Rare earth metal alkyl complexes bearing N,O,P multidentate ligands: Synthesis, characterization and catalysis on the ring-opening polymerization of l-lactide

Article abstract of DOI:10.1016/j.jorganchem.2007.05.032

Alkane elimination reactions of rare earth metal tris(alkyl)s, Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Y, Lu) with the multidentate ligands HL^1-4, afforded a series of new rare earth metal complexes. Yttrium complex 1 supported by flexible amino-imino phenoxide ligand HL¹ was isolated as homoleptic product. In the reaction of rigid phosphino-imino phenoxide ligand HL² with equimolar Ln(CH₂SiMe₃)₃(THF)₂, HL² was deprotonated by the metal alkyl and its imino C{double bond, long}N group was reduced to C-N by intramolecular alkylation, generating THF-solvated mono-alkyl complexes (2a: Ln = Y; 2b: Ln = Lu). The di-ligand chelated yttrium complex 3 without alkyl moiety was isolated when the molar ratio of HL² to Y(CH₂SiMe₃)₃(THF)₂ increased to 2:1. Reaction of steric phosphino β-ketoiminato ligand HL³ with equimolar Ln(CH₂SiMe₃)₃(THF)₂ afforded di-ligated mono-alkyl complexes (4a: Ln = Y; 4b: Ln = Lu) without occurrence of intramolecular alkylation or formation of homoleptic product. Treatment of tetradentate methoxy-amino phenol HL⁴ with Y(CH₂SiMe₃)₃(THF)₂ afforded a monomeric yttrium bis-alkyl complex of THF-free. The resultant complexes were characterized by IR, NMR spectrum and X-ray diffraction analyses. All alkyl complexes exhibited high activity toward the ring-opening polymerization of l-lactide to give isotactic polylactide with controllable molecular weight and narrow to moderate polydispersity.

Full text of DOI:10.1016/j.jorganchem.2007.05.032

Journal of Organometallic Chemistry 692 (2007) 3823–3834
www.elsevier.com/locate/jorganchem
Rare earth metal alkyl complexes bearing N,O,P multidentate
ligands: Synthesis, characterization and catalysis on the
ring-opening polymerization of L-lactide
a,b,
a
Wei Miao a,c, Shihui Li a,c, Dongmei Cui
*, Baotong Huang
a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, 430062, China
b
c
Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
Received 20 April 2007; received in revised form 19 May 2007; accepted 21 May 2007
Available online 2 June 2007
Abstract
Alkane elimination reactions of rare earth metal tris(alkyl)s, Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu) with the multidentate ligands HL1–4
,
afforded a series of new rare earth metal complexes. Yttrium complex 1 supported by flexible amino-imino phenoxide ligand HL1 was iso-
lated as homoleptic product. In the reaction of rigid phosphino-imino phenoxide ligand HL2 with equimolar Ln(CH2SiMe3)3(THF)2, HL2
was deprotonated by the metal alkyl and its imino C@N group was reduced to C–N by intramolecular alkylation, generating THF-solvated
mono-alkyl complexes (2a: Ln = Y; 2b: Ln = Lu). The di-ligand chelated yttrium complex 3 without alkyl moiety was isolated when the
molar ratio of HL2 to Y(CH2SiMe3)3(THF)2 increased to 2:1. Reaction of steric phosphino b-ketoiminato ligand HL3 with equimolar
Ln(CH2SiMe3)3(THF)2 afforded di-ligated mono-alkyl complexes (4a: Ln = Y; 4b: Ln = Lu) without occurrence of intramolecular alkyl-
ation or formation of homoleptic product. Treatment of tetradentate methoxy-amino phenol HL4 with Y(CH2SiMe3)3(THF)2 afforded a
monomeric yttrium bis-alkyl complex of THF-free. The resultant complexes were characterized by IR, NMR spectrum and X-ray diffrac-
tion analyses. All alkyl complexes exhibited high activity toward the ring-opening polymerization of L-lactide to give isotactic polylactide
with controllable molecular weight and narrow to moderate polydispersity.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Rare earth metal; Alkyl complex; Multidentate; Lactide
1. Introduction
alkyls. The extensively developed family is cyclopentadie-
nyl (Cp) and its derivatives [4]. Recently, research efforts
have been directed towards non-Cp ligands by virtue of
their exceptional and tunable steric and electronic features
required for compensating coordinative unsaturation of
metal centers [5–9]. However, rare earth metal alkyl, espe-
cially bis(alkyl) complexes supported by non-Cp ligands
usually encounter salt addition, dimerization or ligand
redistribution due to the highly active and relatively less
steric characters of bis(alkyl) species. Thus, multidentate
ligands have attracted more and more interests, which
can provide proper sterics and electronics for the metal
center to prevent the side-reactions mentioned above via
varying the size and electron donor of the substituents.
There is a growing interest in the past decades in the
investigation of rare earth metal alkyl complexes that have
shown tremendous catalytic activities toward olefin poly-
merization [1], highly regio- or stereo-selective polymeriza-
tions of conjugated monomers [2] and polar monomers [3].
Many ancillary ligands have been utilized to stabilize metal
*
Corresponding author. Address: State Key Laboratory of Polymer
Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China. Tel.: +86 431
85262773; fax: +86 431 85263773.
E-mail address: dmcui@ciac.jl.cn (D. Cui).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.05.032

3824
W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
For example, tridentate phenoxytriamine [10], multidentate
bis(oxazolinate)s [11] and tetradentate triazacyclononane-
amide [12] have been reported to support rare earth metal
amido or alkyl species.
Here, we wish to present the synthesis of a series of rare
earth metal alkyl complexes supported by multidentate
phenol with phosphino, amino or methoxy-amino func-
tionalities and phosphino b-ketoimine auxiliaries. The
dependence of molecular structures of the resultant com-
plexes and their catalytic behavior towards the ring-open-
ing polymerization (ROP) of L-lactide on the ligand
framework will also be discussed.
2. Results and discussion
2.1. Syntheses and characterization of complexes
2.1.1. Synthesis and characterization of complex 1
The protonolysis reaction of Y(CH2SiMe3)3(THF)2 with
N,N,O tridentate ligand HL1 was carried out in hexane at
room temperature overnight to afford yellow solids, which
was not the expected bis(alkyl) product but a tri-ligated
Fig. 1. X-ray structure of 1 with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
1
complex (L1)3Y (1) (Scheme 1). The H NMR and 13C
NMR spectra were indicative to define the formation of
complex 1 by the loss of the signal for phenol proton
around d 13.76. The molecular structure was further con-
firmed by X-ray diffraction to be seven-coordinate
(Fig. 1). The yttrium atom bonded to three ligands, adopt-
ing distorted pentagonal bipyramidal geometry. The imino
nitrogen N(1) and phenoxy oxygen O(2) were axial, while
the phenoxy oxygen atoms O(1) and O(3), imino nitrogen
atoms N(2) and N(3) and the amino nitrogen N(6) occu-
pied the equatorial positions. No coordinated alkyl or
THF moiety was observed. Two of the three ligands
bonded to the yttrium ion in N,O-bidentate modes with
the amino group hanging away from the metal center,
whereas for the third ligand the amino nitrogen combined
with the imino nitrogen and the phenoxy oxygen coordi-
nated to the yttrium ion in N,N,O-tridentate mode. Thus
the bite angles of O(1)–Y(1)–N(1) (74.49(18))ꢁ, O(2)–
Y(1)–N(2) (74.78(18))ꢁ and O(3)–Y(1)–N(3) (72.20(18))ꢁ
were different. This was in agreement with the NMR spec-
trum analysis. The methyl protons of NMe2 group in com-
plex 1 exhibited a broad resonance at d 2.15 ppm, which
was different from the singlet signal at d 2.30 ppm in the
free ligand. The bond distances between yttrium and the
˚
chelated nitrogen atoms Y(1)–N(1) (2.531(6) A), Y(1)–
˚
˚
N(2) (2.476(6) A) and Y(1)–N(3) (2.509(6) A) were close,
which were shorter than that between yttrium and the pen-
˚
dent amino nitrogen atom Y(1)–N(6) (2.714(6) A) (Table
˚
1). The bond distances of N(1)–C(15) (1.276(9) A), N(2)–
˚
˚
C(34) (1.290(9) A) and N(3)–C(53) (1.276(9) A) were com-
parable to the established values observed for C@N double
˚
bond (1.294–1.312 A) [13,14].
2.1.2. Syntheses and characterization of complexes 2a, 2b
and 3
Switching from the flexible ligand HL1 to the rigid and
bulky ligand HL2 was expected to inhibit the formation
of the homoleptic complex. The reaction of HL2 with
tBu
But
tBu
N
N
O
N
N
N
Y
N
But
hexane
+
Y(CH2SiMe3)3(THF)2
O
O
r.t., overnight
OH
N
N
tBu
But
tBu
But
HL1
1
Scheme 1. Synthetic pathway for complex 1.

W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
3825
Table 1
Selected bond lengths (A) and angles (ꢁ) for complex 1
field compared to the singlet resonance around
d
À0.6 ppm in the yttrium tris(alkyl)s. The intramolecular
alkyl migration was confirmed by the formation of group
C(H)(CH2SiMe3)N, which exhibited discrete doublet reso-
nances at d 1.21 and 2.08 ppm assigned to the diastereo-
topic methylene protons. The solid-state structure of
complex 2a was confirmed by X-ray diffraction analysis
to be a monomer of six-coordinate with two solvated
THF molecules. The geometry around the central metal
was described as distorted tetragonal bipyramidal
(Fig. 2). The bond distances between yttrium and THF
˚
Y(1)–N(2)
Y(1)–N(3)
Y(1)–N(1)
Y(1)–N(6)
N(1)–C(15)
2.476(6)
2.509(6)
2.531(6)
2.714(6)
1.276(9)
Y(1)–O(2)
Y(1)–O(1)
Y(1)–O(3)
N(3)–C(53)
N(2)–C(34)
2.161(4)
2.193(5)
2.216(5)
1.276(9)
1.290(9)
O(2)–Y(1)–N(1)
O(1)–Y(1)–N(1)
O(3)–Y(1)–N(1)
N(2)–Y(1)–N(1)
N(3)–Y(1)–N(1)
O(2)–Y(1)–N(6)
O(1)–Y(1)–N(6)
O(3)–Y(1)–N(6)
N(2)–Y(1)–N(6)
N(3)–Y(1)–N(6)
N(1)–Y(1)–N(6)
169.90(18)
74.49(18)
81.95(18)
97.46(19)
102.7(2)
97.78(18)
75.95(18)
132.63(19)
150.3(2)
O(2)–Y(1)–O(1)
O(2)–Y(1)–O(3)
O(1)–Y(1)–O(3)
O(2)–Y(1)–N(2)
O(1)–Y(1)–N(2)
O(3)–Y(1)–N(2)
O(2)–Y(1)–N(3)
O(1)–Y(1)–N(3)
O(3)–Y(1)–N(3)
N(2)–Y(1)–N(3)
97.12(17)
102.16(18)
141.52(18)
74.78(18)
76.54(19)
76.89(19)
87.33(18)
142.23(19)
72.20(19)
139.9(2)
˚
oxygen atoms O(1) and O(3) were 2.355(2) A and
˚
2.397(2) A, respectively, longer than the distance between
˚
yttrium and phenoxy oxygen Y–O(2) (2.119(2) A) (Table
˚
˚
66.3(2)
85.78(19)
2). The bond length of C(19)–N (1.497(4) A) was close to
the normal C–N single bond (1.4904(17) A) [13,14], indi-
cating that the C@N bond of the ligand had been reduced
to a single bond via intramolecular alkylation. The intra-
molecular alkylation has been found in N,O bidentate
Salen titanium or zirconium bis(benzyl) complexes, which
resulted in decomposition of the complexes and the loss
of catalytic activity for ethylene polymerization [16].
The 1H NMR spectrum and the X-ray analysis revealed
that 2b was an analogue of 2a (Fig. 3). The slight difference
between the structures of the two complexes was the bond
angle of O(1)–Ln–O(3) being 174.83(7)ꢁ in 2a, but much
smaller (77.83(16)ꢁ) in 2b, indicating the different arrange-
ment of THF moieties (Table 2).
Treatment of Y(CH2SiMe3)3(THF)2 with 2 equiv. of
HL2 at room temperature for 12 h generated complex 3
in high yields (75%) via abstractions of metal alkyls
(Scheme 2). The existence of the C@N bond was confirmed
by the resonance around d 7.90 ppm in 1H NMR spectrum
assigned to the imino proton of CH@N group. At the same
1 equiv. of Y(CH2SiMe3)3(THF)2 took place immediately
upon addition to afford mono(alkyl) complex 2a. Follow-
ing the similar procedure, the lutetium analogue was
obtained by treatment of HL2 with 1 equiv. of Lu(CH2Si-
Me3)3(THF)2 (Scheme 2). The probable mechanistic path-
way was described as Scheme 2. First, HL2 protonated
one metal alkyl species of Ln(CH2SiMe3)3(THF)2 with
the release of tetramethylsilane to give a bis(alkyl) interme-
diate A. Unfortunately, A was not stable, the imino C@N
group of which was then alkylated by intramolecular
migration of the metal alkyl to generate C(H)(CH2Si-
Me3)N species, leading to the formation of complex 2 even-
tually. The similar protonolysis accompanied by
intramolecular alkyl migration was observed in the isola-
tion of a pyrrolylaldiminato yttrium alkyl complex [15].
The methylene protons of Y–CH2SiMe3 in complex 2a
exhibiting a AB spin around d À0.39 ppm shifted down-
tBu
tBu
tBu
tBu
SiMe3
Ph2
P
O
tBu
N
HC
N
tBu
SiMe3
CH2
SiMe3
(THF)2
OH
1/2 equiv of Ln(CH2SiMe3)3(THF)2
r.t., overnight
N
Ln
O
N
P
THF
Ph2
Ln
O
PPh2
HL2
But
P
Ph2
A
tBu
3: Ln = Y
1 equiv of Ln(CH2SiMe3)3(THF)2
-35 oC, 30 min
HL2
tBu
SiMe3
tBu
O
N
P
(THF)2
SiMe3
Ln
Ph2
2a: Ln = Y
2b: Ln =Lu
Scheme 2. Synthetic pathway for complexes 2a, 2b and 3.

3826
W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
lecular alkyl migration and the formation of C–N bond
reduced from C@N. This might be a two-step process:
firstly, reaction of Y(CH2SiMe3)3(THF)2 with equimolar
HL2 via intramolecular alkyl migration afforded 2a, which
reacted further with the excessive HL2 via alkane elimina-
tion to generate 3. If kept at room temperature for several
days, 2a would slowly disproportionate to 3. Such dispro-
portionation was also observed by Piers et al. in the prep-
aration of yttrium and scandium bis(N,O-bidentate
salicylaldiminato) mono(alkyl) complexes which decom-
posed by 1,3-migration of the alkyl groups to the aldimine
carbon [17]. The molecular structure of complex 3 was fig-
ured out by X-ray analysis to be seven-coordinate with a
solvated THF molecule. No metal-alkyl species existed.
Two ligands were bonded to the yttrium atom to form a
distorted pentagonal bipyramidal geometry around the
metal center (Fig. 4) with P(1), P(2), N(1), N(2) and phen-
oxy oxygen O(2) in equatorial positions, while the other
phenoxy oxygen O(1) and the oxygen atom of THF
Fig. 2. X-ray structure of 2a with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
Table 2
˚
˚
Selected bond lengths (A) and angles (ꢁ) for complexes 2a and 2b
(O(3)) axial. The bond length of N(2)–C(56) (1.321(11) A)
(Table 3) was reasonable for the normal C@N bond,
2a
2b
Ln–O(2)
Ln–O(1)
Ln–O(3)
Ln–N
Ln–C(46)
Ln–P
2.119(2)
2.355(2)
2.397(2)
2.374(2)
2.451(4)
2.9717(8)
1.380(4)
1.497(4)
2.064(4)
2.378(4)
2.434(4)
2.254(5)
2.368(7)
2.9376(16)
1.369(7)
1.491(7)
N–C(18)
N–C(19)
O(2)–Ln–N
O(2)–Ln–C(46)
N–Ln–C(46)
O(2)–Ln–P
N–Ln–P
C(46)–Ln–P
O(1)–Ln–O(3)
85.69(8)
106.36(12)
166.74(11)
152.18(6)
66.54(6)
88.15(16)
103.5(2)
98.8(3)
153.74(12)
69.04(12)
93.06(19)
77.83(16)
101.44(11)
174.83(7)
Fig. 4. X-ray structure of 3 with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) for complex 3
Y(1)–O(2)
Y(1)–O(1)
Y(1)–N(1)
Y(1)–O(3)
Y(1)–N(2)
Y(1)–P(1)
2.130(6)
2.162(6)
2.314(7)
2.444(7)
2.531(8)
3.040(3)
Y(1)–P(2)
3.217(3)
N(1)–C(18)
N(1)–C(19)
N(2)–C(56)
N(2)–C(55)
1.377(11)
1.468(12)
1.321(11)
1.443(11)
O(2)–Y(1)–O(1)
O(2)–Y(1)–N(1)
O(1)–Y(1)–N(1)
O(2)–Y(1)–N(2)
O(1)–Y(1)–N(2)
N(1)–Y(1)–N(2)
O(2)–Y(1)–P(1)
O(1)–Y(1)–P(1)
93.9(2)
81.8(2)
N(1)–Y(1)–P(1)
N(2)–Y(1)–P(1)
O(2)–Y(1)–P(2)
O(1)–Y(1)–P(2)
N(1)–Y(1)–P(2)
N(2)–Y(1)–P(2)
P(1)–Y(1)–P(2)
62.26(17)
78.11(17)
86.29(17)
97.99(18)
147.99(19)
57.43(18)
127.57(8)
Fig. 3. X-ray structure of 2b with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
112.3(3)
138.1(2)
73.5(3)
140.0(2)
143.50(17)
94.05(17)
time, the discrete doublet resonances around d 1.16 and
1.99 ppm arising from the diastereotopic methylene pro-
tons of C(H)(CH2SiMe3)N group confirmed the intramo-

W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
3827
˚
meanwhile the bond length of N(1)–C(19) (1.468(12) A)
˚
was close to the normal C–N bond (1.4904(17) A) [13,14],
indicating that C@N bond of one coordinated ligand had
been reduced to a single bond via intramolecular alkylation
as discussed previously for complex 2a, while the C@N
bond of another ligand retained the double bond nature.
The bond distance between yttrium and the amino nitrogen
˚
Y–Namino (Y–N(1), 2.314(7) A) was slightly shorter than
that between yttrium and the imino nitrogen Y–Nimino
˚
(Y–N(2), 2.531(8) A), both fell in the range of 2.28–
˚
2.62 A observed for Y–N bonds [18]. The six-membered
ring YO(1)NiminoC3 was planar and almost perpendicular
to the six-membered ring YO(2)NaminoC3 that was
much twisted due to the flexible feature. The bond
distances between the yttrium and phenoxy oxygen atoms
˚
˚
Fig. 5. X-ray structure of 4a with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
Y–Ophenoxy (2.130(6) A for Y(1)–O(2) and 2.162(6) A for
Y(1)–O(1)) were much shorter than that between yttrium
˚
and the THF oxygen Y–OTHF (Y–O(3), 2.444(7) A). The
bond angles N(2)–Y–P(2) (57.43(18)ꢁ) and N(1)–Y–P(1)
(62.26(17)ꢁ) were smaller than that found in 2a (N–Y–P,
66.54(6)ꢁ), which was contributed to the crowded environ-
ment in complex 3.
2.1.3. Syntheses and characterization of complexes 4a and 4b
The b-ketoiminato ligand HL3 with sterically bulky 2-
bisphenylphosphine-phenyl group on the nitrogen was pre-
pared, anticipated to stabilize the rare earth metal alkyl
complexes and prevent the intramolecular alkyl migration
and the formation of homoleptic product.
Treatment of HL3 with 1 equiv. of Ln(CH2Si-
Me3)3(THF)2 indeed afforded mono-alkyl bis-ligand com-
plexes (L3)2Ln(CH2SiMe3) (4a: Ln = Y,4b: Ln = Lu)
without alkyl migration and the formation of homoleptic
counterpart (Scheme 3). X-ray diffraction analysis revealed
that the complexes crystallized in the chiral triclinic space
Fig. 6. X-ray structure of 4b with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
ꢀ
group P1, and the primitive unit cell contained two inde-
pendent but geometrically similar molecules. Complexes
4a and 4b were isostructural mono-alkyls of solvent-free
stabilized by two b-ketoiminato ligands in P,N,O-triden-
tate modes. The metal centers adopted distorted pentago-
nal bipyramidal geometry (Figs. 5 and 6) with the enolate
oxygen atoms O(1) and O(2) being axial and the N(1),
N(2), P(1), P(2) and the alkyl carbon C(47) occupying the
equatorial positions. The ligands coordinated to the central
metal ion to form two six-membered rings LnOC3N with
bite angles averaging 74.53ꢁ for 4a and 75.71ꢁ for 4b (Table
4). The two rings arranged in cis-positions and both were
trans to the metal alkyl group. The bond distances of
Ln–Oenolate (av. 2.192 A for 4a, and av. 2.164 A for 4b)
were slightly longer than those of Ln–Ophenoxy found in
˚
˚
˚
˚
˚
2a (2.119 A), 2b (2.064 A) and 3 (av. 2.146 A), but shorter
˚
than Ln–OTHF (2.355–2.444 A) in 2a, 2b and 3. The Ln–N
bond distances (av. 2.470(4) A for 4a, and av. 2.425(4) A for
˚
˚
O
Ph2
P
N
PPh2
hexane
Ln
+ Y(CH2SiMe3)3(THF)2
P
-35oC, 5 min
Ph2
N
N
OH
O
SiMe3
HL 3
4a: Ln = Y
4b: Ln = Lu
Scheme 3. Synthetic pathway for complexes 4a and 4b.

3828
W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
Table 4
˚
Selected bond lengths (A) and angles (ꢁ) for complexes 4a and 4b
4a
4b
Ln–O(1)
Ln–O(2)
Ln–C(47)
Ln–N(1)
Ln–N(2)
Ln–P(2)
Ln–P(1)
2.188(3)
2.196(3)
2.427(5)
2.455(4)
2.486(4)
3.0958(13)
3.1411(13)
2.157(3)
2.170(3)
2.403(5)
2.397(4)
2.453(4)
2.9806(12)
3.1021(12)
O(1)–Ln–O(2)
O(1)–Ln–N(1)
O(2)–Ln–N(2)
N(2)–Ln–P(2)
N(1)–Ln–P(1)
P(2)–Ln–P(1)
168.29(12)
74.64(12)
74.42(12)
59.48(9)
58.93(9)
174.35(3)
169.34(12)
75.95(12)
75.47(13)
60.47(10)
60.36(10)
172.62(4)
Fig. 7. X-ray structure of 5 with 35% probability of thermal ellipsoids,
hydrogen atoms are omitted for clarity.
4b) were slightly shorter than the corresponding Y–Nimino
˚
bond distance in complex 3 (2.531(8) A) but longer than
˚
amino fragment were disordered, and the spatial occupancy
probability for each position was 50%, respectively. The
two alkyl species arranged in cis-positions with one endo-
and another exo- against the O,O,N,O-tetradentate ligand,
generating a distorted tetragonal bipyramidal geometry.
The phenoxy oxygen O(1), methoxy oxygen O(3), an alkyl
carbon C(22) and N atoms occupied the equatorial posi-
tions, while another methoxy oxygen O(2) and another
alkyl carbon C(23) atoms were axial. The bond angle
formed by yttrium ion and pendent methoxy oxygen atoms
O(2)–Y–O(3) was 76.96(7)ꢁ, whereas O(11)–Co–O(15) was
121.31(4)ꢁ for a cobalt complex supported by analogous
phenolate ligand [20]. The distances between yttrium atom
and the pendent methoxy oxygen atoms Y–Omethoxy (Y–
Y–Namino bond distances in 2a and 3 (av. 2.344 A). The
two phosphorus atoms located in trans-positions with
P(1)–Ln–P(2) bond angles of 174.35(3)ꢁ for 4a and
172.62(4)ꢁ for 4b. The bond angles of N–Ln–P (av.
59.205ꢁ for 4a, and av. 60.415ꢁ for 4b) were comparable
to that in complex 3 (av. 59.845ꢁ) but much smaller than
the corresponding those found in 2a (66.54ꢁ) and 2b
(69.04ꢁ).
2.1.4. Synthesis and characterization of complex 5
The above mentioned successful isolations of rare earth
metal alkyl complexes supported by tridentate ligands sug-
gested that the tetradentate bis(methoxyethylene)-amino
phenol HL4 could provide suitable electronic and steric
environment to stabilize rare earth metal bis(alkyl) species.
Reaction of Y(CH2SiMe3)3(THF)2 with equimolar HL4
at À35 ꢁC for 15 min afforded a bis(alkyl) complex
(L4)Y(CH2SiMe3)2 (5) as white solids in quantitative yield
˚
˚
O(2), 2.513(2) A; Y–O(3), 2.4149(19) A) were comparable
to those of Y–OTHF and Y–Oether in a range of 2.45–
˚
2.50 A [21,22], but longer than the distance between yttrium
atom and the phenoxy oxygen atom Y–Ophenoxy (Y–O(1),
1
˚
2.1179(19) A) (Table 5), indicating the similar bond
(Scheme 4). The H NMR analysis of complex 5 displayed
strength of Y–Omethoxy and Y–OTHF. The bond length of
that the methylene protons of Y–CH2SiMe3 exhibited a
doublet around d = À0.45 ppm, different from the singlet
resonance at d = À0.6 ppm in yttrium tri(alkyl)s [19]. The
isolation of single crystals from a mixture of toluene and
hexane (3:1 v/v) at À35 ꢁC allowed us figure out the molec-
ular structure of 5 by means of X-ray analysis (Fig. 7). The
complex was six-coordinate monomer without solvated
THF molecule. Three carbon atoms of one tert-butyl group
and one carbon atom of the pendent methoxy-ethylene
˚
Y–N (2.507(3) A) was comparable to those found in com-
˚
plexes 2a, 2b, 3 and 4a, 4b (2.374–2.486 A). The bond angles
Omethoxy–Y–N averaging 67.945ꢁ were also close to the cor-
responding those found in a yttrium dialkyl complex
[(CH3OCH2CH2)2NCH2-3,5-(CMe3)2-2-C6H2O]Y(CH2-Si-
Me2Ph)2 (65.89(4)–69.30(4)ꢁ) [23]. The bond angle
O
phenoxy–Y–N (78.21(8)ꢁ) was comparable to the corre-
sponding data in a phenoxytriamine yttrium complex
O
O
O
N
O
N
Y
toluene
But
OH
tBu
HL4
But
O
+ Y(CH2SiMe3)3(THF)2
SiMe3
-35 oC, 15 min
tBu
SiMe3
5
Scheme 4. Synthetic pathway for complex 5.

W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
3829
Table 5
Selected bond lengths (A) and angles (ꢁ) for complex 5
Table 7
Polymerization of L-lactide at different monomer-to-initiator ratiosa
˚
Y–C(22)
Y–C(23)
Y–N
2.425(4)
2.421(3)
2.507(3)
Y–O(1)
Y–O(2)
Y–O(3)
2.1179(19)
2.513(2)
2.4149(19)
Entry
Catalyst
[LA] /[Y] Conversion (%)b Mn (10À4 c
)
Mw/Mcn
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2a
2a
5
100
200
300
500
700
1000
300
500
700
1000
99.6
97.8
98.6
97.1
95.5
90.2
98
96
95
88
1.00
1.56
2.00
5.37
7.44
12.9
1.59
2.4
1.43
1.46
1.46
1.39
1.45
1.26
1.65
1.64
1.58
1.59
O(1)–Y–C(23)
O(3)–Y–C(23)
O(1)–Y–C(22)
O(3)–Y–C(22)
C(23)–Y–C(22)
O(1)–Y–N
O(3)–Y–N
C(23)–Y–N
C(22)–Y–N
97.29(9)
92.91(9)
115.39(10)
93.58(10)
101.93(12)
78.21(8)
68.64(8)
96.99(10)
154.70(10)
O(1)–Y–O(3)
O(1)–Y–O(2)
O(3)–Y–O(2)
C(23)–Y–O(2)
C(22)–Y–O(2)
O(2)–Y–N
O(2)–C(17A)–C(16)
O(2)–C(17B)–C(16)
146.26(7)
84.78(8)
76.96(7)
163.44(10)
91.87(11)
67.25(8)
112.5(5)
113.1(7)
5
5
3.63
6.0
10
a
5
Solvent THF, [L-LA] = 1.0 mol/L, 20 ꢁC, 2 h.
Weight of polymer obtained/weight of monomer used.
Measurement by GPC calibrated with standard polystyrene samples,
b
c
and corrected using a factor 0.58.
(O(1)–Y–N(1), 76.2(2)ꢁ) [10], but smaller than those found
in complexes 2a (85.69(8)ꢁ) and 2b (88.15(16)ꢁ). The bond
lengths and angle formed by alkyl species with metal ion,
Y–C (av. 2.423 A) and C–Y–C (101.93(12)ꢁ) were within
the reasonable ranges found in other bis(alkyl) complexes
bearing heteroatom-containing ligands [24].
˚
Table 8
Influence of the reaction medium on the polymerization of L-lactidea
Entry Solvent [L-LA]/
[Y]
t
Conversion
(%)b
Mn
(10À4 c
Mw/
(h)
)
Mnc
2.2. Ring-opening polymerization of L-lactide
1
2
3
THF
CH2Cl2 300
Toluene 300
300
2
5
2
98
31
82
1.59
n.d.
1.21
1.65
n.d.
1.42
Rare earth metal complexes 2a, 2b, 4a, 4b and 5 were
active initiators for the ring-opening polymerization
(ROP) of L-lactide (L-LA) under mild conditions. The cat-
alytic activity was dependent on the molecular structures of
the complexes and the polymerization conditions. The rep-
resentative polymerization data were summarized in Tables
6–8. When mono(alkyl) complexes 2a,b and 5 being initia-
tors, complete conversions of the monomer into polylactide
(PLA) were achieved within 2 h at 20 ꢁC, whereas only 1 h
was needed when complexes 4a and 4b being initiators at
the same conditions. This might be attributed to the coor-
dination of two THF molecules in complexes 2a,b. The less
reactive characteristic of complex 5 might be due to the
coordination of methoxy moieties which played similar
roles as those of THF moieties in complexes 2a,b.
At various monomer-to-initiator ratios ranging from
100 to 1000, polymerization of L-LA initiated by complexes
2a and 5 proceeded smoothly at 20 ꢁC. The molecular
weight of the resulting PLA increased with the ratio while
the molecular weight distribution kept no change obvi-
ously, indicating that the reaction was controllable. When
the ratio was raised over 500, the polymerization was slug-
a
b
c
Complex 5, [L-LA]: 1.0 mol/L, 20 ꢁC.
Weight of polymer obtained/weight of monomer used.
Measurement by GPC calibrated with standard polystyrene samples,
and corrected using a factor 0.58.
gish because of the lower concentration of the initiator, and
complete conversion could not be reached even for long
reaction time. Generally, the molecular weight of the resul-
tant PLA was lower than the theoretic value. In the case of
bis(alkyl) complex 5 being the initiator, molecular weight
of PLA was halved compared with those when using
mono(alkyl) complexes as initiators, suggesting that both
metal alkyl species participated in the initiation, namely,
double-sites, leading to the relatively broader molecular
weight distribution (Table 7).
Reaction medium played important role in the polymer-
ization. THF was the most optimum solvent (Table 8). In
contrast, when the polymerization was performed in
CH2Cl2, the conversion was only 31% for long time (5 h).
This could be imputed to the coordination of CH2Cl2
which changed the steric environment and the Lewis acid-
ity of the central metal [25], consistent with the previously
reported bis(phenolate)yttrium system [26]. When using
toluene as medium, the conversion was relatively higher
than in CH2Cl2 but still lower than in THF due to the poor
solubility of L-LA in toluene.
Table 6
Polymerization of L-lactide with various rare earth meta alkyl complexesa
Entry
Catalyst
t (h)
Mn · (10À4 b
)
Mw/Mn
1
2
3
4
5
a
2a
2b
4a
4b
5
2
2
1
1
2
2.00
2.12
1.97
2.05
1.59
1.46
1.47
1.45
1.48
1.65
1
The H NMR spectrum of PLA displayed a symmetric
quartet resonance at d = 5.15 ppm assigned to the methine
proton, indicating the isotactic microstructure of PLA. No
racemization was detectable.
Solvent: THF, 20 ꢁC, [L-LA]: 1.0 mol/L, [LA]/[Ln] = 300.
Measurement by GPC calibrated with polystyrene standard and cor-
1
b
The H NMR spectrum of PLA oligomer (complex 2a,
rected using a factor of 0.58.
[LA]0/[Y]0 = 20) displayed signals at d = 4.35, 2.66,

3830
W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
1.48 ppm assigned to the end group HOCH(CH3)CO–
[26b,27]. The resonances for the other end group –COCH2-
SiMe3 derived from the coordination-insertion of LA into
metal alkyl species Ln–CH2SiMe3, was weak, whereas, the
end group –COOCH3 gave strong resonance around
d = 3.74 ppm. This might be imputed to the unstable nature
of –COCH2SiMe3 that swiftly transferred into –COOCH3
due to the exchange reaction with terminator CH3OH, in
agreement with the previously reported result [28].
C6H4(HL2) were synthesized according to the literatures
[13,30] by Schiff-base condensation from 3,5-tBu2-
(OH)C6H2CHO and the corresponding amino derivatives.
Phosphino b-ketoimine, 2-PPh2C6H4N@C(Me)CH2C(O)-
CH3 (HL3) was prepared in good yields by condensation
of the corresponding b-diketone with (2-diphenylphos-
phine)aniline in methanol [13,30]. The tetradentate ligand
HL4 was prepared from 2,4-tBu2-C6H2OH, paraformalde-
hyde and (MeOCH2CH2)2NH in refluxing methanol via
Mannich condensation [10].
3. Conclusion
4.2.2. Synthesis of complex 1
We have demonstrated the syntheses and structures of
several lanthanide mono- or bis-alkyl complexes stabilized
by multidentate phenoxide ligands. The protonolysis reac-
tion along with intramolecular alkylation of the C@N
bond of the ligand were observed in some cases. Both the
molecular structure and the catalytic performance are
strongly dependent on the ligand framework. The di-ligand
chelated momo(alkyl) complexes were more active toward
the ROP of L-LA than the mono-ligand chelated
momo(alkyl) complexes of two THF solvate and the
bis(alkyl) complex coordinated by two methoxy moieties,
indicating that the electron-negative metal center decreased
the catalytic activities of the corresponding complexes.
A solution of HL1 (0.123 g, 0.04 mmol) in hexane
(6 mL) was added dropwise to a hexane solution of
Y(CH2SiMe3)3(THF)2 (0.20 g, 0.40 mmol) at À35 ꢁC. The
resulting orange mixture was stirred overnight at room
temperature, and then the volatile was removed in vacuum.
The residue was washed with hexane to afford orange-
yellow solids of complex 1 (0.20 g, 51%). Single crystals
suitable for X-ray analysis grew from a hexane solution
of 1 cooled to À35 ꢁC for several days.1H NMR
(400 MHz, C6D6, 25 ꢁC): d 1.47 (s, 27H, tBu), 1.73
(s, 27H, tBu), 2.15 (br, 18H, NMe2), 2.53 (br, 6H,
CH2NMe2), 3.60 (br, 6H, CH2CH2NMe2), 7.25 (s, 3H,
ArH), 7.73 (s, 3H, ArH), 8.29 (br, 3H, CH@N). 13C
NMR (100 MHz, C6D6, 25 ꢁC): d 30.60 (s, 9C, tBu),
t
4. Experimental
32.31 (s, 9C, Bu), 34.50 (s, 3C, CMe3), 35.99 (s, 3C,
CMe3), 46.29 (br, 6C, NMe2), 47.37 (s, 3C, CH2NMe2),
60.79 (s, 3C, CH2CH2NMe2), 123.05 (s, 3C, p-OC6H2),
130.04 (t, 3C, o-N@CC6H2), 130.73 (s, 3C, p-N@CC6H2),
136.48 (s, 3C, o-OC6H2), 139.68 (s, 3C, ipso-N@CC6H2),
165.18 (s, 3C, ipso-OC6H2), 171.72 (s, 3C, CH@N); IR
(KBr pellets): 638, 703, 745, 788, 814, 838, 876, 900, 929,
978, 1025, 1079, 1164, 1201, 1236, 1257, 1274, 1325,
1359, 1415, 1439, 1462, 1535, 1615, 2767, 2867,
2955 cmÀ1. Anal. Calc. for C58.50H96.50N6O3Y: C, 67.76;
H, 9.16; N, 8.78. Found: C, 67.58; H, 9.03; N, 8.47%.
4.1. General methods
All the syntheses and manipulations of air- and moisture-
sensitive materials were carried out under a dry and oxygen-
free argon atmosphere by using Schlenk techniques or under
a nitrogen atmosphere in an MBRAUN glovebox. All sol-
vents were purified from MBRAUN SPS system. Organo-
metallic samples for NMR spectroscopic measurements
were prepared in a glovebox by using NMR tubes sealed with
paraffin film.1H, 13C NMR spectra were recorded on a Bru-
ker AV400 (400 MHz for 1H, 100 MHz for 13C) spectrome-
ter. NMR assignments were confirmed by the 1H–1H
(COSY) and 1H–13C (HMQC) experiments when necessary.
IR spectra were recorded on a VERTEX 70 FT-IR spec-
trometer. Gel permeation chromatography (GPC) analyses
of polymer samples were carried at 30 ꢁC using THF as an
eluent on a Waters-410 instrument and calibrated using
polystyrene standards. Elemental analyses were performed
at National Analytical Research Centre of Changchun Insti-
tute of Applied Chemistry (CIAC). L-LA (Aldrich) was
recrystallized from CH3COOCH2CH3 was dried over
CaH2, distilled before use. (2-Diphenylphosphino) aniline
was prepared according to the literature [29].
4.2.3. Synthesis of complex 2a
A solution of HL2 (0.20 g, 0.40 mmol) in hexane (5 mL)
was added dropwise to a hexane solution of Y(CH2Si-
Me3)3(THF)2 (0.20 g, 0.400 mmol) at À35 ꢁC. The yellow
reaction mixture was stirred for 30 min and then concen-
trated under reduced pressure to afford yellow solids which
were washed with cold hexane (2 mL · 3) to give 2a (0.28 g,
78%). Single crystals suitable for X-ray analysis grew from
a cold hexane solution of 2a at À35 ꢁC within several days.
1H NMR (400 MHz, C6D6, 25 ꢁC): d À0.39 (AB, 2H,
2JH–H = 11.2 Hz, JH–Y = 2 Hz , YCH2SiMe3), 0.06 (s, 9H,
CH2SiMe3), 0.55 (s, 9H, CHCH2SiMe3), 1.14 (s, 8H,
2
3
THF), 1.21 (d, 1H, JH–H = 12 Hz, JH–H = 4 Hz,
t
t
CHCH2SiMe3), 1.53 (s, 9H, Bu), 1.88 (s, 9H, Bu), 2.08 (t,
1H, JH–H = 13.6 Hz, JH–H = 12.8 Hz, CHCH2SiMe3),
3.75 (s, 8H, THF), 4.82 (d, 1H,3JH–H = 10.8 Hz, CHCH2-
3
3
4.2. Syntheses of complexes
3
4.2.1. Syntheses of ligands
Tridentate ligands 3,5-tBu2-(OH)C6H2CH@N-CH2-
SiMe3), 6.50 (t, 1H, JH–H = 7.2 Hz, m-PC6H4N), 7.07–
7.16 (m, 8H, m,p-PC6H5, m-NC6H4P,C6H2), 7.47–7.50 (m,
3H,
CH2NMe2 (HL1) and 3,5-tBu2-(OH)C6H2CH@N-2-PPh2-

W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
3831
o-PC6H4N, o-NC6H4P and C6H2), 7.72–7.67 (m, 4H,
o-PC6H5). 13C NMR (100 MHz, C6D6, 25 ꢁC): d À0.60 (s,
3C, CHSiMe3), 5.57 (s, 3C, CH2SiMe3), 25.37 (s, 8C,
was stirred for 12 h at room temperature. Removal of vol-
atiles afforded red solids which were recrystallized from
hexane to produce complex 3 (0.37 g, 75%). Orange crys-
tals suitable for X-ray analysis grew by cooling a hexane
solution of complex 3 to À35 ꢁC for several days. 1H
t
THF), 26.09 (s, 2C, YCH2), 31.11 (s, 3C, Bu), 32.71 (s,
t
3C, Bu), 34.71 (s, 1C, CMe3), 36.28 (s, 1C, CMe3), 64.08
(s, 1C, CHCH2SiMe3), 71.38 (s, 8C, THF), 111.84 (s, 1C,
p-PC6H4N), 112.97 (s, 1C, p-NC6H4P), 115.18 (d, 1C,
ipso-PC6H4), 122.36 (s, 1C, o-NC6H4P), 125.80 (s, 1C, o-
PC6H4N), 128.95, 129.24 (d, 4C, m-PC6H5), 129.91 (d,
2C, ipso-PC6H5), 134.18 (s, 1C, m-OC6H2), 134.40 (d, 2C,
p-PC6H5), 134.99 (2C, o,m-OC6H2), 135.37 (d, 4C, o-
C6H5), 136.97 (s, 1C, p-OC6H2), 137.85 (s, 1C, o-
OC6H2-tBu), 159.98 (s, 1C, ipso-NC6H4), 164.66 (d, 1C,
ipso-OC6H2); IR (KBr pellets): 696, 730, 745, 791, 841,
918, 998, 1028, 1069, 1097, 1136, 1163, 1193, 1250, 1330,
1361, 1386, 1436, 1461, 1533, 1596, 1612, 2869, 2904,
2955, 3056 cmÀ1. Anal. Calc. for C49H73NO3PSi2Y: C,
65.38; H, 8.17; N, 1.56. Found: C, 65.41; H, 8.19; N,
1.54%.
NMR (400 MHz, C6D6, 25 ꢁC):
d
À0.33 (s, 2H,
CH2SiMe3), À0.03 (s, 9H, YCH2SiMe3), 0.08 (s, 9H,
CHCH2SiMe3), 1.29 (s, 4H, THF), 1.14 (d, 1H,
t
J = 12 Hz, CHCH2SiMe3), 1.36 (s, 18H, Bu), 1.56 (s,
t
18H, Bu), 2.01 (t, 1H , J = 12 Hz, CHCH2SiMe3), 3.60
(s, 4H, THF), 4.94 (d, 1H, J = 11.6 Hz, CHCH2SiMe3),
6.88–7.00 (m, 2H, m-PC6H4N), 7.14–7.22 (m, 16H, m,p-
PC6H5,m-NC6H4P,C6H2), 7.33–7.45 (m, 6H, o-PC6H4N,
o-NC6H4P and C6H2), 7.60–7.74 (m, 8H, o-PC6H5), 7.90
(s, 1H, CH@N). 13C NMR (100 MHz, C6D6, 25 ꢁC): d
À0.52 (s, 3C, CHCH2SiMe3), 14.77 (s, 1C, CHCH2SiMe3),
25.89 (s, 4C, THF), 23.47 (s, 1C, YCH2), 31.26 (s, 1C,
t
t
YCH2SiMe3), 32.15 (s, 4C, Bu), 32.80 (s, 4C, Bu), 34.70
(d, 2C, J = 23.3 Hz, CMe3), 36.14 (d, 2C, J = 17.5 Hz
CMe3), 64.32 (s, 1C, CHCH2SiMe3), 69.47 (s, 8C, THF),
122.07 (s, 2C, p-PC6H4N), 122.56 (s, 2C, p-NC6H4P),
122.88 (d, 2C, ipso-PC6H4), 123.64 (s, 2C, o-NC6H4P),
124.86 (s, 2C, o-PC6H4N), 127.07, 129.19 (d, 8C, m-
PC6H5), 129.75 (d, 4C, ipso-PC6H5), 132.33 (s, 1C,
CH@N), 132.33 (s, 2C, m-OC6H2), 133.32 (d, 4C, p-
PC6H5), 133.92 (m, 4C, o,m-OC6H2), 135.42 (m, 8C,
o-C6H5), 136.68 (s, 2C, p-OC6H2), 137.69 (s, 2C, o-
OC6H2-tBu), 157.29 (s, 2C, ipso-NC6H4), 165.11 (d, 2C,
ipso-OC6H2); IR (KBr pellets): 696, 743, 766, 790, 841,
918, 999, 1027, 1068, 1094, 1163, 1193, 1249, 1275, 1329,
1360, 1388, 1435, 1461, 1479, 1532, 1591, 1609, 2867,
2905, 2955, 3054, 3211, 3668 cmÀ1. Anal. Calc. for
C74H89N2O3P2SiY: C, 72.06; H, 7.27; N, 2.27. Found: C,
72.29; H, 7.35; N 2.31%.
4.2.4. Synthesis of complex 2b
Following the same procedure described for the synthe-
sis of 2a, treatment of HL2 (0.20 g, 0.40 mmol in 5 mL hex-
ane) with Lu(CH2SiMe3)3(THF)2 (0.23 g, 0.40 mmol)
afforded yellow solids of 2b (0.32 g, 80%). Single crystals
suitable for X-ray analysis grew from a cold solution of
complex 2b in hexane/toluene (1:3 v/v) at À35 ꢁC within
several days.1H NMR (400 MHz, C6D6, 25 ꢁC): d À0.47
(d, 1H, LuCH2SiMe3), 0.06 (s, 9H, LuCH2SiMe3), 0.56 (s,
9H, CHCH2SiMe3), 1.11 (br, 8H, THF), 1.54 (s, 9H,
t
tBu), 1.86 (s, 9H, Bu), 2.35 (t, 1H, LuCH2SiMe3), 3.70
(br, 8H, THF), 4.98 (d, 1 H,3JH–H = 14 Hz, CHCH2-
SiMe3), 6.55 (t, 1H, m-PC6H4N), 7.04–7.11 (m, 2H, o-
NC6H4P,C6H2), 7.15–7.23 (m, 5H, m-PC6H5, o-PC6H4N),
7.49–7.56 (m, 4H, p-P-C6H5, m-NC6H4P and C6H2),
7.81–7.85 (m, 4H, o-PC6H5). 13C NMR (100 MHz, C6D6,
25 ꢁC): d À0.68 (s, 3C, CHSiMe3), 5.35 (s, 3C, CH2SiMe3),
25.37 (s, 8C, THF), 25.78 (s, 2C, YCH2), 31.18 (s, 3C, tBu),
32.71 (s, 3C, tBu), 34.71 (s, 1C, CMe3), 36.22 (s, 1C, CMe3),
63.16 (s, 1C, CHCH2SiMe3), 71.56 (s, 8C, THF), 112.25 (s,
1C, m-NC6H4P), 113.36 (s, 1C, p-NC6H4P), 114.19 (d, 1C,
ipso-PC6H4), 122.46 (s, 1C, ipso-NC6H4), 125.69 (s, 1C, o-
PC6H4N), 129.23, 129.31 (d, 4C, m-PC6H5), 129.73,130.38
(s, 2C, ipso-PC6H5), 134.18 (s, 1C, C6H2), 134.40 (d, 2C,
p-PC6H5), 134.99 (2C, o,m-NC6H2), 135.37 (d, 4C, o-
C6H5), 137.18 (s, 1C, p-OC6H2), 138.09 (s, 1C, o-
OC6H2-tBu), 160.47 (s, 1C, ipso-NC6H4), 165.08 (d, 1C,
ipso-OC6H2); IR (KBr pellets): 696, 744, 791, 841, 918,
999, 1027, 1066, 1094, 1137, 1163, 1191, 1256, 1331, 1360,
1385, 1435, 1460, 1533, 1595, 1612, 2868, 2956,
3054 cmÀ1. Anal. Calc. for C49H73NO3PSi2Lu: C, 59.62;
H, 7.45; N, 1.42. Found: C, 59.71; H, 7.69; N, 1.54%.
4.2.6. Synthesis of complex 4a
To a hexane solution (5.0 mL) of Y(CH2SiMe3)3(THF)2
(0.15 g, 0.30 mmol), 1 equiv. of HL3 (0.13 g, 0.30 mmol in
4 mL toluene) was dropwise added at À35 ꢁC under stir-
ring, and the reaction was kept at À35 ꢁC for 5–10 min.
Removal of the volatiles gave deep red solids, which were
dissolved in 1 mL toluene, and cooled to À35 ꢁC for 2 days
to give colorless crystalline solids. Washed with hexane
carefully and dried in vacuum afforded complex 4a
(0.10 g, 35%). Single crystals suitable for X-ray analysis
grew from a toluene solution of 4a at À35 ꢁC for several
1
days. H NMR (400 MHz, C6D6, 25 ꢁC): d À0.33 (s, 2H,
CH2SiMe3), 0.51 (s, 9H, SiMe3), 1.25 (s, 3H, CH3C@N),
1.71(s, 3H, CH3CO), 4.53 (s, 2H, COCHC@N), 6.64 (br,
2H, o-PC6H4N), 6.91 (br, 2H, o-NC6H4P), 7.10–7.17 (m,
10H, m,p-PC6H5, m,p-NC6H4P), 7.34 (br, 6H, m,p-
PC6H5), 7.70 (br, 4H, o-PC6H5), 8.15 (br, 4H, o-PC6H5).
13C NMR (100 MHz, C6D6, 25 ꢁC): d 5.81 (3C, SiMe3),
22.96 (2C, CH3C=N), 26.3 (s, 2C, CH3C-O), 101.41 (s,
2C, CH), 124.43 (2C, o-PC6H4N), 124.64 (2C, o-NC6H4P),
129.14 (6C, m,p-PC6H5), 130.22 (10C, m,p-PC6H5, m,p-
NC6H4P), 131.73 (s, 2C, ipso-PC6H4), 135.49 (2C,
4.2.5. Synthesis of complex 3
A solution of HL2 (0.40 g, 0.8 mmol) in hexane (5 mL)
was added dropwise to 0.5 equiv. of Y(CH2SiMe3)3(THF)2
(0.20 g, 0.4 mmol) in hexane at À35 ꢁC. The red solution

3832
W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
o-PC6H5), 135.74 (t, 2C, o-PC6H5), 153.37 (s, 2C, ipso-
NC6H4), 169.14 (s, 2C, C@N), 179.58 (s, 2C, C-O); IR
(KBr pellets): 696, 743, 819, 862, 930, 1013, 1068, 1094,
1123, 1157, 1188, 1235, 1262, 1404, 1434, 1457, 1511,
1576, 2954, 3052 cmÀ1. Anal. Calc. for C57H61N2O2P2SiY:
C, 69.50; H, 6.24; N, 2.84. Found: C, 69.52; H, 6.25; N,
2.80%.
1C, CMe3), 36.15 (s, 1C, CMe3), 52.86 (s, 2C,
NCH2CH2), 61.49 (s, 2C, OMe), 63.90 (s, 1C, CH2N),
72.10 (s, 2C, CH2CH2OMe), 123.61, 125.26, 126.01,
137.07, 137.47, 162.53 (s, 6C, aryls); IR (KBr pellets):
616, 644, 669, 694, 744, 840, 861, 877, 1028, 1091, 1114,
1166, 1202, 1248, 1309, 1321, 1361, 1383, 1416, 1442,
1478, 1545, 1604, 2900, 2953, 3418, 3676 cmÀ1. Anal.
Calc. For C29H58NO3Si2Y: C, 56.74; H, 9.52; N, 2.28.
Found: C, 56.71; H, 9.56; N, 2.29%.
4.2.7. Synthesis of complex 4b
To a hexane solution (5.0 mL) of Lu(CH2SiMe3)3(THF)2
(0.18 g, 0.30 mmol), 1 equiv. of HL3 (0.13 g, 0.31 mmol in
4 mL toluene) was dropwise added at À35 ꢁC, and the reac-
tion was kept at À35 ꢁC for 5–10 min. Removal of the vol-
atiles gave deep red solids, which was dissolved in 3 mL
toluene and concentrated to 1 mL, then cooled to À35 ꢁC
for 2 days to give colorless crystals of complex 4b (0.11g,
33%). Single crystals suitable for X-ray analysis grew from
4.3. Polymerization of L-lactide
4.3.1. Typical procedure for polymerization of L-lactide
A typical procedure for polymerization of lactide was
performed in a 25 mL round-bottom flask in a glove-box.
To a stirred THF solution (3.0 mL) of L-lactide (0.50 g,
3.47 mmol) was added a THF solution (1.0 mL) of complex
5 (4.58 mg, 6.94 lmol, [LA]0/[Cat]0 = 500) (Table 7, entry
8). The polymerization was proceeded for 2 h at room tem-
perature (20 ꢁC) and then terminated by 1.0 mL of a mix-
ture of HCl/CH3OH/CHCl3 (1:100:600 v:v) to give white
solids, which were filtered, washed with ethanol, and then
dried at 40 ꢁC for 24 h in vacuo to give PLA (0.48 g,
96%). The molecular weight and the molecular weight dis-
tribution of the resulting polymer were determined by
1
a toluene solution of 4b at À35 ꢁC for several days. H
NMR (400 MHz, C6D6, 25 ꢁC):
d
À0.45 (br, 2H,
CH2SiMe3), 0.49 (s, 9H, SiMe3), 1.24 (s, 3H, CH3C@N),
1.72 (s, 3H, CH3C–O), 4.51 (s, 2H, CH), 6.64 (br, 2H, o-
PC6H4N), 6.97 (br, 2H, o-NC6H4P), 7.11–7.17 (m, 10H,
m,p-PC6H5, m,p-NC6H4P), 7.33 (br, 6H, m,p-PC6H5), 7.71
(br, 4H, o-PC6H5), 8.15 (br, 4H, o-PC6H5). 13C NMR
(100 MHz, C6D6, 25 ꢁC): d 5.95 (3C, SiMe3), 23.09 (2C,
CH3C@N), 26.33 (s, 2C, CH3C–O), 101.75 (s, 2C,
COCHCN), 124.45 (2C, o-PC6H4N), 124.72 (2C, o-
NC6H4P), 129.07 (6C, m,p-PC6H5), 130.15 (10C, m,p-
PC6H5, m,p-NC6H4P), 131.64 (s, 2C, P–C6H4), 135.45
(2C, o-PC6H5), 135.88 (t, 2C, JP–C = 12 Hz, o-PC6H5),
153.41 (s, 2C, N–C6H4), 169.12 (s, 2C, C@N), 179.96 (s,
2C, C–O); IR (KBr pellets): 696, 743, 821, 861, 932, 1015,
1086, 1097, 1124, 1157, 1189, 1235, 1264, 1400, 1435,
1457, 1512, 1578, 2953, 3053 cmÀ1. Anal. Calc. for
C57H61N2O2P2SiLu: C, 63.92; H, 5.74; N, 2.62. Found: C,
63.95; H, 5.75; N, 2.60%.
1
GPC. The microstructure was characterized by H NMR
spectrum.
4.3.2. Oligomer for NMR measurement
To the stirred solution of L-lactide (0.125 g, 0.87 mmol)
in THF (1.5 mL), the solution of complex 2a (41 mg,
0.046 mmol) in THF (0.5 mL) was injected at 20 ꢁC
([LA]0/[Y]0 = 20). The polymerization mixture was stirred
for 2 h and then quenched with hexane/methanol (1:1
v:v). The precipitated oligomer was collected and dried
1
and used for H NMR.
4.2.8. Synthesis of complex 5
4.4. X-ray crystallographic studies
A toluene (2 mL) solution of HL4 (0.18 g, 0.52 mmol)
was added dropwise to 1 equiv. of Y(CH2SiMe3)3(THF)2
(0.26 g, 0.52 mmol) in toluene (2 mL) at À35 ꢁC. The
reaction mixture was stirred for 15 min and then concen-
trated in vacuo to 1 mL. Crude product was precipitated
as white solids, which was washed with hexane carefully
and dried in vacuum to afford complex 5 (0.28 g, 78%).
Colorless single crystals for X-ray analysis grew from a
mixture of toluene and hexane (3:1 v/v) at À35 ꢁC within
Crystals for X-ray analysis were obtained as described
in the preparations. The crystals were manipulated in
the glovebox. Data collections were performed at
À86.5 ꢁC on a Bruker SMART APEX diffractometer with
a CCD area detector, using graphite-monochromated Mo
˚
Ka radiation (k = 0.71073 A). The determination of crys-
tal class and unit cell parameters was carried out by the
SMART program package. The raw frame data were pro-
cessed using SAINT and SADABS to yield the reflection data
file. The structures were solved by using the SHELXTL pro-
gram. Refinement was performed on F2 anisotropically for
all non-hydrogen atoms by the full-matrix least-squares
method. The hydrogen atoms were placed at the calcu-
lated positions and were included in the structure calcula-
tion without further refinement of the parameters. The
crystallographic data and the refinement of complexes 1,
2a, 2b, 3, 4a, 4b, 5 are summarized in Tables 9 and 10,
respectively.
1
4 days. H NMR (400 MHz, C6D6, 25 ꢁC): d À0.45 (d,
4H, J = 2.8 Hz, YCH2SiMe3), 0.58 (s, 18H, YCH2SiMe3),
1.53 (s, 9H, CMe3), 1.64 (t, 2H, J = 12 Hz,
NCH2CH2OMe), 1.92 (s, 9H, CMe3), 2.56 (m, 2H,
NCH2CH2OMe), 2.56 (m, 2H, NCH2CH2OMe), 2.87(t,
2H, J = 16 Hz, NCH2CH2OMe), 3.18 (s, 6H, OMe),
3.56 (s, 2H, CH2N), 7.02 (d, 1H, J = 4 Hz, ArH), 7.69
(d, 1H, J = 4 Hz, ArH); 13C NMR (100 MHz, C6D6,
25 ꢁC): d 5.23 (s, 6C, CH2SiMe3), 30.77 (s, 3C, CMe3),
31.11 (s, 2C, CH2SiMe3), 32.68 (s, 3C, CMe3), 34.70 (s,

W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
3833
Table 9
Summary of crystallographic data for 1, 2a, 2b, and 3
1
2a
2b
3
Formula
C58.50H96.50N6O3Y
C49H73NO3PSi2Y
900.14
Triclinic
C49H74NO3Si2PLu
987.21
Triclinic
C74H89N2O3P2SiY
1233.41
Triclinic
Molecular weight
Crystal system
Space group
1020.83
Monoclinic
P21/C
20.596(2)
18.1472(19)
18.323(2)
90
115.313(2)
90
6190.8(11)
4
ꢀ
ꢀ
ꢀ
P1
P1
P1
˚
a (A)
14.3519(8)
14.5334(9)
14.7334(9)
60.7330(10)
70.3360(10)
79.3630(10)
2523.7(3)
2
10.7026(14)
14.5466(19)
17.401(2)
79.498(2)
75.938(2)
76.767(2)
2535.0(6)
2
14.566(7)
18.193(9)
19.490(10)
104.437(8)
101.602(8)
109.253(8)
4489(4)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g/cm3)
1.095
Mo Ka (0.71073)
52.16
1.184
Mo Ka (0.71073)
51.76
1.293
Mo Ka (0.71073)
52.12
0.912
Mo Ka (0.71073)
50.50
˚
Radiation (k), A
2h
(ꢁ)
)
max
l (cmÀ1
9.85
12.72
20.64
7.34
F(000)
2210
960
1026
1308
Number of observed reflections
35477
9739
9806
15891
Number of parameters refined
12215
521
508
758
Goodness-of-fit
R1
wR2
0.935
0.0922
0.2522
1.032
0.0489
0.1323
0.998
0.0581
0.1149
0.961
0.1158
0.3723
Table 10
Summary of crystallographic data for 4a, 4b and 5
4a
57H61N2O2P2SiY
4b
5
Formula
C
C57H61N2O2P2SiLu
C29H58NO3Si2Y Æ (C7H8)1/ 2
Molecular weight
Crystal system
Space group
985.02
Triclinic
1071.08
Triclinic
659.92
Monoclinic
P2(1)/n
15.3297(11)
14.1719(10)
18.4192(13)
90.00
103.5960(10)
90.00
3889.5(5)
4
ꢀ
ꢀ
P1
P1
˚
a (A)
13.7291(8)
20.0609(12)
20.9137(13)
105.7750(10)
105.2150(10)
93.2490(10)
5297.7(6)
4
13.6540(12)
20.1181(19)
20.8781(19)
105.7280(10)
105.0980(10)
93.2430(10)
5279.6(8)
4
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g/cm3)
1.235
1.347
1.127
˚
Radiation (k), A
Mo Ka (0.71073)
51.10
12.25
Mo Ka (0.71073)
52.06
19.95
Mo Ka (0.71073)
52.08
15.89
2hmax (ꢁ)
l (cmÀ1
F(000)
)
2064
2192
1420
Number of observed reflections
20364
20324
7660
Number of parameters refined
1073
1153
411
Goodness-of-fit
R1
wR2
1.046
0.0704
0.2120
1.006
0.0428
0.1056
0.899
0.0463
0.1059
5. Supplementary material
Acknowledgements
CCDC 607920, 607922, 622505, 607921, 619192, 619722
and 637044 contain the supplementary crystallographic data
for 1, 2a, 2b, 3, 4a, 4b and 5. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
We thank financial supports from Jilin Provincial Sci-
ence and Technology Bureau for Project No. 20050555;
The National Natural Science Foundation of China for
Project Nos. 20571072 and 20674081; The Ministry of
Science and Technology of China for project No.
2005CB623802; ‘‘Hundred Talent Scientist Program’’ of
CAS.

3834
W. Miao et al. / Journal of Organometallic Chemistry 692 (2007) 3823–3834
(b) X.P. Xu, M.T. Ma, Y.M. Yao, Y. Zhang, Q. Shen, Eur. J. Inorg.
References
Chem. (2005) 676;
(c) P.N. O’Shaughnessy, P.D. Knight, C. Morton, K.M. Gillespie, P.
Scott, Chem. Commun. (2003) 1770;
(d) D.V. Gribkov, K.C. Hultzsch, F. Hampell, Chem. Eur. J. 9
(2003) 4796;
(e) C.X. Cai, L. Toupet, C.W. Lehmann, J.F. Carpentier, J.
Organomet. Chem. 683 (2003) 131;
(f) C.X. Cai, A. Amgoune, C.W. Lehmann, J.F. Carpentier, Chem.
Commun. (2004) 330.
[1] (a) See reviews: Z. Hou, Y. Wakatsuki, Coord. Chem. Rev. 231
(2002) 1;
(b) J. Okuda, Dalton Trans. (2003) 2367;
(c) J. Gromada, J.F. Carpentier, A. Mortreux, Coord. Chem. Rev.
248 (2004) 397;
(d) J.Y. Hyeon, J. Gottfriendsen, F.T. Edelmann, Coord. Chem.
Rev. 249 (2005) 2787;
(e) P.M. Zeimentz, S. Arndt, B.R. Elvidge, J. Okuda, Chem. Rev.
106 (2006) 2404 (and the references therein);
[10] I. Westmoreland, J. Arnold, Dalton Trans. (2006) 4155.
[11] H.W. Gorlitzer, M. Spiegler, R. Anwander, Dalton Trans. (1999) 4287.
[12] S. Bambirra, A. Meetsma, B. Hessen, A.P. Bruins, Organometallics
25 (2006) 3486.
[13] P.A. Cameron, V.C. Gibson, C. Redshaw, J.A. Segal, A.J.P. White,
D.J. Williams, Dalton Trans. (2002) 415.
[14] H. Tsurugi, Y. Matsuo, T. Yamagata, K. Mashima, Organometallics
23 (2004) 2797.
[15] Y. Yang, S. Li, D. Cui, X. Chen, X. Jing, Organometallics 26 (2007)
671.
[16] P.D. Knight, A.J. Clarke, B.S. Kimberley, R.A. Jackson, P. Scott,
Chem. Commun. (2002) 352.
[17] D.J.H. Emslie, W.E. Piers, M. Parvez, Dalton Trans. (2003) 2615.
[18] (a) D.P. Long, A. Chandrasekaran, R.O. Day, P.A. Bianconi, A.L.
Rheingold, Inorg. Chem. 39 (2000) 4476;
(f) Y.J. Luo, M. Nishiura, Z.M. Hou, J. Organomet. Chem. 692
(2007) 536.
[2] (a) Y. Zhang, Z. Hou, Y. Wakatsuki, Macromolecules 31 (1998)
8650;
(b) S. Kaita, Z. Hou, Y. Wakatsuki, Macromolecules 32 (1999) 9078;
(c) Z. Hou, Y. Zhang, H. Tezuka, P. Xie, O. Tardif, T.A. Koizumi,
H. Yamazaki, Y. Wakatsuki, J. Am. Chem. Soc. 122 (2000) 10533;
(d) K.C. Hultzsch, P. Voth, K. Beckerle, T. Spaniol, J. Okuda,
Organometallics 19 (2000) 228;
(e) Z. Hou, S. Kaita, Y. Wakatsuki, Macromolecules 34 (2001) 1539;
(f) Z. Hou, S. Kaita, Y. Wakatsuki, Pure Appl. Chem. 73 (2001) 291;
(g) Y. Luo, Y. Yao, Q. Shen, Macromolecules 35 (2002) 8670;
(h) Y. Luo, J. Baldamus, Z. Hou, J. Am. Chem. Soc. 126 (2004)
13910;
(i) E. Kirillov, C.W. Lehmann, A. Razavi, J.F. Carpentier, J. Am.
Chem. Soc. 126 (2004) 12240;
(j) X. Li, Z. Hou, Macromolecules 38 (2005) 6767;
(k) X. Li, J. Baldamus, Z. Hou, Angew. Chem., Int. Ed.Engl. 44
(2005) 962;
(b) P.L. Arnold, S.A. Mungur, A.J. Blake, C. Wilson, Angew.
Chem., Int. Ed. 42 (2003) 5981;
(c) M. E.G. Skinner, P. Mountford, Dalton Trans. (2002) 1694;
(d) J. Wang, X.D. Zhang, D.M. Fan, X. Ling, Chin. J. Inorg. Chem.
17 (2001) 755.
[19] During we were carrying out this work, complex 5 was reported by
Bercaw et al. without crystallographic information and catalytic
activity toward the polymerization of LA: S.C. Marinescu, T.A.
Gapie, M.W. Day, J.E. Bercaw, Organometallics 26 (2007) 1178.
[20] A.R.F. Cox, V.C. Gibson, E.L. Marshall, A.J.P. White, D. Yeldon,
Dalton Trans. (2006) 5014.
[21] (a) H. Ma, T.P. Spaniol, J. Okuda, Dalton Trans. (2003) 4770;
(b) O. Runte, T. Priermeier, R. Anwander, Chem. Commun. (1996)
1385.
[22] A. Amgoune, C.M. Thomas, T. Roisnel, J.F. Carpentier, Chem. Eur.
J. 12 (2006) 169.
[23] F.M. Kerton, A.C. Whitewood, C.E. Willans, Dalton Trans. (2004)
2237.
(l) L. Zhang, Y. Luo, Z. Hou, J. Am. Chem. Soc. 127 (2005) 14562;
(m) O. Tardif, M. Nishiura, Z. Hou, Organometallics 22 (2003) 1171.
[3] (a) H. Yasuda, H. Yamamoto, K. Yokota, A. Nakamura, J. Am.
Chem. Soc. 114 (1992) 4908;
(b) E. Ihara, M. Morimoto, H. Yasuda, Macromolecules 28 (1995)
7886;
(c) H. Yasuda, K. Yamamoto, W.E. Piers, D.J.H. Emslie, Coord.
Chem. Rev. 233 (2002) 131;
(d) O.D. Cabaret, B.M. Vaca, D. Bourissou, Chem. Rev. 104 (2004)
6147;
(e) D. Cui, M. Nishiura, Z. Hou, Macromolecules 38 (2005) 4089;
(f) H. Yasuda, J. Organomet. Chem. 647 (2002) 128.
[4] (a) G. Jeske, E. Laurel, P. Schock, P.N. Swepston, H. Schumann,
T.J. Marks, J. Am. Chem. Soc. 107 (1985) 8103;
(b) W. Roll, H-H. Brintzinger, B. Rieger, R. Zolk, Angew. Chem.,
Int. Ed. 29 (1990) 279;
[24] (a) C.G.J. Tazelaar, S. Bambirra, D. van Leusen, A. Meetsman, B.
Hessen, J.H. Teuben, Organometallics 23 (2004) 936;
(b) S. Bambirra, S.J. Boot, D. van Leusen, A. Meetsman, B. Hessen,
Organometallics 23 (2004) 1891;
(c) H. Schumann, J.A. Meese-Marktscheffel, L. Esser, Chem. Rev. 95
(1995) 865 (and references therein);
(d) S. Arndt, J. Okuda, Chem. Rev. 102 (2002) 1953 (and references
therein);
(c) P.G. Hayes, G.C. Welch, D.J.H. Emslie, C.L. Noack, W.E. Piers,
M. Parvez, Organometallics 22 (2003) 1577.
[25] The NMR spectrum for complex 5 in CD2Cl2 showed a different pattern
from that in C6D6, especially the yttrium alkyl group, shifting up-field
from d = À0.45 ppm to d = À0.62, indicating the change of geometry
of the metal center. Resonances assigned to other groups are listed as
follows: d = À 0.61 (br, 4H, YCH2SiMe3), 0.40 (s, 18H, YCH2SiMe3),
1.40 (s, 9H, CMe3), 1.57 (t, 2H, NCH2CH2OMe), 1.76 (s, 9H, CMe3),
2.49 (m, 2H, NCH2CH2OMe), 2.49 (m, 2H, NCH2CH2OMe), 2.77 (t,
2H, NCH2CH2OMe), 3.10 (s, 6H, OMe), 3.44 (s, 2H, CH2N), 6.88 (d,
1H, J = 4 Hz, ArH), 7.53 (d, 1H, J = 4 Hz, ArH).
[26] (a) X. Liu, X. Shang, T. Tang, N. Hu, F. Pei, D. Cui, X. Chen, X.
Jing, Organometallics 26 (2007) 2747;
(e) O. Tardif, M. Nishiura, Z. Hou, Tetrahedron 59 (2003) 10525.
[5] S. Bambirra, A. Meetsma, B. Hessen, J.H. Teuben, Organometallics
20 (2001) 782.
[6] (a) R. Duchateau, C.T. van Wee, A. Meetsma, J.H. Teuben, J. Am.
Chem. Soc. 115 (1993) 4931;
(b) P.J. Bailey, S. Pace, Coord. Chem. Rev. 214 (2001) 91;
(c) G.R. Giesbrecht, G.D. Whitener, J. Arnold, Dalton Trans. 6
(2001) 923;
(d) Z. Lu, G.P.A. Yap, D.S. Richeson, Organometallics 20 (2001) 706.
[7] P.G. Hayes, W.E. Piers, L.W.M. Lee, L.K. Knight, M. Parvez,
M.R.J. Elsegood, W. Clegg, Organometallics 20 (2001) 2533.
[8] (a) D.J.H. Emslie, W.E. Piers, R. McDonald, Dalton Trans. (2002)
293;
(b) H. Ma, J. Okuda, Macromolecules 38 (2005) 2665.
[27] M. Save, M. Schappacher, A. Soum, Macromol. Chem. Phys. 203
(2002) 889.
[28] B. Liu, D. Cui, J. Ma, X. Chen, X. Jing, Chem. Eur. J. 13 (2007) 834.
[29] X. Dai, A. Wong, S.C. Virgil, J. Org. Chem. 63 (1998) 2597.
[30] D. Simon, G.K. Julian, H.S. Tom, R.J.E. Mark, C. William, J.
Organomet. Chem. 650 (2002) 231.
(b) D.J.H. Emslie, W.E. Piers, M. Parvez, R. McDonald, Organo-
metallics 21 (2002) 4226.
[9] (a) Y.M. Yao, M.T. Ma, X.P. Xu, Y. Zhang, Q. Shen, W.T. Wong,
Organometallics 24 (2005) 4014;