Al and Zn complexes bearing N,N,N-tridentate quinolinyl anilido-imine ligands: Synthesis, characterization and catalysis in l-lactide polymerization

Article abstract of DOI:10.1039/c2dt30594a

Reactions of N,N,N-tridentate quinolinyl anilido-imine ligands with AlMe₃ afford mononuclear aluminum complexes {κ³-[{2- [ArNC(H)]C₆H₄}N(8-C₉H₆N)]}AlMe ₂ (Ar = 2,6-Me₂

Full text of DOI:10.1039/c2dt30594a

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PAPER
Al and Zn complexes bearing N,N,N-tridentate quinolinyl anilido-imine
ligands: synthesis, characterization and catalysis in L-lactide polymerization†
Nan Yang,a Lan Xin,a Wei Gao,*a Jingshun Zhang,a Xuyang Luo,b Xiaoming Liu*a and Ying Mu*a
Received 15th March 2012, Accepted 16th July 2012
DOI: 10.1039/c2dt30594a
Reactions of N,N,N-tridentate quinolinyl anilido-imine ligands with AlMe3 afford mononuclear aluminum
complexes {κ3-[{2-[ArNvC(H)]C6H4}N(8-C9H6N)]}AlMe2 (Ar = 2,6-Me2C6H3 (1a), 2,6-Et2C6H3 (1b),
2,6-iPr2C6H3 (1c)) or dinuclear complexes AlMe3{κ1-[{2-[ArNvC(H)C6H4]N(8-C9H6N)}-κ2]AlMe2
(R = 2,6-Me2C6H3 (2a), 2,6-Et2C6H3 (2b), 2,6-iPr2C6H3 (2c)) depending on the ratios of reactants used.
Similar reactions of ZnEt2 with these ligands give the monoligated ethyl zinc complexes {κ3-[{2-
[ArNvC(H)]C6H4}N(8-C9H6N)]}ZnEt (Ar = 2,6-Me2C6H3 (3a), 2,6-Et2C6H3 (3b), 2,6-iPr2C6H3 (3c))
or bisligated complexes {κ3-[{2-[ArNvC(H)]C6H4}N(8-C9H6N)]}Zn{κ2-[{2-[ArNvC(H)]C6H4}-
N(8-C9H6N)]} (Ar = 2,6-Me2C6H3 (4a), 2,6-Et2C6H3 (4b), 2,6-iPr2C6H3 (4c)). These complexes were
well characterized by NMR and the structures of 1a, 2a, 2c, 3b and 4c were confirmed by X-ray
diffraction analysis. The aluminum and zinc complexes were tested to initiate lactide polymerization in
which the zinc complexes show moderate to high activities in the presence of benzyl alcohol.
productivities for ε-caprolactone and L-lactide ring-opening
Introduction
polymerization in the presence of benzyl alcohol.11 Side-arms
The polylactides (PLAs) prepared from renewable sources have
been widely used in the biomedical, pharmaceutical and agricul-
tural fields as promising alternatives to synthetic petrochemical-
based polymers.1 PLAs are usually prepared by ring-opening
polymerization of lactides with metal alkoxides as initiators. A
large number of alkoxides of Sn,2 Al,3 Zn,3d,4 Mg,5,4d Fe,6 Ti,7
In,8 and some rare-earth metals9 with or without ligands have
been extensively investigated in lactide polymerization. The
aluminum- and zinc-based systems seem to be the best candi-
dates in the preparation of PLAs due to their high Lewis acidity
and low toxicity. The ancillary ligands in the complexes were
proven to play an important role in preventing the epimerization
and transesterification which leads to uncontrolled molecular
weight, broad molecular weight distributions and formation of
macrocycles or oligomers. Over the past decades significant
efforts have been devoted to the design and synthesis of appro-
priate ancillary ligands to improve the performance of the com-
plexes in polymerization. It has been reported that some Al and
Zn complexes with salen/salan and β-diketiminate ligands show
high activities for the ring-opening polymerization of lactides.10
Some anilido-imine ligands were also used to support the alumi-
num or zinc complexes and the complexes show high
were introduced into the anilido-imine ligands to strengthen the
steric control abilities. The rare-earth metal alkyls bearing meth-
oxyl group decorated anilido-imine tridentate ligands can initiate
the polymerization of lactide in a single-site manner. The meth-
oxyl group was believed to prevent the back-biting reaction in
polymerization.12 Recently we have introduced the rigid quinoli-
nyl group into the anilido-imine ligand and the rare-earth metal
alkyl complexes with such tridentate ligands can catalyze the
ε-caprolactone polymerization.13 The coordination of the quino-
linyl group to the metal center inhibits the transesterification and
the polymerization is performed in a living manner.
Herein, we wish to report the synthesis and characterization of
a series of aluminum and zinc complexes supported by quinoli-
nyl anilido-imine ligands. Their catalytic behavior as initiators
for the ring-opening polymerization of L-lactide were also
investigated.
Results and discussions
Synthesis and characterization of aluminum complexes
The quinolinyl anilido-imine ligands were prepared according to
the literature13 and were well characterized by NMR spec-
1
troscopy. All the H NMR spectra of the ligands show a reson-
aState Key Laboratory for Supramolecular Structure and Materials, Jilin
University, 2699 Qianjin Street, Changchun 130012, People’s Republic
of China. E-mail: gw@jlu.edu.cn; Tel: (+86)-431-5168472
bSchool of Chemistry, Jilin University, 2699 Qianjin Street, Changchun
130012, People’s Republic of China
ance at 12.2 ppm for the amine proton suggesting the existence
of a hydrogen bond between NH and imine groups.14 Addition
of 1 equiv of AlMe3 to the toluene solution of the ligands at
0 °C resulted in immediate color change from orange to deep
red. Evaporation of the solvent and recrystallization of the
residue with hexane and toluene afforded the mononuclear
†CCDC 871557–871561. For crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30594a
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Scheme 1 Synthetic route for the mononuclear and dinuclear alumi-
num complexes.
Fig. 1 Stacked plot of variable-temperature 1H NMR spectra of 1a.
aluminum complexes 1a–1c as orange powders in moderate
yields as shown in Scheme 1.
The aluminum complexes 1a–1c were characterized by
1
elemental analysis and NMR spectroscopy. The H NMR and
13C{1H} NMR spectra of these complexes at room temperature
exhibit similar patterns. For all complexes, the singlet for NH in
free ligands disappeared and the singlet resonances for the
HCvN proton at 8.14–8.16 ppm were observed, which shift to
high field with respect to the free ligands. It is worth noting that
1
the H NMR spectra of these complexes show two sets of broad
singlets in the range of −1.45 to −1.42 ppm and −0.72 to
−0.69 ppm for the two methyl groups attached to the aluminum
center (Me–Al). This behavior indicates that the two methyl
groups are in non-equivalent environments and a fluxional
exchange between two isomers may exist in solution at room
temperature. The fluxional behavior of 1a was further studied by
variable-temperature 1H NMR spectroscopy (Fig. 1). The VT
NMR analysis showed that when the temperature was decreased
to −30 °C, the singlet at 2.30 ppm for the two methyl groups of
N-aryl moiety (Me–Ar) broadened and became resolved into two
singlets, probably owing to the lack of rotation of the C–Nimino
bond at low temperature. Upon increasing the temperature from
10 °C, the resonances of the two Me–Al methyl groups broad-
ened and eventually fused together as one broad signal at 60 °C.
This may be attributed to the fast exchange of the molecule
between two isomers at high temperature (see Fig. 1). Similar
phenomena was also observed in rare-earth metal alkyl com-
plexes bearing such ligand.13 In fact the X-ray diffraction analy-
sis of 1a shows that the two isomers do exist in the solid state.
Crystals of 1a suitable for X-ray diffraction analysis were
grown from hexane. The molecular structure is depicted in
Fig. 2. Crystallographic data and selected bond distances and
angles are given in Tables 1 and 2. It can be seen that the Al
atom is five-coordinated and the geometry around it can be best
described as a distorted trigonal bipyramid. The two methyls
bonded to the Al center and the Namido atom are in the equatorial
plane and the other two nitrogen atoms are in apical positions.
The Al atom lies out of the coordinated plane (N1–C1–C2–C16–
N3) by 0.9436(31) Å, but is essentially coplanar with the quino-
linyl ring. The dihedral angle between the plane (N1–C1–C2–
C16–N3) and quinolinyl ring is 42.65(9)°. The Al–Namido bond
lengths of 1.920(3) Å are slightly shorter than the Al–Nimine
Fig. 2 Perspective view of complex 1a with thermal ellipsoids drawn
at 30% probability level. Uncoordinated solvents and hydrogens are
omitted for clarity.
(2.203(2) Å) and Al–Nquinolinyl (2.117(2) Å), but is longer than
those of 1.870(4)–1.890(4) Å for Al–N bonds in similar anilido-
imino Al complexes.11a
Additions of one equiv of AlMe3 to 1a–1c or treatments of
two equivalent of AlMe3 with the ligands, after the appropriate
workup and recrystallization from hexane, afford pale yellow
powders which were proven to be dinuclear complexes 2a–2c
(Scheme 1). The 1H NMR spectra of 2a–2c exhibit a singlet
(−1.37 to −1.39 ppm) for the methyl protons of AlMe3 coordi-
nated to the Nimino atoms. It is worth noting that the spectra
showed two separated singlets (−0.53 to −0.58 ppm) for the
methyl protons of AlMe2 chelated by the aminoquinolinyl
moiety, along with two sets of signals for the ortho groups of the
N-aryl moiety, suggesting that both of the two methyls are
bonded to the aluminum center (Me–Al) and the two ortho sub-
stituents of the N-aryl moiety (R–Ar) are in non-equivalent
environments respectively. The coordination of AlMe3 to Nimino
atoms may inhibit the rotations about the C–Nimino and C–Namido
bond. The molecular structures of 2a and 2c were established by
X-ray diffraction studies and their molecular structures are
depicted in Fig. 3 and 4 respectively. Complexes 2a and 2c are
isostructural dinuclear complexes in which one aluminum is
chelated by the Namido atom and Nquinolinyl atom while the other
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Table 1 Crystal data and structure refinement for complexes 1a, 2a, 2c, 3b and 4c
1a
2a
2c
3b
4c
Formula
Fw
Cryst syst
Space group
a (Å)
b (Å)
c (Å)
C59H60Al2N6
907.09
Monoclinic
C2/c
32.1381(16)
8.4504(4)
18.4874(9)
90
93.3180(10)
90
C29H35Al2N3
479.56
C39H57Al2N3
621.84
C28H29N3Zn
472.91
C115H119N12Zn2
1799.96
Triclinic
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
ˉ
P1
P1
P1
P1
9.7793(8)
11.1335(9)
14.3337(11)
71.6540(10)
72.7750(10)
79.3680(10)
1407.67(19)
2
12.1624(7)
12.6833(7)
13.7183(8)
80.7440(10)
78.2440(10)
73.3270(10)
1972.82(19)
2
10.7628(7)
10.9313(7)
11.4905(8)
69.8200(10)
88.4390(10)
66.2230(10)
1151.30(13)
2
11.438(2)
13.475(3)
17.838(4)
101.71(3)
94.18(3)
108.97(3)
2517.2(9)
1
α (°)
β (°)
γ (°)
v (Å3)
Z
5012.4(4)
4
0.103
μ (mm−1
Rint
)
0.124
0.0161
0.101
0.0154
1.087
0.0116
0.530
0.1271
0.0204
GOOF
1.033
0.0558
0.1464
1.025
0.0544
0.1343
1.027
0.0545
0.1347
1.063
0.0327
0.0833
1.018
0.1131
0.2288
a
R1
b
wR2
2 2
2 2
a R1 = ∑kFo| − |Fck/∑|Fo|. b wR2 = [∑[w(Fo2 − Fc ) ]/∑[w(Fo ) ]]1/2
.
Table 2 Selected bond lengths (Å) and angles (°) for 1a, 2a, 2c, 3b
and 4c
1a
Al(1)–N(1)
Al(1)–N(2)
Al(1)–N(3)
Al(1)–C(25)
Al(1)–C(26)
2a
1.920(3)
2.203(2)
2.117(2)
1.978(3)
1.979(3)
N(1)–Al(1)–N(3)
N(1)–Al(1)–N(2)
N(2)–Al(1)–N(3)
C(25)–Al(1)–C(26)
85.41(9)
78.20(10)
161.48(10)
125.57(14)
Al(1)–N(1)
Al(1)–N(2)
Al(1)–C(23)
Al(1)–C(24)
Al(2)–N(3)
2c
Al(1)–N(1)
Al(1)–N(2)
Al(1)–C(23)
Al(1)–C(24)
Al(2)–N(3)
3b
1.891(2)
1.973(2)
1.947(3)
1.958(3)
2.0418(19)
Al(2)–C(25)
Al(2)–C(26)
Al(2)–C(27)
N(1)–Al(1)–N(2)
C(23)–Al(1)–C(24)
1.971(3)
1.981(3)
1.974(3)
84.73(9)
119.04(13)
1.892(2)
1.980(3)
1.953(3)
1.954(3)
2.045(2)
Al(2)–C(25)
Al(2)–C(26)
Al(2)–C(27)
N(1)–Al(1)–N(2)
C(23)–Al(1)–C(24)
1.977(3)
1.973(3)
1.976(3)
84.29(10)
121.82(15)
Fig. 3 Perspective view of complex 2a with thermal ellipsoids drawn
at 30% probability level. Hydrogens are omitted for clarity.
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–(27)
4c
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–N(4)
Zn(1)–N(5)
1.9927(16)
2.1018(17)
2.1335(16)
1.975(2)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–N(3)
N(2)–Zn(1)–N(3)
80.52(7)
85.75(6)
115.50(6)
1.988(6)
2.093(7)
2.131(6)
1.968(6)
2.167(6)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–N(3)
N(2)–Zn(1)–N(3)
N(4)–Zn(1)–N(5)
78.8(3)
83.2(2)
125.3(3)
80.3(2)
is coordinated by the Nimine atom. The Al(1) is essentially copla-
nar with the quinolinyl group with the deviation of 0.0591(23) Å
in 2a and 0.0942(25) Å in 2c. The Al–Namido bond lengths of
1.891(2) Å in 2a and 1.892(2) Å in 2c are comparable to that of
1.920(3) Å in 1a. The quinolinyl ring is almost vertical to the
phenyl ring (C1 to C6) with a large dihedral angle of 74.0° in 2a
Fig. 4 Perspective view of complex 2c with thermal ellipsoids drawn
at 30% probability level. Uncoordinated solvent and hydrogens are
omitted for clarity.
1
and 70.0° in 2c. The H NMR analysis reveal that 2a–2c are not
Synthesis and characterization of zinc complexes
stable in the solid state and the AlMe3 attached to the imino
group can be released gradually forming the corresponding
mononuclear complexes.
The N,N,N-tridentate zinc complexes were also prepared in a
similar procedure to that for the aluminum complexes as shown
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Fig. 5 Perspective view of complex 3b with thermal ellipsoids drawn
at 30% probability level. Hydrogens and uncoordinated solvent are
omitted for clarity.
Scheme 2 Synthetic route for the zinc complexes.
in Scheme 2. Reactions of the ligands with one equivalent of
ZnEt2 give the complexes 3a–3c in high yields. The identities of
1
these complexes were determined by H NMR, 13C{1H} NMR
analysis and were corroborated by elemental analyses. X-ray
quality crystals of 3b were obtained via recrystallization in
hexane and its structure was elucidated (Fig. 5). The zinc atom
in 3b is in a distorted tetrahedral geometry ligated by the N,N,N-
tridentate ligand and an ethyl group. The zinc atom is essentially
coplanar with the aminoquinolinyl plane with a maximum
deviation of 0.1069(36) Å, and the Zn–N bond distances
(1.9927(16)–2.1335(16) Å) are comparable to those in β-diketi-
minate zinc alkyl complexes.15 The other bond distances and
angles for this complexes are unexceptional and fall in the range
of values reported for related zinc complexes.
Fig. 6 Perspective view of complex 4c with thermal ellipsoids drawn
at 30% probability level. Hydrogens and uncoordinated solvent are
omitted for clarity.
Monitoring by 1H NMR spectroscopy showed that mono-
ligated complexes 3a and 3b are stable in hexane or toluene
solutions. Whereas 3c is unstable and disproportionates in solution
into bisligated complex 4c over a period of several days at room
temperature. Similar ligand rearrangement was also observed in
the zinc complexes supported by β-diketiminato,16 fluorous
imino-alkoxide,17 tris(pyrazolyl)borate ligands,18 and NCN
pincer ligands.19 Alternatively the bisligated complexes 4a–4c
can be synthesized in high yields by deprotonation of the ligands
with 0.5 equivalent of ZnEt2 in toluene. It is worth noting that
Polymerization of L-lactide
The aluminum and zinc complexes were investigated as initiators
for the ring-opening polymerization of L-lactide. The polymeriz-
ations were carried out in toluene at 70 °C and the representative
polymerization results are summarized in Table 3. When used as
single component, all the aluminum complexes are almost inert
for lactide polymerization and no polymer was obtained even at
high temperature. While when activated with benzyl alcohol, the
activities were improved and about 75–95% conversions were
achieved in 24 h. It is worth noting that in both catalytic systems
(1a–1c)–BnOH and (2a–2c)–BnOH, the activities and Mw of the
resulted polymer show no dependence on the bulkiness of ortho-
groups of N-aryl moieties. The monoligated zinc complexes
show varied activities in the absence of benzyl alcohol and the
conversions of 9.1–63.26% were achieved within 30 min, afford-
ing high molecular weight polymer with broad PDI (entries
7–9). The low activities and uncontrollable behavior may due to
the poor nucleophilicity of the alkyl groups and inefficient
initiation.20 Whereas the bisligated complexes 4a–4c are more
active than the corresponding monoligated complexes under the
same conditions. This may be attributed to the different initial
1
the H NMR spectra of these complexes show broad signals for
ortho groups of N-aryl moieties at room temperature suggesting
that some kind of fluxional exchange may exist in solution.
1
However, the variable-temperature H NMR study of 4c shows
no obvious changes with the variation of the temperature. The
molecular structure of 4c was established by an X-ray diffraction
study (Fig. 6). In 4c, the five-coordinated zinc atom is in a trigo-
nal bipyramidal geometry with one N-aryl moiety coordinating
to the zinc atom and the other N-aryl moiety dangling away. No
detectable difference between the two N-aryl moieties was
observed in the VT NMR, suggesting that in solution the bisli-
gated zinc complexes may exist in a four-coordinated form with
both of the two N-aryl moieties dangling away from the metal
center.
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Table 3 Polymerization of L-lactide initiated with zinc complexesa
Entry
Cat
[M]/[I]/[BnOH]
Time (min)
Yieldb (%)
Mc c(×104)
Mn d(×104)
PDId
1
2
3
4
5
6
7
8
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3a
3a
3a
3a
3a
3a
3a
100/1/2
100/1/2
100/1/2
100/1/5
100/1/5
100/1/5
100/1/0
100/1/0
100/1/0
100/1/0
100/1/0
100/1/0
100/1/1
100/1/1
100/1/1
100/1/1
100/1/1
100/1/1
300/1/1
500/1/1
700/1/1
900/1/1
100/1/2
100/1/3
500/1/5
500/1/10
1440
1440
1440
1440
1440
1440
30
30
30
30
30
30
30
30
30
30
30
30
77.79
75.73
76.08
90.60
94.68
94.62
63.26
27.50
9.10
85.90
49.50
54.80
95.18
94.00
92.12
99.38
98.32
90.03
100
0.56
0.55
0.55
0.26
0.27
0.27
0.91
0.40
0.13
1.24
0.71
0.79
1.37
1.35
1.33
1.43
1.42
1.30
4.33
7.21
10.09
12.25
0.68
0.46
1.34
0.71
0.87
0.80
0.76
0.27
0.39
0.38
10.94
8.03
2.36
3.67
10.8
10.8
1.39
1.35
1.36
1.29
1.50
1.51
4.15
6.58
9.65
11.35
0.87
0.60
1.67
0.87
1.13
1.14
1.12
1.13
1.21
1.23
1.09
1.29
1.26
1.87
1.23
1.21
1.16
1.13
1.09
1.34
1.17
1.10
1.22
1.27
1.30
1.35
1.29
1.29
1.15
1.09
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
120
120
120
120
30
30
30
100
100
94.40
94.37
96.20
93.03
97.83
30
a Polymerization conditions: 30 μmol of initiator; T = 70 °C; 15 mL toluene. b Determined by 1H NMR. c Calculated from ([LA]0/[I]0) × conversion ×
144 or ([LA]0/[BnOH]0) × conversion × 144. d Determined by GPC against polystyrene standards in THF, multiplied by 0.58.22
groups used in the two kinds complexes. The Zn–N bonds in the
bisligated complexes are believed to be more active than Zn–C
bonds in the monoligated complexes. Additionally, the activities
of the catalytic systems are strongly affected by the ortho substi-
tuents (entries 7–12). In both cases, there is a sharp decrease in
activity as the ortho substituent goes from 2,6-methyl to isopro-
pyl, probably owing to the growing bulkiness at the metal
centers which inhibits the insertion of the monomer to the Zn–C
and Zn–N bonds. Upon activation with benzyl alcohol (BnOH),
the productivities of the zinc complexes are all dramatically
improved and up to 99.38% conversion can be achieved within
30 min (entries 13–18). The Mn values of the polymers obtained
from these binary systems are very close to the Mc values calcu-
lated according to the monomer/initiator (M/I) molar ratio. At
the same time, the PDIs are all quite narrow ranging from 1.09
to 1.17, which is indicative of single-site catalyst or initiator
systems. The performances of the binary catalytic system were
further investigated by using 3a–BnOH. Polymerizations with
different monomer/initiator (M/I) molar ratios were carried out to
test the control abilities of the catalytic system (entries 13,
19–22). It was found that the Mn of the resulted polymer
increases with the increase of the M/I molar ratio and the PDI
gets slightly broader. The relatively broad PDIs may be attributed
to intermolecular transesterification and intramolecular trans-
esterification (cyclization). Similar intermolecular and intramole-
cular transesterifications have also been observed in magnesium
phenoxides5b and silylamido lanthanide21 initiating systems.
Studies on the relationship of molecular weight vs. conversion
were also conducted with a 3a–BnOH binary system (as shown
in Fig. 7). It shows that the molecular weight of the resultant
polymer increases linearly with the increase of the monomer
Fig. 7 Plot of molecular weight Mn vs. monomer conversion for 3a–
BnOH initiated polymerization of L-lactide. Reaction conditions:
30 μmol of initiator, M/I/B = 100 : 1 : 1, 10 mL toluene, 70 °C.
conversion and the PDIs of these polymer are kept in a narrow
range (1.05–1.14) suggesting the living feature of the binary
system. It was found that the molecular weight (Mn) of the resul-
tant polymer decreases gradually with the increase of the BnOH/
M molar ratios (entries 20, 25–26). The corresponding PDIs are
kept narrow and the measured Mn values fit well with the calcu-
lated values according to the M/BnOH molar ratio, indicating
somewhat “immortal” behavior of the binary system.
In order to further understand the polymerization reaction of
L-lactide initiated by these binary systems, the reaction of com-
1
plexes with BnOH was monitored in situ by H NMR at room
1
temperature. The H NMR results reveal that when BnOH was
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Conclusions
A series of mononuclear/binuclear aluminum complexes bearing
N,N,N-tridentate quinolinyl anilido-imine ligands have been syn-
thesized and structurally characterized. Mononuclear aluminum
complexes exhibit a trigonal bipyramidal coordination around
the metal center, while in binuclear complexes all the aluminum
atoms are in a tetrahedral geometry. Analogous monoligated/
bisligated zinc complexes were also prepared. The zinc atoms in
monoligated complexes are in a tetrahedral geometry, while in
bisligated complexes they have trigonal bipyramidal geometries.
The Zn complexes are efficient initiators for L-lactide ring-
opening polymerization in the presence of benzyl alcohol, and
the polymerization can be performed in an immortal manner.
Fig. 8 1H NMR spectrum of a polymer sample obtained from the 3a–
BnOH system with M/I/B = 25 : 1 : 1 (in CDCl3).
Experiment section
All manipulations involving air and moisture-sensitive com-
pounds were carried out under an atmosphere of dried and
purified nitrogen using standard Schlenk or dry box techniques.
Toluene and hexane were dried over sodium–benzophenone and
distilled under nitrogen prior to use. Elemental analyses were
performed on a Varian EL microanalyzer. NMR spectra were
recorded on a Varian Mercury-300 NMR spectrometer at room
temperature in CDCl3. The molecular weight and molecular
weight distribution of the polymers were measured on a TOSOH
HLC 8220 GPC at 40 °C using THF as eluent against poly-
styrene standards. N-(2-((2,6-diisopropylphenylimino)methyl)
phenyl)quinolin-8-amine,13 C6H4F(CHvNC6H3Me2-2,6)11 and
C6H4F(CHvNC6H3Et2-2,6)11 were synthesized according to lit-
erature procedures. 2,6-Dimethylaniline, 2,6-diethylaniline, 2,6-
diisopropylaniline, AlMe3 and ZnEt2 were purchased from
Aldrich.
N-(2-((2,6-Dimethylphenylimino)methyl)phenyl)quinolin-8-
amine
Scheme 3 Proposed mechanism for the ROP of L-lactide initiated by
3a–3c–BnOH.
n
An BuLi solution of hexane (15 mL, 25.0 mmol) was added to
a THF (40 mL) solution of 8-aminoquinoline (2.0 g, 22 mmol)
at −50 °C, and the mixture was allowed to warm to room temp-
erature overnight. The resulting solution of LiNHAr was
added, the aluminum complexes (1a–1c and 2a–2c) decompose
to corresponding free ligands along with the formation of the
benzyloxy aluminum complexes. This result is well consistent
with the independence of the catalytic behaviour on the ancillary
ligands. When the zinc complexes were reacted with benzyl
cannula-transferred
into
a
solution
of
o-C6H4F
(CHvNC6H3Me2-2,6) (4.0 g, 20.0 mmol) in 45 mL of THF at
room temperature. After stirring for 4 h at 60 °C, the reaction
mixture was quenched with 10 mL of H2O, extracted with
n-hexane, and evaporated to dryness in vacuo and the aimed
1
alcohol, the H NMR spectrum shows the disappearance of the
1
resonances for protons of ZnCH2CH3 in the high-field region
and the appearance of a broad PhCH2OZn signal in the region of
4.0–4.5 ppm, demonstrating the formation of a benzyloxy zinc
complex. The initiation mechanism was elucidated by end-group
analysis of the oligomer of L-lactide, which was synthesized by
the polymerization of the lactide at low monomer-to-initiator
ratio (M/I/B = 25 : 1 : 1) (Fig. 8). End-group analysis shows
that the polymer chains are capped with a benzyl ester group,
indicating that the alkyl zinc complex has been converted to ben-
zyloxy zinc species at the beginning of the polymerization and
the real initiator is the benzyloxy zinc species as shown in
Scheme 3.
product was obtained by chromography in 58% yield. H NMR
(300 MHz, CDCl3, 25 °C): δ 2.32 (s, 6H, CH3), 6.88–6.98 (m,
2H), 7.36–7.52 (m, 5H), 7.09–7.12 (m, 2H), 7.89 (d, 2H,
m-C6H3), 8.11 (dd, JH–H = 8.3, 1.7 Hz, 1H, 3-quinolinyl), 8.41
(s, 1H, HCvN), 8.78 (dd, JH–H = 4.1, 1.7 Hz, 1H, 1-quinolinyl),
12.12 (s, 1H, NH). 13C NMR (75 MHz, CDCl3, 25 °C): δ 18.50
(s, CH3), 113.13 (s, Ar), 114.80 (s, 1C, Ar), 118.47 (s, Ar),
119.26 (s, Ar), 121.01 (s, Ar), 121.45 (s, Ar), 123.77 (s, Ar),
126.56 (s, Ar), 128.03 (s, Ar), 128.06 (s, Ar), 129.03 (s, Ar),
131.57 (s, Ar), 134.69 (s, Ar), 135.76 (s, Ar), 138.92 (s, Ar),
140.67 (s, Ar), 144.12 (s, Ar), 148.22 (s, Ar), 150.73 (s, 1-qui-
nolinyl), 164.89 (s, CHvN) ppm.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 11454–11463 | 11459

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N-(2-((2,6-Diethylphenylimino)methyl)phenyl)quinolin-8-amine
(t, JH–H = 6.0 Hz, 6H, Ar–CH2CH3), 2.60 (q, 4H, JH–H = 6.0 Hz,
Ar–CH2CH3), 6.77 (t, JH–H = 6.0 Hz, 1H, Ar–H), 7.10–7.55 (m,
8H, Ar–H), 7.69–7.72 (m, 2H, Ar–H), 8.16 (s, 1H, CHvN),
8.23 (dd, JH–H = 8.3, 1.5 Hz, 1H, 3-quinolinyl), 8.53 (dd, JH–H
= 4.5, 1.5 Hz, 1H, 1-quinolinyl) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C): δ −7.87 (s, Al–CH3), −3.82 (s, Al–CH3), 15.36
(s, Ar–CH2CH3), 24.88 (s, Al–CH2CH3), 117.18 (s, 7-quinoli-
nyl), 117.24 (s, Ar), 117.74 (s, Ar), 118.02 (s, Ar), 121.05 (s,
3-quinolinyl), 122.08 (s, 5-quinolinyl), 126.16 (s, Ar), 126.32
(s, Ar), 128.58 (s, Ar), 129.17 (s, Ar), 133.58 (s, Ar), 134.90
(s, Ar), 136.80 (s, Ar), 137.49 (s, Ar), 141.60 (s, 4-quinolinyl),
144.67 (s, Ar), 146.64 (s, Ar), 149.26 (s, Ar), 153.38 (s, 1-qui-
nolinyl), 168.52 (s, NvCH) ppm.
n
An BuLi solution of hexane (15 mL, 25.0 mmol) was added to
a THF (40 mL) solution of 8-aminoquinoline (2.0 g, 22.0 mmol)
at −50 °C, and the mixture was allowed to warm to room temp-
erature overnight. The resulting solution of LiNHAr was
cannula-transferred into a solution of o-C6H4F(CHvNC6H3Et2-
2,6) (4.0 g, 20.0 mmol) in 45 mL of THF at room temperature.
After stirring for 4 h at 60 °C, the reaction mixture was quenched
with 10 mL of H2O, extracted with n-hexane, and evaporated to
dryness in vacuo and the target product was obtained by chrom-
1
atography in 62% yield. H NMR (300 MHz, CDCl3, 25 °C):
δ 1.17 (t, JH–H = 9.0 Hz, 6H, CH2CH3), 2.67 (q, JH–H = 9.0 Hz,
4H, CH2CH3), 6.93–7.12 (m, 4H), 7.35–7.51 (m, 5H), 7.88
(d, 2H m-C6H3), 8.10 (dd, JH–H = 8.3, 1.7 Hz, 1H, 3-quinolinyl),
8.41 (s, 1H, HCvN), 8.72 (dd, JH–H = 4.1, 1.7 Hz, 1H, 1-quino-
linyl), 12.07 (s, 1H, NH). 13C NMR (75 MHz, CDCl3, 25 °C):
δ 14.61 (s, CH2CH3), 24.81 (s, CH2CH3), 113.42 (s, Ar), 114.78
(s, Ar), 118.52 (s, Ar), 119.41 (s, Ar), 120.98 (s, Ar), 121.50
(s, Ar), 124.09 (s, Ar), 126.21 (s, Ar), 126.60 (s, Ar), 128.99 (s,
Ar), 131.70 (s, Ar), 133.93 (s, Ar), 134.84 (s, Ar), 135.77 (s,
9-quinolinyl), 139.94 (s, Ar), 140.79 (s, Ar), 148.35 (s, 8-quino-
linyl), 150.28 (s, 1-quinolinyl), 164.75 (s, CHvN) ppm.
Synthesis of complex 1c. Complex 1c was synthesized in the
same manner as 1a with N-(2-((2,6-diisopropylphenylimino)
methyl)phenyl)quinolin-8-amine (0.46 g, 1.14 mmol), and
AlMe3 (2.3 mL, 0.5 M in toluene, 1.15 mmol) as starting
materials or reagents. Pure 1c was obtained by recrystallization
in hexane–toluene as a yellow microcrystalline solid. Yield:
0.32 g (61%). Single crystals for X-ray diffraction analysis were
obtained from hexane at −30 °C. Anal. Calcd for C30H34AlN3
(%): C, 77.72; H, 7.39; N, 9.06. Found: C, 77.52; H, 7.47; N,
1
9.19. H NMR (300 MHz, CDCl3, 25 °C): δ −1.42 (s, 3H, Al–
Synthesis of complex 1a. AlMe3 solution in toluene (2.7 mL,
0.5 M in toluene, 1.35 mmol) was added dropwise to a solution
of N-(2-((2,6-dimethylphenylimino)methyl)phenyl)quinolin-8-
amine (0.47 g, 1.35 mmol) in 20 mL of toluene at 0 °C with stir-
ring. The mixture was allowed to warm to room temperature
gradually and stirred overnight. The solvents were removed
under reduced pressure, and 5 mL hexane was added. The
volume of the solvent was reduced to precipitate the product as a
yellow powder. Recrystallization from hexane–toluene gave 1a
as yellow microcrystalline solid. Yield: 0.25 g (46%). Anal.
Calcd for C26H26AlN3 (%): C, 76.64; H, 6.43; N, 10.31. Found:
C, 76.58; H, 6.48; N, 10.23. 1H NMR (300 MHz, CDCl3,
25 °C): δ −1.45 (s, 3H, Al–CH3), −0.70 (s, 3H, Al–CH3), 2.30
(s, 6H, Ar–CH3), 6.77 (t, JH–H = 6.0 Hz, 1H, p-C6H3), 7.06 (m,
3H, Ar–H), 7.21–7.26 (m, 1H, Ar–H), 7.33–7.37 (m, 2H, Ar–
H), 7.71 (d, JH–H = 9 Hz, 2H, Ar–H), 8.14 (s, 1H, CHvN), 8.24
(dd, JH–H = 8.3, 1.5 Hz, 1H, 3-quinolinyl), 8.53 (dd, JH–H = 4.5,
1.5 Hz, 1H, 1-quinolinyl) ppm. 13C NMR (75 MHz, CDCl3,
25 °C): δ −7.25 (s, Al–CH3), −4.04 (s, Al–CH3), 19.18 (s, Ar–
CH3), 117.14 (s, 7-quinolinyl), 117.26 (s, Ar), 117.70 (s, Ar),
118.04 (s, Ar), 121.21 (s, 3-quinolinyl), 122.11 (s, 5-quinolinyl),
125.73 (s, Ar), 128.30 (s, Ar), 128.57 (s, Ar), 129.19 (s, Ar),
130.95 (s, Ar), 133.61 (s, Ar), 134.97 (s, Ar), 137.51 (s, Ar),
141.61 (s, 4-quinolinyl), 144.66 (s, Ar), 146.62 (s, Ar), 150.36
(s, 8-quinolinyl), 153.36 (s, 1-quinolinyl), 168.48 (s, NvCH)
ppm.
CH3), −0.72 (s, 3H, Al–CH3), 1.13 (d, JH–H = 6.0 Hz, 12H, Ar–
CH(CH3)2), 3.19 (b, 2H, Ar–CH(CH3)2), 6.77 (t, JH–H = 6.0 Hz,
1H, p-C6H3), 7.14–7.55 (m, 8H, Ar–H), 7.67–7.70 (m, 2H, Ar–
H), 8.15 (s, 1H, CHvN), 8.24 (dd, JH–H = 8.3, 1.5 Hz, 1H, 3-
quinolinyl), 8.54 (dd, JH–H = 4.6, 1.5 Hz, 1H, 1-quinolinyl)
ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ −8.93 (s, Al–CH3),
−3.65 (s, Al–CH3), 23.97 (s, Ar–CH(CH3)2), 24.85 (s, Ar–CH
(CH3)2), 28.30 (s, Ar–CH(CH3)2), 15.36 (s, Ar–CH2CH3), 24.88
(s, Al–CH2CH3), 117.17 (s, 7-quinolinyl), 117.28 (s, Ar), 117.85
(s, Ar), 118.00 (s, Ar), 121.01 (s, 3-quinolinyl), 122.09 (s, 5-qui-
nolinyl), 123.60 (s, Ar), 126.48 (s, Ar), 128.63 (s, Ar), 129.23
(s, Ar), 133.59 (s, Ar), 134.94 (s, Ar), 137.55 (s, Ar), 141.58 (s,
4-quinolinyl), 144.76 (s, Ar), 146.70 (s, Ar), 147.96 (s, Ar),
153.49 (s, 1-quinolinyl), 168.27 (s, NvCH) ppm.
Synthesis of complex 2a. Complex 2a was synthesized in the
same manner as 1a with N-(2-((2,6-dimethylphenylimino)-
methyl)phenyl)quinolin-8-amine (0.42 g, 1.21 mmol) and
AlMe3 (4.8 mL, 0.5 M in toluene, 2.42 mmol) as starting
materials or reagents. Recrystallization from hexane gave 2a as a
yellow microcrystalline solid. Yield: 0.32 g (55%). Single cryst-
als for X-ray diffraction analysis were obtained from hexane at
−30 °C. Anal. Calcd for C29H35Al2N3 (%): C, 72.63; H, 7.36;
N, 8.76. Found: C, 72.55; H, 7.28; N, 8.65. 1H NMR (300 MHz,
CDCl3, 25 °C): δ −1.38 (s, 9H, Al(CH3)3), −0.58 (s, 3H,
AlCH3), −0.54 (s, 3H, AlCH3), 2.05 (s, 3H, Ar–CH3), 2.15 (s,
3H, Ar–CH3), 6.22 (d, JH–H = 6.0 Hz, 1H, Ar–H), 6.85–6.88 (m,
1H, Ar–H), 6.97 (m, 1H, 6-quinolinyl), 7.06–7.14 (m, 4H, Ar–
Synthesis of complex 1b. Complex 1b was synthesized in the
same manner as 1a with N-(2-((2,6-diethylphenylimino)methyl)
phenyl)quinolin-8-amine (0.44 g, 1.17 mmol), and AlMe3
(2.3 mL, 0.5 M in toluene, 1.17 mmol) as starting materials or
reagents. Recrystallization from hexane–toluene gave 1b as a
yellow microcrystalline solid. Yield: 0.26 g (52%). Anal. Calcd
for C28H30AlN3 (%): C, 77.21; H, 6.94; N, 9.65. Found: C,
H), 7.27 (d, JH–H = 6.0 Hz, 1H, 7-quinolinyl), 7.35 (t, JH–H
=
6.0 Hz, 1H, 2-quinolinyl), 7.59 (m, 1H, Ar–H), 7.68–7.72 (m,
1H, Ar–H), 8.47 (d, JH–H = 9.0 Hz, 1H, 3-quinolinyl), 8.62 (d,
JH–H = 6.0 Hz, 1H, 1-quinolinyl), 8.94 (s, 1H, CHvN) ppm.
13C NMR (75 MHz, CDCl3, 25 °C): δ −9.55 (s, Al(CH3)3),
−8.59 (s, AlCH3), −8.26 (s, AlCH3), 18.44 (s, Ar–CH3), 18.47
(s, Ar–CH3), 108.36 (s, Ar), 112.13 (s, Ar), 122.21 (s, Ar),
1
77.15; H, 6.87; N, 9.71. H NMR (300 MHz, CDCl3, 25 °C):
δ −1.45 (s, 3H, Al–CH3), −0.70 (s, 3H, Al–CH3), 1.14
11460 | Dalton Trans., 2012, 41, 11454–11463
This journal is © The Royal Society of Chemistry 2012

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125.70 (s, Ar), 126.76 (s, Ar), 128.25 (s, Ar), 128.28 (s, Ar),
128.62 (s, Ar), 128.91 (s, Ar), 129.12 (s, Ar), 129.52 (s, Ar),
129.60 (s, Ar), 129.87 (s, Ar), 130.12 (s, Ar), 131.16 (s, Ar),
136.72 (s, Ar), 141.35 (s, Ar), 143.75 (s, Ar), 151.65 (s, 8-qui-
nolinyl), 152.06 (s, 1-quinolinyl), 167.75 (s, NvCH) ppm.
(s, Ar), 138.17 (s, Ar), 139.72 (s, Ar), 141.35 (s, Ar), 143.75
(s, Ar), 151.97 (s, 8-quinolinyl), 152.42 (s, 1-quinolinyl),
168.55 (s, NvCH) ppm.
Synthesis of complex 3a. ZnEt2 solution in toluene (1.6 mL,
0.5 M in toluene, 0.83 mmol) was added dropwise to a toluene
(20 mL) solution of N-(2-((2,6-dimethylphenylimino)methyl)-
phenyl)quinolin-8-amine (0.25 g, 0.80 mmol) at 0 °C. The
mixture was allowed to warm to room temperature gradually and
stirred overnight. Evaporating the solvents to dryness afforded
the product as a red powder. Recrystallization in hexane gave 3a
as microcrystalline solid. Yield: 0.26 g (75%). Anal. Calcd for
C26H25N3Zn (%): C, 70.19; H, 5.66; N, 9.45. Found: C, 70.23;
Synthesis of complex 2b. Complex 2b was synthesized in the
same manner as 2a with N-(2-((2,6-diethylphenylimino)methyl)-
phenyl)quinolin-8-amine (0.45 g, 1.20 mmol), and AlMe3
(4.8 mL, 0.5 M in toluene, 2.40 mmol) as starting materials or
reagents. Pure 2b was obtained by recrystallization in hexane as
a yellow microcrystalline solid. Yield: 0.36 g (60%). Single crys-
tals for X-ray diffraction analysis were obtained from hexane at
−30 °C. Anal. Calcd for C31H39Al2N3 (%): C, 73.35; H, 7.74;
N, 8.28. Found: C, 73.51; H, 7.64; N, 8.19. 1H NMR (300 MHz,
CDCl3, 25 °C): δ −1.39 (s, 9H, Al(CH3)3), −0.58 (s, 3H,
AlCH3), −0.53 (s, 3H, AlCH3), 1.08–1.15 (m, 6H, Ar–
CH2CH3), 2.25–2.35 (m, 2H, Ar–CH2CH3), 2.55–2.63 (m, 2H,
Ar–CH2CH3), 6.20 (dd, JH–H = 6.0 Hz, 1H, Ar–H), 6.81–6.84
(m, 1H, Ar–H), 6.94 (m, 1H, Ar–H), 7.07–7.09 (m, 1H, Ar–H),
7.19–7.31 (d, JH–H = 6.0 Hz, 4H, Ar–H), 7.36 (t, JH–H = 9.0 Hz,
1H, Ar–H), 7.55–7.60 (m, 1H, Ar–H), 7.68–7.72 (m, 1H, Ar–
H), 8.48 (dd JH–H = 8.3, 1.5 Hz, 1H, 3-quinolinyl), 8.62 (dd,
JH–H = 4.5, 1.5 Hz, 1H, 1-quinolinyl), 8.97 (s, 1H, CHvN)
ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ −9.67 (s, Al
(CH3)3), −8.54 (s, AlCH3), −8.36 (s, AlCH3), 13.89 (s, Ar–
CH2CH3), 14.16 (s, Ar–CH2CH3), 23.75(s, Ar–CH2CH3), 24.08
(s, Ar–CH2CH3), 108.28 (s, 7-quinolinyl), 112.12 (s, Ar),
122.04 (s, Ar), 122.21 (s, Ar), 125.39 (s, Ar), 126.08 (s, Ar),
126.50 (s, Ar), 126.67 (s, Ar), 127.10 (s, Ar), 128.58 (s, Ar),
129.87 (s, Ar), 130.14 (s, Ar), 130.34 (s, Ar), 131.05 (s, Ar),
133.54 (s, Ar), 134.71 (s, Ar), 136.71 (s, Ar), 141.35 (s, Ar),
143.75 (s, Ar), 151.73 (s, 8-quinolinyl), 152.18 (s, 1-quinolinyl),
168.27 (s, NvCH) ppm.
1
H, 5.64; N, 9.51. H NMR (300 MHz, CDCl3, 25 °C): δ 0.07
(q, JH–H = 6.0 Hz, 2H, Zn–CH2CH3), 1.01 (t, JH–H = 6.0 Hz,
3H, Zn–CH2CH3), 1.98 (s, 6H, CH3), 6.91–7.50 (m, 9H, Ar–H),
7.58 (d, JH–H = 9.0 Hz, 1H, Ar–H), 7.87 (d, JH–H = 9.0 Hz, 1H,
Ar–H), 8.05 (s, 1H, CHvN), 8.18 (d, JH–H = 8.1 Hz, 1H, 3-quino-
linyl), 8.45 (dd, JH–H = 4.4, 1.3 Hz, 1H, 1-quinolinyl) ppm. 13C
NMR (75 MHz, CDCl3, 25 °C): δ −0.61 (s, Zn–CH2CH3),
12.96 (s, Zn–CH2CH3), 18.40 (s, Ar–CH3), 105.40 (s, Ar),
110.95 (s, 7-quinolinyl), 119.27 (s, Ar), 121.31 (s, 5-quinolinyl),
124.53 (s, Ar), 125.41 (s, Ar), 126.02 (s, Ar), 127.78 (s, Ar),
128.52 (s, Ar), 129.49 (s, Ar), 129.73 (s, Ar), 130.24 (s, Ar),
130.82 (s, Ar), 132.59 (s, Ar), 136.04 (s, Ar), 138.69 (s, 4-quino-
linyl), 145.07 (s, 8-quinolinyl), 149.56 (s, 1-quinolinyl), 168.92
(s, NvCH) ppm.
Synthesis of complex 3b. Following the same procedure
described for the formation of 3a, treatment of N-(2-((2,6-
diethylphenylimino)methyl)phenyl)quinolin-8-amine (0.45 g,
1.20 mmol) in 20 mL toluene solution with ZnEt2 (2.4 mL, 0.5 M
in toluene, 1.20 mmol) yielded complex 3b as a red-orange
microcrystalline solid after recrystallization with hexane. Yield:
0.46 g (80%). Anal. Calcd for C28H29N3Zn (%): C, 71.11;
1
H, 6.18; N, 8.88. Found: C, 71.19; H, 6.22; N, 8.81. H NMR
Synthesis of complex 2c. Complex 2c was synthesized in the
same manner as 1a with N-(2-((2,6-diisopropylphenylimino)-
methyl)phenyl)quinolin-8-amine (0.49 g, 1.20 mmol), and
AlMe3 (4.8 mL, 0.5 M in toluene, 2.40 mmol) as starting
materials or reagents. Recrystallization from hexane gave pure 2c
as a yellow microcrystalline solid. Yield: 0.41 g (64%). Single
crystals for X-ray diffraction analysis were obtained from hexane
at −30 °C. Anal. Calcd for C33H43Al2N3 (%): C, 73.99; H, 8.09;
N, 7.84. Found: C, 74.15; H, 8.00; N, 7.95. 1H NMR (300 MHz,
CDCl3, 25 °C): δ −1.38 (s, 9H, Al(CH3)3), −0.57 (s, 3H,
AlCH3), −0.53 (s, 3H, AlCH3), 0.86–1.26 (m, 12H, Ar–CH
(CH3)2), 2.76 (m, 1H, Ar–CH(CH3)2), 2.99 (m, 1H, Ar–CH
(CH3)2), 6.00 (d, JH–H = 6.0 Hz, 1H, Ar–H), 6.76–6.78 (m, 1H,
Ar–H), 6.91–6.96 (m, 1H, Ar–H), 7.16–7.37 (m, 6H, Ar–H),
7.52–7.60 (m, 1H, Ar–H), 7.68–7.72 (m, 1H, Ar–H), 8.48 (d,
JH–H = 6.0 Hz, 1H, 3-quinolinyl), 8.62 (d, JH–H = 6.0 Hz, 1H,
1-quinolinyl), 9.00 (s, 1H, CHvN) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C): δ −9.77 (s, Al(CH3)3), −8.39 (s, Al–CH3),
23.75 (s, Ar–CH(CH3)2), 24.25 (s, Ar–CH(CH3)2), 24.33 (s, Ar–
CH(CH3)2), 24.41 (s, Ar–CH(CH3)2), 27.96 (s, Ar–CH(CH3)2),
28.62 (s, Ar–CH(CH3)2), 107.98 (s, Ar), 112.14 (s, Ar), 122.01
(s, Ar), 122.22 (s, Ar), 123.54 (s, Ar), 124.67 (s, Ar), 124.92
(s, Ar), 125.05 (s, Ar), 127.61 (s, Ar), 128.43 (s, Ar), 129.89
(s, Ar), 130.27 (s, Ar), 130.95 (s, Ar), 131.28 (s, Ar), 136.84
(300 MHz, CDCl3, 25 °C): δ 0.07 (q, JH–H = 6.0 Hz, 2H, Zn–
CH2CH3), 0.94 (t, JH–H = 4.0 Hz, 6H, CH2CH3), 1.00 (t, JH–H
=
6.0 Hz, 3H, Zn–CH2CH3), 2.35 (q, JH–H = 4.0 Hz, 4H,
CH2CH3), 6.91–7.50 (m, 9H, Ar–H), 7.58 (d, JH–H = 9.0 Hz,
1H, Ar–H), 7.87 (d, JH–H = 9.0 Hz, 1H, Ar–H), 8.05 (s, 1H,
CHvN), 8.18 (dd, JH–H = 8.3, 1.6 Hz, 1H, JH–H = 4.4, 1.3 Hz,
3-quinolinyl), 8.45 (dd, JH–H = 4.5, 1.5 Hz, 1H, 1-quinolinyl)
ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ −0.66 (s, Zn–
CH2CH3), 12.93 (s, Zn–CH2CH3), 15.10 (s, Ar–CH2CH3),
24.17 (s, Ar–CH2CH3), 105.55 (s, 6-quinolinyl), 110.95 (s, Ar),
119.31 (s, Ar), 121.15 (s, Ar), 124.59 (s, 5-quinolinyl), 125.44
(s, Ar), 125.75 (s, Ar), 126.37 (s, Ar), 129.68 (s, Ar), 129.68
(s, Ar), 130.73 (s, Ar), 132.53 (s, Ar), 135.40 (s, Ar), 135.98
(s, Ar), 138.61 (s, 4-quinolinyl), 145.10 (s, Ar), 148.58 (s, Ar),
148.78 (s, 8-quinolinyl), 154.57 (s, 1-quinolinyl), 168.91
(s, NvCH) ppm.
Synthesis of complex 3c. Following the same procedure
described for the formation of 3a, treatment of N-(2-((2,6-diiso-
propylphenylimino)methyl)phenyl)quinolin-8-amine (0.49 g,
1.20 mmol) in 20 mL toluene solution with ZnEt2 (2.4 mL, 0.5
M in toluene, 1.21 mmol) and subsequent recrystallization with
hexane gave 3c as a red powder. Yield: 0.49 g (82%). Anal.
Calcd for C30H33N3Zn (%): C, 71.92; H, 6.64; N, 8.39. Found:
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C, 71.89; H, 6.59; N, 8.43. 1H NMR (300 MHz, CDCl3, 25 °C):
δ 0.11 (q, JH–H = 6.0 Hz, 2H, Zn–CH2CH3), 0.91 (d, 12H, Ar–
CH(CH3)2), 1.04 (t, JH–H = 6.0 Hz, 3H, Zn–CH2CH3), 2.83 (m,
2H, Ar–CH(CH3)2), 6.90–7.00 (m, 3H, Ar–H), 7.10–7.20 (m,
3H, Ar–H), 7.32–7.40 (m, 2H, Ar–H), 7.45–7.52 (m, 2H, Ar–
H), 7.90 (d, JH–H = 9.0 Hz, 1H, Ar–H), 8.06 (s, 1H, CHvN),
8.18 (d, JH–H = 7.4 Hz, 1H, 3-quinolinyl), 8.41 (d, JH–H = 3.5
Hz, 1H, 1-quinolinyl) ppm. 13C NMR (75 MHz, CDCl3, 25 °C):
δ −1.40 (s, ZnCH2CH3), 13.06 (s, ZnCH2CH3), 24.21 (s, Ar–
CH(CH3)2), 27.96 (s, Ar–CH(CH3)2), 105.50 (s, 7-quinolinyl),
110.74 (s, Ar), 119.56 (s, Ar), 121.08 (s, 3-quinolinyl), 122.78
(s, 5-quinolinyl), 123.66 (s, Ar), 125.04 (s, Ar), 125.73 (s, Ar),
126.05 (s, Ar), 129.75 (s, Ar), 130.78 (s, Ar), 132.47 (s, Ar),
135.82 (s, Ar), 138.65 (s, Ar), 140.34 (s, 4-quinolinyl), 145.24
(s, Ar), 147.05 (s, Ar), 148.95 (s, 8-quinolinyl), 154.52 (s, 1-quino-
linyl), 168.36 (s, NvCH) ppm.
hexane gave 4c as a red powder. Yield: 0.37 g (70%). Anal.
Calcd for C56H56N6Zn (%): C, 76.56; H, 6.43; N, 9.57. Found:
C, 76.49; H, 6.53; N, 9.61. 1H NMR (300 MHz, CDCl3, 25 °C):
δ 0.68 (b, 24H, Ar–CH(CH3)2), 2.71 (b, 4H, Ar–CH(CH3)2),
6.72–7.07 (m, 17H, Ar–H), 7.18–7.25 (m, 5H, Ar–H), 7.53 (b,
2H, Ar–H), 7.96–7.97 (m, 4H, CHvN + Ar–H) ppm. 13C NMR
(75 MHz, CDCl3, 25 °C): δ 23.04 (s, Ar–CH(CH3)2), 27.79 (s,
Ar–CH(CH3)2), 106.82 (s, Ar), 108.95 (s, C6H3), 120.69 (s, Ar),
122.76 (s, Ar), 124.50 (s, C6H3), 130.09 (s, Ar), 130.67 (s, Ar),
132.50 (s, Ar), 138.28 (s, Ar), 138.76 (s, C6H3), 139.50 (s, Ar),
143.74 (s, Ar), 148.87 (b, Ar), 154.14 (s, Ar), 165.38
(b, NvCH) ppm.
Lactide polymerization
In a typical polymerization experiment, an aluminum or zinc
complex (30 μmol), the required amount of L-lactide and benzyl
alcohol in toluene (15 mL) were loaded in a flame-dried vessel
containing a magnetic bar. The vessel was placed in an oil bath
thermostated at 70 °C. After a certain reaction time, the polymer
was isolated by precipitation with cold methanol. The precipitate
was collected and dried under vacuum at 40 °C for 24 h. For
some polymerization reactions, samples were taken for determin-
ing the monomer conversion by 1H NMR during the reaction.
Synthesis of complex 4a. ZnEt2 solution in toluene (1.60 mL,
0.5 M in toluene, 0.80 mmol) was added dropwise to a toluene
(20 mL) solution of N-(2-((2,6-dimethylphenylimino)methyl)-
phenyl)quinolin-8-amine (0.50 g, 1.60 mmol) at 0 °C. The
mixture was allowed to warm to room temperature gradually and
stirred overnight. Evaporating the solvents to dryness and recrys-
tallizing the residue with hexane afforded the product as a red
powder. Yield: 0.51 g (80%). Anal. Calcd for C48H40N6Zn (%):
C, 75.24; H, 5.26; N, 10.97. Found: C, 75.39; H, 5.28; N, 10.85.
1H NMR (300 MHz, CDCl3, 25 °C): δ 1.64 (s, 12H, Ar–CH3),
6.72–7.06 (m, 18H, Ar–H), 7.21–7.27 (m, 2H, Ar–H),
7.41–7.42 (m, 2H, Ar–H), 7.54 (b, 2H, Ar–H), 7.94–7.96 (m,
4H, 2 CHvN + 2 Ar–H). 13C NMR (75 MHz, CDCl3, 25 °C):
δ 18.13 (s, Ar–CH3), 107.23 (s, Ar), 109.28 (s, C6H3), 119.72
(s, C6H3), 120.63 (s, Ar), 123.82 (s, Ar), 124.19 (b, C6H3),
127.74 (s, Ar), 128.52 (s, Ar), 129.91 (s, C6H3), 132.51 (s, Ar),
138.36 (s, Ar), 139.45 (s, Ar), 143.62 (s, Ar), 150.70 (s, Ar),
154.47 (s, Ar), 166.43 (s, NvCH) ppm.
Crystal structure determination
The crystals were mounted on a glass fiber using an oil drop.
Data obtained with the ω–2θ scan mode were collected on a
Bruker SMART 1000 CCD diffractometer with graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å). The structures
were solved by direct methods, and refined with full-matrix
least-squares on F2. All non-hydrogen atoms were refined aniso-
tropically and hydrogen atoms were introduced in calculated pos-
itions with the displacement factors of the host carbon atoms.
All calculations were performed using the SHELXTL crystallo-
graphic software packages.
Synthesis of complex 4b. Following the same procedure
described for the formation of 4a, treatment of N-(2-((2,6-
diethylphenylimino)methyl)phenyl)quinolin-8-amine (0.46 g,
1.20 mmol) in 20 mL toluene solution with ZnEt2 (1.2 mL, 0.5 M
in toluene, 0.60 mmol) and subsequent recrystallization with
hexane gave 4b as a red powder. Yield: 0.39 g (81%). Anal.
Calcd for C52H48N6Zn (%): C, 75.95; H, 5.88; N, 10.22. Found:
C, 75.83; H, 5.81; N, 10.18. 1H NMR (300 MHz, CDCl3,
25 °C): δ 0.68 (b, 12H, Ar–CH2CH3), 2.03(b, 8H, Ar–CH2CH3),
6.70–6.90 (m, 15H, Ar–H), 7.03 (m, 2H, Ar–H), 7.19–7.33 (m,
5H, Ar–H), 7.56 (b, 2H, Ar–H), 7.93–7.96 (4H, CHvN + Ar–
H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ 14.13 (s, Ar–
CH2CH3), 24.34 (s, Ar–CH2CH3), 107.19 (s, Ar), 109.17
(s, C6H3), 119.83 (s, C6H3), 120.65 (s, Ar), 123.82 (s, Ar),
124.17 (s, C6H3), 125.65 (s, Ar), 129.96 (s, Ar), 130.44
(s, C6H3), 131.06 (s, Ar), 132.54 (s, Ar), 134.40 (s, Ar), 138.26
(s, Ar), 143.72 (s, Ar), 150.02 (s, Ar), 154.45 (s, Ar), 166.16
(s, NvCH) ppm.
Acknowledgements
We thank the National Natural Science Foundation of China for
financial support (Nos. 20904013 and 21074043).
Notes and references
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