL PAPER
Pyridine-stabilized RhNPs in the
ionic liquid [BMIM][BF4] were
evaluated in the reduction of
arylketones and nitroarenes with
dihydrogen and formic acid-Et3N
azeotrope, respectively. RhNPs
were applied in the synthesis of
amines and pyrroles under transfer
hydrogenation conditions, through
the sequential multistep processes
Alejandro Serrano-Maldonado, Erika
Martin, Itzel Guerrero-Ríos*
Page No. – Page No.
Pyridine-stabilized rhodium
nanoparticles in ionic liquid as
selective catalysts in
hydrogenation and transfer
hydrogenation processes.
N-reductive
nitroarene
alkylation
reduction/Paal-Knorr
and
condensation.
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