Synthesis and characterization of magnesium alkoxides incorporated into bulky aluminium tetraphenolate helices and application in the ring-opening polymerization of lactides

Article abstract of DOI:10.1002/ejic.201300284

Two aluminium and magnesium heterobimetallic complexes of [(LAl)MgOBn] ₂ (2) and [(LAl)Mg(OC₂H₄OCH₃)] ₂ (3), in which L = N¹,N¹,N²,N ²-tetrakis(2-hydroxy-3,

Full text of DOI:10.1002/ejic.201300284

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DOI:10.1002/ejic.201300284
Synthesis and Characterization of Magnesium Alkoxides
Incorporated into Bulky Aluminium Tetraphenolate
Helices and Application in the Ring-Opening
Polymerization of Lactides
Chunli Jian,[a][‡] Jinjin Zhang,[a][‡] Zhongran Dai,[a] Yuan Gao,[a]
Ning Tang,[a] and Jincai Wu*[a]
Keywords: Ring-opening polymerization / Kinetics / Heterobimetallic complexes / Magnesium / Aluminium / Lactides
Two aluminium and magnesium heterobimetallic complexes
rare helical heterobimetallic initiator, can effectively initiate
the ring-opening polymerization (ROP) of -lactide ( -LA)
leading to polymers with good molecular weight control and
narrow molecular weight distributions. Kinetic studies have
shown the overall rate expression is –d[lactide]/dt = k[lacti-
of [(LAl)MgOBn]2 (2) and [(LAl)Mg(OC2H4OCH3)]2 (3), in
L
L
which
L
=
N1,N1,N2,N2-tetrakis(2-hydroxy-3,5-dimeth-
ylbenzyl)-1,2-ethanediamine, have been synthesized and
characterized by NMR spectroscopy, X-ray crystallography,
and elemental analysis. The magnesium alkoxide of complex
2 supported by the bulky aluminium tetraphenolates, as a
de][complex] for the ROP of L-LA. Furthermore, complex 2
shows modest isotactic selectivity in the ROP of rac-lactide.
kali-metal–aluminium synergic effects.[5] A series of pion-
eering works reported by Mulvey and co-workers also
showed some special synergetic effects in the heterobimetal-
lic complexes in some organic chemical reactions.[6]
Introduction
The design and synthesis of heterobimetallic complexes
have attracted considerable attention because sometimes
they show multiple functionalities and prominent catalytic
activity, selectivity over monometallic complexes, and can
potentially even achieve chemical transformations that are
unprecedented with monometallic catalysts.[1] For example,
heterobimetallic complexes (e.g., Fe, Mo, and Co) at the
active site of the enzyme of nitrogenase play a key role in
the fixation of atmospheric nitrogen gas.[2] Hosokawa et al.
have proposed and isolated Pd–Cu heterobimetallic species
participating in the famous Wacker reaction with PdCl2 and
CuCl2 as catalysts; the catalytic cycle would come to a halt
in the absence of PdCl2 or CuCl2.[3] Heterobimetallic lan-
thanide–alkali-metal complexes based on binol are versatile
catalysts in a wide range of asymmetric reactions that have
never been possible with monometallic lanthanide cata-
lysts.[4] Finally, the heterobimetallic complex of “iBu3Al-
(tmp)Li” [tmp(H) = 2,2,6,6-tetramethylpiperidine] exhibits
high chemo- and regioselectivity in proton abstraction reac-
tions of functionalized aromatic substrates because of al-
Recently we have focused our research on the develop-
ment of multi-metallic catalysts for the ring-opening poly-
merization (ROP) of lactide to obtain biorenewable, biode-
gradable, and biocompatible polylactide (PLA).[7] Up to
now, many metal complexes have been successfully used to
initiate/catalyze the ROP of lactide, giving polymers with
the desired molecular weights and narrow molecular weight
distributions. Among them, many initiators/catalysts, like
diketiminate–zinc,[8] Salan–Al,[9] titanium alkoxides,[10] and
Salen–Ln,[11] show high heterotactic selectivities in the ROP
of lactide,[12] whereas up to now only Salen, Salan, or Salen-
like aluminium complexes have shown excellent isotactic
selectivities (Pm Ͼ 0.9) in the ROP of rac-lactide.[13] Salen–
Al and Salen-like Al systems suffer from low activation and
usually a high temperature (Ͼ70 °C) is necessary for the
polymerization reaction. Although magnesium,[14] cal-
cium,[15] and other complexes[16] show high activity in the
ROP of lactide, only modest isotactic selectivities can be
achieved. The highly active yttrium phospha-salen initiators
very recently reported by Auffrant and Williams and their
co-workers showed high isoselectivities (Pm = 0.84) for the
ring-opening polymerization of rac-lactide.[17] Nevertheless,
finding other excellent, highly active systems with high iso-
tactic selectivities is still a challenge. Thus, two aluminium
and magnesium heterobimetallic bulky complexes were syn-
thesized in this research in which the highly Lewis acidic
aluminium metal acts as an auxiliary to fix the bulky and
[a] State Key Laboratory of Applied Organic Chemistry, Key
Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province, College of Chemistry and
Chemical Engineering, Lanzhou University,
Lanzhou 730000, People’s Republic of China
E-mail: wujc@lzu.edu.cn
Homepage: http://chem.lzu.edu.cn/LZCHEM/visit/szdw/
Teacher_One.aspx?TeacherNO=51
[‡] These authors contributed equally to this work.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300284.
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chiral environment around the active center. Meanwhile, the possible existence of diastereoisomers, which will be dis-
magnesium metal with alkoxide could act as a highly active cussed below. Both complexes have low solubility in hexane
initiator. To chelate both metal ions of AlIII and MgII and and modest solubility in toluene and thf, and complex 2 is
inhibit side-reactions of the ring-opening polymerization of more soluble than complex 3 in CH2Cl2. When the methyl
lactide, the peculiar quadruply charged, sterically hindered groups were replaced by tert-butyl groups in this kind of
ligand of tetraphenolate was chosen. Although the selectiv- ligand, the 1H NMR spectra tended to be a mess, even after
ity is not as high as we expected, it presents another pos- purification, which indicates that the related complexes can-
sible approach to the design of a new type of heterobimetal- not be obtained in an acceptable yield, that is, the greater
lic complex for highly active and selective ring-opening po- steric hindrance of the ligand possibly inhibits the forma-
lymerization of racemic lactide.
tion of this type of complex.
Single crystals of complex 2 suitable for structural char-
acterization were obtained from slow cooling of a toluene
solution; complex 2 crystallizes as dimers in the centrosym-
metric monoclinic space group C2/c. An ORTEP drawing
of complex 2 is shown in Figure 1. Al1 is six-coordinated
Results and Discussion
Synthesis and Characterization of Complexes 2 and 3
Complex 1 was synthesized by heating a mixture of by the four oxygen atoms and two nitrogen atoms of the
AlMe3 and LH4 for 20 hours at reflux according to a litera- tetraphenol ligand and its geometry is a distorted octahe-
ture method but by replacing Al(OiPr)3 with AlMe3.[18] The dron. Mg1 is five-coordinated by three oxygen atoms of one
above reaction mixture reacts directly with 1 equivalent of ligand and two oxygen atoms from the benzyloxy groups
Mg(OBn)2 at 25 °C for 12 hours to afford complex 2 in 30% and adopts a distorted trigonal-bipyramidal geometry, and
yield after recrystallization (Scheme 1). Complex 2 has been the two monomers are bridged through the oxygen atoms
well characterized by NMR spectroscopy and elemental of two benzyloxy groups. This complex is a racemic com-
analysis. It is interesting that complex 4 cannot be obtained pound with two enantiomers in the unit cell with (Λ,Λ)-
by the reversed addition sequence of aluminium and mag- [(LAl)MgOBn]2 having the handedness of the two red heli-
nesium, which also gave complex 2 as the main product, as ces shown in Figure 1. Figure 2 illustrates the chirality at
verified by NMR spectroscopy and X-ray diffraction. This one aluminium center: Two triangular faces of O2–O3–O4
self-assembly behavior may indicate that complex 2 is more at the front (shown in blue) and N1–O1–N2 at the back
stable. To expand this reaction, complex 3 was obtained by (shown in pink) can be chosen, and the three-bladed propel-
the reaction of LH4, tris(methoxyethoxy)aluminium, and lers of O2–N1, O3–O1, and O4–N2 by looking at the mole-
Mg(nBu)2 in 25% yield after recrystallization. The results cule along the Mg–Al direction with anticlockwise charac-
of the elemental analysis agreed well with the structure, ter indicate that the chirality of the aluminium center can
1
whereas the H NMR spectrum is complicated because of be designated as Λ. Because the dimer can be generated
Scheme 1. Synthesis of complexes 2 and 3.
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from the asymmetric unit through a C2 rotation, the config-
urations of both aluminium centers in one dimer are Λ.
Therefore the chirality of the dimer shown in Figure 1 can
be labeled as (Λ,Λ), and the other enantiomer of (Δ,Δ)-
[(LAl)MgOBn]2 can also be found in the unit cell, for which
the chirality around Al3+ is demonstrated in Figure 3. Note
that the clear single set of resonances in the 1H NMR spec-
trum of complex 2 in CDCl3 indicates the nonexistence of
the meso complex in the final product after recrystalli-
zation. As far as we know, this kind of structure with two
magnesium alkoxides as the active centers located in a chi-
ral helical environment is the first to have been reported.
Single crystals suitable for the structural characterization
of complex 3 were also acquired by slow cooling of a tolu-
ene solution. Its molecular structure is depicted in Figure 4
with selected bond lengths given in the caption. In contrast
to complex 2, Mg1 is five-coordinated by two oxygen atoms
of the tetraphenol ligand and three oxygen atoms of the 2-
methoxyethoxy groups. The replacement of one phenolic
oxygen atom by the methoxy group of 2-methoxyethoxy in-
dicates the potential coordination position of lactide in the
ring-opening polymerization with complex 2 as the initiator.
However, complex 3 is a meso compound composed of two
Figure 1. Molecular structure of (Λ,Λ)-[(LAl)MgOBn]2 (2) wiith
ellipsoids drawn at the 30% level. Selected bond lengths [Å]: Al(1)–
O(1) 1.823(7), Al(1)–O(2) 1.857(6), Al(1)–O(3) 1.841(3), Al(1)–O(4)
2.003(0), Al(1)–N(1) 2.060(6), Al(1)–N(2) 2.048(0). Mg(1)–O(2)
2.201(5), Mg(1)–O(3) 2.100(4), Mg(1)–O(4) 2.012(1), Mg(1)–O(5)
1.993(2), Mg(1)–O(5)i 1.9578(12). Symmetry code: i: 2 – x, y,
1.5 – z.
Figure 2. Chirality at the Al3+ center in (Λ,Λ)-[(LAl)MgOBn]2, top view along the Mg–Al direction.
Figure 3. Chirality at the Al3+ center in (Δ,Δ)-[(LAl)MgOBn]2, top view along the Mg–Al direction.
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Table 1. Bond valence sums for complexes 2 and 3.
Complex 2
Complex 3
Mg(1)–O(3)
sij
sij
sij
sij
Al(1)–O(1)
Al(1)–O(2)
Al(1)–O(3)
Al(1)–O(4)
Al(1)–N(1)
Al(1)–N(2)
zj = 3.051
0.578
0.527
0.550
0.355
0.512
0.529
Mg(1)–O(2)
Mg(1)–O(3)
Mg(1)–O(4)
Mg(1)–O(5)
Mg(1)–O(5)i
0.253
0.333
0.422
0.444
0.489
Al(1)–O(1)
0.603
0.623
0.534
0.411
0.389
0.480
0.384
0.400
0.439
0.280
0.540
Al(1)–O(2)
Al(1)–O(3)
Al(1)–O(4)
Al(1)–N(1)
Al(1)–N(2)
zj = 3.040
Mg(1)–O(4)
Mg(1)–O(5)
Mg(1)–O(6)
Mg(1)–O(5)i
zj = 1.941
zj = 2.043
different aluminium chiral centers (Δ,Λ) related by an inver- to the theoretical values of the oxidation states presented in
sion center. Although the structural analysis demonstrates Table 1, which indicates that the structures of complexes 2
complex 3 to be a meso compound, the existence of chiral and 3 are accurate and credible. After the alternation of
(Δ,Δ) and (Λ,Λ) diastereoisomers in the purified solid can- Al3+ and Mg2+, bond valence sums will seriously deviate
not be ruled out due to the complicated 1H NMR spectrum from the theoretical values of the oxidation states. ICP-OES
of the final recrystallized product in CDCl3 solution (see (inductively coupled plasma optical emission spectrometry)
Figure S3 in the Supporting Information). That is, it is diffi- experiments have also been conducted and the results con-
cult to separate the air-sensitive diastereoisomers because firmed the presence of both Al3+ and Mg2+ in the correct
of their possible similar solubility.
ratio of 1:1 for Al3+/Mg2+. The polymerization reactions
reported below can also certify the correct presumption be-
cause the two complexes are highly active, like the magne-
sium alkoxides reported in the literature and unlike the us-
ual poorly active aluminium alkoxide.
ROP of L-Lactide and Kinetic Studies
The ROP of l-lactide initiated by complex 2 was carried
out in dichloromethane at 25 °C. As shown in Table 2, com-
plex 2 was found to be an efficient initiator, giving almost
complete conversions in about 2 hours for the polymeriza-
tion of l-lactide with [lactide]0/[initiator]0 ratios from 100
to 400. In comparison with aluminium alkoxides, the high
activity of complex 2 implies the active center is the alkoxy
group coordinated to magnesium. The initiator system
showed controllable polymerization performance as evi-
denced by the linear correlations between the number
average molar mass (Mn) and monomer/initiator ratio, and
the close agreement between experimental and calculated
Figure 4. Molecular structure of 3 drawn with ellipsoids at the 30%
probability level. Selected bond lengths [Å]: Al(1)–O(1) 1.807(4),
Al(1)–O(2) 1.795(7), Al(1)–O(3) 1.852(6), Al(1)–O(4) 1.949(4),
Al(1)–N(1) 2.162(6), Al(1)–N(2) 2.084(6). Mg(1)–O(3) 2.047(3),
Mg(1)–O(4) 2.032(1), Mg(1)–O(5) 1.998(2), Mg(1)–O(5)i 1.922(4).
Symmetry code: i: –x, 1 – y, –z.
In fact, the sites of Al3+ and Mg2+ in one molecular com-
plex are difficult to identify for they have the same number
of electrons, fortunately bond valence principle gives us an
opportunity to distinguish them based on bond lengths ob-
tained from good single-crystal structures. The bond val-
ence sums offer a relatively simple method for determining
the oxidation state and assessing the correctness of reported
structures. The oxidation state, zj, can be calculated from
the sum of the individual bond valences, sij, as shown in
Equation (1). sij can be calculated from the observed bond
lengths, Rij, by using Equation (2) provided the constants b
and Ro are known. The constant b is 0.37. This value was
determined by Brown and Altermatt[19] and is generally ac-
cepted.[20]
zj = Σsij
(1)
sij = exp[(Ro– Rij)/b]
(2)
Figure 5. Relationship between Mn (᭿) and PDI (᭡) of the poly-
mer formed by using complex 2 and the initial molar ratios [LA]0/
The bond valence sums of Al and Mg based on the posi-
tions of Al3+ and Mg2+ in the structures 2 and 3 are close [I]0 (entries 1–6 in Table 2).
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Table 2. Polymerization of l-LA using complexes 2 and 3.[a]
[h]
Entry
[M]o/[I]o
Complex
Time
Conv.[%]
Mn(calcd)[f]
Mn(exp.)[g]
PDI
Pm
1
2
3
4
5
6
100:1
150:1
200:1
250:1
350:1
400: 1
215:1
215:1
215:1
215:1
100:1
100:1
100:1
100:1
2
2
2
2
2
2
2
2
2
2
2
2
3
3
2 h
2 h
2 h
2 h
2 h
96
98
95
96
98
97
41
78
87
93
82
70
36
93
7000
10700
13800
17400
24800
28000
6400
12200
13600
14500
6000
5400
8300
12400
13300
23000
23700
5200
10400
11500
12500
5700
1.13
1.12
1.09
1.06
1.09
1.13
1.12
1.12
1.08
1.09
1.05
1.08
1.21
1.68
2 h
7[b]
8[b]
9[b]
10[b]
11[c]
12[d]
13
14[e]
5 min
15 min
20 min
30 min
2 h
10 h
6 h
0.55
0.58
5100
2700
6800
4900
4700
7300
3 h
[a] Conditions: [I]0 = 5 mm at 25 °C in dichloromethane. [b] [I]0 = 7 mm, [LA]0 = 1.5 m at 25 °C in CDCl3. [c] Racemic lactide, [I]0
=
4 mm at room temperature in CH2Cl2. [d] Racemic lactide, [I]0 = 4 mm at 0 °C in CH2Cl2. [e] [I]0 = 5 mm at 80 °C in toluene. [f] Calculated
from the molecular weight of l-LA ϫ [l-LA]0/2[1] ϫ conversion + molecular weight of BnOH. [g] GPC data in thf vs. polystyrene
1
standards using a correction factor of 0.58.[22] [h] Obtained from the homonuclear-decoupled H NMR spectrum.
values of Mn (Figure 5). Analysis by 1H NMR of PLA- methine region. Peaks at δ = 7.32 (C6H5CH2) and 4.38 ppm
100:1 (entry 1) produced at an initial [LA]0/[initiator]0 ratio (HOCHMe) with an integral ratio of 5:1 between He and
of 100:1 shows a characteristic quadruple methine peak Hc indicate the polymer chain is capped by one benzyl ester
(Figure 6) at δ = 5.16 ppm, which indicates no serious epi- and one hydroxy group, which suggests that polymerization
merization of the chiral centers in the polymers, confirmed occurs by the insertion of l-lactide into the metal–benz-
by the homonuclear-decoupled 1H NMR spectra of the yloxy bond. Complex 3 was also used as an initiator for the
1
Figure 6. H NMR spectrum of PLA-100 initiated by complex 2 (entry 1 in Table 2).
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ROP of lactide with a [lactide]0/[initiator]0 ratio of 100:1
(entries 13 and 14). Only 36% of lactide was converted into
polylactide in CH2Cl2 in 6 hours giving a polymer with high
molecular weight. The lower activity of complex 3 can poss-
ibly be ascribed to the difficult coordination of lactide to
magnesium at the position occupied by the methoxy group,
which leads to the slow initiation of lactide ROP at room
temperature. The higher molecular weight and broad PDI
can be attributed to the slower initiation of lactide ROP
relative to propagation and the possible existence of dif-
ferent active diastereoisomers in complex 3. Although 93%
conversion of lactide can be achieved in toluene in 3 hours
at 80 °C (entry 14), the molecular weight distribution of
1.68 is very broad.
Kinetic studies of l-lactide polymerization with complex
2 as initiator were conducted to establish the reaction order
in monomer and metal concentration. The conversions of
l-lactide with time at various concentrations of complex 2
Figure 8. First-order kinetic plots for polymerizations using initia-
tors 2. Reagents and conditions: [LA]0 = 1.5 molL–1; (᭢) [I]0
=
([LA]0 = 1.5 m, [I]0 = 7, 5, 3, and 2.5 mm) in CDCl3 were 0.007 molL–1, [LA]0/[I]0 = 215, kobs = 9.92ϫ10–2 min–1; (᭹) [I]0 =
0.005 molL–1, [LA]0/[I]0 = 300, kobs = 5.98ϫ10–2 min–1; (᭿) [I]0 =
1
monitored by H NMR spectroscopy at 25 °C. Note that
0.003 molL–1, [LA]0/[I]0 = 500, kobs = 1.91ϫ10–2 min–1; (᭤) [I]0 =
the initiator system in CDCl3 also showed good control of
polymerization, as evidenced by the linear increase in Mn
0.0025 molL–1, [LA]0/[I]0 = 600, kobs = 0.77ϫ10–2 min–1; 25 °C.
CDCl3.
with conversion and narrow PDIs (1.08–1.12; Figure 7). In
each case, plots of ln[(LA)0/(LA)t] versus time are linear,
which indicates polymerization proceeds with a first-order
dependence on monomer concentration (Figure 8, [LA]0 =
1.5 molL–1, [I]0
= 7 mm, [LA]0/[I]0 = 215, kobs =
9.92ϫ10–2 min–1). Thus, the rate of polymerization can be
written as –d[LA]/dt = kobs[LA] in which kobs = k[I]x and k
is the rate constant. The linear relationship between kobs
and [I]0 (Figure 9) reveals the reaction is first order in the
initiator. Therefore the overall rate equation is –d[LA]/dt =
k[LA][I] (k = 12.65 Lmol–1 min–1). The rate law of –d[LA]/
dt = k[LA][I] is the same as that found in other initiator
systems,[12b,14b,14d,21] but different to similar dimeric magne-
sium alkoxide systems reported by us.[14d]
Figure 9. Linear plot of kobs vs. [I]0 for the polymerization of l-
LA with complex 2. Reagents and conditions: [LA]0 = 1.5 molL–1,
CDCl3, 25 °C, k = 12.65 Lmol–1 min–1.
Ring-Opening Polymerization of rac-Lactide
The polymerization of rac-lactide initiated by complex 2
was also conducted in CH2Cl2, toluene, and thf at room
temperature. Complex 2 induces almost no tacticity in the
ROP of lactide in toluene and thf, whereas modest selectivi-
ties were obtained with CH2Cl2 as solvent. The homonu-
1
clear-decoupled H NMR spectrum in the methine region
of PLA derived from entry 11 shows isotactic predomi-
nance with Pm = 0.55,[23] a slighter better selectivity of Pm
= 0.58 can be obtained by changing the reaction tempera-
ture to 0 °C (see Figure S4 in the Supporting Information).
The lower selectivity than expected can be attributed to the
possible dissociation of the dimer to monomer because lact-
ide must pass through a chiral channel to the active magne-
Figure 7. Plot of molecular weight (Mn) vs. conversion of monomer
with complex 2 as initiator for the polymerization of l-lactide. Rea-
gents and conditions: [LA]0 = 1.5 molL–1, [I]0 = 0.007 molL–1,
CDCl3, 25 °C.
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stirred for 12 h at room temperature. The volatiles were removed
in vacuo and then the residue was recrystallized to give complex 1
as colorless crystals, yield 0.23 g (30%). 1H NMR (600 MHz,
CDCl3, 298 K): δ = 7.30 (m, 4 H, ArH), 6.98 (s, 2 H, ArH), 6.71
(s, 2 H, ArH), 6.62 (s, 8 H, ArH), 6.49 (s, 3 H, ArH), 6.38 (s, 3 H,
sium center in the dimer of complex 2 whereas the mono-
mer only gives an open chiral surrounding around the
active magnesium center for less steric handrance. Although
the selectivity is not as high as we expected, these kinds of
heterobimetallic complexes can provide insights into alter-
native methods for finding highly active and highly isotactic
selective initiator systems.
2
ArH), 6.25 (s, 4 H, ArH), 4.64 (br., 2 H, NCH2Ph), 4.39 (d, J1H-
= 12.0 Hz, 2 H, OCH2Ph), 4.34(br., 2 H, NCH2), 3.94 (d, 4 H,
1H
2
NCH2), 3.67 (br., 2 H, CH2), 3.29 (d, J1H-1H = 12.0 Hz, 2 H,
2
OCH2Ph), 3.11 (m, 4 H, CH2), 2.71 (d, J1H-1H = 11.5 Hz, 2 H,
CH2), 2.63 (br., 2 H, CH2), 2.51 (dd, J1H-1H = 11.5 Hz, 2 H, CH2),
2.30 (s, 6 H, CH3), 2.25 (s, 6 H, CH3), 2.11 (s, 12 H, CH3), 2.03
(m, 4 H, CH2), 2.00(s, 12 H, CH3), 1.76 (s, 6 H, CH3), 1.56 (s, 6
H, CH3) ppm. 13C NMR (600 MHz, CDCl3, 298 K): δ = 156.95,
154.77, 154.48, 152.45, 131.98, 131.71, 130.96, 130.01, 127.61,
127.30, 126.96, 126.79, 126.62, 126.09, 125.71, 125.13, 124.23,
123.73, 122.56, 121.32, 120.42, 65.92, 65.32, 64.83, 63.10, 62.65,
57.16, 56.14, 21.41, 20.58, 20.38, 17.81, 16.33, 15.71, 14.48 ppm.
C90H102Al2Mg2N4O10·0.5CH2Cl2 (1544.83): calcd. C 70.36, H 6.72,
N 3.63; found C 69.82, H 6.85, N 3.21.
Conclusions
Two new kinds of aluminium and magnesium heterobi-
metallic alkoxides 2 and 3 have been synthesized and char-
acterized. The molecular structures of complexes 2 and 3
were confirmed by single-crystal X-ray diffraction tech-
niques. Experimental results indicate that the racemic heli-
cal complex 2 is an efficient initiator of the controlled ring-
opening polymerization of l-lactide leading to polymers
with good molecular weight control and narrow molecular
weight distributions. It also shows modest isotactic selectiv-
ity for the ROP of rac-lactides.
Method 2: MgnBu2 (1.0 m in hexane, 1.1 mL, 1.1 mmol) was added
dropwise to a stirred solution of LH4 (0.596 g, 1.0 mmol) in toluene
(15.0 mL) at 0 °C. The slurry was warmed slowly to room tempera-
ture and stirred for 12 h, and the solution became clear. A mixture
of AlMe3 solution (1.0 m solution in hexane, 1.1 mL, 1.1 mmol)
and BnOH (1 m in toluene, 3.3 mL, 3.3 mmol) in toluene (8 mL)
was added dropwise to this solution at 0 °C. The reaction mixture
was stirred for 20 h at 110 °C. The volatiles were removed in vacuo
and then the residue was recrystallized to give complex 2 as color-
less crystals.
Experimental Section
Materials and Methods: All the syntheses were performed under
dry nitrogen using standard Schlenk techniques. Reagents were
purified by standard methods: toluene, n-hexane, and thf were dis-
tilled under argon from sodium/benzophenone ketyl prior to use,
CH2Cl2 was distilled from P2O5, BnOH and CH3OCH2CH2OH
were distilled from CaH, l-LA and rac-LA were purchased from
Daigang BIO Engineer Limited Co. of China and recrystallized
from toluene, Mg(nBu)2 and Al(CH3)3 were purchased from Acros
Synthesis of Complex 3: A mixture of AlMe3 (1.0 m solution in
hexane, 1.1 mL, 1.1 mmol) and CH3OCH2CH2OH (1 m in toluene,
3.3 mL, 3.3 mmol) in toluene (8 mL) was added to a rapidly stirred
solution of LH4 (0.596 g, 1.0 mmol) in toluene (15.0 mL) at 0 °C.
The solution was warmed slowly to room temperature and stirred
for 20 h at 110 °C. This solution was then cooled to room tempera-
ture and added dropwise to a solution of MgnBu2 (1.0 m in hexane,
1.1 mL, 1.1 mmol). The reaction was continued for 12 h at room
temperature, the volatiles were removed in vacuo, and the residue
recrystallized to give complex 3 as a white powder, yield 0.18 g
(25.0%). The 1H NMR spectrum is complicated possibly indicating
it is a mixture of diastereoisomers (see Figure S3 in the Supporting
Information). C82H102Al2Mg2N4O12 (1438.30): calcd. C 68.48, H
7.15, N 3.90; found C 68.20, H 7.18, N 3.56.
1
Company. H and 13C NMR were recorded with Varian Mercury
Plus 300 and 600 MHz spectrometers. 1H NMR chemical shifts are
reported in ppm versus residual protons in CDCl3: δ = 7.26 ppm.
13C NMR chemical shifts are reported in ppm versus residual 13C
in CDCl: δ = 77.2 ppm. GPC analyses were performed with a
Waters instrument (M510 pump, U6K injector) equipped with
Waters 2414 and Milton Roy differential refractive index detectors
and Waters Styrage® HR 4E THF 7.8ϫ300 mm column in series.
The GPC columns were eluted with tetrahydrofuran at 45 °C at
1.0 mL/min and calibrated with monodisperse polystyrene as a
standard reference.
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-
OES): Complex 1 (10 mg) was decomposed with 65% HNO3 (v/v,
2 mL) and make aqueous solution of HNO3 (100 mL, 5%). The
amounts of aluminium and magnesium were then determined by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES, IRIS Advantage, TJA, USA).
Synthesis of Ligand LH4: The ligand LH4 was prepared according
to the literature.[24] 2,4-Dimethylphenol (6 equiv., 60 mmol), 95%
paraformaldehyde (4 equiv.), and ethylenediamine (1 equiv.) were
heated in a 100 mL pressure flask at 80 °C with stirring for 3 days.
The reaction was allowed to cool and methanol (ca. 50 mL) was
added to the reaction mixture. This mixture was stirred overnight,
the precipitate was isolated by vacuum filtration, and the product
was isolated as a white solid, yield 4.47 g (75%).
Polymerization of L-Lactide: A typical polymerization procedure is
exemplified by the synthesis of PLA ([LA]0/[I]0 = 100:1; Table 2,
entry 1) at room temperature. Complex 1 (0.030 g, 0.02 mmol) was
added to a rapidly stirred solution of l-lactide (0.288 g) in dichloro-
methane (4 mL). The reaction mixture was stirred for 2 h at room
temperature and then quenched with distilled water (0.5 mL). The
solution was concentrated in vacuo and the polymer was redis-
solved in dichloromethane and then precipitated with excess hexane
to give a white crystalline solid. The polymer was then dried in
vacuo to a constant weight. Then the molecular weight and poly-
dispersity index (PDI) were determined by GPC.
Synthesis of Complex 2
Method 1: AlMe3 solution (1.1 mL, 1.1 mmol, 1.0 m solution in
hexane) was added dropwise to a stirred solution of LH4 (0.596 g,
1.0 mmol) in toluene (15.0 mL) at 0 °C. The slurry was warmed
slowly to room temperature and stirred for 20 h at 110 °C, and
the solution became clear. This solution was then cooled to room
temperature and added dropwise to a mixture of MgnBu2 (1.0 m
in hexane, 1.1 mL, 1.1 mmol) and BnOH (1 m in toluene, 2.2 mL,
Kinetics of
L-Lactide Polymerization with Complex 2: A CDCl3
2.2 mmol) in toluene (8 mL) at 0 °C. The reaction mixture was then solution of monomer was added to a solution of the complex. The
Eur. J. Inorg. Chem. 2013, 3533–3541
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FULL PAPER
mixture was then stirred at 25 °C under N2. After appropriate inter-
vals of time, a certain amount of the reaction solution was removed
and quenched with distilled water (1 drop). The aliquots were then
dried to a constant weight in vacuo and analyzed by 1H NMR
spectroscopy. When the initial concentration of the initiator ([I]0)
was 0.007 molL–1, the aliquots were concentrated in vacuo and the
polymer was redissolved in dichloromethane and precipitated with
excess hexane to give a white crystalline solid. The polymer was
then dried in vacuo to constant weight. The Mn and PDI of the
polymer were obtained by GPC analysis.
21271092), the State Education Ministry and Fundamental Re-
search Funds of the Central Universities of China (grant number
lzujbky-2012-67), and the Project for National Basic Science Per-
sonnel Training Fund (grant number J1103307) are gratefully ac-
knowledged.
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3
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Formula
Mn
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C117H118Al2Mg2N4O10C48H51AlMgN2O6
1842.73
296(2)
monoclinic
C2/c
803.20
293(2)
monoclinic
P21/c
19.618(5)
22.593(6)
23.691(6)
90.00
94.828(3)
90.00
15.560(10)
15.096(9)
24.099(11)
90.00
126.68(3)
90.00
γ [°]
V [Å3]
10463(4)
4
1.170
4540(4)
4
1.175
Z
ρcalcd. [gcm–3]
Abs. coeff. [mm–1]
F(000)
0.100
3912
0.107
1704
θ range
Index ranges
2.50–25.15
–23ϽhϽ24
–27ϽkϽ27
–28ϽlϽ28
2.19–23.47
–17ϽhϽ17
–9ϽkϽ16
–24ϽlϽ25
6285/565/547
1.021
Data/restr./parameters 9945/48/616
GOF
1.033
R [IϾ2σ(I)]
wR
0.0615
0.1604
0.0746
0.1117
Peak, hole [eÅ–3]
0.719, –0.458
0.329, –0.191
CCDC-918271 (for 2) and -918272 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): CIF for 2 and 3, NMR spectrum of complex 2, and homonu-
clear-decoupled H NMR spectrum.
1
Acknowledgments
Financial support from the National Natural Science Foundation
of China (NSFC) (grant numbers 21071069, 21171078 and
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Received: February 28, 2013
Published Online: June 4, 2013
Eur. J. Inorg. Chem. 2013, 3533–3541
3541
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim