Neutral and cationic aluminum complexes supported by acetamidate and thioacetamidate heteroscorpionate ligands as initiators for ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1021/om1010676

The synthesis, structures, and ring-opening polymerization (ROP) activity of heteroscorpionate aluminum alkyl and aryloxide complexes are reported. The reactions of the acetamide and thioacetamide heteroscorpionate protio ligands pbptamH (pbptamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide), pbpamH (pbpamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), and (S)-mbpamH ((S)-mbpamH = (S)-(-)-N-α-methylbenzyl-2,2-bis(3,5- dimethylpyrazol-1-yl)acetamide) with 1 equiv of AlR₃ (R = Me, Et, ⁱBu) proceed in very high yields to give the neutral heteroscorpionate dialkyl aluminum complexes [AlR₂{κ ²-pbptam}] (R = Me (1), Et (2)), [AlR₂{κ ²-pbptam}] (R = Me (3), Et (4), ⁱBu (5)), [AlR ₂{κ²-pbptam}] (R = Me (6), Et (7), ⁱBu (8)), and [AlR₂{κ²-(S)-mbpam}] (R = Me (9), Et (10)). In the solid state, complexes 1-10 adopt a tetrahedral structure with the heteroscorpionate ligands arranged in a κ² coordination mode; in the case of the thioacetamidate derivatives 1 and 2 a κ²NN coordination mode is observed, whereas the acetamidate derivatives 3-10 present a κ²NO coordination mode. The structures in solution of 1-10 were investigated by VT NMR spectroscopy, and fluxional exchange between coordinated and noncoordinated pyrazole rings was observed, producing interconversion between the different isomers. Compounds 7 and 10 were used as convenient starting materials for the synthesis of the aryloxide aluminum compounds [Al(OR)₂{κ²-sbpam}] (11) and [Al(OR) ₂{κ²-(S)-mbpam}] (12) (R = 2,6-Me₂C ₆H₃O) by reaction with the corresponding 2,6-dimethylphenol. The complexes [AlMe{κ³-pbptam}][MeB(C ₆F₅)₃] (13), [AlMe{κ³-pbptam}] [B(C₆F₅)₄] (14), [AlEt{κ³- pbptam}][B(C₆F₅)₄] (15), [AlEt{κ ³-sbpam}][B(C₆F₅)₄] (16), and [AlEt{κ³-(S)-mbpam}][B(C₆F₅) ₄] (17) are derived from the ionization of the neutral dialkyl aluminum complexes 1, 2, 7, and 10 with the alkyl abstracting reagent B(C ₆F₅)₃ or [Ph₃C][B(C ₆F₅)₄]. The NMR data were consistent with an overall C_s-symmetric structure for 13-15 and C₁-symmetric structure for 16 and 17, which indicates an effective κ³ coordination of the corresponding heteroscorpionate ligand to the cationic aluminum center. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 1, 2, and 7 were also established. Finally, exhaustive comparative catalytic studies of the aluminum complexes 1-10, 13, and 14 in the ring-opening polymerization of rac-lactide, l-lactide, and ε-caprolactone are also described.

Full text of DOI:10.1021/om1010676

ARTICLE
pubs.acs.org/Organometallics
Neutral and Cationic Aluminum Complexes Supported by
Acetamidate and Thioacetamidate Heteroscorpionate Ligands as
Initiators for Ring-Opening Polymerization of Cyclic Esters
Antonio Otero,*,† Agustín Lara-Sꢀanchez,*,† Juan Fernꢀandez-Baeza,† Carlos Alonso-Moreno,†
Jose A. Castro-Osma,† I. Mꢀarquez-Segovia,† Luis F. Sꢀanchez-Barba,‡ Ana M. Rodríguez,† and
Joaquin C. Garcia-Martinez†
†Departamento de Química Inorgꢀanica, Orgꢀanica y Bioquímica, Universidad de Castilla-La Mancha, Campus Universitario,
13071 Ciudad Real, Spain
‡Departamento de Química Inorgꢀanica y Analítica, Universidad Rey Juan Carlos, Mꢀostoles 28933 Madrid, Spain
S Supporting Information
b
ABSTRACT:
The synthesis, structures, and ring-opening polymerization (ROP) activity of heteroscorpionate aluminum alkyl and aryloxide
complexes are reported. The reactions of the acetamide and thioacetamide heteroscorpionate protio ligands pbptamH (pbptamH =
N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide), pbpamH (pbpamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)-
acetamide), sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), and (S)-mbpamH ((S)-mbpamH =
(S)-(-)-N-R-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide) with 1 equiv of AlR3 (R = Me, Et, iBu) proceed in very
high yields to give the neutral heteroscorpionate dialkyl aluminum complexes [AlR2{κ2-pbptam}] (R = Me (1), Et (2)), [AlR2{κ2-
pbptam}] (R = Me (3), Et (4), iBu (5)), [AlR2{κ2-pbptam}] (R = Me (6), Et (7), iBu (8)), and [AlR2{κ2-(S)-mbpam}] (R = Me
(9), Et (10)). In the solid state, complexes 1-10 adopt a tetrahedral structure with the heteroscorpionate ligands arranged in a κ2
coordination mode; in the case of the thioacetamidate derivatives 1 and 2 a κ2NN coordination mode is observed, whereas the
acetamidate derivatives 3-10 present a κ2NO coordination mode. The structures in solution of 1-10 were investigated by VT
NMR spectroscopy, and fluxional exchange between coordinated and noncoordinated pyrazole rings was observed, producing
interconversion between the different isomers. Compounds 7 and 10 were used as convenient starting materials for the synthesis of
the aryloxide aluminum compounds [Al(OR)2{κ2-sbpam}] (11) and [Al(OR)2{κ2-(S)-mbpam}] (12) (R = 2,6-Me2C6H3O) by
reaction with the corresponding 2,6-dimethylphenol. The complexes [AlMe{κ3-pbptam}][MeB(C6F5)3] (13), [AlMe{κ3-
pbptam}][B(C6F5)4] (14), [AlEt{κ3-pbptam}][B(C6F5)4] (15), [AlEt{κ3-sbpam}][B(C6F5)4] (16), and [AlEt{κ3-(S)-mbpam}]-
[B(C6F5)4] (17) are derived from the ionization of the neutral dialkyl aluminum complexes 1, 2, 7, and 10 with the alkyl abstracting
reagent B(C6F5)3 or [Ph3C][B(C6F5)4]. The NMR data were consistent with an overall Cs-symmetric structure for 13-15 and C1-
symmetric structure for 16 and 17, which indicates an effective κ3 coordination of the corresponding heteroscorpionate ligand to the
cationic aluminum center. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal
structures of 1, 2, and 7 were also established. Finally, exhaustive comparative catalytic studies of the aluminum complexes 1-10, 13,
and 14 in the ring-opening polymerization of rac-lactide, L-lactide, and ε-caprolactone are also described.
’ INTRODUCTION
properties. In order to achieve this goal, the microstructure of the
polymeric chains, including their molecular weight and polydis-
persity, needs to be controlled and this can be performed using
catalysis. In this context, interest in the past decade in the catalytic
Ecologically acceptable complements to saturated polyolefins—
the consumption of which has risen steadily since the specta-
cular discovery of alkene polymerization—include biode-
gradable polymers such as polylactides and polylactones.1,2
The scientific interest in well-defined polymer architectures
results from the need to obtain polymers with good mechanical
Received: November 12, 2010
Published: February 28, 2011
r
2011 American Chemical Society
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ring-opening polymerization (ROP) of lactones and lactides
has spurred several review articles.3 Numerous single-site cata-
lysts bearing electrophilic metal ions, such as magnesium,4
calcium,4,5 titanium,6 lanthanide,7 iron,8 zinc,9 tin,10 and alumi-
num,11 are well-known. Aluminum complexes are among the
most efficient catalysts for lactone and lactide polymerizations
and are generally established using N- and/or O-supported
ligands. Among the variety of catalysts, many chiral aluminum
alkoxides supported by tetradentate Schiff base ligands have been
highlighted as highly selective in the polymerization of rac-lactide
or meso-lactide.11c,e,g-i,k The nature of the ancillary ligand in the
aluminum coordination sphere is an obvious key parameter that
determines the steric and electronic properties and, in turn, the
catalytic performances of these complexes. As a result, there is an
ongoing search for such new ancillaries. Heteroscorpionate
ligands represent one of the most versatile types of anionic
tridentate ligands that can coordinate to a variety of elements,
e.g., from transition metals to main-group metals.12 In recent years,
chemistry based on the design of this particular type of intriguing
heteroscorpionate ligand has been extended considerably in
terms of coordination chemistry13 and catalytic applications.14
In the past decade a number of research groups have contributed
widely to this field, designing new heteroscorpionate ligands
related to the bis(pyrazol-1-yl)methane system and incorporat-
ing several pendant donor arms bearing an anionic functional
donor group, such as carboxylate, dithiocarboxylate, aryloxide,
alkoxide, amide, cyclopentadienyl, acetamidate, thioacetamidate,
and amidinate.15 Furthermore, some of these ligands have been
designed as chiral ligands with high enantiopurity, which is an
important goal in organometallic chemistry in order to prepare
metal-based reagents through efficient stereochemical control in
asymmetric processes.16 In this field, our research group and
others are interested in extending the scope of this chemistry to
the synthesis of heteroscorpionate complexes of the main-group
metals, such as lithium, magnesium, calcium, and aluminum,17
Scheme 1. Summary of Reactions Leading to Complexes
1-10
heteroscorpionate moiety), and this makes them an interesting
type of ligand for the preparation of aluminum complexes, due
to their wide variety of coordination modes.20 We previously
observed that thioacetamide ligands, which contain both hard
and soft donor atoms, bond to the hard Lewis acid Al(III)
by the hard donor atom (N) of the thioacetamidate moiety.19
Bearing this information in mind, we reacted the corresponding
trialkylaluminum with the protonated acetamide or thioaceta-
mide heteroscorpionate precursors20 pbptamH (pbptamH = N-
phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide), pbpamH
(pbpamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide),
sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)-
acetamide), and (S)-mbpamH ((S)-mbpamH = (S)-(-)-N-R-
methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide) in a 1:1
molar ratio in toluene at 0 °C. These reactions gave the mononuclear
dialkyl aluminum complexes [AlR2{κ2-pbptam}] (R = Me (1), Et
(2)), [AlR2{κ2-pbpam}] (R = Me (3), Et (4), iBu (5)), [AlR2{κ2-
sbpam}] (R = Me(6), Et(7), iBu (8)) and[AlR2{κ2-(S)-mbpam}]
(R = Me (9), Et (10)) with vigorous elimination of the correspond-
ing alkane (Scheme 1). Compounds 1 and 2 were isolated as yellow
solids, and compounds 3-10 were isolated as white solids, all in
good yields, after the appropriate workup procedure. All of the
complexes were isolated as chiral compounds, and 9 and 10 were
obtained as enantiopure compounds.
the chemistry of which in terms of scorpionate ligands is based
0
mainly on Trofimenko’s borate (TpRR ) ligands.18 Some of these
scorpionate and heteroscorpionate aluminum complexes are
active initiators in the ring-opening polymerization of cyclic
esters. We have already focused on the possibility of designing
organoaluminum entities in order to study their synthetic
accessibility, structural arrangements, and catalytic behavior in
the ring-opening polymerization of polar monomers such as
ε-caprolactone and lactide. We previously reported the first alumi-
num complexes with thioacetamidate heteroscorpionate ligands,
and these exhibit high versatility in terms of coordination modes due
to the existence of three possible tautomers.19 Bearing in mind the
potential applications of the element aluminun in catalysis, it was of
interest to prepare new neutral and cationic scorpionate aluminum
complexes with potential as high-activity initiators for cyclic ester
polymerization. Herein, we describe in detail the synthesis and
characterization of a series of chiral heteroscorpionate alkyl and
aryloxide aluminum complexes, including cationic derivatives and
some enantiopure complexes. The performance of these complexes
in the polymerization of ε-caprolactone and L-/rac-lactide was
explored under well-controlled conditions.
The 13C NMR signals of the carbonyl and thiocarbonyl groups
in these complexes have been good indicators of the bonding
mode of acetamidate or thioacetamidate moieties of the ligands.
The acetamidate carbon resonance, RNCO, is shifted to higher
field with respect to that of neutral ligands20a (see the Experi-
mental Section), indicating that the acetamidate moiety is
coordinated to the aluminum center through the O atom (see
Scheme 1). In contrast, the corresponding signal for the thioa-
cetamidate carbon resonance, RNCS, is shifted to lower field with
respect to that in the neutral ligand20a (see the Experimental
’ RESULTS AND DISCUSSION
Syntheses and Structural Characterization. Acetamide or
thioacetamide heteroscorpionate precursors contain Lewis basic
coordinating groups (N and O or S centers in the “arm” of the
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Figure 1. 1H NMR spectrum of [AlEt2{κ2-(S)-mbpam}] (10).
irradiation in compounds 3-10 did not lead to any enhancement
of the N-R group. 1H-13C heteronuclear correlation (g-HSQC)
experiments were carried out, and these allowed us to assign the
resonances corresponding to C4, Me3, and Me5 of the pyrazole
rings and alkyl groups. The data support a tetrahedral disposition
for the aluminum atom with a κ2NN coordination mode of the
thioacetamidate heteroscorpionate ligand, whereas the acetami-
date derivatives have a κ2NO coordination mode, as depicted in
Scheme 1.
Scheme 2
Two examples of variable-temperature NMR studies are
discussed below for complexes with an achiral and a chiral
heteroscorpionate ligand, respectively. In the case of complex 2
the VT NMR analysis showed that the resonances of the ethyl
groups directly bonded to the aluminum center, as well as those
of the pyrazole rings, broaden and become resolved into two
separate peaks at -70 °C. A stacked plot of the relevant sections
Section), indicating that the thioacetamidate moiety is coordi-
nated to the metal center through the N atom (see Scheme 1).
However, a small amount of delocalized E-C-N bond probably
exists in the acetamidate or thioacetamidate moieties of the
heteroscorpionate ligands.
1
of the variable-temperature H NMR spectra of 2 is shown in
The 1H and 13C{1H} NMR spectra of 1-5 (without a chiral
carbon) at room temperature show a singlet for each of the H4,
Me3, and Me5 pyrazole protons and carbons, indicating that the
pyrazoles are equivalent, along with one resonance for the two
equivalent alkyl ligands. It is worth noting that some pyrazole
proton signals appear as broad resonances, a finding that
indicates fluxional behavior (Scheme 2), which is thought to be
due to an exchange process between a coordinated and a
noncoordinated pyrazole ring (see discussion below). Further-
Figure 2. At room temperature the exchange is too fast to be
detected on the NMR time scale and the two pyrazole rings
therefore appear equivalent. Thus, an exchange process between
a coordinated and noncoordinated pyrazole ring is taking place
and this involves interconversion from one stereoisomer to the
other. It is worth noting that, given the coordination mode of the
ligands, all the complexes are chiral compounds regardless of
the existence of a chiral center in the heteroscorpionate ligand
used as the scaffold.
1
more, the H and 13C{1H} NMR spectra of 6-10 (which do
The variable-temperature NMR study for complex 6 provides
a good example of a compound bearing a chiral heteroscorpio-
contain a stereogenic carbon) show two singlets for each of the
H4, Me3, and Me5 pyrazole protons and carbons, some of them as
broad resonances (see Figure 1). Moreover, the signals of the
alkyl groups at the aluminum center appear as a single broad
resonance. These data also confirm the presence of an exchange
process between the coordinated and the noncoordinated pyr-
azole rings (Scheme 2).
nate ligand. In the 1H NMR spectra shown in Figure 3 signals can
0
be observed for the CH, H4, H4 , and CH* protons of the
heteroscorpionate ligand at room temperature. When the tem-
perature was decreased to below -80 °C, two signals (relative
intensities 2:1) were observed for each of the diastereoisomers
present (see Scheme 3). For complexes 6-8, which contain a
racemic heteroscorpionate ligand, there are four stereoisomers,
while for complexes 9 and 10, which contain an enantiopure
heteroscorpionate ligand, there are only two diastereoisomers.
The VT NMR analyses carried out on complexes 1-10 (from
þ40 to -80 °C) allowed us to evaluate a series of kinetic
parameters. The rate constants kex at each temperature were
NOESY-1D NMR experiments permitted the unequivocal
1
assignment most of the H NMR resonances. The response in
1
the H NOESY-1D experiment from the ortho protons of the
phenyl ring from the thioacetamidate moiety on irradiating each
of the alkyl groups suggests coordination by the N atom of the
thioacetamidate moiety in compounds 1 and 2. However, similar
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Figure 2. Variable-temperature 1H NMR spectra of compound 2 in toluene-d8.
Figure 3. Variable-temperature 1H NMR spectra in the region from 6.5 to 4.0 ppm for compound 6.
estimated by visual matching of line shapes of simulated and
experimental spectra for complexes 1-10 (see Table 1).21 The
activation enthalpies, activation entropies, and Gibbs free en-
ergies at 20 °C were evaluated from the Eyring plot (see Figures
S1-S9 in the Supporting Information).21
The data collected in Table 1 allow several trends to be
highlighted. Compound 1 has the lowest activation barrier, and
at -80 °C it is close to the coalescence point, which precluded an
evaluation of the kinetic parameters for this compound.
Furthermore, it can be observed that aluminum compounds 1,
3, and 9 (bearing a methyl ligand) show the fastest site exchange
by a significant margin, while complexes 5 and 8 (bearing an
isobutyl ligand) exhibit the slowest site exchange (see the rate
constant kex values in Table 1). It is clear that the replacement of
the methyl or ethyl ligand by the bulkier isobutyl ligand leads to a
significant deceleration of kex in the compounds, evidently the
result of increasing steric hindrance on the aluminum with the
bulkiness of the substituents (see Figure S10 in the Supporting
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ARTICLE
Information). Additionally, the increase in the activation enthalpy
and the slight variation in the activation entropy on increasing the
bulk of the alkyl substituent support an intramolecular associative
displacement mechanism in which the pyrazole ring that is
outside the coordination sphere of the metal center displaces
the coordinated pyrazole ring through a pentacoordinate transi-
tion state. The approach of the free pyrazole ring is necessarily
from the opposite side of the coordinated pyrazole ring; there-
fore, inversion of configuration at the metal center occurs (see
Scheme 2). A similar exchange mechanism for coordinated and
uncoordinated pyrazole groups was previously observed in tris-
(pyrazol-1-yl)hydroborato and other heteroscorpionate alumi-
num complexes.17g,18d,18f
The molecular structures of complexes 1, 2, and 7 were
determined by X-ray diffraction. The corresponding ORTEP
drawings are depicted in Figure 4. In all cases, the heteroscor-
pionate ligand is κ2 coordinated to Al, κ2NN for thioacetamidate
complexes and κ2NO coordinated for the acetamidate complex,
forming pseudotetrahedral complexes with C1 symmetry. The
crystallographic data are given in Table S1 (Supporting In-
formation), and selected interatomic distances and angles are
given in Table 2. The solid-state structures are consistent with
those proposed in Scheme 1 on the basis of solution NMR and
other analytical data. Given the coordination mode of the
heteroscorpionate ligand, complexes 1, 2, and 7 are chiral
compounds. These complexes crystallize as a racemic mixture
with both enantiomers included in the unit cells, which belong to
centrosymmetric space groups. The geometry around the Al
center can be described for all complexes as distorted tetrahedral,
with the dihedral angle between the N1-Al-E and C-Al-C
planes (89.2, 85.7, and 85.22° for 1, 2, and 7, respectively)
consistent with a tetrahedral geometry. Furthermore, the angles
around the Al atom show considerable deviation from ideal
values (range 92.7(4)-116.1(4)° for 1, 92.5(4)-116.9(4)° for
2, 94.6(2)-119.1(8)° for 7), and in all compounds the most
acute angle of 92.7(4)° for 1, 92.5(4)° for 2, and 94.6(2)° for 7 is
observed for N1-Al-E, which is constrained by the bite of the
heteroscorpionate ligand. The distances between N(4) or N(3)
and Al of 3.34(1) Å for 1, 3.298(4) Å for 2, and 3.434(5) Å for 7
are too long to be considered as bonding or an interaction
between N(4) or N(3) and the Al atom, probably due to
the steric hindrance caused by the two alkyl groups. The Al-C
distances (range 1.953(5)-2.00(3) Å) are in good agreement
with literature data.17f,g,18,19 The Al-N1 bond distances of
1.941(9) Å for 1, 1.935 (4) Å for 2, and 1.953(5) Å for 7 are
similar to those in Al/pyrazolyl complexes.17f,g,18,19 The bond
distance of 1.775(4) Å for Al-O1 of 7 is shorter than the
Al-N5 distances of 1.932(9) and 1.918(4) Å for 1 and 2,
respectively.
Scheme 3. Proposed Structures for the Four Enantiomers of
Compound 6
It was envisaged that the new aluminum alkyls would serve as
starting materials to prepare the corresponding alkoxide com-
plexes, which are usually better initiators for ROP. Unfortunately,
reaction of 1-10 with either isopropyl or ethyl alcohol yielded
mixtures of protio ligand, residual metal alkyl, and Al(OR)3
species in toluene at room temperature; in alternative solvents
and/or at high dilution the reactions gave similar mixtures. Thus,
evidence for the formation of alkoxide aluminum species was not
observed. While the reaction of the alkyl organoaluminums 1-10
with 1 or 2 equiv of a source of aliphatic alcohol consistently
yielded intractable mixtures, the reaction of 7 and 10 with 2 equiv
Table 1. Summary of Kinetic Parameters for the Isomerization of Compounds 1-10
complex
Tcoalescence (°C)
kex(20 °C) (s-1
)
ΔGq(20 °C) (kcal mol-1
)
ΔHq (kcal mol-1
)
ΔSq (cal mol-1 K-1
)
1
2
-90
-40
-50
-20
30
n.d.
n.d.
n.d.
n.d.
8300 ( 200
8900 ( 200
4100 ( 150
350 ( 20
4300 ( 150
1800 ( 80
80 ( 10
7.1 ( 0.1
3.5 ( 0.1
3.8 ( 0.1
4.1 ( 0.1
8.4 ( 0.1
8.8 ( 0.1
9.7 ( 0.5
5.2 ( 0.1
5.4 ( 0.2
7.4 ( 0.5
11.0 ( 0.7
11.8 ( 0.6
12.4 ( 0.7
6.8 ( 0.9
7.0 ( 0.8
7.5 ( 0.6
9.0 ( 0.7
9.8 ( 0.8
-24.2 ( 2.5
-11.8 ( 2.7
-12.9 ( 2.6
-13.8 ( 2.7
-28.6 ( 3.8
-30.0 ( 3.2
-33.0 ( 2.2
-17.8 ( 3.0
-18.3 ( 3.6
3
4
5
6
-20
0
7
8
30
9
-40
-10
9200 ( 200
6100 ( 100
10
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Figure 4. ORTEP drawings of compounds 1 (a), 2 (b), and 7 (c). Thermal ellipsoids are set at the 30% probability level, and hydrogen atoms are
omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1, 2, and 7
1
molecule 1
molecule 2
2
7
Al(1)-N(1)
Al(1)-N(5)
Al(1)-C(19)
Al(1)-C(20)
S(1)-C(12)
1.941(9)
1.932(9)
1.97(1)
1.98(1)
1.69(1)
1.51(1)
1.32(1)
1.42(1)
92.7(4)
129.9(8)
113.0(8)
118(1)
Al(2)-N(6)
Al(2)-N(10)
Al(2)-C(40)
Al(2)-C(41)
S(2)-C(32)
C(31)-C(32)
1.93(1)
1.929(8)
1.96(1)
1.97(1)
1.68(1)
1.55(1)
1.30(1)
1.45(1)
92.5(4)
132.4(8)
111.1(7)
116.6(9)
N(1)-Al(2)
N(5)-Al(2)
C(22)-Al(2)
C(24)-Al(2)
C(15)-S(1)
C(14)-C(15)
C(15)-N(5)
C(16)-N(5)
1.935(4)
1.918(4)
1.953(5)
1.971(5)
1.672(4)
1.537(5)
1.332(5)
1.444(5)
94.8(1)
Al(1)-N(1)
Al(1)-O(1)
Al(1)-C(19)
Al(1)-C(17)
O(1)-C(12)
C(11)-C(12)
N(5)-C(12)
N(5)-C(13)
1.953(5)
1.775(4)
1.962(6)
2.00(3)
1.318(6)
1.517(7)
1.273(7)
1.51(2)
C(11)-C(12)
N(5)-C(12)
N(10)-C(32)
N(5)-C(13)
N(10)-C(33)
N(5)-Al(1)-N(1)
C(12)-N(5)-Al(1)
C(13)-N(5)-Al(1)
N(5)-C(12)-C(11)
N(10)-Al(2)-N(6)
C(32)-N(10)-Al(2)
C(33)-N(10)-Al(2)
N(10)-C(32)-C(31)
N(5)-Al(2)-N(1)
C(15)-N(5)-Al(2)
C(16)-N(5)-Al(2)
N(5)-C(15)-C(14)
O(1)-Al(1)-N(1)
N(5)-C(12)-O(1)
N(5)-C(12)-C(11)
O(1)-C(12)-C(11)
C(12)-O(1)-Al(1)
94.6(2)
130.0(3)
112.1(3)
117.1(4)
127.1(5)
115.2(5)
117.4(5)
131.4(3)
of 2,6-dimethylphenol afforded the corresponding bis-aryloxide
aluminum derivatives (Scheme 4). The reactions were carried out
at room temperature in toluene solution for 16 h to give very good
yields of [Al(OR)2{κ2-sbpam}] (11) and [Al(OR)2{κ2-(S)-
mbpam}] (12) (R = 2,6-Me2C6H3O) (>90%). Complex 12
was obtained as an enantiopure compound.
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Scheme 4. Summary of the Reaction Leading to Complexes 11 and 12
Scheme 5. Summary of Reactions Leading to Cationic Complexes 13-17
The room-temperature 1H and 13C{1H} NMR spectra of 11
and 12, both of which have a stereogenic carbon, show two
singlets for some of the H4, Me3, and Me5 pyrazole protons and
carbons, some of which appear broad. Moreover, resonances for
the o-methyl groups of the aryloxide substituents appear as broad
singlets. The broadness of some 1H NMR signals of the hetero-
scorpionate ligand and aryloxide groups in compounds 11 and 12
suggest that dynamic behavior must exist between pyrazole rings,
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Figure 5. Room-temperature 1H NMR spectrum of the complex [AlMe{κ3-pbptam}][MeB(C6F5)3] (13).
and different isomers might be present in slow exchange as with
the bis(alkyl) precursors. The variable-temperature (VT) NMR
analysis shows that at subambient temperature the resonances of
the methyl group on the aryloxide ligands, along with those of the
pyrazole rings and the resonance of H4 of the same rings, broaden
and resolve into two separate peaks. At -100 °C, the resonances
become well-resolved and the NMR spectra become consistent
with a tetrahedral structural disposition with a κ2NO coordina-
tion mode of the acetamidate heteroscorpionate ligand, with two
aryloxide ligands occupying the other two coordination positions
(see Scheme 4).
with both pyrazole rings being coordinated to the cationic center,
forming a tetracoordinate alkyl aluminum cation with a Cs-
symmetric structure, which is consistent with a κ3NNN coordi-
nation of the heteroscorpionate ligand (Scheme 5). Additionally,
responses in the 1H NOESY-1D experiments from protons in the
R-N moiety and Me3 of the pyrazole rings on irradiating the
Al-CH3 protons support the κ3NNN coordination mode and
the lack of response in the 1H NOESY-1D experiment from the
methyl group of the anion on irradiating the Al-CH3 protons or
on irradiating the Me3 protons of the pyrazole rings supports the
noncoordinating nature of the [MeB(C6F5)3]- anion.
Cationic aluminum complexes are of particular interest, due to
the enhancement of the Lewis acidity of the aluminum center,
a characteristic that can be associated with higher ROP activity.
In this context, cationic complexes have already found applica-
tion in lactide and lactone polymerization.17g,22 Tetracoordinate
cationic heteroscorpionate complexes can be particularly attrac-
tive, due to the combination of cationic charge and low coordina-
tion number producing increased electrophilic character at the
metal. The ionization of dialkyl aluminum complexes 1, 2, 7, and
9 were studied with the alkyl abstracting reagents B(C6F5)3 and
[Ph3C][B(C6F5)4]. Addition of 1 equiv of the borane B(C6F5)3
to the methyl derivative 1 in THF-d8 gave the cationic species
[AlMe{κ3-pbptam}][MeB(C6F5)3] (13) (Scheme 5). The salt,
which was stable for days in THF-d8, could not be isolated in a
pure form due to its oily nature. The solution structure of
aluminum cation 13 was deduced from 1H, 13C, and 19F NMR
The use of the trityl salt [Ph3C][B(C6F5)4] allowed access to
well-defined aluminum cation moieties derived from 1, 2, 7, and
10. The reaction of Al-methyl 1 and the diethyl aluminum com-
plexes 2, 7, and 10 with [Ph3C][B(C6F5)4] led to the quantita-
tive formation of the aluminum cations [AlMe{κ3-pbptam}]þ
(14), [AlEt{κ3-pbptam}]þ (15), [AlEt{κ3-sbpam}]þ (16), and
[AlEt{κ3-(S)-mbpam}]þ (17), respectively (Scheme 5). These
compounds were fully dissociated [B(C6F5)4]- salts, as evidenced
1
by H, 13C, and 19F NMR spectroscopy (see the Experimental
Section). The salt species 14-17 are stable for days in THF-d8 at
room temperature. The 1H and 13C{1H} NMR spectra of 14 and
15 (nonchiral compounds) exhibit a sharp singlet for each of the
H4, Me3, and Me5 pyrazole protons, indicating that the pyrazole
rings are equivalent. However, the 1H NMR and 13C{1H} NMR
spectra of complexes 16 and 17 (chiral compounds) show two
sharp singlets for each of the H4, Me3, and Me5 pyrazole protons.
The spectroscopic data obtained under the conditions investigated
are consistent with an overall C1-symmetric structure with an
effective κ3 coordination of the corresponding heteroscorpionate
ligand to the cationic aluminum center (Scheme 5). In addition, the
AlMe or AlEt resonances are shifted upfield relative to the
corresponding resonances in dialkyl complexes (see the Experi-
mental Section), and this is the result of the cationic charge on the
aluminum center. In a way similar to that for complex 13,
resonances for cationic derivatives 14-17 are sharp and
well-resolved even at low temperature, indicating the elevated
coordination energy of the ligand to the metal center and the
absence of any fluxional equilibrium involving the neutral moiety
of the ligand.
1
spectroscopic data (THF-d8 at room temperature). The H
NMR spectrum of 13 (Figure 5) at room temperature shows a
singlet for each of the H4, Me3, and Me5 pyrazole protons,
indicating that the pyrazoles are equivalent, and a sharp singlet
for the methyl ligand. It is worth noting that the pyrazole proton
signals are sharp and well-resolved even at low temperatures
(ca. -90 °C), indicating the elevated coordination energy of the
ligand with the aluminum center and the absence of any fluxional
equilibrium involving the neutral moiety of the ligand. The signal
for the methyl group of the [MeB(C6F5)3]- anion appeared at
0.70 ppm, indicating that the anion was substantially not
coordinated to the aluminum cation.17g The noncoordinating
nature of the [MeB(C6F5)3]- anion is further confirmed by the
small chemical shift difference (Δδ = 3.7 ppm) between the m-
and p-fluorine 19F NMR resonances.23 These data are consistent
Polymerization Studies. A good initiator for the ROP
of cyclic esters requires a redox-inactive metal, an inorganic
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monomeric ethyl acetamidate derivatives 4, 7, and 10 were of
the same order of magnitude and roughly 1 order higher than the
kapp value found for the ethyl thioacetamidate complex 2. The
methyl derivatives 1 and 6 presented the lowest kapp values under
the same conditions. In the case of the isobutyl derivatives 5 and
8, the initiation rate must be very slow and only traces of polymer
were formed, probably due to the steric hindrance of the isobutyl
groups on aluminum retarding the interaction between the Al
center and lactones.
Ethyl initiators 2, 4, 7, and 10 acted as efficient single-
component catalysts for the polymerization of ε-CL to give
medium-high molecular weight polymers; the results of these
experiments are collected in Table 3. A variety of polymerization
conditions were explored for compound 7. Complex 7 initiated
very rapid polymerization of ε-CL at 70 °C (entry 7) and gave
40% conversion of 500 equiv of ε-CL in 0.66 h (see Mn vs
conversion in Figure S11 in the Supporting Information). The
polymerization provided a medium-high molecular weight poly-
mer with a medium broad polydispersity (Mn = 29 460, Mw/Mn =
1.17). When the reaction time was extended to 2 h (entry 11),
almost complete conversion was observed (85%) with an
increase in the molecular weight distribution (Mw/Mn = 1.38,
entry 11). Not unexpectedly, an increase in the temperature up to
130 °C led to complete conversion of the monomer in 0.033 h
with a higher polydispersity index (Mw/Mn = 2.01, entry 12). As
expected, when the reaction solvent was changed to THF, only
35% of the monomer was converted in 4 h and a decrease in
molecular weight was observed (Mn = 24 900, entry 13). These
changes are due to coordination of the THF molecule, a process
that competes with monomer molecules in the polymerization
process. In these tests polymer molecular weights were obtained
by using a monomer to initiator ratio of 500:1. An increase in this
ratio by a factor of 2 gave polymer with a significantly higher
molecular weight, while the molecular weight distribution was
more or less maintained (entry 14). Two possibilities can
account for the relatively moderate to broad polydispersities
observed with the ethyl derivatives at 70 °C. First, in principle the
use of an Al-alkyl group as an initiator, which is known to be less
nucleophilic than alkoxide, leads to a delay in comparison to
propagation.25 Second, back-biting reactions or transesterifica-
tions can take place as side reactions and these result in the
formation of macrocycles with a wide range of molecular weight
distributions. The Mn values measured by gel permeation
chromatography (GPC) were substantially higher than those
predicted on the basis of conversion and on the assumption that
each aluminum center is catalytically active. This deviation could
be consistent with poor rates of initiation (ε-CL initiation in
the Al-Et bond) compared to propagation, which is a well-
established feature of metal alkyl initiators.9b,26
Figure 6. First-order kinetic plots for CL polymerizations in toluene at
70 °C with [CL]/[Al] = 500 and [Al] = 4.5 ꢀ 10-3 mol L-1: 1, kapp
=
2.0 ꢀ 10-3 s-1 (linear fit, R = 0.997); 2, kapp = 5.9 ꢀ 10-3 s-1 (linear fit,
R = 0.998); 3, kapp = 6.5 ꢀ 10-3 s-1 (linear fit, R = 0.997); 4, kapp = 9.0 ꢀ
10-3 s-1 (linear fit, R = 0.997); 6, kapp = 2.0 ꢀ 10-3 s-1 (linear fit, R =
0.997); 7, kapp = 11.9 ꢀ 10-3 s-1 (linear fit, R = 0.997); 9, kapp = 4.5 ꢀ
10-3 s-1 (linear fit, R = 0.997); 10, kapp = 9.6 ꢀ 10-3 s-1 (linear fit, R =
0.997).
template LnM that is inert with respect to undesirable reac-
tions, and a labile ligand able to undergo an insertion reaction
with C-X multiple bonds. In this context, β-diketiminate and
Schiff base aluminum complexes have displayed good catalytic
activity in the ROP of cyclic esters, and most of these examples
are aluminum alkoxide or alkyl complexes plus a cocatalyst
such as BnOH or iPrOH. To the best of our knowledge, very
few alkyl complexes without a cocatalyst have been used as
successful initiators for the ROP of cyclic esters.4a,11l,17h,18g,24
Given that the alkyl compounds 1-10 are thermally robust
and resistant toward hydrolysis, properties which suggest that
these compounds might be useful in catalytic processes that
require relatively high temperatures, we investigated them as
initiators for the ROP of lactide and lactones. We first studied the
catalytic performance—namely activity and degree of control—
in the assessment of the ring-opening polymerization of ε-CL.
The activities of these compounds were evaluated by comparison
of the kinetics parameters. For this purpose, the polymerization
of ε-CL was monitored in time by manual sampling followed by
1H NMR analysis to determine the degree of monomer conver-
sion. The polymerization kinetics were studied for complexes 1-
10 with [ε-CL]0/[Al] = 500 and [Al] = 4.5 ꢀ 10-3 M at 70 °C
using toluene as solvent and without cocatalyst or activator. A
semilogarithmic plot of ln([ε-CL]0/[ε-CL]t) versus reaction
time is shown in Figure 6, where [ε-CL]0 is the initial lactone
monomer concentration and [ε-CL]t the lactone concentration
at a given reaction time t. In all cases, the linearity of the plot
shows that the propagation was first order with respect to lactone
monomer when polymerized at 70 °C in toluene and an
induction period was not observed. The absence of an induction
period indicates that the initiators were reactive from the
beginning; i.e., rearrangement of initiator aggregates was not
necessary to form active species. The linearity of the plot also
illustrates that termination reactions did not occur during the
polymerization. The fastest polymerization for ε-CL was ob-
served for the ethyl derivatives. The kapp values for the
The most active organoaluminum initiators in the ROP of
ε-CL (ethyl derivatives 2, 4, 7, and 10) were tested in lactide
polymerization (Table 4). Derivatives 2, 4, 7, and 10 were
examined for polymerization of L-LA at 110 °C in toluene
without cocatalyst or activator. The resulting PLA had a molec-
ular weight in close agreement with the calculated value for one
polymer chain per metal center (entries 1-4), and the GPC data
of the polyesters obtained exhibit a monomodal weight distribu-
tion. Although the values of Mw/Mn are somewhat higher than
those expected for a purely living polymerization (Mw/Mn < 1.1),
the overall results are consistent with a controlled polymerization
model (see Mn vs conversion in Figure S12 in the Supporting
Information). The polymerization occurred without observable
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Table 3. Polymerization of ε-Caprolactone Catalyzed by Alkyl Complexes 1-10 and Cationic Complexes 13 and 14a
entry
initiator
time (h)
conversn (%)e
Mn(theor) (Da)f
Mn(exptl) (Da)g
Mw/Mn
1
1
14
8
95
54 150
39 900
41 040
47 880
n.d.h
74 780
45 320
43 940
65 736
n.d.
1.45
1.36
1.32
1.47
n.d.
2
2
70
3
3
4
72
4
4
3
84
5
5
16
1.5
0.66
16
5
traces
47
6
6
26 790
22 800
n.d.
44 170
29 460
n.d.
1.42
1.17
n.d.
7
7
40
8
8
traces
56
9
9
31 920
49 590
48 450
55 860
19 950
110 580
22 800
28 500
36 072
74 970
53 887
82 030
24 900
127 734
57 130
61 790
1.28
1.49
1.38
2.01
1.26
1.41
1.63
1.65
10
11
12
13
14
15
16
10
7
3.5
2
87
85
7b
7c
7d
13
14
0.033
4
98
35
1.5
0.66
0.66
97
40
50
a Polymerization conditions: 90 μmol of initiator, 20 mL of toluene as solvent, [ε-CL]0/[initiator]0 = 500, 70 °C. b 130 °C. c THF as solvent.
d [ε-CL]0/[initiator]0 = 1000. e Percentage conversion of the monomer: (weight of polymer recovered)/(weight of monomer) ꢀ 100. f Theoretical Mn =
(monomer/initiator) ꢀ (% conversion) ꢀ (Mw of ε-CL). g Determined by GPC relative to polystyrene standards in tetrahydrofuran. The experimental
Mn was calculated considering Mark-Houwink’s corrections for Mn (Mn(exptl) = 0.56[Mn(GPC)]). h n.d. = not determined.
Table 4. Polymerization of Lactide Catalyzed by Complexes 2, 4, 7, and 10a
entry
initiator
monomer
time (h)
conversn (%)d
Mn(theor) (Da)e
Mn(exptl) (Da)f
Mw/Mn
1
2
L-LA
3
60
80
82
72
90
98
64
83
90
85
30
17 280
23 040
23 616
20 736
25 920
28 224
18 432
23 904
25 920
24 480
8 640
20 520
30 100
21 950
22 300
34 410
32 880
28 740
26 090
51 010
21 980
12 580
1.21
1.26
1.16
1.15
1.32
1.26
1.23
1.17
1.53
2.47
1.11
2
4
L-LA
3
3
7
L-LA
2
4
10
7b
2
L-LA
3
5
L-LA
2
6
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
15
6.5
6
7
4
8
7
9
7b
7c
10
2
10
0.15
11
16
a Polymerization conditions: 90 μmol of initiator, 20 mL of toluene as solvent, [LA]0/[initiator]0 = 200, 110 °C. b 130 °C. c Solvent free. d Percentage
conversion of the monomer: (weight of polymer recovered)/(weight of monomer) ꢀ 100. e Theoretical Mn = (monomer/initiator) ꢀ (% conversion) ꢀ
f
(Mw of LA). Determined by GPC relative to polystyrene standards in tetrahydrofuran. The experimental Mn was calculated considering Mark-
Houwink’s corrections for Mn (Mn(exptl) = 0.56[Mn(GPC)]).
epimerization reactions at the chiral centers, as evidenced by the
after 6 h (entry 8) and produced medium molecular weight
material with a very narrow polydispersity (Mn = 26 090, Mw/Mn =
1.17), while 90% of the polymer was recovered after 2 h at 130 °C
with a broader polydispersity (Mw/Mn = 1.53, entry 9). We can
attribute this broad molecular weight distribution to a small
degree of transesterification and/or slow and incomplete initia-
tion relative to propagation. In all cases (entries 6-11), low-
melting materials were obtained with the Tm ranging from 127 to
140 °C. As far as we are aware, very few examples of aluminum
alkyl initiators without cocatalyst have been reported to act as
active single-site catalysts for the ROP of rac-lactide.11l,18g
27
single resonance at δ 5.2 ppm in the methine region (Pm
>
0.90) in the homonuclear decoupled 1H NMR spectrum (Figure
S13 in the Supporting Information), and afforded highly crystal-
line, isotactic polymers with Tm values in the range 168-172 °C.
The low level of stereochemical imperfections was also revealed
in the poly(L-lactide) with Mn > 15 000, where the optical activity
remained almost constant: [R]D22 = 145-148°. It is important
to note that the PLAs obtained had the same tacticities regardless
of the heteroscorpionate ligand used as a scaffold. An increase in
the temperature up to 130 °C for compound 7 led to higher
productivity without any observable change in tacticity. Once
again, acetamidate derivatives gave higher conversions than the
thioacetamidate counterparts (e.g., entry 1 vs 2).
1
Microstructural analysis of the poly(rac-lactide) by H NMR
spectroscopy revealed that these initiators exert a low degree of
stereoselectivity (Pm values of around 0.50).28 This behavior
during the propagation is most probably the result of the low
steric demand of the methyl substituents in the two pyrazole
rings. This leads to sterically less congested, more flexible, and
Initiators 2, 4, 7, and 10 were also tested in the polymerization
of rac-lactide in toluene at 110 °C without cocatalyst or activator
(entries 6-11). Derivative 7 gave 83% conversion of 200 equiv
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therefore less selective active centers to the incoming lactide.
These results parallel those reported by Sadow et al.,18g who
described systems that polymerized rac-lactide to atactic poly-
(lactide) by means of alkyl aluminum initiators without the
addition of cocatalysts.
molecular weight distribution as well as the fact that no peaks
associated to internal transesterification reaction (back-biting) are
found in the mass spectra of these polymers. The mass spectrum
also reveals that the chemical composition of the polymer is in
agreement with an ethyl initiator group and a methoxy end group,
as [M]þ and [M þ Na]þ ions are detected in the measurements.
As an example, Figure S14 in the Supporting Information shows
peaks at 1844.59 Da ([M]þ) and 1867.58 Da ([M þ Na]þ) that
correspond to a polymer containing 24 lactate repeating units, an
ethyl initiator group, and a methoxy end group.
PLAs can be synthesized by both solution polymerization and
bulk polymerization. However, the solution polymerization
suffers from certain disadvantages such as being susceptible to
the impurity level. The bulk polymerization is thus often
preferred for the large-scale production of PLAs.29 With regard
to the design of new initiators for our ROP studies, we became
interested in exploring the organoaluminum derivate 7 as an
initiator for the bulk polymerization of rac-lactide. In the
polymerization procedure the monomer rac-lactide and the
initiator (rac-LA to Al ratio of 200) were heated to 130 °C.
Complex 7 initiated very rapid polymerization of rac-LA under
bulk conditions (entry 10) and gave 85% conversion of 200 equiv
of rac-LA in 0.15 h. Under bulk conditions complex 7 afforded
lower molecular weight and higher molecular weight distribu-
tions than in solution. This change may be caused by a high level
of side reactions such as interchain and intrachain transesterifica-
tion , which result in a chain-transfer reaction. The Pm value of 0.5
implies that the use of this complex under bulk conditions does
not affect the tacticity of the resulting polymer. Unfortunately,
the aryloxide derivatives [Al(OR)2{κ2-sbpam}] (11) and
[Al(OR)2{κ2-(S)-mbpam}] (12) (R = 2,6-Me2C6H4O) did
not initiate the ROP of lactones and lactides under the same
conditions.
The structure and chemical composition of PLA polymers
obtained by 7 were also studied by matrix-assisted laser desorption
time-of-flight mass spectrometry (MALDI-TOF MS), using di-
thranolasmatrix. Inordertoavoidmultiple species formation, pure
methanol was used to quench the polymerization after reaction on
polymers for MALDI-TOF analysis. Spectra were recorded in
reflection mode using a positive ion extraction. As this is a soft
technique, polymers can be detected as intact molecular adducts;
MALDI-TOF MS spectra can be used to determine the size of
repeat units as well as initiator-group and end-group masses, Mn
and Mw, and thus Mw/Mn, as long as the polymer molecular weight
distribution is not too high. However, detectors usually induce
mass discrimination effects at high masses and the ionization of
species is not independent of their size. As a result, accurate
estimates of average molecular weights by MALDI-TOF are
limited to nearly monodisperse polymer samples. In our case,
PLA polymers prepared are out of the determination range of
Mw/Mn by MALDI-TOF; however, the structural composition of the
polymer can be characterized by this technique. Typical MALDI-
TOF MS spectra of PLA polymer prepared with this catalyst show
a monotonic decrease distribution of peaks because of the already
mentioned high molecular weight distribution (Figure S14 in the
Supporting Information). The mass spectrum shows a cluster of
homologous peaks separated by a molecular mass corresponding
to the repeatingunit of the analyzed polymer. Thispeak separation
was found to be 72.0 Da and corresponds to just one lactate unit
(C3H4O2). The rac-lactide molecule contains two lactate units,
and in principle, it would be expected that peak separation in the
mass spectrum would be 144.0 Da instead of 72.0 Da. Never-
theless, as previously mentioned, an intermolecular transesterifica-
tion reaction will result in a fast interchange of the chain length and
consequently the formation of not only the even numbers of
lactate repeating units but also all possible combinations. This high
degree of intermolecular transesterification could justify the high
The cationic complexes [AlMe{κ3-pbptam}][MeB(C6F5)3]
(13) and [AlEt{κ3-pbptam}][B(C6F5)4] (14), generated in situ
by addition of the abstracting reagents B(C6F5)3 and [Ph3C]-
[B(C6F5)4] to [AlMe2{κ2-pbptam}] (1), were tested as initia-
tors for the ROP of of ε-CL. These compounds initiated very
rapid polymerization at 70 °C and gave almost 50% conversions
of 500 equiv of ε-CL in 45 min (entries 15 and 16 in Table 3).
Compounds 13 and 14 were significantly more active initiators in
the ROP of ε-CL than the dialkyl aluminum precursor 1. This
marked difference in reactivity between the types of species is
presumably a consequence of the fundamental mechanistic
differences in their polymerization performances.30 It has been
established that polymerization can be envisaged to occur by
either transfer of the nucleophilic alkyl ligand to the monomer,
with formation of a metal alkoxide propagating species, or ring
opening and propagation through a cationic mechanism.31 The
higher polydispersities obtained with complexes 13 and 14 point
to a cationic mechanism, where the initial formation of a
carbocationic species is followed by a propagation step. Finally,
the cationic species 13 and 14 did not show activity in the ROP of
lactides.
’ CONCLUSIONS
In conclusion, we report here the facile synthesis of a new
family of dialkyl heteroscorpionate aluminum compounds bear-
ing an acetamidate or thioacetamidate as pendant donor arms.
NMR, VT NMR, and X-ray single-crystal studies allowed κ2NO
and κ2NN coordination modes to be established for the acet-
amidate- and thioacetamidate-containing complexes, respec-
tively. The solid-state structure is not retained in solution, and
fluxional exchange between coordinated and noncoordinated
pyrazole rings of the heteroscorpionate ligands is observed.
Subsequent reaction of some of the dialkyl compounds with a
phenol or alkyl abstracting reagents gave rise to the correspond-
ing aryloxide and cationic derivatives. A series of detailed studies
of ROP processes involving the different types of isolated
complexes were undertaken. ε-Caprolactone was polymerized
to give medium molecular weight polymers with moderate to
broad polydispersities. The broader polydisperities are due to the
poor rates of initiation compared to propagation, which is a well-
established feature of metal alkyl initiators. Not surprisingly, the
polymerization of LA occurred more slowly than that of ε-CL but
offered better control. L-Lactide afforded highly crystalline,
isotactic PLA materials with medium molecular weights, narrow
polydispersities, and very high melting temperatures. Propaga-
tion occurred without observable epimerization with all families
of initiators. Polymerization of rac-lactide did not result in
appreciable levels of selectivity and produced atactic PLAs.
Further iterations of ligand design will be required to find
catalysts capable of achieving stereochemical control under
solution ROP conditions.
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’ EXPERIMENTAL SECTION
126.9 (Cm NPh), 126.4 (Cp NPh), 107.5 (C4), 75.8 (CH), 13.0 (Me5),
11.0 (Me3), 9.5 (AlCH2CH3), 0.9 (AlCH2CH3).
All manipulations were performed under nitrogen, using standard
Schlenk techniques. Solvents were predried over sodium wire (toluene,
n-hexane) and distilled under nitrogen from sodium (toluene) or sodium-
potassium alloy (n-hexane). Deuterated solvents were stored over
activated 4 Å molecular sieves and degassed by several freeze-thaw
cycles. Microanalyses were carried out with a Perkin-Elmer 2400 CHN
analyzer. 1H and 13C NMR spectra were recorded on a Varian Inova FT-
500 spectrometer and referenced to the residual deuterated solvent. The
NOESY-1D spectra were recorded with the following acquisition param-
eters: irradiation time 2 s and number of scans 256, using standard
VARIANT-FT software. Two-dimensional NMR spectra were acquired
using standard VARIANT-FT software and processed using an IPC-Sun
computer. AlMe3, AlEt3, AliBu3, rac-lactide, L-lactide, B(C6F5)3, and
2,4,6-trimethylphenol were purchased from Aldrich. ε-Caprolactone
was purchased from Alfa-Aesar and [Ph3C][B(C6F5)4] from STREM.
The ligands pbpamH, sbpamH, (S)-mbpamH, and pbptamH were
prepared according to literature procedures.20 ε-Caprolactone was dried
by stirring over fresh CaH2 for 48 h and then distilled under reduced
pressure and stored over activated 4 Å molecular sieves. L-Lactide and
rac-lactide were sublimed three times, recrystallized from THF, and
finally sublimed again prior to use. Gel permeation chromatography
(GPC) measurements were performed on a Polymer Laboratories PL-
GPC-220 instrument equipped with a TSK-GEL G3000H column and
an ELSD-LTII light-scattering detector. The GPC column was eluted
with THF at 50 °C at 1 mL/min and was calibrated using eight
monodisperse polystyrene standards in the range 580-483 000 Da.
PLA melting temperatures were measured using a SMP10 melting point
apparatus. The sample was heated to 100 °C and then heated at a rate of
1 °C/min to 185 °C. The specific rotation [R]D22 was measured at 22 °C
on a Perkin-Elmer 241 polarimeter equipped with a Na lamp operating
at 589 nm with a light path length of 10 cm. The MALDI-TOF spectra
were acquired using a Bruker Autoflex II TOF/TOF spectrometer using
dithranol (1,8,9-trihydroxyanthracene) as matrix material. Samples cocrys-
tallized with matrix in a 100:1 ratio on the probe were ionized in positive
reflector mode. External calibration was performed by using Peptide
Calibration Standard II (covered mass range: 700-3200 Da) and Protein
Calibration Standard I (covered mass range: 5000-17 500 Da).
Synthesis of 3. The synthesis of 3 was carried out in a manner
identical with that for 1, using pbpamH (pbpamH = N-phenyl-2,2-
bis(3,5-dimethylpyrazol-1-yl)acetamide; 0.79 g, 2.50 mmol) and AlMe3
(2 M in toluene; 1.25 mL, 2.50 mmol). Yield: 0.82 g, 86%. Anal. Calcd
for C20H26AlN5O: C, 63.31; H, 6.91; N, 18.46. Found: C, 63.11; H, 6.69;
N, 18.13. 1H NMR (C6D6, 297 K; δ (ppm)): 7.64 (d, 3JH-H = 7.0 Hz,
2H, Ho NPh), 7.28 (t, 3JH-H = 6.9 Hz, 2H, Hm NPh), 7.02 (t, 3JH-H
=
6.9 Hz, 1H, Hp NPh), 6.63 (s, 1H, CH), 5.37 (s, 2H, H4), 1.96 (s, 6H,
Me3), 1.90 (s, 6H, Me5), -0.25 (s, 6H, AlCH3). 13C{1H} NMR (C6D 6,
297 K), δ (ppm): 155.3 (NCdO), 149.4, 147.8, 140.4 (C3 or 5, Cipso
NPh), 128.8 (Cm NPh), 124.3 (Co NPh), 124.1 (C p NPh), 107.6 (C4),
71.7 (CH), 13.5 (Me3), 10.7 (Me5), -8.9 (AlCH 3).
Synthesis of 4. The synthesis of 4 was carried out in a manner
identical with that for 1, using pbpamH (pbpamH = N-phenyl-2,2-
bis(3,5-dimethylpyrazol-1-yl)acetamide; 0.79 g, 2.50 mmol) and AlEt3
(1 M in hexane; 2.50 mL, 2.50 mmol). Yield: 0.91 g, 89%. Anal. Calcd for
C22H30AlN5O: C, 64.84; H, 7.42; N, 17.19. Found: C, 64.98; H, 7.39; N,
17.05. 1H NMR (C6D6, 297 K; δ (ppm)): 7.67 (d, 3JH-H = 7.0 Hz, 2H,
Ho NPh), 7.29 (t, 3JH-H = 6.9 Hz, 2H, Hm NPh), 7.04 (t, 3JH-H = 6.9 Hz,
1H, Hp NPh), 6.63 (s, 1H, CH), 5.41 (s, 2 H, H4), 1.99 (s, 6H, Me3),
3
1.88 (s, 6H, Me5), 1.33 (t, J
= 7.9 Hz, 6H, AlCH2CH3), 0.40
H-H
(m, 4H, AlCH2CH3). 13C{1H} NMR (C6D6, 297 K; δ (ppm)): 154.3
(NC dO), 149.4, 147.9, 140.4 (C3 or 5, Cipso NPh), 128.8 (Cm NPh),
124.3 (Co NPh), 124.1 (Cp NPh), 107.6 (C4), 71.7 (CH), 13.4 (Me3),
10.8 (Me5), 9.2 (AlCH2CH3), 0.6 (AlCH2CH3).
Synthesis of 5. The synthesis of 5 was carried out in a manner
identical with that for 1, using pbpamH (pbpamH = N-phenyl-2,2-
bis(3,5-dimethylpyrazol-1-yl)acetamide; 0.79 g, 2.50 mmol) and AliBu3
(1 M in hexane; 2.50 mL, 2.50 mmol). Yield: 1.02 g, 88%. Anal. Calcd for
C26H38AlN5O: C, 67.36; H, 8.26; N, 15.11. Found: C, 67.11; H, 8.39; N,
15.40. 1H NMR (C6D6, 297 K; δ (ppm)): 7.63 (d, 3JH-H = 7.0 Hz, 2H,
Ho NPh), 7.28 (t, 3JH-H = 6.9 Hz, 2H, Hm NPh), 7.00 (t, 3JH-H = 6.9 Hz,
1H, Hp NPh), 6.64 (s, 1H, CH), 5.40 (br s, 2H, H4), 2.12 (br s, 2H,
AlCH2CH(CH3)2), 2.10 (br s, 6H, Me3), 1.91 (br s, 6H, Me5), 1.16
(br s, 12H, AlCH2CH(CH3)2), 0.40 (br s, 4H, AlCH2CH(CH3)2).
13C{1H} NMR (C6D6, 297 K; δ (ppm)): 153.9 (NCdO), 149.4, 147.7,
142.4 (C3 or 5, Cipso NPh), 128.6 (Cm NPh), 124.3 (Co NPh), 124.0 (Cp
NPh), 107.6 (C4), 71.5 (CH), 28.4 (AlCH2CH(CH3)2), 26.3
[AlCH2CH(CH3)2), 13.6 (AlCH2CH(CH3)2), 13.4 (Me3), 10.8 (Me5).
Synthesis of 6. The synthesis of 6was carried out in a manner identical
with that for 1, using sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethyl-
pyrazol-1-yl)acetamide; 0.76 g, 2.50 mmol) and AlMe3 (2 M in toluene;
1.25 mL, 2.50 mmol). Yield: 0.76 g, 85%. Anal. Calcd for C18H30AlN5O: C,
Synthesis of 1. In a 250 cm3 Schlenk tube, pbptamH (pbptam = N-
phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide; 0.85 g, 2.50
mmol) was dissolved in dry toluene (70 mL) and cooled to 0 °C. A
solution of AlMe3 (2 M in toluene; 1.25 mL, 2.50 mmol) was added, and
the mixture was warmed to room temperature and stirred for 1 h. The
solution was concentrated and recrystallized at -26 °C to give com-
pound 1 as yellow crystals. Yield: 0.89 g, 90%. Anal. Calcd for
C20H26AlN5S: C, 60.74; H, 6.63; N, 17.71. Found: C, 60.95; H, 6.84;
N, 17.58. 1H NMR (C6D6, 297 K; δ (ppm)): 7.44 (s, 1H, CH), 7.38 (d,
3JH-H = 7.0 Hz, 2H, Ho NPh), 7.20 (t, 3JH-H = 6.9 Hz, 2H, Hm NPh),
7.03 (t, 3JH-H = 6.8 Hz, 1H, Hp NPh), 5.34 (s, 2H, H4), 2.05 (s, 6H,
Me3), 1.94 (s, 6H, Me5), -0.18 (s, 6H, AlCH3). 13C{1H} NMR (C6D6,
297 K; δ (ppm)): 194.9 (NCdS), 146.5, 150.4, 142.3 (C3 or 5, Cipso
NPh), 129.2 (Co NPh), 127.3 (Cm NPh), 126.3 (Cp NPh), 107.5 (C4),
75.8 (CH), 13.1 (Me3), 11.2 (Me5), -8.1 (Al CH3).
1
60.15; H, 8.41; N, 19.48. Found: C, 60.31; H, 8.19; N, 19.31. H NMR
0
(C6D6, 297 K; δ (ppm)): 6.52 (s, 1H, CH), 5.41, 5.35 (s, 2H, H4,4 ), 4.18
0
(m, 1H, NCH(CH3)(CH2CH3)), 1.99, 1.98 (s, 6H, Me3,3 ), 1.92, 1.82
0
(s,6H,Me5,5 ),1.70,1.60(m,4H,NCH(CH3)(CH2CH3)),1.30(d,3JH-H
=
3
7.9 Hz, 3H, NCH(CH3)(CH2CH3)), 1.01 (t, JH-H = 7.9 Hz, 3H,
NCH(CH3)(CH2CH3)), -0.15 (s, 6H, AlCH3). 13C{1H}0 NMR
0
(C6D6, 297 K; δ (ppm)): 153.5 (NCdO), 149.4, 149.3 (C3,3 or 5,5 ),
0
107.4, 107.2 (C4,4 ), 71.7 (CH), 51.7 (NCH(CH3)(CH2CH3)), 31.6
(NCH0(CH3)(CH2CH3)), 21.4 (NCH(CH3)(CH2CH3)), 13.1, 13.0
Synthesis of 2. The synthesis of 2 was carried out in a manner
identical with that for 1, using pbptamH (pbptam = N-phenyl-2,2-
bis(3,5-dimethylpyrazol-1-yl)thioacetamide; 0.85 g, 2.50 mmol) and
AlEt3 (1 M in hexane; 2.50 mL, 2.50 mmol). Yield: 0.95 g, 90%. Anal.
Calcd for C22H30AlN5S: C, 62.39; H, 7.14; N, 16.53. Found: C, 61.98;
0
(Me3,3 ), 11.3 (NCH(CH3)(CH2CH3)), 10.5, 10.6 (Me5,5 ), -8.8
(AlCH3).
Synthesis of 7. The synthesis of 7 was carried out in a manner
identical with that for 1, using sbpamH (sbpamH = N-sec-butyl-2,2-
bis(3,5-dimethylpyrazol-1-yl)acetamide; 0.76 g, 2.50 mmol) and AlEt3
(1 M in hexane; 2.50 mL, 2.50 mmol). Yield: 0.89 g, 92%. Anal. Calcd for
C20H34AlN5O: C, 61.99; H, 8.84; N, 18.07. Found: C, 61.71; H, 8.59; N,
H, 7.34; N, 16.31. 1H NMR (C6D6, 297 K; δ (ppm)): 7.44 (d, 3JH-H
=
7.2 Hz, 2H, Ho NPh), 7.43 (s, 1H, CH), 7.22 (t, 3JH-H = 6.9 Hz, 2H, Hm
NPh), 7.04 (t, 3JH-H = 6.8 Hz, 1H, Hp NPh), 5.34 (s, 2 H, H4), 2.09 (s,
6H, Me5), 1.91 (s, 6H, Me3), 1.23 (t, 3JH-H = 8.0 Hz, 6H, AlCH2CH3),
0.50 (m, 4 H, AlCH2CH3). 13C{1H} NMR (C6D6, 297 K; δ (ppm)):
195.1 (NC dS), 150.5, 149.4, 142.5 (C3 or 5, Cipso NPh), 129.2 (Co NPh),
17.95. 1H NMR (C6D6, 297 K; δ (ppm)): 6.51 (s, 1H, CH), 5.42, 5.37
0
(s, 2H, H4,4 ), 4.22 (m, 1H, NCH(CH3)(CH2CH3)), 2.01 (s, 6H,
0
0
Me3,3 ), 1.85 (s, 6H, Me5,5 ), 1.65 (m, 2H, NCH(CH3)(CH2CH3)),
1518
dx.doi.org/10.1021/om1010676 |Organometallics 2011, 30, 1507–1522

Organometallics
ARTICLE
1.44 (t, 3JH-H = 7.9 Hz, 6H, AlCH2CH3), 1.31 (d, 3JH-H = 7.9 Hz, 3H,
7.40; N, 12.25. Found: C, 67.54; H, 7.42; N, 12.09. 1H NMR (C6D6, 297
3
3
NCH(CH3)(CH2CH3)), 1.00 (t, JH-H = 7.9 Hz, 3H, NCH(CH3)-
K; δ (ppm)): 7.56 (s, 1H, CH), 7.01 (d, JH-H = 7.0 Hz, 4H, Hm
(CH2CH3)), 0.47 (m, 4H, 3JH-H = 7.9 Hz, AlCH2CH3). 13C{1H} NMR
OMe2Ph), 6.77 (t, JH-H = 7.0 Hz, 2H, Hp OMe2Ph), 5.45 (brs, 2H
3
0
0
0
(C6D6, 297 K; δ (ppm)): 153.6 (NCdO), 149.5, 149.3 (C3,3 and 5,5 ),
H4,4 ), 4.05 (m, 1H, NCH(CH3)(CH2CH3)), 2.33 (br s, 12H,
0
0
0
107.3, 107.2 (C4,4 ), 71.6 (CH), 51.6 (NCH(CH3)(CH2CH3)),
31.5 (NCH(CH3)(CH2CH3)), 21.2 (NCH(CH3)(CH2CH3)), 13.0
OMe2Ph), 2.01, 2.22 (br s, 12H, Me3,3
Me5,5 ), 1.30 (m, 2H,
and
3
NCH(CH3)(CH2CH33)), 1.00 (d, JH-H = 7.9 Hz, 3H, NCH(CH3)-
(CH2CH3)), 0.79 (t, JH-H = 7.9 Hz, 3H, NCH(CH3)(CH2CH3)).
13C{1H} NMR (C6D6, 297 K; δ (ppm)): 155.5 (N CdO), 161.9-
0
0
(Me3,3 ), 11.1 (NCH(CH3)(CH2CH3)), 10.6 (Me5,5 ), 9.3 (AlCH2CH3),
0.8 (AlCH2CH3).
0
0
0
142.5 (C3,3 and 5,5 , C
OMe Ph), 107.2, 106.9 (C4,4 ), 76.2 (CH),
ipso
Synthesis of 8. The synthesis of 8 was carried out in a manner
identical with that for 1, using sbpamH (sbpamH = N-sec-butyl-2,2-
bis(3,5-dimethylpyrazol-1-yl)acetamide;0.79 g, 2.50 mmol) and AliBu3
(1 M in hexane; 2.50 mL, 2.50 mmol). Yield: 0.95 g, 86%. Anal. Calcd for
C24H42AlN5O: C, 64.98; H, 9.54; N, 15.79. Found: C, 65.19; H, 9.22; N,
2
51.6 (NCH(CH3)(CH2CH3)), 31.5 (NCH(CH3)(CH2CH3)), 21.2
(NCH(CH3)(CH2CH3)), 11.1 (NCH(CH3)(CH2CH3)), 13.0, 12.9
0
0
(Me3,3 ), 10.6, 10.5 (Me5,5 ), 127.1 (Cm OMe 2Ph), 125.6 (Co OMe 2Ph),
114.2 (Cp OMe 2Ph), 18.2 (O Me2Ph).
15.55. 1H NMR (C6D6, 297 K; δ (ppm)): 6.85 (s, 1H, CH), 5.48, 5.19
Synthesis of 12. The synthesis of 12 was carried out in a manner
identical with that for 11, using [AlEt2{κ2-(S)-mbpam}] (10; 1.12 g,
2.57 mmol) and 2,6-dimethylphenol (0.63 g, 5.14 mmol). Yield: 1.46 g,
0
(s, 2H, H4,4 ), 4.09 (m, 1H, NCH(CH3)(CH2CH3)), 2.220 (br s, 2H,
0
AlCH2CH(CH3)2)), 2.01 (s, 6H, Me3,3 ), 1.85 (s, 6H, Me5,5 ), 1.75 (m,
2H, NCH(CH3)(CH2CH3)), 1.27 (br s, 12H, AlCH2CH(CH3)2)),
25
92%. [R]D = -35.4° (c 0.10, CD2Cl2). Anal. Calcd for C36H42Al-
1.26 (d, 3JH-H = 7.9 Hz, 3H, NCH(CH3)(CH2CH3)), 0.90 (t, 3JH-H
=
N5O3: C, 69.77; H, 6.83; N, 11.30. Found: C, 69.87; H, 6.94; N, 11.12.
7.9 Hz, 3H, NCH(CH3)(CH2CH3)), 0.47 (br s, 4H, AlCH2CH-
1H NMR (C6D6, 297 K; δ (ppm)): 7.55 (s, 1H, CH), 7.26 (d, 3JH-H
=
(CH3)2)). 13C{1H} NMR (C6D6, 297 K; δ 0(ppm)): 153.4 (NCdO),
7.0 Hz, 2H, Ho NCH(CH3)Ph), 7.11 (t, JH-H = 6.9 Hz, 2H, Hm
NCH(CH3)Ph), 7.07 (t, 3JH-H = 6.9 Hz, 1H, Hp NCH(CH3)Ph), 7.01
3
0
0
149.4, 149.3 (C3,3 and 5,5 ), 107.1, 106.9 (C4,4 ), 71.5 (CH), 51.6 (NCH
(CH3)(CH2CH3)), 31.7 (NCH(CH3)(CH2CH3)), 26.6 (AlCH2CH
(d, 3JH-H = 7.0 Hz, 4H, Hm OMe2Ph), 6.77 (t, 3JH-H = 7.0 Hz, 2H, Hp
0
(CH3)2), 26.0 (AlCH2CH(CH3)2), 23.4(NCH(CH3)(CH2CH3)), 13.9,
OMe2Ph), 5.46, 5.39 (s, 2H, H4,4 ), 5.46 (m, 1H, NCH(CH3)Ph), 2.22,
0
0
0
13.8 (Me3,3 ), 13.7 (AlCH2CH(CH3)2), 11.2 (NCH(CH3)(CH2CH3)),
2.10 (br s, 12H, Me3,3 and Me5,5 ), 1.98 (br s, 12H, OMe2Ph), 1.39 (d,
3JH-H = 7.9 Hz, 3H, NCH(CH3)Ph). 13C{1H} NMR (C6D6, 297 K; δ
0
11.1, 11.0 (Me5,5 ).
0
0
(ppm)): 154.6 (NCdO), 162.8-141.5 (C3,3 and 5,5 , Cipso OMe2Ph,
Synthesis of 9. The synthesis of 9 was carried out in a manner
identical with that for 1, using (S)-mbpamH ((S)-mbpamH = (S)-(-)-
N-R-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide; 0.79 g,
2.50 mmol) and AlMe3 (2 M in toluene; 1.25 mL, 2.50 mmol). Yield:
Cipso NCH(CH3)Ph), 128.3 (Cp NCH(CH3)Ph), 127.9 (Cm NCH-
0
(CH3)Ph), 125.4 (Co NCH(CH3)Ph), 106.3 (C4,4 ), 75.4 (CH), 55.0
0
(NCH(CH3)Ph), 20.7 (NCH( CH3)Ph), 16.45, 13.5, 10.4, 10.2 (Me3,3
0
and Me5,5 ), 127.6 (Cm OMe2Ph), 123.3 (Co OMe2Ph), 117.9 (Cp
OMe2Ph), 12.5, 12.4 (OMe2Ph),
25
0.88 g, 86%. [R]D = -21.3° (c 0.10, toluene). Anal. Calcd for
C22H30AlN5O: C, 64.84; H, 7.42; N, 17.19. Found: C, 64.71; H, 7.39;
N, 17.45. 1H NMR (C6D6, 297 K), δ 7.64 (d, 3JH-H = 7.0 Hz, 2H, Ho
NCH(CH3)Ph), 7.26 (t, 3JH-H = 6.9 Hz, 2H, Hm NCH(CH3)Ph)), 7.12
(t, 3JH-H = 6.9 Hz, 1H, Hp NCH(CH3)Ph), 6.53 (s, 1H, CH), 5.42 (m,
Synthesis of 13. In a glovebox, equimolar amounts of [AlMe2{κ2-
pbptam}] (1; 0.051 g, 0.13 mmol) and B(C6F5)3 (0.067 g, 0.13 mmol)
were added to a sample vial and dissolved in 0.6 mL of THF-d8. The
resulting pale yellow solution was transferred to an NMR tube (10 mm o.
0
0
1H, NCH(CH3)Ph), 5.38, 5.30 (s, 2H, H4,4 ), 1.99, 1.94 (s, 6H, Me3,3 ),
0
1.86, 1.77 (s, 6H, Me5,5 ), 1.59 (d, 3JH-H = 7.9 Hz, 3H, NCH(CH3)Ph),
-0.17 (s, 6H, AlCH3). 13C{1H} NMR (C6D6, 297 K), δ 153.6
d.) and analyzed. 1H NMR (THF-d8, 297 K; δ (ppm)): 7.45 (d, 3JH-H
=
7.0 Hz, 2H, Ho NPh), 7.43 (s, 1H, CH), 7.39 (t, 3JH-H = 6.9 Hz, 2H,
Hm NPh), 7.23 (t, 3JH-H = 6.8 Hz, 1H, Hp NPh), 6.20 (s, 2H, H4), 2.49
(s, 6H, Me5), 2.35 (s, 6H, Me3), 0.70 (br s, 3H, MeB(C6F5)3), 0.06 (s,
3H, AlCH3). 13C{1H} NMR (THF-d8, 297 K; δ (ppm)): 192.8
(NCdS), 154.5, 147,5, 146.5 (C3 or 5, Cipso NPh), 129.2 (Co NPh),
127.3 (Cm NPh), 126.3 (Cp NPh), 111.1 (C4), 77.7 (CH), 14.6 (Me3),
11.8 (Me5), -5.0 (AlCH 3), 10.8 (MeB(C6F5) 3), 140.0-123.4 (MeB
0
0
(NCdO), 149.6, 149.3, 147.5 (C3,3 or 5,5 , Cipso NCH(CH3)Ph), 128.3
(Cm NCH(CH3)Ph), 127.3 (Co NCH(CH3)Ph), 126.4 (Cp NCH-
0
(CH3)Ph), 107.4 (C4,4 ), 71.6 (CH), 54.6 (NCH(CH3)Ph), 24.4
0
0
(NCH(CH3)Ph), 13.0 (Me3,3 ), 10.5 (Me5,5 ), -8.8 (AlCH3).
Synthesis of 10. The synthesis of 10 was carried out in a manner
identical with that for 1, using (S)-mbpamH ((S)-mbpamH = (S)-(-)-
N-R-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide; 0.79 g,
2.50 mmol) and AlEt3 (1 M in hexane; 2.50 mL, 2.50 mmol). Yield: 1.01
g, 93%. [R]D25 = -22.4° (c 0.10, toluene). Anal. Calcd for C24H34Al-
N5O: C, 66.18; H, 7.87; N, 16.08. Found: C, 66.23; H, 7.85; N, 15.98. 1H
( C6F5)3). 19F NMR (THF-d8, 297 K; δ (ppm)): -132.4 (d, 3JF-F
=
18.5 Hz, 2F, o-C6F5), -164.1 (t, 3JF-F = 18.5 Hz, 1F, p-C6F5), -167.8
(t, 3JF-F = 19.1 Hz, 2F, m-C6F5).
Synthesis of 14. In a glovebox, equimolar amounts of [AlMe2{κ2-
pbptam}] (1; 0.051 g, 0.13 mmol) and [Ph3C][B(C6F5)4] (0.12 g, 0.13
mmol) were added to a sample vial and dissolved in 0.6 mL of THF-d8.
The resulting pale yellow solution was transferred to an NMR tube (10
3
NMR (C6D6, 297 K; δ (ppm)): 7.64 (d, JH-H = 7.0 Hz, 2H, Ho
NCH(CH 3)Ph), 7.26 (t, 3JH-H = 6.9 Hz, 2H, Hm NCH(CH 3)Ph), 7.11
(t, 3JH-H = 6.9 Hz, 1H, Hp NCH(CH 3)Ph), 6.52 (s, 1H, CH), 5.47 (m,
0
0
1
1H, N CH(CH3)Ph), 5.40, 5.32 (s, 1H, H4,4 ), 2.01, 1.96 (s, 6H, Me3,3 ),
mm o.d.) and analyzed. H NMR (THF-d8, 297 K; δ (ppm)): 7.60-
0
7.00 (NPh), 7.43 (s, 1H, CH), 6.56 (s, 2H, H4), 2.53 (s, 6H, Me3), 2.41
(s, 6H, Me5), -0.62 (s, 3H, AlCH3), 7.60-7.00 (Ph3CCH3), 2.20 (s,
3H, Ph3CCH3). 13C{1H} NMR (THF-d8, 297 K; δ (ppm)): 191.1
(NCdS), 150.5, 149.3, 147,5 (C3 or 5, Cipso NPh) 129.2 (Co NPh), 126.9
(Cm NPh), 126.4 (Cp NPh), 106.9 (C4), 75.8 (CH), 12.7 (Me3), 9.8
(Me5), -5.5 (AlCH3), 150.5-122.0 (Ph3CCH3), 56.5 (Ph3CCH3),
12.8 (Ph3CCH3), 145.3-122.4 (B(C6F5)4). 19F NMR (THF-d8, 297 K;
1.80 (br s, 6H, Me5,5 ), 1.61 (d, 3JH-H = 7.9 Hz, 3H, NCH(C H3)Ph),
1.39 (br s, 6H, AlCH2C H3), 0.44 (br s, 4 H, AlCH CH3). 13C{1H}
2
NMR (C6D6, 297 K; δ (ppm)): 153.8 (NCdO), 149.7, 149.4, 147.4
0
0
(C3,3 or 5,5 , Cipso NCH(CH3)Ph), 128.3 (Cm NCH(CH3)Ph), 127.3 (Co
0
NCH(CH3)Ph), 126.4 (Cp NCH(CH3)Ph), 107.4 (C4,4 ), 71.7(CH),
0
54.6 (NCH(CH3)Ph), 24.4 (NCH(CH3)Ph), 13.0 (Me3,3 ), 10.6
0
(Me5,5 ), 9.3 (AlCH2CH3), 0.7 (AlCH2CH3).
δ (ppm)): -131.0 (d, 3JF-F = 16.5 Hz, 2F, o-C6F5), -162.9 (t, 3 JF-F
13.5 Hz, 1F, p-C 6F5), -169.8 (t, 3JF-F = 18.2 Hz, 2F, m-C 6F5).
=
Synthesis of 11. In a 100 cm3 Schlenk tube, [AlEt2{κ2-sbpam}] (7;
1.00 g, 2.57 mmol) was dissolved in dry toluene (25 mL) at room
temperature. A solution of 2,6-dimethylphenol (0.63 g, 5.14 mmol) was
added, and the mixture was stirred at room temperature for 16 h.
Removal of the volatiles under reduced pressure yielded 11 as a white
solid. Yield: 1.32 g, 92%. Anal. Calcd for C32H42AlN5O3: C, 67.23; H,
Synthesis of 15. In a glovebox, equimolar amounts of [AlEt2{κ2-
pbptam}] (2; 0.055 g, 0.13 mmol) and [Ph3C][B(C6F5)4] (0.12 g, 0.13
mmol) were added to a sample vial and dissolved in 0.6 mL of THF-d8.
The resulting pale yellow solution was transferred to an NMR tube
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Organometallics
ARTICLE
(10 mm o.d.) and analyzed. 1H NMR (THF-d8, 297 K; δ (ppm)): 7.67
(s, 1H, CH), 7.60-7.00 (NPh), 6.32 (s, 2H, H4), 2.57, (s, 6H, Me5),
2.36 (s, 6H, Me3), 1.09 (t, 3JH-H = 7.9 Hz, 3H,AlCH2CH3), 0.05 (m,
3JH-H = 7.9 Hz, 2H, AlCH2CH3), 7.40-7.00 (Ph3CCH2CH3), 2.59 (m,
Polymerizations of L-lactide and rac-lactide (LA) were performed on a
Schlenk line in a flame-dried Schlenk tube equipped with a magnetic
stirrer. The Schlenk tubes were charged in the glovebox with the
required amount of rac-lactide and initiator, separately, and then
attached to the vacuum line. The initiator and monomer were dissolved
in the appropriate amount of solvent, and temperature equilibration was
ensured in both Schlenk flasks by stirring the solutions for 15 min in a
bath. Polymerizations were stopped by injecting a solution of acetic acid
(5 vol %) in methanol. Polymers were precipitated in methanol, filtered
off, dissolved in THF, reprecipitated in methanol, and dried in vacuo to
constant weight.
General Procedure for Solvent-Free (Melt) Polymerization
of rac-LA. A Schlenk flask was charged with catalyst and rac-LA in the
desired ratio and heated to 130 °C for 9 min with stirring. The mixture
was cooled to room temperature, wet acidic THF (10 mL) was added,
and the resulting solution was evaporated to dryness to give the crude
polymer.
X-ray Crystallographic Structure Determination. Yellow
(for 1 and 2) and colorless (7) crystals were obtained by diffusion of
toluene/hexane. X-ray data for 1, 2, and 7 were collected on a Bruker X8
APEX II CCD area detector diffractometer at T = 180 K using graphite-
monochromated Mo KR radiation (λ = 0.710 73 Å, sealed X-ray tube).
Data were integrated using SAINT,32 and an absorption correction was
performed with the program SADABS.33 The structures were solved by
direct methods using SHELXTL34 and refined by full-matrix least-
squares methods based on F2. Non-hydrogen atoms were refined
anisotropically. All of the hydrogen atoms were placed in calculated
positions and thereafter treated as riding. Salient crystallographic data are
summarized inTable 2, and further details can be foundin theSupporting
Information in CIF format. Selected geometric data are presented in
Table 3. For compound 1, only crystals of low quality could be grown.
Because of the weak diffraction, reflections have been included only up to
θ = 22.0° for the refinement; however, the data were of sufficient quality
to determine the molecular and crystal structure. The asymmetric unit
contains two independent molecules. In the crystal structure of com-
pound 7, the acetamidate and ethyl groups are disordered over two
positions and were modeled as a 50:50 isotropic mixture.
3
2H, Ph3CCH2CH3), 0.87 (t, JH-H = 6.9 Hz, 3H, Ph3CCH2CH3).
13C{1H} NMR (THF-d8, 297 K; δ (ppm)): 191.9 (NCdS), 157.8,
150.2, 147,8 (C3 or 5, Cipso NPh), 124.3 (Co NPh), 128.8 (Cm NPh),
124.1 (Cp NPh), 110.4 (C4), 76.3 (CH), 13.1 (Me3), 10.2 (Me5), 9.2
(AlCH2CH3), 0.6 (AlCH2CH3), 150.5-128.0 (Ph3CCH2CH3), 54.1
(Ph3CCH2CH3), 26.9 (Ph3CCH2CH3), 8.1 (Ph3CCH2CH3). 144.8-
122.0 (B(C6F5)4). 19F NMR (THF-d8, 297 K; δ (ppm)): -132.5
3
3
(d, JF-F = 17.1 Hz, 2F, o-C6F5), -164.2 (t, JF-F = 15.3 Hz, 1F,
p-C 6F5), -167.7 (t, 3JF-F = 16.2 Hz, 2F, m-C 6F5).
Synthesis of 16. In a glovebox, equimolar amounts of [AlEt2{κ2-
sbpam}] (7; 0.050 g, 0.13 mmol) and [Ph3C][B(C6F5)4] (0.12 g, 0.13
mmol) were added to a sample vial and dissolved in 0.6 mL of THF-d8. The
resulting pale yellow solution was transferred to an NMR tube (10 mm o.d.)
and analyzed. 1H NMR (THF-d8, 297 K; δ (ppm)): 7.02 (s, 1H, CH), 6.25,
0
6.16(s, 2H, H4,4 ), 4.13(m, 1H, NCH(CH3)(CH2CH3)), 2.37, 2.28 (s, 6H,
0
0
Me5,5 ), 2.32, 2.25 (s, 6H, Me3,3 ), 1.60 (m, 2H, NCH(CH3)(CH2CH3)),
1.27 (d, 3JH-H = 7.9 Hz, 3H, NCH(CH3)(CH2CH3)), 0.94 (t, 3JH-H = 7.9
3
Hz, 3H, NCH(CH3)(CH2CH3)), 0.92 (t, JH-H = 9.0 Hz, 3H,
3
AlCH2CH3), 0.02 (m, JH-H = 7.9 Hz, 2H, AlCH2CH3), 7.40-7.00
(Ph3CCH2CH3), 2.48 (m, 2H, Ph3CCH2CH3), 0.90 (t, 3JH-H = 6.8 Hz,
3H, Ph3CCH2CH3). 13C{1H} NMR (THF-d8, 297 K; δ (ppm)): 166.6
0
0
0
(NCdO), 154.3, 153.2, 145.6, 144.3 (C3,3 and 5,5 ), 110.2, 110.0 (C4,4 ), 67.9
(CH), 51.6 (NCH(CH3)(CH2CH3)), 29.3 (NCH(CH3)(CH2CH3)),
19.1 (NCH(CH3)(CH2CH3)),0 11.0 (NCH(CH3)(CH2CH3)), 13.4,
0
13.3 (Me3,3 ), 10.9, 10.4 (Me5,5 ), 8.9 (AlCH2CH3), 1.0 (AlCH2CH3),
150.5-128.0 (Ph3CCH2CH3), 53.2 (Ph3CCH2CH3), 31.0 (Ph3CCH2
CH3), 9.1 (Ph3CCH2CH3). 148.8-121.0 (B(C6F5)4). 19F NMR (THF-
d8, 297 K; δ (ppm)): -133.1 (d, 3JF-F = 16.8 Hz, 2F, o-C6F5), -163.9
(t, 3 JF-F = 18.3 Hz, 1F, p-C 6F5), -169.7 (t, 3JF-F = 17.2 Hz, 2F, m-C 6F5).
Synthesis of 17. In a glovebox, equimolar amounts of [AlEt2{κ2-(S)-
mbpam}] (10; 0.057 g, 0.13 mmol) and [Ph3C][B(C6F5)4] (0.12 g, 0.13
mmol) were added to a sample vial and dissolved in 0.6 mL of THF-d8. The
resulting pale yellow solution was transferred to an NMR tube (10 mm o.d.)
and analyzed. 1H NMR (THF-d8, 297 K; δ (ppm)): 7.40-7.00 (NCH(CH
0
3)Ph), 7.05 (s, 1H, CH), 6.22, 6.12 (s, 2H, H4,4 ), 5.30 (m, 3J H-H = 6.9 Hz,
1H, NCH(CH3)Ph), 1.60 (d, 3JH-H = 7.9 Hz, 3H, NCH(CH3)Ph), 2.38,
’ ASSOCIATED CONTENT
0
0
2.20 (s, 6H, Me5,5 ), 2.30, 2.25 (s, 6H, Me3,3 ), 0.92 (s, 3JH-H = 7.9 Hz, 3H,
AlCH2CH3), 0.03 (m, 3JH-H = 7.9 Hz, 2H, AlCH2CH3), 7.40-7.00 (Ph
3CCH2CH3), 2.55 (m, 2H, Ph3CC H2CH3), 0.85 (t, 3 JH-H = 6.9 Hz, 3H,
Ph3CCH2CH ). 13C{1H} NMR (THF-d , 297 K; δ (ppm)): 157.4
S
Supporting Information. Text, figures, tables and CIF
b
files giving VT NMR analyses and kinetic parameters carried out
for complexes 1-10 (from þ40 to -80 °C) and details of data
collection, refinement, atom coordinates, anisotropic displace-
ment parameters, and bond lengths and angles for complexes 1,
2, and 7. This material is available free of charge via the Internet at
http://pubs.acs.org.
3
8
0
(NCdO), 151.2, 149.3, 147.5, 139.3, 137.3 (C3,3 or 5,5 , Cipso NCH-
(CH3)Ph), 128.3 (Cm NCH(CH3)Ph), 127.3 (Co NCH(CH3)Ph), 126.4
(Cp NCH(CH3)Ph), 54.1 (NCH(CH3)Ph), 21.7 (NCH(CH3)Ph),
0
0
0
110.1, 109.8 (C4,4 ), 69.1 (CH), 14.4, 14.1 (Me3,3 ), 12.2, 11.8 (Me5,5 ),
10.6 (AlCH2CH3), 0.6 (AlCH2CH3), 150.5-128.0 (Ph3CCH2CH3), 51.6
(Ph3CCH2CH3), 28.9 (Ph3CCH2CH3), 8.2 (Ph3CCH2CH3), 146.8-
120.0 (B(C6F5)4). 19F NMR (THF-d8, 297 K; δ (ppm)): -133.8
3
3
’ AUTHOR INFORMATION
(d, JF-F = 16.2 Hz, 2F, o-C6F5), -163.4 (t, JF-F = 18.2 Hz, 1F,
p-C6F5), -169.3 (t, 3JF-F = 17.8 Hz, 2F, m-C6F5).
Corresponding Author
*E-mail: Antonio.Otero@uclm.es (A.O.). Tel: þ34926295300.
General Procedure for Solution Polymerization of ε-Ca-
prolactone, L-LA, and rac-LA. Polymerizations of ε-caprolactone
(CL) were carried out on a Schlenk line in a flame-dried round-
bottomed flask equipped with a magnetic stirrer. In a typical procedure,
the initiator was dissolved in the appropriate amount of solvent and
temperature equilibration was ensured by stirring the solution for 15 min
in a temperature-controlled bath. ε-CL was injected, and polymerization
times were measured from that point. Polymerizations were terminated
by the addition of acetic acid (5 vol %) in methanol. Polymers
were precipitated in methanol, filtered off, dissolved in THF, repre-
cipitated in methanol, and dried in vacuo to constant weight.
Fax: þ34926295318.
’ ACKNOWLEDGMENT
We gratefully acknowledge financial support from the Minis-
terio de Ciencia e Innovaciꢀon (MICINN) of Spain (Grant Nos.
CTQ2008-00318/BQU and Consolider-Ingenio 2010 ORFEO
CSD2007-00006) and the Junta de Comunidades de Castilla-La
Mancha, Spain (Grant No. PCI08-0010).
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ARTICLE
’ REFERENCES
(1) (a) Ikada, Y.; Tsuji, T. Macromol. Rapid Commun. 2000, 21, 117–132.
(b) Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 1078–1085.
(2) (a) Shalaby, S. W., Burg, K. J. L., Eds. Absorbable and Biodegrad-
able Polymers; CRC Press: Boca Raton, FL, 2003; Advances in Polymeric
Materials. (b) Domb, A. J., Wiseman, D. M., Eds. Handbook of Biodegrad-
able Polymers; CRC Press: Boca Raton, FL, 1998.
(3) (a) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem.
Rev. 2004, 104, 6147–6176. (b) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C.
Coord. Chem. Rev. 2006, 250, 602–626. (c) Williams, C. K. Chem. Soc.
Rev. 2007, 36, 1573–1580. (d) Wheaton, C. A.; Hayes, P. G.; Ireland,
B. J. Dalton Trans. 2009, 4832–4846.
(4) See for example:(a)Chamberlain, B. M.; Cheng, M.; Moore, D. R.;
Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001,
123, 3229–3238. (b) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg.
Chem. 2005, 44, 8004–8010. (c) Poirier, V.; Roisnel, T.; Carpentier, J.-F.;
Sarazin, Y. Dalton Trans. 2009, 9820–9827. (d) Shen, M.-Y.; Peng, Y.-L.;
Hung, W.-C.; Lin, C.-C. Dalton Trans. 2009, 9906–9913.
(5) See for example: (a) Chisholm, M. H.; Gallucci, J. C.;
Phomphrai, K. Inorg. Chem. 2004, 43, 6717–6725. (b) Piao, L.; Deng,
M.; Chen, X.; Jiang, L.; Jing, X. Polymer 2003, 44, 2331–2336. (c) Ho,
S.-M.; Hsiao, C.-S.; Datta, A.; Hung, C.-H.; Chang, L.-C.; Lee, T.-Y.;
Huang, J.-H. Inorg. Chem. 2009, 48, 8004–8011.
(6) See for example: (a) Gowda, R. R.; Chakraborty, D.; Ramkumar,
V. Eur. J. Inorg. Chem. 2009, 2981–2993. (b) Postigo, L.; Sꢀanchez-
Nieves, J.; Royo, P.; Mosquera, M. E. G. Dalton Trans. 2009, 3756–3765.
(c) Gornshtein, F.; Kapon, M.; Botoshansky, M.; Eisen, M. S. Organo-
metallics 2007, 26, 497–507.
(7) See for example: (a) Platel, R. H.; White, J. P. A.; Williams, C. K.
Inorg. Chem. 2008, 47, 6840–6849. (b) Cai, C.-X.; Amgoune, A.;
Lehmann, C. W.; Carpentier, J.-F. Chem. Commun. 2004, 3, 330–331.
(c) Ma, H.; Okuda, J. Macromolecules 2005, 38, 2665–2673. (d) Bonnet,
F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44, 9046–9055. (e)
Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Chem. Eur. J.
2006, 12, 169–179. (f) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem.,
Int. Ed. 2006, 45, 7818–7821. (g) Liu, X.; Shang, X.; Tang, T.; Hu, N.;
Pei, F.; Cui, D.; Chen, X.; Jing, X. Organometallics 2007, 26, 2747–2757.
(h) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Macromol. Rapid
Commun. 2007, 28, 693–697. (i) Ma, H.; Spaniol, T. P.; Okuda, J. Inorg.
Chem. 2008, 47, 3328–3339.
(8) See for example: (a) Chen, M.-Z.; Sun, H.-M.; Li, W.-F.; Wang,
Z.-G.; Shen, Q.; Zhang, Y. J. Organomet. Chem. 2006, 691, 2489–2494.
(b) Gibson, V. C.; Marshall, E. L.; Navarro-Llobet, D.; White, A. J. P.;
Williams, D. J. Dalton Trans. 2002, 4321–4322. (c) O’Keefe, B. J.;
Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2002,
124, 4384–4393.
(9) See for example: (a) Cheng, M.; Attygalle, A.; Lobkovsky, E.;
Coates, G. W. J. Am. Chem. Soc. 1999, 121, 11583–11584. (b)
Chamberlain, B.; Cheng, M.; Moore, D.; Ovitt, T.; Lobkovsky, E.;
Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229–3238. (c) Chen, H.-Y.;
Huang, B.-H.; Lin, C.- C. Macromolecules 2005, 38, 5400–5405.
(10) See for example: (a) Kricheldorf, H. R.; Kreiser-Saunders, I.;
Stricker, A. Macromolecules 2000, 33, 702–709. (b) Nimitsiriwat, N.;
Gibson, V. C.; Marshall, E. L.; Elsegood, M. R. J. Inorg. Chem. 2008,
47, 5417–5424. (c) Dove, A. P.; Gibson, V. C.; Marshall, E. L.Rzepa,
H. S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834–9843.
(d) Kowalski, A.; Libiszowski, J.; Majerska, K.; Duda, A.; Penczeck, S.
Polymer 2007, 48, 3952–3960. (e) Jiang, G.; Jones, I. A.; Rudd, C. D.;
Walker, G. S. J. Appl. Polym. Sci. 2009, 114, 658–662.
(11) See for example: (a) Spassky, N.; Wisniewski, M.; Pluta, C.;
LeBorgne, A. Macromol. Chem. Phys. 1996, 197, 2627–2637. (b) No-
mura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc. 2002,
124, 5938–5939. (c) Tang, Z.; Chen, X.; Pang, X.; Yang, Y.; Zhang, X.;
Jing, X. Biomacromolecules 2004, 5, 965–970. (d) Zhong, Z.; Dijkstra,
P. J.; Feijen, J. Angew. Chem., Int. Ed. 2002, 41, 4510–4513. (e) Zhong, Z.;
Dijkstra, P. J.; Feijen, J. J. Am. Chem. Soc. 2003, 125, 11291–11298.
(f) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072–4073.
(g) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316–1326.
(h) Ovitt, T. M.; Coates, G. W. J. Polym. Sci., Part A: Polym. Chem. 2000,
38, 4686–4692. (i) Iwasa, N.; Katao, S.; Liu, J.; Fujiki, M.; Furukawa, Y.;
Nomura, K. Organometallics 2009, 28, 2179–2187. (j) Haddad, M.;
Laghzaoui, M.; Welter, R.; Dagorne, S. Organometallics 2009, 28,
4584–4592. (k) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Angew. Chem.
2002, 114, 4692–4695. (l) Du, H.; Pang, X.; Yu., H.; Zhuang, X.; Chen,
X.; Cui, D.; Wang, X.; Jing, X. Macromolecules 2007, 40, 1904–1913.
(m) Thibault, M.-H.; Fontaine, F.-G. Dalton Trans. 2010, 39, 5688–5697.
(12) See for example: (a) Trofimenko, S. Scorpionates: The Coordi-
nation Chemistry of Polypyrazolylborate Ligands; Imperial College Press:
London, 1998. (b) Pettinari, C. Scorpionate II: Chelating Borate
Ligands; Imperial College Press: London, 2008. (c) Marques, N.;
Sella, A.; Takats, J. Chem. Rev. 2002, 102, 2137–2160. (d) Otero, A.;
Fernꢀandez-Baeza, J.; Anti~nolo, A.; Tejeda, J.; Lara-Sꢀanchez, A. Dalton
Trans. 2004, 1499–1510. (e) Parkin, G. Chem. Rev. 2004, 104, 699–768.
(f) Pettinari, C.; Pettinari, R. Coord. Chem. Rev. 2005, 249, 663–691.
(g) Bigmore, H. R.; Lawrence, S. C.; Mountford, P.; Tredget, C. S.
Dalton Trans. 2005, 635–651.
(13) See for example: (a) Santillan, G. A.; Carrano, C. J. Dalton
Trans. 2009, 6599–6605. (b) Godau, T.; Platzmann, F.; Heinemann,
F. W.; Burzlaff, N. Dalton Trans. 2009, 254–255. (c) Fischer, N. V.;
Heinemann, F. W.; Burzlaff, N. Eur. J. Inorg. Chem. 2009, 3960–3965.
(d) Otero, A.; Fernꢀandez-Baeza, J.; Anti~nolo, A.; Tejeda, J.; Lara-
Sꢀanchez, A.; Sꢀanchez-Barba, L.; Expꢀoxito, M. T.; Rodríguez, A. M.
Dalton Trans. 2003, 1614–1619. (e) Otero, A.; Fernꢀandez-Baeza;
Anti~nolo, A.; Tejeda, J.; Lara-Sꢀanchez, A.; Sꢀanchez-Barba, L.; Fernꢀandez-
Lꢀopez, M.; Lꢀopez-Solera, I. Inorg. Chem. 2004, 43, 1350–1358. (f) Otero,
A.; Fernꢀandez-Baeza, J.; Tejeda, J.; Lara-Sꢀanchez, A.; Sꢀanchez-Molina, M.;
Franco, S.; Lꢀopez-Solera, I.; Rodríguez, A. M.; Sꢀanchez-Barba, L. F.;
Morante-Zarco, S.; Garcꢀes, A. Inorg. Chem. 2009, 48, 5540–5554.
(g) Howe, R. G.; Tredget, C. S.; Lawrence, S. C.; Subongkoj, S.; Cowley,
A. R.; Mountford, P. Chem. Commun. 2006, 223–225.
(14) See for example: (a) Kopf, H.; Holzberger, B.; Pietraszuk, C.;
H€ubner, E.; Burzlaff, N. Organometallics 2008, 27, 5894–5905. (b)
Bigmore, H. R.; Lawrence, S. C.; Mountford, P.; Tredget, C. S. Dalton
Trans. 2005, 635–651. (c) Lawrence, S. C.; Ward, B. D.; Dubberley,
S. R.; Kozak, C. M.; Mountford, P. Chem. Commun. 2003, 1176–1177.
(d) Milione, S.; Bertolasi, V.; Cuenca, T.; Grassi, A. Organometallics
2005, 24, 4915–4925. (e) Milione, S.; Montefusco, C.; Cuenca, T.;
Grassi, A. Chem. Commun. 2003, 1176–1177. (f) Cuomo, C.; Milione,
S.; Grassi, A. Macromol. Rapid Commun. 2006, 27, 611–618. (g) Otero,
A.; Fernꢀandez-Baeza, J.; Anti~nolo, A.; Carrillo-Hermosilla, F.; Tejeda, J.;
Diez-Barra, E.; Lara-Sꢀanchez, A.; Sꢀanchez-Barba, L.; Lꢀopez-Solera, I.
Organometallics 2001, 20, 2428–2430. (h) Otero, A.; Fernꢀandez-Baeza,
J.; Anti~nolo, A.; Lara-Sꢀanchez, A.; Tejeda, J.; Sꢀanchez-Barba, L.;
Martínez-Caballero, E.; Lꢀopez-Solera, I. Organometallics 2008, 27,
976–983. (i) Otero, A.; Fernꢀandez-Baeza, J.; Lara-Sꢀanchez, A.;
Anti~nolo, A.; Tejeda, J.; Martínez-Caballero, E.; Mꢀarquez-Segovia, I.;
Lꢀopez-Solera, I.; Sꢀanchez-Barba, L.; Alonso-Moreno, C. Inorg. Chem.
2008, 47, 4996–5005.
(15) See for example: (a) Ghosh, P.; Parkin, G. J. Chem. Commun.
1998, 37–38. (b) Otero, A.; Fernꢀandez-Baeza, J.; Anti~nolo, A.; Tejeda, J.;
Carrillo-Hermosilla, F.; Diez-Barra, E.; Lara-Sanchez, A.; Fernꢀandez-
Lꢀopez, M.; Lanfranchi, M.; Pellinghelli, M. A. J. Chem. Soc., Dalton Trans.
1999, 3537–3539. (c) Otero, A.; Fernꢀandez-Baeza, J.; Anti~nolo, A.;
Tejeda, J.; Carrillo-Hermosilla, F.; Lara-Sanchez, A.; Sꢀanchez-Barba, L.;
Fernꢀandez-Lꢀopez, M.; Rodríguez, A. M.; Lꢀopez-Solera, I. Inorg. Chem.
2002, 41, 5193–5202. (d) Smith, J. N.; Shirin, Z.; Carrano, C. J. J. Am.
Chem. Soc. 2003, 125, 868–869. (e) Otero, A.; Fernꢀandez-Baeza, J.;
Anti~nolo, A.; Tejeda, J.; Lara-Sanchez, A.; Sꢀanchez-Barba, L.; Rodríguez,
A. M.; Maestro, M. A. J. Am. Chem. Soc. 2004, 126, 1330–1331. (f)
Hammes, B. S.; Chohan, B. S.; Hoffman, J. T.; Einw€achter, S.; Carrano,
C. J. Inorg. Chem. 2004, 43, 7800–7806. (g) Adhikari, D.; Zhao, G.;
Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Inorg. Chem.
2006, 45, 1604–1610. (h) Kopf, H.; Pietraszuk, C.; H€ubner, E.; Burzlaff,
N. Organometallics 2006, 25, 2533–2546. (i) Otero, A.; Fernꢀandez-
Baeza, J.; Anti~nolo, A.; Tejeda, J.; Lara-Sanchez, A.; Sꢀanchez-Barba, L.;
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Organometallics
ARTICLE
Sꢀanchez-Molina, M.; Franco, S.; Lꢀopez-Solera, I.; Rodríguez, A. M.
Dalton Trans. 2006, 4359–4370. (j) Otero, A.; Fernꢀandez-Baeza, J.;
Anti~nolo, A.; Tejeda, J.; Lara-Sꢀanchez, A.; Sꢀanchez-Barba, L.; Lꢀopez-
Solera, I.; Rodríguez, A. M. Inorg. Chem. 2007, 46, 1760–1770.
(16) See for example: (a) Otero, A.; Fernꢀandez-Baeza, J.; Lara-
Sꢀanchez, A.; Tejeda, J.; Sꢀanchez-Barba, L. F. Eur. J. Inorg. Chem.
2008, 5309–5326 (Microreview). (b) Elflein, J.; Platzmann, F.; Burzlaff,
N. Eur. J. Inorg. Chem. 2007, 5173–5176. (c) Hegelmann, I.; Burzlaff, N.
Eur. J. Inorg. Chem. 2003, 409–411. (d) Beck, A.; Weibert, B.; Burzlaff, N.
Eur. J. Inorg. Chem. 2001, 521–527. (e) Gennari, M.; Tegoni, M.;
Lanfranchi, M.; Pellinghelli, M. A.; Marchiꢁo, L. Inorg. Chem. 2007,
46, 3367–3377.
Organometallics 2004, 23, 1411–1415. (c) Kim, Y.; Verkade, J. G.
Organometallics 2002, 21, 2395–2399.
(26) Alonso-Moreno, C.; Garcꢀes, A.; Sꢀanchez-Barba, L. F.; Fajardo,
M.; Fernꢀandez-Baeza, J.; Otero, A.; Anti~nolo, A.; Lara-Sꢀanchez, A.;
Lꢀopez-Solera, I. Organometallics 2008, 27, 1310–1321.
(27) Coudane, J.; Ustariz-Peyret, C.; Schwach, G.; Vert, M. J. Polym.
Sci., Polym. Chem. Ed. 1997, 35, 1651–1658.
(28) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W.
J. Am. Chem. Soc. 1999, 121, 11583–11584. (b) Chisholm, M. H.;
Huffman, J. C.; Phomphrai, K. Dalton Trans. 2001, 222–224. (c) Jensen,
T. R.; Schaller, C. P.; Hillmyer, M. A.; Tolman, W. B. J. Organomet.
Chem. 2005, 690, 5881–5891.
(17) See for example: (a) Otero, A.; Fernꢀandez-Baeza, J.; Anti~nolo,
A.; Tejeda, J.; Lara-Sanchez, A.; Sꢀanchez-Barba, L.; Sꢀanchez-Molina, M.;
Franco, S.; Lꢀopez-Solera, I.; Rodríguez, A. M. Eur. J. Inorg. Chem.
2006, 707–710. (b) Sꢀanchez-Barba, L. F.; Garcꢀes, A.; Fajardo, M.;
Alonso-Moreno, C.; Fernꢀandez-Baeza, J.; Otero, A.; Anti~nolo, A.;
Tejeda, J.; Lara-Sanchez, A.; Lꢀopez-Solera, I. Organometallics 2007,
26, 6403–6411. (c) Fischer, N. V.; Heinemann, F. W.; Burzlaff, N.
Eur. J. Inorg. Chem. 2009, 3960–3965. (d) Godau, T.; Platzmann, F.;
Heinemann, F. W.; Burzlaff, N. Dalton Trans. 2009, 254–255. (e)
Schofield, A. D.; Barros, M. L.; Cushion, M. G.; Schwarz, A. D.;
Mountford, P. Dalton Trans. 2009, 85–96. (f) Cushion, M. G.; Meyer,
J.; Heath, A.; Schwarz, A. D.; Fernꢀandez, I.; Breher, F.; Mountford, P.
Organometallics 2010, 29, 1174–1190. (g) Milione, S.; Grisi, F.; Centore,
R.; Tuzi, A. Organometallics 2006, 25, 266–274. (h) Silvestri, A.; Grisi, F.;
Milione, S. J. Polym. Sci., Polym. Chem. 2010, 48, 3632–3639.
(18) See for example: (a) Chisholm, M. H.; Eilerts, N. W.; Huffman,
J. C.; Iyer, S. S.; Pacold, M.; Phomphrai, K. J. Am. Chem. Soc. 2000,
122, 11845–11854. (b) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K.
Inorg. Chem. 2004, 43, 6717–6725. (c) Cowley, A. H.; Carrano, C. J.;
Geerts, R. L.; Jones, R. A.; Nunn, C. M. Angew. Chem., Int. Ed. Engl. 1988,
27, 277–278. (d) Looney, A.; Parkin, G. Polyhedron 1990, 9, 265–276.
(e) Trofimenko, S.; Calabrese, J. C.; Thompson, J. S. Inorg. Chem. 1987,
26, 1507–1514. (f) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C. Inorg.
Chem. 1996, 35, 445–450. (g) Dunne, J. F.; Manna, K.; Wiench, J. W.;
Ellern, A.; Pruski, M.; Sadow, A. D. Dalton Trans. 2010, 39, 641–653.
(19) Otero, A.; Lara-Sanchez, A.; Fernꢀandez-Baeza, J.; Alonso-
Moreno, C.; Tejeda, J.; Castro-Osma, J. A.; Mꢀarquez-Segovia, I.;
Sꢀanchez-Barba, L. F.; Rodríguez, A. M.; Gꢀomez., M. V. Chem. Eur. J.
2010, 16, 8615–8619.
(29) Garlotta, D. J. Polym. Environ. 2001, 9, 63–84.
(30) In contrast to the polymerization of ε-caprolactone carried out
with 1-10, which probably proceeds via a coordination-insertion
mechanism, those with 13 and 14 are assumed to occur via a cationic
mechanism, although this was not definitively established in the present
case. See for example: (a) Lian, B.; Thomas, C. M.; Casagrande, O. L., Jr.;
Lehmann, C. W.; Roisnel, T.; Carpentier, J.-F. Inorg. Chem. 2007,
46, 328–340. (b) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem.
Rev. 2006, 250, 602–626.
(31) Sꢀanchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.;
Bochmann, M. Organometallics 2005, 24, 3792–3799.
(32) SAINTþ v7.12a: Area-Detector Integration Program; Bruker-
Nonius AXS, Madison, WI, 2004.
(33) Sheldrick, G. M. SADABS version 2004/1: A Program for
Empirical Absorption Correction; University of G€ottingen, G€ottingen,
Germany, 2004.
(34) SHELXTL-NT version 6.12. Structure Determination Package;
Bruker-Nonius AXS, Madison, WI, 2001.
(20) (a) Otero, A.; Lara-Sꢀanchez, A.; Fernꢀandez-Baeza, J.; Martínez-
Caballero, E.; Mꢀarquez-Segovia, I.; Alonso-Moreno, C.; Sꢀanchez-Barba,
L. F.; Rodríguez, A. M.; Lꢀopez-Solera, I. Dalton Trans. 2010,
39, 930–940. (b) Otero, A.; Fernꢀandez-Baeza, J.; Lara-Sꢀanchez, A.;
Alonso-Moreno, C.; Mꢀarquez-Segovia, I.; Sꢀanchez-Barba, L. F.; Rodrí-
guez, A. M. Angew. Chem., Int. Ed. 2009, 48, 2176–2179.
(21) (a) Eyring, H. Chem. Rev. 1935, 17, 65–77.(b) Budzelaar,
P. H. M. gNMR 5.1, 2005.
(22) (a) Dagorne, S.; Le Bideau, F.; Welter, R.; Bellemin-Laponnaz, S.;
Maisse-Francois, A. Chem. Eur. J. 2007, 13, 3202–3217. (b) Lian, B.; Ma, H.;
Spaniol, T.; Okuda, J. Dalton Trans. 2009, 9033–9042. (c) Haddad, M.;
Laghzaoui, M.;Welter, R.;Dagorne, S.Organometallics 2009, 28, 4584–4592.
(d) Issenhuth, J.-T.; Pluvinage, J.; Welter, R.; Bellemin-Laponnaz, S.;
Dagorne, S. Eur. J. Inorg. Chem. 2009, 4701–4709. (e) Wheaton, C. A.;
Hayes, P. G. Chem. Commun. 2010, 46, 8404–8406. (f) Clark, L.; Cushion,
M. G.; Dyer, H. D.; Schwarz, A. D.; Duchateau, R.; Mountford, P. Chem.
Commun. 2010, 46, 273–275. (g) Sarazin, Y.; Poirier, V.; Roisnel, T.;
Carpentir, J.-F. Eur. J. Inorg. Chem. 2010, 3423–3428.
(23) Cameron, P.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce,
M.; White, A. J. P.; Williams, D. J. Dalton Trans. 2002, 415–422.
(24) See some of the few examples of alkyl initiators: (a) Yu, R.-C.;
Hung, C.-H.; Huang, J.-H.; Lee, H.-Y.; Chen, J.-T. Inorg. Chem. 2002,
41, 6450–6455. (b) Chakraborty, D.; Chen, E. Y.-X. Organometallics
2002, 21, 1439–1442. (c) Shen, M.; Zhang, W.; Nomura, K.; Sun, W.-H.
Dalton Trans. 2009, 9000–9009.
(25) (a) Wang, X.; Liao, K.; Quan, D.; Wu, Q. Macromolecules 2005,
38, 4611–4617. (b) Li, H.; Wang, C.; Bai, F.; Yue, J.; Woo, H.-G.
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dx.doi.org/10.1021/om1010676 |Organometallics 2011, 30, 1507–1522