Aluminum methyl and isopropoxide complexes with ketiminate ligands: Synthesis, structural characterization and ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1016/j.poly.2014.05.055

Mono(ketiminate) aluminum dimethyl complexes 1a-5a and bis(ketiminate) aluminum methyl complex 2b were synthesized via the reactions of AlMe ₃ with various N-aryl substituted ketimine compounds in a 1:1 or 1:2 molar ratio, respectively. The rea

Full text of DOI:10.1016/j.poly.2014.05.055

Accepted Manuscript
Aluminum Methyl and Isopropoxide Complexes with Ketiminate Ligands: Syn-
thesis, Structural Characterization and Ring-Opening Polymerization of Cyclic
Esters
Cigdem Tuc Altaf, Haobing Wang, Maryam Keram, Yang Yang, Haiyan Ma
PII:
S0277-5387(14)00372-6
DOI:
http://dx.doi.org/10.1016/j.poly.2014.05.055
POLY 10770
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
25 March 2014
22 May 2014
Please cite this article as: C.T. Altaf, H. Wang, M. Keram, Y. Yang, H. Ma, Aluminum Methyl and Isopropoxide
Complexes with Ketiminate Ligands: Synthesis, Structural Characterization and Ring-Opening Polymerization of
Cyclic Esters, Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.05.055
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Aluminum Methyl and Isopropoxide Complexes with Ketiminate Ligands: Synthesis, Structural
Characterization and Ring-Opening Polymerization of Cyclic Esters
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Cigdem Tuc Altaf, Haobing Wang, Maryam Keram, Yang Yang, Haiyan Maa,*
Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic
Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P.
R. China.
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* Corresponding author. Tel/Fax:: +86-21-64253519; EM: haiyanma@ecust.edu.cn
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Submitted to Polyhedron
Revised and re-submitted in
Revised and re-submitted in
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Abstract
Mono(ketiminate) aluminum dimethyl complexes 1a-5a and bis(ketiminate) aluminum methyl
complex 2b were synthesized via the reactions of AlMe3 with various N-aryl substituted ketimine
compounds in a 1:1 or 1:2 molar ratio, respectively. The reaction of Al(OiPr)3 with the ketimine
compound L2H resulted in a bis(ketiminate) aluminum isopropoxide complex 2c. All the
complexes were characterized by H and 13C{1H} NMR spectroscopy, either EA or HRMS
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analysis. X-ray diffraction measurement of complexes 2a and 3a confirmed that in the solid state
both 2a and 3a exist as a tetrahedron structure with the aluminum atom center surrounded by the
oxygen and nitrogen donors of the bis-chelating ketiminate ligand as well as two methyl groups.
Besides, a triligated aluminum complex 3 (Al(L3)3) with a six-coordinated metal core and a
dinuclear aluminum isopropoxide complex 6d [(L6)2Al(µ-OiPr)2Al(OiPr)2] with C2-symmetry
were isolated and characterized by X-ray diffraction studies. All aluminum methyl complexes
behaved as active initiators in the ring-opening polymerization (ROP) of rac-lactide (rac-LA),
affording isotactic-enriched polylactides (PLAs) with high molecular weights and broad molecular
weight distributions at 70 °C. The steric and electronic characters of the ancillary ligands show
significant effects both on the activity and the stereoselectivity. The introduction of an electron-
withdrawing group on the para-position of N-aryl ring of the ligand resulted in an increase of the
catalytic activity. However, the electronic character of ancillary ligands showed a neglectable
effect on the ROP of ε-CL initiated by these aluminum complexes. For both monomers,
bis(ketiminate) aluminum complex 2b was found to be more active than the corresponding
mono(ketiminate) complex 2a of the same ligand. Aluminum isopropoxide complex 2c exerted
better control for the polymerizations of both monomers, providing polymers with narrower

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molecular weight distributions and smaller molecular weights which are comparable to the
theoretical values.
Keywords: aluminum, ketiminate ligand, catalyst, cyclic ester, ring-opening polymerization

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1. Introduction
Polylactides (PLAs) and related polyesters have gained great attention in the past decades for
wide applications in medical and pharmaceutical fields due to their biodegradable, biocompatible,
and permeable properties [1-8]. Nowadays, polylactides are further considered to be ideal
alternatives for commercial olefinic polymers regarding the fact that they are produced from
completely renewable resources in addition to possessing excellent physical/mechanical
properties. Catalytic ring-opening polymerization (ROP) of related cyclic esters is the major
method adopted to synthesize these polymers [9-12]. During the polymerization process, higher
activities and better control can be expected in due course by further optimizing the initiator
structure.
Studies have shown that numerous aluminum complexes display good catalytic performance
toward the ring-opening polymerization of lactide (LA) or ε-caprolactone (ε-CL), most of them
are aluminum alkyl[13-23] and alkoxide complexes [24-43] supported by multidentate ancillary
ligands. As expected, the surrounding coordinated ancillary ligands affect both the stability of
the complexes and their catalytic performance in the ROP of cyclic esters. Among them,
aluminum complexes based on salen [24, 25, 34-36, 38-48], salan [49, 50], or related {ONNO}2−
[50-52], {ONO}2− [21,54] ligand frameworks are the most attractive, since they are capable of
producing highly isotactic polylactides from racemic lactide monomers, which are of high interest
due to the enhanced thermal and mechanical properties by forming stereocomplexes [55].
Due to the versatile modification possibilities of the ligand framework, aluminum complexes
of β-diketiminate ligands and analogues belong to another widely studied branch [56-60]. Our
previous work showed that aluminum ethyl complexes bearing β-diketiminate ligands with bulky
N-aryl substituents could initiate the ROP of ε-CL with moderate activities, affording

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polycaprolactones with broad molecular weight distributions [56]. Recently, Peng and
coworkers reported that aluminum alkyl complexes bearing aliphatic N-substituted β-diketiminate
ligands also showed activities toward the polymerization of ε-CL in the absence of alcohol [60].
Unfortunately, none of these β-diketiminate aluminum complexes are active for the ROP of rac-
lactide (rac-LA). The steric bulkiness of β-diketimine ligand is suggested to be disadvantageous
in the polymerization of rac-LA because that it may significantly hinder the coordination/insertion
of lactide monomer to the metal center. Previously, Huang and coworkers [14] found that
aluminum complexes with a less steric bulky ketiminate ligand {κ2-OCMeCHCMeN-(2,6-
iPr2C6H4)}AlR2 (R = Me, Et, Cl) could initiate the ring-opening polymerization of ε-CL at room
temperature and 60 °C to yield high molecular weight PCLs with broad PDIs (1.29–2.80).
However, the study did not include the electronic and steric effect of ketiminate ligands on the
polymerization as well as the potential catalysis for the ROP of lactide. In view of the less steric
bulkiness in comparison with β-diketiminate ligands, ketiminate ligands may serve as a suitable
ligand framework to develop active aluminum initiators for the ROP of rac-lactide. Therefore,
in this work, ketiminate ligands bearing various electron-donating and electron-withdrawing
groups were used to synthesize aluminum ketiminate complexes, which were further evaluated for
the ROP of rac-LA and ε-CL monomers.
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2. Experimental
2.1. General methods
All manipulations were carried out under a dry argon atmosphere using standard Schlenk
techniques and a glove box unless otherwise indicated. Toluene, n-hexane, light petroleum ether
were refluxed over sodium-benzophenone prior to use. Chloroform-d was dried over calcium
hydride. Corresponding ketimine compounds, 2-(N-2,6-dimethylphenyl)aminopent-2-en-4-one
(L1H), 2-(N-2,6-dichlorophenyl)aminopent-2-en-4-one (L2H), 2-(N-4-tertbutylphenyl)aminopent-
2-en-4-one
(L3H),
2-(N-4-methoxyphenyl)aminopent-2-en-4-one
(L4H),
2-(N-4-
chlorophenyl)aminopent-2-en-4-one (L5H), 2-(N-4-triflouromethylphenyl)aminopent-2-en-4-one
(L6H) were prepared according to the previously published procedure [56]. NMR spectra were
recorded on a Bruker AVANCE-400 spectrometer with CDCl3 as solvent (1H: 400 MHz; 13C
{1H}: 101 MHz). Chemical shifts for H and C {H} NMR spectra were referenced internally
using the residual solvent resonances and reported relative to tetramethylsilane (TMS).
Elemental analyses were performed on an EA-1106 instrument. Gel permeation chromatography
(GPC) analyses were carried out on a Waters instrument (M515 pump, Optilab Rex injector) in
THF at 25 °C, at a flow rate of 1 mL·min−1. Calibration standards were commercially available
narrowly distributed linear polystyrene samples that cover a broad range of molar masses (103 <
M < 2 × 106 g· mol−1).
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2.2. Synthesis of aluminum complexes
2.2.1. Synthesis of N-(2,6-dimethylphenyl)ketiminate aluminum dimethyl complex (κ2-
OCMeCHCMeN-2,6-Me2C6H3)AlMe2 (1a)
AlMe3 (1.2 mL, 2.0 M, 2.3 mmol) was added to a toluene solution (20 mL) of L1H (0.406 g,
2.30 mmol) at 0 °C in a glove box. The mixture was then allowed to warm to room temperature

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and stirred for additional 3 h. The resulting mixture was dried under vacuum to yield a pale
yellow oily product. Recrystallization with toluene at −20 °C resulted in colorless crystalline
solids (0.300 g, 51%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.08 (m, 3H, m, p-H of Ar), 5.28
(s, 1H, CMeCHCMe), 2.12 (s, 6H, Ar-CH3), 2.07 (s, 3H, CH3), 1.67 (s, 3H, CH3), −0.91 (s, 6H,
Al-CH3). 13C{H} NMR (100 MHz, CDCl3, 25 °C): δ 181.3 (C=O), 175.5 (C-N), 141.7 (Ar-C),
132.3 (Ar-C), 128.7 (Ar-C), 126.3 (Ar-C), 99.6 (CH), 25.8 (CH3), 22.2 (CH3), 18.3 (CH3), −10.2
(Al-CH3). Anal. Calcd. for C15H22AlNO: C, 69.47; H, 8.55; N, 5.40. Found: C, 68.74; H, 8.45;
N, 5.11%. Calcd. HRMS for C15H22AlNO: 259.1517; found: 259.1514. m/z (%): 258.1413 [trace,
+
+
M+], 244.1246 [100, M − CH3 ], 188.1066 [2.2, 2,6-CH3C6H3N=C(CH3)OC(CH3) − CH3 ].
2.2.2. Synthesis of N-(2,6-dichlorolphenyl)ketiminate aluminum dimethyl complex (κ2-
OCMeCHCMeN-2,6-Cl2C6H3)AlMe2 (2a)
Similar reaction route as that of 1a was adopted. AlMe3 (2.0 mL, 2.0 M, 4.0 mmol) was
added to a stirred toluene solution of L2H (0.980 g, 4.00 mmol) at 0 °C in a glove box.
Recrystallization of the crude product with a mixture of toluene and hexane resulted in colorless
crystals (0.780 g, 65%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.39 (d, 2H, J = 8.0 Hz, m-H of
Ar), 7.16 (t, 1H, J = 8.0 Hz, p-H of Ar), 5.36 (s, 1H, CMeCHCMe), 2.11 (s, 3H, CH3), 1.77 (s,
3H, CH3), −0.87 (s, 6H, Al-CH3). 13C{H} NMR (100 MHz, CDCl3, 25 °C): δ 183.3 (C=O),
175.5 (C-N), 138.5 (Ar-C), 130.8 (Ar-C), 127.9 (Ar-C), 126.9 (Ar-C), 99.0 (CH), 25.1 (CH3),
21.7 (CH3), −11.1 (Al-CH3). Anal. Calcd. for C13H16AlCl2NO: C, 52.02; H, 5.37; N, 4.67.
Found: C, 51.80; H, 5.17; N, 4.86%.
2.2.3. Synthesis of N-(p-isopropylphenyl)ketiminate aluminum dimethyl complex (κ2-
OCMeCHCMeN-p-CH(CH3)2C6H4)AlMe2 (3a)

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AlMe3 (2.0 mL, 2.0 M, 4.0 mmol) was added to a toluene solution (20 mL) of L3H (0.870 g,
4.00 mmol) at 0 °C in a glove box. The mixture was then allowed to warm to room temperature
and stirred overnight. The mixture was vacuum-dried to yield a yellow oily product.
Recrystallization of the crude product with a mixture of toluene and hexane resulted in colorless
crystals (0.480 g, 44%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.20 (d, 2H, J = 8.0 Hz, o-H of
Ar), 6.85 (d, 2H, J = 8.0 Hz, m-H of Ar), 5.18 (s, 1H, CMeCHCMe), 2.91 (septet, J = 6.8 Hz,
1H, CH(CH3)2), 2.03 (s, 3H, CH3), 1.79 (s, 3H, CH3), 1.26 (d, J = 6.8 Hz, 6H, CH(CH3)2), −0.91
(s, 6H, Al-CH3). 13C{H} NMR (100 MHz, CDCl3): δ 179.6 (C=O), 173.5 (C-N), 146.0 (Ar-C),
140.5 (Ar-C), 126.3 (Ar-C), 123.6 (Ar-C), 98.5 (CH), 32.6 (CH(CH3)2), 24.7 (CH3), 23.0 (CH3),
21.7 (CH3), −11.4 (Al-CH3). Anal. Calcd. for C16H24AlNO: C, 70.30; H, 8.85; N, 5.12. Found:
C, 69.25; H, 8.89; N, 4.89%. Calcd. HRMS for C16H24AlNO: 273.1673; found: 273.1682. m/z
(%): 272.2 ([M−H]+, 15.9), 258.2 ([M−CH3]+, 100), 216.2([L]+, 23.7).
2.2.4. Synthesis of N-(p-methoxyphenyl)ketiminate aluminum dimethyl complex (κ2-
OCMeCHCMeN-p-OCH3C6H4)AlMe2 (4a)
AlMe3 (1.0 mL, 2.0 M, 2.0 mmol) was added to toluene solution of L4H (0.410 g, 2.00 mmol)
at 0 °C in a glove box and was stirred overnight. The resulting reaction mixture was dried under
vacuum to yield a pale yellow oily product. Recrystallization of the crude product with toluene
resulted in colorless crystals (0.280 g, 54%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 6.90 (d,
2H, J = 9.3 Hz, o-H of Ar), 6.85 (d, 2H, J = 9.3 Hz, m-H of Ar), 5.17 (s, 1H, CMeCHCMe),
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3.81 (s, 3H, Ar-OCH3), 2.03 (s, 3H, CH3), 1.79 (s, 3H, CH3), −1.03 (s, 6H, Al(CH3)2). C{H}
NMR (100 MHz, CDCl3, 25 °C): δ 179.8 (O=C), 173.9 (N-C), 157.0 (Ar-C), 135.7 (Ar-C),
124.8 (Ar-CH), 113.6 (Ar-C), 98.5 (CH), 54.4 (OCH3), 24.8 (CH3), 21.6 (CH3), −11.4 (Al-CH3).
Anal. Calcd. for C14H20AlNO2: C, 64.35; H, 7.71; N, 5.36. Found: C, 64.46; H, 7.98; N, 5.10%.

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2.2.5. Synthesis of N-(p-chlorophenyl)ketiminate aluminum dimethyl complex (κ2-
OCMeCHCMeN-p-ClC6H4)AlMe2 (5a)
AlMe3 (2.0 mL, 2.0 M, 4.0 mmol) was added to toluene solution of L5H (0.839 g, 4.00 mmol)
and the reaction mixture was stirred overnight. The reaction mixture was then dried under
vacuum to yield a pale yellow oily product. Recrystallization of the crude product with toluene
resulted in colorless crystals (0.450 g, 42%). 1H NMR (400 MHz, CDCl3, 25°C): δ 7.35 (d, 2H,
J = 8.5 Hz, o-H of Ar), 6.89 (d, 2H, J = 8.5 Hz, m-H of Ar), 5.21 (s, 1H, CMeCHCMe), 2.04 (s,
3H, CH3), 1.79 (s, 3H, CH3), −0.92 (s, 6H, Al-CH3). 13C{H} NMR (100 MHz, CDCl3, 25°C): δ
180.9 (O=C), 173.5 (N-C), 141.6 (Ar-C), 131.2 (Ar-C), 128.6 (Ar-C), 125.4 (Ar-C), 98.7 (CH),
24.8 (CH3), 21.7 (CH3), −11.4 (Al-CH3). Anal. Calcd. for C13H17AlClNO: C, 58.76; H, 6.45; N,
5.27. Found: C, 58.30; H, 6.59; N, 5.34%.
2.2.6. Synthesis of bis{N,-(2,6-dichlorophenyl)ketiminate} aluminum methyl complex (κ2-
OCMeCHCMeN-2,6-Cl2C6H3)2AlMe (2b)
AlMe3 (1.0 mL, 2.0 M, 2.0 mmol) was added to a stirred toluene solution (20 mL) of L2H
(0.980 g, 4.00 mmol) at 0 °C. The reaction mixture was stirred in an oil bath at 90 °C for 3
days. The resulting reaction mixture was dried under vacuum to yield a pale yellow oily product.
Recrystallization of the crude product with toluene resulted in colorless crystalline solids (0.450 g,
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42%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.33 (dd, 2H, J = 1.3 Hz, J = 8.0 Hz, m-H of
Ar), 7.29 (dd, 2H, 4J = 1.3 Hz, 3J = 8.0 Hz, m-H of Ar), 7.03 (t, 2H, J = 8.0 Hz, p-H of Ar), 5.15
(s, 2H, CMeCHCMe), 1.65 (s, 6H, CH3), 1.35 (s, 6H, CH3), −0.88 (s, 3H, Al-CH3). 13C{H}
NMR (100 MHz, CDCl3, 25 °C): δ 180.7 (O-C), 174.1 (C-N), 145.1 (Ar-C), 131.1 (Ar-C), 130.6
(Ar-C), 127.0 (Ar-C), 126.6 (Ar-C), 124.5 (Ar-C), 98.0 (CH), 23.7 (CH3), 22.8 (CH3), −8.5 (Al-

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CH3). Anal. Calcd. for C23H23AlCl4N2O2: C, 52.30; H, 4.39; N, 5.30. Found: C, 52.21; H, 4.24;
N, 5.11%.
2.2.7. The reaction of AlMe3 with 2 equiv. of ketimine L3H
AlMe3 (2 mmol, 1.0 mL, 2.0 M) was added dropwise to a solution of L3H (0.870 g, 4.00
mmol) dissolved in 20 mL of toluene. The reaction mixture was stirred in an oil bath at 90 °C
for 3 days. The color of the solution turned to reddish brown. The mixture was dried under
vacuum to yield a foam-like crude product. White powder could be isolated after recrystallization
with light petroleum ether, which was characterized to be complex 3. 1H NMR (400 MHz,
CDCl3, 25 °C): δ 7.23 (d, 1H, J = 8.4 Hz, o-H of Ar), 7.14-6.98 (m, 5H, o,m-H of Ar), 6.90 (d,
1H, J = 7.2 Hz, m-H of Ar), 6.82 (d, 1H, J = 8.0 Hz, m-H of Ar), 6.73 (d, 1H, J = 7.6 Hz, m-H
of Ar), 6.66 (d, 1H, J = 6.8 Hz, o-H of Ar), 6.59 (d, 1H, J = 6.8 Hz, m-H of Ar), 4.99 (s, 1H,
CMeCHCMe), 4.82 (s, 1H, CMeCHCMe), 4.53 (d, 1H, J = 7.6 Hz, o-H of Ar), 4.37 (s, 1H,
CMeCHCMe), 2.93 (septet, 1H, J = 6.8 Hz, CH(CH3)2), 2.87 (septet, 1H, J = 7.2 Hz,
CH(CH3)2), 2.75 (septet, 1H, J = 6.8 Hz, CH(CH3)2), 1.95 (s, 3H, CH3), 1.54 (s, 3H, CH3), 1.52
(s, 3H, CH3), 1.35 (s, 3H, CH3), 1.32-1.26 (m, 6H, CH(CH3)2), 1.27 (s, 3H, CH3), 1.23 (d, 6H, J
= 6.8 Hz, CH(CH3)2), 1.21 (s, 3H, CH3), 1.16 (d, 3H, J = 6.8 Hz, CH(CH3)2), 1.14 (d, 3H, J =
7.2 Hz, CH(CH3)2). Anal. Calcd. for C42H54AlN3O3: C, 74.64; H, 8.05; N, 6.22. Found: C,
73.02; H, 7.89; N, 5.36%.
2.2.8. Synthesis of bis[N-(2,6-dichlorophenyl)ketiminate] aluminum isopropoxide complex (κ2-
OCMeCHCMeN-2,6-Cl2C6H3)2AlOCH(CH3)2 (2c)
A toluene solution of L2H (0.980 g, 4.00 mmol, 20 mL) was mixed with a toluene solution of
Al(OiPr)3 (2.00 mmol, 0.41 g, 0.5 M). The reaction mixture was stirred in an oil bath at 90 °C
for 3 days. All the volatiles were removed under vacuum to yield a brown oily product.

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Colorless crystals could be isolated after recrystallization with toluene at −20 °C (0.646 g, 67%).
1H NMR (400 MHz, CDCl3, 25 °C): δ 7.36 (d, 2H, J = 8.0 Hz, m-ArH), 7.29 (d, 2H, J = 8.0 Hz,
m-ArH), 7.05 (t, 2H, J = 8.0 Hz, p-ArH), 5.21 (s, 4H, γ-CH), 3.74 (septet, 2H, J = 6.0 Hz,
CH(CH3)2), 1.63 (s, 6H, CH3), 1.38 (s, 6H, CH3), 1.01 (d, 3H, J = 6.0 Hz, CH(CH3)2), 0.99 (d,
3H, J = 6.0 Hz, CH(CH3)2). 13C{H} NMR (100 MHz, CDCl3, 25 °C): δ 183.79 (OC), 175.93
(NC), 145.87 (Ar-C), 132.78 (Ar-C), 132.19 (Ar-C), 128.33 (Ar-C), 127.56 (Ar-C), 125.94 (Ar-
C), 99.35 (γ-CH), 62.66 (OCH(CH3)2), 27.46 (CH3), 27.20 (CH3), 24.79 (CH3), 24.14 (CH3).
Anal. Calcd. for C25H27AlCl4N2O3: C, 52.47; H, 4.76; N, 4.89. Found: C, 52.62; H, 4.72; N,
4.69%.
2.3. X-ray crystallographic determination
Single crystals suitable for X-ray diffraction studies were obtained from a saturated
toluene/hexane mixture for 2a or 3a, or a petroleum ether solution for 3, or a toluene solution for
6d respectively. The crystallographic data of complexes 2a, 3a, 3 and 6d were collected on a
Bruker AXSD8 diffractometer with a graphite-monochromatic Mo-Kα (λ = 0.71073 Å) radiation.
All data were collected at 20 °C using omega-scan techniques. The structures of the complexes
were solved by direct methods and refined using Fourier techniques. An absorption correction
based on SADABS was applied for complex 2a [61]. All non-hydrogen atoms were refined by
full-matrix least-squares on F2 using the SHELXTL program package [62]. Hydrogen atoms
were located and refined by the geometry method. The cell refinement, data collection, and
reduction were done using Bruker SAINT [63]. The structure solution and refinement were
performed with SHELXS-97 [64] and SHELXL-97 [65] respectively. Molecule structures were
generated using ORTEP [66].
2.4. Ring-opening polymerization of cyclic esters

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2.4.1. Typical polymerization procedure for rac-LA
To a solution of rac-lactide (288 mg, 2.00 mmol) in toluene (1.5 mL), the solution of
aluminum complex (0.02 mmol) in toluene (0.5 mL) was added. The total volume was 2.0 mL.
The mixture was then immerged into an oil bath at intended temperature and stirred for desired
time interval. Polymerization aliquot was withdrawn under the protection of argon and
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quenched with methanol. After removal of the volatiles, the residue was subjected to H NMR
analysis. Monomer conversion was determined by observing the methine resonance integration
of monomer vs. polymer in the 1H NMR (CDCl3, 400 MHz) spectrum. The bulk polymerization
mixture was quenched at the same time with methanol. The purification of the polymer in each
case was managed by dissolving the crude sample in CH2Cl2 and precipitating the polymer
solution with methanol. The obtained polymers were further dried in a vacuum oven at 60 ºC for
24 h and subjected to GPC analysis and homonuclear decoupled 1H NMR measurement.
2.4.2. Typical polymerization procedure for ε-CL
To a solution of ε-caprolactone (1.0 mL, 2.0 mmol) in toluene at 70 ºC, the solution of
aluminum complex (0.02 mmol) in toluene (1.0 mL) was added. The mixture was then stirred
for desired time interval at this temperature. Polymerization aliquot was withdrawn under the
protection of argon and quenched with methanol. After removal of the volatiles, the residue was
1
subjected to H NMR analysis. Monomer conversion was determined by observing the
1
integration of monomer vs. polymer methylene resonance in the H NMR (CDCl3, 400 MHz)
spectrum. The bulk polymerization mixture was quenched at the same time by addition of
methanol. The purification of the polymer in each case was managed by dissolving the crude
samples in CH2Cl2 and precipitating the polymer solution with methanol. The obtained polymers
were further dried in a vacuum oven at 60 ºC for 24 h and subjected to GPC analysis.

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3. Results and discussion
3.1. Synthesis of ketiminate aluminum complexes
Various substituted ketimines L1-6H could be easily prepared according to the published
procedure by reacting a 1:1 ratio of 2, 4-pentandione and the corresponding substituted aniline in
toluene under reflux conditions in the presence of para-toluenesulfonic acid as a catalyst [56,67-
69]. Reactions of ketimines L1-5H with trimethylaluminum in 1:1 molar ratio generated the
target four-coordinated aluminum complexes (κ2-OCMeCHCMeNAr)AlMe2 (1a-5a) as illustrated
in Scheme 1. The reactions proceeded smoothly along with the elimination of 1 equiv. of
methane. The crude products were recrystallized by dissolving in minimum amount of toluene or
a hexane-toluene mixture and kept at −20 °C for a few days. Analytically pure colorless
powders or crystals of 1a-5a could be obtained in moderate yields after filtration and repeated
washing with hexane for several times. All the synthesized ketiminate aluminum dimethyl
complexes 1a-5a were characterized by H and C{1H} NMR spectroscopy. While complexes
2a, 4a and 5a were characterized by EA, complexes 1a and 3a were further characterized by
HRMS since satisfied elemental analysis results could not be obtained even after several repeated
recrystallization processes.
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In the 1H and C NMR spectra of complexes 1a-5a, the signals for proton and carbon of the
methine group of the backbone serve as excellent indicators for evaluating the purity of these
metal complexes. All the purified complexes show only one resonance attributable to the
1
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methine group of the backbone at ca. δ 5.2 ppm and ca. δ 98.0 ppm in the H and C NMR
spectra, respectively. Besides, complexes 1a-5a show a characteristic resonance for Al-CH3

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group at ca. δ −0.9 ppm and an integral ratio of Al-CH3 and γ-CH being 6:1 in the H NMR
spectra, indicating the formation of mono(ketiminate) aluminum dimethyl complexes.
The reaction of ketimine L6H bearing a para-CF3 group on the N-phenyl ring with
trimethylaluminum in 1:1 ratio was also attempted. The 1H NMR spectrum of the resulting solid
product indicated the presence of the target mono(ketiminate) aluminum dimethyl complex along
with the bis(ketiminate) aluminum complex, which hindered the isolation of the pure target
aluminum dimethyl complex. This might be due to the strong electron-withdrawing ability of
CF3 group causing significant electron deficiency on the phenyl ring and being in favor of the
further reaction.
Similar reaction route was further applied to synthesize the corresponding bisligated aluminum
complexes of proligands L1-6H by allowing AlMe3 to react with these ketimines in 1:2 ratio. It
was found that harsh conditions of stirring at 90°C for 3 days had to be adopted. However, only
ketimine L2H bearing an N-2, 6-dichlorophenyl was converted to the target bisligated aluminum
1
complex 2b successfully (Scheme 2). In the H NMR spectrum of complex 2b, one singlet for
the two γ-CH at δ 5.16 ppm and two double of doublets along with a triplet in the low field
region are displayed, indicating a symmetric coordination mode of two ketiminate ligands. The
existence of two dd signals accounting for protons in the meta-positions implies that for each
phenyl ring the two meta-protons are chemically inequivalent, likely due to the restricted rotation
of the phenyl ring. The characteristic resonance of Al-CH3 protons for 2b can be observed at δ
−0.88 ppm, which is in 3:2 ratio with the γ-CH resonance.
From the reaction mixture of ketimine L1H and AlMe3 in 1:2 molar ratio, colorless crystals
could be isolated which however proved to be a mixture of mono(ketiminate) complex 1a and the
target bis(ketiminate) complex 1b as well as a small amount of proligand L1H as characterized by

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1H NMR spectroscopy. Due to similar solubilities of three components, the isolation of
analytically pure bisligated aluminum complex 1b was failed.
The 1:2 ratio reaction of AlMe3 with ketimine L3H bearing an N-p-isopropylphenyl resulted in
a reddish-brown solution after 3 days’ reaction at 90 °C. After work-up, a white powder could
1
be isolated from a petroleum ether solution. Unexpectedly, the H NMR spectrum of this
product shows no Al-CH3 resonance, but displays more than expected signals (Fig. 1). The
characteristic three types of γ-H resonances in 1:1:1 ratio together with three multiple resonances
at 2.0~3.0 ppm accounting for the methine proton of the isopropyl group indicate clearly the
presence of three types of ketiminate ligands. We therefore suggest that the stoichiometric
structure of this product might be in the form of “Al(L3)3” (3, Scheme 3). To our surprise, in the
same region of γ-H there is an additional doublet (4.5 ppm, J = 7.6 Hz) with an integral of one
proton. Furthermore, only eleven aromatic protons instead of twelve protons are detected in the
low field region. The doublet at 4.5 ppm has a similar coupling constant with these aromatic
protons. All these features suggest that this doublet might be assignable to an aromatic proton
as well. To further confirm the structure of complex 3, single crystals suitable for X-ray
diffraction study were obtained. The solid state structure of complex 3 (vide post) is in consistent
with our assumption, where three ketiminate ligands surround the aluminum center in a mer-
configuration[70] and an obvious shielding effect of one aromatic proton at the ortho-position by
the adjacent phenyl ring is observed clearly. It is worthy of noting that, although the formation
of aluminum complexes ligated with three N-thiophenol substituted ketiminate ligands was
reported previously [71], a structural characterization of tri(ketiminate) aluminum complex and
such unusual shielding effect have never been reported previously. Similar triligated aluminum
1
complexes were also detected in the H NMR spectra of the crude products of the reactions of

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ketimines L4-6H with AlMe3. However due to the formation of a mixture of products, no
analytically pure complex could be isolated. Possibly, ketimines with bulkier N-aryl substituent,
for instance, 2,6-dichlorophenyl, favor the formation of bisligated aluminum complex, while
ketimines with the less steric p-substituted N-aryl group create triligated complexes under harsh
reaction conditions [72].
In order to obtain aluminum alkoxide complexes with ketiminate ligands, Al(OiPr)3 was used
to react with these ketimine proligands at elevated temperature. From the reaction of proligand
L2H and Al(OiPr)3, the target bis(ketiminate) aluminum isopropoxide complex 2c could be
isolated as colorless crystalline solids in a moderate yield after recrystallization from a toluene
solution at −20 °C. The 1H NMR spectrum of 2c shows a triplet and two doublets belonging to
the aromatic protons in the range of δ 7.05~7.37 ppm. The resonance of two γ-H appears as a
singlet at 5.26 ppm, indicating symmetric coordination geometry. The methine proton and
methyl group of isopropoxide could be clearly identified. The integral ratio of isopropoxy group
and γ-H of the ketiminate ligand is 1 : 2, giving evidence to the formation of bis(ketiminate)
aluminum isopropoxide complex. Based on the literature reports regarding the molecular
structures of many aluminum isopropoxide complexes [70], a dimeric structure bridged by two
isopropoxyl groups is suggested for 2c (Scheme 4).
The reaction of proligands L1H, L3-6H with Al(OiPr)3 did not afford the target bis(ketiminate)
aluminum isopropoxide complexes. The 1H NMR spectra of isolated solids showed no presence
of isopropoxide group in the structures. Instead, three singlets in 1:1:1 molar ratio account for
three γ-CH protons appeared, indicating the formation of “AlL3” complexes again. The solids
obtained however were not clean enough, further recrystallization failed to provide analytical pure
complexes. From the mother liquor of the reaction between Al(OiPr)3 and ketimine L6H, several

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colorless single crystals were obtained, which were characterized by X-ray diffraction
determination to be a binuclear aluminum isopropoxide complex 6d (vide post), where the two
aluminum centers are in different coordination environment. In order to obtain corresponding
bis(ketiminate) aluminum isopropoxide complexes of proligands L1H, L3-6H, different reaction
conditions were adopted, but all proved to be unsuccessful. By monitoring the NMR scale
reaction of L6H and Al(OiPr)3 in a 2:1 molar ratio, the formation of triligated complex 6 as the
major product companied by a small amount of 6d was detected upon gentle warming the
reaction mixture, but no reaction took place at ambient temperature, indicating that the target
aluminum isopropoxide complex “(L6)2Al(OiPr)” is either thermodynamically unstable or more
reactive than the staring precursor which leads to the formation of the more stable triligated
complex 6 as the major product.
3.2. Crystal structures of aluminum complexes
Single crystals of complexes 2a, 3a and 3 were obtained from a saturated toluene solution or a
hexane-toluene mixture, or a saturated petroleum ether solution at −20 °C, respectively. The
unanticipated complex 6d was isolated from the reaction mixture of Al(OiPr)3 and L6H, and
suitable single crystals were successfully obtained from a toluene solution at −20°C. A summary
of crystal and refinement data for 2a, 3a, 3 and 6d are listed in Table 1.
Crystallographic data indicates that 2a possesses a tetra-coordinate metal core surrounded by
the oxygen and nitrogen atoms of the chelating ketiminate ligand and two methyl groups in a
distorted tetrahedral environment (Fig. 2). The biting angle of the ketiminate ligand, N(1)–
Al(1)–O(1) = 95.19(8)°, is significantly smaller than the regular tetrahedral bond angle of 109.5°.
However, it is still in the normal range of 86.76~100.04° for the tetra-coordinate β-diketiminate
metal complexes. The bond lengths between atoms of the ketiminate backbone are all in the

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same ranges, which are comparable to those of aluminum ketiminate complexes reported by
Huang’s group [14]. The C(2)–Al(1)–C(1) bond angle of 117.79(16)° and lengths of Al1−O1
(1.7836(18) Å), and Al1−N1 (1.9438(18) Å) are slightly smaller than the corresponding bond
angle and lengths of mono(ketiminate) aluminum complex bearing N-2,6-isopropylphenyl
substitution reported by Huang [15]. The significant shortness of Al1−O1 bond length in
comparison with Al1-N1 bond length indicates higher covalent nature of this bond.
In the solid state, the crystal of complex 3a belongs to the orthorhombic crystal system. The
aluminum center, being surrounded by the nitrogen and oxygen donors of the chelating ketiminate
ligand and two methyl groups, possesses a distorted tetrahedral geometry as observed in 2a (Fig.
3). All the structure parameters are quite similar to those of 2a, except that the length difference
between Al1−O1 and Al1−N1 bonding is slightly smaller than that of 2a, indicative of a more
averaged donation effect likely attributable to the N-aryl with an electron donating isopropyl
group. The bond length of N1–C8 in 3a is 1.446(3) Å, which is also slightly longer than the one
in 2a (N1–C8 =1.427(2) Å).
The molecular structure of 3 features a six coordinate aluminum center in a slightly distorted
octahedral geometry with the bond angles O(2)−Al(1)−O(3) = 178.80(5)°, N(1)−Al(1)−N(2) =
177.25(5)° and O(1)−Al(1)−N(3) = 174.32° deviating slightly from 180° (Fig. 4). Expectedly,
the higher electronegativity of oxygen atom results in more covalent nature of the Al−O bonds
than that of Al−N bonds, and thus shorter bond lengths are observed for Al−O bonds (Al1−O1
1.8764(10) Å, Al1−O2 1.8321(9) Å, Al1−O3 1.8488(9) Å; Al1−N1 2.0403(11) Å, Al1−N2
2.0524(11) Å, Al1−N3 2.0671(11) Å). However, Al1−O bond lengths and the angles are still
comparable with previously reported octahedral Al(acac)3 (Al−O = 1.892 Å) [69] and Al(hfacac)3

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(Al−O = 1.873-1.883 Å) complexes [73]. As shown in Fig. 4, three ketiminate ligands in
complex 3 surround the aluminum center in a mer-configuration, which is supposed to reduce the
steric repulsion among the ligands more effectively than the likely fac-isomer.
As shown in Fig. 5, the crystal structure of the dinuclear aluminum complex 6d exhibits six-
and four-coordinated aluminum centers bridged by two µ-isopropoxide groups. The whole
molecular has C2-symmetry with an axis defined by the two aluminum centers. The aluminum
center bearing two ketiminate ligands lies in a distorted octahedral environment with the bond
angles significantly deviated from the ideal values of 90° and 180° for octahedral geometry
(O1−Al1−O1* 98.90(13)°, O1−Al1−O2* 167.81(9)°, N1−Al1−N1* 168.48(13)°). Consistently,
Al−O (ketiminate) bonds are more covalent than Al−N bonds because of the more electronegative
nature of oxygen atom (Al1−O1/Al1-O1* 1.8255(19) Å; Al1−N1/Al1-N1* 2.080(2) Å). The
other aluminum center bearing two isopropoxide groups lies in a tetrahedral environment with the
bond angles also deviated from the regular bond angle of 109.5° (O3−Al2−O3* 116.67(18)°,
O3−Al2−O2 118.38(11)°, O2−Al2−O2* 81.14(12)°). The distances between the octahedral
aluminum center and the oxygen atoms of the bridged isopropoxide groups (Al1−O2/Al1−O2*,
1.9231(19) Å) are significantly longer than those between the tetrahedral aluminum center and the
µ-oxygen atoms (Al2−O2/Al2-O2*, 1.791(2) Å) owing to the less electrophilic nature of the
former aluminum center. The spatial distance between two metal centers in 6d (Al1····Al2 =
2.89 Å) is similar to that in the previously reported aluminum complex [(hfacac)Al(µ-
OiPr)2Al(hfacac)] (Al(1)····Al(2) = 2.86 Å) where two aluminum centers with the same
coordination geometry are involved [70].
3.3. Ring-opening polymerization of rac-LA

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Aluminum methyl complexes 1a-5a and 2b were employed in the ROP of rac-LA as single
component initiator and were proved to be active catalysts for the ROP of rac-LA at 70 °C, but
inactive below that temperature. The position and nature of substituents on the N-phenyl ring
have considerable influence on the catalytic activity and selectivity of the corresponding aluminum
complexes.
As shown in Table 2, the polymerization of rac-LA initiated by aluminum dimethyl complex
1a with N-(2,6-dimethlyphenyl)ketiminate ligand reached 56% of monomer conversion in 72 h,
which was found to be the least active catalyst in this group (run 2). On the contrary, complex
2a could achieve almost full monomer conversion within 72 h. The introduction of
electronegative Cl atoms at 2,6-positions of phenyl ring instead of methyl groups resulted in a
significant increase in the catalytic activity. In other words, methyl as an electron-donating
group on 2,6-positions of the phenyl ring decreased the electrophilicity of the aluminum center
through the chelating π-system of ancillary ligand, and thus became a disadvantage for the
coordination of monomer, leading to a decrease in the activity. Activities of complexes 3a (p-
iPr), 4a (p-OMe) and 5a (p-Cl) were investigated in the same group and an increasing tendency of
catalytic activities was found as 3a (p-OMe) < 4a (p-iPr) < 5a (p-Cl). Complex 5a was also the
most active catalyst among 1a-5a by reaching 97% of monomer conversion in 48 h (Run 10).
The data is consistent with the polymerization results reported by Huang [14], the bisligated
aluminum complex 2b was proved to be more active than the corresponding mono(ketiminate)
complex 2a, by converting 90% of monomer in 24 h.
The polymerization of rac-LA initiated by aluminum complexes 1a-5a and 2b afforded high
molecular weight polymers with broad polydispersities. In most of the cases, the measured
number average molecular weights deviate significantly from the theoretical values. As indicated

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in Table 2, all the complexes resulted in isotactic-enriched PLAs at 70 °C. PLAs produced by 1a
and 2a (Pm = 0.70-0.72) display higher isotacticities than those obtained by aluminum complexes
3a-5a bearing ketiminates of N-para-substituted phenyl (Pm = 0.57~0.60). In other words, the
introduction of a substituent on the para-position instead of ortho-positions decreases the
isotactic selectivity. This can be explained by the steric effect of substituents on ortho-positions,
causing bulkiness and blocking partially the coordination sphere of the metal center to direct the
monomer insertion. Besides, the bis(ketiminate) aluminum complex 2b resulted in polylactides
with similar isotacticity as the mono(ketiminate) aluminum complex 2a of the same ligand.
A significant increase of catalytic activities was observed for these complexes with the
addition of isopropanol into the system (See ESI, Table S1). The polymers obtained showed
1
hydroxyl and isopropoxide signals in 1:6 ratio in their H NMR spectra, indicating the formation
of linear PLAs end-chapped with hydroxyl and isopropoxide terminals. The crucial effect of
1
iPrOH on the polymerization prompted us to monitor the reaction via H NMR spectroscopy.
After addition of lactide monomer and isopropanol to 2a in CDCl3 at r.t. ([rac-LA]:[iPrOH]:[2a]
1
= 10:1:1), signals attributable to the free ligand L2H appeared in the H NMR spectrum along
with Al-OCH(CH3)2 proton signals and a new Al-CH3 signal at −0.78 ppm. Besides, small
amount of 2a and some other species in low content also existed. The signals of lactide
monomer were unchanged until the mixture was heated at 50 °C for 24 h to display the
characteristic resonances of PLA. It was clear that the added isopropanol reacted with the
aluminum complex to kick out the free ligand as a dominant pathway and meanwhile new
aluminum species such as “AlMe2(OiPr)” and “AlMe(OiPr)2” were formed, which initiated the
ROP of rac-lactide instead. Similar result was reported previously for amidinate aluminum

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complexes [74]. The presence of multiple active species also gave the explanation on the broad
molecular weight distributions obtained even with the addition of alcohol.
Aluminum alkoxide complexes can be classified into a group which includes the most useful
catalysts for lactide polymerization [10-12,27]. Supporting the previous studies, as a single
component aluminum alkoxide initiator, aluminum isopropoxide complex 2c exhibited
significantly higher catalytic activity than aluminum methyl complexes 1a-5a and 2b for the ROP
o
of rac-LA at 70 C in toluene, and high monomer conversions were observed within 12 h,
indicating that the metal-alkoxide bond is very active for ROP by exerting sufficient nucleophilic
attack on the monomer in initiation. From the data listed in Table 2, it is found that the
polymerization still afforded PLA with relatively broad molecular weight distribution (Mw/Mn =
1.6), but the molecular weight of the polymer sample is comparable to the calculated one. Being
different from the bis(ketiminate) aluminum methyl complex 2b, complex 2c is less stereoselective
for the ROP of rac-LA and provides PLA with dominantly atactic microstructure (Pm = 0.56).
Obviously different active species should be involved in the polymerization, and the dimeric
structure of complex 2c is likely to be responsible for the decrease of the isotactic selectivity.
Furthermore, the polymer obtained from the polymerization of rac-LA initiated by complex 2c
1
shows clearly hydroxyl and isopropoxide signals in 1:6 ratio in the H NMR spectrum, indicating
the formation of linear PLA end-chapped with hydroxyl and isopropoxide terminals.
3.4. Ring-opening polymerization of ε-CL
Complexes 1a-5a, 2b and 2c are also active for the ROP of ε-CL. As shown in Table 3, in
general the polymerizations of ε-CL initiated by these ketiminate aluminum complexes were faster
than those of rac-LA, and could be completed within 12 hours at 70 °C. Unlike the
polymerization of rac-LA, the electronic effect of substituents at the N-phenyl ring of the ligand

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framework on the catalytic activity of the corresponding aluminum complex is not conclusive.
Complexes 1a (o-Me2) and 2a (o-Cl2) with an ortho-substituted N-phenyl group exhibited almost
the same activity. However, the bis(ketiminate) aluminum methyl complex 2b was found to be
significantly more active than all the other aluminum complexes by completing the polymerization
in 5 h under otherwise identical conditions. By using complex 2a (o-Me2) as initiator, ε-CL
conversion up to 65% could be obtained in 3 days at 25 °C, while 98% of monomer conversion
was obtained in 1 day at 50 °C.
It is found that the measured molecular weights of resultant polymers obtained by complexes
1a-5a and 2b significantly deviate from the theoretical values and the molecular weight
distributions are rather broad (PDI = 1.8~2.4). In contrast, aluminum isopropoxide complex 2c
affords polymers with relatively narrow molecular weight distribution (PDI = 1.3) and the
molecular weight is much more comparable to the theoretical value. All these suggest that ROPs
of ε-CL initiated by these aluminum methyl complexes are not well controlled in comparison to
those initiated by aluminum isopropoxide complex.
4. Conclusions
Aluminum methyl complexes 1a-5a and 2b bearing ketiminate ligands were synthesized from
the reactions of trimethylaluminum and the corresponding ketimines. Two of them were further
identified by X-ray crystallography. Bis(ketiminate) aluminum isopropoxide complex 2c was
also obtained from the reaction of the corresponding ketimine with Al(OiPr)3 under harsh
conditions. The reactions of AlMe3 or Al(OiPr)3 with ketimines bearing para-substituted N-
phenyl group often led to the formation of triligated aluminum complexes as the major products,
with typical complex 3 characterized by a X-ray diffraction study. All the aluminum methyl

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complexes were proved to be active catalysts in the ring-opening polymerization of rac-LA and ε-
CL at high temperature to yield high molecular weight polymers with broad polydispersities.
While the substitution on the ketiminate ligand affected the reactivity and stereoselectivity of these
complexes for rac-LA polymerization, it did not show a conclusive effect on the polymerization
of ε-CL monomer. Aluminum isopropoxide complex 2c exerted better control for the ROP of
rac-LA and ε-CL, producing polymers with relatively narrow molecular weight distribution and
smaller molecular weights which are comparable to the theoretical values.
522
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Appendix A. Supplementary data
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CCDC 984641-984644 contain the supplementary crystallographic data for 2a, 3a, 3 and 6d,
respectively.
These
data
can
be
obtained
free
of
charge
via
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 email:
deposit@ccdc.cam.ac.uk.
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Acknowledgments
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This work is subsidized by the National Natural Science Foundation of China (NNSFC,
20604009 and 21074032), and the Fundamental Research Funds for the Central Universities
(WK1214048, WD1113011). All the financial supports are gratefully acknowledged. H. Ma
also thanks the very kind donation of a Braun glove-box by AvH foundation.
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