A series of aluminium complexes based on a β-diketiminate ligand: Synthesis, structures and their application to ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.poly.2019.03.054

A series of aluminum complexes based on a β-diketiminate ligand HL (HL = N{-4-[(2,6-diisopropyl-phenylamino)pent-3-en-2-ylidene]-6-methyl}pyridin-2- amine) have been prepared and characterized spectroscopically and structurally. Reactions of the HL ligand

Full text of DOI:10.1016/j.poly.2019.03.054

Accepted Manuscript
A series of aluminium complexes based on a β-diketiminate ligand: Synthesis,
structures and their application to ring-opening polymerization of ε-caprolac-
tone
Huifen Pan, Yafei Li, Yalan Wang, Yu Ge, Shengwang Xia, Yahong Li
PII:
S0277-5387(19)30234-7
https://doi.org/10.1016/j.poly.2019.03.054
POLY 13855
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 January 2019
8 March 2019
27 March 2019
Please cite this article as: H. Pan, Y. Li, Y. Wang, Y. Ge, S. Xia, Y. Li, A series of aluminium complexes based on
a β-diketiminate ligand: Synthesis, structures and their application to ring-opening polymerization of ε-caprolactone,
Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.03.054
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A series of aluminium complexes based on a β-diketiminate
ligand: Synthesis, structures and their application to
ring-opening polymerization of ε-caprolactone
Huifen Pan, Yafei Li, Yalan Wang, Yu Ge, Shengwang Xia, Yahong Li*
College of Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou 215123
*Corresponding author. Tel: +86-512-65880323, Fax: +86-512-65880089, E-mail
address: liyahong@suda.edu.cn (Y. H. Li)
Abstract: A series of aluminum complexes based on a β-diketiminate ligand HL (HL
=
N{-4-[(2,6-diisopropyl-phenylamino)pent-3-en-2-ylidene]-6-methyl}pyridin-2-
amine) have been prepared and characterized spectroscopically and structurally.
Reactions of the HL ligand with one equivalent of AlMe3, AlEt3 and AlEt2Cl,
respectively, in toluene, gave complexes [LAlMe2] (1), [LAlEt2] (2) and [LAlEtCl] (3)
in high yields. Complexes 1 and 2 contain four-coordinated mononuclear aluminum
center. The aluminum center of 3 is five-coordinated. Complexes 1−3 were used to
catalyze the ring-opening polymerization of ε-caprolactone. Complexes 1−3 are very
rare cases of aluminum alkyl compounds supported by β-diketiminate ligands, which
show moderate activity toward the ring-opening polymerization (ROP) of
ε-caprolactone (CL) in the absence of benzyl alcohol. However, in the presence of
benzyl alcohol, the polymerization rate is remarkably accelerated and the conversions
are greatly improved. The monomer conversion and number average molecular
weight of poly(ε-caprolactone) (PCL) catalyzed by 1−3 shows a linear relationship.
This result indicates that the polymerizations mediated by 1−3 exhibited controllable
manner.
Keywords: β-Diketiminate ligand • Aluminum complexes • ε-Caprolactone •
Ring-opening polymerization • Catalyze
1. Introduction

The desire to alleviate the environmental problem caused by the petrochemical
plastics, which could not decompose naturally within a short time, has led to
sustainable efforts to explore ecological friendly materials [1-2]. Biodegradable
poly(ε-caprolactone) (PCL) and poly(lactide) (PLA) were developed to address this
pollution challenge. Poly(ε-caprolactone) (PCL) and poly(lactide) (PLA) are very
popular polyesters due to their bio-degradable, bio-compatible, and permeable
properties and broad applications in various fields [3-10]. Ring-opening
polymerization (ROP) of ε-caprolactone and lactide mediated by metal catalysts has
proven to be a most promising approach to produce PCL and PLA. The key issue for
designing catalysts for ring-opening polymerization (ROP) of ε-caprolactone depends
on the choice of well-defined ligands and the employment of appropriate metal ions.
A number of metal complexes, including K [11-12], Zn [13-16], Mg [17-20], Ca
[21-23], Al [24-27], In [28-30], Sn [31-32], group IV metals [33-36], and rare earth
metals [37-40] have been utilized as catalysts. Aluminum compounds have attracted
intense attention over the past 20 years due to their strong Lewis acidity, easy
availability and low toxicity. Ligand design is of vital importance for the catalysts
because their catalytic activity is strongly affected by steric and electronic properties
of the ligands. A plethora of organic moieties, e.g. O,O- [41,42], N,O- [43-46], N,N-
[47-49], pyrrolide type-ligands [50-51], etc., have been employed as the ligands for
the polymerization catalysts. Among these ligand skeletons, β-diketiminates have
proved to be appealing ligands which allow for adjusting the steric features by
varying the amide moieties. Currently, the reported β-diketiminate ligands mainly
focus on the symmetric N-substituent moieties with a C2 symmetry [52-59], whereas
the unsymmetrical ligand systems possessing adjustable steric and electronic features
may coordinate with metal ions to generate the metal complexes with better catalytic
activity [60-62]. Literature survey results indicate that aluminum complexes
supported by unsymmetrical β-diketiminate ligands have rarely been reported [60-62].
With these considerations in mind, we designed an unsymmetrical β-diketiminate
ligand HL (HL = N{-4-[(2,6-diisopropyl-phenylamino)pent-3-en-2-ylidene]-6-methyl}
pyridin-2-amine) and prepared three aluminum complexes [LAlMe2] (1), [LAlEt2] (2)

and [LAlEtCl] (3). Herein, we report the syntheses and structures of 1−3. Their
catalytic properties toward ring-opening polymerization of ε-caprolactone have also
been presented.
2. Results and discussion
2.1. Synthesis and characterization
The ligand HL (HL = N{-4-[(2,6-diisopropylphenylamino)pent-3-en-2-ylidene]
-6-methyl}pyridin-2-amine) was prepared by the condensation reaction of
pentane-2,4-dione with 2,6-diisopropylaniline and 6-methyl-2-aminopyridine. Firstly,
the condensation reaction between pentane-2,4-dione with 2,6-diisopropylaniline gave
4-((2,6-diisopropylphenyl)imino)pentan-2-one. Secondly, the reaction between
4-((2,6-diisopropylphenyl)imino)pentan-2-one
and
6-methyl-2-aminopyridine
generated the ligand N{-4-[(2,6-diisopropylphenylamino)pent-3-en-2-ylidene]
-6-methyl}pyridin-2-amine (HL) as a yellow solid. Reactions of the ligand (HL) with
one equivalent of AlMe3, AlEt3 and AlEt2Cl, respectively, in toluene, gave complexes
[LAlMe2] (1), [LAlEt2] (2) and [LAlEtCl] (3) in high yields (Scheme 1).
N
Al
N
AlMe3
Tol
N
Me
Me
1
N
Al
Et Et
N
AlEt3
Tol
N
HN
N
N
2
N
Al
Et Cl
N
AlEt2Cl
Tol
N
3
Scheme 1. Synthesis of aluminum complexes 1−3.
The structures of the HL ligand and complexes 1−3 have been characterized by 1H
and 13C NMR spectra with an Agilent-600 spectrometer. The crystals of the HL ligand

and 1−3 were grown in toluene for two weeks. The structures of the HL ligand and the
aluminum complexes 1−3 were determined by X-ray diffraction analysis and are
shown in Figs. 1−4. The crystallographic data of the HL ligand and complexes 1−3
are summarized in Table 1.
Table 1 Crystallographic data of the HL ligand and complexes 1−3.
HL
1
2
3
Empirical formula
C23H31N3
349.51
120.02
monoclinic
P21/c
C25H36AlN3
405.55
C27H40AlN3
433.60
296.15
monoclinic
P21/c
C25H35AlClN3
439.99
296.15
monoclinic
P21/c
Formula weight [g mol–1]
Temperature [K]
Crystal system
Space group
a/Å
296.15
triclinic
P-1
18.2426(8)
10.7017(5)
21.5388(8)
90
10.4738(11)
10.4887(11)
11.8208(12)
82.597(3)
87.367(3)
71.679(3)
1222.5(2)
2
14.1775(9)
9.2046(5)
20.5039(12)
90
14.1635(16)
10.9365(12)
16.3706(19)
90
b/Å
c/Å
α/°
β/°
95.264(1)
90
100.330(2)
90
101.274(4)
90
γ/°
Volume/Å3
4187.2(3)
4
2632.4(3)
4
2486.9(5)
4
Z
ρcalcg/cm3
μ/mm-1
F(000)
1.109
1.102
1.094
1.175
0.065
0.098
0.095
0.205
1520.0
440.0
944.0
944.0
Crystal size/mm3
0.5 × 0.5 × 0.2 0.6 × 0.5 × 0.4 0.5 × 0.4 × 0.2 0.6 × 0.6 × 0.5
Radiation
Mo-Kα
Mo-Kα
Mo-Kα
Mo-Kα
(λ = 0.71073)
4.23
(λ = 0.71073)
4.822
(λ = 0.71073)
4.54
(λ = 0.71073)
4.74
2Θ range for data
collection/°
to 55.014
-23 ≤ h ≤ 23,
-13 ≤ k ≤ 13,
-26 ≤ l ≤ 27
61198
to 55.186
-13 ≤ h ≤ 13,
-13 ≤ k ≤ 13,
-15 ≤ l ≤ 15
46377
to 55.114
to 55.122
-18 ≤ h ≤ 18,
-14 ≤ k ≤ 14,
-21 ≤ l ≤ 21
59787
Index ranges
-18 ≤ h ≤ 18,
-11 ≤ k ≤ 11,
-25 ≤ l ≤ 26
48090
Reflections collected
Independent reflections
9536
5615
6067
5727
Rint = 0.0751
Rint = 0.0297
5615/0/271
1.025
Rint = 0.0830
6067/0/289
1.045
Rint = 0.0607
5727/0/279
1.051
Data/restraints/parameters 9536/0/483
Goodness-of-fit on F2
Final R indexes
[I>=2σ (I)]
1.101
R1 = 0.0541,
wR2 = 0.1452
R1 = 0.0848,
wR2 = 0.1795
0.53/-0.48
R1 = 0.0414,
wR2 = 0.1184
R1 = 0.0483,
wR2 = 0.1238
0.31/-0.24
R1 = 0.0659,
wR2 = 0.1310
R1 = 0.1094,
wR2 = 0.1465
0.28/-0.28
R1 = 0.0512,
wR2 = 0.1439
R1 = 0.0636,
wR2 = 0.1514
0.68/-0.43
Final R indexes
[all data]
Largest diff.
peak/hole / e Å-3

Fig. 1. ORTEP drawing of the ligand HL with thermal ellipsoids drawn at 30%
probability level. Hydrogen atoms are omitted for clarity.
(a)
(b)
Fig. 2. (a) ORTEP drawing of 1 with thermal ellipsoids drawn at 30% probability
level. Hydrogen atoms are omitted for clarity. (b) Coordination polyhedron of AlIII.
(a)
(b)
Fig. 3. (a) ORTEP drawing of 2 with thermal ellipsoids drawn at 30% probability
level. Hydrogen atoms are omitted for clarity. (b) Coordination polyhedron of AlIII.

(a)
(b)
Fig. 4. (a) ORTEP drawing of 3 with thermal ellipsoids drawn at 30% probability
level. Hydrogen atoms are omitted for clarity. (b) Coordination polyhedron of AlIII.
The structure of 1 and the coordination polyhedron of AlIII ion are illustrated in Fig.
2. Complex 1 exists as twisted tetrahedral structure with the central AlIII ion being
surrounded by two nitrogen atoms of chelating β-diketiminate and two methyl groups.
The nitrogen atom of pyridine ring does not involve in the coordination. The angle of
N1–Al–N2 (89.42(4)°) is smaller than the regular tetrahedral bond angle of 109.28°.
The average Al–C and Al–N bond lengths are 1.966(5) and 1.963(7) Å, respectively,
which are similar to those observed in the aluminum complexes stabilized by
β-diketiminate ligands [63].
Complex 2 crystallizes in the monoclinic space P21/c and the coordination
polyhedron of AlIII (Fig. 3 (b)) ion is similar to that of 1. Complex 2 displays twisted
tetrahedral structure with the central AlIII ion being coordinated by two nitrogen atoms
of chelating β-diketiminate and two ethyl groups. Wherein the nitrogen atom of
pyridine doesn’t participate in the coordination either. The angle of N1–Al–N2 of 2 is
91.81(8)° and it is larger than the angle of N1–Al–N2 of 1. The average Al–C and
Al–N bond lengths of 2 are 1.967(2) and 1.938(8) Å.
Complex 3 exhibits five-coordinated trigonal bipyramidal structure, with the central
AlIII ion being surrounded by two nitrogen atoms of β-diketiminate motif, one pyridyl
nitrogen atom, one carbon atom of ethyl group, and one chlorine atom. The angle of
N3–Al–N2 of 3 (88.69(7)°) is smaller than those of 1 (89.42(4)°) and 2 (91.81(8)°).

The average Al–C and Al–N bond lengths are 2.0022(18) and 1.969(8) Å, respectively,
which are longer than those observed in complexes 1 and 2. The Al–Npyridy bond
length is 2.1745(18) Å and the Al–Cl bond length is 2.1935(8) Å.
It is worth to mention that the structure of 3 is slightly different from those of 1 and
2. While the AlIII centers of 1 and 2 are four-coordinated and display tetrahedral
geometry, the AlIII ion of 3 is five-coordinated and shows trigonal bipyramidal
geometry. We assume that the stereo hindrance of chloride ion is smaller than those of
methyl- and ethyl-group, leading to the further coordination of the nitrogen atom of
pyridine ring to the metal center.
Complexes 1–3 join a small family of aluminum complexes supported by
β-diketiminate ligands [52-62]. However, the AlIII complexes supported by
unsymmetrical β-diketiminate ligands are very rare [26, 62].
2.2. ROP of ε-caprolactone initiated by 1–3
The ring-opening polymerization of ε-caprolactone in toluene using complexes 1–3
as the catalysts under different conditions were investigated. The reaction of the
ring-opening polymerization of ε-caprolactone was shown in Scheme 2.
O
O
O
Catalyst
n
O
n
toluene, 60℃
ε-caprolactone
Polycaprolactone
Scheme 2. The ring-opening polymerization of ε-caprolactone initiated by 1–3.
Initially, the catalytic activity of 1-3 in the absence of benzyl alcohol were tested.
The polymerizations were conducted in toluene, and the molar ratio of ε-caprolactone
to initiator was fixed to 200:1. The results were summarized in Table 2. As can be
seen from Table 2, the complexes 1, 2, and 3 all can catalyze the ring-opening
polymerization of ε-caprolactone and the polymerization product has a relatively high
molecular weight. At the same temperature, the conversion rate of the monomer
increases with the polymerization time (Table 2, entries 3−6, 8, 9). However, the
polymers have broad molecular weight distribution (PDI = 1.62-2.12). The catalytic

activity of 2 is better than those of 1 and 3, which are consistent with the literature
reported results [63-65]. Complexes 1 and 3 exhibit moderate activity (Table 2, entries
1−4, 8-11) according to the evaluation standard established by Redshaw [66].
Table 2 ROP of ε-Caprolactone initiated by 1, 2, and 3[a]
Temp.
(℃)
60
Time
(min)
60
Conv.
(%)[b]
17.4
24.5
23.1
54.2
87.3
99.9
99.8
43.8
76.3
45.2
51.3
[c]
[c]
Entry Initiator CL:Al Solvent
Mw
Mn
PDI[d]
1
2
1
1
1
1
2
2
2
3
3
3
3
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
26511
47074
31019
40254
79979
52514
95705
18417
45238
11283
18552
15185
28706
14667
19708
38268
32295
49526
9827
1.746
1.640
2.115
2.043
2.090
1.626
1.932
1.874
2.031
1.971
1.405
80
60
3
100
100
60
60
4
80
5
60
6
60
180
120
60
7
80
8
60
9
60
120
60
22275
5726
10
11
80
100
60
13202
[a] Reaction conditions: All polymerizations were carried out with toluene as solvent and under nitrogen
1
atmosphere. [b] Measured by H NMR spectroscopy. [c] Calibration was performed with standard polystyrene
samples and measured by gel permeation chromatography (GPC) in THF. [d] PDI =Polydispersity index.
There are few examples of ring-opening polymerization of ε-caprolactone initiated
by aluminum alkyl complexes in the absence of benzyl alcohol because the catalytic
activity of aluminum alkyl compounds for ring-opening polymerization of
ε-caprolactone is very low [63, 67]. Complexes 1–3 are special cases which exhibit
catalytic properties for ring-opening polymerization of ε-caprolactone in the absence
of benzyl alcohol.
It is reported that the reaction of aluminum alkyl compounds with benzyl alcohol
can form aluminum oxide, which can better induce ROP of ε-caprolactone [26].
Therefore, the catalytic activity of 1–3 towards ROP in the presence of benzyl alcohol
was examined. It is found that complexes 1-3 showed excellent activity when the
molar ratios of ε-caprolactone, catalyst and benzyl alcohol (ε-CL/Al/OH) were set to
200:1:1 and 400:1:1 (Table S2 entries 1, 2 and 14) and the conversion of the monomer
reached 99.6%. In order to compare the activities of 1, 2 and 3 and also investigate the
dynamic behavior of the catalytic reactions, the polymerizations initiated by 1, 2 and

3, respectively, under the molar ratio of 600:1:1 (ε-CL/Al/OH) at 60°C were explored
(Table 3). As can be seen from Table 3, even at the molar ratio of 600:1:1, complexes
1-3 still show very good activity. As the reaction time was extended, both conversions
and molecular weights increased (Table 3, entries 1-12). Different from the results
that complex 2 showed better activity in the absence of benzyl alcohol, complex 3
displayed high activity than those of 1 and 2. For complex 1, the conversions of the
monomer increased from 42.9% to 73.5% (Table 3, entries 1-4). As complex 3 was
employed as initiator, the conversions of the monomer increased from 66.5% to
92.0% (Table 3, entries 9-12). These results agree with the proposal that the
polymerization mechanism in the presence of an alcohol is different from that of
absent of the alcohol (Schemes S1 and S2) [26]. The catalytic activity of 1-3 are
comparable to some reported symmetric β-diketiminate supported aluminum
complexes [26, 61].
Table 3. ROP of ε-caprolactone initiated by 1, 2, and 3 in the presence of BnOH[a].
Temp. Time Conv.
[c]
[c]
Entry Initiator CL:Al:BnOH Solvent
Mw
Mn
PDI[d]
(℃)
60
60
60
60
60
60
60
60
60
60
60
60
(min) (%)[b]
1
2
1
1
1
1
2
2
2
2
3
3
3
3
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
600:1:1
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
60
90
42.9
53.9
68.4
73.6
42.9
53.9
59.4
64.9
66.5
71.9
87.5
92.0
10828
11494
11777
11706
10606
14323
15161
19057
9413
8297
8973
9913
1.305
1.281
1.188
3
120
150
60
4
10000 1.171
9378 1.131
5
6
90
12056 1.188
13014 1.165
14682 1.298
7
120
150
60
8
9
8052
1.169
10
11
12
90
17523
39709
49279
13765 1.273
30926 1.284
38620 1.276
120
150
[a] Reaction conditions: All polymerizations were carried out with toluene as solvent and under nitrogen
1
atmosphere. [b] Measured by H NMR spectroscopy. [c] Calibration was performed with standard polystyrene
samples and measured by gel permeation chromatography (GPC) in THF. [d] PDI =Polydispersity index.
To further study the dynamic behavior of the polymerization of ε-caprolactone
initiated by 1, 2 and 3, respectively, the relationship between number average
molecular weights (Mn) of the polymer and the monomer conversion, as well as the

correlation between PDI value and the monomer conversion, were investigated
(600:1:1 at 60 °C). The results are depicted in Figures 5–7.
10
10000
9
8
9500
7
9000
8500
8000
7500
7000
6
5
4
3
2
1
0
20
30
40
50
60
70
80
Conv./%
Fig. 5. Plot of Mn (■ obtained from GPC analysis) and PDI (●, Mw/Mn) in the ROP of
ε-CL initiated by 1/BnOH.
10
18000
16000
14000
12000
10000
8000
9
8
7
6
5
4
3
2
1
0
40
45
50
55
60
65
70
75
Conv./%
Fig. 6. Plot of Mn (■ obtained from GPC analysis) and PDI (●, Mw/Mn) in the ROP of
ε-CL initiated by 2/BnOH.

10
9
8
7
6
5
4
3
2
1
0
40000
35000
30000
25000
20000
15000
10000
5000
65
70
75
80
85
90
95
Conv./%
Fig. 7. Plot of Mn (■ obtained from GPC analysis) and PDI (●, Mw/Mn) in the ROP of
ε-CL initiated by 3/BnOH.
As illustrated in Figures 5-7, there is a linear relationship between monomer
conversion and number average molecular weight (Mn), and the PDI values are about
1. These results indicate that the polymerization reactions initiated by complexes 1–3
are controllable in the presence of benzyl alcohol.
3. Conclusions
The synthesis, structures and reactivity of three aluminum complexes supported by
a novel asymmetric β-diketiminate ligand containing pyridine group, namely,
[LAlMe2] (1), [Al(L)Et2] (2), and [Al(L)EtCl] (3) are depicted. The structures of the
1
ligand LH and complexes 1–3 were characterized by X-ray diffraction analysis, H
and 13C NMR spectroscopy, and element analysis. The ROP of ε-caprolactone
initiated by 1–3 were executed. Their catalytic activities for ε-caprolactone ROP with
benzyl alcohol and without benzyl alcohol were compared. In the presence of BnOH,
complexes 1–3 show higher activity toward the ring-opening polymerization, and the
polymerization reactions exhibit controllable manner. The aim for synthesizing
aluminum compounds with higher catalytic activity is achieved.
4. Experimental section
4.1. General details

All operations were carried out under dry nitrogen atmosphere using glove-box
techniques and standard Schlenk-line. THF, n-hexane, toluene and dichloromethane
were dried over sodium/benzophenone. AlMe3, AlEt3 and AlEt2Cl were used as
received from commercial supplier. ε-Caprolactone (CL) was stirred over CaH2 for
24h, and distilled. NMR spectra and elemental analysis were performed on Bruker
Avance-III 600 MHz NMR spectrometer and Perkin-Elmer 2400 analyser,
respectively. The GPC analysis was carried out at 40 ℃ in THF on PL-GPC50 gel
permeation chromatograph. Single crystal X-ray diffraction data were collected at
293(2) K with a Bruker APEX II CCD diffractometer.
4.2. Synthesis of the ligand (HL)
Acetylacetone (10.00 g, 100.00 mmol) and 2,6-diisopropylaniline (DIPA, 18.02 g,
100.00 mmol) in toluene were reacted with a small amount of p-toluenesulfonic acid
(0.20 g, 1.00 mmol) as catalysts, and the reaction was heated under reflux overnight.
The solvent was evaporated to give a white solid. The resulting solid was reacted with
triethyloxonium tetrafluoroborate (9.59 g, 50mmol) in CH2Cl2 and stirred at room
temperature for one day, then one equivalent of 6-methyl-2-aminopyridine(10.81 g,
100.00 mmol) was added. The reaction mixture was further stirred for three days. The
solvent was spun dry to give a yellow solid. Yield: 21.7 g (62%). 1H NMR (600 MHz,
Chloroform-d): δ = 13.21 (s, 1H), 7.33 (t, J = 7.7 Hz, 1H), 7.14 (d, J = 7.6 Hz, 2H),
7.11 – 7.06 (m, 1H), 6.65 (d, J = 7.3 Hz, 1H), 6.42 (d, J = 8.1 Hz, 1H), 4.93 (s, 1H),
2.91 (h, J = 6.9 Hz, 2H), 2.49 (s, 3H), 2.43 (s, 3H), 1.70 (d, J = 1.1 Hz, 3H), 1.19 (d, J
= 6.9 Hz, 6H), 1.12 (d, J = 6.8 Hz, 6H) ppm. 13C NMR (151 MHz, Chloroform-d): δ =
165.79, 156.87, 155.07, 153.06, 144.80, 138.73, 137.57, 123.61, 122.87, 115.81,
110.84, 99.34, 28.23, 24.41, 23.93, 22.58, 22.46, 21.68 ppm. Elemental analysis calcd
(%) for C23H31N3 (349.51): C 79.10, H 8.76, N 12.24; found: C 79.04, H 8.94, N
12.02.
4.3. Synthesis of LAlMe2 (1)
A solution of LH (0.349 g, 1.0 mmol) in Tol was added dropwise to AlMe3 solution
(1.00 mL, 1 M in hexane, 1.0 mmol) at -35 ℃. The mixture was stirred overnight at
room temperature. Volatile materials were removed under vacuum to give a yellow

solid. Yield: 0.369 g (91%). Crystal were cultured in toluene at -35 ℃ for two weeks.
1H NMR (600 MHz, Chloroform-d): δ = 7.50 (t, J = 7.8 Hz, 1H), 7.19 (dd, J = 8.4, 6.8
Hz, 1H), 7.14 – 7.12 (m, 2H), 6.81 (d, J = 7.5 Hz, 1H), 6.75 (d, J = 8.0 Hz, 1H), 5.15
(s, 1H), 3.03 (p, J = 6.8 Hz, 2H), 2.44 (s, 3H), 2.19 (s, 3H), 1.77 (s, 3H), 1.13 (t, J =
6.5 Hz, 12H), -1.00 (s, 6H). 13C NMR (151 MHz, Chloroform-d): δ = 170.71, 163.19,
157.62, 157.11, 143.18, 141.95, 137.69, 126.15, 123.75, 118.04, 113.96, 100.17,
27.81, 24.49, 23.72, 23.55, 23.12, -10.12. Elemental analysis calcd (%) for
C25H36AlN3 (405.55): C 74.72, H 8.33, N 10.39; found: C 74.04, H 8.31, N 10.36.
4.4. Synthesis of LAlEt2 (2)
A solution of LH (0.349 g, 1.0 mmol) in Tol was added dropwise to AlEt3 solution
(1.00 mL, 1 M in hexane, 1.0 mmol) at -35 ℃. The mixture was stirred overnight at
room temperature. Volatile materials were removed under vacuum to give a yellow
solid. Yield: 0.412 g (95%). Crystal were cultured in toluene at -35 ℃ for two weeks.
1H NMR (600 MHz, Chloroform-d): δ = 7.50 (t, J = 7.8 Hz, 1H), 7.19 (dd, J = 8.4, 6.9
Hz, 1H), 7.14 – 7.11 (m, 2H), 6.84 (d, J = 7.5 Hz, 1H), 6.72 (d, J = 7.9 Hz, 1H), 5.15
(s, 1H), 3.01 (hept, J = 6.8 Hz, 2H), 2.48 (s, 3H), 2.15 (s, 3H), 1.76 (s, 3H), 1.13 (dd,
J = 6.8, 2.2 Hz, 12H), 0.81 (t, J = 8.1 Hz, 6H), -0.28 (dq, J = 14.0, 8.1 Hz, 2H), -0.37
13
(dq, J = 14.0, 8.1 Hz, 2H). C NMR (151 MHz, Chloroform-d): δ = 170.75, 164.39,
157.87, 157.57, 143.32, 141.62, 137.28, 126.23, 123.74, 118.42, 114.85, 100.62,
27.80, 24.49, 24.35, 24.05, 23.42, 22.88, 9.89, -0.11. Elemental analysis calcd (%) for
C27H40AlN3 (433.60): C 74.54, H 8.93, N 9.43; found: C 74.79, H 9.30, N 9.69.
4.5. Synthesis of LAlEtCl (3)
A solution of LH (0.349 g, 1.0 mmol) in Tol was added dropwise to AlEt2Cl
solution (1.00 mL, 1 M in hexane, 1.0 mmol) at -35 ℃. The mixture was stirred
overnight at room temperature. Volatile materials were removed under vacuum to
give a yellow solid. Yield: 0.409 g (93%). Crystal were cultured in toluene at -35 ℃
for two weeks. 1H NMR (600 MHz, Chloroform-d): δ = 7.56 (t, J = 7.8 Hz, 1H), 7.23
– 7.11 (m, 3H), 6.94 (d, J = 8.2 Hz, 1H), 6.85 (d, J = 7.5 Hz, 1H), 5.26 (s, 1H), 3.30
(hept, J = 6.8 Hz, 1H), 2.86 (hept, J = 7.0 Hz, 1H), 2.49 (s, 3H), 2.27 (s, 3H), 1.81 (s,
3H), 1.25 (d, J = 6.7 Hz, 3H), 1.15 (d, J = 6.8 Hz, 6H), 1.06 (d, J = 6.8 Hz, 3H), 0.60

(t, J = 8.1 Hz, 3H), -0.31 (qd, J = 8.1, 2.5 Hz, 2H). 13C NMR (151 MHz,
Chloroform-d): δ = 172.53, 162.96, 157.30, 156.72, 143.93, 142.75, 141.42, 138.78,
126.76, 124.37, 123.68, 118.92, 112.90, 101.19, 28.20, 27.93, 25.40, 24.75, 24.62,
24.44, 23.87, 23.34, 23.05, 8.92, 0.44. Elemental analysis calcd (%) for C25H35AlClN3
(439.99): C 67.89, H 8.30, N 9.72; found: C 68.24, H 8.02, N 9.55.
4.6. Ring-opening polymerization of ε-caprolactone
In the glove box, a solution of ε-caprolactone (0.228 g, 2 mmol) and complex 1
(0.01 mmol) in toluene (2 mL) was placed in a Schlenk flask equipped with a
magnetic stirring bar. The reaction mixture was heated, out of glove-box, to the
desired temperature and quenched with a mixture of HCl/MeOH (2 mL, 1:5 v/v) to
give a white solid. The solid was dissolved in THF and washed with MeOH.
4.7. Ring-opening polymerization of ε-caprolactone in the presence of BnOH
In the glove box, a solution of ε-caprolactone (0.228 g, 2 mmol), complex 1 (0.01
mmol) in toluene (2 mL) and one equivalent of BnOH was placed in a Schlenk flask
equipped with a magnetic stirring bar. The reaction mixture was heated, out of the
glove box, to the desired temperature and quenched with a mixture of HCl/MeOH (2
mL, 1:5 v/v) to give a white solid. The solid was dissolved in THF and washed with
MeOH.
Acknowledgments
The authors appreciate the financial supports from Natural Science Foundation of
China (21772140), Natural Science Foundation of Jiangsu Province of China
(BK20171213), a Project Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institution, and the project of scientific and technologic
infrastructure of Suzhou (SZS201708).
Appendix A. Supplementary data
CCDC <1890189, 1878871, 1878873 and 1878872> contains the supplementary
crystallographic data for <LH, 1, 2 and 3>. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge

Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with NMR spectra of HL, complexes 1–3 and PCL, selected bond lengths and angles
of ligand HL and complexes 1−3, proposed mechanism for the ROP of ε-CL and
MALDI-TOF mass spectrum of Poly(ε-caprolactone).
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A series of aluminium complexes based on a β-diketiminate
ligand: Synthesis, structures and their application to
ring-opening polymerization of ε-caprolactone
Huifen Pan, Yafei Li, Yalan Wang, Yu Ge, Shengwang Xia, Yahong Li*
Three aluminum complexes based on a β-diketiminate ligand have been prepared and
characterized. Complexes 1–3 showed moderate catalytic activity toward
ring-opening polymerization (ROP) of ε-caprolactone in the absence of benzyl
alcohol. The polymerization accelerated remarkably in the presence of benzyl alcohol
(BnOH).