Bridged bis(amidinate) lanthanide complexes: Synthesis, molecular structure and reactivity

Article abstract of DOI:10.1016/j.poly.2008.03.006

The yttrium chloride with the bridged bis(amidinate) L (L = Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃) LYCl(DME) (2) was synthesized and structurally characterized. Treatment of LLnCl(sol)_x (Ln = Yb, sol = THF, x = 2 1; Ln = Y, sol = DME, x = 1 2) with the dilithium salt Li₂L(THF)_0.5 afforded the novel bimetallic lanthanide complexes supported by three ligands, Ln₂(μ₂-L)₃ · DME (Ln = Yb 3, Y 4; DME = dimethylether), instead of the designed complex LLn(μ₂-L)LnL via the ligand redistribution reaction. Complexes 3 and 4 were fully characterized including X-ray analysis and ¹H NMR spectrum for 4. Reaction of LnCl₃ (Ln = Yb, Y) with 2 equiv. of Li₂L(THF)_0.5 gave the anionic complexes [Li(DME)₃][L₂Ln] (Ln = Yb 5, Y 6), which were confirmed by a crystal structure determination. The further study indicated that complexes 3 and 4 can also be synthesized by reaction of LnCl₃ (Ln = Yb, Y) with 1.5 equiv. of Li₂L(THF)_0.5 or reaction of 1 and 2 with anionic complexes 5 and 6. Complexes 3, 4, 5 and 6 were found to be high active catalysts for ring-opening polymerization of ε-caprolactone (CL).

Full text of DOI:10.1016/j.poly.2008.03.006

Polyhedron 27 (2008) 1977–1982
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Bridged bis(amidinate) lanthanide complexes: Synthesis, molecular structure
and reactivity
*
Junfeng Wang, Hongmei Sun, Yingming Yao, Yong Zhang, Qi Shen
Key Laboratory of Organic Synthesis of Jiangsu Province, Department of Chemistry and Chemical Engineering, Dushu Lake Campus, Suzhou University,
Suzhou 215123, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The yttrium chloride with the bridged bis(amidinate) L (L = Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3) LYCl-
(DME) (2) was synthesized and structurally characterized. Treatment of LLnCl(sol)x (Ln = Yb, sol = THF,
x = 2 1; Ln = Y, sol = DME, x = 1 2) with the dilithium salt Li2L(THF)0.5 afforded the novel bimetallic lantha-
nide complexes supported by three ligands, Ln2(l2-L)3 ꢀ DME (Ln = Yb 3, Y 4; DME = dimethylether),
instead of the designed complex LLn(l2-L)LnL via the ligand redistribution reaction. Complexes 3 and 4
were fully characterized including X-ray analysis and 1H NMR spectrum for 4. Reaction of LnCl3 (Ln = Yb,
Y) with 2 equiv. of Li2L(THF)0.5 gave the anionic complexes [Li(DME)3][L2Ln] (Ln = Yb 5, Y 6), which were
confirmed by a crystal structure determination. The further study indicated that complexes 3 and 4 can
also be synthesized by reaction of LnCl3 (Ln = Yb, Y) with 1.5 equiv. of Li2L(THF)0.5 or reaction of 1 and 2
with anionic complexes 5 and 6. Complexes 3, 4, 5 and 6 were found to be high active catalysts for ring-
opening polymerization of e-caprolactone (CL).
Received 18 December 2007
Accepted 11 March 2008
Available online 21 April 2008
Keywords:
Bis(amidinate)
Lanthanide
Synthesis
Structure
Polymerization
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
active initiators for the ring-opening polymerization of e-caprolac-
tone (e-CL), but, the systems gave the polymers with rather broad
Bridged bisanionic ligands have been known to have wide
applications in homogeneous catalysis, as they provide more open
coordination environment around the central metal and more
controllable molecular structure in comparison with the related
monoanionic ligands. Amidinate ligands, as one of the most useful
alternatives to cyclopentadienyl ligands, have attracted increasing
attention in organolanthanide chemistry as their electronic
and steric effects can be easily modified by variation of the substit-
uents on N-atoms [1]. However, the application of bridged bis(ami-
dinate) ligand has been limited [2]. The first yttrium complexes
with a chelating tethered bis(amidinate) ligand, YLCl(THF)2 and
YL[CH(TMS)2](THF), L = Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3, were
reported in 2001 [3]. Then, we have synthesized a serious of ytter-
bium derivatives with the same ligand, including YbLCl(THF)2,
YbLCp(DME), YbLNHAr (Ar = 2,6-Me2C6H3 and 2,6-iPr2C6H3) and
(YbNTMS(l-L)2YbNTMS), and found that different coordination
modes which lead to substantial difference in geometry and reac-
tivity can be achieved by this ligand [4]. These results encouraged
us to further investigate the synthesis, molecular structure and
reactivity of lanthanide derivatives with the same ligand in hopes
to understanding the coordination chemistry of the ligand. Homo-
leptic triamidinate lanthanide complexes were found to be very
molecular weight distributions caused from the multi-active sites
of three Ln–amidinate bonds [1a,1b,1c]. Considering the facts that
linking two amidinate anions together by a bridge should result
in limiting amidinate mobility, and the reactivity of an anionic lan-
thanide aryloxide for polymerization of e-CL is higher than that of
the related neutral complex [5], the synthesis of the complex with
a chelating L and a bridging L, (YbL)2(l-L), and the anionic complex
LiLn(L)2 were tried in hope to improve the catalytic behavior of tri-
amidinate lanthanide complex. It was found that the reaction of
LLnCl(sol)x (Ln = Yb, sol = THF, x = 2 1; Ln = Y, sol = DME, x = 1 2)
with the dilithium salt, Li2L(THF)0.5 yielded the bimetallic com-
plexes with three bridged L ligands, Ln2(l2-L)3 ꢀ DME (Ln = Yb 3; Y
4), in stand of the designed complexes LLn(l2-L)LnL, while the reac-
tion of LnCl3 with 2 equiv. of Li2L(THF)0.5 afforded the anionic com-
plexes Li(DME)3LnL2 (Ln = Yb 5, Y 6) in high yields. Complexes 3, 4,
5, and 6 were found to be high active catalysts for ring-opening
polymerization of e-CL. Here, we would like to report the results.
2. Experimental
2.1. General considerations
All manipulations and reactions were performed under a puri-
fied argon atmosphere using standard Schlenk techniques. Sol-
vents were degassed and distilled from sodium benzophenone
* Corresponding author. Tel.: +86 512 65880329; fax: +86 512 65880305.
E-mail address: qshen@suda.edu.cn (Q. Shen).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.03.006

1978
J. Wang et al. / Polyhedron 27 (2008) 1977–1982
ketyl before use. e-CL was purchased from Acros, dried by stirring
with CaH2 for 48 h, and then distilled under reduced pressure. The
complexes Li2L(THF)0.5 [3], LYbCl(THF)2 (1) [4a] and anhydrous
LnCl3 [6] were prepared according to the literature procedures.
All other reagents were purchased from Acros and used as received
without further purification. IR spectra were recorded on a Magna-
IR 550 spectrometer. 1H NMR spectra were measured on a Unity
Inova-400 spectrometer. Melting points were determined in sealed
capillary tube, and uncorrected. Lanthanide analyses were carried
out by complexometric titration. Carbon, hydrogen and nitrogen
analyses were preformed by direct combustion with a Carlo-Erba
EA 1110 instrument. Molecular weight and molecular weight dis-
tributions were determined against polystyrene standard by gel
permeation chromatography (GPC) on a Waters 1515 apparatus
with three HR columns (HR-1, HR-2 and HR-4), THF was used as
an eluent at 30 °C.
come a clear solution. The volatiles were removed under reduced
pressure and the residue was extracted with toluene (40 mL) and
evaporated to dry. The solid was dissolved in DME (5 mL) and fil-
tered. Cooling the solution to 0 °C afforded 4 as colorless crystals.
Yield: 0.42 g (55%), melt point: 170–173 °C. 1H NMR (400 MHz,
C6D6, 25 °C): d 7.44–7.07 (m, 30H, Ph), 3.64 (m, 4H, OCH2CH2O),
3.44 (m, 6H, OCH3), 3.23 (m, 12H, CH2N), 1.64 (m, 3H, CHH), 1.45
(m, 3H, CHH), 0.26 (s, 54H, SiMe3) ppm. IR (KBr pellet, cmꢁ1):
2955 (m), 1643 (m), 1605 (s), 1559 (s), 1381 (w), 1250 (w), 903
(w), 833 (w), 779 (w), 702 (m). Anal. Calc. for C73H112N12O2Si6Y2
(MW: 1536.11): C, 57.07; H, 7.36; N, 10.94; Y, 11.58. Found: C,
56.93; H, 7.05; N, 11.21; Y, 11.24%.
Method B. A THF (40 mL) solution of LLi2(THF)0.5 (1.41 g,
3 mmol) was added to a stirring suspension of YCl3 (1.41 g,
3 mmol) in THF (30 mL). Cooling to 0 °C afforded Complex 4 was
isolated as colorless crystals by the same procedure as outlined
for method A. Yield: 1.06 g (69%).
2.2. Synthesis of metal complexes
Method C. A clear solution of 6 (1.21 g, 1 mmol) in THF (25 mL)
was added to a stirring solution of 2 (0.64 g, 1 mmol) in THF
(20 ml). Complex 4 was isolated as colorless crystals by the same
procedure as outlined for method A. Yield: 0.95 g (62%).
2.2.1. LYCl(DME) (2)
Method A. A solution of LLi2(THF)0.5 (0.99 g, 2.1 mmol) in THF
(15 mL) was added to a stirring suspension of YCl3 (0.41 g,
2.1 mmol) in THF (30 mL). After stirring at r.t. for 48 h the solvent
was removed in vacuo. The residue was extracted with toluene
(30 mL) and the volume of the extract was reduced to 10 mL fol-
lowed by addition of DME (1 mL). Cooling to 0 °C afforded 2 as col-
orless crystals. Yield: 0.80 g (60%), melt point: 120–123 °C. 1H NMR
(400 MHz, C6D6, 25 °C): d 7.78–6.95 (m, 10H, Ph), 3.79 (m, 4H,
OCH2CH2O), 3.34 (m, 6H, OCH3), 3.12 (m, 4H, CH2N), 1.23 (m, 2H,
CHH), 1.01 (m, 2H, CHH), 0.31 (s, 18H, SiMe3) ppm. IR (KBr pellet,
cmꢁ1): 2956 (m), 1635 (s), 1568 (m), 1446 (m), 1383 (m), 1251
(w), 1206 (w), 1062 (w), 892 (w), 841 (w), 781 (w), 700 (m). Anal.
Calc. for C27H44Cl1N4O2Si2Y (MW: 637.20): C, 50.89; H, 6.97; N,
8.79; Y, 13.95. Found: C, 51.05; H, 6.84; N, 8.96; Y, 13.78%.
Method B. A clear solution of 6 (1.21 g, 1 mmol) in THF (25 mL)
was added to a stirring solution of YCl3 (0.47 g, 1 mmol) in THF
(20 ml). Complex 2 was isolated as colorless crystals by the same
procedure as outlined for method A. Yield: 0.48 g (75%).
2.2.4. [Li(DME)3][L2Yb] (5)
Method A. A stirred suspension of YbCl3 (0.42 g, 1.5 mmol) in
THF (20 mL) was treated with LLi2(THF)0.5 (1.41 g, 3 mmol) in
THF (15 mL). After the reaction mixture was stirred at r.t. for
48 h, the solvent was removed in vacuo. The residue was extracted
with toluene (40 mL) and the volume of the extract was reduced to
10 mL followed by addition of DME (1 mL). Cooling to 0 °C afforded
5 as colorless crystals. Yield: 1.46 g (75%), melt point: 116–118 °C.
IR (KBr pellet, cmꢁ1): 2963 (m), 1628 (m), 1543 (m), 1443 (m),
1381 (m), 1258 (w), 1204 (w), 1095 (w), 1026 (w), 895 (w), 841
(w), 787 (w), 702 (m). Anal. Calc. for C58H98LiN8O6Si4Yb (MW
:
1295.78): C, 53.76; H, 7.64; N, 8.65; Yb, 13.35. Found: C, 53.91;
H, 7.35; N, 8.86; Yb, 13.54%.
Method B. A stirring solution of LYbCl(THF)2 (0.78 g, 1 mmol) in
THF (20 ml) was treated with LLi2(THF)0.5 (0.47 g, 1 mmol) in THF.
Complex 5 was isolated as colorless crystals by the same procedure
as outlined for method A (0.91 g, 70%).
2.2.2. Yb2(l2-L)3 ꢀ DME (3)
Method A. A solution of LLi2(THF)0.5 (0.47 g, 1 mmol) in THF
(15 mL ) was added to a stirring solution of 1 (1.55 g, 2 mmol) in
THF (20 ml). The reaction mixture was stirred for 48 h at r.t. to be-
come a clear solution. The volatiles were removed under reduced
pressure and the residue was extracted with toluene (40 mL) and
evaporated to dry. The solid was dissolved in DME (10 mL) and fil-
tered. Cooling the solution to 0 °C afforded 3 as colorless crystals.
Yield: 1.28 g (75%), melt point: 284–287 °C. IR (KBr pellet, cmꢁ1):
2959 (m), 1628 (m), 1570 (m), 1543 (m), 1385 (m), 1253 (w),
1060 (w), 891 (w), 844 (w), 783 (w), 756 (w), 702 (m). Anal. Calc.
for C73H112N12O2Si6Yb2 (MW: 1704.37): C, 51.44; H, 6.64; N, 9.86;
Yb, 20.31. Found: C, 51.64; H, 6.21; N, 9.97; Yb, 20.03%.
Method B. A solution of LLi2(THF)0.5 (1.41 g, 3 mmol) in THF
(30 mL) was added to a stirring suspension of YbCl3 (0.56 g,
2 mmol) in THF (25 mL). Complex 3 was obtained as colorless crys-
tals by the same procedure as outlined for method A. Yield: 1.02 g
(60%).
2.2.5. [Li(DME)3][L2Y] (6)
Method A. A stirred suspension of YCl3 (0.20 g, 1 mmol) in THF
(20 mL) was treated with LLi2(THF)0.5 (0.94 g 2 mmol) in THF
(20 mL). The reaction mixture was stirred at r.t. for 48 h, the sol-
vent was removed in vacuo. The residue was extracted with tolu-
ene (50 mL) and the volume of the extract was reduced to 5 mL
followed by addition of DME (1 mL). Cooling to 0 °C afforded 6 as
colorless crystals. Yield: 0.91 g (75%), melt point: 109–112 °C. 1H
NMR (400 MHz, C6D6, 25 °C): d 7.85–6.97 (m, 20H, Ph), 3.87 (m,
12H, OCH2CH2O), 3.58 (m, 18H, OCH3), 3.20 (m, 8H, CH2N), 1.46
(m, 2H, CHH), 1.12 (m, 2H, CHH), 0.31 (m, 36H, SiMe3) ppm. IR
(KBr pellet, cmꢁ1): 2948 (m), 1643 (m), 1605 (s) 1559 (m), 1489
(m), 1443 (m), 1373 (m), 1250 (w), 1196 (w), 980 (w), 903 (w),
833 (w), 779 (w), 702 (m). Anal. Calc. for C58H98LiN8O6Si4Y (MW
1211.65): C, 57.49; H, 8.17; N, 9.25; Y, 7.34. Found: C, 57.74; H,
8.05; N, 9.51; Y, 7.62%.
Method B. A stirring solution of 2 (0.64 g, 1 mmol) in THF
(20 ml) was treated with LLi2(THF)0.5 (0.47 g, 1 mmol) in THF.
Complex 6 was isolated as colorless crystals by the same procedure
as outlined for method A (Yield: 0.95 g (78%)).
:
Method C. A clear solution of 5 (2.59 g, 2 mmol) in THF (25 mL)
was added to a stirring solution of 1 (2 mmol) in THF (20 ml). Com-
plex 3 was isolated as colorless crystals by the same procedure as
outlined for method A. Yield: 2.08 g (61%).
2.3. X-ray crystallographic study
2.2.3. Y2(l2-L)3 ꢀ DME (4)
Method A. A solution of LLi2(THF)0.5 (0.24 g, 0.5 mmol) in THF
(20 mL ) was added to a stirring solution of 2 (0.64 g, 1 mmol) in
THF (20 ml). The reaction mixture was stirred for 48 h at r.t. to be-
Suitable crystals of complexes 2–6 for X-ray diffraction were
sealed in a thin-walled glass capillary filled under argon for struc-
tural analysis. Diffraction data were collected on a Rigaku Mercury

J. Wang et al. / Polyhedron 27 (2008) 1977–1982
1979
CCD area detector. The structures were solved by direct methods
N
Ln
N
N
2
N
N
N
N
and refined by full-matrix least-squares procedures based on jFj .
N
N
N
N
LLn(μ2-L)LnL
Ln
All non-hydrogen atoms were refined with anisotropic displace-
ment coefficients [7]. Hydrogen atoms were treated as idealized
contributions. Crystal and refinement data are listed in Table 1.
)
F
5
.
0
H
T
(
x
L
i
L
2
N
5
.
0
expected products
LLnCl(sol)x
Ln= Yb 1, Y 2
0
.
5
L
i
L
(
T
H
2
F
)
2.4. Polymerization of e-CL
N
Ln
N
N
0
.
5
N
N
N
N
N
N
N
N
Ln
Ln2( μ2-L)3
To a stirred solution of e-CL in toluene, a toluene solution of ini-
tiator was added with a syringe. The polymerization mixture was
stirred for a definite time at the desired temperature and then
quenched with an ethanol solution containing a small amount of
hydrochloric acid. Polymer was precipitated from ethanol and
washed with ethanol three times and dried under vacuum.
N
N
N
N
N
N
N
N
=
L=
Ph
Ph
N
Ln= Yb 3, Y4
Me3Si
SiMe3
Scheme 1.
3. Results and discussion
to the procedure reported previously [4a] (Eq. (1)). The molecular
structure of 2 was further determined by X-ray diffraction to be
a monomeric structure with L as a chelating ligand (see the later
part of this paragraph). Reaction of 2 with 0.5 equiv. of the dilith-
ium salt went smoothly. After workup, colorless crystals were iso-
lated in good yield. The crystals were fully characterized including
elemental analysis, 1H NMR and X-ray technique to be the analo-
gous bimetallic complex Y2(l2-L)3 ꢀ DME (4) (Scheme 1). The re-
sults indicated that the occurrence of the ligand redistribution
during the metathesis reaction here is independent on the metal
used.
3.1. Synthesis and molecular structure of LYClDME (2) and Ln2(l2-
L)3 ꢀ DME (Ln = Yb 3; Y 4)
Complex 1 [4a] was synthesized by the literature procedure as
shown in Eq. (1). Treatment of 1 with 0.5 equiv. of LLi2(THF)0.5, fol-
lowed by extraction with toluene, afforded colorless crystals upon
crystallization from toluene. The full characterization by elemental
analysis and X-ray crystal structural determination indicated that
the product was the bimetallic complex Yb2(l2-L)3 ꢀ DME (3), not
the expected complex LYb(l2-L)YbL as shown in Scheme 1
The pathway for the formations of 3 and 4 was proposed to be
the same way that supposed for the preparation of the bimetallic
lanthanocene hydride published [8]. The attack of lithium amidi-
nate results in the cleavage of one of chelated Ln–bis(amidinate)
bonds, followed by redistribution to another Ln metal. The same li-
gand redistribution, derived from the reaction of 1 with NaN(TMS)2
was also described [4b]. These results indicate that the ligand
redistribution of a linked bis(amidinate) ligand with a flexible
bridge could be taken place under certain conditions and could be-
come a strategy to various structure motifs by choice of right re-
agents and ligands.
Further study found that 3 and 4 can also be prepared directly
by reaction of LnCl3 with the dilithium salt in molar ratio of
1:1.5 (Scheme 2) in high yield. Its worth notice that 3(4) was never
isolated from the metathesis reaction of LnCl3 with dilithium salt
in 1:1 molar ratio, by prolonging reaction time and raising reaction
LnCl3 þ Li2LðTHFÞ0:5 ! LLnCLðsolÞxLn ¼ Yb; sol ¼ THF; x
¼ 2; 1; Ln ¼ Y; sol ¼ DME; x ¼ 1; 2
ð1Þ
The isolation of 3, instead of the complex LYb(l2-L)YbL, is some-
what surprised to us because the coordination mode of the L in 3 is
quite different from that in LYb(l2-L)YbL. In 3 each of the three L
acts as a bidented bridging ligand to link the two metals together,
while each of the two L works as a chelating ligand and the third L
as a bridging ligand to link the two YbL parts together in LYb(l2-
L)YbL. Obviously, the formation of 3 should be included the change
of coordination mode of L from a chelating fashion to a bridging
one, i.e. the cleavage of two chelating Yb–amidinate bonds and
then ligand redistribution during the reaction. In order to see
whether this is the special case for Yb metal, the same reaction
with Y was conducted. Complex 2 was then synthesized according
Table 1
Principal crystallographic data and parameters for complexes 2–6
Compound
2
3
4
5
6
Chemical formula
Formula weight
T (K)
C
27H44Cl1N4O2Si2Y
C73H112N12O2Si6Yb2
1704.37
193(2)
C73H112N12O2Si6Y2
1536.11
153(2)
C58H98LiN8O6Si4Yb
1295.78
193(2)
C58H98LiN8O6Si4Y
1211.65
173(2)
637.20
173(2)
Crystal system
Space group
monoclinic
P21/c
triclinic
P1
triclinic
P1
trigonal
P32
trigonal
P32
ꢀ
ꢀ
Unit cell dimensions
a (Å)
b (Å)
c (Å)
a (°)
b (°)
17.7341(15)
17.1032(13)
21.9121(18)
90.00
96.275(2)
90.00
6606.3(9)
8
1.281
12.2154(14)
17.160(2)
21.107(3)
107.239
95.836(3)
92.402(3)
4191.6(9)
2
12.1658(14)
17.189(2)
21.114(3)
107.233(3)
95.831(3)
92.204(3)
4184.4(9)
2
15.679(2)
15.679(2)
24.222(3)
90.00
15.5959(8)
15.5959(8)
24.3310(13)
90.00
90.00
120.00
5125.2(5)
3
1.178
90.00
c (°)
120.00
5156.9(12)
3
Volume (Å3)
Z
Dcalc (mg mꢁ3
)
1.350
1.219
1.252
h Range for data collection (°)
Reflections collected
Independent reflections
Goodness-of-fit on F2
R
3.03–25.35
12101
10020
1.173
0.0600
3.05–25.35
15285
13715
1.098
0.0266
3.05–25.35
15236
12613
1.081
0.0492
3.00–27.48
15424
13685
1.079
0.0523
0.1340.
3.02–25.31
12262
10862
1.105
0.0536
0.1375
wR2
0.1060
0.0579
0.0915

1980
J. Wang et al. / Polyhedron 27 (2008) 1977–1982
Table 2
Selected bond distances (Å) and angles (°) for complex 2
N
Ln
N
N
LnCl3 1.5 Li L(THF)
+
N
N
N
N
2
0.5
N
N
N
N
Y(1)–N(1)
Y(1)–N(3)
Y(1)–C(1)
2.360(3)
2.379(3)
2.695(4)
Y(1)–N(2)
Y(1)–N(4)
Y(1)–C(2)
2.365(3)
2.375(3)
2.765(4)
Ln
N
Li2L(THF)0.5
N(2)–Y(1)–N(1)
57.1(1)
N(4)–Y(1)–N(3)
56.8(1)
Ln= Yb 3, Y 4
0.5Li2L(THF)0.5
LLnCl(sol)x
Ln = Yb 1, Y 2
Scheme 2.
temperature. Therefore, the formation of 3(4) in this procedure
may supposed through two steps: the metathesis reaction affords
1(2) first and then 1(2) reacts further with the dilithium salt to
form 3(4) via ligand redistribution reaction as shown in Scheme 2.
Complexes 2, 3 and 4, have been characterized by IR, single-
crystal X-ray diffraction and also 1H NMR spectroscopy for 2 and
4. No 1H NMR data were obtained for 3, due to the paramagnetism
of the ytterbium. In their IR spectra a strong absorption of C@N
stretch at approximate 1640 cmꢁ1 for both complexes was ob-
served indicating the delocalized double bond of the N–C–N link-
age [9].
The molecular structure of 2 is shown in Fig. 1. Selected bond
distances and angles are listed in Table 2. Complex 2 has a mono-
meric structure, in which the yttrium atom is ligated by two linked
bidentate amidinate moieties through nitrogen atoms, one chlo-
rine atom, and two oxygen atoms of a coordinated DME molecule.
The coordination geometry about the yttrium atom can be best de-
scribed as a distorted trigonal bipyramid with each chelating
bidentate amidinate ligand to occupy one coordination vertex,
which is similar to that of 1 reported previously. The bond lengths
of all Y–N are around 2.370(3) Å, which is comparable to that found
in 1 if the deviation in ion radium between Y and Yb was
considered.
The crystal structures of 3 and 4 are shown in Fig. 2. Selected
bond distances and angles are listed in Table 3. Complexes 3 and
4 are isostructure having a free DME molecule in the unit cell. In
the two complexes the central metals are connected together by
three bridged bis(amidinate)s and each central metal coordinates
to three amidinates in dihapto fashion through the nitrogen atoms
with the Ln–N–C–N units only deviating slight from planarity (the
largest one is 0.0003 Å from the plane Y1–N5–C24–N6). The whole
molecule has a cage-like structure with the two planes formed by
Fig. 2. Molecular structure of the complexes 3 (Ln = Yb) and 4 (Ln = Y). Thermal
ellipsoids are drawn at the 30% level. Hydrogen atoms and a free DME molecule are
omitted for clarity.
LnCCC occupying on top (Ln1–C1–C24–C47) and bottom plane
(Ln2–C2–C25–C48). The coordinated geometries about each metal
can be best described as a distorted octahedron in which four
nitrogen atoms can be considered to occupy equatorial positions
(N1, N2, N5 and N10; N3, N4, N7 and N12) with the angles sum
about the lanthanide center of RN–Ln–N = 363.4° for Yb1 and
363.8° for Y1; 365.8° for Yb2 and 366.3° for Y2, while another
two nitrogen atoms occupy axial positions and the N–Ln–N angle
is distorted away from the idealized 180° (N6–Yb1–N9, 158.7°
and N8–Yb2–N11, 159.2°; N6–Y1–N9, 157.6° and N8–Y2–N11,
158.6°). The angles of N–Ln–N in the Ln–N–C–N units are almost
equal for the two complexes and the average value is 58.3° for 3
and 57.5° for 4, which are comparable to the values found in homo-
leptic lanthanide tri(amidinates) [1a]. The bond lengths of Ln–N
range from 2.352(2) to 2.392(2) Å for 3, and 2.307(2) to
2.354(2) Å for 4. The average Ln–N bond length of 3, 2.361(2) Å,
is comparable to 2.325(2) Å found in 4, if the difference in ionic ra-
dii between the two metals is considered. The values are also com-
parable with 2.342(8) Å for [CyNC(Me)Cy]3Yb [1a] and 2.326(5) Å
for [CyNC(Ph)Cy]3Yb [1a]. The bond lengths of Ln–C on N–C–N
are around 2.765(3) Å for 3, and 2.731(3) Å for 4 which are compa-
rable to the values for lanthanide amidinate complexes published
[1a]. All C–N bond distances within the chelating NCN units are
nearly equal with the average value 1.332(4) Å for both complexes,
which reflects the delocalization of the p bond in the N–C–N unit.
3.2. Synthesis, molecular structure and reactivity of anionic complexes
[Li(DME)3][L2Ln] (Ln = Yb 5;Y 6)
Treatment of YbCl3 with Li2L(THF)0.5 in a molar ratio of 1:2, after
workup afforded colorless crystals in good yield. The crystals were
full characterized including X-ray analysis to be the target product
[Li(DME)3][L2Yb] (5) (Scheme 3). The same reaction with YCl3 gave
Fig. 1. Molecular structure of the complex 2. Thermal ellipsoids are drawn at the
20% level. Hydrogen atoms are omitted for clarity.

J. Wang et al. / Polyhedron 27 (2008) 1977–1982
1981
Table 3
Selected bond distances (Å) and angles (°) for the complexes 3 and 4
Compound
3
4
3
4
Ln(1)–N(1)
Ln(1)–N(2)
Ln(1)–N(5)
Ln(1)–N(6)
Ln(1)–N(9)
Ln(1)–N(10)
Ln(1)–C(1)
Ln(1)–C(24)
Ln(1)–C(47)
2.321(2)
2.329(2)
2.332(2)
2.307(2)
2.324(2)
2.325(2)
2.736(3)
2.727(3)
2.725(3)
2.352(2)
2.364(2)
2.365(2)
2.350(2)
2.360(2)
2.358(2)
2.769(3)
2.765(3)
2.760(3)
Ln(2)–N(3)
Ln(2)–N(4)
Ln(2)–N(7)
Ln(2)–N(8)
Ln(2)–N(11)
Ln(2)–N(12)
Ln(2)–C(2)
Ln(2)–C(25)
Ln(2)–C(48)
2.328(2)
2.320(2)
2.336(2)
2.310(2)
2.316(2)
2.354(2)
2.730(3)
2.727(3)
2.739(3)
2.365(2)
2.355(2)
2.373(2)
2.351(2)
2.351(2)
2.392(2)
2.768(3)
2.762(3)
2.768(3)
N(1)–Ln(1)–N(2)
N(6)–Ln(1)–N(5)
N(9)–Ln(1)–N(10)
58.25(8)
58.13(8)
58.52(8)
57.46(8)
57.28(9)
57.73(8)
N(4)–Ln(2)–N(3)
N(8)–Ln(2)–N(7)
N(11)–Ln(2)–N(12)
58.37(8)
58.17(8)
58.07(8)
57.56(8)
57.34(8)
57.29(8)
length of the Ln–N (2.444(7) Å for 5 and 2.475(4) Å for 6) and the
bond lengths of the Ln– C on the N–C–N (around 2.869(7) Å for 5
and 2.890(5) Å for 6) are longer than the corresponding values
found in 1 and 2. All these bond parameters reflect that four of
amidinate groups make the coordinate sphere around the central
metal more crowded. The coordinate geometry of the cation is
normal.
2 Li2L(THF)0.5
+
-
LnCl3
N
N
N
N
N
N
Li(DME)3
Ln
1 Li2L(THF)0.5
N
N
1 and 2
Ln= Yb 5, Y 6
To see whether the complex LLn(l2-L)LnL (Ln = Yb, Y) could be
prepared by the reaction of 5 and 6 with 1 and 2 respectively,
the two reactions at room temperature were tried, respectively.
Complexes 5 and 6 do react with 1 and 2 smoothly in THF, after
workup, to afford colorless crystals in good yield for both metals,
which were fully characterized to be complexes 3 and 4, respec-
tively, not the complexes LLn(l2-L)LnL as shown in Scheme 4.
The results indicate again that complexes 3 and 4 are really much
more stable than LLn(l2-L)LnL, which leads to the ligand redistri-
bution reaction occurred easily during the present metathesis
reaction.
Scheme 3.
the analogous complex [Li(DME)3][L2Y] (6) as colorless crystals in
good yield (Scheme 3). 1H NMR spectrum of 6 shows the signals
of bis(amidinate) ligands [3], but no 1H NMR data were obtained
for5 due to the paramagnetism of the ytterbium. In their IR spectra
a strong absorption of C@N stretch at approximate 1640 cmꢁ1 for
both complexes was observed indicating the delocalized double
bond of the N–C–N linkage [9].
Complexes 5 and 6 can also be prepared by reaction of 1 and 2,
respectively, with 1 equiv. of Li2L(THF)0.5 (Scheme 3).
Complexes 5 and 6 can react with LnCl3 to afford complexes 1
and 2, respectively (Scheme 4).
The molecule structures of complexes 5 and 6 are shown in
Fig. 3. Selected bond distances and angles are listed in Table 4.
Complexes 5 and 6 have the same structure composed of a pairs
of discrete cation and anion. The Ln atom in anion coordinates to
two bis(amidinate)s to form a distorted tetrahedron, if each chelat-
ing amidinate ligand is considered to occupy a single coordination
site. All four-membered rings Ln–N–C–N are essentially planar. The
average bond angle of the N–Ln–N in the Ln–N–C–N unit, 54.8(2)°,
for 5 is comparable to 54.4(1)° in 6, but smaller than the corre-
sponding values found in complexes 3 and 4. The average bond
3.3. Catalytic activity for ring-opening polymerization of e-CL
The catalytic activity of 3, 4, 5 and 6 for ring-opening polymer-
ization of e-CL was examined under variety of conditions. The pre-
liminary results are listed in Table 5. In comparison the reactivity
of Li2L was also tested, and no reactivity was found under the same
conditions (Table 5, entry 8). It can be seen that all complexes are
active initiators under mild conditions. All systems yield the poly-
mers with a unimodal molecular weight distribution, indicating
they function as single component catalysts. The anionic com-
plexes of 5 and 6 are less active related to 3 and 4. This may be con-
tributed to the crowded coordination environment around the
lanthanide metal by two bridged bis(amidinate) ligands. The cen-
ter metal has profoundly effect on the activity and the active se-
quence of Yb < Y was observed for both the anionic complexes (5
and 6) and the bimetallic complexes (3 and 4). This may result
from the more crowded coordination environment around the
smaller Yb metal than Y metal, which leads to the coordination
of monomer to metal and then insertion of activated monomer
more difficulty. Among the four complexes 4 is the most efficient
catalyst. For example, quantitative yield was obtained within
30 min at room temperature with 4 at the molar ratio of monomer
to catalyst of 2500:1, while the yields were only 87% and 57% for 3
and 6, even the molar ratio of monomer to catalyst decreased to
2000:1 (Table 5, entries 1, 3 and 7). The polymerization system
with 4 could still give 55% yield within 30 min, when the molar ra-
tio of monomer to initiator increased to 4000:1 (Table 5, entry 2).
All polymerization systems gave the polymers with rather broad
molecular weight distributions. The polydispersity index Mw/Mn
Fig. 3. Molecular structure of the complexes 5 (Ln = Yb) and 6 (Ln = Y). Thermal
ellipsoids are drawn at the 30% level. Hydrogen atoms are omitted for clarity.

1982
J. Wang et al. / Polyhedron 27 (2008) 1977–1982
Table 4
Selected bond distances (Å) and angles (°) for the complexes 5 and 6
5
6
5
6
Ln(1)–N(1)
Ln(1)–N(2)
Ln(1)–N(3)
Ln(1)–N(4)
Ln(1)–C(1)
Ln(1)–C(24)
2.492(10)
2.372(5)
2.519(11)
2.373(5)
2.851(6)
2.886(7)
2.534(5)
2.415(4)
2.526(5)
2.414(4)
2.886(5)
2.898(5)
Ln(1)–N(5)
Ln(1)–N(6)
Ln(1)–N(7)
Ln(1)–N(8)
Ln(1)–C(2)
Ln(1)–C(25)
2.525(6)
2.377(7)
2.521(6)
2.374(7)
2.861(6)
2.878(7)
2.542(4)
2.415(4)
2.540(4)
2.417(4)
2.877(5)
2.896(5)
N(2)–Ln(1)–N(1)
N(4)–Ln(1)–N(3)
54.8(2)
54.7(2)
54.3(1)
54.4(1)
N(6)–Ln(1)–N(5)
N(8)–Ln(1)–N(7)
54.6(2)
55.0(2)
54.4(1)
54.5(1)
Appendix A. Supplementary data
LnCl3
1 (2)
3 (4)
CCDC 612924, 612922, 612923, 612925 and 612926 contain the
supplementary crystallographic data for LYCl(DME) (2), Yb2(l2-
L)3 ꢀ DME (3), Y2(l2-L)3 ꢀ DME (4), [L2Yb][Li(DME)3] (5), and
[L2Y][Li(DME)3] (6). These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:depo-
sit@ccdc.cam.ac.uk Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2008.03.006.
5 (6)
1 (2)
Scheme 4.
Table 5
Polymerization of e-CL initiated by complexes 3, 4, 5 and 6a
Entry
Initiator
[M]/[I]
Time (min)
Yieldb (%)
Mn(ꢂ104)
PDIc
1
2
3
4
5
6
7
8
3
4
4
5
5
6
6
Li2L
2000:1
2500:1
4000:1
500:1
700:1
1500
30
30
30
30
30
30
30
30
87
100
55
84
65
73
57
0
13.1
10.4
6.9
0.66
0.96
3.23
4.11
–
1.87
2.22
2.80
2.25
2.66
2.41
2.50
–
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