Pyrazole-3-carboxylates assisted N-heterocyclic carbene palladium complexes: synthesis, characterization, and catalytic activities towards arylation of azoles with arylsulfonyl hydrazides

Article abstract of DOI:10.1002/aoc.5450

Four mononuclear and dinuclear pyrazole-3-carboxylates assisted NHC–Pd complexes have been synthesized and characterized. Notably, the bridge-cleavage reactions of [Pd(μ-Cl)(Cl)(NHC)]₂ with 1H-pyrazole-3-carboxylic acid afforded dinuclear complexes [(NHC)Pd(μ-1H-pyrazolato-3-carboxylate)]₂, in which the 1H-pyrazolato-3-carboxylate was employed as a N^N^O dianionic chelating and bridging ligand. To further explore the structural features and catalytic properties of the complexes, 1-methyl-1H-pyrazole-3-carboxylic acid was introduced into the coordination with [Pd(μ-Cl)(Cl)(NHC)]₂ and the corresponding mononuclear complexes (NHC)PdCl(1-methyl-1H-pyrazole-3-carboxylate) were obtained. The catalytic properties of the complexes in desulfitative arylation of azoles with arylsulfonyl hydrazides were initially investigated.

Full text of DOI:10.1002/aoc.5450

Received: 7 October 2019
DOI: 10.1002/aoc.5450
Revised: 25 November 2019
Accepted: 4 December 2019
F U L L P A P E R
Pyrazole-3-carboxylates assisted N-heterocyclic carbene
palladium complexes: synthesis, characterization, and
catalytic activities towards arylation of azoles with
arylsulfonyl hydrazides
Jin Yang
Department of Chemistry, Huaibei
Four mononuclear and dinuclear pyrazole-3-carboxylates assisted NHC–Pd
Normal University, Huaibei, Anhui,
235000, People's Republic of China
complexes have been synthesized and characterized. Notably, the bridge-
cleavage reactions of [Pd(μ-Cl)(Cl)(NHC)] with 1H-pyrazole-3-carboxylic acid
2
Correspondence
Jin Yang, Department of Chemistry,
Huaibei Normal University, Huaibei,
Anhui 235000, P. R. China.
Email: yangjinlz@163.com
afforded dinuclear complexes [(NHC)Pd(μ-1H-pyrazolato-3-carboxylate)] , in
2
which the 1H-pyrazolato-3-carboxylate was employed as a N ^N ^O dianionic
chelating and bridging ligand. To further explore the structural features and
catalytic properties of the complexes, 1-methyl-1H-pyrazole-3-carboxylic acid
was introduced into the coordination with [Pd(μ-Cl)(Cl)(NHC)]2 and the
corresponding mononuclear complexes (NHC)PdCl(1-methyl-1H-pyrazole-
3-carboxylate) were obtained. The catalytic properties of the complexes in
desulfitative arylation of azoles with arylsulfonyl hydrazides were initially
investigated.
Funding information
Natural Science Foundation of Anhui
Provincial Education Department, Grant/
Award Number: KJ2019A0600; Scientific
Research Project of Anhui Provincial
Education Department, Grant/Award
Number: KJ2015TD002
K E Y W O R D S
desulfitative arylation reaction, N-heterocyclic carbene palladium complexes, pyrazole-
3
-carboxylic acid
[
4]
1
| INTRODUCTION
base in the presence of a Pd salt. However, in situ syn-
thesized NHC–Pd complexes often lead to uncertainty
about both the rate of catalyst formation and the
Since the first discovery by Öfele, Wanzlick, and later
characterization by Arduengo, N-heterocyclic carbenes
[
5]
structure of the catalyst.
Base on this, Nolan,
(
NHCs) have attracted extensive interest as functional
Organ and others committed to developing well-defined
[1]
ligands in coordination chemistry. Subsequent studies
by Herrmann revealed that NHCs are effective ligands,
and they have emerged as a new field of organometallic
chemistry. Owing to their increased σ-donor nature
and unique steric properties, NHC–Pd complexes have
displayed efficient catalytic activities in Pd-catalyzed cou-
pling reactions. However, the high reaction activity of
the free carbenes usually necessitates their operation
under strict conditions. Consequently, much NHC–Pd
catalysts have been prepared by an in situ synthesis
method, in which an imdazolium salt is stirred with a
NHC–Pd catalysts, some of which have shown high activ-
[
6]
ities in Pd-catalyzed reactions.
In order to further
explore the well-defined NHC–Pd complexes, a series of
ancillary ligands has been introduced into the NHC–Pd
complexes and the corresponding catalysts have been
used for various types of organic reactions. Recently, the
monodentate nitrogen ligands have been commonly used
[2]
[3]
[7]
as the ancillary ligands to modify NHC–Pd complexes.
A further elaboration is the combination of NHC–Pd
complexes with multidentate ligands, giving rise to
[8]
highly preorganized heteroleptic Pd complexes.
We
Appl Organometal Chem. 2020;e5450.
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1 of 9
https://doi.org/10.1002/aoc.5450

2
of 9
YANG
and
others
have
developed
pyrazole
and
we report the first example of NHC–Pd complexes cata-
lyzing desulfitative arylation of azoles with arylsulfonyl
hydrazides.
[9]
1
-methylpyrazole assisted NHC–Pd complexes. In our
continuing efforts to further explore new types of well-
defined NHC–Pd complexes and as an extension of this
work, a related system based on pyrazole-3-carboxylate
assisted NHC–Pd complexes has been synthesized. We
envision incorporation of the multidentate ligand into
the NHC–Pd complexes to form a new class of N ^O or
N ^N ^O chelate Pd complexes.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and characterization of
1H-pyrazole-3-carboxylates assisted NHC–
Pd complexes
The azole moiety, which is an important building
unit, is present in many bioactive molecules.
[10]
Develop-
ing efficient synthetic approaches to construct
substituted azoles is an important aspect of organic syn-
thesis. In this regard, direct arylation of azoles has
emerged in recent years as an attractive alternative to
The addition of [Pd(μ-Cl)(Cl)(NHC)] to the N ^O anionic
chelate ligand, which was formed in situ from
2
deprotonation of N-heterocyclic carboxylic acid, is an
[16]
useful way to yield well-defined NHC–Pd complexes.
[11]
traditional methods.
Since Tian reported a type of
To assess the coordination mode of 1H-pyrazole-
3-carboxylic acid towards NHC–Pd complexes, treatment
of [Pd(μ-Cl)(Cl)(NHC)]2 with 1H-pyrazole-3-carboxylic
desulfitative coupling reaction using arylsulfonyl hydra-
zines as new arylating reagents, such compounds have
been recognized as new aryl sources as they are readily
t
acid in refluxing tetrahydrofuran (THF) with KO Bu as
[12]
accessible and compatible with moisture.
Wan, You
base was carried out. The corresponding dinuclear
and Kwong independently reported the Pd-catalyzed
desulfitative arylation of azoles with arylsulfonyl hydra-
pyrazolato-3-carboxylates assisted NHC–Pd complexes
[(NHC)Pd(μ-1H-pyrazolato-3-carboxylate)] (1a and 1b)
2
[13]
zides.
Meanwhile, Wang reported the Pd-catalyzed
were obtained in good yields (Scheme 1). Furthermore,
in order to compare the influence of catalyst structures
on their catalytic performance, 1-methyl-1H-pyrazole-
3-carboxylic acid was also investigated as a N ^O chelate
ligand to coordinate with NHC–Pd units. When the
same reactions were carried out, mononuclear complexes
desulfitative arylation of azoles using sodium
[14]
arylsulfinates as aryl sources.
Moreover, Soulé and
Doucet have comprehensively summarized the Pd-
0
catalyzed desulfitative arylation of azoles using RSO R
derivatives as aryl sources.
2
[15]
Despite these successes,
the limitations of these reagents have been noted, such
as high catalyst loading and excess ancillary ligand. Here
[(NHC)PdCl(1-methyl-1H-pyrazole-3-carboxylate)]
and 2b) were achieved. The H NMR spectra of the
(2a
1
SCHEME 1 Synthesis of pyrazole-
-carboxylates assisted NHC–Pd complexes
3

N-HETEROCYCLIC CARBENE PALLADIUM COMPLEXES
3 of 9
complexes showed distinctive stoichiometric proton sig-
nal resonances of NHCs and (1-methyl-)1H-pyrazole-
Table 1. As shown in Figure 1, dinuclear complexes 1a
and 1b contain centro-symmetric [M L ] metallocycle
2
2
3
-carboxylate in a 1:1 ratio, which supports the formation
structures with essentially square-planar Pd centers. The
central structural motifs are six-membered Pd N rings
13
of the complexes. In their C NMR spectra, the charac-
teristic peaks of the carbonic carbons (170.16 and
1
2
4
including four nitrogen atoms from two pyrazolatos and
two Pd centers. As commonly observed for square-planar
NHC complexes, the Pd centers were coordinated by an
NHC, N ^O -chelating donors from 1H-pyrazolato-3-car-
boxylate, and an N-donor from another ligand. The
NHCs are located trans to the nitrogen atoms of the N ^O -
chelating donors, while the carboxylate oxygens are situ-
ated in the cis position to the NHCs and opposite the
nitrogen atoms from other ligands. Here, the neighboring
N ^O donors of 1H-pyrazolato-3-carboxylate, acting as a
chelating ligand, form the five-membered chelate ring
68.71 ppm for 1a and 2a, 194.24 and 183.62 ppm for 2a
and 2b) could be observed.
2
.2 | Crystal structures
The solid-state structures of the complexes were further
characterized by X-ray crystallography. A summary of
the crystallographic data, data collection, and refinement
parameters for the complexes is provided in Table S1,
and the selected bond lengths and angles are listed in
(PdC NO) and another N-donor links to the neighboring
2
Pd center. In fact, 1H-pyrazolato-3-carboxylate acts both
as a chelating ligand to form the five-membered chelate
rings and as a bridging ligand to link the two Pd centers
through the N,N-donors to form the central six-
membered Pd N macrocycles. The five-membered che-
o
TABLE 1 Selected bond lengths (Å) and angles ( ) for
complexes 1a, 1b, 2a, and 2b
2 2 2
[0.5(1a)] •2CH Cl
2
4
Pd1–C1
Pd1–N4
Pd2–C32
Pd2–N8
1.946(7)
1.997(7)
1.950(8)
1.998(6)
93.9(3)
96.7(3)
92.8(3)
98.2(3)
Pd1–N3
1.942(7)
1.976(6)
1.964(7)
1.976(6)
87.6(3)
81.8(3)
88.0(3)
81.0(2)
late rings (PdC NO) are quasi-coplanar with the six-
2
membered Pd N macrocycles, with a largest dihedral
Pd1–O1
2 4
ꢀ
angle of 2.37 , and oriented approximately perpendicu-
larly to the carbene ring planes, with a smallest dihedral
Pd2–N7
Pd2–O3
ꢀ
angle of 71.05 . As usual in NHC–Pd complexes, the Pd–
C1–Pd1–N3
N3–Pd1–N4
C32–Pd2–N7
N7–Pd2–N8
C1–Pd1–O1
N4–Pd1–O1
C32–Pd2–O3
N8–Pd2–O3
CNHC distances fall into the range of single bond
[1.946(7)–1.989(3) Å] and are within the expected range.
In contrast to the dinuclear complexes 1a and 1b,
when the ancillary ligand was changed to 1-methyl-1H-
pyrazole-3-carboxylate, the mononuclear complexes 2a
and 2b were obtained. As shown in Figure 2, the Pd cen-
ters are surrounded by an NHC, N ^O -chelating donors,
and a chloride, which shows slightly distorted square-
planar geometries. Due to the trans effect, the pyrazole
nitrogen atoms are located trans to the NHCs, while the
carboxylate oxygens are situated in the cis position to the
NHCs. The lengths of the Pd–CNHC bonds in 2a and 2b
are 1.959(7) and 1.969(5) Å in solid crystals, respectively,
which is similar to the related pyridine-2-carboxylate
assisted NHC–Pd complexes.16a
2 2 2
[0.5(1b)] •2CH Cl
Pd1–C1
Pd1–N4
Pd2–C32
Pd2–N8
1.973(3)
2.035(3)
1.989(3)
2.040(3)
92.84(12)
99.20(12)
93.55(12)
97.75(11)
Pd1–N3
2.030(3)
2.059(3)
2.004(3)
2.036(2)
87.35(11)
80.63(11)
87.68(11)
81.04(10)
Pd1–O1
Pd2–N7
Pd2–O3
C1–Pd1–N3
N3–Pd1–N4
C32–Pd2–N7
N7–Pd2–N8
C1–Pd1–O1
N4–Pd1–O1
C32–Pd2–O3
N8–Pd2–O3
2
2 2
a•2CH Cl
Pd1–C1
Pd1–O1
1.959(7)
2.026(5)
89.4(3)
90.1(2)
Pd1–N2
2.084(6)
2.263(2)
79.4(2)
2
.3 | Catalytic investigation
Pd1–Cl1
C1–Pd1–O1
C1–Pd1–Cl1
O1–Pd1–N2
N2–Pd1–Cl1
The catalytic activities of the NHC–Pd complexes in the
desulfitative arylation of azoles with arylsulfonyl hydra-
zides were investigated. The initial studies of these reac-
101.1(2)
2
2 2
b•2CH Cl
tions were performed using TsNHNH and benzoxazole
as model substrates, in the presence of complex 1a, and
Pd1–C1
Pd1–O1
1.969(5)
2.040(4)
89.34(18)
90.68(15)
Pd1–N2
2.102(4)
2.2740(16)
79.34(17)
100.64(14)
2
Pd1–Cl1
the results are summarized in Table 2. Following the pre-
C1–Pd1–O1
C1–Pd1–Cl1
O1–Pd1–N2
N2–Pd1–Cl1
[
13]
vious reported results,
complex 1a (1.0 mol%) was ini-
tially selected as the catalyst, Cu(OAc) (2 equiv) was
2

4
of 9
YANG
FIGURE 1 ORTEP
diagrams of 1a (a) and 1b
(
b) with thermal displacement
parameters drawn at 30%
probability. Parts of the
hydrogen atoms and solvent
molecules (CH Cl ) have been
2
2
omitted for clarity. There are
two independent half molecules
in the unit cell of the solvates
[
0.5(1a)]
0.5(1b)]
2
•2CH
•2CH
2
Cl
Cl
2
and
[
2
2
2
, respectively,
only one molecule is shown.
A
Symmetry codes: 2 - x, 1 - y,
B
1
– z; - x, 1 - y, 1 - z
FIGURE 2 ORTEP
diagrams of 2a (a) and 2b
(
b) with thermal displacement
parameters drawn at 30%
probability. Parts of the
hydrogen atoms and solvent
2 2
molecules (CH Cl ) have been
omitted for clarity. There are two
and four independent molecules
in the unit cell of the solvates
2 2 2 2
2a•2CH Cl and 2b•2CH Cl ,
respectively, only one molecule
A
is shown. Symmetry codes: x,
0.5 - y, z
used as the oxidant, and the effect of solvent was exam-
ined. The coupled product was obtained in 56% yield
when the reaction was performed in THF. It is notewor-
thy that significant improvement was achieved (90%
yield) when 1,4-dioxane was employed as solvent. How-
ever, only 25% yield of product was obtained when the
reaction was performed in dimethyl sulfoxide (DMSO)
and no product was observed when reaction was per-
formed in 1,2-dichloroethane (DCE). On the basis of
these preliminary optimized results, the next studies were
focused on the effect of additives on the model reaction.
Among the additives examined, it was found that tetra-n-
butylammonium bromide (TBAB) was the least effective.
Inorganic bases were then screened and the results indi-
cate that the bases failed to promote the coupling reac-
tion. It is worth noting that the inorganic bases were
effective additions in the C–H activation of azoles with
with the arylating reagent arylsulfonyl hydrazide. The
amount of the catalyst loading was also investigated:
when the catalyst loading was reduced to 0.5 mol%, the
yield reduced to 65%. However, the yields increased
slightly when the catalyst loading was increased. The
reaction time and temperature were then investigated
and the reaction was completed within 12 hr when
ꢀ
1.0 mol% loading of complexes was used at 100 C. With
the increased temperature, the reaction ratio tended to
an invariant value while the decomposition rate and
quantity of the catalyst and substrate were evidently
accelerated. On the basis of the screening, 1,4-dioxane
and Cu(OAc) proved to be the most efficient solvent and
2
ꢀ
oxidant, respectively, with 1.0 mol% complex 1a at 100 C
for 12 hr, thus giving the optimal coupling result.
In order to further examine the catalytic properties of
the complexes, the coupling reactions of TsNHNH with
2
[17]
aryl halides,
but had no effect on our system, which
various substituted benzoxazoles (4-Me, 5-Me, 6-Me, 5-Cl,
and 6-Cl) were investigated using the optimized
indicates that there is a different reaction mechanism

N-HETEROCYCLIC CARBENE PALLADIUM COMPLEXES
5 of 9
a,b
TABLE 2 Optimization of the reaction conditions for complex 1a catalyzed desulfitative arylation reaction
b
Entry
Catalyst
1a (1.0%)
1a (1.0%)
1a (1.0%)
1a (1.0%)
1a (1.0%)
1a (1.0%)
1a (1.0%)
1a (0.5%)
1a (2.0%)
1a (1.0%)
1a (1.0%)
1a (1.0%)
Solvent
Additive
Yield (%)
1
2
3
4
5
6
7
8
THF
–
56
90
25
1,4-dioxane
DMSO
–
–
DCE
–
n.r.
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
TBAB
90
88
90
65
93
92
85
K
2
CO
3
Na
2
CO
3
–
9
–
c
10
11
12
–
d
e
–
–
n.r.
n.r., no reaction.
a
Reaction conditions: benzoxazole (0.5 mmol), TsNHNH
2
(0.75 mmol), complex 1a (0.005 mmol, 6.0 mg), Cu (OAc)
2
(1.0 mmol, 182 mg), solvent (2.0 mL) at
ꢀ
1
b
00 C for 12 hr.
Isolated yield.
c
Performed for 24 hr.
d
ꢀ
Performed at 110 C.
e
2
Without Cu (OAc) .
conditions. Mononuclear complexes 2a and 2b were also
evaluated using the same reaction. In order to maintain
the consistency of the catalyst loading, 1.0 mol% was used
in the catalytic reactions for 1a, 1b and 2.0 mol% was used
for 2a, 2b. The results indicate that the obtained complexes
exhibit good catalytic activities for desulfitative arylation
slightly better catalytic activities than the mononuclear
analogues, therefore implying that the dinuclear com-
plexes have a positive effect on the catalytic activities. This
benefit could not be attributed to the synergy effect due to
the long distances between the two metal centers
(3.909–3.994 Å). The probable reason for the catalytic out-
comes might be that the multidentate chelating coordina-
tion of 1H-pyrazolato-3-carboxylate increased the bonding
interaction between the metal and ancillary ligand,
resulting in a higher tendency of the multidentate ligand
of TsNHNH with benzoxazoles. In addition, the coupling
2
of TsNHNH with benzothiazole was also achieved, but
2
the yields were slightly lower than for coupling with
benzoxazole. One possible reason for this is that the sub-
strates bearing sulfur atoms may decrease the activities of
the catalysts. The use of arylsulfonyl hydrazides was also
explored for the reaction. Benzenesulfonyl hydrazide and
0
to recoordinate to the [NHC-Pd ] species, stabilizing the
active species and leading to an easier catalytic cycle. The
catalytic activities of the NHC–Pd complexes depend ulti-
mately on the balance of the ancillary ligands detaching
4
-chlorobenzenesulfonyl hydrazide all smoothly undergo
0
coupling reactions with benzoxazole and the
products were isolated in 82–93% yields. Besides the
azoles mentioned above, 1-methyl-1H-benzoimidazole
and 1-(p-tolyl)-1H-benzoimidazole could also undergo the
from and reattaching to the [NHC-Pd ] species. On the
whole, these results indicate that the ancillary ligands play
a very important role in stabilizing the catalysts and
forming the catalytically active species.
desulfitative arylation reaction with TsNHNH , although
A proposed catalytic cycle for the Pd-precatalyst cata-
lyzed desulfitative arylation of azoles with arylsulfonyl
2
slightly lower yields were obtained. As can be seen from
the data shown in Table 3, although there is no significant
difference in the yields of the products between the mono-
and dinuclear complex catalysts, when using the same
amount of Pd units, the dinuclear catalysts afforded
[
13]
hydrazides has been reported.
It is uncertain whether
the copper salts take part in the desulfitative oxidation of
II
arylsulfonyl hydrazide, affording the NHC−Pd −Ar
intermediate, but the reaction of NHC−Pd complexes

6
of 9
YANG
a,b
TABLE 3 Desulfinative arylation of azoles with arylsulfonyl hydrazides catalyzed by NHC–Pd complexes
a
Reaction conditions: azole (0.50 mmol), arylsulfonyl hydrazide (0.75 mmol), NHC–Pd (0.005 mmol for 1a and 1b; 0.01 mmol for 2a and 2b), Cu (OAc)
2
ꢀ
(
1.0 mmol), 1,4-dioxane (2.0 mL) at 100 C for 12 hr.
b
Isolated yield.
with arylsulfonyl hydrazide, which gives the NHC
only used as the oxidant, but is also involved in the for-
mation of the organocopper(I) species.
II
−
Pd −Ar intermediate and liberates N and SO gas, is a
2
12,13,15]
2
[
well-known and accepted process.
The generated
active Pd(II) species were stabilized by the presence of
the NHC ligands. Meanwhile, the reaction of Cu(I) spe-
cies, which are generated from either the in situ reduc-
3 | CONCLUSIONS
tion or disproportionation of Cu(OAc) with azoles, gives
In summary, we have prepared and characterized four
new pyrazole-3-carboxylates assisted NHC–Pd complexes.
The catalytic activities of the obtained mono- and din-
uclear complexes in the desulfitative arylation of azoles
with arylsulfonyl hydrazides were initially investigated.
The catalytic results show that the dinuclear complexes
introduce some benefits into the catalytic reaction
2
[18]
the azole−Cu(I) species.
Subsequent transmetallation
II
of azole−Cu(I) species to the NHC−Pd −Ar intermediate
followed by reductive elimination of the resulting inter-
mediate azole−Pd−Ar gives the desired coupling product.
The catalytic cycle is completed when the Pd(0) species is
oxidized to Pd(II) by Cu(OAc) . Here, Cu(OAc) is not
2
2

N-HETEROCYCLIC CARBENE PALLADIUM COMPLEXES
7 of 9
outcomes when compared to their mononuclear ana-
logues. Therefore, the ancillary ligand is a potential
source to adjust the structure of NHC–Pd complexes and
then facilitate the catalytic activities of the complexes.
Further studies on the structure–activity relationships of
such complexes are currently in progress.
12H, CH (CH ) ), 0.85 (d, J = 6.5 Hz, 12H, CH (CH ) ).
3
2
3 2
−
13
C NMR (CDCl , 125 MHz) δ (ppm): 170.16 (COO ),
3
163.97 (Ccarbene), 147.69, 147.18, 144.66, 140.50, 133.98,
130.39, 125.08, 124.64, 124.05, 106.48, 29.03 (CH (CH ) ),
3
2
28.57 (CH (CH ) ), 26.54 (CH (CH ) ), 22.41 (CH (CH ) ),
3
2
3 2
3 2
22.02 (CH (CH ) ). HR-MS (ESI): calcd for
3
2
+
C H N O Pd [M + H] 1211.4141; found 1211.4157;
62
77
8
4
2
calcd
for
[(IPr)Pd(μ-1H-pyrazolato-3-carboxylate)]2
4
4
| EXPERIMENTAL
.1 | General remarks
(C H N O Pd ): C, 61.53; H, 6.33; N, 9.26%; found C,
62
76
8
4
2
61.78; H, 6.61; N, 9.12%.
The chemicals were purchased from commercial sup-
pliers and used without further purification. NMR spec-
tra were recorded on Bruker Avance 400 or 500 NMR
4.4 | [(SIPr)Pd(μ-1H-pyrazolato-3-
carboxylate)] (1b)
2
1
13
spectrometers. H and C NMR were performed in
The procedure yielded 530 mg (87%) of the pure 1b as a
1
CDCl with tetramethylsilane (TMS) as an internal stan-
yellow powder. H NMR (CDCl , 500 MHz) δ (ppm): 7.34
3
3
dard. High-resolution mass spectra were recorded with
an Agilent 6550 iFunnel Q-TOF MS system. The C, H,
and N analyses were performed with a Vario El III
Elementar. Flash column chromatography was carried
(t, J = 7.5 Hz, 4H, p-CH-phenyl), 7.27 (dd, J = 7.5 and
1.5 Hz, 4H, m-CH-phenyl), 7.13 (dd, J = 7.5 and 1.5 Hz,
4H, m-CH-phenyl), 7.00 (d, J = 1.5 Hz, 2H, CH-pyrazole),
6.21 (d, J = 2.0 Hz, 2H, CH-pyrazole), 4.13 (br, 4H, N-
CH CH -N), 4.11 (br, 4H, N-CH CH -N), 3.32 (sept,
out using 300–400 mesh silica gel. [NHC = IPr and SIPr;
2
2
2
2
0
IPr
2
=
N,N -bis-(2,6-di (isopropyl)phenyl)imidazol-
J = 6.5 Hz, 4H, CH (CH ) ), 2.98 (sept, J = 6.5 Hz, 4H,
3 2
CH (CH ) ), 1.50 (d, J = 6.0 Hz, 12H, CH (CH ) ), 1.18 (d,
3 2 3 2
0
-ylidene; SIPr = N,N -bis- (2,6-di (isopropyl)phenyl)
imidazolidin-2-ylidene].
J = 6.5 Hz, 12H, CH (CH ) ), 1.15 (d, J = 6.5 Hz, 12H,
3 2
1
3
CH (CH ) ), 0.81 (d, J = 6.5 Hz, 12H, CH (CH ) ),
C
3
2
3 2
NMR (CDCl , 125 MHz) δ (ppm): 194.24 (Ccarbene),
3
−
4
.2 | Synthesis of complexes 1a, 1b, 2a,
170.22 (COO ), 148.11, 147.14, 146.04, 140.34, 129.59,
and 2b
125.04, 124.26, 106.54, 53.95 (N-CH CH -N), 29.19
2
2
(
CH(CH ) ), 28.85 (CH(CH ) ), 27.23 (CH(CH ) ), 26.75
3 2 3 2 3 2
A
mixture of [Pd(μ-Cl)(Cl)(NHC)]2 (0.5 mmol),
(CH(CH ) ), 23.44 (CH(CH ) ), 22.51 (CH(CH ) ). HR-
3 2 3 2 3 2
MS (ESI): calcd for C H N O Pd [M + H] 1215.4455;
62 81 8 4 2
found 1215.4468: calcd for [(SIPr)Pd(μ-1H-pyrazolato-
+
(1-methyl-)1H-pyrazole-3-carboxylic acid (1.0 mmol) and
t
KO Bu (2.4 mmol for 1a, 1b and 1.2 mmol for 2a, 2b) was
dissolved in anhydrous THF (10.0 ml). After stirring for
3-carboxylate)] (C H N O Pd ): C, 61.33; H, 6.64; N,
9.23%; found: C, 61.61; H, 6.87; N, 9.08%.
2
62 80
8
4
2
ꢀ
1
2 hr at 70 C, the mixture was purified by flash column
chromatography on short silica gel with CH Cl as elu-
2
2
ent. The residue was reduced under vacuum and rec-
rystallized from n-hexane/CH Cl solution to afford the
desired products.
4.5 | (IPr)PdCl(1-methyl-1H-pyrazole-3-
carboxylate) (2a)
2
2
The procedure yielded 590 mg (90%) of the pure 2a as a
1
4.3 | [(IPr)Pd(μ-1H-pyrazolato-3-
yellow powder. H NMR (CDCl , 500 MHz) δ (ppm): 7.51
3
carboxylate)] (1a)
(t, J = 8.0 Hz, 2H, p-CH-phenyl), 7.36 (d, J = 8.0 Hz, 4H,
m-CH-phenyl), 7.14 (s, 2H, N-CHCH-N), 7.08 (d,
J = 2.5 Hz, 1H, CH-pyrazole), 6.43 (d, J = 2.5 Hz, 1H,
2
The procedure yielded 515 mg (87%) of the pure 1a as a
1
yellow powder. H NMR (CDCl , 500 MHz) δ (ppm): 7.44
CH-pyrazole), 3.84 (s, 3H, N-CH ), 2.94 (br, 4H, CH
3
3
(
t, J = 7.5 Hz, 4H, p-CH-phenyl), 7.31 (d, J = 7.5 Hz, 4H,
(CH ) ), 1.41 (d, J = 6.5 Hz, 12H, CH (CH ) ), 1.12 (d,
3
2
3 2
13
m-CH-phenyl), 7.24 (s, 4H, N-CHCH-N), 7.22 (d,
J = 7.5 Hz, 4H, m-CH-phenyl), 6.41 (d, J = 2.5 Hz, 2H,
CH-pyrazole), 6.15 (d, J = 2.0 Hz, 2H, CH-pyrazole), 2.87
J = 7.0 Hz, 12H, CH (CH ) ). C NMR (CDCl3,
3 2
−
125 MHz) δ (ppm): 168.71 (COO ), 152.53 (Ccarbene),
148.43, 146.94, 134.42, 133.00, 130.37, 125.09, 124.06,
106.81, 39.59 (N-CH ), 28.58 (CH (CH ) ), 26.22 (CH
(sept, J = 6.5 Hz, 4H, CH (CH ) ), 2.66 (sept, J = 6.5 Hz,
3 2
3
3 2
4
1
H, CH (CH ) ), 1.39 (d, J = 6.5 Hz, 12H, CH (CH ) ),
(CH ) ), 22.92 (CH (CH ) ). HR-MS (ESI): calcd for
3 2 3 2
C H N O Pd [M–Cl] 619.2264; found 619.2277: calcd
32 41 4 2
3
2
3 2
+
.12 (d, J = 6.5 Hz, 12H, CH (CH ) ), 0.99 (d, J = 6.5 Hz,
3
2

8
of 9
YANG
for
(IPr)PdCl(1-methyl-1H-pyrazole-3-carboxylate)
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5
C H ClN O Pd): C, 58.63; H, 6.30; N, 8.55%; found: C,
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32
41
4 2
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−
(ppm): 183.67 (Ccarbene), 168.76 (COO ), 148.37, 134.59,
1
33.02, 129.62, 124.65, 124.45, 106.78, 53.61 (N-CH CH -
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2
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3
3 2
3 2
6.64 (CH (CH ) ), 24.03 (CH (CH ) ), 23.76 (CH (CH ) ).
3 2 3 2 3 2
+
32
43 4 2
pyrazole-3-carboxylate) (C H ClN O Pd): C, 58.45; H,
32
43
4 2
6
.59; N, 8.52%; found: C, 58.74; H, 6.85; N, 8.30%.
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.7 | General procedure for the NHC–Pd
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_
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2
[
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ACKNOWLEDGEMENTS
Financial support from the Natural Science Foundation
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Anhui
Provincial
Education
Department
2
(KJ2019A0600) and the Scientific Research Project of
[
Anhui Provincial Education Department (KJ2015TD002)
is gratefully acknowledged.
1
[
[
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ORCID
Jin Yang https://orcid.org/0000-0003-0757-7155

N-HETEROCYCLIC CARBENE PALLADIUM COMPLEXES
9 of 9
Organomet. Chem. 2018, 32, e4394. (d)W. Chen, J. Yang, J.
Organometal. Chem. 2018, 872, 24.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
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How to cite this article: Yang J. Pyrazole-3-
carboxylates assisted N-heterocyclic carbene
palladium complexes: synthesis, characterization,
and catalytic activities towards arylation of azoles
with arylsulfonyl hydrazides. Appl Organometal
Chem. 2020;e5450. https://doi.org/10.1002/aoc.
5450
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