Synthesis, characterization and l-lactide polymerization behavior of bis(amidinate) rare earth metal amide complexes

Article abstract of DOI:10.1016/j.ica.2010.07.036

A family of neutral and solvent-free bis(amidinate) rare earth metal amide complexes with a general formula [RC(N-2,6-Me₂C₆H ₃)₂]₂LnN(SiMe₃)₂ (R = phenyl (Ph), Ln = Y (1), Nd (2); R = cyclohexyl (Cy), Ln = Y (3), Nd (4)) were synthesized in high yields by one-pot salt metathesis reaction of anhydrous LnCl₃, amidinate lithium salt [RC(N-2,6-Me₂C ₆H₃)₂]Li, and NaN(SiMe₃)₂ in THF at room temperature. Single crystal structural determination of complexes 1, 2 and 4 revealed that the central metal adopts distorted pyramidal geometry. In the presence of 1 equivalent of ⁱPr-OH, all these complexes were active for l-lactide polymerization in toluene at 70 °C to give high molecular weight (M_n > 10⁴) polymers.

Full text of DOI:10.1016/j.ica.2010.07.036

Inorganica Chimica Acta 363 (2010) 3597–3601
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Inorganica Chimica Acta
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Synthesis, characterization and L-lactide polymerization behavior
of bis(amidinate) rare earth metal amide complexes
Yunjie Luo a, , Ping Xu a,b, Yinlin Lei a, Yong Zhang b, Yaorong Wang b,
*
**
a Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China
b College of Chemistry, Chemical Engineering and Material Science, Suzhou University, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2010
Received in revised form 2 July 2010
Accepted 12 July 2010
Available online 21 August 2010
A family of neutral and solvent-free bis(amidinate) rare earth metal amide complexes with a general for-
mula [RC(N-2,6-Me2C6H3)2]2LnN(SiMe3)2 (R = phenyl (Ph), Ln = Y (1), Nd (2); R = cyclohexyl (Cy), Ln = Y
(3), Nd (4)) were synthesized in high yields by one-pot salt metathesis reaction of anhydrous LnCl3, ami-
dinate lithium salt [RC(N-2,6-Me2C6H3)2]Li, and NaN(SiMe3)2 in THF at room temperature. Single crystal
structural determination of complexes 1, 2 and 4 revealed that the central metal adopts distorted pyra-
i
midal geometry. In the presence of 1 equivalent of Pr-OH, all these complexes were active for
L
-lactide
Keywords:
polymerization in toluene at 70 °C to give high molecular weight (Mn > 104) polymers.
Bis(amidinate) rare earth amide complex
Synthesis
Ó 2010 Elsevier B.V. All rights reserved.
Crystal structure
L
-lactide polymerization
1. Introduction
equivalent of n-BuLi in THF), followed by addition of 1 equivalent
of NaN(SiMe3)2 in THF at room temperature afforded the corre-
sponding bis(amidinate) rare earth metal amide complexes
[RC(N-2,6-Me2C6H3)2]2LnN(SiMe3)2 (R = Ph, Ln = Y (1), Nd (2);
R = Cy, Ln = Y (3), Nd (4)) in 77–87% isolated yields (Scheme 1).
Although salt metathesis strategy was employed as synthetic
approach and THF was used as reaction solvent in these cases,
complexes 1–4 were isolated as neutral, mononuclear, and sol-
vent-free species. These complexes are air- and moisture-sensitive,
soluble in THF, toluene, diethyl ether, but insoluble in aliphatic sol-
vents such as hexane and pentane. They were characterized by ele-
mental analysis, FT-IR, NMR spectroscopy (except for 2 and 4 for
their strong paramagnetic property). Complexes 1, 2 and 4 were
also subjected to X-ray single crystal structure determination.
Room-temperature 1H NMR spectra of 1 and 3 in C6D6 showed a
singlet peak for the methyl resonances of Y–N(SiMe3)2. Restricted
rotation about the N–Caryl bonds was not observed because the
methyl groups on the aryl rings showed only one singlet
resonance. In 1 and 3, the main difference is the substituents at
the carbon atom of ligating NCN moiety (phenyl group for 1
and cyclohexyl group for 3), however, the methyl resonances
of Y–N(SiMe3)2 for 1 are found at d 0.24 ppm (1H NMR) and
d 4.2 ppm (13C NMR), while those for 3 appear at d 0.31 ppm
(1H NMR) and d 3.6 ppm (13C NMR), suggesting a similar electronic
yttrium environment.
Amidinate anions [RC(NR0)2]ꢀ can be easily modified by tuning
the C- and N-substituents for tailing the steric and electronic prop-
erties. They have been extensively employed as ancillary ligands to
form complexes with a variety of metal centers across the periodic
table [1]. Over the past decades, considerable attention has also
been paid to the synthesis and reactivity of amidinate rare earth
metal complexes, which could serve as reagents in organic synthe-
sis and as initiators/catalysts for polymerization [2–6]. In contrast,
there are only rare examples of amidinate-incorporated rare earth
metal amide complexes [7,8]. Herein, we would like to report the
synthesis and characterization of a novel family of bis(amidinate)
rare earth metal amide complexes, as well as their activity towards
L-lactide polymerization.
2. Results and discussion
2.1. Synthesis and characterization of bis(amidinate) rare earth metal
complexes
The one-pot salt metathesis reaction of anhydrous LnCl3 with 2
equivalents of amidinate lithium salt [RC(N-2,6-Me2C6H3)2]Li
(formed in situ by treatment of [RC(N-2,6-Me2C6H3)2]H with 1
Single crystals of complexes 1, 2 and 4 suitable for X-ray diffrac-
tion were grown from a mixture solution of hexane and THF at
ꢀ30 °C. The molecular structures of 1 and 2 are shown in Fig. 1,
and that of 4 is given in Fig. 2. The crystallographic data and the
* Corresponding author. Tel.: +86 574 88130085; fax: +86 574 88130130.
** Corresponding author. Tel.: +86 512 65882806.
E-mail addresses: lyj@nit.zju.edu.cn (Y. Luo), yrwang@suda.edu.cn (Y. Wang).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.036

3598
Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601
R Me
Me
Me
N
Me
Me
Me
N
N(SiMe3)2
Me
N
N
0.5 NaN(SiMe3)2
THF, rt
Ln
1) n-BuLi, THF
R
Me
Me
H
2) 0.5 LnCl3, THF, rt
N
N
Me
Me
R
Me
R =
Ln = Y (1), Nd (2)
Ln = Y (3), Nd (4)
Scheme 1. Synthesis of bis(amidinate) rare earth metal amide complexes.
selected bond distances and angles are summarized in Table 1 and
Table 2, respectively. The overall structural features of 1, 2 and 4,
except for the difference of residue solvent molecule in the lattice
(these solvent molecules could be removed under vacuum), are
quite similar: the central metal is five-coordinated by two biden-
tate amidinate ligands through nitrogen atoms, and one amide
group to form a distorted pyramidal geometry. Therefore, only
the structure of 1 is discussed here. Complex 1 is a C2 symmetric
molecule in the solid state. It is found that the phenyl group on
the central carbon atom of NCN moiety provides steric protection
in the plane of the ligand as well as above and below that plane
(dihedral angle of N(2)C(1)N(1)Y(1) and the plane formed by phe-
nyl group is 53.29°). The dihedral angle of N(2)C(1)N(1)Y(1) and
the plane formed by Si(1)Y(1)N(3)Si(1A) is 66.03°), this orientation
minimizes the interaction between the SiMe3 groups of the amide
group and the 2,6-Me2C6H3 groups of the amidinate ligand. The Y–
N(SiMe3)2 distance in 1 is 2.187(5) Å, which is slightly shorter than
those in (dimb)Y[N(SiMe3)2]2 (dimb = N,N0-diisopropyl(2,6-dimesi-
tyl)benzamidinate) (av. 2.24 Å) [7], and is identical to that in
(SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av.
2.19 Å) [8], if the ionic radius is considered [9]. The distances be-
tween yttrium and the ligating nitrogen atoms (Y–N = 2.369(3)
and 2.376(3) Å) in 1 are consistent with those found in (dimb)Y[N(-
SiMe3)2]2 (av. 2.34 Å) [7], but are somewhat longer than those in
(SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN(SiMe3)2 (av.
2.30 Å) [8] The bite angle of N–Y–N is 56.39°, which is smaller than
those in (SiMe3)2NYb[Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YbN-
(SiMe3)2 (av. 59°) [8].
Fig. 2. Molecular structure of
Hydrogen atoms have been omitted for clarity.
4 with thermal ellipsoids at 20% probability.
2.2. L-lactide polymerization initiated by bis(amidinate) rare earth
metal amide complexes
Polylactides (PLAs) are the promising biodegradable and bio-
compatible synthetic macromolecules, and have been widely ap-
plied in medicine, pharmaceutics, as well as in tissue engineering
[10]. The most effective method to prepare PLAs is the ring-open-
ing polymerization (ROP) of lactides by metal-based catalysts/initi-
ators, and it has been proven that metal alkoxides usually possess
high efficiency [11]. Some rare earth metal complexes also exhib-
ited good performance for lactide polymerization [5,8,12–16]. In
order to understand the polymerization behavior of bis(amidinate)
rare earth metal amide complexes, complexes 1–4 were tested for
the ring-opening polymerization of L-lactide, and the preliminary
polymerization results are listed in Table 3.
All of these neutral complexes alone showed very poor activity
towards -lactide polymerization at 70 °C in toluene. However,
L
i
when these complexes were treated with 1 equivalent of Pr-OH,
they exhibited improved activity, albeit still with rather low activ-
ity compared with other amidinate rare earth metal initiators [5,8].
This might be contributed to that the steric demanding amidinate
ligands in 1–4 hampered the coordination of monomer to central
metal, sequentially resulted in decreasing the initiation capability
Fig. 1. Molecular structure of 1 (left) and 2 (right) with thermal ellipsoids at 10% probability. Hydrogen atoms have been omitted for clarity.

Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601
3599
Table 1
Details of the crystallographic date and refinements for 1, 2 and 4.
1
2
4
Formula
FW
T (K)
Crystal system
Space group
a (Å)
C
52H64N5Si2Yꢂ(C6H14
)
C52H64N5NdSi2ꢂ3(C4H8O)
1175.81
223(2)
monoclinic
P 21/n
17.6303(10)
16.7689(9)
21.5620(11)
90.00
C52H76N5NdSi2
971.60
293(2)
monoclinic
P 21/n
12.3407(12)
19.0488(17)
22.866(2)
90.00
990.34
293(2)
monoclinic
C 2/c
21.434(6)
11.627(3)
23.872(6)
90.00
b (Å)
c (Å)
a
(°)
b (°)
106.272(8)
90.00
5711(2)
4
1.152
1.102
92.6300(10)
90.00
6367.9(6)
4
1.226
0.899
96.766(3)
90.00
5337.8(9)
4
1.209
1.054
c
(°)
V (Å3)
Z
Dcalc (g cmꢀ3
)
l
(mmꢀ1
)
F (0 0 0)
2112
2476
2044
Crystal size (mm)
hmax (°)
0.80 ꢁ 0.60 ꢁ 0.50
0.80 ꢁ 0.50 ꢁ 0.40
0.80 ꢁ 0.80 ꢁ 0.45
25.35
25.5
25.35
Reflections collected
Independent reflections
No. of variables
269 48
5209
299
321 48
11760
649
1.076
497 40
9751
520
Goodness-of-fit GOF on F2
1.185
1.174
R [I > 2
wR
r(I)]
0.0775
0.1690
0.0465
0.1232
0.0611
0.1146
Table 2
Selected bond distances and bond angles for 1, 2, 4.
1
2
4
Bond distances (Å)
Y1–N1
Y1–N1A
Y1–N2
Y1–N2A
Y1–N3
C1–N1
C1–N1A
C1–N2
C1–N2A
Y1–C1
2.369(3)
2.369(3)
2.376(3)
2.376(3)
2.187(5)
1.331(5)
1.331(5)
1.325(5)
1.325(5)
2.802(4)
2.802(4)
Nd1–N1
Nd1–N2
Nd1–N3
Nd1–N4
Nd1–N5
C1–N1
2.462(3)
Nd1–N1
Nd1–N2
Nd1–N3
Nd1–N4
Nd1–N5
C1–N1
2.454(4)
2.448(4)
2.472(4)
2.451(4)
2.284(4)
1.336(6)
1.330(6)
1.333(6)
1.339(6)
2.917(5)
2.921(5)
2.484(3)
2.456(3)
2.473(3)
2.277(3)
1.346(5)
1.336(5)
1.342(5)
1.338(5)
2.921(4)
2.909(4)
C1–N2
C1–N2
C24–N3
C24–N4
Nd1–C1
C24–N3
C24–N4
Nd1–C1
Nd1–C24
Y1–C1A
Nd1–C24
Bond angles (°)
N2–Y1–N1
N2A–Y1–N1A
N2–C1–N1
56.39(11)
56.39(11)
115.2(4)
115.2(4)
N2–Nd1–N1
N4–Nd1–N3
N2–C1–N1
54.38(10)
54.60(10)
114.9(3)
115.0(3)
N2–Nd1–N1
N4–Nd1–N3
N2–C1–N1
53.96(14)
54.09(13)
113.1(4)
113.8(4)
N2A–C1–N1A
N4–C24–N3
N4–C24–N3
Table 3
L-lactide polymerization with amidinate rare earth metal amide complexes.a
for L-lactide polymerization. The influence of organic substituents
at the carbon atoms of the amidinate moiety on the polymerization
activity was observed, and 1 and 2 showed a little bit higher activ-
ity than 3 and 4. This phenomenon could be reasoned that the
cyclohexyl group is much more fluxional than the rigid phenyl
ring. The influence of central metal on the polymerization activity
is obvious, and the activity increases with the increase of the effec-
tive ionic radii (Nd3+ (1.123 Å) > Y3+ (1.040 Å)) [9].
O
O
O
Initiator/iPr-OH
O
n
O
O
n
O
O
L- Lactide
iPr-OH/Ln
(molar ratio)
Yieldb (%)
Mn ꢁ 10ꢀ4
Mw/Mn
c
c
Run
Initiator
1
2
3
4
5
6
7
8
1
2
3
4
1
2
3
4
0
0
0
0
1
1
1
1
trace
trace
trace
trace
74
85
61
73
3. Conclusion
2.17
1.19
2.59
1.08
1.88
2.10
1.98
1.85
In summary, a family of neutral and solvent-free bis(amidinate)
rare earth metal amide complexes were prepared by one-pot salt
metathesis reaction. These complexes were active for L-lactide
polymerization in the presence of one equivalent amount of Pr-
OH to give high molecular weight (Mn > 104) polymers. The poly-
merization activity is dependent on the ionic radii of rare earth
metals, as well as the substituent at the carbon atom of the amidi-
nate moiety.
i
a
Polymerization conditions: [LA]/[Ln] = 100, [LA] = 0.5 M, in toluene, t = 10 h,
70 °C.
Isolated yields of PLA.
Determined by GPC at 40 °C in THF relative to polystyrene standards; corrected
by the Mark–Houwink equation [Mn,obsd = 0.58Mn (GPC)] [17].
b
c

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Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601
934 (m), 841 (s), 777 (s). Anal. Calc. for C52H64N5NdSi2: C, 65.09;
H, 6.72; N, 7.30. Found: C, 65.35; H, 6.56; N, 7.41%.
4. Experimental
4.1. Materials and procedures
4.4. Synthesis of [CyC(N-2,6-Me2C6H3)2]2YN(SiMe3)2 (3)
All manipulations were performed under pure argon with rigor-
ous exclusion of air and moisture using standard Schlenk tech-
niques and a argon-filled glovebox operating at less than 1 ppm
oxygen and 1 ppm moisture. Solvents (toluene, hexane, and THF)
were distilled from sodium/benzophenone ketyl, degassed by the
freeze-pump-thaw method, and dried over fresh Na chips in the
glovebox. Dichloromethane was died by stirring with CaH2, and
distilled before use. Anhydrous LnCl3 were purchased from STREM.
P2O5 was purchased from Sinopharm Chemical Reagent Co., Ltd.
Hexamethyldisiloxane, 2,6-dimethylaniline, cyclohexanecarbox-
ylic acid, benzoic acid, and n-BuLi (1.0 M in hexane solution) were
n-BuLi (1 mL, 1 mmol) was added drop by drop to a THF solu-
tion (30 mL) of [CyC(N-2,6-Me2C6H3)2]H (0.334 g, 1.0 mmol) at
room temperature to give a clear yellow solution. After 15 min,
the reaction mixture (containing [CyC(N-2,6-Me2C6H3)2]Li formed
in situ) was added to a THF slurry of YCl3 (0.098 g, 0.5 mmol).
The mixture was stirred at room temperature for 2 h to afford a
cloudy solution, to which NaN(SiMe3)2 (0.3 mL, 0.5 mmol) was
added via a pipet. After the resulting reaction mixture was stirred
at room temperature for 4 h, the solvents were removed under re-
duced pressure to give brown oily product. The crude product was
washed by hexane, and the residue was extracted by toluene
(2 ꢁ 10 mL). Drying up the solvents afforded yellow powder.
Recrystallization from a mixture solution of hexane and THF affor-
ded 3 as colorless block crystals (0.35 g, 77%). 1H NMR d: 0.31 (s,
18H, SiMe3), 0.52–0.67 (m, 8H, Cy-CH2), 1.09 (d, 4H, Cy-CH2), 1.27
(d, 4H, Cy-CH2), 1.81 (d, 4H, Cy-CH2), 2.24 (m, 2H, Cy-CH), 2.47
(s, 24H, Ph-Me), 6.94–7.01 (12H, Ar-H). 13C NMR (100 HMz, C6D6)
d: 3.6 (SiTMS), 20.6 (Ph-Me), 25.4, 25.9, 29.7 (Cy-CH2), 44.5 (Cy-
CH), 123.4, 128.7, 133.3, 146.4 (Ar-C), 182.4 (NCN). FT-IR (KBr,
cmꢀ1): 3344 (m), 2934 (s), 1637 (s), 1589 (s), 1473 (s), 1388 (s),
1275 (m), 1030 (m), 934 (m), 842 (s), 777 (s). Anal. Calc. for
purchased from Acros, and used as received. L-lactide was pur-
chased from TCI, and recrystallized from hot toluene. Deuterated
solvents (CDCl3, C6D6) were obtained from CIL. Polyphosphoric acid
trimethylsilyl ester (PPSE) [18,19], [CyC(N-2,6-Me2C6H3)2]H [19],
and [PhC(N-2,6-Me2C6H3)2]H [19] were prepared according to the
literature.
Samples of organo rare earth metal complexes for NMR spectro-
scopic measurements were prepared in the glovebox using J.
Young valve NMR tubes. NMR (1H, 13C) spectra were recorded on
a Bruker AVANCE III spectrometer at 25 °C, and referenced inter-
nally to residual solvent resonances unless otherwise stated. Car-
bon, hydrogen, and nitrogen analyses were performed by direct
combustion on a Carlo-Erba EA-1110 instrument, quoted data are
the average of at least two independent determinations. FT-IR
spectra were recorded on a Bruker TENSOR 27 spectrometer.
Molecular weight and molecular weight distribution of the poly-
mers were measured by PL GPC 50 at 40 °C using THF as eluent
against polystyrene standards, flow rate: 1 mL/min, sample con-
centration: 1 mg/mL.
C52H76N5Si2Y: C, 68.16; H, 8.36; N, 7.64. Found: C, 68.43; H, 8.76;
N, 7.71%.
4.5. Synthesis of [CyC(N-2,6-Me2C6H3)2]2NdN(SiMe3)2 (4)
Complex 4 was prepared by a procedure similar to that of 3.
Using n-BuLi (1 mL, 1 mmol), [CyC(N-2,6-Me2C6H3)2]H (0.334 g,
1.0 mmol), NdCl3 (0.125 g, 0.5 mmol). 4 was produced as blue
block crystals (0.49 g, 87%). FT-IR (KBr, cmꢀ1): 3342 (m), 2954
(s), 1625 (s), 1589 (s), 1472 (s), 1360 (s), 1254 (m), 1093 (m),
931 (m), 840 (s), 776 (s). Anal. Calc. for C52H76N5NdSi2: C, 64.28;
H, 7.88; N, 7.21. Found: C, 64.59; H, 7.56; N, 7.08%.
4.2. Synthesis of [PhC(N-2,6-Me2C6H3)2]2YN(SiMe3)2 (1)
n-BuLi (1 mL, 1 mmol, 1 M in hexane)) was added drop by drop
to a THF solution (30 mL) of [PhC(N-2,6-Me2C6H3)2]H (0.328 g,
1.0 mmol) at room temperature. After 15 min, the reaction mixture
(containing [PhC(N-2,6-Me2C6H3)2]Li formed in situ) was added
slowly to a THF slurry of YCl3 (0.098 g, 0.5 mmol). The mixture
was stirred at room temperature for 2 h to afford a cloudy solution,
to which NaN(SiMe3)2 (0.3 mL, 0.5 mmol) was added via a pipet.
After the resulting reaction mixture was stirred at room tempera-
ture for 4 h, solvents were removed under reduced pressure to give
brown oily product. The crude product was washed by hexane, and
the residue was extracted by toluene (2 ꢁ 10 mL). Removal of vola-
tiles yielded yellow powder. Recrystallization from a mixture solu-
tion of hexane and THF gave 1 as colorless block crystals (0.45 g,
85%). 1H NMR (400 MHz, C6D6): d 0.26 (s, 18H, SiMe3), 2.30 (s,
24H, CH3), 6.59–6.83 (m, 22H, Ar-H). 13C NMR (100 HMz, C6D6): d
4.2 (Si(CH3)3) 20.3 (CH3), 124.1, 127.1, 127.9, 128.5, 129.4, 132.6,
134.5, 146.7 (Ar-C), 177.6 (NCN). FT-IR (KBr, cmꢀ1): 3343 (m),
2953 (s), 1625 (s), 1589 (s), 1468 (s), 1374 (s), 1256 (m), 1095
(m), 979 (m), 840 (s), 764 (s). Anal. Calc. for C52H64N5Si2Y: C,
69.07; H, 7.13; N, 7.75. Found: C, 69.25; H, 7.41; N, 7.83%.
4.6. A typical procedure for
L
-lactide polymerization
In a 50 mL Schleck flask, 1 (18 mg, 20
l
mol), -lactide (288 mg,
L
2.0 mmol), and toluene (4.0 mL) were stirred at 70 °C for 10 h (run
5, Table 3). The polymerization was terminated by quenching with
excess ethanol containing 5% aq HCl. The polymer was collected by
filtration, and dried under vacuum at 60 °C to constant weight
(213 mg, 74%).
4.7. X-ray crystallographic study
Suitable single crystals of complexes were sealed in a thin-
walled glass capillary for determining the single-crystal structure.
Intensity data were collected with a Rigaku Mercury CCD area
detector in
x scan mode using Mo Ka radiation (k = 0.71070 Å).
The diffracted intensities were corrected for Lorentz polarization
effects and empirical absorption corrections. The structures were
solved by direct methods and refined by full-matrix least-squares
procedures based on |F|2. All the non-hydrogen atoms were refined
anisotropically. The structures were solved and refined using SHE-
LEXL-97 program.
4.3. Synthesis of [PhC(N-2,6-Me2C6H3)2]2NdN(SiMe3)2 (2)
Complex 2 was prepared by a procedure similar to that of 1.
Using n-BuLi (1 mL, 1 mmol), [PhC(N-2,6-Me2C6H3)2]H (0.328 g,
1.0 mmol), NdCl3 (0.125 g, 0.5 mmol). 2 was produced as blue
block crystals (0.48 g, 84%). FT-IR (KBr, cmꢀ1): 3344 (m), 2932
(s), 1638 (s), 1589 (s), 1473 (s), 1388 (s), 1252 (m), 1091 (w),
5. Supplementary material
CCDC 763877, 763878 and 763879 contain the supplementary
crystallographic data for 1, 2, and 4, respectively. These data can

Y. Luo et al. / Inorganica Chimica Acta 363 (2010) 3597–3601
3601
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This work was partly financial supported by the National Natu-
ral Science Foundation of China (20604023), Zhejiang Provincial
Natural Science Foundation (Y4090617), State Key Laboratory of
Polymer Physics and Chemistry (200902), and Key Laboratory of
Organic Synthesis of Jiangsu Province (KJS0907).
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