Exploring Steric Effects of Zinc Complexes Bearing Achiral Benzoxazolyl Aminophenolate Ligands in Isoselective Polymerization of rac-Lactide

Article abstract of DOI:10.1021/acs.inorgchem.8b01839

A series of tridentate achiral benzoxazolyl-based aminophenolate zinc complexes, LZnN(SiMe₃)₂ (L = 2-{[benzoxazoly-CH₂N(R³)-]CH₂}-6-R¹-4-R²-C₆H₂O, R¹

Full text of DOI:10.1021/acs.inorgchem.8b01839

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Exploring Steric Effects of Zinc Complexes Bearing Achiral
Benzoxazolyl Aminophenolate Ligands in Isoselective
Polymerization of rac-Lactide
Jianwen Hu, Chao Kan, and Haiyan Ma*
Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, School of Chemistry and
Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China
S
* Supporting Information
ABSTRACT: A series of tridentate achiral benzoxazolyl-
based aminophenolate zinc complexes, LZnN(SiMe3)2 (L = 2-
{[benzoxazoly-CH2N(R3)-]CH2}-6-R1-4-R2-C6H2O, R1 = R2
= Cl, R3 = Bn (1); R1 = R2 = tBu, R3 = Bn (2); R1 = trityl, R2
= Me: R3 = Bn (3); R3 = phenethyl (4); R3 = 3-methylbutyl
(7); R3 = n-hexyl (8); R3 = cyclopentyl (9); R3 = cyclooctyl
(11); R3 = 1-adamantyl (12)), was synthesized via the
reactions of Zn[N(SiMe3)2]2 and 1 equiv of the correspond-
ing aminophenol proligands. All of the complexes were
obtained as racemates, and the X-ray diffraction studies
confirmed the monomeric structures of typical complexes 11
and 12, where the metal center is tetra-coordinated by three donors of the aminophenolate ligand and one silylamido group. All
of the complexes proved to be efficient initiators for the ring-opening polymerization of rac-lactide (rac-LA) at ambient
temperature, and the polymerizations were better controlled in the presence of 2-propanol. The substituents on the ortho-
position of the phenoxide unit of the ligand and the skeleton nitrogen atom show significant influences on the stereoselectivity
of the corresponding complex toward the polymerization of rac-LA, leading to the production of heterotactic biased polylactide
(PLA) by complexes 1 and 2 (Pm = 0.40−0.44) and moderately to highly isotactic PLA by complexes 3−12 (Pm = 0.74−0.89).
Detailed mechanism studies and microstructure analysis of typical PLA samples revealed that these zinc initiators afforded
isotactic stereoblock PLAs via a chain-end control mechanism, and there is no obvious polymer exchange process during the
polymerization process.
limited catalyst systems proved to be isoselective.12,13 Among
INTRODUCTION
■
them, aluminum complexes ligated by salen ligands or their
Polylactide (PLA) derived from renewable resources has
derivatives have been extensively investigated.14,15 Although
attracted considerable interest due to its biodegradable and
these complexes show impressive degrees of stereocontrol,
biocompatible advantages. Being regarded as a potential
alternative of petroleum-based plastics, PLA possesses
extensive applications in medicine, packaging, agriculture,
they often suffer from low activities and thus generally require
unacceptably high catalyst loadings and a prolonged reaction
and tissue engineering fields,1−6 which benefit mainly from
the specific chain microstructures of PLA.7,8 Atactic PLA is an
amorphous polymer with a glass transition temperature of 60
°C.6 On the other hand, isotactic PLA is crystalline, and the Tm
value of homochiral poly(L-lactide) (PLLA) or poly(D-lactide)
(PDLA) is 170 °C; while that of the stereocomplexed PLA
derived from the equivalent mixture of PLLA and PDLA is
around 230 °C.9−11 Stereocomplexed PLAs with elevated Tm
(170−220 °C) can also be achieved from the highly
isoselective ring-opening polymerization (ROP) of rac-lactide
(rac-LA)2 that is a much cheaper and accessible starting
material than D-LA (D-LA) based on potential industrial
applications; thus, development of new initiators capable of
producing isotactic stereocomplexed PLA from rac-LA is very
promising.
time (24 h or longer) at elevated temperatures (70−100
°C).16−18 Isoselective catalysts based on complexes of
zinc,19−24 calcium,25,26 indium,27−29 potassium,30−36 gallium,37
group IV metals,38−40 copper,41 and rare-earth metals42−45
have been rarely reported. Among these nonaluminum-based
isoselective catalysts, the o-(9-anthryl)phenoxide potassium
complex bearing a crown ether ancillary ligand reported by Wu
and coworkers afforded PLA with the highest Pm value of 0.94,
but a less accessible reaction condition of −70 °C was
required.33 Meanwhile, the potassium complex bearing a
similar phenoxide ligand with an ortho-xanthenyl group
reported by the same group displayed the highest activity,
which could convert 400 equiv of rac-LA to a conversion of
76% within 5 min at an extremely low temperature of −60 °C
Despite intensive recent research, it is still a fascinating
challenge to prepare isotactic PLAs from rac-LA, and only
Received: July 3, 2018
© XXXX American Chemical Society
A
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a
Scheme 1. Synthetic Route of Aminophenol Proligands and Zinc Complexes
a
The reported proligands L5H, L6H, and L10H and complexes 5, 6, and 10 are listed for comparison.47
and produce highly isotactic PLA (Pm = 0.87).30 However,
these alkali-metal catalysts suffer from relatively low
isoselectivities (Pm = 0.64−0.77) at ambient temperature.
Generally, it still takes several hours or even days for those
nonaluminum-based isoselective catalysts to reach high
monomer conversions. In 2014, Du’s group prepared the
amido-oxazolinate zinc complexes yielding highly isotactic PLA
(Pm = 0.91), but the full conversion of monomer was reached
after 44 h at 23 °C when adopting an initiator/monomer ratio
of 1:100.22 In 2017, Xu and coworkers45 reported the yttrium
bis(phenolate) ether complex which produced PLA with a Pm
value of 0.90 and a significantly improved activity (1.5 h, 200
equiv. −15 °C); Williams’ group reported the phosphasalen
indium complex capable of converting 500 equiv of rac-LA to
highly isotacitc PLA (16 h, 500 equiv., −15 °C, Pm = 0.92),46
and phenylene-Salalen zirconium complex was also reported by
Kol’s group to catalyze the highly isoselective ROP of rac-LA
but with a relatively lower activity (24 h, 300 equiv., 50 °C, Pm
= 0.91).40 Despite these exhilarating breakthroughs, the
isoselectivities and activities of currently reported catalysts/
initiators are still not practical, and the mechanism on
stereocontrol as well as the structure−performance relation-
ship of given catalysts are far from well-understood and need to
be further explored with the aim of discovering initiators
combining both high rates and excellent stereocontrol for rac-
LA polymerization.
stereoselectivity of this series of zinc complexes possessing
multiple stereogenic centers appears to be governed essentially
by the steric effects of the aminophenolate ligand. Inspired by
this work, we focused on constructing ligand skeletons with
less stereogenic centers. In 2017, we reported several zinc
complexes bearing chiral oxazolinyl- or achiral benzoxazolyl-
based aminophenolate ligands, which displayed a remarkable
combination of high isoselectivity and high activity toward the
ROP of rac-LA (1.5 h, 200 equiv., −20 °C, Pm = 0.92). A
chain-end stereocontrol mechanism proved to be involved
solely in the formation of multiblock isotactic PLAs by these
complexes, regardless of the existence of the ligand chirality.47
Because complexes bearing achiral ligands can be synthesized
from chemicals much cheaper than those for chiral ones and
therefore are of benefit to industrial applications, in this work,
we further expanded the achiral benzoxazolyl-aminophenolate
ligand systems by varying substituents both on the ortho-
position of the phenoxide ring and on the center skeleton
nitrogen atom and studied in detail the effect of substituents
on the performance of the corresponding zinc complexes on
the stereoselective polymerization of rac-LA.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Aminophenolate
Zinc Complexes. According to the procedure we reported
previously,47 a series of achiral benzoxazolyl-aminophenol
proligands with various substituents was prepared via three-
step reactions, as illustrated in Scheme 1. The corresponding
zinc silylamido complexes were readily synthesized via the
reactions of the aminophenol proligands with Zn[N(SiMe3)2]2
in a 1:1 molar ratio in toluene at ambient temperature and
were isolated as white powders from toluene/n-hexane
solutions in 41−66% yields. The previously reported
Our group is interested in developing zinc and magnesium
complexes stabilized by achiral or chiral aminophenolate
ligands for the ring-opening polymerization of rac-LA. In
2013, our group reported some zinc complexes supported by
tridentate aminophenolate ligands containing a chiral pyrroli-
dinyl moiety for the ROP of rac-LA, which yielded isotactic
PLAs with Pm values up to 0.84 at −38 °C.19 The
B
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complexes 5, 6, and 10 are also listed for comparison
purposes.47 The reaction of Zn[N(SiMe3)2]2 with proligand
L13H having a bulky trityl group on the skeleton nitrogen atom
failed to afford the target zinc complex (L13)ZnN(SiMe3)2
even at 60 °C; likely, the steric bulkiness significantly hindered
the reaction.
All of the newly synthesized zinc silylamido complexes were
characterized by 1H and 13C{1H} NMR spectroscopy as well as
elemental analysis methods. In the 1H NMR spectra (C6D6) of
these zinc complexes, the resonances were fully assigned, and
only one set of signals accounting for the multidentate ligand
and the silylamido group is shown. Therefore, all of these zinc
complexes were obtained as a pair of racemates. The two
protons of each methylene group in Ar−CH2−N and N−
CH2−CN units of a given complex are inequivalent and give
rise to two doublets, as compared to the singlets in the
spectrum of the free ligand, indicating the participation of all
three donors of the aminophenolate ligand in coordinating
with the zinc center. The coupling constants of Ar−CH2−N
and N−CH2−CN protons are normally large. For instance,
four doublets at 4.36, 3.73, 2.83, and 2.77 ppm with coupling
Figure 1. Molecular structure of 11 (ellipsoids drawn at the 30%
probability, H atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn1−O1 1.911(2), Zn1−N1 2.089(3), Zn1−N2
2.252(2) Zn1−N3 1.913(2), O1−Zn1−N3 119.38(10), O1−Zn1−
N1 102.56(9), N3−Zn1−N1 126.53(11), O1−Zn1−N2 97.95(9),
N3−Zn1−N2 122.12(10), N1−Zn1−N2 78.95(10).
2
2
constants of J = 18.2 Hz or J = 12.0 Hz are displayed in the
1H NMR spectrum of complex 7, suggesting that the difference
in the surrounding environments of the two protons of each
methylene group is distinct, which is also reflected by the
significant separation of two doublets of each methylene group.
All of these indicate strong coordination interactions of these
N donors with the zinc center. Moreover, for complexes 1−3,
the methylene protons of the pendant N-benzyl group display
1
two doublets with very close chemical shifts in the H NMR
spectra, which just look like one sharp peak. Whereas, in the
case of complex 4, the two protons of each methylene group in
the pendant NCH2CH2Ph unit become obviously inequivalent
and give rise to four td resonances, which could be attributed
to the restricted rotation of the NCH2CH2Ph unit. From the
above description, it is suggested that the tetra-coordinating
mode of these zinc complexes is maintained in solution.
Molecular Structures of Complexes 11, 12, and 3d.
Single crystals of complexes 11 and 12 were obtained from a
mixture of toluene/n-hexane solution at room temperature.
Surprisingly, homoleptic complex 3d with a bis-ligated
structure was obtained when trying to get crystals of the
corresponding silylamido complex 3, likely resulted from
certain Schlenk balanced reactions. All of these complexes were
then characterized by X-ray diffraction studies. Detailed crystal
and refinement data are reported in Table S1.
As shown in Figures 1 and 2, complexes 11 and 12 possess a
monomeric structure in the solid state, where the zinc atom is
four-coordinated by three donors of the aminophenolate ligand
and one silylamido group, adopting a distorted tetrahedron
geometry around the metal center. The bond distances of Zn−
N1 and Zn−N2 range from 2.089(3) to 2.2620(12) Å,
showing that both the skeleton N atom and the imine N atom
of the benzoxazolyl ring have strong coordination interactions
with the metal center, which is consistent with the structure in
solution inferred from the 1H NMR spectroscopic studies. Due
to the steric repulsion between the bis(trimethylsilyl)amido
group and the aminophenolate ligand, the corresponding
angles of N3−Zn1−O1 (116.99(5)−119.38(10)°), N3−Zn1−
Figure 2. Molecular structure of 12 (ellipsoids drawn at the 30%
probability, H atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn1−O1 1.9146(12), Zn1−N1 2.2620(12), Zn1−
N2 2.1124(13) Zn1−N3 1.9172(13); O1−Zn1−N3 116.99(5), O1−
Zn1−N1 94.97(5), N3−Zn1−N1 125.49(5), O1−Zn1−N2
105.51(5), N3−Zn1−N2 125.50(6), N1−Zn1−N2 79.90(5).
aminophenolate ligand becomes chiral upon complexation;
meanwhile, the metal zinc center is also chiral because of the
chelation of the tridentate ligand. However, only one pair of
racemates is observed in the primitive cell of each complex,
showing a close relation of these two stereogenic centers.
As depicted in Figure 3, in the structure of the homoleptic
complex 3d, the zinc atom is five-coordinated by two
multidentate ligands with one benzoxazolyl ring located far
away from the metal center. The two axial positions of the
trigonal bipyramid are occupied by the skeleton N atoms of the
two ligands, and both of the skeleton N atoms possess S-
configuration. The equatorial positions are occupied by the
two phenoxy oxygen atoms and one imine N atom of the two
ligands. Being obtained as a pair of racemates, the structure
N
skeleton (125.49(5)−126.53(11)°), and N3−Zn1−N benzoxazolyl
(122.12(10)−125.50(6)° ) in these zinc complexes are all
deviated from ideal 109.47°. It should be further noted that, in
these two structures, the skeleton nitrogen atom of the
C
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2, these zinc silylamido complexes are efficient initiators for the
ROP of rac-LA and the polymerization runs can reach
completion within a few minutes or hours at 25 °C depending
on the structures of the complexes, giving PLAs with high
molecular weights and narrow to broad molecular weight
distributions (Mw/Mn = 1.08−1.88).47,48 Moreover, most of
these zinc complexes exhibit moderate to high isoselectivities
(Pm = 0.74−0.89) toward the polymerization of rac-LA.
The substituent at the ortho-position of the phenolate ring of
the ligand exerts significant influence both on the catalytic
activity and on the stereoselectivity of the corresponding zinc
complex. When the polymerization was carried out in toluene,
complex 1 with an o-chloro substituent on the phenolate ring
required 65 min to convert 500 equiv of rac-LA to 94%
monomer conversion in the presence of 2-propanol (Table 1,
run 2), showing the lowest activity among complexes 1−3
bearing different ortho-substituents as indicated by the
preliminary kinetic studies (Table 1 and Figure S19).49−51
Meanwhile, the stereoselectivity of this complex toward the
ROP of rac-LA is also the lowest, and polymers with
heterotactic preference were obtained (Pm = 0.40−0.41).
The introduction of an o-tert-butyl group in complex 2 resulted
in a surge of the activity, and a high conversion of 90% of the
same amount of monomer could be reached just within several
minutes under otherwise identical conditions (Table 1, run 4).
Complex 2 is also the most active initiator among all the
complexes in this work. In comparison to the significant
increase in the catalytic activity, the stereoselectivity of
complex 2 improved only slightly (Pm = 0.44). When the
ortho-substituent of the phenolate ring was changed to a trityl
group, an obvious decrease in activity was observed for
complex 3 (33 min, 92%, run 6), but it is still higher than that
of complex 1. The steric effect of o-trityl on the stereo-
Figure 3. Molecular structure of 3d (ellipsoids drawn at the 30%
probability, H atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn1−O1 1.9576(16), Zn1−O3 1.9452(16), Zn1−
N1 2.3027(19), Zn1−N2 2.1674(19), Zn1−N4 2.166(2); O1−Zn1−
N1 128.34(7), O1−Zn1−N2 89.56(7), O1−Zn1−N4 92.87(7), O3−
Zn1−O1 127.69(7), O3−Zn1−N1 102.69(7), O3−Zn1−N2
91.75(7), O3−Zn1−N4 91.57(7), N2−Zn1−N1 76.96(7), N4−
Zn1−N1 96.80(7), N4−Zn1−N2 173.45(7).
with R-configuration on both the skeleton N atoms is also
observed in the primitive cell.
Polymerization of rac-LA. The newly synthesized zinc
complexes were evaluated as initiators for the ROP of rac-LA
in toluene and tetrahydrofuran (THF) at ambient temperature.
The polymerization results of the reported complexes 5, 6, and
10 are cited herein for comparison.47 As shown in Tables 1 and
a
Table 1. Polymerization of rac-LA Initiated by Zinc Complexes 1−12 in Toluene
a
b
c
d
[rac-LA]0 = 1.0 M, 25 °C. Determined by 1H NMR spectroscopy. Mn,calcd = ([rac-LA]0/[cat.]0) × 144.14 × conv %. Determined by GPC with
e
f
polystyrenes as standards. Pm is probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR spectroscopy. See Figures
S19−S21 for standard deviations. The polymerization results of complexes 5, 6, and 10 were cited for comparison.47 These kapp data were
g
h
obtained with a molar ratio of [rac-LA]0/[iPrOH]0/[Zn]0 = 200:1:1, [rac-LA]0 = 1.0 M.
D
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a
Table 2. Polymerization of rac-LA Initiated by Zinc Complexes 1−12 in THF
a
b
c
d
[rac-LA]0 = 1.0 M, 25 °C. Determined by 1H NMR spectroscopy. Mn,calcd = ([rac-LA]0/[cat.]0) × 144.14 × conv %. Determined by GPC with
e
f
1
polystyrenes as standards. Pm is probability of forming a new m-dyad, determined by homonuclear decoupled H NMR spectroscopy. The
polymerization results of complexes 5, 6 and 10 were cited for comparison.47 −20 °C.
g
selectivity is even more profound. Complex 3 exhibited
moderate isoselectivity toward the polymerization of rac-LA
(Pm = 0.74), which is in a sharp contrast to those of complexes
1 and 2. By comparing the polymerization data of complexes
1−3, it is clear that the electron withdrawing nature of the
chloro substituent exerts an effect only on the catalytic activity,
and the stereoselectivity of complexes 1−3 is determined
essentially by the steric bulkiness of the ortho-substituent. In
parallel to the steric hindrance of the ortho substitution, a clear
increasing tendency of the isoselectivity toward the ROP of
rac-LA is observed in the order of 3 (CPh3, Pm = 0.74) > 2
(tBu, Pm = 0.44) > 1 (Cl, Pm = 0.41).
Keeping the superior o-trityl group unchanged, the
substituent on the skeleton N atom of the ligand was varied
systematically, which proved to play an important role in
influencing the polymerization activity and more significantly
the isoselectivity of these zinc complexes. By replacing the N-
benzyl on the skeleton N atom to a N-phenethyl group,
complex 4 showed a slightly lower activity but the same
isoselectivity when compared to those of complex 3 (Table 1,
run 6 vs 8). However, when either an acyclic or a cyclic
aliphatic group was introduced, the isoselectivities of the
corresponding zinc complexes 5−12 all improved significantly
(Pm = 0.80−0.89), although accompanied with somewhat
increased or decreased activities. Except for complex 6 with a
tert-butyl group, complexes 5, 7, and 8 with an acyclic
substituent on the skeleton N atom all exhibit enhanced
activities toward the polymerization in comparison with
complex 3 (Table 1, Figure S20). The flexible aliphatic chain
shows benefits to not only the activity but also the
isoselectivity. Among them, complexes 5 and 8 with a linear
activities of complexes 9−12, with complex 12 having a bulky
1-adamantyl group displaying an extremely low activity. A
decreasing tendency of the activity was further observed in the
order of 9 (cyclopentyl) > 10 (cyclohexyl) > 11 (cyclooctyl) >
12 (1-adamantyl) (Table 1, Figure S21). Obviously, the
enhanced steric bulkiness of the substituent on the skeleton N
brings remarkable steric hindrance to the coordination sphere
of the zinc center and is thus unfavorable for the coordination/
insertion of the incoming monomer. Nevertheless, a different
tendency was observed for the isoselectivity, wherein complex
10 with a medium cyclohexyl group exhibits the highest
isoselectivity of Pm = 0.88−0.89. Neither decreasing nor
increasing the ring size provides higher isoselectivities.
Currently we have no convincing explanation on the effects
of different types of substituents. We tentatively attribute these
to the steric hindrance that these substituents can create either
by their rigid structures or via free rotation, and substituents
with medium steric hindrance are beneficial for the
isoselectivity. Nevertheless, the electronic effects of N-benzyl
and N-phenethyl might not be ruled out (vide post).
As shown in Tables 1 and 2, except for complex 1, all the
complexes exhibit higher activities in toluene than in THF. For
example, a monomer conversion of 90% could be reached
within 6 min in toluene when using 2 as the initiator, whereas
9 min was needed to achieve a similar conversion of 94% in
THF (run 4 in Tables 1 and 2). The decrease in activity in
THF is somewhat common for zinc complexes bearing bulky
multidentate ligands and is usually attributed to the competed
coordination of the solvent molecules with the monomer.52,53
A reversed trend is observed for complex 1 with chloro
substituents, which shows higher activities in THF than in
toluene. Probably the coordination of THF molecule(s) would
reduce the electrophilicity of the zinc center that is an
unfavorable factor in the case of complex 1 with electron
aliphatic chain exhibit slightly higher isoselectivities (Pm
=
0.83−0.84). Introduction of a rigid cyclic substituent to the
skeleton N atom led to considerable decrease in the catalytic
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withdrawing substituents and hence enhance the nucleophil-
icity of (Zn)−OR group, favoring the insertion of lactide
monomer. It is further noticed that the influence of solvent on
stereoselectivity depends on the type of the substituent on the
skeleton N atom in these complexes. Complexes 1−4 either
with a benzyl group or a phenethyl group show slightly
enhanced Pm values in THF, whereas complexes 5−12 with an
aliphatic group display in principle the same isoselectivities in
both solvents. It seems that the pendant phenyl group of the
skeleton N-substituent in complexes 1−4 might exert certain
unfavorable electronic effect on the stereoselectivity and the
presence of coordinative THF molecules would diminish such
effect.
Upon addition of 1 equiv of 2-propanol, the activities of all
zinc silylamido complexes increased significantly, giving PLAs
in a more controlled manner as indicated by the matched
molecular weights with the calculated ones as well as narrower
molecular weight distributions (Mw/Mn = 1.07−1.44). This
phenomenon is in line with the cases reported by us and other
groups utilizing silylamido complexes/2-propanol catalytic
systems during the ROP of LA.19,24,47,52 The relative broad
distributions in some cases should be attributed to the
transesterification side reactions occurred at the late stage of
the polymerization when high monomer conversions were
reached.47,48 The polymerization process becomes even better
controlled at a lower reaction temperature. For instance, the
polymerization run carried at −20 °C using complex 11 as the
initiator in the presence of 2-propanol afforded PLA with
improved isoselectivity of Pm = 0.89 and a narrower
distribution (Mw/Mn = 1.08) when compared with those
carried out at ambient temperature (Table 2, runs 22 and 23).
DSC measurement of this sample further indicates the
formation of a stereocomplexed structure (Tm = 183 °C,
Figure S29).
about 1:1:1, which suggests the formation of predominantly
isotactic stereoblocks. Although these features may imply the
operation of a chain-end control mechanism, an enantiomor-
phic site control integrated with a polymer exchange process
may be also applicable because racemic initiators are
thoroughly involved.
To have a further understanding of the mechanism of rac-LA
polymerization catalyzed by this series of zinc complexes to
afford isotactic stereoblock PLAs, we wanted to know whether
there exists a polymer exchange process between different
active centers during the polymerization. Then, the following
controlled polymerization was performed initially. At the first
stage, the highly isoselective complex 10 was chosen to
catalyze the polymerization of 200 equiv of rac-LA in the
presence of 2-propanol in toluene at 25 °C ([rac-LA]0:[10]0:
[iPrOH]0 = 200:1:1), and the polymerization reaction was
carried out for 30 min to ensure a complete conversion of the
monomer.47 At the second stage, another 500 equiv of rac-LA
and one equiv of 2-propanol, together with the slightly
heteroselective complex 2, were added to the above polymer-
ization mixture ([rac-LA]0:[2]0:[iPrOH]0 = 500:1:1). Because
the activity of complex 2 is significantly higher than that of
complex 10, the monomer added at the second stage would be
mainly converted by complex 2. It is assumed that, if there
exists a polymer exchange process during the polymerization
catalyzed by these two complexes, the molecular weight of the
resulting polymer will be more or less averaged, thus affording
PLA with a broad molecular weight distribution but in
unimodal type; however, if the polymer exchange process is
absent or negligible, the high polymerization rate of complex 2
at the second stage will produce PLA with high molecular
weight, which shows significant difference from that of PLA
produced by the less active complex 10, therefore leading to
PLAs in bimodal distribution. The resultant polymer sample
was subjected to GPC measurement, which shows a bimodal
distribution with molecular weights of 2.04 × 104 and 6.83 ×
104 g/mol just corresponding to the calculated values (Figure
S32), indicating that there was no obvious polymer exchange
process during the polymerization. Moreover, a melting point
at 174 °C could be observed in the DSC curve of this sample
(Figure S33), which should be attributed to the long isotactic
chains produced by complex 10.
To further verify the presence of a polymer exchange process
or not, we chose a combination of complexes 1 and 10 with
similar catalytic activities but significantly different stereo-
selectivities to catalyze the polymerization of 400 equiv of rac-
LA in toluene at 25 °C ([rac-LA]0:[1]0:[10]0:[iPrOH]0 =
400:1:1:2, with both complexes added at the same time). Due
to the similar activities of these two complexes, it is expected
that a PLA sample with a unimodal distributed molecular
weight will be obtained. It is also assumed that, if there is a
polymer exchange process between the active centers
generated from complexes 1 and 10, respectively, the resulting
polymer chains will contain both isotactic and heterotactic-
biased blocks, and the isotactic blocks would tend to be shorter
when compared with those produced solely by complex 10,
thus likely leading to the vanishing of the melting point peak.
However, if the polymer exchange process is absent or
negligible, a mixture of isotactic-block polymer chains and
approximately atactic polymer chains will be obtained, which
will still have a melting point. The result of this controlled
polymerization shows that a PLA sample with a molecular
weight of 2.76 × 104 g/mol and a unimodal molecular weight
From the 1H NMR spectra of the resultant polymer samples
obtained by these zinc silylamido complexes, no clear end-
groups could be distinguished. However, in the MALDI-TOF
mass spectrum of a typical oligomer sample obtained by
complex 10 with [rac-LA]0: [Zn]0 = 20:1, a series of signals
end-capped with N(SiMe3)2 and a hydroxyl group are
dominant (Figure S30), indicative of the initiation with the
N(SiMe3)2 group and therefore a coordination−insertion
polymerization by a single-site catalyst. Still, the intra- and
intermolecular transesterifications are inevitable, as evident
from the MALDI-TOF mass spectrum. To acquire some
information about PLA initiated by [Zn]/iPrOH system,
polymerization with [rac-LA]0: [10]0: [iPrOH]0 = 20:1:1 was
1
conducted. The H NMR spectrum of the isolated oligomer
indicated the existence of oligomer chains capped by
i
−CH(CH3)OH and PrOCC(O)− termini (Figure S31).
Isoselective Polymerization Mechanism Studies. To
have some insight into the isoselective polymerization
mechanism conducted by most of these zinc complexes, the
kinetics of rac-, D-, and L-lactide polymerizations catalyzed by
complex 11 were further investigated in this work. It is found
that, as previously reported for complex 6,47 as a racemic
mixture complex 11 exerts no difference in the polymerization
rates of D-lactide (D-LA) and L-lactide (L-LA) but a nearly 2-
fold rate preference for D-LA/L-LA polymerization relative to
that of rac-LA in THF at 25 °C (Figure S22). Moreover, the
homonuclear-decoupled 1H NMR spectrum of a typical
polymer sample produced by complex 11 (Figure S27)
shows that the rmm:mmr:mrm tetrad signal intensity ratio is
F
DOI: 10.1021/acs.inorgchem.8b01839
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
polymerization were dried in an oven at 120 °C overnight and
exposed to a vacuum−argon cycle three times.
distribution (Mw/Mn = 1.12) was obtained (Figure S34). The
melting point of this sample measured by DSC is 172 °C,
further indicating the existence of long isotactic chains (Figure
S35). To verify whether a polymeric mixture was obtained, this
polymer sample was washed with a large amount of methanol
(20 mg with 80 mL of methanol at rt) on the basis of different
solubilities of isotactic PLA and atactic PLA in methanol. Both
the soluble and the insoluble fractions were collected and
NMR spectra were recorded on a Bruker AVANCE-400
spectrometer at 25 °C (1H, 400 MHz; 13C, 100 MHz) unless
1
otherwise stated. Chemical shifts for H and 13C{1H} NMR spectra
were referenced internally using the residual solvent resonances and
reported relative to TMS. 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 a Waters instrument (M1515 pump,
Optilab Rex injector) in THF at 35 °C, at a flow rate of 1 mL/min.
Calibration standards were commercially available narrowly dis-
tributed linear polystyrene samples that cover a broad range of molar
masses (4 × 103 g/mol < Mn < 4.4 × 105 g/mol).
1
subjected to the homonuclear decoupled H NMR measure-
ments. It is found that the spectrum of the original polymer
sample indicates a Pm value of 0.66 (Figure S36). The soluble
fraction proved to consist of most atactic PLA and a small
amount of isotactic PLA, and its Pm value is 0.51 (Figure S37),
while the insoluble fraction consists mainly of the isotactic PLA
and a small amount of atactic PLA with the Pm value being
0.80 (Figure S38). Thus, it is clear that a polymeric mixture
was obtained in this controlled polymerization, and there is no
obvious polymer exchange process during the polymerization.
On the basis of kinetic studies and homonuclear-decoupled
1H NMR spectra of typical polymers as well as the results of
the above controlled polymerizations, we believe that a chain-
end control mechanism is involved in producing isotactic
stereoblock PLAs by this series of racemic zinc complexes, and
there is no obvious polymer exchange process during the
polymerization.
Syntheses. 2-{N-Benzyl-N-[2-(benzoxazolyl)methyl]-
aminomethyl}-4,6- dichlorophenol (L1H). 2-(Chloromethyl)-
benzoxazole (1.51 g, 9.00 mmol) was dissolved in 20 mL of DMF,
and then the solution was slowly added to a mixture of benzylamine
(9.64 g, 90.0 mmol) and K2CO3 (1.37 g, 9.90 mmol) within about 2
h. The mixture was poured into water and extracted with ethyl acetate
three times, and the organic phase was dried over anhydrous MgSO4.
Evaporation of the solvent gave the target secondary amine as a
viscous oil with a purity of about 80%, which was used directly for the
next step. The viscous oil (2.04 g, ca. 8.6 mmol) was dissolved in 10
mL of DMF, and K2CO3 (1.30 g, 9.41 mmol) was added. Then, 2-
bromomethyl-4, 6-dichlorophenol (2.19 g, 8.55 mmol) was added
slowly in 10 min to the above mixture. The mixture was stirred for 2 h
and then poured into water and extracted with ethyl acetate three
times. The organic phase was dried over anhydrous MgSO4.
Evaporation of the solvent gave a viscous oil, which was purified by
column chromatography (silica gel 200−300 Merck, petroleum ether/
ethyl acetate/triethylamine = 50:1:0.01) to provide white solids (2.16
g, 61%) after removal of all the volatiles. 1H NMR (CDCl3, 400 MHz,
298 K): δ 11.03 (s, 1H, OH), 7.80−7.74 (m, 1H, ArH), 7.57−7.52
(m, 1H, ArH), 7.39−7.34 (m, 6H, ArH), 7.32−7.26 (m, 1H, ArH),
7.27 (d, 1H, 4J = 2.5 Hz, ArH), 6.96 (d, 1H, 4J = 2.5 Hz, ArH), 3.96
(s, 2H, ArCH2), 3.95 (s, 2H, NCH2CN), 3.80 (s, 2H, PhCH2).
13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.3 (OCN), 152.3,
CONCLUSION
■
A series of zinc complexes supported by achiral benzoxazolyl
aminophenolate ligands was synthesized and structurally
characterized. Most of these complexes can serve as highly
isoselective and active initiators for the ring-opening polymer-
ization of rac-LA. The substituents of the ancillary ligand, both
at the ortho-position of the phenolate ring and on the skeleton
N atom of the ligand framework, have a profound influence on
the catalytic activity and stereoselectivity. In general, a less
bulky group at the ortho-position of the phenoxy moiety and a
less bulky group on the skeleton N atom of the ligand would
benefit the catalytic activity of the corresponding zinc
complexes. High isoselectivities were observed for complexes
with a trityl group at the ortho-position of the phenolate ring
and a rigid cyclic substituent on the skeleton N atom of the
ligand. Detailed mechanism studies and microstructure analysis
of typical PLA samples revealed that these zinc initiators
afforded isotactic stereoblock PLAs via a chain-end control
mechanism, and there is no obvious polymer exchange process
during the polymerization.
150.9, 140.6, 135.9, 129.6, 129.3, 129.0, 128.2, 127.8, 125.6, 124.8,
124.0, 123.8, 122.0, 120.3, 110.9 (all Ar-C), 57.9 (ArCH2), 57.0
(PhCH2), 49.3 (NCH2CN). Anal. Calcd for C22H18Cl2N2O2: C,
63.93; H, 4.39; N, 6.78. Found: C, 63.75; H, 4.47; N, 6.53%.
2-{N-Benzyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4,6-di-
tert-butylphenol (L2H). The procedure was the same as that of L1H,
except that 2-bromomethyl-4, 6-di-tert-butylphenol (2.54 g, 8.49
mmol), N-(2-benzoxazolylmethyl)benzylamine (2.02 g, ca. 8.5
mmol), and K2CO3 (1.29 g, 9.34 mmol) were used in the second
step. White solids (2.45 g, 63%) were obtained through column
chromatography (silica gel 200−300 Merck, petroleum ether/ethyl
acetate/triethylamine = 20:1:0.01) after removal of all the volatiles.
1H NMR (CDCl3, 400 MHz, 298 K): δ 10.13 (s, 1H, OH), 7.78−7.72
(m, 1H, ArH), 7.58−7.52 (m, 1H, ArH), 7.43−7.31 (m, 6H, ArH),
7.31−7.27 (m, 1H, ArH), 7.25 (d, 1H, 4J = 2.0 Hz, ArH), 6.90 (d, 1H,
4J = 2.0 Hz, ArH), 3.98 (s, 2H, ArCH2), 3.97 (s, 2H, NCH2CN),
3.81 (s, 2H, PhCH2), 1.49 (s, 9H, C(CH3)3), 1.28 (s, 9H, C(CH3)3).
13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 162.6 (OCN), 154.0,
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 the glovebox.
rac-Lactide from Jinan Daigang Biomaterial Co., Ltd. was recrystal-
lized with dry toluene and then sublimed twice under vacuum at 100
°C. 2-Propanol was dried over calcium hydride under argon prior to
distillation. K[N(SiMe3)2] and Zn[N(SiMe3)2]2 were synthesized
according to the literature method.54 2-Bromomethyl-4,6-dichlor-
ophenol,55 2-bromomethyl-4,6-di-tert-butylphenol,56 2-bromomethyl-
4-methyl-6- (triphenylmethyl)phenol,57,58 and 2-chloromethylbenzox-
azole59 were synthesized according to the reported literature
procedures. All other chemicals were commercially available and
used after appropriate purification. Glassware and vials used in the
151.0, 141.0, 136.6, 136.0, 129.9, 128.7, 127.9, 125.3, 124.6, 124.4,
123.5, 120.9, 120.3, 110.9 (all Ar-C), 58.9 (ArCH2), 57.2 (PhCH2),
49.1 (NCH2CN), 35.1 (C(CH3)3), 34.3 (C(CH3)3), 31.8 (C-
(CH3)3), 29.8 (C(CH3)3). Anal. Calcd for C30H36N2O2: C, 78.91; H,
7.95; N, 6.13. Found: C, 78.66; H, 7.87; N, 5.99%.
2-{N-Benzyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-meth-
yl-6-(triphenylmethyl)phenol (L3H). The procedure was the same as
that of L1H, except that 2-bromomethyl-4-methyl-6-
(triphenylmethyl)phenol (3.91 g, 8.83 mmol), N-(2-
benzoxazolylmethyl)benzylamine (2.23 g, ca. 8.8 mmol), and
K2CO3 (1.34 g, 9.71 mmol) were used in the second step. Light
yellow solids (2.94 g, 56%) were obtained through column
chromatography (silica gel 200−300 Merck, petroleum ether/ethyl
acetate/triethylamine = 100:1:0.01) after removal of all the volatiles.
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3
1H NMR (CDCl3, 400 MHz, 298 K): δ 9.83 (s, 1H, OH), 7.72−7.70
3H, J = 8.0 Hz, CH2CH3). 13C{1H} NMR (CDCl3, 100 MHz, 298
K): δ 162.3 (OCN), 154.1, 150.9, 146.2, 141.0, 134.2, 131.2, 131.1,
129.1, 127.1, 125.5, 125.2, 124.5, 122.0, 120.2, 111.0 (all Ar-C), 63.3
(Ph3C), 58.5 (ArCH2), 52.6 (NCH2CH2), 48.7 (NCH2CN), 31.7
(CH2 of n-hexyl), 26.9 (CH2 of n-hexyl), 26.6 (CH2 of n-hexyl), 22.6
(CH2 of n-hexyl), 21.1 (ArCH3), 14.2 (CH2CH3). Anal. Calcd for
C41H42N2O2: C, 82.79; H, 7.12; N, 4.71. Found: C, 82.59; H, 7.27;
4.75%.
(m, 1H, ArH), 7.50−7.48 (m, 1H, ArH), 7.37−7.32 (m, 2H, ArH),
7.26−7.19 (m, 15H, ArH), 7.16−7.12 (m, 3H, ArH), 6.93−6.91 (m,
3H, ArH), 6.82 (br s, 1H, ArH), 3.93 (s, 2H, ArCH2), 3.75 (s, 2H,
PhCH2), 3.55 (s, 2H, NCH2CN), 2.16 (s, 3H, ArCH3). 13C{1H}
NMR (CDCl3, 100 MHz, 298 K): δ 162.0 (OCN), 153.8, 150.7,
146.0, 140.9, 136.0, 134.2, 131.2, 130.0, 129.3, 128.5, 127.6, 127.1,
125.5, 125.1, 124.4, 121.7, 120.1, 110.8 (all Ar-C), 63.3 (Ph3C), 58.3
(ArCH2), 55.9 (PhCH2), 48.2 (NCH2CN), 21.0 (ArCH3). Anal.
Calcd for C42H36N2O2: C, 83.97; H, 6.04; N, 4.66. Found: C, 83.93;
H, 6.19; N, 4.69%.
2-{N-Phenethyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-
methyl-6-(triphenylmethyl)phenol (L4H). The procedure was the
same as that of L1H, except that phenethylamine (10.91 g, 90.0
mmol) was used in the first step, and 2-bromomethyl-4-methyl-6-
(triphenylmethyl)phenol (3.91 g, 8.83 mmol), N-(2-
benzoxazolylmethyl)phenethylamine (2.23 g, ca. 8.8 mmol), and
K2CO3 (1.34 g, 9.71 mmol) were used in the second step to provide
L4H as colorless crystals via recrystallization from dichloromethane
and petroleum ether (3.14 g, 58%). 1H NMR (400 MHz, CDCl3, 298
K): δ 9.69 (s, 1H, ArOH), 7.75−7.69 (m, 1H, ArH), 7.53−7.47 (m,
1H, ArH), 7.39−7.33 (m, 2H, ArH), 7.25−7.18 (m, 13H, ArH),
7.17−7.09 (m, 5H, ArH), 7.03−6.98 (m, 2H, ArH), 6.95 (d, 1H, 4J =
1.7 Hz, ArH), 6.82 (d, 1H, 4J = 1.7 Hz, ArH), 3.94 (s, 2H, ArCH2N),
3.92 (s, 2H, NCH2CN), 2.74−2.67 (m, 2H, NCH2CH2), 2.67−
2.60 (m, 2H, NCH2CH2), 2.18 (s, 3H, ArCH3). 13C{1H} NMR
(CDCl3, 100 MHz, 298 K): δ 162.2 (OCN), 154.0, 150.9, 146.1,
141.0, 139.1, 134.3, 131.2, 129.2, 128.8, 128.5, 127.3, 127.1, 126.4,
125.5, 125.3, 124.6, 121.9, 120.2, 111.0 (all Ar-C), 63.4 (Ph3C), 58.5
(ArCH2), 54.4 (NCH2CH2), 49.1 (NCH2CN), 33.3 (NCH2CH2),
21.1 (ArCH3). Anal. Calcd for C43H38N2O2: C, 84.01; H, 6.23; N,
4.56. Found: C, 84.10; H, 6.24; N, 4.43%.
2-{N-3-Methylbutyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-
methyl-6-(triphenylmethyl)phenol (L7H). The procedure was the
same as that of L1H, except that 3-methylbutylamine (7.84 g, 90.0
mmol) was used in the first step, and 2-bromomethyl-4-methyl-6-
(triphenylmethyl)phenol (3.81 g, 8.59 mmol), N-(2-benzoxazolyl-
methyl)-3-methylbutylamine (1.87 g, ca. 8.6 mmol), and K2CO3 (1.31
g, 9.45 mmol) were used in the second step to provide L7H as
colorless crystals via recrystallization from dichloromethane and
petroleum ether (3.05 g, 61%). 1H NMR (400 MHz, CDCl3, 298 K):
δ 9.88 (s, 1H, OH), 7.74−7.67 (m, 1H, ArH), 7.52−7.46 (m, 1H,
ArH), 7.39−7.31 (m, 2H, ArH), 7.25−7.16 (m, 12H, ArH), 7.15−
7.08 (m, 3H, ArH), 6.91 (d, 1H, 4J = 1.6 Hz, ArH), 6.81 (d, 1H, 4J =
1.6 Hz, ArH), 3.88 (s, 2H, ArCH2), 3.84 (s, 2H, NCH2CN), 2.46−
2.38 (m, 2H, NCH2CH2), 2.17 (s, 3H, ArCH3), 1.42−1.33 (m, 1H,
NCH2CH2CH(CH3)2), 1.32−1.24 (m, 2H, CH2 of NCH2CH2CH-
(CH3)2), 0.78 (d, 6H, 3J = 6.5 Hz, CH(CH3)2). 13C{1H} NMR
(CDCl3, 100 MHz, 298 K): δ 162.4 (OCN), 154.1, 150.9, 146.2,
140.9, 134.2, 131.2, 131.1, 129.1, 127.1, 125.5, 125.2, 124.5, 121.9,
120.2, 110.9 (all Ar-C), 63.3 (Ph3C), 58.4 (ArCH2), 51.1
(NCH2CH2), 48.8 (NCH2CN), 35.3 (NCH2CH2), 26.7
(CHCH3), 22.7 (CHCH3), 21.0 (ArCH3). Anal. Calcd for
C40H40N2O2: C, 82.72; H, 6.94; N, 4.82. Found: C, 82.85; H, 6.93;
N, 4.63%.
2-{N-Cyclopentyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-
methyl-6-(triphenylmethyl)phenol (L9H). The procedure was the
same as that of L1H, except that cyclopentylamine (7.66 g, 90.0
mmol) was used in the first step, and 2-bromomethyl-4-methyl-6-
(triphenylmethyl)phenol (3.84 g, 8.67 mmol), N-(2-
benzoxazolylmethyl)cyclopentylamine (1.88 g, ca. 8.7 mmol), and
K2CO3 (1.32 g, 9.54 mmol) were used in the second step to provide
L9H as colorless crystals via recrystallization from dichloromethane
and petroleum ether (3.21 g, 64%). 1H NMR (400 MHz, CDCl3, 298
K): δ 9.95 (s, 1H, OH), 7.73−7.67 (m, 1H, ArH), 7.52−7.46 (m, 1H,
ArH), 7.38−7.31 (m, 2H, ArH), 7.26−7.17 (m, 12H, ArH), 7.16−
7.10 (m, 3H, ArH), 6.89 (d, 1H, 4J = 1.6 Hz, ArH), 6.85 (d, 1H, 4J =
1.6 Hz, ArH), 3.98 (s, 2H, ArCH2), 3.81 (s, 2H, NCH2CN), 3.02−
2.91 (m, 1H, NCH of cyclopentyl), 2.18 (s, 3H, ArCH3), 1.82−1.71
(m, 2H, CH2 of cyclopentyl), 1.61−1.55 (m, 2H, CH2 of
cyclopentyl), 1.49−1.37 (m, 2H, CH2 of cyclopentyl), 1.36−1.24
(m, 2H, CH2 of cyclopentyl). 13C{1H} NMR (CDCl3, 100 MHz, 298
K): δ 162.9 (OCN), 154.1, 150.8, 146.2, 140.9, 134.3, 131.2, 131.0,
128.9, 127.12, 127.08, 125.4, 125.1, 124.5, 122.3, 120.1, 110.9 (all Ar-
C), 63.4 (Ph3C), 63.1 (ArCH2), 55.9 (NCH), 47.4 (NCH2CN),
29.6 (CH2), 23.8 (CH2), 21.1 (ArCH3). Anal. Calcd for C40H38N2O2:
C, 83.01; H, 6.62; N, 4.84. Found: C, 82.91; H, 6.56; N, 4.73%.
2-{N-Cyclooctyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-
methyl-6-(triphenylmethyl)phenol (L11H). The procedure was the
same as that of L1H, except that cyclooctylamine (11.45 g, 90.0
mmol) was used in the first step, and 2-bromomethyl-4-methyl-6-
(triphenylmethyl)phenol (3.94 g, 8.89 mmol), N-(2-
benzoxazolylmethyl)cyclooctylamine (2.30 g, ca. 8.9 mmol), and
K2CO3 (1.35 g, 9.78 mmol) were used in the second step to provide
L11H as colorless crystals via recrystallization from dichloromethane
and petroleum ether (3.82 g, 62%). 1H NMR (CDCl3, 400 MHz, 298
K): δ 9.91 (s, 1H, ArOH), 7.67−7.60 (m, 1H, ArH), 7.42−7.37 (m,
1H, ArH), 7.35−7.29 (m, 2H, ArH), 7.24−7.13 (m, 12H, ArH),
7.11−7.05 (m, 3H, ArH), 6.89 (d, 1H, 4J = 1.6 Hz, ArH), 6.78 (d, 1H,
4J = 1.6 Hz, ArH), 3.83 (s, 2H, ArCH2), 3.78 (s, 2H, NCH2CN),
2.80−2.71 (m, 1H, NCH of cyclooctyl), 2.16 (s, 3H, ArCH3), 1.73−
1.57 (m, 5H, CH2 of cyclooctyl), 1.50−1.36 (m, 6H, CH2 of
cyclooctyl), 1.36−1.27 (m, 2H, CH2 of cyclooctyl), 1.25−1.16 (m,
1H, CH2 of cyclooctyl). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ
163.8 (OCN), 154.3, 151.0, 146.2, 141.0, 133.9, 131.3, 131.1,
129.1, 127.0, 126.8, 125.4, 125.1, 124.4, 121.6, 120.0, 111.1 (all Ar-C),
63.4 (Ph3C), 58.5 (ArCH2), 54.3 (NCH), 46.4 (NCH2CN), 29.0
(CH2 of cyclooctyl), 26.4 (CH2 of cyclooctyl), 26.3 (CH2 of
cyclooctyl), 25.5 (CH2 of cyclooctyl). Anal. Calcd for C43H44N2O2:
C, 83.19; H, 7.14; N, 4.51. Found: C, 83.26; H, 7.14; N, 4.46%.
2-{N-1-Adamantyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-
methyl-6-(triphenylmethyl)phenol (L12H). The procedure was the
same as that of L1H, except that 1-adamantylamine (13.61 g, 90.0
mmol) was used in the first step, and 2-bromomethyl-4-methyl-6-
(triphenylmethyl)phenol (3.83 g, 8.64 mmol), N-(2-benzoxazolyl-
methyl)-1-adamantylamine (2.44 g, ca. 8.6 mmol), and K2CO3 (1.31
g, 9.50 mmol) were used in the second step to provide L12H as
colorless crystals via recrystallization from dichloromethane and
petroleum ether (4.06 g, 73%). 1H NMR (CDCl3, 400 MHz, 298 K):
δ 10.19 (br s, 1H, OH), 7.69−7.67 (m, 1H, ArH), 7.49−7.47 (m, 1H,
ArH), 7.37−7.32 (m, 2H, ArH), 7.20−7.11 (m, 15H, ArH), 6.86 (s,
1H, ArH), 6.83 (s, 1H, ArH), 4.20 (s, 2H, ArCH2), 3.85 (s, 2H,
NCH2CN), 2.16 (s, 3H, ArCH3), 1.97 (br s, 3H, CH(CH2)3),
1.60−1.46 (m, 12H, CHCH2). 13C{1H} NMR (CDCl3, 100 MHz,
298 K): δ 164.9 (OCN), 154.3, 150.7, 146.2, 140.9, 134.2, 131.34,
131.26, 130.1, 128.4, 127.0, 126.9, 125.4, 125.1, 124.5, 123.3, 120.1,
111.1 (all Ar-C), 63.4 (Ph3C), 56.8 (ArCH2), 50.2 (NC(CH2)3), 43.3
2-{N-nHexyl-N-[2-(benzoxazolyl)methyl]aminomethyl}-4-methyl-
6-(triphenylmethyl)phenol (L8H). The procedure was the same as
that of L1H, except that hexylamine (9.11 g, 90.0 mmol) was used in
the first step, and 2-bromomethyl-4-methyl-6-(triphenylmethyl)-
phenol (3.87 g, 8.73 mmol), N-(2-benzoxazolylmethyl)hexylamine
(2.03 g, ca. 8.7 mmol), and K2CO3 (1.33 g, 9.60 mmol) were used in
the second step to provide L8H as colorless crystals via
recrystallization from dichloromethane and petroleum ether (3.15 g,
1
61%). H NMR (CDCl3, 400 MHz, 298 K): δ 9.88 (s, 1H, OH),
7.74−7.68 (m, 1H, ArH), 7.52−7.46 (m, 1H, ArH), 7.39−7.32 (m,
2H, ArH), 7.24−7.17 (m, 12H, ArH), 7.15−7.09 (m, 3H, ArH), 6.91
(d, 1H, 4J = 1.6 Hz, ArH), 6.80 (d, 1H, 4J = 1.6 Hz, ArH), 3.89 (s, 2H,
ArCH2), 3.86 (s, 2H, NCH2CN), 2.42−2.34 (m, 2H, NCH2CH2),
2.17 (s, 3H, ArCH3), 1.41−1.31 (m, 2H, CH2 of n-hexyl), 1.28−1.13
(m, 4H, CH2 of n-hexyl), 1.12−1.02 (m, 2H, CH2 of n-hexyl), 0.85 (t,
H
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Inorganic Chemistry
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(NCH2CN), 38.8 (CH(CH2)3), 36.3 (C(CH2)3), 29.6
(CHCH2CH), 21.1 (ArCH3). Anal. Calcd for C45H44N2O2·0.1
CH2Cl2: C, 82.91; H, 6.82; N, 4.29. Found: C, 82.73; H, 6.77; N,
4.13%.
[(L3)ZnN(SiMe3)2] (3). The procedure was the same as that of
complex 1, except that L3H (0.601 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 3 as
1
white solids (0.563 g, 68.2%). H NMR (C6D6, 400 MHz, 298 K): δ
7.51 (d, 6H, 3J = 7.6 Hz, ArH), 7.28 (d, 1H, 3J = 7.6 Hz, ArH), 7.24
(d, 1H, 4J = 2.0 Hz, ArH), 7.10−7.02 (m, 3H, ArH), 6.98 (t, 6H, 3J =
7.6 Hz, ArH), 6.93−6.89 (m, 1H, ArH), 6.82−6.77 (m, 7H, ArH),
2-{N-Triphenylmethyl-N-[2-(benzoxazolyl)methyl]-
aminomethyl}-4-methyl-6- (triphenylmethyl)phenol (L13H). The
procedure was the same as that of L1H, except that triphenylme-
thylmine (23.34 g, 90.0 mmol) was used in the first step, and 2-
bromomethyl-4-methyl-6-(triphenylmethyl)phenol (2.95 g, 6.66
mmol), N-(2-benzoxazolylmethyl)triphenylmethylamine (2.60 g, ca.
6.7 mmol), and K2CO3 (1.01 g, 7.33 mmol) were used in the second
step to provide L13H as colorless crystals via recrystallization from
dichloromethane and petroleum ether (3.44 g, 69%). 1H NMR
4
2
6.24 (d, 1H, J = 2.0 Hz, ArH), 4.60 (d, 1H, J = 12.0 Hz, ArCH2),
2
2
4.27 (d, 1H, J = 14.0 Hz, NCH2CN), 3.90 (d, 1H, J = 14.0 Hz,
NCH2CN), 3.50 (s, 2H, PhCH2), 3.19 (d, 1H, 2J = 12.0 Hz,
ArCH2), 1.99 (s, 3H, ArCH3), 1.28−1.19 (m, 8H × 0.25, hexane),
0.88 (t, 6H × 0.25, 3J = 6.8 Hz, hexane), 0.28 (s, 18H,
N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 168.0
(OCN), 151.2, 147.9, 136.8, 136.0, 133.7, 131.90, 131.87, 131.7,
131.54, 131.47, 129.0, 127.06, 127.03, 126.4, 125.7, 125.2, 120.91,
120.85, 120.3, 110.8 (all Ar-C), 64.1 (Ph3C), 61.2 (ArCH2), 60.1
(PhCH2), 45.8 (NCH2CN), 20.8 (ArCH3), 6.42 (N(Si(CH3)3)2).
Anal. Calcd for C48H53N3O2Si2Zn·0.25 C6H14: C, 70.19; H, 6.72; N,
4.96. Found: C, 69.75; H, 6.56; N, 5.00%.
3
(CDCl3, 400 MHz, 298 K): δ 7.32 (d, 6H, J = 7.3 Hz, ArH), 7.24−
7.16 (m, 10H, ArH), 7.02−6.95 (m, 14H, ArH), 6.84 (td, 1H, 3J = 7.2
4
Hz, J = 1.2 Hz, ArH), 6.80 (br s, 1H, ArH), 6.76 (br s, 1H, ArH),
6.73 (td, 1H, 3J = 7.2 Hz, 4J = 1.2 Hz, ArH), 6.67 (d, 1H, 3J = 7.2 Hz,
3
2
ArH), 6.60 (d, 1H, J = 7.2 Hz, ArH), 4.58 (d, 1H, J = 16.0 Hz,
2
ArCH2), 4.40 (d, 1H, J = 16.0 Hz, ArCH2), 2.31−2.26 (m, 1H,
[(L4)ZnN(SiMe3)2] (4). The procedure was the similar to that of
complex 1, except that L4H (0.615 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 4 as
white solids (0.379 g, 45.4%) after recrystallization with a mixture of
NCH2CN), 2.14 (s, 3H, ArCH3), 2.13−2.08 (m, 1H, NCH2C
N). 13C{1H} NMR (CDCl3, 100 MHz, 298 K): δ 149.8 (OCN),
149.5, 146.3, 146.1, 137.6, 135.4, 131.8, 131.6, 130.8, 130.4, 129.6,
129.1, 128.2, 127.5, 126.6, 126.3, 125.9, 122.0, 121.3, 120.7, 118.1,
109.6, 109.2 (all Ar-C), 70.7 (NCPh3), 63.5 (CPh3), 48.7 (ArCH2),
43.9 (NCH2CN), 21.0 (ArCH3). Anal. Calcd for C42H42N2O2·0.3
CH2Cl2: C, 83.78; H, 5.85; N, 3.34. Found: C, 83.85; H, 5.79; N,
3.37%.
1
THF and n-hexane. H NMR (C6D6, 400 MHz, 298 K): δ 7.48 (d,
6H, 3J = 7.6 Hz, ArH), 7.35 (d, 1H, 3J = 7.6 Hz, ArH), 7.32 (d, 1H, 4J
3
= 2.0 Hz, ArH), 7.11−7.02 (m, 5H, ArH), 6.96 (t, 6H, J = 7.6 Hz,
ArH), 6.94−6.88 (m, 1H, ArH), 6.87−6.80 (m, 2H, ArH), 6.77−6.71
(m, 3H, ArH), 6.61 (d, 1H, 4J = 2.0 Hz, ArH), 4.50 (d, 1H, 2J = 12.0
Hz, ArCH2), 3.79 (d, 1H, 2J = 18.0 Hz, NCH2CN), 3.59−3.54 (m,
4H × 0.1, THF), 3.08−2.98 (m, 1H, NCH2CH2), 2.93−2.86 (m, 1H,
[(L1)ZnN(SiMe3)2] (1). In a glovebox, the aminophenol L1H (0.413
g, 1.00 mmol) was dissolved in toluene (5 mL) and was added
dropwise to a solution of Zn[N(SiMe3)2]2 (0.390 g, 1.01 mmol) in
toluene (3 mL). The reaction mixture was stirred at room
temperature overnight, and all the volatiles were removed under
vacuum to afford a white solid which was then recrystallized with a
mixture of n-hexane and toluene. Colorless crystals of complex 1 were
2
NCH2CH2), 2.84 (d, 1H, J = 12.0 Hz, ArCH2), 2.81−2.77 (m, 1H,
NCH2CH2), 2.75 (d, 1H, 2J = 18.0 Hz, NCH2CN), 2.60−2.51 (m,
1H, NCH2CH2), 2.18 (s, 3H, ArCH3), 1.41−1.38 (m, 4H × 0.1,
THF), 0.23 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100
MHz, 298 K): δ 167.8 (OCN), 164.4, 151.1, 147.8, 138.4, 137.3,
136.2, 133.8, 131.7, 131.6, 129.01, 128.96, 127.0, 126.4, 125.6, 125.2,
121.1, 120.8, 120.4, 110.8 (all Ar-C), 67.8 (THF), 64.1 (Ph3C), 61.6
(ArCH2), 61.2 (NCH2CH2), 49.0 (NCH2CN), 31.7 (PhCH2), 25.8
(THF), 21.0 (ArCH3), 6.36 (N(Si(CH3)3)2). Anal. Calcd for:
C49H55N3O2Si2Zn·0.1 C4H8O: C, 70.07; H, 6.64; N, 4.96. Found:
C, 69.94; H, 6.75; N, 4.72%.
1
obtained in 67.7% yield (0.223 g). H NMR (C6D6, 400 MHz, 298
K): δ 7.56 (d, 1H, 3J = 8.0 Hz, ArH), 7.15−7.12 (m, 3H, ArH), 6.97−
6.93 (m, 2H, ArH), 6.85−6.80 (m, 2H, ArH), 6.71−6.66 (m, 2H,
4
3
ArH), 6.13 (d, 1H, J = 2.6 Hz, ArH), 4.01 (d, 1H, J = 14.2 Hz,
3
2
PhCH2), 3.97 (d, 1H, J = 14.2 Hz, PhCH2), 3.81 (d, 1H, J = 11.8
Hz, ArCH2), 3.51 (d, 1H, 2J = 16.9 Hz, NCH2CN), 2.74 (d, 1H, 2J
2
= 16.9 Hz, NCH2CN), 2.59 (d, 1H, J = 11.8 Hz, ArCH2), 1.27−
[(L7)ZnN(SiMe3)2] (7). The procedure was the same as that of
complex 1, except that L7H (0.581 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 7 as
3
1.19 (m, 8H × 0.1, hexane), 0.88 (t, 6H × 0.1, J = 6.8 Hz, hexane),
0.50 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298
K): δ 166.1 (OCN), 162.1, 151.0, 135.2, 132.1, 131.5, 130.0, 129.3,
129.1, 128.6, 126.7, 126.4, 125.8, 123.2, 119.5, 117.3, 110.8 (all Ar-C),
63.1 (ArCH2), 59.8 (PhCH2), 50.7 (NCH2CN), 32.0 (hexane),
23.1 (hexane), 14.4 (hexane), 6.18 (N(Si(CH3)3)2). Anal. Calcd for:
C28H35Cl2N3O2Si2Zn·0.1 C6H14: C, 53.12; H, 5.67; N, 6.50. Found:
C, 52.80; H, 5.61; N, 6.43%.
1
white solids (0.463 g, 57.4%). H NMR (C6D6, 400 MHz, 298 K): δ
3
7.48 (d, 6H, J = 7.2 Hz, ArH), 7.34−7.28 (m, 2H, ArH), 7.13−7.02
(m, 5H × 0.8, toluene), 6.96 (t, 6H, 3J = 7.2 Hz, ArH), 6.94−6.89 (m,
3
1H, ArH), 6.85−6.80 (m, 2H, ArH), 6.75 (t, 3H, J = 7.2 Hz, ArH),
4
2
6.60 (d, 1H, J = 2.0 Hz, ArH), 4.36 (d, 1H, J = 12.0 Hz, ArCH2),
2
2
3.73 (d, 1H, J = 18.2 Hz, NCH2CN), 2.83 (d, 1H, J = 12.0 Hz,
ArCH2), 2.77 (d, 1H, 2J = 18.2 Hz, NCH2CN), 2.71−2.61 (m, 1H,
NCH2CH2), 2.45−2.35 (m, 1H, NCH2CH2), 2.18 (s, 3H, ArCH3),
2.10 (s, 3H × 0.8, toluene), 1.69−1.56 (m, 1H, CH2 of
NCH2CH2CH), 1.35−1.24 (m, 2H, CH2 of NCH2CH2CH), 0.84
[(L2)ZnN(SiMe3)2] (2). The procedure was the same as that of
complex 1, except that L2H (0.457 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 2 as
1
white solids (0.253 g, 55.4%). H NMR (C6D6, 400 MHz, 298 K): δ
7.53 (d, 1H, 3J = 8.0 Hz, ArH), 7.10−7.02 (m, 5H, ArH), 7.04 (d, 1H,
3
3
(d, 3H, J = 6.2 Hz, CHCH3), 0.77 (d, 3H, J = 6.2 Hz, CHCH3),
0.22 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298
K): δ 167.9 (OCN), 164.6, 151.1, 147.8, 137.1, 136.1, 133.7, 131.7,
131.5, 129.3 (toluene), 128.6 (toluene), 127.0, 126.4, 125.7
(toluene), 125.6, 125.2, 121.1, 120.7, 120.6, 110.8 (all Ar-C), 64.1
(Ph3C), 60.8 (ArCH2), 58.5 (NCH2CH2), 48.6 (NCH2CN), 32.9
(NCH2CH2), 27.1 (CHCH3), 22.9 (CHCH3), 22.4 (CHCH3), 21.5
(ArCH3), 21.1 (toluene), 6.33 (N(Si(CH3)3)2). Anal. Calcd for:
C46H57N3O2Si2Zn·0.8 C7H8: C, 70.49; H, 7.27; N, 4.78. Found:
C,70.32; H, 7.18; N, 4.71%.
4J = 2.0 Hz, ArH), 6.80−6.74 (m, 1H, ArH), 6.68−6.57 (m, 2H,
4
2
ArH), 6.53 (d, 1H, J = 2.0 Hz, ArH), 4.22 (d, 1H, J = 11.2 Hz,
2
2
ArCH2), 4.15 (d, 1H, J = 13.8 Hz, PhCH2), 4.11 (d, 1H, J = 13.8
Hz, PhCH2), 3.70 (d, 1H, 2J = 17.2 Hz, NCH2CN), 3.30 (d, 1H, 2J
= 17.2 Hz, NCH2CN), 3.01 (d, 1H, 2J = 11.2 Hz, ArCH2), 1.56 (s,
9H, C(CH3)3), 1.29−1.23 (m, 8H × 0.1, hexane), 1.17 (s, 9H,
3
C(CH3)3), 0.88 (t, 6H × 0.1, J = 7.2 Hz, hexane), 0.55 (s, 18H,
N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 166.8
(OCN), 164.7, 151.1, 138.0, 135.5, 135.3, 132.7, 131.6, 129.1,
126.3, 125.6, 125.1, 124.4, 120.2, 119.9, 110.8 (all Ar-C), 63.3
(ArCH2), 62.1 (PhCH2), 50.5 (NCH2CN), 35.5 (C(CH3)3), 33.9
(C(CH3)3), 32.2 (hexane), 32.0 (C(CH3)3), 30.2 (C(CH3)3), 23.0
(hexane), 14.3 (hexane), 6.36 (N(Si(CH3)3)2). Anal. Calcd for
C36H53N3O2Si2Zn·0.1 C6H14: C, 63.71; H, 7.95; N, 6.09. Found: C,
63.15; H, 7.63; N, 5.93%.
[(L8)ZnN(SiMe3)2] (8). The procedure was the same as that of
complex 1, except that L8H (0.595 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 8 as
1
white solids (0.382 g, 46.5%). H NMR (C6D6, 400 MHz, 298 K): δ
7.47 (d, 6H, 3J = 7.2 Hz, ArH), 7.34 (d, 1H, 3J = 8.0 Hz, ArH), 7.31
(d, 1H, 4J = 2.4 Hz, ArH), 7.13−7.03 (m, 5H × 0.2, toluene), 6.95 (t,
I
DOI: 10.1021/acs.inorgchem.8b01839
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
6H, 3J = 8.0 Hz, ArH), 6.86−6.81 (m, 3H, ArH), 6.71 (t, 3H, 3J = 7.2
Hz, ArH), 6.62 (d, 1H, 4J = 2.4 Hz, ArH), 4.44 (d, 1H, 2J = 12.0 Hz,
Hz, NCH2CN), 3.04 (d, 1H, J = 11.7 Hz, ArCH2), 2.19 (s, 3H,
ArCH3), 2.11 (s, 3H × 0.2, toluene), 1.83 (br s, 3H, CH(CH2)3),
1.69−1.61 (m, 6H, NC(CH2)3), 1.43−1.39 (m, 7.6 H, 6H of
CHCH2CH and 4H × 0.4 of THF), 1.29−1.23 (m, 8H × 0.2,
hexane), 0.88 (t, 6H × 0.2, 3J = 7.2 Hz, hexane), 0.31 (s, 18H,
N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 169.6
(OCN), 164.6, 150.8, 147.9, 137.9 (toluene), 136.5, 136.2, 133.6,
131.9, 131.6, 129.3 (toluene), 128.6 (toluene), 127.4, 127.2, 127.1,
126.2, 125.7 (toluene), 125.5, 125.1, 121.4, 121.0, 120.7, 110.7 (all
Ar-C), 67.8 (THF), 64.1 (Ph3C), 60.4 (ArCH2), 53.2 (NCH2CN),
42.7 (NC(CH2)3), 37.7 (CH(CH2)3), 36.1 (C(CH2)3), 32.0
(hexane), 29.8 (CHCH2CH), 25.7 (THF), 23.0 (hexane), 21.1
(ArCH3 and toluene), 14.3 (hexane), 6.74 (N(Si(CH3)3)2). Anal.
Calcd for C51H61N3O2Si2Zn·0.2C7H8, ·0.4C4H8O, and ·0.2C6H14: C,
70.98; H, 7.40; N, 4.50. Found: C,70.72; H, 7.25; N, 4.54%.
2
2
ArCH2), 3.77 (d, 1H, J = 18.0 Hz, NCH2CN), 2.82 (d, 1H, J =
2
12.0 Hz, ArCH2), 2.70 (d, 1H, J = 18.0 Hz, NCH2CN), 2.56 (td,
1H, 3J = 12.5 Hz, 4J = 4.0 Hz, NCH2CH2), 2.27−2.20 (m, 1H,
NCH2CH2), 2.19 (s, 3H, ArCH3), 2.10 (s, 3H × 0.2, toluene), 1.89−
1.76 (m, 1H, CH2 of n-hexyl), 1.50−1.37 (m, 1H, CH2 of n-hexyl),
1.32−1.25 (m, 2H, CH2 of n-hexyl), 1.17−1.06 (m, 2H, CH2 of n-
hexyl), 1.05−0.96 (m, 2H, CH2 of n-hexyl), 0.90 (t, 3H, 3J = 6.9 Hz,
CH2CH3), 0.22 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (C6D6, 100
MHz, 298 K): δ 168.0 (OCN), 164.4, 151.2, 147.8, 137.3
(toluene), 136.2, 133.7, 131.7, 131.6, 129.3 (toluene), 128.6
(toluene), 127.0, 126.4, 125.6 (toluene), 125.1, 121.1, 120.7, 120.6,
110.8 (all Ar-C), 64.1 (Ph3C), 60.6 (ArCH2), 60.2 (NCH2CH2), 48.8
(NCH2CN), 31.9 (CH2 of n-hexyl), 27.4 (CH2 of n-hexyl), 24.9
(CH2 of n-hexyl), 23.0 (CH2 of n-hexyl), 21.0 (ArCH3), 14.3
(CH2CH3), 6.26 (N(Si(CH3)3)2). Anal. Calcd for: C47H59N3O2Si2Zn·
0.2 C7H8: C, 69.37; H, 7.29; N, 5.01. Found: C, 68.88; H, 7.25; N,
4.98%.
X-ray Crystallography. The X-ray diffraction measurements of
single crystals of complexes 11, 12, and 3d were performed on a
Bruker SMART APEX II diffractometer with graphite-monochro-
mated Mo Kα (λ= 0.71073 Å) radiation. All data were collected at
293 K using the ω-scan techniques. All structures were solved by
direct methods and refined using Fourier techniques. An absorption
correction based on SADABS was applied.60 All non-hydrogen atoms
were refined by full-matrix least-squares on F2 using the SHELXTL
program package.61 Hydrogen atoms were located and refined by the
geometry method. The cell refinement and data collection and
reduction were done by Bruker SAINT.62 The structure solution and
refinement were performed by SHELXS-9763 and SHELXL-97,64
respectively. For further crystal data and details of measurements, see
Table S1 in the Supporting Information. Molecular structures were
generated using the ORTEP program.65
Typical Polymerization Procedure. In a glovebox, an initiator
solution (0.5 mL, 10 mmol/mL) from a stock solution in toluene or
THF was injected sequentially into a series of 10 mL vials loaded with
rac-LA (0.144 g, 1.00 mmol) and a suitable amount (0.5 mL) of the
same dry solvent. The mixture was stirred at room temperature and
quenched at specific time intervals by adding an excess amount of
normal light petroleum ether. After being dissolved with dichloro-
methane, a small amount of an aliquot of the bulk solution was
withdrawn and dried under reduced pressure for monomer conversion
determination via 1H NMR spectroscopy. The bulk solution was
slightly concentrated and the polymer was precipitated from
dichloromethane via the addition of excess methanol. The collected
polymer sample was further dried in a vacuum oven at 60 °C for 16 h
to a constant weight for GPC and 1H and homonuclear-decoupled 1H
NMR analyses. In the cases where 2-propanol was used, the monomer
solution was treated at first with a solution of 2-propanol for 5 min,
and then a solution of the initiator was injected into the mixture.
Otherwise, the procedures were the same.
[(L9)ZnN(SiMe3)2] (9). The procedure was the same as that of
complex 1, except that L9H (0.579 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 9 as
1
white solids (0.449 g, 55.9%). H NMR (C6D6, 400 MHz, 298 K): δ
3
7.48 (d, 6H, 3J = 7.6 Hz, ArH), 7.33 (d, 1H, J = 7.6 Hz, ArH), 7.29
(d, 1H, 4J = 2.0 Hz, ArH), 6.97 (t, 6H, 3J = 7.6 Hz, ArH), 6.94−6.89
3
(m, 1H, ArH), 6.86−6.80 (m, 2H, ArH), 6.76 (t, 3H, J = 7.6 Hz,
4
2
ArH), 6.59 (d, J = 2.0 Hz, 1H, ArH), 4.45 (d, 1H, J = 12.0 Hz,
ArCH2), 3.74 (d, 1H, 2J = 18.2 Hz, NCH2CN), 2.97−2.84 (m, 3H,
1H of ArCH2, 1H of NCH2CN, 1H of NCH), 2.16 (s, 3H, ArCH3),
1.77−1.67 (m, 1H, CH2 of cyclopentyl), 1.59−1.50 (m, 1H, CH2 of
cyclopentyl), 1.48−1.31 (m, 4H, CH2 of cyclopentyl), 1.28−1.16 (m,
2H, CH2 of cyclopentyl), 0.27 (s, 18H, N(Si(CH3)3)2). 13C{1H}
NMR (C6D6, 100 MHz, 298 K): δ 168.4 (OCN), 164.6, 151.0,
147.9, 136.8, 136.2, 133.6, 131.7, 131.4, 127.1, 126.3, 125.6, 125.1,
121.2, 120.6, 120.5, 110.7 (All Ar-C), 69.2 (ArCH2), 64.1 (Ph3C),
59.7 (NCH), 47.3 (NCH2CN), 28.3 (CH2), 28.1 (CH2), 23.8
(CH2), 23.6 (CH2), 21.0 (ArCH3), 6.43 (N(Si(CH3)3)2). Anal. Calcd
for: C46H55N3O2Si2Zn: C, 68.76; H, 6.90; N, 5.23. Found: C, 68.59;
H, 6.91; N, 5.07%.
[(L11)ZnN(SiMe3)2] (11). The procedure was the same as that of
complex 1, except that L11H (0.621 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 11 as
1
white solids (0.423 g, 50.1%). H NMR (C6D6, 400 MHz, 298 K): δ
4
7.51 (d, 6H, 3J = 7.6 Hz, ArH), 7.25 (d, 1H, J = 2.0 Hz, ArH), 7.21
(d, 1H, 3J = 7.6 Hz, ArH), 7.03 (t, 6H, 3J = 7.6 Hz, ArH), 6.92−6.87
3
(m, 1H, ArH), 6.84 (t, 3H, J = 7.6 Hz, ArH), 6.77−6.74 (m, 2H,
4
2
ArH), 6.57 (d, J = 2.0 Hz, 1H, ArH), 4.25 (d, 1H, J = 12.0 Hz,
2
2
ArCH2), 3.55 (d, 1H, J = 18.2 Hz, NCH2CN), 3.09 (d, 1H, J =
2
18.2 Hz, NCH2CN), 3.03 (d, 1H, J = 12.0 Hz, ArCH2), 2.12 (s,
ASSOCIATED CONTENT
3H, ArCH3), 2.07−1.97 (m, 1H, NCH of cyclooctyl), 1.64−1.38 (m,
6H, CH2 of cyclooctyl), 1.37−1.18 (m, 7H, CH2 of cyclooctyl), 1.06−
0.94 (m, 1H, CH2 of cyclooctyl), 0.27 (s, 18H, N(Si(CH3)3)2).
13C{1H} NMR (C6D6, 100 MHz, 298 K): δ 168.3 (OCN), 165.3,
150.9, 147.9, 135.9, 135.8, 133.7, 131.6, 131.2, 127.2, 126.2, 125.7,
125.1, 121.0, 120.62, 120.60, 110.6 (All Ar-C), 65.2 (Ph3C), 64.1
(ArCH2), 57.6 (NCH), 46.0 (NCH2CN), 30.8 (CH2 of cyclooctyl),
30.3 (CH2 of cyclooctyl), 29.5 (CH2 of cyclooctyl), 26.7 (CH2 of
cyclooctyl), 26.1 (CH2 of cyclooctyl), 26.0 (CH2 of cyclooctyl), 25.7
(CH2 of cyclooctyl), 21.0 (ArCH3), 6.32 (N(Si(CH3)3)2). Anal. Calcd
for C49H61N3O2Si2Zn: C, 69.60; H, 7.27; N, 4.97. Found: C, 69.65;
H, 7.43; N, 4.74%.
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01839.
1H and 13C NMR spectra of all complexes and related
NMR reactions, homonuclear decoupled 1H NMR
spectra of typical polymer samples, plots of kinetic
studies of rac-LA polymerization by complexes 1−12,
GPC trace of the PLA samples, and DSC measurement
of the typical samples (PDF)
[(L12)ZnN(SiMe3)2] (12). The procedure was the same as that of
complex 1, except that L12H (0.645 g, 1.00 mmol) and Zn[N-
(SiMe3)2]2 (0.390 g, 1.01 mmol) were used to afford complex 12 as
Accession Codes
CCDC 1852679−1852681 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
white solids (0.479 g, 55.1%). H NMR (C6D6, 400 MHz, 298 K): δ
4
7.50−7.45 (m, 7H, ArH), 7.33 (d, 1H, J = 2.0 Hz, ArH), 7.14−7.11
(m, 2H × 0.2, toluene), 7.08−7.00 (m, 3H × 0.2, toluene), 6.99−6.91
4
(m, 7H, ArH), 6.85−6.78 (m, 5H, ArH), 6.63 (d, 1H, J = 2.0 Hz,
ArH), 4.52 (d, 1H, 2J = 11.7 Hz, ArCH2), 3.88 (d, 1H, 2J = 18.7 Hz,
NCH2CN), 3.58−3.55 (m, 4H × 0.4, THF), 3.41 (d, 1H, 2J = 18.7
J
DOI: 10.1021/acs.inorgchem.8b01839
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Type Ligands: Highly Stereoselective Ring-Opening Polymerization
of rac-Lactide. Organometallics 2012, 31, 2016−2025.
(16) Stanford, M. J.; Dove, A. P. Stereocontrolled Ring-Opening
Polymerisation of Lactide. Chem. Soc. Rev. 2010, 39, 486−494.
(17) Thomas, C. M. Stereocontrolled Ring-opening Polymerization
of Cyclic Esters: Synthesis of New Polyester Microstructures. Chem.
Soc. Rev. 2010, 39, 165−173.
(18) Dijkstra, P. J.; Du, H.; Feijen, J. Single Site Catalysts for
Stereoselective Ring-Opening Polymerization of Lactides. Polym.
Chem. 2011, 2, 520−527.
(19) Wang, H.; Ma, H. Highly Diastereoselective Synthesis of Chiral
Aminophenolate Zinc Complexes and Isoselective Polymerization of
rac-Lactide. Chem. Commun. 2013, 49, 8686−8688.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: haiyanma@ecust.edu.cn; Fax/Tel.: +86 21 64253519.
ORCID
Haiyan Ma: 0000-0003-0810-2493
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was subsidized by the National Natural Science
Foundation of China (NNSFC, Grant 21474028) and the
Fundamental Research Funds for the Central Universities
(Grant WD1113011). All financial support is gratefully
acknowledged. H.M. also thanks the very kind donation of a
Braun glovebox by AvH foundation.
́
́
(20) Honrado, M.; Otero, A.; Fernandez-Baeza, J.; Sanchez-Barba, L.
́
́
F.; Garces, A.; Lara-Sanchez, A.; Rodríguez, A. M. Efficient Synthesis
of an Unprecedented Enantiopure Hybrid Scorpionate/Cyclo-
pentadienyl by Diastereoselective Nucleophilic Addition to a Fulvene.
Organometallics 2013, 32, 3437−3440.
́
́
(21) Honrado, M.; Otero, A.; Fernandez-Baeza, J.; Sanchez-Barba, L.
́
́
F.; Garces, A.; Lara-Sanchez, A.; Rodríguez, A. M. Stereoselective
ROP of rac-Lactide Mediated by Enantiopure NNO-Scorpionate Zinc
Initiators. Organometallics 2014, 33, 1859−1866.
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