Mono- and dinuclear N-heterocyclic carbene palladium complexes with diazine ligands and their catalytic activities toward the Mizoroki–Heck reaction

Article abstract of DOI:10.1080/00958972.2017.1395416

Mono- and dinuclear N-heterocyclic carbene palladium complexes with diazine ligands were synthesized and characterized through adjusting the stoichiometric ratio of the reactants. The catalytic properties of all complexes were further studied in the Mizor

Full text of DOI:10.1080/00958972.2017.1395416

Journal of Coordination Chemistry
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Mono- and dinuclear N-heterocyclic carbene
palladium complexes with diazine ligands and
their catalytic activities toward the Mizoroki–Heck
reaction
Jin Yang
To cite this article: Jin Yang (2017): Mono- and dinuclear N-heterocyclic carbene palladium
complexes with diazine ligands and their catalytic activities toward the Mizoroki–Heck reaction,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1395416
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1395416
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Date: 06 November 2017, At: 23:53

Journal of Coordination Chemistry, 2017
ꢁꢁpꢂ://ꢃꢄꢅ.ꢄꢆg/10.1080/00958972.2017.1395416
ꢀ
Mono- and dinuclear N-heterocyclic carbene palladium
complexes with diazine ligands and their catalytic activities
toward the Mizoroki–Heck reaction
Jin Yang
dꢇpꢈꢆꢁꢉꢇꢊꢁ ꢄꢋ Cꢀꢇꢉꢅꢂꢁꢆꢌ, hꢍꢈꢅbꢇꢅ nꢄꢆꢉꢈꢎ uꢊꢅvꢇꢆꢂꢅꢁꢌ, hꢍꢈꢅbꢇꢅ, P r Cꢀꢅꢊꢈ
ABSTRACT
ARTICLE HISTORY
rꢇcꢇꢅvꢇꢃ 9 fꢇbꢆꢍꢈꢆꢌ 2017
accꢇpꢁꢇꢃ 24 sꢇpꢁꢇꢉbꢇꢆ 2017
Mono- and dinuclear N-heterocyclic carbene palladium complexes
with diazine ligands were synthesized and characterized through
adjusting the stoichiometric ratio of the reactants. The catalytic
properties of all complexes were further studied in the Mizoroki–Heck
reaction. The results indicated that the dinuclear complexes induced
some benefits in catalytic behavior.
KEYWORDS
n-hꢇꢁꢇꢆꢄcꢌcꢎꢅc cꢈꢆbꢇꢊꢇ;
pꢈꢎꢎꢈꢃꢅꢍꢉ cꢄꢉpꢎꢇxꢇꢂ;
ꢃꢅꢈzꢅꢊꢇ ꢎꢅgꢈꢊꢃꢂ; mꢅzꢄꢆꢄkꢅ–
hꢇck ꢆꢇꢈcꢁꢅꢄꢊ
1
. Introduction
Since the discovery of N-heterocyclic carbenes (NHCs) [1], NHCs have emerged as efficient
ligands and their transition metal complexes have been widely applied in organometallic
catalysts [2–8]. In particular, NHC–Pd complexes have been extensively utilized in coupling
reactions [2–4]. In many cases, NHC–Pd complexes are formed in situ, which sometimes gave
different results compared to preformed compounds [9–12]. As a result, a series of well-
defined NHC–Pd complexes were developed and their catalytic activities were fully evaluated
in organic transformation [13–18]. A family of well-defined NHC–Pd complexes (NHC)PdCl2
(
3-chloropyridine) was reported by Organ. In this system, 3-chloropyridine stabilizes the Pd
center, while in the catalytic process the Pd center can lose the pyridine ligand, opening up
a coordination site for substrate [19]. The coordination and dissociation of ancillary ligands
considerably affect the catalytic activity of the complexes.
CONTACT Jꢅꢊ yꢈꢊg
ꢌꢈꢊgjꢅꢊꢎz@163.cꢄꢉ
sꢍppꢎꢇꢉꢇꢊꢁꢈꢎ ꢃꢈꢁꢈ ꢋꢄꢆ ꢁꢀꢅꢂ ꢈꢆꢁꢅcꢎꢇ cꢈꢊ bꢇ ꢈccꢇꢂꢂꢇꢃ ꢈꢁ ꢀꢁꢁpꢂ://ꢃꢄꢅ.ꢄꢆg/10.1080/00958972.2017.1395416.
©
2017 iꢊꢋꢄꢆꢉꢈ uK lꢅꢉꢅꢁꢇꢃ, ꢁꢆꢈꢃꢅꢊg ꢈꢂ tꢈꢌꢎꢄꢆ & fꢆꢈꢊcꢅꢂ Gꢆꢄꢍp

2
J. YANG
Scheme 1. sꢌꢊꢁꢀꢇꢁꢅc ꢆꢄꢍꢁꢇ ꢄꢋ ꢁꢀꢇ ꢉꢄꢊꢄ- ꢈꢊꢃ ꢃꢅꢊꢍcꢎꢇꢈꢆ nhC–Pꢃ cꢄꢉpꢎꢇxꢇꢂ.
Despite excellent progress in mononuclear complexes, the properties of dinuclear NHC-Pd
complexes are less studied. Suitable ligands join two or more NHC–Pd units to form multinu-
clear complexes and provide models to test catalytic activities of the multi-metallic catalysts
[20–28]. Hor and Peris have reported that the catalytic performance of dinuclear NHC-Pd
complexes could be improved by cooperative effect of two metals [21, 24]. This prompted
our study of new dinuclear NHC–Pd complexes with bridging heterocycle linkers. Pyrazine-
bridged dinuclear NHC–Pd complexes based on unsaturated imidazol-2-ylidenes have been
employed as precatalysts for the Hiyama reaction [23]. However, its saturated analogs have
not been described. Saturated imidazolidin-2-ylidenes were superior to their unsaturated
analogs in some catalytic reactions [29–31]. To compare catalytic properties of the NHC–Pd
complexes, herein the synthesis of mono- and dinuclear NHC–Pd complexes based on satu-
rated imidazolidin-2-ylidene and diazines (pyrazine, pyrimidine, and pyridazine) was attempted
and their catalytic potential toward the Mizoroki–Heck reaction was explored (Scheme 1).
2
. Experimental
2
.1. General considerations
All reactions were carried out under air. The chemicals were purchased from commercial
suppliers and used without purification. [Pd(μ-Cl)(Cl)(SIPr)] was prepared according to the
2
literature method [32].
2
.2. Preparation of the mononuclear NHC–Pd complexes (1a–1c)
A mixture of [Pd(μ-Cl)(Cl)(SIPr)] (0.10 mmol, 113.6 mg) and diazine (0.20 mmol, 16.0 mg)
2
was stirred in CH Cl (5.0 mL) at ambient temperature for 6 h to give a yellow solution. The
2
2
solvent was reduced and the resulting residue was washed with diethyl ether to give
mononuclear NHC–Pd complexes.
2
.2.1. [(SIPr)PdCl (pyrazine)] (1a)
2
1
The procedure yielded 110 mg (85%) of 1a. H NMR (CDCl , 400 MHz) δ (ppm) = 8.61 (br, 2H,
3
pyrazine-H), 8.42 (br, 2H, pyrazine-H), 7.42–7.41 (m, 2H, Ar–H), 7.31–7.30 (m, 4H, Ar–H), 4.09
(
s, 4H, NCH CH N), 3.57 (br, 4H, CH(CH ) ), 1.59 (br, 12H, CH(CH ) ), 1.10 (br 12H, CH(CH ) ).
2 2 3 2 3 2 3 2
1
3
C NMR (CDCl , 100 MHz) δ (ppm) = 184.2 (Ccarbene), 147.4, 145.6, 145.2, 135.0, 129.5, 124.4,
3

JOURNAL OF COORDINATION CHEMISTRY
3
5
3.8 (NCH CH N), 28.7 (CH(CH ) ), 26.7 (CH(CH ) ), 24.1 (CH(CH ) ). HR-MS (ESI): calcd for
2 2 3 2 3 2
3 2
−1
−
+
C H ClN Pd [M − Cl ] 611.2133; found 611.2141. IR (KBr, cm ): 3180, 2951, 2916, 1604,
3
1
42
4
1480, 1419, 1408, 1378, 1326, 1279, 1228, 1134, 1062, 928, 860, 740. Anal. Calcd for [(SIPr)
PdCl (pyrazine)] (C H Cl N Pd): C, 57.46; H, 6.53; N, 8.65%. Found: C, 57.72; H, 6.75; N, 8.38%.
2
31 42
2
4
2
.2.2. [(SIPr)PdCl (pyrimidine)] (1b)
2
1
The procedure yielded 115 mg (89%) of 1b. H NMR (CDCl , 400 MHz) δ (ppm) = 9.24 (s, 1H,
3
pyrimidine-H), 8.76 (d, 1H, J = 4.0 Hz, pyrimidine-H), 8.54 (d, 1H, J = 2.4 Hz, pyrimidine-H),
7
.44–7.40 (m, 2H, Ar–H), 7.32–7.26 (m, 4H, Ar–H), 7.13–7.10 (m, 1H, pyrimidine-H), 4.09 (s, 4H,
NCH CH N), 3.58 (sept, J = 6.8 Hz, 4H, CH(CH ) ), 1.56 (d, J = 6.4 Hz, 12H, CH(CH )2), 1.28 (d,
2
2
3 2
3
1
3
J = 6.8 Hz, 12H, CH(CH ) ). C NMR (CDCl , 100 MHz) δ (ppm) = 184.6 (Ccarbene), 159.9, 157.8,
3
2
3
1
57.6, 147.5, 135.1, 129.5, 124.4, 120.8, 53.8 (NCH CH N), 28.7 (CH(CH ) ), 26.8 (CH(CH ) ),
2 2 3 2 3 2
−
+
24.1 (CH(CH ) ). HR-MS (ESI): calcd for C H ClN Pd [M − Cl ] 611.2133; found 611.2144. IR
3
2
31 42
4
−
1
(
KBr, cm ): 3133, 2966, 2866, 1604, 1462, 1418, 1380, 1350, 1288, 1210, 1162, 1128, 1070,
9
46, 802, 763, 757. Anal. Calcd for [(SIPr)PdCl (pyrimidine)] (C H Cl N Pd): C, 57.46; H, 6.53;
2 31 42 2 4
N, 8.65%. Found: C, 57.68; H, 6.80; N, 8.36%.
2
.2.3. [(SIPr)PdCl (pyridazine)] (1c)
2
1
The procedure yielded 102 mg (79%) of 1c. H NMR (CDCl , 400 MHz) δ (ppm) = 8.87 (d,
3
J = 5.2 Hz, 1H, pyridazine-H), 8.77 (d, J = 4.0 Hz, 1H, pyridazine-H), 7.42–7.38 (m, 2H, Ar–H),
7
4
1
1
.32–7.30 (m, 4H, Ar–H), 7.24–7.21 (m, 1H, pyridazine-H), 7.19–7.16 (m, 1H, pyridazine-H),
.06 (s, 4H, NCH CH N), 3.58 (sept, J = 6.8 Hz, 4H, CH(CH ) ), 1.58 (d, J = 6.8 Hz, 12H, CH(CH ) ),
2
2
3 2
3 2
13
.27 (d, J = 6.8 Hz, 12H, CH(CH ) ). C NMR (CDCl , 100 MHz) δ (ppm) = 185.9 (Ccarbene), 153.9,
3
2
3
51.5, 147.5, 135.3, 129.3, 128.6, 127.7, 124.3, 53.8 (NCH CH N), 28.7 (CH(CH ) ), 26.7
2
2
3 2
−
+
(CH(CH ) ), 24.1 (CH(CH ) ). HR-MS (ESI): calcd for C H ClN Pd [M − Cl ] 611.2133; found
3 2 3 2 31 42 4
−1
6
1
8
11.2132. IR (KBr, cm ): 3145, 3106, 3090, 2960, 1606, 1482, 1470, 1380, 1330, 1230, 1188,
054, 849, 740. Anal. Calcd for [(SIPr)PdCl (pyridazine)] (C H Cl N Pd): C, 57.46; H, 6.53; N,
.65%. Found: C, 57.28; H, 6.35; N, 8.83%.
2
31 42
2
4
2
.3. Preparation of dinuclear NHC–Pd compounds (2a and 2b)
Procedure 1: [Pd(μ-Cl)(Cl)(SIPr)] (0.10 mmol, 113.6 mg) and diazine (0.10 mmol, 8.0 mg) were
2
stirred in CH Cl (5.0 mL) at ambient temperature for 6 h to give a yellow solution. The solvent
2
2
was reduced and the resulting residue was washed with diethyl ether to give the dinuclear
NHC-Pd complexes.
Procedure 2: A mixture of 1a or 1b (0.10 mmol, 64.8 mg) and [Pd(μ-Cl)(Cl)(SIPr)]2
(
0.05 mmol, 56.8 mg) was stirred in CH Cl (5.0 mL) at ambient temperature for 3 h. The
2 2
solvent was reduced and the resulting residue was washed with ether to give dinuclear
NHC–Pd complexes 2a and 2b.
2
.3.1. [(SIPr)PdCl ] (μ-pyrazine) (2a)
2 2
1
The procedure yielded 98 mg (81%) of 2a. H NMR (CDCl , 400 MHz) δ (ppm) = 8.52 (br, 4H,
3
pyrazine-H), 7.29 (br, 4H, Ar–H), 7.27 (br, 8H, Ar–H), 4.07 (s, 8H, NCH CH N), 3.50 (sept,
2
2
J = 6.8 Hz, 8H, CH(CH ) ), 1.48 (d, J = 6.8 Hz, 12H, CH(CH ) ), 1.25 (d, J = 6.8 Hz, 12H, CH(CH ) ).
3
2
3 2
3 2
13
C NMR (CDCl , 100 MHz) δ (ppm) = 183.9 (Ccarbene), 147.4, 145.9, 134.9, 129.5, 124.3, 53.8
3
(
NCH CH N), 28.6 (CH(CH ) ), 26.7 (CH(CH ) ), 24.1 (CH(CH ) ). HR-MS (ESI): calcd for
2 2 3 2 3 2 3 2

4
J. YANG
+
+
−1
C H Cl N Pd [M + H ] 1217.3321; found 1217.3324. IR (KBr, cm ): 3128, 3070, 2970, 1616,
58
81
4
6
2
1590, 1466, 1445, 1380, 1366, 1348, 1269, 1214, 1126, 1070, 1060, 946, 801, 757, 746. Anal.
Calcd for [(SIPr)PdCl ] (μ-pyrazine) (C H Cl N Pd ): C, 57.29; H, 6.63; N, 6.91%. Found: C,
2
2
58 80
4
6
2
57.44; H, 6.87; N, 6.70%.
2
.3.2. [(SIPr)PdCl ] (μ-pyrimidine) (2b)
2
2
1
The procedure yielded 102 mg (84%) of 2b. H NMR (CDCl , 400 MHz) δ (ppm) = 9.20 (s, 1H,
3
pyrimidine-H), 8.72 (d, 2H, J = 5.6 Hz, pyrimidine-H), 7.41–7.37 (m, 4H, Ar–H), 7.28–7.26 (m, 8H,
Ar–H), 6.90 (t, J = 5.6 Hz, 1H, pyrimidine-H), 4.06 (s, 8H, NCH CH N), 3.53 (sept, J = 6.8 Hz, 4H,
2
2
13
CH(CH ) ), 1.51 (d, J = 6.8 Hz, 12H, CH(CH ) ), 1.25 (d, J = 6.8 Hz, 12H, CH(CH ) ). C NMR (CDCl ,
3
2
3 2
3 2
3
100 MHz) δ (ppm) = 184.6 (Ccarbene), 159.9, 157.8, 157.6, 147.5, 135.1, 129.5, 124.4, 120.8, 53.8
(
NCH CH N), 28.7 (CH(CH ) ), 26.8 (CH(CH ) ), 24.1 (CH(CH ) ). HR-MS (ESI): calcd for C H Cl N Pd
2
2
+
3 2
3 2
3 2
58 81
4
6
2
+
−1
[M + H ] 1217.3321; found 1217.3341. IR (KBr, cm ): 3130, 3089, 2969, 1608, 1600, 1580, 1460,
1
440, 1378, 1365, 1344, 1280, 1210, 1136, 1080, 1059, 940, 810, 740. Anal. Calcd for [(SIPr)PdCl ] (μ-
2 2
pyrimidine) (C H Cl N Pd ): C, 57.29; H, 6.63; N, 6.91%. Found: C, 57.11; H, 6.41; N, 6.69%.
58
80
4
6
2
2
.4. General procedure for the Mizoroki–Heck reaction
In a reaction vessel, aryl bromide (0.50 mmol), styrene (0.60 mmol, 62.4 mg), Cs CO
2
3
(
0.75 mmol, 245 mg), and NHC–Pd complex (0.25 or 0.5 mol%, about 3.0 mg) were mixed in
DMF (2.0 mL). The mixture was stirred at 140 °C for 3 h. After cooling to room temperature,
the filtrate was concentrated and the residue was subjected to purification via flash column
chromatography to give the pure product.
2
.5. Crystallography
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 [33]. Structure solutions and refinements
were performed with the SHELX-97 package [34]. All non-hydrogen atoms were refined ani-
2
sotropically by full-matrix least-squares on F . Hydrogens on carbon were included in idealized
geometric positions with thermal parameters equivalent to 1.2 times those of carbon.
3
. Results and discussion
3
.1. Synthesis and characterization of the complexes
To assess the coordination of diazines (pyrazine, pyrimidine, and pyridazine) toward NHC–Pd
complexes, reactions of [Pd(μ-Cl)(Cl)(SIPr)] with diazines in various stoichiometric ratios
2
were done. When [Pd(μ-Cl)(Cl)(SIPr)] reacted with diazines (pyrazine, pyrimidine, and pyri-
2
dazine) in the stoichiometric ratio of 1:2, mononuclear complexes 1a–1c were achieved,
while reactions of [Pd(μ-Cl)(Cl)(SIPr)] with diazines (pyrazine and pyrimidine) in the stoichi-
2
ometric ratio of 1:1 afforded dinuclear complexes 2a and 2b. The dinuclear complexes 2a
and 2b could be also obtained through reaction of 1a and 1b with [Pd(μ-Cl)(Cl)(SIPr)] . The
2
only exception is the dinuclear complexes with pyridazine. Due to the steric congestion, the
bidentate pyridazine could not bridge two SIPr–Pd units to form the dinuclear compound.

JOURNAL OF COORDINATION CHEMISTRY
5
The formation of the mono- and dinuclear complexes is evident from the distinctive stoi-
1
chiometric proton signal resonances of SIPr with diazines in H NMR spectra. The single
resonances at 4.06–4.09 ppm for methylene protons of NCH CH N are in agreement with
2
2
1
3
the related SIPr-Pd complexes. The number of signals observed in C NMR spectra of the
complexes was as expected and the carbene carbons were observed at 183.9–185.9 ppm,
similar with SIPr-Pd complexes. FT-IR spectra of the NHC-Pd complexes were in agreement
1
with the results of the H NMR analysis. IR spectra of the free diazine ligands show medium
−1
absorptions for C=N stretches at 1618–1605 cm , while the complexes show strong C=N
−1
stretches at 1580 and 1598 cm . The red shift and reduced intensity indicate that the diazine
ligands are coordinated to Pd(II) in the latter cases [35–38]. Formation is confirmed by their
−
+
+
[
M − Cl ] or [M + H] fragments observed in high-resolution mass spectra.
3
.2. Structural analyses
In agreement with the spectral data, the crystal structures of 1a–1c show mononuclear com-
plexes with each palladium coordinated by a SIPr, a nitrogen donor from diazines and two
chlorides in an essentially square planar geometry (Figure 1). As expected, the SIPr and the
Figure 1. mꢄꢎꢇcꢍꢎꢈꢆ ꢂꢁꢆꢍcꢁꢍꢆꢇꢂ ꢄꢋ 1a–1c ꢈꢊꢃ 2a–2b. sꢄꢉꢇ ꢀꢌꢃꢆꢄgꢇꢊꢂ ꢈꢊꢃ ꢂꢄꢎvꢇꢊꢁ ꢉꢄꢎꢇcꢍꢎꢇꢂ (Ch Cꢎ ꢈꢊꢃ
2
2
n-C h ) ꢀꢈvꢇ bꢇꢇꢊ ꢄꢉꢅꢁꢁꢇꢃ ꢋꢄꢆ cꢎꢈꢆꢅꢁꢌ.
6
14
a
B
nꢄꢁꢇꢂ: sꢌꢉꢉꢇꢁꢆꢌ cꢄꢃꢇꢂ: −x, 2 − y, −z; 1 − x, −y, z.

6
J. YANG
diazines are trans to each other. The dihedral angles between the carbene ring planes and
the PdCNCl planes are 60.91 to 74.71°, which are typical for complexes to reduce steric con-
2
gestion. Although 1a, 1b, and 1c are isostructural, there are some differences. For example,
the dihedral angles between the PdCNCl planes and the diazines in 1a and 1b (13.06–45.90°)
2
are shorter than that found in 1c (62.72°); the diazines which are tilted by 16.54–88.30° relative
to the carbene ring planes in 1a and 1b are larger than that in 1c (6.08°). The Pd–C bonds in
1
a–1c from 1.956(3) to 1.978(3) Å are somewhat shorter than that found in (SIPr)PdCl (3-
2
chloropyridine) [1.990(3) Å]. The Pd–N bonds in 1a and 1b [2.103(3)–2.152(3) Å] are compa-
rable to (SIPr)PdCl (3-chloropyridine) [2.108(3) Å], but are slightly longer than that found in
2
1c [2.043(3) Å]. For 2a and 2b, the diazines link two SIPr–Pd units to form dinuclear frameworks
with Pd⋯Pd separations from 6.70 to 11.13 Å. The Pd–C [1.964(3) and 1.969(4) Å] and Pd–N
bond distances [2.127(2) and 2.159(3) Å] are slightly longer than their unsaturated analog,
[
(IPr)PdCl ] (μ-pyrazine) [23]. The carbene ring planes are twisted out of the PdCNCl planes
2
2
2
with dihedral angles of 72.61 to 74.63° in order to avoid intra-ligand repulsion. The diazines
are tilted by 25.07–35.25° relative to the PdCNCl planes, while carbene ring planes are approx-
2
imately perpendicular to the diazines with dihedral angles of 73.97 and 81.39°.
3
.3. Catalytic reactions
The palladium-catalyzed Mizoroki–Heck reaction is a powerful method for preparation of
biaryl derivatives [39]. Much effort has been done to develop efficient NHC–Pd systems for
the reaction [40–44]. With the synthesis of this series of mono- and dinuclear NHC–Pd com-
plexes, we were able to test and compare their catalytic activities. Since the bases and sol-
vents as well as Pd-precatalysts have been employed, herein, the model reaction between
4-bromoanisole and styrene was directly performed in common polar solvents (Table 1).
Table 1. opꢁꢅꢉꢅzꢈꢁꢅꢄꢊ ꢄꢋ ꢁꢀꢇ ꢆꢇꢈcꢁꢅꢄꢊ cꢄꢊꢃꢅꢁꢅꢄꢊꢂ ꢋꢄꢆ ꢁꢀꢇ mꢅzꢄꢆꢄkꢅ–hꢇck ꢆꢇꢈcꢁꢅꢄꢊꢈ.
Yield (%)b
Entry
[Pd]
Solvent
Base
1
2
3
4
5
6
7
8
9
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (1%)
1a (0.5%)
1a (0.25%)
1a (0.5%)
1a (0.5%)
dmf
K2Co3
K2Co3
K2Co3
K2Co3
K2Co3
93
92
80
50
45
96
90
70
74
72
85
95
70
72
90
dma
dmso
1,4-ꢃꢅxꢄꢊꢇ
thf
dmf
dmf
dmf
dmf
dmf
dmf
dmf
dmf
dmf
dmf
Cꢂ Co
2
3
nꢈ Co
2
3
Koh
nꢈoh
t
10
11
12
13
14
15
Ko Bꢍ
Koac
Cꢂ Co
2
3
Cꢂ Co
2
3
c
Cꢂ Co
2
3
3
ꢃ
Cꢂ Co
2
ꢈrꢇꢈcꢁꢅꢄꢊ cꢄꢊꢃꢅꢁꢅꢄꢊꢂ: 4-bꢆꢄꢉꢄꢈꢊꢅꢂꢄꢎꢇ (0.50 ꢉꢉꢄꢎ), ꢂꢁꢌꢆꢇꢊꢇ (0.60 ꢉꢉꢄꢎ), nhC–Pꢃ (0.25–1.0% ꢉꢄꢎ), bꢈꢂꢇ (0.75 ꢉꢉꢄꢎ),
ꢂꢄꢎvꢇꢊꢁ (2.0 ꢉl) ꢈꢁ 140 °C ꢋꢄꢆ 3 ꢀ.
b
iꢂꢄꢎꢈꢁꢇꢃ ꢌꢅꢇꢎꢃ.
c
tꢀꢇ ꢆꢇꢈcꢁꢅꢄꢊ wꢈꢂ pꢇꢆꢋꢄꢆꢉꢇꢃ ꢈꢁ 120 °C ꢋꢄꢆ 3 ꢀ.
ꢃ
tꢀꢇ ꢆꢇꢈcꢁꢅꢄꢊ wꢈꢂ pꢇꢆꢋꢄꢆꢉꢇꢃ ꢈꢁ 120 °C ꢋꢄꢆ 8 ꢀ.

JOURNAL OF COORDINATION CHEMISTRY
7
Initially, the effect of solvent on the reaction was examined, and a significant solvent effect
was observed. High yields were obtained when the reactions were performed in DMF and
DMA. Among the bases examined, all reactions took place smoothly to give product in
moderate to good yields within 3 h; the best result can be achieved with Cs CO . To further
2
3
examine the catalytic efficiency of 1a, a variation of the catalyst loading from 1.0–0.5 to
.25 mol% within 3 h was performed to give the product in 93, 91, and 80% yields, respec-
0
tively. The reaction with low catalyst loading results in incomplete conversion. On the basis
of the above preliminary optimized results, the next study focused on reaction temperature.
Among the temperatures examined, 140 °C was the optimum reaction temperature.
Reducing the temperature to 120 °C resulted in decreased product yield. However, the reac-
tion can be also finished at 120 °C through increasing the reaction time to 8 h.
With the optimized reaction condition, catalytic results between a variety of aryl bromides
and styrene are summarized in Scheme 2. In order to maintain consistency of the catalyst
loading, 0.5 mol % for 1a–1c and 0.25 mol % for 2a and 2b were used in the catalytic reactions.
All of the catalysts displayed moderate to good outcomes in coupling reactions. Different
substituents on aryl bromide were tolerated, and good yields of the products were obtained.
The reactions of aryl bromides with electron-withdrawing groups such as CN and CF at the
3
para-position are more efficient. The ortho-substituents of aryl bromides, 2-bromotoluene
proceeds well and generated the corresponding products in good yields. On the whole, the
activities of the dinuclear NHC–Pd complexes as catalysts in the Mizoroki–Heck reactions are
similar to those of related mononuclear complexes. This suggests that the catalytic activity
of the NHC–Pd complexes is mainly dependent on the NHC ligand. The dinuclear complexes
as catalysts do not display outstanding superiority. The reason may be that the distances
between two palladium centers are too large for cooperative interactions.
To further compare the catalytic efficiency between mono- and dinuclear complexes,
kinetic experiments for 1a and 2a were performed by GC analysis. As shown in Figure 2, the
reactions turned over immediately and were complete within 3 h. Complexes 1a and 2a
Scheme 2. mꢅzꢄꢆꢄkꢅ–hꢇck ꢆꢇꢈcꢁꢅꢄꢊꢂ cꢈꢁꢈꢎꢌzꢇꢃ bꢌ 1a–1c ꢈꢊꢃ 2a–2b[ꢈ,b].
nꢄꢁꢇꢂ: [ rꢇꢈcꢁꢅꢄꢊ cꢄꢊꢃꢅꢁꢅꢄꢊꢂ: 4-bꢆꢄꢉꢄꢈꢊꢅꢂꢄꢎꢇ (0.50 ꢉꢉꢄꢎ), ꢂꢁꢌꢆꢇꢊꢇ (0.60 ꢉꢉꢄꢎ), nhC–Pꢃ (0.25–0.5 % ꢉꢄꢎ), Cꢂ Co (0.75 ꢉꢉꢄꢎ),
ꢈ]
2
3
[b]
dmf (2.0 ꢉl) ꢈꢁ 140 °C ꢋꢄꢆ 3 ꢀ. iꢂꢄꢎꢈꢁꢇꢃ ꢌꢅꢇꢎꢃ.

8
J. YANG
Figure 2. rꢇꢈcꢁꢅꢄꢊ ꢄꢋ 4-bꢆꢄꢉꢄꢈꢊꢅꢂꢄꢎꢇ ꢈꢊꢃ ꢂꢁꢌꢆꢇꢊꢇ pꢆꢄꢉꢄꢁꢇꢃ bꢌ 1a ꢈꢊꢃ 2a.
demonstrated similar catalytic activities. However, the induction period for these complexes
was slightly different, a short time was present for 2a relative to 1a, which may be related
to reduction to the active Pd species. The observance of short induction period for 2a pos-
sibly indicates cooperative effects to some extent. Although the complexes showed good
catalytic activities, there is no obvious advantage in catalyst amount and reaction activity
compared to recently reported NHC–Pd complexes. As ancillary ligands, the diazine ligands
indeed adjust the σ-donation property of the NHC to some extent. However, the real role of
the nitrogen donors in the catalytic process needs to be further investigated.
4
. Conclusion
Five mono- and dinuclear heteroleptic palladium complexes containing NHC and diazines
were synthesized and characterized. The catalytic performance of the complexes in the
Mizoroki–Heck reaction exhibited good catalytic activities. Although mono- and dinuclear
complexes display similar catalytic activities, dinuclear complexes introduce some benefits
into the catalytic reaction with shorter induction periods than their mononuclear counter-
parts, which might be attributed to cooperative effects.
Supplementary material
CCDC 1520583–1520587 contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.
ac.uk/data_request/cif.
Disclosure statement
No potential conflict of interest was reported by the author.

JOURNAL OF COORDINATION CHEMISTRY
9
Funding
Financial supports from the National Natural Science Foundation of China [grant number 21301061].
References
[
[
[
[
[
[
[
[
1] A.J. Arduengo, R.L. Harlow, M. Kline. J. Am. Chem. Soc., 113, 36 (1991).
2] S. Würtz, F. Glorius. Acc. Chem. Res., 41, 1523 (2008).
3] S. Di ́e z-Gonza ́l ez, N. Marion, S.P. Nolan. Chem. Rev., 109, 3612 (2009).
4] G.C. Fortman, S.P. Nolan. Chem. Soc. Rev., 40, 5151 (2011).
5] M. Poyatos, J.A. Mata, E. Peris. Chem. Rev., 109, 3677 (2009).
6] G.C. Vougioukalakis, R.H. Grubbs. Chem. Rev., 110, 1746 (2010).
7] S.P. Nolan. Acc. Chem. Res., 44, 91 (2011).
8] K. Riener, S. Haslinger, A. Raba, M.P. Högerl, M. Cokoja, W.A. Herrmann, F.E. Kühn. Chem. Rev., 114,
5215 (2014).
[9] H. Lebel, M.K. Janes, A.B. Charette, S.P. Nolan. J. Am. Chem. Soc., 126, 5046 (2004).
[
[
[
[
10] Y. Kong, L. Wen, H. Song, S. Xu, M. Yang, B. Liu, B. Wang. Organometallics, 30, 153 (2011).
11] T. Guo, S. Dechert, F. Meyer. Organometallics, 33, 5145 (2014).
12] J.-Y. Lee, J.-Y. Lee, Y.-Y. Chang, C.-H. Hu, N.M. Wang, H.M. Lee. Organometallics, 34, 4359 (2015).
13] C.J. O’Brien, E.A.B. Kantchev, N. Hadei, C. Valente, G.A. Chass, J.C. Nasielski, A. Lough, A.C. Hopkinson,
M.G. Organ. Chem. Eur. J., 12, 4743 (2006).
[
[
[
[
[
[
14] S.B. Moosun, M.G. Bhowon, E.C. Hosten, S. Jhaumeer-Laulloo. J. Coord. Chem., 69, 2736 (2016).
15] P. Anitha, R. Manikandan, P. Viswanathamurthi. J. Coord. Chem., 68, 3537 (2015).
16] S. Hussain, I.H. Bukhari, S. Ali, S. Shahzadi, M. Shahid, K.S. Munawar. J. Coord. Chem., 68, 662 (2015).
17] M.B. Ibrahim, S.M. Shakil Hussain, A. Fazal, M. Fettouhi, B. El Ali. J. Coord. Chem., 68, 432 (2015).
18] J. Yang. J. Coord. Chem., 70, 441 (2017).
19] J. Nasielski, N. Hadei, G. Achonduh, E.A.B. Kantchev, C.J. O’Brien, A. Lough, M.G. Organ. Chem. Eur.
J., 16, 10844 (2010).
[
[
[
[
[
[
[
[
[
[
[
[
[
[
20] Y. Han, H.V. Huynh, G.K. Tan. Organometallics, 26, 6447 (2007).
21] S.K. Yen, L.L. Koh, H.V. Huynh, T.S.A. Hor. Chem. Asian J., 3, 1649 (2008).
22] D. Yuan, H.V. Huynh. Organometallics, 29, 6020 (2010).
23] J. Yang, L. Wang. Dalton Trans., 41, 12031 (2012).
24] G. Guisado-Barrios, J. Hiller, E. Peris. Chem. Eur. J., 19, 10405 (2013).
25] Q.-X. Liu, W. Zhang, X.-J. Zhao, Z.-X. Zhao, M.-C. Shi, X.-G. Wang. Eur. J. Org. Chem., 2013, 1253 (2013).
26] J. Yang, P. Li, Y. Zhang, L. Wang. Dalton Trans., 43, 7166 (2014).
27] J. Yang, P. Li, Y. Zhang, L. Wang. J. Organomet. Chem., 766, 73 (2014).
28] T. Guo, S. Dechert, F. Meyer. Organometallics, 33, 5145 (2014).
29] N. Marion, O. Navarro, J. Mei, E.D. Stevens, N.M. Scott, S.P. Nolan. J. Am. Chem. Soc., 128, 4101 (2006).
30] C.E. Hartmann, S.P. Nolan, C.S.J. Cazin. Organometallics, 28, 2915 (2009).
31] C.W. Gallop, C. Zinser, D. Guest, O. Navarro. Synlett, 25, 2225 (2014).
32] M.R.L. Furst, C.S.J. Cazin. Chem. Commun., 46, 6924 (2010).
33] G.M. Sheldrick. SHELXL-97, Program for X-ray Crystal Structure Refinement, University of Göttingen,
Germany (1997).
[
[
[
[
[
[
34] Bruker Advanced X-ray Solutions. SMART for WNT/2000 (Version 5.34), Bruker AXS Inc., Madison,
Wisconsin, USA (1997).
35] S. Shujah, Zia-ur-Rehman, N. Muhammad, A. Shah, S. Ali, N. Khalid, A. Meetsma. J. Organomet.
Chem., 741–742, 59 (2013).
36] M. Iqbal, S. Ali, Zia-Ur Rehman, N. Muhammad, M. Sohail, V. Pandarinathan. J. Coord. Chem., 67,
1731 (2014).
37] S. Khan, M.K. Amir, M.M. Naseer, R. Abbasi, K. Mazhar, M.N. Tahir, I.Z. Awan, Zia-Ur Rehman. J.
Coord. Chem., 68, 2539 (2015).
38] M.K. Amir, Zia-ur-Rehman, F. Hayat, S.Z. Khan, G. Hogarth, T. Kondratyuk, J.M. Pezzuto, M.N. Tahir.
RSC Adv., 6, 110517 (2016).
39] I.P. Beletskaya, A.V. Cheprakov. Chem. Rev., 100, 3009 (2000).

10
J. YANG
[
[
[
[
[
40] W.A. Herrmann, M. Elison, J. Fischer, C. Köcher, G.R.J. Artus. Angew. Chem. Int. Ed., 34, 2371 (1995).
41] M.B. Andrus, C. Song, J. Zhang. Org. Lett., 4, 2079 (2002).
42] E.A.B. Kantchev, G.-R. Peh, C. Zhang, J.Y. Ying. Org. Lett., 10, 3949 (2008).
43] S. Guo, H.V. Huynh. Organometallics, 31, 4565 (2012).
44] T.-T. Gao, A.-P. Jin, L.-X. Shao. Beilstein J. Org. Chem., 8, 1916 (2012).