Structural and catalytic studies of zinc complexes containing amido-oxazolinate ligands

Article abstract of DOI:10.1039/c1dt10974j

Several zinc complexes bearing amido-oxazolinate ligands are described. Reactions of ligand precursors, HNC₂^EOxa (HNC ₂^EOxa = HNC₂^MeOxa, HNC ₂^OMeOxa, HNC₂

Full text of DOI:10.1039/c1dt10974j

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PAPER
Structural and catalytic studies of zinc complexes containing
amido-oxazolinate ligands†
Ming-Tsz Chen and Chi-Tien Chen*
Received 25th May 2011, Accepted 12th September 2011
DOI: 10.1039/c1dt10974j
Several zinc complexes bearing amido-oxazolinate ligands are described. Reactions of ligand
E
E
precursors, HNC2 Oxa (HNC2 Oxa = HNC2MeOxa, HNC2OMeOxa, HNC2StBuOxa and HNC2NMe2Oxa)
or HNPhSMeOxa, with half or one molar equivalent of ZnEt2 afford bis(chelate) zinc complexes,
E
E
E
E
(NC2 Oxa)2Zn [C2 = propyl, (1); C2 = 2-methoxyethyl, (2); C2 = 2-N,N¢-dimethylethyl, (3)] or zinc
ethyl complexes, (NC2StBuOxa)ZnEt(4) and (NPhSMeOxa)ZnEt(5), using tetrahydrofuran or hexane as
E
E
E
solvents. The zinc benzyl oxide complexes, [(NC2 Oxa)Zn(m-OBn)]2 [C2 = propyl, (6); C2
=
E
2-methoxyethyl, (7); C2 = 2-tert-butylthioethyl, (8)], are obtained from the reactions of ligand
E
precursors, HNC2 Oxa, with one molar equivalent of ZnEt(OBn) (generated in situ on 1 : 1 ratio of
ZnEt2 and BnOH) in tetrahydrofuran. The molecular structures are reported for compounds 1, 3, 5, 6
and 7. All eight compounds were assessed as efficient catalyst precursors towards the ring-opening
polymerization of L-lactide and e-caprolactone.
complexes have been synthesized and their catalytic activities in
ring opening polymerization of L-lactide or e-caprolactone in the
Introduction
Biodegradable polymers such as poly(e-caprolactone) (PCL) and
poly(lactide) (PLA), as well as their copolymers, are useful for
their application for tissue engineering, drug delivery, and environ-
mentally friendly wrapping materials.1 The major polymerization
method for synthesizing biodegradable polymers using metal-
based initiators/catalysts for ring opening polymerization has
been proven to provide greater control over the molecular char-
acteristics of the polymers.2 Therefore metal complexes bearing
auxiliary ligands for ring opening polymerization have attracted
great interest, mainly because of their promising activities and
great success in preparing the well-defined polyesters. Due to
the promising catalytic application of metal b-diketiminate com-
plexes in ring opening polymerization, metal centres including
magnesium, calcium and zinc, with structurally-related ligands
were synthesized and some of them have been examined for their
catalytic activities in ring opening polymerisation.3 The efficient
performance of these metal complexes encourages us to investigate
ligand precursors bearing similar chelating systems and iso-
electronic feature related to b-diketiminate ligands. According to
our previous catalytic studies on metal anilido-oxazolinate3k,3o,3p or
anilido-pyrazolate3q complexes and the catalytic activities demon-
strated by some zinc complexes bearing b-diketiminate ligands in
ring opening polymerization,4 zinc amido-oxazolinate complexes
are expected to be useful initiators/catalysts in ring opening
polymerization. In this paper, several zinc amido-oxazolinate
presence of benzyl alcohol are also investigated.
Results and discussion
The preparation of ligand precursor HNPhSMeOxa3o has
been reported by us using palladium-catalyzed amination.5
According to the previous established procedure, similar
ligand precursors with aliphatic functionalities were pre-
pared straightforward by using copper-catalyzed amination6
of 2-(2-bromophenyl)-4,4-dimethyl-2-oxazoline7 with suitable
amine or hydrochloride salt of amine (propylamine for
HNC2MeOxa;3p 2-methoxyethylamine for HNC2OMeOxa;3p 2-tert-
butylthioethylamine hydrochloride8 for HNC2StBuOxa;3p unsym-
dimethyl-ethylenediamine for HNC2NMe2Oxa) in the presence of
CuI, L-proline and K3PO4 in DMSO at 110 ◦C for 14 h. All of
these ligand precursors are characterized by NMR spectroscopy as
well as elemental analyses. Syntheses and the proposed structures
are summarized in Scheme 1. In view of the promising catalytic
activities of zinc b-diketiminate complexes,3,4 preparation of zinc
amido-oxazolinate complexes was examined using one molar
equivalent of ligand precursors with one molar equivalent of
ZnEt2 resulting in the isolation of zinc ethyl complexes, 4 or 5
as yellow solid. Repeated reactions using one molar equivalent
E
of HNC2 Oxa with half molar equivalent of ZnEt2 in hexane
yielded zinc bis(amido-oxazolinate) complexes 1–3. Syntheses of
zinc benzyl oxide complexes 6–8 can be achieved by the reactions
of HNC2MeOxa, HNC2OMeOxa or HNC2StBuOxa with one molar
equivalent of ZnEt(OBn) (generated in situ on 1 : 1 ratio of ZnEt2
and BnOH) in tetrahydrofuran solution. Alternative route for
Department of Chemistry, National Chung Hsing University, Taichung 402,
Taiwan, Republic of China. E-mail: ctchen@dragon.nchu.edu.tw
† CCDC reference numbers 827166–827170. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt10974j
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Scheme 1 Structures of 1–8.
the preparation of zinc benzyl oxide complex 8, was achieved
by the reaction of 4 with one equivalent of benzyl alcohol
in tetrahydrofuran solution. The spectroscopic data of 1–8 are
consistent with the structures proposed in Scheme 1.
Suitable crystals of 1, 3, 5, 6 and 7 for structural determi-
nation are obtained from concentrated hexane or concentrated
tetrahydrofuran solutions. The molecular structures are depicted
in Fig. 1–5. The geometry at the zinc centre of 1 can be described
as distorted tetrahedral geometry with two chelates coordinating
with bite angles of 94.25(7)◦ and 94.67(7)◦, which are smaller than
those (94.8(2)–99.67(1)◦) found in zinc b-diketiminate complexes
with alkyl oxide bridge functionalities.4a–c,4f,g The Zn–Namido bond
˚
lengths (1.9468(18) and 1.9448(17) A) are shorter than Zn–Noxazoline
˚
bond lengths (2.0113(16) and 2.0077(16) A), which might result
from the p-donation ability of the anionic amido nitrogen.9 For
3, similar to those discussed above, two bite angles, 94.50(7)
and 93.99(7)◦, are similar to those discussed for 1, and the Zn–
˚
Fig. 1 Molecular structure of 1. Selected bond lengths (A) and bond an-
˚
Namido bond lengths (1.9459(18) and 1.9491(18) A) are also shorter
gles (◦): Zn–N(1), 2.0113(16); Zn–N(2), 1.9468(18); Zn–N(3), 2.0077(16);
Zn–N(4), 1.9448(17); C(1)–N(2), 1.348(3); C(10)–N(1), 1.283(3);
C(15)–N(4), 1.348(3); C(24)–N(3), 1.292(3); N(2)–Zn–N(4), 127.01(8);
N(4)–Zn–N(3), 94.25(7); N(1)–Zn–N(4), 114.76(7); N(2)–Zn–N(3),
114.34(7); N(1)–Zn–N(3), 112.87(7); N(1)–Zn–N(2), 94.67(7). Hydrogen
atoms on carbon atoms omitted for clarity. Perspective view of 1 with
probability ellipsoids at 20% level.
˚
than the Zn–Noxazoline bond lengths (2.0069(18) and 2.0110(19) A).
For 5, a distorted trigonal planar geometry can be described
with zinc metal center coordinated with two nitrogen atoms
from ligand and one carbon atom from ethyl group. The bite
angle (93.36(11)◦) is smaller than those discussed above and the
˚
Zn–Namido bond length (1.944(3) A) is shorter than Zn–Noxazoline
˚
bond length (1.991(3) A). The molecular structures of 6 and 7
exist as benzoxyl-bridged dimers with a core planar Zn–O–Zn–
O ring in each case and their molecular structure diagrams can
be described as distorted tetrahedron geometry for zinc centre.
The dimers lie about inversion centres in the crystal structures.
For 6, the planar core features two similar Zn–Obenzoxyl distances
˚
(1.9793(13) and 1.9637(13) A) with ZnOZn and OZnO angles
of 99.00(16) and 81.00(6)◦. Each zinc atom is four-coordinate,
which is coordinated with one nitrogen atom of the oxazoline
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˚
˚
Fig. 3 Molecular structure of 5. Selected bond lengths (A) and
Fig. 2 Molecular structure of 3. Selected bond lengths (A) and bond
bond angles (◦): Zn–N(1), 1.991(3); Zn–N(2), 1.944(3); C(19)–C(20),
1.428(7); C(14)–N(1), 1.285(4); C(12)–S, 1.766(3); C(13)–S, 1.791(4);
N(2)–Zn–N(1), 93.36(11); C(19)–Zn–N(1), 129.30(15); C(19)–Zn–N(2),
137.16(15). Hydrogen atoms on carbon atoms omitted for clarity. Per-
spective view of 5 with probability ellipsoids at 20% level.
angles (◦): Zn–N(1), 2.0069(18); Zn–N(2), 1.9459(18); Zn–N(4), 2.0110(7);
Zn–N(5), 1.9491(18); C(1)–N(2), 1.355(3); C(11)–N(1), 1.283(3);
C(16)–N(5), 1.347(3); C(26)–N(4), 1.285(3); N(1)–Zn–N(2), 94.50(19);
N(4)–Zn–N(5), 93.99(7); N(1)–Zn–N(5), 114.21(8); N(2)–Zn–N(4),
117.60(8); N(2)–Zn–N(5), 125.24(8); N(1)–Zn–N(4), 112.46(8). Hydrogen
atoms on carbon atoms omitted for clarity. Perspective view of 3 with
probability ellipsoids at 20% level.
˚
group with Zn–N(1) bond length of 1.9795(16) A, one nitrogen
atom of the amido group with a Zn–N(2) bond length of
˚
1.9414(17) A and two oxygen atoms from two bridged benzyl
oxide groups. These data are in the range of known distances
and angles for zinc b-diketiminate complexes4 and structurally-
related zinc complexes.3a–3c,3e,10 Basically, compound 7 is quite
similar to compound 6 with 2-methoxyethyl substituent instead
of propyl substituent on the amido group. Similar to 6, two zinc
metal centres are bridged via Zn–Obenzoxyl bonds (1.9753(15) and
˚
1.9637(16) A) to form a planar core ring Zn–O(3)–ZnA–O(3A)
with the angles subtended at the zinc atoms (81.01(6)◦) narrower
than those (98.99(6)◦) at oxygen atoms. Bond lengths and bond
angles are similar to those discussed above for 6.
Polymerization studies
˚
Fig. 4 Molecular structure of 6. Selected bond lengths (A) and bond an-
Several zinc b-diketiminate complexes4 are known as efficient
initiators/catalysts in the ring opening polymerization (ROP).
Polymerization of L-lactide and e-caprolactone employing 1–
8 as catalysts has been systematically investigated under a dry
nitrogen atmosphere. Representative results are collected in Table
1 and 2 for L-lactide and e-caprolactone, respectively. Optimized
gles (◦): Zn–N(1), 1.9795(16); Zn–N(2), 1.9414(17); Zn–O(2), 1.9793(13);
Zn–O(2A), 1.9637(13); O(2)–C(15), 1.404(2); N(1)–C(10), 1.300(2);
N(2)–C(1), 1.354(3); N(1)–Zn–N(2), 96.52(7); N(2)–Zn–O(2A), 120.57(7);
O(2A)–Zn–O(2), 81.00(6); O(2)–Zn–N(1), 120.93(7); N(2)–Zn–O(2),
120.82(7); N(1)–Zn–O(2A), 119.32(6). Zn–O(2)–Zn(A), 99.00(6). Hydro-
gen atoms on carbon atoms omitted for clarity. Perspective view of 6 with
probability ellipsoids at 20% level. Symmetry code A = 2-x,-y, -z.
◦
conditions were found to be toluene at 50 C in the presence of
benzyl alcohol after several trials on running polymerization with
dichloromethane, tetrahydrofuran and toluene for polymerization
of L-lactide or e-caprolactone. The same conditions were applied
to examine the catalytic activities of the other seven catalysts.
Typical polymerization reaction was carried out at 50 ◦C in
10 mL toluene solution for L-lactide, or in 15 mL toluene
solution for e-caprolactone. Experimental results show compound
7 demonstrates similar activities to 8 within the same period
(entries 7–8), whereas 4 and 5 exhibit poor conversion with
time up to 60 min (entries 4–5). Zinc benzyl oxide complexes
6–8 behave in a controlled manner and show better activities
than our previous report with aromatic functionality.3k These
good conversions demonstrated by 6–8 might result from the
tuning of Lewis acidity of metal centre with pendant functionality
or decrease of the rigidity by the alkyl group, which make
coordination of monomer to the metal centre easily, leading to an
increase in propagation. Due to the ease of preparation and better
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those foundinthe reactions without the addition of benzyl alcohol.
The end group analysis is demonstrated by the 1H NMR spectrum
of the polymer produced from L-lactide and 6 ([M]o/[BnOH] =
50). Peaks are assignable to the corresponding protons in the
proposed structure with capped benzyl alkoxyl group, as shown
in Fig. 7. Complexes 1–8 were also investigated to explore their
catalytic behavior in the ROP of e-caprolactone. Representative
results are collected in Table 2. Experimental results indicate
that complexes have lower activity in catalyzing ROP of e-
caprolactone than that in catalyzing ROP of L-lactide. These
reactions give PCLs with reasonable number-average molecular
weight (Mn) and narrow PDI values (1.05–1.13). The plot of
Mn vs. ([M]0/[I]0) demonstrated by those data initiated by 6
exhibits a linear relationship indicating the “living” character of
the polymerization process, as shown in Fig. 8 (entries 6, 9–11).
1
Based on the H NMR spectroscopy, polymers are capped with
benzyl alkoxyl group, as shown in Fig. 9.
˚
Fig. 5 Molecular structure of 7. Selected bond lengths (A) and bond
In conclusion, eight zinc complexes have been prepared and
fully characterized. Under optimized conditions, zinc benzyl
oxide complexes bearing amido-oxazolinate ligands demonstrate
efficient activities for the ring opening polymerization of both L-
lactide and e-caprolactone than other zinc ethyl or zinc bis(chelate)
complexes in the presence of benzyl alcohol. Zinc benzyl oxide
complexes bearing aliphatic functionalities show better catalytic
activities than those bearing aromatic ones which have been
reported by us previously. Those results demonstrate the Lewis
acidity of metal centre could be tuned via the substitution of
functionality on the amino group with aliphatic substituents
resulting in the enhancement of catalytic activities. Furthermore
the decrease of ligand’s rigidity caused by the aliphatic group
might make the coordination of monomer to the metal centre
more easily, leading to an increase in propagation. Preliminary
studies on fine-tuning of ligand precursors and further applica-
tion of metal complexes to the catalytic reactions are currently
underway.
angles (◦): Zn–N(1), 1.9855(18); Zn–N(2), 1.396(18); Zn–O(3), 1.9753(15);
Zn–O(3A), 1.9637(16); O(3)–C(15), 1.418(3); N(1)–C(10), 1.287(3);
N(2)–C(1), 1.358(3); N(1)–Zn–N(2), 95.93(8); N(2)–Zn–O(3A), 121.66(7);
O(3A)–Zn–O(3), 81.01(6); O(3)–Zn–N(1), 121.12(7); N(2)–Zn–O(3),
121.11(8); N(1)–Zn–O(3A), 117.44(7). Zn–O(3)–Zn(A), 98.99(6). Hydro-
gen atoms on carbon atoms omitted for clarity. Perspective view of 7 with
probability ellipsoids at 20% level. Symmetry code A = 1-x,-y, -z.
controlled character, compound 6 was introduced to examine the
living and immortal characters under optimized conditions. The
linear relationship between the number-average molecular weight
(Mn) and the monomer-to-initiator ratio ([M]0/[I]0) demonstrated
in Fig. 6 (entries 6, 9–11) implies the “living” character of the
polymerization process. The ‘immortal’ character was examined
using one equiv. ratio (on [M]o/[Zn]o) of benzyl alcohol as the
chain transfer agent (entry 12). The Mn of the polymers created
from these polymerization reactions became half or one third of
Fig. 6 Polymerization of L-lactide catalyzed by 6 in toluene at 50 ◦C.
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Table 1 Polymerization of L-lactide using compounds 1–8 as catalysts at 50 ◦Ca
Entry
Catalyst
{[M]0:[Zn]0}:[BnOH]
Time (min)
Mn (obsd)b
Mn (calcd)c
Conv. (%)d
Yield (%)e
Mw/Mnb
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
6
6
6
6
50 : 1
50 : 1
50 : 1
50 : 1
50 : 1
50 : 0
50 : 0
50 : 0
100 : 0
150 : 0
200 : 0
100 : 1
60
60
60
60
60
10
15
15
25
40
55
10
14600 (8500)
13800 (8000)
10400 (6000)
—
6600
6100
6000
—
90
83
97
54
76
90
88
90
96
98
98
97
81
74
85
—
—
83
77
80
85
90
89
83
1.09
1.08
1.71
—
—
—
—
14300 (8300)
19800 (11500)
12900 (7500)
33200 (19300)
44200 (25600)
64200 (37200)
16200 (9400)
6600
6500
6600
14000
21300
28400
7100
1.14
1.12
1.11
1.15
1.14
1.15
1.08
9
10
11
12
a In 10 ml toluene, [Zn]0 = 0.05 M; [BnOH] = 0.05M. b Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are
the values obtained from GPC times 0.58. c Calculated from [M(lactide) ¥ [M]0/[Zn]0 ¥ conversion yield/([BnOH]eq)] + M(BnOH). d Obtained from 1H
NMR analysis. e Isolated yield.
Table 2 Polymerization of e-caprolactone using compounds 1–8 as catalysts at 50 ◦Ca
Entry
Catalyst
{[M]0:[Zn]0}:[BnOH]
Time (min)
Mn (obsd)b
Mn (calcd)c
Conv. (%)d
Yield (%)e
Mw/Mnb
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
6
6
6
6
50 : 1
50 : 1
50 : 1
50 : 1
50 : 1
50 : 0
50 : 0
50 : 0
100 : 0
150 : 0
200 : 0
100 : 1
90
90
90
70
70
30
30
30
30
30
40
30
11400 (6400)
8100 (4500)
6400
4600
6500
6200
5400
5800
5500
5700
11400
15700
22000
5800
92
77
91
90
93
99
94
98
99
91
96
99
83
63
81
82
85
85
81
84
89
85
82
87
1.06
1.05
1.17
1.17
1.16
1.13
1.15
1.16
1.10
1.13
1.13
1.13
11500 (6400)
13200 (7400)
12400 (6900)
12400 (6900)
14000 (7800)
13000 (7300)
23800 (13300)
33200 (18600)
47700 (26700)
12700 (7100)
9
10
11
12
a In 15 ml toluene. [Zn]0 = 0.125 M; [BnOH] = 0.125M. b Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are
the values obtained from GPC times 0.56. c Calculated from [M(e-caprolactone) ¥ [M]0/[Zn]0 ¥ conversion yield/([BnOH]eq)] + M(BnOH). d Obtained
from 1H NMR analysis. e Isolated yield.
Experimental
All manipulations were carried out under an atmosphere of
dinitrogen using standard Schlenk-line or drybox techniques.
Solvents were refluxed over the appropriate drying agent and
distilled prior to use. DMSO (dimethyl sulfoxide, TEDIA) was
used as supplied. Deuterated solvents were dried over molecular
sieves.
1
1H and 13C{ H} NMR spectra were recorded either on Varian
Mercury-400 (400 MHz) or Varian Inova-600 (600 MHz) spec-
trometers in chloroform-d at ambient temperature unless stated
otherwise and referenced internally to the residual solvent peak
and reported as parts per million relative to tetramethylsilane.
Elemental analyses were performed by Elementar Vario ELIV
instrument. The GPC measurements were performed in THF
at 35 ◦C with a Waters 1515 isocratic HPLC pump, a Waters
2414 refractive index detector, and Waters Styragel column
(HR4E). Molecular weights and molecular weight distributions
were calculated using polystyrene as standard.
Diethylzinc (1 M in hexane, Aldrich), CuI (Strem), K3PO4 (Lan-
caster), L-proline (Alfa) and unsym-dimethyl-ethylenediamine
(Acros) were used as supplied. Ligand precursors such
as 2-(2-bromophenyl)-4,4-dimethyl-2-oxazoline,11 HNC2MeOxa,3p
HNC2OMeOxa,3p HNC2StBuOxa,3p or HNPhSMeOxa3o were prepared
Fig. 7 1H NMR spectrum of PLA-50 catalyzed by 6 in toluene at 50 ◦C.
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Fig. 8 Polymerization of e-caprolactone catalyzed by 6 in toluene at 50 ◦C.
All volatiles were removed in vacuum to yield a yellow solid.
The yellow solid was purified by flash column chromatography
on silica gel (ethyl acetate/hexane 1 : 8 then methanol). The
final band (pale yellow) was collected. Solvents were removed in
vacuum to give a white solid; yield 0.61 g, 78%. 1H NMR (CDCl3,
600 MHz): d 1.35(s, oxazoline-CH3, 6H), 2.32(s, N(CH3)2, 6H)
2.61(t, CH2, 2H, J = 6.6 Hz), 3.33 (q, CH2, 2H, J = 6.2 Hz),
3.97(s, oxazoline-CH2, 2H), 6.59(t, CH-Ph, 1H, J = 7.5 Hz),
6.67(d, CH-Ph, 1H, J = 8.4 Hz), 7.27(d, CH-Ph, 1H, J = 7.8
1
Hz), 7.70(d, CH-Ph, 1H, J = 6.6 Hz), 8.54(s, NH, 1H). 13C { H}
NMR (CDCl3, 150 MHz): d 8.7(oxazoline-CH3), 41.7(CH2),
45.7(N(CH3)2), 58.2(CH2), 67.7(C(CH3)2), 76.8(oxazoline-CH2),
110.1, 114.1, 129.6, 132.0(CH-Ph), 108.7, 148.9, 162.2(one
tert-C-oxazoline and two tert-C-Ph). Anal. Calc. for C15H23N3O:
C, 68.93; H, 8.87; N, 16.08. Found C, 68.88; H, 8.66; N, 16.28.
(NC2MeOxa)2Zn (1). To a flask containing HNC2MeOxa (0.93
g, 4 mmol) and 15 mL hexane, 3.12 mL ZnEt2 (1.0 M in hexane,
3.12 mmol) was added at 0 ◦C. The reaction mixture was allowed
to warm to room temperature and reacted overnight. After 14
h of stirring, the yellow suspension was filtered and washed with
hexane to afford a yellow solid. Yield 0.47 g, 49%. 1H NMR (C6D6,
600 MHz): d 0.79(s, CH2CH3, 3H), 0.93(t, oxazoline-CH3, 6H, J =
9.3 Hz), 1.67(m, CH2, 1H), 2.09(m, CH2, 1H), 3.33(d, oxazoline-
CH2, 1H, J = 8.4 Hz), 3.38 (d, oxazoline-CH2, 1H, J = 8.4 Hz),
3.48(m, CH2, 1H), 3.57(m, CH2, 1H), 6.43(t, CH-Ph, 1H, J =
7.2 Hz), 6.92(d, CH-Ph, 1H, J = 9 Hz), 7.26(t, CH-Ph, 1H, J =
Fig. 9 1H NMR spectrum of PCL-50 catalyzed by 6 in toluene at 50 ◦C.
using the literature method. Benzyl alcohol (TEDIA) was dried
over magnesium sulfate and distilled before use. e-Caprolactone
(Acros) was dried over magnesium sulfate and distilled under
reduced pressure. L-lactide was recrystallized from toluene prior
to use.
1
9 Hz), 8.14(d, CH-Ph, 1H, J = 8.4 Hz). 13C{ H} NMR (C6D6,
Preparations
150 MHz): d 11.9(CH2CH3), 22.9(CH2), 27.7(oxazoline-CH3),
27.8(oxazoline-CH3), 53.7(CH2), 77.0 (oxazoline-CH2), 110.1,
113.9, 132.6, 134.4(CH-Ph), 66.4, 104.9, 159.7, 167.1 (two tert-
C-oxazoline and two tert-C-Ph). Anal. Calc. for C28H38N4O2Zn:
C, 63.69; H, 7.25; N, 10.61. Found: C, 64.04; H, 7.20; N, 10.64.
HNC2NMe2Oxa. To
Bromophenyl)-4,4-dimethyl-2-oxazoline (0.76 g,
a
Schlenk flask containing 2-(2-
mmol),
3
unsym-dimethyl-ethylenediamine (0.98 mL, 9 mmol), CuI (0.06 g,
0.3 mmol), L-proline (0.07 g, 0.6 mmol), K3PO4 (1.27 g, 6 mmol)
and DMSO (3.5 mL) was added at room temperature under
nitrogen. The reaction mixture was heated to 110 ◦C for 14 h.
The resulting dark-brown solution was extracted with EA/H2O
three times. The organic layer was dried over Na2SO4 and filtered.
(NC2OMeOxa)2Zn (2). The procedure for the preparation of
2 was similar to that used for 1 but with HNC2OMeOxa (0.99
g, 4 mmol), 2.8 mL ZnEt2 (1.0 M in hexane, 2.8 mmol) and
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15 mL hexane. A yellow solution was obtained. Yield 0.75 g,
67%. 1H NMR (CDCl3, 600 MHz): d 1.14(s, oxazoline-CH3, 3H),
1.24(s, oxazoline-CH3, 3H), 3.32(s, OCH3, 3H), 3.50(m, CH2, 1H),
3.58(m, CH2, 1H), 3.67(m, CH2, 2H), 3.97(d, oxazoline-CH2, 1H,
J = 8.4 Hz), 4.06 (d, oxazoline-CH2, 1H, J = 7.8 Hz), 6.28(t, CH-
Ph, 1H, J = 7.5 Hz), 6.83(d, CH-Ph, 1H, J = 9 Hz), 7.21(t, CH-Ph,
NMR (C6D6, 150 MHz): d –0.4(ZnCH2CH3), 14.8(ZnCH2CH3),
28.1(oxazoline-CH3), 77.1(oxazoline-CH2), 113.2, 115.5, 124.5,
126.1, 127.0, 131.5, 134.4(CH-Ph), 66.0, 105.2, 135.8, 149.3, 157.2,
167.7(two tert-C-oxazoline and four tert-C-Ph). Anal. Calc. for
C20H24N2OSZn: C, 59.18; H, 5.96; N, 6.90. Found: C, 58.79; H,
5.49; N, 6.79.
1
1H, J = 7.8 Hz), 7.75(d, CH-Ph, 1H, J = 8.4 Hz). 13C{ H} NMR
[(NC2MeOxa)Zn(l-OBn)]2 (6). To a flask containing 2.2 mL
ZnEt2 (1.0 M in hexane, 2.2 mmol), 20 mL THF, and BnOH
(0.23 mL, 2.2 mmol) was added at 0 ◦C. The reaction mixture
was allowed to warm to room temperature and reacted for 3h.
The colorless solution was cooled down to 0 ◦C and NHC2MeOxa
(0.47 g, 2 mmol) was added. After additional 3 h of stirring,
the yellow solution was pumped to dryness and the residue was
washed with 10 mL hexane to afford a yellow solid. Yield 0.59
(CDCl3, 150 MHz): d 28.0(oxazoline-CH3), 28.1(oxazoline-CH3),
49.9(CH2), 58.8(OCH3), 71.1(CH2), 77.3(oxazoline-CH2), 109.7,
113.2, 132.1, 133.9(CH-Ph), 66.7, 104.7, 159.3, 166.8(two tert-C-
oxazoline and two tert-C-Ph). Anal. Calc. for C28H38N4O4Zn: C,
60.05; H, 6.84; N, 10.00. Found: C, 59.72; H, 6.86; N, 10.18.
(NC2NMe2Oxa)2Zn (3). The procedure for the preparation of
3 was similar to that used for 1 but with HNC2NMe2Oxa (1.05
g, 4 mmol), 3.12 mL ZnEt2 (1.0 M in hexane, 3.12 mmol) and
15 mL hexane. A yellow solution was obtained. Yield 0.58 g,
49%. 1H NMR (CDCl3, 600 MHz): d 1.14(s, oxazoline-CH3,
3H), 1.25(s, oxazoline-CH3, 3H), 2.29(s, N(CH3)2, 6H), 2.46(m,
CH2, 1H), 2.67(m, CH2, 1H), 3.97(d, oxazoline-CH2, 1H, J =
8.4 Hz), 4.07 (d, oxazoline-CH2, 1H, J = 7.8 Hz), 6.27(t, CH-
Ph, 1H, J = 7.2 Hz), 6.80(d, CH-Ph, 1H, J = 8.4 Hz), 7.23(t,
1
g, 73%. H NMR (C6D6, 600 MHz): d 0.95(s, CH3, 12H, J =
7.5 Hz), 0.98(s, oxazoline-CH3, 12H), 1.83(m, CH2, 4H), 3.35(s,
oxazoline-CH2, 4H), 3.61(m, CH2, 4H), 4.78 (d, OCH2Ph, 2H,
J = 5.7 Hz), 4.86(d, OCH2Ph, 2H, J = 5.7 Hz), 6.53(t, CH-
Ph, 2H, J = 7.5 Hz), 6.95(q, CH-Ph, 2H, J = 7.4 Hz), 6.99(m,
CH-Ph, 4H), 7.26(m, CH-Ph, 4H), 7.34(m, CH-Ph, 2H), 8.19(m,
1
CH-Ph, 2H). 13C { H} NMR (CDCl3, 150 MHz): d 11.7(CH3),
1
CH-Ph, 1H, J = 6.9 Hz), 7.76(d, CH-Ph, 1H, J = 6 Hz). 13C{ H}
22.6(CH2), 27.9(CH3), 52.5(CH2), 70.1(CH2), 76.8(CH2), 110.5,
113.7, 127.1, 128.4, 129.2, 132.7, 134.4(CH-Ph), 66.0, 104.7, 144.4,
159.7, 168.1(two tert-C-oxazoline and three tert-C-Ph). Anal.
Calc. for C42H52N4O4Zn2: C, 62.46; H, 6.49; N, 6.94. Found: C,
61.88; H, 6.86; N, 6.94.
NMR (CDCl3, 150 MHz): d 28.0(oxazoline-CH3), 28.2(oxazoline-
CH3), 46.0(N(CH3)2), 46.4(CH2), 57.8(CH2), 77.3(oxazoline-
CH2), 109.5, 113.4, 132.1, 133.8(CH-Ph), 66.7, 104.6, 159.2,
166.9(two tert-C-oxazoline and two tert-C-Ph). Anal. Calc. for
C30H44N6O2Zn: C, 61.48; H, 7.57; N, 14.34. Found: C, 61.43; H,
7.69; N, 13.97.
[(NC2OMeOxa)Zn(l-OBn)]2 (7). The procedure for the prepa-
ration of 7 was similar to that used for 6 but with HNC2OMeOxa
(0.5 g, 2 mmol), 2.2 mL ZnEt2 (1.0 M in hexane, 2.2 mmol), BnOH
(0.23 mL, 2.2 mmol) and 20 mL THF. A yellow solid was obtained.
Yield 0.32 g, 38%. 1H NMR (C6D6, 600 MHz): d 0.83(s, oxazoline-
CH3, 4H), 1.00(s, oxazoline-CH3, 8H), 3.07(s, OCH3, 4H), 3.15(s,
OCH3, 2H), 3.29(d, oxazoline-CH2, 2H, J = 11.4 Hz), 3.48(t, CH2,
2H, J = 7.5 Hz), 3.66(t, CH2, 2H, J = 6.6 Hz), 3.91(t, CH2, 2H,
J = 7.5 Hz), 4.03(t, CH2, 2H, J = 6.6 Hz), 4.72(d, OCH2Ph, 2H,
J = 10.8 Hz), 4.78(d, OCH2Ph, 2H, J = 10.8 Hz), 6.42(q, CH-
Ph, 2H, J = 7.5 Hz), 6.84(t, CH-Ph, 4H, J = 7.4 Hz), 6.91(t,
CH-Ph, 4H, J = 7.5 Hz), 7.03(m, CH-Ph, 4H), 7.20(t, CH-Ph,
(NC2StBuOxa)ZnEt (4). To a flask containing HNC2StBuOxa
(0.61 g, 2 mmol) and 15 mL THF, 2.2 mL ZnEt2 (1 M in hexane, 2.2
◦
mmol) was added at 0 C under nitrogen. The reaction mixture
was allowed to warm to room temperature and reacted for 12
h or overnight. Volatile materials were removed under vacuum
to yield yellow powder. The powder was washed with hexane
and the yellow powder was obtained after filtration. Yield 0.71
1
g, 89%. H NMR (C6D6, 600 MHz): d 0.72(q, 2H, J = 8 Hz,
ZnCH2CH3), 0.91(s, 6H, oxazoline-CH3), 1.21(s, S(CH3)3, 9H),
1.68(t, ZnCH2CH3, 3H, J = 7.8 Hz), 2.80(t, CH2, 2H, J = 7.2 Hz),
3.32(s, oxazoline-CH2, 2H), 3.84(t, CH2, 2H, J = 7.2 Hz), 6.51(t,
CH-Ph, 1H, J = 6.9 Hz), 6.91(d, CH-Ph, 1H, J = 9 Hz), 7.25(t,
1
4H, J = 8.4 Hz), 8.10(t, CH-Ph, 2H, J = 9.6 Hz). 13C { H} NMR
(C6D6, 150 MHz): d 27.9(CH3), 28.1(CH3), 49.5(CH2), 50.0(CH2),
28.5(CH3), 58.5(OCH3), 59.0(OCH3), 70.1(CH2), 71.2(CH2),
72.0(CH2), 77.0(CH2), 110.7, 113.5, 114.1, 127.1, 129.0, 129.43,
132.7, 134.6(CH-Ph), 58.5, 104.9, 144.5, 160.2, 168.0(two tert-C-
oxazoline and three tert-C-Ph). Anal. Calc. for C42H52N4O6Zn2:
C, 60.08; H, 6.24; N, 6.67. Found: C, 59.48; H, 5.85; N, 6.55.
1
CH-Ph, 1H, J = 6.9 Hz), 8.13(d, CH-Ph, 1H, J = 6 Hz). 13C{ H}
NMR (C6D6, 150 MHz): d -0.1(ZnCH2CH3), 13.5(ZnCH2CH3),
28.2(oxazoline-CH3), 29.6(CH2), 31.1(C(CH3)3), 42.1(C(CH3)3),
50.8(CH2), 76.9(oxazoline-CH2), 111.7, 113.4, 132.2, 134.6(CH-
Ph), 65.9, 105.6, 158.2, 167.5(two tert-C-oxazoline and two tert-
C-Ph). Anal. Calc. for C19H30N2OSZn: C, 57.06; H, 7.56; N, 7.00.
Found: C, 57.17; H, 7.07; N, 7.52.
[(NC2StBuOxa)Zn(l-OBn)]2 (8). The procedure for the prepa-
ration of 8 was similar to that used for 6 but with HNC2StBuOxa
(1.16 g, 3.79 mmol), 4.92 mL ZnEt2 (1.0 M in hexane, 4.92 mmol)
BnOH (0.51 mL, 4.92 mmol) and 20 mL THF. A yellow solid
(NPhSMeOxa)ZnEt (5). The procedure for the preparation of
5 was similar to that used for 4 but with HNPhSMeOxa (0.31 g,
1 mmol), 1.1 mL ZnEt2 (1.0 M in hexane, 1.1 mmol) and 15
1
was obtained. Yield 1.3 g, 70%. H NMR (C6D6, 600 MHz): d
1
mL THF. A yellow solid was obtained. Yield 0.38 g, 82%. H
1.21(s, oxazoline-CH3, 6H), 1.30(s, SC(CH3)3, 18H), 2.91(t, CH2,
4H, J = 8.4 Hz), 3.38(s, oxazoline-CH2, 4H), 3.94(t, CH2, 4H,
J = 8.4 Hz), 4.86 (d, OCH2Ph, 2H, J = 5.4 Hz), 4.87(d, OCH2Ph,
2H, J = 5.7 Hz), 6.51(t, CH-Ph, 2H, J = 7.5 Hz), 6.92(t, CH-
Ph, 2H, J = 7.4 Hz), 6.99(t, CH-Ph, 4H, J = 7.5 Hz), 7.10(d,
CH-Ph, 2H, J = 9 Hz), 7.32(m, CH-Ph, 6H), 8.17(d, CH-Ph,
NMR (C6D6, 400 MHz): d 0.57(q, ZnCH2CH3, 2H, J = 8.1
Hz), 0.88(s, oxazoline-CH3, 6H), 1.34(t, ZnCH2CH3, 3H, J =
8.2 Hz), 1.92(s, SCH3, 3H), 3.32(s, oxazoline-CH2, 2H), 6.50(t,
CH-Ph, 1H, J = 7.4 Hz), 6.86(m, CH-Ph, 1H), 6.99(m, CH-
Ph, 2H), 7.70(t, CH-Ph, 1H), 7.11(m, CH-Ph, 2H), 7.24(d, CH-
1
1
Ph, 1H, J = 7.6 Hz), 8.11(d, CH-Ph, 1H, J = 9.6 Hz). 13C{ H}
2H, J = 7.8 Hz). 13C { H} NMR (C6D6, 150 MHz): d 27.5(CH2),
12892 | Dalton Trans., 2011, 40, 12886–12894
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Table 3 Summary of crystal data for compounds 1, 3, 5, 6 and 7
1
3
5
6
7
Formula
Fw
T (K)
C34H52N4O2Zn
614.17
297(2)
C30H44N6O2Zn
586.08
297(2)
Monoclinic
P21/c
11.1690(11)
8.6171(8)
32.893(3)
90
94.650(2)
90
3155.3(5)
4
1.234
C20H24N2OSZn
405.84
297(2)
Orthorhombic
Pbca
14.5692(11)
15.4862(12)
17.1701(13)
90
90
90
3873.9(5)
8
1.392
1.385
20817
C42H52N4O4Zn2
807.62
297(2)
Monoclinic
P21/n
11.0773(7)
16.3723(10)
11.3458(7)
90
99.0590(10)
90
2032.0(2)
2
1.287
C42H52N4O6Zn2
839.62
297(2)
Monoclinic
P21/c
10.8346(9)
19.8535(17)
9.8078(8)
90
103.536(2)
90
2051.1(3)
2
1.359
Crystal system
Triclinic
¯
Space group
P1
˚
a (A)
9.9372(8)
11.1773(9)
17.8036(14)
73.9740(10)
87.5980(10)
66.0250(10)
1731.1(2)
2
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
˚
V (A)
Z
rc (Mg m-3)
1.178
0.743
9785
370
m(Mo-Ka) (mm-1)
Reflections collected
No. of parameters
0.813
17161
352
1.225
11282
235
1.220
11474
244
226
Indep. reflns (Rint
)
6686 (0.0180)
0.0395, 0.1126
0.0469, 0.1180
1.022
6184 (0.0406)
0.0380, 0.1037
0.0507, 0.1104
1.005
3821 (0.0629)
0.0411, 0.0987
0.0809, 0.1159
0.933
3978 (0.0244)
0.0315, 0.0987
0.0461, 0.1077
1.031
4036 (0.0273)
0.0340, 0.0959
0.0460, 0.1033
1.014
a
Final R indices R1a, wR2
R indices (all data)
GoFb
a R1 = [R(|Fo| - |Fc|]/R |Fo|], wR2 = [R w(Fo - Fc2)2/R w(Fo2)2]1/2, w = 0.10. b GoF = [Rw(Fo -Fc2)2/(Nrflns–Nparams)]1/2
.
2
2
28.5(CH3), 31.3(CH3), 50.1(CH2), 70.1(CH2), 77.0(CH2), 105.3,
110.0, 113.5, 127.2, 128.9, 132.8, 134.7(CH-Ph), 41.8, 50.1, 144.2,
Acknowledgements
We would like to thank the National Science Council of the
Republic of China for financial support (grant number NSC 98-
2113-M-005-002-MY3).
159.5, 168.0, 187.1(two tert-C-oxazoline, one SC(CH3)3 and three
tert-C-Ph). Anal. Calc. for C42H52N4O4S2Zn2: C, 60.31; H, 6.75;
N, 5.86. Found: C, 59.73; H, 6.71; N, 5.56.
Polymerization procedure for L-lactide and e-caprolactone
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50 mL n-heptane to precipitate polymers. Crude products were
recrystallized from THF–hexane and dried in vacuo up to a
constant weight.
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Crystal structure data
Crystals were grown from concentrated hexane solutions (1, 3,
5) or tetrahydrofuran/hexane solution (6, 7), and isolated by
filtration. Suitable crystals of 1, 3, 5, 6 or 7 were sealed in thin-
walled glass capillaries under a nitrogen atmosphere and mounted
on a Bruker AXS SMART 1000 diffractometer. The absorption
correction was based on the symmetry equivalent reflections using
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and was confirmed using the structure solution. The structure
was solved by direct methods using a SHELXTL package.13 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. Some details of the
data collection and refinement are given in Table 3.
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12894 | Dalton Trans., 2011, 40, 12886–12894
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The Royal Society of Chemistry 2011
©