Preparation and characterization of aluminum alkoxides coordinated on salen-type ligands: Highly stereoselective ring-opening polymerization of rac -lactide

Article abstract of DOI:10.1021/om201281w

A series of salen-type ligands (L¹H₂-L ⁶H₂) with sterically bulky cumyl groups have been synthesized. Reaction of these ligands with AlMe₃ yields the mononuclear aluminum complexes [LAlMe] (1-3) or dinuclear species [L ₂Al₂Me₄] (4-6), respectively. Further reaction of [LAlMe] (1-3) with benzyl alcohol produces [LAl(OBn)] (1a-3a), respectively. Solid-state structural studies reveal that complexes 1a and 2a are mononuclear; however, complex 6 is a dinuclear species. Aluminum alkoxides 1a-3a are highly stereoselective in the ROP of rac-lactide, producing polylactide (PLA) with 94-97% enantiomeric selectivity (P_m) at high conversion. Their high enantioselectivity leads to PLA with high T_m (205 °C). The polymerization of l-lactide by these complexes also shows good living features with narrow PDI values (M_w/M_n = 1.06-1.25) signaling less or no transesterification, which can be further verified by MALDI-TOF mass spectrometric analysis.

Full text of DOI:10.1021/om201281w

Article
pubs.acs.org/Organometallics
Preparation and Characterization of Aluminum Alkoxides
Coordinated on salen-Type Ligands: Highly Stereoselective Ring-
Opening Polymerization of rac-Lactide
Hsiao-Li Chen, Saikat Dutta, Pei-Ying Huang, and Chu-Chieh Lin*
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, Republic of China
S
* Supporting Information
ABSTRACT: A series of salen-type ligands (L1H2−L6H2) with
sterically bulky cumyl groups have been synthesized. Reaction of
these ligands with AlMe3 yields the mononuclear aluminum
complexes [LAlMe] (1−3) or dinuclear species [L2Al2Me4] (4−
6), respectively. Further reaction of [LAlMe] (1−3) with benzyl
alcohol produces [LAl(OBn)] (1a−3a), respectively. Solid-state
structural studies reveal that complexes 1a and 2a are
mononuclear; however, complex 6 is a dinuclear species.
Aluminum alkoxides 1a−3a are highly stereoselective in the
ROP of rac-lactide, producing polylactide (PLA) with 94−97%
enantiomeric selectivity (Pm) at high conversion. Their high
enantioselectivity leads to PLA with high Tm (205 °C). The
polymerization of L-lactide by these complexes also shows good
living features with narrow PDI values (Mw/Mn = 1.06−1.25) signaling less or no transesterification, which can be further verified
by MALDI-TOF mass spectrometric analysis.
alkoxides supported on bisphenolate have been prepared in our
group and these complexes have demonstrated good catalytic
activities toward polymerization of ε-caprolactone and L-
lactide.7 Aluminum salen and half-salen complexes are easily
prepared,8 and most of them have shown highly stereocatalytic
activity toward the ROP of rac-lactide.9 We report herein the
preparation of a series of aluminum salen complexes with cumyl
groups on ortho and para positions and their activities toward
the ROP of lactides.
INTRODUCTION
■
Polylactides (PLAs) have been widely used in many
applications, ranging from packing to biomedical devices, due
to their unique biodegradable, biocompatible, and permeable
properties.1 They can be prepared by direct polycondensation
of lactic acid or by ring-opening polymerization (ROP) of
lactides using a suitable homogeneous catalyst.2 Among the
various preparation methods of PLAs, ring-opening polymer-
ization of lactides (LAs) using metal complexes as catalysts/
initiators has been proven to be the most effective method.
Over the past decades, numerous metal complexes have been
used as catalysts/initiators for ROP of lactides and many of
them have been proven to have high activity, low toxicity, and
low cost and are able to produce high-molecular-weight
polymers with low polydispersity.3 Recently, there has been
greater demand for the control of the stereochemistry of
insertion of the lactide monomer into the PLA chain, since the
mechanical properties of PLAs rely on its microstructure.4
However, catalysts for the ROP of rac-lactide offering good
control of the microstructure of PLA are relatively rare as
compared to those for ROP of L-LA.
RESULTS AND DISCUSSION
■
Syntheses and Characterization. Though many alumi-
num salen complexes have shown great stereoselectivity toward
ring-opening polymerization of rac-lactide, the influence of
substituents and linkers on salen affecting the stereoselectivity
toward the ROP of rac-lactide remains an interesting topic. As a
result, a series of salen-type ligands with sterically bulky cumyl
groups (−CMe2Ph) at the ortho and para positions have been
synthesized in moderate to high yield (Scheme 1). The reaction
of L1H2−L3H2 with 1 mol equiv of AlMe3 yields [LAlMe] (1−
3), respectively. Further reaction of compounds 1−3 with a
stoichiometric amount of benzyl alcohol (BnOH) gives the
corresponding aluminum alkoxide complexes [LAl(OBn)]
(1a−3a). All six complexes have been fully characterized. For
instance, the 1H NMR spectrum of aluminum alkoxide complex
Aluminum complexes constitute the most effective and
versatile synthetic strategy for preparing amorphous to
semicrystalline PLAs with a wide range of physical, mechanical,
and degradation properties.5 Over the past decade, several
aluminum species with N,O-donor ligands have been emerged
as ring-opening polymerization catalysts, among which alkoxido
and amino−alkoxido derivatives have been found to be
particularly efficient.6 Previously, a series of the aluminum
Received: December 27, 2011
Published: February 27, 2012
© 2012 American Chemical Society
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Scheme 1. Preparation of salen-Type Ligands and Their Aluminum Complexes
a
1
Table 1. Comparison of H NMR Data for Ligands L1H2−L3H2, 1−3, and 1a−3a
b
L1H2
L2H2
L3H2
1
2
3
1a
2a
3a
−HCN
8.14 (s)
8.41 (s)
8.23 (s)
4.67 (s)
8.24 (s)
8.22 (s)
3.44(m)
8.06 (s)
8.16 (s)
8.10 (s)
4.79 (d)
4.07 (d)
7.90 (s)
8.21 (s)
7.97 (s)
8.16 (s)
7.96 (s)
CNCH2−
3.26(s)
3.27 (d)
3.12 (d)
3.47 (d)
3.06 (d)
3.97 (br)
4.81 (d)
4.06 (t)
4.19 (d)
3.44 (t)
−OCH2Ph
Al−Me
3.81−3.59(m)
4.33−4.30(m)
−1.42 (s)
−1.61 (s)
a
b
1
All peaks of the ligands and aluminum complexes were characterized by H NMR. Not isolated and determined.
1
1a displays a broad signal at δ 3.97 ppm corresponding to CH2
groups on the benzyl alkoxy group. The imine proton (N
CH) appears at 7.90 ppm, which is about 0.2 ppm upfield from
that of the free salen ligand, L1H2. It is worth noting that peaks
for NCH2C split from a sharp singlet peak in the free ligand 1
to two doublets in complex 1a due to the formation of an
AlNCCCN six-membered ring, resulting in an AB pattern of
NCH2 hydrogens at the backbone. In addition, two CH3 groups
on the amine backbone in L1-H2 also split into two singlet
peaks at δ 0.87 and 0.73, respectively. The phenomenon is the
result of the AlNCCCN six-membered ring causing different
chemical environments between the inner and outer ring. This
different environment can be verified by X-ray crystal structure
studies. Similar results were found for compounds 2a and 3a.
For instance, the methylene peak of alkoxide is a broad signal
from δ 3.81 to 3.59 for complex 2a, and the signals
corresponding to NCH2N protons appear at δ 4.81 and 4.19
ppm, respectively. A comparison of H NMR data for ligands
L1H2−L3H2, 1−3, and 1a−3a are summarized in Table 1.
However, ligands L4H2 and L5H2 react with 2 molar equiv of
AlMe3, yielding the metal complexes [LnAl2Me4] (4 and 5), as
shown in Scheme 1. Unfortunately, further reaction of 4 and 5
with 4 mol equiv of BnOH did not successfully give aluminum
1
alkoxide complexes. The H NMR spectrum of the methyl
group on the metal center displays two singlet peaks at δ −1.32
and −1.33 ppm for 4 and two doublet peaks at −1.05 and
−1.36 ppm for 5. The reaction of L6H2 with trimethylalumi-
num in toluene at 70 °C gives a mixture of the dinuclear species
[L6Al2Me4] (6) and the momonuclear species [L6AlMe] (6′).
After the reaction, complete removal of the solvent under
vacuum results in a sticky yellow residue. Compound 6′ was
precipitated by adding dry n-hexane to the sticky residue and
stirring for 30 min. Interestingly, yellow crystals of compound 6
were obtained from the filtrate of the n-hexane extract after
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storage overnight at room temperature and it was found that 6
was the major product in the mixture of 6′ and 6. It is evident
that the higher solubility of the compound 6 as compared to
that of compound 6′ in n-hexane results in single crystals of 6.
Fresh samples of complex 6′, dissolved in chloroform-d, display
an intense upfield signal at δ −1.40 corresponding to the Al−
1
CH3 moiety, similar to the H NMR spectral pattern of salen
aluminum alkyl complexes.10 Signals corresponding to NCH
protons appear at δ 8.27 and 8.03 ppm, integrated as 1:3 with
the signal corresponding to the Al−CH3 moiety. This confirms
a difference in the chemical environment of the NCH
protons in comparison to that of 6. Similar differences in the
1H NMR pattern are recorded for signals corresponding to the
NCH protons of the cyclohexyl ring, in which case complex 6′
displays two signals, one at δ 3.02 resolving as a multiplet and
the other at δ 2.55 resolving as a doublet, suggesting chemically
nonequivalent protons and confirming a tetradentate donor
arrangement of salen around the central Al. In contrast, protons
of the NCH moiety of the cyclohexyl unit display chemically
equivalent signals at δ 3.45 ppm in the case of complex 6.
Complexes 1a−3a, 4−6, and 6′ have been fully characterized by
1H and 13C NMR and elemental analysis. Fortunately, crystals
of aluminum complexes suitable for X-ray structure determi-
nations could be obtained from a hexane/toluene mixed
solution.
Figure 2. Molecular structure of [L2Al(OBn)] (2a) with thermal
ellipsoids drawn at the 50% probability level. All hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
Al−O(3) = 1.7330(11), Al−O(2) = 1.7863(12), Al−O(1) =
1.8339(12), Al−N(1) = 1.9734(14), Al−N(2) = 2.0606(14); O(3)−
Al−O(2) = 124.35(6), O(3)−Al−O(1) = 100.41(6), O(2)−Al−O(1)
= 89.98(5), O(3)−Al−N(1) = 110.20(6), O(2)−Al−N(1) =
124.41(6), O(1)−Al−N(1) = 90.17(5), O(3)−Al−N(2) = 85.44(5),
O(2)−Al−N(2) = 87.78(5), O(1)−Al−N(2) = 173.99(6), N(1)−Al−
N(2) = 86.50(5).
Crystal Structure Determination. The molecular struc-
tures of 1a, 2a, and 6 were determined by the X-ray diffraction
methods (for crystallographic data see Table S1 in the
Supporting Information) and the structures were solved by
direct methods using the SHELXTL package.11 The molecular
structures of 1a and 2a are depicted in Figures 1 and 2,
values of 1a and 2a are 0.822 and 0.826, respectively, indicating
the geometries around the Al center of both complexes are
distorted trigonal bipyramidal (tbp) rather than square
pyramidal (sqp).
As depicted in Figure 3a, complex 6 shows a dinuclear
feature in the solid state in which two Al atoms are four-
coordinated, bonding to one N, one O, and two C atoms of the
methyl anion groups adopting a tetrahedral geometry around
both Al centers. The O(1) and N(1) atoms and (in another
set) N(2) and O(2) of the salen are bonded to Al(1) and Al(2)
atoms, forming two distorted-boat-shaped rings consisting of
N(1)−C(7)−C(6)-C(1)−O(1)−Al(1) and N(2)−C(14)−
C(15)-C(20)−O(2)−Al(2) units. In compound 6, Al atoms
are located completely away from the ONNO plane of the
salen, which is in stark contrast to the geometry of the Al alkyl
or alkoxy complexes consisting of tetradentate ONNO donor
salen. This unusual formation of a dinuclear geometry is
probably due to the rigid chair conformation of the cyclohexyl
ring and trans arrangement of the NN unit and also subsequent
steric congestion contributed by the phenyl groups of the ortho
and para cumyl groups. It is important to note the N(1)−
C(8)−C(13)−N(2) connectivity, in which N(1) and N(2)
exist in different planes with a the bond angles N(1)−C(8)−
C(13) = 109.08(15)° and C(8)−C(13)−N(2) = 109.04(15)°.
In compound 6, the methyl anions (C(57)−C(60)) bonded to
the Al(1) and Al(2) centers are oriented toward the outer side
of the six-membered rings consisting of Al(1)−O(1)−C(1)−
C(6)−C(7)−N(1) and Al(2)−O(2)−C(20)−C(15)−C(14)−
N(2). These methyl anions are placed in two perpendicular
planes that are also perpendicular to the plane formed by the
slightly distorted six-membered rings. The Al(1) center displays
angles C(58)−Al(1)−N(1) = 114.36(9)° and C(57)−Al(1)−
N(1) = 105.76(9)° and a similarly perpendicular arrangement
of the methyl in the other six-membered rings consisting of
Al(2) center with angles C(60)−Al(2)−N(2) = 106.25(9)° and
C(59)−Al(2)−N(2) = 112.60(8)°, respectively. Figure 3b
displays a chair conformation exhibited by the cyclohexyl unit
Figure 1. Molecular structure of [L1Al(OBn)] (1a) with thermal
ellipsoids drawn at the 50% probability level. All hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
Al(1)−O(3) = 1.7381(11), Al(1)−O(1) = 1.7788(10), Al(1)−O(2) =
1.8289(10), Al(1)−N(1) = 1.9828(12), Al(1)−N(2) = 2.0227(12);
O(3)−Al(1)−O(1) = 122.59(5), O(3)−Al(1)−O(2) = 100.03(5),
O(1)−Al(1)−O(2) = 91.50(5), O(3)−Al(1)−N(1) = 115.78(5),
O(1)−Al(1)−N(1) = 120.49(5), O(2)−Al(1)−N(1) = 89.02(5),
O(3)−Al(1)−N(2) = 86.28(5), O(1)−Al(1)−N(2) = 89.31(5),
O(2)−Al(1)−N(2) = 171.91(5), N(1)−Al(1)−N(2) = 83.64(5).
respectively. The structure shows that complexes 1a and 2a are
both mononuclear with a five-coordinate aluminum center
bonding to a benzyl alkoxide and an N,N,O,O-tetradentate
salen ligand. The two largest angles in each complex are O(2)−
Al(1)−N(2) = 171.91(5)° and O(3)−Al(1)−O(1)° =
122.59(5) in 1a, while the other angles in 2a are O(1)−Al−
N(2) = 173.99(6)° and O(2)−Al−N(1) = 124.41(6)°. The τ
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Figure 3. (a) Molecular structure of [L6Al2Me4] (6) with thermal ellipsoids drawn at the 50% probability level. All hydrogen atoms and solvent
molecules are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Al(1)−O(1) = 1.7786(13), Al(1)−C(57) = 1.948(2), Al(1)−
C(58) = 1.954(2), Al(1)−N(1) = 1.9738(16), Al(2)−O(2) = 1.7706(14), Al(2)−C(60) = 1.953(2), Al(2)−C(59) = 1.957(2), Al(2)−N(2) =
1.9807(16); O(1)−Al(1)−C(57) = 111.96(8), O(1)−Al(1)−C(58) = 110.25(8), C(57)−Al(1)−C(58) = 118.06(10), O(1)−Al(1)−N(1) =
93.71(6), C(57)−Al(1)−N(1) = 105.78(8), C(58)−Al(1)−N(1) = 114.43(8), O(2)−Al(2)−C(60) = 113.41(8), O(2)−Al(2)−C(59) = 105.95(8),
C(60)−Al(2)−C(59) = 121.38(9), O(2)−Al(2)−N(2) = 93.87(6), C(60)−Al(2)−N(2) = 106.29(8), C(59)−Al(2)−N(2) = 112.60(8). (b) Chair
conformation of the cyclohexyl unit and Al(1) and Al(2) in different planes. (c) Two six-membered rings with AlMe2 units in different planes.
a
Table 2. Ring-Opening Polymerization of L-Lactide Initiated by 1a−3a
b
c
d
e
entry
cat.
time (h)
[I]0/[M]0
conversn (%)
Mn(GPC)
Mn(calcd)
PDI
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
2a
2a
2a
2a
3a
12
12
12
12
12
12
12
12
12
1/50
96
96
96
97
92
96
97
98
72
11 600 (6 700)
24 000 (13 900)
37 600 (21 800)
46 200 (26 800)
11 500 (6 700)
18 100 (10 500)
25 200 (14 600)
36 200 (21 000)
7 000
14 000
21 000
28 000
6 700
1.11
1.10
1.10
1.11
1.08
1.08
1.09
1.07
1.06
1/100
1/150
1/200
1/50
1/75
10 400
14 000
21 000
10 500
1/125
1/150
1/100
18 800 (10 900)
a
b
c
Reaction conditions: 70 °C, toluene (10.0 mL), [I] = 5 mM. Obtained from the 1H NMR analysis. Obtained from GPC analysis and calibrated by
polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.13 Calculated from the molecular weight of Mw(LA) ×
[M]0/[BnOH]0 × conversion + Mw(BnOH). Obtained from GPC.
d
e
of the complex 6. The average Al−Me bond length in complex
6 is ∼1.95 Å, which is consistent with the same in reports of
similar aluminum methyl complexes.10,12
Ring-Opening Polymerization of L-Lactide. Ring-open-
ing polymerization of L-lactide using complexes 1a−3a (5.0
mM) as an initiator has been systematically studied at 70 °C
(Table 2). Each of these complexes displayed good activities for
the polymerization of L-lactide with 72−97% conversion within
12 h with good molecular weight control between the molar
masses determined by GPC analysis (Mn(GPC)) and those
calculated (Mn(calcd)), as well as narrow distributions ranging
from 1.06 to 1.10. The linear relationship of 1a and 2a between
the Mn and [M]0/[I]0 ratio and the low polydispersity index
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(PDI) of the polymer simply point to the highly controlled and
“living” character of the polymerization process (Figure 4).
Figure 5. MALDI-TOF-MS spectrum of poly(L-LA) prepared by ROP
of L-LA using 1a as an initiator.
Figure 4. Polymerization of L-LA catalyzed by 1a and 2a in toluene at
70 °C. The relationship between Mn versus PDI of polymer and the
initial mole ratio [LA]0/[I]0 is shown.
associated with the internal transesterification reaction (72.0
Da).
Stereoselective Polymerization of rac-Lactide. It has
been known that the physical and degradation properties of
PLA are dramatically dependent on the stereochemistry of the
polymer chain. Since Spassky et al. illustrated highly stereo-
selective polymerization of rac-lactide initiated by aluminum
Schiff base complexes,15 many salen-based aluminum alkoxides
acting as catalysts with high stereocontrol in the ring-opening
polymerization of rac-lactide have been reported.6g,h,16 To
investigate the influence of the ligand geometry and metal ions
on the stereochemistry of the polymerization process, ROP of
rac-LA was performed using both aluminum alkoxide
complexes 1a−3a and aluminum methyl complexes 4−6 in
the presence of benzyl alcohol (Table 4). Complexes 1a and 2a
exhibit very high isotactic selectivity: Pm = 0.97 in 1a and Pm =
ROP of L-lactide initiated by complexes 4−6 in the presence
of BnOH with a [I]/[M]/[BnOH] ratio of 1/200/4 are
summarized in Table 3. Experimental results indicate that 6 is
an efficient initiator for the polymerization of L-lactide, in which
case >90% conversion was noted within 9 h at 70 °C. Further
investigations of 6 by varying the monomer to initiator ratio
([LA]0/[BnOH]0) from 50 to 200 showed that the livingness
of the polymerization is maintained even at different ratios
(Table 3). However, complexes 4 and 5 have less catalytic
activity, probably due to the sterically bulky substituent on the
diamine retarding the reaction rate.
MALDI-TOF Mass Spectroscopic Studies. MALDI-TOF
(matrix-assisted laser desorption/ionization time-of-flight) mass
spectra have been widely used to determine the size of
repeating units as well as the initiator group and end group
masses.14 In order to understand the structure and composition
of the PLA obtained, a MALDI-TOF spectrum of PLA (entry
1, Table 3) was performed using HABA as a matrix, as shown in
Figure 5. In the results, accurate estimates of Mn by MALDI-
TOF are limited to nearly monodisperse polymer samples and
the mass spectrum shows a cluster of homologous peaks
separated by a molecular mass of ∼144 Da corresponding to
one L-lactide repeating unit, indicating that the intermolecular
transesterification did not happen in polymerization. Further,
no peaks are found in the mass spectra of the polymer which is
1
0.94 in 2a, estimated from a homodecoupled H NMR, with
conversion >90% at 70 °C in toluene. The slight selectivity
difference between 1a and 2a might be due to the flexibility of
the backbone, in which 2,2-dimethyl-1,3-diamine is more
flexible than 2-(aminomethyl)aniline. This hypothesis can be
verified by the X-ray structure results. The N(1)−N(2)
distance is 2.671 Å in 1a and 2.765 Å in 2a, indicating that
harsh and crowded conditions near the active metal center
induce higher selectivity. To produce the greater selectivity,
complex 3a is designed with a ligand bearing a huge steric
a
Table 3. Ring-Opening Polymerization of L-Lactide Initiated by 4−6
b
c
d
e
entry
cat.
time (h)
[I]0/[M]0/[BnOH]0
conversn (%)
Mn(GPC)
Mn(calcd)
PDI
1
2
3
4
5
6
7
8
9
10
4
5
6
4
5
6
6
6
6
6
12
12
8
1/200/4
1/200/4
1/200/4
1/200/4
1/200/4
1/200/4
1/100/2
1/200/2
1/300/2
57
61
99
72
94
99
95
94
91
88
6 400 (3 700)
7 600 (4 200)
4 200
4 400
7 200
5 300
6 800
7 200
7 000
13 600
19 800
25 500
1.08
1.08
1.25
1.08
1.13
1.25
1.21
1.24
1.29
1.26
13 800 (8 000)
7 900 (4 600)
10 700 (6 200)
13 800 (8 000)
12 800 (7 400)
24 100 (14 000)
34 500 (20 100)
24
55
8
8
9
9
10
1/400/2
46 200 (26 800)
a
b
c
Reaction conditions: 70 °C, toluene (10.0 mL), [I] = 5 mM. Obtained from the 1H NMR analysis. Obtained from GPC analysis and calibrated by
polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.13 Calculated from the molecular weight of Mw(LA)x [M]0/
[BnOH]0 x conversion + Mw(BnOH). Obtained from GPC.
d
e
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a
Table 4. Ring-Opening Polymerization of rac-Lactide
b
c
d
e
entry
cat.
time (h)
[I]0/[M]0/ [BnOH]0
conversn (%)
Mn(GPC)
Mn(calcd)
PDI
Pm
Tm (°C)
1
2
3
4
5
6
7
1a
2a
3a
4
12
12
24
39
55
18
18
1/100/0
1/100/0
1/100/0
1/200/4
1/200/4
1/100/2
1/100/2
94
90
57
45
70
93
21 000 (12 200)
20 000 (11 600)
10 900 (6 300)
f
13 500
13 000
6 400
3 240
5 000
6 800
1.07
1.08
1.06
f
0.97
0.94
0.97
f
203
185
205
5
6 300 (3 700)
13 400 (7 800)
10 700 (6 200)
1.10
1.27
1.28
0.33
0.60
0.76
6
6′
73
5 400
150
a
b
c
Reaction conditions: 70 °C, toluene (10.0 mL), [I] = 5 mM. Obtained from the 1H NMR analysis. Obtained from GPC analysis and calibrated by
polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.13 Calculated from the molecular weight of Mw(LA) ×
[M]0/[BnOH]0 × conversion + Mw(BnOH). Obtained from GPC. (The original GPC data have been placed in the Supporting Information in
Table S4.) Not determined.
d
e
f
Figure 6. (a) Linear plots of [LA]0/[LA] versus time, demonstrating the first-order dependence on monomer concentration ([LA]0 = 0.25 M; [1a]0
= 4, 5, 6, 7, 9, 11 mM in d-toluene at 70 °C). (b) Linear plot of ln kp versus ln [1a] for the polymerization of L-lactide with 1a as an initiator (d-
toluene, 70 °C, [LA]0 = 0.25 M).
cumyl substituent and flexible amine backbone to induce a
suitable geometry for rac-lactide polymerization.
commercial homochiral PLLA is 170 °C; however, the melting
point of poly(rac-LA) with high isotacticity (Pm > 0.9) can
increase to 190 °C or even >200 °C (Pm > 0.95), which is a
superior material. As shown in Table 4, the melting point of
poly(rac-LA) obtained from 3a (entry 3, Table 4) with high Pm
> 0.97 is up to 205 °C on the basis of DSC analysis. Thermal
experimental results suggest that the obtained PLAs can be
categorized as the multiblock stereocopolymer (PLLA-
PDLA)n.17
Kinetic Studies of Polymerization of Lactides by 1a. In
order to understand the mechanism for polymerization of L-LA
by 1a−3a, kinetic studies of 1a have been systematically
investigated with various concentrations of 1a (4, 5, 6, 7, 9, and
11 mol L−1, respectively) with respect to a constant
concentration [LA] (0.25 M) at 70 °C. In each case, the plot
of ln ([LA]0/[LA]) versus time (min) is linear, indicating that
polymerization proceeds with the first-order dependence on
monomer concentration (Figure 6a). Hence, the rate of
polymerization may be written as −d[LA]/dt = kobs[LA]x,
where kobs = kp[1a]y, in which kp is the propagation rate
constant. To determine the order in aluminum complex (y), the
linear relationship between ln kpversus ln [1a] (Figure 6b)
reveals that the order in the initiator (slope) is ca. 1.0 (1.099)
and the polymerization rate constant, kp, is 1.192 M−2 min−1,
Although the conversion did not reach 90% during the 12 h,
it shows superior selectivity with Pm = 0.97. Aluminum methyl
complexes 4 and 5 for rac-LA polymerization in the presence of
benzyl alcohol have good molecular weight control but
unfortunately do not have selectivity in the reaction. In
addition, complex 6 exhibits >90% conversion at 70 °C in
toluene with a low isotactic selectivity (Pm = 59−60%);
however, an improvement of the isotactic selectivity (Pm
=
76%) was noted in the polymerization of rac-LA catalyzed by 6′
in the presence of BnOH. The different results of 6 and 6′ in
catalytic systems are consistent with the trend of the highly
isotactic preference displayed by a large number of aluminum
alkoxide and methyl complexes of a wide range of salen ligands.
In general, the mononuclear complexes 1a−3a and 6 have
higher selectivity than dinuclear aluminum complexes 4−6,
probably because the mononuclear complex can provide a
special geometry space for the monomer L-LA and D-LA to
enter and react with the active mental center.
The stereoselective ring-opening polymerization (ROP) of
rac-lactide is an important target of research in academia as well
as in industry because different micro sequences result in
different polymers. For instance, the melting point (Tm) of the
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instrument. GPC measurements were performed on a Jasco PU-2080
PLUS HPLC pump system equipped with a differential Jasco RI-2031
PLUS refractive index detector using THF (HPLC grade) as an eluent
(flow rate 1.0 mL/min, at 40 °C). The chromatographic column was
determined by the y intercept of the regression line. Therefore,
the overall rate equation is −d[LA]/dt = k[LA][1a]. This rate
law is consistent with a mechanism involving coordinative
insertion at a single Al site. The alkoxide group as an initiator
for ROP was analyzed by 1H spectroscopic studies.18 Due to its
high isotactic selectivity, it is expected that the polymerization
rate of rac-lactide will be the same as that for L-lactide. Indeed,
experimental results show that kobs = 0.058 h−1 for rac-lactide is
exactly the same as that for L-lactide of kobs = 0.054 h−1, as
shown in Figure 7.
̂
JORDI Gel DVB 103 Å, and the calibration curve was made by
primary polystyrene standards to calculate Mn(GPC). Differential
scanning calorimetry (DSC) was carried out with a Perkin-Elmer
STA6000 instrument. The sample was heated from 30 to 300 °C at a
rate of 20 °C/min. Matrix-assisted laser desorption ionization-time-of-
flight mass spectrometry (MALDI-TOF MS) analyses were carried out
with a Bruker Autoflex III TOF/TOF equipped with an MCP
detector. The sample was dissolved in THF, and the matrix was 2,5-
dihydroxybenzoic acid (HABA). Ions formed by a pulsed UV laser
beam with 3 ns bandwidth (nitrogen laser λ was 337 nm) were
accelerated through 20 kV, and the detection voltage was set at 1.7 kV.
General Procedures for Ligands L1-H2−L6-H2. Ligands were
prepared by the reaction of 3,5-bis(α,α-dimethylbenzyl)-2-hydrox-
ybenzaldehyde (7.16 g, 20 mmol) with the corresponding diamine
(10.5 mmol) in refluxing ethanol (50.0 mL) for 12 h, and the reaction
mixture was then cooled to room temperature, giving a yellow
precipitate. The yellow powder was collected by filtration and dried
under vacuum.
N,N′-3,5-Bis(α,α-dimethylbenzyl)-2-hydroxysalicylidene-2,2-di-
1
methyl-1,3-diamine (L1-H2). Yield: 6.2 g (75%). H NMR (CD3Cl,
400 MHz, ppm): δ 13.35 (2H, s, OH), 8.14 (2H, s, NCH), 7.32−
7.01 (24H, m, ArH), 3.26 (4H, s, CH2C(CH3)2CH2), 1.71 (12H, s,
C(CH3)), 1.68 (12H, s, C(CH3)), 0.89 (3H, s, CH2C(CH3)2CH2),
0.88 (3H, s, CH2C(CH3)2CH2). 13C NMR (CDCl3, 100 MHz, ppm):
δ 166.2 (ArCN), 157.8 (ArCOH), 150.7 (ArCC(CH3)2), 150.5
(ArCC(CH3)2), 139.4 (ArC cumyl), 136.0 (ArC cumyl), 129.1, 127.9,
127.7, 126.7, 125.7, 125.6, 124.9 (ArC), 117.9 (ArCCN), 68.3
(NCH2CMe2), 42.4 (C(CH3)2), 42.2 (C(CH3)2), 36.2
(CH2CMe2CH2), 30.9 (C(CH3)2), 29.3 (C(CH3)2), 24.3 (CH2C-
(CH3)CH2). Anal. Calcd (found) for C55H62N2O2: N, 3.58 (3.53); C,
84.36 (84.11); H, 7.98 (7.79).
Figure 7. Comparison of kinetic data for polymerizations of rac-LA
and L-LA by complex 1a. Conditions: [LA]0 = 0.25 M; [1a]0 = 7 mM
in d-toluene at 70 °C.
N,N′-3,5-Bis(α,α-dimethylbenzyl)-2-hydroxysalicylidene-2-
CONCLUSIONS
1
(aminomethyl)aniline (L2-H2). Yield: 6.7 g (80%). H NMR (CD3Cl,
■
A series of aluminum alkyl and aluminum alkoxide complexes
have been synthesized and fully characterized. Complexes 1a−
3a show efficient activity for ROP of L-lactide. In addition,
complexes 1a−3a display high stereoselectivity toward rac-
lactide with Pm values up to 97%, where the highest melting
point could be 205 °C. At the same time, these aluminum
complexes can avoid transesterfication effectively and in the
MALDI-TOF spectrum show separate homologous peaks
(144.0 Da). The kinetic of the polymerization of L-lactide,
catalyzed by 1a, was investigated using in situ 1H NMR
spectroscopy and revealed a first-order dependence for both the
L-lactide and 1a.
400 MHz, ppm): δ 13.33 (1H, s, OH), 13.11 (1H, s, OH), 8.41 (1H, s,
NCH), 8.23 (1H, s, NCH), 7.38−6.96 (28H, m, ArH), 4.67 (2H,
s, CNCH2Ar), 1.70 (6H, s, CH3), 1.68 (6H, s, CH3), 1.67 (6H, s,
CH3), 1.62 (6H, s, CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ 166.7
(ArCN), 164.2 (ArCN), 157.9 (ArCOH), 157.8 (ArCOH), 151.8,
150.9, 150.7, 150.6, 150.4 (ArCC(CH3)2), 147.2 (ArCNC), 140.4,
139.5 (ArCC(CH3)2), 136.6, 136.1 (ArCC(CH3)2), 132.0
(ArCCH2N), 130.4, 130.4, 129.2, 128.8, 128.5, 128.2, 128.1, 127.9,
126.8, 125.8, 125.7, 125.7, 125.24, 125.1 (ArC), 118.6 (ArCCN),
118.5 (ArC), 118.2 (ArCCN), 58.9 (ArCH2NC), 42.4, 42.3, 42.1,
42.0 (C(CH3)2), 30.9, 30.8, 29.3, 29.2 (C(CH3)2). Anal. Calcd
(found) for C57H58N2O2: N, 3.49 (3.67); C, 85.25 (85.03); H, 7.28
(7.43).
N,N′-3,5-Bis(α,α-dimethylbenzyl)-2-hydroxysalicylidene-1,3-pen-
tanediamine (L3-H2). Yield: 6.4 g (78%). 1H NMR (CD3Cl, 400 MHz,
ppm): δ 13.61 (1H, s, OH), 13.45 (1H, s, OH), 8.24 (1H, s, NCH),
8.22 (1H, s, NCH), 7.33−7.21 (22H, m, ArH), 7.15 (1H, d, J = 2.0
Hz, ArH), 7.09 (1H, d, J = 2.4 Hz, ArH), 3.44 (2H, m, CNCH2),
3.06−2.98(1H, m, CH2CH2CH(C2H5)), 1.95 (2H, m, CH2CH2CH-
(C2H5)), 1.84−1.98 (24H, m, C(CH3)2), 1.64 (2H, m, CHCH2CH3),
0.87 (3H, t, CHCH2CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ
166.6, 164.6 (ArCN), 157.8 (ArCOH), 150.7, 150.7, 150.6, 150.6
(ArCC(CH3)2), 139.4, 139.3 (ArC cumyl), 136.1, 135.9 (ArC cumyl),
129.1, 128.8, 127.9, 127.8, 127.7, 127.6, 127.6, 126.7, 125.6, 124.9 (Ar,
ArCHN), 117.9, 117.7 (ArCCN), 69.1 (NCH(C2H5)), 56.1
(NCH2CH2CH(C2H5)), 42.4, 42.2, 42.09 (C(CH3)2), 36.7
(NCH2CH2CH(C2H5)), 30.9, 30.9 (C(CH3)2), 29.5, 29.4, 29.4
(C(CH3)2), 29.2, 29.1 (NCH2CH2CH(CH2CH3)), 10.7 (NCH-
(CH2CH3)). Anal. Calcd (found) for C55H62N2O2: N, 3.58 (3.58);
C, 84.36 (84.53); H, 7.98 (7.95) .
EXPERIMENTAL SECTION
■
General Methods and Materials. All manipulations were carried
out under a dry nitrogen atmosphere. Solvents were dried by refluxing
at least 24 h over sodium/benzophenone (n-hexane, toluene, THF),
phosphorus pentoxide (CH2Cl2), or anhydrous magnesium sulfate
(benzyl alcohol). L-Lactide was purchased from the Bio Invigor Corp.
and recrystallized from a toluene solution prior to use. Deuterated
solvents (Aldrich) were dried over molecular sieves. Other reagents
were purchased from Aldrich or Acros and used without further
purification. 3,5-Bis(α,α-dimethylbenzyl)-2-hydroxybenzaldehyde19
and trans-11,12-diamino-9,10-dihydro-9,10-ethanoanthracene20 were
prepared according to the literature methods.
1
Measurements. H and 13C NMR spectra were recorded on a
1
Varian Mercury 400 (400 MHz for H and 100 MHz for 13C) or 600
1
(600 MHz for H and 150 MHz for 13C) spectrometer with chemical
shifts given in parts per million (ppm) from internal TMS.
Microanalyses were performed using a Heraeus CHN-O-RAPID
N,N′-3,5-Bis(α,α-dimethylbenzyl)-2-hydroxysalicylidene-3-
1
(aminomethyl)benzylamine (L4-H2). Yield: 5.6 g (65%). H NMR
instrument. Melting points were determined using a Buchi 535m
(CD3Cl, 400 MHz, ppm): δ 13.33 (2H, s, OH), 8.29 (2H, s, NCH),
̈
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7.35−7.03 (27H, m, ArH), 4.60 (4H, s, NCH2Ar), 1.71 (12H, s,
C(CH3)2), 1.65 (12H, s, C(CH3)2). 13C NMR (CDCl3, 100 MHz,
ppm): δ 166.1 (ArCN), 157.6 (ArCOH), 150.7, 150.5 (ArCC-
(CH3)2), 139.5 (ArC cumyl), 138.3 (ArCCH2NC), 136.1 (ArC
cumyl), 129.0, 128.9, 128.7, 128.0, 127.9, 127.7, 127.6, 126.9, 126.7,
125.7, 125.6, 125.5, 125.0 (Ar, ArCHN), 118.0 (ArCCN), 62.8
(NCH2Ar), 42.3 (C(CH3)2), 42.1 (C(CH3)2), 30.9 ((CH3)2), 29.3
(C(CH3)2). Anal. Calcd (found) for C58H60N2O2: N, 3.43 (3.47); C,
85.25 (84.84); H, 7.40 (7.37).
(Al−CH3). Anal. Calcd (found) for C58H59AlN2O2: N, 3.32 (3.42); C,
82.63 (81.93); H, 7.05 (7.58).
(L1-AlOBn) (1a). A solution of AlMe3 (1.05 mL, 2.1 mmol) in
hexane was added slowly to a solution with vigorously stirred ice-cold
toluene (30.0 mL) containing L1-H2 (1.56 g, 2.0 mmol). The mixture
was then refluxed overnight and cooled to room temperature. BnOH
(0.21 mL, 2 mmol) was then added, and the mixture was refluxed for
12 h, during which time the solution color changed from yellow to
pale green. The solution was cooled to 25 °C and the supernatant
removed by filtration. The resulting solid was dried under vacuum,
N,N′-3,5-Bis(α,α-dimethylbenzyl)-2-hydroxysalicylidene-trans-
11,12-diamino-9,10-dihydro-9,10-ethanoanthracene (L5-H2). Yield:
7.2 g (75%). 1H NMR (CD3Cl, 600 MHz, ppm): δ 12.30 (2H, s, OH),
8.15 (2H, s, NCH), 7.31−7.06 (30H, m, ArH), 6.94 (2H, m, ArH),
4.10 (2H, s, ArHCNCHCHAr), 3.45 (2H, s, ArHCNCH), 1.68−
1.67 (18H, m, C(CH3)2), 1.60 (6H, m, C(CH3)2). 13C NMR (CDCl3,
150 MHz, ppm): δ 165.4 (ArCN), 157.4 (ArC−OH), 150.9
(ArCC(CH3)2), 150.3 (ArCC(CH3)2), 140.6 (ArC cumyl), 139.6
(quaternary carbon atom of backbone), 139.5 (quaternary carbon
atom of backbone), 136.1 (ArC- cumyl), 129.3,128.0, 127.9, 127.6,
126.6, 126.5, 126.2, 125.8, 125.6, 125.6, 124.9, 123.8 (ArC), 117.7
(ArCCN), 76.3 (ArCHCHNC), 51.9 (ArCHCHNC), 42.3,
42.1 (C(CH3)2), 30.8, 29.8, 28.7 (C(CH3)2). Anal. Calcd (found) for
C66H64N2O2: N, 3.05 (3.08); C, 86.42 (86.34); H, 7.03 (6.91).
N,N′-3,5-Bis(α,α-dimethylbenzyl)-2-hydroxysalicylidene-
( )-trans-1,2-cyclohexanediamine (L6-H2). Yield: 5.3 g (63%). 1H
NMR (CD3Cl, 400 MHz, ppm): δ 13.23 (2H, s, OH), 8.07 (2H, s,
HCN), 7.09−7.27 (20H, m, ArH), 6.91 (4H, s, ArH), 3.12 (2H, d,
CH), 1.73 (4H, m, CH), 1.65 (18H, s, C(CH3)2), 1.53 (6H, s,
C(CH3)2), 1.51 (m, 2H, CH), 1.26 (m, 2H, CH). 13C NMR (CDCl3,
100 MHz, ppm): δ 165.5 (ArCN), 157.9 (ArCOH), 150.9
(ArCC(CH3)2), 139.6 (ArC cumyl), 135.6, 129.5, 128.2, 127.9,
126.9, 125.8, 125.8, 125.2 (ArC), 118.2(ArCCN), 72.4 (CyC),
42.6 (C(CH3)2Ph), 31.2 (CyC), 30.5 (CyC), 28.9 (CyC), 24.5 (CyC).
Anal. Calcd (found) for C52H62N2O2: C, 84.59 (84.66); H, 7.86
(8.09); N, 3.52 (3.18).
1
yielding a pale green powder. Yield: 1.55 g (85%). H NMR (CD3Cl,
600 MHz, ppm): δ 7.90 (2H, s, NCHAr), 7.26−6.83 (29H, m,
ArH), 3.97 (2H, br, OCH2Ph), 3.47 (2H, d, J = 11.4 Hz,
NCH2C(CH3)2), 3.06 (2H, d, J = 12.0 Hz, NCH2C(CH3)2), 1.94
(6H, s, C(CH3)2), 1.71 (6H, s, C(CH3)2), 1.56 (12H, d, J = 3.6 Hz,
C(CH3)2), 0.87 (3H, s, CH2C(CH3)2CH2), 0.73 (3H, s, CH2C-
(CH3)2CH2). 13C NMR (CDCl3, 100 MHz, ppm): δ 169.6 (ArCN),
163.1 (ArCOH), 150.6 (ArCC(CH3)2), 150.5 (ArCC(CH3)2), 145.7
(Al−OCH2CAr), 139.9 (ArC cumyl), 137.3 (ArC cumyl), 134.5,
129.0, 127.9, 127.7, 127.4, 126.8, 126.7, 125.6, 125.4, 125.1 (ArC),
118.5 (ArCCN), 68.4 (NCH2CMe2), 65.2 (OCH2Ph), 43.3, 42.2
(C(CH3)2), 35.7 (CH2CMe2CH2), 30.6 (C(CH3)2), 30.5 (C(CH3)2),
29.8 (C(CH3)2), 28.0 (C(CH3)2), 25.4 (CH2C(CH3)CH2), 25.2
(CH2C(CH3)CH2). Anal. Calcd (found) for C62H67AlN2O3: N, 3.03
(3.09); C, 81.37 (80.73); H, 7.38 (6.99).
(L2-AlOBn) (2a). The synthetic procedures for complex 2a were the
same as those for complex 1a. Yield: 1.53 g (82%). 1H NMR (CD3Cl,
400 MHz, ppm): δ 8.21 (1H, s, NCHAr), 8.16 (1H, s, NCHAr),
7.52−6.77 (31H, m, ArH), 6.37 (2H, m, ArH), 4.81 (1H, d, J = 13.2
Hz, CNCH2Ar), 4.19 (1H, d, J = 12.8 Hz, CNCH2Ar), 3.81−3.59
(2H, m, OCH2Ph), 2.03 (3H, s, C(CH3)2), 1.94 (6H, s, C(CH3)2),
1.67 (3H, s, C(CH3)2), 1.58 (6H, s, C(CH3)2), 1.53 (6H, s, C(CH3)2).
13C NMR (CDCl3, 150 MHz, ppm): δ 172.3, 168.1 (ArCN), 164.5,
162.5 (ArCOH), 150.5, 150.3, 150.2 (ArCC(CH3)2), 149.1 (ArCN),
145.7 (Al−OCH2CAr), 140.5, 139.4 (ArC cumyl), 138.4, 136.9 (ArC
cumyl), 135.4, 135.3 (ArC), 130.4 (ArCCH2N), 130.0, 129.7, 129.0,
128.2, 127.9, 127.8, 127.6, 127.0, 126.9, 126.8, 126.7, 126.7, 126.6,
126.0, 125.5, 125.4, 125.3, 125.1, 124.8, 121.6 (ArC), 119.1, 117.9
(ArCCN), 64.5 (OCH2Ph), 63.5 (ArCH2NC), 43.6, 43.0, 42.0
(C(CH3)2), 31.6, 30.6, 30.5, 30.1, 28.8, 28.1, 27.6 (C(CH3)2). Anal.
Calcd (found) for C64H63AlN2O3: N, 3.00 (3.06); C, 82.20 (83.49); H,
6.79 (7.12).
(L1-AlMe) (1). A solution of AlMe3 (1.05 mL, 2.1 mmol) in hexane
was added slowly to a solution with vigorously stirred ice-cold toluene
(30.0 mL) containing L1-H2 (1.56 g, 2.0 mmol), and the mixture was
then refluxed for 12 h. The solution was cooled to 25 °C, and the
supernatant was removed by filtration. The resulting solid was dried
under vacuum, yielding a light yellow powder. Yield: 1.32 g (80%). 1H
NMR (CD3Cl, 400 MHz, ppm): δ 8.06 (2H, s, NCHAr), 7.40−7.19
(20H, m, ArH), 7.09 (2H, d, J = 2.8 Hz, ArH), 7.01 (2H, d, J = 2.8 Hz,
ArH), 3.27 (2H, d, J = 11.6 Hz, NCH2C(CH3)2), 3.12 (2H, d, J = 12.0
Hz, NCH2C(CH3)2), 2.00 (6H, s, C(CH3)2), 1.85 (6H, s, C(CH3)2),
1.70 (12H, s, C(CH3)2), 1.16 (3H, s, CH2C(CH3)2CH2), 1.04 (3H, s,
CH2C(CH3)2CH2), −1.42 (3H, s, AlCH3). 13C NMR (CDCl3, 100
MHz, ppm): δ 169.2 (ArCN), 163.0 (ArC−OH), 150.8, 150.6
(ArCC(CH3)2), 140.5 (ArC cumyl), 136.7 (ArC cumyl), 134.1, 128.8,
127.9, 127.5, 126.7, 126.6, 125.4, 124.9 (ArC), 118.4 (ArCCN), 67.4
(NCH2CMe2), 43.2, 42.0 (C(CH3)2), 36.2 (CH2CMe2CH2), 31.6
(C(CH3)2), 29.4 (C(CH3)2), 28.0 (C(CH3)2), 25.9, 25.5 (CH2C-
(CH3)CH2), −8.8 (Al−CH3). Anal. Calcd (found) for C56H63AlN2O2:
N, 7.71 (7.77); C, 81.72 (79.39); H, 7.71 (7.77).
(L3-AlOBn) (3a). The synthetic procedure was similar to that for
complex 1a. Yield: 1.73 g (75%). 1H NMR (CD3Cl, 400 MHz, ppm):
δ 7.97 (1H, s, NCHAr), 7.96 (1H, s, NCHAr), 7.26−6.81 (29H,
m, ArH), 4.33−4.30 (2H, m, OCH2Ph), 4.06 (1H, t, CNCH2), 3.44
(1H, t, CNCH2), 3.26−2.98 (1H, m, CH2CH2CH(C2H5)), 1.85−
1.44 (26H, m, CH2CH2CH(C2H5), C(CH3)2, (2H, m, CHCH2CH3)),
0.62 (3H, t, CHCH2CH3). 13C NMR (CDCl3, 150 MHz, ppm): δ
168.9, 167.6 (ArCN), 163.5, 163.1 (ArCOH), 151.1, 150.9, 150.6,
150.5 (ArCC(CH3)2), 146.2 (Al−OCH2CAr), 139.9, 139.8 (ArC
cumyl), 137.2, 137.0 (ArC cumyl), 134.8, 133.5, 129.0, 128.9, 128.5,
128.2, 127.9, 127.8, 127.7, 127.6, 127.5, 127.4, 126.8, 126.6, 126.5,
126.5, 126.2, 125.4, 125.3, 125.1, 124.8 (ArC), 118.4, 118.3 (ArCC
N), 72.3 (NCH(C2H5)), 64.9 (OCH2Ph), 55.9 (NCH2CH2CH-
(C2H5)), 43.4, 43.0 (C(CH3)2), 42.0, 41.9(C(CH3)2), 33.4
(NCH2CH2CH(C2H5)), 30.9, 30.9 (C(CH3)2), 29.5, 29.5, 29.4
(C(CH3)2), 29.2, 29.1 (NCH2CH2CH(CH2CH3)), 10.83 (NCH-
(CH2CH3)). Anal. Calcd (found) for C62H67AlN2O3:N, 3.06 (2.89);
C, 81.37 (81.54); H, 7.38 (7.68).
(L2-AlMe) (2). The synthetic method of complex 2 is the same as
1
for complex 1. Yield: 1.43 g (85%). H NMR (CD3Cl, 600 MHz,
ppm): δ 8.16 (1H, s, NCHAr), 8.10 (1H, s, NCHAr), 7.38−7.35
(1H, m, ArH), 7.29−7.17 (20H, m, ArH), 7.11−7.05 (4H, m, ArH),
7.00 (2H, br, ArH), 6.91 (1H, br, ArH), 4.79 (1H, d, J = 12.6.Hz,
ArCH2N), 4.07 (1H, d, J = 13.2 Hz, ArCH2N), 2.04 (6H, s, C(CH3)2),
2.04 (6H, s, C(CH3)2), 1.95 (4H, s, C(CH3)2), 1.74 (8H, s, C(CH3)2),
1.69 (6H, s, C(CH3)2), −1.61 (3H, s, AlCH3). 13C NMR (CDCl3, 150
MHz, ppm): δ 172.3, 166.2 (ArCN), 164.6, 162.1 (ArCOH), 150.8,
150.6, 150.5, 150.3 (ArCC(CH3)2), 148.5 (ArCN), 141.0, 140.0 (ArC
cumyl), 137.9, 136.3 (ArC cumyl), 134.8, 134.7 (ArC), 130.6
(ArCCH2N), 129.9, 129.7, 129.4, 129.0, 128.2, 127.9, 127.6, 127.4,
127.1, 126.8, 126.7, 126.6, 126.3, 125.5, 125.3 124.8, 121.0 (ArC),
119.3, 117.8 (ArCCN), 63.4 (ArCH2NC), 43.4, 42.8, 42.1, 42.0
(C(CH3)2), 31.4, 30.6, 30.6, 30.5, 29.3, 27.5, 27.3 (C(CH3)2), −7.8
(L4-Al2Me4) (4). A solution of AlMe3 in toluene (2.05 mL, 4.10
mmol) was added slowly to a yellow solution of L4-H2 (1.63 g, 2
mmol) in toluene (20.0 mL) at room temperature under N2, and the
mixture was then refluxed for 12 h. The volatile materials were
removed under vacuum, and the resulting sticky yellow residue was
dried completely under vacuum. Yield: 1.73 g (93%). 1H NMR
(CD3Cl, 400 MHz, ppm): δ 8.01 (2H, s, NCH), 7.43−6.95 (27H,
m, ArH), 4.55 (4H, s, NCH2Ar), 1.69 (12H, s, C(CH3)2), 1.57 (12H,
s, C(CH3)2), −1.32, −1.33 (12H, s, Al(CH3)2). 13C NMR (CDCl3,
100 MHz, ppm): δ 171.3 (ArCN), 161.0 (ArCOH), 150.6, 150.4
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(ArC-C(CH3)2), 140.6 138.3 (ArCCH2NC), 135.6 (ArC cumyl),
133.5, 130.1, 129.7, 129.4, 128.1, 127.5, 126.7, 125.7, 125.5, 124.7,
118.0 (ArC), 117.9 (ArCCN), 60.1 (NCH2Ar), 42.7, 42.3, 42.2, 42.0
(C(CH3)2), 30.8, 28.8 (C(CH3)2), −10.5 (Al-CH3). Anal. Calcd
(found) for C62H70Al2N2O2: N, 3.01 (2.98); C, 80.14 (80.15); H, 7.59
(7.29).
the isolated compound were compared with those of 6, which gave a
complete match.
Typical Polymerization Procedures of L-Lactide for 1a−3a. A
mixture of 1a (0.05 mmol) and L-lactide (0.720 g, 5.0 mmol) in
toluene (10.0 mL) was stirred at 70 °C for 12 h. The reaction was
quenched by the addition of water (0.5 mL), and n-hexane (100.0 mL)
was added to give a white solid.
Typical Polymerization Procedures of L-Lactide for 4−6. L-
Lactide (0.720 g, 5.0 mmol) was added to a mixture of the aluminum
methyl complex 4 (0.05 mmol) and BnOH (0.20 mmol) in toluene
(10.0 mL), and the resulting solution was stirred at 70 °C for 12 h.
The reaction was quenched by the addition of water (0.5 mL), and n-
hexane (100.0 mL) was added to give a white solid.
Typical Polymerization Procedures of rac-Lactide for 1a−3a.
A mixture of 1a (0.05 mmol) and rac-lactide (0.72 g, 5.0 mmol) in
toluene (10 mL) was stirred at 70 °C for 12 h, and the reaction was
then quenched by the addition of water (1.0 mL). On addition of n-
hexane (90.0 mL), the polymer precipitated as a white crystalline solid.
The solid was then filtered, washed with cold n-hexane (10.0 mL)
twice, and dried under vacuum.
Typical Polymerization Procedures of rac-Lactide for 4−6
and 6′. A mixture of complex 4 (0.05 mmol), BnOH (0.20 mmol),
and rac-lactide (0.72 g, 5.0 mmol) in toluene (10.0 mL) was stirred at
70 °C for 18 h. The reaction was then quenched by the addition of
water (0.5 mL) and n-hexane (100.0 mL), and the polymer was
precipitated to give a white crystalline solid.
X-ray Crystallographic Studies. Suitable crystals of 1a, 2a, and 6
were covered with perfluoropolyether vacuum oil (Aldrich, Fombliny)
held on a glass fiber and mounted on CryoLoop (Hampton Research)
and cooled rapidly under a stream of cold nitrogen gas using an
Oxford Diffraction Ltd. Gemint R and S. The space group
determination was based on a check of the Laue symmetry and
systematic absences and was confirmed using the structure solution.
The structure was solved by direct methods using the SHELXTL
package. All non-H atoms were located from successive Fourier maps,
and hydrogen atoms were refined using a riding model. Anisotropic
thermal parameters were used for all non-H atoms, and fixed isotropic
parameters were used for H atoms. Crystallographic data for
complexes 1a, 2a, and 6 are given in in the Supporting Information.
(L5-Al2Me4) (5). Methods for complex 5 were the same as those for
1
4. Yield: 1.65 g (90%). H NMR (CD3Cl, 600 MHz, ppm): δ 7.44−
7.06 (28H, m, ArH), 6.27 (2H, s, NCH), 6.21 (2H, s, ArH) 4.35
(2H, s, ArHCNCH), 3.85 (2H, s, ArHCNCHCHAr), 1.66 (6H,
s, C(CH3)2), 1.65 (6H, s, C(CH3)2), 1.59 (12H, s, C(CH3)2), −1.05
(6H, d, J = 1.2 Hz, Al(CH3)2), −1.36 (6H, d, J = 1.2 Hz, Al(CH3)2).
13C NMR (CDCl3, 100 MHz, ppm): δ 168.4 (ArCN), 161.1
(ArCOH), 151.6 (ArCC(CH3)2), 150.2 (ArCC(CH3)2), 140.4 (ArC
cumyl), 138.1 (quaternary carbon atom of backbone), 137.0
(quaternary carbon atom of backbone), 133.9, 130.5, 128.0, 127.7,
127.5, 127.4, 126.7, 125.7, 125.5, 124.7, 124.6 (ArC), 117.7 (ArCC
N), 65.4 (ArCHCHNC), 51.1 (ArCHCHNC), 42.2, 42.0
(C(CH3)2), 30.8, 30.5, 29.1, 28.5 (C(CH3)2), −10.1, −11.0 (Al−
CH3). Anal. Calcd (found) for C70H74Al2N2O2: C, 81.68 (80.21); H,
7.25 (7.95); N, 2.72 (2.95).
Preparation of [L6Al2(Me)4] (6) and [L6AlMe] (6′). A solution of
trimethylaluminum in toluene (2.25 mL, 4.50 mmol) was added to a
yellow solution of the ligand L6-H2 (1.19 g, 1.50 mmol) in toluene
(20.0 mL) at room temperature under N2, and the mixture was stirred
for 4 h. Then the mixture was stirred at 70 °C for 12 h. The solvent
and excess of AlMe3 were removed under vacuum, and the resulting
sticky yellow residue was completely dried. To this was added dry n-
hexane (20.0 mL), and the mixture was stirred for 30 min. The
resulting light yellow precipitate of [L6AlMe] was collected by vacuum
filtration. The solid was further washed with dry n-hexane (10.0 mL ×
2) and dried under vacuum. Yield: 0.37 g (30%). The filtrate of the n-
hexane extract was stored for 2 days at room temperature, which
resulted in yellow single crystals and crystalline solids of [L6Al2Me4]
which crashed out of the solution. Solids were collected by vacuum
filtration and dried under vacuum. Yield: 0.41 g (46%).
1
[L6Al2Me4] (6). H NMR (CD3Cl, 400 MHz, ppm): δ 7.77 (2H, s,
HCN), 7.06−7.28 (20H, m, ArH), 6.80 (4H, s, ArH), 3.45 (2H, dd,
J = 6.2 Hz, CH), 2.13 (2H, m, CH), 1.89 (2H, m, CH), 1.78 (12H, s,
C(CH3)2), 1.62 (6H, m, CH3), 1.42 (6H, s, CH3), 1.30 (4H, m, CH),
−0.77 (6H, s, Al−CH3), −1.40 (6H, s, Al−CH3). 13C NMR (CDCl3,
100 MHz, ppm): δ 173.4, 161.5, 150.7, 150.6, 140.5, 138.5, 134.7,
129.6, 128.3, 127.8, 127.0, 125.9, 125.7, 125.1, 117.6 (HCN, ArC),
71.7 (CyC), 42.4 (C(CH3)2Ph), 42.1 (C(CH3)2Ph), 32.2 (C-
(CH3)2Ph), 31.8 (C(CH3)2Ph), 30.9 (C(CH3)2Ph), 27.1 (CyC),
24.6 (CyC), 22.9 (CyC), 14.3 (CyC), −7.4 (Al−CH3), −8.3 (Al−
CH3). Anal. Calcd (found) for C60H72Al2N2O2: C, 79.44 (79.48); H,
8.00 (8.35); N, 3.09 (3.65).
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files, text, figures, and tables giving details of the crystal
structure determination of compounds 1a, 2a, and 6 and
additional experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cchlin@mail.nchu.edu.tw. Tel: +886-4-22840411. Fax:
+886-4-22862547.
■
1
[L6AlMe] (6′). H NMR (CD3Cl, 400 MHz, ppm): δ 8.27 (1H, s,
HCN), 8.03 (1H, s, HCN), 7.11−7.35 (20H, m, ArH), 6.95 (1H,
s, ArH), 6.83 (2H, s, ArH), 3.56 (1H, m, CH), 3.02 (1H, m, CH), 2.55
(1H, m, CH), 2.35 (1H, m, CH), 2.11 (2H, m, CH), 2.08 (3H, s,
CH3), 2.04 (3H, s, CH3), 1.98 (3H, s, CH3), 1.80 (3H, s, CH3), 1.69
(6H, s, CH3), 1.59 (6H, s, CH3), 1.45 (2H, m, CH), 1.35 (2H, m,
CH), −1.40 (s, 3H, Al−CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ
168.6, 161.4, 155.9, 151.1, 150.9, 140.7, 137.3, 136.6, 134.8, 129.8,
128.1, 127.9, 126.9, 125.7, 125.1, 118.9, 118.4 (HCN, ArC), 66.2 (s,
NCH), 62.4 (NCH), 44.1 (C(CH3)2Ph), 43.4 (C(CH3)2Ph), 42.3
(C(CH3)2Ph), 42.2 (C(CH3)2Ph), 31.8 (CyC), 28.6 (CyC), 27.3
(CyC), 24.0 (CyC), 14.3 (Al−CH3). Anal. Calcd (found) for
C57H63AlN2O2: N, 3.35 (3.18); C, 81.98 (81.66); H, 7.60 (7.10).
Conversion of 6′ to 6. To a toluene solution (10.0 mL) of 6′
(0.360 mmol) was added a trimethylaluminum solution (0.718 mmol)
at room temperature under N2, and the mixture was stirred for 2 h.
Then the mixture was stirred for 12 h at 70 °C. The solvent was
evaporated to dryness to obtain a yellow residue. To this was added
dry n-hexane (20.0 mL), and the mixture was stored for 12 h at room
temperature. The precipitated yellow solid was filtered under vacuum
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the National Science Council of the
Republic of China is gratefully appreciated.
■
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