Synthesis, characterization and catalytic activity of magnesium and zinc aminophenoxide complexes: Catalysts for ring-opening polymerization of l-lactide

Article abstract of DOI:10.1039/c1dt11080b

A series of novel magnesium and zinc aminophenoxide complexes were successfully synthesized and one zinc complex was characterized by X-ray crystallography. They were also investigated as initiators for the ring opening polymerization of l-lactide. The co

Full text of DOI:10.1039/c1dt11080b

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PAPER
Synthesis, characterization and catalytic activity of magnesium and zinc
aminophenoxide complexes: Catalysts for ring-opening polymerization of
L-lactide†
Hui-Ju Chuang,a Shi-Feng Weng,a Chia-Chih Chang,a Chu-Chieh Lin*a and Hsuan-Ying Chen*b
Received 9th June 2011, Accepted 11th July 2011
DOI: 10.1039/c1dt11080b
A series of novel magnesium and zinc aminophenoxide complexes were successfully synthesized and
one zinc complex was characterized by X-ray crystallography. They were also investigated as initiators
for the ring opening polymerization of L-lactide. The complexes are effective in forming polylactides
with good conversions. The nature and steric bulk of the ligands coordinated to the central metal ions
enormously influenced the polymer properties. Among all the complexes, the zinc aminophenoxide
complexes as initiators produced polymers with good molecular weight control and relatively narrow
PDIs.
had different variation in their characters and activities towards
Introduction
ROP of LA.7e Thus the authors confirmed that changing the
Due to their biodegradability, biocompatibility, and permeable
coordination numbers of the ligands affects the catalyst character
properties, aliphatic polyesters, such as poly(e-caprolactone)
(PCL), poly(lactide) (PLA), and their copolymers show potential
and its activity towards cycloester ROP.
Formerly Tolman et al. reported a single-site zinc complex
applications in a variety of fields.1 Accordingly, the preparation of
PCL and PLA has drawn intensive scrutiny recently. The major
polymerization method used to synthesize these polymers has been
the ring-opening polymerization (ROP) of lactide (LA) and e-
caprolactone (CL) and functionally related compounds. Currently
metal alkoxides (e.g., Al,2 Li,3 Mg,4 Fe,5 Sn,6 or Zn7) are used to
initiate the ring-opening polymerization of LA and related cyclic
esters.8 The undesired backbiting/transesterification reactions can
be minimized by using a bulky ligand coordinately attached with
an active metal center which provides a steric barrier. Recently,
many coordinately bulky metal complexes have been synthesized
in order to increase the active site of the mononuclear form.2,6,7
Schiff base supported metal complexes have been used as
catalysts in many applications, due to their diverse forms and
ease of preparation.9 However, their application in ROP was not
explored thoroughly. The first Schiff base zinc complex used as
an initiator for polymerization of L-lactide was developed by
Chisholm et al.7c The authors changed the Schiff base ligands
from bidentate to tridentate and found that the zinc complexes
supported by N,N,O-tridentate aminophenol ligands7d (Scheme
1) and they exhibit good control in the polymerization of LA
to yield high molecular weight PLA at rates faster than other
Zn systems reported to till date. Interestingly, the variations
occurred by changing ligands from tridentate to bidentate or multi-
dentate type. Herewith we describe the preparation of a series
of aminophenol ligands (Scheme 2) with different coordination
numbers and sizes of amino group. The catalytic activities of zinc
and magnesium alkoxides with different substituents on the amino
group of aminophenol ligands were discussed in detail.
Scheme 1
aDepartment of Chemistry, National Chung Hsing University, Taichung 402,
Taiwan, R.O.C. E-mail: cchlin@mail.nchu.edu.tw
Results and discussion
bDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical
University, 100, Shih-Chuan 1st Road, Kaohsiung, 80708, Kaohsiung,
Taiwan, R.O.C. E-mail: hchen@kmu.edu.tw
Synthesis and characterization of Zn and Mg Complexes
All ligands L1–H–L5–H were prepared by refluxing a mixture
of dialkylamine, para-formaldehyde, and 2,4-di-tert-butylphenol.
Followed by the reaction of L1–H–L5–H, a stoichiometric amount
of di-n-butylmagnesium in THF was added for 3 h at 0 ◦C,
and then 1 equiv of 2-(dimethylamino)ethanol was added to
† Electronic supplementary information (ESI) available: Table giving fur-
ther details of the crystal structure determination, atomic coordinates and
isotropic thermal parameters, bond lengths and angles, and anisotropic
displacement parameters for 2b. CCDC reference number 816057. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11080b
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Scheme 2 Preparation of a series of aminophenol ligands.
form magnesium alkoxide compounds (1a–5a) in moderate yield
(Scheme 3). Similarly, reaction of L L1–H–L5–H with ZnEt2 in
THF for 3 h at 0 ◦C and then with benzyl alcohol (BnOH)
afforded the benzylalkoxide complexes (1b–5b). To compare with
zinc benzylalkoxide complexes, the magnesium benzylalkoxide
complex (2c) with L2–H was synthesized. During the reaction
of aminophenol with MgnBu2 or ZnEt2, an intermediate was
observed as monomeric [LMgnBu] or [LZnEt]. We also obtained
the alkyl magnesium (2a¢) and zinc (2b¢) by ligand L2–H. The
formula and structure are conformed on the basis of 1H and
13C NMR spectra and X-ray crystal analysis. The single crystal
of complex 2b in Fig. 1 and the supporting information (Table
S1, ESI†) shows that the zinc complex is the dimer lying
about an inversion centre in the crystal structure with the four-
coordinate Zn(II) ion adopting a distorted tetrahedral geometry.
The angles between O(1)–Zn–O(2) (123.74(6)◦) and O(2)–Zn–
O(2A) (82.42(5)◦) and the distances between the Zn atom and
O(1), O(2), O(2A) and N were 1.9028(13), 1.9766(13), 1.9827(13)
Scheme 3 Synthesis of magnesium and zinc complexes 1b–9b.
(Chart 1) existing in solution based on the 1H NMR studies of 2b.
To prove which two isomers are possible, freezing point depression
studies of 2b using naphthalene as the solvent was performed. The
dissociation percentage of the dimeric species can be calculated
from eqn (1) which DTf is the melting point depressed,
(1)
DTf = i ¥ Kf ¥ Cm
i is the number of particles measured number of particles added,
◦
Kf is 6.9 C mol-1 kg for naphthalene, and Cm is the molality of
the solute (mol kg-1).
˚
and 2.1015(15) A, confirming the distortion of the structure from
an ideal tetrahedral topology.
Chart 1 Two possible isomers for 2b.
Experimental results reveal the activity i = 1.66 0.07, indicating
the existence of ca. 34% of dinuclear species and 66% of
mononuclear species.
Ring-opening polymerization of L-lactide
Polymerization of L-lactide using 1a (5 mM) as an initiator
was systematically investigated in toluene under a dry nitrogen
atmosphere (Table 1, entry 1–4). From Table 1, it was found that
complex 1a is an efficient initiator for the polymerization of L-
lactide. The polymerization was accomplished within 4 min at
room temperature and the scope of these poly(L-lactide)’s (PLLA)
polydispersity index (PDI, Mw/Mn) (1.01–1.37) is acceptable.
The molecular weight of PLLA was not easy to control due to
its high activity. In Fig. 2, the 1H NMR spectrum of PLLA
prepared using a [LA]0/[initiator] ratio of 25 showed one ethyl
group and one hydroxy chain end with an integral ratio of
2 : 1 among Hc and Hd, suggesting that the initiation occurred
through the insertion of the 2-(dimethylamino)ethanolate group
from 1a into L-lactide. There are no significant influences on
the activity of magnesium complexes 2a–5a towards ROP of
L-lactide by modifying the steric and coordination number of
Fig.
1
Molecular structure of [L2Zn(m-Obn)]2 (2b) as 20% ellip-
soids (hydrogen atoms and four cyclohexyl groups are omitted
for clarity). Selected bond lengths (A) and angles (◦): Zn–O(1)
˚
1.9028(13), Zn–O(2) 1.9766(13), Zn–O(2a)1 1.9827(13), Zn–N 2.1015(15),
O(1)–Zn–O(2) 123.74(6), O(1)–Zn–O(2a)1 108.43(6), O(2)–Zn–O(2a)1
82.42(5), O(1)–Zn–N 100.07(6), O(2)–Zn–N 124.13(6), O(2a)1–Zn–N
117.25(6). The symmetry operation implied by the additional “A” letters
in the atom labels is (-x, -y, -z).
Melting point depression study of 2b
Although the crystal structure study of 2b indicates that complex
2b is a dinuclear species, there are two possible isomers for 2b
9602 | Dalton Trans., 2011, 40, 9601–9607
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Table 1 Polymerization of L-lactide using complexes 1a–5a, 1b–5b, and 2c as initiators at room temperature in toluene (15 mL)
Entry
[M]/ [Init.]
T (min)
Conv (%)a
Mn (calcd)b
Mn (GPC)c
Mn (nmr)a
PDI
1
2
3
4
5
6
7
8
25 (1a)
4
4
4
4
3
6
5
3
70
70
70
70
70
9
60
73
24
60
94
88
98
98
98
98
93
97
95
87
83
92
92
97
90
95
94
80
3400
6400
9400 (5500)
4500
1.37
1.22
1.21
1.01
1.27
1.31
1.15
1.16
1.07
1.09
1.06
1.05
1.05
1.07
1.02
1.03
1.17
1.17
50 (1a)
26 100 (15 100)
33 400 (19 400)
22 700 (13 200)
27 400 (15 900)
27 300 (15 800)
34 600 (20 100)
22 700 (16 100)
2900 (1700)
12 300
21 600
18 400
75 (1a)
10 700
14 200
14 200
14 200
13 500
14 200
1600
3000
5300
6400
13 400
14 100
13 100
13 800
13 600
13 100
100 (1a)
100 (2a)
100 (3a)
100 (4a)
150 (5a)
12.5 (1b)
25 (1b)
d
—
—
—
—
d
d
d
9
1800
3300
5600
7500
10
11
12
13
14
15
16
17
18
5400 (3100)
9300 (5400)
37.5 (1b)
50 (1b)
12 900 (7500)
25 800 (15 000)
76 800 (44 500)
32 500 (18 900)
28 600 (16 600)
31 700 (18 400)
29 800 (17 300)
d
100 (1b)
100 (2b)
100 (3b)
100 (4b)
100 (5b)
100 (2c)
—
—
—
—
—
—
d
d
d
d
d
a Obtained from 1H NMR analysis. b Calculated from the molecular weight of LA ¥ [M]0/[RO-]0 ¥ conversion yield + Mw(ROH). c Obtained from GPC
analysis and calibrated by a polystyrene standard. Values in parentheses are the values obtained from GPC ¥ 0.58.10 d Not available.
Fig. 2 1H NMR spectrum of PLLA-25 (25 indicates [LA]0/[initiator]0 =
25) by 1a.
Fig. 3 Polymerization of LA catalyzed by 1b in toluene at room
the ligands (Table 1, entry 5–8). Experimental results show that
temperature. The relationship between Mn ( ), PDI ( ) of polymer and
all these complexes efficiently initiate the polymerization of L-
the initial mole ratio [LA]0/[initiator] is shown.
lactide during 3–6 min. Ring-opening polymerization of L-lactide
employing zinc complex 1b as an initiator in toluene completed
within 70 min at room temperature. On the basis of the molecular
weight of PLLA and the [LA]0/[1b] ratio (Table 1, entry 9–13),
we conclude that the benzylalkoxides could be used as a good
initiators. The linear increase in Mn with conversion and the low
PDI of the polymers revealed that the level of polymerization
control was high (Fig. 3). The 1H NMR spectrum of PLLA (Fig.
4) indicated that the polymer chain should be capped with one
benzyl ester and one hydroxy end; this result suggested that the
polymerization occurred through the insertion of a BnO- group
into the L-lactide. When the amino group of the catalysts was
substituted to 2b–5b, the activities were changed (Table 1, entry 14–
17). Experimental results showed that all Zn complexes efficiently
initiate the polymerization of L-lactide, and PLLA was obtained
with the expected molecular weight and with low polydispersity
except when using 2b. The complex 2b can also polymerize
L-lactides with narrow molecular weight distribution and high
activity (only 9 min). From Table 1 (entry 13–17), the reactivity
order of the catalysts is 2b > 5b > 3b ≥ 1b ≥ 4b. The driving
force from dimer to monomer is the key factor, presumably the
Fig. 4 1H NMR spectrum of PLLA-25 (25 indicates [LA]0/[BnO-]0 = 25)
by 1b.
monomer with three coordination numbers were more effective
than the dimer with four coordination numbers. It makes the
complex 2b dissociate to be the monomer easily and its own high
activity is due to the bulky dicyclohexyl group. The complex 5b also
has a similar cause, as the labile ‘sulfur’ atom of the ligand can
stabilize the intermediate between the monomer and the dimer.
When the magnesium benzylalkoxide complex 2c was used as
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the initiator, the L-LA polymerization approached to 80% within
60 min at room temperature (entry 18), which is less effective than
2a and 2b. The reason for the activity of 2a > 2c was that the amino
group of 2-(dimethylamino)ethanolate can stabilize the Mg center
during the polymerization with L-LA dissociation and association.
The activity of complex 2b was more than that of 2c, probably due
to Mg–OBn being a stronger bond than Zn–OBn, thus 2c needs
more energy to break the Mg–OBn bond to initiate ROP of L-LA.
to 2-(dimethylamino)ethanolate. However for zinc complexes, the
activity increases by changing the amino groups toward huge alkyl
or multi-dentate labile group.
Experimental section
General
Standard Schlenk techniques and a N2-filled glove-box were
used throughout the isolation and handling of all the com-
pounds. Solvents, L-lactide, and deuterated solvents were purified
prior to use. Naphthalene, diethylamine, 2,4-di-tert-butylphenol,
dicyclohexylamine, piperidine, 1-methylpiperazine, bis(thiophen-
2-ylmethyl)amine, and para-formaldehyde were purchased and
used without further purification. 2,2¢-Methylenebis(4-chloro-
6-isopropyl-3-methylphenol) (L3–H2) was prepared by acid-
catalyzed condensation following literature procedures.2d 1H and
13C NMR spectra were recorded on a Varian Mercury-400 (400
Mechanistic studies of polymerization of L-lactide
Although 2b is a dimeric four-coordinated compound in the solid
state, it exists in equilibrium between a dimer and a monomer
1
in solution according to the H NMR spectrum. Therefore, the
reaction mechanism was proposed as shown in Scheme 4. L-
Lactide coordinates to the monomeric three coordinated metal
alkoxide a, yielding a penta-coordinated intermediate b, followed
by the insertion of an alkoxide group into L-lactide to produce a
new initiator c. There exists the equilibrium between the monomer
c and dimer d, except that the bulky steric or multi-dentate
amino group attached tends to monomer c. When more L-lactides
coordinate to the monomer c, this step is the chain propagation
step for the production of PLLA and finally PLLA was obtained.
1
MHz for H and 100 MHz for 13C) spectrometer with chemical
shifts given in ppm from the internal TMS or center line of CDCl3.
Microanalyses were performed using a Heraeus CHN-O-RAPID
instrument.
Synthesis and characterization of ligands (L1–H–L5–H)
Synthesis of 2,4-di-tert-butyl-6-((diethylamino)methyl)phenol
(L1–H)11. Diethylamine (7.31 g, 100.0 mmol), para-
formaldehyde (3.00 g, 100.0 mmol), and 2,4-di-tert-butylphenol
(10.31 g, 50 mmol) were dissolved in ethanol (20 mL). The
solution was refluxed for 24 h under nitrogen and then cooled
to room temperature. Volatile materials were removed under
vacuum to yield a light yellow oil. The oil was dissolved in
CH2Cl2 (100 mL) and the solution was washed with water (2 ¥
100 ml) and the solvent removed at reduced pressure to give the
1
white powders (10.67 g, 73.2%). H NMR (400 MHz, CDCl3,
ppm): 7.20, 6.82 (s, Ph-H, 2H); 3.74 (s, PhCH2N, 1H); 2.61 (q,
N(CH2CH3)2, J = 7.2 Hz, 4H); 1.42, 1.28 (s, C(CH3)3, 18H); 1.09
(t, N(CH2CH3)2, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3,
ppm): 154.69, 140.18, 135.42, 123.20, 122.54, 121.44 (Ph); 57.86
(PhCH2N); 46.16 (NCH2CH3); 34.83, 34.11 (CCH3); 31.71, 29.59
(CCH3); 11.22 (NCH2CH3).
Synthesis of 2,4-di-tert-butyl-6-((dicyclohexylamino)methyl)-
1
phenol (L2–H)12. Using a method similar to that for L1–H. H
NMR (400 MHz, CDCl3, ppm): 7.16, 6.80 (s, Ph-H, 2H); 3.91 (s,
PhCH2N, 2H); 2.66 (quintet, CH2N(CH)2, 2H); 1.85–1.58, 1.26–
0.99 (m, Cy-H, 22H). 13C NMR (100 MHz, CDCl3, ppm): 155.07,
139.94, 135.19, 123.07, 122.26, 122.04 (Ph); 57.74 (PhCH2N);
50.41 (NCH2CH3); 34.78, 34.11 (CCH3); 30.69, 29.54 (CCH3);
31.74, 26.22, 26.04 (Cy).
Scheme 4 Proposed mechanism for the ROP of L-lactide by Mg or Zn
complexes.
Conclusions
Synthesis of 2,4-di-tert-butyl-6-(piperidin-1-ylmethyl)phenol
A series of aminophenol ligands (L1–H–L5–H) and their mag-
nesium and zinc alkoxide complexes were synthesized. Through
modifying the amino group with different coordination numbers
and size, the activity of the system was adapted for the poly-
merization of L-LA. For magnesium complexes, there are slight
diversities with the different amino groups although the activity
dramatically increases by changing the initiator from benzyl oxide
(L3–H)11. Using a method similar to that for L1–H. H NMR
1
(400 MHz, CDCl3, ppm): 7.20, 6.81 (s, Ph-H, 2H); 3.63 (s,
PhCH2N, 2H); 2.58, 1.62 (m, N(C5H10), 10H); 1.42, 1.28 (s,
C(CH3)3, 18H). 13C NMR (100 MHz, CDCl3, ppm): 154.46,
140.20, 135.34, 123.25, 122.60, 121.00 (Ph); 62.87 (PhCH2N);
53.67, 25.78, 24.08 (N(C5H10)r); 34.81, 34.07 (C(CH3)3); 31.67,
29.56 (C(CH3)3).
9604 | Dalton Trans., 2011, 40, 9601–9607
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Synthesis
of
2,4-di-tert-butyl-6-((4-methylpiperazin-1-yl)-
(NCH3); 57.36 (OCH2CH2N); 54.99 (PhCH2N), 45.77, 35.82,
34.11 (N(C5H10)); 32.62, 32.56 (C(CH3)3); 32.48, 30.02 (C(CH3)3).
Anal. Calc. For (C24H42MgN2O2): C, 69.47%; H, 10.20%; N,
6.75%. Found: C, 70.51%; H, 9.92%; N, 7.25%. Mp: 170–172 ◦C.
methyl)phenol (L4–H)13. Using a method similar to that for
1
L1–H. H NMR (400 MHz, CDCl3, ppm): 7.22, 6.84 (s, Ph-H,
2H); 3.69 (s, PhCH2N, 2H); 2.54–2.34 (br, N(C4H8), 8H); 2.32
(s, NCH3, 3H); 1.41, 1.28 (s, CCH3, 18H). 13C NMR (100 MHz,
CDCl3, ppm): 154.13, 140.29, 135.51, 123.45, 122.95, 120.50 (Ph);
62.10 (PhCH2N); 54.90, 52.25 (N(C4H8)N); 45.82 (NCH3); 34.84,
34.12 (CCH3); 31.67, 29.59 (CCH3).
Synthesis of L4MgOC2H4NMe2 (4a). Using a method similar
to that for 1a. Yield: 1.76 g (82.%). 1H NMR (400 MHz,
C6D6, ppm): 7.63, 7.06 (s, PhH, 2H); 3.53 (s, PhCH2N,
2H); 3.02 (br, OCH2, 2H); 2.22 (br, CH2NMe2, 2H); 2.38–
2.22, 2.11–2.08 (br, ArH, NCH3, 17H);1.85, 1.49 (s, C(CH3)3,
18H). 13C NMR (100 MHz, CDCl3, ppm): 164.54, 135.84,
132.20, 125.70, 123.65, 120.30 (Ph); 62.86 (OCH2CH2N); 57.47
(OCH2CH2N); 52.29, 45.20 (Ar); 35.13 (NCH3); 33.63 (ArNCH3);
31.87, 29.92 (C(CH3)3); 31.61, 29.52 (C(CH3)3). Anal. Calc. For
(C24H43MgN3O2): C, 67.05%; H, 10.08%; N, 9.77%. Found: C,
66.69%; H, 10.18%; N, 9.60%. Mp: 176–182 ◦C.
Synthesis of 2-((bis(thiophen-2-ylmethyl)amino)methyl)-4,6-di-
tert-butylphenol (L5–H). Using a method similar to that for L1–
1
H. H NMR (400 MHz, CDCl3, ppm): 7.22, 6.87 (s, Ph-H, 2H);
7.30 (d, Ar-H, 2H); 7.01–6.80 (m, Ar-H, 4H); 3.88 (s, NCH2,
2H); 3.82 (s, PhCH2N, 2H); 1.45, 1.28 (s, CCH3, 18H). 13C NMR
(100 MHz, CDCl3, ppm): 154.07, 140.77, 138.70, 135.84, 123.13,
120.71 (Ph); 127.80, 126.76, 125.75 (Ar); 57.49 (PhCH2N); 49.95
(NCH2); 34.94, 34.15 (CCH3); 31.68, 29.65 (CCH3). Anal. Calc.
for (C25H33NOS2): C, 70.21%; H, 7.78%; N, 3.28%; S, 15.00%.
Found: C, 70.21%; H, 7.49%; N, 3.19%; S, 14.80%
Synthesis of L5MgOC2H4NMe2 (5a). Using a method similar
1
to that for 1a. Yield: 1.16 g (43%). H NMR (400 MHz, C6D6,
ppm): 7.51–6.70 (m, ArH, 8H); 3.73 (s, PhCH2N, 2H); 1.76, 1.38 (s,
C(CH3)3, 18H). 13C NMR (100 MHz, C6D6, ppm): 154.87, 140.99,
136.29, 126.34, 123.35, 121.36 (Ph); 139.16, 126.84, 124.47 (Ar);
57.59 (PhCH2N); 50.14 (NCH2Ar); 46.40 (OCH2CH2NMe2);
35.39 (OCH2CH2NMe2); 32.76 (NCH3); 35.77, 34.35 (CCH3);
31.94, 30.02 (CCH3). Anal. Calc. For (C29H42MgN2O2S2): C,
64.61%; H, 7.85%; N, 5.20%. Found: C, 62.03%; H, 8.25%; N,
6.31%. Mp: 182–184 ◦C.
Synthesis of magnesium complexes
Synthesis of L1MgOC2H4NMe2
(1a). MgnBu2
(5.5
mL, 1.0 in hexane, 5.50 mmol) was added slowly
M
to an ice cold (0 ◦C) solution of L1–H (1.45 g,
5.00 mmol) in THF (15 mL). The mixture was stirred for
3 h and then 2-(dimethylamino)ethanol (0.6 mL, 10.0 M,
6.0 mmol) was added and stirred for 3 h at room temperature.
Volatile materials were removed under vacuum to yield yellow
oil. The oil was washed with hexane (20 mL) and a white powder
was obtained after filtration. Yield: 1.45 g (72%). 1H NMR
(400 MHz, CDCl3, ppm): 7.17, 6.79 (s, Ph-H, 2H); 3.79–3.56
(br, PhCH2NEt2, OCH2CH2, 4H); 2.73 (t, OCH2CH2NMe2, J =
14 Hz, 2H); 2.33 (br, N(CH2CH3)2, N(CH3)2, 10H); 1.47, 1.28 (s,
C(CH3)3, 18H); 1.09 (t, N(CH2CH3)2, 6H). 13C NMR (100 MHz,
CDCl3, ppm): 164.7, 135.6, 131.8, 125.6, 123.3, 121.0 (Ph); 62.7
(OCH2CH2NMe2); 59.4 (OCH2CH2NMe2); 57.4 (PhCH2N); 45.0
(NCH2CH3); 43.2 (NCH3); 35.1, 33.6 (CCH3); 32.0, 30.0 (CCH3);
29.5 (NCH2CH3). Anal. Calc. for (C23H42MgN2O2): C, 68.56%;
H, 10.51%; N, 6.95%. Found: C, 67.94%; H, 9.86%; N, 6.81%.
Mp: 177–178 ◦C.
Synthesis of L2MgnBu (2a¢). MgnBu2 (5.5 mL, 1.0 M in hexane,
5.50 mmol) was added slowly to an ice cold (0 ◦C) solution
of L2–H (2.00 g, 5.00 mmol) in THF (15 mL). The mixture
was stirred for 3 h, and then 2-(dimethylamino)ethanol (0.6 mL,
10.0 M, 6.0 mmol) was added to be stirred for 3 h at room
temperature. Volatile materials were removed under vacuum to
yield a yellow oil. The oil was washed with hexane (20 mL) and a
white powder was obtained after filtration. Yield: 2.12 g (83.4%).
1H NMR (400 MHz, C6D6, ppm): 7.55, 7.05 (s, PhH, 2H); 4.40,
3.75 (dd, PhCH2, J = 12.8 Hz, 2H); 2.73–1.78, 1.78–1.38, 1.38–
0.026 (m, Cy-H, MgC4H9, 31H); 1.78, 1.38 (s, C(CH3)3, 18H).
13C NMR (100 MHz, C6D6, ppm): 157.77, 140.80, 140.30, 127.37,
126.96, 125.57 (Ph); 59.96 (PhCH2N); 35.98, 34.16 (CCH3); 31.72,
30.47 (CCH3); 64.19, 31.89, 26.82, 27.81 (Cy); 30.46, 26.13, 14.35,
12.64 (MgC4H9). Anal. Calc. For (C31H53MgNO): C, 77.56%; H,
11.13%; N, 2.92%.. Found: C, 77.45%; H, 11.59%; N, 2.99%. Mp:
212–214 ◦C.
Synthesis of L2MgOC2H4NMe2 (2a). Using a method similar
to that for 1a. Yield: 1.20 g (47.1%). 1H NMR (400 MHz, C6D6,
ppm): 7.49, 6.98 (s, PhH, 2H); 3.73 (s, PhCH2N, 2H); 3.41 (br,
OCH2, 2H); 2.08 (br, CH2NMe2, 2H); 1.89 (s, N(CH3)2, 6H);
2.58–2.54, 1.73–1.53, 1.18–0.29 (m, Cy-H, 22H); 1.76, 1.38 (s,
C(CH3)3, 18H). 13C NMR (100 MHz, C6D6, ppm): 155.88, 140.38,
135.84, 123.34, 122.80, 122.54 (Ph); 57.80, 32.04, 26.34, 26.18
(Cy); 50.77 (NCH3); 35.31 (CH2N); 34.35 (OCH2CH2N); 31.90
(OCH2CH2N); 30.87, 30.00 (C(CH3)3); 22.98, 14.26 (C(CH3)3).
Anal. Calc. for (C31H54MgN2O): C, 72.85%; H, 10.65%; N, 5.48%.
Found: C, 57.11%; H, 9.44%; N, 9.52%. Mp: 173–174 ◦C.
Synthesis of L2MgOBn (2c). MgnBu2 (5.5 mL, 1.0 M in hexane,
5.50 mmol) was added slowly to an ice cold (0 ◦C) solution of L2–H
(2.00 g, 5.00 mmol) in THF (15 mL). The mixture was stirred for 3
h, and then benzyl alcohol (0.6 mL, 10.0 M, 6.0 mmol) was added
to be stirred for 3 h at room temperature. Volatile materials were
removed under vacuum to yield a light yellow oil. The oil was
washed with hexane (20 mL) and a white powder was obtained
1
after filtration. Yield: 2.26 g (81%). H NMR (400 MHz, C6D6,
Synthesis of L3MgOC2H4NMe2 (3a). Using a method similar
to that for 1a. Yield: 1.39 g (70%). H NMR (400 MHz, C6D6,
ppm): 7.63, 7.07 (s, PhH, 2H); 3.32 (s, PhCH2N, 2H); 2.93 (br,
OCH2, 2H); 2.10 (br, CH2NMe2, 2H); 2.05 (s, N(CH3)2, 6H);
2.05–1.87, 1.87–1.51, 1.51–1.22 (m, N(C5H10), 10H); 1.87, 1.22 (s,
C(CH3)3, 18H). 13C NMR (100 MHz, C6D6, ppm):165.37, 136.74,
132.94, 125.64, 124.14, 121.33 (Ph); 64.99 (OCH2CH2N); 63.46
ppm):7.62–6.98 (m, PhH, 7H); 5.10 (d, OCH2Ph, J = 77.6 Hz, 2H);
3.55 (s, PhCH2N, 2H); 2.52–2.10, 1.91–1.53, 1.38–0.73 (m, Cy-H,
22H); 1.96, 1.49 (s, 18H, C(CH3)3). 13C NMR (100 MHz, C6D6,
ppm): 164.16, 155.88, 144.57, 140.39, 136.93, 134.54, 126.84,
126.53, 123.47, 122.54 (Ph); 66.61 (OCH2Ph); 57.43 (PhCH2N);
35.81, 34.15 (CCH3); 30.87, 30.48 (CCH3); 61.80, 32.35, 26.95,
26.34 (Cy). Anal. Calc. For (C34H51MgNO2): C, 77.04%; H, 9.70%;
1
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N, 2.64%. Found: C, 76.17%; H, 9.59%; N, 2.96%. Mp: 170–
172 ◦C.
6.60 (m, 13H, ArH); 4.43 (d, OCH2Ph, J = 13.6 Hz, 2H); 3.86–3.66
(m, 6H, NCH2); 1.61, 1.26 (s, C(CH3)3, 18H). 13C NMR (100 MHz,
CDCl3, ppm): 163.45, 144.26, 137.63, 134.74, 134.19, 133.96,
131.16, 131.00, 128.33, 127.33, 126.98, 126.66, 124.22, 118.31 (Ar);
68.80 (OCH2Ph); 59.83 (PhCH2N); 49.10 (NCH2); 35.52, 33.89
(CCH3); 31.85, 29.82 (CCH3). Anal. Calc. For (C32H39NO2S2Zn):
C, 71.50%; H, 9.00%; N, 2.45%. Found: C, 70.83%; H, 8.82%; N,
2.96%. Mp: 176–178 ◦C.
Synthesis of zinc complexes
Synthesis of L1ZnOBn (1b). ZnEt2 (5.5 mL, 1.0 M in hexane,
5.50 mmol) was added slowly to an ice cold (0 ◦C) solution of L1–
H2 (2.91 g, 5.00 mmol) in THF (15 mL). The mixture was stirred
for 3 h, and then benzyl alcohol (0.6 mL, 10.0 M, 6.0 mmol) was
added to be stirred for 3 h at room temperature. Volatile materials
were removed under vacuum to yield a light yellow oil. The oil
was washed with hexane (20 mL) and a white powder was obtained
after filtration. Yield: 1.89 g (81.6%). 1H NMR (400 MHz, CDCl3,
ppm): 7.37, 7.28, 7.16, 6.72 (s, Ph-H, 7H); 4.82 (s, OCH2Ph, 2H);
3.42 (s, CH2NEt2, 2H); 2.39 (septet, N(CH2CH3)2, J = 6.8 Hz, 4H);
1.58, 1.27 (s, C(CH3)3, 18H); 0.96 (t, N(CH2CH3)2, J = 6.8 Hz,
6H). 13C NMR (100 MHz, CDCl3, ppm): 163.64, 144.09, 137.38,
134.74, 128.28, 127.61, 127.02, 125.54, 124.00, 119.48 (Ph); 69.78
(OCH2Ph); 59.54 (PhCH2NEt2); 43.94 (NCH2CH3); 35.36, 33.81
(PhCCH3); 31.85, 29.61 (PhCCH3); 7.87 (NCH2CH3). Anal. Calc.
For (C26H39NO2Zn): C, 67.45%; H, 8.49%; N, 3.03%. Found: C,
67.21%; H, 8.03%; N, 3.48%. Mp: 180–184 ◦C.
Synthesis of L2ZnEt (2b¢). ZnEt2 (5.5 mL, 1.0 M in hexane,
5.50 mmol) was added slowly to an ice cold (0 ◦C) solution of L2–H
(2.00 g, 5.00 mmol) in THF (15 mL). The mixture was stirred for 3
h, and then 2-(dimethylamino)ethanol (0.6 mL, 10.0 M, 6.0 mmol)
was added to be stirred for 3 h at room temperature. Volatile
materials were removed under vacuum to yield a light yellow oil.
The oil was washed with hexane (20 mL) and a white powder was
obtained after filtration. Yield: 2.12 g (81%). 1H NMR (400 MHz,
CDCl3, ppm): 7.22, 6.87 (s, PhH, 2H); 3.90 (s, PhCH2N, 2H); 2.83–
1.54, 1.40–1.31, 1.26–1.01 (m, Cy-H, ZnCH2CH3, 25H); 1.43, 1.28
(s, C(CH3)3, 18H); 0.36 (q, ZnCH2CH3, J = 5.3 Hz, 2H). 13C
NMR (100 MHz, CDCl3, ppm):161.46, 138.02, 136.55, 125.43,
123.49, 123.03 (Ph); 51.93 (PhCH2N); 35.04, 33.89 (CCH3); 29.65,
26.68 (CCH3); 62.73, 31.79, 26.00, 25.70 (Cy); 12.04 (ZnCH2CH3);
1.05 (ZnCH2CH3). Anal. Calc. For (C29H49NOZn): C, 70.64%; H,
10.02%; N, 2.84%. Found: C, 69.59%; H, 9.58%; N, 2.70%. Mp:
202–204 ◦C.
Synthesis of L2ZnOBn (2b). Using a method similar to that
1
for 1b. Yield: 2.19 g (77%). H NMR (400 MHz, CDCl3, ppm):
7.43–6.77 (m, PhH, 7H); 5.13, 4.87, 4.94 (s, OCH2Ph, 2H);3.76(s,
PhCH2, 2H); 2.69–1.62, 1.56–1.27, 1.23–0.75(m, Cy-H, 22H);
1.59, 1.25 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3, ppm):
164.12, 144.29, 137.17, 134.34, 128.93, 127.91, 126.36, 126.18,
123.21, 121.36 (Ph); 68.12 (OCH2Ph); 56.79 (PhCH2N); 35.37,
33.79 (CCH3); 31.90, 29.92 (CCH3); 63.41, 31.45, 30.73, 26.75,
25.74 (Cy). Anal. Calc. For (C34H51NO2Zn): C, 71.50%; H, 9.00%;
N, 2.45%. Found: C, 70.83%; H, 8.82%; N, 2.96%. Mp: 186–
190 ◦C.
X-ray crystallographic studies
A suitable crystal of complex 2b was sealed in a thin-walled glass
capillary under a dry nitrogen atmosphere and mounted on a
Bruker AXS SMART 1000 diffractometer. Intensity data were
collected in 1350 frames with increasing w (width of 0.3◦ per
frame). The absorption correction was based on the symmetry-
equivalent reflections using the SADABS program. The space
group determination was based on a check of the Laue symmetry
and systematic absences and confirmed by using the structure
solution. The structure was solved by the direct method using
the SHELXTL package. All non-H atoms were located from
successive Fourier maps, and hydrogen atoms were refined using
a riding model. Anisotropic thermal parameters were used for
all non-H atoms, and fixed isotropic parameters were used for H
atoms.
Synthesis of L3ZnOBn (3b). Using a method similar to that
for 1b. Yield: 2.00 g (84.4%). 1H NMR (400 MHz, CDCl3, ppm):
7.36, 7.30, 7.14, 7.06, 6.68 (s, Ph-H, 7H); 4.87(br, OCH2Ph, 2H);
3.63 (s, PhCH2N, 2H); 2.68–1.65 (m, N(C5H10), 10H); 1.65, 1.29
(s, C(CH3)3, 18H). 13C NMR (100 MHz, CDCl3, ppm): 164.58,
164.09, 137.81, 137.15, 134.84, 128.30, 127.04, 125.89, 124.24,
119.44 (Ph); 65.58 (OCH2Ph); 55.86 (PhCH2); 56.37, 24.91, 23.58
(N(C5H10)); 35.36, 33.86 (C(CH3)3); 31.91, 29.73 (C(CH3)3). Anal.
Calc. For (C27H39NO2Zn): C, 68.27%; H, 8.28%; N, 2.95%. Found:
C, 67.85%; H, 8.02%; N, 2.90%. Mp: 216–220 ◦C.
Synthesis of L4ZnOBn (4b). Using a method similar to that
for 1b. Yield: 1.57 g (64.1%). 1H NMR (400 MHz, CDCl3,
ppm): 7.41, 7.32, 7.11, 6.69 (s, PhH, 7H); 4.88 (br, OCH2Ph,
2H); 3.30 (s, PhCH2N, 2H); 2.71–2.46, 2.32–2.09 (m, N(C5H10),
10H); 2.40 (s, NCH3, 3H); 1.64, 1.29 (s, C(CH3)3, 18H). 13C
NMR (100 MHz, CDCl3, ppm): 163.68, 137.83, 137.17, 135.17,
129.02, 128.47, 128.21, 127.06, 125.96, 125.29, 124.43, 118.88 (Ph);
68.92 (OCH2Ph); 64.92 (PhCH2); 54.94, 53.84 (N(C5H10)); 45.82
(NCH3); 35.56, 33.86 (CCH3); 31.88, 29.88 (CCH3). Anal. Calc.
For (C27H40N2O2Zn): C, 66.18%; H, 8.23%; N, 5.72%. Found: C,
66.54%; H, 8.35%; N, 5.46%. Mp: 228–230 ◦C.
General procedures for the polymerization of L-lactides
A typical polymerization procedure was exemplified by the
synthesis of entry 12 (Table 1) using complex 1b as an initiator.
1
The polymerization conversion was analyzed by H NMR spec-
troscopic studies. Toluene (10.0 mL) was added to a mixture of
complex 1b (0.0291 g, 0.05 mmol) and L-lactide (0.36 g, 2.5 mmol)
at 25 ◦C. After the solution was stirred for 70 min, the reaction was
then quenched by adding to a drop of water, and the polymer was
precipitated by pouring into n-hexane (100.0 mL) to give white
solids. The white solid was dissolved in CH2Cl2 (10.0 mL) and
then n-hexane (100.0 mL) was added to give a white crystalline
solid. Yield: 0.54 g (75%).
Synthesis of L5ZnOBn (5b). Using a method similar to that for
1b. Yield: 1.32 g (44.1%). 1H NMR (400 MHz, CDCl3, ppm): 7.34–
9606 | Dalton Trans., 2011, 40, 9601–9607
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GPC measurements
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The GPC measurements were performed on a Waters 1515 Iso-
cratic HPLC Pump system equipped with a difference Waters 2414
Refractive Index detector using THF (HPLC grade) as an eluent.
The chromatographic column was a Waters Styragel Column
(HR4E) and the calibration curve was made by polystyrene
standards to calculate Mn(GPC). The solution (25 mL) was then
injected into the GPC and the flow eluent rate was 1 mL min-1. The
results were calculated by SISC chromatography data solution 1.0
edition.
Acknowledgements
Financial support from the National Science Council of Republic
of China is gratefully acknowledged.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9601–9607 | 9607
©