Fe2O3/NGr@C- and Co-Co3O4/NGr@C-catalysed hydrogenation of nitroarenes under mild conditions

Article abstract of DOI:10.1039/c5cy01925g

An improved hydrogenation of nitroarenes using nano-structured iron- and cobalt-based catalysts is presented. Modifications of the heterogeneous catalysts by N-doped graphene-flakes are crucial for the success of selective reductions. The use of polar solvents and basic additives has a significant positive influence on the rate of reduction of nitroarenes. This allows performing non-noble metal-catalysed hydrogenations under very mild reaction conditions (e.g. 70 °C and 20 bar). On the basis of the obtained catalytic results a heterolytic mechanism for the hydrogenation process is postulated, too.

Full text of DOI:10.1039/c5cy01925g

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Fe2O3/NGr@C- and Co-Co3O4/NGr@C-catalysed hydrogenation of
nitroarenes under mild conditions
Dario Formenti,a,b Christoph Topf,b Kathrin Junge,b Fabio Ragainia and Matthias Beller*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
AN IMPROVED HYDROGENATION OF NITROARENES USING NANO-STRUCTURED surrounded by a Co3O4 layer, whereas in the Fe-congener
IRON- AND COBALT-BASED CATALYSTS IS PRESENTED. MODIFICATIONS OF THE (Fe2O3/NGr@C) almost only an oxidic phase composed of
HETEROGENEOUS CATALYSTS BY N-DOPED GRAPHENE-FLAKES ARE CRUCIAL FOR Fe2O3 is present. In both cases, the oxidic shell is decorated
THE SUCCESS OF SELECTIVE REDUCTIONS. THE USE OF POLAR SOLVENTS AND BASIC with layers (“flakes”) of nitrogen-doped graphene (NGr)
ADDITIVES HAS A SIGNIFICANT POSITIVE INFLUENCE ON THE RATE OF REDUCTION OF derived from the thermal decomposition of the ligated Phen
NITROARENES. THIS ALLOWS PERFORMING NON-NOBLE METAL-CATALYSED (see ESI). These two catalytic materials were successfully
HYDROGENATIONS UNDER VERY MILD REACTION CONDITIONS (E.G. 70 °C AND 20 applied in many reactions ranging from reductions,[4,6]
BAR). ON THE BASIS OF THE OBTAINED CATALYTIC RESULTS
A
HETEROLYTIC reductive amination[6] and oxidative transformations.[7b,8]
MECHANISM FOR THE HYDROGENATION PROCESS IS POSTULATED, TOO.
Concerning hydrogenations, both catalysts showed excellent
chemoselectivities towards the reduction of
a variety
Catalytic systems based on non-precious, bio-relevant and
abundant metals are the base of a virtuous development of
cost-effective and sustainable chemical processes.[1] In the last
decades many efforts have been made to discover and to
improve efficient and competitive catalysts based on non-
noble metals.[2] In addition to well-defined organometallic
complexes, the combination of cheap metals with suitable
supports allows producing robust heterogeneous catalysts
which are recyclable, thus enhancing the global efficiency of
the chemical transformation.[3] In this context, the
replacement of expensive noble metal-based hydrogenation
systems for more economic versions is still a great challenge.
In the last three years, we reported versatile novel catalytic
applications with different nanoscale heterogeneous carbon
supported catalysts based on Co and Fe.[4] These catalysts are
prepared by pyrolysis of in situ generated complexes of Co and
Fe acetate with 1,10-phenanthroline (Phen) using Vulcan® XC
72R as carbon support.[5] These nanocomposites exhibit a
core-shell architecture which constitutes the structural
prerequisite for catalytic activity. In the case of the Co-based
catalyst (Co-Co3O4/NGr@C), an inner metallic core is
nitroarenes bearing reducible and/or poisoning-capable
functional groups. Nevertheless the original reaction
conditions were quite harsh (>120°C, 50 bar H2).
Here, we demonstrate for the first time that the use of polar
solvents and the addition of bases allow to perform such
hydrogenations under considerably milder reaction conditions.
We commenced our study with testing various solvents in the
hydrogenation of nitrobenzene. As shown in Figure 1, the use
of polar solvent mixtures results in an increasing of the
conversions compared to apolar or low-polar ones. In
particular, alcoholic solvents such as ethanol or methanol give
higher conversions. Furthermore, addition of water leads to
improvements in almost every case (see Figures S5 and S6 for
overall results). Merely, with MeOH as solvent, addition of
water does not affect conversion or selectivity (Figures 1(b)
and S7). Thus, EtOH-H2O and MeOH were chosen as solvents,
which are in accordance with guidelines as green and
sustainable choice for chemical transformations.[9]
As shown in Table S3 and S4, the catalysts prepared without
Phen (CoxOy@C and FexOy@C) do not exhibit any catalytic
activity. Thus, the basicity provided by the NGr is crucial for
the activity. This evidence is in agreement with a heterolytic
cleavage of the dihydrogen molecule in which the N present in
a.
Dipartimento di Chimica – Università degli Studi di Milano, Via C. Golgi
19, 20133 Milano, Italy.
Leibniz Institut fü katalyse e.V. an der universität Rostock, Albert
b.
the graphitic matrix can act as proton acceptor.
As a
Einstein Strasse 29a, 18059 Rostock, Germany.
consequence, an additional base present in the liquid phase
Electronic Supplementary Information (ESI) available: catalysts preparation,
additional catalytic data. See DOI: 10.1039/x0xx00000x
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should enhance the catalytic activity. In order to confirm this which makes the use of chromatographic separation methods
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DOI: 10.1039/C5CY01925G
assumption, we tested the model reaction in the presence of a for its removal from the reaction mixture indispensable.
variety of inorganic and organic bases (1 equiv. with respect to Therefore, aqueous ammonia was chosen instead of DMAP
PhNO2). For both catalysts, although high conversions were since it is less toxic and possesses a negligible environmental
achieved, poor aniline selectivities were observed when impact.[11] Advantageously, ammonia is gaseous and thus
inorganic bases (except NH3) were added to the reaction readily removed from the reaction mixture. Since bases and
mixture. In this case, azobenzene as well as azoxybenzene alcoholic solvents are known to act as transfer hydrogenation
were identified as major side products (see Figure S8 and S9 agents, control experiments were carried out in order to rule
for further details).
out this reaction pathway (Table S3 and S4). As expected, no
substrate conversion was detected in each case. Following our
initial hypothesis, acidic additives were also examined (Figure
S10 and S11).
Figure 1. Influence of the solvent on conversion and selectivity for Co-Co3O4/NGr@C (a)
and Fe2O3/NGr@C (b). Reaction conditions are based on previous optimizations: 0.5
mol % Co (5 mg), 70 °C, 20 bar H2, 13 h or 4.5 mol % Fe (42 mg), 120 °C, 50 bar H2, 4 h.
In fact it is known that alcohols in combination with strong
inorganic bases act as reducing agents towards nitro
compounds, giving rise to undesired coupling products.[10] On
the contrary, when the catalytic transformation was
conducted in the presence of an organic base, higher
conversions were achieved compared to the reaction that was
carried out without additives. Noteworthy, unwanted by-
products did not form and the selectivity towards aniline
remained very good (>95 %), (Figures 2(a) and (b)). Among the
Figure 2. Influence of added organic base on conversion and selectivity for Co-
Co3O4/NGr@C (a) and Fe2O3/NGr@C (b). Reaction conditions are based on previous
optimizations: 0.5 mol % Co (5 mg), 70 °C, 20 bar H2, 13 h or 4.5 mol % Fe (42 mg), 120
°C, 50 bar H2, 4 h.
The results clearly indicate that addition of Lewis acids has a
detrimental effect for the reaction outcome. For both
heterogeneous materials the addition of metal triflates led to a
marked decrease of catalyst performance. This behaviour is
attributed to the interaction of acidic additives with the basic
N-sites in the NGr that inhibit the catalytic activity. Next, we
proceeded to evaluate the optimum amount of base. We
conducted the reactions applying different equivalents of Et3N
tested
bases
triethylamine
(Et3N)
and
4-
dimethylaminopyridine (DMAP) were found to be the best
additives
for
Co-Co3O4/NGr@C
and
Fe2O3/NGr@C,
respectively. However, DMAP is a toxic and corrosive solid,[11]
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and NH3 for Co-Co3O4/NGr@C and Fe2O3/NGr@C, respectively. decorated with halogen atoms are smoothly converted to the
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The results are summarised in Figures S12 and S13. Even a corresponding anilines
small amount of base (0.05 and 0.1 equiv. of Et3N and NH3 for hydrodehalogenation. Furthermore, both catalysts tolerate
Co-Co3O4/NGr@C and Fe2O3/NGr@C, respectively) is able to C=C double bonds (2c), nitriles (2d), carbonyl (2e), esters (2n
( ) without any
2a and D2ObI: 10.1039/C5CY01925G
)
accelerate the reaction significantly. However, best and amides (2i). In no case any further undesired reduction
conversions were achieved with 1 equiv. of base with respect product was detected. In the case of 2n, a reaction carried out
to the substrate. Using amount of bases higher than 1 equiv. with addition of Et3N produced mono- and di-ethyl esters from
(Co-Co3O4/NGr@C) or 1.5 equiv. (Fe2O3/NGr@C), resulted in a the methyl congener by transesterification (see Figure S14 and
slight decrease of both conversion and selectivity. In the S15) after 20 h (conversion >99 %). The ratio between the two
presence of an optimal base amount, catalytic hydrogenations compounds was around 2:1 in favour of the di-ethylated
were performed under milder conditions (Table 1). In case of product. In order to avoid this side-reaction, Co-catalysed
Co-Co3O4/NGr@C a prolonged reaction time allowed for hydrogenation of 2n was run in the absence of any base. As
complete conversion at 70 °C and 20 bar (Table 1, entry 3), expected, no transesterification product was detected and
while with Fe2O3/NGr@C 90 °C and 30 bar hydrogen pressure thus an excellent yield of the desired aniline derivative was
are required (Table 1, entry 7). Nevertheless, to the best of our achieved. Sterically demanding substrates such as 2-
knowledge this latter example represents the first Fe-catalyzed nitrobiphenyl (1j) and 2,4,6-tri-t-butylnitrobenzene (1m) were
hydrogenation of nitroarenes at such low temperature and hydrogenated upon applying a prolonged reaction time
pressure.
without affecting the selectivity and hence the corresponding
Then, the recyclability of the catalysts was explored (Figure 3). yield.
In the case of the Co catalyst, a slight decreasing of the
conversion was detected in the third run while for Fe after an
initial decrease after the first run, the activity stayed almost
constant. ICP analysis of the liquid phase after each run reveals
no metal leaching for both the catalysts (detection limit < 0.5
ppm).
Table 1. Hydrogenation of nitrobenzene to aniline: reaching mild conditions.a)
Entry
Catalyst
T
P
t
Conv.
[%]b)
85
95
>99
79
65
86
>99
Sel.
[%]b)
>99
>99
>99
99
>99
>99
>99
[°C]
70
70
70
120
90
90
90
[bar]
20
20
20
50
30
30
30
[h]
13
18
20
4
20
24
28
1
2
3
4
5
6
7
[Co]
[Co]
[Co]
[Fe]
[Fe]
[Fe]
[Fe]
a) Reaction conditions for [Co]: 0.5 mmol PhNO2, 0.5 mol % [Co] (5 mg), 2 mL EtOH +
100 μL H2O, 1 equiv. of Et3N; reaction conditions for [Fe]: 0.5 mmol PhNO2, 4.5 mol %
[Fe] (42 mg), 3 mL MeOH, 1 equiv. of NH3; b) GC conversions and selectivities using n-
hexadecane as internal standard.
In addition, Maitlis’ hot filtration test ruled out any catalytic
activity provided by soluble Co or Fe metal species (Table S5).
Elemental analyses of both materials after the fifth run are
within the experimental error the same of the pristine ones. In
addition, the found values are very similar to those reported in
the two original papers.[4a,b] These findings, coupled with ICP
analyses, clearly demonstrates that the catalysts maintained
their original composition even after the recycles.
Figure 3. Recycling experiments for Co-Co3O4/NGr@C (a) and Fe2O3/NGr@C (b).
Reaction conditions for [Co] and [Fe] are reported in Table 1, entries
respectively.
3 and 7
To evaluate the scope of the new catalytic protocol, we tested
various substituted aromatic nitro compounds bearing
reducing-labile functional groups (Scheme 1). Substrates
Based on the presented results we conclude that: (1) a
correlation between higher solvent polarity and higher
conversion is present, (2) basic additives increase the catalyst
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Table 2. Hydrogenation of 1-decene.a)
activity (even at low concentration) whereas (3) acid additives
are detrimental for the conversion. On the base of these
outcomes it is clear that the reaction needs a basic and polar
environment. These observations support our working
hypothesis that the dihydrogen molecule is activated by
heterolytic cleavage. This assumption is in agreement with
observations by Sánchez-Delgado and co-workers for Ru and
Rh NPs supported on MgO.[12]
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Entry
Catalyst
Additive
Conv.
[%]a)
1
2
3
4
5
6
7
8
CoxOy@C
Co-Co3O4/NGr@C
CoxOy@C
Co-Co3O4/NGr@C
FexOy@C
Fe2O3/NGr@C
FexOy@C
-
-
9
9
Et3N (1 equiv.)
Et3N (1 equiv.)
-
-
NH3 (1 equiv.)
NH3 (1 equiv.)
11
11
11
16
12
19
Scheme 1. Co- and Fe-catalyzed hydrogenation of nitroarenes: Substrate scope.a)
Fe2O3/NGr@C
a) Reaction conditions: 0.5 mmol 1-decene, 0.5 mol % Co (5 mg) or 4.5 mol % Fe (42
mg). b) GC conversions using n-hexadecane as internal standard.
Moreover, addition of basic promoters does not change the
rate of the reaction (Table 2, entries 3 and 4). Applying Fe-
based catalysts a similar trend is observed. Carbonyl and cyano
groups and quinolines were completely unreactive with these
catalysts (Scheme 1). This is not surprising because these
substrates are generally activated by Lewis acids and the
catalysts herein presented are composed by a carbonaceous
support that completely lacks any acidic sites. In keeping with
this, some of us recently reported that Co-based catalysts
similarly prepared using ceria[15] or α-alumina[16] as supports
instead of carbon were active in the hydrogenation of ketones
to alcohols and quinolines to 1,2,3,4-tetrahydroquinolines,
respectively. In comparison, the materials prepared with
Vulcan® carbon were much less active in the same
transformations. On the contrary, nitroarenes are unlikely to
require a Lewis acid as an activator, since they are among the
weakest nucleophiles known. This results in a very high
selectivity towards the reduction of the nitro functionality with
respect to other reducible groups when the carbonaceous
support is employed.
a) Reaction conditions for [Co]: 0.5 mol % Co (5 mg), 0.5 mmol ArNO2, 70 °C, 20 bar H2,
1 equiv. Et3N, 2 mL EtOH + 100 μL H2O. Reaction conditions for [Fe]: 4.5 mol % Fe (42
mg), 0.5 mmol ArNO2, 90 °C, 30 bar H2, 1 equiv. aqueous NH3, 3 mL MeOH. Yields were
calculated by GC analysis (calibration curve, n-hexadecane as internal standard) using
commercially available anilines. In all cases, complete conversions were observed. b)
Reaction carried out without Et3N. c) 1 mol % Co was used. d) 6 mol % Fe was used.
Conclusions
In conclusion, we presented the hydrogenation of nitroarenes
catalysed by N-doped graphene coated Co- and Fe-
nanocomposites under improved mild conditions. Crucial for
success were the use of polar solvents and/or the addition of
base to the reaction mixture. Under optimal conditions
demanding substrates gave excellent yields and selectivities.
Furthermore, recyclability of the heterogeneous catalysts was
explored and maintained even in the presence of additives. On
the basis of the obtained results, an ionic hydrogenation
mechanism involving heterolytic activation of H2 is likely to be
involved.
In this process the dihydrogen molecule is formally cleaved
into a hydride and a proton.[13] The hydride atom is bound to
the metal-based nanoparticle whereas the proton is attached
to the basic N atoms in close proximity to the nanoparticle or
by the basic additive present in the liquid phase. In order to
further understand this heterolytic activation mechanism, we
decided to hydrogenate non-polar 1-decene. In principle, this
terminal olefin is less prone to heterolytic hydrogenations
compared to nitrobenzene.[14] Accordingly, Co-based catalysts
prepared with and without Phen, do not show any difference
in the hydrogenation of 1-decene (Table 2, entries 1 and 2).
Acknowledgements
Dr. Tobias Stemmler and Dr. Annette-Enrica Surkus are
gratefully acknowledged for helpful discussions. One of us
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aromatic nitro compounds see: (c) R. V. Jagadeesh, G.
DOI: 10.1039/C5CY01925G
Wienhofer, F. A. Westerhaus, A.-E. Surkus, M.-M. Pohl, H.
(D.F.) thanks Milan University and the Erasmus+ Programme
for a fellowship.
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Junge, K. Junge, M. Beller, Chem. Commun., 2011, 47,
10972; (d) R. V. Jagadeesh, K. Natte, H. Junge, M. Beller,
ACS Catal., 2015, 5, 1526-1529.
Notes and references
‡
Standard catalytic reaction: An 8 mL reaction vial was charged
7
8
9
For Fe-catalysed reactions see: (a) T. Stemmler, A.-E.
Surkus, M.-M. Pohl, K. Junge, M. Beller, ChemSusChem,
2014, 7, 3012; (b) R. V. Jagadeesh, H. Junge, M. Beller,
ChemSusChem, 2015, 8, 92. For Co-catalysed reactions
see: (a) T. Stemmler, F. A. Westerhaus, A.-E. Surkus, M.-
M. Pohl, K. Junge, M. Beller, Green Chem., 2014, 16, 4535;
(d) S. Pisiewicz, T. Stemmler, A.-E. Surkus, K. Junge, M.
Beller, ChemCatChem, 2015, 7, 62.
(a) R. V. Jagadeesh, H. Junge, M.-M. Pohl, J. Radnik, A.
Brückner, M. Beller, J. Am. Chem. Soc., 2013, 135, 10776;
(b) D. Banerjee, R. V. Jagadeesh, K. Junge, M.-M. Pohl, J.
Radnik, A. Brückner, M. Beller, Angew. Chem. Int. Ed.,
2014, 53, 4359; (c) R. V. Jagadeesh, H. Junge, M. Beller,
Nat. Commun., 2014, 5, 4123; (d) X. Cui, Y. Li, S.
Bachmann, M. Scalone, A.-E Surkus, K. Junge, C. Topf, M.
Beller, J. Am. Chem. Soc., 2015, 137, 10652.
with the substrate, internal standard (n-hexadecane), catalyst, ad-
ditive (if stated) and solvent. Then, the vial was closed with a
screw cap equipped with septum and needle. The reaction vessels
(up to 7) were placed into a 300 mL Parr autoclave. The latter
was flushed with hydrogen twice at 10 bar and the loaded with
the desired pressure. After that it was placed into a pre-heated
aluminium block. At the end of the reaction, the autoclave was
quickly cooled down using an ice bath and vented. Finally, the
samples were removed, diluted with a suitable solvent, filtered
using a Pasteur pipette filled with celite and analysed by GC. For
catalysts preparation, recycling experiments and Maitlis’ test pro-
cedures see ESI.
1
(a) Catalysis without Precious Metals, Wiley-VCH Verlag,
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2
For very recent reviews and reports see: (a) B. Su, Z.-C.
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11 R. K. Henderson, A. P. Hill, A. M. Redman, H. F. Sneddon,
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13 Ionic hydrogenation mechanisms are well-known in
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14 B. S. Jursic in Theoretical Organic Chemistry, ed. C.
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4
For Co-based catalyst see: (a) F. A. Westerhaus, R. V.
Jagadeesh, G. Wienhöfer, M.-M. Pohl, J. Radnik, A.-E.
Surkus, J. Rabeah, K. Junge, H. Junge, M. Nielsen, A.
Brückner, M. Beller, Nat. Chem., 2013, 5, 537. For Fe-
based catalyst see: (b) R. V. Jagadeesh, A.-E. Surkus, H.
Junge, M.-M. Pohl, J. Radnik, J. Rabeah, H. Huan, V.
Schünemann, A. Brückner, M. Beller, Science, 2013, 342,
1073. For the first report of Fe/Phen system as the
precursor for this kind of catalyst see: (c) M. Lefèvre, E.
Proietti, F. Jaouen, J.-P. Dodelet, Science, 2009, 324, 71.
For laboratory protocols concering the preparation of Fe-
and Co-based catalysts see respectively: (a) R. V.
Jagadeesh, T. Stemmler, A.-E. Surkus, H. Junge, K. Junge,
M. Beller, Nat. Protoc., 2015, 10, 548; (b) R. V. Jagadeesh,
T. Stemmler, A.-E. Surkus, M. Bauer, M.-M. Pohl, J. Radnik,
K. Junge, H. Junge, A. Bruckner, M. Beller, Nat. Protoc.,
2015, 10, 916.
5
6
For Co-catalysed reduction of aromatic nitro compounds
see: (a) F. A. Westerhaus, I. Sorribes, G. Wienhöfer, K.
Junge, M. Beller, Synlett, 2015, 26, 313; (b) R. V.
Jagadeesh, D. Banerjee, P. B. Arockiam, H. Junge, K. Junge,
M.-M. Pohl, J. Radnik, A. Bruckner, M. Beller, Green
Chem., 2015, 17, 898 . For Fe-catalysed readuction of
15 T. Stemmler, F. Chen, S. Pisiewicz, A.-E. Surkus, M.-M.
Pohl, C. Topf, M. Beller, J. Mater. Chem. A, 2015, 3, 17728.
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11718.
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