Aminoarenethiolate aluminum complexes: Synthesis, characterization, and use in l -lactide polymerization

Article abstract of DOI:10.1021/om400111f

Reaction of AlMe₃ with S(SiMe₃)(C₆H ₃-2-CH₂NRR′-5-^tBu) (RR′ = C ₅H₁₀ (1a), C₄H₈ (1b), Me₂ (1c)), at ambient temperature, affords the a

Full text of DOI:10.1021/om400111f

Article
pubs.acs.org/Organometallics
Aminoarenethiolate Aluminum Complexes: Synthesis,
Characterization, and Use in L‑Lactide Polymerization
́
Lorena Postigo, Marıa del Carmen Maestre, Marta E. G. Mosquera, Tomas Cuenca,*
́
and Gerardo Jimenez*
́
́
́ ́ ́
Departamento de Quımica Inorganica, Universidad de Alcala, Campus Universitario, E-28871 Alcala de Henares (Madrid), Spain
S
* Supporting Information
ABSTRACT: Reaction of AlMe3 with S(SiMe3)(C6H3-2-
CH2NRR′-5-tBu) (RR′ = C5H10 (1a), C4H8 (1b), Me2
(1c)), at ambient temperature, affords the amino adducts
[AlMe3{S(SiMe3)(C6H3-2-CH2NRR′-5-tBu)}-κN] (RR′ =
C5H10 (2a), C4H8 (2b), Me2 (2c)), which undergo TMS
elimination upon heating to give the monomeric amino-
arenethiolate aluminum complexes [AlMe2{S(C6H3-2-
CH2NRR′-5-tBu)-κ2S,N}] (RR′ = C5H10 (3a), C4H8 (3b),
Me2 (3c)). Following the same procedure, treatment of
AlCl2Me and AlCl3 with 1 yields analogous aminoarenethiolate aluminum complexes with different degrees of methylation, the
chloro methyl and dichloro complexes [AlClMe{S(C6H3-2-CH2NRR′-5-tBu)-κ2S,N}] (RR′ = C5H10 (4a), C4H8 (4b), Me2 (4c))
and [AlCl2{S(C6H3-2-CH2NRR′-5-tBu)}-κ2S,N] (RR′ = C5H10 (5a), C4H8 (5b) Me2 (5c)), respectively. These complexes have
been characterized by multinuclear NMR spectroscopy and elemental analysis. Moreover, the molecular structures of 3a,b have
been determined by X-ray diffraction methods. Aluminum complexes 3 have been investigated for the ring-opening
polymerization (ROP) of L-lactide, achieving high conversions in relatively short periods of time. The PLAs obtained feature an
aminoarenethiolate end functionality, as inferred from MALDI-TOF mass analysis.
anionic or neutral, coordinated to a hard metal center are
scarce, even though recent reports have shown that the use of
ancillary ligands with softer donors could affect the Lewis
acidity of the metal center, with remarkable effects on catalytic
performance.7
In contrast, the synthesis of well-defined aliphatic polyesters
with precisely controlled end-group functionalities is a target of
current interest. In this regard, aluminum thiolates have proved
excellent initiators to prepare polyesters with an end thioester, a
functional group that may next be utilized in further chemical
reactions, making them useful candidates in a wide variety of
applications.8 For instance, dimethylaluminum thiolates
AlMe2(SR) initiate polymerization of lactones in a living
fashion, leading to the formation of heterobifuntional polyesters
(i.e., one hydroxy functional end and one thioester, −COSR′,
end group), which further reacts with poly(ethylene glycol)
(PEG) derivatives, affording a PCL-PEG copolymer.8b
With this in mind, we decided to explore the formation of
aluminum derivatives by coordinating different bidentate
aminothiolate ligands to be tested as a catalyst/initiator in
the polymerization of L-lactide. Herein, we report the synthesis
and structural characterization of novel aluminum complexes
bearing an aminoarenethiolate ligand with the aim of studying
their catalytic performance in the ring-opening polymerization
of L-lactide and analyze the role played by the thiolate group in
INTRODUCTION
■
Polylactic acid (PLA) is a highly versatile and biodegradable
aliphatic polyester derived from 100% renewable resources,
such as corn and sugar beets.1 The combination of such
characteristics and its well-suited mechanical and physical
properties make PLA an environmentally friendly and
sustainable alternative to traditional petrochemical-based
plastics, mainly in packaging, coating, and fiber applications.2
In addition, PLA is suitable for more sophisticated fields such as
scaffolds in tissue repair and engineering, in drug-controlled
delivery systems, and in microelectronics.3
Among the wide variety of ways exploited, the ring-opening
polymerization (ROP) of lactide (LA) promoted by organo-
metallic compounds via a coordination−insertion mechanism
represents the most efficient and versatile method to prepare
PLAs with controlled microstructural properties such as
molecular weight, polydispersity, and stereoregularity.2a,3b,4
Although the activity of aluminum species is significantly
lower than that of other metals, typically used in these
processes, they generally exert good polymerization control.
Thus, a wide variety of aluminum(III) compounds, particularly
those supported by Salen or Salan ancillary ligands, have been
demonstrated to show excellent molecular weight and stereo-
chemical control in the ROP of lactide.5
The majority of organometallic complexes tested in ROP
bearing multidentate ligands contain, in various combinations,
nitrogen and oxygen hard donor atoms.6 In contrast, thus far,
monoanionic chelating ligands incorporating soft donors, either
Received: February 8, 2013
© XXXX American Chemical Society
A
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Scheme 1. Reactions of AlMe3 with Trimethylsilyl-Protected Aminoarenethiolates
Figure 1. X-ray structures of 3a,b. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
the process, either as a supporting ligand or as an initiator
group.
On heating the aforementioned mixture at 130 °C for 3 days,
the amino adducts 2 evolve SiMe4, affording the corresponding
aminoarenethiolate dimethyl complexes 3 in a quantitative
manner, as judged by NMR spectroscopy (Scheme 1). After
workup, complexes 3 are isolated as white crystalline solids in
high yield (78−82%). Complexes 3 are conveniently prepared
when the reaction of 1 with AlMe3 is carried out at 130 °C.
Although compounds 2 and 3 are very moisture sensitive in
solution, they are thermally stable at room temperature and
their solutions can be kept under rigorously dry conditions for a
long period of time without noticeable decomposition, enabling
suitable crystals for X-ray analysis to be obtained for some of
them. Both types of complexes are quite soluble in most
common solvents.
RESULTS AND DISCUSSION
■
Synthesis of Aminoarenethiolate Aluminum Com-
plexes. Following the protocol established by van Koten et al.
to synthesize arenethiolate copper9 and zinc10 derivatives, we
decided to explore the synthesis of the targeted complexes
using trimethylsilyl-protected aminoarenethiolate proligands
S(SiMe3)(C6H3-2-CH2NRR′-5-tBu) (RR′ = C5H10 (1a), C4H8
(1b), Me2 (1c)).9a
Reaction of AlMe3 with an equimolar amount of the
corresponding trimethylsilyl-protected aminoarenethiolate pro-
ligands 1 at room temperature affords a mixture of two new
aluminum derivatives in the molar ratio 9:1 (as observed by 1H
NMR). The major component of the reaction mixture is the
four-coordinated adduct [AlMe3{S(SiMe3)(C6H3-2-CH2NRR′-
5-tBu)}-κN] (RR′ = C5H10 (2a), C4H8 (2b), Me2 (2c)), with
the amine nitrogen coordinated to the aluminum atom, as
inferred from the NMR study. Such behavior is in agreement
with the strong tendency shown by AlMe3 to form four-
coordinate adducts, as reported elsewhere.5d The minor
compound is the corresponding monomeric dimethyl alumi-
num complex [AlMe2{S(C6H3-2-CH2NRR′-5-tBu)}-κ2S,N]
(RR′ = C5H10 (3a) C4H8 (3b), Me2 (3c)), which bears a
chelating monoanionic aminoarenethiolate ligand, stemming
from the elimination of SiMe4 (Scheme 1).
The spectroscopic behavior of compounds 2 and 3 is in
agreement with Cs symmetry in solution, consistent with the
1
proposed structures. A striking spectroscopic feature of the H
NMR spectra of the amino adduct 2 is the presence of two
sharp singlets at high field of equal intensity, each integrating
for nine protons, confirming the coordination of the AlMe3
moiety (δ −0.16 (2a), −0.30 (2b), −0.30 (2c)) along with
retention of the SiMe3 group (δ 0.15 (2a), −0.09 (2b), 0.07
(2c)) in the molecule. According to the aforementioned
symmetry, protons ascribed to the Ar−CH2−N methylene
group are equivalent, as are each pair of methylene (in 2a,b) or
methyl groups (in 2c) of the fragment “RR” attached to
nitrogen. However, unlike the case for the free ligand, each pair
of protons CH2 of the piperidinyl and pirrodinyl rings is
Compounds 2 can be isolated from the reaction mixture by
fractional crystallization from n-hexane, in only modest to poor
yields (40−45%). Unfortunately, although these compounds
are obtained as NMR pure compounds, their microanalyses
values are not optimal, due to their oily nature.
1
1
nonequivalent, as shown by H−13C HMQC and H-TOCSY
measurements, a consequence of the coordination of the
nitrogen atom to aluminum. Complexation with aluminum
blocks the inversion of the tertiary nitrogen and differentiates
B
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the geminal protons of all piperidinyl and pirrodinyl methylene
moieties.11
The features of the H NMR spectra of complexes 3 are
similar to those observed in the spectra of compounds 2, except
for the disappearance of the singlet ascribed to the SiMe3 group
and a one-third decrease of the relative intensity of the
resonance attributed to magnetically equivalent methyls on
aluminum, which confirms the elimination of TMS and the
formation of the desired thiolate complex.
tested the reaction of AlClMe2 with an equimolar amount of
the appropriate compound 1; these reactions actually proceed
at room temperature and occur much faster than previously
described. However, such reactions are not specific and along
with the targeted compound 3 the formation of the new chloro
methyl aminoarenethiolate complex [AlClMe{S(C6H3-2-
CH2NRR′-5-tBu)}-κ2S,N] (RR′ = C5H10 (4a) C4H8 (4b),
Me2 (4c)) is observed, in a low molar ratio, ∼8 (3):2 (4). The
formation of complexes 4 involves the competitive elimination
of SiMe4 (Scheme 2). A similar outcome is reached when
AlCl2Me is used as the starting material; in this case the major
component of the reaction mixture is the expected complex 4,
contaminated with a minimal amount (below a molar ratio of
5%) of the corresponding dichloro derivatives [AlCl2{S(C6H3-
2-CH2NRR′-5-tBu)}-κ2S,N] (RR′ = C5H10 (5a), C4H8 (5b),
Me2 (5c)). To verify the nature of the latter, complexes 5 are
specifically prepared, on an NMR tube scale, by treatment of
AlCl3 with a stoichiometric amount of the appropriate
compound 1. Consequently, the dichloro aminoarenethiolate
complexes 5 have been characterized only by NMR spectros-
copy.
1
The 13C NMR spectra of these complexes are consistent with
those predicted, in agreement with the Cs symmetry of these
species in solution. A relevant feature of these spectra is the
number of resonances for the carbon atoms of the piperidinyl
and pyrrodinyl rings, three (21.5, 22.6, 51.3, 2a; 21.5, 24.7,
53.8, 3a) and two (24.4, 52.0, 2b; 22.0, 55.8, 3b), respectively,
which confirms the equivalence of each pair of the methylene
groups of these heterocycles.
Single crystals of 3a,b suitable for X-ray diffraction analysis
were grown in hexane solutions at −10 °C, and the molecular
structure was established by X-ray diffraction studies. Molecular
structures of 3a,b are illustrated in Figure 1, and selected bond
lengths and angles are given in Table 1. These complexes
crystallize in the monoclinic system.
None of these reactions afforded the transient amino adduct
analogous to those isolated in the reactions with AlMe3,
probably because the higher acidity of the aluminum atom in
the chloro derivatives favors the next substitution process, as
inferred from the increased rate and greater facility with which
these reactions proceed at room temperature. Complexes 4 are
obtained as analytically pure white solids by fractional
crystallization from toluene, in moderate yields (60−65%).
Table 1. Selected Bond Distances (Å) and Angles (deg) for
3a,b
3a
3b
Al(1)−S(1)
Al(1)−N(1)
Al(1)−C(1)
Al(1)−C(2)
S(1)−C(10)
2.2712(11)
2.059(2)
1.967(3)
1.965(3)
1.771(3)
2.2673(10)
2.041(2)
1.965(3)
1.953(3)
1.778(2)
1
Due to the chirality shown by complexes 4, their H NMR
spectra feature more complexity than those of complexes 3.
Thus, the arene−CH2 protons display two doublets (J = 12 Hz)
in the range 3−4 ppm and, additionally, all the pairs of the
methylene (in 4a,b) or methyl groups (in 4c) of the fragment
“RR′” attached to nitrogen are diastereotopic. Rather, the
spectroscopic behavior of complexes 5 is parallel to that
analyzed for complexes 3, apart from the absence of resonances
at high field attributed to the methyl groups bound to
aluminum. The 13C NMR spectra of these complexes are
consistent with those predicted. Thus, whereas the 13C NMR
spectra of complexes 5a−c show only three (20.9, 22.0, 54.0),
two (21.7, 55.6), and one resonances (46.7), respectively, for
the RR′ moiety attached to the nitrogen atom, those of
compounds 4 feature five (21.0, 21.2, 22.3, 53.1, 54.4), four
(21.7, 21.8, 54.4, 55.7), and two resonances (44.0, 47.0) for
4a−c, respectively.
Since alkoxide ligands have been habitually used as initiator
groups for lactide polymerization, we attempted to replace the
methyl groups, in complexes 3, with alkoxide ligands by
treatment of these with alcohols, a procedure that has proved a
convenient synthetic strategy for aluminum alkoxide deriva-
tives.13 Unfortunately, all these reactions caused the elimination
of the thiolate ligand with the formation of intractable mixtures
of various aluminum species.13a In view of such results, the
aminoarenethiolate chelating ligand seems to be more easily
released than the methyl groups. However, we decided to test
directly the dimethyl derivatives 3 in the polymerization of L-
lactide and to explore the role of the S,N-chelating moiety in
such processes, either as a supporting ligand or as an initiating
group.
N(1)−Al(1)−S(1)
C(1)−Al(1)−S(1)
C(1)−Al(1)−C(2)
C(1)−Al(1)−N(1)
C(10)−S(1)−Al(1)
101.38(7)
109.37(11)
120.07(15)
108.33(12)
96.74(9)
99.50(6)
110.36(11)
117.72(16)
109.70(11)
93.30(8)
X-ray diffraction analysis establishes the monomeric nature of
both complexes and confirms that the aluminum atom bears a
monoanionic S,N-chelating arenethiolate ligand. The resulting
Al(1)−S(1)−C(10)−C(9)−C(8)−N(1) six-membered metal-
lacycle displays a twisted-boat conformation, with the sulfur and
methylenic carbon atoms occupying the upper positions.
Instead, the piperidinyl and pirrodinyl rings adopt chair and
half-chair conformations, respectively, with the aluminum atom
located in an axial disposition.
The tetracoordinated aluminum atom has a distorted-
tetrahedral geometry, which is apparent from the N−Al−S
bite angles of 101.38(7) and 99.50(6)° in 3a,b, respectively.
Also, the angles C−S−Al of 96.74(9) and 93.30(8)° in 3a,b,
respectively, are too acute for an sp3-hybridized sulfur atom.
The bond distances of Al−C (1.967(3), 1.965(3) Å in 3a and
1.953(3), 1.965(3) Å in 3b), Al−S (2.2712(11) Å in 3a and
2.2673(10) Å in 3b) and Al−N (2.059(3) Å in 3a and 2.041(2)
Å in 3b) are all within the expected range for tetracoordinated
aluminum complexes.12
In an attempt to improve the synthetic strategy for the
dimethyl aluminum complexes 3, we envisioned that the use of
AlClMe2 would allow more convenient experimental con-
ditions, via a classical SiClMe3 elimination route. Thus, we
L-Lactide Polymerization Initiated by Complexes 3.
The dimethyl aminoarenethiolate complexes 3 are assessed in
the ring-opening polymerization of L-lactide (L-LA). Polymer-
C
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Scheme 2. Reactions of 1 with Different Chloro Aluminum Precursors
Table 2. L-Lactide Polymerization Initiated by Complexes 3
b
c
d
d
entry
initiator
[L-LA]/[Al]
time (h)
conversn (%)
kapp (min−1
)
Mn(calcd)
Mn(obsd)
Mw/Mn
1
2
3
3a
3b
3c
100
100
100
4
4
4
85
80
80
8.00 × 10−3
7.64 × 10−3
7.11 × 10−3
12240
11520
11520
12100
13800
22400
1.54
2.34
1.29
a
b
Conditions: all polymerizations were carried out in toluene solution (7 mL) at 130 °C, 0.031 mmol of catalyst, 0.45 g of L-lactide. Measured by 1H
c
d
NMR. Calculated for one growing chain per aluminum atom (Mn(calcd) = (L-LA/Al) × conversion × 144). Determined by gel permeation
chromatography, calibrated with polystyrene standards in tetrahydrofuran and corrected by the Mark−Houuwink factor of 0.58.14
izations are carried out in toluene solution at 130 °C with an
equivalent molar ratio of monomer to initiator of 100 ([M]0/
[Al]0). All three complexes proved significantly active in
comparison with previously reported aluminum thiolate
compounds, achieving higher conversions in shorter periods
of time.8b Representative results are summarized in Table 2.
The kinetics of the ring-opening polymerization of L-LA with
complexes 3 by quenching samples from reactions, up to a
min (Figure 2). In fact, after 30 min of reaction the conversion
reached by 3a is double that achieved by complexes 3b,c.
Semilogarithmic plots of ln ([LA]/[LA]0) versus polymer-
ization time are linear, indicating that the polymerization
followed pseudo-first-order kinetics in lactide concentration
(Figure 3). Apparent polymerization rate constants of Kapp
=
8.00 × 10−3, 7.64 × 10−3, and 7.11 × 10−3 min−1 are found for
3a−c, respectively.
The GPC data for the polymers obtained exhibit monomodal
molecular weight distributions along with high number-average
molecular weights (Table 2). However, these efficient catalysts
afford poor control over ROP parameters, as inferred from the
1
conversion of ∼80%, have been followed by H NMR analysis
(Figure 2). Although all three values are very close, it is
important to note that while the polymerization process
promoted by 3a lacks an induction period, those initiated by
complexes 3b,c possess a short induction period of ca. 15−20
Figure 3. First-order kinetics plots for the polymerization of L-lactide
promoted by complexes 3.
Figure 2. Kinetics of L-lactide conversion with complexes 3.
D
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mass spectra were recorded in positive mode. 1,8,9-Anthracenetriol
was used as the matrix, and trifluoroacetic acid was added as a
cationization agent.
fairly broad molecular weight distribution (Mw/Mn = 2.34)
shown by the polymers obtained with 3b and the significant
deviation between observed and calculated (on the basis of
monomer conversion) molecular weight for those obtained
with 3c, which can be accounted for by a partial deactivation of
the aluminum complex during the polymerization process. In
addition, a further corroboration of this is the lack of linear
dependence between the number-average molecular weight and
the monomer to initiator ratio.
Synthesis of [AlMe3{S(SiMe3)(C6H3-2-CH2NC5H10-5-tBu)}-κN]
(2a). A solution of 0.30 g (0.88 mmol) of S(SiMe3)(C6H3-2-
CH2NC5H10-5-tBu) in toluene (10 mL) was added dropwise to a
toluene solution (10 mL) of AlMe3 (0.48 mL, 0.96 mmol) at room
temperature. The reaction mixture was stirred for 2 h. After the
removal of solvent under vacuum, the resulting residue was extracted
into hexane (10 mL) and isolated by fractional crystallization from n-
1
hexane to give 2a as a white oil (0.15 g, 0.35 mmol, 40%). H NMR
To establish the role played by the aminoarenethiolate ligand
in these ROP polymerization experiments, we performed an
(C6D6, 400 MHz): δ −0.16 (s, 9H, AlMe3), 0.15 (s, 9H, SiMe3), 0.37,
1
1.05, 2.01, 2.24, 3.35 (m, 1H, 3H, 3 × 2H, NC5H10), 1.20 (s, 9H,
end-group analysis by H NMR spectroscopy and MALDI-
3
CMe3), 4.50 (s, 2H, NCH2), 6.95 (d, 1H, J = 8 Hz, C6H3), 7.07 (dd,
TOF mass spectrometry. Although, unfortunately, NMR
spectroscopy did not identify this group, MALDI-TOF mass
spectrometry confirmed the formation of polymers capped with
an aminoarenethiolate group at one end. Thus, the amino-
arenethiolate chelating moiety acts as an initiating group in
these ROP processes of L-lactide.
3
4
4
1H, J = 8 Hz, J = 2 Hz, C6H3), 7.62 (d, 1H, J = 2 Hz, C6H3).
13C{1H} NMR (C6D6, 100.6 MHz): δ −5.6 (AlMe3), 0.67 (SiMe3),
21.5, 22.6, 51.3 (NC5H10), 30.2 (CMe3), 34.4 (CMe3), 53.8 (NCH2),
124.5, 134.3, 136.2 (C6H3), 130.4, 142.4, 152.8 (C6H3-ipso). Anal.
Calcd for C22H42AlNSSi: C, 64.79; H, 10.40; N, 3.43. Found: C, 66.45;
H, 11.73; N, 3.29.
Synthesis of [AlMe3{S(SiMe3)(C6H3-2-CH2NC4H8-5-tBu)}-κN]
(2b). A procedure similar to that described above for 2a was adopted
using S(SiMe3)(C6H3-2-CH2NC4H8-5-tBu) (0.32 g, 0.99 mmol) and
AlMe3 (0.50 mL, 1.09 mmol) to give the adduct 2b as a pale yellow oil
CONCLUSIONS
■
Herein, we have described suitable and straightforward
synthetic routes to prepare aluminum aminoarenethiolate
derivatives with different degrees of methylation. The present
work shows that bidentate aminothiolate ligands, which
combine a soft thiolate donor with a hard amino donor, are
suitable ligands in the coordination chemistry of aluminum-
(III), a hard Lewis acid.
1
(0.17 g, 0.44 mmol, 45%). H NMR (C6D6, 400 MHz): δ −0.30 (s,
9H, AlMe3), −0.09 (s, 9H, SiMe3), 0.98, 1.31, 2.83, 3.14 (m, 4 × 2H,
NC4H8), 1.09 (s, 9H, CMe3), 4.35 (s, 2H, NCH2), 6.90 (d, 1H, 3J = 8
Hz, C6H3), 7.03 (dd, 1H, 3J = 8 Hz, 4J = 2 Hz, C6H3), 7.64 (d, 1H, 4J =
2 Hz, C6H3). 13C{1H} NMR (C6D6, 100.6 MHz): δ −8.2 (AlMe3),
0.61 (SiMe3), 24.4, 52.0 (NC4H8), 31.0 (CMe3), 34.3 (CMe3), 61.9
(NCH2), 124.4, 133.5, and 135.0 (C6H3), 132.2, 142.1, and 152.2
(C6H3-ipso). Anal. Calcd for C21H40AlNSSi: C, 64.05; H, 10.26; N,
3.55. Found: C, 66.11; H, 10.97; N, 3.22.
The catalytic performances of dimethyl complexes 3 in L-
lactide ring-opening polymerization have been explored. All
three complexes have been shown as efficient catalysts,
achieving high conversions in relatively short periods of time.
Polymerization kinetic studies revealed a first-order dependence
on monomer concentration. Although these catalysts afford
poor control over ROP parameters, the thioester end group
shown in the polyesters obtained may provide a suitable
method for further functional procedures of such polymers.
Synthesis of [AlMe3{S(SiMe3)(C6H3-2-CH2NMe2-5-tBu)}-κN]
(2c). A procedure similar to that described above for 2a was adopted
using S(SiMe3)(C6H3-2-CH2NMe2-5-tBu) (0.32 g, 1.08 mmol) and
AlMe3 (0.60 mL, 1.19 mmol) to give the adduct 2c as a white oil (0.15
1
g, 0.43 mmol, 40%). H NMR (C6D6, 400 MHz): δ −0.30 (s, 9 H,
AlMe3), 0.07 (s, 9 H, SiMe3), 1.16 (s, 9 H, CMe3), 2.08 (s, 6 H, NMe2),
4.27 (s, 2 H, NCH2), 6.81 (d, 1 H, 3J = 8 Hz, C6H3), 7.05 (dd, 1 H, 3J
4
4
= 8 Hz, J = 2 Hz, C6H3), 7.65 (d, 1 H, J = 2 Hz, C6H3). 13C{1H}
NMR (C6D6, 100.6 MHz): δ −10.2 (AlMe3), 0.65 (SiMe3), 31.2
(CMe3), 34.4 (CMe3), 42.2 (NMe2), 57.1 (NCH2), 124.0, 133.4, 135.0
(C6H3), 131.2, 141.8, 152.8 (C6H3-ipso). Anal. Calcd for
C19H38AlNSSi: C, 62.05; H, 10.43; N, 3.80. Found: C, 63.33; H,
10.98; N, 3.10.
EXPERIMENTAL SECTION
■
All manipulations were performed under argon using Schlenk and
high-vacuum-line techniques or in a Model HE-63 glovebox. Solvents
were dried and purified with an MBRAUN solvent purification system.
Deuterated solvents were stored over activated 4 Å molecular sieves
and degassed by several freeze−thaw cycles. AlMe3 (2.0 M solution in
toluene), AlClMe2 (1.0 M solution in hexane), AlCl2Me (1.0 M
solution in hexane), and AlCl3 were purchased from commercial
sources (Aldrich) and used as received. L-Lactide (Aldrich) was
recrystallized from toluene, washed with diethyl ether, and sublimed at
110 °C prior to use. C, H, and N microanalyses were performed on a
Perkin-Elmer 240B and/or Heraeus CHN-O-Rapid microanalyzer.
The elemental analyses for complexes 2 deviated from acceptable
limits, since hexane could not be completely removed due to the oily
nature of these complexes, which explains the high carbon values.
NMR spectra were recorded on a Bruker AV400 instrument (1H NMR
Synthesis of [AlMe2{S(C6H3-2-CH2NC5H10-5-tBu)}-κ2S,N] (3a).
A solution of S(SiMe3)(C6H3-2-CH2NC5H10-5-tBu) (0.96 g, 2.86
mmol) in toluene (30 mL) was added dropwise to a solution of AlMe3
(1.60 mL, 3.14 mmol) in toluene (30 mL) at room temperature. The
reaction mixture was heated to 130 °C and stirred for 4 days. After the
mixture was cooled to room temperature, the solvent was removed
under vacuum to yield a white residue that was extracted into a
mixture of hexane and toluene (15 and 5 mL, respectively). The
resulting solution was concentrated under vacuum to ca. 10 mL and
cooled to −20 °C to give 3a as a white solid (0.72 g, 2.28 mmol, 80%).
1H NMR (C6D6, 400 MHz): δ −0.40 (s, 6H, AlMe2), 0.81, 0.90, 1.06,
1.81, 2.59 (m, 2 × 1H, 4H, 2 × 2H, NC5H10), 1.18 (s, 9H, CMe3), 3.43
(s, 2H, NCH2), 6.79 (d, 1H, 3J = 8 Hz, C6H3), 7.07 (dd, 1H, 3J = 8 Hz,
1
at 400 MHz, 13C NMR at 100.6 MHz MHz) at 25 °C. H and 13C
4
4J = 2 Hz, C6H3), 7.97 (d, 1H, J = 2 Hz, C6H3). 13C{1H} NMR
NMR chemical shifts (δ) were determined using residual signals of the
deuterated solvents and were calibrated versus TMS. The assignment
of resonances was carried out by using 1D (1H, 13C{1H}) and 2D
(HMQC, HMBC) NMR experiments.
Gel permeation chromatography (GPC) analysis was performed at
25 °C on a Varian HPLC system, using THF as the mobile phase. Gel
permeation chromatography (GPC) analyses of polymer samples were
carried out in THF at 25 °C (Waters GPCV-2000). The calibration
curves were made using different polystyrene standards. MALDI-TOF
MAS analyses were performed using an Agilent MALDI-TOF LC/MS
instrument; the ionization source was a Masstech AP/MALDI. The
(C6D6, 100.6 MHz): δ −7.2 (AlMe2), 21.5, 24.7, 53.8 (NC5H10), 32.3
(CMe3), 34.7 (CMe3), 61.5 (NCH2), 121.4, 130.1, 132.4 (C6H3),
130.4, 142.4, 152.8 (C6H3-ipso). Anal. Calcd for C18H30AlNS: C,
67.67; H, 9.39; N, 4.38. Found: C, 67.45; H, 9.89; N, 4.29.
Synthesis of [AlMe2{S(C6H3-2-CH2NC4H8-5-tBu)}-κ2S,N] (3b). A
procedure similar to that described above for 3a was used with
S(SiMe3)(C6H3-2-CH2NC4H8-5-tBu) (0.94 g, 2.92 mmol) and AlMe3
(1.60 mL, 3.21 mmol) to afford 3b as an orange solid (0.70 g, 2.27
1
mmol, 78%). H NMR (C6D6, 400 MHz): δ −0.45 (s, 6H, AlMe2),
1.10, 1.84, 2.56 (m, 4H, 2 × 2H, NC4H8), 1.18 (s, 9H, CMe3), 3.33 (s,
E
dx.doi.org/10.1021/om400111f | Organometallics XXXX, XXX, XXX−XXX

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Article
2H, NCH2), 6.70 (d, 1H, 3J = 8 Hz, C6H3), 7.05 (dd, 1H, 3J = 8 Hz, 4J
= 2 Hz, C6H3), 7.97 (d, 1H, 4J = 2 Hz, C6H3). 13C{1H} NMR (C6D6,
100.6 MHz): δ −8.5 (AlMe2), 22.0, 55.8 (NC4H8), 31.3 (CMe3), 34.5
(CMe3), 61.9 (NCH2), 120.9, 130.1, 132.2 (C6H3), 131.7, 141.9, 152.7
(C6H3-ipso). Anal. Calcd for C17H28AlNS: C, 66.84; H, 9.17; N, 4.59.
Found: C, 66.57; H, 9.04; N, 4.46.
(NC5H10), 31.0 (CMe3), 34.6 (CMe3), 61.2 (NCH2), 122.4, 131.2,
131.7 (C6H3), 128.4, 137.4, 153.8 (C6H3-ipso).
Synthesis of [AlCl2{S(C6H3-2-CH2NC4H8-5-tBu)}-κ2S,N] (5b). A
procedure similar to that described above for 5a was adopted using 1b
(0.030 g, 0.093 mmol) and AlCl3 (0.014 g, 0.112 mmol) to give 5b. 1H
NMR (C6D6, 400 MHz): δ 1.06, 1.34, 1.99, 2.96 (m, 4 × 2H, NC4H8),
3
Synthesis of [AlMe2{S(C6H3-2-CH2NMe2-5-tBu)}-κ2S,N] (3c). A
procedure similar to that described above for 3a was applied using
S(SiMe3)(C6H3-2-CH2NMe2-5-tBu) (0.90 g, 3.04 mmol) and AlMe3
(1.67 mL, 3.34 mmol) to give 3c as a white solid (0.70 g, 2.29 mmol,
82%) after 7 days of reaction. 1H NMR (C6D6, 400 MHz): δ −0.52 (s,
6H, AlMe2), 1.17 (s, 9H, CMe3), 1.61 (s, 6H, NMe2) 3.14 (s, 2H,
NCH2), 6.67 (d, 1H, 3J = 8 Hz, C6H3), 7.12 (dd, 1H, 3J = 8 Hz, 4J = 2
1.08 (s, 9H, CMe3), 3.35 (s, 2H, NCH2), 6.59 (d, 1H, J = 8 Hz,
C6H3), 7.03 (dd, 1H, 3J = 8 Hz, 4J = 2 Hz, C6H3), 7.78 (d, 1H, 4J = 2
Hz, C6H3). 13C{1H} NMR (C6D6, 100.6 MHz): δ 21.7, 55.6 (NC4H8),
31.1 (CMe3), 34.5 (CMe3), 61.3 (NCH2), 122.3, 130.6, 131.9 (C6H3),
129.8, 136.7, 153.7 (C6H3-ipso).
Synthesis of [[AlCl2{S(SiMe3)(C6H3-2-CH2NMe2-5-tBu)}-κ2S,N]
(5c). A procedure similar to that described above for 5a was adopted
using 1c (0.030 g, 0.101 mmol) and AlCl3 (0.016 g, 0.121 mmol) to
4
Hz, C6H3), 7.96 (d, 1H, J = 2 Hz, C6H3). 13C{1H} NMR (C6D6,
1
give 5c. H NMR (C6D6, 400 MHz): δ 1.11 (s, 9H, CMe3), 1.79 (s,
100.6 MHz): δ −8.7 (AlMe2), 31.2 (CMe3), 34.5 (CMe3), 46.1
(NMe2), 66.1 (NCH2), 121.0, 130.6, 132.4 (C6H3), 131.2, 141.8, 152.8
(C6H3-ipso). Anal. Calcd for C15H26AlNS: C, 64.48; H, 9.38; N, 5.01.
Found: C, 64.12; H, 9.45; N, 4.95.
3
6H, NMe2), 3.13 (s, 2H, NCH2), 6.53 (d, 1H, J = 8 Hz, C6H3), 7.01
3
4
4
(dd, 1H, J = 8 Hz, J = 2 Hz, C6H3), 7.74 (d, 1H, J = 2 Hz, C6H3).
13C{1H} NMR (C6D6, 100.6 MHz): δ 31.0 (CMe3), 34.5 (CMe3), 46.7
Synthesis of [AlClMe{S(C6H3-2-CH2NC5H10-5-tBu)}-κ2S,N] (4a).
A solution of S(SiMe3)(C6H3-2-CH2NC5H10-5-tBu) (0.20 g, 0.59
mmol) in toluene (10 mL) was added dropwise to a solution of
AlCl2Me (0.65 mL, 0.65 mmol). The reaction mixture was stirred for 2
h at room temperature, at which point the volatiles were pumped off
and the resulting solid was washed with hexane (3 × 5 mL) and
(NMe2), 65.7 (NCH2), 122.5, 130.9, 132.1 (C6H3), 129.2, 136.5, 153.9
(C6H3-ipso).
Typical L-Lactide Polymerization Procedure. A 0.45 g portion
(3.12 mmol) of L-lactide was added to a Teflon-valved Schlenk tube
loaded with the aluminum complex (0.031 mmol) dissolved in toluene
(7 mL) and equipped with a magnetic stirrer. The resulting mixture
was immersed in an oil bath preset at 130 °C, and the polymerization
time was measured from this point. At certain time intervals, an aliquot
of the reaction mixture was taken out using a syringe to determine
monomer conversion by 1H NMR. Finally, when a conversion of over
80% was reached, the polymerization was quenched by the addition of
5 mL of acidified MeOH (HCl, 10 wt %). Then, the polymer was
precipitated into 150 mL of methanol and washed thoroughly. The
polymer was dissolved in acetone, precipitated in methanol at 0 °C,
collected by filtration, and dried to constant weight in a vacuum oven
at 50 °C.
1
recrystallized from toluene to give 4a (0.12 g, 0.35 mmol, 60%). H
NMR (C6D6, 400 MHz): δ −0.26 (s, 3H, AlMe), 0.87, 0.98, 1.39, 1.83,
2.10, 2.48, 3.02 (m, 2H, 3H, 5 × 1H, NC5H10), 1.14 (s, 9H, CMe3),
2
3
3.28, 3.69 (d, 2 × 1H, J = 12 Hz, NCH2), 6.71 (d, 1H, J = 8 Hz,
C6H3), 7.01 (dd, 1H, 3J = 8 Hz, 4J = 2 Hz, C6H3), 7.85 (d, 1H, 4J = 2
Hz, C6H3). 13C{1H} NMR (C6D6, 100.6 MHz): δ 21.0, 21.2, 22.3,
53.1, 54.4 (NC5H10), 31.0 (CMe3), 34.5 (CMe3), 61.4 (NCH2), 121.5,
130.8, 131.8 (C6H3), 129.3, 139.6, 153.3 (C6H3-ipso). Anal. Calcd for
C17H27AlClNS: C, 60.06; H, 7.94; N, 4.11. Found: C, 60.75; H, 8.02;
N, 4.29.
Synthesis of [AlClMe{S(C6H3-2-CH2NC4H8-5-tBu)}-κ2S,N] (4b).
A procedure similar to that described above for 4a was adopted using
S(SiMe3)(C6H3-2-CH2NC4H8-5-tBu) (0.23 g, 0.71 mmol) and
AlCl2Me (0.80 mL, 0.78 mmol) to obtain complex 4b (0.15 g, 0.46
Single-Crystal X-ray Structure Determination of Com-
pounds 3a,b. Details of the X-ray experiment, data reduction, and
final structure refinement calculations are summarized in the
Supporting Information. Suitable single crystals of 3a,b for the X-ray
diffraction study were selected. Data collection was performed at
200(2) K, with the crystals covered with perfluorinated ether oil. The
crystals were mounted on a Bruker-Nonius Kappa CCD single-crystal
diffractometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). Multiscan16 absorption correction
procedures were applied to the data. The structures were solved,
using the WINGX package,17 by direct methods (SHELXS-97) and
refined using full-matrix least squares against F2 (SHELXL-97).18 All
non-hydrogen atoms were anisotropically refined. Hydrogen atoms
were geometrically placed and left riding on their parent atoms. C17 in
3a and some carbon atoms in 3b (C16, C17, and C15) show some
positional disorder that was left untreated. Full-matrix least-squares
1
mmol, 61%). H NMR (C6D6, 400 MHz): δ −0.34 (s, 3H, AlMe),
1.06, 1.35, 1.81, 2.03, 2.41, 3.11 (m, 3H, 5 × 1H, NC4H8), 1.07 (s, 9H,
CMe3), 3.11, 3.56 (d, 2 × 1H, 2J = 12 Hz, NCH2), 6.63 (d, 1H, 3J = 8
Hz, C6H3), 7.01 (dd, 1H, 3J = 8 Hz, 4J = 2 Hz, C6H3), 7.86 (d, 1H, 4J =
2 Hz, C6H3). 13C{1H} NMR (C6D6, 100.6 MHz): δ 21.7, 21.8, 54.4,
55.7 (NC4H8), 31.1 (CMe3), 34.5 (CMe3), 61.4 (NCH2), 121.6, 130.7,
132.1 (C6H3), 129.3, 139.0, 153.2 (C6H3-ipso). Anal. Calcd for
C16H25AlClNS: C, 58.96; H, 7.67; N, 4.29. Found: C, 59.19; H, 7.84;
N, 4.16.
Synthesis of [AlClMe{S(C6H3-2-CH2NMe2-5-tBu)}-κ2S,N] (4c).
A procedure similar to that described above for 4a was adopted using
S(SiMe3)(C6H3-2-CH2NMe2-5-tBu) (0.27 g, 0.91 mmol) and AlCl2Me
(1 mL, 1.00 mmol) to obtain complex 4c (0.17 g, 0.59 mmol, 65%).
1H NMR (C6D6, 400 MHz): δ −0.40 (s, 3H, AlMe), 1.13 (s, 9H,
2
refinements were carried out by minimizing ∑w(Fo − Fc2)2 with the
SHELXL-97 weighting scheme and stopped at shift/err <0.001. The
final residual electron density maps showed no remarkable features.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC-923240 (3a) and CCDC-923241 (3b). Copies of the data
can be obtained free of charge on application to the CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk).
2
CMe3), 1.53, 1.90 (s, 2 × 3H, NMe2), 2.93, 3.39 (d, 2 × 1H, J = 12
Hz, NCH2), 6.58 (d, 1H, 3J = 8 Hz, C6H3), 7.02 (dd, 1H, 3J = 8 Hz, 4J
= 2 Hz, C6H3), 7.84 (d, 1H, 4J = 2 Hz, C6H3). 13C{1H} NMR (C6D6,
100.6 MHz): δ 31.2 (CMe3), 34.5 (CMe3), 44.0, 47.0 (NMe2), 65.7
(NCH2), 121.8, 130.7, 132.2 (C6H3), 130.1, 138.8, 153.4 (C6H3-ipso).
Anal. Calcd for C14H23AlClNS: C, 56.07; H, 7.69; N, 4.66. Found: C,
56.63; H, 7.95; N, 4.20.
Synthesis of [AlCl2{S(C6H3-2-CH2NC5H10-5-tBu)}-κ2S,N] (5a). A
0.5 mL portion of C6D6 was added to a mixture of 1a (0.030 g, 0.089
mmol) and AlCl3 (0.014 g, 0.107 mmol), and the resulting solution
was injected into a Teflon-valved NMR tube. The reaction was
monitored by NMR spectroscopy at 25 °C, and 5a evolved as the
unique product, after 2 h of reaction time. 1H NMR (C6D6, 400
MHz): δ 0.77, 0.86, 1.01, 1.30, 2.01, 2.95 (m, 2 × 1H, 4 × 2H,
NC5H10), 1.20 (s, 9H, CMe3), 3.50 (s, 2H, NCH2), 6.67 (d, 1H, 3J = 8
Hz, C6H3), 7.03 (dd, 1H, 3J = 8 Hz, 4J = 2 Hz, C6H3), 7.77 (d, 1H, 4J =
2 Hz, C6H3). 13C{1H} NMR (C6D6, 100.6 MHz): δ 20.9, 22.0, 54.0
ASSOCIATED CONTENT
■
S
* Supporting Information
A table and CIF files giving crystallographic data, including
fractional coordinates, bond lengths and angles, anisotropic
displacement parameters, and hydrogen atom coordinates, for
complexes 3a,b and figures giving 1H NMR spectra and
MALDI-TOF mass spectra of PLA obtained by 3a. This
F
dx.doi.org/10.1021/om400111f | Organometallics XXXX, XXX, XXX−XXX

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Article
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material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*G.J.: tels, 34918854767; fax, 34 918854683; e-mail, gerardo.
jimenez@uah.es.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
L.P. acknowledges the Universidad de Alcala
́
for a fellowship.
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