Zinc and aluminum complexes supported by quinoline-based N,N,N-chelate ligands: Synthesis, characterization, and catalysis in the ring-opening polymerization of ε-caprolactone and rac-lactide

Article abstract of DOI:10.1021/om200423g

A series of zinc and aluminum complexes supported by quinoline-based N,N,N-chelate ligands were synthesized and characterized. The reaction of 2-PyCH₂PPh₂ or ArN=C(Ph)CH₂PPh₂ (Ar = Ph, p-MeC₆H₄, p-MeOC₆H₄) with 8-azidoquinoline in dichloromethane gave the iminophosphoranes 2-PyCH ₂P(Ph₂)=N(8-C₈H₆N) (1; C ₈H₆N = quinolyl) and ArN=C(Ph)CH₂P(Ph ₂)=N(8-C₈H₆N), respectively. Treatment of the iminophosphoranes with 1 equiv of ZnEt₂ afforded the corresponding zinc complexes [Zn(Et){2-PyCHP(Ph₂)=N(8-C₈H ₆N)}] (2) and [Zn(Et){ArNC(Ph)=CHP(Ph₂)=N(8-C ₈H₆N)}] (5a, Ar = Ph; 5b, Ar = p-MeC₆H ₄; 5c, Ar = p-MeOC₆H₄). Similar reactions between the iminophosphoranes and an equimolar amount of AlMe₃ generated the aluminum complexes [Al(Me₂){2-PyCHP(Ph ₂)=N(8-C₈H₆N)}] (3) and [Al(Me ₂){ArNC(Ph)=CHP(Ph₂)=N(8-C₈H₆N)}] (6a, Ar = Ph; 6b, Ar = p-MeC₆H₄). Compounds 1-6 were all characterized by ¹H, ¹³C, and ³¹P NMR spectroscopy and elemental analysis. The molecular structures of complexes 2, 3, 5a, and 6b were determined by single-crystal X-ray diffraction techniques. In the presence of benzyl alcohol (BnOH) each of the zinc and aluminum complexes is the active catalyst in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL), leading to polymers with good molecular weight control and narrow molecular weight distribution. The zinc complexes catalyze the ROP of rac-lactide (rac-LA) efficiently in the presence of BnOH, and the polymerizations are well controlled. However, the aluminum complexes are inactive toward the ROP of rac-lactide under the same conditions.

Full text of DOI:10.1021/om200423g

ARTICLE
pubs.acs.org/Organometallics
Zinc and Aluminum Complexes Supported by Quinoline-Based
N,N,N-Chelate Ligands: Synthesis, Characterization, and Catalysis in
the Ring-Opening Polymerization of ε-Caprolactone and rac-Lactide
Wen-An Ma and Zhong-Xia Wang*
CAS Key Laboratory of Soft Matter Chemistry, Joint Laboratory of Green Synthetic Chemistry, and Department of Chemistry,
University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
S Supporting Information
b
ABSTRACT: A series of zinc and aluminum complexes sup-
ported by quinoline-based N,N,N-chelate ligands were synthe-
sized and characterized. The reaction of 2-PyCH2PPh2 or
ArNdC(Ph)CH2PPh2 (Ar = Ph, p-MeC6H4, p-MeOC6H4)
with 8-azidoquinoline in dichloromethane gave the iminopho-
sphoranes 2-PyCH2P(Ph2)dN(8-C8H6N) (1; C8H6N = quinolyl)
and ArNdC(Ph)CH2P(Ph2)dN(8-C8H6N), respectively. Treat-
ment of the iminophosphoranes with 1 equiv of ZnEt2 afforded the
corresponding zinc complexes [Zn(Et){2-PyCHP(Ph2)dN(8-
C8H6N)}] (2) and [Zn(Et){ArNC(Ph)dCHP(Ph2)dN(8-C8H6N)}] (5a, Ar = Ph; 5b, Ar = p-MeC6H4; 5c, Ar = p-MeOC6H4).
Similar reactions between the iminophosphoranes and an equimolar amount of AlMe3 generated the aluminum complexes
[Al(Me2){2-PyCHP(Ph2)dN(8-C8H6N)}] (3) and [Al(Me2){ArNC(Ph)dCHP(Ph2)dN(8-C8H6N)}] (6a, Ar = Ph; 6b, Ar =
p-MeC6H4). Compounds 1À6 were all characterized by 1H, 13C, and 31P NMR spectroscopy and elemental analysis. The molecular
structures ofcomplexes2, 3, 5a, and 6b weredetermined by single-crystalX-raydiffraction techniques. Inthe presence of benzyl alcohol
(BnOH) each of the zinc and aluminum complexes is the active catalyst in the ring-opening polymerization (ROP) of ε-caprolactone
(ε-CL), leading to polymers with good molecular weight control and narrow molecular weight distribution. The zinc complexes
catalyze the ROP of rac-lactide (rac-LA) efficiently in the presence of BnOH, and the polymerizations are well controlled. However, the
aluminum complexes are inactive toward the ROP of rac-lactide under the same conditions.
’ INTRODUCTION
Tridentate N,N,N- and N,N,O-chelate aluminum and zinc com-
plexes were also proven to be efficient catalysts for the ROP of
ε-CL or LA.5 Recently, we reported aluminum and tin(II)
complexes supported by quinoline-based N,N,O-chelate ligands
and found that the aluminum complexes have good catalytic
activity in the ROP of ε-CL, giving polymers with good control
over the molecular weight and distribution.6 Sun and co-workers
also found several aluminum complexes bearing quinoline-based
ligands to be active catalysts for the ROP of ε-CL.7 We wished to
further investigate the potential of quinoline based ligand
stabilized aluminum and zinc complexes in catalyzing the ROP
of ε-CL or rac-LA. Hence, we designed quinoline-based N,N,N-
tridentate ligands, synthesized their aluminum and zinc com-
plexes, and studied catalysis of the complexes in the ROP of ε-CL
and rac-LA. Herein we report the results.
Synthetic aliphatic polyesters such as poly(lactide) (PLA),
poly(glycolide) (PGA), poly(ε-caprolactone) (PCL), and their
copolymers have attracted considerable attention due to their
biocompatible, biodegradable, and permeable properties, which
are important in biomedical and pharmaceutical applications.1
Among these polymers, PLA, whose starting materials can be
derived from renewable resources, has found wide applicability,
including medical, agricultural, and packaging applications,1,2
while PCL is ideally suited for long-term drug delivery due to
its slow degradation in comparison to other polymers.3 The most
promising method for the synthesis of polyesters is the ring-
opening polymerization (ROP) of ε-CL and LA catalyzed by
metal complexes.1,2 Numerous catalyst systems for the polymer-
izations have been developed. However, different catalysts can
lead to different microstructures and physical and mechanical
properties of the polymers. Therefore, it is still of great interest to
develop new catalysts for the preparation of PCL and PLA. Zinc
and aluminum complexes have been among the most extensively
studied catalysts for this purpose.1,2 For example, a range of
salen-Al complexes were found to catalyze the stereoselective
polymerization of rac-LA.2c β-Diiminate zinc complexes cata-
lyzed the polymerization of rac-LA in high activity and selectivity.4
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Compounds 1À6. The
synthesis of compounds 1À3 is summarized in Scheme 1.
Received: May 19, 2011
Published: June 27, 2011
r
2011 American Chemical Society
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Organometallics
ARTICLE
Scheme 1. Synthesis of Compounds 1À3
Figure 2. ORTEP diagram of complex 3 (30% probability thermal
ellipsoids). Selected bond lengths (Å) and angles (deg): Al(1)ÀN(1) =
2.189(3), Al(1)ÀN(2) = 1.952(3), Al(1)ÀN(3) = 2.100(3), Al(1)À
C(16) = 1.987(5), Al(1)ÀC(17) = 1.982(4), P(1)ÀN(2) = 1.638(3),
P(1)ÀC(10) = 1.710(4), C(10)ÀC(11) = 1.400(5); N(2)ÀAl(1)À
C(17) = 121.84(17), N(2)ÀAl(1)ÀC(16) = 111.87(17), C(17)À
Al(1)ÀC(16) = 124.9(2), N(2)ÀAl(1)ÀN(3) = 93.03(12), C(17)À
Al(1)ÀN(3) = 92.94(17), C(16)ÀAl(1)ÀN(3) = 95.83(17),
N(2)ÀAl(1)ÀN(1) = 77.53(12), C(17)ÀAl(1)ÀN(1) = 91.54(17),
C(16)ÀAl(1)ÀN(1) = 88.43(16), N(1)ÀAl(1)ÀN(3) = 170.54(12),
P(1)ÀN(2)ÀAl(1) = 122.38(15), C(6)ÀN(2)ÀAl(1) = 115.6(2),
C(5)ÀN(1)ÀAl(1) = 110.3(2), C(11)ÀN(3)ÀAl(1) = 122.1(2).
complex 3 exhibits only a single AlÀMe signal at δ À0.08 ppm,
implying the molecule undergoes possibly fast structural isomer-
ization in solution. The 31P NMR spectra of the complexes show
resonance signals at δ 16.78 ppm for complex 2 and δ 22.57
ppm for complex 3.
The structures of complexes 2 and 3 were further character-
ized by single crystal X-ray diffraction techniques. The ORTEP
drawing of 2 is displayed in Figure 1, along with selected bond
lengths and angles. The ligand bonds to the central zinc atom in a
tridentate manner. The central zinc atom has a distorted-tetra-
hedral coordination geometry with an acute N(quinolyl)ÀZnÀ
N(bridge) angle (N(1)ÀZn(1)ÀN(2) = 81.38(13)°). The
N(2)ÀZnÀN(3) angle is wider (N(2)ÀZn(1)ÀN(3) =
94.60(14)°) than that of N(1)ÀZn(1)ÀN(2), which corre-
sponds with the metal ring sizes. The ZnÀN(1) distance of
2.093(3) Å is shorter than the ZnÀN(quinolyl) distance in
[Zn(Et){ButNdP(Pri)2CH(8-C8H6N)}] (2.1651(17) Å), while
the Zn(1)ÀN(2) distance (2.068(3) Å) is comparable to the ZnÀ
N(PdN) distance in [Zn(Et){ButNdP(Pri)2CH(8-C8H6N)}]
(2.0584(15) Å).8 The Zn(1)ÀN(3) distance (2.148(4) Å) is
longer than the ZnÀN(pyridyl) distances in [Zn(Et){2-(3,5-
Me2C3HN2)-6-{N(SiMe3)C(Ph)dCH}C5H3N}] (2.064(4) Å)
and [Zn(Et){2-{N(SiMe3)dP(Ph)2}-6-{N(SiMe3)P(Ph)2dCH}-
C5H3N}] (2.103(2) Å) but is still within the normal range.9
The P(1)ÀN(1) distance of 1.630(3) Å is slightly longer than a
typical PÀN double bond10 but is normal for a coordinated
iminophosphorane,11 while the P(1)ÀC(22) distance of 1.704(4)
Å is between a single and double bond.10
Figure 1. ORTEP diagram of complex 2 (30% probability thermal
ellipsoids). Selected bond lengths (Å) and angles (deg): Zn(1)ÀN(1) =
2.093(3), Zn(1)ÀN(2) = 2.068(3), Zn(1)ÀN(3) = 2.148(4), Zn(1)À
C(28) = 1.953(5), P(1)ÀN(2) = 1.630(3), P(1)ÀC(22) = 1.704(4),
C(22)ÀC(23) = 1.413(6); C(28)ÀZn(1)ÀN(2) = 137.7(2), C(28)À
Zn(1)ÀN(1) = 120.1(2), N(1)ÀZn(1)ÀN(2) = 81.38(13), C(28)À
Zn(1)ÀN(3) = 114.8(2), N(2)ÀZn(1)ÀN(3) = 94.60(14), N(1)ÀZn-
(1)ÀN(3) = 99.16(14), C(6)ÀN(2)ÀP(1) = 121.1(3), C(6)ÀN-
(2)ÀZn(1) = 110.3(3), P(1)ÀN(2)ÀZn(1) = 104.17(17).
Compound 1 was prepared in high yield by reaction of
8-azidoquinoline with 2-PyCH2PPh2 in CH2Cl2. Treatment
of 1 with ZnEt2 in toluene gave a zinc complex (2) in 80%
yield. In order to make the reaction to go to completion, the
reaction mixture was heated at 110 °C for 12 h after it was
stirred at room temperature overnight. Treating 1 with AlMe3
in toluene at room temperature and then refluxing for 12 h
generated aluminum complex 3 in 76% yield. Both 2 and 3 are
air-sensitive yellow crystals and are stable under a nitrogen
atmosphere. They are soluble in toluene and can be purified by
recrystallization from toluene.
The ORTEP drawing of 3 is shown in Figure 2, along with
selected bond lengths and angles. The central aluminum atom is
five-coordinated and has a distorted-trigonal-bipyramidal geo-
metry in which the iminophosphoranyl nitrogen atom and two
methyl carbon atoms occupy the equatorial positions and the
quinolyl nitrogen atom and the pyridyl nitrogen atom occupy the
axial positions. The arrangement of the N(1)Al(1)N(3) atoms is
Compounds 1À3 were all characterized by elemental analysis
and 1H, 13C, and 31P NMR spectroscopy. The analytical results
1
are in accord with their respective formulas. In the H NMR
spectrum of compound 1 one set of proton signals was observed
and is consistent with the structure of 1. The 31P NMR spectrum
exhibits a single signal at δ 15.15 ppm. The 1H NMR spectrum of
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Organometallics
ARTICLE
Scheme 2. Synthesis of Compounds 4À6
Figure 3. ORTEP diagram of complex 5a (30% probability thermal
ellipsoids; the toluene molecule is omitted). Selected bond lengths (Å)
and angles (deg): Zn(1)ÀN(1) = 2.108(3), Zn(1)ÀN(2) = 2.062(3),
Zn(1)ÀN(3) = 2.037(3), Zn(1)ÀC(36) = 1.993(4), P(1)ÀN(2) =
1.615(3), P(1)ÀC(22) = 1.740(3), C(22)ÀC(23) = 1.390(4),
N(3)ÀC(23) = 1.343(4); C(36)ÀZn(1)ÀN(3) = 117.54(14), C(36)À
Zn(1)ÀN(2) = 134.00(15), N(3)ÀZn(1)ÀN(2) = 96.23(113), C(36)À
Zn(1)ÀN(1) = 112.03(15), N(3)ÀZn(1)ÀN(1) = 110.99(10),
N(2)ÀZn(1)ÀN(1) = 79.93(11), P(1)ÀN(2)ÀZn(1) = 116.27(15),
C(23)ÀC(22)ÀP(1) = 126.4(3), N(3)ÀC(23)ÀC(22) = 123.5(3),
C(23)ÀN(3)ÀZn(1) = 117.1(2).
close to a line, the N(1)ÀAl(1)ÀN(3) angle being 170.54(12)°.
The N(2)Al(1)C(16)C(17) atoms are approximately coplanar,
the sum of the angles at aluminum being 358.61°. The AlÀN(1)
distance of 2.189(3) Å is slightly longer than the AlÀN(quinolyl)
distance in [Al(Me2){OC(But)dCHP(Ph2)dN(8-C9H6N)}]
(2.162(3) Å) and in [Al(Me2){OC(Me)dCHC(Me)dN(8-
C9H6N)}] (2.1448(19) Å).6 The AlÀN(2) distance of 1.952(3)
Å is normal for an iminophosphorane-coordinated aluminum
complex.6,12 The AlÀN(3) distance of 2.100(3) Å is shorer than
the AlÀN(pyridyl) distance in [Al(Me2){3,5-But2-2-(O)-
C6H2CHdN(2-C5H4N)}] (2.254(2) Å)13 but is still within
the normal range.9a,14 The P(1)ÀN(2) and P(1)ÀC(10)
distances are comparable to the corresponding distances in
complex 2.
The synthesis of compounds 4À6 is shown in Scheme 2.
Compounds 4aÀc were synthesized through reaction of 8-azi-
doquinoline with ArNdC(Ph)CH2PPh2 (Ar = Ph, p-MeC6H4,
p-MeOC6H4) under the same conditions as for 1. Treatment of
4aÀc with ZnEt2 in toluene gave zinc complexes 5aÀc as yellow
crystalline solids. In order to drive the reactions to reach
completion, it is necessary to heat the reaction mixture for a
few hours. The complexes were purified by recrystallization from
toluene (for 5a,c) or diethyl ether (for 5b). Reaction of 4a,b with
AlMe3 in toluene gave the yellow aluminum complexes 6a,b,
respectively. Complex 6a can be recrystallized from toluene.
Complex 6b is very soluble in toluene and was recrystallized from
diethyl ether. Compounds 4aÀc are air stable and gave satisfac-
Figure 4. ORTEP diagram of complex 6b (30% probability thermal
ellipsoids; benzene molecules are omitted). Selected bond lengths (Å)
and angles (deg): Al(1)ÀC(37) = 1.996(6), Al(1)ÀC(38) = 1.985(6),
Al(1)ÀN(1) = 2.222(6), Al(1)ÀN(2) = 1.986(5), Al(1)ÀN(3) =
2.044(6), P(1)ÀN(2) = 1.634(5), P(1)ÀC(10) = 1.719(6), C(10)ÀC-
(11) = 1.376(7), N(3)ÀC(11) = 1.372(7); C(38)ÀAl(1)ÀN(2) =
121.4(3), C(38)ÀAl(1)ÀC(37) = 123.5(3), N(2)ÀAl(1)ÀC(37) =
112.1(3), C(38)ÀAl(1)ÀN(3) = 96.2(2), N(2)ÀAl(1)ÀN(3) =
1
tory microanalytical results. The H NMR spectral data reveal
that each of the compounds exists in an enamine form. The
chemical shifts of NH at 12.46, 12.43, and 12.48 ppm, respec-
tively, prove the presence of a hydrogen bond in each compound.
The 13C and 31P NMR spectral data are also consistent with their
respective structures. Complexes 5aÀc and 6a,b were character-
ized by elemental analyses and 1H, 13C, and 31P NMR spectros-
copy. The 1H NMR spectra of complexes 5aÀc show that in each
molecule the chemical shift of the CH2 group appears in the
low-frequency region, proving the presence of a zincÀC bond.
The 13C NMR spectra give results consistent with the 1H NMR
spectra. The 31P NMR spectrum appears as a single signal at
about δ 16.30 ppm for each complex, shifting slightly toward low
92.7(2), C(37)ÀAl(1)ÀN(3)
=
98.2(3), C(38)ÀAl(1)ÀN(1)
=
89.4(3), N(2)ÀAl(1)ÀN(1) = 75.7(2), C(37)ÀAl(1)ÀN(1) = 87.1(2),
N(3)ÀAl(1)ÀN(1) = 168.3(3), C(5)ÀN(1)ÀAl(1) = 112.2(5),
C(6)ÀN(2)ÀAl(1) = 116.8(4), P(1)ÀN(2)ÀAl(1) = 118.9(3),
C(11)ÀN(3)ÀAl(1) = 120.5(4), C(11)ÀC(10)ÀP(1) = 120.8(5),
N(3)ÀC(11)ÀC(10) = 118.7(6).
frequency compared with those of 4aÀc and being very close to
1
that of 2. The H NMR spectra of 6a,b both exhibit a single
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Organometallics
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Table 1. Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Complexes 2, 3, 5aÀc, and 6a,ba
c
d
e
entry
cat.
T (°C)
time (min)
conversn (%)c
yield (%)
104Mn,calc
104Mn,NMR
104Mn,GPC
PDI f
1
2
90
70
70
50
30
70
50
30
30
90
70
90
70
90
70
70
50
30
70
50
30
10
30
30
75
220
8
96
98
15
96
35
89
86
78
89
93
53
97
98
96
78
95
88
98
93
81
88
92
94
2.20
2.25
1.74
2.11
1.62
2.19
1.37
1.37
2
3b
2
2
4
2
93
2.20
2.23
2.00
1.38
5
2
6
3
84
75
69
85
85
48
93
95
93
76
85
80
95
92
75
82
2.04
1.97
1.79
2.04
2.13
1.22
2.22
2.25
2.20
1.79
2.18
2.02
2.25
2.13
1.86
2.02
2.04
1.69
1.61
2.00
2.05
0.99
2.33
2.24
2.01
1.83
2.11
2.15
2.23
2.76
3.06
1.84
2.32
1.95
1.69
2.35
1.93
1.22
1.87
2.11
2.13
3.63
2.74
2.78
2.55
3.31
2.51
2.88
1.10
1.10
1.14
1.14
1.33
1.38
1.33
1.33
1.34
1.23
1.10
1.08
1.08
1.11
1.06
1.06
7
3
30
75
90
120
180
15
45
30
60
8
8
3
9
3
10
11
12
13
14
15
16
17
18
19
19
20
5a
5a
5b
5b
5c
5c
6a
6a
6a
6b
6b
6b
18
75
6
9
75
a All polymerizations were carried out in toluene. Conditions: [ε-CL]0 = 2 M, [ε-CL]0:[cat.]0:[BnOH]0 = 200:1:1, except for entry 3. b BnOH was not
c
employed: [ε-CL]0:[cat.]0 = 200:1. Calculated from the molecular weight of ε-CL times the conversion of monomer and the ratio of [ε-CL]0 to
[BnOH]0 plus the molecular weight of BnOH. d Measured by H NMR spectra. Obtained from GPC analysis and calibrated against polystyrene
1
e
standard, multiplied by 0.56.15 f Obtained from GPC analysis.
resonance at δ 0.09 ppm. In the 13C NMR spectra, the signals for
the methyls appear at À3.03 ppm (for 6a) and À3.07 ppm (for
6b), respectively. These results are consistent with those of
complex 3. The 31P NMR spectra of complexes 6a,b show
resonance signals at 21.62 and 21.57 ppm, respectively, being
very close to that of 3 at 22.57 ppm.
the plane composed of N(2), C(37), and C(38) atoms, and the
N(1)ÀAl(1)ÀN(3) angle of 168.3(3)° is slightly smaller than
the corresponding angle in complex 3 (170.54(12)°). The
AlÀN(PdN) and AlÀN(quinolyl) distances (1.986(5) Å and
2.222(6) Å, respectively) are both longer than the corresponding
distances in complex 3. The AlÀN(enamide) distance of
2.044(6) Å is shorter than the AlÀN(pyridyl) distance
(2.100(3) Å) in complex 3 but longer than the AlÀN(enamide)
distance (1.901(3) Å) in [Al(Me2){N(Ph)C(Ph)dCHP(Ph2)d
N(p-MeC6H4)}].12 The P(1)ÀN(2) and P(1)ÀC(10) distances
are very close to the corresponding distances in complex 3.
ROP of ε-CL and rac-LA Catalyzed by the Zinc or Aluminum
Complexes. The catalysis of the zinc and aluminum complexes
toward the ROP of ε-CL was evaluated first, and the results are
given in Table 1. Each of the complexes initiated the ROP of
200 equiv of ε-CL at elevated temperature in the presence of
BnOH. The polymerization catalyzed by 2/BnOH at 90 °C
proceeded rapidly. The monomer conversion reached 96% in
10 min (entry 1, Table 1). At 70 °C the monomer conversion
reached 98% in 30 min using the same catalyst system (entry 2,
Table 1). In sharp contrast, in the absence of BnOH complex 2
led to only 15% monomer conversion at 70 °C in 30 min (entry 3,
Table 1). At lower temperature the polymerization is much
slower. At 30 °C only 35% of the monomer was converted to
polymer in 220 min using 2/BnOH as the catalyst (entry 5,
Table 1). The complex 3/BnOH system exhibits higher catalytic
activity than 2/BnOH. The former led to 89% conversion of
ε-CL in 8 min at 70 °C and 78% monomer conversion in 75 min
at 30 °C (entries 6 and 8, Table 1). 5aÀc show different catalytic
activities. Among them, 5b exhibits the highest catalytic activity
and 5a shows the lowest activity (entries 10À15, Table 1). It
The structures of complexes 5a and 6b were also determined
by single-crystal X-ray diffraction techniques. The ORTEP
diagram of complex 5a is depicted in Figure 3, along with
selected bond lengths and angles. The skeletal structure is similar
to that of complex 2. The central zinc atom has a distorted-
tetrahedral geometry. The acute N(quinolyl)ÀZnÀN(bridge)
angle of 79.93(11)° is close to the corresponding angle in
complex 2 (81.38(13)°). The ZnÀN(PdN) and ZnÀN-
(quinolyl) distances of 2.062(3) and 2.108(3) Å, respectively,
are comparable to those in complex 2. However, the ZnÀN(3)
distance of 2.037(3) Å is shorter than that in complex 2
(2.148(4) Å). The former is slightly longer than the ZnÀN-
(enamide) distance in [Zn(Et){2-(3,5-Me2C3HN2)-6-{N-
(SiMe3)C(Ph)dCH}C5H3N}] (2.014(4) Å).9a The P(1)À
N(2) distanceof1.615(3) Å is slightly shorterthanthat in complex
2 (1.630(3) Å), and the former is closer to the distance of a PÀN
double bond.10
Complex 6b crystallizes with two molecules in the asymmetric
unit (Figure 4; one molecule is presented). The skeletal structure
of 6b is similar to that of complex 3. The central aluminum atom
has a distorted-trigonal-bipyramidal geometry with the imino-
phosphorane nitrogen atom and two methyl carbon atoms
occupying the equatorial positions and the quinolyl nitrogen
atom and the enamide nitrogen atoms occupying the axial
positions. However, the aluminum atom deviates slightly from
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Figure 6. Plots of PCL Mn (9, obtained from GPC analysis) and
polydispersity (O, Mw/Mn) as a function of ε-CL conversion using
complex 2 at 40 °C. Conditions: [M]0:[Al]0:[BnOH]0 = 200:1:1;
[M]0 = 2 M; solvent toluene.
Figure 5. Plots of ln([M]0/[M]) versus time for the polymerization of
ε-CL catalyzed by 2 (9) and 5b (2). Conditions: [M]0/[Al]/[BnOH]0 =
200:1:1; [M]0 = 2 M; solvent toluene; polymerization temperature
40 °C for 2 and 65 °C for 5b.
seems that the p-MeC6H4 group on the imine nitrogen atom in
5b causes the ligand to provide a proper electronic environment
at the metal center for catalysis. However, the activities of
complexes 5aÀc are all lower than that of complex 2. For example,
5b/BnOH drives 98% monomer conversion in 45 min at 70 °C,
while 2/BnOH requires 30 min for the same conversion at 70 °C.
Complex 6a exhibits almost the same activity as 6b at 70 °C (entries
16 and 19, Table 1). However, at 30 °C complex 6a is slightly more
active than 6b (entries 18 and 21, Table 1). It is also noted that 6a,b
are both more active than either 3 or 5b. Hence, an approximate
activity order is 6a g 6b > 3 > 2 > 5b > 5c > 5a. In other words, the
aluminum complexes exhibit higher catalytic activity than the zinc
complexes in the ROP of ε-CL. In most cases the determined
molecular weights of the polymers by GPC closely match the
calculated values. The polydispersities are also relatively narrow,
ranging from 1.08 to 1.38. These results imply that the catalytically
active species are quite stable during the reaction process and the
polymerizations are well controlled. In addition, the aluminum
complexes 3 and 6a,b show higher catalytic activity and better
control than the N,N,O-chelate aluminum complexes 7a,b that we
previously reported.6
Figure 7. Plots of PCL Mn (9 ,obtained from GPC analysis) and
polydispersity (O, Mw/Mn) as a function of ε-CL conversion using
complex 5b at 65 °C. Conditions: [M]0:[Al]0:[BnOH]0 = 200:1:1;
[M]0 = 2 M; solvent toluene.
controlled. The controlled polymerization is further proven by
the linear relationship of molecular weight with monomer
conversion in each catalytic reaction, along with relatively low
PDI values (Figures 6 and 7).
We next evaluated catalysis of the zinc and aluminum com-
plexes toward the ROP of rac-lactide. Surprisingly, the aluminum
complexes are inactive in the presence or absence of BnOH. Each
zinc complex is an active catalyst toward the ROP of rac-lactide in
the presence of BnOH at elevated temperature (Table 2). At
70 °C the complex 2/BnOH system exhibits high catalytic
activity. It drives 50À300 equiv of rac-LA to polymerize within
15 min, giving 93À97% monomer conversions (entries 1À6,
Table 2). The PDIs of polyesters ranging from 1.18 to 1.38 are
relatively narrow. A linear relationship between the number
average molecular weight (Mn) and ([LA]0 À [LA])/[Zn]0 as
shown in Figure 8 implies the “living” character of the polymer-
ization process. However, the polylactide molecular weights are
lower than expected at higher monomer loading. In the presence
of 2 equiv of BnOH complex 2 exhibited a catalytic activity
In order to establish the reaction order in monomer and metal
concentration, we also carried out kinetic studies of ε-CL
polymerization catalyzed by complexes 2 and 5b in the presence
of BnOH. Plots of ln([CL]0/[CL]) versus time using each
catalyst exhibits a good linear relationship (Figure 5), which
shows that the polymerization proceeds with first-order depen-
dence on monomer concentration. The reaction rate remains
constant with reaction time, indicating a constant number of
active sites throughout the polymerization. This implies that
the polymerizations catalyzed by 2 and 5b, respectively, are
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Table 2. Ring-Opening Polymerization of rac-Lactide Catalyzed by Complexes 2 and 5aÀca
c
d
e
entry
cat.
[cat.]0/[BnOH]0/[ LA]0
conversn (%)c
104Mn,calc
104Mn,NMR
104Mn,GPC
PDIf
1
2
1:1:50
97
97
97
96
94
93
97
95
71
96
95
83
88
95
96
95
96
0.71
1.41
2.11
2.78
3.40
4.03
2.78
1.38
2.06
1.39
2.75
3.00
2.55
1.38
2.09
2.75
3.47
0.58
0.75
1.43
1.90
2.10
3.20
1.42
1.85
1.66
1.80
2.17
2.64
1.67
2.37
2.86
3.73
3.56
1.01
1.20
1.49
1.82
1.89
2.05
1.18
1.38
1.33
1.26
1.28
1.22
2
2
1:1:100
1:1:150
1:1:200
1:1:250
1:1:300
1:2:200
1:1:100
1:1:200
1:1:100
1:1:200
1:1:250
1:1:200
1:1:100
1:1:150
1:1:200
1:1:250
3
2
4
2
5
2
6
2
7
2
8
5a
5a
5b
5b
5b
5c
5c
5c
5c
5c
1.34
2.00
1.41
2.16
1.63
1.16
1.85
2.12
2.84
3.09
1.16
1.18
1.21
1.23
1.10
1.08
1.19
1.20
1.22
1.22
9
10
11
12
13b
14
15
16
17
a All polymerizations were carried out in toluene at 70 °C and run for 15 min, except for entry 12. Conditions: [LA]0 = 0.5 M. b Polymerization was
carried out at 20 °C and run for 216 min. c Calculated from the molecular weight of LA times the conversion of monomer and the ratio of
[LA]0/[BnOH]0 plus the molecular weight of BnOH. d Measured by 1H NMR spectra. e Obtained from GPC analysis and calibrated against polystyrene
standard, multiplied by 0.58.17 f Obtained from GPC analysis.
Figure 8. Plot of Mn versus ([LA]0 À [LA])/[Zn]0 for the polymer-
ization of LA by 2. Conditions: [LA]0 = 0.5 M, toluene, 70 °C.
Figure 9. Plot of Mn versus ([LA]0 À [LA])/[Zn]0 for the polymer-
ization of LA by 5c. Conditions: [LA]0 = 0.5 M, toluene, 70 °C.
microstructure, identified by 1H NMR spectroscopic analysis.16
In addition, complex 5c/BnOH is also able to catalyze the ROP
of rac-LA at 20 °C (entry 13, Table 1), but the reaction is much
slower than that at a higher temperature.
similar to that using 1 equiv of BnOH. However, the molecular
weights of the polymers determined by 1H NMR spectroscopy
are about half of the theoretical values (entry 7, Table 2). This is
attributed to the fact that all the added alcohol molecules
contribute to the immortal polymerization. When 100 equiv of
rac-LA was employed, complexes 5aÀc all displayed excellent
catalytic activity. They led to 95À96% monomer conversion in
15 min at 70 °C. However, when more rac-LA was loaded,
complex 5c showed higher catalytic activity than either 5a or 5b.
Good molecular weight control using 5c is demonstrated by a
linear increase in Mn with LA conversion (Figure 9) and
relatively narrow molecular weight distributions of the polymers
(polydispersity index, PDI = 1.19À1.22). It should be noted
that the PLAs obtained by these zinc catalysts have an atactic
’ CONCLUSIONS
We have synthesized and characterized a series of zinc and
aluminum complexes supported by novel quinoline-based N,N,
N-chelate ligands. In the presence of benzyl alcohol all the zinc
and aluminum complexes are able to catalyze the ROP of ε-CL
and the reactions lead to polymers with good molecular weight
control and narrow molecular weight distribution. The zinc
complexes catalyze the ROP of rac-lactide efficiently in the
presence of BnOH. The polymerizations are well controlled
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and give PLAs with an atactic microstructure. However, the
aluminum complexes are inactive toward the ROP of rac-lactide
under the same conditions.
3H, Ar), 7.05À7.11 (m, 1H, Ar), 7.15À7.35 (m, 6H, Ar), 7.45 (dd, J =
1.5, 8.4 Hz, 1H, Ar), 7.65À8.80 (b, 4H, Ar), 7.88À7.95 (m, 1H, Ar), 8.22
(dd, J = 1.5, 8.4 Hz, 1H, Ar). 13C NMR (C6D6): δ À1.13, 13.72, 48.73
(d, J = 132.3 Hz), 108.08, 116.65, 120.16 (d, J = 8.8 Hz), 120.69 (d, J =
17.4 Hz), 121.04, 128.77 (d, J = 11.8 Hz), 130.13, 131.41, 133.00, 134.61
(d, J = 3.6 Hz), 138.37, 145.92, 146.34. 31P NMR (C6D6): δ 16.78 (m).
Synthesis of [Al(Me2){2-PyCHP(Ph2)dN(8-C8H6N)}] (3).
AlMe3 (0.35 mL, 2.3 M solution in hexane, 0.81 mmol) was added
dropwise to a stirred solution of 1 (0.30 g, 0.72 mmol) in toluene
(10 mL) at room temperature. The mixture was stirred overnight at
room temperature and then refluxed for 12 h. The resulting solution was
cooled to room temperature and filtered. Concentration of the filtrate
formed yellow crystals of 3 (0.2585 g, 76%), mp 222À224 °C. Anal.
Calcd for C29H27AlN3P: C, 73.25; H, 5.72; N, 8.84. Found: C, 73.01; H,
5.68; N, 8.74. 1H NMR (C6D6): δ À0.08 (s, 6H, AlCH3), 3.54 (d, J = 24
Hz, 1H, PCH), 5.90À5.99 (m, 1H, Ar), 6.48 (d, J = 8.4 Hz, 1H, Ar),
6.57À6.81 (m, 5H, Ar), 6.85À7.19 (m, 6H, Ar), 7.33 (dd, J = 1.5, 8.1 Hz,
1H, Ar), 7.86À8.00 (m, 4H, Ar), 8.23 (d, J = 6 Hz, 1H, Ar), 8.45 (dd, J =
1.5, 4.5 Hz, 1H, Ar). 13C NMR (C6D6): δ À3.17, 53.14 (d, J = 141.86
Hz), 109.02, 115.81, 116.30 (d, J = 9.4 Hz), 119.80 (d, J = 17.3 Hz),
121.95, 128.02, 128.44, 128.60, 128.98, 130.15, 131.42, 131.58 (d, J = 2.5
Hz), 133.32 (d, J = 10.1 Hz), 135.24 (d, J = 3.2 Hz), 136.99, 144.00,
145.54, 164.75. 31P NMR (C6D6): δ 22.57 (m).
Synthesis of PhNdC(Ph)CH2P(Ph2)dN(8-C8H6N) (4a). The
synthesis of 4a followed the same procedure as for 1. Thus, reaction of
8-azidoquinoline (0.9866 g, 5.798 mmol) with PhNdC(Ph)CH2PPh2
(2.00 g, 5.271 mmol) in CH2Cl2 (30 mL) gave 4a (2.31 g, 84%), mp
176À178 °C. The product was pure enough for the next step. The
sample used for analysis was further purified by recrystallization of the
crude product from diethyl ether. Anal. Calcd for C35H28N3P: C, 80.60;
H, 5.41; N, 8.06. Found: C, 80.36; H, 5.42; N, 7.94. 1H NMR (CDCl3):
δ 4.50 (d, J = 24.3 Hz, 1H, PCH), 6.72À6.90 (m, 2H, Ar), 6.94À7.14
(m, 6H, Ar), 7.16À7.30 (m, 4H, Ar), 7.35À7.54 (m, 8H, Ar), 7.78À8.00
(m, 5H, Ar), 8.70À8.81 (m, 1H, Ar), 12.46 (s, 1H, NH). 13C NMR
(CDCl3): δ 86.67 (d, J = 134.6 Hz), 115.68, 117.16 (d, J = 14 Hz),
120.81, 121.68, 123.02, 127.10, 128.23 (d, J = 5 Hz), 128.47, 128.54,
128.62, 128.94, 129.83, 131.28, 131.31, 132.07 (d, J = 9.9 Hz), 133.31,
135.89, 139.13 (d, J = 16.5 Hz), 142.53, 147.50, 160.22. 31P NMR
(CDCl3): δ 5.63 (m).
Synthesis of p-MeC6H4NdC(Ph)CH2P(Ph2)dN(8-C8H6N) (4b).
The synthesis of 4b followed the same procedure as for 1. Treatment of
8-azidoquinoline (0.9515 g, 5.591 mmol) with p-MeC6H4NdC(Ph)CH2-
PPh2 (2.00 g, 5.083 mmol) in CH2Cl2 (30 mL) generated compound 4b
(2.48 g, 91%), mp 158À160 °C. The product was pure enough for the next
step. The sample used for analysis was further purified by recrystallization of
the crude product from diethyl ether. Anal. Calcd for C36H30N3P: C, 80.73;
H, 5.65; N, 7.85. Found: C, 80.51; H, 5.66; N, 7.80. 1H NMR (CDCl3): δ
2.18 (s, 3H, CH3), 4.42 (d, J = 24.6 Hz, 1H, PCH), 6.75 (d, J = 7.2 Hz, 1H,
Ar), 6.90 (s, 4H, Ar), 6.95À7.06 (m, 2H, Ar), 7.15À7.27 (m, 5H, Ar),
7.30À7.47 (m, 7H, Ar), 7.79À7.96 (m, 5H, Ar), 8.68À8.76 (m, 1H, Ar),
12.43 (s, 1H, NH). 13C NMR (CDCl3): δ 20.80, 85.54 (d, J = 135.4 Hz),
115.57, 117.07 (d, J = 13.9 Hz), 120.75, 123.12, 127.05, 128.18, 128.41,
128.55, 128.80, 131.20, 131.22, 132.03 (d, J= 9.9 Hz), 135.82, 139.96, 147.50,
160.49. 31P NMR (CDCl3): δ 5.72 (m).
Synthesis of p-MeOC6H4NdC(Ph)CH2P(Ph2)dN(8-C8H6N)
(4c). LiBun (3.9 mL, 2.5 M solution in hexane, 9.75 mmol) was added
dropwise to a solution of diisopropylamine (0.9936 g, 9.819 mmol) in
THF (20 mL) at À20 °C. The mixture was stirred at that temperature
for 20 min and then added dropwise to a solution of 4-methoxy-N-(1-
phenylethylidene)benzenamine (2.00 g, 8.877 mmol) in THF (15 mL)
at about À80 °C. The mixture was stirred for an additional 2 h at that
temperature. A solution of chlorodiphenylphosphine (1.76 mL, 9.803
mmol) in THF (10 mL) was added dropwise to the reaction mixture.
The resulting solution was warmed to room temperature and stirred for
’ EXPERIMENTAL SECTION
General Procedures. All air- or moisture-sensitive manipulations
were performed under a nitrogen atmosphere using standard Schlenk
and vacuum-line techniques. Solvents were distilled under nitrogen over
sodium (toluene), sodium/benzophenone (n-hexane, THF, and Et2O),
or CaH2 (CH2Cl2) and degassed prior to use. Chlorodiphenylpho-
sphine was purchased from Acros Organics and distilled prior to use.
Diisopropylamine was dried with NaOH and distilled prior to use.
LiBun, ZnEt2, and AlMe3 were purchased from Acros Organics or Alfa-
Aesar and used as received. CDCl3 and C6D6, purchased from
Cambridge Isotope Laboratories, Inc., were degassed and stored over
Na/K alloy (C6D6) or 4 Å molecular sieves (CDCl3). ε-Caprolactone,
purchased from Acros Organics, was stirred over CaH2 for 24 h and
distilled under vacuum. rac-Lactide was purchased from Beijing Yuan-
shengrong, Inc. and recrystallized three times from toluene prior to use.
8-Azidoquinoline,18 2-((diphenylphosphino)methyl)pyridine,19 N-(1-
phenyl-2-(diphenylphosphino)ethylidene)benzenamine,12 and 4-meth-
yl-N-(1-phenyl-2-(diphenylphosphino)ethylidene)benzenamine12 were
prepared using the procedures described in the literature. NMR spectra
were recorded on a Bruker av300 spectrometer at ambient temperature.
1
The chemical shifts of H and 13C NMR spectra were referenced to
internal solvent resonances or TMS; the 31P NMR spectra were
referenced to external 85% H3PO4. Elemental analysis was performed
by the Analytical Center of the University of Science and Technology of
China. Gel permeation chromatograph (GPC) measurements were
performed on a Waters 150C instrument equipped with UltraStyragel
columns (103, 104, and 105 Å) and 410 refractive index detector, using
monodispersed polystyrene as calibration standard. THF was used as
eluent at a flow rate of 1 mL/min.
Synthesis of 2-PyCH2P(Ph2)dN(8-C8H6N) (1). A solution of
8-azidoquinoline (1.35 g, 7.933 mmol) in CH2Cl2 (10 mL) was added
dropwise to a stirred solution of 2-PyCH2PPh2 (2.00 g, 7.212 mmol) in
CH2Cl2 (20 mL) at 0 °C. The mixture was stirred at 0 °C for 30 min and
then at room temperature for 4 h. Solvent was removed under reduced
pressure. The residue was washed with n-hexane and dried in vacuo to
give a brown powder of 1 (2.632 g, 87%), mp146À148 °C. The product
was pure enough for the next step. The sample for analysis was further
purified by recrystallization of the crude product from diethyl ether.
Anal. Calcd for C27H22N3P: C, 77.31; H, 5.29; N, 10.02. Found: C,
76.91; H, 5.24; N, 10.18. 1H NMR (CDCl3): δ 4.49 (d, J = 14.6 Hz, 2H,
CH2), 6.88À7.00 (m, 2H, Ar), 7.05À7.15 (m, 2H, Ar), 7.23À7.43 (m,
9H, Ar), 7.74À7.86 (m, 4H, Ar), 7.96 (dd, J = 1.5, 8.4 Hz, 1H, Ar),
8.22À8.32 (m, 2H, Ar). 13C NMR (CDCl3): δ 40.99 (d, J = 64.5 Hz),
113.83 (d, J = 3.7 Hz), 119.43, 120.72, 121.75, 123.29, 127.18, 128.47,
128.64, 128.89 (d, J = 13 Hz), 131.92 (d, J = 10.2 Hz), 132.27 (d, J = 2.5
Hz), 133.01, 136.28, 136.35, 137.81, 148.10, 149.17. 31P NMR
(CDCl3): δ 15.15 (m).
Synthesis of [Zn(Et){2-PyCHP(Ph2)dN(8-C8H6N)}] (2).
ZnEt2 (0.79 mL, 1 M solution in hexane, 0.79 mmol) was added
dropwise to a stirred solution of 1 (0.30 g, 0.72 mmol) in toluene
(10 mL) at about À80 °C. The mixture was warmed to room
temperature, stirred overnight at that temperature, and heated to
110 °C (bath temperature) for 12 h. The resulting solution was cooled
to room temperature and filtered. Concentration of the filtrate generated
yellow crystals of 2 (0.2935 g, 80%), mp 174À176 °C. Anal. Calcd for
C29H26N3PZn 0.5C7H8: C, 69.83; H, 5.41; N, 7.52. Found: C, 69.87;
3
H, 5.42; N, 7.58. 1H NMR (C6D6): δ 0.69 (q, J = 8.1 Hz, 2H, CH2), 1.61
(t, J = 8.1 Hz, 3H, CH3), 2.24 (s, PhCH3), 3.94 (d, J = 22.5 Hz, 1H,
PCH), 6.03À6.08 (m, 1H, Ar), 6.56À6.66 (m, 2H, Ar), 6.77À6.87 (m,
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16 h. Volatiles were removed under reduced pressure, and the residue
was extracted with CH2Cl2 (60 mL). The extract was concentrated to
about 30 mL and was added dropwise to a solution of 8-azidoquinoline
(1.6617 g, 9.765 mmol) in CH2Cl2 (30 mL) at 0 °C. The mixture was
stirred at 0 °C for 30 min and then at room temperature for 4 h. Solvents
were removed under vacuum, and the residue was dissolved in diethyl
ether (40 mL). The resulting solution was filtered. Concentration of the
filtrate generated light brown crystals of 4c (2.4971 g, 51% based on
4-methoxy-N-(1-phenylethylidene)benzenamine), mp 168À170 °C.
Anal. Calcd for C36H30N3OP: C, 78.39; H, 5.48; N, 7.62. Found: C,
78.34; H, 5.61; N, 7.48. 1H NMR (CDCl3): δ 3.68 (s, 3H, OCH3), 4.36
(d, J = 24.6 Hz, 1H, PCH), 6.66 (d, J = 8.7 Hz, 2H, C6H4), 6.74 (d, J = 6.9
Hz, 1H, Ar), 6.95 (d, J = 8.7 Hz, 2H, C6H4), 6.98À7.10 (m, 2H, Ar),
7.17À7.25 (m, 4H, Ar), 7.33À7.47 (m, 8H, Ar), 7.85 (dd, J = 7.8, 11.7
Hz, 4H, Ar), 7.94 (d, J = 8.4 Hz, 1H, Ar), 8.63À8.70 (m, 1H, Ar), 12.48
(s, 1H, NH). 13C NMR (CDCl3): δ 55.45, 84.00 (d, J = 136.4 Hz),
113.56, 115.55, 116.98 (d, J = 13.3 Hz), 120.77, 124.90, 127.08, 127.97,
128.16, 128.44, 128.59, 128.66, 128.73, 129.81, 131.23, 132.07 (d, J=9.8Hz),
135.87, 147.63, 148.98, 155.02, 161.01. 31P NMR (CDCl3): δ 5.72 (m).
Synthesis of [Zn(Et){PhNC(Ph)dCHP(Ph2)dN(8-C8H6N)}]
(5a). ZnEt2 (0.64 mL, 1 M solution in hexane, 0.64 mmol) was added
dropwise to a stirred solution of 4a (0.30 g, 0.58 mmol) in toluene
(10 mL) at about À80 °C. The mixture was warmed to room
temperature, stirred overnight at room temperature, and heated to
110 °C for 8 h. The resulting solution was cooled to room temperature
and then filtered. Concentration of the filtrate gave yellow crystals of 5a
(t, J = 8.1 Hz, 3H, CH3), 2.11 (s, PhCH3), 3.27 (s, 3H, OCH3), 4.08 (d,
J = 26.6 Hz, 1H, PCH), 6.50 (dd, J = 4.5, 8.3 Hz, 1H, Ar), 6.70 (d, J = 8.8
Hz, 3H, Ar), 6.74 (d, J = 8.3 Hz, 1H, Ar), 6.86À7.17 (m, 12H, Ar), 7.33
(dd, J = 1.4, 8.3 Hz, 1H, Ar), 7.53À7.62 (m, 2H, Ar), 7.69À8.24 (b, 4H,
Ar), 8.36 (dd, J = 1.4, 4.5 Hz, 1H, Ar). 13C NMR (C6D6): δ 0.73, 14.49,
21.42, 54.89, 70.20 (d, J = 126.4 Hz), 114.31, 116.68, 119.46 (d, J = 8.8
Hz), 121.32, 125.32, 125.70, 127.92, 128.16, 128.57, 128.68, 128.84,
129.34, 130.21, 131.46, 132.37 (d, J = 9.2 Hz), 138.02, 147.04, 154.26.
31P NMR (C6D6): δ 16.28 (m).
Synthesis of [Al(Me2){PhNC(Ph)dCHP(Ph2)dN(8-C8H6N)}]
(6a). AlMe3 (0.28 mL, 2.3 M solution in hexane, 0.64 mmol) was added
dropwise to a stirred solution of 4a (0.30 g, 0.58 mmol) in toluene
(10 mL) at room temperature. The mixture was stirred overnight at
room temperature and heated to 110 °C for 12 h. The resulting solution
was cooled to room temperature and filtered. Concentration of the
filtrate afforded yellow crystals of 6a (0.1561 g, 47%), mp 258À260 °C.
Anal. Calcd for C37H33AlN3P: C, 76.93; H, 5.76; N, 7.27. Found: C,
76.66; H, 5.77; N, 7.01. 1H NMR (C6D6): δ 0.09 (s, 6H, AlCH3), 4.43
(d, J = 30.9 Hz, 1H, PCH), 6.58 (dd, J = 2.4, 6.3 Hz, 1H, Ar), 6.60À6.67
(m, 1H, Ar), 6.71 (dd, J = 4.5, 8.1 Hz, 1H, Ar), 6.76À6.85 (m, 4H, Ar),
6.86À7.00 (m, 9H, Ar), 7.03 (dd, J = 1.8, 7.5 Hz, 2H, Ar), 7.38 (dd, J =
1.2, 8.1 Hz, 1H, Ar), 7.57 (d, J = 6.9 Hz, 2H, Ar), 7.87 (d, J = 6.9 Hz, 2H,
Ar), 7.91 (d, J = 6.9 Hz, 2H, Ar), 8.70 (dd, J = 1.2, 4.5 Hz, 1H, Ar). 13C
NMR (C6D6): δ À3.03, 75.53 (d, J = 128.5 Hz), 115.93 (d, J = 9.4 Hz),
116.62, 122.11, 127.62, 128.60, 128.76, 130.46, 132.07 (d, J = 2.3 Hz),
133.43 (d, J = 10.2 Hz), 136.46, 144.73, 152.74. 31P NMR (C6D6): δ
21.62 (m).
(0.2052 g, 58%), mp 218À220 °C. Anal. Calcd for C37H32N3PZn 0.8
3
C7H8: C, 74.29; H, 5.62; N, 6.10. Found: C, 74.34; H, 5.44; N, 6.12. 1H
NMR (C6D6): δ 0.98 (q, J = 8.1 Hz, 2H, CH2), 1.83 (t, J = 8.1 Hz, 3H,
CH3), 2.11 (s, PhCH3), 4.16 (d, J = 27 Hz, 1H, PCH), 6.48 (dd, J = 4.5,
8.4 Hz, 1H, Ar), 6.64À6.78 (m, 3H, Ar), 6.84À7.21 (m, 16H,
Ar+PhCH3), 7.30 (d, J = 8.1 Hz, 1H, Ar), 7.55À7.64 (m, 2H, Ar),
7.67À8.13 (b, 4H, Ar), 8.40 (dd, J = 1.2, 4.2 Hz, 1H, Ar). 13C NMR
(C6D6): δ 0.78, 14.59, 21.42, 73.51 (d, J = 127.7 Hz), 116.71, 119.22 (d,
J = 9 Hz), 119.77, 121.39, 124.36, 125.70, 127.99, 128.28, 128.57, 128.71,
128.82, 129.34, 130.28, 131.47, 132.29 (d, J = 9.8 Hz), 138.00, 147.19,
153.47. 31P NMR (C6D6): δ 16.30 (m).
Synthesis of [Al(Me2){p-MeC6H4NC(Ph)dCHP(Ph2)dN(8-
C8H6N)}] (6b). AlMe3 (0.27 mL, 2.3 M solution in hexane, 0.62 mmol)
was added dropwise to a stirred solution of 4b (0.30 g, 0.56 mmol) in
toluene (10 mL) at room temperature. The mixture was stirred over-
night at room temperature and heated to 110 °C for 12 h. The resulting
solution was cooled to room temperature and filtered. Solvents were
removed under vacuum. Recrystallization of the residue from diethyl
ether formed yellow crystals of 6b (0.1856 g, 56%), mp 248À250 °C. A
single crystal of 6b suitable for an X-ray diffraction determination was
grown from benzene. Anal. Calcd for C38H35AlN3P 0.3C4H10O: C,
3
1
76.69; H, 6.24; N, 6.84. Found: C, 76.31; H, 5.92; N, 7.06. H NMR
Synthesis of [Zn(Et){p-MeC6H4NC(Ph)dCHP(Ph2)dN(8-
C8H6N)}] (5b). ZnEt2 (0.62 mL, 1 M solution in hexane, 0.62 mmol)
was added dropwise to a stirred solution of 4b (0.30 g, 0.56 mmol) in
toluene (10 mL) at about À80 °C. The mixture was stirred overnight at
room temperature and heated to 110 °C for 8 h. The resulting solution
was cooled to room temperature and filtered. Solvents were removed
under vacuum. Recrystallization of the residue from diethyl ether
afforded yellow crystals of 5b (0.1973 g, 56%), mp 202À204 °C. Anal.
Calcd for C38H34N3PZn: C, 72.55; H, 5.45; N, 6.68. Found: C, 72.11; H,
5.44; N, 6.68. 1H NMR (C6D6): δ 0.97 (q, J = 8.1 Hz, 2H, CH2), 1.83 (t,
J = 8.1 Hz, 3H, CH3), 2.09 (s, 3H, CH3), 4.12 (d, J = 27 Hz, 1H, PCH),
6.50 (dd, J = 4.5, 8.1 Hz, 1H, Ar), 6.64À6.78 (m, 2H, Ar), 6.82À7.15 (m,
14H, Ar), 7.31 (d, J = 8.1 Hz, 1H, Ar), 7.54À7.64 (m, 2H, Ar),
7.67À8.21 (b, 4H, Ar), 8.42 (d, J = 4.5 Hz, 1H, Ar). 13C NMR
(C6D6): δ 0.78, 14.57, 20.79, 72.13 (d, J = 127.4 Hz), 116.66, 119.36
(d, J = 9 Hz), 121.35, 124.38, 127.94, 128.68, 128.82, 129.49, 130.26,
131.43, 132.32 (d, J = 9.7 Hz), 137.98, 147.15. 31P NMR (C6D6): δ
16.30 (m).
(C6D6): δ 0.09 (s, 6H, AlCH3), 1.88 (s, 3H, CH3), 4.41 (d, J = 30.9 Hz,
1H, PCH), 6.57 (dd, J = 2.4, 6.3 Hz, 1H, Ar), 6.65À6.82 (m, 7H, Ar),
6.83À6.91 (m, 1H, Ar), 6.92À7.01 (m, 6H, Ar), 7.05 (dd, J = 1.8, 7.2 Hz,
2H, Ar), 7.39 (dd, J = 1.5, 8.4 Hz, 1H, Ar), 7.60 (d, J = 6.9 Hz, 2H, Ar),
7.88 (d, J = 7.2 Hz, 2H, Ar), 7.92 (d, J = 6.9 Hz, 2H, Ar), 8.70 (dd, J = 1.5,
4.5 Hz, 1H, Ar). 13C NMR (C6D6): δ À3.07, 20.79, 75.01 (d, J = 128.8
Hz), 115.95 (d, J = 9.3 Hz), 116.60, 122.09, 127.51, 127.62, 127.66,
128.40, 128.59, 128.75, 130.44, 132.03 (d, J = 2.6 Hz), 133.45 (d, J = 10.1
Hz), 136.44, 144.74, 150.10. 31P NMR (C6D6): δ 21.57 (m).
X-ray Crystallography. Single crystals of complexes 2, 3, 5a and
6b were respectively mounted in Lindemann capillaries under nitrogen.
Diffraction data of complexes 2, 3, and 5a were collected on an Oxford
Diffraction Gemini S Ultra diffractometer with mirror-monochromated
Cu KR radiation (λ = 1.541 84 Å) (for 2 and 3) or graphite-mono-
chromated Mo KR radiation (λ = 0.710 73 Å) (for 5a). Diffraction data
of complex 6b were collected on a Bruker Smart CCD area detector with
graphite-monochromated Mo KR radiation (λ = 0.710 73 Å). The
structures were solved by direct methods using SHELXS-9720 and
refined against F2 by full-matrix least squares using SHELXL-97.21
The disordered solvent molecules in complexes 2 and 3 were removed
from the diffraction data using the SQUEEZE program. Hydrogen
atoms were placed in calculated positions. Crystal data and experimental
details of the structure determinations are given in Table 3.
Synthesis of [Zn(Et){p-MeOC6H4NC(Ph)dCHP(Ph2)dN(8-
C8H6N)}] (5c). ZnEt2 (1.0 mL, 1 M solution in hexane, 1.0 mmol) was
added dropwise to a stirred solution of 4c (0.50 g, 0.91 mmol) in toluene
(20 mL) at about À80 °C. The mixture was stirred overnight at room
temperature and heated to 110 °C for 8 h. The resulting solution was
cooled to room temperature and filtered. Concentration of the filtrate
gave yellow crystals of 5c (0.4861 g, 83%), mp 160À162 °C. Anal. Calcd
Polymerization of ε-Caprolactone Catalyzed by 2, 3, 5aÀc,
and 6a,b. A typical polymerization procedure was exemplified by the
synthesis of PCL using complex 2 as a catalyst in the presence of an
for C38H34N3OPZn C7H8: C, 73.32; H, 5.74; N, 5.70. Found: C, 73.19;
3
H, 5.76; N, 5.68. 1H NMR (C6D6): δ 0.91 (q, J = 8.1 Hz, 2H, CH2),1.78
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ARTICLE
Table 3. Details of the X-ray Structure Deteminations of Complexes 2, 3, 5a, and 6b
2
3
5a C7H8
6b 2C6H6
3
3
empirical formula
fw
C29H26N3PZn
512.87
C29H27AlN3P
475.49
C44H40N3PZn
707.13
C50H47AlN3P
747.86
T (K)
291(2)
291(2)
150(2)
298(2)
cryst syst
space group
a (Å)
triclinic
triclinic
triclinic
monoclinic
Cc
P1
P1
P1
10.1130(4)
11.3801(5)
13.7665(6)
85.789(4)
68.959(4)
73.559(4)
1417.54(10)
2
9.749(5)
10.560(5)
18.134(5)
104.816(5)
95.061(5)
102.460(5)
1741.9(13)
2
11.6230(4)
12.7857(6)
14.1984(6)
86.513(4)
68.431(4)
85.898(3)
1955.86(14)
2
22.133(2)
11.2770(10)
36.196(3)
90
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
107.595(2)
90
8611.7(14)
8
Dcalcd (g cmÀ3
)
1.202
0.907
1.201
1.154
F(000)
532
500
740
3168
μ (mmÀ1
)
1.878
1.061
0.701
0.121
θ range for data collecn (deg)
no. of rflns collected
4.05À68.25
11 836
2.55À69.48
15 087
2.84À26.37
16 681
1.93À25.02
21 334
no. of indep rflns (Rint
)
5122 (0.0237)
5122/12/308
1.052
6395 (0.0207)
6395/0/305
1.004
7992 (0.0462)
7992/1/444
1.041
11 433 (0.0614)
11 433/2/997
0.854
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R1
0.0433
0.1063
0.0578
0.1565
0.0578
0.1473
0.0476
0.0504
wR2
R indices (all data)
R1
0.0491
0.0664
0.0813
0.1455
wR2
0.1087
0.1591
0.1592
0.0627
largest diff peak and hole (e ÅÀ3
)
0.716, À0.949
0.50, À0.22
0.635, À0.419
0.140, À0.165
equimolar amount of benzyl alcohol. The conversion of ε-CL was
determined by H NMR spectroscopic analyses. Complex 2 (0.0174 g,
0.034 mmol) and complex 2 (0.0171 g, in 5.0 mL of toluene, 0.033
mmol) were added successively into the mixture. After the solution was
stirred for 15 min, the polymerization was terminated by addition of
several drops of glacial acetic acid. After it was stirred for 0.5 h at room
temperature, the resulting solution was dropped into cool methanol with
stirring. The viscous precipitate was collected and dissolved in dichlor-
omethane, and the solution was dropped into n-hexane. The white
precipitate was collected by filtration, washed with methanol, and dried
under vacuum, giving a white solid (0.7201 g, 75%). For GPC analysis,
the sample was dissolved in dichloromethane, passed through a short
neutral aluminum oxide column, precipitated in methanol, and dried
under vacuum.
Kinetic Studies. A typical kinetic study procedure was exemplified
by the polymerization of 200 equiv of ε-CL using complex 2 as a catalyst
in the presence of an equimolar amount of benzyl alcohol. Complex 2
(0.0174 g, 0.034 mmol) and toluene (3.04 mL) were added successively
into a Schlenk tube. After the complex dissolved, the Schlenk tube was
placed in an oil bath, the temperature of which was preset at 40 °C. The
mixture was stirred for 10 min, and then benzyl alcohol (0.34 mL, 0.1 M
in toluene, 0.034 mmol) and ε-CL (0.776 g, 6.8 mmol) were added
successively into the Schlenk tube. Samples were taken from the reaction
mixture using a syringe at a 15 min interval. The polymerizations of the
samples taken from the Schlenk tube were terminated by addition of a
drop of glacial acetic acid. Each of the samples was measured by 1H NMR
to calculate the conversion of ε-CL and the value of ln([M]0/[M]). The
other part of each sample was diluted with dichloromethane and passed
through a short neutral aluminum oxide column. The eluate was
concentrated and then dropped into cool methanol with stirring.
1
0.034 mmol) and toluene (3.04 mL) were added successively into a
Schlenk tube. Then the Schlenk tube was placed in an oil bath, the
temperature of which was preset at 70 °C. After the complex dissolved,
benzyl alcohol (0.34 mL, 0.1 M in toluene, 0.034 mmol) and ε-CL
(0.7725 g, 6.77 mmol) were added successively into the Schlenk tube.
After the solution was stirred for 30 min, the polymerization was
terminated by addition of several drops of glacial acetic acid. After it
was stirred for 0.5 h at room temperature, the resulting viscous solution
was diluted with dichloromethane and then dropped into cool methanol
with stirring. The white precipitate was collected by filtration under
reduced pressure, washed once with cool methanol, and dried under
vacuum, giving a white solid (0.7269 g, 94%). For GPC analysis, the
sample was dissolved in dichloromethane, passed through a short neutral
aluminum oxide column, precipitated in methanol, and dried under
vacuum.
Polymerization of rac-Lactide Catalyzed by 2 and 5aÀc. All
the polymerizations were conducted under the same concentration of
monomer, which was 0.5 M. A typical polymerization procedure was
exemplified by the synthesis of PLA using complex 2 as a catalyst in the
presence of an equimolar amount of benzyl alcohol. The conversion of
rac-lactide was determined by 1H NMR spectroscopic analyses. rac-LA
(0.9611 g, 6.67 mmol) and toluene (8.0 mL) were added into a Schlenk
tube. The Schlenk tube was heated, and the rac-LA dissolved. When the
solution was cooled to about 70 °C, the Schlenk tube was placed in an oil
bath, the temperature of which was preset at 70 °C. After the mixture was
stirred at 70 °C for 30 min, benzyl alcohol (0.34 mL, 0.1 M in toluene,
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Organometallics
ARTICLE
The white precipitates were collected by filtration under reduced
pressure, washed with cool methanol, and dried under vacuum. The
dried polymer was used for GPC analysis.
(13) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; White,
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’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving X-ray crystal
b
structure data for complexes 2, 3, 5a, and 6b. This material is
(17) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S.
Macromol. Rapid Commun. 1997, 18, 325.
available free of charge via the Internet at http://pubs.acs.org.
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Bruce, M. R. M.; Bruce, A. E. Inorg. Chem. 1993, 32, 2202.
(19) Akermark, B.; Krakenberger, B.; Hansson, S. Organometallics
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(21) Sheldrick, G. M. SHELXL97, Programs for Structure Refinement;
Universit€at G€ottingen, G€ottingen, Germany, 1997.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: zxwang@ustc.edu.cn.
’ ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China
(Grant No. 20972146) and the Foundation for Talents of Anhui
Province (Grant No. 2008Z011) for financial support. We also
thank Prof. D.-Q. Wang and Dr. S.-M. Zhou for the crystal
structure determinination and Prof. S.-Y. Liu for the GPC
analysis.
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