A highly Lewis-acidic Pd(iv) surface on Pd@SiO2 nanocatalysts for hydroalkoxylation reactions

Article abstract of DOI:10.1039/c4cc05112b

The Pd(iv) species, known to be critical intermediates in homogeneous catalysis, were successfully generated on the surface of Pd nanocatalysts via oxidation with iodobenzene dichloride (PhICl₂) or N-chlorosuccinimide (NCS). In particular, the

Full text of DOI:10.1039/c4cc05112b

Volume 50 Number 95 11 December 2014 Pages 14907–15110
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Hyunwoo Kim, Hyunjoon Song et al.
A highly Lewis-acidic Pd(IV) surface on Pd@SiO nanocatalysts for
2
hydroalkoxylation reactions

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A highly Lewis-acidic Pd(IV) surface on Pd@SiO2
nanocatalysts for hydroalkoxylation reactions†
Cite this: Chem. Commun., 2014,
0, 14938
5
Mijong Kim,‡ Suyeon Lee,‡ Kiseong Kim, Dongwoo Shin, Hyunwoo Kim* and
Received 3rd July 2014,
Accepted 15th September 2014
Hyunjoon Song*
DOI: 10.1039/c4cc05112b
www.rsc.org/chemcomm
22
The Pd(IV) species, known to be critical intermediates in homo- by the Pt(II) or Pt(IV) species on the NP surface. It appeared that
geneous catalysis, were successfully generated on the surface of Pd on a Pd surface a species that was more Lewis-acidic than Pd(II)
nanocatalysts via oxidation with iodobenzene dichloride (PhICl
N-chlorosuccinimide (NCS). In particular, the Pd@SiO yolk–shell existence of the Pd(IV) species on the Pd NP surface is question-
nanocatalyst activated by PhICl showed high reactivity and super- able because in homogeneous catalysis they are considered to be
ior stability to the other Pd-based catalysts for hydroalkoxylation transient intermediates in Pd(II)-catalyzed C–H functionalization
2
) or could be generated, such as the Pd(IV) species. However, the
2
2
2
5–27
reactions even at 25 8C.
reactions.
Thus, we have investigated the hydroalkoxylation
yolk–shell nanocatalysts under
reaction using Pd and Pd@SiO
2
Well-defined nanostructured materials have emerged as a new oxidative conditions in order to identify the actual active species
class of heterogeneous catalysts, the reactivity and selectivity of on the surface. In the present study, we envisioned that (a) the
which can be rationally modulated by their size, surface structure, Pd(IV) species were predominantly generated on the surface via
1–4
and composition.
2
For instance, the heterogeneous Pd species the oxidation with PhICl ; (b) the surface species were effectively
have been demonstrated to be effective catalysts in a variety of modulated via the oxidation with N-chlorosuccinimide (NCS) and
5
,6
7,8
reaction systems, including C–C bond coupling and C–H bond PhI(OAc)
2
, which largely changed the catalytic activity; and (c) the
nanocatalysts with surface oxidation were the only Pd
9
functionalization reactions. Such efficient nanoscale heterogeneous Pd@SiO
2
1
0–15
catalysts are now classified as ‘‘nanocatalysts’’.
various designs of nanocatalysts, metal/metal oxide bicomponent
systems, i.e. Pt@SiO core–shell nanoparticles, can successfully precursor, Pd(acac) , in the presence of oleylamine and trioctyl-
Among the catalysts active for the reactions at 25 1C.
Pd NPs were synthesized via thermal decomposition of a Pd
2
2
28
mimic bifunctional heterogeneous catalysts, and have exhibited phosphine. The particles were spherical with an average particle
16,17
remarkable stability in high temperature reactions.
also introduced Ni@SiO and Pd@SiO yolk–shell nanostructures was formed using the water-in-oil microemulsion method, and by
which, as highly active catalysts for hydrogen transfer and Suzuki partial dissolution of the silica shells under hydrothermal condi-
coupling reactions, showed higher stability and reusability than tions. After heat treatment under H , the resulting yolk–shell
However, the structure showed an average diameter of 30 Æ 2.6 nm, bearing
active species of the nanocatalysts were limited to only a neutral Pd cores with an average size of 5.1 Æ 0.4 nm and large vacancies
We have diameter of 5.0 Æ 0.3 nm (Fig. 1a). The Pd@SiO
2
yolk–shell structure
2
2
2
18,19
their homogeneous counterparts could achieve.
5
–9,20
state, metal(0),
high-valent transition metals.
Recently, Toste, Somorjai, and coworkers disclosed a new treated in situ with PhICl
concept of applying heterogeneous catalysts to known homo- 2-phenylethynylphenol (1) as shown in Fig. 1c.
geneous catalytic reactions through the generation of electro- A survey of homogeneous and heterogeneous reactions
They at elevated temperature (140 1C) is summarized in Table 1.
showed that p-bond activation reactions were efficiently promoted Consistent with previous reports, homogeneous Pd(OAc) and
albeit numerous catalytic reactions involved in the SiO shells, identified in the transmission electron micro-
2
21
scopy (TEM) image (Fig. 1b). The Pd and Pd@SiO NPs were
2
2
, and used for hydroalkoxylation of
2
2–24
philic surfaces on noble metal nanoparticles (NPs).
2
PdCl resulted in the formation of only 10 and 7% of 2-phenyl-
benzofuran (2), respectively (Table 1, entries 1 and 2). When we
2
Department of Chemistry, Korea Advanced Institute of Science and Technology, and
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (ibs),
Daejeon, 305-701, Korea. E-mail: hwkim@kaist.edu, hsong@kaist.ac.kr
used heterogeneous Pd catalysts, Pd and Pd@SiO yolk–shell NPs,
2
both catalysts were inactive in the absence of PhICl
2
. (Table 1,
†
1
Electronic supplementary information (ESI) available: Experimental, XPS, and
entries 3 and 5). However, in the presence of 25 mol% of PhICl ,
2
H NMR spectra of 1, 2, and 3. See DOI: 10.1039/c4cc05112b
‡
M.K. and S.L. contributed equally to this work.
the Pd catalyst (1 mol%) successfully promoted the reaction with
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Fig. 1 TEM images of (a) Pd NPs and (b) Pd@SiO yolk–shell nanocatalysts,
and (c) a reaction scheme for hydroalkoxylation of 2-phenylethynylphenol
1) to 2 and electrophilic chlorination of 2 to 3 (see text).
(
Table 1 Hydroalkoxylation of 1 using homogeneous and heterogeneous
a
Pd catalysts at 140 1C
b
b
Catalyst
Oxidant
Cyclization (%) Conversion (%)
No. (1 mol%) (X mol%)
(2 + 3)
(2 : 3)
Fig. 2 XPS spectra of the Pd 3d level (pink) and deconvolution peaks of
the Pd species, Pd(0) (black), Pd(II) (red), and Pd(IV) (blue); (a) for pristine
1
2
3
4
5
6
7
8
9
Pd(OAc)
PdCl
Pd NPs
Pd NPs
Pd@SiO
Pd@SiO
Pd@SiO
Pd NPs
2
—
—
—
PhICl
—
10
7
0
100
0
100
0
10 : 0
7 : 0
—
80 : 20
—
76 : 24
—
Pd NPs and (b) for Pd NPs after treatment with PhICl
for 1 h; fraction of the Pd species after treatment with variable amounts of
PhICl (c) for 1 h and (d) for 12 h; XPS spectra of the Pd 3d level for Pd NPs
after treatment (e) with PhI(OAc) (2.4 equiv.) and (f) with NCS (2.4 equiv.).
2
(2.4 equiv.) at 140 1C
2
2
(25)
(25)
2
2
2
2
2
PhICl
2
PhI(OAc)
NCS (25)
NCS (25)
2
(100)
100
100
90 : 10
94 : 6
2
the Pd surface oxidation by PhICl mainly generated the Pd(IV)
Pd@SiO
2
species, and not Pd(II). The oxidized surface was stable enough to
conduct the measurement in air without significant degradation.
The intensity of the Cl 2p3/2 peak at 199.0 eV was roughly
a
b
1
Reactions in a sealed vial in toluene for 12 h. Determined by H NMR.
100% conversion (Table 1, entries 4 and 6). In our experiments, correlated with that of the Pd peaks, revealing that the coordi-
we identified 3-chloro-2-phenylbenzofuran (3) as a side product nation of Cl atoms could stabilize the high oxidation states of
2
3
(Fig. S1, ESI†) and demonstrated that the chlorination of 2 pro- the surface atoms (Fig. S5, ESI†).
ceeded via reaction with PhICl
2
even without Pd catalysts, while no
After stirring in toluene at 140 1C for 1 h, the Pd(IV) fraction
reaction took place between 1 and PhICl
2
(Fig. S2, ESI†). Thus, rapidly increased and reached the maximum value of 80%,
2
we could utilize the conversion of cyclization by combining the along with an increase in the amount of PhICl up to 1.2 equiv.
yields of 2 and 3.
To investigate oxidants other than PhICl
(Fig. 2c and Fig. S6, ESI†). This observation is surprising because
2
for use in catalyst the formation of stable organometallic Pd(IV) complexes has rarely
25–27
activation, PhI(OAc) and N-chlorosuccinimide (NCS) were tested. been reported in homogeneous reactions,
which is in contrast
2
Under the reaction conditions tested, PhI(OAc) could not activate to the stable Pt(IV) species which are known to be active in many
2
31
the Pd@SiO
using a large amount (Table 1, entry 7). In contrast, NCS facilitated significantly diminished the Pd(IV) fraction to 24% (Fig. 2d and
the Pd catalysts, Pd and Pd@SiO yolk–shell NPs to provide Fig. S7, ESI†), indicating that the highly electrophilic Pd(IV) species
cyclized products as efficiently as PhICl
X-ray photoelectron spectroscopy (XPS) is one of the most
effective tools to identify the Pd species on heterogeneous but apparently, other oxidants need to be surveyed for knowing
2 2
catalyst for the hydroalkoxylation reaction even upon catalytic reactions. A prolonged treatment of PhICl for 12 h
2
32
2
(Table 1, entries 8 and 9). had slowly degraded into the Pd(0) and Pd(II) species.
PhICl is an excellent oxidant for generating the Pd(IV) species,
2
29,30
surfaces.
In the Pd NPs without treatment, the Pd 3d core- their potential for expansion of reaction scopes. After treatment
level lines were fitted with two main peaks, where the Pd 3d5/2 with PhI(OAc) under the experimental conditions, the Pd 3d
2
5/2
peak at 335.8 eV was assignable to the Pd(0) species (Fig. 2a). peak appeared at 337.0 eV, corresponding to the Pd(II) species
Remarkably, after the treatment with PhICl , the major Pd 3d5/2 (Fig. 2e). As shown in entry 7 of Table 1, no hydroalkoxylation
2
peak shifted to the higher binding energy at 338.2 eV, which could reaction occurred, directly indicating that the Pd(II) species
be assigned to the Pd(IV) species. This clear change indicated that were not acidic enough to catalyse the reaction. In contrast,
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treatment with NCS generated the Pd(IV) species in a significant surface (Fig. 3a). However, prolonged treatment by PhICl (10 equiv.)
fraction (18%), as determined by quantitative analysis (Fig. 2f). for 12 h successfully provided Pd(IV) as the major species (69%,
This elucidated the effective conversion into the products Fig. 3b, and Fig. S8, ESI†). Although the Pd(IV) species were effectively
indicated in entries 8 and 9 of Table 1. In a homogeneous generated at 25 1C, the Pd NPs were inactive, due to the surfactant
system, NCS successfully generated a stable Pd(IV) compound passivation on the surface. In contrast, all the surfactants in the
3
3
via oxidative addition to the Pd(II) center.
At an elevated temperature (140 1C) in toluene, the Pd and under a flow of H
Pd@SiO yolk–shell NPs exhibited similar activities. However, particle agglomeration due to the protective SiO
at 25 1C, only the Pd@SiO catalyst with PhICl was remarkably the surfactant-free Pd@SiO catalyst was effective at 25 1C. As
active for the hydroalkoxylation reaction. The homogeneous expected, the addition of surfactants to the Pd@SiO catalyst
Pd(OAc) and PdCl catalysts yielded 9% and trace amounts of completely eliminated its catalytic activity (Table 2, entry 6).
Pd@SiO
2
catalyst were perfectly removed by heat treatment at 500 1C
2
, while the catalyst structure was stable against
34
2
2
shell. Thus, only
2
2
2
2
2
2
products, respectively (Table 2, entries 1 and 2). While 1 mol%
Another advantage of the Pd@SiO yolk–shell catalyst is its high
2
Pd NPs with 25 mol% of PhICl did not yield any products at stability after reaction. After the treatment with PhICl at 140 1C
2
2
2
5 1C, distinctively, 1 mol% Pd@SiO
PhICl
Moreover, the product selectivity increased to 95 : 5 when the with PhICl
amounts of PhICl were reduced to 10 mol% (Table 2, entry 5). the reaction at 140 1C for 12 h (Table 1, entry 6, and Fig. 3d).
XPS analysis showed that oxidation of the catalyst surface by PhICl The hydroalkoxylation reactions using the Pd@SiO catalyst
2
yolk–shell NPs with 25 mol% (Table 1, entry 4), the Pd NPs were severely agglomerated and formed
2
exhibited complete conversion (Table 2, entries 3 and 4). large clusters (Fig. 3c). In contrast, the Pd@SiO
2
catalysts activated
2
were very stable without any structural deformation after
2
2
2
at 25 1C for 1 h, was not as effective as that at 140 1C, in which proceeded in a heterogeneous way. The negative effect of the
significant amounts of the Pd(II) species were generated on the surfactants on the Pd@SiO yolk–shell catalyst (Table 2, entry 6)
2
indicated that the reaction actually occurred on the heterogeneous
Pd surface. To gain more evidence of this, the catalyst was filtered
after the reaction in entry 6 of Table 1, and the filtrates were
Table 2 Hydroalkoxylation of 1 using homogeneous and heterogeneous
Pd catalysts at 25 1C
a
analysed using ICP-OES (inductively coupled plasma-optical emis-
b
b
sion spectrometry). The Pd species were undetected at the detection
limit of the measurement (0.003 ppm), revealing that the catalysts
Catalyst
(1 mol%)
PhICl
(mol%)
2
Cyclization (%)
(2 + 3)
Conversion (%)
(2 : 3)
No.
were very stable against leaching during the reaction. The Pd@SiO2
1
2
3
4
5
Pd(OAc)
PdCl
Pd NPs
Pd@SiO
Pd@SiO
Pd@SiO
2
0
0
25
25
10
10
9
o1
0
100
100
0
9 : 0
—
—
78 : 22
95 : 5
—
2
catalyst could be recycled six times using a simple decantation
procedure, and was used for the hydroalkoxylation reaction without
a significant decrease of activity (Fig. S3, ESI†).
2
2
2
c
6
In summary, the Pd and Pd@SiO
as the primary surface species by oxidation with PhICl
2
nanocatalysts generated Pd(IV)
2
a
b
. The Pd(IV)
Reactions in a sealed vial in toluene for 12 h. Determined by
1
c
H NMR. Excess surfactants were added.
species were highly active for the hydroalkoxylation reaction of 1,
and their activity was even higher than that of the homogeneous Pd
catalysts. Two products, 2 and 3, were identified, which were
selectively yielded by using different amounts of PhICl . The catalyst
2
surface was also activated to form Pd(IV) by the addition of NCS,
leading to a high activity of the hydroalkoxylation reaction, whereas
PhI(OAc)
2
formed primarily Pd(II) on the surface, which was not
yolk–shell
efficient for the reaction. In particular, the Pd@SiO
2
catalyst exhibited a remarkably high activity, higher than those of
other homogeneous and heterogeneous catalysts at 25 1C, with the
additional advantage of excellent stability. Consequently, by the
combination of appropriate activation reagents, this type of nano-
catalyst system bearing various metal NPs may be applicable for a
wide range of valuable organic reactions. Such combinations should
provide excellent stability, high levels of activity, and superior
selectivity to homogeneous systems.
This work was supported by IBS-R004-D1, and the National
Research Foundation of Korea (NRF) funded by the Korean Govern-
ment (MSIP) (2012-005624, R11-2007-050-00000-0, 2011-0014197).
Notes and references
Fig. 3 Fraction of the Pd species, Pd(0) (black), Pd(II) (red), and Pd(IV) (blue),
after treatment with variable amounts of PhICl at 25 1C for (a) 1 h and (b) 12 h;
TEM images of (c) Pd NPs and (d) Pd@SiO yolk–shell catalyst recovered after
the reactions of entries 4 and 6 in Table 1.
2
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