Aluminum chelates supported by β-quinolyl enolate ligands: Synthesis and ROP of ?-CL

Article abstract of DOI:10.1039/c6dt00001k

Treatment of tautomers of β-quinolyl ketone-enaminones 1a-6a with AlMe₃ afforded β-quinolyl enolato dialkylaluminium complexes LAlMe₂1b-6b (L = [(2-C₉H₆N)-CHC(R)-O-], R = CH₃ (1b), ^tBu (2b), Ph (3b), o-tolyl (4b), p-tolyl (5b), p-OMePh (6b)), respectively. 2b reacted with benzyl alcohol to generate the corresponding LAl(OBn)₂ complex 2c. Complexes 1b-6b and 2c were characterized by ¹H and ¹³C NMR spectroscopy, elemental analyses and single crystal X-ray diffraction analyses. All complexes were tested as catalyst precursors for ring-opening polymerization of ?-caprolactone (?-CL). The results indicated that LAlMe₂ (1b-6b) exhibited good activity towards the ROP of ?-CL in the presence of benzyl alcohol at 80 °C, and LAl(OBn)₂2c exhibited higher catalytic activity in the absence of alcohol than 1b-6b for the ROP of ?-CL. However, both polymerizations were less controlled. Kinetic studies showed that the polymerization reaction catalyzed by 1b-6b and 2c proceeded with first-order dependence on the monomer and took place through coordination-insertion.

Full text of DOI:10.1039/c6dt00001k

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ARTICLE
Aluminum Chelates Supported by β-Quinolyl Enolate Ligands:
Synthesis and ROP of ε-CL†
Peng Wang,a Xiaomin Hao,a Jianhua Cheng,b Jianbin Chaoc and Xia Chena*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Treatment of tautomers of β-quinolyl ketone-enaminones 1a-6a with AlMe3 afforded the aluminium β-quinolyl enolato
t
dialkylaluminium complexes LAlMe2 1b-6b (L = [(2-C9H6N)-CH=C(R)-O-] (R = CH3 (1b), Bu (2b), Ph (3b), o-Tolyl (4b), p-Tolyl
www.rsc.org/
(5b), p-OMePh (6b)), respectively. 2b reacted with benzyl alcohol to generate the corresponding LAl(OBn)2 complex 2c.
1
Complexes 1b-6b and 2c were characterized by H and 13C NMR spectroscopy, elemental analyses and single crystal X-ray
diffraction analyses. All complexes were tested as catalyst precursers for ring-opening polymerization of ε-caprolactone (ε-
CL). The results indicated that LAlMe2 (1b-6b) were good activity towards the ROP of ε-CL in the presence of benzyl alcohol
at 80 °C, and LAl(OBn)2 2c exhibited higher catalytic activity in the absence of alchol than 1b-6b for the ROP of ε-CL.
However, both polymerizations were less controlled. Kinetic studies showed the polymerization reaction catalyzed by 1b-
6b and 2c proceeded with first-order dependence on monomer and performanced through coordination-insertion.
Introduction
R1
R2
R2
R3
N
O
N
O
Biodegradable polyesters such as polycaprolactone (PCL),
polylactide (PLA) and their copolymers have attracted
considerable interest from industrial and academic research
groups.1 Although they can be produced by condensation
reactions, these polymers are effectively prepared by ring-
opening polymerization (ROP) via a coordination-insertion
mechanism initiated by metal-based complexes due to the
advantages of well-controlled molecular weight and
regularity.2 To date, a huge number of metal-based catalysts
have been developed, e.g. zinc,3 magnesium,4 aluminium,5
stannous,6 calcium,7 iron8 and rare earth,9 and their catalytic
performances have been widely investigated. Among these
metal-based ROP initiators, aluminum alkoxides have attracted
considerable attention due to the oxophilicity and Lewis
acidity of the metal center.
In recent years, aluminum β-diketiminate,5c-e Salen,5f-g
phenolate,5h-i or amidinate5j-k complexes have proved to be
efficient and excellent initiators for the ROP of cyclic
esters.5a,5b Relevant to these aluminum initiators, the
relationships between structure and activity have been
discussed in the ROP process. The steric hindrance and
electronic effect of ligand seems to be crucial in determining
N
O
R1
R1
R2
ketiminate
quinolyl enolate
(this work)
Schiff-base
Chart 1 Different types of N,O ligands.
the catalytic performances of ROP initiators.10 Among these
metal complexes, steric bulky ligands usually play a crucial
roles in chain-end controlled ROP processes, so that subtle
modifications of ligand or metal may result in great effects on
stereoselectivity.9k,10c Most of the current reports with
alkoxide-based ligands are focused on ketiminate and Schiff
base,11 surprisingly, few studies have involved the aluminium
complexes bearing quinolyl-enolate ligands where the imine
donor is part of a heteroaromatic ring system (Chart 1).
Recently, Bildstein and his co-workers have synthesized
pyridyl/chinolyl [N,O] β-ketiminato and [N,N] β-diketiminato
ligands by modifying literature procedure, which utilizes the
reaction of picolyl/quinolyl-lithium with acetonitrile or
benzonitrile followed by acidic hydrolysis.12 We have reported
the insertion reaction of lithiated picoline with nitriles gave
[N,N] β-diketiminato complexes13 and lithium quinolylamide
with nitriles afford quinolylgunidinates.14
a.School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan,
030006, China. E-mail: chenxia@sxu.edu.cn
With this mind, we decided to explore the preparation of
aluminum derivatives coordinated by bidentate quinolyl-
enolate ligands and to mediate as a catalyst in the polymeriza-
b.State Key Laboratory of Polymer Physics and Chemistry
，
Changchun Institute of
Applied Chemistry
，Chinese Academy of Sciences，Changchun 130022, China
c. Scientific Instrument Center, Shanxi University, Taiyuan, 030006, China
† Electronic Supplementary Informaꢀon (ESI) available: CCDC 1411759-1411762
and 1463852. For ESI and crystallographic data in CIF or other electronic format
See DOI: 10.1039/x0xx00000x
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Journal Name
R
nBuLi
THF
Li
RCN
THF
N
N
N
N
Li
R
R
R
H+/H2O
THF
O
N
O
N
N
O
H
H
Ketone (I)
1a-6a
Enol (II)
Enaminone (III)
R
1b R = CH3
2b R = tBu
AlMe3
toluene, 120 oC,12 h
N
O
3b R = Ph
Al
4b R = o-Tolyl
5b R = p-Tolyl
6b R = p-OMePh
1b-6b
Scheme 1 Synthesis of aluminium β-quinolyl enolates 1b-6b
changed according to the polarity of the solvent. For example,
the proligand 3a exists in CDCl3 as a mixture of 80%
enaminone (III) and 20% ketone (I) whereas in (D3C)2SO only
enaminone is present. In the solid state, both of 4a and 6a
exist in their enaminone (III) tautomer, which is consistent
with the previous report.12
Synthesis and characterization of aluminum complexes 1b-6b.
Aluminum complexes 1b-6b were obtained in good yields by
the stoichiometric reactions of AlMe3 with the corresponding
ligands under mild conditions and isolated as yellow solid from
toluene in 70-91% yield (Scheme 1). These complexes are air
and moisture sensitive, they are soluble in aromatic
hydrocarbons (benzene, toluene) except for complex 6b which
is slightly soluble in toluene but readily soluble in THF. All
Fig. 1 X-ray structure of 1b with thermal ellipsoids represented at the 30% probability
level. Hydrogen atoms are omitted for clarity.
tion of cyclic esters. In this contribution, we report the
synthesis and characterization of some novel aluminum
1
aluminum complexes were characterized by H and 13C NMR
spectroscopy and elemental analysis. Compared to the ligands
6a, the 1H NMR spectra of 1b
1a- -6b show no proton signals
complexes containing differently substituted
β-quinolyl
enolate [N,O] ligands, and develop their application for the
ROP of ε-CL.
appeared in low field regions (δ 14.95 to 15.70 ppm) and the
resonances for the protons of Al–CH3 appeared in the high-
field regions (δ -0.60 to -0.48 ppm). The 1H and 13C NMR
spectra of all the complexes 1b-6b show one resonance for the
Results and discussion
methine CH resonances of ketiminate framework at ca. 5.44-
6.19 and 93.89-100.44 ppm, which confirmed that the metal
complexes are pure.
Synthesis and Characterization
Synthesis of ligand precursors 1a-6a. As depicted in Scheme 1,
the proligands of quinolyl β-ketiminate were synthesized
according to the improved literature procedures.12 Firstly, the
addition of quinolyl-lithium with aliphatic or aromatic nitriles
yielded quinolyl azaazyl-lithiums. Then, the mixture followed
Complexes 1b, 2b, 5b and 6b were further characterized by
single crystal X-ray diffraction. Their molecular structures are
displayed in Fig. 1-4, and selected bond lengths and angles are
showed in Table 1. Complexes 1b
geometry. Each of complexes contains
metallacycle AlNCCCO, where the six-membered ring is almost
coplanar with the quinolyl group. For the structure of 1b 2b
,
2b
,
5b and 6b have a similar
a
six-membered
by acid hydrolysis gave quinolyl-enolates 1a
(68-88%). The proligands 1a 6a were slightly air-sensitive or
stable as yellow solids. Compounds 1a 6a were characterized
by NMR spectroscopy, which showed three different
tautomers: quinolyl-ketone ( ), -enol (II) and -enaminone (III).
-6a in good yields
-
,
,
-
5b and 6b, the aluminum atom is four-coordinated bearing
bidentate ligands in a distorted-tetrahedral geometry.The
biting angles of enolate ligands, N-Al-O, are in the range of
96.63(6)-97.57(5)°, smaller than regular tetrahedral bond
angle of 109.28°. The bond lengths of Al-O and Al-N in these
I
In genral, enaminones are favored over enols for 2-substituted
quinolines probably due to stabilization by hydrogen
bonding.15 The equilibria between these tautomers may be
2 | J. Name., 2012, 00, 1-3
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Table 1 Selected bond lengths (Å) and angles (°) for complexes 1b, 2b, 5b and 6b
1b
2b
5b
6b
Al(1)-O(1)
1.7814(11)
1.9682(12)
1.3086(18)
1.356(2)
97.57(5)
1.7786(15)
1.9761(16)
1.312(2)
1.358(3)
96.63(6)
1.769(2)
1.967(3)
1.309(4)
1.358(4)
97.53(11)
1.7809(12)
1.9699(13)
1.3154(18)
1.365(2)
Al(1)-N(1)
O(1)-C(11)
C(11)-C(10)
O(1)-Al(1)-N(1)
97.44(5)
.
O
N
Al
Bn
OBn
2BnOH
THF
N
O
O
O
Al
Bn
BnO
Al
N
O
2b
Scheme 2 Synthesis of aluminium benzyloxide 2c
2c
Fig. 2 X-ray structure of 2b with thermal ellipsoids represented at the 30% probability
level. Hydrogen atoms are omitted for clarity.
1.356(2)-1.1.358(3) and 1.308(2)-1.315(2) Å, being indicative of
enolate structure, which is consistent with those observed in
enolates [Al(Me2){OCR=CHP(Ph2)=N(8-C9H6N)}] [C-C = 1.365(5)
Å, C-O = 1.284(5) Å].16
Reactivity of aluminium β-quinolyl enolates toward alcohols
We attempted to prepare the aluminium benzyloxide
complexes of
derivatives are ideal catalysts for ROP of
β
-quinolyl enolates 1b-6b, because alkoxide
-CL, which are
ε
formed in situ by the reaction between an exogenous alcohol
and an enolate. Treatment of 1b, 3b-6b with 2 equivalent of
BnOH in THF solvent over room temperature, respectively,
resulted in intractable mixtures due to the poor solubility in
Fig. 3 X-ray structure of 5b with thermal ellipsoids represented at the 30% probability
level. Hydrogen atoms are omitted for clarity. Symmetry codes: (i): x, 0.5-y, z.
1
THF which were hardly purified, and no clear evidence of H
NMR spectra were obtained. However, conducting the same
reaction of 2b with BnOH, the product 2c dissolved completely
in THF (Scheme 2). 2c was isolated in 73% yield as yellow
crystals suitable for X-ray diffraction from a concentrated
toluene solution at room temperature for 2 days.
1
The H NMR spectrum of 2c in CDCl3 at room temperature
shows two sets of resonance peaks with a ratio of 1:1,
indicating the existence of dimeric species. It was mainly
identified by the appearance of two doublet resonances of the
quinolyl protons (8.98 and 9.29 ppm) and two singlet
resonances of the tertbutyl protons (1.06 and 1.33 ppm).
Other aluminium benzyloxides resulted in intractable mixtures
due to the poor solubility and no clear evidences were gained.
The X-ray structure of 2c (Fig. 5) illustrates a dimeric
behaviour with two symmetrical pentacoordinated aluminium
center bridging through the oxygen atom of the benzyl alkoxy
Fig. 4 X-ray structure of 6b with thermal ellipsoids represented at the 30% probability
level. Hydrogen atoms are omitted for clarity.
complexes are in the range of 1.769(2)-1.781(2) and 1.967(3)-
1.976(2) Å, which are comparable to those in the phenoxy-
imine aluminum complexes [Al-O (1.772(2)-1.861(2), Al-N
(1.970(2)-2.027(3) Å] reported by Carpentier5h and ketiminates group. The geometry around Al atoms are distorted triangular
[1.788(2)-1.820(2), 1.954(6)-2.044(2)] described by Huang.5c
For the enolate backbone of complexes 1b, 2b, 5b and 6b, the
C10-C11 and C11-O1 distances are all in the same ranges of
bipyramidal with an compressed axial O3-Al1-N1 169.22(6)
º
and the equatorial O2-Al1-O1, O2-Al1-O3 and O1-Al1-O3 are
115.49(7), 129.17(6) and 113.80(6)º (Table 2). The Al1ꢀO3
distances of bridging benzyloxide groups ranging from
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Table 2 Selected bond lengths (Å) and angles (°) for complexes 2c
Al(1)-O(1)
1.7682(12)
1.7362(12)
1.8236(11)
1.9141(11)
2.0487(14)
1.304(2)
Al(1)-O(2)
Al(1)-O(3)
Al(1)-O(3)i
Al(1)-N(1)
O(1)-C(11)
C(10)-C(11)
O(1)-Al(1)-O(2)
O(1)-Al(1)-O(3)
O(3)-Al(1)-N(1)
O(2)-Al(1)-O(3)
Al(1)-O(3)-Al(1)i
1.355(3)
115.49(7)
113.80(6)
169.22(6)
129.17(6)
104.23(5)
1 equiv. benzyl alcohol (Table 3, entries 1-6) and their PDIs
were in the range of 1.47-1.64. 5b was used as an example to
study the influence of temperature and the amount of
monomer and BnOH on polymerization reaction. The
polymerization reaction proceeded by 5b (in a ratio of [CL]0 :
[Al]0 : [BnOH]0 = 200 : 1 : 1) showed a high rate at 80 °C in 60
min giving 94% monomer conversion (Table 3, entry 7).
Meanwhile, at lower reaction temperature the monomer
conversion also decreased (Table 3, entries 8-10). For example,
Fig. 5 X-ray structure of 2c with thermal ellipsoids represented at the 30% probability
level. Hydrogen atoms are omitted for clarity. Symmetry codes: (i): 1ꢀx, ꢀy, 1ꢀz.
1.8236(11) to 1.9141(11) Å are longer than Al1-O1 [1.7682(12)
Å] and Al1-O2 [1.7362(12) Å], which are comparable to those
in the dimeric aluminium ketiminate (from 1.8445(15) to
1.9091(16) Å) reported by Chen.5l
at 20 ºC in 420 min the polymerization reaction was very
Ring opening polymerization of ε-caprolactone
slowly, and the monomer conversion only reached 20% (Table
3, entry 8). As many reported Al complexes, the presence of
benzyl alcohol in the system can improve the catalytic activity
and control PDI.18 In the absence of benzyl alcohol, 5b
displayed poor activity with trace conversion after 60 min at
The ring-opening polymerization of ε-CL using Al complexes
1b-6b and 2c as the initiators was conducted in different
conditions, and the results were listed in Table3. The monomer
conversion was measured by 1H NMR spectroscopy and the
polymer was characterized by GPC. According to the results,
80
ºC (Table 3, entry 11). And in the presence of 2 equiv. or 4
equiv. benzyl alcohol, 5b exhibited good catalytic activity
1b-6b showed good activity within 60 min at 80 ºC with adding
Table 3 The ROP of ε-CL catalyzed by complexes 1b-6b and 2c
Entry
1
Complex
1b
2b
3b
4b
5b
6b
5b
5b
5b
5b
5b
5b
5b
5b
5b
5b
5b
2c
CL:Al:BnOH
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
200:1:1
200:1:1
200:1:1
200:1:1
100:1:0
100:1:2
100:1:4
200:1:4
400:1:4
800:1:4
800:1:8
100:1:0
200:1:0
400:1:0
T/ºC
Time/min
Conv.b(%)
96
Mn,calcc (10-4)
1.11
Mn,GPCd(10-4)
0.93 (0.52)
1.09 (0.61)
0.92 (0.52)
1.04 (0.58)
1.05 (0.59)
1.10 (0.62)
2.04 (1.14)
0.66 (0.37)
1.64 (0.92)
2.24 (1.25)
PDIe
1.52
1.57
1.51
1.52
1.47
1.64
1.38
1.07
1.40
1.21
80
80
80
80
80
80
80
20
40
60
80
80
80
80
80
80
80
80
80
80
60
60
2
92
1.06
3
60
91
1.04
4
60
92
1.06
5
60
95
1.10
6
120
60
92
1.06
7
94
2.16
8
420
300
180
60
20
0.47
9
61
1.41
10
11
12
13
14
15
16
17
18
19
20
95
2.18
Trace
94
60
0.55
0.28
0.58
1.14
2.27
0.90
1.13
2.05
2.84
0.69 (0.39)
0.42 (0.24)
1.10 (0.62)
1.63 (0.91)
2.45 (1.37)
1.81 (1.01)
0.91 (0.51)
1.37 (0.77)
2.10 (1.18)
1.52
1.34
1.66
1.86
2.75
1.57
1.59
1.80
2.12
60
93
60
99
60
99
60
99
60
78
98
16
2c
16
89
2c
16
62
a Unless otherwise specified, the polymerizations were carried out in toluene. b Measured by 1H NMR spectroscopy. c Mn,calc = 114.14 × ([CL]0/([BnOH]0) × conv. (%) +
108.13 (1b-6b) or Mn,calc = 114.14 × ([CL]0/([Al]0) × conv. (%) + 108.13 (2c). d Obtained from GPC analysis in THF using polystyrene standard. Values in parentheses are
the values obtained from GPC multiplied by 0.56. e Obtained from GPC analysis.
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a
Table 4 Catalytic activity of k(obs) Determined for the ROP of ε-CL with Al complexes
Complex
[CL]0/[Al]/[BnOH]
100/1/1
kobs (min-1)
1b
0.0555(1)
2b
3b
4b
5b
6b
2c
100/1/1
100/1/1
100/1/1
100/1/1
100/1/1
100/1/0
0.0423(5)
0.0357(6)
0.0425(6)
0.0503
0.0220(3)
0.2334(9)
Fig. 7 Relationship between Ln([CL]0/[CL]) and time of the polymerization initiated by
2c at 80 °C with a ratio of [CL]0 : [Al]0 = 100 : 1 ( [CL]0 = 1 M).
aAll reactions were performed at 80 °C with [CL]0 = 1.0 M in toluene.
(Table 3, entries 12 and 13) with similar conversion to that in
the presence of 1 equiv. benzyl alcohol. When 4 equiv. BnOH
was used, with the monomer to Al ratio varying from 100 to
400, the polymerization performed smoothly to give PCL with
an increasing molecular weight, close to theoretic values. Note
that, when the ratio was over 800, the polymerization was
rapid and the polymerization system became viscous, which
resulted in the deviation of the molecular weight of the
resultant PCL from the theoretic value and the broadened PDI
(Table 3, entries 13-16). With an increase of BnOH to 8 equiv.,
Fig.8 Relationship between Mn (●, obtained from GPC analysis) or polydispersity (∎,
the monomer conversion was slower (Table 3, entry 17). The Mw/Mn) with ε-CL conversion using 2c in toluene at 80 °C with a ratio of [CL]0 : [Al]0
=
100 : 1.
molecular weight was proportional decreased with the
increment of benzyl alcohol, while the molecular weight
Accordingly, once aluminium benzyloxide 2c was thoroughly
characterized, it was also explored for its catalytic activity
toward ROP of ε-CL. Aluminum alkoxides are ideal initiators
especially for the ROP of cyclic ester, because they can mimic
the propagation species better than alkyls. The results of the
polymerization reactions are listed in Table 3, entries 18-20.
distribution remained in
a controlled range. This was
consistent with the immortal polymerization initiated by
aluminum complexes based on pyridine substituted alcohols
and phenoxy-thioether aluminum complexes.19
The results of methyl groups complexes 1b-6b in Table 3,
entries 1-16, revealed the Mn,GPC of these polymers
proportionally were smaller than Mn,calc. and their
polydispersity index (PDI) was larger (1.47-1.64), which
indicated that the Al/BnOH system may contain multiple active
The reactions were conducted in toluene at 80 ºC, without the
addition of a cocatalyst. 2c was found to be highly active and
polymerized monomer ε-CL with 98% conversion within 16
minutes, when the [CL]/[Al] ratio was 100/1 (Table 3, entry 18).
species, such as LAl(Me)OBn and LAl(OBn)2, or include side Even when the ratio of [CL]0 : [Al]0 was increased to 400 : 1,
the polymerization can be completed up to 62% in 16 min at
80 C (Table 3, entry 20).
In order to investigate the process of the polymerization
reactions such as transesterification.
º
catalyzed by these complexes, the kinetic studies of ε-CL poly-
merization initiated by 1b-6b and were carried out in the
presence of BnOH at 80 ºC ([CL]0 : [Al]0 : [BnOH]0 = 100 : 1 : 1)
(Fig. 6). The catalytic activity of k(obs) determined for the ROP of
ε-CL with Al complexes were listed in Table 4. From the results,
we can obtain the activity order is 1b
> 5b > 4b > 2b > 3b > 6b.
The result demonstrated that the catalytic activity of k(obs)
was influenced by electronic effect. The ligands with electron
withdrawing groups, such as 6b, can decrease the catalytic
activity.
In addition, the kinetic study of ε-CL poly-merization
initiated by 2c was carried out in the presence of BnOH at 80
°C ([CL]0 : [Al]0 = 100 : 1). A good liner relationship between
Ln([CL]0/[CL]) and time were showed in Fig. 7, which proved
that the polymerization was carried out with first-order
dependency on the monomer concentration. The
Fig. 6 Relationship between Ln([CL]0/[CL]) and time of the polymerization initiated by
1b-6b in the presence of BnOH in toluene at 80 °C with a ratio of [CL]0 : [Al]0 : [BnOH]0 =
100 : 1 : 1 ( [CL]0 = 1 M).
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Journal Name
within benzyl alcohol followed first-order kinetics. End group
analyses of the polymer indicated polymerization was
performed in accordance with
a classical coordination-
insertion mechanism. However, the methyl groups of
aluminium complexes 1b-6b are not real initiators. The real
catalytic species, LAl(Me)OBn and LAl(OBn)2 were formed by
the reaction of LAlMe2 and BnOH. Dimeric benzyloxide
LAl(OBn)2 2c was isolated, which has higher activity toward
ROP of ε-CL than methyl Al complexes. All the polymerizations
of ε-CL using 1b-6b system or 2c were accompanied by a
certain degree of side reaction such as transesterification.
Although we could not find an efficient catalyst in a living
polymerization fashion, these results clearly indicated that the
benzyloxyl group plays an essential role in the polymerization.
Further studies are required to understand the dinuclear
benzyloxide [LAl(OBn)2]2 complexes with bridging benzyloxide
and terminal benzyloxide to Al, resulting in the ability to
initiate ROP.
Fig. 9 The 1H NMR spectrum of PCL initiated by 5b/BnOH in the ratio of [CL]0 : [Al]0
:
[BnOH]0 = 40 : 1 : 1 ([CL]0 = 1 M) in toluene at 60 °C for 60 min.
corresponding observed rate constant was kobs =
0.2334(9) min-
1
(Table 4). This value of kobs is significantly larger than those of
1b-6b. The molecular weights versus conversation also showed
a good liner relationship, however, the PDIs became broad
after certain monomer conversions [e.g. PDI = 1.12 (conversion
35%), 1.18 (conversion 58%), 1.33 (conversion 83%), 1.52
(conversion 93%), 1.59 (conversion 98%), Fig. 8, Table S2].
Similar results were observed in the ROPs of ε-CL using the Al
complexes 1b-6b (Figs. S15-17). The results suggested that a
certain degree of transesterfication accompanied the
propagation in the ROPs under these conditions.
Experimental
General remarks
All the reactions were carried out under nitrogen, using
standard Schlenk techniques. All the solvents (toluene and THF)
were dried and distilled from sodium, stored with 4
Å
molecular sieves. AlMe3 (1.0 M solution in toluene) and ε-
Caprolactone were purchased from Alfa Aesar . ε-Caprolactone
was dried by CaH2 for 48 h, and then distilled under reduced
pressure. 1H and 13C NMR spectra were recorded on Bruker
To understand the polymerization mechanism of ε-CL by
these aluminium complexes 1b-6b, PCL-40 was produced from
the polymerization reaction initiated by 5b/BnOH ([CL]0 : [Al]0 :
[BnOH]0 = 40 : 1 : 1) at 60 ºC for 60 min. And the end group DRX 300 and 600 instruments. Elemental analyses were
1
analysis of the PCL was determined by H NMR spectrum (Fig.
9). As shown in the result, the peak ratio of PhCH2O (Hb) and
CH2OH (Hg) was close to 1. All the results implied benzyloxy
group and hydroxymethyl group were located at the ends of
PCL and as many literatures reported the polymerization
reaction may initiate through coordination-insertion
menchanism.16,18a,20
performed with a Vario EL–III instrument. The molecular
weight and PDI of the polymers were measured by a TOSOH
HLC8220 GPC at 40 °C using THF as eluent (0.35 mL/min),
calibrated by polystyrene standards. Ligands 1a-6a were
prepared according to the published methods.12
Synthesis of Complexes
In addition, we also used 1b and 5b to initiate the ROP of
rac-lactide: the result showed no polymer was produced with
Synthesis of LAlMe2 (1b). To a stirred solution of AlMe3 (2.1
mL of a 1.0 M solution in toluene, 2.1 mmol) was added 1a
(0.371 g, 2.0 mmol) in toluene (10 mL). The yellow solution
was heated at 120 °C overnight. And after that the resulting
solution was concentrated and gave pale yellow crystals (0.434
g, 90%). Anal. Calcd. for C14H16AlNO: C, 69.70; H, 6.68; N, 5.81.
or without adding BnOH after 24 h at 80 ºC, less than 40%
monomer conversions were achieved within 72 h at 80
did not make further studies.
ºC. We
1
Found: C, 69.74; H, 6.53; N, 5.74. H NMR (CDCl3, 300 MHz): δ
Conclusions
8.12 (d, J = 8.7 Hz, 1H, ArH), 7.96 (d, J = 8.7 Hz, 1H, ArH), 7.67
(t, J = 7.8 Hz, 2H, ArH), 7.43 (t, J = 7.5 Hz, 1H, ArH), 6.95 (d, J =
8.7 Hz, 1H, ArH), 5.44 (s, 1H, -CH), 2.10 (s, 3H, -CH 3), -0.56 (s,
6H, AlCH3). 13C NMR (CDCl3, 75 MHz): δ 175.58, 158.45, 143.03,
139.38, 130.94, 128.45, 125.80, 125.18, 123.22, 122.47, 98.38,
22.58, -8.46.
A series of four coordinated Al quinolyl-enolate complexes
were synthesized and characterized by NMR spectroscopy and
element analysis. The structure of 1b, 2b, 5b, 6b and 2c were
confirmed by X-ray crystallography. All the complexes showed
good activity towards the ROP of ε-CL in the presence of
benzyl alcohol at high temperature. Higher polymerization
activity was achieved by the complexes with weaker electron-
donating substituents at the para-position of aryl ring.
Furthermore, the kinetic studies of 1b-6b and 2c revealed that
polymerization reactions initiated by these Al complexes
Synthesis of LAlMe2 (2b). Complex 2b was synthesized using
the same procedure as for 1b. Reaction of 2a (0.455 g, 2 mmol)
with AlMe3 (2.1 mL of a 1.0 M solution in toluene, 2.1 mmol) in
toluene (10 mL) yielded pale yellow crystals (0.482 g, 85%).
6 | J. Name., 2012, 00, 1-3
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Anal. Calcd. for C17H22AlNO: C, 72.06; H, 7.83; N, 4.94. Found: (CDCl3, 75 MHz): δ 169.74, 161.77, 158.71, 143.24, 139.00,
C, 72.01; H, 7.93; N, 5.01. 1H NMR (CDCl3, 300 MHz): δ 8.10 (d, 130.94, 130.40, 128.60, 128.43, 125.87, 125.12, 124.20,
J = 8.4 Hz, 1H, ArH), 7.95(d, J = 9.0 Hz, 1H, ArH), 7.63-7.68 (m, 122.48, 113.76, 95.32, 55.53, -8.39.
2H, ArH), 7.41 (t, J = 7.5 Hz, 1H, ArH), 7.02 (d, J = 8.7 Hz, 1H,
ArH), 5.54 (s, 1H, -CH), 1.23 (s, 9H, -(CH3)3), -0.60 (s, 6H, AlCH3).
13C NMR (CDCl3, 75 MHz): δ 185.61, 159.23, 140.08, 138.98,
130.80, 128.34, 125.79, 125.06, 124.04, 122.57, 93.89, 38.93,
28.26, -8.66.
Synthesis of [LAl(μ-OBn)2]2 (2c). To a stirred solution of 2b
(0.567 g, 2.0 mmol) in THF (10 mL) was added benzyl alcohol
(0.42 mL, 4.0 mmol), and the mixture was stirred at room
temperature for 24 h. Then the volatile materials were
removed under vacuum and the residues were redissolved in
hot toluene (10 mL). The solution was cooled to temperature
affording a yellow crystalline solid after 2 d (0.683 g, 73%).
Anal. Calcd. for C58H60Al2N2O6: C, 74.50; H, 6.47; N, 3.00.
Synthesis of LAlMe2 (3b). Complex 3b was synthesized using
the same procedure as for 1b. Reaction of 3a (0.495 g, 2 mmol)
with AlMe3 (2.1 mL of a 1.0 M solution in toluene, 2.1 mmol) in
toluene (10 mL) yielded yellow solid (0.516 g, 85%). Anal.
Calcd. for C19H18AlNO: C, 75.23; H, 5.98; N, 4.62. Found: C,
75.11; H, 6.13; N, 4.71. 1H NMR (CDCl3, 300 MHz): δ 8.18 (d, J =
8.7 Hz, 1H, ArH), 8.04 (d, J = 8.7 Hz, 1H, ArH), 7.95-7.96 (m, 2H,
ArH), 7.71 (t , J = 8.1 Hz, 2H, ArH), 7.44-7.49 (m, 4H, ArH), 7.18
1
Found: C, 74.59; H, 6.48; N, 2.95. H NMR (CDCl3, 600 MHz): δ
9.32 (d, J = 9.0 Hz, 1H, ArH), 9.01 (d, J = 8.4 Hz, 1H, ArH), 7.77-
6.58 (m, 30H, ArH), 5.12-5.09 (m, 3H, CH2 and -CH), 5.05 (s, 1H,
-CH), 4.39 (d, J = 13.8, 1H, CH2), 4.34 (d, J = 13.8, 1H, CH2),
4.12-3.98 (m, 4H, CH2), 1.36 (s, 9H, -(CH3)3), 1.08 (s, 9H, -(CH3)3).
13C NMR (CDCl3, 150 MHz): δ 182.36, 182.10, 159.42, 159.18,
146.98, 146.02, 143.47, 143.34, 142.13, 142.02, 137.68, 137.30,
131.11, 130.27, 127.82, 127.74, 127.65, 127.57, 127.50, 127.32,
126.61, 126.48, 126.07, 125.93, 125.84, 125.71, 125.57, 125.46,
125.37, 125.22, 125.16, 124.92, 124.68, 124.61, 124.45, 124.13,
95.22, 94.84, 65.83, 65.60, 64.78, 64.46, 38.69, 38.57, 28.37,
28.24.
(d, J =9.0 Hz, 1H, ArH), 6.19 (s, 1H, -CH), -0.51 (s, 6H, AlCH3). 13
C
NMR (CDCl3, 75 MHz): δ 169.67, 158.61, 143.14, 139.37,
137.92, 131.11, 130.50, 128.46, 126.84, 126.06, 125.44,
124.17, 122.65, 96.49, -8.33.
Synthesis of LAlMe2 (4b). Complex 4b was synthesized using
the same procedure as for 1b. Reaction of 4a (0.522 g, 2 mmol)
with AlMe3 (2.1 mL of a 1.0 M solution in toluene, 2.1 mmol) in
toluene (10 mL) yielded yellow solid (0.539 g, 86%). Anal.
Calcd. for C20H20AlNO: C, 75.69; H, 6.35; N, 4.41. Found: C,
75.60; H, 6.53; N, 4.41. 1H NMR (CDCl3, 300 MHz): δ 8.22 (d, J =
7.2 Hz, 1H, ArH), 8.08 (d, J = 8.1 Hz, 1H, ArH), 7.72-7.77 (m, 2H,
ArH), 7.48-7.53 (m, 2H, ArH), 7.24-7.30 (m, 3H, ArH)), 7.13 (d, J
= 7.2 Hz, 1H, ArH), 5.79 (s, 1H, -CH), 2.55 (s, 3H, -CH3Ph), -0.48
(s, 6H, AlCH3). 13C NMR (CDCl3, 75 MHz): δ 174.36, 158.57,
143.12, 139.54, 136.77, 131.15, 129.34, 128.52, 128.18,
126.13, 125.67, 125.51, 123.90, 122.72, 100.44, 20.62, -8.48.
Synthesis of LAlMe2 (5b). Complex 5b was synthesized using
the same procedure as for 1b. Reaction of 5a (0.522 g, 2 mmol)
with AlMe3 (2.1 mL of a 1.0 M solution in toluene, 2.1 mmol) in
toluene (10 mL) yielded yellow crystals (0.578 g, 91%). Anal.
X-ray crystallography
Diffraction data of 1b, 2b, 5b, 6b and 2c were collected on a
Bruker Smart Apex CCD diffractometer with Mo Kα radiation
(λ=0.71073 Å). A total of N reflections were collected by using
ω scan mode. The structures were solved by direct methods
(SHELXS-97)21 and refined against F2 by full-matrix least-
squares using SHELXL-97.22 All non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen
atoms were placed in calculated positions. Crystal data and
experimental details of all complexes were showed in Table 5.
Polymerization of ε-CL catalyzed by complexes 1b-6b
Complex 5b was used as an example of typical polymerization
procedure by in the presence of 1 equiv. BnOH (Table 2, entry
5). In a Schlenk tube, complex 5b (0.016 g, 0.05 mmol) was
dissolved in 4.5 mL toluene and an equiv. benzyl alcohol (0.5
mL, 0.1 M in toluene, 0.05 mmol) was added. The mixture was
stirred for 30 min at 80 °C. Then ε-CL (0.571 g, 5 mmol) was
added with stirring. After required time (60 min), several drops
of glacial acetic were added to quench the polymerization
reaction and the resulting solution was poured into MeOH
(100 mL) with stirring 12 h. The PCL was procured through
filtration and dried under vacuum.
Calcd. for C19H18AlNO: C, 75.69; H, 6.35; N, 4.41. Found: C,
1
75.68; H, 6.38; N, 4.53. H NMR (CDCl3, 300 MHz)
： δ 8.17 (d,
J= 8.7 Hz, 1H, ArH), 8.01 (d, J = 8.7 Hz, 1H, ArH), 7.86 (d, J = 7.8
Hz, 2H, ArH), 7.70 (t , J = 7.8 Hz, 2H, ArH), 7.45 (t , J = 7.5 Hz,
1H, ArH), 7.24 (d, J = 8.1 Hz, 2H, ArH), 7.14 (d, J = 8.7 Hz, 1H,
ArH), 6.16 (s, 1H, -CH), 2.41 (s, 3H, -CH3Ph), -0.52 (s, 6H,
AlCH3). 13C NMR (CDCl3, 75 MHz): δ 169.95, 158.68, 143.21,
140.88, 139.18, 135.11, 131.02, 129.16, 128.44, 126.84,
125.95, 125.27, 124.20, 122.58, 95.95, 21.63, -8.35.
Synthesis of LAlMe2 (6b). Complex 6b was synthesized using
the same procedure as for 1b. Reaction of 6a (0.555 g, 2 mmol)
with AlMe3 (2.1 mL of a 1.0 M solution in toluene, 2.1 mmol) in
toluene (10 mL) yielded yellow solid and recrystallized from
THF (0.467 g, 70%). Anal. Calcd. for C20H20AlNO2: C, 72.06; H,
6.05; N, 4.20. Found: C, 71.98; H, 6.18; N, 4.23. 1H NMR (CDCl3,
300 MHz): δ 8.16 (d, J = 8.1 Hz, 1H, ArH), 7.91-7.98 (m, 3H,
ArH), 7.67-7.70 (m, 2H, ArH), 7.42 (t, J = 7.2 Hz, 1H, ArH), 7.10
(d, J = 8.1 Hz, 1H, ArH), 6.94 (d, J = 8.4 Hz, 2H, ArH), 6.10 (s, 1H,
-CH), 3.86 (s, 3H, -OCH3Ph), -0.51 (s, 6H, AlCH3). 13C NMR
Acknowledgements
We thank the National Natural Science Foundation of China
(grant no. 21072120), Shanxi Scholarship Council of China
(2013010), Shanxi International Science and Technology
Cooperation Program (2015081050) and Open Research Fund
of State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Table 5 Details of the X-ray structure determination of complexes 1b, 2b, 5b, 6b and 2c
Complex
Formula
Mw(gmol-1)
T(K)
1b
2b
5b
6b
2c
C
14H16AlNO
C17H22AlNO
283.34
200(2) K
Monoclinic
P21/n
C20H20AlNO
317.35
200(2) K
Orthorhombic
Pnma
C20H20AlNO2
333.35
200(2) K
Monoclinic,
P21/n
C58H60Al2N2O6
935.04
299(2) K
Monoclinic
P21/n
241.26
200(2) K
Monoclinic
P21/n
Crystal system
Space group
a(Å)
7.0753(3)
15.6291(6)
11.8798(4)
90
8.4914(17)
13.955(3)
14.174(3)
90
19.546(4)
7.1870(14)
12.178(2)
90
10.9124(4)
11.7506(4)
13.8700(5)
90
10.5275(4)
18.9855(8)
13.0549(5)
90
b(Å)
c(Å)
α(°)
β(°)
90.9470(10)
90
101.02(3)
90
90
99.8360(10)
90
103.3240(10)
90
γ(°)
V(Å3)
90
1313.50(9)
4
1648.6(6)
4
1710.8(6)
4
1752.37(11)
4
2539.05(17)
2
Z
Dcalcd(gcm–3
)
1.220
1.142
1.232
1.264
1.223
μ (mm–1
)
0.138
0.119
0.122
0.127
0.110
F(000)
512
608
672
704
992
Reflections collected
9697
27725
2915
9283
12522
18027
Independent(Rint)reflections
2328
1652
3089
4485
(0.0275)
1.044
(0.0346)
1.028
(0.0342)
1.056
(0.0264)
1.028
(0.0292)
1.038
Goodness of fit on F2
Final R indices [I>2σ(I)]
R1
0.0357,
0.0977
0.0418,
0.1338
0.0563,
0.1675
0.0392,
0.1249
0.0417
0.1005
wR2
R indices (all data)
R1
0.0385,
0.1011
0.0559,
0.1576
0.0614,
0.1778
0.0428,
0.1344
0.0543
0.1095
0.231 and
-0.270
wR2
Largest diff. peak and hole [e Å−3
]
0.234 and
-0.271
0.231 and
-0.330
0.324 and
-0.416
0.253 and
-0.353
4
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DOI: 10.1039/C6DT00001K
Graphic abstract
The ROP towards ε-CL initiated by six aluminum β-quinolyl-enolates and their benzyloxids was
reported.