Zinc complexes supported by maltolato ligands: Synthesis, structure, solution behavior, and application in ring-opening polymerization of lactides

Article abstract of DOI:10.1021/om300321h

A series of novel zinc alkoxides supported by chelating maltolato (MalO; MalOH = maltol) ligands were successfully synthesized and characterized. Reaction of MalOH with ZnEt₂ (3:4) gives a trinuclear cluster [Zn₃(Et)₂(MalO)₄] (1), which spontaneously disproportionates in solution to mononuclear species [Zn(MalO)₂] (1a) and [Zn(Et)(MalO)] (1b); (1a) and (1b) form [Zn(MalO)₂(py)] (2), [Zn(Et)(MalO)(py)] (3), [Zn(MalO)₂]₂ ((1a)₂), [Zn(MalO)(OBn)]₂ ((1c)₂), and [Zn₄(Et) ₂(OEt)₂(MalO)₄] (4) on addition of pyridine, benzyl alcohol (BnOH), or dry O₂, respectively. Compounds 1, 2, (1a)₂, and 4 were characterized by elemental analysis, NMR, ESI-MS, and single-crystal X-ray structural analysis. Variable-temperature NMR experiments showed that (1a)₂ and (1c)₂ are in equilibrium with the monomeric form in solution. The addition of l-lactide (l-LA) to a combination of 1 and 2 equiv of BnOH in dichloromethane at room temperature in different molar ratios leads to rapid and efficient generation of poly(l-LA) with end-capped BnO groups. According to kinetic studies, propagation by [Zn(OBn)(MalO)] (1c) is first-order with respect to both the monomer and 1c concentrations; 1a in ring-opening polymerization of l-LA shows no activity. These results suggest a single-site active species in the ring-opening polymerization of l-LA.

Full text of DOI:10.1021/om300321h

Article
pubs.acs.org/Organometallics
Zinc Complexes Supported by Maltolato Ligands: Synthesis,
Structure, Solution Behavior, and Application in Ring-Opening
Polymerization of Lactides
Rafał Petrus and Piotr Sobota*
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
S
* Supporting Information
ABSTRACT: A series of novel zinc alkoxides supported by
chelating maltolato (MalO; MalOH = maltol) ligands were
successfully synthesized and characterized. Reaction of MalOH
with ZnEt2 (3:4) gives a trinuclear cluster [Zn3(Et)2(MalO)4]
(1), which spontaneously disproportionates in solution to
mononuclear species [Zn(MalO)2] (1a) and [Zn(Et)(MalO)]
(1b); (1a) and (1b) form [Zn(MalO)2(py)] (2), [Zn(Et)-
(MalO)(py)] (3), [Zn(MalO)2]2 ((1a)2), [Zn(MalO)-
(OBn)]2 ((1c)2), and [Zn4(Et)2(OEt)2(MalO)4] (4) on
addition of pyridine, benzyl alcohol (BnOH), or dry O2,
respectively. Compounds 1, 2, (1a)2, and 4 were characterized
by elemental analysis, NMR, ESI-MS, and single-crystal X-ray
structural analysis. Variable-temperature NMR experiments
showed that (1a)2 and (1c)2 are in equilibrium with the monomeric form in solution. The addition of L-lactide (L-LA) to a
combination of 1 and 2 equiv of BnOH in dichloromethane at room temperature in different molar ratios leads to rapid and
efficient generation of poly(L-LA) with end-capped BnO groups. According to kinetic studies, propagation by
[Zn(OBn)(MalO)] (1c) is first-order with respect to both the monomer and 1c concentrations; 1a in ring-opening
polymerization of L-LA shows no activity. These results suggest a single-site active species in the ring-opening polymerization of
L-LA.
context, magnesium,4−6 calcium,4,7 zinc,4,5 and aluminum4,8 are
INTRODUCTION
Polylactide (PLA) is a biodegradable, biocompatible, aliphatic
polyester derived from renewable resources, such as corn, sugar
■
attractive metals of low toxicity. For example, β-diketiminate,5,9
bisphenolate,10 Schiff’s bases,11 phenoxyamine,12 and tris-
(pyrazolyl)hydroborate13 zinc complexes exhibit excellent
activities for ROP of L-LA. Our group recently developed the
aminophenolate zinc compound [Zn(tbpca)2] (tbpcaH = N-
[methyl(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-cyclo-
hexylamine), which showed high activity in L-LA polymer-
ization.14
beets, and agricultural waste.1 PLA has found numerous
specialty applications in the biomedical industry, such as
biodegradable screws and sutures, scaffolds for tissue engineer-
ing, matrixes for controlled drug delivery systems,2 and as
environmentally friendly bulk packing biodegradable materials.3
The concept of PLA synthesis is very well established. The ring-
opening polymerization (ROP) of L-lactide (L-LA) for the
preparation of PLA, the method used in this study, has been
investigated extensively.4 This process, in the presence of metal
alkoxides (M−OR), is thought to occur via a coordination−
insertion mechanism, whereby the metal center activates the
carbonyl group of the incoming L-LA molecule toward attack
by the alkoxide group. This is followed by insertion of an L-LA
molecule into the M−OR bond with cleavage of the acyl
oxygen bond of the monomer.4 In industry, PLA is synthesized
by ROP using tin(II) bis(2-ethylhexanate) as a catalyst.
Although Sn(Oct)2 has been accepted as a food additive by
the U.S. Food and Drug Administration, the toxicity associated
with most tin compounds is a considerable drawback in the
case of biomedical applications.4 There has, therefore, been
much research devoted to finding well-defined complexes of
high activity containing biologically benign metals.4−8 In this
We turned our attention toward zinc compounds supported
by MalO (MalOH = maltol) ligands, which are excellent for the
production of polymers with low polydispersities and narrow
molecular weight distributions.4 Maltol is a very attractive
ligand and is easily deprotonated to give an O,O′-bidentate
chelating system for a number of biologically active metal ions,
to form thermodynamically stable and neutral complexes.15 For
instance, metal−maltol complexes, such as [Fe(MalO)3],16
[Al(MalO)3],17 [Ga(MalO)3],18 and [VO(MalO)2],19 have
found numerous medical applications as metallotherapeutic
drugs and metal-based diagnostic agents.15 We also chose
maltol for its commercial availability and low cost, and because
it is harmless to humans.20
Received: April 19, 2012
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Zinc Complexes 1−4
We describe here our results on the synthesis and
characterization of
a zinc−maltol polynuclear
[Zn3(Et)2(MalO)4] cluster, which takes an active part in L-LA
polymerization, and formation of compounds 2−4 (Scheme 1)
and (1a)2−(1c)2 (Scheme 2) by reaction with pyridine, benzyl
alcohol (BnOH), or dry oxygen.
Scheme 2. Synthesis of Zinc Complexes (1a)2 and (1c)2
Figure 1. Molecular structure of [Zn3(Et)2(MalO)4]·toluene (1). The
displacement ellipsoids are drawn at the 30% probability level.
Toluene molecule and hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Zn1−O1 2.053(3), Zn1−
O2 2.136(3), Zn1−O7 2.115(3), Zn1−O8 2.183(4), Zn1−O23
2.10(3), Zn1−O24 2.055(4), Zn2−O2 2.078(4), Zn2−O8 2.562(4),
Zn2−O15 2.157(4), Zn2−O16 2.069(3), Zn2−C13 1.992(4), Zn3−
O8 2.066(4), Zn3−O16 2.006(3), Zn3−O24 2.034(3), Zn3−C21
1.961(4), O1−Zn1−O8 111.6(4), O1−Zn1−O23 92.5(4), O1−Zn1−
O24 161.8(4), O2−Zn1−O7 149.2(4), O8−Zn1−O23 151.8(4),
O8−Zn1−O24 80.6(4), O23−Zn1−O24 80.1(4), O2−Zn2−O16
110.8(4), O2−Zn2−C13 116.2(4), O8−Zn2−O15 140.9(4), O16−
Zn2−C13 131.4(4), O8−Zn3−O16 83.5(4), O8−Zn3−O24 83.9(4),
O8−Zn3−C21 123.8(6), O16−Zn3−C21 137.5(6), O16−Zn3−O24
93.9(5), O24−Zn3−C21 118.6(7).
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Zinc Complexes 1−
4. As shown in Scheme 1, the zinc atom easily forms
coordination compounds by reaction of the hydroxyl-group-
containing MalOH with an active organometallic species, such
as diethylzinc. Treatment of 4 equiv of MalOH with 3 equiv of
ZnEt2 in toluene at room temperature affords the cluster
complex [Zn3(Et)2(MalO)4] (1, 75%). The compound was
isolated as colorless crystals, which are readily soluble in
hydrocarbons, chlorinated solvents, and THF. Its structure was
confirmed by elemental analysis, X-ray crystallography, NMR
spectroscopy, and ESI-MS. The molecular structure of 1 is
shown in Figure 1, and selected bond distances and angles are
given in the figure legend. Single-crystal X-ray diffraction
(XRD) data showed that solid 1 is a trinuclear cluster
containing an incomplete cubane Zn3O4 core with different
coordination modes around the zinc atoms. The complex
consists of one [Zn(MalO)2] and two [Zn(Et)(MalO)]
subunits. The metal ions are held together by three μ2-
O(alkoxo) and one μ3-O(alkoxo) oxygen atom. The Zn1 ion
forms a six-coordinated distorted octahedron with O6 donor
sets. The geometry around the Zn2 ion is a distorted trigonal
bipyramid surrounded by O4Et groups, and the Zn3 ion
geometry is a distorted tetrahedron with surrounding O3Et
B
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donor ligands. The 1H NMR spectrum of 1 in C6D6 shows one
chemical equivalent of zinc-bonded ethyl groups and one
environment for MalO ligands (Supporting Information, Figure
S1), suggesting a dynamic process occurring in solution. We
decided to investigate using a variable-temperature (VT) NMR
study. Compound 1 shows VT 1H NMR (Supporting
Information, Figure S5) behavior consistent with the dynamic
monomer−trimer equilibrium shown in Scheme 1. Moreover,
we conducted cryoscopic molecular weight determinations,
which showed that compound 1 undergoes dissociation in p-
xylene solution. It is worth to note that, when the reaction of
MalOH with ZnEt2 was carried out at a 1:1 molar ratio, the
mixture of compounds 1 (isolated in crystalline form and
identified by measuring the unit cell parameters), 1b, and free
ZnEt2 was observed (Supporting Information, Figure S7). To
trap the disproportionation products of 1, pyridine was added,
forming colorless crystals of complex [Zn(MalO)2(py)] (2,
26%). XRD analysis showed that 2 possesses a square-
pyramidal geometry with four oxygen atoms of the chelating
MalO ligands and one pyridine nitrogen atom occupying an
apical position (Figure 2). The NMR data are fully consistent
c r y s t a l s o f t h e t e t r a n u c l e a r c o m p o u n d
[Zn4(Et)2(OEt)2(MalO)4] (4, 39%). Single-crystal XRD
measurements showed that 4 exists as a tetrameric cluster,
which contains two environmentally different zinc centers, as
shown in Figure 3. The zinc ions and bridging alkoxo groups
Figure 3. Molecular structure of [Zn4(Et)2(OEt)2(MalO)4]·toluene
(4). The displacement ellipsoids are drawn at the 30% probability
level. Toluene molecule and hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Zn1−O1 2.087(4), Zn1−
O2 2.124(4), Zn1−O9 2.084(4), Zn1−O9A 2.137(4), Zn1−O11
2.104(3), Zn1−O12 2.109(4), Zn2−O2 2.055(4), Zn2−O9 2.034(3),
Zn2−O12A 2.048(4), Zn2−C7 1.983(4), O1−Zn1−O2 78.5(5), O1−
Zn1−O9 156.4(4), O1−Zn1−O12 102.9(5), O2−Zn1−O9 78.9(4),
O2−Zn1−O12 177.9(4), O2−Zn1−O12 177.9(4), O9−Zn1−O12
99.9(5), O9A−Zn1−O11 156.7(4), O2−Zn2−O9 81.7(5), O2−Zn2−
O12A 98.1(5), O2−Zn2−C7 122.4(6), O9−Zn2−O12A 82.7(4),
O9−Zn2−C7 134(5), O12A−Zn2−C7 125.1(5).
Figure 2. Molecular structure of [Zn(MalO)2(py)] (2). The
displacement ellipsoids are drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Zn1−O1 2.140(2), Zn1−O2 1.980(2), Zn1−N7
2.034(3), O1−Zn1−O1A 160.3(3), O1−Zn1−O2 81.2(3), O1−
Zn1−O2A 91.5(3), O2−Zn1−O2A 137.1(3), O1−Zn1−N7
100.1(2), O2−Zn1−N7 111.5(2).
are arranged in a “double-open” face-sharing dicubane-like core
with two missing vertices. The Zn1 adopts an octahedral
coordination environment with O6 donor sets. In contrast, Zn2
forms a distorted tetrahedral coordination geometry consisting
of two μ2-O bridging MalO oxygen atoms, supported by one
ethoxo and one ethyl group. The identity of 4 was additionally
determined by NMR spectroscopic studies (Supporting
Information, Figures S19−S22).
In the ROP of LA, it is very well established that the use of
external alcohols in situ to activate alkylzinc species leads to the
formation of active alkoxide complexes, with rapid and efficient
chain transfer, to generate PLA end-capped with ester groups
from the added alcohol.22 To prepare zinc−alkoxide complexes
useful for ROP catalysis,23 alcoholysis of the ethylzinc complex
1 was investigated. The treatment of 1 with 2 equiv of BnOH in
dichloromethane, followed by partial solvent removal, results in
a crystalline material [Zn(MalO)2]2 ((1a)2, 28%), which is
weakly soluble in conventional solvents. Single-crystal X-ray
analysis showed that (1a)2 is a centrosymmetric dimeric
complex, in which alkoxide groups bridge heavily distorted
square-pyramidal zinc centers (Figure 4). The complex
[Zn(OBn)(MalO)]2 ((1c)2, 59%) was isolated from the filtrate
as a yellow solid by precipitation with hexane. Attempts to grow
single crystals of (1c)2 for XRD analysis were unsuccessful. The
with the anticipated formula (Supporting Information, Figures
S8−S11). Complex [Zn(Et)(MalO)(py)] (3, 47%) was
isolated from the filtrate as a dark green solid by precipitation
with hexane. Although we were not able to obtain single
1
crystals, the H NMR spectra of 3 over a wide temperature
range (Supporting Information, Figure S17) show a set of
symmetric ligand resonances, indicating a simple structure,
attributed to the tetrahedral geometry of [Zn(Et)(MalO)(py)].
ESI-MS studies support the proposed formula, giving a simple
molecular ion at 268 m/z, corresponding to [Zn(MalO)(py)]+
moieties (Supporting Information, Figure S18). To confirm the
existence in solution of the tricoordinated intermediate
[Zn(Et)(MalO)] (1b), we carried out a reaction with dry O2,
as shown in Scheme 1. It is well known that oxygenation of
three-coordinated alkylzinc species in solution affords alkylzinc
alkoxide moieties.21 A toluene solution of 1 was treated with an
excess of dry O2 at 0 °C, the reaction was stirred for 10 min,
and then excess O2 was removed. Work up gave colorless
C
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1
confirmed by VT H, 13C, COSY, and HMQC spectroscopic
studies (Supporting Information, Figures S32−S36). Studies
carried out using (1a)2 in the ROP of L-LA in the presence of
BnOH showed complete inactivity of this complex in the
polymerization process. These results indicate that only 1c acts
as a single-site catalyst for the preparation of poly(L-LA)
(PLLA). The polymerization of L-LA using 1c as an initiator
was systematically examined. Representative results obtained
using this binary system are presented in Table 1. The standard
procedure for the preparation of PLLA is outlined in Scheme 3.
Scheme 3. Ring-Opening Polymerization of L-Lactide
Initiated by in Situ Generated 1c
Figure 4. Molecular structure of [Zn(MalO)2]2·2CH2Cl2 ((1a)2). The
displacement ellipsoids are drawn at the 30% probability level.
Dichloromethane molecule and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Zn1−O1
2.037(3), Zn1−O2 2.006(3), Zn1−O7 2.014(3), Zn1−O8 2.168(3),
Zn1−O8A 2.003(3), O1−Zn1−O2 83.4(2), O1−Zn1−O7 136.9(2),
O1−Zn1−O8 93.6(2), O1−Zn1−O8A 105.7(2), O2−Zn1−O7
91(2), O2−Zn1−O8 162.5(2), O2−Zn1−O8A 113.1(2), O7−Zn1−
O8A 115.6(2).
1H NMR spectrum of (1c)2 has no complexity and shows a
dimeric species in which the two [Zn(MalO)]+ units are
bridged by two μ2-O oxygen atoms of benzyl alkoxides
(Supporting Information, Figure S29). This is clearly shown
by the aromatic and methylene resonances of the bridging BnO
groups shifting upfield to 7.25 and 4.56 ppm, respectively, as
compared with 7.68 and 5.27 ppm for the terminally
coordinated BnO groups in the monomeric species 1c
(Supporting Information, S32). Further studies have shown
that (1c)2 tends to disproportionate into complex (1a)2 and
Zn(OBn)2, as shown in Scheme 2. Such disproportionation of
heteroleptic zinc alkoxides has previously been noted by
Carpentier and co-workers, who proved that dimeric [Zn-
(OiPr)(ONPh,Bn)]2 is transformed into Zn(OiPr)2 and [Zn-
(ONPh,Bn)2] [HONPh,Bn = (E)-4-(benzylimino)-1,1,1-trifluoro-
2-(trifluoromethyl)-4-phenylbutan-2-ol].24
The addition of L-LA to a combination of 1 and 2 equiv of
BnOH in dichloromethane at room temperature in different
molar ratios led to the rapid and efficient generation of PLLA
with end-capped BnO groups. The monomer conversion and
number-average molecular weight (Mn) values were determined
1
using H NMR spectroscopy and calculated from the ratio of
the aromatic peaks from the BnO group at ∼7.04 ppm to the
methine peak from the polymer backbone (Figure 5;
Supporting Information, Figures S37−S42). Furthermore, the
Mn and polydispersity index (PDI) values were determined by
GPC (see Table 1). To identify the end groups using 1H NMR,
the polymerization was conducted with a low ratio, L-LA/Zn1c
= 10 (“oligomerization”), for 0.5 h at room temperature (Table
1, entry 1; Figure 5), and then quenched with methanol. The
end groups of the obtained oligomer were analyzed and
Ring-Opening Polymerization of L-Lactide. Because of
the relative instability of the benzyl alkoxide (1c)2 and the
difficulty of isolating the pure compound in amounts large
enough to carry out extensive catalysis studies, we investigated
combinations of ethylzinc compound 1 with BnOH to generate
[Zn(MalO)2] (1a) and [Zn(MalO)(OBn)] (1c) in situ, as
1
1
confirmed using H NMR and ESI-MS. From the H NMR
spectrum, the number-average degree of polymerization of the
L-LA units was determined by comparing the integrals of the
signals corresponding to the benzyl ester groups with methine
protons from the polymer backbone (labeled g and e, e″ in
Table 1. Polymerization of L-Lactide Using in Situ Generated 1c at 25 °C in Dichloromethane
a
b
c
c
d
entry
[L-LA]/[Zn1c]
t (h)
Mw/Mn
Mn(GPC)
Mn(calcd)
Mn(NMR)
C (%)
Tm (°C)
1
2
3
4
5
10
25
0.5
1
1500
3600
1600
3800
99
98
136.8
145.1
150.8
159.0
169.4
162.8
1.26
1.30
1.12
1.16
2.15
3600
6200
50
2.5
6
7000
6700
96
100
200
50
14 800
23 700
9100
14 400
27 200
7300
14 800
28 700
9700
99
12
2
94
e
6
100
a
b
Obtained from GPC analysis and calibrated by polystyrene standards. Calculated from the molecular weight of L-LA × [L-LA]/[Zn1c] ×
c
d
1
conversion yield plus Mw(BnOH). Obtained from H NMR analysis. Obtained from DSC analysis (Supporting Information, Figures S44−S49).
e
Obtained in bulk polymerization at 90 °C.
D
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along with the data obtained using ESI-MS for the BnO−
PLLA10 oligomer (Figures 5 and 6) is consistent with an
initiation mechanism in which the MalO ligand remains bound
to the zinc center while the BnO ligand initiates polymerization.
CONCLUSION
■
This work demonstrates the application of the chelating
MalOH ligand in L-LA polymerization. The advantages of the
MalOH ligand are that it is readily available commercially,
cheap, and harmless to humans. We have shown that the direct
reaction of MalOH with ZnEt2 (3:4) in toluene gave a
trinuclear cluster [Zn3(Et)2(MalO)4] (1), which spontaneously
disproportionates in solution to the mononuclear species
[Zn(MalO)2] (1a) and [Zn(Et)(MalO)] (1b); these can be
1
detected by monitoring the reaction mixture using VT H
1
Figure 5. H NMR spectrum of BnO−PLLA10 oligomer in C6D6.
NMR spectroscopy. Furthermore, the disproportionation
products of 1 were trapped by the addition of pyridine,
BnOH, or dry O2, resulting in the formation of [Zn-
(MalO)2(py)] (2), [Zn(Et)(MalO)(py)] (3), [Zn(MalO)2]2
( ( 1 a ) 2 ) , [ Z n ( M a l O ) ( O B n ) ] 2 ( ( 1 c ) 2 ) , a n d
[Zn4(Et)2(OEt)2(MalO)4] (4), respectively (Schemes 1 and
2). We also demonstrated that the heteroleptic complex (1c)2
is unstable in solution and easily disproportionates to the
homoleptic (1a)2 and Zn(OBn)2. The experimental results
reported in this paper strongly suggest that the monometallic
zinc complex 1c mediates the insertion of the L-LA monomer
and efficiently generates PLLA with end-capped BnO groups.
The polymerization kinetic studies show a first-order depend-
ence on both [L-LA] and [1c], leading us to propose a single-
site active species. Finally, our observations represent an
important advance in understanding the dynamic processes
occurring in solution and should be considered in the design of
new efficient zinc catalysts for the polymerization of
heterocyclic monomers.
Figure 5). The calculated number-average degree of polymer-
ization was 10. ESI-MS confirmed the proposed structure
(Figure 6). Since the monoisotopic masses of the BnO moiety
(107.05 Da) and of an L-LA unit (144.04 Da) are known, the
molecular weight of the BnO−PLLAn polymer at any given
value of n can be calculated. For example, a sodium-cationized
oligomer with n = 10 has a monoisotopic mass of 1571.5 Da
(Figure 6). Using the masses corresponding to the individual
peaks and their respective intensities, the Mn and weight-
average molecular weight (Mw) of BnO−PLLAn can be
calculated from the ESI-MS.25 The calculated Mn value
corresponds to a number-average degree of L-LA units of 9.
The results from 1H NMR spectroscopy and ESI-MS show the
presence of one benzyl ester group per polymer chain and a
degree of polymerization approximately that of the L-LA/Zn1c
quotient. Furthermore, a plot of the PLLA Mn obtained from
1H NMR analysis and polystyrene standards GPC as a function
of [L-LA]/[Zn1c] = 25−200 (Figure 7) demonstrates good
polymerization control. The linear nature of this plot, in
conjunction with the relatively narrow polydispersities (Mw/Mn
< 1.3), suggests that the polymerization is living and proceeds
by a coordination−insertion mechanism.26 Additionally, the
agreement between the observed and theoretical values of Mn
EXPERIMENTAL SECTION
■
Materials and Methods. All the syntheses were performed under
a dry nitrogen atmosphere using standard Schlenk techniques.
Reagents were purified by standard methods: toluene, hexane, C6D6,
and C7D8 were distilled from Na; CH2Cl2 was distilled from CaH2 and
Figure 6. ESI-MS of BnO−PLLA10 oligomer.
E
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Figure 7. Plot of PLLA Mn obtained from 1H NMR analysis and polystyrene standards GPC vs [L-LA]/[Zn1c], with PDIs marked as open triangles.
then P2O5; BnOH was distilled from P2O5; L-LA was recrystallized
from toluene and sublimed; and pyridine was predried and then
distilled over solid KOH. MalOH, BnOH, L-LA, and ZnEt2 were
purchased from Aldrich (St Louis, MO). 1H, 13C, COSY, and HMQC
NMR spectra were recorded at room temperature using Bruker
Avance 300 and 500 MHz spectrometers. Chemical shifts are reported
in parts per million and referenced to the residual protons in
deuterated solvents. ESI-MS data were collected on a Bruker
micrOTOF-Q mass spectrometer. The thermal analysis of polymers
was performed using a Universal V4.5A differential scanning
calorimeter (TA Instruments, New Castle, DE) at a heating rate of
2 °C/min. Elemental analysis was determined on a PerkingElmer 2400
CHN Elemental Analyzer. The GPC measurements were performed
on a Viscotek TDA 305 Triple detector array GPCmax. The GPC
columns were eluted with dichloromethane at 35 °C at 1 mL/min and
calibrated with polystyrene standards over a peak average molecular
weight (Mp) range of 600−3 000 000 Da.
off, washed with hexane (3 × 5 mL), and dried under vacuum. Yield:
0.56 g (47%). Anal. Calcd for C13H15NO3Zn: C, 52.28; H, 5.06; N,
4.69. Found: C, 52.33; H, 5.12; N, 4.63. 1H NMR (C6D6, 300 MHz): δ
8.48 (2H, m, py), 6.87 (1H, tt, J = 7.7, 1.8 Hz, py), 6.74 (1H, d, J = 5.1
Hz, CH−CO), 6.56 (2H, m, py), 6.15 (1H, d, J = 5.1 Hz, 
CH−O), 2.41 (3H, s, CH3MalO), 1.72 (3H, t, J = 8.1 Hz, CH3Et), 0.84
(2H, k, J = 8.1, CH2Et). 13C NMR (C6D6, 75 MHz): δ 179.3 (1C, C
O), 152.9−151.6 (2C, C−CH3, C−O−Zn), 152.2 (1C, CH−O),
149.4 (2C, py), 136.8 (1C, py), 124 (2C, py), 111 (2C, CH−C
O), 15.1 (1C, CH3MalO), 13.7(CH3Et), −1.71 (1C, CH2).
Synthesis of [Zn4(Et)2(OEt)2(MalO)4] (4). A solution of 1 (1 g,
1.33 mmol) in toluene (100 mL) at 0 °C was exposed to an excess of
dry O2 for 10 min. After oxygenation, the excess O2 was removed, and
nitrogen was flowed through the system. The orange mixture was
stirred for 1 h and allowed to warm to room temperature. After 3 h,
the volume was reduced to 60 mL. Colorless crystals were obtained by
slow evaporation from the toluene solution. Yield: 0.70 g (39%). Anal.
Calcd for C32H40O14Zn4: C, 42.22; H, 4.43. Found: C, 42.30; H, 4.50.
1H NMR (C6D6, 500 MHz): δ 6.65 (4H, m, CH−O), 6.10 (4H, m,
Synthesis of [Zn3(Et)2(MalO)4] (1). ZnEt2 (11.90 mL, 11.90
mmol) was added dropwise to a solution of MalOH (2 g, 15.86 mmol)
in toluene (100 mL). The light green mixture was stirred at room
temperature. After 1 h, the volume was reduced to 30 mL, and the
light green precipitate was filtered off, washed with hexane (3 × 10
mL), and dried under vacuum. The light green solid was dissolved in
hot toluene (100 mL) and recrystallized to yield colorless crystals.
Yield: 2.25 g (75%). Anal. Calcd for C28H30O12Zn3: C, 44.56; H, 4.01.
Found: C, 44.90; H, 4.20. 1H NMR (C6D6, 500 MHz): δ 6.60 (4H, d,
J = 5.1 Hz, CH−O), 6.02 (4H, d, J = 5.1 Hz, CH−CO), 2.43
(s, 12H, CH3MalO), 1.50 (6H, t, J = 7.9 Hz, CH3Et), 0.61 (4H, k, J = 7.9,
CH2). 13C NMR (C6D6, 125 MHz): δ 178.6 (4C, CO), 152.9 (4C,
CH−O), 152.2−149.9 (8C, C−CH3, C−O−Zn), 111.5 (4C,
=CH−CO), 15.5 (4C, CH3MalO), 13.3 (2C, CH3Et), 0.1 (2C, CH2).
Synthesis of [Zn(MalO)2(py)] (2). Pyridine (6 equiv, 0.64 mL,
7.95 mmol) was added dropwise to a solution of 1 (1 g, 1.33 mmol) in
toluene (100 mL). The dark orange mixture was stirred at room
temperature. After 4 h, the volume was reduced to 60 mL under
vacuum. Single crystals for XRD analysis were obtained from the
concentrated mother liquor at room temperature. The crystals were
filtered off, washed with hexane (3 × 10 mL), and dried under vacuum.
Yield: 0.41 g (26%). Anal. Calcd for C17H15NO6Zn: C, 51.73; H, 3.83;
CH−CO), 4.48 (4H, m, CH2OEt), 2.42 (12H, s, CH3MalO), 1.56−
1.36 (12H, m, CH3OEt, CH3Et), 0.80 (4H, m, CH2Et). 13C NMR (C6D6,
125 MHz): δ 178.7 (4C, CO), 152.2 (4C, C−CH3), 151.8−150.8
(8C, CH−O, C−O−Zn), 110.9 (4C, CH−CO), 62 (2C,
CH2OEt), 21.8 (4C, CH3OEt, CH3Et), 15.2 (4C, CH3MalO), 6.9 (2C,
CH2Et).
Characterization of [Zn(MalO)2] (1a) and [Zn(OBn)(MalO)]
(1c). BnOH (0.55 mL, 5.30 mmol) was added to a solution of 1 (2 g,
2.65 mmol) in dichloromethane (40 mL). After the yellow mixture
had been stirred at room temperature for 1 h, the volume was reduced
to 10 mL, and the yellow precipitate was filtered off, washed with
hexane (3 × 10 mL), and dried under vacuum. Yield: 2.22 g (92%).
Anal. Calcd for C38H34O14Zn3: C, 50.11; H, 3.76. Found: C, 50.60; H,
1
3.85. H NMR (C6D6, 300 MHz): δ 7.65 (4H, m, m-ArHBnO), 7.13−
6.99 (6H, m, o/p-ArHBnO), 6.73 (4H, d, J = 4.8, Hz, CH−O), 6.08
(4H, d, J = 4.8 Hz, CH−CO), 5.27 (4H, s, CH2), 2.37 (12H, s,
CH3). 13C NMR (C6D6, 75 MHz): δ 178.6 (4C, CO), 152.4 (4C,
CH−O), 151.1 (4C, C−CH3), 145.8 (4C, Zn−O−C), 129.3−126.2
(10C, ArHBnO), 110.7 (4C, CH−CO), 68.3 (2C, CH2), 15.3 (4C,
CH3).
1
N, 3.55. Found: C, 51.72; H, 3.73; N, 3.25. H NMR (CD2Cl2, 500
Synthesis of [Zn(MalO)2]2 ((1a)2). BnOH (0.28 mL, 2.66 mmol)
was added to a solution of 1 (1 g, 1.33 mmol) in dichloromethane (50
mL). The reaction mixture was stirred for 2 h. The volume was
reduced to 10 mL under vacuum to yield colorless crystals. The
crystals were filtered off, washed with hexane (3 × 10 mL), and dried
under vacuum. Yield: 0.29 g (28%). Anal. Calcd for C24H20O12Zn2: C,
45.67; H, 3.19. Found: C, 45.9; H, 3.21. 1H NMR (CD2Cl2, 300
MHz): δ 7.73 (4H, d, J = 5.1 Hz, CH−O), 6.49 (4H, d, J = 5.1 Hz,
CH−CO), 2.39 (s, 12H, CH3). 13C NMR (CD2Cl2, 75 MHz): δ
MHz): δ 8.57 (2H, m, py), 7.82 (1H, tt, J = 7.7, 1.6 Hz, py), 7.74 (2H,
d, J = 5.1 Hz, CH−CO), 7.40 (2H, m, py), 6.50 (2H, d, J = 5.1
Hz, CH−O), 2.42 (6H, s, CH3). 13C NMR (CD2Cl2, 125 MHz): δ
178.2 (2C, CO), 152.7 (2C, CH−O), 151.8−151.5 (4C, C−
CH3, C−O−Zn), 149.7 (2C, py), 138.7 (1C, py), 125.1 (2C, py),
110.9 (2C, CH−CO), 15 (2C, CH3).
Synthesis of [Zn(Et)(MalO)(py)] (3). Hexane (50 mL) was added
to the filtrate from 2 to give a dark green precipitate, which was filtered
F
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Article
178.1 (4C, CO), 153.5 (4C, CH−O), 153.3−150.4 (8C, C−
CH3, Zn−O−C), 111 (4C, CH−CO), 15.2 (4C, CH3).
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Polymerization Procedure. A typical polymerization procedure is
exemplified by the synthesis of PLLA at room temperature. BnOH (16
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures giving selected NMR and ESI-MS spectra of
1
compounds 1−4; H NMR and DSC spectra of polymers
obtained using in situ generated 1c; kinetic study; and a CIF file
giving crystallographic data for 1, (1a)2, 2, and 4. This material
is available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data for the structural analyses reported in this
paper have also been deposited with the Cambridge Crystallo-
graphic Data Centre (CCDC), nos. 873545−873548. Copies of
the information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk;
home page: http://www.ccdc.cam.ac.uk).
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AUTHOR INFORMATION
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Corresponding Author
*Tel: (+48) 71 375 73 06. Fax: (+48) 71 328 23 48. E-mail:
piotr.sobota@chem.uni.wroc.pl.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support by the Wrocław Research
Centre EIT+, under the project “Biotechnologies and advanced
medical technologies − BioMed” (POIG.01.01.02-02-003/08),
financed by the European Regional Development Fund
(Operational Programme Innovative Economy, 1.1.2)
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