Synthesis and structural determination of zinc complexes based on an anilido-aldimine ligand containing an O-donor pendant arm: Zinc alkoxide derivative as an efficient initiator for ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1039/c3dt50756d

Zinc complexes bearing the anilido-aldiminate AA^OMe ligand (AA^OMe-H = (E)-2,6-diisopropyl-N-(2-(((2-methoxyethyl)imino)methyl) phenyl)aniline) were synthesized in a stepwise method and were structurally characterized. The reaction of AA^OMe-H (1) with one equivalent of diethyl zinc (ZnEt₂) furnishes a three-coordinated and mononuclear zinc complex [(AA^OMe)ZnEt] (2). Further reaction of 2 with a stoichiometric amount of benzyl alcohol (BnOH) affords a four-coordinated and dinuclear zinc benzylalkoxide complex [(AA^OMe)Zn(μ-OBn)] ₂ (3). In the presence of two equivalents of AA^OMe-H with ZnEt₂, a homoleptic and four-coordinated zinc complex [(AA ^OMe)₂Zn] (4) is formed. The geometry around the zinc centres of 3 and 4 are both distorted tetrahedrals, while 2 adopts a different coordination mode with a slightly distorted trigonal planar geometry. The variable-temperature ¹H NMR studies of 3 illustrate that 3 exhibits a dinuclear structure in solution at low temperature as well as in the solid state. While raising the temperature, it drifts towards dissociation to form a mononuclear zinc benzylalkoxide species, which coexists in solution. The ring-opening polymerizations of ε-caprolactone (ε-CL) and β-butyrolactone (β-BL) catalyzed by complexes 3 and 4 are investigated. The ε-CL and β-BL polymerizations initiated by zinc alkoxide 3 were demonstrated to have living characteristics and to proceed in a controlled manner with narrow polydispersity indices (PDIs < 1.12). An efficient catalytic performance for the β-BL polymerization with a high monomer-to-initiator ratio (1200/1) initiated by 3 has also been reported. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt50756d

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Synthesis and structural determination of zinc
complexes based on an anilido-aldimine ligand
containing an O-donor pendant arm: zinc alkoxide
derivative as an efficient initiator for ring-opening
polymerization of cyclic esters†
Cite this: DOI: 10.1039/c3dt50756d
Chao-Hsiang Wang,a Chen-Yu Li,a Bor-Hunn Huang,*b Chu-Chieh Lin*b and
Bao-Tsan Ko*b
Zinc complexes bearing the anilido-aldiminate AAOMe ligand (AAOMe-H = (E)-2,6-diisopropyl-N-(2-(((2-
methoxyethyl)imino)methyl)phenyl)aniline) were synthesized in a stepwise method and were structurally
characterized. The reaction of AAOMe-H (1) with one equivalent of diethyl zinc (ZnEt2) furnishes a three-
coordinated and mononuclear zinc complex [(AAOMe)ZnEt] (2). Further reaction of 2 with a stoichio-
metric amount of benzyl alcohol (BnOH) affords a four-coordinated and dinuclear zinc benzylalkoxide
complex [(AAOMe)Zn(μ-OBn)]2 (3). In the presence of two equivalents of AAOMe-H with ZnEt2, a homolep-
tic and four-coordinated zinc complex [(AAOMe)2Zn] (4) is formed. The geometry around the zinc centres
of 3 and 4 are both distorted tetrahedrals, while 2 adopts a different coordination mode with a slightly
distorted trigonal planar geometry. The variable-temperature 1H NMR studies of 3 illustrate that 3 exhi-
bits a dinuclear structure in solution at low temperature as well as in the solid state. While raising the
temperature, it drifts towards dissociation to form a mononuclear zinc benzylalkoxide species, which
coexists in solution. The ring-opening polymerizations of ε-caprolactone (ε-CL) and β-butyrolactone (β-BL)
catalyzed by complexes 3 and 4 are investigated. The ε-CL and β-BL polymerizations initiated by zinc alk-
oxide 3 were demonstrated to have living characteristics and to proceed in a controlled manner with
narrow polydispersity indices (PDIs < 1.12). An efficient catalytic performance for the β-BL polymerization
with a high monomer-to-initiator ratio (1200/1) initiated by 3 has also been reported.
Received 20th March 2013,
Accepted 27th May 2013
DOI: 10.1039/c3dt50756d
www.rsc.org/dalton
resembling polyurethane.3 As
introduce a single-active site into metal complexes supported
a result, systems which
Introduction
Using well-defined metal complexes for the evolution of cata- by a variety of ancillary ligands such as Schiff base, β-diketimi-
lytic systems toward ring-opening polymerizations (ROP) has nate, bisphenolate, amino-bis(phenolate),4 pyrazolone ketimi-
had considerable attention because ROP provides a promising nate,5 benzotriazole phenolate,6 and many other ligands have
method for producing biodegradable polymers such as poly(ε- been reported to achieve high catalytic activities in a well-
caprolactone) (PCL), poly(lactide) (PLA) and poly(β-butyrolactone) controlled fashion. In particular, the anilido-aldiminate (AA)
(PHB), as well as their copolymers.1 Among them, PHB is a kind system attracts increasing attention due to the easily achiev-
of thermoplastic polymer with specific properties (Tm ∼ 180 °C able single-site catalysts.7 For instance, rare-earth metal com-
and Tg ∼ 2 °C)2 which has a stretchable behavior similar plexes bearing the N,N,N-tridentate AA ligand and introducing
to a BOPP film, whilst performing in an elastomeric manner the pendant arm of the quinolinyl group on the amido nitro-
gen have demonstrated a single active site nature which
efficiently initiates the polymerization of ε-caprolactone
(ε-CL).7b
aDepartment of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan
bDepartment of Chemistry, National Chung Hsing University, Taichung 402, Taiwan.
Based on the inspiration of the efficient catalytic systems
supported by AA ligands, our current focus has been to
develop such ligands with a pendant arm attached via the
E-mail: btko@dragon.nchu.edu.tw, gaia0824@gmail.com, cchlin@mail.nchu.edu.tw;
Fax: +886-4-22862547; Tel: +886-4-22840411-724
†Electronic supplementary information (ESI) available. CCDC 928698–928701.
imine nitrogen and to complex these to aluminium, mag-
nesium and zinc precursors as catalysts for CO2/epoxide
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50756d
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Chart 1 Several types of anilido-aldimine ligands containing N- or O-donor
pendant arms.
coupling and ROP of cyclic esters. Recently, a useful synthetic
route for the preparation of bi-, tri- or multi-dentate AA was
established8 and the purification method is simple in that the
products are readily obtained via only a recrystallization and
without any chromatographic technique. Moreover, we have
synthesized and characterized several magnesium and zinc
complexes derived from N,N,N-tridentate anilido-aldimine (AA)
ligands (type I–II, Chart 1).9 The ROPs of L-lactide, ε-CL and
β-butyrolactone in the presence of alcohol catalyzed by these
complexes have illustrated a high performance with a living
characteristic, furnishing polyesters with narrow polydispersity
indices (PDIs < 1.30). To extend the investigations into AA
derivatives and to compare differences in the polymerization
Scheme 1 Synthetic routes of (a) the ligand AAOMe-H (1) and (b) the com-
plexes (2)–(4).
efficiency,
a new anilido-aldimine ligand containing an
O-donor pendant arm (type III, Chart 1) has been designed
and synthesized. We envisaged that the minor changes in the
pendant arm of the O-donor functional group might lead to
differences in the bonding modes and the synthesis of well-
defined ROP metal alkoxide initiators, [(AA)M(OR)]n (M = Zn or
Mg; OR = alkoxy; n = 1 or 2). In this article, we describe the
synthesis, structure, and ROP catalytic studies of zinc deriva-
tives bearing the anilido-aldimine ligand of this kind. We also
report the use of an effective zinc alkoxide derivative to
prepare PHB with a large molecular weight.
Results and discussion
Syntheses and crystal structure
The new anilido-aldimine ligand containing the O-donor func-
tional group, (E)-2,6-diisopropyl-N-(2-(((2-methoxyethyl)imino)-
methyl)phenyl)aniline (AAOMe-H, 1), was synthesized in an
84% yield based on methods from the literature8 as shown in
Scheme 1(a). Compound 1 was isolated as a crystalline solid
Fig. 1 ORTEP drawing of AAOMe-H (1) with probability ellipsoids drawn at the
30% level. Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.357(3), N(1)–
C(7) 1.422(3), N(2)–C(19) 1.266(3), N(2)–C(20) 1.465(4), N(1)⋯N(2) 2.704(3).
and was characterized by 1H and 13C NMR spectra. The that of the sum of the van der Waals radii (3.10 Å), probably
1H NMR spectrum of the ligand exhibited signals at 8.46 ppm suggesting a hydrogen bond exists between the N–H(aniline)
for the aldimine proton of HCvN, and the corresponding and the N(imine). The mean planes of two phenyl rings are
13C NMR signal was around 165.76 ppm, indicating the for- approximately perpendicular to each other with a dihedral
mation of the desired AAOMe-H ligand. The downfield N–H angle equal to 86.7(5)°. As shown in Scheme 1(b), the reaction
signal at 10.48 ppm implies a hydrogen bond exists between of compound 1 with one equivalent of diethyl zinc (ZnEt2) in
the N–H and the N(imine), which was further verified by X-ray n-hexane gave a yellow crystalline solid, [(AAOMe)ZnEt] (2) in a
structural determination. Suitable crystals for X-ray diffraction good yield (85%). Complex 2 was also characterized by spectro-
analysis of 1 were obtained from a saturated n-hexane solution, scopic studies. The disappearance of the N–H signal of 1 and
as shown in Fig. 1. The molecular structure of 1 reveals that the appearance of the resonance of Zn–Et (0.85 and 0.12 ppm)
the distance between N(1)⋯N(2) (2.704(3) Å) is shorter than reveals that complex 2 was successfully synthesized. To prepare
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the well-defined ROP Zn initiator [(AA)Zn(OR)]n (OR = alkoxy; and the appearance of two doublet resonances of the methy-
n = 1 or 2), the presence of 2 and a stoichiometric amount of lene protons from the benzylalkoxy group (4.52 and 4.30 ppm).
benzyl alcohol in THF afforded the zinc benzylalkoxide Alternatively, 3 could also be prepared directly by the reaction
complex [(AAOMe)Zn(μ-OBn)]2 (3). It was mainly identified by of AAOMe-H with 1.0 mol equiv. of ZnEt2, followed by the
the disappearance of the aniline group (10.48 ppm), the addition of BnOH (one equiv.) in a good yield. Furthermore,
upfield shift from 8.46 to 7.90 ppm of the aldimine proton treatment of 1 and half an equivalence of ZnEt2 in refluxing
toluene solution produced
a bis-adduct zinc complex
[(AAOMe)2Zn] (4). The spectroscopic verification showed the dis-
appearance of the aniline proton (10.48 ppm) and the methyl
protons of the isopropyl group had split from the signal at
1.13 ppm into four doublet signals (1.10, 0.94, 0.77 and
0.39 ppm). All Zn complexes were isolated as yellow crystalline
solids and were characterized by microanalysis as well as X-ray
diffraction.
Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawings illus-
trating selected bond lengths and angles of the molecular
structures of 2, 3 and 4 are shown in Fig. 2–4, respectively. The
crystal structure of 2 displays that it is a three-coordinate
monomeric complex and has a slightly distorted trigonal
planar geometry around the Zn atom (ca. 359.7(2)°). The zinc
atom slightly diverges from the mean plane of the six-mem-
bered ring composed of Zn/N(1)/C(1)/C(2)/C(19/)N(2) by about
0.107 Å. It also slightly deviates from the triangle mean plane
(N(1)/N(2)/C(23)) by about 0.061 Å. The bond lengths around
the Zn centre (Zn–N(1) = 1.938(2) Å and Zn–N(2) = 1.996(2) Å)
are shorter than the related distances in the three-coordinate
complex [(AABn)ZnEt]10 (Zn(1)–N(1) = 1.950(3) Å and Zn(1)–N(2) =
2.015(3) Å), which might disclose the stronger bonding
between the zinc and the ancillary ligand owing to the less
steric hindrance of the methoxy group. On the basis of the
Fig. 2 ORTEP drawing of complex 2 with probability ellipsoids drawn at the
50% level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (°): Zn–N(1) 1.938(2), Zn–N(2) 1.996(2), Zn–C(23) 1.965(2), N(2)–
C(19) 1.290(3), N(1)–Zn–N(2) 94.62(7), N(1)–Zn–C(23) 134.55(9), C(23)–Zn–N(2)
130.52(9).
Fig. 3 ORTEP drawing of complex 3 with probability ellipsoids drawn at the 50% level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Zn(1)–N(1) 2.005(2), Zn(1)–N(2) 1.959(2), Zn(1)–O(2) 1.984(2), Zn(1)–O(2A) 1.951(2), N(2)–Zn(1)–N(1) 95.77(6), N(2)–Zn(1)–O(2) 123.49(6), O(2A)–Zn(1)–
N(1) 114.40(6), O(2A)–Zn(1)–N(2) 129.24(6), O(2A)–Zn(1)–O(2) 79.82(5), O(2)–Zn(1)–N(1) 115.84(5).
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Fig. 4 ORTEP drawing of complex 4 with probability ellipsoids drawn at the
50% level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (°): Zn–N(1) 1.967(2), Zn(1)–N(2) 2.068(2), Zn–N(3) 2.034(2), Zn–N(4)
1.965(2), N(1)–Zn–N(2) 94.19(5), N(1)–Zn–N(3) 113.45(5), N(1)–Zn–N(4) 132.40(5),
N(2)–Zn–N(3) 97.16(5), N(2)–Zn–N(4) 119.45(5), N(3)–Zn–N(4) 95.81(5).
Fig. 5 Variable-temperature 1H NMR studies of complex 3 in CDCl3.
centre, and the pendant oxygen atom does not coordinate to
the zinc centre.
single crystal X-ray analysis for 3, the complex has been con-
structed as a symmetric structure bearing two bridging benzyl-
alkoxy groups and two ancillary ligands with an inversion
centre between two zinc atoms. Complex 3 demonstrates a
four-coordinate dimmeric zinc benzylalkoxide complex and a
distorted tetrahedral geometry around the zinc atoms with the
two included angles, O(2A)–Zn(1)–N(2) = 129.24(6)° and O(2A)–
Zn(1)–O(2) = 79.82(5)°. Notably, the different bond distances
between the zinc and nitrogen atoms were observed as Zn(1)–
N(2) = 1.959(2) Å and Zn(1)–N(1) = 2.005(2) Å. Because of the
resonance effect of the aniline substituent on the phenyl ring
with respect to the lack of electronic density, the bond strength
of Zn(1)–N(2) is slightly stronger than Zn(1)–N(1). This differs
from the chelating mode of the zinc complex reported by
Coates et al., [(BDI-3)Zn(μ-OiPr)]2,11 with the corresponding
bond lengths of Zn(1)–N(1) = 2.008(2) Å and Zn(1)–N(2) =
1.996(2) Å. Furthermore, recrystallization from a saturated
toluene solution generates complex 4 as transparent yellow
crystals. Crystallographic analysis indicates that 4 is a homo-
leptic and a four-coordinate complex lying on a distorted tetra-
hedral geometry around the zinc centre with the bond angles
ranging from 94.15(9)° to 132.44(9)°. The Zn-containing bond
lengths, Zn–N(1) = 1.967(2) Å, Zn–N(2) = 2.069(2) Å, Zn–N(3) =
2.034(2) Å and Zn–N(4) = 1.963(2) Å, show that 4 is built of two
covalent bonds and two dative bonds around the zinc centre,
suggesting an iso-structure compared with [(AABn)2Zn].10
Notably, two six-membered rings consisting of a Zn centre
bonded to bidentate AAOMe ligands in 4 are formed in twisted
conformations. It is interesting to note that all AAOMe ligands
in complexes 2–4 assume a N,N-bidentate bonding mode with
the amido and imine nitrogen atoms coordinated to the metal
Variable-temperature 1H NMR studies of complex 3
1
On the basis of the H NMR spectrum of complex 3 (Fig. S1,
ESI†), the signal pattern demonstrates the existence of two
different conformations in solution at 20 °C. To figure it out, a
variable-temperature 1H NMR experiment was accomplished
in CDCl3 as shown in Fig. 5. One set of splitting peaks
(4.18–4.46 ppm) for 3 at −55 °C was found since the two
methylene protons of the benzylalkoxy group were fixed and
were chemically non-equivalent at the lower temperature. As
the temperature was raised (∼10 °C), the signal at
4.42–4.48 ppm (e + e′) began to broaden gradually. As the
temperature was raised to 50 °C, these resonances split signifi-
cantly, while the peak of the benzyl group (e′) shows a singlet
peak at ca. 4.45 ppm, indicating another species has appeared.
The new species can be attributed to a mononuclear zinc alk-
oxide complex dissociated from the dimeric Zn complex 3 in
the solution state. It is also believed that this mononuclear
zinc complex is the active species during the ring-opening
polymerization of cyclic esters. These results are similar to
those previously reported for zinc and magnesium complexes
with N,N,O-tridentate Schiff base ligands.12
Ring-opening polymerization of ε-caprolactone and
β-butyrolactone
The ring-opening polymerization (ROP) of ε-caprolactone
(ε-CL) was generally executed in toluene under a dry nitrogen
atmosphere and was systematically studied as listed in Table 1.
As shown in entries 1–3, the optimized ε-CL polymerization
conditions initiated by 3 with a monomer-to-initiator ratio of
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Table 1 Ring-opening polymerization of ε-caprolactone initiated by 3 and 4/BnOH
Entry
Cat.
[ε-CL]0/[Zn]0/[BnOH]0
t (min)
Conv.c (%)
Mn (calcd)d
Mn (obsd)e
Mn (NMR)f
PDIe
1a
2b
3b
4b
5b
6b
7b
3
3
3
3
3
3
200/1/0
200/1/0
200/1/0
25/1/0
50/1/0
100/1/0
200/1/1
180
50
70
70
70
70
120
85
87
99
99
99
99
99
19 500
20 000
22 700
2900
5800
11 400
22 700
25 000 (14 000)
29 100 (16 300)
33 900 (19 000)
5700 (3200)
10 200 (5700)
18 300 (10 200)
19 600 (11 000)
23 700
25 200
26 400
3400
7200
14 100
28 800
1.05
1.05
1.07
1.09
1.04
1.05
1.27
4/BnOH
a [Zn]0 = 5 mM, 10 mL toluene, 30 °C. b [Zn]0 = 5 mM, 10 mL toluene, 55 °C. c Obtained from 1H NMR determination. d Calculated from the
molecular weight of ε-caprolactone (114.14 g mol−1) multiplied by [ε-CL]0/[Zn]0 multipled by the conversion yield plus the molecular weight of
benzyl alcohol (108.14 g mol−1). e Obtained from GPC analysis and calibrated by a polystyrene standard. Values in parentheses are the values
obtained from GPC multiplied by 0.56.15 f Obtained from 1H NMR analysis.
200 were found to be [Zn]0 = 5 mM in toluene at 55 °C, and the 3–6). As illustrated in Fig. S3, ESI,† a linear relationship
conversion yields proceed to completion within 70 min. The between Mn versus [β-BL]0/[3]0 and narrow polydispersity
experimental results of entries 3–6 demonstrate that the indices ranging from 1.05 to 1.11 also indicate a “controlled”
complex 3 has good catalytic performances in a range of manner during the ROP of β-BL. The end group determination
monomer/initiator molar ratios, [ε-CL]0/[Zn]0, from 25 to 200. from the 1H NMR spectrum of PHB-25 (Fig. 7) revealed that
The linear relationship between Mn versus [ε-CL]0/[Zn]0 and the the polymer chain end was capped with a benzyl ester and a
low polydispersity indices (<1.10), as shown in Fig. S2, ESI,† hydroxyl group with the integration ratio close to 5 : 2 : 1
exhibit a living characteristic and a well-controlled manner for between Ha, Hc and Hd. It is worth noting that 3 demonstrates
1
the polymerization process. The H NMR spectrum of PCL-25 excellent catalytic activity up to a 1200-fold ratio, [β-BL]0/[3]0 =
displays that the polymer chain end was capped with a benzyl 1200, generating a high molecular weight of PHB (Mn
=
ester and a hydroxyl group with the integration ratio close to 49 200 g mol−1), and its PDI remains narrow (∼1.08). The cata-
5 : 2 : 2 between Ha, Hb and Hd (Fig. 6). Moreover, based on the lytic performance of the β-BL polymerizations by zinc alkoxide
successful ROP of lactones polymerized by the [(AABn)2Zn]/ 3 seems to be better than that of our previously reported tri-
9-AnOH catalytic system,10 the bis-adduct Zn 4 was expected to dentate anilido-aldimine zinc ethyl catalyst [(AAPip)ZnEt], in
be an active catalyst for the ε-CL polymerization in the pres- which a 5-fold equiv. of catalyst loading towards alcohol is
ence of alcohol. With the addition of a stoichiometric amount required to reach a high conversion yield and a large Mn under
of BnOH under the similar conditions of the previously the higher monomer-to-initiator ratio (>500).9b
described entries, 4 can also perform with a good catalytic
Based on the aforementioned results, the mechanism of
activity toward the polymerization of ε-CL. It was found that ROP catalyzed by 3, as shown in Scheme 2, could be proposed
99% monomer conversion was achieved within 120 min, as the following: the dissociation of 3 affords the active
however, with a slightly larger PDI (∼1.27) than 3. This may be species, the mononuclear zinc benzylalkoxide complex (A), fol-
due to the larger steric hindrance around the zinc centre in lowed by the coordination of the cyclic ester to the zinc centre
complex 4, retarding the coordination of the monomers to the forming an intermediate (B). Thereby, the insertion of the Zn–
zinc atoms, which further decreases the rate of propagation. It OBn to the carbonyl group of the monomers forms a ring-
was believed that the 4/BnOH system might undergo an “acti- opening species (C). Finally, proceeding with the propagation
vated-monomer” route, where ε-CL or BnOH was activated by generates the polyester (D), end-capped by a benzylalkoxy and
the Zn complex 4 and BnOH behaved as a nucleophile.6b
As a result of the good performance in the ROP of ε-CL, 3
has been further applied to catalyze the ring-opening polymer-
ization of β-butyrolactone (β-BL) in order to produce poly-
(3-hydroxybutyrate) (PHB). All polymerizations were also per-
formed under a dry N2 atmosphere in toluene. As depicted by
entries 1–2 in Table 2, the Zn complex 3 can polymerize β-BL
in toluene with ∼94% conversion yields within 8 h at 55 °C,
yielding PHB with a very low PDI (<1.10). Under these optimal
conditions, the polymerization of β-BL was then systematically
studied to explore the catalytic properties (Table 2, entries
a hydroxyl group.
Conclusions
A new anilido-aldimine ligand, AAOMe-H (1), modified by an
O-donor pendent-arm substituent, and three novel zinc com-
plexes 2–4 bearing such AA ligands have been synthesized and
fully characterized by spectroscopic studies as well as micro-
analysis. All compounds were also structurally confirmed
by single crystal X-ray diffraction determination. Variable-
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Fig. 6 1H NMR spectrum of PCL-25 (Table 1, entry 4) initiated by 3.
Table 2 Ring-opening polymerization of β-butyrolactone initiated by 3
Entry
[β-BL]0/[3]0
t (h)
Conv.c (%)
Mn (calcd)d
Mn (obsd)e
Mn (NMR)f
PDIe
1a
2b
3b
4b
5b
6g
400/1
400/1
50/1
100/1
200/1
1200/1
28.5
63
94
98
99
99
98
11 000
16 300
2200
4400
8600
10 700
15 400
3800
6000
11 000
49 200
9100
21 100
3000
1.06
1.08
1.11
1.09
1.05
1.08
8
8
8
8
8
5400
11 300
h
50 700
—
a [3]0 = 5 mM, 5 mL toluene, 30 °C. b [3]0 = 5 mM, 5 mL toluene, 55 °C. c Obtained from 1H NMR determination. d Calculated from the molecular
weight of β-butyrolactone (86.09 g mol−1) multiplied by [β-BL]0/2·[3]0 multiplied by the conversion yield plus the molecular weight of benzyl
alcohol (108.14 g mol−1 e Obtained from GPC analysis and calibrated by a polystyrene standard. f Obtained from 1H NMR analysis. g Condition:
)
.
[3]0 = 10 mM in 10 mL toluene, 55 °C. h Not determination.
1
temperature H NMR studies of 3 display it has a dissociative catalytic activity of complex 3 also enabled the preparation of a
behavior in the solution state as the temperature is raised high molecular weight of atactic-PHB with a very narrow PDI
from −55 to 50 °C. 3 is a four-coordinate dinuclear Zn alkoxide (<1.1).
complex at low temperatures, and dissociates to form a three-
coordinate monomeric zinc alkoxide complex, which is con-
sidered as an active species during the ROP process, at rela-
tively high temperatures. Experimental results illustrate that
the zinc benzylalkoxide 3 initiates the ROP of ε-CL and β-BL in
Experimental
General considerations
a living fashion, furnishing PCL and PHB with controlled
molecular weights and very narrow PDIs. The excellent
All manipulations were carried out under a dry nitrogen
atmosphere. Solvents and reagents were dried by refluxing for
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Fig. 7 1H NMR spectrum of PHB-25 (Table 2, entry 3) initiated by 3.
Scheme 2 Proposed mechanism of ROP initiated by 3.
at least 24 h over sodium/benzophenone (hexane, toluene, p-toluenesulfonic acid (Acros), Pd(OAc)2 (Lancaster), sodium
tetrahydrofuran (THF)), or over phosphorus pentoxide tert-butoxide (Acros), 2,6-diisopropylaniline (Acros), trifluoro-
(CH2Cl2). β-BL and benzyl alcohol were mixed with CaH2 acetic acid (Acros), and 2-methoxyethylamine (Acros) were pur-
and filtered, followed by distillation under reduced pressure. chased and used without further purification. 1H NMR and
Deuterated solvents and ε-caprolactone (ε-CL) were dried 13C NMR spectra were recorded on Bruker Aveance (300 and
over 4 Å molecular sieves. ZnEt2 (1.0 M in hexane) (Aldrich), 400 MHz) spectrometers with chemical shifts given in parts
2-bromobenzaldehyde (Acros), ethylene glycol (Acros), per million from the peak of the internal TMS. Microanalyses
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were performed using a Heraeus CHN-O-RAPID instrument. then stirred for 6 h while the temperature was slowly increased
Gel permeation chromatography (GPC) measurements were to ambient temperature. Volatile materials were removed
performed on a Jasco PU-2080 plus system equipped with a under vacuum to give yellow solids. The residues were re-
RI-2031 detector using THF (HPLC grade) as an eluent. The dissolved in 30 mL of tetrahydrofuran and thence benzyl
chromatographic column was Phenomenex Phenogel 5μ 103 Å alcohol (0.21 mL, 2.0 mmol) was added slowly to this ice cold
and the calibration curve used to calculate Mn(GPC) was pro- THF solution. The mixture was then stirred for another 6 h
duced from polystyrene standards. The GPC results were cal- while the temperature was slowly increased to ambient temp-
culated using the Scientific Information Service Corporation erature. Volatile materials were removed under vacuum and
(SISC) chromatography data solution 3.1 edition.
the residues were recrystallized from 20 mL of n-hexane to give
a yellow crystalline solid. Yield: 0.69 g (68%). Suitable crystals
for X-ray diffraction analysis were obtained from a saturated
toluene solution. Anal. calc. for C58H72N4O4Zn2: N, 5.49; C,
68.29; H, 7.11. Found: N, 5.22; C, 67.64; H, 7.09%. 1H NMR
(CDCl3, 223 K, ppm): δ 7.93 (1H, s, HCvN), 7.29–5.87 (12H, m,
PhH), 4.45 (1H, d, OCH2Ph), 4.22 (1H, d, OCH2Ph), 3.13 (3H, s,
OCH3), 3.03–3.10 (2H, m, CH(CH3)2), 3.00 (2H, t, NCH2CH2O),
2.54 (2H, t, NCH2CH2O), 1.05 (6H, d, CH(CH3)2), 0.83 (6H, d,
CH(CH3)2).
Synthesis of (E)-2,6-diisopropyl-N-(2-(((2-methoxyethyl)imino)-
methyl)phenyl)aniline (AAOMe-H, 1)
A mixture of 2-methoxyethylamine (0.36 g, 3.4 mmol) and
2-(2,6-diisopropyl-phenylamino)benzaldehyde
(0.95
g,
3.4 mmol) were stirred in refluxing methanol (30 mL) for 12 h.
The solution was condensed to ca. 10 mL, and then 40 mL of
n-hexane was introduced to precipitate the crude product.
Volatile materials were removed under vacuum to give brown
solids. Suitable crystals for X-ray diffraction analysis were
obtained from a saturated n-hexane solution. Yield: 0.96 g
(84%). Anal. calc. for C22H30N2O: N, 8.28; C, 78.06; H, 8.93.
Found: N, 7.99; C, 77.82; H, 9.08%. 1H NMR (CDCl3, ppm):
δ 10.48 (1H, s, PhNH), 8.46 (1H, s, HCvN), 7.29–6.18 (7H, m,
PhH), 3.78 (2H, t, NCH2CH2O), 3.64 (2H, t, NCH2CH2O), 3.34
(3H, s, OCH3), 3.10 (2H, m, CH(CH3)2), 1.13 (12H, m,
CH(CH3)2). 13C NMR (CDCl3, ppm): δ 165.76 (HCvN), 149.48,
147.54, 135.24, 133.64, 131.10, 127.17, 123.68, 116.62, 114.92,
111.66 (Ar), 72.74 (NCH2CH2O), 61.12 (NCH2CH2O), 58.97
(OCH3), 28.35 (CH(CH3)2), 24.88, 22.84 (CH(CH3)2).
Synthesis of [(AAOMe)2Zn] (4)
To an ice cold solution of 1 (0.68 g, 2.0 mmol) in 30 mL
toluene was slowly added ZnEt2 (1.20 mL, 1.0 M in n-hexane,
1.20 mmol) under a dry nitrogen atmosphere. The mixture was
then stirred for 36 h in refluxing toluene. Volatile materials
were removed under vacuum and the residues were recrystal-
lized from n-hexane to give a yellow crystalline solid. Suitable
crystals for X-ray diffraction analysis were obtained from a
saturated toluene solution. Yield: 0.58 g (79%). Anal. calc. for
C44H58N4O2Zn: N, 7.57; C, 71.38; H, 7.90. Found: N, 7.34; C,
71.62; H, 8.15%. 1H NMR (CDCl3, ppm): δ 8.11 (1H, s, HCvN),
7.12–6.95 (4H, m, PhH), 6.83 (1H, t, PhH), 6.30 (1H, t, PhH),
6.01 (1H, d, PhH), 3.64 (1H, m, NCH2CH2O), 3.39–3.27 (3H, m,
NCH2CH2O, NCH2CH2O), 3.21 (3H, s, OCH3), 3.17–2.96 (2H,
m, CH(CH3)2), 1.10, 0.94, 0.77, 0.39 (12H, d, CH(CH3)2).
13C NMR (CDCl3, ppm): δ 170.99 (HCvN), 160.26, 147.10,
145.16, 143.96, 137.28, 132.37, 124.24, 124.21, 123.66, 119.65,
115.59, 111.85 (Ar), 71.56 (NCH2CH2O), 58.55 (NCH2CH2O),
57.96 (OCH3), 28.21, 27.44 (CH(CH3)2), 25.43, 24.77, 24.17,
22.77 (CH(CH3)2).
Synthesis of [(AAOMe)ZnEt] (2)
To an ice cold solution of 1 (0.68 g, 2.0 mmol) in 30 mL
n-hexane was slowly added ZnEt2 (2.20 mL, 1.0 M in n-hexane,
2.20 mmol) under a dry nitrogen atmosphere. The mixture was
then stirred for 6 h while the temperature was slowly increased
to ambient temperature. Volatile materials were removed
under vacuum and the residues were recrystallized from a satu-
rated n-hexane solution to give yellow crystals suitable for X-ray
diffraction analysis. Yield: 0.73 g (85%). Anal. calc. for
C24H34N2OZn: N, 6.49; C, 66.73; H, 7.93. Found: N, 6.99; C,
66.50; H, 7.54%. 1H NMR (CDCl3, ppm): δ 8.32 (1H, s, HCvN),
7.23–7.18 (4H, m, PhH), 7.01 (1H, t, PhH), 6.40 (1H, t, PhH), Polymerization of ε-caprolactone catalyzed by 3
6.22 (1H, d, PhH), 3.88 (2H, t, NCH2CH2O), 3.66 (2H, t,
A typical polymerization procedure was exemplified by the syn-
NCH2CH2O), 3.35 (3H, s, OCH3), 3.00 (2H, m, CH(CH3)2), 1.10
(6H, d, CH(CH3)2), 1.03 (6H, d, CH(CH3)2), 0.85 (3H, t,
ZnCH2CH3), 0.12 (2H, q, ZnCH2CH3). 13C NMR (CDCl3, ppm):
δ 170.21 (HCvN), 157.23, 144.83, 143.67, 137.16, 133.56,
124.80, 123.65, 116.05, 113.99, 112.35 (Ar), 72.54 (NCH2CH2O),
60.73 (NCH2CH2O), 59.08 (OCH3), 27.08 (CH(CH3)2), 24.20,
24.03 (CH(CH3)2), 11.88 (ZnCH2CH3), −2.08 (ZnCH2CH3).
thesis of PCL-200 (the number 200 indicates the designed
[ε-CL]0/[Zn]0). To a rapidly stirring solution of [(AAOMe)Zn-
(μ-OBn)]2 (3) (0.051 g, 0.050 mmol) in toluene (10 mL) was
added ε-caprolactone (2.20 mL, 20.0 mmol) under a dry nitro-
gen atmosphere. The reaction mixture was then stirred at
55 °C for 70 min. The conversion yield (99%) of PCL-200 was
1
analyzed by H NMR spectroscopic studies. After the reaction
was quenched by the addition of excess water (1.0 mL), the
polymer was precipitated into n-hexane (100 mL). The final
Synthesis of [(AAOMe)Zn(μ-OBn)]2 (3)
To an ice cold solution of 1 (0.68 g, 2.0 mmol) in 30 mL polymer was then dissolved in THF (5.0 mL) and purified on
n-hexane was slowly added ZnEt2 (2.20 mL, 1.0 M in n-hexane, precipitation again in methanol (50 mL), collected, and dried
2.20 mmol) under a dry nitrogen atmosphere. The mixture was under vacuum. Yield: 2.09 g (92%).
Dalton Trans.
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Table 3 Crystallographic data of complexes 1–4
1
2
3
4
Empirical formula
C
22H30N2O
C24H34N2OZn
431.90
100(2) K
Monoclinic
P21/c
10.0897(3)
21.5824(7)
10.7341(4)
90
102.916(2)
90
2278.32(13)
4
1.259
1.094
920
C58H72N4O4Zn2
1019.94
100(2) K
Triclinic
ˉ
P1
9.4830(3)
12.5950(4)
12.6403(4)
104.2100(10)
108.1830(10)
103.9290(10)
1305.32(7)
1
C44H58N4O2Zn
740.31
100(2) K
Triclinic
ˉ
P1
10.6979(2)
13.3403(2)
14.2243(2)
92.6920(10)
96.9920(10)
104.5650(10)
1943.78(5)
2
Formula weight
Temp (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
338.48
296(2) K
Monoclinic
C2/c
31.4201(10)
8.0500(3)
8.0500(3)
90
90.222(2)
90
4157.0(3)
8
α (°)
β (°)
γ (°)
V (Å3)
Z
Dcalc (Mg m−3
)
1.082
0.066
1472
1.297
0.968
540
1.265
0.673
792
μ (Mo Kα) (mm−1
F(000)
)
Reflections collected
No. of parameters
14 550
227
4091 (0.0634)
0.0788
0.1831
1.023
20 551
253
5609 (0.0432)
0.0393
0.0827
23 060
312
6458 (0.0197)
0.0359
0.0942
32 685
460
9617 (0.0242)
0.0312
0.0735
Indep. reflns (Rint
)
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
Goodness-of-fit on F2
1.008
1.050
1.032
Polymerization of ε-caprolactone catalyzed by 4 in the
presence of BnOH
and dried under vacuum to give a yellow colloid. Yield: 1.51 g
(93%).
A typical polymerization procedure was exemplified by the syn-
thesis of PCL-200 (the number 200 indicates the designed
[ε-CL]0/[BnOH]0). To a rapidly stirring solution of [(AAOMe)2Zn]
(4) (0.037 g, 0.050 mmol) and benzyl alcohol (0.05 mL,
0.05 mmol, 1 M in toluene) in toluene (10 mL) was added
ε-caprolactone (1.10 mL, 10.0 mmol) under a dry nitrogen
atmosphere. The reaction mixture was then stirred at 25 °C for
120 min. The conversion yield (99%) of PCL-200 was analyzed
by 1H NMR spectroscopic studies. After the reaction was
quenched by the addition of excess water (1.0 mL), the
polymer was precipitated into n-hexane (100 mL). The final
polymer was then dissolved in THF (5.0 mL) and purified on
precipitation again in methanol (50 mL), collected, and dried
under vacuum. Yield: 0.97 g (85%).
X-ray crystallographic studies
Suitable crystals of 1 was sealed in thin-walled glass capillaries
and mounted on a Bruker APEX2 diffractometer to collect the
diffraction data at 296 K. Suitable crystals of complexes 2–4
were mounted onto glass fiber using perfluoropolyether oil
and cooled rapidly in a stream of cold nitrogen gas to collect
the diffraction data at 100 K using a Bruker APEX2 diffracto-
meter. Intensity data were collected in 1350 frames with
increasing w (width of 0.5° per frame). The absorption correc-
tion was based on the symmetry-equivalent reflections using
the SADABS program.13a The space group determination was
based on a check of the Laue symmetry and systematic
absence, and was confirmed by the structure solution. The
structures were solved with direct methods using a SHELXTL
package.13b,c All non-H atoms were located from successive
Fourier maps, and hydrogen atoms were treated as a riding
model on their parent C atoms. Anisotropic thermal para-
meters were used for all non-H atoms, and fixed isotropic para-
meters were used for H-atoms. Drawing of the molecules was
done using Oak Ridge Thermal Ellipsoid Plots (ORTEP).14
Crystallographic data of complexes 1–4 are summarized in
Table 3.
Synthesis of polyhydroxybutyrate with benzylalkoxy chain-end
group catalyzed by 3
A typical polymerization procedure was exemplified by the syn-
thesis of PHB-200 (the number 200 indicates the designed
[ε-CL]0/[Zn]0). To a rapidly stirring solution of [(AAOMe)-
Zn(μ-OBn)]2 (3) (0.051 g, 0.050 mmol) in toluene (5 mL) was
added ε-caprolactone (2.20 mL, 20.0 mmol) under a dry nitro-
gen atmosphere. The reaction mixture was then stirred at
55 °C for 8 h. The conversion yield (94%) of PHB-200 was ana-
Acknowledgements
1
lyzed by H NMR spectroscopic studies. After the reaction was
quenched by the addition of excess water (1.0 mL), the We gratefully acknowledge the financial support from the
polymer was precipitated into n-hexane (100 mL). The final National Science Council, Taiwan (NSC101-2113-M-005-020-
polymer was then dissolved in dichloromethane (5.0 mL) and MY3 to B.-T. Ko and NSC101-2113-M-005-010-MY3 to C.-C.
purified on precipitation again in n-hexane (50 mL), collected, Lin).
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Dalton Transactions
Y.
Mu,
Organometallics,
2008,
27,
5889–5893;
References
(c) L. E. N. Allan, J. A. Bélanger, L. M. Callaghan,
D. J. A. Cameron, A. Decken and M. P. Shaver, J. Organomet.
Chem., 2012, 706–707, 106–112.
8 Y.-C. Liu, C.-H. Lin, H.-L. Chen and B.-T. Ko, Acta Crystal-
logr., Sect. E: Struct. Rep. Online., 2009, 65, o2791.
9 (a) Y.-H. Tsai, C.-H. Lin, C.-C. Lin and B.-T. Ko, J. Polym.
Sci., Part A: Polym. Chem., 2009, 47, 4927–4936;
(b) Y.-C. Liu, C.-H. Lin, B.-T. Ko and R.-M. Ho, J. Polym. Sci.,
Part A: Polym. Chem., 2010, 48, 5339–5347.
1 (a) E. Chiellini and R. Solaro, Adv. Mater., 1996, 8, 305–313;
(b) R. E. Drumright, P. R. Gruber and D. E. Henton, Adv.
Mater., 2000, 12, 1841–1846.
2 A. Steinbüchel, Biopolymers, Wiley-VCH, 2002, ISBN 3-527-
30290-5.
3 R. A. Gross and B. Kalra, Science, 2002, 297, 803–806.
4 (a) B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem.
Soc., Dalton Trans., 2001, 2215–2224; (b) A. C. Albertsson
and I. K. Varma, Biomacromolecules, 2003, 4, 1466–1486; 10 C.-H. Wang, C.-Y. Li, C.-H. Lin, Y.-C. Liu and B.-T. Ko,
(c) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Inorg. Chem. Commun., 2011, 14, 1456–1460.
Chem. Rev., 2004, 104, 6147–6176; (d) J. Wu, T.-L. Yu, 11 B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt,
C.-T. Chen and C.-C. Lin, Coord. Chem. Rev., 2006, 250,
602–626, and references therein; (e) C. A. Wheaton,
E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001,
123, 3229–3238.
P. G. Hayes and B. J. Ireland, Dalton Trans., 2009, 4832– 12 (a) H.-Y. Chen, H.-Y. Tang and C.-C. Lin, Macromolecules,
4846; (f) A. K. Sutar, T. Maharana, S. Dutta, C.-T. Chen and
C.-C. Lin, Chem. Soc. Rev., 2010, 39, 1724–1746, and refer-
ences therein.
5 (a) Y. Huang, W.-C. Hung, M.-Y. Liao, T.-E. Tsai, Y.-L. Peng
and C.-C. Lin, J. Polym. Sci., Part A: Polym. Chem., 2009, 47,
2318–2329; (b) H.-J. Chuang, H.-L. Chen, J.-L. Ye,
Z.-Y. Chen, P.-L. Huang, T.-T. Liao, T.-E. Tsai and C.-C. Lin,
J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 696–707;
(c) H.-J. Chuang, H.-L. Chen, B.-H. Huang, T.-E. Tsai,
2006, 39, 3745–3752; (b) W.-C. Hung and C.-C. Lin, Inorg.
Chem., 2009, 48, 728–734.
13 (a) G. M. Sheldrick, SADABS, Program for area detector
absorption correction, Institute for Inorganic Chemistry,
University
of
Göttingen,
Germany,
1996;
(b) G. M. Sheldrick, SHELXTL-97, Program for refinement of
crystal structures, University of Göttingen, Germany, 1997;
(c) G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crys-
tallogr., 2008, 64, 112–122.
P.-L. Huang, T.-T. Liao and C.-C. Lin, J. Polym. Sci., Part A: 14 (a) C. K. Johnson, ORTEP: A FORTRAN Thermal-Ellipsoid
Polym. Chem., 2013, 51, 1185–1196; (d) M.-W. Hsiao and
C.-C. Lin, Dalton Trans., 2013, 42, 2041–2051.
6 (a) C.-Y. Li, C.-Y. Tsai, C.-H. Lin and B.-T. Ko, Dalton Trans.,
2011, 1880–1887; (b) Y.-E. Tai, C.-Y. Li, C.-H. Lin, Y.-C. Liu,
B.-T. Ko and Y.-S. Sun, J. Polym. Sci., Part A: Polym. Chem.,
2011, 49, 4027–4036; (c) C.-Y. Sung, C.-Y. Li, J.-K. Su,
Plot Program for Crystal Structure Illustrations, ORNL-3794
(Rev.), Union Carbide Corp., Oak Ridge Natl. Lab., June
1965; (b) C. K. Johnson, ORTEP-II: A FORTRAN Thermal-
Ellipsoid Plot Program for Crystal Structure Illustrations,
ORNL-5138, Union Carbide Corp., Oak Ridge Natl. Lab.,
March 1976.
T.-Y. Chen, C.-H. Lin and B.-T. Ko, Dalton Trans., 2012, 41, 15 The Mn (GPC) value is multiplied by a factor of 0.56, giving
953–961; (d) C.-Y. Li, C.-R. Wu, Y.-C. Liu and B.-T. Ko,
Chem. Commun., 2012, 48, 9628–9630; (e) C.-Y. Li, C.-J. Yu
and B.-T. Ko, Organometallics, 2013, 32, 172–180.
7 (a) W. Yao, Y. Mu, A. Gao, W. Gao and L. Ye, Dalton Trans.,
2008, 3199–3206; (b) W. Gao, D. Cui, X. Liu, Y. Zhang and
the actual Mn of the poly(ε-caprolactone), see: (a) J. Baran,
A. Duda, A. Kowalski, R. Szymanski and S. Penczek, Macro-
mol. Rapid Commun., 1997, 18, 325–333; (b) M. Save,
M. Schappacher and A. Soum, Macromol. Chem. Phys.,
2002, 203, 889–899.
Dalton Trans.
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