A simple procedure for the covalent grafting of triphenylphosphine ligands on silica: Application in the palladium catalyzed Suzuki reaction

Article abstract of DOI:10.1039/b820449g

An organic-inorganic hybrid material derived from covalently grafting a palladium complex of the type (L)₂PdCl₂ (L = Si(OMe) ₃ functionalized PPh₃) on silica was synthesized, characterized and sucessfully applie

Full text of DOI:10.1039/b820449g

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A simple procedure for the covalent grafting of triphenylphosphine ligands
on silica: application in the palladium catalyzed Suzuki reaction†
Lei Wang,a,b Andreas Reis,a Andreas Seifert,c Thomas Philippi,a Stefan Ernst,a Mingjun Jiab and
Werner R. Thiel*a
Received 17th November 2008, Accepted 12th February 2009
First published as an Advance Article on the web 11th March 2009
DOI: 10.1039/b820449g
An organic–inorganic hybrid material derived from covalently grafting a palladium complex of the type
(L)2PdCl2 (L = Si(OMe)3 functionalized PPh3) on silica was synthesized, characterized and sucessfully
applied as a catalyst for the Suzuki reaction.
Palladium triphenylphosphine complexes have often been used
Introduction
for Heck and Suzuki couplings, mainly due to the simple
The palladium catalyzed Suzuki cross-coupling of aryl halides
handling of these complexes and the cheap phosphine donor
with arylboronic acids to form biaryls has emerged as an extremely
ligand. Depending on the substrate they show good activity and
powerful tool in organic synthesis.1 However, in homogeneous
selectivity. Additionally, some heterogeneous catalysts require the
catalysis separation and recovery of the catalyst are difficult,
addition of phosphine ligands such as PPh3 to obtain reasonable
activity.9 It therefore is not surprising that the heterogenization
of phosphine ligands on solid supports was taken as a tool
for the heterogenization of palladium. In an early report Trost
et al. described polymer-supported palladium catalysts based
on diphenylphosphanyl functionalized polystyrene.10 In the fol-
lowing years a variety polymer-supported palladiumphosphine
complexes were synthesized and used for coupling reactions.11
However, to the best of our knowledge no procedure for the
covalent heterogenization of palladiumphosphine complexes on
porous silica as the support has been reported.
In the present work, we describe the synthesis of a novel
heterogeneous cross-coupling catalyst by immobilization of a
functionalized palladium triphenyphosphine complex on the
surface of silica gel. The material was characterized by solid
state 29Si, 13C and 29P CP-MAS NMR spectroscopy, SEM and
EDX, surface area BET measurements, IR and elemental analysis.
Furthermore, the catalytic properties of the palladium containing
material was studied exemplarily for the Suzuki cross-coupling
between aryl halides and arylboronic acid.
which is of special disadvantage for large-scale preparations.
Therefore, a large number of heterogeneous palladium containing
catalysts have been developed during the last years. These solids
can simply be recovered from the reaction mixture by filtration
and reused.2 Organic polymers and inorganic oxides (e.g., silica
gel) are commonly used as supports for the synthesis of such
catalysts.2,3 Different palladium complexes, bearing ligands such
as carbenes, amines or thiols, have been immobilized on silica
supports.4 In a detailed study, Clark and Paul investigated the
relationship between chemical structure and catalytic activity in
the Suzuki coupling reaction for a wide variety of bidentate ligands
grafted on silica including N,N-, N,S- and N,O-donating systems.5
They found that lowering the bond energy between the palladium
centre and the ligand results in an increase in reactivity. Grudden
et al. reported that thiol modified mesoporous materials are
remarkable scavengers for palladium, and the resulting palladium
encapsulated materials catalyze Suzuki type coupling reactions
with virtually no leaching of palladium.6 Corma et al. immobilized
an oxime-carbapalladacycle complex on mercaptopropyl modified
silica. This solid is an active and stable heterogeneous catalyst for
the Suzuki coupling reaction of aryl bromides and chlorides in
water.7
Results and discussion
Although significant efforts have been achieved to develop
heterogenized palladium containing catalysts, such systems still
suffer from high catalyst loadings and a lower efficiency compared
to their analogues in the homogeneous phase. Moreover, the
catalytic activity of heterogeneous palladium catalysts sometimes
happens to decrease or even disappear after use.8
Synthesis and characterization of the palladium catalysts
It should be mentioned at this point that the usage of diarylalkyl
phosphines like Ph2P(CH2)3Si(OEt)3 has already been reported
for the synthesis of phosphine functionalized silica materials.12
However, these ligands are much more sensitive towards oxidation
than triarylphosphines.13 Additionally, the steric demand of the
alkyl chain is moderate and the linker between the phosphorus
donor and the silicon based anchoring group is short, which
might favour additional interactions between the framework walls
and the active site. We therefore focused on immobilizing a
triphenylphosphine based system onto silica via an alkyl chain
linker attached to one of the aryl rings of the phosphine ligand.
In general, there are two ways to immobilize a catalytically active
complex covalently on the surface of a solid support: on one hand,
aTechnische Universita¨t Kaiserslautern, Fachbereich Chemie, Erwin-
Schro¨dinger-Str. Geb. 54, D-67663, Kaiserslautern, Germany. E-mail:
thiel@chemie.uni-kl.de; Fax: +49 631 2054676; Tel: +49 631 2052752
bKey Laboratory of Surface and Interface Chemistry of Jilin Province,
College of Chemistry, Jilin University, Changchun, 130012, China
cTechnische Universita¨t Chemnitz, Institut fu¨r Chemie, Straße der Nationen
62, D-09111, Chemnitz, Germany
† Electronic supplementary information (ESI) available: Additional exper-
imental data. See DOI: 10.1039/b820449g
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the active complex can be prepared prior to the grafting procedure,
on the other hand, the ligand system can be grafted first followed
by treatment of this hybrid material with an appropriate source
of the active metal centre. For our palladium system we chose
the first route, since it determines the molar ratio of palladium
and phosphine to 1 : 2, the palladium complex can be purified
and spectroscopically characterized before starting the anchoring
procedure and the two phosphines will be anchored in an ideal
position for coordination to one palladium site on the surface.
This prevents the generation of palladium centres coordinated to
one or three phosphine molecules.
Starting from 4-diphenylphosphinylbenzenecarboxylicacid
methylester (1)14 the Si(OR)3 function required for the grafting
was introduced by a solvent free NaOMe catalyzed thermal
aminolysis with 3-trimethoxysilylpropylamine (Scheme 1). The
oily product 2 was characterized by NMR spectroscopy: three
sets of signals at high field in the 1H NMR spectrum (3.46,
1.76, 0.73 ppm), which are typical for the silicon functionalized
propylene chain, a sharp singlet (3.58 ppm) for the Si(OMe)3
group, one broad singlet at 6.53 ppm for the NH group
and a quite complex pattern of resonances between 7.3 and
7.7 ppm for the five magnetically inequivalent protons of the
aryl rings, which are all coupling with the phosphorus atom.
1
The 31P{ H} NMR spectrum shows one sharp resonance at
-4.10 ppm, typical for a triaryl substituted phosphorus atom.
For the synthesis of the palladium(II) complex 3, ligand 2 was
dissolved in CH2Cl2 and treated with half an equivalent of
di(benzonitrile)dichloropalladium(II), giving the desired product
in almost quantitative yields. The resonance of the phosphorus
atom is shifted to 24.4 ppm due to the formation of the complex.15
In the last step of the synthesis, the complex was covalently
anchored on a commercially available mesoporous but not
ordered silica gel (Aldrich, TLC standard grade).
After grafting of the palladium(II) complex 3, the palladium
content in the solid material 4 was determined by elemental
analysis. Additionally, to these data Table 1 shows the results
of the BET measurements for the parent silica and palladium
functionalized silica 4. Although 0.4 mmol of 3 per gramme of
SiO2 were applied in the grafting procedure, elemental analysis
shows that only 0.18 mmol g-1 of palladium complex 3 were
finally immobilized on the silica surface. For solid 4, the expected
reduction of the specific surface area, the pore volume and pore
diameter with respect to the parent support could be quantified.
This indicates that the surface modification indeed takes place
inside the primary mesopores of the silica gel.
The presence of palladium in the compound 4 was confirmed by
EDX analysis (see ESI).† As revealed by SEM done at a resolution
of 50 nm no larger palladium particles (d ≥ 50 nm) are present on
the surface of the support (Fig. 1). This does not change after a
fourfold reuse of the catalyst (see ESI).† Both observations confirm
a uniform dispersion of the active sites.
Scheme 1 Synthesis of the hetrogenized palladium catalyst 4.
Fig. 1 SEM image of the freshly prepared catalyst 4.
Table 1 Characterization of the parent silica gel and the hybrid material 4
Chem. composition (wt%)
Material
S
BET/m2 g-1
Pore volume/cm3 g-1
BJH pore diameter/nm
C
H
N
Immobilized Pda/mmol g-1
Neat silica
4
500
247
0.75
0.43
6.0
5.5
—
10.1
—
1.4
—
0.5
—
0.18
a Calculated according to the nitrogen content the elemental analysis.
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The analysis of the nitrogen adsorption and desorption data
exhibited a type IV isotherm (definition by IUPAC) with a
hysteresis characteristic for mesoporous materials possessing pore
diameters between 2 and 50 nm (Fig. 2).16 This indicates that
the overall mesoporosity is maintained after the introduction of
palladium complex.
The 13C CP-MAS NMR spectrum of 4 (Fig. 4) shows signals
for the aryl groups in the range from 145.1 to 120.1 ppm. Three
resonances at 42.8, 22.6, 9.0 ppm can be correlated with the
resonances of the propylene linker in the precursor 3 (42.4, 22.7
=
and 6.6 ppm). A signal at 168.0 ppm can be assigned to C O
group of the linker. It is found at an almost similar chemical shift
as for 3, which indicates, that the interaction between the amide
group of the linker and the silica framework is only weak.19 The
dominant resonance at 48.9 ppm can be assigned to organosilica
C-SiOCH3 groups, implying that the hydrolysis/condensation of
3 is not fully completed18 as already mentioned in the discussion
of the 29Si CP-MAS NMR spectrum.
Fig. 4 13C CP-MAS NMR spectrum of the hybrid material 4.
Fig. 2 (A) N2 adsorption/desorption isotherms at 77 K and (B) pore
size distribution of 4; adsorption points are marked by solid squares and
desorption points by empty squares.
The structural retention of the palladium(II) complex during
the immobilization process is further proved by the 31P CP-MAS
NMR of 4. As shown in Fig. 5, the 31P-NMR signal of 4 in
the solid state (23.6 ppm) corresponds well with the chemical
shift of the palladium complex in solution (24.4 ppm). There is
no signal of uncoordinated phosphine and phosphineoxide. The
unsymmetrical phosphorus resonance is due to differences in the
chemical environment of the phosphine and the palladium sites
which are located in close proximity to the surface.
The 29Si CP-MAS NMR spectrum (Fig. 3) of the hybrid material
4 provides direct evidence for the covalent incorporation of the
phosphinepalladium(II) complex 3. According to the literature, the
signals at -110, -101, and -92 ppm correspond to the Si(OSi)4 (Q4),
HOSi(OSi)3 (Q3), and (HO)2Si(OSi)2 (Q2) sites of the inorganic
silica framework.17 Covalent grafting of 3 on the silica surface
makes the Q2 signal of the neat silica almost disappear, which is
due to the consumption of highly reactive geminal Si–OH groups
during the condensation reaction.18 For the modified material 4,
three additional broad and overlapping signals appear at -49, -58,
and -67 ppm, which can be assigned to R–Si(RO)2(OSi) (T1), R-
Si(RO)(OSi)2 (T2), and R-Si(OSi)3 (T3) organosiloxane sites. Since
the T1 resonance is dominant over T2 and T3, the covalent grafting
of the silylated phosphine is mainly due to R–Si(RO)2(OSi) sites.
Fig. 5 31P CP-MAS NMR spectrum of the hybrid material 4; the asterisks
denote rotational side bands.
The presence of the palladium complex on the silica surface is
indicated in the IR spectrum by a weak amide band at 1539 cm-1,
which is found at the same wavenumber as for the precursor 3
(see supporting information). A second amide band at 1645 cm-1,
which is observed for 3, is overlapped by an absorption of the
support at 1636 cm-1.
Fig. 3 13Si CP-MAS NMR spectrum of the hybrid material 4.
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Table 3 Suzuki reactions between aryl halides and aryboronic acidsa
Application for the Suzuki reaction
To evaluate the general activity of catalyst 4, the Suzuki reaction
of phenylboronic acid with an aryl halide (benzene bromide) was
examined (Scheme 2 and Table 2). Initially, an optimization of the
base was performed using Cs2CO3, K3PO4, K2CO3 and KF under
identical conditions. These bases have frequently been applied
for homogeneous and heterogeneous catalyzed Suzuki coupling
reactions.20 Cs2CO3 and K3PO4 turned out to be highly efficient
under the given conditions.
Halide
Arylboronic acids
Product
Yield (%)b
99
24
98c
Traces
Scheme 2 Suzuki coupling of aryl bromide and phenylboronic acid.
Table 2 Suzuki reaction of bromobenzene and phenylboronic acid with
different basesa
a Reaction conditions: aryl halide (1 mmol); arylboronic acid (1.5 mmol);
K3PO4 (1.2 mmol); Pd (1 mol% relative to aryl halide); dioxane (5 mL),
80 ◦C. b Determined by GC-MS with n-decane as the internal standard.
c GC-MS and 1H NMR.
Base
Cs2CO3
90
K3PO4
92
K2CO3
76
KF
71
Yield (%)b
a Reaction conditions: PhBr (1 mmol); PhB(OH)2 (1.5 mmol); base
(1.2 mmol); Pd (1 mol% relative to aryl halide); dioxane (5 mL), 80 ◦C.
b Determined by GC-MS with n-decane as the internal standard.
Table 4 Study on the reusability of 4 in the Suzuki reactiona
Yield (%)b
The activity of the heterogenized catalyst system is in general
comparable to that of (PPh3)2PdCl2 and of a related system
where PdCl2 was heterogenized using a polymer (PS-DEAM resin)
bound phosphine ligand of the type Ph2PPh-CONHR.21
Base
1st run
2nd run
88
3rd run
89
4th run
82
Cs2CO3
90
a Reaction conditions: PhBr (1 mmol); PhB(OH)2 (1.5 mmol); base
(1.2 mmol); Pd (1 mol% relative to aryl halide); dioxane (5 mL), 80 ◦C.
b Determined by GC-MS with n-decane as the internal standard.
Based upon these results, a series of different substrates were
tested in the presence of catalyst 4 (Table 3). The coupling of
iodobenzene and arylboronic acid afforded the expected biaryl
in very good yield (entry 1). However, for the less reactive
chlorobenzene, a pronounced reduction in activity was observed
particles with a diameter larger than 50 nm could be detected
(see ESI).† We suppose that both the silica matrix and the
organic ligand contribute to a stabilization of small palladium
nanoparticles. To clarify the role of the phosphine ligand, we set up
a control experiment, with a catalyst prepared by directly loading
PdCl2(PhCN)2 on silica. The overall palladium loading was similar
to that of 4. Running a Suzuki reaction between phenylboronic
acid and bromobenzene with this catalyst under identical reaction
conditions as before gave less than 50% of conversion after 24 h in
the first run. The activity of this catalyst was completely lost after
the first run. This suggests that the grafted phosphine ligand plays
an important role for the activity and stability of catalyst.
A further objective of our studies was to determine whether
4 acts as a truly heterogeneous catalyst. To test this, the coupling
reaction was carried out for 1 h until a conversion of about 30% was
reached. Then the catalyst was removed by filtration at the reaction
temperature. The filtrate was treated with another 1.2 equiv. of
Cs2CO3 and heated for additional 23 h at 80 ◦C. The reaction
continued although the conversion did not reach the level obtained
with 4, which means that at least a part of the catalytic activity of
4 must be assigned to a homogeneous reaction. To determine the
degree of leaching of the metal from the heterogeneous catalyst,
the catalyst was removed by filtration at 80 ◦C after the reaction
was completed (24 h) and the palladium content of the filtrate was
determined by atomic absorption spectrometry (AAS). We found a
◦
(entry 2). Rising the reaction temperature up to 120 C did not
increase the conversion but resulted in a yield of only 8% after 24 h,
probably due to rapid catalyst deactivation. Additionally sterically
hindered substrates were investigated: the coupling of 1-bromo-2-
methylnaphthalene and boronic acid almost reached completion.
In contrast, the reaction between 1-bromo-2-methylnaphthalene
and 2-methylnaphthylboronic acid, which would be interesting for
the synthesis of 1,1¢-dimethyl-2,2¢-binaphthyl, afforded only traces
of the product, probably due to inherent difficulties in coupling
two sterically hindered arenes.
Reusability is an important feature that determines the appli-
cability of a heterogeneous catalysts. We examined the reusability
of 4 with Cs2CO3 as the external base. After the first use of the
catalyst in the Suzuki reaction, the reaction mixture was treated
as described in the experimental part. The recovered catalyst
was used successfully for four subsequent reactions and exhibited
consistent catalytic activity, indicating the excellent reusability of
this heterogeneous catalyst (Table 4).
It should also be pointed out here that the initially yellow
coloured catalyst turned to grey after the first run, and the
colour darkened gradually with further recycling. This is possible
by the formation of metallic nanoparticles during the catalytic
transformation. A similar observation was reported by Trilla
et al.22 However, by microscopic investigations no palladium
3318 | Dalton Trans., 2009, 3315–3320
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◦
8
13
=
palladium contamination of about 0.4% relative to the original Pd
loading of 4, which implies that leaching is not a serious problem
for this catalyst. This is also confirmed by the excellent reusability
of this catalytically active material.
H ). C{1H} NMR (150.92 MHz, 25 C, CDCl3) d 167.1 (s, C O),
141.8 (d, 1JPC = 13.9 Hz, C4), 136.5 (d, 1JPC = 11.1 Hz, Ci), 134.9
(s, C1), 133.9 (d, JPC = 20.3 Hz, C0), 133.5 (d, JPC = 18.5, C3),
129.0 (s, Cp), 128.6 (d, 3JPC = 6.5 Hz, Cm), 126.8 (d, 3JPC = 6.5 Hz,
C2), 50.3 (s, C9), 42.4 (s, C6), 22.7 (s, C7), 6.7 (s, C8). 31P {1H}
NMR (161.98 MHz, 25 ◦C, CDCl3) d -4.10. IR (KBr, cm-1) 3315
m, 3052 w, 2939 m, 2838 m, 1636 s, 1597 m, 1541 s, 1481 m, 1434
m, 1390 w, 1305 m, 1191 m, 1087 s, 1018 w, 815 s, 745 s, 697 s, 505
w.
2
2
Experimental
Materials
The silica gel used for the heterogenization was obtained from
Aldrich (TLC standard grade, without binder, catalogue no.
28 850-0), all other chemicals were supplied by Acros. The solvents
used for the preparations were dried by standard methods. All
reactions were performed under inert gas conditions (N2).
Dichloridobis[(4-diphenylphosphinylbenzenecarboxylicacid-4N-
(3-trimethoxysilypropyl)amide]palladium(II)
(3). 178
mg
(0.46 mmol) of di(benzonitrile)dichloropalladium(II) were dis-
solved in 30 mL of dry CH2Cl2. This solution was added to
a solution of 430 mg (0.92 mmol) of 2 in 10 mL of CH2Cl2.
The reaction mixture was stirred for 4 h at room temperature,
Characterization
R
then filtrated through a Whatmanꢀ filter and the filtrate was
Elemental analyses were carried out at the Department of
Chemistry (TU Kaiserslautern). Infrared spectra were recorded
with a Perkin-Elmer FT-IR 1000 spectrometer. NMR spectra
were recorded with a Bruker Avance 400 or 600 spectrometer.
The infrared spectra (KBr) were recorded using a Jasco FT/IR-
6100 spectrometer in a frequency range between 4000–400 cm-1.
Nitrogen adsorption/desorption isotherms were measured at the
liquid nitrogen temperature, using a Quantachrome Autosorb 1
sorption analyzer. The specific surface areas were calculated by the
Brunauer–Emmett–Teller (BET) equation at a relative pressure of
0.9 (P/P0), and the pore size distribution curves were analyzed
with the desorption branch by the BJH method. 13C CP-MAS,
31P CP-MAS and 29Si CP-MAS NMR spectra were obtained on
a Bruker DSX Avance spectrometer at resonance frequencies of
100.6, 162.0 and 79.5 MHz, respectively. The analyses of catalysts
surfaces before or after the reaction were done by scanning electron
microscopy (SEM) with JSM-6490LA and energy dispersive X-ray
analysis (EDX). The atomic adsorption spectrum was measured
by a Perkin Elmer AAnalyst 300.
evaporated to dryness under reduced pressure. The residue was
washed with ether (3 ¥ 10 mL) to give a pale yellow solid. Yield:
511 mg (99%). Anal. calcd for C50H60N2Cl2O8P2PdSi2 (1112.5) C
1
53.96, H 5.40, N 2.52, found (%) C 53.26, H 5.38, N 2.53. H
◦
NMR (400.13 MHz, 25 C, CDCl3): d 7.73–7.34 (m, 14H, Har),
6.87 (br, NH), 3.52 (s, 9H, H9), 3.35–3.34 (m, 2H, H6), 1.69–1.66
(m, 2H, H7), 0.65 (t, JHH= 8.3 Hz, 2H, H8). 13C{1H} NMR
◦
i
=
(150.92 MHz, 25 C, CDCl3) d 166.7 (s, C O), 136.7 (s, C ), 135.0
(m, C0), 134.8 (m, C3), 133.3 (m, C4), 131.5 (s, Cp), 129.0 (m, C1),
128.3 (m, Cm), 126.5 (m, C2). 50.5 (s, C9), 42.4 (s, C6), 22.7 (s, C7),
6.6 (s, C8). 31P{1H} NMR (161.98 MHz, 25 ◦C, CDCl3) d 24.43.
IR (KBr, cm-1) 3383 m, 3054 w, 2939 m, 2838 w, 1644 m, 1538 m,
1483 w, 1435 m, 1304 w, 1190 m, 1093 s, 813 w, 747 w, 693 m,
514 m.
Hybrid catalyst 4. 3 was immobilized on SiO2 according to a
standard procedure: 150 mg (0.13 mmol) of 3 dissolved in 4 mL of
dry CH2Cl2 was added to a suspension of 340 mg of silica in 40 mL
of dry toluene. The mixture was stirred for 12 h at 100 ◦C. The solid
was filtered off and extracted with CH2Cl2 in a Soxhlet apparatus
for 24 h. Finally, the solid was dried in vacuo at 50 ◦C to obtain 4.
Catalyst synthesis
4-Diphenylphosphinylbenzenecarboxylicacid-4N-(3-trimethoxy-
silypropyl)amide (2). 1.27 g (3.97 mmol) of 1 and 0.021 g (0.397
mmol) of NaOMe were mixed with 0.71 g (3.97 mmol) of 3-
Catalytic Suzuki coupling. A dried Schlenk tube equipped
with a magnetic stirring bar was charged under an inert gas
atmosphere of nitrogen with the aryl halide (1.0 mmol), the
arylboronic acid (1.5 mmol), Cs2CO3 (1.2 mmol), 4 (60 mg,
0.01 mmol Pd) and 5 mL of dioxane. The mixture was heated
at 80 ◦C for 24 h. After cooling to room temperature, the mixture
was diluted with Et2O and filtered. The solid residual catalyst was
washed with water (3 ¥ 10 mL) and Et2O (3 ¥ 10 mL) and then
◦
trimethoxysilylpropylamine and heated to 170 C for 4 h. After
cooling to room temperature, all volatiles were removed in vacuo,
R
the residue was dissolved in CH2Cl2, filtered over a Whatmanꢀ
filter to remove NaOMe. After removing the solvent, 1.67 g
(90%) of 2 were obtained as a yellow oily residue. The numbering
scheme for the NMR assignment is given in Scheme 3. 1H NMR
◦
dried at 50 C. After this, it could be used for the next run. The
organic phase was separated and dried over MgSO4 and the GC
yield was determined using decane as an internal standard.
◦
(400.13 MHz, 25 C, CDCl3): d 7.72, 7.74 (2 ¥ s, 2H, ratio 1 : 1,
2¥H2), 7.30–7.38 (m, 12H, Har), 6.53 (br, NH), 3.58 (s, 9H, H9),
3.45–3.49 (m, 2H, H6), 1.78–1.74 (m, 2H, H7), 0.71–0.75 (m, 2H,
Conclusion
We synthesized a heterogeneous material by covalently anchoring
the triphenylphosphine palladium complex on the surface of a
silica gel. This material performed excellent activity and reusability
in the Suzuki coupling reaction. Further studies of other reactions
with this material as well as further development of the catalyst
system are currently in progress.
Scheme 3 Assignment of the NMR spectra for 2 and 3.
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Acknowledgements
Lei Wang thanks the China Scholarship Council for financial
support.
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3320 | Dalton Trans., 2009, 3315–3320
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