Aluminium alkyl complexes supported by [OSSO] type bisphenolato ligands: Synthesis, characterization and living polymerization of rac-lactide

Article abstract of DOI:10.1039/b416875e

Aluminium alkyl complexes [(OSSO)AlR] (1-3: R = Me, Et) were isolated in good yields from the protonolysis reaction of AlR₃ with the corresponding tetradentate 1.ω-dithiaalkanediyl-bridged bisphenols (1,4-dithiabutanediyl-bis(6-tert-butyl-4-methylphenol), etbmpH₂; ortho-xylylenedithio-bis(6-tert-butyl-4-methylphenol), xytbmpH₂). The monomeric structures of all three complexes were confirmed by X-ray diffraction studies. Complexes 1 and 2 have an isotypic packing arrangement. The aluminium center is coordinated by the etbmp ligand and one alkyl group with distorted trigonal bipyramidal geometry. Complex 3 shows C, symmetry with square pyramidal geometry around the metal center. Substitution reaction of complex 1 with trityl alcohol gave the monomeric alkoxide complex [(etbmp)Al(OCPh₃)] 4, which has a similar trigonal bipyramidal geometry around the aluminium atom as complex 1. In the presence of isopropanol, complexes 1-3 initiated the living ring-opening polymerization of rac-lactide (PDI = 1.03-1.06, M _w/M_n). The ligand structure influenced the tacticity of the obtained polymer, with complex 3 giving heterotactic-enriched polylactides. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416875e

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F U L L P A P E R
Aluminium alkyl complexes supported by [OSSO] type bisphenolato
ligands: synthesis, characterization and living polymerization of
rac-lactide†
Haiyan Ma,a Gianluca Melillo,b Leone Oliva,b Thomas P. Spaniol,a Ulli Englerta and
Jun Okuda*a
a Institute of Inorganic Chemistry, Aachen University of Technology (RWTH),
Professor-Pirlet-Strasse 1, D-52056 Aachen, Germany.
E-mail: jun.okuda@ac.rwth-aachen.de; Fax: +49 241 8092644
b Dipartimento di Chimica, Universita´ di Salerno, Via S. Allende, I-84081 Baronissi (SA), Italy
Received 4th November 2004, Accepted 15th December 2004
First published as an Advance Article on the web 12th January 2005
Aluminium alkyl complexes [(OSSO)AlR] (1–3: R = Me, Et) were isolated in good yields from the protonolysis
reaction of AlR3 with the corresponding tetradentate 1,x-dithiaalkanediyl-bridged bisphenols (1,4-dithiabutanediyl-
bis(6-tert-butyl-4-methylphenol), etbmpH2; ortho-xylylenedithio-bis(6-tert-butyl-4-methylphenol), xytbmpH2). The
monomeric structures of all three complexes were confirmed by X-ray diffraction studies. Complexes 1 and 2 have an
isotypic packing arrangement. The aluminium center is coordinated by the etbmp ligand and one alkyl group with
distorted trigonal bipyramidal geometry. Complex 3 shows Cs symmetry with square pyramidal geometry around the
metal center. Substitution reaction of complex 1 with trityl alcohol gave the monomeric alkoxide complex
[(etbmp)Al(OCPh3)] 4, which has a similar trigonal bipyramidal geometry around the aluminium atom as complex 1.
In the presence of isopropanol, complexes 1–3 initiated the living ring-opening polymerization of rac-lactide (PDI =
1.03–1.06, Mw/Mn). The ligand structure influenced the tacticity of the obtained polymer, with complex 3 giving
heterotactic-enriched polylactides.
Introduction
Results and discussion
Polylactides (PLA) prepared from renewable sources are im-
portant biodegradable materials for biomedical, pharmaceutical
and agricultural applications.1 Ring-opening polymerization of
lactides initiated by alkoxides of Sn, Al, Zn, Mg, Ca, Fe, Li
and the rare earth metals2–4 is one efficient way to produce
high molecular weight PLAs.5–13 Besides isotactic PLAs pre-
pared from enantiomerically pure L- or D-lactide, heterotactic,9
syndiotactic,10 and stereoblock isotactic11–13 PLAs are also
accessible from rac- or meso-lactide by employing chain-end
control or enantiomorphic site control.14
Synthesis of aluminium bisphenolate complexes
1,4-Dithiabutanediyl-bis(6-tert-butyl-4-methylphenol) (etbmpH2)
reacted rapidly with AlMe3 in hydrocarbons to afford the
aluminium methyl complex 1 as colorless crystals in moderate
yield after recrystallization from n-hexane. Spectroscopic data
and elemental analysis are consistent with the structure shown
in Scheme 1. The monomeric structure was further confirmed
by X-ray diffraction studies (Fig. 1, vide infra). The isolation of
pure complex 1 was accompanied by the precipitation of a white
powder (1a) even under extremely strict exclusion of air and
moisture, which showed a complicated resonance pattern in the
1H NMR spectrum including six signals for the 4-methyl group
but no Al-methyl resonance. Spectroscopic data of the product
obtained under these experimental conditions do not indicate
the presence of any dinuclear complex [(etbmp)2Al2Me2] which
was obtained earlier.17
For structurally well-defined initiators for lactide polymeriza-
tion, nitrogen-containing ligands such as imino-phenolates,10–13
8,9b,c
7,9a,d
amino-phenolates,
and b-diketiminates dominate.
Espe-
cially for initiators based on aluminium, the ligands involved
9b,10–13
are mainly salen- or salan-type bisphenolates.
Recently,
we have developed tetradentate 1,x-dithiaalkanediyl-bridged
bisphenolato ligands to support Group 3 and 4 metals which
feature hemilabile sulfur donors.15,16 The Group 3 complexes
were shown to be highly active initiators for the ring-opening
polymerization of L-lactide, which, however, was at the most
controlled (Mw/Mn = 1.15–1.41). By using the potentially more
electrophilic aluminium center and by modifying the bridge
between the two sulfur atoms, we intended to influence the
hemilabile nature of the sulfur donors within the chelate ligand
and consequently the polymerization behavior. We report here
aluminium complexes containing a dianionic OSSO-type ligand
which act as efficient initiators for the living polymerization of
rac-lactide in the presence of isopropanol.
Scheme 1
The ethyl analogue 2 formed without any side products
(Fig. 2). When ortho-xylylenedithio-bis(6-tert-butyl-4-methyl-
phenol) (xytbmpH2) reacted with AlMe3, under dilute condi-
tions, the crude product after evaporation of the volatiles in the
reaction mixture was spectroscopically characterized as complex
† Electronic supplementary information (ESI) available: Details about
complex 1a, kinetic studies about the ring-opening polymerization of
rac-lactide by complexes 1–3/iPrOH, 1H NMR and homodecoupled
proton spectra of poly(rac-lactide). See http://www.rsc.org/suppdata/
dt/b4/b416875e/
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7
7 2 1
©

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isopropanol in n-pentane afforded a white precipitate, which
showed broad signals in the 1H NMR spectra (CDCl3, toluene-
d8) at r.t. corresponding to the structure of “[(etbmp)Al(OiPr)]”.
This was further confirmed by elemental analysis. Upon cooling
to −20 ◦C, complicated resonance patterns for the ligand
1
moiety were displayed in the H NMR spectrum (toluene-d8);
however the existence of three multiplets at 5.54, 5.24, 5.03 ppm
accounting for the methine proton of OCH(CH3)2 group clearly
indicated the aggregated structure of this aluminium isopropox-
ide complex. Unlike the Group 3 complexes with the same
type of ligands,16 excess amount of isopropanol resulted in the
alcoholysis of the bisphenolate ligand in complex 1 producing
free phenol. Similar results were obtained for complex 3 from the
NMR scale reactions. To avoid aggregation, trityl alcohol was
used to react with complex 1. As expected, monomeric alkoxide
complex [(etbmp)Al(OCPh3)] 4 was isolated in moderate yield
(Fig. 4).
Fig. 1 ORTEP diagram of the molecular structure of [(etbmp)AlMe]
(1). Displacement ellipsoids are drawn at the 50% probability level.
Hydrogen atoms are omitted for clarity.
Fig. 2 ORTEP diagram of the molecular structure of [(etbmp)AlEt]
(2). Displacement ellipsoids are drawn at the 50% probability level.
Hydrogen atoms are omitted for clarity.
Fig. 4 ORTEP diagram of the molecular structure of [(etbmp)-
Al(OCPh3)] (4). Displacement ellipsoids are drawn at the 50% probabil-
ity level. Hydrogen atoms are omitted for clarity. For tert-butyl groups,
only the carbon atoms connected to phenyl rings are shown.
3.18 After recrystallization from n-hexane, prismatic crystals were
obtained (Fig. 3). Complexes 1–3 were fully characterized by
spectroscopic methods and elemental analysis. Noticeably, for
1–3, the methyl group or the methylene group attached to the Al
atom could not be detected by 13C NMR spectroscopy at room
temperature.
Crystal structures of 1–4
Single crystals suitable for X-ray diffraction studies were ob-
tained from n-hexane solution at −40 ◦C for 1 or by slow evapo-
ration of the saturated n-hexane solutions at room temperature
for 2–4. Crystallographic data and results of refinements are
summarized in Table 1. Selected bond lengths and angles are
listed in Tables 2 and 3.
As depicted in Fig. 1, the X-ray diffraction analysis revealed
the monomeric structure of complex 1 and the distorted trigonal
bipyramidal geometry at the aluminium center. C25, S1 and O2
form the equatorial plane from which the Al atom deviates by ca.
˚
0.279(1) A in the direction of O1. Compared to S1, S2 is weakly
coordinated to the aluminium atom with the distance of Al–S2 =
˚
2.8181(9) A regarding the sum of metal radius rm(Al) and van der
Waals radius rv(S) [rm(Al) + rv(S) = 1.43 + 1.80 = 3.23 A],19 and
˚
Fig. 3 ORTEP diagram of the molecular structure of [(xytbmp)AlMe]
(3). Displacement ellipsoids are drawn at the 50% probability level.
Hydrogen atoms are omitted for clarity.
occupies the axial positions together with O1. With the exception
of this long axial contact, the other coordination distances
are expected, the axial Al–O1 [1.787(2) A] bond length being
˚
Anticipated as effective initiators for the ring-opening poly-
merization of lactides, aluminium alkoxide complexes were
synthesized by protonolysis of the methyl group in complexes
1 and 3 with alcohol. The reaction of 1 with one equiv. of
slightly longer than the equatorial counterpart Al–O2 [1.753(1)
˚
A] which are typical of those in five-coordinate bisphenolate
˚
9b,13,20
aluminium complexes (1.754–1.809 A).
These phenomena
were also observed in an aluminium complex with tetradentate
7 2 2
D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7

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Table 1 Crystallographic data for 1–4
1
2
3
4
Formula
M
C25H35AlO2S2
458.63
120(2)
0.53 × 0.26 × 0.24
Orthorhombic
P212121 (no.19)
8.829(1)
15.074(2)
18.759(3)
90
90
90
2496.5(6)
4
1.220
C26H37AlO2S2
472.66
248(2)
0.5 × 0.3 × 0.2
Orthorhombic
P212121 (no.19)
8.952(4)
15.202(6)
18.987(7)
90
90
90
2584(2)
4
C31H39AlO2S2
534.72
120(2)
0.47 × 0.19 × 0.10
Orthorhombic
Cmc21 (no. 36)
17.001(2)
17.967(2)
10.009(1)
90
90
90
3057.3(6)
4
1.162
C43H47AlO3S2
702.91
153(2)
0.35 × 0.20 × 0.08
Monoclinic
P21/c (no. 14)
14.766(7)
11.823(6)
22.17(1)
90
Temperature/K
Crystal size/mm
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
103.26(1)
90
3767(3)
3
˚
U/A
Z
4
Dc/g cm−3
1.215
0.260
1.239
0.203
l/mm−1
0.267
0.227
F(000)
984
1016
1144
1496
h Range/◦
1.73–28.36
11, −19 to 20, 25
29929
6235
0.0627
3.13–25.99
−10 to 11, 18, 23
15321
5045
0.0700
1.65–28.33
22, 23, 13
20943
3933
0.0560
1.42–25.20
17, 14, 26
47730
6727
0.2207
Data collected (hkl)
No. of reflections collected
No. of independent reflections
Rint
Final R1, wR2[I > 2r(I)]
R1, wR2 (all data)
Goodness of fit on F2
0.0387, 0.0894
0.0467, 0.0929
1.056
0.0420, 0.0871
0.0575, 0.0939
1.061
0.0394, 0.0875
0.0435, 0.0899
1.076
0.0763, 0.1693
0.1477, 0.2104
0.987
0.701, −0.602
−3
˚
Dqmax, min/e A
0.405, −0.205
0.287, −0.228
0.359, −0.224
Flack parameter
0.01(6)
−0.02(7)
0.09(8)
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) in 1, 2, 4
S1–(CH2)2–S2 chelate in 1 is also indicated by the rather small
angles of Al–S1–C12 (102.56(7)◦) and Al–S2–C13 (100.61(7)◦)
as compared to those in complex 3 (Al–S–C12 = 108.10(6)◦).
Complexes 1 and 2 are structurally closely related. In both cases,
crystallization from the racemic mixture leads to conglomerates,
i.e. the two enantiomers crystallize separately. The individual
crystal chosen for the X-ray diffraction experiment of 2 is
isotypic to the enantiomorph of the crystal structure of 1 (Fig. 1,
Fig. 2).
1
2
4
Al–O1
Al–O2
Al–S1
Al–S2
Al–Xa
S1–C12
1.787(2)
1.753(1)
2.4462(8)
2.8181(9)
1.943(2)
1.815(2)
1.784(2)
1.757(2)
2.450(1)
2.822(1)
1.947(3)
1.819(3)
1.748(3)
1.721(3)
2.360(2)
2.961(2)
1.670(3)
1.817(5)
The modification of the bridging part in the ligand led to
a dramatic change in the coordination geometry of the metal
center. In complex 3, the Al atom is coordinated by four donor
atoms of the ligand as well as the axial methyl group to give
a square pyramidal geometry (Fig. 3). The molecule shows Cs-
O1–Al–S1
O1–Al–S2
O2–Al–S1
O2–Al–S2
O1–Al–O2
S1–Al–S2
O1–Al–Xa
O2–Al–Xa
S1–Al–Xa
S2–Al–Xa
Al–S1–C12
Al–S2–C13
81.59(5)
157.88(5)
104.34(5)
76.99(5)
101.64(7)
77.48(2)
107.72(9)
126.25(9)
123.51(7)
89.94(7)
102.56(7)
100.61(7)
81.73(7)
157.57(7)
104.82(7)
76.48(7)
101.66(9)
77.28(4)
104.3(1)
128.6(1)
122.2(1)
93.7(1)
87.1(1)
155.0(1)
112.9(1)
73.2(1)
102.3(2)
72.82(5)
114.8(2)
125.7(2)
107.4(1)
86.1(1)
˚
symmetry. Noticeably, the two Al–S bond lengths (2.6209(7) A)
are equal in complex 3, which are nearly the average value of
˚
those in complex 1 ([2.4462(8) +2.8181(9)]/2 = 2.63 A). The
other bond distances are comparable to those in complexes 1
and 2. The aromatic ring of the bridge is located opposite to the
axial methyl group, although no interaction with the aluminium
metal center is implied.
102.5(1)
100.9(1)
98.7(2)
96.9(2)
a For 1 and 2, X corresponds to C25; for 4, X corresponds to O3.
All the aluminium–carbon bond lengths in complexes 1–3 are
˚
rather similar (1.943(2)–1.948(3) A), which, however, are no-
ticeably shorter than those in the corresponding five-coordinate
aluminium complexes with nitrogen-containing ligands (1.955–
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) in 3
˚
9b,13
1.972 A).
The weaker coordination ability of the sulfur atoms
Al–O
1.755(1)
1.948(3)
1.391(3)
1.389(5)
1.848(2)
Al–S
2.6209(7)
1.411(4)
1.377(3)
1.488(3)
at the aluminium center as compared to the nitrogen donors
could explain this.
Al–C16
C13–C14
C15–C15*
S–C12
C13–C13*
C14–C15
C12–C13
Fig. 4 shows the aluminium alkoxide complex 4. Although
the methyl ligand is replaced by the bulky trityl alkoxide group,
the coordination environment around the aluminium center
is maintained as in complex 1. The aluminium atom is five-
coordinate showing a distorted trigonal bipyramidal geometry.
O1 and weakly coordinated S2 occupy the axial positions,
while the other three donors form the equatorial plane. Due
to the steric hindrance between the bulky trityl group and the
ortho-tert-butyl groups of the bisphenolate ligand, the steric
arrangement of the ligand is slightly “compressed”, as evidenced
by the smaller angle of O1–Al–S2 (155.0(1)◦ vs. 157.88(5)◦ in
1). A comparison of the molecular structures of complexes 1,
3 and 4 shows that the replacement of an alkyl group by an
O–Al–S
80.88(4)
95.86(9)
114.63(8)
108.10(6)
O–Al–S*
S–Al–S*
S–Al–C16
146.13(6)
83.52(3)
96.93(7)
O–Al–O*
O–Al–C16
Al–S–C12
9b
aminophenolato ligand. The unexpected long contact of S2 to
Al in complex 1 is obviously caused by the considerable strain
imposed by the ethylene bridge between two sulfur donors. No
similar effect is observed in complex 3 (Fig. 3) where the strain is
relieved partly by a longer bridging group. The rigidity of the Al–
D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7
7 2 3

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Table 4 Polymerization of rac-lactide with 1–3a
Time/h Convb (%) Mn(SEC) Mw/Mn Mn(NMR) Prc
alkoxide ligand does not significantly influence the coordination
geometry at the aluminium center.
Structures in solution
1
1
2
3
3
1
10
10
22
118
31
92
93
33
92
7826
20430
20337
10580
20500
1.06
1.04
1.04
1.08
1.07
4384
13464
12888
5104
0.49
0.50
0.49
0.64
0.65
Despite the asymmetric structure in the solid state, the 1H
NMR spectra of 1 in toluene-d8 show symmetric patterns for
the two aromatic moieties as well as the methyl ligand in the
temperature range of −60 to 80 ◦C, indicating either a rapid
exchange between trigonal bipyramidal [trans-(O1, O2), C2-
symmetry; or cis-(O1,O2), C1-symmetry] and square pyramidal
(Cs-symmetry) configurations on the NMR time scale or a pre-
dominantly square pyramidal configuration. The four protons
of the ethylene bridge display two multiplets in 1 : 1 ratio at 2.46
and 2.22 ppm at room temperature, consistent with the sulfur
coordination mode of complex 1 in the solid state. With increas-
ing temperature these signals broaden and eventually coalesce
12744
a [LA]0 : [Al]0 : [iPrOH]0 = 100 : 1 : 1, [LA]0 = 0.5 M, toluene, 70 ◦C.
b Monomer conversion, determined by 1H NMR spectroscopy. c Pr is the
probability of forming a new r-dyad, determined by homodecoupled 1H
NMR.
isopropanol to generate in situ the active alkoxide initiators for
the ring-opening polymerization of rac-lactide. All polymeriza-
tion runs were carried out at 70 ◦C in toluene. As shown in
Table 4, 1/iPrOH system polymerized rac-lactide smoothly; at
a [LA]0/[Al]0 ratio of 100 with [LA]0 = 0.5 M, 90% conversion
to PLA was reached after 8 h. Preliminary studies indicated
a first order dependence on the monomer concentration with
kapp = 9.57 × 10−5 s−1 (Fig. 6). The polymerization proceeded
in a living fashion over the entire conversion range, as shown
by the linear increase of the Mn with the conversion and
low polydispersity indices (PDI = 1.03–1.06, Mw/Mn) of the
obtained polymers up to 95% conversion (Fig. 7). Since Mn
estimated from the 1H NMR spectra agreed well with Mn
(theory), the deviation of Mn (SEC) from Mn (theory) was
◦
at 75 C (Fig. 5). It is plausible to assume that the two sulfur
donors coordinate to the aluminium center in an alternating way
(for each sulfur donor, strong and weak in turn) in solution. This
process is sufficiently slow to be frozen out on the NMR time
scale below 75 ◦C. Applying the Eyring equation, the activation
energy for this process of 1 could be calculated as DG‡
=
c
69.2 kJ mol−1 (Tc = 75 ◦C, Dm = 135 Hz). Similar activation
energies were calculated for the corresponding scandium and
lutetium complexes ([(etbmp)Ln{N(SiHMe2)2}(THF)], Ln =
Sc, DG‡ = 69.8 kJ mol−1; Ln = Lu, DG‡ = 63.5 kJ mol−1).16
c
c
Fig. 6 Semilogarithmic plots of rac-lactide conversion vs. time using
1/iPrOH in toluene at 70 ◦C, [LA]0 : [Al]0 : [iPrOH]0 = 100 : 1 : 1,
[LA]eq = 0.02 mol L−1
.
Fig. 5 Variable temperature 1H NMR of complex 1 (toluene-d8). The
–SCH2CH2S– moiety is shown.
The ethylene bridge of complex 4 displays the same pattern as
1
complex 1 in the H NMR spectrum. As in the solid state, the
bulky trityl alkoxide group of complex 4 does not change the
coordination geometry at the aluminium atom in solution. For
complex 3, a clear symmetric pattern is observed for the ligand
and the methyl group in the 1H NMR spectrum and the chemical
shift of CH2 protons of the bridge is low-field shifted (4.03 vs.
3.70 ppm in free bisphenol). Therefore the square pyramidal
coordination geometry around the aluminium center is retained
in solution.
Fig. 7 Plots of Mn vs. monomer conversion (determined by 1H NMR)
using 1 for rac-lactide polymerization. –ꢀ–: Mn (SEC, PDI indicated
in parentheses; polystyrene standards), –᭺–: Mn (theory). Conditions:
[LA]0 : [Al]0 : [iPrOH]0 = 100 : 1 : 1, [LA]0 = 0.5 M, toluene, 70 ◦C.
Ring-opening polymerization of rac-lactide
In order to investigate the influence of the ligand structure on
the polymerization process, complexes 1–3 were reacted with
7 2 4
D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7

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1
most likely due to polystyrene as standards.13,21 The H NMR
orated to dryness. The residue was extracted with toluene and
water, the organic phase was dried over anhydrous magnesium
sulfate. The clear solution was concentrated to dryness and the
obtained thick oil was recrystallized with hexane (150 mL) to
afford colorless crystals in 84% yield (10.86 g, two crops). Anal.
Calcd. for C30H38O2S2: C 72.83, H 7.74, S 12.96%; found: C
72.68, H 7.83, S 12.93%. dH (400 MHz, CDCl3): 7.04 (dd, 2 H,
3J = 5.5 Hz, 4J = 3.5 Hz, Bz), 7.03 (s, 2 H, 5-H), 6.98 (s, 2 H,
spectra of the polylactides showed that the polymer chains were
systemically end-capped with isopropyl ester and a hydroxyl
group respectively indicative of a coordination–insertion mech-
anism. The homonuclear decoupled 1H NMR analysis showed
that atactic polylactides were obtained by using 1/iPrOH. As
expected, 2/iPrOH polymerized rac-lactide at the same rate as
1/iPrOH under the same conditions (kapp = 9.61 × 10−5 s−1) and
produced atactic polylactides.
3
4
3-H), 6.86 (s, 2 H, OH), 6.82 (dd, 2 H, J = 5.5 Hz, J =
3.5 Hz, Bz), 3.76 (s, 4 H, SCH2), 2.18 (s, 6 H, 4-CH3), 1.30 [s,
18 H, 6-C(CH3)3]; dC (100 MHz, CDCl3): 153.4 (Ar–C1), 135.4
(Ar–C6), 135.3 (Bz), 133.9 (Bz), 130.3 (Ar–C3), 129.5 (Ar–C5),
128.5 (Bz), 127.5 (Ar–C4), 117.8 (Ar–C2), 38.1 (SCH2), 34.6
[6-C(CH3)3], 29.1 [6-C(CH3)3], 20.4 (4-CH3).
Compared to 1/iPrOH system, polymerization using 3/
iPrOH was considerably slow (Table 4, kapp = 0.83 × 10−5 s−1),
and was accompanied by an induction period. The polymeriza-
tion was still well-controlled after the induction period, a linear
relationship of Mn (SEC) vs. conversion was obtained along
with narrow molecular weight distributions (PDI = 1.06–1.09).
[(etbmp)AlMe] (1). To a suspension of 1,4-dithiabutanediyl-
bis(6-tert-butyl-4-methylphenol) (etbmpH2) (1.67 g, 4.0 mmol)
in n-hexane (30 mL), 2.0 M solution of trimethylaluminium in
n-hexane (2 mL, 1.37 g, 4.0 mmol) was added dropwise via a sy-
ringe at room temperature. Instantaneous evolution of methane
and almost full dissolution of the suspension were observed. The
colorless reaction mixture was stirred overnight. After filtration
to remove trace amounts of the unreacted bisphenol, the filtrate
was concentrated to dryness and recrystallized with hexane at
1
Homonuclear decoupled H NMR spectroscopy showed that
heterotactic-enriched polylactides were obtained, independent
of the monomer conversion.
Although the ethylene bridge in complexes 1 and 2 might
have introduced some asymmetric character to the aluminium
center under the polymerization conditions (the two signals
coalesce at 75 ◦C, Fig. 5), the interconversion of the two
chiral conformations is obviously too fast to induce efficient
stereoselectivity. Complex 3 is achiral in solution and the slight
stereocontrol could have been induced by the chain-end control
mechanism which is possibly favored by the more bulky ligand
environment.9,12,13
◦
−40 C, colorless prismatic crystals were isolated in 71% yield
(1.3 g). Anal. Calcd. for C25H35As2O2: C 65.47, H 7.69, S 13.98%;
found: C 65.08, H 7.80, S 14.03%. dH (400 MHz, CDCl3): 7.13
(d, 2 H, 4J = 2 Hz, 5-H), 7.06 (d, 2 H, 4J = 2 Hz, 3-H), 3.13 (m, 4
H, SCH2), 2.21 (s, 6 H, 4-CH3), 1.35 [s, 18 H, 6-C(CH3)3], −0.34
(s, 3 H, Al–CH3); dC (100 MHz, CDCl3): 158.6 (Ar–C1), 139.2
(Ar–C6), 131.7 (Ar–C3), 130.5 (Ar–C5), 126.9 (Ar–C4), 117.2
(Ar–C2), 39.4 (SCH2CH2S), 35.2 [6-C(CH3)3], 29.3 [6-C(CH3)3],
20.6 (4-CH3).
Conclusion
Through the protonolysis reaction several aluminium alkyl com-
plexes containing tetradentate 1,x-dithiaalkanediyl-bridged
bisphenolato ligands were obtained, which in the presence of
isopropanol, acted as active initiators for the living ring-opening
polymerization of rac-lactide. The modification of the ligand
structure significantly influenced the coordination geometry at
the metal center of the initiator as well as the tacticity of the
obtained polylactide. Further studies are underway to elucidate
the influence of the ligand structure on the stereocontrol during
the rac-lactide polymerization by this new family of complexes.
[(etbmp)AlEt] (2). The procedure was similar to that of
complex 1, except that 1,4-dithiabutanediyl-bis(6-tert-butyl-4-
methylphenol) (etbmpH2) (0.209 g, 0.500 mmol) and 1.0 M
solution of triethylaluminium in hexane (0.5 mL, 0.346 g,
0.500 mmol) were used, colorless prismatic crystals were isolated
in 78% yield (0.185 g, two crops). Anal. Calcd. for C26H37AlS2O2:
C 66.06, H 7.89, S 13.57%; found: C 65.94, H 7.99, S 13.54%. dH
(400 MHz, CDCl3): 7.13 (d, 2 H, 4J = 2 Hz, 5-H), 7.06 (d, 2 H,
4J = 2 Hz, 3-H), 3.13 (br d, 4 H, SCH2), 2.21 (s, 6 H, 4-CH3), 1.36
[s, 18 H, 6-C(CH3)3], 1.10 (t, 3 H, 3J = 8 Hz, AlCH2CH3), 0.33
(q, 2 H, 3J = 8 Hz, Al–CH2CH3); dC (100 MHz, CDCl3): 158.8
(Ar–C1), 139.2 (Ar–C6), 131.6 (Ar–C3), 130.5 (Ar–C5), 126.8
(Ar–C4), 117.1 (Ar–C2), 39.3 (SCH2CH2S), 35.2 [6-C(CH3)3],
29.3 [6-C(CH3)3], 20.6 (4-CH3), 9.0 (Al–CH2CH3).
Experimental
General considerations
All operations were performed under an inert atmosphere
of argon using standard Schlenk-line or glovebox techniques.
Toluene and n-hexane were distilled under argon from sodium/
benzophenone ketyl prior to use. n-Pentane was purified by
distillation from sodium/triglyme benzophenone ketyl. Alu-
minium alkyls (Aldrich) were used as received. 1,4-Dithia-
butanediyl-bis(6-tert-butyl-4-methylphenol) was synthesized ac-
cording to a modification of the methods in the literature.15
All other chemicals were commercially available and used after
appropriate purification. NMR spectra were recorded on Bruker
DRX 400, Varian 200 or 500 spectrometers at 25 ◦C unless
otherwise stated. Chemical shifts for 1H and 13C NMR spectra
were referenced internally using the residual solvent resonances
(CDCl3, 7.24 ppm; C6D6, 7.15 ppm) and reported relative to
tetramethylsilane. Elemental analyses were performed by the
Microanalytical Laboratory in University of Mainz.
[(xytbmp)AlMe] (3). The procedure was similar to above, ex-
cept that ortho-xylylenedithio-bis(6-tert-butyl-4-methylphenol)
(xytbmpH2) (0.247 g, 0.500 mmol) and 2.0 M solution of
trimethylaluminium in hexane (0.25 mL, 0.171 g, 0.500 mmol)
were used, colorless prismatic crystals were isolated in 79% yield
(0.210 g, two crops). Anal. Calcd. for C31H39AlS2O2: C 69.63, H
7.35, S 11.99%; found: C 69.67, H 7.87, S 12.35%. dH (500 MHz,
CDCl3): 7.28 (d, 2 H, 4J = 2 Hz, 5-H), 7.12 (d, 2 H, 4J = 2 Hz,
3-H), 7.04 (dd, 2 H, 3J = 5.8 Hz, 4J = 3.3 Hz, Bz), 6.69 (dd, 2 H,
3J = 5.8 Hz, 4J = 3.3 Hz, Bz), 4.03 (s, 4 H, SCH2), 2.31 (s, 6 H,
4-CH3), 1.32 [s, 18 H, 6-C(CH3)3], −0.80 (s, 3 H, Al–CH3); dC
(125 MHz, CDCl3): 158.1 (Ar–C1), 139.2 (Ar–C6), 133.9 (Bz),
130.8 (Ar–C3), 130.4 (Ar–C5), 130.1 (Bz), 128.7 (Bz), 127.0
(Ar–C4), 118.3 (Ar–C2), 42.0 (SCH2), 35.1 [6-C(CH3)3], 29.2
[6-C(CH3)3], 20.8 (4-CH3).
Synthesis of the complexes
xytbmpH2. To the solution of 2-tert-butyl-6-mercapto-4-
methylphenol (10.287 g, 52.50 mmol) in methanol (10 mL),
sodium hydroxide (2.099 g, 52.50 mmol) was added as solid.
The mixture was warmed gently until no solid was left. Then the
solution of 1, 2-bis(bromomethyl)benzene (6.927 g, 26.30 mmol)
in toluene (20 mL) was added dropwise at 60 ◦C. The reaction
mixture was refluxed for 3 h. All the volatiles were rotary evap-
[(etbmp)Al(OiPr)]. To the solution of complex 1 (0.229 g,
0.500 mmol) in n-pentane (10 mL), a solution of isopropanol
(0.030 g, 0.50 mmol) in n-pentane (2 mL) was added dropwise
via a syringe. After addition a clear solution was obtained which
turned turbid immediately. The white suspension was stirred for
16 h. After filtration, the white solid was washed with n-pentane
(3 × 5 mL) and dried under vacuum (in quantitative yield). Anal.
D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7
7 2 5

View Article Online
Calcd. for [C27H39AlS2O3]n: C 64.51, H 7.82, S 12.76%; found:
C 64.46, H 7.94, S 13.19%. dH (200 MHz, CDCl3): 6.99–6.93
(br d, 4 H, Ar–H), 4.90 [br s, H, OCH(CH3)2], 2.67 (br s, 4 H,
SCH2), 2.17 (s, 6 H, 4-CH3), 1.60–0.80 [br m, 24 H, 6-C(CH3)3,
OCH(CH3)2].
For the graphical representation, the program ORTEP-III was
used as implemented in the program system WINGX.
CCDC reference numbers 244970, 244971, 244972 and 254740
for complexes 1–4.
See http://www.rsc.org/suppdata/dt/b4/b416875e/ for cry-
stallographic data in CIF or other electronic format.
[(etbmp)Al(OCPh3)] (4). To a suspension of Ph3COH
(0.027 g, 0.10 mmol) in n-hexane (3 mL) was added dropwise a
solution of complex 1 (0.050 g, 0.011 mmol) in n-hexane (2 mL).
After some minutes the reaction solution became pale yellow and
a white solid precipitated gradually. The suspension was stirred
for 3 h. After filtration, the white precipitates were washed with
1 mL of n-hexane and dried under vacuum to give white powders
in 52% yield (40 mg). Anal. Calcd. for C43H47AlS2O2: C 73.47,
H 6.74, S 9.12%; found: C 72.90, H 7.50, S 8.53%. dH (400 MHz,
CDCl3): 7.38–7.41 (dd, 6 H, J = 8 Hz, J = 2 Hz, m-Ph–H),
7.10–7.15 (m, 9 H, o, p-Ph–H), 7.04 (d, 4 H, 3,5-Ar–H), 3.01–
2.98 (br m, 4 H, SCH2), 2.22 (s, 6 H, 4-CH3), 1.28 [s, 18 H,
6-C(CH3)3]; dC (100.6 MHz, CDCl3): 157.6 (Ar–C1), 149.6 (Ph-
C), 138.9 (Ar–C6), 130.9 (Ar–C3), 130.3 (Ar–C5), 128.0 (Ph-
C), 127.2 (Ph-C), 126.7 (Ar–C4), 125.9 (Ph-C), 117.0 (Ar–C2),
82.7(CPh3), 39.0 (SCH2), 35.0 [6-C(CH3)3], 29.1 [6-C(CH3)3],
20.4 (4-CH3).
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (DFG) and the
Fonds der Chemischen Industrie for financial support.
References
3
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rac-Lactide (Aldrich) was recrystallized with dry toluene and
sublimated twice under vacuum at 50 ◦C. Spectroscopic analysis
of polymers was performed in CDCl3. Isopropanol was refluxed
over CaH2 and distilled under argon. Molecular weights and
polydispersities were determined by size-exclusion chromatog-
◦
raphy (SEC) in THF at 35 C, at a flow rate of 1 mL min−1
utilizing an Agilent 1100 Series HPLC, G1310A isocratic pump,
an Agilent 1100 Series refractive index detector and 8 × 600 mm,
8 × 300 mm, 8 × 50 mm PSS SDV linear M columns. Calibration
standards were commercially available narrowly distributed
linear polystyrene samples which cover a broad range of molar
masses (103 < M < 2 × 106 g mol−1).
To a solution of rac-lactide (1.081 g, 7.5 mmol) and iso-
propanol (4.5 mg, 0.075 mmol) in toluene (13.2 mL), the solution
of aluminium complex 1 (34.4 mg, 0.075 mmol) in toluene
(1.8 mL) was injected. The total volume was 15 mL. The mixture
was then immerged into an oil bath of 70 ◦C for polymerization.
1 mL of polymerization aliquots were withdrawn at appropriate
time intervals, and quenched with cold pentane. Monomer
conversion was monitored by integration of monomer vs.
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into methanol. The co◦llected polymers were further dried in
1
a vacuum oven at 60 C for 16 h and analyzed by H, 13C,
homonuclear decoupled 1H NMR.
Crystal structure analysis of 1–4
Relevant crystallographic data are summarized in Table 1. The
data of complexes 1, 3 and 4 were collected with a Bruker AXS
diffractometer and reduced with the program system SMART.22
The data of 2 were collected with an Enraf Nonius CAD4
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D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7

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only complex 3 was isolated in high yield. Attempts to obtain the
molecular weight of this white precipitate by mass spectrometry
failed. 1H NMR: dH (CDCl3, 200 MHz): 7.00 (s, 4 H), 6.65 (m,
2 H), 6.20 (s, 2 H), 4.19 (br s, 4 H), 1.95 (s, 6 H), 1.41 (s, 18 H), −0.45
(s, 3 H).
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D a l t o n T r a n s . , 2 0 0 5 , 7 2 1 – 7 2 7
7 2 7