Pd nanoparticle-silica nanotubes (Pd@SNTs) as an efficient catalyst for Suzuki-Miyaura coupling and sp2 C-H arylation in water

Article abstract of DOI:10.1039/c3gc41672k

Silica nanotubes (SNTs) functionalized with Pd-NPs on the inner surface performed as efficient nano-reactors for C-C coupling in water; the nano-confinement offers minimized leaching of Pd and yet efficient mass transfer for Suzuki-Miyaura coupling and C-H arylation of thiazoles in water with very high TON.

Full text of DOI:10.1039/c3gc41672k

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Pd nanoparticle-silica nanotubes (Pd@SNTs) as an
efficient catalyst for Suzuki–Miyaura coupling and
sp2 C–H arylation in water†
Cite this: Green Chem., 2013, 15, 3468
Ginam Park,a Sanghee Lee,b Sang Jun Son*b and Seunghoon Shin*a
Received 16th August 2013,
Accepted 2nd October 2013
Silica nanotubes (SNTs) functionalized with Pd-NPs on the inner surface performed as efficient nano-reac-
tors for C–C coupling in water; the nano-confinement offers minimized leaching of Pd and yet efficient
mass transfer for Suzuki–Miyaura coupling and C–H arylation of thiazoles in water with very high TON.
DOI: 10.1039/c3gc41672k
www.rsc.org/greenchem
Recently, silica nanotubes (SNTs) have gained a great deal of
Herein we disclose a synthesis of Pd-nanoparticles immobi-
interest due to their multi-faceted applications in sensors, tar- lized on SNTs (Pd@SNTs) and their applications in Suzuki–
geted delivery of genes and drugs, hydrogen storage and bio- Miyaura coupling and sp2 C–H arylation of thiazoles in neat
separation.1 On the other hand, immobilization of catalytically water as the benchmark reactions. The Pd@SNT nanocompo-
active transition metals on the heterogeneous support has site described herein showed excellent catalytic activity with
drawn significant interest.2 The combination of these 910 000 TON and maintains stability toward the additives such
approaches, i.e. employing SNTs as a catalyst support,3 would as bases or oxidants and against leaching under the reaction
be highly promising because of their ultra-high surface area, conditions. Furthermore, we presented preliminary results on
tunable size and ease of differential functionalization of inner/ fine-tuning the heterogeneous catalyst by straightforward
outer surfaces of SNTs.4 The latter aspect is particularly modification of inner channels to further optimize its catalytic
appealing for catalytic applications in water,5 because the efficiency.
tailor-made inner surface of SNTs can provide a catalytic site
We turned to SNTs grown on anodized aluminum
for organic substrates while the unmodified outer silica oxide (AAO)9 because of the advantages over soft template
surface ensures phase compatibility as a colloidal suspension methods,3,10 such as uniform and precisely controllable
in water. While cheap, non-toxic and non-flammable, the dimension, ease of functionalization and mechanical strength
greenness of water as a reaction medium remains a debatable that are ideally suited for catalytic applications. The prepa-
issue. However, aqueous phase organometallic catalysis offers ration of Pd@SNT nanocomposite is schematically illustrated
a facile separation of catalyst and organic product phases and in Scheme 1.11 After deposition of a silica layer on AAO by a
furthermore shows enhanced activity and selectivity in a sol–gel method, the inner surface of SNTs was functionalized
number of cases.13
by soaking it in a mixture of toluene–diethylene triamine
Particularly a semi-heterogeneous nanoparticle system that
does not require expensive supporting ligands or a capping
agent, such as imidazole, N-heterocyclic carbene (NHC), phos-
phines or dendrimers, is highly desirable.6,7 Nevertheless,
achieving both high catalytic activity and easy catalyst separ-
ation/reuse in aqueous media without significant catalyst leach-
ing8 still remains a prominent challenge and will contribute
towards greener C–C bond formations.
aDepartment of Chemistry and Institute for Natural Sciences, Hanyang Unversity,
17 Haengdong-dong, Seongdong-gu, Seoul 133-791, Korea.
E-mail: sshin@hanyang.ac.kr
bDepartment of Chemistry, Gachon University, Bokjeong-dong, Sujung-gu, Seongnam,
Gyeonggi-do 461-701, Korea. E-mail: sjson@gachon.ac.kr
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3gc41672k
Scheme 1 Preparation of Pd@SNTs and their TEM image.
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Table 1 Optimization of Suzuki–Miyaura coupling conditionsa
Table 2 Suzuki–Miyaura coupling using Pd@SNT in watera,b
Pdb
(mol%)
Yieldc
(%)
SNT
Entry
Base
Condition
(by TEM)d
1
2
3
4
5
6
7
0.001
0.001
0.001
0.001
0.001
0.1
K2CO3
K2CO3
K2CO3
NaHCO3
Et3N
NaHCO3
NaHCO3
25 °C, 30 h
80 °C, 2 h
105 °C, 0.5 h
80 °C, 5.5 h
80 °C, 30 h
80 °C, 1.5 h
80 °C, 36 h
70
81
87
83
0
99
91
—
X
X
O
—
O
O
0.0001e
3ba (6 h, 90)c
3ca (8 h, 89)c
3ab (6 h, 98)c
3ac (12 h, 90)c
3da (2 h, 82)c
3cc (15 h, 96)d
3ad (4 h, 91)d
3bd (8 h, 84)c
3cd (4 h, 78)d
3dc (10 h, 90)d
3ea (10 h, 81)c
3ae (28 h, 78)d
3ee (14 h, 65)d
3af (4 h, 97)d
3ag (24 h, 94)d
3ah (5 h, 98)d
3ai (18 h, 81)d
3df (5 h, 89)d
3cg (16 h, 94)d
3dg (15 h, 84)d
a Conditions: Phenylboronic acid (1a, 1.2 equiv.), 4-iodophenol (2a, 1.0
equiv.), 3.0 equiv. of base and Pd stock solution in water (0.1 M with
respect to 2a). b Pd loading. c Isolated yields of 3aa. d Morphology of
the SNT after reaction (X: disrupted; O: intact). e APTES-Pd@SNTs were
used.
a Arylboronic acids 1 (1.2 equiv.), aryl halide 2 (1.0 equiv.), NaHCO3
(3.0 equiv.) and Pd stock solution in water (0.1 M). b Isolated yield after
chromatographic purification. c 0.01 mol% Pd. d 0.1 mol% Pd.
(DETA)–diisopropylethyl amine (DIEA), followed by treatment
with a solution of Na2PdCl4, and then baking at 120 °C for the
thermal reduction.10c Dissolving the template by phosphoric
acid produced Pd@SNTs which were collected by filtration and
were suspended in water. HR-TEM (transmission electron
microscopy) images of the thus formed Pd@SNT composite
(Scheme 1) showed that the SNTs had a uniform dimension of
70 nm (i.d.) × 3.0 μm (length) and the Pd nanoparticles were
evenly dispersed on the inner surface of SNTs and had an
average diameter of 3.6 nm 0.9. For catalytic reactions, the
Pd@SNT stock solution was prepared in water at a con-
centration of 1.0 × 103 ppm Pd (ICP-MS).
The catalytic performance of Pd-nanoparticles deposited
inside the SNTs (Pd@SNTs) was first tested in Suzuki–Miyaura
reaction employing phenylboronic acid (1a) and 4-iodophenol
(2a) as substrates in neat water (Table 1). The aqueous suspen-
sion of Pd-SNTs (0.001 mol% Pd) with 3 equiv. of K2CO3 gave
the product 3aa in 87% after 30 min at 105 °C (entry 3). In con-
trast to the stability of Pd@SNTs in acidic medium, as indi-
cated in its preparation, the morphology of SNTs by TEM
analysis after this run showed significant disruption of SNTs
with leaching of Pd nanoclusters outside of SNTs under these
highly basic conditions. Gratifyingly, NaHCO3 as a base did
not show any disruption of SNTs or aggregation of Pd after the
reaction (Fig. S2 in the ESI†) and was chosen as an optimal
base (entry 4). After further optimization of the inner surface
of SNTs (vide infra, APTES-modification), TON of 9.1 × 105 and
TOF of 2.5 × 104 h−1 were observed (entry 7).12
We continued to inspect the generality of this catalytic
system under Suzuki–Miyaura reaction conditions (Table 2).
The reactions of aryl boronic acids (1a–e) with aryl
iodides (2a–e) and bromides (2f–i) employing Pd@SNTs
(0.1–0.01 mol% Pd) as a catalyst underwent smoothly at
80 °C in water. Most water-insoluble substrates completely
solubilized during the progress of the reaction at 80 °C. Inter-
estingly, even hydrophobic aryl halides that remained insolu-
ble in neat water (i.e. on-water13) throughout the reaction
showed satisfactory conversions (>95%). However, aryl
chlorides were incompetent to react under the current
conditions.14
Table 3 C–H arylation using Pd@SNT in watera,b
5aj (85)
5bl (70)
5ar (85)
5ao (82)
5al (95)
5ck (99)
5bj (79)
5aq (63)
5an (72)
5ak (91)
5cj (54)
5as (75)
5ap (67)
5am (93)
5cl (82)
a Thiazoles 4 (1.0 equiv.) and aryl iodides 2 (1.2 equiv.) in water
(0.1 M). b Isolated after chromatographic purification.
Interestingly, the high reactivity of the Pd@SNT system was
also observed in a direct C–H functionalization/cross coupling
of thiazoles in water. To the best of our knowledge, sp2 C–H
functionalization occurring in water employing a hetero-
geneous catalyst is unprecedented.15 As reported by Greaney
et al. using a homogeneous system,15c omission of phosphine
or silver salts led to no reaction or inferior conversion
(Table S1†). Notably, the use of Pd@SNTs (0.5 mol% Pd) gave a
comparable conversion to that of a homogeneous system at
higher loading (PdCl2(dppf), 5 mol%). Use of neat water
proved beneficial compared to acetonitrile (91% of 5ak in
water vs. 14% of 5ak in CH3CN).13 The generality of the C–H
arylation of thiazoles is demonstrated in Table 3 and the scope
of aryl iodides covers electron-rich, electron-poor and sterically
demanding ones. Notably, even a hydrophobic substrate such
as 2s underwent an efficient cross-coupling in water to give
5as.
To investigate the possibility that the catalytic activity is
manifested by the homogeneous Pd ions leached out of SNTs,
we performed ES-MS analysis of the Suzuki coupling mixture
of aryl bromides: we could not observe any species that have a
characteristic isotope pattern corresponding to PdBr2 or
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Table 4 ICP-MS measurement for determination of Pd leaching
Before Suzuki
reactiona
After Suzuki
reactiona
[Pd] (with no pre-treatment)b
[Pd] (with pre-treatment)b,c
Total Pd leachingd
33.6 ppb
58.3 ppb
0.58 wt%
165.4 ppb
189.8 ppb
1.9 wt%
a Suzuki–Miyaura reaction was conducted employing 1a and 2a and
0.1 mol% (10 ppm) of Pd@SNTs; the error limit of ICP-MS
measurement was less than 0.02 ppb. b The solution containing
Pd@SNTs was filtered through a Millipore® filter (pore size of
110 nm) before and after the Suzuki reaction. c The filtrate was pre-
treated with 2 mL of HF and 5 mL of HCl–HNO3 (3 : 1) and the
resulting solution was heated at 180 °C for 3 h before ICP-MS analysis.
d The total Pd leaching (%) as soluble Pd ions and Pd nanoclusters.
Fig. 1 Tests for heterogeneity: (a) three-phase test, (b) hot-filtration test and
the Suzuki reaction. Most probably, this amount resulted from
the catalytically inactive (see Fig. 1b), soluble Pd-nanoclusters
liberated from SNT which could not be detected by ICP-MS
without acid pre-treatment.
(c) Hg-poisoning test.
−
PdBr3 either in positive or in negative mode. To further
From these experiments, it is clear that the catalytic reac-
confirm the absence of Pd leaching out of SNT, we tested the tion occurred on the inner surface of Pd@SNT (Fig. 1a).
following experiments shown in Fig. 1.8b In a three-phase test However, the distinction between homogeneous and hetero-
(Fig. 1a),16 Wang resin-bound aryl iodides would not react with geneous mechanisms is not trivial,8,19 i.e. the Pd-NP could
aryl boronic acids unless the catalytic Pd species leach out of merely serve as a reservoir of the catalytically active Pd ions in
SNTs, because a larger diameter (37–74 μm) of the resin would a homeopathic amount19c or the Pd-nanoparticle itself could
not diffuse inside SNTs (70 nm, i.d.).11 Indeed, the resin- catalyse the C–C coupling at the low coordination number
bound 4-iodobenzoic acid derivatives failed to react with 1a–c vertex and edge atoms. The ICP-MS data in Table 4 indicated
with Pd@SNTs (0.1 mol% Pd) and both 1a–c and 4-iodo- that a small amount (<2%) of water-soluble Pd ions (or
benzoic acid were recovered unreacted after treatment with colloidal Pd) may form, but they were shown to be catalytically
CF3COOH. A control experiment with Pd2(dba)3 (1 mol% Pd) inactive (hot-filtration test in Fig. 1b). Furthermore, higher
in DMF showed uneventful coupling in 61% yield of 3ad after rates were observed as the concentration of Pd@SNT increased
detachment from the resin.11 In the hot filtration test,17
a
in the range of 0.0001–1 mol% Pd, contrary to the monomeric
mixture of 1a and 2a in the presence of Pd@SNT (0.005 mol% Pd system.19c These, taken together, point to the Pd-NP as the
Pd) in water at 80 °C was hot-filtered (to avoid Pd agglomera- active catalyst for the Suzuki coupling. These suggest that the
tion) after 20 min incubation time through a 110 nm effective suppression of leaching of Pd-NPs without the need
Millipore® membrane (Fig. 1b). The filtrate was incompetent for supporting ligands7c–f was presumably due to spatial con-
to provide further conversion, while the undisturbed control finement of Pd-NPs within the SNTs and their protection from
showed a continuous increase in the Suzuki coupled product, convectional and shear forces while stirring and heating.
which indicated that the filtrate solution did not contain a
To examine the ability of organic substrates to diffuse
catalytically relevant Pd species. When a mercury droplet18 was inside SNTs, we prepared Pd@SNTs with longer (20 μm) and
added to the reaction mixture of 1a, 2a and Pd@SNT shorter (0.5 μm) lengths of SNTs than standard 3.0 μm SNTs
(0.005 mol% Pd) in water at ∼50% conversion, an immediate and their catalytic rates were compared.11 The Pd@SNTs of
suppression of activity was noted, pointing to the hetero- 0.5 μm length showed significant leaching of Pd even by TEM
geneous nature of the present system (Fig. 1c).8a
inspection (Fig. S3†). On the other hand, the Pd@SNTs of
The amount of the leached Pd was quantitatively measured 20 μm length showed only ∼20% decrease in the catalytic
by ICP-MS analysis (Table 4). The Pd@SNT solution in water efficiency compared to the 3 μm Pd@SNTs (both at 0.005 mol%
was prepared at a concentration of 10 ppm Pd and the suspen- Pd), further supporting an effective diffusion of substrates
sion was filtered through Millipore® before and after the toward the Pd-NPs residing in a deeper inner channel of SNTs
Suzuki reaction. To accurately determine the amount of total (Fig. S4†).
Pd both as ionic and soluble nanocluster forms, the filtrate
One potential advantage of the Pd@SNT system is the ease
was pre-treated with HF–HCl–HNO3. The result showed that of surface modification in a fashion similar to an optimization
0.58 wt% Pd leached before the reaction and 1.9 wt% Pd after process in homogeneous catalysis, such as with regard to sol-
the Suzuki coupling, respectively. The difference in the vents and ligands, which is generally a difficult task for a
measured amount of Pd between those with and without acid heterogeneous catalytic system. We have chosen three different
pre-treatment was almost identical (∼25 ppb) before and after surface modification agents having distinct surface
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Fig. 3 (a) Comparison of the rate of Suzuki-coupling (1a and 2a). (b) The initial
rates of Suzuki coupling vs. the contact angles of the modified glass surfaces.
910 000 (entry 7, Table 1). The C–H arylation (72% 5ck with
APTES-Pd@SNT vs. 54% 5ck with unmodified Pd@SNTs) was
also significantly accelerated by the APTES modification
(Table S8†).
The rate enhancement in APTES-Pd@SNTs could be attribu-
ted mostly to the smallest average size of Pd-NPs and thus the
highest number of Pd atoms at the edges and vertexes.6a,b The
observed rate enhancement in PEG- and C18-Pd@SNTs,
having more hydrophilic and hydrophobic inner surfaces,
respectively, than pristine Pd@SNTs is not well-understood at
this point. Nevertheless, it could be due to a change in
effective micromolarity of catalytic Pd species and/or sub-
strates on the silica inner surface.23 In addition, the amino
group (APTES) or ethylene glycol unit (PEG) might have func-
tioned as an anchoring ligand for Pd-NPs inside SNTs.7g
Although, further studies must be awaited to systematically
evaluate these factors and to further improve the catalytic
activity, it is clear that the performance of the catalyst could be
tuned by straightforward modification of the inner surface of
Pd@SNTs.
Lastly, catalyst recovery was tested with the APTES-modified
Pd@SNTs (0.1 mol% Pd) employing 1a and 2a as substrates
and using three different methods, i.e. Method A: filtration
through a Millipore® filter, Method B: precipitation by ultra-
centrifugation (5000 rpm, 10 min) and re-suspension and
Method C: extraction of the aqueous layer with ether. The first
two methods resulted in a partial loss of catalytic activity after
the 4th and 5th cycle, presumably due to the adsorption
of Pd@SNTs inside the polymeric filter and precipitation of
SNTs during ultracentrifugation, respectively. Among these
methods, Method C allowed the most efficient recycling of
catalytic aqueous layer and at the 10th cycle, the recovered
aqueous phase retained an excellent catalytic activity produ-
cing 89% of 3aa in 2 h (Table S9†).
Fig. 2 TEM images and size distribution of modified Pd@SNTs: (a) APTES–
Pd@SNTs. (b) PEG–Pd@SNTs. (c) C18–Pd@SNTs (scale bars: 100 nm). Average
size of particles: (a) 2.20 0.86 nm, (b) 3.64 0.89 nm and (c) 3.65 0.95 nm.
characteristics, i.e. APTES [(3-aminopropyl)triethoxy silane)],
PEG (2-[methoxy-(polyethylenoxy)propyl]trimethoxy silane)
and C18 (trimethoxy octadecyl silane).11 AAO-bound Pd@SNTs
prepared as above were treated with these surface modifying
reagents and were baked at 120 °C before being dissolved
out of the AAO-template to finally provide each modified
Pd@SNTs (denoted as APTES-, PEG- and C18-Pd@SNTs,
respectively). The TEM analysis showed that there was signifi-
cant size re-distribution of the Pd-NPs during the modification
proess (Fig. 2), which presumably resulted from an equili-
brium between soluble Pd ions with Pd-NPs8b or via assembly–
deassembly between Pd-NPs with a relatively low melting
temperature.20
The modified Pd@SNTs were characterized as follows: the
absence of primary amino groups inside the unmodified
Pd@SNTs was first checked by the Kaiser test (Fig. S5†),21 indi-
cating that primary amines from DETA were consumed during
the Pd-NP formation at 120 °C. Thus the primary amino
residues in APTES-Pd@SNTs, as observed by an optical
microscopy with the aid of a fluorescence probe (Fig. S6†),22
originated solely from the ATPES-modification. To further
interrogate the inner-surface property of the modified
Pd@SNTs, a plane glass functionalized with Pd-NPs (denoted
as Pd@glass, in comparison to Pd@SNT) was treated with the
above modifying agents to simulate the modified inner sur-
faces of SNTs.11 The measurement of their contact angles
showed the expected order of hydrophilicity: APTES-Pd@glass
Conclusions
(θ
= 27.76°), PEG-Pd@glass (θ = 45.06°), followed by
C18-Pd@glass (θ = 75.40°), which is in general accord with the
observed order of initial rates (at 5 min) of Suzuki coupling
using these modified Pd@SNTs (all at 0.005 mol% Pd load-
ings) (Fig. 3). Among these, the APTES-Pd@SNTs showed ca.
×1.8 fold increase in the rates of Suzuki–Miyaura coupling rela-
tive to the unmodified Pd@SNT with a maximum TON of
In summary, we have described herein preparation of Pd nano-
particles imbedded inside silica nanotubes (Pd@SNTs) and
their applications in cross-coupling reactions in water without
organic co-solvents. The current Pd@SNT system opens an
exceptional opportunity for heterogeneous catalysis in water in
that it affords an effective mass transfer of hydrophobic
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substrates into the inner channel of SNTs, excellent catalytic
efficiency and yet prevents leaching in water. Also, the catalytic
inner surface could be further tailored to optimize its catalytic
activity. Recyclability of Pd-NPs as well as the first sp2 C–H
functionalization of heteroarenes in water employing the
heterogeneous system has been demonstrated.
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Acknowledgements
The authors thank the Pioneer Research Center Program
(2011-0002138, MEST) and the NRF (NRF-2010-0022936, 11 See ESI† for details.
NRF-2012-015662 and NRF-2012M3A7B4049653) for generous 12 For review of Suzuki reactions in water:
a
financial support. This paper is dedicated to Professor
Junghun Suh on the occasion of his honourable retirement.
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