Yttrium phosphasalen initiators for rac-lactide polymerization

Article abstract of DOI:10.1021/om301129k

A series of highly active yttrium phosphasalen initiators for the heteroselective ring-opening polymerization of rac-lactide are reported. The initiators are yttrium alkoxide complexes ligated by iminophosphorane analogues of the popular "salen" ligand, t

Full text of DOI:10.1021/om301129k

Article
pubs.acs.org/Organometallics
Yttrium Phosphasalen Initiators for rac-Lactide Polymerization
Clare Bakewell,†,§ Thi-Phuong-Anh Cao,‡,§ Xavier F. Le Goff,‡ Nicholas J. Long,† Audrey Auffrant,*,‡
and Charlotte K. Williams*,†
†Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.
‡Laboratoire Heteroelements et Coordination, CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France
S
* Supporting Information
ABSTRACT: A series of highly active yttrium phosphasalen
initiators for the heteroselective ring-opening polymerization
of rac-lactide are reported. The initiators are yttrium alkoxide
complexes ligated by iminophosphorane analogues of the
popular “salen” ligand, termed “phosphasalens”. A series of
novel phosphasalens have been synthesized, with varying
substituents on the phenoxide rings and ethylene, propylene,
rac-cyclohexylene, R,R-cyclohexylene, phenylene, and 2,2-
dimethylpropylene groups linking the iminophosphorane
moieties. Changing the substituents on the phosphasalen
ligands results in changes to the rates of polymerization (kobs
)
and to the PLA heterotacticity (Ps = 0.87). Generally, the initiators have high rates, excellent polymerization control, and a
tolerance to low loadings.
diamides, which show high activities and good control.8 By
INTRODUCTION
■
changes in the substituents on the phosphorus atoms it was
possible to control the complexes’ nuclearity in THF solutions;
mononuclear complexes showed high heteroselectivity (Ps =
0.8), while dinuclear complexes (dimers) lacked any stereo-
control.8e We also discovered that more electron donating
substituents led to qualitatively higher rates of polymer-
ization.8d This led to the development and application of
electron-donating iminophosphorane analogues of the popular
salen ligand class phosphasalens.10 In early 2012, we reported a
new phosphasalen yttrium complex, complex A (Scheme 1), for
use as an initiator in lactide polymerization.10 It showed rates
among the highest reported combined with high hetero-
selectivity (Ps = 0.9). Complex A is also capable of high activity
even at very low catalyst loadings (0.02 mol %) and using
The ring-opening polymerization (ROP) of rac-lactide (rac-
LA) is of interest because the product, polylactide (PLA), is a
degradable and biocompatible material.1 The monomer, lactide,
is obtained from lactic acid, which, in turn, is produced from
the fermentation of sugars. Lactide undergoes ring-opening
polymerization (ROP) to produce PLA, a process which can be
initiated by metal complexes, organo-catalysts, or enzymes.1c,2
Metal alkoxides or amide complexes can be desirable due to
their high rates (activities), high selectivities, and facility to use
low loadings, thereby producing high-molecular-weight poly-
mers. The selection of the initiator can also influence the
polymer tacticity; in particular, controlling the stereochemistry
of rac-LA ROP is important.1c,2b,3 Yttrium initiators have
attracted considerable attention due to their very high rates of
polymerization, high degrees of polymerization control, and
facility to control the polymer tacticity via modification of the
ancillary ligands. The earliest reports of yttrium initiators
focused on homoleptic yttrium alkoxide complexes.4 These
showed excellent activities but needed to be prepared in situ to
prevent redistribution reactions and/or formation of μ-oxo
clusters. Subsequently, a range of ancillary ligands have been
widely investigated, including diphenolate diimines/diamines
(Salen or Salan ligands),5 amine bis(phenolates),6 dithiaalkane
bis(phenolates),7 and bis(phosphinic)diamides,8 as well as a
range of other ligands with amido, amidinato, phosphido, and
oxazoline donors.9 Some of these successful ligands can be used
to effect stereocontrolled ROP of rac-LA, especially to produce
heterotactic PLA.6−9 There are also three reports of yttrium
initiators which can produce isotactic, stereoblock PLA from
rac-LA.9g,h,k We have previously investigated phosphinic
Scheme 1. Structures of Yttrium Phosphasalen Initiators A
and B, Which Are Heteroselective and Isoselective for the
ROP of rac-LA, Respectively10,11
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: November 26, 2012
Published: February 26, 2013
© 2013 American Chemical Society
1475
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
unpurified lactide. Very recently, complex B has been reported,
which enables good isoselectivity (Pi = 0.8) while maintaining
high rates.11 Thus, the iminophosphorane ligand appears to be
useful for this type of catalysis and a greater understanding of
the structure−activity relationships is merited. Herein, we
describe a series of new yttrium phosphasalen complexes where
the groups linking the imino functionalities and/or the
phenolate substituent differ. There is an investigation of the
underlying influences of the ligand structures on rate and
heteroselectivity. In particular, we were motivated to target
complexes which would exist as discrete (mononuclear) species
in solution, as dinuclear yttrium phosphasalen initiators have
earlier been shown to be less efficient.8e,10
The proligands were deprotonated by reaction with 4 equiv
of potassium bis(trimethylsilyl)amide; quantitative conversion
was verified by 31P{1H} NMR spectroscopy. Yttrium
phosphasalen chloride complexes were formed by reaction
with [YCl3(THF)3.5]; this led to a shift to lower field in the
singlet signal observed by 31P{1H} NMR spectroscopy (δP ∼30
ppm). Addition of potassium alkoxide (ethoxide/tert-butoxide)
to the yttrium halide complexes led to the formation of the
yttrium phosphasalen alkoxide compounds 1−6, which, in
general, showed a slight upfield shift in 31P{1H} NMR spectra
(Scheme 2). All of the complexes were fully characterized by
NMR experiments and elemental analyses. In some cases,
crystals suitable for X-ray crystal diffraction have been obtained.
Compound 6 has recently been communicated,11 but its
performance is included here, along with that of compound
A,10 for purposes of comparison.
X-ray Crystallography. Crystals suitable for single-crystal
X-ray diffraction experiments were isolated from a cyclohexane
solution of compound 1 (Table 1 and Figure 1). The complex
cocrystallized with equal proportions of the (R,R)- and (S,S)-
1,2-diiminocyclohexylene backbone. Compound 2, synthesized
from the resolved (R,R)-1,2-diaminocyclohexane, shows the
same structural data as 1 (Figure S1 and Table S1 (Supporting
Information)). The yttrium center has an octahedral coordina-
tion geometry with the two nitrogen and two oxygen atoms of
the phosphasalen ligand occupying the equatorial positions and
the tert-butoxide group and a THF molecule occupying the
axial positions. The Y−N1,2 bond lengths in complex 1 are
closely comparable with those observed in A (Y−N1,2 bond
lengths 2.380(7) and 2.391(2) Å for 1 compared with 2.375(2)
and 2.324(2) Å for A), and all other bond lengths for complex
1 are similar, within error, to those for A.10 Only the Y−O
bonds experience a slight shortening (Y−O bonds 2.234(2) Å
for 1 compared with 2.167(2) and 2.170(2) Å for A). On
comparison of the structures of complexes 1 and 2 featuring a
cyclohexylene linkage with that of A (ethylene linkage), there
are differences in the orientation of the phenyl rings attached to
the phosphorus atoms which could be due to steric interactions
between these rings and the cyclohexylene ring atoms. This
steric hindrance may “open” the coordination sphere of yttrium
and enable THF coordination at the sixth coordination site.
The same rationale may explain why compound 5, which has a
1,2-phenylenediimine linker, also has a coordinated THF
molecule (Figure 2 and Table 1). It adopts a distorted-
octahedral geometry, with the phosphasalen ligand occupying
the equatorial positions and the axial positions being occupied
by the tert-butoxide group and a THF molecule. Once again,
the Y−O1,2 and Y−N1,2 bond length values are in close
agreement with those obtained for complexes A, 1, and 2.10
The phosphasalen ligand with a 2,2-dimethylpropylene
diimine backbone would also be expected to be less sterically
congested than A, and indeed, compound 4 also has a
coordinated THF molecule (Figure 3).
RESULTS AND DISCUSSION
■
Synthesis of Compounds 1−6. To form discrete
mononuclear species, bulky substituents at the ortho positions
of the phenoxide rings are essential. Thus, tert-butyl
substituents at the ortho phenolate positions were used
throughout this study. Furthermore, the group linking the
two imino groups must be controlled to ensure that only a
single yttrium center is coordinated. Therefore, linkers
comprising two to three carbons were selected. The proligands
L1−L6 were synthesized by reacting 2,4-di-tert-butyl-6-phos-
phinophenol, via the Kisanov reaction, with bromine and the
appropriate diamine (Scheme 2 and Supporting Information).
This method gave the phosphasalen proligands L1−L6 in high
yields (Scheme 2 and Scheme S1 (Supporting Information)).
All of the new compounds were fully characterized by
multinuclear NMR spectroscopy and elemental analyses.
Scheme 2. General Synthetic Route to Achieving Proligands
a
L1−L6 and Compounds 1−6
The elemental analyses of complexes 1−5 confirm the
presence of a THF molecule in the coordination sphere of
yttrium, in the solid state, even after prolonged periods of
drying. X-ray crystal structures for compounds 1, 2, 4, and 5
also show a bound THF molecule, despite the crystals being
grown from cyclohexane solutions. In order to understand
a
Reagents and conditions: (i) (a) Br2 (1 equiv), CH2Cl2, 195−298 K,
1 h; (b) DABCO (0.5 equiv), diamine (0.5 equiv), CH2Cl2, 195−298
K, 16 h, yields L1 75%, L2 57%, L3 71%, L4 65%, L5 72%, L6 69%; (ii)
(a) KHDMS (4 equiv), THF, 298 K, 4 h, (b) [YCl3(THF)3.5], THF,
298 K, 4 h, (c) KOR′, THF, 298 K, 7 h, yields 1 79%, 2 87%, 3 74%, 4
88%, 4a 81%, 5 87%, 6 88%.
1
whether THF remained coordinated in solution, H NMR
spectra were recorded in d8-toluene; however, these all showed
that the THF resonances were at almost exactly the same
chemical shifts as those of free THF. This indicates that THF is
1476
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compounds 1, 4, and 6
t
Y1−O1
Y1−O2
Y1−O3 Bu
Y1−O4 THF
Y1−N1
Y1−N2
P1−N1
P2−N2
1
4
5
2.234(2)
2.234(2)
2.191(3)
2.220(3)
2.058(2)
2.072(3)
2.040(3)
2.518(2)
2.462(3)
2.467(3)
2.391(2)
2.432(4)
2.389(4)
2.391(2)
2.402(4)
2.406(3)
1.588(2)
1.597(4)
1.606(4)
1.588(2)
1.599(4)
2.232(3)
2.243(3)
1.604(4)
O1−Y1−N1
N1−Y1−N2
N2−Y1−O2
O1−Y1−O2
O2−Y1−O3
O2−Y1−O4
O3−Y1−O4
1
101.47(8)
84.0(1)
69.9(1)
80.8(1)
68.3(1)
82.17(6)
83.2(1)
80.9(1)
120.1(1)
109.3(1)
126.1(1)
82.18(5)
95.6(1)
95.5(1)
97.10(6)
78.3(1)
95.5(1)
178.5(1)
173.5(1)
160.6(1)
4
5
80.8(1)
Figure 1. ORTEP view of the solid-state structure of compound 1. The R,R (a) and S,S (b) enantionmers are both illustrated. Thermal ellipsoids are
drawn at the 50% probability level, and hydrogen atoms are omitted for clarity.
a slight excess of pyridine and removal of all solvents under
vacuum. When the sample was redissolved, the 1H NMR
spectrum (d8-toluene) showed that pyridine was present, but
again, the resonances were not shifted significantly in
comparison to those of free pyridine. Taken together, these
findings provide tentative support for the notion that THF is
coordinated to the yttrium center for complexes 1−5. In
contrast, complex 6 does not have any THF coordinated/
1
associated with it, either by H NMR spectroscopy or by
elemental analysis. This is in line with the characterization data
for the related compound A. Taking into account its structural
similarity to complex A (ethylene backbone), it is proposed that
complex 6 also has a pentacoordinate ytrrium center.
Figure 2. ORTEP view of the structure of compound 5 with thermal
rac-Lactide Polymerization. Compounds 1−6 were
ellipsoids drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
tested as initiators for rac-lactide ring-opening polymerization
(Scheme 3, Table 2).
a
Scheme 3. General Polymerization Procedure
a
Reagents and conditions: (i) [I]/[LA] = 1/500, THF, 298 K, 1 M
[LA].
The polymerizations were conducted under a standard set of
conditions, using a 1 M solution of lactide in THF at 298 K.
The solvent selection is important: the initiators decompose in
methylene chloride solutions, and toluene can only be applied
at elevated temperatures (>343 K). The polymerizations were
monitored by taking regular aliquots which were analyzed by
Figure 3. ORTEP view of the solid-state structure of compound 4,
with thermal ellipsoids drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity.
either not coordinated or that rapid exchange occurs on the
NMR time scale. Additional NMR experiments, including low-
temperature studies (243 K) and NOE NMR, failed to provide
conclusive evidence of THF coordination. However, it was
possible to remove the THF from the complexes by addition of
NMR spectroscopy to determine the conversion and by GPC-
MALLS (gel permeation chromatography-multiple angle laser
light scattering) to determine the evolution of the number-
averaged molecular weight. The polymerization kinetics were
monitored for each initiator, showing a first-order dependence
1477
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
Table 2. Polymerization Data Using Initiators 1−6
a
b
c
d
d
e
initiator
I
[LA]0/[I]
time (min)
conversion (%)
kobs × 10−3 (s−1
)
Mn(exptl) (g mol−1
)
Mn(calcd) (g mol−1
)
PDI
Ps
1
500/1
500/1
500/1
500/1
500/1
500/1
500/1
1000/1
1000/1
4
4
89
92
95
86
91
89
90
86
9.4
10.7
12.3
3.7
84000
45600
64100
66200
68400
61900
65500
64100
64800
124000
1.05
1.04
1.07
1.07
1.02
1.01
1.05
1.5
0.75
0.75
0.75
0.67
0.7
f
1
2
4
120500
66800
3
9
4
20
22
6
2.0
76500
4a
5
1.5
70900
0.69
0.84
0.87
0.86
6.7
196200
165000
223000
6
0.33
0.75
A
97
79
140000
1.34
a
b
1
Polymerization conditions: THF, 298 K, 1 M [LA]. Determined by integration of the methine region of the H NMR spectrum (LA 4.98−5.04
c
ppm; PLA 5.08−5.22 ppm). Determined from the gradients of the plots of ln{[LA]0/[LA]t} versus time, where the average errors are 5−12%
(determined using initiators 4a and 5). Determined by gel permeation chromatography (GPC) in THF, using multiangle laser light scattering (gel
permeation chromatography-multiple angle laser light scattering, GPC-MALLS; see the Experimental Section). Determined by analysis of all the
tetrad signals in the methine region of the homonuclear decoupled H NMR spectrum. Polymerization conducted with 1 equiv of PrOH.
d
e
12
f
1
i
on lactide concentration in every case; the pseudo-first-order
rate constants, kobs, were obtained as the gradient of the linear
fits to plots of ln([LA]0/[LA]t) versus time (Figure 4). The
tacticity of the resulting PLA was assessed by integration of the
methyne region of the homonuclear decoupled proton NMR
spectrum and by application of Bernoullian statistics to predict
the tetrad probabilities, according to the method described by
Coudane et al.12
contrast to previous results from our group using bis-
(phosphinic)diamido yttrium initiators, where C3-bridged
complexes were significantly faster than C2-bridged analogues.8c
Okuda has discovered that for the dithia bis(phenoxide)
yttrium initiators C3-bridged complexes are both faster and
more heteroselective; this was attributed to increased ligand
flexibility, facilitating configurational isomerization.7c,e Alumi-
num salan complexes also showed enhanced selectivities and
rate with C3 vs C2 bridging groups.14
Compounds 1−6 were all highly active initiators in the ROP
of rac-LA (Table 2). Initiator 6, which is structurally analogous
to compound A, was extremely active toward the ROP of rac-
LA, with the polymerization proceeding to nearly full
conversion in <20 s (1 M solution in LA, 1/1000 6/LA). It
was not possible to obtain a kobs value for this initiator, as the
reaction proceeded too quickly to monitor by the conventional
aliquot technique, but it is worth noting that compound 6
initiates at a faster rate, kobs = 7.9 × 10−2 s−1, than compound
A.10 In contrast, compounds 1−5 were less active than A.
Compounds 1, 2, and 5, which each have a C2 bridge, were
approximately 2−3 times faster than compounds 3, 4, and 4a,
each having a C3 bridge (Table 2). The activities of these
yttrium phosphasalen complexes outstrip those of many other
yttrium complexes, particularly those coordinated by salen/
salan ancillary ligands.5a,c,d,g,h We have hypothesized that the
iminophosphorane group, which is more electron donating
than an imine moiety, accelerates the rate of polymerization by
increasing the lability of the yttrium alkoxide propagating group
Polymerization Control. All polymerizations showed a
linear evolution of Mn against conversion and narrow
polydispersity indices, indicative of good control. The C3-
bridged compounds showed Mn values closer to those
predicted on the basis of initiator concentration (Figure 5).
In contrast, C2-bridged compounds (1, 2, and 5) showed
molecular weights higher than predicted. This is attributed to
the qualitatively higher rates for the C2-bridged initiators
combined with the reduced initiating capability of the tert-
butoxide group (note both 6 and A, which are significantly
faster, still show further reduced control). When 1 equiv of
isopropyl alcohol is added to these reactions, the Mn values
match much more closely those predicted on the basis of initial
monomer concentration (Table 2, line 2); this finding confirms
that the lack of control relates to the poor initiating ability of
the sterically congested tert-butyl alkoxide. Nevertheless, the
rate of propagation is consistently faster than the rate of
initiation, leading to products with narrow PDI in all cases.
All of the new initiators are heteroselective, but the degree of
stereocontrol is reduced according to the nature of the diimine
linker group; i.e., A > 6 > 5 > 1 = 2 > 4 > 3. The tacticity is
controlled by a chain end control mechanism, whereby the
stereochemistry of the last inserted monomer unit controls the
binding/ring opening of the subsequent lactide unit. Such
heteroselectivity is rather unusual for this type of yttrium
complex, as related salen/salan yttrium initiators show little or
no heteroselectivity (Ps < 0.7).5a,c,d,g,h It is proposed that the
stereocontrol may arise from the increased steric shielding of
the active site by the iminophosphorane group, in particular
from the two phenyl substituents on the phosphorus atoms. A
possible rationale for the stereoselectivity might relate to the
differing degrees of steric crowding of the active sites: more
open active sites, such as those where THF is coordinated in
the solid state, show lower degrees of control vs more
congested active sites, where THF remains unbound, e.g. A and
6. We, and others, have previously observed that the rates and
and thereby increasing the rate of insertion of lactide.8c,10,11
A
related hypothesis has been proposed, and tested by detailed
kinetic studies, by Tolman and Hillmyer to rationalize activity
trends for a series of phenoxy aluminum ROP initiators.13
Our results also indicate that the length and nature of the
bridging group between the phosphinoimino moieties
influences the rates, which decrease in the order 6 > A ≫ 2
> 1 > 5 > 3 > 4, 4a. It is pertinent to consider the different
structures of compounds A and 6 vs 1−5, as these account for
the largest differences in rate. All of the complexes exhibit
mononuclear structures in the solid state and in solution;
however, initiators A and 6 have pentacoordinate, square-
pyramidal geometries at yttrium, while 1−5 have hexacoordi-
nate, octahedral geometries at yttrium, with a molecule of THF
coordinated. These differing coordination geometries may
account for the stark differences in rate between A and 6 and
1−5. Within the series of hexacoordinate yttrium complexes 1−
5, those with C2 bridge groups are faster. This finding is in
1478
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
Figure 4. Plots of ln([LA]0/[LA]t) vs time: (i) initiators 1−3 and 5; (ii) initiators 4 and 4a. Conditions: [LA]0 = 1 M, 1/500 [I]/[LA], THF, 298 K.
stereocontrol are higher when polymerizations are conducted in
THF in comparison to other solvents.1c,2d,6o,q,7b,c,8a,e,h,9c These
phosphasalen initiators also show reduced heteroselectivity
using toluene, at 348 K, as the reaction medium. For example,
initiator 4a has a Ps value of 0.60 in toluene versus 0.69 in THF.
The observed rates cannot, however, be easily compared due to
the difference in temperature required for homogeneous
polymerizations in toluene: 348 K (toluene) versus 298 K
(THF). In order to compare the polymerizations at the same
temperature, slurry phase polymerizations, in toluene, were
conducted at 298 K; here the reaction was heterogeneous at the
start of the polymerization but became homogeneous as the
polymerization progressed. This is attributed to the enhanced
solubility of PLA in toluene (298 K) in comparison to rac-
lactide (298 K). Under slurry conditions, a marked reduction in
heteroselectivity was observed for compounds 1 and 5, where
Ps = 0.69 (toluene) vs 0.75 (THF) and Ps = 0.67 (toluene) vs
0.84 (THF), respectively. It is worth highlighting that this result
is less likely due to the slurry conditions, as when identical
experiments were conducted with the highly isoselective
compound B, identical stereoselectivities were observed (Pi =
0.76, 298 K, in both toluene and THF). This finding suggests
that high heteroselectivity depends both on the ligand
substituents and on the use of THF as the solvent. This
could be due to THF coordination accelerating coordination/
insertion of a particular enantiomer of rac-lactide. However,
other physical/chemical phenomena could also be implicated:
e.g., differences in solvent polarity. This solvent dependence
will be the subject of further, future investigations.
1479
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
Figure 5. Evolution of Mn versus percentage conversion for the polymerization using initiator 4 (circles) and initiator 4a (squares) and PDI versus
percentage conversion of initiator 4a (diamonds). Conditions: [LA]0 = 1 M, 1/500 [I]/[LA], THF, 298 K.
prepared following literature procedure.15 Ligand syntheses are
described in the Supporting Information.
CONCLUSIONS
■
A series of phosphasalen yttrium alkoxide compounds, 1−6,
were synthesized and characterized. All compounds were fully
characterized by multinuclear NMR spectroscopy and
elemental analysis and, in some cases, X-ray diffraction. The
complexes show mononuclear structures in both the solid state
(X-ray diffraction) and solution (31P, 13C NMR spectroscopic
interpretation). All new compounds are viable initiators for rac-
lactide ROP, showing very high rates. Within the series of
phophasalen ligands, the more electron donating ligand shows
the highest rate and the diimine linker group also influences the
rate in the order ethylene > cyclohexylene > phenylene >
propylene. The initiators exhibit good polymerization control,
with a linear evolution of molecular weight and narrow
polydispersity indices being observed in all cases. The degree of
molecular weight control improves in the inverse order to rate.
The initiators all yield heterotactic PLA from rac-LA; the Ps
values decrease according to the nature of the diimine linker
group in the order ethylene > cyclohexylene > phenylene >
propylene. Thus, the fastest initiators are also the most
heteroselective. It is proposed that these factors are both
controlled by the steric congestion at the active site, which can
be easily tuned by modification of the phosphasalen ancillary
ligand.
Nuclear magnetic resonance (NMR) spectra were recorded on a
1
Bruker Av300 spectrometer operating at 300 MHz for H, 75.5 MHz
for 13C{1H}, and 121.5 MHz for 31P{1H}. Solvent peaks were used as
1
internal references for H and 13C chemical shifts (ppm). 31P peaks
were referenced to external 85% H3PO4. When needed, higher
resolution 31P{1H} NMR and 1H{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.
Elemental analyses were determined by Mr. Stephen Boyer at
London Metropolitan University, Science Centre. PLA number
averaged molecular weights, Mn, and polydispersity indexes (Mw/Mn;
PDI) were determined using gel permeation chromatography,
conducted on a Polymer Laboratories GPC-50 instrument 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 at 35 °C. The light scattering
detector was a triple-angle detector (Dawn 8, Wyatt Technology), and
the 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−1.16
General Procedure for the Synthesis of the Yttrium
Complexes 1−6. Potassium bis(trimethylsilyl)amide (240 mg, 1.2
mmol) was added into a slurry of the phosphasalen proligands Lx
(0.30 mmol) in THF (20 mL). After 4 h, a cloudy solution was
obtained. The completion of the deprotonation reaction was verified
by 31P{1H} NMR spectroscopy. The insoluble potassium salt was
removed by centrifugation, and [YCl3(THF)3.5] (134 mg, 0.30 mmol)
was added. After 4 h of stirring at 298 K, potassium tert-butoxide (34
mg, 0.30 mmol) was added into the mixture, giving a cloudy solution.
Stirring was continued for 7 h, and the solid was removed by
centrifugation. The solvent was evaporated in vacuo, and the residue
was crystallized in cyclohexane (5 mL), giving 1−5 as colorless crystals
or solids.
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. Tetrahydrofuran and petroleum ether for ligand and complex
synthesis were directly taken from a MBraun MB-SPS 800 Solvent
Purification Machine. rac-Lactide was recrystallized from anhydrous
toluene and sublimed three times prior to use. [YCl3(THF)3.5] was
Compound 1: L1 (316 mg, 0.30 mmol); 31P{1H} NMR spectrum
after deprotonation δ 18 ppm, after the addition of [YCl3(THF)3.5] δ
22.6 ppm, 27.5 ppm (equal intensities); yield 265 mg (2.4 mmol,
1480
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
79%); 1H NMR (300 MHz, THF-d8; δ (ppm)) 7.89 (ddd, 3JP,H =10.6
CIV(CH3)3). Anal. Calcd for C66H87N2O4P2Y: C, 70.57; H, 7.81; N,
2.49. Found: C, 70.47; H, 7.97; N, 2.53.
Hz, 3JH,H= 7.5 Hz, 4JH,H= 1.0 Hz, 2H, o-CH(PPh2)), 7.75 (ddd, 3JP,H
=
Compound 3: L3 (304 mg, 0.30 mmol); 31P{1H} NMR spectrum
after the deprotonation δ 23.8 ppm, after the addition of
12.3 Hz, 3JH,H = 7.5 Hz, 4JH,H = 1.0 Hz, 2H, o-CH(PPh2)), 7.62 (ddd,
3JP,H = 11.2 Hz, 3JH,H = 7.5 Hz, 4JH,H = 1.0 Hz, 2H, o-CH(PPh2)), 7.56
1
3
3
4
[YCl3(THF)3.5] δ 36.9 ppm; yield 240 mg (0.22 mmol, 74%); H
(ddd, JP,H = 11.0 Hz, JH,H = 7.5 Hz, JH,H = 1.0 Hz, 2H, o-
NMR (300 MHz, THF-d8; δ (ppm)) 7.63 (dd, 3JH,H = 7.0 Hz, 3JP,H
=
CH(PPh2)), 7.51−7.42 (m, 5H, p-CH(PPh2) + m-CH(PPh2)), 7.38
3
3
4
10.5 Hz, 4H, o-CH(PPh2)), 7.50 (dd, JH,H = 7.0 Hz, JP,H = 10.5 Hz,
4H, o-CH(PPh2)), 7.48 (m, 2H, p-CH(PPh2)), 7.41 (m, 4H, m-
CH(PPh2)), 7.40 (m, 2H, p-CH(PPh2)), 7.32 (m, 4H, m-CH(PPh2)),
7.29 (d, 4JH,H = 2.5 Hz, 2H, CbH), 6.37 (dd, 4JH,H = 2.5 Hz, 3JP,H = 15.5
Hz, 2H, CdH), 3.51 (m, 2H, N-CH2-CH2-CH2-N), 3.16 (m, 2H, N-
CH2-CH2-CH2-N), 1.90 (m, 1H, N-CH2-CH2-CH2-N), 1.63 (m, 1H,
N-CH2-CH2-CH2-N), 1.42 (s, 18H, C(CH3)3), 1.09 (s, 18H,
C(CH3)3), 0.86 (s, 9H, O−C(CH3)3); 31P{1H} NMR (121.5 MHz,
THF-d8; δ (ppm)) 33.8 (s); 13C{1H} NMR (75 MHz, THF-d8; δ
(m, 2H, m-CH(PPh2)), 7.32 (m, 2H, m-CH(PPh2)), 7.27 (d, JH,H
=
2.0 Hz, 1H, CbH), 7.21 (d, 4JH,H = 2.0 Hz, 1H, CbH), 7.17 (m, 1H, p-
CH(PPh2)), 6.84 (dd, 4JH,H = 2.0 Hz, 3JP,H = 16.0 Hz, 1H, CdH), 6.82
(m, 2H, m-CH(PPh2)), 6.47 (dd, JH,H = 2.0 Hz, 3JP,H = 16.0 Hz, 1H,
4
4
CdH), 4.62 (m, 1H, N-CH-CH-N), 2.97 (m, JP,H = 3.0 Hz, 1H, N-
IV
CH-CH-N), 1.57 (m, 2H, CH2(cyclohexane)), 1.31 (s, 9H, Cc,a
-
CIV(CH3)3), 1.12 (m, 2H, CH2(cyclohexane)), 1.12 (s, b, 9H, O−
CIV(CH3)3), 1.10 (s, 9H, Cc,aIV- CIV(CH3)3), 1.09 (s, 9H, Cc,a
-
IV
CIV(CH3)3), 0.96 (s, 9H, Cc,aIV-CIV(CH3)3), 0.82 (m, 2H,
CH2(cyclohexane)), 0.66 (m, 1H, CH2(cyclohexane)), 0.48 (m, 1H,
CH2(cyclohexane)); 31P{1H} NMR (121.5 Hz, THF-d8; δ(ppm)):
27.6 (s, PV), 20.8 (s, PV); 13C{1H} NMR (75 MHz, THF-d8; δ (ppm))
3
3
(ppm)) 169.3 (d, JP,C = 1.5 Hz, CIV-O), 139.0 (d, JP,C = 8.0 Hz,
Cc,aIV), 134.3 (d, 2/3JP,C = 9.0 Hz, m-or o-CH(PPh2)), 134.1 (d, 2/3JP,C
=
9.0 Hz, m-or o-CH(PPh2), 133.9 (d, JP4,C = 10.5 Hz, Cc,aIV), 132.6 (d,
1JP,C = 87.8 Hz, CIV-PPh2), 131.9 (d, JP,C = 1.0 Hz, p-CH(PPh2)),
131.8 (d, 4JP,C = 1.0 Hz, p-CH(PPh2)), 128.9 (d, 2/3JP,C = 11.5 Hz, m-
3
δ 169.1 (m, CIV-O), 168.7 (m, CIV-O), 140.1 (d, JP,C = 8.0 Hz, CcIV),
3
139.5 (d, JP,C = 8.0 Hz, CcIV),137.3 (d, JP,C = 83.2 Hz, CIV(PPh2)),
3
1
2
137.1 (d, 1JP,C = 85.5 Hz, CIV(PPh2)), 134.9 (d, 3JP,C = 13.5 Hz, CaIV),
or o-CH(PPh2)), 128.4 (s, CbH), 128.1 (d, JP,C = 14.0 Hz, CdH),
113.3 (d, 1JP,C = 121.0 Hz, CIV(PPh2)), 69.8 (s, O-CIV(CH3)3), 47.1 (d,
2/3
135.2 (d,
J
= 9.0 Hz, m-or o-CH(PPh2)), 134.4 (d, 2/3JP,C= 9.0 Hz,
P,C
2JP,C = 7.0 Hz, N-CH2-CH2-CH2-N), 36.4 (t, JP,C = 9.0 Hz, N-CH2-
3
2/3
m-or o-CH(PPh2)), 134.0 (d,
J
= 8.2 Hz, m-or o-CH(PPh2));
P,C
CH2-CH2-N), 34.6 (s, O-CIV(CH3)3), 31.9 (s, Cc,aIV-CIV(CH3)3), 31.7
2/3
3
133.9 (d,
J
= 8.2 Hz, m-or o-CH(PPh2)), 133.8 (d, JP,C = 14.3
P,C
(s, Cc,aIV-CIV(CH3)3), 31.4 (s, Cc,aIV-CIV(CH3)3), 30.4 (s, Cc,a
-
IV
Hz, CaIV), 132.4 (d, JP,C = 86.2 Hz, CIV(PPh2)), 132.1 (d, JP,C = 2.1
1
4
CIV(CH3)3). Anal. Calcd for C63H83N2O4P2Y: C, 69.86; H, 7.72; N,
2.59. Found: C, 69.72; H, 7.66; N, 2.67.
4
Hz, p-CH(PPh2)), 132.0 (d, JP,C = 2.2 Hz, p-CH(PPh2)), 131.9 (d,
4JP,C = 2.2 Hz, p-CH(PPh2)), 131.8 (d, JP,C = 87.0 Hz,
1
Compound 4: L4 (312 mg, 0.30 mmol); 31P{1H} NMR spectrum
after the deprotonation δ 22.3 ppm, after the addition of
2/3
CIV(PPh2)),129.2 (d,
J
= 11.2 Hz, m-or o-CH(PPh2)), 129.1 (d,
P,C
2/3
J
= 11.2 Hz, m-or o-CH(PPh2)), 128.9 (d, 2/3JP,C = 11.2 Hz, m-or
P,C
1
[YCl3(THF)3.5] δ 36.5 ppm; yield 295 mg (0.26 mmol, 88%); H
2
o-CH(PPh2)), 128.8 (d, JP,C = 11.8 Hz, CdH), 128.6 (s, CbH), 128.3
NMR (300 MHz, THF-d8; δ (ppm)) 7.70 (ddd, 4JH,H = 1.5 Hz, 3JH,H
=
2
1
(s, CbH), 127.6 (d, JP,C = 13.5 Hz, CdH), 116.0 (d, JP,C = 118.4 Hz,
CIV(PPh2)), 113.2 (d, 1JP,C = 118.4 Hz, CIV(PPh2)), 61.3 (br s, N-CH-
CH-N), 54.0 (s, CH2(cyclohexane ring)), 49.0 (s, CH2(cyclohexane
ring)), 36.1 (s, CIV(CH3)3), 35.9 (s, CIV(CH3)3), 34.9 (s, CIV(CH3)3),
34.6 (s, CIV(CH3)3), 32.2 (s, CIV(CH3)3), 32.0 (s, CIV(CH3)3), 30.2 (s,
CIV(CH3)3). Anal. Calcd for C66H87N2O4P2Y: C, 70.57; H, 7.81; N,
2.49. Found: C, 70.66; H, 7.65; N, 2.59.
3
4
8.0 Hz, JP,H = 10.5 Hz, 4H, o-CH(PPh2)), 7.49 (ddd, JH,H = 1.5 Hz,
3JH,H = 8.0 Hz, JP,H = 10.5 Hz, 4H, o-CH(PPh2)), 7.42 (m, 4H, m-
3
CH(PPh2)), 7.34 (m, 4H, m-CH(PPh2)), 7.29 (d, 4JH,H = 2.0 Hz, 2H,
CbH), 7.15 (m, 2H, p-CH(PPh2)), 7.07 (m, 2H, p-CH(PPh2)), 6.43
4
3
3
(dd, JH,H = 2.0 Hz, JP,H = 15.5 Hz, 2H, CdH), 3.39 (d, JP,H = 12.5
3
Hz, 1H, N−CH2), 3.34 (d, JP,H = 12.5 Hz, 1H, N−CH2), 2.85 (d,
3JP,H = 12.0 Hz, 1H, N-CH2), 2.79 (d, JP,H = 12.5 Hz, 1H, N-CH2),
3
Compound 2: L2 (316 mg, 0.30 mmol); 31P{1H} NMR spectrum
after deprotonation δ 18 ppm, after addition of [YCl3(THF)3.5] δ 22.6
ppm, 27.5 ppm (equal intensities); yield 293 mg (0.26 mmol, 87%);
1.36 (s, 18H, Cc,aIV-CIV(CH3)3), 1.02 (s, 18H, Cc,aIV-CIV(CH3)3), 0.80
(s, 9H, O-CIV(CH3)3), 0.62 (s, 3H, N-CH2-CIV(CH3)2), 0.50 (s, 3H,
N-CH2-CIV(CH3)2); 31P{1H} NMR (121.5 MHz, THF-d8; δ (ppm))
33.4 (s); 13C{1H} NMR (75 MHz, THF-d8; δ (ppm)) 169.8 (m, CIV-
3
1H NMR (300 MHz, THF-d8; δ (ppm)) 7.89 (ddd, JP,H = 10.6 Hz,
4
3
3JH,H = 7.5 Hz, JH,H = 1.0 Hz, 2H, o-CH(PPh2)), 7.75 (ddd, JP,H
=
O), 137.9 (d, 3JP,C = 7.7 Hz, Cc,aIV), 133.8 (d, 2/3JP,C = 8.5 Hz, m-or o-
12.3 Hz, 3JH,H = 7.5 Hz, 4JH,H = 1.0 Hz, 2H, o-CH(PPh2)), 7.62 (ddd,
3JP,H = 11.2 Hz, 3JH,H = 7.5 Hz, 4JH,H = 1.0 Hz, 2H, o-CH(PPh2)), 7.56
(ddd, JP,H = 11.0 Hz, JH,H = 7.5 Hz, JH,H = 1.0 Hz, 2H, o-
2/3
CH(PPh2)), 133.6 (d,
J
= 8.5 Hz, m-or o-CH(PPh2)), 132.8 (d,
P,C
1JP,C = 97.0 Hz, CIV(PPh2)), 131.7 (d, JP,C = 87.4 Hz, CIV(PPh2)),
1
3
3
4
130.9 (s, p-CH(PPh2)), 130.8 (s, p-CH(PPh2)), 127.9 (d, 2/3JP,C = 11.0
CH(PPh2)), 7.51−7.42 (m, 5H, p-CH(PPh2) + m-CH(PPh2)), 7.38
2/3
Hz, m-or o-CH(PPh2)), 128.8 (d,
J
= 11.0 Hz, m-or o-
P,C
4
(m, 2H, m-CH(PPh2)), 7.32 (m, 2H, m-CH(PPh2)), 7.27 (d, JH,H
=
2
CH(PPh2)), 127.5 (s, CbH), 127.5 (d, JP,C = 12.5 Hz, CdH), 112.4
2.0 Hz, 1H, CbH), 7.21 (d, 4JH,H = 2.0 Hz, 1H, CbH), 7.17 (m, 1H, p-
CH(PPh2)), 6.84 (dd, 4JH,H = 2.0 Hz, 3JP,H = 16.0 Hz, 1H, CdH), 6.82
(d, 1JP,C = 121.0 Hz, CIV-PPh2), 69.2 (s, O-CIV(CH3)3), 58.1 (d, 2JP,C
=
7.5 Hz, N-CH2), 38.1 (t, 3JP,C = 12.5 Hz, N-CH2-CIV-CH2-N), 35.1 (s,
Cc,aIV-CIV(CH3)3), 33.7 (s, O-CIV(CH3)), 33.5 (s, Cc,aIV-CIV(CH3)3),
31.0 (s, Cc,aIV-CIV(CH3)3), 29.6 (s, Cc,aIV-CIV(CH3)3), 26.1 (s,
CIV(CH3)2), 25.2 (s, N-CH2-CIV(CH3)2). Anal. Calcd for
C65H86N2O4P2Y: C, 70.32; H, 7.81; N, 2.52. Found: C, 70.40; H,
7.74; N, 2.61.
(m, 2H, m-CH(PPh2)), 6.47 (dd, JH,H = 2.0 Hz, 3JP,H = 16.0 Hz, 1H,
4
CdH), 4.62 (m, 1H, N-CH-CH-N), 2.97 (m, 1H, N-CH-CH-N), 1.57
(m, 2H, CH2(cyclohexane)), 1.31 (s, 9H, Cc,aIV-CIV(CH3)3), 1.12 (m,
2H, CH2(cyclohexane)), 1.12 (s, b, 9H, O-CIV(CH3)3), 1.10 (s, 9H,
IV
Cc,aIV-CIV(CH3)3), 1.09 (s, 9H, Cc,aIV-CIV(CH3)3), 0.96 (s, 9H, Cc,a
-
CIV(CH3)3), 0.82 (m, 2H, CH2(cyclohexane)), 0.66 (m, 1H,
CH2(cyclohexane)), 0.48 (m, 1H, CH2(cyclohexane)); 31P{1H}
NMR (121.5 MHz, THF-d8; δ(ppm)) 27.6 (s, PV), 20.8 (s, PV);
13C{1H} NMR (75 MHz, THF-d8; δ (ppm)) 169.1 (m, CIV-O), 140.1
(d, 3JP,C = 8.0 Hz, CcIV), 134.0 (d, 2/3JP,C = 8.2 Hz, m-or o-CH(PPh2));
Compound 4a: L4 (312 mg, 0.30 mmol), KOEt (25.2 mg, 0.3
mmol) was used instead of KOtBu; 31P{1H}NMR spectrum after
deprotonation δ 22.3 ppm, after addition of [YCl3(THF)3.5] δ 36.5
ppm; yield 265 mg (0.25 mmol, 81%); 1H NMR (300 MHz, THF-d8;
4
3
3
δ (ppm)): 7.75 (ddd, JH,H = 1.5 Hz, JH,H = 7.0 Hz, JP,H = 11.5 Hz,
2/3
3
4
3
3
133.9 (d,
J
= 8.2 Hz, m-or o-CH(PPh2)), 133.8 (d, JP,C = 14.3
P,C
4H, o-CH(PPh2)), 7.58 (ddd, JH,H = 1.5 Hz, JH,H = 7.0 Hz, JP,H
=
Hz, CaIV), 132.4 (d, JP,C = 86.2 Hz, CIV(PPh2)), 132.0 (d, JP,C = 2.2
1
4
11.5 Hz, 4H, o-CH(PPh2)), 7.45 (m, 8H, m-CH(PPh2) + p-
4
4
4
Hz, p-CH(PPh2)), 131.9 (d, JP,C = 2.2 Hz, p-CH(PPh2)), 131.8 (d,
CH(PPh2)), 7.35 (d, JH,H = 2.0 Hz, 2H, CbH), 7.33 (vtd, JP,H
=
=
2/3
1JP,C = 87.0 Hz, CIV(PPh2)), 129.2 (d,
J
= 11.2 Hz, m-or o-
= 11.2 Hz, m-or o-CH(PPh2)), 128.6 (s,
CbH), 127.6 (d, JP,C = 13.5 Hz, CdH), 113.2 (d, JP,C = 118.4 Hz,
CIV(PPh2)), 61.3 (br s,, N-CH-CH-N), 54.0 (s, CH2(cyclohexane
ring)), 49.0 (s, CH2(cyclohexane ring)), 34.9 (s, CIV(CH3)3), 34.6 (s,
CIV(CH3)3), 32.2 (s, CIV(CH3)3), 32.0 (s, CIV(CH3)3), 30.2 (s,
3
3
4
2.5 Hz, JH,H = J′H,H = 7.0 Hz, 4H, m-CH(PPh2)), 6.77 (dd, JH,H
P,C
2/3
2.0 Hz, 3JP,H = 15.0 Hz, 2H, CdH), 4.03 (t, 3JH,H = 6.0 Hz, 1H, O-CH2-
CH(PPh2)), 129.1 (d,
J
P,C
2
1
3
3
CH3), 4.01 (t, JH,H = 6.0 Hz, 1H, O-CH2-CH3), 3.69 (d, JP,H= 15.0
Hz, 1H, N-CH2), 3.64 (d, 3JP,H = 15.0 Hz, 1H, N-CH2), 2.68 (d, JP,H
3
= 12.5 Hz, 1H, N-CH2), 2.61 (d, 3JP,H = 12.5 Hz, 1H, N-CH2), 1.29 (s,
18H, Cc,aIV-CIV(CH3)3), 1.19 (s, 18H, Cc,aIV-CIV(CH3)3), 0.90 (m, 3H,
1481
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
O-CH2-CH3), 0.46 (s, 3H, N-CH2-CIV(CH3)2), 0.44 (s, 3H, N-CH2-
CIV(CH3)2); 31P{1H} NMR (121.5 MHz, THF-d8; δ (ppm)) 33.4 (s);
13C{1H} NMR (75 MHz, THF-d8; δ (ppm)) 169.1 (m, CIV-O), 139.0
(d, 3JP,C = 7.5 Hz, Cc,aIV), 134.8 (d, 2/3JP,C = 9.0 Hz, m-or o-CH(PPh2)),
spectroscopy and GPC. The conversion of LA to PLA was determined
by integration of the methine proton peaks of the 1H NMR spectra” δ
5.00−5.30. The Ps value was determined by integration of the methine
1
region of the homonuclear decoupled H NMR spectrum: δ 5.1−
5.24.12 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). The refractive angle increment for
polylactide (dn/dc) in THF was 0.042 mL g−1.16
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 9717 and Shelxl-97.18 ORTEP drawings were made
using ORTEP III for Windows.17,19 X-ray crystallographic data for 1,
4, and 6 are given in Table 3.
2/3
3
134.3 (d,
J
= 9.0 Hz, m-or o-CH(PPh2)), 134.0 (d, JP,C = 14.5
P,C
Hz, Cc,aIV), 133.1 (d, 1JP,C = 88.5 Hz, CIV(PPh2)), 131.7 (d, 1JP,C = 90.5
Hz, CIV(PPh2)), 131.7 (s, p-CH(PPh2) + CbH), 127.9 (d, JP,C = 13.0
2
Hz, CdH), 114.4 (d, 1JP,C = 120.0 Hz, CIV-PPh2), 61.6 (s, O-CH2-CH3),
2
3
58.2 (d, JP,C = 7.5 Hz, N-CH2), 37.5 (t, JP,C = 12.5 Hz, N-CH2-CIV-
CH2-N), 34.9 (s, Cc,aIV-CIV(CH3)3), 33.5 (s, Cc,aIV-CIV(CH3)3), 31.3 (s,
O-CH2-CH3), 30.9 (s, Cc,aIV-CIV(CH3)3), 29.3 (s, Cc,aIV-CIV(CH3)3),
26.2 (s, CIV(CH3)2), 22.3 (s, N-CH2-CIV(CH3)2). Anal. Calcd for
C63H82N2O4P2Y: C, 69.92; H, 7.64; N, 2.59. Found: C, 69.96; H, 7.66;
N, 2.65.
Compound 5: L5 (314 mg, 0.30 mmol); 31P{1H} NMR spectrum
after deprotonation δ 13.2 ppm, after addition of [YCl3(THF)3.5] δ
1
27.3 ppm; yield 290 mg (0.26 mmol, 87%); H NMR (300 MHz,
Table 3. X-ray Crystallographic Data for Compounds 1, 4,
and 6
4
3
3
THF-d8; δ (ppm)): 7.71 (ddd, JH,H = 1.5 Hz, JH,H = 8.0 Hz, JP,H
=
12.0 Hz, 4H, o-CH(PPh2)), 7.58 (ddd, JH,H = 1.5 Hz, 3JH,H = 8.0 Hz,
4
3JP,H = 12.0 Hz, 4H, o-CH(PPh2)), 7.58 (m, 2H, p-CH(PPh2)), 7.51
1
4
6
4
(m, 4H, m-CH(PPh2)), 7.45 (m, 2H, p-CH(PPh2)), 7.31 (d, JH,H
=
formula
C66H87N2O4P2Y
C65H87N2O4P2Y C66H81N2O4P2Y,
2(C6H12)
4
3.0 Hz, 2H, CbH), 7.30 (m, 4H, m-CH(PPh2)), 6.37 (dd, JH,H = 3.0
3
3
4
Hz, JP,H = 15.5 Hz, 2H, CdH), 6.27 (ddd, JH,H = 6.0 Hz, JH,H = 3.5
Mr
1123.23
1111.22
1285.49
Hz, 4JP,H = 3.0 Hz, 2H, CeH), 6.01 (dd, 3JH,H = 6.0 Hz, 4JH,H = 3.5 Hz,
space
group
P21/m
P21/c
C2/c
2H, CfH), 1.27 (s, 18H, Cc,aIV-CIV(CH3)3), 1.04 (s, 18H, Cc,a
-
IV
CIV(CH3)3), 0.56 (b, 9H, O-CIV(CH3)3); 31P{1H} NMR (121.5 MHz,
THF-d8; δ (ppm)) 24.9 (s); 13C{1H} NMR (75 MHz, THF-d8; δ
(ppm)) 170.1 (br s, CIV-O), 144.8 (dd, 2JP,C = 19.0 Hz, 3JP,C = 4.0 Hz,
T (°C)
λ (Å)
−123
−123
−123
́
0.71069
10.652(1)
26.052(1)
11.446(1)
90.00
0.71069
11.409(1)
27.849(1)
21.019(1)
90.00
0.71069
35.816(1)
19.399(1)
25.180(1)
90.00
a (Å)
CIV-N), 139.0 (d, JP,C = 7.5 Hz, Cc,aIV), 134.4 (d, JP,C = 16.0 Hz,
3
3
b (Å)
Cc,aIV), 134.0 (d,
J = 10.0 Hz, m-or o-CH(PPh2)), 132.3 (s, p-
2/3
P,C
1
c (Å)
CH(PPh2)), 132.0 (s, p-CH(PPh2)), 132.0 (d, JP,C = 88.5 Hz,
CIV(PPh2)), 131.1 (d, JP,C = 87.0 Hz, CIV(PPh2)), 129.3 (d,
J
=
1
2/3
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
P,C
2/3
107.083(1)
90.00
113.905(5)
90.00
123.852(1)
90.00
11.5 Hz, m-or o-CH(PPh2)), 128.8 (s, CbH), 128.7 (d,
J
= 11.5
P,C
Hz, m-or o-CH(PPh2)), 128.4 (d, 2JP,C = 14.0 Hz, CdH), 122.2 (d, 3JP,C
= 10.0 Hz, CeH), 118.3 (s, Cf H), 114.8 (d, 1JP,C = 124.0 Hz, CIV-PPh2),
35.7 (s, Cc,aIV-CIV(CH3)3), 34.3 (s, Cc,aIV-CIV(CH3)3), 33.9 (b, O-
CIV(CH3)3), 31.8 (s, (b, O-CIV(CH3)3), 31.7 (s, Cc,aIV-CIV(CH3)3),
30.3 (s, Cc,aIV-CIV(CH3)3). Anal. Calcd for C66H80N2O4P2Y: C, 71.02;
H, 7.22; N, 2.51. Found: C, 70.85; H, 7.43; N, 2.55.
3036.2(4)
2
6105.5(6)
4
14529.2(10)
8
d (g cm−3
)
1.229
1.209
1.175
μ (cm−1
)
1.061
1.054
0.895
a
R1
0.0512
0.11277
0.0783
0.0702
Compound 6:11 L6 (295 mg, 0.30 mmol); the 31P{1H} NMR
spectrum after deprotonation showed only one singlet at +20.3 ppm;
the product was isolated as colorless crystals; yield 250 mg (0.26
mmol, 88%);11 1H NMR (300 MHz, THF-d8; δ (ppm)) 7.63 (dd, 3JP,H
b
wR2
0.1588
0.1679
a
b
2
2
R1 =∑||Fo| − |Fc||/∑|Fo|. wR2 ={∑[w(Fo − Fc )2]/
2
2
∑[w(Fo )2]}1/2; w−1 = σ2(Fo ) + (aP)2 + bP.
3
= 11.5 Hz, JH,H = 7.0 Hz, 4H, o-CH(PPh2)), 7.54 (m, 2H, p-
CH(PPh2)), 7.48 (m, 5H, p-CH(PPh2) + m-CH(PPh2)), 7.40 (dd,
3JP,H = 11.5 Hz, JH,H = 7.0 Hz, 4H, o-CH(PPh2)), 7.39 (m, 1H, m-
3
4
3
CH(PPh2)), 6.92 (d, JH,H = 3.0 Hz, 2H, CbH), 5.94 (dd, JP,H = 15.5
4
Hz, JH,H = 3.0 Hz, 2H, CdH), 3.35 (s, 6H, -OCH3), 3.31 (m, 2H, N-
ASSOCIATED CONTENT
* Supporting Information
CH2-CH2-N), 3.18 (m, b, 2H, N-CH2-CH2-N), 1.39 (s, 18H, CaIV
-
■
CIV(CH3)3), 0.84 (s, b, 9H, O-CIV(CH3)3); 31P{1H} NMR (121.5
MHz, THF-d8; δ (ppm)) 31.6 (s); 13C{1H} NMR (75 MHz, THF-d8;
S
CIF files, tables, figures, and text giving crystallographic data for
1, 2, 4, and 6, additional experimental details, and character-
ization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
δ (ppm)) 168.8 (br s, CIV-O), 148.2 (d, JP,C = 20.0 Hz, CcIV), 141.1
3
(d, 3JP,C = 9.5 Hz, CaIV), 134.1 (d, 2JP,C = 9.0 Hz, o-CH(PPh2)), 133.6
(d, 2JP,C = 9.0 Hz, o-CH(PPh2)), 132.3 (d, 1JP,C = 88.0 Hz, CIV(PPh2)),
4
1
132.0 (d, JP,C = 1.5 Hz, p-CH(PPh2)), 131.7 (d, JP,C = 90.0 Hz,
CIV(PPh2)), 131.7 (d, JP,C = 1.5 Hz, p-CH(PPh2)), 129.0 (d, JP,C
=
4
3
AUTHOR INFORMATION
Corresponding Author
*E-mail: c.k.williams@imperial.ac.uk (C.K.W.).
3
10.5 Hz, m-CH(PPh2)), 128.9 (d, JP,C = 10.5 Hz, m-CH(PPh2)),
■
120.3 (d, JP,C = 1.0 Hz, CbH), 114.2 (d, 2JP,C = 14.5 Hz, CdH), 111.6
4
(d, 1JP,C = 123.0 Hz, CIV-PPh2), 55.5 (s, O-CH3), 52.1 (dd, 2/3JP,C = 6.5
3/3
Hz,
J
= 18.0 Hz, N-CH2-CH2-N), 35.7 (s, CaIV-CIV(CH3)3), 33.3
P,C
Author Contributions
(s, O−CIV(CH3)3), 30.2 (s, CaIV-CIV(CH3)3), 26.0 (s, O-CIV(CH3)3).
Anal. Calcd for C52H61N2O5P2Y: C, 66.10; H, 6.51; N, 2.96. Found: C,
65.89; H, 6.42; N, 2.85.
§These authors contributed equally.
Notes
General Polymerization Procedure. In a glovebox, rac-lactide
(288 mg, 2 mmol) was dissolved in THF (1.8 mL). A stock solution of
initiator (0.2 mL, 0.02 M) was injected into the reaction mixture, such
that the overall concentration of lactide was 1 M and that of initiator
was 2 mM. Aliquots were taken from the reaction under a nitrogen
atmosphere and quenched with hexane (1−2 mL), and the solvent was
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The CNRS, the Ecole Polytechnique, and the EPSRC (DTG
studentship to C.B., EP/C544846/1 and EP/C544838/1) are
acknowledged for funding this research.
1
allowed to evaporate. The crude product was analyzed by H NMR
1482
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483

Organometallics
Article
J. 2011, 17, 1872−1883. (p) Zhang, J.; Qiu, J.; Yao, Y.; Zhang, Y.;
Wang, Y.; Shen, Q. Organometallics 2012, 31, 3138−3148. (q) Nie, K.;
Fang, L.; Yao, Y.; Zhang, Y.; Shen, Q.; Wang, Y. Inorg. Chem. 2012, 51,
11133−11143.
REFERENCES
■
(1) (a) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek,
G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.;
Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T.
Science 2006, 311, 484−489. (b) Williams, C. K.; Hillmyer, M. A.
Polym. Rev. 2008, 48, 1−10. (c) Platel, R. H.; Hodgson, L. M.;
Williams, C. K. Polym. Rev. 2008, 48, 11−63. (d) Dove, A. P. Chem.
Commun. 2008, 6446−6470. (e) Inkinen, S.; Hakkarainen, M.;
Albertsson, A. C.; Sodergard, A. Biomacromolecules 2011, 12, 523−532.
(2) (a) Gross, R. A.; Kumar, A.; Kalra, B. Chem. Rev. 2001, 101,
2097−2124. (b) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D.
Chem. Rev. 2004, 104, 6147−6176. (c) Kamber, N. E.; Jeong, W.;
Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L.
Chem. Rev. 2007, 107, 5813−5840. (d) Ajellal, N.; Carpentier, J. F.;
Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.;
Trifonov, A. Dalton Trans. 2010, 39, 8363−8376.
(7) (a) Ma, H.; Okuda, J. Macromolecules 2005, 38, 2665−2673.
(b) Ma, H. Y.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2006,
45, 7818−7821. (c) Ma, H.; Spaniol, T. P.; Okuda, J. Inorg. Chem.
2008, 47, 3328−3339. (d) Buffet, J.-C.; Kapelski, A.; Okuda, J.
Macromolecules 2010, 43, 10201−10203. (e) Kapelski, A.; Buffet, J.-C.;
Spaniol, T. P.; Okuda, J. Chem. Asian J. 2012, 7, 1320−1330.
(8) (a) Hodgson, L. M.; White, A. J. P.; Williams, C. K. J. Polym. Sci.,
Part A: Polym. Chem. 2006, 44, 6646−6651. (b) Platel, R. H.;
Hodgson, L. M.; White, A. J. P.; Williams, C. K. Organometallics 2007,
26, 4955−4963. (c) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg.
Chem. 2008, 47, 6840−6849. (d) Hodgson, L. M.; Platel, R. H.; White,
A. J. P.; Williams, C. K. Macromolecules 2008, 41, 8603−8607.
(e) Platel, R. H.; White, A. J. P.; Williams, C. K. Chem. Commun. 2009,
4115−4117. (f) Buchard, A.; Auffrant, A.; Ricard, L.; Le Goff, X. F.;
Platel, R. H.; Williams, C. K.; Le Floch, P. Dalton Trans. 2009, 10219−
10222. (g) Buchard, A.; Platel, R. H.; Auffrant, A.; Le Goff, X. F.; Le
Floch, P.; Williams, C. K. Organometallics 2010, 29, 2892−2900.
(h) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2011, 50,
7718−7728.
(3) (a) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486−
494. (b) Dijkstra, P. J.; Du, H. Z.; Feijen, J. Polym. Chem. 2011, 2,
520−527.
(4) (a) McClain, S. J.; Ford, T. M.; Drysdale, N. E. Polym. Prep. 1992,
463−464. (b) Stevels, W. M.; Ankone, M. J. K.; Dijkstra, P. J.; Feijen,
J. Macromol. Chem. Phys. 1995, 196, 1153−1161. (c) Stevels, W. M.;
Ankone, M. J. K.; Dijkstra, P. J.; Feijen, J. Macromolecules 1996, 29,
3332−3333. (d) Simic, V.; Spassky, N.; Hubert-Pfalzgraf, L. G.
Macromolecules 1997, 30, 7338−7340. (e) Simic, V.; Girardon, V.;
Spassky, N.; Hubert-Pfalzfraf, L. G.; Duda, A. Polym. Degrad. Stab.
1998, 59, 227−229. (f) Chamberlain, B. M.; Jazdzewski, B. A.; Pink,
M.; Hillmyer, M. A.; Tolman, W. B. Macromolecules 2000, 33, 3970−
3977. (g) Chamberlain, B. M.; Sun, Y.; Hagadorn, J. R.; Hemmesch, E.
W.; Young, V. G., Jr.; Pink, M.; Hillmyer, M. A.; Tolman, W. B.
Macromolecules 1999, 32, 2400−2402.
(5) (a) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121,
4072−4073. (b) Ovitt, T. M.; Coates, G. W. J. Polym. Sci., Part A:
Polym. Chem. 2000, 38, 4686. (c) Broderick, E. M.; Thuy-Boun, P. S.;
Guo, N.; Vogel, C. S.; Sutter, J.; Miller, J. T.; Meyer, K.; Diaconescu, P.
L. Inorg. Chem. 2011, 50, 2870−2877. (d) Broderick, E. M.; Guo, N.;
Vogel, C. S.; Xu, C.; Sutter, J. r.; Miller, J. T.; Meyer, K.;
Mehrkhodavandi, P.; Diaconescu, P. L. J. Am. Chem. Soc. 2011, 133,
9278−9281. (e) Li, W.; Zhang, Z.; Yao, Y.; Zhang, Y.; Shen, Q.
Organometallics 2012, 31, 3499−3511. (f) Zhang, Z. J.; Xu, X. P.; Li,
W. Y.; Yao, Y. M.; Zhang, Y.; Shen, Q.; Luo, Y. J. Inorg. Chem. 2009,
48, 5715−5724. (g) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.;
Carpentier, J. F. Organometallics 2009, 28, 1469−1475. (h) Grunova,
E.; Kirillov, E.; Roisnel, T.; Carpentier, J. F. Organometallics 2008, 27,
5691−5698.
(9) (a) Wang, L.; Liu, D.; Cui, D. Organometallics 2012, 31, 6014−
6021. (b) Shen, X.; Xue, M.; Jiao, R.; Ma, Y.; Zhang, Y.; Shen, Q.
Organometallics 2012, 31, 6222−6230. (c) Li, G.; Lamberti, M.;
Mazzeo, M.; Pappalardo, D.; Roviello, G.; Pellecchia, C. Organo-
metallics 2012, 31, 1180−1188. (d) Zhang, Z.; Cui, D. Chem. Eur. J.
2011, 17, 11520−11526. (e) D’Auria, I.; Mazzeo, M.; Pappalardo, D.;
Lamberti, M.; Pellecchia, C. J. Polym. Sci., Part A: Polym. Chem. 2011,
49, 403−413. (f) Mahrova, T. V.; Fukin, G. K.; Cherkasov, A. V.;
Trifonov, A. A.; Ajellal, N.; Carpentier, J. F. Inorg. Chem. 2009, 48,
4258−4266. (g) Arnold, P. L.; Buffet, J. C.; Blaudeck, R.; Sujecki, S.;
Wilson, C. Chem. Eur. J. 2009, 15, 8241−8250. (h) Arnold, P. L.;
Buffet, J. C.; Blaudeck, R. P.; Sujecki, S.; Blake, A. J.; Wilson, C. Angew.
Chem., Int. Ed. 2008, 47, 6033−6036. (i) Ajellal, N.; Lyubov, D. M.;
Sinenkov, M. A.; Fukin, G. K.; Cherkasov, A. V.; Thomas, C. M.;
Carpentier, J. F.; Trifonov, A. A. Chem. Eur. J. 2008, 14, 5440−5448.
(j) Skvortsov, G. G.; Yakovenko, M. V.; Castro, P. M.; Fukin, G. K.;
Cherkasov, A. V.; Carpentier, J. F.; Trifonov, A. A. Eur. J. Inorg. Chem.
2007, 3260−3267. (k) Heck, R.; Schulz, E.; Collin, J.; Carpentier, J. F.
J. Mol. Catal. A: Chem. 2007, 268, 163−168. (l) Alaaeddine, A.;
Amgoune, A.; Thomas, C. M.; Dagorne, S.; Bellemin-Laponnaz, S.;
Carpentier, J.-F. Eur. J. Inorg. Chem. 2006, 3652−3658.
(10) Cao, T.-P.-A.; Buchard, A.; Le Goff, X. F.; Auffrant, A.; Williams,
C. K. Inorg. Chem. 2012, 51, 2157−2169.
(6) (a) Cai, C. X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J. F.
Chem. Commun. 2004, 330−331. (b) Bonnet, F.; Cowley, A. R.;
Mountford, P. Inorg. Chem. 2005, 44, 9046−9055. (c) Amgoune, A.;
Thomas, C. M.; Roisnel, T.; Carpentier, J. F. Chem. Eur. J. 2006, 12,
169−179. (d) Amgoune, A.; Thomas, C. M.; Carpentier, J. F.
Macromol. Rapid Commun. 2007, 28, 693−697. (e) Amgoune, A.;
Thomas, C. M.; Carpentier, J. F. Pure Appl. Chem. 2007, 79, 2013−
2030. (f) Miao, W.; Li, S. H.; Zhang, H. X.; Cui, D.; Wang, Y. R.;
Huang, B. T. J. Organomet. Chem. 2007, 692, 4828−4834. (g) Liu, X.
L.; Shang, X. M.; Tang, T.; Hu, N. H.; Pei, F. K.; Cui, D. M.; Chen, X.
S.; Jing, X. B. Organometallics 2007, 26, 2747−2757. (h) Dyer, H. E.;
Huijser, S.; Schwarz, A. D.; Wang, C.; Duchateau, R.; Mountford, P.
Dalton Trans. 2008, 32−35. (i) Dyer, H. E.; Huijser, S.; Susperregui,
N.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford,
P. Organometallics 2010, 29, 3602−3621. (j) Clark, L.; Deacon, G. B.;
Forsyth, C. M.; Junk, P. C.; Mountford, P.; Townley, J. P. Dalton
Trans. 2010, 39, 6693−6704. (k) Clark, L.; Cushion, M. G.; Dyer, H.
E.; Schwarz, A. D.; Duchateau, R.; Mountford, P. Chem. Commun.
2010, 46, 273−275. (l) Nie, K.; Gu, X. Y.; Yao, Y. M.; Zhang, Y.; Shen,
Q. Dalton Trans. 2010, 39, 6832−6840. (m) Zhao, W.; Cui, D. M.;
Liu, X. L.; Chen, X. S. Macromolecules 2010, 43, 6678−6684.
(n) Sinenkov, M.; Kirillov, E.; Roisnel, T.; Fukin, G.; Trifonov, A.;
Carpentier, J.-F. Organometallics 2011, 30, 5509−5523. (o) Bouyahyi,
M.; Ajellal, N.; Kirillov, E.; Thomas, C. M.; Carpentier, J. F. Chem. Eur.
(11) Bakewell, C.; Cao, T.-P.-A.; Long, N.; Le Goff, X. F.; Auffrant,
A.; Williams, C. K. J. Am. Chem. Soc. 2012, 134, 20577−20580.
(12) Coudane, J.; Ustariz-Peyret, C.; Schwach, G.; Vert, M. J. Polym.
Sci., Part A: Polym. Chem. 1997, 35, 1651−1658.
(13) Ding, K.; Miranda, M. O.; Moscato-Goodpaster, B.; Ajellal, N.;
Breyfogle, L. E.; Hermes, E. D.; Schaller, C. P.; Roe, S. E.; Cramer, C.
J.; Hillmyer, M. A.; Tolman, W. B. Macromolecules 2012, 45, 5387−
5396.
(14) (a) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; Pugh, R. I.;
White, A. J. P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15343−15348.
(b) Du, H.; Pang, X.; Yu, H.; Zhuang, X.; Chen, X.; Cui, D.; Wang, X.;
Jing, X. Macromolecules 2007, 40, 1904.
(15) Sobota, P.; Utko, J.; Szafert, S. Inorg. Chem. 1994, 33, 5203−
5206.
(16) Dagorne, S.; Le Bideau, F.; Welter, R.; Bellemin-Laponnaz, S.;
Maisse-Franco̧ is, A. Chem. Eur. J. 2007, 13, 3202−3217.
(17) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119.
(18) Sheldrick, G. M., SHELXL-97; Universitat Gottingen,
̈
̈
Gottingen, Germany, 1997.
̈
(19) Farrugia, L. J. ORTEP III; Department of Chemistry, University
of Glasgow, Glasgow, Scotland, 2001.
1483
dx.doi.org/10.1021/om301129k | Organometallics 2013, 32, 1475−1483