Highly efficient aqueous phase reduction of nitroarenes catalyzed by phosphine-decorated polymer immobilized ionic liquid stabilized PdNPs

Article abstract of DOI:10.1039/c7cy02557b

Palladium nanoparticles stabilized by lightly cross-linked phosphine-decorated polymer immobilized ionic liquids (PIIL) and their PEGylated counterparts (PEGPIIL) are highly effective catalysts for the aqueous phase hydrogenation and sodium borohydride-based reduction of a wide range of nitroaromatic and heteroaromatic compounds under mild conditions with low catalyst loadings. Introduction of extensive cross-linking with tris(4-vinylphenyl)phosphine to isolate the phosphine-based heteroatom and limit the number of surface Pd?P interactions did not have a significant influence on catalyst performance. Comparative testing revealed PdNPs immobilized on lightly cross-linked phsophine-decoarted PEGylated polymer to be a highly efficient catalyst for the aqueous phase reduction of nitroarenes with a TON of 36000 (TOF = 2580 h^-1) for hydrogenation and a TON of 274000 (TOF = 17125 h^-1) for transfer hydrogenation. Even though these reactions occur under diffusion control due the poor solubility of the substrate these values are the highest to be reported for the room temperature aqueous phase reduction of nitroarenes catalyzed by a nanoparticle-based system. A continuous flow reduction of nitrobenzene in a packed bed reactor operated over a period of 250 min with no sign of catalyst deactivation and the corresponding space-time-yield of 0.738 g L^-1 min^-1 is a marked improvement on that of 0.384 g L^-1 min^-1 obtained in batch. The same system also catalyzes a tandem Suzuki-Miyaura cross coupling-nitroarene reduction sequence to afford high yields of biaryl amine in an operationally straightforward single-pot procedure. This is a highly versatile protocol which will enable the aromatic nitro fragment to be introduced as a nitro-substituted aryl or heteroaryl halide and as such will lend itself to rapid diversification for the synthesis of a wide range of amines.

Full text of DOI:10.1039/c7cy02557b

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Highly efficient aqueous phase reduction of
nitroarenes catalyzed by phosphine-decorated
Cite this: DOI: 10.1039/c7cy02557b
polymer immobilized ionic liquid stabilized
PdNPs†
a
a J. G. Knight,
*
T. Backhouse,a A. Bradford,a F. Saunders,a
*
*
*
S. Doherty,
R. A. Bourne,
b T. W. Chamberlain,
b R. Stones,b
A. Clayton b and K. Lovelock
c
Palladium nanoparticles stabilized by lightly cross-linked phosphine-decorated polymer immobilized ionic
liquids (PIIL) and their PEGylated counterparts (PEGPIIL) are highly effective catalysts for the aqueous phase
hydrogenation and sodium borohydride-based reduction of a wide range of nitroaromatic and hetero-
aromatic compounds under mild conditions with low catalyst loadings. Introduction of extensive cross-
linking with trisIJ4-vinylphenyl)phosphine to isolate the phosphine-based heteroatom and limit the number
of surface Pd⋯P interactions did not have a significant influence on catalyst performance. Comparative
testing revealed PdNPs immobilized on lightly cross-linked phsophine-decoarted PEGylated polymer to be
a highly efficient catalyst for the aqueous phase reduction of nitroarenes with a TON of 36000 (TOF = 2580
h
−1) for hydrogenation and a TON of 274 000 (TOF = 17125 h−1) for transfer hydrogenation. Even though
these reactions occur under diffusion control due the poor solubility of the substrate these values are the
highest to be reported for the room temperature aqueous phase reduction of nitroarenes catalyzed by a
nanoparticle-based system. A continuous flow reduction of nitrobenzene in a packed bed reactor operated
over a period of 250 min with no sign of catalyst deactivation and the corresponding space-time-yield of
0.738 g L−1 min−1 is a marked improvement on that of 0.384 g L−1 min−1 obtained in batch. The same system
also catalyzes a tandem Suzuki–Miyaura cross coupling-nitroarene reduction sequence to afford high yields
of biaryl amine in an operationally straightforward single-pot procedure. This is a highly versatile protocol
which will enable the aromatic nitro fragment to be introduced as a nitro-substituted aryl or heteroaryl ha-
lide and as such will lend itself to rapid diversification for the synthesis of a wide range of amines.
Received 18th December 2017,
Accepted 7th February 2018
DOI: 10.1039/c7cy02557b
rsc.li/catalysis
chemicals, pigments, dyes, polymers and additives.1 While
there are a number of synthetic pathways available to this
Introduction
Aromatic amines are key motifs in a wide range of bioactive
molecules and intermediates in the production of agro-
class of compound, including N-arylation2 and catalytic reduc-
tion of imines or nitriles,3 the reduction of nitro compounds
is perhaps the most versatile and widely used.4 Despite the
popularity of this approach, there are a number of drawbacks
including the need for either stoichiometric amounts of earth
abundant metal reagent,5 often in combination with acid,
high catalyst loadings6 and/or the use of toxic reducing
agents.7 In addition, harsh reaction conditions, the use of
harmful organic solvent, poor functional group tolerance, par-
tial reduction to hydroxylamines, hydrazones, azoarenes and
azoxyarenes and contamination of the product with metal all
limit the potential applications of this technology. As such
there is considerable interest in developing selective catalysts
that operate under mild conditions at low catalyst loadings in
environmentally green solvents.
a NUCAT, School of Chemistry, Bedson Building, Newcastle University, Newcastle
upon Tyne, NE1 7RU, UK. E-mail: simon.doherty@ncl.ac.uk;
Tel: +44(0)191 208 6537
b Institute of Process Research and Development, School of Chemistry and School
of Chemical and Process Engineering, University of Leeds, LS2 9JT, UK
c School of Chemistry, Food and Pharmacy, University of Reading, Reading, RG6
6AP, UK
† Electronic supplementary information (ESI) available: Synthesis and characteri-
sation of imidazolium-based monomers, co-polymers 3a–8a, PdCl4−loaded co-
polymers 1b–8b and polymer immobilised ionic liquid stabilised PdNPs 3c–8c;
TGA and DSC curves for 3a–c to 8a–c, SEM images for 3a–8a, 3b–8b and 3c–8c,
TEM images of 1c–8c, FTIR traces, X-ray photoelectron spectra for 3b–8b and
3c–8c and FD-XAS spectra for 1b–c, 2b–c, 3b–c and 4b–c. See DOI: 10.1039/
c7cy02557b
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In this regard, metal nanoparticles stabilized by either in-
able physicochemical properties and functionalization, with
the advantages of attachment to a solid support, which will
limit loss of ionic liquid, facilitate separation and recovery
and reduce the volume of ionic liquid as the catalyst would
be retained in a small amount of polymer immobilized ionic
liquid. In addition to improving the long term stability and
recyclability of the nanoparticles, incorporation of a hetero-
atom donor into a polymer immobilized ionic liquid presents
a number of additional benefits including modification of
the electronic structure of the metal surface, control of nano-
particle size and morphology and the capacity to tune the hy-
drophilicity of the ligand environment.28 Gratifyingly, our ini-
tial foray in this area demonstrated that PEG-modified
phosphine-decorated polymer immobilized ionic liquid stabi-
lized palladium nanoparticles (PdNP@PPh2-PEGPIILP) are re-
markably active and selective catalysts for the aqueous phase
hydrogenation of α,β-unsaturated aldehydes, ketones, am-
ides, esters and nitriles giving high conversions and up to
100% selectivity for reduction of the CC double bond.29
Interestingly, both the heteroatom donor phosphine and the
immobilized ionic liquid appear to be essential to achieving
the high activity and selectivity. With the aim of further ex-
ploring the efficacy of PdNP@PIILP-based systems and inves-
tigating whether the same features determine their perfor-
mance as catalysts in other reductions, we extended our
studies to include a comparative study of the aqueous phase
hydrogenation and transfer hydrogenation of nitroarenes.
This study also aimed to investigate the effect on catalyst
properties/performance of introducing three-fold cross-
linking in the form of trisIJp-vinylphenyl)phosphine30 on the
basis that it should isolate the heteroatom donors and
thereby influence the number and type of metal-donor inter-
actions; in this manner we hoped to obtain a more robust
catalyst as well as demonstrate whether Pd⋯PPh2 interac-
tions control NP formation. In this regard, Yuan recently
demonstrated that the size, size distribution and stability of
a range of metal clusters could be controlled by an unusual
synergy between the ‘conventional’ stabilization provided by
heterocyclic cations in polyionic liquids and an in situ gener-
ated polycarbene; a record high for the catalytic performance
in the methanolysis of ammonia borane was reported.31
Thus, it would be eminently reasonable to expect this mecha-
nism of metal cluster stabilization to translate to the
phosphine-decorated polymer immobilized ionic liquids de-
scribed herein. The polymers developed in this project and
the associated nomenclature designed to identify their com-
position are shown in Fig. 1.
organic or carbon-based materials are evolving into an effi-
cient class of catalyst for the reduction of aromatic nitro com-
pounds as they have been shown to have high activity and
selectivity and allow for recycling and scale-up using continu-
ous flow technology.8 Examples of systems with encouraging
reaction credentials for either hydrogenation or transfer hy-
drogenation of nitroarenes include palladium nanoparticles
supported on amino-functionalized mesocellular foam,9 size-
controlled palladium nanoparticles immobilized on carbon
nanospheres,10 PVP-stabilized palladium nanoparticles,11
magnetically separable Fe–Ni bimetallic nanoparticles12 and
gold nanoparticles supported by imidazolium-based porous
organic polymers.13 Other recently developed systems with
promising performance profiles include palladium nano-
particles stabilized by polymethylhydrosiloxane,14 Nb2O5
nanowire,15 amine-functionalized hollow magnetic nano-
particles,16 Fe3O4 magnetic nanocomposites combined with
graphene oxide and carbon nanotubes,17 phosphine-
functionalized ionic liquids,18 multi-walled carbon nano-
tubes,19 nitrogen functionalized active carbon20 and gold
nanoparticles supported by imidazolium based porous or-
ganic polymers or graphene oxide/carbon nanotubes.21 While
some of these systems possess desirable reaction credentials,
very few combine all the criteria that are considered essential
for an environmentally green and sustainable process, i.e.
low catalyst loadings, short reaction times, operationally
straightforward protocols, high selectivity, elimination of or-
ganic solvent, efficient recyclability and mild conditions.
Thus, there is immense potential and opportunity to develop
new catalyst technology to address each of these issues and
identify improved catalysts. One such system based on Fe/
ppm Pd nanoparticles and a PEG-containing designer surfac-
tant appears to meet the majority of these criteria by deliver-
ing safe, sustainable, environmentally responsible and selec-
tive reduction of nitro groups in water at room
temperature.22
Ionic liquids are an intriguing class of solvent that has
also been widely used in catalysis and more recently as both
solvent and stabilizer for transition metal nanoparticles.23
However, while this stabilization is believed to result from
weak electrostatic interactions that are easily displaced to
generate the active site, they are often not sufficient to pre-
vent nanoparticle aggregation under the conditions of cataly-
sis.24 To this end, metal binding heteroatom donors have
been incorporated into ionic liquids on the basis that the
heteroatom would bind to the nanoparticle and supplement
the weak electrostatic interactions and provide long term sta-
bility for use in catalysis.25 Although this strategy has been
successfully applied in a number of cases,26 the large vol-
umes required for catalysis and leaching during work-up are
major issues that will hamper their implementation and
applications.
While PdNPs stabilized by ionic liquids immobilized on
either a porous polymer32 or a support such as graphene
oxide-based nanocomposites,33 magnetic core shell nano-
particles26r or silica34 catalyze the reduction of nitroarenes,
the majority of these systems suffer limitations such as the
need for high catalyst loadings, elevated reaction tempera-
tures or long reaction times, the use of organic solvents or a
large excess of reducing agent. Herein, we report that PdNPs
stabilized by PEG-modified phosphine-decorated polymer-
We have been exploring the concept of polymer
immobilized ionic liquid phase (PIILP) catalysis27 to combine
favourable characteristics of ionic liquids, such as their tun-
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PEGPIILP (2b) and their corresponding PIILP-stabilized
PdNPs, 1c and 2c, respectively.29 Threefold phosphine cross-
linked PIILP 3a was prepared by AIBN initiated radical poly-
merization of the corresponding imidazolium monomer with
PIJ4-vinylphenyl)3 in a 1 : 1 mixture of THF/ethanol at 80 °C
and its PEGylated counterpart 4a was prepared in a similar
manner
from
PIJ4-vinylphenyl)3
and
2-methyl-1-
(2,5,8,11,14,17,20,23-octaoxapentacosan-25-yl)-3-(4-vinylbenzyl)-
1H-3λ4-imidazolium chloride with the aim of increasing hy-
drophilicity and water solubility/compatibility for use in
aqueous phase catalysis. As for 1a and 2a, a 1 : 2 ratio of PIJ4-
vinylphenyl)3 to imidazolium monomer was retained to en-
sure that complete exchange of halide for the tetra-
chloropalladate anion would result in a palladium to phos-
phine ratio of one; this would then enable a direct and
meaningful comparison with our earlier systems. The corre-
sponding PIIL-stabilized PdNPs were prepared by sodium bo-
rohydride reduction of 3b and 4b, respectively, and isolated
as black powders in good yield. Polymers 5a–9a (vide infra)
were designed to examine the influence on catalyst perfor-
mance of the polymer components polyethyleneglycol (PEG),
ionic liquid and phosphine and were prepared from the con-
stituent monomers according to the procedure described for
1a–4a. Full details of the synthesis and compound characteri-
zation data are provided in the ESI.†
The solid state 31P NMR spectra of PdCl4@PPh2-PIILP (1b),
PdCl4@PPh2-PEGPIILP (2b), PdNP@PPh2-PIILP (1c) and
PdNP@PPh2-PEGPIILP (2c) confirm the presence of Pd⋯P in-
teractions, which are clearly apparent from the low-field
chemical shifts of δ 26.8, 27.9, 29.5 and 28.8 ppm, respec-
tively; these are similar to previously reported data.35 In addi-
tion, the absence of any high-field signals in the region of δ
−5 to −7 ppm indicates that there is no uncoordinated PPh2.
In stark contrast, the corresponding spectra for 3b–4b and
3c–4c reveal that a small fraction (<10%) of the phosphine
remains uncoordinated as evidenced by minor signals at δ
−4.1 and −4.8 ppm, respectively (ESI,† Fig. S36, S41, S64 and
S70). Surface characterization of the palladium in 1c–4c was
undertaken with X-ray photoelectron spectroscopy (XPS) by
analysis of the Pd 3d3/2 and Pd 3d5/2 doublets (Fig. 2a–d). The
peaks with binding energies (BE) between 339.1–340.7 eV (Pd
3d3/2) and 333.8–335.5 eV (Pd 3d5/2) are attributed to Pd(0)
species while those with binding energies of 341.1–342.7 eV
(Pd 3d3/2) and 335.9–337.4 eV (Pd 3d5/2) correspond to PdIJII)
species; the latter most likely results from reoxidation of
Pd(0). The X-ray photoelectron spectra of 1b–4b contain char-
acteristic Pd 3d doublets with binding energies of 336.6–
337.5 eV and 341.8–342.8 eV, which is consistent with the
presence of PdIJII) associated with phosphine coordinated
chloropalladate. Together, the solid state NMR data and pal-
ladium binding energies (3d5/2 and 3d3/2) are consistent with
the phosphine providing the primary stabilization of the
nanoparticles; this is in agreement with data reported for
other phosphine-stabilized palladium nanoparticles.26q,36
Fluorescence-detected X-ray absorption spectroscopy (FD
XAS) showed a general increase in the Pd LIII edge energy
Fig. 1 Composition and formulation of polymer immobilized ionic
liquids 1a–4a and their [PdCl4]2− loaded counterparts (1b–4b) and
PdNPs (1c–4c).
immobilized ionic liquids are remarkably efficient and selec-
tive catalysts for the aqueous phase hydrogenation of nitro-
arenes under exceptionally mild conditions as well as NaBH4-
mediated hydrogenations in batch and continuous flow. To
this end, while the stabilization of palladium nanoparticles
for use in aqueous phase catalysis is a burgeoning area of
interest there are surprisingly few reports of the reduction of
nitroarenes in water and, to the best of our knowledge, the
optimum system described herein is one of the most efficient
to be reported for this class of substrate, even though reac-
tions occur under diffusion control. Indeed, the newly devel-
oped catalyst operates with high efficiency at ambient tem-
perature and gives excellent yields in short reaction times
with a low catalyst loading, either at low hydrogenation pres-
sure or with only a slight excess of reducing agent. Moreover,
the same system also catalyzes a tandem aqueous phase
Suzuki–Miyaura cross-coupling-nitroarene reduction se-
quence to afford high yields of biaryl amine in an operation-
ally straightforward single-pot protocol.
Results and discussion
Synthesis and characterization of heteroatom-donor stabi-
lized palladium nanoparticles
Polymers 1a and 2a were prepared as previously described as
were precursors PdCl4@PPh2-PIILP (1b) and PdCl4@PPh2-
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Fig. 2 Pd 3d core level XPS spectrum of 1c–4c (a–d respectively)
referenced to the hydrocarbon C 1s. X-ray source: monochromatic Al
Kα.
relative to the Pd precursor, Na2ijPdCl4], and Pd/C controls for
1b–4b and 1c–4c materials, respectively, indicating a small re-
duction of the electron density on the palladium metal
within the polymer supports (ESI,† Fig. S103–S106). However,
there is a significantly smaller change in white line intensity
for catalyst 2c and 4c upon reduction compared to 1c and 3c,
implying that the addition of the PEG to the polymer support
influences the amount of electron density located on the pal-
ladium metal in the final catalyst materials. This observation
is slightly at odds with the XPS data, which might be related
to surface sensitive nature of XPS, however, this requires fur-
ther investigation. Thermogravimetric analysis of 1a–4a
shows that these polymers are stable up to 225–300 °C (ESI,†
Fig. S6 and S11) and the initial weight loss before the onset
of decomposition is attributed to removal of absorbed water
molecules trapped in the ionic environment. These TGA pro-
files confirm that 1a–4a should be ideally suited for use as
catalyst supports even under harsh reactions conditions.
TEM micrographs revealed that 1c and 2c consist of small,
near monodisperse, nanoparticles with average diameters of
2.29 0.96 and 1.93 0.67 nm, respectively, while the nano-
particles in the highly cross-linked systems 3c and 4c are
larger with average diameters of 3.67 0.67 and 4.01 1.11
nm, respectively; the micrographs and associated distribution
histograms based on >100 particles are shown in Fig. 3.
Interestingly, at this stage it appears that site isolation of the
heteroatom by extensive cross-linking with trisIJ4-
vinylphenyl)phosphine has a marked influence on particle
size and distribution as the palladium to phosphine ratio in
each of these systems is the same by virtue of the 2 to 1 ratio
of phosphine to imidazolium-based monomer. Further stud-
ies are currently underway to vary the palladium to phos-
phine ratio as well as the extent of site isolation in order to
explore and quantify how the Pd⋯P interactions in the
palladate-based precursor influence the size and distribution
Fig. 3 HRTEM images of (a) PdNP@PIILP (1c), (c) PdNP@PPh2-PEGP
IILP (2c), (e) PdNP@PSty3-PIILP (3c) and (g) PdNP@PSty3-PEGPIILP (4c)
and (b, d, f and h) corresponding particle size distributions determined
by counting >100 particles. Mean nanoparticle diameters are 2.29
0.96 nm, 1.93 0.67 nm, 3.67 0.67 nm and 4.02 1.11 nm for 1c–4c
respectively. Black and white scale bars are 5 and 10 nm respectively.
of the nanoparticles and the kinetics of formation. As a com-
parison, palladium nanoparticles with mean diameters be-
tween 2.7 nm and 3.6 nm have recently been prepared by re-
duction of [PdIJacac)IJCOD)]ijBF4] and PdIJacac)2 in the presence
of phosphine-functionalized ionophilic ligands while in the
absence of ligand aggregates formed.26p,q
Hydrogenation of nitroarenes
Preliminary optimizations were conducted in a stirred bench-
top reactor under mild conditions (25 °C) using a 0.47 mol%
loading of 2c to examine the effect of catalyst, solvent and pres-
sure on conversion and selectivity, full details are presented in
Table 1 and Fig. 4. A survey of solvent showed that high conver-
sions were obtained in water and water/ethanol while reactions
conducted in conventional organic solvents gave much lower
conversions (entries 1–6). The practical advantages associated
with aqueous phase catalysis together with the environmentally
benign properties of water and the potential benefits of the ‘hy-
drophobic effect’ prompted us to perform the remainder of our
studies in this solvent.
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Table 1 Hydrogenation of nitrobenzene as a function of catalyst and
sion of 15% and 24% were obtained at 70 psi and 315 psi of
hydrogen, respectively, after only 30 min at room tempera-
ture; this corresponds to TOFs of 580 h−1 and 1021 h−1, re-
spectively, (measured as moles of product per mole of cata-
lyst). However, as high conversions could be obtained at
room temperature under a hydrogen pressure as low as 70
psi, all further reactions were conducted at this pressure by
extending the reaction time where necessary. Under these
conditions, comparative catalyst testing of 1c–4c revealed that
modification by removal of the PEG resulted in a marked re-
duction in performance as 1c only gave 60% conversion in
the same time (entry 8), while the introduction of the phos-
phine as a cross-linker also resulted in a slight reduction in
activity as 3c and 4c gave conversions of 81% and 72%, re-
spectively (entries 9 and 10). For comparison, 0.47 mol% Pd/
C catalysed this hydrogenation under the same conditions
but only reached 61% conversion after 1.5 h (entry 7). The ef-
ficacy of 2c was further tested by reducing the catalyst load-
ing even further to 0.0018 mol% and under otherwise identi-
cal conditions the conversion of 65% after 15 h corresponds
to a total turnover number (TON, measured as total moles of
product per mole of catalyst) of 36 100 and a TOF of 2400
h−1. This is probably more representative of the potential in-
trinsic turnover rate of the catalyst, particularly as a further
decrease in catalyst loading to 0.001 mol% resulted in a mar-
ginal increase in TON to 37 300, in the same time. The effi-
cacy of 2c compared with 1c is most likely associated with
the hydrophilicity of the PEGylated support increasing the
dispersibility of the catalyst and thereby facilitating access of
the substrate to the active site.
A survey of the relevant literature reveals that even though
these reactions occur under diffusion control 2c outperforms
existing systems and, in the majority of cases, by quite some
margin.17,19,20,32,34 For example, the TOF of 1021 h−1 obtained
with 2c in water at room temperature under 315 psi of hydro-
gen is significantly higher than 499 h−1 for an assembly of
PdNPs in SiO2-supported ionic liquid brushes,34 and is a sub-
stantial improvement on that of 30 h−1 for PdNPs supported
on graphene oxide/carbon nanotubes-Fe3O4 at 60 °C,17 50 h−1
for PdNPs immobilized in porous imidazolium-based ionic
liquid polymers at 50 °C,30 30 h−1 for Pd–Pt bimetallic nano-
particles on functionalized multi-wall-carbon nanotubes19
and 130 h−1 for PdNPs supported on nitrogen functionalized
active carbon.20 Good conversions have also been achieved
with a range of other PdNP-based systems but reactions were
typically conducted in organic solvents such as methanol,
ethanol, toluene, ionic liquid or ethyl acetate and often re-
quired elevated temperatures which precludes a viable com-
parison; even so the TOFs obtained with these systems are
typically lower than those for 2c.5b,9a,16,18,37
solventa
Catalyst
Solvent
Time (min)
% Yieldb (d)c
TOF (h−1 d
)
2c
2c
2c
2c
2c
2c
Pd/C
1c
3c
Water
90
90
90
90
90
90
90
90
90
90
100 (−)
99 (1)
83 (5)
18 (2)
5 (2)
<142
140
117
26
7
10
86
35
120
59
Water/EtOH
Ethanol
Ethyl acetate
Toluene
MeCN
7 (2)
Watere
Water
Water
Water
61 (4)
60 (2)
81 (3)
72 (4)
4c
a
Reaction conditions: 1.0 mmol nitrobenzene, mol% Pd in 1c (0.89
mol%), 2c (0.47 mol%), 3c (0.45 mol%), 4c (0.81 mol%), 10 mL
solvent, 25 °C, 70 psi H2 pressure, time. Yields determined by 1H
b
NMR spectroscopy using dioxane as internal standard and gas
chromatography using decane as internal standard. Average of three
c
¯
¯
¯
¯
runs. Average deviation (d) = (|x1 − x| + |x2 − x| + |x3 + x|)/3 where x
d
= (x1 + x2 + x3)/3 and xn is the conversion for run n. Moles product
per mole catalyst per hour based on total palladium content. 1.0
e
mmol nitrobenzene, 0.47 mol% Pd/C, 70 psi H2, 25 °C, 10 mL water.
A study of the variation in conversion/composition as a
function of pressure for the hydrogenation of nitrobenzene
catalysed by 2c in water at room temperature for 30 min
showed that conversions increased with increasing pressure
up to 68% at 315 psi (Fig. 4). While this profile is a clear indi-
cation of mass transfer limited dissolution of hydrogen in
water the reaction will most likely still be under diffusion
control, even under a high pressure of hydrogen, due to the
poor solubility of the substrate. In addition, the short reac-
tion time required for this study revealed the presence of
varying quantities of hydroxylamine as the only observable
intermediate, which suggests that reduction of this species is
slow; however, full conversion to aniline as the sole product
was obtained when the reaction time was extended to 90
min. A reduction in the catalyst loading to 0.047 mol% also
resulted in a significant increase in activity such that conver-
The conditions identified above have also been applied to
the hydrogenation of a range of substituted aromatic and
heteroaromatic nitro compounds to assess the scope and effi-
cacy of 2c (Table 2). Good conversions were obtained for
nitroarenes substituted with electron-donating groups such
as methoxy, hydroxy and methyl (entries 1–3) and electron
Fig.
4
Reaction profile as
a
function of pressure for the
hydrogenation of nitrobenzene. Reaction conditions: 1.0 mmol
nitrobenzene, mol% Pd in 2c (0.47 mol%), 10 mL water, 30 min at 25
°C.
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withdrawing groups such as ester, cyano and ketone, all of
which were reduced to the corresponding aniline at room
temperature in reasonably short reaction times (entries 3–6).
Catalyst 2c is functional group tolerant and highly
chemoselective as substrates bearing reducible substituents
such as cyano, ester and ketone all gave the corresponding
amine as the sole product (entries 4–6). Surprisingly, hydro-
genation
of
4-chloro-1-nitrobenzene
and
2-chloro-1-
nitrobenzene resulted in efficient hydrodechlorination to af-
ford aniline as the sole product in 98% and 99% yield, re-
spectively, after 12 h (entries 7–8). Such an efficient and se-
lective hydrodechlorination is quite remarkable and has not
previously been documented and studies are currently under-
way to elucidate the pathway for this process and to establish
what factors influence selectivity. In this regard, hydro-
dehalogenation is a common competing reaction during the
hydrogenation of halo-substituted nitroarenes catalysed by
palladium nanoparticles but in such cases aniline is generally
a relatively minor by-product.38 Sterically hindered substrates
such as 2-substituted nitrobenzenes also gave good conver-
sions at room temperature and in reasonably short reaction
times (entries 9–11) with the exception of 1-nitronaphthalene
which required significantly longer but eventually reached
73% conversion after 16 h (entry 12). The same protocol was
extended to the reduction of heterocyclic nitroarenes includ-
ing 2- and 3-nitropyridine and 6- and 8-nitroquinoline which
gave the corresponding amines in high yield after short reac-
tion times (entries 13–16). In stark contrast, 4-nitroindole
proved to be a much more challenging substrate and only
reached 27% conversion after 150 min, however, practical
conversions could be obtained by extending the reaction time
accordingly. Finally, 6-nitrobenzothiazole proved to be
completely resistant to hydrogenation by 2c as the substrate
was recovered in quantitative yield even after a reaction time
of 16 h. At this stage, we suggest that benzothiazole poisons
or inhibits the catalyst by coordination of the sulfur donor
atom to the surface of the nanoparticle, on the basis that a
competition reaction containing 6-nitrobenzothiazole and ni-
trobenzene catalysed by 2c only gave 7% conversion to ani-
line after 90 min, compared with the quantitative conversion
obtained in the absence of 6-nitrobenzothiazole. Rather sur-
prisingly, even though a number of the substrates in Table 2
are solids and either poorly soluble or insoluble in water,
good yields of amine were obtained despite the fact that dis-
solution will limit reaction efficiency.
The influence on catalyst performance of the surface ionic
liquid, phosphine and PEG components has been systemati-
cally examined by comparing the performance of
PdNP@PPh2-PEGPIILP (2c) against PdNP@PPh2-PIILP (1c),
PdNP@PPh2-PEGstyrene (5c) and PdNP@PEGPIILP (6c) under
the standard conditions described above (see Table S1 in the
ESI† for structural details). The data in Table 3 shows that se-
lective removal of PEG results in a decrease in TOF from 142
h−1 for 2c to 43 h−1 for 1c, which most probably reflects an
improvement in dispersibility for the PEG-based system in
water, particularly since the mean diameters of the nano-
particles in both systems are similar. Interestingly selective
removal of the PPh2 also resulted in a significant drop in ac-
tivity to 44 h−1 for ionic liquid-PEG based 6c. Since a PEG-
ionic liquid based system is expected to be highly dispersed
and water soluble the drop in activity may well be associated
Table
2 Hydrogenation of aromatic and heteroaromatic nitroarenes
catalysed by PdNP@PPh2-PEGPIILP (2c)a
Substrate
Solvent
Time (min)
90
% Yieldb (d)c
Water/ethanol
98 (2)
Water
Water
Water
150
150
60
98 (1)
93 (3)
99 (1)
Water/ethanol
90
93 (3)
Water
Water
Water
150
720
720
86 (4)
98d (1)
99d (1)
Water
210
86 (4)
Water
Water
150
150
74 (5)
85 (5)
Water
960
73 (4)
Water
Water
Water
150
240
150
98 (1)
84 (5)
89 (4)
Water
Water
150
150
93 (3)
27 (3)
a
Reaction conditions: 1 mmol nitrobenzene, 0.47 mol% 2c, 10 mL
solvent, 25 °C, H2 pressure 70 psi, time. Yields determined by 1H
b
NMR spectroscopy using dioxane as internal standard and gas
chromatography using decane as internal standard. Average of at
c
¯
¯
least three runs. Average deviation (d) = (|x1 − x| + |x2 − x| + |x3
+
¯
¯
x|)/3 where x = (x1 + x2 + x3)/3 and xn is the conversion for run n.
Yield of aniline resulting from complete hydrodechlorination.
d
with the larger size of the NPs in 6c (3.23
0.61 nm)
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Table
3 Hydrogenation of nitrobenzene as a function of catalyst
benzene and re-pressurizing the reactor with hydrogen. The
data in Fig. 5a shows a drop in yield over the first three cycles
after which the conversion profile remains stable. ICP analy-
sis of the aqueous phase collected after run five revealed that
the palladium content had dropped by 39% from 42 ppm
(0.5 mol%) to 26 ppm which might account for the gradual
decrease in conversion over several recycles. These loses may
result from adhesion of insoluble catalyst to the surface of
the reactor and glassware during the separation-recovery pro-
tocol as ICP analysis on the combined organic extracts con-
firmed that leaching was negligible (<0.1 ppm). However, at-
trition was visually most evident during the first three runs
after which the reaction mixture appeared homogeneous and
thus we cannot discount the possibility that soluble catalyst
is responsible for the observed conversions. In this regard,
TEM analysis of the solution after the sixth run revealed an
compositiona
Catalyst
YieldIJ%)/TOF (h−1 b,c
)
Catalyst
% yield/TOF (h−1 b,c
)
1c
2c
3c
4c
38/43
5c
6c
7c
8c
34/69
38/44
21/62
44/23
66/140
55/122
45/56
a
Reaction conditions: 1 mmol nitrobenzene, catalyst 1c–8c, 10 mL
water, 25 °C, 70 psi H2, 60 min. Yields determined by 1H NMR
b
spectroscopy using dioxane as internal standard. Average of at least
three runs. Moles product per mole catalyst per hour based on total
palladium content.
c
compared with 2c (1.93 0.67 nm). Finally, selective removal
of the ionic liquid component also results in a drop in TOF
to 69 h−1 for 5c; this may also reflect the water solubilizing ef-
fect of the ionic liquid component as the mean diameter of
the NPs in 5c (1.83 0.44 nm) are similar to that for 2c. Al-
though it is clear that each component has a direct and dra-
matic effect on catalyst performance further studies will be
required to establish the role of the heteroatom donor i.e.
whether it influences NP formation and size and/or surface
properties or modifies catalyst dispersibility and solubility.
Similarly, the efficiency of catalysts with combinations of two
components removed have also been examined, namely
PPh2-styrene co-polymer, PdNP@PPh2-styrene (7c) and
imidazolium-based polyionic liquid PdNP@PIILP (8c) and
both were shown to be markedly less efficient than 2c under
the same conditions.
increase in the mean particle diameter to 3.05
0.86 nm
compared with that of 1.93 0.67 nm measured for a freshly
prepared sample of 2c (Fig. 5b and c); this corresponds to a
decrease in surface area of ca. 35% which appears to corre-
late with the drop in conversion.
Transfer hydrogenation of nitroarenes
The encouraging performance of 2c as a catalyst for the hy-
drogenation of nitroarenes prompted us to extend our stud-
ies to include a comparison of its efficacy as a catalyst for the
aqueous phase reduction of nitroaromatic compounds using
sodium borohydride as the reducing agent. Relying on recent
literature protocols as a lead, a series of catalytic reactions
were first conducted with nitrobenzene as the benchmark
substrate as this reduction has recently been catalyzed by
PdNP-based systems stabilized by a host of supports includ-
ing PVP,11 graphene oxide/polyIJimidazole/imidazolium) nano-
composites,33 carbon nitride,39 PEGylated imidazolium-based
phosphinite ionic liquid-modified magnetic silica core shell
Reasoning that an aqueous phase compatible catalyst
could be well-suited to facile separation and recovery, a recy-
cle experiment was conducted by extracting the product and
unreacted substrate into ethyl acetate before recharging the
aqueous solution of catalyst with a further portion of nitro-
nanoparticles,26r
carbon
nanospheres,10
poly-
methylhydrosiloxane,14 Nb2O5 nanowires,15 Fe3O4@dextran
particles40 and trisIJtriazolyl)-polyethylene glycol.41 Batch reac-
tions first explored the effect of time, solvent, substrate :
NaBH4 ratio and rate of addition on conversion, full details
of which are presented in Table 4. Complete conversion of ni-
trobenzene and 100% selectivity for aniline was obtained in
water after 2 h at 25 °C using a 0.047 mol% loading of 2c
and a substrate : NaBH4 ratio of 2.5, whereas markedly lower
yields and selectivities were obtained under the same condi-
tions in ethanol, methanol and a 1 : 1 mixture of alcohol and
water (entries 1–5). The low yields and variable selectivities
for aniline in these solvents were associated with incomplete
reduction to intermediates such N-phenylhydroxylamine and
azoxybenzene (vide infra). Conversions increased quite dra-
matically with increasing temperature (entries 6–9) such that
near complete reduction to aniline in >99% selectivity could
be obtained after only 20 min at 80 °C. In comparison, reac-
tions conducted at lower temperatures resulted in incomplete
reduction and were less selective for aniline; under these
Fig. 5 (a) Recycle study for the hydrogenation of nitrobenzene in
water catalysed by PdNP@PPh2-PEGPIILP (2c) and (b) HRTEM image of
catalyst after 6th run and (c) particle size distribution for 2c after five
recycles showing an average NP size of 3.05
white scale bars are 10 nm and 5 nm, respectively.
0.86 nm. Black and
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Table 4 Reduction of nitrobenzene by NaBH4 as a function of catalyst,
68% selectivity with hydroxylamine as the only significant by-
product. A control reaction for the reduction of nitrobenzene
conducted in water in the absence of 2c but with 2.5 equiva-
lents of NaBH4 gave no conversion even after 6 h which con-
firmed the active role of the catalyst. For comparison, the re-
duction of nitrobenzene catalyzed by 0.047 mol% Pd/C under
the same conditions reached 90% conversion but only 63%
selectivity for aniline after 2 h (entry 17). Finally, reduction of
the catalyst loading to 0.00035 mol% gave aniline in 96%
yield and 96% selectivity after 16 h at 50 °C; this corresponds
to a TON of 274 000 and a TOF of 17 125 h−1. Although the
poor solubility of the substrate undoubtedly limits this TOF,
comparison with related NP-based systems that also operate
under mass transfer control10,14,15,17,19,33,40,41,43 reveals it to
be the most active NP-based system to be reported for the
aqueous phase transfer hydrogenation of nitroarenes. While
selectivities at low conversion varied quite dramatically due
to incomplete reduction, aniline was obtained as the sole
product at complete conversion; as such reactions to explore
substrate scope were run to full conversion to ensure high se-
lectivity (vide infra). During the preparation of this manu-
script, Uberman and Martin disclosed an elegant and com-
prehensive study on highly efficient PVP-Pd nanoparticles
generated by electrochemical methods.11 The TOF of 960 h−1
(97% conversion) reported for the reduction of nitrobenzene
required 4.0 equivalents of sodium borohydride and reac-
tions were conducted in water/ethanol whereas the TOF of
1000 h−1 obtained with 2c only required 2.5 equivalents of
NaBH4 and the reaction was conducted in water. A more
meaningful comparison was obtained by performing the re-
duction of nitrobenzene with 2c under the same conditions
described for PVP-PdNP (4.0 equivalents NaBH4, 6,1 ethanol,
water, room temperature, 1 h). Gratifyingly, the TOF of 1810
h−1 at 85% conversion is a marked improvement on the 960
h−1 reported for the PVP-PdNPs; however, both systems most
likely operate under diffusion control and the TOF of 17 125
h−1 obtained at 50 °C (vide supra) is probably more represen-
tative of the potential activity of 2c.
solvent, temperature and hydrogen sourcea
NaBH4
equiv.
Temp Time
Conv
(%)b
Select
(%)c
Catalyst Solvent
(°C)
(min)
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
1c
3c
4c
Pd/C
Water
Ethanol
Methanol
EtOH/water
MeOH/water 2.5
2.5
2.5
2.5
2.5
25
25
25
25
25
20
40
60
80
25
25
25
25
25
25
25
25
25
25
25
25
120
120
120
120
120
20
20
20
20
120
120
120
120
20
20
20
20
100
40
33
77
91
26
51
66
100
11
35
76
81
37
67
78
86
>99
82
92
63
100
14
27
17
27
38
65
73
>99
60
64
64
85
48
62
69
70
>99
82
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
2.5
2.5
2.5
2.5
0.5
1.0
1.5
2.0
2.5
5.0
10.0
25.0
2.5
2.5
2.5
2.5
120
120
120
120
92
63
a
Reaction conditions: 1 mmol nitrobenzene, mol% Pd in 1c (0.89
mol%), 2c (0.47 mol%), 3c (0.45 mol%), 4c (0.81 mol%), 2 mL
b
solvent, 0.5–2.5 mmol NaBH4, time, temperature.
Yields
determined by 1H NMR spectroscopy using dioxane as internal
standard and gas chromatography using decane as internal standard.
Average of at least three runs. Selectivity for aniline [% aniline/(%
c
aniline
+ % N-phenylhydroxylamine + % azoxybenzene + %
azobenzene.
conditions the major product of partial reduction was
N-phenylhydroxylamine. Variation of the NaBH4 : substrate ra-
tio revealed that conversions and selectivities dropped quite
dramatically as the ratio was reduced below 2.5 (entries 10–
12), while an increase in this ratio to 5 or 10 improved both
conversion and selectivity (entries 14–16) and a further in-
crease to 25 resulted in 86% conversion to afford aniline in
Two mechanisms have been proposed for the hydrogena-
tion of nitroarenes,1d,42 the first involves direct
Scheme 1 General mechanisms for the hydrogenation of nitroarenes (a) direct pathway (b) condensation.
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hydrogenation via nitroso and hydroxylamine intermediates I
and II, respectively, while the second is a condensation route
involving azoxy and azo intermediates III and IV, respectively
(Scheme 1). While palladium-based catalysts generally oper-
ate via direct hydrogenation,1d,37,42a Uberman et al. have re-
cently provided convincing evidence that electrochemically
generated PdNPs stabilized by PVP catalyse the hydrogena-
tion of 4-chloronitrobenzene via the condensation pathway;11
moreover, high selectivity for the azoxy intermediate could be
obtained at low palladium loading and low NaBH4 addition.
Interested in exploring the reaction profile for 2c, the reduc-
tion of nitrobenzene was monitored and intermediates iden-
tified and quantified using 1H NMR spectroscopy and GC,
the results of which are presented graphically in
Fig. 6a and b. Under the standard conditions described
above, N-phenylhydroxylamine (II) was identified as the major
intermediate (43%) and only trace quantities of azoxy (III)
(4%) and azo intermediates (IV) (3%) were detected i.e. 86%
selective for hydroxylamine (Fig. 6a). This composition-time
profile is consistent with reaction via the direct pathway in
contrast to the PdNP-PVP system described above in which
azoxy III was identified as the major intermediate.11 While it
is difficult to provide a definitive explanation for this differ-
ence, the surface of the PdNP may well be heavily populated
with PPh2 and such crowding could disfavour condensation
of two molecules of chemisorbed Ar–N–O–H. In comparison,
the reaction was markedly slower in the presence of a re-
duced amount of sodium borohydride (1.0 equivalent) and in
this case a significant amount of azoxybenzene (up to 21%)
was present suggesting that condensation is dominant under
these conditions (Fig. 6b). In this regard, we note that low
concentrations of hydrogen have previously been reported to
favour the condensation pathway.43 Thus, it may well ulti-
mately be possible to identify an optimum catalyst and condi-
tions to achieve high yields and selectivities for commercially
important azoxy-based compounds.
A series of kinetic studies conducted on the hydrogenation
of 4-nitrophenol were used to quantify and compare the per-
formance of catalysts 1c–4c. This transformation is typically
chosen to obtain rate data because the reaction can be moni-
tored by UV-vis spectroscopy as the 4-nitrophenolate anion
has a characteristic absorption band at 400 nm which de-
creases with time and the 4-aminophenolate product has a
distinctive absorption band at 300 nm. The associated plots
of lnIJCt/C0) against time for the reduction of 4-nitrophenolate
catalyzed by 1c–4c appear to be linear which is consistent
with related studies that report pseudo first order kinetics for
this reduction (Fig. 7);44 even though this data appears to be
a reasonable fit deviations from linearity may well be attrib-
uted to mass transfer effects. The associated rate constants
together with the corresponding activity parameters κ, the ra-
tio between the rate constant and the moles of catalyst, are
summarized in Table 5. The activity parameter allows a more
meaningful comparison to be undertaken as it takes the cata-
lyst loading into account and since the reaction volume is
constant it provides a more realistic measure of the perfor-
mance.40 To this end, the data in Table 5 confirms that 2c
has the highest activity parameter, while that for 3c is mar-
ginally lower which is consistent with the conversions
obtained in the optimization studies described above
(Table 4).
The protocol developed above has also been applied to the
aqueous phase reduction of a selection of nitroarenes to com-
pare the performance of pre-reduced PdNP@PPh2-PEGPIILP
(2c) against catalyst generated from its precursor
PdCl4@PPh2-PEGPIILP (1c) as well as to explore the scope of
substrates and evaluate the efficiency against existing
Fig. 6 (a) Reaction profile as a function of time for the reduction of
nitrobenzene in water at 25 °C using 2.5 mole equivalents of NaBH4
and a 0.05 mol% loading of 2c, showing direct hydrogenation via N-
phenylhydroxylamine to be the major pathway, (b) reaction
composition as a function of time for the reduction of nitrobenzene in
water using 1 mole equivalent of NaBH4 and a 0.05 mol% loading of
catalyst.
Fig. 7 Plots of lnIJCt/C0) versus time for the reduction of 4-nitrophenol
(5.0 mmol) using 2.5 equivalents of NaBH4 catalysed by 1c (0.089
mol%), 2c (0.047 mol%), 3c (0.045 mol%), 4c (0.081 mol%).
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Table 5 Summary of kobs and activity parameters (κ) for catalysts 1c–4c
1-aminonaphthalene could be obtained in the same time
when the reaction temperature was raised to 50 °C. Hydroge-
nation of 1-chloro-4-nitrobenzene occurred with high selectiv-
ity to afford 4-chloroaniline in 93% yield together with a mi-
Parameter/catalyst
kobs (min−1
1c
2c
3c
4c
)
0.025
5620
0.027
0.025
0.026
6420
κ (min−1 mol−1
)
11 500
11 110
nor
amount
of
aniline
(6%)
resulting
from
hydrodechlorination. In contrast, hydrodechlorination of
2-chloro-1-nitrobenzene was markedly more facile under the
same conditions such that a 39 : 61 mixture of 2-chloroaniline
and aniline was obtained after 6 h. Unfortunately, 2c was un-
able to reduce heteroaromatic nitro compounds such as 2-
and 3-nitropyridine or 6- and 8-nitroquinoline as quantitative
amounts of starting material were consistently recovered even
after an extended reaction time of 4 h. This is perhaps not
too surprising as there are only a handful of reports of the re-
duction of nitro-substituted heteroaromatics catalysed by
PdNP-based systems; notably these typically use alternative
reducing agents such as ethanol or polymethylhydrosiloxane
and require reaction temperatures as high as 80 °C as well as
high catalyst loadings and/or long reaction times.9b,14 One ex-
ception is a ligand free Fe/ppm Pd nanoparticles system sta-
bilized by the PEG containing designer surfactant TPGS-750-
M developed by Lipshutz which catalyses the reduction of
several nitro containing heterocycles to give good yields of
amine.22 In an attempt to investigate the extent to which
pyridine-based substrates may poison/inhibit the catalyst or
modify the course of the reaction the reduction of nitroben-
zene was monitored as a function of time after pre-treating
the catalyst with one equivalent of pyridine for 10 minutes
immediately prior to addition of the substrate. The resulting
composition-time profile in Fig. 8 reveals a marked difference
compared with that in the absence of pyridine (Fig. 6a) as sig-
nificant quantities of azoxy and azo intermediates were iden-
tified together with N-phenylhydroxylamine i.e. in the pres-
ence of pyridine condensation appears to compete with
direct hydrogenation. However, further studies will be re-
quired to understand why 2c does not catalyse the reduction
of N-heterocycles. To this end, a catalytic reaction mixture
containing 2-nitrotoluene and 5-nitroindole gave 4% conver-
sion to 2-nitrotoluene after 1 h whereas the corresponding
systems; all reactions were run to high conversion in order to
obtain the corresponding amine as the sole product. The
data in Table 6 shows that 2c efficiently reduces nitroarenes
containing electron-donating and electron-withdrawing
groups to afford the corresponding aniline as the sole prod-
uct in high yield at room temperature after relatively short re-
action times; typically 1–6 h (entries 2–6). The high selectivity
for the aniline over other intermediate reduction products
such as azoxyarenes, hydroxylamines, oximes and nitrones
enabled a straightforward isolation and purification protocol
to be developed. Nitroarenes substituted at the 2-position
were also converted to the corresponding aniline in high
yield and 100% selectivity, albeit after longer reaction times
due to steric hindrance inhibiting access of the substrate to
the catalyst surface. Similarly, reduction of 1-nitro-
naphthalene was sluggish under the same conditions with a
yield of 79% after 6 h, however, complete conversion to
Table 6 Reduction of nitroarenes by NaBH4 using PdCl4@PPh2-PEGPIILP
(2b) or PdNP@PPh2-PEGPIILP (2c)a
Substrate
Time (min) % Yieldb (d)c 2b % Yieldb (d)c 2c
120
120
120
120
360
300
99 (1)
99 (1)
100 (−)
87 (5)
93d (3)
86 (3)
99 (1)
99 (1)
90 (3)
86 (4)
74e (3)
97 (1)
120
360
99 (1)
97 (2)
39f (4)
34g (4)
360
79 (4)
94 (2)
a
Reaction conditions: 1 mmol nitrobenzene, 0.05 mol% 2b or 2c, 10
mL water, 2.5 mmol NaBH4, 25 °C, time. Yields determined by 1H
b
NMR spectroscopy using dioxane as internal standard and gas
Fig. 8 Reaction composition as a function of time for the reduction of
nitrobenzene (1.0 mmol) in water at 25 °C using 2.5 mole equivalents
of NaBH4, and 0.05 mol% 2c in the presence of one equivalent (1.0
mmol) of pyridine showing that the condensation pathway competes
with direct reduction via N-phenylhydroxylamine.
chromatography using decane as internal standard. Average of at
c
¯
¯
least three runs. Average deviation (d) = (|x1 − x| + |x2 − x| + |x3
+
¯
¯
x|)/3 where x = (x1 + x2 + x3)/3 and xn is the conversion for run n.
6% aniline formed. 7% aniline formed. 61% aniline formed.
66% aniline formed.
d
g
e
f
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reaction conducted in the absence of 5-nitroindole gave 46%
conversion in the same time. Moreover, addition of
5-nitroindole to a sodium borohydride mediated reduction of
2-nitrotoluene after a reaction time of 1 h resulted in immedi-
ate deactivation as the conversion remained at 46% even af-
ter the reaction time was extended to 2 h.
While the TOF's obtained under transfer hydrogenation
with 2c are markedly higher than those for hydrogenation,
the former protocol is not tolerant towards reducible groups
such as ketones and esters or heteroaromatic compounds
which appear to deactivate the catalyst. However, 2c is an ef-
ficient catalyst for both protocols under mild operating con-
ditions which will provide a platform for further studies to
develop improved systems.
As 2c is generated in an aqueous phase sodium borohydride
reduction of PdCl4@PPh2-PEGPIILP (2b), we became interested
in exploring whether this precursor could be used to generate
catalyst in situ immediately prior to addition of the substrate.
Thus, a series of comparative reactions were conducted using a
0.05 mol% loading of 2b and, gratifyingly, for each substrate
tested yields were comparable to those obtained with 2c, for
the same reaction time (Table 6). This protocol could be poten-
tially advantageous as the precatalyst has a long shelf-life and it
would also eliminate the need to prepare, isolate and store
nanoparticle catalysts. TEM analysis of in situ generated 2c and
4c revealed that the resultant nanoparticles are monodisperse
with average particle diameters of 3.36 0.61 nm and 2.55
0.97 nm, respectively, compared with 1.93 0.96 nm and 4.02
1.11 nm for their pre-reduced counterparts (Fig. 9a–d). Interest-
ingly, this change of ca. 40% for both does not appear to be
reflected in the conversions as 2b and 2c both gave 99% con-
version for the transfer hydrogenation of nitrobenzene while
4b and 4c gave conversion of 93% and 92%, respectively, i.e.
surface modification by the phosphine may well dominate ac-
tivity rather than particle size, although further studies will be
required to substantiate the extent to which Pd⋯P interactions
modify the electronic properties and catalyst efficacy. However,
at this stage we cannot exclude the possibility that the observed
conversions are dispersion controlled, particularly based on a
comparison of the TOFs obtained with 2c and 5c as the NPs in
both systems have similar mean diameters (vide infra).
The efficacy of 2c for the hydrogenation of nitroarenes un-
der mild conditions prompted us to explore the potential for
use in a continuous flow process as this should overcome the
problem of catalyst loss, allow straightforward product sepa-
ration and, moreover, be amenable to scale-up. Three alter-
ations to the reaction conditions were required to success-
fully transfer the process to continuous flow: (i) a 1 : 1 solvent
mixture of ethanol and water was used to solubilise reac-
tants/products; (ii) NaBH4 was dissolved in 1 M NaOH(aq) (pH
14) to reduce the rate of hydrolysis (half-life = 426 days com-
pared to 3.7 seconds at pH 7);45 (iii) the reaction temperature
was increased to 60 °C to compensate for the reduced rate of
reduction observed when using an ethanol–water solvent mix-
ture. The packed-bed reactor consisted of a 2.5 mL alumin-
ium tube packed with catalyst 2c (92 mg, 3.81 wt% Pd) and
sand. A schematic of the set-up is shown in Scheme 2 and
further details are provided in the ESI. In summary, a 99%
yield of the desired aniline product was achieved under these
conditions, with no sign of catalyst deactivation after a period
of 250 min (20 reactor volumes). The space-time yield (STY)
of the process was almost twice that of its batch counterpart
(0.738 cf. 0.384 g L−1 min−1). Following these preliminary re-
sults, work is currently underway to develop an automated
system for the optimisation of reaction conditions for pro-
ductivity and green metrics.
The efficiency of 2c as a hydrogenation catalyst prompted
us to explore its potential to catalyse an aqueous phase tan-
dem Suzuki–Miyaura cross coupling-hydrogenation sequence
as a practical single-pot protocol for the synthesis of biaryl
amines (Scheme 3). In order for this tandem sequence to be
realized the conditions for each individual reaction must be
compatible. Fortunately, to this end, a preliminary control
Suzuki–Miyaura cross-coupling between 3- or 4-bromo-
Fig. 9 HRTEM images of in situ generated (a) PdNP@PPh2-PEGPIILP
(2c) and (b) PdNP@PSty3-PEGPIILP (4c) and (c and d) corresponding
particle size distributions determined by counting >100 particles.
Mean nanoparticle diameters are 3.36 0.61 nm and 2.55 0.97 nm
for 2c and 4c, respectively. Black and white scale bars are 5 and 10
nm, respectively.
Scheme 2 Flow set-up for the transfer hydrogenation of nitroben-
zene using sodium borohydride and
a PdNP@PPh2-PEGPIILP (2c)
packed bed reactor. aAniline yield determined by HPLC analysis using
1,3,5-trimethoxybenzene as internal standard.
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atom donor influences nucleation and NP growth; however,
further studies will be required to firmly establish the role
of the heteroatom donor in nanoparticle formation and
whether it influences/modifies surface electronic structure
and reactivity. To this end, during the review of this manu-
script, Yang reported that platinum nanoparticles stabilised
by triphenylphosphine modified silica are highly
chemoselective catalysts for the hydrogenation of acetophen-
one and phenylacetylene; the efficacy of this system was
proposed to arise from a strong interaction between the
phosphine donor and platinum NPs.47 While batch recycle
studies suffered from catalyst loss during the recovery pro-
cedure, preliminary studies using a packed-bed reactor un-
der continuous flow operation were highly encouraging as
good yields and a stable catalyst profile were achieved over
4 h and, gratifyingly, the corresponding space-time-yield was
twice that obtained in batch. The optimum system has been
used to develop a tandem Suzuki–Miyaura cross coupling
nitro reduction sequence and the corresponding biaryl
amines were obtained in excellent yields from an operation-
ally straightforward single-pot procedure. In summary, PEG-
based phosphine-modified polymer immobilized ionic
liquid-stabilized PdNPs catalyse the reduction of a range of
aromatic and heteroaromatic nitro compounds under ex-
tremely mild operating condition; moreover, catalyst gener-
ated in situ either competes with or outperforms its
preformed counterpart which offers a number of practical
benefits. The concept of heteroatom donor-modified PIIL-
stabilised NPs will provide a strong platform to develop al-
ternative PIILP-based metal nanoparticles and bimetallic
systems to diversify the range of catalytic transformations
and applications. The current emphasis/challenge is to iden-
tify and develop a system that will enable catalyst rates to
be determined.
Scheme 3 Single-pot Suzuki–Miyaura cross coupling hydrogenation
sequence for the synthesis of biarylamines catalysed by 2c.
benzene and phenylboronic acid gave a near quantitative
yield of product after only 3 h under these optimum condi-
tions. After this time, an aqueous solution of NaBH4 (2.5
equiv.) was added and the resulting mixture maintained at
room temperature for a further 8 h. Gratifyingly, the two sub-
strates examined 3- and 4-bromo-1-nitrobenzene gave the 3-
and 4-aminobiphenyl, respectively, in excellent isolated yield.
These are the first examples of single-pot tandem Suzuki–
Miyaura
coupling-hydrogenation
reaction
sequences
conducted at room temperature in aqueous media and future
studies will explore the range of aryl boronic acids as well as
complimentary reaction combinations between nitrobenzene
boronic acids and aryl/heteroaryl bromides. There are two re-
ports of this type of consecutive reaction but in each case the
reaction was conducted in ethanol or an ethanol/water mix-
ture and either a markedly higher catalyst loading and/or ele-
vated reaction temperature was required; however, good con-
versions were obtained in both cases.11,46
Conclusions
This study has demonstrated that PEG-modified phosphine-
decorated polymer immobilised ionic liquid-stabilised palla-
dium nanoparticles are highly efficient and chemoselective
catalysts for the aqueous phase hydrogenation of a range of
aromatic and heteroaromatic nitroarenes at low catalyst
loadings under mild conditions. While the same systems
also catalyse the aqueous phase sodium borohydride-
mediated reduction of nitroaromatics with remarkable effi-
cacy, they do not catalyse the transfer hydrogenation of
heteroaromatic nitroarenes or nitroarenes with reducible
groups such as ketones or esters. Moreover, even though re-
actions have been shown to occur under diffusion control,
PdNP@PPh2-PIILP is the most efficient NP-based system to
be reported for the aqueous phase hydrogenation and trans-
fer hydrogenation of nitroarenes under mild conditions. Dif-
ferences in catalyst efficacy arising from selective modifica-
tion of the support have been attributed to changes in the
balance of hydrophilicity/hydrophobicity which affects cata-
lyst dispersibility and facilitates access to the active site. Al-
though it is not possible to fully deconvolute how each
component influences catalyst performance, for instance,
dispersibility, solubility, NP size and surface modification,
this study has shown that each of the ionic liquid, PEG and
phosphine components is necessary to achieve optimum ef-
ficiency. Interestingly, larger nanoparticles formed when the
heteroatom donor was either isolated in the cross-linker or
completely absent which may well indicate that the hetero-
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge Newcastle University for financial
support (TB). Solid state 31P and 13C NMR spectra were
obtained at the EPSRC UK National Solid State NMR Service
at Durham University, high resolution mass spectra were
obtained at the ESPRC National Mass Spectrometry Service in
Swansea, XPS data was obtained at NEXUS the National
EPSRC XPS Users Service at Newcastle University. FD-XAS was
performed at BM28 at the ESRF (Experiment no. CH-5045)
with help from Dr Laurence Bouchenoire and Dr Paul Thomp-
son. We also thank Dr Maggie White and Dr Rantej Kler for
collecting the powder X-ray diffraction data (Newcastle) and
Dr Kathryn White for the SEM images (Faculty of Medical Sci-
ences, Newcastle University). The authors also sincerely thank
reviewer 2 for providing constructive, insightful, well-directed
and highly informative feedback and comments.
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