Preparation of zirconium and hafnium complexes containing chiral N atoms from asymmetric tertiary amine ligands, and their catalytic properties for polymerization of rac-lactide

Article abstract of DOI:10.1039/c6cy02618d

A series of N-(2-pyridylmethyl)-N,N-bis(2′-hydroxy-benzyl) amine ligands were synthesized, and the new asymmetric tertiary amine ligands reacted with ZrCl₄ and HfCl₄ to give complexes 1-6. The chiral N atoms were created in the complexes upon coordination of the asymmetric tertiary amine ligands to the metal centers. The complexes were used as catalysts for ring-opening polymerization of rac-lactide. The results show that the substituents and structure of the ligands have effects on the activity and stereoselectivity of the catalyst. The stereoselectivity was promoted by the complex containing the ligand with the electron-withdrawing group. Complex 3, with bulkier cumyl groups on the ligand, possesses better stereoselectivity. The complexes are able to catalyze the living control ring-opening polymerization of rac-lactide to afford isotactic-rich polylactide with narrow molecular weight distribution. Kinetic studies reveal that polymerization is of the first order in both monomer and catalyst. The microstructure of the resulting polymer is determined by ¹H NMR homonuclear decoupled spectra. The results show that the isotactic-rich chain is dominant in the prepared polylactide. The P_m value is in the range of 0.67-0.81.

Full text of DOI:10.1039/c6cy02618d

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Volume 6 Number 1 7 January 2016 Pages 1–308
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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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ARTICLE
Preparation of zirconium and hafnium complexes containing
chiral N atoms from unsymmetric tertiary amine ligands, and their
catalytic properties for polymerization of rac-lacꢀde†
Received 00th January 20xx,
*
a
a
a
a
*a
a
Minggang Hu, Fuzhong Han, Wenzhi Zhang, Wenhui Ma, Qigang Deng, Weiming Song,
b
a
Hailong Yan and Guohua Dong
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of N-(2-Pyridylmethyl)-N,N-bis(2’-hydroxy-benzyl) amine ligands were synthesized and the new unsymmetric
tertiary amine ligands reacted with ZrCl and HfCl to give complexes 1-6. The chiral N atoms were created in the
4
4
complexes upon coordination of the unsymmetric tertiary amine ligands to the metal centers. The complexes are used as
catalysts for ring-opening polymerization of rac-lactide. The results show substitutes and structure of the ligands have
effects on activity and stereoselectivity of the catalyst. The stereoselectivity was promoted using complex containing the
ligand with electron-withdrawing group. Complex
3 with bulkier cumyl groups on the ligand possesses better
stereoselectivity. The complexes are able of catalyzing living control ring-opening polymerization of rac-lactide to afford
isotactic-rich polylactide with narrow molecular weight distribution. The Kinetic studies reveal the polymerization are the
1
first order in both monomer and catalyst. The microstructure of resulting polymer is determined by H NMR homonuclear
m
decoupled spectra. The results show the isotactic-rich chain is dominant in the prepared polylactide. The P value is in the
range of 0.67-0.81.
3
9-44
complexes exhibit heteroselectivity,
4
complexes processes isoselectivity.
and only a few group 4
5-48
Introduction
Stereoregular polylactide has excellent mechanical
properties and degrades rather slowly. Therefore, the
production of the stereoregular PLA via control of
stereochemistry has recently become research focus. The
stereoselective polymerization of lactide can be achieved
through two different strategies. Complexes with achiral
ligands induce stereoselectivity most probably via a chain-end
control mechanism, and complexes with chiral ligands induce
enantiomorphic site control in the lactide ring-opening
Environmental friendly and biodegradable polymer materials
from renewable resources have attracted more and more
attention, as environmental problem and sustainable
1
,2
development are intensively considered. Polylactide (PLA)
derived from biorenewable resources such as corn and wheat
becomes a focus on the field, and the biocompatible and
biodegradabile material as a substitute for oil-based polymer
has been exploited for vast applications such as packaging,
3
-6
surgery, and pharmacology. PLA is mainly prepared via the
ring-opening polymerization (ROP) of lactide (LA). The
4
9-53
polymerization.
It is notable that chiral complexes can be
7
,8
numerous metal complexes (Li, Na,
9,10
11,12
13
Ca, Mg,
14-16 obtained from achiral ligands, i.e. chirality can be obtain by
coordination of achiral ligand to metal center. The chiral
K,
1
7,18
19,20
21,22
23,24
25,26
27,28
Zn,
Al,
Ga,
In,
Fe,
rare earth
) are used as
induction is one of the most interesting fields because of its
applications in asymmetric catalysis and stereochemical
reaction. Kojima reported a chiral induction upon coordination
initiators for the ROP of LA. Group 4 metal complexes as good
lewis acid catalysts have been attractting particular interest,
and the complexes have been used widely among the
interesting transformations in synthesis and show excellent
3
to form an enantiomeric Ru complex with an achiral C -
5
4
2
9-32
symmetric tertiary amine, and some complexes containing
-symmetric tertiary amines have been used in the
catalysis properties in many reactions.
In recent years,
C
3
some Ti, Zr and Hf complexes were used as catalysts for the
5
5
stereoselective ring-opening polymerization of lactide.
Davidson used Zr and Hf complexes with C -symmetric tertiary
amine ligands to initiate heterotactic polymerization of rac-
ring-opening polymerization of rac-lactide, and stereoregular
3
3-38
3
polymers were obtained.
However, most of group 4
5
6
lactide (P
with C -symmetric amine-bis(benzotriazole phenolate), and
the complex exhibit isotactic secectivity (P
r
= 0.98). In addition, Ko synthesized a Zr complex
2
5
7
m
= 0.71). We are
interested in the tertiary amine ligand complexes and
anticipate the chiral N can be formed with the introduction of
unsymmetric tertiary amine ligand to metal center. Here we
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Br
report the synthesis of unsymmetric tertiary amine ligands and
their Zr and Hf complexes. N atom of the tertiary amine is
linked by three different groups, and the coordinating of ligand
to central metal results the creating of charlity on nitrogen due
to inhibiting conformation inversion of the unsymmetric
tertiary amine. The complexes containing chiral N centers are
effective catalysts for the ROP of LA in a controlled fashion,
and isotactic-rich PLA with very narrow polydispersity indexes
can be obtained.
R
Br
R
R
Br
O
O
OH
R
N
L1-4
2
, NaH
N
1) H
M
N
OH
N
toluene
N
Br
N
2) MCl
4
O
O
R
R
Br
H
H
H
H
2
2
2
2
L1: R = Me;
Br
L2: R = tBu;
L3: R = Cumyl;
1: M = Hf; L
2: M = Hf; L
1
2
L4: R = Cl
3
4
5
6
: M = Hf; L3
: M = Hf; L4
: M = Zr; L1
: M = Zr; L2
Results and discussion
Scheme 2 synthesis of complex 1-6.
Synthesis of ligands and complexes
1
5
-4
The unsymmetric N
modified literature procedures.
substituted salicylaldehyde
aminomethylpyridine in EtOH, and then the in-situ reduction
of the Schiff base obtained using NaBH gave secondary amine
intermediate (A). Finally the target product unsymmetric N
2
O
2
-ligand H
2
L
was synthesized via a
8-60
Salicylaldehyde or
reacted with 2-
Polymerization of lactide with complexes 1-6
The ring-opening polymerizations of rac-LA were carried out
by complexes in toluene with molar ratio of
LA]/[Cat]/[MeOH] = 100:100:1, and the conversions of rac-LA
1-6
a
4
[
2
2
O -
1
are determined by
representative results were shown in table 1. The conversions
of monomer reach 91–96% in 3 h with Hf complexes as
catalysts. While complexes and are used as catalysts, the
conversions of monomer are 78 and 86% in the same reaction
time. It is shown that Hf complexes ( ) are more active than
Zr complexes ( ). The activity order is opposite to that of
most of reported Zr and Hf complexes.
H NMR spectra (Fig. S1). The
ligand was prepared by the reaction of the secondary amine
with paraformaldehyde and substituted phenol at 110 °C.
1-4
(Scheme 1).
5
6
Complexes
1
-
6
were prepared by the reactions of Group 4
-ligands.
2 2
tetrahalide with sodium salts of unsymmetric N O
1-4
Firstly, about 4 equiv of NaH was added to the solution of the
unsymmetric ligand in toluene and the ligand was converted to
its disodium derivative, and then Group 4 tetrahalide was
added to the mixture in ice-water bath at 0 °C, the reaction
mixture was stirred for about 24 h, and the product was
5, 6
5
7,61
The possible reason
for the opposite activity trend of the complexes is not clear
currently.
To explore the control degree of polymerization, the ring-
obtained (Scheme 2). Complexes 1-6 were characterized by
1
opening polymerization of rac-LA by complex 3 was further
HR-MS and H NMR. The HR-MS spectra show that the
complexes are eight-coordinate mononuclear complexes
investigated. Analysis of gel permeation chromatography
implies the number-average molecular weight of resultant PLA
rises in proportion with the increase of monomer conversion
2
-
1
coordinated by two L ligands. The H NMR spectra of
complexes display doublets at δ = 3.18-5.33 ppm for the
protons of six CH groups linked N atom of tertiary amine,
1-6
(
Fig. 1), and the polydispersity is approximately close to a
constant (1.1). The linear varying of number-average
molecular weight with monomer conversion and the narrow
polydispersity (M /M = 1.03-1.14) show complex catalyzes a
2
1
1
4
while H NMR spetrum of H
2
L -H
.72-3.94 ppm for the six CH protons. Two doublets at δ =
0.20-8.47 ppm in the H NMR spectrum of the complexes
2
L display three singlets at δ =
3
1
2
1
w
n
3
controlled polymerization process and the ring-opening
polymerization of LA possesses a living nature.
could be assigned to the ortho-proton in each pyridyl group.
These two doublets shift downfield, as compared to the signal
at δ = 8.50-8.71 ppm for the corresponding pyridyl proton in
the free ligand.
Kinetics of LA polymerization
The kinetics of ring-opening polymerization of rac-LA with
OH
OH
complexes 1-6 was studied. The semilogarithmic plots for the
NH
2
Br
CHO
+
Br
N
polymerizations are shown in Fig. 2. The apparent propagation
-1
rate constants (k ) are 1.74 ± 0.05 h for
1
) rt, EtOH
H
N
-1
, 1.52 ± 0.04 h for
3
N
2) NaBH
4
app
1
-
-1
, 1.09 ± 0.03 h for
-1
, 0.82 ± 0.01 h for
Br
Br
2
, 1.36 ± 0.04 h for
4
1
A
-1
6
, and 0.64 ± 0.01 h for
5
, respectively. The studies on
R
kinetics suggest that the polymerization rates increase with
3
OH
Br
the increase of size of substituted group, complex 3 ((L ) Hf)>
2
R
OH
2
1
complex 2 ((L ) Hf)> complex 1 ((L ) Hf) (Fig. 2). In addition, the
2 2
R
,
PA
N
OH
induction period occurred for the polymerizations, which is
similar to that previously reported for polymerizations
62
Br
R
N
1
10

catalyzed by group 4 complexes. The induction period for
ROP of lactide depends on the ability of the monomer to
63
coordinate to the initiator. The mononuclear complexes
1
2
t
3
4
2 2 2 2
H L : R = Bu; H
L
:
R
=
M
e
;
H
L
:
R
=
C
u
m
y
l
;
H
L
:
R
=
C
l
1
4
2 2
Scheme 1. synthesis of H L -H L
2
| J. Name., 2012, 00, 1-3
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2
-
coordinated by two L ligands possess the steric hindrance
around the active site and the coordination of monomer to
metal center is not facile, which maybe results in appearance
of the induction period.




2
2
1
1
0
0
.5
.0
.5
.0
.5
.0






Further, Using methanol as initiator, the kinetics of ring-


opening polymerization of rac-LA with complex
3 and 2 under
a series of ratio of monomer to catalyst was studied in detail.
The results were shown in Fig. 3 and 4. In each case, the linear
0 t
relationship between ln ([LA] /[LA] ) and reaction time was
observed, implying that the polymerization is first order
kinetics in monmer. The processes of rac-lactide
polymerizations with complex
law:
3
and
2
follow first order rate
0
.0
0.5
1.0
1.5
2.0
2.5
3.0
-d[LA]/dt = kapp[LA]
(1)
Time (h)
Fig. 2 Kinetics of rac-LA polymerization using complexes
catalysts in toluene at 130 °C with [LA]/[cat]/[MeOH] = 100:1:1
and [LA] = 1.0mol/L: Ⅰ, complex , kapp = 1.74 ± 0.05 (R =
0.998);Ⅱ, complex , kapp = 1.52 ± 0.04 (R = 0.998); Ⅲ, complex
, kapp = 1.36 ± 0.04 (R = 0.998); Ⅳ, complex , kapp = 1.09 ± 0.03
R = 0.998); Ⅴ, complex , kapp = 0.82 ± 0.01 (R = 0.999); Ⅵ,
complex , kapp = 0.64 ± 0.01 (R = 0.999).
1-6 as
a
Table 1 The ROP of rac-LA by complexes 1-6
c
b
c
d
Run Cat T (°C) t (h) Conv(%)
M
n
M
w
/M
n
P
m
3
1
1
1
2
2
2
3
3
3
4
4
5
5
6
6
130
130
130
130
70
1.5
3
66
91
88
93
52
92
96
60
80
92
43
78
53
86
8500
1.04
1.08
1.19
1.21
1.02
1.03
1.14
1.02
1.23
1.25
1.12
1.14
1.06
1.09
0.71
0.67
0.73
0.68
0.80
0.79
0.72
0.81
0.78
0.77
0.76
0.73
0.77
0.74
2
4
(
1
2
3
4
5
6
7
8
9
12100
9900
6
1.5
3
5
10300
7300
6.5
1.5
3
130
130
70
11900
12400
8600
3
2
1
0
5


130
130
130
130
130
130
1.5
3
9700


1
0
1
2
3
4
10500
4400


1
1
1
1
1.5
3
9800


1.5
3
7200
10800
a
b
1
c
[
rac-LA]/[Cat]/[MeOH] = 100:1:1.
measured by H NMR.
determined by GPC, calibrated with polystyrene standard and
P is the probability
m
of isotactic chain forming, calculated by analysis of proportion
between different tetrad resonance of methine of PLA in the
6
4 d
multiplied by the correcting factor of 0.58.
0
1
2
3
4
5
Time (h)
1
65
homonuclear decoupled H NMR spectra.
Fig. 3 Kinetics of rac-LA polymerization using complex 3 as a
catalyst in toluene at 130 °C with [LA] = 1.0mol/L: Ⅰ,
1
5000
2000
2.0
1.8
1.6
1.4
1.2
1.0
[LA]/[
LA]/[
[LA]/[
LA]/[
3
3
3
3
]/[MeOH] = 100/1/1, kapp = 1.74 ± 0.05 (R = 0.998);Ⅱ,
]/[MeOH] = 150/1/1, kapp = 1.16 ± 0.03 (R = 0.998); Ⅲ,
]/[MeOH] = 200/1/1, kapp = 0.82 ± 0.01 (R = 0.999); Ⅳ,
]/[MeOH] = 300/1/1, kapp = 0.57 ± 0.02 (R = 0.997).
[
1
[
9
6
3
000
000
000
0
x
[Hf] , and where kapp and k
Where kapp = k
p
p
are the apparent
propagation rate constant and the propagation rate constant.
As shown in Fig. 3, the kapp for declines with the increase of
the ratio of monomer to catalyst. The kapp values are 1.74 ±
3
-1
-1
-1
-1
0
.05 h , 1.16 ± 0.03 h , 0.82 ± 0.01 h and 0.57 ± 0.02 h with
LA]/[ ]/[MeOH] = 100:1:1, 150:1:1, 200:1:1, and 300:1:1,
respectively. The similar results were oberserved for the ring-
opening of rac-lactide with complex (Fig. 4). The apparent
propagation rate constant (kapp) decreases from 1.52 ± 0.04 h
0
20
40
60
80
100
[
3
conversion (%)
2
-1
Fig. 1 Plots of M
vs. monomer conversion for ring-opening polymerization of
rac-LA at 130 in toluene with [LA]/[ ]/[MeOH] = 100:1:1.
n w n
and molecular weight distribution (M /M )
-1
-1
-1
to 1.03 ± 0.052 h to 0.73 ± 0.01 h and to 0.49 ± 0.02 h ,
℃
3
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while the [LA]/[
2
]/[MeOH] ratio changes from 100:1:1 to
The effect of the temperature on the rate of lactide
polymerization with and methanol was also investigated. The
To explore the order in metal, the dependence of apparent polymerization rate of rac-LA decreased with the decrease of
150:1:1 to 200:1:1 and to 300:1:1.
3
-1
0
propagation rate constants on [Hf] was investigated. The plots temperature. The values of kapp were 1.74 ± 0.05 h at 130 °C,
-1
-1
of ln kapp versus ln [Hf]
increase of ln kapp with ln [Hf]
From the polts, the order in catalysts are 1.02 ± 0.04 for
0
were shown in Fig. 5 and 6. The linear 0.87 ± 0.01 h at 110 °C, 0.52 ± 0.01 h at 90 °C, and 0.21 ±
-1
0
implies a first-order in catalyst. 0.01 h at 70 °C, respectively (Fig. S2). The similar results were
and observed in polymerization of L-LA using and methanol. The
3
3
-1
-1
-1
values of kapp decreased from 2.19 ± 0.03 h to 1.68 ± 0.03 h
-1
1
.03 ± 0.03 for
2
, and the k
p
values of 0.0532 ± 0.001 L·mol ·s
-1
-1
values of 0.0477 ± 0.001 L·mol ·s for 2 were to 1.21 ± 0.03 h , and to 0.75 ± 0.02 h , while the temperature
-1
-1
for 3 and the k
p
determined. Therefore, the overall kinetic law of ROP of declined from 130 °C to 110 °C to 90 °C and to 70 °C,
lactide with complex
eq 2 and eq 3:
3
or
2
can be represented in the following respectively (Fig. S3). Furthermore, the values of k
temperature were deduced according to the relationship
between k and kapp. By means of the Arrhenius equation, the
activation energy of the polymerization of lactide by and
methanol was calculated from plot of ln k versus 1/T (Fig. 7).
for the rac-LA polymerization was
p
at different
1
.02
.03
-d[LA]/dt = k
d[LA]/dt = k
p
[Hf] [LA]
(2)
(3)
p
1
-
p
[Hf] [LA]
3
P
The activation energy E
a
3
-7.6
-7.8
-8.0
-8.2
-8.4
-8.6
-8.8




2
1
0




0
1
2
3
4
5
Time (h)
-9.0
-
5.8
-5.6
-5.4
-5.2
-5.0
-4.8
-4.6
ln [Hf]
Fig. 4 Kinetics of rac-LA polymerization using complex 2 as a
catalyst in toluene at 130 °C with [LA] = 1.0mol/L: Ⅰ ,
[
[
[
[
LA]/[
LA]/[
LA]/[
LA]/[
2
2
2
2
]/[MeOH] = 100/1/1, kapp = 1.52 ± 0.04 (R = 0.998); Ⅰ , Fig. 6 Polymerization of rac-LA with complex 2 and relationship
]/[MeOH] = 150/1/1, kapp = 1.03 ± 0.02 (R = 0.999); Ⅰ , between ln kapp and the ln [Hf] (slope 1.03 ± 0.03, and R =
]/[MeOH] = 200/1/1, kapp = 0.73 ± 0.01 (R = 0.999); Ⅰ , 0.999).
]/[MeOH] = 300/1/1, kapp = 0.49 ± 0.02 (R = 0.996).
-
2.5
3.0
-7.6
-7.8
-8.0
-8.2
-8.4
-8.6
-8.8
-




-3.5
-4.0
-4.5
-5.0
-5.8
-5.6
-5.4
-5.2
-5.0
-4.8
-4.6
2.4
2.5
2.6
2.7
2.8
2.9
3.0
-
1000/T(K )
1
ln [Hf]
Fig. 5 Polymerization of rac-LA with complex
3 and relationship Fig. 7 Arrhenius plot of ln k versus 1/T for the polymerization
p
between ln kapp and the ln [Hf] (slope 1.02 ± 0.04, and R = of L- and rac-LA at different temperatures by using
3/methanol
0.998).
(Ⅰ , L-LA; Ⅰ , rac-LA).
4
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-
1
3
9.5 kJ·mol , which is slightly higher than that for the ring- ligand with electron-withdrawing group, the isotactic
opening polymerization of L-LA under the same condition (20.5 selectivity of rac-lactide polymerization was promoted. The
4
((L )
-
1
kJ·mol ) (Fig. 7). The similar results were observed in the complex
4
2
Hf) with chlorine on the phenolate moieties of
polymerization of lactide by
2
and methanol. The ligands displayed better isotactic selectivity than complexes
2
((L )
1
-
1
1
were 46.2 kJ·mol for rac- ((L )
polymerization activation energy E
a
2
Hf) and
2
2
Hf) with methyl and tert-butyl groups on
-1
LA and 30.8 kJ·mol for the L-LA (Fig. S6).
the phenolate moieties of ligands. Additionly, compared with
Hf complexes ( and ), Zr complexes ( and ) with the same
ligands possess high stereoselectivities.
1
2
5
6
Seteroselectivity of rac-lactide polymerization
The stereochemistry of the polymerization of rac-lactide
was also supported by activation parameters. The
setereoselectivity of the polymerization can be identified by
The stereochemical microstructures of the resultant PLA
obtained from rac-LA with complexes 1-6 were analyzed from
1
H NMR spectra of methine resonances with the homonuclear
decoupling of PLA methyl resonances. Several resonances were
‡
‡
the activation entropy difference (ΔS ―ΔS ) and activation
m
r
‡
‡
1
observed in the homonuclear decoupled H NMR spectra
r
enthalpy difference (ΔH m―ΔH ) between homopropagation
m r
(k ) and cross-propagation (k ). According to first-order
because of different linking mode of the chiral carbon atoms of
the prepared PLA (Fig. S7-S9). The linking can be represented
by array of tetrads, i.e. mmm, mmr, mrm, rmm and rmr, and
6
7
Markovian statistics and absolute reaction rate theory, eq 4
can be used to determine the activation entropy difference
and activation enthalpy difference.
m
the probability of isotactic-rich chain of polymer (P ) can be
‡
‡
r
‡
‡
r
P /(1―P
m
m
) = k
m
/k
r
= esp[(ΔS
m
―ΔS
)/R―(ΔH
m
―ΔH
)/RT] (4)
calculated by the ratio of these resonances. The homonuclear
1
decoupled H NMR spectra of all polymers prepared by
complexes 1-6 reveal the mmm tetrad resonances from the
isotactic linking of adjacent chiral carbon atoms of the polymer
chain are predominant, which shows the isotactic-rich chains
1
1
.5
.4
6
are preferentially produced in polymers. The C NMR
6
13
spectra of polymers prepared from rac-lactide and L-lactide
(Fig. S10 and S11) were also analyzed. The results show
1.3
isotactic-rich PLA is obtained in the ring-opening
1
1
.2
.1
polymerization of rac-lactide. In previous reports, complexes
containing C
3
-symmetry tertiary amine ligands were used in
5
5,56
ROP of rac-LA, and heterotactic polymers were obtained.
The mechanism of the heteroselectivity has been assumed
that inversion of axial chirality during chain propagation can
lead to alternating stereochemistry at the metal center and
the heteroselective polymereization is controlled by the
1.0
.9
0
2
.4
2.5
2.6
2.7
2.8
-1
2.9
3.0
dynamic chirality at the metal. The complexes 1-6 supported
1
000/T (K )
by the unsymmetric tertiary amine ligands are capable of
catalyzing isoselective polymerization of rac-LA. Although
detailed mechanism of stereoselectivity is not clearly
understood to date, it seems that the coordination of
unsymmetric tertiary amine ligand to a metal center and the
nitrogen chiral center in the complex may have influence on
m m
Fig. 8 Plot of ln (P /(1-P )) versus 1/T for the polymerization of
rac-LA at different temperatures by using 3/methanol.
stereoslectivity of polymerization. The P
range of 0.67-0.81 for the ring-opening polymerization of rac-
LA catalyzed by complexes in toluene (table 1).
m
values are in the
1.5
1
.4
.3
1-6
1
The effects of structure and substituted group of ligands on
seteroselectivity of rac-lactide polymerization were
investigated. The results show that the steric hindrance and
electronic effects of different substituents on phenol rings of
ligands have influence on seteroselectivity of rac-lactide
1.2
1.1
1.0
0.9
0.8
3
2
2 2
polymerization. Complexes 3 ((L ) Hf) and 2 ((L ) Hf) with bulky
t
Bu and Cumyl groups at the ortho and para positions on the
phenolate moieties of ligands exhibit better seteroselectivity.
1
((L )
Complex
1
2
Hf) with smaller size methylsubstituent
0.7
2.4
2.5
2.6
2.7
2.8
2.9
3.0
possesses lower seteroselectivity. The reason maybe is that
ligands with bulky steric hindrance substituents give a shelter
to improve stereoselective coordination of a chiral monomer
-
000/T (K )
1
1
to metal center. Furthermore, using complex containing the Fig. 9 Plot of ln (P /(1-P
m
m
)) versus 1/T for the polymerization of
/methanol.
rac-LA at different temperatures by using
2
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ARTICLE
Journal Name
i.e. complex
3 is able to catalyze isoselective ROP of rac-LA to
give isotactic-rich PLA, which is in agreement with the
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
observed isoselectivity in the ROP of rac-LA.




Mechanism of Lactide Polymerization
There are two main mechanisms for the ROP of lactide: the
coordination insertion mechanism and the activated monomer
6
8,69
mechanism.
polymerization, the PLA oligomer was prepared with complex
, using methol as an initiator under the low ratio of monomer
To explore the mechanism of lactide
3
to initiator ([LA]/[Cat]/[MeOH] = 20:1:1). The mass spectrum
analysis of the final oligomer demonstrated the polymer chain
has an end group of methyl ester (Fig. S12), which can be
0
.0
0.4
0.8
1.2
1.6
2.0
verified by
a series of peaks at 32 + 72n + 23
Time (h)
+
CH O(COCHMeO) H·Na ). The H NMR spectrum of the
1
(
3
n
oligomer showed a quartet at 4.34 ppm and a singlet at 3.74
ppm (Fig. 11). These peaks can be assigned to the methine
proton adjacent to the hydroxyl end group and the protons of
Fig. 10 Semi-log figure as time changes for polymerizations of
L- and rac-LA with in toluene at 130 °C: Ⅰ , L-LA, kapp = 2.19 ±
.03 (R = 0.999) ; Ⅰ , rac-LA, kapp = 1.74 ± 0.05 (R = 0.998).
3
0
1
methyl ester end group, respectively. The H NMR spectrum
shows the resultant oligomer contain one hydroxyl end group
and one methyl ester end group. Furthermore, the reactions of
1
alcohol with the complexes were investigated by H NMR
spectroscopy. The complexes are stabile in the presence of
excess of BnOH (5 equiv; in toluene-d , 70 °C, 7 h) or MeOH (5
8
equiv; in toluene-d , 60 °C, 10 h). On the basis of these findings
8
combined with end-group analyses of the PLA oligomers
obtained, we propose that ROP reaction of lactide catalyzed by
70
the complex, proceed via an activated monomer mechanism.
Conclusions
Zirconium and hafnium complexes containing unsymmetric
tertiary amine ligands were synthesized. Complexes are
1-6
active catalysts for the ring-opening polymerization of lactide.
The studies on kinetics imply the ring-opening polymerization
of lactide follow first order in both monomer and catalyst. The
rate of polymerization with the complex containing bulky
ligand is faster. The microstructure analysis of resultant
polymer shows the isotactic enriched chain is formed. The
structure of catalyst has a considerable effect on the
1
Fig. 11 H NMR spectrum of PLA by
[LA]/[cat]/[MeOH] = 20/1/1).
3 in toluene at 130 °C
(
Plots of ln [P /(1―P )] vs. 1/T were drawn according to the
m
m
values of Pm of polylactide prepared by
3 in different
temperature (Fig. 8). From the slope of the plot, an activation
‡
enthalpy difference (ΔH m―ΔH ) of -10.2 kJ mol was
‡
-1
stereoselectivity in the polymerization. Complex 3 with big
r
cumyl groups at ortho and para positions on phenolate
obtained, and from the intercept of the plot, an activation
‡
entropy difference (ΔS m―ΔS ) of -17.8 J K mol was
‡
r
-1
-1
moieties of ligand exhibits better stereocontrol. The
stereoselectivity in the polymerization increases as complex
4
deduced, which implies the preference of isotactic
with electron-withdrawing Cl groups at ortho and para
positions on phenolate moieties of ligand is used as a catalyst.
The kinetics on polymerization L-LA and rac-LA suggest the
polymerization rate of L-LA is higher than that of rac-LA, which
is consistent with the observed fact that the complexes show
isoselectivity in polymerization of rac-LA.
enchainment in the ring-opening of rac-LA. The similar result
‡
‡
‡
-1
‡
was obtained by
m r m r
2 (ΔH ―ΔH = -12.4 kJ mol , ΔS ―ΔS = -
-
1
2
4.7 J K , Fig. 9).
In addition, the studies on kinetics of the polymerizations of
rac-LA and L-LA further prove the complexes possess
isoselectivity in ROP of rac-LA. The polymerization of rac-LA
-
1
with complex
3 exhibits the kapp of 1.74 ± 0.05 h , which is
lower than that determined in the polymerization of L-LA
Experimental
-1
under the same reaction condition (2.19 ± 0.03 h ) (Fig. 10).
General procedures
The lower chain propagation reaction rate for rac-LA
demonstrates complex
3
prefers to promote same All operation involved compounds sensitive to air and water
enantiomeric monomer to link together in chain propagation, was carried out under standard schlenk line techniques.
6
| J. Name., 2012, 00, 1-3
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Journal Name
Cat Pa ll ey as si es dS oc ni eo nt ca ed j &u s Tt me ca hr gn i on sl ogy
View Article Online
DOI: 10.1039/C6CY02618D
ARTICLE
t
Solvent toluene and hexane were distilled from 1.27 (s, 9H, Bu). HR-MS: m/z = 591.1170 (Calcd 591.1045)
+
2 2 2
sodium/benzophenone ketyl under nitrogen atmosphere [M+H] . Anal. Calcd for C28H34Br N O : C, 56.96; H, 5.80; N,
before using. Dichloromethane was distilled from calcium 4.74. Found: C, 56.87; H, 5.75; N, 4.69.
hydride under nitrogen atmosphere. L-lactide and rac-lactide
3
2
(Aldrich) were recrystallized from dry toluene and were Preparation of ligand H L
sublimed twice in vacuo at 55 °C prior to use. Anhydrous
zirconium chloride and hafnium chloride (Alfa Aesar), and
other commercially available reagents were used without
purification. Glassware involved in polymerization was dried
for 12 h in an oven at 120 °C, and was exchanged three times
under vacuum-nitrogen line. NMR spectra were recorded on
Bruker DRX 600 and Varian 400 spectrometers at ambient
temperature. Mass spectra were recorded on an ESI-Q-TOFMS
3
Ligand H
0.551 g, 1.48 mmol), paraformaldehyde (0.044 g, 1.48 mmol)
and 2,4-bis(α,α-dimethylbenzyl)phenol (0.489 g, 1.48 mmol)
2
L was prepared from intermediate secondary amine
(
1
according to a similar method described above for ligand H
2
L .
, 293 K): δ = 8.50
d, 1H, Py, J = 4.8 Hz), 7.72−6.76 (17H, Ph and Py), 3.77 (s, 2H,
N-CH -Ph), 3.72 (2s, 4H, N-CH -Py), 1.68, (s, 6H, 2Me), 1.66 (s,
H, 2Me). HR-MS: m/z = 715.1370 (Calcd 715.1358) [M+H] .
Anal. Calcd for C38 : C, 63.88; H, 5.36; N, 3.92. Found:
1
yield: 0.855 g (81%). H NMR (600 MHz, CDCl
3
(
2
2
+
6
(Micro) instrument. Elemental analyses were performed with
2 2 2
H38Br N O
an Elementar Vario EL III elemental analyzer. The gel
permeation chromatography (GPC) measurements of the
synthesized polymers were performed on a PL-GPC 220
instrument. All polymer samples were analyzed at 30 °C using
THF as solvent at a flow rate of 1.0 mL/min. The number-
C, 63.83; H, 5.32; N, 3.89.
4
2
Preparation of ligand H L
4
L was prepared from intermediate secondary amine
Ligand H
2
(
0.826 g, 2.22 mmol), paraformaldehyde (0.067 g, 2.22 mmol)
average molecular weight (M
n w n
) and polydispersities (M /M ) of
and 2,4-dichlorolphenol (0.362 g, 2.22 mmol) according to a
1
L . yield: 0.862 g
the polymers were calibrated against polystyrene standards,
similar method described above for ligand H
2
and the M
4
n
value was corrected by a correcting coefficient of
1
(71%). H NMR (600 MHz, CDCl , 293 K): δ = 8.71 (d, 1H, Py, J =
6
3
0.58.
4
.8 Hz), 7.87−6.98 (7H, Ph and Py), 3.94 (s, 2H, N-CH
2
-Ph), 3.84,
1
(s, 2H, N-CH -Py), 3.83 (s, 2H, N-CH -Py). HR-MS: m/z =
2
2
Preparation of ligand H
2
L
+
46.9038 (Calcd 546.9013) [M+H] . Anal. Calcd for
5
3
,5-Dibromosalicylaldehyde (6.158 g, 0.022 mol), 2-
C
20
H16Br
2
Cl
2
N
2
O
2
: C, 43.91; H, 2.95; N, 5.12. Found: C, 43.88; H,
aminomethylpyridine (2.357 g, 0.022 mol) and 40 mL EtOH
were added to a flask. The mixture was stirred 4 h at room
2.92; N, 5.08.
temperature. Then NaBH
°C in ice bath, and the mixture was stirred 1 h. Solvent was
removed under reduced pressure, and 30 mL water was added. A schlenk flask was charged with ligand H
The mixture was neutralized by ice acetic acid and was mmol) in toluene (20 mL). NaH (0.104 g, 4.32 mmol) was
extracted by CH Cl . Solvent was removed in vacuo. After added the solution, and the mixture was stirred for 4 h at
recrystallizing in MeOH, white intermediate secondary amine room temperature. The schlenk flask was placed in ice-water
A) was obtained (Scheme 1). Yield: 7.039 g (86%). bath at 0 °C and HfCl (0.173 g, 0.54 mmol) was added, and
The intermediate secondary amine (0.923 g, 2.48 mmol), then the reaction mixture was warmed up to room
4
(0.832 g, 0.022 mol ) was added at
1
Preparation of complex 1 ((L )
2
Hf)
0
1
2
L (0.547 g, 1.08
2
2
(
4
paraformaldehyde (0.074 g, 2.48 mmol) and 2,4- temperature and was stirred for 24 h. The suspension was
dimethylphenol (30 uL, 2.48 mmol) were loaded in a flask filtrated under nitiogen and the filtrate was concentrated to
placed in oil bath. The mixture was stirred 9 h at 110 °C, and dryness. The residue was washed with cold hexane. An off-
1
1
was cooled to room temperature. The resulting solid (H
2
L ) white solid was obtained. Yield: 0.494 g (77%). H NMR (600
, 293 K): δ = 10.10 (d, 1H, Py, J = 4.2 Hz), 9.86 (d, 1H,
, 293 K): δ = 8.71 Py, J = 4.2 Hz), 7.51−6.46 (m, 14H, Ph and Py), 4.82 (d, 2H, N-
d, 1H, Py, J = 4.8 Hz), 7.80−6.69 (7H, Ph and Py), 3.92 (s, 2H, N- CH
-Ph, J = 12.0 Hz), 4.80, (d, 2H, N-CH -Py, J = 12.0 Hz), 3.97 (d,
CH -Ph), 3.80, (s, 2H, N-CH -Py), 3.78 (s, 2H, N-CH -Py), 2.21 (s, 2H, N-CH -Py, J = 13.2 Hz), 3.93 (d, 2H, N-CH -Py, J = 13.2 Hz),
H, 2Me). HR-MS: m/z = 507.0255 (Calcd 507.0106) [M+H] . 3.66 (d, 2H, N-CH -Py, J = 12.6 Hz), 3.62 (d, 2H, N-CH -Py, J =
was recrystallized from MeOH and a white solid was obtained. MHz, CDCl
3
1
Yield: 1.055 g (84%). H NMR (600 MHz, CDCl
3
(
2
2
2
2
2
2
2
+
6
2
2
Anal. Calcd for C22
H
22Br
2
N
2
O
2
: C, 52.20; H, 4.38; N, 5.53. Found: 12.6 Hz), 1.27 (s, 6H, 2Me), 1.25 (s, 6H, 2Me). HR-MS: m/z =
+
1188.9563 (Calcd 1188.9286) [M+H] . Anal. Calcd for
C, 52.16; H, 4.35; N, 5.49.
C
44
H
40Br
4
HfN
4
O
4
: C, 44.52; H, 3.40; N, 4.72. Found: C, 44.48; H,
2
2
Preparation of ligand H L
3.37; N, 4.69.
2
L was prepared from intermediate secondary amine
Ligand H
2
2
Preparation of complex 2 ((L )
2
Hf)
(
1.012 g, 2.72 mmol), paraformaldehyde (0.082 g, 2.72 mmol)
2
and 2,4-di-tert-butylphenol (0.561 g, 2.72 mmol) according to A schlenk flask was charged with ligand H
2
L (0.721 g, 1.22
L . yield: 1.405 g mmol) in toluene (25 mL). To the solution was added NaH
, 293 K): δ = 8.71 (d, 1H, Py, J = (0.117 g, 4.88 mmol), and the mixture was stirred for 4 h at
.8 Hz), 7.83−6.83 (7H, Ph and Py), 3.90 (s, 2H, N-CH
-Ph), 3.83, room temperature. The schlenk flask was placed in ice-water
s, 2H, N-CH -Py), 3.82 (s, 2H, N-CH -Py), 1.41 (s, 9H, Bu) and bath at 0 °C and HfCl
1
a similar method described above for ligand H
2
1
(87%). H NMR (600 MHz, CDCl
3
4
2
t
(
2
2
4
(0.195 g, 0.61 mmol) was added, and
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then the reaction mixture was warmed up to room
Journal Name
1
2
temperature and was stirred for 24 h. The suspension was Preparation of complex 5 ((L ) Zr)
filtrated under nitiogen and the filtrate was concentrated
1
L (0.445 g, 0.88
A schlenk flask was charged with ligand H
2
under reduced pressure. The resulting solid was recrystallized
from hexane and dichloromethane to give complex as an off-
white solid. yield: 0.595 g (72%). H NMR (600 MHz, CDCl , 293
K): δ = 9.27 (d, 1H, Py, J = 4.8 Hz), 8.60 (d, 1H, Py, J = 4.8 Hz),
.79−6.42 (m, 14H, Ph and Py), 4.70 (d, 2H, N-CH
-Ph, J = 15.6
Hz), 4.66, (d, 2H, N-CH -Py, J = 15.6 Hz), 4.26 (d, 2H, N-CH -Py, J
15.0 Hz), 4.22 (d, 2H, N-CH -Py, J = 15.0 Hz), 3.80 (d, 2H, N-
-Py, J = 12.0 Hz), 3.75 (d, 2H, N-CH -Py, J = 12.0 Hz), 1.31 (s,
mmol) in toluene (15 mL). To the solution was added NaH
0.085 g, 3.52 mmol), and the mixture was stirred for 4 h at
room temperature. The schlenk flask was placed in ice-water
bath at 0 °C and ZrCl (0.103 g, 0.44 mmol) was added, and
2
(
1
3
4
7
2
then the reaction mixture was warmed up to room
temperature and was stirred for 24 h. The suspension was
filtrated under nitiogen and the filtrate was concentrated to
dryness. The residue was washed with cold hexane. An off-
2
2
=
2
CH
2
2
t
t
18H, 2 Bu), 1.22 (s, 18H, 2 Bu). HR-MS: m/z = 1357.1557 (Calcd
+
1
white solid was obtained. yield: 0.358 g (74%). H NMR (600
4 4 4
1357.1164) [M+H] . Anal. Calcd for C56H64Br HfN O : C, 49.63;
MHz, CDCl
Py, J = 4.2 Hz), 7.60−6.51 (m, 14H, Ph and Py), 5.02 (d, 2H, N-
CH -Ph, J = 6.6 Hz), 4.97, (d, 2H, N-CH -Py, J = 6.6 Hz), 4.94 (d,
H, N-CH -Py, J = 13.2 Hz), 4.92 (d, 2H, N-CH -Py, J = 13.2 Hz),
L (0.843 g, 1.18 3.64 (d, 2H, N-CH -Py, J = 12.6 Hz), 3.61 (d, 2H, N-CH -Py, J =
3
, 293 K): δ = 10.02 (d, 1H, Py, J = 4.8 Hz), 9.77 (d, 1H,
H, 4.76; N, 4.13. Found: C, 49.59; H, 4.72; N, 4.09.
2
2
3
Preparation of complex 3 ((L )
2
Hf)
2
2
2
3
A schlenk flask was charged with ligand H
2
2
2
mmol) in toluene (20 mL). NaH (0.113 g, 4.72 mmol) was 12.6 Hz), 1.28 (s, 6H, 2Me), 1.26 (s, 6H, 2Me). HR-MS: m/z =
+
added to the suspension, and the mixture was stirred for 4 h at 1098.9886 (Calcd 1098.8867) [M+H] . Anal. Calcd for
room temperature. The schlenk flask was placed in ice-water
bath at 0 °C and HfCl (0.189 g, 0.59 mmol) was added, and 3.64; N, 5.06.
then the reaction mixture was warmed up to room
C
44
H
40Br
4
N
4
O
4
Zr: C, 48.06; H, 3.67; N, 5.09. Found: C, 48.03; H,
4
2
temperature and was stirred for 24 h. The suspension was Preparation of complex 6 ((L )
2
Zr)
filtrated under nitiogen and the filtrate was concentrated. The
2
2
A schlenk flask was charged with ligand H L (0.673 g, 1.14
resulting solid was recrystallized by diffusion of hexane into
dichloromethane to give complex as a white solid. yield:
.738 g (78%). H NMR (600 MHz, CDCl , 293 K): δ = 10.11 (d,
H, Py, J = 4.8 Hz), 9.68 (d, 1H, Py, J = 4.8 Hz), 8.52−6.29 (m,
4H, Ph and Py), 4.70 (d, 2H, N-CH -Ph, J = 13.8 Hz), 4.63, (d,
H, N-CH -Py, J = 13.8 Hz), 3.88 (d, 2H, N-CH -Py, J = 16.2 Hz),
.83 (d, 2H, N-CH -Py, J = 16.2 Hz), 3.29 (d, 2H, N-CH -Py, J =
4.2 Hz), 3.18 (d, 2H, N-CH -Py, J = 14.2 Hz), 1.25 (s, 12H, 4Me),
.16 (s, 12H, 4Me). HR-MS: m/z = 1605.1768 (Calcd 1605.1790)
mmol) in toluene (20 mL). NaH (0.109 g, 4.56 mmol) was
added to the solution, and the mixture was stirred for 4 h at
room temperature. The schlenk flask was placed in ice-water
3
1
0
1
3
2
3
1
1
3
4
bath at 0 °C and ZrCl (0.133 g, 0.57 mmol) was added, and
2
then the reaction mixture was warmed up to room
temperature and was stirred for 24 h. The suspension was
filtrated under nitiogen and the filtrate was concentrated to
dryness. The residue was washed with cold hexane. A white
2
2
2
2
2
1
solid was obtained. yield: 0.469 g (65%). H NMR (600 MHz,
+
[M+H] . Anal. Calcd for C76
H72Br
4
HfN
4
O
4
: C, 56.93; H, 4.53; N,
CDCl
3
, 293 K): δ = 9.27 (d, 1H, Py, J = 4.8 Hz), 8.47 (d, 1H, Py, J =
.8 Hz), 7.59−6.49 (m, 14H, Ph and Py), 4.59 (d, 2H, N-CH
-Ph, J
13.8 Hz), 4.43, (d, 2H, N-CH -Py, J = 13.8 Hz), 4.18 (d, 2H, N-
-Py, J = 8.4 Hz), 4.01 (d, 2H, N-CH -Py, J = 8.4 Hz), 3.57 (d,
-Py, J = 9.6 Hz), 3.42 (d, 2H, N-CH -Py, J = 9.6 Hz),
3.49. Found: C, 56.89; H, 4.49; N, 3.47.
4
=
2
2
4
Preparation of complex 4 ((L )
2
Hf)
CH
2
2
4
A schlenk flask was charged with ligand H
2
L (0.744 g, 1.36 2H, N-CH
2
2
t
t
mmol) in toluene (25 mL). NaH (0.131 g, 5.44 mmol) was 1.25 (s, 18H, 2 Bu), 1.16 (s, 18H, 2 Bu). HR-MS: m/z =
+
added to the suspension, and the mixture was stirred for 4 h at 1267.1037 (Calcd 1267.0745) [M+H] . Anal. Calcd for
room temperature. The schlenk flask was placed in ice-water
bath at 0 °C and HfCl (0.218 g, 0.68 mmol) was added, and 5.06; N, 4.38.
then the reaction mixture was warmed up to room
56 4 4 4
C H64Br N O Zr: C, 53.05; H, 5.09; N, 4.42. Found: C, 53.01; H,
4
temperature and was stirred for 24 h. The suspension was Ring-opening polymerization of lactide
filtrated under nitiogen and the filtrate was concentrated. The
The toluene solution of lactide was injected sequentially to a
series of Schlenk tubes loaded with complexes . These
resulting solid was recrystallized by slowly volatilization of
dichloromethane to give complex as a white solid. yield:
.587 g (68%). H NMR (600 MHz, CDCl , 293 K): δ = 10.20 (d,
H, Py, J = 4.2 Hz), 9.84 (d, 1H, Py, J = 4.2 Hz), 7.83−6.14 (m,
4H, Ph and Py), 5.33 (d, 2H, N-CH -Ph, J = 18.0 Hz), 5.30, (d,
H, N-CH -Py, J = 18.0 Hz), 4.64 (d, 2H, N-CH -Py, J = 13.2 Hz),
.60 (d, 2H, N-CH -Py, J = 13.2 Hz), 4.15 (d, 2H, N-CH -Py, J =
2.0 Hz), 4.11 (d, 2H, N-CH -Py, J = 12.0 Hz). HR-MS: m/z =
270.7375 (Calcd 1270.7071) [M+H] . Anal. Calcd for
Cl HfN : C, 37.87; H, 2.22; N, 4.42. Found: C, 37.84;
H, 2.19; N, 4.38.
1-6
4
tubes were placed in a silicon oil bath at appropriate
temperature. After the appropriate time, these reactions were
terminated by the addition of methanol. A sample was taken
from the reaction mixture and the percentage conversion of
1
0
1
1
2
4
1
1
3
2
1
2
2
monomer was determined by the ratio of integration of the H
2
2
NMR signal for the methine proton of polylactide to that of
lactide. The solvent was removed in vacuo, and the crude
2
+
product was dissolved in
a
minimum amount of
C
40
H28Br
4
4
4 4
O
dichloromethane, and then cold wet methanol was added. The
resulting polymers were precipitated and were collected. The
8
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ARTICLE
product was washed with 3
×
under high vacuum at 50 °C for 12 h.
30 mL methanol and was dried 24 I. Yu, A. Acosta-Ramirez, P. Mehrkhodavandi, J. Am. Chem.
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Acknowledgements
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This work was supported by the Project Sponsored by the
Scientific Research Foundation for the Returned Overseas
Chinese Scholars, State Education Ministry (Grant No. [2015]
3
11), the National Natural Science Foundation of China (Grant 29 A. Sauer, J.-C. Buffet, T. P. Spaniol, H. Nagae, K. Mashima, J.
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Catalysis Science & Technology
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DOI: 10.1039/C6CY02618D
a table of contents entry
Preparation of zirconium and hafnium complexes containing chiral N atoms from unsymmetric tertiary amine
ligands, and their catalytic properties for polymerization of rac-lactide.
-
-
-
-
-
-
2.5
3.0
3.5
4.0
4.5
5.0
O
R
O
O
R O
O
R
O
-
1
R
n
E =20.5 kJ mol
O
a
D-LA
O
+
Br
R
Br
R
O
O
N
+
N
M
N
-
1
N
rac-LA
L-LA
E
a
=39.5 kJ mol
O
O
O
O
R
R
O
Br
Br
S O
S
O
S O
n
O
O
isotactic PLA
S
L-LA
2
.4
2.5
2.6
2.7
2.8
2.9
3.0
-
1
1
000/T(K )