Catalytic Hydrogenation of Arenes in Water Over In Situ Generated Ruthenium Nanoparticles Immobilized on Carbon

Article abstract of DOI:10.1002/cctc.201700056

We describe a tandem process to generate active Ru nanoparticles (≈7 nm) immobilised in situ on carbon from an organometallic precursor and formic acid to afford the hydrogenation of a wide range of arenes and heteroarenes in yields up to 72 % with high conversions and selectivities for the desired products. The hydrogenation of several substrates analogous to lignin-derived fragments to the corresponding alicyclic products was also achieved. Our experimental investigations evidenced that the observed enhanced activity for arene hydrogenation was driven by the unique structural advantages of the organometallic precursor to activate formic acid, in which the presence of a nitrogen ligand is crucial to achieve a high catalytic activity. TEM analysis revealed the formation of Ru⁰ nanoparticles, and Hg⁰ poisoning experiments support the heterogeneous nature of the active catalyst.

Full text of DOI:10.1002/cctc.201700056

Accepted Article
Title: Catalytic hydrogenation of arenes in water over in situ generated
ruthenium nanoparticles immobilized on carbon
Authors: Ambikesh Dhar Dwivedi, Rohit Kumar Rai, Kavita Gupta, and
Sanjay Kumar Singh
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ChemCatChem
10.1002/cctc.201700056
Catalytic hydrogenation of arenes in water over in situ generated
ruthenium nanoparticles immobilized on carbon
[a]
[a]
[a]
Ambikesh Dhar Dwivedi, Rohit Kumar Rai, Kavita Gupta, and Sanjay
[a,b]
Kumar Singh*
[
a]
[b]
Discipline of Chemistry, School of Basic Sciences, Discipline of Metallurgy Engineering
and Materials Science, Indian Institute of Technology Indore, Simrol 453552, Indore, India.
E-mail: sksingh@iiti.ac.in. Tel: 91 0731 2438 774
Abstract
We describe a tandem process to in situ generate active ruthenium nanoparticles (ca 7 nm)
immobilized on carbon, from an organometallic precursor and formic acid, to afford
hydrogenation of a wide range of arenes and heteroarenes in yields up to 72%, with high
conversions and selectivities for the desired products. Hydrogenation of several substrates
analogous to lignin-derived fragments, to corresponding alicyclic products was also achieved.
Our experimental investigations evidenced that the observed enhanced activity for arene
hydrogenation was driven by the unique structural advantage with the organometallic
precursor to activate formic acid, where the presence of a nitrogen ligand is crucial to achieve
higher catalytic activity. TEM analysis revealed the formation of Ru(0) nanoparticles and
Hg(0) poisoning experiments supporting the heterogeneous nature of the active catalyst.
1
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ChemCatChem
10.1002/cctc.201700056
Introduction
Aromatic and heteroaromatic ring hydrogenation in one of the most important industrially
applicable methodology because it provide a convenient way to synthesize a wide range of
[
1-4]
cyclic hydrocarbons.
Till now, fossil raw materials remains the main source of origin for
the aromatic compounds. Therefore, the development of efficient processes for the
conversion of renewable lignocellulosic biomass to value-added fine chemicals attracts
[5]
considerable recent attention. In this regard, selective hydrogenation of liginin-derived
[6-7]
aromatic compounds to corresponding alicyclic products is of particular importance.
Since, functionalization of cyclohexane is challenging due to the low reactivity of
cyclohexane, synthesis of cyclohexane and its derivative by hydrogenation of aromatic
[6,8-16]
precursors attracted extensive scientific and industrial attention.
For instance,
chemoselective hydrogenation of benzoic acid to cyclohexanecarboxylic acid has gained
great attention, as the hydrogenated product cyclohexanecarboxylic acid is an important
organic intermediate in pharmaceuticals for the synthesis of drugs like praziquantel and
[17]
ansatrienin. However, selective hydrogenation of aromatic ring is challenging particularly
in the presence of several reducible functional group, because of it is an energy consuming
reaction due to the resonance energy of aromatic ring and also the aromatic ring
hydrogenation is least preferred in the presence of other reducible functional groups.
Hydrogenation of benzoic acid and its derivative were reported earlier using
[
18]
stoichiometric amount of Na-K alloys.
However, being environmentally friendly and
efficient (recyclable catalyst) catalytic hydrogenation process has gained advantages over
stoichiometric hydrogenation reaction. Therefore, several efficient catalysts based on
monometallic Pd, Pt, Rh, Ru, and several alloys were explored for aromatic ring
[
19-31]
hydrogenation of benzoic acid and its derivatives.
However most of the catalytic
2
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ChemCatChem
10.1002/cctc.201700056
reactions were performed usually under high pressure of H
2
, which may have several safety
concerns. Among these catalysts, ruthenium catalyzed catalysts are attractive, because of its
[6,8,10,12,15,19,24-
low cost and almost comparable activity than those of highly active Pd catalyst.
2
7,31]
Interestingly, most of the active catalysts are those which are supported on carbonaceous
[
6,9,19-21,24,25,27,28,30,31]
materials.
Notably, recent findings inferred that stabilizers, such as
surfactants or ligands, significantly control the catalytic properties of metal
[
15,26,30,31]
nanoparticles.
Recently, Beller et al. reported highly selective hydrogenation of
[
6]
arenes over ruthenium nanoparticle catalysts modified with a carbon-nitrogen matrix. More
recently, Li et al. also reported ruthenium nanoparticles immobilized on mesoporous carbon
as highly active catalyst for the hydrogenation of benzoic acid to cyclohexanecarboxylic
[
27]
acid. Hence, several synthetic procedures for the synthesis of Ru/C nanoparticles have
o
been explored, but in most of the cases high temperature (up to 450 C), high pressure (up to
6
2
0 bar) of H gas, and externally added carbon supports were used to synthesise Ru/C
[
32-36]
nanoparticles.
On the other hand designing effective organometallic catalyst for the hydrogenation
of (hetero)arenes has also received significant attention in recent years. Zeng et al. have
recently reported selective hydrogenation of aromatic ring of ketone and phenols using
[
37]
cyclic(amino)alkyl carbene rhodium complexes using H
2
(5 bar) gas. Notably, ruthenium
arene complexes based on nitrogen donor ligands, such as ethylenediamine derived ligands
[38-42]
has also been identified as a well established class of transfer hydrogenation catalysts,
and therefore we anticipated that these complexes could be a good candidate to achieve arene
ring hydrogenation under a precisely tuned reaction condition. Reaction of the precursor,
ruthenium-arene dimer, with ethylenediamine (en) in a suitable solvent led to the
6
+
[43-44]
straightforward synthesis of cationic [( -arene)Ru(en)Cl] complex.
We have recently
explored such complexes for a tandem catalytic transformation of furan to ketoacid and
3
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ChemCatChem
10.1002/cctc.201700056
[
44]
diketones with an aid of formic acid as H-source. Recently, Fan et al. have also explored
analogues complexes for enantioselective hydrogenation of naphthalene derivatives in the
[45-46]
presence of H
2
gas.
Notably, the above and plenty of several other transfer
hydrogenation/hydrogenation reactions catalyzed by (arene)Ru-en based complex are
[
38-42,44-46]
targeted mostly towards C=O or C=N bonds.
100°C) using high pressure H gas (10  60 bar), hydrogenation of C=C bond (aromatic
and others) were also successfully attempted using ruthenium complexes.
However, at elevated temperature (60

2
[
48-50]
Striking work of Finke et al. established that at higher temperature (110 °C and 60
atm H
2
) catalytic reaction performed using ruthenium metal complexes, instead of ruthenium
complex, traces of Ru(0) nanoparticles, generated in situ, are the active catalytic species for
[50-52]
the hydrogenation of benzene.
These and other analogous classical findings suggested
that under tuned condition deliberate in situ generation of active metal nanoparticles from
[
53]
metal complexes may resulted in high catalytic activity.
facile hydrogenation of a wide range of arenes and heteroarenes by in situ generated active
ruthenium nanoparticles immobilized on carbon and H gas from an organometallic
Therefore, we report herein a
2
6
+
precursor, [( -benzene)Ru(en)Cl] , and formic acid, respectively via a tandem catalytic
process in water. We also investigated the effect of ligands to achieve high catalytic activity
for ruthenium catalyst. Using this methodology, several substrates analogous to lignin-
derived fragments were also successfully hydrogenated to corresponding alicyclic products.
Results and Discussion
At the outset of our experiments, hydrogenation of arenes was performed using benzoic acid
6
as a model substrate (Scheme 1). A teflon coated autoclave containing [( -
6
benzene)Ru(en)Cl]PF (5 mol %) dissolved in 10 mL water was added to benzoic acid and
formic acid. The sealed autoclave was heated at a constant temperature of 150 °C for 16 h.
4
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ChemCatChem
10.1002/cctc.201700056
Results inferred a complete conversion of benzoic acid (1) to cyclohexanecarboxylic acid
1a) with >99% selectivity.
(
Scheme 1. Catalytic hydrogenation of benzoic acid to cyclohexanecarboxylic acid
using several ruthenium precursors in water
1
13
Complete disappearance of aromatic peaks in H and C NMR and the formation of
new peak corresponding to aliphatic C-H, further confirmed the formation of
cyclohexanecarboxylic acid (1a). HRMS analysis is also in good agreement with that of
+
cyclohexanecarboxylic acid (m/z for cyclohexanecarboxylic acid + Na 151.0730 (calcd.) and
1
3 2
51.0734 (found)). Interestingly, when reaction was conducted using RuCl .xH O, instead of
6
[
( -benzene)Ru(en)Cl]PF
6
, reaction could not proceed (Scheme 1, and Table S1, Supporting
Information). Moreover, reaction performed with a phosphine containing (arene)Ru complex,
6
6
[
( -benzene)RuPPh
3
Cl
2
], or in the presence of (arene)Ru dimer [{( -benzene)RuCl}
2
], also
resulted in no conversion of benzoic acid (Scheme 1 and Table S1, Supporting Information).
These results inferred that it is not just the Ru metal but also the overall structural ingredients
6
of the [( -benzene)Ru(en)Cl]PF
6
which play a crucial role in the observed high catalytic
activity towards benzoic acid (1) to cyclohexanecarboxylic acid (1b) conversion. Notably, in
5
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ChemCatChem
10.1002/cctc.201700056
transfer hydrogenation catalysts, ligands having N-H bonds play determining role particularly
[
38-42]
to help in the activation of H-source.
ruthenium based catalysts explored for HCOOH to H
HCOOH, the most active catalysts are one those having nitrogen based ligand.
Presence of nitrogen was found to be advantageous in these complexes, because of its crucial
involvement in providing protic (–NH) hydrogen and subsequently in the facile release of H
gas by stabilising various intermediate by H-bond. For instance in a very recent report, Huang
et al. demonstrated H production from HCOOH catalysed by a Ru complex having diamine
based nitrogen ligands.
It is evident from the literature that among the
2
conversion or hydrogenation using
[
44,54-55]
2
2
[
54]
Moreover, result showed that reaction pathway following the
participation of –NH is more favourable. Moreover, we have also investigated several Ru
complexes with ligands having –NH bonds, and observed that these catalysts demonstrated
[44,55]
high catalytic active for hydrogenation reaction using HCOOH as H-source.
Therefore,
6
+
presence of nitrogen ligands in the studied ruthenium complex [( -C
6
H
6
)Ru(en)Cl] is
crucial to achieve high catalytic activity.
To gather further evidence to support the crucial role of formic acid as H-source in the
hydrogenation of benzoic acid, catalytic reaction was performed by replacing formic acid
with other organic acids such as acetic acid, propionic acid and isobutyric acid (Figure 1a,
and Table S2, Supporting Information). Contrary to the high conversion of benzoic acid to
cyclohexanecarboxylic acid achieved in the presence of formic acid, reaction with other
organic acid resulted in no conversion of benzoic acid. Furthermore, decreasing the formic
acid contents (<18 equiv.), resulted in the decrease in the conversion of benzoic acid.
Notably, in the absence of formic acid reaction could not proceed (Figure 1b, and Table S3,
Supporting Information). These observations attributed to a strong structure-activity
6
+
relationship of the [( -benzene)Ru(en)Cl] having N-H bonds and the crucial role of formic
[
44,55]
6
+
acid as a H-source.
To investigate if [( -benzene)Ru(en)Cl] can also catalyze the
6
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ChemCatChem
10.1002/cctc.201700056
6
decomposition of HCOOH to H
2
, experiment was performed [( -C
H
6 6
)Ru(en)Cl]PF
6
and
HCOOH (S/C = 20) in water at 150 °C (in the absence of the substrate). Results inferred that
6
+
indeed the [( -C
6
H
6
)Ru(en)Cl] is catalytically active for the decomposition of HCOOH to
-1
H
2
, and complete conversion of HCOOH to H
2 2
and CO was achieved in 4 h (TOF = 5.0 h )
(Figure S1, Supporting Information). Moreover, catalytic hydrogenation of benzoic acid at
lower reaction temperature of 120 °C or 100 °C, resulted in the very poor conversion or
ended with no reaction at all. However, at 170 °C, complete conversion of benzoic acid to
cyclohexanecarboxylic acid was achieved in lesser reaction time (10 h) (Figures S2 and
Tables S4-S5, Supporting Information).
(a)
(b)
Figure 1. Ruthenium catalyzed hydrogenation of benzoic acid to
cyclohexanecarboxylic acid with (a) several organic acids (18 equiv.), and (b) varying
amount of formic acid (024 equiv). Reaction conditions: (a) benzoic acid (1.0
mmol), catalyst (5 mol%), carboxylic acid and water (10 mL) at 150 °C for 16 h.
Notably, during the catalytic reaction, initial dark brown colour of the reaction
mixture turned to black turbid suspension. The resulting black particles were separated by
centrifugation and were analysed further (Figure 2, and Figure S3, Supporting Information).
Powder X-ray diffraction pattern (XRD) showed the formation of hexagonal closed pack
(
hcp) Ru nanoparticles with a prominent (2Ɵ) peak at 44° indexed to (101) lattice planes of
[24]
Ru (JCPDS 06-0663). Moreover, presence of a broad peek centred at a 2Ɵ value of 25 °C,
[
24,27]
suggesting formation of a carbon supported Ru nanoparticle (Ru/C).
The TEM image of
7
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Ru/C nanoparticles showed the presence of ca 7 nm sized, nearly spherical nanoparticles
Figure 2a) consistent with XRD, selective area electron diffraction (SAED) pattern also
(
revealed the presence of (101) lattice plane of hcp Ru nanoparticles. Energy-dispersive X-ray
spectroscopy (EDX) (Figure 2) and elemental mapping (Figure S3, Supporting Information)
further support the presence of well dispersed Ru nanoparticles. In agreement with the above
analyses, ICP-IES analysis also confirmed the presence of Ru metal. Moreover, the
1
supernatant obtained after catalytic reaction was also analysed by H NMR and mass spectral
analysis (Figure S4, Supporting Information). Results revealed that the characteristic peaks of
6
1
organometallic-Ru complex [( -benzene)Ru(en)Cl]PF
6
were completely vanished in H
6
+
NMR, and also the molecular ion peak of the [( -benzene)Ru(en)Cl] at m/z 275.2 was
absent in the supernatant. The above observations suggested the complete conversion of
organometallic-Ru complex to Ru/C nanoparticles.
8
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b
c
a
5
1nm
2
0 nm
e
f
Ru
70 nm
d
Ru
4
00 nm
g
Ru
Ru
20
0
5
10
Energy (keV)
15
20
40
2 ( )
60
80
o
Figure 2. Characterization of ruthenium nanoparticle catalyst. (a) TEM with
histogram (inset). (b) SAED. (c-d) Elemental mapping of corresponding TEM image.
(e) SEM-EDX line scan and (f) point analysis. (g) Powder XRD pattern.
To further establish the nature of the active catalyst, hydrogenation experiments were
performed by using pre-synthesised Ru/C nanoparticles, obtained from the organometallic
6
precursor ([( -benzene)Ru(en)Cl]PF
6
), in the presence of external H
2
gas and HCOOH,
respectively (Scheme 2). The pre-synthesised Ru/C nanoparticles exhibited good catalytic
activity toward the hydrogenation reaction in the presence of external hydrogen gas, but
6
could not catalyze the hydrogenation reaction in the presence of HCOOH. However, the [( -
6 2
benzene)Ru(en)Cl]PF was not able to catalyze arene hydrogenation in the presence of H
gas. Worthy to mention, that the organometallic complex catalyzed the facile transformation
[44,54-55]
of HCOOH to H
2
gas by transfer hydrogenation.
. Therefore, our experimental findings
2
clearly demonstrated that the organometallic Ru complex catalysed the generation of H gas
9
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from formic acid, and subsequently transformed into Ru/C, with the aid of H
which further catalyzes the hydrogenation of benzoic acid using H
2
gas produced,
2
(in-situ/externally
added)(Scheme S1 and Figure S1, Supporting Information). These result clearly inferred that
pre-synthesised Ru/C nanoparticle from organometallic-Ru precursor is the active form of
catalyst for arene ring hydrogenation of benzoic acid to cyclohexanecarboxylic acid with the
2
aid of in situ generated H gas.
Scheme 2. Catalytic hydrogenation of benzoic acid to cyclohexanecarboxylic acid
o
conducted in the presence of various ruthenium catalysts at 170 C for 10 h.
Therefore, based on above findings, we anticipated that the reaction proceeds through
6
+
an initial activation of formic acid to H
2
gas by the [( -benzene)Ru(en)Cl] complex, where
the presence of N-H bond is a crucial factor and is in consistence with our earlier findings,
6
+
where [( -benzene)Ru(en)Cl] complex catalyzed the hydrogenation of aldehydes with the
aid of formic acid in water. The Hg(0) poisoning experiments support the homogenous nature
of the catalyst during the activation of formic acid and the formation of Ru-hydride
1
0
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ChemCatChem
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6
+ [44]
complexes, [( -benzene)Ru(H)(en)] . However, performing the catalytic reaction at high
temperature may transfer this highly active in situ generated Ru-hydride complex into a
[
51-52]
carbon supported Ru nanoparticles (Ru/C).
Notably, we observed very low conversions
for the catalytic reactions performed at lower temperature. Also, the colour of the reaction
mixture remains unchanged at lower temperature suggesting that only a small portion of pre-
catalyst has been converted into the Ru(0) metal nanoparticles (Tables S4-S5, Supporting
1
Information). Notably, H NMR and mass spectral analysis of the supernatant obtained after
the reaction at 170 ºC inferred no characteristic peaks corresponding to the organometallic Ru
precursor, suggesting the complete conversion of organometallic Ru precursor to Ru(0)
nanoparticles (Figure S4, Supporting Information). A similar metal-hydride complex were
also reported during the formation of Ru(0) and Rh(0) from the respective organometallic
[
50-52]
complexes by Fink et al.
Formation of Ru/C nanoparticles in our reaction system is
indeed in consistence with the famous Fink scheme “A→B, then A + B→2B” demonstrating
[
52]
the heterogeneous nucleation process. Therefore, the in situ generated Ru/C nanoparticle
catalyst facilitates the hydrogenation of benzoic acid to cyclohexanecarboxylic acid using
6
available in situ generated H
2
gas (from formic acid activated by the [( -
+
benzene)Ru(en)Cl] ). Hg(0) poisoning experiments performed by conducting the catalytic
experiments in the presence of Hg(0), inferred that catalytic conversion is drastically
suppressed (Table S6, Supporting Information). This result is consistence with earlier reports
[
50-53]
which strongly support the heterogeneous nature of the catalyst.
It is worth mentioning
here, that reaction performed by using pre-synthesised Ru/C nanoparticles resulted in no
conversion in the presence of formic acid (Table S1, Supporting Information). These findings
further suggested that the crucial activation of formic acid to generate H
2
gas, is only
6
+
facilitated by the unique structural advantage with [( -benzene)Ru(en)Cl] complex. Thus
6
+
the hydrogenation of arene using the pre-catalyst [( -benzene)Ru(en)Cl] represents a
1
1
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ChemCatChem
10.1002/cctc.201700056
6
+
unique combination of homogeneous (activation of formic acid by [( -benzene)Ru(en)Cl] )
and heterogeneous catalytic (hydrogenation of arenes by in situ generated Ru/C) cycles
(Scheme S1).
Notably, during the transformation of benzoic acid (1) to cyclohexanecarboxylic acid
1a), no intermediates were observed. To gain further insights into the ruthenium catalyzed
(
hydrogenation of arenes and to understand the mode of hydrogenation reaction, catalytic
hydrogenation of arenes having at least two phenyl rings, such as diphenylmethane (2), was
investigated under the optimized reaction conditions. Analyzing the hydrogenation reaction
1
by H NMR revealed a stepwise process for the hydrogenation of phenyl rings of
diphenymethane (2) (Figure 3). In the initial 6 h, partially hydrogenated product
(cyclohexylmethyl)benzene (2b) was observed with 82% selectivity, along with the
completely hydrogenated product, dicyclohexylmethane (2a) (18% selectivity) (Figure 3 and
Table 1, entry 2). Further extending the reaction to 10 h, resulted in the compete conversion
of diphenylmethane (2) selectively to dicyclohexylmethane (2a) in 72% yield (Table 1, entry
2
).
1
Figure 3. H NMR monitoring for the ruthenium catalyzed hydrogenation of
diphenylmethane (2) in water.
1
2
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We further performed the catalytic hydrogenation reaction using a range of several
other arenes and heteroarenes under the optimized reaction conditions (Table 1 summarized
the results). As inferred from the results, a wide range of aromatic compounds, including
those having more than one phenyl rings, biphenyl, phenylpyridine, benzophenone and
naphthalene, can be transformed to corresponding hydrogenated form with good selectivity
(Table 1, entries 1-17). Biphenyl (3) was hydrogenated to 1,1’-bicyclohexane (3a) and
cyclohexylbenzene (3b) with 1:1 selectivity (Table 1, entry 3). Analogous to
diphenylmethane, benzophenone (4) also showed the stepwise hydrogenation of two phenyl
ring, where hydrogenation of ketonic group was also observed (Table 1, entry 4). Notably,
complete hydrogenation of ketonic group of benzophenone (2) resulted in the formation of
(cyclohexylmethyl)benzene (2b) as a major product, as observed with the hydrogenation of
diphenylmethane (2). Aromatic compounds having reducible substituents, such as
acetophenone (5), cinnamaldehyde (6), benzonitrile (7), benzamide (11) and others were also
transformed into corresponding hydrogenated products, with the hydrogenation of arene ring
and the reducible functional groups (Table 1, entries 5-13). Benzonitrile (7) undergoes facile
hydrogenation to form 1-cyclohexylmethylamine (7a) and benzylamine (7b) by the
hydrogenation of benzene ring and nitrile group (Table 1, entry 7). Hydrogenation of aniline
(
8) to cyclohexylamine (8a) was also achieved in 58% yield (Table 1, entry 8). Analogous to
aniline hydrogenation, hydrogenation of nitrobenzene (9) also led to the formation of
cyclohexylamine (8a) in 41% yield (Table 1, entry 9). Hydrogenation of benzylalcohol (12)
to 1-cyclohexylmethanol (12a) was also achieved with high selectivity (Table 1, entry 12).
Hydrogenation of phenol to cyclohexanone is an industrially important challenging task due
[
56]
to its high demand for nylon 6 and nylon 66 production. Under the reaction conditions,
phenol (13) was reduced to form cyclohexanone (13b) by the hydrogenation of benzene ring
[
57,58]
of phenol (Table 1, entry 13).
However, the major fraction of the hydrogenated product
1
3
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10.1002/cctc.201700056
is cyclohexanol (13a) formed by complete hydrogenation of phenol (Table 1, entry 13).
Unlike biphenyl, fused ring arenes, such as naphthalene (14), showed the formation of only
single ring hydrogenated product (14a) albeit with poor conversion (Table 1, entry 14).
Table 1. Ruthenium catalyzed hydrogenation of (hetero)arenes
[
a]
Entry
1
Substrate
Product
Conv./Sel. (%) [yield]
>99/>99 [61% (1a)]
2
>99/>99 [72% (2a)]
[
b]
6
3/18:82 (2a/2b)
3
4
89/51:49 (3a/3b)
40% (3a)]
[
>99/75:11:7:7
2b/2/4a/4b)
(
5
6
>99/>99 [62% (5a)]
>99/75:25 (6a/6b)
7
8
81/22:78 (7a/7b)
>99/>99 [58% (8a)]
1
4
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9
>99/ >99 (8)
99/ >99[41% (8a)]
[
d]
>
1
1
0
1
81/53:47 (1a/1)
78/67:33 (11a/1a)
[
37% (11a)]
1
1
2
3
81/>99
99/85:15(13a/134b)
1
1
1
4
5
6
33/>99
98/33:67 (15a/15b)
[
c]
>
99/>99 [57% (15a)]
>99/>99[57% (16a)]
1
7
>64.5/>99[33% (17a)]
[
d]
>
99/>99[67% (17a)]
[a] Reaction condition: arene (1.0 mmol), ruthenium catalyst (precursor complex) (5
mol%), formic acid (18 mmol), water (10 ml) at 170 °C for 10 h. [b] 6 h, [c] 20 h, [d] 24 h.
Interestingly with phenylpyridine (15), hydrogenation of pyridine ring was favoured
over phenyl ring. After 10 h of the reaction, 2-cyclohexylpiperidine (15a) and 2-
phenylpiperidine (15b) was observed respectively with 33% and 67% selectivities. Complete
1
5
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10.1002/cctc.201700056
conversion of phenylpyridine selectively to 2-cyclohexylpiperidine (15a) in high yield (57%)
was achieved in 20 h (Table 1, entry 15). Analogous to phenylpyridine, facile hydrogenation
[57]
of pyridine to piperidine was also observed in 57% yield (Table 1, entry 16). Notably
under acetic condition, pyridine gets protonated which activates pyridine for
[
58]
hydrogenation. Piperidine is one of the most common scaffold in natural products and
[
58]
several biologically important pharmaceutical and drug molecules.
Unlike pyridine,
hydrogenation of 2-aminopyridine (17) resulted in the formation of 3,4,5,6-tetrahydopyridin-
2
-amine (17a) in lower yield 33% (Table 1, entry 17), because 17a gains resistance toward
[
59]
further hydrogenation under acidic condition.
It is important to mention that,
diphenylmethane (2), biphenyl (3), acetophenone (5), benzylalcohol (12), phenol (13) and
cinnamaldehyde (6), which are analogous to lignin-derived fragments, have also been
[
6,7]
successfully hydrogenated to corresponding alicyclic reaction products (Scheme 3).
Scheme 3. Ruthenium catalyzed hydrogenation of several substrates analogous to
lignin-derived fragments
Further to enhance the viability of this process, gram scale reaction for the ruthenium
catalyzed hydrogenation of benzoic acid (1) to cyclohexanecarboxylic acid (1a) was also
conducted under the optimised reaction conditions at 170 °C for 20 h (Scheme 4). Results
1
6
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ChemCatChem
10.1002/cctc.201700056
inferred a facile conversion of 1.22 g of benzoic acid (1) to cyclohexanecarboxylic acid (1a)
in 60% yield (0.76 g).
Scheme 4. Gram-scale reaction for the ruthenium catalyzed hydrogenation of
benzoic acid
Recyclability of in-situ generated Ru/C catalysts for the transformation of benzoic
acid to cyclohexanecarboxylic acid was also performed under optimised reaction condition.
After each catalytic runs, in situ generated Ru/C nanoparticles were separated from reaction
mixture by centrifugation, and reused for the next catalytic run (conversions for three
st
nd
rd
consecutive catalytic runs were >99% (1 run), >99% (2 run) and 80% (3 run)) by adding
specified amounts of the benzoic acid, water and external H
Supporting Information).
2
gas as H-source (Figure S5,
Conclusion
In conclusion, an efficient catalytic system was developed, where the cooperative
6
+
synergy of the [( -benzene)Ru(en)Cl] complex and formic acid resulted in the in situ
generation of active ruthenium nanoparticles immobilized on carbon (Ru/C) and H gas to
2
achieved facile tandem catalytic hydrogenation of a wide range of arenes and heteroarenes in
water. Catalytic activity of the reported catalytic system was attributed to the unique
6
+
structural-activity relationship of the precursor complex [( -benzene)Ru(en)Cl] , where the
1
7
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ChemCatChem
10.1002/cctc.201700056
presence of N-H bond plays a crucial role in the initial activation of formic acid to generate
gas. Further the reaction temperature enhances the rapid reduction of the precursor
H
2
complex Ru(II) complex to Ru(0). TEM observations of the presence of Ru/C nanoparticles
and the Hg(0) poisoning experiments suggesting the heterogeneous nature of the catalyst.
Moreover, a wide range of aromatic compounds analogous to lignin-derived fragments were
also successfully hydrogenated to corresponding alicyclic products, and thus the present
catalytic system may also finds application in biomass transformation reactions. Further
exploration in this direction is underway.
Acknowledgements
Authors thank IIT Indore, CSIR, New Delhi and SERB-DST, New Delhi for financial
support. SIC, IIT Indore and SAIF, IIT Bombay is acknowledged for instrumental facility.
A.D.D. thanks MHRD and CSIR, New Delhi, K.G. and R.K.R. thank CSIR, New Delhi, for
their fellowships.
Keywords: Hydrogenation, Arenes, Heteroarenes, Ruthenium, Formic acid,
Cooperative catalysis, Tandem reactions
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(
Table-of-Contents)
Catalytic hydrogenation of arenes in water over in situ generated
ruthenium nanoparticles immobilized on carbon
[a]
[a]
[a]
Ambikesh Dhar Dwivedi, Rohit Kumar Rai, Kavita Gupta, and Sanjay
[a,b]
Kumar Singh*
Selective hydrogenation of a wide range of arenes and heteroarenes, including substrates
analogous to lignin-derived fragments, to corresponding alicyclic products in yields up to 72
%
were achieved over an in situ generated active ruthenium nanoparticles immobilized on
carbon, where the unique structural advantages with the organometallic ruthenium precursor
to activate formic acid was found to be crucial for generating active ruthenium nanoparticles
immobilized on carbon and to achieve enhanced activity via a tandem catalytic process.
in situ generated Rutheniumnanoparticlesimmobilized on carbon
2
4
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