Magnesium and calcium complexes containing biphenyl-based tridentate iminophenolate ligands for ring-opening polymerization of rac -lactide

Article abstract of DOI:10.1021/ic4012668

A series of racemic 2-[(2′-methoxybiphenyl-2-ylimino)methyl]-4-R ²-6-R¹-phenols (L¹H-L⁸H) were reacted with {Mg[N(SiMe₃)₂]₂}₂ and Ca[N(SiMe₃)₂]₂·2THF (THF = tetrahydrofuran), respectively, to provide nine heteroleptic magnesium complexes L^1-8MgN(SiMe₃)₂ [R¹ = ⁱPr, R² = H (1a); R¹ = ^tBu, R ² = Me (2a and 2a·THF); R¹ = R² = ^tBu (3a); R¹ = R² = CMe₂Ph (4a); R¹ = CPh₃, R² = ^tBu (5a); R ¹ = 1-piperidinylmethyl, R² = ^tBu (6a); R ¹ = Cl, R² = ^tBu (7a); R¹ = Br, R² = ^tBu (8a)], two homoleptic calcium complexes (L ^2,5)₂Ca (2b and 5b), and one heteroleptic calcium complex [(L⁴)CaN(SiMe₃)₂·THF] (4b), which have been fully characterized. In the solid state, magnesium complexes 2a and 6a are isostructural, and each possesses a monomeric structure, while magnesium complexes 7a and 8a are dimeric, where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. Although bearing the same iminophenoloate ligand, the molecular structures of complexes 2a and 2a·THF are different from each other. In complex 2a·THF, the coordination of one molecule of THF to the magnesium atom leads to dissociation of the methoxy group of the ligand from the metal center. The homoleptic calcium complex 2b has a six-coordinate metal core ligated by all six donor atoms of two iminophenolate ligands. The heteroleptic magnesium complexes 1a-8a and calcium complex 4b proved to be efficient initiators for the ring-opening polymerization of rac-lactide at ambient temperature in THF or at 70 C in toluene, and the polymerizations were better controlled in the presence of 2-propanol. The introduction of a bulky ortho substituent on the phenoxy unit of the ligand resulted in an increase of the catalytic activity of the corresponding metal complex. Microstructure analysis of the resultant poly(rac-lactide) samples via homonuclear-decoupled ¹H NMR spectroscopy revealed P_r values ranging from 0.60 to 0.81, which closely depended on the employed catalyst and polymerization conditions.

Full text of DOI:10.1021/ic4012668

Article
pubs.acs.org/IC
Magnesium and Calcium Complexes Containing Biphenyl-Based
Tridentate Iminophenolate Ligands for Ring-Opening Polymerization
of rac-Lactide
Wei Yi and Haiyan Ma*
Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, East China University of
Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Repulbic of China
S
* Supporting Information
ABSTRACT: A series of racemic 2-[(2′-methoxybiphenyl-2-
ylimino)methyl]-4-R2-6-R1-phenols (L1H-L8H) were reacted
with {Mg[N(SiMe3)2]2}2 and Ca[N(SiMe3)2]2·2THF (THF =
tetrahydrofuran), respectively, to provide nine heteroleptic
i
magnesium complexes L1−8MgN(SiMe3)2 [R1 = Pr, R2 = H
(1a); R1 = tBu, R2 = Me (2a and 2a·THF); R1 = R2 = tBu (3a);
t
R1 = R2 = CMe2Ph (4a); R1 = CPh3, R2 = Bu (5a); R1 = 1-
t
t
piperidinylmethyl, R2 = Bu (6a); R1 = Cl, R2 = Bu (7a); R1 =
Br, R2 = tBu (8a)], two homoleptic calcium complexes (L2,5)2Ca
(2b and 5b), and one heteroleptic calcium complex [(L4)CaN-
(SiMe3)2·THF] (4b), which have been fully characterized. In
the solid state, magnesium complexes 2a and 6a are
isostructural, and each possesses a monomeric structure, while
magnesium complexes 7a and 8a are dimeric, where the two
metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium
center in these complexes can be best described as a distorted tetrahedral geometry. Although bearing the same iminophenoloate
ligand, the molecular structures of complexes 2a and 2a·THF are different from each other. In complex 2a·THF, the
coordination of one molecule of THF to the magnesium atom leads to dissociation of the methoxy group of the ligand from the
metal center. The homoleptic calcium complex 2b has a six-coordinate metal core ligated by all six donor atoms of two
iminophenolate ligands. The heteroleptic magnesium complexes 1a−8a and calcium complex 4b proved to be efficient initiators
for the ring-opening polymerization of rac-lactide at ambient temperature in THF or at 70 °C in toluene, and the polymerizations
were better controlled in the presence of 2-propanol. The introduction of a bulky ortho substituent on the phenoxy unit of the
ligand resulted in an increase of the catalytic activity of the corresponding metal complex. Microstructure analysis of the resultant
poly(rac-lactide) samples via homonuclear-decoupled 1H NMR spectroscopy revealed Pr values ranging from 0.60 to 0.81, which
closely depended on the employed catalyst and polymerization conditions.
Alkaline-earth metal complexes supported by iminopheno-
loate ligands have been used as catalysts in many applications
because of the diverse forms and easy preparation of
iminophenol proligands.15 The first magnesium iminopheno-
late complex [(SalenOMe)Mg(OBn)]2 used as an initiator for
the polymerization of rac-LA was developed by Lin and co-
workers.16 The complex exhibited good catalytic activity and
yielded PLAs with very narrow molecular weight distributions.
However, it only afforded heterotactic bias PLAs (Pr = 0.57) in
tetrahydrofuran (THF) and atactic polymers in toluene at
room temperature. Recently, the same group17 also reported a
series of magnesium complexes supported by NNO-tridentate
iminophenolate ligands for the ROP of L-LA. The results
showed that the introduction of an electron-withdrawing
substituent at the para position of the phenoxy unit decreased
INTRODUCTION
■
Polylactides (PLAs) as biodegradable and biocompatible
materials have been widely used in the biomedical and
pharmaceutical fields and more recently are considered as
environmentally friendly alternatives to olefinic polymers.1
Accordingly, the preparation of PLA has been drawn intensive
scrutiny, and the use of metal-based catalysts for the ring-
opening polymerization (ROP) of lactides (LAs) proves to be
the most effective.2 Many discrete complexes of various metals,
such as aluminum,3 indium,4 tin,5 sodium,6 zinc,7 magnesium,8
calcium,9 titanium, zirconium,10 and rare-earth metals,11 have
been synthesized and evaluated for this issue. Although a large
variety of metal derivatives could effectively catalyze the ROP of
LAs, it is preferable to use initiators based on biocompatible
metals such as magnesium and calcium because PLAs are more
widely utilized as surgical sutures,12 drug-delivery vehicles,13
and artificial tissue matrices.14
Received: May 22, 2013
© XXXX American Chemical Society
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a
Scheme 1.
a
(a) (1) −78 °C, n-BuLi/B(OMe)3; (2) HCl. (b) −5 to 0 °C, HCl/NaNO2/KI. (c) PdCl2(PPh3)2/K2CO3/iPrOH; 12 h. (d) Sn/HCl, ethanol,
reflux 5 h. (e) ethanol, reflux, 5h.
Scheme 2
the catalytic activity of the corresponding complex. More
recently, Darensbourg and Karroonnirun7j assessed a zinc
silylamido complex supported by chiral NNO-tridentate
iminophenolate ligand for the polymerization of rac-LA in
dichloromethane (DCM), and predominantly heterotactic
PLAs (Pr = 0.68−0.89) were obtained. Unfortunately,
analogous magnesium and calcium complexes ligated with the
same ligand are not reported. By looking into the structure of
the mentioned chiral zinc silylamido complex, we find that the
chiral unit is far away from the zinc center, which might not be
capable of inducing a specific chirality at the metal center.
Biphenyl derivants are widely used in asymmetric synthesis.18 It
is considerable that the axial chirality of the biphenyl framework
may serve as an excellent chiral resource to induce a specific
chirality at the metal center of the target complex, and when it
is used in racemic form, the resultant racemic complex would
be more favorable for the stereoselective ROP of rac-LA.
Herein we describe the preparation of a series of magnesium
and calcium complexes supported by novel tridentate
imonophenolate ligands based on a racemic biphenyl frame-
work. The catalytic performances of the silylamido complexes
toward the ROP of rac-LA are reported in detail.
B
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Scheme 3
configuration at the magnesium center. As shown in the
following figures (vide post), in the solid state complexes 1a−
6a are monomeric, while complexes 7a and 8a with o-halogen
substitution each possess a dimeric structure. Attempts at
recrystallization of magnesium complex 2a with a THF/n-
hexane mixture afforded a THF-coordinated magnesium
complex, 2a·THF, as pale-yellow needlelike crystals, which
was also characterized via X-ray diffraction study to be
monomeric.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Magnesium and
Calcium Complexes. Scheme 1 illustrates the synthetic
strategies to prepare the desired biphenyl-based iminophenol
proligands. The synthesis of 2-methoxy-2′-nitrobiphenyl was
based on a modified procedure reported in the literature.19 Via
a Suzuki coupling reaction between 1-iodo-2-nitrobenzene and
2-methoxyphenylboronic acid, the mentioned compound could
be isolated in high yield of 94%. 2′-Methoxybiphenyl-2-amine
was obtained by a reduction of the above coupling product in
the existence of tin. Condensation reactions of this amine with
different substituted salicylaldehyde derivatives under reflux in
ethanol yielded the target biphenyl-based iminophenol
proligands L1−8H. All of the obtained iminophenols except
for L4H are yellow-to-orange crystalline solids, whereas L4H is
isolated as an orange viscous oil. All proligands were
characterized via 1H and 13C{1H} NMR spectroscopy and
high-resolution mass spectroscopy (HRMS).
The heteroleptic magnesium complexes 1a−8a were
prepared in moderate yields from the reaction of {Mg[N-
(SiMe3)2]2}2 and 1 equiv of the corresponding proligand in
toluene at room temperature, respectively (Scheme 2). The
recrystallization process accounting for further purification was
somewhat challenging because these complexes are highly
soluble in toluene or THF but almost insoluble in n-hexane.
Finally, all of the magnesium complexes were successfully
recrystallized in a mixture of toluene/n-hexane at room
temperature or at −38 °C to give yellow-green crystalline
solids. For all complexes, the spectroscopic data and elemental
analysis are consistent with the stoichiometric structure of one
iminophenolate ligand and one bis(trimethylsilyl)amido group
chelating to the metal center. Although there exist both an
axially chiral biphenyl moiety and a stereogenic metal center in
Similar reactions of selected proligands L2H, L4H, and L5H
with Ca[N(SiMe3)2]2·2THF were carried out to synthesize the
corresponding calcium silylamido complexes. Unexpectedly, the
reaction of equimolar amounts of L2H and Ca[N(SiMe3)2]2·
2THF did not give the target heteroleptic calcium complex
(L2)CaN(SiMe3)2, but the bisligated complex (L2)2Ca (2b)
isolated in moderate yield. Therefore, the reaction of L2H with
an excess of Ca[N(SiMe3)2]2·2THF in a ratio of 1:1.5 was
monitored via 1H NMR spectroscopy, which indicated that the
bisligated calcium complex 2b was formed immediately after
the mix of two reactants (less than 30 min) accompanied by the
release of 2 equiv of free HN(SiMe3)2 (Figure S1 in the
Supporting Information, SI). No signal assignable to the target
heterolepic calcium complex could be observed, suggesting that
the formation of 2b did not arise from a rearrangement upon
workup. Further attempts in carrying the reaction at −78 °C
throughout did not move the equilibrium to the heteroleptic
product. Complex 2b could also be quantitatively prepared
from the reaction of L2H and Ca[N(SiMe3)2]2·2THF in a
molar ratio of 2:1. Such behavior was in stark contrast with that
of the reaction between L4H and Ca[N(SiMe3)2]2·2THF,
which readily led to the formation of the corresponding
heteroleptic complex (L4)CaN(SiMe3)2·THF (4b; Scheme 3).
To prove whether it is due to a steric effect of the ortho
substituent on the phenoxy ring, the reaction of L5H (bearing
an o-trityl group) and Ca[N(SiMe3)2]2·2THF was conducted,
which, however, led to the bisligated calcium complex 5b
1
each complex, no diastereomer could be observed in the H
NMR spectra of these complexes, suggesting that the axial
chirality of biphenyl may have induced exclusively a certain
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beyond our expectation. It seems that a much more
complicated situation is encountered, where the steric/
electronic effects of the ligand substituents as well as solubility
differences between the homolepic and heterolepic complexes
may contribute together to the formation of one specific
product.
structure. As to the third structure of 8a appearing below −40
°C, probably it is a diastereomer of the minor isomer.
The singlet of the methoxy protons of magnesium complex
2a·THF appears at 3.28 ppm, which is upfield-shifted from that
of complex 2a (3.57 ppm) and is closer to the corresponding
resonance of the free ligand L2H, indicating that the
coordination of THF leads to dissociation of the methoxy
group in 2a·THF. Darensbourg and co-workers7j also de-
claimed that the structure of a zinc diaminophenolate complex
is different in a coordinative solvent or not. To examine the
All of these magnesium and calcium complexes have been
fully characterized, and their structures in solution were
1
investigated in detail via H NMR spectroscopy with the aim
of understanding the coordination environment around the
metal center. As shown in Table 1, the resonance of the
1
universality in this series of complexes, we measured the H
NMR spectra of the rest of the magnesium complexes in
benzene-d6 with the addition of one tiny drop of THF (around
1−3 equiv). As expected, a significant upfield shift of the
methoxy resonance could be observed for most of the
complexes, suggesting that the methoxy group is dissociated
from the magnesium center in the presence of THF. Complex
5a showed a different situation, where the resonance of the
methoxy protons is hardly shifted. Likely, the methoxy group is
still coordinated to the metal center, probably because of
protection by the bulky o-trityl group.
Crystal Structures of Magnesium Complexes 2a, 2a·
THF, 6a, 7a, and 8a and Calcium Complex 2b. Complexes
2a, 2a·THF, 6a, 7a, 8a, and 2b were further characterized by
single-crystal X-ray diffraction studies. The single crystals were
obtained by slightly cooling a saturated toluene/n-hexane
mixture or a THF/n-hexane mixture, respectively. Crystallo-
graphic data and results of the refinements are summarized in
Table S1 in the SI, and selected bond lengths and angles are
listed in Tables 2 and 3.
1
Table 1. Chemical Shifts of Methoxy Resonances in the H
NMR Spectra of Magnesium Complexes and Corresponding
a
Proligands
Ar-OCH3
C6D6 with THF
Ar-OCH3
b
complex
C6D6
ligand
C6D6
1a
2a
3a
4a
5a
6a
7a
8a
3.61
3.57
3.59
3.45
3.40
3.65
3.33
3.28
3.33
3.09
3.35
3.47
3.39
3.29
L1H
L2H
L3H
L4H
L5H
L6H
L7H
L8H
3.34
3.35
3.37
3.16
3.10
3.35
3.37
3.37
c
d
3.61
3.62
a
b
In ppm. One tiny drop of THF was added to the solution of the
magnesium complex in C6D6. The corresponding resonance of
complex 2a·THF in C6D6. The corresponding resonance of the major
c
d
structure in C6D6.
As depicted in Figure 1, complex 2a has a monomeric
structure in the solid state where the magnesium atom is four-
coordinated by three heteroatom donors of the tridentate
ligand and one bis(trimethylsilyl)amido group, adopting a
distorted tetrahedral geometry. The molecule shows C1
symmetry, and both enantiomers are found in the centrosym-
methoxy protons of magnesium complex 1a appears at 3.61
ppm in the H NMR spectrum, which is 0.27 ppm downfield-
1
shifted from that of the free ligand L1H (3.34 ppm). A similar
situation could also be found for magnesium complexes 2a−6a
and calcium complex 4b, which proves that in solution these
complexes are monomeric and the methoxy group of the
iminophenolate ligand in these complexes is still coordinated to
the metal center. This assignment is also supported by X-ray
diffraction analyses of 2a and 6a.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in 2a,
2a·THF, and 6a
[(L2)MgN(SiMe3)2·toluene] (2a)
Two sets of signals accounting for the stoichiometric
1
Mg1−O2
Mg1−O1
1.889(4)
2.062(4)
Mg1−N1
Mg1−N2
1.985(4)
2.106(4)
structure are displayed in the H NMR spectrum of 7a in
benzene-d6, represented by the methoxy resonances at 3.61 and
3.29 ppm (with a ratio of 4:1). It is worth noting that, although
the X-ray diffraction determination indicates a dimeric structure
of complex 7a in the solid state where the methoxy group of the
biphenyl moiety is not coordinated to the magnesium center, a
significant downfield shift of the methoxy resonance attribut-
able to the major structure is also observed compared to that of
the free ligand. The relevant resonance of the minor structure,
however, resembles the one of the free ligand. Similar features
are also observed in the 1H NMR spectrum of 8a in toluene-d8
at 25 °C (Figure S2a in the SI). With an increase of the
temperature from 25 to 105 °C, the broad signals of the minor
structure of 8a disappeared completely above 75 °C. In the
low-temperature region (−60 to +20 °C), no significant
increase of the signals of the minor structure with a decrease of
the temperature could be observed (Figure S2b in the SI);
instead, a very small amount of a new structure with a CH3O−
chemical shift at 3.04 ppm and a N[Si(CH3)3]2 resonance at
0.15 ppm appeared below −40 °C. It is, therefore, suggested
that the dominant structures of 7a and 8a in solution are still
monomeric and the minor isomer should have a dimeric
O2−Mg1−N1
N1−Mg1−O1
N1−Mg1−N2
119.75(18)
105.64(16)
131.84(17)
O2−Mg1−O1
O2−Mg1−N2
O1−Mg1−N2
120.80(16)
89.54(16)
86.45(15)
[(L2)MgN(SiMe3)2·THF] (2a·THF)
Mg1−O1
Mg1−O3
Si1−N2
1.9109(17)
2.025(2)
1.702(2)
Mg1−N2
Mg1−N1
Si2−N2
1.987(2)
2.097(2)
1.693(2)
O1−Mg1−N2
N2−Mg1−O3
N2−Mg1−N1
Si2−N2−Si1
123.80(8)
114.66(9)
118.41(9)
125.00(12)
O1−Mg1−O3
O1−Mg1−N1
O3−Mg1−N1
96.93(8)
90.65(7)
108.35(8)
[(L6)MgN(SiMe3)2] (6a)
Mg1−O1
Mg1−O2
1.905(2)
2.047(2)
Mg1−N3
Mg1−N1
1.985(3)
2.111(3)
O1−Mg1−N3
N3−Mg1−O2
N3−Mg1−N1
117.35(11)
103.37(11)
136.79(11)
O1−Mg1−O2
O1−Mg1−N1
O2−Mg1−N1
120.40(11)
88.55(10)
89.76(10)
D
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methoxy group to the magnesium center. The molecular
structure of complex 2a·THF (Figure 2) is different from that
Table 3. Selected Bond Lengths (Å) and Angles (deg) in 7a,
8a, and 2b
[(L7)MgN(SiMe3)2] (7a)
Mg1−O2
Mg1−N3
Mg2−N4
Mg2−N2
1.9953(18)
2.000(2)
1.994(2)
2.139(2)
Mg1−O1
Mg1−N1
Mg2−O1
Mg2−O2
1.9981(18)
2.143(2)
1.9935(18)
2.0016(19)
O2−Mg1−O1
O1−Mg1−N3
O1−Mg2−N4
N4−Mg2−O2
80.05(7)
O2−Mg1−N3
O2−Mg1−N1
O1−Mg2−O2
O1−Mg2−N2
131.11(9)
111.33(8)
80.01(7)
127.03(9)
130.87(9)
125.41(9)
108.23(8)
[(L8)MgN(SiMe3)2] (8a)
Mg1−O2
Mg1−N3
Mg2−N2
Mg2−N4
1.993(4)
1.998(5)
2.151(5)
2.000(5)
Mg1−O1
Mg1−N1
Mg2−O1
Mg2−O2
1.999(4)
2.150(5)
2.000(4)
2.004(4)
O2−Mg1−O1
O1−Mg1−N3
O1−Mg2−N4
N4−Mg2−O2
79.52(15)
124.46(19)
129.11(18)
127.31(19)
O2−Mg1−N3
O2−Mg1−N1
O1−Mg2−O2
O1−Mg2−N2
129.58(18)
111.51(17)
79.22(15)
114.23(17)
Figure 2. ORTEP diagram of the molecular structure of 2a·THF.
Thermal ellipsoids are drawn at the 30% probability level. Hydrogen
atoms are omitted for clarity.
[(L2)2Ca] (2b)
Ca1−O2
Ca1−N2
Ca1−O3
2.410(5)
2.481(6)
2.184(5)
Ca1−N1
Ca1−O4
Ca1−O1
2.472(6)
2.553(6)
2.203(5)
of complex 2a, with the oxygen atom of the methoxy group in
2a·THF dissociating from the metal center. In complex 2a·
THF, the magnesium atom is still four-coordinated by two
heteroatom donors of the tridentate ligand, one bis-
(trimethylsilyl)amido group, and one molecule of THF,
adopting a distorted tetrahedral geometry, instead of a five-
coordinate structure usually adopted by common magnesium
complexes.20a,22 More interestingly, it is found that, in complex
2a·THF, the Ra configuration of the biphenyl moiety in the
iminophenolate ligand leads to an R configuration of the
magnesium center instead, which is in contrast to the situation
in 2a. It seemed that, during the recrystallization process of 2a
with THF, the coordinated methoxy group in 2a was
substituted by a THF molecule via a “SN2”-type reaction to
afford a reversed configuration at the metal center.
Furthermore, as expected the dihedral angle of the biphenyl
moiety in complex 2a·THF is 70.82°, obviously larger than that
of complex 2a.
O1−Ca1−O2
O1−Ca1−N1
O2−Ca1−N2
O3−Ca1−O4
115.9(2)
74.46(19)
88.1(2)
O3−Ca1−N1
O2−Ca1−N1
O2−Ca1−O4
O1−Ca1−O4
160.4(2)
77.05(19)
148.56(19)
88.01(19)
108.09(18)
The molecular structure of complex 6a (Figure 3) is similar
to that described for complex 2a. The introduction of an
additional donor atom in the piperidinyl group hardly
influences the coordination geometry in complex 6a because
the N2 atom of piperidinyl is located far away and not
coordinated to the magnesium center. In addition, the angle of
N3−Mg−N1 = 136.79(11)° in complex 6a is significantly more
open than that in complex 2a [131.84(17)°]. A slightly larger
dihedral angle of biphenyl (60.02°) than that of 2a is also
observed. Similar to 2a, the Ra configuration of the biphenyl
moiety in 6a also leads to an S configuration of the magnesium
center or vice versa.
The ORTEP drawing of the molecular structure of 7a given
in Figure 4 shows that in the solid state complex 7a possesses a
dimeric structure with two metal centers bridged by the two
phenolato oxygen atoms of the ligands, and the whole molecule
has C1 symmetry, as indicated by slight differences in the
corresponding bond lengths and angles. Each magnesium
center adopts a distorted tetrahedral geometry. The methoxy
groups of both iminophenolate ligands are not coordinated to
Figure 1. ORTEP diagram of the molecular structure of 2a. Thermal
ellipsoids are drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity.
metric crystal structure. Without exception, the Ra config-
uration of the biphenyl moiety in the iminophenolate ligand
leads to an S configuration of the magnesium center, and vice
versa Sa leads to an R configuration. The bond length between
the magnesium atom and silylamido nitrogen atom (Mg1−N1)
in complex 2a is 1.985(4) Å, which is in the normal range of
1.980−2.023 Å reported in the literature.10d,20 The dihedral
angle of the biphenyl moiety being 57.6°, obviously smaller
than that of the free ligand,21 is likely due to coordination of the
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Figure 3. ORTEP diagram of the molecular structure of 6a. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted
for clarity.
Figure 5. ORTEP diagram of the molecular structure of 8a. Thermal
ellipsoids are drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity.
Figure 4. ORTEP diagram of the molecular structure of 7a. Thermal
ellipsoids are drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity.
two corresponding magnesium−halogen distances deviate from
each other significantly (Mg1−Cl2 = 3.862 Å vs Mg2−Cl1 =
magnesium centers. Both magnesium centers are still chiral
because the biphenyl moieties of two ligands possess opposite
chiral configurations. The Mg−N (silylamido) bond lengths in
complex 7a [Mg1−N3 = 2.000(2) Å; Mg2−N4 =1.994(2) Å]
are longer than that in complex 2a [1.985(4) Å] but still in the
normal range reported in the literature (1.980−2.023 Å).10d,20
The Mg2−O1−Mg1 angle is about 100.05(8)°, and the
distance of Mg···Mg is 3.0588(12) Å, slightly longer than the
3.538 Å for 7a; Mg1−Br2 = 3.469 Å vs Mg2−Br1 = 3.871 Å for
8a), probably indicating somewhat weak van der Waals
interaction between one magnesium center and a halogen
atom, especially for 8a with a Mg1−Br2 distance shorter than
the sum of the metal radius rm(Mg) and the van der Waals
radius rv(Br) [rm(Mg) + rv(Br) = 1.60 + 2.00 = 3.60 Å]. Likely,
it is the van der Waals force that breaks the symmetry of the
molecule.
As shown in Figure 6, calcium complex 2b possesses a
monomeric structure in the solid state and has a six-coordinate
calcium core, adopting a distorted octahedral geometry. The
whole molecule shows C1 symmetry, with the two biphenyl
moieties possessing the same axial chirality. The distances
between the calcium atom and O1 and O3 of two phenoxy
units are 2.203(5) and 2.184(5) Å, respectively. The angle of
O3−Ca1−N1 being 160.4(2)° is the largest, and thus O3, Ca1,
and N1 construct the axis of the octahedral structure. The other
donor atoms of O1, O2, O4, and N2 are in the equatorial
position. The distance between the calcium atom and the (O1,
16
reported values for [(SalenMe)Mg(OBn)]2 (2.999 Å).
Similar to complex 2a·THF, the dihedral angles of the biphenyl
moieties in 7a being 74.82° and 74.44° are also larger than
those of 2a and 6a with a coordinated methoxy group. The
solid-state structure of complex 8a (Figure 5) is shown to be
similar, which is also dimeric. Each magnesium center adopts a
distorted tetrahedral geometry, and the configurations of two
biphenyl moieties in one molecule are mainly opposite.23
Significantly larger dihedral angles of the biphenyl units in 8a
(82.70° and 75.55°) than those of all of the other complexes
are observed. It is worth noting that, in complexes 7a and 8a,
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o-piperidinylmethyl substituent displayed a relatively lower
activity, which might be ascribed to the existence of an
additional donor group in the piperidinyl ring, unfavorable for
the coordination/insertion of monomer. Complexes 7a and 8a
with an o-halogen substituent on the phenolate ring also
displayed lower activities. A similar effect was previously
reported for magnesium complexes supported by NNO-
tridentate iminophenolate17 or β-ketiminato24 ligands, where
the electron-withdrawing group was substituted at the para
position of the phenolate ring or the 1 position of the
ketiminato framework. The authors suggested that the
introduction of an electron-withdrawing group caused the
magnesium atom to be more acidic, resulting in a stronger
Mg−OR bond during polymerization, which retarded the
insertion reaction of monomer. It is therefore conceivable that
the electronic-withdrawing effect of an o-halogen substituent in
complexes 7a and 8a may also play a dominant role during
polymerization. Besides, on the basis of the fact that a dimeric
structure is even obtained for magnesium silylamido complexes
7a and 8a in the solid state, the steric effect of a smaller o-
halogen group in 7a and 8a, which is not sufficient to prevent
the active magnesium center (in the Mg−OR form during
polymerization) from aggregating, could not be ruled out.
To investigate the exact influence of ligand substituents on
the polymerization rate of rac-LA, preliminary kinetic studies
were carried out for representative magnesium complexes with
an initiator/monomer ratio of 1:25 ([LA]0 = 0.20 mol·L−1) at
25 °C in benzene-d6. The semilogarithmic plots of ln([LA]0/
[LA]t) versus time are presented in Figure 7. A clear decreasing
tendency of the apparent propagation rate is observed in the
Figure 6. ORTEP diagram of the molecular structure of 2b. Thermal
ellipsoids are drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity.
O2, and N2) plane is 0.034 Å. The O2 and O4 atoms of two
methoxy groups are in the reverse positions, as indicated by the
corresponding angle of O2−Ca−O4 = 148.56(19)°.
ROP of rac-LA. All of the synthesized magnesium
complexes 1a−8a and calcium complexes 2b, 4b, and 5b
were evaluated as initiators for the ROP of rac-LA in THF at
room temperature or in toluene at 70 °C. The representative
polymerization data are summarized in Table 4. Except for the
homoleptic calcium complexes 2b and 5b, all of these alkaline-
earth silylamido complexes can effectively initiate the ROP of
rac-LA either alone or in the presence of 2-propanol, giving
PLAs with high molecular weights and relatively broad
molecular weight distributions (Mw/Mn = 1.06−1.75).
order of 5a (CPh3; kobs = 0.11 min−1) > 4a (cumyl; kobs
0.0245 min−1) > 2a (tBu; kobs = 0.011 min−1) > 1a (iPr; kobs
=
=
0.0043 min−1) > 8a (Br; kobs = 0.0016 min−1). Among them,
magnesium complex 5a bearing the most bulky trityl group at
the ortho position of the phenolate unit exhibits the highest
rate in the polymerization. Complex 1a with an o-isopropyl
group shows the lowest activity among those bearing electron-
donating substituents but is still more active than complex 8a
with an electron-withdrawing bromo group at the ortho
position of the phenolate ring.
The structure of the ancillary ligand has a considerable
influence on the catalytic performance of the corresponding
metal complex. For instance, magnesium complex 5a with an o-
trityl group on the phenolate ring displayed the highest catalytic
activity among this series of magnesium complexes. The
polymerization went to 94% monomer conversion in 1.5 min at
room temperature in THF (entry 17), whereas the rest of the
complexes achieved moderate-to-high monomer conversions
within 30 min under otherwise identical polymerization
conditions. A general increasing tendency of catalytic activity
with an increase of the steric bulkiness of the ortho substituent
on the phenolate ring could be observed for magnesium
complexes 1a−5a, that is, 5a (CPh3) > 4a (cumyl) > 3a (tBu)
≈ 2a (tBu) > 1a (iPr). Lin et al.24 reported that by replacing
1,5-dimethyl groups with steric bulkier tert-butyl groups in the
tridentate β-ketiminatomagnesium alkoxide complexes [LMg-
(μ-OBn)]2, the catalytic activity of the resultant complex
increased dramatically. Recently, Okuda and co-workers10i also
reported a series of group 4 metal complexes containing
bridged bis(phenolate) ligands of the (OSSO) type; it was
founded that the complex with a bulky o-cumyl group displayed
higher activity for meso-LA polymerization than the one with an
o-tert-butyl group. All of these can be attributed to the fact that
the introduction of steric bulkier groups, especially at the ortho
position of the anionic donor atom (in this work, the
phenoxide), could protect more efficiently the metal center of
active metal alkoxide species from aggregation and therefore
lead to an increase of the catalytic activity. In comparison with
magnesium complexes 1a−5a, magnesium complex 6a with an
When calcium complex 4b was allowed to polymerize 200
equiv of rac-LA in THF at room temperature, a monomer
conversion up to 89% could be achieved in 25 min (entry 34),
indicating that 4b was less active than the corresponding
magnesium complex 4a ligated with the same ligand (entry 13).
Similar reactivity order was also found for calcium and
magnesium complexes with a tripyrazolylborate ligand in the
ROP of rac-LA.25 The relatively low reactivity of calcium
complex 4b is probably related with a large radius of the
calcium ion, which could not be effectively prevented by the
tridentate ligand from aggregation or ligand scrambling.
As shown in Table 4, the solvent has a great impact on the
catalytic activity as well. All of the complexes show much higher
activity in THF than in toluene. For instance, using complex 1a
as the initiator, a monomer conversion of 90% could be reached
within 30 min in THF (entry 1), whereas the yield was just
17% in toluene at room temperature after 420 min. When the
reaction temperature was increased to 70 °C, the conversion
could be added to 96% in 330 min (entry 3). Therefore, all of
the polymerization reactions in toluene were preferably
conducted at 70 °C. In toluene, the significantly superior
activity of complex 5a with a trityl group was no longer
observed; a more or less similar activity was obtained compared
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a
Table 4. ROP of rac-LA Initiated by Complexes 1a−8a and 4b
b
c
d
d
e
run
cat.
[LA]0/[M]0/[iPrOH]0
solvent
temp (°C)
time (min)
conv (%)
Mn,calcd (×104)
Mn (×104)
Mw/Mn
Pr
1
1a
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
200:1:1
200:1:0
THF
THF
Tol
25
25
70
70
25
25
70
70
25
25
70
70
25
25
70
70
25
25
−38
70
70
25
25
70
70
25
25
70
70
25
25
70
70
25
25
70
70
30
60
90
92
96
90
95
96
90
97
93
95
90
97
92
94
94
96
94
94
92
95
98
82
92
94
90
82
90
92
93
86
84
92
95
89
97
94
94
2.59
2.65
2.77
2.60
2.74
2.77
2.60
2.80
2.68
2.74
2.60
2.80
2.65
2.71
2.71
2.77
2.71
2.91
2.65
2.72
2.82
2.36
2.65
2.71
2.60
2.36
2.59
2.65
2.68
2.48
2.42
2.65
2.74
2.56
2.80
2.71
2.71
3.24
1.93
3.41
2.27
4.45
2.02
3.50
2.57
3.68
2.42
2.80
2.11
4.27
3.06
4.15
3.05
11.9
3.04
21.9
4.73
3.75
2.98
1.42
1.92
2.90
2.80
2.32
2.60
1.89
2.75
1.99
2.70
2.64
2.89
2.69
2.61
1.59
1.06
1.64
1.51
1.75
1.30
1.48
1.45
1.60
1.40
1.54
1.45
1.58
1.12
1.47
1.33
1.48
1.37
1.44
1.46
1.50
1.63
1.36
1.51
1.21
1.67
1.19
1.46
1.27
1.43
1.20
1.50
1.50
1.52
1.19
1.22
1.23
0.68
0.63
0.48
0.46
0.70
0.69
0.46
0.45
0.72
0.71
0.41
0.40
0.69
0.63
0.46
0.46
0.72
0.71
0.81
0.49
0.49
0.60
0.60
0.50
0.49
0.73
0.68
0.47
0.50
0.75
0.69
0.50
0.49
0.53
0.53
0.43
0.44
2
3
330
120
30
4
Tol
5
2a
3a
4a
5a
THF
THF
Tol
6
60
7
120
30
8
Tol
9
THF
THF
Tol
30
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
60
120
30
Tol
THF
THF
Tol
20
55
120
30
Tol
THF
THF
THF
Tol
1.5
2
2d
120
30
Tol
6a
7a
8a
4b
THF
THF
Tol
30
60
240
120
30
Tol
THF
THF
Tol
480
600
300
30
Tol
THF
THF
Tol
270
480
300
25
Tol
THF
THF
Tol
60
60
200:1:1
Tol
30
2.27
a
b
c
d
1
[rac-LA]0 = 1.0 M. Determined by H NMR spectroscopy. Mn,calcd = ([LA]0/[M]0) × 144.13 × conv %. Determined by GPC, without
e
1
correction. Pr is the probability of forming a new r-dyad, determined by homonuclear-decoupled H NMR spectroscopy.
to the activities of complexes 2a−4a with a tert-butyl or cumyl
group (entry 20 vs entries 7, 11, and 15). As we discussed
previously, complex 5a may possess a different coordination
environment around the metal center in THF in comparison
with the other magnesium complexes, which might account for
the significant enhancement of its catalytic activity in THF.
Furthermore, consistent with the tendency in THF, magnesium
complexes 7a and 8a displayed very low activities in toluene
with monomer conversions up to 92% achieved in 480 or 600
min.
Generally, the ROPs of rac-LA initiated by these alkaline-
earth silylamido complexes are not well-controlled, giving
moderately distributed polymers (Mw/Mn = 1.12−1.75)
because the silylamido group is less nucleophilic than alkoxide
and is believed to cause a relatively slow initiation. By using
complex 1a as the initiator to catalyze the ROP of rac-LA in
THF, the relationship of Mn of the obtained PLA sample versus
conversion was plotted (Figure 8), which, however, indicated a
linear increase of Mn with conversion. Although the measured
Mn values deviate somewhat from the theoretical data and the
molecular weight distributions are relatively broad (Mw/Mn =
1.46−1.52), a moderately controlled polymerization process is
suggested, where the rate of initiation is not so slow compared
to the rate of propagation.
To understand the differences between the initiation by an
amide group and an alkoxide group in our system, the rac-LA
polymerization was carried out with complexes 1a−8a and 4b
in the presence of 2-propanol. Before conducting systematic
polymerization studies, the NMR tube reaction of typical
complexes 2a, 2a·THF, and 5a with 2-propanol in benzene-d6
was monitored, respectively. The 1:1 ratio reaction of 2a or 2a·
THF with 2-propanol generated the same iminophenolato-
magnesium alkoxide “[L2MgOiPr]”, characterized by the
proton resonance peaks at 3.29, 2.13, and 1.70 ppm assignable
to methoxy, methyl, and tert-butyl of the ligand as well as a
multiplet at 4.17 ppm and a doublet at 1.20 ppm assignable to
the isopropoxyl group (Figure S3 in the SI). In comparison
with the original silylamido complex 2a, the methoxy proton
resonance of the formed “[L2MgOiPr]” species is significantly
upfield-shifted, indicating a dissociated state of this group.
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further addition of 1 equiv more of 2-propanol, however, led to
partial decomposition of the formed magnesium isorpopoxide,
as indicated by the appearance of free ligand L5H.
On the basis of the above studies, the polymerization of rac-
LA was initiated by the in situ generated magnesium
isopropoxide obtained by strictly controlling the molar ratio
of the magnesium silylamido complex to 2-propanol at 1:1. As
listed in Table 4, the polymerizations of rac-LA initiated by
complexes 1a−8a and 4b/2-propanol are well-controlled,
affording polymers with relatively narrow molecular weight
distributions (Mw/Mn = 1.06−1.40). Meanwhile, the measured
molecular weights (Mn) are close to the calculated ones. It is
worth noting that, in contrast to our previous results,7f the
addition of 2-propanol has a different influence on the
polymerization conducted in THF or in toluene. In THF, the
polymerizations of rac-LA initiated by complexes 1a−8a and 4b
in the presence of 2-propanol are unexpectedly slow compared
to those without the addition of 2-propanol, while the order in
toluene is still consistent with literature reports.7f,g For example,
using complex 1a as the initiator, the polymerization proceeded
to 90% conversion within 30 min in THF (entry 1), whereas
the yield just reached 92% after 60 min (entry 2) in the
presence of 1a/2-propanol under the same polymerization
conditions. In toluene, a monomer conversion of 96% could be
achieved by 1a in 330 min; the addition of 2-propanol
significantly shortened the polymerization time to 120 min to
reach a similar conversion of 90% (entries 3 and 4). It is thus
suggested that, although an alkoxide group is superior to an
amide group in initiation, a coordinative solvent may bring a
complicated effect during the polymerization by either
facilitating dissociation of the in situ formed dimeric metal
alkoxide species or blocking the coordination site via a
competitive coordination to the metal center.
Figure 7. Semilogarithmic plots of LA monomer conversion versus
time, [LA]0/[Mg]0 = 25, [LA]0 = 0.20 M, T = 25 °C, C6D6 (0.5 mL):
rac-LA polymerization using 1a (black ), kobs = 0.0043 min−1, R2 =
■
−1
2
●
▲
0.997, 2a (red ), kobs = 0.011 min , R = 0.996, 4a (blue ), kobs
=
0.0245 min , R = 0.983, 5a (green ), kobs = 0.11 min−1, R2 = 0.999,
−1
2
▼
and 8a (brown ), kobs = 0.0016 min−1, R2 = 0.967.
◆
To acquire some insight into the polymerization mechanism
of the magnesium complex/2-propanol system, the NMR-scale
polymerization was conducted with 2a·THF in the presence of
1 equiv of 2-propanol with [rac-LA]0:[2a·THF]0:[iPrOH]0 =
20:1:1. The polymerization started instantaneously, and the
active oligomer “{L2Mg[(OCH(CH3)CO)nOiPr]}” could be
identified unambiguously (Figure S6 in the SI). Meanwhile, a
Figure 8. Relationship of Mn and the polydispersity index (PDI) of the
PLA sample versus monomer conversion catalyzed by complex 1a
([rac-LA]0= 1.0 M, [rac-LA]0:[1a]0= 200:1, 25 °C, in THF).
1
small amount of (L2)2Mg could also be observed in the H
NMR spectrum, which is inactive for the polymerization. End-
group analysis by 1H NMR spectroscopy of the purified
oligomer sample showed clearly the existence of both terminal
groups of isopropoxy and HOCH(CH3)CO− according to the
resonances at about 1.22 and 5.01 ppm for the former and 1.48
and 4.36 ppm for the latter (Figure S7 in the SI). Electrospray
ionization time-of-flight mass spectroscopy (ESI-TOF-MS)
further revealed that the oligomers end-capped with
(CH3)2CHO− and HOCH(CH3)CO− groups were formed,
although there were some transesterification side reactions
during the polymerization evidenced by the existence of
oligomers with an odd number of lactate units. Accordingly,
the polymerization initiated by the magnesium complex/2-
propanol system is suggested to proceed via a common
“coordination−insertion” mechanism.
Microstructure analyses of PLAs were achieved through
inspection of the methine region of homonuclear-decoupled 1H
NMR spectra of the resultant polymers. All of the magnesium
complexes 1a−8a displayed moderate heterotactic selectivity
(Pr = 0.6−0.75) for the ROP of rac-LA in THF, whereas atactic
or isotactic bias PLAs (Pr = 0.50−0.40) were obtained in
toluene. Carpentier and co-workers27 reported that yttrium
Therefore, it is suggested that the in situ generated magnesium
isopropoxide “[L2MgOiPr]” should have a dimeric structure,
where each magnesium center is four-coordinated, instead of a
monomeric structure with a three-coordinated metal center. It
is worth noting that, besides the major “[L2MgOiPr]” product,
there is also a set of signals at 3.19, 2.15, and 1.75 ppm in small
amounts in the 1H NMR spectrum, which is in consistent with
the resonances of the bisligated magnesium complex
(L2)2Mg.26 The percentage of the bisligated complex increased
significantly upon standing overnight (in the case of 2a·THF,
from 1:4.5 to 1:1.2), indicative of a relatively fast rearrangement
process in solution.
The reaction of 5a with 2-propanol in a 1:1 ratio generated
the expected magnesium isopropoxide “[L5MgOiPr]” as the
single product (Figures S4 and S5a,b in the SI), suggesting that
an increase of the steric hindrance of the ortho substituent
could protect efficiently the formed magnesium isopropoxide
species from rearrangement or ligand scrambling. The methoxy
proton resonance appears at 3.19 ppm, which is 0.21 ppm
upfield-shifted from that of complex 5a. Obviously, the
methoxy group in “[L5MgOiPr]” is also dissociated from the
metal center, and a dimeric structure is therefore suggested. A
I
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complexes supported by dianionic alkoxyaminobis(phenolate)
ligands showed high heteroselectivity for rac-LA polymerization
in THF, whereas the propagating species in toluene generated
nearly atactic materials. A similar solvent effect was also found
by Chisholm and co-workers28 that magnesium complexes
bearing 1,5,9-trimesityldipyrromethene ligands showed much
higher stereoselectivity in THF than in a toluene/DCM
mixture. On the basis of the results of a computational analysis,
Rzepa and co-workers29 proposed that the high heteroselectiv-
ity obtained in the ROP of rac-LA by magnesium β-
diketiminate complexes in a coordinative solvent might be
attributed to a solvent effect, which serves to balance the system
entropically.
In comparison with the crucial effect of the solvent, variation
of the ligand structure represented by the ortho substituent at
the phenoxy moiety has a slight influence on the stereo-
selectivity. PLAs with nearly the same heteroselectivities were
obtained by complexes 1a−5a. Compared to complexes 1a−6a,
complexes 7a and 8a with a halo substituent displayed a slightly
enhanced preference for the heterotactic dyad enchainment in
THF (Pr = 0.73 and 0.75, entries 26 and 30; Figure S9 in the
SI). At the present stage, we have no reasonable explanation
about this point, but it is suggested that a steric effect arising
from a certain structure of these two complexes in solution
should be responsible for the observed selectivity. Moreover,
the only heteroleptic calcium complex 4b hardly showed
selectivity for the ROP of rac-LA in both solvents, resulting in
atactic to isotactic bias PLAs of high molecular weight, which
was in contrast to the moderately heteroselective magnesium
complex 4a with the same ligand (Pr = 0.69, in THF, entry 13).
It is implied that the coordination site in calcium complex 4b is
not sufficiently surrounded to impart any stereoselectivity in
the ROP event. The Pr values of the obtained polymers also
varied slightly upon a change in the polymerization temper-
ature. As listed in Table 4, entry 19, the Pr value of PLA
obtained by 5a in THF increased to 0.81 when the
polymerization temperature was decreased to −38 °C (Figure
S10 in the SI).
°C, producing heterotactic-enriched PLAs with Pr values up to
0.81.
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations were carried out
under a dry argon atmosphere using standard Schlenk-line or glovebox
techniques. Toluene and n-hexane were refluxed over sodium
benzophenone ketyl prior to use. Benzene-d6, chloroform-d, and
other reagents were carefully dried and stored in a glovebox.
{Mg[N(SiMe3)2]2}2,30 Ca[N(SiMe3)2]2·2THF,31 2-hydroxy-3-isopro-
pylbenzaldehyde,32 3-tert-butyl-2-hydroxy-5-methylbenzaldehyde,32
3,5-di-tert-butyl-2-hydroxybenzaldehyde,32 2-hydroxy-3,5-dicumylben-
zaldehyde,32 5-tert-butyl-2-hydroxy-3-tritylbenzaldehyde,33 5-tert-butyl-
3-chloro-2-hydroxybenzaldehyde,34 and 5-tert-butyl-3-bromo-2-hy-
droxybenzaldehyde,35 were synthesized according to literature
methods. rac-LA (Aldrich) was recrystallized with dry toluene and
then sublimed twice under vacuum at 80 °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 a vacuum−argon cycle three
times.
Measurements. NMR spectra were recorded on a Bruker
AVANCE-400 spectrometer at 25 °C (1H, 400 MHz; 13C, 100
1
MHz) unless otherwise stated. Chemical shifts for H and 13C NMR
spectra were referenced internally using the residual solvent
resonances and reported relative to tetramethylsilane. Elemental
analyses were performed on an EA-1106 instrument. Spectroscopic
analyses of polymers were performed in CDCl3. Gel permeation
chromatography (GPC) analyses were carried out on an Agilent
instrument (L1200 pump, Optilab Rex injector) in THF at 25 °C at a
flow rate of 1 mL·min−1. 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). The Mn
values were reported without correction.
Syntheses. 2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-6-isopro-
pylphenol (L1H). 2-Hydroxy-3-isopropylbenzaldehyde (0.821 g, 5.00
mmol) was mixed with 2-methoxy-2′-amidobiphenyl (0.996 g, 5.00
mmol) in ethanol (50 mL). The mixture was refluxed at 80 °C and
stirred for 5 h at this temperature. After cooling to room temperature,
the solution was concentrated to about 20 mL and kept at −20 °C.
Yellow crystalline solids could be obtained in 80% yield (1.209 g). 1H
NMR (CDCl3, 400 MHz, 298 K): δ 13.05 (s, 1H, OH), 8.49 (s, 1H,
NCHAr), 7.38 (m, 2H, ArH), 7.32 (m, 2H, ArH), 7.22 (t, 2H, J = 7.2
Hz, ArH), 7.17 (d, 1H, J = 7.8 Hz, ArH), 7.13 (d, 1H, J = 7.5 Hz,
ArH), 6.99 (t, 1H, J = 7.5 Hz, ArH), 6.91 (d, 1H, J = 8.0 Hz, ArH),
6.82 (t, 1H, J = 7.6 Hz, ArH), 3.68 (s, 3H, CH3OAr), 3.30 (sept, 1H, J
= 6.9 Hz, ArCH(CH3)2), 1.19 (d, 6H, J = 6.9 Hz, CH(CH3)2).
13C{1H} NMR (CDCl3, 100 MHz): δ 162.7, 158.6, 156.4, 147.5,
136.3, 133.7, 131.3, 131.1, 129.6, 129.4, 129.0, 128.5, 128.4, 126.4,
120.4, 118.6, 118.28, 118.27, 110.6 (NCHAr and all ArC), 55.2
(CH3OAr), 26.4 (ArCH(CH3)2), 22.2 (ArCH(CH3)2). HRMS. Calcd
for C23H23NO2: 345.1729. Found: 345.1726.
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4-methyl-6-tert-butyl-
phenol (L2H). The procedure was the same as that of L1H, except that
3-tert-butyl-2-hydroxy-5-methylbenzaldehyde (0.961 g, 5.00 mmol)
and 2-methoxy-2′-amidobiphenyl (0.996 g, 5.00 mmol) were used to
afford ligand L2H as yellow crystalline solids in 80% yield (1.494 g).
1H NMR (CDCl3, 400 MHz, 298 K): δ 13.35 (s, 1H, OH), 8.47 (s,
CONCLUSIONS
■
A series of racemic [ONO]-type iminophenols (L1H−L8H)
based on the biphenyl skeleton and their magnesium and
calcium silylamido complexes were synthesized and structurally
characterized. X-ray diffraction studies of typical magnesium
complexes revealed that, when a chloro or bromo substituent is
introduced to the ortho position of the phenoxide unit, the
corresponding magnesium complexes 7a and 8a crystallize
unexpectedly as dimers, with two metal centers bridging
through the oxygen atoms of two phenolato ligands, while the
other magnesium complexes are still monomeric in the solid
state, possessing a four-coordinate metal core. All of these
magnesium and calcium silylamido complexes exhibit good
catalytic activity toward rac-LA polymerization, and the
observed activity increases with an increase in the steric
bulkiness of the ortho substituent on the phenoxide unit.
Furthermore, calcium complex 4b is less active than magnesium
complex 4a with the same ligand, and complex 8a shows a
much lower activity among these complexes but the best
stereoselectivity for ROP of rac-LA in THF at room
temperature (Pr = 0.75). Additionally, the most sterically
hindered initiator 5a gains an enhanced heterotactic bias in the
polymerization of rac-LA in THF at a low temperature of −38
1H, NCHAr), 7.41 (m, 2H, ArH), 7.34 (t, 2H, J = 7.5 Hz, ArH), 7.22
(m, 2H, ArH), 7.12 (s, 1H, ArH), 7.00 (t, 1H, J = 7.4 Hz, ArH), 6.96
(m, 2H, ArH), 3.73 (s, 3H, CH3OAr), 2.28 (s, 3H, CH3Ar), 1.37 (s,
9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.7,
158.3, 156.5, 147.3, 137.2, 133.9, 131.3, 131.1, 131.0, 130.1, 129.0,
128.5, 128.4, 126.5, 126.4, 120.3, 118.8, 118.1, 110.5 (NCHAr and all
ArC), 55.2 (CH3OAr), 34.7 (C(CH3)3), 29.2 (C(CH3)3), 20.6
(CH3Ar). HRMS. Calcd for C25H27NO2: 373.2042. Found: 373.2041.
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4,6-di-tert-butylphe-
nol (L3H). The procedure was the same as that of L1H, except that 3,5-
J
dx.doi.org/10.1021/ic4012668 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
di-tert-butyl-2-hydroxybenzaldehyde (1.172 g, 5.000 mmol) and 2-
methoxy-2′-amidobiphenyl (0.996 g, 5.00 mmol) were used to afford
3.73 (s, 3H, CH3OAr), 1.30 (s, 9H, C(CH3)3). 13C{1H} NMR
(CDCl3, 100 MHz, 298 K): δ 162.0, 156.4, 155.5, 147.0, 142.0, 133.9,
131.3, 131.2, 130.5, 129.3, 128.5, 128.1, 127.0, 126.8, 121.0, 120.6,
119.5, 118.3, 110.9, (NCHAr and all ArC), 55.4 (CH3OAr), 34.1
(C(CH3)3), 31.2 (C(CH3)3). HRMS. Calcd for C24H24ClNO2:
393.1496. Found: 393.1488.
1
ligand L3H as yellow crystalline solids in 82% yield (1.704 g). H
NMR (CDCl3, 400 MHz, 298 K): δ 13.40 (s, 1H, OH), 8.53 (s, 1H,
NCHAr), 7.43−7.38 (m, 3H, ArH), 7.34 (m, 2H, ArH), 7.21 (m, 2H,
ArH), 7.15 (d, 1H, J = 2.3 Hz, ArH), 6.99 (t, 1H, J = 7.4 Hz, ArH),
6.95 (d, 1H, J = 8.3 Hz, ArH), 3.77 (s, 3H, CH3OAr), 1.39 (s, 9H,
C(CH3)3), 1.31 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz,
298 K): δ 163.1, 158.2, 156.5, 147.4, 140.0, 136.7, 134.1, 131.3, 131.0,
128.9, 128.50, 128.46, 127.6, 126.42, 126.37, 120.3, 118.3, 118.0,
110.5, (NCHAr and all ArC), 55.2 (CH3OAr), 35.0 (C(CH3)3), 34.1
(C(CH3)3), 31.4 (C(CH3)3), 29.3 (C(CH3)3). HRMS. Calcd for
C28H33NO2: 415.2511. Found: 415.2509.
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4-tert-butyl-6-bromo-
phenol (L8H). The procedure was same as that of L1H, except that 5-
tert-butyl-3-bromo-2-hydroxybenzaldehyde (1.286 g, 5.000 mmol) and
2-methoxy-2′-amidobiphenyl (0.996 g, 5.00 mmol) were used to afford
1
ligand L8H as orange crystalline solids in 89% yield (1.951 g). H
NMR (CDCl3, 400 MHz, 298 K): δ 13.30 (s, 1H, OH), 8.47 (s, 1H,
NCHAr), 7.59 (d, 1H, J = 2.3 Hz, ArH), 7.45−7.32 (m, 4H, ArH),
7.25 (d, 1H, J = 2.3 Hz, ArH), 7.21 (m, 2H, ArH), 7.01 (t, 1H, J = 7.4
Hz, ArH), 6.95 (d, 1H, J = 8.3 Hz, ArH), 3.73 (s, 3H, CH3OAr), 1.30
(s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.0,
156.3, 155.5, 147.0, 142.5, 133.9, 133.5, 131.3, 131.2, 129.3, 128.5,
128.1, 127.9, 126.8, 120.6, 119.4, 118.4, 110.9, 110.5 (NCHAr and all
ArC), 55.3 (CH3OAr), 34.1 (C(CH3)3), 31.3 (C(CH3)3). HRMS.
Calcd for C24H25BrNO2 [M + H]: 438.1069. Found: 438.1063.
[(L1)MgN(SiMe3)2] (1a). In a glovebox, the ligand L1H (0.345 g,
1.00 mmol) was added slowly to a solution of {Mg[N(SiMe3)2]2}2
(0.345 g, 0.500 mmol) in toluene (20 mL). The reaction solution was
stirred at room temperature for 24 h. All of the volatiles were removed
under vacuum. The resultant yellow solids were recrystallized with a
mixture of toluene and n-hexane at −38 °C to afford yellow crystalline
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4,6-dicumylphenol
(L4H). The procedure was the same as that of L1H, except that 2-
hydroxy-3,5-dicumylbenzaldehyde (1.792 g, 5.000 mmol) and 2-
methoxy-2′-amidobiphenyl (0.996 g, 5.00 mmol) were used to afford
1
ligand L4H as an orange viscous oil in 75% yield (2.024 g). H NMR
(CDCl3, 400 MHz, 298 K): δ 13.09 (s, 1H, OH), 8.39 (s, 1H,
NCHAr), 7.36−7.28 (m, 5H, ArH), 7.25 (m, 3H, ArH), 7.24−7.18 (m,
4H, ArH), 7.13 (m, 4H, ArH), 7.04 (m, 2H, ArH), 6.88 (t, 1H, J = 7.4
Hz, ArH), 6.61 (d, 1H, J = 8.3 Hz, ArH), 3.35 (s, 3H, CH3OAr), 1.69
(s, 6H, C(CH3)2Ph), 1.58 (s, 6H, C(CH3)2Ph). 13C{1H} NMR
(CDCl3, 100 MHz, 298 K): δ 162.2, 157.7, 156.1, 150.7, 146.9, 139.4,
136.4, 134.2, 131.1, 130.8, 129.2, 128.7, 128.4, 128.2, 128.0, 127.5,
126.7, 126.5, 125.7, 125.6, 124.7, 120.0, 118.3, 117.6, 110.5 (NCHAr
and all ArC), 54.8 (CH3OAr), 42.4 (C(CH3)2Ph), 41.9 (C(CH3)2Ph),
30.9 (C(CH3)2Ph), 29.0 (C(CH3)2Ph). HRMS. Calcd for C38H37NO2:
539.2824. Found: 539.2827.
1
solids in 53% yield (0.280 g). H NMR (C6D6, 400 MHz, 298 K): δ
7.64 (s, 1H, NCHAr), 7.16 (d, 1H, J = 7.2 Hz, ArH), 7.09 (m, 1H,
ArH), 7.01 (m, 2H, ArH), 6.86 (d, 1H, J = 7.7 Hz, ArH), 6.78 (d, 2H, J
= 7.3 Hz, ArH), 6.67 (m, 3H, ArH), 6.46 (t, 1H, J = 7.5 Hz, ArH), 3.82
(sept, 1H, J = 6.9 Hz, CH(CH3)2), 3.61 (s, 3H, CH3OAr), 1.37 (d,
3H, J = 6.9 Hz, CH(CH3)2), 1.25 (d, 3H, J = 6.9 Hz, CH(CH3)2), 0.39
(s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ
173.6, 169.3, 153.4, 150.1, 141.7, 133.6, 132.4, 132.3, 132.1, 131.5,
130.2, 130.24, 130.04, 127.4, 126.8, 124.0, 121.7, 119.2, 114.4
(NCHAr and all ArC), 66.1 (CH3OAr), 27.2 (CH(CH3)2), 23.0
(CH(CH3)2), 22.3 (CH(CH3)2), 5.7 (N(Si(CH3)3)2). Anal. Calcd for
C29H40MgN2O2Si2: C, 65.83; H, 7.62; N, 5.29. Found: C, 65.73; H,
7.65; N, 5.22.
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4-tert-butyl-6-trityl-
phenol (L5H). The procedure was the same as that of L1H, except that
5-tert-butyl-2-hydroxy-3-tritylbenzaldehyde (2.101 g, 5.000 mmol) and
2-methoxy-2′-amidobiphenyl (0.996 g, 5.00 mmol) were used to afford
1
ligand L5H as yellow crystalline solids in 72% yield (2.165 g). H
NMR (CDCl3, 400 MHz, 298 K): δ 13.10 (s, 1H, OH), 8.43 (s, 1H,
NCHAr), 7.38 (m, 2H, ArH), 7.29 (m, 2H, ArH), 7.18−7.10 (m, 18H,
ArH), 7.04 (dd, 1H, J = 7.3 and 1.2 Hz, ArH), 6.88 (t, 1H, J = 7.3 Hz,
ArH), 6.56 (d, 1H, J = 8.0 Hz, ArH), 3.24 (s, 3H, CH3OAr), 1.70 (s,
9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.6,
158.0, 155.9, 147.2, 145.5, 139.7, 134.1, 134.0, 132.2, 131.1, 131.0,
130.84, 130.78, 129.0, 128.8, 128.4, 128.2, 128.0, 127.3, 127.0, 126.4,
125.3, 120.0, 118.5, 118.2, 110.7 (NCHAr and all ArC), 63.4
(ArCPh3), 54.8 (CH3OAr), 34.0 (C(CH3)3), 31.3 (C(CH3)3).
HRMS. Calcd for C43H39NO2: 601.2981. Found: 601.2980.
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4-tert-butyl-6-(piperi-
din-1-ylmethyl)phenol (L6H). The procedure was the same as that of
L1H, except that 5-tert-butyl-2-hydroxy-3-[(piperidin-1-yl)methyl]-
benzaldehyde (1.377 g, 5.000 mmol) and 2-methoxy-2′-amidobiphenyl
(0.996 g, 5.00 mmol) were used to afford ligand L6H as yellow
crystalline solids in 86% yield (1.963 g). 1H NMR (CDCl3, 400 MHz,
298 K): δ 8.57 (s, 1H, NCHAr), 7.40 (m, 3H, ArH), 7.32 (m, 2H,
ArH), 7.26 (s, 1H, ArH), 7.23 (dd, 1H, J = 7.2 and 1.6 Hz, ArH), 7.19
(d, 1H, J = 8.0 Hz, ArH), 7.00 (t, 1H, J = 7.4 Hz, ArH), 6.92 (d, 1H, J
= 8.2 Hz, ArH), 3.72 (s, 3H, CH3OAr), 3.57 (s, 2H, NCH2Ar), 2.44
(br, 4H, NCH2CH2), 1.60 (m, 4H, NCH2CH2), 1.44 (m, 2H,
CH2CH2CH2), 1.30 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100
MHz, 298 K): δ 162.0, 157.2, 156.5, 148.2, 140.6, 133.7, 131.3, 131.2,
131.0, 128.8, 128.6, 128.4, 126.5, 126.1, 124.7, 120.3, 118.8, 118.2,
110.7 (NCHAr and all ArC), 57.1 (NCH2Ar), 55.3 (CH3OAr), 54.2
(NCH2CH2), 33.9 (C(CH3)3), 31.4 (C(CH3)3), 26.0 (NCH2CH2),
24.3 (CH2CH2CH2). HRMS. Calcd for C30H36N2O2: 456.2777.
Found: 456.2774.
[(L2)MgN(SiMe3)2] (2a). Following a procedure similar to that
described for 1a, L2H (0.373 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Yellow crystals
could be obtained after recrystallization from a mixture of toluene and
1
n-hexane at −38 °C in 52% yield (0.290 g). H NMR (C6D6, 400
MHz, 298 K): δ 7.60 (s, 1H, NCHAr), 7.20 (s, 1H, ArH), 7.15 (s, 2H,
ArH), 7.12 (m, 2H, ArH), 7.01 (m, 5H, ArH), 6.82 (d, 1H, J = 7.3 Hz,
ArH), 6.79 (d, 1H, J = 7.3 Hz, ArH), 6.71 (m, 2H, ArH), 6.37 (s, 1H,
ArH), 3.57 (s, 3H, CH3OAr), 2.10 (s, 3H, CH3Tol), 2.06 (s, 3H,
CH3Ar), 1.64 (s, 9H, C(CH3)3), 0.40 (s, 18H, N(Si(CH3)3)2).
13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 173.6, 169.0, 153.5, 150.4,
141.8, 134.7, 133.7, 132.6, 132.2, 131.6, 130.2, 130.0, 129.3, 128.5,
127.9, 127.5, 126.7, 125.6, 124.0, 122.0, 121.7, 120.9, 119.8 (NCHAr
and all ArC), 66.0 (CH3OAr), 35.4 (C(CH3)3), 29.7 (C(CH3)3), 21.4
(CH3Tol), 20.5 (CH3Ar), 5.7 (N(Si(CH3)3)2). Anal. Calcd for
C31H44MgN2O2Si2·C7H8: C, 70.29; H, 8.07; N, 4.31. Found: C,
70.48; H, 8.07; N, 4.15.
[(L2)MgN(SiMe3)2·THF] (2a·THF). Complex 2a·THF was obtained
as yellow crystals by recrystallizing complex 2a in a THF/n-hexane
mixture at −38 °C. 1H NMR (C6D6, 400 MHz, 298 K): δ 7.86 (s, 1H,
NCHAr), 7.24 (d, 1H, J = 2.2 Hz, ArH), 7.18 (m, 1H, ArH), 7.13−
7.07 (m, 2H, ArH), 7.03 (m, 2H, ArH), 6.83 (t, 1H, J = 7.5 Hz, ArH),
6.73 (t, 1H, J = 7.5 Hz, ArH), 6.62 (d, 1H, J = 8.0 Hz, ArH), 6.46 (s,
1H, ArH), 3.49 (br, 4H, THF), 3.38 (s, 3H, CH3OAr), 2.09 (s, 3H,
CH3Ar), 1.62 (s, 9H, C(CH3)3), 1.31 (br, 4H, THF), 0.37 (s, 18H,
N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 174.7,
168.8, 155.3, 150.7, 141.5, 134.5, 134.0, 132.1, 132.0, 131.2, 130.8,
129.7, 129.3, 126.4, 124.6, 123.8, 121.9, 119.9, 115.9 (NCHAr and all
ArC), 68.4 (THF), 60.0 (CH3OAr), 35.4 (C(CH3)3), 29.8 (C(CH3)3),
25.4 (THF), 20.6 (CH3Ar), 5.9 (N(Si(CH3)3)2). Anal. Calcd for
2-[(2′-Methoxybiphenyl-2-ylimino)methyl]-4-tert-butyl-6-chloro-
phenol (L7H). The procedure was the same as that of L1H, except that
5-tert-butyl-3-chloro-2-hydroxybenzaldehyde (1.063 g, 5.000 mmol)
and 2-methoxy-2′-amidobiphenyl (0.996 g, 5.00 mmol) were used to
afford ligand L7H as orange crystalline solids in 86% yield (1.694 g).
1H NMR (CDCl3, 400 MHz, 298 K): δ 13.17 (s, 1H, OH), 8.51 (s,
1H, NCHAr), 7.43 (m, 2H, ArH), 7.36 (m, 3H, ArH), 7.21 (m, 3H,
ArH), 7.01 (t, 1H, J = 7.4 Hz, ArH), 6.95 (d, 1H, J = 8.3 Hz, ArH),
K
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Inorganic Chemistry
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C31H44MgN2O2Si2·C4H8O: C, 66.91; H, 8.18; N, 4.46. Found: C,
66.42; H, 8.40; N, 4.21.
(CH3OAr), 57.7 (NCH2Ar), 55.1 (NCH2CH2), 33.8 (C(CH3)3),
31.5 (C(CH3)3), 26.8 (NCH2CH2), 25.1 (CH2CH2CH2), 5.8
(N(Si(CH3)3)2). Anal. Calcd for C36H53MgN3O2Si2: C, 67.53; H,
8.34; N, 6.56. Found: C, 67.98; H, 8.06; N, 6.67.
[(L3)MgN(SiMe3)2] (3a). Following a procedure similar to that
described for 1a, L3H (0.415 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Recrystallization
with a mixture of toluene and n-hexane at −38 °C afforded yellow
[(L7)MgN(SiMe3)2]2 (7a). Following a procedure similar to that
described for 1a, L7H (0.394 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Recrystallization
with a mixture of toluene and n-hexane at room temperature afforded
yellow crystals in 52% yield (0.299 g). 1H NMR (C6D6, 400 MHz, 298
K): δ 7.54 (s, 1H, NCHAr), 7.52 (d, 1H, J = 2.6 Hz, ArH), 7.09 (m,
1H, ArH), 7.02 (m, 2H, ArH), 6.82 (m, 2H, ArH), 6.73 (m, 1H, ArH),
6.71−6.67 (m, 2H, ArH), 6.58 (d, 1H, J = 2.6 Hz, ArH), 3.61 (s, 3H,
CH3OAr), 1.01 (s, 9H, ArC(CH3)3), 0.41 (s, 18H, N(Si(CH3)3)2).
13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 173.3, 164.1, 153.2, 149.7,
136.6, 133.9, 132.2, 132.0, 131.5, 130.5, 130.2, 129.9, 129.7, 127.4,
127.1, 123.8, 121.9, 121.6, 119.4 (NCHAr and all ArC), 66.1
(CH3OAr), 33.6 (C(CH3)3), 31.2 (C(CH3)3), 5.8 (N(Si(CH3)3)2).
Anal. Calcd for C60H82Cl2Mg2N4O4Si4: C, 62.38; H, 7.15; N, 4.85.
Found: C, 61.75; H, 7.11; N, 4.82.
1
crystalline solids in 55% yield (0.329 g). H NMR (C6D6, 400 MHz,
298 K): δ 7.67 (s, 1H, NCHAr), 7.57 (s, 1H, ArH), 7.11 (t, 1H, J = 8.0
Hz, ArH), 7.02 (m, 2H, ArH), 6.92 (d, 1H, J = 7.6 Hz, ArH), 6.83 (m,
2H, ArH), 6.70 (m, 3H, ArH), 3.59 (s, 3H, CH3OAr), 1.67 (s, 9H,
C(CH3)3), 1.22 (s, 9H, C(CH3)3), 0.39 (s, 18H, N(Si(CH3)3)2).
13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 174.0, 169.0, 153.5, 150.4,
141.5, 135.5, 132.6, 132.2, 131.6, 131.2, 130.2, 130.0, 129.97, 129.94,
127.6, 126.7, 124.1, 121.7, 119.3 (NCHAr and all ArC), 66.0
(CH3OAr), 35.8 (C(CH3)3), 33.9 (C(CH3)3), 31.5 (C(CH3)3), 29.7
(C(CH3)3), 5.8 (N(Si(CH3)3)2). Anal. Calcd for C34H50MgN2O2Si2:
C, 68.15; H, 8.41; N, 4.67. Found: C, 68.14; H, 8.51; N, 4.54.
[(L4)MgN(SiMe3)2] (4a). Following a procedure similar to that
described for 1a, L4H (0.540 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Recrystallization
with a mixture of toluene and n-hexane at −38 °C afforded yellow
[(L8)MgN(SiMe3)2] (8a). Following a procedure similar to that
described for 1a, L8H (0.438 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Recrystallization
with a mixture of toluene and n-hexane at room temperature afforded
yellow crystals in 52% yield (0.322 g). 1H NMR (C6D6, 400 MHz, 298
K): δ 7.73 (d, 1H, J = 2.5 Hz, ArH), 7.52 (s, 1H, NCHAr), 7.11−7.07
(m, 1H, ArH), 7.02 (m, 2H, ArH), 6.82 (m, 2H, ArH), 6.76 (dd, 1H, J
= 7.7 and 1.4 Hz, ArH), 6.72−6.65 (br m, 2H, ArH), 6.62 (d, 1H, J =
2.5 Hz, ArH), 3.62 (s, 3H, CH3OAr), 1.00 (s, 9H, C(CH3)3), 0.41 (s,
18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ
173.3, 153.3, 149.7, 137.3, 137.2, 132.2, 132.0, 131.6, 130.9, 130.6,
130.1, 129.9, 129.5, 127.4, 127.1, 123.9, 121.9, 119.1, 119.0 (NCHAr
and all ArC), 66.1 (CH3OAr), 33.5 (C(CH3)3), 31.2 (C(CH3)3), 5.8
(N(Si(CH3)3)2). Anal. Calcd for C60H82Br2Mg2N4O4Si4: C, 57.93; H,
6.64; N, 4.50. Found: C, 57.40; H, 6.60; N, 4.31.
1
crystalline solids in 56% yield (0.405 g). H NMR (C6D6, 400 MHz,
298 K): δ 7.64 (s, 1H, NCHAr), 7.41 (d, 1H, J = 2.6 Hz, ArH), 7.26
(d, 2H, J = 7.0 Hz, ArH), 7.18 (m, 4H, ArH), 7.09 (m, 4H, ArH), 7.00
(m, 3H, ArH), 6.91 (d, 1H, J = 7.8 Hz, ArH), 6.77 (m, 3H, ArH), 6.66
(t, 1H, J = 7.0 Hz, ArH), 6.53 (d, 1H, J = 8.0 Hz, ArH), 3.45 (s, 3H,
CH3OAr), 2.00 (br s, 3H, C(CH3)2Ph), 1.54 (s, 9H, C(CH3)2Ph),
0.26 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298
K): δ 173.3, 168.4, 153.3, 151.9, 151.1, 150.3, 141.4, 135.2, 133.8,
132.3, 131.9, 131.7, 131.6, 130.0, 129.9, 129.8, 127.5, 127.1, 126.7,
126.2, 125.8, 124.5, 124.0, 122.1, 119.2 (NCHAr and all ArC), 65.8
(CH3OAr), 43.0 (C(CH3)2Ph), 42.4 (C(CH3)2Ph), 31.9 (C-
(CH3)2Ph), 31.0 (C(CH3)2Ph), 5.8 (N(Si(CH3)3)2). Anal. Calcd for
C44H54MgN2O2Si2: C, 73.05; H, 7.52; N, 3.87. Found: C, 72.52; H,
7.46; N, 3.62.
[(L2)2Ca] (2b). The ligand L2H (0.373 g, 1.00 mmol) was added
slowly to a solution of Ca[N(SiMe3)2]2·2THF (0.507 g, 1.00 mmol) in
toluene (20 mL), and the reaction solution was stirred at room
temperature for 24 h. All of the volatiles were removed under vacuum.
The resultant solids were recrystallized with a mixture of toluene and
[(L5)MgN(SiMe3)2] (5a). Following a procedure similar to that
described for 1a, L5H (0.602 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Recrystallization
with a mixture of toluene and n-hexane at −38 °C afforded yellow
1
n-hexane to give yellow crystals in 55% yield (0.216 g). H NMR
1
crystalline solids in 50% yield (0.395 g). H NMR (C6D6, 400 MHz,
(C6D6, 400 MHz, 298 K): δ 7.59 (s, 2H, NCHAr), 7.26 (s, 2H, ArH),
7.12, (br, 2H, ArH), 7.04 (br, 4H, ArH), 7.01−6.96 (m, 4H, ArH),
6.68 (br, 4H, ArH), 6.50 (s, 2H, ArH), 6.23 (br, 2H, ArH), 3.38 (s,
6H, CH3OAr), 2.12 (s, 6H, CH3Ar), 1.84 (s, 18H, C(CH3)3). 13C{1H}
NMR (C6D6, 100 MHz, 298 K): δ 168.5, 159.0, 152.1, 151.9, 140.8,
134.9, 133.7, 133.0, 132.5, 132.1, 131.2, 130.5, 130.3, 129.2, 129.0,
125.0, 122.7, 121.5, 119.4 (NCHAr and all ArC), 43.6 (CH3OAr), 31.9
(C(CH3)3), 30.2 (C(CH3)3), 23.0 (CH3Ar).
298 K): δ 7.63 (s, 1H, NCHAr), 7.59 (d, 1H, J = 2.7 Hz, ArH), 7.49
(m, 6H, ArH), 7.17 (m, 8H, ArH), 7.09 (m, 4H, ArH), 7.01 (m, 2H,
ArH), 6.80 (d, 1H, J = 7.6 Hz, ArH), 6.74 (d, 1H, J = 2.6 Hz, ArH),
6.69 (t, 1H, J = 7.6 Hz, ArH), 6.60 (d, 1H, J = 7.8 Hz, ArH), 3.40 (s,
3H, CH3OAr), 1.05 (s, 3H, C(CH3)3), 0.15 (s, 18H, N(Si(CH3)3)2).
13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 173.6, 168.0, 153.5, 150.6,
139.7, 136.7, 135.1, 132.3, 131.8, 131.6, 131.2, 129.8, 129.6, 127.5,
126.8, 125.5, 123.9, 122.2, 119.6 (NCHAr and all ArC; ArC signals
could not be fully detected because of the poor solubility of 5a in
C6D6), 65.9 (CH3OAr), 64.4 (ArCPh3), 33.8 (C(CH3)3), 31.2
(C(CH3)3), 5.8 (N(Si(CH3)3)2). Anal. Calcd for C49H56MgN2O2Si2:
C, 74.93; H, 7.19; N, 3.57. Found: C, 74.68; H, 7.19; N, 3.34.
[(L6)MgN(SiMe3)2] (6a). Following a procedure similar to that
described for 1a, L6H (0.456 g, 1.00 mmol) was treated with
{Mg[N(SiMe3)2]2}2 (0.345 g, 0.500 mmol) in toluene (20 mL) at
room temperature to give yellow solids after workup. Recrystallization
with a mixture of toluene and n-hexane at room temperature afforded
yellow crystals in 53% yield (0.339 g). 1H NMR (C6D6, 400 MHz, 298
K): δ 7.84 (d, 1H, J = 2.6 Hz, ArH), 7.68 (s, 1H, NCHAr), 7.12 (m,
1H, ArH), 7.05 (m, 2H, ArH), 6.90 (d, 1H, J = 7.6 Hz, ArH), 6.85−
6.80 (br, 2H, ArH), 6.72 (m, 3H, ArH), 3.87 (br s, 1H, NCH2Ar), 3.81
(br s, 1H, NCH2Ar), 3.65 (s, 3H, CH3OAr), 2.50 (br, 4H,
NCH2CH2−), 1.57 (br, 4H, NCH2CH2−), 1.38 (br, 2H,
CH2CH2CH2), 1.23 (s, 9H, C(CH3)3), 0.39 (s, 18H, N(Si(CH3)3)2).
13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 173.6, 168.5, 153.5, 150.3,
136.1, 134.7, 132.5, 132.3, 131.72, 131.68, 130.2, 130.0, 129.9, 129.6,
127.4, 126.8, 124.2, 121.8, 118.3 (NCHAr and all ArC), 66.1
[(L4)CaN(SiMe3)2·THF] (4b). Following a procedure similar to that
described for 1a, L4H (0.540 g, 1.00 mmol) was treated with
Ca[N(SiMe3)2]2·2THF (0.507 g, 1.00 mmol) in toluene (20 mL) at
room temperature to give white solids after evaporation to dryness.
Recrystallization with a mixture of toluene and n-hexane at −38 °C
1
afforded yellow crystals in 56% yield (0.454 g). H NMR (C6D6, 400
MHz, 298 K): δ 7.70 (s, 1H, NCHAr), 7.49 (d, 1H, J = 2.1 Hz, ArH),
7.42 (d, 2H, J = 7.8 Hz, ArH), 7.28 (d, 2H, J = 7.8 Hz, ArH), 7.17 (br,
4H, ArH), 7.08−6.96 (br, 5H, ArH), 6.98 (m, 2H, ArH), 6.76 (m, 2H,
ArH), 6.68 (d, 1H, J = 7.8 Hz, ArH), 6.64 (d, 1H, J = 7.8 Hz, ArH),
3.57 (s, 3H, CH3OAr), 3.02 (m, 4H, THF), 1.86 (br, 3H,
C(CH3)2Ph), 1.72 (br, 3H, C(CH3)2Ph), 1.62 (s, 6H, C(CH3)2Ph),
1.02 (m, 4H THF), 0.30 (s, 18H, N(Si(CH3)3)2). 13C NMR (C6D6,
100 MHz, 298 K): δ 171.7, 168.2, 154.8, 152.9, 152.6, 151.8, 140.3,
133.4, 132.9, 132.5, 132.4, 132.0, 131.4, 130.4, 129.5, 129.3, 127.2,
126.6, 126.4, 125.8, 125.7, 125.4, 124.4, 124.1, 120.8 (NCHAr and all
ArC), 68.6 (THF), 64.3 (CH3OAr), 42.7 (C(CH3)2Ph), 42.4
(C(CH3)2Ph), 31.9 (C(CH3)2Ph), 31.1 (C(CH3)2Ph), 25.0 (THF),
5.9 (N(Si(CH3)3)2). Anal. Calcd for C48H62CaN2O3Si2: C, 71.06; H,
7.70; N, 3.45. Found: C, 71.10; H, 7.99; N, 3.32.
L
dx.doi.org/10.1021/ic4012668 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[(L5)2Ca] (5b). The proligand L5H (0.602 g, 1.00 mmol) was added
slowly to a solution of Ca[N(SiMe3)2]2·2THF (0.507 g, 1.00 mmol) in
toluene (20 mL), and the reaction solution was stirred at room
temperature for 24 h. All of the volatiles were removed under vacuum.
The resultant solids were recrystallized with a mixture of toluene and
99.012(2)°, γ = 96.196(2)°, Z = 2. For 6a, C39.50H56.50MgN3O2Si2,
triclinic, P1; a = 12.8011(19) Å, b = 13.056(2) Å, c = 15.255(2) Å, α =
̅
66.387(2)°, β = 74.955(3)°, γ = 61.162(3)°, Z = 2. For complex 7a,
C60H82Cl2Mg2N4O4Si4, monoclinic, P2(1)/c; a = 16.8799(10) Å, b =
13.7086(8) Å, c = 29.2964(17) Å, α = γ = 90°, β = 101.5630(10)°, Z =
4. For complex 8a, C60H82Br2Mg2N4O4Si4, monoclinic, P2(1)/c; a =
17.023(2) Å, b = 13.7491(18) Å, c = 29.338(4) Å, α = γ = 90°, β =
1
n-hexane to give yellow crystals in 60% yield (0.372 g). H NMR
(C6D6, 400 MHz, 298 K): δ 7.45−7.35 (br, 16H, ArH), 7.11 (d, 2H, J
= 7.2 Hz, toluene), 7.10−6.82 (br, 28H, ArH), 6.75 (t, 3H, J = 7.2 Hz,
toluene), 6.65−6.25 (br, 4H, ArH), 6.20−5.80 (br, 4H, ArH), 2.90−
2.46 (br s, 6H, CH3OAr), 2.10 (s, 3H, CH3Tol), 1.20 (s, 18H,
C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 168.7, 146.4,
145.5, 138.0, 135.0, 134.7, 131.3, 131.0, 130.9, 130.8, 128.8, 128.6,
127.0, 126.7, 124.5, 124.1, 120.7, 118.0 (NCHAr and all ArC), 63.7
(CH3OAr), 54.8 (CPh3Ar), 33.7 (C(CH3)3), 31.3 (C(CH3)3). Anal.
Calcd for C86H76CaN2O4·C7H8: C, 83.75; H, 6.35; N, 2.10. Found: C,
84.27; H, 6.45; N, 1.77.
101.930(2)°, Z = 4. For 2b, C60.50H64.50CaN O , triclinic, P1; a =
̅
2
4
13.118(4) Å, b = 13.922(5) Å, c = 17.106(5) Å, α = 112.018(5)°, β =
96.288(6)°, γ = 107.016(5)°, Z = 2.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Crystallographic data for 2a, 2a·THF, 6a, 7a, 8a, and 2b, H
NMR spectrum of the reaction of L2H and Ca[N(SiMe3)2]2·
2THF in a ratio of 1:1.5, variable-temperature 1H NMR spectra
Typical Polymerization Experiments. In a Braun Labstar
glovebox, an initiator solution from a stock solution in THF or
toluene was injected sequentially into a series of 10 mL vials loaded
with rac-LA and suitable amounts of dry solvent. After specified time
intervals, each vial was taken out of the glovebox; an aliquot was
withdrawn and quenched quickly with wet light petroleum ether, the
reaction mixture was quenched at the same time by adding an excess
amount of light petroleum ether and one drop of water. All of the
volatiles in the aliquots were removed, and the residue was subjected
to monomer conversion determination, which was monitored by
1
of 8a in toluene-d8, H NMR trace spectra of the reaction
between complex 2a, 2a·THF, or 5a and 2-propanol, 1H NMR
spectrum and ESI-TOF MS spectrum of the PLA oligomer
obtained by 2a·THF/2-propanol, homonuclear-decoupled H
1
NMR spectra of PLAs by complexes 5a and 8a, and X-ray
crystallographic data of complexes 2a, 2a·THF, 6a, 7a, 8a, and
2b in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
integration of monomer versus polymer methine resonances in H
AUTHOR INFORMATION
Corresponding Author
*E-mail: haiyanma@ecust.edu.cn. Fax/Tel.: +86 21 64253519.
NMR (CDCl3, 400 MHz, 298 K). The precipitates collected from the
bulk mixture were dried in air, dissolved with DCM, and sequentially
precipitated into methanol. The obtained polymer was further dried in
a vacuum oven at 50 °C for 16 h. Each reaction was used as one data
point. In the cases where 2-propanol was used, the solution of initiator
was injected into the solution of rac-LA and 2-propanol in toluene or
THF. Otherwise, the procedures were the same.
■
Notes
The authors declare no competing financial interest.
Kinetic Studies of rac-LA Polymerization. A solution of the
magnesium silylamido complex (0.016 mmol) in 1 mL of C6D6 was
prepared. An NMR tube was charged with rac-LA (14.4 mg, 0.1
mmol), and then 0.3 mL of C6D6 ([LA]0 = 0.2 M) and 0.2 mL of the
catalyst solution were injected sequentially. The monomer conversion
was continuously analyzed by 1H NMR spectroscopy at room
temperature.
ACKNOWLEDGMENTS
■
This work is subsidized by the National Natural Science
Foundation of China (Grant 21074032), the Program for New
Century Excellent Talents in University (Grant NCET-06-0413
to H.M.), and the Fundamental Research Funds for the Central
Universities (Grants WD1113011 and WK1214048). All
financial support is gratefully acknowledged. H.M. also is
thankful for the very kind donation of a Braun glovebox by the
AvH Foundation.
Oligomerization Experiment. Oligomerization of rac-LA was
carried out in C6D6 at 25 °C with complex 2a·THF as the initiator
under the condition of [rac-LA]0:[2a·THF]0:[iPrOH]0 = 20:1:1. The
1
reaction mixture was stirred for 0.5 h and monitored with H NMR
spectroscopy. The mixture was then quenched by adding wet n-
REFERENCES
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17.275(12) Å, α = γ = 90°, β = 96.899(12)°, Z = 4. For 2a·THF,
C H MgN O Si ·C H O, triclinic, P1; a = 9.4272(8) Å, b =
̅
31 44
2
2
2
4
8
12.9769(10) Å, c = 17.2741(14) Å, α = 100.008(2)°, β =
M
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dx.doi.org/10.1021/ic4012668 | Inorg. Chem. XXXX, XXX, XXX−XXX

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