How the substitution faraway from NHCs affects the structural features and catalytic activity of dicarbene dipalladium complexes

Article abstract of DOI:10.1002/zaac.201300344

A series of [PdPyCl₂]₂(di-NHC) complexes were prepared (di-NHC are two 1-(2,6-dimethylphenyl)imidazolidene molecules bridged by an aliphatic -(CH₂)_n- linker (n = 3, 4, 5, 6, and 10)). All complexes were fully characterized by NMR spectroscopy and elemental analyses. The crystal structures of four complexes (n = 3, 4, 5 and 6) were determined by X-ray diffraction. The influence of the distant methyl group on the structural features and catalytic activity with increasing of length of linker was investigated by comparing the results of these 2,6-dimethylphenyl palladium complexes with those of their known mesityl analogues. X-ray studies show the distant methyl substitution has big impact on the structure feature of the complexes with the shorter linker between two NHC (ethylene and propylene), but has a little or no effect on that of the complexes with longer linker (butylene and hexylene). Catalytic results of the arylation of styrene show that the remote substitute has big effect on the regioselectivity of the product in all complexes with shorter and longer linkers, but has a limited effect on the yield. Copyright

Full text of DOI:10.1002/zaac.201300344

ARTICLE
DOI: 10.1002/zaac.201300344
How the Substitution Faraway from NHCs Affects the Structural Features
and Catalytic Activity of Dicarbene Dipalladium Complexes
Kaiqi Ge,[a] Changsheng Cao,*[a] Gang Liu,[a] Yuling Li,[a] Juan Zhang,[a]
Guangsheng Pang,[b] and Yanhui Shi*[a]
Keywords: Dinuclear; Palladium; N-heterocyclic carbene; Crystal structure; Heck reaction
Abstract. A series of [PdPyCl2]2(di-NHC) complexes were prepared their known mesityl analogues. X-ray studies show the distant methyl
(di-NHC are two 1-(2,6-dimethylphenyl)imidazolidene molecules substitution has big impact on the structure feature of the complexes
bridged by an aliphatic –(CH2)n– linker (n = 3, 4, 5, 6, and 10)). All with the shorter linker between two NHC (ethylene and propylene),
complexes were fully characterized by NMR spectroscopy and ele-
but has a little or no effect on that of the complexes with longer linker
mental analyses. The crystal structures of four complexes (n = 3, 4, 5 (butylene and hexylene). Catalytic results of the arylation of styrene
and 6) were determined by X-ray diffraction. The influence of the show that the remote substitute has big effect on the regioselectivity
distant methyl group on the structural features and catalytic activity
with increasing of length of linker was investigated by comparing the
results of these 2,6-dimethylphenyl palladium complexes with those of
of the product in all complexes with shorter and longer linkers, but
has a limited effect on the yield.
Introduction
Over the past few decades, the transition metal complexes
of N-heterocyclic carbenes (NHCs) have attracted considerable
attention due to their structural variety, easy access, and sta-
bility toward heat, moisture and air.[1] In the family of N-het-
erocyclic carbenes, a large number of bidentate bis-NHC li-
gands and their many complexes have been studied. In these
ligands, a pair of NHC moieties are connected by either an
aliphatic linker, such as alkyl[2] and ether chain,[3] or aromatic
linker, such as phenyl[4] and pyridyl chain.[5] Three types of
metal complexes are mainly formed (Scheme 1), which are re-
lated to the flexibility and size of the linkers, steric hindrance
of N substitutes of the NHC, and types of NHC rings. Among
the bis-NHC ligands, di-NHCs with an aliphatic –(CH2)n–
linker are the first ones to be prepared.[6] The easy preparation
of this type of ligand has allowed a controlled study of their
coordination to metals by modifying the length of the linker.
We have prepared a series of di-NHC dipalladium com-
plexes of the type III with different –(CH2)n– bridges and dif-
ferent N-substitutes on NHC; and investigated the influence of
the bridging linker and N-substitutes on the structure and cata-
Scheme 1. The conformations of NHC metal complexes.
lytic reactivity of the Pd complexes.[7] In previous study, we
also found that 4-substitution on phenyl, which is quite far
away from NHC ring has some effect on the structure and
catalytic reactivity of complexes with ethylene bridged linker
(Scheme 2). For example, 2,6-dimethylphenyl complex 1a
adopts a cis configuration with a torsion angle of –88.28° in-
volving backbone atoms of N1–C1–C4–N2, however, the mes-
ityl complex 1b adopts a trans configuration with 179.59° of
torsion angle (Figure 1).[7a,7c] Furthermore, in the catalytic
arylation of styrene with bromobenzene under the same reac-
tion conditions, 1a gave 93% yield with 11:1 of E/Z ratio of
the product, whilst 1b gave 96% yield with an E/Z ratio of
22:1. To further understand how the faraway substitution from
NHC affects the structure and catalytic activity of palladium
complexes with increasing of the length of bridge, we synthe-
sized a new series of 2,6-dimethylphenyl palladium complexes
with –(CH2)n– bridges (n = 3–6 and 10; 2a–6a). In the paper,
we investigated their solid-state structures and catalytic activi-
ties in arylation reactions, and compared the results with those
of their known mesityl analogues (2b–6b).
* Prof. Dr. C. Cao
E-Mail: chshcao@jsnu.edu.cn
* Prof. Dr. Y. Shi
E-Mail: yhshi@jsnu.edu.cn
[a] College of Chemistry and Chemical Engineering and Jiangsu Key
Laboratory of Green Synthetic Chemistry for Functional Materials
Jiangsu Normal University
Xuzhou, Jiangsu, 221116, P. R. China
[b] State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry
Jilin University
Changchun, Jilin, 130012, P. R. China
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Z. Anorg. Allg. Chem. 2014, 640, (2), 444–450

Structural Features and Catalytic Activity of Dicarbene Dipalladium Complexes
Figure 1. Crystal structures of complexes 1a and 1b.
shifted downfield relative to that of the imidazolium NCHN
peak of the starting ligand precursors (ca. 137 ppm).[8]
Solid State Structures of the Palladium Complexes
The molecular structures of 2a–5a were determined by
means of X-ray diffraction studies. The molecular diagrams of
2a–5a are shown in Figure 2, Figure 3, Figure 4, and Figure 5.
Scheme 2. Di-NHC dipalladium complexes.
Results and Discussion
Synthesis and Characterization of the di-NHC Dipalladium
Complexes (2a–6a)
The synthesis of di-NHC dipalladium complexes 2a–6a was
achieved by refluxing of the corresponding imidazolium chlor-
ide with palladium chloride in the presence of K2CO3 in pyr-
idine in good yield (Scheme 3). The palladium complexes were
fully characterized by NMR spectroscopy and gave satisfac-
tory elemental analyses. The complexes are air- and moisture
stable and can be stored in an air atmosphere in solid state for
more than 6 months without any noticeable decomposition.
The proton signal of NCHC of imidazolium chlorides (ca.
Figure 2. ORTEP structure of complex 2a (n = 3) with the probability
ellipsoids drawn at the 30% level. Hydrogen atoms are omitted for
clarity.
1
10 ppm) was absent in the H NMR of palladium complexes,
confirming carbene generation. In addition, 13C NMR provides
direct evidence of the metalation of the ligand, as seen by the
signal at ca. 150 ppm, which is assigned to the Pd–C resonance
Figure 3. ORTEP structure of complex 3a (n = 4) with the probability
ellipsoids drawn at the 30% level. Hydrogen atoms are omitted for
clarity.
Scheme 3. Synthesis of complexes 2a–6a.
Z. Anorg. Allg. Chem. 2014, 444–450
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445

C. Cao, Y. Shi et al.
ARTICLE
Figure 5. ORTEP structure of complex 5a (n = 6) with the probability
ellipsoids drawn at the 30% level. Hydrogen atoms and solvent are
omitted for clarity.
Figure 4. ORTEP structure of complex 4a (n = 5) with the probability
ellipsoids drawn at the 30% level. Hydrogen atoms and solvent are
omitted for clarity.
Selected bond lengths and angles are given in Table 1. All plexes are similar and comparable to those shown in their mes-
complexes show slightly distorted square-planar arrangements ityl analogues. The distance of Pd(1)–Pd(2) in a molecule was
around two central palladium atoms, which are surrounded by supposed to increase with the increasing the length of linker
imidazolylidene, two chloro ligands in a trans configuration, between two NHCs. However, the distance in complex 2a with
and a pyridine. Interestingly, the structure of complexes 2a and propylene linker is 6.569 Å, whilst it is 4.816 Å in 3a with
3a with shorter bridge (n = 3, 4) consists of two pseudo- butylene linker. The shortness of the Pd–Pd distance in 3a is
square-planar subunits in a cis configuration, however, the due to a face-to-face π–π stacking between two pyridines,
structure of complexes 4a and 5a with longer bridge (n = 5, which pulls the two central palladium atoms much closer.
6) consists of two subunits in a trans configuration. All bond
The structure feature of 2a with a propylene bridge is very
lengths and angles lie in the expected ranges (Table 1). The different from its methyl-substituted analogue 2b. For 2a, the
bond lengths of Pd–Ccarbene and Pd–Npyridine in these com- dihedral angle of two imidazole and pyridine rings is 61.05°
Table 1. Selected bond lengths /Å and angles /° for complexes 2a–5a.
Distance
2a
3a
Angle
2a
3a
Pd(1)–C(1)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
Pd(1)–N(1)
C(1)–N(4)
C(1)–N(3)
Pd(2)–C(4)
Pd(2)–Cl(3)
Pd(2)–Cl(4)
Pd(2)–N(2)
C(4)–N(6)
C(4)–N(12)
Pd(1)–Pd(2)
1.968(5)
2.3048(17)
2.3075(16)
2.136(5)
1.356(7)
1.340(7)
1.963(5)
2.2893(17)
2.3131(17)
2.093(4)
1.345(7)
1.358(7)
6.569
1.969(3)
2.3084(10)
2.2949(10)
2.097(3)
1.348(4)
1.347(4)
Cl(1)–Pd(1)–Cl(2)
C(1)–Pd(1)–N(1)
C(4)–Pd(2)–N(2)
Cl(3)–Pd(2)–Cl(4)
N(4)–C(1)–N(3)
N(6)–C(4)–N(12)
N(4)–C(1)–Pd(1)
N(3)–C(1)–Pd(1)
N(6)–C(4)–Pd(2)
N(12)–C(4)–Pd(2)
Imidazole dihedral angle
174.95(6)
177.4(2)
174.8(2)
179.00(6)
105.7(5)
105.0(5)
128.8(4)
125.3(4)
124.8(4)
129.9(4)
61.05
176.27(4)
178.77(13)
105.6(3)
126.7(2)
127.6(3)
a)
b)
49.63
9.62
29.49
Pyridine dihedral angle
35.89
–22.62
c)
4.816
Torsion angle
Distance
4a
5a
Angle
4a
5a
Pd(1)–C(21)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
Pd(1)–N(1)
C(21)–N(3)
C(21)–N(4)
Pd(2)–C(29)
Pd(2)–Cl(3)
Pd(2)–Cl(4)
Pd(2)–N(2)
C(29)–N(5)
C(29)–N(6)
Pd(1)–Pd(2)
1.974(6)
2.3034(16)
2.2915(15)
2.102(5)
1.338(7)
1.353(7)
1.960(5)
2.2927(18)
2.3013(17)
2.111(5)
1.333(7)
1.362(7)
8.233
1.963(3)
2.2990(10)
2.2919(10)
2.102(3)
1.347(4)
1.350(4)
Cl(1)–Pd(1)–Cl(2)
C(21)–Pd(1)–N(1)
N(4)–C(21)–N(3)
Cl(3)–Pd(2)–Cl(4)
C(29)–Pd(2)–N(2)
N(5)–C(29)–N(6)
N(3)–C(21)–Pd(1)
N(4)–C(21)–Pd(1)
N(5)–C(29)–Pd(2)
N(6)–C(29)–Pd(2)
Imidazole dihedral angle
177.61(6)
179.2(2)
105.7(5)
174.82(8)
178.2(2)
104.4(4)
127.1(4)
127.1(4)
129.9(4)
125.7(4)
37.08
174.14(4)
176.93(11)
105.3(3)
128.2(2)
126.5(2)
a)
b)
0.00
0.00
180.00
Pyridine dihedral angle
55.26
158.25
c)
11.325
Torsion angle
a) Imidazole dihedral angle = angle between LS planes of imidazole rings. b) Pyridine dihedral angle = angle between LS planes of pyridine
rings. c) Torsion angle = torsion angle involving the backbone atoms N(pyridine)–C(carbene)–C(carbene)–N(pyridine).
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Structural Features and Catalytic Activity of Dicarbene Dipalladium Complexes
and 35.89°, respectively. However, the dihedral angle de-
creases to 32.83° and 0.44° for 2b, respectively. In addition,
the two NHC–Pd–Py subunits adopt the cis configuration with
–22.62° of torsion angle involving of the backbone atoms
N(pyridine)–C(carbene)–C(carbene)–N(pyridine) in 2a, whilst
two subunits adopt a surprising X-shaped configuration with
109.88° in 2b.[7b] The reason for the different configuration of
these two complexes is due to different face-to-face π–π stack-
ing involving in the two complexes. In 2a, there is a strong π–
π stacking in two intermolecular pyridine rings with 3.458 Å
(N1–C72) of the shortest atom-to-atom distance and 3.467 Å
of the distance of the centers of the two pyridine rings (Fig-
Figure 7. π–π stacking among pyridine rings in complex 3a.
ure 6). However, the π–π stacking exits in two intramolecular
imidazole rings and two intermolecular pyridines in 2b.[7b] The
different steric effect due to the substitution might be the
reason for the different π–π stacking ability in complexes 2a
and 2b.
ene and propylene), has a little or no effect on that of the
complexes with longer linker (butylene and hexylene).
Catalytic Activity for Mizoroki-Heck Cross-coupling
Reaction
The palladium-catalyzed arylation of olefins has found wide
application in organic synthesis. The activity of complexes 2a–
6a in the catalytic arylation of styrene was tested in order to
elucidate the influence of the ligands. The reactions were con-
ducted in a vial in the presence of 0.5 mol% of palladium
catalysts 2a–6a and K2CO3 in a mixture of bromobenzene and
styrene in an argon atmosphere. The results are given in
Table 2. From the results, we can see the complexes of 2a–6a
Figure 6. π–π stacking between two pyridine rings in complex 2a.
Table 2. Results of the arylation of styrene with bromobenzene.
The molecular arrangements in solid-state structure of 3a
and 3b with butylene linker are quite similar. For 3a, the dihe-
dral angle of two imidazole and pyridine rings is 49.64° and
9.62°, respectively, and the two NHC-Pd-Py subunits adopt a
cis configuration with 29.49° of torsion angle involving of the
backbone atoms N(pyridine)–C(carbene)–C(carbene)–N(pyrid-
ine). For 3b, the dihedral angle of two imidazole and pyridine
rings is respectively 48.52° and 17.40°, and the two NHC-Pd-
Py subunits adopt the cis configuration with 32.17° of torsion
angle. The face-to-face π–π stacking between two intramolecu-
lar and intermolecular pyridines is the reason for this special
configuration in these two complexes. In 3a, the shortest dis-
tance between two intramolecular and intermolecular pyridines
is 3.364 Å (N1–C8A) and 3.356 Å (N1–C17A), respectively.
The distance of the centers of the two intramolecular and inter-
molecular pyridine rings is 3.601 Å and 3.562 Å, respectively
(Figure 7).
For 4a with pentylene linker, the two subunits adopt a trans
configuration with 158.25° of torsion angle, and the dihedral
angle of two imidazole and pyridine rings is 37.08° and 55.26°,
respectively. Unfortunately, the qualified crystal of 4b is not
available for comparison. The molecular arrangement of com-
plexes 5a and 5b are the same, with trans configuration and
parallel pyridine and imidazole rings. Therefore, the 4-substi-
tute far from NHC has big impact on the solid-state structure
a)
a)
a)
b)
Entry
Catalyst
R
Conversion
/%
Yield
/%
E/Z ratio
1
2
3
4
5
6
7
8
2a
3a
4a
5a
6a
2a
3a
4a
5a
6a
2a
3a
4a
5a
6a
2a
3a
4a
5a
6a
H
H
H
H
H
100
100
100
100
100
95
93
95
92
94
91
93
95
94
96
95
90
93
90
89
40
63
44
39
49
17:1
16:1
12:1
15:1
16:1
6:1
5:1
10:1
9:1
10:1
11:1
17:1
14:1
16:1
15:1
19:1
4:1
p-OMe 98
p-OMe 100
p-OMe 100
p-OMe 100
p-OMe 100
p-F
p-F
p-F
p-F
p-F
o-OMe 54
o-OMe 65
o-OMe 46
o-OMe 47
o-OMe 56
9
10
11
12
13
14
15
16
17
18
19
20
100
96
100
97
92
2:1
20:1
3:1
a) Reaction conditions: 0.5 mmol of aryl bromide, 0.6 mmol of styr-
ene, and 0.75 mmol of K2CO3 in 1 mL of DMF. b) Determined by GC
of the complexes with shorter linker between two NHC (ethyl- using dodecane as internal standard.
Z. Anorg. Allg. Chem. 2014, 444–450
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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447

C. Cao, Y. Shi et al.
ARTICLE
have shown good catalysis for the arylation of olefins with less The yield was decreased with less catalyst loading insignifi-
hindered bromobenzene with both electron-withdrawing and cantly, and 13% of yield was found with 0.2 mol% of 5a.
electron-donating substitutes. However, only moderate yields
were obtained with more hindered o-methoxy bromobenzene.
Conclusions
The length of the bridged ligands has limited effect on the
yield and regioselectivity of the products and it is hard to ratio-
nalize the effect on the current stage.
A series of bi-palladium dicarbene complexes 2a–6a with
different length of linker was synthesized. The solid-state
structures of 2a–5a and the catalytic activities of 2a–6a were
studied. The influence of the 4-methyl-group of phenyl on the
structural features and catalytic activities of these complexes
was investigated by comparing the results of 2a–6a with those
of their known mesityl analogues 2b–6b. X-ray studies show
that the distant substitute has big impact on the structural fea-
tures of the complexes with the shorter linker between two
NHC (ethylene and propylene), but has a little or no effect on
that of the complexes with longer linker (butylene and hexy-
lene). Catalytic results of arylation of styrene show that the
distant substitute has great effect on the regioselectivity of the
product in all complexes with shorter and longer linkers, but
has limited effect on the yield.
Compared to their mesityl analogues 2b–6b, the reaction
catalyzed by complexes 2a–6a generally provided the similar
yield but poor regioselectivity in all complexes whether the
linker is shorter or longer. For example, complex 2b with
shorter propylene linker gave 93% of yield with 25:1 of E/Z
ratio of product in the reaction of bromobenzene with styrene
(95% with 17:1 for p-methoxy bromobenzene and 93% with
19:1 for p-fluoro bromobenzene),[7b] whereas, 2a gave 95%
yield with 17:1 ratio in the reaction of bromobenzene (91%
with 6:1 for bromobenzene and 95% with 11:1 for p-fluoro
bromobenzene). Furthermore, complex 5b with longer decy-
lene linker gave 93% of yield with 23:1 of E/Z ratio of product
in the reaction of bromobenzene with styrene (91% with 11:1
for p-methoxy bromobenzene and 93% with 20:1 for p-fluoro
bromobenzene),[7b] whereas, 5a gave 92% yield with 15:1 ra-
tio in the reaction of bromobenzene (94% with 9:1 for bromo-
benzene and 90% with 16:1 for p-fluoro bromobenzene). The
reason for the good regioselectivity of mesityl Pd complexes
is not very clear on current stage, probably due to the combina-
tive electronic and steric effects.
Experimental Section
Materials and General Procedures: All manipulations were carried
out using standard Schlenk techniques. Solvents were purified and de-
gassed by standard procedures. The corresponding imidazolium chlor-
ides were prepared according to the previous method.[8] Pyridine was
distilled from calcium hydride under argon atmosphere. Potassium car-
bonate was ground to a fine powder prior to use. All other chemicals
were obtained from common suppliers and used without further purifi-
Further experiments to investigate the effect of different sol-
vents, bases, and catalyst loadings with catalyst 5a were per-
formed (Table 3). The results show that among the bases em-
ployed, K2CO3 is the most suitable base. The relatively low
yield was detected with other bases, like Cs2CO3 and K3PO4,
whereas no product was observed with organic base DMAP.
Furthermore, DMF is the best solvent tested, and relatively
low yield was observed in 1,4-dioxane, DMSO, and toluene.
1
cation. H and 13C spectra were recorded with a Bruker AV 400 MHz
spectrometer at room temperature and referenced to the residual sig-
nals of the solvent. GC-MS was performed with an Agilent 6890-
5973N system with electron ionization (EI) mass spectrometry. Ele-
mental analyses were performed with an EuroVektor Euro EA-300 ele-
mental analyzer.
Table 3. Arylation of styrene catalyzed by 5a under different condi-
tions.
Representative Procedure for the Synthesis of Complex 2a: To a
mixture of 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-(1,3-propanediyl)bisimi-
dazolium dichloride (1.372 g, 3.0 mmol), PdCl2 (1.069 g, 6.0 mmol),
and K2CO3 (8.292 g, 60 mmol) in a 50 mL round-bottomed flask was
added pyridine (35.5 mL). The reaction mixture was heated at 85 °C
for 18 h, after which time the mixture was filtered through Celite and
washed with DCM. The solvent was removed under vacuum, and the
crude was washed by diethyl ether (15 mL). The pure compound was
obtained as yellow solid by recrystallization with DCM / diethyl ether
a)
b)
Entry
Catalyst load- Base
ing /mol%
Solvent
Yield
E/Z
ratio
1
in 64% yield. (1.723 g). H NMR (CDCl3, 400 MHz): δ = 8.79 (d, J
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.4
0.3
0.2
0.02
Cs2CO3
DMF
DMF
DMF
DMF
36
39
0
12:1
15:1
0
= 5.6 Hz, 4 H, Py-H), 7.70 (t, J = 7.6 Hz, 2 H, Py-H), 7.56 (d, J =
1.2 Hz, 2 H, NCH), 7.35–7.21 (m, 6 H, Ar-H, 4 H, Py-H), 6.82 (d, 2
H, J = 1.6 Hz, NCH), 4.97 (t, J = 6.8 Hz, 4 H, NCH2), 3.45 (quin, J
= 7.2 Hz, 2 H, CH2), 2.30 (s, 12 H, CH3). 13C NMR (CDCl3,
100 MHz): δ = 151.3, 150.1, 137.9, 137.3, 136.7, 129.4, 128.5, 124.3,
123.8, 123.1, 49.4, 30.7, 19.1. C35H38Cl4N6Pd2 (897.37 g·mol–1):
calcd. C 46.85; H 4.27; N 9.37%; found: C 46.27; H 3.79; N 9.89%.
K3PO4
DMAP
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
92
15:1
20:1
14:1
18:1
13:1
Ͼ99:1
Ͼ99:1
0
1,4-dioxane 51
toluene
DMSO
DMF
DMF
DMF
83
72
79
35
13
0
9
10
11
Synthesis of Complex 3a: The complex was synthesized by a similar
procedure as described for 2a using 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-
(1,4-butanediyl)bisimidazolium dichloride (1.414 g, 3 mmol) instead
of 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-(1,3-propanediyl)bisimidazolium
dichloride. Yield: 78% (2.133 g). 1H NMR (CDCl3, 400 MHz): δ =
DMF
a) Reaction conditions: 0.5 mmol of phenylbromide, 0.6 mmol of styr-
ene, and 0.75 mmol of base in 1 mL of solvent. b) Determined by GC
using dodecane as internal standard.
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Structural Features and Catalytic Activity of Dicarbene Dipalladium Complexes
8.78 (d, J = 5.2 Hz, 4 H, Py-H), 7.66 (t, J = 7.6 Hz, 2 H, Py-H), 7.35–
(1,10-decanediyl)bisimidazolium dichloride (1.667 g, 3 mmol) instead
7.20 (m, 2 H, NCH, 6 H, Ar-H, 4 H, Py-H), 6.88 (d, J = 1.2 Hz, NCH), of 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-(1,3-propanediyl)bisimidazolium
4.93 (t, J = 5.6 Hz, 4 H, NCH2), 2.48 (quin, J = 7.2 Hz, 4 H, CH2), dichloride. Yield: 87% (2.598 g). 1H NMR (CDCl3, 400 MHz): δ =
2.30 (s,12 H, CH3). 13C NMR (CDCl3, 100 MHz): δ = 151.4, 150.2,
8.75 (d, J = 5.2 Hz, 4 H, Py-H), 7.66 (d, J = 7.6 Hz, 2 H, Py-H), 7.33–
137.7, 137.3, 136.7, 129.4, 128.4, 124.2, 123.9, 122.2, 50.8, 27.3, 19.0. 7.19 (m, 2 H, NCH, 4 H, Ar-H, 4 H, Py-H), 7.14 (d, J = 1.6 Hz, 2 H,
C36H40Cl4N6Pd2 (911.40 g·mol–1): calcd. C 47.44; H 4.42; N 9.22%; Ar-H), 6.88 (d, J = 2.0 Hz, 2 H, NCH), 4.74 (d, J = 7.6 Hz, 4 H,
found: C 47.04; H 4.11; N 9.67%.
NCH2), 2.29 (s, 12 H, CH3), 2.19 (quin, J = 7.2 Hz, 4 H, CH2), 1.48
(m, 8 H, CH2), 1.37 (quin, J = 7.2 Hz, 4 H, CH2). 13C NMR (CDCl3,
100 MHz): δ = 151.3, 150.2, 137.7, 137.4, 136.8, 129.3, 128.4, 124.2,
123.7, 121.7, 51.5, 30.5, 29.3, 29.1, 26.6, 19.0. C42H52Cl4N6Pd2
(995.55 g·mol–1): calcd. C 50.67; H 5.26; N 8.44%; found: C 50.35;
H 4.96; N 8.83%.
Synthesis of Complex 4a: The complex was synthesized by a similar
procedure as described for 2a using 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-
(1,5-pentanediyl)bisimidazolium dichloride (1.456 g, 3 mmol) instead
of 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-(1,3-propanediyl)bisimidazolium
dichloride. Yield: 69% (1.916 g). 1H NMR (CDCl3, 400 MHz): δ =
8.77 (d, J = 5.2 Hz, 4 H, Py-H), 7.67 (t, J = 7.2 Hz, 2 H, Py-H), 7.34–
7.20 (m, 2 H, NCH, 6 H, Ar-H, 4 H, Py-H), 6.81 (d, J = 2.0 Hz, 2 H,
NCH), 4.83 (t, J = 7.2 Hz, 4 H, NCH2), 2.43 (quin, J = 7.2 Hz, 4 H,
CH2), 2.28 (s, 12 H, CH3), 1.69 (quin, J = 7.0 Hz, 2 H, CH2). 13C
NMR (CDCl3, 100 MHz): δ = 151.3, 150.1, 137.7, 137.4, 136.8, 129.3,
128.5, 124.2, 123.6, 122.3, 51.0, 29.8, 23.3, 19.1. C37H42Cl4N6Pd2
(925.42 g·mol–1): calcd. C 48.02; H 4.57; N 9.08%; found: C 47.78;
H 4.23; N 9.38%.
Procedure for the Heck Coupling Reaction: An oven-dried 4 mL
vials containing
a stirrer bar was charged with aryl bromide
(0.50 mmol), catalyst (0.5 mol%), and K2CO3 (103.7 mg, 0.75 mmol)
in the glove box and sealed with a cap containing a PTFE septum.
DMF (1 mL) and styrene (70 μL, 0.60 mmol) were injected sequen-
tially. The mixture was stirred at 110 °C for 23 h. Yield was deter-
mined by GC using dodecane as internal standard.
X-ray Crystallography: Intensity data were collected with a Rigaku
Mercury CCD area detector in ω scan mode by using Mo-Kα radiation
(λ = 0.71075 Å). The diffracted intensities were corrected for Lorentz
polarization effects and empirical absorption corrections. Details of the
intensity data collection and crystal data are given by full-matrix least-
squares procedures based on F2.[9] All the non-hydrogen atoms were
refined anisotropically. The hydrogen atoms in these complexes were
all generated geometrically (C–H bond lengths fixed at 0.95 Å), as-
signed appropriate isotropic thermal parameters, and allowed to ride
on their parent carbon atoms. All the hydrogen atoms were held sta-
tionary and included in the structure factor calculation in the final stage
of full-matrix least-squares refinement. The structure was solved by
directed methods using the SHELXS-97 program and absorption cor-
Synthesis of Complex 5a: The complex was synthesized by a similar
procedure as described for 2a using 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-
(1,6-hexanediyl)bisimidazolium dichloride (1.498 g, 3 mmol) instead
of 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј-(1,3-propanediyl)bisimidazolium
dichloride. Yield: 69% (1.945 g). 1H NMR (CDCl3, 400 MHz): δ =
8.76 (d, J = 5.2 Hz, 4 H, Py-H), 7.65 (t, J = 7.2 Hz, 2 H, Py-H), 7.33–
7.19 (m, 2 H, NCH, 6 H, Ar-H, 4 H, Py-H), 6.86 (d, J = 1.6 Hz, 2 H,
NCH), 4.75 (t, J = 7.6 Hz, 4 H, NCH2), 2.29 (m, 12 H, CH3, 4 H, CH2),
1.70 (quin, J = 7.2 Hz, 4 H, CH2),. 13C NMR (CDCl3, 100 MHz): δ
= 151.3, 150.1, 137.7, 137.4, 136.8, 129.3, 128.4, 124.2, 123.7, 122.0,
51.2, 30.1, 26.0, 19.0. C38H44Cl4N6Pd2 (939.45 g·mol–1): calcd. C
48.0258; H 4.72; N 8.95%; found: C 48.41; H 4.56; N 9.32%.
Synthesis of Complex 6a: The complex was synthesized by a similar rection was performed by SADABS program. Selected crystallo-
procedure as described for 2a using 1,1Ј-bis(2,6-dimethylphenyl)-3,3Ј- graphic data are shown in Table 4. Crystals suitable of complexes 2a–
Table 4. Crystallographic data for complexes 2a–5a.
2a
3a
4a
5a
Empirical formula
Formula weight
C35H38Cl4N6Pd2
897.35
C36H40Cl4N6Pd2
911.34
C38H44Cl6N6Pd2
1010.29
C40H48Cl8N6Pd2
1109.24
Temperature /K
293(2)
293(2)
298(2)
298(2)
Crystal system
Space group
orthorhombic
Pbca
monoclinic
C2/c
triclinic
P1
monoclinic
P21/c
Crystal size /mm
a /Å
b /Å
c /Å
α /°
β /°
0.30ϫ0.20ϫ0.10
16.3672(13)
13.4879(12)
34.333(4)
90
0.55ϫ0.40ϫ0.33
20.079(2)
22.699(4)
14.3096(18)
90
132.956(7)
90
4773.3(11)
4
0.40ϫ0.30ϫ0.20
11.7805(4)
15.2507(5)
15.3214(5)
90.156(2)
103.480(2)
105.546(2)
2572.64(15)
2
0.40ϫ0.30ϫ0.20
13.0124(15)
13.5202(16)
14.5259(17)
90
112.232(2)
90
2365.6(5)
2
90
90
γ /°
V /Å3
7579.3(13)
8
1.573
1.264
3600
Z
Dcalcd. /mg·cm–3
Absorption coefficient /mm–1
F(000)
1.268
1.005
1832
1.304
1.039
1016
1.557
1.247
1116
θ range /°
1.72 to 25.01°
19586 / 5848
5848 / 0 / 424
1.184
R1 = 0.0402
wR2 = 0.0917
R1 = 0.0579
wR2 = 0.1064
1.79 to 28.32°
8255 / 5254
5254 / 0 / 217
1.082
R1 = 0.0515
wR2 = 0.1835
R1 = 0.0775
wR2 = 0.2258
2.02 to 25.00°
38229 / 9003
9003 / 29 / 466
1.028
R1 = 0.0552
wR2 = 0.1729
R1 = 0.0686
wR2 = 0.1913
1.69 to 25.00°
27130 / 4149
3314/0/210
1.286
R1 = 0.0290
wR2 = 0.0820
R1 = 0.0342
wR2 = 0.1049
Reflections collected / unique
Data / restrains / parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ (I)]
R indices (all data)
Z. Anorg. Allg. Chem. 2014, 444–450
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
449

C. Cao, Y. Shi et al.
ARTICLE
M. M. Luo, Organometallics 2009, 28, 3093–3099; c) C. L. Lund,
M. J. Sgro, D. W. Stephan, Organometallics 2012, 31, 580–587; d)
W. W. N. O, A. J. Lough, R. H. Morris, Organometallics 2010, 29,
570–581; e) J. W. Wang, Q. S. Li, F. B. Xu, H. B. Song, Z. Z.
Zhang, Eur. J. Org. Chem. 2006, 1310–1316.
5a for X-ray diffraction analysis were obtained by slow evaporation
of a saturated DCM solution at room temperature.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-948181 (3a), CCDC-948182 (4a), and CCDC-948183
(5a) (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk,
http://www.ccdc.cam.ac.uk).
[4] a) M. A. Iqbal, R. A. Haque, S. Budagumpi, M. B. Khadeer Ah-
amed, A. M. S. Abdul Majid, Inorg. Chem. Commun. 2013, 28, 64–
69; b) M. Baron, C. Tubaro, M. Basato, M. M. Natile, C. Graiff,
J. Organomet. Chem. 2013, 723, 108–114; c) E. Alcalde, R. M.
Ceder, C. López, N. Mesquida, G. Muller, S. Rodríguez, Dalton
Trans. 2007, 25, 2696–2706; d) S. K. Schneider, J. Schwarz, G. D.
Frey, E. Herdtweck, W. A. Herrmann, J. Organomet. Chem. 2007,
692, 4560–4568; e) M. Raynal, C. S. J. Cazin, C. Vallée, H. Oliv-
ier-Bourbigou, P. Braunstein, Dalton Trans. 2009, 19, 3824–3832;
f) A. G. Tennyson, E. L. Rosen, M. S. Collins, V. M. Lynch, C. W.
Bielawski, Inorg. Chem. 2009, 48, 6924–6933; g) D. M. Khramov,
A. J. Boydston, C. W. Bielawski, Org. Lett. 2006, 8, 1831–1834;
h) J. A. V. Er, A. G. Tennyson, J. W. Kamplain, V. M. Lynch, C. W.
Bielawski, Eur. J. Inorg. Chem. 2009, 1729–1738; i) A. G. Tenny-
son, D. M. Khramov, C. D. Varnado, P. T. Creswell, J. W. Kam-
plain, V. M. Lynch, C. W. Bielawski, Organometallics 2009, 28,
5142–5147; j) F. E. Hahn, M. C. Jahnke, T. Pape, Organometallics
2007, 26, 150–154.
[5] a) B. Liu, W. Chen, S. Jin, Organometallics 2007, 26, 3660–3667;
b) D. H. Brown, G. L. Nealon, P. V. Simpson, B. W. Skelton, Z.
Wang, Organometallics 2009, 28, 1965–1968; c) A. T. Norm, K. J.
Cavell, Eur. J. Inorg. Chem. 2008, 6, 2781–2880; d) D. Pugh, J. A.
Wright, S. Freeman, A. A. Danopoulos, Dalton Trans. 2006, 775–
782; e) M. Hernandez-Juarez, M. Vaquero, E. Alvarez, V. Salazar,
A. Suarez, Dalton Trans. 2013, 42, 351–354; f) A. A. D. Tulloch,
A. A. Danopoulos, G. J. Tizzard, S. J. Coles, M. B. Hursthouse,
R. S. H. Motherwell, W. B. Motherwell, Chem. Commun. 2001,
1270–1271.
[6] a) W. A. Herrmann, J. Schwarz, M. G. Gardiner, M. Spiegler, J.
Organomet. Chem. 1999, 575, 80–86; b) W. A. Herrmann, M. Eli-
son, J. Fischer, C. Koecher, G. R. J. Artus, Angew. Chem. Int. Ed.
Engl. 1995, 34, 2371–2374; c) M. Muehlhofer, T. Strassner, W. A.
Herrmann, Angew. Chem. Int. Ed. 2002, 41, 1745–1747; d) C.-M.
Jin, B. Twamley, J. M. Shreeve, Organometallics 2005, 24, 3020–
3023.
[7] a) C. Cao, Y. Zhuang, J. Zhao, Y. Peng, X. Li, Z. Shi, G. Pang, Y.
Shi, Inorg. Chim. Acta 2010, 363, 3914–3918; b) J. Zhao, L. Yang,
K. Ge, Q. Chen, Y. Zhuang, C. Cao, Y. Shi, Inorg. Chem. Commun.
2012, 20, 326–329; c) L. Yang, J. F. Zhao, Y. F. Li, K. Q. Ge, Y. Z.
Zhuang, C. Cao, Y. Shi, Inorg. Chem. Commun. 2012, 22, 33–36.
[8] C. Cao, Y. Zhuang, J. Zhao, G. Pang, Y. Shi, J. Chem. Res. 2011,
35, 320–322.
Acknowledgements
We gratefully acknowledge National Natural Science Foundation of
China (NSFC) (21071121, 21172188, and 21104064), Scientific Re-
search Foundation (SRF) for the Returned Overseas Chinese Scholars
(ROCS), State Education Ministry (SEM), the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions (PAPD),
the 333 Project for the Cultivation of High-Level Talents
(33GC10002), and the State Key Laboratory of Inorganic Synthesis
and Preparative Chemistry at Jilin University (2013–03) for financial
support.
References
[1] a) S. Díez-González, N. Marion, S. P. Nolan, Chem. Rev. 2009,
109, 3612–3676; b) S. Würtz, F. Glorius, Acc. Chem. Res. 2008,
41, 1523–1533; c) E. Peris, R. H. Crabtree, Coord. Chem. Rev.
2004, 248, 2239–2246; d) W. A. Herrmann, Angew. Chem. Int. Ed.
2002, 41, 1290–1309; e) T. M. Trnka, R. H. Grubbs, Acc. Chem.
Res. 2001, 34, 18–29; f) P. L. Arnold, I. J. Casely, Chem. Rev.
2009, 109, 3599–3611; g) X. Bugaut, F. Glorius, Chem. Soc. Rev.
2012, 41, 3511–3522; h) G. C. Vougioukalakis, R. H. Grubbs,
Chem. Rev. 2010, 110, 1746–1787; i) C. Samoj1owicz, M. Bieniek,
K. Grela, Chem. Rev. 2009, 109, 3708–3742.
[2] a) C. Marshall, M. F. Ward, W. T. A. Harrison, J. Organomet.
Chem. 2005, 690, 3970–3975; b) M. Micksch, T. Strassner, Eur. J.
Inorg. Chem. 2012, 5872–5880; c) M. S. Jeletic, I. Ghiviriga, K. A.
Abboud, A. S. Veige, Organometallics 2007, 26, 5267–5270; d)
L. G. Bonnet, R. E. Douthwaite, R. Hodgson, Organometallics
2003, 22, 4384–4386; e) J. A. Mata, A. R. Chianese, J. R. Mieczni-
kowski, M. Poyatos, E. Peris, J. W. Faller, R. H. Crabtree, Organo-
metallics 2004, 23, 1253–1263; f) M. T. Zamora, M. J. Ferguson,
R. McDonald, M. Cowie, Dalton Trans. 2009, 7269–7287; g) M.
Raynal, C. S. J. Cazin, C. Vallée, H. Oivier-Bourbigou, P.
Braunstein, Organometallics 2009, 28, 2460–2470.
[9] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
[3] a) D. J. Nielsen, K. J. Cavell, B. W. Skelton, A. H. White, Organo-
metallics 2006, 25, 4850–4856; b) X. Q. Zhang, Y. P. Qiu, B. Rao,
Received: June 12, 2013
Published Online: October 15, 2013
450
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 444–450