Rapid and controlled polymerization of rac-lactide using N,N,O-chelate zinc enolate catalysts

Article abstract of DOI:10.1021/om400193z

The synthesis and characterization of N,N,O-chelate zinc enolate complexes and the catalysis of the complexes for the ROP of rac-lactide are reported. The pyrazole-based ligand precursors o-(3,5-Me₂C₃HN ₂)C₆H

Full text of DOI:10.1021/om400193z

Article
pubs.acs.org/Organometallics
Rapid and Controlled Polymerization of rac-Lactide Using
N,N,O-Chelate Zinc Enolate Catalysts
Xiao-Feng Yu, Cheng Zhang,† and Zhong-Xia Wang*
CAS Key Laboratory of Soft Matter Chemistry and Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui 230026, People’s Republic of China
S
* Supporting Information
ABSTRACT: The synthesis and characterization of N,N,O-chelate
zinc enolate complexes and the catalysis of the complexes for the
ROP of rac-lactide are reported. The pyrazole-based ligand
precursors o-(3,5-Me2C3HN2)C6H4NC(Me)CHC(OH)R1
(R1 = Me, 1; R1 = Ph, 2; R1 = t-Bu, 3; R1 = CF3, 4) were
synthesized by reaction of 2-(3,5-dimethyl-1H-pyrazol-1-yl)-
benzenamine with 1,3-diketones, including pentane-2,4-dione,
1-phenylbutane-1,3-dione, 5,5-dimethylhexane-2,4-dione, and
1,1,1-trifluoropentane-2,4-dione. Treatment of 1−4 with ZnEt2
generated the N,N,O-coordinated zinc complexes [Zn(Et)-
{o-(OC(R1)CHC(Me)N)C6H4(3,5-Me2C3HN2)}] (R1
=
Me, 5; R1 = Ph, 6; R1 = t-Bu, 7; R1 = CF3, 8). The imino-
phosphoranyl-moiety-containing ligand precursors o-(3,5-
Me2C3HN2)C6H4NP(Ph2)CH2C(O)R2 (R2 = Ph, 9;
R2 = t-Bu, 10) were synthesized by reaction of 1-(2-azidophenyl)-3,5-dimethyl-1H-pyrazole with 1-phenyl-2-(diphenylphosphino)-
ethanone and 3,3-dimethyl-1-(diphenylphosphino)butan-2-one, respectively. Treatment of 9 and 10 with ZnEt2 afforded the zinc
complexes [Zn(Et){o-(OC(R2)CHP(Ph2)N)C6H4(3,5-Me2C3HN2)}] (R2 = Ph, 11; R2 = t-Bu, 12). The ligand precursors and
complexes were characterized by NMR spectroscopy and elemental analyses. Complexes 5 and 11 were also characterized by single-
crystal X-ray diffraction techniques. In the presence of BnOH complexes 5−8 efficiently catalyzed the ring-opening polymerization of
rac-lactide in a controlled fashion, whereas complexes 11 and 12 showed much lower catalytic activity under the same conditions.
skeleton to tune the catalytic properties of their metal com-
plexes in the ROP of cyclic esters. For this reason we designed
two classes of pyrazole-based N,N,O-tridentate enolate ligands
containing a ketiminate moiety and an iminophosphoranyl
moiety, respectively, synthesized and characterized zinc com-
plexes of the ligands, and studied the catalysis of the complexes
in the ROP of rac-lactide. Herein we report the results.
INTRODUCTION
■
Polylactide (PLA) as an important class of biodegradable and
biocompatible polymer has attracted considerable attention due
to its biomedical, pharmaceutical, and agricultural applications.1
PLA is also a promising and practical material for an attractive
alternative to petrochemical-derived polyolefin because its mono-
mer is derived from naturally renewable resources such as corn
and sugar beets.2 PLA is typically produced by the ROP of
lactide catalyzed by metal complexes such as Sn,3 Al,4 Zn,5 Ca,6
and Mg5a,b,7 complexes. In this context, zinc-based catalysts
have been extensively employed and are among the highly
active metal-based catalysts used to date for the controlled
polymerization of lactide. For instance, in 2003 Tolman et al.
found a highly active zinc catalyst supported by an amino-
phenolate-based N,N,O-tridentate ligand, which can lead to
96% conversion of rac-lactide in 13 min at 25 °C at a monomer
to catalyst ratio of 1000.8 Lin et al. developed a catalyst system
and demonstrated a zinc complex bearing an N,N,O-tridentate
Schiff base ligand with hindered substituents to lead to good
stereocontrol at low temperature, producing highly heterotactic
PLA from the polymerization of rac-lactide.9 Several other
N,N,O-chelate zinc complexes were also proven to be active
catalysts for the ROP of lactides.10 We wished to modify
N,N,O-ligands by changing the coordination groups or ligand
RESULTS AND DISCUSSION
■
Synthesis and Characterization of N,N,O-Chelate Zinc
Enolate Complexes. As shown in Scheme 1, ligand precursors
1−3 were synthesized in good yields through reactions of
2-(3,5-dimethyl-1H-pyrazol-1-yl)benzenamine with the corre-
sponding 1,3-diketones in refluxing toluene in the presence of 4
Å molecular sieves and a catalytic amount of formic acid. The
crude products were purified by column chromatography. Com-
pound 4 was synthesized by grinding a mixture of 2-(3,5-dimethyl-
1H-pyrazol-1-yl)benzenamine and 1,1,1-trifluoropentane-2,4-
dione using sulfuric acid as catalyst under solventless conditions.
Respective treatment of 1−4 with ZnEt2 in toluene at room
temperature generated N,N,O-chelate zinc complexes 5−8.
Received: March 9, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Zinc Complexes 5−8
Figure 1. ORTEP drawing (30% probability) of complex 5. Selected
bond lengths (Å) and angles (deg): Zn(1)−C(17) = 1.973(6), Zn(1)−
O(1) = 1.991(4), Zn(1)−N(1) = 2.051(4), Zn(1)−N(3) = 2.159(4),
O(1)−C(2) = 1.265(7), N(1)−C(4) = 1.299(7), N(1)−C(6) =
1.427(7), C(2)−C(3) = 1.395(8); C(17)−Zn(1)−O(1) = 118.5(2),
C(17)−Zn(1)−N(1) = 129.3(2), O(1)−Zn(1)−N(1) = 92.17(17),
C(17)−Zn(1)−N(3) = 116.5(2), O(1)−Zn(1)−N(3) = 110.01(18),
N(1)−Zn(1)−N(3) = 84.13(16), C(2)−O(1)−Zn(1) = 123.6(4).
Compounds 1, 3, and 4 are pale yellow (1 and 3) or purplish
pink oils (4), and 2 is yellow crystals. They gave satisfactory
1
elemental analytical results. The H and 13C NMR spectra
match their respective structures. The 19F NMR spectrum of
compound 4 displayed a single signal at δ −76.85 ppm. This
showed that the CF3 group is adjacent to the carbonyl carbon
rather than the imine carbon, in comparison with the 19F NMR
spectra of similar systems (the chemical shift of 19F NMR
spectra in CF3C(NAr)CH(OH)R is about −63 ppm).11 It
distance of 1.973(6) Å is slightly shorter than those in LZnEt
(1.997(4) Å)8 and [L1ZnEt]2 (1.985(4) Å)9a and slightly longer
12
than that in [Zn(Me)(bpatBu ,Me )] (1.962(8) Å). The C2−C3
distance of 1.395(8) Å is slightly longer than a typical C−C double
bond due to electron delocalization.
2
2
The synthesis of ligand precursors 9 and 10 and complexes 11
and 12 is shown in Scheme 2. The reaction of 1-(2-azidophenyl)-
1
should be noted that the H NMR spectra showed each of the
compounds to exist as an equilibrium mixture of keto and enol
forms in CDCl3 solution, the ratio of keto to enol form ranging
from about 1:5 (1, 2, and 4) to 1:15 (3). Complexes 5−8 are
air-sensitive pale yellow (5 and 7), yellow (6), or yellowish gray
crystals (8). They were characterized by elemental analyses and
1H and 13C NMR spectroscopy. In the 1H NMR spectra of the
complexes, the OH signals in the ligand precursors were not
observed and ZnEt signals appeared at 0.65−0.93 and 1.51−
1.61 ppm, respectively. 13C NMR spectra of complexes 5−8
showed that the signals of ZnCH2 appeared at high field
(−2.09, −2.04, −2.16, and −2.34 ppm, respectively). The 19F
NMR spectrum of complex 8 was also determined, the
chemical shift being −74.57 ppm. The structure of complex 5
was additionally characterized by single-crystal X-ray diffraction.
The ORTEP drawing is depicted in Figure 1, along with
selected bond lengths and angles. The central zinc atom is four-
coordinate and has a distorted-tetrahedral geometry. The dif-
ference between the angles C17−Zn−N1 (129.3(2)°) and
N1−Zn−N3 (84.13(16)°) is illustrative of the distortion
from an ideal tetrahedral topology. In comparison with several
N,N,O-coordinate zinc complexes, the Zn−O distance of 1.991(4)
Å in complex 5 is longer than the corresponding distance in
LZnEt (L = 2,4-di-tert-butyl-6-{[(2′-dimethylaminoethyl)-
methylamino]methyl}phenolate) (1.956(2) Å).8 The Zn−N1
distance of 2.051(4) Å is shorter than the Zn−N(imine) distance
in [L1ZnEt]2 (L1 = 2-[(2-(dimethylamino)ethylimino)methyl]-4-
chlorophenolate) (2.168(3) Å).9a The longer bond distance of the
latter is probably due to the higher coordination number of the zinc
atom. The Zn−N3 distance of 2.159(4) Å in complex 5 is longer
Scheme 2. Synthesis of Zinc Complexes 11 and 12
3,5-dimethyl-1H-pyrazole with 1-phenyl-2-(diphenylphosphino)-
ethanone and 3,3-dimethyl-1-(diphenylphosphino)butan-2-one,
respectively, in CH2Cl2 afforded compounds 9 and 10. Treatment
of 9 and 10 with ZnEt2 in toluene gave the corresponding zinc
complexes 11 and 12. Compounds 9 and 10 are yellow crystals or
powders, and 11 and 12 are colorless crystals. Both 11 and 12
are air sensitive and stable under a nitrogen atomsphere. Each
1
compound was characterized by H, 13C, and 31P NMR spec-
troscopy and elemental analysis. The 1H NMR spectra of 9 and
10 showed that each of them also exists as an equilibrium
mixture of keto and enol forms in solution and the enol form is
predominant. The 1H NMR spectra of 11 and 12 revealed ZnEt
than the Zn−N(pyrazolyl) distances in [Zn(Me)(bpatBu ,Me )]
2
2
(bpatBu ,Me = (3,5-di-tert-butylpyrazol-1-yl)(3′,5′-dimethylpyr-
2
2
azol-1-yl)acetate) (2.085(7) and 2.104(6) Å).12 The Zn−C
B
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signals. The 31P NMR spectra showed the chemical shifts of the
complexes slightly shifted to higher fields in comparison with
those of the corresponding ligand precursors in enol form. The
coordination mode of complex 11 was confirmed by single-
crystal X-ray diffraction analysis. An ORTEP drawing is pre-
sented in Figure 2, along with selected bond lengths and angles.
Table 1. Ring-Opening Polymerization of rac-Lactide
Catalyzed by Complexes 5−8, 11, and 12
a
b
time
(min)
conversn
(%)
c
d
e
entry cat.
Mn(calcd) Mn(GPC)
PDI
1
2
3
4
5
6
7
5
1
1
96
83
97
90
98
92
93
13901
12114
14045
13137
14200
13324
13440
11200
10400
13500
10000
13000
11500
11900
1.17
1.08
1.06
1.11
1.07
1.08
1.11
6
6
3
7
1
8
1
11
12
858
1668
a
All polymerizations were carried out in CH2Cl2 at 25 °C; [LA]0 = 0.5 M
b
and [LA]0/[Zn]/[BnOH] = 100/1/1. Measured by 1H NMR
spectra. Calculated from the molecular weight of LA times the
c
conversion of monomer plus the molecular weight of BnOH.
d
Obtained from GPC analysis and calibrated against a polystyrene
standard, multiplied by 0.58.15 Obtained from GPC analysis.
e
[Zn(OBn){OC6H4{2-(CHNCH2CH2NMe2)}}]2, drives
96% conversion of rac-LA in 4 h at 25 °C at a monomer to
catalyst ratio of 100;9a [(IMes)Zn(OBn)2]2 (IMes = 1,3-di-
mesitylimidazol-2-ylidene) initiates 96% rac-LA monomer
consumption after 20 min at 25 °C at a monomer to catalyst
ratio of 130.13 The lower activity of complex 7 in comparison to
that of 5 may be due to the steric hindrance of the t-Bu group
in complex 7, which hampers the incorporation of monomer
into the growing polymer chain. Complex 8 showed activity
close to that of complex 5. Hence, it does not seem that the
electronic effect difference of the ligands is the decisive factor
for the catalytic activity in the present catalytic systems.9,11,14
However, the fact that complex 6 has the lowest activity among
the complexes cannot be reasonably explained on the basis of
electronic effects. It is probably a composite result of steric and
electronic effects. The PDIs of polymers, ranging from 1.06 to
1.17, are narrow. The molecular weights of the polymers
determined by GPC approximately match those calculated
(entries 1−5, Table 1). These facts show that the polymerizations
are controlled. A linear relationship between the number average
molecular weight (Mn) and ([LA]0-[LA])/[Zn]0, as shown in
Figure 3, implies the “living” character of the polymerization
process catalyzed by 5-BnOH. The polylactide molecular weight
in each rac-LA to catalyst ratio also approximately matches the
theoretical value (Table 2). However, it was also noted that Mn
values determined by GPC were somewhat lower than the
predicted polymer molecular weights. This might imply the
Figure 2. ORTEP drawing (30% probability) of complex 11. Selected
bond lengths (Å) and angles (deg): Zn(1)−N(1) = 2.0336(17),
Zn(1)−N(3) = 2.141(2), Zn(1)−C(12) = 1.982(3), Zn(1)−O(1) =
2.0006(17), P(1)−N(1) = 1.6100(18), C(14)−C(21) = 1.369(3);
C(12)−Zn(1)−O(1) = 114.58(10), C(12)−Zn(1)−N(1) =
128.57(10), C(12)−Zn(1)−N(3) = 124.79(10), O(1)−Zn(1)−
N(1) = 97.65(7), O(1)−Zn(1)−N(3) = 95.45(8), N(1)−Zn(1)−
N(3) = 88.05(7).
The structural skeleton is similar to that of complex 5. Thus,
the central zinc atom is surrounded by N, N, O, and C atoms
and has a distorted-tetrahedral geometry. The C12−Zn−N1
angle (128.57(10)°) is much wider than that of N1−Zn−N3
(88.05(7)°). The N3−Zn−O angle of 95.45(8)° is narrower
than the corresponding angle in complex 5, which is 110.01(18)°.
The Zn−O distance of 2.0006(17) Å and Zn−C distance of
1.982(3) Å are slightly longer than the corresponding distances
in complex 5 [(1.991(4) and 1.973(6) Å, respectively), and the
Zn−C distance is close to that of [L1ZnEt]2 (1.985(4) Å).9a
The Zn−N distances of 2.0336(17) and 2.141(2) Å are shorter
than the corresponding distances in complex 5 (2.051(4) and
2.159(4) Å, respectively). The C14−C21 distance of 1.369(3)
Å is indicative of a C−C double bond.
Catalysis of the Zinc Complexes in the ROP of rac-
Lactide. Catalysis of zinc complexes 5−8, 11, and 12 toward
the ROP of rac-lactide was evaluated, and the results showed
that each of these zinc complexes can catalyze the polymer-
ization of rac-lactide at ambient temperature in the presence of
benzyl alcohol (Table 1). Complexes 5−8 exhibited high
activity. They led to polymerization of 100 equiv of rac-LA in
1−3 min, giving 90−98% monomer conversions (entries 1−5,
Table 1). Such activity is lower than that of the amino-
phenolate-based N,N,O-chelate zinc complex reported by
Tolman and co-workers, which leads to 96% conversion of
rac-LA in 5 min at 25 °C at a monomer to catalyst ratio of 650.8
However, the activity of 5−8 is comparable to or higher than
those of other known zinc-based catalyst systems.5,9 For example,
a zinc complex bearing an N,N,O-tridentate Schiff base ligand,
1
possibility of a certain degree of chain transfer. The H NMR
spectra of PLAs indicated that the polymer chain was capped
with a benzyl ester on one end and a hydroxyl group on the
other end (Figures S1 and S2 in the Supporting Information).
The observed results suggest that the polymerization may be
initiated through insertion of the benzyloxy group to LA,
followed by ring opening via acyl oxygen cleavage, and the
BnOZnL formed in situ is the real catalyst (Scheme S1 in the
Supporting Information). The polymers so obtained are atactic,
1
as indicated by H NMR spectral analysis. Lin et al. indicated
that the tacticity of the polymer was significantly influenced by
the steric hindrance of the ligand in N,N,O-chelate Schiff base
zinc catalyst systems, a sterically bulky ligand resulting in an
increase in Pr.9a These tactics may also be suitable for our
catalytic systems. Further investigation is in progress.
The catalytic activity of complexes 11 and 12 is much lower
than that of complexes 5−8, as seen in Table 1. The polymeri-
zation went to completion in hours using complex 11 or 12 in
C
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groups. The determined molecular weights of the PLAs obtained
by catalysis with 11 and 12 in the presence of BnOH are close to
the calculated values, and the polymers have a low polydispersity
(1.08 and 1.11, respectively; Table 1). A plot of ln([LA]0/[LA])
versus time in a 11/BnOH-catalyzed rac-LA polymerization
exhibits a linear relationship (Figure 4), indicating that the poly-
merization proceeds with first-order dependence on monomer
concentration. The number average molecular weight (Mn)
follows a linear relationship in monomer conversion, and poly-
dispersity (PDI) values remain low (Figure 5), proving the
◆
( , obtained from GPC analysis) and
Figure 3. Plots of Mn
▲
polydispersity ( , Mw/Mn) as a function of ([LA]0 − [LA])/[Zn] for
the polymerization catalyzed by 5/BnOH at 25 °C. Conditions:
[Zn]0/[BnOH]0 = 1/1; [LA]0 = 0.5 M; solvent CH2Cl2.
Table 2. Ring-Opening Polymerization of rac-Lactide
a
Catalyzed by Complex 5/BnOH
[LA]0/
[Zn]
time
(min)
conversn
b
c
d
e
entry
(%)
Mn(calcd) Mn(GPC)
PDI
1
2
3
4
5
50
100
150
200
250
0.5
1
96
96
96
90
90
7055
13901
20927
26079
32608
6800
11200
18000
24400
31500
1.09
1.17
1.11
1.19
1.17
3
■
Figure 5. Plots of Mn ( , obtained from GPC analysis) and
4
▲
polydispersity ( , Mw/Mn) as a function of rac-LA conversion using
7.5
complex 11/BnOH as the catalyst. Conditions: [LA]0/[Al]0/[BnOH]0
= 100/1/1; [LA]0 = 0.5 M; solvent CH2Cl2; polymerization
temperature 38 °C.
a
Polymerizations were carried out in CH2Cl2 at 25 °C; [LA]0 = 0.5 M
b
and [Zn]/[BnOH] = 1/1. Measured by 1H NMR spectra.
c
Calculated from the molecular weight of LA times the conversion
of monomer plus the molecular weight of BnOH. Obtained from
d
controlled character of the polymerization.17 The polymers
formed by catalysis with 11 and 12 are also atactic.
GPC analysis and calibrated against a polystyrene standard, multiplied
by 0.58.15 Obtained from GPC analysis.
e
CONCLUSIONS
the presence of benzyl alcohol (entries 6 and 7, Table 1). This
contrast of catalytic activity is probably caused by the difference
in electronic effects between iminophosphoranyl and imino
groups in the two classes of ligands.16 It seems that the com-
bination of imino, pyrazolyl, and alkenoxy groups provides a
proper electronic environment at the metal center for the catalysis.
The lower activity of complex 12 in comparison to that of 11 is
ascribed to the greater steric hindrance of the ligand in complex 12
and the difference of electron effects between phenyl and tert-butyl
■
In summary, we have synthesized and characterized several zinc
complexes supported by novel pyrazolyl-based NNO-tridentate
enolate ligands. In the presence of BnOH the complexes bear-
ing ligands containing a ketiminate unit (5−8) are extremely
active catalysts for the ring-opening polymerization of rac-
lactide, whereas the complexes bearing ligands containing an
iminophosphoranyl unit (11 and 12) show much lower cata-
lytic activity. The approximate activity order is 5 ≈ 8 > 7 > 6 ≫
11 > 12. The electron effect difference between the two types
of ligands may be the decisive factor determining the catalytic
activity of the complexes. The polymerizations are controlled
and give PLAs with an atactic microstructure. Future work will
involve further modification of ligands to improve stereocontrol.
EXPERIMENTAL SECTION
■
All air- or moisture-sensitive manipulations were performed under dry
N2 using standard Schlenk techniques. Solvents were distilled under
nitrogen over sodium (toluene), sodium/benzophenone (n-hexane
and diethyl ether), or calcium hydride (CH2Cl2). ZnEt2 was purchased
from Acros Organics and used as received. CDCl3 and C6D6 were
purchased from Cambridge Isotope Laboratories and stored over
activated molecular sieves (CDCl3) or Na/K alloy (C6D6). 1-(2′-
Aminophenyl)-3,5-dimethylpyrazole,18 1-(2′-azidophenyl)-3,5-dime-
thylpyrazole,19 and 1-phenyl-2-(diphenylphosphino)ethanone20 were
prepared according to reported methods. NMR spectra were recorded
on a Bruker av300 spectrometer at ambient temperature. The chemical
shifts of 1H and 13C NMR spectra were referenced to internal solvent
resonances; the 31P NMR spectra were referenced to external 85%
Figure 4. Plot of ln([LA]0/[LA]) versus time for the polymerization
of rac-lactide catalyzed by 11/BnOH. Conditions: [LA]0/[11]/
[BnOH] = 100/1/1; [LA]0 = 0.5 M; solvent CH2Cl2; polymerization
temperature 38 °C.
D
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H3PO4, and the 19F NMR spectra were referenced to external
CF3COOH. Elemental analysis was performed by the Analytical Center
of the University of Science and Technology of China. Molecular weights
and molecular weight distributions were determined on a Waters 150C
gel permeation chromatograph (GPC) equipped with UltraStyragel
columns (103, 104, and 105 Å) and a 410 refractive index detector,
using monodispersed polystyrene as the calibration standard. THF
(HPLC grade) was used as eluent at a flow rate of 1 mL/min.
Synthesis of o-(3,5-Me2C3HN2)C6H4NC(Me)CHC(OH)Me
(1). A mixture of 1-(2′-aminophenyl)-3,5-dimethylpyrazole (2.68 g,
14.31 mmol), 2,4-pentanedione (2.86 g, 28.56 mmol), activated 4 Å
molecular sieves (30 g), toluene (50 mL), and several drops of formic
acid was refluxed for 40 h and then cooled to room temperature. The
molecular sieves were filtered out and washed with CH2Cl2 (3 × 20 mL).
The resulting solution was washed two times with water. The organic
layer was dried over MgSO4, filtered, and concentrated under vacuum
to give an oil. The crude product was purified by column chroma-
tography (petroleum ether/ethyl acetate 1/1) to afford a light yellow
oil (2.63 g, 70%). 1H NMR (CDCl3): δ 1.83 (s, 3H, Me), 2.03 (s, 3H,
Me), 2.13 (s, 3H, Me), 2.28 (s, 3H, Me), 5.09 (s, 1H, CH), 5.97 (s,
1H, pyrazolyl), 7.23−7.42 (m, 4H, C6H4), 12.05 (s, 1H, OH). 13C
NMR (CDCl3): δ 11.13, 13.41, 19.47, 28.97, 98.24, 105.87, 126.16,
126.35, 128.83, 128.86, 134.67, 135.46, 140.69, 149.53, 159.41, 195.95.
Anal. Calcd for C16H19N3O: C, 71.35; H, 7.11; N, 15.60. Found: C,
71.86; H, 7.18; N, 15.72.
solvent was removed under vacuum to afford an oil (1.10 g, 64%). 1H
NMR (CDCl3, 400 MHz): δ 1.95 (s, 3H, Me), 2.14 (s, 3H, Me), 2.26
(s, 3H, Me), 5.42 (s, 1H, CH), 5.98 (s, 1H, CH), 7.32 (d, J = 7.6 Hz,
1H, C6H4), 7.42−7.51 (m, 3H, C6H4), 12.18 (s, 1H, OH). 13C NMR
(CDCl3, 100 MHz): δ 11.46, 13.47, 20.31, 91.23, 106.61, 117.40 (q,
J = 287 Hz), 127.28, 128.28, 128.87, 129.33, 134.05, 135.36, 140.76,
150.33, 167.95, 176.82 (q, J = 33 Hz). 19F NMR (CDCl3, 376 MHz):
δ −76.85. Anal. Calcd for C16H16F3N3O: C, 59.44; H, 4.99; N, 13.00.
Found: C, 59.58; H, 4.82; N, 13.20.
Synthesis of [Zn(Et){o-(OC(Me)CHC(Me)N)C6H4(3,5-
Me2C3HN2)}] (5). ZnEt2 (1.90 mL, 1 M solution in hexane, 1.90
mmol) was added dropwise to a stirred solution of 1 (0.50 g, 1.86
mmol) in toluene (10 mL) at about −80 °C. The mixture was warmed
to room temperature, stirred overnight at that temperature, and
concentrated under vacuum. A small amount of diethyl ether was
added to the concentrated solution to give pale yellow crystals of 5
1
(0.57 g, 84%), mp 156−157 °C. H NMR (C6D6): δ 0.65−0.82 (m,
2H, ZnEt), 1.50 (s, 3H), 1.58 (t, J = 8 Hz, 3H, ZnEt), 1.68 (s, 3H,
Me), 2.01 (s, 3H, Me), 2.29 (s, 3H, Me), 4.82 (s, 1H, CH), 5.45 (s,
1H, pyrazolyl), 6.62 (d, J = 8.1 Hz, 1H, Ar), 6.68−6.75 (m, 1H, ArH),
6.82 (d, J = 7.8 Hz, 1H, Ar), 6.87−6.95 (m, 1H, Ar). 13C NMR
(C6D6): δ −2.09, 11.90, 13.24, 13.78, 21.25, 28.37, 97.34, 106.97,
123.90, 125.81, 126.34, 128.64, 130.88, 141.10, 145.39, 149.99, 166.99,
185.58. Anal. Calcd for C18H23N3OZn: C, 59.59; H, 6.39; N, 11.58.
Found: C, 59.12; H, 6.14; N, 11.94.
Synthesis of o-(3,5-Me2C3HN2)C6H4NC(Me)CHC(OH)Ph
(2). A mixture of 1-(2′-aminophenyl)-3,5-dimethylpyrazole (2.00 g,
10.68 mmol), benzoylacetone (2.07 g, 12.76 mmol), activated 4 Å
molecular sieves (30 g), toluene (50 mL), and several drops of formic
acid was refluxed for 40 h and then cooled to room temperature.
The molecular sieves were filtered out and washed with CH2Cl2
(3 × 20 mL). The solution was washed two times with water. The
organic layer was dried over MgSO4, filtered, and concentrated under
vacuum. The crude product was recrystallized from toluene to give
yellow crystals of 2 (3.06 g, 86%), mp 130−132 °C. 1H NMR
(CDCl3): δ 1.91 (s, 3H), 2.15 (s, 3H), 2.28 (s, 3H), 5.78 (s, 1H, CH),
5.94 (s, 1H, pyrazolyl), 7.30−7.45 (m, 7H, Ar), 7.84−7.88 (m, 2H,
Ar), 12.70 (s, 1H, OH). 13C NMR (CDCl3): δ 11.60, 13.72, 20.26,
94.71, 106.31, 127.06, 127.25, 128.35, 129.17, 131.03, 135.49, 135.59,
140.09, 141.10, 150.02, 162.34, 188.89. Anal. Calcd for C21H21N3O: C,
76.11; H, 6.39; N, 12.68. Found: C, 76.00; H, 6.39; N, 12.64.
Synthesis of o-(3,5-Me2C3HN2)C6H4NC(Me)CHC(OH)t-Bu
(3). A mixture of 1-(2′-aminophenyl)-3,5-dimethylpyrazole (2.20 g,
11.75 mmol), 5,5-dimethylhexane-2,4-dione (1.96 g, 13.78 mmol),
activated 4 Å molecular sieves (30 g), toluene (50 mL), and several
drops of formic acid was refluxed for 40 h and then cooled to room
temperature. The molecular sieves were filtered out and washed with
CH2Cl2 (3 × 20 mL). The solution was washed two times with water.
The organic layer was dried over MgSO4, filtered, and concentrated
under vacuum to give an oil. The crude product was purified by
column chromatography (petroleum ether/ethyl acetate 1/1) to afford
Synthesis of [Zn(Et){o-(OC(Ph)CHC(Me)N)C6H4(3,5-Me2-
C3HN2)}] (6). ZnEt2 (1.20 mL, 1 M solution in hexane, 1.20 mmol)
was added dropwise to a stirred solution of 2 (0.38 g, 1.15 mmol) in
toluene (10 mL) at about −80 °C. The mixture was warmed to room
temperature, stirred overnight, and concentrated under vacuum. A
small amount of n-hexane was added to the solution to give yellow
1
crystals of 6 (0.38 g, 78%), mp 229−230 °C. H NMR (C6D6): δ
0.73−0.93 (m, 2H, ZnEt), 1.56 (s, 3H, Me), 1.61 (t, J = 8.1 Hz, 3H,
ZnCH2CH3), 1.68 (s, 3H, Me), 2.28 (s, 3H, Me), 5.39 (s, 1H, CH),
5.57 (s, 1H, pyrazole), 6.63 (dd, J = 1.2, 7.8 Hz, 1H, Ar), 6.74 (dt, J =
1.2, 7.2 Hz, 1H, Ar), 6.83 (dd, J = 1.5, 7.8 Hz, 1H, Ar), 6.94 (dt, J =
1.2, 7.5 Hz, 1H, Ar), 7.12−7.25 (m, 3H, Ar), 8.09 (dd, J = 1.6, 7.8 Hz,
2H, Ar). 13C NMR (C6D6): δ −2.04, 11.90, 13.18, 13.74, 21.72, 94.86,
106.65, 127.17, 127.55, 127.85, 128.32, 129.38 130.42, 140.67, 142.09,
145.02, 149.64, 167.78, 178.77. Anal. Calcd for C23H25N3OZn: C,
65.02; H, 5.93; N, 9.89. Found: C, 64.90; H, 5.91; N, 9.68.
Synthesis of [Zn(Et){o-(OC(t-Bu)CHC(Me)N)C6H4(3,5-
Me2C3HN2)}] (7). ZnEt2 (1.90 mL, 1 M solution in hexane, 1.90 mmol)
was added dropwise to a stirred solution of 3 (0.55 g, 1.77 mmol) in
toluene (10 mL) at about −80 °C. The mixture was warmed to room
temperature and stirred overnight. The resulting solution was
concentrated to afford pale yellow crystals of 7 (0.67 g, 93%)., mp
1
111−113 °C. H NMR (C6D6): δ 0.74−0.79 (m, 2H, ZnCH2CH3),
1.26 (s, 9H, t-Bu), 1.47 (s, 3H, Me), 1.51 (t, J = 8.2 Hz, 3H, ZnEt),
1.57 (s, 3H, Me), 2.24 (s, 3H, Me), 5.02 (s, 1H, CH), 5.37 (s, 1H,
pyrazolyl), 6.54 (d, J = 8 Hz, 2H, Ar), 6.64 (t, J = 7.6 Hz, 1H, Ar), 6.77
(d, J = 8 Hz, 1H, Ar), 6.84 (t, J = 7.6 Hz, 1H, Ar). 13C NMR (C6D6): δ
−2.16, 11.72, 11.87, 13.12, 13.70, 21.87, 28.73, 29.03, 40.99, 92.19,
106.82, 125.59, 125.50, 125.98, 127.19, 128.41, 140.99, 145.75, 149.74,
167.78, 194.45. Anal. Calcd for C21H29N3OZn: C, 62.30 H, 7.22; N,
10.38. Found: C, 61.92; H, 7.17; N, 10.70.
1
a light yellow oil (2.53 g, 69%). H NMR (CDCl3): δ 1.12 (s, 9H,
t-Bu), 1.77 (s, 3H, Me), 2.09 (s, 3H, Me), 2.27 (s, 3H, Me), 5.23 (s,
1H, CH), 5.91 (s, 1H, pyrazolyl), 7.24−7.27 (m, 1H, Ar), 7.30−7.33
(m, 1H, Ar), 7.36−7.41 (m, 2H, Ar), 12.22 (s, 1H, OH). 13C NMR
(CDCl3): δ 11.53, 13.68, 20.04, 27.78, 41.72, 93.28, 106.09, 126.69,
127.32, 128.93, 129.01, 129.52, 135.48, 135.68, 141.05, 149.82, 160.97.
Anal. Calcd for C19H25N3O·0.1CH3COOEt: C, 72.76; H, 8.12; N,
13.12. Found: C, 72.52; H, 8.09; N, 13.07.
Synthesis of o-(3,5-Me2C3HN2)C6H4NC(Me)CHC(OH)CF3
(4). 1-(2′-Aminophenyl)-3,5-dimethylpyrazole (1.0 g, 5.34 mmol) and
1,1,1-trifluoropentane-2,4-dione (0.86 g, 5.58 mmol) were mixed in a
mortar. To the mixture was added 2 drops of concentrated sulfuric
acid. The mixture was ground for 10 min and left to stand for 10 min
under an infrared lamp. Then 1,1,1-trifluoropentane-2,4-dione (0.86 g,
5.58 mmol) was added to the mortar and grinding was continued for
an additional 10 min. After that, the process of addition of 1,1,1-
trifluoropentane-2,4-dione (0.86 g, 5.58 mmol) and grinding was
repeated once again. The resultant species was extracted with CH2Cl2
(20 mL) and filtered. The filtrate was dried over MgSO4, and then
Synthesis of [Zn(Et){o-(OC(CF3)CHC(Me)N)C6H4(3,5-
Me2C3HN2)}] (8). ZnEt2 (2.5 mL, a 1.5 M solution in toluene, 3.75
mmol) was added dropwise to a stirred solution of 4 (0.79 g, 2.46
mmol) in toluene (10 mL) at about −80 °C. The mixture was then
warmed to room temperature and stirred overnight at that tem-
perature. The resulting solution was concentrated in vacuo. n-Hexane
(20 mL) was added to the concentrated solution to afford complex 8
1
as a yellowish gray solid (0.93 g, 91%), mp 116−118 °C. H NMR
(C6D6): δ 0.64−0.80 (m, 2H, ZnCH2), 1.23 (s, 3H, Me), 1.46 (t, J = 8
Hz, 3H, ZnCH2CH3), 1.58 (s, 3H, Me), 2.18 (s, 3H, Me), 5.26 (s, 1H,
CH), 5.33 (s, 1H, CH), 6.50 (dd, J = 1.2, 8.0 Hz, 1H, C6H4), 6.56 (d, J
= 1.2, 8.0 Hz, 1H, C6H4), 6.68 (dt, J = 1.2, 8.0 Hz, 1H, C6H4), 6.84
(dt, J = 1.2, 8.0 Hz, 1H, C6H4). 13C NMR (C6D6): δ −2.34, 11.79,
12.91, 13.35, 21.05, 93.52 (q, J = 2.6 Hz), 107.35, 120.76 (q, J = 282.6
E
dx.doi.org/10.1021/om400193z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Hz), 124.91, 125.14, 125.79, 127.94, 128.87, 129.96, 141.37, 143.62,
150.59, 166.02 (q, J = 30.9 Hz), 170.81. 19F NMR (C6D6): δ −74.57.
Anal. Calcd for C18H20F3N3OZn: C, 51.88; H, 4.84; N, 10.08. Found:
C, 51.59; H, 4.68; N, 9.83.
1.79 (t, J = 8.1 Hz, 3H, ZnEt), 2.53 (s, 3H, Me), 3.79 (d, J = 24.3 Hz,
1H, PCH), 5.57 (s, 1H, pyrazolyl), 6.48−6.56 (m, 2H, Ar), 6.71−6.79
(m, 1H, Ar), 6.97 (b, 6H, Ar), 7.15−7.24 (m, 1H, Ar), 7.58 (b, 4H,
Ar). 13C NMR (C6D6): δ −1.52, 12.34, 13.55, 14.34, 29.25, 41.46 (d, J
= 13.7 Hz), 63.94 (d, J = 132.8 Hz), 106.82, 120.87 (d, J = 2.9 Hz),
126.29 (d, J = 1.8 Hz), 128.26, 128.58, 128.61, 128.64, 128.72, 131.02
(d, J = 1.7 Hz), 132.29 (d, J = 9.7 Hz), 133.69 (d, J = 10.6 Hz), 141.04,
144.89 (d, J = 5.9 Hz), 150.21, 195.81 (d, J = 1.9 Hz). 31P NMR
(C6D6): δ 18.17. Anal. Calcd for C31H36N3OPZn: C, 66.13; H, 6.45;
N, 7.46. Found: C, 65.71; H, 6.42; N, 7.55.
X-ray Crystallography. Single crystals of complexes 5 and 11
were respectively mounted in Lindemann capillaries under nitrogen.
Diffraction data were collected at 294(2) K on a Bruker Smart CCD
area detector with graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å). The structures were solved by direct methods using
SHELXS-9721 and refined against F2 by full-matrix least squares using
SHELXL-97.22 Hydrogen atoms were placed in calculated positions.
Typical Procedure for the ROP of rac-Lactide. A typical
polymerization procedure is exemplified by the synthesis of PLA using
complex 5 as a catalyst precursor. BnOH (0.78 mL, 0.1 M solution in
toluene, 0.078 mmol) was added dropwise to a solution of 5 (28.4 mg,
0.078 mmol) in toluene (2 mL), and the mixture was stirred for 2 h.
Solvent was removed in vacuo, and the residue was dissolved in
CH2Cl2 (5 mL) to form a catalyst solution. The catalyst solution
(2.5 mL) was injected into a stirred solution of rac-LA (0.562 g,
3.90 mmol) in CH2Cl2 (5.3 mL) at 25 °C. The solution was stirred for
1 min, and the reaction was quenched by adding several drops of
glacial acetic acid into the reaction mixture. A small amount of sample
was taken from the mixture to measure the conversion of rac-LA by a
1H NMR spectrum. The remaining sample was dropped into n-hexane
to form white precipitates. The precipitates were collected by filtration
and dried under vacuum to give a white solid for GPC analysis.
Kinetic Studies of Polymerization of rac-LA Catalyzed by 11-
BnOH. A method similar to that used in the typical polymerization
procedure was used to prepare a catalyst solution (0.0234 M in
CH2Cl2) through reaction of 11 with BnOH. A Schlenk tube con-
taining a solution of rac-LA (1.35 g, 9.36 mmol) in CH2Cl2 (14.7 mL)
was kept in an oil bath preset at 38 °C. To the stirred solution was
added the catalyst solution (4.0 mL). Samples were taken from the
reaction mixture using a syringe at 20 min intervals. The conversion of
rac-LA in each of the samples was measured by 1H NMR spectra, and
the polymer was precipitated in n-hexane. The precipitates were
collected by filtration and dried under vacuum to afford a white solid
for GPC analysis.
Synthesis of o-(3,5-Me2C3HN2)C6H4NP(Ph2)CH2C(O)Ph (9).
A solution of 1-(2′-azidophenyl)-3,5-dimethylpyrazole (1.80 g, 8.44 mmol)
in CH2Cl2 (10 mL) was added dropwise to a stirred solution of
Ph2PCH2C(O)Ph (2.56 g, 8.41 mmol) in CH2Cl2 (10 mL) at 0 °C.
The mixture was warmed to room temperature and stirred overnight.
Solvent was removed under reduced pressure. The residue was
dissolved in diethyl ether (20 mL), and the solution was concentrated
1
to give yellow crystals of 9 (3.38 g, 82%), mp 64−66 °C. H NMR
(CDCl3): δ 2.01 (s, Me), 2.14 (s, Me), 2.23 (s, Me), 4.07 (d, J = 15.3
Hz, CH), 4.37 (d, J = 15.3 Hz, CH), 5.79 (s, pyrazolyl), 5.95 (s,
pyrazolyl), 6.40 (d, J = 7.8 Hz, Ar), 6.63 (t, J = 7.5 Hz, Ar), 6.84 (t, J =
7.5 Hz, Ar), 6.96 (t, J = 7.8 Hz, Ar), 7.07 (d, J = 7.5 Hz, Ar), 7.10−7.17
(m, Ar), 7.19−7.26 (m, Ar), 7.29−7.44 (m, Ar), 7.51−7.62 (m, Ar),
7.69−7.80 (m, Ar). 13C NMR (CDCl3): δ 11.66, 12.10, 13.52, 13.86,
40.77 (d, J = 50 Hz), 50.94 (d, J = 125 Hz), 104.29, 106.77, 118.18,
120.60 (d, J = 4.6 Hz), 121.54, 123.62 (d, J = 10.6 Hz), 126.47, 126.84,
127.74, 128.28, 128.33, 128.37, 128.40, 128.58, 128.73, 128.93, 129.10,
129.48, 129.69, 129.89, 129.99, 131.65, 131.80, 132.07 (d, J = 10 Hz),
132.22 (d, J = 2.6 Hz), 133.22, 137.10, 141.27 (d, J = 14.6 Hz), 141.81,
141.96, 146.96, 147.61, 149.92, 185.63. 31P NMR (CDCl3): δ −4.42,
23.71. Anal. Calcd for C31H28N3OP: C, 76.06; H, 5.76; N, 8.58.
Found: C, 75.74; H, 5.71; N, 8.85.
Synthesis of o-(3,5-Me2C3HN2)C6H4NP(Ph2)CH2C(O)-t-Bu
(10). A solution of 1-(2′-azidophenyl)-3,5-dimethylpyrazole (2.00 g,
9.38 mmol) in CH2Cl2 (10 mL) was added dropwise to a stirred
solution of Ph2PCH2C(O)tBu (2.67g, 9.39 mmol) in CH2Cl2 (10 mL)
at 0 °C. The mixture was warmed to room temperature and stirred
overnight. Solvent was removed under reduced pressure, and the
residue was dissolved in n-hexane. The resulting solution was
concentrated and kept at −20 °C to form a yellow powder of 10
1
(3.00 g, 68%), mp 110−112 °C. H NMR (CDCl3): δ 0.78 (s, t-Bu),
0.92 (s, t-Bu), 2.13 (s, Me), 2.18 (s, Me), 2.23 (s, Me), 3.59 (d, J =
14.7 Hz, CH), 3.72 (d, J = 32.4 Hz, CH), 5.87 (s, pyrazolyl), 5.93 (b,
pyrazolyl), 6.37 (d, J = 7.8 Hz, Ar), 6.51−6.82 (m, Ar), 6.88 (t, J = 7.2
Hz, Ar), 6.92−7.80 (m, Ar). 13C NMR (CDCl3): δ 11.85, 12.10, 13.56,
13.91, 25.70, 28.50, 38.29 (d, J = 55.6 Hz), 47.50 (d, J = 122.5 Hz),
104.35, 106.73, 118.11, 120.95 (d, J = 4.5 Hz), 121.23, 123.79 (d, J =
10.6 Hz), 126.49, 127.90, 128.50 (d, J = 12.1 Hz), 128.85, 128.97 (d, J
= 12.4 Hz), 131.54 (d, J = 10.5 Hz), 131.86 (d, J = 4.7 Hz), 132.33 (d,
J = 0.6 Hz), 137.61, 141.90, 147.39, 147.64, 149.77, 179.86. 31P NMR
(CDCl3): δ −2.25, 24.44. Anal. Calcd for C29H32N3OP: C, 74.18; H,
6.87; N, 8.95. Found: C, 74.06; H, 7.04; N, 8.69.
ASSOCIATED CONTENT
* Supporting Information
■
Synthesis of [Zn(Et){o-(OC(Ph)CHP(Ph2)N)C6H4(3,5-
Me2C3HN2)}] (11). ZnEt2 (1.60 mL, 1 M solution in hexane, 1.60
mmol) was added dropwise to a stirred solution of 9 (0.73 g, 1.50
mmol) in toluene (10 mL) at about −80 °C. The mixture was warmed
to room temperature and stirred overnight. The resulting solution was
concentrated under vacuum. A small amount of n-hexane was added to
the solution to give colorless crystals of 11 (0.78 g, 89%), mp 190−192
S
CIF files, figures, and a table giving X-ray crystallographic data
1
for complexes 5 and 11, H NMR spectra of PLAs, and the
proposed mechanism for the catalytic polymerization. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
°C. H NMR (C6D6): δ 0.95 (q, J = 8.1 Hz, 2H, ZnEt), 1.46 (s, 3H,
Me), 1.80 (t, J = 8.1 Hz, 3H, ZnEt), 2.49 (s, 3H, Me), 4.43 (d, J = 23.4
Hz, 1H, PCH), 5.51 (s, 1H, pyrazolyl), 6.53 (d, J = 3.9 Hz, 2H, Ar),
6.72−6.80 (m, 2H, Ar), 6.97 (b, 5H, Ar), 7.13−7.20 (m, 5H, Ar), 7.58
(b, 3H, Ar), 8.01−8.06 (m, 2H, Ar). 13C NMR (C6D6): δ −1.41,
12.27, 13.61, 14.26, 68.11 (d, J = 133.9 Hz), 107.04, 120.94, 126.30,
127.94, 128.18, 128.60, 128.67, 129.27, 131.24 (d, J = 1 Hz), 132.48
(d, J = 9.8 Hz), 133.82 (d, J = 10.8 Hz), 141.02, 142.88 (d, J = 16.2
Hz), 144.86 (d, J = 5.5 Hz), 150.50, 180.81. 31P NMR (C6D6): δ
21.87. Anal. Calcd for C33H32N3OPZn: C, 67.99; H, 5.53; N, 7.21.
Found: C, 68.03; H, 5.46; N, 7.32.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zxwang@ustc.edu.cn.
■
Present Address
†School of Engineering Technology, Beijing Normal University
at Zhuhai, Guangdong 519087, People’s Republic of China.
Notes
The authors declare no competing financial interest.
Synthesis of [Zn(Et){o-(OC(t-Bu)CHP(Ph2)N)C6H4(3,5-
Me2C3HN2)}] (12). The same procedure as for 11 was used. Thus,
reaction of 10 (0.35 g, 0.75 mmol) with ZnEt2 (0.80 mL, 1 M solution
in hexane, 0.80 mmol) in toluene afforded, after workup, colorless
crystals of 12 (0.32 g, 76%), mp 161−163 °C. H NMR (C6D6): δ
0.88 (q, J = 8.1 Hz, 2H, ZnEt), 1.33 (s, 9H, t-Bu), 1.43 (s, 3H, Me),
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (20972146) and the Foundation for
Talents of Anhui Province (Grant No. 2008Z011). We thank
1
F
dx.doi.org/10.1021/om400193z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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dx.doi.org/10.1021/om400193z | Organometallics XXXX, XXX, XXX−XXX