Synthesis, structural characterization and reactivity of aluminium complexes supported by benzotriazole phenoxide ligands: Air-stable alumoxane as an efficient catalyst for ring-opening polymerization of l-lactide

Article abstract of DOI:10.1039/c0dt01108h

Aluminium complexes bearing sterically bulky benzotriazole-phenoxide ligands are synthesized and characterized structurally. The reaction of 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol ( ^CMe2PhBTP-H) or 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol (^t-BuBTP-H) with AlMe₃ (1.2 molar equiv.) in toluene yields [(^CMe2PhBTP)AlMe₂] (1) or [(^t-BuBTP) AlMe₂] (2) as a four-coordinated monomeric aluminium complex. Compound 1 reacts further with ^CMe2PhBTP-H in a stoichiometric proportion, affording penta-coordinated monomeric aluminium methyl complex [(^CMe2PhBTP)₂AlMe] (3). Complex 3 is also obtained directly upon treatment of AlMe₃ with ^CMe2PhBTP-H (two molar equiv.) in refluxing toluene in high yield. In the presence of H ₂O (half a molar equiv.), hydrolysis of 3 in a mixed solvent of THF and toluene at ambient temperature affords [{(^CMe2PhBTP) ₂Al}₂(μ-O)] (4), in which the oxo ligand acts as a chelating group linearly bridging two aluminium centers. Air-stable alumoxane 4 is an efficient catalyst for the ring-opening polymerization of l-lactide (l-LA) in the presence of 9-anthracenemethanol (9-AnOH). Complex 4 catalyzes the polymerization of l-LA in a controlled manner, yielding PLLAs with the expected molecular weights and narrow polydispersity indices (PDIs). The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt01108h

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PAPER
Synthesis, structural characterization and reactivity of aluminium complexes
supported by benzotriazole phenoxide ligands: air-stable alumoxane as an
efficient catalyst for ring-opening polymerization of L-lactide†
Chen-Yu Li, Chen-Yen Tsai, Chia-Her Lin and Bao-Tsan Ko*
Received 26th August 2010, Accepted 6th December 2010
DOI: 10.1039/c0dt01108h
Aluminium complexes bearing sterically bulky benzotriazole-phenoxide ligands are synthesized and
characterized structurally. The reaction of 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-
phenylethyl)phenol (CMe2PhBTP-H) or 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol (t-BuBTP-H) with
AlMe3 (1.2 molar equiv.) in toluene yields [(CMe2PhBTP)AlMe2] (1) or [(t-BuBTP)AlMe2] (2) as a
four-coordinated monomeric aluminium complex. Compound 1 reacts further with CMe2PhBTP-H in a
stoichiometric proportion, affording penta-coordinated monomeric aluminium methyl complex
[(CMe2PhBTP)2AlMe] (3). Complex 3 is also obtained directly upon treatment of AlMe3 with CMe2PhBTP-H
(two molar equiv.) in refluxing toluene in high yield. In the presence of H2O (half a molar equiv.),
hydrolysis of 3 in a mixed solvent of THF and toluene at ambient temperature affords
[{(CMe2PhBTP)2Al}2(m-O)] (4), in which the oxo ligand acts as a chelating group linearly bridging two
aluminium centers. Air-stable alumoxane 4 is an efficient catalyst for the ring-opening polymerization
of L-lactide (L-LA) in the presence of 9-anthracenemethanol (9-AnOH). Complex 4 catalyzes the
polymerization of L-LA in a controlled manner, yielding PLLAs with the expected molecular weights
and narrow polydispersity indices (PDIs).
reaction with an ammonia–toluene two-phase liquid system or
hydrogen elimination.
Introduction
The structural characterization and investigation of the catalytic
Biodegradable polymers such as poly(e-caprolactone) (PCL),
activity of alumoxanes were stimulated by the discovery of
poly(lactide) (PLA) and their copolymers attract interest due to
methylalumoxane (MAO) as an excellent cocatalyst for olefin poly-
their wide use in the biomedical, pharmaceutical and environ-
merization by group 4 metallocenes.1 Alumoxanes are well-known
to have the general formula [RAlO]n or [R2AlOAlR2]n, which can
be synthesized by the controlled hydrolysis of organoaluminium
compounds using water, or H2O contained in hydrated metal
salts.2 Alumoxanes readily undergo association to form oligomeric
structures,3 resulting in difficulty of preparing molecular alu-
moxanes with low degrees of association in a pure, crystalline
product. To avoid the oligomerization of hydrolytic aluminium
methyl compounds, several attempts to synthesize discrete molec-
ular organoalumoxanes on introducing bulky bidentate chelated
groups instead of methyl groups have been demonstrated.4 For
instance, aluminium complex [LAlX2] (L = HC[(CMe)N(2,6-
mental fields.6 The major chemical synthetic route to PCL and
PLA is the ring-opening polymerization (ROP) of cyclic esters
using discrete complexes7 of aluminium,8 calcium,9 magnesium,10
rare-earth metals11 and zinc derivatives.12 These metal complexes
with a single active site supported by various ancillary ligands
such as b-diketiminate, Schiff base, amino-bis(phenolate) and
many others have been demonstrated to achieve great activity
with molecular weight in a controlled manner. Although most
have been reported to be effective initiators or catalysts for the
preparation of polyesters with controlled molecular weights and
narrow polydispersity indices (PDIs), air-stable metal complexes
with well defined structures that catalyze ROP efficiently are rare.
To achieve these criteria, alumoxanes seem to be prospective
candidates, and only a few studies of their reactivity toward ROP
of cyclic esters have been reported.13
iPr2C6H3)]2- or HC[(CMe)N(2,4,6-Me3C6H2)]2 ; X = halide or H)5
-
incorporating a sterically hampered b-diketiminate ligand, L, is
readily transformed into the alumoxane hydroxide either upon
During our tests with sterically bulky ligands of modi-
fied aluminium methyl [(CMe2PhBTP)2AlMe] (CMe2PhBTP = 2-(2H-
benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)-phenolate), we
observed that this compound reacts readily with water, forming
stable Al–O–Al moieties under mild conditions. Our current
interest lies in the preparation and characterization of alumoxanes
Department of Chemistry, Chung Yuan Christian University, Chung-
Li, 32023, Taiwan. E-mail: btko@cycu.edu.tw; Fax: +886-3-2653399;
Tel: +886-3-2653327
† CCDC reference numbers 790451–790453. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt01108h
1880 | Dalton Trans., 2011, 40, 1880–1887
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Scheme 1 Synthetic routes to complexes (1)–(4).
incorporating sterically bulky BTP and applying these complexes
as catalysts for the ROP of lactones and lactides. We report
here the synthesis, characterization and catalytic studies of novel
aluminium derivatives [(CMe2PhBTP)AlMe2] (1), [(t-BuBTP)AlMe2]
(2), [(CMe2PhBTP)2AlMe] (3), and [{(CMe2PhBTP)2Al}2(m-O)] (4).
for 2 and -0.81 ppm for 3. Complex 3 reacts smoothly with
H2O (half a molar equiv.) in the mixed solvent THF–toluene
at ambient temperature to afford [{(CMe2PhBTP)2Al}2(m-O)] (4).
Compound 4 is stable for over one year in the solid state when
placed in air and is soluble in common organic solvents (toluene,
CH2Cl2, THF) but insoluble in hexane and pentane. All these
compounds were isolated as pale yellow crystalline solids and
characterized by spectra as well as microanalyses. Compound 2–
4 are further confirmed by X-ray single crystal measurements.
Although attempts to obtain the structure of a single crystal of
compound 1 were unsuccessful, its structure in the solid state was
verified with compound 2 through their isostructural relation.
Suitable crystals for X-ray structural determination of com-
plexes 2 and 3 were obtained from their saturated toluene
solutions. Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagrams
displaying selected bond lengths and angles of the molecular
structures of 2 and 3 are presented in Fig. 1 and 2, respectively. The
structure of 2 is monomeric, coordinated by the N, O-bidentate
ligand in an unsymmetrical fashion; the geometry around Al
is distorted from tetrahedral with bond distances Al–O(1) =
(
t-BuBTP = 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenolate)
Results and discussion
Syntheses and crystal structure
In terms of coordination chemistry, the benzotriazole-phenolate
group can provide N,O-bidentate chelation to stabilize transition-
metal or main-group metal complexes, especially with sterically
bulky substitutes at the 6-position of the phenol group. The
monomeric aluminium compound [(CMe2PhBTP)AlMe2] (1) is syn-
thesized (yield 84%) through the reaction of 2-(2H-benzotriazol-
2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (CMe2PhBTP-H) with
AlMe3 (1.2 molar equiv.), as shown in Scheme 1. [(t-BuBTP)AlMe2]
(2) was prepared by employing 2-(2H-benzotriazol-2-yl)-4,6-di-
tert-butylphenol (t-BuBTP-H) as the ligand according to the same
synthetic route. Compound 1 reacts further with CMe2PhBTP-
H in a stoichiometric proportion to yield penta-coordinated
monomeric aluminium complex [(CMe2PhBTP)2AlMe] (3). Complex
3 was also prepared directly in 88% yield upon reaction of AlMe3
with CMe2PhBTP-H (two mol equiv.) under refluxing toluene. The
formation of expected complexes 1–3 was demonstrated by the
disappearance of the O–H signal of CMe2PhBTP-H or t-BuBTP-H
group (~11.60 ppm) and the appearance of the resonance for
alkyl protons of CH3 in the region -0.97 ppm for 1, -0.58 ppm
˚
˚
˚
1.7562(18) A, Al–N(1) = 1.972(2) A, Al–C(21) = 1.946(3) A, and
˚
Al–C(22) = 1.940(3) A, which are all comparable with bond lengths
observed for a four-coordinated aluminium complex containing
a Schiff-base derivative.14 Because of a bulky substituent at the
6-position of the BTP ligand, the monomeric four-coordinate
geometry is formed instead of a dimeric penta-coordinate feature
observed in an aluminium complex with a less bulky BTP ligand.15
The solid structure of 3 also reveals a monomeric Al(III) com-
plex, containing two N,O-bidentate BTP ligands. The geometry
around the Al center is penta-coordinate with an almost ideal
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distances are well within the normal range for bonds reported for
the Schiff base Al complex with five-coordinate geometry.14
Single crystals of complex 4 suitable for X-ray structural analysis
were obtained on slow cooling of a saturated toluene solution.
Complex 4 crystallizes in monoclinic space group P21/c with one
molecule of 4 and 1.5 molecules of toluene in the asymmetric
unit. In this alumoxane compound, each Al atom exhibits a
distorted trigonal-bipyramidal geometry with two oxygen atoms
and two nitrogen atoms from the benzotriazole-phenolate ligand
and one (m-O) unit as shown in Fig. 3. The coordination mode
of m-O from an Al–O–Al moiety is attributed to the linear type
(Al(2)–O(5)–Al(1) = 178.45(10)◦) without a center of symmetry
in the solid state. The averaged bond distances between the Al
atom and O(phenoxy) and N(benzotriazole) are 1.7598(18) and
˚
2.0621(18) A, which are all comparable with the bond lengths
observed for five-coordinated aluminium complex 3. The Al-(m-
˚
Fig. 1 ORTEP drawing of complex 2 with probability ellipsoids drawn
at level 30%. Hydrogen atoms and the disordered carbon atoms of
O) distances in 4 (1.6916(17) and 1.6886(17) A) are only slightly
shorter than those reported for [(m-O){iPrLAl(m-O)}2(AlMe)] (av.
˚
-
1.715 (2) A, iPrL = HC[(CMe)N(2,6-iPr2C6H3)]2 ),16 but are similar
t-butyl groups are omitted for clarity. Selected bond lengths (A) and
˚
angles (◦): Al–O(1) 1.7562(18), Al–N(1) 1.972(2), Al–C(21) 1.946(3),
Al–C(22) 1.940(3), O(1)–Al–N(1) 91.00(8), O(1)–Al–C(22) 113.57(14),
O(1)–Al–C(21) 114.20(12), C(22)–Al–N(1) 108.29(13), C(21)–Al–N(1)
107.27(12), C(22)–Al–C(21) 118.48(16).
iPr
16
˚
to those in [{ LAl(OH)}2(m-O)] (1.698(3) and 1.694(3) A)
and [{ LAl(OH)}2(m-O)] (1.691(2) and 1.701(2) A, MeL =
Me
˚
-
HC[(CMe)N(2,4,6-Me3C6H2)]2 ).17
Fig. 2 ORTEP drawing of complex 3 with probability ellipsoids drawn at
level 30%. Hydrogen atoms and toluene◦molecule are omitted for clarity.
˚
Selected bond lengths (A) and angles ( ): Al–O(1) 1.7673(15), Al–O(2)
1.7621(15), Al–N(1) 2.0854(18), Al–N(4) 2.0884(17), Al–C(61) 1.961(2),
O(2)–Al–O(1) 119.35(7), O(2)–Al–C(61) 120.30(10), O(1)–Al–C(61)
120.35(10), O(2)–Al–N(1) 90.11(7), O(1)–Al–N(1) 87.65(7), N(1)–Al–N(4)
174.07(7), O(2)–Al–N(4) 87.37(7), O(1)–Al–N(4) 88.90(7), C(61)–Al–N(4)
94.13(9), C(61)–Al–N(1) 91.78(9).
Fig. 3 ORTEP drawing of complex 4 with probability ellipsoids
drawn at level 30%. Carbon atoms and phenyl rings of the CMe2PhBTP-
groups, hydrogen atoms and toluene molecules are omitted for clarity.
Selected bond lengths (A) and angles (◦): Al(1)–O(1) 1.7573(18),
˚
Al(1)–O(2) 1.7578(17), Al(1)–O(5) 1.6916(17), Al(1)–N(1) 2.0550(18),
Al(1)–N(4) 2.0568(18), Al(2)–O(3) 1.7635(18), Al(2)–O(4) 1.7607(17),
Al(2)–O(5) 1.6886(17), Al(2)–N(10) 2.0685(18), Al(2)–N(7) 2.0683(18),
O(1)–Al(1)–O(2) 116.99(9), O(5)–Al(1)–O(1) 121.66(9), O(5)–Al(1)–O(2)
121.35(9), O(5)–Al(1)–N(1) 91.37(8), O(1)–Al(1)–N(1) 88.30(8),
O(2)–Al(1)–N(1) 89.89(8), O(5)–Al(1)–N(4) 92.49(8), O(1)–Al(1)–N(4)
89.52(8), O(2)–Al(1)–N(4) 88.26(7), N(1)–Al(1)–N(4) 176.14(9),
O(5)–Al(2)–O(4) 119.04(9), O(5)–Al(2)–O(3) 121.02(9), O(4)–Al(2)–O(3)
119.94(9), O(5)–Al(2)–N(10) 91.99(8), O(4)–Al(2)–N(10) 88.25(8),
O(3)–Al(2)–N(10) 89.31(8), O(5)–Al(2)–N(7) 92.76(8), O(4)–Al(2)–N(7)
89.82(8), O(3)–Al(2)–N(7) 87.87(8), N(10)–Al(2)–N(7) 175.23(9),
Al(2)–O(5)–Al(1) 178.45(10).
trigonal-bipyramidal environment in which aryloxy oxygen atoms
O(1) and O(2) and carbon atom C(61) occupy the equatorial plane.
The sums of the bond angles around the Al center in this plane
are 360.00(9)◦, and the angle (N(1)–Al–N(4)) formed by the axial
bonds is 174.07(7)◦. The coordination of chelated ligands to the
aluminium atom results in the formation of two six-membered
rings displaying two nearly coplanar conformations with mean
˚
˚
deviations 0.051 A and 0.067 A. The distances between the Al
atom and atoms O(1), O(2), N(1), N(4) and C(61) are 1.7673(15),
˚
1.7621(15), 2.0854(18), 2.0884(17), 1.961(2) A, respectively. These
1882 | Dalton Trans., 2011, 40, 1880–1887
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An analysis of short intramolecular contacts in 4 indicated that
p–p stacking interactions play an important role in the crystal
packing and stability of the linear alumoxane. As is discernible
from Fig. 4, p–p stacking interactions of two kinds are observed.
For the benzotriazole groups (Fig. 4a), each pair of neighboring
Ring-opening polymerization of L-lactides
Based on lactone polymerization catalyzed by an amine
bis(phenolate) aluminium complex,19 Ln5(m-O)(OiPr)13 (Ln =
Y, La, Sm, Yb)20 or Fe5(m5-O)(OEt)13,21 the bis(benzotriazole-
phenolate) ligand incorporating Al methyl complex 3 and oxo-
bridged Al complex 4 were expected to behave as catalysts
toward the ROP of L-lactide (L-LA) in the presence of alcohol.
In general, the polymerization was performed in toluene with
a prescribed equivalent ratio on the catalyst, monomers and 9-
anthracenemethanol (9-AnOH) for 48 h. The 9-AnOH is a pale
yellow solid and can be easily handled while very small amounts
of initiators (several mg) should be precisely weighed. After the
reaction was quenched by the addition of dilute HCl(aq), the
polymer was precipitated into MeOH. The experimental results
of the polymerization of L-LA catalyzed with 3 or 4 in the
presence of 9-AnOH under varied conditions are outlined in Table
1. Alumoxane complex 4 was used to investigate the effects of
reaction conditions in detail because it was more stable to air
than the analogous bis(benzotriazole-phenolate) Al complex 3.
According to trials of polymerization with the effects of the ratio
of the catalyst to 9-AnOH, temperature and concentration using
4 as the catalyst (Table 1, entries 1–4), optimized conditions had a
catalyst concentration (0.005 M) and 9-AnOH (two molar equiv.)
in toluene (10 mL) at 110 ◦C. Polymerization of L-LA under these
optimal conditions was then systematically investigated (Table 1,
entries 4–7). The conversion yield attained >92% in toluene within
two days at 110 ◦C with the ratio [L-LA]0/[9-AnOH]0 in the range
25–100. The number-average molecular weight (Mn determined
from 1H NMR) of the obtained polymers is close to the molecular
weight calculated from the molar ratio of monomer to 9-AnOH.
The PDIs of poly(L-lactide)s produced are allnarrow, ranging from
1.04 to 1.16, and a linear relationship between Mn (determined
˚
benzotriazole rings is parallel and separated by 3.605–3.721 A
(distances between the ring centroids), indicating the presence
of significant face-to-face p–p stacking interactions.18 A p–p
interaction of a second type exists between the five-membered
rings of the benzotriazole groups and phenyl rings of CMe2Ph
groups; the distances are slightly longer than those in the p–
˚
p interaction of the first type, ranging from 3.779 to 3.924 A
(Fig. 4b). The short interplanar p–p separations of the aromatic
moieties (benzotriazole or phenyl) are believed to provide an
additional intramolecular interaction to enhance the stability of
our alumoxane, resulting in a product that is stable to air in the
solid state.
1
from GPC) and [LA]0/[9-AnOH]0 exists (Fig. 5). The H NMR
spectrum of PLLA-50 (‘50’ indicates the designed [LA]0/[9-
AnOH]0 ratio) (Fig. 6) displays that the polymer chain is capped
with one 9-anthracenemethyl ester and one hydroxyl chain end.
This result indicates that the [{(CMe2PhBTP)2Al}2(m-O)]/9-AnOH
system might undergo an ‘activated-monomer’ path, whereas the
monomer or 9-AnOH is activated by 4 and 9-AnOH behaves as
Fig. 4 (a) p–p interactions between adjacent aromatic rings of ben-
zotriazole groups in 4. (b) p–p interactions between aromatic rings of
benzotriazole groups and phenyl rings of CMe2Ph substitutes in 4.
Fig. 5 Polymerization of L-LA catalyzed by 4 in toluene at 110 ◦C. The
relationship between Mn(ꢀ), PDI(ꢁ) of the polymer and the initial molar
ratio [LA]0/[9-AnOH]0 is shown.
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Table 1 Ring-opening polymerization of L-lactide (L-LA) catalyzed by complexes 3 and 4 in the presence of 9-AnOH
e
f
g
Entry
Cat.
[LLA]0/[Cat.]0/[9-AnOH]0
Conv.d (%)
Mn (calcd.)
Mn (obsd.)
Mn (NMR)
PDIf
i
1a
2a
3b
4c
5c
6c
7c
8c
4
4
4
4
4
4
4
3
200/1/0
200/1/2
200/1/2
200/1/2
50/1/2
100/1/2
150/1/2
200/1/2
35
71
85
94
92
92
94
47
10 100h
10 200
12 200
13 500
3300
6600
10 000
6700
12 200 (7100)
16 000 (9300)
21 000 (12 000)
22 300 (13 000)
4900 (2800)
9200 (5300)
16 000 (9300)
8000 (4600)
—
1.16
1.06
1.05
1.04
1.16
1.13
1.06
1.12
11 900
12 500
13 000
3400
7500
11 500
7000
a 0.05 mmol complexes, 15 mL toluene, 90 ◦C, 48 h. b 0.05 mmol complexes, 15 mL toluene, 110 ◦C, 48 h. c 0.05 mmol complexes, 10 mL toluene, 110 ◦C,
48 h. d Obtained from 1H NMR determination. e Calculated from the molecular weight of L-lactide times [LA]0/[9-AnOH]0 times conversion yield plus
the molecular weight of 9-AnOH. f Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained
from GPC times 0.58.23 g Obtained from 1H NMR analysis. h Calculated from the molecular weight of L-lactide times 200 times conversion yield. i Not
available.
Fig. 6 1H NMR spectra of PLLA-50 (Table 1, entry 6) in CDCl3.
a nucleophile.22 In comparison, complex 3 demonstrated poor
activity but a similar control of PDI toward the ROP of L-LA
(Table 1, entry 8 vs. 4) under the optimal conditions of complex 4.
efficiently the ‘controlled’ ROP of L-lactide in the presence of 9-
anthracenemethanol.
Experimental
Conclusions
General conditions
Four novel aluminium complexes of sterically bulky benzotriazole-
phenoxide ligands have been synthesized and fully characterized
by spectroscopic methods as well as X-ray single crystal struc-
tural determination. On the basis of the short interplanar p–p
interaction of the aromatic moieties (benzotriazole or phenyl) in
the crystal structure of 4, additional intramolecular p–p stacking
interactions are believed to enhance the stability of the linear
alumoxane 4, resulting in that complex becoming stable to air.
Air-stable alumoxane 4 has also been demonstrated to catalyze
All manipulations were carried out under a dry nitrogen atmo-
sphere. Solvents and reagents were dried by refluxing for at least 24
h over sodium/benzophenone (hexane, toluene, tetrahydrofuran
(THF)), or over phosphorus pentoxide (CH2Cl2). Deuterated
˚
solvents were dried over 4 A molecular sieves. L-Lactide was
recrystallized from a toluene solution prior to use. AlMe3
(2.0 M in toluene), 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-
1-phenylethyl)phenol
4,6-di-tert-butylphenol (t-BuBTP-H) and 9-anthracenemethanol
(
CMe2PhBTP-H), 2-(2H-benzotriazol-2-yl)-
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(9-AnOH) were purchased and used without further purification.
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance (300 MHz) spectrometer with chemical shifts given in parts
per million from the peak of internal TMS. Microanalyses were
performed using a Heraeus CHN-O-RAPID instrument. Gel per-
meation chromatography (GPC) measurements were performed
on a Jasco PU-2080 plus system equipped with a RI-2031 detector
using THF (HPLC grade) as an eluent. The chromatographic
calc. for C61H59AlN6O2: N, 8.99; C, 78.35; H, 6.36%. Found: N,
9.15; C, 78.36; H, 6.57%. 1H NMR (CDCl3, ppm): d 8.38 (s, 2H,
JH-H = 3 Hz, Ar-H); 7.96 (br, 4H, Ar-H); 7.53–7.21 (m, 16H, Ar-
H); 6.36 (t, 6H, JH-H = 3 Hz, Ar-H); 6.26 (t, 4H, JH-H = 3 Hz,
Ar-H); 1.87 (d, 12H, (C(CH3)2); 1.46, 1.13 (s, 12H, (C(CH3)2);
-0.81 (s, 3H, Al(CH3)). 13C NMR (CDCl3, ppm): d 151.0, 149.7,
149.3, 140.1, 139.9, 138.5, 137.9, 129.1, 128.3, 128.1, 127.9, 127.2,
126.7, 126.5, 126.3, 126.0, 125.7, 125.3, 124.4, 123.6, 123.3, 119.2,
118.8, 118.1 (Ph); 42.7, 42.6 (C(CH3)2); 34.5, 31.0, 26.6 (C(CH3)2);
-3.8 (AlMe).
˚
column was Phenomenex Phenogel 5 m 103 A and the calibration
curve used to calculate Mn (GPC) was produced from polystyrene
standards. The GPC results were calculated using the Scientific
Information Service Corporation (SISC) chromatography data
solution 3.1 edition.
Synthesis of complex [{(CMe2PhBTP)2Al}2(l-O)] (4)
◦
To an ice-cold solution (0 C) of 3 (0.93 g, 1.0 mmol) in toluene
(20 mL) was slowly added H2O (5.0 mL, 0.1 M in THF, 0.5 mmol).
The mixture solution was stirred at 25 ◦C for 16 h and was
dried under vacuum to give a yellow solid. The resulting solid
was extracted with hot toluene (30 mL) and the extract was
concentrated to ca. 20 mL. Pale yellow crystals were obtained
on cooling to 0 ◦C overnight. Yield: 0.66 g (72%). Anal. calc. for
Synthesis of complex [(CMe2PhBTP)AlMe2] (1)
To an ice-cold solution (0 ◦C) of CMe2PhBTP-H (0.45 g, 1.0 mmol)
in toluene (20 mL) was slowly added AlMe3 (0.6 mL, 2.0 M in
toluene, 1.2 mmol). The mixture was stirred at room temperature
for 4 h and the volatile components were removed in vacuo. The
residue was washed with hexane twice (2 ¥ 20 mL) and the
resulting precipitate was collected by filtration and then dried
under vacuum to give a yellow solid. Yield: 0.42 g (84%). Anal.
calc. for C32H34AlN3O: N, 8.34; C, 76.32; H, 6.80%. Found: N,
8.40; C, 76.12; H, 7.16%. 1H NMR (CDCl3, ppm): d 8.23 (s, 1H,
Ar-H); 8.01 (d, 1H, JH-H = 6 Hz, Ar-H); 7.71 (d, 1H, JH-H = 6 Hz,
Ar-H); 7.54 (t, 1H, JH-H = 9 Hz, Ar-H); 7.53 (t, 1H, JH-H = 9 Hz,
Ar-H); 7.39 (d, 1H, JH-H = 3 Hz, Ar-H); 7.33–7.10 (m, 11H, Ar-
C120H112Al2N12O5: N, 9.05; C, 77.64; H, 6.08%. Found: N, 8.72; C,
1
77.85; H, 5.75%. H NMR (CDCl3, ppm): d 7.88 (s, 4H, JH-H
3 Hz, Ar-H); 7.72 (d, 4H, JH-H = 9 Hz, Ar-H); 7.57 (d, 4H, JH-H
=
=
9 Hz, Ar-H); 7.50–7.22 (m, 24H, Ar-H); 6.95 (t, 4H, JH-H = 9 Hz,
Ar-H); 6.42 (t, 4H, JH-H = 9 Hz, Ar-H); 6.18 (t, 12H, JH-H = 3 Hz,
Ar-H); 6.04 (t, 8H, JH-H = 3 Hz, Ar-H); 1.88, 1.45, 1.14 (s, 48H,
(C(CH3)2). 13C NMR (CDCl3, ppm): d 151.4, 149.8, 149.0, 140.7,
139.9, 138.6, 137.9, 137.4, 129.1, 128.2, 128.0, 126.6, 126.5, 126.3,
126.2, 126.2, 126.0, 125.6, 125.3, 124.2, 123.3, 118.8, 116.9, 116.8
(Ph); 42.6, 42.3 (C(CH3)2); 34.0, 31.1, 31.0, 26.2 (C(CH3)2).
H); 1.80, 1.64(s, 12H, Ar-C(CH3)2); -0.97 (s, 6H, Al(CH3)2). 13
C
NMR (CDCl3, ppm): d 150.9, 150.6, 149.7, 142.4, 142.2, 139.6,
139.3, 130.2, 129.1, 128.8, 128.3, 128.2, 128.1, 127.9, 127.7, 126.8,
126.4, 125.8, 125.6, 124.9, 118.9, 118.0, 117.5, 114.2 (Ph); 42.9,
42.6 (C(CH3)2); 31.0, 29.2 (C(CH3)2); -10.7 (AlMe2).
Polymerization of L-lactide catalyzed by linear alumoxane
complex 4
A typical polymerization procedure was exemplified by the
synthesis of PLLA-100 (the number 100 indicates the designed
[LLA]0/[9-AnOH]0). Polymerization was carried out under a dry
nitrogen atmosphere, except that the catalyst was handled in air.
A rapidly stirring solution of [{(CMe2PhBTP)2Al}2(m-O)] (4) (0.094
g, 0.05 mmol) and 9-AnOH (0.020 g, 0.1 mmol) in toluene (5 mL)
was heated to 110 ◦C. After 3 h, the activated species were added
to L-lactide (1.44 g, 10.0 mmol) in toluene (5 mL) and the reaction
mixture was stirred at 110 ◦C for 48 h. The conversion yield
(94%) of PLLA-100 was analyzed by 1H NMR spectroscopic
determination. Volatile materials were removed in vacuo, and the
residue was redissolved in THF (10 mL). The mixture was then
quenched by the addition of 3 N HCl solution (10 mL), and the
polymer was then precipitated into MeOH (50 mL) to give a white
crystalline solid. Yield: 1.15 g (80%).
Synthesis of complex [(t-BuBTP)AlMe2] (2)
To an ice-cold solution (0 ◦C) of t-BuBTP-H (0.32 g, 1.0 mmol)
in toluene (20 mL) was slowly added AlMe3 (0.6 mL, 2.0 M in
toluene, 1.2 mmol). The mixture was stirred at room temperature
for 4 h and concentrated under vacuum to remove all volatile
materials. The residue was washed with hexane (2 ¥ 20 mL) and
the resulting precipitate was collected by filtration and then dried
in vacuo to give a yellow solid. Yield: 0.31 g (88%). Anal. calc. for
C20H30AlN3O: N, 11.07; C, 69.63; H, 7.97%. Found: N, 11.23; C,
1
69.18; H, 7.64%. H NMR (CDCl3, ppm): d 8.23 (s, 1H, Ar-H);
8.08 (d, 1H, Ar-H); 7.86 (d, 1H, Ar-H); 7.63–7.55 (m, 2H, Ar-H);
7.49 (s, 1H, Ar-H); 1.51 (s, 9H, C(CH3)3); 1.40 (s, 9H, C(CH3)3);
-0.58 (s, 6H, Al(CH3)2). 13C NMR (CDCl3, ppm): d 150.1, 142.5,
142.0, 140.0, 139.7, 130.2, 127.9, 126.8, 126.6, 118.9, 116.6, 114.3
(Ph); 35.9, 34.5 (C(CH3)3); 31.5, 29.6 (C(CH3)3); -10.2 (AlMe2).
X-ray crystallographic studies
Synthesis of complex [(CMe2PhBTP)2AlMe] (3)
Suitable crystals of complex 2–4 were sealed in thin-walled glass
capillaries under a nitrogen atmosphere and were mounted on
a Bruker APEX2 diffractometer. Intensity data were collected
in 1350 frames with increasing w (width of 0.5 per frame). The
absorption correction was based on the symmetry-equivalent
reflections using SADABS. The space group determination was
based on a check of the Laue symmetry and systematic absence,
and was confirmed using the structure solution. The structures
To an ice-cold solution (0 ◦C) of C Me2PhBTP-H (0.90 g, 2.0 mmol)
in toluene (20 mL) was slowly added AlMe3 (0.6 mL, 2.0 M in
toluene, 1.2 mmol). The mixture solution was refluxed for 16 h
and then the volatile components were removed in vacuo. The
resulting solid was extracted with hot toluene (15 mL) and the
extract was concentrated to ca. 5 mL. Pale yellow crystals were
obtained on cooling to 0 ◦C overnight. Yield: 0.82 g (88%). Anal.
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Table 2 Crystallographic data of complexes 2–4
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2
3·C7H8
4·1.5C7H8
Empirical formula
Formula weight
Temp.
C22H30AlN3O C68H67AlN6O2 C130.5H124Al2N12O5
379.47
1027.26
296(2) K
Triclinic
1994.38
296(2) K
Monoclinic
P21/c
21.7765(8)
22.1620(8)
24.9183(8)
90
112.629(2)
90
11100.0(7)
4
1.193
0.088
4228
108206
1331
296(2) K
Triclinic
¯
P1
Crystal system
¯
Space group
P1
˚
a (A)
6.7876(3)
10.5043(4)
17.6083(9)
102.867(3)
93.234(3)
106.608(2)
1163.14(9)
2
13.4849(5)
13.9078(5)
17.8632(6)
78.436(2)
70.643(2)
66.815(2)
2896.04(18)
2
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
3
˚
V (A )
Z
Dcalc (Mg m-3)
m (Mo Ka) (mm-1)
F(000)
1.083
1.178
0.102
408
0.085
1092
Reflections collected 18267
No. of parameters 259
48084
704
Indep. reflns (Rint
)
5617 (0.0501) 14 376 (0.0426) 27 858 (0.0869)
R1[I > 2s(I)]
0.0651
0.1749
0.0593
0.1458
1.013
0.0638
0.1181
1.002
wR2 [I > 2s(I)]
Goodness-of-fit on F2 1.032
were solved with direct methods using a SHELXTL package. All
non-H atoms were located from successive Fourier maps, and
hydrogen atoms were refined using a rigid model. Anisotropic
thermal parameters were used for all non-H atoms, and fixed
isotropic parameters were used for H-atoms. For complex 2, the
DFIX instruction was applied to restrain bonds of C17–C18,
C17–C19, C17–C20, C17–C18¢, C17–C19¢ and C17–C20¢ for the
disordered t-Bu group. In addition, the EADP instruction was
also applied on the terminal atoms of C18, C19, C20, C18¢,
C19¢ and C20¢ for the disordered t-Bu group. For complex 4,
the DFIX instruction was applied to restrain the C1S–C2S bond
for the toluene molecule. The disordered toluene molecules were
also treated as isotropic to prevent the unsatisfied refinement.
Crystallographic data of complexes 2–4 are summarized in Table
2.
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Acknowledgements
We gratefully acknowledge the financial support from the National
Science Council, Taiwan (NSC99-2113-M-033-007-MY2). We
also thank Professor Chu-Chieh Lin for valuable discussions.
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This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 1880–1887 | 1887
©