Palladium nanoparticles in catalytic carbon nanoreactors: The effect of confinement on Suzuki-Miyaura reactions

Article abstract of DOI:10.1039/c4ta06953f

We explore the construction and performance of a range of catalytic nanoreactors based on palladium nanoparticles encapsulated in hollow graphitised nanofibres. The optimum catalytic material, with small palladium nanoparticles located almost exclusively at the graphitic step-edges within nanoreactors, exhibits attractive catalytic properties in Suzuki-Miyaura cross-coupling reactions. Confinement of nanoparticles at the step-edges facilitates retention of catalytic centres and recycling of catalytic nanoreactors without any significant loss of activity or selectivity over multiple catalytic cycles. Furthermore, careful comparison of the catalytic properties of palladium nanoparticles either on or in nanoreactors reveals that nanoscale confinement of catalysts fundamentally affects the pathways of the Suzuki-Miyaura reaction, with the yield and selectivity for the cross-coupled product critically dependent on the steric properties of the aryl iodide reactant, whereas no effects of confinement are observed for aryl boronic acid reactants possessing substituents in different positions. These results indicate that the oxidative addition step of the Suzuki-Miyaura reaction occurs at the step-edge of nanofibres, where the mechanisms and kinetics of chemical reactions are known to be sensitive to nanoscale confinement, and thus the extent of confinement in carbon nanoreactors can be discretely controlled by careful selection of the aryl iodide reactant.

Full text of DOI:10.1039/c4ta06953f

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Palladium nanoparticles in catalytic carbon
nanoreactors: the effect of confinement on
Suzuki–Miyaura reactions†
Cite this: DOI: 10.1039/c4ta06953f
B. Cornelio,ab A. R. Saunders,c W. A. Solomonsz,c M. Laronze-Cochard,b A. Fontana,a
c
J. Sapi,b A. N. Khlobystovcd and G. A. Rance
*
We explore the construction and performance of a range of catalytic nanoreactors based on palladium
nanoparticles encapsulated in hollow graphitised nanofibres. The optimum catalytic material, with small
palladium nanoparticles located almost exclusively at the graphitic step-edges within nanoreactors,
exhibits attractive catalytic properties in Suzuki–Miyaura cross-coupling reactions. Confinement of
nanoparticles at the step-edges facilitates retention of catalytic centres and recycling of catalytic
nanoreactors without any significant loss of activity or selectivity over multiple catalytic cycles.
Furthermore, careful comparison of the catalytic properties of palladium nanoparticles either on or in
nanoreactors reveals that nanoscale confinement of catalysts fundamentally affects the pathways of the
Suzuki–Miyaura reaction, with the yield and selectivity for the cross-coupled product critically
dependent on the steric properties of the aryl iodide reactant, whereas no effects of confinement are
observed for aryl boronic acid reactants possessing substituents in different positions. These results
indicate that the oxidative addition step of the Suzuki–Miyaura reaction occurs at the step-edge of
nanofibres, where the mechanisms and kinetics of chemical reactions are known to be sensitive to
nanoscale confinement, and thus the extent of confinement in carbon nanoreactors can be discretely
controlled by careful selection of the aryl iodide reactant.
Received 17th December 2014
Accepted 5th January 2015
DOI: 10.1039/c4ta06953f
www.rsc.org/MaterialsA
interactions to facilitate transport of the reactants into the
internal channel.15–21 For these reasons, they have the potential
Introduction
to outperform previously studied hollow nanoscale materials
and permit exploration of new chemical processes.15,21–23 In our
recent studies, we have evaluated the properties of catalytic
chemical reactions conned within hollow graphitised nano-
bres (GNF).24–27 This unique carbon nanomaterial possesses a
topologically complex nanoscale structure comprising a series
of stacked cups encased within concentric tubes. The internal
diameters are typically between 50 and 70 nm, an order of
magnitude greater than the dimensions of simple molecular
species, and the nanobre tips are always open, thus allowing
efficient transport of molecules through the internal structure.
Furthermore, the distinctive internal herringbone structure
results in a succession of 3–4 nm high steps formed by rolled-up
sheets of graphene that provide effective “anchoring points” for
molecules and nanoparticles.28 This creates localised nanoscale
reaction environments, different to the bulk phase, where the
local concentration of aromatic reactants is maximised through
attractive non-covalent interactions between the reactant and
the underlying graphitic surface of the nanoreactor24–27 and the
activity,25,27 selectivity24,26,27 and stability25 of conned metal
nanoparticle catalysts are enhanced across a range of prepara-
tive chemical transformations. Thus, GNF represents the ideal
The connement of chemical reactions inside carbon nano-
tubes (CNT) is emerging as a highly versatile tool to control the
pathways of molecular transformations and is leading to the
formation of new molecules and materials inaccessible by other
means.1–4 Although the use of molecular species, such as
cyclodextrins, calixarenes, cucurbiturils, supramolecular and
coordination cages, micelles and bilayer vesicles,5–10 and
extended porous solids, including zeolites,6,11 to hold chemical
reactions has been well established over the last three decades,
carbon nanotubes and related hollow carbon nanostructures
possess superior chemical,12 thermal13 and mechanical
stability13,14 and rely solely upon ubiquitous van der Waals
a
`
Dipartimento di Farmacia, Universita G. d'Annunzio, Via dei Vestini, I-66100 Chieti,
Italy
b
´
´
Institut de Chimie Moleculaire de Reims, CNRS UMR 7312, Universite de Reims
Champagne-Ardenne, UFR Pharmacie, 51 Rue Cognacq-Jay, F-51096 Reims Cedex,
France
cSchool of Chemistry, University of Nottingham, University Park, Nottingham, NG7
2RD, UK. E-mail: graham.rance@nottingham.ac.uk; Fax: +44 115 9513563
dNottingham Nanotechnology and Nanoscience Centre, University of Nottingham,
University Park, Nottingham, NG7 2RD, UK
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta06953f
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nanoscale container to further probe the effects of connement glassware was cleaned with a mixture of hydrochloric and nitric
on chemical reactions. acid (3 : 1 v/v, ‘aqua regia’) and rinsed thoroughly with deion-
In this study, we explore the effect of connement in GNF on ised water, cleaned with potassium hydroxide in isopropyl
the palladium-catalysed Suzuki–Miyaura reaction.29 This versa- alcohol and nally rinsed with deionised water. 1H and 13C
tile cross-coupling reaction represents one of the most powerful NMR spectra were obtained using a Bruker DPX-300 (300.13 and
carbon–carbon bond forming methodologies30,31 and is appli- 75.48 MHz respectively) spectrometer at 298 K using CDCl3 as
cable to the broadest spectrum of possible substrates.32 Today, the solvent. Mass spectrometry was conducted on either a
it is one of the most inuential methods utilised in the Bruker Apex IV (ESI) or a Waters Autospec (EI) in positive mode.
synthesis of natural products and pharmaceuticals,33–35 with its Thermogravimetric analysis was performed using a TA Instru-
importance recognised by the award of the 2010 Nobel Prize in ments SDT Q600 under a ow of air at a rate of 90 mL minꢀ1 at a
Chemistry to its pioneering discoverer.36 However, despite the heating rate of 10 ꢁC minꢀ1 from room temperature to 1000 ꢁC.
vast number of studies probing the inuence of both molecular Transmission electron microscopy (TEM) was performed using
and nanoparticulate catalysts on the mechanism and properties a JEOL 2100F TEM (eld emission gun source, information
of the Suzuki–Miyaura reaction,31,37–40 to the best of our knowl- limit <0.19 nm, 200 kV accelerating voltage) at room tempera-
edge only a small number of studies have probed the inuence ture. Statistical analysis of nanoparticle size (N > 50) and loca-
of nanoscale connement on this important cross-coupling tion was conducted using Gatan DigitalMicrograph soware.
reaction.41–45 Oen conducted within the pores of mesoporous Acquisition of tilt series was performed using a Gatan 916
silica,41–44 the conned reactions catalysed by palladium nano- tomography holder and a Gatan 646 Double tilt holder. This
particles (PdNP) oen proceed with the attractive catalytic analysis involved tilting individual nanoreactor catalysts along
properties of near selective formation in high yields of the cross- the GNF growth axis and was used to determine the location of
coupled product. Yet, as no comparison to the same catalytic catalytic centres (inside or outside nanoreactors). Statistical
species located on the exterior surface of the support material relevance was ensured by analysing multiple nanoreactors, in
was possible, the sole effect of connement on the properties of different regions of the specimen and averaging across the
both the catalyst and the reaction could not be disclosed. entire sample. Energy dispersive X-ray analysis was performed
Through systematic investigation of the assembly and catalytic using an Oxford Instruments INCA 560 X-ray microanalysis
properties of a range of different palladium nanoparticle– system. TEM samples were prepared by drop-drying methanolic
graphitised nanobre composites, we have procured suitable solutions onto a copper grid mounted “lacey” carbon lms.
materials that allow us to denitively discriminate between the
effects of support and connement. Our ideal catalytic nano-
reactor, comprising small palladium catalytic centres located
nearly exclusively at the internal step-edges within GNF,
exhibits relatively good activity, selectivity and stability in the
carbon–carbon bond forming reactions of electron decient
aryl iodides and phenylboronic acids. Furthermore, we
demonstrate through exploration of a range of aryl iodides and
boronic acids, using both competitive and non-competitive
methodologies, that both the yields and the specic selectivity
for the cross-coupled product depend critically on the steric
properties of the aryl iodide reactant. As the conned reactions
are seemingly independent of the properties of the boronic acid,
we propose that nanoscale connement in GNF affects only the
oxidative addition step at the nanoreactor step-edge, with the
reactivity of the afforded intermediate dependant on the steric
congestion at the reaction locus. Thus, we show that the
desirable properties of connement can be modulated through
considered selection of appropriate aryl iodide reactants.
Preparation of PdNP@GNF nanoreactors
PdNP@GNF were prepared by modication of the method
described by Zhi et al.42 A solution of potassium tetra-
chloropalladate (3.83 mg, 0.0117 mmol) in deionised water (1
mL) was added dropwise to graphitised nanobres (25 mg, GNF,
ꢁ
heated for 1 h at 450 C in air). The combined dispersion was
stirred at room temperature for 1 h and then dried at room
temperature under vacuum for 15 h. The obtained solid was
heated in a tube furnace under an atmosphere of hydrogen/
argon (10/90) at 400 ꢁC for 3 h and nally cooled at room
temperature under an inert atmosphere of argon to yield a black
solid. Additional catalytic nanoreactors and supported catalysts
were prepared according to modied literature procedures and
novel protocols (S1 and S2, ESI†).26,46–49
The Suzuki–Miyaura cross-coupling reaction
The catalytic properties of nanoreactors and supported catalysts
were determined using the Suzuki–Miyaura cross-coupling
reaction (S3, ESI†).52 In a typical procedure, to the aryl iodide
1a–e (0.041–0.056 mmol, 1.0 eq.), the boronic acid 2a–e (0.053–
0.075 mmol, 1.3 eq.), sodium acetate (0.9–17.2 mg, 0.094–0.13
Experimental
General experimental
All reagents were purchased from Sigma-Aldrich, UK and used mmol, 2.3 eq.) and the catalyst (0.2–4.7 mg, 2 mol%) in a two-
without further purication. Carbon nanostructures were necked round-bottomed ask under an inert atmosphere of
purchased from a range of commercial suppliers: SWNT (helix argon was added via cannula a degassed solution of methanol (5
SWNT from Helix Material Solutions, made by arc discharge), mL) and the resulting suspension stirred at 70 ꢁC for 24 h. Aer
MWNT (PD30L520 MWNT from NanoLab, made by chemical cooling to room temperature, the dispersion was ltered
vapour deposition) and GNF (Pyrograf PR19 GNF from Applied through a PTFE membrane lter (pore diameter 0.2 mm) and
Science Inc., made by chemical vapour deposition). All concentrated under vacuum. The products were identied by 1H
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NMR spectroscopy of the crude mixture or isolated by ash or in aqueous suspension, with an aqueous solution of potas-
column chromatography on a silica gel (ccsg). Competitive sium tetrachloropalladate, followed by removal of the solvent
Suzuki–Miyaura reactions were performed in an analogous and treatment with molecular hydrogen to form ligand-free,
fashion to the method described above (S3, ESI†).
and therefore highly reactant accessible, metal nano-
particles.42,46,47 PdNP@GNF-III was prepared by gas-phase
impregnation of GNF with palladium bis(acetylacetonate) and
subsequent hydrogen-mediated reduction48 and PdNP@GNF-IV
by the encapsulation of pre-formed dodecanethiolate-stabilised
palladium nanoparticles49 using scCO2 as a transport and pro-
cessing medium.26
Comprehensive transmission electron microscopy investi-
gation of the structure of the composite materials reveals that
PdNP@GNF-I, -II and -III (Fig. 1(b–d) and S1(a–c), ESI†) were not
suitable to study the conned Suzuki–Miyaura reactions as the
majority of the palladium catalytic centres appear on the
external surfaces of nanobres, with the catalytically active
surface passivated by graphitic carbon shells in some cases
(Fig. S1(c), ESI†). Conversely, nanoparticle catalysts in
PdNP@GNF (Fig. 1(a), 3(a, b) and S2, ESI†) and PdNP@GNF-IV
(Fig. 1(e) and S1(d), ESI†) are nearly exclusively located at the
internal corrugations within GNF and hence represent more
attractive materials to explore the properties of conned cross-
coupling reactions.
Results and discussion
Graphitised nanobres are an ideal material to compare the
effects of catalyst support and connement on the properties of
the Suzuki–Miyaura cross-coupling reaction given the signi-
cant difference between their external and internal surfaces.
This should permit discrete control of the location of catalyst
centres through careful consideration of catalyst assembly
conditions. Furthermore, the cost of GNF is relatively low and
the presence of nanobre synthesis catalyst, which has the
potential to adversely affect the desired properties of the
nanocatalyst, is negligible. In our preliminary experiments, a
range of novel catalysts comprising palladium nanoparticles
conned for the rst time within the internal channel of GNF
were prepared based on adaptation to literature procedures for
the encapsulation of palladium in CNT26,46–49 and mesoporous
silica.42 The loading of palladium precursors was xed across all
samples and the structural properties of the obtained
composites were evaluated by extensive transmission electron
microscopy imaging (Fig. 1 and 3, Table 1 and Fig. S1 and S2,
ESI†) and thermogravimetric analysis (Table 1 and Fig. S3,
ESI†).
The catalytic performance of palladium nanoparticle–
graphitised nanobre nanoreactors was appraised using the
reaction of 1-iodo-4-nitrobenzene 1a and phenylboronic acid 2a
(Scheme 1 and Table 2).
Although the assembly of nanoreactors by insertion of pre-
formed nanoparticles is generally preferred to methodologies
PdNP@GNF, PdNP@GNF-I and PdNP@GNF-II were
prepared by liquid-phase impregnation of GNF, either as a solid
Fig. 1 TEM images of the catalytic carbon nanoreactors: (a) PdNP@GNF possessing small PdNP (dNP ¼ 7.0 ꢂ 2.2 nm) which nucleate and grow at
the graphitic step-edges of GNF nanoreactors; (b) PdNP@GNF-I comprising a bimodal distribution of small pseudo-spherical PdNP (dNP ¼ 15.3 ꢂ
8.9 nm) and larger faceted nanoparticles which are mostly located on the nanofibre exterior; (c) PdNP@GNF-II possessing PdNP (dNP ¼ 9.4 ꢂ 6.4
nm) mainly located on the outer surface of GNF, owing to poor impregnation of the GNF suspension with the palladium precursor solution; (d)
PdNP@GNF-III assembled using gas-phase impregnation of GNF with the palladium precursor, leading to the formation of PdNP (dNP ¼ 12.9 ꢂ
3.9 nm) poorly inserted inside GNF and covered with carbon shells and (e) PdNP@GNF-IV characterised by ultra-small PdNP (dNP ¼ 1.1 ꢂ 0.3 nm)
pre-formed and selectively encapsulated inside GNF by using scCO2 as transport and process medium. Scale bars are (a) 50, (b–d) 100 and (e)
20 nm.
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Table 1 Structural characterisation of the nanoreactor catalysts by transmission electron microscopy (TEM) and thermogravimetric analysis
(TGA)
TEM analysis
TGA analysis
PdNP locationb
Toxid /ꢁC
Pd900c/%
c
Entry
Catalyst
dNPa/nm
1
2
3
4
5
PdNP@GNF
7.0 ꢂ 2.2
15.3 ꢂ 8.9
9.4 ꢂ 6.4
12.9 ꢂ 3.9
1.1 ꢂ 0.3
Mostly in (>80%)
Mostly out
Mostly out
Mostly out
Mostly in (>90%)
705
711
691
737
711
7.8
9.5
6.2
8.2
4.1
PdNP@GNF-I
PdNP@GNF-II
PdNP@GNF-III
PdNP@GNF-IV
a
dNP refers to the mean nanoparticle diameter as determined by TEM. b PdNP location was estimated by tilt analysis (Fig. S2, ESI). c Toxid and Pd900
refer to the temperature of primary oxidation in air and the percentage of residual palladium metal at 900 ꢁC respectively as determined by TGA.
Scheme 1 The palladium catalysed reactions of aryl iodides 1 and phenylboronic acids 2 leading to the products of Suzuki–Miyaura cross-
coupling 3, dehalogenation 4 and Ullmann coupling 5.
Table 2 Comparison of the selectivity and activity of catalytic nanoreactors in the Suzuki–Miyaura cross-coupling of 1-iodo-4-nitrobenzene 1a
and phenylboronic acid 2aa
Entry
Catalyst
t/h
24
24
24
Conversionb/%
Selectivity for 3aa (4 : 5a)b/%
1
2
3
4
5
PdNP@GNF
83
98
82
36
40
87 (10 : 3)
95 (0 : 5)
96 (0 : 4)
93 (2 : 5)
0 (100 : 0)
PdNP@GNF-I
PdNP@GNF-II
PdNP@GNF-III
PdNP@GNF-IV
24
144
a
Standard conditions: 1-iodo-4-nitrobenzene 1a (0.056 mmol, 1.0 eq.), phenylboronic acid 2a (0.075 mmol, 1.3 eq.), sodium acetate (0.13 mmol, 2.3
eq.), catalyst (2 mol%), methanol (5 mL), 70 ꢁC, 24 h. b Conversion and selectivity for 4-nitrobiphenyl 3aa, nitrobenzene 4 and 4,40-dinitrobiphenyl
5a, the products of cross-coupling, dehalogenation and Ullmann coupling respectively, were determined by 1H NMR spectroscopy of the crude
mixture.
involving direct formation of nanoparticles in nanoreactors, combination of deep penetration of small nanoparticles at the
due to the retention of structural properties of the pre-formed step-edge reaction locus and the presence of a sterically-con-
catalysts regardless of their explicit location (inside or outside gesting nanoparticle stabilisation agent, means that these
nanoreactors), the reaction catalysed by PdNP@GNF-IV was catalytic materials were unsuitable for further use. The reac-
sluggish, resulting in low conversion of starting materials at tions catalysed by the ligand-free catalysts PdNP@GNF-I and -II
extended reaction times and affording only the by-product of resulted in high conversion and selectivity for the cross-coupled
dehalogenation 4 (Table 2, entry 5). The signicant reduction in product 3aa (Table 2, entries 2 and 3, respectively) but do not
catalytic performance of PdNP@GNF-IV, likely associated with allow investigation of the effects of nanoscale connement due
the poor accessibility of reactants to the catalyst due to a to the location of the catalytic centres almost exclusively on the
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exterior surfaces, whereas the passivation of the catalyst by relative to classical molecular catalysts which cannot be
carbon shells resulted in the poor catalytic properties of recycled.
PdNP@GNF-III (Table 2, entry 4). Conversely, the reaction cat-
HR-TEM imaging of the PdNP@GNF structures aer one
alysed by PdNP@GNF proceeded with good conversion and catalytic cycle (Fig. S4, ESI†) illustrates that the initial small
selectivity for the elected cross-coupled product (Table 2, entry PdNP (dNP ¼ 7.0 ꢂ 2.2 nm) undergo a process of growth into
1), demonstrating catalytic properties comparable to more larger nanoparticles (dNP ¼ 21.1 ꢂ 9.8 nm), modulated by Ost-
traditional molecular palladium catalysts, e.g. PdCl2(PPh3)2.50 wald ripening under the so-called “dissolution-precipitation
Furthermore, and most signicantly, recycling of the catalyst in equilibrium” typically observed in nanoparticle catalysis. This
the reaction of 1-iodo-4-cyanobenzene and phenylboronic acid explains the slight decrease in activity aer the rst cycle, but
indicated that connement of nanoparticles at the nanobre critically the nanoparticle catalysts remain anchored to the step-
step-edges preserves the selectivity for the cross-coupled edges within GNF and therefore are available for subsequent
product across catalytic cycles and, despite a moderate drop in reactions. Indeed, nanoparticle size (dNP ¼ 21.8 ꢂ 14.8 nm) does
activity aer the initial reaction, retains good activity cycle-to- not seemingly change aer ve catalytic cycles (Fig. 3(c, d) and
cycle over at least ve catalytic cycles (Fig. 2 and S3.2, ESI†). S4, ESI†), highlighting the high stability of the catalyst, impar-
Thus, PdNP@GNF represents a superior catalytic material ted by connement at nanoreactor step-edges, which is reec-
ted in the consistent activities observed aer cycle one.
As the PdNP@GNF composite exhibited effective control of
catalyst positioning, good stability and promising catalytic
properties in preliminary cross-coupling reactions, we decided
to fabricate the corresponding material with nanoparticles
deposited on the atomically-smooth exterior surface of GNF
(PdNP/GNF) in order to facilitate investigation of the effect of
nanoscale connement on the Suzuki–Miyaura reaction
(Fig. S5(d), ESI†). Furthermore, we explored the versatility of the
preparation protocol of PdNP@GNF for the encapsulation of
catalyst nanoparticles in narrower carbon nanostructures, such
as single- and multi-walled carbon nanotubes (SWNT and
MWNT, respectively). Connement of palladium nanoparticles
failed within SWNT likely owing to the small internal diameter
of these narrow nanotubes and the consequent high resistance
to the transport of precursor molecules into the internal
channel (Fig. S5(c), ESI†). Indeed, palladium nanoparticles were
observed within MWNT but, the catalytic material almost
Fig. 2 Evaluation of the conversion in the Suzuki–Miyaura cross-
coupling of 1-iodo-4-cyanobenzene and phenylboronic acid after 16
h using PdNP@GNF nanoreactors over five catalytic cycles. The
completely blocked the inner channel of the nanotube, thus
inhibiting the penetration of molecular reactants (Fig. S5(b),
selectivity for the cross-coupled product 4-cyanobiphenyl was 100%
across all catalytic cycles.
Fig. 3 TEM analysis of PdNP@GNF catalytic nanoreactors (a and b) before catalysis and (c and d) after five catalytic cycles showing PdNP growth
from 7.0 ꢂ 2.2 to 21.8 ꢂ 14.8 nm. The corrugated interior of GNF facilitates favourable van der Waals interactions with encapsulated nano-
particles, modulating the controlled growth of nanoparticle catalytic centres and restricting the loss of precious metal from the internal channel
of the nanoreactor under the conditions of the Suzuki–Miyaura reaction. Inset: schematic representation of the location of palladium nano-
particles at the internal step-edges of GNF. The proposed reaction locus is denoted by a red circle at the apex of the two graphitic surfaces of the
nanofibre step-edge and the catalytic centre. Scale bars are (a and d) 50 and (b and c) 10 nm.
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Table 3 Comparison of the activity and selectivity of PdNP@GNF and PdNP/GNF catalysts in the Suzuki–Miyaura cross-coupling reaction of aryl
iodides 1a–e and phenylboronic acids 2a–ea
Selectivity for 3
Entry
Catalyst
R1
R2
Conversionb/%
(4 : 5a–b)b/%
1
2
3
4
5
6
7
8
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
3-NO2
3-NO2
2-NO2
2-NO2
H
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2a
2a
2a
2a
2b
2b
2b
2b
H
H
83
78
77
65
74
74
58
58
83
81
99
98
80
87
42
44
23
24
3aa
3aa
3ab
3ab
3ac
3ac
3ad
3ad
3ae
3ae
3ba
3ba
3ca
3ca
3db
3db
3eb
3eb
87 (10 : 3)
80 (17 : 3)
94 (3 : 3)
92 (4 : 4)
97 (1 : 2)
97 (1 : 2)
68 (32 : 0)
67 (33 : 0)
96 (2 : 2)
95 (3 : 2)
71 (28 : 1)
79 (19 : 2)
64 (36 : 0)
99 (1 : 0)
100 (0 : 0)
100 (0 : 0)
100 (0 : 0)
100 (0 : 0)
4-Me
4-Me
3-Me
3-Me
2-Me
2-Me
3-OMe
3-OMe
H
H
H
H
4-Me
4-Me
4-Me
4-Me
9
10
11
12
13
14
15
16
17
18
H
4-NH2
4-NH2
a
Standard conditions: aryl iodides 1a–e (0.041–0.056 mmol, 1.0 eq.), boronic acids 2a–e (0.053–0.075 mmol, 1.3 eq.), sodium acetate (0.094–0.13
mmol, 2.3 eq.), catalyst (2 mol%), methanol (5 mL), 70 ꢁC, 24 h. b Conversion and selectivity for biphenyls 3aa–eb, nitrobenzene 4 and 4,40- and 3,30-
dinitrobiphenyls 5a and 5b, respectively, the products of cross-coupling, dehalogenation and Ullmann coupling respectively were determined by 1H
NMR spectroscopy of the crude mixture.
ESI†). Hence, only GNF-based nanoscale catalysts were further compared to the cross-couplings of 4- and 3-tolylboronic acids
explored.
2b and c, presumably due to increased steric hindrance of the
To probe the inuence of nanoscale connement in GNF on incoming boronic acid to the oxidative addition intermediate,
the properties of the Suzuki–Miyaura cross-coupling reaction, this behaviour was independent of the reaction locus (i.e. inside
we conducted a comprehensive study where the steric and or outside nanoreactors) whereas for the sterically hindered 1-
electronic properties of both the aryl iodide and boronic acid iodo-2-nitrobenzene 1c the PdNP/GNF supported catalyst was a
coupling partners were systematically varied and the yields and superior catalyst than the PdNP@GNF conned catalyst (Table
distribution of products compared for reactions catalysed by 3, entry 13 vs. 14).
PdNP@GNF nanoreactors and PdNP/GNF supported catalysts
(Scheme 1 and Table 3).
In order to investigate whether nanoscale connement
affected solely the reaction pathway of sterically hindered aryl
The effect of substituent position on the aryl iodide was iodides, we also explored the effect of the nature of the
studied by performing the reactions of 1-iodo-4-, 1-iodo-3- or 1- substituent of both the aryl iodide and the boronic acid and
iodo-2-nitrobenzene 1a–c with phenylboronic acid 2a (Table 3, thus the potential inuence of electronic effects on the cross-
entries 1 vs. 2, 11 vs. 12 and 13 vs. 14). Subtle enhancements in coupling reactions. Comparison of the catalysed reactions of 1-
the conversion and selectivity for the cross-coupled product in iodo-4-nitrobenzene 1a, iodobenzene 1d and 4-iodoaniline 1e
the reaction of the non-sterically hindered 1-iodo-4-nitroben- with 4-tolylboronic acid 2b (Table 3, entries 1 vs. 2, 15 vs. 16 and
zene 1a and 2a were observed for reactions catalysed by 17 vs. 18) evidences that connement does not change the
PdNP@GNF. Slight improvements in conversion were seen for pathways of these reactions, with activities following the known
the conned reaction of 1-iodo-3-nitrobenzene 1b and 2a. reactivity proles (i.e. NO2 > H > NH2) and selectivities accord-
However, the reaction of the sterically congested 1-iodo-2- ing to the selectivity-reactivity principle for cross-coupling
nitrobenzene 1c with 2a was seemingly more sensitive to cata- reactions51 using both PdNP@GNF and PdNP/GNF. Further
lyst location, with a subtle reduction in conversion and a evidence of the scarce inuence of both substituent position
notable decrease in selectivity for the corresponding cross- and electronic effect on the boronic acid was obtained by per-
coupled product 3ca observed for the conned reaction. Inter- forming the reaction of 1a with 3-methoxybenzene 2e (Table 3,
estingly, no effect of connement was observed when entry 9 vs. 10). We have observed that irrespective of the boronic
comparing the catalyst-conned and catalyst-supported reac- acid employed, neither the electronic or steric features led to
tions of 1-iodo-4-nitrobenzene 1a with 4-, 3- and 2-tolylboronic any signicant changes in the properties of the reaction
acids 2b–d (Table 3, entries 3 vs. 4, 5 vs. 6 and 7 vs. 8). It is induced by connement. It is interesting to note that Ullmann
important to note that, although the reaction of bulky 2-tol- homo-coupling products 5a–b were also detected in our explo-
ylboronic acid 2d was less selective and proceeded more slowly ration of catalyst properties; however, these products are
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generally only present in minor quantities, particularly for the halide), which undergo further transformations, the rate and
reactions of activated and non-bulky aryl iodides 1a–b, and their nature of which are inuenced by the experimental conditions,
formation critically is seemingly unaffected by nanoscale the concentrations of specic components of the reaction
connement in nanoreactors.
mixture and the properties of the catalyst.
In order to rationalise the behaviour observed in our study, it
As the catalytic centre in PdNP@GNF is located at the internal
is important to carefully consider the mechanism of the palla- step-edge, it follows logically that the rst step of the catalytic
dium nanoparticle-mediated Suzuki–Miyaura cross-coupling cycle, the oxidative addition, must take place at the nanoparticle–
reaction. Whilst the specic nature of the active species and nanobre interface. Here, p–p interactions between the
mode of operation remains somewhat controversial in the incoming reactant and the graphitic surfaces of the nanoreactor
scientic literature,52 with leading researchers providing step-edge are prevalent and facilitate high local concentrations
contradictory evidence to support homogeneous53–55 and and pre-organisation of the substrate prior to oxidative addi-
heterogeneous56–58 mechanistic pathways, it is becoming tion.24–27 Our measurements provide compelling evidence of the
increasingly accepted that nanoparticle-catalysed cross- prevailing role of the oxidative addition step in the nanoparticle-
coupling reactions follow a quasi-homogeneous pathway,59 mediated Suzuki–Miyaura cross-coupling reaction. We propose
wherein the catalytic cycle is initiated by the oxidative addition that the observed increase in activity in nanoreactors for non-
of low coordinate surface Pd atoms to the aryl halide reactant. sterically demanding aryl iodide substrates (Table 3, entries 1 vs.
This results in leaching of soluble palladium species into 2 and 3 vs. 4) is associated with enhanced concentration of both
solution, such as [ArPdX] and [ArPdX2]ꢀ (where Ar ¼ aryl, X ¼ aryl iodide reactants and soluble palladium species at the
Scheme 2 The fate of the intermediate following oxidative addition yields the products of cross-coupling or dehalogenation depending on the
competitive reactions with boronic acid or methanol respectively. The dehalogenation reaction is initiated by displacement of iodide from the
oxidative addition intermediate by methanol. Subsequent to deprotonation, b-hydride elimination affords the [ArPdH] intermediate from which
the product of dehalogenation ArH is obtained.64 It is important to note that both mechanistic pathways result in the elimination of Pd(0), which is
retained within the nanoreactor channel and re-deposits onto existing nanoparticles confined at the internal step-edges. Thus, the observed
Ostwald ripening of nanoparticles subsequent to catalysis provides further evidence supporting the quasi-homogeneous nature of nanoparticle-
catalysed reactions.
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nanobre step-edge; conversely, the conversion for reactions leached oxidative addition intermediate remains trapped in the
performed outside nanobres is higher for more sterically-con- restricted volume of the step-edge and therefore its coordination
gested structural analogues (Table 3, entry 13 vs. 14) due to to the incoming boronic acid becomes partially impeded.
unrestrained aryl concentration outside the nanobres. More Consequently, rapid diffusion of methanol to the step-edge, due
interestingly, the selectivity for the products of cross-coupling to its reduced molecular size becomes more favourable,
3ba and 3ca and dehalogenation 4 also appears to be dependent promoting the formation of the product of dehalogenation
on the steric properties of the aryl iodide reactants, with more (Scheme 2).60–64
congested aryl iodides promoting the dehalogenation process,
Indeed, for highly congested step-edge environments, such
particularly for the cross-coupling of bulky 1-iodo-2-nitrobenzene as those using PdNP@GNF-IV, only the product of dehaloge-
1c (Table 3, entries 11 vs. 12 and 13 vs. 14). This unexpected nation is observed (Table 2, entry 5). This provides further
behaviour is attributed to signicant molecular crowding for evidence for the importance of the step-edge environment in
sterically congested aryl iodides inside nanoreactors. Here, the modulating the pathways of chemical reactions, with the rate of
Scheme 3 The competitive Suzuki–Miyaura reactions of (a) two aromatic boronic acids with a common aryl iodide and (b) two aryl iodides with a
common aromatic boronic acid.
Table 4 Comparison of the activity and selectivity of PdNP nanoreactors (PdNP@GNF) and supported PdNP catalysts (PdNP/GNF) in the
competitive Suzuki–Miyaura cross-coupling reactionsa
Reactant II
Reactant
I
Selectivity for cross-coupled productsb/%
(C : D)
Entry
Catalyst
A
B
Conversionb/%
1
2
3
4
5
6
7
8
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
PdNP@GNF
PdNP/GNF
1a
1a
1a
1a
2b
2b
2b
2b
6
6
6
6
10
10
10
10
7
7
63
63
67
68
94
89
52
53
28 : 72 (8 : 9)
28 : 72 (8 : 9)
2a
2a
11
11
1d
1d
63 : 37 (8 : 3aa)
64 : 36 (8 : 3aa)
15 : 85 (12 : 13)
42 : 58 (12 : 13)
100 : 0 (12 : 3db)
80 : 20 (12 : 3db)
a
Standard conditions: 1-iodo-4-nitrobenzene 1a, 1-naphthalene boronic acid 6, 2-naphthalene boronic acid 7, phenylboronic acid 2a, 4-tolylboronic
acid 2b, 1-iodonaphthalene 10, 2-iodonaphthalene 11 and iodobenzene 1d (0.065 mmol, 1 eq.), sodium acetate (0.15 mmol, 2.3 eq.), catalyst (2
b
1
mol%), methanol (5 mL), 70 ꢁC, 24 h. Determined by H NMR spectroscopy of the crude mixture.
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conned reactions determined by oxidative addition and the palladium nanoparticles within the interior of graphitised
selectivity critically dependent on the steric properties of the nanobres and the resultant catalytic nanoreactor exhibited
aryl halide. This implies that, subsequent to oxidative addition, attractive catalytic properties in the Suzuki–Miyaura cross-
further reactions do not occur within the step-edge, but rather coupling reaction. A slight modication of this procedure
within the internal cavity of the nanoreactor. In this larger allowed us to prepare relevant catalysts with nanoparticles
reaction volume, the pathway of the reaction will therefore be deposited on the exterior of the nanoreactors in order to prop-
unaffected by the steric properties of the boronic acid and erly investigate the effect of connement in cross-coupling
hence the effects of connement on activity and selectivity will reactions. Through systematic comparison of the yields and
not be observed.
distributions of products of reactions using a series of
To further probe this phenomenon, we performed a series of substituted aryl iodides and boronic acids, we have demon-
competitive Suzuki–Miyaura reactions, where either two strated that the activity and selectivity of the conned reactions
aromatic boronic acids compete for a common aryl iodide or are sensitive to the steric congestion of the aryl iodides, whereas
two aryl iodides for a common boronic acid (Scheme 3 and no inuence of connement was observed for the reaction of
Table 4).
bulky boronic acids. The competitive Suzuki–Miyaura reactions,
The reaction of 1-iodo-4-nitrobenzene 1a and either 1- or 2- where two reagents compete for a common coupling agent,
naphthalene boronic acids, 6 and 7 respectively, exhibited the indicated that the connement effects are only prevalent for two
same conversion of 1a and selectivity ratio between the afforded aryl iodides competing for one boronic acid and reside in the
cross-coupled products 8 and 9 using both the PdNP catalytic mechanism of conned cross-couplings. We have shown that
nanoreactor and the supported catalyst (Table 4, entry 1 vs. 2). the oxidative addition step of the catalytic cycle occurs at the
Furthermore, the cross-coupling of 1a with either 1-naphthalene GNF step-edge, where the local concentration and organisation
or phenylboronic acids, 6 or 2a respectively, was similarly unaf- of the aryl iodide reactant are critically inuenced by nanoscale
fected by connement (Table 4, entry 3 vs. 4). These results connement, with the reactions of non-sterically demanding
conrm that connement effects are independent of the nature substrates promoted in this unusual reaction environment.
of the boronic acid. In contrast, when two aryl iodides compete Controlling the properties of organic transformations utilising
for a common boronic acid, signicant effects induced by the effects of catalyst connement in graphitised nanobres is
connement were observed. Whilst the reactions of 4-tolylbor- becoming an increasingly versatile tool for synthetic chemistry,
onic acid 2b and a mixture of 1- and 2-iodonaphthalenes, 10 and particularly as the unique step-edge reaction site has the
11 respectively, showed comparable conversion of 2b irrespective potential to enhance both the activity and selectivity of the
of the catalyst location, connement of the catalyst resulted in a catalyst and promotes the recycle and reuse of the material
2.8-fold decrease in selectivity towards the formation of the cross- multiples times with retention of elected catalyst properties.
coupled product with bulky 1-iodonaphthalene 10 compared to
the reaction catalysed by PdNP/GNF (Table 4, entry 5 vs. 6). In
addition, comparison of the properties of the conned and
Acknowledgements
supported reactions of 4-tolylboronic acid 2b with both 1-iodo- The authors would like to thank the European Research Council
naphthalene 10 and iodobenzene 1d (Table 4, entry 7 vs. 8), (ERC), the Engineering and Physical Sciences Research Council
indicated that a higher selectivity for the cross-coupling of bulky (EPSRC), the Italian Ministry of Education, University and
1-iodonaphthalene 10 was afforded. Clearly, the interactions of Research (MIUR, PRIN 2010–2011 – prot. 2010N3T9M4) for
the naphthalene system with the graphitic surfaces became more supporting this research. BC is grateful for nancial support
important and maximal at the step-edges where the high local from “Italian-French University” (Vinci 2011), University of
concentration of 1-iodonaphthalene 10 promoted a 6.7-fold Chieti-Pescara, “Centre National de la Research Scientique
increase in the selectivity towards the formation of the cross- (CNRS)” and University of Reims. The authors thank the Not-
coupled product 12 within the catalytic nanoreactor (Table 4, tingham Nanotechnology and Nanoscience Centre (NNNC) for
entry 5 vs. 7) in contrast to the 1.9-fold increase obtained for the access to transmission electron microscopy facilities.
reaction catalysed by supported PdNP (Table 4, entry 6 vs. 8).
These results validate the hypothesis that the nature of the aryl
iodides in conned reactions is crucial only if the oxidative
Notes and references
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