Synthesis and structural characterization of various N,O,N-chelated aluminum and gallium complexes for the efficient ROP of cyclic esters and carbonates: How do aluminum and gallium derivatives compare ?

Article abstract of DOI:10.1021/om3011068

The novel Al and Ga coordination compounds 4, 5, 7, and 8 of the type (NON)AlX (NON^2- = {RNC₆H₄}₂O ^2-; 4, R = Cy, X = Me; 5, R = C₅H₉, X = OCH₂Ph; 7, R = C₆F₅, X = Me) and (NON)GaX (NON^2- = {R₂NHC₆H₄} ₂O^2-; 8, R = C₆F₅, X = Me) have been synthesized via, in the case of 4, 7, and 8, a methane elimination reaction between the corresponding protio ligands (NON)H₂ and MMe₃ precursors (M = Al, Ga). The Al alkoxide derivative 5 was prepared via an alcoholysis reaction between κ³-N,O,N-{(C₅H ₉)NC₆H₄}₂O}₂AlNMe ₂ and PhCH₂OH. The tetracoordinate Al-THF adduct κ²-N,N′-{(C₅H₉)NC₆H ₄}₂O}₂Al(Me)(THF) (6) was prepared via a methane elimination reaction between the protio ligand TfNONH₂ (Tf = CF₃SO₂, 1c) and AlMe₃ in a CH ₂Cl₂/THF solvent mixture. As determined from X-ray studies, complexes 4, 7, and 8 are monomeric in the solid state and feature a central tetracoordinate metal center effectively κ³-N,O, N′ chelated and adopting a trigonal-monopyramidal geometry. The presence of Ga···F(o-C₆F₅) contacts in the solid-state structure of compound 8 are likely to reflect the Lewis acidity of the Ga center. The Al alkoxide derivative [κ³-N,O,N-{(C ₅H₉)NC₆H₄}₂O} ₂AlOCH₂Ph]₂ (5) was isolated as a dimer containing two five-coordinate (trigonal-pyramidal) Al centers and retains its dimeric structure in solution, while the Al-THF adduct 6 crystallizes in a monomeric form with an Al center in a tetrahedral geometry. The catalytic performances of the Al and Ga species 4, 5, 7, and 8 as ROP initiators of rac-lactide (rac-LA), ε-caprolactone (ε-CL), and trimethylene carbonate (TMC) were estimated, and most of them efficiently mediate the controlled and immortal ROP of these three monomers in the presence of BnOH, such an alcohol acting as an effective chain transfer agent. Of particular interest, the Ga amido species κ³-N,O,N-{(C₅H ₉)NC₆H₄}₂O}₂Ga-NMe ₂ (3) outperforms its Al counterpart κ³-N,O,N- {(C₅H₉)NC₆H₄}₂O} ₂AlNMe₂ (2) in the ROP of rac-LA, whether in terms of control or activity, to afford isotactically enriched PLA (P_m = 0.7). In contrast, all the Al derivatives are more efficient catalysts for the polymerization of ε-CL or TMC than the Ga analogues. For the ROP of TMC initiated by the Al and Ga complexes 2-5, an increased Lewis acidity of the metal center is clearly beneficial to both the activity and the ROP control. Notably, the C₆F₅N Ga species 8 was found to be inactive in the ROP of rac-LA, ε-CL, and TMC, which may be related to both electronic (C-F···Ga interactions) and steric factors.

Full text of DOI:10.1021/om3011068

Article
pubs.acs.org/Organometallics
Synthesis and Structural Characterization of Various N,O,N-Chelated
Aluminum and Gallium Complexes for the Efficient ROP of Cyclic
Esters and Carbonates: How Do Aluminum and Gallium Derivatives
Compare ?
Frederic Hild,† Nirvana Neehaul,† Frederic Bier,† Morgane Wirsum,† Christophe Gourlaouen,‡
́
́
́
́
and Samuel Dagorne*,†
†Institut de Chimie de Strasbourg, CNRS-Universite
́
de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France
‡Laboratoire de Chimie Quantique, Institut de Chimie de Strasbourg, CNRS-Universite
Strasbourg, France
́
de Strasbourg, 1 rue Blaise Pascal, 67000
S
* Supporting Information
ABSTRACT: The novel Al and Ga coordination compounds
4, 5, 7, and 8 of the type (NON)AlX (NON2−
=
{RNC6H4}2O2−; 4, R = Cy, X = Me; 5, R = C5H9, X =
OCH2Ph; 7, R = C6F5, X = Me) and (NON)GaX (NON2−
=
{R2NHC6H4}2O2−; 8, R = C6F5, X = Me) have been
synthesized via, in the case of 4, 7, and 8, a methane
elimination reaction between the corresponding protio ligands
(NON)H2 and MMe3 precursors (M = Al, Ga). The Al
alkoxide derivative 5 was prepared via an alcoholysis reaction
between κ3-N,O,N-{(C5H9)NC6H4}2O}2AlNMe2 and
PhCH2OH. The tetracoordinate Al−THF adduct κ2-N,N′-{(C5H9)NC6H4}2O}2Al(Me)(THF) (6) was prepared via a methane
elimination reaction between the protio ligand TfNONH2 (Tf = CF3SO2, 1c) and AlMe3 in a CH2Cl2/THF solvent mixture. As
determined from X-ray studies, complexes 4, 7, and 8 are monomeric in the solid state and feature a central tetracoordinate metal
center effectively κ3-N,O,N′ chelated and adopting a trigonal-monopyramidal geometry. The presence of Ga···F(o-C6F5) contacts
in the solid-state structure of compound 8 are likely to reflect the Lewis acidity of the Ga center. The Al alkoxide derivative [κ3-
N,O,N-{(C5H9)NC6H4}2O}2AlOCH2Ph]2 (5) was isolated as a dimer containing two five-coordinate (trigonal-pyramidal) Al
centers and retains its dimeric structure in solution, while the Al−THF adduct 6 crystallizes in a monomeric form with an Al
center in a tetrahedral geometry. The catalytic performances of the Al and Ga species 4, 5, 7, and 8 as ROP initiators of rac-
lactide (rac-LA), ε-caprolactone (ε-CL), and trimethylene carbonate (TMC) were estimated, and most of them efficiently
mediate the controlled and immortal ROP of these three monomers in the presence of BnOH, such an alcohol acting as an
effective chain transfer agent. Of particular interest, the Ga amido species κ3-N,O,N-{(C5H9)NC6H4}2O}2Ga-NMe2 (3)
outperforms its Al counterpart κ3-N,O,N-{(C5H9)NC6H4}2O}2AlNMe2 (2) in the ROP of rac-LA, whether in terms of control or
activity, to afford isotactically enriched PLA (Pm = 0.7). In contrast, all the Al derivatives are more efficient catalysts for the
polymerization of ε-CL or TMC than the Ga analogues. For the ROP of TMC initiated by the Al and Ga complexes 2−5, an
increased Lewis acidity of the metal center is clearly beneficial to both the activity and the ROP control. Notably, the C6F5N Ga
species 8 was found to be inactive in the ROP of rac-LA, ε-CL, and TMC, which may be related to both electronic (C−F···Ga
interactions) and steric factors.
as cyclic ester and carbonate ROP initiators, as they typically
mediate such polymerization processes in a controlled (and
possibly stereocontrolled) manner for the production of tailor-
made and narrowly disperse (and possibly stereoregular)
polymeric materials.2 Among these metal-based ROP initiators,
Al(III) alkoxide derivatives historically hold a special place as,
for instance, they were the first metal species shown to initiate
the controlled ROP of lactide (for the production of high-
INTRODUCTION
■
The ring-opening polymerization (ROP) of cyclic esters, such
as lactide (LA) and ε-caprolactone (ε-CL), and cyclic
carbonates, such as trimethylene carbonate (TMC), initiated
by metal alkoxide compounds has been the subject of
numerous studies over the past 10−15 years, which is due to
the biodegradability and biocompatibility of the resulting
materials, such as polylactic acid (PLA) and poly(trimethylene
carbonate) (PTMC).1,2 In this area, discrete ligand-supported
complexes of Lewis acidic and oxophilic metals (M = Al(III),
Sn(II), Zn(II), Mg(II), Ca(II), Ln(III)) have met great success
Received: November 19, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
molecular-weight PLA) via the now well-established ROP
mechanism.3 Also, pioneering reports by Spassky and others on
the possibility of using ligand-supported Al alkoxide initiators
such as (salen)AlOR-type initiators for the stereoselective ROP
of rac-lactide constituted important breakthroughs in the field.4
Yet, despite their relative low cost and ready availability, Al
derivatives for use as ROP initiators of cyclic esters typically
feature a lower ROP activity than other oxophilic metal
complexes and are rather moisture sensitive.5 In addition, the
controversial toxicity of Al(III) is likely to severely limit the
potential usefulness of Al(III)-based ROP initiators for
industrial applications.
(NON)H2 protio ligands bearing electron-withdrawing N
ubstituents. To this end, aside from the (NON)H2 ligands
{(C6H11)NHC6H4}2O (1a) and {(C5H9)NHC6H4}2O (1b),8a
two novel (NON)H2 ligands {(Tf)NHC6H4}2O (1c;Tf =
SO2CF3) and {(C6F5)NH-C6H4}2O (1d) were prepared in
reasonable yields by adapting reported literature procedures,
and their synthesis is depicted in Scheme 1.9 Also, the synthesis
Scheme 1
Contrasting with the numerous studies on Al-based ROP
initiators of cyclic esters and carbonates, the use of metal
initiators of the lower group 13 metals, i.e. Ga(III) and In(III),
remains much less studied, with, in the case of Ga, only a
couple of recent reports on lactide ROP.6,7 These investigations
clearly evidenced that ligand-supported Ga(III) and In(III)
alkoxides may initiate the controlled polymerization of rac-
lactide with, in some cases, some degree of stereocontrol.6,7
Focusing on Ga, key attractive features include a better
biocompatibility of Ga(III) vs Al(III) and a higher stability of
Ga(III) alkyl/amido/alkoxide compounds in protic and polar
media, which is of interest in polymerization catalysis. While
the use of Ga(III) initiators for the ROP of cyclic esters/
carbonates is possibly promising, the potential usefulness and
the particularities of Ga(III) species as ROP initiators deserve
further study. In particular, it may be useful to assess how such
derivatives perform in comparison to their Al(III) counterparts.
We have become interested in the synthesis and potential use
in catalysis of tetracoordinate Al(III) and Ga(III) species
supported by adequate N,O,N diamido-ether dianionic ligands
forcing the group 13 metal center into a trigonal-monopyr-
amidal coordination geometry.8 Such distorted N,O,N-
supported Al and Ga complexes, whose increased reactivity
arises from a destabilizing ligand-defined geometry rendering
the metal center more Lewis acidic, appear as promising
candidates for the efficient mediation of catalytic trans-
formations, whether in polymerization catalysis or in the
production of fine chemicals. On that matter, in preliminary
studies, the Al amido derivative κ3-N,O,N-{(C5H9)-
NC6H4O}2AlNMe2 was observed to be highly active in the
ROP of trimethylene carbonate in a controlled and immortal
manner,8a while its Ga analogue was shown to be an effective
catalyst for the hydroamination of terminal alkynes.8b The latter
constitutes a rare instance of group 13 catalyzed hydro-
amination reactions, illustrating the reactivity potential of these
N,O,N-bearing group 13 systems.
and characterization of (NON)MX species incorporating
various MX groups, i.e. X = Me, NMe2, OBn, were carried
out for further use in polymerization catalysis.
Earlier work on the coordination chemistry of ligands 1a,b
with Al(III) and Ga(III) was restricted to the synthesis of the
corresponding MNMe2 derivatives κ3 -N, O, N-
{(C5H9NC6H4)2O}MNMe2 (2, M = Al; 3, M = Ga) via an
amine elimination reaction between 1a,b and M2(NMe2)6 (M =
Al, Ga), thus with both the Al and Ga amido precursors being
suitable for the preparation of the desired (NON)MNMe2
derivatives.8 In contrast, we found in the present study that
access to the corresponding MMe derivatives via a methane
elimination route (reaction of (NON)H2 with MMe3) only to
be successful for Al(III). Thus, while the Al methyl analogue κ3-
N,O,N-{(CyNC6H4)2O}AlMe (4) was prepared in good yield
via reaction of ligand 1a with AlMe3 (120 °C, 5 days, toluene;
Scheme 2), the formation of the Ga analogue (from a 1/1
mixture of GaMe3 and ligand 1b) was never observed even after
a prolonged reaction time (120−150 °C, up to 7 days in
Scheme 2
To further probe and exploit the potential in polymerization
catalysis of these reactive N,O,N-chelated group 13 systems and
to gain insight into the relative performance in polymerization
catalysis of Al vs Ga analogues, the present contribution reports
on the synthesis and structural characterization of various
N,O,N-supported Al and Ga derivatives and their use in the
ROP of cyclic esters (rac-LA, ε-CL) and trimethylene
carbonate.
RESULTS AND DISCUSSION
■
Synthesis and Structure of a Variety of N,O,N-Bearing
Al and Ga Complexes. A goal of the present study was the
synthesis of various highly Lewis acidic tetracoordinate Al and
Ga species of the type (NON)MX (for subsequent use as ROP
initiators of cyclic esters/carbonates) through the use of
B
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
toluene). An NMR monitoring experiment of the latter reaction
(C6D6, 120 °C, overnight) indicated the quantitative formation
of a single species identified as the amido amino ether Ga
dimethyl species κ3-N,O,N-{(C5H9)NC6H4OC6H4NH-
Scheme 3
1
(C5H9)}GaMe2 (4′; Scheme 2) on the basis of H and 13C
1
NMR data. In particular, the H NMR spectrum for species 4′
contains a characteristic NH-Cy doublet resonance and two
CH-C5H9 signals, while the 13C NMR spectrum exhibits 2 Ga-
Me resonances and 12 signals in the aromatic region. These
NMR data are thus consistent with an overall C1-symmetric
structure at room temperature.
lated as an analytically pure solid and formulated as a dimer on
the basis of solid-state and solution data.
In the solid state and as determined by X-ray crystallographic
studies (Figure 2), compound 5 indeed crystallizes as a dimer
Compound 4′, which arises from a methane elimination
reaction between GaMe3 and only one of the amino groups of
ligand 1b, surprisingly yielded an intractable mixture of
compounds upon further heating, which possibly arises from
thermolysis reactions involving the ligand backbone (prior to a
clean aminolysis of a Ga−C bond).
The molecular structure of the Al methyl complex 4 was
determined by X-ray crystallographic studies, confirming the
formation of an effective κ3-N,O,N-(NON)Al chelate with an Al
center in a trigonal-monopyramidal (tmp) geometry (Figure 1).
Figure 2. Molecular structure of the Al complex 5. Hydrogen atoms
and the Ph groups of the PhCH2OAl groups are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Al(1)−N(1) = 1.870(3),
Al(1)−N(2) = 1.846(3), Al(1)−O(1) = 2.027(2), Al(1)−O(4) =
1.878(2), Al(2)−N(3) = 1.858(3), Al(2)−N(4) = 1.836(3), Al(2)−
O(2) = 1.858(2), Al(2)−O(3) = 2.016(2); O(1)−Al(1)−O(4) =
163.6(1), N(1)−Al(1)−N(2) = 120.4(1), N(3)−Al(2)−N(4) =
121.0(1), O(2)−Al(2)−O(3) = 166.4(1).
and may be seen as two κ3-N,O,N-{(C5H9NC6H4)2O}Al units
being linked to one another via two μ-OBn alkoxide units,
which results into a pseudo C2-symmetric compound (with a
pseudo-C2 axis of symmetry orthogonal to the Al2O2
quadrilateral and going through its center; Figure 2), and
with both five-coordinate Al centers adopting nearly ideal
trigonal-bipyramidal (tbp) geometries (see, in particular, the
angle values in Figure 2). The Al−N amido and Al−(μ-O)
bond distances (average 1.852(5) and 1.868(4) Å, respectively)
are within the expected range for these types of bonds.10,11
Overall, the bonding parameters for species 5 are rather normal
and, with the exception of its dimeric nature, comparable to
those in the Al methyl and amido analogues 2 and 4. It is
noteworthy that the dimeric vs monomeric nature of the Al
benzyloxide species 5 vs compounds 2 and 4 further illustrates
the known propensity of group 13 alkoxide species to readily
aggregate. In solution (C6D6, room temperature), the NMR
data for species 5 agree with an effective C2h-symmetric
structure (with an effective C2-symmetric axis and an
orthogonal plane of symmetry). In particular, the 13C{1H}
spectrum of 5 contains two singlet resonances (δ 69.1 and
57.1) assigned to the AlO-CH2Ph and the CH-C5H9 moieties,
respectively, while two other 13C signals were attributed to the
Figure 1. Molecular structure of the Al complex 4. Hydrogens atoms
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Al−N(1) = 1.837(2), Al−N(2) = 1.854(2), Al−C(1) = 1.947(3), Al−
O = 1.937(2); N(1)−Al−-N(2) = 122.2(1), N(1)−Al−C(1) =
118.4(1), N(2)−Al−C(1) = 115.6(1), C(1)−Al−O = 117.0(1).
Overall, the structural and bonding parameters for species 4 are
closely related to those for its AlNMe2 analogue 2 with, in
particular, the presence of an apical vacant site well-disposed for
substrate coordination to the Lewis acidic metal center. The
solution NMR data for complex 4 agree with its solid-state
structure being retained in solution and with an effective Cs-
symmetric structure (room temperature, C6D6).
Due to the well-known suitability of discrete Al alkoxide
species as ROP initiators of cyclic esters, the (NON)AlOR
derivative 5 was also prepared for subsequent polymerization
studies (Scheme 3). Thus, the amine elimination reaction of the
AlNMe2 species 2 with 1 equiv of BnOH (80 °C, toluene,
overnight) yielded the formation of the corresponding AlOBn
complex [κ3-N,O,N-{(C5H9NC6H4)2O}AlOCH2Ph]2 (5), iso-
C
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
CH2-C5H9 groups. Complex 5 appears to retain its solid-state
dimeric nature in solution, as confirmed by DOSY (diffusion
ordered spectroscopy) measurements (C6D6, room temper-
ature). From the corresponding diffusion coefficient (D = 5.23
× 10−10 m2 s−1), a hydrodynamic radius of 6.6(6) Å was
estimated for 5. This value is to be compared with the mean
radius of 5 (7.37 Å) evaluated from its solid-state structure
using a standard set of van der Waals radii.12 The DOSY-
estimated hydrodynamic radius and the calculated mean radius
thus match reasonably well, taking into account experimental
and calculations margins of error. Overall, the NMR and DOSY
data are consistent with the solid-state structure of 5 being
retained in solution.
Going from N-cycloalkyl to N-Tf substituents in proligands
(NON)H2 (1a,b vs 1c) was found to dramatically alter the
reactivity/structure of the resulting Al(III) complexes. Thus,
despite the screening of various temperature and solvent
conditions, the reaction of 1c with the amido precursors
M2(NMe2)6 (M = Al, Ga) consistently afforded complicated
mixtures of unidentified products. In contrast and unlike
ligands 1a,b, ligand 1c reacts quickly with 1 equiv of AlMe3 at
low temperature (−35 to 0 °C, 10 min, 1/1 CH2Cl2/THF
solvent mixture) to yield the quantitative formation of the Al−
THF adduct chelate κ2-N,N-{(CF3SO2)NC6H4}2OAl(Me)-
(THF) (6; Scheme 4), which was isolated as a highly air-
Figure 3. Molecular structure of the Al complex 6. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Al−N(1) = 1.900(4), Al−N(2) = 1.890(4), Al−O(1) = 1.857(4), Al−
C(1) = 1.926(5); N(1)−Al−N(2) = 108.1(2), N(1)−Al−C(1) =
110.3(2), N(2)−Al−C(1) = 121.8(2).
N(2) bite angle (108.1(2)°) of the Al chelate is significantly
more acute than that in the related η3-N,O,N-(NON)Al chelate
4 (N(1)−Al−N(2) = 122.2(1)°). Such a smaller N−Al−N
bond angle likely arises from the tetrahedral vs tmp geometry at
Al, which somewhat pushes away the central Ph−O−Ph donor
atom (as reflected by the long Al···O(2) distance of 2.694(2)
Å) to minimize ring strain and thus certainly disfavors the
formation of a κ3-N,O,N-(NON)Al chelate. As a comparison,
the related tetracoordinate Al complex κ3-N,N′,N″-
{(TfNCHiPrCH2)2NBn}AlMe, in which the Al center adopts
a tmp geometry and is κ3 supported by a diamido amino ligand,
has been reported.14 In our case, the supporting NON2−
dianionic ligand incorporates a central ether donor that is less
σ-donating (to Al(III)) than an amine. In the case of species 6,
the coordination of a THF molecule onto Al to yield a four-
coordinate tetrahedral Al center apparently provides a superior
stability to the resulting Al methyl complex.15 Also, in
compound 6, the Al−OTHF and Al−C(1) bond distances
(1.857(4) and 1.926(5) Å, respectively) are a bit shorter than
those observed in related neutral κ2-diamido Al−THF methyl
complexes while being comparable to those in cationic Al
methyl adducts,16,17 suggesting an important ionic character in
these bonds and presumably reflecting a fairly electrophilic Al
center in 6. The NMR data for complex 6 agree with a C1-
symmetric structure in solution under the studied conditions
(CD2Cl2, room temperature) and the effective coordination of
THF onto Al, as observed in the solid state. In particular, the
19F NMR spectrum of 6 exhibits two rather sharp singlet CF3
resonances (δ −74.7, −77.4), indicating that the two CF3
groups are not equivalent (Figure S1, Supporting Information).
Such a low-symmetry structure for 6 is most likely due to a slow
ring inversion of the eight-membered Al metallacycle at room
temperature on the NMR time scale. To probe this issue
further, high-temperature 19F NMR studies of species 6 were
performed, resulting in the observation of a coalescence of the
two CF3 signals at 50 °C (C6D6) with a subsequent sharpening
of the single CF3 resonance as the temperature is further raised
(Figures S2 and S3, Supporting Information). These data also
Scheme 4
sensitive compound, soluble in CH2Cl2 and THF. Species 6 is
thermally unstable even in the solid state and decomposes over
weeks at low temperature (−35 °C) under N2. However,
carrying out the reaction of 1c with AlMe3 in a noncoordinative
solvent (CH2Cl2) afforded, in good yield after purification of
the crude product, a colorless solid residue insoluble in all
common solvents but THF and formulated as “{(CF3SO2)-
NC6H4}2OAl(Me)” on the basis of elemental analysis. The
insolubility of the latter in nearly all solvents (even upon
heating) and the fact that it solubilizes in THF to quantitatively
yield the Al−THF adduct 6 suggest the formation of
{(CF3SO2)NC6H4}2OAl(Me)-based Al polynuclear aggregates
upon reacting a 1/1 1c/AlMe3 mixture in CH2Cl2. These
observations presumably reflect the instability of the putative
κ3-N,O,N-{(CF3SO2)NC6H4}2OAlMe under the studied con-
ditions, this being likely due to the highly Lewis acidic Al metal
center in such a derivative.
The molecular structure of the Al−THF adduct 6 was
determined by X-ray crystallographic analysis, and it is depicted
in Figure 3. Unlike compounds 2, 4, and 5, which all feature a
κ3-N,O,N-{(C5H9)NC6H4}2OAl chelate with a tmp geometry at
Al, the four-coordinate Al center in species 6 adopts a distorted-
tetrahedral geometry and is supported by the dianionic diamido
ligand [{(CF3SO2)NC6H4}2O]2− in a κ2-N,N′ fashion (with
thus no coordination of the NON ligand central ether donor to
Al) to yield the formation of an eight-membered-ring Al
metallacycle in a boat−boat conformation.13 The N(1)−Al−
D
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
allowed an estimation of the free enthalpy of activation of the
dynamic process (ΔG⧧ = 14(1) kcal mol−1), a value in line with
energy barriers observed for the ring inversion of heteroatom-
containing eight-membered rings.10 The poor stability of
derivative 6 overall suggested that such NTf-ligated group 13
metal alkyl compounds may be of limited efficiency in
polymerization catalysis and the coordination chemistry of
ligand 1c was not extended to Ga(III).
In contrast, the use of the C6F5NH-incorporating diamino
ether protio ligand 1d provided access to robust Al(III) and
Ga(III) Lewis acidic species. Thus, the methane elimination
reaction of ligand 1d with 1 equiv of MMe3 (M = Al, Ga; 80 or
130 °C, toluene, overnight) readily afforded the corresponding
(NON)-MMe chelates κ3-N,O,N-{(C6F5NC6H4)2O}MMe (7,
M = Al; 8, M = Ga; Scheme 5), which were isolated as highly
contacts between the Ga center and two o-fluorine atoms (one
per C6F5 ring) constitutes a noteworthy structural feature, with
the Ga(1)−F(1) and Ga(1)−F(6) distances (2.77 and 2.95 Å,
respectively, Figure 4) well below the sum of the van der Waals
radii for Ga and F (i.e., 3.34 Å).18 Such interactions clearly
reflect the highly Lewis acidic nature of the Ga center in 8 and,
in view of the robustness of 8, may play a key role (although
not anticipated) in the stability of the Ga species 8. In addition,
the room-temperature 19F NMR spectrum of species 8 contains
a coalescence signal for the o-C6F5 group, while that at −30 °C
features two decoalesced o-C6F5 resonances in a 1/1 ratio (each
corresponding to 2F) indicating the presence of two types of o-
C6F5 fluorines (Figures S4 and S5, Supporting Information).
These data are consistent with a restricted/slow rotation
around the N−C6F5 bonds on the NMR time scale under the
studied conditions. Although such a restrained rotation around
the N−C6F5 bonds may be related to the persistence of Ga···F
contacts in solution, steric factors may also be at play. As a
comparison, the 19F NMR data for the Al analogue 7 from
room temperature down to −30 °C only displays sharp
resonances for the C6F5 groups. Despite numerous attempts, no
suitable X-ray-quality single crystals of the Al compound 7
could be grown. Of relevance to the present C−F···Ga contacts,
it may be noted that analogous C−F···M interactions have been
observed and studied in group 4 complexes supported by
tripodal amido ligands.19
Scheme 5
The nature of the Ga···F interactions in species 8 was studied
by theoretical methods (see the Experimental Section for
further details). Figure 5 features the weak interactions in
air-sensitive yet thermally stable colorless solids. Aside from
elemental analysis and NMR data, the proposed formulation
was confirmed by a X-ray-determined solid-state structure in
the case of the Ga derivative 8 (Figure 4). As was somewhat
expected, species 8 crystallizes as discrete κ3-N,O,N-{(C6F5N−
C6H4)2O}GaMe molecules in which the tetracoordinate Ga
metal center adopts a tmp geometry imposed by the κ3-
coordinated NON2− ligand. The presence of very short Ga···F
Figure 5. Noncovalent interactions in complex 8. The strongly
repulsive interactions are shown in red and the strongly attractive ones
in blue. The green basins correspond to the purely van der Waals
(dispersive) interactions.
complex 8. Around the gallium metal center, large repulsive
annuluses represent the repulsive interactions between the lone
pairs of the chelating NON ligand. Between the phenyls, there
are large areas corresponding to van der Waals interactions
between the rings. The NCI approach shows that the
interaction between the gallium and fluorine atoms is weak
and essentially of van der Waals type, though an ionic
contribution could have been expected between the electro-
philic Ga center and the negatively charged fluorines. A close
inspection shows the presence of a small ionic contribution,
highlighted by a small blue spot in the middle of the green area.
Figure 4. Molecular structure of the Ga complex 8. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ga(1)−N(1) = 1.898(2), Ga(1)−N(2) = 1.923(2), Ga(1)−C(25) =
1.927(3), Ga(1)−O(1) = 2.104(1); N(1)−Ga(1)−N(2) = 110.2(7),
N(1)−Ga(1)−C(25) = 125.6(9), C(25)−Ga(1)−N(2) = 123.5(9),
C(25)−Ga(1)−O(1) = 111.9(8).
E
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 1. ROP of rac-LA Initiated by the Al and Ga Complexes 2−5 and 7
b
b
c
d
e
f
g
h
entry
cat.
rac-LA
BnOH
t (h)
conversn (%)
Mn(theor)
Mn(cor)
PDI
Pm
i
1
2
100
100
100
100
100
100
100
100
20
1
3
45
80
92
100
31
75
0
6480
11520
2650
14200
12400
2700
3.10
1.66
1.15
1.12
1.09
1.11
i
2
2
j
3
2
5
17
1
0.70
0.70
i
4
3
14400
4464
12400
3800
k
5
3
5
k
6
3
17
15
15
7
10800
8200
l
7
4
m
8
4
1
1
95
93
42
61
79
98
98
0
13680
2678
8100
1700
6200
7700
7900
15600
3000
1.30
1.20
1.25
1.32
1.23
1.19
1.22
m
9
4
i
10
11
12
13
14
15
5
100
100
100
100
100
100
1
6048
0.62
0.62
0.62
i
5
2
8784
i
5
3
11376
14400
2822
m
7
1
5
5
15
15
15
m
7
m
8
a
b
c
d
e
[rac-lactide]0 = 1 M. In equiv versus initiator. Reaction time. Monomer conversion. Calculated using Mn(theor) = [rac-lactide]0/[BnOH]0 ×
f
Mrac‑LA × conversn Measured by GPC in THF (30 °C) using PS standards and corrected by applying the appropriate correcting factor (0.58).
g
h
i
j
k
l
Measured by GPC in THF (30 °C). Determined by 1H NMR. Toluene, 80 °C. Toluene, 60 °C. Dichloromethane, 40 °C. Dichloromethane, 25
m
°C. Toluene, 90 °C.
However, the dispersive forces mainly dominate the Ga···F
interactions.
Ring-Opening Polymerization of rac-Lactide by the
(NON)M−X Complexes 2−5 and 7. The Al and Ga
compounds 2−8 were all tested as ROP initiators of rac-lactide
and, with the exception of derivatives 6 and 8, were essentially
found to polymerize rac-lactide in a controlled manner. The
lack of any ROP activity for species 6 is certainly to be ascribed
to its thermal instability with a probable decomposition of 6
under polymerization conditions prior to any ROP activity. The
polymerization results are presented in Table 1 for various
reaction conditions (toluene or CH2Cl2, 25−90 °C, 1−17 h,
with/without BnOH). As stated in the Introduction, a primary
goal of the present investigations was aimed at comparing Al vs
Ga catalytic performances. Thus, remarkably, while the
GaNMe2 species 3 efficiently polymerizes rac-lactide (100
equiv of rac-LA, entries 4−6,Table 1) in a controlled manner,
the Al amido analogue is less active under identical conditions
and produces a broadly disperse PLA material (entries 1 and 2,
Table 1), this being consistent with a poorly controlled ROP
process. For the ROPs initiated by the Ga species, all
polydispersities (PDI) are below 1.15, with SEC data featuring
in all cases monomodal symmetric peaks (Figure S6,
Supporting Information). This, together with a linear
correlation between the molecular weight of the polymer
chain and the monomer to polymer conversion, supports a
well-controlled polymerization process (Figure 6; for additional
kinetic data, see Figure S7, Supporting Information).
For the rac-LA ROP carried out in CH2Cl2 at 40 °C (entry 5,
Table 1), the resulting PLA material was also analyzed by
MALDI-TOF spectrometry (Figure S8, Supporting Informa-
tion), unambiguously establishing the presence of an NMe2
moiety at the ester end of the formed PLA and the absence of
substantial transesterification reactions as the ROP proceeds. In
addition, under these conditions, the production of isotactically
enriched PLA (Pm = 0.70) was clearly evidenced from
decoupled 1H NMR data (Figure S9, Supporting Information).
Hence, all data suggest that the ROP of rac-LA by the Ga
species 3 proceeds via a classical coordination−insertion
mechanism.
Figure 6. Plot of Mn as a function of monomer conversion in the ROP
of rac-lactide by the Ga species 3. Conditions: 100 equiv of rac-lactide
(vs 3), [rac-lactide]0 = 1 M, CH2Cl2, 40 °C.
Going from the Al amido derivative 2 to its alkoxide analogue
5 dramatically improves rac-LA polymerization control (but
with a ROP activity remaining lower than that of the Ga amido
species 3), thereby illustrating that the nature of the AlX
initiating moiety may be crucial for a well-behaved ROP
process. Thus, reasonably narrowly disperse, chain-length-
controlled, and OBn-ester-ended PLA is produced upon
using compound 5 as an initiator (entries 10−12, Table 1;
Figures S10−S13, Supporting Information). While being
transesterified (as evidenced by MALDI-TOF; Figure S13,
Supporting Information), the resulting PLA features a slight
isotactic bias (Pm = 0.62, Figure S14, Supporting Information).
As for the (NON)MMe derivatives 4, 7, and 8, they expectedly
were found to be inactive in the ROP of rac-LA as single-
component initiators, since the MMe moiety is typically a poor
initiating group for the ROP of cyclic esters/carbonates.
Performing the ROP of rac-LA using a 1/1 4/BnOH mixture
(for an anticipated in situ generation of a ROP initiator of the
type (NON)AlOBn, entries 8 and 9, Table 1) produced a
mixture of PLA material consisting of OBn-ester-ended and
NON-ligand functionalized PLA chains, as deduced from
MALDI-TOF analysis (Figure S15, Supporting Information).
This indicates that, under these ROP conditions (toluene, 90
F
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
°C), the ROP of rac-LA may proceed either via an initial LA
insertion into the Al−N amido bonds (of (NON)AlX) or into
the in situ formed Al−OBn bond to form, after subsequent
chain growth, the observed mixture of PLA chains. These
results further illustrate that the reaction conditions frequently
required for an in situ alcoholysis (of Al alkyl species) are such
that competing reactions may also occur.
immortal manner, allowing the production of narrowly disperse
and chain-length-controlled PCL under rather mild conditions.
All experimental data (SEC, MALDI-TOF and kinetic data)
support controlled and immortal ROP processes in all cases,
with BnOH behaving as an effective chain transfer agent
(Figures S22−S28, Supporting Information). In particular, all
data agree with PCLs bearing OBn-ester-end groups (as
deduced from MALDI-TOF data), with the exception of the
PCL produced via the ROP of ε-CL initiated by the Al species
2. For the latter, the MALDI-TOF data agree with the
obtainment of two different chain-ended PCLs: OBn- and
NMe2-ester-ended groups (Figure S24, Supporting Informa-
tion). The presence of PCL chains bearing NMe2-ester-end
groups reflects that the ROP of ε-CL initiated by 2 proceeds, at
least to some extent, via a coordination−insertion mechanism.
However, the presence of OBn-ester-ended groups along with
the fact that no reaction is observed between species 2 and
BnOH at room temperature (thus under polymerization
conditions) suggest that an activated monomer mechanism is
also at play.22
The Al compound 2 (in the presence of 5 equiv of BnOH)
exhibits the best ROP performances with the quantitative
conversion of 100 equiv of ε-CL to well-defined PCL within 10
min at room temperature (entry 1, Table 2) and lies among the
most efficient group 13 based developed thus far for the
polymerization of ε-CL.2g As for the Ga amido species 3, while
displaying a good ROP activity and control (entries 3 and 4,
Table 2), it is less active than its Al analogue 2 with a 41%
conversion to PCL of 100 equiv of ε-CL within 10 min at room
temperature. The (C6F5)N-based Al species 7, although less
efficient than the AlNMe2 2, exhibits a good ROP activity,
whereas the Ga species 8 is inactive under identical conditions
(entry 6 vs 8, Table 2). The lower performances of the Ga vs Al
analogues may be ascribed to the less Lewis acidic character of
Ga vs that of Al, this being apparently the key factor
determining an efficient ROP of ε-CL. In line with this and
within the Al compounds, the more Lewis acidic AlMe species
7 performs significantly better than its analogue 4 (entry 6 vs 5,
Table 2). For all metal complexes, the ROP of ε-CL proceeds
in a highly controlled manner.
Ring-Opening Polymerization of TMC by the
(NON)MX Complexes 2−4 and 7. Well-defined metal
initiators able to ring-open polymerize cyclic carbonates such
as TMC to produce chain-length-controlled PTMC in an
efficient/controlled manner and under mild reaction conditions
remain surprisingly rare.23 In preliminary studies on group 13
(NON)MX-type complexes, the Al amido 2 was found to be
highly active at room temperature in the ROP of TMC for the
production of narrowly disperse and chain-length-controlled
PTMC (entries 1−3, Table 3).8a In the present section, we
detail our TMC polymerization studies carried out with the Ga
analogues 3 and 8 and with the Al methyl/alkoxide species 4, 5,
and 7.
As summarized in Table 3, with the exception of the Ga
species 8, all tested group 13 species were found to readily
mediate the ROP of TMC at room temperature in the presence
of the chain transfer agent BnOH. All available experimental
data (SEC, MALDI-TOF, and kinetic data, Figures S29−S41,
Supporting Information) support fairly well-controlled, immor-
tal ROP processes with the formation of OBn-ester-end PTMC
material, except for the PTMC resulting from the TMC ROP
initiated by the Ga amido species 3. Thus, for the latter, the
MALDI-TOF spectrum of the prepared PTMC contain peaks
The group 13 complexes 2, 7, and 8 were also tested for rac-
lactide ROP activity in the presence of various amounts of an
alcohol source such as BnOH so as to probe their suitability for
the immortal ROP of rac-LA.20 As depicted in Table 1 (entries
3 and 13−15), though the Ga compound 8 is inactive, the Al
species 2 and 7 were found to efficiently mediate the controlled
and immortal ROP of rac-LA in the presence of BnOH (acting
as a chain transfer agent) to afford narrowly disperse, chain-
length-controlled and, in the case of initiator 2, isotactic-
enriched PLA (Pm = 0.70). In particular, for the 2/BnOH
initiating system, the Mn(calcd) values match well the expected
Mn(theor) values (based on the initial [rac-LA]0/[BnOH]0
ratio, Figure S16, Supporting Information). Overall, character-
ization and polymerization kinetic data for systems 2/BnOH
and 7/BnOH (Figures S17−S21, Supporting Information) are
consistent with controlled alcohol-mediated chain transfer
reactions during these ROP reactions. One may note that the
Al derivative 2 and the presumably more Lewis acidic analogue
7 (in the presence of 5 equiv of BnOH) exhibit comparable
ROP activities under the studied (unoptimized) conditions
with the nearly quantitative polymerization of 100 equiv of rac-
LA within 15 h (60 °C, toluene; entries 3 and 14, Table 1).
Also, the lack of ROP activity of the Ga species 8 in the
presence of excess BnOH (entries 16 and 17, Table 1)
obviously contrasts with the good activity of the other two
compounds, which may be related to the presence of Ga···F
close contacts (vide supra) that, along with steric factors,
apparently severely hinder monomer approach and coordina-
tion.
The higher ROP activity of the Ga analogue 3 versus the Al
species 2 and 5 may be related to the less Lewis acidic character
of Ga vs Al, which is expected to result in the formation of less
robust κ2-(lactate)M adducts for Ga as the ROP proceeds (A,
Chart 1) that are thus more susceptible to chain growth.
Chart 1
Though attempts to cleanly generate κ2-(lactate)M-type
adducts (M = Al, Ga) with our systems (by an equimolar
reaction of 2 or 3 with (S)-methyl lactate) all were
unsuccessful, several reactivity studies of Al complexes toward
lactide have shown that the formation of robust η2-(lactate)Al-
type adducts may significantly hinder and even obstruct
subsequent chain growth.21
Ring-Opening Polymerization of ε-CL by the
(NON)M−X Complexes 2−4 and 7. As summarized in
Table 2, the Al and Ga complexes 2−4 and 7 in the presence of
BnOH readily mediate the ROP of ε-CL in a controlled and
G
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 2. ROP of ε-CL Initiated by the Al and Ga Complexes 2−4 and 7
b
b
c
d
e
f
g
entry
cat.
ε-CL
BnOH
t
conversn (%)
Mn(theor)
Mn(cor)
PDI
1
2
3
4
5
6
7
8
2
2
3
3
4
7
7
8
100
600
100
600
100
100
100
100
5
5
5
5
5
5
5
5
10 min
3 h
100
100
41
95
15
46
95
0
2280
13697
936
2300
14400
700
1.26
1.29
1.15
1.05
1.09
1.06
1.40
10 min
3 h
13012
432
12500
500
15 h
10 min
30 min
10 min
1050
2166
600
2400
a
b
c
d
e
Conditions: [ε-caprolactone]0 = 1 M, room temperature, CH2Cl2. In equiv versus initiator. Reaction time. Monomer conversion. Calculated
f
using Mn(theor) = [ε-caprolactone]0/[BnOH]0 × Mε‑CL × conversn. Measured by GPC in THF (30 °C) using PS standards and corrected by
applying the appropriate correcting factor (0.56). Measured by GPC in THF (30 °C).
g
a
Table 3. ROP of TMC Initiated by the Al and Ga Complexes 2−5, 7, and 8
b
b
c
d
e
f
g
entry
cat.
TMC
BnOH
t (min)
conversn (%)
Mn(theor)
Mn(cor)
PDI
1
2
2
2
3
3
3
3
4
4
4
5
5
5
7
7
7
7
8
100
300
600
100
100
300
600
100
100
600
100
300
600
100
300
600
600
100
5
3
3
5
5
3
3
5
5
3
5
3
3
5
3
3
3
5
5
90
100
100
19
1944
10308
20508
495
1500
9500
20700
400
1.12
1.18
1.12
1.03
1.15
1.21
1.35
1.13
1.15
1.47
1.21
1.26
1.32
1.12
1.33
1.17
1.21
/
h
2
30
h
3
30
4
5
30
5
98
2107
6738
15500
2148
1332
6642
1809
7760
15420
1373
5310
6024
8676
2300
9600
22400
2500
1000
12800
1600
12800
25900
1100
1600
1900
3100
6
180
300
65
7
75
h
8
10
100
60
9
1
10
11
12
13
14
15
16
17
18
30
h
32
30
100
100
100
62
h
30
h
30
8
15
15
30
8
51
29
42
0
0
/
a
b
c
d
Polymerization conditions: [TMC]0 = 1 M, dichloromethane, room temperature. In equiv versus initiator. Reaction time. Monomer conversion.
e
f
Calculated using Mn(theor) = [TMC]0/[BnOH]0 × MTMC × conversn. Measured by GPC in THF (30 °C) using PS standards and corrected by
applying the appropriate correcting factor (0.57, 0.76, or 0.88).24 Measured by GPC in THF (30 °C). The reaction time was not optimized.
g
h
for both OBn-ester-ended and NMe2-amido-end polymer
chains (Figure S33, Supporting Information), suggesting, as
discussed earlier, that two different mechanisms are involved in
the ROP of TMC by species 3: the classical coordination/
insertion mechanism (yielding the NMe2-capped PTMC) and
the “activated monomer” mechanism (yielding the OBn-capped
PTMC). As a comparison, under identical ROP conditions,
only OBn-capped PTMC is produced with the Al amido
analogue 2.
As deduced from Table 3 (entries 4−7), the activity of the
Ga amido 3 is much lower than that of the Al analogue 2 (19%
vs quantitative conversion to PTMC of 100 equiv of TMC, 5
equiv of BnOH, 5 min, room temperature) but the conversion
of 100 equiv of TMC is nearly quantitative after 30 min with
initiator 3. Kinetic studies performed with the Ga initiator 3
Figure 7. Plot of the molecular weight number (Mn) of the formed
(300 equiv of TMC, 3 equiv of BnOH) are all consistent with a
PTMC as a function of [3]0/[BnOH]0 in the ROP of TMC by a 3/
ROP proceeding in a controlled manner. In particular, a linear
BnOH initiating system. Conditions: 300 equiv of TMC (vs 3),
correlation between the Mn(cor) value of the formed PTMC
and the monomer conversion during the polymerization
reaction is observed (Figure S30, Supporting Information).
Also, as depicted in Figure 7, despite Mn values higher than
expected, varying the initial Ga initiator/BnOH ratio (for a
given amount of monomer) was found to be linearly related to
the molecular weight number (Mn) of the resulting polymers.
[TMC]0 = 1 M, CH2Cl2, room temperature, 5 h, quantitative
conversion.
For a given set of conditions (100 equiv of TMC, 5 equiv of
BnOH), an apparent first-order kinetics in monomer was
established for the ROP of TMC by 3/BnOH (Figure S31,
H
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Supporting Information) with a rate constant (kobs = 0.006
min−1) consistent with a ROP process being much slower in
comparison to that initiated by the Al system 2/BnOH system
(kobs = 0.152 min−1; Figure S32, Supporting Information): thus,
of ε-CL or TMC. Nevertheless, the results obtained here with
the Ga initiator 3 first shows that discrete Ga derivatives are
suitable candidates for the controlled and immortal ROP of ε-
CL and TMC. Interestingly, in the case of the ROP of TMC
and with these (NON)MX systems, an increased Lewis acidity
of the metal center is clearly beneficial to both the activity and
the ROP control. Though the present results do not allow any
broad conclusion to be drawn, they suggest that, in addition to
their biocompatibility, Ga(III) initiators may, in some instances,
complement their Al(III) congeners.
k
obs(Al)/kobs(Ga) ≈ 25. In addition, in the case of 3 and unlike
what is observed for species 2, a loss of chain-length control for
the resulting PTMC is observed with increasing monomer
loadings (entry 3 vs 7, Table 3). Altogether, in the present
systems, the obtained results in the ROP of TMC suggest that
going from the Al to the less Lewis acidic Ga counterpart is
detrimental both to the catalytic activity and, to a lesser extent,
to the chain-length control of the resulting material.
EXPERIMENTAL SECTION
■
Within the Al series, the AlMe and AlOCH2Ph compounds 4
and 5 both efficiently polymerize TMC at room temperature in
the presence of the chain-transfer agent BnOH to afford chain-
length-controlled PTMC with, for the most part, molecular
numbers (Mn) in good agreement with the monomer/BnOH
initial ratios (entries 8−13, Table 3). While exhibiting TMC
ROP performances comparable to those of 2, the Al alkoxide is
significantly more active than its Al methyl analogue 4, with a
quantitative vs 32% conversion of 600 equiv of TMC to PTMC
within 30 min at room temperature (entry 13 vs 10, Table 3).
Overall, these data likely reflect the key importance of Lewis
acidity for high activity and control in the ROP of TMC. The
more Lewis acidic Al center in the amido and alkoxide
complexes 2 and 5 vs that in species 4 may rationalize the
better ROP activity and control.
General Procedures. All experiments were carried out under N2
using standard Schlenk techniques or in a MBraun Unilab glovebox.
Toluene, pentane, and dichloromethane were collected after going
through drying columns (SPS apparatus, MBraun) and stored over
activated molecular sieves (4 Å) for 24 h in a glovebox prior to use.
Tetrahydrofuran was distilled over Na/benzophenone and stored over
activated molecular sieves (4 Å) for 24 h in a glovebox prior to use.
CH2Cl2, CD2Cl2, and C6D6 were distilled from CaH2, degassed under
a N2 flow, and stored over activated molecular sieves (4 Å) in a
glovebox prior to use. All deuterated solvents were obtained from
Eurisotop (CEA, Saclay, France). All other chemicals were purchased
from Aldrich and were used as received, with the exception of
trimethylene carbonate purchased from either TCI Europe Corpo-
ration or Boehringer: it was dried over CaH2 in THF for 24 h and
precipitated with pentane prior to use. The NMR spectra were
recorded on Bruker AC 300, 400, and 500 MHz NMR spectrometers
in Teflon-valved J. Young NMR tubes at ambient temperature. 1H and
13C chemical shifts are reported vs SiMe4 and were determined by
reference to the residual 1H and 13C solvent peaks. Elemental analyses
for all compounds were performed at the Service de Microanalyse of
SUMMARY AND CONCLUSIONS
■
Through the use of different N-substituted (NON)H2 protio
ligands and studies of their coordination chemistry toward
Al(III) and Ga(III) precursors, a family of structurally diverse
Al and Ga complexes of the type (NON)MX has been
synthesized and characterized. Four of these group 13
derivatives were isolated as mononuclear species featuring a
central tetracoordinate metal center adopting a tmp geometry
(species 3, 4, 7, and 8), whereas the Al alkoxide derivative 5 was
isolated as a dimer containing two five-coordinate and tbp Al
centers. Remarkably, the reactivity/stability balance is quite
subtle in such species and may greatly be impacted by the
nature of the N substituents. Thus, while the C6F5N-based
species 7 and 8 were initially thought of and anticipated to
afford highly Lewis acidic and thus very reactive Al and Ga
(NON)MX species, the presence of M···F(o-C6F5) interactions
in these species is proposed to significantly quench the Lewis
acidity of the metal center, as deduced from structural and
catalytic studies. Using the Tf-based (NON)H2 protio ligand 1c
(reacted with AlMe3) allowed the isolation of the Al−THF
adduct 7, in which the Al center is four-coordinate and adopts a
preferred tetrahedral geometry. Due to its presumed instability,
the sought after TfN-based species η3-N,O,N-(NON)Al-Me
was never observed.
́
the Universite de Strasbourg (Strasbourg, France). GPC analyses were
performed on a system equipped with a Shimadzu RID10A
refractometer detector using HPLC-grade THF as the eluent.
Molecular weights and polydispersity indices (PDIs) were calculated
using polystyrene standards. In the case of molecular weight number
(Mn), these were corrected with appropriate correcting factors for the
Mn values. MALDI-TOF mass spectroscopic analyses were performed
́
at the Service de Spectrometrie de Masse de l’Institut de Chimie de
Strasbourg and run in a positive mode: samples were prepared by
mixing a solution of the polymers in CH2Cl2 with a 0.5 mg/100 mL
concentration; 2,5-dihydroxybenzoic acid (DHB) was used as the
matrix in a 5:1 volume ratio. The compounds Al2(NMe2)6,
Ga2(NMe2)6, bis-amino-ether protio ligands {(C6H11)NHC6H4}2O
(1a) and {(C5H9)NH-C6H4}2O (1b), the Al complex κ3-N,O,N-
{(C5H9)NC6H4O}2AlNMe2 (2), and the Ga complex κ3-N,O,N-
{(C5H9)NC6H4O}2GaNMe2 (3) were all synthesized according to
reported literature procedures.8,25
Computational Details. DFT calculations on experimental
geometries were performed using the B3LYP functional as
implemented in the GAUSSIAN 09 package26 witha 6-31G basis set
for all atoms except for gallium. For the latter, the small core
pseudopotential from Dolg27 and the associated basis set were used.28
Weak interactions were studied by analysis of the obtained wave
function by the NCIPLOT package.29
In addition to the structural patterns and diversity within the
(NON)MX series, the catalytic performances of Al and Ga
analogues in the ROP of rac-LA, ε-CL, and TMC were
estimated and the vast majority of these derivatives efficiently
mediate the controlled and immortal ROP of these three
monomers in the presence of BnOH acting an effective chain
transfer agent. Of particular interest, the Ga amido species 3
was found to exhibit performance superior to that of its Al
counterpart 3 in the ROP of rac-LA, whether in terms of
control or activity, to afford isotactically enriched PLA. In
contrast, the Al derivatives perform better in the polymerization
{(CF3SO2)NH-C6H4}2O (1c). In a glovebox, 2,2′-oxydianiline
(940.0 mg, 4.700 mmol) was charged into a 250 mL round-
bottom one-necked flask containing a Teflon-sealed stir bar and
was dissolved in dry dichloromethane (10 mL). In another
Schlenk flask, NaHCO3 (4 equiv, 1,580 g, 18.80 mmol) was
charged and dissolved in dry dichloromethane (10 mL). The
sodium carbonate CH2Cl2 solution was then added via cannula
to the diamine solution. The resulting mixture was cooled to 0
°C, and triflic anhydride (O(CF3SO2)2; 2.1 equiv, 2.820 g,
1,680 mL, 10.00 mmol) was then added dropwise via a syringe
at 0 °C. Upon addition, the solution turned brown and then
I
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
1
purple. After the addition was complete, the solution was
stirred at 0 °C for 15 min and then at room temperature for 2
days. The reaction mixture was quenched with water (200 mL)
and diluted with ethyl acetate (400 mL), and brine (200 mL)
was added. The phases were separated, the organic phase was
dried over MgSO4, and the solvent was evaporated. A green oil
was obtained. Ether and pentane were added, and the mixture
was kept at −35 °C. The precipitate was filtered, and a white
solid was obtained (400.0 mg, 18% yield). Anal. Calcd for
C14H10F6N2O5S2 (464.36): C, 36.21; H, 2.17. Found: C, 36.56;
H, 2.05. 1H NMR (300 MHz, CDCl3): δ 7.32 (m, 2H, Ar), 6.60
(m, 4H, Ar), 6.38 (m, 2H, Ar). 19F{1H} NMR (282.4 MHz,
CDCl3): δ −76.7 (s, CF3). 13C{1H} NMR (75 MHz, C6D6): δ
149.7 (Cipso-O), 129.0 (Ar), 126.2 (Ar), 125.8 (Ar), 125.4 (Ar),
deduced from H NMR data. H NMR (C6D6, 400 MHz): δ
7.63 (d, 3JHH = 8.4 Hz, Ar, 1H), 7.00 (t, 3JHH = 8.0 Hz, Ar, 1H),
3
3
6.95 (d, JHH = 8.0 Hz, Ar, 1H), 6.91 (d, JHH = 8.0 Hz, Ar,
1H), 6.86 (t, 3JHH = 8.4 Hz, Ar, 1H), 6.52 (t, 3JHH3 = 8.8 Hz, Ar,
3
2H), 6.45 (t, JHH = 7.2 Hz, Ar, 1H), 4.57 (d, JHH = 8.2 Hz,
3
3
NH, 1H), 3.73 (dq, JHH = 8.5 Hz, JHH = 6.5 Hz, CH, 1H),
3.68 (q, JHH = 6.5 Hz, CH, 1H), 1.08−2.08 (m, C5H9, 16H),
3
0.29 (s, CH3, 6H). 13C{1H} NMR (C6D6, 100 MHz): δ 160.8
(Cipso), 151.0 (Cipso), 137.0 (Cipso), 135.7 (Cipso), 124.8 (Ar),
123.2 (Ar), 122.0 (Ar), 119.1 (Ar), 118.2 (Ar), 117.4 (Ar),
113.1 (Ar), 112.0 (Ar), 69.3 (CH-C5H9), 57.4 (CH-C5H9), 34.9
(C5H9), 33.5 (C5H9), 31.5 (C5H9), 30.6 (C5H9), 24.4 (C5H9),
24.2 (C5H9), 24.1 (C5H9), 23.9 (C5H9), −3.4 (Me), −4.4 (Me).
[κ3-N,O,N-{(C5H9)NC6H4}2O}2AlOCH2Ph]2 (5). Under a
nitrogen atmosphere, the Al amido complex 2 (420.0 mg,
0.519 mmol) was dissolved in anhydrous toluene. Benzylic
alcohol (1 equiv, 109 μL) was then added and the mixture
heated to 80 °C (oil bath) overnight with stirring. The volatiles
were subsequently evaporated to yield a brown oily residue.
Recrystallization of the latter from a 1/1 Et2O/pentane mixture
(cooled to −35 °C) afforded compound 5 as an analytically
pure white solid (200.0 mg, 0.428 mmol, 44% yield). Anal.
Calcd for C29H33AlN2O2: C, 74.34; H, 7.10; N, 5.98. Found: C,
1
122.8 (q, JCF = 290 Hz, CF3), 118.6 (Ar).
{(C6F5)NH-C6H4}2O (1d). Under a dry nitrogen atmosphere,
2,2′-oxydianiline (1.08 g, 5.38 mmol) was dissolved in dry THF
(15 mL) and added to a hexamethyldisilazane lithium salt
suspension (4.050 g, 24.00 mmol, 4.5 equiv) in THF (10 mL)
at −78 °C. The mixture was stirred until room temperature was
reached and then cooled again to −78 °C. Hexafluorobenzene
(2.5 equiv, 2.500 g, 13.45 mmol) was then slowly added, and
the reaction mixture was stirred at room temperature for 3 days.
The reaction mixture was quenched with water (100 mL), and
the product was extracted with diethyl ether (3 × 100 mL).
The combined organic layers were dried over K2CO3, and the
solvents were evaporated. The resulting brown oil was purified
by column chromatography (SiO2, pentane/dichloromethane
7/3 as eluent) to afford analytically pure 1d (1.320 g, 2.480
mmol, 46% yield). Anal. Calcd for C24H10F10N2O: C, 54.15; H,
1.89; N, 5.26. Found: C, 54.14; H, 2.07; N, 5.27. 1H NMR (300
1
74.56; H, 7.01; N, 5.54. H NMR (300 MHz, CD2Cl2): δ 7.62
(dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 2H), 7.28 (dt, 3JHH = 6.9 Hz,
4JHH = 1.5 Hz, 2H), 7.25 (m, 5H), 6.82 (dd, 3JHH = 8.1 Hz, 4JHH
= 1.5 Hz, 2H), 6.44 (dt, 3JHH = 7.2 Hz, 4JHH = 1.5 Hz, 2H), 5.21
(s, CH2Ph), 4.18 (q, 3JHH = 8.7 Hz, 2H), 1.36−2.30 (m, 16H).
13C{1H} NMR (75 MHz, CD2Cl2): δ 145.6 (Ar), 144.8 (Ar),
139.8 (Ar), 128.1 (Ar), 126.5 (Ar), 126.1 (Ar), 124.5 (Ar),
117.0 (Ar), 115.5 (Ar), 112.6 (Ar), 69.1 (OCH2), 57.1 (CH-
C5H9), 28.4 (C5H9), 25.3 (C5H9).
3
4
MHz, C6D6): δ 6.83 (dt, JHH = 8.7 Hz, JHH = 2.1 Hz, 2H),
3
4
3
κ2-N,N-{(CF3SO2)NC6H4}2OAlMe (6). Under a nitrogen
atmosphere, a precooled pentane solution of AlMe3 (88.5 mg,
1.23 mmol) was quickly syringed into a precooled (−35 °C)
CH2Cl2 solution of the bis-triflate diamino-ether ligand 1c
(570.0 mg, 1.23 mmol) with vigorous stirring. Upon addition of
AlMe3, immediate bubbling took place (formation of methane)
and the colorless solution was warmed to room temperature
and stirred overnight. After this time, it was evaporated to
dryness to yield a colorless crude residue found to be insoluble
in aromatic solvents and CH2Cl2. Dissolution of the latter crude
product in a minimal amount of a 1/1 THF/Et2O solvent
mixture and subsequent storage at −35 °C afforded analytically
pure compound 6 as colorless crystals (577 mg, 72% yield).
Anal. Calcd for C19H19AlF6N2O6S2 (576.04): C, 39.59; H 3.32;
6.70 (dt, JHH = 8.1 Hz, JHH = 2.1 Hz, 2H), 6.66 (dd, JHH
=
7.5 Hz, 4JHH = 1.8 Hz, 2H), 6.47 (dd, 3JHH = 8.1 Hz, 4JHH = 2.1
Hz, 2H), 5.35 (s, NH, 2H). 19F{1H} NMR (282.4 MHz,
3
4
C6D6): δ −148.7 (dd, JFF = 25.2 Hz, JFF = 6.9 Hz, 4F),
−161.6 (t, 3JFF = 22.8 Hz, 2F), −163.1 (dt, 3JFF = 24.0 Hz, 4JFF
= 6.9 Hz, 4F). 13C{1H} NMR (75 MHz, C6D6): δ 144.7 (Cipso),
142.6 (d, 1JCF = 249 Hz, C6F5), 141.8 (d, 1JCF = 242 Hz, C6F5),
1
138.0 (d, JCF = 256 Hz, C6F5), 134.4 (Cipso), 124.6 (CH),
121.4 (CH), 118.4 (CH), 114.7 (CH), 67.5 (Cipso), 25.5
(C6F5).
κ3-N,O,N-{(C6H11)NC6H4}2OAlMe (4). In a glovebox, AlMe3
(130.0 g, 1.76 mmol) was added to a precooled (−35 °C)
toluene solution of the bis-amino ligand 1a (640.0 mg, 1.76
mmol). The resulting mixture was stirred at 120 °C for 5 days.
After evaporation to dryness, the crude product was
precipitated in pentane to give a white powder (0.42 g, 1.03
× 10−3 mol, 63% yield). Anal. Calcd for C25H33AlN2O: C,
1
N, 4.86. Found: C, 40.17; H, 3.56; N, 5.32. H NMR (400
MHz, C6D6): δ 8.00 (br, 1H, Ar), 7.55 (m, 1H, Ar), 6.96 (m,
6H, Ar), 3.94 (br, 4H, THF), 1.16 (br, 4H, THF), −0.43 (s,
3H, Al-Me). 19F{1H} NMR (376 MHz, C6D6): δ -74.7 (s, CF3),
−77.40 (s, CF3). 19F{1H} NMR (376 MHz, C6D6, 333 K): δ −
76.0 (s, CF3). 13C{1H} NMR (75 MHz, C6D6): δ 150.9 (br,
Cipso-O), 133.9 (br, Ar), 126.3 (Ar), 125.6 (Ar), 123.2 (Ar),
121.8 (br, CF3), 117.7 (Ar), 74.7 (THF), 24.6 (THF), −12.3
(Al-Me).
1
74.23; H, 8.22; N, 6.93. Found: C, 73.94; H, 8.01; N, 6.57. H
NMR (400 MHz, C6D6): δ 7.22 (dd, 3JHH = 8.2 Hz, 4JHH = 1.3
3
4
Hz, 2H), 7.02 (dt, JHH = 6.6 Hz, JHH = 1 Hz, 2H), 6.63 (dd,
3JHH = 8.2 Hz, 4JHH = 1.3 Hz, 2H), 6.36 (dt, 3JHH = 7.5 Hz, 4JHH
= 1.5 Hz, 2H), 3.18 (m, 2H), 2.23 (m, 4H), 1.69−1.57 (m,
6H), 1.39−1.12 (m, 10H), −0.20 (s, 3H). 13C{1H} NMR (100
MHz, C6D6): δ 147.8 (Ar), 145.0 (Ar), 128.2 (Ar), 118.3 (Ar),
112.8 (Ar), 112.1 (Ar), 53.9 (CH-Cy), 35.4 (Cy), 35.2 (Cy),
26.7 (Cy), 26.3 (Cy), 26.1 (Cy), −7.6 (AlMe).
κ3-N,O,N-{(C6F5)NC6H4}2OAlMe (7). In a glovebox, AlMe3
(27.0 mg, 0.380 mmol) was added to a toluene solution of the
bis-amino ligand 1d (200.0 mg, 0.380 mmol) and the resulting
greenish yellow solution was stirred at 80 °C overnight. After
evaporation to dryness, the resulting green solid was dissolved
in a minimum amount of dichloromethane and then
precipitated in pentane. The mixture was filtered and further
dried in vacuo to afford compound 7 in a pure form as a white
κ2-N,O-{(CyNC6H4)O(C6H4NHCy)}GaMe2 (4′). In a glove-
box, GaMe3 (0.07 g, 0.59 mmol) was added to a precooled
(−35 °C) toluene solution of the bis-amino ligand 1b (0.20 g,
0.59 mmol). The resulting mixture was stirred at 130 °C for 6
days to yield the quantitative conversion to complex 4′, as
J
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
solid (200.0 mg, 0.350 mmol, 92% yield). Anal. Calcd for
C25H11AlF10N2O (572.05): C, 52.46; H, 1.94; N, 4.89. Found:
C, 51.07; H, 2.25; N, 4.77. 1H NMR (300 MHz, C6D6): δ 6.98
F.H. acknowledges the ADEME and the Reg
Ph.D. fellowship.
́
ion Alsace for a
3
3
REFERENCES
(d, JHH = 7.8 Hz, 2H), 6.85 (t, JHH = 7.9 Hz, 2H), 6.45 (dd,
■
3JHH = 8.6 Hz, 4JHH = 7.5 Hz, 4H), 0.31 (s, 3H, Al-Me). 19F{1H}
(1) (a) Lindblad, M. S.; Liu, Y.; Albertsson, A.-C.; Ranucci, E.;
Karlsson, S. Adv. Polym. Sci. 2002, 157, 139. (b) Vert, M.
Biomacromolecules 2005, 6, 538. (c) Nair, L. S.; Laurencin, C. T.
Prog. Polym. Sci. 2007, 32, 762.
NMR (282.4 MHz, C6D6): δ −148.6 (dd, 3JFF = 16.5 Hz, 4JFF
=
3
4
2.1 Hz, 4F), −163.2 (td, JFF = 25.5 Hz, JFF = 6.3 Hz, 4F),
−164.4 (t, 3JFF = 23 Hz, 2F). 13C{1H} NMR (125 MHz, C6D6):
δ 146.7 (Cipso-O), 143.5 (d, 1JCF = 252 Hz, C6F51), 139.0 (d, 1JCF
= 251 Hz, C6F5), 140.0 (Cipso-N), 137.5 (d, JCF = 251 Hz,
C6F5), 125.2 (CH-Ar), 122.1 (CH-Ar), 120.7 (Cipso-C6F5),
116.1 (CH-Ar), 115.4 (CH−Ar), −15.0 (Al-Me).
(2) Selected reviews: (a) O’Keefe, B. J.; Hillmeyer, M. A.; Tolman,
W. B. Dalton Trans. 2001, 2215. (b) Dechy-Cabaret, O.; Martin-Vaca,
B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (c) Wu, J.; Yu, T.-L.;
Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006, 205, 602. (d) Dove, A.
P. Chem. Commun. 2008, 6446. (e) Platel, R. H.; Hodgson, L. M.;
Williams, C. K. Polym. Rev. 2008, 48, 11. (f) Wheaton, C. A.; Hayes, P.
G.; Ireland, B. J. Dalton Trans 2009, 4832. (g) Thomas, C. M. Chem.
Soc. Rev. 2010, 39, 165. (h) Arbaoui, A.; Redshaw, C. Polym. Chem.
2010, 1, 801. (i) Buffet, J.-C.; Okuda, J. Polym. Chem. 2011, 2, 2758.
(3) (a) Trofimoff, L.; Aida, T.; Inoue, S. Chem. Lett. 1987, 991.
(b) Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules 1988,
κ3-N,O,N-{(C6F5)NC6H4}2OGaMe (8). In a glovebox, GaMe3
(68.2 mg, 0.590 mmol) was added to a toluene solution of the
bis-amino ligand 1d (158.0 mg, 0.300 mmol) and the resulting
solution was stirred at 130 °C for 3 days. After evaporation to
dryness, the resulting black solid was washed with pentane and
dried in vacuo to afford compound 8 in a pure form as a white
solid (100.0 mg, 0.160 mmol, 53% yield). Anal. Calcd for
C25H11GaF10N2O (615.07): C, 48.82; H, 1.80; N, 4.55. Found:
C, 48.54; H, 1.97; N, 4.62. 1H NMR (300 MHz, C6D6): δ 7.06
21, 286. (c) Dubois, P.; Jacobs, C.; Jer
Macromolecules 1991, 24, 2266. (d) Dubois, P.; Ropson, N.; Jer
R.; Teyssie,
́
o
̂
me, R.; Teyssie,
́
P.
me,
́
o
̂
́
P. Macromolecules 1996, 29, 1965. (e) Kowalski, A.; Duda,
A.; Penczek, S. Macromolecules 1998, 31, 2114.
3
3
4
(d, JHH = 8.1 Hz, 2H), 6.86 (t, JHH = 7.9 Hz, JHH = 1.5 Hz,
2H), 6.49 (m, 4H), 0.01 (s, 3H, Ga-Me). 19F{1H} NMR (282.4
MHz, C6D6): δ −148.4 (dd, 3JFF = 23.5 Hz, 4JFF = 3.7 Hz, 4F),
(4) (a) Le Borgne, A.; Vincens, V.; Jouglard, M.; Spassky, N.
Makromol. Chem., Macromol. Symp. 1993, 73, 37. (b) Spassky, N.;
Wisniewski, M.; Pluta, C.; Le Borgne, A. Macromol. Chem. Phys. 1996,
197, 2627. (c) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121,
4072. (d) Radano, C. P.; Baker, G. L.; Smith, M. R., III J. Am. Chem.
Soc. 2000, 122, 1552. (e) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J.
Am. Chem. Soc. 2002, 124, 5938. (f) Zhong, Z.; Dijkstra, P. J.; Feijen, J.
J. Am. Chem. Soc. 2003, 125, 11291. (g) Hormnirun, P.; Marshall, E.
L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc.
2004, 126, 13598.
(5) For recent and representative Al-mediated ROP of cyclic esters,
see: (a) Che, H. L.; Dutta, S.; Huang, P. Y.; Lin, C. C. Organometallics
2012, 31, 2016. (b) Bouyahyi, M.; Roisnel, T.; Carpentier, J.-F.
Organometallics 2012, 31, 1458. (c) Lamberti, M.; D’Auria, I.; Mazzeo,
M.; Milione, S.; Bertolasi, V.; Pappalardo, D. Organometallics 2012, 31,
5551. (d) Bakewell, C.; Platel, R. H.; Cary, S. K.; Hubbard, S. M.; Roaf,
J. M.; Levine, A. C.; White, A. J. P.; Long, N. J.; Haaf, M.; Williams, C.
K. Organometallics 2012, 31, 4729. (e) Whitelaw, E. L.; Loraine, G.;
Mahon, M. F.; Jones, M. D. Dalton Trans. 2011, 40, 11469. (f) Li, C.
Y.; Tsai, C. Y.; Lin, C. H.; Ko, B. T. Dalton Trans. 2011, 40, 1880.
−163.0 (td, 3JFF = 22.3 Hz, 4JFF = 4.2 Hz, 4F), −164.4 (t, 3JFF
=
-
22.6 Hz, 2F). 13C{1H} NMR (125 MHz, C6D6): δ 146.6 (Cipso
1
O), 143.6 (d, JCF = 238 Hz, C6F5), 139.3 (Cipso-N), 137.5 (d,
1JCF = 239 Hz, C6F5), 127.2 (CH-Ar), 119.2 (CH-Ar), 118.5
(CH-Ar), 115.6 (CH-Ar), −11.3 (Ga-Me).
Typical Procedure for the ROP of rac-LA, ε-CL, and
TMC. In a glovebox, the catalyst (6.11 mg, 1.04 × 10−5 mol)
was charged in a vial equipped with a Teflon screw cap and a
dichloromethane or toluene solution (1 M) of the appropriate
amount of monomer was added via a syringe all at once. The
resulting solution was vigorously stirred (at room temperature
or heated) for the appropriate time. The vial was then removed
from the glovebox, and the reaction mixture was quenched with
MeOH, provoking the precipitation of the polymer, which was
washed several times with MeOH, dried in vacuo until constant
1
weight, and subsequently analyzed by H NMR, SEC, and
(g) Otero, A.; Lara-San
C.; Castro-Osma, J. A.; Mar
́
chez, A.; Fernan
́
dez-Baeza, J.; Alonso-Moreno,
chez-Barba, L. F.;
MALDI-TOF spectrometry.
́
quez-Segovia, I.; San
́
Rodríguez, A. M.; Garcia-Martinez, J. C. Organometallics 2011, 30,
1507. (h) Darensbourg, D. J.; Karroonnirun, O.; Wilson, S. J. Inorg.
Chem. 2011, 50, 6775. (i) Shen, M.; Huang, W.; Zhang, W.; Hao, X.;
Sun, W.-H.; Redshaw, C. Dalton Trans. 2010, 39, 9912. (j) Bouyahyi,
M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2010, 29, 491.
(k) Thibault, M. H.; Fontaine, F. G. Dalton Trans. 2010, 39, 5688.
(l) Nomura, N.; Akita, A.; Ishii, R.; Mizuno, M. J. Am. Chem. Soc. 2010,
132, 1750. (m) Haddad, M.; Laghzaoui, M.; Welter, R.; Dagorne, S.
Organometallics 2009, 28, 4584. (n) Issenhuth, J.-T.; Pluvinage, J.;
Welter, R.; Bellemin-Laponnaz, S.; Dagorne, S. Eur. J. Inorg. Chem.
2009, 4701. (o) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.;
Carpentier, J.-F. Organometallics 2009, 28, 1469. (p) Du, H. Z.;
Velders, A. H.; Dijkstra, P. J.; Sun, J. R.; Zhong, Z. Y.; Chen, X. S.;
Feijen, J. Chem. Eur. J. 2009, 15, 9836. (q) Schwarz, A. D.; Chu, Z. Y.;
Mountford, P. Organometallics 2010, 29, 1246.
(6) For the ROP of cyclic esters mediated by Ga initiators, see:
(a) Horeglad, P.; Kruk, P.; Pecaut, J. Organometallics 2010, 29, 3729.
(b) Horeglad, P.; Szczepaniak, G.; Dranka, M.; Zachara, J. Chem.
Commun. 2012, 48, 1171.
(7) For selected examples of the ROP of cyclic esters mediated by In
species, see: (a) Hsieh, I. P.; Huang, C. H.; Lee, H. M.; Kuo, P. C.;
Huang, J. H.; Lee, H. I.; Cheng, J. T.; Lee, G. H. Inorg. Chim. Acta
2006, 359, 497. (b) Douglas, A. F.; Patrick, B. O.; Mehrkhodavandi, P.
Angew. Chem., Int. Ed. 2008, 47, 2290. (c) Peckermann, L.; Kapelski,
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving characterization data for all
polymers, including MALDI-TOF SEC data, selected kinetic
data for the carried ROP reactions, and crystal and refinement
data for compounds 4−6 and 8. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dagorne@unistra.fr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CNRS and The University of Strasbourg for
financial support. Lydia Brelot and Corinne Bailly (Service de
Crystallographie, Institut de Chimie de Strasbourg) are
gratefully acknowledged for the X-ray analysis of all complexes.
K
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
A.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2009, 48, 5526.
(d) Pietrangelo, A.; Hillmeyer, M. A.; Tolman, W. B. Chem. Commun.
2009, 2736. (e) Buffet, J.-C.; Okuda, J.; Arnold, P. L. Inorg. Chem.
2010, 49, 419. (f) Hu, M. G.; Wang, M.; Zhang, P. L.; Wang, L.; Zhu,
F. J.; Sun, L. C. Inorg. Chem. Commun. 2010, 13, 968. (g) Blake, M. P.;
Schwarz, A. D.; Mountford, P. Organometallics 2011, 30, 1202.
(h) Acosta-Ramirez, A.; Douglas, A. F.; Yu, I.; Patrick, B. O.;
Diaconescu, P. L.; Mehrkhodavanti, P. Inorg. Chem. 2010, 49, 5444.
(i) Yu, I.; Acosta-Ramirez, A.; Mehrkhodavanti, P. J. Am. Chem. Soc.
2012, 134, 12758. (j) Normand, M.; Kirillov, E.; Roisnel, T.;
Carpentier, J.-F. Organometallics 2012, 31, 5511.
(8) (a) Hild, F.; Brelot, L.; Dagorne, S. Organometallics 2011, 30,
5457. (b) Hild, F.; Dagorne, S. Organometallics 2012, 31, 1189.
(9) (a) Huang, P.; Ramphal, J.; Wei, J.; Liang, C.; Jallal, B.;
McMahon, G.; Tang, C. Bioorg. Med. Chem. 2003, 11, 1835.
(b) Khusniyarov, M. M.; Harms, K.; Burghaus, O.; Sundermeyer, J.;
Sarkar, B.; Kaim, W.; van Slageren, J.; Duboc, C.; Fiedler, J. Dalton
Trans. 2008, 1355.
Welter, R.; Bellemin-Laponnaz, S.; Maisse-Franco
2007, 13, 3202.
̧
is, A. Chem. Eur. J.
(22) Endo, T. General Mechanisms in Ring-Opening Polymerization.
In Handbook of Ring-Opening Polymerization; Dubois, P., Coulembier,
O., Raquez, J.-M., Eds.; Wiley-VCH: Weinheim, Germany, 2009; pp
53−64.
(23) See, for instance, ref 8a and: Helou, M.; Miserque, O.; Brusson,
J.-M.; Carpentier, J.-F.; Guillaume, S. M. ChemCatChem 2010, 2, 306.
(24) (a) Palard, I.; Schappacher, M.; Belloncle, B.; Soum, A.;
Guillaume, S. Chem. Eur. J. 2007, 13, 1511. (b) Delcroix, D.; Martin-
Vaca, B.; Bourissou, D.; Navarro, C. Macromolecules 2010, 43, 8828.
(25) (a) Benaissa, M.; Gonsalves, K. E.; Rangaranjan, S. P. Appl. Phys.
Lett. 1997, 71, 3685. (b) Waggoner, K. M.; Olmstead, M. M.; Power,
P. P. Polyhedron 1990, 9, 257.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
(10) For representative X-ray-characterized Al compounds featuring
terminal Al−N amido bonds, see: (a) Krossing, I.; Noth, H.; Schwenk-
̈
Kircher, H. Eur. J. Inorg. Chem. 1998, 927. (b) Wehmschulte, R. J.;
Power, P. P. Inorg.Chem. 1998, 37, 6906. (c) Liang, L.-C.; Huang, M.-
H.; Hung, C.-H. Inorg. Chem. 2004, 43, 2166. (d) Olson, J. A.; Boyd,
R.; Wilson, J. W.; Foley, S. R. Organometallics 2008, 27, 5333.
(e) Maisse-Franco̧ is, A.; Azor, L.; Schmitt, A.-L.; Coquel, A.; Brelot, L.;
Welter, R.; Bellemin-Laponnaz, S.; Dagorne, S. J. Organomet. Chem.
2012, 696, 4248.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
(11) For representative X-ray-characterized Al compounds featuring
bridging Al−(μ-O) benzyloxide moieties, see: (a) Cetinkaya, B.;
Hitchcock, P. B.; Jasim, H. A.; Lappert, M. F.; Williams, H. D.
Polyhedron 1990, 9, 239. (b) Liu, Y.-C.; Ko, B.-T.; Huang, B.-H.; Lin,
C.-C. Organometallics 2002, 21, 2066. (c) Nomura, N.; Ishii, R.;
Yamamoto, Y.; Kondo, T. Chem. Eur. J. 2007, 13, 4433.
(12) Average between a maximum value Rmax = 17.81 Å
corresponding to the longest distance from molecule’s baricenter
and a minimum value Rmin = (3V/4π)1/3 = 5.84 Å derived from the
molecular volume V = 834.34 Å3, estimated according to: Gavezotti, A.
J. Am. Chem. Soc. 1983, 105, 5220. van der Waals radii used: H, 1.2 Å;
C, 1.7 Å; O, 1.52 Å; Al, 2.00 Å.18
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision B.01; Gaussian, Inc., Wallingford, CT, 2009.
(27) Metz, B.; Stoll, H.; Dolg, M. J. Chem. Phys. 2000, 113, 2563.
(28) Peterson, K. A. J. Chem. Phys. 2003, 119, 11099.
(29) Contreras-Garcia, J.; Johnson, E. R.; Keinan, S.; Chaudret, R.;
Piquemal, J.-P.; Beratan, D. N.; Yang, W. J. Chem. Theory Comput.
2011, 7, 625−632.
(13) For more details on ring conformations and dynamics of eight-
membered rings, see: Anet, F. A. L. Dynamics of Eight-Membered
Rings in the Cyclooctane Class. In Topics in Current Chemistry;
Boschke, F., Ed.; Springer-Verlag: New York, 1974; Vol. 45, pp 169−
220.
(14) Nelson, S. G.; Kim, B.-K.; Peelen, T. J. J. Am. Chem. Soc. 2000,
122, 9318.
(15) As a comparison, one may note that the N-cycloalkyl-ligated Al-
Me species 4 does not readily react with THF (no reaction with 5
equiv of THF, C6D6, room temperature, overnight).
(16) For examples of X-ray-characterized κ2-diamido Al-THF methyl
complexes, see: (a) Yang, Z.; Ma, X.; Roesky, H. W.; Yang, Y.;
́ ́
Jimenez-Perez, V. M.; Magull, J.; Ringe, A.; Jones, P. G. Eur. J. Inorg.
Chem. 2007, 4919. (b) Riehm, T.; Wadepohl, H.; Gade, L. H. Inorg.
Chem. 2008, 47, 11467.
(17) For structural parameters of cationic Al methyl complexes, see:
Dagorne, S.; Atwood, D. A. Chem. Rev. 2008, 108, 4037.
(18) For vdW radii values of main-group elements, see: (a) Mantina,
M.; Chamberlin, A. C.; Valero, R.; Cramer, C. J.; Truhlar, D. G. J. Phys.
Chem. A 2009, 113, 5806. (b) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(19) (a) Memmler, H.; Walsh, K.; Gade, L. H.; Lauher, J. W. Inorg.
Chem. 1995, 34, 4062. (b) Gade, L. H. Acc. Chem. Res. 2002, 35, 575.
(20) For reviews of immortal metal-catalyzed ROP of cyclic esters
and carbonates, see: (a) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29,
39. (b) Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.;
Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39,
8363.
(21) (a) Lewin
́ ́
ski, J.; Horeglad, P.; Wojcik, K.; Justyniak, I.
Organometallics 2005, 24, 4588. (b) Dagorne, S.; Le Bideau, F.;
L
dx.doi.org/10.1021/om3011068 | Organometallics XXXX, XXX, XXX−XXX