Phosphasalen yttrium complexes: Highly active and stereoselective initiators for lactide polymerization

Article abstract of DOI:10.1021/ic202015z

Preparation and characterization of three yttrium alkoxide complexes with new phosphasalen ligands are reported. The phosphasalens are analogues of the well-known salen ligands but with iminophosphorane donors replacing the imine functionality. The three yttrium alkoxide complexes show mono- and dinuclear structures in the solid state, depending on the substituents on the ligand. The new ligands and complexes are characterized using multinuclear NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction experiments. The complexes are all rapid initiators for lactide polymerization; they show excellent polymerization control on addition of exogeneous alcohol. The mononuclear complex shows extremely rapid rates and a high degree of stereocontrol in rac-lactide polymerization, yielding heterotactic PLA (P _s of 0.9). The phosphasalens are, therefore, excellent ligands for lactide ring-opening polymerization catalysis showing superior rates and stereocontrol versus salen ligands, which may be related to their excellent donating ability and the high degrees of steric protection they can confer.

Full text of DOI:10.1021/ic202015z

Article
pubs.acs.org/IC
Phosphasalen Yttrium Complexes: Highly Active and Stereoselective
Initiators for Lactide Polymerization
Thi-Phuong-Anh Cao,† Antoine Buchard,‡ Xavier F. Le Goff,† Audrey Auffrant,*,†
and Charlotte K. Williams*,‡
†Laboratoire Het
́ ́ ́ ́
eroelements et Coordination, Ecole Polytechnique, CNRS, 91128 Palaiseau Cedex, France
‡Department of Chemistry, Imperial College London, London, SW7 2AZ, United Kingdom
S
* Supporting Information
ABSTRACT: Preparation and characterization of three yttrium
alkoxide complexes with new phosphasalen ligands are reported.
The phosphasalens are analogues of the well-known salen ligands
but with iminophosphorane donors replacing the imine function-
ality. The three yttrium alkoxide complexes show mono- and
dinuclear structures in the solid state, depending on the substituents
on the ligand. The new ligands and complexes are characterized
using multinuclear NMR spectroscopy, mass spectrometry,
elemental analysis, and single-crystal X-ray diffraction experiments.
The complexes are all rapid initiators for lactide polymerization;
they show excellent polymerization control on addition of
exogeneous alcohol. The mononuclear complex shows extremely
rapid rates and a high degree of stereocontrol in rac-lactide
polymerization, yielding heterotactic PLA (Ps of 0.9). The phosphasalens are, therefore, excellent ligands for lactide ring-opening
polymerization catalysis showing superior rates and stereocontrol versus salen ligands, which may be related to their excellent
donating ability and the high degrees of steric protection they can confer.
initiators for ring-opening polymerizations, we prepared several
new yttrium alkoxide complexes with the phosphasalen ligand;
these complexes are studied for lactide polymerization, where
they show superior properties to the salen analogues.
INTRODUCTION
■
The tetradentate Schiff base ligand N,N-bis(salicylidine)-
ethylenediamine or salen is a ubiquitous ancillary ligand in
catalysis.1 Perhaps the most well-known application of this
ligand is in the metal-catalyzed asymmetric epoxidation and
kinetic resolution of epoxides.1a−c The salen ligand class has
also been very successfully applied in polymerization catalysis,
for example, in the stereoselective ring-opening polymerization
of lactide2 and epoxides,3 and copolymerization of epoxides and
carbon dioxide.4 Optimization of the catalytic activity and
selectivity has led to new generations of salen ligands, where
changes to the diimine linker, phenol ring substituents, and
even reduction of the one/both of the imine groups to amine
(so-called salan ligands) has been achieved.4a−f This is a fruitful
area for catalysis but also a crowded one. Surprisingly, there
have been very few reports of heteroatom substitution on the
backbone of the salen ligand.5 On the other hand,
iminophosphorane moieties are now quite well-established
ligands in coordination chemistry and even in catalysis.6,7
Iminophosphorane ligands are of interest because, in contrast
to imine moieties, they lack π-accepting ability due to the
absence of any π system. Therefore, this ligand class behaves as
strong σ and π donors due to the presence of two lone pairs on
the N atom. Some of us have already reported the preparation
of phosphasalen complexes of Ni(II) and Pd(II) which differ
from conventional salen complexes in both their electronic and
their steric properties.8 Motivated by interest in developing new
Ring-opening polymerization of lactide (LA) is an interesting
process not the least of which is because the product,
polylactide (PLA), is a degradable polymer suitable for
applications in medicine and as a degradable commodity
plastic.9 The monomer, lactide, derives from lactic acid which is
produced by fermentation of sugars, produced from high starch
content plants. Therefore, PLA is widely investigated and
commercially produced as a sustainable alternative to
petrochemicals. The most efficient synthesis, and the
commercial route to PLA, involves ring-opening polymerization
of lactide.9d,10 This can be accomplished by a wide range of
metal complexes, most usually metal alkoxide or amido species,
or by organocatalysts and even by enzymes.10,11 Application of
metal catalysts is attractive as it can lead to high rates, good
degrees of polymerization and stereocontrol, and production of
block copolymers.9d,10a Selection of the metal catalyst is critical
in controlling the properties of PLA. Yttrium and other Group
3 complexes are particularly promising as they exhibit high rates
and, through careful selection of the ancillary ligand, can enable
stereocontrol.2b,12 Yttrium salen complexes were first inves-
Received: September 14, 2011
Published: February 7, 2012
© 2012 American Chemical Society
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a
tigated nearly a decade ago by Ovitt and Coates; they showed
moderate rates but lack polymerization or tacticity con-
trol.2b,12ad Carpentier and colleagues also studied alkoxide/
phenolate ligands, related to salens, which were slow initiators
and produced atactic PLA.12q Recently, Diaconescu and co-
workers reported a salen derivative with a ferrocenediyl unit in
the imine backbone; it showed faster rates.12r Phenolate−amine
ligands have generally proved quite effective; for example, the
groups of Carpentier, Mountford, and others have widely
investigated application of amino−diphenolate yttrium initia-
tors, which show good/excellent rates and high degrees of
stereocontrol.12a−h,13 Okuda pioneered the use of dithia−
alkanediyl-bridged diphenolate Group 3 complexes, some of
which showed excellent heteroselectivity via a dynamic
monomer recognition process controlled by interconversion
of initiator enantiomers.12s−u The complexes also yield highly
syndiotactic PLA from meso-LA.12n Arnold and co-workers
recently reported an interesting initiating system comprising a
racemic mixture of alkoxide−phosphine oxide yttrium com-
plexes, which yielded isotactic PLA from rac-LA.12w,x Finally,
our research group prepared bis(phosphinic/thiophosphinic)-
diamido yttrium complexes which show very high rates of
polymerization and, in some cases, good heteroselectivity (Ps =
0.8).12i−m
Scheme 2. Synthesis of Phosphasalen−Yttrium Complexes
RESULTS AND DISCUSSION
■
a
Reagents and conditions: (i) [YCl3(THF)3.5] (1 equiv), THF, 298 K,
1−2 h, then t-BuOK (1 equiv) or EtOK (1 equiv), 298 K, 7 h.
Labeling schemes of the complexes are used for NMR characterization.
Synthesis and Characterization of Yttrium−Phospha-
salen Complexes. Our previous lactide polymerization
catalyst development work utilized phosphorus-substituted
ancillary ligands, in particular, phosphinic amido ligands and
iminophosphoranes.7,12i−m These showed high rates for lactide
ring-opening polymerization. It was, therefore, of interest to
continue to develop this type of ligand and study its catalytic
polymerization activity. Given the ubiquity and utility of salen
ligands, we were interested in targeting an iminophosphorane
analogue (phosphasalen) of this ligand class. Two new
phosphasalen ligands were prepared (Scheme 1).
issues) using potassium hexamethyldisilazide. This synthetic
strategy is quite useful as it produces high yields of pure
product and is compatible with a range of substituents on the
phenoxide ring or diamine backbone. Hence, from the related
phosphinophenol, the phosphasalen ligands L1 and L2 were
obtained, respectively, in 64 and 55% yield overall within two
steps.
Reaction of ligand L1 with [YCl3(THF)3.5] in tetrahydrofur-
an resulted in immediate precipitation of a white product. The
31P{1H}NMR spectrum showed complete consumption of
ligand, as evidenced by the disappearance of the signal at 18.6
ppm. The total insolubility of the product prevented any further
analysis; the product is believed to be a cluster/polymer of the
ligand−yttrium−chloride. A similar effect has been observed by
Diaconescu and co-workers using related yttrium chloride
Schiff base complexes.5a,12r Despite the insolubility of the
product, addition of an equivalent of potassium tert-butoxide/
ethoxide into the reaction mixture induced slow dissolution.
The 31P{1H}NMR spectrum of the solution presented a single
signal at significantly lower field compared to the dianionic
ligand, indicating coordination of the phosphasalen ligand to
the yttrium center. After removal of the potassium salt, the
ethoxide and tert-butoxide phosphasalen−yttrium complexes
(complexes 1 and 2, respectively) were isolated as white solids,
which were characterized by multinuclear NMR spectroscopy,
elemental analyses, and single-crystal X-ray diffraction experi-
ments (Table 1).
a
Scheme 1. Synthesis of Phosphasalen Ligands
a
Reagents and conditions: (i) Br2 (1 equiv), CH2Cl2, 45 min, then
ethylenediamine (0.5 equiv), Bu3N (1 equiv), 16 h; (ii) KHMDS (4
equiv), THF, 298 K, 16 h.
Both complexes 1 and 2 adopt a dimeric structure in the
solid state (Figures 1 and 2 and Tables 2 and 3) with the
yttrium centers exhibiting a distorted octahedral geometry
formed by the tetradentate phosphasalen ligand, the alkoxide
group, and a phenoxide/alkoxide bridge. Both dimers are
symmetric: there is no difference between the two yttrium
The diphenoldiphosphonium salts S1 and S2 were obtained
by a Kirsanov reaction between ethylenediamine and o-
diphenylphosphinophenol, Scheme 2. They were deprotonated
in essentially quantitative yield (except for L2 due to solubility
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Table 1. Crystallographic Data for Compounds 1−3
1
2
3
formula
Mr
C80H74N4O6P4Y2, 2(C7H8)
1673.40
C84H82N4O6P4Y2,3(C7H8)
1821.64
P-1
C58H73N2O3P2Y
997.03
space group
T (°C)
λ (Å)
P-1
C2/c
−123
0.71069
−123
−123
0.71069
10.530(1)
14.575(1)
16.430(1)
65.436(1)
87.959(1)
89.298(1)
2291.9(3)
1
0.71069
20.514(1)
27.064(1)
22.120(1)
90.00
a (Å)
12.950(1)
13.948(1)
14.744(1)
102.175(1)
108.382(1)
112.817(1)
2153.9(3)
1
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
117.231(1)
90.00
10919.7(8)
8
d (g cm−3
)
1.290
1.320
1.213
μ (cm−1
)
1.469
1.387
1.170
a
R1
0.0539
0.0562
0.0458
b
wR2
0.1368
0.1134
0.0813
CCDC no.
836603
836604
836605
a
b
2
2
2
R1 = ∑∥Fo| − |Fc∥/∑|Fo|. wR2 = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2; w−1 = σ2(Fo ) + (aP)2 + bP.
Figure 2. ORTEP view of the solid-state structure of complex 2, with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
Figure 1. ORTEP view of the solid-state structure of complex 1, with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
than that in the equatorial position (Y−O1) (2.213(3) vs
2.182(3) Å), whereas the Y−O(ethoxide) (Y−O3#2) bond in
the axial position is slightly shorter than that in the equatorial
site (Y−O3) (2.243(3) vs 2.273(3) Å). Both yttrium−alkoxide
bonds are longer than the yttrium−phenoxide bonds (2.243(3)
and 2.273(3) Å vs 2.182(3) and 2.213(3) Å), probably because
of their bridging coordination modes. There is almost no
difference between the bond lengths of Y−N1 and Y−N2 nor
between N1−P1 and N2−P2, respectively. However, the
distance Y−P1 is significantly longer than Y−P2 (3.561(1) vs
3.414(1) Å), and the Y−N1−P1 angle is larger than Y−N2−P2
centers in either complex; thus, all selected bond lengths and
angles are represented for only one yttrium center.
In complex 1, the ethoxide groups bridge between the two
metal centers, forming a [Y2O2] four-membered ring (Figure
1). The ligand adopts a cis-β-configuration around the metal:
one phenoxide (O2) and one bridging ethoxide group (O3#2)
occupy axial coordination sites at the yttrium center. The
yttrium center and the other four coordinated atoms (O1, N1,
N2, O3) are almost coplanar (dihedral angle (O1, N1, N2, O3)
= 1.52(22)°, (Y, N1, O1, O3) = 0.11(25)°). The yttrium−
O(phenoxide) (Y−O2) bond, in the axial site, is slightly longer
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Table 2. Selected Bond Lengths (Angstroms) and Angles (degrees) for Compound 1
Y1−Y1#2
Y1−O1
Y1−O2
Y1−O3
Y1−O3#2
Y1−N1
3.6492(7)
2.182(3)
2.213(3)
2.273(3)
2.243(3)
2.415(3)
Y1−N2
P1−N1
P2−N2
O1−Y1−N1
O2−Y1−N1
N1−Y1−N2
2.438(3)
1.598(3)
1.595(3)
78.8(1)
N2−Y1−O2
O1−Y1−O2
O2−Y1−O3
O2−Y1−O3#2
N1−Y1−N2−O2
O1−Y1−N2−N1
81.4(1)
97.1(1)
90.6(1)
156.1(1)
86.40(11)
4.05(25)
90.5(1)
70.4(1)
Table 3. Selected Bond Lengths (Angstroms) and Angles (degrees) for Compound 2
Y1−O1
Y1−O2
Y1−O3
Y1−O2#2
Y1−N1
Y1−N2
P1−N1
2.203(3)
2.338(2)
2.079(3)
2.312(2)
2.398(3)
2.372(3)
1.596(3)
P2−N2
1.586(3)
78.6(1)
O2#2−Y1−O3
148.8(1)
9.39(24)
6.29(26)
88.49(11)
82.27(10)
O1−Y1−N1
O2−Y1−N1
N1−Y1−N2
N2−Y1−O2
O1−Y1−O2
O2−Y1−O3
Y1−O2−N2−N1
Y1−O1−N1−N2
O1−N1−Y1−O3
O1−N1−Y1−O2#2
143.7(1)
70.4(1)
74.9(1)
133.0(1)
89.2(1)
Figure 3. 31P{1H} NMR spectra of complexes 1 and 2 recorded in THF on (a) 300 and (b) 500 MHz instruments.
(123.81(17)° vs 114.05(17)°). These differences clearly show
that the two phosphorus atoms are in slightly different
coordination environments, in accordance with them being
linked to two phenoxide groups in differing environments (axial
vs equatorial positions).
Complex 2 is also a dimer in the solid state; however,
changing the alkoxide coligand from ethoxide to tert-butoxide
changes the structure of the complex (Figure 2). The two
yttrium centers remain in distorted octahedral geometries, but
the tert-butoxide ligands now occupy the axial positions,
presumably due to steric hindrance preventing them from
adopting bridging coordination modes. The complex adopts a
trans coordination geometry with a single μ-phenoxide
coordination mode per ligand. The Y−N and P−N bond
lengths are comparable to those in complex 1, but the Y−O(μ-
phenoxide) bond is longer (2.338(2) vs 2.203(3) Å) and the
Y−O(alkoxide) bond is significantly shorter (2.079(3) vs
2.243(3) Å). Although the two phenoxide oxygen atoms adopt
different coordination modes, the two phosphorus atoms are in
almost identical environments, as shown by close similarity in
As a consequence, the 31P{1H} NMR spectrum of complex 1
in THF recorded on a 500 MHz instrument exhibits two signals
of equal intensity, very close to each other (Figure 3). These
signals are not part of a doublet as they are separated by 25 Hz,
2
which is significantly larger than previously reported JYP
coupling constants for similar complexes.5a,12i−m,14 When the
spectrum was recorded on a lower frequency (300 MHz)
instrument, the signals were not resolved and merged into a
single broad signal.
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the P−Y distances (3.490(1) vs 3.507(1) Å) and the P−N−Y
bond angles (121.55(18)° vs 122.48(18)°). Indeed, in the
31P{1H} NMR spectrum of complex 2, in THF, only a single
doublet is observed due to coupling with the yttrium center.
The value of the coupling constant (2JYP = 3.4 Hz) is
and yttrium−oxygen bonds in complex 3 are slightly shorter
compared to the corresponding values in complexes 1 and 2,
probably as a result of the pentacoordination mode.
Lactide Ring-Opening Polymerization. All three com-
plexes were highly active initiators for ring-opening polymer-
ization (ROP) of lactide, Table 5. Selection of the polymer-
ization solvent was critical, with initiator decomposition
occurring in methylene chloride and the partial solubility of
lactide in toluene (at ambient temperature) leading to
heterogeneous polymerization, albeit extremely rapidly. The
optimum solvent for use at ambient temperature (25 °C) was
THF; all experiments were conducted using an initial
concentration of lactide of 1 M, so as to enable accurate
comparison between the different initiators and additives.
Initiator 3. Complex 3 is extremely active; it is even able to
polymerize unpurified lactide. Near quantitative conversion is
reached within 2 min at low initiator loadings ([3] = 1 mM).
The polymerization rate was analyzed by taking aliquots; the
pseudo-first-order rate constant, kobs, was 0.08 s−1 at [3] = 1
mM (Figure 6). The rate of polymerization is thus among the
highest values reported for this reaction, easily comparing with
related yttrium systems and outstripping many other metal
centers.9d,10 Figure 5 and Table 6 compare the polymerization
results with other yttrium salen initiators.
Initiator 3 is significantly faster at lower loadings than
comparable yttrium salen analogues. Although caution should
be applied in comparing values as the experiments were not run
under identical conditions, it is clear that initiator 3 shows
remarkably enhanced rates, even under significantly lower
loadings than literature yttrium complexes, Figure 5. For
example, complex 62d took 14 h to reach complete conversion
at 100:1 loading of lactide:initiator, complex 513c took 1 h to
reach complete conversion at a loading of 300:1, and the
ferrocenyl salen complex 412r took 40 min to reach high
conversions at a loading of 500:1. In contrast, complex 3 was
able to completely polymerize as many as 5000 equiv of lactide
in 30 min and 1000 equiv in less than 1 min. It is also notable
that initiator 3 can be used at rather low loadings, down to
[LA]:[3] = 5000, which is useful for production of high
molecular weight PLA (up to 700 kg/mol). Finally, it is
apparent that the other yttrium salen complexes do not allow
significant stereocontrol in rac-lactide ROP; in contrast,
complex 3 enables a high degree of heteroselectivity in rac-
LA ROP.
2
comparable with other reported values for JYP in similar
complexes.12i−k,m This provides some indication that the solid-
state structure may be maintained in solution.
Complex 3 was prepared from a phosphasalen ligand with
sterically hindered tert-butyl substituents on the phenoxide ring
and with a tert-butyl alkoxide coligand (Scheme 2). This ligand
was deliberately targeted so as to prevent dimerization and
enable isolation of a mononuclear yttrium alkoxide complex.
The 31P{1H} NMR spectrum, in THF, showed a single doublet,
at 31.6 ppm, with a coupling constant of 3.0 Hz, suggesting that
complex 3 adopts a trans configuration in an analogous manner
to complex 2. On examination of the NMR spectra of
complexes 2 and 3 some clear differences were observed. In
particular, the 13C{1H} NMR spectra showed that the
quaternary phenolate carbon atoms are equivalent in the
spectrum of compound 3, whereas they are different in the case
of compound 2 (and compound 1), giving rise to two doublets,
likely due to the bridging and nonbridging coordination modes
of the phenolate. This supported the notion that complex 3 has
a mononuclear structure. Definitive evidence concerning the
structure of complex 3 was obtained by X-ray diffraction
analysis of monocrystals grown from a saturated solution of 3 in
cyclohexane (Figure 4 and Table 4). The complex adopts
Figure 4. ORTEP view of the solid-state structure of complex 3 with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
The LA ROP using initiator 3 is quite well controlled with
the number-averaged molecular weight (Mn) of the PLA
increasing linearly with percentage conversion (Figure 7) and
polydispersity indices remaining narrow throughout polymer-
ization (<1.4). The experimentally determined values for the
number-averaged molecular weight exceed those calculated (on
the basis of initiator concentration and conversion). This could
be due to either incomplete initiation from the tert-butyl
alkoxide groups or partial deactivation of the initiator. In order
square-based pyramidal geometry, with the four coordinating
atoms of the phosphasalen ligands forming the basal plane, as
shown by the dihedral angle O1−N1−N2−O2 (4.13(0.11)°).
The yttrium center is slightly out of the medial plane of these
four atoms (0.7472 (0.0011) Å). The angle at O3 atom is close
to 180°, which probably results from the steric hindrance of the
phosphasalen ligand in the basal plane. All yttrium−nitrogen
Table 4. Selected Bond Lengths (Angstroms) and Angles (degrees) for Compound 3
Y1−O1
Y1−O2
Y1−O3
Y1−N1
Y1−N2
P1−N1
2.1671(318)
2.1704(18)
2.025(2)
P2−N2
1.584(2)
80.51(7)
139.87(8)
71.78(8)
79.68(7)
102.03(7)
O2−Y1−O3
O1−Y1−O3
C55−O3−Y1
O1−N1−N2−O2
O1−N1−N2−Y1
109.16(8)
115.03(8)
171.40 (19)
4.13(0.11)
25.13(0.10)
O1−Y1−N1
O2−Y1−N1
N1−Y1−N2
N2−Y1−O2
O1−Y1−O2
2.375(2)
2.324(2)
1.595(2)
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Table 5. Data for the ROP of Lactide Using Yttrium Initiators 1−3 [LA]0 = 1 M
a
b
c
c
d
e
initiator (I) [LA]0:[I] or [LA]0:[I]:[iPrOH]
time
conversion /% Mn(calcd) /kg mol−1 Mn(exp) /kg mol−1 PDI
Ps
kobs
f
f
1
2
3
3
3
3
3
3
200:1
3360 s(56 min)
90
91
98
97
80
99
99
98
25.9
26.2
28.8
140
115
144
288
720
27.1
57.6
64.6
223
105
332
330
700
1.36
0.78
0.78
0.86
0.86
0.90
0.87
0.87
0.88
0.0031
0.2522
200:1
40 s
5 s
1.42
1.20
1.34
1.08
1.31
1.35
1.23
l
l
l
200:1
g
1000:1
1000:1:1
1000:1
2000:1
5000:1
45 s
70 s
0.0799
h
i
k
120 s
k
10 min
j
k
30 min
a
Conversions were determined from the 1H NMR spectra (CDCl3) of the crude products by integration of the methine resonance assigned to LA (δ
b
c
= 4.92 ppm) and PLA (δ = 5.00−5.30 ppm). Mn(calcd) = 144[LA]0/[I] × % conversion LA. Mn(exp) was determined using GPC, in THF, using
multiangle laser light scattering (GPC-MALLS) according to the method described earlier.12i,j The polydispersity index was also determined from
d
GPC, PDI = Mw/Mn. Ps is the probability of racemic linkages between monomer units and is determined by comparison of the integrals of tetrads in
the homonuclear decoupled 1H NMR spectrum with those calculated using Bernouillan statistics according to the method described by Coudane et
al.15 Determined from the gradients of the ln{[LA]0/[LA]t} (for first-order reaction) or 1/[LA]t (for second-order reaction) versus time plots.
e
f
g
h
Expressed in M−1s−1. The reaction is second order in lactide. Expressed in s−1. The reaction is first order in lactide, [I] = 1 mM. Reaction using
i
j
k
unpurified lactide. Slurry of 2 mmol of lactide in 1 mL THF at 25 °C. Slurry of 5 mmol of lactide in 1 mL THF at 25 °C. Reaction time
unoptimized. This experiment showed a rapid increase of PDI and decrease of Ps over time.
l
Figure 5. Relevant reported yttrium salen and semisalen initiators.2d,12q,r,13c
PDI being below 1.10 in almost all samples (Figure 7). These
results indicate that isopropyl alcohol is a viable chain transfer
agent and a more effective initiator. Examination of the
MALDI-ToF MS of the PLA produced from the initiating
system comprising 3 and iPrOH shows two series of chains,
one end-capped with tert-butoxyester and hydroxyl groups and
the other end capped with isopropylester and hydroxyl groups
(Figure S7 of the Supporting Information).
VT-NMR experiments were carried out to investigate
formation of a yttrium isopropoxide complex upon addition
of isopropanol (1 equiv) and its exchange with the alcohol in
solution; detailed results are presented in the Supporting
Information (Figures S12−15). Formation of an LYOiPr
complex and its rapid exchange with free tBuOH was
demonstrated. As isopropoxide is a better initiating group
than tert-butoxide, the equilibrium between LYOiPr and
LYOtBu is driven toward formation of the isopropoxide
complex during reaction. Moreover, HOtBu is a poor chain
transfer agent. Taking into account all data, polymerizations are
expected to be dominated by the yttrium isopropoxide initiator.
Initiators 1 and 2. Complexes 1 and 2 also showed good
activity for the ROP of lactide, although they were somewhat
slower than complex 3. Comparing the time taken to reach
complete conversion at 5 mM initiator loading complex 1
required 3500 s, 2 took 40 s, and 3 needed just 5 s.
Polymerizations using 1 and 2 were analyzed by taking aliquots
throughout polymerizations.
Analysis of the kinetics for polymerization was undertaken
using initiator 2 (Figure 8). This species showed complex
kinetic behavior, with a second-order dependence on lactide
concentration and a third-order dependence on yttrium
concentration. Such data are indicative of substantial
Figure 6. ln{[LA]0/[LA]t} versus time for LA ROP using initiator 3.
Polymerization conditions: [LA]0 = 1 M, [3] = 1 mM, THF, 25 °C.
to test this and to bring the experimental Mn close to its
calculated values, an experiment was carried out where an
equivalent of isopropyl alcohol was added. A yttrium isopropyl
alkoxide complex is expected to be a better initiator due to its
reduced steric hindrance and is also a good model for the
secondary alkoxide propagating species. Addition of an
equivalent of isopropyl alcohol led to improved control, as
evidenced by the excellent agreement between the exper-
imentally determined and the calculated values for Mn and the
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Table 6. Data for the ROP of Lactide Using Yttrium Initiators 3−6
a
b
c
c
d
initiator (I)
[LA]0:[I]
time
45 s
conversion / %
Mn(calcd) /kg mol−1
Mn(exp) /kg mol−1
PDI
Ps
3
1000:1
2000:1
5000:1
500:1
300:1
100:1
100:1
97
140
288
720
72
223
330
700
93
1.34
1.35
1.23
1.13
1.64
0.86
0.87
0.88
e
g
g
3
f
10 min
30 min
40 min
60 min
14 h
99
3
98
412r
513c
62d
90
>98
>98
80
43
37
0.65
0.72
712q
3 days
12
10
1.18
a
Conversions were determined from the 1H NMR spectra (CDCl3) of the crude products by integration of the methine resonance assigned to LA (δ
b
c
= 4.92 ppm) and PLA (δ = 5.00−5.30 ppm). Mn(calcd) = 144[LA]0/[I] × % conversion LA. Mn(exp) was determined using GPC, in THF, using
multiangle laser light scattering (GPC-MALLS) according to the method described earlier.12i,j The polydispersity index was also determined from
d
GPC, PDI = Mw/Mn. Ps is the probability of racemic linkages between monomer units and determined by comparison of the integrals of tetrads in
the homonuclear decoupled 1H NMR spectrum with those calculated using Bernouillan statistics, according to the method described by Coudane et
al.15 Slurry of 2 mmol of lactide in 1 mL of THF at 25 °C. Slurry of 5 mmol of lactide in 1 mL of THF at 25 °C. Reaction time unoptimized
e
f
g
behaviors of 1 and 2 are in direct contrast with complex 3, with
bulky substituents on the phosphasalen ligand, which
accelerates the rate and prevents catalyst aggregation.
PLA Tacticity. The polylactide produced from rac-lactide
using the three initiators shows a heterotactic bias (Ps).
Determination of Ps was carried out using the probabilities
calculated by Coudane et al.;15 every tetrad signal was analyzed
and the average value for Ps reported (for all spectra see Figures
9 and S8−10 in the Supporting Information). PLA produced
using initiators 1 and 2 had Ps values in the range 0.78−0.81,
regardless of the rac-LA conversion or initiator concentration.
Complex 3 shows considerably enhanced heteroselectivity
with Ps 0.86−0.90. Once again, the probability of heterotactic
enchainment was independent of the conversion of rac-LA,
addition of isopropyl alcohol, or concentration of the initiator.
It is quite interesting to observe that highly heterotactic PLA
was produced from either purified or crude LA. As an
illustration of the high degrees of heteroselectivity conferred
1
by this initiator, the methyne region of the H NMR spectrum
of PLA (Figure 9, at 98% conversion of LA) is illustrated; it is
dominated by two quartets, assigned to heterotactic PLA
(Figure 9 also shows the homonuclear decoupled NMR
spectrum with the tetrad assignment and Ps value).
Figure 7. Evolution of Mn (kg/mol) versus conversion (%) for LA
ROP using 3 (filled triangles) and with an extra equivalent of iPrOH
(hollow triangles). Also plotted is Mn(calcd) (filled circles). Polymer-
ization conditions: [LA]0 = 1 M, [3] = [iPrOH] = 1 mM, THF, 25 °C.
Discussion. Complexes 1 and 2 initiate and propagate ROP
via a coordination insertion mechanism. The MALDI-ToF MS
of the PLA produced using these initiators shows chains end
capped with ethyl ester and tert-butyl ester groups, respectively
(as well as the expected hydroxyl group at the opposite chain
end, consistent with this mechanism, see Figures S4 and S5,
Supporting Information). Both complexes exhibit dinuclear
structures in the solid state, with complex 1 having bridging
ethoxide groups while complex 2 bridges through a phenolate
group from each phosphasalen ligand. Both complexes are
viable initiators, but 1 is significantly slower than 2. Indeed, at 5
mM initiator concentration, complete conversion was observed
after approximately 1 h using complex 1 and after only 40 s
using complex 2. Both initiators display complex polymer-
ization kinetics, consistent with second-order dependencies on
lactide concentration and aggregation of the yttrium complexes.
This motivated study of more sterically hindered phosphasalen
ligands.
aggregation occurring under the conditions of the catalysis and
underscore the importance of the phosphasalen ancillary ligand
structure. The kinetics were not monitored in detail for
complex 1, due to its relatively slower rate, but preliminary
investigations showed a related second-order dependence on
lactide concentration (Figure S1, Supporting Information).
Polymerizations using complex 1 were well controlled, as
shown by the linear evolution in the molecular weight with
conversion and the narrow PDI for the PLA (Figure S2,
Supporting Information). There was a good agreement
between Mn(calcd) and Mn(exp), suggesting both alkoxide
groups initiate polymerization. On the other hand, for complex
2 the Mn(exp) values were approximately double the Mn(calcd)
values on the basis of a dinuclear structure (Figure S3,
Supporting Information). In situ 31P{1H} NMR spectroscopy
was used to monitor a mixture of complex 2 and 25 equiv of
LA. The spectrum was quite different from complex 2 alone; it
showed four nonequivalent phosphorus nuclei (Figure S11,
Supporting Information), data which could support aggregation
under the conditions of the catalysis. Currently, we are unable
to unambiguously assign the solution structure of complex 2
under the polymerization conditions. However, the complex
Complex 3 was prepared from a phosphasalen ligand having
sterically hindered tert-butyl substituents on the phenol ring.
On the basis of NMR data and the solid-state structure, it is
proposed that this species retains a mononuclear structure in
solution. Polymerizations using this initiator showed extremely
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Figure 8. Plot showing 1/[LA]t (M−1) versus time (s) for LA ROP using 2 over the concentration range 2.5−5 mM. Polymerization conditions:
[LA]0 = 1 M, THF, 25 °C, [2] = 5 mM (filled triangles, kobs = 0.252 M−1 s−1); [2] = 3.3 mM (filled circles, kobs = 0.0933 M−1 s−1); [2] = 2.9 mM
(open circles, kobs = 0.0768 M−1 s−1); [2] = 2.5 mM (open triangles, kobs = 0.0491 M−1 s−1). RHS: Plot of kobs versus [Y], polymerization conditions;
[LA]0 = 1 M, THF, 25 °C, [2] = 1−5 mM.
spectroscopy; it showed that the ligand remained coordinated
to the yttrium centers, ruling out ligand dissociation during
catalysis. Furthermore, complex 3 showed excellent hetero-
selectivity for rac-LA ROP. This interesting finding further
confirms earlier experiments using bis(phosphinic amido)
yttrium complexes, where high degrees of heteroselectivity
were only observed from a mononuclear complex.12j Stereo-
control is proposed to arise via a chain end control mechanism,
that is, where the stereochemistry of the last inserted lactide
unit controls the stereochemistry of the coordination/insertion
into the next lactide unit. Our group and others previously
observed enhanced stereocontrol for yttrium initiators in
THF.12g,j,t,21 An NMR-scale experiment involving addition of
increasing quantities of THF to a solution of 3 in toluene did
not lead to a significant shift in the THF resonances due to
either rapid exchange or a lack of coordination. It is, however,
clear that the THF solvent is important for the high
stereocontrol as an experiment conducted in toluene ([LA] =
1 M, [3] = 1 mM, in toluene, 80 °C, 91% conversion) yielded a
PLA with much lower heteroselectivity (Ps = 0.73 vs 0.89 in
pure THF).
Coates2d and Carpentier12q both studied yttrium complexes
of (substituted) tetradentate salen ligands (Figure 5, 6 and 7).
The yttrium complexes, either as mono- or dinuclear complexes
(bridging through the alkoxide group), were not particularly
active in lactide ROP, and limited stereocontrol was reported.
These phosphasalen yttrium complexes show markedly differ-
ent behavior, giving very high activities for lactide polymer-
Figure 9. {1H} and 1H{1H} NMR spectrum of PLA in CDCl3: [LA]0
ization and high heteroselectivity for rac-LA ROP. The
= 1M, [3] = 1 mM, [iPrOH] = 1.2 mM, 98% conversion of LA, THF,
electronic properties of the iminophosphorane functional
25 °C, Ps = 0.88.20
group in the phosphasalen ligand differ notably from the
imine moiety in the salen analogue. NBO (natural bond
orbital) analyses were performed on the structures of model
compounds optimized using density functional theory with
B3PW91 as the functional and the 6-31G* basis set; these
rapid rates; indeed, the pseudo-first-order rate constant (at [3]
= 1 mM) is very high (kobs = 0.08 s−1), and the rate depends to
the first order on the concentration of LA. A mixture of
complex 3 and 25 equiv of LA was analyzed by 31P{1H} NMR
calculations underline the divergent electronic properties of the
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ligands (Figure 10). Whereas the charge difference between the
C and the N of an imine remain low (|Δq| = 0.51), there is a
The iminophosphorane functionality is significantly more
electron donating than the imine analogue in the salen ligand.
In lactide ROP, there is a balance between requiring a Lewis-
acidic yttrium center to facilitate lactide coordination while also
requiring a labile yttrium alkoxide bond to accelerate insertion
of the lactide unit. Thus, it seems that a better donor ligand is
desirable; this may be due to weakening or labilizing of the
yttrium−alkoxide bond and acceleration of the insertion step.
Diaconescu and co-workers recently reported a switchable
activity in lactide ROP using an yttrium alkoxide complex with
a tetradentate ligand featuring in the backbone an iminophos-
phorane linked to a ferrocene.5b Whereas the complex bearing
the neutral (reduced) ligand with an electron-rich nitrogen
donor group is active for the ROP of lactide, the oxidized
complex (featuring a ferrocenium) is completely inactive. Thus,
polymerization activity is switched ‘off’ when the electron
density at the nitrogen decreases. It was hypothesized that
although insertion of the monomer to the oxidized complex is
possible, propagation from such a species is unfavorable, since
the alkoxide would be connected to a more electron-deficient
metal. This observation strongly supports the benefit of a good
donor ligand for the ROP of lactide. The flexibility of the ligand
could also play an important role as the coordination geometry
at yttrium is expected to change during the coordination−
insertion steps in ROP, and a more flexible ligand could be
beneficial; some theoretical studies supporting this notion have
been reported.23
Figure 10. Comparisons of the Wiberg indices and NBO charge for
various model compounds calculated using Gaussian09 using the
B3PW91 functional and 6-31G* basis set.
pronounced difference between the phosphorus and the
nitrogen atoms of an iminophosphorane (|Δq| = 2.29). This
indicates that the iminophosphorane adopts a dipolar structure,
which is further confirmed by the Wiberg bond indices, which
were calculated at 1.34 for the P−N bond vs 2.02 for the C−N
of the imine. Therefore, there is no double bond and no π
system in an iminophosphorane. Thus, they should be seen as
P+−N− instead of the conventional PN, the P−N bonding
being enforced by negative hyperconjugation.22 Upon coordi-
nation, the iminophosphorane ligands behave as strong π and σ
donors.
With this in mind, it is interesting to re-examine a series of
initiators our group previously studied: the bis(phosphinic)-
diamido yttrium complexes. The compounds show excellent
activities in lactide polymerization.12i,j,l All complexes were
highly active; furthermore, a monomeric complex gave
heterotactic PLA (Ps = 0.85). All complexes have tetradentate
bis(phosphinic amido) ancillary ligands coordinated to the
yttrium centers via the N and O atoms. NBO analysis
performed on a model of diphenylphosphinic amido derivative
shows some similarities with iminophosphorane derivatives
(Figure 10). Indeed, the NBO charges at P, N, and O were
calculated at +2.13, −1.18, and −1.14, respectively. Moreover,
both the P−N and the P−O bond indices are close to 1. This is
in agreement with the existence of two resonance forms for this
functionality, one of which corresponds to an iminophosphor-
ane (NP−O−). The X-ray crystal structures of these
complexes show that the P−N bond lengths and geometries
at the P and N centers fit with the ligands having some degree
of iminophosphorane functionality (Figure 11, Table 7).
Experimentally, it appears that this iminophosphorane ligand
motif accelerates ROP at the yttrium centers, leading to much
enhanced activities and even enabling stereochemical control.
CONCLUSIONS
■
Three yttrium alkoxide complexes featuring a new class of
phosphasalen ligands have been synthesized and characterized
by multinuclear NMR techniques, elemental analysis, and X-ray
diffraction experiments. All three complexes are highly active
initiators for rac-lactide ring-opening polymerization and
display good polymerization control and good/excellent
heteroselectivity. The order of activity of the complexes spans
3 > 2 ≫ 1 with all complexes displaying excellent rates but
complex 3 being particularly fast and able to polymerize
unpurified lactide to complete conversion within seconds. It is
notable that all phosphasalen yttrium complexes display
significantly greater activity than the analogous yttrium salen
complexes. All polymerizations are well controlled; the number-
averaged molecular weight increases linearly with conversion,
show good fits with the calculated molecular weights, and the
polydispersity indices are moderate/narrow. Furthermore, all
complexes produce heterotactic PLA, with Ps values up to 0.9 in
the case of 3. The high polymerization activities can be
rationalized by the electronic properties of the ligands:
iminophosphorane ligands are considerably more electron
donating than their salen analogues. In lactide polymerization
there are two important processes that the metal active site
Figure 11. Dimeric bis(phosphinic amido)12l,i yttrium complexes.
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Table 7. Structural Data for Dimeric Bis(phosphinic amido)12i,l and Phosphasalen Yttrium Complexes (Y−N and Y−O
distances relative to ancillary ligand)
complex
P−N (Å)
Y−N (Å)
Y−O (Å)
812i
912i
1012i
1112l
1
1.576(2), 1.568(2)
1.5907(18), 1.5989(18)
1.5922(13), 1.5821(12)
1.596(6), 1.582(6)
1.598(3), 1.595(3)
1.596(3), 1.586(3)
1.595(2), 1.584(2)
2.336(2), 2.395(2)
2.3560(17), 2.3348(17)
2.3681(12), 2.3542(12),
2.216(6), 2.345(6)
2.415(3), 2.438(3)
2.398(3), 2.372(3)
2.375(2), 2.324(2)
2.3336(17), 2.4444(16)
2.3409(15), 2.4720(13)
2.3548(10), 2.4621(10)
2.315(5), 2.524(5)
2.182(3), 2.213(3),
2
2.203(3), 2.338(2),
3
2.1671(318), 2.1704(18)
Synthesis of S1 and S2. At −78 °C, bromine (200 μL, 3.88 mmol)
was added dropwise to a solution of the appropriate phenolphosphine
(1.52 g, 3.88 mmol) in CH2Cl2 (30 mL). The cold bath was removed,
and stirring was continued for 45 min at 25 °C; subsequently, the
solution was cooled to −78 °C. Bu3N (926 μL, 3.88 mmol) was added,
followed by ethylene diamine (130 μL, 1.94 mmol), and the solution
was allowed to warm to 25 °C. After 16 h, a cloudy yellow solution
was formed, from which the solvents were evaporated and THF (15
mL) was added. A white slurry was formed with sonification, and the
white product was isolated by filtration, washed with THF (5 × 7 mL),
and dried in vacuum.
controls: coordination of the lactide, which is governed by the
Lewis acidity of the metal center, and insertion of the lactide
into the metal alkoxide bond, which is governed by the lability/
nucleophilicity of that alkoxide. It is therefore of significant
interest to note that more electron-donating ligands, such as
these phosphasalen ligands or the bis(phosphinic amido)
ligands previously described by our group, are very well suited
to preparation of highly active polymerization initiators. This
could be due to an increase in the lability of the yttrium
alkoxide bond; further investigations will aim to uncover the
structure−activity relationships of this interesting new class of
ligand. Furthermore, metal−salen complexes have found a very
wide range of catalytic applications, far beyond lactide
polymerization; this report highlights the potential impact of
phosphasalen analogues on a range of other catalyzes.
S1: 0.98 g, 1.26 mmol, 65%. 31P{1H} NMR (CDCl3): δ 39.3 (s, PV).
1H NMR (CDCl3): δ 7.67−7.59 (12H, m, p-CH(PPh2), o-CH(PPh2)),
7.52−7.46 (12H, m, m-CH(PPh2), CaH, CbH), 6.86−6.75 (4H, m,
3
3
CcH, NH), 6.57 (2H, dd, JH,H = 8.0 Hz, JP,H = 14.5 Hz, CdH), 3.43
(4H, d, JP,H = 10.5 Hz, CH2). 13C{1H} NMR (CDCl3): δ 159.7 (d,
3
2JP,C = 0.5 Hz, OCIV), 136.1 (d, 4JP,C = 1.0 Hz, CbH), 133.3 (d, 2JP,C
11.5 Hz, CdH), 133.1 (d, 4JP,C = 3.0 Hz, p-CH(PPh2)), 131.9 (d, 2JP,C
=
=
11.5 Hz, o-CH(PPh2)), 128.5 (d, 3JP,C = 13.5 Hz, m-CH(PPh2)), 121.4
(d, 1JP,C = 106.5 Hz, CIV(PPh2)), 119.6 (d, 3JP,C = 13.5 Hz, CcH), 117.7
(d, 3JP,C = 7.0 Hz, CaH), 106.8 (d, 2JP,C = 104.0 Hz, PPh2CIV), 43.3 (d,
2JP,C = 1.5 Hz, CH2). HR-EI-MS: 307.1132 ([M − 2Br]2+;
C38H36N2O2P22+; calcd 307.1126.
EXPERIMENTAL SECTION
■
Materials and Methods. All reactions were conducted under an
atmosphere of dry nitrogen or argon using standard Schlenk and
glovebox techniques. Solvents and reagents were obtained from
commercial sources. Tetrahydrofuran, toluene, pentane, hexane, and
petroleum ether were distilled from sodium/benzophenone under dry
nitrogen. Dichloromethane was distilled from CaH2 under dry
nitrogen. rac-Lactide was recrystallized from anhydrous toluene and
sublimed three times prior to use. [YCl3(THF)3.5]24 and phosphino-
phenols25 were prepared following literature procedure.
S2·THF: 1.47 g, 1.37 mmol, 71%. 31P{1H} NMR (CDCl3): δ 40.3
(s, PV). 1H NMR (CDCl3): δ 8.15(b, 2H, CH(PPh2)), 7.75−7.56 (m,
18H, CH(PPh2)), 6.93 (b, 2H, CbH), 6.44 (dd, 4JH,H = 2.1 Hz, 3JP,H
=
15.2 Hz, 2H, CH), 3.78 (m, 4H, O−CH2 (THF)), 3.74 (b, 4H, N−
CH2−CH2−N), 1.85 (m, 4H, O−CH2−CH2 (THF)), 1.59 (b, 2H,
OH), 1.52 (s, 18H, Ca−C(CH3)3), 1.08 (s, 18H, Cc−C(CH3)3).
The insolubility of the product in all regular deuterated solvents
prevents 13C{1H} NMR characterization.
Measurements. Nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker Av300 spectrometer operating at 300 MHz for
1H, 75.5 MHz for 13C{1H}, and 121.5 MHz for 31P{1H}. Solvent peaks
were used as internal references for 1H and 13C chemical shifts (ppm).
31P peaks were referenced to external 85% H3PO4. When needed,
Synthesis of L1 and L2. KHMDS (7.0 g, 35.1 mmol) was added to
a suspension of the aminophosphonium salt (8.8 mmol) in THF (320
mL) at ambient temperature. After 16 h, a cloudy solution was formed.
The white potassium bromide byproduct was removed by
centrifugation. Solvent was removed in vacuo, and the residue was
dissolved in petroleum ether (60 mL). After 5 min of sonification, a
white slurry was formed. The white solid was separated by
centrifugation, washed with petroleum ether (20 mL), and dried in
vacuo.
1
higher resolution 31P{1H} NMR and H{1H} NMR (homodecoupled
spectroscopy) experiments were performed on a Bruker Av500
spectrometer, equipped with a z-gradient bbo/5 mm tunable probe
and a BSMS GAB 10 A gradient amplifier providing a maximum
gradient output of 5.35 G/cmA. 1H NMR spectra for all lactide
polymerizations were performed on a Bruker Av400 instrument. The
following abbreviations are used: br, broad; s, singlet; d, doublet; dd,
doublet of doublets; t, triplet; m, multiplet; v, virtual. The labeling
schemes of the phosphasalen ligands and complexes are given in
Schemes 1 and 2.
Elemental analyses were determined by Mr. Stephen Boyer at
London Metropolitan University, Science Centre, 29 Hornsey Road,
London N7 7DD. Mass spectra were performed on a Micromass
MALDI-ToF micro MX using potassium salts for ionization. The PLA
number-averaged molecular weight, Mn, and polydispersity index (Mw/
Mn; PDI) were determined using gel permeation chromatography
equipped with multiangle laser light scattering (GPC-MALLS). Two
Polymer Laboratories Mixed D columns were used in series with THF
as the eluent at a flow rate of 1 mL min−1 on a Polymer Laboratories
PL GPC-50 instrument at 35 °C. The light scattering detector was a
triple-angle detector (Dawn 8, Wyatt Technology), and data were
analyzed using Astra V version 5.3.4.18. The refractive angle increment
for polylactide (dn/dc) in THF was 0.042 mL g−1.26
L1: 6.0 g, 8.7 mmol, 99%. 31P{1H} NMR (THF-d8): δ 18.6 (s, PV).
1H NMR (THF-d8): δ 7.73−7.67 (m, 8H, o-CH(PPh2)), 7.62−7.57
5
4
(m, JP,H = 1.5 Hz, 4H, p-CH(PPh2)), 7.55−7.49 (d, JP,H = 2.0 Hz,
8H, m-CH(PPh2), 7.09 (m, 2H, 3JH,H = 8.5 Hz, 3JH,H = 7.0 Hz, 4JH,H
=
1.5 Hz, 2H, CbH), 6.6.61 (m, 2H, 3JH,H = 8.5 Hz, 2H, CaH), 6.47 (ddd,
4JH,H = 1.5 Hz, JH,H = 7.0 Hz, JP,H = 16.0 Hz, 2H, CdH), 6.05 (ddd,
3
3
3JH,H = 3J′H,H = 7.0 Hz, JP,H = 3.0 Hz, 2H, CcH), 3.42 (dd, 3JP,H = 6.0
4
Hz, 3J′P,H = 6.5 Hz, 4H, N−CH2−CH2−N). 13C{1H} NMR (THF-d8):
δ 174.3 (d, JP,C = 3.5 Hz, O−CIV), 132.9 (d, JP,C = 13.0 Hz, CdH),
2
2
132.8 (d, 1JP,C = 77.0 Hz, CIV(PPh2)), 132.1 (d, JP,C = 1.3 Hz, CbH),
4
131.7 (2JP,C = 8.0 Hz, o-CH(PPh2)), 129.0 (d, JP,C = 2.0 Hz, p-
4
CH(PPh2)), 127.0 (d, 3JP,C = 10.5 Hz, m-CH(PPh2)), 121.8 (d, 3JP,C
=
=
9.0 Hz, CaH), 112.5 (d, 1JP,2C = 128.7 Hz, CIV−PPh2), 107.0 (d, 3JP,C
3
15.2 Hz, CcH), 50.4 (dd, JP,C = 27.0 Hz, JP,C = 8.0 Hz, N−CH2−
CH2−N). Anal. Calcd for C38H32K2N2O2P2: C, 66.26; H, 4.68; N,
4.07. Found: C, 66.19; H, 4.73; N, 3.99.
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L2t·2THF: 7.2 g, 6.8 mmol, 78%. 31P{1H} NMR (THF-d8): δ 20.6
C84H82N4O6P4Y2: C, 65.29; H, 5.35; N, 3.63. Found: C, 65.27; H,
5.25; N, 3.76.
1
(s, PV). H NMR (toluene-d8): δ 7.52−7.50 (m, 10H, CH(PPh2) +
CbH), 7.05−7.01 (m, 12H, CH(PPh2)), 6.30 (dd, 4JH,H = 2.5 Hz, 3JP,H
= 17.0 Hz, 2H, CdH), 3.41−3.38 (m, 8H, O−CH2(THF)), 3.20 (vt,
JP,H = J′P,H = 5.0 Hz, 4H, N−CH2−CH2−N), 1.70 (s, 18H, Ca−
C(CH3)3), 1.36 (m, 8H, O−CH2−CH2(THF)), 1.08 (s, 18H, Cc−
C(CH3)3). 13C{1H} NMR (THF-d8): δ 173.4 (d, 2JP1,C = 4.0 Hz, CIV−
Complex 3. [YCl3(THF)3.5] (313 mg, 0.7 mmol) was added to a
solution of L2 (745 mg, 0.7 mmol) in THF (25 mL) at −40 °C. After
2 h of stirring at room temperature, potassium tert-butoxide (78.6 mg,
0.7 mmol) was added into the mixture, giving a cloudy solution.
Stirring was continued for 7 h, and the solid was removed by
centrifugation. Solvents were removed from the filtrate, yielding a pale
yellow oil, which transformed into a white solid after 1 week of storing
at room temperature (680 mg, 0.68 mmol, 97%). Monocrystals
suitable for X-ray diffraction were grown from a saturated solution of
complex 3 in cyclohexane.
O), 140.1 (d, JP,C = 9.5 Hz, CcIV), 134.2 (d, JP,C = 81.5 Hz,
3
2/3
CIV(PPh2)), 133.7 (d,
J
P,C
= 7.5 Hz, o- or m-CH(PPh2)), 130.5 (d,
4JP,C = 2.0 Hz, p-CH(PPh2)), 128.8 (d, JP,C = 15.5 Hz, CdH), 128.6
2
(d, JP,C = 17.0 Hz, CaIV), 128.4 (d,
J
= 10.5 Hz, o- or m-
3
2/3
P,C
CH(PPh2)), 127.4 (d, 4JP,C = 1.0 Hz, CbH), 112.8 (d, 1JP,C = 134.0 Hz,
CIV−PPh2), 52.1 (dd, JP,C = 9.5 Hz, J′P,C = 29.5 Hz, N−CH2−CH2−
1
3: 31P{1H} NMR (THF-d8): δ 30.8 (s, PV). H NMR (THF-d8): δ
3
3
N), 36.0 (d, JP,C = 2.0 Hz, CaIV−C(CH3)3), 34.0 (d, JP,C = 0.5 Hz,
CcIV−C(CH3)3), 31.9 (s, C(CH3)3), 29.9 (s, C(CH3)3). Anal. Calcd
for C62H80K2N2O4P2: C, 70.42; H, 7.63; N, 2.65. Found: C, 70.57; H,
7.80; N, 2.49.
4
4
7.57 (d, JH,H = 7.0 Hz, JP,H = 11.2 Hz, 4H, o-CH(PPh2)), 7.61 (vt,
3JH,H = 7.0 Hz, 5JP,H = 1.0 Hz, 2H, p-CH(PPh2)), 7.43 (vt, 3JH,H = 3J′H,H
= 7.0 Hz, 4H, m-CH(PPh2)), 7.42 (dd, 3JH,H = 7.0 Hz, 3JP,H = 10.7 Hz,
3
4H, o-CH(PPh2)), 7.42 (vt, JH,H = 7.0 Hz, 2H, p-CH(PPh2)), 7.34
3
3
4
(vtd, JH,H = J′H,H = 7.0 Hz, JP,H = 2.5 Hz, 4H, m-CH(PPh2)), 7.28
(d, 3JH,H = 2.5 Hz, 2H, CbH), 6.34 (dd, 3JH,H = 2.5 Hz, 3JP,H = 15.9 Hz,
2H, CdH), 3.31 (m, 2H, N−CH2−CH2−N), 3.18 (m, 2H, N−CH2−
CH2−N), 1.37 (s, 18H, Ca−C(CH3)3), 0.98 (s, 18H, Cc−C(CH3)3),
0.68 (br. s, 9H, O−C(CH3)3). 13C{1H} NMR (THF-d8): δ 169.7 (d,
Synthesis of Yttrium Complexes. Complex 1. [YCl3(THF)3.5]
(313 mg, 0.7 mmol) was added to a solution of L1 (491 mg, 0.7
mmol) in THF (30 mL) at −40 °C. A white slurry was formed
after 2 min; stirring was continued at room temperature for 1 h.
Potassium ethoxide (59 mg, 0.7 mmol) was added, giving a
cloudy solution after 7 h. The solid was removed by
centrifugation, and the filtrate was evaporated to give an off-
white solid. Crystallization from toluene yielded the product as
colorless crystals (430 mg, 0.59 mmol, 84%)
3JP,C = 2.7 Hz, CIV−O), 139.0 (d, 3JP,C = 8.2 Hz, CcIV); 134.0 (d, 3JP,C
=
15.4 Hz, CaIV), 134.1 (d,
J
= 8.7 Hz, o-CH(PPh2)), 133.5 (d,
2/3
P,C
2/3
1
J
= 8.9 Hz, o-CH(PPh2)), 132.7 (d, JP,C = 86.6 Hz, CIV(PPh2)),
P,C
131.7 (d, JP,C = 89.0 Hz, CIV(PPh2)),131.9 (d, JP,C = 1.7 Hz, p-
1
4
1
1: 31P{1H} NMR (THF-d8): δ 29.3 (s, PV). H NMR (THF-d8): δ
CH(PPh2)), 131.5 (d, 4JP,C = 1.8 Hz, p-CH(PPh2)), 128.9 (d, 2/3JP,C
=
3
3
2/3
7.77−7.24 (m, 20H, CH(PPh2)), 7.09 (dddd, JH,H = 8.7 Hz, JH,H
=
9.6 Hz, m-CH(PPh2)), 128.8 (d,
J
= 10.0 Hz, m-CH(PPh2)),
P,C
7.0 Hz, 4JH,H = 1.7 Hz, 5JP,H = 1.2 Hz, 2H,CbH), 6.79 (ddd, 3JH,H = 7.7
Hz, JH,H = 1.7 Hz, JP,H = 13.9 Hz, 2H,CdH), 6.52 (ddd, JH,H = 8.7
Hz, JH,H = 0.7 Hz, JP,H = 8.3 Hz, 2H, CaH), 6.31 (dddd, JH,H = 7.7
128.3 (s, CbH), 127.8(d, JP,C = 13.9 Hz, CdH), 112.5(d, 1JP,C = 122.2
2
4
3
3
Hz, CIV−PPh2), 47.3 (dd, JP,C = 7.5 Hz, J′P,C = 19.0 Hz, N−CH2−
4
4
3
CH2−N), 35.8(s, CaIV−C(CH3)3 + O−C(CH3)3), 34.3 (s, CcIV
−
−
3
4
4
C(CH3)3), 31.7 (s, CcIV−C(CH3)3 + O−C(CH3)3), 30.4 (s, CaIV
Hz, JH,H = 7.0 Hz, JH,H = 0.7 Hz, JP,H = 3.5 Hz, 2H, CcH), 3.42 (b,
2H, O−CH2−CH3), 3.08 (b, 2H, N−CH2−CH2−N), 2.84 (b, 2H,
N−CH2−CH2−N), 1.01 (b, 1H, O−CH2−CH3); 0.62 (b, 2H, O−
C(CH3)3). Anal. Calcd for C58H73N2O3P2Y: C, 69.87; H, 7.38; N,
2.81. Found: C, 69.55; H, 7.26; N, 2.76.
CH2−CH3). 13C{1H} NMR (THF-d8): δ 172.1 (d, JP,C = 2.3 Hz,
3
General Procedure for Lactide Polymerization. To a rapidly
stirred solution of rac-lactide (288 mg, 2 mmol) in THF, in a vial in
the glovebox, was added the appropriate quantity of a solution of the
initiator in tetrahydofuran (0.01 M for complex 3, 0.02 M for complex
2, 0.1 M for complex 1); the overall concentration of rac-lactide was
therefore kept at 1 M. Aliquots of the reaction mixture were taken at
regular intervals, precipitated in hexane, and quenched in air. Solvent
was allowed to evaporate slowly in air, yielding a crude product, which
CIV−O), 172.0 (d, JP,C = 2.7 Hz, CIV−O), 134.0 (s, p-CH(PPh2)),
3
2/3
4
134.0(d,
J
= 9.5 Hz, o- or m-CH(PPh2)), 133.1 (d, JP,C = 10.9
P,C
Hz, CbH), 132.0 (m, CdH); 131.8 (m, C(Ar)), 129.1 (m, C(Ar)),
122.7 (d, 3JP,C = 7.5 Hz, CaH), 114.9 (d, 1JP,C = 113.8 Hz, CIV−PPh2),
113.0 (d, 3JP,C = 14.9 Hz, CcH), 51.3 (dd, JP,C = 6.3 Hz, J′P,C = 13.1 Hz,
N−CH2−CH2−N), 68.0 (s, O−CH2−CH3), 25.1 (s, O−CH2−CH3).
Anal. Calcd for C80H74N4O6P4Y2: C, 64.52; H, 5.01; N, 3.76. Found:
C, 64.44; H, 5.07; N, 3.72.
1
was analyzed by H NMR spectroscopy (to determine % conversion)
Complex 2. [YCl3(THF)3.5] (313 mg, 0.7 mmol) was added to a
solution of L1 (491 mg, 0.7 mmol) in THF (50 mL) at −40 °C. A
white slurry was formed after 2 min; stirring was continued at room
temperature for 1 h. Potassium tert-butoxide (78.6 mg, 0.7 mmol) was
added into the mixture, giving a cloudy solution after 7 h. Solid was
removed by centrifugation. The solution was concentrated (to 10 mL).
The product precipitated from solution as a white solid (380 mg, 0.49
mmol, 70%). Crystals suitable for X-ray diffraction experiments were
obtained from a solution of complex 2 in THF/toluene (1/3 volume).
and GPC (to determine the Mn and PDI). The probability of
syndiotactic linkages between monomer units (Ps) is determined from
the homonuclear decoupled 1H NMR spectrum: δ 5.14 and 5.22 ppm
in CDCl3 for heterotactic PLA.15
Kinetic Analyses. Reactions were monitored by taking aliquots
and quenching with a large excess of hexane (which precipitates both
PLA and any unreacted LA). This precipitation is expected to inhibit
further polymerization (in combination with the high dilution, any
polymerization which occurred after this quenching would be expected
to do so very slowly). The vial was then immediately removed from
the glovebox and exposed to air. Such conditions are sufficient to
hydrolyze yttrium−alkoxide bonds (by reference to exposure of NMR
samples of initiators to air) and stop polymerizations completely.
Solvents were evaporated in air (overnight). Vacuum was not used to
remove the solvents, so as to prevent any unintentional loss of lactide
(through sublimation).
X-ray Crystallography. Data were collected at 150 K on a Bruker
Kappa APEX II diffractometer using a Mo Kα (λ = 0.71069 Å) X-ray
source and a graphite monochromator. The crystal structure was
solved using SIR 9727 and Shelxl-97.28 ORTEP drawings were made
using ORTEP III for Windows.29
1
2: 31P{1H} NMR (THF-d8): δ 30.4 (s, PV). H NMR (THF-d8): δ
3
3
7.73−7.30 (m, 20H, CH(PPh2)), 7.08 (dddd, JH,H = 8.7 Hz, JH,H
=
7.0 Hz, 4JH,H = 1.7 Hz, 5JP,H = 1.0 Hz, 2H,CbH), 6.53 (ddd, 3JH,H = 8.0
Hz, JH,H = 1.7 Hz, JP,H = 14.6 Hz, 2H,CdH), 6.50 (ddd, JH,H = 8.7
Hz, JH,H = 1.0 Hz, JP,H = 5.5 Hz, 2H, CaH), 6.18 (dddd, JH,H = 8.0
4
3
3
4
4
3
3
4
4
Hz, JH,H = 7.0 Hz, JH,H = 1.0 Hz, JP,H = 3.5 Hz, 2H, CcH), 3.42 (b,
2H, O−CH2−CH3), 3.22 (b, 2H, N−CH2−CH2−N), 2.99 (b, 2H,
N−CH2−CH2−N), 0.88 (b, 9H, O−CIV(CH3)3). 13C{1H} NMR
(THF-d8): δ 172.7 (d, 3JP,C = 2.3 Hz, CIV−O), 172.6 (d, 3JP,C = 2.5 Hz,
CIV−O), 134.0(d, 2/3JP,C = 9.2 Hz, o- or m-CH(PPh2)), 134.0(d, 2/3JP,C
4
= 8.6 Hz, o- or m-CH(PPh2)),133.1 (d, JP,C = 12.6 Hz, CdH), 131.8
4
1
(d, JP,C = 2.3 Hz, p-CH(PPh2)),131.6 (d, JP,C = 74.6 Hz,
CIV(PPh2)),131.5 (d, JP,C = 2.8 Hz, CbH), 131.5 (d, JP,C = 75.0
4
1
Hz, CIV(PPh2)), 128.9 (d, 2/3JP,C = 11.5 Hz, o- or m-CH(PPh2)), 128.7
ASSOCIATED CONTENT
2/3
3
■
(d,
J
= 10.7 Hz, o- or m-CH(PPh2)), 122.0 (d, JP,C = 8.0 Hz,
P,C
1
3
S
CaH), 114.2 (d, JP,C = 121.3 Hz, CIV−P(Ph2)), 112.4 (d, JP,C = 15.0
Hz, CcH), 68.0 (s, O−CIV(CH3)3), 51.9 (dd, JP,C = 5.2 Hz, J′P,C = 15.5
Hz, N−CH2−CH2−N), 32.7(br. s, O−CIV(CH3)3). Anal. Calcd for
* Supporting Information
Additional kinetics plots; MALDI-ToF MS and 1H{1H} spectra
of synthesized PLA; NMR studies on addition of isopropanol
2167
dx.doi.org/10.1021/ic202015z | Inorg. Chem. 2012, 51, 2157−2169

Inorganic Chemistry
Article
Am. Chem. Soc. 2005, 127 (31), 10869−10878. (r) Cohen, C. T.;
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Trans. 2006, 1, 237−249. (s) Darensbourg, D. J.; Ganguly, P.; Choi,
W. Inorg. Chem. 2006, 45 (10), 3831−3833. (t) Li, B.; Zhang, R.; Lu,
X.-B. Macromolecules 2007, 40 (7), 2303−2307. (u) Darensbourg, D.
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This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: audrey.auffrant@polytechnique.edu (A.A.); c.k.
williams@imperial.ac.uk (C.K.W.).
Notes
The authors declare no competing financial interest.
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