Palladacycles bearing COOH-/ester-functionalized N-heterocyclic carbenes: Divergent syntheses and catalytic applications

Article abstract of DOI:10.1002/aoc.4703

Two new [C^N]-type palladacyclic dinuclear complexes bearing carboxylate-containing N-heterocyclic carbenes (NHCs) were synthesized, and in both cases the carboxylato-NHC ligand adopts a bridging mode. Both complexes proved to be suitable precursors, whic

Full text of DOI:10.1002/aoc.4703

Received: 17 August 2018
DOI: 10.1002/aoc.4703
Revised: 18 October 2018
Accepted: 22 October 2018
F U L L P A P E R
Palladacycles bearing COOH‐/ester‐functionalized N‐
heterocyclic carbenes: Divergent syntheses and catalytic
applications
Yanyan Hu | Shuai Guo
Department of Chemistry, Capital Normal
Two new [C^N]‐type palladacyclic dinuclear complexes bearing carboxylate‐
University, Beijing, People's Republic of
China
containing N‐heterocyclic carbenes (NHCs) were synthesized, and in both
cases the carboxylato‐NHC ligand adopts a bridging mode. Both complexes
Correspondence
proved to be suitable precursors, which can be used to divergently access
palladacycles bearing ester‐ or COOH‐functionalized NHCs upon esterification
or acidolysis. In the esterification reactions, alkyl halides are found to selec-
tively react with the carboxylato moieties, and the palladacycle scaffold is
retained even when excess haloalkane is employed. In the acidolysis reactions,
the desired COOH‐tethered complexes can only be obtained when stoichiomet-
ric acid (with respect to Pd) is used, while excess acid destroys the metallacycle
scaffold. Finally, a preliminary catalytic study reveals the good performances of
all newly synthesized complexes in direct aromatic C─H functionalization
reactions with alkynes. Poisoning experiments indicate that these
hydroarylation reactions are likely to be homogeneously catalyzed.
Shuai Guo, Department of Chemistry,
Capital Normal University, Beijing,
People's Republic of China.
Email: guoshuai@cnu.edu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21502122;
Beijing Natural Science Foundation,
Grant/Award Number: 2164057
KEYWORDS
N‐heterocyclic carbene, palladacycle
1 | INTRODUCTION
typically spectating) donor atom in a single ligand can
in principle assist the stabilization of metal centers,
Over the past decade, the chemistry of N‐heterocyclic
carbenes (NHCs) has experienced a rapid develop-
ment.[1] In contrast to the extensively studied phos-
phines, one of the notable advantages of NHCs is the
relative ease of ligand functionalizations.[2] The incorpo-
ration of additional donors to the arms of NHCs confers
on the corresponding complexes enhanced stabilities
and tunable stereoelectronic properties.[3] In addition,
functionalized NHCs also render feasible the immobili-
zation of carbene complexes onto various solid supports,
which can be employed to access heterogeneous cata-
lysts.[4] In this regard, the attachment of O‐containing
moieties (e.g. carboxylates, esters, ethers, etc.) to NHCs
is particularly attractive, since such a combination of a
hard (oxygen, typically hemilabile) and soft (carbon,
which has found manifold applications especially in
metal‐mediated catalysis. Recently, this research area
has been comprehensively reviewed by Braunstein and
co‐workers.2f
NHC‐based palladacycles of type I (Figure 1), combin-
ing the structural benefits of metallacycle scaffolds[5] and
NHCs, can be regarded as analogues of well‐established
phosphine‐based palladacycle precatalysts (e.g. Buchwald
type,[6] Indolese–Studer type,[7] etc.) and have attracted
rapidly growing interest in recent years. Such motifs
reported by Nolan et al.[8] and others[9] have exhibited
outstanding catalytic performances in various organic
transformations.
Notably, despite the fact that donor‐functionalized
NHCs (df‐NHCs) can impart potential advantages to
Appl Organometal Chem. 2019;e4703.
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https://doi.org/10.1002/aoc.4703

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HU AND GUO
FIGURE 1 NHC‐based palladacycle I, and comparison of pre‐functionalization route A versus divergent functionalization route B to
synthesize the target complexes II
corresponding complexes and facilitate their applications
2.1 | Syntheses of carboxylate‐NHC
(vide supra), the syntheses and catalysis of palladacycles I
bearing df‐NHCs have received little attention, and only a
small number of scattered examples[10] have been hith-
erto known. These prior works inspired us to explore
the syntheses of such palladacycles bearing carboxylic
acid‐/ester‐functionalized NHCs (II, Figure 1). The
choice of carboxylate‐based groups is not only due to
the benefit brought by O‐containing functionalities (vide
supra) but also because of their rich organic chemistry
easing their further potential modifications.
palladium precursors
Two COOH‐functionalized imidazolium salts 1a and 1b
were prepared (Scheme 1). Compound 1a was accessed
following a slightly modified procedure,[15] and salt 1b
was newly synthesized and further fully characterized.
Notably, the COOH proton survives from the elimination
with the chloride anion, and no formation of betaine‐like
species was observed.
Subsequent palladations with acetato‐bridged dimeric
precursor [Pd(μ‐CH3COO)(Ppy)]2 (2; Ppy
=
2‐(2‐
Based on the background discussed above and our
continuing interest in carbene chemistry, herein we
report the syntheses of palladacycles bearing
carboxylate‐anchored NHCs. Their reactivities in
acidolyses/esterifications were also systematically investi-
gated. Comparative catalytic studies including poisoning
experiments were also carried out employing the newly
synthesized df‐NHC palladacycles as precatalysts in direct
C─H functionalizations with alkynes.
pyridinyl)phenyl‐κC,κN) in the presence of K2CO3 led to
the formation of two dinuclear complexes 3a and 3b, in
which the carboxylate‐tethered imidazolin‐2‐ylidenes
serve as bridging ligands. Notably, in previous studies,
carboxylate ion‐anchored NHCs predominantly adopt
monodentate (carbene‐ligated) or chelating coordination
fashions.[16] Such bridging modes have been reported
for df‐NHCs bearing other types of functionalities such
as oxazolines and pyridines.[17]
Complexes 3a and 3b were isolated as pale yellow
solids, which are considerably stable even in solution. In
addition, 3a and 3b show poor solubility in common
organic solvents such as dimethylsulfoxide (DMSO),
CH2Cl2 and CH3OH, probably due to their rigid dimeric
2 | RESULTS AND DISCUSSION
Although the target complexes (II) can be accessed via a
traditional ‘pre‐functionalization’ route (route A,
1
structure. In the H NMR spectrum of 3a in CD3OD, the
Figure 1),[11]
a
more attractive ‘post‐modification’
methylene protons of the NHC arms give four sets of
broad signals at 5.10, 4.28, 2.87 and 2.68 ppm with an inte-
gration ratio of 1:1:1:1, which implies that the methylene
protons become diastereotopic due to the loss of free rota-
tion of the NHC arms. In addition, the broadness of these
resonances indicates the presence of dynamic behavior in
the solution of 3a. Similar signals were observed as well in
the case of 3b with N‐phenyl substituents.
route[12] (route B, Figure 1) will be explored herein, since
this strategy is highly modular and divergent (route B ver-
sus route A). However, it should also be noted that the
acidolyses/esterifications[13] designed in route B may
undermine the metallacycle rending this route less
straightforward, since palladacyclic complexes especially
those bearing strongly electron‐releasing carbanions are
known to undergo various competing reactions (e.g.
acidolyses, oxidative additions, etc.).[14]
The identity of 3a was unambiguously confirmed
using X‐ray diffraction analysis of a single crystal

HU AND GUO
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SCHEME 1 Syntheses of carboxylate‐
NHC bridged dinuclear palladacycles 3a
and 3b
obtained by slow evaporation of a solution of 3a in
CH2Cl2. The molecular structure is depicted in Figure 2.
The molecule contains two palladium centers, each of
which essentially adopting square planar geometry. The
carboxylate‐functionalized imidazolin‐2‐ylidenes serve as
the bridges interconnecting two metal centers leading
the formation of a 14‐membered metallocycle. In addi-
tion, the carbene is cis to the phenyl donor of [C^N]‐che-
lator, the coordination fashion of which has been
observed in some previously reported mononuclear
NHC‐based palladacycles.[8,9]
representative and treated with CH3I as electrophile to
investigate this reactivity (Scheme 2). Gratifyingly, the
reaction proceeded smoothly and afforded ester‐anchored
complex 4a in excellent yield.
Such observation prompted us to further expand the
scope of alkyl halides (Scheme 3). It was found that
alkyl bromides and chlorides can undergo bond scis-
sions cleanly producing
a wide range of ester‐
functionalized NHC palladacycles in good to excellent
yields. Note that in the cases of chloro‐ or
2.2 | Esterifications of carboxylate‐NHC
complexes 3a and 3b
The carboxylate‐NHC complexes 3a and 3b may serve as
potential precursors to access ester‐ or COOH‐
functionalized complexes upon esterification or
acidolysis. As an initial attempt, 3a was selected as a
SCHEME 2 Reaction of dinuclear complex 3a with CH3I
FIGURE 2 Molecular structure of
complex 3a showing 50% probability
ellipsoids. Hydrogen atoms and another
crystallographically independent molecule
have been omitted for clarity

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HU AND GUO
SCHEME 3 Divergent syntheses of
ester‐/COOH‐functionalized NHC
palladacycles starting from 3a or 3b
bromoalkanes, a more forcing condition (90°C in
CH3CN) needs to be applied to achieve a higher conver-
sion. In all cases, no formation of palladium black was
observed.
Finally, several attempts to use aryl halides to achieve
esterification were to no avail even at an elevated reaction
temperature, and a non‐negligible amount of palladium
black was observed.
Additionally, 3a was used as a representative and
treated with (3‐chloropropyl)trimethoxysilane. This reac-
tion successfully afforded trimethoxysilyl‐functionalized
complex 7a as the product, which can be potentially fur-
ther grafted onto solid supports such as silica to access
heterogeneous catalysts.
For the ester‐functionalized complexes 4a–7a, the
protons of N‐ethylene moieties give two sets of compli-
cated multiplets falling in the range 4.57–4.90 and 3.02–
3.30 ppm, confirming the presence of geminal couplings
brought by the diastereotopic ethylene protons. This sug-
gests that although the carboxylate‐NHC bridges of 3a
and 3b were cleaved, the free rotation of ester‐anchored
2.3 | Acidolyses of carboxylate‐NHC
complexes 3a and 3b
Then, the reactivity of 3a and 3b towards hydrochloric
acid was examined (Scheme 3). Gratifyingly, when
treated with two equivalents of HCl (in aqueous solu-
tion), 3a and 3b underwent regioselective acidolyses
affording COOH‐tethered NHC palladacycle 8a and 8b
as the sole product. Similar to ester‐tethered analogues,
the N‐ethylene protons are found to be diastereotopic.
The 13C NMR signals of carbonyl group in 8a and 8b
show significant upfield shift (Δδ > 7 ppm) compared to
those for ester‐functionalized counterparts 4a–6b.
Finally, in all esterification reactions (4a–7a), no
other side reactions occurred despite large excess of alkyl
halides being utilized. However, in the cases of acidolysis
reactions, 8a and 8b can be accessed only when two
equivalents of HCl are used. By contrast, in the presence
of four equivalents of HCl, the protonation of aryl carban-
ion was observed as evidenced by the formation of free 2‐
phenylpyridine in a yield of 48%, which is expected to be
formed via the protonation of aryl carbanion followed by
ligand dissociation. Finally, in all attempts, no formation
of imidazolium salt generated from protonation of
carbene was found. This clearly demonstrates the marked
N‐substituents is restricted.
A positive correlation
between the rotation barrier and the size of halido
coligand has been observed in a previous study.[18] Since
6a bears the smallest chlorido coligand (versus Br and
I), the corresponding rotation barrier is expected to be
the lowest. In view of this, 6a was chosen as a representa-
tive and further studied by variable‐temperature 1H NMR
experiments. It was found that the ethylene signals did
not show coalescence even at 353 K, which indicates a
high rotation barrier for 6a and other analogues reported
herein. The carbene signals of 4a–6b were found to fall in
a narrow range of 173.4–174.6 ppm and the resonances at
172.2–172.8 ppm are assigned to Ccarbonyl atoms, which
were confirmed by 2D HMBC experiments.

HU AND GUO
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resistance of Pd─Ccarbene bonds towards acid, which indi-
cates the potential use of such complexes in catalysis
especially those requiring acidic conditions (vide infra).
Complexes 4a, 5a, 6b and 8b were further character-
ized using X‐ray diffraction analyses of their single crys-
tals (Figure 3). Selected crystallographic data are listed
in Table SI‐1 (supporting information). The bite angles
ranging from 80.96° to 81.95° are comparable to those
reported in non‐functionalized[8,9] or functionalized[10]
NHC palladacycles. In the solid‐state structure of 8b, it
was found that two molecules dimerize through hydro-
gen bonds, which is similar to the case of acetic acid.[19]
an iodide ligand source cleanly regenerated 4a demon-
strating the revisable coordination–decoordination
behavior (hemilability) of the carboxylate‐based function-
ality. This may also account for the exhibited superiority
of the functionalized NHC complexes synthesized herein
over the non‐functionalized counterpart in catalytic reac-
tions (vide infra).
2.5 | Catalytic studies
The observed robustness of Pd─Ccarbene bonds in
acidolysis reactions (vide supra) encouraged us to exam-
ine the catalytic performances of the complexes reported
herein in certain reactions under acidic conditions. In a
preliminary study, direct aromatic C─H functionalization
with alkynes[22] was chosen. In a model reaction,
mesitylene (9) and ethyl propiolate (10) were employed
as the substrates, and trifluoroacetic acid was used as
the solvent (Table 1). In the very first attempt, the
hydroarylation reaction was carried out at 60°C affording
ethyl (Z)‐2,4,6‐trimethylphenylacrylate (11) and the bis
addition product 12 in a total yield of 84% (entry 1). This
reaction was found to proceed smoothly at a milder con-
dition (at 25°C) but for a prolonged time, which can also
give a comparable yield and selectivity.
2.4 | Hemilability of functionalities
The ester/COOH functionalities incorporated in the
newly synthesized complexes are potentially hemilabile
groups. The oxygen atom in the NHC arm may serve as
a donor to stabilize an unsaturated metal center, and dis-
sociate in the presence of a stronger incoming ligand.
Such behavior is believed to bring stabilization benefits
during metal‐mediated catalysis.[2,20] As a representative,
4a was treated with 1 equivalent of AgBF4 as a halide
scavenger, and this reaction gave the formation of a ten-
tatively assigned O‐coordinated chelating complex A[21]
1
as evidenced by H NMR spectroscopy (Figure SI‐1 in
Under this condition, all other ester‐ or COOH‐
functionalized palladacyclic complexes were subjected to
the supporting information). Further addition of NaI as
FIGURE 3 Molecular structures of complexes 4a, 5a, 6b and 8b showing 50% probability ellipsoids. Hydrogen atoms (except for COOH
hydrogen in 8b), solvent molecules, another crystallographically independent molecule (for 4a) and the disordered ethyl group (for 5a)
have been omitted for clarity

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HU AND GUO
TABLE 1 Hydroarylation of ethyl propiolate (10) with mesitylene (9)a
Yield (%)b
Total
Cat.
loading (%)
Temp.
(°C)
Time
(h)
Poisoning
agent
Entry
Cat.
11
12
1
3a
3a
3b
4a
4a
4a
4b
5a
5b
6a
6b
8a
8b
0.5
0.5
0.5
1
60
25
25
25
25
25
25
25
25
25
25
25
25
2
24
24
24
24
24
24
24
24
24
24
24
24
—
86
88
82
82
84
65
67
88
86
88
80
95
92
54
58
50
62
64
53
55
61
61
61
60
60
61
32
30
32
20
20
12
12
27
25
27
20
35
30
2
—
3
—
4
—
5c
6d
7
1
Hg
PPh3
—
1
1
8
1
—
9
1
—
10
11
12
13
1
—
1
—
1
—
1
—
aReaction conditions: (i) 1.5 mmol of mesitylene (9), 1.0 mmol of ethyl propiolate (10), 1 mol% (for mononuclear complexes) or 0.5 mol% (for dinuclear com-
plexes) of precatalyst, trifluoroacetic acid (1 ml).
bYields (with respect to ethyl propiolate) were determined by 1H NMR spectroscopy with 1,3,5‐trimethoxylbenzene as an internal standard.
cTwo drops of mercury were added into the reaction tube after 1 h.
d0.0003 mmol (3% with respect to catalyst loading) of PPh3 was added into the reaction tube after 1 h.
this hydroarylation reaction, revealing the good perfor-
mances of all precatalysts. The highest selectivity was
observed in the cases of iodido complexes 4a and 4b,
the reason for which is currently not clear yet. Addition-
ally, complexes bearing NHCs with N‐methyl substituents
are generally slightly superior to or comparable with their
counterparts with N‐phenyl substituents (e.g. entry 10
versus entry 11). It should be additionally noted that
under an acidic condition, the ester groups of the
precatalysts may undergo acidolyses generating structur-
ally similar species.13b This may account for the small dif-
ferences of catalytic activities for most complexes.
The hemilability of carboxylate‐based functionalities
in the NHC arms may bring potential benefits to the cata-
lytic reactions (vide supra). To give a deeper insight, a non‐
functionalized NHC counterpart [PdI(C^N)(Ph,Buimy)]
(C^N = 2‐(2‐pyridinyl)phenyl‐C,N, Ph,Buimy = 1‐phenyl‐
4‐nbutylimdazolin‐2‐ylidene; B) was used to make a com-
parison with its structurally comparable analogue 4b.
The former precatalyst gives the formation of the product
11 as the predominant product in a yield of 40% (2 h), while
the yield can reach 53% in the case with 4b as the
precatalyst. The superior catalytic behavior of the latter is
tentatively attributed to the positive effects brought by
the oxygen‐containing functionality, which remains to be
further investigated in greater detail in future.
In addition, a few poisoning experiments were carried
out to distinguish the homogeneity/heterogeneity of the
present catalytic system. Elemental mercury[23] is known
to stall or markedly undermine catalytic activity if the
palladium‐mediated reaction is heterogeneously cata-
lyzed. As a supplementary experiment, 3 mol% (with
respect to the catalyst) of PPh3 was also applied as a poi-
soning agent. This method, although less widely used
compared to Hg poisoning, was proposed by Widegren
and Finke[24] and has been used by some others.[25] It
has been suggested that only a few active metal sites on
the surface of a heterogeneous catalyst can be sufficiently
poisoned by <<1.0 equivalent (with respect to the cata-
lyst) of a ligand such as PPh3. It was found that no sharp
drop of yield was observed for the catalysis in the pres-
ence of these poisoning agents ( entries 4–6), which indi-
cates the hydroarylation reactions are likely to be
homogeneously catalyzed.

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TABLE 2 Hydroarylation of alkynes with 8b as precatalysta
Entry
Arene
Alkyne
Product (yield, %)
1
2
3
4
5
6
7
aReaction conditions: (i) 1.5 mmol of mesitylene (9), 1.0 mmol of ethyl propiolate (10), 1 mol% of 8b, trifluoroacetic acid (1 ml), 25°C, 24 h.
bYields (with respect to ethyl propiolate) were determined by 1H NMR spectroscopy with 1,3,5‐trimethoxylbenzene as an internal standard.
With 8b as the representative precatalyst, a selection
of electronically and sterically varied alkynes and arenes
were employed to further expand the substrate scope
(Table 2). When employing ethyl propiolate as the alkyne
substrate, good yields can be obtained in both cases of
mesitylene or pentamethylbenzene (entries 1 and 3).
However, moderate yields were observed when a more
challenging alkyne substrate, phenylacetylene, was used
(entries 2 and 4). To further investigate the functionality
tolerance, arene substrates with various functional groups
(e.g. Br, OH) were employed, and the catalytic reactions
gave the desired products in moderate yields (entries 5–
7). In a particular case also accompanying a cyclization
process, a lactone product can be achieved in a low yield
of 34% (entry 5).
3 | CONCLUSIONS
In summary, this study reports two dinuclear
palladacyclic complexes, which bear carboxylate‐tethered
NHC ligands adopting bridging modes. These complexes
proved to be suitable precursors to access carboxylic
acid‐/ester‐functionalized NHC palladacycles in
a

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HU AND GUO
divergent and expedient manner. No side reactions
involving aryl‐based carbanions occurred in the cases of
esterification even when excess aryl halide is employed,
while the reactivity in acidolysis reactions is found to be
dependent on the amount of HCl used. Finally and in a
preliminary study, all complexes are found to exhibit
good catalytic behaviors in direct aromatic C─H
functionalizations with alkynes. The superior catalytic
performance exhibited by the df‐NHC complex over its
non‐functionalized counterpart has been tentatively
attributed to the hemilability of the carboxylate‐based
NHC substituents. Poisoning experiments with Hg or
PPh3 suggest a homogeneous nature of the catalytic
system.
This report provides a series of new examples of less
studied df‐NHC‐containing palladacycles. To the best of
our knowledge, this is also the first report demonstrating
that esterification/acidolysis of NHC arms can be
achieved without affecting palladacyclic motifs. The
divergent synthetic route and catalytic studies explored
herein will facilitate the syntheses and applications of
NHC palladacycles and related complexes. Further
expanding the structural diversity of target complexes
and the substrate scope of catalytic hydroarylations is cur-
rently underway in our laboratory.
4.2 | Synthesis of 1b
A
25 ml Schlenk tube was charged with 1‐
phenylimidazole (506 μl, 4.0 mmol), 3‐chloroproionic
acid (446 mg, 4.1 mmol) and toluene (5 ml). The reaction
mixture was heated under reflux for 12 h. After cooling to
ambient temperature, the precipitate was collected by
centrifugation and washed with ethyl acetate (3 ml × 5).
The residue was fully dried under vacuum affording the
product as a white solid in a yield of 86%. 1H NMR
(DMSO‐d6, 600 MHz, δ, ppm): 12.74 (s, 1H, COOH),
9.97 (s, 1H, NCN), 8.34 (s, 1H, Ar─H), 8.05 (s, 1H,
Ar─H), 7.81–7.79 (m, 2H, Ar─H), 7.68–7.66 (m, 2H,
Ar─H), 7.60–7.58 (m, 1H, Ar─H), 4.46 (t, 2H, NCH2,
J = 6.6 Hz), 3.03 (s, 2H, NCH2CH2, J = 6.6 Hz). 13C
NMR (DMSO‐d6, 175 MHz, δ, ppm): 171.6 (C═O), 136.0,
134.7, 130.2, 129.6, 123.6, 121.6, 120.7 (Ar─C), 45.2
(NCH2), 33.8 (NCH2CH2).
4.3 | General procedure for syntheses of
3a and 3b
A 25 ml Schlenk tube was charged with compound 2
(0.5 mmol, 320 mg) and imidazolium salt (1.0 mmol).
Then, K2CO3 (5 mmol, 690 mg) and CH3CN (5 ml) were
added. The reaction mixture was heated for 24 h at 100°C.
All the volatiles were removed under vacuum yielding a
yellow solid. The desired product was extracted using
CH2Cl2 (3 ml × 3) and purified using column chromatog-
raphy (SiO2).
4 | EXPERIMENTAL
4.1 | General considerations
Unless otherwise stated, all the manipulations were car-
ried out without taking precautions to exclude air and
moisture. All chemicals and solvents were used as
received without further purification if not mentioned
Complex 3a was obtained as a pale yellow solid in a
yield of 78%. H NMR (CD3OD, 600 MHz, δ, ppm): 8.58
1
(br s, 2H, Pyr─H), 7.94–7.88 (m, 4H, Ar─H), 7.62 (d,
2H, Ar─H, J = 7.2 Hz), 7.32–7.25 (m, 6H, Ar─H), 7.06–
7.04 (m, 2H, Ar─H), 6.91 (t, 2H, Ar─H, J = 7.8 Hz),
6.52 (br s, 2H, Ar─H), 5.10, 4.28 (br s, 4H, NCH2), 3.86
(s, 6H, NCH3), 2.87, 2.68 (br s, 4H, NCH2CH2). MS
1
otherwise. H NMR and 13C NMR spectra were recorded
with a Varian 600 MHz spectrometer at Capital Normal
University, a Bruker 400 MHz spectrometer at Beijing
University of Chemical Technology, or
a
Bruker
(ESI) m/z 414 [M/2
+
H]+. Anal. Calcd for
700 MHz spectrometer at Beijing Institute of Technology.
The chemical shifts were internally referenced to the
residual solvent signals relative to (CH3)4Si (1H, 13C).
ESI mass spectra were measured using an Agilent 6540
Q‐TOF mass spectrometer at Beijing University of Chem-
ical Technology. X‐ray single crystal diffraction analyses
were done using a SuperNova, Dual, Cu at zero, AtlasS2
diffractometer at Beijing Normal University or a Rigaku
XtaLAB P200 MM003 diffractometer at Capital Normal
University or an Agilent Xcalibur Eos Gemini diffractom-
eter at Beijing University of Chemical Technology. Ele-
mental analyses were done with a Vario EL cube
elemental analyzer at Beijing University of Chemical
Technology.
C36H34N6O4Pd2 (%): C, 52.25; H, 4.14; N, 10.16. Found
(%): C, 51.97; H, 4.36; N, 10.38.
Complex 3b was obtained as a pale yellow solid in a
1
yield of 72%. H NMR (CD3OD, 600 MHz, δ, ppm): 8.69
(br s, 2H, Pyr─H), 8.04–8.03 (m, 4H, Ar─H), 7.96–7.93
(m, 2H, Ar─H), 7.84–7.82 (m, 2H, Ar─H), 7.68 (s, 2H,
Ar─H), 7.58 (s, 2H, Ar─H), 7.45–7.44 (m, 3H, Ar─H),
7.36–7.34 (m, 5H, Ar─H), 7.26–7.24 (m, 2H, Ar─H),
6.85–6.83 (m, 2H, Ar─H), 6.57 (br s, 2H, Ar─H), 6.33
(br s, 2H, Ar─H), 5.46, 4.39 (br s, 4H, NCH2), 2.92–2.89
(m, 2H, NCH2CH2), 2.80–2.75 (m, 2H, NCH2CH2). MS
(ESI) m/z 476 [M/2
+
H]+. Anal. Calcd for
C46H38N6O4Pd2 (%): C, 58.05; H, 4.02; N, 8.83. Found
(%): C, 57.88; H, 3.95; N, 8.46.

HU AND GUO
9 of 12
Complex 5a was obtained as a pale yellow solid in a
yield of 96%. H NMR (CDCl3, 600 MHz, δ, ppm): 9.48
4.4 | General procedure for syntheses of
4a and 4b
1
(dd, 1H,J1 = 6.6 Hz, J2 = 1.2 Hz, Pyr─H), 7.81 (td, 1H,
J1 = 1.8 Hz, J2 = 7.8 Hz, Ar─H), 7.73–7.71 (m, 1H, Ar─H),
7.56 (dd, 1H, J1 = 0.6 Hz, J2 = 7.8 Hz, Ar─H), 7.22–7.19 (m,
2H, Ar─H), 7.19 (d, 1H, J = 1.8 Hz, Ar─H), 7.08 (td, 1H,
J1 = 7.2 Hz, J2 = 0.6 Hz, Ar─H), 6.97 (d, 1H, J = 1.8 Hz,
Ar─H), 6.90 (td, 1H, J1 = 7.8 Hz, J2 = 1.2 Hz, Ar─H),
6.02 (d, 1H, J = 7.2 Hz, Ar─H), 4.70–4.61 (m, 2H, NCH2),
4.09 (q, 2H, J = 6.0 Hz, OCH2), 3.94 (s, 3H, NCH3), 3.14–
3.02 (m, 2H, NCH2CH2), 1.19 (t, 3H, J = 7.2 Hz,
OCH2CH3). 13C NMR (CDCl3, 150 MHz, δ, ppm): 174.1
(NCN), 172.4 (C═O), 164.9, 156.2, 151.8, 147.3, 139.1,
136.7, 130.5, 124.9, 124.4, 123.2, 123.1, 122.6, 118.7 (Ar─C),
61.4 (OCH2), 47.3 (NCH2), 39.3 (NCH3), 35.9 (NCH2CH2),
14.8 (OCH2CH3). MS (ESI) m/z 442 [M − I]+. Anal. Calcd
for C20H22BrN3O2Pd (%): C, 45.95; H, 4.24; N, 8.04. Found
(%): C, 45.72; H, 4.56; N, 8.38.
Complex 5b was obtained as a pale yellow solid in a
yield of 90%. 1H NMR (CDCl3, 600 MHz, δ, ppm): 9.45 (s,
1H, Pyr─H), 7.97–7.96 (m, 2H, Ar─H), 7.72 (s, 1H, Ar─H),
7.60 (s, 1H, Ar─H), 7.40–7.26 (m, 6H, Ar─H), 7.14 (s, 1H,
Ar─H), 6.98 (s, 1H, Ar─H), 6.84 (s, 1H, Ar─H), 6.03 (s,
1H, Ar─H), 4.85, 4.77 (br s, 2H, NCH2), 4.10 (br s, 2H,
OCH2CH3), 3.19, 3.13 (br s, 2H, NCH2CH2), 1.20 (br s,
3H, OCH2CH3). 13C NMR (CDCl3, 150 MHz, δ, ppm):
174.2 (NCN), 172.5 (C═O), 164.9, 156.3, 151.9, 146.9,
140.8, 138.9, 136.7, 130.2, 129.7, 128.7, 125.8, 124.6, 124.2,
124.0, 122.9, 122.5, 118.6 (Ar─C), 61.5 (OCH2), 47.8
(NCH2), 35.6 (NCH2CH2), 14.8 (OCH2CH3). MS (ESI) m/
z 504 [M − I]+. Anal. Calcd for C25H24BrN3O2Pd (%): C,
51.35; H, 4.14%; N, 7.19%. Found (%): C, 51.16%; H,
4.56%; N, 6.93%.
A 25 ml Schlenk tube was charged with compound 3a or
3b (0.02 mmol) and acetone (1 ml). To this mixture, CH3I
(0.6 mmol) was added. Then the reaction mixture was
stirred for 12 h at 40°C. All the volatiles were evaporated
under vacuum, and the crude product was washed with
diethyl ether (1 ml × 3). The residue was fully dried under
vacuum affording the product as a yellow solid.
Complex 4a was obtained in a yield of 99%.1 H NMR
(CDCl3, 600 MHz, δ, ppm): 9.70 (s, 1H, Pyr─H), 7.80–
7.72 (m, 2H, Ar─H), 7.56–7.55 (m, 1H, Ar─H), 7.19–
7.18 (m, 2H, Ar─H), 7.09–7.07 (m, 1H, Ar─H), 6.99–
6.98 (m, 1H, Ar─H), 6.94–6.91 (m, 1H, Ar─H), 5.92 (d,
1H, Ar─H, J = 6.0 Hz), 4.65–4.57(m, 2H, NCH2), 3.88
(s, 3H, NCH3), 3.62 (s, 3H, OCH3), 3.13–3.03 (m, 2H,
NCH2CH2). 13C NMR (CDCl3, 150 MHz, δ, ppm): 173.4
(NCN), 172.7 (C═O), 164.8, 157.8, 154.7, 147.4, 138.8,
136.0, 130.5, 125.0, 124.5, 123.5, 123.2, 122.8, 119.0
(Ar─C), 52.5 (OCH3), 47.3 (NCH2), 39.4 (NCH3), 35.2
(NCH2CH2). MS (ESI): m/z 428 [M − I]+. Anal. Calcd
for C19H20IN3O2Pd (%): C, 41.07; H, 3.63; N, 7.56. Found
(%): C, 41.02; H, 3.75; N, 7.28.
1
Complex 4b was obtained in a yield of 98%. H NMR
(CDCl3, 600 MHz, δ, ppm): 9.68 (d, 1H, Pyr─H,
J = 6 Hz), 7.93 (d, 2H, Ar─H, J = 12 Hz), 7.72 (t, 1H, Ar─H,
J = 6 Hz), 7.64–7.62 (m, 1H, Ar─H), 7.46 (d, 1H, Ar─H,
J = 6 Hz), 7.40 (s, 1H, Ar─H), 7.36–7.34 (m, 2H, Ar─H),
7.28–7.27 (m, 2H, Ar─H), 7.11 (t, 1H, Ar─H, J = 6.0 Hz),
7.03 (t, 1H, Ar─H, J = 6.0 Hz), 6.91 (t, 1H, Ar─H,
J = 6.0 Hz), 6.03 (d, 1H, Ar─H, J = 6.0 Hz), 4.88–4.81 (m,
1H, NCH2), 4.76–4.72 (m, 1H, NCH2), 3.65 (s, 3H,
OCH3), 3.24–3.19 (m, 1H, NCH2CH2), 3.14–3.10 (m, 1H,
NCH2CH2). 13C NMR (CDCl3, 150 MHz, δ, ppm): 173.6
(NCN), 172.8 (C═O), 164.9, 157.9, 154.9, 147.1, 140.8,
138.7, 136.2, 130.3, 129.7, 128.8, 125.9, 124.8, 124.3, 124.0,
123.3, 122.9, 118.9 (Ar─C), 52.5 (OCH3), 47.9 (NCH2),
35.0 (NCH2CH2). MS (ESI): m/z 490 [M − I]+. Anal. Calcd
for C24H22IN3O2Pd (%): C, 46.66; H, 3.59; N, 6.80. Found
(%): C, 46.42; H, 3.78; N, 6.47.
4.6 | General procedure for the syntheses
of 6a and 6b
A 25 ml Schlenk tube was charged with compound 3a or
3b (0.02 mmol) and CH3CN (1 ml). To this mixture,
PhCH2Cl (0.6 mmol) was added. Then the reaction mix-
ture was stirred for 24 h at 90°C. All the volatiles were
evaporated under vacuum, and the crude product was
washed with diethyl ether (1 ml × 3). The residue was
fully dried under vacuum affording the product as a pale
yellow solid.
4.5 | General procedure for syntheses of
5a and 5b
1
A 25 ml Schlenk tube was charged with compound 3a or
3b (0.02 mmol) and CH3CN (1 ml). To this mixture,
CH3CH2Br (0.6 mmol) was added. Then the reaction mix-
ture was stirred for 24 h at 90°C. All the volatiles were
evaporated under vacuum, and the crude product was
washed with diethyl ether (1 ml × 3). The residue was
fully dried under vacuum affording the product.
Complex 6a was obtained in a yield of 93%. H NMR
(CDCl3, 700 MHz, δ, ppm): 9.36 (s, 1H, Pyr─H), 7.85–
7.84 (m, 1H, Ar─H), 7.76–7.75 (m, 1H, Ar─H), 7.60–
7.59 (m, 1H, Ar─H), 7.36–7.26 (m, 6H, Ar─H), 7.17 (s,
1H, Ar─H), 7.10–7.08 (m, 1H, Ar─H), 6.98 (s, 1H, Ar─H),
6.89–6.88 (m, 1H, Ar─H), 6.10 (d, 1H, J = 7.0 Hz, Ar─H),
5.12, 5.09 (d, 2H, J = 12.0 Hz, OCH2Ph), 4.78–4.69 (m,

10 of 12
HU AND GUO
2H, NCH2CH2), 3.99 (s, 3H, NCH3), 3.24–3.14 (m, 2H,
NCH2CH2). 13C NMR (CDCl3, 175 MHz, δ, ppm): 174.5
(NCN), 172.2 (C═O), 164.8, 155.2, 150.3, 147.3, 139.2,
137.1, 136.2, 130.5, 129.2, 128.9, 124.8, 124.4, 123.2,
122.8, 122.5, 118.6 (Ar─C), 67.2 (OCH2Ph), 47.2 (NCH2),
39.2 (NCH3), 36.2 (NCH2CH2). MS (ESI): m/z 504
[M − Cl]+. Anal. Calcd for C25H24ClN3O2Pd (%): C,
55.57; H, 4.48; N, 7.78. Found (%): C, 55.09; H, 4.18; N,
7.45.
(OCH2CH2CH2). MS (ESI): m/z 576 [M − Cl]+. Anal.
Calcd for C24H32ClN3O5PdSi (%): C, 47.06; H, 5.27; N,
6.86. Found (%): C, 46.75; H, 5.14; N, 6.71.
4.8 | General procedure for syntheses of
8a and 8b
A 25 ml Schlenk tube was charged with 3a or 3b
(0.06 mmol) and CH2Cl2 (3 ml). To this mixture, an aque-
ous solution of HCl (0.12 mmol) was added. The mixture
was stirred at 40°C for 24 h. All the volatiles were evapo-
rated under vacuum affording the product as a bright yel-
low solid.
1
Complex 6b was obtained in a yield of 91%. H NMR
(CDCl3, 400 MHz, δ, ppm): 9.28 (d, 1H, J = 4.0 Hz,
Pyr─H), 7.97 (d, 2H, J = 7.6 Hz, Ar─H), 7.67 (td, 1H,
J1 = 1.2 Hz, J2 = 7.6 Hz, Ar─H), 7.53 (m, 1H, Ar─H),
7.34–7.26 (m, 9H, Ar─H), 7.23–7.20 (m, 2H, Ar─H),
7.13–7.10 (m, 1H, Ar─H), 6.91 (t, 1H, J = 7.2 Hz, Ar─H),
6.75 (td, 1H, J1 = 0.8 Hz, J2 = 7.6 Hz, Ar─H), 6.02 (d, 1H,
1
Complex 8a was obtained in a yield of 96%. H NMR
(CDCl3, 600 MHz, δ, ppm): 9.19 (br s, 1H, Pyr─H),
7.80–7.77 (m, 1H, Ar─H), 7.69–7.68 (m, 1H, Ar─H),
7.53 (d, 1H, J = 7.8 Hz, Ar─H), 7.20–7.18 (m, 1H, Ar─H),
7.14 (br s, 1H, Ar─H), 7.05–7.02 (m, 1H, Ar─H), 6.96–
6.95 (m, 1H, Ar─H), 6.89–6.86 (m, 1H, Ar─H), 6.07 (br
s, 1H, Ar─H), 4.63, 4.53 (br s, 2H, NCH2), 3.90 (s, 3H,
NCH3), 3.14–3.10, 2.96–2.94 (m, 2H, NCH2CH2). The sig-
nal of COOH proton was not observed. 13C NMR (CDCl3,
150 MHz, δ, ppm): 173.9 (NCN), 164.6 (C═O), 150.0,
147.1, 139.3, 137.2, 130.5, 124.9, 124.4, 123.0, 122.9,
122.8, 118.7 (Ar─C), 54.1 (NCH2), 47.4 (NCH2CH2), 39.1
(NCH3). MS (ESI): m/z 414 [M − Cl]+. Anal. Calcd for
C18H18ClN3O2Pd (%): C, 48.02; H, 4.03; N, 9.33. Found
(%): C, 47.78; H, 4.34; N, 8.91.
2
J = 7.2 Hz, Ar─H), 5.11, 5.05 (d, 2H, JH,H = 12.4 Hz,
OCH2), 4.90–4.74 (NCH2), 3.30–3.16 (m, 2H, NCH2CH2).
13C NMR (CDCl3, 100 MHz, δ, ppm): 174.6 (NCN),
172.3 (C═O), 164.8, 155.3, 150.3, 146.8, 140.7, 139.0,
136.9, 136.2, 130.1, 129.7, 129.1, 128.8, 128.7, 125.6,
124.4, 124.1, 124.0, 122.5, 122.2, 118.5 (Ar─C), 67.1
(OCH2), 47.7 (NCH2), 35.8 (NCH2CH2). MS (ESI) m/z
566 [M − Cl]+. Anal. Calcd for C30H26ClN3O2Pd (%): C,
59.81; H, 4.35; N, 6.98. Found (%): C, 59.45; H, 4.14; N,
7.40.
4.7 | Synthesis of 7a
Complex 8b was obtained in a yield of 92%.1H NMR
(DMSO‐d6, 600 MHz, δ, ppm): 9.14 (d, 1H, J = 5.4 Hz,
Pyr─H), 8.09–8.08 (m, 2H, Ar─H), 8.00–7.97 (m, 2H,
Ar─H), 7.84 (s, 1H, Ar─H), 7.72 (s, 1H, Ar─H), 7.65–
7.63 (m, 1H, Ar─H), 7.45–7.41 (m, 3H, Ar─H), 7.31 (t,
1H, J = 7.2 Hz, Ar─H), 6.96 (t, 1H, J = 7.2 Hz, Ar─H),
6.82 (t, 1H, J = 7.2 Hz, Ar─H), 5.96 (d, 1H, J = 7.2 Hz,
Ar─H), 4.67–4.58 (m, 2H, NCH2), 3.01, 2.94 (br s, 2H,
NCH2CH2). 13C NMR (CDCl3, 150 MHz, δ, ppm): 171.8
(NCN), 163.7 (C═O), 154.6, 148.8, 145.9, 139.9, 139.6,
135.9, 129.7, 129.0, 127.8, 124.5, 124.0, 123.9, 123.4,
122.6, 122.5, 118.8 (Ar─C), 54.9 (NCH2, NCH2CH2). MS
Complex 3a (0.1 mmol, 82.7 mg), (3‐choloropropyl)
trimethoxysilane (1.6 mmol, 296 μl) and CH3CN were
mixed in a Schlenk tube under nitrogen atmosphere.
The tube was sealed and the reaction mixture was stirred
and heated under reflux for 24 h. All the volatiles were
evaporated under vacuum. The obtained yellow residue
was purified using column chromatography (SiO2)
affording the product as a pale yellow solid in a yield of
1
20%. H NMR (CDCl3, 600 MHz, δ, ppm): 9.31–9.30 (m,
1H, Ar─H), 7.80 (td, 1H, J1 = 8.4 Hz, J2 = 1.8 Hz, Ar─H),
7.71–7.70 (m, 1H, Ar─H), 7.54 (dd, 1H, J1 = 7.8 Hz,
J2 = 1.2 Hz), 7.22–7.18 (m, 2H, Ar─H), 7.05 (td, 1H,
J1 = 7.8 Hz, J2 = 0.6 Hz, Ar─H), 6.96–6.95 (m, 1H,
Ar─H), 6.88 (td, 1H, J1 = 7.8 Hz, J2 = 1.2 Hz, Ar─H),
6.05–6.04 (m, 1H, Ar─H), 4.71–4.61(m, 2H, NCH2), 3.98
(t, 2H, J = 6.6 Hz, OCH2), 3.94 (s, 3H, NCH3), 3.53 (s,
9H, SiOCH3), 3.13–3.02 (m, 2H, NCH2CH2), 1.69–1.64
(ESI): m/z 476 [M
−
Cl]+. Anal. Calcd for
C23H20ClN3O2Pd (%): C, 53.92; H, 3.93; N, 8.20. Found
(%): C, 53.83; H, 4.16; N, 8.51.
4.9 | Synthesis of B
(m,
2H,
OCH2CH2CH2),
0.60–0.56
(m,
2H,
Complex B was synthesized by treating compound 2 with
1‐phenyl‐3‐nbutylimidazolium iodide salt and K2CO3 fol-
lowing a procedure similar to that for accessing 3a and
3b. Complex B was obtained as a yellow solid in a yield
of 70%. H NMR (CDCl3, 600 MHz, δ, ppm): 9.67 (dd,
1H, Pyr─H, J1 = 5.4 Hz, J2 = 0.6 Hz), 7.95–7.93 (m, 2H,
OCH2CH2CH2). 13C NMR (CDCl3, 150 MHz, δ, ppm):
174.5 (NCN), 172.4 (C═O), 164.8, 155.2, 150.3, 147.2,
139.2, 137.0, 130.4, 124.8, 123.2, 122.8, 122.5, 118.6
(Ar─C), 67.3 (OCH2), 51.2 (SiOCH3), 47.2 (NCH2), 39.2
(NCH3), 36.1 (NCH2CH2), 22.5 (OCH2CH2), 5.9
1

HU AND GUO
11 of 12
Ar─H), 7.66 (td, 1H, Ar─H, J1 = 7.8 Hz, J2 = 1.2 Hz),
7.57–7.56 (m, 1H, Ar─H), 7.40 (dd, 1H, Ar─H,
J1 = 7.8 Hz, J2 = 1.2 Hz), 7.33–7.28 (m, 3H, Ar─H),
7.24–7.21 (m, 1H, Ar─H), 7.18 (d, 1H, Ar─H,
J = 2.6 Hz), 7.07–7.04 (m, 1H, Ar─H), 6.99 (td, 1H,
Ar─H, J1 = 7.8 Hz, J2 = 1.2 Hz), 6.89 (dd, 1H, Ar─H,
J1 = 7.8 Hz, J2 = 1.8 Hz), 6.08 (dd, 1H, Ar─H,
J1 = 7.2 Hz, J2 = 0.6 Hz), 4.61–4.57 (m, 1H, NCH2),
4.39–4.34 (m, 1H, NCH2), 2.04–1.96 (m, 1H, NCH2CH2),
1.89–1.81 (m, 1H, NCH2CH2), 1.40–1.31 (m, 2H,
NCH2CH2CH2), 0.85 (t, 3H, NCH2CH2CH2CH3,
J = 7.8 Hz). 13C NMR (CDCl3, 150 MHz, δ, ppm): 172.9
(NCN), 164.7, 158.0, 154.8, 147.0, 140.8, 138.6, 136.3,
130.1, 129.5, 128.6, 125.8, 124.6, 124.2, 123.2, 122.9,
122.3, 118.8 (Ar─C), 52.5 (NCH2), 32.3 (NCH2CH2), 20.6
(NCH2CH2CH2), 14.3 (NCH2CH2CH2CH3). MS (ESI): m/
z 460 [M − I]+. Anal. Calcd for C46H38N6O4Pd2 (%): C,
49.04; H, 4.12; N, 7.15. Found (%): C, 48.76; H, 3.88; N,
7.55.
ACKNOWLEDGEMENTS
The authors thank the National Natural Science Founda-
tion of China (grant no. 21502122) and Beijing Natural
Science Foundation (grant no. 2164057) for financial
support.
CONFLICT OF INTEREST
There are no conflicts to declare.
ORCID
Shuai Guo https://orcid.org/0000-0001-7729-0980
REFERENCES
[1] For selected reviews, see (a) F. E. Hahn, M. C. Jahnke, Angew.
Chem. Int. Ed. 2008, 47, 3122. (b) S. Gu, C. Chen, W. Chen,
Curr. Org. Chem. 2011, 15, 3291. (c) W. Zhao, V. Ferro, M. V.
Baker, Coord. Chem. Rev. 2017, 339, 1. (d) D. Bourissou, O.
Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev. 2000, 100, 39.
(e) J. C. Y. Lin, R. T. W. Huang, C. S. Lee, A. Bhattacharyya,
W. S. Hwang, I. J. B. Lin, Chem. Rev. 2009, 109, 3561. (f) S.
Bellemin‐Laponnaz, S. Dagorne, Chem. Rev. 2014, 114, 8747.
(g) R. H. Crabtree, Coord. Chem. Rev. 2013, 257, 755. (h) P. L.
Arnold, S. Pearson, Coord. Chem. Rev. 2007, 251, 596. (i) O.
chuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem.
Rev. 2009, 109, 3445. (j) S. Díez‐González, N. Marion, S. P.
Nolan, Chem. Rev. 2009, 109, 3612. (k) E. Peris, Chem. Rev.
2017, 118, 9988. https://doi.org/10.1021/acs.chemrev.6b00695
(l) M. Melaimi, R. Jazzar, M. Soleilhavoup, G. Bertrand, Angew.
Chem. Int. Ed. 2017, 56, 10046. (m) L.‐A. Schaper, S. J. Hock,
W. A. Herrmann, F. E. Kühn, Angew. Chem. Int. Ed. 2013, 52,
270. (n) D. G. Gusev, Organometallics 2009, 28, 6458. (o) D.
Zhang, G. Zi, Chem. Soc. Rev. 2015, 44, 1898.
4.10 | General procedure for catalytic
hydroarylations
A 25 ml Schlenk tube was charged with precatalyst
(0.01 mmol for mononuclear complex and 0.005 mmol
for dinuclear complex) and a magnetic stirrer. Then,
mesitylene (1.5 mmol, 208 μl), ethyl propiolate (1.0 mmol,
101 μl) and trifluoroacetic acid (1000 μl) were added. The
tube was sealed and the reaction mixture was stirred at
the temperature indicated in Table 1. CH2Cl2 (5 ml) and
deionized water (5 ml) were added. The organic phase
was washed with water (5 ml × 3) and collected. 1,3,5‐
Trimethoxylbenzene (1.0 mmol) as an internal standard
was added. All the volatiles were removed in vacuo, and
the crude product was analyzed by 1H NMR spectroscopy.
[2] (a) O. Kühl, Chem. Soc. Rev. 2007, 36, 592. (b) A. T. Normand,
K. J. Cavell, Eur. J. Inorg. Chem. 2008, 2781. (c) D. Yuan, H. V.
Huynh, Molecules 2012, 17, 2491. (d) H. M. Lee, C. C. Lee, P. Y.
Cheng, Curr. Org. Chem. 2007, 11, 1491. (e) A. John, P. Ghosh,
Dalton Trans. 2010, 39, 7183. (f) S. Hameury, P. de Frémont, P.
Braunstein, Chem. Soc. Rev. 2017, 46, 632.
[3] For a recent review on the electronic properties of NHCs, see H.
4.11 | X‐ray diffraction studies
V. Huynh, Chem. Rev. 2018, 118, 9457.
[4] (a) K. V. S. Ranganath, S. Onitsuka, A. K. Kumar, J. Inanaga,
Catal. Sci. Technol. 2013, 3, 2161. (b) W. J. Sommer, M. Weck,
Coord. Chem. Rev. 2007, 251, 860. (c) R. Zhong, A. C. Lindhorst,
F. J. Groche, F. E. Kühn, Chem. Rev. 2017, 117, 1960.
Structural solution was carried out with the Olex2 pro-
gram.[26] The structure was solved using direct methods.
All non‐hydrogen atoms were generally given anisotropic
displacement parameters in the final model. All hydrogen
atoms were put at calculated positions. A summary of
selected important crystallographic data is given in Table
SI‐1 (supporting information). CCDC 1847062 and
1847064–1847067 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[5] For selected reviews on palladacycles, see (a) J. Dupont, C. S.
Consorti, J. Spencer, Chem. Rev. 2005, 105, 2527. (b) R. B. Bed-
ford, Chem. Commun. 2003, 1787. (c) M. Albrecht, G. van
Koten, Angew. Chem. Int. Ed. 2001, 40, 3750. (d) M. Q. Slagt,
D. A. P. van Zwieten, A. J. C. M. Moerkerk, R. J. M. Klein
Gebbink, G. van Koten, Coord. Chem. Rev. 2004, 248, 2275.
(e) A. Bruneau, M. Roche, M. Alami, S. Messaoudi, ACS Catal.
2015, 5, 1386.

12 of 12
HU AND GUO
[6] M. R. Biscoe, B. P. Fors, S. L. Buchwald, J. Am. Chem. Soc.
2008, 130, 6686.
J. M. Asensio, P. Gómez‐Sal, R. Andrés, E. de Jesús, Dalton
Trans. 2017, 46, 6785. (c) S. Horn, M. Albrecht, Chem.
Commun. 2011, 47, 8802. (d) L. R. Moore, S. M. Cooks, M. S.
Anderson, H.‐J. Schanz, S. T. Griffin, R. D. Rogers, M. C. Kirk,
K. H. Shaughnessy, Organometallics 2006, 25, 5151. (e) A. A.
Danopoulos, P. Cole, S. P. Downing, D. Pugh, J. Organometal.
Chem. 2008, 693, 3369.
[7] A. Schnyder, A. F. Indolese, M. Studer, H.‐U. Blaser, Angew.
Chem. Int. Ed. 2002, 41, 3668.
[8] For the original work by Nolan et al., see (a) O. Navarro, R. A.
Kelly III, S. P. Nolan, J. Am. Chem. Soc. 2003, 125, 16194. (b) M.
S. Viciu, R. A. Kelly III, E. D. Stevens, F. Naud, M. Studer, S. P.
Nolan, Org. Lett. 2003, 5, 1479.
[17] For selected examples, see (a) W. A. Herrmann, L. J. Goossen,
M. Spiegler, Organometallics 1998, 17, 2162. (b) A. A. D.
Tulloch, A. A. Danopoulos, S. Kleinhenz, M. E. Light, M. B.
Hursthouse, G. Eastham, Organometallics 2001, 20, 2027.
[9] For selected examples, see (a) G.‐R. Peh, E. A. B. Kantchev, J.‐
C. Er, J. Y. Ying, Chem. Eur. J. 2010, 16, 4010. (b) J. Nasielski,
N. Hadei, G. Achonduh, E. A. B. Kantchev, C. J. O. Brien, A.
Lough, M. G. Organ, Chem. Eur. J. 2010, 16, 10844. For a
related review, see (c) P. G. Gildner, T. J. Colacot, Organome-
tallics 2015, 34, 5497.
[18] X. Xie, H. V. Huynh, Org. Chem. Front. 2015, 2, 1598.
[19] R. E. Jones, D. H. Templeton, Acta Crystallogr. 1958, 11, 484.
[20] For selected articles studying this concept, see (a) K. Riener, M.
J. Bitzer, A. Pöthig, A. Raba, M. Cokoja, W. A. Herrmann, F. E.
Kühn, Inorg. Chem. 2014, 53. 12767 (b) H. V. Huynh, C. H. Yeo,
Y. X. Chew, Organometallics 2010, 29, 1479.
[10] (a) P. L. Arnold, M. S. Sanford, S. M. Pearson, J. Am. Chem. Soc.
2009, 131, 13912. (b) M. Li, H. Song, B. Wang, Organometallics
2015, 34, 1969. For other analogues, see (c) L. Dang, J. Guo, H.
Song, B. Liu, B. Wang, Dalton Trans. 2014, 43. 17177 (d) L.
Dang, H. Song, B. Wang, Organometallics 2014, 33, 6812.
1
[21] Based on the H NMR spectroscopy and literature reports, we
proposed the structure of complex A with a C═O oxygen atom
coordinating to the metal (see the supporting information). For
structurally related complexes, see references [2f] and [13a].
[11] For an original work from Herrmann, see (a) W. A. Herrmann,
L. J. Gooβen, M. Spiegler, J. Organometal. Chem. 1997, 547,
357. For other selected examples, see (b) D. S. McGuinness,
K. J. Cavell, Organometallics 2000, 19, 741. (c) H. V. Huynh,
J. Wu, J. Organometal. Chem. 2009, 694, 323. (d) E. Steeples,
A. Kelling, U. Schilde, D. Esposito, New J. Chem. 2016, 40,
4922. (e) E. K. Bullough, M. A. Little, C. E. Willans, Organome-
tallics 2013, 32, 570. (f) A. Meyer, M. A. Taige, T. Strassner,
J. Organometal. Chem. 2009, 694, 1861.
[22] For selected examples employing NHC‐based precatalysts in
this type of reactions, see (a) M. S. Viciu, E. D. Stevens, J. L.
Petersen, S. P. Nolan, Organometallics 2004, 23, 3752. (b) R.
Saravanakumar, V. Ramkumar, S. Sankararaman, Organome-
tallics 2011, 30, 1689. (c) P. K. Hota, A. Jose, S. K. Mandal,
Organometallics 2017, 36, 4422. (d) A. Biffis, C. Tubaro, G.
Buscemi, M. Basatoa, Adv. Synth. Catal. 2008, 350, 189. (e) S.
Gonell, M. Poyatos, J. A. Mata, E. Peris, Organometallics
2011, 30, 5985.
[12] For post‐modification‐related examples, see (a) H. V. Huynh,
Q. Teng, Chem. Commun. 2013, 49, 4244. (b) F. Cisnetti, C.
Gibard, A. Gautier, J. Organometal. Chem. 2015, 782, 22. (c)
V. H. Nguyen, B. M. El Ali, H. V. Huynh, Organometallics
2018, 37, 2358. (d) V. H. Nguyen, M. B. Ibrahim, W. W.
Mansour, B. M. E. Ali, H. V. Huynh, Organometallics 2017,
36, 2345. (e) J. C. Bernhammer, H. Singh, H. V. Huynh, Organ-
ometallics 2014, 33, 4295. (f) Q. Teng, D. Upmann, S. A. Z. Ng
Wijaya, H. V. Huynh, Organometallics 2014, 33, 3373. (g) D.
Yuan, Q. Teng, H. V. Huynh, Organometallics 2014, 33, 1794.
[23] (a) D. R. Anton, R. H. Crabtree, Organometallics 1983, 2, 855.
(b) J. P. Collman, K. M. Kosydar, M. Bressan, W. Lamanna,
T. Garrett, J. Am. Chem. Soc. 1984, 106, 2569. (c) L. N. Lewis,
N. Lewis, J. Am. Chem. Soc. 1986, 108, 7228.
[24] J. A. Widegren, R. G. Finke, J. Mol. Catal. A 2003, 198, 317.
[25] For selected examples, see (a) M. R. Eberhard, Org. Lett. 2004,
6, 2125. (b) B. Inés, R. SanMartin, M. J. Moure, E. Domínguez,
Adv. Synth. Catal. 2009, 351, 2124. (c) N. T. S. Phan, M. Van
Der Sluys, C. W. Jones, Adv. Synth. Catal. 2006, 348, 609.
[13] For selected examples applying acidolyses/esterifications of
carboxylate ion‐based NHC complexes, see (a) A. Mangalum,
Y. Htet, D. A. Roe, C. D. McMillen, A. G. Tennyson, Inorg.
Chim. Acta 2015, 435, 320. (b) V. Leigh, W. Ghattas, H.
Mueller‐Bunz, M. Albrecht, J. Organometal. Chem. 2014, 771,
33.
[26] O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard,
H. Puschmann, J. Appl. Crystallogr. 2009, 42, 339.
SUPPORTING INFORMATION
[14] For selected examples or reviews, see (a) A. E. Poulain, D.
Canseco‐Gonzalez, R. Hynes‐Roche, H. Müller‐Bunz, O.
Schuster, H. Stoeckli‐Evans, A. Neels, M. Albrecht, Organome-
tallics 2011, 30, 1021. (b) P. Sehnal, R. J. K. Taylor, I. J. S.
Fairlamb, Chem. Rev. 2010, 110, 824. (c) L.‐M. Xu, B.‐J. Li, Z.
Yang, Z.‐J. Shi, Chem. Soc. Rev. 2010, 39, 712.
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
[15] W. Song, H. Han, X. Liao, R. Sun, J. Wu, M. Xie, Macromole-
cules 2014, 47, 6181.
How to cite this article: Hu Y, Guo S.
Palladacycles bearing COOH‐/ester‐functionalized
N‐heterocyclic carbenes: Divergent syntheses and
catalytic applications. Appl Organometal Chem.
2019;e4703. https://doi.org/10.1002/aoc.4703
[16] To the best of our knowledge, examples bearing bridging
carboxylate‐tethered NHCs are rare. For such an examples,
see (a) C. Y. Chan, P. A. Pellegrini, I. Greguric, P. J. Barnard,
Inorg. Chem. 2014, 53. 10862 For selected examples bearing
chelating or mono‐ligated carboxylate‐tethered NHCs, see (b)