CHEMCATCHEM
COMMUNICATIONS
DOI: 10.1002/cctc.201300821
Platinum N-Heterocyclic Carbene Nanoparticles as New
and Effective Catalysts for the Selective Hydrogenation of
Nitroaromatics
[
a]
[a]
[b, c]
[b, c]
Patricia Lara,* Andrꢀs Suꢁrez, Vincent Colliꢂre,
Karine Philippot,
and
[d]
Bruno Chaudret
th
Dedicated to Professor Ernesto Carmona on the occasion of his 65 birthday.
Abstract: In this communication, we report the first synthesis
of Pt NPs stabilized with NHC ligands and their investigation as
catalysts in the chemoselective hydrogenation of nitroarenes.
The results in catalysis show that by a proper choice of the
NHC stabilizer and the adjustment of the NHC/metal ratio,
these NHC-capped Pt NPs exhibit high levels of activity and se-
lectivity in the hydrogenation reactions. In particular, Pt NPs
(NHC) ligands to stabilize metallic NPs has been demonstrated
[5]
by us and other groups, although current examples are limit-
[6]
ed to a few metals such as Ru, Pd, and Au. In particular, NHC-
stabilized Ru NPs have been found to efficiently catalyze differ-
[7]
ent hydrogenation reactions.
The reduction of nitroarenes is a transformation of great in-
terest as the resulting anilines are important intermediates in
the industrial production of polymers, dyestuffs, agrochemicals,
0.2
stabilized with 2 equiv. of IPr carbene (PtIPr ) catalyze the che-
moselective reduction of a series of functionalized nitroarenes
[8]
and pharmaceuticals. Conventional methods for the reduc-
tion of nitroarenes usually involve the use of reductants such
as Fe (Bꢀchamp process), Sn and Zn in acid media, and sulfides
under mild conditions (1 bar H , 308C). This catalyst tolerates
2
the presence of a range of functional groups including hydrox-
yl, benzyloxy, carbonyl and olefinic moeities as well as halo-
gens.
[9]
(H S or NaSH). However, as the use of H as a reducing agent
2
2
results in environmentally preferable processes, the develop-
ment of metal catalysts for the hydrogenation of nitroarenes is
of high interest. A significant challenge in the development of
new catalysts resides in the chemoselective reduction of the
nitro group if other reducible functionalities, such as carbonyl,
cyano, and alkenyl or halide groups, are present in the nitroar-
omatic substrates. A number of catalysts, both homogene-
Transition-metal nanoparticles (NPs) are widely used in multi-
ple research areas such as optoelectronics, sensing, medicine,
[
1]
catalysis, etc. In catalysis, interest in them stems from their
particular electronic configuration and much larger surface
area in comparison with the bulk metals, which gives rise to
[10]
[11]
ous and heterogeneous, based on different metals have
shown significant levels of chemoselectivity, although in many
cases this is accompanied by an intrinsically low catalytic activi-
ty. For example, following pioneering work by Corma et al.,
supported Au NPs on SiO , TiO , and Fe O have been revealed
[
2]
numerous active sites. Metal NPs are widely applied in a ple-
thora of catalytic reactions such as CÀC coupling and hydroge-
[
3]
nation. Several types of ligands have been reported to stabi-
[
4]
lize metal NPs. Recently, the ability of N-heterocyclic carbene
2
2
2
3
as very selective hydrogenation catalysts of nitroarenes, how-
ever, to attain high reaction rates these catalysts usually re-
[
a] Dr. P. Lara, Dr. A. Suꢀrez
[12]
quire harsh conditions (T>1008C, P>10 bar H2). In addition,
Instituto de Investigaciones Quꢁmicas (IIQ)
and Departamento de Quꢁmica Inorgꢀnica
Consejo Superior de Investigaciones Cientꢁficas (CSIC)
and Universidad de Sevilla
Avda. Amꢂrico Vespucio, 49, 41092 Sevilla (Spain)
Fax: (+34)954-46-01-65
Beller and co-workers have recently developed first-row-metal
[10c]
catalysts based on homogeneous iron phosphine
and sup-
[13]
ported cobalt oxide complexes. These catalytic systems tol-
erate the presence of different functional groups, which in-
clude olefin and carbonyl moieties and halogens, although
they require the use of relatively high temperatures (110–
E-mail: patricia@iiq.csic.es
[b] V. Colliꢃre, Dr. K. Philippot
Laboratoire de Chimie de Coordination; CNRS; LCC
1
208C) and pressures (P>20 bar). Hence, the development of
2
05 Route de Narbonne, 31077 Toulouse (France)
chemoselective catalysts for nitroarene hydrogenation able to
operate under mild conditions (room temperature, atmospher-
[
c] V. Colliꢃre, Dr. K. Philippot
Universitꢂ de Toulouse
UPS, INPT; LCC
ic pressure or 1 bar H ) is highly desirable. Under these condi-
2
F-31077 Toulouse (France)
tions, the accumulation of reaction intermediates such as hy-
droxylamines, which are prone to exothermic disproportiona-
tion, as well as azo and azoxy derivatives that result from the
condensation of hydroxylamines and nitroso species, is a po-
tential difficulty. Recently, several groups have reported the
[
d] Dr. B. Chaudret
LPCNO; Laboratoire de Physique et Chimie de Nano-Objets du CNRS, 135,
UMR 5215 INSA-CNRS-UPS
Institut National des Sciences Appliquꢂes
1
35 Avenue de Rangueil, 31077 Toulouse (France)
[14a,b]
[14c]
use of Pd
and bimetallic RhNi
NPs as selective catalysts
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300821.
for the reduction of nitro groups in the presence of halogen
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 87 – 90 87