Achiral lanthanide alkyl complexes bearing N,O multidentate ligands. Synthesis and catalysis of highly heteroselective ring-opening polymerization of rac-lactide

Article abstract of DOI:10.1021/om0700359

Alkane elimination reactions of amino-amino-bis(phenols) H ₂L^1-4, Salan H₂L⁵, and methoxy-β-diimines HL^6,7 with lanthanide tris(alkyl)s, Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Y, Lu), respectively, afforded a series of lanthanide alkyl complexes 1-8 with the release of tetramethylsilane. Complexes 1-6 are THF-solvated mono(alkyl)s stabilized by O,N,N,O-tetradentate ligands. Complexes 1-3 and 5 adopt twisted octahedral geometry, whereas 4 contains a tetragonal bipyramidal core. Bearing a monoanionic moiety L⁶ (L⁷), complex 7 (8) is a THF-free bis(alkyl). In complex 7, the O,N,N-tridentate ligand combined with two alkyl species forms a tetrahedral coordination core. Complexes 1, 2, and 3 displayed modest activity but high stereoselectivity for the polymerization of rac-lactide to give heterotactic polylactide with the racemic enchainment of monomer units P_r ranging from 0.95 to 0.99, the highest value reached to date. Complex 5 exhibited almost the same level of activity albeit with relatively low selectivity. In contrast, dramatic decreases in activity and stereoselectivity were found for complex 4. The Salan yttrium alkyl complex 6 was active but nonselective. Bis(alkyl) complexes 7 and 8 were more active than 1-3 toward polymerization of rac-LA, however, to afford atactic polylactides due to di-active sites. The ligand framework, especially the bridge between the two nitrogen atoms, played a significant role in governing the selectivity of the corresponding complexes via changing the geometry of the metal center. The NMR spectrum of the active species of the rac-lactide oligomer attached to complex 1 demonstrated a coordination-insertion mechanism. In addition it also confirmed that the geometry of the metal center of complex 1 in the solid state was retained in solution (THF) during the polymerization, which contributed significantly to the high selectivity of the complex.

Full text of DOI:10.1021/om0700359

Organometallics 2007, 26, 2747-2757
2747
Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate
Ligands. Synthesis and Catalysis of Highly Heteroselective
Ring-Opening Polymerization of rac-Lactide
Xinli Liu,† Xiaomin Shang,†,‡ Tao Tang,† Ninghai Hu,† Fengkui Pei,† Dongmei Cui,*,†
Xuesi Chen,† and Xiabin Jing†
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and
Department of Chemical Engineering, Changchun UniVersity of Technology,
Changchun 130012, People’s Republic of China
ReceiVed January 12, 2007
Alkane elimination reactions of amino-amino-bis(phenols) H2L1-4, Salan H2L5, and methoxy-â-diimines
HL6,7 with lanthanide tris(alkyl)s, Ln(CH2SiMe3)3(THF)2 (Ln ) Y, Lu), respectively, afforded a series
of lanthanide alkyl complexes 1-8 with the release of tetramethylsilane. Complexes 1-6 are THF-
solvated mono(alkyl)s stabilized by O,N,N,O-tetradentate ligands. Complexes 1-3 and 5 adopt twisted
octahedral geometry, whereas 4 contains a tetragonal bipyramidal core. Bearing a monoanionic moiety
L6 (L7), complex 7 (8) is a THF-free bis(alkyl). In complex 7, the O,N,N-tridentate ligand combined
with two alkyl species forms a tetrahedral coordination core. Complexes 1, 2, and 3 displayed modest
activity but high stereoselectivity for the polymerization of rac-lactide to give heterotactic polylactide
with the racemic enchainment of monomer units Pr ranging from 0.95 to 0.99, the highest value reached
to date. Complex 5 exhibited almost the same level of activity albeit with relatively low selectivity. In
contrast, dramatic decreases in activity and stereoselectivity were found for complex 4. The Salan yttrium
alkyl complex 6 was active but nonselective. Bis(alkyl) complexes 7 and 8 were more active than 1-3
toward polymerization of rac-LA, however, to afford atactic polylactides due to di-active sites. The ligand
framework, especially the “bridge” between the two nitrogen atoms, played a significant role in governing
the selectivity of the corresponding complexes via changing the geometry of the metal center. The NMR
spectrum of the active species of the rac-lactide oligomer attached to complex 1 demonstrated a
coordination-insertion mechanism. In addition it also confirmed that the geometry of the metal center
of complex 1 in the solid state was retained in solution (THF) during the polymerization, which contributed
significantly to the high selectivity of the complex.
single-site complexes of the formula LmMR.4 Lm is the ancillary
ligand, the steric and electronic character of which tune the
properties of the metal center, the catalytic activity, and the
selectivity of the complexes; R is the initiation group, usually
being alkoxide, phenoxide, and amide, or in some cases alkyl
species, which is supposed to affect the activity of the
complexes. Appropriate combinations of Lm with M and R have
generated highly active initiators for the ROP of LA, such as
Introduction
Polylactides (PLAs) are the most promising biodegradable
and biocompatible synthetic macromolecules, possessing ver-
satile physical properties and having been widely used in
medicine, pharmaceutics, and tissue engineering such as media
for the controlled release of drugs, scaffolds, and the delivery
of antibodies and genes.1 The applications are strongly depend-
ent on the microstructures of PLAs. Ring-opening polymeriza-
tion (ROP) of lactide (LA) by single-site catalysts is the most
efficient manner to obtain PLAs with predicted molecular weight
and narrow molecular weight distribution and, significantly,
tacticity. The past two decades have witnessed the development
of catalysts for such aims. Metal alkoxides are well-known
highly active initiators albeit with less stereocontrol due to the
aggregation nature of the active species.2,3 Multidentate ligands
containing heteroatoms are intended to encapsulate metal ions
and thus control the geometry of complexes to form discrete
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* Corresponding author. E-mail: dmcui@ciac.jl.cn. Fax: (+86) 431
85262773. Tel: +86 431 85262773.
† Changchun Institute of Applied Chemistry.
‡ Changchun University of Technology.
(1) (a) Finne, A.; Albertsson, A.-C. Biomacromolecules 2003, 3, 684-
690. (b) Kowals ki, A.; Duda, A.; Penczek, S. Macromolecules 2000, 33,
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10.1021/om0700359 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/14/2007

2748 Organometallics, Vol. 26, No. 10, 2007
Liu et al.
Schiff-base (Salen)-ligated aluminum alkoxides,5 â-diiminate
(BDI)-stabilized zinc, magnesium, and lanthanide alkoxides,6
N,N-bidentate guandinate-7 or amidinate8-supported lanthanide
complexes, and N,N,N-multidentate calcium and zinc alkoxides.9
The phenolate10 and bisphenolate11 ligated metal complexes also
occupy important positions in the catalyst list. Most of catalysts
mentioned above have been explored to initiate the ROP of
optically pure D-LA or L-LA to yield isotactic PLA with the
retention of configuration; however, for the racemic lactide (rac-
LA), atactic PLA is usually isolated. Stereoselective polymer-
ization of rac-LA is initially accessed by employing an optically
pure Salen aluminum initiator to afford isotactic PLA when the
conversion is below 50%.5a Thorough developments have been
achieved by using homochiral or racemic Salen aluminum
initiators that can completely transfer rac-LA to stereoblock P(L-
LA)-b-P(D-LA) via ligand exchange or polymer chain transfer
mechanism.5b-h Recent investigations demonstrate strikingly that
the stereoblock PLA can be obtained by using achiral Salen
aluminum alkoxides via polymer-chain-end control. The alkyl
bridge between the two imido nitrogen atoms in the ligands is
crucial to govern the selectivity of the complexes.5i-k Contrarily,
studies on the synthesis of heterotactic PLA have remained less
explored. This polymerization is extremely sensitive to the
sterics of the ligands, because the environment of the last unit
of the propagating active species must be sterically proper to
incorporate the configurationally opposite enantiomer. â-Di-
iminate zinc or magnesium alkoxide or amide6a-c and Salan
(CdN reduction derivative of Salen) aluminum alkyls12 repre-
sent rare examples of heteroselective initiators. Complexes based
Scheme 1. Preparation of Complexes 1-4
Scheme 2. Preparation of Complex 5
Scheme 3. Preparation of Complex 6
on rare earth metals, unfortunately, have demonstrated to be
less stereocontrolled.5f Breakthroughs have been achieved
very recently by Carpentier,13 Mountford,14 and Okuda,15 who
employed modified bis(phenolate)-ligated lanthanide amide,
alkoxide, or borohydride to realize heteroselective polymeriza-
tion of rac-LA. The tacticity of the resulting PLA varies from
medium to very high level depending on the ligand framework.
Recently, our group has been engaged in preparing a new family
of rare earth metal alkyl complexes stabilized by non-cyclo-
pentadienyl (nonCp) ligands.16 The amino-amino-bisphenols are
one type of moiety we used, which have been applied to prepare
group 4 metal complexes that catalyze living polymerization
of olefins17 and cyclic esters18 and stereoselective polymerization
of rac-LA.19 Here, we wish to report that the combination of
these ligands with lanthanide alkyl units afforded initiators for
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Achiral Lanthanide Alkyl Complexes
Organometallics, Vol. 26, No. 10, 2007 2749
Figure 1. ORTEP drawing of complex 1 with 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 1-5
1‚C6H14
(Ln ) Y)
2‚C6H14
(Ln ) Y)
3‚C6H14
(Ln ) Lu)
4
5‚C6H14
(Ln ) Y)
(Ln ) Y)
Ln-O(1)
2.1379(16)
2.1441(17)
2.3866(17)
2.424(3)
2.574(2)
2.588(2)
150.94(7)
83.87(6)
81.98(6)
102.92(8)
105.32(8)
105.94(8)
78.08(6)
79.52(6)
99.86(7)
154.15(8)
97.72(7)
91.27(7)
167.99(7)
85.37(8)
69.01(6)
2.138(2)
2.168(2)
2.375(3)
2.432(3)
2.576(3)
2.1279(10)
2.1239(9)
2.3596(9)
2.3593(15)
2.5118(11)
2.5838(11)
151.03(4)
80.39(4)
2.154(2)
2.141(2)
2.378(2)
2.423(3)
2.614(2)
2.536(3)
155.66(8)
84.14(8)
88.83(8)
109.32(10)
93.71(10)
88.52(11)
79.62(7)
76.96(7)
88.81(8)
170.35(10)
91.43(8)
93.02(9)
172.96(8)
98.13(12)
84.99(9)
2.113(5)
2.159(5)
2.354(5)
2.445(7)
2.582(6)
2.525(6)
152.57(18)
85.39(18)
83.16(18)
102.2(2)
105.0(2)
107.4(2)
77.07(17)
79.92(17)
97.91(18)
154.6(2)
102.28(19)
81.88(18)
160.25(19)
88.9(2)
Ln-O(2)
Ln-O(3)
Ln-C(3)
Ln-N(1)
Ln-N(2)
2.651(3)
O(1)-Ln-O(2)
O(1)-Ln-O(3)
O(2)-Ln-O(3)
O(1)-Ln-C(3)
O(2)-Ln-C(3)
O(3)-Ln-C(3)
O(1)-Ln-N(1)
O(2)-Ln-N(1)
O(3)-Ln-N(1)
C(3)-Ln-N(1)
O(1)-Ln-N(2)
O(2)-Ln-N(2)
O(3)-Ln-N(2)
C(3)-Ln-N(2)
N(1)-Ln-N(2)
148.77(10)
81.57(10)
80.65(9)
102.44(11)
106.21(11)
101.89(11)
78.90(9)
80.92(3)
103.50(4)
102.60(5)
104.99(4)
81.86(3)
79.64(4)
99.55(3)
155.41(4)
95.87(4)
98.40(3)
169.17(4)
85.73(4)
69.77(3)
79.58(9)
101.03(10)
156.98(11)
92.83(10)
100.13(10)
169.23(10)
88.26(11)
68.73(9)
66.78(19)
ROP of rac-LA with unprecedented heteroselectivity. The
resulting PLA has a Pr of 0.99, the highest value reached to
date, as far as we are aware. (Pr: probability of racemic
enchainment of monomer units calculated according to the
methine region of the homonuclear decoupled 1H NMR
spectrum.) The syntheses and full characterizations of these
complexes and investigations on the correlation between
catalytic performances and molecular structure, and the mech-
anism of polymerization, will be presented. The primary
polymerization results by using the rare earth metal bis(alkyl)s
bearing modified â-diiminate ligands are also described as a
comparison.
amino-amino-bisphenols, H2L1-4, and lanthanide tris(alkyl)s,
Ln(CH2SiMe3)3(THF)2 (Ln ) Y, Lu), took place immediately
upon addition by release of tetramethylsilane. The reaction was
maintained at -30 °C for 12 h in hexane, then the volatiles
were removed under reduced pressure to afford oily residues.
The residues were carefully washed with a minimum amount
of cold hexane to extrude unreacted reagents to give white
crystalline solids of lanthanide mono(alkyl) complexes 1-5
(Schemes 1 and 2). It is crucial to control the reaction
temperature below -25 °C and avoid prolonging the reaction
time to increase the yield; otherwise, some unknown complexes
would be produced. Following a similar procedure, treatment
of Y(CH2SiMe3)3(THF)2 with Salan compound H2L5 afforded
complex 6 selectively (Scheme 3). All complexes 1-6 except
5 have good solubility in solvents such as THF, ether, toluene,
benzene, and hexane. Therefore, pure solids or good single
crystals for X-ray analysis were isolated only from a concen-
trated mixture of toluene and hexane (1:10 v/v) at -30 °C within
several days. An alternative pathway for the preparation of
complexes 1-6 is through metathesis reaction of lithium salts
Results and Discussion
Synthesis and Characterization of Lanthanide Alkyl
Complexes 1-8. Reactions between the neutral compounds of
(19) Chumura, A.; Davidson, M.; Jones, M.; Lunn, M.; Mahon, M.;
Johnson, A.; Khunkamchoo, P.; Roberts, S.; Wong, S. Macromolecules
2006, 39, 7250-7257.

2750 Organometallics, Vol. 26, No. 10, 2007
Liu et al.
Figure 2. ORTEP drawings of the molecule cores of complexes 1-5.
Scheme 4. Preparation of Complexes 7 and 8
of the ligands with lanthanide trichlorides followed by alkyla-
tion. However, lithium adduct or ate complexes are usually
isolated. The NMR spectra for complexes 1-6 were informative
for the formation of mono(alkyl) species. The methylene protons
of the metal alkyl species in 1-6 gave resonances around δ
0-0.5, downfield shifted compared to that at δ -0.60 in metal
tris(alkyl)s. The pendent nitrogen atom was found to attach a
metal center in solution, because the proton resonances of the
groups -N(CH3)2 in 1 and 4, -N(CH2CH3)2 in 2 and 3, and
-NC5H4 in 5 shifted approximately 0.3-0.5 ppm, respectively,
compared to those in the free ligands H2L1-H2L4. The benzylic
Ln-Nterminal (av 2.286 Å).13b,21 The bond distance of Ln-C,
with an average of 2.417 Å, is typical for those found in nonCp-
ligated rare earth metal alkyl complexes.24 The bond angle of
O(1)-Y-O(2), 155.66(8)°, in 4 is larger than 152.57(18)° in 5
and the average of 150.25° in 1-3, while the bond angle of
O(3)-Ln-N(1) is 99.86(7)° for 1, 101.03(10)° for 2, 99.55-
(3)° for 3, 97.91(18)° for 5, and only 88.81(8)° for 4 (Table 1);
an obvious difference is found for the deviation angle of the
alkyl ligand from the LnONO plane, being 29.39° for 1, 29.97°
for 2, 31.45° for 3, and a slightly smaller 26.77° for 5; however,
a much smaller 2.72° was found for 4 (Figure 2). This suggested
that the short ethylene bridge between the two nitrogen atoms
N(1) and N(2) in 1-3 and the stiff piconyl bridge in 5 cause a
more distorted geometry of these molecular cores than the
longer, flexible trimethylene bridge in 4, which might contribute
significantly to the different activity and selectivity among these
complexes (Vide infra).
1
protons in each complex were diastereotopic, giving a H AB
spin in contrast to the singlet resonance in free ligands;
moreover, the methylene protons were equivalent, showing
doublets with coupling constants of 3-5 Hz in all yttrium
complexes 1, 2, 4, and 5, indicating monomeric and Cs
symmetric molecular structures in solution.
X-ray diffraction analyses displayed that the overall structures
of 1-5 are analogous six-coordinate monomers (Figure 1,
ORTEP plot of complex 1).20 For complexes 1-3 and 5, each
metal ion bonds to an alkyl moiety, a THF molecule, and the
dianionic amino-amino-modified bisphenolato ligand in an
O,N,N,O-tetradentate mode, adopting a twisted octahedral
geometry. In complex 4, the O,N,N,O-tetradentate ligand
combined with a THF molecule and the alkyl species forms a
tetragonal bipyramidal geometry core. The pendent nitrogen and
THF oxygen atoms occupy the axial positions, while the
phenolate oxygen, the nitrogen, and the carbon atoms are located
at equatorial positions. The average bond distance of Ln-Ophenol
(Ln-O(1): av 2.134 Å; Ln-O(2): av 2.147 Å) is comparable
to that formed by a lanthanide metal ion and phenol oxy-
gen,13b,21,22 which is noticeably shorter than Ln-O(3), averaging
2.371 Å (Table 1), whereas the bond distances from lanthanide
ions to the equatorial nitrogen (Ln-N(1)) and the pendent
nitrogen (Ln-N(2)) atoms are similar, falling in a narrow range
of 2.5118(11)-2.651(3) Å, which are comparable to other
yttrium complexes containing these groups,21,23 but longer than
Treatment of ligands HL6 with Y(CH2SiMe3)3(THF)2 gener-
ated the corresponding yttrium bis(alkyl) complex 7 (Scheme
4). The molecular structure of 7 is shown in Figure 3. The
yttrium atom coordinates to the N,N,O-tridentate ligand and two
alkyl species arranged in cis-positions, generating a tetrahedral
geometry core. The complex is solvent-free. The bond lengths
of Y(1)-N(1) and Y(1)-N(2) (2.310(2) and 2.338(2) Å) are
(24) (a) Bambirra, S.; Meetsma, A.; Hessen, B.; Teuben, J. Organome-
tallics 2000, 16, 3197-3205. (b) Bambirra, S.; Meetsma, A.; Hessen, B.;
Teuben, J. Organometallics 2001, 20, 782-785. (c) Bambirra, S.; van
Leusen, D.; Meetsman, A.; Hessen, B.; Teuben, J. H. Chem. Commun. 2001,
637-638. (d) Emslie, D. J. H.; Piers, W. E.; Parvez, M.; McDonald, R.
Organometallics 2002, 21, 4226-4240. (e) Kirillow, E.; Toupet, L.;
Lehmann, C. W.; Razavi, A.; Carpentier, J. Organometallics 2003, 22,
4467-4479. (f) Bambirra, S.; van Leusen, D.; Meetsman, A.; Hessen, B.;
Teuben, J. H. Chem. Commun. 2003, 522-523. (g) Hayes, P. G.; Welch,
G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W. E.; Parvez, M.
Organometallics 2003, 22, 1577-1579. (h) Bambirra, S.; Boot, S. J.; van
Leusen, D.; Meetsma, A.; Hessen, B. Organometallics 2004, 23, 1891-
1898.
(20) The 1H and 13C NMR spectra for complexes 6-8, crystallographic
data and structure refinements of complexes 1-5 and 7, and ORTEP
drawings of complexes 2-5; see Supporting Information.
(21) Emslie, D.; Piers, W.; MacDonald, R. Dalton Trans. 2002, 293-
294.
(22) Cai, C.; Toupet, L.; Lehmann, C.; Carpentier, J. J. Organomet.
Chem. 2003, 683, 131-136.
(23) (a) Runte, O.; Preiermeier, T.; Anwander, R. Chem. Commun. 1996,
1358-1359. (b) Evans, W.; Fujimoto, C.; Ziller, J. Chem. Commun. 1999,
311-312.

Achiral Lanthanide Alkyl Complexes
Organometallics, Vol. 26, No. 10, 2007 2751
Figure 3. ORTEP drawing of complex 7 with 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
slightly shorter than Ln-N, averaging 2.572 Å in complexes
1-5, but the bond length of Y(1)-O(1) (2.4394(16) Å) is longer
than Ln-O(1) (av 2.134 Å) in complexes 1-5. The bond angle
of C(1)-Y(1)-C(2), 115.92(9)°, is larger than those found in
bis(alkyl) yttrium complexes stabilized by anilido imine (108.90-
(6)°),24g tetradentate triamine (av 101.4°),24b,c,h and Salen
compounds (110.97(7)°),24d but smaller than that in the amidi-
nato yttrium bis(alkyl) complex (140.56(6)°).24f Complex 7 is
asymmetric in the solid state, consistent with that in solution,
which is evidenced in the NMR spectrum, because the meth-
ylene protons YCH2SiMe3 are diastereotopic, showing four
discrete signals that further split into a doublet due to coupling
with the Y atom (SI, Figure 7). Treatment of Y(CH2SiMe3)3-
(THF)2 with HL7 afforded complex 8 (Scheme 4). The 1H and
13C NMR spectral analyses indicate a symmetric monomer of
bis(alkyl) species in solution.20
Ring-Opening Polymerization of rac-Lactide. All these
lanthanide alkyl complexes 1-8 can initiate the ROP of rac-
LA. The catalytic activity and stereoselectivity are strongly
dependent on the molecular structures of the complexes and
the polymerization conditions. For mono(alkyl) complexes 1-3,
5, and 6, almost complete conversion was reached within 1 h
at room temperature. The molecular weights of the resulting
PLAs were close to the theoretic values, and the molecular
weight distributions were narrow. Strikingly, complexes 1-3
showed similar high stereoselectivity for rac-LA to afford
heterotactic PLA with Pr over 0.94 (Table 2, entries 1, 4, 6).6b,25
Complex 5 showed the same level of catalytic activity as
complexes 1-3, albeit with relatively low selectivity (Table 2,
entry 9). In contrast, dramatic decreases in catalytic activity and
stereocontrol ability were found for complex 4 even though it
is an analogue of complex 1 (Table 2, entries 7, 8). The Salan-
ligated mono(alkyl) complex 6 showed no selectivity (Pr ) 0.65,
Table 2, entry 10), although its aluminum counterpart was
reported to be highly heteroselective (Pr ) 0.96).12 Using bis-
(alkyl) complexes 7 and 8 as initiators, a complete conversion
could be achieved in 30 min. The molecular weight of the
resultant PLA was close to the theoretic value calculated on
the basis of di-active sites. Thus, we suggested that both metal
alkyl species participated in the initiation, leading to the
relatively broad molecular weight distribution and atactic
microstructure of the isolated PLA (Table 2, entries 11, 12).
At various monomer-to-initiator ratios ranging from 100 to
500, polymerization of rac-lactide initiated by complexes 1 and
2 proceeded smoothly. The molecular weight of the resulting
PLA increased with the ratio, while the molecular weight
distribution did not change obviously (PDI ) 1.25-1.48) (Table
3, entries 1-4 and 7-10). When the ratio was raised to over
500, the reaction became sluggish because of too low concentra-
tion of the initiator, and complete conversion could not be
reached even for long reaction times (entries 5, 6, 11). Strikingly,
the high heteroselectivity of complexes 1 and 2 was not
influenced or even slightly increased with the ratio and also
showed no relationship with the conversion. The resulting
heterotactic PLA had a Pr of 0.99, the highest value reached up
to date, as far as we aware. This result indicated the absence of
transesterification that would cause a decrease in selectivity.
Reaction conditions also affected the catalytic performance
of the complexes. With complex 2, if the polymerization was
performed at 70 °C, 85% conversion could be achieved in 5
min (Table 4, entry 1), whereas, the heterotacticity of the
resultant PLA dropped slightly (Pr ) 0.93). This also indicated
that 2 was stable at high temperature, at least, in the initiation
time scale. Polymerization medium played an important role in
influencing the activity and stereocontrol ability of the catalysts.
THF was the optimum solvent (Table 4, entry 2).9e,f,12,15
Contrarily, polymerization performed in CH2Cl2 at room tem-
perature for a long time (12 h) led to low conversion and
isolation of low-tacticity PLA (Pr ) 0.75, Table 4, entry 3).
This could be due to the competitive coordination between the
polar CH2Cl2 molecule and monomer lactide, which changed
the environment of the initiator.26 Toluene was such a poor
(25) Zell, M.; Padden, B.; Paterick, A.; Thakur, K.; Kean, R.; Hillmyer,
M.; Munson, E. Macromolecules 2002, 35, 7700-7707.

2752 Organometallics, Vol. 26, No. 10, 2007
Table 2. Polymerization of rac-Lactide with Various Lanthanide Alkyl Complexesa
Liu et al.
[LA]
entry
initiator [Ln]
(mol‚L-1
)
[LA]/[Ln]
t (h)
T (°C)
conv
Mcalcd ×10-4 b
Mn ×10-4 c
PDI
Prd
1
2
3
4
5
6
7
8
1
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.0
1.0
300
300
300
300
300
300
300
300
300
300
500
300
1
1
24
1
2
1
6
1
1
1
20
20
20
20
20
20
20
70
20
20
20
20
100
96
trace
100
68
100
trace
70
4.32
4.15
n.d.f
4.32
2.93
4.32
n.d.
3.02
3.89
4.32
3.60g
2.16g
5.66
4.69
n.d.
3.96
1.54
3.86
n.d.
2.10
2.12
3.74
4.57
2.67
1.32
1.44
n.d.
1.43
1.29
1.59
n.d.
1.44
1.51
1.64
1.72
1.96
0.96
0.95
0.94
0.97
0.90
0.94
n.d.
0.75
0.90
0.65
0.69
0.64
1 + C6H5NH2 (1:1)
1 + HOiPr (1:1)
2
2-Me e
3
4
4
5
6
7
8
9
91
10
11
12
100
100
100
0.5
0.5
b
c
d
a Polymerizations were performed in THF. Calculated by ([LA]/[Ln]) × 144.14 × X (X ) conv). Determined by GPC against polystyrene standard. Pr
is the probability of racemic linkages between monomer units determined from the methane region of the homonuclear decoupled 1H NMR spectrum.
eComplex 2-Me is an analogue of 2 with methyl substituents on the ortho-positions of phenyl rings. n.d.: not determined. Calculated by ([LA]/2[Ln]) ×
f
g
144.14 × X (X ) conv).
medium that almost no polymerization of rac-LA took place at
room temperature. The reaction became obviously rapid at 70
°C, but gave atactic PLA with low molecular weight and broad
molecular weight distribution (Table 4, entries 4, 5). This is in
striking contrast to the methoxy-amino-bisphenolato lanthanide
alkoxide systems, which initiate the equally rapid ROP of rac-
LA in toluene and THF.13
Much higher selectivity has been found for the dimeric
L4-stabilized yttrium borohydrides (Pr ) 0.87).14
Mechanistic Aspect. A computational investigation of the
mechanism for the stereoselective ROP of rac-LA with (BDI)-
Mg(OiPr) reported by Rzepa demonstrates that the clashes
between the methyl of the first insertion lactide and the ortho
substituent of N-aryl of the BDI ligand represent the most
significant contribution to the stereoselectivity.29 The larger the
ortho substituent is, the higher the selectivity of the complex.
This can well explain some experimental results. For instance,
substituting the ortho CH3 unit of the N-aryl in (BDI)Zn(OiPr)
by a larger iPr group or substituting the ortho CH3 group of
the bisphenolate in the lanthanide alkoxide complexes by a larger
C(CH3)2Ph group leads to a dramatic increase in the selectivity
of the corresponding complexes.6b,13b For our system, the sterics
of the ortho position, indeed, influenced the selectivity, however
not as obviously as the above-mentioned systems. The less bulky
complex 2-Me 30 (ortho CH3) showed slightly lower selectivity
(Pr ) 0.90, Table 2, entry 5) than the spacially steric analogue
The effect of the type of initiator was also investigated. L1-
ligated yttrium amide and alkoxide initiators, in situ generated
by addition of equivalent aniline and 2-propanol to yttrium alkyl
complex 1, respectively, were employed to catalyze the polym-
erization of rac-LA. Both exhibited similar selectivity to the
parent alkyl species. The slight drop in heterotacticity for the
resulting PLAs might be attributed to the impurities in the
systems, such as the unreacted lanthanide tris(alkyl)s and the
ligand-redistribution byproduct, which were active initiators but
nonselective (Table 2, entries 2, 3). It should be noted that the
alkoxide species displayed extremely low activity; only a trace
amount of polymer could be isolated from the polymerization
carried out for 24 h at room temperature, which might be due
to the too electron-negative metal center. This is in contrast to
the literature results that lanthanide alkoxide species are slightly
less or equally active initiators compared to the corresponding
alkyl congeners and initiate very controllable polymerization.13b,15
In fact, the catalytic performance of a complex is affected
by the concerted influence of the ligand framework and Lewis
acidity of the central metal27 and the nucleophilicity of initiation
species.11d,e Complex L1-OMeY(CH2SiMe3)(THF) containing a
pendent OMe functional group in the ligand skeleton displays
modest selectivity13b compared to complex 1 (L1Y(CH2SiMe3)-
(THF)), having a pendent N(CH3)2 functionality. The same L1-
attached ytterbium methyl shows activity to caprolactone,28
whereas the yttrium amido counterpart is highly active to rac-
LA but ambiguous in selectivity owing to the existence of
competitive initiators (impurities).13b The titanium bis(alkoxo)
complexes bearing a L1 moiety can initiate the polymerization
of caprolactone and L-LA.18 L4-attached titanium bis(alkoxide)s
are active toward the ROP of rac-LA, although it is nonselective,
whereas their zirconium and hafnium counterparts belong to
the rare examples of group 4 metal complexes that show modest
iso- or heteroselectivity (Pr/Pm ) 0.3/0.7; Pr/Pm ) 0.5/0.5).19
(26) The NMR spectrum (see Supporting Information) for complex 5 in
CD2Cl2 showed a different pattern from that in C6D6 especially the yttrium
alkyl group, shifting to lower-field region at δ ) 0.09 from -0.6 ppm,
indicating the change of geometry of the metal center.
(27) The Lewis acidity of the central metal ion influences the catalytic
activity of the complex toward the polymerization of cyclohexene oxide
with CO2; see: (a) Cui, D.; Nishiura, M.; Hou, Z. Macromolecules 2005,
38, 4089-4095. (b) Darensbourg, D. J.; Niezgoda, S. A.; Draper, J. D.;
Reibenspies, J. H. J. Am. Chem. Soc. 1998, 120, 4690-4698. (c)
Darensbourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124, 6335-
6342.
(28) Yao, Y.; Ma, M.; Xu, X.; Zhang, Y.; Shen, Q.; Wong, W.
Organometallics 2005, 24, 4014-4020.
(29) Marshall, E.; Gibson, V.; Rzepa, H. J. Am. Chem. Soc. 2005, 127,
6048-6051.
(30) Methyl-substituted phenol reacted with dimethylamino ethylene
amine to afford H2L2-Me, treatment of which by Y(CH2SiMe3)2THF2
gave L2-MeY(CH2SiMe3)(THF) (2-Me), an analogue of 2. NMR data: δ
7.11 (s, 1H, C6H2), 6.88 (s, 1H, C6H2), 6.86 (s, 1H, C6H2), 6.82 (s, 1H,
C6H2), 5.42 (s, 1H, ArCH2N), 5.39(s, 1H, ArCH2N), 5.07(s, 1H, ArCH2N),
5.04(s, 1H, ArCH2N), 3.69 (m, 4H, THF), 3.38 (s, 1H, ArCH2N), 3.35
(s, 1H, ArCH2N), 3.08 (s, 1H, ArCH2N), 3.05 (s, 1H, ArCH2N), 2.80
(m, 2H, N(CH2CH3)2), 2.66 (m, 2H, N(CH2)2N), 2.51 (s, 3H, ArCH3), 2.44
(s, 3H, ArCH3), 2.40 (s, 3H, ArCH3), 2.32 (m, 2H, N(CH2)2N), 2.22
(m, 2H, N(CH2CH3)2), 2.20 (s, 3H, ArCH3), 1.53 (m, 4H, THF), 0.69 (br,
3H, N(CH2CH3)2), 0.43 (br, 3H, N(CH2CH3)2, 9H, CH2Si(CH3)3), -0.10,
-0.57 (AB, d, J(H, H) ) 12 Hz, J(Y, H) ) 3.2 Hz, 2H, CH2Si(CH3)3)).

Achiral Lanthanide Alkyl Complexes
Organometallics, Vol. 26, No. 10, 2007 2753
Table 3. Polymerization of rac-Lactide with Complexes 1 and 2 at Various Monomer-to-Initiator Ratiosa
entry
initiator [Ln]
[LA] (mol‚L-1
)
[LA]/[Ln]
t (h)
conv
Mcal × 10-4 b
Mn × 10-4 c
PDI
Prd
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
2
2
2
2
2
1.7
1.7
1.7
1.7
1.7
2.2
1.0
1.0
1.7
1.0
1.7
100
200
300
500
700
1000
100
200
300
500
700
1
1
1
1
3
18
1
1
1
1
100
100
100
100
86
1.44
2.88
4.32
7.20
8.67
5.18
1.44
2.88
4.32
7.20
10.08
1.93
2.11
5.66
9.25
3.77
3.51
1.78
2.68
3.96
6.69
8.04
1.25
1.41
1.32
1.33
1.48
1.47
1.38
1.28
1.43
1.39
1.59
0.95
0.97
0.96
0.96
0.99
0.98
0.96
0.97
0.97
0.97
0.98
36
100
100
100
100
95
10
11
3
b
c
a Polymerizations were performed in THF at 20 °C. Calculated by ([LA]/[Ln]) × 144.14 × X (X ) conv). Determined by GPC against polystyrene
d
1
standard. Pr is the probability of racemic linkages between monomer units determined from the methane region of the homonuclear decoupled H NMR
spectrum.
Table 4. Polymerization of rac-Lactide with Complex 2: Influence of the Reaction Conditions
entry
solvent
[LA] (mol‚L-1
)
[LA]/[Ln]
t (h)
T (°C)
conv
Mcal × 10-4 a
Mn × 10-4 b
PDI
Prc
1
2
3
4
5
THF
THF
CH2Cl2
toluene
toluene
1.0
1.7
1.0
0.5
0.5
300
300
300
300
300
5 min
1
12
12
1
70
20
20
20
70
85
100
70
trace
86
3.67
4.32
4.32
n.d.d
4.32
3.07
3.96
3.98
n.d.
1.47
1.43
1.57
n.d.
0.93
0.97
0.75
n.d.
1.83
1.67
0.63
b
c
a Calculated by ([LA]/[Ln]) × 144.14 × X (X ) conv). Determined by GPC against polystyrene standard. Pr is the probability of racemic linkages
1
d
between monomer units determined from the methane region of the homonuclear decoupled H NMR spectrum. n.d. ) not determined.
2 (ortho tBu, Pr ) 0.97, Table 2, entry 4). This result was
consistent with the O,S,S,O-type scandium alkoxide systems
reported by Okuda et al. that showed that the sterics of the ortho
substituent of the bisphenolate does not greatly influence the
selectivity.15 They attribute the high selectivity to a dynamic
monomer-recognition process involving interconversion of the
ligand configuration. The structure of Salan yttrium complex 6
is much close to these O,S,S,O-type scandium alkoxide systems;
however, it is a nonselective initiator, which might be due to
the lesser constraint of the ligand framework and the absence
of the exchange of ligand configuration.
It was noteworthy that the major differences in molecular
structures of complexes 1-5 were the bridge between the two
nitrogen atoms of the ligand, the type of lanthanide element,
and the pendent amino alkyl group. X-ray crystallographic data
had shown that the bridge significantly influenced the geometry
of the metal center. The more flexible trimethylene bridge
resulted in a less distorted tetragonal bipyramid in 4, while the
short ethylene in 1-3 and the rigid pyconyl in 5 generated a
more twisted octahedral geometry. This was reflected in the
deviation angles of the alkyl ligand from the LnONO plane,
being 29.39° for 1, 29.97° for 2, 31.45° for 3, and a slightly
smaller 26.77° for 5, but very small 2.72° for 4 (Vide supra), in
a trend of 1 ≈ 2 ≈ 3 > 5 . 4, which was fairly coincident
with the catalytic activity31 and stereoselectivity. This indicated
that changing the substituent of the pendent nitrogen atom from
methyl (1) to ethyl (2), or switching the central metal from
yttrium (2) to lutetium (3), caused negligible differences in
catalytic behavior. In contrast, the bridge between the two
nitrogen atoms was crucial in governing the catalytic perfor-
mance via tuning the geometry of the metal center.
isolated the active propagating species of rac-LA oligomer
attached to complex 1. The process could be described as shown
in Scheme 5. The S,S-LA (or R,R-LA) enantiomer first
coordinated to a yttrium ion via its Oacyl from the back-side of
the alkyl moiety with respect to the steric environment of the
metal center in the cases of complexes 1-3 and 5 being
catalysts. For complex 4, the coordination of monomer was more
random due to the less distorted geometry of the metal center.
The following nucleophilic addition of Y-CH2SiMe3 species
to the Ccabonyl led to the ring-opening of S,S-LA (or R,R-LA) to
form five-membered metal-lactate active species with C(O)-
CH2SiMe3 as the end group. The NMR spectrum was clear and
informative (Figure 4). There were two sets of ideal quartet/
doublet resonances at 5.01/1.59 and 4.82/1.53,11e respectively,
with an integral intensity ratio of 1:3 and a J(H, H) of 3.6 Hz,
which were assigned to the methine (Ha, Ha′) and methyl (Hb,
Hb′) protons derived from two different enantiomeric lactate
units bonding to the yttrium ion. This indicated that two types
of active propagating species were present, consistent with the
alternating arrangement of two enantiomers in the polymer
chain. More strikingly, the ligand also showed two sets of
chemical shifts. The benzylic proton Hg (PhCH2N) gave two
1H AB spins compared to one in complex 1 (SI, Figure 12);
the amino methyl protons Hi (-N(CH3)2) displayed two singlet
resonances compared to one in complex 1. This suggested that
the ligand and the pendent nitrogen atom kept attaching to the
metal ion and the geometry of the central metal of complex 1
did not collapse but remained in solution upon monomer
coordination and insertion. It further confirmed that the metal
ion was bonded to two different enantiomeric lactate units. Thus
the environment of the resulting propagating sites was spacially
steric. Incorporation of a configurationally opposite enantiomer
R,R-LA (S,S-LA) should be favored to lower this transition-
state energy. The following nucleophilic addition of the newly
formed metal alkoxy propagating species to the Ccabonyl led to
the ring-opening of R,R-LA (S,S-LA) to form a racemic
enchainment. This coordination-insertion mechanism was
proved by the presence of the end group C(O)CH2SiMe3, giving
a singlet resonance at δ 1.49 assignable to the methylene protons
Obviously, it is important to know whether the geometry of
the molecular core is retained in solution especially during the
polymerization process and what are the mechanistic aspects
of the ROP of rac-LA initiated by these Ln-C species. Thus,
we designed a polymerization performed in C4D8O at a
monomer-to-initiator ratio of 15:1 (10 min) and successfully
(31) The activities of complexes 1-5 were 73.5, 73.5, 73.5, 0, and 66.9
kg‚mol-1‚h-1, respectively.

2754 Organometallics, Vol. 26, No. 10, 2007
Scheme 5 . Formation of Active Propagating Species of rac-LA Oligomer Initiated by Complex 1
Liu et al.
Figure 4. 1HNMR spectrum (400 MHz) of the active species of rac-lactide oligomer initiated by complex 1 (*C4D8O, [LA]0:[Y] ) 15:1,
25 °C).
Hk, which was comparable with C(O)CH2SiMe3 in [(C5Me4-
SiMe3)Y(OC(O)CH2SiMe3)2]2,27a and also evidenced in the 13
atoms of the ligand played a critical role in governing the
stereoselectivity via tuning the geometry of the molecular core
of the complexes, while the mono(alkyl) complex bearing a
Salan ligand was an active initiator, unfortunately without
stereocontrol due to the less constrained ligand. The monoan-
ionic â-diiminate-ligated lanthanide bis(alkyl) complexes were
also active but nonselective due to the di-active sites. A more
important implication of the current work is that single-site
catalysts offer new opportunities for microstructural control of
polymers if new reaction pathways are accessed through catalyst
design and control of reaction conditions.
C
DEPT spectrum (SI, Figures 13 and 15). During the process,
the THF molecule might be extruded from or rebonded to the
Y ion to satisfy a six-coordination sphere and also contributed
partially to lower the transition-state energy.29 This might be
the reason that THF was the optimum medium for the
polymerization.
Conclusion
We have demonstrated that the combinations of amino-amine-
modified bisphenolate ligands with lanthanide alkyl units
generated unprecedented stereoselective initiators for the ring-
opening polymerization of rac-LA to give heterotactic poly-
lactide with Pr up to 0.99. The bridge between the two nitrogen
Experimental Section
General Methods. All reactions were carried out under a dry
and oxygen-free argon atmosphere by using Schlenk techniques or

Achiral Lanthanide Alkyl Complexes
Organometallics, Vol. 26, No. 10, 2007 2755
under a nitrogen atmosphere in an MBraun glovebox. Solvents were
purified by an MBraun SPS system. Ligands H2L1-H2L5, HL6
and HL7 were synthesized according to modified literature
procedures.6e,17b,32 The phenols and amines were purchased from
Aldrich or Fluka. All liquids were dried over 4 Å molecular sieves
for a week and distilled before use, and solid materials were used
without purification. The synthesis of lanthanide tris(alkyl)s fol-
lowed the established method with minor alteration.33
(m), 808(m), 781(w), 668(m) cm-1. Anal. Calcd for C42H73N2O3-
SiY (%): C, 65.43; H, 9.54; N, 3.63. Found: C, 65.35; H, 9.48;
N, 3.56.
L2Y(CH2SiMe3)(THF) (2). Following a similar procedure to that
described for the preparation of complex 1, treatment of Y(CH2-
SiMe3)3(THF)2 (0.181 g, 0.37 mmol) with equimolar H2L2 (0.20
g, 0.37 mmol) afforded complex 2 as white powders (0.18 g, 64%).
1H NMR (400 MHz, C6D6, 25 °C): δ 7.71 (d, 4J(H, H) ) 2.7 Hz,
4
Instruments and Measurements. Organometallic samples for
2H, C6H2), 7.23 (d, J(H, H) ) 2.7 Hz, 2H, C6H2), 4.05 (m, 4H,
2
2
NMR spectroscopic measurements were prepared in a glovebox
THF), 3.87 (d, J(H, H) ) 12.0 Hz, 2H, ArCH2N), 3.02 (d, J(H,
H) ) 12.0 Hz, 2H, ArCH2N), 2.75 (m, 2H, N(CH2CH3)2), 2.47
(br, 2H, N(CH2)2N), 2.22 (br, 2H, N(CH2CH3)2), 1.99 (br, 2H,
N(CH2)2N), 1.91 (s, 18H, C(CH3)3), 1.56 (s, 18H, C(CH3)3), 1.32
(m, 4H, THF), 0.63 (s, 9H, CH2Si(CH3)3), 0.56 (t, 3J(H, H) ) 7.3
1
by use of NMR tubes and then sealed by paraffin film. H, 13C
NMR spectra were recorded on a Bruker AV400 (FT, 400 MHz
1
for H; 100 MHz for 13C) spectrometer. NMR assignments were
1
1
confirmed by H-1H (COSY), H-13C (HMQC), and 13C NMR
(DEPT) experiments when necessary. The molecular weight and
molecular weight distribution of the polymers were measured by a
GPC Waters 410. Crystals for X-ray analysis were obtained as
described in the Experimental Section. The crystals were manipu-
lated in a glovebox. Data collections were performed at -86.5 °C
on a Bruker SMART APEX diffractometer with a CCD area
detector, using graphite-monochromated Mo KR radiation (λ )
0.71073 Å). The determination of crystal class and unit cell
parameters was carried out by the SMART program package. The
raw frame data were processed using SAINT and SADABS to yield
the reflection data file. The structures were solved by using the
SHELXTL program. Refinement was performed on F2 anisotro-
pically for all non-hydrogen atoms by the full-matrix least-squares
method. The hydrogen atoms were placed at the calculated positions
and were included in the structure calculation without further
refinement of the parameters. IR spectra were obtained on a Bruker
Vertex 70 FTIR spectrometer. Elemental analyses were performed
at National Analytical Research Centre of Changchun Institute of
Applied Chemistry.
2
Hz, 6H, N(CH2CH3)2), -0.29 (d, J(Y, H) ) 3.2 Hz, 2H, CH2Si-
(CH3)3) ppm. 13C NMR (100 MHz, C6D6, 25 °C): 162.2 (2C, ipso-
2,4-tBu2C6H2O), 137.4 (2C, ipso-2,4-tBu2C6H2O), 136.9 (2C, ipso-
2,4-tBu2C6H2O), 126.2 (2C, 2,4-tBu2-C6H2O), 125.2 (2C, 2,4-tBu2-
C6H2O), 125.1 (2C, ipso-2,4-tBu2-C6H2O), 72.1 (2C, THF), 66.0
(2C, ArCH2N), 52.8 (1C, N(CH2)2N), 49.0 (1C, N(CH2)2N), 45.9
(2C, N(CH2CH3)2), 36.2 (2C, C(CH3)3), 34.7 (2C, C(CH3)3), 32.7
(6C, C(CH3)3), 31.3 (6C, C(CH3)3), 26.3 (1C, CH2SiMe3), 25.6 (2C,
THF), 9.0 (2C, N(CH2CH3)2), 5.5 (3C, Si(CH3)3) ppm. IR (KBr
pellet): ν 2954(m), 2901(m), 2867(m), 1603(m), 1559(w), 1478-
(m), 1443(m), 1415(m), 1385(m), 1361(m), 1304(m), 1248(m),
1238(m), 1203(m), 1167(m), 1134(m), 1115(w), 1089(w), 1059-
(w), 1026(m), 959(w), 912(m), 876(m), 862(m), 838(m), 806(m),
782(w), 694(m) cm-1. Anal. Calcd for C44H77N2O3SiY (%): C,
66.13; H, 9.71; N, 3.51. Found: C, 65.78; H, 9.66; N, 3.40.
L2Lu(CH2SiMe3)(THF) (3). Following the same procedure as
that of complex 1, complex 3 was isolated by treatment of Lu-
(CH2SiMe3)3(THF)2 (0.213 g, 0.37 mmol) with equimolar H2L2
1
(0.20 g, 0.37 mmol) as white powders (0.22 g, 71%). H NMR
L1Y(CH2SiMe3)(THF) (1). To a hexane (5 mL) solution of
Y(CH2SiMe3)3(THF)2 (0.181 g, 0.37 mmol) was added an equimolar
amount of H2L1 (0.19 g, 0.37 mmol, in 10 mL of hexane). The
reaction mixture was stirred at -30 °C for 12 h. Removal of the
volatiles afforded a residue, which was carefully washed with cold
hexane to remove nonreacted lanthanide tris(alkyl)s and then cooled
to -30 °C again. White solids of complex 1 were deposited on the
bottom of the flask in 1 day (0.19 g, 70%). Good crystals for X-ray
analysis were isolated by recrystallizing the solids from a mixture
of toluene (0.5 mL) and hexane (2.5 mL) at -30 °C for several
(400 MHz, C6D6, 25 °C): δ 7.74 (d, 4J(H, H) ) 2.8 Hz, 2H, C6H2),
4
7.22 (d, J(H, H) ) 2.8 Hz, 2H, C6H2), 4.09 (m, 4H, THF), 3.91
2
2
(d, J(H, H) ) 12.0 Hz, 2H, ArCH2N), 3.00 (d, J(H, H) ) 12.0
Hz, 2H, ArCH2N), 2.79 (m, 2H, N(CH2CH3)2), 2.46 (br, 2H,
N(CH2)2N), 2.27 (br, 2H, N(CH2CH3)2), 2.02 (br, 2H, N(CH2)2N),
1.90 (s, 18H, C(CH3)3), 1.56 (s, 18H, C(CH3)3), 1.28 (m, 4H, THF),
0.62 (s, 9H, CH2Si(CH3)3), 0.55 (t, 3J(H, H) ) 7.3 Hz, 6H,
N(CH2CH3)2), -0.41 (br, 2H, CH2Si(CH3)3) ppm. 13C NMR(100
MHz, C6D6, 25 °C): δ 162.6(2C, ipso-2,4-tBu2C6H2O), 137.4 (2C,
ipso-2,4-tBu2C6H2O), 137.3 (2C, ipso-2,4-tBu2C6H2O), 126.0 (2C,
2,4-tBu2-C6H2O), 125.2 (2C, 2,4-tBu2-C6H2O), 125.0 (2C, ipso-
2,4-tBu2-C6H2O), 72.5 (2C, THF), 66.0 (2C, ArCH2N), 52.7 (1C,
N(CH2)2N), 49.3 (1C, N(CH2)2N), 46.5 (2C, N(CH2CH3)2), 36.2
(2C, C(CH3)3), 34.7 (2C, C(CH3)3), 32.7 (6C, C(CH3)3), 31.4 (6C,
C(CH3)3), 31.0 (1C, CH2SiMe3), 25.7 (2C, THF), 9.1 (2C,
N(CH2CH3)2), 5.6 (3C, CH2Si(CH3)3) ppm. IR (KBr pellets): ν
2954(m), 2901(m), 2865(m), 1605(m), 1477(m), 1445(m), 1415-
(m), 1383(m), 1361(m), 1308(m), 1240(m), 1203(m), 1167(m),
1133(m), 1115(w), 1071(w), 1018(m), 913(m), 876(m), 839(m),
809(m), 772(w), 670(m) cm-1. Anal. Calcd for C44H77N2O3SiLu
(%): C, 59.70; H, 8.77; N, 3.16. Found: C, 59.55; H, 8.78; N,
3.11.
1
4
days. H NMR (400 MHz, C6D6, 25 °C): δ 7.72 (d, J(H, H) )
4
2.7 Hz, 2H, C6H2), 7.23 (d, J(H, H) ) 2.72 Hz, 2H, C6H2), 4.06
2
(br, 4H, THF), 3.86 (d, J(H, H) ) 12.0 Hz, 2H, ArCH2N), 3.00
2
(d, J(H, H) ) 12.0 Hz, 2H, ArCH2N), 2.39 (br, 2H, N(CH2)2N),
1.90 (s, 18H, C(CH3)3), 1.83 (s, 6H, N(CH3)2), 1.70 (br, 2H,
N(CH2)2N), 1.57 (s, 18H, C(CH3)3), 1.27 (br, 4H, THF), 0.62 (s,
2
9H, CH2Si(CH3)3), -0.35 (d, J(Y, H) ) 3.2 Hz, 2H, CH2SiMe3)
ppm. 13C NMR (100 MHz, C6D6, 25 °C): δ 162.3 (2C, ipso-2,4-
tBu2-C6H2), 137.4 (2C, ipso-2,4-tBu2-C6H2), 136.9 (2C, ipso-2,4-
tBu2-C6H2), 126.3 (2C, 2,4-tBu2-C6H2), 125.2(2C, 2,4-tBu2-C6H2)
125.1 (2C, ipso-2,4-tBu2-C6H2), 72.3 (2C, THF), 65.9 (2C, ArCH2N),
60.47 (1C, N(CH2)2N), 48.9 (1C, N(CH2)2N), 46.8 (2C, N(CH3)2),
36.2 (2C, C(CH3)3), 34.7 (2C, C(CH3)3), 32.7 (6C, C(CH3)3), 31.1
(6C, C(CH3)3), 26.1 (1C, CH2SiMe3), 25.6 (2C, THF), 5.4 (3C,
Si(CH3)3) ppm. IR (KBr pellet): ν 2956(m), 2903(m), 2870(m),
1606(m), 1480(m), 1446(m), 1417(m), 1388(m), 1363(m), 1307-
(m), 1255(m), 1240(m), 1205(m), 1169(m), 1136(m), 1113(w),
1087(w), 1031(m), 996(w), 963(w), 940(w), 915(m), 879(m), 839-
L3Y(CH2SiMe3)(THF) (4). Reaction of Y(CH2SiMe3)3(THF)2
(0.181 g, 0.37 mmol) with equimolar H2L3 (0.199 g, 0.37 mmol)
by the same manner as described previously for the formation of
complex 1 afforded complex 4 (0.08g, 30.7%). 1H NMR (400 MHz,
4
C6D6, 25 °C): δ 7.72 (d, J(H, H) ) 2.8 Hz, 2H, C6H2), 7.23 (d,
2
4J(H, H) ) 2.8 Hz, 2H, C6H2), 3.99 (m, 4H, THF), 3.89 (d, J(H,
2
H) ) 12.0 Hz, 2H, ArCH2N), 2.88 (d, J(H, H) ) 12.0 Hz, 2H,
(32) (a) Burke, W.; Smith, R.; Weatherbee, C. J. Am. Chem. Soc. 1952,
74, 602-605. (b) Burke, W.; Kolbezen, M.; Stephens, C. J. Am. Chem.
Soc. 1952, 74, 3601-3605. (c) Carey, D.; Cope-Eatough, E.; Vilaplana-
Mafe´Mair, E.; Mair, F.; Pritchard, R.; Warren, J.; Wood, R. Dalton Trans.
2003, 1083-1093.
ArCH2N), 2.55 (m, 2H, NCH2CH2CH2N(CH3)2), 2.09 (s, 6H,
N(CH3)2, 1.90 (s, 18H, C(CH3)3), 1.57 (s, 18H, C(CH3)3), 1.50 (s,
2H, NCH2CH2CH2N(CH3)2), 1.32 (m, 4H, THF), 0.90 (br, 2H,
NCH2CH2CH2N(CH3)2), 0.59 (s, 9H, CH2Si(CH3)3), -0.39 (d, 2J(Y,
H) ) 3.2 Hz, 2H, CH2Si(CH3)3) ppm. 13C NMR(100 MHz, C6D6,
(33) Lappert, M.; Pearce, R. Chem. Commun. 1973, 126-127.

2756 Organometallics, Vol. 26, No. 10, 2007
Liu et al.
25 °C): 162.4 (2C, ipso-2,4-tBu2C6H2O), 137.3 (2C, ipso-2,4-
tBu2C6H2O), 136.6 (2C, ipso-2,4-tBu2C6H2O), 126.5 (2C, 2,4-tBu2-
C6H2O), 125.6 (2C, ipso-2,4-tBu2-C6H2O), 124.9 (2C, 2,4-tBu2-
C6H2O), 71.9 (2C, THF), 66.7 (2C, ArCH2N), 63.4 (1C, NCH2CH2-
CH2N(CH3)2), 56.6 (1C, NCH2CH2CH2N(CH3)2), 48.0 (2C, N(CH3)2),
36.2 (2C, C(CH3)3), 34.7 (2C, C(CH3)3), 32.7 (6C, C(CH3)3), 31.5
(6C, C(CH3)3), 25.6 (2C, THF), 25.0 (1C, CH2SiMe3), 14.9 (1C,
CH2CH2CH2), 5.4 (3C, CH2Si(CH3)3) ppm. IR (KBr pellet): ν
2954(m), 2866(m), 2784(m), 1604(m), 1478(m), 1442(m), 1415-
(m), 1386(m), 1361(m), 1305(m), 1266(m), 1238(m), 1202(m),
1167(m), 1134(m), 1099(w), 1057(m), 1027(m), 970(w), 914(m),
876(m), 838(m), 808(m), 777(w), 694(m) cm-1. Anal. Calcd for
C43H75N2O3SiY (%): C, 65.79; H, 9.63; N, 3.57. Found: C, 65.69;
H, 9.61; N, 3.57.
946(w), 915(w), 876(m), 839(m), 809(m), 795(w), 772(w), 680-
(m) cm-1. Anal. Calcd for C42H73N2O3SiLu (%): C, 58.86; H, 8.59;
N, 3.27. Found: C, 58.65; H, 8.55; N, 3.20.
L6Y(CH2SiMe3)2 (7). To a stirred suspension of ligand HL6 (0.26
g, 0.84 mmol) in hexane (20 mL) was added Y(CH2SiMe3)3(THF)2
(0.41 g, 0.84 mmol) in hexane (5 mL) at 0 °C. The mixture was
reacted until a clear solution was obtained in 0.5 h. Then, the
solution was concentrated to half-volume and cooled to -30 °C.
Bright yellow single crystals of complex 7 were isolated after
several hours (0.17 g, 75%). 1H NMR (400 MHz, C6D6, 25 °C): δ
7.22-7.19 (br, 3H, 2,6-iPr2-C6H3), 6.9 (t, 3J(H, H) ) 7.2 Hz, 1H,
3
2-OMe-C6H4-âH), 6.82 (t, J(H, H) ) 8.0 Hz, 1H, 2-OMe-C6H4-
γH), 6.74 (dd, 3J(H, H) ) 1.2 Hz, 1.2 Hz, 1H, 2-OMe-C6H4-RH),
3
6.58 (d, J(H, H) ) 8.0 Hz, 1H, 2-OMe-C6H4-δH), 5.18 (s, 1H,
L4Y(CH2SiMe3)(THF) (5). To a stirred solution of ligand H2L4
(0.22 g, 0.04 mmol) in toluene (10 mL) was added a toluene (5
mL) solution of Y(CH2SiMe3)3(THF)2 (0.20 g, 0.04 mmol) at -30
°C. The reaction mixture was stirred for 12 h, and the volatiles
were removed under vacuum. The residue was washed with hexane
and dried under vacuum, to give complex 5 as white powders (0.24
g, 75%). Good crystals for X-ray analysis were grown from a
HC(C(CH3)NAr)2), 3.86 (s, 3H, ArOCH3), 3.31 (m, 2H, ArCH-
(CH3)2), 2.04 (s, 3H, C(CH3)NAr), 1.72 (s, 3H, C(CH3)NAr), 1.41
(d, 3J(H, H) ) 8.0 Hz, 6H, ArCH(CH3)2), 1.23 (d, 3J(H, H) ) 8.0
Hz, 6H, ArCH(CH3)2), 0.23 (s, 18H, CH2Si(CH3)3), -0.30, -0.33
2
2
(d, d, J(H, H) ) 12.0 Hz, J(Y, H) ) 4.0 Hz, 2H, CH2SiMe3),
2
2
-0.45, -0.48 (d, d, J(H, H) ) 12.0 Hz, J(Y, H) ) 4.0 Hz, 2H,
CH2SiMe3). 13C NMR(100 MHz, C6D6, 25 °C): δ 168.8 (1C, HC-
(C(Me)NAr)2), 160.7 (1C, HC(C(Me)NAr)2), 151.4 (1C, ipso-2-
OMe-C6H4), 143.2 (1C, ipso-2,6-iPr2-C6H3), 142.5 (2C, ipso-2,6-
iPr2-C6H3), 138.5 (1C, ipso-2-OMe-C6H4), 127.0 (1C, 2,6-iPr2-
C6H3), 124.9 (2C, 2,6- iPr2-C6H3), 124.0 (1C, 2-OMe-C6H4), 123.8
(1C, 2-OMe-C6H4), 123.5 (1C, 2-OMe-C6H4), 111.8 (1C, 2-OMe-
C6H4), 100.9 (1C, HC(C(Me)NAr)2), 58.9 (3C, ArOCH3), 36.9 (1C,
CH2SiMe3), 36.5 (1C, CH2SiMe3), 29.3 (2C, ArCH(CH3)2), 25.5
(2C, ArCH(CH3)2), 24.7 (2C, ArCH(CH3)2), 23.9 (1C, C(CH3)
NAr), 23.3 (1C, C(CH3) NAr), 4.6 (6C, CH2Si(CH3)3). IR (KBr
pellet): ν 3057(m), 2959(m), 2866(m), 2836(w), 1637(m), 1597-
(m), 1560(m), 1516(m), 1478(m), 1462(m), 1437(m), 1384(m),
1363(m), 1327(m), 1284(m), 1241(m), 1227(m), 1177(m), 1117-
1
toluene solution. H NMR (400 MHz, C6D6, 25 °C): δ 8.73 (d,
4
3J(H, H) ) 5.1 Hz, 1H, Py-RH), 7.46 (d, J(H, H) ) 2.5 Hz, 2H,
C6H2), 7.09 (d, 4J(H, H) ) 2.5 Hz, 2H, C6H2), 6.55 (d, d, d, 3J(H,
3
H) ) 1.6 Hz, 1.7 Hz, 1.5 Hz, 1H, â-Py), 6.26 (t, J(H, H) ) 6.4
3
Hz, 1H, γ-Py), 5.90 (d, J(H, H) ) 8.0 Hz, 1H, δ-Py), 4.10 (br,
2
4H, THF), 4.06 (d, J(H, H) ) 12.0 Hz, 2H, ArCH2N), 3.36 (s,
2
2H, PyCH2N), 3.06 (d, J(H, H) ) 12.0 Hz, 2H, ArCH2N), 1.82
(s, 18H, C(CH3)3), 1.52 (s, 18H, C(CH3)3), 1.36 (m, 4H, THF),
2
0.70 (s, 9H, CH2Si(CH3)3), 0.09 (d, J(Y, H) ) 3.0 Hz, 2H, CH2-
SiMe3) ppm. 13C NMR(100 Hz, C6D6, 25 °C): 162.6 (2C, ipso-
2,4-tBu2C6H2O), 160.5 (1C, ipso-C6H4N), 149.8 (1C, C6H4N), 138.1
(2C, ipso-2,4-tBu2C6H2O), 136.8 (2C, ipso-2,4-tBu2C6H2O), 126.1
(1C, C6H4N), 126.0 (2C, ipso-2,4-tBu2-C6H2O), 124.6 (2C, 2,4-
tBu2-C6H2O), 124.1 (2C, ipso-2,4-tBu2-C6H2O), 122.5 (1C, C6H4N),
121.7 (1C, C6H4N), 72.0 (2C, THF), 64.9 (2C, ArCH2N), 57.5 (1C,
PyCH2N), 36.0 (2C, C(CH3)3), 34.6 (2C, C(CH3)3), 32.8 (6C,
C(CH3)3), 30.8 (6C, C(CH3)3), 26.8 (1C, CH2SiMe3), 25.6 (2C,
THF), 5.40 (3C, CH2Si(CH3)3) ppm. IR (KBr pellet): ν 2955(m),
2905(m), 2869(m), 1607(m), 1574(m), 1520(w), 1479(m), 1443-
(m), 1417(m), 1388(m), 1362(m), 1307(m), 1240(m), 1204(m),
1169(m), 1156(m), 1136(m), 1101(w), 1070(w), 1057(m), 1029-
(w), 1016(w), 975(w), 932(w), 915(w), 877(m), 838(m), 809(m),
754(m), 697(m) cm-1. Anal. Calcd for C44H69N2O3SiY (%): C,
66.81; H, 8.79; N, 3.54. Found: C, 66.66; H, 8.71; N, 3.50.
(m), 1050(w), 1031(m), 935(m), 789(m), 750(m), 735(m) cm-1
.
Anal. Calcd for C32H53N2OSi2Y (%): C, 61.31; H, 8.52; N, 4.47.
Found: C, 61.10; H, 8.44; N, 4.39.
L7Y(CH2SiMe3)2 (8). Complex 8 was prepared according to the
same procedure as that for 7 by reaction of Y(CH2SiMe3)3(THF)2
(0.41 g, 0.84 mmol) and HL7 as light yellow crystals (0.36 g, 85%).
3
1H NMR (400 MHz, C6D6, 25 °C): δ 6.91-6.85 (td, J(H, H) )
1.7 Hz, 1.4 Hz, 1.6 Hz, 2H, 2-OMe-C6H4-âH), 6.83-6.78 (td, 3J(H,
H) ) 1.2 Hz, 1.1 Hz, 1.2 Hz, 2H, 2-OMe-C6H4-γH), 6.65 (d, 3J(H,
H) ) 8.0 Hz, 4H, 2-OMe-C6H4-R,δH), 5.19 (s, 1H, HC (C(CH3)-
NAr)2), 3.87 (s, 6H, ArOCH3), 2.01 (s, 6H, C(CH3) NAr), 0.11 (s,
18H, CH2Si(CH3)3), -0.49 (br, 4H, CH2SiMe3). 13C NMR (100
MHz, C6D6, 25 °C): δ 164.0 (2C, HC(C(Me)NAr)2), 151.7 (2C,
ipso-2-OMe-C6H4), 138.2 (2C, ipso-2-OMe-C6H4), 125.2 (2C,
2-OMe-C6H4), 124.4 (2C, 2-OMe-C6H4), 123.3 (2C, 2-OMe-C6H4),
111.5 (2C, 2-OMe-C6H4), 104.1 (1C, HC(C(Me)NAr)2). 57.6 (2C,
ArOCH3), 30.6 (1C, CH2SiMe3), 30.2 (1C, CH2SiMe3), 23.4 (2C,
C(CH3) NAr), 4.3 (6C, CH2Si(CH3)3). IR (KBr pellet): ν 3061-
(w), 2948(m), 2839(w), 1630(w), 1527(m), 1482(m), 1451(m),
1388(m), 1308(w), 1272(m), 1236(m), 1193(w), 1175(m), 1114-
(m), 1046(w), 1017(m), 930(m), 858(m), 790(m), 745(m), 695(w)
cm-1. Anal. Calcd for C27H43N2O2Si2Y (%): C, 56.62; H, 7.57;
N, 4.89. Found: C, 56.33; H, 7.48; N, 4.64.
L5Y(CH2SiMe3)(THF) (6). Treatment of Y(CH2SiMe3)3(THF)2
(0.181 g, 0.37 mmol) with Salan ligand H2L5 (0.20 g, 0.37 mmol)
according to the procedure described for the preparation of 1
afforded complex 6 as white powders (0.18 g, 64%). 1H NMR (400
MHz, C6D6, 25 °C): δ 7.76 (s, 2H, C6H2), 7.06 (s, 2H, C6H2),
4.70 (br, 1H, THF), 4.09 (br, 4H, ArCH2N), 3.75 (br, 1H, THF),
2.95 (br, 2H, THF), 2.83 (br, 2H, THF), 2.17 (br, 3H, NCH3), 1.94
(br, 7H, NCH3, N(CH2)2N), 1.82 (s, 18H, C(CH3)3), 1.53 (s, 18H,
C(CH3)3), 1.24 (br, 1H, THF), 1.05 (br, 1H, THF), 0.44 (s, 9H,
2
2
CH2Si(CH3)3), -0.35, -0.38 (AB, J(H, H) ) 12.0 Hz, J(Y, H)
) 4.0 Hz, 2H, CH2SiMe3) ppm. 13C NMR (100 MHz, C6D6, 25
°C): δ 162.4 (1C, ipso-2,4-tBu2-C6H2), 161.6 (1C, ipso-2,4-tBu2-
C6H2), 138.3 (1C, ipso-2,4-tBu2-C6H2), 137.2 (2C, ipso-2,4-tBu2-
C6H2), 136.7 (1C, ipso-2,4-tBu2-C6H2), 126.7 (2C, 2,4-tBu2-C6H2),
125.7 (2C, 2,4-tBu2-C6H2), 124.7 (2C, ipso-2,4-tBu2-C6H2), 72.3
(2C, THF), 65.7 (2C, ArCH2N), 64.7 (1C, N(CH2)2N), 52.0 (1C,
N(CH2)2N), 46.0 (1C, NCH3), 45.0 (1C, NCH3), 36.1 (2C, C(CH3)3),
34.7 (2C, C(CH3)3), 32.7 (6C, C(CH3)3), 31.0 (6C, C(CH3)3), 29.7
(1C, CH2SiMe3), 25.6 (2C, THF), 5.2 (3C, Si(CH3)3). IR (KBr
pellet): ν 2955(m), 2902(m), 2865(m), 1605(m), 1477(m), 1444-
(m), 1415(m), 1389(m), 1361(m), 1308(m), 1283(m), 1239(m),
1203(m), 1167(m), 1133(m), 1071(w), 1014(w), 981(w), 963(w),
Polymerization of rac-Lactide. A typical procedure for polym-
erization of rac-LA was performed in a 25 mL round flask in a
glovebox. To a stirred solution of D,L-LA (0.50 g, 3.47 mmol) in
1.50 mL of THF was added a THF solution (0.5 mL) of complex
1 (13.37 mg, 0.017 mmol, [LA]/[Y] ) 200:1, [LA] ) 1.75 mol/
L). The polymerization took place immediately at room temperature.
The system became viscous in a few minutes and kept stirring for
1 h and then was terminated by 1.0 mL of HCl/CH3OH/CHCl3
(0.1/10/60 v/v). The viscous solution was quenched by an excess
amount of ethanol, filtered, washed with ethanol, and then dried at
40 °C for 24 h in Vacuo to give polymer product (0.50 g, 100%).

Achiral Lanthanide Alkyl Complexes
Organometallics, Vol. 26, No. 10, 2007 2757
The molecular weight and the molecular weight distribution of the
resulting polymer were determined by GPC. The tacticity of the
PLA was calculated according to the methine region homonuclear
Supporting Information Available: CIF file and crystal-
lographic data for complexes 1-5 and 7 including atomic coordi-
nates and anisotropic thermal parameters, ORTEP drawings of
X-ray structures for complexes 2-4, 1H and 13C NMR spectra for
complexes 6-8, which have not been characterized by X-ray
1
decoupling H NMR spectrum.
1
diffraction analysis, H-1H COSY and DEPT 13C NMR spectra
Acknowledgment. The authors are grateful for financial
support from Jilin Provincial Science and Technology Bureau
for project no. 20050555; The National Natural Science
Foundation of China for projects nos. 20571072 and 20674081;
The Ministry of Science and Technology of China for project
no. 2005CB623802; and “Hundred Talent Scientist Program”
of the Chinese Academy of Sciences.
for active species of racemic lactide oligomer attaching to complex
1
1, and GPC curves and homonuclear decoupled H NMR spectra
of the represented samples are available free of charge via the
Internet at http://pubs.acs.org.
OM0700359