Heteroatom Donor-Decorated Polymer-Immobilized Ionic Liquid Stabilized Palladium Nanoparticles: Efficient Catalysts for Room-Temperature Suzuki-Miyaura Cross-Coupling in Aqueous Media

Article abstract of DOI:10.1002/adsc.201800561

Palladium nanoparticles stabilized by heteroatom donor-modified polystyrene-based polymer immobilized ionic liquids (PdNP@HAD-PIILP; HAD-PPh₂, OMe, NH₂, CN, pyrrolidone) are highly efficient catalysts for the Suzuki-Miyaura cross-cou

Full text of DOI:10.1002/adsc.201800561

Title: Heteroatom Donor-Decorated Polymer-Immobilized Ionic Liquid
Stabilized PdNPs: Efficient Catalysts for Room-Temperature
Suzuki-Miyaura Cross-Coupling in Aqueous Media
Authors: Simon Doherty, Julian Knight, Tom Backhouse, Einas Abood,
Hind alshaikh, Ashley Clemmet, Jack Ellison, Richard Bourne,
Thomas Chamberlain, Rebecca Stones, Nicholas Warren, Ian
Fairlamb, and Kevin lovelock
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800561
Link to VoR: http://dx.doi.org/10.1002/adsc.201800561

10.1002/adsc.201800561
Advanced Synthesis & Catalysis
Very Important Publication
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Heteroatom Donor-Decorated Polymer-Immobilized Ionic
Liquid Stabilized Palladium Nanoparticles: Efficient Catalysts
for Room-Temperature Suzuki-Miyaura Cross-Coupling in
Aqueous Media
S. Doherty,a,* J. G. Knight,a,* T. Backhouse,a E. Abood,a H. Al-shaikh,a A.R.
Clemmet,a J. R. Ellison,a R. A. Bourne,b T. W. Chamberlain,b R. Stones,b N. J. Warren,b
I. A. J. Fairlambc and K. R. J. Lovelockd
a
NUCAT, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK. Tel:
+44 (0) 191 208 6537; Fax +44 (0) 191 208 6929; E-mail: simon.doherty@ncl.ac.uk
Institute of Process Research & Development, School of Chemistry and School of Chemical and Process Engineering,
b
University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
School of Chemistry, Food and Pharmacy, University of Reading, Reading, RG6 6AT, UK
c
d
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######
Abstract. Palladium nanoparticles stabilized by heteroatom benchmark as well as commercial samples of Pd/C. Catalyst
donor-modified polystyrene-based polymer immobilized generated in situ from either [PdCl4]@PPh2-PIILP or its
ionic liquids (PdNP@HAD-PIILP; HAD = PPh2, OMe, NH2, PEGylated counterpart [PdCl4]@PPh2-PEGPIILP, by
CN, pyrrolidone) are highly efficient catalysts for the Suzuki- reduction with phenylboronic acid, outperformed their pre-
Miyaura cross-coupling in aqueous media under mild formed counterparts for the vast majority of substrates
conditions. Catalyst modified with phosphine was examined. The turnover frequency of 16,300 h-1 obtained at
consistently the most efficient as it gave high yields across a room temperature is one of the highest to be reported for
range of substrates under mild conditions at low catalyst palladium nanoparticle-catalysed Suzuki-Miyaura cross-
loadings. Incorporation of polyethylene glycol into the coupling between 4-bromoacetophenone and phenylboronic
phosphine modified immobilised ionic liquid support acid in aqueous media under such mild conditions.
improved catalyst efficacy by improving dispersibility and
facilitating access to the active site. Moreover, each of the
heteroatom modified catalysts was more active than the
corresponding unsubstituted imidazolium-based polystyrene
Keywords: Palladium nanoparticles; immobilised ionic
liquids; heteroatom donor; stabilisation; Suzuki-Miyaura
cross coupling; aqueous media
of the product with palladium and difficulty recovering
the catalyst for purification and recycling. To this end,
transition metal nanoparticles are evolving into a
Introduction
Transition metal-catalysed aryl-aryl and aryl- highly versatile class of catalyst for the Suzuki-
heteroaryl bond formation is a powerful tool in Miyaura cross-coupling,[7] in addition to a host of other
synthesis as the resulting biaryls are key motifs in a useful organic transformations.[8] The high activity
host of important bioactive natural products, associated with these nanoparticles has been attributed
pharmaceutical intermediates, fine chemicals and to the number of catalytically active atoms per unit area
functional materials.[1] While high activities have been of the surface i.e. the high surface to volume ratio and
achieved for the coupling of aryl chlorides with aryl quantum confinement effects.[9] However, the high
boronic acids using homogeneous palladium catalysts surface energy of small nanoparticles drives their
based on sterically demanding electron-rich aggregation towards larger species which are less
phosphines
such
as
Buchwald’s
biaryl active and/or selective.[10] One solution to this problem
monophosphines,[2] Beller’s cataCXium,[3] Stradiotto’s has been to impregnate the nanoparticles into supports
Dalphos,[4]
the
Doherty-Knight
KITPHOS such as mesoporous silicas,[11] metal oxides,[12]
monophosphines[5] and Kwong’s indolyl-based zeolites,[13] porous carbon structures,[14] metal organic
monophosphines,[6] these systems suffer numerous frameworks[15] and polymers[16] to provide steric
drawbacks including the use of expensive oxygen stabilization. More recently, ionic liquids have been
sensitive phosphines, organic solvents, contamination shown to act as both solvent and stabilizer for a host of
1
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
transition metal nanoparticles and the ability to modify liquids that could result from modifying the surface
and tailor their physicochemical properties and electronic structure, the hydrophilicity and/or steric
functionality offers immense potential for developing properties of the ligand as well as the ionic
new catalyst technology.[17] Unfortunately, the use of microenvironment. For example, RuNP stabilised by
ionic liquids also suffers several practical limitations amino-modified alumina (RuNP-NH2--Al2O3) are
including high cost compared with water or a markedly more efficient catalysts for the
traditional solvent, high viscosity and leaching of the hydrogenation of levulinic acid than Ru/-Al2O3; this
ionic liquid during work-up or continuous flow was attributed to the highly dispersed electron rich
operation.[18] In addition, while the primary ruthenium centres[33] and ultra-small PtNPs stabilised
stabilisation of nanoparticles by ionic liquids is by triphenylphosphine-modified silica are significantly
believed to result from weak electrostatic interactions more chemoselective for the catalytic hydrogenation of
that are easily displaced to allow access to the active acetophenone and styrene than its unmodified
site,[19] they do not always provide sufficient counterpart, the NPs of which are much larger.[34] In
stabilization against aggregation under working other recent examples, phosphorus donors have been
conditions. This problem has been addressed by shown to control the size, shape and electronic
incorporating a metal-binding heteroatom donor such properties of rhodium, ruthenium and gold
as an amine,[20] phosphine,[21] nitrile,[22] thiol,[23] nanoparticles and thereby their performance as
hydroxyl[24] and bipyridine[25] into the ionic liquid on selective catalysts for the hydrogenation of arenes and
the basis that the additional covalent interactions -unsaturated
aldehydes,[35]
and
surface
would improve the long term stability of the modification by N-heterocyclic carbenes has been
nanoparticles as well as control the kinetics of shown to; (i) influence the performance and stability of
formation.[26] Indeed, this strategy has proven ruthenium and palladium nanoparticles as catalysts for
successful and there have been reports of marked oxidations and reductions,[36] (ii) activate unreactive
improvements in catalyst performance including; (i) Pd/Al2O3 for the Buchwald-Hartwig amination[37] and
enhancements in activity and selectivity for (iii) tune reactivity and selectivity of Ru/K-Al2O3 for
hydrogenations catalysed by PdNPs stabilised with chemoselective hydrogenation.[38]
either ionophilic phosphine-based ligands[27a,b] or 2,2ʹ-
Interested in exploring the influence of the number
dipyridylamine-functionalised ionic liquids,[27c] (ii) and type of heteroatom donors on the formation and
improved recyclability for NiNPs stabilised by an efficacy of polymer immobilised ionic liquid-stabilised
amino-modified imidazolium-based ionic liquid[28a] palladium nanoparticles, we have undertaken a
and (iii) switchable chemoselectivity for the comparison of the efficiency of palladium
hydrogenation of aryl ketones and aldehydes catalysed nanoparticles stabilised by various heteroatom donor-
by RuNPs stabilised with phosphine-functionalised modified polymer immobilised ionic liquids as catalyst
ionic liquids.[29] However, the large volumes often for the Suzuki-Miyaura cross-coupling. While initially
required, their high cost and the difficulty associated cautious that the use of the large excess of capping
with recovering and purifying the ionic liquid after agent required to maximize surface coverage might
catalysis are major issues that severely limit their restrict or block access to the active site,[39] a handful
implementation.
of recent studies provide convincing evidence that such
a strategy could actually improve catalyst
Polymer Immobilized Ionic Liquids (PIILs)[30] are
an emerging class of functional material that combine performance.[40] For example, Yuan and co-workers
the favourable properties of a polymer support with the demonstrated that the combination of a polymer
well-documented advantages of an ionic liquid immobilized ionic liquid and a polytriazolium-derived
environment such as catalyst stabilization, facile poly(N-heterocyclic carbene) act synergistically to
catalyst activation, ease of recycling and enhancements stabilize a range of metal nanoparticles with precisely
in rate and selectivity.[31] Moreover, covalent controlled sizes of 1 nm and a record high catalytic
attachment of an ionic liquid to a polymer has several performance for rhodium-catalysed methanolysis of
additional benefits as it prevents leaching of the ionic ammonia borane was reported,[40a] while Huang
liquid, improves durability, facilitates separation and obtained a marked enhancement in selectivity for the
recovery of the catalyst and reduces the volume of liquid phase aerobic oxidation of benzyl alcohol
ionic liquid required to a single or double layer rather catalysed by silica supported Au nanoparticles capped
than bulk solvent; this could ultimately result in a with chemisorbed polyvinylpyrrolidone.[40b] Herein,
significant cost saving. We have recently initiated a we describe the results of this comparison which has
program to develop heteroatom donor-decorated shown that (i) PdNPs stabilised by heteroatom donor-
polymer immobilised ionic liquids with the intention modified polymer immobilised ionic liquids
of combining the stabilization provided by a outperform their unmodified counterpart, (ii) catalyst
heteroatom donor and an ionic liquid with a polymer stabilised by phosphine-based PIIL is more efficient
support to explore whether the nature of the heteroatom than those stabilized by other heteroatom donors such
donor and its loading influences nanoparticle as nitrile, methoxy, amide, and hydroxyl, (iii) catalyst
formation, specifically their size, morphology and stabilised by PEGylated phosphine-decorated polymer
efficacy as catalysts.[32] Moreover, there may well be immobilised ionic liquid consistently outperformed its
additional benefits associated with incorporating non-PEGylated counterpart and, moreover, is among
heteroatom donors into polymer immobilised ionic the most active PdNP-based system to be reported for
2
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
Suzuki-Miyaura cross-coupling in aqueous media at
Polymers were typically characterised by solution
room temperature and (iv) catalysts generated in situ NMR spectroscopy, TGA, SEM and IR spectroscopy
by reduction of tetrachloropalladate-loaded polymer while palladium-loaded systems were characterised
immobilised ionic liquid (PdCl4@HAD-PIIL) are using a combination of solid state NMR spectroscopy,
typically more efficient than their preformed IR, XPS, TEM, SEM and ICP-OES; full details are
counterparts (PdNP@HAD-PIIL) across the majority provided in the ESI. The thermal stability of polymers
of substrates examined.
1a-f was investigated by thermogravimetric analysis
and differential scanning calorimetry. The TGA
profiles of 1a-f showed an initial weight loss just below
100 °C due to removal of physisorbed water and
ethanol; this was followed by two or three main
degradation pathways between 240-300 °C confirming
that these polymers are entirely suitable for use as
supports to stabilise metal nanoparticles for use in
catalysis. The palladium loadings of 2a-f and 3a-f were
determined to be 0.15-2.1 mmol g-1 and 0.14-3.0 mmol
g-1, respectively, using ICP-OES. The 13C CP/MAS
NMR spectra of 1a-f and 2a-f contain characteristic
signals between  120-145 ppm attributed to the
imidazolium ring and the aromatic carbon atoms as
well as an additional signal at higher field which
corresponds to the methyl group on the imidazolium
ring. Additional polymer specific signals associated
with the heteroatom donor were also evident and
appear at  75 and 59 ppm (CH2OMe),  24 ppm
(CH2CN),  176 ppm (C=O, pyrrolidone) and  49 ppm
(CH2NH2). Surface characterization of the
tetrachloropalladate-loaded precursors 2a-f was
undertaken by X-ray photoelectron spectroscopy
(XPS) and a single Pd 3d3/2 and Pd 3d5/2 doublet with
binding energies of 337.0-337.8 eV and 342.2-343.2
eV is attributed to the Pd2+ ion. The X-ray
photoelectron spectra of 3a-f typically contained two
pairs of 3d3/2 and Pd 3d5/2 doublets; peaks with binding
energies of 335.0-335.5 eV and 340.1-340.7 eV
correspond to Pd(0) while those with binding energies
of 336.1-338.1 eV and 341.2-343.3 eV correspond to
surface Pd2+ ions (see ESI). Scanning electron
microscopy revealed a stark difference in surface
morphology of the polymer after exchange of the
chloride with palladate (see ESI) as the surface of 1a-f
has a rough granular texture compared with the
smoother flat surfaces of 2a-f. Powder X-ray
diffraction data was also collected on 3a-f to
examine/confirm the crystal phase and to compare the
mean size of PdNPs determined using the
Debye-Scherrer formula with the sizes obtained from
analysis of the TEM micrographs. The XRD patterns
for 3a, 3c, 3d and 3f each contained diffraction peaks
at 2 = 40.1, 46.3, 68.5, 82.1 and 86.0 which index to
the (111), (200), (220), (311) and (222) lattice planes
of the face centered cubic phase of palladium (JCPDS
No. 46-1043), consistent with reported literature.[41] In
comparison, no diffraction peaks for metallic
palladium were detected for 3a-PEG, 3b or 3e which
suggests that the palladium particles are highly
dispersed with sizes < 2.5 nm.42 TEM micrographs of
catalysts 3a-f revealed that the palladium nanoparticles
were near monodisperse with heteroatom-dependent
average diameters ranging from 1.7 ± 0.78 to 3.20 ±
0.62 nm, details of which are presented in Table S2;
representative micrographs and associated distribution
Figure 1. Composition of polymer immobilised ionic
liquids 1a-f and 1a-PEG used in this study.
Results and Discussion
Heteroatom donor-decorated polymers 1a-f were
prepared by AIBN-initiated radical polymerisation of
imidazolium-modified styrene monomer, heteroatom
donor modified styrene and dicationic cross-linker in a
1.86:1:0.14 ratio (Scheme 1). The ratio of heteroatom
donor-based monomer to imidazolium co-
monomer/cross-linker was chosen such that complete
exchange of bromide for tetrachloropalladate would
correspond to a heteroatom donor to palladium ratio of
one. Polymers 1a-f were impregnated with
tetrachloropalladate by ion exchange in water to afford
PdCl4@HAD-PIILP (2a-f) as brown-red solids in near
quantitative yield. The corresponding PIIL-stabilised
nanoparticles PdNP@HAD-PIILP (3a-f) were
prepared by sodium borohydride reduction of 2a-f in
ethanol and isolated as black powders in high yield
(Scheme 1).
Scheme 1. Synthesis of polymers 1a-f, [PdCl4]2-
impregnated polymers 2a-f and PIILP-stabilized palladium
nanoparticles 3a-f.
3
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
histograms for 3a and 3a-PEG based on > 100 cross-coupling between 4-bromoacetophenone and
particles are shown in Figure 2 and those for 3b-f are phenylboronic acid as this is the accepted benchmark
presented in the ESI (see ESI).
transformation for catalyst evaluation, full details of
which are presented in Table 1. Reactions were
initially conducted using 0.05 mol% catalyst in
ethanol/water at room temperature and under these
conditions high conversions (78-98%) were obtained
with 3a-e after 30 min; for comparison a conversion of
only 58% was obtained with the same loading of 3f
which does not contain a stabilising heteroatom donor.
As the highest conversion was obtained with catalyst
3a, the efficacy of its PEGylated 3a-PEG counterpart
was also examined with the aim of introducing
additional weak stabilising interactions and enhancing
water compatibility and solubility for use in aqueous
phase catalysis.[44] To this end, there are numerous
reports of efficient catalysis of the Suzuki-Miyaura
cross-coupling
with
palladium
nanoparticles
Figure 2. HRTEM images of (a) 3a and (c) 3a-PEG, and (b
and d) corresponding particle size distributions determined
by counting >100 particles. Mean nanoparticle diameters
are 2.29 ± 0.96 nm and 1.93 ± 0.67 nm for 3a and 3a-PEG,
respectively. Black and white scale bars are 25 and 10 nm,
respectively.
immobilised on PEG-modified stabilisers such as
Fe3O4@poly(ethylene
glycol),[45]
tris(triazolyl)-
polyethylene glycol,[46] Fe3O4@poly(ethylene glycol)-
bridged amine-functionalised imidazolium ionic
liquids,[47] PEG-substituted phosphine oxides,[48] and
PEG-tagged
azamacrocycles,[49]
PEGylated
imidazolium-based phosphinite ionic liquid-modified
-Fe2O3@SiO2,[50] as well as with size-controlled
palladium NPs generated in the presence of varying
concentrations of unmodified PEG in water.[51]
Gratifyingly, 3a and its PEGylated counterpart, 3a-
PEG, appear to outperform each of these systems and
in most cases under much milder conditions. As
complete conversion was also obtained with 3a-PEG
both systems were taken forward for further
optimisation and evaluation.
Interestingly, the disparate sizes of the nanoparticles
in 3a-f may be due to the influence of the heteroatom
donor on nucleation and growth of the palladium
nanoparticles as the heteroatom to palladium ratio in
each of the catalysts is the same by virtue of the 1 to 2
ratio of monomers. To this end, there have been a
number of reports providing definitive evidence that
the nature of heteroatom can influence nanoparticle
size. For example, Rossi obtained smaller palladium
nanoparticles in the presence of a phosphine-
functionalized support (2.9 ± 0.6 nm) compared with
the corresponding amine-modified system (3.8 ± 1.2
nm).[43a,b] More recently, Yang prepared platinum NPs
stabilized on triphenylphosphine-modified silica that
were smaller than those on unmodified silica and also
markedly more chemoselective for the hydrogenation
of acetophenone and phenylacetylene; spectroscopic
studies also confirmed the presence of a strong Pt-P
interaction between the PPh3 and PtNPs.[34] The size of
palladium nanoparticles stabilized on linker-modified
Al-MIL-53 have also been shown to be dependent on
the nature of the heteroatom modifier[43c] while the size
of ruthenium NPs stabilized by secondary phosphine
oxides are substituent dependent.[43d] Stable small and
homogenously dispersed ruthenium NPs have been
prepared in imidazolium-based ILs in the presence of
primary alkyl amines; the mean size of 1.2 nm was
independent of the alkyl chain length of the
imidazolium ring. In contrast, in the absence of amine
the size of the NPs was highly dependent on the length
of the alkyl chain; spectroscopic studies provided
convincing evidence that the size control was the result
of coordination of the amine to the nanoparticle.[43e]
A comparison of the various heteroatom donor-
based catalysts 3a-f was initially undertaken to identify
the most efficient system for further optimisation.
Preliminary screening focused on the Suzuki-Miyaura
Table 1. Comparison of the palladium-catalysed Suzuki-
Miyaura cross-coupling between 4-bromoacetophenone and
phenylboronic acid using 3a-f.[a]
Entry
Catalyst
HAD
Conv
TOF
(%)[b] (h-1)[c]
1
2
3
4
5
6
7
PPh2
98
78
90
94
94
58
100
3920
3120
3600
3760
3760
3220
4000
3a
3b
3c
3d
3e
CH2NH2
CH2CN
CH2OMe
pyrrolidone
H
3f
PPh2
3a-PEG
[a]
Reaction conditions: 1.0 mmol 4-bromoacetophenone, 1.13
mmol phenylboronic acid, 1.2 mmol K2CO3, 0.05 mol% 3a-f or 3a-
PEG, 2.4 mL ethanol/water 1/1, room temp., 30 min. [b] Yields
determined by gas chromatography using decane as internal
[c]
standard. Average of three runs. Moles of product per mole of
catalyst per hour based on total palladium content.
A series of batch reactions with 0.05 mol% of ex-
situ prepared 3a or 3a-PEG explored the effect of
solvent and base on catalyst efficacy in order to
4
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
identify optimum conditions for substrate screening. enabling different reducing agents and/or conditions to
The results in Table 2 reveal that solvent has a dramatic be used where appropriate and facilitate rapid catalyst
effect on catalyst efficacy with 3a and 3a-PEG both and reaction screening. To this end, comparative
giving high conversions in a water/ethanol mixture catalyst testing was undertaken by stirring 0.05 mol%
(1/1) (entry 7) while reactions conducted in neat 2a and 2a-PEG with phenylboronic acid for 10 min
ethanol or water (entries 1-2) or conventional organic immediately prior to addition of 4-bromoacetophenone
solvent (entries 3-5) gave much lower conversions. and under the same conditions the yields of 1-([1,1'-
Interestingly, while THF-water has been used the biphenyl]-4-yl)ethan-1-one were comparable to or
solvent of choice for Suzuki-Miyaura cross-couplings higher than those obtained with ex-situ prepared
catalysed by homogeneous systems,[52] it gave poor catalyst; for example, catalyst generated in-situ from
conversions with 3a and 3a-PEG (entry 6). As the use 2a and 2a-PEG gave 96% and 100% conversion,
of water as an additive/co-solvent to promote the respectively, after only 30 min which matched the
Suzuki-Miyaura cross-coupling is well-documented, a conversions obtained with ex-situ prepared 3a and
series of reactions were conducted to determine the 3a.PEG. TEM analysis of in-situ generated 2a and 2a-
optimum volumetric ratio of ethanol-water (entries 7- PEG revealed that the palladium nanoparticles are
9). While reactions conducted in neat ethanol were monodisperse with mean diameters of 2.61 ± 0.38 and
slow and only reached 28% conversion after 30 min, 3.36 ± 0.61 nm, respectively (Figure 3); for
addition of water resulted in a dramatic improvement comparison ex-situ prepared 3a and 3a-PEG have
in efficiency such that the optimum volumetric ratio of smaller mean diameters of 2.29 ± 0.96 and 1.93 ± 0.67
1/1 gave complete conversion with no evidence for nm, respectively.
homocoupling of the aryl bromide (entry 7). This
appears to be primarily related to the dispersion of the
catalyst in the reaction mixture, with dynamic light
scattering (DLS) measurements revealing large
aggregates (500 nm to 10 µm) in pure water and
ethanol as compared to a clear solution, with ~100 nm
particles, observed in a 50:50 water/ethanol mixture
(see ESI Figure S66 for full details). The potential
green benefits and practical advantages associated with
conducting catalysis in aqueous media prompted us to
use a 1/1 ethanol-water mixture for the remainder of
our studies. While the initial choice of K2CO3 as base
was guided by literature precedent, a survey of a
limited range of alternative bases was undertaken to
establish which was the most effective as the correct Figure 3 HRTEM images of (a) 3a and (c) 3a-PEG
combination of base and solvent is crucial to achieving generated in-situ, and (b and d) corresponding particle size
good conversions. As expected, high conversions were distributions determined by counting >100 particles. Mean
also obtained with sodium and caesium carbonate as nanoparticle diameters are 2.61 ± 0.38 nm and 3.36 ± 0.61
well as potassium phosphate (entries 10-12) while nm for 3a and 3a-PEG formed in-situ, respectively. Black
other inorganic bases such as caesium fluoride and and white scale bars are 25 and 10 nm, respectively.
alkali-metal acetates (entries 13-15) gave much lower
conversions as did tributylamine (entry 16). Finally,
the efficiency of 3a and 3a-PEG was compared with
The robustness of 3a-PEG was tested by reducing the
commercially available samples of 5 mol% Pd/C from catalyst loading to 0.001 mol% and under otherwise
various suppliers in order to determine the relative identical conditions a conversion of 98% was obtained
merits of the new catalysts. Three samples were after 6 h; this corresponds to a total turnover number
examined and under the same conditions, 0.1 mol% (TON) of 98,000 and an average turnover frequency
Pd/C catalysed the Suzuki-Miyaura cross-coupling (TOF) of 16,300 h-1. Remarkably, 3a-PEG outperforms
between 4-bromobenzophenone and phenylboronic tris(triazolyl)-polyethylene glycol stabilised palladium
acid but only reached 9%, 13 % and 6% conversion, nanoparticles recently developed by Astruc and co-
respectively, in the same time (entries 17-19).
workers which reached a TON 89,000 and a TOF of 2,700
As 3a and 3a-PEG are generated from their h-1 for the Suzuki-Miyaura cross-coupling between
tetrachloropalladate-based precursors, PdCl4@PPh2- bromobenzene and phenyl boronic acid in an
PIILP (2a) and PdCl4@PPh2-PEGPIILP (2a-PEG), ethanol/water mixture;44 in addition, a significantly
respectively, by reduction with sodium borohydride, higher temperature of 80 °C was required to achieve this
we examined the possibility of using phenylboronic efficiency whereas 3a-PEG operates at room temperature.
acid to reduce the precursor in-situ immediately prior A survey of the literature revealed that 3a-PEG also
to addition of substrate. There would be several outperforms the vast majority of PdNP-based catalysts for
practical advantages associated with this strategy as it the Suzuki-Miyaura cross-coupling between 4-
would eliminate the need to prepare, isolate and store bromoacetophenone and phenylboronic and is even
the nanoparticle catalysts, and as such it would comparable to water soluble diaminocyclohexane-
streamline the protocol, improve versatility by cyclodextrin-supported PdNPs which gave a TOF of
5
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
Table 2. Optimization of the Suzuki-Miyaura cross-coupling between 4-bromoacetophenone and phenylboronic acid using
3a and 3a-PEG [a]
Entry
Catalyst
3a-PEG
Solvent
Base
Yield (%)[b]
3a
24
10
3
TOF (h-1)[c]
3a-PEG
28
32
3
3a
3a-PEG
1120
1280
120
1
2
3
4
5
6
7
8
ethanol
H2O
toluene
THF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
K3PO4
CsF
NaOAc
CsOAc
NBu3
K2CO3
K2CO3
K2CO3
896
400
120
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
3a-PEG
2
1
80
40
DMF
12
10
98
70
29
83
91
62
22
9
11
2
Pd/C
Pd/C
Pd/C
20
15
100
66
39
74
75
66
27
7
11
5
9
13
6
480
800
THF/H2O
EtOH/H2O (1/1)
EtOH/H2O (3/1)
EtOH/H2O (1/3)
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
400
600
3920
2800
1160
3320
3640
2480
880
4000
2640
1560
2960
3000
2640
1080
280
9
10
11
12
13
14
15
16
17
18
19
360
440
440
80
200
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
360
520
240
[a] Reaction conditions: 1.0 mmol 4-bromoacetophenone, 1.13 mmol phenylboronic acid, 1.2 mmol base, 0.05 mol% 3a-f
and Pd/C, 2.4 mL solvent, room temp., 30 min. [b] Yields determined by gas chromatography using decane as internal
standard. Average of three runs. [c] Moles of product per mole of catalyst per hour based on total palladium content.
16,000 h-1 based on 32% conversion after 2h with a catalyst good conversions can be obtained for the Suzuki-Miyaura
loading of 0.001 mol%.[53] For comparison, the TOF cross-coupling of activated and unactivated aryl bromides as
obtained with 3a-PEG in water/ethanol at room temperature well as sterically challenging substrates. Both ex-situ
is also a substantial improvement on that of 950 h-1 for tris- prepared catalysts, 3a and 3a-PEG, gave high conversions
imidazolium-stabilised PdNPs at 90 °C,[54] 132 h-1 for water with activated aryl bromides such as 3- and 4-
soluble PdNPs stabilised by PEG-tagged click-derived tris- bromobenzonitrile (entry 1-2), 1-bromo-4-nitrobenzene and
triazoles at 100 °C,[46] 198 h-1 for PdNPs distributed in the 1-bromo-3-nitrobenzene
(entry
4-5)
and
4-
framework of ionic liquid-modified ordered mesoporous bromobenzaldehyde (entry 6), with 3a-PEG generally
organosilica at 60°C,[55] and 310 h-1 for magnetically outperforming its non-PEGylated counterpart 3a, albeit by
separable
palladium nanoparticles at 65°C;[45] while each of the above between
systems required elevated temperatures 3a-PEG operated phenylboronic acid was also investigated as it is a sterically
efficiently at room temperature. Other PdNP-based systems demanding substrate that would afford biaryl
Fe3O4@PEG-iminophosphine-supported only a small margin in some cases. The cross-coupling
diethyl (2-bromophenyl)phosphonate and
a
that catalyse this transformation include supports and monophosphonate which could be further elaborated to a
stabilisers such as graphene and graphene oxide,[14a] biaryl monophosphine; gratifyingly a high conversion was
starch,[56]
ionic
liquid-grafted
poly(p-phenylene) obtained after only 6 h (entry 10). Electron-rich and
microspheres,[31d] thiazolidine-based mesoporous silica sterically-demanding coupling partners typically required
gel,[57] phosphine-functionalised silica surface,[43a] amino- longer reaction times (5-16 h) to reach comparable
functionalised metal-organic frameworks,[15g] multi-layered conversions at room temperature (entries 3, 7-9 and 11-16),
covalently supported ionic liquids,[58] PdNP@benonite,[59] however, reaction times could be reduced quite significantly
hybrid pyrazoles,[60] and amino-modified silica gel;[11e] at elevated temperatures. Inspection of the conversions in
however, the TOF obtained in each case was markedly lower Table 3 clearly shows that catalyst generated in-situ by
than that obtained with 3a-PEG.
reduction of 2a or 2a-PEG outperformed their ex-situ
Having identified optimum conditions and obtained an prepared counterparts across the range of electron-poor and
encouraging conversion for the benchmark coupling, sterically challenging substrates examined; this is most
catalyst testing was extended to a range of aryl bromides to evident
for
4-bromoanisole
(entries
11),
2-
explore and assess the scope and limitations of these systems. bromoacetophenone (entry 9) and 3-bromotoluene (entry
The data in Table 3 compares the performance of catalysts 16). For example, catalyst generated in-situ from 2a-PEG
generated in-situ from 2a and 2a-PEG with their ex-situ gave 99% conversion for the cross-coupling between 2-
prepared counterparts 3a and 3a-PEG and clearly shows that bromoacetophenone and phenylboronic acid while its ex-
6
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
situ prepared counterpart 3a-PEG only reached 91% elucidate the nature of the coordination environment at
conversion in the same time; similarly the corresponding palladium in 3a and 3a-PEG as well as the influence of
non-PEGylated systems 2a and 3a reached 58% and 34%, added donor atoms under conditions of catalysis.
respectively, for the same cross-coupling (entry 9). This
comparison also highlights the improvement in performance
80
60
40
20
0
that can be achieved by attaching a PEG-substituent to the
stabilising support as in-situ generated and ex-situ prepared
PdNP@PPh2-PEGPIILP gave markedly higher conversions
than their respective PdNP@PPh2-PIILP counterparts.
Finally, the same protocol was extended to the cross-
coupling of 2-bromopyridine and 2-bromopyrimidine
(entries 17-18) with phenylboronic acid but, unfortunately,
only low yields of the corresponding heterobiaryl were
obtained, even after extended reaction times. Thus, it
appears that heteroaryl bromides may either deactivate or
poison the catalyst through coordination of the nitrogen
donor atom to the surface of the nanoparticle as catalyst
0
60
180
360
480
Time of pyridine addition (min)
Figure 4. Conversion as a function of pyridine addition time
for the Suzuki-Miyaura cross-coupling between 2-
bromotoluene and phenylboronic acid, catalysed by 0.1
mol% ex-situ prepared 3a-PEG; each reaction was worked
mixtures containing 0.05 mol% 3a-PEG, phenylboronic up after 8 h.
acid and pre-stirred with either 1 mmol of 2-bromopyridine
and 2-bromopyrimidine were inactive for the cross-coupling
A preliminary study of the influence on catalyst
of 4-bromoacetophenone. Even though the phosphines in
performance of the surface ionic liquid (IL),
diphenylphosphine donor (PPh2) and PEG has been
undertaken by comparing the efficiency of a series of
catalysts containing varying combinations of each
component.32 The data summarised in Table 4 demonstrates
the marked influence of the phosphino donor on catalyst
efficacy as selective removal of this component results in a
dramatic drop in TOF from 9840 h-1 and 8760 h-1 for
PdNP@PPh2-PEGPIILP and PdNP@PPh2-PIILP to 1560 h-
3a-PEG appear to be covalently attached to the surface of
the palladium nanoparticles, as evidenced by solid state 31
P
NMR spectroscopy, the efficacy of this catalyst suggests that
either the surface palladium atoms are not all entirely
coordinatively saturated or phosphine dissociation is a facile
process as the substrate must be able to access the active
surface sites. However, the large excess of nitrogen donor-
based heteroaromatic substrate may well saturate the surface
palladium atoms and deactivate the catalyst. In preliminary
work a sample of 3a-PEG was treated with pyridine to
assess the effect of nitrogen donor groups on the activity of
the catalyst (see supporting information). A reduction in
catalytic activity was observed after pre-treating 3a-PEG
with 1 mmol of pyridine for either 1 h and 16 h as both gave
conversions of only 6% (c.f. 100% before addition of
1
and 360 h-1 for PdNP@PEG-PIILP and PdNP@PIILP,
respectively. In addition, selective removal of the ionic
liquid also results in a decrease in TOF to 6600 h-1 for
PdNP@PPh2-PEGstyrene compared with 9840 h-1 for
PdNP@PPh2-PEGPIILP. Although it is clear that each
component has a direct effect on catalyst performance,
further studies will be required to deconvolute how each
component influences catalyst performance, in particular,
whether the heteroatom donor influences nanoparticle
formation and size and/or surface electronic structure.
Moreover, as removal of either the ionic liquid or PEG
pyridine)
for
the
cross-coupling
between
4-
bromoacetophenone and phenylboronic acid; this further
suggests that coordination of the heteroatom donor to the
surface of the nanoparticle may well be responsible for the
deactivation/passivation. The variation in conversion as a
function of pyridine addition time for the Suzuki-Miyaura
cross-coupling between 2-bromotoluene and phenylboronic
acid catalysed by 0.1 mol% 3a-PEG was also monitored by
running a series of reactions in parallel, adding 1 mmol of
pyridine at different time intervals and working the reaction
up after 8 h to obtain conversion as a function of pyridine
quenching time. The profile shown in Figure 4 clearly
reveals that addition of pyridine results in instantaneous
deactivation of the catalyst as the conversions-time profile
maps closely to that in Figure 6 for the same reaction
conducted in the absence of pyridine (vide infra).
Fluorescence-detected X-ray absorption spectroscopy
(FD-XAS) of the catalyst after pyridine treatment for 1 h
showed an increase in the Pd LIII edge energy relative to the
untreated 3a-PEG catalyst of 0.32 ± 0.13 eV which is
indicative of a small reduction of the electron density on the
palladium metal within the nanoparticles upon pyridine
addition (see ESI Figure S67 and Table S3 for full
details),[61] However, there is no real change in white line
intensity or shape of the edge after addition. Further in-situ
surface investigations are currently underway to fully
results in
a
drop in activity the balance of
hydrophilicity/hydrophobicity clearly effects dispersibility
and/or swelling and thereby access to the active site.
Table 4. Suzuki-Miyaura cross-coupling between 4-
bromoacetophenone and phenyl boronic acid as a function
of catalyst composition.
Components present TOF (h-1)
Name
PdNP@PPh2-PEGPIILP
PdNP@PPh2-PIILP
PdNP@PIILP
PPh2, IL, PEG
PPh2, IL
IL
9840
8760
360
PdNP@PPh2-styrene
PdNP@PEGPIILP
PdNP@PPh2-PEGstyrene PPh2, PEG
PPh2
IL, PEG
4800
1560
6600
[a]
Reaction conditions: 1.0 mmol 4-bromoacetophenone, 1.13
mmol phenylboronic acid, 1.2 mmol K2CO3, 0.05 mol% ex-situ
prepared catalyst, 2.4 mL EtOH/H2O (1/1), room temp, time 10
min.
7
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
Table 3. Suzuki-Miyaura cross-coupling between phenylboronic acid and a range of aryl bromides catalyzed by in-situ
generated and ex-situ prepared 3a and 3a-PEG [a]
Yield (%)[b] (TOF (h-1))[c]
Entry
Ar-Br
Time (h)
0.5
2a
95 (3800)
2a-PEG
99 (3960)
3a
96 (3840)
3a-PEG
99 (3960)
1
2
3
3
80 (534)
99 (660)
96 (640)
99 (660)
5
2
81 (324)
89 (890)
98 (392)
91 (910)
95 (380)
96 (960)
99 (396)
98 (980)
4
5
1
96 (1920)
99 (1980)
95 (1900)
99 (1980)
0.5
95 (3800)
97 (3880)
97 (3880)
99 (3960)
6
16
16
93 (116)
95 (120)
98 (122)
99 (124)
83 (104)
81 (102)
92 (116)
99 (120)
7
8
16
58 (82)
99 (124)
34 (42)
91 (114)
9
6
5
89 (296)
62 (248)
99 (330)
97 (388)
57 (190)
45 (180)
92 (306)
93 (372)
10
11
5
31 (124)
89 (112)
89 (356)
99 (124)
31 (124)
91 (114)
96 (384)
94 (118)
12
13
16
6
96 (320)
67 (84)
90 (300)
80 (100)
85 (284)
60 (76)
89 (296)
78 (98)
14
15
16
6
80 (266)
49 (164)
87 (290)
69 (230)
16
16
16
22 (128)
34 (42)
20 (26)
28 (36)
17 (22)
19 (24)
23 (28)
17
18
28 (136)
[a] Reaction conditions: 1.0 mmol aryl bromide, 1.13 mmol phenylboronic acid, 1.2 mmol K2CO3, 0.05 mol% catalyst
generated in-situ from 2a or 2a-PEG or ex-situ prepared 3a and 3a-PEG, 2.4 mL EtOH/H2O (1/1), room temp, time (h). [b]
[c]
Yields determined by gas chromatography using decane as internal standard. Average of three runs. Moles product per
mole catalyst per hour based on total palladium content.
8
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
Interested in exploring the influence of introducing 3a-PEG, respectively. A comparative study of the
a PEG substituent on the reaction profile, the Suzuki- variation in conversion as a function of time for the
Miyaura cross-coupling between 4-bromobenzonitrile Suzuki-Miyaura
cross-coupling
between
2-
and phenylboronic acid, catalysed by either 0.1 mol% bromotoluene and phenylboronic acid catalysed by a
of ex-situ prepared PdNP@PPh2-PIILP (3a) or 0.1 mol% loading of in-situ generated and ex-situ
PdNP@PPh2-PEGPIILP (3a-PEG) was monitored as prepared 3a-PEG at room temperature shows that the
a function of time. The resulting composition-time composition-time profiles are qualitatively similar and
profile revealed that the latter reaches complete that the optimum conversion is essentially reached
conversion after only 10 min whereas the former after 4 h (Figure 6).
requires 15 min to reach completion (Figure 5a-b, ●).
Similarly, the corresponding catalyst generated in situ
PdNP@PPh2-PEGPIILP
PdCl4@PPh2-PEGPIILP
from 0.1 mol% PdCl4@PPh2-PIILP (2a) and
PdCl4@PPh2-PEGPIILP (2a-PEG) also show
qualitatively similar conversion-time profiles (Figures
5a-b, ▲) suggesting that reduction is both facile and
does not influence the reaction kinetics. In addition, the
data in Figure 5a clearly shows that both PdNP@PPh2-
PIILP systems (i.e. ex-situ prepared and in-situ
generated) experience an induction period of
approximately 5 min whereas the PdNP@PPh2-
PEGPIILP system turns over immediately; this may
well be due to poorer solvation of 3a compared with its
more hydrophilic PEG-based counterpart. However,
the rates of 3a and 3a-PEG during the ‘active period’
appear to be similar.
80
60
40
20
0
0
200
400
600
800
(Time min)
Figure 6. Reaction profile as a function of time for the
Suzuki-Miyaura cross-coupling between 2-bromotoluene
and phenylboronic acid catalysed by in-situ generated (▲)
and ex-situ prepared (●) 3a-PEG.
(a)
PdCl4@PPh2-PIILP
PdNP@PPh2-PIILP
100
80
60
40
20
0
TEM examination of the nanoparticles isolated
from the reaction catalysed by ex situ prepared
PdNP@PPh2-PEGPIILP revealed that while the
palladium nanoparticles remained near monodisperse
there has been a significant increase in size with a mean
diameter of 3.72 ± 0.58 nm, compared to the sample
examined before catalysis which has a mean diameter
of 1.93 ± 0.67 nm; the micrograph and associated
distribution histogram based on > 100 particles are
shown in Figure 7.
0
5
10
15
20
25
30
Time (min)
(b)
PdCl4@PPh2-PEGPIILP
PdNP@PPh2-PEGPIILP
100
80
60
40
20
0
0
5
10
15
20
25
30
Time (min)
Figure 7. (a) HRTEM image of ex-situ prepared
PdNP@PPh2-PEGPIILP (3a-PEG) after use in the Suzuki-
Miyaura cross-coupling between 2-bromotoluene and
phenylboronic acid for 6 h and (b) the corresponding particle
size distribution determined by counting >100 particles.
Mean nanoparticle diameters are 3.72 ± 0.58 nm. Black and
white scale bars are 25 and 10 nm, respectively.
Figure 5. Reaction profile as a function of time for the
Suzuki-Miyaura cross-coupling between 4-
bromobenzonitrile and phenylboronic acid catalysed by (a)
in-situ generated (▲) and ex-situ prepared (●) 3a and (b) in-
situ generated (▲) and ex-situ prepared (●) 3a-PEG.
Although some of the more challenging substrates
required long reaction times, the majority reached
good conversions within 16 h, with the exception of 2-
bromotoluene which only reached 80% and 78 %
conversion with in-situ generated and ex-situ prepared
The reusability of 3a.PEG was investigated for
the benchmark Suzuki-Miyaura coupling between 4-
bromoacetophenone and phenylboronic acid to assess
the robustness and longevity of the catalyst and the
9
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
potential for incorporation into a flow process. The
practical problems associated with recovering the
small amount of catalyst (0.05 mol%, 1.0 mg) by
filtration prevented us from performing conventional
recycle experiments. Instead, reuse experiments were
conducted by extracting the product and unreacted
starting material with diethyl ether before recharging
the aqueous phase with ethanol, 4-bromoacetophenone
and phenylboronic acid.
A
preliminary reuse
experiment conducted for 30 min revealed a slight but
gradual drop in conversion from 99% to 81% after the
5th run. However, conversions improved when the
reaction time was extended to 60 min (Figure 8).
Analysis of the organic phase collected after each reuse
revealed that the palladium content was too low to be
detected by ICP-OES confirming that the drop in
activity was not due to leaching of palladium during
the extraction. TEM analysis of the aqueous phase
remaining after the 5th 60 min run revealed that the
palladium nanoparticles were near monodisperse but
significantly larger with a mean diameter of 4.9 ± 1.0
nm, compared with 1.93 ± 0.67 nm for freshly prepared
catalyst (Figure 9). While this increase in size may
account for the drop in activity, we should consider the
possibility that conversions may also become mass
transfer limited as the reaction mixture becomes
progressively more difficult to stir due to the increasing
amounts of precipitates that from during reuse. While
further studies are clearly required to distinguish
between these factors, successful reuse of 3a.PEG is
encouraging and suggests that this system may well be
sufficiently robust for use in a scale-up continuous
flow process. Interestingly, a reuse experiment for the
Figure 9. (a) HRTEM image of ex-situ prepared
PdNP@PPh2-PEGPIILP (3a.PEG) after 5 reuses in the
Suzuki-Miyaura
cross-coupling
between
4-
bromoacetophenone and phenyl boronic acid and (b) the
corresponding particle size distribution determined by
counting >100 particles. Mean nanoparticle diameters are
4.9 ± 1.0 nm. Black and white scale bars are 25 and 10 nm,
respectively.
The nature of the active species in palladium
nanoparticle-catalysed
Suzuki-Miyaura
cross-
couplings remains controversial due to the difficulty
associated with obtaining definitive unambiguous
information.[7a,62] While many reports suggest that
nanoparticles act as a reservoir that leaches to liberate
a soluble active palladium species there is also a
significant body of evidence for catalysis at defect sites
on the surface of the nanoparticles i.e. a heterogeneous
process.[63] Determination of the palladium content
remaining in solution after reaction may be misleading
as it will not distinguish between a pathway involving
leaching and re-deposition (release and capture
mechanism). Thus, a series of mercury poisoning tests
were conducted for the cross-coupling between 4-
bromotoluene and phenylboronic acid using in-situ
generated and ex-situ prepared 3a-PEG in order to
probe the pathway.[62] In these reactions a mixture
containing either PdCl4@PPh2-PEGPIILP (2a-PEG)
or PdNP@PPh2-PEGPIILP (3a-PEG), phenylboronic
acid (to ensure reduction of [PdCl4]2-) and potassium
carbonate in water/ethanol was pre-stirred with 400
equivalents of mercury prior to initiating reaction by
addition of 4-bromotoluene. A range of pre-stirring
times were investigated and the resulting conversion-
mercury poisoning time profile (Figure 10) revealed
that both catalyst systems experience a marked
reduction in conversion from ca. 90% in the absence of
mercury to 46-50% upon direct addition of mercury
with no pre-stirring; conversions continued to drop to
20-22% for 60 min pre-stirring and ultimately to 5%
after the pre-stirring was increased to 24 h. Although
the drop in activity resulting from the addition of
mercury may be taken as evidence for catalysis by
surface active palladium,[64] this interpretation must be
treated with caution as poisoning is often reported to
result in an immediate quench of the reaction. In
addition, there have also been a few reports that
mercury can poison molecular Pd(0) species[65] and,
thus, gradual deactivation/passivation of the catalyst
surface with respect to leaching of active soluble
palladium species may also explain the observed
cross-coupling
between
4-bromotoluene
and
phenylboronic acid catalysed by 3a-PEG was also
conducted and the conversions of 89% and 87% for the
first and second runs, respectively, confirmed that the
catalyst remained highly active even after 6 h; again
this may suggest that the increase in NP size described
in Figure 7 does not significantly impact catalyst
activity.
30 min run
60 min run
100
80
60
40
20
0
1
2
3
4
5
Cycle number
Figure 8. Reuse study for the Suzuki-Miyaura cross-
coupling between 4-bromoacetophenone and phenylboronic
acid catalysed by ex-situ prepared 3a.PEG for reaction times
of 30 min and 60 min.
10
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
conversion-pre-stirring time profile. The gradual drop
in conversion as a function of pre-stirring time may
well be associated with the efficacy of mixing as the
mercury must be well-dispersed in the mixture to
poison the catalyst and the use of water/ethanol may
limit the efficacy of this process.
PdCl4-PEGPPh2
PdNP-PEGPPh2
100
80
60
40
20
0
no Hg
0
20
40
60
1440
Pre-stirring time (min)
Figure 12. Schematic showing a possible pathway involving
capture of leached palladium as an active mono-coordinate
species.
Figure 10. Conversion as a function of Hg(0) pre-stirring
time for the Suzuki-Miyaura cross-coupling between 4-
bromotoluene and phenylboronic acid catalysed by in-situ
generated and ex-situ prepared 3a-PEG; all reactions were
run for 6 h.
Conclusion
Reaction dilution studies for the cross-coupling
between 4-bromotoluene and phenylboronic acid
catalysed by in-situ generated and ex-situ prepared 3a-
PEG under optimum conditions resulted in a slight in
increase in conversion as the reaction was diluted from
1.2 mL to 10.0 mL (Figure 11).
In conclusion, palladium nanoparticles stabilised by
heteroatom donor-decorated polymer immobilised
ionic liquids catalyse the Suzuki-Miyaura cross-
coupling between aryl bromides and phenyl boronic
acid with remarkable efficacy in aqueous media under
exceptionally mild conditions. Catalyst stabilised by
phosphino-decorated polymer immobilised ionic
liquid (PdNP@PPh2-PIILP) and its PEGylated
PdCl4-PEGPPh2
PdNP-PEGPPh2
100
80
60
40
20
0
counterpart
(PdNP@PPh2-PEGPIILP)
were
consistently more efficient than each of the other
heteroatom donor-modified systems examined, across
the range of substrates tested. Improvements in catalyst
performance arising from the introduction of PEG are
attributed to an increase in dispersibility and/or
solubility facilitating access to more exposed active
site. Catalysts generated in-situ from PdCl4@PPh2-
PIILP or PdCl4@PPh2-PEGPIILP either compete with
or outperform their ex-situ generated counterparts
which offers numerous practical advantages for
optimisation as well as substrate and reaction screening.
The TONs obtained for the benchmark coupling are
among the highest to be reported for PdNP-based
catalysts in aqueous media under such mild conditions.
As such, these results are highly encouraging and
provide a strong platform to further study and explore
the influence of the heteroatom donor on the nucleation
and growth of nanoparticles and their efficiency as
catalysts. Moreover, the modular construction of the
PIIL support will also enable properties such as surface
characteristics, ionic microenvironment, heteroatom to
metal ratio, hydrophilicity and porosity to be modified
and thereby, ultimately, catalyst surface interactions,
substrate accessibility and catalyst efficacy to be
optimized.
1,2
2,4
3,6
4,8
6
10
Solvent volume (mL)
Figure 11. Conversion as a function of reaction volume
(dilution) for the Suzuki-Miyaura cross-coupling between 4-
bromotoluene and phenylboronic acid catalysed by in-situ
generated and ex-situ prepared 3a-PEG.
While this is certainly not definitive evidence for
catalysis by a heterogeneous system it does eliminate a
purely homogeneous pathway. Moreover, at this stage
we cannot exclude a pathway involving solubilisation
of a surface palladium atom that is captured by a
proximal phosphine to afford an active supported
mono-coordinate LPd type species (Figure 12).
11
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10.1002/adsc.201800561
Advanced Synthesis & Catalysis
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4157; j) S. Doherty, J. G. Knight, C. R. Addyman, C. H.
Smyth, N. A. B. Ward, R. W. Harrington,
Organometallics 2011, 30, 6010–6016.
We gratefully acknowledge Newcastle University for financial
support (TB) and King Abdulaziz University, Saudi Arabia
(Scholarship to H.F.A). 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 Thompson. We also thank Dr
Maggie White for collecting the powder X-ray diffraction data
(Newcastle) and Dr Kathryn White for the SEM images (Faculty of
Medical Sciences, Newcastle University).
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FULL PAPER
Heteroatom Donor-Decorated Polymer-
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