Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(rac-lactide) Synthesis

Article abstract of DOI:10.1021/acs.inorgchem.5b02709

A high degree of chain end control in the isoselective ring-opening polymerization (ROP) of rac-lactide is a challenging research goal. In this work, eight highly active sodium and potassium phenolates as highly isoselective catalysts for the ROP of rac-l

Full text of DOI:10.1021/acs.inorgchem.5b02709

Article
pubs.acs.org/IC
Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(rac-
lactide) Synthesis
Yangyang Sun, Jiao Xiong, Zhongran Dai, Xiaobo Pan, Ning Tang, and Jincai Wu*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Key Laboratory of Nonferrous
Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou University, Lanzhou 730000, People’s Republic of China
S
* Supporting Information
ABSTRACT: A high degree of chain end control in the
isoselective ring-opening polymerization (ROP) of rac-lactide
is a challenging research goal. In this work, eight highly active
sodium and potassium phenolates as highly isoselective
catalysts for the ROP of rac-lactide are reported. The best
isoselectivity value of Pm = 0.94 is achieved. The isoselective
mechanism is chain-end control through the analysis of the
stereoerrors in the microstructure of a final polymer; thus,
isotactic multiblock structure polymers are obtained, and the
highest melt point can reach 192.5 °C. The donating group in phenolate can clearly accelerate the ROP reaction, potassium
complexes are more active than the analogous sodium complexes, and the big spacial hindrance of the ligand can decrease the
activity. The high isoselectivities of these complexes mostly result from their sandwich structure constructed by the plane of the
crown and the plane of the anthryl group.
suffer from their low activities.6 The isoselectivities of new
INTRODUCTION
■
emerging systems, such as yttrium phosphasalen alkoxides,7
Stereoselective polymerization of chiral and prochiral mono-
zinc amidooxazolinates,8 and other complexes,9 need to be
mers is a valuable research subject because different tactic
improved further.
polymers will own different chemical and physical characters.1
Sodium and potassium are nontoxic and cheap elements, and
Obviously, catalysts play a key role in this area, among which a
are suitable for the catalytic synthesis of polylactides,10
metal catalyst with a suitable ligand is very attractive for the
especially in medical-related fields. Recently, we discovered
high efficiency, high selectivity, and easy tuning of the ligand.2
that several sodium and potassium phenolate crown ether
Despite the fact that many stereoselective metal catalysts have
been reported, sodium and potassium complexes as stereo-
selective catalysts with the real active center exclusively on the
sodium or potassium ions have not been well developed. In the
complexes can stereoselectively catalyze the ROP of rac-lactide,
giving isotactic polylactide (A−C in Chart 1).11 However,
compared with the highly isoselective aluminum−Salen or
aluminum−Salan system, the isoselectivities are not very high.
1980s, Donald J. Cram et al. reported that several chiral crown
Thus, in this work we try to improve the sodium/potassium
ether potassium complexes can catalyze Michael addition
phenolate crown ether system further with an attempt to get
reactions with high asymmetric induction and the polymer-
better isotacticities. We also want to understand this good
ization of methacrylate esters, giving isotactic polymers,3 which
system in detail via the study of the structure factors and
changed our bias on the ability of sodium and potassium
reaction mechanism.
complexes in stereoselective reactions and encouraged us to
explore more stereoselective sodium or potassium metal
complexes as catalysts for polymerization reactions or organic
reactions.
Herein we extend the application of sodium/potassium
complexes to the stereoselective synthesis of polylactide (PLA),
because polylactide is an important bioderived polymer in wide
fields such as packaging, fiber technology, and medicine.4 As we
know, the physical and chemical properties of polylactide highly
associate with its stereoregularity. The stereocontrolled ring-
opening polymerization (ROP) of rac-lactide is still a valuable
and challenging research goal.5 Especially, the isoselective ROP
of rac-lactide needs to be explored further because some
problems have not been overcome. For example, the excellent
isoselective aluminum−Salen or aluminum−Salan initiators
RESULTS AND DISCUSSION
■
Synthesis and Structures of Sandwich-Type Alkali-
Metal Phenolates 1−8. Our previous results show the bent
xanthenyl plane of the ortho position in the phenol ligand is
important for the isoselectivity of the ROP of rac-lactide (A−C
in Chart 1).11 Here, we changed the previous bent xanthenyl
plane to a standard planar substituted group of anthryl (D in
Chart 1, Scheme 1) to adjust the subtle confined space between
the plane of the crown and the plane of the substituted group of
phenol; a stronger interaction between the monomer and the
Received: August 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02709
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Article
a
Chart 1. Alkali-Metal Crown Ether Complexes
a
The planar xanthenyl/anthryl groups are replaced with red lines in
the right molecular structures to present their different extended
orientations. Crown = 15-crown-5 or 18-crown-6. M = K or Na.
Figure 1. Molecular structures of 1, 3, 4, and 8 with probability
ellipsoids at 30% (all of the hydrogen atoms are omitted for clarity).
Selected bond lengths (Å): complex 1, K1−O1 = 2.5737(18), C1−O1
= 1.289(3); complex 3, K1−O1 = 2.444(5), C1−O1 = 1.289(6);
complex 4, K1−O1 = 2.498(2), C1−O1 = 1.290(4); complex 8, Na1−
O1 = 2.1478(18), C1−O1 = 1.287(3).
Scheme 1. Synthesis of Complexes 1−8
substituent group. The Na1−O1 bond distance of 2.1478(18)
Å in sodium analogue 8 is shorter than the K1−O1 bond
distance of 2.498(2) Å in complex 4. The four crystal structures
demonstrate a very important feature of these complexes, that
all K+/Na+ active centers are partially sandwiched between the
plane of the crown ether and the plane of the anthryl group.
Comparatively, the surroundings of the active K1 center of
complexes 1−4 are less bulky than that in the potassium
complex of A and more crowded than that in the potassium
complexes of B and C due to the different extended orientation
of the substituted planar group (Chart 1).11 As we know, the
catalytic activity and stereoselectivity are sensitive to the
environment of the active center. The visually different
distances of K1/Na1−O1 and spacial orientations of the
crown ether and planar anthryl group may lead to different
catalytic behaviors in the ROP of rac-lactide.
Controllability of the ROP of rac-Lactide and
Structure Influences of the Catalysts. Complex 1 is a
highly active catalyst for the polymerization of rac-lactide
because the polymerization with a 100/1/1 [rac-LA]0/[1]0/
[BnOH]0 ratio can be accomplished in 93% conversion within
2 min at room temperature in 5 mL of toluene (Table 1, entry
2, [cat.]0 = 2.0 mM). Only 68% of the monomer was consumed
in THF within 30 min, the slow reaction rate can be attributed
to the competitive coordination of THF to inhibit the
coordination of lactide to K+ (Table 1, entry 1). All complexes
show high activities (Table 1, entries 3−9) with toluene as the
solvent. The sequence of activities is 3 > 2 > 1 for the three
similar potassium complexes (Table 1, entries 2−4), which
suggests the electron-donating group can accelerate the ROP
reaction. In other words, the more negatively charged phenoxy
anion is more active. Compared with complex 2, the low
active end of the polymer chain can be expected in this special
confined space. Four phenol ligands, H1L−H4L, were
synthesized according to a literature method.12 Complexes
1−8 were obtained in high yields via the reactions of the
corresponding ligand with a stoichiometric amount of 18-
crown-6/15-crown-5 and KN(SiMe3)2/NaN(SiMe3)2 in THF
(Scheme 1). Crystals of complexes 1, 3, 4, and 8 suitable for
single-crystal X-ray diffraction were isolated from toluene,
bezene, or glycol dimethyl ether solutions. Their molecular
structures are shown in Figure 1. Compared to complex 1, O1
of the phenoxy anion in complexes 3 and 4 is more negatively
charged due to the electronic-donating ability of the methoxy
group and two tert-butyl groups, which can give rise to a
stronger interaction between K1 and O1. Thus, the K1−O1
bond distances of 2.444(5) Å in complex 3 and 2.498(2) Å in
complex 4 are shorter than that of 2.5737(18) Å in complex 1.
Despite the repulsion between the crown ether and tert-butyl
group at the ortho position, the K1−O1 bond in complex 4 is
still shorter than that in complex 1, which indicates the K1−O1
bond is affected significantly by the electronic nature of the
B
DOI: 10.1021/acs.inorgchem.5b02709
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a
Table 1. rac-Lactide Polymerization Catalyzed by 1−8
b
c
d
e
entry
cat.
[cat.]0/[rac-LA]0/[BnOH]0
t (min)
conversion (%)
Mn,obsd (g/mol)
Mn,calcd (g/mol)
PDI
Pm
f
1
1
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/20/1
30
2
68
93
99
99
99
99
85
92
99
95
92
93
96
99
96
94
7200
12800
11600
13500
14400
17200
12700
13300
14100
2500
9900
13500
14400
14400
14400
14400
12300
13400
14400
2800
1.12
1.11
1.12
1.11
1.14
1.22
1.15
1.19
1.35
1.19
1.12
1.10
1.03
1.08
1.39
1.46
0.70
0.72
0.73
0.65
0.72
0.67
0.66
0.64
0.67
0.71
0.73
0.74
0.75
0.70
0.70
0.71
2
3
2
4
1.5
5
5
6
3
7
2
8
2
9
5
10
11
12
13
2
1/200/1
1/300/1
1/400/1
1/1000/50
1/20/0
3
26600
39400
53100
2600
26600
40300
55400
3000
3
3
g
14
3
15
16
17
2
14900
18600
2800
1/50/0
2
6800
h
h
h
h
h
1/100/0
2
92 (96)
12600 (19000)
13400 (13800)
1.44 (1.30)
0.71 (0.72)
a
Conditions: reaction carried out under a dry nitrogen atmosphere, 0.01 mmol of catalyst, 5 mL of toluene, at room temperature, in addition to the
b
c
1
annotated conditions. Determined by H NMR spectroscopy. Experimental Mn and PDI determined by GPC in THF against polystyrene
standards and corrected using the factor 0.58.18 Calculated from the molecular weight of rac-LA × [LA]0/[BnOH]0 × conversion yield + MBnOH
.
d
e
f
g
Determined by analysis of all of the tetrad signals in the methine region of the homonuclear-decoupled 1H NMR spectrum.7,13 In THF. In 10 mL
h
of toluene. The data in parentheses were obtained from parallel experiments.
a
Table 2. rac-Lactide Polymerization Catalyzed by 1−8 at −70 °C
b
c
d
e
f
entry
cat.
[cat.]0/[rac-LA]0/[BnOH]0
t (h)
conversion (%)
Mn,obsd (g/mol)
Mn,calcd (g/mol)
PDI
Pm
Tm (°C)
1
1
1
1
1
2
3
4
5
6
7
8
1/20/1
10
8
92
92
90
93
94
94
96
92
86
77
95
2300
12300
12000
26500
14000
15200
11300
12500
12000
10300
13900
2800
13400
13100
26900
13600
13600
14000
13400
12500
11200
13800
1.09
1.06
1.05
1.05
1.07
1.04
1.09
1.05
1.06
1.09
1.05
0.89
0.90
0.92
0.94
0.88
0.91
0.89
0.86
0.85
0.82
0.81
185.2
186.3
190.4
192.5
184.9
189.4
185.5
167.7
161.3
(155.4)
(151.6)
g
2
1/100/1
1/100/1
1/200/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
3
4
10
16
6
5
6
4
7
12
18
12
8
8
9
10
11
20
a
Conditions: reactions carried out under a dry nitrogen atmosphere, 0.01 mmol of catalyst, 5 mL of toluene, at −70 °C, in addition to the annotated
b
c
1
conditions. Determined by H NMR spectroscopy. Experimental Mn and PDI determined by GPC in THF against polystyrene standards and
corrected using the factor 0.58.18 Calculated from the molecular weight of rac-LA × [LA]0/[BnOH]0 × conversion yield + MBnOH. Determined by
d
e
analysis of all of the tetrad signals in the methine region of the homonuclear-decoupled 1H NMR spectrum.7,13 Measured by DSC. Tm values were
f
recorded in the second run. The data in parentheses were obtained in the first run because the Tm could not be detected in the second run due to
g
slow crystallization. At −60 °C.
activity of complex 4 illustrates the bulky tert-butyl group at
another ortho position will slightly hinder the incoming lactide
and decrease the ROP reaction rate (Table 1, entry 5). The
same trend can also be found in the sodium analogues 5−8
(Table 1, entries 6−9). Compared to potassium complexes 1−
4, sodium complexes 5−8 are slightly less active. The
environment around the sodium ion is more crowded than
that around the potassium ion in complexes 1−4 for the smaller
sodium ion radius. Consequently, the transport of lactide or
BnOH to the active center will be slightly difficult in the
sodium complexes. On the other hand, the oxyen of the
phenoxy anion in the sodium complexes is less negatively
charged because the Na−O bond is less ionic than the K−O
bond, which can also lead to sodium complexes being less
active. This trend is similar to that of other analogous sodium
and potassium complexes.11b,c The polymerizations catalyzed
by complexes 1−8 are living, which can be proved by the
controllable molecular weights and low molecular weight
distributions (Table 1, entries 1−9) and can be confirmed
further by the fact that the molecular weights of polymers with
complex 1 as a catalyst have a linear relationship with the ratio
of [rac-LA]0 to [BnOH]0 (Table 1, entries 2 and 10−13, Figure
S1). Complex 1 also can catalyze the immortal ROP of 1000
equiv of rac-lactide in the presence of 50 equiv of external
C
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BnOH as a co-initiator, affording a controlled molecular weight
and a relatively narrow PDI (Table 1, entry 14). In the absence
of a co-initiator of BnOH, the ROP of rac-LA still proceeds
quickly, but the molecular weights are not under control and
the polydispersity is high (Table 1, entries 15−17).
temperature. All the above catalytic results show that the
activity and isoselectivity of these types of potassium/sodium
complexes are sensitive to the subtle confined space between
the crown ether and the planar anthryl group. A best Pm value
of 0.94 was achieved (Table 2, entry 4) for complex 1 when the
ratio [rac-LA]0/[1]0/[BnOH]0 was 200/1/1. This value is the
highest recorded for alkali-metal complexes until now, which
means this system is essentially comparable to the good Salen−
Al system.
Stereoselectivity of the ROP of rac-Lactide, Structure
Influences of the Catalysts, and Microstructure of
Poly(rac-lactide). As shown in Table 1, modest isoselective
Pm values ranging from 0.64 to 0.75 for complexes 1−8 can be
achieved at room temperature in toluene, which are higher than
the Pm value of 0.55 for a similar potassium crown ether
complex supported with a simple 2,4-di-tert-butylphenol ligand
(Table S1, entry 1). Compared with THF, toluene is chosen as
the solvent because a higher activity can be obtained in toluene
using complex 1 as a catalyst, which can allow us to study the
catalytic behavior at low temperature (Table 1, entries 1 and 2).
The isoselectivities of potassium complexes are better than
those of sodium complexes at room temperature possibly
because the confined space of the sodium complexes is smaller
due to the smaller plane of 15-crown-5 than 18-crown-6. The
isoselectivities of complexes 1, 2, and 4 are similar, and the Pm
value of 0.65 for complex 3 is the lowest one in the four
potassium complexes. Compared to those of complexes 1, 2,
and 4, the ROP reaction is very quick for complex 3 due to the
strong electron-donating ability of the methoxy group, which
hints that the energy barrier of ROP is very low.
1
Typical H NMR, 13C NMR, and homonuclear-decoupled
spectra of the methine region of poly(rac-lactide) [poly(rac-
LA)] using 1 as a catalyst are shown in Figure 2A and Figure S2
(Table 2, entry 4).7,13 The high isotacticities of the polymer can
also be confirmed by high melting points as shown in Table 2;
for example, the polymer with an isotacity of Pm = 0.94
demonstrates a high Tm of 192.5 °C (Figure 2B). It should be
noted that the Pm value determined from 13C NMR for this
With an attempt to improve the isoselectivity of the ROP of
rac-lactide, we decreased the reaction temperature to −70 °C,
and the reaction rate decreased remarkably. All polymers
obtained are isotactically enriched with enhanced Pm values of
0.88−0.94 when using potassium complexes 1−4 as the
catalysts (Table 2, entries 1−7). The isoselectivity of complex
3 becomes similar to those of other potassium complexes. The
o-tert-butyl group in complex 4 initially was designed to
sandwich the active center well to increase the isoselectivity;
however, compared to the similar complex 2, the catalytic result
sees no remarkable increase in the isoselectivity. This indicates
that the effect of the tert-butyl group at the ortho position is not
significant enough to increase the isoselectivity and the high
isoselectivity mostly results from the sandwich structure
constructed by the plane of the crown and the plane of the
anthryl group. The simple potassium crown ether complex
supported with a simple 2,4-di-tert-butylphenol ligand also
exhibited a high isoselectivity of Pm = 0.83 at −70 °C (Table
S1, entry 2), which clearly indicates the sandwich structure is
vital in this system again. However, the almost atactic polymers
with Pm = 0.55 obtained at room temperature (Table S1, entry
1) suggest the steric hindrance provided by the substituted
group at the ortho position is also important. The isoselectivity
of Pm = 0.90 for complex 1 is better than Pm = 0.86 for
potassium complex A in Chart 1 at −60 °C under the same
conditions (Table 2, entry 2). What is more, because the
surroundings of the active center are too crowded, the ROP of
lactide cannot proceed at room temperature with complex A as
a catalyst, while complexes 1−8 can overcome the anti-
Arrhenius behavior of potassium complex A.11a In addition,
changing the previous bent xanthenyl plane to a standard planar
substituted group of anthryl, the active center is sandwiched
well and the high activity, which was sacrificed in systems B and
C (Chart 1), is reserved even at −70 °C.11b,c The
isoselectivities of sodium complexes 5−8 also increased sharply
to above 0.80, although the Pm values are still lower than those
of the potassium analogues, the same as the trend at room
Figure 2. Characterization of poly(rac-lactide) by (A) the methine
region of the (a) 1H NMR spectrum and (b) homonuclear-decoupled
1H NMR spectrum (Table 2, entry 4)7,13 and (B) DSC thermograms
of the second heating cycles of the samples of PLLA and poly(rac-
lactide)s with different Pm values [(a) PLLA, (b) Pm = 0.88 (Table 2,
entry 5), (c) Pm = 0.91 (Table 2, entry 6), (d) Pm = 0.94 (Table 2,
entry 4)]. (C) Pm values determined for all tetrads on the basis of
Bernoulli statistics. Their average values were used (Table 2, entry 4).
(D) Relationship between the Pm values and the melting points.
D
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Article
sample is 0.92, which is comparable to the result from the
determination by homonuclear-decoupled 1H NMR. The
stereoerrors in the microscopic structure of a final polymer
with a Pm of 0.94 were analyzed by the tetrad signals in the
Tabernero and Cano et al.10k These results suggest that the
mechanism in the absence of an alcohol should be different
from the reaction in the presence of alcohol. As Tabernero and
Cano et al. pointed out, in the absence of an alcohol, ROP
proceeds via a coordination−insertion mechanisum, while the
ligand-assisted monomer-activated mechanism is suitable for
the controllable ROP reaction in the presence of BnOH.
1
homonuclear-decoupled H NMR spectrum (Table 2, entry 4;
Figure 2C).6q,8,11c The signal intensity ratio of rmr, rmm, mmr,
and mrm tetrad peaks was about 1/11/11/12. The isoslectivity
calculation was based on the Bernoullian statistics of tetrads
according to refs 6e, 6q, 13c, and 14. The theoretical ratios
between these four tetrad peaks are close to the experimetal
values. For example, the experimental value of [rmr]/[mrm] =
1/12 (8.3%) agrees well with the value of [rmr]/[mrm] = [(1
CONCLUSIONS
■
The highly isoselective and living polymerization of rac-LA
giving isotactic multiblock poly(rac-LA) catalyzed by a series of
sodium and potassium o-(9-anthryl)phenolates was reported in
this work. The donating group in phenolate can clearly
accelerate the ROP reaction. The high isoselectivities of these
complexes mostly result from their sandwich structure
constructed by the plane of the crown and the plane of the
anthryl group. The best isotacticity of Pm = 0.94 achieved here
was the highest recorded value for alkali-metal complexes,
which demonstrates that this system is comparable to that of
known stereoselective aluminum, rare-earth-metal, and zinc
catalysis systems. These catalyst structures presented a new
catalyst design feature for the stereoselective polymerization of
cyclic esters: associating a crown ether (dipole interaction) and
a bulky phenoxide (ionic interaction) with a Lewis acidic
monocation, Na+ or K+, and embedding the active center in a
confined space. Considering sodium and potassium are
innocuous, abundant elements in the human body, it is
valuable to improve this system for the ROP of rac-lactide, and
we believe this type of catalyst can potentially be applied in
future industry after a suitable elevation of this system.
− Pm)2/2]/[(1 − Pm)/2] = 1 − Pm (8%) obtained from the 13
C
NMR determination and is also very close to the 1 − Pm value
obtained from 1H NMR determination. Therefore, the
isoselective mechanism of this system can be considered as a
chain end control mechanism.15 It should be noted that the
high Tm values accord well with the empirical function
relationship between the isotacticity and melting temperature
of the multiblock stereocopolymer of poly(rac-lactide) reported
by Nomura et al. (Figure 2D),16 which also can prove that the
chain end control stereoselective mechanism is suitable for this
system. Both the microscopic structure and the typical high Tm
values hint that these final poly(rac-lactide)s are multiblock
stereocopolymers. The calculation based on the best isotacticity
of Pm = 0.94 suggests the average isotactic length can reach 15
(30 lactic acid units).16
End group analysis of the final polymer by 1H NMR
spectroscopy revealed that all polymer chains are end-capped
by one benzyl ester and a hydroxyl (Table 2, entry 1; Figure
S3). The ESI mass spectrum of a final polymer (Table 2, entry
1; Figure S4) confirmed this further by a series of peaks at
144m + 108 + 18 with a charge of +1, which can be assigned to
EXPERIMENTAL SECTION
General Considerations. All syntheses and manipulations of air-
and moisture-sensitive materials were performed under a dry nitrogen
■
+
m(C6H8O4) + BnOH + NH4 . It should be noted that there is
1
another series of peaks at 0.5(144m + 108 + 18 + 18) with a
charge of +2, which can be attributed to m(C6H8O4) + BnOH
atmosphere using standard Schlenk techniques or in a glovebox. H
NMR and 13C NMR spectra were recorded on a Varian Mercury Plus
300 MHz spectrometer, a JNM-ECS 400 MHz spectrometer, and a
+
+
+ NH4 + NH4 . Both series of peaks with a difference in
molecular mass of ∼144 Da suggest no serious trans-
esterification reaction happens during this polymerization
process. This information indicates that BnOH is a real
initiator for the ROP of lactide. Therefore, as the monomer-
activated mechanism is supposed for most alkali-metal
phenoxides, the lactide can be activated after being coordinated
to K+ or Na+, and consequently, the activated BnOH can attack
the carbonyl group to initiate the ROP reaction.10h,j However,
in this mechanism the role of the phenoxy is not well explained.
Another ligand-assisted monomer-activated mechanism pro-
posed by Davidson et al. in sodium aryloxide complex
systems10e and Carpentier et al. in rare-earth-metal and
alkaline-earth-metal cationic complex systems17 seems to be
suitable for this system because we found that the ROP reaction
rate and stereoselectivity is highly related to the metal ion and
phenoxy structure. For example, the electron-donating groups
can remarkably increase the ROP reaction rate. It is reasonably
explained that the phenoxy with an electron-donating group is
more basic and more easily actives an alcohol via a hydrogen
bond to accelerate the ROP reaction. The immortal ROP
results also match this mechanism well. It should be noted that,
in the absence of BnOH, no end groups were observed in the
1HNMR spectrum of the resultant PLA (Figure S5; Table 1,
entry 16). The MALDI-TOF experiment suggested the
polymer (Table 1, entry 16) is cyclic polylactide (Figure S6).
This phenomenon has been reported and explained by
1
Varian INOVA 600 MHz spectrometer. H NMR chemical shifts are
reported in parts per million versus residual protons in deuterated
solvents as follows: δ 7.26 ppm for chloroform-d, δ 7.16 ppm for
benzene-d6, δ 2.08 ppm for toluene-d8. 13C NMR chemical shifts are
reported in parts per million versus residual 13C in the solvent: δ 77.0
ppm for chloroform-d, δ 128.06 ppm for benzene-d6, δ 137.48 ppm for
toluene-d8. The elemental compositions of the complexes were
measured using an Elemental Vario EL series CHN analyzer with
the samples under a nitrogen atmosphere. The molecular weights (Mn
and Mw) and the molecular weight distributions (Mw/Mn) of the
polymer samples were measured by gel permeation chromatography
(GPC) at 25 °C using THF as a solvent, an eluent flow rate of 1 mL/
min, and narrow polystyrene standards as reference samples. The
measurements were performed using a Waters 1525 binary system that
was equipped with a Waters 2414 RI detector using two Styragel
columns (102−106 kg/mol). Each reported value was corrected using a
factor of 0.58 for polylactide according to the literature.18 Calorimetric
measurements were conducted using a Sapphire DSC apparatus
manufactured by PerkinElmer Instruments. Polymer samples (5.0 mg)
were placed in aluminum pans and heated/cooled at a rate of 10 °C
min−1 under a nitrogen atmosphere. The previous thermal history of
the samples was erased by heating them to 200 °C before cooling
them to 0 °C. Measurements were performed between 30 and 220 °C.
The melting temperatures were evaluated as the maxima of the melting
endotherms. The ESI mass spectroscopic data were obtained using a
Thermo Scientific Orbitrap Elite mass spectrometer (LTQ Orbitrap
Elite). The MALDI-TOF mass spectroscopic data were obtained using
α-cyano-4-hydroxycinnamic acid as the matrix in a Bruker Daltonics,
Inc. BIFLEX III MALDI-TOF mass spectrometer.
E
DOI: 10.1021/acs.inorgchem.5b02709
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Materials. Toluene, THF, and hexane were dried by refluxing with
sodium and benzophenone ketyl; the latter serves as an indicator.
CH2Cl2 was distilled from P2O5. rac-LA was purchased from Daigang
BIO Engineer Ltd. of China and was recrystallized from toluene three
times. CDCl3 was purchased from J&K Scientific, Ltd. in Beijing and
was dried over activated molecular sieves. KN(SiMe3)2 and NaN-
(SiMe3)2 were purchased from J&K Scientific Ltd. and used as
received. 18-Crown-6 and 15-crown-5 were purchased from local
companies and were used as received.
was slowly added KN(SiMe3)2 (1.0 mL of a 1.00 M solution in THF,
1.0 mmol) at 0 °C under a nitrogen atmosphere. During this process,
the colorless solution changed to red. After the solution was stirred for
4 h at room temperature, the red precipitate formed was separated by
filtration. The solid residue was washed with 20 mL of hexane and
dried in vacuo to give complex 1 as a red powder (0.438 g, 77%). Red
crystals of 1 suitable for X-ray diffraction studies were obtained from a
1
benzene solution at room temperature. H NMR (400 MHz, toluene-
d8, 25 °C): δ 8.52 (d, J = 8.8 Hz, Ar-H, 1H), 8.10 (s, Ar-H, 1H), 7.81
(d, J = 8.4 Hz, Ar-H, 2H), 7.51 (t, J = 7.6 Hz, Ar-H, 1H), 7.33−7.22
(m, Ar-H, 3H), 7.21−7.10 (m, Ar-H, 3H), 6.71 (t, J = 7.0 Hz, 1H),
2.92 (s, crown ether-H, 24H). 13C NMR (100 MHz, toluene-d8, 50
°C): δ 132.91, 132.71, 132.06, 131.12, 129.93, 126.46, 123.63, 123.40,
121.08, 70.05. Anal. Calcd for C32H37KO7: C, 67.37; H, 6.23. Found:
C, 67.11; H, 6.51.
Syntheses. Synthesis of 2-(9-Anthryl)phenol (H1L). A sample of 2-
bromophenol (1.88 g, 11 mmol) was dissolved in 80 mL of THF and
cooled to −20 °C using a low-temperature reactor. Under a nitrogen
n
atmosphere 22 mmol of BuLi (8.8 mL, 2.5 M hexane solution) was
added. The solution became white and was slowly warmed to room
temperature over the course of 2 h. To this solution was slowly added
11 mmol of anthrone (2.13 g dissolved in 100 mL of THF), and the
yellow solution which formed was stirred overnight. The solution was
hydrolyzed with 30 mL of 6 M HCl and stirred for 1 h. The mixture
was extracted with diethyl ether. The organic layer was washed three
times with 500 mL of water, dried with MgSO4, and evaporated to
dryness, affording a brown solid. This crude product was purified by
column chromatography over silica gel (CH2Cl2/petroleum ether = 1/
5), affording 0.96 g (32%) of 2-(9-anthryl)phenol as an off-white solid.
1H NMR (300 MHz, chloroform-d, 25 °C): δ 8.52 (s, Ar-H, 1H), 8.03
(d, J = 8.4 Hz, Ar-H, 2H), 7.65 (d, J = 8.7 Hz, Ar-H, 2H), 7.53−7.35
(m, Ar-H, 5H), 7.25 (dd, J = 7.5, 1.8 Hz, Ar-H, 1H), 7.17−7.09 (m,
Ar-H, 2H), 4.49 (s, −OH, 1H). 13C NMR (100 MHz, chloroform-d,
25 °C): δ 153.73, 132.12, 131.50, 130.79, 129.83, 129.63, 128.56,
127.96, 126.33, 126.01, 125.50, 123.95, 120.69, 115.70. Anal. Calcd for
C20H14O: C, 88.33; H, 4.99. Found: C, 88.76; H, 5.22.
Synthesis of Complex 2. According to the procedure described
above for 1, H2L (0.326 g, 1.0 mmol) was converted to complex 2 as a
1
red brown powder (0.502 g, 80%). H NMR (300 MHz, benzene-d6,
25 °C): δ 8.78 (d, J = 8.5 Hz, Ar-H, 2H), 8.17 (s, Ar-H, 1H), 7.88 (d, J
= 9.5 Hz, Ar-H, 2H), 7.72 (dd, J = 8.6, 2.9 Hz, Ar-H, 1H), 7.48 (d, J =
2.9 Hz, Ar-H, 1H), 7.30 (ddd, J = 8.3, 6.4, 1.4 Hz, Ar-H, 2H), 7.22
(ddd, J = 8.6, 6.4, 1.5 Hz, Ar-H, 2H), 7.19 (s, Ar-H, 1H), 2.85 (s,
crown ether-H, 24H), 1.50 (s, ArC(CH3)3, 9H). 13C NMR (100 MHz,
benzene-d6, 50 °C): δ 132.75, 132.14, 131.39, 129.73, 126.39, 125.49,
124.86, 123.41, 120.35, 69.94, 33.93, 32.69. Anal. Calcd for
C36H45KO7: C, 68.89; H, 7.50. Found: C, 68.76; H, 7.21.
Synthesis of Complex 3. According to the procedure described
above for 1, H3L (0.300 g, 1.0 mmol) was converted to complex 3 as a
deep red powder (0.510 g, 85%). Clear red crystals of 3 suitable for X-
ray diffraction studies were obtained from a toluene solution at room
1
Synthesis of 2-(9-Anthryl)-4-tert-butylphenol (H2L). According to
the procedure described above for H1L, 2-bromo-4-tert-butylphenol
(1.14g, 5 mmol) was converted to 2-(9-anthryl)-4-tert-butylphenol as
temperature. H NMR (300 MHz, benzene-d6, 25 °C): δ 8.56 (d, J =
8.4 Hz, Ar-H, 2H), 8.20 (s, Ar-H, 1H), 7.87 (d, J = 8.0 Hz, Ar-H, 2H),
7.47−7.30 (m, Ar-H, 1H), 7.37−7.19 (m, Ar-H, 5H), 7.07 (d, J = 2.8
Hz, Ar-H, 1H), 3.54 (s, −OCH3, 3H), 2.99 (s, crown ether-H, 24H).
13C NMR (100 MHz, benzene-d6, 50 °C): δ 132.53, 131.71, 130.38,
125.57, 125.05, 124.68, 124.20, 118.62, 116.78, 70.21, 56.24. Anal.
Calcd for C33H39KO8: C, 65.99; H, 6.60. Found: C, 65.76; H, 6.52.
Synthesis of Complex 4. According to the procedure described
above for 1, H4L (0.382 g, 1.0 mmol) was converted to complex 4 as a
deep red powder (0.455 g, 67%). Red crystals of 4 suitable for X-ray
diffraction studies were obtained from a benzene solution at room
temperature. Because of the poor solubility of 4, the expected
1
an off-white powder (0.62 g, 38%). H NMR (300 MHz, chloroform-
d, 25 °C): δ 8.57 (s, Ar-H, 1H), 8.08 (d, J = 8.4 Hz, Ar-H, 2H), 7.70
(d, J = 8.9 Hz, Ar-H, 2H), 7.43−7. 57 (m, Ar-H, 3H), 7.42 (ddd, J =
8.6, 6.5, 1.4 Hz, Ar-H, 2H), 7.27 (d, J = 2.5 Hz, Ar-H, 1H), 7.09 (d, J =
8.6 Hz, Ar-H, 1H), 4.37 (s, −OH, 1H), 1.34 (s, ArC(CH3)3, 9H). 13
C
NMR (75 MHz, chloroform-d, 25 °C): δ 151.38, 143.40, 131.56,
130.82, 130.53, 129.03, 128.57, 127.78, 126.58, 126.26, 126.20, 125.47,
123.22, 115.04, 34.25, 31.62. Anal. Calcd for C24H22O: C, 88.66; H,
7.08. Found: C, 88.31; H, 6.79.
Synthesis of 2-(9-Anthryl)-4-methoxyphenol (H3L). According to
the procedure described above for H1L, 2-bromo-4-methoxyphenol
(1.01 g, 5 mmol) was converted to 2-(9-anthryl)-4-methoxyphenol as
a light yellow powder (0.56 g, 37%). 1H NMR (300 MHz, chloroform-
d, 25 °C): δ 8.54 (s, Ar-H, 1H), 8.05 (d, J = 8.3 Hz, Ar-H, 2H), 7.70
(d, J = 8.6 Hz, Ar-H, 2H), 7.48 (ddd, J = 8.3, 6.5, 1.4 Hz, Ar-H, 2H),
7.41 (ddd, J = 8.1, 6.5, 1.5 Hz, Ar-H, 2H), 7.09 (d, J = 8.9 Hz, Ar-H,
1H), 7.02 (dd, J = 8.9, 2.9 Hz, Ar-H, 1H), 6.81 (d, J = 2.8 Hz, Ar-H,
1H), 4.21 (s, −OH, 1H), 3.77 (s, −OCH3, 3H). 13C NMR (75 MHz,
chloroform-d, 25 °C): δ 153.47, 147.79, 131.50, 130.64, 129.73,
128.57, 127.99, 126.38, 126.03, 125.52, 124.49, 116.54, 116.46, 115.65,
55.76. Anal. Calcd for C20H16O: C, 88.63; H, 5.10. Found: C, 83.98;
H, 5.37.
information on 13C NMR was not achieved. H NMR (300 MHz,
1
benzene-d6, 25 °C): δ 8.76 (d, J = 8.7 Hz, Ar-H, 2H), 8.07 (s, Ar-H,
1H), 7.88 (d, J = 2.9 Hz, Ar-H, 1H), 7.82 (d, J = 7.6 Hz, Ar-H, 2H),
7.28 (d, J = 2.9 Hz, Ar-H, 1H), 7.24 (ddd, J = 8.2, 6.4, 1.4 Hz, Ar-H,
2H), 7.13−7.19 (m, Ar-H, 2H), 2.75 (s, crown ether-H, 24H), 2.02 (s,
ArC(CH3)3, 9H), 1.55 (s, ArC(CH3)3, 9H). Anal. Calcd for
C40H53KO7: C, 70.48; H, 7.92. Found: C, 70.14; H, 7.80.
Synthesis of Complex 5. According to the procedure described
abuove for 1, H1L (0.270 g, 1.0 mmol) was converted to give complex
5 as a brown powder (0.250 g, 49%). 1H NMR (300 MHz, benzene-d6,
25 °C): δ 8.64 (dd, J = 8.7, 1.2 Hz, 2H), 8.16 (s, Ar-H, 1H), 7.81−7.91
(m, Ar-H, 2H), 7.69 (ddd, J = 8.2, 7.0, 2.0 Hz, Ar-H, 1H), 7.46 (dd, J =
7.3, 2.0 Hz, Ar-H, 1H), 7.33 (dd, J = 8.3, 1.2 Hz, Ar-H, 1H), 7.24−7.30
(m, Ar-H, 2H), 7.20 (d, J = 1.4 Hz, Ar-H, 1H), 7.17−7.18 (m, Ar-H,
1H), 6.88 (td, J = 7.2, 1.3 Hz, Ar-H, 1H), 2.76 (s, crown ether-H,
20H). 13C NMR (100 MHz, benzene-d6, 50 °C): δ 132.90, 132.63,
131.97, 130.84, 129.91, 126.75, 124.92, 123.80, 123.68, 121.12, 68.94.
Anal. Calcd for C30H33NaO6: C, 70.12; H, 6.25. Found: C, 70.30; H,
6.49.
Synthesis of Complex 6. According to the procedure described
above for 1, H2L (0.326 g, 1.0 mmol) was converted to complex 6 as a
dark red powder (0.396 g, 70%). 1H NMR (300 MHz, benzene-d6, 25
°C): δ 8.70 (d, J = 8.9 Hz, Ar-H, 2H), 8.17 (s, Ar-H, 1H), 7.87 (d, J =
8.5 Hz, Ar-H, 2H), 7.70 (d, J = 10.6 Hz, Ar-H, 1H), 7.48 (d, J = 2.7
Hz, Ar-H, 1H), 7.16−7.35 (m, Ar-H, 5H), 2.73 (s, crown ether-H,
20H), 1.48 (s, ArC(CH3)3, 9H). 13C NMR (100 MHz, benzene-d6, 50
°C): δ 132.68, 132.02, 131.06, 130.88, 129.83, 129.64, 128.52, 126.55,
126.37, 125.58, 125.14, 124.73, 123.95, 123.77, 123.54, 123.41, 120.57,
Synthesis of 2-(9-Anthryl)-4,6-di-tert-butylphenol (H4L). Accord-
ing to the procedure described above for H1L, 2-bromo-4,6-di-tert-
butylphenol (1.42 g, 5 mmol) was converted to 2-(9-anthryl)-4,6-di-
tert-butylphenol as a pale brown powder (0.57 g, 30%). 1H NMR (300
MHz, chloroform-d, 25 °C): δ 8.52 (s, 1H), 8.02 (d, J = 7.9 Hz, Ar-H,
2H), 7.71 (d, J = 8.8 Hz, Ar-H, 2H), 7.51 (d, J = 2.4 Hz, Ar-H, 1H),
7.45 (ddd, J = 8.3, 6.6, 1.3 Hz, Ar-H, 2H), 7.39 (ddd, J = 8.2, 6.6, 1.6
Hz, Ar-H, 2H), 7.11 (d, J = 2.4 Hz, Ar-H, 1H), 4.54 (s, −OH, 1H),
1.50 (s, 9H, ArC(CH3)3), 1.34 (s, 9H, ArC(CH3)3). 13C NMR (100
MHz, chloroform-d, 25 °C): δ 149.91, 142.13, 135.36, 131.65, 131.10,
131.02, 128.58, 127.82, 126.50, 126.37, 126.27, 125.49, 123.81, 123.70,
35.17, 34.45, 31.77, 29.77. Anal. Calcd for C28H30O: C, 87.72; H, 7.68.
Found: C, 87.91; H, 7.90.
Synthesis of Complex 1. To a solution of H1L (0.270 g, 1.0 mmol)
and 18-crown-6 ether (0.264 g, 1.0 mmol) in tetrahydrofuran (20 mL)
F
DOI: 10.1021/acs.inorgchem.5b02709
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Inorganic Chemistry
Article
120.18, 69.26, 33.95, 32.69, 32.49. Anal. Calcd for C34H41NaO6: C,
71.03; H, 7.12. Found: C, 71.51; H, 7.27.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wujc@lzu.edu.cn.
Notes
■
Synthesis of Complex 7. According to the procedure described
above for 1, H3L (0.300 g, 1.0 mmol) was converted to give complex 7
1
as a red powder (0.385 g, 71%). H NMR (300 MHz, benzene-d6, 25
°C): δ 8.61 (d, J = 8.1 Hz, Ar-H, 2H), 8.17 (s, Ar-H, 1H), 7.87 (d, J =
8.3 Hz, Ar-H, 2H), 7.38 (dd, J = 8.8, 3.0 Hz, Ar-H, 1H), 7.24−7.34 (m,
Ar-H, 3H), 7.21 (d, J = 7.7 Hz, Ar-H, 2H), 7.10 (d, J = 3.0 Hz, Ar-H,
1H), 3.57 (s, −OCH3, 3H), 2.72 (s, crown ether-H, 20H). 13C NMR
(100 MHz, benzene-d6, 50 °C): δ 132.36, 131.52, 129.15, 127.76,
125.77, 125.24, 124.90, 119.65, 118.14, 116.38, 69.96, 55.81. Anal.
Calcd for C31H35NaO7: C, 67.86; H, 6.72. Found: C, 68.62; H, 6.50.
Synthesis of Complex 8. According to the procedure described
above for 1, H4L (0.382 g, 1.0 mmol) was converted to complex 8 as a
red-brown powder (0.468 g, 78%). Clear brown crystals of 8 suitable
for X-ray diffraction studies were obtained from a glycol dimethyl ether
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Grants 21271092, 21171078, and
21401161), the Science Foundation of Gansu Province of
China (Grant 1308RJ2A121), the project for the National Basic
Science Personnel Training Fund (Grant J1103307), and the
ScGrid of the Supercomputing Center of the Chinese Academy
of Sciences is gratefully acknowledged. We thank Dr. Qingfeng
Wu at the Institute of Modern Physics Chinese Academy of
Sciences for the MALDI-TOF mass measurement.
1
solution at room temperature. H NMR (300 MHz, benzene-d6, 25
°C): δ 8.69 (d, J = 8.7 Hz, 2H), 8.08 (s, Ar-H, 1H), 7.88 (d, J = 2.8 Hz,
Ar-H, 1H), 7.82 (d, J = 8.4 Hz, Ar-H, 2H), 7.27 (d, J = 2.8 Hz, Ar-H,
1H), 7.24 (ddd, J = 8.2, 6.4, 1.3 Hz, Ar-H, 2H), 7.13 (dd, J = 6.5, 1.4
Hz, Ar-H, 2H), 2.47−2.75 (m, crown ether-H, 20H), 2.01 (s,
ArC(CH3)3), 9H), 1.54 (s, ArC(CH3)3), 9H). 13C NMR (100 MHz,
benzene-d6, 50 °C): δ 136.19, 132.86, 132.26, 131.57, 128.51, 126.23,
124.87, 123.36, 122.81, 68.61, 36.15, 34.26, 32.84, 30.74. Anal. Calcd
for C38H49NaO6: C, 72.97; H, 7.83. Found: C, 73.05; H, 7.91.
General Procedure for Polymerization of rac-Lactide. A
typical polymerization procedure is illustrated by the synthesis of PLA
([LA]0/[cat.]0/[BnOH]0 = 100/1/1; Table 1, entry 2). rac-Lactide
(0.144 g, 1.0 mmol) was added to a toluene solution of 1 (0.0057 g,
0.01 mmol) and BnOH (0.1 mL, 0.01 mmol, 0.1 M in toluene). The
solution then was then rapidly stirred at room temperature for 3 min
and quenched by a few drops of water. The polymer was precipitated
by adding hexane (20 mL). A white crystalline solid was obtained by
recrystallization from a CH2Cl2/hexane mixed solvent and dried under
vacuum. It should be noted that just partial lactide can be soluble in
toluene at room temperature or at −70 °C, but the reaction solution
becomes clear when the polymerization proceeds and the monomer
can be wholly consumed.
Crystallographic Studies. The data were collected using a
SuperNova (Dual) X-ray diffractometer equipped with a graphite-
monochromated Cu/Mo Kα radiation source (λ = 1.54184/0.71073
Å). The structures were solved by direct methods using the Siemens
SHELXL-97 program.19 Non-hydrogen atoms were refined with
anisotropic displacement parameters during the final cycles. All
hydrogen atoms were placed by geometrical considerations and were
added to the structure factor calculations. The crystal data and
refinement results are summarized in Table S1.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02709.
Experimental details of the synthesis and characterization
of all compounds H1L−H4L and 1−8, NMR spectra of
complexes 1−8, and description of the polymerization
studies (PDF)
Crystallographic data for complex 1 with CCDC
reference number 1404985 (CIF)
Crystallographic data for complex 3 with CCDC
reference number 1404986 (CIF)
Crystallographic data for complex 4 with CCDC
reference number 1404987 (CIF)
Crystallographic data for complex 8 with CCDC
reference number 1404988 (CIF)
G
DOI: 10.1021/acs.inorgchem.5b02709
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DOI: 10.1021/acs.inorgchem.5b02709
Inorg. Chem. XXXX, XXX, XXX−XXX