Dinuclear NHC-palladium complexes containing phosphine spacers: Synthesis, X-ray structures and their catalytic activities towards the Hiyama coupling reaction

Article abstract of DOI:10.1039/c4dt00180j

Six dinuclear N-heterocyclic carbene (NHC) palladium complexes, [PdCl ₂(IMes)]₂(μ-dppe) (1), [PdCl₂(IPr)] ₂(μ-dppe) (2), [PdCl₂(IMes)]₂(μ-dppb) (3), [PdCl₂(IPr)]₂(μ-dppb) (4), [PdCl ₂(IMes)]₂(μ-dpph) (5), and [PdCl₂(IPr)] ₂(μ-dpph) (6) [IMes = N,N′-bis-(2,4,6-trimethylphenyl) imidazol-2-ylidene; IPr = N,N′-bis-(2,6-di(iso-propyl)phenyl)imidazol-2- ylidene; dppe = 1,2-bis(diphenylphosphino)ethane, dppb = 1,4- bis(diphenylphosphino)butane; and dpph = 1,6-bis(diphenylphosphino)hexane], have been synthesized through bridge-cleavage reactions of chloro-bridged dimeric compounds, [Pd(μ-Cl)(Cl)(NHC)]₂, with the corresponding diphosphine ligands. The obtained compounds were fully characterized by ¹H NMR, ¹³C NMR and ³¹P NMR spectroscopy, FT-IR, elemental analysis and single-crystal X-ray crystallography. Moreover, further explorations of the catalytic potential of the dinuclear carbene palladium complexes as catalysts for the Pd-catalyzed transformations have been performed under microwave irradiation conditions, and the complexes exhibited moderate to good catalytic activity in the Hiyama coupling reaction of trimethoxyphenylsilane with aryl chlorides.

Full text of DOI:10.1039/c4dt00180j

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Dinuclear NHC–palladium complexes containing
phosphine spacers: synthesis, X-ray structures and
their catalytic activities towards the Hiyama
coupling reaction†
Cite this: Dalton Trans., 2014, 43,
7166
a
a
a
a,b
Jin Yang, Pinhua Li, Yicheng Zhang and Lei Wang*
Six dinuclear N-heterocyclic carbene (NHC) palladium complexes, [PdCl
2
(IMes)]
(µ-dppb) (4), [PdCl (IMes)]
(µ-dpph) (6) [IMes = N,N’-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr = N,N’-bis-
2
(µ-dppe) (1),
2
[PdCl (IPr)]
2
(µ-dppe) (2), [PdCl
2
(IMes)]
2
(µ-dppb) (3), [PdCl
2
(IPr)]
2
2
2
(µ-dpph) (5),
and [PdCl
2
(IPr)]
2
(
(
2,6-di(iso-propyl)phenyl)imidazol-2-ylidene; dppe = 1,2-bis(diphenylphosphino)ethane, dppb = 1,4-bis-
diphenylphosphino)butane; and dpph = 1,6-bis(diphenylphosphino)hexane], have been synthesized
through bridge-cleavage reactions of chloro-bridged dimeric compounds, [Pd(μ-Cl)(Cl)(NHC)]
2
, with the
13
1
corresponding diphosphine ligands. The obtained compounds were fully characterized by H NMR,
C
3
1
NMR and P NMR spectroscopy, FT-IR, elemental analysis and single-crystal X-ray crystallography. More-
over, further explorations of the catalytic potential of the dinuclear carbene palladium complexes as cata-
lysts for the Pd-catalyzed transformations have been performed under microwave irradiation conditions,
and the complexes exhibited moderate to good catalytic activity in the Hiyama coupling reaction of tri-
methoxyphenylsilane with aryl chlorides.
Received 18th January 2014,
Accepted 26th February 2014
DOI: 10.1039/c4dt00180j
www.rsc.org/dalton
metal in solution and probably render the intermediate cat-
Introduction
ionic palladium compounds poorly electrophilic, thereby
3
Since the successful isolation of the first stable carbene by disfavoring the nucleophilic addition step. Therefore,
Arguengo in 1991, N-heterocyclic carbenes (NHCs) have different strategies have been adopted in order to modify the
attracted increasing attention and have been intensively carbene complexes. One strategy consists in tuning the elec-
1
explored as spectator ligands in transition metals. The strong tronic properties of the carbene ligands by introducing suit-
metal–carbene σ-bond provides numerous opportunities for able functional groups in the carbene ligands. Recently, a
the development of transition metal catalysts. In particular, number of modified carbene ligands with various functional
carbene palladium complexes derived from imidazolium pre- groups and the corresponding palladium complexes have been
4
cursors have been successfully developed as highly active pre- efficiently prepared and applied in cross-coupling reactions.
catalysts in a wide range of organic transformations including Another strategy is to introduce an ancillary ligand into the
Buchwald–Hartwig amination, Heck olefin arylation, and metal core, thereby enhancing the potential for hemilability
Suzuki, Kumada, Hiyama, Stille, Negishi and Sonogashira and heterometallation of the carbene ligands. The introduc-
2
coupling reactions of chloroarenes. However, NHC ligands in tion of an ancillary ligand at the metal core could be a syn-
palladium complexes, which have been compared to the thetically convenient strategy for tuning the electronic and
classic trialkylphosphine ligands, appear to be stronger coordi- steric properties of the coordination sphere. Recent studies
nating ligands which undergo little dissociation from the have illustrated that the provision of a functional ancillary
ligand to complement the strongly binding NHCs could
promote a reversible coordination and dissociation mechan-
ism which is highly suitable for catalytic applications.
a
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000,
5
PR China. E-mail: leiwang@chnu.edu.cn; Fax: +86-561-309-0518;
Towards this aim, various heteroatom donors have been used
to join in the coordination with the NHC–Pd complexes.
Tel: +86-561-380-2069
b
6
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China
Organ has reported the first pyridine type ligands modified
carbene palladium complexes as active catalysts for a series of
cross-coupling (Suzuki, Negishi, Kumada, Stille and Buch-
†
Electronic supplementary information (ESI) available: Selected bond lengths
1
13
31
and angles for compounds 1–6 and H, C and P NMR spectra of compounds
–6. CCDC 950139–950142, 971319, 971320. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4dt00180j
1
7
wald–Hartwig amination) reactions. Nolan and Cazin have
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1
3
31
reported a series of tertiary phosphine modified carbene palla- the selected C NMR and P NMR data were summarized in
dium complexes as active catalysts for Suzuki cross-coupling Table 1. The formation of the dinuclear complexes was evident
8
and other reactions. Hahn has reported a series of novel from the distinctive stoichiometric proton signal resonances of
macrocyclic and multinuclear carbene complexes which were the N-heterocyclic carbene ligands and diphosphine ligands in
9
1
13
modified by phosphine ligands. The use of heteroatom the H NMR spectra. In addition, the C NMR spectra revealed
donors to support and activate the metal for catalysis is an the appearance of diagnostic carbene carbon doublet peaks
emerging idea in coordination catalysis. Furthermore, follow- (171.0 and 168.9 ppm for 1, 173.6 and 171.7 ppm for 2, 171.6
ing the initial reports of mononuclear N-heterocyclic carbene and 169.6 ppm for 3, 173.7 and 171.7 ppm for 4, 171.4 and
palladium complexes, di- and multi-nuclear N-heterocyclic 169.5 ppm for 5, 173.6 and 171.7 ppm for 6, respectively),
carbene complexes containing various bridging ligands con- which were split by the adjacent P donors. These values were
1
0
tinue to attract a great deal of attention. The use of different significantly shifted downfield relative to that of the imidazoli-
spacers with different liabilities in a multinuclear framework nium NCHN peak of the starting ligand precursors (141.0 ppm
1
0k
could provide a model to test the catalytic benefits of dissocia- for IMes·HCl and 145.0 ppm for IPr·HCl),
tive ligand and non-interacting multi-metallic catalytic sites. expected close to the values found in the mononuclear
We have recently reported a simple route to the synthesis of N-heterocyclic carbene (NHC)/PPh palladium complex
dinuclear NHC–palladium complexes with bridging N-hetero- PdCl (IPr)PPh (169.8 and 172.4 ppm). The important
but were as
3
8
a
2
2
3
J
CP
1
0k
cyclic ligands. Following our interest in the construction of coupling constant (196.1 to 208.7 Hz) is characteristic of the
functionalized complexes that can enter an array of di- and trans-position of the phosphine ligands with respect to the
8
a,11
31
multi-nuclear systems, herein we extended the methodology to NHCs.
introduction of bridging diphosphine ligands into NHC–palla- showed a sharp singlet (13.0 to 16.5 ppm) shifted upfield
dium complexes and reported the synthesis and structural when compared to the mononuclear complex PdCl (IPr)PPh
Furthermore, P NMR spectra of all complexes
2
3
characterization of six diphosphine ligands bridged dinuclear (20.4 ppm), attributed probably to the more electron-rich of
N-heterocyclic carbene palladium complexes. Furthermore, the the PPh3.
catalytic applications of the dinuclear NHC–palladium com-
plexes in the Hiyama coupling reaction of trimethoxyphenyl-
silane with a range of aryl chlorides under microwave
Crystal structures
irradiation conditions were studied.
The molecular structures of 1–6 have been further character-
ized by single-crystal X-ray diffraction studies. A summary of
the crystallographic data is provided in Table 2 and selected
bond lengths and angles are given in Table S1 (ESI†). As
expected, the crystal structures of 1–6 showed a dinuclear
framework with two palladium(II) centers that were held
Results and discussion
Synthesis of NHC–palladium complexes
Following the methods for the synthesis of mononuclear N- together by a bridging diphosphine ligand. Each palladium
8
a,11
heterocyclic carbene (NHC)/PR3 palladium(II) complexes,
reactions of the chloro-bridged dimeric compounds [Pd(μ-Cl)- phosphorus atom and two chloro ions in a trans-arrangement,
Cl)(NHC)] with the bridging diphosphine ligands in CH Cl at giving rise to a four coordinate environment (Scheme 1). In
the ambient temperature were carried out, and the corres- agreement with the spectral data, the incoming P donors were
ponding dinuclear NHC–palladium complexes [PdCl (IMes)]
invariably trans to the NHC ligands which are similar to that
µ-dppe) (1), [PdCl (IPr)] (µ-dppe) (2), [PdCl (IMes)] (µ-dppb) (3), observed in the mononuclear N-heterocyclic carbene palla-
center was coordinated by an N-heterocyclic carbene ligand, a
(
2
2
2
2
2
-
(
2
2
2
2
[
[
PdCl (IPr)] (µ-dppb) (4), [PdCl (IMes)] (µ-dpph) (5), and dium complexes. Both of the compounds 1 and 2 crystallize as
2
2
2
2
PdCl
2
(IPr)]
2
(µ-dpph) (6) were obtained in good yields. The syn- CH Cl solvates. The structure of solvate 1·2CH Cl crystallizes
2
2
2
2
thetic route is straightforward, and all complexes have been iso- in the monoclinic space group P2(1)/c, while the i-Pr-substi-
lated as yellow and air-stable solids. Moreover, we have tuted analogue of the solvate 2·2CH Cl belongs to the triclinic
2
2
attempted to carry out the N-heterocyclic carbene palladium
complex formation through one-pot reactions from their
respective imidazolium chloride salts by reaction with PdCl2
13
31
Table 1 Selected C and P NMR spectral data of 1–6
and the bridging diphosphine ligands in the presence of K
CO
2 3
13
a
31
a
C NMR (Ccarbene
)
2 2 n 2
P NMR [(Ph P(CH ) ]
as a base in various solvents (e.g., THF, CH CN, toluene and
3
2
Complex
δ
C
J
CP
δ
P
dioxane), but the reaction resulted in rapid decomposition of
the reactants. Thus, it appeared that the one-pot reaction is
unsuitable for the synthesis of the diphosphine-bridged di-
nuclear N-heterocyclic carbene palladium complexes.
1
2
3
4
5
6
170.0
172.6
170.6
172.7
170.4
172.7
208.7
196.9
197.4
196.1
197.4
196.4
16.5
16.5
13.7
13.5
13.5
13.0
NMR studies
The diphosphine-bridged N-heterocyclic carbene palladium
a 13
31
3
C and P NMR spectra were recorded in CDCl at 298 K (δ in ppm,
complexes 1–6 were first characterized by NMR analysis and J in Hz).
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Table 2 Crystallographic data for compounds 1–6
Compound
1·2CH
2
Cl
2
2·2CH Cl
2 2
3
4
5
2
6·H O
Formula
fw
C
70
H
76Cl
8 4
N P
2
Pd
2
C
82
H
100Cl
8
4 2
N P Pd
2
C
70
H
76Cl
4 4
N P
Pd
2 2
C
82
H
100Cl
4
N
4 2
P Pd
2
C
72
H
80Cl
4 4
N P
Pd
2 2
84 4 4 2 2
C H106Cl N OP Pd
1604.27
1531.69
1700.00
1389.89
1558.20
1417.94
Crystal system
Space group
Monoclinic
P2(1)/c
12.0625(11)
24.805(2)
13.2469(13)
90.00
114.600(2)
90.00
3603.9(6)
2
Triclinic
Monoclinic
P2(1)/n
10.996(8)
20.067(15)
15.377(11)
90.00
104.080(12)
90.00
3291(4)
2
Orthorhombic
Pna21
29.940(3)
13.7039(14)
21.536(2)
90.00
90.00
90.00
8835.9(15)
4
1.171
Orthorhombic
Pccn
11.988(7)
35.97(2)
15.244(9)
90.00
90.00
90.00
6573(7)
4
1.433
Monoclinic
P2(1)/n
9.8882(16)
20.486(3)
19.902(3)
90.00
94.177(3)
90.00
4020.8(11)
2
Pˉ1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
13.168(6)
13.479(7)
28.285(13)
99.380(7)
90.260(8)
119.210(7)
4303(4)
2
3
V/Å
Z
D
calc./cm−
3
1.411
1.312
1.403
1.325
F(000)
µ/mm−
GOF
Reflections
collected
1564
0.882
1.078
17 895
1756
0.746
1.146
21 896
1428
0.801
1.025
15 769
3240
0.604
1.114
55 671
2920
0.804
1.176
30 014
1672
0.666
1.049
20 356
1
Independent
reflections (Rint
Observed
6318 (0.0424)
4728
14 904 (0.2229)
10 500
5729 (0.0480)
4156
15 517 (0.0395)
13 055
5682 (0.0904)
4614
7060 (0.0284)
5943
)
reflections
[
I > 2σ(I)]
Refined
410
875
364
863
361
411
parameters
R
1
[I > 2σ(I)]
0.0447
0.1283
0.0828
0.2676
0.0582
0.2064
0.0445
0.1327
0.1191
0.2634
0.0334
0.0860
wR (all data)
2
Scheme 1 Overview of the synthesis of the dinuclear N-heterocyclic
carbene (NHC)–palladium complexes 1–6.
space group P ˉ1 . There are two independent molecules in a
unit cell of the solvate 2·2CH Cl , only one molecule is shown
2
2
in Fig. 2. The molecular structures of 1 and 2 which are com-
posed of P–C–C–P skeletons are quite similar except the substi-
tuted groups of the carbene ring. The predominant molecular
Fig. 1 ORTEP diagram of 1 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms and solvent molecules
shape for 1 and 2 is a zigzag chain structure of the P–C–C–P (CH Cl ) have been omitted for clarity. The suffix A in 1 denotes sym-
2
2
metry operation 1 − x, 1 − y, 1 − z.
skeleton in which the NHC groups are oriented to opposite
directions of each other. The two carbene planes adopt a paral-
lel but non-coplanar orientation with the vertical separation of
2
.62 Å for 1 and 2.31 Å, 2.36 Å for 2. As shown in Fig. 1 and 2, to 93.83(5)°. The dihedral angles between the carbene ring
each Pd center is four-coordinated by an N-heterocyclic planes and the PdCNCl coordination planes amount to
2
carbene ligand, a P donor from the diphosphine ligand and 69.53°, 77.33° and 77.50°, which are typical for NHC com-
two chloro ions in a slightly distorted square planar geometry, plexes to relieve steric congestion. The phosphine ligand acts
with angles between adjacent ligands ranging from 85.73(4)° as bridges linking the palladium centers to form dinuclear
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Fig. 3 ORTEP diagram of 3 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms have been omitted for clarity.
The suffix A in 3 denotes symmetry operation 1 − x, −y, −z.
Fig. 2 ORTEP diagram of 2 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms and solvent molecules
(
2 2
CH Cl ) have been omitted for clarity. The suffix A in 2 denotes sym-
metry operation 1 − x, 1 − y, 1 − z.
NHC–palladium complexes with Pd⋯Pd separation of 6.97 Å
for 1 and 7.21 Å, 7.24 Å for 2, respectively. However, there are
some minor structural differences between 1 and 2. The di-
hedral angle between the carbene ring plane and the plane
defined by Pd1P1P1APd1A in compound 1 is only 38.34°,
which is significantly shorter than that in compound 2 (69.93°
and 70.12°), and might be caused by the steric repulsion of the
i-Pr groups. The corresponding dihedral angle between the
PdCNCl
compound 1 (72.16°) is significantly larger than that found in
(33.52° and 33.57°).
2
coordination plane and the Pd1P1P1APd1A plane in
2
The reactions of the dimeric compounds [Pd(μ-Cl)(Cl)- Fig. 4 ORTEP diagram of 4 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms have been omitted for clarity.
2
(NHC)] with the bridging diphosphine ligand dppb yield the
compounds 3 and 4. The Mes-substituted compound 3 crystal-
lizes in the monoclinic space group P2(1)/c and the i-Pr-substi-
tuted analogue 4 crystallizes in the orthorhombic space group dihedral angle of 72.22°. In compound 3, the Pd1P1P1APd1A
Pna21. X-Ray structure analysis also shows zigzag chain struc- plane is oriented at dihedral angles of 30.89° and 67.23° with
tures of the dppb with the distances of two corresponding respect to the carbene ring plane and the PdCNCl2 coordi-
Pd⋯Pd being 8.52 Å for 3 and 8.93 Å for 4. The dihedral nation plane. These values are somewhat shorter than that in
angles between the carbene ring planes and the PdCNCl2 compound 1 (38.34° and 72.16°). In compound 4, the
coordination planes amount to 82.55° for compound 3, and Pd1P1P1APd1A adopts a non-coplanar orientation with a
6
8.69°, 78.51° for 4, respectively (Fig. 3 and 4). Although com- torsion angle of 44.34°.
pounds 3 and 4 possess the same four hydrocarbon chains of
For the diphosphine ligand with a P–C–C–C–C–C–C–P back-
the diphosphine ligand, the vectors of the two NHC–Pd groups bone, single crystals were obtained for the Mes and i-Pr-substi-
for these molecules are significantly different. In compound 3, tuted derivatives 5 and 6, which confirmed the dinuclear
the carbene ring planes adopt a parallel orientation (a vertical NHC–palladium structures. The Mes-substituted compound 5
separation of 4.03 Å) with opposite directions, which is similar crystallizes in the orthorhombic space group Pccn, and the
to that observed in compounds 1 and 2. The molecular struc- i-Pr-substituted analogue 6 crystallizes as H O solvates 6·H O,
2
2
ture of 4 which possesses the i-Pr-substituted ligand is also which belongs to the monoclinic space group P2(1)/n. X-Ray
composed of a zigzag chain structure of the P–C–C–C–C–P structure analysis also shows zigzag chain structures of the
backbone; however, the two carbene ring planes point to the dpph with the distances of two corresponding Pd⋯Pd being
same direction and are arranged in a twisted orientation with 9.87 Å for 5 and 11.43 Å for 6 (Fig. 5 and 6). The two NHC–Pd
a dihedral angle of 33.39°. The two corresponding PdCPCl
2
groups in 5 and 6 are bridged by one dpph ligand and are
coordination planes also point to the same direction with a oriented to opposite directions of each other, respectively,
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plane with the dihedral angle of 89.96°, which is significantly
larger than that found in compound 5. Moreover, the dihedral
angle between the PdCNCl
2
coordination plane and the
Pd1P1P1APd1A plane in compound 5 (62.84°) is somewhat
shorter than that found in compounds 1 and 3, but is signifi-
cantly larger than that found in compound 6 (30.46°), and as
mentioned above, these differences might be attributed to the
larger steric hindrance of the ortho iso-propyl groups in 6 than
the methyl groups in analogue 5.
The detailed structural analysis indicates that the structural
parameters around the palladium centers, such as the bond
angles and bond distances, are in a range similar to those for
Fig. 5 ORTEP diagram of 5 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms have been omitted for clarity. mononuclear NHC–palladium complexes. The Pd–Ccarbene
The suffix A in 5 denotes symmetry operation −x, −y, 1 − z.
bonds [2.043(5) Å for 1, 2.032(10), 2.036(9) Å for 2, 2.031(6) Å
for 3, 2.047(5), 2.041(5) Å for 4, 1.992(11) Å for 5, and 2.038(3)
Å for 6] and Pd–P bonds [2.3177(12) Å for 1, 2.288(3), 2.307(3)
Å for 2, 2.317(2) Å for 3, 2.3107(15) Å, 2.3067(15) Å for 4,
2.278(3) Å for 5, and 2.3004(8) Å for 6] in compounds 1–6 are
comparable to those found in mononuclear phosphine/
N-heterocyclic carbene–palladium complexes PdCl (NHC)PPh
2
3
[
e.g., (Pd–Ccarbene: 2.028(5) Å and Pd–P: 2.3137(6) Å for
PdCl (SIMes)-PPh (SIMes = N,N′-bis(2,4,6-trimethylphenyl)-
imidazolidin-2-ylidene)
P: 2.3054(13) Å for PdCl
2
3
1
1b
and Pd–Ccarbene: 2.032(5) Å and Pd–
7
a
2
3
(IPr)PPh ]. The Pd–Cl bonds fall in a
narrow range of 2.193(4)–2.309(3) Å and are comparable to
those found in the N-heterocyclic carbene–palladium com-
8
plexes. Furthermore, the carbene ring planes of all complexes
2
are twisted out of the adjacent PdCPCl coordination planes
with the dihedral angles ranging from 59.73° to 82.55° in
order to avoid intra-ligand repulsion. However, compared to
the mononuclear complexes, there was no clear trend for the
steric hindrance influence in dinuclear complexes 1–6. For
example, the dihedral angle of 69.53° between the carbene
ring plane and the PdCNCl
2
plane in compound 1 is somewhat
shorter than that in the mononuclear complex PdCl (SIMes)
2
1
1b
PPh
3
(74.91°).
With the same Mes-substituent, the PdCPCl
2
coordination plane of the compound 3 is oriented approxi-
mately perpendicular to the carbene ring plane with a dihedral
angle of 82.55°, which is significantly larger than the mono-
nuclear complex PdCl
2 3
(SIMes)PPh . In compound 5, the di-
Fig. 6 ORTEP diagram of 6 with thermal displacement parameters
hedral angle between the carbene ring plane and the PdCNCl2
drawn at 30% probability. Hydrogen atoms and solvent molecules (H
have been omitted for clarity. The suffix A in 6 denotes symmetry oper- plane (76.96°) is slightly larger than that in the mononuclear
ation −x, 2 − y, −z.
2
O)
complex PdCl (SIMes)PPh . With the i-Pr-substituent NHC–
palladium complexes, the dihedral angles between the
PdCPCl coordination plane and the carbene ring are signifi-
2
3
2
which are similar to that found in compounds 1–3. The verti- cantly different, for example, the dihedral angles between the
cal distance between the two parallel carbene ring planes in carbene ring plane and the PdCNCl2 plane in compound 2
compound 5 (2.40 Å) is significantly shorter than that in com- (77.33° and 77.50°), which fall in the range of 1 (69.53°) and 3
pound 6 (9.42 Å), which is attributed to the more extended di- (82.55°), but are significantly larger than that in the mono-
hedral angle between the carbene ring plane and the nuclear i-Pr-substituent NHC–palladium complex PdCl (IPr)-
2
7
a
Pd1P1P1APd1A plane of compound 6. In compound 5, the di- PPh3 (67.78°). In compound 4, although the two carbene
hedral angles between the carbene ring plane and the ring segments have seemingly identical chemical environ-
Pd1P1P1APd1A plane are significantly shorter (14.62°) than ments, the coordination geometries of the two Pd atoms are
that found in the Mes-substituted analogues 1 (38.34°) and 3 slightly different, with the dihedral angles between carbene
(30.89°). However, in compound 6, the carbene ring plane is ring planes and the coordination planes amounting to 68.69°
oriented approximately perpendicular to the Pd1P1P1APd1A and 78.51°, respectively. For the compound 6, the dihedral
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angle between the carbene ring plane and the PdCNCl
2
plane Table 3 Hiyama reaction of trimethoxyphenylsilane with aryl chlorides
a
catalyzed by 1–6
(59.73°) is shorter than that for all the above mentioned mono-
and dinuclear complexes.
Catalytic tests
The palladium-catalyzed Hiyama coupling reaction is one of
the most powerful methods for the preparation of biaryl
derivatives. A great deal of effort has been directed towards the
development of efficient catalytic systems for the Hiyama reac-
Cat. loading
[mol%]
b
Entry
Aryl chloride
[Pd]
Yield (%)
1
2
3
4
5
6
7
8
9
R = 4-NO
R = 4-NO2
2
1
2
3
4
5
6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
84
88
83
90
87
89
92
82
85
84
88
87
88
90
78
83
80
82
84
83
67
70
65
73
63
69
55
60
63
62
57
60
1
2
tion. Recently, some N-heterocyclic carbene palladium com-
R = 4-NO
R = 4-NO
R = 4-NO
R = 4-NO
R = 4-NO
R = 4-CN
R = 4-CN
R = 4-CN
R = 4-CN
R = 4-CN
R = 4-CN
R = 4-CN
2
2
2
2
2
plexes have been used in the Hiyama coupling reaction with
1
3
high activities.
Microwave-assisted organic synthesis is
advantageous for enabling rapid, reproducible, and scalable
chemistry development, which has been extensively used for
c
1
1
2
3
4
5
6
1
4
carrying out chemical reactions. It was envisaged that micro-
wave irradiation would enhance the rate of reaction, thereby
reducing the reaction time. Herein, the catalytic activities of
10
11
12
the dinuclear NHC–Pd complexes for the Hiyama coupling 13
reaction were examined. Compounds 1–6 are catalytically
active towards the Hiyama coupling reaction of aryl chloride
with trimethoxyphenylsilane in the presence of TBAF (tetra- 17
butylammonium fluoride) in toluene under microwave
irradiation conditions, yielding the corresponding biphenyl
products given in Table 3. In general, all the compounds 21
give rise to highly active catalysts for the aryl chloride sub-
strates with electron-withdrawing groups, such as 4-chloroni-
trobenzene giving good yields within 30 min at 120 °C under 25
microwave irradiation (Table 3, entries 1–6). When the reaction
was carried out with oil-bath heating at 120 °C, a slightly
higher yield (92%) was obtained; however, the reaction time 29
must be prolonged to more than 6 h (Table 3, entry 7). For the
activated substrate, 4-chlorobenzonitrile and 4-chloroacetophe-
none, as expected, gave better coupling yields (78–88%) than
chlorobenzene (63–73%) (Table 3, entries 8–26). For the less
active electron-donating 4-chlorotoluene, the yields of the pro-
ducts dropped to 55–63% (Table 3, entries 27–32). It has to be for 30 min. Isolated yield. The reaction was carried out with an oil-
pointed out that the difference in yield according to the sub-
c
14
15
16
1
R = 4-COCH
R = 4-COCH
R = 4-COCH
R = 4-COCH
R = 4-COCH
R = 4-COCH
R = H
3
3
3
3
3
3
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
1
2
8
9
0
22
23
24
R = H
R = H
R = H
R = H
26
27
28
R = H
R = 4-CH
R = 4-CH
R = 4-CH
R = 4-CH
R = 4-CH
R = 4-CH
3
3
3
3
3
3
3
3
3
0
1
2
a
Reaction conditions: trimethoxyphenylsilane (0.30 mmol), aryl
chloride (0.25 mmol), [Pd] catalyst (containing 1.0 mol% Pd), TBAF
0.50 mmol) in toluene (1.0 mL) at 120 °C under microwave irradiation
(
b
c
bath heating at 120 °C and stirred for 6 h.
strate results from the electrophilic character of the aryl chlor-
ides. Highest yields are achieved for the aryl chlorides with
electron-withdrawing groups. In addition, the sterically bulky
di(iso-propyl) derivatives (compounds 2, 4 and 6) consistently
performed better with higher yields than their trimethyl
NHC–palladium complexes, this kind of dinuclear complexes
would have potential uses in catalysis.
counterparts (compounds 1, 3 and 5), which is in agreement
1
5
with the result reported in the literature.
Experimental
General considerations
Conclusions
2
The dimeric palladium complexes [Pd(μ-Cl)(Cl)(NHC)] were
In summary, a series of dinuclear N-heterocyclic carbene prepared using a method previously reported in the litera-
1
6
(
NHC)–palladium complexes 1–6 bridged by the diphosphine ture. The chemicals were purchased from commercial suppli-
ligands have been prepared by reacting the chloro-bridged ers and were used without purification prior to use except
1
13
31
dimeric carbene complexes with the respective bridging where otherwise indicated. All H, C and P NMR were per-
ligand. The catalytic behaviors of the diphosphine-bridged formed in CDCl and recorded on a Bruker Avance 400 NMR
3
dinuclear NHC–palladium complexes in the Hiyama coupling spectrometer with tetramethylsilane (TMS) as an internal stan-
reaction were investigated and generated the corresponding dard. IR spectra were recorded on a Bruker IFS 120 HR spectro-
products in moderate to good yields. As a derivative of the meter as KBr disks. Elemental analyses were performed on a
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7166–7175 | 7171

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Paper
Dalton Transactions
1
3
2 2
Vario El III elementar. Flash column chromatography was 2.19 (br, 4H, CH ), 1.94 (br, 4H, CH ). C NMR (100 MHz,
2
carried out using a 300–400 mesh silica gel.
CDCl ): δ = 170.6 (d, J = 197.4 Hz, Ccarbene), 138.5 (o-CH -C
3
C,P
3
4
Ar), 136.3 (d, JC,P = 3.4 Hz, N-CAr), 135.5 (p-CH
C,P = 10.2 Hz, CH Ph), 130.8 (d,
PdCl (IMes)] (μ-dppe) (1). A mixture of the dimeric 129.6 (d, J = 2.1 Hz, CH Ph), 128.9 (CH Ph), 127.7 (d, J
3
-C Ar), 133.6
Synthesis
2
1
(
d,
J
J
C,P = 42.5 Hz, CPh),
4
4
[
=
C,P
2
2
C,P
4
complex [Pd(μ-Cl)(Cl)(IMes)]
2
(96 mg, 0.10 mmol) and dppe 10.0 Hz, CH Ph), 123.1 (d, JC,P = 5.8 Hz, N–CHvCH–N), 30.9
3
1
(40 mg, 0.10 mmol) was dissolved in CH
2
Cl
2
(5 mL) and (PCH
2
CH
2
), 21.2 (p-CH
3
), 19.0 (o-CH
3
), 14.1 (PCH
2
CH
2
).
P
−1
stirred at ambient temperature overnight. The reaction mixture NMR (121.5 MHz, CDCl ): δ = 13.7. IR (KBr, cm ): 2964, 2867,
3
was filtered over Celite, the solvent was reduced under vacuum 1486, 1433, 1403, 1380, 1331, 1276, 1179, 1105, 1058, 1025,
and a yellow precipitate formed on addition of n-hexane 945, 909, 849, 799. Anal. Calc. for [PdCl
2 2
(IMes)] (μ-dppb)
(
10 mL). The solid was filtered off, washed with n-hexane, and (C H Cl N P Pd ): C, 60.49; H, 5.51; N, 4.03%. Found: C,
7
0
76
4
4
2
2
dried under vacuum; yield 117 mg (86%). Crystals suitable for 60.74; H, 5.17; N, 4.15%.
X-ray analysis were obtained by a slow diffusion of n-hexane [PdCl (IPr)] (μ-dppb) (4). Compound 4 was synthesized by a
into a dichloromethane solution of the product. H NMR similar method for the synthesis of 2 except that dppb was
400 MHz, CDCl ): δ = 7.31–7.24 (m, 12H), 7.17 (t, J = 7.2 Hz, used instead of dppe. Yield: 134 mg (86%). Crystals suitable
H), 7.02 (s, 4H), 6.96 (s, 8H), 2.29–2.28 (m, 40H). C NMR for X-ray analysis were obtained by a slow diffusion of
2
2
1
(
8
3
1
3
2
1
(100 MHz, CDCl ): δ = 170.0 (d, J = 208.7 Hz, Ccarbene), 138.4 n-hexane into a dichloromethane solution of the product. H
3 C,P
2
(
o-CH
3
-CAr), 136.2 (N-CAr), 135.4 (p-CH
0.4 Hz, CH Ph), 130.3 (d, JC,P = 42.4 Hz, CPh), 129.4 (CH Ph), 7.2 Hz, 8H), 7.11 (s, 4H), 3.52–3.47 (m, 4H, PCH
28.8 (CH Ph), 127.5 (d, J = 9.9 Hz, CH Ph), 123.0 (d, J
.6 Hz, N–CHvCH–N), 21.6 (p-CH
3
-CAr), 133.6 (d, JC,P
=
NMR (400 MHz, CDCl
3
): δ = 7.37–7.24 (m, 24H), 7.20 (t, J =
CH ), 3.12
(sept, J = 6.4 Hz, 8H, CH(CH ) ), 1.83 (br, 4H, PCH CH ), 1.27
1
1
1
5
2
2
2
4
=
C,P
C,P
3 2
2
2
3
), 18.9 (o-CH
3
), 15.2 (PCH
2
). (d, J = 6.4 Hz, 24H, CH
3
), 1.07 (d, J = 6.8 Hz, 24H, CH
3
).
3
1
−1
13
2
P NMR (121.5 MHz, CDCl
3
): δ = 16.5. IR (KBr, cm ): 2920,
C NMR (100 MHz, CDCl ): δ = 172.7 (d, JC,P = 196.1 Hz,
3
2
2
858, 1488, 1435, 1406, 1330, 1280, 1230, 1106, 914, 854, 727. Ccarbene), 146.6 (iPr-CAr), 135.3 (N-CAr), 133.5 (d, JC,P =
1
Anal. Calc. for [PdCl
5
2
(IMes)]
2
(μ-dppe) (C68
H
72Cl
4
N
4
P
2
Pd
2
): C, 10.0 Hz, CH Ph), 130.8 (d, JC,P = 42.4 Hz, CPh), 129.6 (CH Ph),
4
9.97; H, 5.33; N, 4.11%. Found: C, 59.72; H, 5.07; N, 4.18%.
PdCl (IPr)] (μ-dppe) (2). A mixture of the dimeric complex 123.9 (d, J = 5.8 Hz, N–CHvCH–N), 123.5 (CH Ar), 28.5 (CH
129.4 (d, JC,P = 2.1 Hz, CH Ph), 127.6 (CH Ph), 127.5 (CH Ph),
4
[
2
2
C,P
3
1
[
Pd(μ-Cl)(Cl)(IPr)]
.10 mmol) was dissolved in CH
ambient temperature overnight. The reaction mixture was fil- 1466, 1433, 1406, 1380, 1364, 1274, 1120, 1100, 1058, 942, 908,
tered over Celite, the solvent was reduced under vacuum and a 800, 755, 736. Anal. Calc. for [PdCl (IPr)] (μ-dppb)
yellow precipitate formed on addition of n-hexane (10 mL). (C82 Pd ): C, 63.20; H, 6.47; N, 3.60%. Found: C,
The solid was filtered off, washed with n-hexane, and dried 62.97; H, 6.72; N, 3.43%.
under vacuum; yield 130 mg (85%). Crystals suitable for X-ray [PdCl (IMes)] (μ-dpph) (5). Compound 5 was synthesized by
2
(113 mg, 0.10 mmol) and dppe (40 mg, (CH
3
)
2
), 26.2 (CH(CH
3
)
2
), 22.8 (CH(CH
3 2 2 2
) ), 15.2 (PCH CH ). P
−
1
0
2
Cl
(5 mL) and stirred at NMR (121.5 MHz, CDCl ): δ = 13.5. IR (KBr, cm ): 2964, 2864,
2 3
2
2
H
100Cl
4
N
4
P
2
2
2
2
analysis were obtained by a slow diffusion of n-hexane into a a similar method for the synthesis of 1 except that dpph was
1
dichloromethane solution of the product. H NMR (400 MHz, used instead of dppe. Yield: 128 mg (91%). Crystals suitable
3
CDCl ): δ = 7.39 (t, J = 7.6 Hz, 4H), 7.27–7.19 (m, 20H), 7.14 (t, for X-ray analysis were obtained by a slow diffusion of
1
J = 7.2 Hz, 8H), 7.06 (s, 4H), 3.12 (sept, J = 6.4 Hz, 8H, n-hexane into a dichloromethane solution of the product. H
CH(CH ) ), 2.00 (br, 4H, CH ), 1.20 (d, J = 6.4 Hz, 24H, CH ), NMR (400 MHz, CDCl ): δ = 7.39–7.37 (m, 12H), 7.24 (t, J =
3
2
2
3
3
1
3
1
1
(
.03 (d, J = 6.8 Hz, 24H, CH
72.6 (d,
N-CAr), 133.9 (d, JC,P = 10.5 Hz, CH Ph), 130.3 (d, JC,P
2.8 Hz, CPh), 130.1 (d, JC,P = 1.8 Hz, CH Ph), 129.4 (d, JC,P
3
). C NMR (100 MHz, CDCl
3
): δ = 7.2 Hz, 8H), 7.05–7.03 (m, 12H), 2.41 (s, 12H, p-CH
), 2.07–2.00 (m, 4H, PCH CH CH ), 1.22 (br, 4H,
PCH CH CH ), 1.10 (br, 4H, PCH CH CH ).
3
), 2.32 (s,
2
J
C,P = 196.9 Hz, Ccarbene), 146.6 (iPr-CAr), 135.3 24H, o-CH
3
2
2
2
2
1
13
=
=
C
NMR
2
2
2
2
2
2
4
4
2
4
(100 MHz, CDCl ): δ = 170.4 (d, JC,P = 197.4 Hz, Ccarbene), 138.4
-CAr), 136.2 (N-CAr), 135.5 (p-CH
3
4
2
5
.5 Hz, CH Ph), 127.4 (d,
J
C,P = 9.9 Hz, CH Ph), 123.8 (d, (o-CH
3
3
-CAr), 133.5 (d, JC,P
=
4
1
4
JC,P = 5.5 Hz, N–CHvCH–N), 123.4 (CH Ar), 28.4 (CH(CH ) ), 10.1 Hz, CH Ph), 130.9 (d, J = 42.4 Hz, CPh), 129.5 (d, J
3 2
C,P
C,P
31
2
2
(
1
6.2 (CH(CH
3
)
2
), 22.8 (CH(CH
): δ = 16.5. IR (KBr, cm ): 2919, 2857, 1484, Ph), 123.0 (d,
435, 1407, 1329, 1275, 1229, 1103, 1028, 910, 853, 732. Anal. (PCH CH CH ), 22.6 (PCH CH CH ), 21.2 (p-CH3), 18.9
3 2 2
) ), 19.2 (PCH ). P NMR = 2.4 Hz, CH Ph), 128.8 (CH Ph), 127.5 (d, JC,P = 9.8 Hz, CH
−
1
4
121.5 MHz, CDCl
3
J
C,P
=
5.7 Hz, N–CHvCH–N), 30.5
2
2
2
2
2
2
3
1
Calc. for [PdCl
2
(IPr)]
2
(μ-dppe) (C80
H
96Cl
4
N
4
P
2
Pd
2
): C, 62.79; H, (o-CH
13.5. IR (KBr, cm ): 2964, 2868, 1465, 1431, 1380, 1331, 1278,
PdCl (IMes)] (μ-dppb) (3). Compound 3 was synthesized by 1170, 1125, 1069, 946, 819, 802, 736. Anal. Calc. for
3 2 2 2 3
), 14.0 (PCH CH CH ). P NMR (121.5 MHz, CDCl ): δ =
−1
6
.32; N, 3.66%. Found: C, 62.58; H, 6.17; N, 3.34%.
[
2
2
a similar method for the synthesis of 1 except that dppb was [PdCl
used instead of dppe. Yield: 125 mg (90%). Crystals for X-ray N, 3.95%. Found: C, 60.63; H, 5.47; N, 4.04%.
diffraction were obtained by a slow diffusion of diethyl ether [PdCl (IPr)] (μ-dpph) (6). Compound 6 was synthesized by a
2 2 4 4 2 2
(IMes)] (μ-dpph) (C72H80Cl N P Pd ): C, 60.98; H, 5.69;
2
2
1
into a dichloromethane solution of the product. H NMR similar method for the synthesis of 2 except that dppb was
400 MHz, CDCl ): δ = 7.35–7.31 (m, 12H), 7.23–7.19 (m, 8H), used instead of dppe. Yield: 141 mg (89%). Crystals suitable
.03–7.00 (m, 12H), 2.36 (s, 12H, p-CH ), 2.30 (s, 24H, o-CH ), for X-ray analysis were obtained by a slow diffusion of diethyl
(
3
7
3
3
7172 | Dalton Trans., 2014, 43, 7166–7175
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Dalton Transactions
Paper
1
ether into a dichloromethane solution of the product. H NMR data, data collection, and refinement parameters for com-
400 MHz, CDCl ): δ = 7.50 (t, J = 7.6 Hz, 4H), 7.35–7.31 (m, plexes 1–6 is provided in Table 2.
(
3
2
8
6
0H), 7.19 (t, J = 7.2 Hz, 8H), 7.13 (s, 4H), 3.12 (sept, J = 6.8 Hz,
H, CH(CH ), 2.00–1.93 (m, 4H, PCH CH CH ), 1.30 (d, J =
.8 Hz, 24H, CH ), 1.10 (d, J = 7.2 Hz, 24H, CH ), 0.95–0.90 (m,
3
)
2
2
2
2
3
3
Acknowledgements
1
3
8
J
H, PCH
2
2 2 3
CH CH ). C NMR (100 MHz, CDCl ): δ = 172.7 (d,
2
C,P = 196.4 Hz, Ccarbene), 146.6 (iPr-CAr), 135.4 (N-CAr), 133.5 Financial support from the National Natural Science Foun-
3
1
(d, J = 9.9 Hz, CH Ph), 130.9 (d, J = 42.2 Hz, CPh), 129.6 dation of China (no. 21172092 and 21301061) and the National
C,P C,P
4
2
(
CH Ph), 129.4 (d, JC,P = 2.4 Hz, CH Ph), 127.5 (d, JC,P = 9.8 Science Foundation of Anhui Educational Bureau (no.
4
Hz, CH Ph), 123.9 (d, JC,P = 5.9 Hz, N–CHvCH–N), 123.5 (CH KJ2013B251) is gratefully acknowledged.
Ar), 31.5 (PCH CH CH ), 28.5(CH(CH ) ), 26.2(CH(CH ) ), 22.8
2
2
2
3 2
3 2
3
1
(
(
1
PCH
121.5 MHz, CDCl
435, 1405, 1331, 1276, 1105, 1058, 1025, 945, 914, 849, 799.
Anal. Calc. for [PdCl (IPr)] (μ-dpph) (C84 Pd ): C,
3.60; H, 6.61; N, 3.53%. Found: C, 63.77; H, 6.42; N, 3.34%.
2
CH
2
CH
2
), 22.6 (CH(CH
3
)
2
), 14.0(PCH
2 2 2
CH CH ). P NMR
−
1
3
): δ = 13.0. IR (KBr, cm ): 2964, 2866, 1487,
Notes and references
2
2
H
104Cl
4
N
4
P
2
2
1 (a) F. Glorius, N-Heterocyclic Carbenes in Transition Metal
Catalysis, Springer, Berlin, 2007; (b) S. P. Nolan, N-Hetero-
cyclic Carbenes in Synthesis, Wiley-VCH, Weinheim, 2006;
6
General procedure for the NHC–Pd catalyzed Hiyama reaction
(
c) A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am.
The aryl chloride (0.25 mmol), trimethoxyphenylsilane
Chem. Soc., 1991, 113, 361; (d) W. A. Herrmann, Angew.
Chem., Int. Ed., 2002, 41, 1290; (e) F. E. Hahn, Angew.
Chem., Int. Ed., 2006, 45, 1348; (f) F. E. Hahn, Dalton
Trans., 2009, 6893 and following papers in this issue;
(g) C. S. J. Cazin, Dalton Trans., 2013, 42, 7254 and follow-
ing papers in this issue.
(0.30 mmol), NHC–Pd complex (containing 1.0 mol% Pd),
TBAF (0.50 mmol) and dry toluene (1.0 mL) were added into
an oven-dried microwave vial. The reaction mixture was irra-
diated in a microwave apparatus at 120 °C for 30 min. After the
reaction mixture was cooled to ambient 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.
2 For selected examples, see: (a) S. Díez-González, N. Marion
and S. P. Nolan, Chem. Rev., 2009, 109, 3612;
(b) G. C. Fortman and S. P. Nolan, Chem. Soc. Rev., 2011,
40, 5151; (c) S. P. Nolan and H. Clavier, Chem. Soc. Rev.,
2010, 39, 3305; (d) X. Bugaut and F. Glorius, Chem. Soc.
Rev., 2012, 41, 3511; (e) H. D. Velazquez and F. Verpoort,
Chem. Soc. Rev., 2012, 41, 7032; (f) S. J. Ryan, L. Candish
and D. W. Lupton, Chem. Soc. Rev., 2013, 42, 4906;
(g) Y. Ma, C. Song, W. Jiang, G. Xue, J. F. Cannon, X. Wang
and M. B. Andrus, Org. Lett., 2003, 5, 4635; (h) C. Yang,
H. M. Lee and S. P. Nolan, Org. Lett., 2001, 3, 1511;
(i) G. A. Grasa, A. C. Hillier and S. P. Nolan, Org. Lett., 2001,
3, 1077; ( j) H. M. Lee and S. P. Nolan, Org. Lett., 2000, 2,
2053; (k) G. Le Duc, S. Meiries and S. P. Nolan, Organo-
metallics, 2013, 32, 7547.
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).
1
7
Multi-scan corrections were applied using SADABS. Structure
solutions and refinements were performed with the SHELX-97
1
8
package. All non-hydrogen atoms were refined anisotropi-
2
cally by full-matrix least-squares on F . The hydrogen atoms to
carbon were included in idealized geometric positions with
thermal parameters equivalent to 1.2 times those of carbon
atoms. In compound 1, a disordered co-crystallized solvent
3 (a) M. J. Lappert, J. Organomet. Chem., 1975, 100, 139;
(b) T. Weskamp, F. J. Kohl, W. Hieringer, D. Gleich and
W. A. Herrmann, Angew. Chem., Int. Ed., 1999, 38, 2416;
(c) J. Schwarz, V. P. W. Böhm, M. G. Gardiner, M. Grosche,
W. A. Herrmann, W. Hieringerand and G. Raudaschl-
Sieber, Chem.–Eur. J., 2000, 6, 1773.
2 2
molecule CH Cl was refined over two positions with occu-
pancies of 0.57/0.43. In compound 4, the crystal lattice con-
3
tains solvent accessible voids of 327 Å ; however, the final
difference electron density map contained no chemically sig-
−
3
nificant peaks, the highest peak being 0.90 e Å at a distance
of 0.89 Å from the Pd atom and no model for any solvent could
be found. In compound 6, the unit cell includes disordered
solvent water molecules, which could not be modeled as dis-
crete atomic sites. We employed PLATON/SQUEEZE to calcu-
late the diffraction contribution of the solvent water molecules
and, thereby, to produce a set of solvent-free diffraction inten-
4 For selected examples, see: (a) J. C. C. Chen and I. J. B. Lin,
Dalton Trans., 2000, 839; (b) D. S. McGuinness and
K. J. Cavell, Organometallics, 2000, 19, 741; (c) J. C. C. Chen
and I. J. B. Lin, Organometallics, 2000, 19, 5113;
(d) X. Zhang, Q. Xia and W. Chen, Dalton Trans., 2009,
7045; (e) Y. Zhou and W. Chen, Organometallics, 2007, 26,
2742; (f) J. Ye, X. Zhang, W. Chen and S. Shimada, Organo-
metallics, 2008, 27, 4166; (g) X. Zhang, Z. Xi, A. Liu and
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sities. The SQUEEZE calculations showed a total solvent
3
accessible area volume of 200.3 Å and the residual electron
density amounted to 20 e per unit cell, corresponding to
nearly 2 molecules of H O. A summary of the crystallographic
2
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