Aluminum, indium, and mixed yttrium-lithium complexes supported by a chiral binap-based fluorinated dialkoxide: Structural features and heteroselective ROP of lactide

Article abstract of DOI:10.1021/om500458g

The new chiral Binap-based fluorinated dialcohol proteo-ligand {ONNO}H₂ has been prepared under its enantiomerically pure and racemic forms following reaction of (R)- or (rac)-1,1′-binaphthyl-2,2′-diamine with 5,5,5-trifluoro-4-hydroxy-4-(trifluoromethyl)pentan-2-one. The racemic proteo-ligand reacts with the trivalent metallic precursors AlEt₂Cl and In(CH₂SiMe₃)₃ to yield the complexes (rac)-{ONNO}AlCl (1) and (rac)-{ONNO}In(CH₂SiMe₃) (2), while the reaction of (R)-{ONNO}H₂ with a 1:1 mixture of Y(N(SiMe₃)₂)₃ and LiN(SiMe₃)₂ gives the enantiomerically pure heterobimetallic [((R)-{ONNO})₂Y·Li] (3). Complex 3 acts as a single-component initiator for the near-perfect heteroselective ring-opening polymerization (ROP) of racemic lactide, yielding polymers with P_r up to 0.99 under mild conditions. It also produces syndiotactic-enriched polylactide (P_r = 0.80) by ROP of meso lactide.

Full text of DOI:10.1021/om500458g

Article
pubs.acs.org/Organometallics
Aluminum, Indium, and Mixed Yttrium−Lithium Complexes
Supported by a Chiral Binap-Based Fluorinated Dialkoxide: Structural
Features and Heteroselective ROP of Lactide
Nicolas Maudoux,† Thierry Roisnel,‡ Jean-Francois Carpentier,*,† and Yann Sarazin*,†
̧
†Institut des Sciences Chimiques de Rennes, Organometallics: Materials and Catalysis Laboratories, UMR 6226 CNRS-Universite
Rennes 1, F-35042 Rennes Cedex, France
́
de
‡Institut des Sciences Chimiques de Rennes, Centre de Diffraction X, UMR 6226 CNRS-Universite
́
de Rennes 1, F-35042 Rennes
Cedex, France
S
* Supporting Information
ABSTRACT: The new chiral Binap-based fluorinated dial-
cohol proteo-ligand {ONNO}H2 has been prepared under its
enantiomerically pure and racemic forms following reaction of
(R)- or (rac)-1,1′-binaphthyl-2,2′-diamine with 5,5,5-trifluoro-
4-hydroxy-4-(trifluoromethyl)pentan-2-one. The racemic pro-
teo-ligand reacts with the trivalent metallic precursors AlEt2Cl
and In(CH2SiMe3)3 to yield the complexes (rac)-{ONNO}-
AlCl (1) and (rac)-{ONNO}In(CH2SiMe3) (2), while the
reaction of (R)-{ONNO}H2 with a 1:1 mixture of Y(N-
(SiMe3)2)3 and LiN(SiMe3)2 gives the enantiomerically pure heterobimetallic [((R)-{ONNO})2Y·Li] (3). Complex 3 acts as a
single-component initiator for the near-perfect heteroselective ring-opening polymerization (ROP) of racemic lactide, yielding
polymers with Pr up to 0.99 under mild conditions. It also produces syndiotactic-enriched polylactide (Pr = 0.80) by ROP of meso
lactide.
the Oalkoxide atoms16 afford complexes that show comparable
efficiency as ROP catalysts to their aryloxide analogues.11a,b,d,17
We report here on three aluminum, indium, and mixed
yttrium−lithium complexes supported by a new chiral Binap-
based fluorinated dialkoxide. It is shown that the Y−Li complex
is a catalyst for the heteroselective ROP of rac-LA that yields
near-perfect hPLA and also generates syndiotactic-enriched
PLA by ROP of meso-LA.
INTRODUCTION
■
Isotactic poly(lactide) (iPLA), a crystalline polymer accessed by
ring-opening polymerization (ROP) of L-lactide, is the
archetype of a “green” polymer featuring good physical
properties.1 Because it can produce iPLA with improved
properties (Tm up to 220 °C) compared to commercially
available PLAs (Tm = 170−180 °C), the isoselective ROP of
racemic lactide (rac-LA) has become the focus of intense
investigations.2 Catalysts affording isospecific ROP reactions
leading to iPLA have been sought for 20 years. Breakthroughs
have been achieved with aluminum-salen initiators,3 and
recently promising results have also been obtained with
indium,4 gallium,5 zinc,6 yttrium,7 and group 4 metals.8 The
supporting ligands are often dianionic aryloxides (salen,3 salan,9
or salalen10), and the use of a chiral SalBinap backbone in
Spassky’s and Coates’ seminal contributions gave isoselective
catalysts that have not been surpassed.11
We and others have developed rare-earth complexes that
display high heteroselectivity toward the ROP of rac-LA using a
variety of bis(aryloxide) ligands incorporating amino12 or
dithiaalkanediyl bridges.13,14 Although amorphous heterotactic
PLA (hPLA) is not used yet for any specific application, highly
heteroselective catalysts for the ROP of rac-LA are of interest
because they can also produce crystalline (Tm = 152 °C)
syndiotactic PLA (sPLA) by stereocontrolled polymerization of
meso lactide (meso-LA).3b,15 We have also shown that
fluorinated (di)alkoxides with CF3 groups in α-position to
RESULTS AND DISCUSSION
■
Synthesis and Structure of Fluorinated Diol Proligand
and Related Al, In, and Y Complexes. The enantiomerically
pure Binap-based proteo-ligand (R)-{ONNO}H2 was isolated
in 45% yield upon reaction of the commercially available (R)-
(+)-1,1′-binaphthyl-2,2′-diamine with 5,5,5-trifluoro-4-hydroxy-
4-(trifluoromethyl)pentan-2-one (prepared in 79% yield by
nucleophilic addition of acetone onto hexafluoroacetone under
forcing conditions)18 in refluxing toluene (Scheme 1). The
identity and purity of (R)-{ONNO}H2 were established by
NMR spectroscopy, elemental analysis, mass spectrometry, and
X-ray diffraction analysis. The methylene CHH hydrogen
atoms are diastereotopic, giving rise to an AB spin system (2JHH
Special Issue: Catalytic and Organometallic Chemistry of Earth-
Abundant Metals
Received: April 30, 2014
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of (R)-{ONNO}H2
= 16.6 Hz). Due to the chiral backbone, the two CF3 groups
carried by the same Csp3 atom are not equivalent, and each
gives rise to a quartet (4JFF = 9.8 Hz) in the 19F{1H} NMR
spectrum of the proteo-ligand. The two halves of the proteo-
ligand are otherwise magnetically equivalent, as a single set of
resonances is detected in each of the 1H, 19F{1H}, and 13C{1H}
NMR spectra. The racemic version of the proteo-ligand, (rac)-
{ONNO}H2, was obtained from (rac)-( )-1,1′-binaphthyl-
2,2′-diamine following the same procedure, and the analytical
data for the racemate were strictly identical to those of (R)-
{ONNO}H2.
ethane (Scheme 2). Complex 1 was characterized by NMR
spectroscopy and combustion analysis. At room temperature,
the 1H and 19F{1H} NMR spectra feature very broad
resonances, indicative of fluxional behavior. The dynamic
1
phenomenon was frozen at 213 K. At this temperature, the H
NMR spectrum shows the absence of symmetry in this
complex, with in particular two singlets for the nonequivalent
CH3 groups and two sets of resonances for independent AB
systems (2JHH = 15.0 and 16.3 Hz) assigned to methylene CHH
hydrogens. This is corroborated by the 19F{1H} NMR data,
where four distinct quartets indicate that all CF3 (4JFF = 9.0 and
10.3 Hz) are nonequivalent. Reaction of 1 with KOiPr failed to
deliver the targeted (rac)-{ONNO}Al(OiPr).
The molecular solid-state structure of (S)-{ONNO}H2
obtained from a scalemic mixture is depicted in Figure 1. It
Single crystals of 1 suitable for X-ray diffraction analysis were
obtained by recrystallization from dichloromethane. Both
enantiomers of the complex are found in the asymmetric unit
in a 1:1 ratio. The representation of the R enantiomer displayed
in Figure 2 shows the aluminum atom to rest in a five-
coordinated trigonal bipyramidal environment in this mono-
metallic complex, with one Oalkoxide and one Nimine atom
occupying the apical positions. However, poor refinement
precludes discussion of the metric parameters in this complex.
The hydrocarbyl complex (rac)-{ONNO}In(CH2SiMe3) (2)
was isolated in 66% yield19 upon reaction of In(CH2SiMe3)3
with 1 equiv of (rac)-{ONNO}H2 and concomitant release of
SiMe4. The NMR data recorded in benzene-d6 or THF-d8 show
that 2 is highly fluxional in solution at 298 K. In particular, in
addition to a sharp singlet, the 19F{1H} NMR spectrum in
THF-d8 exhibits a very broad, almost flat resonance spreading
over nearly 8 ppm. The fluxional behavior is frozen at 213 K,
where the 19F{1H} NMR spectrum shows four broad singlets of
Figure 1. ORTEP representation of the molecular solid-state structure
of (S)-{ONNO}H2. Ellipsoids are drawn at the 50% probability level.
Only one of the two independent and crystallographically equivalent
molecules in the asymmetric unit is depicted. Hydrogen atoms (except
OH) are omitted for clarity. Color code: red, O; blue, N; black, C;
dark green, F. Selected bond lengths (Å): N(1)−C(5) = 1.263(11),
N(1)−C(7) = 1.427(10), C(2)−C(4) = 1.539(13), C(4)−C(5) =
1.513(11), O(1)−C(2) = 1.395(10).
1
equal intensities (Figure 3). The H NMR spectrum of the
complex is well resolved at this temperature and features an AB
spin system at high field for the InCHHSiMe3 hydrogens (2JHH
= 12.0 Hz) and two other AB systems for the CHHC(CF3)2
(2JHH = 14.4 and 14.7 Hz). All hydrogens are in a unique
environment, and thus the pseudochiral molecule has no
symmetry at this temperature.
The NMR-scale reaction of (R)-{ONNO}H2 with strictly 1
equiv of Y(N(SiMe3)2)3 afforded an unidentified compound
(C) featuring three distinct ligand environments on the basis of
NMR spectroscopic data, but which a priori did not correspond
to a putative (R)-{ONNO}Y(N(SiMe3)2). Besides, resonances
for free HN(SiMe3)2 and unreacted Y(N(SiMe3)2)3 were
detected in the NMR spectrum of the crude product, while all
the proteo-ligand had reacted.18 The spectroscopic data
suggested the presence of a protic NH or OH moiety could
not be ruled out, yet we were unable to reconcile them with a
zwitterion of the type {ONNO}Y−{ONN(H)+O} akin to the
bis(aryloxide)amine samarium(III) zwitterion reported by
Mountford and co-workers.20
shows the proteo-ligand to be in the S configuration, with an
angle of 79.5° between the two mean planes defined by the
naphthalene units. The imino-alcohol tethers, which exhibit
typical internal O−H···N hydrogen bonding, are located away
from each other to minimize steric congestion.
The stoichiometric reactions of (R)-{ONNO}H2 or (rac)-
{ONNO}H2 with Al2Me6, [Al(OiPr)3]4, or AlMe2(OiPr) failed
to give clean species, as unidentifiable products or intractable
mixtures of compounds were obtained regardless of the
reaction conditions. By contrast, (rac)-{ONNO}H2 reacted
cleanly and quantitatively with AlEt2Cl to afford the expected
(rac)-{ONNO}AlCl (1) upon protonolysis and release of
B
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Scheme 2. Synthesis of Complexes 1−3 by Protonolysis Reactions
indicating partial symmetry in solution; this is further attested
by 1H NMR spectroscopy (only two AB systems for all
backbone methylene CHH hydrogens and two singlets for all
CH3 moieties). The molecular solid-state structure of 3 was
elucidated by X-ray crystallography (Figure 4). The two,
nonequivalent ligand fragments in the complex retain their
original R configuration. One of these is κ2-coordinated to the
yttrium atom (corresponding to O(1) and O(48)), while the
other one (viz., O(51) and O(98)) acts as a μ2:κ2,κ2 ligand
bridging the two metal atoms. The six-coordinated yttrium
atom lies in a slightly distorted octahedral environment, with all
four Oalkoxide atoms located at approximately the same distance
from the metal, in the range 2.162(2)−2.209(2) Å. The lithium
atom is also six-coordinated, with two Li−Oalkoxide and two Li−
Nimine bonds being complemented by two nonequivalent Li···F
secondary interactions (2.175(6) and 2.412(6) Å); the
geometry about the Li atom forms a much distorted
tetrahedron. The Y−Li distance is too long for potential Y···
Li interactions.21 NMR data for 3 recorded in C6D6 or THF-d8
did not provide evidence that the Li···F interactions are
maintained in solution, but DOSY NMR measurements and
diffusion molecular weight analysis (D-fw) demonstrated that
the heterobimetallic structure of the complex is preserved in
THF-d8.18,22 This is confirmed by comparison of the
hydrodynamic radii of the complex determined by PGSE
NMR spectroscopy (rPGSE = 6.72 Å) and from the crystallo-
graphic data (rXRD = 7.28 Å).18
Figure 2. ORTEP representation of the molecular solid-state structure
of the R enantiomer of (rac)-{ONNO}AlCl (1). Hydrogen atoms are
omitted for clarity. Color code: red, O; blue, N; black, C; dark green,
F; bright green, Cl; lavender, Al. Ellipsoids are drawn at the 50%
probability level. Discussion of metric parameters is not pertinent
owing to R1 = 17.2%.
The reaction of Y(N(SiMe3)2)3 with 2 equiv of (R)-
{ONNO}H2 gave an intractable mixture of compounds and
did not provide useful information toward the identification of
compound C. On the other hand, the one-pot reaction of (R)-
{ONNO}H2, Y(N(SiMe3)2)3, and LiN(SiMe3)2 in 2:1:1
proportions returned cleanly and quantitatively the hetero-
bimetallic [((R)-{ONNO})2Y·Li] (3) (see Scheme 1). The
formulation for 3 was established on the basis of spectroscopic
(1H, 19F, and 7Li NMR in C6D6) and crystallographic data, and
its purity was corroborated by elemental analysis. The 7Li NMR
Ring-Opening Polymerization Catalysis. Although by
itself, or in combination with benzyl alcohol (BnOH), the
aluminum complex 1 does not polymerize rac-LA, we have
found that both the indium and yttrium−lithium complexes 2
and 3 mediate the heteroselective ROP of rac-LA under mild
conditions (Table 1). Complex 2 requires the addition of 1
equiv of BnOH to catalyze the polymerization of 100 equiv of
spectrum of 3 shows the presence of a single resonance at δ7Li
=
0.86 ppm. In the 19F{1H} NMR spectrum, four resonances of
equal intensity (three broad signals and a sharp quartet) are
identified, each corresponding to two CF3 groups and
C
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Figure 3. 19F{1H} NMR spectra of (rac)-{ONNO}In(CH2SiMe3) (2) (THF-d8, 376.49 MHz) recorded at 298 K (top) and 213 K (bottom).
Figure 4. ORTEP representation of the molecular solid-state structure of [((R)-{ONNO})2Y·Li] (3). Hydrogen atoms are omitted for clarity. Color
code: red, O; blue, N; black, C; dark green, F; lavender, Li; violet, Y. Ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and
angles (deg): Y(1)−O(48) = 2.162(2), Y(1)−O(1) = 2.170(2), Y(1)−O(98) = 2.1899(19), Y(1)−O(51) = 2.209(2), Y(1)−N(14) = 2.518(3),
Y(1)−N(35) = 2.531(2), Y(1)−Li(1) = 3.075(6), O(51)−Li(1) = 1.998(5), F(59)−Li(1) = 2.412(6), N(64)−Li(1) = 2.121(6), N(85)−Li(1) =
2.089(6), F(91)−Li(1) = 2.175(6), O(98)−Li(1) = 2.108(6); O(48)−Y(1)−O(1) = 164.91(8), O(48)−Y(1)−O(98) = 92.09(8), O(1)−Y(1)−
O(98) = 99.08(8), O(48)−Y(1)−O(51) = 96.61(8), O(1)−Y(1)−O(51) = 94.71(8), O(98)−Y(1)−O(51) = 83.53(7), O(48)−Y(1)−N(14) =
94.64(8), O(1)−Y(1)−N(14) = 73.94(8), O(98)−Y(1)−N(14) = 97.61(8), O(51)−Y(1)−N(14) = 168.64(8), O(48)−Y(1)−N(35) = 74.37(8),
O(1)−Y(1)−N(35) = 93.56(8), O(98)−Y(1)−N(35) = 165.82(8), O(51)−Y(1)−N(35) = 101.91(7), N(14)−Y(1)−N(35) = 79.66(8), O(51)−
Li(1)−N(85) = 151.0(3), O(98)−Li(1)−N(64) = 155.0(3), F(91)−Li(1)−F(59) = 161.5(3).
a
Table 1. Ring-Opening Polymerization Data
b
c
d
entry
cat
LA
[LA]0/[Cat]0/[BnOH]0
solv
T (°C)
t (h)
conv (%)
Mn,theo (g·mol−1
)
Mn,SEC (g·mol−1
)
Mw/Mn
Pr
1
2
2
2
3
3
3
3
3
rac-
rac-
rac-
rac-
L-
100:1:1
100:1:1
170:1:0
300:1:0
170:1:0
170:1:0
300:1:0
Tol
70
70
30
30
30
30
30
7
7
5
5
7
7
5
57
68
8300
9900
5000
5900
21 900
19 500
nd
1.04
1.05
1.45
1.74
nd
0.53
0.70
0.99
0.99
0.00
0.00
0.80
THF
THF
THF
THF
THF
THF
e
3
100
75
24 600
32 400
4400
4
5
6
7
18
D-
16
3900
nd
nd
meso-
100
43 200
20 800
1.16
a
b
[rac-LA]0 = 0.8 M; see the SI for complementary results. Mn,theo = [rac-LA]0/[Cat]0 × conversion ×144.13 (+108.14 when BnOH was employed);
c
reactions quenched with MeOH unless otherwise specified. Determined by size exclusion chromatography vs polystyrene standards and corrected
d
e
by a factor of 0.58. Probability of racemic enchainments for consecutive lactide units, determined by selective 1H{1H} NMR spectroscopy. Reaction
quenched with EtOH. nd: not determined.
D
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rac-LA (entries 1, 2), as otherwise no controlled reaction is
taking place (vide infra). Partial conversions (50−70%) are
obtained after 7 h with moderate control of the molecular
weights but very narrow molecular weight distributions. End-
group analysis by NMR spectroscopy confirms the presence of
the expected termini, BnOC(O)CH(CH3)− and −CH-
(CH3)OH. The reaction is slightly faster in THF than in
toluene, an observation already made with other indium-based
ROP initiators.4 Interestingly, reactions in THF afford PLA
with a heterotactic bias (Pr = 0.70),23 whereas the polymers
produced in toluene are atactic, suggesting that THF may play a
noninnocent role as a coligand.
An independent NMR experiment showed the total absence
of reaction between 2 and BnOH (1−10 equiv) in THF-d8, at
either 25 or 70 °C; the [In]−CH2SiMe3 moiety did not
undergo any alcoholysis, and complex 2 remained stable for
several hours under these conditions. Similar observations have
been reported before with related indium-carbyl complex-
es11d,24 and strongly suggest that the 2/BnOH system here
operates via an activated monomer mechanism, in which 2 acts
as a Lewis acidic catalyst that activates the monomer toward
nucleophilic attack by external (macro)alcohol molecules.
The polymerization of rac-LA mediated by 3 takes place in
THF under mild conditions (30 °C, [rac-LA]0 = 0.8 M)
without the need for any exogenous nucleophile (Table 1,
entries 3, 4), readily converting 170−300 equiv of monomer.
Under these conditions, the reactions are not controlled, as
experimental Mn,SEC values are lower than the calculated Mn,theo
ones and the molecular weights of the resulting polymers are
not narrowly dispersed. Monomer consumption follows first-
order kinetics in [rac-LA]. Monitoring of an NMR-scale
polymerization of rac-LA25 showed that a long induction
period preceded the steady-state regime, which in this case
prevented accurate and reliable determination of the observed
rate constant by a simple first-order analysis.18 The initiation
and propagation rate constants ki = (1.83 0.20) × 10−4 L·
mol−1·s−1 and kp = (156.6 12.5) × 10−4 L·mol−1·s−1 were
determined by fitting the experimental data to a nonlinear
regression model for living ROP reactions.18,26 Most gratify-
ingly, near-perfect heterospecific hPLA is obtained (Figure 5),
with values of Pr very close to 1 (entries 3, 4 in Table 1); this
compares favorably with other heteroselective catalysts for the
ROP of rac-LA disclosed in the literature.12−14 Complex 3
polymerizes rac-LA faster than the enantiomerically pure L-LA
(compare entries 3 and 5), consistently with the hetero-
selectivity displayed by the initiator toward rac-LA. Moreover,
the polymerization of D-LA, the other enantiomer of lactide,
takes place at the same rate as that of L-LA (entries 5 and 6).
Note that the value of Pr decreases to 0.90−0.95 if the
polymerization is carried in CH2Cl2 instead of THF, if the
reaction temperature is increased to 60−80 °C (to enhance the
reaction rate), or if the reaction is not quenched as soon as full
monomer consumption is reached (presumably due to
deleterious transesterification processes).18,27
1
Figure 5. Methine region of the H homodecoupled NMR spectrum
(500.13 MHz, CDCl3 with calibration at 7.26 ppm, 298 K) of a PLA
sample prepared by polymerization of rac-LA mediated by 3 (Table 1,
entry 4; Pr = 0.99).
followed by (ii) transesterification of the initial chain-end in the
presence of methanol during quenching of the reaction and
precipitation of the polymer, both carried out with methanol.28
In fact, in a control experiment, quenching of the polymer-
ization with EtOH instead of the customary MeOH resulted in
the formation of an EtOC(O)CH(CH3)− end-capped
polylactide.18 Our attempts to isolate or unambiguously identify
a putative {ONNO}-capped PLA before quenching were
unsuccessful.
It has been shown that highly heteroselective catalysts
polymerize meso-LA to syndiotactic sPLA.15 The polymer-
ization of meso-LA promoted by 3 is stereoselective, as it
generates narrowly dispersed syndiotactic sPLA (Table 1, entry
7, Pr = 0.80; Figure 7). Yet, the level of selectivity is lower for
meso-LA than for rac-LA, which as observed by Hillmyer and
Tolman suggests that the configuration of both stereocenters of
the last inserted lactide molecule is determinant for the
stereochemistry of the insertion of the next monomer.14g
Qualitative comparison of the data for entries 4 and 7 in Table
1 indicates that meso-LA is polymerized a little more rapidly
than rac-LA; in similar cases, it has been said that this could
result from the greater ring strain in meso-LA.15e Again, the
kinetics of the polymerization exhibit first-order dependence in
monomer concentration, with kobs = 7.00 × 10−5 s−1.
Monitoring of an NMR-scale reaction did not show the
presence of a detectable induction period, ki = (304.7 127.7)
× 10−4 L·mol−1·s−1 and kp = (143.4 1.19) × 10−4 L·mol−1·
s−1. Comparisons of the plots of monomer consumption and of
the respective values of ki and kp under otherwise strictly
identical conditions indicates that the ROP of meso-LA is
overall marginally faster than that of rac-LA, but essentially in
the present scenario due to the long induction period detected
in the case of rac-LA (see above).18,24
1
End-group analysis by H NMR spectroscopy and MALDI-
ToF MS demonstrated the presence of −CH(CH3)OH and
MeOC(O)CH(CH3)− termini (Figure 6 and the SI). If the
former is to be expected, a MeOC(O)CH(CH3)− chain-end
is unusual in the present context. We surmise that these
observations result from (i) initial (i.e., for the first insertion)
nucleophilic attack of the coordinated monomer by one of the
Oalkoxide atoms in 3, where therefore the ligand would act not
only as an ancillary group but also as a reactive nucleophile,
CONCLUSION
■
In summary, we have shown that the heterobimetallic complex
[((R)-{ONNO})2Y·Li] (3) bearing a fluorinated dialkoxo
ligand is available quantitatively and under its enantiomerically
pure form by simple concomitant, one-pot protonolysis
E
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1
Figure 6. H NMR spectrum (500.13 MHz, CDCl3, 298 K) of an hPLA prepared by ROP of rac-LA mediated by 3 and quenched by MeOH.
argon from sodium mirror/benzophenone ketyl prior to use. All
deuterated solvents (Eurisotop, Saclay, France) were stored in sealed
ampules over activated 3 Å molecular sieves and were thoroughly
degassed by several freeze−thaw−vacuum cycles. Racemic lactide
(Acros) was purified by recrystallization from a hot (80 °C),
concentrated iPrOH solution, followed by two subsequent recrystal-
lizations in hot (105 °C) toluene; after purification, it was stored at
−30 °C under the inert atmosphere of the glovebox. Benzyl alcohol
and 2-propanol (Aldrich) were dried and distilled over magnesium
turnings and stored over 3 Å molecular sieves.
NMR spectroscopic data were recorded on Bruker AM-400 and
AM-500 spectrometers. All 1H and 13C{1H} chemicals shifts were
determined using residual signals of the deuterated solvents and were
calibrated vs SiMe4. Assignment of the signals was carried out using
1D (1H, 13C{1H}) and 2D (COSY, HMBC, HMQC) NMR
experiments. Coupling constants are given in hertz. 19F{1H} chemical
shifts were determined by external reference to an aqueous solution of
NaBF4. DOSY NMR experiments were carried out on a Bruker Avance
III 400 MHz spectrometer equipped with a BBOF pulsed field-
gradient probe using a bipolar gradient pulse stimulated echo
sequence. Each experiment was performed on a 0.021 M solution at
298 K using a spectral width of 4807 Hz, a 90° pulse width of 11.5 μs,
a diffusion delay time of 0.05 s, and a total diffusion-encoding pulse
1
width of 0.0016 s. The diffusion encoding pulse strength was arrayed
from 0 to 35 G·cm−2 over 12 or 16 increments with four dummy scans
and eight scans per increment.
Elemental analyses were performed on a Carlo Erba 1108 elemental
analyzer instrument at the London Metropolitan University by
Stephen Boyer and were the average of a minimum of two
independent measurements.
Size exclusion chromatography (SEC) measurements were
performed on an Agilent PL-GPC50 equipped with two PLgel 5 Å
MIXED-C columns and a refractive index detector. The column was
eluted with THF at 30 °C at 1.0 mL·min−1 and was calibrated using 11
monodisperse polystyrene standards in the range 580 to 380 000 g·
mol−1. The molecular weights of all PLAs were corrected by a factor of
0.58.29
Typical Schlenk-Scale Polymerization Procedure. In the
glovebox, the metal catalyst (ca. 5.0−15.0 mg) was placed in a
Schlenk flask together with the monomer (ca. 0.1−1.0 g). The Schlenk
flask was sealed and removed from the glovebox. All subsequent
operations were carried out on a vacuum manifold using Schlenk
techniques. The required amount of solvent (THF or toluene) was
added with a syringe to the catalyst and the monomer, followed when
Figure 7. Methine region of the H homodecoupled NMR spectrum
(500.13 MHz, CDCl3 with calibration at 7.26 ppm, 298 K) of an sPLA
sample prepared by polymerization of meso-LA mediated by 3 (Table
1, entry 7; Pr = 0.80).
reactions. This complex polymerizes racemic lactide to an
unusual12 near-perfect heterotactic polylactide with Pr = 0.99,
while it also gives syndiotactic polylactide (Pr = 0.80) from meso
lactide. Although the reactions are not entirely controlled and
the mechanisms have not been fully unraveled, these results
open up new directions in the area of ligand design for
stereoselective ROP catalysis.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under an
inert atmosphere using standard Schlenk techniques or in a dry,
solvent-free glovebox (Jacomex; O2 < 1 ppm, H2O < 5 ppm) for
catalyst loading. Solvents (THF, Et2O, CH2Cl2, pentane, and toluene)
were purified and dried (water contents all below 10 ppm) over
alumina columns (MBraun SPS). THF was further distilled under
F
dx.doi.org/10.1021/om500458g | Organometallics XXXX, XXX, XXX−XXX

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Article
necessary by addition of alcohol (iPrOH or BnOH, 3−10 μL). The
resulting mixture was immerged in an oil bath preset at the desired
temperature, and the polymerization time was measured from this
point. The reaction was terminated by addition of MeOH, and the
polymer was precipitated in methanol or a methanol/pentane mixture.
The polymer was then dried to constant weight in a vacuum oven at
55 °C under dynamic vacuum (<5 × 10−2 mbar).
Typical NMR-Scale Polymerization Procedure. All NMR-scale
ROP reactions were conducted in THF-d8. In a typical experiment, the
catalyst and the monomer were loaded in an NMR tube in the
glovebox. The NMR tube was placed in a Schlenk flask, which was
then removed from the glovebox and connected to the vacuum
manifold. All subsequent operations were performed using Schlenk
techniques. The appropriate amount of THF-d8 was added to the
NMR tube in this order at room temperature. The NMR tube was
then sealed, gently heated to ensure complete dissolution of the
monomer, and introduced in the spectrometer preset at the desired
temperature. Time measurement started at this point. Data points
were collected at regular intervals (typically 30−480 s, with D1 = 0.5 s
and NS = 8−16 scans) until conversion of the monomer stopped (this
usually coincided with near-full conversion). The conversion was
reliably determined by integrating the methine region of PLA vs that
of the monomer.
NMR (CDCl3, 298 K, 100.62 MHz): δ 209 (s, CO), 122.6 (q, 1JCF
2
= 287 Hz, CF3), 76.2 (hept, JCF = 30 Hz, C(CF3)2), 37.0 (s, CH2),
32.0 (s, CH3) ppm.
(R)-{ONNO}H2. 5,5,5-Trifluoro-4-hydroxy-4-(trifluoromethyl)-
pentan-2-one (1.64 g, 7.32 mmol) was dissolved in dry toluene (ca.
45 mL). (R)-(+)-1,1′-Binaphthyl-2,2′-diamine (0.91 g, 3.18 mmol)
and then p-toluenesulfonic acid (36 mg, 6 mol %) were added. The
mixture was refluxed at 140 °C for 4.5 days in a setup equipped with a
Dean−Stark apparatus. The volatiles were then removed in vacuo to
afford a brown solid, which was extracted with Et2O. Filtration
followed by evaporation of the solvent gave a solid, which was purified
by chromatography over silica (petroleum ether/ethyl acetate = 92:8).
Recrystallization in petroleum ether yielded the enantiomerically pure
1
title compound as an off-white solid (1.0 g, 45%). H NMR (C6D6,
298 K, 400.16 MHz): δ 8.04 (s, 2H, OH), 7.70 (d, 4H, 3JHH = 8.5 Hz,
3
3
CaromH), 7.19 (t, 2H, JHH = 7.7 Hz, CaromH), 7.10 (d, 2H, JHH = 8.4
Hz, CaromH), 6.97 (t, 2H, 3JHH = 8.7 Hz, CaromH), 6.75 (d, 2H, 3JHH
=
2
8.7 Hz, CaromH), 2.12−1.96 (AB system, 4H, JHH = 16.6 Hz, CH2),
1.37 (s, 6H, CH3) ppm. 19F{1H} NMR (C6D6, 298 K, 376.49 MHz): δ
4
4
−77.8 (q, JFF = 9.8 Hz, CF3), −78.4 (q, JFF = 9.8 Hz, CF3) ppm.
13C{1H} NMR (C6D6, 298 K, 100.62 MHz): δ 171.3 (CN), 143.3,
133.6, 132.1, 129.6, 128.7, 127.2, 125.7, 125.6 (all Carom), 123.6 (q, 1JCF
2
= 286 Hz, CF3), 124.8 (Carom), 120.5 (Carom), 76.5 (hept, JCF = 29.4
Curve-Fitting Methods. For kinetic experiments, the values of ki
and kp were determined with the software Origin and Datafit by curve-
fitting of experimental data as described in ref 24.
Hz, C(CF3)2), 33.5 (CH2), 22.7 (CH3) ppm. Anal. Calcd for
C32H24F12N2O2 (696.53 g·mol−1): C, 55.2; H, 3.5; N, 4.0. Found:
C, 54.7; H, 3.4; N, 4.0. ESI-HRMS: m/z calcd for [M + H]+
697.17189; found 697.1721 (0 ppm).31
X-ray Diffraction Crystallography. Single crystals of (R)-
{ONNO}H2 and [((R)-{ONNO})2Y·Li] (3) suitable for X-ray
diffraction analysis were obtained by recrystallization of the purified
compound. Diffraction data were collected at 150(2) K using a Bruker
APEX CCD diffractometer with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). A combination of ω and Φ scans was
carried out to obtain at least a unique data set. The crystal structures
were solved by direct methods; remaining atoms were located from
difference Fourier synthesis followed by full-matrix least-squares
refinement based on F2 (programs SIR97 and SHELXL-97).30
Carbon- and oxygen-bound hydrogen atoms were placed at calculated
positions and forced to ride on the attached atom. All non-hydrogen
atoms were refined with anisotropic displacement parameters. The
locations of the largest peaks in the final difference Fourier map
calculation as well as the magnitude of the residual electron densities
were of no chemical significance. Crystal data and details of data
collection and structure refinement for all compounds structurally
characterized (CCDC 997544 and 997545) can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
MALDI-ToF Mass Spectrometry. MALDI-TOF mass spectra
were recorded at the CESAMO (Bordeaux, France) on a Voyager
mass spectrometer (Applied Biosystems) equipped with a pulsed N2
laser source (337 nm) and a time-delayed extracted ion source. Spectra
were recorded in the positive-ion mode using the reflectron mode and
with an accelerating voltage of 20 kV. A THF solution (1 mL) of the
matrix (trans-3-indoleacrylic acid (Aldrich, 99%)) and a MeOH
solution of the cationization agent (NaI, 10 mg·mL−1) were prepared.
A fresh solution of the polymer samples in THF (10 mg·mL−1) was
then prepared. The three solutions were then rapidly combined in a
1:1:10 volume ratio of matrix-to-sample-to-cationization agent. One to
two microliters of the resulting solution was deposited onto the sample
target and vacuum-dried.
(rac)-{ONNO}Al(Cl) (1). (rac)-{ONNO}H2 (150 mg, 0.22 mmol)
was dissolved in toluene (3 mL) and added dropwise to a solution of
AlEt2Cl (240 μL, 0.22 mmol) in toluene (1.5 mL). The reaction
mixture was stirred at 40 °C for 3 h, when a colorless solid
precipitated. The solid was isolated by filtration, washed with pentane
(3 × 5 mL), and dried in vacuo to afford 1 as a colorless powder (150
1
mg, 92%). H NMR (CD2Cl2, 213 K, 400.16 MHz): δ 8.08−8.05 (m,
3
4
2H, CaromH), 8.00 (dd, 2H, JHH = 8.2 Hz, JHH = 3.0 Hz, CaromH),
7.58−7.52 (m, 2H, CaromH), 7.39−7.30 (m, 3H, CaromH), 7.23 (m, 2H,
3
CaromH), 7.14 (d, 1H, JHH = 8.7 Hz, CaromH), 3.06 and 2.68 (AB
system, 2H, 2JHH = 16.3 Hz, CH2), 2.91 and 2.81 (AB system, 2H, 2JHH
= 15.0 Hz, CH2), 1.93 (s, 3H, CH3), 1.79 (s, 3H, CH3) ppm. 19F{1H}
4
NMR (CD2Cl2, 213 K, 376.49 MHz): δ −75.68 (q, JFF = 10.3 Hz,
CF3), −76.93 (q, 4JFF = 10.3 Hz, CF3), −79.28 (q, 4JFF = 9.0 Hz, CF3),
4
−80.50 (q, JFF = 9.0 Hz, CF3) ppm. 13C{1H} NMR (CD2Cl2, 298 K,
125.75 MHz): δ 133.38, 132.05, 129.38, 129.18, 127.90, 127.08,
125.88 (all Carom), 30.1 (br, CH2), 26.3 (br, CH3) ppm. The
resonances for NC, C(CF3)3, and C(CF3)3 are not visible at 298 K.
Anal. Calcd for C32H22AlClF12N2O2 (756.94 g·mol−1): C, 50.8; H, 2.9;
N, 3.7. Found: C, 50.9; H, 3.0; N, 3.8.
(rac)-{ONNO}In(CH2SiMe3) (2). (rac)-{ONNO}H2 (200 mg, 0.29
mmol) and In(CH2SiMe3)3 (109 mg, 0.29 mmol) were dissolved in 5
mL of THF, and the reaction medium was stirred at 80 °C for 15 h.
Evaporation of the volatiles gave a solid, which was washed with
pentane (3 × 2 mL) to afford 2 as a colorless powder (170 mg, 66%).
1H NMR (THF-d8, 213 K, 500.13 MHz): δ 8.17 (d, 1H, 3JHH = 8.7 Hz,
C
aromH), 8.13 (d, 1H, 3JHH = 8.7 Hz, CaromH), 8.06 (d, 2H, 3JHH = 7.9
Hz, CaromH), 7.55−7.45 (m, 2H, CaromH), 7.36−7.24 (m, 4H, CaromH),
3
3
7.13 (d, 1H, JHH = 8.7 Hz, CaromH), 7.06 (d, 1H, JHH = 8.4 Hz,
2
CaromH), 3.07 and 2.58 (AB system, 2H, JHH = 14.4 Hz,
CH2C(CF3)2), 2.93 and 2.54 (AB system, 2H, JHH = 14.7 Hz,
2
CH2C(CF3)2), 2.07 (s, 3H, CH3), 1.72 (s, 3H, CH3), 0.13 (s, 9H,
Si(CH3)3), −0.05 and −0.30 (AB system, 2H, 2JHH = 12.0 Hz, InCH2)
ppm. 19F{1H} NMR (THF-d8, 213 K, 376.49 MHz): δ −75.13 (q, 4JFF
5,5,5-Trifluoro-4-hydroxy-4-(trifluoromethyl)pentan-2-one.
A sealed 100 mL stainless steel autoclave was charged with
hexafluoroacetone sesquihydrate (12.5 g, 64.7 mmol) and acetone
(57.5 mL, 775 mmol). The autoclave was heated at 155 °C for 90 h
and then cooled to room temperature. Diethyl ether (20 mL) was
added, the mixture was dried over Na2SO4 and filtered, and the
solvents were pumped off. The residual oil was distilled under reduced
pressure (2 mmHg, 32 °C) to give the desired product as a colorless
4
= 8.4 Hz, CF3), −77.73 (q, JFF = 9.0 Hz, CF3), −78.37 (q, 4JFF = 9.0
Hz, CF3), −79.73 (q, 4JFF = 8.4 Hz, CF3) ppm. 13C{1H} NMR (THF-
d8, 213 K, 100.25 MHz): δ 184.8 (NC), 178.2 (NC), 142.6,
141.7, 133.7, 133.6, 133.0, 132.6, 131.1, 131.0, 129.3 (all Carom),
129.0−123.0 (2 q overlapping with other resonances, CF3), 128.6,
128.4, 128.2, 127.6, 126.6, 126.5, 126.4, 126.2, 122.7, 122.1 (all Carom),
90.0−79.4 (2 overlapping hept, C(CF3)2), 41.1 (CH2C(CF3)2), 38.1
(CH2C(CF3)2), 26.0 (CH3), 25.0 (CH3), 2.2 (Si(CH3)3), 1.2 (InCH2)
1
liquid. Yield: 11.4 g, 79%. H NMR (CDCl3, 298 K, 400.16 MHz): δ
6.72 (s, 1H, OH), 2.97 (s, 2H, CH2), 2.36 (s, 3H, CH3) ppm. 19F{1H}
NMR (CDCl3, 298 K, 376.49 MHz): δ −78.3 (s, CF3) ppm. 13C{1H}
G
dx.doi.org/10.1021/om500458g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ppm. Anal. Calcd for C36H33F12InN2O2Si (896.54 g·mol−1): C, 49.7;
H, 3.8; N, 3.2. Found: C, 49.8; H, 4.3; N, 3.5.
2006, 103, 15343. (i) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T.
Chem.Eur. J. 2007, 13, 4433.
[((R)-{ONNO})2Y·Li] (3). (R)-{ONNO}H2 (201 mg, 0.29 mmol),
Y(N(SiMe3)2)3 (82 mg, 0.14 mmol), and LiN(SiMe3)2 (24 mg, 0.14
mmol) were dissolved in toluene (3 mL) and stirred at 40 °C for 3 h.
The volatiles were then pumped off to give a solid, which was washed
with pentane (3 × 3 mL) and dried in vacuo to afford 3 as a colorless
powder (206 mg, 96%). 1H NMR (C6D6, 298 K, 500.13 MHz): δ 7.77
(4) Douglas, A. F.; Patrick, B. O.; Mehrkhodavandi, P. Angew. Chem.,
Int. Ed. 2008, 47, 2290.
(5) Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. Inorg.
Chem. 2013, 52, 12561.
(6) (a) Wang, H.; Ma, H. Chem. Commun. 2013, 49, 8686.
(b) Honrado, M.; Otero, A.; Fernandez-Baeza, J.; Sanchez-Barba, L. F.;
Lara-Sanchez, A.; Tejeda, J.; Caarrion, M. P.; Martinez-Ferrer, J.;
Garces, A.; Rodriguez, A. M. Organometallics 2013, 32, 3437.
(7) Bakewell, C.; Cao, T.-P.-A.; Long, N.; Le Goff, X. F.; Auffrant, A.;
Williams, C. K. J. Am. Chem. Soc. 2012, 134, 20577.
3
(d, 2H, JHH = 8.7 Hz, CaromH), 7.69−7.65 (m, 4H, CaromH), 7.61 (d,
3
3
2H, JHH = 8.6 Hz), 7.28 (d, 2H, JHH = 8.7 Hz, CaromH), 7.21−7.11
(m, 4H, CaromH), 7.09−7.00 (m, 6H, CaromH), 6.95 (m, 2H, CaromH),
6.89 (m, 2H, CaromH), 3.00 and 2.63 (AX system, 4H, 2JHH = 14.2 Hz,
2
CH2), 2.44 and 2.29 (AB system, 4H, JHH = 16.3 Hz, CH2), 1.32 (s,
(8) Whitelaw, E. L.; Davidson, M. G.; Jones, M. D. Chem. Commun.
6H, CH3), 1.20 (s, 6H, CH3) ppm. 19F{1H} NMR (C6D6, 298 K,
376.49 MHz): δ −78.4 (br, CF3), −77.9 (q, 4JFF = 9.7 Hz, CF3), −74.9
(br, CF3), −74.3 (br, CF3) ppm. 13C{1H} NMR (C6D6, 298 K, 100.25
MHz): δ 179.7 (NC), 172.0 (NC), 145.8, 145.1, 133.7, 133.5,
132.9, 131.8, 130.4, 129.8, 129.0, 128.9 (all Carom), 127.0−120.9 (4 q
overlapping CF3 + other Carom resonances) 126.9, 126.5, 126.4, 126.0,
125.2, 125.1, 123.5, 123.1, 122.3, 121.2 (all CaromH), 81.0 (2
overlapping hept, C(CF3)2), 41.3 (CH2), 40.5 (CH2), 25.4 (CH3),
23.3 (CH3). 7Li NMR (C6D6, 298 K, 155.51 MHz): δ 0.86 ppm. Anal.
Calcd for C64H44F24LiN4O4Y (1484.87 g·mol−1): C, 51.8; H, 3.0; N,
3.8. Found: C, 51.8; H, 3.1; N, 3.8.
2011, 47, 10004.
(9) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 2004, 126, 2688.
(10) (a) Whitelaw, E. L.; Loraine, G.; Mahon, M. F.; Jones, M. D.
Dalton Trans. 2011, 40, 11469. (b) Hancock, S. L.; Mahon, M. F.;
Jones, M. D. Dalton Trans. 2013, 42, 9279.
(11) For other Al-ligand systems leading to isotactic PLA, see:
(a) Bouyahi, M.; Grunova, E.; Marquet, N.; Kirillov, E.; Thomas, C.
M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2008, 27, 5815.
(b) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F.
Organometallics 2009, 28, 1469. (c) Darensbourg, D. J.; Karroonnirun,
O.; Wilson, S. J. Inorg. Chem. 2011, 50, 6775. (d) Normand, M.;
Dorcet, V.; Kirillov, E.; Carpentier, J.-F. Organometallics 2013, 32,
1694.
ASSOCIATED CONTENT
■
S
* Supporting Information
(12) (a) Cai, C. X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J. F.
Chem. Commun. 2004, 40, 330. (b) Amgoune, A.; Thomas, C. M.;
Roisnel, T.; Carpentier, J.-F. Chem.Eur. J. 2006, 12, 169.
(c) Amgoune, A.; Thomas, C. M.; Carpentier, J. F. Macromol. Rapid
Commun. 2007, 28, 693. (d) 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. (e) Clark, L.; Cushion, M. G.; Dyer, H. E.; Schwarz, A.
D.; Duchateau, R.; Mountford, P. Chem. Commun. 2010, 46, 273.
(f) Zhao, W.; Cui, D.; Liu, X.; Chen, X. Macromolecules 2010, 43,
6678. (g) Zhao, W.; Cui, D.; Liu, X.; Chen, X. Macromolecules 2010,
43, 6678. (h) Nie, K.; Gu, X.; Yao, Y.; Zhang, Y.; Shen, Q. Dalton
Trans. 2010, 39, 6832. (i) Dyer, H. E.; Huijser, S.; Susperregui, N.;
Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford, P.
Organometallics 2010, 29, 3602. (j) Bouyahyi, M.; Ajellal, N.; Kirillov,
E.; Thomas, C. M.; Carpentier, J.-F. Chem.Eur. J. 2011, 17, 1872.
(k) Nie, K.; Fang, L.; Yao, Y.; Zhang, Y.; Shen, Q.; Wang, Y. Inorg.
Chem. 2012, 51, 11133.
General experimental procedures; table of X-ray diffraction
data; NMR data; full polymerization data; DOSY NMR
measurements; MALDI-ToF MS data; ROP kinetic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jean-francois.carpentier@univ-rennes1.fr.
*E-mail: yann.sarazin@univ-rennes1
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(13) (a) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem. 2006, 118,
7982. (b) Ma, H.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2008, 47,
3328.
Assistance from Stephen Boyer (elemental analysis, London
Metropolitan University) and financial support from Total
Raffinage-Chimie (research grant to N.M.) are most
appreciated. We thank Dr. Jean-Michel Brusson, Dr. Martine
Slawinski, and Dr. Alexandre Welle (Total) for stimulating
discussions.
(14) For other heteroselective ROP initiators, see ref 5 and:
(a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 1999, 121, 11583. (b) Chamberlain, B. M.; Cheng, M.;
Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2001, 123, 3229. (c) Chisholm, M. H.; Gallucci, J.;
Phomphrai, K. Chem. Commun. 2003, 48. (d) Chmura, A. J.; Chuck, C.
J.; Davidson, M. G.; Jones, M. D.; Lunn, M. D.; Bull, S. D.; Mahon, M.
F. Angew. Chem., Int. Ed. 2007, 46, 2280. (e) Chmura, A. J.; Davidson,
M. G.; Frankis, C. J.; Jones, M. D.; Lunn, M. D. Chem. Commun. 2008,
1293. (f) Platel, R. H.; White, A. J. P.; Williams, C. K. Chem. Commun.
2009, 4115. (g) Pietrangelo, A.; Knight, S. C.; Gupta, A. K.; Yao, L. J.;
Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2010, 132, 11649.
(h) Cushion, M. G.; Mountford, P. Chem. Commun. 2011, 47, 2276.
(i) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2011, 50,
7718. (j) Yang, S.; Nie, K.; Zhang, Y.; Xue, M.; Yao, Y.; Shen, Q. Inorg.
Chem. 2014, 53, 105. (k) Bakewell, C.; Cao, T.-P.-A.; Le Goff, X. F.;
Long, N. J.; Auffrant, A.; Williams, C. K. Organometallics 2013, 32,
1475. (l) Mou, Z.; Liu, B.; Liu, X.; Xie, H.; Rong, W.; Li, L.; Li, S.; Cui,
D. Macromolecules 2014, 47, 2233.
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(18) See the Supporting Information for details.
(19) The reaction is essentially quantitative. The lower isolated yield
in the In-alkyl complex 2 in comparison with the other two complexes
1 and 3 is simply due to its much greater solubility in hydrocarbons,
probably resulting in substantial losses during washings with pentane.
(20) Dyer, H. E.; Huijser, S.; Schwarz, A. D.; Wang, C.; Duchateau,
R.; Mountford, P. Dalton Trans. 2008, 32.
(21) Evans, W. J.; Shreeve, J. L.; Broomhall-Dillard, R. N. R.; Ziller, J.
W. J. Organomet. Chem. 1995, 501, 7.
(22) (a) Li, D.; Keresztes, I.; Hopson, R.; Williard, P. G. Acc. Chem.
Res. 2009, 42, 270. (b) Kagan, G.; Li, W.; Li, D.; Hopson, R.; Williard,
P. G. J. Am. Chem. Soc. 2011, 133, 6596.
(23) Pr, the probability of a racemic enchainment between two
consecutive LA units, is the measure of heterotacticity. Pr is
determined by examination of the methine region of the
1
homodecoupled H NMR spectrum. (a) Thakur, K. A. M.; Kean, R.
T.; Hall, E. S.; Kolstad, J. J.; Lindgren, T. A.; Doscotch, M. A.;
Siepmann, J. I.; Munson, E. J. Macromolecules 1997, 30, 2422.
(b) Chisholm, M. H.; Iyer, S. S.; Matison, M. E.; McCollum, D. G.;
Pagel, M. Chem. Commun. 1997, 1999.
(24) Maudoux, N.; Roisnel, T.; Dorcet, V.; Carpentier, J.-F.; Sarazin,
Y. Chem.Eur. J. 2014, 20, 6131.
(25) Conditions: [lactide]0 = 0.8 M in THF-d8, [lactide]0/[3]0 =
200:1, T = 30 °C.
(26) Wang, L.; Bochmann, M.; Cannon, R. D.; Carpentier, J.-F.;
Roisnel, T.; Sarazin, Y. Eur. J. Inorg. Chem. 2013, 5896.
(27) Comparison of entries 1 and 2 in Table S5 shows that when the
reaction is carried out for an unduly long time after complete
monomer conversion, Pm decreases from 0.91 to 0.85 and the PDI
increases. Besides, the analogous reaction carried out under more
controlled conditions at 40 °C for only 1.5 h yields a polymer featuring
Pm = 0.94 in 96% yield.
(28) Note that the addition of 1 equiv of BnOH co-initiator to 3
suppresses all ROP catalytic activity.
(29) Save, M.; Schappacher, M.; Soum, A. Macromol. Chem. Phys.
2002, 203, 889.
(30) (a) Sheldrick, G. M. SHELXS-97, Program for the Determination
of Crystal Structures; University of Goettingen: Germany, 1997.
(b) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Goettingen: Germany, 1997.
(31) The racemic (rac)-{ONNO}H2 was obtained similarly and gave
identical spectral and analytical data.
I
dx.doi.org/10.1021/om500458g | Organometallics XXXX, XXX, XXX−XXX