Synthesis of tetra- and pentacoordinate palladium complexes of functionalized N-heterocyclic carbenes and a comparative study of their catalytic activities

Article abstract of DOI:10.1039/b903285a

Palladium complexes, [Pd(1-butyl-3-((7-methyl-1,8-naphthyridin-2-yl)methyl) imidazolylidene)₂](PF₆)₂ (1), [Pd(3-methyl-1-(pyrimidin-2-yl)imidazolylidene)₂(CH ₃CN)](PF₆)₂ (4), [Pd(3-benzyl-1-(pyrimidin-2- yl)imidazolylidene)₂(CH₃CN)](PF₆)₂ (5), [Pd(3-ethyl-1-(pyrimidin-2-yl)imidazolylidene)₂Cl](PF ₆) (6), and [Pd(3-benzyl-1-(pyrimidin-2-yl)imidazolylidene)Cl ₂] (7), have been prepared via transmetallation of corresponding in situ generated silver-carbene complexes. These palladium complexes have been characterized by NMR spectroscopy and elemental analyses, and their structures were determined by X-ray single-crystal diffraction. Complexes 1 and 7 are tetracoordinate displaying square-planar geometry. Complexes 4-6 are pentacoordinate showing uncommon square-pyramidal geometry with acetonitrile or chloride at the equatorial position. The Suzuki and Hiyama coupling reactions of a wide range of aryl bromides and chlorides have been comparatively investigated by using tetracoordinate moncarbene, tetracoordinate dicarbene, and pentacoordinate dicarbene palladium complexes as catalysts. The tetracoordinate moncarbene palladium complex showed the highest catalytic activity for both coupling reactions.

Full text of DOI:10.1039/b903285a

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This paper is published as part of a Dalton Transactions themed issue on:
N-Heterocyclic Carbenes
Guest Editor Ekkehardt Hahn
Westfälische Wilhelms-Universität, Münster, Germany
Published in issue 35, 2009 of Dalton Transactions
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www.rsc.org/dalton | Dalton Transactions
Synthesis of tetra- and pentacoordinate palladium complexes of functionalized
N-heterocyclic carbenes and a comparative study of their catalytic activities†
Xiaoming Zhang, Qinqin Xia and Wanzhi Chen*
Received 17th February 2009, Accepted 4th June 2009
First published as an Advance Article on the web 29th June 2009
DOI: 10.1039/b903285a
Palladium complexes, [Pd(1-butyl-3-((7-methyl-1,8-naphthyridin-2-yl)methyl)imidazolylidene)2](PF6)2
(1), [Pd(3-methyl-1-(pyrimidin-2-yl)imidazolylidene)2(CH3CN)](PF6)2 (4), [Pd(3-benzyl-1-(pyrimidin-
2-yl)imidazolylidene)2(CH3CN)](PF6)2 (5), [Pd(3-ethyl-1-(pyrimidin-2-yl)imidazolylidene)2Cl](PF6) (6),
and [Pd(3-benzyl-1-(pyrimidin-2-yl)imidazolylidene)Cl2] (7), have been prepared via transmetallation of
corresponding in situ generated silver-carbene complexes. These palladium complexes have been
characterized by NMR spectroscopy and elemental analyses, and their structures were determined by
X-ray single-crystal diffraction. Complexes 1 and 7 are tetracoordinate displaying square-planar
geometry. Complexes 4–6 are pentacoordinate showing uncommon square-pyramidal geometry with
acetonitrile or chloride at the equatorial position. The Suzuki and Hiyama coupling reactions of a wide
range of aryl bromides and chlorides have been comparatively investigated by using tetracoordinate
moncarbene, tetracoordinate dicarbene, and pentacoordinate dicarbene palladium complexes as
catalysts. The tetracoordinate moncarbene palladium complex showed the highest catalytic activity for
both coupling reactions.
clarification of the reaction mechanism. Here in this paper, we
Introduction
report the synthesis and structural characterization of a few tetra-
Transition metal complexes of N-heterocyclic carbenes (NHCs)
and pentacoordinate palladium complexes of functionalized NHC
have been attracting great interest because of their usefulness
ligands, their catalytic behavior in Suzuki and Hiyama reactions
as ancillary ligands in metal-catalyzed reactions, primarily based
of a range of aryl bromides and chlorides were comparatively
on the analogy between NHCs and tertiary phosphines in the
studied.
past decades.1 A number of metal/NHC complexes have shown
their enhanced catalytic activities in a wide range of organic
Results and discussion
transformations including C–C2 and C–N3 bond formation, olefin
metathesis,4 oligomerization and polymerization of alkenes.5 It is
generally accepted that the provision of monodentate or multiden-
tate chelate functional groups to complement the strongly binding
NHCs could promote a reversible coordination and dissociation
mechanism which is highly desirable for catalytic applications.6
In addition, the presence of sterically bulky groups bound to the
N-atoms favors reductive elimination of the product from central
metals.
Synthesis and characterization of tetracoordinate
[Pd(L1)2](PF6)2 (1).
The naphthyridine-functionlized imidazolium salt, HL1·PF6, was
prepared by refluxing 2-bromomethyl-7-methyl-1,8-naphthyridine
and N-n-butylimidazole in toluene and subsequent anion ex-
change in water (Scheme 1). Deprotonation of the imidazolium
salts with Ag2O in CH3CN proceeded smoothly at 50 ◦C, and
subsequent addition of Pd(cod)Cl2 to the colorless filtrate led to
the isolation of 1 as a white powder in 73% yield. The formulation
of 1 was determined by elemental analysis and further confirmed
by NMR observations. 1H NMR spectrum of 1 in DMSO-d6
shows complete disappearance of the acidic NCHN signal which
appears at 9.36 ppm in its precursor HL1·PF6, and a downfield
resonance of carbene carbon at 168.1 ppm was observed in its
13C NMR spectrum. These strongly indicate that the two ligands
are coordinated to the palladium center. These two ligands are
magnetically equivalent and other chemical shifts are similar to
its parent imidazolium salt, HL1·PF6. Complex 1 is quite stable
towards air and moisture. ESI-MS spectrum of 1 in acetonitrile
displays peaks at 811.02, 665.27 and 386.15 amu which can be
assigned to [Pd(L1)2(PF6)]+ (calcd 811.20 amu), [Pd(L1)2]+ (calcd
666.24 amu), and [PdL1]+ (calcd 386.07 amu), respectively.
The most successful application at present appears to be
the use of palladium complexes of NHC ligands in C–C and
C–heteroatom coupling reactions.7 Among the known Pd-NHC
complexes, most of them are tetracoordinate containing one
or two chelate ligands in square planar geometry.8 We have
recently reported the synthesis and structures of a family of
silver clusters,9 palladium,10 and nickel complexes11 stabilized
by pyridine-, pyrazole-, and naphthyridine-functionalized NHC
ligands, and these palladium and nickel complexes were found
to be good catalysts for C–C coupling reactions. The struc-
tural characterization and catalytic studies of non-square planar
palladium complexes would provide useful information for the
Department of Chemistry, Zhejiang University, Xixi Campus, Hangzhou
310028, China. E-mail: chenwzz@zju.edu.cn; Fax: 86-571-88273314
† CCDC reference numbers 720713–720717. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b903285a
The solid state structure of 1 was further confirmed by X-ray
diffraction. The asymmetric unit of 1 contains two independent
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7045–7054 | 7045
©

Chart 1
heteroarene group in all these three complexes. The flexibility of
the ligand allows a palladium ion to be surrounded by two ligands
forming six-membered rings in boat conformation, and thus usual
square-planar complexes could be obtained. When the heteroarene
is directly attached to the NHC ring, two rigid bidentate ligands are
impossible to locate in a coordination plane due to steric repulsion.
This steric effect can be clearly illustrated by the isolation and
structural characterization of pentacoordinate complexes 4–6.
Scheme 1 Synthesis of 1.
molecules which are essentially the same, and thus only one
is shown. As shown in Fig. 1, the palladium atom is bonded
to two carbene carbons and two naphthyridyl nitrogen atoms
adopting a square planar geometry. The two chelates adopt a
trans arrangement around the palladium atom, such that there
is a Ci symmetry at the palladium center. Both trans-positioned
NHC rings and naphthyridine rings are virtually coplanar. The
imidazolylidene and naphthyridine rings are both bisected by
the coordinated plane with dihedral angles of 47.24 and 51.49◦,
Synthesis and characterization of tetracoordinate
[Pd(L4)2(CH3CN)](PF6)2 (4), [Pd(L5)2(CH3CN)](PF6)2 (5), and
[Pd(L6)2Cl](PF6) (6)
Imidazolium salts, 1-alkyl-3-pyrimidylimidazolium (R = Me,
Bz, Et) were obtained by the condensation reaction of
2-chloropyrimidine and corresponding N-alkylimidazole. Treat-
ment of these imidazolium salts with Ag2O and then Pd(COD)Cl2
afforded palladium(II)-NHC complexes 4, 5, and 6 in 53 to 76%
yields (Scheme 2). Complexes 4 and 5 are pentacoordinate having
the same formulation [Pd(L)2(CH3CN)](PF6)2. These complexes
have been characterized by NMR spectroscopy and elemental
analyses. The formation of the Pd-NHC complexes 4 and 5 was
confirmed by the absence of the 1H NMR resonance for the acidic
NCHN protons at 10.13 and 10.4 ppm, where the resonance
signals of HL4·PF6 and HL5·PF6 were found, respectively. In the
13C NMR spectra of these two complexes, the resonances ascribed
to the carbenic carbon atoms appear at 160.1 and 159.9 ppm,
˚
˚
respectively. The Pd–C (2.009(6) A) and Pd–N (2.039(5) A)
bond distances are quite normal as compared to many known
tetracoordinate palladium(II)-NHC complexes.8 It should be
noted that NHC donors are usually cis-arranged in the previously
reported dicarbene palladium complexes due to larger trans effect
of carbenes.8 So far only a few structurally characterized Pd-
dicarbene complexes having a trans-conformation have already
been reported.12
Fig. 1 ORTEP drawing of the cationic section of 1 with thermal ellipsoids
drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.
Only one of the two independent molecules is shown. Selected bond
◦
˚
distances (A) and angles ( ): Pd(1)–C(1) 2.008(7), Pd(1)–N(3) 2.037(6),
C(1)–Pd(1)–C(1A) 180.0, C(1A)–Pd(1)–N(3) 84.7(3), C(1)–Pd(1)–N(3)
95.3(3), N(3)–Pd(1)–N(3A) 180.0. Symmetry code: #A -x + 2, -y + 1,
-z + 1.
We have recently reported the structures of the naphthyridine
and pyrimidine functionalized NHC palladium complexes 210a
and 310d (Chart 1). These two complexes are also tetracoordinate
adopting a square planar geometry, but the two ligands are cis
arranged around the palladium center. We note that there is a
methylene group between the NHC ring and the coordinated
Scheme 2 Synthesis of 4–6.
7046 | Dalton Trans., 2009, 7045–7054
This journal is
The Royal Society of Chemistry 2009
©

respectively, which fall in the range of 182.4–149.5 ppm for
Pd-NHC complexes, depending upon the ancillary ligands.13 Al-
though the two pentacoordinate complexes contain two magnet-
ically inequivalent 1-aryl-3-(2-pyrimidyl)imidazolylidene ligands
result indicates that the two ligands are magnetically equivalent
in solution due to dynamic behaviour. The ease of dissociation
of the acetonitrile molecule allows facile exchange of two ligands.
ESI-MS of complexes 4 and 5 in acetonitrile solution were studied.
The most intense peaks at 465.17 and 617.20 amu corresponding to
[Pd(L4)2(CH3CN)]+ (calcd 467.07 amu) and [Pd(L5)2(CH3CN)]+
(calcd 619.14 amu), respectively, are observed suggesting that these
complexes are stable in solution, and the cationic cores supported
by NHC ligands are maintained in the gas phase and solution.
Complexes 4 and 5 have been further characterized by X-ray
diffraction analyses. The molecular structures of the cationic
section are depicted in Fig. 2. Both complexes are pentacoordinate
with distorted square pyramidal geometry. The coordination
geometry is completed by two C,N-chelate ligands and one
acetonitrile molecule. The solvent molecules are located at the
equatorial plane and arranged trans to NHC rings in 4 and 5.
The second carbene is trans to the pyrimidine group of a different
ligand, and the apical position is occupied by another pyrimidine.
1
in their solid state (see Fig. 2), their H and 13C NMR spectra
show only one set of resonance signals assigned to the ligand. This
˚
˚
The equatorial Pd–C (1.973–1.996 A) and Pd–N (2.062–2.083 A)
bond distances are quite normal and comparable to the reported
values in the Pd-NHC complexes,8,13 whereas the two axial Pd–N
˚
bond distances are significantly longer (2.805(5), 2.834(4) A)
than normal Pd–N bonds. The long Pd–N bond distances in a
Pd–NHC complex recently reported is 2.762(6) A.10d The presence
˚
of the pentacoordinate sphere is suggested assuming the weak
apical Pd–N interaction on the basis of the sum of van der Walls
˚
radii of two atoms is 3.18 A. Pd(II) complexes tends to adopt
square-planar coordination geometry. Pentacoordinate Pd(II)
complexes are uncommon. So far only a few such complexes in
distorted trigonal bipyramidal or square pyramidal environments
supported by phenanthroline and polyphosphine ligands are
known, and all of them show long apical Pd–N bonds ranging
14
˚
from 2.34 to 2.81 A.
Complex 6 was obtained by the carbene transfer reaction
of HL6·PF6 and Pd(COD)Cl2. Interestingly, complex 6 has a
formulation of [Pd(L6)2Cl](PF6) which is determined by elemental
analysis and further confirmed by NMR observations. The
absence of an acidic CH downfield proton resonance signal which
appears at 10.19 ppm in its carbene precursor HL6·PF6 illustrates
that the ligand is coordinated to palladium atom. In the 1H
NMR spectrum of 6, two sets of signals assignable to L6 were
observed revealing that two ligands around the palladium atom are
magnetically inequivalent. 13C NMR spectrum of 6 also exhibits
two singlets at 161.8 and 160.7 ppm due to two NHC carbene
carbon resonances trans to chloride and pyrimidine, respectively.
The dynamically inert nature of the Pd–Cl bond inhibits the ligand
exchange process as observed for 4 and 5. The ESI-MS spectrum
of 6 in acetonitrile also shows the most intense peak at 489.27 amu
which can be ascribed to [Pd(L6)2(CH3CN)]+ (calcd 489.05 amu).
The solid state structure of 6 determined by X-ray diffraction
analysis is shown in Fig. 3. The palladium atom is penta-
coordinate and has the same square pyramidal geometry as
[Pd(L)2(CH3CN)](PF6)2 analogy. However, when much bulkier
ligands having 2,6-diisopropylphenyl or 2,4,6-trimethylphenyl
groups were employed, no NHC-Pd complex could be obtained.
Probably the steric repulsion of ligands does not allow two ligands
to coordinate to the same palladium ion. It should be noted that
pentacoordinate palladium complexes are uncommon. To the best
of our knowledge, only one example of a structurally characterized
Fig. 2 ORTEP drawing of the cationic section of 4 (a) and 5 (b). Thermal
ellipsoids are drawn at 30% probability level. Hydrogen atoms are omitted
◦
˚
for clarity. Selected bond distances (A) and angles ( ) for 4 (a): Pd(1)–C(9)
1.973(4), Pd(1)–C(1) 1.983(4), Pd(1)–N(9) 2.066(4), Pd(1)–N(3) 2.078(4),
Pd(1)–N(7) 2.813(4), C(1)–Pd(1)–C(9) 95.45(17), C(1)–Pd(1)–N(9)
172.56(16), C(9)–Pd(1)–N(9) 89.55(15), C(1)–Pd(1)–N(3) 80.17(16), C(9)–
Pd(1)–N(3) 174.94(15), N(9)–Pd(1)–N(3) 95.05(14), C(1)–Pd(1)–N(7)
96.46(15), C(9)–Pd(1)–N(7) 69.04(15), N(9)–Pd(1)–N(7) 90.47(14),
N(3)–Pd(1)–N(7) 108.74(13). For 5 (b): Pd(1)–C(1) 1.974(4), Pd(1)–C(15)
1.996(4), Pd(1)–N(9) 2.062(4), Pd(1)–N(3) 2.075(4), Pd(1)–N(7) 2.834(4),
C(1)–Pd(1)–C(15) 95.73(17), C(1)–Pd(1)–N(9) 174.66(16), C(15)–Pd(1)–
N(9) 89.63(16), C(1)–Pd(1)–N(3) 79.85(16), C(15)–Pd(1)–N(3) 174.66(16),
N(9)–Pd(1)–N(3) 94.79(15), C(1)–Pd(1)–N(7) 99.51(15), C(15)–Pd(1)–
N(7) 68.42(16), N(9)–Pd(1)–N(7) 82.56(14), N(3)–Pd(1)–N(7) 115.08(14).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7045–7054 | 7047
©

Fig.
4 ORTEP drawing of the molecule of 7 with thermal el-
lipsoids are drawn at 30% probability level. Hydrogen atoms are
omitted for clarity. Only one of the two independent molecules
◦
˚
is shown. Selected bond distances (A) and angles ( ): Pd(1)–C(1)
1.971(6), Pd(1)–N(3) 2.056(5), Pd(1)–Cl(1) 2.287(2), Pd(1)–Cl(2) 2.353(2),
C(1)–Pd(1)–N(3) 79.8(2), C(1)–Pd(1)–Cl(1) 98.60(19), N(3)–Pd(1)–Cl(1)
176.88(14), C(1)–Pd(1)–Cl(2) 172.25(19), N(3)–Pd(1)–Cl(2) 92.77(14),
Cl(1)–Pd(1)–Cl(2) 88.90(7).
Fig. 3 ORTEP drawing of the cationic section of 6. Thermal ellip-
soids are shown at 30% probability level with hydrogen atoms omitted
◦
˚
for clarity. Selected bond distances (A) and angles ( ): Pd(1)–C(10)
1.978(5), Pd(1)–C(1) 1.993(6), Pd(1)–N(3) 2.083(4), Pd(1)–Cl(1) 2.327(2),
Pd(1)–N(7) 2.948(5), C(10)–Pd(1)–C(1) 98.2(2), C(10)–Pd(1)–N(3)
177.5(2), C(1)–Pd(1)–N(3) 79.86(19), C(10)–Pd(1)–Cl(1) 88.04(17),
C(1)–Pd(1)–Cl(1) 173.66(15), N(3)–Pd(1)–Cl(1) 93.91(14), C(10)–Pd(1)–
N(7) 66.36(19), C(1)–Pd(1)–N(7) 87.86(19), N(3)–Pd(1)–N(7) 114.89(15),
Cl(1)–Pd(1)–N(7) 95.89(14).
˚
Pd–Cl trans to NHC ligand (2.353(2) A) is longer than that of
˚
Pd–Cl trans to the pyrimidine group (2.287(2) A) due to a larger
trans effect of the carbene ligand. The benzyl ring plane is twisted
out of the coordination plane, while the imidazolylidene ring and
the pyrimidyl ring are essentially coplanar. The structure of 8 has
been briefly described by our group recently.11d
palladium complex of NHC ligands has been recently reported by
our group.10d
Computational studies
Synthesis and characterization of tetracoordinate [Pd(L5) Cl2] (7)
We have recently shown that the rigidity and bulkiness of
substituents play an important role in determining the struc-
ture of a nickel–carbene complex.11f Full molecular geometry
optimizations have been carried out at the Becke3LYP (B3LYP)
level of density functional theory (DFT) for the palladium
complexes. We optimized their conformation on the assumption
that the coordination sphere of these [Pd(L)2]2+ intermediates is
square planar. We found that when the chelates form a rigid
five-membered palladacycle, the steric repulsion between the
substituent groups will force the coordination geometry distorted
(Scheme 4). Actually the geometrically optimized structure is a
distorted tetrahedron (Fig. 5(b)) in the case of pyrimidylimida-
zolylidene. However, the geometrically optimized structures of
those complexes containing flexible six-membered palladacycle are
in the case of (pyridylmethyl)imidazolylidene still square planar
(compound 3, Fig. 5(a)). The tetrahedral conformation of the 16e
configuration is not energetically preferred and the space brought
about due to steric repulsion would allow an additional donor,
yielding more stable palladium complexes of higher coordination
numbers (Scheme 4).
Reactions of HL5·Cl with Ag2O and subsequent Pd(cod)Cl2
afforded 7 in 70% yield (Scheme 3). Because the chloride atom
could coordinate to the central palladium atom, no matter what
the R substituents were, palladium complexes with the same
formulation of PdLCl2 were obtained. The formulation of complex
7 was also confirmed by elemental analysis and NMR spectra.
The 13C resonance peak of the carbenic carbon appears at
156.9 ppm, which is consistent with those of known palladium-
NHC complexes.8,13 ESI-MS spectra of complexes 7 in acetonitrile
show peaks at 378.49, 342.85, and 238.31 amu which can be
ascribed to [Pd(L7)Cl]+ (calcd 376.98 amu), [Pd(L7)]+ (calcd
342.01 amu), and [L7]+ (calcd 236.10 amu), respectively.
Scheme 3 Synthesis of 7.
The structural determination shows that the geometry of
complex 7 is square-planar. The asymmetric unit of 7 contains
two independent molecules which are essentially the same, and
thus only one is shown in Fig. 4. The Pd–C and Pd–Cl bond
distances are normal and consistent with those of palladium(II)
halide complexes containing NHC ligands. The bond distance of
Scheme 4 Formation of pentacoordinate complexes.
7048 | Dalton Trans., 2009, 7045–7054
This journal is
The Royal Society of Chemistry 2009
©

Table 1 DE values (kcal mol-1) in [PdL2S]2+ (S = CH3CN)
Complexes
DE
[Pd(L4)2(CH3CN)]2+
[Pd(L5)2(CH3CN)]2+
-20.5
-19.0
The reaction energies for the formation of more stable
[Pd(L)2S]2+ (DE) complexes from tetrahedron [Pd(L)2]2+ interme-
diates with a solvent molecule was calculated, and the results are
given in Table 1. The theoretically calculated reaction energies DE
are all negative, illustrating that the pentacoordinate palladium
complexes are more stable than the distorted tetrahedral com-
plexes.
Fig. 6 Comparison of the catalytic activities of 1–8 for Suzuki reaction
of 4-tolyl bromide with phenylboronic acid. Yields are determined by gas
chromatography after 1 h.
complexes based on the yields after 1 h follows: tetracoordinate
monocarbene > pentacoordinate dicarbene > tetracoordinate
dicarbene. The most active complexes 3, 5, and 8 are selected as the
representatives of tetracoordinate dicarbene, pentacoordinate di-
carbene, and tetracoordinate monocarbene palladium complexes,
respectively. The Suzuki coupling reactions of a wide range of
aryl bromides and chlorides with phenylboronic acid were further
studied to compare the activities of complexes 3, 5, and 8 in detail
and the results are listed in Table 2.
Table 2 Suzuki coupling reactions catalyzed by complexes 3, 5, and 8a
GC/Isolated
Yield (%)
Entry
R
X
Cat.
Time (h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26b
27b,c
28 b
4-C(O)CH3
4-C(O)CH3
4-C(O)CH3
H
H
H
4-Me
4-Me
4-Me
4-MeO
4-MeO
4-MeO
2-Me
2-Me
2-Me
1-bromo naphthalene Br
1-bromo naphthalene Br
1-bromo naphthalene Br
4-C(O)CH3
4-C(O)CH3
4-C(O)CH3
4-CN
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
3
5
8
3
5
8
3
5
8
3
5
8
3
5
8
3
5
8
3
5
8
3
5
8
8
8
8
8
6
2
0.5
6
100/93
100/95
100/92
93/87
100/91
100/97
71/63
99/92
99/95
80/78
98/90
99/93
65/58
99/91
98/94
62/51
100/97
98/92
64/60
93/90
95/90
11
2
0.5
12
3
0.5
12
6
0.5
12
3
1
12
3
1
12
6
2
12
6
2
12
6
Fig. 5 Optimized structures of 3 (a) and [Pd(L4)2]2+ (b).
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Catalytic studies
The catalytic activities of the tetra- and pentacoordinate palladium
complexes were studied for C–C formation reactions. Suzuki
coupling reactions have been the most versatile and important
method for the synthesis of unsymmetrically substituted biaryl
compounds.15 The coupling reaction of 4-tolyl bromide with
phenylboronic acid in a mixed solvent DMF/H2O (10:1) with
1 mol% catalyst loading using Cs2CO3 as base at 80 ◦C was
studied.16 Complexes 1–8 are all active, and Fig. 6 demonstrates
their differences under the same reaction conditions. The results
show that the order of catalytic activity of these Pd–NHC
4-CN
4-CN
H
H
4-Me
4-MeO
95/91
92/89
31
87/82
53
6
12
15
a Reaction conditions: aryl halide 1.0 mmol, phenylboronic acid 1.5 mmol,
◦
Pd 1 mol%, Cs2CO3 1.2 mmol, DMF/H2O (3/0.3 mL), 80 C. b 120 ◦C.
c 4-Tolylboronic acid was used instead of phenylboronic acid.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7045–7054 | 7049
©

Table 3 Hiyama reactions catalyzed by complex 8a
All three catalysts are effective towards aryl bromides bearing
either electron-withdrawing or electron-donating substituents, and
activated aryl chlorides. The data clearly show that monocarbene
complex 8 is more active than 3 and 5, and the Suzuki coupling
of aryl bromides and activated chlorides with phenylboronic acids
by using 1 mol% of 8 could be completed within 0.5–2 h to give
nearly quantitative yields (entries 3, 6, 9, 12, 21, and 24). The
coupling of sterically hindered substrates, 2-bromotoluene and
1-bromonaphthalene, also gave excellent yields (entries 15 and
18). The dicarbene pentacoordinate complex 5 shows intermediate
catalytic activities, and the yields of the target product are also
excellent when the reaction time was prolonged to 2–6 h. The
dicarbene tetracoordinate complex 3 is the least efficient. The
coupling yields are relatively lower than in the cases of 5 and 8
even the reaction time was extended to 6–12 h. Complex 5 also
shows good activities for electron deficient aryl chlorides such as
4-acetylphenyl chloride and 4-chlorobenzonitrile (entries 20 and
23). However, both complexes 3 and 5 are totally inefficient for
the coupling of unactivated and deactivated aryl chlorides. When
1 mol% of 8 was employed, the coupling reaction of chlorobenzene
with 4-tolylboronic acid could proceed affording only a 31% yield
of 4-methylbiphenyl at 80 ◦C (entry 25). However, the yield could
be improved to 87% when the reaction temperature was elevated
to 120 ◦C (entry 26). The coupling of deactivated substrate,
2-chlorotoluene also gave a moderate yield of 53% (entry 27).
Activation of C–Cl bond is still a challenging subject in organic
chemistry. Complex 8 exhibits good catalytic activities towards a
range of aryl chlorides. Taking account of its ease of preparation
and robustness towards air and moisture, complex 8 would be
expected to be practically applicable.
Entry
R
X
Time (h)
GC/Isolated Yield (%)
1
2
3
4
5
6
7
8
9
4-C(O)CH3
H
4-Me
4-MeO
2-Me
4-C(O)CH3
4-CN
H
4-Me
4-MeO
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
2
6
6
6
6
6
6
12
12
12
100/95
98/93
71/66
88/81
68/62
95/91
83/80
51/47
Trace
Trace
10
a Reaction conditions: aryl halide 1.0 mmol, trimethoxy(phenyl)silane
2.0 mmol, cat. 1 mol%, Cs2CO3 2.0 mmol, TBAF 2.0 mmol, dioxane
3 mL, 80 ◦C.
had to be prolonged to 12 h (entry 8). Unfortunately, when
deactivated substrates 4-chlorotoluene and 4-chloroanisole were
involved, only trace amounts of products were observed.
Conclusion
In summary, we have successfully prepared a series of tetra-
and penta-coordinate Pd-NHC complexes bearing mono- and
dicarbene ligands and their structures have been definitely
determined by X-ray crystallography. The catalytic behaviour
of tetracoordinate moncarbene, tetracoordinate dicarbene, and
pentacoordinate dicarbene palladium complexes in Suzuki and
Hiyama coupling reactions were comparatively studied. The
tetracoordinate monocarbene complex was found to be the most
efficient catalyst precursor for both coupling reactions of aryl
bromides and chlorides. Dicarbene palladium complexes are much
less active than the monocarbene palladium complex. It seems
that the additional donor blocks one of the coordination sites at
the metal inhibiting approach of substrate to metal center, thus
reducing its catalytic activity.
Recently, organosilanes have become a good choice for C–C
cross-coupling reaction because they are environmentally benign
and stable to various reaction conditions.17 The so-called Hiyama
coupling reactions have been widely applied for the preparation
of biphenyl derivatives. However, Hiyama reactions promoted
by metal–NHC complexes are relatively scarce.18 The coupling
reactions of aryl halides with trimethoxy(phenyl)silane were also
tested by using complexes 3, 5, and 8 as catalysts in dioxane at
Experimental
◦
80 C in the presence of TBAF which is a well known promoter
General procedures
for the Hiyama coupling reaction in organic solvents.19 The results
are summarized in Table 3. To our surprise, complexes 3 and 5
are totally inefficient even for the coupling reaction of activated
4-bromoacetophenone and trimethoxy(phenyl)silane. However,
complex 8 is an excellent catalyst for Hiyama coupling of aryl
bromides and activated aryl chlorides. These results again illustrate
that the monocarbene complex is much more active than the
dicarbene complexes for C–C formation reactions. When 1 mol%
of 8 was employed, both electron-rich and electron-deficient aryl
bromide substrates gave excellent yields of the corresponding
biphenyl products within 6 h (entries 1–4). The complex also
applies to steric hindered substrate such as 2-bromotoluene, but
the yield of the corresponding product is a bit lower (entry 5).
The coupling reaction of activated 4-chloroacetophenone and
4-chlorobenzonitrile also proceeded smoothly to give 95% and
83% yields (entries 6 and 7), respectively. For the less active
chlorobenzene the yield dropped to 51%, and the reaction time
All the chemicals were obtained from commercial suppliers and
used without further purification. N-picolylimidazole,20 and 2-
(bromomethyl)-7-methyl-1,8-naphthyridine21 were prepared ac-
cording to the known procedures. The C, H, and N elemental
analyses were carried out with a Vario EL III elemental analyzer.
1H and 13C NMR spectra were recorded on a Bruker Avance-400
(400 MHz) spectrometer. Chemical shifts (d) are expressed in ppm
downfield to TMS at d = 0 ppm and coupling constants (J) are
expressed in Hz.
1-Butyl-3-((7-methyl-1,8-naphthyridin-2-yl)methyl)imidazolium
hexafluorophosphate (HL1·PF6)
2-(bromomethyl)-7-methyl-1,8-naphthyridine (2.36 g, 10 mmol)
was added to a solution of N-n-butylimidazole (2.48 g, 20 mmol) in
100 mL of toluene. The mixture was refluxed for 48 h. The resulting
7050 | Dalton Trans., 2009, 7045–7054
This journal is
The Royal Society of Chemistry 2009
©

precipitate was isolated and washed with toluene (2 ¥ 5 mL) and
Et2O (2 ¥ 5 mL), which was then dissolved in water (20 mL).
To the aqueous solution was added an excess of NH4PF6 (3.3 g,
20 mmol) resulting in an immediate light yellow precipitation. The
yellow solid was collected and washed with water, Et2O and dried
under vacuum. Yield: 3.75 g (89%), light yellow powder. 1H NMR
(400 MHz, DMSO-d6): d 9.36 (s, NCHN, 1H), 8.51, 8.37 (both
d, J = 8.8, naphthyridine, each 1H), 7.88 (m, naphthyridine, 2H),
7.67, 7.55 (both d, J = 8.0, imidazolium, each 1H), 5.85 (s, CH2,
2H), 4.29 (t, J = 7.6, CH2, 2H), 2.69 (s, CH3, 3H), 1.84, 1.33 (both
m, CH2, each 2H), 0.94 (t, J = 7.6, CH3, 3H). 13C NMR (100 MHz,
DMSO-d6): d 163.5, 157.5, 154.9, 139.1, 137.7, 137.6, 124.1, 123.8,
122.7, 120.4, 120.3, 53.7, 49.1, 31.8, 25.4, 19.2, 13.7. Anal. Calcd
(%) for C17H21N4F6P: C, 47.89; H, 4.96; N, 13.14. Found: C, 48.02;
H, 4.98; N, 13.39.
2.07 (s, 2CH3CN, 6H). 13C NMR (100 MHz, DMSO-d6): d 160.1,
155.4, 155.3, 126.7, 121.6, 120.2, 118.5, 38.2, 1.5. Anal. Calcd
for C18H19N9F12P2Pd·CH3CN·H2O: C, 29.41; H, 2.96; N, 17.15.
Found: C, 29.02; H, 2.99; N, 17.32.
3-Benzyl-1-(pyrimidin-2-yl)imidazolium hexafluorophosphate
(HL5·PF6)
The compound was prepared according to the same procedure as
for HL1·(PF6) starting from N-benzylimidazole (1.58 g, 10 mmol)
and 2-chloropyrimidine (2.28 g, 20 mmol). Yield: 3.45 g (90%),
white powder. 1H NMR (400 MHz, DMSO-d6): 10.4 (s, NCHN,
1H), 9.06 (d, J = 4.8, pyrimidine, 2H), 8.49, 8.02 (both s,
imidazolium, each 1H), 7.76 (t, J = 4.8, pyrimidine, 1H), 7.54
(d, J = 7.2, phenyl, 1H), 7.43 (m, pyrimidine and phenyl, 3H),
5.59 (s, CH2, 2H), 13C NMR (100 MHz, DMSO-d6): 160.4, 152.6,
136.6, 134.8, 129.5, 129.4, 128.9, 124.1, 122.9, 120.1, 53.1. Anal.
Calcd for C14H13N4F6P: C, 43.99; H, 3.43; N, 14.66. Found: C,
44.21; H, 3.32; N, 14.93.
[Pd(1-butyl-3-((7-methyl-1,8-naphthyridin-2-
yl)methyl)imidazolylidene)2](PF6)2 (1)
To a solution of HL1·PF6 (213 mg, 0.5 mmol) in CH3CN (10 mL)
was added Ag2O (64 mg, 0.275 mmol), and the mixture was stirred
at room temperature for 5 h. To the filtrate was added Pd(cod)Cl2
(73 mg, 0.25 mmol). After stirring overnight, the mixture was
[Pd(3-benzyl-1-(pyrimidin-2-yl)imidazolylidene)2(CH3CN)](PF6)2
(5)
R
filtered through Celiteꢀ, and the filtrate was then concentrated to
The compound was prepared according to the same procedure
as for 1 starting from HL5·PF6 (191 mg, 0.5 mmol), Ag2O
(64 mg, 0.275mmol) and Pd(cod)Cl2 (73 mg, 0.25 mmol). Yield:
ca. 2 mL. Addition of Et2O (20 mL) to the filtrate afforded a white
precipitate immediately. The precipitate was collected and washed
with Et2O (20 mL) to afford a white powder. Yield: 164 mg (73%).
X-ray quality crystals were obtained by slow diffusion of Et2O into
the CH3CN solution of the complex. 1H NMR (400 MHz, DMSO-
d6): d 8.99, 8.60, 8.28, 7.75 (all d, J = 8.0, naphthyridine, each 2H),
7.60, 7.15 (both d, J = 2.0, imidazolylidene, each 2H), 6.54, 6.28
(both d, J = 15.2, CH2, each 2H), 3.00 (t, J = 8.0, CH2, 4H), 2.54
(s, CH3, 6H), 1.18, 0.64, 0.45, 0.10 (all m, CH2, each 2H), 0.46 (m,
CH3, 6H). 13C NMR (100 MHz, DMSO-d6): d 168.1, 165.9, 161.4,
152.8, 143.7, 138.9, 126.0, 124.3, 122.7, 122.1, 121.5, 56.0, 48.7,
31.5, 25.6, 19.4, 13.6. Anal. Calcd (%) for C34H40N8F12P2Pd·H2O:
C, 41.88; H, 4.34; N, 11.49. Found: C, 41.61; H, 4.52; N, 11.41.
1
154 mg (68%), white powders. H NMR (400 MHz, DMSO-d6):
8.97 (d, J = 5.6, pyrimidine, 4H), 8.33, 7.86 (both d, J = 1.6,
imidazolylidene, each 2H), 7.75 (t, J = 5.2, pyrimidine, 2H), 7.18,
7.01 (both m, benzyl, 10H), 5.11, 4.73 (both d, J = 14.8, CH2, each
2H), 2.05 (s, 2CH3CN, 6H). 13C NMR (100 MHz, DMSO-d6):
159.3, 155.3, 154.5, 134.5, 128.5, 128.1, 126.7, 125.5, 121.1, 120.3,
118.0, 53.8, 1.04. Anal. Calcd for C30H27N9F12P2Pd·CH3CN·H2O:
C, 39.66; H, 3.33; N, 14.45. Found: C, 39.28; H, 3.41; N, 14.42.
3-Ethyl-1-(pyrimidin-2-yl)imidazolium hexafluorophosphate
(HL6·PF6)
This compound was prepared following the procedure described
for HL1·PF6 by starting from 2-chloropyrimidine (1.14 g,
10 mmol) and N-ethylimidazole (1.92 g, 20 mmol). Yield: 2.78 g
(87%), white powder. 1H NMR (400 MHz, DMSO-d6): d 10.19 (s,
NCHN, 1H), 9.06 (d, J = 4.8, pyrimidine, 2H), 8.49, 8.05 (both s,
imidazolium, each 1H), 7.77 (t, J = 4.8, pyrimidine, 1H), 4.37
(m, CH2, 2H), 1.52 (t, J = 7.6, CH3, 3H). 13C NMR (100 MHz,
DMSO-d6): d 160.4, 152.6, 135.3, 122.8, 122.1, 119.9, 53.9, 22.5.
Anal. Calcd (%) for C9H11N4F6P: C, 33.76; H, 3.46; N, 17.50.
Found: C, 33.91; H, 3.55; N, 17.82.
3-Methyl-1-(pyrimidin-2-yl)imidazolium hexafluorophosphate
(HL4·PF6)
The compound was prepared following the procedure described
for HL1·(PF6) by starting from 2-chloropyrimidine (1.14 g,
10 mmol) and N-methylimidazole (1.64 g, 20 mmol). Yield: 2.75 g
(90%), white powder. 1H NMR (400 MHz, DMSO-d6): d 10.13 (s,
NCHN, 1H), 9.04 (d, J = 4.8, pyrimidine, 2H), 8.46, 7.94 (both s,
imidazolium, each 1H), 7.76 (t, J = 4.8, pyrimidine, 1H), 4.01
(s, CH3, 3H). 13C NMR (100 MHz, DMSO-d6): d 160.4, 152.5,
136.8, 125.4, 122.7, 119.2, 36.8. Anal. Calcd (%) for C8H9N4F6P:
C, 31.39; H, 2.96; N, 18.30. Found: C, 31.31; H, 3.04; N, 18.11.
[Pd(3-ethyl-1-(pyrimidin-2-yl)-1-imidazolylidene)2Cl](PF6) (6)
The compound was prepared according to the same procedure as
for 1 starting from HL6·(PF6) (160 mg, 0.5 mmol), Ag2O (64 mg,
0.275 mmol), and Pd(cod)Cl2 (73 mg, 0.25 mmol). Yield: 84 mg
[Pd(3-methyl-1-(pyrimidin-2-yl)imidazolylidene)2(CH3CN)](PF6)2
(4)
1
The compound was prepared according to the same procedure as
for 1 starting from HL4·PF6 (153 mg, 0.5 mmol), Ag2O (64 mg,
0.275mmol), and Pd(cod)Cl2 (73 mg, 0.25 mmol). Yield: 142 mg
(53%), white powder. H NMR (400 MHz, CD3CN): 9.35, 9.03
(m, pyrimidine, each 1H), 8.79 (d, J = 4.8, pyrimidine, 2H), 8.31,
7.94, 7.59, 7.27 (all d, J = 2.0, imidazolylidene, each 1H), 7.68,
7.49 (both t, J = 4.8, pyrimidine, each 1H), 4.62, 4.51, 3.39, 3.14
(all m, CH2, each 1H), 1.57, 0.99 (both t, J = 7.6, CH3, each
3H). 13C NMR (100 MHz, CD3CN): d 161.8, 160.7, 159.1, 157.6,
1
(76%), white powder. H NMR (400 MHz, DMSO-d6): 9.05 (d,
J = 4.8, pyrimidine, 4H), 8.36 (d, J = 2.0, imidazolylidene, 2H),
7.79 (m, pyrimidine and imidazolylidene, 4H), 3.57 (s, CH3, 6H),
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7045–7054 | 7051
©

156.9, 154.9, 123.4, 122.7, 121.7, 121.1, 120.4, 117.6, 47.9, 45.6,
14.8, 14.7. Anal. Calcd (%) for C18H20N8F6PClPd·2H2O: C, 32.21;
H, 3.60; N, 16.69. Found: C, 32.30; H, 3.81; N, 17.03.
C, 38.87; H, 3.35; N, 13.41. Crystals suitable for X-ray diffraction
were grown by slow diffusion of Et2O into its CH3CN solution.
General procedure for Suzuki reactions with 8
4-Bromotoluene (170 mg, 1.0 mmol), phenylboronic acid (184 mg,
1.5 mmol), 8 (4 mg, 1 mmol%), Cs2CO3 (400 mg, 1.2 mmol), and
DMF/H2O (3/0.3 mL) were subsequently added to a Schlenk
tube. The solution was heated to 80 ◦C under an atmosphere
of N2 for 0.5 h. The mixture was then allowed to cool to room
temperature and added to 20 mL of water. The product was
extracted with CH2Cl2 (3 ¥ 10 mL). The combined organic layer
was dried over anhydrous MgSO4 and concentrated in vacuo. The
residue was purified by flash chromatography on silica gel to give
the desired product.
3-Benzyl-1-(pyrimidin-2-yl)-1H-imidazol-3-ium chloride (HL5·Cl)
2-Chloropyrimidine (2.28 g, 20 mmol) was added to a solution
of N-benzylimidazole (1.58 g, 10 mmol) in 30 mL of toluene. The
mixture was refluxed for 24 h. The resulting precipitate was isolated
and washed with toluene (2 ¥ 5 mL) and Et2O (2 ¥ 5 mL). Yield:
2.44 g (90%), light yellow powder. 1H NMR (400 MHz, DMSO-
d6): d 10.58 (s, NCHN, 1H), 9.06 (d, J = 5.2, pyrimidine, 2H), 8.46,
8.16 (both s, imidazolium, each 1H), 7.78 (t, J = 4.8, pyrimidine,
1H), 7.60 (d, J = 7.6, benzyl, 2H), 7.60 (d, J = 7.6, benzyl, 2H),
7.49 (m, benzyl, 3H), 5.70 (s, CH2, 2H). 13C NMR (100 MHz,
DMSO-d6): d 160.4, 152.6, 136.5, 135.0, 129.3, 129.2, 129.0, 124.2,
122.8, 119.9, 52.7. Anal. Calcd (%) for C14H13N4Cl·H2O: C, 57.83;
H, 5.20; N, 19.27. Found: C, 57.61; H, 5.32; N, 19.30.
General procedure for Hiyama reactions with 8
4-Bromoacetophenone (199 mg, 1.0 mmol), trimethoxy(phenyl)-
silane (396 mg, 2.0 mmol), 8 (4 mg, 1 mmol%), Cs2CO3 2.0 mmol,
TBAF (520 mg, 2.0 mmol), dioxane 3 mL were subsequently added
to a Schlenk tube. The solution was heated to 80 ◦C under an
atmosphere of N2 for 2 h. The mixture was then allowed to cool to
room temperature and added to 20 mL of water. The product was
extracted with CH2Cl2 (3 ¥ 10 mL). The combined organic layer
was dried over anhydrous MgSO4 and concentrated in vacuo. The
residue was purified by flash chromatography on silica gel to give
the desired product.
Pd(3-benzyl-1-(pyrimidin-2-yl)-1-imidazolylidene)Cl2 (7)
A solution of HL7·Cl (136 mg, 0.5 mmol) in CH3CN (10 ml) was
treated with Ag2O (64 mg, 0.275 mmol). The mixture was stirred
at room temperature for 2 h with exclusion of light. Then the
mixture was filtered and Pd(cod)Cl2 (145 mg, 0.5 mmol) was added
to the filtrate. The mixture was allowed to react for 2 h at room
R
temperature. After filtration through a plug of Celiteꢀ, the solvent
was removed in vacuo and the residue was washed with diethyl
ether. Complex 7 was obtained as a yellow powder. Yield: 144 mg,
70%.1H NMR (DMSO-d6): 9.44, 9.07 (both br, pyrimidine, each
1H), 8.12, 7.63 (both d, J = 2.0, imidazolylidene, each 1H), 7.73
(t, J = 4.8, pyrimidine, 1H), 7.50 (d, J = 8.0, benzyl, 2H), 7.37
(m, benzyl, 3H), 6.02 (s, CH2, 2H). 13C NMR (DMSO-d6): 156.9,
152.0, 136.7, 129.0, 128.5, 128.3, 125.1, 120.5, 118.2, 52.1. Anal.
Calcd for C14H12Cl2N4Pd·H2O: C, 38.96; H, 3.27; N, 12.98. Found:
X-Ray structural determination
Single-crystal X-ray diffraction data were collected at 298(2) K
on a Siemens Smart-CCD area-detector diffractometer with a
˚
Mo Ka radiation (l = 0.71073 A) by using an w-2q scan
mode. Unit-cell dimensions were obtained with least-squares
refinement. Data collection and reduction were performed using
the SMART and SAINT software.22 The structures were solved
Table 4 Summary of X-ray crystallographic data for complexes 1 and 4–7
1
4
5
6
7
Formula
Fw
C34H40F12N8P2Pd
957.08
monoclinic
P21/c
17.283(2)
11.3693(13)
22.236(3)
90
109.000(2)
90
4131.2(8)
4
1.539
0.617
1936
1.93–25.01
20274
7269, 0.0532
1.111
0.0572, 0.1234
0.1276, 0.1700
0.751, -0.517
C20H22F12N10P2Pd
798.82
triclinic
C32H30F12N10P2Pd
951.00
triclinic
C18H24ClF6N8O2PPd
671.27
monoclinic
P21/c
12.0874(14)
7.4205(11)
29.316(3)
90
90.932(2)
90
2629.2(6)
4
1.696
0.943
1344
1.39–25.01
12718
4614, 0.0335
1.101
0.0504, 0.1037
0.0692, 0.1120
0.583, -0.749
C14H12Cl2N4Pd
413.58
monoclinic
P21/c
5.1207(14)
24.454(3)
23.766(3)
90
93.217(2)
90
2971.3(9)
8
1.849
1.605
1632
1.67–25.01
15239
5218, 0.0458
1.138
0.0509, 0.0916
0.0677, 0.0971
0.702, -0.995
Crystal system
Space group
P-1
P-1
˚
a, A
11.0400(10)
12.9230(13)
13.5160(16)
111.148(2)
101.0820(10)
107.4680(10)
1614.7(3)
2
1.643
0.773
792
1.72–25.01
8405
13.2820(13)
13.3260(13)
13.4030(15)
67.8290(10)
65.292(2)
72.3640(10)
1965.8(3)
2
1.607
0.649
952
1.71–25.01
10131
˚
b, A
˚
c, A
a, deg
b, deg
g , deg
3
˚
V, A
Z
D
calc, Mg m-3
m, mm-1
F(000)
q range
Reflections collected
Reflections Unique, Rint
Goodness-of-fit on F2
R, [I > 2sI]
R, (all data)
5582, 0.0179
1.025
0.0459, 0.1050
0.0662, 0.1171
0.744, -0.314
6820, 0.0202
1.059
0.0500, 0.1297
0.0658, 0.1486
0.869, -0.576
-3
˚
Largest diff. Peak and hole, e A
7052 | Dalton Trans., 2009, 7045–7054
This journal is
The Royal Society of Chemistry 2009
©

by direct methods, and the non-hydrogen atoms were subjected to
anisotropic refinement by full-matrix least squares on F2 using the
SHELXTXL package.23 Hydrogen atom positions for all of the
structures were calculated and allowed to ride on their respective
R. A. Kelly, E. D. Stevens, F. Naud, M. Studer and S. P. Nolan, Org.
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Ed. Engl., 1998, 37, 2490; (e) R. E. Giudici and A. H. Hoveyda, J. Am.
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T. T. Salguero, M. W. Day and R. H. Grubbs, J. Am. Chem. Soc., 2007,
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J. Am. Chem. Soc., 2002, 124, 914; (b) W. Li, H. Sun, M. Chen, Z.
Wang, D. Hu, Q. Shen and Y. Zhang, Organometallics, 2005, 24, 5925;
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1996, 118, 784; (d) E. Jellema, P. H. M. Budzelaar, J. N. H. Reek and
B. de Bruin, J. Am. Chem. Soc., 2007, 129, 11631; (e) G. W. Nyce, T.
Glauser, E. F. Connor, A. Mock, R. M. Waymouth and J. L. Hedrick,
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˚
C atoms with C–H distances of 0.93–0.97 A and Uiso(H) = -1.2–
1.5Ueq(C). For complex 1, several restraints have been used to
model the butyl groups, and the carbon atoms of butyl groups
and fluorine atoms of the anions have been isotropically refined
with occupancies of 1.0. For complex 4, disordered acetonitrile
molecules in its lattice could not be modelled successfully and
were removed from the reflection data with SQUEEZE24 with a
3
˚
solvent accessible void volume 305.2 A . Further details of the
structural analyses are summarized in Table 4.
Computational details
Full molecular geometry optimizations have been carried out
at the Becke3LYP (B3LYP) level of density functional theory
(DFT).25 Frequency calculations at the same level of theory have
also been performed. The effective core potentials (ECPs) of Hay
and Wadt with double-f valence basis sets (LanL2DZ)26 were used
to describe Pd. Polarization functions were added for C (z(d) =
0.8), H (z(d) = 0.11).27 The 6-31G basis set was used for all the
atoms except Pd.28 All the calculations were performed with the
GAUSSIAN 98 software package.29 In all the energy profiles, the
calculated electronic energies were used to describe the energetic
aspects. The computational method and the basis sets used in this
work have been widely recognized in theoretically investigating
structures, bonding and reaction mechanisms of organometallic
systems.
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Acknowledgements
We are grateful to the support of the NSF of China (20872129
and J0830413), the Ph.D. Programs Foundation of Ministry of
Education of China (200803350011), and Zhejiang University
K. P. Chao’s High Technology Development Foundation for
financial support.
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