Ytterbium amides of linked bis(amidinate): Synthesis, molecular structures, and reactivity for the polymerization of l-lactide

Article abstract of DOI:10.1039/b709310a

The steric effect of an amide group on the synthesis, molecular structures and reactivity of ytterbium amides supported by linked bis(amidinate) L (L = [Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃]) is reported. Reaction of LYbCl(THF)₂ with equimolar NaNHAr′ and NaNHAr (Ar′ = 2,6-Me₂C₆H₃; Ar = 2,6- ⁱPr₂C₆H₃), respectively, gave the corresponding monometallic amide complexes LYb(NHAr′)(DME) 1 and LYb(NHAr)(DME) 2, in which the linked bis(amidinate) is coordinated to the metal center as a chelating ligand. The similar reaction with NaN(SiMe ₃)₂ afforded a bimetallic amide complex (TMS) ₂NYb(L)₂YbN(TMS)₂ 3 formed through the rearrangement reaction of L induced by the bulky N(SiMe₃)₂ group. In complex 3 the two linked bis(amidinate)s act as bridging ancillary ligands to link two YbN(TMS)₂ species in one molecule. The definite molecular structures of 1-3 were provided by single-crystal X-ray analysis. Complexes 1-3 are efficient initiators for the polymerization of l-lactide, and their catalytic performance is highly dependent on the amido groups and molecular structures. The polymerizations initiated by complexes 1 and 2 proceeded in a living fashion as evidenced by the narrow polydispersities of the resulting polymers, together with the linear natures of the number average molecular weight versus conversion plots, while the polymerization system with complex 3 provided polymers with rather broad molecular weight distributions. This journal is The Royal Society of Chemistry.

Full text of DOI:10.1039/b709310a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Ytterbium amides of linked bis(amidinate): synthesis, molecular structures,
and reactivity for the polymerization of L-lactide†
Junfeng Wang, Tao Cai, Yingming Yao, Yong Zhang and Qi Shen*
Received 19th June 2007, Accepted 31st August 2007
First published as an Advance Article on the web 10th September 2007
DOI: 10.1039/b709310a
The steric effect of an amide group on the synthesis, molecular structures and reactivity of ytterbium
amides supported by linked bis(amidinate) L (L = [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]) is
reported. Reaction of LYbCl(THF)2 with equimolar NaNHArꢀ and NaNHAr (Arꢀ = 2,6-Me2C6H3;
Ar = 2,6-iPr2C6H3), respectively, gave the corresponding monometallic amide complexes
LYb(NHArꢀ)(DME) 1 and LYb(NHAr)(DME) 2, in which the linked bis(amidinate) is coordinated to
the metal center as a chelating ligand. The similar reaction with NaN(SiMe3)2 afforded a bimetallic
amide complex (TMS)2NYb(L)2YbN(TMS)2 3 formed through the rearrangement reaction of L
induced by the bulky N(SiMe3)2 group. In complex 3 the two linked bis(amidinate)s act as bridging
ancillary ligands to link two YbN(TMS)2 species in one molecule. The definite molecular structures of
1–3 were provided by single-crystal X-ray analysis. Complexes 1–3 are efficient initiators for the
polymerization of L-lactide, and their catalytic performance is highly dependent on the amido groups
and molecular structures. The polymerizations initiated by complexes 1 and 2 proceeded in a living
fashion as evidenced by the narrow polydispersities of the resulting polymers, together with the linear
natures of the number average molecular weight versus conversion plots, while the polymerization
system with complex 3 provided polymers with rather broad molecular weight distributions.
of its steric and electronic effect. So, there is much scope for
designing novel lanthanide amides as efficient initiators in lactide
Introduction
There is currently considerable interest in developing structurally
polymerization.
well-characterized lanthanide complexes as homogeneous cat-
Linked bis(amidinate)s are an alternative “NNNN” tetraden-
alysts for the ring-opening polymerization of lactide, due to
the biodegradable and biocompatible nature of polylactides and
tate ligand. The synthesis of the monomeric yttrium alkyl
complex, LYCH(SiMe3)2(THF)2 (L = [Me3SiNC(Ph)N(CH2)3NC-
(Ph)NSiMe3]), was first reported in 2001,4a then the analogous
cyclopentadienyl ytterbium complex, LYbCp(DME), was synthe-
sized by us.4b However, the application of this class of ligand in
designing lanthanide complexes as efficient initiators has been
ignored to date. Here we would like to report novel lanthanide
amide initiators for controlled polymerization of L-lactide by the
combination of a linked bis(amidinate) ligand with a ytterbium
amide unit. The synthesis of these amide complexes and the effect
of the amido group on the molecular structures will be presented.
their wide applications in medicine, pharmaceutics and tissue
engineering.1 Lanthanide alkoxides are well-known to be highly
efficient initiators for the ring-opening polymerization of lactide.2
In particular, lanthanide alkoxides supported by polydentate lig-
ands, such as tetradentate bis(phenolate) containing heteroatoms
“ONNO”,2c “ONOO”,2d,2e exhibit a high degree of control over
the polymerization of lactide especially of the stereochemistry of
the lactide polymerization.2d,2e
Lanthanide amide complexes are another class of initiators
which have recently attracted much attention.2d,2e,3 A growing
number of structurally characterized silylamide complexes were
synthesized and found to initiate lactide polymerization with high
reactivity.2d,2e,3a–3f However, only some of the lanthanide amide
initiators afforded polylactides with predicted molecular weights
and narrow molecular weight distributions. These were the amides
bearing tetradentate bis(phenolate) “ONOO”,2d,3d “ONOO”,2d
and “OSSO”.3e,3f The catalytic performance of lanthanide amides
is well-known to depend on both amide, being the initiating
group, and the ancillary ligand around the metal center which
tunes the catalytic activity, controllability and selectivity by virtue
Results and discussion
Synthesis and characterizations of complexes 1–3
The reaction of LYbCl(THF)2,4b which was synthesized according
to the literature method, with 1 equiv. of sodium amide, NaNHArꢀ
(Arꢀ = 2,6-Me2C6H3), took place smoothly and a color change
from light yellow to orange was immediately observed. After
removal of NaCl and treatment with DME the reaction mixture
afforded the DME-solvated amide complex 1 as yellow crystals
in 55% yield upon crystallization (Scheme 1). The addition of
DME facilitated crystallization. The same metathesis reaction
with the more bulky amide salt, NaNHAr (Ar = 2,6-iPr2C6H3),
yielded the analogous amide complex 2 as orange crystals in 60%
yield (Scheme 1). IR spectra of both complexes showed a strong
Key Laboratory of Organic Synthesis of Jiangsu Province, Department of
Chemistry and Chemical Engineering, Dushu Lake Campus, Suzhou Uni-
versity, Suzhou 215123, People’s Republic of China. E-mail: qshen@suda.
edu.cn; Fax: +86-512-65880305; Tel: +86-512-65880306
† CCDC reference numbers 622179, 651215 and 651216. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b709310a
absorption of C N stretch at approximately 1640 cm−1 indicating
=
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Scheme 1
the delocalized double bond of the N–C–N linkage.5 1H NMR
spectra for both complexes were not available as ytterbium(III) is
paramagnetic.
The same structural motif, with
a
chelating linked
bis(amidinate) ligand, for 1 and 2 was further confirmed by X-ray
diffraction analysis. Their solid state structures are shown in
Fig. 1 and 2, respectively. Selected bond parameters are listed
in Table 1. The central metal in each complex coordinates to four
Fig. 2 OPTEP diagram of complex 2 showing atom-numbering scheme.
Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms
are omitted for clarity.
nitrogen atoms from the chelating linked bis(amidinate) ligand,
one nitrogen atom from the amide group and two oxygen atoms
from a solvated DME molecule to adopt a distorted trigonal
bipyramidal geometry, if the amidinate ligand is considered to
occupy a single coordination vertex. The centers of the two
amidinate groups and the oxygen atom O(1) occupy equatorial
positions, while the nitrogen atom N(5) and the other oxygen
atom O(2) are located at axial positions. The N(5)–Yb(1)–O(2)
angle is distorted from the idealized 180◦ to 149.91(16)◦ for 1 and
147.63(12)◦ for 2. Almost no difference in the Yb–N(amidinate)
bond distance between 1 and 2 was observed. In both complexes
the amidinate groups are each unsymmetrically bound to the
2
central metal in g fashion through the nitrogen atoms in the
Yb–N–C–N plane with a slight deviation (largest deviation is
˚
0.0970 A of plane Yb(1)–N(1)–C(1)–N(2) in complex 2). A
substantial difference in Yb–N bond distance for the amidinate
nitrogens attached to the bridge compared to those attached to
the SiMe3 group was observed for both complexes (the former
Fig. 1 OPTEP diagram of complex 1 showing atom-numbering scheme.
Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms
are omitted for clarity.
˚
˚
being 0.171 A shorter for 1, 0.166 A for 2). The angle between the
two Yb–N–C–N planes is 30.8◦ for 1 and 37.2◦ for 2 indicating
a more open sphere around Yb both in 1 and 2 due to the
chelating ancillary ligand. The same feature was found in the
related complexes LY[CH(SiMe3)2](THF),4a LYbC5H5(DME)4b
and a bridged aminotroponiminate lanthanum complex4c reported
◦
˚
Table 1 Selected bond lengths (A) and bond angles ( ) for complexes 1
and 2
1
2
Yb(1)–N(1)
Yb(1)–N(2)
Yb(1)–N(3)
Yb(1)–N(4)
Yb(1)–N(5)
Yb(1)–C(1)
Yb(1)–C(2)
N(1)–C(1)
N(2)–C(1)
N(3)–C(2)
N(4)–C(2)
2.491(5)
2.301(5)
2.448(5)
2.307(4)
2.229(4)
2.776(5)
2.780(5)
1.343(7)
1.312(7)
1.341(7)
1.324(7)
2.432(4)
2.317(3)
2.514(4)
2.287(4)
2.214(3)
2.738(4)
2.814(4)
1.344(6)
1.317(6)
1.333(5)
1.317(6)
˚
previously. The Yb–N(amide) bond distance of 2.214(3) A for 1
˚
and 2.229(4) A for 2 is slightly shorter than those for the ytterbium
amide complexes reported previously,6 when the difference in ionic
radii amongst Yb metals with different coordination numbers is
considered.
Treatment of LYbCl(THF)2 with the most bulky amide salt
NaN(SiMe3)2 in comparison with NaNHArꢀ and NaNHAr (Arꢀ =
2,6-Me2C6H3; Ar = 2,6-iPr2C6H3) mentioned above generated the
bimetallic amide complex 3 as colorless crystals in 56% yield
upon crystallization (Scheme 1). The identity of 3 was established
N(2)–Yb(1)–N(1)
N(4)–Yb(1)–N(3)
N(5)–Yb(1)–O(2)
56.11(15)
56.62(15)
149.91(16)
56.88(12)
55.48(12)
147.63(12)
1
by X-ray crystallography (discussed below), although H NMR
spectra of 3 were unavailable due to the paramagnetism of
5276 | Dalton Trans., 2007, 5275–5281
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ytterbium(III). Obviously, the formation of 3 indicates that a ligand
rearrangement reaction occurred during the metathesis reaction.
A ligand rearrangement reaction for a bridged bis(amidinate)
ligand has never been reported before, although a similar lig-
and rearrangement reaction for a linked bis(cyclopentadienyl)
ligand was described previously.7,8 The occurrence of the ligand
rearrangement reaction in our case may be attributed to the
overcrowded environment around the central metal which would
result upon formation of the corresponding monometallic amide,
LYbN(SiMe3)2, due to the steric hindrance induced by the
bulky N(SiMe3)2 group. The same reaction pathway as that
described for the bimetallic lanthanocene hydride7,8 was proposed
to explain the formation of 3. The attack of NaN(SiMe3)2
results in the cleavage of one of the linked bis(amidinate)–Yb
bonds, followed by rearrangement to another Yb atom. The
linked bis(amidinate) ligand here acts as bridging linked dianions
to link the two Yb-NTMS moieties in one molecule. Such a
coordination mode for a linked bis(amidinate) with a flexible –
CH2CH2– bridge was found previously in a bimetallic titanium
complex meso/rac-{CpTi(Me)2[N(tBu)C(Me)N(CH2CH2)–]}2.9
In that case the bimetallic titanium complex was formed from the
reaction of an a,x-bis(carbodiimide) with two equiv. of CpTiMe3
by CH4 elimination.
which the two Yb-NTMS groups are linked together by the
two bridged bis(amidinate) ligands. The overall geometry of the
molecule can be described as a boat-like structure with two amide
groups lying on the same up sites. Each metal coordinates to two
amidinate groups from the two bridging linked bis(amidinate)
ligands and one nitrogen atom from the amide group to form
a trigonal planar structure with the bisector of the amidinate
ligands (Yb(1A)–C(1A) and Yb(1A)–C(24A); Yb(2A)–C(2A) and
Yb(2A)–C(25A) vectors) defining two vertices and the metal–
amide linkage defining the third vertex. The angles defined by
these vectors sum to 360◦. All Yb–N distances for the amidinate
˚
nitrogen attached to the bridge and to SiMe3 are around 2.29 A
˚
for Yb(1A) and 2.30 A for Yb(2A), no significant difference
between them is observed, which is a marked contrast with
those in 1 and 2, as well as in an alkyl yttrium complex,4a but
quite similar to those found in the bis(amidinate) complex, [(p-
MeOC6H4)C(NSiMe3)2]2Y[CH(SiMe3)2] 4, supported by the two
monoanionic amidinate ligands.9 The angles between the planes
of Yb(1A)–N(1A)–C(1A)–N(2A) and Yb(1A)–N(5A)–C(24A)–
N(6A) and the planes of Yb(2A)–N(7A)–C(25A)–N(8A) and
Yb(2A)–N(3A)–C(2A)–N(4A) are 83.6◦ and 85.2◦, respectively,
which are much larger than 30.8◦ in 1 and 37.2◦ in 2. The angles
are also larger than 75.6◦ for the related monometallic complex
4.10 The much larger angles found in complex 3 might be attributed
to the demand of the bimetallic structure. This may also be the
reason as to why 3 is a solvent free complex, while complexes 1 and
2 allow a coordinated DME molecule. The two Y–N(amide) bond
Crystals of 3 suitable for an X-ray structure determination
were obtained from a mixture of hexane–Et2O. The molecular
structure of 3 is shown in Fig. 3. Selected bond parameters
are listed in Table 2. Complex 3 has a bimetallic structure, in
˚
distances are almost equal giving an average of 2.19 A which is
˚
apparently shorter than 2.34 A found in monometallic guanidinate
amide [C6H11NC(N(SiMe3)2)NC6H11]2YbN(SiMe3)2,11 but longer
˚
˚
than that found in complexes 1 (2.21 A) and 2 (2.23 A), taking
into account the difference in ionic radii resulting from the
different coordination number. The bimetallic structure of 3 in
comparison with 1 and 2 shows that coordinate geometry of a
linked bis(amidinate) can be modified by using a different steric
bulky reagent.
Polymerization of L-lactide initiated by complexes 1–3
All complexes have been tested as initiators for the polymerization
of L-lactide. The results obtained under various conditions are
listed in Table 3. All the complexes were found to be highly active
initiators for the polymerization of L-lactide in toluene. However,
these complexes showed much lower reactivity in THF compared
with that in toluene (Table 3, entries 7, 15, and 19), presumably
because of competitive coordination between the monomer and
THF.2d,3c,3d
Fig. 3 OPTEP diagram of complex 3 showing atom-numbering scheme.
Thermal ellipsoids are drawn at the 10% probability level. Hydrogen atoms
and free solvent are omitted for clarity.
To further address the influence of the amido group on the
polymerization characterizations of lanthanide amides, a series
of experiments was carried out at 60 ◦C in toluene. Complex 2
allowed complete conversion of 300 equiv. of L-lactide to polymer
◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) for complex 3
◦
within 1 min at 60 C in toluene at a concentration of L-lactide
of 1 mol L−1 (entry 8), while complexes 1 and 3 afforded the
conversion of 54% and trace amounts respectively under the same
conditions (entries 2 and 16). For complex 2 the polymer yield
was still as high as 95% within 7 min though the molar ratio
of monomer to initiator increased to 500 (entry 13). The activity
sequence for the amido group is –N(SiMe3)2 < –NHArꢀ < –NHAr.
The influence of the amido groups on the molecular weight
Yb(1A)–N(1A)
Yb(1A)–N(2A)
Yb(1A)–N(5A)
Yb(1A)–N(6A)
Yb(1A)–N(9A)
2.30(1)
2.30(1)
2.28(1)
2.29(1)
2.18(1)
Yb(2A)–N(3A)
Yb(2A)–N(4A)
Yb(2A)–N(7A)
Yb(2A)–N(8A)
Yb(2A)–N(10A)
2.26(1)
2.31(1)
2.30(1)
2.31(1)
2.19(1)
N(2A)–Yb(1A)–N(1A)
N(5A)–Yb(1A)–N(6A)
59.0(4)
58.8(4)
N(3A)–Yb(2A)–N(4A)
N(7A)–Yb(2A)–N(8A)
59.7(4)
58.6(4)
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Table 3 Polymerization of L-lactide initiated by complexes 1–3a
T/◦C
[M]0 : [I]0
t/min
Yield (%)c
Mn × 104d
Mn × 104e
PDI
b
Entry
Initiator [I]
1
2
3
4
5
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
3
3
3
3
60
60
60
60
70
80
60
60
60
60
60
60
60
60
60
60
60
60
60
300
300
300
300
300
300
300
300
500
500
500
500
500
600
300
300
300
300
300
0.5
1
1.5
3
1
1
1
1
20 s
1.5
3
5
7
5
1
1
5
10
10
30
54
65
92
70
85
—
100
15
50
75
89
95
23
—
Trace
60
100
—
3.30
6.10
6.90
10.9
7.77
9.20
—
4.10
0.83
3.80
5.80
6.32
6.50
2.79
—
1.30
2.33
2.81
3.97
3.02
3.67
—
4.32
1.08
3.60
5.40
6.41
6.84
1.98
—
1.32
1.33
1.32
1.35
1.32
1.25
—
1.31
1.30
1.30
1.30
1.26
1.29
1.22
—
6
7f
8
9
10
11
12
13
14
15f
16
17
18
19f
—
—
—
3.30
6.00
—
2.59
4.32
—
1.89
2.01
—
a General polymerization conditions: in toluene, [LLA] = 1 mol L−1
conv. × [M]0/[I]0. f In THF.
.
b [M]0 : [I]0 = [monomer] : [initiator]. c Yield: weight of polymer obtained/weight of
monomer used. d Measured by GPC calibrated with standard polystyrene samples. e Mn value calculated from the relationship: molecular weight of M ×
and molecular weight distribution of the resulting polymers was
also observed. All the polymers obtained by complexes 1–3
have unimodal molecular weight distributions. However, the
molecular weight distribution indices of the polymers are diverse,
ranging from 1.22–2.01 depending on the complex used. The
polymerization systems with complexes 1 and 2 gave polymers
with narrow molecular weight distributions ranging from 1.22
to 1.35 (entries 1–6 and 8–14), while the polymerization by
complex 3 yielded polymers with rather broad molecular weight
distributions (1.89–2.01, entries 17 and 18). The reason for this
may be because the former two complexes acted as single-
active site initiators, but complex 3 played the role of a double-
active site initiator due to the two Yb–N(SiMe3)2 active groups
participating in the initiation. The polydispersity indices of the
polymers resulting from systems using initiators 1 and 2 are
lower than the usual values found for lanthanide amides (ranging
from 1.32–1.72)3b indicating that the two systems have “living
Fig. 4 Plot of reaction time versus ln[M]0/[M] initiated by 1 and 2.
character”. Moreover, a difference in the molecular weights of
Conditions: [M]0/[I]0 = 300 for 1; [M]0/[I]0 = 500 for 2, [LA]0 = 1 mol,
the polymers resulting from the system using complex 1 compared
to those obtained with complex 2 was observed. The experimental
number molecular weights obtained by complex 2 are close to the
calculated values, while the experimental values for the case of
complex 1 are larger than the calculated values (entries 1–6 and
8–14). This result indicated that polymerization with complex 2
proceeded in better controlled mode compared to the system with
complex 1.
60 ◦C, Ar.
polymers increased proportionally with the monomer conversions,
while the molecular weight distributions (Mw/Mn) of the resulting
polymers remained narrow (Mw/Mn ≤ 1.35) and intact (Fig. 5).
All the results indicated that the polymerization of lactide initiated
by 1 and/or 2 proceeded in a controlled mode.
The controlled polymerization of lactide with lanthanide amides
without addition of alcohol as an initiator was negligible. These
results indicated that substantial differences in geometry and
catalytic behavior can be achieved by the combination of a linked
bis(amidinate) ligand and lanthanide active species.
The polymerization kinetics at 60 ◦C in toluene with the
molar ratio of monomer : initiator of 300 for 1 and 500 for
2, respectively, were conducted. The conversion increased with
polymerization time and the pseudo first order kinetic plots
of ln([M]0/[M]) versus polymerization time were observed for
both systems as shown in Fig. 4. The linear semi logarithmic
plots indicate the concentration of the catalytically active species
remained constant throughout the polymerization. In both cases
the number average molecular weights (Mn) of the resulting
Conclusion
We have synthesized and structural characterized ytterbium
amides complexes 1–3 bearing chelating or bridged bis(amidinate)
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temperature. The volatiles were removed under reduced pressure
and the residue was extracted with hot DME (2 × 50 mL) and
the volume of the extract reduced to 40 mL. Cooling the solution
◦
to 0 C afforded the yellow crystals (0.89 g, 55% based on Yb),
mp 163–165 ◦C (Found: C, 52.41; H, 6.95; N, 8.35; Yb, 21.39.
C35H54N5O2Si2Yb requires C, 52.15; H, 6.75; N, 8.69; Yb, 21.47);
mmax/cm−1 2960 m, 1642 s, 1608 s, 1569 s, 1541 s, 1488 m, 1438 m,
1384 s, 1084 s, 899 w, 782 m, 746 m, 700 m (KBr pellet).
Synthesis of LYbNHAr (DME) (Ar = 2,6-iPr2C6H3) (2)
Following a procedure similar to that describe for the preparation
of complex 1, treatment of a solution of LYbCl(THF)2 (1.55 g,
2 mmol) in THF (30 mL) with one equiv. of NaNHAr (2 mmol,
0.60 M) in THF afforded complex 2 as orange crystals (1.03 g,
60% based on Yb), mp 190–192 ◦C (Found: C, 54.61; H, 7.66; N,
8.41; Yb, 20.19. C39H62N5O2Si2Yb: C, 54.33; H, 7.25; N, 8.12; Yb,
20.07); mmax/cm−1 2947 m, 1642 s, 1607 s, 1567 m, 1541 s, 1467 m,
1437 m, 1384 s, 1084 s, 899 w, 781 m, 700 m (KBr pellet).
Fig. 5 Plot of polymer yield versus Mn and Mw/Mn initiated by 1 and 2.
Conditions are the same as in Fig. 4.
ligands depending on the amido group. For amido groups HNArꢀ
and HNAr the monometallic complexes 1 and 2 with a linked
bis(amidinate) ligand were synthesized, while for the bulky amido
group N(SiMe3)2 the bimetallic complex 3 supported by two
bridged bis(amidinate) ligands was prepared. Complexes 1 and
2 were found to initiate the polymerization of L-lactide in a con-
trolled mode which was confirmed by the results of polymerization
kinetics, andthelinear relationshipbetween molecular weights and
conversions, and the narrow molecular weight distributions of the
resulting polymers (Mw/Mn = 1.22–1.35). The results presented
here indicated linked bis(amidinate) ligand systems may have
potential applications in designing controlled lanthanide catalyst
for homogeneous catalysis.
Synthesis of (TMS)2NYb(L)2YbN(TMS)2 (3)
A solution of NaN(TMS)2 (2 mmol, 1 M) in THF was added to
a stirring solution of LYbCl(THF)2 (1.55 g, 2 mmol) in THF
(30 mL). The reaction mixture was stirred for 48 h at room
temperature. The volatiles were removed under reduced pressure
and the residue was extracted with Et2O (2 × 40 mL). The volume
of the extract was reduced to 10 mL followed by addition of
1 mL hexane. Cooling the solution to 0 ◦C for crystallization
afforded complex 3 as colorless crystals (0.85 g, 56% based on
Yb), mp 95–97 ◦C (Found: C, 46.56; H, 7.18; N, 8.91; Yb, 22.55.
C58H104N10Si8Yb2: C, 46.06; H, 6.93; N, 9.26; Yb, 22.88); mmax/cm−1
2955 m, 1635 s, 1559 s, 1497 m, 1381 s, 1250 m, 1211 w, 964 w,
833 m, 779 m, 748 m, 702 m (KBr pellet).
Experimental
Polymerization of L-lactide
General
A typical procedure for polymerization of L-lactide was performed
in a 25 mL round-bottom flask. To a stirred solution of L-lactide
(0.43 g, 3 mmol) in toluene (2.4 mL), a toluene solution of complex
2 (0.6 mL, [M]0 : [I]0 = 500 : 1) was added with a syringe. The
reaction mixture was stirred for 7 min at 60 ◦C (Table 3, entry 13)
and then quenched with an ethanol solution containing a small
amount of hydrochloric acid (5%). The polymer precipitated from
ethanol and was washed with ethanol three times and dried under
vacuum (0.41 g, 95%).
All manipulations and reactions were performed under a purified
argon atmosphere using standard Schlenk techniques. Solvents
were degassed and distilled from sodium benzophenone ketyl
under argon before use. L-Lactide was purchased from Acros.
4b
LYbCl(THF)2 was prepared according to the literature proce-
dure. All other reagents were purchased from Acros and used
as received without further purification. The IR spectra were
recorded on a Magna-IR 550 spectrometer. Melting points were
determined in a sealed Ar-filled capillary tube, and uncorrected.
Lanthanide analyses were carried out by complexometric titra-
tions. Carbon, hydrogen and nitrogen analyses were preformed by
direct combustion with a Carlo-Erba EA 1110 instrument. Molec-
ular weight and molecular weight distributions were determined
against polystyrene standard by gel permeation chromatography
(GPC) on a Waters 1515 apparatus with three HR columns (HR-1,
HR-2 and HR-4). THF was used as an eluent at 30 ◦C.
X-Ray crystallography†
Diffraction data were collected on a Rigaku Mercury CCD area
˚
detector in x scan mode using Mo-Ka radiation (k = 0.71070 A).
X-Ray quality crystals of 1, 2 and 3·0.75(Et2O)·0.25(C6H14) were
obtained directly from the above preparations. A yellow block of
1 with dimensions 0.60 × 0.42 × 0.30 mm, an orange block of 2
with dimensions 0.62 × 0.43 × 0.17 mm and a colorless block of 3
with dimensions 0.24 × 0.15 × 0.14 mm were each mounted in a
sealed capillary. Diffraction data were collected using the x mode
with a detector distance of 35 mm to the crystals. A total of 720
oscillation images for each were collected in the range 6.01◦ < 2h <
50.70◦ for 1, 6.01◦ < 2h < 50.70◦ for 2 and 6.06◦ < 2h < 50.70◦
Synthesis of LYb(NHArꢀ)(DME) (Arꢀ = 2,6-Me2C6H3) (1)
To a stirred solution of LYbCl(THF)2 (1.55 g, 2 mmol) in THF
(30 mL) was added a solution of NaNHArꢀ (2 mmol, 0.50 M)
in THF. The reaction mixture was stirred for 48 h at room
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Table 4 Details of the crystallographic data and refinements for complexes 1–3
1
2
3·0.75(Et2O)·0.25(C6H14)
Empirical formula
FW
T/K
C35H54N5O2Si2Yb
806.05
223(2)
Monoclinic
P21/c
23.288(2)
8.6725(7)
20.4441(19)
90
111.162(2)
90
3850.6(6)
4
1.390
C39H62N5O2Si2Yb
862.16
193(2)
Monoclinic
P21/c
24.670(3)
8.9342(11)
19.672(3)
90
104.525(3)
90
4197.1(9)
4
1.364
C62.5H115N10O0.75Si8Yb2
1589.36
193(2)
Triclinic
¯
P1
17.806(2)
18.419(3)
29.290(5)
80.666(8)
72.924(8)
66.204(7)
8392(2)
4
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/mg m−3
1.290
0.0888
58 140
R(int)
0.0403
36 132
0.0481
39 439
No. of reflns. collcd.
No. of indep. collcd.
7051
1.189
0.0477
0.0945
7639
1.154
0.0368
0.0746
30 041
1.091
0.1022
0.1740
GOF
R
wR
for 3·0.75(Et2O)·0.25(C6H14). The collected data were reduced by
the program CrystalClear (Rigaku and MSC, ver. 1.3, 2001), and
absorption corrections (multi-scan) were applied, which resulted
in transmission factors ranging from 0.264 to 0.469 for 1, 0.3269
to 0.6935 for 2 and 0.540 to 0.718 for 3·0.75(Et2O)·0.25(C6H14).
The diffracted intensities were corrected for Lorentz polarization
effects and empirical absorption corrections.
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The structures were solved by direct methods12 and refined
by full-matrix least-squares procedures based on |F|2.13 When
crystal 3·0.75(Et2O)·0.25(C6H14) was separated from its mother
liquor, rapid evaporation of part of the solvated molecules in the
crystal was observed. Although numerous attempts were made,
this crystal always weakly diffracted, especially at high angles,
which made the final R value relatively higher. Therefore, one
of the Et2O and n-C6H14 molecules in 3·0.75(Et2O)·0.25(C6H14)
were refined with occupancy factors of 0.5 to give reasonable
temperature factors. For 1 and 2, C(4) atoms were found to be
disordered over two positions and were refined using disorder
models with ratios of 0.58 : 0.42 for 1 and 0.65 : 0.35 for
2. All the non-hydrogen atoms, except the solvent molecules
in 3·0.75(Et2O)·0.25(C6H14), were refined anisotropically. The
hydrogen atoms on C3, C4, C5 in 1, 2 and the solvent molecules in
3·0.75(Et2O)·0.25(C6H14) were not located, other hydrogen atoms
˚
were placed in geometrically idealized positions (C–H = 0.981 A
˚
for methyl groups, C–H = 0.99 A for methylene groups and C–H =
˚
0.95 A for phenyl groups) and constrained to ride on their parent
atoms with Uiso(H) = 1.5 Ueq(C) for methyl groups and Uiso(H) =
1.2 Ueq(C) for methylene and phenyl groups. All the calculations
were performed on a PC computer using a SHELXL-97 software
package.12 Details of the intensity data collection and crystal data
are given in Table 4.
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Acknowledgements
Financial support from the National Nature Science Foundation
of China is gratefully acknowledged.
5280 | Dalton Trans., 2007, 5275–5281
This journal is
The Royal Society of Chemistry 2007
©

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This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5275–5281 | 5281
©