Steric and chelating ring concerns on the l-lactide polymerization by asymmetric β-diketiminato zinc complexes

Article abstract of DOI:10.1039/c6ra07453g

A series of new asymmetrical N-aryl-N′-alkyl β-diketimines bearing a pendant pyridyl group (L¹H-L⁴H) were synthesized in a two steps reaction from acetylacetone and the corresponding appropriate amines. The reaction of these β-diketi

Full text of DOI:10.1039/c6ra07453g

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Steric and chelating ring concerns on the L-lactide
polymerization by asymmetric b-diketiminato zinc
complexes†
Cite this: RSC Adv., 2016, 6, 36705
Wan-Jung Chuang,a Hsing-Yin Chen,a Wei-Ting Chen,a Heng-Yi Chang,a
a
a
Michael Y. Chiang,ab Hsuan-Ying Chen and Sodio C. N. Hsu
*
*
A series of new asymmetrical N-aryl-N0-alkyl b-diketimines bearing a pendant pyridyl group (L1H–L4H) were
synthesized in a two steps reaction from acetylacetone and the corresponding appropriate amines. The
reaction of these b-diketimines (LH) with ZnEt2 afforded the corresponding ethyl zinc complexes. The
ethyl zinc complexes were characterized by NMR spectroscopy and single crystal X-ray diffraction to
confirm their structure and ligand binding mode. Diffusion-ordered nuclear magnetic resonance
spectroscopy (DOSY) experiments yielded diffusion coefficients suggesting that all ethyl zinc complexes
are monomeric in solution. All these zinc complexes demonstrate catalytic capabilities towards the ring-
opening polymerization of L-lactide. The rate of polymerization depended heavily on the ligand favoring
steric bulky aryl substituents (1,3,5-trimethylphenyl) and short pendant arms (pyridylmethyl).
Received 22nd March 2016
Accepted 4th April 2016
DOI: 10.1039/c6ra07453g
www.rsc.org/advances
properties of the N-aryl groups were examined by Lin et al.7 A
few NNN-tridentate b-diketiminato zinc complexes have been
reported in studies concerning the lactide ROP reaction.9,10
Introduction
Due to environmental concerns, petrolic plastics are gradually
being replaced with biodegradable materials such as aliphatic
polyester.1 Polylactide (PLA), produced by the ring-opening
polymerization (ROP) of lactide (LA),2 is a leading biodegrad-
able and biocompatible polyester, which has attracted consid-
erable attention due to its suitability for biomedical and
pharmaceutical applications.3 Zinc complexes bearing ligands
including b-diketiminate,4–10 bis(imino)aryl NCN pincer,11
bis(pyrazol-yl)methane-base NNO-donor scorpinate,12 Schiff
base,13 anilido-aldimine,14 amido-oxazolinate,15 diimine,16
maltolate,17 and phosphido pincer ligand18 have proven to be
efficient catalysts for the ROP of lactides. These ROP catalysts
resulted in polymers with well-controlled molecular weight and
narrow molecular weight distribution.
Zinc complexes bearing bidentate ligands with nitrogen
donors have been extensively applied to the studies on ROP of
lactides. The substituent effects of the polymerization of LA by
zinc complexes bearing bidentate b-diketiminate ligands were
previously studied by Coates et al.3,4 and the investigations on
the electronic effects of the substituents on the electronic
Chen reported the rst asymmetric NNN-tridentate b-diketimi-
nato zinc complex and implied the exible pendant arm may
affect ROP activity.9 Recently, an asymmetric N-aryl-N0-alkyl b-
diketiminato zinc complex bearing pendant pyridyl group was
shown to be a highly active and good catalyst in the ROP of LA.10
Despite the extensive studies previously, two important issues
remain unanswered: (1) how the length of exible pendant arm
govern the zinc complexes formation and ROP activity? (2) Do
the steric bulkiness of the substituents effect the polymeriza-
tion result? To answer these questions, four different N-aryl-N0-
alkyl b-diketiminato ligands bearing aryl substituents of
different size or pendant pyridyl arm with different length
(Chart 1) were designed and reported herein on their coordi-
nation behavior. Furthermore, we reported the synthesis of
a series of zinc complexes ligated by these asymmetrical b-
diketiminate ligands as well as their structure–reactivity rela-
tionship in the ROP of L-lactide.
Results and discussion
Ligand synthesis and characterization
aDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung 807, Taiwan. E-mail: hchen@kmu.edu.tw; sodiohsu@kmu.edu.tw; Fax: 2-((2,6-Diisopropylphenyl)imido)-2-penten-4-one and 2-((2,4,6-
+886-7-3125339; Tel: +886-7-3121101-2585; +886-7-3121101-6984
bDepartment of Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
trimethylphenyl)imido)-2-penten-4-one were prepared by
simple condensation of acetylacetone with an equimolar
amount of corresponding aniline in the presence of a catalytic
amount p-toluenesulfonic acid (TsOH) and water was removed
azeotropically during the reaction. The products were subse-
quently treated with appropriate amine in toluene and in the
† Electronic supplementary information (ESI) available: Text containing
additional gures (Fig. S1–S30), tables (Tables S1 and S2), and crystallographic
data in CIF format for the structure determinations of L3H, zinc complexes 1–4.
CCDC 1434846–1434850. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6ra07453g
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Chart 1 Representation of the N-aryl-N0-alkyl b-diketimine ligand sets.
Scheme 1 Synthesis of the N-aryl-N0-alkyl b-diketimine ligand sets.
34
˚
˚
presence of an equimolar amount of TsOH, to afford the desired 0.01 A) or clear bond alternation (DC–C ¼ 0.08 A, DC–N
¼
1
4
3
N-aryl-N0-alkyl b-diketimine ligands (L H–L H; Scheme 1) in 50– 0.06 A).35,37 The obvious differences in the L H (DC–C ¼ 0.064 A,
˚
˚
60% yield. The L3H was synthesized recently by similar two-step DC–N ¼ 0.045 A) showed the localized bond feature in the imine-
˚
procedure but no crystal structure was reported.10,19 These enamine backbone.
ligands were characterized by 1H NMR, 13C{1H} NMR and mass
spectroscopy as well as elemental analysis. Crystal suitable for
X-ray diffraction study of L3H was obtained from hexane at
ꢀ20 ꢁC. Fig. 1 shows the expected planar conformation of an b-
diketimine (NCCCN) backbone of L3H with an almost perpen-
dicular 2,6-diisopropylphenyl ring (the inter-planar angle
between the phenyl ring and the NCCCN plane is 98.7ꢁ). The
pyridylmethyl arm was found to swing away from the NCCCN
backbone. According to the literature data on b-diketimine
compounds,20–25 the bond length within NCCCN backbone can
vary signicantly. In the case of 2-amino-4-iminopent-2-enes,
Synthesis and structure characterization of asymmetrical b-
diketiminato zinc complexes
The reactions of b-diketimine ligands L1H–L4H with diethyl zinc
in a 1 : 1 molar ratio in n-hexane at ꢀ94 ꢁC readily produced the
corresponding zinc complexes [L1ZnEt]2 (1), [L2ZnEt] (2),
[L3ZnEt]2 (3), [L4ZnEt] (4) (Scheme 2), with the elimination of
ethane.
Single crystals of zinc complexes [L1ZnEt]2 (1), [L2ZnEt] (2),
[L3ZnEt]2 (3), and [L4ZnEt] (4) suitable for X-ray diffraction
characterization were obtained from saturated hexane solution
at ꢀ20 ꢁC. Selected bond lengths and angles are summarized in
Table 1 for comparison. The crystallographic analysis results
showed that complexes 1 and 3 are dinuclear, both consist
of two b-diketiminato zinc subunits bridged by two pyr-
idylmethyleneamide linkers forming a ten-membered metal-
lamacrocycle (Fig. 2 and 3). Each Zn center assumes distorted
tetrahedral coordination geometry. Only half of complex 1 or 3
is observed in the asymmetric unit which is related to other half
by a crystallographic imposed C2 symmetry. Report on crystal
structure of complex 3 appeared during the preparation of the
current manuscript.10 We kept the structural plot (Fig. 3) for the
sake of easy comparison. For mononuclear complexes 2 and 4,
the zinc centers assume distorted tetrahedral geometry coor-
dinated by tridentate b-diketiminato ligand and an ethyl group
(Fig. 4 and 5). The major structural difference between pair 1 & 3
and pair 2 & 4, other than the obvious different nuclearity, is
their chain length of the pyridine arm on the b-diketiminato
ligands. Complex 1 & 3 have shorter C1 (one methylene) pyridine
arm while complex 2 & 4 have longer C2 (two methylenes)
pyridine arm. From Table 1, one can see that both 2 & 4 bearing
the bonding mode can be either delocalization (DC–C, DC–N
<
Fig. 1 ORTEP representation of the X-ray structure of L3H at the 50%
probability level. Hydrogen atoms were not shown for clarity. Selected
bond lengths (A˚): N(2)–C(8) 1.350(4), C(8)–C(9) 1.367(5), C(9)–C(10)
1.431(5), C(10)–N(3) 1.305(4).
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Scheme 2 Synthesis procedure of the zinc complexes.
Table 1 Structural parameters of complexes 1, 2, 3, and 4 ab
bond with zinc center than the C1 pyridine arm. This stronger
Zn–Npy bond plus the ring strain of six-membered coordina-
tion ring in complexes 2 & 4 caused a higher out-of-plane
(the NCCCN mean plane of b-diketiminato ligand) distance
of the zinc atoms (Table 1, zinc out-of-plane distances: 0.375 &
1
2
3
4
a
Zn–Naryl
2.012(6)
2.009(6)
2.305(7)
1.972(8)
0.121
94.2(2)
101.4(2)
88.8(2)
2.025(2)
2.017(2)
2.174(2)
2.003(3)
0.375
93.61(10)
114.36(10)
88.20(9)
119.85(12)
133.79(12)
103.47(12)
2.0379(17)
2.0208(17)
2.2672(18)
1.994(2)
0.239
94.14(7)
103.04(7)
90.58(7)
116.52(9)
130.62(9)
116.20(9)
2.023(2)
2.012(2)
2.170(3)
1.995(3)
0.560
93.34(9)
109.91(10)
89.33(11)
125.72(12)
123.49(12)
108.01(13)
Zn–Nalkyl
Zn–Npy
ꢀ
ꢀ
0.560 A for 2 & 4 vs. 0.121 & 0.239 A for 1 & 3). All four zinc
complexes were characterized by H NMR, 13C{1H} NMR and
1
Zn–Ceth
NCCN / Znc
elemental analysis.
b
N
N
N
N
N
N
aryl–Zn–Nalkyl
aryl–Zn–Npy
alkyl–Zn–Npy
aryl–Zn–Ceth
alkyl–Zn–Ceth
py–Zn–Ceth
Solution behavior of asymmetrical b-diketiminato zinc
complexes
116.8(3)
133.8(3)
115.2(3)
In order to investigate if the solution structures of zinc
complexes 1–4 are the same as their solid state structures, the
diffusion-ordered spectroscopy (DOSY) technique was per-
formed to gain insight into the behavior of these complexes in
solution. DOSY is a powerful tool to obtain molecular parame-
ters such as formula weight (FW) and hydrodynamic radii.26–30
Each time an NMR tube with toluene-d8 solvent was loaded
with zinc complex and three internal references. Diffusion
a
b
Bond length are listed in A. Bond angles are given in degrees. c The
˚
distance between zinc atom and the NCCCN mean plane.
C2 pyridine arm showed shorter Zn–Npy bond length compared
with that of the C1 arm bearing complexes (1 and 3). This
means that the C2 pyridine arm has a stronger coordination
Fig. 2 ORTEP drawing of 1 [L1ZnEt]2 (non-hydrogen atoms) with ellipsoids drawn at the 50% probability level. Selected bond lengths (A˚) and
bond angles (deg): Zn(1)–N(1) 2.012(6), Zn(1)–N(2) 2.009(6), Zn(1)–N(3) 2.305(7), Zn(1)–C(1) 1.972(8), N(1)–Zn(1)–N(2) 94.2(12), N(1)–Zn(1)–N(3)
101.4(2), N(2)–Zn(1)–N(3) 88.8(2), N(1)–Zn(1)–C(1) 116.8(3), N(2)–Zn(1)–C(1) 133.8(3), and N(3)–Zn–C(1) 115.2(3).
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Fig. 3 ORTEP drawing of 3 [L3ZnEt]2 (non-hydrogen atoms) with ellipsoids drawn at the 50% probability level. Selected bond lengths (A˚) and
bond angles (deg): Zn(1)–N(1) 2.0379(17), Zn(1)–N(2) 2.0208(17), Zn(1)–N(3) 2.2672(18), Zn(1)–C(24) 1.994(2), N(1)–Zn(1)–N(2) 94.14(7), N(1)–
Zn(1)–N(3) 103.04(7), N(1)–Zn(1)–N(2) 90.58(7), N(1)–Zn(1)–C(24) 116.52(9), N(2)–Zn(1)–C(24) 130.62(9), N(3)–Zn(1)–C(24) 116.20(9).
coefficient (D) and FW values of zinc complexes 1–4 are given in mononuclear zinc complex instead of the dinuclear structure in
the ESI.† The diffusion coefficient of 1.15 ꢂ 10ꢀ9 m2 sꢀ1 for 2 solid state. The formation of the monomeric form in solution is
(FW of 425.09) and 1.06 ꢂ 10ꢀ9 m2 sꢀ1 for 4 (FW of 457.78) are possibly connected to more stable chelation binding mode than
both in agreement with the monomeric form as seen from the bridged dimerization binding mode in comparisons of the Zn–
˚
X-ray studies (2.54% error for 2 and 2.33% error for 4). On the Npy distance of solid state (2.267 and 2.305 A for dimeric
other hand, the diffusion coefficients for 1 and 3 are 1.22 ꢂ 10ꢀ9 complex 1 and 3; 2.170 and 2.174 A for monomeric complex 2
˚
m2 sꢀ1 (FW of 431.84; 7.71% error) and 0.82 ꢂ 10ꢀ9 m2 sꢀ1 (FW and 4). Therefore, dimeric complexes 1 and 3 may be easier to
of 501.17; 13.4% error), respectively. The DOSY experiments of 1 dissociate their bridged pyridyl methylene arms than mono-
and 3 suggested that their solution state structure should be meric complexes 2 and 4.
Fig. 4 ORTEP drawing of 2 [L2ZnEt] (non-hydrogen atoms) with ellipsoids drawn at the 50% probability level. Selected bond lengths (A˚) and bond
angles (deg): Zn–N(1) 2.025(2), Zn–N(2) 2.017(2), Zn–N(3) 2.174(2), Zn–C(22) 2.003(3), N(1)–Zn(1)–N(2) 93.61(10), N(1)–Zn(2)–N(3) 114.36(10),
N(2)–Zn(1)–N(3) 88.20(9), N(1)–Zn(1)–C(22) 119.85(12), N(2)–Zn(1)–C(22) 133.79(12), N(3)–Zn(1)–C(22) 103.47(12).
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postulation that on–off coordination of the pyridine arm
occurred in complex 1 but not in 2. Similar case on the different
chemical shis of pyridine a-protons between pyridine disso-
ciation and association in Zn complexes have been reported
previously.31–33 Based on these observations, the C1 pyridyl arm
may be easier dissociation than the C2 pyridyl arm, which is also
consistent with crystallographic Zn–Npy distances shown above.
More importantly, only one singlet is displayed for the methy-
lene protons of the C1 pyridyl arm of complexes 1 and 3, while
two triplets are displayed for the CH2CH2 of the C2 pyridyl arm
of complexes 2 and 4, which however seem to be inconsistent
with the coordination of pyridyl arm to the zinc metal center. A
stable coordination of pyridyl arm to zinc center would cause
the two methylene protons of C1 pyridyl arm to be diasterotopic,
therefore displayed as two doublets; a similar situation will
occur for the CH2CH2 of the C2 pyridyl arm, with more
complicated coupling modes being expected. Another possi-
bility for the monomeric pyridyl arm chelating complex 2 may
undergo a six-member ring uxional mechanism to cause the
broadening signals of CH2CH2 in the C2 pyridyl arm region.
Talking all these into consideration, probably some dynamic
processes involving reversible dissociation and association of
the C1 pyridyl arm in the complexes 1 and 3 may take place in
solution at ambient temperature. In fact, in the VT NMR
spectra, broadening of these signals are observed at low
temperatures, with that of complex 1 more signicant, sug-
gesting a stable coordination state will be reached soon. The C2
pyridyl arm complexes 2 and 4 remain them chelating mode in
the solution due to the unchanged pyridine a-protons in VT
NMR. Therefore, based on the above discussions we assume the
possible solution conformations for complexes 1–4 in Scheme 2
and Fig. 6.
Fig. 5 ORTEP drawing of 4 [L4ZnEt] (non-hydrogen atoms) with
ellipsoids drawn at the 50% probability level. Selected bond lengths (A˚)
and bond angles (deg): Zn(1)–N(1) 2.023(2), Zn(1)–N(2) 2.012(2), Zn(1)–
N(3) 2.170(3), Zn(1)–C(25) 1.995(3), N(1)–Zn(1)–N(2) 93.34(9), N(1)–
Zn(1)–N(3) 109.91(10), N(2)–Zn(1)–N(3) 89.33(11), N(1)–Zn(1)–C(25)
125.72(12), N(2)–Zn(1)–C(25) 123.49(12), N(3)–Zn(1)–C(25) 108.01(13).
To further support the monomeric behavior of solution for
1
all ethyl Zn complexes, the variable temperature H NMR (VT
NMR) of complexes 1 and 2 were examined and shown in Fig. 6,
S29 and S30.† The signal of the pyridine a-proton of complex 1
was obviously shied from 8.02 ppm (at 293 K) to 7.59 ppm (at
183 K) while that of complex 2 was essentially unchanged (7.87
to 7.90 ppm). The VT NMR results were in accord with the
Fig. 6 Variable temperature 1H NMR experiment of the pyridine a-proton of complexes 1 and 2 (7.50–8.05 ppm).
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Ring-opening polymerization of L-lactide
Complexes 1–4 were used as the catalysts with benzyl alcohol
(BnOH) as a initiator for the ring-opening polymerization of L-
lactide in CH2Cl2 at room temperature. The polymerization
results are listed in Table 2, in which complex 1 was the most
active catalyst for the polymerization of L-lactide producing
polymers with narrow polydispersity index (PDI). The observed
Mn(GPC) values (entries 1–4, Table 2) were smaller than the
calculated Mn values, which were similar to the Mn(NMR) (Fig. 7).
This implies that intra-molecular transesterication occurred
even though the Mn(GPC) value slightly increased with an
increase in the ratio of [LA]0/[BnOH]. Other catalysts displayed
similar effects. In addition, complex 1 revealed higher catalytic
activity than Zn complex bearing NNO-tridenate b-diketiminato
([LA]0/[Zn] ¼ 100 : 1, in CH2Cl2, at ambient temperature, 40
min, conversion ¼ 83%).10 It implied that benzyl alkoxide is the
better initiator than phenolate.
Fig. 7 Linear plot of Mn(GPC), Mn(NMR), and Mn(calcd) vs. [LA]0 ꢂ conv./
[BnOH] (red circles are Mn(calcd), blue triangles are Mn(NMR), and black
squares are Mn(GPC)).
Table 2 clearly indicated a decreasing order of catalytic
activity from complexes 1 to 4 (1 > 2 [ 3 > 4). This indicates
that the most inuencing factor of the reactivity is the steric
repelling feature on the substituents of phenyl groups. The
bulky isopropyl substituents on phenyl groups in 3 and 4 are
a disadvantage to catalytic activity. We believe the introduction
of steric bulky groups to the phenyl rings decreases the space
around the zinc center and blocks the coordination between LA
and zinc center. In addition, the short-arm complexes 1 and 3
showed the greater catalytic activity than their long-arm
analogues 2 and 4, due to the instability of shorter chelating
ring. Indeed, crystallographic data revealed that zinc complexes
with the pyridin-2-ylethyl group (2 & 4) tend to form six-
membered ring mononuclear form while a ten-membered
ring dimeric structure was obtained with ligands of pyridin-2-
ylmethyl group (1 & 3). According to DOSY studies of all four
zinc complexes, however, they are all mononuclear in solution.
It was also reported in the literature10 that the dimeric ethyl Zn
analogue bearing the same b-diketiminato ligands with pyridin-
2-ylmethyl group (C1 pyridyl arm) reacted with phenol and
became monomeric Zn phenolate complexes. This further
indicates that ve-membered-ring zinc complexes 1 and 3 with
pyridin-2-ylmethyl group may be easier to dissociate than zinc
complexes 2 and 4. Therefore we postulate that during the
polymerization process, zinc complexes 1 and 3 reacted with
BnOH to form monomeric benzyl alkoxide Zn complexes also (B
in Scheme 3). Since in the monomeric form the shorter C1
pyridyl arm in b-diketiminato ligand forms ve-membered
chelate ring while the longer C2 pyridyl arm forms the six-
membered ring, it is easy to understand the shorter arm
complexes (1 & 3) have higher tendency to open up the more
strained ve-membered ring than the longer-arm complexes (2
& 4). Because of the easier dissociation of pyridin-2-ylmethyl
arm in complexes 1 and 3, it is easy for the coordination of L-
lactide with zinc atom to form C in Scheme 3. Following the
initiation of benzyl alkoxide to L-lactides, three-coordinate zinc
complex (D in Scheme 3) was formed with dangling pyridin-2-
ylmethyl arm due to the open-up of the unstable ve-
membered-ring. As for complexes 2 and 4, they initially also
transformed into benzyl alkoxide zinc complexes E in Scheme 3
aer reacting with BnOH. However the following step is
different. Since the six-membered ring zinc complexes with
pyridin-2-ylethyl arm are stable, it is reluctant to open up.
Therefore, upon L-lactide coordination, a 5-coordinate zinc
Table 2 With L-LA polymerization initiated by complexes 1–4 at room temperature
Conv.c (%)
Mn(GPC)
Mn(calcd)
Mn(NMR)
PDIa
kobsd (sꢀ1
)
a
b
c
Entry
Complex
T (min)
1
1
1
1
1
2
3
4
15
15
15
15
50
>99
>99
>99
>99
>99
93
12 900
9300
14 400
7200
16 100
9700
20 100
28 000
11 700
8300
1.06
1.04
1.01
1.01
1.01
1.01
1.02
0.299
—
—
2e
3f
4g
5
14 100
16 000
11 900
12 600
11 700
21 500
28 600
14 400
13 500
6500
—
0.100
0.006
0.005
6
7
360
360
89
9200
a
b
Obtained from GPC analysis and calibrated according to polystyrene standards. Calculated from the molecular weight of L-lactide ꢂ [M]0/
[BnOH]0 ꢂ conversion yield plus Mw(BnOH). Obtained from 1H NMR analysis. [LA]0/[Zn + BnOH] ¼ 100; [LA]0 ¼ 1.00 M in CH2Cl2 for 1–3 or
c
d
e
f
[LA]0/[Zn + BnOH] ¼ 50; [LA]0 ¼ 1.32 M in CDCl3 for 4. [LA]0/[cat.]/[BnOH] ratio was 50 : 0.5 : 1. [LA]0/[cat.]/[BnOH] ratio was 150 : 0.5 : 1.
g
[LA]0/[cat.]/[BnOH] ratio was 200 : 0.5 : 1.
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Scheme 3 Differences between dimeric and monomeric Zn complexes during the L-lactide polymerization.
complex (F in Scheme 3) could be formed. Following the attack
of benzyl alkoxide to L-lactides, a four-coordinate zinc complex
(G in Scheme 3) could be formed keeping the coordination of
pyridin-2-ylethyl arm intact. Because the six-membered ring was
stabilized by the intra-chelation of the pyridin-2-ylethyl arm,
there was less space around zinc center for G than the three-
coordinate D. This decreased the efficacy of L-lactide coordina-
tion in G provide a good rationale for the lower catalytic activity
for 2 and 4.
Kinetic study of the polymerization of L-lactide by 1–4
A kinetic study was performed in order to determine the reac-
tion order of L-lactide in the polymerization of L-lactide with
zinc complexes and BnOH. Conversion of L-lactide over time
was recorded by 1H NMR for zinc complexes and BnOH of
various concentrations at 25 ꢁC until L-lactide consumption was
completed ([LA]0/[Zn + BnOH] ¼ 100; [LA]0 ¼ 1.00 M in CH2Cl2
for 1–3 or [LA]0/[Zn + BnOH] ¼ 50; [LA]0 ¼ 1.32 M in CDCl3 for
4). Plots of ln([LA]0/[LA]) vs. time in a wide range of [LA]0 are
linear, showing that polymerization proceeds with a rst-order
dependence on [L-lactide] (Fig. 8, kobs ¼ 0.299 sꢀ1 for 1, 0.100 sꢀ1
for 2, 0.006 sꢀ1 for 3, and 0.005 sꢀ1 for 4). Therefore the rate of
polymerization can be described as ꢀd[LA]/dt ¼ kobs[LA]. The
kinetic data showed that the steric effect increased the catalytic
activity by more than 50 folds and the chelating effect increased
the catalytic activity by more than 2 fold.
Conclusions
Zinc complexes 1–4 supported by asymmetrical b-diketiminato
ligands were readily synthesized. The dimeric 1, 3 and mono-
meric 2, 4 were elucidated by X-ray diffraction techniques.
Diffusion-ordered NMR spectroscopy (DOSY) experiments,
however, have shown that all zinc complexes are mononuclear
in solution state. The effect of the ligand structure on the ROP of
L-lactide proved to be signicant. It was found that steric bulky
substituents hindered L-lactide coordination to Zn and hence
decreased the catalytic activity of the corresponding Zn
Fig. 8 First-order kinetic plots for L-LA polymerizations with time with
1–4.
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complex. The ligands with the longer pyridin-2-ylethyl arm can 122.30, 120.97, 95.71, 49.67, 21.89, 21.62, 19.72, 19.29. Anal.
form a stable six-membered ring by the arm and lessen the calcd for C20H25N3: C, 78.14; H, 8.20; N, 13.67. Found: C, 78.20;
chance for L-lactide coordination. Conversely, the ligands with H, 8.16; N, 13.61. ESI-MS m/z ¼ 308.23 ([M + H]+).
the shorter pyridin-2-ylmethyl arm form mononuclear zinc
L2H. The reaction of 2-((2,4,6-trimethylphenyl)imido)-2-
complexes in solution state that is prone to dissociation. penten-4-one (2.10 g, 9.65 mmol), 2-(aminoethyl)pyridine (1.18
Freeing the coordination site causes higher chance for L-lactide g, 9.65 mmol), and p-toluenesulfonic acid (1.84 g, 9.65 mmol)
coordination and thus increases the catalytic activity for the were carried out following a procedure similar to L1H synthesis.
shorter arm complexes.
Yield: 2.02 g (66%). 1H NMR (C6D6, 400 MHz, 298 K): d 11.08 (bs,
1H, NH), 8.40 (d, 1H, J ¼ 4.9 Hz, 1.8 Hz, 1.0 Hz, Py-Ha), 6.92–6.54
(m, 6H, Ar-H and Py-H), 4.63 (s, 1H, backbone-CH), 3.44 (t, 2H,
J ¼ 3.6 Hz, CH2CH2Py), 2.75 (t, 2H, J ¼ 6.4 Hz, CH2CH2Py), 2.27
(s, 3H, para PhCH3), 2.10 (s, 6H, ortho PhCH3), 1.62 (s, 3H,
backbone-CH3), 1.60 (s, 3H, backbone-CH3). 13C{1H} NMR
(C6D6, 100.06 MHz, 298 K): d 166.79, 160.15, 156.07, 150.43,
148.39, 136.39, 131.55, 129.50, 128.37, 124.30, 121.90, 94.41,
43.66, 40.23, 21.80, 21.66, 19.75, 19.29. Anal. calcd for C21H27N3:
C, 78.46; H, 8.47; N, 13.07. Found: C, 78.49; H, 8.50; N, 13.06.
ESI-MS m/z ¼ 322.25 ([M + H]+).
Experimental section
All manipulations were carried out under an atmosphere of
puried dinitrogen in the dry box, or using standard Schlenk
techniques. Chemical reagents were purchased from Aldrich
Chemical Co. Ltd., Lancaster Chemicals Ltd., or Fluka Ltd. All
the reagents were used without further purication, apart from
all solvents that were dried over Na (Et2O, THF) or CaH2
(CH2Cl2, CH3CN) and then thoroughly degassed before use.
2-((2,6-Diisopropylphenyl)imido)-2-penten-4-one34 and 2-((2,4,6-
trimethylphenyl)imido)-2-penten-4-one35 were synthesized by
following the published procedure. 1H and 13C{1H }NMR
spectra were recorded using a Varian Gemini-200 or Varian
Unity Plus-400 FT-NMR spectrometer. Proton and carbon shis
are relative to internal Me4Si or solvent resonance. ESI mass
spectra were collected on a Waters ZQ 4000 mass spectrometer.
Microanalysis was performed using a Heraeus CHN-O-RAPID
instrument. GPC measurements were performed on a Jasco
PU-2080 PLUS HPLC pump system equipped with a differential
Jasco RI-2031 PLUS refractive index detector using THF (HPLC
grade) as an eluent (ow rate 1.0 mL minꢀ1, at 40 ꢁC). The
L3H. 2-(2,6-Diisopropylphenylimido)-2-pentene-4-one (10 g,
38.6 mmol), 2-(aminomethyl)pyridine (4.17 g, 38.6 mmol), and
p-toluenesulfonic acid (7.34 g, 38.6 mmol) were carried out
following a procedure similar to L1H synthesis. Recrystalliza-
tion of the crude product from hexane gave pure L3H as a yellow
solid (7.35 g, 55% yield). 1H NMR (C6D6, 400 MHz, 298 K):
d 11.52 (bs, 1H, NH), 8.38 (ddd, 1H, J ¼ 4.5 Hz, 2.0 Hz, 1.2 Hz, Py-
Ha), 7.07–6.56 (m, 6H, Ar-H and Py-H), 4.74 (s, 1H, backbone-
CH), 4.36 (s, 2H, CH2Py), 3.17 (septet, 2H, J ¼ 7 Hz, ArCH(CH3)2),
1.68 (s, 3H, backbone-CH3), 1.66 (s, 3H, backbone-CH3), 1.23 (d,
6H, J ¼ 7 Hz, ArCH(CH3)2), 1.18 (d, 6H, J ¼ 7 Hz, ArCH(CH3)2).
13C{1H} NMR (C6D6, 100.06 MHz, 298 K): d 167.17, 160.76,
156.03, 149.98, 147.68, 138.58, 136.63, 123.85, 123.63, 122.11,
120.73, 95.35, 49.42, 28.92, 24.50, 23.51, 22.13, 19.46. Anal.
calcd for C23H31N3: C, 79.04; H, 8.94; N, 12.02. Found: C, 79.00;
H, 8.97; N, 11.95. ESI-MS m/z ¼ 350.41 ([M + H]+).
˚
chromatographic column was JORDI Gel DVB 103 A, and the
calibration curve was made according to primary polystyrene
standards to calculation of Mn(GPC).
L4H. The reaction of 2-(2,6-diisopropylphenylimido)-2-
pentene-4-one (2.65 g, 10.3 mmol), 2-(aminoethyl)pyridine
Ligand synthesis
The synthesis of L1H–L4H were carried out using similar (1.26 g, 10.3 mmol), and p-toluenesulfonic acid (1.96 g, 10.3
procedures. As a representative example the synthesis of L1H is mmol) were carried out following a procedure similar to L1H
given below in detail.
synthesis. Yield: 2.13 g (57%). 1H NMR (C6D6, 400 MHz, 298 K):
L1H. The reaction of 2-((2,4,6-trimethylphenyl)imido)-2- d 11.14 (bs, 1H, NH), 8.39 (ddd, 1H, J ¼ 4.0 Hz, 1.9 Hz, 1.0 Hz, Py-
penten-4-one (8.38 g, 38.6 mmol), 2-(aminomethyl)pyridine Ha), 7.13–6.56 (m, 6H, Ar-H and Py-H), 4.64 (s, 1H, backbone-
(4.17 g, 38.6 mmol), and p-toluenesulfonic acid (7.34 g, 38.6 CH), 3.49 (t, 2H, J ¼ 7 Hz, CH2CH2Py), 3.11 (septet, 2H, J ¼ 7 Hz,
mmol) were dissolved in toluene (60 mL) and reuxed for 18 h ArCH(CH3)2), 2.83 (t, 2H, J ¼ 7 Hz, CH2CH2Py), 1.65 (s, 3H,
using a Dean–Stark apparatus to collect water. The volatiles backbone-CH3), 1.63 (s, 3H, backbone-CH3), 1.22 (d, 6H, J ¼ 7
were removed under vacuum to give a yellow solid that was Hz, ArCH(CH3)2), 1.20 (d, 6H, J ¼ 7 Hz, ArCH(CH3)2).13C{1H}
extracted with dichloromethane (100 mL) and the resultant NMR (C6D6, 100.06 MHz, 298 K): d 166.75, 159.75, 155.88,
solution was stirred for 1 h with a saturated solution of sodium 150.64, 147.93, 138.59, 136.16, 123.72, 123.62, 123.56, 123.68,
carbonate (100 mL). The organic layer was separated, dried over 94.22, 43.33, 40.24, 28.86, 24.50, 23.52, 22.10, 19.51. Anal. calcd
MgSO4, and concentrated to give crude product. Recrystalliza- for C24H33N3: C, 79.29; H, 9.15; N, 11.56. Found: C, 79.22; H,
tion of the crude product from hexane gave pure L1H as a yellow 9.16; N, 11.51. ESI-MS m/z ¼ 364.12 ([M + H]+).
solid (6.68 g, 56% yield). 1H NMR (C6D6, 400 MHz, 298 K):
d 11.44 (bs, 1H, NH), 8.39 (ddd, 1H, J ¼ 4.8 Hz, 1.8 Hz, 1.0 Hz,
Zinc complexes synthesis
Py-Ha), 7.10–6.55 (m, 6H, Ar-H and Py-H), 4.73 (s, 1H, backbone-
CH), 4.32 (s, 2H, CH2Py), 2.25 (s, 3H, para PhCH3), 2.20 (s, 6H,
[L1ZnEt]2 (1). In an inert atmosphere a solution of dieth-
ortho PhCH3), 1.65 (s, 3H, backbone-CH3), 1.64 (s, 3H, back- ylzinc solution (3.26 mmol) was added dropwise into the L1H
bone-CH3). 13C{1H} NMR (C6D6, 100.06 MHz, 298 K): d 167.38, (1.0 g, 3.26 mmol) solution in 3 mL of hexane at ꢀ94 C. The
ꢁ
161.48, 156.22, 150.20, 148.30, 136.92, 131.88, 129.75, 128.26, color changed from yellow to red during warm-up. Aer stirring
36712 | RSC Adv., 2016, 6, 36705–36714
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at room temperature for 12 h, the solvent was removed under 50.46, 39.74, 28.18, 24.46, 24.14, 23.56, 21.64, 13.37, ꢀ0.23.
vacuum and the residue extracted with 75 mL of hexane and Anal. calcd for C26H37N3Zn: C, 68.34; H, 8.16; N, 9.20. Found: C,
ltered through a plug of Celite. The volume was reduced and 68.39; H, 8.06; N, 9.15.
ꢁ
the solution was placed at ꢀ20 C overnight, yielding red crys-
1
talline solids (1.18 g, 90%). H NMR (C6D6, 200 MHz, 298 K):
d 8.02 (d, J ¼ 5.3 Hz, 1H, Py1), d 6.94 (s, 2H, Ph), d 6.85 (td, J ¼ 7.6
Hz, 1H, Py3), d 6.47 (d, J ¼ 7.9 Hz, 1H, Py4), d 6.41 (d, J ¼ 6.6 Hz,
1H, Py2), d 4.81 (s, 1H, CH(C]N)2), d 4.47 (s, 2H, CH2Py), d 2.29
(s, 6H, ortho PhCH3), d 2.23 (s, 3H, para PhCH3), d 1.80 (s, 3H,
CH3(C]N)), d 1.71 (s, 3H, CH3(C]N)), d 1.25 (t, J ¼ 8.1 Hz, 3H,
ZnCH2CH3), d 0.44 (q, J ¼ 8.1 Hz, 2H, ZnCH2CH3). 13C NMR
(C6D6, 101 MHz, 298 K) d 165.54, 164.71, 159.30, 148.30, 147.09,
136.79, 132.83, 131.55, 129.25, 122.05, 121.70, 94.69, 54.42,
22.64, 21.94, 21.05, 18.85, 13.71, ꢀ0.90. Anal. calcd for
Typical polymerization procedure
A typical polymerization procedure was exemplied by the
synthesis of PLA (entry 1, Table 2) at room temperature. The
conversion yield (>99%) of PLA was analyzed by H NMR spec-
1
troscopic studies. A mixture of the catalyst (0.1 mmol) and
L-lactide (1.44 g, 10.0 mmol) in CH2Cl2 (10.0 mL) was stirred at
room temperature for 15 min. Volatile materials were removed
under vacuum, and the residue was redissolved in THF
(5.0 mL). The mixture was then quenched through the addition
of EtOH, and the polymer was precipitated by pouring it into
n-hexane (80.0 mL) to yield white crystalline solids. Yield: 1.34 g
(93%).
C
44H58N6Zn2: C, 65.92; H, 7.29; N, 10.48. Found: C, 65.84; H,
7.25; N, 10.32.
[L2ZnEt] (2). Following the procedure described for 1, reac-
tion of diethylzinc (3.26 mmol) and L2H (1.05 g, 3.26 mmol)
gave 2 as orange crystalline solids (1.29 g, 96%). 1H NMR (C6D6,
400 MHz, 298 K): d 7.94 (ddd, J ¼ 5.1, 1.8, 0.9 Hz, 1H, Py1), d 6.95
(s, 2H, Ph), d 6.83 (td, J ¼ 7.7, 1.8 Hz, 1H, Py3), d 6.45 (d, J ¼ 7.7
Hz, 1H, Py4), d 6.36 (ddd, J ¼ 7.6, 5.1, 1.2 Hz, 1H, Py2), d 4.70 (s,
1H, CH(C]N)2), d 3.38 (t, J ¼ 6.9, 5.0 Hz, 2H, CH2CH2Py), d 2.77
(t, J ¼ 6.9, 5.0 Hz, 2H, CH2CH2Py), d 2.26 (s, 6H, ortho PhCH3),
d 2.25 (s, 3H, para PhCH3), d 1.72 (s, 3H, CH3(C]N)), d 1.67 (s,
3H, CH3(C]N)), d 1.49 (t, J ¼ 8.1 Hz, 3H, ZnCH2CH3), d 0.62 (q, J
¼ 8.1 Hz, 2H, ZnCH2CH3). 13C NMR (C6D6, 101 MHz, 298 K)
d 166.70, 164.36, 160.64, 148.39, 147.40, 137.23, 132.63, 132.05,
129.30, 124.43, 121.41, 94.29, 49.05, 37.19, 22.78, 21.54, 21.09,
18.86, 14.03, 1.28. Anal. calcd for C23H31N3Zn: C, 66.58; H, 7.53;
N, 10.13. Found: C, 65.54; H, 7.55; N, 10.10.
Kinetic study of polymerization of L-LA by 1
L-LA (10.0 mmol) was added to a solution of 1 (0.1 mmol) and
BnOH (0.1 mmol) in CH2Cl2 (10 mL). The mixture was then
stirred at room temperature under N2. At appropriate time
intervals, 0.2 mL aliquots were removed and then dried to
1
a constant weight under vacuum and analyzed by H NMR.
X-ray crystal structure determinations
All single-crystal X-ray diffraction data were measured on
a Bruker Nonius Kappa CCD diffractometer using Mo Ka radi-
˚
ation (l ¼ 0.71073 A). The data collection was executed using
the SMART program.36 Cell renement and data reduction were
made with the SAINT program.37 The structure was determined
using the SHELXTL/PC program38 and rened using full-matrix
least-squares. All non-hydrogen atoms were rened anisotrop-
ically, whereas hydrogen atoms were placed at the calculated
positions and included in the nal stage of renements with
xed parameters. Further details are given in Tables S1 and S2.†
[L3ZnEt]2 (3). Following the procedure described for 1,
reaction of diethylzinc (2.86 mmol) and L3H (1.0 g, 2.86 mmol)
gave 3 as orange crystalline solids (1.11 g, 88%). 1H NMR (C6D6,
200 MHz, 298 K): d 8.27 (d, 1H, J ¼ 4.7, Py1), d 7.19 (d, 3H, J ¼
0.5, Ph), d 6.94 (td, 1H, J ¼ 7.8, 6.1, Py3), d 6.60 (d, 1H, J ¼ 7.6,
Py4), d 6.51 (m, Py2), d 4.84 (s, 1H, CH(C]N)2), d 4.55 (s, 2H,
CH2Py), d 3.41 (hept, 2H, J ¼ 7.3, PhCH(CH3)2), d 1.80 (s, 3H,
CH3(C]N)), d 1.74 (s, 3H, CH3(C]N)), d 1.33 (d, 6H, J ¼ 6.8,
PhCH(CH3)2), d 1.17–1.12 (m, 9H, PhCH(CH3)2 + ZnCH2CH3),
d 0.37 (q, 2H, J ¼ 8.1, ZnCH2CH3). 13C NMR (C6D6, 101 MHz, 298
K): d 166.44, 165.91, 159.68, 148.84, 146.33, 142.20, 136.53,
125.37, 123.72, 121.90, 121.54, 95.32, 54.90, 28.25, 24.65, 24.02,
23.57, 22.08, 13.26, ꢀ0.71. Anal. calcd for C50H70N6Zn2: C,
67.83; H, 7.98; N, 9.39. Found: C, 67.79; H, 7.96; N, 9.49.
[L4ZnEt] (4). Following the procedure described for 1, reac-
tion of diethylzinc (2.96 mmol) and L4H (1.03 g, 2.96 mmol)
gave 4 as orange crystalline solids (1.24 g, 92%). 1H NMR (C6D6,
200 MHz, 298 K): d 8.23 (d, 1H, J ¼ 4.0, Py1), d 7.16 (s, 3H, Ph),
d 6.94 (t, 1H, J ¼ 7.6, Py3), d 6.62 (d, 1H, J ¼ 7.6, Py4), d 6.55–6.44
(m, 1H, Py2), d 4.73 (s, 1H, CH(C]N)2), d 3.67 (t, 2H, J ¼ 6.7,
CH2CH2Py), d 3.31 (hept, 2H, J ¼ 6, PhCH2(CH3)2), d 2.89 (t, 2H,
J ¼ 6.6, CH2CH2Py), d 1.77 (s, 3H, CH3(C]N)), d 1.69 (s, 3H,
CH3(C]N)), d 1.35 (t, 3H, J ¼ 8.0, ZnCH2CH3), d 1.20 (dd, 12H, J
Acknowledgements
Financial support of the Ministry of Science and Technology of
Taiwan (102-2113-M-037-008-MY3, 104-2113-M-037-010) and
NSYSU-KMU Joint Research Project (NSYSUKMU103-I004,
NSYSUKMU 104-P006) are highly appreciated. We thank Mr
Ting-Shen Kuo, National Taiwan Normal University for X-ray
structural determinations and Mr Min-Yuan Hung, Center for
Research Resources and Development of KMU for their
facilities.
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36714 | RSC Adv., 2016, 6, 36705–36714
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