Syntheses and structures of lanthanide borohydrides supported by a bridged bis(amidinate) ligand and their high activity for controlled polymerization of ε-caprolactone, l-lactide and rac-lactide

Article abstract of DOI:10.1039/c2dt30096f

Metathesis reaction of LLnCl(THF)₂ [L = (Me ₃SiNC(C₆H₅)N)₂(CH₂) ₃] with NaBH₄ in a 1:1.5 molar ratio in THF (THF = tetrahydrofuran) at 60 °C afforded the monoborohydride LLn(BH ₄)(DME) [Ln = Y (1), Nd (2), Sm(3) and Yb(4)] crystallized from DME solution (DME = dimethoxyethane). Crystal structure analyses revealed 1-4 are monomers, in which each metal is ligated by one L ligand, one η³-BH₄ group and one DME molecule in a trigonal bipyramid geometry. Complexes 1-4 were found to be very active single-site initiators for the controlled ring opening polymerization of ε-caprolactone (ε-CL) and l-lactide (l-LA) as judged by relatively narrow molecular weight distributions (M_w/M_n: 1.34-1.50) and experimental values M_n(exp) were in good agreement with theoretic values M _n(theo). The highest activity and the best control over the molecular weight for both monomers were found for the system with 2. These monoborohydride complexes can also initiate the ring opening polymerization of rac-LA to gave heterotactically enriched polyLA with Pr (heterotactic enrichment) values in a range of 0.69-0.85 depending on the lanthanide metals and the most effective heterotactic enrichment (Pr) was found for 1 (Pr = 0.85). Moreover, complex 1 initiated the polymerization of rac-LA in a living fashion.

Full text of DOI:10.1039/c2dt30096f

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PAPER
Syntheses and structures of lanthanide borohydrides supported by a bridged
bis(amidinate) ligand and their high activity for controlled polymerization of
ε-caprolactone, L-lactide and rac-lactide†
Wenbo Li,a Mingqiang Xue,a Jing Tu,a Yong Zhanga and Qi Shen*a,b
Received 13th January 2012, Accepted 5th April 2012
DOI: 10.1039/c2dt30096f
Metathesis reaction of LLnCl(THF)2 [L = (Me3SiNC(C6H5)N)2(CH2)3] with NaBH4 in a 1 : 1.5 molar
ratio in THF (THF = tetrahydrofuran) at 60 °C afforded the monoborohydride LLn(BH4)(DME) [Ln = Y
(1), Nd (2), Sm(3) and Yb(4)] crystallized from DME solution (DME = dimethoxyethane). Crystal
structure analyses revealed 1–4 are monomers, in which each metal is ligated by one L ligand, one η3-
BH4 group and one DME molecule in a trigonal bipyramid geometry. Complexes 1–4 were found to be
very active single-site initiators for the controlled ring opening polymerization of ε-caprolactone (ε-CL)
and L-lactide (L-LA) as judged by relatively narrow molecular weight distributions (Mw/Mn: 1.34–1.50)
and experimental values Mn(exp) were in good agreement with theoretic values Mn(theo). The highest
activity and the best control over the molecular weight for both monomers were found for the system with
2. These monoborohydride complexes can also initiate the ring opening polymerization of rac-LA to gave
heterotactically enriched polyLAwith Pr (heterotactic enrichment) values in a range of 0.69–0.85
depending on the lanthanide metals and the most effective heterotactic enrichment (Pr) was found for 1
(Pr = 0.85). Moreover, complex 1 initiated the polymerization of rac-LA in a living fashion.
The initiating active group X in these catalysts is, typically, an
aryloxide, alkoxide, secondary amide, or alkyl moiety. The pio-
Introduction
Polyesters have wide applications in the medical and material
fields as biodegradable surgical sutures or a delivery medium for
control release of drugs and as thermoplastic materials due to
their biodegradable, biocompatible and permeable properties.1
The controlled ring opening polymerization of cyclic esters,
such as ε-CL, LA and cyclic carbonate, is an efficient route for
the synthesis of aliphatic polyesters with predictable molecular
weight, low polydispersity index (PDI) and control over stereo-
chemistry, as well as co-monomer incorporation.2 Thus, signifi-
cant efforts over the past few decades have been placed toward
the design and synthesis of well-characterized single site lantha-
nide catalysts L′mLnX (L′ = ancillary ligand, m = 1 or 2, X =
initiating group), as single site lanthanide catalysts can initiate
well-controlled polymerization of cyclic esters and may also
allow the characterization of the initiator precursor or the active
species and thus further mechanistic studies.
neering work by Guillaume et al. has revealed borohydride is
also a valuable active group either without supporting ancillary
ligands or as the metallocene complex.3 Since then, great atten-
tion has been focused on the development of lanthanide mono-
borohydride complexes with various ancillary ligands as a single
site initiator in ring opening polymerization of cyclic esters.4
This is because the ancillary ligands around the Ln–BH4 species
play a key role in determining the efficiency and degree of
control of lanthanide borohydride species in the ring-opening
polymerization of cyclic esters.
The samarium monoborohydride complex [Sm(C5Me5)2(BH4)-
(THF)] has first been used for the controlled ring opening
polymerization of ε-CL. The system yielded polymers with a
molecular weight distribution index (Mw/Mn) of around 1.30 and
deviation between Mn(exp) and Mn(theo) was observed when the
polymerization gave a molar ratio of [ε-CL]0/[I]0 of more than
400.3c The polymerization of ε-CL with bis(phosphinimino)-
methanide borohydride complexes at 0 °C in THF afforded poly-
mers with narrower distributions (Mw/Mn: 1.06–1.11).4i The
polymerizations of rac-LA catalyzed by borohydride lanthanide
complexes supported by bulky guanidinate ligands provided
atactic polymers with a good degree of control Mw/Mn.4d It is
worth noting that the lanthanide monoborohydride complexes
bearing tetradentate bis(phenolate) ligands showed combined
high activity, good degree of control and selectivity. The
polymerization with these complexes gave heterotactically
aKey Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou 215123, People’s Republic of China. E-mail:
qshen@suda.edu.cn; Fax: +86-512-65880305; Tel: +86-512-65880306
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032,
People’s Republic of China
†Electronic supplementary information (ESI) available. CCDC 843489
(for 1), 843490 (for 2), 843491 (for 3) and 843492 (for 4). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2dt30096f
7258 | Dalton Trans., 2012, 41, 7258–7265
This journal is © The Royal Society of Chemistry 2012

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enriched poly(rac-LA) with Pr values up to 0.87.4f Recent
results reveal the stereoselective polymerization of rac-LA is
highly sensitive to ligand sterics.5 Therefore, we were interested
in the synthesis of lanthanide monoborohydride with new
ligands and investigating the effect of the lanthanide borohydride
architecture on activity and selectivity of lactide polymerization.
Bridged bis(amidinate) dianions can be viewed as one kind of
tetradentate “NNNN” ligands, which have the advantages of
tunable electronic and steric effects. However, the usage of the
bridged bis(amidinate) ligands in the design of single site lantha-
nide catalysts has still been limited to date. The bis(amidinate)
ligand L (L = (Me3SiNC(C6H5)N)2(CH2)3) has first been used
for the synthesis of a lanthanide alkyl complex.6 Recently, we
reported that the ligand provides a useful coordination environ-
ment, which allows the synthesis of isolable lanthanide amides,7
alkoxides and aryloxides,8 and these derivatives can all serve as
highly active single site initiators to initiate the polymerization
of L-LA and ε-CL in a living fashion.7,8
Taking this background into account, we attempt to synthesize
lanthanide monoborohydride stabilized by this ligand L and then
evaluate the performance of the new monoborohydride com-
plexes as single-site initiators for ring-opening polymerization of
cyclic esters. It was found that the lanthanide monoborohydride
supported by L, LLn(BH4)(DME) [Ln = Y (1), Nd (2), Sm (3)
and Yb (4)], could be prepared in reasonable yields and all these
complexes can serve as highly active initiators for the ring-
opening polymerization of ε-CL, L-LA, and rac-LA in a living
manner. The polymerization of rac-LA with 1 in THF at room
temperature gave heterotactically enriched polyLA with Pr value
up to 0.85. Here we would like to report the results.
colorless crystals in 71% yield upon crystallization from DME at
−15 °C (Scheme 1).
The 1H NMR spectra of 1 showed a clear signal around
0.63 ppm assigned to all the hydrogen atoms of the BH4 group,
besides the expected sets of resonances for the amidinate ligand
and the coordinated DME molecule. The signals for the free
DME molecule in the unit cell could not be detected as the
sample was dried under vacuum for a long time and this DME
molecule was removed. The IR spectra of 1 exhibited two strong
absorption bands at 2200–2400 cm−1 assigned to η3-bridging
borohydride stretching vibration and terminal borohydride
stretching vibration.3,4,10 This is consistent with the crystal struc-
tural characterization.
The success in the synthesis of 1 promoted us to synthesize
the related lanthanide borohydrides by use of this procedure. The
reactions of LLnCl(THF)2 (Ln = Nd, Sm, Yb) with NaBH4 in a
1 : 1.5 molar ratio were carried out. All reactions went smoothly
and afforded the corresponding complexes LLn(BH4)(DME)
[Ln = Nd (2), Sm (3), Yb (4)] as purple crystals for 2, light
yellow crystals for 3 and yellow crystals for 4 in 69, 73 and 76%
yields, respectively (Scheme 1).
The IR spectra of 2–4 are similar to that of 1 and all show, in
the region 2200–2400 cm–1, two strong absorptions diagnostic
of the η3-bridging borohydride stretching vibration and terminal
borohydride stretching vibration.3,4,10 The 1H and 13C NMR
spectra of 3 in d8-THF at 25 °C show the expected sets of reso-
nances for the amidinate ligand and the coordinated DME mole-
cule. The free DME molecule could not be found as a volatile.
The borohydride group appeared as a multiplet at δ = −2.07 to
−2.4 ppm. Complexes 2–4 were further confirmed by X-ray
crystal structural analyses.
During the preparation of our manuscript, we have found an
online paper concerning the synthesis of naphthalene linked bis
(amidinate) lanthanide borohydride complexes [1,8-C10H6{NC-
(tBu)N-2,6-Me2-C6H3}2]Ln(BH4)(μ-BH4)Li(THF)2 (Ln = Nd,
Sm) and the catalytic activity for ring-opening polymerization of
rac-LA at 20 °C as single-site di-initiators with two borohydride
groups operative, however atactic polyLAs were obtained.9
Complexes 1–4 are soluble in THF and DME, but almost
insoluble in toluene. All complexes are sensitive to air and
moisture, and decompose immediately and release bubble expo-
sing in air, whilst they can be stored for several weeks without
decomposition at room temperature under dry inert gas.
Molecular structures of 1–4
Results and discussion
Complexes 1–4 were isolated as solvates 1·DME,2·DME,
3·DME and 4·DME. The monomeric structures of 1–4 are
shown in Fig. 1 as they are isostructural and isomorphorphic.
Selected bond distances and angles are listed in Table 1.
Synthesis of [LLn(BH4)(DME)] (1–4)
The reaction of lanthanide tris(borohydrides) Ln(BH4)3(THF)n
with anionic reagents is an efficient method for new derivatives.4
Thus, the preparation of LY(BH4) by use of Y(BH4)3(THF)n as a
starting reagent was tried. The reaction of Y(BH4)3(THF)n with
an equivalent of Li2L was conducted in THF at 60 °C for 2 days.
However, the reaction afforded LY(BH4)(DME) (1) in a low
The central metal in each complex coordinates to four nitrogen
atoms from the chelating L ligand, three bridged H atoms of the
tridentate BH4 group and two oxygen atoms from a DME mol-
ecule. The coordinate geometry around the lanthanum metal can
be described as a distorted trigonal bipyramid, if the BH4 group
and each amidinate ligand are all considered to occupy one site.
The centers of the two amidinate groups and the oxygen atom
O(2) occupy equatorial positions with the sum of the angles of
C–Ln–C and the two C–Ln–O of 347.68° for 1, 346.1° for 2,
346.35° for 3 and 347.96° for 4. The boron atom B(1) and the
other oxygen atom O(1) are located in the apices in a distorted
setup forming a B(1)–Ln–O(1) angle of 165.0(3)° for 1, 162.2(3)°
for 2, 161.18(14)° for 3 and 163.68(18)° for 4. The Ln–N bond
distance for the amidinate nitrogens attached to the bridge is
shorter than that of the amidinate nitrogens attached to the
yield (45%) upon crystallization from
(Scheme 1).
a DME solution
The metathesis reaction of LYCl(THF)2 with NaBH4 in a
1 : 1 molar ratio was then tried again in THF at 60 °C for 2 days
with the aim of getting a higher yield of 1. However, the reaction
went sluggishly and considerable amounts of the starting chlo-
ride and NaBH4 remained, which hampered the isolation and
purification of the target complex. Thus, the same reaction with
an increased amount of NaBH4 to 0.5-fold molar excess was
tried. Indeed, the reaction went smoothly and afforded 1 as
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Scheme 1 Syntheses of complexes 1–4.
Table 1 Selected bond distances (Å) and angles (°) for complexes 1–4
1
2
3
4
Ln–N(1)
Ln–N(2)
Ln–N(3)
Ln–N(4)
Ln–B(1)
Ln–C(1)
Ln–C(2)
Ln–H
2.477(5)
2.325(5)
2.466(5)
2.302(5)
2.503(8)
2.816(6)
2.780(6)
2.27
2.554(9)
2.409(8)
2.534(8)
2.384(9)
2.636(11)
2.877(11)
2.867(11)
2.49
2.527(5)
2.381(5)
2.510(5)
2.358(5)
2.630(4)
2.875(6)
2.852(5)
2.36
2.455(5)
2.300(5)
2.439(4)
2.272(5)
2.527(5)
2.790(6)
2.760(6)
2.463
N(1)–C(1)
N(2)–C(1)
N(3)–C(2)
N(4)–C(2)
N(2)–Ln–N(1)
N(4)–Ln–N(3)
B(1)–Ln–O(1)
N(1)–Ln–N(3)
1.324(8)
1.315(7)
1.314(8)
1.307(7)
55.63(17)
55.73(17)
165.0(3)
166.33(18)
1.297(13)
1.290(13)
1.335(14)
1.300(13)
53.1(3)
54.1(3)
162.2(3)
166.5(3)
1.308(8)
1.305(7)
1.319(7)
1.322(7)
53.74(16)
54.64(15)
161.18(14)
167.23(18)
1.318(7)
1.303(7)
1.309(8)
1.305(7)
55.75(17)
55.99(17)
163.68(18)
166.75(17)
Fig. 1 ORTEP diagram of the molecular structures of complexes 1–4.
Thermal ellipsoids are drawn at 30% probability level. All hydrogens
except for H atoms attached to B atom, free solvent molecules are
omitted for clarity.
To assess the catalytic behavior of the new 1–4 synthesized
above, their catalytic activity as single site initiators for the
polymerization of ε-CL, L-LA, and rac-LAwas studied.
SiMe3 group in 1–4. The same situation is observed in the other
derivatives bearing a bridged bis(amidinate) ligand.11 The
average Ln–N(amidinate) bond distances of 1–4 are 2.393,
2.470, 2.444 and 2.367 Å, respectively, which are comparable
with each other when the differences in ionic radii among these
metals were considered. The two N–C bond distances of each
amidinate ligand in each complex are almost consistent
(Table 1), reflecting the electron delocalization within the
anionic NCN unit. The distance of Ln to B (2.503(8) Å for 1,
2.636(11) Å for 2, 2.630(4) Å for 3 and 2.527(5) Å for 4) are
significantly shorter than the corresponding Ln–B distances of a
η2-BH4 group,4d which are characteristic of η3-BH4 ligands. The
average Ln–H bond distances of complexes 1–4 are 2.27, 2.49,
2.36 and 2.463 Å, respectively, which are comparable with the
complexes reported.4d Hence, the boron atoms in 1–4 are linked
to the lanthanide metal center via three bridging hydrogen
atoms.
Ring opening polymerization of ε-CL by 1–4
The catalytic activity of 1–4 toward the polymerization of ε-CL
was assessed in a toluene solution at 20 °C with a molar ratio of
monomer to initiator of 1000 ([M]0 = 0.5 M (M = mol L−1)).
The polymerization is fast, as shown in Table 2. The monomer
conversion is quantitative within 1 min for 2, 86% for 3, 79%
for 1 and 71% yield for 4, respectively. The observed decreasing
order of activity of Nd > Sm > Y > Yb is consistent with the
order of the ion radius, which is also observed for the boro-
hydride complexes with a tetradentate bis(phenolate) ligand.4f
Toluene is a better solvent as compared to THF (runs 2 and 5).
The PDIs (Mw/Mn) of the poly(ε-CL) formed with these com-
plexes, which were evaluated by GPC, are in the range of
1.34–1.37 (Table 2, runs 1–4). The measured number molecular
weights (Mn) of the resulting polymers with 1–4 are in accord-
ance with the calculated ones (Table 2, runs 1–4) for one
polymer chain growing per metal center. To gain further insight
into the degree of control of the ring opening polymerization
with 2, additional four polymerizations were carried out with
[ε-CL]0/[2]0 ratios of 200 : 1, 500 : 1, 800 : 1, and 1500 : 1 in
toluene at room temperature. All polymerizations proceeded fast
Catalytic activity of 1–4 for polymerization of ε-CL, L-LA and
rac-LA
The yttrium borohydride complex [Ph(NSiMe3)2]2Y(BH4)(THF)
supported by monoanionic amidinate ligands was reported by
Teuben and co-workers,12 however no catalytic activity for
polymerization of cyclic lactones has been featured.
7260 | Dalton Trans., 2012, 41, 7258–7265
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Table 2 Polymerization of ε-CL initiated by 1–4a
Run
Init.
[M]0/[I]0
Time (min)
Yieldb (%)
Mn(theo)c (104)
Mn(exp)d (104)
PDId
1
2
3
4
5e
6
7
8
9
1
2
3
4
2
2
2
2
2
1000
1000
1000
1000
1000
200
500
800
1500
1
1
1
1
2
1
1
1
2
79
>99
86
71
77
>99
>99
>99
>99
9.0
11.4
9.8
8.1
8.8
2.3
5.7
9.1
17.1
9.3
1.35
1.34
1.37
1.34
1.41
1.50
1.51
1.52
1.46
12.1
10.2
8.7
7.7
3.6
6.4
9.7
16.8
a General polymerization conditions: [M]0 = 0.5 M, solvent = toluene, T = 20 °C. b Yield: weight of polymer obtained/weight of monomer used.
c Theoretical value: 114 × [M]0/[I]0 × conv. d Measured by GPC relative to poly-MMA standards. e Solvent = THF.
Mn(exp) are in good agreement with Mn(theo) for one polymer
chain growing per metal center at the [L-LA]0/[2]0 ratio up to
1500. However, a slight deviation from the Mn(theo) was
observed when the ratio went up to 2000. This may be because
of the transesterification side reaction.
Fig. 4 shows semi-logarithmic plots of L-LA conversion
versus time for 2. Under these conditions, the reaction kinetics
displays a first-order dependence on the monomer concentration,
without any induction period. A linear increase of Mn with L-LA
conversion was also observed (Fig. 5), indicating that the
polymerization proceeds in a living fashion.
To investigate the stereochemistry of the polymerization
process, the ring opening polymerization of rac-LA by 1–4 was
also tested. Polymerizations were carried out either in THF or in
Tol at 20 °C. All complexes can serve as highly active initiators
and the same activity dependence on the central metals as that
for polymerization of L-LA was observed (Table 4, runs 1–4). It
is unexpected that the solvent has less effect on the polymer
yields (Table 4 runs 1 and 5). All PLAs obtained with 1–4
showed monomodal GPC traces with polydispersity values in
the range of 1.32–1.48 indicative of a single-site character under
these conditions. Polymerization of rac-LA with 1 also follows
first-order kinetic dependence on the concentration of rac-LA
without induction time (Fig. 6). The Mn(exp) increased linearly
with the conversion of the monomer recorded and the PDIs kept
in the range of 1.36–1.41 (Fig. 7), indicating the polymerization
occurs in a living fashion.
Fig. 2 Mn versus [M]0/[I]0 initiated by 2. Condition: solvent = Tol,
[M]0 = 0.5 M, T = 20 °C.
and completed within 2 min to give polymers with relatively
narrow PDIs (1.46–1.52, runs 6–9). The Mn(exp) of the poly-
mers increased linearly with the ratio of [ε-CL]0/[2]0 and
Mn(exp) are in good agreement with Mn(theo) as illustrated in
Fig. 2, indicating 2 is an efficient initiator for controlled
polymerization of ε-CL, and negligible chain-transfer reactions
occur under the reaction conditions tested. However, the reaction
kinetics could not be determined as the polymerization is
too fast.
The homonuclear decoupled 1H NMR spectra13 at 20 °C
show that PALs synthesized in THF are heterotactic-enriched up
to 0.85 for 1, while 0.80 for 4, 0.71 for 3 and 0.69 for 2. A
similar dependence of Pr on the central metals was also found in
the polymerization systems with tetradentate bis(phenolkate)
lanthanide borohydrides.4f These Pr values (0.82–0.85) are com-
petitive with some of the best systems so far reported in the lit-
erature for lanthanide monoborohydride initiators.4d,f,14 Almost
no dependence of Pr on the conversion (Fig. 7) and the amount
of the initiator (Table 4, runs 1 and 6) was observed. However,
the Pr values are influenced by the solvents: the polymerization
in toluene gave atactic PLA (Table 4, run 5). The same feature
has also been found in the other systems reported5c–e,k and the
reason for it has been demonstrated previously.15
Ring opening polymerization of L-LA and rac-LA
The polymerizations of L-LA with 1–4 were conducted either in
THF or in toluene at room temperature. All complexes are
efficient initiators for the ring opening polymerization of L-LA.
The influence of the central metal and polymerization solvent on
the activity is observed. With 1 the reaction proceeded faster in
toluene than in THF (Table 3, runs 1 and 2). This may be attribu-
ted to the competitive coordination to the metal of THF mol-
ecules. All the PolyLAs obtained by 1–4 showed monomodal
GPC traces with relatively narrow PDI values in the range of
1.36–1.49 (Table 3). The Mn(exp) of the resulting polymers with
2 increased linearly with [L-LA]0/[2]0 as illustrated in Fig. 3 and
The end groups of the low molecular weight poly(ε-CL) and
poly(L-LA) obtained with 2 were analyzed by 1H NMR in
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Table 3 Polymerization of L-LA initiated by 1–4a
Run
Init.
[M]0/[I]0
Time (min)
Yieldb (%)
Mn(theo)c (104)
Mn(exp)d (104)
PDId
1e
2f
3
4
5
6
7
8
9
10
11
12
13
14
1
1
1
1
2
3
4
2
2
2
2
2
2
2
1000
1000
1000
1000
1000
1000
1000
200
300
500
800
1000
1500
2000
60
10
8
5
5
5
5
5
7
77
70
86
53
93
83
35
>99
>99
>99
>99
>99
>99
>99
11.1
10.1
12.4
7.6
13.4
12.0
5.0
2.8
4.3
7.2
11.5
14.4
21.6
28.8
6.43
9.8
12.00
7.7
12.4
13.62
12.2
2.74
4.58
6.81
11.34
14.81
22.51
30.30
1.37
1.36
1.37
1.36
1.46
1.45
1.39
1.43
1.46
1.44
1.49
1.44
1.38
1.39
9
10
10
12
15
a General polymerization conditions: solvent = toluene, [M]0 = 0.75 M, T = 20 °C. b Yield: weight of polymer obtained/weight of monomer used.
c Theoretical value: 144.13 × [L-LA]0/[I]0 × conv. d Measured by GPC relative to poly-MMA standards. e Solvent = THF, [M]0 = 0.5 M. f [M]0
=
0.5 M.
Fig. 5 Yield versus Mn and Mw/Mn for the polymerization of L-LA
initiated by 2. Condition: solvent = Tol, [M]0 = 0.75 M, T = 20 °C.
Fig. 3 Mn versus [M]0/[I]0 initiated by 2. Condition: [M]0/[I]0 = 1500,
solvent = toluene, [M]0 = 0.75 M, T = 20 °C.
CDCl3. The 1H NMR spectrum of the low molecular weight
poly(ε-CL) shows, besides the polymer chain peaks, two triplet
signals at δ 3.5 ppm and 3.6 ppm assigned to the protons of
–CH2OH chain end. No other signals assignable to any other
terminal groups were observed indicating that both chain ends of
the polymers, which were prepared by 2 are hydroxyl groups
1
formed after the hydrolysis of the metal–alkoxide bond. The H
NMR spectrum of low molecular weight PLA also shows only
the quartet characteristics of the CH(Me)OH terminal group at
δ = 4.36 ppm, which was formed by hydrolysis of the metal–
alkoxide bond. The end group analyses demonstrate the ring-
opening process for both monomers of ε-CL and L-LA here
occurs via a classical coordination/insertion mechanism with an
initial ring-opening through acyl–oxygen bond cleavage.3 The
[HBH3] functionality in the polymerization acts as both an initi-
ating group and a reducing agent yielding α,ω-telechelic poly(ε-
CL) diols by reduction of the aldehyde formed after the insertion
of the first monomer into the metal–hydride bond as described
by Guillaume and co-workers in the ring opening polymerization
of ε-CL with (C5Me5)2SmBH4,3,16 and the ring opening
Fig. 4 Polymerization time versus ln[L-LA]0/[L-LA] initiated by 2.
Condition: [M]0/[I]0 = 1500, solvent = Tol, [M]0 = 0.75 M, T = 20 °C.
7262 | Dalton Trans., 2012, 41, 7258–7265
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Table 4 Polymerization of rac-LA initiated by 1–4a
Run
Init.
[M]0/[I]0
Time (min)
Yieldb (%)
Mn(theo)c (104)
Mn(exp)d (104)
PDId
Pre
1
2
3
4
5f
6
1
2
3
4
1
1
1000
1000
1000
1000
1000
500
30
30
30
30
30
30
71
88
78
68
73
>99
10.2
12.7
11.2
9.8
10.5
7.2
10.6
11.5
11.7
10.8
10.2
6.2
1.34
1.44
1.38
1.32
1.32
1.48
0.85
0.69
0.71
0.80
0.72
0.85
a General polymerization conditions: solvent = THF, [M]0 = 1 M, T = 20 °C. b Yield: weight of polymer obtained/weight of monomer used. c Theoretic
value: 144.13 × [M]0/[I]0 × conv. d Measured by GPC relative to polystyrene standards. e Determined by 1H NMR. f Solvent = toluene.
Fig. 7 Mn and Mw/Mn versus conversion for the polymerization of rac-
LA initiated with complexes 1. Conditions: [M]0/[I]0 = 1000, solvent =
THF, [M]0 = 1.0 M, T = 20 °C.
Fig. 6 Polymerization time versus ln[M]0/[M] for the polymerization
of rac-LA initiated by complexes 1. Conditions: [M]0/[I]0 = 1000,
solvent = THF, [M]0 = 1.0 M, T = 20 °C.
polymerization of rac- and L-LA with other lanthanide borohy-
used throughout all manipulations of air- and moisture-sensitive
dride initiators.4d,f
complexes. THF, toluene and DME were deoxygenated, dried
over sodium benzophenone ketyl and distilled just before use.
Li2L and LLnCl(THF)2 were prepared according to the literature
Conclusion
procedure.6 ε-CL (Aldrich) was dried over CaH2 (at least 3 days)
The bridged bis(amidinate) ligand L provides a suitable coordi-
nation environment that allows synthesis of isolable monomeric
monoborohydride complexes [LLn(BH4)(DME)] [Ln = Y(1), Nd
(2), Sm(3), Yb(4)] in reasonable yields. Complexes 1–4 can
serve as single-site initiators for the ring-opening polymerization
of ε-CL, L-LA and rac-LAwith high activity. Complex 2 initiates
the polymerization of ε-CL and L-LA in a living fashion, provid-
ing the polymers with controlled molecular weights and rela-
tively narrow polydispersities in a wide range of monomer/
initiator ratio. The controlled ring opening polymerization of
rac-LA can be realized by 1 to give heterotactic-enriched PLAs
(Pr up to 85%).
then distilled under reduced pressure before use. L-LA and
rac-LAwere sublimed under vacuum at 90 °C and then recrystal-
lized from dry ethyl acetate. Lanthanide analyses were performed
by EDTA titration with a xylenol orange indicator and hexamine
buffer. Carbon, hydrogen, and nitrogen analyses were performed
by direct combustion using a Carlo-Erba (EA) 1110 instrument.
The uncorrected melting points of crystalline samples in
1
sealed capillaries (under argon) are reported as ranges. H NMR
spectra and 13C NMR spectra were recorded on a Unity Inova-
400 spectroscopy instrument and processed using NUTS
software. The IR spectra were recorded with a Nicolet-550
FTIR spectrometer as KBr pellets. Molecular weights and mole-
cular weight distributions were determined against a poly-
MMA (methyl methacrylate) standard by gel permeation
chromatography (GPC) on PL 50 apparatus, THF was used as an
eluent at a flow rate of 1.0 ml min−1 at 40 °C. Microstructures
of PLAs were measured by homodecoupling 1H NMR spec-
troscopy at 20 °C in CDCl3 on a Unity Inova-400 spectroscopy
instrument.
Experimental
Materials and methods
Standard Schlenk techniques use an inert atmosphere of purified
argon and vacuum atmospheres. An N2-filled glove box was
This journal is © The Royal Society of Chemistry 2012
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Table 5 Crystallographic data for 1–4
1 DME
2 DME
3 DME
4 DME
Formula
Mol. wt
λ (Å)
Cryst syst
Space group
Cryst size (mm)
a (Å)
b (Å)
c (Å)
α (°)
C31H58BN4O4Si2Y
706.71
C31H58BN4O4Si2Nd
762.04
0.71075
Orthorhombic
P212121
0.40 × 0.13 × 0.10
8.7384(7)
20.0777(19)
23.162(2)
90
C31H58BN4O4Si2Sm
768.15
0.71075
Orthorhombic
P212121
0.75 × 0.30 × 0.20
8.6970(5)
19.9369(11)
23.1964(13)
90
C31H58BN4O4Si2Yb
790.84
0.71075
Orthorhombic
P212121
0.40 × 0.30 × 0.28
8.6469(12)
19.790(3)
23.214(3)
90
0.71075
Orthorhombic
P212121
0.80 × 0.40 × 0.35
8.683(2)
19.888(6)
23.197(6)
90
β (°)
90
90
90
90
4063.7(6)
4, 1.246
1.371
90
90
4022.1(4)
4, 1.269
1.554
90
90
γ (°)
V (Å3)
4005.9(18)
4, 1.172
1.550
3972.4(10)
4, 1.322
2.450
Z(Å3), Dcalcd (g mL−1
)
μ (mm−1
F (000)
)
1504
1588
1596
1628
θ range (°)
3.10–25.50
8666
6724
3.09–27.47
20 514
9028
3.10–27.48
27 690
9171
3.11–27.47
16 387
8853
Total no. of reflns
No. of indep reflns
Rint
0.0340
0.0828
0.0360
0.0383
GOF
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
0.843
0.0551, 0.0996
0.0885, 0.1130
1.067
0.0861, 0.1389
0.1293, 0.1632
1.093
0.0459, 0.1105
0.0497, 0.1133
0.957
0.0421, 0.0776
0.0521, 0.0823
Syntheses
(m), 1091 (m), 990 (m), 916 (s), 833 (s), 779 (m), 700 (s), 625
(m), 513 (m) cm−1
.
LY(BH4)(DME) (1). Into a THF solution of LYCl(THF)2
(3.74 g, 5.40 mmol) which was prepared in situ by the reaction
of YCl3 with Li2L, was added NaBH4 (0.31 g, 8.10 mmol) with
stirring. The suspension was allowed to stir at 60 °C overnight to
give a white suspension. Removing NaCl by centrifugation led
to a clean colorless solution. The volatiles were removed under
reduced pressure, and the residue was extracted with DME
(about 30 ml). Concentrating the DME solution to about 15 ml
and centrifugation led to a colorless solution, from which 1 was
obtained as colorless crystals upon crystallization at −15 °C after
several days. Yield: 2.7 g, 71%. m.p. 149–150 °C. 1H NMR
(400 MHz, C4D8O) δ = 7.28 (4 H, t, J = 7.2 Hz, ArH), 7.21 (2
H, t, J = 7.3 Hz, ArH), 7.06–7.00 (4 H, m, ArH), 3.47–3.43 (4
H, m, O–CH2), 3.31 (6 H, s, O–CH3), 1.74–1.70 (6 H, m,
(CH2)3), 0.65–0.60 (4 H, m, BH4), −0.25(18 H, s, Si(CH3)3).
13C NMR (400 MHz, C4D8O): δ = 182.13 (CvN), 143.33 (Ar),
131.75 (Ar), 131.03 (Ar), 129.85 (Ar), 71.13 (O–CH2), 51.93
(O–CH3), 35.24 (N–CH2), 29.25 (N–CH2–CH2), 6.26 (Si-
(CH3)3). C31H58BN4O4Si2Y (706.71): calcd. C, 52.69; H, 8.27;
N, 7.93; Y, 12.58; found C, 51.89; H, 7.92; N, 7.25; Y, 13.12. IR
(KBr): 2949 (s), 2900 (m), 2287 (s), 2223 (m), 1610 (s), 1568
(s), 1429 (s), 1356 (s), 1247 (m), 1159 (m), 1201 (m), 1089 (m),
1022 (m), 928 (s), 873 (s), 846 (s), 779 (m), 748 (m), 700 (s),
LSm(BH4)(DME) (3). The synthesis of 3 was carried out by
the same way as that for the synthesis of 1, but LSmCl(THF)2
(4.49 g, 5.97 mmol) and NaBH4 (0.34 g, 8.96 mmol) were used.
Light yellow crystals of 3 were isolated. Yield: 3.34 g, 73%. m.
1
p. 175–177 °C. H NMR (400 MHz, C4D8O): δ = 7.27 (4 H, s,
ArH), 7.04–6.89 (6 H, m, ArH), 3.70–3.64 (4 H, m, O–CH2),
3.50 (6 H, s, O–CH3), 1.93–1.90 (6 H, m, (CH2)3), 0.23 (18 H,
s, Si(CH3)3), −2.07 to −2.4 (4 H, m, BH4). 13C NMR
(400 MHz, C4D8O): δ = 207.16 (CvN), 143.16 (Ar), 130.34
(Ar), 130.10 (Ar), 128.61 (Ar), 68.23 (O–CH2), 61.73 (O–CH3),
35.95 (N–CH2), 26.41 (N–CH2–CH2), 4.49 (Si(CH3)3).
C31H58BN4O4Si2Sm (768.15): calcd. C, 48.47; H, 7.61; N, 7.29;
Sm, 19.57; found C, 48.21; H, 7.58; N, 7.55; Sm, 18.96. IR
(KBr): 2953 (s), 2285 (s), 2220 (m), 1607 (s), 1570 (s), 1431
(s), 1156 (m), 1089 (m), 966 (m), 905 (s), 873 (s), 847 (s), 779
(m), 748 (m), 700 (s), 556 (m) cm−1
.
LYb(BH4)(DME) (4). The synthesis of 4 was carried out in
the same way as that for the synthesis of 1, but LYbCl(THF)2
(4.51 g, 5.82 mmol) and NaBH4 (0.33 g, 8.73 mmol) were used.
Light yellow crystals of 4 were isolated. Yield: 3.49 g, 76%. m.
p. 151–153 °C. C31H58BN4O4Si2Yb (790.84): calcd. C, 47.08;
H, 7.39; N, 7.08; Yb, 21.88; found C, 46.12; H, 7.56; N, 7.67;
Yb, 22.46. IR (KBr): 2954 (s), 2291 (s), 2225 (m), 1607 (s),
1570 (s), 1492 (s), 1378 (s), 1250 (m), 1157 (m), 1125 (m),
1074 (m), 1020 (m), 899 (s), 846 (s), 779 (m), 750 (m), 700 (s),
624 (m), 513 (m) cm−1
.
613 (m), 507 (m) cm−1
.
LNd(BH4)(DME) (2). The synthesis of 2 was carried out in
the same way as that for the synthesis of 1, but LNdCl(THF)2
(4.01 g, 5.35 mmol) and NaBH4 (0.30 g, 8.03 mmol) were used.
Light purple crystals of 2 were isolated. Yield: 2.8 g, 69%. m.
p. 189–191 °C. C31H58BN4O4Si2Nd (762.04): calcd. C, 48.86;
H, 7.67; N, 7.35; Nd, 18.93; found C, 48.12; H, 7.66; N, 7.48;
Nd, 19.06. IR (KBr): 2951 (s), 2894 (m), 2301 (s), 2221 (m),
1624 (s), 1575 (s), 1427 (s), 1354 (s), 1241 (m), 1156 (m), 1201
Typical procedure for the polymerization of ε-CL, L-LA and
rac-LA
The procedures for the polymerizations of ε-CL, L-LA and rac-
LA initiated by 1–4 were similar, and a typical polymerization
procedure is given below. A 50 mL Schlenk flask equipped with
7264 | Dalton Trans., 2012, 41, 7258–7265
This journal is © The Royal Society of Chemistry 2012

View Article Online
Macromolecules, 2005, 38, 6888; (c) I. Palard, A. Soum and S.
M. Guillaume, Chem.–Eur. J., 2004, 10, 4054.
a magnetic stirring bar was charged with the desired amount of
monomer and solvent (toluene/THF). The contents of the flask
were then stirred at setting temperature for 5 min. Into the flask a
definite initiator solution in toluene (THF) was added using a
syringe. The mixture was stirred vigorously for the desired time,
during which time an increase in the viscosity of the solution
was observed. The polymerization solution was quenched by an
addition of acidic ethanol and poured into ethanol to precipitate
the polymer. The polymer was collected and dried under vacuum
until constant weight and weighed.
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X-Ray crystallographic structure determinations
Crystals suitable for X-ray diffraction of 1–4 were sealed,
respectively, in a thin-walled glass capillary filled with argon.
Diffraction data were collected on a Rigaku Mercury CCD area
detector in the ω scan mode using MoKα radiation (λ =
0.71075 Å) at 223(2) K. The diffracted intensities were corrected
for Lorentz-polarization effects and empirical absorption correc-
tions. The structures were solved by direct methods and refined
by full-matrix least-squares procedures based on |F|2. All of the
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms in these complexes were all generated geometrically,
appropriate isotropic thermal parameters assigned, and were
allowed to ride on their parent carbon atoms. All of the hydrogen
atoms were held stationary and included in the structure factor
calculations in the final stage of full-matrix least-squares refine-
ment. The structures were refined using SHELXL-97 programs.
Details of the intensity data collection and crystal data are given
in Table 5. CCDC-843489 (for 1), -843490 (for 2), -843491 (for
3) and -843492 (for 4) contain the supplementary crystallo-
graphic data for this paper.†
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744.
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F. Carpentier, Inorg. Chim. Acta, 2012, 383, 137.
Acknowledgements
10 M. Ephritikhine, Chem. Rev., 1997, 97, 2193.
We are grateful to the National Natural Science Foundation of
China (Grants 21132002 and 20972107) for financial support.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7258–7265 | 7265