Heteroleptic palladium(II) complexes containing N-heterocyclic carbenes and 4-phenyl-1H-1,2,3-triazole: Synthesis, characterization, and catalytic application

Article abstract of DOI:10.1080/00958972.2018.1476972

Reaction of the dimeric N-heterocyclic carbene (NHC) palladium compounds [Pd(μ-Cl)(Cl)(NHC)]₂ with 4-phenyl-1H-1,2,3-triazole gave four mono- and dinuclear complexes 1–4. Mononuclear complexes 1 and 2 [(NHC)PdCl₂(4-phenyl-1H-1,2,3-triazole)] were obtained when the reactions were performed in CH₂Cl₂, whereas dinuclear complexes 3 and 4 [Pd₂(μ-Cl)(μ-4-phenyl-1H-1,2,3-triazole)Cl₂(NHC)₂] were obtained when the reactions were performed in THF in reflux with Et₃N as the base. Further explorations of the catalytic properties of 1–4 for Pd-catalyzed transformations have been performed and these complexes exhibited moderate to high catalytic activities for Suzuki–Miyaura coupling and arylation of benzoxazoles with aryl bromides.

Full text of DOI:10.1080/00958972.2018.1476972

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Heteroleptic palladium(II) complexes containing
N-heterocyclic carbenes and 4-phenyl-1H-1,2,3-
triazole: Synthesis, characterization, and catalytic
application
Wei Chen & Jin Yang
To cite this article: Wei Chen & Jin Yang (2018): Heteroleptic palladium(II) complexes containing
N-heterocyclic carbenes and 4-phenyl-1H-1,2,3-triazole: Synthesis, characterization, and catalytic
application, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1476972
To link to this article: https://doi.org/10.1080/00958972.2018.1476972
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Journal of Coordination Chemistry, 2018
https://doi.org/10.1080/00958972.2018.1476972
Heteroleptic palladium(II) complexes containing
N-heterocyclic carbenes and 4-phenyl-1H-1,2,3-triazole:
Synthesis, characterization, and catalytic application
Wei Chen and Jin Yang
department of Chemistry, huaibei normal university, huaibei, Pr China
ABSTRACT
ARTICLE HISTORY
received 1 february 2018
accepted 18 april 2018
Reaction of the dimeric N-heterocyclic carbene (NHC) palladium
compounds [Pd(μ-Cl)(Cl)(NHC)]2 with 4-phenyl-1H-1,2,3-triazole gave
four mono- and dinuclear complexes 1–4. Mononuclear complexes 1
and 2 [(NHC)PdCl2(4-phenyl-1H-1,2,3-triazole)] were obtained when
the reactions were performed in CH2Cl2, whereas dinuclear complexes
KEYWORDS
N-heterocyclic carbene
palladium; 4-Phenyl-1H-
1,2,3-triazole; suzuki–
miyaura coupling; direct
arylation
3
and 4 [Pd2(μ-Cl)(μ-4-phenyl-1H-1,2,3-triazole)Cl2(NHC)2] were
obtained when the reactions were performed in THF in reflux with
Et3N as the base. Further explorations of the catalytic properties of
1–4 for Pd-catalyzed transformations have been performed and these
complexes exhibited moderate to high catalytic activities for Suzuki–
Miyaura coupling and arylation of benzoxazoles with aryl bromides.
1. Introduction
Since earlier studies by Wanzlick [1], Öfele [2], and after their isolation and characterization
by Arduengo [3], N-heterocyclic carbenes (NHCs) have been widely studied as multifunc-
tional ligands in homogeneous catalysis. A great number of NHC–Pd complexes have been
prepared and employed as effective catalysts in organic transformations [4–7]. The early
stage study usually focused on the NHC–Pd precatalysts, which were prepared in situ by
CONTACT Jin yang
yangjinlz@163.com
supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2018.1476972.
© 2018 informa uK limited, trading as taylor & francis Group

2
W. CHEN AND J. YANG
reaction of metal precursors with imidazolium salts. Recently, many well-defined NHC–Pd
complexes have been synthesized and displayed remarkable catalytic activities [8–10].
Among these, a group of NHC–Pd complexes named Pd–PEPPSI–NHC, introduced by Organ,
has received great attention [11, 12]. Extensive studies suggested that the catalytic activities
of these well-defined complexes are mainly dependent on the electronic properties and
steric hindrance of NHC moieties, however, the ancillary ligands also affected the catalytic
activity to some extent [13]. Based on this, a number of well-defined NHC–Pd complexes
with various ancillary ligands have been developed and shown promising catalytic activities
[14–17].
Because the coordination behaviors of the sp2-hybridized nitrogen atoms in azoles are
basically identical with pyridine ligands, various azoles have been introduced into the coor-
dination with NHC–Pd complexes [18–23]. The type of azoles significantly affected the struc-
tures of NHC–Pd complexes. As recently reported by our group, when 1H-tetrazole,
1-phenyl-1H-tetrazole and 1H-benzotriazole were used as ancillary ligands, mononuclear
[(NHC)PdCl2(azoles)] complexes were achieved. However, when 5-phenyl-1H-tetrazole was
employed, tetrazole bridged dinuclear NHC‒Pd complexes were formed through deproto-
nating the tetrazole ligand into the corresponding azolate anion [24, 25]. To obtain more
information about the deprotonation of azole ligands and compare their catalytic properties
of these well-defined NHC–Pd complexes, herein, we selected a weaker acidic azole, 4-phe-
nyl-1H-1,2,3-triazole, as ancillary ligand to coordinate with NHC–Pd complexes. Through
adjusting the deprotonation of 4-phenyl-1H-1,2,3-triazole, the mono- and dinuclear NHC–Pd
complexes were achieved, respectively. Furthermore, their catalytic activities towards Suzuki–
Miyaura coupling and arylation of benzoxazole with aryl bromides were preliminarily inves-
tigated (Scheme 1).
2. Experimental
2.1. General remarks
All the reactions and manipulations were carried out under air atmosphere. 4-Phenyl-1H-
1,2,3-triazole [26] and dimeric complexes [Pd(μ-Cl)(Cl)(IPr)]2 and [Pd(μ-Cl)(Cl)(SIPr)]2 [IPr =
N,N′-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene; SIPr = N,N′-bis-(2,6-diisopropylphenyl)
imidazolidin-2-ylidene] [27] were prepared according to literature method. NMR spectra
were recorded in CDCl3 at 400 MHz (for 1H NMR) and 100 MHz (for 13C‒{1H} NMR) on a Bruker
Avance 400 NMR spectrometer. High-resolution mass spectra were recorded on an Agilent
6550 iFunnel Q-TOF MS System. IR spectra were recorded on a Bruker IFS 120HR spectrometer
using KBr disks. The C, H, and N analyses were performed with a Vario El III elementar. Flash
column chromatography was carried out using 230–400 mesh silica gel.
2.2. Preparation of mononuclear NHC–Pd complexes 1 and 2
A mixture of [Pd(μ-Cl)(Cl)(NHC)]2 (0.10 mmol, 113 mg) and 4-phenyl-1H-1,2,3-triazole
(0.20 mmol, 29 mg) were stirred in CH2Cl2 (5.0 mL) at room temperature for 6 h to give a
yellow solution. The solvent was reduced to about 1.0 mL and the resulting residue was
washed with diethyl ether (10.0 mL) and recrystallized from n-hexane/CH2Cl2 to give the
mononuclear NHC–Pd complexes.

JOURNAL OF COORDINATION CHEMISTRY
3
2.2.1. [PdCl2(IPr)(5-phenyl-1H-1,2,3-triazole)] (1)
The procedure yielded 125 mg (88%) of the product 1. 1H NMR (400 MHz, CDCl3):
δ (ppm) = 12.94 (s, 1H, NH), 8.27 (s, 1H, CH-azole), 7.67 (dd, J = 8.0 Hz and 1.2 Hz, 2H,
CH-benzene), 7.53 (t, J = 7.6 Hz, 2H, CH-benzene), 7.38–7.36 (m, 5H), 7.34–7.30 (m, 2H), 7.18
(s, 2H), 3.12, (sept, 4H, J = 6.8 Hz, CH(CH3)2), 1.48 (d, J = 6.4 Hz, 12H, CH(CH3)2), 1.15 (d,
J = 6.8 Hz, 12H, CH(CH3)2). 13C‒{1H} NMR (100 MHz, CDCl3): δ (ppm) = 151.6 (Ccarbene), 146.6
(N–CAr), 134.7 (iPr-C), 130.4 (CAr), 130.3 (CAr), 128.9 (CAr), 128.8 (CAr), 126.0 (N–CH=CH–N), 125.3
(CAr), 124.1 (CAr), 28.7 (CH(CH3)2), 26.3 (CH(CH3)2), 23.1 (CH(CH3)2). HR-MS (ESI): calcd for
C35H43ClN5Pd [M–Cl–]+ 674.2242; found 674.2199. IR (KBr, cm−1): 3250, 2965, 2865, 1652, 1630,
1576, 1557, 1539, 1480, 1455, 1384, 1362, 1300, 1269, 1171, 1091, 1056, 1017, 970, 914, 805.
Anal. Calcd for [PdCl2(IPr)(4-phenyl-1H-1,2,3-triazole)] (C35H43Cl2N5Pd): C, 59.12; H, 6.10; N,
9.85%. Found: C, 59.44; H, 6.39; N, 9.65%.
2.2.2. [PdCl2(SIPr)(5-phenyl-1H-1,2,3-triazole)] (2)
The procedure yielded 130 mg (91%) of the product 2 as a yellow powder. 1H NMR (400 MHz,
CDCl3): δ (ppm) = 12.94 (s, 1H, NH), 8.23 (s, 1H, CH-azole), 7.65 (d, J = 6.8 Hz, 2H, CH-benzene),
7.43 (t, J = 7.6 Hz, 2H, CH-benzene), 7.35–7.30 (m, 7H), 4.11 (s, 4H, NCH2CH2 N), 3.55 (sept, 4H,
J = 6.8 Hz, CH(CH3)2), 1.55 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.29 (d, J = 7.2 Hz, 12H, CH(CH3)2).
13C‒{1H} NMR (100 MHz, CDCl3): δ (ppm) = 183.2 (Ccarbene), 147.4 (N–CAr), 134.9 (iPr-C), 130.2
(CAr), 129.6 (CAr), 128.9 (CAr), 128.8 (CAr), 125.9 (CAr), 124.5 (CAr), 53.8 (N–CH2CH2–N), 28.8
(CH(CH3)2), 26.8 (CH(CH3)2), 24.1 (CH(CH3)2). HR-MS (ESI): calcd for C35H45ClN5Pd [M − Cl−]+
676.2398; found 676.2379. IR (KBr, cm−1): 3151, 3067, 2968, 1640, 1589, 1566, 1480, 1464,
1410, 1383, 1372, 1310, 1277, 1237, 1204, 1175, 1120, 1085, 1061, 971, 929, 804. Anal. Calc.
for [PdCl2(SIPr)(4-phenyl-1H-1,2,3-triazole)] (C35H45Cl2N5Pd): C, 58.95; H, 6.36; N, 9.82%. Found:
C, 58.65; H, 6.16; N, 9.65%.
2.3. Preparation of the dinuclear NHC–Pd complexes 3 and 4
A mixture of [Pd(μ-Cl)(Cl)(NHC)]2 (0.10 mmol, 113 mg), 4-phenyl-1H-1,2,3-triazole (0.10 mmol,
14.5 mg), and Et3N (0.20 mmol, 21 mg) were stirred in THF (5.0 mL) under reflux for 6 h to
give a yellow solution. After cooling, the solvent was reduced to about 1.0 mL, the resulting
residue was washed with diethyl ether (10.0 mL) and recrystallized from n-hexane/CH2Cl2
to give the dinuclear NHC–Pd complexes.
2.3.1. [Pd2(μ-Cl)(μ-4-phenyl-1H-1,2,3-triazole)Cl2(IPr)2] (3)
The procedure yielded 105 mg (85%) of the product 3 as a yellow powder. 1H NMR (400 MHz,
CDCl3) δ (ppm) = 7.93 (s, 1H, CH-azole), 7.55–7.53 (m, 2H), 7.51–7.46 (m, 4H), 7.35 (d, J = 6.4 Hz,
2H, CH-benzene), 7.28–7.25 (m, 6H), 7.22 (dd, J = 8.0 Hz and 1.2 Hz, 2H, CH-benzene), 7.18
(br, 3H), 7.08 (s, 2H, NCH=CHN), 3.43 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.08 (sept, J = 6.8 Hz, 2H,
CH(CH3)2), 2.78 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.41 (m, 12H, CH(CH3)2), 1.31 (d, J = 6.4 Hz, 6H,
CH(CH3)2), 1.12 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.02 (d, J = 7.2 Hz, 12H, CH(CH3)2), 0.88
(d, J = 6.8 Hz, 6H, CH(CH3)2), 0.30 (d, J = 6.4 Hz, 6H, CH(CH3)2). 13C‒{1H} NMR (100 MHz, CDCl3):
δ (ppm) = 156.7 (trans-Ccarbene), 154.7 (cis-Ccarbene), 147.4 (N–CAr), 146.4 (N–CAr), 146.2 (Cazole),
143.8 (Cazole), 135.1 (iPr-CAr), 134.4 (iPr-CAr), 131.7 (Cazole-Ar), 130.1 (CAr), 130.1 (CAr), 129.9 (CAr),
128.3 (CAr), 126.8 (CAr), 125.2 (N–CH=CH–N), 124.9 (N–CH=CH–N), 124.5 (Cazole-Ar), 124.4
(Cazole-Ar), 124.0 (CAr), 123.8 (CAr), 28.9 (CH(CH3)2), 28.6 (CH(CH3)2), 27.4 (CH(CH3)2), 26.6

4
W. CHEN AND J. YANG
(CH(CH3)2), 26.3 (CH(CH3)2), 26.3 (CH(CH3)2), 25.8 (CH(CH3)2), 23.4 (CH(CH3)2), 23.3 (CH(CH3)2),
22.8 (CH(CH3)2), 21.8 (CH(CH3)2). HR-MS (ESI): calcd for C62H79Cl3N7Pd2 [M + H+]+ 1240.3536;
found 1240.3551. IR (KBr, cm−1): 3138, 3070, 2927, 2866, 1699, 1629, 1465, 1407, 1383, 1364,
1341, 1328, 1253, 1203, 1180, 1120, 1090, 1020, 977, 936, 799. Anal. Calcd for [Pd2(μ-Cl)(μ-4-
phenyl-1H-1,2,3-triazole)Cl2(IPr)2] (C62H78Cl3N7Pd2): C, 60.03; H, 6.34; N, 7.90%. Found: C, 60.26;
H, 6.55; N, 8.18%.
2.3.2. [Pd2(μ-Cl)(μ-4-phenyl-1H-1,2,3-triazole)Cl2(SIPr)2] (4)
The procedure yielded 108 mg (87%) of the product 4 as a yellow powder. 1H NMR (400 MHz,
CDCl3): δ (ppm) = 7.92 (s, 1H, CH-azole), 7.68 (d, J = 7.2 Hz, 2H, CH-benzene), 7.43 (t, J = 7.6 Hz,
2H, CH-benzene), 7.37–7.29 (m, 6H), 7.22 (d, J = 7.2 Hz, 1H, p-CH-aniline), 7.16 (d, J = 7.6 Hz,
4H, m-CH-aniline), 7.11 (dd, J = 7.6 and 1.2 Hz, 2H, m-CH-aniline), 4.22 (s, 2H, NCH2CH2 N),
4.08 (s, 2H, NCH2CH2 N), 3.96 (s, 2H, NCH2CH2 N), 3.94 (s, 2H, NCH2CH2 N), 3.63 (sept, J = 6.8 Hz,
2H, CH(CH3)2), 3.46 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.39 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.12
(sept, J = 6.8 Hz, 2H, CH(CH3)2), 1.50 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.41 (d, J = 6.8 Hz, 6H,
CH(CH3)2), 1.32 (d, J = 6.4 Hz, 6H, CH(CH3)2), 1.22 (dd, J = 6.8 and 1.6 Hz, 12H, CH(CH3)2), 1.13
(d, J = 6.8 Hz, 6H, CH(CH3)2), 0.85 (d, J = 6.8 Hz, 6H, CH(CH3)2), 0.28 (d, J = 6.8 Hz, 6H, CH(CH3)2).
13C‒{1H} NMR (100 MHz, CDCl3): δ (ppm) = 187.2 (trans-Ccarbene), 185.8 (cis-Ccarbene), 147.9
(N–CAr), 147.5 (N–CAr), 147.1 (Cazole), 146.9 (Cazole), 143.9 (iPr-CAr), 135.4 (iPr-CAr), 134.6 (Cazole-Ar),
132.0 (CAr), 130.2 (CAr), 129.3 (CAr), 128.5 (CAr), 126.8 (CAr), 124.7 (Cazole-Ar), 124.4 (Cazole-Ar), 124.4
(CAr), 124.3 (CAr), 124.3 (CAr), 53.8 (N–CH2CH2–N), 53.8 (N–CH2CH2–N), 28.9 (CH(CH3)2), 28.6
(CH(CH3)2), 27.5 (CH(CH3)2), 26.9 (CH(CH3)2), 26.8 (CH(CH3)2), 25.8 (CH(CH3)2), 24.3 (CH(CH3)2),
24.2 (CH(CH3)2), 23.9 (CH(CH3)2), 22.2 (CH(CH3)2). HR-MS (ESI): calcd for C62H83Cl3N7Pd2
[M + H+]+ 1244.3849; found 1244.3867. IR (KBr, cm−1): 3150, 3068, 2962, 2866, 1480, 1456,
1417, 1384, 1363, 1322, 1298, 1269, 1089, 1056, 1017, 977, 801. Anal. Calc. for [Pd2(μ-Cl)(μ-4-
phenyl-1H-1,2,3-triazole)Cl2(SIPr)2] (C62H82Cl3N7Pd2): C, 59.83; H, 6.64; N, 7.88%. Found: C,
60.11; H, 6.81; N, 8.17%.
2.4. General procedures for Suzuki–Miyaura coupling
In an air atmosphere, a sealable reaction tube was charged with aryl bromide (0.5 mmol),
phenylboronic acid (0.6 mmol), K3PO4 (0.75 mmol, 159 mg), NHC–Pd complex (0.5% mol,
1.8 mg for 1 and 2; 0.25% mol, 1.6 mg for 3 and 4), and EtOH (2.0 mL). The mixture was stirred
for 2 h at room temperature. Then the solvent was evaporated under vacuum and the residue
was purified via column chromatography with petroleum ether–EtOAc (20:1) as eluent to
give the products.
2.5. General procedure for arylation of benzoxazoles
In an air atmosphere, a sealable reaction tube was charged with benzoxazole (0.5 mmol),
aryl bromide (0.6 mmol), K3PO4 (0.75 mmol), NHC‒Pd complex (1.0% mol, 3.6 mg for 1 and
2; 0.5% mol, 3.2 mg for 3 and 4), and DMF (2.0 mL). The mixture was stirred for 12 h at
130 °C. Then the mixture was cooled to room temperature and evaporated under vacuum.
The residue was subjected to purification via column chromatography with petroleum ether–
EtOAc (20:1) as eluent to give the products.

JOURNAL OF COORDINATION CHEMISTRY
5
2.6. X-ray crystallography
Single crystals for X-ray diffraction analysis were obtained by slow diffusion of n-hexane into
their CH2Cl2 solvents. Data collection was performed on a Bruker-AXS SMART CMOS area
detector diffractometer at 296 K using ω rotation scans with a scan width of 0.5° and Mo-Kα
radiation (λ = 0.71073 Å). Multi-scan corrections were applied using SADABS [28]. Structure
solutions and refinements were performed with the SHELX-2014 package [29]. All non-hy-
drogen atoms were refined anisotropically by full-matrix least-squares on F2. The hydrogen
atoms to carbon were included in idealized geometric positions with thermal parameters
equivalent to 1.2 times those of carbon atoms. CCDC 1534746, 1534747, 1534748 and
1534749 contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data Center via https://www.
ccdc.cam.ac.uk/structures/.
3. Results and discussion
3.1. Preparation and characterization of the mono- and dinuclear NHC–Pd
complexes
To assess the coordination of 4-phenyl-1H-1,2,3-triazole towards NHC–Pd complexes, reac-
tion of the triazole with [Pd(μ-Cl)(Cl)(NHC)]2 was performed. Mono- and dinuclear complexes
were achieved, which are closely related to the reaction conditions. Mononuclear complexes
1 and 2 were synthesized through reaction of [Pd(μ-Cl)(Cl)(NHC)]2 with 4-phenyl-1H-1,2,3-
triazole in CH2Cl2 at room temperature, while dinuclear complexes 3 and 4 were achieved
when the reaction was performed in THF at reflux with Et3N as the base. In this procedure,
4-phenyl-1H-1,2,3-triazole, as a weak acid, can be easily deprotonated to form the azolate
anion by addition of Et3 N. Moreover, the nitrogen atom of 4-phenyl-1H-1,2,3-triazole coor-
dinated to the palladium center also increases the acidity of the remaining NH moiety and
facilitates its deprotonation. We have also attempted to avoid the addition of base, but failed
to isolate the products. On the other hand, inorganic bases (Na2CO3, K2CO3 and Cs2CO3) can
be also used in this system to give the dinuclear compounds. The obtained complexes were
fully characterized by NMR, Fourier transform infrared (FT-IR) spectroscopies, elemental anal-
ysis and X-ray crystallography. For mononuclear complexes 1 and 2, the stoichiometric pro-
ton signal resonances of NHCs and triazole with a ratio of 1:1 were observed in their 1H NMR
spectra. Furthermore, the 1H NMR spectra revealed the appearance of diagnostic NH peaks
at 12.94 ppm for 1 and 2, which indicated 4-phenyl-1H-1,2,3-triazole, as a neutral ligand,
only coordinated to the metal center and did not deprotonate to form the azolate anion.
Furthermore, the characteristic peaks of the carbene carbons (151.6 ppm for 1 and 183.2 ppm
for 2) were found in their 13C NMR spectra. For dinuclear complexes 3 and 4, due to the poor
symmetry, the NMR spectra are more complicated compared to their corresponding mon-
onuclear complexes. However, the characteristic peaks of the carbonic carbons were clearly
found in their 13C NMR spectra. Moreover, the singlet resonances of the carbonic carbon in
trans arrangement (156.7 ppm for 3 and 187.2 ppm for 4) were shifted downfield relative to
that of the resonances of carbonic carbon in cis arrangement (154.7 ppm for 3 and 185.8 ppm
for 4) [30, 31].

6
W. CHEN AND J. YANG
Table 1. Crystallographic data for 1–4.
1
2•CH2Cl2
3
4
formula
fw
Crystal system
space group
C35h43Cl2n5Pd
C36h47Cl4n5Pd
797.98
monoclinic
P 21/c
11.8821(5)
20.5243(8)
16.6345(7)
90.00
104.4530(10)
90.00
3928.3(3)
4
C62h78Cl3n7Pd2
1240.46
monoclinic
P 21/n
19.992(3)
15.168(2)
20.436(3)
90.00
92.361(5)
90
6191.7(15)
4
C62h82Cl3n7Pd2
1244.49
monoclinic
P 21/n
20.0317(6)
15.1629(5)
20.4755(7)
90.00
93.3180(10)
90.00
6208.8(3)
4
711.04
monoclinic
P 21/c
20.576(4)
18.817(3)
28.165(5)
90.00
100.951(3)
90.00
10,706(3)
12
a/Å
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
dcalc/g cm−3
F(000)
1.323
4416
0.700
1.022
1.349
1648
0.775
1.089
1.331
2568
0.753
1.117
1.331
2584
0.751
1.159
μ/mm−1
Gof
reflections collected
53,807
18,840
(0.0353)
13,886
1175
0.0331
0.0790
60,814
6957
(0.0251)
6191
424
0.0424
0.1228
96,603
10,987
(0.0380)
9154
683
0.0378
0.0901
93,043
11,025
(0.0196)
9199
683
0.0432
0.0930
ind. reflections (Rint
)
obs. reflections [I >2σ(I)]
refined parameters
R1 [I > 2σ(I)]
wR2 (all data)
Figure 1. Crystal structures of 1 and 2 with thermal ellipsoids drawn at the 30% probability level.
hydrogens are omitted for clarity.
3.2. Crystal structures
The structures of the complexes have been further characterized by X-ray diffraction analyses
and the summary of the crystallographic data is provided inTable 1. The molecular structures
are shown in Figures 1 and 2. There are three independent molecules in the unit cell of 1;
only one molecule is shown in Figure 1. Complex 2 crystallizes as CH2Cl2 solvates (2•CH2Cl2).
As commonly observed for square planar palladium complexes, each of the mononuclear
complexes 1 and 2 contains an essentially square-planar palladium center, which is coordi-
nated by a NHC, two chlorides and the nitrogen atom of 4-phenyl-1H-1,2,3-triazole in a trans
fashion. In 1 and 2, the PdCNCl2 coordination planes are oriented approximately perpendic-
ular to the carbene rings with dihedral angles in a range of 67.90–79.86° and are twisted

JOURNAL OF COORDINATION CHEMISTRY
7
from the triazole rings by 2.92–23.83°. Furthermore, the triazole ring planes are tilted by
16.15–23.09° relative to the phenyl ring planes. As usual in NHC-bearing complexes, the
Pd–C bond lengths [1.955(3)–1.962(3) Å] are in the range of a single bond, which are slightly
shorter than that found in (IPr)PdCl2(3-chloropyridine) [1.969(3) Å] [11]. The Pd–N bonds
[2.098(2) − 2.107(3) Å] are also slightly shorter than that of the (IPr)PdCl2(3-chloropyridine),
which indicated the stronger Pd–N bond interactions. Moreover, although the three inde-
pendent molecules in 1 have the same structure, further analysis indicated that there are
also some differences in the bond lengths and angles. For example, the Pd–C and Pd–N bond
lengths are basically identical, however, the dihedral angles between the PdCNCl2 coordi-
nation planes and triazole ring planes are different, one of the dihedral angles (23.83°) are
significantly larger than the other two (2.92 and 3.04°). Accordingly, one of the dihedral
angles between the carbene ring and the triazole ring plane (46.05°) is smaller than the
others (71.78 and 82.58°).
In contrary to the mononuclear complexes 1 and 2, dinuclear complexes 3 and 4 are
structurally similar with the reported 5-phenyl-1H-tetrazole stabilized NHC–Pd complexes.
As shown in Figure 2, the azolate anions and chlorides bridged two adjacent NHC−Pd units
to form the dinuclear structures. The dinuclear complexes contain two essentially square-pla-
nar palladium centers, each of which is coordinated with a NHC, a nitrogen atom from 4-phe-
nyl-1H-1,2,3-triazole and two (one μ2-bridging and one terminal) chlorides. The central
Figure 2. Crystal structures of 3 and 4 with thermal ellipsoids drawn at the 30% probability level.
hydrogens are omitted for clarity.
Scheme 1. triazole ligand stabilized nhC–Pd complexes.

8
W. CHEN AND J. YANG
structural motifs Pd2N2Cl are not coplanar with the bridging chlorides deviating from the
Pd2N2 planes by 0.692 and 0.653 Å, respectively. As expected, the triazole planes are oriented
almost coplanar to the PdCNCl2 coordination planes with the maximum dihedral angle of
17.60°. Furthermore, the PdCNCl2 coordination planes are oriented approximately perpen-
dicularly to the carbene rings with dihedral angles ranging from 72.97–75.13°. Due to the
steric hindrance of the NHCs, the relative disposition of the ligands about two metal centers
is cis and trans, respectively. As usual in NHC-bearing complexes, the Pd–C bonds in 3 and
4 [1.976(3) and 1.965(3) Å for 3; 1.975(4) and 1.962(4) Å for 4] lie within the same range as
those observed for above-mentioned mononuclear complexes. Moreover, due to the oppo-
site arrangement of the palladium centers, the cis effect of the carbon atoms resulted in
shorter Pd–C bonds than the carbon atoms in the trans-arrangement.
3.3. Catalytic properties
Since Herrmann demonstrated that NHC−Pd complexes possess good stability and are suit-
able for use in catalysis, considerable effort has been made to modify the NHC−Pd complexes
that lead to improved catalytic performance [32–38]. Herein, we want to examine the change
of the ancillary ligand in NHC−Pd complexes bringing about the reactive catalysts capable
of difficult reactions. Firstly, the catalytic activities of the obtained complexes in the Suzuki–
Miyaura coupling reaction were investigated. Mononuclear complex 1 was selected as pre-
catalyst to optimize the reaction condition (Table S1 in Supporting Information for details).
Different combinations of bases and solvent were explored in the presence of 1. Among the
solvents screened, EtOH was most effective and gave product in high yield of 95%. The bases
also had an influence on the reaction, K2CO3 was comparable with Cs2CO3 but the use of
K3PO4 dramatically improved the yield. Other bases, such as Na2CO3, KOAc, and KOH, led to
inferior yields. The loading of the catalyst was also examined. The use of 0.25 mol% Pd catalyst
only gave the product in 65% yield. Increasing the amount of catalyst loading up to 0.5 mol%,
high yield was obtained. However, the yields were not improved significantly upon increasing
the amount of catalyst loading up to 1.0 mol%. In view of the above analysis, we obtained
the optimized catalytic conditions for the reaction: EtOH as solvent, K3PO4 as base, 0.5 mol
% of 1, and at room temperature for 2 h. We next examined the versatility of this protocol;
the catalytic results in Suzuki–Miyaura coupling when using obtained catalysts are summa-
rized in Table 2. Although the activities of the obtained complexes are lower than those of
recently reported NHC-based catalysts [17], they establish a valid platform for comparison.
In general, the reactions between aryl bromides and phenylboronic acid were found to
proceed smoothly to obtain the products. A wide array of electronically diverse aryl bromides
in the reaction with phenylboronic acid was examined. These conditions allowed the cou-
pling of ortho, meta, and para-substituted aryl bromides to afford high yields within 2 h.
Moreover, aryl bromides bearing electron withdrawing and donating groups could couple
efficiently with phenylboronic acid, and generated the products in good yields. Also, the
substituted phenylboronic acids were able to undergo the reactions and generated the
desired products.
To be further testing the catalytic properties of the obtained complexes, we decided to
study another model reaction: direct arylation of benzoxazoles with aryl bromides. Since
Ohta first reported the Pd-catalyzed direct arylation of heterocycles with aryl halides, the
transition metal-catalyzed arylation of benzoxazoles has proved to be an effective method

JOURNAL OF COORDINATION CHEMISTRY
9
Table 2. suzuki–miyaura coupling catalyzed by 1–4a.
R1
R1= h
R2
R2= h
Catalyst (mol%)
Yield (%)b
TONc
Entry
1
2
3
4
5
6
7
8
1 (0.5%)
2 (0.5%)
95
95
83
80
95
90
85
82
95
94
86
83
88
90
82
80
95
92
90
90
95
95
88
90
94
92
87
85
190
190
332
320
190
180
340
328
190
188
344
332
176
190
328
320
190
184
360
360
190
190
352
360
188
184
348
340
3 (0.25%)
4 (0.25%)
1 (0.5%)
R1 = h
R2 = 4–oCh3
R2 = 3–oCh3
R2 = 2–oCh3
R2 = 4–f
2 (0.5%)
3 (0.25%)
4 (0.25%)
1 (0.5%)
9
R1 = h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
2 (0.5%)
3 (0.25%)
4 (0.25%)
1 (0.5%)
R1 = h
2 (0.5%)
3 (0.25%)
4 (0.25%)
1 (0.5%)
R1 = h
2 (0.5%)
3 (0.25%)
4 (0.25%)
1 (0.5%)
R1 = 4–oCh3
R1 = 4–oCh3
R2 = 4–oCh3
R2 = 4–tBu
2 (0.5%)
3 (0.25%)
4 (0.25%)
1a (0.5%)
1b (0.5%)
2a (0.25%)
2b (0.25%)
areaction conditions: aryl bromide (0.50 mmol), aryl boronic acid (0.60 mmol), K3Po4 (0.75 mmol), nhC‒Pd complex (0.5%
mol for 1‒2; 0.25% mol for 3‒4) in etoh (2.0 ml) at 25 °C for 2 h.
bisolated yield.
cmol of aryl bromide converted/mol of catalyst.
in organic transformation [39–41]. Despite the interest in this reaction, only a few examples
of NHC‒Pd catalyzed arylation of benzoxazoles using aryl halides have been reported
[42–44]. Herein, we would like to further investigate their catalytic activity toward the reac-
tion. Gratifyingly, we were able to perform the arylation of benzoxazoles with aryl bromides
under the optimized reaction conditions [1.0 mol% (based on the concentration of the
metal), K3PO4, DMF, 130 °C, 12 h, seeTable S2 in Supporting Information for details]. As shown
in Table 3, in general, all the catalysts displayed good outcomes in arylation of aryl bromides.
Substituents on the aryl bromides did not affect the reaction. For instance, the direct coupling
reactions of benzoxazole with bromobenzene, 4-bromoanisole, 4-bromofluorobenzene and
4-bromotoluene proceeded smoothly to generate the corresponding 2-arylated benzoxa-
zoles in good yields. Sterically hindered substituents on the aryl bromides have less effect
on the reaction. Aryl bromides with a methyl group on ortho, meta, para-position of the
phenyl ring gave the corresponding products in good yields. Furthermore, 5-methoxy-ben-
zoxazole has also been arylated in good yields. From the catalytic results, we can see that
the mononuclear and dinuclear complexes displayed similar catalytic activities for the aryl-
ation of benzoxazoles.

10
W. CHEN AND J. YANG
Table 3. arylation of benzoxazoles with aryl bromides catalyzed by 1–4a.
R1
R1= h
R2
R2= h
Catalyst (mol%)
Yield (%)b
TONc
Entry
1
2
3
4
5
6
7
8
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
1 (1.0%)
2 (1.0%)
3 (0.5%)
4 (0.5%)
90
88
90
87
90
92
92
90
90
92
88
86
88
85
89
90
85
87
90
85
85
82
84
80
95
92
96
95
90
88
180
174
90
R1 = h
R2 = 4–oCh3
R2 = 4–f
92
184
180
90
9
R1 = h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
92
176
172
88
R1 = h
R2 = 4–Ch3
R2 = 3–Ch3
R2 = 2–Ch3
R2 = 4–Ch3
85
178
180
85
R1 = h
87
180
170
85
R1 = h
83
168
160
95
92
192
190
R1 = 5–oCh3
areaction conditions: benzoxazole (0.50 mmol), aryl bromide (0.60 mmol), K3Po4 (0.75 mmol), nhC‒Pd complex (1% mol for
1‒2; 0.5% mol for 3‒4) in dmf (2.0 ml) at 130 °C for 12 h.
bisolated yield.
cmol of aryl bromide converted/mol of catalyst.
Following this preliminary screening, the mononuclear complexes achieved similar or
higher catalytic activities for both reactions. There is no advantage in catalytic activity of the
dinuclear complexes compared to their mononuclear analogs. The dinuclear complexes, as
a general trend, when using the same amount of catalyst loading, afford lower catalytic
activities than their mononuclear analogs. The reduction of the catalytic activities shown by
the dinuclear complexes might be due to the strong bonding interaction between the metal
and the azolate anions, which made the complexes to be difficult to eliminate the N-donors
and the following formation of the [(NHC)-Pd(0)] species [14, 15]. Furthermore, during this
catalytic reaction, the same type of mononuclear NHC-Pd(0) species are formed and therefore
show similar catalytic activity. The reactions were really performed with the same type of
catalysts. Cooperative effects of the dinuclear complexes have not been deomonstrated. A
more detailed study is needed to clarify the real role of the triazole ligand in the catalytic
process.

JOURNAL OF COORDINATION CHEMISTRY
11
4. Conclusion
Four mononuclear and dinuclear heteroleptic palladium(II) complexes containing
N-heterocyclic carbenes and 4-phenyl-1H-1,2,3-triazole have been conveniently synthesized
and fully characterized. These complexes have been utilized as catalysts for Suzuki–Miyaura
coupling and arylation of benzoxazoles with aryl bromides in air atmosphere. Although the
catalytic activities of the obtained complexes are lower than those recently reported NHC-
based catalysts, they provide a method to adjust the coordination mode of the NHC–Pd
complexes with functionalized ancillary ligand and establish a platform for comparison of
their catalytic activities. Further modification of the NHC‒Pd complexes and their application
are in progress in our laboratory.
Acknowledgements
Financial supports from the National Natural Science Foundation of China (No. 21301061) and
the National Science Foundation of Anhui Educational Bureau (No. KJ2016A881) are gratefully
acknowledged.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
Financial supports from the National Natural Science Foundation of China [grant number 21301061];
the National Science Foundation of Anhui Educational Bureau [grant number KJ2016A881].
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