Synthesis and structural studies of heterobimetallic alkoxide complexes supported by bis(phenolate) ligands: Efficient catalysts for ring-opening polymerization of l-lactide

Article abstract of DOI:10.1021/ic901938e

A series of heteroblmetallic titanium(IV) complexes [LTi(O′Pr)(μ- O′Pr)₂Li(THF)₂], [LTi(O′Pr)(μ-O′Pr) ₂Na(THF)₂], [LTi(μ-O′Pr)₂Zn(O′ Pr)₂], and [LTi(μ-O′Pr)₂Mg(O′Pr) ₂] (where L = bidentate bisphenol ligands) have been synthesized and characterized including a structural determination of [L¹Ti(μ ₂-O′Pr)₂(O′Pr)Li(THF)₂] (1a). These complexes were investigated for their utility in the ring-opening polymerization (ROP) of L-lactide (LA). Polymerization activities have been shown to correlate with the electronic properties of the substituent within the bisphenol ligand. In contrast to monometallic titanium Initiator 1e, all the heterobimetallic titanium initiators (Ti-Li, Ti-Na, Ti-Zn, and Ti-Mg) show enhanced catalytic activity toward ring-opening polymerization (ROP) of L-LA. In addition, the use of electron-donating methoxy or methylphenylsulfonyl functional ligands reveals the highest activity. The bisphenol bimetallic complexes give rise to controlled ring-opening polymerization, as shown by the linear relationship between the percentage conversion and the number-average molecular weight. The polymerization kinetics using 2c as an initiator were also studied, and the experimental results indicate that the reaction rate is first-order with respect to both monomer and catalyst concentration with a polymerization rate constant, k = 81.64 M^-1 min^-1.

Full text of DOI:10.1021/ic901938e

Inorg. Chem. 2010, 49, 665–674 665
DOI: 10.1021/ic901938e
Synthesis and Structural Studies of Heterobimetallic Alkoxide Complexes
Supported by Bis(phenolate) Ligands: Efficient Catalysts for Ring-Opening
Polymerization of -Lactide
L
Hsuan-Ying Chen, Mei-Yu Liu, Alekha Kumar Sutar, and Chu-Chieh Lin*
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, Republic of China
Received October 2, 2009
A series of heterobimetallic titanium(IV) complexes [LTi(OiPr)(μ-OiPr)2Li(THF)2], [LTi(OiPr)(μ-OiPr)2Na(THF)2],
[LTi(μ-OiPr)2Zn(OiPr)2], and [LTi(μ-OiPr)2Mg(OiPr)2] (where L = bidentate bisphenol ligands) have been synthe-
sized and characterized including a structural determination of [L1Ti(μ2-OiPr)2(OiPr)Li(THF)2] (1a). These complexes
were investigated for their utility in the ring-opening polymerization (ROP) of L-lactide (LA). Polymerization activities
have been shown to correlate with the electronic properties of the substituent within the bisphenol ligand. In contrast to
monometallic titanium initiator 1e, all the heterobimetallic titanium initiators (Ti-Li, Ti-Na, Ti-Zn, and Ti-Mg) show
enhanced catalytic activity toward ring-opening polymerization (ROP) of L-LA. In addition, the use of electron-donating
methoxy or methylphenylsulfonyl functional ligands reveals the highest activity. The bisphenol bimetallic complexes
give rise to controlled ring-opening polymerization, as shown by the linear relationship between the percentage
conversion and the number-average molecular weight. The polymerization kinetics using 2c as an initiator were also
studied, and the experimental results indicate that the reaction rate is first-order with respect to both monomer and
catalyst concentration with a polymerization rate constant, k = 81.64 M-1 min-1
.
Introduction
Among the variety of initiators, a large number of metal
alkoxide complexes of aluminum,5 zinc,6 magnesium,7 iron,8
lanthanide,9 lithium,10 and titanium11 have been active as LA
polymerization catalysts affording polymers with controlled
molecular weights and narrow molecular weight distribution.
Despite the fact that several excellent initiators have been
As a biodegradable and biocompatible polymeric material
derived from annually biorenewable natural resources poly-
(L-lactide) (PLLA) has attracted considerable attention as a
promising alternative to synthetic petrochemical-based poly-
mers.1 Mechanical and physical properties of PLLAs, along
with their biodegradable and biocompatible nature, make
them prospective thermoplastics with broad applications
such as single-use packaging materials, medical sutures,
and drug delivery systems.2 Ring-opening polymerization
(ROP) of lactide, for example, by metal initiators, proved to
be the most efficient method for the preparation of PLAs
with controlled molecular weight and narrow molecular-
weight distribution.3,4 Over the past few decades, there has
been a particular emphasis on the synthesis of discrete, well-
defined metal complexes as active ring-opening polymeriza-
tion initiators.
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666 Inorganic Chemistry, Vol. 49, No. 2, 2010
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opening polymerization of
L-lactide in bulk phase with
narrow dispersities.16 Living ring-opening polymerization
of ε-caprotactone was achieved using well-defined titanium
alkoxide complexes.17
However, titanium alkoxides have low activity toward
ROP because of their high positive charges and strong π
bonding effect between Ti and alkoxides and consequently
decreases the rate of polymerization. To overcome this
barrier, Bochmann et al. developed heterobimetallic complex
of titanium-sodium supported by a multidentate amino-
bis(phenol) ligand (Scheme 1, B).11h This complex revealed a
higher reactivity for ROP of ε-captorlactone than the mon-
omeric complex A. The higher activity of bimetallic complex
B, which achieves almost complete conversion of 200 equiv of
CL over 2 h, is probably due to the coordination of sodium
metal reducing the electron density on alkoxo Ti-O bond
and therefore decreases π donor ability.
On the other hand, alkali metal alkoxide complexes have
shown excellent catalytic activity toward ROP of lactones
and lactide when the complexes are supported by sterically
demanding ligands. We reported Li,10e Na,18 Mg,7e,h and
Zn6e complexes supported by 2,20-ethylidene-bis(4,6-di-tert-
butylphenol) (EDBP-H2) ligands as shown in Scheme 2, and
all of them demonstrated excellent catalytic activity for the
ROP of L-lactide.
On the basis of the experiences in synthesis and ROP
activity of the alkali metal complexes, Mg, and Zn-alkoxide
complexes and the scope of development of the titanium
based initiators, we thus believed it would be interesting to
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Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 667
synthesize a series of titanium based heterobimetallic
complexes supported by sterically protected bis(phenolate)
ligands with a systematic variation of substitutents as alkyl,
halide, and sulfonate on the ligand and subsequently varying
the other metal with Li, Na, Mg, and Zn. Such well-defined
heterobimetallic species can then be examined for their ROP
the reported ligands, the data was in accord with the
literature reports. The H NMR signals for the corre-
1
sponding aromatic rings, as well as the tert-butyl and
methine protons of L2-H2 and L4-H2 appeared at δ 1.37
(18H, s, C(CH3)3), 1.16 (18H, s, C(CH3)3), 5.92 (1H, s,
CH) and (18H, s, C(CH3)3), 1.16 (18H, s, p-C(CH3)3), and
6.11 (1H, s, HCPh3), respectively, supporting the identical
chemical environments of these groups and structure of
the ligands.
activity of L-lactide, and the results can be analyzed in terms
of the electronics of the ligand and the effect of the four
different metals (Li, Na, Mg, and Zn) on the polymerization
activity.
Synthesis of L1-ligand supported magnesium(II), sodium-
(I), lithium(I), and zinc(II) complexes and their ROP activ-
ity have been reported in the literature.6e,7e,h,10e,18 Herein,
we extended the chemistry of L1 for the preparation of
monometallic titanium(IV) complex 1e following the
literature methods.11,13 Further, we attempted the syn-
thesis of the heterobimetallic titanium(IV) alkoxide
complexes using L1-L4-H2 ligands. The synthesis was
carried out in two steps: first, condensation of the pro-
ligand Ln-H2 with equivalent metal alkyl or hydride to
produce the monometallic complexes followed by the
treatment of the monometallic complexes with Ti(OiPr)4
in 1:1 ratio to produce heterobimetallic complexes with
reasonable yields. In the reaction of the bis(phenolate)
ligands with LinBu, NaH, MgnBu2, and ZnEt2 in THF
readily generated corresponding tetracoordinated com-
plexes [(L-H)Li(THF)3], [(L-H)Na(THF)3], [LMg]2, and
[LZn]2, respectively (Scheme 4). For all the bimetallic
complexes, pale yellow crystals were obtained from white
suspension in THF; recrystallization gives moderate to
good yields. These bimetallic complexes 1a-1d, 2c-2d,
3c, and 4c-4d were fully characterized by NMR spectral
and elemental analysis.
Results and Discussion
Synthesis of Bisphenol Supported Mixed Metal Com-
plexes. Bisphenol ligands Ln-H2 (n = 1-4) were synthe-
sized by the condensation of 2 equiv of the substituted
phenol with the appropriate aldehyde under reflux con-
ditions (Scheme 3).5d The ligands were obtained as white
crystalline powder and in good yield (>70%). They were
characterized by 1H NMR spectroscopy and in the case of
Scheme 3. Synthesis of Bisphenol Ligands L-H2 (1-4)
The 1H NMR resonances indicate that chemical envir-
onments of aromatic rings, as well as two bridging iso-
propoxide groups are identical in complexes 1a and 1b.
The chemical shifts corresponding to the methine and
methyl protons of the metal bound isopropoxide in
Scheme 4. Synthesis of Metal Complexes

668 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chen et al.
Table 1. Polymerization of
L
-Lactide in Toluene (15 mL) Using Complexes
1a-1d (0.05 mmol) as Initiators at 30 °C
[M]/ time conv
(h)
Mn
Mn
entry cat [cat]
(%)a (calcd)b Mn (obs)c (NMR)d PDI
1
2
3
4
5e
6
7
1a 150
1b 150
1c 100
1d 100
1d 100
1e 100
1e 100
94
94
0.5
3.5
1.5
74
80
91
89
94
27
76
5400
9100(5300) 5300 1.07
5800 10600(6100) 6000 1.05
6600 15200(8800) 6800 1.27
6500 10800(6300) 6900 1.28
6800 13600(7900) 5900 1.18
f
5500
11
94
f
f
9300(5400) 5900 1.08
f
a Obtained from 1H NMR analysis. b Calculated from the molecular
weight of LAꢁ[M]0/[OiPr-]0 ꢁ conversion yield þ Mw(iPrOH). c Ob-
tained from GPC analysis and calibrated by polystyrene standard.
Values in parentheses are the values obtained from GPC times 0.58.19
d Obtained from 1H NMR analysis. e The reaction temperature is 70 °C.
f Not available.
Figure 1. Molecular structure of EDBPTi(μ-OPri)3Li(THF)2 1a as 20%
ellipsoids (hydrogen atoms and tBu groups are omitted for clarity).
˚
Selected bond lengths (A): Ti-O(1) 1.876(3), Ti-O(2) 1.898(3), Ti-O(3)
are also involved in μ-oxo bridges between the lithium
and titanium atoms. The lithium is four-coordinate with
coordination of THF molecules and two bridging oxygen
atoms with distorted tetrahedral geometry. Two mean
bond lengths of titanium-oxygen (aryloxide) [Ti-O(1)
1.760(3), Ti-O(4) 1.932(3), Ti-O(5) 1.924(3), Ti-H(7A) 2.55(4), Li-O-
(4) 1.891(9), Li-O(5) 1.922(9), Li-O(6) 1.990(10), Li-O(7) 1.989(9).
Bond angles (deg): O(1)-Ti-O(2) 90.0. (2), O(1)-Ti-O(3) 99.7(2),
O(1)-Ti-O(5) 89.7(2), O(2)-Ti-O(3) 105.2(2), O(2)-Ti-O(4)
88.7(2), O(3)-Ti-O(4) 101.9(2), O(3)-Ti-O(5) 109.8(2), O(4)-Li-
O(5) 80.0(3), O(4)-Li-O(6) 116.5(5), O(4)-Li-O(7) 122.5(5),
O(5)-Li-O(6) 115.7(5), O(5)-Li-O(7) 120.5(5), O(6)-Li-O(7)
101.9(4), O(3)-Ti-H(7A) 166.8(10).
˚
and Ti-O(2) type bonds are 1.876(3) and 1.898(3) A] and
titanium-oxygen (bridging isopropoxide) [Ti-O(4) and
˚
Ti-O(5) type bonds are 1.932(3) and 1.924(3)A] in 1a are
complex 1a appeared at δ 5.11 and 1.28, respectively,
which is identical to that of 1b’s at δ 5.11 and 1.25 ppm,
respectively.
significantly longer than that of titanium-oxygen
(terminal isopropoxide) bond of 1.762(3) A. This differ-
˚
ence is due to significant π-bonding in Ti-O (terminal
isopropoxide), while in the case of bridging isopropoxide
groups, reduction in the extent of the π-bonding resulted
in longer Ti-O distances. This is corroborated by a much
wider Ti-O-C angle in 1a in the case of the terminal OPri
ligand [mean Ti-O(3)-C(32) is 159.7(5)° against Ti-O-
(4)-C(35) 128.7(3)° and Ti-O(5)-C(38) 130.9(3)°]. Si-
In contrast, complexes 1c and 1d containing two brid-
ging and two terminal iso-propoxide groups exhibit
different H NMR resonances from that of 1a and 1b.
1
Two sets of methine proton resonances appeared at δ 4.86
and 4.34 for 1c and δ 4.82 and 4.19 for 1d suggesting
inequivalent chemical environments for the bridging iso-
propoxides. Similarly, the methine signals of the bridging
isopropoxide appreared at δ 5.02 and 4.31 for 2c and δ
milar structural features were reported for
a
titanium-sodium bimetallic complex.11h Interestingly, a
closer inspection of the 3D structure of 1a reveals close
association of the hydrogen atom at C(7)of the EDBP
ligand in the Ti empty coordination site, making the
1
4.99 and 4.20 for 2d. For compound 3c, an unusual H
NMR pattern was recorded showing three sets of methine
signals of the coordinated isopropoxide at δ 4.93, 4.55,
and 4.25 ppm. More strangely, four separate methine
signals were observed at δ 5.14, 4.35, 4.03, 3.49 and δ 5.10,
4.25, 4.11, 3.41 for 4c and 4d, respectively, which is
probably due to the highly fluxional nature of the term-
inal and bridging isopropoxides. The 1H NMR data
analysis revealed that complexes 1c-2c, and 1d-2d,
contain two diastereotopic methine protons in each iso-
propoxide group and the twelve methyl protons
(-OCH(CH3)2) resonated at one signal whereas the other
twelve of the methyl protons resolved as two separate
multiplets of dq pattern indicating that the bulky ortho-
tert-butyl substituents restricted the free rotation of the
phenyl moiety on the NMR time scale.
Molecular Structure of Bimetallic Complex 1a. Single
crystals of the bimetallic titanium-lithium complex 1a
[L1Ti(μ2-OiPr)2(OiPr)Li(THF)2] suitable for X-ray dif-
fraction measurement were obtained by the slow cooling
of a hot hexane solution. The molecular structure of
1a and selected bond lengths and angles are shown in
Figure 1. The titanium atom bears a distorted octahedral
geometry having a terminal isopropoxide group lying on
the axial position and the other four oxygen atoms on the
basal position with the bond angles ranging from 88.7 to
109.8°. However, two oxygen atoms from isopropoxides
titanium acquire a distorted octahedral geometry in
˚
which the Ti
H(7A) distance is 2.550 A and the
3 3 3 3
C(7)-H(7A)-Ti angle is 132.2°. It could be argued that
the EDBP ligand is rocked in a direction that would
essentially increase the Ti H agostic type of interac-
tion. Such movement of hemilabile EDBP ligand could
accommodate an incoming monomer for coordination to
the Ti center.
3 3 3
Ring-Opening Polymerization of L-Lactide. Ring-open-
ing polymerization of L-lactide initiated by bimetallic
alkoxide complexes 1a-1e, 2c-2d, 3c, and 4c-4d were
carried out in toluene at a range of temperatures of 30-
70 °C, and the polymerization results are deposited in
Tables 1 and 2. It is evidenced that the heterobimetallic
complexes are stable in toluene within this temperature
range and remain intact during the course of polymeri-
zation process. All complexes displayed high activities in
ring-opening polymerization with narrow molecular
weight distribution and great control of molecular
weights. From the careful analysis of the polymerization,
runs performed in Tables 1 and 2, several structure-
activity trends can be drawn. Complexes 1a-1e as ini-
tiator in toluene at 30 °C (Table 1, entries 1 and 2) show no
difference in activity when replacing lithium 1a with

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 669
Table 2. Polymerization of
2c, 2d, 3c, 4c, 4d (0.05 mmol) as Initiators
L
-Lactide in Toluene (15 mL) Using Complexes 1c, 1d,
that two of these four iPrO- groups could be involved in
initiation. The linear increase in Mn with conversion and
the low polydispersity index (PDI, Mw/Mn) of the poly-
mers revealed controlled nature of polymerization
[M]/ temp time conv
entry ini [ini] (°C)
Mn
Mn
Mn
(h) (%)a (calcd)b (obs)c (NMR)d PDI
1
(Figure 2). The H NMR spectrum of PLLA (Figure 3)
1
2
3
4
5
6
7
8
1c
1c
1d
1d
1d
2c
2c
2c
2c
2d
2d
2d
2d
2d
3c
3c
4c
4d
4d
4d
100 30
100 30
100 70
100 30
100 50
50 30
100 30
150 30
200 30
100 30
50 50
100 50
150 50
200 50
100 30
100 30
100 30
100 30
100 50
100 50
0.25
0.5
1.5
3.5
1
0.5
0.5
0.5
0.5
1
1
1
1
1
0.25
0.5
0.25
1
0.5
1
27
91
94
89
56
89
94
94
95
13
85
92
91
91
5
indicated that the polymer chain is capped with one
isopropyl ester and one hydroxyl end due to the insertion
of an isopropyl alkoxy group into the lactide in PLLA.
The integration of the methine and methyl protons of the
isopropyl end-capped polyester reveals the molecular
weight Mn(NMR) which has good agreement with the
calculated molecular weights and GPC (Mn(obs)) analy-
sis. The complex 2d also polymerized L-lactide with
molecular weight control and narrow molecular weight
distribution, but the polymerization rate was slower than
that of 2c.
From entries 1, 5, 7, and 12 in Table 2, we found that the
Ti-Zn and Ti-Mg complexes supported by the L2-H2
ligand were more effective than L1-H2. When the methyl
was replaced with the methylphenylsulfonyl group, the
reactivity of complexes 4c or 4d increased due to the high
electron donor ability of the ligand (Table 2, entries 17
and 20). Summarized here from the results in Table 2, a
prominent decrease in catalytic activity is found in the
order 4c >2c > 1c > 3c for Ti-Zn complexes and 4d>
2d>1d for Ti-Mg complexes, indicating that electron-
donating substituents at the ortho-position are the ad-
vantage to the polymerization process.
The comparative ROP activity of the heterobimetallic
complexes of Ti with the main group metal initiators
shows the introduction of the Mg and zinc has enhanced
the rate of the reaction to a large extent. The results
showed Ti-Zn isopropoxide systems are quite compar-
able to the BDI (2-((2,6-diisopropylphenyl)amido)-
4-((2,6-diisopropylphenyl)-imino)-2-pentene) supported
Coates Zn-isopropoxide initiators in terms of rate and
conversion.20 The rate of the reaction of L-LA using
Ti-Mg and Ti-Zn complexes was slower than the highly
active homobimetallic Mg and Zn initiators supported by
Schiff base ligands.7h,21,6d,21,22 The ROP of the L-LA
using isopropoxy bridged heterobimetallic Ti-Li com-
plexes was very slow for a 70-80% conversion. However,
lithium forms dimeric complexes supported by the bis-
(phenolate) ligand which are more active for the ROP of
lactide.10e
6600
6800
6500
8800
7900
6300
6800 1.27
5900 1.18
6900 1.28
3300
6800
3600
7800
3700 1.13
7200 1.15
10200 11400 10500 1.16
13700 14800 14000 1.15
9
10
11
12
13
14
15
16
17
18
19
20
3100
6700
3900
8700
3300 1.23
6800 1.19
9900 12200 10600 1.15
13200 16200 13600 1.12
94
93
32
47
95
3400
6800
3800
7700
3700 1.17
7900 1.29
6900
5000
5100 1.29
a Obtained from 1H NMR analysis. b Calculated from the molecular
weight of LAꢁ[M]0/[OiPr-]0 ꢁ conversion yieldþMw(iPrOH). c Ob-
tained from GPC analysis and calibrated by polystyrene standard.19
d Obtained from 1H NMR analysis.
sodium 1b. In contrast, when the magnesium was intro-
duced in place of lithium or sodium (Table 1 entries 1
and 2) in 1d, the polymerization rate drastically enhanced
(Table 1, entry 4). Remarkably faster reaction was noted
when magnesium was replaced with zinc in complex 1c
(Table 1, entry 3). This is probably due to the difference in
electronic configurations and charge density of zinc and
magnesium, which controls the interactions and electron
transfer process in the formation of the metal-lactide
complexes. It is known that the charge density of magne-
sium is higher than that of zinc, resulting in a stronger
Mg-OR bond and therefore making it more difficult to
cleave the Mg-O bond causing decrease in the polymeri-
zation rate and resulting in longer reaction time.
Expected molecular weights (Mn) of the polymer ob-
tained from GPC analysis calibrated by polystyrene
standard for all the entries in Table 1. Similarly, molec-
ular weights Mn(NMR) of the polymers were obtained
1
from the H NMR data analysis by integrating methine
Kinetic Studies of Polymerization of L-Lactide by 2c.
Kinetic studies were performed to establish the reaction
order with respect to monomer and initiator for the
polymerization of L-lactide using 2c. Conversion of L-
lactide ([LA] = 0.167 M in 10 mL of toluene) with time
was monitored by 1H NMR for various concentrations of
2c ([2c] = 3.33, 2.57, and 2.09 mM, respectively) at 30 °C.
Plots of ln[LA]0-ln[LA] vs time for a wide range of [2c]
were linear as shown in Figure 4a, indicating the first-
order dependence on monomer concentration. Thus, the
rate expression can be written as-d[LA]/dt = kobs[LA]1 =
k[2c]x[LA]1, where kobs = k[2c]x and k = polymerization
rate constant. A plot of ln(kobs) vs ln[2c] (Figure 4b) allows
and methyl signals of the polymer chain end-capped with
an isopropyl ester group.19 The obtained Mn(NMR)
showed good agreement with the calculated Mn(calcd)
and Mn(obs). By analysis of the ROP results of the
bimetallic complexes 1a-1e in Table 1, we found that
Ti-Zn or Ti-Mg mixed metal complexes are more
effective in polymerization of L-LA in comparison to
the bimetallic Ti-Li, Ti-Na or monometallic Ti com-
plexes. Polymerization of L-lactide using complexes 2c,
2d, 3c, 4c, and 4d as an initiator was systematically
investigated in toluene (Table 2). In the case of the
complex 2c, the polymerization completed within 30
min at 30 °C. On the basis of the molecular weight of
poly(L-lactide) (PLLA) and [LA]0/[2c] ratio, we believed
(20) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229–3238.
(21) Wu, J.; Huang., B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-C. Polymer
2005, 46, 9784–9792.
(19) Mn(GPC) is multiplied by a factor of 0.58, giving the actual Mn of
poly(lactide). Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S.
Macromol. Rapid Commun. 1997, 18, 325–333.
(22) Hung, W.-C.; Lin, C.-C. Inorg. Chem. 2009, 48, 728–734.

670 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chen et al.
Figure 2. Plot of Mn(GPC) vs [LA]/[2c] and [LA]/[2d]. The values in parentheses are PDI values obtained from GPC analysis.
Figure 3. 1H NMR spectrum of PLLA-50 (50 indicates [LA]0/[2c]0 = 50).
the determination of k and x (the order in [2c] con-
centration). From the calculation of the y-intercept of
the regression line, polymerization rate constant k is
calculated to be 81.64 M-1 min-1. The slope (x) of the
fitted line calculated to be one nearest whole number and
thus the reaction is first order in catalyst 2c suggesting that
one molecule is involved in the transition state for the
propagation event.
Thus, the overall rate expression is -d[LA]/dt = k-
[LA]1[2c]1. Also from Figure 4, we found that the reaction
was at rest in initial 10 min, and then, polymerization
started quickly. This may be due to the formation of a new
active species by the mixing of catalyst with LA which
needs a certain time to develop, and the process can be
monitored by the 1H NMR studies. We recorded the 1H
NMR of the mixture of 2c and LA from 10 to 60 min at
regular intervals at 30 °C and a stack plot of the spectra is
shown in Figure 5. It is interesting to note that the rate law
-d[LA]/dt = k[LA]1[2c]1 is in good agreement with the
previously described Mg and Zn-dimeric initiators re-
ported by us.7d,e,8
A slow polymerization rate with long reaction time
(from 4 to 15 h) was reported for the well-defined titanium
alkoxide initiators.23 Less than 50% conversion of L-LA
to PLA using Ti(O-i-Pr)4, TiCl(O-i-Pr)3, TiCl2(O-i-Pr)2,
(23) Kim, Y; Janeshwara, G. K.; Verkade, G. Inorg. Chem. 2003, 42,
1437–1447.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 671
Figure 4. (a) First-order kinetic plots for L-lactidepolymerizations withtimeintoluene with differentconcentrationof[2c] as aninitiator. (b) Linearplotof
ln kobs vs ln[2c] with [LA]0 = 0.167 M in toluene.
Figure 5. 1H NMR spectrum of a mixture of LA and 2c in CDCl3 with time.
and TiCl3(O-i-Pr) as initiator requires 30 min at 130 °C. In
our systems, a dramatic increase in ROP reactivity is
observed when zinc or magnesium is introduced in bime-
tallic Ti-Zn and Ti-Mg (1c, 1d) at room temperature.
Comparatively, a slow conversion rate of L-LA to PLA
using bimetallic Ti-Li/Na complexes (1a, 1b) is recorded
which resembles the conversion rate for the bis-
(phenolate) ligand supported Li and Na alkoxides with
aggregates as initiators.10e,18,24 Coates and co-workers
previously reported remarkable reactivity of the well-
defined Zn(O-i-Pr) complexes for the ROP of lactide.19
Similarly, we observed the notable enhancement of the
rate of reaction on introduction of Zn in titanium based
heterobimetallic initiators. Similarly, Okuda and co-
workers have reported that yittrium-lithium hetero-
metallic complexes are efficient initiators for the ROP of
L-lactide and ε-caprolatone where a mechanism similar to
radical chain polymerization is tentatively assumed based
on the kinetic results.25
Conclusion
In conclusion, we have synthesized a series of bisphenol
ligands (L1-H2∼L4-H2) and their titanium-based heterobi-
metallic complexes. These complexes all show good activities
for the controlled ring-opening polymerization of L-lactide.
(25) (a) Hultzsch, K. C.; Okuda, J. Macromol. Rapid Commun. 1997, 18,
809–815. (b) Beckerle, K.; Hultzsch, K. C.; Okuda, J. Macromol. Chem. Phys.
1999, 200, 1702–1707.
(24) Huang, B.-H.; Ko, B.-T.; Athar, T.; Lin, C.-C. Inorg. Chem. 2006, 45,
7348–7356.

672 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chen et al.
Among this series of bimetallic complexes, Ti-Zn and
Ti-Mg systems have demonstrated high activities toward
the controlled polymerization of LA as compared with the
other titanium initiators reported in the literature. This is
probably a convenient synthetic strategy to increase the ROP
ability of titanium based complexes. Furthermore, complexes
with ligands bearing electron donating groups such as L2-H2
or L4-H2 show significantly higher activities than others
because these ligands stabilize higher oxidation states of
titanium. We have also found that the polymerization reac-
tion proceeds with first-order rate dependence on both the
monomer and initiator concentrations. Mechanistic studies
of polymerization in order to elucidate the role of the metals
are continued in our laboratories.
concentrated to 10 mL. White crystalline solids were obtained after
3 days at -20 °C. Yield: 4.69 g (70%). Anal calcd for C42H54O5S: C,
75.19; H, 8.11%. Found: C, 74.60; H, 7.90%. 1H NMR (CDCl3,
ppm): δ7.87 (2H, d, J = 7.6 Hz, o-SPh-H), 7.34 (2H, d, J= 8.0 Hz,
m-SPh-H), 7.28 (4H, br, HOPh-H þ 2,4-SOPh-H), 7.08 (1H, br, 3-
SOPh-H), 7.00 (1H, br, 1-SOPh-H), 6.50 (2H, s, HOPh-H), 6.11
(1H, s, HCPh3), 5.23 (2H, s, HOPh), 2.45 (3H, s, PhCH3), 1.44
(18H, s, o-C(CH3)3), 1.16 (18H, s, p-C(CH3)3). 13C NMR (CDCl3,
ppm): δ 150.36, 147.38, 145.64, 142.17, 136.64, 135.74, 132.51,
130.69, 129.89, 128.52, 128.43, 127.52, 126.23, 124.24, 122.83,
122.76 (Ph); 40.18 (CHPh3), 34.95 (p- PhC(CH3)3), 34.17 (o-PhC-
(CH3)3), 31.38 (o-PhC(CH3)3), 29.87 (p-PhC(CH3)3), 21.65
(PhCH3).
Synthesis of [L1Ti(μ2-OiPr)2(OiPr)Li(THF)2] (1a). nBuLi (4.4
mL, 2.5 M in hexane, 11.0 mmol) was added slowly to an ice cold
(0 °C) solution of L1-H2 (4.386 g, 10.0 mmol) in THF (40 mL).
The mixture was stirred for 3 h, and then titanium(IV) isoprop-
oxide (2.84 g, 10.00 mmol) was added and stirred for 12 h at
room temperature. Volatile materials were removed under
vacuum to yield a red oil. The oil was washed with hexane
(30 mL), and a light yellow powder was obtained after filtration.
Yield: 4.69 g (58%). Anal calcd for C47H81LiO7Ti: C, 69.44; H,
10.04%. Found: C, 69.81; H, 10.06%. 1H NMR (C6D6, ppm):
δ 7.48, 7.30 (4H, s, Ph-H), 5.11 (2H, m, J = 6 Hz, OCH(CH3)2),
4.72 (1H, q, J = 6.8 Hz, PhCH(CH3)Ph), 3.37 (8H, t, J = 4.8
Hz, O(CH2CH2)2), 1.98 (3H, d, J = 6.8 Hz, PhCH(CH3)Ph),
1.72, (18H, s, PhC(CH3)3), 1.38 (26H, br, O(CH2CH2)2 þ PhC-
(CH3)3), 1.28 (12H, d, J = 6 Hz, OCH(CH3)2). 13C NMR
(C6D6, ppm): δ 161.15, 137.55, 137.07, 121.38, 120.46, 120.24,
(Ph), 73.23 (OCH(CH3)2), 67.99 (O(CH2CH2)2), 36.41
(PhCH(CH3)Ph), 35.64 (p-PhC(CH3)3), 34.53 (o-PhC(CH3)3),
32.07 (OCH(CH3)2), 30.60 (O(CH2CH2)2), 27.27 (p-PhC-
(CH3)3), 26.60 (PhCH(CH3)Ph), 25.48 (o-PhC(CH3)3). Mp:
173-174 °C.
Experimental Section
General Conditions. All manipulations were carried out under
a dry nitrogen atmosphere. Solvents, L-lactide, and deuterated
solvents were purified before used. 2, 20-Ethylidenebis (4,6-di-
tert-butylphenol), 2,4-di-tert-butylphenol, 2-methoxybenzalde-
hyde, 4-chloro-2-isopropyl-5-methylphenol, formaldehyde, 2-hy-
droxybenzaldehyde, and 4-methylbenzene-1-sulfonyl chloride
1
were purchased and used without further purification. H and
13C NMR spectra were recorded on a Varian Mercury-400
spectrometer with chemical shifts given in ppm from the internal
TMS or center line of CDCl3. Microanalyses were performed
using a HeraeusCHN-O-RAPID instrument. The GPC measure-
ments were performed on a Postnova PN1122 solvent delivery
system with TriSEC GPC software using THF (HPLC grade) as
an eluent. Molecular weight and molecular weight distributions
were calculated using polystyrene as the standard. 2, 20-Methyl-
enebis (4-chloro-6-isopropyl-3-methylphenol) (L3-H2)5d were
prepared by acid-catalyzed condensation following literature
procedures.
Synthesis of [L1Ti(μ2-OiPr)2(OiPr)Na(THF)2] (1b). A mix-
ture of L1-H2 (4.386 g, 10.0 mmol) and NaH (5.00 g, 60%, 11.0
mmol) was stirred in a mixed solvent of THF (40 mL) at room
temperature for 3 h. Residual NaH was removed by filtration
and then titanium(IV) isopropoxide (2.84 g, 10.00 mmol) was
added to be stirred for 12 h at room temperature. Volatile
materials were removed under vacuum to yield red solid. The
solid was washed with hexane (30 mL) and a yellow powder was
obtained after filtration. Yield: 2.50 g (30%). Anal calcd for
C47H81NaO7Ti: C, 68.09; H, 9.85%. Found: C, 68.92; H, 9.62%.
1H NMR (C6D6, ppm): δ 7.46, 7.28 (4H, s, Ph-H), 5.11 (3H, q,
J = 6Hz, OCH(CH3)2), 4.74 (1H, q, J =6.8 Hz, PhCH(CH3)Ph),
3.38 (8H, t, J = 5.2 Hz, O(CH2CH2)2), 1.97 (3H, d, J = 6.8 Hz,
PhCH(CH3)Ph), 1.72, (18H, s, PhC(CH3)3), 1.38 (18H, s, PhC-
(CH3)3), 1.30 (8H, m, J = 5.2 Hz, O(CH2CH2)2), 1.25 (12H, d,
J =6 Hz, OCH(CH3)2). 13CNMR(C6D6, ppm):δ161.17, 138.62,
137.04, 121.348, 120.43, 120.22, (Ph), 73.24 (OCH(CH3)2), 68.02
(O(CH2CH2)2), 35.80 (PhCH(CH3)Ph), 35.63 (p-PhC(CH3)3),
34.53 (o-PhCCH3), 32.08 (OCH(CH3)2), 30.80 (O(CH2CH2)2),
27.25 (p-PhC(CH3)3), 26.61 (PhCH(CH3)Ph), 25.48 (o-PhC-
(CH3)3). Mp: 178-179 °C.
Synthesis of 6,60-((2-Methoxyphenyl)methylene)bis(2,4-di-
tert-butylphenol) (L2-H2). A mixture of 2,4-di-tert-butylphenol
(4.12 g, 20 mmol), o-anisaldehyde (1.45 mL, 10 mmol), and
benzenesulfonic acid (0.40 mL) in hexane (50 mL) was refluxed
for 30 h. After neutralization with aqueous NaOH solution (0.1
N, 20 mL), the organic layer was extracted with water (30 mL)
twice. The hexane layer was then dried over MgSO4 and con-
centrated to 15 mL. White crystalline solids were obtained after
2 days at-20 °C. Yield: 3.97 g (75%). Anal calcd for C36H50O3:C,
81.46; H, 9.49%. Found: C, 81.16; H, 9.74%. 1H NMR (CDCl3,
ppm): δ 7.31-6.71 (8H, m, Ph-H), 5.92 (1H, s, CH), 4.95 (2H,
br, OH), 3.80 (3H, s, OCH3), 1.37 (18H, s, C(CH3)3), 1.16 (18H, s,
C(CH3)3). 13C NMR (CDCl3, ppm): δ 156.78, 150.70, 142.28,
136.50, 130.46, 128.65, 128.63, 126.96, 123.96, 122.80, 121.19,
110.92 (Ph), 55.75 (OCH3), 39.66, 31.52 (C(CH3)3), 34.98, 34.29
(C(CH3)3), 29.84 (CH). Mp: 152-154 °C.
Synthesis of 2-(Bis(3,5-di-tert-butyl-2-hydroxyphenyl)methyl)-
phenyl-4-methyl Benzenesulfonate (L4-H2). Triethylamine (40 mL)
was added to a rapidly stirred solution of 2-hydroxybenzaldehyde
(12.20 g, 100 mmol) in 40 mL of CH2Cl2. The mixture was
cooled to 0 °C, and a solution of 4-methylbenzene sulfonyl chloride
(19.0 g, 100 mmol) in 20 mL of CH2Cl2 was added dropwise over
20 min. After the addition was complete, the mixture was allowed
to warm to room temperature and stirred overnight. The resulting
solution was washed with water (60 mL) three times, and the
solvent was removed at reduced pressure. 2-Formylphenyl-
4-methylbenzenesulfonate was isolated in 95% yield (26.22 g,
95 mmol). Then, a mixture of 2,4-di-tert-butylphenol (4.12 g,
20 mmol), 2-formylphenyl-4-methylbenzenesulfonate (2.76 g,
10 mmol), and benzenesulfonic acid (0.40 mL) in toluene (30 mL)
was refluxed for 12 h. After neutralization with aqueous NaOH
solution (0.1 N, 20 mL), the organic layer was extracted with water
(30 mL) twice. The toluene layer was then dried over MgSO4 and
Synthesis of [L1Ti(μ2-OiPr)2Zn(OiPr)2] (1c). A diethylzinc
solution (5.5 mL, 1.0 M in hexane, 5.50 mmol) was added slowly
to an ice cold solution (0 °C) of L1-H2 (2.193 g, 5.00 mmol) in
THF (40 mL). The mixture was stirred for 3 h at room
temperature and then titanium(IV) isopropoxide (1.42 g, 5.00
mmol) was added and stirred for 12 h at room temperature.
Volatile materials were removed under vacuum to yield an
orange red solid. The solid was washed with hexane (40 mL),
and a yellow powder was obtained after filtration. Yield:
2.59 g (66%). Anal calcd for C42H72O6TiZn: C, 64.16; H,
1
9.23%. Found: C, 64.26; H, 8.84%. H NMR (CDCl3, ppm):
δ 7.28-7.01 (4H, m, Ph-H), 4.90 (1H, q, J = 6 Hz, PhCH-
(CH3)Ph), 4.86 (2H, m, J = 6 Hz, OCH(CH3)2), 4.34 (2H, m,
J = 6 Hz, OCH’(CH3)2), 1.62 (3H, d, J = 6 Hz, PhCH(CH3)Ph),

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 673
1.41, 1.27 (36H, s, PhC(CH3)3), 1.44 (6H, d, J = 6 Hz, OCH-
(CH3)2), 1.29 (12H, d, J = 6 Hz, OCH(CH3)2), 0.76(6H,
d, J = 6 Hz, OCH(CH3)2). 13C NMR (CDCl3, ppm): δ 160.55,
141.59, 136.19, 133.89, 120.82, 120.27 (Ph), 79.84, 71.85 (OCH-
(CH3)2), 66.74 (PhCH(CH3)Ph), 35.26, 34.43 (PhC(CH3)3),
31.66, 30.52 (PhC(CH3)3), 29.31 (PhCH(CH3)Ph), 27.18, 25.33,
21.56 (OCH(CH3)2). Mp: 170-172 °C.
PhCH(CH3)2), 1.26 (6H, d, J = 6 Hz, PhCH(CH3)2), 1.16 (12H,
J = 6.4 Hz, OCH(CH3)2), 1.15 (12H, d, J = 6.8 Hz, OCH-
(CH3)2). 13C NMR (CDCl3, ppm): δ 161.67, 134.34, 131.59,
130.93, 125.50, 123.63 (Ph), 71.30, 66.82 (OCH(CH3)2), 28.78,
28.12 (OCH(CH3)2), 26.86, 26.76 (PhCH(CH3)2), 25.71
(PhCH2Ph), 23.15, 22.90 (PhCH(CH3)2), 17.34 (PhCH3). Mp:
204-206 °C.
Synthesis of [L4Ti(μ2-OiPr)2Zn(OiPr)2] (4c). The procedures
are similar to that for 1c with L4-H2 (6.700 g, 10.00 mmol) being
used. Yield: 8.12 g (80%). Anal calcd for C54H80O9STiZn: C,
63.68; H, 7.92%. Found: C, 62.39; H, 7.15%. 1H NMR (CDCl3,
ppm): δ 7.52 (2H, d, J = 7.2 Hz, o-SPh-H), 7.26-6.92 (10H, m,
Ph-H), 5.14 (1H, m, OCH(CH3)2), 5.08 (1H, br, HCPh3), 4.35
(1H, m, OCH(CH3)2), 4.03 (1H, m, OCH(CH3)2), 3.49 (1H, m,
OCH(CH3)2), 2.37 (3H, s, PhCH3), 1.47 (18H, s, o-C(CH3)3), 1.38
(12H, d, J = 3.2, OCH(CH3)2), 1.20 (18H, s, p-C(CH3)3), 0.86
(6H, br, OCH(CH3)2), 0.30 (6H, d, J = 6.0, OCH(CH3)2).
13C NMR (CDCl3, ppm): δ 162.53, 149.90, 143.82, 141.07,
135.62, 134.25, 134.05, 130.75, 130.31, 129.45, 127.99, 126.65,
124.67, 123.95, 120.25, 117.29 (Ph), 78.83, 72.79, 66.78 (OCH-
(CH3)2), 36.31 (CHPh3), 35.37 (p- PhC(CH3)3), 34.26 (o-PhC-
(CH3)3), 31.59 (o-PhC(CH3)3), 30.93 (p-PhC(CH3)3), 29.51, 27.22,
26.99, 24.64, 22.63, 21.49, 14.07 (OCH(CH3)2). Mp: 220-221 °C.
Synthesis of [L4Ti(μ2-OiPr)2Mg(OiPr)2] (4d). The procedures
are similar to that for 1d with L4-H2 (6.700 g, 10.00 mmol) being
used. Yield: 2.83 g (29%). 1H NMR (CDCl3, ppm): δ 7.52 (2H,
br, o-SPh-H), 7.25-6.94 (10H, m, Ph-H), 5.10 (1H, br, OCH-
(CH3)2), 5.07 (1H, br, HCPh3), 4.25 (1H, br, OCH(CH3)2), 4.11
(1H, br, OCH(CH3)2), 3.41 (1H, br, OCH(CH3)2), 2.34 (3H, s,
PhCH3), 1.45 (24H, s, o-C(CH3)3), 1.42 (6H, br, OCH(CH3)2),
1.28 (6H, br, OCH(CH3)2), 1.22 (18H, s, p-C(CH3)3), 0.89 (6H,
br, OCH(CH3)2), 0.31 (6H, br, OCH(CH3)2). 13C NMR (CDCl3,
ppm): δ 162.66, 149.90, 143.79, 141.05, 135.62, 134.32, 133.98,
130.74, 130.25, 129.43, 127.97, 126.62, 124.67, 123.92, 120.25,
117.24 (Ph), 78.65, 71.67, 63.50, 63.28 (OCH(CH3)2), 35.34 (p-
PhC(CH3)3), 34.24 (o-PhC(CH3)3), 31.58 (o-PhC(CH3)3), 30.94
(p-PhC(CH3)3), 30.45 (CHPh3) 29.69, 29.02, 27.20, 26.99, 24.64,
22.62, 21.47, 14.05 (OCH(CH3)2). Mp: 220-221 °C.
Synthesis of [L1Ti(μ2-OiPr)2Mg(OiPr)2] (1d). To an ice cold
solution (0 °C) of L1-H2 (2.193 g, 5.0 mmol) in THF (40 mL) was
slowly added a MgnBu2 (5.50 mL, 1.0 M in heptane, 5.50 mmol)
solution. The mixture was stirred for 3 h at room temperature
and then titanium(IV) isopropoxide (1.42 g, 5.00 mmol) was
added and stirred for 12 h at room temperature. Volatile
materials were removed under vacuum to yield an orange yellow
solid. The solid was washed with hexane (40 mL), and an orange
powder was obtained after filtration. Yield: 2.01 g (54%). Anal
calcd for C42H72O6TiMg: C, 67.69; H, 9.74%. Found: C, 67.63;
H, 9.64%. 1H NMR (CDCl3, ppm): δ 7.34-7.04 (4H, m, Ph-H),
4.96 (1H, q, J = 6 Hz, PhCH(CH3)Ph), 4.82 (2H, m, J = 6 Hz,
OCH(CH3)2), 4.19 (2H, m, J = 6 Hz, OCH’(CH3)2), 1.62 (3H, d,
J = 6 Hz, PhCH(CH3)Ph), 1.41, 128 (36H, s, PhC(CH3)3), 1.38
(6H, d, J = 6 Hz, OCH(CH3)2), 1.31 (12H, d, J = 6 Hz, OCH-
(CH3)2), 0.63(6H, d, J = 6 Hz, OCH(CH3)2). 13C NMR (CDCl3,
ppm):δ 160.80, 141.66, 135.55, 133.73, 120.97, 120.27 (Ph), 79.57,
70.94 (OCH(CH3)2), 63.51 (PhCH(CH3)Ph), 35.26, 34.43 (PhC-
(CH3)3), 31.66, 30.56 (PhC(CH3)3), 29.59 (PhCH(CH3)Ph),
27.16, 25.13, 21.94 (OCH(CH3)2). Mp: 182-184 °C.
Synthesis of [L1Ti(OiPr)2] (1e). 1e were prepared by the
reaction of L1-H2 and titanium(IV) isopropoxide following
literature procedures.6a
Synthesis of [L2Ti(μ2-OiPr)2Zn(OiPr)2] (2c). The procedures
are similar to that for 1c with L2-H2 (2.654 g, 5.00 mmol) being
used. Yield: 2.39 g (54%). Anal calcd for C48H76O7TiZn: C,
65.63; H, 8.72%. Found: C, 65.60; H, 8.72%. 1H NMR (CDCl3,
ppm): δ 7.50-6.72 (8H, m, Ph-H), 6.74 (1H, s, PhCHPh), 5.02
(2H, m, J = 6 Hz, OCH(CH3)2), 4.31 (1H, m, J = 6 Hz, OCH-
(CH3)2), 3.57 (1H, m, J = 6 Hz, OCH(CH3)2), 3.34 (3H, s,
OCH3), 1.41, 1.22 (36H, s, PhC(CH3)3), 1.46 (6H, d, J = 6 Hz,
OCH(CH3)2), 1.34 (12H, d, J = 6 Hz, OCH(CH3)2), 0.33 (6H, d,
J = 6 Hz, OCH(CH3)2). 13C NMR (CDCl3, ppm): δ 162.19,
158.30, 140.94, 134.59, 133.06, 131.52, 128.81, 126.48, 124.48,
119.49, 119.29, 111.44 (Ph), 78.87, 72.36, 66.71 (OCH(CH3)2),
55.12 (OCH3), 35.25 (PhCHPh), 36.01, 34.25 (PhC(CH3)3),
31.63, 30.74 (PhC(CH3)3), 29.49, 27.13, 24.69 (OCH(CH3)2).
Mp: 214-216 °C.
Typical Polymerization Procedure. A typical polymerization
procedure was exemplified by the synthesis of PLA-100 using 2c
as an initiator at 30 °C. The conversion yield (95%) of PLA was
1
analyzed by H NMR spectroscopic studies. A mixture of 2c
(0.0439 g, 0.05 mmol) and L-lactide (1.44 g, 10 mmol) in toluene
(15 mL) was stirred at 30 °C for 30 min. Volatile materials
were removed in vacuo, and the residue was redissoved in THF
(10 mL). The mixture was then quenched by the addition of an
aqueous acetic acid solution (0.35 N, 10 mL), and the polymer
was precipitated on pouring into n-hexane (40 mL) to give
a white crystalline solid. Yield: 1.04 g (72%).
Synthesis of [L2Ti(μ2-OiPr)2Mg(OiPr)2] (2d). The procedures
are similar to that for 1d with L2-H2 (2.654 g, 5.00 mmol) being
used. Yield: 2.18 g (52%). Anal calcd for C48H76O7TiMg:
C, 68.86; H, 9.15%. Found: C, 68.38; H, 8.73%. 1H NMR
(CDCl3, ppm): δ 7.50-6.72 (8H, m, Ph-H), 6.77 (1H, s,
PhCHPh), 4.99 (2H, m, J = 6 Hz, OCH(CH3)2), 4.20 (1H, m,
J = 6 Hz, OCH0(CH3)2), 3.47 (1H, m, J = 6 Hz, OCH00(CH3)2),
3.34 (3H, s, OCH3), 1.41, 1.22 (36H, s, PhC(CH3)3), 1.37 (6H, d,
J = 6 Hz, OCH(CH3)2), 1.35 (12H, d, J = 6 Hz, OCH(CH3)2),
0.31(6H, d, J = 6 Hz, OCH(CH3)2). 13C NMR (CDCl3, ppm):
δ 162.40, 158.37, 140.99, 134.71, 133.04, 131.52, 128.84,
126.51, 124.52, 119.53, 119.32, 115.53 (Ph), 78.76, 71.38, 63.49
(OCH(CH3)2), 55.11 (OCH3), 35.26 (PhCHPh), 36.12, 34.27
(PhC(CH3)3), 31.64, 30.81 (PhC(CH3)3), 29.71, 27.17, 24.72
(OCH(CH3)2). Mp: 214-216 °C.
X-ray Crystallographic Studies. Suitable crystals for X-ray
structural determination were sealed in thin-walled glass capil-
laries under a nitrogen atmosphere and mounted on a Bruker
AXS SMART 1000 diffractometer. Intensity data were col-
lected in 1350 frames with increasing w (width of 0.3° per frame).
The absorption correction was based on the symmetry equiva-
lent reflections using the SADABS program.26 The space group
determination was based on a check of the Laue symmetry and
systematic absences and was confirmed using the structure
solution. The structure was solved by direct methods using a
SHELXTL package.27 All non-H atoms were located from
successive Fourier maps, and hydrogen atoms were refined
using a riding model. Anisotropic thermal parameters were used
for all non-H atoms, and fixed isotropic parameters were used
Synthesis of [L3Ti(μ2-OiPr)2Zn(OiPr)2] (3c). The procedures
are similar to that for 1c with L3-H2 (3.814 g, 10.00 mmol) being
used. Yield: 5.15 g (71%). Anal calcd for C33H52Cl2O6TiZn: C,
54.37; H, 7.19%. Found: C, 54.21; H, 7.19%. 1H NMR (CDCl3,
ppm): δ 6.99 (2H, s, Ph-H), 4.93 (1H, m, J = 6.8 Hz, OCH-
(CH3)2), 4.55 (2H, m, J = 6.4 Hz, OCH(CH3)2), 4.25 (1H, m,
J = 6.8 Hz, OCH(CH3)2), 4.17 (1H, d, J = 15 Hz, PhCHH’Ph),
3.70 (1H, d, J = 15 Hz, PhCHH’Ph), 3.41 (2H, q, J = 6 Hz,
PhCH(CH3)2), 2.33 (6H, s, PhCH3), 1.31 (6H, d, J = 6 Hz,
(26) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,
Germany, 1998.
(27) Sheldrick, G., Sheldrick, G. M., SHELXTL-Plus, NT Crystallo-
graphic System, release 5.1; Bruker Analytical X-ray Systems: Madison, WI,
1998.

674 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chen et al.
for H atoms. Due to the disorder of THF and the t-Bu group of
the EDBP ligand, the bond lengths within THF and the t-Bu
groups are not reliable.
are greatly acknowledged. Helpful comments from the reviewers
are also appreciated.
Supporting Information Available: Summary of X-ray crys-
tallography data and crystallographic information file. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. Financial support from the National
Science Council (NSC95-2113-M-005-016-MY3) and Minister
of Economic Affair (97-EC-17-A-S1-111) of Republic of China