Rare-earth metal complexes supported by 1,ω-dithiaalkanediyl-bridged bis(phenolato) ligands: Synthesis, structure, and heteroselective ring-opening polymerization of rac-lactide

Article abstract of DOI:10.1021/ic702312b

Monomeric yttrium and lutetium bis(phenolato) complexes [Ln(OSSO){N(SiHMe₂)₂}(THF)] (Ln = Y, Lu) were prepared from the reaction of silylamido complexes [Ln{N(SiHMe₂) ₂}₃(THF)₂] with 1 equiv of tetradentate 1,ω-dithiaalkanediyl-bridged bis(phenol) (OSSO)H₂ 1-9 in moderate to high yields. In contrast to the rigid configuration of scandium analogues, the yttrium complexes 2b and 3b and the lutetium complex 3c that contain a C₂ bridge between the two sulfur donors of the ligand are symmetric in solution. The monomeric nature of these complexes was indicated by an X-ray diffraction study of the yttrium complex 6b. The yttrium center in 6b is coordinated to the tetradentate [OSSO]-type ligand, one silylamido group and one THF ligand with the two oxygen donors of the [OSSO]-type ligand located trans. Corresponding bis(phenolato) silylamido complexes of larger rare-earth metals could not be obtained from similar reactions: Reaction of [La{N(SiHMe₂)₂}₃(THF)₂] with 1,2-xylylene-linked bis(phenol) gave a dinuclear lanthanum complex 6d of the formula [La₂(OSSO)₃] with two inequivalent eight-coordinate metal centers. The yttrium and lutetium complexes efficiently initiated the ring-opening polymerization (ROP) of lactides in THF. The heteroselectivity during the ROP of rac-lactide was enhanced when the steric demand of the bis(phenolato) ligand was increased, either by extending the bridge length or by introducing bulky orirtosubstituents in the phenoxy units. A C₃ bridge within the ligand backbone is essential to allow configurational interconversion of the active site between Λ and Δ configuration during polymerization, allowing accommodation of both enantiomers of the monomer in an alternating fashion.

Full text of DOI:10.1021/ic702312b

Inorg. Chem. 2008, 47, 3328-3339
Rare-Earth Metal Complexes Supported by 1,ω-Dithiaalkanediyl-Bridged
Bis(phenolato) Ligands: Synthesis, Structure, and Heteroselective
Ring-Opening Polymerization of rac-Lactide
Haiyan Ma,† Thomas P. Spaniol, and Jun Okuda*
Institute of Inorganic Chemistry, RWTH Aachen UniVersity, Landoltweg 1,
D-52056 Aachen, Germany
Received November 25, 2007
Monomeric yttrium and lutetium bis(phenolato) complexes [Ln(OSSO){N(SiHMe2)2}(THF)] (Ln ) Y, Lu) were prepared
from the reaction of silylamido complexes [Ln{N(SiHMe2)2}3(THF)2] with 1 equiv of tetradentate 1,ω-dithiaalkanediyl-
bridged bis(phenol) (OSSO)H2 1–9 in moderate to high yields. In contrast to the rigid configuration of scandium
analogues, the yttrium complexes 2b and 3b and the lutetium complex 3c that contain a C2 bridge between the
two sulfur donors of the ligand are symmetric in solution. The monomeric nature of these complexes was indicated
by an X-ray diffraction study of the yttrium complex 6b. The yttrium center in 6b is coordinated to the tetradentate
[OSSO]-type ligand, one silylamido group and one THF ligand with the two oxygen donors of the [OSSO]-type
ligand located trans. Corresponding bis(phenolato) silylamido complexes of larger rare-earth metals could not be
obtained from similar reactions: Reaction of [La{N(SiHMe2)2}3(THF)2] with 1,2-xylylene-linked bis(phenol) gave a
dinuclear lanthanum complex 6d of the formula [La2(OSSO)3] with two inequivalent eight-coordinate metal centers.
The yttrium and lutetium complexes efficiently initiated the ring-opening polymerization (ROP) of lactides in THF.
The heteroselectivity during the ROP of rac-lactide was enhanced when the steric demand of the bis(phenolato)
ligand was increased, either by extending the bridge length or by introducing bulky ortho-substituents in the phenoxy
units. A C3 bridge within the ligand backbone is essential to allow configurational interconversion of the active site
between Λ and ∆ configuration during polymerization, allowing accommodation of both enantiomers of the monomer
in an alternating fashion.
Introduction
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As biodegradable and biocompatible polymeric materials
derived from renewable natural resources, poly(L-lactide)
(PLLA) has recently attracted considerable attention.1,2
Moreover, a PLA stereocomplex formed by blending PLLA
with its enantiomer poly(D-lactide) (PDLA) shows a higher
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* To whom correspondence should be addressed. E-mail: jun.okuda@
ac.rwth-aachen.de.
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† Current address: Laboratory of Organometallic Chemistry, East China
University of Science and Technology, 130 Meilong Road, 200237
Shanghai, PR China.
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3328 Inorganic Chemistry, Vol. 47, No. 8, 2008
10.1021/ic702312b CCC: $40.75  2008 American Chemical Society
Published on Web 03/04/2008

StereoselectiWe ROP of rac-Lactide
stereocomplexation processes restrict practical application.3
Metal-catalyzed stereoselective ring-opening polymerization
(ROP) of lactides is expected to provide new methods to
control the microstructure of PLA formed during polymer-
ization.4 So far, PLAs with various microstructures ranging
from isotactic5,6 and heterotactic7–12 to syndiotactic13 can
be obtained from metal-initiated stereoselective ROP of rac-
lactide and meso-lactide. Aluminum complexes with chiral-
bridged bis(iminophenolato) ligands have been reported to
polymerize rac-LA to isotactic or stereoblock/stereogradient
PLA5a–h and to polymerize meso-LA to syndiotactic PLA
via enantiomorphic site control.13a Achiral aluminum bis-
(iminophenolato) or bis(aminophenolato) complexes produce
isotactic5i–q or heterotactic PLA7 from rac-LA via a chain-
end control mechanism. Zinc,8 magnesium,9 calcium,10 and
rare-earth metal11 complexes are highly active for ROP of
rac-LA, in some cases showing significant preference for
heterotactic dyad enchainment. Although the nature of the
metal center and the ancillary ligand sphere critically
influence the polymerization behavior,5–14 the factors that
govern the stereocontrol during the ROP of lactides are still
not well understood. It is believed that a proper design of
the ancillary ligand will eventually allow fine-tuning of the
stereoselectivity. Recently, we have introduced a series of
1,ω-dithiaalkanediyl-bridged bis(phenol)s (Scheme 1) as
ancillary ligands for scandium complexes (Scheme 2).15 By
variation of the ligand architecture, we have obtained
moderate to high heteroselectivity for the ROP of rac-LA.11d
A dynamic monomer recognition step based on the ancillary
ligand’s fluxionality was suggested to be responsible for the
high heteroselectivity. In order to further investigate the
influence of this ligand framework on the polymerization
behavior of the resulting complexes in the ROP of lactides,
we have now extended this ligand system to other rare-earth
metals.
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Results and Discussion
Synthesis. Similar to the previously reported yttrium
complexes 1b and 5b,15a complex 2b was synthesized by
amine elimination reaction of yttrium silylamide
[Y{N(SiHMe2)2}3(THF)2] and 1 equiv of 1,4-dithiabutane-
diyl-2,2′-bis{4,6-di(2-phenyl-2-propyl)phenol} (etccpH2, 2)
in toluene at 50 °C (Scheme 2). Due to the bulky cumyl
substituent in the bis(phenol) framework, complete com-
plexation was reached only after 8 days. Workup of the
reaction mixture afforded analytically pure 2b as colorless
microcrystals, which were characterized by NMR spectros-
copy and elemental analysis. Attempts to obtain single
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45, 474–475. (b) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White,
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Inorganic Chemistry, Vol. 47, No. 8, 2008 3329

Ma et al.
Scheme 1. Linked Bis(phenol)s (OSSO)H2, 1-9 (ada ) 1-adamantyl)
Scheme 2
crystals by recrystallization from aliphatic hydrocarbons led
to partial decomposition of the product. Similarly, 7b was
isolated in quantitative yield by simple evaporation of all
volatiles from the reaction mixture of [Y{N(SiHMe2)2}3
(THF)2] with ortho-xylylenedithiobis{4,6-di(2-phenyl-2-pro-
pyl)phenol} (xytccpH2, 7).
When [Y{N(SiHMe2)2}3(THF)2] was treated with rac-(2,3-
trans-butanediyl-1,4-dithiabutanediyl)-2,2′-bis{6-tert-butyl-
4-methylphenol} (cytbmpH2, rac-3), ortho-xylylenedithio-
bis(6-tert-butyl-4-methylphenol) (xytbmpH2, 6), or rac-(3,4-
trans-butanediyl-1,6-dithiahexanediyl)-2,2′-bis(6-tert-butyl-
4-methylphenol) (cmtbmpH2, rac-9) in n-pentane at ambient
3330 Inorganic Chemistry, Vol. 47, No. 8, 2008

StereoselectiWe ROP of rac-Lactide
SiMe protons in complexes 3b, 3c, and 9b, as the chiral
trans-cyclohexanediyl backbone of the bis(phenolato) ligands
renders them diastereotopic.
The 1,ω-dithiaalkanediyl-bridged bis(phenol)s were also
tested to coordinate at other rare-earth metals. The NMR
scale reaction of 1 equiv of bis(phenol) 6 with samarium
silylamide [Sm{N(SiHMe2)2}3(THF)2] in C6D6 showed the
formation of the expected product, but no product was
obtained after workup using n-hexane. Similar NMR scale
reaction with [La{N(SiHMe2)2}3(THF)2] also suggested the
formation of [La(xytbmp){N(SiHMe2)2}]. On a synthetic
scale, recrystallization from toluene/pentane only gave a
small amount of colorless prisms, which show unidentifiable
signals in the 1H NMR spectrum in pure C6D6. When a drop
of THF-d8 is added to C6D6, this compound shows two sets
of signals in a 2:1 ratio for the bis(phenolato) ligand and
resonances for the silylamido group are absent.17 A single-
crystal X-ray diffraction study revealed a dinuclear structure
that contains three bis(phenolato) ligands, [La2(xytbmp)3]
(6d) (Scheme 3 and Figure 2). This product could be obtained
in better yield by reacting 2 equiv of [La{N(SiHMe2)2}3
(THF)2] with 3 equiv of bis(phenol) ligand. Satisfactory
elemental analysis could not be obtained due to the presence
of other unidentifiable substances. Treatment of 6d with
[La{N(SiHMe2)2}3(THF)2] in C6D6 led to the formation of
[La(xytbmp){N(SiHMe2)2}] but was accompanied by sig-
nificant decomposition as indicated by formation of HN(Si-
HMe2)2.
Figure 1. ORTEP diagram of the molecular structure of
[Y(xytbmp){N(SiHMe2)2}(THF)] (6b). Thermal ellipsoids are drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
temperature, colorless crystals of 3b, 6b, and 9b were isolated
in moderate to good yields after recrystallization from
aliphatic hydrocarbons. Lutetium complexes 3c and 6c were
synthesized in good yields by reacting [Lu{N(SiHMe2)2}3
(THF)2] with the corresponding bis(phenol), following a
procedure analogous to that to prepare the yttrium complexes.
1
According to its H NMR spectrum in C6D6, the solid
obtained by simple evaporation of the reaction mixture of
[Y{N(SiHMe2)2}3(THF)2] and ortho-xylylenedithiobis(6-
adamantyl-4-methylphenol) (xytampH2, 8) contains one
bis(phenolato) ligand, one bis(dimethylsilyl)amido group, and
one-half of an equivalent of THF ligand. Attempts to isolate
pure product by recrystallization of the residue from aliphatic
hydrocarbons were unsuccessful. It seems that the incorpo-
rated THF ligand is highly labile, leading to decomposition
of the product during solvent extraction.
Crystal Structure of 6b and 6d. Single crystals of 6b
and 6d suitable for X-ray diffraction were obtained by
slightly cooling the saturated n-hexane solution or toluene/
pentane mixture. Crystallographic data and results of the
refinements are summarized in Table 1, and selected bond
lengths and angles are listed in Tables 2 and 3.
As depicted in Figure 1, the yttrium center in 6b is six-
coordinate, ligated to the tetradentate bis(phenolato) [OSSO]-
type ligand, one bis(dimethylsilyl)amido group, and one
coordinated THF ligand and adopts a distorted octahedral
coordination geometry. The silylamido ligand is located cis
to THF and trans to one of the sulfur atoms; the two oxygen
donors of the bis(phenolato) ligand are arranged trans to each
other, as indicated by the corresponding angles N-Y-O3
98.13(6), N-Y-S2 174.73(4), and O1-Y-O2 148.64(5)°.
Compared to the scandium analogue 6a,11d all of these angles
are slightly decreased, possibly due to the larger radius of
the yttrium atom. Both enantiomers of the C1-symmetric
molecule are found in the centrosymmetric crystal structure.
The Y-O(ligand) bond lengths of 2.1609(14) and 2.1682(14)
Å in 6b fall into the range of terminal Y-O bond lengths in
Spectroscopic data and elemental analyses of all the
yttrium and lutetium complexes are consistent with the
structure consisting of one bis(phenolato) ligand, one bis-
(dimethylsilyl)amido group, and one coordinated THF mol-
ecule at the metal center. Their monomeric nature was further
confirmed by an X-ray diffraction study on a single crystal
of the yttrium complex 6b (Figure 1). In contrast to the rigid
C1 configuration of the scandium complexes with a C2
bridge,11d,15a complexes 2b, 3b, and 3c possess high sym-
metry in solution. The fast dissociation of THF on the NMR
time scale leads to a pseudo-five-coordinated environment
around the metal center with either C2-(trigonal bipyramidal)
or Cs-symmetry (square pyramidal).15a,16 This renders both
phenyl moieties of the same ligand chemically equivalent,
1
and only one set of signals is observed in the H NMR
spectra. Two sets of doubled signals are displayed for the
(17) [La2(xytbmp)3], 6d: 1H NMR (C6D6 with a small amount of THF-d8,
4
4
200 MHz): δ 7.39 (d, 4H, JHH ) 1.8 Hz, 5-H), 7.21 (4, 2H, JHH
)
(16) (a) Aubrecht, K. B.; Chang, K.; Hillmyer, M. A.; Tolman, W. B. J.
Polym. Sci., A: Polym. Chem. 2001, 39, 284–293. (b) Arndt, S.; Voth,
P.; Spaniol, T. P.; Okuda, J. Organometallics 2000, 19, 4690–4700.
(c) Gountchev, T. I.; Tilley, T. D. Organometallics 1999, 18, 5661–
5667.
1.8 Hz, 3-H), 7.12 (m, 2H, overlapped), 7.06 (m, 2H), 7.00 (m, 4H,
Ar-H), 6.85 (m, 2H, Ar-H), 6.67 (m, 4H, Ar-H), 6.35 (m, 2H, Ar-H),
3.98 (s, 8H, SCH2), 3.76 (s, 4H, SCH2), 2.32 (s, 12H, 4-CH3), 2.10
(s, 6H, 4-CH3), 1.69 (s, 36H, 6-C(CH3)3), 1.65 (s, 18H, 6-C(CH3)3).
Inorganic Chemistry, Vol. 47, No. 8, 2008 3331

Ma et al.
Scheme 3
Table 1. Crystallographic Data for 6b and 6d
6b
6d
C103H126La2O6S6
formula
Mr
C38H58NO3S2Si2Y
786.10
1930.22
temperature/K
crystal size/mm
crystal system
space group
a/Å
130(2)
120(2)
0.80 × 0.70 × 0.60
monoclinic
P21/n
9.975(3)
23.333(8)
17.589(6)
98.237(7)
4052(2)
0.42 × 0.16 × 0.15
monoclinic
C2/c
48.696(3)
20.2759(13)
21.3906(13)
104.676(3)
20431(2)
8
b/Å
c/Å
ꢀ/deg
U/Å3
Z
4
Dc/g cm-3
1.289
1.636
1664
1.255
0.997
8016
µ/mm-1
F(000)
θ range/deg
data collected
(hkl)
no. of reflection
collected
2.10–28.37
(13, –31 to 26,
-20 to 23
27108
0.86–26.14
-58 to 60,
( 25, ( 26
101915
Figure 2. ORTEP diagram of the molecular structure of [La2(xytbmp)3]
(6d). Thermal ellipsoids are drawn at the 30% probability level. Hydrogen
atoms, methyl groups of the tert-butyl substituents (as well as the solvent
molecules toluene and pentane) are omitted for clarity.
no. of independent
reflns
10032
20279
Rint
0.0433
0.0362, 0.0896
0.0903
0.0889, 0.1853
octahedral complexes (2.104–2.177 Å)18 and are slightly
longer than those in five-coordinate complexes, such as
[Y{O-SiHMe2-calix[4]arene}(THF)]2 (2.061–2.069 Å),19
final R1, wR2
[I > 2σ(I)]
R1, wR2 (all data)
goodness of fit on F2
∆Fmax,min/e Å-3
0.0495, 0.0954
1.017
0.874, -0.684
0.1327, 0.2019
1.161
1.546, -0.950
t
[Y(1,3-(SiMe3)2-2-Ph-ꢀ-diketiminato)2(OC6H2 Bu2-2,6-Me-
4)] (2.075(2) Å),16a and [Y{2-(Et2NCH2CH2)2NCH2-4,6-tBu2-
C6H2O}{N(SiHMe2)2}2] (2.055(5) Å).20 In contrast to the
yttrium complex 5b (Y-O(ligand) ) 2.132(5) and 2.151(5)
Å; Y-O(THF) ) 2.382(5) Å), which has a trigonal-prismatic
coordination geometry and cis-oriented oxygen donors of the
bis(phenolato) ligand,15a complex 6b possesses slightly
longer Y-O(ligand) bond lengths and a shorter Y-O(THF)
bond length of 2.3069(16) Å. Nevertheless, both the Y-N
bond lengths in 5b and 6b are comparable, matching the
values (2.229(4)-2.276(4) Å) found for the corresponding
precursor.21 The Y-S1 and Y-S2 bond lengths in 6b differ
from each other by about 0.09 Å, with an elongated Y-S2
bond trans to the N(SiHMe2)2 moiety. This can be explained
(18) (a) Runte, O.; Priermeier, T.; Anwander, R. Chem. Commun. 1996,
1385–1386. (b) Lara-Sanchez, A.; Rodriguez, A.; Hughes, D. L.;
Schormann, M.; Bochmann, M. J. Organomet. Chem. 2002, 663, 63–
69. (c) Emslie, D. J. H.; Piers, W. E.; Parvez, M.; McDonal, R.
Organometallics 2002, 21, 4226–4240. (d) Lara-Sanchez, A.; Rod-
riguez, A.; Hughes, D. L.; Schormann, M.; Bochmann, M. J.
Organomet. Chem. 2002, 663, 63–69. (e) Boyd, C. L.; Toupance, T.;
Tyrrell, B. R.; Ward, B. D.; Wilson, C. R.; Cowley, A. R.; Mountford,
P. Organometallics 2005, 24, 309–330. (f) Marinescu, S. C.; Agapie,
T.; Day, M. W.; Bercaw, J. E. Organometallics 2007, 26, 1178–1190.
(19) Anwander, R.; Eppinger, J.; Nagl, I.; Scherer, W.; Tafipolsky, M.;
Sirsch, P. Inorg. Chem. 2000, 39, 4713–4720.
(21) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck,
(20) Westmoreland, I.; Arnold, J. Dalton Trans. 2006, 4155–4163.
E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847–858.
3332 Inorganic Chemistry, Vol. 47, No. 8, 2008

StereoselectiWe ROP of rac-Lactide
Table 2. Selected Bond Lengths (Å) and Angles (deg) in 6b
the other lanthanum center. In each of these ligands, one of
the phenolate oxygen atoms is bridging both metal atoms.
Taking into account the interactions to both oxygen and to
sulfur, 8-fold coordination and thus a double-capped octa-
hedral geometry for each lanthanum center in 6d results. The
molecule displays C1-symmetry in the solid state, but the
bis(phenolato) moiety at La2 nearly possesses Cs-symmetry
relative to a mirror plane defined by La1-O2-La2-O4. The
other two bis(phenolato) ligands are nearly C2-symmetric
with regard to each other, with the symmetry axis along the
La1-La2 axis. The La-S bond lengths in 6d show a broad
variation. Some of them are significantly larger than those
in a reported lanthanum complex with a thiacrown ether
ligand (3.0891(4)-3.1263(4) Å),22 indicating weak coordina-
tion interactions to the sulfur atoms. This explains the
formation of a symmetric structure of 6d in a mixture of
C6D6/THF-d8 (vide supra). The addition of THF apparently
results in the dissociation of sulfur atoms from the lanthanum
centers and thereby facilitates the configurational change of
the bis(phenolato) ligand skeleton to form a symmetric
structure. The terminal La-O bond lengths in 6d (2.242(7)–
2.273(5) Å) are comparable to values reported for terminal
La-O bonds (2.229(3)–2.315(3) Å).11a The La1-La2 dis-
tance of 4.0705(8) Å also falls within the normal range of
3.997–4.096 Å.
Ring-Opening Polymerization of rac-Lactide. As can
been seen from the data compiled in Table 4, the yttrium
complexes 1b-9b and the lutetium complexes 1c-3c were
highly active in the polymerization of rac-LA in THF at
ambient temperature. High conversion of monomer to PLA
was achieved within 1 h. Similar to our previous results,11d,15
complexes with a C2-bridged ligand are generally more active
than those with ligand systems featuring longer links under
the same conditions. All of the yttrium and lutetium
complexes are significantly more active than the correspond-
ing scandium complexes.11d,15a,b
Y-N
2.2345(17)
2.1682(14)
2.1609(14)
2.3069(16)
3.3684(10)
69.72(4)
109.82(5)
158.85(4)
81.01(4)
67.69(4)
78.90(4)
94.619(19)
148.64(5)
89.30(6)
Y-S1
2.8831(10)
2.9736(9)
1.7098(18)
1.7092(18)
3.4809(12)
103.41(6)
107.95(6)
98.13(6)
Y-O1
Y-S2
Y-O2
N-Si1
Y-O3
N-Si2
Y· · ·Si1
Y· · ·Si2
O1-Y-S1
O2-Y-S1
O3-Y-S1
O1-Y-S2
O2-Y-S2
O3-Y-S2
S1-Y-S2
O1-Y-O2
O1-Y-O3
N-Y-O1
N-Y-O2
N-Y-O3
N-Y-S1
N-Y-S2
Y-N-Si1
Y-N-Si2
Si1-N-Si2
O2-Y-O3
89.67(5)
174.73(4)
116.67(9)
123.38(9)
119.65(10)
86.53(6)
Table 3. Selected Bond Lengths (Å) and Angles (deg) in 6d
La1-O1
2.273(5)
2.492(6)
2.242(7)
2.530(5)
2.527(5)
2.473(6)
2.252(6)
2.261(6)
4.0705(8)
62.92(13)
91.80(14)
78.62(14)
145.61(13)
123.14(13)
54.16(13)
103.41(12)
68.96(13)
97.78(18)
100.6(2)
La1-S1
La1-S2
La1-S3
La1-S4
La2-S2
La2-S4
La2-S5
La2-S6
3.187(2)
3.325(2)
3.177(2)
3.302(2)
3.254(2)
3.358(2)
3.162(3)
3.215(3)
La1-O2
La1-O3
La1-O4
La2-O2
La2-O4
La2-O5
La2-O6
La1-La2
O1-La1-S1
O1-La1-S2
O1-La1-S3
O1-La1-S4
O2-La1-S1
O2-La1-S2
O2-La1-S3
O2-La1-S4
O1-La1-O2
O1-La1-O3
O1-La1-O4
O2-La1-O3
O2-La1-O4
O3-La1-O4
O5-La2-S2
O5-La2-S4
O5-La2-S5
O5-La2-S6
O6-La2-S2
O6-La2-S4
O6-La2-S5
O6-La2-S6
O2-La2-O4
O2-La2-O5
O2-La2-O6
O4-La2-O5
O4-La2-O6
O5-La2-O6
La1-O2-La2
O3-La1-S1
O3-La1-S2
O3-La1-S3
O3-La1-S4
O4-La1-S1
O4-La1-S2
O4-La1-S3
O4-La1-S4
S1-La1-S2
S1-La1-S3
S1-La1-S4
S2-La1-S3
S2-La1-S4
S3-La1-S4
O2-La2-S2
O2-La2-S4
O2-La2-S5
O2-La2-S6
O4-La2-S2
O4-La2-S4
O4-La2-S5
O4-La2-S6
S2-La2-S4
S2-La2-S5
S2-La2-S6
S4-La2-S5
S4-La2-S6
S5-La2-S6
La1-O4-La2
81.63(16)
142.20(17)
63.21(17)
85.76(17)
102.72(13)
62.51(12)
126.25(12)
53.42(12)
72.53(7)
121.43(6)
151.01(6)
154.57(6)
103.74(6)
74.28(5)
55.08(13)
67.64(12)
91.55(13)
79.67(13)
64.16(13)
52.79(12)
119.06(13)
144.21(13)
104.05(6)
145.06(6)
82.14(7)
154.01(18)
154.2(2)
71.14(18)
98.3(2)
150.06(19)
75.07(16)
62.94(18)
121.07(19)
78.62(17)
164.56(17)
119.81(19)
62.61(18)
71.51(18)
141.12(19)
123.81(19)
94.7(2)
The resulting PLAs have high number-average molecular
weights that slightly deviate from the theoretical values.
Significant deviations are observed especially for the lutetium
complexes (entries 26, 29). We explain this by partial
deactivation of the rare-earth metal complexes during the
polymerization process. The molecular weight distributions
of the PLAs obtained by these yttrium and lutetium
complexes are narrower compared to those obtained by the
scandium analogues but still broader than that expected for
a living polymerization. A relatively slow initiation step by
the less electrophilic silylamide ligand during the polymer-
ization procedure may account for this.15b,23
66.48(6)
132.62(6)
81.72(6)
117.9(2)
94.9(2)
108.4(2)
108.9(2)
by the stronger electron-donating ability of the silylamido
group that weakens the opposite Y-S2 bond. Further features
such as a Si1-N-Si2 angle of 119.65(10)° that is nearly
ideal for sp2 hybridization and the absence of Y · Si and Y ·H
contacts exclude a ꢀ(Si-H) agostic interaction in complex
6b. This is further supported by the spectroscopic data of
6b, where the septet signal of Si H at 5.00 ppm is virtually
not shifted compared to the chemical shift of 4.99 ppm in
[Y{N(SiHMe2)2}3(THF)2].21
The lanthanum complex 6d contains two unsymmetrically
coordinated lanthanum centers (Figure 2) and can be thought
of consisting of a cationic fragment [Ln(OSSO)]+ coordi-
nated to an anionic fragment [Ln(OSSO)2]-. One of the three
[OSSO]-type ligands is exclusively bonded to only one metal
center (La2). The two remaining ligands are coordinated to
(22) Karmazin, L.; Mazzanti, M.; Pécaut, J. Chem. Commun. 2002, 654–
655.
(23) (a) Chisholm, M. H.; Delbridge, E. E. New J. Chem. 2003, 27, 1177–
1183. (b) Chisholm, M. H.; Delbridge, E. E.; Galucci, J. C. New
J. Chem. 2004, 6, 145–152. (c) Chisholm, M. H.; Galucci, J.;
Krempner, C.; Wiggenhorn, C. Dalton Trans. 2006, 846–851.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3333

Ma et al.
a
Table 4. Ring-Opening Polymerization of Lactides at Room Temperature by Rare-Earth Metal Complexes [Ln(OSSO){N(SiHMe2)2}(THF)n]
i
k
k
entry
init.
monomer
[LA]0/[Ln]0
t (h)
conv.h (%)
Mn,cald (×104)
Mn,exp (×104)
Mw/Mn
Prl
1
2
3
4
5
6
7
8
1b
1b
2b
2b
2b
2b
2b
3b
3b
3b
3b
5b
5b
5b
5b
6b
6b
6b
7b
7b
7b
7b
9b
9b
9b
1c
3c
3c
3c
3c
6c
6c
6c
rac-LA
rac-LA
L-LA
300
1500b
300
750
300
1
2
94
78
94
95
95
98
81
80
64
90
79
97
90
98
97
80
81
89
93
85
93
91
66
89
92
92
63
78
91
93
81
94
98
4.06
16.9
4.06
10.3
4.11
10.6
8.75
3.46
27.7
3.89
34.2
4.19
3.89
2.12j
1.40j
3.46
3.50
38.5
4.02
3.68
4.02
39.3
2.85
3.85
39.8
3.98
2.72
3.37
3.93
40.2
3.50
4.06
10.6
3.14
14.8
6.90
14.0
5.09
10.0
12.4
5.20
26.5
3.71
22.6
2.93
3.53
1.90
1.43
3.34
4.43
21.2
3.65
5.01
4.54
20.4
3.29
3.07
18.3
9.05
10.8
8.47
8.70
51.4
52.4
4.72
8.93
1.66
1.90
1.58
1.59
1.62
1.99
1.59
1.74
1.77
1.60
1.95
1.28
1.52
1.27
1.12
1.28
1.59
1.77
1.31
1.33
1.45
1.74
1.19
1.74
2.07
1.89
1.43
1.64
1.54
1.57
1.43
1.62
1.79
0.68
0.68
-
0.15
0.5
0.15
0.5
2
0.17
6
0.17
6
24
2
0.83
0.3
2
0.5
4
2
L-LA
-
rac-LA
rac-LA
rac-LA
L-LA
0.71
0.71
0.72
-
750
750c
300
9
L- LA
3000b
300
-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
rac-LA
rac-LA
L-LA
rac-LA
rac-LA
rac-LA
L-LA
rac-LA
rac-LA
L-LA
rac-LA
rac-LA
rac-LA
L-LA
rac-LA
rac-LA
rac-LA
L-LA
rac-LA
rac-LA
rac-LA
L-LA
0.72
0.70
-
0.84
0.51
0.51
-
0.85
0.84
-
0.86
0.86
0.85
-
0.88
0.88
0.64
-
0.67
0.67
0.66
-
3000b
300
300
300d, e
300d, f
300
300
3000g
300
300
0.5
1
4
300
3000g
300
2.5
0.75
6
0.2
0.1
0.1
0.5
3
300
3000b
300
300
300
300
3000b
300
2
2
4
rac-LA
rac-LA
300
750
0.82
0.81
a [Ln]0 ) 2.9 mmol/L, THF 1 mL. b [Ln]0 ) 1.45 mmol/L, THF 1 mL. c [Ln]0 ) 1.45 mmol/L, THF 2 mL. d Toluene, 1 mL. e 2-Propanol was added,
[iPrOH]0/[Ln]0 ) 2. 2-Propanol was added, [iPrOH]0/[Ln]0 ) 3. g [Ln]0 ) 0.29 mmol/L, THF 1 mL. h Monomer conversion determined by 1H NMR
f
spectroscopy. i Mn,calcd ) ([LA]0/[Ln]0) × 144.13 × conv %. j Mn,calcd ) (300 × 144.13 × conv %)/n,n ) [iPrOH]0/[Ln]0. k Determined by GPC and corrected
l
1
using the Mark–Houwink factor of 0.58. Probability of forming a new r-dyad, determined by homonuclear decoupled H NMR spectroscopy.
Table 5. Summary of Heterotactic Selectivity during ROP of
The microstructure analysis10b,24 of PLAs formed from
rac-LA with yttrium complexes 1b-9b and lutetium com-
rac-Lactide by Rare-Earth Metal Complexes
[Ln(OSSO){N(SiHMe2)2}(THF)n]a
plexes 1c-3c revealed that the ligand sphere and the nature
of the metal center exert a significant influence on the
tacticity of the growing polymer chain. All the complexes
show substantial heterotactic selectivity with a maximum Pr
of 0.88 observed for the yttrium complex 9b. Similar to the
observations previously made for scandium complexes,11d
the heterotactic selectivity of yttrium complexes 1b–9b is
apparently enhanced when the steric demand of the bis(phe-
nolato) ligand is increased either by the length of the bridge
or by bulky ortho-substituents in the phenoxy unit. The
influence of the bridging moiety is most significant (Table
5). The most notable surge in heterotacticity is observed with
the extension of the bridging unit from C2 to C3, where the
Pr value increases from 0.68 for complex 1b to 0.84 for
complex 5b. Exchanging the ortho-substituent tert-butyl
against cumyl results only in slight improvement to 0.71 for
complex 2b. A further increase in the length of the bridging
unit to a C4 bridge in the bis(phenolato) ligands of complexes
6b-9b did not bring any further improvement in heterose-
lectivity. The chirality of the ligand skeleton in complexes
(24) Polylactide characterization: (a) Chisholm, M. H.; Iyer, S. S.; Matison,
M. E.; McCollum, D. G.; Pagel, M. Chem. Commun. 1997, 1999–
2000. (b) Thakur, K. A. M.; Kean, R. T.; Hall, E. S. Anal. Chem.
1997, 69, 4303–4309. (c) Thakur, K. A. M.; Kean, R. T.; Zell, M. T.;
Padden, B. E.; Munson, E. J. Chem. Commun. 1998, 1913–1914.
a [LA]0/[Ln]0 ) 300, [Ln]0 ) 2.9 mmol/L, THF, refer entries in Table
4 for detail. b Probability of forming a new r-dyad, determined by
homonuclear decoupled H NMR spectroscopy.
1
3334 Inorganic Chemistry, Vol. 47, No. 8, 2008

StereoselectiWe ROP of rac-Lactide
Scheme 4
3b, 3c, and 9b has no influence on the tacticity and thereby
suggests the absence of a normal enantiomorphic site control
during ROP of rac-LA by these rare-earth metal complexes.
In our previous work, a model complex which is believed
to mimic the catalytically active species or intermediate of
the stereoselective ROP of rac-lactide was obtained from
the reaction of [Sc(xytbm){(NSiHMe2)2}(THF)] and (R)-(+)-
butyl lactate.11d On the basis of its structure, a dynamic
monomer recognition process was proposed to explain the
origin of the high heteroselectivity of bis(phenolato) scan-
dium complexes for the ROP of rac-lactide.11d This mech-
anism is also thought to be operative for yttrium and lutetium
complexes supported by the same series of bis(phenolato)
ligands. As illustrated in Scheme 4, after the insertion of
either enantiomer of rac-lactide in the initiation step, the
bulky chiral environment around the metal center generated
by the ligand framework will allow one of the enantiomers
(L-lactide in Scheme 4) to approach and coordinate to the
metal center. After ring-opening of the coordinated monomer,
the steric repulsion between the R-methyl group of the ring-
opened monomer and the ortho-substitutent of the ligand will
induce a transformation between Λ and ∆ configuration of
the ligand. The ∆ configuration will preferentially attack
D-lactide, eventually leading to a heterotactic polymer chain.
To achieve this dynamic monomer recognition, fluxionality
of the ligand framework is a prerequisite. For complexes
bearing a C2-bridged bis(phenolato) ligand, the two protons
of the SCH2 unit are chemically inequivalent as shown by a
typical AB spin pattern in the 1H NMR spectra (THF-d8, 25
°C), which indicates the lack of fluxionality of a C2 bridge.
For complexes bearing C3-bridged bis(phenolato) ligands
such as the yttrium complex 5b, all four protons of two SCH2
units are always chemically equivalent and are recorded as
distinct triplets in the H NMR spectra (C6D6 or THF-d8).
1
This implies that the C3 bridge allows a free configurational
transformation of the ligand framework. In agreement with
this result, a surge of heterotactic selectivity in ROP of rac-
lactide was observed for complex 5b bearing a C3-bridged
ligand as compared to complexes 1b-3b containing the C2-
bridged ligands. For complex 2b, the cumyl group in the
ortho-position of the phenoxy ligand increases the congestion
at the metal center, but the rigid C2 bridge restricts a flexible
configuration transformation, resulting only in slight im-
provement of the heteroselectivity. Only slight increase in
the heteroselectivity during ROP of rac-lactide was observed
for complexes 6b-9b, likely caused by the increase of the
steric bulkiness of the ligands. It is also possible that the
C4-bridged units involved in these ligands are somewhat rigid
compared to aliphatic alkanediyl-linked systems. The influ-
ence of the C3 bridge compared to the C2 bridge was also
observed for aluminum complexes with tetradentate Schiff
base ligands,5i where the high isotactic selectivity for ROP
of rac-lactide was obtained for complexes with the C3-
bridged ligand. However, further increasing the bridge length
in this aluminum system led to a decrease of isoselectivity.
Obviously, different stereocontrol processes may be operative
here.5q
As can be seen from Table 5, the heteroselectivity during
ROP of rac-LA is increased for metal complexes of a given
ligand (1, 3, and 6) in the order Sc (a) > Y (b) > Lu (c).
Compared to yttrium and lutetium, scandium is significantly
smaller. Therefore, a given ligand will form a more crowded
coordination environment at scandium in comparison to the
situation at yttrium and lutetium, resulting in higher het-
Inorganic Chemistry, Vol. 47, No. 8, 2008 3335

Ma et al.
hydride. Anhydrous lanthanide trichlorides (Aldrich or Strem
products) were used as received. Benzene-d6, chloroform-d, and
other reagents were carefully dried and stored in a glovebox.
[Ln{N(SiHMe2)2}3(THF)x] (Ln ) Sc, Y, Lu; x ) 1, 2) were
synthesized according to the literature methods.21,25 4,6-Di(2-
phenyl-2-propyl)-2-thiophenol was synthesized according to a
modification of the method reported in the literature.26 1,4-
Dithiabutanediyl-2,2′-bis{4,6-di(2-phenyl-2-propyl)phenol}, etccpH2
(2),26 ortho-xylylenedithiobis(6-tert-butyl-4-methylphenol), xytb-
mpH2 (6),27 ortho-xylylenedithiobis(6-adamantyl-4-methylphenol),
xytampH2 (8),11d and rac-(3,4-trans-butanediyl-1,6-dithiahexanediyl)-
2,2′-bis(6-tert-butyl-4-methylphenol), cmtbmpH2 (rac-9),11d were
prepared according to reported methods. L-Lactide and rac-lactide
(Aldrich) were recrystallized from dry toluene and sublimed twice
under vacuum at 50 °C. 2-Propanol was dried over calcium
hydride prior to distillation. All other chemicals were commercially
available and used after appropriate purification. Glassware and vials
used in the polymerization were dried in an oven at 120 °C
overnight and exposed to vacuum-argon cycle three times.
NMR spectra were recorded on Bruker DRX 400, Varian 200
and 500 spectrometers at 25 °C (1H: 200, 400, 500 MHz; 13C: 50,
100, 125 MHz) unless otherwise stated. Chemical shifts for 1H and
13C NMR spectra were referenced internally using the residual
solvent resonances and reported relative to tetramethylsilane. NMR
assignments were confirmed by Apt, 1H-1H (COSY), and 1H-13C
(HMQC) when necessary. Elemental analyses were performed by
the Microanalytical Laboratory of University of Mainz, Germany.
Spectroscopic analysis of polymers was performed in CDCl3.
Molecular weights and polydispersities of polymer samples were
determined by size exclusion chromatography (GPC) in THF at
35 °C, at a flow rate of 1 mL/min utilizing an Agilent 1100 Series
HPLC, G1310A isocratic pump, an Agilent 1100 Series refractive
index detector and 8 × 600 mm, 8 × 300 mm, 8 × 50 mm PSS
SDV linear M columns. Calibration standards were commercially
available narrowly distributed linear polystyrene samples that cover
a broad range of molar masses (103 < Mn < 2 × 106 g mol-1).
eroselectivity. As the radius of yttrium is somewhat larger
than that of lutetium, the heteroselectivity could be expected
to be lower. However, the lutetium complexes, generally
more active than the yttrium analogues, show decreased
heteroselectivity.
Yttrium complexes 6b-9b show higher heterotactic
selectivity and are highly efficient for the ROP of rac-lactide.
Remarkably high conversions up to 91% for 3000 equiv of
monomer to PLAs can be achieved within hours using
relatively low initiator concentrations (entries 18, 22, and
25). The tacticities of the obtained polymers are determined
by the initiator, regardless of their high molecular weights
up to 887 000 g mol-1. Compared to the high efficiency
toward rac-lactide, complexes 6b-9b polymerize L-lactide
rather slowly, reflecting the high selectivity for alternating
enchainment of L-lactide and D-lactide (entries 16 vs 17, 19
vs 20, and 23 vs 24). As reported previously,11d the use of
THF as solvent is crucial for the high heteroselectivity and
activity by these complexes during the ROP of rac-LA.
Polymerization runs in toluene only afforded traces of
polymers after several days. The addition of excess alcohol
to the initiator/rac-LA mixture in toluene accelerates the
polymerization to give atactic PLAs in a controlled manner
(entries 14 and 15). The crucial role of THF for high
heterotactic selectivity has also been reported for some well-
defined zinc, calcium, and yttrium initiator systems.9a,10b,11b
More recently, a related alkoxyamine bis(phenolato) yttrium
complex has been reported to show heteroselective and
immortal behavior in the presence of 2-propanol.11f
Conclusions
Utilizing the well-established amine elimination method,
several monomeric yttrium and lutetium complexes supported
by sulfur-bridged bis(phenolato) ligands of [OSSO]-type
were synthesized and structurally characterized. This series
of yttrium and lutetium complexes efficiently initiated the
ring-opening polymerization of rac-lactide in THF. High
heterotacticity was observed for the resultant poly(lactide)
chains. Increase in heteroselectivity during the ROP of rac-
lactide by these complexes was observed with increasing
steric bulk of the bis(phenolato) ligands. The influence of
the bridging moiety is most significant. On the basis of the
structure of a model complex of scandium, “a dynamic
enantiomorphic site control”11d,12g resulting from the con-
figurational interconversion of the active site during the
polymerization was suggested to be responsible for the
observed high heteroselectivity. Thus a C3 or C4 bridge
allows sufficient fluxionality of the ligand framework to
enable the interconversion between Λ to ∆ configurations,
responsible for enantiomer recognition.
rac-(2,3-trans-butanediyl-1,4-dithiabutanediyl)-2,2′-bis(6-tert-
butyl-4-methylphenol), cytbmpH2 (rac-3)28. Solid sodium hy-
droxide (4.908 g, 0.123 mmol) was added to a solution of 4-methyl-
6-tert-butyl-2-thiophenol (24.051 g, 0.123 mmol) in methanol (100
mL). The mixture was exposed to ultrasound until sodium hydroxide
dissolved. Cyclohexene oxide (12.043 g, 0.123 mmol) was added
dropwise via syringe to the yellow solution and the mixture was
heated to reflux for 2 h. All volatiles were removed in vacuum
from the reaction mixture. The solid residue was extracted with
diethyl ether and washed with water (3 × 100 mL). The pale yellow
organic phase was separated, dried over anhydrous sodium sulfate,
and evaporated to dryness to give 4-methyl-6-tert-butyl-2-(trans-
2-hydroxycyclohexyl)thiophenol as a pale yellow viscous liquid
1
(35.134 g, 97%) which was pure by H NMR spectroscopy. 1 H
4
NMR (CDCl3, 200 MHz): δ 7.15 (d, 1H, JHH ) 1.6 Hz), 7.09
(25) (a) Herrmann, W. A.; Anwander, R.; Munck, F. C.; Scherer, W.;
Dufaud, V.; Huber, N. W.; Artus, G. R. J. Z. Naturforsch. B. 1994,
49, 1789–1797. (b) Herrmann, W. A.; Munck, F. C.; Artus, G. R. J.;
Runte, O.; Anwander, R. Organometallics 1997, 16, 682–688.
(26) (a) Capacchione, C.; Proto, A.; Ebeling, H.; Mülhaupt, R.; Möller,
K.; Spaniol, T. P.; Okuda, J. J. Am. Chem. Soc. 2003, 125, 4964–
4965. (b) Capacchione, C.; Manivannan, R.; Barone, M.; Beckerle,
K.; Centore, R.; Oliva, L.; Proto, A.; Tuzi, A.; Spaniol, T. P.; Okuda,
J. Organometallics 2005, 24, 2971–2982.
(27) Ma, H.; Melillo, G.; Oliva, L.; Spaniol, T. P.; Englert, U.; Okuda, J.
Dalton Trans. 2005, 721–727.
(28) Beckerle, K.; Manivannan, R.; Lian, B.; Meppelder, G.-J. M.; Raabe,
G.; Spaniol, T. P.; Ebeling, H.; Pelascini, E.; Mülhaupt, R.; Okuda, J.
Angew. Chem., Int. Ed. 2007, 46, 4790–4793.
Experimental Section
General Considerations. All operations were performed under
an inert atmosphere of argon using standard Schlenk-line or
glovebox techniques. Toluene, n-hexane, and THF were distilled
under argon from sodium/benzophenone ketyl prior to use. n-
Pentane was purified by distillation from sodium/triglyme ben-
zophenone ketyl. Dichloromethane was distilled from calcium
3336 Inorganic Chemistry, Vol. 47, No. 8, 2008

StereoselectiWe ROP of rac-Lactide
4
(d, 1H, JHH ) 1.6 Hz), 3.35 (m, 1H, cyclohexyl-H), 2.56 (m,
153.0 (C1), 150.5, 150.2, 141.4, 135.4 (Ar-C6), 134.3, 132.5, 130.5
(C3), 127.92, 127.87 (C4), 127.78, 127.3, 126.5, 125.52, 125.48,
125.2, 117.1 (C2), 42.4 (SCH2), 42.3 (C(CH3)2), 30.8 (CH3), 29.3
(CH3). Anal. Calcd for C56H58O2S2: C, 81.31; H, 7.07; S, 7.75.
Found: C, 81.22; H, 7.12; S, 7.54.
1H, SCH), 2.26 (s, 3H, CH3), 2.08 (m, 2H, cyclohexyl-H), 1.68
(m, 2H, cyclohexyl-H), 1.40 (s, 9H, C(CH3)), 1.24 (m, 4H,
cyclohexyl-H).
Thionyl chloride (5.542 g, 46.01 mmol) was added dropwise
via syringe to the solution of 4-methyl-6-tert-butyl-2-(trans-2-
hydroxycyclohexyl)thiophenol (11.29 g, 38.34 mmol) in dichlo-
romethane (100 mL) at -30 °C. The reaction mixture was warmed
slowly to room temperature and heated to reflux for 5 h. After
cooling, the reaction mixture was concentrated to dry to remove
the excess SOCl2. The obtained residue was treated with water and
diethyl ether for 10 min. After extraction with diethyl ether and
sequential washing with water, aqueous sodium bicarbonate solution
and water, the organic layer was dried over anhydrous sodium
sulfate. After workup and removal of the solvent under vacuum,
4-methyl-6-tert-butyl-2-(trans-2-chlorocyclohexyl)thiophenol (11.85
g, 98%) was obtained as a viscous, pale orange liquid, which was
[Y(etccp){N(SiHMe2)2}(THF)] (2b). A solution of 1,4-dithi-
abutanediyl-2,2′-bis{4,6-di(2-phenyl-2-propyl)phenol} (etccpH2)
(0.376 g, 0.500 mmol) in toluene (5 mL) was added slowly to a
solution of [Y{N(SiHMe2)2}3(THF)2] (0.315 g, 0.500 mmol) in
toluene (10 mL) at room temperature. The yellow solution was
stirred for 8 days at 50 °C. The obtained slightly pale yellow
solution was concentrated to dryness to afford a foam-like solid,
which was further dried under high vacuum for several hours. The
solid obtained was then dissolved with 10 mL of n-pentane and
filtered. The clear filtrate was concentrated to dryness to give a
1
white powder in quantitative yield. H NMR (C6D6, 500 MHz): δ
7.46 (d, 2H, 4JHH ) 2.0 Hz), 7.28 (br d, 4H), 7.26 (dd, 4H, 3JHH
)
1
1
4
4
pure by H NMR spectroscopy. H NMR (CDCl3, 200 MHz): δ
8.5 Hz, JHH ) 1.5 Hz), 7.21 (d, 2H, JHH ) 2.5 Hz), 7.16–7.10
7.23 (s, 1H, OH), 7.17 (d, 1H, 4 JHH ) 2.2 Hz), 7.10 (d, 1H, 4 JHH
(m, 8H), 7.03 (tt, 2H, 3JHH ) 7.5 Hz, 4JHH ) 1.5 Hz), 6.95 (tt, 2H,
3
3
4
3
3JHH ) 7.5 Hz, JHH) 1.5 Hz), 4.93 (septet, 2H, JHH ) 3.0 Hz,
) 2.2 Hz), 3.84 (td, 1H, Ja,a ) 9.0 Hz, Ja,e ) 4.0 Hz, ClCH),
3
3
2.88 (td, 1H, Ja,a ) 9.0 Hz, Ja,e ) 4.0 Hz, SCH), 2.44–2.28 (m,
1H, cyclohexyl-H), 2.26 (s, 3H, CH3), 2.23–2.08 (m, 1H, cyclo-
hexyl-H), 1.85–1.59 (m, 4H, cyclohexyl-H), 1.40 (s, 9H,
C(CH3)), 1.39 (m, 2H, overlapped, cyclohexyl-H).
SiH), 2.92 (m, 4H, THF), 2.36 (br d, 4H, SCH2), 1.80 (br s, 12H,
3
CH3), 1.62 (s, 12H, CH3), 0.98 (m, 4H, THF), 0.33 (d, 12H, JHH
) 3.0 Hz, Si(CH3)2). 13C{1H} NMR (C6D6, 125 MHz): δ 166 (C1),
152.1 (Ar-C), 151.6 (Ar-C), 138.2 (C6), 137.1 (C4), 130.5 (C3),
128.5(C5), 127.0 (Ar-C), 126.3 (Ar-C), 125.8 (Ar-C), 124.8 (Ar-
C), 118.1 (C2), 70.3 (THF), 43.0 (S CH2), 42.7 (C(CH3)2), 31.3
(CH3), 25.1 (THF), 3.7 (Si(CH3)2). Anal. Calcd for C58H74
NO3S2Si2Y·0.1(C7H8): C, 67.04; H, 7.17; N, 1.33; S, 6.10. Found:
C, 67.56; H, 7.32; N, 1.88; S, 6.35.
Sodium hydroxide (1.037 g, 25.93 mmol) was added to the
solution of 4-methyl-6-tert-butyl-2-thiophenol (5.082 g, 25.93
mmol) in methanol (20 mL). The mixture was exposed to ultrasound
until the sodium hydroxide dissolved completely to give a yellow
solution. A solution of 4-methyl-6-tert-butyl-2-(2-chlorocyclohexyl)-
thiophenol (8.113 g, 25.93 mmol) in the mixture of methanol and
dichloromethane (50 mL/40 mL) was added dropwise to the above
yellow solution. The reaction mixture was heated to reflux for 2 h,
white solids precipitated gradually. After cooling, the white solids
were collected by suction, washed with a small amount of methanol
and dried under vacuum. The solids were dissolved in dichlo-
romethane and filtrated. After removal of solvent, the solid was
recrystallized with hexane to afford needle-like, colorless crystals
[Y(cytbmp){N(SiHMe2)2}(THF)] (3b). Solid cytbmpH2 (0.236
g, 0.500 mmol) was added slowly to a solution of [Y{N
(SiHMe2)2}3(THF)2] (0.315 g, 0.500 mmol) in n-pentane (15 mL)
at room temperature. The colorless solution was stirred for 5 days,
and all the volatiles were removed in vacuo to afford a white solid,
which was further dried under high vacuum for several hours. The
solid obtained was then dissolved with n-hexane. After filtration,
the filtrate was concentrated to about 3 mL and kept at -40 °C to
give colorless crystals (300 mg, 79%, two crops). 1H NMR (C6D6,
1
(13.2 g, 83%). H NMR (CDCl3, 200 MHz): δ 7.40 (s, 2H, OH),
7.18 (d, 2H, 4JHH ) 2.0 Hz), 7.10 (d, 2H, 4JHH ) 2.0 Hz), 2.75 (m,
2H, SCH), 2.25 (s, 6H, CH3), 2.04 (m, 2H, cyclohexyl-H), 1.65
(m, 2H, cyclohexyl-H), 1.41 (s, 18H, C(CH3)), 1.40 (m, 2H,
overlapped, cyclohexyl-H), 1.21 (m, 2H, cyclohexyl-H). 13C{1H}
NMR (CDCl3, 50 MHz): δ 154.0 (Ar-C1), 135.6 (Ar-C6), 134.7
(Ar-C4), 129.7 (Ar-C3), 128.3 (Ar-C5), 116.4 (Ar-C2), 52.8 (SCH),
34.9 (6-C(CH3)), 33.4 (cyclohexyl-C), 29.5 (6-C(CH3)), 25.4
(cyclohexyl-C), 20.7 (4-CH3). Anal. Calcd for C28H40O2S2: C, 71.14;
H, 8.53; S, 13.57. Found: C, 71.02; H, 8.56; S, 13.6.
4
4
500 MHz): δ 7.28 (d, 2H, JHH ) 2.4 Hz, 5-H), 7.17 (d, 2H, JHH
) 2.4 Hz, 3-H), 5.22 (septet, 2H, SiH, 3JHH ) 3.0 Hz), 3.94 (br s,
4H, THF), 2.68 (br s, 2H, SCH), 2.19 (s, 6H, CH3), 1.89 (br s, 2H,
cyclohexyl-H), 1.72 (s, 18H, C(CH3)3), 1.65 (br s, 2H, cyclohexyl-
H), 1.17 (m, 4H, THF), 1.06 (br s, 2H, cyclohexyl-H), 0.43 (d,
3
3
6H, JHH ) 3.0 Hz, Si(CH3)2), 0.41 (d, 6H, JHH ) 3.0 Hz,
Si(CH3)2), 0.39 (br s, 2H, cyclohexyl-H, overlapped). 13C{1H} NMR
(C6D6, 125 MHz): δ 167.4 (C1), 138.5 (C6), 135.5 (C3), 130.8
(C5), 123.7 (C4), 115.7 (C2), 71.2 (THF), 53.2 (S CH2), 35.6 (6-
C(CH3)3), 34.5 (cyclohexyl-C), 29.9 (6-C(CH3)3), 25.5 (cyclohexyl-
C), 25.1 (THF), 20.9 (4-CH3), 3.7 (Si(CH3)2). Anal. Calcd for
C36H60NO3S2Si2Y: C, 56.59; H, 7.91; N, 1.83; S, 8.39. Found: C,
56.92; H, 8.41; N, 2.16; S, 8.42.
ortho-Xylylenedithio-bis{4,6-di(2-phenyl-2-propyl)phenol},
xytccpH2 (7). Under argon, to the solution of 4,6-di(2-phenyl-2-
propyl)-2-thiophenol (9.060 g, 24.9 mmol) in a mixture of methanol
and toluene (25 mL/5 mL) was added slowly solid sodium
hydroxide (0.997 g, 24.9 mmol) at 50 °C. The obtained yellow
solution was further treated dropwise with a solution of 1,2-
bis(bromomethyl)benzene (3.289 g, 12.46 mmol) in toluene (20
mL). The reaction solution was stirred overnight at 60 °C to give
a mixture of white solids, a colorless solution, and a viscous
orange yellow oil. All the volatiles were removed under vacuum.
The residue was dissolved with n-pentane (80 mL) and filtered.
The filtrate was kept at 0 °C for 4 h; colorless prismatic crystals
were obtained (5.95 g, 57%). 1H NMR (CDCl3, 200 MHz): δ
[Lu(cytbmp){N(SiHMe2)2}(THF)] (3c). Solid cytbmpH2 (0.236
g, 0.500 mmol) was added slowly to a solution of [Lu{N
(SiHMe2)2}3(THF)2] (0.357 g, 0.500 mmol) in n-pentane (15 mL)
at room temperature. The colorless solution was stirred for 5 days.
After filtration to remove trace amount of solid, all the volatiles
were removed in vacuo to afford a solid foam, which was dried
under high vacuum for several hours. The solid obtained was then
dissolved in n-hexane (4 mL) and kept at -40 °C to give colorless
crystals (260 mg, 61%, two crops). 1H NMR (C6D6, 500 MHz): δ
4
4
7.32–7.09 (m, 22H, Ar-H), 7.00 (m, 2H, Ar-H), 6.83 (d, 2H, JHH
7.29 (d, 2H, JHH ) 2.1 Hz, 5-H), 7.15 (2H, 3-H, overlapped by
3
) 2.2 Hz, 3-H), 6.66 (m, 2H, Ar-H), 6.41 (s, 2H, OH), 3.45 (s,
solvent signal), 5.2 (septet, 2H, JHH ) 3.0 Hz, SiH), 3.95 (br s,
4H, SCH2), 1.54 (s, 24H, CH3). 13C{1H} NMR (C6D6, 50 MHz): δ
4H, THF), 2.64 (br s, 2H, SCH), 2.19 (s, 6H, CH3), 1.85 (br d, 2H,
Inorganic Chemistry, Vol. 47, No. 8, 2008 3337

Ma et al.
cyclohexyl-H), 1.73 (s, 18H, C(CH3)3), 1.63 (br d, 2H, cyclohexyl-
H), 1.14 (m, 4H, THF), 1.02 (br d, 2H, cyclohexyl-H), 0.46 (d,
6H), 7.29–7.20 (m, 8H), 7.11–7.04 (m, 6H), 7.16–7.10 (m, 8H),
6.93 (m, 2H), 6.82 (m, 2H), 6.55 (m, 2H), 4.80 (septet, 2H, 3JHH
)
3
3
6H, JHH ) 3.0 Hz, Si(CH3)2), 0.43 (d, 6H, JHH ) 3.0 Hz,
Si(CH3)2), 0.36 (br t, 2H, cyclohexyl-H, overlapped). 13C{1H} NMR
(C6D6, 125 MHz): δ 168.3 (C1), 139.2 (C6), 135.5 (C3), 131.0
(C5), 123.6 (C4), 115.1 (C2), 71.6 (THF), 53.2 (S CH2), 35.5 (6-
C(CH3)3), 34.4 (cyclohexyl-C), 30.0 (6-C(CH3)3), 25.5 (cyclohexyl-
C), 25.1 (THF), 20.9 (4-CH3), 3.9 (Si(CH3)2), 3.8 (Si(CH3)2). Anal.
Calcd for C36H60NLuO3S2Si2: C, 50.86; H, 7.11; N, 1.65; S, 7.54.
Found: C, 51.01; H, 7.50; N, 1.94; S, 7.52.
3.0 Hz, SiH), 3.58 (s, 4H, SCH2), 2.95 (m, 4H, THF), 1.74 (12H,
CH3), 1.70 (s, 12H, CH3), 1.06 (m, 4H, THF), 0.27 (d, 12H, JHH
3
) 3.0 Hz, Si(CH3)2). 13C{1H} NMR (C6D6, 50 MHz): δ 166 (C1),
151.8 (Ar-C), 151.6 (Ar-C), 138.0 (C6), 136.3 (C4), 136.2 (Ar-C),
133.6 (C3), 130.9 (C5), 130.2 (Ar-C), 127.1 (Ar-C), 126.7 (Ar-C),
125.9 (Ar-C), 124.9 (Ar-C), 121.0 (Ar-C), 116.5 (C2), 70.1 (THF),
43.3 (SCH2), 42.8 (C(CH3)2), 40.2 (C(CH3)2), 31.4 (CH3), 30.5
(CH3), 25.2 (THF), 3.4 (Si(CH3)2). Anal. Calcd for C64H78
NO3S2Si2Y: C, 68.73; H, 7.03; N, 1.25; S, 5.73. Found: C, 68.78;
H, 7.19; N, 1.62; S, 5.67.
[Y(xytbmp){N(SiHMe2)2}(THF)] (6b). Solid xytbmpH2 (0.247
g, 0.500 mmol) was added slowly to the solution of [Y{N
(SiHMe2)2}3(THF)2] (0.315 g, 0.500 mmol) in n-hexane (10 mL)
at room temperature. The colorless reaction mixture was stirred
for 3 days. Some white solid precipitated and more n-hexane was
added to dissolve the solid. After filtration to remove trace amounts
of impurities, the clear solution was concentrated in vacuo to afford
a white powder, which was dried for several hours. The powder
obtained was recrystallized from n-hexane at -30 °C to afford
[Y(cmtbmp){N(SiHMe2)2}(THF)] (9b). A solution of cmtb-
mpH2 (0.295 g, 0.589 mmol) in n-pentane (5 mL) was added slowly
to a solution of [Y{N(SiHMe2)2}3(THF)2] (0.371 g, 0.589 mmol)
in n-pentane (10 mL) at room temperature. The obtained solution
was stirred for 10 days at room temperature. All the volatiles were
removed in vacuo to afford a foam-like solid, which was further
dried under high vacuum for several hours. The solid was then
recrystallized with n-pentane and kept at -40 °C to give colorless
1
colorless crystals (300 mg, 76%). H NMR (C6D6, 400 MHz): δ
1
4
4
crystals (190 mg, 41%). H NMR (C6D6, 500 MHz): δ 7.17 (d,
7.25 (d, 2H, JHH ) 2.0 Hz, 5-H), 7.24 (d, 2H, JHH ) 2.0 Hz,
3-H), 6.77 (m, 2H, Ar-H), 6.62 (m, 2H, Ar-H), 5.00 (septet, 2H,
3JHH ) 3.0 Hz, SiH), 3.89 (m, 4H, THF), 3.63 (s, 4H, SC H2),
2.28 (s, 6H, 4-CH3), 1.60 (s, 18H, 6-C(CH3)3), 1.19 (m, 4H, THF),
0.30 (d, 12H, 3JHH ) 3.0 Hz, Si(CH3)2). 13C{1H} NMR (C6D6, 100
MHz): δ 165.3 (C1), 138.1 (C6), 133.6 (Ar-C), 132.0 (C3), 130.6
(C5), 129.7(Ar-C), 127.9 (Ar-C), 124.8 (C4), 120.9 (C2), 70.6
(THF), 40.3 (SCH2), 35.5 (6-C(CH3)3), 30.0 (6-C(CH3)3), 25.1
(THF), 20.9 (4-CH3), 3.3 (Si(CH3)2). Anal. Calcd for C38H58
NO3S2Si2Y: C, 58.06; H, 7.44; N, 1.78; S, 8.16. Found: C, 57.85;
H, 7.56; N, 1.96; S, 8.25.
4
4
2H, JHH ) 2.4 Hz, 5-H), 7.12 (2H, JHH ) 2.4 Hz, 3-H), 5.19
(septet, 2H, 3JHH ) 3.0 Hz, SiH), 4.01 (m, 4H, THF), 2.84 (d, 2H,
2JHH ) 13.0 Hz, SCH2), 2.72 (dd, 2H, 2JHH ) 13.0 Hz, 3JHH ) 5.8
Hz, SCH2), 2.23 (s, 6H, 4-CH3), 1.66 (s, 18H, 6-C(CH3)3), 1.66
(2H, cyclohexyl-H, overlapped), 1.32 (m, 4H, THF), 1.27 (br m,
4H, cyclohexyl-H), 0.99 (br m, 4H, cyclohexyl-H), 0.40 (d, 6H,
3
3JHH ) 3.0 Hz, Si(CH3)2), 0.37 (d, 6H, JHH ) 3.0 Hz, Si(CH3)2).
13C{1H} NMR (C6D6, 125 MHz): δ 164.04 (C1), 164.02 (C1′),
137.8 (C6), 130.9 (C3), 128.7 (C5), 125.1 (C4), 123.6 (C2), 71.1
(THF), 44.2 (SCH2), 41.1 (cyclohexyl-C), 35.5 (6-C(CH3)3), 31.9
(cyclohexyl-C), 29.9 (6-C(CH3)3), 25.2 (THF), 25.0 (cyclohexyl-
C), 21.0 (4-CH3), 3.5 (Si(CH3)2), 3.4 (Si(CH3)2). Anal. Calcd for
C38H64NO3S2Si2Y: C, 57.62; H, 8.14; N, 1.77; S, 8.10. Found: C,
58.00; H, 8.26; N, 2.00; S, 8.34.
[Lu(xytbmp){N(SiHMe2)2}(THF)] (6c). Solid xytbmpH2 (0.247
g, 0.500 mmol) was added slowly to the solution of [Lu{N
(SiHMe2)2}3(THF)2] (0.357 g, 0.500 mmol) in n-hexane (15 mL)
at room temperature. The colorless reaction mixture was stirred
for 5 days. White solids precipitated, and more n-hexane was added
to dissolve the solid. After filtration to remove trace amounts of
impurities, the clear solution was concentrated in vacuo to afford
a white powder, which was dried for several hours. The powder
obtained was recrystallized from n-heptane at -30 °C to afford
Typical Polymerization Procedure. In a glovebox, initiator from
a stock solution in THF or toluene was injected sequentially to a
series of 6 mL vials loaded with L-lactide or rac-lactide and a
suitable amount of dry solvent. After specified time intervals, each
vial was taken out of the glovebox; an aliquot was withdrawn and
quenched quickly with n-pentane; the reaction mixture was
quenched at the same time by adding excess amount of n-pentane
and one drop of water. All the volatiles in the aliquots were
removed, and the residue was subjected to monomer conversion
determination which was monitored by integration of monomer
versus polymer methine or methyl resonances in the 1H NMR
spectra (CDCl3, 200 MHz). The precipitates collected from the bulk
mixture were dried in air, dissolved with dichloromethane, and
sequentially precipitated into methanol. The obtained polymer was
further dried in a vacuum oven at 60 °C for 16 h for GPC analysis.
Each reaction was used as one data point. In the cases where
2-propanol was used, the initiator solution in toluene was treated
first with the solution of 2-propanol in the same solvent for 10
min, and then injected to the solution of L-lactide or rac-lactide.
Otherwise the procedures were the same.
1
colorless crystals (320 mg, 74%). H NMR (C6D6, 200 MHz): δ
4
4
7.27 (d, 2H, JHH ) 2.0 Hz, 5-H), 7.26 (d, 2H, JHH ) 2.0 Hz,
3-H), 6.77 (m, 2H, Ar-H), 6.62 (m, 2H, Ar-H), 5.00 (septet, 2H,
3JHH ) 3.0 Hz, SiH), 3.92 (m, 4H, THF), 3.64 (s, 4H, SCH2), 2.27
(s, 6H, 4-CH3), 1.61 (s, 18H, 6-C(CH3)3), 1.14 (m, 4H, THF), 0.30
(d, 12H, 3JHH ) 3.0 Hz, Si(CH3)2). 13C{1H} NMR (C6D6, 50 MHz):
δ 166.0 (C1), 138.9 (C6), 133.2 (Ar-C), 132.5 (C3), 130.7 (C5),
130.0 (Ar-C), 127.8 (Ar-C), 124.6 (C4), 119.9 (C2), 71.4 (THF),
40.8 (SCH2), 35.6 (6-C(CH3)3), 30.1 (6-C(CH3)3), 25.1 (THF), 21.0
(4-CH3), 3.6 (Si(CH3)2). Anal. Calcd for C38H58NLuO3S2Si2: C,
52.33; H, 6.70; N, 1.61; S, 7.35. Found: C, 51.89; H, 7.07; N, 1.97;
S, 7.67.
[Y(xytccp){N(SiHMe2)2}(THF)] (7b). Solid ortho-xylylene-
dithiobis{4,6-di(2-phenyl-2-propyl)phenol} (xytccpH2) (0.414 g,
0.500 mmol) was added slowly to
a solution of [Y{N
Crystal Structure Analysis. X-ray diffraction measurements
were performed on a Bruker AXS diffractometer with Mo KR
radiation using ω-scans. Crystal parameters and results of the
(SiHMe2)2}3(THF)2] (0.315 g, 0.500 mmol) in toluene (20 mL) at
room temperature. The solution was stirred for 3 days at 70 °C.
The obtained slightly pale yellow solution was concentrated to
dryness to afford a foam-like solid, which was further dried under
high vacuum for several hours. The solid obtained was then
dissolved with 10 mL of n-pentane and filtered. The clear filtrate
was concentrated to dryness to give a white powder in quantitative
(29) ASTRO, SAINT and SADABS. Data Collection and Processing
Software for the SMART System; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1996.
(30) Sheldrick, G. M. SHELXS-86: A Program for Crystal Structure
Solution; University of Göttingen: Göttingen, Germany, 1986.
1
yield. H NMR (C6D6, 200 MHz): δ 7.45 (m, 2H), 7.42–7.37 (m,
3338 Inorganic Chemistry, Vol. 47, No. 8, 2008

StereoselectiWe ROP of rac-Lactide
structure refinements are given in Table 1. Absorption corrections
were carried with the multiscan method using SADABS.29 All
structures were solved by direct methods (SHELXS-86)30 and
refined (SHELXS-97)31 against all F2 data. The structure of 6d
contains the cocrystallized solvent molecules toluene and pentane
in the crystal lattice which are partially disordered. For each
molecule of 6d, the unit cell contains 1.5 molecules of toluene and
0.5 molecules of pentane. Disorder is also found in one of the
phenolate units. All non-hydrogen atoms except those in the
disordered groups and also in solvent molecules were refined with
anisotropic displacement parameters. Hydrogen atoms were included
into calculated positions, and only the hydrogen atoms in 6b that
are bound to Si1 and Si2 were located in a difference Fourier map
and are refined in their position. For the graphical representation,
the program ORTEP was used as implied in the program system
WinGX.32 Crystallographic data (excluding structure factors) have
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication nos. CCDC 666471 (6b) and 666472
(6d). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K.
(fax: (+44)1223–336–033; e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk.
Acknowledgment. We gratefully acknowledge the finan-
cial support by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
Supporting Information Available: Crystallographic data for
6b and 6d as cif files and 1H NMR spectra of polylactide samples
(homodecoupled methine region). This material is available free
of charge via the Internet at http://pubs.acs.org.
(31) Sheldrick, G. M. SHELXL-97: A Program for Crystal Structure
Refinement; University of Göttingen: Göttingen, Germany, 1997.
(32) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
IC702312B
Inorganic Chemistry, Vol. 47, No. 8, 2008 3339