Improvement in aluminum complexes bearing a Schiff base in ring-opening polymerization of ε-caprolactone: the synergy of the N,S-Schiff base in a five-membered ring aluminum system

Article abstract of DOI:10.1039/c8dt03285h

A series of five-membered ring aluminum complexes bearing thiol-Schiff base ligands were synthesized, and their application in the ring-opening polymerization of ε-caprolactone (CL) was investigated. The complexes exhibited dramatically higher catalytic a

Full text of DOI:10.1039/c8dt03285h

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Improvement in aluminum complexes bearing a
Schiff base in ring-opening polymerization of
ε-caprolactone: the synergy of the N,S-Schiff base
in a five-membered ring aluminum system†
Cite this: Dalton Trans., 2018, 47,
15565
a
a
a
a
a
Ting-Wei Huang, Rou-Rong Su, Yi-Chen Lin, Hsin-Yu Lai, Chien-Yi Yang,
b
a
a,c
Gopal Chandru Senadi,
Hsuan-Ying Chen
Yi-Chun Lai, Michael Y. Chiang and
a,c,d
*
A series of five-membered ring aluminum complexes bearing thiol-Schiff base ligands were synthesized, and
their application in the ring-opening polymerization of ε-caprolactone (CL) was investigated. The complexes
2
exhibited dramatically higher catalytic activity than the six-membered ring S AlMe
2
complex (approximately
5-Ph
4
- to 10-fold higher) and the five-membered ring L
2
AlMe complex (approximately 7- to 19-fold higher).
Moreover, a shorter induction period was observed when the five-membered ring aluminum complexes
bearing thiol-Schiff base ligands were used compared with the other types of aluminum complexes bearing
Schiff base ligands. The electron-withdrawing groups enhanced the catalytic activity of the Al complexes
compared with the electron donating groups. The thiol-Schiff base ligand and the five-membered ring
aluminum catalysis had a synergistic effect that was stronger than the combination of their individual effects.
Received 10th August 2018,
Accepted 3rd October 2018
DOI: 10.1039/c8dt03285h
rsc.li/dalton
group of ε-caprolactone (CL) for conveniently conducting the
ROP of CLs. Aluminum complexes are commonly selected as
Introduction
Environmental problems are attracting increasing attention, catalysts for ROP because of their strong Lewis acidity, con-
and one such problem is that petrochemical plastics do not venient synthesis, and the low cost of their precursors. Among
7
break down naturally within a short time and cause ecological all aluminum complexes, the aluminum complexes bearing
1
damage. Therefore, the development of convenient and environ- Schiff base ligands have attracted the highest interest, presum-
mentally friendly materials, such as biodegradable plastics, ably because of their many diverse substituent forms and con-
has become the focus of researchers. Poly(ε-caprolactone)s venient synthesis. Our recent studies revealed that using
(PCLs) are widely used for commercial purposes because of aluminum complexes with an N,S-Schiff base resulted in a sig-
their biodegradability, biocompatibility, and permeability; nificantly higher polymerization rate than using aluminum
2
–5
thus, they are extensively utilized in a variety of fields. Ring- complexes with an N,O-Schiff base (5- to 12-fold for CL
8
a
opening polymerization (ROP) is a popular method for the syn- polymerization, Fig. 1A).
Moreover, five-membered ring
thesis of PCLs because the method allows precise molecular aluminum complexes with an N,O-Schiff base resulted in a sig-
weight control of polymers, unlike traditional polycondensa- nificantly higher polymerization rate than six-membered ring
6
tion. Metal catalysts act as Lewis acids to activate the carbonyl aluminum complexes (two- to three-fold for CL polymeriz-
8
b
ation, Fig. 1B). On the basis of our recent research, we
reported the synthesis of aluminum complexes bearing aryl-
ideneaminobenzenethiol (Fig. 1C) and the investigation of
their catalytic reactivity during CL ROP to prove the synergy of
the two aforementioned strategies.
a
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung, Taiwan, 80708, Republic of China. E-mail: hchen@kmu.edu.tw;
Fax: +886-7-3125339; Tel: +886-7-3121101-2585
b
Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur,
Chennai – 603203, India
c
Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, 80424,
Republic of China
d
Results and discussion
Synthesis and characterization of aluminum complexes
Department of Medical Research, Kaohsiung Medical University Hospital,
Kaohsiung 80708, Republic of China
†
Electronic supplementary information (ESI) available: Polymer characterization
Aryl-2,3-dihydrobenzothiazole ligands were synthesized by the
reaction of thioaniline and aryl-2-carbaldehyde in CH Cl .
data and details of the kinetic study. CCDC 1828535. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c8dt03285h
2
2
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Fig. 1 (A) Use of thiol-Schiff base ligands and (B) five-membered ring aluminum complexes with an N,O-Schiff base; (C) synergy of these two
strategies.
Fig. 2 Synthesis of thiol-Schiff base ligands and the associated aluminum complexes.
However, the ligands could not be isolated directly because compounds (Fig. 2A). The formula and the structure were con-
9
1
13
they were further oxidized by O2 to form arylbenzothiazole.
firmed by H and C NMR spectroscopy, elemental analysis,
ClH
The ligand synthesis apparatus was set up in a dry box and and X-ray crystal analysis. L AlMe (4) was crystallized in
monitored using H nuclear magnetic resonance (NMR) toluene by allowing it to stand at −20 °C. The X-ray structure
Fig. S18–S22†). When all the aryl-2-carbaldehyde had been of L AlMe (4) (Fig. 3, CCDC 1828535†) reveals that the Al
2
1
ClH
(
2
consumed, volatile materials were removed under vacuum atom has a distorted tetrahedral geometry with the two methyl
without isolation. All ligands reacted with a stoichiometric groups. The aluminum atom is positioned 0.733 Å above the
quantity of AlMe3 in toluene at 0 °C to produce aluminum phenyl ring plane. The distance between the Al atom and the
15566 | Dalton Trans., 2018, 47, 15565–15573
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ClH
phenyl ring plane of L AlMe
2
(4) is longer than that of
5-Ph
L
AlMe2 (0.443 Å, Table 1), but is shorter than that of
S AlMe (1.129 Å, Table 1). Table 1 reveals that the distances of
2
2
Al–C bonds in the five-membered ring aluminum system are
shorter than those in the six-membered ring aluminum system;
however, the Al–N bonds in the five-membered ring aluminum
system are longer. The N–Al–S (or N–Al–O) angles in the five-
membered ring aluminum system are smaller than those in the
six-membered ring aluminum system. The crystallography data
of aluminum complexes with N,O- and N,S-Schiff base ligands
display similar geometries except that the distance of the Al–O
bond is shorter than that of the Al–S bond.
ClH
2
Fig. 3 Molecular structures of L AlMe (4) with 30% probability ellip-
To compare with the five-membered ring aluminum com-
plexes bearing thiol-Schiff base ligands, 2-benzylideneamino-
soids (all of the hydrogen atoms are omitted for clarity).
H
OMe
phenol (O -H), 2-(2-methoxybenzylidene)aminophenol (O -H),
ClBr
and 2-(4-bromobenzylidene)amino-4-chlorophenol (O -H)
Table 1 Comparisons of the selected bond lengths and angles for the
(
Fig. 2B) and the associated Al complexes were synthesized as dis-
played in Fig. 2B. However, the anticipated Al complexes (L-Al-
Me type) could not be isolated because they were too unstable
and formed disproportionation products, such as O 2AlMe (6),
three aluminum complexes
2
10
H
OMe
ClBr
O
2
AlMe (7), and O
2
AlMe (8). Even when the temperature
H
OMe
of the reactions of AlMe
3
with ligands (O -H, O
-H, respectively) was reduced to −78 °C, the formation of
both products (L-Al-Me and L -Al-Me types) was observed. In
-H, and
ClBr
O
2
2
the purification by using toluene and n-hexane as solvents, the
2
ClH
5-Ph
S AlMe
2
L
AlMe
2
(4)
L
AlMe
2
product of the L-Al-Me type was converted to one of the L -Al-
2
2
Selected bond distances (Å)
Me type. The results revealed that the thiol-Schiff base ligands
prevented the disproportionation to synthesize a highly cataly-
tically active Al form with one ligand and two methyl groups.
Al–S or Al–O
Al–N
Al–C
2.2690(7)
1.9890(15)
1.9655(18)
1.968(2)
2.2904(8)
1.7833(15)
2.0240(17)
1.958(2)
1.961(2)
0.443
2.0074(16)
1.953(2)
1.951(2)
0.733
Al–phenyl plane
1.129
Polymerization of ε-caprolactone
Selected angles (°)
N–Al–S or N–Al–O
C–Al–C
Polymerizations of CL were investigated by using aluminum
122.17(10) complexes and two equivalents of BnOH as initiators in
97.66(6)
119.25(9)
87.46(5)
121.65(10)
85.39(7)
Table 2 Polymerization of CL catalyzed using various Al complexes
a
b
a
c
d
Time
(min)
Conv.
(%)
MnCal
MnNMR
MnGPC
k
min
obs (error)
Induction period
(min)
1
(g mol−1)
(g mol−1)
c
−1
Entry
Cat.
(g mol−
)
PDI
ClBr
1
2
3
4
5
L
AlMe
2
(1)
(2)
(4)
(5)
(3)
12
12
20
25
25
90
150
144 h
145 h
60/561
96
92
92
95
90
90
89
81
5600
5400
5400
5500
5200
5200
5200
4700
5200
5400
4800
5600
5200
7900
5500
5800
5000
7000
8500
6000
5900
7800
6400
8100
5200
5100
4000
5700
11 700
4200
1.26
1.19
1.23
1.23
1.21
1.04
1.17
1.17
1.39
1.25
0.284(12)
0.260(17)
0.136(4)
0.115(8)
0.105(4)
0.028(2)
0.016(1)
—
0.8
1.8
0.9
0.8
2.6
5.0
12.9
—
ClCl
L
AlMe
2
ClH
L
L
L
AlMe
2
HH
AlMe
AlMe
2
ClO
2
8
a
b
2
6
7
8
9
1
S AlMe
2
8
5-Ph
L
AlMe
2
H
O
O
O
2
AlMe (6)
OMe
2
AlMe (7)
AlMe (8)
90
24/92
—
0.005(1)
—
72.4
e
ClBr
0
2
a
1
Reaction conditions: Toluene (5 mL), [M]
0
: [Cat.]
Calculated from the molecular weight of the monomer × [monomer]
and calibration on the basis of the polystyrene standard. MnGPC values are obtained from GPC multiplied by 0.56.
the slope of the first-order kinetic plot of ε-caprolactone polymerization versus time. The conversion of ε-caprolactone with time was monitored
0 0
: [BnOH] = 100 : 1 : 2, [CL] = 2.0 M, at room temperature. Obtained from H NMR analysis.
b
c
0
/[BnOH] × conversion yield + Mw(BnOH). Obtained from GPC analysis
0
14 d
The observed kobs value is
1
e
0 0 0
using H NMR. Toluene (5 mL), [M] : [Cat.] : [BnOH] = 100 : 1 : 2, [CL] = 2.0 M, at room temperature.
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Fig. 4 Comparison of the kobs of CL polymerization for various aluminum complexes.
toluene (Table 2). Table 2 reveals that all five-membered ring rate. Moreover, the five-membered ring aluminum complexes
aluminum complexes bearing thiol-Schiff base ligands exhibi- bearing thiol-Schiff base ligands displayed dramatically higher
2
ted high catalytic activity in CL ROP, and the activity was catalytic activity than those of S AlMe
2
(approximately 4- to
5
-Ph
slightly influenced by the ligand substituent. The catalytic 10-fold) and
L
AlMe
2
(approximately 7- to 19-fold).
rates for CL polymerization decreased in the following order Moreover, the reaction times (induction period) required for
1
0
ClBr
ClCl
ClH
(
L
Fig. 4 and 5): L AlMe
2
(1) > L AlMe
2 2
(2) > L AlMe (4) > forming Al benzyl alkoxide from BnOH and the dimethyl Al
(5) > L AlMe (3). This ranking reveals that the complex for all five-membered ring Al catalysts bearing thiol-
HH
ClO
AlMe
2
2
presence of a greater number of electron-withdrawing groups Schiff base ligands (0–3 min) were significantly shorter than
2
5-Ph
such as chloride and bromide resulted in a higher catalytic the induction periods of S AlMe
13 min). The results strongly suggest that the synergy of using
thiol-Schiff base ligands in a five-membered ring aluminum
2 2
(5 min) and L AlMe
(
H
OMe
system is effective. Moreover, O
AlMe (8) exhibited low catalytic activity in CL polymeriz-
ation at room temperature (conversion is 0% for O 2AlMe (6),
2
AlMe (6), O
2
AlMe (7), and
ClBr
O
2
H
OMe
ClBr
0% for O
2
AlMe (7), and 24% for O
2
AlMe after 1 h by
using the conditions listed in Table 2). This fact proved that
using thiol-Schiff bases as ligands resulted in the formation of
stable and highly catalytically active five-membered ring alumi-
num complexes (L–Al–Me
cally active five-membered ring aluminum complexes bearing
N,O-Schiff base ligands (L –Al–Me type) were obtained.
On the basis of the results, CL polymerization using
2
type). However, only less catalyti-
2
ClBr
L
AlMe (1) with the highest catalytic activity as a catalyst
2
was systematically investigated to study the polymerization
controllability (Table 3). The optimal conditions (entries 1–5,
Table 3) for the optimal catalytic activity and the optimal con-
0
ditions for the polymers were [CL] /[Al]/[BnOH] = 100 : 1 : 3
and [Al] = 10 mM in toluene 5 mL. The experimental results in
Table 3 revealed that the CL polymerization with this ratio
Fig. 5 First-order kinetic plots of ε-caprolactone polymerizations using
various Al complexes against time.
ClBr
Table 3 Results of controlled CL polymerizations using L
2
AlMe (1) as the catalyst
a
MnCal (g mol−1
b
MnNMR (g mol−1
a
MnGPC (g mol−1
c
c
Entry
[CL]/[Al]/[BnOH]
Time (min)
Conv. (%)
)
)
)
PDI
1
2
3
4
5
6
7
8
9
100 : 1 : 1
100 : 1 : 2
100 : 1 : 3
100 : 1 : 4
100 : 1 : 5
50 : 1 : 3
46
12
7
10
10
5
98
96
90
80
76
91
87
92
92
94
87
11 500
5600
3500
2400
1800
1900
2700
6300
7100
12 300
11 700
10 100
4800
5200
4200
3200
2700
3700
7600
8500
11 000
16 700
26 900
5900
4300
2900
2300
1700
2700
6100
7800
13 200
16 700
1.63
1.26
1.14
1.11
1.09
1.07
1.10
1.14
1.19
1.24
1.28
75 : 1 : 3
8
150 : 1 : 3
200 : 1 : 3
250 : 1 : 3
350 : 1 : 3
23
65
110
180
1
1
0
1
ClBr
a
1
b
Reaction conditions: Toluene (5 mL), [L AlMe
2
] = 0.02 M, at room temperature. Obtained from H NMR analysis. Calculated from the mole-
c
cular weight of monomer × [monomer]
0
/[BnOH]
0
× conversion yield + Mw(BnOH). Obtained from the GPC analysis and calibration on the basis
of the polystyrene standard. MnGPC values are obtained from GPC multiplied by 0.56.
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0
Fig. 6 Linear plot of Mn(GPC) versus [CL] × conv./[BnOH], with the PDI
indicated by blue squares (Table 3, entries 3, 6–11).
ClBr
Fig. 8 Linear plot of kobs versus [L
AlMe
2
+ 2 BnOH] for CL polymer-
ization with [CL] = 2.0 M in toluene (5 mL).
(
[Al]/[BnOH] = 1 : 3) was controllable, as confirmed by the
linear relationship between MnGPC and [M] /[BnOH] (Fig. 6)
with acceptable polydispersity indexes (PDIs, 1.07–1.28). The be described as −d[CL]/dt
0
0
=
kobs[CL], where kobs
=
1
ClBr
x
H NMR spectrum of PCL (entry 1 of Table 3) confirmed one kp[L AlMe + 2 BnOH] , and k is the propagation rate con-
2
p
ClBr
benzyl group (peaks a and b) and hydroxyl chain ends (peak stant. By plotting ln kobs against ln[L AlMe2 + 2 BnOH], x
c′) with an integral ratio of 5 : 2 : 2, suggesting that the was found to be 2.58, and the ln k value was 8.474 (M
−
1
p
−
1
ClBr
initiation occurred through a benzyl alkoxide insertion into CL min ) (Fig. 8). Polymerizing CL by using L AlMe2 (1) at
Fig. 7). The MALDI-TOF spectrum of PCL (entry 9 of Table 3) room temperature resulted in the following rate law:
(
also confirmed the presence of BnOH at the chain end of PCL
Fig. S17†).
ꢀd½CLꢁ=dt ¼ 4788 ꢂ ½CLꢁ½L AlMe þ 2 BnOHꢁ2:58:
ClBr
(
2
Kinetic study of ε-caprolactone polymerization using
According to the kinetic study results, the Al complex may
ClBr
L
2
AlMe (1)
aggregate to a new catalytic species after reacting with BnOH
HH
2
The kinetic studies were performed at room temperature to because the order of [L AlMe + 2 BnOH] was larger than
ClBr
examine the effect of the ratio of [CL]
0 2
/[L AlMe + 2 BnOH] 2. To identify the real catalyst in the polymerization process,
HH
([CL] = 2.0 M in 5 mL of toluene) as described in Table S2 and the reaction of L AlMe (5) with 2 eq. of BnOH in CDCl was
2
3
Fig. S1.† The preliminary results indicated a first-order depen- investigated, and the results are displayed in Fig. 9. The moni-
1
dency on [CL] (Fig. S1†), and the rate of polymerization could toring of the disappearance of the NMR signals of the two
1
Fig. 7 H NMR spectrum of PCL (entry 3, Table 3).
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1
HH
HH
HH
2 2 3
Fig. 9 The NMR spectra of (A) L -H, (B) L AlMe (5), (C) BnOH, and (D) a mixture of L AlMe (5) and BnOH (1 : 2) in CDCl .
HH
methyl groups from L AlMe2 (5) after reacting with benzyl Schiff base ligands in 5-membered ring aluminum catalysis
7
l,t,8,10
alcohols and consultation of related studies
benzyl alcohols replace the two methyl groups of L AlMe2
5). However, the imine group (8.82 ppm) of the Al complex
suggest that has proven to have effective synergy.
HH
(
also disappeared, and a few of the ligands appeared with Experimental section
benzyl alkoxide (5.02 ppm). The product could not be isolated
2
Standard Schlenk techniques and a N -filled glovebox were
after the purification. It is unclear so far what the real catalyst
ClBr
used throughout the isolation and handling of all the com-
pounds. Solvents, such as hexane, toluene, and THF, were pur-
ified by distillation after adding sodium metal and benzophe-
none. CH Cl was purified by distillation after adding P O . ε-
2
was in the polymerization, and why the order of [L AlMe + 2
BnOH] was 2.5.
2
2
2 5
Caprolactone was purified by distillation after adding MgSO
4
.
Conclusions
CDCl was used after adding molecular sieves. Deuterated
3
A series of five-membered ring aluminum complexes bearing chloroform, ε-caprolactone and dimethyl aluminum chloride
thiol-Schiff base ligands were synthesized, and their appli- were purchased from Acros. Benzyl alcohol, 2-aminophenol,
cation in CL polymerization was investigated. All the five-mem- 2-aminothiophenol,
bered ring aluminum complexes bearing thiol-Schiff base aldehyde, 4-chlorobenzaldehyde,
ligands exhibited dramatically higher catalytic activity than 4-methoxybenzaldehyde, and trimethylaluminum were pur-
2-amino-4-chlorobenzenethiol,
benz-
4-bromobenzaldehyde,
2
1
13
those of the six-membered ring S AlMe
2
complex (approxi- chased from Alfa Aesar. H and
C NMR spectra were
1
mately 4- to 10-fold higher) and the five-membered ring recorded on a Varian Gemini2000-200 (200 MHz for H and
5
-Ph
13
L
AlMe
2
complex (approximately 7- to 19-fold higher), and a 50 MHz for C) spectrometer with chemical shifts given in
shorter induction period compared with other types of alumi- ppm from the internal TMS or the center line of CDCl
3
.
num complexes bearing Schiff base ligands was also observed. Microanalyses were performed using a Heraeus CHN-O-RAPID
Moreover, the electron-withdrawing groups enhanced the cata- instrument. GPC measurements were performed on a Jasco
lytic activity of the Al complexes compared with the electron PU-2080 PLUS HPLC pump system equipped with a differential
donating groups. Our novel approach of integrating thiol- Jasco RI-2031 PLUS refractive index detector using THF (HPLC
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grade) as an eluent (flow rate: 1.0 mL min− , at 40 °C). The Synthesis of L AlMe
1
ClH
(4)
2
chromatographic column was a JORDI Gel DVB 103 Å, and the
calibration curve was obtained by using primary polystyrene
ClBr
A method similar to that for L AlMe
2
was used, except that
benzaldehyde was used in place of 4-bromobenzaldehyde.
Yield: 2.47 g (54%). H NMR (CDCl , 200 MHz) 8.83 (s, 3H,
CHvN), 7.84 (d, 2H, J = 8 Hz, o-H–NvCPh), 7.74 (t, 1H, J =
Hz, p-H–NvCPh), 7.63 (d, 2H, J = 8 Hz, m-H–NvCPh), 7.47
d, 1H, J = 8 Hz, o-H–Ph–Cl), 7.22–7.17 (m, 2H, Ar–H), −0.68 (s,
3 2 3
) ) ppm. C NMR (CDCl , 50 MHz) 169.20 (CvN),
46.50 (ArC–N), 139.46 (ArCS), 135.02, 133.90, 131.91, 131.00,
H
11 OMe
12
ClBr
standards to calculate Mn(GPC). O -H,
H
O
-H, and O
were prepared following literature procedures. All the
thiol-Schiff base ligands could not be isolated directly because
-
1
3
1
3
8
9
2
they were further oxidized by O to form arylbenzothiazole.
(
6
1
3
H, Al(CH
ClBr
2
Synthesis of L AlMe (1)
1
A mixture of 4-bromobenzaldehyde (2.77 g, 15 mmol) and 129.36, 128.91, 119.38 (Ar), −7.63 (Al(CH ) ) ppm. Elemental
3
2
ClH
2
-amino-4-chlorobenzenethiol (2.40 g, 15 mmol) was stirred anal. found (calcd) for L AlMe
for 18 h in CH Cl
(30 mL) at room temperature in a dry box. (4.61); C, 59.95 (59.31); H, 4.94 (4.97)%. Mp: 79 °C.
CH Cl of the solution was removed under vacuum to give a
2
: C15H15AlClNS: N, 4.55
2
2
2
2
HH
brown powder. Toluene (30 mL) was transferred as a solvent, Synthesis of L AlMe
and the solution was transferred to AlMe (8 mL, 2.0 M,
6 mmol) in toluene (10 mL) at 0 °C. The solution was stirred
(5)
A method similar to that for L AlMe
benzaldehyde was used in place of 4-bromobenzaldehyde, and
-aminothiophenol was used in place of 2-amino-4-chloroben-
2
ClBr
3
was used, except that
2
1
for 18 h. Volatile materials were removed under vacuum to give
a red powder and then hexane (50 mL) was transferred to
the suspension. The reddish orange powder was obtained after
2
1
zenethiol. Yield: 2.54 g (62%). H NMR (CDCl
s, 1H, CHvN), 7.82 (d, 2H, J = 8 Hz, o-H–NvCPh), 7.68 (t, 1H,
J = 8 Hz, p-H–NvCPh), 7.59 (d, 2H, J = 8 Hz, m-H–NvCPh),
.52 (d, 1H, J = 8 Hz, Ar–H), 7.22–6.98 (m, 3H, Ar–H), −0.69 (s,
H, Al(CH ) ) ppm. C NMR (CDCl , 50 MHz) 168.1 (CvN),
3
, 200 MHz) 8.82
(
1
filtering. Yield: 3.46 g (60%). H NMR (CDCl
3
, 200 MHz) 8.75
(
s, 1H, CHvN), 7.77 (d, 2H, J = 10 Hz, m-H-Ph-Br), 7.69 (d, 2H,
J = 10 Hz, o-H-Ph-Br), 7.47 (d, 1H, J = 8 Hz, m-H-Ph-S), 7.24 (d,
H, J = 8 Hz, o-H-Ph-S), 7.17 (s, 1H, m-H-Ph-S), −0.67 (s, 6H, Al
7
6
1
1
13
3
2
3
1
45.85 (ArC–N), 140.73 (Ar C–S), 134.45, 133.00, 132.18, 130.99,
1
3
(
(
1
CH
ArC–N), 142.79 (ArC–S), 134.65, 132.95, 132.35, 131.97, 130.45,
30.25 (C–Ar), 120.08 (ArC–Cl), −7.49 (Al(CH ppm.
14AlBrClNS:
3 2 3
) ) ppm. C NMR (CDCl , 50 MHz) 169.98 (CvN), 146.75
30.72, 130.09, 129.16, 123.73, 119.20 (Ar), −7.68 (Al(CH ) )
3 2
HH
ppm. Elemental anal. found (calcd) for L AlMe : C H AlNS:
2
15 16
3 2
) )
N, 4.96 (5.20); C, 66.99 (66.89); H, 6.37 (5.99)%. Mp: 89 °C.
ClBr
2
Elemental anal. found (calcd) for L AlMe : C15H
H
N, 3.16 (3.66); C, 46.98 (47.08); H, 4.12 (3.69)%. Mp: 240 °C. Synthesis of O 2AlMe (6)
Yield: 1.78 g (27%). Mp: 70 °C.
H
O -H (1.97 g, 10 mmol) was added to toluene (40 mL) and the
solution was transferred to AlMe (2.5 mL, 2.0 M, 5 mmol) at
°C. The solution was stirred for 3 h. Volatile materials were
ClCl
3
Synthesis of L AlMe (2)
2
0
ClBr
A method similar to that for L AlMe was used, except that removed under vacuum to give a deep red powder and
2
4
-chlorobenzaldehyde was used in place of 4-bromobenzalde- then hexane (50 mL) was transferred to the suspension. The
1
hyde. Yield: 4.47 g (88%). H NMR (CDCl
3
, 200 MHz) 8.77 (s, reddish orange powder was obtained after filtering. Yield:
1
1
H, CHvN), 7.78 (d, 2H, J = 10 Hz, m-H–NvCPh-Cl), 7.57 (d, 1.58 g (73%). H NMR (CDCl , 200 MHz) 8.90 (s, 2H, CHvN),
3
2
H, J = 8 Hz, o-H–NvCPh–Cl), 7.41 (d, 1H, J = 10 Hz, o-H-Ph- 8.30 (d, 4H, J = 8 Hz, o-NvCPh–H), 7.52–7.40 (m, 6H, Ar), 7.20
1
3
S), 7.18–7.13 (m, 2H, H–Ar), −0.69 (s, 6H, Al(CH
3
)
2
) ppm.
C
(d, 4H, J = 8 Hz, m-NvCPh–H), 6.93 (d, 2H, J = 8 Hz, o-NPh–H),
1
3
NMR (CDCl , 50 MHz) 169.53 (ArCvN), 146.75(ArC–N), 142.39 6.78 (t, 2H, J = 8 Hz, p-H–PhO), −1.25 (s, 3H, AlCH ) ppm.
C
3
3
(
ArC–S), 134.64, 133.32, 132.09, 131.63, 130.45, 130.36, 129.99, NMR (CDCl , 50 MHz) 160.03 (CvN), 158.68 (ArC–O) 136.54
3
1
3 2
20.33, 120.01 (Ar), −7.56 (Al(CH ) ) ppm. Elemental anal. (ArC–N), 133.77, 132.58, 131.83, 130.77, 128.63, 118.88, 117.52,
ClCl
found (calcd) for L AlMe : C H AlCl NS: N, 4.28 (4.14); C, 115.83 (C–Ar), −8.03 (Al(CH )) ppm. Elemental anal. found
5
2
15 14
2
3
O
2.93 (53.27); H, 4.13 (4.17)%. Mp: 78 °C.
2 27 23 2 2
(calcd) for L AlMe: C H AlN O : N, 6.62 (6.45); C, 74.25
(
74.64); H, 5.19 (5.34)%. Mp: 120 °C.
ClO
Synthesis of L AlMe (3)
2
OMe
ClBr
Synthesis of O
2AlMe (7)
A method similar to that for L AlMe
2
was used, except that
H
4
-methoxybenzaldehyde was used in place of 4-bromobenzal- A method similar to that for O AlMe (6) was used, except that
2
1
OMe
H
1
dehyde. Yield: 1.98 g (47%). H NMR (CDCl
H, CHvN), 7.82 (d, 2H, J = 8 Hz, 2H, o-H–NvCPh–OMe), 7.40 NMR (CDCl
d, 2H, J = 10 Hz, p-H–NPh–Cl), 7.13–7.04 (m, 4H, Ar–H), 3.93 Hz, o-NvCPh–H), 7.49–7.38 (m, 4H, NvCPh–H), 7.20- (t, 2H,
3
, 200 MHz) 8.63 (s,
O
-H was used in place of O -H. Yield: 2.04 g (83%).
H
1
3
, 200 MHz) 9.29 (s, 2H, CHvN), 8.87 (d, 2H, J = 8
(
(
1
3
s, 3H, OCH
3
), −0.65 (s, 6H, Al(CH
3
)
2
) ppm. C NMR (CDCl
3
,
J = 8 Hz, p-NvCPh–H), 7.03–6.89 (m, 6H, CvN–Ar–H), 6.76 (t,
), −1.33 (s, 3H,
38.92 (ArC–S), 134.02, 133.44, 129.09, 128.52, 124.50, 119.20, AlCH ) ppm. C NMR (CDCl , 50 MHz) 160.40 (CvN), 158.84
5
1
1
0 MHz) 167.91 (CvN), 165.32 (ArC–OMe), 146.79 (ArC–N), 2H, J = 8 Hz, o-H–PhO), 3.89 (s, 6H, OCH
3
1
3
3
3
14.81 (Ar), 55.79 (OCH
anal. found (calcd) for L AlMe
3
), −7.36 (Al(CH
3
)
2
) ppm. Elemental (ArC–OMe), 156.47 (ArC–O), 137.32 (ArC–N), 134.31, 132.08,
: C16H17AlClNOS: N, 4.03 130.42, 123.30, 120.87, 118.73, 117.36, 116.40, 110.67 (Ar),
ClBr
2
(4.20); C, 57.08 (57.57); H, 5.11 (5.13)%. Mp: 83 °C.
55.93 (OCH ), −7.64 (AlCH ) ppm. Elemental anal. found
3
3
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Dalton Transactions
OMe
(calcd) for O
2
AlMe: C29
H27AlN
2
O
4
: N, 5.92 (5.66); C, 70.69
3 J. Nicolas, S. Mura, D. Brambilla, N. Mackiewicz and
(70.43); H, 5.22 (5.50)%. Mp: 134 °C.
P. Couvreur, Chem. Soc. Rev., 2013, 42, 1147.
4
D. J. A. Cameron and M. P. Shaver, Chem. Soc. Rev., 2011,
0, 1761.
ClBr
Synthesis of O 2AlMe (8)
4
H
A method similar to that for O AlMe (6) was used, except that
5 (a) W. Mattanavee, O. Suwantong, S. Puthong,
T. Bunaprasert, V. P. Hoven and P. Supaphol, ACS Appl.
Mater. Interfaces, 2009, 1, 1076; (b) N. E. Zander,
J. A. Orlicki, A. M. Rawlett and T. P. Beebe Jr., ACS Appl.
Mater. Interfaces, 2012, 4, 2074; (c) S. Tarafder and S. Bose,
ACS Appl. Mater. Interfaces, 2014, 6, 9955; (d) O. Castaño,
N. Sachot, E. Xuriguera, E. Engel, J. A. Planell, J.-H. Park,
G.-Z. Jin, T.-H. Kim, J.-H. Kim and H.-W. Kim, ACS Appl.
Mater. Interfaces, 2014, 6, 7512; (e) E. A. Rainbolt,
K. E. Washington, M. C. Biewer and M. C. Stefan, Polym.
Chem., 2015, 6, 2369; (f) L. C. Palmer, C. J. Newcomb,
S. R. Kaltz, E. D. Spoerke and S. I. Stupp, Chem. Rev., 2008,
2
ClBr
H
1
O
-H was used in place of O -H. Yield: 2.11 g (64%).
NMR (CDCl , 200 MHz) 8.76 (s, 2H, CHvN), 8.14 (d, 4H, J =
Hz, o-NvCPh–Br), 7.63 (d, 4H, J = 8 Hz, m-NvCPh–Br), 7.37
s, 2H, o-N–Ph–H), 7.17 (d, 2H, J = 8 Hz, p-N–Ph–H), 6.83 (d,
H, m-N–Ph–H), −1.25 (s, 3H, AlCH ppm. Because
AlMe (8) could not dissolve well in CDCl , d -acetone,
H
3
8
(
2
O
3
)
ClBr
2
3
6
1
3
and d
Elemental
C H AlBr Cl N O : N, 3.98 (4.24); C, 49.23 (49.05); H, 3.12
6
-DMSO, the C NMR spectrum was not available.
ClBr
anal.
found
(calcd)
for
O
2
AlMe:
2
7
19
2
2 2 2
(
2.90)%. Mp: 147 °C.
General procedures for the polymerization of ε-caprolactone
108, 4754.
A typical polymerization procedure was exemplified by the syn-
6 (a) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou,
Chem. Rev., 2004, 104, 6147; (b) S. Jacobsen, P. Degee,
H. G. Fritz, P. Dubois and R. Jerome, Polym. Eng. Sci., 1999,
39, 1311.
ClBr
2
thesis of entry 1 (Table 2) using the L AlMe complex as a
1
catalyst. The polymerization conversion was analyzed by
H
NMR spectroscopic studies. Toluene (5.0 mL) was added to a
ClBr
mixture of the
0.2 mmol), and ε-caprolactone (10 mmol) at room tempera-
ture. At indicated time intervals, 0.05 mL aliquots were
L
AlMe
2
complex (0.1 mmol), BnOH
7 (a) J. Lewiński, P. Horeglad, M. Dranka and I. Justyniak,
Inorg. Chem., 2004, 43, 5789; (b) N. Nomura, T. Aoyama,
R. Ishii and T. Kondo, Macromolecules, 2005, 38, 5363;
(c) N. Iwasa, J. Liu and K. Nomura, Catal. Commun., 2008,
9, 1148; (d) J. Liu, N. Iwasa and K. Nomura, Dalton Trans.,
2008, 3978; (e) N. Iwasa, M. Fujiki and K. Nomura, J. Mol.
Catal. A: Chem., 2008, 292, 67; (f) N. Iwasa, S. Katao, J. Liu,
M. Fujiki, Y. Furukawa and K. Nomura, Organometallics,
2009, 28, 2179; (g) J. Wu, X. Pan, N. Tang and C.-C. Lin,
Eur. Polym. J., 2007, 43, 5040; (h) Z.-X. Du, J.-T. Xu, Y. Yang
and Z.-Q. Fan, J. Appl. Polym. Sci., 2007, 105, 771;
(
1
3
removed, trapped with CDCl (1 mL), and analyzed by H
NMR. After the solution was stirred for 12 min, the reaction
was quenched by adding a drop of ethanol, and the polymer
precipitated as a white solid when poured into n-hexane
(60.0 mL). The isolated white solid was dissolved in CH Cl
2 2
(5.0 mL) and then n-hexane (70.0 mL) was added to obtain a
purified crystalline solid. Yield: 0.91 g (80%).
(
i) A. Arbaoui, C. Redshaw and D. L. Hughes, Chem.
Conflicts of interest
Commun., 2008, 4717; ( j) C. Zhang and Z.-X. Wang,
J. Organomet. Chem., 2008, 693, 3151; (k) D. J. Darensbourg
and O. Karroonnirun, Organometallics, 2010, 29, 5627;
The authors declare no competing financial interest.
(
l) D. Pappalardo, L. Annunziata and C. Pellecchia,
Macromolecules, 2009, 42, 6056; (m) N. Iwasa, S. Katao,
J. Liu, M. Fujiki, Y. Furukawa and K. Nomura,
Organometallics, 2009, 28, 2179; (n) D. J. Darensbourg,
O. Karroonnirun and S. J. Wilson, Inorg. Chem., 2011, 50,
Acknowledgements
This study was supported by the Ministry of Science and
Technology of Taiwan (Grant MOST 107-2113-M-037-001) and
6
775; (o) W. Zhang, Y. Wang, W.-H. Sun, L. Wang and
C. Redshaw, Dalton Trans., 2012, 41, 11587;
p) K. Matsubara, C. Terata, H. Sekine, K. Yamatani,
Kaohsiung
Medical
University
“NSYSU-KMU
JOINT
RESEARCH PROJECT” (NSYSUKMU 107-P010). We thank the
Center for Research Resources and Development at Kaohsiung
Medical University for instrumentation and equipment
support.
(
T. Harada, K. Eda, M. Dan, Y. Koga and M. Yasuniwa,
J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 957;
(
(
q) X.-F. Yu and Z.-X. Wang, Dalton Trans., 2013, 42, 3860;
r) H.-L. Han, Y. Liu, J.-Y. Liu, K. Nomurac and Y.-S. Li,
Dalton Trans., 2013, 42, 12346; (s) M. Normand, T. Roisnel,
J.-F. Carpentier and E. Kirillov, Chem. Commun., 2013, 49,
11692; (t) S. Tabthong, T. Nanok, P. Kongsaeree,
S. Prabpaib and P. Hormnirun, Dalton Trans., 2014, 43,
1348; (u) A. Meduri, T. Fuoco, M. Lamberti, C. Pellecchia
and D. Pappalardo, Macromolecules, 2014, 47, 534–543;
(v) Z. Qu, R. Duan, X. Pang, B. Gao, X. Li, Z. Tang, X. Wang
and X. Chen, J. Polym. Sci., Part A: Polym. Chem., 2014, 52,
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