Improvement in Titanium Complexes Bearing Schiff Base Ligands in the Ring-Opening Polymerization of L-Lactide: A Dinuclear System with Hydrazine-Bridging Schiff Base Ligands

Article abstract of DOI:10.1021/acs.inorgchem.5b02590

A series of titanium (Ti) complexes bearing hydrazine-bridging Schiff base ligands were synthesized and investigated as catalysts for the ring-opening polymerization (ROP) of L-lactide (LA). Complexes with electron withdrawing or steric bulky groups reduced the catalytic activity. In addition, the steric bulky substituent on the imine groups reduced the space around the Ti atom and then reduced LA coordination with Ti atom, thereby reducing catalytic activity. All the dinuclear Ti complexes exhibited higher catalytic activity (approximately 10-60-fold) than mononuclear L^Cl-H-TiOPr₂ did. The strategy of bridging dinuclear Ti complexes with isopropoxide groups in the ROP of LA was successful, and adjusting the crowded heptacoordinated transition state by the bridging isopropoxide groups may be the key to our successful strategy.

Full text of DOI:10.1021/acs.inorgchem.5b02590

Article
pubs.acs.org/IC
Improvement in Titanium Complexes Bearing Schiff Base Ligands
in the Ring-Opening Polymerization of L‑Lactide: A Dinuclear System
with Hydrazine-Bridging Schiff Base Ligands
Hsi-Ching Tseng,† Hsing-Yin Chen,† Yen-Tzu Huang,† Wei-Yi Lu,‡ Yu-Lun Chang,†,§
Michael Y. Chiang,†,§ Yi-Chun Lai,† and Hsuan-Ying Chen*,†
†Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan R.O.C.
‡Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan R.O.C.
§Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan R.O.C.
S
* Supporting Information
ABSTRACT: A series of titanium (Ti) complexes bearing hydrazine-
bridging Schiff base ligands were synthesized and investigated as catalysts
for the ring-opening polymerization (ROP) of L-lactide (LA). Complexes
with electron withdrawing or steric bulky groups reduced the catalytic
activity. In addition, the steric bulky substituent on the imine groups
reduced the space around the Ti atom and then reduced LA coordination
with Ti atom, thereby reducing catalytic activity. All the dinuclear Ti
complexes exhibited higher catalytic activity (approximately 10−60-fold)
than mononuclear LCl−H-TiOPr2 did. The strategy of bridging dinuclear Ti
complexes with isopropoxide groups in the ROP of LA was successful, and
adjusting the crowded heptacoordinated transition state by the bridging
isopropoxide groups may be the key to our successful strategy.
and associated Ti complexes were synthesized, and their
application in L-LA polymerization was studied.
1.0. INTRODUCTION
Poly(lactide) (PLA) is an accepted biopolymer designed to
resolve the pollution problem caused by petrochemical plastics.
It is also a common biomaterial in various fields such as MRI
contrast agent,1a humidity detection,1b nanocomposites,1c cell/
tissue antiadhesion,1d blood circulation,1e drug delivery,1f bone
replacement,1g tissue engineering,1h−m and biomedical applica-
tion1n,s because of its biodegradability, biocompatibility, and
permeability. Ring-opening polymerization (ROP) is the main
method of synthesizing PLA with various metal catalysts.2
Because of the problem of cytotoxic metal residues in PLA, the
use of a noncytotoxic metal such as titanium (Ti)3 has been
researched extensively with regard to lactides polymerization.
Ligands are crucial for designing organometallic catalysts because
of their ability to improve reactivity and selectivity. A study on
the creation of Ti complexes3j bearing Schiff base ligands
reported that the different steric effects of these ligands altered
the coordinated form with various catalytic activities of L-lactide
(LA) and ε-caprolactone polymerizations as shown in Figure 1.
Bochmann3k and Lin3l both reported that the catalytic activity
of heterobimetallic Ti complexes bearing bisphenolate ligands
exceeded that the mononuclear Ti complex displayed in Figure 2.
These aforementioned findings inspired us to design the
dinuclear Ti complexes bearing hydrazine-bridging Schiff base
ligands with bridging isopropoxide groups, which may improve
the catalytic activity of Ti complexes bearing Schiff base ligands.
In this study, a series of the hydrazine-bridging Schiff base ligands
2.0. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of Ti Complexes.
Symmetrical hydrazine-bridging Schiff base ligands were
synthesized by condensing the derivatives of 2-hydroxyphenone
or 2-hydroxybenzaldehyde with half the equivalent of hydrazine
hydrate in ethanol, as illustrated in Scheme 1(A). The
unsymmetrical LCl−H-H was synthesized in two steps, as displayed
in Scheme 1(B): (1) Condensation of 4-chlorobenzaldehyde
with one equivalent of hydrazine hydrate in ethanol afforded the
intermediate 4-chlorobenzylidenehydrazide; (2) one equivalent
of salicylaldehyde was reacted with 4-chlorobenzylidenehydrazide
to provide LCl−H-H with reasonable yields. All the symmetrical
ligands reacted with two equivalent of Ti isopropoxide in toluene
to obtain a moderate yield of Ti compounds (Scheme 1(A)). The
crystal of LCH3-Ti2O(OPr)2 was observed from LCH3-Ti(OPr)6
in the nuclear magnetic resonance (NMR) tube in the air after
2 weeks. The reaction of LCH3-Ti2O(OPr)2 synthesis may have
comprised the disproportionation product of LCH3-Ti(OPr)6
reacting with H2O to replace isopropoxide. LCl−H-TiOPr2 was
synthesized from the reaction of LCl−H-H and Ti isopropoxide
(1:1). The reaction of one equivalent of LCl−H-H and two
Received: November 9, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02590
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Figure 1. Structures and their catalytic activity of Ti complexes bearing Schiff base ligands.
Figure 2. Comparison of catalytic activity between heterobimetallic and mononuclear Ti complexes.
Scheme 1. Synthesis of Hydrazine-Bridging Schiff Base Ligands and Associated Ti Complexes
equivalents of Ti isopropoxide was also attempted; however,
only LCl−H-TiOPr2 was observed. ((LCl−H)3Ti3O3) was synthe-
sized from the reaction of LCl−H-TiOPr2 and H2O in THF.
The X-ray structure of LBu-TiOPr6 (Figure 3) reveals the
disordered octahedral geometry of the dinuclear Ti complex with
four terminal isopropoxide groups and two bridging isoprop-
oxide groups. LBu-TiOPr6 possesses C2 symmetry with a C2 axis
through the center between N(2)−N(1) and Ti(2)−Ti(1) and
perpendicular to the lines of N(2)−N(1) and Ti(2)−Ti(1). The
angle between C(15)−N(1)−Ti(1) and C(16)−N(2)−Ti(2)
B
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Figure 4. Molecular structure of LCH3-Ti2O(OPr)2 as 20% probability
ellipsoids (all of the hydrogen atoms were omitted for clarity). Selected
bond lengths (A) and bond angles (deg): Ti(1)−O(2) 1.795(3),
Ti(1)−O(7) 1.836(3), Ti(1)−O(1) 1.892(3), Ti(1)−O(3) 1.931(3),
Ti(1)−N(2) 2.244(3), Ti(1)−N(1) 2.324(3), Ti(2)−O(5) 1.796(3),
Ti(2)−O(7) 1.843(3), Ti(2)−O(4) 1.885(3), Ti(2)−O(6) 1.921(3),
Ti(2)−N(3) 2.255(3), Ti(2)−N(4) 2.293(3), O(2)−Ti(1)−O(7)
100.15(13), O(2)−Ti(1)−O(1) 97.90(13), O(7)−Ti(1)−O(1)
103.51(13), O(2)−Ti(1)−O(3) 100.76(13), O(7)−Ti(1)−O(3)
152.10(12), O(1)−Ti(1)−O(3) 91.67(13), O(2)−Ti(1)−N(2)
94.46(13), O(7)−Ti(1)−N(2) 82.16(12), O(1)−Ti(1)−N(2)
165.20(13), O(3)−Ti(1)−N(2) 78.00(12), O(2)−Ti(1)−N(1)
173.54(13), O(7)−Ti(1)−N(1) 74.31(12), O(1)−Ti(1)−N(1)
80.45(12), O(3)−Ti(1)−N(1) 85.56(12), N(2)−Ti(1)−N(1)
88.13(12), O(5)−Ti(2)−O(7) 101.34(13), O(5)−Ti(2)−O(4)
97.26(13), O(7)−Ti(2)−O(4) 101.60(12), O(5)−Ti(2)−O(6)
100.23(14), O(7)−Ti(2)−O(6) 149.61(12), O(4)−Ti(2)−O(6)
96.61(12), O(5)−Ti(2)−N(3) 176.11(13), O(7)−Ti(2)−N(3)
76.52(12), O(4)−Ti(2)−N(3) 80.10(12), O(6)−Ti(2)−N(3)
82.97(12), O(5)−Ti(2)−N(4) 90.42(13), O(7)−Ti(2)−N(4)
81.00(12), O(4)−Ti(2)−N(4) 171.18(12), O(6)−Ti(2)−N(4)
77.68(12), N(3)−Ti(2)−N(4) 92.43(12).
Figure 3. Molecular structure of LBu-TiOPr6 as 20% probability
ellipsoids (all of the hydrogen atoms were omitted for clarity). Selected
bond lengths (A) and bond angles (deg): Ti(1)−O(2) 1.7812(18),
Ti(1)−O(3) 1.7830(19), Ti(1)−O(1) 1.9347(16), Ti(1)−O(5)
2.0144(15), Ti(1)−O(4) 2.0905(17), Ti(1)−N(1) 2.264(2), Ti(1)−
Ti(2) 3.2415(6), Ti(2)−O(7) 1.7885(17), Ti(2)−O(6) 1.8055(18),
Ti(2)−O(8) 1.9193(16), Ti(2)−O(4) 2.0028(15), Ti(2)−O(5)
2.0600(16), Ti(2)−N(2) 2.316(2), O(2)−Ti(1)−O(3) 102.62(9),
O(2)−Ti(1)−O(1) 96.71(7), O(3)−Ti(1)−O(1) 94.63(8), O(2)−
Ti(1)−O(5) 99.94(7), O(3)−Ti(1)−O(5) 93.62(7), O(1)−Ti(1)−
O(5) 159.32(7), O(2)−Ti(1)−O(4) 96.59(8), O(3)−Ti(1)−O(4)
157.98(7), O(1)−Ti(1)−O(4) 93.69(7), O(5)−Ti(1)−O(4)
72.35(6), O(2)−Ti(1)−N(1) 170.05(8), O(3)−Ti(1)−N(1)
86.96(8), O(1)−Ti(1)−N(1) 79.65(7), O(5)−Ti(1)−N(1) 81.89(7),
O(4)−Ti(1)−N(1) 74.54(7), O(2)−Ti(1)−Ti(2) 109.73(6), O(3)−
Ti(1)−Ti(2) 124.25(6), O(1)−Ti(1)−Ti(2) 124.01(5), O(5)−
Ti(1)−Ti(2) 37.78(5), O(4)−Ti(1)−Ti(2) 36.70(4), N(1)−Ti(1)−
Ti(2) 65.86(5), O(7)−Ti(2)−O(6) 99.67(9), O(7)−Ti(2)−O(8)
94.64(8), O(6)−Ti(2)−O(8) 97.72(8), O(7)−Ti(2)−O(4) 92.77(7),
O(6)−Ti(2)−O(4) 101.13(7), O(8)−Ti(2)−O(4) 158.26(8), O(7)−
Ti(2)−O(5) 162.50(7), O(6)−Ti(2)−O(5) 93.45(7), O(8)−Ti(2)−
O(5) 95.02(7), O(4)−Ti(2)−O(5) 73.23(6), O(7)−Ti(2)−N(2)
93.05(8), O(6)−Ti(2)−N(2) 166.91(8), O(8)−Ti(2)−N(2)
78.03(7), O(4)−Ti(2)−N(2) 81.20(7), O(5)−Ti(2)−N(2) 74.77(7).
Table 1) illustrated that the steric effect in the phenol group
reduced the catalytic activity and L3‑OMe-TiOPr6, L5‑OMe-TiOPr6,
and LBr‑CH3-TiOPr6 (entries 3, ,4 and 8, respectively, Table 1)
displayed that electron donating groups slightly increased the
catalytic activity. In addition, Ti complexes bearing Schiff
base ligands with the substitutes in the phenol group, such
as LBu-TiOPr6, L5‑OMe-TiOPr6, L3‑OMe-TiOPr6, LNap-TiOPr6,
and LBr‑CH3-TiOPr6, exhibited a low controllability of polymer
molecular weight and a broad polydispersity index (PDI). The
planes is 38.33°. The X-ray structure of LCH3-Ti2O(OPr)2
(Figure 4) also displays the disordered octahedral geometry of
the dinuclear Ti complex with two terminal isopropoxide groups
and one bridging oxide. LCH3-Ti2O(OPr)2 possesses C2
symmetry with a C2 axis through the oxide and perpendicular
to the lines of N(2)−N(4), N(1)−N(3), and Ti(2)−Ti(1).
LCl−H-TiOPr2 (Figure 5) reveals a common type I (Figure 1, cis
for N−N) Ti complex in the octahedral form with two terminal
isopropoxide groups and two LCl−H-O− ligands, which exhibits
no coordination of N atoms in the 4-chlorobenzylidene group.
The geometry of Ti atoms in (LCl−H)3Ti3OPr3 (Figure 6) is
similar to that in LCl−H-TiOPr2, except that two terminal isoprop-
oxide groups are placed with one oxide and a six-membered ring,
and a trinuclear form with D3 symmetry is constructed with three
Ti−O units.
2.2. Polymerization of Lactides. LA polymerization using
Ti complexes as initiators in toluene was investigated under
nitrogen at 60 °C (Table 1). As exhibited in entries 1,5, and 6 of
Table 1, Ti complexes with ligands, where R1 were H, methyl
(CH3), and phenyl (ph) groups, exhibited different catalytic
activities according to the substitutes in the imino group, and the
catalytic trend of R1 was H > CH3 > ph. LBu-TiOPr6 (entry 2,
1
spectra of ESI-MS (Figure S25) and H NMR (Figure S26)
indicated the polymer chain should be capped with one
isopropoxide ester and one hydroxy end, suggesting that
back reactions leading to the formation of macrocycles do not
occur. Furthermore, the catalytic activity of all dinuclear Ti
complexes bearing hydrazine-bridging Schiff base ligands was
higher (approximately 10−60-fold) than that of mononuclear
LCl−H-TiOPr2, implying that the strategy of bridging dinuclear Ti
complexes with isopropoxide groups in the ROP of LA was
successful. On the basis of the linear relationship between
MnGPC and ([LA]0 × conv.)/[LH-TiOPr6] exhibited in Figure 7,
polymerizing LA by using LH-TiOPr6 as the catalyst demon-
strated high controllability with a narrow PDI. However, Figure 7
reveals that only four isopropoxides groups in LH-TiOPr6 were
initiators during the polymerization process. In addition, rac-LA
polymerization using these Ti complexes as initiators was also
investigated and shown in Table S3. However, the results of the
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Figure 5. Molecular structure of LCl−H-TiOPr2 as 30% probability
ellipsoids (all of the hydrogen atoms were omitted for clarity). Selected
bond lengths (A) and bond angles (deg): Ti−O(3) 1.8164(13), Ti−
O(4) 1.8221(14), Ti−O(2) 1.8898(15), Ti−O(1) 1.8962(14), Ti−
N(3) 2.3398(17), Ti−N(1) 2.3491(16), O(3)−Ti-O(4) 100.51(6),
O(3)−Ti-O(2) 98.85(6), O(4)−Ti-O(2) 96.46(6), O(3)−Ti-O(1)
96.57(6), O(4)−Ti-O(1) 98.15(6), O(2)−Ti-O(1) 156.41(6), O(3)−
Ti-N(3) 89.06(6), O(4)−Ti-N(3) 170.15(6), O(2)−Ti-N(3) 79.64(6),
O(1)−Ti-N(3) 82.89(6), O(3)−Ti-N(1) 170.99(6), O(4)−Ti-N(1)
88.14(6), O(2)−Ti-N(1) 82.50(6), O(1)−Ti-N(1) 79.58(6), N(3)−
Ti-N(1) 82.40(6).
Figure 6. Molecular structure of (LCl−H)3Ti3OPr3 as 20% probability
ellipsoids (all of the hydrogen atoms were omitted for clarity). Selected
bond lengths (A) and bond angles (deg): Ti(1)−O(9) 1.8043(16),
Ti(1)−O(7) 1.8353(16), Ti(1)−O(2) 1.8942(17), Ti(1)−O(1)
1.8961(18), Ti(1)−N(1) 2.3072(19), Ti(1)−N(3) 2.327(2), Ti(2)−
O(7) 1.8113(16), Ti(2)−O(8) 1.8351(17), Ti(2)−O(3) 1.9021(18),
Ti(2)−O(4) 1.9023(17), Ti(2)−N(5) 2.322(2), Ti(2)−N(7)
2.324(2), Ti(3)−O(8) 1.8173(16), Ti(3)−O(9) 1.8236(16), Ti(3)−
O(6) 1.8927(18), Ti(3)−O(5) 1.9031(17), Ti(3)−N(9) 2.293(2),
Ti(3)−N(11) 2.318(2), O(9)−Ti(1)−O(7) 95.35(7), O(9)−Ti(1)−
O(2) 98.69(8), O(7)−Ti(1)−O(2) 96.48(7), O(9)−Ti(1)−O(1)
98.09(8), O(7)−Ti(1)−O(1) 98.19(8), O(2)−Ti(1)−O(1)
156.52(7), O(9)−Ti(1)−N(1) 87.09(7), O(7)−Ti(1)−N(1)
177.46(7), O(2)−Ti(1)−N(1) 83.82(7), O(1)−Ti(1)−N(1)
80.75(7), O(9)−Ti(1)−N(3) 175.83(7), O(7)−Ti(1)−N(3)
88.62(7), O(2)−Ti(1)−N(3) 79.54(7), O(1)−Ti(1)−N(3) 82.58(7),
N(1)−Ti(1)−N(3) 88.96(7), O(7)−Ti(2)−O(8) 96.25(7), O(7)−
Ti(2)−O(3) 98.07(8), O(8)−Ti(2)−O(3) 100.36(8), O(7)−Ti(2)−
O(4) 99.90(8), O(8)−Ti(2)−O(4) 96.67(8), O(3)−Ti(2)−O(4)
153.65(7), O(7)−Ti(2)−N(5) 88.06(7), O(8)−Ti(2)−N(5)
175.66(7), O(3)−Ti(2)−N(5) 79.55(7), O(4)−Ti(2)−N(5)
81.96(7), O(7)−Ti(2)−N(7) 176.81(7), O(8)−Ti(2)−N(7)
86.94(7), O(3)−Ti(2)−N(7) 81.27(7), O(4)−Ti(2)−N(7) 79.71(7),
N(5)−Ti(2)−N(7) 88.75(7), O(8)−Ti(3)−O(9) 96.00(7), O(8)−
Ti(3)−O(6) 99.93(8), O(9)−Ti(3)−O(6) 96.59(8), O(8)−Ti(3)−
O(5) 96.99(8), O(9)−Ti(3)−O(5) 99.74(7), O(6)−Ti(3)−O(5)
155.04(8), O(8)−Ti(3)−N(9) 90.75(7), O(9)−Ti(3)−N(9)
173.20(7), O(6)−Ti(3)−N(9) 81.45(8), O(5)−Ti(3)−N(9)
80.11(8), O(8)−Ti(3)−N(11) 177.88(7), O(9)−Ti(3)−N(11)
86.11(7), O(6)−Ti(3)−N(11) 79.96(8), O(5)−Ti(3)−N(11)
82.45(8), N(9)−Ti(3)−N(11) 87.14(7).
selectivity of poly-rac-LA were not special (Pr (LNap) = 52%,
Pr (LH) = 38%, Pr (L5‑OMe) = 44%, Pr (LPh) = 54%, Pr (LBr‑CH3) =
50%, Pr (LBu) = 51%).
2.3. Kinetic Studies of LA Polymerization Catalyzed by
LH-TiOPr6. Kinetic studies were performed at 60 °C with respect
to the ratio of [LA]0/[LH-TiOPr6] ([LA] = 2.5 M in 5 mL of
toluene) as exhibited in Table S2 and Figures 8 and 9. The
preliminary results indicated a first-order dependency on [LA]
(Figure 8). By plotting kobs against [LH-TiOPr6], a kprop value of
4.03 (M−1min−1) was obtained (Figure 9). Polymerizing LA by
using LH-TiOPr6 at 60 °C demonstrated the following rate law:
d[LA]/dt = 4.03 × [LA][LH‐TiOPr6]
2.4. Mechanistic Studies of Polymerization. The
polymerization data revealed that only four isopropoxide groups
in Ti complexes were initiators during polymerization. To realize
the role of these six isopropoxide groups, the 1H NMR spectra of
the mixture of one equivalent of LCH3-TiOPr6 and six equivalents
of LA in CDCl3 at 60 °C were studied, as displayed in Figures 10
and S2. Figure 10 reveals that the isopropoxide groups began
initiating LA at 40 °C, but the initiation rate was extremely slow.
After 2 days at 60 °C, the polymerization rate for isopropoxides
groups a and b decreased evidently, but the isopropoxide group c
still existed. Thus, only four terminal isopropoxide groups could
initiate LA polymerization; this is consistent with the polymer-
ization results. This phenomenon was similar to the polymer-
ization achieved by using other dinuclear Ti complexes3m as
catalysts.
According to the polymerization results, kinetic characteristics,
and 1H NMR study, one monomer was consumed in every poly-
merization cycle, and the Ti complexes retained the dinuclear
form because the order of the monomer and Ti complex was 1.
The possible polymerization mechanism is provided in Figure 11.
One of the bridging isopropoxide groups increased the bond with
the Ti atom on the left side and reduced the bond with the Ti
atom on the right side to increase the space around the Ti atom
on the right side. LA bonded to the Ti atom to form a hepta-
coordinated form (transition state A in Figure 11). The crowded
hepta-coordinated transition state enhanced the initiation of
terminal isopropoxide to LA, thereby releasing unstable energy as
well as continually repeating the coordination of monomers and
the initiation of alkoxide to produce the polymer. LCl−H-TiOPr2
exhibited low activity because coordinating the monomer was
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d
Table 1. Polymerizing L-Lactide by Using Each of the Ti Complexes as an Initiator at 60 °C
a
b
c
c
Entry
L-TiOPr6, L=
Time (min)
Conv.
MnCal
MnGPC
PDI
kobs (10−3 min−1
)
1
2
3
4
5
6
7
8
LH
50
110
50
89%
91%
93%
92%
99%
96%
99%
99%
99%
93%
87%
89%
93%
2200
2200
2200
2200
2500
2400
2500
2500
3600
4500
2800
1000
11200
3100
2400
4100
4000
4300
4000
7800
5600
2500
6100
3500
1600
18500
1.25
1.39
1.82
1.94
1.86
1.10
1.34
1.93
1.10
1.31
1.24
1.09
1.29
65.21 (298)
33.64 (162)
70.09 (227)
94.20 (513)
63.31 (157)
12.57 (49)
25.57 (201)
14.17 (82)
1.18 (9)
LBu
L5‑OMe
L3‑OMe
LNap
30
80
LPh
240
150
300
3000
60
LCH3
LBr‑CH3
LCl−H-TiOPr2
LH
e
9
f
10
g
11
LH
60
h
12
LH
60
i
13
LH
180
a
b
Obtained from 1H NMR analysis. Calculated from the molecular weight of monomer × [monomer]0/[iPrO−]0 × conversion yield + Mw(PriO).
c
Obtained from GPC analysis and calibration based on the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.58.
d
e
Reaction condition: toluene (5 mL), [LA] = 2.0 M, [LA]:[Cat] = 100:1. Reaction condition: toluene (5 mL), [LA] = 2.0 M, [LA]:[Cat] = 100:2.
f
g
Reaction condition: toluene (5 mL), [LA] = 2.0 M, [LA]:[Cat] = 100:0.5. Reaction condition: toluene (5 mL), [LA] = 2.0 M, [LA]:[Cat] =
h
i
100:0.75. Reaction condition: toluene (5 mL), [LA] = 2.0 M, [LA]:[Cat] = 100:2. Reaction condition: toluene (5 mL), [LA] = 2.0 M,
[LA]:[Cat] = 100:0.2.
Figure 8. First-order kinetic plots of LA polymerization with various
Figure 7. Linear plot of various Mncal. with the supposed initiators
concentrations of [LH-TiOPr6] plotted against time with [LA] = 2.5 M
in toluene (5 mL).
and MnGPC against [LA]0 × conv./[LH-TiOPr6] (Table 1, entries 1 and
10−13).
difficult without the adjusting of the bridging isopropoxide group
to the stable hepta-coordinated form. In addition, the hydrazine-
bridging Schiff base ligands added more space around the Ti atom
than the other dinuclear Ti complex3m displayed in Figure 12 did.
Figure 12 reveals that the distance of between the two oxygen
atoms of the terminal isopropoxide groups, which were in the
trans-position to the imine groups of LBu-TiOPr6, was 4.429 Å
and longer than that of the other dinuclear Ti complex3m
(3.947 Å). This result demonstrated that LA coordination in Ti
complexes bearing hydrazine-bridging Schiff base ligands was
easier and the catalytic activity was higher (LH-TiOPr6: conv. =
93% in 180 min, [Cat] = 4.0 mM, [LA]:[Cat] = 500:1 at 60 °C;
other dinuclear Ti complex:3m conv. = 89% in 12 h, [Cat] =
4.0 mM, [LA]:[Cat] = 500:1 at 100 °C). To explain the difference
of the catalytic rates of LPh-TiOPr6, LCH3-TiOPr6, and
LH-TiOPr6, the density functional theory (DFT) calculation of
two dinuclear forms of LBu‑Ph-TiOPr6 and LBu‑CH3-TiOPr6
(Figure 13) was studied, and the data were compared with
LBu-TiOPr6. Figure 13 reveals that the distance between two
Figure 9. Linear plot of kobs against [LH-TiOPr6] for LA polymerization
with [LA] = 2.5 M in toluene (5 mL).
E
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that of mononuclear LCl−H-TiOPr2. Overall, the strategy of
bridging dinuclear Ti complexes with isopropoxide groups in the
ROP of LA was successful, and the adjustment of the crowded
heptacoordinated transition state by the bridging isopropoxides
may explain the success of our strategy.
4.0. EXPERIMENTAL SECTION
Standard Schlenk techniques and a N2-filled glovebox were used all over
the isolation and treatment of all the compounds. Solvents, L-lactide,
and deuterated solvents were purified prior to use. Salicylaldehyde,
2-hydroxy-5-methoxybenzaldehyde, 3,5-di-tert-butyl-2-hydroxybenzal-
dehyde, 1-(5-bromo-2-hydroxyphenyl)ethan-1-one, 1-(2-hydroxyphenyl)-
ethan-1-one, 4-chlorobenzaldehyde, 3-hydroxy-2-naphthaldehyde,
hydrazine monohydrate and benzyl alcohol were purchased from
Aldrich. 1H and 13C NMR spectra were recorded on a Varian
Gemini2000−200 (200 MHz for 1H and 50 MHz for 13C) spectrometer
with chemical shifts given in ppm from the internal TMS or center line of
CDCl3. Microanalyses were performed using a Heraeus CHN-O-RAPID
instrument. The gel permeation chromatography (GPC) measurements
were performed on a Waters 1515 Isotratic HPLC pump system
equipped with a differential Waters 2414 refractive index detector using
THF (HPLC grade) as the eluent. The chromatographic column was a
Water Styragel Column (HR4E), and the calibration curve was made by
polystyrene standards to calculate Mn(GPC). Ligands of LH-H,4 Lnap-H,5
LCH3-H,6 LBu-H,7 and LPh-H8 were prepared by acid-catalyzed
condensation following literature procedures. DFT geometry optimiza-
tions were carried out at M06/6-31G* level combined with the D3
version of Grimme’s dispersion correction. Calculations were performed
by Gaussian 09 program.
Figure 10. 1H NMR study of the mixture of one equivalent of
L
CH3-TiOPr6 and six equivalents of LA in CDCl3 at 60 °C.
oxygen atoms of the terminal isopropoxide groups, which were
in the trans-position to the imine groups of LBu-TiOPr6, exceeded
that of LBu‑CH3-TiOPr6 (4.385 Å) and LBu‑Ph-TiOPr6 (4.300 Å)
because of the steric effect between the substituent on the imine
groups. Moreover, compared with LPh-TiOPr6 and LCH3-TiOPr6,
LH-TiOPr6 with more space around the Ti atom displayed
highest catalytic activity.
Synthesis of L3‑OMe-H. Hydrazine monohydrate (1.16 g, 23.0 mmol)
was added dropwise into an ethanol solution (100 mL) of the 2-hydroxy-
3-methoxybenzaldehyde (7.1 g, 46.6 mmol) at room temperature. The
resulting solution was refluxed for 1 day. The product precipitated as a
yellow solid which was filtered. Yield: 5.18 g (69%). 1H NMR (CDCl3,
200 MHz): δ 10.99 (1H, s, OH), 8.68 (2H, s, CHN), 7.04−6.96
(4H, m, ArH), 6.85 (1H, s, ArH), 3.81 (6H, s, OCH3). 13C NMR
spectrum was not available because of low solubility of LOMe-H in CDCl3.
Mp = 188 °C
3.0. CONCLUSIONS
This study synthesized a series of hydrazine-bridging Schiff
base ligands and their corresponding titanium complexes. The
electronic and steric effects of the phenolate group altered LA
polymerization. If the ligands of Ti complexes were electron
withdrawing or steric bulky groups, then the degree of activity
would decrease dramatically. In addition, the steric bulky sub-
stituent on the imine groups reduced the space around the Ti
atom and then reduced LA coordination with Ti atom contents,
thereby reducing catalytic activity. The catalytic activities of all
dinuclear Ti complexes exceeded (approximately 10−60-fold)
Synthesis of L5‑OMe-H. Using a method similar to that for L3‑OMe-H
expect 1-(5-methoxy-2-hydroxyphenyl)ethan-1-one was used in place
1
of 2-hydroxy-3-methoxybenzaldehyde. Yield: 5.25 g (75%). H NMR
(CDCl3, 200 MHz): δ δ 10.99 (1H, s, OH), 8.68 (2H, s, CHN),
Figure 11. Possible mechanism of polymerization using Ti complexes bearing hydrazine-bridging Schiff base ligands as catalysts.
F
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Figure 12. Structural comparison of dinuclear Ti complexes bearing (a) hydrazine-bridging Schiff base ligands and (b) N,N-di(salicylidene)-2-
hydroxyphenylmethanediamine.3m
Figure 13. Structural comparison of dinuclear Ti complexes bearing (a) LBu-TiOPr6, (b) LCH3-TiOPr6, and (c) LPh-TiOPr6.
7.04−6.96 (4H, m, ArH), 6.85 (1H, s, ArH), 3.81 (6H, s, OCH3). 13
C
(76%). 1H NMR (CDCl3, 200 MHz): δ 7.94 (2H, s, CHN), 7.00 (2H,
d, J = 10 Hz, Ar-H), 6.93 (2H, d, J = 7 Hz, Ar-H), 6.66 (2H, t, J = 7 Hz,
Ar-H), 5.02, 4.78, 4.46 (sept, 6H, J = 6.0 Hz, OCH(CH3)2), 3.92
(6H, s, OCH3), 1.40−1.18, (m, 24 H, OCH(CH3)2), 0.88 (br,
12H, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 159.97 (CN),
150.64, 150.51, 121.95, 120.17, 119.16, 113.29 (Ar), 78.25, 77.41,
74.51 (OCH(CH3)2), 55.59 (OCH3), 25.88, 25.28, 25.05, 24.05, 23.55
(OCH(CH3)2). Anal. Calcd (found) for C34H56N2O10Ti2: C, 54.55
(55.05); H, 7.54 (7.49); N, 3.74 (3.76) %. Mp = 122 °C.
NMR spectrum was not available because of low solubility of LOMe-H
in CDCl3. Mp = 210 °C
Synthesis of LBr‑CH3-H. Using a method similar to that for L3‑OMe-H
expect 1-(5-bromo-2-hydroxyphenyl)ethan-1-one was used in place of
1
2-hydroxy-3-methoxybenzaldehyde. Yield: 10.227 g (48%). H NMR
(CDCl3, 200 MHz): δ 7.74 (2H, s, ArH), 7.47, 6.94 (4H, d, J = 10 Hz,
ArH), 2.55 (6H, s, NCCH3). 13C NMR spectrum was not available
because of low solubility of LBr‑CH3-H in CDCl3. Mp = 292 °C
Synthesis of LCl−H-H. 4-chlorobenzaldehyde (7.03 g, 50 mmol) was
added dropwise into an ethanol solution (100 mL) of the hydrazine
monohydrate (5.0 g, 0.10 mol) at 0 °C for 6 h. Volatile materials were
removed under vacuum to give a white powder. 6.625 g of the white
powder was reacted with 2-hydroxybenzaldehyde (5.24 g, 43 mmol) in
ethanol (50 mL) and refluxed for 1 day. The product precipitated as a
yellow solid which was filtered. Yield: 7.12 g (64%). 1H NMR (CDCl3,
200 MHz): δ 877, 8.59 (2H, s, CHN), 7.79, 7.45 (4H, d, J = 8 Hz,
ArH), 7.38−7.34 (2H, m, ArH), 7.05−6.92 (2H, m, ArH). 13C NMR
spectrum was not available because of low solubility of LCl−H-H in
CDCl3. Mp = 112 °C
Synthesis of L5‑OMe-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect L5‑OMe-H was used in place of LH-H. Yield: 1.68 g
(43%). 1H NMR (CDCl3, 200 MHz): δ 7.92 (2H, s, CHN), 7.00 (2H,
d, J = 10 Hz, Ar-H), 6.78 (2H, d, J = 10 Hz, Ar-H), 6.75 (2H, s, Ar-H),
4.98, 4.77, 4.48 (sept, 6H, J = 5.8 Hz, OCH(CH3)2), 3.79 (6H, s,
OCH3), 1.38−1.13, (m, 24 H, OCH(CH3)2), 0.92 (d, 12H, J = 5.8 Hz,
OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 159.97 (CN),
150.64, 150.51, 121.95, 120.17, 119.16, 113.29 (Ar), 78.25, 77.41, 74.51
(OCH(CH3)2), 55.59 (OCH3), 25.88, 25.28, 25.05, 24.05, 23.55
(OCH(CH3)2). Anal. Calcd (found) for C34H56N2O10Ti2: C, 54.55
(55.05); H, 7.54 (7.49); N, 3.74 (3.76) %. Mp = 122 °C.
Synthesis of LH-TiOPr6. A mixture of LH-H (1.20 g, 5 mmol) and
Ti(OiPr)4 (2.843 g, 10 mmol) in THF (30 mL), was stirred for 12 h.
Volatile materials were removed under vacuum to give yellow oil and
then it was washed with hexane (50 mL). The product precipitated as
Synthesis of LBu-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect LBu-H was used in place of LH-H. Yield: 3.42 g
(75%). 1H NMR (CDCl3, 200 MHz): δ 7.95 (2H, s, CHN), 7.44, 7.06
(4H, s, Ar-H), 4.95, 4.83, 4.44 (sept, 6H, J = 5.8 Hz, OCH(CH3)2), 1.52,
1.31 (36H, s, C(CH3)3), 1.30−1.08, (m, 24 H, OCH(CH3)2), 0.83 (d,
12H, J = 5.8 Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 159.29
(CN), 148.16, 139.11, 137.10, 126.57, 125.44, 120.85 (Ar), 78.09
(OCH(CH3)2), 36.01, 33.55 (C(CH3)3), 31.55, 29.79 (C(CH3)3),
26.02, 22.62, 14.11 (OCH(CH3)2). Anal. Calcd (found) for
C48H84N2O8Ti2: C, 63.15 (63.74); H, 9.27 (9.73); N, 3.07 (3.31) %.
Mp = 133 °C.
1
a yellow powder which was filtered. Yield: 2.48 g (72%). H NMR
(CDCl3, 200 MHz): δ 7.94 (2H, s, CHN), 7.36 (2H, t, J = 6 Hz,
Ar-H), 7.26 (2H, d, J = 6 Hz, Ar-H), 6.82 (2H, d, J = 6 Hz, Ar-H),
6.74 (2H, t, J = 6 Hz, Ar-H), 4.99, 4.77, 4.48 (sept, 6H, J = 5.8 Hz,
OCH(CH3)2), 1.40−1.1.12, (m, 24 H, OCH(CH3)2), 0.92, 0.89 (d,
12H, J = 5.8 Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 165.15
(CN), 150.93, 134.07, 131.72, 119.97, 119.52, 116.89 (Ar), 78.56,
77.77, 74.68 (OCH(CH3)2), 25.91, 25.85, 25.35, 25.13, 24.18, 23.59
(OCH(CH3)2). Anal. Calcd (found) for C32H52N2O8Ti2: C, 55.82
(55.45); H, 7.61 (7.62); N, 4.07 (4.67) %. Mp = 122 °C.
Synthesis of LNap-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect LNap-H was used in place of LH-H. Yield: 3.35 g
1
(85%). H NMR (CDCl3, 200 MHz): δ 8.79 (2H, s, CHN), 7.93,
Synthesis of L3‑OMe-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect L3‑OMe-H was used in place of LH-H. Yield: 2.85 g
7.00 (4H, d, J = 10 Hz, Ar-H), 7.73, 7.67 (4H, d, J = 8 Hz, Ar-H), 7.50,
7.24 (4H, t, J = 8 Hz, Ar-H), 4.95, 4.74, 4.41 (sept, 6H, J = 5.8 Hz,
G
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OCH(CH3)2), 1.33, 1.26, 1.14, 0.82, 0.78 (d, 36H, J = 5.8 Hz,
OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 219.76 (CN), 165.72,
147.83, 134.61, 133.49, 129.13, 127.40, 123.05, 122.57, 119.73, 110.46
(Ar), 78.60, 78.25, 74.89 (OCH(CH3)2), 25.82, 25.46, 25.28, 24.47,
23.52 (OCH(CH3)2). Anal. Calcd (found) for C40H56N2O8Ti2: C, 60.92
(60.45); H, 7.16 (7.44); N, 3.55 (3.82) %. Mp = 110 °C.
added to a mixture of complex LH-TiOPr6 (0.068 g, 0.1 mmol) and
LA (1.44 g, 10 mmol) at 60 °C. After the solution was stirred for 50 min,
the reaction was then quenched by adding to a drop of ethanol, and
the polymer was precipitated pouring into n-hexane (70.0 mL) to give
white solids. The white solid was dissolved in CH2Cl2 (5.0 mL) and
then n-hexane (70.0 mL) was added to give white crystalline solid.
Yield: 1.08 g (75%).
Synthesis of LCH3-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect LCH3-H was used in place of LH-H. Yield: 2.90 g
(81%). 1H NMR (CDCl3, 200 MHz): δ 7.33 (2H, t, J = 8 Hz, Ar-H),
7.29 (2H, d, J = 8 Hz, Ar-H), 6.83 (2H, d, J = 8 Hz, Ar-H), 6.74 (2H, t,
J = 8 Hz, Ar-H), 5.00, 4.77, 4.52 (sept, 6H, J = 6.0 Hz, OCH(CH3)2),
2.15 (6H, s, CH3CN) 1.38, 1.30, 1.20, 1.17, 0.92 (d, 12H, J = 6.0 Hz,
OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 162.92 (CN),
161.77, 132.21, 128.52, 124.14, 119.63, 116.64 (Ar), 78.75, 77.29, 74.04
(OCH(CH3)2), 26.18, 26.08, 25.41, 25.10, 24.23, (OCH(CH3)2),
23.57 (CH3CN). Anal. Calcd (found) for C34H56N2O8Ti2: C, 56.99
(56.60); H, 7.88 (7.72); N, 3.91 (3.63) %. Mp = 113 °C.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.5b02590.
■
S
Polymer characterization data, and details of the kinetic
study (PDF)
Synthesis of LCH3-Ti2O(OPr)2. 0.12 g of LCH3-Ti(OPr)6 was
dissolved in CDCl3 (1.0 mL) in NMR tube. After 2 weeks, only the
residual yellow powder was observed in the NMR tube. The crystal of
AUTHOR INFORMATION
Corresponding Author
*Hsuan-Ying Chen, E-mail: hchen@kmu.edu.tw, Tel.:
+886-7-3121101-2585, Fax: +886-7-3125339.
■
L
CH3-Ti2O(OPr)2 was observed from the residual yellow powder.
Synthesis of LBr‑CH3-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect LBr‑CH3-H was used in place of LH-H. Yield: 2.93 g
(67%). H NMR (CDCl3, 200 MHz): δ 7.40 (2H, s, Ar-H), 7.34,
Notes
1
The authors declare no competing financial interest.
6.71 (2H, d, J = 8 Hz, Ar-H), 4.97, 4.73, 4.51 (sept, 6H, J = 6.0 Hz,
OCH(CH3)2), 2.11 (6H, s, CH3CN) 1.36, 1.28, 1.17, 1.14, 0.95, 0.92
(d, 12H, J = 6.0 Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz):
δ 162.01 (CN), 160.70, 134.94, 130.86, 125.40, 121.61, 107.74 (Ar),
79.21, 77.61, 74.12 (OCH(CH3)2), 26.19, 25.30, 25.01, 24.11, 23.43,
(OCH(CH3)2), 18.41 (CH3CN). Anal. Calcd (found) for
C34H54Br2N2O8Ti2: C, 46.71 (46.39); H, 6.23 (6.47); N, 3.20 (3.19)
%. Mp = 192 °C.
ACKNOWLEDGMENTS
■
This study is supported by Kaohsiung Medical University “Aim
for the top 500 universities grant” under Grant No. KMU-
DT103007, NSYSU-KMU JOINT RESEARCH PROJECT
(#NSYSUKMU 104-P006), and the Ministry of Science and
Technology (Grant MOST 104-2113-M-037-010 and 104-2815-
C-037-033-M). We thank Center for Research Resources and
Development at Kaohsiung Medical University for the
instrumentation and equipment support.
Synthesis of LPh-TiOPr6. Using a method similar to that for
LH-TiOiPr6 expect LPh-H was used in place of LH-H. Yield: 2.90 g
1
(81%). H NMR (CDCl3, 200 MHz): δ 7.38−7.34 (4H, m, Ar-H),
7.29−7.16 (8H, m, Ar-H), 6.93 (2H, t, J = 8 Hz, Ar-H), 6.76 (2H, d, J =
8 Hz, Ar-H), 6.49 (2H, t, J = 8 Hz, Ar-H), 5.05, 4.76, 4.48 (sept, 6H, J =
6.0 Hz, OCH(CH3)2), 1.42, 1.36, 1.26, 1.17, 0.90 (d, 12H, J = 6.0 Hz,
OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): δ 162.92 (CN),
161.77, 132.21, 128.52, 124.14, 119.63, 116.64 (Ar), 78.75, 77.29,
74.04 (OCH(CH3)2), 26.18, 26.08, 25.41, 25.10, 24.23, (OCH(CH3)2),
23.57 (CH3CN). Anal. Calcd (found) for C44H60N2O8Ti2: C, 62.86
(63.10); H, 7.19 (7.52); N, 3.33 (3.93) %. Mp = 121 °C.
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and Ti(OiPr)4 (2.843 g, 10 mmol) in THF (30 mL), was stirred for
1 day. Volatile materials were removed under vacuum to give yellow oil
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1
as a yellow powder which was filtered. Yield: 1.64 g (48%). H NMR
(CDCl3, 200 MHz): δ 8.88, 8.08 (4H, s, CHN), 7.35, 7.25 (8H, d,
J = 10 Hz, Ar-H), 7.12, 6.27 (4H, d, J = 8 Hz, Ar-H), 6.90, 6.61 (4H, t,
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1
precipitated as a yellow powder which was filtered. The H NMR
spectrum of the yellow powder revealed there were lots of impurities
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complex LH-TiOPr6 as a catalyst. The polymerization conversion
was analyzed by 1H NMR spectroscopic studies. Toluene (5.0 mL) was
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DOI: 10.1021/acs.inorgchem.5b02590
Inorg. Chem. XXXX, XXX, XXX−XXX