Phenoxy-thioether aluminum complexes as ε-caprolactone and lactide polymerization catalysts

Article abstract of DOI:10.1021/om300505m

A series of new phenoxy-thioether (OS) proligands have been synthesized. They were found to readily react with 1 equiv of AlMe₃ to afford the corresponding Al chelate complexes {4,6-tBu₂-OC₆H ₂-2-CH₂S(2-R-C₆H₄)}AlMe₂ (R = H (1), Br (2), CH₃ (3), CF₃ (4)) in quantitative yields. All the aluminum methyl complexes are stable monomeric species. In the solid state, as determined from X-ray crystallographic studies, complex 2 consists of a four-coordinate aluminum species in which the metal center is chelated by the sulfur and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and l- and rac-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of rac-LA were converted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by 1-4 are first order with respect to monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters, increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide.

Full text of DOI:10.1021/om300505m

Article
pubs.acs.org/Organometallics
Phenoxy-Thioether Aluminum Complexes as ε‑Caprolactone and
Lactide Polymerization Catalysts
Marina Lamberti,*,† Ilaria D’Auria,† Mina Mazzeo,*,† Stefano Milione,† Valerio Bertolasi,‡
and Daniela Pappalardo§
†Dipartimento di Chimica e Biologia, Universita
̀
di Salerno, via Ponte don Melillo, I-84084, Fisciano, Salerno, Italy
di Ferrara, I-44100 Ferrara, Italy
del Sannio, via dei Mulini 59/A, I-82100, Benevento,
‡Dipartimento di Chimica e Centro di Strutturistica Diffrattometrica, Universita
̀
§Dipartimento di Scienze per la Biologia, la Geologia e l’Ambiente, Universita
Italy
̀
S
* Supporting Information
ABSTRACT: A series of new phenoxy-thioether (OS) proligands have
been synthesized. They were found to readily react with 1 equiv of AlMe3
to afford the corresponding Al chelate complexes {4,6-tBu2-OC6H2-2-
CH2S(2-R-C6H4)}AlMe2 (R = H (1), Br (2), CH3 (3), CF3 (4)) in
quantitative yields. All the aluminum methyl complexes are stable
monomeric species. In the solid state, as determined from X-ray
crystallographic studies, complex 2 consists of a four-coordinate
aluminum species in which the metal center is chelated by the sulfur
and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and L- and
rac-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the
cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of rac-LA were
converted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by 1−4 are first order with respect to
monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the
polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho
position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters,
increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide.
initiate the ROP of lactides and lactones.4 In particular, it was
INTRODUCTION
■
shown that the imino substituents strongly affect the perform-
ance of the initiatiors, in some cases providing living catalysts
with high initiation efficiency.6 Notable examples are the related
salen aluminum complexes for their ability to promote
stereoselective polymerization of rac-lactide.7
The interest in the synthesis of aliphatic polyesters stems
mostly from their biodegradability, in light of recent concerns
with the environment, and from their biocompatibility, which
make them suitable materials for medical and pharmaceutical
applications.1
While the majority of these aluminum complexes bearing
multidentate ligands contain nitrogen or oxygen as neutral
donor atoms, anionic chelating ligands incorporating soft
donors such as a phosphorus or sulfur atom have been much
less explored for coordination to the hard aluminum center.8 In
this regard, Okuda described aluminum complexes containing
dianionic tetradentate OSSO-type ligands which acted as
efficient initiators for the living polymerization of rac-lactide
in the presence of isopropyl alcohol.9
We recently reported new monoanionic bidentate ligands,
having a phenoxo group and oxygen or sulfur atoms as
additional neutral donors, for the synthesis of octahedral
bis(chelate) group 4 metal complexes as catalysts for ethylene
and α-olefin polymerization.10
In particular, polycaprolactone (PCL) is degraded by
hydrolysis of its ester bonds under physiological conditions
and has therefore received a great deal of attention for use as an
implantable biomaterial and as a drug delivery system.2
Polylactide (PLA) represents a potential candidate to replace
traditional olefin-based polymers as an ecological thermoplastic
resin from renewable resources.3
Among the various methods exploited, ring-opening
polymerization (ROP) of the related cyclic esters promoted
by metal initiators via a coordination−insertion mechanism is
the most efficient route, allowing the production of polyesters
with good control in terms of molecular weight, microstructure,
and stereoregularity.4
Due to their high Lewis acidity and low toxicity, aluminum
compounds, especially Al alkoxides and aryloxides, are well-
suited initiators for the ROP of cyclic esters such as lactides and
lactones.4,5 Among these, it is worth noting that phenoxy-imine
based aluminum complexes have been found to be able to
Received: June 11, 2012
Published: August 1, 2012
© 2012 American Chemical Society
5551
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
As an extension of our previous work and taking into account
the interest devoted to aluminum compounds bearing phenoxy-
based ligands as catalysts for polar monomer polymerization,
aluminum compounds bearing this new class of ligands were
synthesized, characterized, and then tested as initiators for the
ROP of ε-caprolactone (ε-CL), D,L-lactide (rac-LA), and L-
lactide (L-LA).
RESULTS AND DISCUSSION
■
Synthesis and Structure of Phenoxy-Thioether Dime-
thylaluminum Complexes 1−4. Four ligands are described
in this work, each including a di-tert-butylphenoxo anionic
donor and an aromatic thioether neutral donor differently
substituted on the aromatic ring. The ligands H-L1−H-L4 were
synthesized by reacting the suitable thiophenol with 2-
(bromomethyl)-4,6-di-tert-butylphenol using dry dimethyl
formamide as the solvent according to a published procedure.10
Aluminum complexes 1−4 were obtained, in quantitative
yields, through a classical alkane elimination route with
trimethylaluminum in benzene (Scheme 1). Solution 1H
Scheme 1. Synthetic Route for Complexes 1−4
Figure 1. ORTEP view of the complex 2 showing thermal ellipsoids at
the 30% probability level. Selected bond distances (Å) and angles
(deg): Al(1)−C(1), 1.942(3); Al(1)−C(2), 1.939(4); Al(1)−S(1)
2.514(1); Al(1)−O(1) 1.749(2); C(1)−Al(1)−C(2), 119.2(2);
C(1)−Al(1)−S(1), 107.0(1); C(1)−Al(1)−O(1) 116.7(1); C(2)−
Al(1)−O(1), 113.1(1); C(2)−Al(1)−S(1), 104.5(1); O(1)−Al(1)−
S(1), 91.3(1).
NMR studies of the reactions performed in benzene-d6
indicated that the neutral proligands were deprotonated by
one metal alkyl of trimethylaluminum with the release of 1
equiv of methane. The disappearance of the O−H signal of the
free ligands and the appearance of a resonance for the protons
of two equivalent methyls bound to the aluminum in the high-
field region of the 1H NMR spectra demonstrated the
formation of the desired complexes.
the distortion is the angle-based parameter τ.18 It determines
the extent to which observed geometries are more like
tetrahedral or TMP geometries.19 An ideal tetrahedron has τ
= 0, and an ideal vacant trigonal bipyramid has τ = 1. Using this
measure, the Al atom of complex 2 has τ = 0.51; this indicates
that the Al atom is about halfway between a tetrahedron and an
axially vacant trigonal bipyramid.
In order to get more insight into the electronic structures of
phenoxy-thioether aluminum complexes and to explore the
effect of the substituents of the aromatic thioether ring,
theoretical calculations were undertaken. Density functional
theory (B3LYP) and Møller−Plesset second-order perturbation
theory (MP2) were chosen as quantum chemical methods.
Complex 2 was chosen to evaluate the performance of the
selected methods. The Al−ligand bond lengths in the
optimized structures were measured and compared with
corresponding experimental values from the X-ray structure.
The results show that the B3LYP functional reproduces with
moderate accuracy the geometry of the selected complex. In
particular, the Al−S bond distance is overestimated by 0.10 Å.
MP2 was found to perform much better and led to an
acceptable value for the same Al−S distance, with an
underestimation of 0.01 Å. In order to shed light on the
strength of Al−ligand interactions in this class of complexes,
Mayer bond orders were computed. The results of the
calculations are summarized in Table 1. Very low bond orders
were found for the Al−S bonds (ranging between 0.28 and
0.31), suggesting a small overlap between orbitals (or weak
covalent bonding), whereas for the Al−C bonds the values were
close to 1.
Complexes 1−4 were characterized by elemental analysis,
NMR spectroscopy, and single-crystal X-ray diffraction (2).
Single crystals of 2 were grown from a saturated hexane
solution at −20 °C. An ORTEP11 view and selected bond
distances and angles of complex 2 are shown in Figure 1 (see
Tables S1 and S2 in the Supporting Information for
crystallographic details). X-ray analysis established the mono-
meric nature of complex 2 and revealed a distorted-
tetracoordinate geometry of the Al center effectively κ2-chelated
by the sulfur and oxygen atoms of the bidentate ligand. The
Al−S bond length is 2.514(1) Å: i.e., in the range of other
reported aluminum−thioether bond distances for coordination
number 4.12 The Al−O bond length of 1.749(2) Å is slightly
lower than those of other phenolate−Al complexes. The
Al(1)−O(1)−C(3)−C(8)−C(9)−S1 six-membered chelating
ring displays a screw-boat S6 conformation13 with the following
puckering parameters: QT = 0.872(2) Å, φ2 = −90.9(2)°, θ2 =
66.5(1)°.14 Distorted-tetracoordinate Al complexes tend to
adopt structures that are intermediate between the classical
tetrahedral geometry and the more unusual trigonal-monopyr-
amidal (TMP) geometry. These distortions may produce
enhancement of the Lewis acidity of the metal center in
comparison with that of the tetrahedral analogues, leading to
very promising catalysts.15−17 A quantitative measurement of
We were also interested in evaluating the influence of the
substituent of the aromatic thioether ring on the Lewis acidity
5552
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
The VT NMR analysis of complex 2 showed that, at
subambient temperature, the resonance of methyl groups
bonded to aluminum and that of the methylene bridge broaden
and resolve each into two separate peaks (Figure 2).
Coalescence of the signals was observed at −40 °C; below
this temperature the resonances became sharp and well
Table 1. Results of the Natural Bond Orbital (NBO)
Analysis for the Aluminum Complexes: Bond Distances (d in
Å), Mayer Bond Orders (bo), and Natural Charges at
Aluminum (QAl in e) and Sulfur (QS in e)
complex
dAl−S
bo
QAl
QS
1
1
2
3
4
2.536
2.567
2.540
2.545
0.309
0.282
0.302
0.305
1.797
1.823
1.810
1.801
0.311
0.353
0.327
0.330
resolved and the H NMR spectra are consistent with the
chelation of one bidentate ligand to the metal center and with
the solid-state structure being retained in solution.
Analogous fluxional behavior was observed for complexes 1,
3, and 4, with coalescence temperatures in the range −40 °C to
−50 °C.
of the metal center in complexes 1−4. Two aspects were
considered: the Al−S bond distance and the natural charges on
the S and Al atoms (Table 1). The electronic charge on the Al
atom and the Al−S bond distance can give indirect measure-
ments of the Lewis acidity at the metal center: the greater the
charge (or the longer the bond), the greater the Lewis acidity.
The longest bond distance was observed for complex 2, while
the shortest distance was observed for complex 1. The other
two complexes featured intermediate values. Natural population
analysis for Al and S atoms shows that the partial charges on
these atoms have a similar trend. The phenoxo-thioether Al
complexes reported in this study should display the following
order of increasing Lewis acidity: 1 < 3 ≈ 4 < 2. However, the
changes in electronic charges and bond distances are moderate
and the influence of the substituent of the aromatic thioether is
expected to be modest.
Kinetic parameters were calculated for complexes 1 and 2
1
using line shape analysis of the H NMR data measured over
the temperature range 293−193 K in dichloromethane-d2;
1
selected H NMR spectra and calculated exchange rates are
shown in the Supporting Information. The free energies of
activation for the fluxional processes were calculated from the
line shape analysis of the Al−Me protons to be ΔG⧧ = 11.61
0.90 kcal mol−1 and ΔG⧧ = 11.81 0.09 kcal mol−1 at 293 K
for 1 and 2, respectively. The activation parameters were ΔH⧧
= 8.37 0.08 kcal mol−1 and ΔS⧧ = −11.05 3.36 cal mol−1
K−1 for complex 1 and ΔH⧧ = 6.04
0.26 kcal mol−1 and
ΔS⧧= −19.38 1.19 cal mol−1 K−1 for complex 2. The data are
consistent with a fast conformation change of the six-
membered-ring aluminum metallacycle. A similar fluxional
behavior was observed for other tetracoordinate aluminum
complexes.8,20,21
Solution Structures of Aluminum Complexes 1−4.
1
Formation and Reactivity of Cationic Species. There
has been a growing interest in the synthesis of cationic
aluminum complexes, since the enhanced Lewis acidity of the
aluminum center should yield higher catalytic activity and may
lead to new applications.22
The ionization chemistry of N,N- and N,O-based Al neutral
dialkyls has yielded novel families of highly Lewis acidic Al
species, differently the synthesis and the study of the reactivity
of cationic Al alkyl species supported by “softer” chelating
ligands has still been scarcely explored.8,23 Within this context,
the synthesis of cationic aluminum compounds supported by
bidentate OS ligands could be interesting, as these ligands
combine a hard oxygen donor with a soft sulfur donor for the
coordination to the reactive center.
Thus, the reaction of Al methyl complex 1 was carried out
with 1 equiv of B(C6F5)3, a classical methide abstracting
reagent, to generate the corresponding Al cation.
The addition of 1 equiv of B(C6F5)3 to a benzene-d6 solution
of (OS)AlMe2 (1) afforded the neutral complex (OS)Al-
(C6F5)Me in 24 h. The 1H NMR spectrum exhibited a triplet at
The H NMR spectra of complexes 1−4 at room temperature
showed no complexity and indicated the presence of symmetric
structures; a sharp single resonance assigned to the Al−Me
protons in the range −0.31 to −0.25 ppm, a sharp singlet
ascribed to the S-CH2-Ar methylene protons, and resonances
corresponding to hydrogens of the aromatic rings were
observed.
The uniqueness and the symmetry of these species in
benzene-d6 solution at room temperature are corroborated by
the observation of a single set of resonances in the 13C NMR
spectra. The 13C NMR spectra of complexes 1−4 all exhibited a
single characteristic resonance for the carbon atoms of the
AlMe2 group in the region −8.60 to −7.80 ppm.
These data agree with an overall Cs symmetry for complexes
1−4 on the NMR time scale at room temperature and suggest a
stereochemically nonrigid coordination environment at the
metal center. The fluxionality supposed for the phenoxy-
thioether aluminum complexes 1−4 in solution at room
temperature has been investigated by variable-temperature
(VT) NMR analyses.
1
Figure 2. Variable-temperature H NMR spectra of 2 in dichloromethane-d2.
5553
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
−0.16 ppm (JHF = 1.12 Hz) characteristic of an Al−Me
resonance coupled to the α-fluorines of a coordinated C6F5
group.24 The methylene bridge of the OS fragment gave a well-
resolved AB resonance centered at 3.68 ppm (Δδ = 0.21 ppm, J
= 12.44 Hz); the diasterotopic nature of these protons is
consistent with a strong coordination of the sulfur atom to the
“harder” oxygen of the THF external donor. As a matter of fact,
the signals for the THF coordinated to the aluminum atom
appeared at 4.20 and 2.18 ppm (OCH2 and OCH2CH2,
respectively), downfield with respect to the resonances of the
“free” THF in the same solvent. Analogous results were
previously observed for closely related phenoxy-imine and
anilinotroponato aluminum compounds.28
1
asymmetric aluminum center. Two quintets in the same H
NMR spectrum at 1.33 and 0.96 indicated the presence of
MeB(C6F5)2 and Me2B(C6F5) in a 4:1 molar ratio, respectively.
The 19F NMR spectrum revealed, inter alia, the presence of
The reactivity of the cationic species thus formed toward ε-
caprolactone was successively studied. One equivalent of ε-
caprolactone was added to the same NMR tube, and a H
1
−
MeB(C6F5)3 , suggesting that the species (OS)Al(C6F5)Me is
NMR spectrum was registered. Interestingly, the resonances of
the THF were shifted to higher field (3.90 and 1.93, OCH2 and
OCH2CH2, respectively), toward those of the free THF species
(3.69 and 1.82), while the resonances of the ε-caprolactone
appeared as broad multiplets and were shifted downfield (4.47
ppm, O−CH2; 2.71 ppm, COCH2). Notably, the same
resonances of the “free” ε-caprolactone in dichloromethane-d2
appeared at 4.17 and 2.57 ppm, respectively. Moreover, the
AlMe+ and the B(C6F5)3Me− singlet signals still appear at the
previous chemical shifts, as well as the ligand’s resonances. The
whole picture may suggest a rapid intermolecular exchange
process between the THF and the ε-caprolactone coordinated
to the aluminum cation (Scheme 2). A similar exchange process
between bound and “free” THF molecule was previously
observed by Gibson28a for analogous systems.29
−
formed by the transfer of a C6F5 group from MeB(C6F5)3 by
the first formed coordinatively unsaturated cationic intermedi-
ate.
Analogous results have been previously reported for the
reaction of dialkylaluminum complexes bearing bidentate N,N
or N,O ligands with B(C6F5)3, leading to unstable tricoordinate
alkylaluminum cations which react quite quickly by abstracting
−
a C6F5 group from MeB(C6F5)3 anions to form neutral
LnAl(C6F5)R products.24 Several studies have shown that the
presence of a Lewis base B can stabilize the aluminum cation by
forming LnAlR(B)+ adducts.20,25 Gibson et al. showed that a
pendant donor group, which is weakly bonding or nonbonding
in the neutral aluminum complexes, becomes a normal donor
group in the cationic derivatives upon reaction with B(C6F5)3,
thus stabilizing these species.26
The same sequence of experiments was performed on
compound 3 and produced analogous results.
Polymerization Studies. Ring-Opening Polymerization
of ε-Caprolactone. The reactivity of compounds 1−4 in the
ring-opening polymerization of ε-CL was studied (Scheme 3).
Therefore, the “trapping” of the cationic species was obtained
in the presence of THF as donor molecule in dichloromethane-
d2 solution. When B(C6F5)3 (1 equiv) was added to a
dichloromethane-d2 solution of 1 containing 1 equiv of THF,
a THF-coordinated aluminum methyl cation was formed
(Scheme 2). Characteristic resonances in the 1H NMR
Scheme 3. Ring-Opening Polymerization of ε-Caprolactone
Initiated by Complexes 1−4
Scheme 2. Formation of Cationic Species of Complex 1
Polymerization screenings were performed under a nitrogen
atmosphere in a toluene solution of ε-CL and the proper
aluminum compound. The polymers, precipitated from the
reaction solution by addition of hexane, were analyzed by NMR
and gel permeation chromatography (GPC).
The main results of the polymerization studies performed
with complex 1 are summarized in Table 2.
Aluminum complex 1 exhibits good activity toward the ROP
of ε-CL in the absence of alcohols (runs 1 and 2, Table 2). Two
polymerization experiments carried out at 50 and 70 °C
demonstrated that the conversion increased significantly with
temperature (runs 1 and 2, Table 2). At 50 °C full conversion
was obtained in 180 min. Under these reaction conditions, the
initiation efficiency was 52%, suggesting that only half of the
catalytic centers were active. The broad molecular weight
distribution would reflect, at least in part, the poor initiation
efficiency of the methyl group. On the other hand, when the
same experiment was performed at 70 °C, 250 equiv of
monomer were converted in 40 min with a slightly better
agreement between calculated and experimental molecular
weight values, suggesting that when temperature is increased,
the efficiency of monomer first insertion on the Al−methyl
bond increases.
spectrum are signals at δ −0.31 for Al−Me, appearing at
lower field in comparison with the resonance of the neutral
compound 1 (δ −0.63 in dichloromethane-d2) as a
consequence of the positive charge on the aluminum atom,
and a broad resonance at δ 0.46 for B(C6F5)3Me−, characteristic
of the “free” anion,27 thus suggesting that there are no relevant
interactions between the cation and anion, at least under the
experimental conditions used. The methylene bridge of the OS
fragment gave in this case a singlet at 4.06 ppm, shifted to
higher field with respect to the same signal in the neutral
compound (4.20 ppm): this opposite effect, with respect to the
AlMe+ shift behavior, may be due to a decreased coordination
of the “soft” sulfur atom, supplied by the presence of the
As is well known in the literature, amido and alkyl initiators
are usually inferior initiating groups with respect to metal
5554
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
Table 2. Ring-Opening Polymerization of ε-CL Promoted by Complex 1
a
b
c
d
d
run
amt of MeOH (equiv)
temp (°C)
time (min)
conversn (%)
Mn(th) (×103)
Mn(GPC) (×103)
Mw/Mn
1
2
3
4
5
6
7
50
70
25
50
70
70
70
70
180
40
100
70
41.1
28.8
41.1
41.1
41.1
19.7
3.9
79.6
44.1
50.3
50.8
44.2
23.9
5.2
1.77
1.46
1.40
1.30
1.49
1.26
1.11
1
1
1140
130
30
100
100
100
96
1
2
30
10
1
30
94
e
8
30
100
4.1
2.8
1.31
a
b
1
General conditions: complex 1, 25 μmol; toluene, 4 mL; [ε-CL]/[Al] = 360; MeOH, 1 equiv. Conversion of ε-CL as determined by H NMR
c
d
spectral data. Mn(th) (g mol−1) = 114.14([ε-CL]0/[I]0) × conversion of ε-CL. Experimental Mn (g mol−1) and Mw/Mn values determined by GPC
in THF against polystyrene standards and corrected using the factor 0.56. [ε-CL]/[Al] = 36.
e
alkoxides for the ROP of cyclic esters. The alkoxide initiating
group mimics the propagating groups of the presumed active
species and complexes with these moieties produce polymers of
predictable molecular weights and with narrow molecular
weight distributions. Alkoxide initiators can be generated in situ
by alcoholysis of amido and alkyl complexes with alcohols.5,6a,30
In this context, three polymerization experiments were
conducted at different temperature by adding 1 equiv of
methanol (runs 3−5, Table 2). Aside from confirming the
beneficial effect of the temperature on the polymerization rate,
it was observed that all the obtained PCLs possess quite narrow
molecular weight distributions (Mw/Mn = 1.30−1.49) with
unimodal characteristics. The Mn values determined by GPC
(and corrected by the factor 0.56)31 match excellently those
calculated for a controlled polymerization, on the assumption
that a single PCL chain is produced per metal center through
initiation of the polymerization by the alkoxide group. These
observations reflected the single-site nature of the active species
and confirmed that, under these conditions, all the aluminum
centers are active.
ization process; therefore, a coordination−insertion mecha-
nism, proceeding through acyl−oxygen cleavage of the
monomer, should be operative in this system.33 Moreover,
the agreement between the Mn value determined by GPC
(Mn(GPC) = 2804) and that determined by NMR (Mn =
3424) indicated that the polymerization proceeds exclusively by
the mechanism detailed above.
To evaluate the potential of cationic aluminum compounds
supported by bidentate OS ligands in the ROP of ε-
caprolactone, a polymerization test was performed under the
same reaction conditions used for the neutral species (see run 1
in Table 2). The cationic species, generated in situ by reaction
of complex 1 and 1 equiv of B(C6F5)3, was found to initiate the
ROP of ε-CL with a very low activity in comparison with the
corresponding neutral derivative (a conversion of 15% was
obtained in 180 min). The lack of activity of the cationic
species in ε-CL polymerization under the conditions studied
can be ascribed to the strong Lewis acidity of the Al metal
center of the cationic species that should form a robust Lewis
acid/base adduct with the cyclic ester.
To explore the possibility of achieving immortal polymer-
ization32 with these systems, experiments were conducted in
the presence of an excess of methanol as chain transfer agent.
Representative results are shown in Table 2 (runs 6 and 7).
Increasing the amount of methanol did not significantly affect
the polymerization rate, and the obtained PCLs showed
monomodal, narrow distributions with a good correlation
between the experimental and calculated Mn values. These
observations establish that the metal complex is stable in the
presence of large amounts of free alcohol (i.e., the ligand is not
displaced) and a fast reversible exchange between free alcohol
and growing PCL chains takes place during the polymerization
process. These processes compete with monomer propagation;
however, since the observed PCL polydispersivities are low, this
exchange reaction between alkoxides and free alcohol molecules
takes place faster than chain propagation.32b
In an effort to investigate the effects of modifying the steric
and electronic environment around the reactive center, we
carried out a study of the polymerization behavior of phenoxy-
thioether aluminum complexes 1−4 by performing ε-CL
polymerization under the same reaction conditions (see
Table 3).
The aluminum complexes 1−4 were all effective initiators for
the ROP of ε-CL at 50 °C. Good activities were observed under
these conditions with turnover frequencies (TOF) up to 900
h−1. These activity data compare favorably with those of related
phenoxy-imine and -amine catalysts under the same reaction
Table 3. Ring-Opening Polymerization of ε-CL Initiated by
Complexes 1−4
conversn
Mn(th)
c
Mn(GPC)
d
M /
wd
a
b
run complex
(%)
(×103)
(×103)
Mn
In order to confirm that the alkoxide ligand for this complex
is the true initiatior of the ROP process, low-molecular-weight
PCL was obtained by carrying out a polymerization experiment
at a low CL/Al ratio (run 8, Table 2). 1H NMR analysis of this
PCL sample disclosed the presence of methyl ester end groups
(−COOCH3; 3.65 ppm), generated via insertion of the
monomer unit into the Al−OCH3 bond with cleavage of the
acyl−oxygen bond of the monomer, and hydroxyl end groups
(CH2CH2OH; 3.62 ppm), generated by hydrolysis of the
growing chain. No signals were observed in the aromatic
region. These data confirm that the terminal alkoxide OMe
group is the only initiating moiety involved in the polymer-
9
1
2
3
4
36
82
79
78
14.8
33.7
32.5
32.0
18.2
34.6
38.7
24.0
1.37
1.27
1.39
1.27
10
11
12
a
General conditions: complex, 12.5 μmol; toluene, 2 mL; CL/Al =
360; cocatalyst (MeOH), 1 equiv; temperature, 50 °C; reaction time,
b
20 min. Conversion of ε-CL as determined by 1H NMR spectral data.
c
Mn(th) (g mol−1) = 114.14([ε-CL]0/[I]0) × conversion of ε-CL.
d
Experimental Mn (in g mol−1) and Mw/Mn values determined by
GPC in THF against polystyrene standards and corrected using the
factor 0.56.
5555
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
conditions,6 and result much better than those obtained with
the O,P-phosphinophenolate aluminum complexes.8
From the data reported in Table 3 it is found that the
presence of a substituent in the ortho position of the aromatic
thiophenol ring results in a beneficial effect in terms of catalytic
activity; in fact, complexes 2−4 all were more active than
unsubstituted complex 1, giving similar conversions.
Kinetic investigations were conducted by NMR experiments
to establish the reaction order with respect to monomer
concentration.
could be explained in light of the sterically demanding character
of the monomer lactide and with the increased steric
encumbrance around the reactive center due to the possible
chelation by a carbonyl group of the lactate arm to form a five-
membered-ring η2-lactate−Al unit. This coordination is
frequently observed, for both cationic and neutral aluminum
species.21
All polymerizations proceeded in a living fashion, leading to
polymers with monomodal and very narrow molecular weight
distributions (Mw/Mn = 1.12−1.16). In fact, the experimental
number average molecular weights (Mn) showed a good
agreement with the theoretical values and a linear correlation
with monomer conversion (Figure 4).
The polymerization kinetics were studied with [ε-CL]0/[Al]0
= 100 at 50 °C using toluene-d8 as solvent. For all complexes
the polymerization obeyed first-order kinetic in monomer with
instantaneous initiation (Figure 3). Within the series of
The addition of excess alcohol as an initiator/chain transfer
agent accelerated the reaction and an effective immortal
polymerization was achieved (cf. runs 14 and 17, Table 4).
Complexes 1−4 were active in the polymerization of rac-
lactide (runs 18−21, Table 4). In all cases the homodecoupled
1H NMR spectra of the methine regions of PLAs show atactic
microstructures.
1
The H NMR spectra in CDCl3 of PLAs (see Figure S8 in
the Supporting Information) obtained with methoxide systems
(1−4/MeOH) show a resonance at δ 3.73 ppm for the methyls
of a COOCH3 group and a broadened quartet at δ 4.35 and a
doublet at 2.66 ppm characteristic of the CH(Me)OH terminal
group (the doublet for the methylic protons of the same group
mostly overlaps with those of PLA). The observation of these
end groups suggests that the ROP proceeds via a classical
coordination/insertion mechanism: i.e., initial acyl−oxygen
bond cleavage by the transfer of the nucleophilic methoxide
group of the metal complex to the monomer with the eventual
formation of a metal alkoxide propagating species, which is
hydrolyzed at the end of the reaction. A strictly agreement
between the Mn value determined by GPC (Mn(GPC) =
10400) and that determined by NMR (Mn 1H NMR 10800)
was observed.
Figure 3. Pseudo-first-order kinetic plots for ROP of ε-CL promoted
■
●
by 1 ( ) and 2 ( ). The pseudo-first-order rate constants are (3.80
0.10) × 10−3 s−1 ( ) (R = 0.999) and (6.34 0.13) × 10−3 s−1 ( )
■
●
(R = 0.998). Conditions: [Al] = 1.0 × 10−2 M; [ε-CL]/[Al] = 100; T
= 50 °C; toluene-d8 as solvent.
CONCLUSIONS
complexes, the relative rates correlate well with the activities
observed in the polymerization runs reported in Table 3. The
fastest polymerization was observed for complex 2, having the
pseudo-first-order rate constant kapp = 6.34 × 10−3 s−1. Kinetic
plots for complexes 1 and 2 are reported in Figure 3.
Ring-Opening Polymerization of Lactides. Polymerizations
of L- and rac- lactide were performed with 1−4 in toluene
solution at 80 °C, in the presence of MeOH (Scheme 4).
Polymers were characterized by NMR and GPC.34 The main
results of the polymerization studies are summarized in Table 4.
■
The present work shows that bidentate phenoxy-thioether
(OS) ligands, which combine a hard phenoxide donor with a
soft sulfur donor, are suitable ligands in the coordination
chemistry of the hard Lewis acid aluminum(III), as they readily
form stable and monomeric monoadducts of the type
OSAlMe2. All aluminum complexes have been synthesized via
alkane elimination reactions in almost quantitative yields.
Their catalytic performances in ε-CL, L-LA, and rac-LA ring-
opening polymerization have been explored. In all cases the
polymerizations proceed in a controlled fashion and immortal
polymerization are achieved upon addition of an excess of
methanol, as a chain transfer agent. End group analyses of the
obtained oligomer samples support a classical coordination−
insertion mechanism.
Scheme 4. Ring-Opening Polymerization of Lactide Initiated
by Complexes 1−4
The substituents at the ortho position of the aromatic ring
bound to the sulfur atom have a moderate effect on the catalytic
behavior of the aluminum complexes. In particular, the
presence of a substituent at the ortho position of the
thiophenol aryl ring resulted in an increase of the catalytic
activity in the ε-caprolactone polymerization, regardless of the
size and the electronic character of the substituent, whereas in
the polymerization of lactide a decrease in the catalytic activity
was observed by increasing the size of the substituent.
In the ROP of L-lactide the 1/MeOH system exhibited the
highest catalytic activity among complexes 1−4 under the same
polymerization conditions (cf. runs 13−16, Table 4). The
polymerization data may suggest that the catalytic activity
decreases with the increase of steric hindrance offered by the
ortho substituent on the aromatic ring of the sulfur donor. This
5556
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
Table 4. Polymerization of L- and rac-Lactide Initiated by Complexes 1−4
a
b
c
d
d
run
cat.
amt of MeOH (equiv)
mon
time (h)
conversn (%)
Mn(th) (×103)
Mn(GPC) (×103)
Mw/Mn
13
14
15
16
17
18
19
20
21
1
2
3
4
2
1
2
3
4
1
1
1
1
4
4
4
4
4
L-LA
48
48
48
48
24
24
24
24
24
77
54
57
38
98
98
99
99
97
11.1
7.8
8.2
5.5
3.5
3.5
3.5
3.6
3.5
11.6
8.7
7.3
4.0
3.8
3.1
3.2
3.1
1.14
1.12
1.14
1.16
1.11
1.14
1.16
1.15
1.18
L-LA
L-LA
L-LA
L-LA
rac-LA
rac-LA
rac-LA
rac-LA
3.3
a
b
1
General conditions: complex, 12.5 μmol; toluene, 2 mL; T = 80 °C, [LA]/[Al] = 100. Conversion of LA was determined by H NMR analysis.
c
d
Mn(th) (in g mol−1) =144.14([LA]0/[I]0) × conversion of LA. Experimental Mn (g mol−1) and Mw/Mn values determined by GPC in THF against
polystyrene standards and corrected using the factor 0.58.
NMR tubes equipped with J. Young valves. NMR spectral simulations
were performed using WINDNMR (DNMR71.EXE)35 by taking into
1
account the H chemical shift variation of complex 1 as a function of
temperature: shifts were measured below the exchange region and
extrapolated into the temperature range where simulations were
performed. Final simulated line shapes were obtained via an iterative
parameter search upon the exchange constant k. Estimated standard
deviations (s) in the slope and the y intercept of the Eyring plot
determined the errors in ΔH⧧ and ΔS⧧, respectively. The standard
deviation in ΔG⧧ was determined from the formula σ2(ΔG⧧) =
σ2(ΔH⧧) + T2σ2(ΔS⧧) −2Tσ(ΔH⧧)σ(ΔS⧧).
The molecular weights (Mn and Mw) and the molecular mass
distribution (Mw/Mn) of polymer samples were measured by gel
permeation chromatography (GPC) at 30 °C, using THF as solvent,
an eluant flow rate of 1 mL/min, and narrow polystyrene standards as
reference. The measurements were performed on a Waters 1525
binary system equipped with a Waters 2414 RI detector using four
Styragel columns (range 1 000−1 000 000 Å). Every value was the
average of two independent measurements. It was corrected using the
factor 0.58 for polylactide and 0.56 for polycaprolactone according to
the literature.
Figure 4. Plot of number-averaged molecular weights, Mn (Da), and
PDI values versus conversion for PLLA produced by catalyst 2/MeOH
(1/1) at 80 °C. PDI values are given in parentheses.
Computational Details. All calculations were performed using the
Gaussian 03 program package.36 The complexes were energy-
minimized without symmetry constraints at the B3LYP37 levels of
theory using the standard 6-31G(d) basis set for C and H and the 6-
311G(d,p) basis set for all other atoms. Stationary point geometries
were characterized as local minima on the potential energy surfaces.
The absence of imaginary frequencies verified that structures were true
minima at their respective levels of theory.
The structure optimized at B3LYP levels of theory was used as a
starting point for successive reoptimization at the MP2 level using the
same combination of basis sets.
The chemical bonding in the complexes was analyzed by employing
the natural bond orbital (NBO) partition scheme by Weinhold and co-
workers38 using the natural bond orbital program (NBO 3.1),39 as
implemented in Gaussian 03. The NPA analysis was obtained by
performing MP2 single-point calculations at the previously MP2
optimized structures.
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations of air- and/or water-
sensitive compounds were carried out under a dry nitrogen
atmosphere using a Braun Labmaster drybox or standard Schlenk-
line techniques. Glassware and vials used in the polymerization were
dried in an oven at 120 °C overnight and exposed three times to
vacuum−nitrogen cycles.
Benzene, hexane, and toluene were distilled over sodium
benzophenone. THF was distilled over LiAlH4. Dimethylformamide
and AlMe3 were used as received. Deuterated solvents were dried
using molecular sieves. Lactide was purified by crystallization from dry
toluene. MeOH was purified by distillation over sodium. ε-CL was
dried with CaH2 for 24 h at room temperature and then distilled under
reduced pressure.
All other chemicals were commercially available and used as
received unless otherwise stated.
H-L1 was prepared according to a published procedure.10
Instruments and Measurements. Elemental analyses were
recorded on a Thermo Finningan Flash EA 1112 series C, H, N, S
analyzer in the microanalytical laboratory of the institute.
The NMR spectra were recorded on a Bruker Avance 400
spectrometer (1H, 400.13 MHz; 13C, 100.62 MHz) at 25 °C, unless
otherwise stated. Chemical shifts (δ) are given in parts per million and
Synthesis of Proligands and Complexes. Synthesis of H-L2.
The reaction was performed according the procedure previously
reported in the literature.10 To a stirred solution of 2-bromothiophe-
nol (0.927 g, 5.01 mmol) and K2CO3 in 50 mL of DMF at room
temperature was added a solution of 2-(bromomethyl)-4,6-di-tert-
butylphenol (1.50 g, 5.01 mmol) in 20 mL of DMF. The mixture was
stirred at room temperature for 3 h. The product was extracted with 50
mL of diethyl ether and 50 mL of water. The organic layer was washed
with water (3 × 25 mL). The organic solution was dried over sodium
sulfate. The solvent was removed under vacuum, giving a pale yellow
solid. Yield: 1.12 g; 50%.
1
coupling constants (J) in hertz. H NMR spectra are referenced using
the residual solvent peak at δ 7.16 for C6D6 and δ 7.27 for CDCl3. 13
C
NMR spectra are referenced using the residual solvent peak at δ
128.39 for C6D6 and δ 77.23 for CDCl3. 19F NMR spectra are
referenced to external CFCl3.
1H NMR (400.13 MHz, CDCl3, 298 K): δ 7.57 (d, J = 8 Hz, 1 H,
ArH), 7.37 (d, J = 8 Hz, 1 H, ArH), 7.22 (d, 4J = 2 Hz, 1 H, ArH), 7.08
1
Variable-temperature H NMR experiments were performed with a
4
Bruker AVANCE 400 (operating at 400.13 MHz) in CD2Cl2 using
(t, 1 H, J = 8 Hz, ArH), 7.07 (t, J = 8 Hz, 1 H, ArH), 6.90 (d, J = 2
5557
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
4
Hz, 1 H, ArH), 6.00 (s, 1 H, OH), 4.20 (s, 2 H, CH2), 1.44 (s, 9 H,
CCH3), 1.23 (s, 9 H, CCH3). 13C{1H} NMR (100.62 MHz, CDCl3,
298 K): 151.6 (q, CO), 142.7 (q, CS), 137.2 (q, CBr), 135.42 q, 133.3
(CH), 132.8 (CH), 128.8 (CH), 128.0 (CH), 126.8 q, 125.6 (CH),
124.2 (CH), 121.3 q, 36.7 (CH2), 35.2 (CCH3), 34.4 (CCH3), 31.7
(CCH3), 30.1 (CCH3).
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.56 (d, J = 2.4 Hz, 1H,
ArH), 6.90−6.68 (m, 5H, ArH), 3.73 (s, 2H, CH2), 2.06 (s, 3H, CH3),
1.68 (s, 9 H, CCH3), 1.30 (s, 9 H, CCH3), −0.30 (s, 6 H, AlCH3).
13C{1H} NMR (62.89 MHz, C6D6, 298 K): δ 156.6 (q, CO), 140.7
(q,CS), 139.4 q, 138.9 q, 131.2 (CH), 129.8 (CH), 129.2 (CH), 127.3
(CH), 126.0 (CH), 125.6 (CH), 120.6 q, 38.2 (CH2), 35.9 (CCH3),
34.6 (CCH3), 32.2 (CCH3), 30.9 (CCH3), 20.3 (ArCH3), −8.3
(AlCH3). Anal. Calcd for C24H35AlOS: C, 72.32; H, 8.85; S, 8.04.
Found: C, 72.21; H, 8.74; S, 8.17.
Synthesis of H-L3. The synthesis of H-L3 was performed according
to the same procedure as for H-L2. Yield: 0.858 g; 52%.
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.40 (d, J = 2.5 Hz, 1H,
ArH), 7.16 (m, 2H, ArH), 6.86 (m, 3H, ArH), 6.74 (d, J = 2.5 Hz, 1H,
ArH), 6.16 (s, 1H, ArOH), 3.73 (s, 2H, CH2), 2.14 (s, 3H, CH3), 1.59
(s, 9H, CCH3), 1.22 (s, 9H, CCH3). 13C{1H} NMR (100.62 MHz,
C6D6, 298 K): δ 152.1 q (CO), 142.4 q (CS), 139.6 q, 137.2 q, 133.8
q, 131.7 (CH), 130.4 (CH), 127.4 (CH), 126.7 (CH), 125.7 (CH),
123.8 (CH), 122.3 q, 36.7 (CH2), 35.3 (C(CH3)3), 34.3 (C(CH3)3),
31.7 (CCH3), 30.0 (CCH3), 20.7 (CH3Ar).
Complex 4 (L4AlMe2). To a stirred solution containing AlMe3
(0.036 g, 0.50 mmol) in benzene (2 mL) was added dropwise a
solution of the ligand precursor (200 mg, 0.50 mmol) in benzene (2
mL). The solution was stirred for 1 h at room temperature. The
solvent was removed under vacuum, forming a pale yellow solid that
was pure according to 1H NMR and elemental analysis. Yield: 0.226 g;
95%.
4
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.51 (d, J = 2.3 Hz, 1H,
Synthesis of H-L4. The synthesis of H-L4 was performed according
to the same procedure as for H-L2. Yield: 1.25 g; 52%.
ArH), 7.16 (d, 1H, J = 6.5 Hz, ArH), 6.95 (d, 1H, J = 6.5 Hz, ArH),
6.58 (m, 2H, ArH), 6.53 (d, 2H, 4J = 2.3 Hz, ArH), 3.79 (s, 2H, CH2),
1.64 (s, 9 H, CCH3), 1.19 (s, 9 H, CCH3), −0.31 (s, 6 H, AlCH3).
13C{1H} NMR (62.89 MHz, C6D6, 298 K): δ 155.9 (q, CO), 141.6 (q,
CS), 139.7 q, 134.2 (CH), 132.4 (CH), 129.8 (CH), 127.7 (CH),
127.06 q, 126.3 (CH), 126.0 (CH), 120.7 q, 40.7 (CH2), 35.9 (CCH3),
34.5 (CCH3), 32.1 (CCH3), 31.0 (CCH3), −7.9 (AlCH3). Anal. Calcd
for C22H27F3OS: C, 63.70; H, 7.13; S, 7.09. Found: 63.65; H, 7.06; S,
7.11.
ε-Caprolactone Polymerizations. In a typical polymerization, a
magnetically stirred reactor vessel (50 cm3) was charged sequentially
with a solution of precatalyst (25 μmol in 4 mL of dry toluene) and
monomer (1 mL, 9.0 mmol). Subsequently, 0.25 mL of a solution 0.1
M of methanol in toluene (25 μmol) was added. The mixture was
thermostated at the required temperature and, after the required
polymerization time, poured into hexane. The precipitated polymer
was recovered by filtration and dried at 40 °C in a vacuum oven. The
polymer was characterized by NMR spectroscopy and GPC analysis.
1H NMR (CDCl3, 25 °C): δ 1.34 (m, 2H, −CH2−), 1.62 (m, 4H,
−CH2−), 2.29 (t, 2H, −CH2C(O)O−), 4.04 (t, 2H, −CH2OC(O)−),
3.62 (t, 2H, −CH2OH), 3.65 (s, 3H, −C(O)OCH3). 13C NMR
(CDCl3, 25 °C): δ 24.7, 25.7, 28.5, 34.3, 64.3 (−OCO(CH2)5−), 51.7
(−C(O)OCH3), 62.7 (−CH2OH), 173.7 (−COO−).
Lactide Polymerizations. In a typical polymerization, a magneti-
cally stirred reactor vessel (50 cm3) was charged sequentially with the
monomer (rac- or L-lactide, 350 mg, 2.4 mmol), the precatalyst (25
μmol), and 4 mL of dry toluene. Subsequently, 0.25 mL of a 0.1 M
solution of methanol in toluene (25 μmol) was added. The mixture
was thermostated at the required temperature and, after the required
polymerization time, poured into hexane. The precipitated polymer
was recovered by filtration and dried at 40 °C in a vacuum oven.
Conversions were determined by integration of the monomer vs
polymer methine resonances in the 1H NMR spectrum of crude
product (in CDCl3). The polymer was purified by redissolving in
CH2Cl2 and precipitating from rapidly stirred methanol. The polymer
1H NMR (400.13 MHz, CDCl3, 298 K): δ 7.62 (d, J = 8 Hz, 1H,
ArH), 7.40 (d, J = 8 Hz, 1 H, ArH), 7.36 (t, J = 8 Hz, 1 H, ArH), 7.29
(t, J = 8 Hz, 1 H, ArH), 7.18 (d, J = 2.5 Hz, 1 H, ArH), 6.77 (d, J = 2.5
Hz, 1 H, ArH), 5.87 (s, 1H, ArOH), 4.15 (s, 2H, CH2), 1.40 (s, 9H,
CCH3), 1.16 (s, 9H, CCH3). 13C{1H} NMR (100.62 MHz, C6D6, 298
K): δ 152.1 q (CO), 142.5 q (CS), 137.5 q, 134.3 (CH), 131.7 q, 127.2
(CH), 126.9 (m, CF3), 125.8 (CH), 133.97 q, 124.1 (CH), 121.7 q,
37.6 (CH2), 35.2 (C(CH3)3), 34.2 (C(CH3)3), 31.6 (C(CH3)3), 30.0
(C(CH3)3). 19F{1H} NMR (376.45 MHz, C6D6, 298 K): δ −60.6.
Complex 1 (L1AlMe2). To a stirred solution containing AlMe3
(0.110 g, 1.52 mmol) in benzene (4 mL) was added dropwise a
solution of the ligand precursor (0.500 g, 1.52 mmol) in benzene (3
mL). The solution was stirred for 1 h at room temperature. The
solvent was removed under vacuum, forming a pale yellow solid that
was pure according to 1H NMR and elemental analysis. Yield: 0.584 g;
95%.
4
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.53 (d, J = 2.5 Hz, 1H,
ArH), 6.90 (m, 2H, o-H Ph), 6.78 (m, 3H, m-H, p-H Ph), 6.59 (d, 4J =
2.5 Hz, 1H, ArH), 3.73 (s, 2H, CH2), 1.68 (s, 9 H, CCH3), 1.26 (s, 9
1
H, CCH3), −0.25 (s, 6 H, AlCH3). H NMR (400.13 MHz, CD2Cl2,
298 K): δ 7.32−7.11 (overlapped multiplets, 4H, ArH), 6.47 (d, 1H,
ArH), 4.20 (s, 2H, CH2), 1.42 (s, 9 H, CCH3), 1.17 (s, 9 H, CCH3),
−0.63 (s, 6 H, AlCH3). 13C{1H} NMR (100.62 MHz, C6D6, 298 K): δ
156.1 q (CO), 140.9 q (CS), 139.6 q, 131.7 (2 CH), 129.8 (CH),
129.7 (2 CH), 129.5 q, 126.4 (CH), 125.7 (CH), 121.1 q, 40.3 (CH2),
36.0 (CCH3), 34.6 (CCH3), 32.1 (CCH3), 31.0 (CCH3), −7.8
(AlCH3). Anal. Calcd for C23H33AlOS: C, 71.84; H, 8.65; S, 8.34.
Found: C, 71.69; H, 8.43; S, 8.22.
Complex 2 (L2AlMe2). To a stirred solution containing AlMe3
(0.088 g, 1.23 mmol) in benzene (4 mL) was added dropwise a
solution of the ligand precursor (0.500 g, 1.23 mmol) in benzene (4
mL). The solution was stirred for 1 h at room temperature. The
solvent was removed under vacuum, forming a pale yellow solid that
was pure according to 1H NMR and elemental analysis. Yield: 0.553 g;
97% yield.
1
was characterized by NMR spectroscopy and GPC analysis. H NMR
4
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.07 (d, J = 2.5 Hz, 1H,
(CDCl3, 25 °C): δ 1.56 (m, 6H, −CHCH3−), 3.79 (s, 3H,
−C(O)OCH3), 5.18 (m, 2H, −CHCH3−). 13C NMR (CDCl3, 25
°C): δ 16.8 (−C(O)OCHCH3−), 69.2 (−C(O)OCHCH3−), 169.5,
169.8 (−COO−).
Kinetic Experiments. In a typical experiment carried out in a
Braun Labmaster glovebox, initiator solution, from a stock solution in
toluene-d8, was injected into a in Teflon-valved J. Young NMR tube
loaded with the monomer dissolved in a suitable amount of toluene-d8
as dry solvent. The sample was thermostated at the required
temperature. The polymerization reaction was monitored via 1H
NMR analysis.
The characteristic chemical shift for each monomer in toluene-d8 is
4.12 ppm (q, CH; lactide), and 3.63 ppm (m, CH2; ε-caprolactone).
The characteristic chemical shift for each polymer in toluene-d8 is 5.12
ppm (q, CH; polylactide), and 4.00 ppm (t, CH2; poly-ε-
caprolactone).
ArH), 6.79 (m, 2H, ArH), 6.63 (dt, 1H, ArH), 6.26 (dt, 1H, ArH),
3.80 (s, 2H, CH2), 1.64 (s, 9 H, CCH3), 1.28 (s, 9 H, CCH3), −0.29
(s, 6 H, AlCH3). 13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 157.0
(q, CO), 140.5 (q, CS), 139.4 (q, CBr), 133.7 (CH), 131.2 (CH),
130.5 (CH), 129.5 q, 128.4 (CH), 125.9 (CH), 125.6 (CH), 125.1 q,
119.7 q, 38.5 (CH2), 35.8 (CCH3), 34.6 (CCH3), 32.2 (CCH3), 30.8
(CCH3), −8.7 (AlCH3). Anal. Calcd for C23H32AlBrOS: C, 59.61; H,
6.96; S, 6.9. Found: C, 58.83; H, 6.14; S, 6.77.
Complex 3 (L3AlMe2). To a stirred solution containing AlMe3
(0.042 g, 0.58 mmol) in benzene (2 mL) was added dropwise a
solution of the ligand precursor (0. 200 g, 0.58 mmol) in benzene (2
mL). The solution was stirred for 1 h at room temperature. The
solvent was removed under vacuum, forming a pale yellow solid that
was pure according to 1H NMR and elemental analysis. Yield: 0.220 g,;
95%.
5558
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
Crystal Structure Determination. The crystal data of complex 2
were collected at room temperature using a Nonius Kappa CCD
diffractometer with graphite-monochromated Mo Kα radiation. The
data sets were integrated with the Denzo-SMN package40 and
corrected for Lorentz, polarization, and absorption effects (SOR-
TAV41). The structure was solved by direct methods (SIR9742) and
refined using full-matrix least squares with all non-hydrogen atoms
anisotropic and hydrogens included on calculated positions, riding on
their carrier atoms.
2007, 13, 4433−4451 and references therein. (c) Spassky, N.;
Wisniewski, M.; Pluta, C.; Le Borgne, A. Makromol. Chem. Phys.
1996, 197, 2627−2637.
(8) Haddad, M.; Laghzaoui, M.; Welter, R.; Dagorne, S. Organo-
metallics 2009, 28, 4584−4592.
(9) (a) Ma, H.; Melillo, G.; Oliva, L.; Spaniol, T. P.; Englert, U.;
Okuda, J. Dalton Trans. 2005, 721−727. (b) Lian, B.; Ma, H.; Spaniol,
T. P.; Okuda, J. Dalton Trans. 2009, 9033−9042.
(10) Lamberti, M.; Mazzeo, M.; Pellecchia, C. Dalton Trans. 2009,
8831−8837.
(11) Burnett, M. N.;. Johnson, C. K. ORTEP III, Report ORNL-6895;
Oak Ridge National Laboratory, Oak Ridge, TN, 1996.
(12) Braune, W.; Ma, H.; Spaniol, T. P.; Okuda, J. Organometallics
2005, 24, 1953−1958.
All calculations were performed using SHELXL-9743 and PARST13
implemented in the WINGX44 system of programs. The crystal data
are given in Table S1 (Supporting Information). Selected bond
distances and angles are given in Table S2 (Supporting Information).
Crystallographic data (excluding structure factors) have been
deposited at the Cambridge Crystallographic Data Centre and
allocated the deposition number CCDC 859040. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
or on application to the CCDC, Union Road, Cambridge CB2 1EZ,
U.K. (fax, (+44)1223-336033; e-mail, deposit@ccdc.cam.ac.uk).
(13) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659−659.
(14) Cremer, D.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 1354−1358.
(15) (a) Nelson, S. G.; Kim, B.-K.; Peelen, T. J. J. Am. Chem. Soc.
2000, 122, 9318−9319. (b) Nelson, S. G.; Peelen, T. J.; Wan, Z. J. Am.
Chem. Soc. 1999, 121, 9742−9743.
́
(16) Emig, N.; Nguyen, H.; Krautscheid, H.; Reau, R.; Cazaux, J.-B.;
ASSOCIATED CONTENT
* Supporting Information
Figures, tables, and a CIF file giving H NMR spectra of
■
Bertrand, G. Organometallics 1998, 17, 3599−3608.
S
(17) Hild, F.; Brelot, L.; Dagorne, S. Organometallics 2011, 30,
5457−5462.
1
(18) (a) Addison, A. W.; Rao, Y. N.; Reedijk, J.; Van Rijn, J.;
Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.
(b) Vela, J.; Cirera, J.; Smith, J. M.; Lachicotte, R. J.; Flaschenriem, C.
J.; Alvarez, S.; Holland, P. L. Inorg. Chem. 2007, 46, 60−71.
(19) The value is obtained by dividing the difference between the
sum of the basal ligand−basal ligand angles and the sum of the basal
ligand−axial ligand angles by 90°: τ = [∑(basal−M−basal)/∑(basal−
M−axial) ]/90°.
complexes 1−4, details on variable-temperature experiments,
and crystallographic data for complex 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+39) 089969603. E-mail: mlamberti@unisa.it (M.L.);
mmazzeo@unisa.it (M.M.).
■
(20) Dagorne, S.; Lavanant, L.; Welter, R.; Chassenieux, C.;
Haquette, P.; Jaouen, G. Organometallics 2003, 22, 3732−3741.
A
Notes
quantitative comparison of the two cases is not possible, as the
activation parameters were not reported.
The authors declare no competing financial interest.
(21) Dagorne, S.; Le Bideau, F.; Welter, R.; Bellemin-Laponnaz, S.;
Maisse-Francois, A. Chem. Eur. J. 2007, 13, 3202−3217.
(22) (a) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407−430.
(b) Dagorne, S.; Atwood, D. A. Chem. Rev. 2008, 108, 4037−4071.
ACKNOWLEDGMENTS
■
We thank Dr. Patrizia Oliva for NMR assistance, Dr. Patrizia
Iannece for elemental analyses, and Dr. Mariagrazia Napoli for
GPC measurements. This work was supported by the
University of Salerno (FARB Grant).
(23) (a) Karsch, H. H.; Appelt, A.; Riede, J.; Muller, G.
̈
Organometallics 1987, 6, 316−323. (b) Fryzuk, M. D.; Giesbrecht,
G. R.; Olovsson, G.; Rettig, S. J. Organometallics 1996, 15, 4832−4841.
(c) Fryzuk, M. D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998,
37, 6928−6934. (d) Liang, L.-C.; Huang, M.-H.; Hung, C.-H. Inorg.
Chem. 2004, 43, 2166−2174. (e) Lee, W.-Y.; Liang, L.-C. Dalton
Trans. 2005, 1952−1956.
(24) Qian, B.; Ward, D. L.; Smith, M. R. Organometallics 1998, 17,
3070−3076.
(25) (a) Ihara, E.; Young, V. G.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 8277−8278. (b) Radzewich, C. E.; Guzei, I. A.; Jordan, R. F. J.
Am. Chem. Soc. 1999, 121, 8673−8674.
REFERENCES
■
(1) (a) Albertsson, A. C.; Varma, I. K. Biomacromolecules 2003, 4,
1466−1486. (b) Robert, C.; de Montigny, F.; Thomas, C. M. Nat.
Commun. 2011, 2, 586 DOI: 10.1038/ncomms1596.
(2) Arbaoui, A.; Redshaw, C. Polym. Chem. 2010, 1, 801−826.
(3) (a) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater.
2000, 12, 1841−1846. (b) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev.
2010, 39, 486−494.
(4) Some selected reviews: (a) O’Keefe, B. J.; Hillmyer, M. A.;
Tolman, W. B. Dalton Trans. 2001, 2215−2224. (b) Dechy-Cabaret,
O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147−6176.
(c) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006,
250, 602−626. (d) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165−173.
(e) Dijkstra, P. J.; Du, H.; Feijen, J. Polym. Chem. 2011, 2, 520−527.
(f) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polym. Rev. 2008, 48,
11−63.
(26) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce,
M. D.; White, A. J. P.; Williams, D. J. Dalton Trans. 2002, 415−422.
(27) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015−10031.
(28) (a) Cameron, P. A.; Gibson, V. C.; Redshaw, C. J.; Segal, A.;
Solan, G. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2001, 1472−
1476. (b) Pappalardo, D.; Tedesco, C.; Pellecchia, C. Eur. J. Inorg.
Chem. 2002, 621−628. (c) Pappalardo, D.; Mazzeo, M.; Montefusco,
P.; Tedesco, C.; Pellecchia, C. Eur. J. Inorg. Chem. 2004, 1292−1298.
(29) Following a reviewer’s suggestion, compound 1 was directly
reacted with 1 equiv of B(C6F5)3 in a dichloromethane-d2 solution
containing 1 equiv of ε-CL, and the mixture was analyzed by 1H NMR.
Although the appearance of the broad resonance at δ 0.46 for the
B(C6F5)3Me− indicated the abstraction of the methyl group from the
dimethylaluminum neutral compound 1, the spectrum was complex
and at least two cationic species (δ − 0.29 (major) and − 0.28, AlMe+)
were observed.
(5) Gibson, C. V.; Marshall, E. L. In Comprehensive Coordination
Chemistry II; Elsevier: Amsterdam, 2004; Vol. 9, p 1.
(6) (a) Nomura, N.; Aoyama, T.; Ishii, R.; Kondo, T. Macromolecules
2005, 38, 5363−5366. (b) Liu, J.; Iwasa, N.; Nomura, K. Dalton Trans.
2008, 3978−3988. (c) Iwasa, N.; Liu, J.; Nomura, K. Catal. Commun.
2008, 9, 1148−1152. (d) Pappalardo, D.; Annunziata, L.; Pellecchia,
C. Macromolecules 2009, 42, 6056−6062.
(7) (a) Hormniurun, P.; Marshall, E. D.; Gibson, V. C.; Pugh, R. I.;
White, A. J. P. Proc. Natl. Acad. Sci. U.S.A. 2006, 42, 15343−15348.
(b) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem. Eur. J.
5559
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560

Organometallics
Article
(30) (a) Gong, S.; Ma, H. Dalton Trans. 2008, 3345−3357.
(b) Arbaoui, A.; Redshaw, C.; Hughes, D. L. Chem. Commun. 2008,
4717−4719. (c) Iwasa, N.; Katao, S.; Liu, J. Y.; Fujiki, M.; Furukawa,
Y.; Nomura, K. Organometallics 2009, 28, 2179−2187.
(31) (a) Save, M.; Schappacher, M.; Soum, A. Macromol. Chem. Phys.
2002, 203, 889−899. (b) Palard, I.; Soum, A.; Guillaume, S. M.
Macromolecules 2005, 38, 6888−6894.
(32) (a) Aida, T.; Maekawa, Y.; Asano, S.; Inoue, S. Macromolecules
1988, 21, 1195−1202. (b) Amgoune, A.; Thomas, C. M.; Carpentier,
J.-F. Macromol. Rapid Commun. 2007, 28, 693−697. (c) Ajellal, N.;
Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier,
V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363−8376.
(33) Following a reviewer’s suggestion, in order to discriminate
between the “coordination−insertion” and the “activated monomer”
1
mechanisms, both resulting in the same end groups, the H NMR
monitoring of the reaction of complex 1 with 1 equiv of MeOH was
carried out in C6D6 at room temperature. Although a complex mixture
of products was observed, the formation of methane (signal at 0.16
ppm) suggested the alcoholysis of the aluminum−methyl bond, ruling
out the “activated monomer” mechanism.
(34) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S.
Macromol. Rapid Commun. 1997, 18, 325−333.
(35) Reich, H. J. WINDNMR v7.1.6, 2002.
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; J. W.Ochterski,
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; J. J.Dannenberg,
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
B.05; Gaussian, Inc., Wallingford, CT, 2004.
(37) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−
789. (b) Reiher, M.; Salomon, O.; Hess, B. A. Theor. Chem. Acc. 2001,
107, 48−55. (c) Salomon, O.; Reiher, M.; Hess, B. A. J. Chem. Phys.
2002, 117, 4729−4737.
(38) (a) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural
Bond Orbital Donor-Acceptor Perspective; Cambridge University Press:
London, 2005. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J.
Chem. Phys. 1985, 83, 735−746. (c) Reed, A. E.; Weinhold, F. J. Chem.
Phys. 1985, 83, 1736−1740. (d) Reed, A. E.; Curtiss, L. A.; Weinhold,
F. Chem. Rev. 1988, 88, 899−926.
(39) Glendening, E. D.; Reed, A. E.; Carpentier, J. E.; Weinhold, F.
NBO Version 3.1; Theoretical Chemistry Institute, University of
Wisconsin, Madison, WI.
(40) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter, C.
W., Sweet, R. M., Eds.; Academic Press: London, 1997; Vol. 276, Part
A, pp 307−326.
(41) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33−38.
(42) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna,
R. SIR-97. J. Appl. Crystallogr. 1999, 32, 115−119.
(43) Sheldrick, G. M. SHELX-97, Program for Crystal Structure
Refinement; University of Gottingen, Gottingen, Germany, 1997.
̈
̈
(44) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838.
5560
dx.doi.org/10.1021/om300505m | Organometallics 2012, 31, 5551−5560