Zinc and aluminum complexes of chiral ligands: Synthesis, characterization and application to rac-lactide polymerization

Article abstract of DOI:10.1016/j.jorganchem.2016.12.031

The crystalline complexes L₂Zn (1), L₃Al (2), L′₂Zn (3) and L'AlMe₂(4) were synthesized from the reactions of the corresponding chiral ligands (S)[sbnd]N-(2-pyrrolylmethylene)-1-phenylethylamine (HL) or (S)[sbnd

Full text of DOI:10.1016/j.jorganchem.2016.12.031

Accepted Manuscript
Zinc and aluminum complexes of chiral ligands: Synthesis, characterization and
application to rac-lactide polymerization
Bin Jia, Junsheng Hao, Xuehong Wei, Hongbo Tong, Meisu Zhou, Diansheng Liu
PII:
S0022-328X(16)30592-7
10.1016/j.jorganchem.2016.12.031
JOM 19754
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 8 November 2016
Revised Date: 11 December 2016
Accepted Date: 25 December 2016
Please cite this article as: B. Jia, J. Hao, X. Wei, H. Tong, M. Zhou, D. Liu, Zinc and aluminum
complexes of chiral ligands: Synthesis, characterization and application to rac-lactide polymerization,
Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2016.12.031.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical abstract
Zinc and aluminum complexes of chiral ligands: Synthesis, characterization and application to
rac-lactide polymerization
Bin Jiaa,c*, Junsheng Haob, Xuehong Weib*, Hongbo Tongc, Meisu Zhouc and Diansheng Liuc*
Four novel chiral zinc and aluminum complexes L2Zn2 (1) , L3Al (2), L'2Zn (3) and L'AlMe2 (4)
[L= (S)-2-pyrrolal-1-phenylethylimine, L'= (S)-N-((2-pyrrolyl)methyl)-1-phenylethanamine were
synthesized in moderate or good yield via simple synthetic routes and all of the complexes can be used as
initiators to give the isotactic-rich polylactides
HL
Me
Me
NH
N
N
Me
N
N
ZnEt2
N
N
N
Al
Zn
AlMe3
N
N
Me
N
N
Me
1
NaBH4
Me
2
Me
NH
NH
ZnEt2
Me
N
HN
N
lMe
3
A
Me
Zn
HN
N
NH
Al
Me
Me
Me
4
3
HL'

ACCEPTED MANUSCRIPT
Zinc and aluminum complexes of chiral ligands: Synthesis, characterization and
application to rac-lactide polymerization
Bin Jiaa,c*, Junsheng Haob, Xuehong Weib*, Hongbo Tongc, Meisu Zhouc and Diansheng Liuc*
a School of Basic Medical Science, Shanxi Medical University, Taiyuan 030001, P.R China;
b School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, P. R. China
c Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, P. R. China
Corresponding author, Bin Jia
E-mail:bjia2006@163.com; xhwei@sxu.edu.cn; dsliu@sxu.edu.cn.
T: +86-351-701-8390;
F: +86-351-701-1688
Abstract: The crystalline complexes L2Zn (1), L3Al (2), L'2Zn (3) and L'AlMe2 (4) were
synthesized
from
the
reactions
of
the
corresponding
chiral
ligands
or
(S)-N-(2-pyrrolylmethylene)-1-phenylethylamine
(HL)
(S)-N-(2-pyrrolylmethyl)-1-phenylethanamine (HL') and diethylzinc or trimethylaluminum,
1
respectively. Complexes 1-4 were well characterized by H NMR, 13C NMR spectroscopy,
elemental analyses and single crystal X-ray crystallography. Furthermore, the catalytic activities
toward ring-opening polymerization (ROP) of rac-lactide have been investigated and all of the
complexes can be used as initiators to give the isotactic-rich polylactides.
Key words: Zinc; Aluminum; Chiral ligand; rac-Lactide; Polymerization.
1. Introduction
Metal complexes have attracted significant research attentions due to their novel structure

ACCEPTED MANUSCRIPT
and properties.[1-3] Importantly, some metal complexes exhibit catalytic activity for the chemical
polymer products. Polylactic acid (PLA), a renewable, biocompatible and biodegradable polymer,
has emerged to replace some petrochemical-based polyolefins as environmentally benign plastic
materials [4-6]. Ring-opening polymerization of rac-lactide initiated by discrete metal complexes
of Al, Zn, Mg, Ca, Sn, Fe and rare-earth metals is an efficient way to produce PLA with high
molecular weights as well as various tacticities [7-12]. Among which, organoaluminum and
organozinc complexes bearing salen ligands and their derivatives play an important role for this
purpose [13-17]. However, chiral metal complexes for the polymerization of rac-lactide have been
rarely investigated [18-20]. Recently, Gudong Du’s group used Chiral anilido-oxazolinate
aluminum complexes for the ROP of rac-lactide and showed hetero- and isotactic selectivities by
modulation of the substituent groups[19a]. Okuda and co-workers reported the chiral (OSSO)-type
bis(phenolato) aluminum complexes which are active for the ROP of rac-lactide k[19d]. Other
chiral salen-based aluminum catalysts gave rise to isotactic (or block-isotactic) PLA, but low to
moderate catalytic activity [15a, 17a-b, 20]. Schaper’s group reported the more active chiral
β-diketiminate zinc system that initiated the polymerization of rac-lactide with highly heterotactic
(Pr = 0.84 - 0.87) [20c]. To our knowledge，it still remains a challenge to obtain isotactic
enantiopure or stereoblock polymers from rac-lactide with these chiral zinc or aluminum
complexes.
We report herein the synthesis, structural characterization of well-defined mononuclear zinc
and aluminum complexes ligated by the chiral (S)-N-(-2-pyrrolylmethylene)-1-phenylethylamine
(HL) or (S)-N-(2-pyrrolylmethyl)-1-phenylethanamine (HL'), and their applications as catalysts in
the ring-opening polymerization of rac-lactide.

ACCEPTED MANUSCRIPT
2. Results and discussion
2.1. Synthesis and characterization of complexes 1-4
The two chiral ligands HL and HL′ [HL = C4H3NH{CH=NCH(Me)C6H5}-2, HL′=
C4H3NH{CH2NHCH(Me)C6H5}-2] were readily prepared by the reaction of the appropriate
2-pyrrolecarboxaldehyde and S-methyl benzylamine. The desired crystalline complexes L2Zn (1),
and L3Al (2) were prepared by treatment of ligand HL with diethylzinc (2:1 molar ratio) or
trimethylaluminum (3:1 molar ratio) in hexane at ambient temperature in high yields.
Attempts for the synthesis of the complexes Et2Zn2L2 or Me2AlLfrom the reactions of HL with
one equivalent portion of diethylzinc or trimethylaluminum at ambient temperature were
unsuccessful, respectively. The synthetic procedures and transformation of complexes 1, 2 are
illustrated in Scheme 1.
Me
Me
Me
Me
N
N
N
N
N
ZnEt2
AlMe3
N
N
Zn
N
Al
H
N
N
HL
N
N
Me
1
NaBH4
2
Me
Me
ZnEt2
Me
Me
N
HN
Me
lMe
A
3
Zn
HN
Me
N
N
NH
N
N
H
H
Al
Me
HL'
4
3
. The general synthetic routes for the complexes
Scheme 1
1-4
Similarly, treatment of ligand HL′ with diethylzinc or trimethylaluminum in hexane at
0°C gave complexes L′2Zn (3) and L′AlMe2 (4), respectively. Attempts for the synthesis of the

ACCEPTED MANUSCRIPT
complexes Et2Zn2L′2 or AlL′3 from the reactions of HL′ and one equivalent portion of diethylzinc
or three equivalent portions of trimethylaluminum at ambient temperature failed. The synthetic
procedures of complexes 3 and 4 are illustrated in Scheme 1.
2.2. Molecular structures of complexes 1-4
The single-crystal X-ray analysis reveals that complex 1 crystallizes as a mononuclear
species in the orthorhombic chiral space group P2(1)2(1)2(1) in Fig. 1, and selected bond
lengths and angles are listed in Table 1. The central zinc atom wrapped by two bis-bidentate
chiral Schiff base ligands lies in the distorted tetrahedral coordination geometry. The zinc atom
is approximately coplanar with N1C1C2C3C4C5N2 or N3C14C15C16C17C18N4 with deviations
of 0.100 Å or 0.195 Å, respectively. The dihedral angle of the planes N1ZnN2 and N3ZnN4 is
82.20°. The bond angles of N1-Zn-N2 [84.61(3)°] and N3-Zn-N4 [84.55(2)°] are comparable to
that found in [{C4H3N(CHNCMe3)-2}2Zn] [84.1(1)°] [21], whereas the four interchelate angles are
123.52(13)°, 114.23(12)°, 131.51(13)° and 123.02(13)°, respectively. This difference should
be attributed to the effect of the coordinated amino group on the two symmetric chiral ligands.
The Zn-N distances [1.969(3) Å (Zn-N1), 1.987(3) Å (Zn-N3)] of complex 1 are comparable to
that of the distance of Zn-N (pyrrolyl) in [{C4H3N(CHNCMe3)-2}2Zn] [1.975(2) Å] [21].
(Figure 1)
(Table 1)
In complex 2 (Fig. 2), there are two almost identical independent molecules in the
asymmetric unit. Therefore, bond lengths and angles in one molecule are listed in Table 2. The
six-coordinated aluminum center is located in distorted octahedral environment composed by six

ACCEPTED MANUSCRIPT
N atoms from the three ligands. In complex 1, atoms N1,C1,C2,C3,C4,C5 and N2 are located in
one plane and the distance from Zn atom to this plane is 0.100 Å. The same structural feature
can be found in complex 2 with the distances of Al atom to the planes N1C1C2C3C4C5N2,
N3C14C15C16C17C18N4 and N5C27C28C29C30C31N6 of 0.084, 0.045 and 0.133Å,
respectively. The NAlN planes are approximately vertical to each other. The dihedral angles
between N1AlN2 and N3AlN4, or N5AlN6 are 86.32° and 89.84°, respectively; the dihedral
angle between N3AlN4 and N5AlN6 is 88.07°. The Al-N distances [1.952(3), 1.976(3) Å] are
longer than the distances of Al-N in [{C4H3N(CH2NMe2)-2}AlMe2] [1.8785(13) Å] and
[{[C4H3N(2-CH2NtBu)]Al}2] [1.834(1) Å] [22]. The bond angles of N-Al-N [81.92(11)°,
80.53(12)°, 80.75(11)°] are narrower than that found in [{C4H3N(CH2NMe2)-2}AlMe2]
[85.40(5)°] or [{[C4H3N(2-CH2NtBu)]Al}2] [91.4(1)°] [22].
(Figure 2)
(Table 2)
Crystals suitable for an X-ray diffraction study were obtained for 3 (Fig. 3). There are the
two independent molecules in the asymmetric unit, and the geometry parameters have no
significant differences. Therefore, bond lengths and angles in one molecule are listed in Table 1.
The X-ray diffraction study shows that the central zinc atom lies in the distorted tetrahedral
coordination geometry composed by two trans arranged five-membered rings. The dihedral angle
of planes N1Zn1N2 and N3Zn1N4 is 82.26°, which is comparable to that in complex 1 [82.20°].
The bond angles of N1-Zn1-N2 [84.49(12)°] and N3-Zn1-N4 [84.57(11)°] or the Zn1-N
distances [1.945(2) Å (Zn1-N1), 1.953(2) Å (Zn1-N3)] are also comparable to those found in
complex 1 [84.61(13)°, 84.55(12)°] or [1.969(3) Å, 1.987(3) Å], respectively.

ACCEPTED MANUSCRIPT
(Figure 3)
The molecular structure of complex 4 is presented in Fig. 4, and its bond lengths and angles
are collected in Table 1. The four-coordinate, distorted tetrahedral aluminum atom has its distance
to N1 ca. 0.14 Å longer than that to N2, while the six angles centered at the aluminum atom range
from the N1-Al-N2 at ca. 85° to the C14-Al-C15 at ca. 120°. The Al-N2 distance [1.874(3) Å] is
shorter than those in complex 2 [2.031(3) Å (Al1-N2), 1.952(3) Å(Al1-N4), 2.043(3) Å
(Al1-N6)], respectively. But the Al-N1 distance [2.012(3) Å] is comparable to those found in
complex 2 [1.952(3) Å, 2.071(3) Å or 1.976(3) Å ].
(Figure 4)
2.3. Catalytic behavior of the catalysts 1-4
Complexes 1-4 were used as initiators for catalytic ring-opening polymerization of
rac-lactide. Polymerizations were carried out in Schlenk flask under nitrogen atmosphere strictly
free of moisture and oxygen in toluene, as well as the polymerization factors of molar ratio of
[LM]/ [LA](1:100 or 1: 200) and reaction temperature (room temperature or 70 °C) were tested,
which were summarized in Table 3. The polymerization data show that complexes 1-4 can initiate
the ring opening polymerization (ROP) of rac-lactide under mild conditions. At room temperature
complexes 1-4 were found to be low active at a [LM] / [LA] molar ratio of 1: 100 (Table 3, entries
1, 4, 7 and 10), but at 70 °C, all above complexes showed high and similar levels of activity
(Table 3, entries 2-3, 5-6, 8-9 and 11-12). Molecular weight distributions [Mw/Mn, polydispersity
index, PDI (1.23-1.56)] are broad, indicating not well-controlled polymerization. For complexes 1
and 3, (Table 3, entries 2 to 3 and entries 8 to 9), moderate molecular weight distributions are

ACCEPTED MANUSCRIPT
observed. Molecular weights and polydispersities of the polymers produced ranged from 9.2 to
20.2 kg mol−1 and 1.35 to 1.56, respectively.
For two aluminum initiators (complexes 2 and 4), all gave similar molecular weights,
moderate average molecular weight distributions (entries 5 to 6 and entries 11 to 12). When
complex 2 was used as an initiator, rapid polymerization was observed and over 90% yields
achieved within 3 h, and the polymer molecular weight was highest [(Mn) up to 41.2 kg mol−1
(Table 3, entry 6)]. The instability of metal complex of N-containing ligand towards
alcoholysis/enolysis was also observed in other Al and Zn compounds containing nitrogen and
oxygen atoms [23]. The resulting polylactides are all isotactic-rich (riched in the region of
5.16-5.09 ppm) under our conditions examined, as determined by the methine region of the
1
homonuclear decoupled HNMR experiments in the region of 5.30-5.00 ppm [24]. Those results
show that the catalytic activities of complexes 1-4 resemble those of
{(±)-trans-Cy(NSiMe3)2}Al2Me4 and {(±)-trans-Cy(NSiMe3)2}2Zn4Et4 [25], while the
microstructure of the resulting polylactides is similar to that initiated by RAlMe (R =
PhC(N-2,6-iPr2C6H3)(N-2,6-Me2C6H3)) [26].
(Table 3)
3. Conclusions
In conclusion, four novel chiral zinc and aluminum complexes have been readily prepared
from the reactions of chiral ligands HL or HL′ and diethylzinc or trimethylaluminum. All
complexes are active as initiators in the ring opening polymerization of rac-lactide under mild
conditions, and afford polylactides with similar molecular weights and broad molecular weight

ACCEPTED MANUSCRIPT
1
distributions (PDI = 1.23-1.56). Determination of homonuclear decoupled HNMR on methine
region showed that the polylactides are isotactic-rich in the range of 5.16–5.09 ppm. The
stereoselectivity of the chiral ligand on the ROP of rac-lactide will be reported in due course.
4. Experimental
4.1. General comments
All manipulations were carried out under nitrogen atmosphere in the flamed Schlenk-type
glassware on a dual manifold Schlenk line. Solvents purchased from commercial sources were
distilled over standard drying agents under nitrogen from alkali metals directly and stored over 4
Å molecular sieves. All the chemicals used were of reagent grade, obtained from Aldrich.
rac-Lactide was recrystallized with dry toluene and sublimed twice under vacuo at 50 °C. Melting
points were determined in sealed capillaries under argon on an electrothermal apparatus and are
1
1
uncorrected. H and 13C NMR spectra were recorded on a Bruker DRX 300 (300.1 MHz for H,
75.5 MHz for 13C) instrument and referenced internally to the residual solvent resonances
1
(chemical shift data in δ). The homonuclear decoupled H NMR spectra were recorded on a
Bruker AV 400 spectrometer at 400. Elemental analyses were performed on a Vario EL-III
instrument. X-ray single crystal structures were determined on a Bruker Smart CCD APEX area
detector. Polymerizations were carried out in Schlenk flasks under nitrogen atmosphere strictly
free of moisture and oxygen at room temperature or 70 °C, as well as the polymerization factors of
molar ratio of [LM]/ [LA] and reaction time were tested. Gel permeation chromatography (GPC)
analyses were carried out on a Waters 1515 Breeze Gel Permeation Chromatograph equipped with
differential refractive index detectors. The GPC columns were eluted with tetrahydrofuran with 1

ACCEPTED MANUSCRIPT
mL/min rate at 25 °C and were calibrated with monodisperse polystyrene standards.
5. Preparation
5.1. Preparation of Ligands.
5.1.1. Preparation of (S)-N-(2-pyrrolylmethylene)-1-phenylethylamine (HL)
According to the classical condensation reaction of aldehyde and amine, the Schiff base HL
was isolated as yellow oily liquid. (18.22 g, 95% yield). 1HNMR (300 MHz, CDCl3): δ 1.65, 1.67
(d, 3 H, CH3), 4.59, 4.61 (m, 1 H, CHCH3), 6.28, 6.56, 6.79 (3 H, CH of pyrrolyl), 7.29-7.45 (m, 5
H, Ar-H), 8.26 (s, 1 H, N=CH), 9.50 (s, 1 H, NH). 13CNMR (75.5 MHz, CDCl3): δ 27.1 (CH3),
71.6 (CHCH3), 112.3, 117.4, 124.8, 129.6 (C of pyrrolyl), 128.4, 131.2, 132.9, 147.7 (Ar-C), 153.4
(C=N). Anal. Calcd for C13H14N2: C, 78.75; H, 7.12; N, 14.13%. Found C, 78.84; H, 7.06; N,
14.10%.
5.1.2. Preparation of (S)-N-(2-pyrrolylmethyl)-1-phenylethanamine(HL')
To ethanol solution of C4H3NH{CH=NCH(Me)C6H5}-2 (HL), (9.91 g, 50.0 mmol ) was
slowly added NaBH4 (2.84 g, 75.1 mmol) at 0 °C, after being stirred for additional 8 h at ambient
temperature, the mixture was rotary evaporated under reduced pressure. Then to the mixture was
added water (10.0 mL), the aqueous phase was separated and the organic complex was extracted
with diethyl ether (15.0 mL × 3). The combined organic phase was dried over MgSO4 and rotary
evaporated under reduced pressure, then the product HL' (9.77 g, 98% yield) was obtained as
yellow oil liquid. 1H NMR (300 MHz, CDCl3): δ 1.42, 1.44 (d, 3 H, CH3), 3.67(s, 2 H, CH2), 3.84,
3.86 (m, 1 H, CHCH3), 6.05, 6.19, 6.74 (3 H, CH of pyrrolyl), 7.32-7.41 (m, 5 H, Ar-H), 8.96 (s, 1

ACCEPTED MANUSCRIPT
H, NH of pyrrolyl), 1.78 (s, 1 H, NH). 13CNMR (75.5 MHz, CDCl3): δ 27.6 (CH3), 51.2 (CH2NH),
67.9 (CHCH3), 112.2, 117.2, 124.6, 130.3 (C of pyrrolyl), 127.1, 128.6, 132.1, 146.7 (Ar-C). Anal.
Calcd for C13H16N2: C, 77.96; H, 8.05; N, 13.99%. Found C, 78.05; H, 7.99; N, 13.96%.
5.2. Preparation of complexes 1-4.
5.2.1. Preparation of L2Zn (1)
HL (1.19 g, 6.0 mmol) was dissolved in hexane (25.0 mL), and the solution was cooled to
0 °C. Diethylzinc (3.0 mL, 1 M, 3.0 mmol) was added to this solution slowly, and the mixture was
stirred overnight at room temperature. The resulting precipitate was collected by filtration, washed
with cold hexane (2 x 10 mL), and dried in vacuo (1.11 g, 78%). Crystals suitable for X-ray
crystallographic analysis were crystallized from diethyl ether at ambient temperature. Mp:
1
159-161°C. H NMR (300 MHz, C6D6): δ 1.20 (d, 6 H, CH3), 4.07 (s, 2 H, CHCH3), 6.64, 6.86,
6.87 (m, 6 H, CH of pyrrolyl), 6.94, 6.97, 7.05, 7.20 (m, 10 H, Ar-H), 7.56 (s, 2 H, N=CH). 13C
NMR (75.5 MHz, C6D6): δ 23.0 (CH3), 64.6 (CHCH3), 113.7, 118.2, 121.3, 135.9 (C of pyrrolyl),
127.2, 128.8, 136.6, 143.3 (Ar-C), 158.0 (C=N). Anal. Calcd for C26H26N4Zn: C, 67.90; H, 5.70;
N, 12.18%. Found C, 67.69; H, 5.58; N, 12.31%.
5.2.2. Preparation of L3Al (2).
HL (1.23 g, 6.0 mmol) was dissolved in hexane (25.0 mL), and the solution was cooled to
0 °C. Trimethylaluminum (1.0 mL, 2 M, 2.0 mmol) was added to this solution slowly, and the
mixture was stirred overnight at room temperature. The resulting precipitate was collected by
filtration, washed with cold hexane (2 x 10 mL), and dried in vacuo (0.78 g, 1.3 mmol, 63%).

ACCEPTED MANUSCRIPT
Crystals suitable for X-ray crystallographic analysis were grown from diethyl ether at ambient
temperature. Mp: 179-181 °C. 1H NMR (300 MHz, C6D6): δ 1.14, 1.16, 1.26, 1.28, 1.49, 1.51 (m,
9 H, CH3), 4.14, 4.25, 4.79 (s, 3 H, CHCH3), 6.10, 6.26, 6.73 (m, 9 H, CH of pyrrolyl), 7.09~7.69
(m, 15 H, Ar-H), 7.81, 8.01, 8.08 (s, 3 H, N=CH). 13C NMR (75.5 MHz, C6D6): δ 24.0, 25.1, 25.8
(CH3), 59.4, 60.9, 61.3 (CHCH3), 114.8, 115.4, 118.2, 119.5, 134.3, 135.4 (C of pyrrolyl),
128.7~145.5 (Ar-C), 158.3, 158.9, 159.6 (C=N). Anal. Calcd for C39H39AlN6: C, 75.70; H, 6.35; N,
13.58%. Found C, 75.54; H, 6.29; N, 13.67%.
5.2.3. Preparation of L′2Zn (3).
HL′ (1.19 g, 6.0 mmol) was dissolved in hexane (25.0 mL), and the solution was cooled to
0 °C. Diethylzinc (3.0 mL, 1 M, 3.0 mmol) was added to this solution slowly, and the mixture was
stirred overnight at room temperature. The resulting precipitate was collected by filtration, washed
with cold hexane (2 x 10 mL), and dried in vacuo (1.07 g, 74%). Crystals suitable for X-ray
crystallographic analysis were grown from diethyl ether at ambient temperature . Mp: 155-157 °C.
1H NMR (300 MHz, C6D6): δ 0.55, 0.58 (d, JHH = 5.3 Hz, 6 H, CH3), 2.90~2.96 (t, JHH = 18.7 Hz,
2 H, CHCH3), 3.04~3.15, (m, JHH = 56.5 Hz, 4 H, NCH2), 3.2 (s, 2 H, NH), 6.14, 6.15, 6.58 (m, 6
H, CH of pyrrolyl), 6.16~6.92 (m, 10 H, Ar-H). 13C NMR (75.5 MHz, C6D6): δ 20.4 (CH3), 48.9
(CH2N), 58.9 (CHCH3), 104.7, 110.79, 125.8, 135.1 (C of pyrrolyl), 126.7, 127.6, 129.5, 142.1
(Ar-C), Anal. Calcd for C26H30N4Zn: C, 67.31; H, 6.52; N, 12.08%. Found C, 67.38; H, 6.59; N,
11.91%.
5.2.4. Preparation of L′AlMe2 (4).
HL′ (0.59 g, 3.0 mmol) was dissolved in hexane (25.0 mL), and the solution was cooled to

ACCEPTED MANUSCRIPT
0°C. Trimethyl aluminum (1.5 mL, 2 M, 3.0 mmol) was added to this solution slowly, and the
mixture was stirred overnight at room temperature. The resulting precipitate was collected by
filtration, washed with cold hexane (2 x 10 mL), and dried in vacuo (0.51 g, 68%). Crystals
suitable for X-ray crystallographic analysis were grown from diethyl ether at ambient temperature.
1
Mp: 136-138 °C. H NMR (300 MHz, C6D6): δ -0.52, -0.28 (d, JHH = 30.5 Hz, 6 H, Al(CH3)2),
0.96, 0.98 (d, JHH = 8.8 Hz, 3 H, CH3), 1.37, 1.39 (d, JHH = 7.0 Hz, 3 H, CH3), 3.42 (q, JHH = 8.6
Hz, 2 H, NCH2), 3.62 (q, JHH = 3.4 Hz, 1 H, CHCH3), 6.18, 6.20, 6.61 (m, 3 H, CH of pyrrolyl),
6.69-7.14 (m, 5 H, Ar-H). 13C NMR (75.5 MHz, C6D6): δ -11.4, -8.98 (Al(CH3)2), 23.0 (CH3),
47.8 (NCH2), 60.2 (CHCH3), 104.8, 111.4, 122.8, 139.8 (C of pyrrolyl), 126.9, 129.0, 131.9, 142.2
(Ar-C). Anal. Calcd for C15H21AlN2: C, 70.29; H, 8.26; N, 10.93%. Found C, 70.17; H, 8.22; N,
10.98%.
5.3. General procedure for polymerization of rac-lactide
A Schlenk tube previously dried at 150 °C and cooled under nitrogen, and then the Schlenk
flask was charged with a solution of the initiator in toluene (0.2 mL). To this solution was added
rapidly a slurry of the monomer (rac-lactide) in the appropriate ratio in toluene (10.0 mL). The
mixture was immediately stirred with a magnetic stir bar at room temperature or 70 °C. After the
desired time, the reaction was quenched by adding acidified methanol (ca. 1.0 mL of 1.2 M
CH3COOH solution) and the polymer was precipitated with excess methanol (50.0 mL). The
polymer was filtered and then dried in vacuo to constant weight.
6. X-ray Crystallography.
The single crystals of 1-4 suitable for X-ray diffraction studies were obtained. X-Ray

ACCEPTED MANUSCRIPT
diffraction data were collected with graphite-monochromated Mo-Ka radiation (λ= 0.71073 Å) on
a Bruker Smart Apex CCD diffractometer at 298(2) K or 213(2) K, equipped with an Oxford
Cryosystems CRYOSTREAM device. The collected frames were processed with the proprietary
software SAINT and an absorption correction was applied (SADABS) to the collected reflections
[27]. The structures of these molecules were solved by direct methods and expanded by standard
difference Fourier syntheses using the software SHELXTL [28, 29]. Structure refinements were
made on F2 using the full-matrix least-squares technique. All non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen atoms were placed in their idealized
positions and allowed to ride on the respective parent atoms. Pertinent crystallographic data and
other experimental details are summarized in Table 4. Full crystallographic information for each
complex is available in its CIF.
(Table 4)
Acknowledgements
We thank National Natural Science Foundation of China (No. 21272142 and 21371111) , the
Special Fund for Agroscientific Research in the Public Interest (No. 201303106) and the Research
Fund for the Doctoral Program of Shanxi Medical University [No. 055208] financial support.
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge
Crystallographic Data centre, CCDC no. 854041 for complex 1, CCDC no. 854043 for complex 2 ,
CCDC no. 854044 for complex 3 and CCDC no. 854045 for complex 4. Copies of this
information may be obtained free of charge from The Director, CCDC, 12 Union Road,

ACCEPTED MANUSCRIPT
Cambridge CB2 1EZ, UK (Fax: +44-1223-336003; e-mail: deposit@ccdc.ac.uk or www: http://
www.ccdc.cam.ac.uk).
Reference
[1]. D. Zhang , G. Zi, Chem. Soc. Rev., (44) 2015 1898-1921
[2]. (a) Q. Dong, W. Qu, W. Liang, F. Tai, K. Guo, C.-W. Leung, W.-Y. Wong, J. Mater. Chem.
C, 4 (2016) 5010-5018;
(b) Q. Dong, W. Qu, W. Liang, K. Guo, H. Xue, Y. Guo, Z. Meng, C.-L. Ho, C.-W. Leung,
W.-Y. Wong, Nanoscale, 8 (2016) 7068-7074;
(c) Q. Dong, G. Li, C.-W. Leung, I. Manners, C.-L. Ho, W.-Y. Wong, J. Mater. Chem. C, 3
(2015) 734-741;
(d) Q. Dong, G. Li, C.-L. Ho, C.-W. Leung, P. W.-T. Pong, I. Manners, W.-Y. Wong, Adv.
Funct. Mater. 24 (2014) 857-862;
(e) Q. Dong, G. Li, C.-L. Ho, M. Faisal, C.-W. Leung, P. W.-T. Pong, K. Liu, B.-Z. Tang, I.
Manners , W.-Y. Wong, Adv. Mater. 24 (2012) 1034-1040.
[3]. (a) H.Yanga, L. Zhangb, D.Zanga, Z. Fu , Z. Fan, Catalysis Communications,62 (2015)
104-106;
(b) S.Abedi, M.Abdouss, Applied Catalysis A: General , 475 (2014) 386-409;
(c) D. Peng, X. Yan, C.Yu, S. Zhang , X, Li Polym. Chem., 7 (2016) 2601-2634.
[4]. M. J. Stanford and A. P. Dove, Chem. Soc. Rev. 39 (2010) 486-494.
[5]. (a) M. J.-L.Tschan, E. Brule, P. Haquette, C. M. Thomas, Polym. Chem. 3 (2012) 836-851;
(b) C. A. Wheaton, P. G. Hayes, B. J. Ireland, Dalton Trans. (2009) 4832-4846;

ACCEPTED MANUSCRIPT
(c) S. M. Guillaume, E. Kirillov, Y. Sarazin and J.-F. Carpentier, Chem.-Eur. J. 21(2015) ,
7988-8003;
(d) A. Amgoune, L. Lavanant, C. M. Thomas, Y. Chi, R. Welter, S. Dagorne and J.-F.
Carpentier, Organometallics, 24 (2005) 6279-6282;
(e) T. Artham, M. Doble, Macromol. Biosci. 8 (2008) 14-24.
[6]. R. Langer, J. P. Vacanti, Science, 260 (1993) 920-926.
[7]. (a) Y. Sarazin, J. F. Carpentier, Chem. Rev. 115 (2015) 3564-3614;
(b) S. Dagorne, M. Normand, E. Kirillov, J. F. Carpentier, Coord. Chem. Rev. 257 (2013)
1869-1886.
[8]. (a) Y. Chapurina , T. Roisnel, J.-F. Carpentier , E. Kirillov, Inorg. Chim. Acta, 431 (2015)
161-175;
(b) I. D. Vieira, S. Herres-Pawlis, Eur. J. Inorg. Chem. 2012 (2012) 765-774;
(c) S. Tabthong, T. Nanok,P. Sumrit, P. Kongsaeree, S. Prabpai, P. Chuawong and P.
Hormnirun, Macromolecules, 48 (2015), 6846-6861;
(d) S. Tabthong, T. Nanok,P. Sumrit, P. Kongsaeree, S. Prabpai, P. Chuawong and P.
Hormnirun, Macromolecules, 48 (2015), 6846-6861;
(e) J. C. Buffet, J. Okuda, Polym. Chem. 2 (2011) 2758-2763;
(f) B. Jia, X. Wei, H. Tong, M. Zhou, D. Liu, Inorg. Chim. Acta, 388 (2012) 127-134.
[9] (a) A. K. Sutar, T. Maharana, S. Dutta, C. T. Chen, C. C. Lin, Chem. Soc. Rev. 39(2010)
1724-1746;
(b) C. M. Thomas, Chem. Soc. Rev. 39 (2010) 165-173;
(c) O. D. Cabaret, B. M. Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147-6176.

ACCEPTED MANUSCRIPT
[10]. M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer, M. Pacold and K. Phomphrai, J.
Am. Chem. Soc. 122 (2000) 11845-11854.
[11]. (a) R. D. Köhn, Z. Pan, J. Sun and C. Liang, Catal. Commun. 4 (2003) 33-37;
(b) Q. Dong, X. Ma, J. Guo, X. Wei, M. Zhou, D. Liu, Inorg. Chem. Commun. 11 (2008)
608-611;
(c) K. B. Aubrecht, K. Chang, M. A. Hillmyer and W. B. Tolman, J. Polym. Sci., Part A:
Polym. Chem. 39 (2001) 284-293;
(d) M. H. Chisholm, C. C. Lin, J. C. Gallucci and B.T. Ko, Dalton Trans. (2003) 406-412.
[12]. (a) K. B. Aubrecht, K. Chang, M. A. Hillmyer and W. B. Tolman, J. Polym. Sci., Part A:
Polym. Chem. 39 (2001) 284-293;
(b) B. J. O’Keefe, L. E. Breyfogle, M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc.
124 (2002) 4384-4393.
[13]. (a) H. J. Chuang, H. L.Chen, B. H. Huang, T. E. Tsai, P. L. Huang, T. T. Liao, C. C. Lin, J.
Polym. Sci., Part A: Polym. Chem. 51 (2013) 1185-1196;
(b) H. Wang, Y. Yang and H. Ma, Macromolecules, 47(2014) 7750-7764;
(c) J. B. L. Gallaway, J. R. K. McRae, A. Decken, M. P. Shaver, Can. J. Chem. 90 (2012)
419-426;
(d) J. B. L. Gallaway, J. R. K. McRae, A. Decken, M. P. Shaver, Can. J. Chem. 90 (2012)
419-426;
(e) X. F. Yu, C. Zhang, Z. X. Wang, Organometallics, 32 (2013) 3262-3268.
[14]. (a) H. J. Chuang, S. F. Weng, C. C. Chang, C. C. Lin, H. Y. Chen, Dalton Trans. 40 (2011)
9601-9607;

ACCEPTED MANUSCRIPT
(b) H. Du, X. Pang, H. Yu, X. Zhang, X. Chen, D. Cui, X. Wang, X. Jing,
Macromolecules, 40 (2007) 1904-1913;
(c) Z. Zhong, P. J. Dijkstra and J. Feijen, J. Am. Chem. Soc. 125 (2003) 11291-11298.
[15]. (a) N. Nomura, R. Ishii, M. Akakura and K. Aoi, J. Am. Chem. Soc. 124 (2002)
5938-5839;
(b) K. Majerska and A. Duda, J. Am. Chem. Soc. 126 (2004) 1026-1027;
(c) P. Hormnirun, E. L. Marshall, V. C. Gibson, A. J. P. White and D. J. Williams, J. Am.
Chem. Soc. 126 (2004) 2688-2689.
[16]. M. Wisniewski, A. Le Borgne, N. Spassky, Chem. Phys. 198 (1997) 1227-1238.
[17]. (a) J.F. Greco, M. J. McNevin and J. R. Hagadorn, Organometallics, 24 (2005) 5167-5171;
(b) S. A. Ahmed, M. S. Hill, P. B. Hitchcock, S. M. Mansell and O. S. John,
Organometallics, 26 (2007) 538-549;
(c) M. H. Chisholm, J. C. Gallucci, K. T. Quisenberry and Z. Zhou, Inorg. Chem. 47
(2008) 2613-2624;
(d) A. J. Chmura, D. M. Cousins, M. G. Davidson, M. D. Jones, M. D. Lunn and M. F.
Mahon, Dalton Trans. (2008) 1437-1443.
[18]. Q. Wang, F. Zhang, H. Song, G. Zi, J. Organomet. Chem. 696 (2011) 2186-2192.
[19]. (a) B. Shi, A. Srinivas, Z. l. Lu, K. Edward, and G. Du, Organometallics, 33 (2014)
2489-2495;
(b) C. Bakewell, R. H. Platel, S. K.Cary, S. M. Hubbard, J.M.Roaf, A. C. Levine, A. J. P.
White, N. J. Long, M. Haaf, C. K. Williams, Organometallics, 31 (2012) 4729-4736;
(c) Y. Chapurina, T. Roisnel, J.-F. Carpentier , E. Kirillov , Inorganica Chimica Acta, 431

ACCEPTED MANUSCRIPT
(2015) 161–175;
(d) B. Lian, H. Ma, T.P. Spaniol and J. Okuda, Dalton Trans. (2009) 9033-9042.
[20]. (a) N. Spassky, M. Wisniewski, C. Pluta, A. Le Borgne, Macromol. Chem. Phys. 197 (1996)
2627-2637;
(b) C. P. Radano, G. L. Baker, M. R. Smith, J. Am. Chem. Soc. 122 (2000) 1552-1553;
(c) F. Drouin, P. O. Oguadinma, T. J. J. Whitehorne, R. E. Prud’homme and F. Schaper,
Organometallics, 29(2010) 2139-2147;
(d) J. D. Darensbourg, O. Karroonnirun, Organometallics, 29 (2010) 5627-5634;
(e) H. Du, A. H. Velders, P. J. Dijkstra, Z. Zhong, X. Chen, J. Feijen, Macromolecules, 42
(2009) 1058-1066 ;
(f) F. Drouin, P. O. Oguadinma, T. J. J. Whitehorne, R. E. Prud’homme and F. Schaper,
Organometallics, 33 (2013) 2489-2495;
(g) H. L. Chen, S. Dutta, P. Y. Huang, C. C. Lin, Organometallics, 31 (2012) 2016-2025.
[21]. J. A. Kanters, A. L. Spek, R. Postma, G. C. van Stein and G. van Koten, Acta Cryst. C39
(1983) 999-1001.
[22]. C. F. Tsai, H. Chen, J. Chang, C. Hung, C. Lai, C. Hu and J. Huang, Inorg. Chem. 43
(2004) 2183-2188.
[23]. (a) V. C. Gibson, C. Redshaw, A. J. P. White and D. J. Williams, J. Organomet. Chem. 550
(1998) 453-456;
(b) C. J. Carmalt, J. D. Mileham, A. J. P. White and D. J. Williams, Organometallics, 22
(2003) 1554-1557;
(c) J. Pinkas, J. Löbl, D. Dastych, M. Necas and H. W. Roesky, Inorg. Chem. 41 (2002)

ACCEPTED MANUSCRIPT
6914-6918;
( d) A. P. Dove, V. C. Gibson, E. Marshall, A. J. P. White and D. J. Williams, Dalton
Trans. (2004) 570-578;
(e) Z. Zheng, M. K. Elmkaddem, C. Fischmeister, T. Roisnel, C. M. Thomas, J. F.
Carpentier and J. L. Renaud, New J. Chem. 32 (2008) 2150-2158.
(a) K. Peng and C. Chen, Dalton Trans. (2009) 9800-9806.;
(b) J. E. Kasperczyk, Macromolecules, 28 (1995) 3937-3939;
(c) K. A. M. Thakur, R. T. Kean, E. S. Hall, M. A. Doscotch, J. I. Siepmann,
Macromolecules, 30 (1997) 2422-2428;
[24].
(d) X. Pang, X. Chen, H. Du, X. Wang, X. Jing, J. Organomet. Chem. 692 (2007)
5605-5613;
(e) J. C. Buffet, J. Okuda and P. L. Arnold, Inorg. Chem. 49 (2010) 419-426.
D. Chakraborty and E.Y.-X. Chen, Organometallics 22 (2003) 769-774.
F. Qian, K. Liu and H. Y. Ma, Dalton Trans. 39 (2010) 8071-8083
[25].
[26].
[27]. Bruker SMART (Version 5.0) and SAINT (Version 6.02), Bruker AXS Inc., Madison, WI,
(2000).
[28]. G. M. Sheldrick, SADABS, SHELXS97 and SHELXL97, University of Göttingen,
Germany, (1997).
[29]. G. M. Sheldrick, SHELXTL/PC. Version, 6.10, Bruker AXS Inc., Madison, WI, (1999).

ACCEPTED MANUSCRIPT
Tables
Table 1. Selected bond lengths [Å] and angles [deg] for 1, 3 and 4
Complex 1
1.969(3)
2.062(3)
1.987(3)
2.044(3)
Complex 3
1.945(2)
2.126(2)
1.953(2)
2.179(2)
Complex 4
2.012(3)
1.874(3)
1.950(5)
1.949(5)
Zn1-N1
Zn1-N2
Zn1-N3
Zn1-N4
Al-N1
Al-N2
Al-C14
Al-C15
N1-Zn1-N2
N3-Zn1-N4
N3-Zn1-N2
N1-Zn1-N3
C1-N1-Zn1
C5-N2-Zn1
C14-N3-Zn1
C18-N4-Zn1
84.61(13)
84.55(12)
114.23(12)
131.51(13)
144.8(3)
85.30(9)
C15-Al-C14
C15-Al-N2
N2-Al-N1
C15-Al-N1
N2-Al-C14
C9-N1-Al
C1-N1-Al
C10-N2-Al
119.9(2)
113.06(19)
85.21(14)
111.3(2)
115.6(2)
105.9(2)
117.5(2)
114.5(3)
84.01(9)
135.73(9)
129.47(8)
138.65(13)
105.16(12)
139.65(15)
101.64(12)
107.8(2)
145.2(3)
108.5(3)
Table 2. Selected bond lengths [Å] and angles [°] for 2
Bond lengths
Al1-N1
1.952(3)
2.071(3)
1.976(3)
Al1-N2
Al1-N4
Al1-N6
2.031(3)
1.952(3)
2.043(3)
Al1-N3
Al1-N5
Bond angles
N1-Al1-N4
N4-Al1-N5
N4-Al1-N2
N1-Al1-N6
N5-Al1-N6
N1-Al1-N3
N5-Al1-N3
N6-Al1-N3
170.37(11)
97.40(12)
92.06(12)
94.53(11)
80.75(11)
92.31(11)
174.23(11)
93.92(10)
N1-Al1-N5
N1-Al1-N2
N5-Al1-N2
N4-Al1-N6
N2-Al1-N6
N4-Al1-N3
N2-Al1-N3
C1-N1-C4
90.32(11)
81.92(11)
91.72(11)
92.39(11)
171.68(11)
80.53(12)
93.74(11)
105.6(3)

ACCEPTED MANUSCRIPT
Table 3 Polymerization of rac-lactide catalyzed by chiral complexes 1-4
O
O
O
O
O
O
O
O
Catalyst
+
O
O
O
O
m
O
O
n
O
O
rac-Lactide
Isotactic Polylactide
Catalyst
[LA]/[C]]a
100:1
100:1
200:1
100:1
100:1
200:1
100:1
100:1
200:1
100:1
100:1
200:1
T (°C)
25
Time (h)
Yield (%) b
Mn (g/mol) c
Mw (g/mol) c
–
Mw/Mn c
–
Pm d
–
1
2
1
1
1
2
2
2
3
3
3
4
4
4
8
3
3
8
3
3
8
3
3
8
3
3
36.
90
87
37
96
82
26
92.
79
24
97.
81
–
70
9190
16573
–
13693
22376
–
1.49
1.35
–
0.71
0.69
–
3
70
4
25
5
70
26230
41194
–
35935
56436
–
1.37
1.37
–
0.71
0.72
–
6
70
7
25
8
70
12447
20206
–
18048
31521
–
1.45
1.56
–
0.68
0.69
–
9
70
10
11
12
25
70
11046
19504
13918
23990
1.26
1.23
0.69
0.70
70
a Toluene; b The yields were calculated according to the mass of the polymer was precipitated by the addition of MeOH and was
collected by filtration and dried; c Determined by gel-permeation chromatography, calibrated with polystyrene standards in
tetrahydrofuran; d Pm is the probability of meso linkages between monomer units and is determined from the methine region
(5.30–5.00 ppm) of the homonuclear decoupled 1H NMR spectrum in CDCl3 at 25 °C.

ACCEPTED MANUSCRIPT
Table4 Crystallographic data and structure refinement for complexes 1-4
complex
1
2
3
4
Foumula
C26H26ZnN4
459.88
C39H39AlN6
618.74
C26H30ZnN4
463.91
C15H21N2Al
256.32
Formula weight
Temperature
Crystal system
Space group
a [Å]
203(2)
238(2)
296(2)
223(2)
Orthorhombic
P2(1)2(1)2(1)
10.139(2)
13.180(3)
17.023(4)
90.00
Monoclinic
P2(1)
Triclinic
P1
Monoclinic
P2(1)
10.094(5)
31.345(17)
11.899(8)
90.00
9.335(11)
9.393(11)
15.058(18)
91.223(16)
93.624(16)
113.103(15)
1210(2)
2
6.711(4)
15.724(8)
7.272(4)
90.00
b [Å]
c [Å]
α [deg]
β [deg]
90.00
114.68(5)
90.00
97.065(5)
90.00
γ [deg]
90.00
V [Å3]
2274.8(9)
4
3421(3)
4
761.6(7)
2
Z
Dcalc (g/cm3)
Absorption coefficient (mm-1)
F(000)
1.343
1.201
1.273
1.118
1.100
0.096
1.034
0.119
960
1312
488
276
No. of reflcns collected
No. of ind reflcns
Parameters
Flack parameters
Rint
10634
23927
12728
3031
3940
12071
8438
2489
282
867
563
166
-0.017(15)
0.0338
-0.08(10)
0.0327
0.0011(12)
0.0312
-0.5(3)
0.0202
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
Goodness of fit, F2
0.0335, 0.0813
0.0398, 0.1082
1.133
0.0399, 0.0711
0.0600, 0.0766
1.011
0.0386, 0.0947
0.0477, 0.1000
1.011
0.0644, 0.1658
0.0721, 0.2005
1.139
Largest diff. peak , hole [eÅ–3
]
0.399 to -0.410
0.137 to -0.133
0.321 to -0.365
0.557 to -0.508

ACCEPTED MANUSCRIPT
FIGURE CAPTIONS
Fig. 1. Molecular structure of complex 1 showing thermal ellipsoids are plotted at 30% probability
level, hydrogen atoms are omitted for clarity.
Fig. 2. Molecular structure of complex 2 showing thermal ellipsoids are plotted at 30% probability
level, hydrogen atoms are omitted for clarity

ACCEPTED MANUSCRIPT
Fig. 3. Molecular structure of complex 3 showing thermal ellipsoids are plotted at 30% probability
level, hydrogen atoms are omitted for clarity.
Fig. 4. Molecular structure of complex 4 showing thermal ellipsoids are plotted at 30% probability
level, hydrogen atoms are omitted for clarity.

ACCEPTED MANUSCRIPT
Highlights
ꢀ Four novel chiral zinc and aluminum complexes have been synthesized.
ꢀ Their structures were confirmed by X-ray diffraction analyses.
ꢀ All complexes are active catalysts for the polymerization of rac-lactide.