Anilidopyridyl-pyrrolide and anilidopyridyl-indolide group 3 metal complexes: Highly active initiators for the ring-opening polymerization of rac -lactide

Article abstract of DOI:10.1021/om201251g

Three new group 3 metal complexes, bearing anilidopyridyl-pyrrolide (L ¹) and anilidopyridyl-indolide (L²) as dianionic tridentate ligands, with the general formula LMN(SiHMe₂)₂ were synthesized (complex 1, M = Y, L = L¹; complex 2, M = Sc, L = L¹; complex 3, M = Y, L = L²). All complexes were fully characterized and tested as initiators for the ROPs of rac-lactide. The yttrium complexes 1 and 3 resulted in highly active catalysts (TOF up to 10⁴ mol_lactide mol_Y^-1 h^-1), whereas the scandium complex showed moderate activities. This class of catalysts allowed a good control of the macromolecular architecture of the polymer, namely, the nature of end groups, the molecular weights, and their distribution. Moreover, the obtained PLAs showed P_r values in the range of 0.57-0.84, depending on the nature of the initiator and solvent. Well-controlled and rapid ROPs of rac-lactide were obtained in the solvent-free polymerizations at 130 °C as well, suggesting that complexes 1-3 are stable at high temperature. Finally, in the presence of 2-propanol, complex 1 promoted the immortal ROP of rac-lactide, showing a remarkable TOF of 3.5 × 10⁴ mol _lactide mol_Y^-1h^-1.

Full text of DOI:10.1021/om201251g

Article
pubs.acs.org/Organometallics
Anilidopyridyl-Pyrrolide and Anilidopyridyl-Indolide Group 3 Metal
Complexes: Highly Active Initiators for the Ring-Opening
Polymerization of rac-Lactide
Gang Li,† Marina Lamberti,*,† Mina Mazzeo,† Daniela Pappalardo,‡ Giuseppina Roviello,§
and Claudio Pellecchia†
†Dipartimento di Chimica e Biologia, Universita
̀
di Salerno, I-84084, via Ponte don Melillo, Fisciano, Salerno, Italy
del Sannio, via dei Mulini 59/A, 82100, Benevento, Italy
̀
di Napoli Parthenope, Centro Direzionale Napoli, Isola C4, 80143, Napoli, Italy
‡Dipartimento di Studi Geologici ed Ambientali, Universita
̀
§Dipartimento di Tecnologia, Universita
S
* Supporting Information
ABSTRACT: Three new group 3 metal complexes, bearing
anilidopyridyl-pyrrolide (L1) and anilidopyridyl-indolide (L2) as
dianionic tridentate ligands, with the general formula LMN-
(SiHMe2)2 were synthesized (complex 1, M = Y, L = L1; complex
2, M = Sc, L = L1; complex 3, M = Y, L = L2). All complexes were
fully characterized and tested as initiators for the ROPs of rac-lactide.
The yttrium complexes 1 and 3 resulted in highly active catalysts
(TOF up to 104 mollactide molY−1 h−1), whereas the scandium complex
showed moderate activities. This class of catalysts allowed a good
control of the macromolecular architecture of the polymer, namely,
the nature of end groups, the molecular weights, and their
distribution. Moreover, the obtained PLAs showed Pr values in the
range of 0.57−0.84, depending on the nature of the initiator and
solvent. Well-controlled and rapid ROPs of rac-lactide were obtained
in the solvent-free polymerizations at 130 °C as well, suggesting that complexes 1−3 are stable at high temperature. Finally, in
the presence of 2-propanol, complex 1 promoted the immortal ROP of rac-lactide, showing a remarkable TOF of 3.5 × 104
mollactide molY−1 h−1.
variety of coordination environments have been explored. In
particular, some promising ligand backbones, such as
polydentate phenolate-based ligands, that were previously
applied to prepare group 4 metal complexes for ethylene
and/or α-olefin polymerization with high performances,5 have
also been investigated in combination with main group and
transition metals as initiators for the ROP of cyclic esters with
impressive results.6 In this context, the proper choice of the
ligand framework is crucial, as the reactivity and moreover the
stereoselectivity of the rare-earth metal compounds could be
significantly modulated by tuning the electronic and steric
properties of the coordination environment. Stereoselective
catalysts, based on rare-earth metal complexes have also been
reported, and they mainly featured bulky bisphenolate-type
ligands. For example, scandium complexes bearing a 1,ω-
dithiaalkanediyl-bridged bis(phenolate)ligand7 and yttrium
complexes bearing multidentate bis(phenolate)amino ligands
displayed excellent heteroselectivity in the ROP of rac-lactide.8
By contrast, examples of stereoselective catalysts, based on rare-
earth metal complexes coordinated to ligands other than
INTRODUCTION
■
Biodegradable and biocompatible polylactide (PLA), as a
thermoplastic resin derived from renewable resources, is
attracting considerable attention in both academic and
industrial research as a sustainable alternative to traditional
commodity polymers, and for special applications in agriculture
and medicine.1 The control of the polylactide chains, in terms
of molecular weight, composition, and microstructure, strongly
affect the mechanical and physical properties, biodegradability,
and the end use of the material.2
The ring-opening polymerization (ROP) of lactide promoted
by metal coordination catalysts is the elected method to
produce polymers of high molecular weight, low polydispersity,
and controlled microstructure. To this purpose, the design and
synthesis of single-site catalysts for the ROP of rac-lactide with
high activities and stereoselectivities become a main goal.3
In the past decade, a large number of metal coordination
catalysts bearing polydentate ligands have been explored.
Among various initiators, group 3 metal complexes resulted
of particular interest because of their high polymerization rate,
good polymerization control, and low toxicity.3b Indeed, after
the discovery, by Dupont researchers, of homoleptic yttrium
alkoxides as highly active initiators for the ROP of lactide,4 a
Received: December 16, 2011
Published: January 20, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Group 3 Metal Complexes 1−3
Scheme 2. Synthesis of the Proligand H2L2
bisphenolate, are rare.9 In this regard, we recently described
rare-earth metal bis(alkyl) complexes bearing phosphido pincer
ligands, which showed high catalytic activity in the ROP of D,L-
lactides, although in the absence of stereocontrol.10
we report on their synthesis, characterization, and application in
the ROP of rac-lactide, resulting in high activity, good control,
and heteroselectivity (Pr up to 0.84).
RESULTS AND DISCUSSION
In recent years, ligands with nitrogen donor atoms have been
used for the synthesis of novel rare-earth complexes, as they are
expected to stabilize highly electrophilic metals. Examples
reported in the literature include rare-earth complexes with
bidentate NN ligands, such as amidinates, guanidinates,
aminotroponiminates, β-diketiminate, and chelating diamide.11
On the contrary, the use of a tridentate [−NNN−] ligand is
poorly represented, with just few examples of complexes
bearing symmetric pyridine-diamide type ligands or diketimi-
nato derivative ligands.12 We have recently reported the
synthesis of anilidopyridyl-pyrrolide dianionic [−NNN−]
tridentate ligands that have been already exploited for the
preparation of group 4 metal complexes that are very active in
the production of highly isotactic polypropylene and poly(1-
hexene).13 It is worth to note that such ligands may be suitable
as ancillary ligands in the design of “single-site” group 3 metal
initiators for the ROP of lactone and lactides.3e Indeed,
tridentate dianionic ligands may provide the required rigid
environment for the steric control during the polymerization of
the racemic lactide, leaving the place to a sole labile initiating
group.
■
Synthesis and Characterization. The proligand H2L1 has
been synthesized following a previously published synthetic
procedure.13b For the synthesis of the anilidopyridyl-indolide
compound H2L2, a similar synthetic approach was followed, as
described in Scheme 2. The Suzuki−Miyaura cross-coupling
reaction between 6-bromo-2-pyridine-carboxaldehyde and N-
Boc-indole-2-boronic acid (Boc = tert-butyloxycarbonyl)
afforded compound A, as a yellow solid, in 86% yield. The
condensation reaction between A and 2,6-diisopropylaniline by
the use of 10 equiv of silica gel 60 as a catalyst gave the
indolylpyridylimine precursor B as a light yellow solid (yield:
78%), which, by alkylation with 2-iPrC6H4Li, furnished the
desired product (H2L2) as a yellow solid in 88% yield (Scheme
2). All compounds were fully characterized by H and 13C
1
NMR spectroscopies.
Complexes 1−2 were prepared by acid−base transamination
reactions between the proligand (H2L1) and 1 equiv of
Y(N(SiHMe2)2)3(THF)2 or Sc(N(SiHMe2)2)3(THF) in ben-
zene solution at room temperature overnight. The compounds
were isolated as light yellow powders in good yields (1, 89%; 2,
78%). Recrystallization of products from cold hexane yielded
analytically pure compounds.
In the framework of our current interest in the ROP of
lactone and lactides,10,14 we have synthesized novel yttrium and
scandium complexes bearing anilidopyridyl-pyrrolide (L1) and
anilidopyridyl-indolide (L2) ligands (Scheme 1). In this paper,
Complex 3 was synthesized by reaction of the proligand
H2L2 with 1 equiv of Y(N(SiHMe2)2)3(THF)2 at 50 °C for 2 h.
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The compound was obtained as an orange powder in 76% yield
(Scheme 1).
All complexes were fully characterized by multinuclear NMR
spectroscopy and elemental (C, H, N) analyses (see the
Experimental Section).
Single crystals of 1 and 2, obtained from n-hexane solutions
at 253 K, were characterized by X-ray diffraction, and the
molecular structures are shown in Figures 1 and 2. The
Figure 2. ORTEP-3 view of 2. Thermal ellipsoids are shown at the
30% probability level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (deg): Sc1−N4 2.059(3), Sc1−N3
2.075(3), Sc1−O1 2.184(3), Sc1−N1 2.211(3), Sc1−N2 2.243(3);
N4−Sc1−N3 112.9(1), N4−Sc1−O1 115.4(1), N3−Sc1−O1
104.4(1), N2−Sc1−O1 111.9(1), N4−Sc1−N1 93.0(1), O1−Sc1−
N1 86.0(1), N3−Sc1−N2 72.1(1), N1−Sc1−N2 71.0(1), C15−C14−
N3−C24 76.3(2).
angle Sc−N−Si is smaller [Sc1−N4−Si1 108.2(1)° vs Sc1−
N4−Si2 127.2(1)°]. These features, not so significant for the
similar yttrium complex 1, probably reveal a weak monoagostic
β-Si−H interaction, not rarely observed for similar silylamido
complexes.15b,16
Figure 1. ORTEP-3 view of 1. Thermal ellipsoids are shown at the
30% probability level. Hydrogen atoms and a hexane crystallization
molecule lying on a crystallographic inversion center are omitted for
clarity. Moreover, the statistical alternative positions of two C atoms of
a SiMe2 group are not shown. Selected bond distances (Å) and angles
(deg): Y1−N1 2.388(3), Y1−N2 2.438(3), Y1−N3 2.246(3), Y1−N4
2.286(3), Y1−O1 2.412(3), Y1−O2 2.400(3); N1−Y1−N2 67.1(1),
N2−Y1−N3 68.6(1), N3−Y1−N4 111.6(1), N4−Y1−N1 112.4(1),
O2−Y1−O1 157.9(1), C15−C14−N3−C24 76.0(4).
NMR analysis of complexes 1−3, in [D8] THF at room
temperature, showed in all cases single monomeric species with
C1 symmetry in solution. Particularly diagnostic in the 1H NMR
spectra are two doublets of methyl protons (CH3) of −SiHMe2
groups ranging from −0.13 to 0.004 ppm and six doublets of
methyl protons (CH3) of isopropyl groups ranging from 2.6 to
3.6 ppm. Single resonances and sharp septets, were observed
for the proton of SiH groups. In both the 1H and the 13C NMR
spectra, coordinated THF molecules were detected, as
suggested by the presence of resonances as two multiplets
cleanly shifted from free THF solvent (Δδ (1H) = 0.04 ppm;
Δδ (13C) = 0.38 ppm)17 (see the Supporting Information).
Rare-earth silylamido complexes with β-hydrogens often
show single or multiple agostic interactions with the metal
center.18 Usually these weak interactions are easily destroyed by
the coordination of a strongly coordinating solvent, such as
tetrahydrofuran, so we decided to verify the possible presence
of the interaction by studying the structure of complex 1 in
[D2] methylene dichloride and [D8] toluene solution.
To promote THF dissociation from the metal center,
complex 1 was initially dissolved in methylene dichloride, and
subsequently, the added solvent was removed by evaporation in
vacuo.
complexes crystallize in monoclinic (1, space group P21/c) and
triclinic (2, space group P1) systems, respectively. As far as 1,
̅
the asymmetric unit contains half a hexane crystallization
molecule lying on a crystallographic inversion center.
Complex 1 shows a distorted octahedral geometry around
the metal center due to the steric constraints of the [−NNN−]
tridentate ligand. The four nitrogen atoms lie on a plane
containing also the metal (Y is 0.07(2) Å out of the mean plane
defined by the N atoms). The coordination is completed by
two THF molecules in a trans position, one of them being bent
toward the pyrrole ring to avoid steric repulsions with the bulky
i-Pr group [O1−Y1−O2 = 157.9(1)°].
The coordination geometry of the complex 2 at the metal
center is best described as distorted square-pyramidal. The
metal ion lies 0.578(2) Å out of the plane defined by N1, N2,
N3, and N4. A THF molecule occupies the apical position. The
anilidopyridyl-pyrrolide fragment is planar in both cases. All the
M−N bond distances fall in the range characteristic of similar
complexes.15 The M−N(pyridyl) bond distance is longer than
the other ones, according to the different nature of the metal
bond.15,16 Also the M−O bond lengths are in line with those
found for similar compounds.15b
The NMR analysis performed at room temperature showed
that the structure of complex 1, in both solvents, was consistent
with a single monomeric C1-symmetric species consisting of
one tridentate anilidopyridyl-pyrrolide ligand and a silylamido
moiety, −N(SiHMe2)2, coordinated to the metal center. A
complete THF displacement from the metal center was
observed.
As far as complex 2, it is interesting to note that one of the
silicon atoms of the {N(SiHMe2)2} moiety has a shorter
distance to the metal center than the other one [Sc1−Si1
3.053(1) Å vs Sc1−Si2 3.380(1) Å], and the corresponding
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a
Table 1. Ring-Opening Polymerization of rac-Lactide Initiated by Complexes 1−3
b
c
d
e
run
[I]0
solvent
T (°C)
time (min)
conv %
Mn(th) (× 103)
Mn(GPC) (× 103)
PDI
Pr
1
2
1
1
1
2
2
2
3
3
3
1
1
2
3
1
THF
20
20
5
5
89
88
25.7
25.4
28.8
6.1
25.4
14.9
25.2
9.5
2.23
2.23
1.54
1.58
1.61
1.87
1.94
2.13
2.02
1.70
2.02
1.67
1.99
1.67
0.77
0.57
0.64
0.72
0.73
0.73
0.74
0.73
0.75
0.84
0.59
0.53
0.54
0.69
toluene
CH2Cl2
THF
3
20
2
100
21
4
20
180
960
180
15
5
toluene
CH2Cl2
THF
20
78
22.5
7.2
30.3
6.9
6
20
25
7
20
67
19.3
27.4
28.3
23.9
25.1
19.6
21.6
28.8
21.9
23.9
28.6
37.5
29.7
20.0
27.0
8
toluene
CH2Cl2
THF
20
15
96
9
20
10
98
10
11
12
13
0
120
10
83
in bulk
in bulk
in bulk
toluene
130
130
130
20
87
10
68
10
75
f
14
10
100
23.5
a
b
c
Conditions: [I0], 5 mM; [rac-LA]/[I0], 200; 2 mL of solvent. Reaction times were not necessarily optimized. Calculated Mn of PLA (g/mol) =
d
144.14 × ([rac-LA]/[I0]) × conversion (rac-LA)%. Experimental Mn values (corrected using the Mark−Houwink factor of 0.58) were determined
by GPC analysis in THF using polystyrene standards. Pr value is determined from the methine region of the homonuclear decoupled H NMR
spectrum. Ten equivalents of pyridine was added in the polymerization medium.
e
1
f
1
The JSiH coupling constant is generally a good probe to
evaluate the presence of the metal−β-Si−H agostic interactions.
For complex 1, the 1JSiH was 176 Hz; this value falls in the range
of 160−200 Hz characteristic of the silicon hydrides,19
suggesting the absence of any agostic Si−H interaction with
the yttrium center also in the absence of coordinating solvents.
The same study was extended to complex 2. In this case, the
interaction between the coordinated THF and the more Lewis
acidic metal center resulted in being stronger; thus, the solvent
dissociation was not achieved, even after evaporation in vacuo
for prolonged times.
generally observed for larger rare-earth metal complexes. A
polymerization experiment (run 5) carried out with complex 2
in toluene gave about 78% conversion within 16 h (TOF: 10
−1
mollactide molSc h−1), whereas only low conversions were
obtained in more coordinating solvents, such as THF and
CH2Cl2. Reasonably, for this smaller metal, the competition
between coordinating solvents and lactide for metal coordina-
tion is more significant than that found for yttrium complexes.
GPC analysis of the polymer samples obtained by the
complexes 1−3 displayed monomodal molecular weight
distributions ranging from 1.54 to 2.23. The number-average
molecular mass (Mn) values are in good agreement with the
theoretical ones calculated on the assumption that a single PLA
chain is produced per metal center. A good control of
polymerization by complex 1 was also demonstrated by the
linear relationship between number-average molecular masses
and the monomer-to-metal ratio (Figure 3 and Table S1 in the
Supporting Information). By increasing the monomer-to-
initiator molar ratio up to 1200 (run 15 in Table 3), a good
agreement between theoretical and experimental molecular
weight was still obtained. Most significantly, catalyst 1 was able
to convert 1050 equiv of lactide within 6 min at 20 °C, with a
remarkable TOF of 104 mollactide molY−1 h−1.
The relatively high PDI values can be due to trans-
esterification processes occurring during the polymerization
reactions, a phenomenon that is often observed in the ROP of
cyclic esters promoted by group 3 metal initiators. To explore
this possibility, low-molecular-weight samples were properly
synthesized. The ESI-MS analysis carried out on these products
showed that the most intense peaks are consistent with linear
both even-membered and odd-membered oligomers, confirm-
ing the occurrence of intramolecular transesterification
reactions.
1
The H NMR analysis of 2 in [D2] methylene dichloride
solution showed a 1JSiH of 164 Hz; this value suggested that the
weak Sc−β-Si−H agostic interaction detected in the solid state
by X-ray analysis is too feeble to survive in solution.
Ring-Opening Polymerization of rac-Lactide. Com-
plexes 1−3 were tested as initiators for the ROP of rac-lactide
under a variety of experimental conditions. The main results are
listed in Table 1. The yttrium complexes 1 and 3 resulted in
being highly active in the ROP of rac-lactide, promoting
complete conversion of 200 equiv of monomer within a few
minutes at room temperature (TOF up to 6000 mollactide
molY−1 h−1).3b Their activities were found to be dependent on
the polymerization medium, following the order CH2Cl2 >
toluene ≥ THF (Table 1). A similar trend of the rate of ROP
reactions promoted by group 3 metal complexes has been often
observed.8a,b,20 The yttrium complex 1 (runs 1−3) displayed
slightly higher activities than those of the yttrium complex 3
(runs 7−9), bearing ligand H2L2. This can be ascribable both to
the higher electron-donating character of the indolyl group,
which decreases the Lewis acidity of the metal center, and/or to
the larger steric hindrance of the indolyl moiety, which can
hamper the nucleophilic attack of the monomer to the metal
center. Compared with the yttrium complexes, the scandium
complex 2 displayed lower activities (runs 4−6). This result is
in line with literature data7a,8b showing that higher activities are
For industrial-scale polymerizations, solvent-free conditions
offer several advantages over solution polymerization (e.g., no
solvent is required and the vulnerability to impurity levels and
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literature22, and Pr values were evaluated by integrating the
suitable peaks.23
As shown in Table 1, the microstructure analysis of the
polymer samples obtained by complexes 1−3 showed moderate
to good heterotacticity, depending on both the used catalysts
and the reaction conditions. In particular, the polymerization
medium has a relevant effect on the stereoselectivity of initiator
1. The polymerization in THF was found to be much more
selective than those in toluene and in CH2Cl2. A similar
behavior was previously observed for other catalysts.7b,8b,20,24 A
theoretical study examining this effect suggested that the
solvent plays a key role in stabilizing the transition state, which
dictates the stereochemistry of monomer insertion to give
heterotactic PLAs.25
To get more insights in this coordination effect, a
polymerization run was conducted with catalyst 1, under the
same conditions of run 2, but adding 10 equiv of pyridine, a
markedly strong donor compound. A positive effect on the
degree of control of the catalyst was observed in terms of both
molecular mass and stereoselectivity. In fact, a better agreement
between theoretical and experimental molecular weight and a
narrower molecular weight distribution were observed in the
presence of pyridine (cf. run 2 and run 14); at the same time, a
significant increase of the Pr value was obtained. These
observations support the beneficial effect of the presence of
an additional coordinating donor to the metal center.
The Pr values for the PLAs obtained by the scandium
complex revealed close to the best value obtained by the
yttrium complex 1, regardless of the solvent. This result
probably reflects the influence of the ionic radius of the metal
center. Indeed, the environment around the smaller scandium
metal center is more hindered, resulting in a better selectivity in
each solvent.7c In the same way, the yttrium complex 3
produces heterotactic PLAs (Pr up to 0.75) and the solvent
does not play a big effect on the stereoselectivity of the catalyst.
Reasonably, the metal center in complex 3, being less acidic and
more hindered than yttrium in complex 1, is less prone to
solvent coordination.
Figure 3. Dependence of Mn on monomer-to-yttrium ratio using
complex 1 for rac-LA polymerization in THF at 20 °C. ( )
Experimental data from Table S1 in the Supporting Information
(Mn(GPC) × 0.58). The straight line represents the theoretical trend
(Mn(th)).
●
unwanted side reactions is lower).21 Thus, solvent-free
polymerizations at 130 °C were carried out with complexes
1−3 (runs 11−13 in Table 1). Rapid ROP of rac-lactide with
conversions up to 87% were obtained within 10 min, suggesting
that complexes 1−3 are stable at high temperature. The
activities follow the same order, 1 > 3 > 2, observed in solution
polymerization. Analysis of molecular weights and molecular
weight distributions is consistent with well-controlled polymer-
ization, with levels of control similar to those observed in
solution. These results suggest that the anilidopyridyl-pyrrolide
and anilidopyridyl-indolide dianionic [−NNN−] tridentate
ligands are able to stabilize the metal center even under drastic
reaction conditions.
The stereochemistry of the obtained PLA samples was
determined by recording the homonuclear decoupled 1H NMR
spectra (see Figure 4). The peaks were assigned to the
appropriate tetrads in accordance with the shifts reported in the
Not surprisingly, higher heteroselectivity was obtained at
lower temperature (run 10) while the stereocontrol was lost at
130 °C (runs 11−13).
Detailed microstructural analysis of the PLA produced by
complex 1 (run 10) confirmed a chain-end control mechanism
(Table 2) whereby the stereochemistry of the last inserted
lactide unit controls the binding of the next monomer.23b
Table 2. Tetrad Probabilities Based on Bernoullian Statistic
(th) for a Pr of 0.84 and Experimental Values (exptl) As
Obtained by NMR Analysis
tetrad
formula
exptl
th
2
[mmm]
[mmr]
[rmm]
[rmr]
Pm + PrPm/2
PrPm/2
0.08
0.07
0.07
0.35
0.43
0.09
0.07
0.07
0.35
0.42
PrPm/2
Pr2/2
(Pr2 + PrPm)/2
[mrm]
For a deeper comprehension of the polymerization
mechanism, we analyzed the polymer obtained carrying out
the ROP of rac-lactide at a low monomer-to-initiator molar
ratio ([LA]0/[1]0 = 20:1, run 1 in Table S1 (Supporting
1
Figure 4. Methine region of the homonuclear decoupled H NMR
spectrum (400 MHz, CDCl3, 298 K) of the PLA sample obtained by
complex 1 in THF at 0 °C (run 10 in Table 1).
1
Information)). H NMR analysis of this sample (Figure S13 in
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the Supporting Information) revealed a set of minor resonances
at 0−0.3 and 4.53 ppm, which, according to the literature,26 can
be attributed to the methyl groups and the hydrogen atoms,
respectively, of the silylamido end group (−N(SiHMe2)2),
generated by the insertion of a monomer unit into the metal−
silylamido bond. Other minor resonances at 2.74 ppm (−OH)
and 4.35 ppm (−CH−) are attributable to hydroxyl end groups
(HOCH(CH3)CO−), generated by hydrolysis of the polymer
chain.27 Therefore, these results suggest that the polymer chain
is produced by a coordination−insertion mechanism process in
which the silylamido ligand acts as the initiating group.
As is well known in the literature, amido and alkyl initiators
are usually inferior initiating groups with respect to metal
alkoxides for the ROP of polar monomers. The alkoxide
initiating group mimics the propagating groups of the
presumed active species, and complexes with these moieties
produce polymers of predictable molecular weight and with
narrow molecular weight distributions. Alkoxide initiators can
be generated in situ by alcoholysis of amido and alkyl
complexes.27 With the aim to optimize the control over the
ROP of rac-lactide, polymerization experiments by complex 1
in the presence of 2-propanol were carried out. The main
results are summarized in Table 3. Remarkably, the yttrium
that, in these conditions, the isopropoxide ligand is the true
initiating group for the ROP process. Moreover, the agreement
between the experimental value of Mn (Mn(GPC) = 13 500)
and that determined by NMR (Mn = 14 000) indicated that the
polymerization proceeds exclusively via insertion of the
monomer unit into the Y−OCH(CH3)2 bond.
CONCLUSION
■
Novel group 3 metal complexes bearing anilidopyridyl-
pyrrolide and anilidopyridyl-indolide dianionic [−NNN−]
tridentate ligands were synthesized and fully characterized. All
complexes resulted in being active in the ROP of rac-lactide at
room temperature, with remarkable activity for the yttrium
complexes (TOF up 104 mollactide molY−1 h−1). A good control
of molecular characteristics of the polymer chains, such as end
groups, molecular weight, and molecular weight distribution,
was obtained in all cases. In the presence of 2-propanol, the
immortal ROP of rac-lactide was promoted. Rapid and well-
controlled ROP of rac-lactide was also obtained in the solvent-
free polymerizations at 130 °C, showing that this class of
ligands is able to stabilize the metal center under drastic
reaction conditions, a result that is relevant not only from an
academic point of view but also for the potential industrial
applicability.
Additionally, the obtained PLAs were heterotactic with Pr
values in the range of 0.57−0.84, depending on the nature of
the initiator and solvent. A chain-end control mechanism
enhanced by solvent coordination at the active site was
recognized to be operating in these systems.
Table 3. ROP of rac-Lactide by Initiator 1 in the Presence of
2-Propanol
a
[iPrOH]/
b
c
d
[I]0
time
conv Mn(th)
Mn(GPC)
(× 103)
e
run
(μmol)
(min) (%)
(× 103)
PDI
Pr
In summary, these novel group 3 metal catalysts bearing the
anilidopyridyl-pyrrolide and anilidopyridyl-indolide ligands lead
to a promising combination of activity, controlled character,
and heteroselectivity in the polymerization of rac-lactide.
15
16
17
18
0
1
6
2
2
2
85
98
98
90
147.0
84.8
28.3
14.1
136.1
81.8
28.8
13.5
2.15
2.21
1.19
1.11
0.68
0.66
0.68
0.66
5
10
a
Conditions: [I0] = 1.7 mM; [rac-LA]/[I0] = 1200; 2 mL of THF;
EXPERIMENTAL SECTION
b
■
room temperature. Reaction times were not necessarily optimized.
c
General Considerations. All manipulations of air- and/or water-
sensitive compounds were carried out under a nitrogen atmosphere
using standard Schlenk or glovebox techniques. Glassware and vials
were dried in an oven at 135 °C overnight and exposed to a vacuum−
nitrogen cycle thrice before use. All solvents were purchased from
Carlo Erba. Toluene, THF, and CH2Cl2 were refluxed over sodium/
benzophenone or calcium hydride (CaH2) and then distilled under
nitrogen before use. 1-Bromo-2-isopropylbenzene was purchased from
Alfa Aesar without purification before use. Lanthanide precursors,18b,31
the anilinepyridinepyrrole compound (H2L1),13b and N-(t-butoxycar-
bonyl)-indole-2-boronic acid32 were prepared by literature procedures.
rac-Lactide (Aldrich) was purified by crystallization from dry toluene.
2-Propanol was purified by distillation over sodium. All other
chemicals was purchased from Sigma-Aldrich and used as received
unless otherwise stated.
Calculated Mn of PLA (g/mol) = 144.14 × ([rac-LA]/[I0 + iPrOH])
d
× conversion (rac-LA)%. Experimental Mn values (corrected using
the Mark−Houwink factor of 0.58) were determined by GPC analysis
e
in THF using polystyrene standards. Pr value is determined from the
1
methine region of the homonuclear decoupled H NMR spectrum.
alkoxide, generated by treatment of complex 1 with 2-propanol
in a 1:1 ratio, can polymerize 1200 equiv within 2 min at 20 °C.
The obtained turnover frequency (TOF: 3.5 × 104 mollactide
molY−1 h−1) is, to our knowledge, among the best values
reported so far for the room-temperature ROP of lactide with
metal-based initiatiors.28 The experimental Mn value was in
good agreement with the theoretical Mn value, while the
polydispersity index of the PLA did not change with respect to
the polymerization carried out in the absence of alcohol. As
expected, changing the initiating group from silylamido to
isopropoxide results in no change in the heterotacticity of the
obtained PLA. Increasing the amount of 2-propanol resulted in
immortal polymerization of rac-lactide,29 producing polymers
with molecular weight values proportional to that equivalent of
the 2-propanol added and narrow molecular weight distribu-
tions (PDI = 1.19 and 1.11 for 5 and 10 equiv of alcohol,
respectively). End group analysis of the obtained PLA samples
showed signals at 1.24 and 5.03 ppm attributable, according to
the literature,30 to the methyl and methine protons of the
−OCH(CH3)2, respectively (Figure S14 in the Supporting
Information). The signals of the silylamido group (−N-
(SiHMe2)2) were not detected. These observations confirmed
Instruments and Measurements. The NMR spectra were
recorded on a Bruker Avance 400 spectrometer (1H, 400.00 MHz;
13C, 100.62 MHz) at 25 °C, unless otherwise stated. Deuterated
solvents were purchased from Cambridge Isotope Laboratories, Inc.
and degassed and dried over activated 3 Å molecular sieves prior to
use. Chemical shifts (δ) are listed as parts per million and coupling
constants (J) in hertz. 1H NMR spectra are referenced using the
residual solvent peak at δ 3.58 and 1.73 ppm for THF-d8 and δ 7.26
ppm for CDCl3. 13C NMR spectra are referenced using the residual
solvent peak at δ 67.57 and 25.48 ppm for THF-d8 and δ 77.23 ppm
for CDCl3.
The molecular weights (Mn) and the molecular weight distribution
(Mw/Mn) of polymer samples were measured by GPC at 30 °C. THF
was used as the solvent, the flow rate of the eluant was 1.0 mL/min,
and narrow polystyrene standards were used as a reference. The
measurements were performed on a Waters 1525 binary system
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Organometallics
Article
equipped with a Waters 2414 RI detector using four Styragel columns
(range of 1000−1 000 000 Å). Every value was the average of two
independent measurements. It was corrected using the Mark−
Houwink factor of 0.58 according to the literature.33
(m, 9H, α-CH2, THF and CH(CH3)2), 3.32 (sept, 1H, CH(CH3)2),
2.80 (sept, 1H, CH(CH3)2), 1.77 (m, 8H, β-CH2,THF), 1.40 (d,
J(H,H) = 6.8 Hz, 3H, CH(CH3)2), 1.19 (d, J(H,H) = 6.8 Hz, 3H,
CH(CH3)2), 1.14 (d, J(H,H) = 6.8 Hz, 3H, CH(CH3)2), 1.09 (d,
J(H,H) = 6.8 Hz, 3H, CH(CH3)2), 0.46 (d, J(H,H) = 6.8 Hz, 3H,
CH(CH3)2), 0.01 (d, J(H,H) = 2.8 Hz, 6H, HSi(CH3)2), −0.06 (d,
J(H,H) = 6.8 Hz, 3H, CH(CH3)2), −0.10 ppm (d, J(H,H) = 2.8 Hz,
6H, HSi(CH3)2). 13C NMR (100.62 MHz, [D8]THF, 25 °C): δ =
169.98, 156.27, 152.90, 147.14, 146.89, 146.31, 146.06, 140.62, 139.43,
132.30, 131.02, 129.19, 127.02, 126.33, 125.67, 125.17, 124.44, 123.45,
116.55, 114.14, 110.91, 109.83, 74.09 (NCH), 68.38 (α-CH2, THF),
28.84, 28.42, 28.16, 26.54, 26.37, 24.71 (β-CH2, THF), 23.17(CH-
(CH3)2), 4.34 (HSi(CH3)2), 4.03 ppm (HSi(CH3)2). Elemental
analysis calcd (%) for C35H49N4Si2Y: C, 62.66; H, 7.36; N, 8.35.
Found: C, 62.58; H, 7.29; N, 8.27
X-ray Crystallographic Studies. Single crystals of 1 and 2,
suitable for X-ray analysis, were obtained from n-hexane solutions at
253 K. Data collection was performed in flowing N2 at 173 K on a
Bruker-Nonius κCCD diffractometer (Mo Kα radiation, CCD rotation
images, thick slices, φ scans + ω scans to fill the asymmetric unit). Cell
parameters were determined from 104 reflections in the range of
3.187° ≤ θ ≤ 21.225°, and 191 reflections in the range of 3.945° ≤ θ
≤ 20.708° for 1 and 2, respectively. Semiempirical absorption
corrections (multiscan SADABS)34 were applied. The structure was
solved by direct methods (SIR 97 package)35 and refined by the full-
matrix least-squares method (SHELXL program of SHELX97
package)36 on F2 against all independent measured reflections, using
anisotropic thermal parameters for all non-hydrogen atoms. H atoms
were placed in calculated positions with Ueq equal to those of the
carrier atom and refined by the riding method. For 1, a SiMe2 group
was affected by positional disorder, which was rationalized using
isotropic thermal parameters for the relative C atoms. All plots were
generated by using the program ORTEP-3.37
Synthesis of Complex 2. A solution of Sc[N(SiHMe2)]3(THF)
(113 mg, 0.22 mmol) in benzene (5 mL) was added dropwise into a
stirred solution of H2L1 (100 mg, 0.22 mmol) in benzene (10 mL).
Following by 1H NMR, the solution was stirred for overnight at
ambient temperature. All volatiles were removed under vacuum to
yield a yellow solid. The crude product was washed with pentane. The
1
light yellow solid was obtained in 78% yield. H NMR (400 MHz,
Crystal data and details of the data collections are reported in Table
S1 in the Supporting Information.
CCDC 843706 (1) and 848525 (2) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[D8]THF, 25 °C): δ = 7.50 (t, J(H,H) = 8.0 1H, H-Py), 7.27 (d,
J(H,H) = 8.0 Hz, 1H, H-Py), 7.08−6.84 (m, 8H, Ar-H and NC4H3),
6.64 (d, J(H,H) = 3.2 Hz, 1H, NC4H3), 6.37 (d, J(H,H) = 7.5 Hz, 1H,
H-Py), 6.05 (d, J(H,H) = 1.6 Hz, 1H, NC4H3), 5.76 (s, 1H, NCH),
4.78 (m, 2H, SiH), 3.60 (m, 4H, α-THF), 3.33 (sept, 1H, CH(CH3)2),
3.18 (sept, 1H, CH(CH3)2), 2.64 (sept, 1H, CH(CH3)2), 1.78 (m, 4H,
β-THF), 1.34 (d, J(H,H) = 6.7 Hz, 3H, CH(CH3)2), 1.16 (d, J(H,H)
= 6.7 Hz, 3H, CH(CH3)2), 1.13 (d, J(H,H) = 6.9 Hz, 3H,
CH(CH3)2), 1.07 (d, J(H,H) = 6.9 Hz, 3H, CH(CH3)2), 0.55 (d,
J(H,H) = 6.7 Hz, 3H, CH(CH3)2), 0.03 (d, J(H,H) = 6.7 Hz, 3H,
CH(CH3)2), −0.11 (d, J(H,H) = 2.8 Hz, 6H, HSi(CH3)2), −0.13 ppm
(d, J(H,H) = 2.7 Hz, 6H, HSi(CH3)2). 13C NMR (100.62 MHz,
[D8]THF, 25 °C): δ = 169.58, 155.97, 152.71, 147.54, 146.42, 145.89,
145.03, 140.55, 140.45, 133.75, 130.51, 127.51, 126.56, 125.90, 125.60,
124.00, 116.02, 113.90, 111.10, 109.36, 74.39 (NCH), 68.38 (α-CH2,
THF), 29.04, 28.97, 28.46, 26.54, 26.46, 24.87 (β-CH2, THF), 22.79
(CH(CH3)2), 2.86 (HSi(CH3)2), 2.15 ppm (HSi(CH3)2). Elemental
analysis calcd (%) for C35H49N4Si2Sc: C, 67.05; H, 7.88; N, 8.94.
Found: C, 67.11; H, 7.69; N, 8.87
Synthesis of Complex 3. A solution of Y[N(SiHMe2)]3(THF)2
(120 mg, 0.2 mmol) in benzene (5 mL) was added dropwise into a
stirred solution of H2L2 (100 mg, 0.2 mmol) in benzene (10 mL). The
solution was warmed to 50 °C and stirred for 2 hours. All volatiles
were removed under vacuum to yield a yellow solid. The crude
product was washed with pentane. The orange solid was obtained in
76% yield. 1H NMR (300 MHz, [D8]THF, 25 °C): δ = 7.77−7.70 (m,
2H, Ar-H), 7.59 (t, J(H,H) = 7.8, 1H, H-Py), 7.45 (d, J(H,H) = 7.8
Hz, 1H, H-Py), 7.10−6.75 (m, 10H, Ar-H), 6.55 (d, J(H,H) = 7.5 Hz,
1H, H-Py), 5.93 (s, 1H, NCH), 4.73 (m, 2H, SiH), 3.61 (m, 9H, α-
THF and CH(CH3)2), 3.36 (sept, 1H, CH(CH3)2), 2.78 (sept, 1H,
CH(CH3)2), 1.78 (m, 8H, β-THF), 1.40 (d, J(H,H) = 6.9 Hz, 3H,
CH(CH3)2), 1.21 (d, J(H,H) = 6.6 Hz, 3H, CH(CH3)2), 1.16 (d,
J(H,H) = 6.9 Hz, 3H, CH(CH3)2), 1.11 (d, J(H,H) = 6.9 Hz, 3H,
CH(CH3)2), 0.50 (d, J(H,H)= 6.6 Hz, 3H, CH(CH3)2), 0.04 (d,
J(H,H) = 2.8 Hz, 6H, HSi(CH3)2), −0.03 (d, J(H,H) = 6.9 Hz, 3H,
CH(CH3)2), −0.10 ppm (d, J(H,H) = 2.8 Hz, 6H, HSi(CH3)2). 13C
NMR (75.47 MHz [D8]THF, 25 °C): δ = 170.62, 156.13, 152.97,
147.32, 147.10, 146.56, 145.91, 139.55, 132.30, 131.23, 127.26, 126.47,
125.88, 125.25, 124.67, 123.74, 121.37, 121.03, 119.79, 118.46, 117.69,
116.96, 101.52, 74.26 (NCH), 68.23 (α-CH2, THF), 28.90, 28.46,
28.41, 28.36, 26.54, 26.34, 24.43 (β-CH2, THF), 23.06 (CH(CH3)2),
4.63 (HSi(CH3)2), 3.90 (HSi(CH3)2). Elemental analysis calcd (%) for
C39H51N4Si2Y: C, 64.97; H, 7.13; N, 7.77. Found: C, 64.82; H, 7.01;
N, 7.69
Synthesis of Proligands. Synthesis of H2L2. n-Butyl lithium
(1.45 mL, 3.63 mmol, 2.5 M in hexane) was added dropwise to a
solution of 1-bromo-2-isopropylbenzene (678 mg, 3.41 mmol) in dry
diethyl ether (10 mL) at 0 °C. The colorless solution was warmed to
room temperature and stirred for 3 h. The solution was then added
dropwise to a dry diethyl ether (5 mL) solution of compound B (434
mg, 1.14 mmol) at −78 °C. The yellow solution was warmed to room
temperature and stirred for 30 min. The color turned to red. The
reaction was followed by TLC and then quenched with NH4Cl(aq) at
0 °C. The organic phase was separated and reserved. The aqueous
phase was washed with diethyl ether (3 × 30 mL). The combined
organic phases were extracted with water (2 × 30 mL) and brine (1 ×
30 mL). The organic phase was dried over Na2SO4. The solvent was
distilled off by rotary evaporation. The crude product was purified by
flash column chromatography on silica gel using hexane/diethyl ether
(20/1) as the eluent. The colorless oil was concentrated under
1
vacuum, affording a yellow solid (yield: 88%). H NMR (400 MHz,
CDCl3, 25 °C): δ = 8.97 (s, 1H, indole-NH), 7.71−7.68 (m, 1H, Ar-
H), 7.64 −7.57 (m, 3H, Ar-H), 7.37 (d, J(H,H) = 8 Hz, 1H, Py-H),
7.39−7.28 (m, 3H, Ar-H), 7.21 (m, 1H, Ar-H), 7.11 (d, J(H,H) = 1.2
Hz, 1H, Ar-H), 7.07 (s, 3H, Ar-H), 6.96 (m, 2H, Ar-H), 5.52 (s, 1H,
NCH), 4.19 (br, 1H, NH), 3.02 (sept, 1H, CH(CH3)2), 2.89 (sept,
2H, CH(CH3)2), 1.05 (d, J(H,H) = 7 Hz, 6H, CH(CH3)2), 0.98 (d,
J(H,H) = 7 Hz, 3H, CH(CH3)2), 0.96 ppm (m, 9H, CH(CH3)2). 13
C
NMR (100.62 MHz, CDCl3, 25 °C): δ = 162.48, 149.53, 146.56,
142.96, 142.56, 139.43, 137.30, 136.87, 136.45, 129.31, 127.72, 127.47,
126.17, 125.81, 123.71, 123.63, 123.27, 121.33, 120.26, 119.74, 117.97,
111.54, 100.43, 66.18(NCH), 28.82, 28.00, 24.01, 24.05, 23.97, 24.30
ppm. Elemental analysis calcd (%) for C35H39N3: C, 83.79; H, 7.84; N,
8.38. Found: C, 83.22; H, 7.51; N, 8.29
Synthesis of Complex 1. A solution of Y[N(SiHMe2)]3(THF)2
(315 mg, 0.5 mmol) in benzene (5 mL) was added dropwise into a
stirred solution of H2L1 (226 mg, 0.5 mmol) in benzene (10 mL).
Following by 1H NMR, the solution was stirred for overnight at
ambient temperature. All volatiles were removed under vacuum to
yield a yellow solid. The crude product was washed with pentane. The
1
light yellow solid was obtained in 89% yield. H NMR (400 MHz,
[D8]THF, 25 °C): δ = 7.42 (t, 1H, J(H,H) = 8.0 Hz, H-Py), 7.27 (d,
J(H,H) = 8.0 Hz, 1H, H-Py), 7.12 (dd, 1H, J(H,H) = 8.0 Hz, J(H,H)
= 5.2 Hz, NC4H3), 7.08−6.79 (m, 7H, Ar-H), 6.69 (dd, 1H, J(H,H) =
8 Hz, J(H,H) = 1.2 Hz, NC4H3), 6.31 (d, J(H,H) = 7.6 Hz, 1H, H-Py),
6.09 (m, 1H, NC4H3), 5.86 (s, 1H, NCH), 4.56 (m, 2H, SiH), 3.58
General Polymerization Procedures. A 12 mL vial in a Braun
Labmaster glovebox was charged sequentially with a solution of rac-
lactide in an appropriate ratio in 1.5 mL of dry solvent, and a solution
of the required metal initiator in 0.5 mL of dry solvent was added
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Organometallics
Article
rapidly. The mixture was immediately stirred with a magnetic stir bar
at the desired temperature for the desired time. After a certain time, an
aliquot of the reaction mixture was sampled with a pipet for
determining the monomer conversion by 1H NMR spectroscopy
(CDCl3, 400 MHz). The reaction mixture was quenched by adding
wet n-hexane. The polymer was filtered and dried in a vacuum oven at
60 °C for 16 h.
(c) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Macromol. Rapid
Commun. 2007, 28, 693−697. (d) Bouyahyi, M.; Ajellal, N.; Kirillov,
E.; Thomas, C. M.; Carpentier, J.-F. Chem.Eur. J. 2011, 17, 1872−
1883. (e) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen,
X.; Jing, X. Organometallics 2007, 26, 2747−2757.
(9) Non-bisphenolate yttrium catalysts for stereoselective ROP of
rac-lactide: (a) Arnold, P. L.; Buffet, J. C.; Blaudeck, R. P.; Sujecki, S.;
Blake, A. J.; Wilson, C. Angew. Chem. 2008, 120, 6122−6125;(b)
Angew. Chem., Int. Ed. 2008, 47, 6033−6036. (c) Platel, R. H.; White,
A. J. P.; Williams, C. K. Chem. Commun. 2009, 4115−4117. (d) Zhang,
Z.; Cui, D. Chem.Eur. J. 2011, 17, 11520−11526.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Representative H and 13C NMR spectra for complexes 1−3,
(10) (a) Mazzeo, M.; Lamberti, M.; D’Auria, I.; Milione, S.; Peters, J.
C.; Pellecchia, C. J. Polym. Sci., Part A-1: Polym. Chem. 2010, 48,
1374−1382. (b) D’Auria, I.; Mazzeo, M.; Pappalardo, D.; Lamberti,
M.; Pellecchia, C. J. Polym. Sci., Part A-1: Polym. Chem. 2011, 49, 403−
413.
certain intermediates, and some polymers and a table and CIF
files giving X-ray crystallographic data for complexes 1 and 2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) Piers, W. E.; Emslie, D. J. H. Coord. Chem. Rev. 2002, 233−234,
129.
AUTHOR INFORMATION
■
(12) (a) Estler, F.; Eickerling, G.; Herdtweck, E.; Anwander, R.
Organometallics 2003, 22, 1212−1222. (b) Jie, S.; Diaconescu, P. L.
Organometallics 2010, 29, 1222−1230. (c) Lu, E.; Gan, W.; Chen, Y.
Organometallics 2009, 28, 2318−2324.
(13) (a) Annunziata, L.; Pappalardo, D.; Tedesco, C.; Pellecchia, C.
Macromolecules 2009, 42, 5572−5578. (b) Li, G.; Lamberti, M.;
D’Amora, S.; Pellecchia, C. Macromolecules 2010, 43, 8887−8891.
(14) (a) Pappalardo, D.; Annunziata, L.; Pellecchia, C.; Biesemans,
M.; Willem, R. Macromolecules 2007, 40, 1886−1890. (b) Pappalardo,
D.; Annunziata, L.; Pellecchia, C. Macromolecules 2009, 42, 6056−
6062.
Corresponding Author
*Fax: (+39) 089969562. E-mail: mlamberti@unisa.it.
ACKNOWLEDGMENTS
■
The authors thank Dr. Patrizia Iannece for elemental analyses
and for electrospray ionization-mass spectrum analyses, Dr.
Patrizia Oliva for NMR assistance, and Dr. Mariagrazia Napoli
for GPC measurements. G.R. thanks C.RdC “Nuove
̀
Tecnologie per le Attivita Produttive” of Campania Region
(Italy) for use of the Bruker-Nonius X-ray equipment.
(15) (a) Xu, X.; Chen, Y.; Zou, G.; Sun, J. Dalton Trans. 2010, 39,
3952−3958. (b) Doring, C.; Kempe, R. Eur. J. Inorg. Chem. 2009,
̈
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