Characterization of heterogeneous aryl-Pd(ii)-oxo clusters as active species for C-H arylation

Article abstract of DOI:10.1039/d0cc06716d

C-H arylation with heterogeneous palladium was investigated. The surface oxidation of Pd nanoparticles with a hypervalent iodine reagent, [Ph2I]BF4, resulted in the generation of Pd(ii)-aryl-oxo clusters, which were characterized as the crucial intermediate.

Full text of DOI:10.1039/d0cc06716d

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Characterization of heterogeneous aryl–Pd(II)–
oxo clusters as active species for C–H arylation†
Cite this:DOI: 10.1039/d0cc06716d
a
Taeil Shin,‡a Minjun Kim,‡a Younjae Jung,a Sung June Cho, *b Hyunwoo Kim
*
a
Received 8th October 2020,
Accepted 28th October 2020
and Hyunjoon Song
*
DOI: 10.1039/d0cc06716d
rsc.li/chemcomm
C–H arylation with heterogeneous palladium was investigated. The usually carried out.18–21 We have also characterized high valent
surface oxidation of Pd nanoparticles with a hypervalent iodine nanostructures such as Pd(II), Pd(IV), and Rh(III), and investigated
reagent, [Ph2I]BF4, resulted in the generation of Pd(II)–aryl–oxo various organic transformations, hydroalkoxylation, coupling of
clusters, which were characterized as the crucial intermediate.
epoxides and CO2, and C–H halogenation. Moreover, Pd nanoca-
talysts were used for a tandem C–H halogenation and cross-
There is a long-standing debate for actual catalytic mechanisms coupling reaction, mediated by heterogeneous Pd(0) and Pd(IV)
through homogeneous and heterogeneous pathways in various catalysts.22,23
organic transformations, while even using homogeneous pre-
In the present study, we revisited the Pd NPs-catalyzed C–H
cursors.1–3 The formation of small Pd nanoparticles (NPs) has been arylation reaction of indoles and other heterocycles, and char-
reported during the Pd-catalyzed cross-coupling reactions including acterized the ‘‘heterogeneous’’ surface intermediates. By various
Suzuki–Miyaura, Sonogashira, and Mizoroki–Heck reactions.4–10 spectroscopic analyses, the active species were characterized
The Pd NPs could be yielded by the aggregation of homogeneous to be aryl–Pd(II)–oxo clusters bearing Pd(II)–C bonds on their
Pd precursors, such as Pd(OAc)2 (OAc = acetate) and Pd(dba)2 (dba surface, which were used to demonstrate the plausible hetero-
= dibenzylideneacetone). Recently, in situ formation of Pd NPs has geneous Pd(0)/Pd(II) reaction mechanisms.
been identified after the catalytic C–H bond functionalization.11–14
To identify the heterogeneous species in the Pd NPs-
It has been proposed that the heterogeneous surface potentially catalyzed C–H arylation, we have synthesized uniform Pd NPs
catalyzed the reactions on the basis of multiple control experi- by thermal decomposition of Pd–oleylamine complex according
ments. For instance, Glorius and co-workers reported the regiose- to the literature.24 The transmission electron microscopy (TEM)
lective arylation of thiophenes and polyaromatic hydrocarbons image shows that the particles are uniformly spherical and
using Pd/C in the presence of hypervalent iodine(III) reagents.15,16 highly monodisperse with an average diameter of 4.2 Æ 0.4 nm
The reaction mechanism was proposed to use heterogeneous Pd(0) (Fig. 1a). The X-ray diffraction (XRD) shows a single broad peak
and Pd(II) species similar to the homogeneous counterpart.15–17 at 401, corresponding to the (111) reflection of Pd face-centered-
However, ‘‘Pd(II) NPs’’ is not a proper term for the intermediates cubic structure (JCPDS No. 46-1043) (Fig. S1, ESI†). The average
considering the strong electrostatic repulsion between Pd(II) domain size was estimated to be 4.7 nm from the FWHM of the
species. It is still rarely explored what the actual catalytic species (111) peak by using the Debye–Scherrer equation.25
are in the heterogeneous reactions.
Before the reaction study, proper solvents were selected using
The metal NPs have been commonly used for low-valent the Pd NPs treated with diphenyliodonium salt. In ethanol, the
transition metal catalysis due to their zero-valent nature. Recently, Pd NPs were leached out and agglomerated due to high solvent
Toste and Somorjai expanded the heterogeneous catalysis into polarity (Fig. S2, ESI†). In contrast, the Pd NPs were stable in
high-valent metal chemistry that the homogeneous counterparts THF, maintaining its original colloidal morphology (Fig. 1b).
The reaction conditions of the C–H arylation of indole were
a Department of Chemistry, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 34141, Republic of Korea. E-mail: hwkim@kaist.edu,
hsong@kaist.ac.kr
b Department of Chemical Engineering, Chonnam National University, Gwangju
61186, Republic of Korea. E-mail: sjcho@chonnam.ac.kr
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc06716d
screened in the presence of the Pd NPs and [Ph2I]BF4 (Table 1).
Under the standard condition, the arylation selectively occurred
at the C2-position of indole (1b) with 91% and 81% yields in
ethanol and THF, respectively (Table 1, entries 1 and 2).
Interestingly, homogeneous Pd(II) reagents, PdCl2, Pd(TFA)2,
and Pd(OAc)2 showed superior reactivity to the Pd NPs (Table S1,
ESI†), but provided the products with moderate yields (42–72%).
‡ These authors contributed equally to this work.
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Scheme 1 Substrate scope of indoles, furans, and thiophenes. a Condi-
tions: indole (0.2 mmol), [Ph2I]BF4 (0.26 mmol) and solvent (2.0 mL)
without exclusion of air and moisture. Isolated yields are given.
Fig. 1 (a and b) TEM images and (c and d) XPS spectra at the Pd 3d level of
(a and c) the pristine and (b and d) oxidized Pd NPs after the treatment with
[Ph2I]BF4.
(1a), 1,5,6,7-tetrahydro-4H-indol-4-one (2a), methyl 2-carboxylate
pyrrole (3a), methyl 1H-indole-5-carboxylate (4a) 5-methoxy-1H-
indole (5a) and 5-methyl-1H-indole (6a) transformed into the
corresponding phenylated products in 80%, 84%, 81%, 94%,
85% and 89% yields, respectively. Remarkably, the C2 arylation
products were exclusively generated over the C3 arylation ones
in excellent regioselectivities of 499 : 1 in all reactions. O-Hetero-
cyclic compounds such as benzofuran (7a), 3-methyl benzofuran
(8a), 2-furaldehyde (9a), and 2-pentylfuran (10a) also provided the
C2 arylation products (7b–10b) with excellent regioselectivity in
77%, 84%, 84%, and 52% yields, respectively. In the case of C–H
arylation of sulfur-containing heterocycles, benzo[b]thiophene
(11a) was not effective (25% yield), but 3-methylthiophene (12a)
provided the C3-arylated product (12c) with good yield of 71%.
Both sulfur-containing heterocycles showed the regioselectivity at
the C3 position (499 :1) in accordance with the literature.16
To confirm the involvement of the heterogeneous catalytic
species in the Pd NPs-catalyzed C–H arylation, we performed
hot filtration and mercury poisoning tests.27 The reaction
mixture at 60 1C was filtered, and the filtrate did not contain
Table 1 Arylation of indole with diaryliodonium salt catalyzed by homo-
and heterogeneous Pd sources
Selectivityb (%)
No.a
Catalyst
Solvent
Yield (%)
1b
1c
1d
1
2
3
4
5
6
Pd NPs
Pd NPs
PdCl2
Pd(TFA)2
Pd(OAc)2
Pd(OAc)2
EtOH
THF
THF
THF
THF
EtOH
91
80
42
47
65
72
99
99
80
81
76
76
—
—
10
18
12
7
—
—
10
1
12
17
a
Conditions: indole (0.2 mmol) [Ph2I]BF4 (0.26 mmol) and solvent
b
(2.0 mL) without exclusion of air and moisture. Determined by gas
chromatography-mass spectrometry (GC-MS) analysis. Mesitylene was
used as an internal standard.
However, significant amounts of the arylation also occurred at any trace of Pd species over the detection limit (o0.01 ppm)
the C3 position (1c) and biarylation at the C2 and C3 positions analyzed by inductively coupled plasma-optical emission spec-
(1d) (Table 1, entries 3–6). The homogeneous Pd catalysts trometry (ICP-OES) (Fig. S5, ESI†). The addition of 2a and
showed a large difference in reactivity and selectivity compared [Ph2I]BF4 into the filtrate did not proceed the reaction at all
to the heterogeneous Pd NPs. However, the two reaction path- at 60 1C for 6 h, also supporting the absence of the catalytic Pd
ways are not clearly distinguished because the heterogeneous Pd species in the filtrate. For the mercury poisoning test, a drop of
was formed after the reaction with Pd(TFA)2 or Pd(OAc)2 (Fig. S3, mercury to the reaction mixture of 2b completely stopped the
ESI†).26
reaction (Fig. S6, ESI†). These control experiments indicate that
We next investigated the reaction scope of the Pd NPs- the C–H arylation mainly occurred by the heterogeneous cata-
catalyzed C–H arylation of heterocyclic compounds (Scheme 1). lytic species of the Pd NPs.15,16
The heterocyclic substrate was heated at 60 1C for 24 h with
Generally, the heterogeneous catalyst has a high level of
1.3 equiv. of [Ph2I]BF4 and 5 mol% of Pd NPs in THF. Under the reusability compared to the homogeneous catalyst. After the
reaction condition, N-heterocyclic compounds including indole reaction, the Pd NPs were recovered and reused five more times.
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The recycle experiments provided the arylated product within surface sites exposed outward (Fig. 2c). On the other hand, the
the narrow range of 80–86% conversions under 6 cycles (Fig. S7, oxidized Pd NPs (red) have distinct signals at 1.6–2.2 Å, corres-
ESI†). After the recycling experiments, the morphology of the ponding to the overlaps of Pd–C and Pd–O bonds, prior to the
catalyst was preserved as the original spherical structure well. strong peak for the Pd–Pd bond. Using the average Pd–C
Because only the surface was oxidized to form active species, the and Pd–O bond lengths observed in organometallic com-
original hardy Pd(0) frameworks of the catalysts may lead to a pounds,32,33 the optimal fitting provided the coordination
high degree of reliability (see below).
numbers of Pd–Pd, Pd–C, and Pd–O to be 4.8, 1.2, and 1.1,
Under the present oxidative reaction conditions, Pd(0) spe- respectively (Fig. 2c). Based on these spectroscopic measure-
cies are easily oxidizable to Pd(II) or Pd(IV) species. To clarify the ments, it is reasonable to identify that the oxidized NPs are
oxidation states of Pd NPs during the reaction, the Pd NPs were composed of polymeric aryl–Pd(II)–oxo clusters, –(Pd(II)(Ph)–O)–,
treated with [Ph2I]BF4 in THF at 60 1C without the substrates. bearing both Pd–Ph and Pd–O bonds. The charge of Pd(II) was
The X-ray photoelectron spectroscopy (XPS) shows that the stabilized by the coordination of aryl and oxo ligands. The large
pristine Pd NPs have the major two peaks at 340.2 and 335.8 fraction of the Pd species is still Pd(0), indicating that the
eV in the Pd 3d region, corresponding to Pd(0) species (Fig. 1c, pristine Pd NPs are partially oxidized by [Ph2I]BF4. The oxidized
green).28 After the treatment, two new peaks distinctively domains are potentially located at the surface, in consideration
emerge at 341.8 and 336.7 eV (Fig. 1d, blue), indicating the of the coordination number (4.8) of Pd–Pd bonds nearly iden-
formation of Pd(II) species by surface oxidation.29 The infrared tical to that of the pristine NPs and large Pd(II) peaks in the XPS
spectroscopy of the oxidized NPs showed that a new peak spectrum. These aryl–Pd(II)–oxo clusters are robust in aprotic
appeared at 730 cmÀ1, correlated with the typical out of plane solvents such as THF and retain their nanoscale morphology.
bending peak of monosubstituted rings (Fig. S8, ESI†).30 These
measurements reveal the existence of Ph–Pd(II) species under model reaction, arylation of indole (1a), was selected. To
the reaction conditions. demonstrate the arylation reactions that occurred on the sur-
To propose the catalytic mechanism of C–H arylation, a
X-ray absorption spectroscopy provides direct information face of Pd(II) with phenyl, we performed three control experi-
of the oxidation state and coordination environment at the ments. First, the catalytic arylation was carried out with excess
metal centres.31 In X-ray absorption near-edge structure indole (5 equiv.) in the presence of [Ph2I]BF4 (1 equiv.). The
(XANES), the characteristic white line at 24 370 eV with the arylated product (1c) was isolated in 15% yield. Then the
subsequent pattern at 24 388 eV for oxidized Pd NPs (red) is surface oxidation states of Pd NPs were characterized by XPS.
close to that of the pristine Pd NPs (green), rather than the The spectrum at the Pd 3d level after the arylation showed the
intense peak at 24 371 eV of PdO (black). It indicates that the large decrease of Pd(II) (Fig. 3a and b). It evidenced that
major fraction of the oxidized Pd NPs still involves Pd(0) species coordinated phenyl groups on the Pd(II) surface were used as
(Fig. 2a). The Fourier transform of k2-weighted Pd K-edge arylation sources. Second, the mixture of Pd NPs and [Ph2I]BF4
extended X-ray absorption fine structure (EXAFS), however, were heated to 60 1C for 24 h in THF. After the reaction,
shows significantly distinct features from Pd(0) (Fig. 2b and biphenyl was solely yielded without the original substrates
Fig. S9, ESI†). The pristine Pd NPs show a strong single peak (Fig. S10, ESI†). In comparison, the biphenyl compounds were
(green) identical to that of Pd foils, which corresponds to the not detected without Pd NPs. These results confirmed the
Pd–Pd bond length at 2.7 Å. The coordination number of the formation of Pd(II)–aryl species during the reaction.34 Third,
Pd–Pd bond was simulated to be 5.0 due to the less-coordinated the addition of H2O increased the product yields (Fig. S11, ESI†)
and the fraction of Pd(II) species in the XPS spectrum (Fig. S12,
ESI†). However, the addition of O2 did not affect the yield.
Thus, we could rationally propose that H2O participates in the
catalyst generation and the C–H arylation. As a result, we
propose a plausible catalytic cycle for the C–H arylation on
the heterogeneous Pd (Fig. 3c). The surface of Pd NPs is
oxidized to generate –(Pd(II)(Ph)O)– clusters (II), which mediate
C–H activation to form the intermediate (III). The subsequent
reductive elimination provides the intermediate (IV), which is
further oxidized to the –(Pd(II)(Ph)O)– clusters (II) by the action
of [Ph2I]BF4 and H2O.
In conclusion, we investigated the heterogeneous catalytic
system promoting C–H arylation reactions of heterocycle com-
pounds in high yield and regioselectivity. Multiple spectroscopic
analyses revealed that the oxidized Pd(II) intermediates were
–(Pd(II)(Ph)O)– clusters generated on the Pd(0) surface. Such hetero-
geneous Pd(II) species are expected to additional features superior
Fig. 2 (a) Pd K-edge XANES and (b) FT of k2-weighted Pd K-edge EXAFS
spectra. (c) Structural parameters obtained from the EXAFS refinement.
to homogeneous catalysis, including easy access to regioselectively
demanded substrates. These heterogeneous catalytic systems with
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Conflicts of interest
The authors declare no conflict of interest.
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