Comparative study of lactide polymerization by zinc alkoxide complexes with a β-diketiminato ligand bearing different substituents

Article abstract of DOI:10.1016/j.molcata.2011.02.013

A series of β-diketiminate zinc complexes has been synthesized and their reactivity for the ring-opening polymerization (ROP) of lactide has been studied. The reaction of β-diketimines (LH) with diethyl zinc forms the monomeric [LZnEt] complexes which further react with benzyl alcohol (BnOH) in toluene/hexane yielding dinuclear or trinuclear zinc complexes. These complexes have been characterized by single crystal X-ray diffraction, which showed the tri- and tetra-coordinated zinc complexes in which zinc atoms exhibit trigonal and tetrahedral geometry, respectively. All complexes have been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is heavily dependent on the N-aryl substituents with the order: alkyl group ～ alkoxy > halide group > nitro group. The β-diketiminate zinc complexes allow controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight. On the basis of literature reports, a mechanism for ROP of lactide has been proposed.

Full text of DOI:10.1016/j.molcata.2011.02.013

Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Comparative study of lactide polymerization by zinc alkoxide complexes with a
ˇ-diketiminato ligand bearing different substituents
Hsuan-Ying Chena,∗, Ya-Liu Pengb, Tai-Hsiung Huangb, Alekha Kumar Sutarb,
Stephen A. Millerc, Chu-Chieh Linb,∗∗
a Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, 100, Shih-Chuan 1st Road, Kaohsiung 80708, Taiwan, ROC
b Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
c Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of ˇ-diketiminate zinc complexes has been synthesized and their reactivity for the ring-
opening polymerization (ROP) of lactide has been studied. The reaction of ˇ-diketimines (LH) with
diethyl zinc forms the monomeric [LZnEt] complexes which further react with benzyl alcohol (BnOH)
in toluene/hexane yielding dinuclear or trinuclear zinc complexes. These complexes have been charac-
terized by single crystal X-ray diffraction, which showed the tri- and tetra-coordinated zinc complexes in
which zinc atoms exhibit trigonal and tetrahedral geometry, respectively. All complexes have been tested
as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of poly-
merization is heavily dependent on the N-aryl substituents with the order: alkyl group ∼ alkoxy > halide
group > nitro group. The ˇ-diketiminate zinc complexes allow controlled ring-opening polymerization as
shown by the linear relationship between the percentage conversion and the number-average molecular
weight. On the basis of literature reports, a mechanism for ROP of lactide has been proposed.
© 2011 Elsevier B.V. All rights reserved.
Received 11 January 2011
Received in revised form 16 February 2011
Accepted 18 February 2011
Available online 1 March 2011
Keywords:
Biodegradable biopolymers
Zinc
Lactide
Ring-opening polymerization
ˇ-Diketiminate
1. Introduction
[8], or Ti [9]) have also proven to be active in ROP of lactide.
The neutral three-coordinate chelating diamide aluminum com-
Poly(lactic acid) (PLA) [1], produced by the ring-opening poly-
merization (ROP) of lactide (LA), is a leading biodegradable and
biocompatible polyester and has attracted considerable attention
mainly because of its biomedical and pharmaceutical applications
[2]. In industry, PLA is synthesized by ROP using tin(II) bis(2-
ethylhexanoate) (SnOct2) as a catalyst. Although SnOct2 has been
accepted as a food additive by the U.S. FDA, the toxicity associ-
ated with most tin compounds is a considerable drawback in the
case of biomedical applications [1a]. So scientists all over the world
are exploring novel, well-defined catalysts having the characteris-
tics of good biocompatibility, high catalytic activity, and excellent
stereoselectivity.
Nowadays, zinc complexes have been proven to be efficient cat-
alysts for the ROP of lactide in the presence of alcohol, mainly
due to their great successes in living ring-opening polymeriza-
tion, producing polymers with well-controlled molecular weight
and narrow molecular weight distribution [3]. Besides the zinc
systems, metal alkoxides (e.g., Al [4], Li [5], Mg [6], Fe [7], Sn
plexes were synthesized by Chakraborty and Chen [4j], which
produced telechelic polycaprolactones (PCLs) with high molecular
weights on the order of 106 Da. On the other hand, the magnesium
ˇ-diketiminate complexes were extremely active for the ROP of
rac-lactide, with up to 80% conversion in less than 10 min at room
temperature [10]. Based on the success with aluminum and mag-
nesium metals, Chisholm’s group [11] synthesized ˇ-diketiminate
calcium complexes with very high activity. Zinc complexes bearing
polydentate ligands with nitrogen donors have been extensively
studied for ROP of lactides. To the best of our knowledge, the
substituent effects in the polymerization of LA by zinc complexes
bearing ˇ-diketiminate ligands have been previously studied by
Coates and coworkers [3b,c,j]. However, these investigations were
focused on the steric effects of the substituents on the N-aryl
groups. Also, ˇ-diketiminate ligands have emerged as one of the
most versatile ligand classes in both main group and transition
metal coordination chemistry [12]. Nearly all of the ˇ-diketiminate
ligands involved possess a symmetrical structure [12u], allowing
facile modulation of steric and electronic factors [12v–y] and there-
fore they are ideal ligand sets for the design of novel zinc based
catalysts/initiators for the ROP of lactide.
∗
Corresponding author. Tel.: +886 7 3121101; fax: +886 4 3125339.
Corresponding author. Tel.: +886 4 22840411; fax: +886 4 22862547.
E-mail addresses: hchen@kmu.edu.tw (H.-Y. Chen), cchlin@mail.nchu.edu.tw
Herein, we report the synthesis of a series of zinc complexes
ligated by symmetrical or unsymmetrical ˇ-diketiminate ligands,
and their catalytic behavior for the ROP of lactide. Specifically, the
∗∗
(C.-C. Lin).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.02.013

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H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
direct reaction of ligand 2e (L5H) with diethyl zinc followed by the
addition of BnOH; the structure of 4e was confirmed by the NMR
spectrum, and elemental analysis. The mononuclear complexes 3c
and 3d undergo ligand redistribution to complexes 4i and 4j with
the elimination of one eq. of diethyl zinc within 7 h and 2 days,
respectively, due to the lack of steric bulk at the N-aryl ortho posi-
tions (Scheme 2). It is interesting to note that the complexes 4a–d
are dinuclear in nature whereas 4f and 4g are trinuclear. To under-
stand this variation, we compared all of our complexes with the
ˇ-diketiminate Zn alkoxide complexes reported in the literature
[3b–d,j] and concluded that, dinuclear complexes are produced
when the ligands have alkyl groups at the N-aryl positions whereas,
trinuclear zinc complexes are formed, when electron withdrawing
groups are substituted in the 1- or 3- positions of pentenyl group
steric and electronic effects of these ligands on the polymerization
of lactide were explored.
2. Results and discussion
2.1. Synthesis of ˇ-diketiminate zinc complexes
With the endeavor of assessing substituent effects on polymer-
ization activity, various substituents such as alkyl, alkoxy, halo,
and the nitro group were incorporated to obtain ˇ-diketiminate
ligands with different electron-donating and accepting abilities.
The symmetrical ˇ-diketimines 2a–d (L1H–L4H) were synthe-
sized in two steps as depicted in Scheme 1: (1) condensation
of 2,4-pentanedione with one eq. of primary aromatic amine
in toluene, with p-toluenesulfonic acid as catalyst, afforded the
enaminoketone intermediate 1; (2) another eq. of the same aro-
matic amine was pre-treated with p-toluenesulfonic acid in a
1:1 ratio to afford the p-toluenesulfonate ammonium salt, which
was then reacted with the corresponding enaminoketone 1 to
give ˇ-diketimines 2a–d in reasonable yields. In contrast, it is
simpler to synthesize ˇ-diketimines 2a–d, with the same N-
aryl substituents by refluxing 2.0 eq. of the primary aromatic
amine with 2,4-pentanedione in toluene and p-toluenesulfonic
acid as the catalyst. The reactions of ligands 2a–d with diethyl
zinc in a 1:1 molar ratio in n-hexane/toluene readily gener-
ated the tri-coordinated zinc complexes 3a–d (Scheme 2), with
the elimination of 1 eq. of ethane. Similar two-step condensa-
tion reactions were adopted to synthesize the unsymmetrical
ˇ-diketimines. Successful synthesis of 2e (L5H) was limited to
the reaction of 4-(perfluorophenylamino)pent-3-en-2-one (1e)
with 2-methoxyaniline by using para-toluenesulfonic acid as the
catalyst. Also ˇ-diketimines ligands 2f and 2g (L6H–L7H) were
synthesized by the reaction of 4-(2,6-diisopropylphenyl)amino-
pent-3-en-2-one (1f) with the corresponding aromatic amines
using p-toluenesulfonic acid as the catalyst (Scheme 1). Zinc
complexes 3f and 3g with unsymmetrical ˇ-diketimines were syn-
thesized accordingly via the reactions of ligands 2f and 2g with
diethyl zinc (Scheme 2).
i
following propyl group at the 2,6 N-aryl positions. This may be
due to ␲-resonance effect and/or electron donating effect of sub-
stituent. To clarify this, we prepared ligand 2f (L6H) with the OMe
group at the para position, which produced trinuclear zinc com-
plex. Following this result, we noted that ˇ-diketiminate ligands
2a (L1H), 2c (L3H) and 2e (L5H), which have substituent groups
bearing lone pairs of electrons (Br, Cl, F, and OMe), afford dinu-
clear complexes. This may be due to electron withdrawing groups
Br and Cl in the case of 2a and 2c, respectively, and in case of
2e, the N-pentafluorophenyl group is more electron-withdrawing
than the OMe electron-donating group. For the synthesis of trinu-
clear complex, we prepared ligand 2b (L2H), which still produce
dinuclear complex (4b), then we prepared the NMR tube with
[(L2)2Zn2(ꢀ-OBn)2] (4b) in benzene-d6 and heat it at 60 ◦C for 4
days but no change in it. We heat the mixture with 1 eq. 4b and
1 eq. Zn2(OBn)2 in THF at 80 ◦C for 4 days. From the NMR spectra,
[(L2)2Zn3(ꢀ-OBn)4] seems to appear but mixed with [(L2)2Zn2(ꢀ-
OBn)2] and we cannot purify it (supporting information Fig. 1). Thus
finally we synthesize 2f (L6H) and 2g (L7H) with N-methoxyphenyl
groups which produce trinuclear complexes 4f and 4g (Scheme 2),
which conform by the NMR spectrum, elemental analysis and X-ray
structural determination. Based on the above experiments, a gen-
eralized mechanistic scheme can be proposed (Scheme 3). Besides,
[(L2)2Zn2(ꢀ-OEt)2] (4k) can be synthesized from [L2ZnEt] (3b) by
react with O2 (Scheme 4) which was supported by the literature
review [13] and confirmed by X-ray structural determination. Zn
alkylperoxides are the initial products of the auto-oxidation of
alkylzinc compounds. Although it is certain that the final products
are zinc alkoxides, the mechanisms by which these products are
formed are uncertain. It is likely that reactions between alkylz-
inc compounds and alkylzinc peroxides are involved in the final
reactions, since such a reaction has been observed independently.
The 1H NMR spectra indicate that for zinc complexes 3a–d, the
chemical environments of both aromatic rings as well as the two
backbone methyl groups are identical. Zinc complexes 3f and 3g
with unsymmetrical ˇ-diketiminate ligands exhibit separate reso-
nances in the 1H NMR spectra. The chemical shifts for the methine
protons of the 2-propyl groups (–CHMe2) in complexes 3f and 3g
appear near 3.0 ppm, almost the same as those in the neutral ˇ-
diketimines, but because of restricted N-aryl bond rotation, there
are two separate doublets for the methyl groups (–CHMe2) in com-
plexes 3f and 3g. In addition, 1H NMR spectra of zinc complexes
3a–d and 3f and 3g generally reveal two diastereotopic methylene
protons in each ethyl group (Zn–CH2CH3), resonating at negative
ppm, further implicating restricted N-aryl bond rotation.
Subsequently, attempts to synthesize the corresponding alkox-
ide complexes 4a–d and 4f and 4g were carried out by reacting the
ˇ-diketiminate zinc ethyl complexes 3a–d and 3f and 3g with 1.2
eq. of BnOH at 0 ◦C, respectively, (Scheme 2). But there are lots of
impurities in 3a reaction and it could not be purified. Fortunately
the 3a crystal was got and its structure was identified. A homoleptic
complexes [(L)2Zn] were observed by the NMR spectrum from the
synthesis of 4f and 4g. It is believed that, in the presence of BnOH,
LZnEt reacts with BnOH to yield LZnOBn and the disproportiona-
tion products, L2Zn and ZnOBn2. The disproportionation takes place
when the functional groups of N-aryl substituents are not bulky
and the similar situation was reported in the literature [12z]. The
combination of 2 eq. of [LZnOBn] with Zn(OBn)2 gives [L2Zn3(␮-
OBn)4]. The complex [(L5)2Zn2(␮-OBn)2] (4e) was formed by the
2.2. Crystal structure of zinc complexes
Single crystals of zinc complexes 3f [L6ZnEt], 4a[(L1)2Zn2(ꢀ-
OBn)2], 4b[(L2)2Zn2(ꢀ-OBn)2], and 4k[(L2)2Zn2(ꢀ-OEt)2] suitable
for X-ray diffraction measurement were obtained. Crystallographic
data, the results of structure refinements, and selected bond lengths
and angles are summarized in Figs. 1–4 and supporting information
Table 1. As shown in Fig. 1, being surrounded by two nitro-
gen donors of the chelating ˇ-diketiminate ligand and one ethyl
group, the zinc center in complex 3f possesses a distorted trigo-
nal geometry with bond angles 96.35◦, 130.30◦, and 133.30◦ for
N(1)–Zn–N(2), N(1)–Zn–C(25), and N(2)–Zn–C(25), respectively.
˚
There are close interactions between Zn and N(1) (1.962 A) as
˚
well as between Zn and N(2) (1.961 A). The distance between Zn
˚
and C(25) is 1.949 A. Metrical analysis reveals that 4a (Fig. 2),
4b (Fig. 3), 4k (Fig. 4) are iso-structural-i.e., distorted tetrahedral
geometry with only slight variations in bond lengths and angles. In
4a, the average O(1)–Zn(1)–O(1A) and N(1)–Zn–N(2) bond angles

H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
63
R1
R1
R4
R3
N HN
R2 R2
R4
R3
+
ArNH2
2a (L1H) : R1 = R2 = R3 = Br; R4 = H
2b (L2H) : R1 = R2 = R3 = Me; R4 = H
2c (L3H) : R1 = R2 = R4 = H; R3 = Cl
2d (L4H) : R1 = OMe; R2 = R3 = H; R4 = NO2
toluene
+
ArNH2
O HN
p-TsOH
O
O
Ar
1
R5
R8
R6
R7
N HN
R5
+ Ar`NH2
R9
R6
2e (L5H) : R5 = R6 = R7 = F ; R8 = OMe; R9 = H
2f (L6H) : R5 = iPr; R6 = R7 = R8 = H; R9 = OMe
2g(L7H) : R5 = iPr; R6 = R7 = R9 = H; R8 = OMe
Scheme 1.
are 82.90◦ and 96.80◦. The distances from Zn(1) to O(1), O(1A),
N(1), and N(2) are 1.977, 1.959, 1.989 and 1.998 A, respectively. In
plexes displayed good activities for the polymerization of l-lactide
with narrow distribution and great control of molecular weight. The
structures of the ancillary ligands have a significant influence on the
polymerization behavior of the corresponding zinc complexes. The
discrepancy between the calculated and observed Mn(nmr) values
is small except for 3a and 4b. The reasons for 3a may be the ini-
tiator (BnOH) was additional and it needed time to exchange ethyl
group for BnOH. It made the time delay for the alkoxide formation
and brought about slightly broad PDI. Besides, few of the dispro-
portionation made the part of BnOH unavailable and the Mn(nmr)
value was higher than calculated with. The reason for 4b may be
the polymerization rate of 4b was too fast to cause the transester-
ification.
Comparing the polymerization runs performed in Table 1, sev-
eral structure-activity trends may be drawn. It is found that
for both zinc complexes with symmetrical or unsymmetrical ˇ-
diketiminate ligands, the electronic nature of the substituents of
the phenyl groups exerts great influence on the ROP of l-LA. A clear
increasing tendency of catalytic activity is found for complexes 4b,
4f, 4g in the order 4b > 4g > 4f (entry 2, 7, 6) with time, indicat-
ing that electron-donating substituents at the ortho-position are
an advantage to the catalytic activity. The introduction of electron-
donating groups on the phenyl rings decreases the lewis acidity of
˚
4b, the average O(1)–Zn(1)–O(2), O(1)–Zn(2)–O(2), N(1)–Zn–N(2),
and N(3)–Zn–N(4) bond angles are 81.63◦, 81.39◦, 97.60◦, and96.60,
respectively. The distances from Zn(1) atom to O(1), O(2), N(1) and
˚
N(2) are 1.989, 1.956, 1.983 and 1.982 A. The distances from Zn(2)
˚
atom to O(1), O(2), N(3) and N(4) are 1.960, 1.993, 1.997 and 2.001 A
and with bond angles of 82.91, 82.89,119.03 and 126.19. In 4k,
the Zn atoms with the average compressed O(1)-Zn(1)-O(2), O(1)-
Zn(2)-O(2), N(1)-Zn-N(2) and N(3)-Zn-N(4) bond angles of 83.10◦,
80.90◦, 98.40◦ and 98.10◦. The distance from Zn(1) atom to O(1),
˚
O(2), N(1) and N(2) are 1.985, 1.899, 1.938 and 1.934 A. The dis-
tance from Zn(2) atom to O(1), O(2), N(3) and N(4) are 1.982, 1.989,
˚
2.008 and 2.029 A.
2.3. Ring-opening polymerization of l-lactide
ˇ-Diketiminate Zn alkoxide complexes 3a, 4b–g as well as the
previously reported zinc complex [3d] (BDI-OMe)2Zn3(ꢀ-OBn)4,
initiate the ring-opening polymerization of l-lactide (LA) in toluene
at room temperature (for 3a and 4d the temperature is 100 ◦C and
the concentration of 4d is 1.25 mM because of the low solubility in
toluene). The polymerization results are listed in Table 1, all com-
Ar
Ar
Zn
Ar
Bn
O
BnOH
ZnEt2
N
N
N
N HN
2a -2d
N
N
Zn
4a -4d
Ar
Ar
Ar
Ar
Ar
Zn
O
Bn
N
Ar
3a -3d
Ar`
`Ar
Zn
Ar
Bn
O
N
N
N
BnOH
ZnEt2
4e
Zn
O
Bn
N
N HN
2e - 2g
Ar`
Ar
Ar`
N
`Ar
Bn
Bn
ZnEt2
BnOH
N
N
O
O
Zn
Zn
Zn
Ar
N
N
4f - 4g
Ar
Ar`
O
O
N
Ar
Zn
Bn
Bn
3f - 3g
Scheme 2.

64
H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
Bn
O
N
N
N
N
Zn
Zn
0.5
O
Bn
N
BnOH
Zn
O
Zn(OBn)2
N
Bn
N
N
Zn Et
-ZnEt2
N
N
N
N
2 Zn(OBn)2
0.5
Zn
Δ
ZnEt2
Bn
O
Bn
O
N
N
N
NH
Zn
Zn
Zn
0.5
O
O
N
N
Bn
Bn
Scheme 3. General proposed mechanism of zinc complex formation.
N
O Et
Zn
O
N
N
N
4l
Et
O
N
N
N
O2
O2
Zn
Zn
Zn
0.5
O
Et
O
O
N
Et
4k
N
N
N
N
3b
Zn
Zn
Et
0.5
O
O
4m
Scheme 4.
Table 1
ˇ-Diketiminate Zn alkoxide complexes (2.5 mM) polymerized l-LA yielded PLA in toluene (20 mL) at room temperature and [LA]0/[cat.] ratio was 100:1.
Entry
Cat.
T (min)
120
2
290
360
120
5
t (◦C)
PDI
Mn (GPC)a
Mn (calcd)b
Mn (nmr)c
Conv (%)
1
2
3
4
5
6
7
8f
[L1ZnEt + BnOH]d(3a)
(L2)2Zn2(ꢀ-OBn)2(4b)
(L3)2Zn2(ꢀ-OBn)2(4c)
(L4)2Zn2(ꢀ-OBn)2e(4d)
(L5)2Zn2(ꢀ-OBn)2(4e)
(L6)2Zn3(ꢀ-OBn)4(4f)
(L7)2Zn3(ꢀ-OBn)4(4g)
(BDI-OMe)2Zn3(ꢀ-OBn)4
100
R.T.
R.T.
100
R.T.
R.T.
R.T.
R.T.
1.25
1.05
1.05
1.09
1.06
1.20
1.17
1.09
28,800
9500
6900
6400
12,500
9400
6700
11,600
7800
4400
87
94
87
86
99
96
99
93
15,100
10,000
17,200
7700
6800
9000
12,500
7200
3600
3700
3500
3
10
4300
4000
9200
a
Obtained from GPC analysis and calibrated by polystyrene standard.
b
c
d
e
f
Calculated from the molecular weight of l-lactide × [M]o/[BnO−
o × conversion yield plus Mw(BnOH).
]
Obtained from 1H NMR analysis.
In situ reaction of [LA]/[BnOH] = 75, [L1ZnEt] = 1.67 mM, toluene 30 mL.
[LA]0/[cat.] ratio was 200:1, [(L4)2Zn2(ꢀ-OBn)2] = 1.25 mM.
It was reported on ref. [3h].

H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
65
O
O
O
EWG
δ+
O
EDG
δ−
O
O
O
N
O
N
Zn
b
Zn
a
δ+
O
δ−
O
N
N
Bn
Bn
easy dissociation
EWG
EDG
hard dissociation
Scheme 5. Generalized mechanism for ROP of lactide (EDG = electron donating group and EWG = electron withdrawing group).
the zinc center through the chelating system of the ancillary lig-
and, which is favorable for the breaking of the Zn–alkoxide bond
and leads to an increase in activity (Scheme 5a). Thus as can be seen
in entries 2, 6, 7, 8, all complexes bearing electron donating groups
on the N-aryl positions need less than 10 min for polymerization.
Whereas, the entries for complexes bearing electron withdrawing
groups (entries 1, 3, 4, 5) show that the ring opening rate is very
slow. The lewis acidity of the zinc center is increased with elec-
tron withdrawing groups is strengthened, which causes the rate of
polymerization to decrease (Scheme 5b). The rate of polymeriza-
tion depends on the functional groups at the N-aryl positions in the
following order: alkoxy ∼ alkyl group > halide group > nitro group.
The result is identical to literature report [3l,m,6f,g] and the prin-
ciple is similar to acid dissociation constants of benzoic acid with
rates of alkaline hydrolysis of ethyl benzoates [14]. Besides, the dif-
ference between the dinuclear and trinuclear complex is small for
ROP of l-LA. It seems the electric effect is the key to control the
catalytic rate.
Fig. 2. ORTEP drawing of 4a [(L1)2Zn2(ꢀ-OBn)2] (non-hydrogen atoms) with ellip-
soids drawn at the 20% probability level. Selected bond lengths (Å) and bond
angles (deg): Zn(1)–O(1) 1.977(5), Zn(1)–O(1A) 1.959(5), Zn(1)–N(1) 1.989(6),
Zn(1)–N(2) 1.998(6), N(1)–Zn(1)–N(2) 96.80(3), O(1)–Zn(1)–O(1A) 82.90(2),
Zn(1)–O(1)–Zn(1A) 97.10(2).
2.4. Kinetic studies of polymerization of l-lactide by 4e
A kinetic study was performed in order to obtain reaction
order in monomer and zinc complex for the polymerization of l-
Fig. 3. ORTEP drawing of 4b [(L2)2Zn2(ꢀ-OBn)2] (non-hydrogen atoms) with
ellipsoids drawn at the 20% probability level. Selected bond lengths (Å)
and bond angles (deg): Zn(1)–O(1) 1.989(5), Zn(1)–O(2) 1.956(5), Zn(1)–N(1)
1.983(6), Zn(1)–N(2) 1.982(6), Zn(2)–O(1) 1.960(5), Zn(2)–O(2) 1.993(5), Zn(2)–N(3)
1.997(6), Zn(2)–N(4) 2.001(6), N(1)–Zn(1)–N(2) 97.60(3), N(3)–Zn(2)–N(4) 96.60(3),
O(1)–Zn(1)–O(2) 81.63(18), O(1)–Zn(2)–O(2) 81.39(19), Zn(1)–O(1)–Zn(2) 98.50(2),
Zn(1)–O(2)–Zn(2) 98.50(2).
Fig. 1. ORTEP drawing of 3f [L6ZnEt] (non-hydrogen atoms) with ellipsoids drawn
at the 20% probability level. Selected bond lengths (Å) and bond angles (deg):
Zn–N(1) 1.962(3), Zn–N(2) 1.961(3), Zn–C(25) 1.949(5), N(1)–Zn–N(2) 96.35(12),
N(1)–Zn–C(25) 130.30(2) and N(2)–Zn–C(25) 133.30(2).

66
H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
Fig. 6. Linear plot of kobs vs [4e] for the polymerization of l-LA with [LA]0 = 0.125 M
in toluene. k = 8.37 M−1 s−1
.
Fig. 4. ORTEP drawing of 4k [(L2)2Zn2(ꢀ-OEt)2] (non-hydrogen atoms) with ellip-
soids drawn at the 20% probability level. Selected bond lengths (Å) and bond
angles (deg): Zn(1)–O(1) 1.985(13), Zn(1)–O(2) 1.899(16), Zn(1)–N(1) 1.938(14),
Zn(1)–N(2) 1.934(13), Zn(2)–O(1) 1.982(13), Zn(2)–O(2) 1.989(14), Zn(2)–N(3)
2.008(12), Zn(2)–N(4) 2.029(13), N(1)–Zn(1)–N(2) 98.40(6), N(3)–Zn(2)–N(4)
98.10(6), O(1)–Zn(1)–O(2) 83.10(5), O(1)–Zn(2)–O(2) 80.90(5), Zn(1)–O(1)–Zn(2)
96.70(6), Zn(1)–O(2)–Zn(2) 99.30(7).
uclear complex and allows two monomers to bond the active
center.
3. Conclusions
lactide with compound 4e. Conversion of l-lactide with time was
recorded by 1H NMR for different concentrations of 4e at 25 ◦C
until monomer consumption was completed ([LA]0/[4e] = 200;
[LA]0 = 0.125 M in toluene). To ascertain the kinetic order in [4e], the
dependence of kobs on [LA]0 was analyzed. Plots of ln([LA]0/[LA])
vs time in a wide range of [LA]0 are linear, showing polymer-
ization proceeds with second-order dependence on monomer
concentration (Fig. 5, kobs = 0.0286 M−1 s−1). Therefore the rate of
polymerization can be described as −d[LA]/dt = kobs[LA]1, where
kobs = k[4e]x and k is the rate constant. Plotting ln kobs vs ln[4e] lets
us to determine x, the order in compound 4e concentration. From
the slope of the fitted line as shown in Fig. 6, the rate constant k
is 8.37 M−1 s−1. Therefore, on the basis of this analysis, x = 1. The
reaction is both first order in compound 4e and monomer concen-
trations, and the overall rate equation is −d[LA]/dt = k[LA]1[4e]1.
It is interesting to note that the rate law −d[LA]/dt = k[LA]1[4e]1
is different than the rate law of −d[LA]/dt = k[LA]2[ini]1 found
in the trinuclear initiator’s system [3h]. The reason may be that
the trinuclear complex is less sterically hindered than the din-
Zinc complexes 4a–j supported by symmetrical or unsymmetri-
cal ˇ-diketiminate ligands were readily synthesized. The molecular
structure of complex 3f, 4a, 4b and 4k were further confirmed by
X-ray diffraction techniques. The effect of the ligand substituents
on both the ROP of lactide and di- or trinuclear zinc structure is
significant. It has been found that electron-donating substituents
like methoxy group support the formation of trinuclear complexes.
And also the electron-donating substituents at the phenyl rings
decrease the electrophilicity of the zinc center as well as decrease
the bond strength between zinc and BnO−, and are favorable for
the coordination and insertion of lactide monomers, whereas the
electron withdrawing group gives the adverse effect. This effect
changes the rate of polymerization.
4. Experimental
All manipulations were carried out under a dry nitrogen atmo-
sphere. Solvents, benzyl alcohol, l-lactide and deuterated solvents
were purified before uses. 1H and 13C NMR spectra were recorded
on a Varian Mercury-400 spectrometer with chemical shifts given
in ppm from the internal TMS or center line of CHCl3. Microanaly-
ses were performed using a Heraeus CHN-O-RAPID instrument. The
GPC measurements were performed on a Postnova PN1122 solvent
delivery system with TriSEC GPC software using THF (HPLC grade)
as an eluent. Molecular weight and molecular weight distributions
were calculated using polystyrene as standard.
4.1. Synthesis of 2-((2,4,6-tribromophenyl)amino)-4-
((2,4,6-tribromophenyl)imino)-2-pentene (L1H) (2a)
2,4,6-Tribromoaniline (32.9 g, 100 mmol), 2,4-pentanedione
(5.0 g, 50 mmol), and p-toluenesulfonic acid (0.30 mL) were
refluxed in absolute toluene (30 mL) for 3 days. After being cooled
to room temperature, the solution was passed through silica
gel to remove p-toluenesulfonic acid and volatile materials were
removed under vacuum to obtain a light yellow powder. The solid
was then recrystallized from acetone at 0 ◦C to get white crystals.
Yield: 14.3 g (40%). 1H NMR (CDCl3): ı 12.04 (1H, s, NH), 7.70 (4H,
s, ArH), 5.05 (1H, s, ˇ-CH), 1.81 (6H, s, C CNCH3) ppm. 13C NMR
(CDCl3): ı 162.55 (C N), 142.45, 134.22, 121.73, 118.03 (Ph), 95.22
Fig. 5. First-order kinetic plots for l-LA polymerizations with time in toluene with
different concentration of complex 4e as an initiator.

H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
67
(ˇ-C), 21.03(C CNCH3) ppm. Anal. Calc. (found) for C17H12Br6N2:
C 28.21 (28.42), H 1.67 (1.61), N 3.87 (3.94)%.
127.83, 127.57, 125.88 (Ph), 94.42 (ˇ-C), 68.86 (PhCH2O), 23.02
(C CNCH3), 20.94 (4-CH3Ph), 18.18 (2,6-(CH3)2Ph) ppm. Anal. Calc.
(found) for C60H72N4O2Zn2: C 71.23 (71.33), H 7.17 (6.86), N 5.54
(5.38)%.
4.2. Synthesis of [L1ZnEt] (3a)
To a suspension of (L1H) (3.58 g, 5 mmol) in toluene (15 mL) was
added diethyl zinc (7 mL, 7 mmol). After being stirred at 0 ◦C for
3 h, a light yellow solution was obtained. Volatile materials were
removed in vacuo to yield a light yellow powder. Adding 30 mL
hexane and washing the solid for 2 h, the white powder was col-
lected by filtration. The white solid was dried in vacuo. Yield: 1.27 g
(31%). 1H NMR (CDCl3): ı 7.74 (4H, s, ArH), 5.10 (1H, s, ˇ-CH), 1.82
(6H, s, C CNCH3), 0.65 (3H, t, J = 8.0 Hz, ZnCH2CH3), −0.16 (2H,
q, J = 8.0 Hz, ZnCH2CH3) ppm. 13C NMR (CDCl3): ı 167.87 (C N),
146.02, 134.33, 121.58, 118.09 (Ph), 96.23 (ˇ-C), 23.20 (C CNCH3),
11.48 (ZnCH2CH3), −3.10 (ZnCH2CH3) ppm. Anal. Calc. (found) for
4.7. Synthesis of 2-((4-chlorophenyl)amino)-4-
((4-chlorophenyl)imino)-2-pentene (L3H) (2c)
4-Chloroaniline (12.8 g, 100 mmol), 2,4-pentanedione (5.0 g,
50 mmol), and p-toluenesulfonic acid (0.30 mL) were refluxed in
absolute toluene (30 mL) for 12 days. After being cooled to room
temperature, the solution was passed through silica gel to remove
p-toluenesulfonic acid. Volatile materials were removed under vac-
uum to give a yellow powder. The solid was recrystallized from
ethanol at 0 ◦C to yield yellow crystals. Yield: 9.57 g (60%). 1H NMR
(CDCl3): ı 12.60 (1H, s, NH), 7.21 (4H, d, J = 8.8 Hz, ClC(CHCH)2CN),
6.84 (4H, d, J = 8.8 Hz, ClC(CHCH)2CN), 4.87 (1H, s, ˇ-CH), 1.95 (6H, s,
C19H16Br6N2Zn: C 27.93 (27.86), H 1.97 (1.92), N 3.43 (3.63)%.
C
CNCH3) ppm. 13C NMR (CDCl3): ı 159.51 (C N), 143.96, 128.70,
4.3. Synthesis of [(L1)2Zn2(ꢀ-OBn)2] (4a)
128.39, 123.57 (Ph), 97.83 (ˇ-C), 20.65(C CNCH3) ppm. Anal. Calc.
(found) for C17H16Cl2N2: C 63.96 (63.82), H 5.05 (5.24), N 8.78
(8.54)%.
To a suspension of L1ZnEt (2.04 g, 2.5 mmol) in toluene (15 mL)
was added BnOH (0.31 mL, 3.0 mmol). When the mixture was
stirred for 3 h, the resulting white powder was collected by filtra-
tion. After adding 15 mL toluene and 15 mL THF to the solid, the
solution was stirred at 70 ◦C for 1 day and then filtrated. Colorless
crystals appeared from the filtrate at −18 ◦C. NMR data showed a
mixture of inseparable products. However, a single crystal suitable
for X-ray diffraction analysis could be obtained.
4.8. Synthesis of [(L3)2Zn] (4i)
To a suspension of (L3H) (3.19 g, 10 mmol) in hexane (15 mL)
was added diethyl zinc (12 mL, 12 mmol). After being stirred at 0 ◦C
for 3 h, a light yellow solution was obtained. Volatile materials were
removed in vacuo to yield a yellow powder. From the presence of
ethyl group signals in the NMR spectra the formation of L3ZnEt
could be deduced. However, disproportionation to (L3)2Zn over a
period of 7 h at room temperature could be observed. Yield: 3.16 g
(90%). 1H NMR (CDCl3): ı 7.13 (4H, d, J = 8.4 Hz, ClC(CHCH)2CN),
6.49 (4H, d, J = 8.4 Hz, ClC(CHCH)2CN), 4.54 (1H, s, ˇ-CH), 1.75
(6H, s, C CNCH3) ppm. 13C NMR (CDCl3): ı 166.46 (C N), 148.40,
129.67, 128.44, 125.52 (Ph), 96.15 (ˇ-C), 23.04 (C CNCH3) ppm.
Anal. Calc. (found) for C108H104Cl12N12Zn3 (3 (L3)2Zn. 1 hexane): C
59.19 (59.19), H 4.78 (4.50), N 7.67 (7.31)%.
4.4. Synthesis of 2-((2,4,6-trimethylphenyl)amino)-4-
((2,4,6-trimethylphenyl)imino)-2-pentene (L2-H) (2b)
The ligand was prepared as described in the literature [15].
4.5. Synthesis of [L2ZnEt] (3b)
To a suspension of (L2H) (3.34 g, 10 mmol) in hexane (15 mL) was
added diethyl zinc (12 mL, 12 mmol). After being stirred at 0 ◦C for
3 h, a light yellow solution was obtained. Volatile materials were
removed in vacuo to yield a yellow powder. After adding 15 mL
n-hexane and stirred at 60 ◦C for 2 h, the solid redissoved in hex-
ane. The light yellow powder was collected by filtration and dried
in vacuo after the solution was kept at −18 ◦C for a week. Yield:
1.71 g (40%). 1H NMR (CDCl3): ı 6.87 (4H, s, ArH), 4.95 (1H, s, ˇ-
CH), 2.27 (6H, s, C CNCH3), 2.09 (12H, s, 2,6-(CH3)2Ph), 1.71 (6H, s,
4-CH3Ph), 0.56 (3H, t, J = 8.0 Hz, ZnCH2CH3), −0.26 (2H, q, J = 8.0 Hz,
ZnCH2CH3) ppm. 13C NMR (CDCl3): ı 166.63 (C N), 145.19, 133.36,
130.68, 128.75 (Ph), 94.71 (ˇ-C), 22.47 (C CNCH3), 20.82 (2,6-
(CH3)2Ph), 18.53 (4-CH3Ph), 11.39 (ZnCH2CH3), −3.53 (ZnCH2CH3)
ppm. Anal. Calc. (found) for C25H34N2Zn: C 70.17 (69.51), H 8.01
(7.56), N 6.55 (6.24)%.
4.9. Synthesis of [(L3)2Zn2(ꢀ-OBn)2] (4c)
To a suspension of L3ZnEt (1.07 g, 2.5 mmol) was added BnOH
(0.31 mL, 3.0 mmol) in hexane (15 mL). When the mixture was
stirred for 3 h, the resulting light yellow powder was collected by
filtration. The light yellow solid was dried in vacuo. Yield: 0.49 g
(40%). 1H NMR (CDCl3): ı 7.17, 6.86, (5H, s, C6H5CH2O), 7.07 (4H,
d, J = 4.8 Hz, ClC(CHCH)2CN), 6.43 (4H, d, J = 4.8 Hz, ClC(CHCH)2CN),
4.54 (1H, s, ˇ-CH), 4.41 (2H, s, PhCH2O) 1.61 (6H, s, C CNCH3) ppm.
13C NMR (CDCl3): ı 167.53 (C N), 148.50, 144.44, 129.25,128.72,
128.54, 128.46, 127.95, 127.03, 126.49, 125.96 (Ph), 95.64 (ˇ-C),
68.66 (PhCH2O), 23.72 (C CNCH3) ppm. Anal. Calc. (found) for
C48H44Cl4N4O2Zn2: C 58.74 (57.67), H 4.52 (4.56), N 5.71 (5.55)%.
4.6. Synthesis of [(L2)2Zn2(ꢀ-OBn)2] (4b)
4.10. Synthesis of 2-((2-methoxy-5-nitrophenyl)amino)-4-
((2-methoxy-5-nitrophenyl)imino)-2-pentene (L4H) (2d)
To a suspension of L2ZnEt (1.07 g, 2.5 mmol) in hexane (15 mL)
was added BnOH (0.31 mL, 3.0 mmol). When the mixture was
stirred for 3 h, the resulting white powder was collected by fil-
tration. After adding 15 mL of toluene to the solid, the solution
was stirred for 15 min and then filtrated. The solid has been
recrystallized from solvent hexane by placing at −18 ◦C for days
and dried in vacuo. Colorless crystals appeared from the filtrate
at −18 ◦C. Yield: 0.76 g (60%). 1H NMR (CDCl3): ı 7.11, 7.09,
6.99, 6.97, (5H, m, C6H5CH2O), 6.74 (4H, s, ArH), 4.78 (1H, s,
ˇ-CH),4.42 (2H, s, PhCH2O), 2.36 (6H, s, C CNCH3), 1.61 (12H, s, 2,6-
(CH3)2Ph), 1.41 (6H, s, 4-CH3Ph) ppm. 13C NMR (CDCl3): ı 167.54
(C N),152.62 (MeOCCN), 145.49, 144.25, 132.34, 131.76, 128.93,
2-Methoxy-5-nitroaniline
(16.8 g,
100 mmol),
2,4-
pentanedione (5.0 g, 50 mmol), and p-toluenesulfonic acid
(0.30 mL) were refluxed in absolute toluene (30 mL) for 9 days.
After being cooled to room temperature, the solution was passed
through the silica gel to remove p-toluenesulfonic acid. Volatiles
materials were removed under vacuum to give deep yellow
powder. The solid was recrystallized from 30 mL ethanol and
15 mL chloroform at 0 ◦C to get a deep yellow crystal. Yield: 8.01 g
(40%). 1H NMR (CDCl3): ı 12.97 (1H, s, NH), 7.96 (2H, d, J = 8.8 Hz,
MeOCCHCHCNO2), 7.93 (2H, s, NO2CCHCNC), 6.92 (2H, d, J = 8.8 Hz,
MeOCCHCHCNO2), 5.07 (1H, s, ˇ-CH), 3.90 (6H, s, CH3O), 2.18

68
H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
(6H, s, C CNCH3) ppm. 13C NMR (CDCl3): ı 160.32 (C N),155.63
(MeOCCHCHCNO2), 141.37 (MeOCCHCHCNO2), 135.31 (MeOCCN),
119.63, 116.70, 110.02 (Ph), 100.31 (ˇ-C), 56.30 (CH3O), 21.52
(C CNCH3) ppm. Anal. Calc. (found) for C19H20N4O6: C 57.00
(56.61), H 5.03 (5.49), N 13.99 (14.31)%.
4.14. Synthesis of [(L5)2Zn2(ꢀ-OBn)2] (4e)
To a suspension of L5H (0.37 g, 1.00 mmol) in hexane (20 mL)
was added diethyl zinc (1.2 mL, 1.2 mmol). After being stirred at
0 ◦C for 8 h, a light yellow solution was obtained. Volatile materials
were removed in vacuo to yield yellow oil. To an emulsion of the yel-
low oil in n-hexane (20 mL) was added BnOH (0.11 mL, 1.0 mmol).
After stirring for 8 h, the volatile materials were removed in vacuo
to yield a light yellow powder. The solid has been recrystallized
from solvent toluene by placing at 0 ◦C for 10 days and dried in
vacuo. Yield: 0.34 g (63%). 1H NMR (CDCl3): ı 7.28–6.51 (9H, m,
ArH), 4.75 (1H, s, ˇ-CH), 4.57 (2H, br s, PhCH2O), 3.61 (3H, s,
ArOCH3), 1.61 (3H, s, CH3CNArF5), 1.38 (3H, s, CH3CNArOMe).
13C NMR (CDCl3): ı 171.93 (CH3CNArF5), 166.72 (CH3CNHOMe),
152.02 (MeOCCN), 145.00, 143.29, 140.78, 139.09,138.50, 137.56,
136.65, 136.03, 129.02, 128.21, 127.91, 125.45, 125.29, 119.65 (Ph),
95.90 (ˇ-C), 68.85 (PhCH2O), 55.03 (OCH3), 23.16 (CH3CNArOMe),
20.56 (CH3CNHF5). Anal. Calc. (found) for C52H48F10N4O4Zn2
[(L6)2Zn2(␮-OBn)2. C2H6]: C 56.08 (56.76), H 4.34 (4.98), N 5.03
(5.48)%.
4.11. Synthesis of [(L4)2Zn] (4j)
To a suspension of (L4H) (2.00 g, 5 mmol) in toluene (15 mL) was
added diethyl zinc (6 mL, 6 mmol). After being stirred at 0 ◦C for
3 h, a deep orange solution was obtained. Volatile materials were
removed in vacuo to yield a red powder. From the presence of
ethyl group signals in the NMR spectra the formation of L4ZnEt
could be deduced. However, disproportionation to (L4)2Zn over a
period of 2 days at room temperature could be observed. After being
added toluene (15 mL), stirred until the solid was redissolved and
filtrated, the red crystals appeared from the filtrate at −18 ◦C for 1
day. Yield: 1.17 g (54%). 1H NMR (CDCl3): ı 7.90 (2H, d, J = 9.2 Hz,
MeOCCHCHCNO2), 7.68 (2H, s, NO2CCHCNC), 6.77 (2H, d, J = 9.2 Hz,
MeOCCHCHCNO2), 4.57 (1H, s, ˇ-CH), 3.76 (6H, s, CH3O), 1.70
(6H, s, C CNCH3) ppm. 13C NMR (CDCl3): ı 168.61 (C N), 157.99
(MeOCCHCHCNO2), 141.51 (MeOCCHCHCNO2), 138.91 (MeOCCN),
120.93, 120.74, 110.19 (Ph), 96.70 (ˇ-C), 56.07 (CH3O), 22.95
(C CNCH3) ppm. Anal. Calc. (found) for C38H38N8O12Zn: C 52.82
(52.17), H 4.43 (4.57), N 12.97 (13.17)%.
4.15. Synthesis of 4-(2,6-diisopropylphenyl)
amino-pent-3-en-2-one
2,6-Diisopropylaniline (20 g, 113 mmol) was mixed with 2,4-
pentanedione (14.5 mL, 136 mmol) in 20 mL toluene containing a
catalytic amount (0.1 g) of p-toluenesulfonic acid. The solution was
refluxed for 24 h with continued water removal by a Dean–Stark
trap. The product was purified by column chromatography on sil-
ica gel with n-hexane. Removal of volatiles afforded an orange oil.
This orange oil was recrystallized from n-hexane at −18 ◦C to yield
a white solid. Yield: 21 g (71%). 1H NMR (CDCl3): ı 12.06 (1H, s,
NH), 7.30–7.12 (3H, m, ArH), 5.21 (1H, s, ˇ-CH), 3.03 (1H, sept,
J = 6.8 Hz, CHMe2), 2.12 (1H, s, CH3COC), 1.71 (1H, s, CH3CNHAr),
1.21 (1H, d, J = 6.8 Hz, CH(CH3))2, 1.15 (1H, d, J = 6.8 Hz, CH(CH3))2.
13C NMR (CDCl3): ı 195.67 (C O), 163.02 (HC(CNHAr), 146.09,
133.33, 128.10, 123.54 (Ph), 95.43 (ˇ-C), 28.83 (CH3C O), 28.30
(CHMe2), 24.39, 22.46 (CH(CH3)2), 19.91 (CH3CNHAr).
4.12. Synthesis of [(L4)2Zn2(ꢀ-OBn)2] (4d)
To a suspension of L4ZnEt (0.62 g, 1.25 mmol) was added BnOH
(0.16 mL, 1.5 mmol) in toluene (15 mL). After stirring the mix-
ture was for 3 h, the resulting deep yellow powder was collected
by filtration. The deep yellow solid was dried in vacuo. Yield:
0.45 g (64%). 1H NMR (CDCl3):ı7.83 (2H, br s, MeOCCHCHCNO2),
7.41, 6.89 (5H, br s, C6H5CH2O), 7.26 (2H, br s, NO2CCHCNC),
6.63 (2H, br s, MeOCCHCHCNO2), 4.74 (2H, s, PhCH2O), 4.70
(1H, s, ˇ-CH), 3.82 (6H, s, CH3O), 1.38 (6H, s, C CNCH3) ppm.
13C NMR (CDCl3): ı 169.34 (C N), 158.70 (MeOCCHCHCNO2),
140.83 (MeOCCHCHCNO2), 139.60 (MeOCCN), 128.69, 127.98,
127.03, 123.05, 121.00, 109.47 (Ph), 95.21 (ˇ-C), 69.69 (PhCH2O),
56.28 (CH3O), 23.58 (C CNCH3) ppm. Anal. Calc. (found) for
4.16. Synthesis of 2-((4-methoxyphenyl)imino)-4-
((2,6-diisopropylphenyl)amido)-2-pentene.(L6H) (2f)
C52H52N8O14Zn2: C 54.60 (54.23), H 4.58 (4.50), N 9.80 (9.99)%.
4-Methoxyaniline
(3.8 g,
31 mmol),
4-(2,6-
diisopropylphenyl)amino-pent-3-en-2-one (7.9 g, 31 mmol), and
p-toluenesulfonic acid (0.10 g) were refluxed in absolute toluene
(5 mL) with water removed using a Dean–Stark trap for 2 days. After
being cooled to room temperature, the product was purified by
column chromatography on silica gel with n-hexane then removal
of volatiles afforded a brown solid. The brown solid was recrystal-
lized from the n-hexane to give yellow a solid. Yield: 4.62 g (41%).
1H NMR (CDCl3): ı 12.61 (1H, s, NH), 7.14–7.09 (3H, 2,6-iPr2ArH),
6.90 (2H, d, J = 6.8 Hz, MeOC(CHCH)2CN), 6.81 (2H, d, J = 6.8 Hz,
MeOC(CHCH)2CN), 4.84 (1H, s, ˇ-CH), 3.78 (3H, s, ArOCH3), 2.98
(2H, sept, J = 6.8 Hz, CHMe2), 2.00 (3H, s, CH3CNArOMe), 1.69 (3H,
4.13. Synthesis of 2-(2-methoxyphenylimino)-4-
(pentafluorophenylamido)-2-pentene.(L5H) (2e)
Pentafluoroaniline (21 g, 110 mmol) was mixed with 2,4-
pentanedione (12 g, 120 mmol) in 100 mL toluene containing a
catalytic amount (0.1 g) of p-toluenesulfonic acid. The solution
was refluxed for 24 h with water removed using a Dean–Stark
trap. Volatile materials were removed under vacuum and the sus-
pension in toluene (100 mL) was added 2-methoxyaniline (14 mL,
120 mmol). The solution was refluxed for 48 h with water removed
using a Dean–Stark trap again. After being cooled to room tem-
perature, the product was purified by column chromatography on
silica gel with toluene then removal of volatiles afforded a yel-
low solid. Yield: 29 g (71%). 1H NMR (CDCl3): ı 12.38 (1H, s, NH),
7.15–6.89 (4H, m, ArH), 5.00 (1H, s, ˇ-CH), 3.81 (3H, s, ArOCH3), 2.12
(3H, s, CH3CNArF5), 1.93 (3H, s, CH3CNArOMe). 13C NMR (CDCl3):
ı 171.45 (CH3CNArF5), 155.87 (CH3CNHOMe), 152.31 (MeOCCN),
140.53, 139.09, 138.92, 137.55, 136.74, 136.46, 128.95, 126.08,
125.36, 124.19, 120.28, 111.22 (Ph), 97.54 (ˇ-C), 55.64 (OCH3),
22.23 (CH3CNArOMe), 20.60 (CH3CNHF5). Anal. Calc. (found) for
C18H15F5N2O: C 58.38 (58.73), H 4.08 (4.12), N 7.56 (7.57)%.
s, CH3CNH2,6-iPr2Ar), 1.20, 1.11 (12H, d, J = 6.8 Hz, ArCH(CH3)2). 13
C
NMR (CDCl3): ı 163.52 (CH3CNArOMe), 156.77 (CH3CNH2,6-iPr2Ar),
156.12 (MeOC(CHCH)2CN), 143.49 (NCCCMe2), 140.18 (NCCCMe2),
136.58, 124.71, 123.96, 122.85, 114.03 (Ph), 95.09 (ˇ-C), 55.41
(OCH3), 28.21 (ArCH(CH3)2), 24.04, 22.65 (ArCH(CH3)2), 21.14
(CH3CNArOMe), 20.42 (CH3CNH2,6-iPr2Ar). Anal. Calc. (found) for
C24H32N2O: C 79.08 (78.73), H 8.85 (8.36), N 7.68 (7.08)%.
4.17. Synthesis of L6ZnEt (3f)
To a rapidly stirred solution of L6H (3.64 g, 10 mmol) in
dry n-hexane (15 mL) was added ZnEt2 (12 mL, 12 mmol), and

H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
69
the mixture was stirred at 0 ◦C for 1 h. After stirring for 3 h
4.20. Synthesis of [L7ZnEt] (3g)
at room temperature the solvent was removed in vacuo to
yield an light yellow solids which were redissolved in hex-
ane (15 mL). Recrystallized at −18 ◦C to yield colorless crystals.
Yield: 3.66 g, (80%). 1H NMR (CDCl3): ı 7.14 (3H, m, 2,6-iPr2ArH),
6.91 (2H, d, J = 6.4 Hz, MeOC(CHCH)2CN), 6.83 (2H, d, J = 6.8 Hz,
MeOC(CHCH)2CN), 4.91 (1H, s, ˇ-CH), 3.77 (3H, s, OCH3), 3.00
(2H, sept, J = 6.8 Hz, CHMe2), 1.94(3H, s, CH3CNArOMe), 1.73 (3H, s,
CH3CNH2,6-iPr2Ar), 1.20, 1.13 (12H, d, J = 6.8 Hz, ArCH(CH3)2), 0.58
(3H, t, J = 8 Hz, ZnCH2CH3), −0.14 (2H, q, J = 8 Hz, ZnCH2CH3). 13C
NMR (CDCl3): ı 167.00 (CH3CNArOMe), 166.56 (CH3CNH2,6-iPr2Ar),
156.27 (MeOC(CHCH)2CN), 144.48 (NCCCMe2), 143.24 (NCCCMe2),
141.35 (MeOCCN), 125.38, 125.19, 123.23, 113.79 (Ph), 95.29 (ˇ-
C), 55.29 (OCH3), 27.86(ArCH(CH3)2), 24.08 (ArCH(CH3)2), 23.32
(CH3CNArOMe), 23.21 (CH3CNH2,6-iPr2Ar), 11.60 (ZnCH2CH3),
−2.68 (ZnCH2CH3). Anal. Calc. (found) for C26H36N2OZn: C 68.19
(68.49), H 7.92 (7.66), N 6.12 (6.32)%.
To a rapidly stirred solution of L7H (4.3 g, 11.8 mmol) in
dry n-hexane (30 mL) was added ZnEt2 (14 mL, 1.0 M in hex-
ane, 14mmol), and the mixture was stirred at 0 ◦C for 1 h.
After stirring for 12 h at room temperature the solvent was
removed in vacuo to yield a sticky yellow solid which was
recrystallized from hexane (15 mL) at −18 ◦C to yield a white
microcystalline solid. Yield 4.7 g, (86%). 1H NMR (CDCl3): ␦ 7.14
(3H, m, 2,6-iPr2Ar H), 6.98–6.87 (4H, m, ArH), 4.94 (1H, s, ␤-CH),
3.78 (3H, s, OCH3), 3.05 (2H, sept, JH–H = 7 Hz, CHMe2), 1.94(3H,
s, HC{C(CH3)NAr}2), 1.74 (s, 3H, HC{C(CH3)NAr}2), 1.21 (6H, d,
JH–H = 6.8 Hz, ArCH(CH3)2), 1.14 (6H, d, JH–H = 6.8 Hz, ArCH(CH3)2)
0.50 (3H, t, JH–H = 8.2 Hz, ZnCH2CH3), −0.263 (2H, q, JH–H = 8 Hz,
ZnCH2CH3). 13C NMR (CDCl3): ı 166.96 (HC{C(CH3)NAr}), 166.34
(HC{C(CH3)NAr}), 152.57 (MeOCCN), 144.72 (NCCCMe2), 141.46
(NCCCMe2), 138.92 (MeOCCN), 125.72, 125.07, 125.04, 123.21,
120.44, 111.16 (Ph), 95.17 (␤-C), 55.50 (OCH3), 27.87(ArCH(CH3)2),
24.04 (ArCH(CH3)2), 23.36 (ArCH(CH3)2), 23.27 (HC{C(CH3)NAr}2),
22.96 (HC{C(CH3)NAr}2), 11.42 (ZnCH2CH3), −2.88 (ZnCH2CH3).
4.18. Synthesis of [(L6)2Zn3(ꢀ-OBn)4] (4f)
4.21. Synthesis of [(L7)2Zn3(ꢀ-OBn)4] (4g)
To a rapidly stirred solution of L6ZnEt (2.28 g, 5 mmol) in dry
hexane (30 mL) was slowly added BnOH (0.39 mL, 3.75 mmol)
at 0 ◦C. The mixture was allowed to warm to room tempera-
ture and stirred for 12 h. The yellow powder was collected by
filtration, and washed with hexane (15 mL, 0 ◦C) three times.
The light yellow powder was dried in vacuo. Yield 1.35 g (40%).
1H NMR (CDCl3): ı 7.08 (3H, m, 2,6-iPr2ArH), 7.02, 6.78 (10H,
br s, ArH), 6.45 (2H, d, J = 8.8 Hz, MeOC(CHCH)2CN), 6.40 (2H,
d, J = 8.8 Hz, MeOC(CHCH)2CN), 4.63 (1H, s, ˇ-CH), 4.20 (4H, br
s, PhCH2O), 3.47 (3H, s, OCH3), 2.95(2H, br s, CHMe2), 1.70
(3H, s, CH3CNArOMe), 1.56 (3H, s, CH3CNH2,6-iPr2Ar), 1.00–0.83
(12H, br m, ArCH(CH3)2), 0.77 (6H, s, ArCH(CH3)2). 13C NMR
To a rapidly stirred solution of L7ZnEt (2 g, 4.37 mmol) in dry
hexane (30 mL) was slowly added BnOH (0.55 mL, 5.24 mmol) at
0 ◦C. The mixture was allowed to warm to room temperature
and stirred for 12 h. The resulting white powder was collected
by filtration, and washed with hexane (15 mL) three times. The
white solid was dried in vacuo. The solid was recrystallized from
toluene (15 mL) at −18 ◦C to yield a white microcystalline solid.
Yield 1.2 g (40.7%). 1H NMR (CDCl3): ␦ 7.10–6.45 (17H, m, ArH),
4.69 (1H, s, ␤-CH), 4.14(4H, br s, OCH2Ar), 3.44(3H, s, OCH3),
2.87(2H, br s, CHMe2), 1.64 (3H, s, HC{C(CH3)NAr}2), 1.56 (3H,
s, HC{C(CH3)NAr}2), 0.94 (6H, d, JH–H = 6.8 Hz, ArCH(CH3)2), 0.77
(6H, s, ArCH(CH3)2). 13C NMR (CDCl3): ı 168.10 (HC{C(CH3)NAr}),
167.92 (HC{C(CH3)NAr}), 152.63 (MeOCCN), 144.41 (NCCCMe2),
142.46 (NCCCMe2), 138.77 (MeOCCN), 127.74, 127.02, 125.73,
124.86, 124.64, 123.35, 120.74, 111.70 (Ph), 94.51 (␤-C), 68.30
(PhCH2O), 55.207 (CH3O), 27.45 (ArCH(CH3)2), 24.14 (ArCH(CH3)2),
(ArCH(CH3)2), 23.71 (HC{C(CH3)NAr}2), 22.89 (HC{C(CH3)NAr}2).
Anal. Calc. (found) for C76H90N4O6Zn3·C6H5CH3:C, 69.04 (68.82);
H, 6.84 (6.71); N, 3.88 (3.81)%.
(CDCl3):
ı
168.12 (CH3CNArOMe), 167.04 (CH3CNH2,6-iPr2Ar),
155.76 (MeOC(CHCH)2CN), 144.88 (NCCCMe2), 144.12 (NCCCMe2),
142.72 (MeOC(CHCH)2CN), 142.27, 127.69, 126.94, 125.88, 125.17,
125.03, 123.43, 113.64 (Ph), 94.86 (ˇ-C), 55.05 (OCH3), 27.57
(ArCH(CH3)2), 24.05 (ArCH(CH3)2), 23.69 (CH3CNArOMe), 23.31
(CH3CNH2,6-iPr2Ar). Anal. Calc. (found) for C76H90N4O6Zn3: C 67.53
(67.25), H 6.71 (6.15), N 4.14 (4.24)%.
4.19. Synthesis of 2-((2-methoxyphenyl)imino)4-
4.22. Typical polymerization procedure
((2,6-diisopropylphenyl)amido)-2-pentene(L7-H) (2g).
A typical polymerization procedure was exemplified by the syn-
thesis of PLA (cat.4e = [(L5)2Zn2(ꢀ-OBn)2]) at room temperature.
The conversion yield (99%) of PLA was analyzed by 1H NMR spectro-
scopic studies. A mixture of the catalyst (0.0556 g, 0.05 mmol) and
l-lactide (0.72 g, 5 mmol) in toluene (20 mL) was stirred at room
temperature for 2 h. Volatile materials were removed in vacuo, and
the residue was redissoved in THF (10 mL). The mixture was then
quenched by the addition of an aqueous acetic acid solution (0.35 N,
10 mL), and the polymer was precipitated on pouring into n-hexane
(40 mL) to give white crystalline solids. Yield: 0.50 g (69%).
2-Methoxyaniline
(7.6 g,
62 mmol),
4-(2,6-
diisopropylphenyl)amino-pent-3-en-2-one (15.8 g, 62 mmol),
and p-toluenesulfonic acid (0.10 g) were refluxed in absolute
toluene (5 mL) for 2 days with continued water removal by a
Dean–Stark trap. After being cooled to room temperature, the
product was purified by column chromatography on silica gel with
n-hexane then removal of volatiles afforded a brown solid. The
brown solid was recrystallized from n-hexane to yield a yellow
solid. Yield: 12 g (53%). 1H NMR (CDCl3): ı 12.56 (1H, br s, NH),
7.15–6.97 (3H, 2,6-iPr2Ar H), 6.90–6.81 (4H, m, Ar H), 4.89 (1H, s,
␤-CH), 3.69 (3H, s, ArOCH3), 3.00 (2H, sept, JH–H = 6.8 Hz, CHMe2),
2.12 (3H, s, HC{C(CH3)NAr}2), 1.69 (3H, s, HC{C(CH3)NAr}2),
1.18 (6H, d, JH–H = 6.8 Hz, ArCH(CH3)2), 1.11 (6H, d, JH–H = 6.8 Hz,
4.23. Kinetic studies of polymerization of l-LA by 4e
ArCH(CH3)2). 13C NMR (CDCl3):
ı
163.21 (HC{C(CH3)NAr}),
l-LA (1.25 mmol) was added to a solution of 4e (with 2.5, 12.5,
25.0, and 62.5 mM) in toluene (10 mL). The mixture was then
stirred at room temperature under N2. At appropriate time inter-
vals, 0.2 mL aliquots were removed and quenched with methanol
(1 drop). The aliquots were then dried to constant weight under
vacuum and analyzed by 1H NMR.
155.97 (HC{C(CH3)NAr}), 151.89 (MeOCCN), 143.68 (NCCCMe2),
140.11 (NCCCMe2), 133.01, 123.81, 123.73, 123.19, 120.37,
111.31(Ph), 96.28 (␤-C), 55.53 (OCH3), 28.44 (ArCH(CH3)2), 23.85
(ArCH(CH3)2), 22.70 (ArCH(CH3)2), 21.09 (HC{C(CH3)NAr}2), 20.90
(HC{C(CH3)NAr}2).

70
H.-Y. Chen et al. / Journal of Molecular Catalysis A: Chemical 339 (2011) 61–71
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