A new library of arsine, stibine-stabilized N-heterocyclic carbene palladium complexes: Synthesis, structures and activities in C-C and C-N coupling reactions

Article abstract of DOI:10.1039/c4dt01773k

A series of new N-heterocyclic carbene palladium complexes bearing AsPh₃ and SbPh₃ functions PdCl₂(IMes)(AsPh ₃) (1a), PdCl₂(IMes)(SbPh₃) (1b), PdCl ₂(IPr)(AsPh₃) (1c), PdCl₂(IPr)(SbPh ₃) (1d), PdCl₂(SIMes)(AsPh₃) (2a), PdCl ₂(SIMes)(SbPh₃) (2b), PdCl₂(SIPr)(AsPh ₃) (2c), and PdCl₂(SIPr)(SbPh₃) (2d) (IMes = N,N′-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr = N,N′-bis-(2,6-di(iso-propyl)phenyl)imidazol-2-ylidene; SIMes = N,N′-bis-(2,4,6-trimethylphenyl)imidazolidin-2-ylidene; SIPr = N,N′-bis-(2,6-di(iso-propyl)phenyl)imidazolidin-2-ylidene) was synthesized and fully characterized by ¹H NMR, ¹³C NMR, and FT-IR spectroscopy, elemental analysis and single-crystal X-ray crystallography. The catalytic potential of these arsine and stibine-stabilized carbene palladium complexes for Pd-mediated transformations was investigated with various substrates in the Hiyama cross-coupling reactions and Buchwald-Hartwig aminations. The results demonstrate that these complexes show high catalytic activity and good tolerance to various chemical functions.

Full text of DOI:10.1039/c4dt01773k

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A new library of arsine, stibine-stabilized
N-heterocyclic carbene palladium complexes:
synthesis, structures and activities in C–C and
C–N coupling reactions†
Cite this: Dalton Trans., 2014, 43,
14114
Jin Yang,a Pinhua Li,a Yicheng Zhanga and Lei Wang*a,b
A series of new N-heterocyclic carbene palladium complexes bearing AsPh3 and SbPh3 functions
PdCl2(IMes)(AsPh3) (1a), PdCl2(IMes)(SbPh3) (1b), PdCl2(IPr)(AsPh3) (1c), PdCl2(IPr)(SbPh3) (1d), PdCl2(SIMes)-
(AsPh3) (2a), PdCl2(SIMes)(SbPh3) (2b), PdCl2(SIPr)(AsPh3) (2c), and PdCl2(SIPr)(SbPh3) (2d) (IMes =
N,N’-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr
= N,N’-bis-(2,6-di(iso-propyl)phenyl)imidazol-
2-ylidene; SIMes = N,N’-bis-(2,4,6-trimethylphenyl)imidazolidin-2-ylidene; SIPr = N,N’-bis-(2,6-di(iso-
1
propyl)phenyl)imidazolidin-2-ylidene) was synthesized and fully characterized by H NMR, 13C NMR, and
FT-IR spectroscopy, elemental analysis and single-crystal X-ray crystallography. The catalytic potential of
these arsine and stibine-stabilized carbene palladium complexes for Pd-mediated transformations was
investigated with various substrates in the Hiyama cross-coupling reactions and Buchwald–Hartwig
aminations. The results demonstrate that these complexes show high catalytic activity and good tolerance
to various chemical functions.
Received 15th June 2014,
Accepted 28th July 2014
DOI: 10.1039/c4dt01773k
www.rsc.org/dalton
complexes with mixed-ligand systems.5 This is driven by the
expectation that the inherent hybrid character of such systems
Introduction
Since the discovery of the first stable nucleophilic N-hetero- would be electronically more sensitive and tunable to the
cyclic carbene (NHC) in 1991 by Arduengo,1 N-heterocyclic car- requirements of the substrates. Among the known mixed-
benes and their transition-metal complexes have been the ligand systems, the pairing of N-heterocyclic carbenes (NHCs)
focus of intense research in organometallic chemistry and with nitrogen- and phosphorus-based donor ligands are of
homogeneous catalysis.2 In particular, palladium(II) carbene special interest, because of the established role of nitrogens6
complexes derived from imidazolium precursors have been and phosphines7 in catalysis as well as the resemblance and
successfully developed as highly active pre-catalysts for C–C synergy between these two classes of ligands. For example,
and C–N coupling reactions, such as Suzuki, Heck, Kumada, Organ’s group reported the mixed pyridine type ligands and
Hiyama, Stille, Negishi, Sonogashira couplings, and Buch- N-heterocyclic carbene palladium complexes as active catalysts
wald–Hartwig amination, as well as CO–olefin copolymeriza- for various Pd-mediated C–C and C–N cross-coupling reac-
tion.3 In recent years, the focus of research of the palladium(II) tions. The major feature in the catalyst design is the combined
carbene complexes has been mainly on the development of use of an σ-donating N-heterocyclic carbene and a “throw-
universal catalysts that would be highly efficient, user-friendly, away” pyridyl ligand.6a–e
A similar family of complexes
robust, and could be employed for various Pd-mediated cross- reported by Navarro’s group, in which the pyridine fragment
coupling reactions.4 Based on this, much research effort has was substituted by the more efficient “throw-away” ligand (tri-
been devoted to the study of N-heterocyclic carbene palladium ethylamine), showed higher activities for C–C and C–N bond
formation.6f In addition, Herrmann, Nolan, Cazin, etc.
reported mixed phosphine/N-heterocyclic carbene palladium
aDepartment of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000,
complexes as active catalysts for C–C and C–N coupling reac-
PR China. E-mail: leiwang@chnu.edu.cn; Fax: +86-561-309-0518;
tions.7a–h Among which, Cazin’s group reported a series of PR3
Tel: +86-561-380-2069
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
and P(OR)3-stabilized N-heterocyclic carbene palladium com-
plexes as active catalysts for Suzuki cross-coupling reactions.7d,e
Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China
†Electronic supplementary information (ESI) available: Crystallographic data for
complexes 1a–1d and 2a–2d. 1H and 13C–{1H} NMR spectra for complexes 1a–1d,
2a–2d and the coupling products. CCDC 1003797–1003804. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt01773k
Although the catalytic activities of N-heterocyclic carbene palla-
dium complexes are dependent mainly on the properties of
the NHC, the auxiliary ligands which are attached to the metal
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dimeric compounds [Pd(μ-Cl)(Cl)(NHC)]2 were thus readily
cleaved with AsPh3 and SbPh3 to provide a range of mono-
nuclear N-heterocyclic carbene palladium(II) complexes
[PdCl2(NHC)(EPh3)] (NHC = IMes, IPr, SIMes and SIPr; E = As
and Sb) in good yields. The synthetic route is straightforward,
and all complexes were isolated as yellow and air-stable solids.
Complexes 1a–1d and 2a–2d were fully characterized by 1H
NMR, 13C–{1H} NMR, and FT-IR spectroscopy, elemental analy-
sis, and X-ray crystallography. The formation of the mixed
ligand complexes was evident from the distinctive stoichio-
metric proton signal resonances of the N-heterocyclic carbene
ligands and the corresponding auxiliary ligands in the 1H
NMR spectra. In the unsaturated N-heterocyclic carbene ana-
logues (1a–1d), the methylene protons of the NHC backbones
Scheme 1 Overview the synthesis of the N-heterocyclic carbene
(NHC) palladium complexes [PdCl2(NHC)(EPh3)].
gave rise to
a
singlet integrating for two protons
center can also affect the catalytic activities. In comprehensive
reviews on Pd catalysed C–C and C–N coupling reactions they
have been mentioned that, regardless of the catalyst used, a
Pd(0)–Pd(II) cycle was involved in the coupling.8 In these cata-
lytic systems, the presence of NHC ligands could be effective
leading to the formation of the [(NHC)–Pd(0)] active species
that start the catalytic cycle. The steric bulk of the NHC
ligands enables stabilization of the active Pd(0) species and
favors reductive elimination, while the strong σ-donor charac-
ter facilitates the oxidative addition of aryl halides.3d,f For the
above-mentioned two systems, both of the auxiliary ligands
(nitrogen and phosphite ligands) remain attached to the metal
center after the reduction to Pd(0), however there was a major
difference, as for the former, the “throw-away” ligands (pyridyl
ligands or triethylamine) departed from the metal centers to
yield the active species,6b,e,f while for the latter, the phosphite
ligands, being stronger σ-donors, might remain attached to
the [(NHC)–Pd(0)] after the reduction to Pd(0) during the cata-
lytic cycle, which could stabilized the Pd(0) active species.7e
Although NHCs with N and P donors are relatively common,
those with As and Sb donors still remain relatively unexplored.
Triaryl derivatives of arsenic have been reported as more
(7.09–7.19 ppm), while in the saturated N-heterocyclic carbene
analogues (2a–2d), the methylene protons of the NHC back-
bones gave rise to a singlet integrating for four protons
(4.03–4.14 ppm), characteristic of the saturated carbenes. In
addition, the formation of the metal complexes was evident
from the distinctive Pd–Ccarbene peaks (162.6–166.1 ppm) for
the unsaturated N-heterocyclic carbene palladium analogues
(1a–1d), which appeared at higher fields relative to their 4,5-
dihydro counterparts.4b,7d,e These values indicated that the
electronic environment around the carbene carbon remains
relatively constant in complexes 1a–1d and were slightly
shifted up-field when compared to the mononuclear N-hetero-
cyclic carbene (NHC)/PPh3 palladium complex PdCl2(IPr)PPh3
(171.1 ppm).7d For the saturated N-heterocyclic carbene palla-
dium analogues (2a–2d), the distinctive Pd–Ccarbene peaks
(191.7–194.2 ppm) were also slightly up-field compared to the
mononuclear analogue, PdCl2(SIMes)PPh3 (197.2 ppm).7j
Table 1 summarizes the selected 1H and 13C–{1H} NMR data
associated with complexes 1a–1d and 2a–2d.
Crystallographic structures
The solid-state structures of the complexes were unambigu-
ously determined by X-ray diffraction. Single crystals of the
complexes 1a–1d and 2a–2d suitable for X-ray diffraction analy-
sis were obtained from the evaporation of the dichloro-
methane–n-hexane solutions of the compounds. Complexes
1b, 1c, 1d, 2b and 2c crystallize as CH2Cl2 solvates 1b·CH2Cl2,
(1c)4·CH2Cl2, (1d)2·CH2Cl2, 2b·CH2Cl2, and 2c·0.5CH2Cl2,
respectively. There are two independent molecules in a unit
cell of the solvate (1c)4·CH2Cl2, only one molecule is shown in
Fig. 1. As expected, the Pd(II) centres are essentially square
planar, which were coordinated by an N-heterocyclic carbene
ligand, an As or Sb donor, and two chloro ions with angles
between adjacent ligands ranging from 86.41(4)° to 95.17(9)°.
In agreement with the P donors in the complexes PdCl2(SIMes)-
PPh3 and PdCl2(IPr)PPh3, the incoming As and Sb donors are
invariably trans to the NHC ligands. The Pd–Ccarbene distances,
in the range of a single bond (1.990(6) Å–2.021(6) Å), are
slightly longer than that of the corresponding PPh3 analogues
efficient ligands than their phosphorus analogues in
a
number of transition-metal-catalyzed reactions.9 Considering
that, in addition to the NHC moieties, the ancillary ligands
also played important roles in the catalytic activity of the
NHC–Pd complexes, in order to obtain more information on
this process, we focused our attention on the development of a
new library of catalysts bearing As and Sb donors-stabilized
N-heterocyclic carbene palladium complexes (Scheme 1), which
would enrich the already versatile NHC chemistry. Further-
more, the catalytic activities of the complexes in the C–C and
C–N coupling reactions were also investigated in order to
compare them with the nitrogen or phosphine-bearing catalytic
systems.
Results and discussion
Synthesis of NHC–palladium complexes
Following the method for the synthesis of N-heterocyclic (2.032(5) Å for PdCl2(IPr)PPh3 and 2.028(5) Å for PdCl2(SIMes)-
carbene (NHC)/PR3 palladium(II) complexes,7d the chloro-bridged PPh3, respectively).7d,j The Pd–As bond lengths in complexes
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Table 1 Selected 1H, 13C–{1H} NMR spectral data, bond lengths, dihedral angles and %VBur values for complexes 1a–1d and 2a–2d
1H NMRa
δNCHxCHxN
13C NMRa
δCcarbene
PdCPCl2/carbine
dihedral angle (°)
Complex
Pd–Ccarbene (Å)
Pd–E (Å)
NHC %VBur
1a
1b
1c
7.09
7.10
7.19
162.6
163.3
165.1
2.008(5)
2.018(3)
1.990(6)
1.994(6)
2.024(3)
1.994(3)
2.009(3)
2.017(3)
2.021(6)
2.4216(9)
2.5812(3)
2.4232(8)
2.4290(9)
2.5939(5)
2.4103(7)
2.5881(4)
2.4676(7)
2.5849(7)
74.89
84.99
63.62
69.79
86.11
75.51
84.40
69.51
71.51
32.6
31.7
35.0
34.0
32.4
33.5
32.6
36.1
35.8
1d
2a
2b
2c
2d
7.10
4.03
4.03
4.14
4.12
166.1
191.7
192.1
193.5
194.2
a 1H and 13C–{1H} NMR spectra were recorded in CDCl3 at 298 K (δ in ppm, J in Hz).
Fig. 1 ORTEP diagrams of [PdCl2(IMes)(AsPh3)] (1a), [PdCl2(IMes)(SbPh3)] (1b), [PdCl2(IPr)(AsPh3)] (1c), [PdCl2(IPr)(SbPh3)] (1d), [PdCl2(SIMes)(AsPh3)]
(2a), [PdCl2(SIMes)(SbPh3)] (2b), [PdCl2(SIPr)(AsPh3)] (2c), and [PdCl2(SIPr)(SbPh3)] (2d) with thermal displacement parameters drawn at 30%
probability. Parts of the hydrogen atoms and solvent molecules (CH2Cl2 for 1b, 1c, 1d, 2b and 2c) have been omitted for clarity.
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1a, 1c, 2a and 2c (ranging from 2.4103(7) Å to 2.4676(7) Å) are
longer than the Pd–N bond lengths in the related NHC–nitro-
gen palladium complexes [2.205(4) Å for PdCl2(IPr)(NEt3) and
2.219(2) Å for PdCl2(SIPr)(NEt3)]6f and the Pd–P bond lengths
in the NHC–phosphine palladium complexes [2.3054(13) Å for
PdCl2(IPr)PPh3 and 2.3137(6) Å for PdCl2(SIMes)PPh3, respect-
ively]. A similar disparity can be observed when the SbPh3-
bearing complexes are compared. The Pd–Sb bond lengths in
complexes 1b, 1d, 2b and 2d (ranging from 2.5812(3) Å to
2.5939(5) Å) are longer than the related Pd–As, Pd–P, and Pd–N
bond lengths. The Pd–Cl bonds fall in a narrow range of 2.256(2)–
2.3055(13) Å and are within the expected range relative to
the N-heterocyclic carbene–palladium complexes.7d,e Further-
more, the dihedral angles between the carbene ring planes
and the square-planar Pd coordination planes (PdCPCl2) are in
the range of 63.62–84.99°, which are typical of NHC complexes
to relieve steric congestion. As can be seen when comparing
the values obtained for complexes 1a–1d and 2a–2d, the
nature of the As and Sb donors does not have a large influence
on the bond lengths and angles around the palladium centres.
On the other hand, the steric bulk of the NHC ligands of com-
plexes 1a–1d and 2a–2d was assessed using the SambVca appli-
cation.10 As shown in Table 1, the highest value for the NHC
%VBur is the ligand calculated in the AsPh3-containing saturated
SIPr complex 2c (36.1 for the NHC moiety). Unsaturated Mes
complex 1b, featuring SbPh3, shows the smallest %VBur for the
carbene (31.7). Generally, the saturated N-heterocyclic car-
benes are slightly larger steric hindrances than their unsatu-
rated analogues. This is in accordance with previous
observations made for this ligand.7d,e
Table 2 Hiyama coupling reaction of aryl chlorides with organosilicon
catalyzed by the complexes 1a–1d and 2a–2da
Entry [Pd] catal.
Aryl chloride Organosilicon Yieldb (%)
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1b
87
89c
79d
72e
88f
70g
80h
89
9
1c
1d
2a
2b
2c
2d
92
94
85
87
90
93
90
10
11
12
13
14
15
PdCl2(IPr)(PPh3)
16
17
18
19
20
21
22
23
24
1a
1b
1c
1d
2a
2b
2c
2d
81
82
86
84
83
82
85
86
81
PdCl2(IPr)(PPh3)
25
26
27
28
1a
1b
1c
1d
75
77
81
79
29
30
31
32
1a
1b
1c
1d
80
82
85
86
Catalytic studies
In order to evaluate the effect of these mixed ligations about
palladium centres, catalytic studies were first performed on the
Hiyama coupling reaction, which has been comprehensively
employed for the synthesis of symmetric and non-symmetric
biaryls.11 As shown in Table 2, the arsine and stibine-stabilized
carbene palladium complexes 1a–1d and 2a–2d are catalyti-
cally active towards the coupling reactions of various aryl chlor-
ides with trimethoxyphenylsilane, giving the corresponding
biphenyl products in high yields. According to the general
reaction conditions reported in the literature, the reactions
were performed in dried toluene in the presence of TBAF. The
reaction between 4-chloroanisole and trimethoxyphenylsilane,
which was catalyzed by complex 1a, was first investigated to
optimize the reaction conditions (the amount of catalyst, reac-
33
34
35
36
1a
1b
1c
1d
70
72
75
73
a Reaction conditions: aryl chloride (0.50 mmol), trimethoxyphenylsilane
(0.60 mmol), [Pd] catalyst (containing 0.5 mol% Pd), and TBAF
(1.0 mmol) in dried toluene (2.0 mL) at 110 °C for 4 h. b Isolated yield.
c 0.75 mol% of [Pd] catalyst was used. d 0.25 mol% of [Pd] catalyst was
used. e The reaction time was shortened to 2 h. f The reaction time was
prolonged to 6 h. g The reaction was carried out at 80 °C. h The reaction
was carried out at 80 °C for 12 h.
prolonged to 12 h, the yield of the product was improved to
some extent. From the above results, the reactions were then
tion time and temperature). As shown in Table 2, the use of performed in dried toluene in the presence of TBAF at 110 °C
for 4 h with a Pd-catalyst loading of 0.5 mol%. Under the opti-
mized reaction conditions, the comparison of the catalytic
0.50 mol% Pd catalyst 1a gave the product in 87% yield. The
yield of the product was not improved significantly upon
increasing the amount of the catalyst to 0.75 mol%. However, activities of the complexes 1b–1d and 2a–2d was further
carried out. The reaction of the activated substrate, 4-chloro-
anisole, with trimethoxyphenylsilane generated the corres-
a lower yield was observed when the amount of the catalyst
was decreased (Table 2, entries 1–3). On further investigation
of the reaction time, the model reaction was found to be com- ponding product in yields of 87–94% (Table 2, entries 1 and
8–14). It has to be pointed out that the unsaturated hetero-
cyclic imidazolidin-2-ylidene derivatives 1a–1d and the satu-
pleted in 4 h at 110 °C. When the temperature was lowered
from 110 to 80 °C, the coupling product was obtained in a
lower yield within 4 h. However, as the reaction time was rated heterocyclic imidazolidin-2-ylidene derivatives 2a–2d
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demonstrated a similar level of catalytic activities and the
difference in yields mainly according to the steric bulk of the
NHC ligands. The IPr-based complexes 1c, 1d, 2c and 2d with
the sterically bulky substituent groups gave high yields
(90–94%), and showed similar catalytic activities with the
mixed phosphine/N-heterocyclic carbene palladium complex
PdCl2(IPr)(PPh3) (Table 2, entry 15), but compared better to
the performance of the Mes-based complexes 1a, 1b, 2a and
2b. Furthermore, the aryl chlorides with meta- and ortho-
methoxy on the benzene ring were also used as one of the sub-
strates to react with trimethoxyphenylsilane (Table 2, entries
16–28) and high yields were achieved, whereas yields over 75%
were obtained with the steric hindrance of 2-chloroanisole. In
addition, heterocyclic chlorides, such as 2-chloropyridine and
3-chloropyridine, were also found to be efficient coupling part-
ners in this system and yields over 70% of the corresponding
products were obtained under the present reaction conditions
(Table 2, entries 29–36).
Having obtained good results in C–C coupling reaction, we
then investigated the catalytic application of the complexes in
C–N bond forming reaction. The reaction to be studied was the
Buchwald–Hartwig amination12 for the synthesis of aryl
amines which was catalysed by the complexes 1a–1d and 2a–
2d. According to the general reaction conditions reported in
the literature, the initial experiments with 4-chlorotoluene and
aniline were performed to optimize the reaction conditions
(reaction time and temperature). For the reaction temperature,
110 °C was the best choice; the reactions were generally com-
plete in 4 h at 110 °C. The amount of the palladium catalyst
was also screened, and 0.5 mol% loading of palladium was
found to be optimal. The reactions indeed proceeded smoothly
to afford the corresponding products in good yields when per-
formed in dried dioxane in the presence of KOtBu at 110 °C for
4 h with a Pd-catalyst loading of 0.5 mol%. The results are
summarized in Table 3. In all the cases, anilines could be ary-
lated with electronically diverse aryl chlorides. Both electron-
poor and electron-rich aryl chlorides reacted with amines to
afford the corresponding products in high yields (Table 3
entries 1–21). The arsine, stibine-stabilized N-heterocyclic
carbene palladium complexes demonstrated a similar level of
catalytic activities compared with the phosphine-functiona-
lized N-heterocyclic carbene palladium complex PdCl2(SIMes)-
(PPh3) (Table 3, entries 13 and 22). ortho-Substituted aniline
showed no steric effect on this reaction. As shown in entries
23–34, the reaction of 2,6-dimethylaniline with aryl chlorides,
including 4-methylaniline, 4-methoxyaniline, and 2-chloro-
naphthalene, provided ortho-substituted diarylamines in good
yields.
Table 3 Buchwald–Hartwig amination of aryl chlorides with amines
catalyzed by the complexes 1a–1d and 2a–2da
Entry [Pd] catal.
Aryl chloride
Amine
Yieldb (%)
1
2
3
4
5
6
2a
2a
2a
2a
2a
2b
91
93c
81d
83e
94f
92
7
8
2c
2d
95
94
9
1a
1b
1c
1d
90
92
92
94
10
11
12
13
PdCl2(SIMes)(PPh3)
96
14
15
16
17
18
19
20
21
22
2a
2b
2c
2d
1a
1b
1c
1d
93
93
95
97
94
92
97
96
96
PdCl2(SIMes)(PPh3)
23
24
25
26
2a
2b
2c
2d
92
94
96
95
27
28
29
30
2a
2b
2c
2d
91
93
96
94
31
32
33
34
2a
2b
2c
2d
84
82
88
87
a Reaction conditions: aryl chloride (0.50 mmol), amine (0.60 mmol),
KOtBu (0.75 mmol), [Pd] catalyst (containing 0.5 mol% Pd), and dried
dioxane (2.0 mL) at 110 °C for 4 h. b Isolated yield. c 0.75 mol% of [Pd]
catalyst was used. d 0.25 mol% of [Pd] catalyst was used. e The reaction
time was shortened to 2 h. f The reaction time was prolonged to 6 h.
sterically hindered IPr-based complexes showed the higher cata-
lytic activity, whereas Mes-based complexes showed a slightly
lower activity. Furthermore, similar catalytic activities were
observed for the same N-heterocyclic carbene skeletons with
different auxiliary ligands suggesting the similar properties of
the AsPh3 and SbPh3.
According to the literature,8 palladium-catalyzed cross-
coupling reactions are thought to proceed via a mechanism
that involves a Pd(0)/Pd(II) progress. Initially, the NHC–Pd(II)
catalyst could be effective leading to the formation of the
[(NHC)–Pd(0)] active species that starts the catalytic cycle. Due
to the similar properties of AsPh3 and SbPh3 with the P(OR)3
ligands, as stronger σ-donors, As and Sb might remain
attached to the [(NHC)–Pd(0)] after the reduction to Pd(0),
which could stabilized the Pd(0) active species.7e The next step
In general, the results above-mentioned indicated that
arsine, stibine-stabilized N-heterocyclic carbene palladium
complexes exhibited high catalytic activities as pre-catalysts for
C–C and C–N coupling reactions and performed in a similar way
to the N-heterocyclic carbene palladium complexes bearing
the PPh3. Under the same reaction conditions, the difference
of the catalytic activities between complexes 1a–1d or 2a–2d
can only be attributed to the different NHC skeletons. The
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Paper
involves an oxidative addition reaction of an aryl halide with o-CH3). 13C NMR (CDCl3, 100 MHz) δ (ppm) = 162.6 (Ccarbene),
the Pd(0) centre. Then, a transmetallation reaction yields Pd(II) 138.7 (o-CH3–C Ar), 136.3 (N–C Ar), 135.4 (CH Ph), 134.4
species that contains the two moieties to be coupled. The final (C Ph), 132.6 (p-CH3–C Ar), 129.3 (CH Ph), 129.1 (CH Ph),
step releases the product through a reductive elimination 128.1 (N–CHvCH–N), 123.6 (CH Ar), 21.2 (p-CH3), 19.0
process that simultaneously regenerates the active Pd(0) (o-CH3). IR (KBr, cm−1): 3150, 2917, 2857, 1607, 1482, 1436,
catalyst.
1409, 1378, 1334, 1283, 1232, 1080, 1024, 999, 849. Anal. Calc.
for [PdCl2(IMes)(AsPh3)] (C39H39AsCl2N2Pd): C, 59.44; H, 4.99;
N, 3.56%. Found: C, 59.58; H, 5.14; N, 3.44%.
[PdCl2(IMes)(SbPh3)] (1b). The procedure yielded 78 mg
(93%) of the pure product 1b as a yellow powder. 1H NMR
(CDCl3, 400 MHz) δ (ppm) = 7.46–7.45 (m, 6H), 7.37–7.34
(m, 3H), 7.29–7.25 (m, 6H), 7.12–7.10 (m, 6H), 2.47 (s, 6H,
p-CH3), 2.39 (s, 12H, o-CH3). 13C NMR (CDCl3, 100 MHz)
δ (ppm) = 163.3 (Ccarbene), 138.7 (o-CH3–C Ar), 136.6 (N–C Ar),
136.3 (CH Ph), 135.4 (C Ph), 131.0 (p-CH3–C Ar), 129.5
(CH Ph), 129.1 (CH Ph), 128.6 (N–CHvCH–N), 123.6 (CH Ar),
21.3 (p-CH3), 18.9 (o-CH3). IR (KBr, cm−1): 3152, 3046,
2918, 2858, 1608, 1481, 1433, 1409, 1377, 1334, 1280, 1231,
1034, 1021, 997, 851. Anal. Calc. for [PdCl2(IMes)(SbPh3)]
(C39H39Cl2N2PdSb): C, 56.11; H, 4.71; N, 3.36%. Found:
C, 56.32; H, 4.91; N, 3.24%.
Conclusions
In summary, a new library of arsine, stibine-stabilized N-hetero-
cyclic carbene palladium complexes has been synthesized and
fully characterized by NMR studies combined with X-ray ana-
lyses. The X-ray data obtained also allowed for a full analysis of
the steric properties of the ligands, using the computational
tool SambVca. Furthermore, the catalytic activities of this
series of complexes were surveyed in the C–C formation by the
Hiyama coupling reaction and the C–N bond formation by the
Buchwald–Hartwig reaction. These complexes are catalytically
active in the C–C and C–N formation reactions and the steri-
cally bulky, IPr–Pd(II) complexes bearing the AsPh3 and SbPh3
ligands demonstrated the higher activities, while the auxiliary
ligand moieties (AsPh3 and SbPh3) in the complexes have
similar effect on the catalytic activity. However, the exact role
of the hetero-functionality of the complexes in the catalytic
performance also needs to be better defined.
[PdCl2(IPr)(AsPh3)] (1c). The procedure yielded 78 mg (89%)
of the pure product 1c as a yellow powder. 1H NMR (CDCl3,
400 MHz) δ (ppm) = 7.62–7.58 (m, 2H), 7.42–7.40 (m, 4H),
7.33–7.31 (m, 9H), 7.23–7.19 (m, 8H), 3.24 (sept, J = 6.8 Hz, 4H,
CH(CH3)2), 1.39 (d, J = 6.4 Hz, 12H, CH(CH3)2), 1.14 (d, J =
6.8 Hz, 12H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): δ = 165.1
(Ccarbene), 146.7 (iPr–C Ar), 135.3 (N–C Ar), 134.3 (CH Ph),
132.5 (C Ph), 129.8 (CH Ph), 129.2 (CH Ph), 128.0 (CH Ph),
124.4 (N–CHvCH–N), 123.8 (CH Ar), 28.6 (CH(CH3)2), 26.2
(CH(CH3)2), 23.0 (CH(CH3)2). IR (KBr, cm−1): 3071, 3048, 2963,
2928, 2865, 1482, 1464, 1409, 1342, 1268, 1208, 1024, 999, 934,
909, 882. Anal. Calc. for [PdCl2(IPr)(AsPh3)] (C45H51AsCl2N2Pd):
C, 61.97; H, 5.89; N, 3.21%. Found: C, 62.06; H, 5.91; N, 3.26%.
[PdCl2(IPr)(SbPh3)] (1d). The procedure yielded 80 mg
(87%) of the pure product 1d as a yellow powder. 1H NMR
(CDCl3, 400 MHz) δ (ppm) = 7.60–7.57 (m, 2H), 7.41–7.38
(m, 10H), 7.34–7.30 (m, 3H), 7.25–7.22 (m, 6H), 7.19 (s, 2H),
3.21 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.42 (d, J = 6.8 Hz, 12H,
CH(CH3)2), 1.15 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C NMR
(100 MHz, CDCl3): δ = 166.1 (Ccarbene), 146.6 (iPr–C Ar), 136.5
(N–C Ar), 135.2 (CH Ph), 130.9 (C Ph), 129.9 (CH Ph), 129.4
(CH Ph), 128.5 (CH Ph), 124.4 (N–CHvCH–N), 123.9 (CH Ar),
28.7 (CH(CH3)2), 26.2 (CH(CH3)2), 23.1 (CH(CH3)2). IR (KBr,
Experimental section
General remarks
The chemicals were purchased from commercial suppliers and
were used without purification prior to use unless otherwise
indicated. All 1H and 13C–{1H} NMR spectroscopy was per-
formed in CDCl3 and spectra were recorded on a Bruker
Avance 400 NMR spectrometer with tetramethylsilane (TMS)
as an internal standard. Elemental analyses were performed
on a Vario El III elementar. IR spectra were recorded on
a Bruker IFS 120HR spectrometer using KBr disks. Flash
column chromatography was carried out using 300–400 mesh
silica gel.
Synthesis of complexes 1a–1d and 2a–2d
A
mixture of the dimeric complex [Pd(μ-Cl)(Cl)(NHC)]2 cm−1): 3118, 3067, 3050, 2965, 2927, 2866, 1577, 1479, 1465,
(0.05 mmol) and the appropriate arsine or stibine ligand 1432, 1383, 1362, 1331, 1268, 1207, 1067, 998, 911, 801. Anal.
(0.10 mmol) was dissolved in CH2Cl2 (5.0 mL) and stirred at Calc. for [PdCl2(IPr)(SbPh3)] (C45H51Cl2N2PdSb): C, 58.81;
room temperature for 6 h. The reaction mixture was filtered H, 5.59; N, 3.05%. Found: C, 58.63; H, 5.78; N, 3.26%.
over Celite, the solvent was reduced under vacuum and the
[PdCl2(SIMes)(AsPh3)] (2a). The procedure yielded 72 mg
yellow precipitate was formed by addition of n-hexane. The (91%) of the pure product 2a as a yellow powder. 1H NMR
solid was then filtered off, washed with n-hexane, and dried (CDCl3, 400 MHz) δ (ppm) = 7.34–7.27 (m, 9H), 7.24–7.21 (m,
under vacuum to yield the pure product as a yellow powder.
6H), 7.06 (s, 4H), 4.03 (s, 4H, NCH2CH2N), 2.58 (s, 12H,
[PdCl2(IMes)(AsPh3)] (1a). The procedure yielded 70 mg o-CH3), 2.43 (s, 6H, p-CH3). 13C NMR (100 MHz, CDCl3): δ =
(89%) of the pure product 1a as a yellow powder. 1H NMR 191.7 (Ccarbene), 137.6 (o-CH3–C Ar), 137.2 (N–C Ar), 135.2
(CDCl3, 400 MHz) δ (ppm) = 7.38–7.33 (m, 9H), 7.27–7.23 (p-CH3–C Ar), 134.3 (CH Ph), 132.6 (C Ph), 129.3 (CH Ph),
(m, 6H), 7.11–7.09 (m, 6H), 2.48 (s, 6H, p-CH3), 2.39 (s, 12H, 129.2 (CH Ph), 128.0 (CH Ph), 51.2 (NCH2CH2N), 21.2 (p-CH3),
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19.1 (o-CH3). IR (KBr, cm−1): 3047, 2952, 2916, 1608, 14891, After the reaction mixture was cooled to room temperature, the
1455, 1436, 1375, 1300, 1269, 1183, 1079, 1025, 950, 848. Anal. solution was concentrated with a rotary evaporator, and the
Calc. for [PdCl2(SIMes)(AsPh3)] (C39H41AsCl2N2Pd): C, 59.29; residue was then subjected to purification via flash column
H, 5.23; N, 3.55%. Found: C, 59.41; H, 5.48; N, 3.37%.
chromatography with petroleum ether–EtOAc (15 : 1) as an
[PdCl2(SIMes)(SbPh3)] (2b). The procedure yielded 74 mg eluent to give the corresponding pure products.
(88%) of the pure product 2b as a yellow powder. 1H NMR
(CDCl3, 400 MHz) δ (ppm) = 7.40–7.38 (m, 6H), 7.35–7.31
(m, 3H), 7.26–7.22 (m, 6H), 7.06 (s, 4H), 4.03 (s, 4H, NCH2CH2N),
2.58 (s, 12H, o-CH3), 2.42 (s, 6H, p-CH3). 13C NMR (CDCl3,
100 MHz) δ (ppm) = 192.1 (Ccarbene), 138.0 (o-CH3–C Ar), 137.1
(N–C Ar), 136.6 (p-CH3–C Ar), 135.2 (CH Ph), 131.2 (C Ph),
129.4 (CH Ph), 129.4 (CH Ph), 128.5 (CH Ph), 51.2
(NCH2CH2N), 21.2 (p-CH3), 19.1 (o-CH3). IR (KBr, cm−1): 3048,
2951, 2946, 1606, 1577, 1492, 1452, 1432, 1380, 1333, 1296,
1270, 1068, 1034, 1021, 997, 911, 852. Anal. Calc. for
[PdCl2(SIMes)(SbPh3)] (C39H41Cl2N2PdSb): C, 55.97; H, 4.94;
N, 3.35%. Found: C, 56.23; H, 4.84; N, 3.26%.
[PdCl2(SIPr)(AsPh3)] (2c). The procedure yielded 79 mg
(90%) of the pure product 2c as a yellow powder. 1H NMR
(CDCl3, 400 MHz) δ (ppm) = 7.54–7.51 (m, 2H), 7.38–7.36
(m, 4H), 7.32–7.27 (m, 9H), 7.22–7.18 (m, 6H), 4.14 (s, 4H,
NCH2CH2N), 3.65 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.46 (d, J =
6.4 Hz, 12H, CH(CH3)2), 1.29 (d, J = 6.8 Hz, 12H, CH(CH3)2).
13C NMR (100 MHz, CDCl3): δ = 193.5 (Ccarbene), 147.7 (o-CH3–C
Ar), 135.4 (N–C Ar), 134.3 (p-CH3–C Ar), 132.5 (C Ph),
129.2 (CH Ph), 129.1 (CH Ph), 128.0 (CH Ph), 124.3 (CH Ph),
53.8 (NCH2CH2N), 28.7 (CH(CH3)2), 26.8 (CH(CH3)2), 24.0
(CH(CH3)2). IR (KBr, cm−1): 3054, 2962, 2926, 2867, 1478,
1455, 1436, 1419, 1383, 1327, 1297, 1269, 1243, 1079, 1056,
1024, 999, 907, 803. Anal. Calc. for [PdCl2(SIPr)(AsPh3)]
(C45H53AsCl2N2Pd): C, 61.83; H, 6.11; N, 3.20%. Found:
C, 61.46; H, 6.30; N, 3.26%.
[PdCl2(SIPr)(SbPh3)] (2d). The procedure yielded 85 mg
(92%) of the pure product 2d as a yellow powder. 1H NMR
(CDCl3, 400 MHz) δ (ppm) = 7.54–7.51 (m, 2H), 7.38–7.36
(m, 4H), 7.32–7.27 (m, 9H), 7.22–7.18 (m, 6H), 4.14 (s, 4H,
NCH2CH2N), 3.65 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 1.46 (d, J =
6.4 Hz, 12H, CH(CH3)2), 1.29 (d, J = 6.8 Hz, 12H, CH(CH3)2).
13C NMR (100 MHz, CDCl3): δ = 194.2 (Ccarbene), 147.6 (o-CH3–C
Ar), 136.5 (N–C Ar), 135.4 (p-CH3–C Ar), 131.0 (C Ph), 129.3
(CH Ph), 129.1 (CH Ph), 128.5 (CH Ph), 124.3 (CH Ph),
53.8 (NCH2CH2N), 28.7 (CH(CH3)2), 26.8 (CH(CH3)2), 24.1
(CH(CH3)2). IR (KBr, cm−1): 3050, 2963, 2925, 2866, 1608,
1578, 1480, 1454, 1433, 1327, 1296, 1270, 1244, 1181, 1068,
1021, 998, 909, 803. Anal. Calc. for [PdCl2(SIPr)(SbPh3)]
(C45H53Cl2N2PdSb): C, 58.68; H, 5.80; N, 3.04%. Found:
C, 58.44; H, 5.72; N, 3.26%.
General procedure for the [PdCl2(NHC)(EPh3)] catalysed
Buchwald–Hartwig amination
A sealable reaction tube equipped with a magnetic stir bar was
charged with aryl chloride (0.50 mmol), amine (0.60 mmol),
KOtBu (0.75 mmol), [PdCl2(NHC)(EPh3)] complex (E = As, Sb,
0.0025 mmol), and dry dioxane (2.0 mL). The mixture was
heated in an oil bath at 110 °C and stirred for 4 h. After the
reaction mixture was cooled to room temperature, the product
was filtered over Celite and washed with ethyl acetate. Then
the filtrate was concentrated with a rotary evaporator, and the
residue was then subjected to purification via flash column
chromatography with petroleum ether–EtOAc (10 : 1) as an
eluent to give the corresponding pure products.
X-ray crystallography
Data collection was performed on a Bruker-AXS SMART CCD
area detector diffractometer at 296 K using ω rotation scans
with a scan width of 0.3° and Mo-Kα radiation (λ = 0.71073 Å).
Multi-scan corrections were applied using SADABS.13 Structure
solutions and refinements were performed with the SHELX-97
package.14 All non-hydrogen atoms were refined anisotropi-
cally 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. In complex 2c, the unit cell includes serious disordered
solvent CH2Cl2 molecule, which could not be modelled as dis-
crete atomic sites. We employed PLATON/SQUEEZE to calcu-
late the diffraction contribution of the solvent CH2Cl2
molecule and, thereby, to produce a set of solvent-free diffrac-
tion intensities.15 The SQUEEZE calculation showed a total
solvent accessible area volume of 605.7 Å3 and the residual
electron density amounted to 98 e per unit cell, corresponding
to nearly 0.5 molecule of CH2Cl2. In complex 2d, the crystal
lattice contains very large solvent accessible voids in the
structure, however the final difference electron density map
contained no chemically significant peaks, the highest peak
being 0.53 e Å−3 at a distance of 0.66 Å from H(45) atom
and no model for any solvent could be found. A summary of
the crystallographic data, data collection, and refinement para-
meters for complexes 1a–1d and 2a–2d is provided in
Table S1 (ESI†).
General procedure for the [PdCl2(NHC)(EPh3)] catalysed
Hiyama coupling reaction
Acknowledgements
A sealable reaction tube equipped with a magnetic stir bar was
charged with aryl chloride (0.50 mmol), trimethoxyphenyl- Financial support from the National Natural Science Foun-
silane (0.60 mmol), [PdCl2(NHC)(EPh3)] complex (E = As, Sb, dation of China (no. 21372095, 21172092 and 21301061) and
0.0025 mmol), TBAF (1.0 mmol) and dry toluene (2.0 mL). The the Anhui Provincial Natural Science Foundation (no.
mixture was heated in an oil bath at 110 °C and stirred for 4 h. 1408085QB36) is gratefully acknowledged.
14120 | Dalton Trans., 2014, 43, 14114–14122
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14122 | Dalton Trans., 2014, 43, 14114–14122
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