Titanatranes containing tetradentate ligands with controlled steric hindrance

Article abstract of DOI:10.1016/j.jorganchem.2007.04.031

New monomeric titanatranes {A figure is presented} (Ar = 2,6-di-i-Pr-phenyl; x = 0, 5; x = 1, 6; x = 2, 7; x = 3, 8) were synthesized from the corresponding tetradentate ligands 1-4, respectively, using an equimolar mixture of Ti(O-i-Pr)₄ and 2,6-di-i-Pr-phenol. These compounds show the catalytic activity for the ring opening polymerization of l-lactide.

Full text of DOI:10.1016/j.jorganchem.2007.04.031

Journal of Organometallic Chemistry 692 (2007) 3519–3525
www.elsevier.com/locate/jorganchem
Titanatranes containing tetradentate ligands with controlled
steric hindrance
a
a
a
Sang-deok Mun a,1, Junseong Lee b,1, So Han Kim , Younjin Hong , Young-ho Ko ,
a
c
c
b
Young Kook Shin , Jeong Hak Lim , Chang Seop Hong , Youngkyu Do ,
a,
*
Youngjo Kim
a
Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea
Department of Chemistry, School of Molecular Science BK21, Center for Molecular Design and Synthesis, KAIST, Daejeon 305-701, Republic of Korea
b
c
Department of Chemistry and Center for Electro- and Photo-Responsive Molecules, Korea University, Seoul 136-701, Republic of Korea
Received 21 March 2007; received in revised form 11 April 2007; accepted 24 April 2007
Available online 1 May 2007
Dedicated to Professor John G. Verkade in honor of his 72nd birthday.
Abstract
New monomeric titanatranes
(Ar = 2,6-di-i-Pr-phenyl; x = 0, 5; x = 1, 6; x = 2, 7; x = 3, 8) were
synthesized from the corresponding tetradentate ligands 1–4, respectively, using an equimolar mixture of Ti(O-i-Pr)4 and 2,6-di-i-Pr-phe-
nol. These compounds show the catalytic activity for the ring opening polymerization of l-lactide.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Titanium; Tetradentate ligand; X-ray structure; Ring opening polymerization; Polylactide
1. Introduction
describe new tetradentate trianionic triethoxyamine ligands
(N(CH2CMe2O)x(CH2CH2O)3ꢀx3ꢀ) (x = 1, 2; x = 2, 3;
x = 3, 4) with controlled steric hindrance in a stepwise fash-
ion. In addition, the synthesis, characterization and catalytic
ability for the polymerization of lactide, for four titanatranes
possessing imino-2,20,200-triethanolate (1), imino-2,2-
dimethyl-2,20,200-triethanolate (2), imino-2,2,20,20-tetra-
methyl-2,20,200-triethanolate (3), and imino-2,2,20,20,200,200-
hexamethyl-2,20,200-triethanolate (4) in Scheme 1 will be
demonstrated. The corresponding four titanatranes 5–8 all
possess an axial anionic 2,6-di-i-Pr-phenolate ligand.
Atranes synthesized from triethanolamine feature a wide
variety of metallic and nonmetallic central atoms [1]. Many
studies of new atrane systems have focused on main group
elements such as silicon, phosphorus, aluminum and tin
[1,2]. Among the transition metallic atranes, many
examples of titanium complexes bearing a triethanolate-
amine ligand and its derivatives have been reported in the lit-
erature [3]. While most of the extensive previous work on
titanatrane has been focused on the modification of axial
substituents trans to annular N ! Ti bond [3a,3b,3c,
3d,3e,3f,3i,3j,3k,3l,3m,3p,3q], much less attention has been
directed toward the electronic or steric modification of tri-
ethanolamine in itself [3g,3h,3n,3o]. In this regard, here we
2. Experimental
2.1. General considerations
All reactions were carried out under dinitrogen atmo-
sphere using standard Schlenk and glove box techniques
[4]. All solvents were dried by distilling from sodium/
*
Corresponding author. Tel.: +82 43 261 3395; fax: +82 43 267 2279.
E-mail address: ykim@cbu.ac.kr (Y. Kim).
These authors are equally contributed to this work.
1
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.04.031

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S.-d. Mun et al. / Journal of Organometallic Chemistry 692 (2007) 3519–3525
OTi(OiPr)3
O
HO
HO
OH
O
O
O Ti
N
N
toluene
1
5
OTi(OiPr)3
HO
HO
OH
50oC
O
O
OH
OH
O
O
+
HN
O Ti
N
4 day stirring
toluene
N
2
6
OTi(OiPr)3
O
HO
HO
50oC
O Ti
OH
O
O
O
OH
+
2
H2N
N
4 day stirring
N
toluene
7
3
OTi(OiPr)3
toluene
O
50oC
O Ti
HO
O
O
OH
O
HO
+
NH3
3
4 day stirring
N
N
4
8
Scheme 1.
benzophenone ketyl (toluene, tetrahydrofuran (THF),
diethylether and hexane) or CaH2 (methylene chloride)
under a nitrogen atmosphere and stored over the acti-
vated molecular sieves 3A [5]. All deuterium solvents
were dried over activated molecular sieves (4A) and were
used after vacuum transfer to a Schlenk tube equipped
with J. Young valve. 1H and 13C{1H} NMR spectra
were recorded at ambient temperature on a Bruker
DPX-300 NMR spectrometer using standard parameters.
The chemical shifts are referenced to the residual peaks
of CDCl3 (7.24 ppm, 1H NMR; 77.0 ppm, 13C{1H}
NMR). Elemental analyses and mass data were per-
formed by EA 1110-FISONS(CE) and ICP-MASS HP-
4500, respectively. The thermal properties of polymers
were investigated by Thermal Analyst 200 DSC system
under nitrogen atmosphere at a heating rate of 10 ꢁC/
min. Molecular weights of polymers were determined
by gel permeation chromatography (GPC) and the mea-
surements were carried out at room temperature with
THF as the eluent (1 mL/min) using a Waters 510 pump,
a Waters 717 Plus Autosampler, four Polymer Laborato-
2.2. Synthesis
All chemicals were purchased from Aldrich. Compound
5 was synthesized in the yield of 85% by the modified liter-
ature procedure, [6] in which toluene was used instead of
THF as a solvent.
2.3. Synthesis of 2
Isobutylene oxide (3.61 g, 55.0 mmol) and diethanola-
mine (5.26 g, 50.0 mmol) was added to a 10 mL screw
cap vial containing stirring bar. The vial was tightly sealed
by Teflon tape and paraffin film. The mixture was main-
tained at room temperature for overnight and was then
heated for 4 days at 50 ꢁC. The removal of volatile com-
pounds at reduced pressure gave the desired product 2
(8.60 g, 97%) as colorless oil (see Scheme 1).
1H NMR (CDCl3, 300.13 MHz): d 4.51 (br s, 3H, OH),
3.57 (t, 4H, J = 4.8 Hz, OCH2), 2.64 (t, 4H, J = 4.8 Hz,
OCH2CH2N), 2.41 (s, 2H, OCMe2CH2N), 1.15 (s, 6H,
Me).
4
5
13C{1H} NMR (CDCl3, 75.46 MHz): d 70.7 (OCMe2-
CH2N), 66.8 (OCMe2), 59.9 (OCH2), 59.2 (OCH2-
CH2N), 27.7 (Me).
˚
ries PLgel columns (100, 500, 10 , 10 A) in series, and a
Wyatt Optilab DSP interferometric refractometer as a
detector. The columns were calibrated with polystyrene
standards.
HRMS (EI) m/z calcd: 177.2414. Found: 177.2419.

S.-d. Mun et al. / Journal of Organometallic Chemistry 692 (2007) 3519–3525
3521
2.4. Synthesis of 3
1H NMR (CDCl3, 300.13 MHz): d 6.99 (d, J = 7.5 Hz,
2H, aryl-H), 6.81 (t , J = 7.5 Hz, 1H, aryl-H), 4.45 (t,
J = 5.2 Hz, 2H, OCH2), 3.63 (m, 2H, CHMe2), 3.30–3.17
(m, 6H, CH2CH2N and CMe2CH2N), 1.34(s, 6H, OCMe2),
1.32 (s, 6H, OCMe2), 1.23 (d, J = 6.8 Hz, 12H, CHMe2).
13C{1H} NMR (CDCl3, 75.46 MHz): d 160.1, 137.8,
122.5, 120.0 (aryl), 83.47 (OCMe2), 71.35 (OCMe2CH2),
71.05 (OCH2), 63.82 (OCH2CH2), 31.16 (OCMe2), 30.53
(OCMe2), 26.77 (CHMe2), 23.35 (CHMe2).
Ligand 3 as a colorless oil was prepared in a yield of 97%
(10.0 g) by reacting isobutylene oxide (7.93 g, 110.0 mmol)
with ethanolamine (3.05 g, 50.0 mmol) in a manner analo-
gous to the procedure for the ligand 2 (see Scheme 1).
1H NMR (CDCl3, 300.13 MHz): d 3.95 (br s, 3H, OH),
3.59 (t, 2H, J = 5.3 Hz, OCH2CH2), 2.76 (t, 2H, J = 5.3 Hz,
OCH2CH2), 2.58 (s, 4H, OCMe2CH2N), 1.17 (s, 12H, Me).
13C{1H} NMR (CDCl3, 75.46 MHz): d 71.0 (CMe2CH2N),
68.8 (OCMe2), 61.3 (OCH2), 60.3 (OCH2CH2N), 28.1 (Me).
HRMS (EI) m/z calcd: 205.2946. Found: 205.2948.
Elemental Anal. Calc. for C22H37NO4Ti: C, 61.82; H,
8.78; N, 3.28. Found: C, 62.18; H, 8.66; N, 3.14%.
2.8. Synthesis of 8
2.5. Synthesis of 4
The desired product 8 as a deep yellow crystal was pre-
pared in a yield of 78% in a manner analogous to the pro-
cedure for 6. (See Scheme 1).
Ligand 4 as a colorless solid was prepared in a yield of
98% (4.60 g) by reacting isobutylene oxide (4.76 g,
66.0 mmol) with 2 M solution of ammonia in methanol
(10.0 mL, 20.0 mmol) in a manner analogous to the proce-
dure for the ligand 2 (see Scheme 1).
1H NMR (CDCl3, 300.13 MHz): d 7.00 (d, J = 7.5 Hz,
2H, aryl-H), 6.81 (t, J = 7.5 Hz, 1H, aryl-H), 3.67 (m,
2H, CHMe2), 3.26 (s, 6H, OCMe2CH2), 1.35 (s, 18H,
OCMe2), 1.24 (d, J = 6.8 Hz, 12H, CHMe2).
1H NMR (CDCl3, 300.13 MHz): d 3.38 (br s, 3H, OH),
2.70 (s, 6H, CH2), 1.20 (s, 18H, Me).
13C{1H} NMR (CDCl3, 75.46 MHz): d 160.2, 137.8,
122.4, 119.6 (aryl), 82.80 (OCMe2), 74.74 (OCMe2CH2),
30.34 (OCMe2), 26.73 (CHMe2), 23.35(CHMe2).
13C{1H} NMR (CDCl3, 75.46 MHz): d 70.9 (NCH2),
70.3 (OCMe2), 28.7 (Me).
HRMS (EI) m/z calcd: 233.3477. Found: 233.3480.
Table 1
2.6. Synthesis of 6
Crystallographic data and parameters for 6 and 7
6
7
To a toluene solution composed of 2,6-di-i-Pr-phenol
(0.891 g, 5.00 mmol) in 10 mL of toluene was added drop-
wise at room temperature a solution of Ti(O-i-Pr)4
(1.42 g, 5.00 mmol) in 10 mL of toluene. After 2 h, the reac-
tion solution was added dropwise to the solution of 2
(0.89 g, 5.00 mmol) in 10 mL of toluene. The reaction mix-
ture was stirred at room temperature overnight, and then
the volatiles were evaporated under vacuum, leaving a yel-
low solid, to which was added 15 mL of toluene. The yellow
solution was filtered, and the desired product 6 was isolated
as pale yellow crystals after the solution remained at ꢀ15 ꢁC
in a refrigerator for a few days (1.65 g, 83%) (see Scheme 1).
1H NMR (CDCl3, 300.13 MHz): d 6.99 (d, J = 7.5 Hz,
2H, aryl-H), 6.81 (t , J = 7.5 Hz, 1H, aryl-H), 4.47 (m,
4H, OCH2), 3.62 (m, 2H, CHMe2), 3.25 (m, 4H,
CH2CH2N), 3.22(s, 2H, OCMe2CH2N) 1.35 (s, 6H,
OCMe2), 1.23 (d, J = 6.80 Hz, 12H, CHMe2).
Empirical formula
Formula weight
Temperature (K)
C
399.37
130
0.71073
Monoclinic
P21/c
20H33NO4Ti
C22H37NO4Ti
427.43
100
0.74999
Orthorhombic
P212121
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
˚
a (A)
8.8687(12)
13.4020(18)
18.312(3)
90
103.506(2)
90
2116.4(5)
4
1.253
8.8570(18)
13.965(3)
18.493(4)
90
90
90
2287.4(8)
4
1.241
0.400
920
1.10–28.64
0 6 h 6 11,
ꢀ18 6 k 6 18,
ꢀ24 6 l 6 24
10587
5835 [0.0906]
5835/0/263
1.049
R1 = 0.0603,
wR2 = 0.1632
R1 = 0.0624,
wR2 = 0.1606
0.800 and ꢀ0.961
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
0.427
856
h Range for data collection (ꢁ) 2.29–28.28
Index ranges
ꢀ8 6 h 6 11,
ꢀ8 6 k 6 17,
ꢀ23 6 l 6 23
8168
3869 [0.0495]
3869/0/239
0.723
R1 = 0.0442,
wR2 = 0.0700
R1 = 0.0872,
wR2 = 0.0742
13C{1H} NMR (CDCl3, 75.46 MHz): d 160.1, 137.8,
122.5, 120.2 (aryl), 84.20 (OCMe2), 71.02 (OCH2), 68.26
(OCMe2CH2), 60.05 (OCH2CH2), 30.36 (OCMe2), 26.80
(CHMe2), 23.33 (CHMe2).
Reflections collected
Independent reflections [Rint
Data/restraints/parameters
Goodness-of-fit on F2
]
Elemental Anal. Calc. for C20H33NO4Ti C, 60.15; H,
8.33; N, 3.51. Found: C, 60.19; H, 8.57; N, 3.73%.
Final R indices [I > 2r(I)]
2.7. Synthesis of 7
R indices (all data)
Largest difference in peak and 0.296 and ꢀ0.357
The desired product 7 as a yellow crystal was prepared
in a yield of 82% in a manner analogous to the procedure
for 6 (see Scheme 1).
ꢀ3
˚
hole (e A
)
P
P
P
P
R1 = iFoj–jFci/ jFoj and wR2 = { [w(Fo2–Fc2)2]/ [w(Fo2)2]}1/2
.

3522
S.-d. Mun et al. / Journal of Organometallic Chemistry 692 (2007) 3519–3525
Elemental Anal. Calc. for C24H41NO4Ti: C, 63.29; H,
9.07; N, 3.08. Found: C, 63.52; H, 9.15; N, 2.97%.
yield. These four products in the solid state were stable
in air for a few weeks and, according to H NMR spec-
1
troscopy, they decomposed slightly after a few days at
room temperature in CDCl3 solutions contained in
capped NMR tubes. They are soluble in polar organic
solvents and in toluene. As expected, 5, 6, and 7 are
totally insoluble in alkanes such as n-hexane; however,
compound 8 is unexpectedly soluble even in hydrocarbon
solvents such as n-hexane and n-pentane. All the com-
pounds evaluated as catalysts in the present work were
pre-purified by recrystallization in toluene.
2.9. X-ray structure determination for 6 and 7
The crystallographic measurements were performed at
130 K for 6 using a Bruker CCD-1000 diffractometer with
˚
Mo Ka (k = 0.71073 A) radiation or 100 K for 7 using syn-
˚
chrotron radiation (k = 0.74999 A) on a 4AMXW ADSC
Quantum-210 detector with a silicon double crystal mono-
chromator at the Pohang Accelerator Laboratory, Korea.
Specimens of suitable quality and size (0.1 · 0.1 · 0.1 mm3)
were selected, mounted, and centered in the X-ray or syn-
chrotron beam by using a video camera. The structures were
solved by the direct method and refined by full matrix least-
squares methods using the SHELXTL program package with
anisotropic thermal parameters for all non-hydrogen atoms
[7]. Final refinement based on the reflections (I > 2.0(I)) con-
verged at R1 = 0.0442, wR2 = 0.0700, and GOF = 0.723 for
6 and at R1 = 0.0603, wR2 = 0.1632, and GOF = 1.049 for 7.
Further details are listed in Table 1.
1
The H NMR spectra of ligands 1–4 and complexes 5–
8 display well-defined resonances with their expected inte-
grations. In comparison to the free tetradentate precur-
sors 1–4, all signals in 5–8 are shifted to downfield,
which is a consequence of the complexation with Lewis
1
acidic titanium metal. In the H NMR spectra, the extent
of downfield shift in 1H NMR spectra is greater for
OCH2 resonances (0.9 ppm) than for CH2N resonances
(0.6–0.8 ppm). The greater extent of downfield shifts of
OCH2 NMR resonance than those of CH2N resonance
suggests a strong bond between O atom and Ti atom
and a weak interaction between the N atom and the Ti
atom upon complexation. The solution structure therefore
is consistent with the structure in the solid since the Ti–O
bonds are of normal length and the Ti–N bond is long
and weak. The NMR signals were sharp and variable-
temperature studies showed no evidence of inter- or intra-
molecular ligand exchange at ambient temperature. In
3. Polymerization procedure
Solution polymerizations of LA were carried out by
charging a stirring bar and LA to a 5 mL vial in the glove
box and then the flask was immersed at the oil bath of
100 ꢁC. Polymerization began with the addition of 1 mL
stock toluene solution of the titanium compound. After
10 h, the polymerization was terminated by the addition
of 5 mL of MeOH. The reaction mixture was washed with
excess methanol several times. The precipitated polymer
was collected by filtration, washed with methanol (40 mL,
washing for five times), and dried in a vacuum oven at
40 ꢁC for 12 h. In the case of the bulk polymerization, no
solvent was used and polymerization was carried out in a
manner analogous to the solution polymerization except
for the polymerization temperature of 130 ꢁC.
1
addition, the H and 13C{1H} NMR indicate that com-
plexes show monomeric structural feature in solution.
Thus, the ligands are coordinated to the titanium atom
in a tetradentate manner and this interpretation is sup-
ported by the structural studies described below.
In order to confirm the molecular structure and to elu-
cidate the metal-ligand bonding in this titanatrane, the
single-crystal X-ray diffraction studies for 6 and 7 were
performed. Single-crystal X-ray structure, selected bond
distances, and selected bond angles for 6 and 7 are shown
in Figs. 1 and 2, respectively. In contrast to the often
observed oxygen-bridged dimeric structural feature for tit-
anatranes [3b,3d,3e,3f,3l,3p], the crystal structures of 6
and 7 show mononuclear characters, which are consistent
with their NMR spectra, presumably because of the steric
bulk of the axially located di-i-Pr-phenolate ligand. In
addition to the three anionic oxygens, the titanium atom
in 6 and 7 is ligated via a transannular interaction stem-
ming from the bridgehead amino nitrogen, giving a
slightly distorted trigonal bipyramidal local geometry
around the metal. The amino nitrogen N and oxygen
atom of di-i-Pr-phenolate ligand occupy the axial posi-
tions of a trigonal bipyramidal coordination array. The
three five-membered rings adopt an envelope conforma-
tion similar to all the previously investigated monomeric
atranes and azatranes [3,8]. The sum of the angles around
the equatorial oxygens is 347.70(8)ꢁ for 6 and 347.11(7)ꢁ
4. Result and discussion
Ligands 2–4 were prepared by the reaction between
suitable amine sources and slightly more than appropriate
equivalents of isobutylene oxide as shown in Scheme 1.
They were mixed in a 10 mL screw cap vial and allowed
to stand overnight at room temperature. The mixture
was then heated for several days at 50 ꢁC to ensure com-
plete reaction. NMR studies in benzene-d6 indicate that
this reaction is essentially complete after 3 days at
50 ꢁC. The new tetradentate ligands 2–4 could be isolated
in pure form by removal of the solvent at reduced pres-
sure. 2 and 3 were obtained as sticky oil, but 4 obtained
only as solid state.
The treatment of Ti(O-i-Pr)4 with 1 equiv. of 2,6-di-i-
Pr-phenol and one equiv. of the ligand precursors 1–4
in toluene gave, after workup, the novel titanatranes
5–8 as pale or deep yellow crystals in 78–85% isolated
for 7. As
a
result, the acute Oeq–Ti–N angles

S.-d. Mun et al. / Journal of Organometallic Chemistry 692 (2007) 3519–3525
3523
deviation range 0.20(8)–1.46(3)ꢁ reported for other mono-
nuclear titanatranes [3h,3i,3k,3m]; however, 7 shows
somewhat large deviation compared with the deviations
for other mononuclear titanatranes, although values of
15.5(1)–51.05(8)ꢁ for this angle have been described for
di- or multinuclear oxo-bridged titanatranes [3b,3d,3e,
3f,3j,3l,3p]. The average Ti–O bond distance for all four
˚
˚
oxygens in each of 6 [1.830(2) A] and 7 [1.834(2) A] is sim-
ilar to the average of this distance observed for other
structurally characterized titanatranes [3b,3d,3e,3f,3-
h,3i,3j,3k,3l,3m,3p]. In addition, the transannular Ti–N
˚
˚
bond distance in 6[2.306(2) A] and 7 [2.301(2) A] falls into
the middle range of 2.264(3)–2.400(3) A found in previ-
˚
ously structurally characterized titanium trialkanolamine
derivatives
[3b,3d,3e,3f,3h,3i,3j,3k,3l,3m,3p,3q].
This
observation confirms the existence of the transannular
interaction.
Fig. 1. Molecular drawing of compound 6 and atom labeling. (H atoms
were omitted for clarity) Selected bond distances (A): Ti1–O1 = 1.828(2),
Ti1–O2 = 1.833(2), Ti1–O3 = 1.832(2), Ti1–O4 = 1.827(2), Ti1–N =
2.306(2). Selected bond angle (ꢁ): C1–O1–Ti1 = 163.79(16), O1–Ti1–
Polylactide (PLA) is the biodegradable and renewable
polymeric material as the controlled drug-release devices,
absorbable sutures, medical implants for orthopedic use,
disposable degradable plastic articles and scaffolds for tis-
sue engineering [9]. Until now, a large variety of catalytic
systems based on tin, aluminum, zinc, magnesium, iron,
lanthanide and lithium organometallic complexes contain-
ing initiating groups such as amides, carboxylates, and alk-
oxides for the ring opening polymerization (ROP) of
lactide (LA) have been reported in the literature [10]; how-
ever, in spite of the shortest history for titanium alkoxides
in the field of the ROP of LA, titanium alkoxides have been
extensively studied since 2002 [3q,6,11].
˚
N = 179.59(7),
O1–Ti1–O2 = 101.39(8),
O1–Ti1–O3 = 102.10(8),
O1–Ti1–O4 = 102.04(8),
O2–Ti1–O3 = 114.56(8),
O2–Ti1–O4 =
115.41(8), O3–Ti1–O4 = 117.73(8), O2–Ti1–N = 78.86(7), O3–Ti1–
N = 78.06(7), O4–Ti1–N = 77.56(8).
To investigate the effect of the substituents in tetraden-
tate ligand on the polymerization behavior, the compounds
5–8 are examined as catalysts for the ring opening polymer-
ization of l-LA. Our results on the use of titanium alkoxide
catalysts 5–8 for the bulk and solution polymerization of l-
LA are summarized in Table 2 in terms of the activity of
the catalyst, yield, Tm, and molecular weight. The PLA
samples were analyzed by GPC and DSC.
Because compounds 5–8, regardless of the polymeriza-
tion condition, gave PLA with more than 90% yield within
10 h, they showed slightly higher catalytic activity than pre-
viously reported titanium alkoxides [6,11] but they still
exhibited lower activity than other metal alkoxides in the
ROP of LA [10]. In spite of no big differences in yield,
the catalytic efficiency decreases in the order of
5–6 > 7 > 8 in bulk polymerization and in the order of
5 > 6 > 7 > 8 in solution polymerization, indicating that
the steric effects of the methyl substituents in the tetraden-
tate ligands 1–4 are best optimized when no substituents
are present. Interestingly, no discrimination in activity
between bulk and solution polymerization for 5–8 was
found. Among 5–8, compound 5 gave a PLA with the high-
est molecular weight and also the increase of the methyl
substituents in tetradentate ligands exert a bad influence
on the molecular weight; however, the differences between
the bulk and solution conditions are not clear. So, the
molecular weights of all the resulting polymers obtained
Fig. 2. Molecular drawing of compound 7 and atom labeling. (H atoms
˚
were omitted for clarity) Selected bond distances (A): Ti1–O1 = 1.831(2),
Ti1–O2 = 1.837(2), Ti1–O3 = 1.826(2), Ti1–O4 = 1.841(2), Ti1–N =
2.301(2). Selected bond angle (ꢁ): C1–O1–Ti1 = 162.06(14), O1–Ti1–
N = 178.51(7),
O1–Ti1–O2 = 101.25(8),
O1–Ti1–O3 = 102.55(7),
O1–Ti1–O4 = 102.59(7),
O2–Ti1–O3 = 113.68(7),
O2–Ti1–O4 =
117.89(7), O3–Ti1–O4 = 115.54(7), O2–Ti1–N = 77.39(7), O3–Ti1–
N = 78.62(7), O4–Ti1–N = 77.63(7).
[avg = 78.16(7)ꢁ for 6 and 77.88(7)ꢁ for 7] and the obtuse
Oeq–Ti–Oax angles [avg = 101.84(8)ꢁ for 6 and 102.13(8)ꢁ
for 7] reflect a displacement of the titanium atom toward
the axial oxygen. Furthermore, the Nax–Ti–Oax angle
deviates from linearity by 0.41(7)ꢁ in 6 and by 1.49(7)ꢁ
in 7. This deviation in 6 falls near the short end of the

3524
S.-d. Mun et al. / Journal of Organometallic Chemistry 692 (2007) 3519–3525
Table 2
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk.
Ring opening polymerization of l-LA using 5–8
Entry
Catalyst
Yield (%)c Tm (ꢁC)d Men
Mwe
PDIe
1a
2b
3a
4b
5a
6b
7a
8b
a
5
95.8
95.1
96.2
93.8
93.8
91.0
90.3
89.6
166.8
167.3
161.1
163.9
161.2
163.0
161.0
164.5
10600 23800 2.25
14800 24100 1.63
8300 15300 1.84
7300 13000 1.80
6200 11700 1.90
6900 11900 1.72
6200 12000 1.93
7200 12200 1.69
6
7
8
Acknowledgements
This work was supported by the research grant of the
Chungbuk National University in 2005. The authors
acknowledge PAL for beam line use (Grant 2006-3041-10).
Bulk polymerization condition: Tp = 130 ꢁC, [LA]/[Ti] = 200, cata-
lysts = 0.05 mmol, time = 10 h.
b
References
Solution polymerization condition: Tp = 100 ꢁC, [LA]/[Ti] = 200, cat-
alysts = 0.05 mmol, time = 10 h, toluene 1 ml.
c
Isolated yield.
Determined by DSC.
Determined by GPC.
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