Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-Opening Polymerization of ?-Caprolactone: A Five-Membered-Ring System

Article abstract of DOI:10.1021/acs.organomet.7b00068

A series of five- and six-membered-ring Al complexes bearing Schiff bases was synthesized and their application to the ring-opening polymerization of ?-caprolactone (CL) was studied. The five-membered-ring Al complexes have been shown to have a significan

Full text of DOI:10.1021/acs.organomet.7b00068

Article
pubs.acs.org/Organometallics
Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-
Opening Polymerization of ε‑Caprolactone: A Five-Membered-Ring
System
Chieh-Ling Lee,† Ya-Fan Lin,†,‡ Man-Ting Jiang,† Wei-Yi Lu,† Jaya Kishore Vandavasi,§ Li-Fang Wang,†
Yi-Chun Lai,† Michael Y. Chiang,†,∥ and Hsuan-Ying Chen*,†,⊥
†Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, Republic of China
‡Department of Fragrance and Cosmetic Science, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, Republic of China
§Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada
∥Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, Republic of China.
⊥Department of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung 80708, Taiwan, Republic of China
S
* Supporting Information
ABSTRACT: A series of five- and six-membered-ring Al
complexes bearing Schiff bases was synthesized and their
application to the ring-opening polymerization of ε-caprolac-
tone (CL) was studied. The five-membered-ring Al complexes
have been shown to have a significantly higher polymerization
rate than six-membered-ring Al complexes (2−3 fold for CL
polymerization). The X-ray data revealed that the Al center of
a five-membered-ring Al complex is farther from the Schiff
base ligand than is that of a six-membered-ring Al complex.
The results of density functional theory calculations also
suggest that more space around the Al center of five-
membered-ring Al complexes may reduce the steric repulsion in CL polymerization and increase the catalytic activity of five-
membered-ring Al complexes.
because of the ease of synthesis of diverse substituents of Schiff
base ligands. The results showed that the polymerization rate of
cyclic ester ROP could be influenced through the steric and
electronic effects of a phenolate or phenylimino group on these
Al complexes bearing Schiff base ligands. In general, Al
complexes bearing sterically bulky4b,l,o,q,u or electron-with-
drawing4b−d,k−n,p substituents displayed higher catalytic activity.
However, no study has reported the influence of the geometric
structure of Al catalysts bearing Schiff base ligands on the
catalytic activity of cyclic ester ROP.
INTRODUCTION
■
Poly-ε-caprolactones (PCLs) are commercially available
biomaterials and are extensively used in assorted fields because
of their biodegradability, biocompatibility, and permeability.1
PCL is typically synthesized through ring-opening polymer-
ization (ROP), as ROP can offer greater molecular weight
control of polymers than traditional polycondensation.2 Many
metal complexes3 have been used as Lewis acids to increase the
positive charge of carbonyl groups for easy access to the ROP
of cyclic esters (Scheme 1). Among them, Al complexes4−10 are
suitable catalysts for ROP because of their strong Lewis acidity,
ease of synthesis, and low-cost precursors. There have been
several reports for the catalytic activity of cyclic ester ROP by
utilizing Al complexes4 bearing Schiff base ligands as catalysts
Our group studied Ti complexes11 with various Schiff base
ligands and found that the different steric effects of these
ligands altered the coordinated form and further influenced the
monomer coordination and hence the catalytic activities of L-
lactide (LA) and ε-caprolactone (CL) polymerizations. In
continuing our research, we extend our work to Al complexes
and found that Al complexes with Schiff base ligands are mostly
six-membered-ring structures (Figure 1a). We wonder if the
catalytic behavior of CL polymerization should be influenced by
switching to five-membered-ring Al complexes bearing Schiff
base ligands (Figure 1b). Herein, we report the synthesis of
Scheme 1. Role of a Metal Catalyst in the ROP of Cyclic
Esters
Received: January 28, 2017
© XXXX American Chemical Society
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Figure 1. (a) Six-membered-ring and (b) five-membered-ring
structures of Al complexes bearing Schiff base ligands.
various Al complexes with Schiff base ligands in both five- and
six-membered-ring structures and the study of their structure−
function relationship in catalytic ROP.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Al Complexes. 2-
Amino-4,6-di-tert-butylphenol12 was prepared through the
reduction of 2,4-di-tert-butyl-6-nitrophenol, which was synthe-
sized through the nitration of 2,4-di-tert-butylphenol (Figure 2).
The five-membered-ring Schiff base ligands were formed after
condensation reactions of 2-amino-4,6-di-tert-butylphenol with
various aldehydes. All ligands reacted with a stoichiometric
amount of trimethylaluminum in toluene to produce Al
compounds (Figure 2). The Al complexes of six-membered-
ring form were also synthesized through the aforementioned
method for comparison. L6‑ThioAlMe2 could not be isolated as
pure product because precipitation was hard due to its high
solubility in hexane. However, we were lucky to get crystals
from NMR tube by simple evaporation. Its formula and
Figure 3. Molecular plot of L5‑PhAlMe2 with 30% probability ellipsoids
(all of the hydrogen atoms are omitted for clarity). Selected distances
and angles: d(Al−O) = 1.7833(15) Å, d(Al−N) = 2.0240(17) Å,
d(Al−C22) = 1.961(2) Å, d(Al−C23) = 1.958(2) Å; ∠O−Al−N =
85.39(7)°, ∠C22−Al−C23 = 122.17(10)°].
the distorted-tetrahedral geometry of the Al complex with two
methyl groups. The Al atom sits 0.443 Å above the phenyl ring
plane, which falls in the range of that of the six-membered-ring
Schiff base Al complexes4 (0.00−0.927 Å). The distances
between the Al atom and N, O, C(22), and C(23) are 2.024(2),
1.783(2), 1.961(2), and 1.958(2) Å, respectively. The N−Al−
O and C(22)−Al−C(23) angles are 85.39(7) and 122.17(10)°,
1
structure were confirmed through H and 13C NMR spectra,
elemental analysis, and X-ray crystal analysis. The X-ray
structure of L5‑PhAlMe2 (Figure 3, CCDC 1487513) illustrated
Figure 2. Synthesis of ligands and their Al complexes.
B
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respectively. The X-ray structure of L5‑OMeAlMe2 (Figure 4,
CCDC 1487514) showed bond angles and bond lengths similar
∠O−Al−N. A significant difference between five- and six-
membered-ring dimethyl Al complexes bearing Schiff base
ligands is that the ∠O−Al−N angle of five-membered-ring Al
complexes was smaller than that of six-membered-ring Al
complexes by approximately 6°. This implies that there is more
vacant space around the Al center of five-membered-ring Al
complexes for potential substrate binding. This might
contribute to the enhanced catalytic activity for five-
membered-ring Al complexes (vide infra).
Polymerization of ε-Caprolactone. Polymerizations of
CL were studied by using all Al complexes except L6‑ThioAlMe2
because of its low purity. Two equivalents of BnOH in toluene
was used as an initiator (Table 1). As shown in entries 1−3 of
Table 1, the positive effect of L5‑PhAlMe2 over L6‑PhAlMe2 and
L6‑BnAlMe2 is evident, as only 2.5 h was required to reach 89%
conversion of PCL for catalyst L5‑PhAlMe2, whereas times of 7
and 9 h were required for L6‑PhAlMe2 and L6‑BnAlMe2,
respectively. In addition, L5‑PhAlMe2 afforded greater control
of polymer molecular weight at levels comparable with
Mn(calcd), Mn(NMR), and Mn(GPC) and narrow polydisper-
sity indexes (PDI) (Mn(NMR) = 5000, Mn(GPC) = 4000, PDI
= 1.17). Moreover, the PCL of entry 1 synthesized by
L6‑PhAlMe2 gave a larger Mn(GPC) in comparison to
Mn(NMR) and a broad PDI (1.73). Although L6‑PhAlMe2
had higher catalytic activity than L6‑BnAlMe2, the controllability
of L6‑PhAlMe2 was lower than that of L6‑BnAlMe2. This also
showed that the catalytic activity of six-membered Al complexes
could be influenced greatly through different N substituents of
Schiff base ligands. On the basis of these results, we decided to
choose L5‑PhAlMe2 for catalytic PCL studies and scrutinized it
with various substituted anilines such as Me, pyridine, OTs,
benzyl, furan, and thiophene. A comparison of five-membered-
ring Al complexes bearing Schiff base ligands with various
substituents on an N-aromatic ring revealed that L5‑OTsAlMe2 is
the most effective catalyst. We compared one of the anilines
substituted with a methoxy group with six-membered
L6‑OMeAlMe2 (entry 4, Table 1), and it also showed lower
efficiency in comparison with five-membered L5‑OMeAlMe2
(entry 5, Table 1). These results strongly suggest that the
five-membered-ring complexes are favored for PCL.
Figure 4. Molecular plot of L5‑OMeAlMe2 with 30% probability
ellipsoids (all of the hydrogen atoms are omitted for clarity). Selected
distances and angles: d(Al−O) = 1.7932(19) Å, d(Al−N) = 2.007(2)
Å, d(Al−C23) = 1.952(3) Å, d(Al−C24) = 1.958(3) Å; ∠O−Al−N =
85.92(8)°, ∠C23−Al−C24 = 120.70(13)°.
to those of L5‑PhAlMe2 (d(Al−N) = 2.007(2) Å, d(Al−O(1)) =
1.793(2) Å, d(Al−C23) = 1.952(3) Å, d(Al−C24) = 1.958(3)
Å, ∠O(1)−Al−N = 85.92(8)°, ∠C(23)−Al−C(24) =
120.70(13)°). In addition, there is no interaction between the
Al atom and the methoxy group (d(Al−O(2)) = 4.947 Å). For
the six-membered-ring L6‑ThioAlMe2 (Figure 5, CCDC
The satisfactory results of CL polymerization using
L5‑PhAlMe2 are evident from the linear relationship between
Mn(GPC) and [monomer]0 × conversn/[BnOH]0 (entries 3
and 10−13 in Table 1 and Figure 6), as well as from the low
1
PDIs (1.12−1.27). The H NMR spectrum of PCL (entry 9,
Table 1) confirmed one benzyl group (phenyl group for peak a
and methylene group for peak b) and hydroxyl chain ends
(peak c) with an integral ratio of 5:2:2, suggesting that
initiation occurred through BnOH insertion into CL (Figure
7).
Figure 5. Molecular plot of L6‑ThioAlMe2 with 30% probability
ellipsoids (all of the hydrogen atoms are omitted for clarity), Selected
distances and angles: d(Al−O) = 1.7815(15) Å, d(Al−N) =
1.9714(19) Å, d(Al−C21) = 1.956(2) Å, d(Al−C22) = 1.960(2) Å;
∠O−Al−N = 93.96(7)°, ∠C21−Al−C22 = 119.06(11)°.
Kinetic Study of ε-Caprolactone Polymerization by
Five- and Six-Membered-Ring Al Complexes. The results
of the kinetic study on CL polymerization at room temperature
by five- and six-membered-ring Al complexes are presented in
Table 2 and Figures S1 and S2 in the Supporting Information.
The five-membered-ring Al catalysts showed a significantly
higher polymerization rate than did the six-membered-ring Al
catalysts (2−3-fold for CL polymerization). In addition, the
reaction time required from BnOH and dimethyl Al complex to
Al benzyl alkoxide (induction period)6b of five-membered-ring
Al catalysts (1−35 min) is significantly shorter than that of six-
membered-ring Al catalysts (109−227 min). The higher
catalytic activity and shorter induction period when five-
1487512), the distances between the Al atom and N, O,
C(21), and C(22) are 1.971(2), 1.782(2), 1.956(2), and
1.960(2) Å, respectively. The N−Al−O and C(21)−Al−C(22)
angles are 93.96(7) and 119.06(11)°, respectively. These values
are comparable to the reported values in the literature for six-
membered-ring Al complexes bearing Schiff base ligands: bond
lengths range from 1.9344q to 1.983 Å4s for the Al−CH3 bond,
from 1.75613 to 1.796 Å4r for the Al−O bond, and from 1.9544s
to 2.027 Å4q for the Al−N bond. The angles are from 112.944o
to 123.25°4r for ∠C−Al−C and from 91.894r to 95.74°4r for
C
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a
Table 1. Polymerization of CL Catalyzed by Five- and Six-Membered-Ring Al Complexes
b
c
b
d
d
L of LAlMe2
time (min)
conversn (%)
Mn(calcd) (g mol−1
)
Mn(NMR) (g mol−1
)
Mn(GPC) (g mol−1
)
PDI
1
L6‑Ph
L6‑Bn
L5‑Ph
L6‑OMe
L5‑OMe
L5‑Py
50/540
70/1500
60/150
60/500
60/240
60/540
60/120
60/1640
50/1460
120
10/90
11/94
50/89
0/95
32/94
10/91
54/90
5/75
10/85
>99
5200
5500
5200
5500
5500
5300
5200
4300
5000
2900
8500
11400
25800
5900
3800
5000
6100
7600
11000
5300
4700
3800
2700
9000
13000
35800
7700
3900
4000
7700
7000
13700
3100
3300
3500
2800
9700
13900
41600
1.73
1.13
1.17
1.44
1.23
1.41
1.08
1.10
1.21
1.12
1.20
1.27
1.29
2
3
4
5
6
7
L5‑OTs
L5‑Ful
L5‑Thio
L5‑Ph
8
9
e
f
10
11
12
13
L5‑Ph
200
>99
g
h
L5‑Ph
240
87
L5‑Ph
1520
90
a
b
Reaction conditions unless specified otherwise: toluene (5 mL), [M]0/[cat.]0/[BnOH]0 = 100/1/2, [CL] = 2.0 M, at room temperature. Obtained
c
d
from 1H NMR analysis. Calculated from (molecular weight of monomer) × [monomer]0/[BnOH]0 × (conversion yield) + Mw(BnOH). Obtained
from GPC analysis and calibration based on the polystyrene standard. Values of Mn(GPC) are the values obtained from GPC times 0.56. Reaction
conditions: toluene (5 mL), [M]0/[cat.]0/[BnOH]0 = 50/1/2, [CL] = 1.0 M, at room temperature. Reaction conditions: toluene (5 mL), [M]0/
[cat.]0/[BnOH]0 = 150/1/2, [CL] = 3.0 M, at room temperature. Reaction conditions: toluene (5 mL), [M]0/[cat.]0/[BnOH]0 = 200/1/2, [CL] =
e
f
g
h
4.0 M, at room temperature. Reaction conditions: toluene (5 mL), [M]0/[cat.]0/[BnOH]0 = 500/1/2, [CL] = 10.0 M, at room temperature.
Figure 6. Linear plot of Mn(GPC) vs [CL]0 × conversn/[BnOH],
with PDI values indicated by blue dots (entries 3 and 10−13 in Table
1).
Figure 7. 1H NMR spectrum of PCL (entry 9 of Table 1) with BnOH
as an initiator.
membered-ring Al catalysts are used may be attributed to the
larger empty space around its Al center, making coordination of
CL and BnOH to the Al center much easier. This facilitates the
initiation of CL and increases the rate of benzyl alkoxide
formation. To prove our theory, the reactions of 2 equiv of
BnOH and 1 equiv of Al complex (L5‑OMeAlMe2 and
L6‑OMeAlMe2, respectively) were monitored by 1H NMR
(Figures S26 and S27 in the Supporting Information) to give
insight into the induction period in PCL. The peak of the
methyl group on Al atom (−9.23 ppm) disappeared after 30
min for L5‑OMeAlMe2 but after 240 min for L6‑OMeAlMe2. This
clearly showed that five-membered-ring Al complexes could
transform the dimethyl Al complex to the Al benzyl alkoxide
much more quickly. It is noted, however, for five-membered-
ring Al catalysts bearing N substituents with chelating
properties such as L5‑FulAlMe2, L5‑ThioAlMe2, and L5‑PyAlMe2,
lower polymerization rates yet still shorter induction periods
are observed. One possible reason is that the chelating groups
may coordinate to an Al atom, weaken the methyl−Al bond,
and facilitate benzyl alkoxide formation to give a shorter
induction period. However, the same chelating feature may
hinder the initial coordination of CL and hence give a lower
polymerization rate in comparison to unsubstituted L5‑PhAlMe2.
In fact, such an argument on the reduction of CL coordination
to the Al center by chelating groups has been reported
previously.6b Only L5‑OTsAlMe2 has a higher catalytic activity
than does L5‑PhAlMe2 and, at this moment, the reason is
unclear and further studies are in progress. In addition, the
catalytic activity of L6‑OMeAlMe2 is higher than that of
L6‑PhAlMe2 and L6‑BnAlMe2, and the induction period of
L6‑OMeAlMe2 is longer than that of L6‑PhAlMe2 and L6‑BnAlMe2.
This is totally the opposite of what is observed in five-
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Table 2. Observed Rate Constants (kobs) for Polymerization
of CL Catalyzed by Five- and Six-Membered-Ring Al
Complexes with 2 equiv of BnOH in Toluene at Room
a
Temperature
induction period (error),
entry L in LAlMe2 kobs (error), 10−3 min−1
min
1
2
3
4
5
6
7
8
9
L6‑Ph
5.51(34)
2.07(60)
15.79(92)
8.65(88)
9.02(10)
5.95(2)
129(23)
147(27)
12(5)
227(37)
13(4)
35(8)
14(5)
10(7)
0
L6‑Bn
L5‑Ph
L6‑OMe
L5‑OMe
L5‑Py
L5‑OTs
L5‑Ful
L5‑Thio
19.52(130)
1.30(3)
1.28(2)
a
The observed kobs value is the slope of the first-order kinetic plot of ε-
caprolactone polymerization with time. The conversion of ε-
caprolactone with time was monitored by H NMR.
Figure 9. Linear plot of kobs against [L5‑OMeAlMe2 + 2 BnOH] for CL
polymerization with [CL] = 2.0 M in toluene (5 mL).
1
membered-ring Al catalysts. This implies that substituted
groups have different effects on these Al complexes with
different ring sizes. A possible scenario that may explain why
the catalytic activity of L6‑OMeAlMe2 is higher than that of
L6‑PhAlMe2 and L6‑BnAlMe2 is that the extra pendant group
could induce the formation of more active dinuclear Al
complexes.4i,q−s,5e However, more work is required to verify
these details.
Furthermore, kinetic studies were performed at room
temperature to examine the effect of the [CL]0/[L5‑OMeAlMe2
+ 2 BnOH] ratio ([CL] = 2.0 M in 5 mL of toluene), as
described in Table S2 in the Supporting Information and Figure
8. The preliminary results indicated a first-order dependence on
[CL] (Figure 8). When kobs was plotted against [L5‑OMeAlMe2
+ 2 BnOH], a kprop value of 0.296 M−1 min−1 was obtained
(Figure 9). Polymerizing CL using L5‑OMeAlMe2 at room
temperature demonstrated the following rate law:
Mechanistic Study of ε-Caprolactone Polymerization
by Five- and Six-Membered-Ring Al Complexes.
According to the literature,14 a possible mechanism is proposed
in Figure 10. Formation of the benzyl alkoxide by attack of the
methyl groups on the Al center by BnOH is believed to be what
happened in the induction period. Monitoring the disappear-
1
ance of H NMR signals on the methyl group (Figure S26 in
the Supporting Information) of L5‑OMeAlMe2 on reaction with
BnOH (1:2) suggests that methyl groups of L5‑OMeAlMe2 are
replaced by benzyl alcohols to form the real catalysts,
L5‑PhAl(OBn)2, before polymerization starts.4l,t,6b,14 The
lactone can coordinate to Al via carbonyl oxygen, resulting in
the formation of the trigonal-bipyramidal five-coordinated Al
complex Al-CL. One of the benzyl alkoxides undertakes a
nucleophilic attack on a carbonyl carbon to form the
intermediate Al-CL-OBn. This species then rotates to form
Al-CL-OBn′ and further chelates to the Al center in the
bidentate mode, forming AlC‑O‑Al‑O. This is immediately
d[CL]/dt = 0.296 × [CL][L5‐OMeAlMe2 + 2BnOH]
Figure 8. First-order kinetic plots of CL polymerization with various concentrations of [L5‑OMeAlMe2 + 2 BnOH] plotted against time with [CL] =
2.0 M in toluene (5 mL).
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Figure 10. Mechanistic study on CL polymerization by using L5‑PhAlMe2 as a catalyst.
Figure 11. Geometries of five- and six-membered-ring Al complexes bearing Schiff base ligands.
Figure 12. Optimized structures: (a) L5‑PhAl(OBn)2, d(Al−O) = 1.789 Å, d(Al−N) = 1.998 Å, ∠O−Al−N = 87.17°; (b) L6‑PhAl(OBn)2, d(Al−O)
= 1.791 Å, d(Al−N) = 1.943 Å, ∠O−Al−N = 95.05°; (c) CL-Al(L5‑Ph)(OBn)2, d(Al−O) = 1.818 Å, d(Al−N) = 2.145 Å, d(Al−OCL) = 1.951 Å,
∠O−Al−N = 80.33°; (d) CL-Al(L6‑Ph )(OBn)2, d(Al−O) = 1.815 Å, d(Al−N) = 2.062 Å, d(Al−OCL) = 1.985 Å, ∠O−Al−N = 88.01°. Hydrogen
atoms are omitted for clarity.
followed by a ring-opening step and recovery of the active state
Al-OR. The tetrahedral four-coordinated complex Al-OR can
accommodate another lactone to repeat the ring-opening
polymerization. The comparison of X-ray structures between
five- and six-membered-ring Al complexes revealed that the bite
angles (∠O−Al−N) of six-membered-ring dimethyl Al
complexes4r bearing Schiff base ligands are larger (91.89−
95.74°) than those of five-membered-ring complexes (85.39°
for L5‑PhAlMe2 and 85.92° for L5‑OMeAlMe2). The smaller bite
angle ∠O−Al−N makes the five-membered ring significantly
more puckered than the six-membered ring. The result is that
the Al center of a five-membered-ring Al complex is farther
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from the Schiff base ligand than is that of a six-membered-ring
Al complex. With L6‑ThioAlMe2 as an example, the distance
between the Al atom and the quaternary carbon atom on the
ortho position of the phenolate group is 4.624 Å, and the
distance between the Al atom and the methylene group of the
thiophen-2-ylmethyl group is 3.013 Å (Figure 11). However,
the Al center of L5‑PhAlMe2 is far from the Schiff base ligand
(distance between the Al atom and the quaternary carbon atom
on the ortho position of the phenolate group 4.690 Å; distance
between the Al atom and the methylene group of the benzyl
group 3.037 Å). It is reasonable to assume that CL-OBn in the
CL polymerization process by using a five-membered-ring Al
complex is more stable than that of a six-membered-ring Al
complex because of less steric repulsion, and this stability
increases the polymerization rate.
data revealed that the Al center of a five-membered-ring Al
complex is farther away from the Schiff base ligand in
comparison with the six-membered-ring Al complex. This
phenomenon could reduce the steric repulsion of the transition
state CL-OBn in the CL polymerization process and increase
the catalytic activity of the five-membered-ring Al complex. The
reported new strategy can be used to design Al complexes
bearing Schiff base ligands with high catalytic activity for CL
polymerization. We anticipate this work may inspire other
scientists to design new Al catalysts with a five-membered-ring
form for ring-opening polymerization.
EXPERIMENTAL SECTION
■
Standard Schlenk techniques and a N2-filled glovebox were used
throughout the isolation and handling of all the compounds. Solvents,
ε-caprolactone, and deuterated solvents were purified prior to use.
Deuterated chloroform and ε-caprolactone were purchased from
Acros. Benzyl alcohol, trimethylaluminum, 4,6-di-tert-butylphenol, 3,5-
di-tert-butyl-2-hydroxybenzaldehyde, palladium on carbon (10%),
furfural, 2-pyridinecarboxaldehyde, 2-methoxybenzaldehyde, benzalde-
hyde, p-toluenesulfonyl chloride, salicyaldehyde, o-anisidine, aniline,
To shed light on the influence of the steric congestion, we
further accessed the structural features and stabilities of five-
and six-membered-ring Al complexes by means of density
functional theory (DFT) calculations. ROP catalysts
L5‑PhAl(OBn)2 and L6‑PhAl(OBn)2 were optimized at the
B3LYP-D3/6-31g*/PCM(toluene) level. The calculated O−
Al−N bite angles for L5‑PhAl(OBn)2 and L6‑PhAl(OBn)2 are
respectively similar to their dimethyl analogues (Figure 12a,b).
To measure the occupied space, the percent buried volume (%
Vbur)15 was theoretically analyzed with the SambVca suite.16
The results show that L6‑PhAl(OBn)2 is more sterically crowded
than L5‑PhAl(OBn)2, as their %Vbur values are 74.3 and 60.1,
respectively. Since the CL-coordinated complex plays a crucial
role in the polymerization process, the bond dissociation
energies (BDEs) of corresponding CL-Al(L)(OBn)2 com-
plexes were computed. Both optimized structures of CL-
Al(L5‑Ph)(OBn)2 and CL-Al(L6‑Ph)(OBn)2 are best described
as a distorted TBP with an Al−OCL distance of 1.951 Å in CL-
Al(L5‑Ph)(OBn)2 and 1.985 Å in CL-Al(L6‑Ph)(OBn)2 (Figure
12c,d). The BDEs of Al−CL are 13.4 and 11.8 kcal/mol for
CL-Al(L5‑Ph)(OBn)2 and CL-Al(L6‑Ph)(OBn)2, respectively.
The crowded environment of CL-Al(L6‑Ph)(OBn)2 situates the
CL far away from the Al atom and forms a weaker Al−CL bond
than that of CL-Al(L5‑Ph)(OBn)2. In comparison with the six-
membered-ring L6‑PhAl(OBn)2, L5‑PhAl(OBn) is less bulky and
hence favors the CL coordination to afford CL-Al-
(L5‑Ph)(OBn)2. Moreover, the resultant Al−CL bond is harder
to break, and ROP therefore proceeds more readily. Without a
doubt, five-membered-ring Al catalysts showed greater catalytic
activities and shorter induction periods in comparison to those
of six-membered-ring Al catalysts. However, this can be
attributed to geometry, electronic effects, or both. To confirm
the real effect influencing catalytic activity and induction period,
the charge on the aluminum center was calculated through
natural population analysis (NPA). The values of the charge on
aluminum center are almost identical: 2.050 and 2.055 for
L5‑PhAl(OBn)2 (Figure 12a) and L6‑PhAl(OBn)2 (Figure 12b),
respectively. From the NPA results, we conclude that only
steric effects influence the catalytic activity and induction
period of five- and six-membered-ring Al catalysts in CL ROP.
and benzylamine were purchased from Alfa Aesar. H and 13C NMR
1
spectra were recorded on a Varian Gemini 2000-200 (200 MHz for 1H
and 50 MHz for 13C) spectrometer with chemical shifts given in ppm
from the internal TMS or center line of CDCl3. Microanalyses were
performed using a Heraeus CHN-O-RAPID 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 JORDI
Gel DVB 103 Å, and the calibration curve was made by primary
polystyrene standards to calculate Mn(GPC). 2-Amino-4,6-di-tert-
butylphenol,12 2-formylphenyl-4-methylbenzenesulfonate,17 L5‑Ph-H,18
L5‑Py-H,19 L5‑ful-H,20 L5‑Thio-H,20 L6‑Ph-H,21 L6‑OMe-H,22 and L6‑Bn-H23
were prepared following literature procedures.
Synthesis of L5‑OMe-H. A mixture of 2-amino-4,6-di-tert-
butylphenol (2.21 g, 10 mmol) and 2-methoxybenzaldehyde (1.36 g,
10 mmol) was refluxed for 1 day in ethanol (20 mL). The solution was
removed under vacuum to give a yellow mud, and then cold hexane
(20 mL) was transferred to the washed red powder three times to give
1
a light yellow powder. Yield: 2.41 g (71%). H NMR (CDCl3, 200
MHz): δ 9.13 (1H, s, CHN), 7.23 (1H, s, p-Ar H), 7.20 (1H, s, o-Ar
H), 7.45 (1H, t, p-Ar HOMe, J = 8.0 Hz), 7.00 (1H, t, m-Ar HOMe, J
= 8.0 Hz), 8.17 (1H, d, o-Ar HOMe, J = 8.0 Hz), 7.08 (1H, d, m-Ar
HOMe, J = 8.0 Hz), 3.94 (3H, s, OCH3), 1.45 (9H, s, C(CH3)3), 1.34
(9H, s, C(CH3)3). 13C NMR (CDCl3, 50 MHz): δ 159.47 (CN),
151.76, 148.73, 141.23, 135.40, 134.89, 132.61, 127.29, 124.71, 122.91,
120.81, 111.17, 110.23 (Ar), 55.53 (OCH3), 34.89, 34.55 (C(CH3)3),
31.67, 29.47 (C(CH3)3). Anal. Found (calcd) for L5‑OMe-H,
C22H29NO2: N, 4.23 (4.13); C, 77.92 (77.84); H, 8.54 (8.61).
Synthesis of L5‑OTs-H. This compound was preprared using a
method similar to that for L5‑OMe-H except that 1-(5-bromo-2-
hydroxyphenyl)ethan-1-one was used in place of 2-methoxybenzalde-
hyde. Yield: 3.45 g (72%). 1H NMR (CDCl3, 200 MHz): δ 8.47 (1H,
s, CHN), 8.06 (1H, d, Ar H, J = 6 Hz), 7.27−7.65 (6H, m, Ar H),
7.13 (2H, d, ArH, J = 6 Hz), 6.96 (1H, s, Ar H), 2.20 (3H, s, CH3),
1.44 (9H, s, C(CH3)3), 1.36 (9H, s, C(CH3)3). 13C NMR (CDCl3, 50
MHz): δ 149.38 (CN), 149.04, 148.75, 146.08, 141.23, 135.19,
132.07, 129.89, 129.48, 128.13, 127.65, 127.43, 123.85, 109.97 (Ar),
21.45 (CH3), 34.91, 34.53 (C(CH3)3), 31.62, 29.42 (C(CH3)3). Anal.
Found (calcd) for L5‑OTs-H, C28H33NO4S: N, 2.83 (2.92); C, 70.51
(70.12); H, 6.99 (6.94).
CONCLUSIONS
■
A series of five- and six-membered-ring Al complexes bearing
Schiff base ligands were synthesized, and their CL polymer-
ization was studied. The five-membered-ring Al complexes
revealed an appreciably higher (2−3-fold) polymerization rate
than the six-membered-ring Al complexes in the CL polymer-
ization with a shorter induction period. Geometric structure
Synthesis of L6‑Thio-H. A mixture of 3,5-di-tert-butyl-2-hydrox-
ybenzaldehyde (2.34 g, 10 mmol) and thiophen-2-ylmethanamine
(1.13 g, 10 mmol) was refluxed for 1 day in ethanol (20 mL). L6‑Thio-H
crystallized after the solution was kept at −20 °C for 1 day. Yield: 2.33
g (71%).1H NMR (CDCl3, 200 MHz): δ 8.35 (1H, s, CHN), 7.32,
7.02 (2H, s, Ar H), 7.19, 6.91 (3H, br, ThioH), 4.88 (2H, s, NCH2),
G
DOI: 10.1021/acs.organomet.7b00068
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Organometallics
Article
1.38 (9H, s, C(CH3)3), 1.24 (9H, s, C(CH3)3). 13C NMR (CDCl3, 50
MHz): δ 166.77 (CN), 157.97, 140.89, 140.11, 136.73, 127.16,
126.92, 126.12, 125.38, 124.91, 117.79 (Ar), 57.22 (NCH2), 35.03,
34.10 (C(CH3)3), 31.50, 29.45 (C(CH3)3). Anal. Found (calcd) for
124.93, 110.46, 109.87 (Ar), 35.24, 34.76 (C(CH3)3) 31.65, 29.14
(C(CH3)3), 21.62 (CH3), −9.99 (Al(CH3)2). Anal. Found (calcd) for
L
5‑OMeAlMe2, C24H34AlNO2: N, 3.22 (3.54); C, 72.53 (72.88); H, 8.51
(8.66). Mp: 170 °C.
L
6‑Thio-H, C20H27NOS: N, 4.58 (4.25); C, 72.44 (72.90); H, 8.01
Synthesis of L6‑PhAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L6‑Ph-H was used in
place of L5‑Ph-H. Yield: 2.41 g (66%). 1H NMR (CDCl3, 200 MHz): δ
8.24 (1H, s, CHN), 7.54, 7.04 (2H, s, Ar H), 7.23−7.43 (5H, m, Ar
H), 1.38 (9H, s, C(CH3)3), 1.25 (9H, s, C(CH3)3), −0.79 (6H, s,
Al(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 170.45 (CN), 162.34,
147.00, 140.71, 139.26, 132.94, 129.77, 129.20, 127.70, 122.19, 118.66
(Ar), 31.28, 29.28 (C(CH3)3), 35.31, 34.07 (C(CH3)3), −9.23
(Al(CH3)2). Anal. Found (calcd) for L6‑PhAlMe2, C23H32AlNO: N,
3.87 (3.83); C, 75.77 (75.58); H, 8.90 (8.83). Mp: 90 °C.
(8.26).
Synthesis of L5‑PhAlMe2. A mixture of L5‑Ph-H (3.08 g, 5 mmol)
and AlMe3 (6 mL, 2.0 M, 12 mmol) in toluene (15 mL) was stirred for
3 h at 0 °C. Volatile materials were removed under vacuum to give a
yellow powder, and then hexane (30 mL) was transferred to the
suspension. A yellow powder was obtained after filtering. Yield: 2.63 g
1
(72%). H NMR (CDCl3, 200 MHz): δ 8.82 (1H, s, CHN), 7.90
(2H, d, Ar H, J = 6 Hz), 7.53−7.62 (3H, m, Ar H), 7.34 (1H, s, Ar H),
7.24 (1H, s, Ar H), 1.44 (9H, s, m-C(CH3)3), 1.35 (9H, s, C(CH3)3),
−0.74 (6H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 170.45
(CN), 162.34, 147.00, 140.71, 139.26, 132.94, 129.77, 129.20,
127.70, 122.19, 118.66 (Ar), 31.28, 29.28 (C(CH3)3), 35.31, 34.07
(C(CH3)3), −9.23 (Al(CH3)2). Anal. Found (calcd) for L5‑PhAlMe2,
C23H32AlNO: N, 3.72 (3.83); C, 75.40 (75.58); H, 8.90 (8.83). Mp:
140 °C.
Synthesis of L6‑BnAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L6‑Bn-H was used in
place of L5‑Ph-H. Yield: 2.39 g (63%). 1H NMR (CDCl3, 200 MHz): δ
8.15 (1H, s, CHN), 7.52, 6.97 (2H, s, Ar H), 7.29−7.40 (5H, m, Ar
H), 4.71 (2H, s, NCH2), 1.39 (9H, s, C(CH3)3), 1.28 (9H, s,
C(CH3)3), −0.95 (6H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): δ
171.75 (CN), 140.40, 138.77, 134.99, 131.83, 129.05, 128.97,
128.51, 120.34, 118.11 (Ar), 60.40 (CH2), 35.25, 34.02 (C(CH3)3),
31.32, 29.25 (C(CH3)3), −10.00 (Al(CH3)2). Anal. Found (calcd) for
Synthesis of L5‑PyAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L5‑Py-H was used in
place of L5‑Ph-H. Yield: 2.79 g (76%). 1H NMR (CDCl3, 200 MHz): δ
8.69 (1H, d, J = 8.0 Hz, o-pyridine H), 8.62 (1H, s, NCH), 7.98 (1H, t,
J = 8.0 Hz, pyridine H), 7.72 (1H, d, J = 8.0 Hz, pyridine H), 7.52
(1H, t, J = 8.0 Hz, pyridine H), 7.36−7.34 (2H, m, Ar H), 1.46 (s, 9H,
CH3), 1.33 (s, 9H, CH3), −0.86 (s, 6H, Al(CH3)2). 13C NMR (CDCl3,
50 MHz): δ 162.45 (CN), 140.68, 140.12, 138.96, 129.11, 125.35,
109.18, 147.71, 147.51, 137.23, 128.48, 124.42 (Ar), 35.31, 34.42
(C(CH3)3), 31.42, 28.96 (C(CH3)3), −9.01 (Al(CH3)2). Anal. Found
(calcd) for L5‑PyAlMe2, C22H31AlN2O: N, 7.78 (7.64); C, 72.24
(72.10); H, 8.21 (8.53). Mp: 180 °C.
L
6‑PhAlMe2, C24H34AlNO: N, 3.51 (3.69); C, 75.70 (75.95); H, 8.95
(9.03). Mp: 108 °C.
Synthesis of L6‑OMeAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L6‑OMe-H was used
in place of L5‑Ph-H. Yield: 2.93 g (74%). 1H NMR (CDCl3, 200 MHz):
δ 8.39 (1H, s, CHN), 7.55, 7.08 (2H, s, p-Ar H), 6.96−7.35 (4H, m,
Ar HOMe), 3.89 (3H, s, OCH3), 1.44 (9H, s, m−C(CH3)3), 1.31 (9H,
s, C(CH3)3), −0.84 (6H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz):
δ 169.03 (CN), 163.87, 151.82, 140.80, 138.60, 135.93, 132.64,
128.73, 128.56, 121.60, 121.54, 119.11, 111.22 (Ar), 54.92 (OCH3),
35.27, 34.03 (C(CH3)3), 31.27, 29.32 (C(CH3)3), −10.48 (Al(CH3)2).
Anal. Found (calcd) for L6‑OMeAlMe2, C24H34AlNO2: N, 3.85 (3.54);
C, 73.23 (72.88); H, 8.89 (8.66). Mp: 130 °C.
Synthesis of L5‑FulAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L5‑Ful-H was used in
place of L5‑Ph-H. Yield: 2.06 g (58%).1H NMR (CDCl3, 200 MHz): δ
8.49 (1H, s, NCH), 7.77, 7.30 (2H, s, Ar H), 7.23−7.17 (2H, m, Ful
H), 6.70 (1H, t, J = 2.0 Hz, Ful H), 1.44 (s, 9H, CH3), 1.33 (s, 9H,
CH3), −0.74 (s, 6H, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): δ
157.68 (CN), 147.73, 139.90, 126.18, 121.53, 113.88, 108.45,
148.76, 139.35, 138.48, 132.00 (Ar), 35.27, 34.54 (C(CH3)3), 31.61,
29.09 (C(CH3)3), −9.92 (Al(CH3)2). Anal. Found (calcd) for
Synthesis of L6‑ThioAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L6‑Thio-H was used
in place of L5‑Ph-H. This substance could not be purified with hexane
1
because it dissolves in hexane. The H NMR spectrum was complex
and revealed that the disproportionation product was also produced. A
single crystal was obtained in an NMR tube when CDCl3 was
vaporized.
L
5‑FulAlMe2, C21H30AlNO2: N, 3.79 (3.94); C, 71.12 (70.96); H,
8.46 (8.51). Mp: 144 °C.
Synthesis of L5‑OMeAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L5‑OMe-H was used
in place of L5‑Ph-H. Yield: 2.69 g (68%).1H NMR (CDCl3, 200 MHz):
δ 9.11 (1H, s, CHN), 7.89 (1H, d, o-Ar HOMe, J = 8 Hz), 7.62
(1H, t, m-Ar HOMe, J = 8 Hz), 7.32, 7.20 (2H, s, Ar H), 6.99−7.13
(2H, m, Ar HOMe), 3.94 (3H, s, OCH3), 1.44 (9H, s, C(CH3)3), 1.34
(9H, s, C(CH3)3), −0.80 (6H, s, Al(CH3)2). 13C NMR (CDCl3, 50
MHz): δ 159.84 (CN), 156.48, 138.75, 138.57, 135.19, 125.69,
121.88, 157.25, 134.86, 129.66, 120.86, 111.36, 110.21 (Ar), 55.80
(OCH3), 35.17, 34.47 (C(CH3)3), 31.65, 29.09 (C(CH3)3), −9.91
(Al(CH3)2). Anal. Found (calcd) for L5‑OMeAlMe2, C24H34AlNO2: N,
3.36 (3.54); C, 72.66 (72.88); H, 8.68 (8.66). Mp: 160 °C.
General Procedures for the Polymerization of ε-Caprolac-
tone. A typical polymerization procedure is exemplified by the
synthesis of entry 3 (Table 1) using complex L5‑PhAlMe2 as a catalyst.
The polymerization conversion was analyzed by 1H NMR
spectroscopic studies. Toluene (5.0 mL) was added to a mixture of
complex L5‑PhAlMe2 (0.1 mmol), BnOH (0.2 mmol), and ε-
caprolactone (10 mmol) at room temperature. At indicated time
intervals, 0.05 mL aliquots were removed, trapped with CDCl3 (1
mL), and analyzed by 1H NMR. After the solution was stirred for 150
min, the reaction was then quenched by adding a drop of isopropyl
alcohol, and the polymer precipitated as a white solid when it was
poured into n-hexane (30.0 mL). The isolated white solid was
dissolved in CH2Cl2 (5.0 mL), and then n-hexane (70.0 mL) was
added to give a purified crystalline solid. Yield: 0.92 g (81%).
Synthesis of L5‑ThioAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L5‑Thio-H was used
in place of L5‑Ph-H. Yield: 2.75 g (74%).1H NMR (CDCl3, 200 MHz):
δ 8.86 (1H, s, NCH), 7.82−7.80 (2H, br, Ar H), 7.30−7.21 (3H, m, Ar
H), 1.44 (s, 9H, CH3), 1.35 (s, 9H, CH3), −0.61 (s, 6H, Al(CH3)2).
13C NMR (CDCl3, 50 MHz): δ 170.36 (CN), 162.27, 146.90,
140.60, 139.19, 132.87, 129.68, 129.28, 128.95, 128.14, 127.64, 122.06
(Ar), 35.28, 34.00 (C(CH3)3), 31.29, 29.35 (C(CH3)3), −9.00
(Al(CH3)2). Anal. Found (calcd) for L5‑ThioAlMe2, C21H30AlNOS:
N, 4.01 (3.77); C, 67.91 (67.89); H, 8.49 (8.14). Mp: 164 °C.
Synthesis of L5‑OTsAlMe2. This compound was preprared using a
method similar to that for L5‑PhAlMe2 except that L5‑OTs-H was used in
place of L5‑Ph-H. Yield: 3.59 g (67%).1H NMR (CDCl3, 200 MHz) δ.
13C NMR (CDCl3, 50 MHz) δ152.57 (CN), 151.54, 149.99, 146.68,
134.49, 133.26, 130.33, 130.12, 128.78, 128.09, 127.86, 127.47, 126.71,
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00068.
Polymer characterization data and details of the kinetic
study (PDF)
Accession Codes
CCDC 1487512−1487514 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
H
DOI: 10.1021/acs.organomet.7b00068
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*H.-Y.C.: e-mail, hchen@kmu.edu.tw; tel, +886-7-3121101-
2585; fax, +886-7-3125339.
■
ORCID
Hsuan-Ying Chen: 0000-0003-1424-3540
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by Kaohsiung Medical University
“Aim for the top 500 universities grant” under Grant No.
KMU-DT105009, NSYSU-KMU Joint Research Project
(#NSYSUKMU 104-P006), and the Ministry of Science and
Technology (Grant MOST 105-2113-M-037-007). We thank
the Center for Research Resources and Development at
Kaohsiung Medical University for instrumentation and equip-
ment support.
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