Tetranuclear Palladium Complexes of Abnormal N-Heterocyclic Carbene Ligands and their Catalytic Activities in Mizoroki-Heck Coupling Reaction of Electron-Rich Aryl Chlorides

Article abstract of DOI:10.1002/adsc.201900805

Based on the ligand scaffold of an imidazolyl/benziimidazolyl moiety and a N-CH₂C(=O)NHPh substituent, two series of ligand precursors with ortho hydroxy groups incorporated on the N-phenyl rings were prepared. The structural fine tuning of the ligand scaffold allowed the synthesis of tetranuclear palladium complexes with abnormal N-heterocyclic carbene (aNHC) ligands. For precursors with C2-methyl blocking groups, pyridine-assisted C?H bond activation led to the formation of mononuclear tridentate palladium aNHC complexes or tetranuclear complexes with tridentate CNO donors. Representative mononuclear and tetranuclearpalladium aNHC complexes were structurally characterized by X-ray diffraction studies, revealing very short Pd?C bond distances. The tetranuclear palladium aNHC complexes were very effective in catalyzing Mizoroki-Heck coupling reaction, and were capable of employing a range of aryl chlorides including deactivated substrates with low palladium loading of 0.2 mol%. (Figure presented.).

Full text of DOI:10.1002/adsc.201900805

Title: Tetranuclear Palladium Complexes of Abnormal <i>N</i>-
Heterocyclic Carbene Ligands and their Catalytic Activities in
Mizoroki-Heck Coupling Reaction of Electron-Rich Aryl Chlorides
Authors: Jhen-Yi Lee, Yong-Siang Su, Yu-Shan Wang, and Hon Man
Lee
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900805
Link to VoR: http://dx.doi.org/10.1002/adsc.201900805

10.1002/adsc.201900805
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Tetranuclear Palladium Complexes of Abnormal N-
Heterocyclic Carbene Ligands and their Catalytic Activities in
Mizoroki-Heck Coupling Reaction of Electron-Rich Aryl
Chlorides
Jhen-Yi Lee,a Yong-Siang Su,a Yu-Shan Wang,a and Hon Man Leea,*
a
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
Phone: 886-4-7232105 ext. 3523; E-mail: leehm@cc.ncue.edu.tw
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. Based on the ligand scaffold of an palladium aNHC complexes were structurally characterized
imidazolyl/benziimidazolyl moiety and a N-CH2C(=O)NHPh by X-ray diffraction studies, revealing very short Pd─C
substituent, two series of ligand precursors with ortho bond distances. The tetranuclear palladium aNHC
hydroxy groups incorporated on the N-phenyl rings were complexes were very effective in catalyzing Mizoroki-Heck
prepared. The structural fine tuning of the ligand scaffold coupling reaction, and were capable of employing a range
allowed the synthesis of tetranuclear palladium complexes of aryl chlorides including deactivated substrates with low
with abnormal N-heterocyclic carbene (aNHC) ligands. For palladium loading of 0.2 mol%.
precursors with C2-methyl blocking groups, pyridine-
assisted C—H bond activation led to the formation of
mononuclear tridentate palladium aNHC complexes or
tetranuclear complexes with tridentate CNO donors.
Representative mononuclear and tetranuclear
Keywords: Carbene ligands; Palladium; C—C coupling;
Heck reaction; Tridentate ligands
metalation of dimethyaminoarene derivatives showed
excellent catalytic activities with high turnover
number (TON) and high turnover frequencies (TOF)
in cross-coupling reactions of aryl bromides at room
temperature.[4e] However, the use of unreactive aryl
chlorides was not reported.
Introduction
Mononuclear palladium complexes are ubiquitous in
the literature because of their numerous catalytic
applications in synthetic organic chemistry.[1] In
particular, monopalladium complexes with N-
heterocyclic carbene (NHC) ligands have proved
highly versatile in the last two decades for catalyzing
various cross-coupling reactions.[2] Because of the
recent interest in supramolecular coordination
complexes (SCC), the chemistry of multinuclear
palladium complexes has attracted significant
attention.[3] Nevertheless, tetranuclear palladium
complexes are rarely used in cross-coupling
reactions.[4] Huynh et al. investigated the catalytic
activity of a tetranuclear [Pd4S4] macrocyclic square
based on NHC ligand in aqueous Suzuki-Miyaura
coupling reaction.[4a] A series of dendritic palladium
catalysts including a soluble tetranuclear complex,
some of which were effective in catalyzing various
cross-coupling reactions, were reported by Astruc et
al.[4b] Song and Yin reported that tetranuclear
palladium complexes containing C,N-bidentate
furoylhydrazone exhibited good activity in Suzuki-
Miyaura coupling reaction of activated aryl
chlorides.[4d] Knölker et al. found that several
tetranuclear palladium complexes obtained from the
It has been shown that imidazole-based NHC
ligands can have alternative binding modes via C4/5
atoms, instead of the commonly observed
coordination via the C2 atom. These variants of NHC
ligands are called “abnormal NHCs (aNHCs)”.[5]
Because they are more electron-rich than their normal
counterparts,[5a,6] improved catalytic activities have
been reported.[6d,6e,7] Considering this information, we
reasoned that tetranuclear palladium complexes
featuring aNHC ligands as catalysts in cross-coupling
reactions would be worthy of investigation. To our
knowledge, these kinds of complexes are still
unknown in the literature. Previously, we reported
various palladium(II) complexes with a ligand
scaffold bearing an imidazolyl moiety and a N-
CH2C(=O)NRPh sidearm (R = H or Me) (Figure 1).[8]
The ligand scaffold possesses various sites that may
undergo C–H or N–H cleavage, resulting in different
palladium complexes. These include C,C-type
palladacycle A and B,[8a] palladalactam C,[8a] C,C-
type palladacycle D,[8a] and palladium aNHC
complex E.[8b] In this work, we demonstrate that the
1
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10.1002/adsc.201900805
Advanced Synthesis & Catalysis
introduction of an ortho hydroxyl group on the N-
phenyl ring of the ligand scaffold allow a facile
preparation of novel complexes (1 － 3), including
mononuclear and tetranuclear palladium complexes
bearing tridentate aNHC ligands.
－3 in Mizoroki-Heck coupling reaction conducted in
a molten salt. We found that the tetranuclar palladium
complex with aNHC ligand was extremely effective
with a wide range of aryl chlorides including
deactivated substrates, affording quantitative yields
of coupled products with a low Pd loading in most
cases.
Results and Discussion
The synthesis of the two series of target ligand
precursors is shown in Scheme 1. The key feature in
these precursors is an ortho OH group on the N-
phenyl rings; the ligands are potentially tridentate
after triple deprotonation of the OH, NH(C=O), and
one of the protons either on the imidazole ring or C2-
methyl/phenyl groups. For ligand precursors La–e,
the 2-position of the heterocyclic ring is blocked by a
phenyl or a methyl group, preventing the possibility
of normal NHC coordination. However, aNHC
coordination is possible after deprotonation at the C5
position of the imidazole ring. For ligand precursors
Lf–g, the imidazole ring is replaced with a
benziimidazole, thus eliminating the possibilities of
normal and abnormal NHC coordination. Hence,
deprotonation can only occur on the C2-methyl group
(see D in Figure 1).
Imidazolium salts La–e were straightforwardly
obtained through the reactions of appropriate
imidazole
derivatives
with
2-chloro-N-(2-
hydroxyphenyl)acetamide in DMF at high
temperature with modest yields in the range of 50－
78%. Notably, the reactions proceeded smoothly
without interference from the free hydroxy groups
and thus their protection was not necessary. Ligand
precursors Lf–g featuring a benziimidazole instead of
an imidazole moiety were similarly obtained with
mediocre yields of 53 and 49%, respectively. All the
new non-hygroscopic white solids were characterized
using NMR spectroscopy and high-resolution
electron-spray mass spectrometry. Typically, two
Figure 1. Palladium(II) complexes with a ligand scaffold
based on an imidazolyl moiety and an amido flanking
group.
1
downfield broad signals were observed in their H
The Pd-catalyzed Mizoroki-Heck coupling reaction
is one of the most powerful and versatile tools
currently used in synthetic organic chemistry to
construct C—C bonds.[9] Among aryl halides, the use
of aryl chlorides as substrates is desirable in
industrial applications because they are inexpensive
and widely available. However, highly effective
palladium catalyst systems are needed for the
activation of stronger Ar—Cl bonds. While
remarkable progress has been made with the effective
use of even deactivated aryl chlorides by phosphine-
assisted catalyst systems,[10] it is still necessary to
develop cost-effective phosphine-free catalysts with
low Pd loading in green reaction media. Although
mononuclear Pd–NHC complexes are shown to be
highly versatile in catalyzing the reaction, [2] most of
these systems suffer from poor performance of
deactivated and electron-rich aryl chloride substrates.
[2l] Herein, we also present the catalytic activities of 1
NMR spectra. With reference to our previous work
on structurally related compounds,[11] the most
downfield signal at ca. 10 ppm was attributed to the
OH proton, while the second most downfield signal at
ca. 9.8 ppm was attributed to the NH proton.
2
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10.1002/adsc.201900805
Advanced Synthesis & Catalysis
Scheme 1. Synthesis of ligand precursors.
The coordination chemistry between the new
ligand precursors and palladium dichloride was
thoroughly investigated. Reactions of the ligand
precursors La–c featuring a C2-phenyl group with
PdCl2 using potassium carbonate as the base in DMF
at room temperature afforded pure compounds of
tetranuclear palladium complexes featuring tridentate
aNHC ligands (Scheme 2a). The successful formation
of the tetranuclear palladium complexes was
unambiguously confirmed by the representative ESI-
MS analysis of two of these compounds in solution
and single-crystal X-ray diffraction studies (vide
infra). For 2a, the molecular peak of the compound
was observed as the base peak at m/z = 1951.2
(theoretical m/z = 1951.2). The base peak of 2b at m/z
= 2072.3 was attributable to the [M+H]+ ion
(theoretical m/z = 2072.3). The characteristic
abnormal carbene carbon resonances were observed
at ca. 131 ppm, which were comparable to those of
relevant palladium aNHC complexes.[12] Notably, the
reaction between the ligand precursors and PdCl2
yielded only palladium aNHC complexes. The C－H
activation at the C2-phenyl ring, as seen in A (see
Figure 1), was not observed. Also, the μ-phenoxide
bridge in 2 was strong, as attempts to heat a solution
of the tetranuclear complex with pyridine or PPh3 did
not yield any mononuclear complex such as 1d and
1e (vide infra).
Scheme 2. Synthesis of mononuclear and tetranuclear
palladium complexes.
However, the reactions of ligand precursors Ld
and Le bearing C2-methyl protecting groups with
PdCl2 under the same reaction conditions, failed to
deliver the desired tetranuclear palladium aNHC
complexes. We projected that C2-methyl substituted
imidazolium compounds Ld and Le would be more
electron-rich than C2-phenyl substituted compounds
La and Lb and therefore hypothesized that adding
pyridine as an extra organic base and conducting the
complexation reactions at elevated temperature might
assist deprotonation at the C5 position. Indeed, pure
products of 1d and 1e were obtained (Scheme 2b).
Subsequent characterization including single-crystal
X-ray diffraction studies revealed that complexes
1d,e were mononuclear palladium aNHC complexes.
The aNHC ligand was chelated in a tridentate CNO
fashion with pyridine occupying the fourth
coordination site. Thus, in addition to acting as a base,
the coordination property of pyridine also plays a
crucial role in the formation of these mononuclear
complexes. Noteworthy, when pyridine was added to
the aforementioned preparation of complexes La-c, a
mixture of products was obtained, presumably due to
the formation of both tetranuclear and mononuclear
complexes of aNHCs.
In the ligand precursors Lf and Lg, the possibilities
of both normal and abnormal NHC coordination were
eliminated. The only possible metal coordination of
the third donor group was via CH2 coordination after
methyl group deprotonation. We have previously
reported CH3 activation and the formation of carbon-
coordinated palladium complexes (see D in Figure
3
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10.1002/adsc.201900805
Advanced Synthesis & Catalysis
1).[8a] We carried out the reaction between ligand
precursor Lf and Lg with PdCl2 under identical
reaction conditions for 2a–c, however, no pure
product was obtained. We next added pyridine and
carried out the reaction at 80 °C with the aim of
assisting deprotonation of the methyl group (Scheme
2c),[8a] tetranuclear complexes 3f and 3g were
successfully obtained. The multinuclear nature of
these tetranuclear complexes was confirmed by a
crude X-ray structural analysis of 3g. Despite the
limited quality of the structural data (R1 > 0.1) due to
poor crystal quality, the connectivity of atoms was
unambiguously confirmed (see Figure S1 in
from 1.951 Å[13] to 2.022 Å.[7c] The Pd—C bond
distances in 2b were even shorter (vide infra), while
the Pd—N bond distance from pyridine N atom
(2.037(7) Å) was slightly longer than from the
amidate N atom (2.022(7) Å), reflecting stronger
coordination from the latter donor.
The tetranuclear nature of 2b was unambiguously
established in the structural studies (Figure 3). Each
palladium center in 2b adopts distorted square planar
coordination environment and is coordinated by a
tridentate ligand via an abnormal carbenic C atom,
amidate N atom, and μ-phenolic oxygen atom. The
remaining coordination site is occupied by a bridging
phenolic O atom from an adjacent ligand. The four
palladium centers are not coplanar. The interatomic
axis between Pd2 and Pd3 atoms is above and
approximately normal to that between Pd1 and Pd4
atoms. Interestingly, Pd2 interacts weakly with Pd3
atom with a contact distance of 3.1784(7) Å (sum of
van der Waals radii of Pd = 3.26 Å).[14] Such
interaction is absent between Pd1 and Pd4 atoms;
where the corresponding interatomic distance is ca
3.27 Å. The very short Pd—C distances involving
Pd1, Pd2, and Pd3 ranged from 1.920(6)—1.928(6) Å.
The corresponding distance involving Pd4 was
slightly longer (1.947(7) Å).
Electronic
Supporting
Information).
The
aforementioned ESI-MS analysis for 2a and 2b
showed the molecular peaks as base peaks, indicating
high stability in solution. In contrast, the ESI-MS
analysis of 3f revealed a complex spectrum with
multiple signals in a wide range of m/z 700–1800
(theoretical m/z for M+ = 1903.2), implying much
lower stability in solution. Notably, a comparison of
Scheme 2b vs. 2c clearly showed that aNHC
coordination in 1d and 1e was more favorable than
the methylene coordination as found in 3f and 3g. In
contrast to the mononuclear 1d and 1e, tetranuclear
complexes 3f and 3g were formed even when their
complexation reactions were carried out in the
presence of pyridine.
Figure 3. Molecular structure of 2b at 50 % probability
level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (): Pd1—O2, 2.016(5);
Pd1—N4, 2.022(7); Pd1—C1, 1.9308(8); Pd1—N3,
2.037(7); O2—Pd1—N4, 84.9(2); C1—Pd1—N4, 94.8(3);
C1—Pd1—N3, 88.9(3); O2—Pd1—N3, 91.4(2); N3—
Pd1—N4, 176.3(3); C1—Pd1—O2, 179.6(3).
Figure 2. Molecular structure of 1d at 50 % probability
level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (): Pd1—O2, 2.016(5);
Pd1—N4, 2.022(7); Pd1—C1, 1.9308(8); Pd1—N3,
2.037(7); O2—Pd1—N4, 84.9(2); C1—Pd1—N4, 94.8(3);
C1—Pd1—N3, 88.9(3); O2—Pd1—N3, 91.4(2); N3—
Pd1—N4, 176.3(3); C1—Pd1—O2, 179.6(3).
Complex 2c crystallizes in an orthorhombic space
group, where the tetranuclear complex is situated on a
special position along a crystallographic 2-fold axis
along the c-axis (Figure 4). The structural features of
2c are similar to those of 2b. There is also a weak
interaction in 2b between a pair of Pd atoms with a
contact distance of 3.1759(11) Å, comparable to that
of 2c. The interatomic distance between the other pair
of Pd atoms is much larger (3.301 Å). The bond
distances between the Pd atom and the aNHC
moieties are ca. 1.932 Å.
The structures of 1d, 2b, and 2c have been
successfully established in single-crystal X-ray
diffraction studies. Table S1 in the Supporting
Information provides the crystallographic data. The
tridentate coordination of the ligand involving an
aNHC coordination in 1d was confirmed with the
palladium center adopting a slightly distorted square-
planar coordination environment (Figure 2).
Generally, the most striking feature of all the new
structures are the very short Pd—C bond distances.
To the best of our knowledge, the Pd—C bond
distance of 1.9308(8) Å in 1d is the shortest of those
reported in palladium imidazole-based aNHC
complexes, which have Pd—C bond distance ranging
4
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10.1002/adsc.201900805
Advanced Synthesis & Catalysis
comparable to the TOF of 427 h-1 of the tetranuclear
palladium complex that was reported by Knölker et al.
for the same reaction.[4e]
Table 1. Mizoroki-Heck reaction of 4-
chloroacetophenone with styrene a)
Entry
Complex
mol
% Pd
atom
X
Time
Yield
(%)
TON TOF
100
(96:4)
65
(97:3)
64
(97:3)
100
(96:4)
86
(97:3)
85
(96:4)
83
(96:4)
17
54
1
2
3
4
5
6
1d
1e
2a
2b
2c
3f
0.2
0.2
0.2
0.2
0.2
0.2
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
500
325
320
500
430
425
250
163
160
250
215
213
Figure 4. (a) Molecular structure of 2c at 50 % probability
level. Hydrogen atoms are omitted for clarity. (b) A
simplified view of molecular structure of 2c showing the
connectivity around palladium centers. Hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and
angles (°): Pd1—O4, 2.064(5); Pd1—O3A, 2.054(5);
Pd1—N3A, 1.989(6); Pd1—C3A, 1.932(8); Pd2—O3,
2.053(5); Pd2—O4, 2.060(5); Pd2—N6, 2.002(6); Pd1—
C33, 1.931(8); O4—Pd1—O3A, 92.1(2); O3A—Pd1—
N3A, 83.5(2); C3A—Pd1—N3A, 95.2(3); O4—Pd1—
C3A, 89.2(3); O4—Pd1—N3A, 175.5(3); C3A—Pd1—
O3A, 178.7(3); O3—Pd1—C33, 89.1(3); O3—Pd2—O4,
92.7(2); O4—Pd2—N6, 83.1(2); C33—Pd1—N6, 95.1(3);
O3—Pd1—N6, 175.7(2); C32—Pd1—O4, 177.1(3).
Nonbonding distance (Å): Pd1···Pd1A, 3.1759(11).
Symmetry code: A, 0.5 – x, –y, z.
7
8
9
3g
1d
2b
0.2
0.1
0.1
Cl
Cl
Cl
2
2
2
415
170
540
208
85
270
(97:3)
10
11
2b
2b
0.2
0.1
Cl
Br
4
2
0 b)
--
840
--
420
84
94
(96:4)
18
12
13
2b
2b
0.05
0.02
Br
Br
4
4
1880
900
470
225
a) Reaction conditions: 1.4 mmol of styrene, 1 mmol of
aryl halide, 1.1 mmol of NaOAc, 2 g of TBAB, Pd cat.,
140 ℃. Yield determined by NMR using 1,3,5-
trimethoxybenzene as internal standard, anti/gem ratio in
parenthesis. b) H2O/NMP, K2CO3, 100 oC.
The catalytic activity of new complexes in the
Mizoroki-Heck reaction was investigated (Table 1).
The use of highly polar molten salts (the so-called
nonaqueous ionic liquids) as green reaction media in
Mizoroki-Heck reaction is believed to facilitate
cationic mechanism in the reaction and stabilize
underligated Pd(0) species by forming anionic
complexes with halide ions,[9c,15] leading to enhanced
PhCl reactivity.[9c,15a,16] We previously conducted
successful Mizoroki-Heck reactions in molten n-
tetrabutylammonium bromide (TBAB).[12a,17] To
screen for the most efficient complex, a benchmark
reaction between 4-chloroacetophenone and styrene
(4) using 0.2 mol% of Pd atom loading (i.e., 0.05
mol% of a tetranuclear complex) was carried out
using our reported reaction conditions (NaOAc as
base, molten TBAB as solvent, 140 °C, 2 h).[17f] Of
the new complexes (entries 1‒7), both mononuclear
aNHC complex 1d and tetranuclear aNHC complex
2b afforded quantitative yields of product (entries 1
and 4). Reducing the Pd atom loading to 0.1 mol%
revealed that tetranuclear complex 2b was more
effective than mononuclear complex 1d, resulting in
an improved 54 % product yield (entries 8 vs 9).
Using H2O/NMP as a solvent and K2CO3 as a base as
reported by Knölker et al,[4e] the catalytic reaction
was totally inhibited (entry 10). As expected, the use
of more reactive bromide substrate produced higher
TONs and TOFs (entries 11‒13). A small 0.05 mol%
Pd loading of complex 2b was sufficient for
delivering a good TOF of 470 h-1 (entry 12). This is
After the most efficient tetranuclear complex 2b
was identified, the substrate scope of the catalyst
system was investigated (Table 2). The complex
proved to be highly efficient for the coupling reaction
of activated aryl chlorides such as 4-
chloronitrobenzne, 4-chlorocyanobenzene, 1-chloro-
4-(trifluoromethyl)benzene, 4-chlorobenzaldehyde,
and 4-chloroacetophenone with styrene, producing
quantitative yields of the corresponding coupled
products in 2 ‒ 6 h (entries 1 ‒ 5). Increasing the
reaction time to 12 h, complex 2b was also highly
effective for the utilization of electron-neutral
substrates such as chlorobenzene and 4-chlorotoluene,
affording quantitative yields (entries 6 and 7). A
quantitative product yield was also achieved with the
unreactive electron-rich 4-chloroanisole (entry 8).
Next, we tested the viability of the catalyst system
for handling sterically hindered substrates. The
system proved highly effective, producing
quantitative yields with slightly hindered meta-
substituted aryl chlorides (entries 9, 11, 12, and 13).
Notably, entry 13 represents the first time that 1-
chloro-3,5-dimethoxybenzene can be applied as a
substrate in the Mizoroki-Heck coupling reaction
with styrene. In previous reports, more reactive
alkene partners than simple styrene, such as 1-
5
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10.1002/adsc.201900805
Advanced Synthesis & Catalysis
alkenyltrifluoroborate[18] or 1-alkenylboronic acid,[19]
were required for the formation of 5hh when using
the deactivated aryl chloride as a substrate. However,
compared with examples in the literature,[10c,16] the
catalyst system was less effective at handling ortho-
substituted aryl chlorides; the use of 2-
chlorobenzonitrile produced only a poor 36% yield of
coupled product with styrene (entry 10).
Even though there are two possible sites for C–C
bond formation when 1,4-dichlorobenzene was used
as substrate, the reaction was highly selective,
producing only the mono-alkene product (entry 14).
Double Heck coupling was not observed even if
styrene was in excess. The quantitative yield of 5i is
the highest among reports of the same catalytic
reaction.[20]
The substrate scope was then examined with
respect to the alkenes partners. Entry 15 shows that 4-
chloroacetophenone can react with 4-chlorostyene
with a quantitative product yield within 2 h. However,
when a slightly electron-rich 4-methystyrene was
used, a longer reaction time of 6 h was required
(entry 16). When aryl chloride substrates with
electron-donating substituents were used (entries 17
and 18), an even longer reaction time of 12 h was
required. The quantitative production of coupled
product 7g in entry 17 represents the best reported
yield of the same coupling reaction.[21] For the
coupling reaction between 4-chloroanisole and 4-
methystyrene, only one paper reported satisfactory
yields of coupled product (62–70%), with a higher 1
mol% Pd loading and a prolonged reaction time of 44
h.[21b] In contrast, our catalyst system was much more
effective, delivering a quantitative yield of 7h with a
mild 0.2 mol% Pd loading in 12 h (entry 18). Entries
19–20 show that 4-methoxystyrene can also be
smoothly applied to reactions with electron-rich and
electron-neutral aryl chlorides. However, in reactions
with deactivated 4-chloroanisole, a poor 40% yield of
coupled product 8h was obtained (entry 21). The only
existing paper on the coupling reaction between 4-
chloroanisole and 4-methoxystyrene reported that the
compound 8h had a good 83% yield; however, a
higher Pd loading of 1 mol% was required.[22] We
repeated the reaction with a 1 mol% of Pd loading
and a comparable 79% of product yield was obtained.
Table 2. Mizoroki-Heck coupling reaction of aryl
chlorides catalyzed by tetranuclear complex 2b a)
Entry
Ar─X
Product
Time Yield
(%)
(h)
6
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10.1002/adsc.201900805
Advanced Synthesis & Catalysis
Also, n-butyl acrylate was successfully employed
as a substrate, resulting in quantitative yields of
products with electron-rich and electron-neutral aryl
chlorides (entries 22 and 23). However, a slightly
lower yield of 82% was obtained with 4-
chloroanisole (entry 24). Interestingly, when methyl
acrylate was used as a substrate, a tandem
Heck/transesterification reaction occurred, producing
a quantitative yield of the coupled product 9h (entry
25). The transesterification involving the TBAB
solvent was previously observed by us.[17f]
Scheme 3. The Mizoroki-Heck reaction of 4-chloroanisole
and styrene in a preparative scale.
Finally, to demonstrate the utility of the catalyst
system based on 2b in synthetic organic chemistry,
we conducted a reaction between deactivated 4-
chloroanisole (10 mmol) and styrene (14 mmol) in a
preparative scale, which successfully resulted in a
quantitative yield of 2.1 g of the coupled product 5h
(Scheme 3).
Conclusion
We designed and prepared two novel series of ligand
precursors with an ortho hydroxy group incorporated
on the N-phenyl ring of the ligand scaffold based on
an imidazolyl/benziimidazoly moiety and a N-
CH2C(=O)NR'Ph side-arm. This structural fine tuning
of the ligand scaffold allowed for the synthesis of
tetranuclear palladium complexes including the first
example of such multinuclear complexes with aNHC
ligands. As evidenced from the extremely short
Pd─C bond distances in representative crystal
structures, the electron-donating aNHC moiety bind
strongly to the palladium centers in both the
mononuclear and tetranuclear complexes. The
extremely effective catalytic activities of the
tetranuclear palladium aNHC complexes in the
Mizoroki-Heck coupling reaction seem to correlate
well with their short Pd─carbene bond distances. The
robust tetranuclear complex 2b was capable of using
a wide range of aryl chlorides including unreactive
deactivated substrates; in many examples,
quantitative yields of coupled products were obtained.
Interestingly, the catalytic activity of the tetranuclear
complex is superior to that of its mononuclear
counterpart. The rate-determining step of the reaction
is believed to be the oxidative addition of aryl
chlorides. The highly effective catalytic activity of
the tetranuclear aNHC complex 2b may be related to
a higher local concentration of active electron-rich
LPd(0) which favors the oxidative addition of the
a) Reaction conditions: 1.4 mmol of styrene, 1 mmol
of aryl halide, 1.1 mmol of NaOAc, 2 g of TBAB,
0.05 mol% 2b (i.e. 0.2 mol% of Pd atom loading),
140 C. Isolated yield. b) Methyl acrylate was used as
the alkene coupling partner.
7
This article is protected by copyright. All rights reserved.

10.1002/adsc.201900805
Advanced Synthesis & Catalysis
3
7.74 (m, 5H, Ar H), 8.06 (d, J(HH) = 3.0 Hz, 2H, Ar H),
9.75 (s, 1H, NH), 10.08 ppm (s, 1H, OH); 13C{1H} NMR
([D6]DMSO): δ 51.2 (CH2), 51.4 (CH2), 116.1, 116.2 (d,
2J(CF) = 21.9 Hz, Ar C), 119.1, 121.3 (quaternary C),
unreactive C—Cl bonds. The molten TBAB solvent
may stabilize the active species, contributing also to
the enhanced activity of aryl chlorides.
3
122.9, 124.6, 125.5, 130.0, 130.4 (d, J(CF) = 8.3 Hz, Ar
C), 130.9, 131.0 (quaternary C), 131.1 (quaternary C),
133.1, 145.6 (quaternary C), 148.8 (quaternary C), 162.4 (d,
1J(CF) = 245.3 Hz, CF), 164.1 ppm (C=O); HRMS (ESI)
m/z calcd for C24H21FN3O2 [M–Cl]+ 402.1617; found
402.1605.
Experimental Section
General Information
All manipulations were performed under a dry nitrogen
atmosphere using standard Schlenk techniques. Solvents
were dried with standard procedures. Starting chemicals
were purchased from commercial source and used as
Synthesis of 1-Benzyl-3-(2-((2-hydroxyphenyl)amino)-
2-oxoethyl)-2-methyl-1H-imidazol-3-ium Chloride (Ld)
The compound was prepared with a similar procedure to
that of La. A mixture of 1-benzyl-2-methyl-1H-imidazole
1
received. H and 13C{1H} NMR spectra were recorded at
300.13 and 75.47 MHz, respectively, on a Bruker AV-300
spectrometer. Elemental analyses were performed on a
Thermo Flash 2000 CHN-O elemental analyzer. ESI-MS
was carried out on a Finnigan/Thermo Quest MAT 95XL
mass spectrometer at National Chung Hsing University
(Taiwan).
(1.41
g,
8.19
mmol)
and
2-chloro-N-(2-
hydroxyphenyl)acetamide (1.52 g, 8.19 mmol) in DMF (20
mL) was used. A white solid was obtained. Yield: 1.46 g
1
(50 %). M.p. = 250.6–251.2 °C; H NMR ([D6]DMSO): δ
2.59 (s, 3H, CH3), 5.27 (s, 2H, CH2), 5.49 (s, 2H, CH2),
3
6.74 (t, 1H, J(HH) = 6.0 Hz, Ar H), 6.94 (s, 2H, imi H),
7.31‒7.46 (m, 5H, Ar H), 7.79 (d, 3H, 3J(HH) = 9.0 Hz, Ar
H), 9.98 ppm (br s, 2H, NH, OH); 13C{1H} NMR
([D6]DMSO): δ 10.2 (CH3), 51.0 (CH2), 51.1 (CH2), 116.1,
119.2, 121.9, 122.9, 123.5, 125.6, 125.7 (quaternary C),
128.1, 129.0, 129.5, 135.0 (quaternary C), 146.2
(quaternary C), 148.8 (quaternary C), 164.1 ppm (C=O);
HRMS (ESI) m/z calcd for C19H20N3O2 [M–Cl]+ 322.1555;
found 322.1557.
Synthesis of 1-Benzyl-3-(2-((2-hydroxyphenyl)amino)-
2-oxoethyl)-2-phenyl-1H-imidazol-3-ium Chloride (La)
A mixture of 1-benzyl-2-phenyl-1H-imidazole (1.01 g,
4.31 mmol) and (0.80 g, 4.31 mmol) in DMF (20 mL) was
placed in a Schlenk flask. The mixture was heated to 120
oC overnight. After cooling, the white solid was collected
on a frit, washed with THF, and dried under vacuum.
1
Yield: 1.2 g (67 %). M.p. = 246.1–246.5 °C; H NMR
Synthesis
of
3-(2-((2-Hydroxyphenyl)amino)-2-
([D6]DMSO): δ 5.14 (s, 2H, CH2), 5.35 (s, 2H, CH2), 6.71
oxoethyl)-1-(4-methoxybenzyl)-2-methyl-1H-imidazol-
3
3
(t, J(HH) = 6.0 Hz, 1H, Ar H), 6.92 (t, J(HH) = 6.0 Hz,
1H, Ar H), 7.00–7.07 (m, 3H, Ar H), 7.30 (s, 3H, imi H,
Ar H), 7.60–7.70 (m, 6H, Ar H), 8.15 (d, 3J(HH) = 9.0 Hz,
2H, Ar H), 9.90 (s, 1H, NH), 10.26 ppm (s, 1H, OH);
13C{1H} NMR ([D6]DMSO): δ=51.5 (CH2), 51.9 (CH2),
116.1, 119.1, 121.3 (quaternary C), 122.8, 122.9, 124.7,
125.5, 125.6 (quaternary C), 127.9, 128.9, 129.3, 130.0,
130.9, 133.1, 134.9 (quaternary C), 145.7 (quaternary C),
148.8 (quaternary C), 164.2 ppm (C=O); HRMS (ESI) m/z
calcd for C24H22N3O2 [M–Cl]+ 384.1711; found 384.1706.
3-ium Chloride (Le)
The compound was prepared with a similar procedure to
that of La. A mixture of 1-(4-methoxybenzyl)-2-methyl-
1H-imidazole (0.91 g, 4.50 mmol) and 2-chloro-N-(2-
hydroxyphenyl)acetamide (0.84 g, 4.50 mmol) was used. A
white solid was obtained. Yield: 1.37 g (78 %). M.p. =
1
223.8–224.2 °C; H NMR ([D6]DMSO): δ 2.61 (s, 3H,
CH3), 3.74 (s, 3H, OCH3), 5.29 (s, 2H, CH2), 5.41 (s, 2H,
3
CH2), 6.73 (t, 1H, J(HH) = 6.0 Hz, Ar H), 6.91–6.98 (m,
3
4H, Ar H, imi H), 7.33 (d, 2H, J(HH) = 9.0 Hz, Ar H),
Synthesis
of
3-(2-((2-Hydroxyphenyl)amino)-2-
7.77–7.79 (m, 3H, Ar H), 10.0 (s, 1H, NH), 10.2 ppm (s,
1H, OH); 13C{1H} NMR ([D6]DMSO): δ 10.2 (CH3), 50.7
(CH2), 50.9 (CH2), 55.7 (OCH3), 114.8, 116.1, 119.2,
121.6, 122.7, 123.4, 125.5, 125.8 (quaternary C), 126.8
(quaternary C), 130.0, 145.9 (quaternary C), 148.7
(quaternary C), 159.8 (quaternary C), 164.0 ppm (C=O);
HRMS (ESI) m/z calcd for C20H22N3O3 [M–Cl]+ 352.1661;
found 352.1659.
oxoethyl)-1-(3-methoxybenzyl)-2-phenyl-1H-imidazol-
3-ium Chloride (Lb)
The compound was prepared with a similar procedure to
that of La. A mixture of 1-(3-methoxybenzyl)-2-phenyl-
1H-imidazole (1.1 g, 4.16 mmol) and 2-chloro-N-(2-
hydroxyphenyl)acetamide (0.77 g, 4.16 mmol) was used. A
white solid was obtained. Yield: 1.0 g (55 %); M.p. =
1
195.1–195.8 °C; H NMR ([D6]DMSO): δ 3.67 (s, 3H,
Synthesis of 1-Benzyl-3-(2-((2-hydroxyphenyl)amino)-
OCH3), 5.13 (s, 2H, CH2), 5.31 (s, 2H, CH2), 6.61–6.73 (m, 2-oxoethyl)-2-methyl-1H-benzo[d]imidazol-3-ium
3
3H, Ar H), 6.86–7.01 (m, 3H, Ar H), 7.23 (t, J(HH) = 9.0
Hz, 1H, Ar H), 7.61–7.69 (m, 6H, imi H, Ar H), 8.13 (d,
3J(HH) = 15.0 Hz, 2H, Ar H), 9.87 (s, 1H, NH), 10.21 ppm
(s, 1H, OH); 13C{1H} NMR ([D6]DMSO): δ 51.5 (CH2),
51.8 (CH2), 55.6 (OCH3), 113.6, 114.4, 116.1, 119.1, 119.9,
121.3 (quatenary C), 122.8, 122.9, 124.7, 125.6, 130.0,
130.5, 130.9, 133.1, 136.4 (quatenary C), 145.7 (quatenary
C), 148.8 (quatenary C), 159.9 (quatenary C), 164.2 ppm
(C=O); HRMS (ESI) m/z calcd for C25H24N3O3 [M–Cl]+
414.1817; found 414.1821.
Chloride (Lf)
The compound was prepared with a similar procedure to
that of La.
A
mixture of 1-benzyl-2-methyl-1H-
benzo[d]imidazole (1.1 g, 4.94 mmol) and 2-chloro-N-(2-
hydroxyphenyl)acetamide (0.92 g, 4.94 mmol) was used. A
white solid was obtained. Yield: 0.97 g (53 %). M.p.
1
248.5–249.1 °C; H NMR ([D6]DMSO): δ 2.98 (s, 3H,
CH3), 5.69 (s, 2H, CH2), 5.90 (s, 2H, CH2), 6.72–6.75 (m,
1H, Ar H), 6.97 (d, 3J(HH) = 3.0 Hz, Ar H), 7.32–7.40 (m,
3
5H, Ar H), 7.64 (t, J(HH) = 6.0 Hz, 2H, Ar H), 7.82 (d,
Synthesis of 1-(4-Fluorobenzyl)-3-(2-((2-
hydroxyphenyl)amino)-2-oxoethyl)-2-phenyl-1H-
imidazol-3-ium Chloride (Lc)
3J(HH) = 9.0 Hz, 1H, Ar H), 7.98 (d, 3J(HH) = 9.0 Hz, 1H,
3
Ar H), 8.04 (d, J(HH) = 9.0 Hz, 1H, Ar H), 10.09 (s, 1H,
NH), 10.20 ppm (s, 1H, OH); 13C{1H} NMR ([D6]DMSO):
δ 11.5 (CH3), 48.3 (CH2), 48.6 (CH2), 113.4, 113.8, 116.1,
119.2, 122.6, 125.6, 125.8 (quaternary C), 126.9, 127.0,
127.7, 128.9, 129.5, 131.1 (quaternary C), 131.9
(quaternary C), 134.6 (quaternary C), 148.6 (quaternary C),
153.8 (quaternary C), 164.0 ppm (C=O); HRMS (ESI) m/z
calcd for C23H22N3O2 [M–Cl]+ 372.1711; found 372.1701.
The compound was prepared with a similar procedure to
that of La. A mixture of 1-(4-fluorobenzyl)-2-phenyl-1H-
imidazole (1.0 g, 3.96 mmol) and 2-chloro-N-(2-
hydroxyphenyl)acetamide (0.74 g, 3.96 mmol) was used. A
white solid was obtained. Yield: 1.1 g (71 %). M.p. =
1
114.2–114.6 °C; H NMR ([D6]DMSO): δ 5.08 (s, 2H,
3
CH2), 5.32 (s, 2H, CH2), 6.73 (t, J(HH) = 6.0 Hz, 1H, Ar
H), 6.92 (s, 2H, imi H), 7.12–7.21 (m, 5H, Ar H), 7.57–
8
This article is protected by copyright. All rights reserved.

10.1002/adsc.201900805
Advanced Synthesis & Catalysis
Synthesis
of
3-(2-((2-Hydroxyphenyl)amino)-2-
To a 20 mL Schlenk flask, PdCl2 (0.042 g, 0.28 mmol), La
(0.1 g, 0.24 mmol) and K2CO3 (0.13 g, 0.96 mmol) were
dissolved in dry DMF (10 mL) under nitrogen atmosphere.
The solution was allowed to stir at room temperature. The
residual was extracted with DCM/H2O twice. The extract
was dried over anhydrous MgSO4 and evaporated to
dryness under vacuum to give a yellow solid, which was
washed with diethyl ether and dried under vacuum. Yield:
oxoethyl)-1-(4-methoxybenzyl)-2-methyl-1H-
benzo[d]imidazol-3-ium Chloride (Lg)
The compound was prepared with a similar procedure to
that of La. A mixture of 1-(4-methoxybenzyl)-2-methyl-
1H-benzo[d]imidazole (1.0 g, 4.16 mmol) and 2-chloro-N-
(2-hydroxyphenyl)acetamide (0.77 g, 4.16 mmol) was used.
A white solid was obtained. Yield: 0.89 g (49 %). M.p.
1
0.092 g (79 %). M.p. 251.9–252.3 °C (dec.); H NMR
1
(CDCl3): δ 4.46–4.67 (m, 16H, CH2), 6.14 (s, 4H, imi H),
238.4–239.2 °C; H NMR ([D6]DMSO): δ 2.98 (s, 3H,
3
6.37 (t, J(HH) = 6.0 Hz, 4H, Ar H), 6.65–6.98 (m, 16H,
CH3), 3.73 (s, 3H, OCH3), 5.66 (s, 2H, CH2), 5.80 (s, 2H,
CH2), 6.70–6.76 (m, 1H, Ar H), 6.94–6.96 (m, 4H, Ar H),
7.32 (d, 3J(HH) = 6.0 Hz, 2H, Ar H), 7.60–7.65 (m, 2H, Ar
H), 7.82 (d, 3J(HH) = 9.0 Hz, 1H, Ar H), 8.01 (d, 3J(HH) =
9.0 Hz, 2H, Ar H), 10.04 (s, 1H, NH), 10.10 ppm (s, 1H,
OH); 13C{1H} NMR ([D6]DMSO): δ 11.5 (CH3), 48.2
(CH2), 55.7 (OCH3), 113.3, 113.9, 114.9, 116.1, 119.2,
122.5, 125.6, 125.8 (quaternary C), 126.5 (quaternary C),
126.8, 127.0, 129.5, 131.0 (quaternary C), 131.9
(quaternary C), 148.6 (quaternary C), 153.6 (quaternary C),
159.7 (quaternary C), 164.0 ppm (C=O); HRMS (ESI) m/z
calcd for C24H24N3O3 [M–Cl]+ 402.1817; found 402.1808.
Ar H), 7.17–7.51 (m, 28H, Ar H), 7.86 (d, 3J(HH) = 9.0 Hz,
3
4H. Ar H), 8.48 ppm (d, J(HH) = 9.0 Hz, 4H, Ar H);
13C{1H} NMR (CDCl3): δ 51.2 (CH2), 55.7 (CH2), 115.4,
120.6 (quaternary C), 122.6, 123.9, 127.0 (quaternary C),
127.2, 128.1, 128.5, 128.8, 129.1, 129.4, 129.8, 129.9,
130.3, 130.6, 130.8, 131.3 (Pd─C), 134.8, 135.1, 135.2,
139.9 (quaternary C), 142.4 (quaternary C), 161.5
(quaternary C), 165.1 ppm (C=O); Elemental analysis
calcd (%) for C96H76N12O8Pd4: C, 59.13; H, 3.93; N, 8.62;
found: C, 59.01; H, 3.89; N, 8.43 %.
Synthesis of Tetrakis[-1-[(3-methoxyphenyl)methyl]-
2-phenyl-3-[2-oxo-2-[(2-phenolato-κO)amino-κN]ethyl]-
1H-imidazoliumato-κC4]tetrapalladium (2b)
Synthesis
of
[1-Benzyl-2-methyl-3-[2-oxo-2-[(2-
phenolato-κO)amino-κN]ethyl]-1H-imidazoliumato-
κC4](pyridine)palladium (1d)
The complex was prepared with a similar procedure to that
of 2a. A mixture of PdCl2 (0.039 g, 0.22 mmol), Lb (0.10
g, 0.22 mmol) and K2CO3 (0.11 g, 0.88 mmol) was used. A
yellow solid was obtained. Yield: 0.096 g (84 %). M.p.
To a 20 mL Schlenk flask, PdCl2 (0.050 g, 0.28 mmol), Ld
(0.1 g, 0.28 mmol), pyridine (22.6 μL, 0.28 mmol), and
K2CO3 (0.15 g, 1.1 mmol) were dissolved in dry DMF (10
mL) under nitrogen atmosphere. The solution was allowed
1
269.4–270.1 °C (dec.); H NMR (CDCl3): δ 3.62 (s, 12H,
o
CH3), 4.47–4.70 (m, 16H, CH2), 6.15 (s, 4H, imi H), 6.31–
6.40 (m, 16H, Ar H), 6.65–6.72 (m, 8H, Ar H), 7.08 (t,
3J(HH) = 9.0 Hz, 4H, Ar H), 7.51 (br s, 16H, Ar H), 7.87
(d, 3J(HH) = 9.0 Hz, 4H, Ar H), 8.51 ppm (d, 3J(HH) = 9.0
Hz, 4H, Ar H); 13C{1H} NMR (CDCl3): δ 51.1 (CH2), 55.2
(CH3), 55.7 (CH2), 112.3, 114.0, 115.4, 119.3, 120.6
(quaternary C), 121.9, 122.6, 123.9, 129.4, 129.7, 129.9,
130.4, 131.3 (Pd−C), 136.3 (quaternary C), 139.9
(quaternary C), 142.5 (quaternary C), 159.8 (quaternary C),
161.4 (quaternary C), 165.1 ppm (C=O); Elemental
analysis calcd (%) for C100H84N12O12Pd4: C, 58.02; H,
4.09; N, 8.12; found: C, 58.10; H, 3.60; N, 8.05 %.
to stir at 50 C overnight. The residual was extracted with
DCM/H2O twice. The extract was dried over anhydrous
MgSO4 and evaporated to dryness under vacuum to give a
solid. Diethyl ether was added and the yellowish solid
formed was collected on frit and dried under vacuum.
1
Yield: 0.12 g (85 %). M.p. 225.1–225.4 °C (dec.); H
NMR ([D6]DMSO): δ 2.59 (s, 3H, CH3), 4.70 (s, 2H, CH2),
5.34 (s, 2H, CH2), 6.19–6.40 (m, 2H, Ar H), 6.56 (t,
3J(HH) = 9.0 Hz, 1H, Ar H), 7.10 (s, 1H, imi H), 7.23–7.39
(m, 4H, py H, Ar H), 7.53–7.66 (m, 2H, Ar H), 7.79 (t,
3J(HH) = 9.0 Hz, 1H, py H), 8.00–8.18 (m, 2H, Ar H),
8.77 ppm (d, J(HH) = 6.0 Hz, 2H, py H); 13C{1H} NMR
3
(CDCl3): δ 14.5 (CH3), 50.9 (two overlapping CH2), 120.1,
120.3 (quaternary C), 124.8 (py C), 127.2, 128.5, 128.6,
129.3, 129.9 (quaternary C), 130.8 (Pd ─ C), 134.3
(quaternary C), 138.4 (py C), 146.1 (quaternary C), 153.2
(Py C), 163.1 ppm (C=O); Elemental analysis calcd (%)
for C24H22N4O2Pd: C, 57.13; H, 4.39; N, 11.11; found: C,
57.47; H, 4.59; N, 10.94 %.
Synthesis of Tetrakis[-1-[(4-fluorophenyl)methyl]-2-
phenyl-3-[2-oxo-2-[(2-phenolato-κO)amino-κN]ethyl]-
1H-imidazoliumato-κC4]tetrapalladium (2c)
The complex was prepared with a similar procedure to that
of 2a. A mixture of PdCl2 (0.044 g, 0.25 mmol), Lc (0.10 g,
0.25 mmol) and K2CO3 (0.14 g, 1.0 mmol) was used. A
yellow solid was obtained. Yield: 0.063 g (50 %). M.p.
Synthesis of [1-[(4-Methoxyphenyl)methyl]-2-methyl-3-
[2-oxo-2-[(2-phenolato-κO)amino-κN]ethyl]-1H-
imidazoliumato-κC4](pyridine)palladium (1e)
1
239.5–240.2 °C (dec.); H NMR (CDCl3): δ 4.47–4.65 (m,
3
16H, CH2), 6.10 (s, 4H, imi H), 6.38 (t, J(HH) = 6.0 Hz,
4H, Ar H), 6.65–6.96 (m, 24H, Ar H), 7.53 (br s, 16H, Ar
3
H), 7.85 (d, J(HH) = 9.0 Hz, 4H, Ar H), 8.46 ppm (d,
The complex was prepared with a similar procedure to that
of 1d. A mixture of PdCl2 (0.046 g, 0.26 mmol), Le (0.10
g, 0.26 mmol), pyridine (20.9 μL, 0.26 mmol), and K2CO3
(0.14 g, 1.0 mmol) was used. A yellow solid was obtained.
3J(HH) = 6.0 Hz, 4H, Ar H); 13C{1H} NMR (DMSO-d6): δ
50.0 (CH2), 55.5 (CH2), 115.9 (d, 2J(CF) = 20.4 Hz, Ar C),
120.3, 121.1, 121.9, 122.4 (quaternary C), 124.1, 129.2,
3
1
129.9 (d, J(CF) = 7.5 Hz, Ar C), 130.1, 130.8, 131.0,
Yield: 0.095 g (69 %). M.p. 226.9–227.1 °C (dec.); H
131.7 (quaternary C), 131.9 (Pd─C), 140.1 (quaternary C),
142.6 (quaternary C), 161.2 (quaternary C), 162.1 (d,
1J(CF) = 243.0 Hz, CF), 164.8 ppm (C=O); elemental
analysis calcd (%) for C96H72F4N12O8Pd4: C, 57.02; H,
3.59; N, 8.31; found: C, 57.31; H, 3.87; N, 7.96 %
NMR (CDCl3): δ 2.19 (s, 3H, CH3), 3.74 (s, 3H, OCH3),
3
4.57 (s, 2H, CH2), 4.97 (s, 2H, CH2), 6.40 (t, J(HH) = 9.0
Hz, 1H, Ar H), 6.68–6.85 (m, 7H, imi H, py H, Ar H), 7.41
3
3
(t, J(HH) = 6.0 Hz, 2H, Ar H), 7.80 (t, J(HH) = 9.0 Hz,
1H, py H), 8.24 (d, 3J(HH) = 6.0 Hz, 1H, Ar H), 8.97 ppm
3
(d, J(HH) = 6.0 Hz, 2H, py H); 13C{1H} NMR (CDCl3):
δ=10.1 (CH3), 50.6 (CH2), 55.4 (OCH3), 56.1 (CH2), 113.5,
114.5, 116.3, 121.4, 123.1, 123.5, 125.4 (py C), 125.9
(quaternary C), 128.3, 129.9 (quaternary C), 131.6 (Pd─
C), 137.9 (Py C), 145.1 (quaternary C), 149.9 (quaternary
C), 153.1 (py C), 159.7 (quaternary C), 164.2 ppm (C=O);
elemental analysis calcd (%) for C25H24N4O3Pd: C, 56.17;
H, 4.52; N, 10.48; found: C, 56.48; H, 4.97; N, 10.66 %.
Synthesis of Tetrakis[-2-[2-(3-benzyl-2-methanidyl-
C-benzimidazolium-1-yl)acetyl]azanidyl-N-
phenolato-O]tetrapalladium (3f)
To a 20 mL Schlenk flask, PdCl2 (0.050 g, 0.27 mmol), Lf
(0.15 g, 0.40 mmol), pyridine (21.6 μL, 0.27 mmol), and
K2CO3 (0.22 g, 1.61 mmol) were dissolved in dry DMF
(10 mL) under nitrogen atmosphere. The solution was
allowed to stir at 80 oC overnight. The residual was
extracted with DCM/H2O twice. The extract was dried
over anhydrous MgSO4 and evaporated to dryness under
vacuum to give a solid. MeOH was added and the orange
Synthesis of Tetrakis[-1-benzyl-2-phenyl-3-[2-oxo-2-
[(2-phenolato-κO)amino-κN]ethyl]-1H-imidazoliumato-
κC4]tetrapalladium (2a)
9
This article is protected by copyright. All rights reserved.

10.1002/adsc.201900805
Advanced Synthesis & Catalysis
solid formed was collected on frit and dried under vacuum.
void volume of 2,536 Å (589 electrons). This volume is in
a good agreement of the presence of three CH2Cl2
molecules per molecule of 2c (643 Å, 147 electrons).
CCDC files 1845482 (1d), 1845487 (2b), and 1845488
(2c) contain supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
1
Yield: 0.061 g (48 %). M.p. = 245.2–245.5 °C (dec.). H
NMR (CDCl3): δ 1.98–2.02 (m, 4H, PdCHaHb), 2.20–2.24
2
(m, 4H, PdCHaHb), 4.62 (s, 8H, CH2), 4.93 (d, J(HH) =
2
15.0 Hz, 4H, CHaHb), 5.20 (d, J(HH) = 18.0 Hz, 4H,
CHaHb), 6.46–6.49 (m, 8H, Ar H), 6.98 (br s, 8H, Ar H),
7.14-7.20 (m, 12H, Ar H), 7.31–7.36 (m, 8H, Ar H), 7.60
Cambridge
Crystallographic
Data
Centre
via
3
3
(d, J(HH) = 9.0 Hz, 4H, Ar H), 8.00 (d, J(HH) = 6.0 Hz,
www.ccdc.cam.ac.uk/data_request/cif.
3
4H, Ar H), 8.13 ppm (d, J(HH) = 6.0 Hz, 4H, Ar H).
13C{1H} NMR ([D6]DMSO): δ 6.2 (PdCH2), 47.4 (CH2),
50.7 (CH2), 111.7, 112.3, 113.0, 115.9, 122.3, 123.6, 125.4
(quaternary C), 127.4, 127.6, 128.6, 129.4, 131.4
(quaternary C), 132.1 (quaternary C), 135.4 (quaternary C),
143.8 (quaternary C), 162.9 (C=O), 166.5 ppm (quaternary
C). Elemental analysis calcd (%) for C92H76N12O8Pd4: C,
58.09; H, 4.03; N, 8.84; found: C, 57.79; H, 4.39; N,
8.89 %.
Acknowledgements
We are grateful to the Ministry of Science and Technology of
Taiwan (MOST 107-2113-M-018-002) for financial support of
this work.
Synthesis of Tetrakis[-2-[2-[2-methanidyl-C-3-[(4-
methoxyphenyl)methyl]benzimidazolium-1-
yl]acetyl]azanidyl-N-phenolato-O]tetrapalladium
(3g)
References
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The complex was prepared with a similar procedure to that
of 3f. A mixture of PdCl2 (0.046 g, 0.25 mmol), Lg (0.10 g,
0.25 mmol) and K2CO3 (0.14 g, 1.0 mmol) was used. A
orange solid was obtained. Yield: 0.049 g (39 %). M.p. =
1
249.5–249.9 °C (dec.). H NMR (CDCl3): δ 1.96–2.00 (m,
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4H, PdCHaHb), 2.12–2.17 (m, 4H, PdCHaHb), 3.75 (s, 12H,
CH3), 4.63 (ABq, ΔAB = 18.7 Hz, JAB = 15.0 Hz, 8H, CH2),
2
2
4.86 (d, J(HH) = 18.0 Hz, 4H, CHaHb), 5.13 (d, J(HH) =
15.0 Hz, 4H, CHaHb), 6.45–6.55 (m, 8H, Ar H), 6.69 (d,
3J(HH) = 9.0 Hz, 8H, Ar H), 6.82–6.94 (m, 8H, Ar H),
7.04–7.21 (m, 4H, Ar H), 7.29–7.39 (m, 8H, Ar H), 7.58 (d,
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3
NMR (CDCl3): δ 29.7 (PdCH2), 47.6 (two overlapping
CH2), 51.3 (OCH3), 110.9, 111.2, 115.9, 116.2, 116.3,
118.3, 121.7, 124.5 (quaternary C), 124.8 (quaternary C),
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analysis calcd (%) for C96H84N12O12Pd4: C, 57.02; H, 4.19;
N, 8.31; found: C, 57.33; H, 3.85; N, 8.62 %.
General Procedure for Mizoroki−Heck Reaction
In a typical reaction, a mixture of aryl halide (1 mmol),
styrene (1.4 mmol), base (1.1 equiv), and palladium
catalyst (0.2–0.02 mol% Pd atom loading) in TBAB (2 g)
was stirred at 140 °C for an appropriate period of time
(2−16 h). After the reaction was cooled, the product was
extracted with ethyl ether (3×10 mL) and the organic layer
was dried with anhydrous MgSO4. The solvent was
removed completely under high vacuum to give a crude
product. NMR yield of the crude product was determined
by using 1,3,5-trimethoxybenzene as internal standard. The
pure product was isolated by column chromatography.
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Advanced Synthesis & Catalysis
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11
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Advanced Synthesis & Catalysis
FULL PAPER
Tetranuclear Palladium Complexes of Abnormal N-
Heterocyclic Carbene Ligands and their Catalytic
Activities in Mizoroki-Heck Coupling Reaction of
Electron-Rich Aryl Chlorides
Adv. Synth. Catal. Year, Volume, Page – Page
Jhen-Yi Lee, Yong-Siang Su, Yu-Shan Wang and
Hon Man Lee*
12
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