A tridentate CNO-donor palladium(II) complex as efficient catalyst for direct C―H arylation: Application in preparation of imidazole-based push–pull chromophores

Article abstract of DOI:10.1002/aoc.3956

A series of imidazolium chlorides for the formation of tridentate CNO-donor palladium(II) complexes featuring N-heterocyclic carbene moieties have been developed from cheap and readily available starting materials with high yields. Their palladium complex

Full text of DOI:10.1002/aoc.3956

Received: 22 March 2017
DOI: 10.1002/aoc.3956
Revised: 9 May 2017
Accepted: 17 June 2017
F U L L P A P E R
A tridentate CNO‐donor palladium(II) complex as efficient
catalyst for direct C―H arylation: Application in
preparation of imidazole‐based push–pull chromophores
Hui‐Hong Li | Ratnava Maitra | Ya‐Ting Kuo | Jie‐Hong Chen | Ching‐Han Hu |
Hon Man Lee
Department of Chemistry, National
A series of imidazolium chlorides for the formation of tridentate CNO‐donor
Changhua University of Education,
Changhua 50058, Taiwan, ROC
palladium(II) complexes featuring N‐heterocyclic carbene moieties have been
developed from cheap and readily available starting materials with high yields.
Correspondence
Their palladium complexes were prepared by reactions between the ligand
precursors and PdCl2 using K2CO3 as base in pyridine with reasonable yields.
These air‐stable metal complexes were characterized using H NMR and 13C
Hon Man Lee, Department of Chemistry,
National Changhua University of
Education, Changhua 50058, Taiwan,
ROC.
1
Email: leehm@cc.ncue.edu.tw
{1H} NMR spectroscopy and elemental analyses. Heteronuclear multiple bond
correlation experiments were performed to identify key NMR signals of these
compounds. The structures of two of the complexes were also established by
single‐crystal X‐ray diffraction analysis. One of these complexes was
successfully applied in the direct C―H functionalization reactions between
heterocyclic compounds and aryl bromides, producing excellent yields
of coupled products. The coupling reactions were scalable, allowing grams of
coupled products to be obtained with a mere 2 mol% of Pd loading. The catalyst
system developed allowed the large‐scale preparation of several push–pull chro-
mophores straightforwardly. Photophysical properties based on UV–visible and
fluorescence spectroscopy for these chromophores were investigated. Deep blue
photoluminescence with moderate quantum efficiency and twisted intramolec-
ular charge transfer excited state were observed for these chromophores. Den-
sity functional theory (DFT) and time‐dependent DFT calculations were
performed to support the experimental results.
Funding information
Ministry of Science and Technology,
Taiwan, Grant/Award Number: MOST
105‐2119‐M‐018‐001
KEYWORDS
C―H functionalization, homogeneous catalysis, N‐heterocyclic carbene, push–pull chromophore
1 | INTRODUCTION
tridentate ligands featuring NHCs as at least one of the
donor groups has been demonstrated by numerous
N‐heterocyclic carbene (NHC) ligands and their transition
metal complexes have attracted considerable interest
because of their accessibility, high thermal stability and
remarkable catalytic activities in a wide variety organic
reactions.[1] The importance of palladium complexes with
reports published over the past ten years.[2] Among these
reported complexes, palladium complexes with tridentate
NHC, amidate and alkoxide donor groups have been
shown to have great potential in catalysis. For example,
Lee et al. reported palladium(II) complexes featuring such
Appl Organometal Chem. 2017;e3956.
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Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3956

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LI ET AL.
tridentate CNO ligands and they were highly promising
for C―H activation of various hydrocarbons under mild
conditions.[3] Jung and co‐workers reported chiral
palladium complexes also bearing similar tridentate
CNO ligand frameworks and these complexes were
successfully applied in asymmetric oxidative Heck‐type
reaction between arylboronic acids and acyclic alkenes,
delivering high enantioselectvities.[4] We have been
involved in research on the exploration of new palladium
complexes for catalyzing direct C―H arylation reactions.
Such methodology to construct biaryls without the need
of a preliminary organometallic reagent is highly
desirable because of the reduced waste and fewer reaction
steps.[5] In this regard, we had successfully developed a
palladium(II) acetate complex bearing PCy3 and
monodentate NHC ligands (Pd(L)(PCy3)(OAc)2; L = 1,3‐
dibenzylimidazol‐2‐ylidene).[6] The complex delivered
excellent catalytic activity in direct C―H arylation
reaction between imidazole derivatives and aryl chlorides.
However, the catalyst system involved the use of expen-
sive and air‐sensitive PCy3 ligand. To develop effective
phosphine‐free palladium precatalysts, we envisioned
that palladium complexes bearing tridentate NHC,
amidate and phenoxide coordinating moieties could be
highly promising for mediating direct C―H arylation
reactions. The replacement of alkoxide with phenoxide
donor groups should impart even higher stability to
the palladium complexes.
The photophysical properties of these ICT‐type chromo-
phores and their theoretical calculations are also
presented.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of ligand precursors
Taking into consideration of cost and easy availability of
starting materials, number of reaction steps and overall
yields, imidazolium salts 1a, 1b and 2a were
designed (Scheme 1). Compounds 1a and 1b feature five‐
membered imidazole rings, whereas 2a is similar to 1a
but with an enlarged benzimidazole ring instead. These
compounds are ligand precursors for the formation of
tridentate CNO‐donor ligands. They were prepared
straightforwardly by quaternization reactions between
the simple intermediate 2‐chloro‐N‐(2‐hydroxyphenyl)
acetamide with corresponding imidazole derivatives in tet-
rahydrofuran (THF) at 75°C overnight. Good to excellent
yields in the range 80–92% were obtained. These new com-
pounds are white solids with poor solubility in THF and
chloroform. They dissolve more readily in high‐polarity
solvents such as dimethylsulfoxide (DMSO) and
dimethylformamide (DMF). Unlike other imidazolium
salts reported by us earlier,2e they are non‐hygroscopic
and can be stored in air without any precaution. They were
1
characterized using H NMR and 13C NMR spectroscopy
Luminescent organic materials have attracted con-
siderable interest for their application in organic light
emitting diodes as panel displays and light sources.[7]
We are interested in the construction of heteroaromatic
biaryls via direct C―H arylation reactions since the
methodology allows easy access to push–pull chromo-
phores based on heteroaromatic biaryls. These chromo-
phores possess π‐conjugate systems which are end‐
capped with donor (D) and acceptor (A) groups. An
intramolecular charge transfer (ICT) from the D unit
to the A unit results in absorption at long wavelength.[8]
By careful design and selection of the D and A subunits
and the π‐conjugate system in these D–π–A molecules,
tuning of the ICT emission for desired optical and elec-
tronic properties can be achieved.[9]
In this paper, we present the synthesis and characteri-
zation of novel tridentate CNO‐donor palladium
complexes. These new phosphine‐free complexes can be
successfully applied as precatalysts in direct C―H
arylation between imidazole derivatives and aryl
bromides. The catalytic reactions are scalable, capable of
delivering coupled products on the gram scale. The
practicability of these complexes in catalyzing organic
transformations is illustrated by large‐scale and one‐step
preparations of imidazole‐based push–pull chromophores.
and electrospray ionization mass spectrometry (ESI‐MS).
In each of their 1H NMR spectra, three downfield signals
were observed in the range 9.2–10.2 ppm, corresponding
to NCHN, NH or OH protons. Deuterium exchange exper-
iments showed that the two most downfield signals col-
lapsed whereas the signal at ca 9.3 ppm for 1a and 1b
remained intact (see Figures S20–S23 in the supporting
information). The result established that the signal at 9.3
ppm should be due to the less acidic NCHN protons of
the heterocyclic rings. To identity the remaining two
downfield signals of NH and OH protons, heteronuclear
multiple‐bond correlation (HMBC) experiments for 1a,
1b and 2a were conducted. The two‐dimensional NMR
technique correlates chemical shifts between protons and
carbons that are two or three bonds away. As an example,
the experiment for 1b revealed a cross peak between the
carbonyl carbon at 164.4 ppm and the signal at ca 10.0
ppm. The result indicates that this middle signal at 10.0
ppm was assignable to the NH proton. Thus the most
downfield signals in each case were deduced to be the most
acidic OH resonances. In line with the presence of acidic
OH protons, negative mode ESI‐MS spectra in DMF exhib-
ited m/z peaks due to the [M − H]− phenoxide ions. Inter-
estingly, in the mass spectra of 1a and 1b, the base peaks
were due to [M − H + 2H2O]− at m/z 378.2 and 302.3,

LI ET AL.
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SCHEME 1 Synthesis of imidazolium salts.
respectively. The water molecules presumably came from
the moisture present in DMF solvent. The phenoxide O
and the carbonyl O atoms in these solvated negative ions
were probably involved in hydrogen bonding interactions
with the water molecules. Such supramolecular interac-
tion also occurred in the solid‐state structure of complex
3b (discussed below).
analysis. The absence of the three downfield NH, NCHN
and OH signals in the 1H NMR spectra reflects the
successful tridentate chelation of the ligands. In both the
13C{1H} NMR spectra of 3a and 3b, there are two signals
located very closely at ca 165 ppm. In contrast, two
well‐separated signals at 164.8 and 169.9 ppm were
observed in the spectrum of 4a. To identify the key signals
of C―O, C¼O and carbene carbons, HMBC experiments
were again performed. In the two‐dimensional NMR
spectrum of 3b, the signal at ca 157 ppm correlates with
the methyl protons (at 3.09 ppm) and methylene protons
(at 4.80 ppm), revealing its carbene carbon identity. The
two close signals at 165 ppm are due to C¼O and C―O
carbons, based on the evidence of cross peaks of these
signals with the methylene and aromatic carbons. In
sharp contrast, the carbene carbon signal in the spectrum
of 4a bearing benzimidazole ring was downfield shifted to
169.9 ppm. Its assignment was confirmed by its cross peak
with the methylene protons at ca 5.2 ppm. This relatively
downfield carbene resonance is in line with those in
similar palladium complexes with NHC ligands featuring
2.2 | Synthesis of palladium complexes
Palladium tridentate NHC/amidate/phenoxide complexes
3a, 3b and 4a were prepared by reacting the ligand
precursors 1a, 1b and 2a, respectively, with PdCl2 in
pyridine in the presence of K2CO3 as base at 50°C
(Scheme 2), affording the desired products as yellow
solids. Reasonable yields of products (57–75%) were
obtained. These solids were air stable and showed good
solubility only in high‐polarity solvents such as DMSO
1
and DMF. They were characterized using H NMR and
13C{1H} NMR spectroscopy, elemental analysis and, in
the case of 3a and 3b, single‐crystal X‐ray diffraction
SCHEME 2 Synthesis of palladium NHC complexes.

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LI ET AL.
similar benzimidazole rings.[10] Interestingly, the signal at
164.7 ppm was due to an accidental overlapping of C¼O
and C―O signals, reflecting from the evidences of its
larger‐than‐normal signal intensity and its correlation
peaks with both methylene and aromatic protons. It is
worth noting that the carbene carbon signal at 157.8
ppm of 3a was 12 ppm upfield than that of 169.9 ppm of
4a with almost identical structure but benzimidazole in
4a instead of imidazole ring in 3a. This fact indicates that
the electronic properties of 4a with an enlarged heterocy-
clic ring were quite different from those of 3a. Such
difference in electronic properties between 3a and 4a
was in line with their marked difference in catalytic
performance.
To understand the thermal stability of these air‐stable
solids, thermogravimetric analysis (TGA) was performed.
As shown in Figure S1 in the supporting information,
these complexes are thermally robust; they do not decom-
pose up to ca 200°C. Complexes 3a and 3b are slightly less
thermally stable than 4a. Their onset decomposition
temperatures are 198.1, 207.5 and 219.9°C, respectively.
2.3 | Molecular structures
The structures of 3a and 3b have been established from
X‐ray diffraction studies. Crystallographic data are listed
in Table S1 in the supporting information, while
selected bond distances and angles are given in Table 1.
The tridentate coordination of the ligands in these
complexes were confirmed (Figure 1). In both structures,
the palladium center adopted slightly distorted square
planar geometry with the ∠O―Pd―C (171.5(2)°) and
∠N―Pd―N (168.7(2)°) bond angles in 3a and the
corresponding ∠O―Pd―C (171.9(2)°) and ∠N―Pd―N
(173.9(2)°) in 3b deviating from the ideal 180°. The
Pd―C bond distances in 3a and 3b were 1.962(7) and
1.973(7) Å, respectively, whereas the trans Pd―O bond
FIGURE 1 Molecular structures of (a) 3a and (b) 3b with 50%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
distances in 3a and 3b were 2.020(4) and 2.050(4) Å,
respectively. The Pd―C bond distances are slightly longer
than those in related palladium tridentate palladium NCO
complexes bearing o‐aryloxide and NHC moieties (1.912–
1.946 Å).[11] Due to the strong amidate coordination, the
Pd―N bond distances from the amidate groups (1.997(6)
Å in 3a and 1.986(5) Å in 2b) were significantly shorter
than those from the trans pyridine ligands (2.046(6) Å in
3a and 2.026(6) Å in 3b). In each of the two structures,
the five‐member chelate ring and the phenyl ring are
nearly coplanar, whereas the six‐member chelate ring is
puckered. The pyridine rings were twisted from the metal
coordination planes with dihedral angles of 56.8° in 3a
and 53.1° in 3b. In the structure of 3b, two water
molecules form moderately strong hydrogen bonding
interactions with the coordinated oxygen and carbonyl
oxygen atoms. The corresponding O⋅⋅⋅O distances are
2.780 and 2.757 Å, respectively. Unfortunately, crystals
of 4a suitable for X‐ray diffraction analysis could not be
obtained.
TABLE 1 Selected bond distances (Å) and angles (°) around Pd in
3a and 3b
3a
3b
Pd1–C1
1.962(7)
2.046(6)
1.997(6)
2.020(4)
97.7(2)
Pd1–C6
1.973(7)
2.026(6)
1.986(5)
2.050(4)
93.7(2)
Pd1–N4
Pd1–N1
Pd1–N3
Pd1–N4
Pd1–O2
Pd1–O2
C1–Pd1–N4
C1–Pd1–N3
O2–Pd1–N4
O2–Pd1–N3
O2–Pd1–C1
N4–Pd1–N3
C6–Pd1–N1
C6–Pd1–N4
O2–Pd1–N1
O2–Pd1–N4
O2–Pd1–C6
N1–Pd1–N4
90.7(3)
90.9(2)
88.5(2)
92.9(2)
83.9(2)
82.93(19)
171.9(2)
173.9(2)
2.4 | Direct C―H arylation reactions
171.5(2)
168.7(2)
The catalytic performances of the new complexes in direct
arylation reactions of imidazole derivatives with aryl

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TABLE 2 Screening of palladium complexesa
Entry
Catalyst
3a
Yield (%)
TON
TOF (h−1
)
1
46
23
1.3
2.1
2.8
1.8
1.3
1.8
2.8
0.6
1.2
2
3
4
5
6
7
8
9
3b
74
100
65
37
50
33
23
32
50
11
21
4a
PdCl2/2[L3H3⋅Cl]
Pd(OAc)2/2[L3H3⋅Cl]
Pd(OAc)2/2PCy3
Pd(IPr)(3‐ClPy)Cl2
PdCl2
45
64
100
22
Pd(OAc)2
42
aReaction conditions: 1.7 mmol 1,2‐dimethylimidazole, 1.0 mmol aryl halide, 2.0 mmol KOAc, 5 ml DMA, 2.0 mol% Pd catalyst, 140°C, 18 h, isolated yield.
halides were explored. The coupling reaction between
1,2‐dimethylimidazole and 4‐bromoacetophenone was
taken as the benchmark reaction. Taking into
consideration previous works,[12] the initial investigation
begins with the following conditions: palladium
complexes (2.0 mol%), 1,2‐dimethylimidazole (1.7 equiv.),
4‐bromoacetophenone (1 equiv.), KOAc as base (2 equiv.)
in DMA at 140°C, and reaction time of 18 h. The three
new palladium pincer‐like complexes as well as other
palladium catalyst systems were firstly screened
(Table 2). Entry 3 indicates that complex 4a featuring
NHC moiety based on benzimidazole ring was the most
reactive precatalyst, which was followed by complex 3b
(entry 2). Complex 3a, which is structurally similar to
4a, was much less reactive (entry 1). It would be
convenient if the in situ catalyst system involving metal
and ligand precursors could be directly applied in the
catalytic reaction. However, entries 4 versus 3 clearly
show that an inferior yield of 65%, instead of quantitative
yield from 4a, was obtained using the in situ catalyst
system of PdCl2/2a. Changing the metal precursor to Pd
(OAc)2 led to further decrease in product yield (entry 5).
Electron‐donating phosphines such as PCy3 are
generally applied in C―C cross‐coupling reactions.
However, the in situ catalyst system of Pd(OAc)2/2PCy3
delivered a lower yield of 64% (entry 6). To gauge the
catalytic performance of 4a, its activity was also compared
with that of a PEPPSI complex, which was highly effective
in many cross‐coupling reactions.[13] Entry 7 shows that
Pd(IPr)(3‐ClPy)Cl2 featuring a monodenate IPr carbene
ligand was essential as free PdCl2 and Pd(OAc)2 were
much less effective (entries 8 and 9). Complex 4a was less
active than the aforementioned Pd(L)(PCy3)(OAc)2,[6] and
our previously reported preformed palladium(0) complex
featuring bidentate NHC and PPh2 moieties[15] which
was capable of utilizing aryl chlorides as substrates. It
should be noted that due to the unfavorable electronic
repulsion between the lone pair on the N3 and the
Pd―C(4) bond in the catalytic intermediate,[16] the
general reactivity trend of imidazole is that C5 is much
more reactive than C4 position.[17] Thus p‐(1,2‐
dimethylimidazol‐5‐yl)acetophenone (5) was the sole
product from the coupling reaction. Similarly, in all of
the following reactions involving 1,2‐dimethylimidazole
and imidazo[1,2‐a]pyridine, C5‐arylated products were
obtained exclusively. Attempts to recycle the palladium
complex (entry 3) for the next catalytic run were
unsuccessful.
After identifying 4a as the most reactive pincer‐like
complex, the reaction conditions were also re‐examined,
aiming at further optimizing the product yield (Table 3).
Entries 1–3 indicate that 2 mol% of palladium loading
was optimum. The product yield is markedly influenced
by the choice of solvent as demonstrated in entries 2
versus 4–6. DMA was the solvent of choice, whereas
switching to DMF, THF and toluene led to significant
reduction of yields. Lowering the reaction temperature
from 140 to 120°C resulted in a large decrease in product
yield from 100 to 54% (entries
2
versus 7).
Potassium acetate was the best base for the reaction, as
sodium acetate, potassium phosphate and potassium
carbonate gave much lower yields (entries 2 versus 8–10).
With complex 4a and the optimized conditions in
hand, the substrate scope of the reaction was then
investigated. As indicated in Table 4 (entries 1–3),
ligand was also highly effective in delivering
a
quantitative product yield. Noteworthy, in a very recent
report by Ke and co‐workers sterically encumbered
tetraarylimidazolium carbene Pd–PEPPSI complexes were
also shown to be highly efficient.[14] The presence of

6 of 16
LI ET AL.
TABLE 3 Screening of conditionsa
Entry
Solvent
Base
Catalyst (mol%)
T (°C)
Yield (%)
TON
TOF (h−1
)
1
DMA
KOAc
3
140
100
33
1.8
2.8
1.7
1.8
0.4
0.0
1.5
0.3
0.0
0.4
2
DMA
DMA
DMF
THF
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
NaOAc
K3PO4
K2CO3
2
1
2
2
2
2
2
2
2
140
140
140
140
140
120
140
140
140
100
30
64
14
0
50
30
32
7
3
4
5
6
Toluene
DMA
DMA
DMA
DMA
0
7
54
11
0
27
6
8
9
0
10
15
8
aReaction conditions: 1.7 mmol 1,2‐dimethylimidazole, 1.0 mmol aryl halide, 2.0 mmol KOAc, 5 ml DMA, 1–3 mol% 4a, 140°C, 18 h, isolated yield.
TABLE 4 Substrate scopea
Entry
Reactant
Product
Yield (%)b
TONb
TOF (h−1 b
)
1
92 (92)
46 (46)
2.6 (2.6)
2
90 (90)
45 (45)
2.5 (2.5)
3
4
5
6
100 (85)
91 (40)
90 (35)
78 (0)
50 (43)
46 (20)
45 (18)
35 (0)
2.8 (2.4)
2.6 (1.1)
2.5 (1.0)
1.9 (0.0)
7
100 (90)
50 (45)
2.8 (2.5)
(Continues)

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TABLE 4 (Continued)
Entry
Reactant
Product
Yield (%)b
TONb
TOF (h−1 b
2.4 (2.2)
)
8
85 (78)
43 (39)
9
0
0
0
0
0
0
0
10
11
0
0
12
13
14
15
16
100 (100)
100 (100)
89 (85)
50 (50)
50 (50)
45 (43)
50 (50)
50 (48)
2.8 (2.8)
2.8 (2.8)
2.5 (2.4)
2.8 (2.8)
2.8 (2.7)
100 (100)
100 (95)
aReaction conditions: 1.7 mmol 1,2‐dimethylimidazole, 1.0 mmol aryl halide, 2.0 mmol KOAc, 5 ml DMA, 2.0 mol% 4a, 140°C, 18 h, isolated yield.
bReaction performed under N2. Values in parentheses relate to reactions performed in air.
electron‐withdrawing aryl bromide substrates reacted
favorably with 1,2‐dimethylimidazole, affording excellent
product yields (90–100%). For electron‐poor aryl bromide,
steric hindrance on the phenyl ring did not affect the
coupling reaction (entries 1 versus 2). Electron‐neutral
and electron‐rich substrates also reacted well, providing
good yields of products (entries 4, 5, 7). Steric effect from
these substrates, however, can influence the product
yields. A comparison between entries 5 and 6 clearly
indicates that the sterically hindered 2‐bromotoluene
afforded an inferior yield (78%) to that from
4‐bromotoluene (90%). Entry 8 shows that the use of
3‐bromoanisole gave an 85% product yield, which was
lower than that of quantitative yield from 4‐bromoanisole
(entry 7). The reaction, however, did not proceed when
the most sterically demanding isomer, 2‐bromoanisole,
was used as substrate (entry 9). Placing a tert‐butyl group
on the C4 position of 2‐bromotoluene also led to failure of

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LI ET AL.
the reaction (entries 10 versus 6). No coupled product was
obtained when the sterically bulky 2‐bromo‐1,3,5‐
trimethylbenzene (entry 11) or aryl chlorides were used
substrate.
(Table 4, entries 1 and 2). The decrease in yields is
much obvious for electron‐donating and sterically
hindered substrates. For example, for the electron‐poor
4‐bromoacetophenone, the product yield decreased
moderately from 100 to 85% (entry 3). In contrast, while
a 78% product yield was obtained under nitrogen when
2‐bromotoluene was used as substrate, this reaction was
completely shut down in air (entry 6). Pleasingly, for the
reactions with electron‐withdrawing imidazo[1,2‐a]pyri-
dine, they could be conducted in air without loss in
product yields (entries 13–16). These results clearly
indicate the possible involvement of air‐sensitive Pd
(0) species. For electron‐releasing heteroaromatic
compounds, the reactions should be slow such that pro-
tection of the air‐sensitive intermediates is needed. But
for the reactions with electron‐withdrawing imidazole
derivatives, the turnover is fast and thus the protection
of reactive intermediates becomes unimportant.
Preliminary studies of the use of benzothiophene as
substrate are also promising (Table 5). Based on our previ-
ous report,[20] the initial conditions of 4a (2 mol%), PivOH
(30 mol%), K2CO3 (1.5 equiv.), aryl halide (1.0 equiv.), het-
erocycle (1.7 equiv.) and DMA as solvent at 110°C for 18 h
were employed. As seen from entry 1, benzothiophene
coupled favorably with 4‐bromoacetophenone, producing
an excellent yield of 94% of C2‐coupled product. A similar
yield of 95% could also be obtained with the electron‐poor
4‐bromobenzonitrile substrate (entry 2). 4‐Bromotoluene
Besides electron‐releasing 1,2‐dimethylimidazole,
electron‐withdrawing imidazo[1,2‐a]pyridine was also
tested as coupling partner. Imidazo[1,2‐a]pyridine and
its derivatives are important in the pharmaceutical
industry because of their wide range of biological activi-
ties.[18] These compounds have also been shown to have
great potential in optoelectronic applications.[19] In gen-
eral, higher yields were obtained with imidazo[1,2‐a]pyr-
idine as coupling partner. Both electron‐poor
4‐bromobenzonitrile and 4‐bromoacetophenone coupled
well with imidazo[1,2‐a]pyridine, delivering quantitative
product yields (entries 12 and 13). Electron‐rich substrates
can also be used effectively; quantitative yield of products
was also obtained with electron‐rich bromoanisoles
(entries 15 and 16), whereas using 4‐bromotoluene, a
good 89% yield was obtained (entry 14).
All the aforementioned catalytic reactions were
carried out under nitrogen. Since complex 4a is air stable,
it would be convenient if the catalytic reactions could be
carried out without the necessity of nitrogen protection.
In the case of the reaction with electron‐releasing 1,2‐
dimethylimidazole, product yields generally decreased
when the reactions were conducted in air with the
exception of using bromobenzonitriles as substrate
TABLE 5 Using other heteroyclic compounds as coupling partnera
Entry
1
Aryl halide
Product
Yield (%)
94
2
3
4
5
95
100
64
42
aReaction conditions: 1.7 mmol benzo[b]thiophene, 1.0 mmol aryl halide, 1.5 mmol K2CO3, 5 ml DMA, 30 mol% PivOH, 2.0 mol% 4a, 110°C, 18 h, isolated yield.

LI ET AL.
9 of 16
gave an excellent quantitative yield (entry 3). 4‐
Bromoanisole and 3‐bromoanisole were also successfully
applied as substrates albeit with lower product yields of
64 and 42%, respectively (entries 4 and 5).
involving 1,2‐dimethylimidazole and the phenoxide O
atom. Subsequent reductive elimination in D affords
the arylated imidazole and regenerates the active
species. It should be noted that the possibility of a
‘cocktail’‐like mixture of active catalysts[22] involving
both homogeneous and heterogeneous species cannot
be excluded as well since the yield for the benchmark
coupling reaction between 1,2‐dimethylimidazole and
4‐bromoacetophenone decreased from 100 to 63% when
the reaction was conducted in the presence of a drop
of Hg.
2.5 | Competitive Reactions
To gain further insight into the coupling reaction, com-
petitive reactions were performed (Scheme 3). A mixture
of 1,2‐dimethylimidazole and imidazo[1,2‐a]pyridine
was allowed to compete for electron‐rich 4‐bromoanisole,
yielding the couple products 9a and 15a with 10 and 82%
isolated yields, respectively.
A similar competitive
2.6 | Large‐scale preparation of push–pull
reaction employing electron‐poor 4‐bromoacetophenone
produced a quantitative production of 13 only. The fact
that electron‐poor imidazo[1,2‐a]pyridine prevails over
electron‐rich 1,2‐dimethylimidazole in the coupling reac-
tion with aryl bromides strongly suggests that the C―H
arylation reaction proceeds via a concerted metalation–
deprotonation (CMD) rather than an electrophilic
aromatic substitution (SEAr) pathway.[21] Noteworthy,
density functional theory (DFT) calculation confirmed
the electron‐donating nature of the 1,2‐dimethylimidazole
unit and the electron‐accepting property of the imidazo
[1,2‐a]pyridine moiety and thus validated the competitive
reactions and the CMD proposition of the catalytic path-
way (discussed below).
A proposed catalytic cycle is shown in Scheme 4. It
involves the active palladium(0) species A with a vacant
coordination site. Oxidative addition of aryl bromide to
the palladium(0) complex gives the intermediate B.
The bromide in B is substituted to form intermediate
C, releasing an acetate acid molecule and KBr. This
intermediate transforms into a CMD transition state
chromophores
Compound 6a is a D–π–A molecule with a 1,2‐
dimethylimidazolyl donor group and a cyano acceptor
group (Figure 2). The practicability of using complex
4a to catalyze direct C―H arylation reactions was
demonstrated by a large‐scale preparation of such a
push–pull chromophore. The small‐scale synthesis
between 1,2‐dimethylimidazole and 4‐bromobenzonitrile
using 1 mmol of the aryl bromide yielded 92% of 6a
(Table 4, entry 1). On upscaling the amount of aryl bro-
mide substrate to 10 mmol, a 70% yield with 1.4 g of 6a
was obtained. A similar upscaling reaction for com-
pound 15a was achieved, producing 1.4 g of product in
66% yield (Table 4, entry 15 for small‐scale synthesis).
Compound 15a is also a D–π–A molecule but with a
reversed direction of charge transfer; The A and D units
should be the imidazo[1,2‐a]pyridine and the methoxy
groups, respectively. For tuning the photophysical prop-
erties, we were also interested in the preparation of
compound 20 which is similar to 15a but with an extra
phenyl ring. A direct C―H arylation reaction between
imidazo[1,2‐a]pyridine and 4‐bromo‐4′‐methoxy‐1,1′‐
biphenyl using 4a as precatalyst successfully produced
2.3 g of 20 (77% yield). It should be noted that from
the standpoint of atom economy and overall cost, the
low 2 mol% Pd loading is highly desirable. In many
cases of C―H activation reactions, 5–10 mol%
palladium loadings are required.5b
2.7 | Photophysical properties of push–
pull chromophores
Cyrański and co‐workers reported that imidazo[1,2‐a]pyr-
idines possessing aryl substituents at the 2‐position
displayed strong emission bands in the blue region.[23]
The photophysical properties of the three compounds
6a, 15a and 20 with aryl substituents at the 4‐position
have never been reported and the results of our investiga-
tion are presented in Table 6. Chromophore 6a showed a
SCHEME 3 Competitive direct C―H arylation reactions.

10 of 16
LI ET AL.
SCHEME 4 Proposed mechanism for
the direct arylation reaction of 1,2‐
dimethylimidazole catalyzed by 4a.
FIGURE 2 Large‐scale synthesis of
push‐pull chromophores via direct C―H
arylation reactions.
single broad absorption at 311nm in THF with a high molar
extinction coefficient of 17 900 M−1 cm−1 (Figure 3). Con-
trastingly, two absorption bands were observed in the spec-
tra of 15a and 20. For 15a, the two broad absorption bands
were observed at 257 and 293 nm. The molar extinction
coefficient for the low‐energy band was only 7900 M−1 cm
−1, which was 10 000 M−1 cm−1 lower than that for 6a. For
20 with an extra phenyl ring, the two absorption bands

LI ET AL.
11 of 16
TABLE 6 Photophysical properties of push–pull chromophoresa
b
Chromophore
6a
Solvent
Absorption, λmax (nm)
ε (M−1 cm−1
)
Emission λmax (nm)
Stokes shift (cm−1
)
ФF
THF
311
17 900
383
5 942
0.29
0.26
0.29
0.24
0.04
0.04
0.00
0.03
0.27
0.24
0.21
0.19
DCM
DMSO
ACN
307
311
307
293
293
295
292
315
312
319
311
15 200
16 400
18 600
7 900
382
412
404
427
428
436
429
384
422
426
425
6 290
7 883
7 820
10 710
10 765
10 963
10 937
5 704
8 355
7 874
8 625
15a
20
THF
DCM
DMSO
ACN
9 100
9 800
11 700
23 500
21 100
22 700
36 300
THF
DCM
DMSO
ACN
aConcentration for absorption and emission measurements was 10−5 M. The excitation wavelength was at the absorption λmax
bDetermined with reference to anthracene in methanol (ФF = 0.27).
.
shifted to longer wavelengths of 271 and 315 nm. The
molar extinction coefficient for the band at 315 nm
became much larger (23 500 M−1 cm−1). The ICT
emission peak for 6a in THF was observed at 383 nm
with a moderate quantum yield of 0.29. In contrast, com-
pound 15a exhibited dual emissions in THF, consisting
of a weak blue emission at 427 nm with a low quantum
efficiency of 0.04 and an even weaker high‐energy band
at ca 343 nm. The low‐ and high‐energy transitions could
be attributed to the twisted intramolecular charge trans-
fer (TICT)[24] and the localized excited state transitions,
respectively. The Stokes shift of ca 11 000 cm−1 for the
TICT emission is very large. With an extra phenyl ring
in 20, the ICT emission maximum was blue‐shifted to
384 with a shoulder observed at 411 nm. A moderate
quantum yield of 0.27 was observed.
The absorption maxima for the three compounds were,
in general, unresponsive to a change in solvent polarity
(Figure 4, and Figure S2 in the supporting information),
indicating the relatively non‐polar nature of their ground
states. In contrast, significant bathochromatic shifts were
observed for the emission maxima of 6a and 20 in solvents
of higher polarity, indicating the more polarized nature of
their ICT excited states. The emission bands for 6a were
observed in the range 382–412 nm (Figure S2 in the
supporting information). In nonpolar THF solvent,
the emission peak for 20 was observed at 384 nm. But in
the more polar solvents DCM, DMSO and ACN, the emis-
sion maxima red‐shifted to the deep blue region in
the range 422–426 nm (Figure 4). Contrastingly, the
solvatochromic shift for 15a was less pronounced. In polar
DMSO solvent, the emission was quenched due to the non‐
FIGURE 3 UV absorption and emission spectra of 6a, 15a and 20
in THF.

12 of 16
LI ET AL.
2.8 | Theoretical calculations
To understand the details of electronic transitions of these
chromophores, theoretical calculations were performed
using DFT with the B3LYP functional[27]/6‐31G(d) basis
set. The UV spectra were calculated using time‐dependent
DFT (TD‐DFT) at the B3LYP/6‐31G(d) and M06[28]/6‐31G
(d) level of theory with conductor‐like polarizable contin-
uum model (CPCM)[29] in THF. The optimized geometries
of 6a, 15a and 20 are shown in Figure S4 in the supporting
information. Compound 6 is nonplanar with the aryl ring
twisted ca 37.3° from the 1,2‐dimethylimidazole unit. The
twisting was even larger in 15a with a dihedral angle of ca
45.2° between the imidazo[1,2‐a]pyridine and aryl units.
The large initial twist angle is consistent with the fact that
the closer the initial angle to the ideal 90°, the easier the
population of the TICT kinetics. Compound 20 is also
nonplanar with the central phenyl ring forming dihedral
angles of ca 42.6° and 36.1° with the imidazo[1,2‐a]pyri-
dine and aryl rings, respectively. The computed transitions
using both B3LYP and M06 functionals were agreed well
with those from the experimental results (Table S2 in the
supporting information). For all the three compounds,
the lowest electronic transitions mainly involve the HOMO
! LUMO transitions. Frontier orbitals of these compounds
are shown in Figure S5 in the supporting information. For
compound 6, the HOMO lies predominately on the 1,2‐
dimethylimidazaole moiety, whereas the LUMO is largely
localized on the acceptor group and the phenyl ring next
to it. Thus the ICT nature of the HOMO ! LUMO
transition was confirmed. For compound 15a, the HOMO
is distributed over the entire molecule on the donor atom,
the phenyl ring and the heterocyclic unit. But the LUMO
is well localized on the pyridine subunit of the imidazo
[1,2‐a]pyridine moiety. For compound 20, the HOMO is
similarly spread over the entire molecule with a large elec-
tronic contribution from the five‐member ring of the
imidazo[1,2‐a]pyridine moiety, whereas the LUMO is
largely localized on the pyridine unit of the heterocyclic
moiety and the central phenyl ring. The LUMO+1 is mostly
localized on theimidazo[1,2‐a]pyridine moiety. The energy
difference between LUMO and LUMO+1 is small and both
HOMO ! LUMO and HOMO ! LUMO+1 contribute to
the low‐energy transition. Compounds 6, 15a and 20 have
an energy gap of 4.21, 4.42 and 4.24 eV, respectively. The
presence of the extra phenyl from 15a to 20 avoided the
TICT excited state and reduced the energy gap by 0.18 eV.
FIGURE 4 UV absorption and emission spectra of 20 in various
solvents.
radiative decay of the TICT excited state (Figure S2 in the
supporting information).
To calculate the dipole moment of the excited states,
Lippert–Mataga plots were obtained (Figure S3 in the
supporting information), which are essentially plots
of the Stokes shift (Δνem) versus orientation polarity of
solvents (Δf).[25] From the slope of the plot, the change
of dipole moment between the ground state and excited
state (Δμ) can be calculated. The positive slopes for all
the chromophores reflect that the excited ICT states
are more polar than the ground states. Using Onsager
radii of 5.7, 5.7 and 6.7 Å for 6a, 15a and 20, respec-
tively, which were estimated from their optimized struc-
tures by theoretical calculations, their respective
calculated Δμ were found to be 19.4, 6.8 and 23.7 D.
The calculated dipole moments of their ground states
were 6.2, 4.4 and 5.0 D and thus the corresponding
dipole moments of excited states were found to be
25.6, 11.2 and 28.7 D, respectively. For compound 15a,
in line with the involvement of a TICT excited state,[26]
an even better fit was obtained if replotting Δνem against
Δf′ (Figure S3 in the supporting information).
3 | CONCLUSIONS
A series of tridentate CNO‐donor palladium complexes
featuring NHC/amidate/phenoxide moieties have been
successfully synthesized from cheap starting materials

LI ET AL.
13 of 16
with high yields. These air‐stable complexes effectively cat-
alyze direct C―H functionalization reactions between het-
erocycles and aryl halides with a low 2 mol% of Pd loading.
The complex with NHC moiety based on benzimidazole
was much more efficient than those with a simple imidaz-
ole ring. Catalytic activities comparable to those of PEPPSI
complexes were achieved. Competitive experiments indi-
cate a probable CMD mechanistic pathway. The coupling
reactions are scalable, capable of delivering gram‐scale of
coupled products. Large‐scale preparations of three
push–pull chromophores were successfully carried out.
These imidazole‐based ICT‐type molecules exhibit inter-
esting photophysical properties with moderate quantum
efficiencies. The new catalyst allows a cheap and quick
access to large amount of these interesting luminescent
organic solids for potential optoelectronic applications.
in THF (30 ml) was placed in a Schlenk flask. The mixture
was heated under reflux for 24 h. After cooling, the white
solid was collected on a frit, washed with THF and dried
1
under vacuum. Yield: 3.4 g (80%). H NMR (DMSO‐d6,
δ, ppm): 5.29 (s, 2H, CH2), 5.50 (s, 2H, CH2), 6.72–6.78
(m, 1H, Ar H), 6.95 (d, 2H, J = 6.0 Hz, Ar H), 7.39–7.44
(m, 5H, Ar H), 7.80–7.84 (m, 3H, Ar H), 9.36 (s, 1H,
NCHN), 9.93 (s, 1H, OH), 10.13 (s, 1H, NH). 13C{1H}
NMR (DMSO‐d6, δ, ppm): 51.9 (CH2), 52.3 (CH2), 116.1,
119.2, 122.4, 122.7, 124.8, 125.4, 125.8 (quaternary C),
128.8, 129.2, 129.5, 135.4 (quaternary C), 138.1 (NCHN),
148.7 (quaternary C), 164.3 (C―O). M.p. 206.0–206.3°C.
HRMS (ESI): m/z calcd for C18H17O2N3Cl [M − H]−
342.1004; found 342.1014.
4.1.2 | Synthesis of 1b
4 | EXPERIMENTAL
The compound was prepared with a procedure similar to
that for 1a. A mixture of 1‐methyl‐1H‐imidazole (0.82 g,
10 mmol) and 2‐chloro‐N‐(2‐hydroxyphenyl)acetamide
All manipulations were performed under a dry nitrogen
atmosphere using standard Schlenk techniques. Unless
otherwise noted, all the catalytic reactions were carried
out in thick‐walled sealable tubes. All the solvents were
directly used as received without any further purification
unless otherwise specified. Common starting chemicals
were purchased from commercial sources and used as
received. 1H NMR and 13C{1H} NMR spectra, unless
otherwise specified, were recorded at 300.13 and 75.47
MHz, respectively, in CDCl3 or DMSO‐d6 as solvents. Flash
chromatography was performed using 230–400 mesh silica
gel. ESI‐MS was carried out with a sector field mass
spectrometer. Elemental analysis was performed with a
CHN‐O elemental analyzer. TGA was performed with
a PerkinElmer Pyris 6, under flowing N2 gas (20 ml min
−1), and the heating rate was 20°C min−1. The UV–visible
spectra of the compounds (10 μM) were recorded using a
Thermo Scientific Evolution 220 UV–visible spectropho-
tometer at room temperature with a cell of 1 cm path
length. Photoluminescence emission spectra of the com-
pounds (10 μM) were recorded with a Horiba FluoroMax‐
4 spectrofluorometer at room temperature using a cell of 1
cm path length. The fluorescence quantum yields (Φf) were
determined using anthracene as a reference with known Φf
(0.27) in methanol. 2‐Chloro‐N‐(2‐hydroxyphenyl)acet-
amide[30] and 4‐bromo‐4′‐methoxy‐1,1′‐biphenyl[31] were
prepared according to a literature procedure.
1
(1.9 g, 10.0 mmol) was used. Yield: 1.8 g (82%). H NMR
(DMSO‐d6, δ, ppm): 3.91 (s, 3H, CH3), 5.31 (s, 2H, CH2),
6.74 (t, 1H, J = 6.0 Hz, Ar H), 6.91–7.02 (m, 2H, Ar H),
7.75–7.79 (m, 3H, Ar H), 9.26 (s, 1H, NCHN), 10.02 (s,
1H, NH), 10.22 (s, 1H, OH). 13C{1H} NMR (DMSO‐d6, δ,
ppm): 36.3 (CH3), 51.7 (CH2), 116,1, 119.2, 122.6, 123.5,
124.3, 125.4, 125.8 (quaternary C), 138.3 (NCHN), 148.6
(quaternary C), 164.4 (C―O). M.p. 211.2–211.6°C. HRMS
(ESI): m/z calcd for C12H13ClN3O2 [M − H]− 266.0696;
found 266.0682.
4.1.3 | Synthesis of 2a
The compound was prepared with a procedure similar to
that for 1a. A mixture of 1‐benzyl‐1H‐benzo[d]imidazole
(2.1 g, 10.0 mmol) and 2‐chloro‐N‐(2‐hydroxyphenyl)acet-
1
amide (1.9 g, 10.0 mmol) was used. Yield: 4.0 g (92%). H
NMR (DMSO‐d6, δ, ppm): 5.65 (s, 2H, CH2), 5.87 (s, 2H,
CH2), 6.72–6.77 (m, 1H, Ar H), 6.95–6.96 (m, 2H, Ar H),
7.36–7.46 (m, 3H, Ar H), 7.51–7.53 (m, 2H, Ar
H), 7.63–7.71 (m, 2H, Ar H), 7.82 (d, 1H, J = 6.0 Hz,
Ar H), 7.99–8.05 (m, 2H, Ar H), 10.02 (s, 1H, NCHN),
10.05 (s, 1H, NH), 10.14 (s, 1H, OH). 13C{1H} NMR
(DMSO‐d6, δ, ppm): 49.7 (CH2), 50.3 (CH2), 114.3, 114.4,
116.1, 119.2, 122.7, 125.6, 125.8 (quaternary C), 127.1,
127.4, 128.7, 129.2, 129.5, 130.9 (quaternary C), 132.4
(quaternary C), 134.4 (quaternary C), 144.3 (NCHN),
148.7 (quaternary C), 164.1 (C―O). M.p. 190.4–190.6°C.
HRMS (ESI): m/z calcd for C22H19O2N3Cl [M − H]−
392.1160; found 392.1169.
4.1 | Syntheses
4.1.1 | Synthesis of 1a
A mixture of 1‐benzyl‐1H‐imidazole (1.6 g, 10 mmol) and
2‐chloro‐N‐(2‐hydroxyphenyl)acetamide (1.9 g, 10 mmol)

14 of 16
LI ET AL.
dissolved in dry pyridine (10 ml) under nitrogen
atmosphere. The solution was allowed to stir at 50°C for
2 days. The workup procedure was similar to that for 3a.
A yellowish solid was obtained. Yield: 0.115 g (73%). M.
p. 190.5–190.9°C. Anal. Calcd for C27H22N4O2Pd (%): C,
59.95; H, 4.10; N, 10.36. Found (%): C, 59.73; H, 4.33; N,
4.1.4 | Synthesis of 3a
To a 20 ml Schlenk flask, PdCl2 (0.052 g, 0.29 mmol), 1a
(0.099 g, 0.29 mmol) and K2CO3 (0.16 g, 1.2 mmol) were
dissolved in dry pyridine (10 ml) under nitrogen atmo-
sphere. The solution was allowed to stir at 50°C for 12 h.
The solvent was removed completely under vacuum.
Water (20 ml) was added and the aqueous layer was
extracted with DCM (2 × 10 ml). 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 a frit and dried
under vacuum. Yield: 0.082 g (57%). M.p. 232.4–232.8°C.
Anal. Calcd for C23H20N4O2Pd (%): C, 56.28; H, 4.11; N,
1
10.64. H NMR (DMSO‐d6, δ, ppm): 5.19 (br s, 4H, CH2),
6.17–6.32 (m, 1H, Ar H), 6.43 (d, 1H, J = 9.0 Hz, Ar
H), 6.59 (t, 1H, J = 6.0 Hz, Ar H), 6.93 (br s, 2H, Ar H),
7.17–7.44 (m, 8H, Py H, Ar H), 7.74–7.98 (m, 3H, Py H,
Ar H), 8.76 (d, 2H, J = 3.0 Hz, Py H). 13C{1H} NMR
(DMSO‐d6, δ, ppm): 50.2 (CH2), 53.2 (CH2), 112.4, 113.1,
115.6, 123.2, 123.9, 124.2, 124.4, 124.6, 126.3, 127.9,
128.9, 133.8, 135.9, 139.7, 142.6, 150.1, 152.6, 164.8
(C―O and C=O), 169.9 (Pd―C).
1
11.41. Found (%): C, 56.55; H, 4.40; N, 11.40. H NMR
(DMSO‐d6, δ, ppm): 4.70 (s, 2H, CH2), 4.85 (s, 2H, CH2),
6.19 (t, 1H, J = 6.0 Hz, Ar H), 6.33–6.40 (m, 1H, Ar
H), 6.54 (t, 1H, J = 6.0 Hz, Ar H), 6.89–6.90 (m, 2H, Ar
H), 7.21–7.42 (m, 6H, imi H, Py H, Ar H), 7.59 (s, 1H, imi
H), 7.74 (d, 1H, J = 6.0 Hz, Ar H), 7.92 (t, 1H, J = 6.0 Hz,
Py H), 8.67 (d, 2H, J = 3.0 Hz, Py H). 13C{1H} NMR
(DMSO‐d6, δ, ppm): 52.3 (CH2), 56.7 (CH2), 112.9, 115.6,
122.7, 123.1, 123.7, 126.3, 126.7, 128.1, 129.0, 136.9, 139.7,
142.7, 150.1, 152.7, 157.8 (Pd―C), 164.7, 164.9. Crystals
suitable for X‐ray diffraction studies were grown by vapor
diffusion of diethyl ether into a DMF solution of 3a.
4.2 | General Procedure for Direct C―H
Arylation between Aryl Halides and
Imidazole Derivatives
Typically, a mixture of aryl halide (1.0 mmol), imidazole
(1.7 mmol), KOAc (2.0 mmol) and Pd precatalyst (0.02
mmol) was dissolved in DMA (5 ml) under a nitrogen or
air atmosphere. The reaction mixture was stirred at 140°
C in a preheated oil bath for 18 h. After the mixture was
cooled, ethyl acetate (10 ml) was added. The solution
was poured into water (50 ml) and extracted with ethyl
acetate (3 × 25 ml). The combined extracts were
washed with brine, dried over anhydrous MgSO4 and
evaporated to dryness under vacuum to give the crude
product, which was purified by column chromatography.
Products were identified by comparison of NMR data with
those in the literature (see supporting information).
4.1.5 | Synthesis of 3b
To a 20 ml Schlenk flask, PdCl2 (0.052 g, 0.29 mmol), 1b
(0.064 g, 0.29 mmol) and K2CO3 (0.16 g, 1.2 mmol) were
dissolved in dry pyridine (10 ml) under nitrogen
atmosphere. The solution was allowed to stir at 50°C for
12 h. The workup procedure was similar to that for 3a.
A yellowish solid was obtained. Yield: 0.142 g (75%). M.
p. 220.4–221.0°C. Anal. Calcd for C17H16N4O2Pd (%): C,
49.23; H, 3.89; N, 13.51. Found (%): C, 49.15; H, 4.24; N,
1
4.3 | General Procedure for Direct C―H
Arylation between Aryl Halides and Benzo
[b]thiophene
13.62. H NMR (DMSO‐d6, δ, ppm): 3.09 (s, 3H. CH3),
4.80 (s, 2H, CH2), 6.20 (t, 1H, J = 6.0 Hz, Ar H), 6.40 (d,
1H, J = 6.0 Hz, Ar H), 6.56 (t, 1H, J = 6.0 Hz, Ar H),
7.28 (s, 1H, imi H), 7.51 (s, 1H, imi H), 7.65 (t, 2H, J =
6.0 Hz, py H), 7.80 (d, 1H, J = 6.0 Hz, Ar H), 8.12 (t, 1H,
J = 9.0 Hz, py H), 8.80 (d, 2H, J = 6.0 Hz, py H). 13C{1H}
NMR (DMSO‐d6, δ, ppm): 36.6 (CH3), 56.7 (CH2), 112.8,
115.6, 122.1, 123.0, 123.5, 123.6, 126.6, 139.9, 142.7,
152.9, 157.1 (Pd―C), 164.7, 164.9. Crystals suitable for
X‐ray diffraction studies were grown by vapor diffusion
of diethyl ether into a DMF solution of 3b.
Typically, a mixture of aryl halide (1.0 mmol), benzo[b]
thiophene (1.7 mmol), K2CO3 (1.5 mmol) and Pd
precatalyst (0.02 mmol) was dissolved in DMA (5 ml)
under nitrogen. The reaction mixture was stirred at 110°
C in a preheated oil bath for 18 h. After the mixture was
cooled, ethyl acetate (10 ml) was added. The solution
was poured into water (50 ml) and extracted with ethyl
acetate (3 × 25 ml). The combined extracts were washed
with brine, dried over anhydrous MgSO4 and evaporated
to dryness under vacuum to give the crude product, which
was purified by column chromatography. Products were
identified by comparison of NMR data with those in the
literature (see supporting information).
4.1.6 | Synthesis of 4a
To a 20 ml Schlenk flask, PdCl2 (0.052 g, 0.29 mmol), 2a
(0.144 g, 0.29 mmol) and K2CO3 (0.16 g, 1.2 mmol) were

LI ET AL.
15 of 16
Nolan, J. Organometal. Chem. 2002, 653, 69. c) W. A. Herr-
mann, K. Öfele, D. von Preysing, S. K. Schneider,
J. Organometal. Chem. 2003, 687, 229. d) U. Christmann, R.
Vilar, Angew. Chem. Int. Ed. 2005, 44, 366. e) E. A. B. Kantchev,
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Richter, M. Schedler, F. Glorius, Nature 2014, 510, 485.
4.4 | General procedure for large‐scale
preparation of push–pull chromophores
The preparation was similar to the small‐scale general
procedure. A mixture of aryl halide (10 mmol), imidazole
derivative (17 mmol), KOAc (20 mmol) and Pd precatalyst
(0.2 mmol) in DMA (20 ml) was used.
4.5 | Single‐crystal x‐ray diffraction
[2] a)E. Peris, R. H. Crabtree, Coord. Chem. Rev. 2004, 248, 2239. b)E.
Peris, J. A. Loch, J. Mata, R. H. Crabtree, Chem. Commun. 2001,
201. c) J. A. Loch, M. Albrecht, E. Peris, J. Mata, J. W. Faller, R.
H. Crabtree, Organometallics 2002, 21, 700. d) S. Gründemann,
M. Albrecht, J. A. Loch, J. W. Faller, R. H. Crabtree, Organometal-
lics 2001, 20, 5485. e) J.‐Y. Lee, J.‐Y. Lee, Y.‐Y. Chang, C.‐H. Hu,
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CCDC files 1515104 (3a) and 1515105 (3b) contain
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ACKNOWLEDGMENT
We are grateful to the Ministry of Science and Technol-
ogy, Taiwan for financial support of this work (grant no.
MOST 105‐2119‐M‐018‐001).
ORCID
Hon Man Lee http://orcid.org/0000-0002-9557-3914
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How to cite this article: Li H‐H, Maitra R, Kuo
Y‐T, Chen J‐H, Hu C‐H, Lee HM. A tridentate
CNO‐donor palladium(II) complex as efficient
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