Synthesis and structural characterization of well-defined anionic aluminium alkoxide complexes supported by NON-type diamido ether tridentate ligands and their use for the controlled ROP of lactide

Article abstract of DOI:10.1039/b911802k

The tridentate proligands (RNH-o-C₆H₄)₂O (1a, R = C₅H₉; 1b, R = Cy) were found to readily react with LiAlH₄ to yield the corresponding lithium aluminium dihydrido salt species [η²-N,N-{(RN-o-C₆H₄) ₂O}AlH(μ-H)Li(THF)]₂ (2a, R = C₅H ₉; 2b, R = Cy) in 50% and 42% yield, respectively. The solid-state structure of both complexes 2a and 2b were determined by X-ray crystallographic studies. Compounds 2a and 2b readily react with one equivalent of benzaldehyde to afford the corresponding mono-benzyloxide species η²-N,N-{RN- o-C₆H₄)₂O}Al(H)(μ -OCH₂Ph)Li(THF) ₂ (4a, R = C₄H₉; 4b, R = Cy), as confirmed by X-ray studies in the case of 4b. In a similar manner, when compounds 2a and 2b are reacted with two equivalents of benzaldehyde the bis-benzyloxide derivatives 5a and 5bη³-N,N,O-{RN-o-C₆H₄) ₂O}Al(μ -OCH₂Ph)₂Li(THF)₂ (5a, R = C₄H₉; 5b, R = Cy) may be prepared. While the lithium Al mono-alkoxide species 4a-b are inactive in the ring-opening polymerization (ROP) of lactide, their bis-alkoxide Al analogues 5a-b polymerize rac-lactide and (S)-lactide at room temperature, which is rather uncommon for Al-based alkoxide systems. Kinetic studies of the lactide ROP initiated by compound 5a suggest a strong preference for racemic enchainment during the ROP chain growth; the resulting PLAs are however moderately heterotactic due to detrimental transesterification processes occurring as the chain grows. The Royal Society of Chemistry.

Full text of DOI:10.1039/b911802k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and structural characterization of well-defined anionic aluminium
alkoxide complexes supported by NON-type diamido ether tridentate ligands
and their use for the controlled ROP of lactide†
Fre´de´ric Hild,a Pierre Haquette,b Lydia Brelotc and Samuel Dagorne*a
Received 16th June 2009, Accepted 1st September 2009
First published as an Advance Article on the web 22nd September 2009
DOI: 10.1039/b911802k
The tridentate proligands (RNH-o-C6H4)2O (1a, R = C5H9; 1b, R = Cy) were found to readily react
2
with LiAlH4 to yield the corresponding lithium aluminium dihydrido salt species [h -N,N-{(RN-o-
C6H4)2O}AlH(m-H)Li(THF)]2 (2a, R = C5H9; 2b, R = Cy) in 50% and 42% yield, respectively. The
solid-state structure of both complexes 2a and 2b were determined by X-ray crystallographic studies.
Compounds 2a and 2b readily react with one equivalent of benzaldehyde to afford the corresponding
2
mono-benzyloxide species h -N,N-{RN-o-C6H4)2O}Al(H)(m -OCH2Ph)Li(THF)2 (4a, R = C4H9; 4b,
R = Cy), as confirmed by X-ray studies in the case of 4b. In a similar manner, when compounds 2a and
2b are reacted with two equivalents of benzaldehyde the bis-benzyloxide derivatives 5a and 5b
3
h -N,N,O-{RN-o-C6H4)2O}Al(m -OCH2Ph)2Li(THF)2 (5a, R = C4H9; 5b, R = Cy) may be prepared.
While the lithium Al mono-alkoxide species 4a–b are inactive in the ring-opening polymerization (ROP)
of lactide, their bis-alkoxide Al analogues 5a–b polymerize rac-lactide and (S)-lactide at room
temperature, which is rather uncommon for Al-based alkoxide systems. Kinetic studies of the lactide
ROP initiated by compound 5a suggest a strong preference for racemic enchainment during the ROP
chain growth; the resulting PLAs are however moderately heterotactic due to detrimental
transesterification processes occurring as the chain grows.
To improve the catalytic performance of Al-based ROP initia-
tors of rac-lactide, we have become interested in the synthesis of
formally charged aluminium alkoxide complexes. In this regard,
Introduction
The ring-opening polymerization (ROP) of rac-lactide, a dimer of
the renewable resource lactic acid, initiated by well-defined metal
while cationic aluminium alkoxides may be too Lewis acidic for the
alkoxide complexes has been the subject of numerous studies over
ROP chain-growth step to proceed, thereby precluding the poly-
merization process to occur,3 the expected enhanced nucleophilic-
ity of the Al-OR- alkoxide moiety in anionic aluminium alkoxide
derivatives should favour both the polymerization initiation and
subsequent chain-growth steps. Also, in such anionic systems, a
counteranion such as Li+ may well act as a weak Lewis acid able
to activate the lactide monomer toward nucleophilic attack by the
Al-OR- alkoxide.
In general, despite their interesting features, well-defined
anionic metal alkoxide complexes have rarely been used as
ROP initiators of rac-lactide.4 For aluminium, the use of
the commercially available reagent Red-Al, formulated as
Na+[AlH2(OCH2CH2OCH3)2]-, for the ROP of (L)-lactide con-
stitutes the only example on this matter. In the latter case, rather
harsh conditions are however required for the ROP to proceed
(110–130 ◦C, 2–3 days), which may result from a high level of
aggregation in Red-Al.5 The use of an appropriate supporting
ligand for coordination to Al may provide access to well-defined
species, limit aggregation processes, and thereby be beneficial to
catalytic activity. The potentially N,O,N tridentate bis(amido)-
ether ligand of type A (Scheme 1) appeared suited for the synthesis
of aluminium alkoxide anions of the type {LX2}Al(OR)2- because
its steric nature may easily be tuned via the N-R substituent and
the central oxygen ether atom may provide an extra-electronic sta-
bilization of the N,N-chelated Al metal centre and thus disfavour
aggregation. In addition and apart from their straightforward
the past fifteen years as the derived polymers, poly(lactic) acid
(PLA), are biodegradable and biocompatible.1 Among these metal
alkoxide species, neutral aluminium derivatives supported by var-
ious tetradentate ligands such as, for instance, Salen- and Salan-
type chelate ligands have been widely investigated and several
of these systems were found to stereoselectively polymerize rac-
lactide to afford highly stereoregular polylactide materials, which
are of particular interest as they exhibit improved thermal and
mechanical properties versus those of atactic PLA.2 Nevertheless,
lactide polymerization initiated by Al alkoxide compounds is
usually slow compared to other metal alkoxide initiators, which
greatly limits the potential usefulness of these systems despite the
low cost of aluminium.2e
aLaboratoire DECOMET, Institut de Chimie (UMR CNRS 7177), Univer-
site´ de Strasbourg, 4 rue Blaise Pascal, 67000, Strasbourg, France
bLaboratoire Charles Friedel, UMR CNRS 7223, Ecole Nationale
Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75005, Paris,
France
cService de Radiocristallographie, Institut de Chimie (UMR CNRS 7177),
Universite´ de Strasbourg, 4 rue Blaise Pascal, 67000, Strasbourg, France
† Electronic supplementary information (ESI) available: The MALDI-
TOF mass spectrum of a PLA sample and an ORTEP drawing along with
a summary of crystallographic data for compound 2b. CCDC reference
numbers 735904–735907. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b911802k
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 533–540 | 533
©

Scheme 1
synthesis, ligands of type A have been shown to be effective
ancillary ligands for high-oxidation-state and oxophilic group 4
metals.6
Here we report the synthesis, structures of anionic aluminium
alkoxide complexes bearing a bis(amido)-ether ligand of type A
as well as their applications in the controlled ROP of rac-lactide.
Along the way, the synthesis and characterization of a couple of
anionic hydrido Al species were carried out and are thus also
discussed herein.
Fig. 1 Molecular structure (ORTEP drawing) of the lithium aluminium
hydrido species 2a with partial atom labelling. The hydrogen atoms,
except those bonded to Al and Li, are omitted for clarity. Selected
˚
Results and discussion
bond distances (A): Al–N(1) = 1.865(1), Al–N(2) = 1.908(1), Al–H(1) =
1.55(2), Al–H(6) = 1.59(2), Li–O(1) = 2.025(3), Li–O(2) = 1.930(3),
Li–N(2) = 2.246(3), Li–H(6) = 2.21(2); selected bond angles (◦):
N(1)–Al–N(2) = 114.17(5), N(1)–Al–H(1) = 110.4(9), H(1)–Al–H(6) =
109.2(11), O(2)–Li–O(1) = 114.15(14), O(1)–Li–N(2) = 75.82(9).
Access to Al alkoxide anionic species chelated by a dianionic
chelating ligand of type A was achieved through the synthesis
of the corresponding Al hydrido anions, which were then subse-
quently converted to the Al alkoxide derivatives.
drido aluminate dimers.7 Within each [h -N,N-{((C5H9)N-o-
2
Synthesis and structure of the lithium Al hydrido complexes 2a–b
C6H4)2O}AlH2Li(THF)] moiety, the Al atom adopts a distorted
2
tetrahedral structure and is effectively h -N,N-chelated by the
The proligands (RNH-o-C6H4)2O (1a, R = C5H9; 1b, R = Cy,
Scheme 2) were readily synthesized in a one-step procedure
according to a literature report from the commercially available
bis(amino)-ether (NH2-o-C6H4)2O.6 The reaction of proligands
1a–b with one equivalent of LiAlH4 (THF, -35 ◦C to room tem-
perature) affords the corresponding lithium Al hydrido complexes
supporting NON2- ligand with a relatively large bite angle (N(1)–
Al–N(2) = 114.17(5)◦). The coordination sphere of the Al centre is
then completed by a terminal and a bridging hydride; the Al metal
centre apparently does not interact with the central oxygen donor
atom of the NON2- ligand. The Al–N bond distances (1.886(1) A
˚
average) as well as both the terminal and bridging Al–H distances
2
[h -N,N-{(RN-o-C6H4)2O}AlH(m-H)Li(THF)]2 (2a, R = C5H9;
˚
(1.57 A average), albeit of low accuracy for the Al–H distances, are
2b, R = Cy, Scheme 2) in 50% and 42% yield, respectively. The
molecular structures of both 2a and 2b were determined by X-ray
crystallographic studies and that of 2a is depicted in Fig. 1, (see
Table 1 for a summary of crystallographic data and ESI† for the
molecular structure of 2b). As expected, compounds 2a and 2b
exhibit very similar structural features in the solid-state and these
will only be discussed for 2a.
consistent with other examples reported in the literature.8 Both
2
Li atoms are h -N,O-chelated by a NON2- ligand with a rather
acute O(1)–Li–N(2) bite angle (75.82(9)◦), which results in the
formation of a Al–(m-N)–Li bridge. The sphere of coordination of
the Li atoms is completed by two bridging hydrides and a THF
molecule, both exhibiting normal bonding parameters to Li.
The NMR data for both salt compounds 2a and 2b (CD2Cl2,
room temperature) are consistent with effective C2v-symmetric
structures under the studied conditions, implying some degree
of fluxionality that was not further investigated. Compound 2b is
unstable in CD2Cl2 solution at room temperature and reacts with
the solvent to slowly and quantitatively yield the corresponding
neutral Al hydrido complex {(CyN-o-C6H4)2O}AlH(THF) (3b)
along with formation of CHD2Cl and LiCl. Thus, apart from
resonances assigned to 3b, the 1H NMR spectrum of the reaction
mixture exhibits a quintet signal (d 2.99, 2JHD = 1.6 Hz) assigned to
the proton resonance in CHD2Cl.9 Compound 3b was synthesized
on a preparative scale allowing its isolation; IR and elemental
analysis data further confirmed its identity. Although not studied,
it appears very likely that the closely related salt species 2a
undergoes a similar decomposition.
Scheme 2
As illustrated in Fig. 1, the salt compound 2a is dimeric in
2
the solid state and is best described as two [h -N,N-{((C5H9)N-o-
C6H4)2O}AlH2Li(THF)] units connected by two Al–H–Li bridges.
This results in the formation of a central four-membered Li2H2
ring, a frequently observed structural element in lithium hy-
One should mention that the nature of the tridentate supporting
ligand NON2- may play a crucial role in the relatively good
stability of the lithium hydrido Al species 2a and 2b; as a
534 | Dalton Trans., 2010, 39, 533–540
This journal is
The Royal Society of Chemistry 2010
©

Table 1 Summary of crystallographic data for compounds 2a, 4b and 5a
Compound
2a·CH2Cl2
4b
5a
Formula
C27H38AlCl2LiN2O2
527.41
triclinic
C39H54AlLiN2O4
648.76
monoclinic
C 2/c
23.5757(14)
22.3191(16)
17.1128(7)
90
124.518(3)
90
7419.3(8)
8
1.162
0.095
2800
C44H56AlLiN2O5
726.83
monoclinic
P 21
8.7739(4)
20.3007(11)
11.7890(6)
90
107.960(3)
90
1997.49(17)
2
1.208
0.098
780
Formula weight
Crystal system
Space group
P-1
˚
a (A)
9.531(1)
12.933(1)
13.383(1)
102.921(2)
109.009(2)
107.777(2)
1386.31(9)
2
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
V (A )
Z
3
˚
Density (g cm-3)
m (Mo Ka) (mm-1)
F(000)
1.263
0.292
560
Crystal Size [mm]
0.10 ¥ 0.10 ¥ 0.10
0.50 ¥ 0.50 ¥ 0.30
0.30 ¥ 0.15 ¥ 0.10
Data collection
Temperature (K)
Radiation (A)
Theta min-max
173(2)
MoKa-0.71073
1.72-30.00
173(2)
MoKa-0.71073
1.39-27.46
173(2)
MoKa-0.71073
1.82-27.47
˚
Dataset [h, k, l]
Tot., Uniq. Data, R(int)
Observed data (I>2s(I))
-13/13, -17/18, -18/17
11345, 8002, 0.0170
6720
-20/30, -26/28, -22/17
21934, 8428, 0.0889
4825
-11/8, -26/24, -11/15
10906, 4707, 0.0807
2780
Refinement
N
reflections, Nparameters
8002, 324
8428, 381
4707, 467
R2(all), R1(I>2s(I)), wR2(all), 0.0614, 0.0476, 0.1588, 0.1359, 1.114 0.1556, 0.0920, 0.2899, 0.2436, 1.058 0.1554, 0.0890, 0.2392, 0.2010, 1.026
wR1(I>2s(I)), Goof
Max. and Av. Shift/Error
Min, Max. Resd Dens. (e A )
0.002, 0.000
-0.608, 0.927
0.000, 0.000
-0.703, 1.138
0.000, 0.000
-0.412, 0.675
-3
˚
comparison, the reaction of tridentate triamino proligands of the
type (RNHCH2CH2)2NR’ with LiAlH4 directly yields the corre-
3
sponding diamido amino Al neutral hydrido species {h -N,N,N-
(RNHCH2CH2)2NR’}AlH;10 in the latter case, the flexibility of
the ligand backbone along with the L-type amino central donor
atom apparently strongly favour the formation of the neutral Al
hydrido derivative.
Synthesis and structure of the lithium Al alkoxide complexes 4a–b
and 5a–b
While the reaction of the lithium Al hydrido species 2a and 2b with
one or two equivalents of an alcohol source such as PhCH2OH
consistently yielded untractable mixtures of species, that with ben-
zaldehyde afforded the corresponding Al mono- and bis-alkoxide
derivatives. Thus, both compounds 2a and 2b readily react with
◦
one equiv. of benzaldehyde (THF, -35 C to room temperature)
to afford the corresponding lithium Al monoalkoxide species
2
h -N,N-{RN-o-C6H4)2O}Al(H)(m -OCH2Ph)Li(THF)2 (4a, R =
C4H9; 4b, R = Cy; Scheme 3).
Scheme 3
The solid-state molecular structure of 4b was determined by
X-ray crystallographic studies. As depicted in Fig. 2, compound
4b crystallizes as associated Al anions and Li cations and may
structure as a result of the rather wide {NN}Al bite angle (N(1)–
Al(1)–N(2) = 116.8(1)◦) and is connected to the Li cation via
a m-O benzyloxide bridging moiety, with an Al–O bond distance
(Al(1)–O(2) = 1.790(2) A) in the expected range for Al alkoxides.
+
be formally seen as a “Li(THF)2 ” cationic fragment being O,O-
2
2
h chelated by the “h -N, N -{RN-o-C6H4)2O}Al(H)(OCH2Ph)-”
11
˚
Al anionic moiety. The Al atom adopts a distorded tetrahedral
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 533–540 | 535
©

Fig. 2 Molecular structure (ORTEP drawing) of the lithium aluminium
hydrido mono-benzyloxide species 4b with partial atom labelling. With the
exception of H(1a), the hydrogen atoms are omitted for clarity. Selected
Fig. 3 Molecular structure (ORTEP drawing) of the lithium alu-
minium bis-benzyloxide species 5a with partial atom labelling. The
hydrogen atoms are omitted for clarity. The disordered part of the
carbon atoms C(20), C(21) and those in the two THF molecules are
˚
bond distances (A): Al(1)–N(1) = 1.892(3), Al(1)–N(2) = 1.882(3),
Al(1)–O(2) = 1.790(2), Al(1)–H(1a) = 1.44(3), Li(1)–O(1) = 2.096(7),
Li(1)–O(2) = 1.877(7), Li(1)–O(3) = 1.942(8), Li(1)–O(4) = 1.899(8); se-
lected bond angles (◦): N(2)–Al(1)–N(1) = 116.85(12), Al(1)–O(2)–Li(1) =
113.5(2), O(1)–Li(1)–O(2) = 83.2(3), O(3)–Li(1)–O(4) = 102.2(3).
˚
also omitted for clarity. Selected bond distances (A): Al(1)–O(1) =
2.177(4), Al(1)–O(2) = 1.808(4), Al(1)–O(3) = 1.769(4), Al(1)–N(1) =
1.892(5), Al(1)–N(2) = 1.893(5), Li(1)–O(2) = 1.918(12), Li(1)–O(3) =
1.897(11), Li(1)–O(4) = 1.968(13), Li(1)–O(5) = 1.935(13); selected bond
angles (◦): N(1)–Al(1)–N(2) = 121.1(2), O(2)–Al(1)–O(3) = 87.9(2),
O(2)–Li(1)–O(3) = 81.2(5), O(5)–Li(1)–O(4) = 103.9(6).
The coordination sphere of Al is completed by a terminal hydride
˚
(Al–H(1a) = 1.44(3) A). The Li atom is bonded to four oxygens:
two THF molecules (O(3) and O(4)), the central donor oxygen of
the NON2- chelating ligand (O(1)) and the Al-m-O-Li benzyloxide
oxygen (O(1)). The geometrical constraints imposed by the NON
ligand backbone most likely account for the distorted tetrahedral
geometry at Li(O(1)–Li(1)–O(2) = 83.2(3)◦) as well as the longer
is best described as trigonal bipyramidal with the NON2- amido
nitrogens (N(1) and N(2)) and a bridging m-O benzyloxide (O(3))
located at the equatorial positions, while the other m-O benzyloxide
oxygen (O(2)) along with the NON central donor atom (O(1))
occupy the axial coordination sites. Thus, despite an expected
decreased Lewis acidity for the formally anionic Al metal center,
the Al centre is penta-coordinated, which is presumably favoured
by the presence of the NON central donor oxygen atom O(1) in
its vicinity. Nevertheless, while the Al alkoxides and amido bond
˚
Li(1)–O(1) bond distance (2.096(7) A) versus other Li–O distances
˚
(1.91(1) A average). Such constraints are likely to result from the
significant ring strain associated with the eight-membered-ring
{NON)Al metallacycle, forced to adopt an unpreferred “boat-
like” conformation for coordination of both the central NON and
the m-O benzyloxide oxygens to Li (Fig. 2).12
˚
˚
distances (1.89(1) A and 1.892(9) A average, respectively) are in
The NMR data for compounds 4a and 4b (CD2Cl2, room
temperature) agrees with the overall Cs-symmetric structures,
which implies, given that 4b adopts a C1-symmetric structure in
the solid state, a fluxional behaviour of these species in solution
under the studied conditions; this was not further investigated.
The identity of 4a and 4b was confirmed by elemental analysis
and IR data. In particular, the IR spectra of 4a and 4b exhibit an
absorption peak at 1805 and 1811 cm-1, respectively, characteristic
of an Al–H bond.
˚
the expected range, the Al(1)–O(1) bond distance (2.177(4)) A) is
much longer than the classical Al–ether bond distance (from 1.85
13
˚
to 2 A).
The NMR data for analytically pure compounds 5a and 5b
(CD2Cl2, room temperature) are consistent with C2v-symmetric
structures under the studied conditions. In particular, the 1H and
13C NMR spectra each contain one set of resonances for the
NON2- chelating ligand, a singlet signal for the PhCH2O moiety
and agree with the presence of two molecules of THF. Based on
the solid-state structure of 5a, the observed C2v-symmetry for
5a and 5b may result from a fast face exchange (via decoordi-
nation/coordination) of the Al-coordinated NON-oxygen atom
(O(1) in Fig. 4) on the NMR time scale.
3
The lithium bis-alkoxide Al derivatives h -N,N,O-{RN-o-
C6H4)2O}Al(m -OCH2Ph)2Li(THF)2 (5a, R = C4H9; 5b, R = Cy;
Scheme 3) were synthesized in an analogous manner to that used
for 4a and 4b, but using two equivalents of benzaldehyde. The
molecular structure of 5a was determined by X-ray crystallo-
graphic studies and is illustrated in Fig. 3. The solid-state structure
of the lithium bis-alkoxide Al species 5a is quite different to that
of its mono-alkoxide analog 4b. In compound 5a, the Al anionic
ROP of lactide by the lithium Al benzyloxide species 5a and 5b
+
fragment is connected to the “Li(THF)2 ” cationic moiety via two
As stated in the Introduction, the use of well-defined anionic metal
alkoxide complexes as initiators of the ROP of lactide has not been
much studied despite the potential higher activity. The synthesized
m-O benzyloxide oxygens, resulting in the formation of a nearly
planar AlO2Li structural motif. The geometry at the Al center
536 | Dalton Trans., 2010, 39, 533–540
This journal is
The Royal Society of Chemistry 2010
©

Table 2 Polymerization of rac-lactide initiated by compound 5aa
Conv. (%)b
Time (h)
Mn(obsd)c
PDI
Mn(calcd)d
35
50
60
70
82
5
7
8
10
16
3950
5530
5790
6730
8060
1.04
1.05
1.06
1.04
1.03
5800
8290
9950
11600
13590
a Polymerization conditions: CH2Cl2, room temperature, [rac-LA]0/Al =
230, [rac-LA]0 = 1.6 M. b As determined by 1H NMR spectroscopy.
c Measured by SEC at 25 ◦C in THF relative to polystyrene standards
with Mark–Houwink corrections for Mn [Mn(obsd) = 0.56 Mn(SEC)].16
d Calculated for two growing chains per aluminium atom and thus from
[(MWrac-LA ¥ 230 ¥ conversion)]/2.
Fig. 4 Plot of monomer (rac-lactide) conversion versus Mn values of
PLAs. Conditions: complex 5a as the initiator, 230 equiv. of rac-LA,
[rac-LA]0 = 1.6 M, CH2Cl2, room temperature.
to cyclic PLAs, which suggests that back-biting reactions take
place, to some extent, during chain growth. One should however
point out that all SEC chromatograms of the synthesized PLAs
display a symmetrical peak with no shoulder, which suggests that
the generated cyclic PLA chains have very similar hydrodynamic
radii to those in linear PLAs. As for their stereoregularity, the
formed PLAs are moderately heterotactic (Pr = 0.67 and 0.65
at 60% and 85% conversion, respectively), as determined by
1H-homodecoupled NMR experiments.14 This apparent prefer-
ence for racemic enchainment was further confirmed by com-
paring the relative conversion rate of rac-LA versus (S)-LA to
the corresponding PLA materials. Thus, as illustrated in Fig. 6,
the ROP of rac-LA initiated by 5a proceeds much faster than
that of (S)-LA (krac/ks ~ 20). Thus, the moderate heterotacticity
of the isolated PLAs (from rac-LA) is certainly to be ascribed
to detrimental transesterification processes. One should finally
mention that the ROP of (S)-lactide initiated by compound 5a
only yielded the obtainment of poly-(S)-lactide, indicating the
absence of epimerization during polymerization.
NON-supported Al alkoxide complexes 4a–b and 5a–b were thus
tested for their activity in lactide ROP.
While the mono-alkoxide Al derivatives 4a and 4b were found
to be inactive in rac-lactide (rac-LA) ROP in CH2Cl2 at room
temperature, both bis-alkoxide Al species 5a and 5b are active
under these mild conditions (CH2Cl2, room temperature, 16 h,
230 equiv. of rac-LA, 82% and 85% conversion to PLA for 5a and
5b, respectively). As initiators 5a and 5b are structurally close and
thus exhibit a similar lactide ROP activity, the polymerization
of lactide was thoroughly studied only in the case of 5a; all
experimental data support a rather well-controlled rac-LA ROP
process; these include: (i) all PLAs, isolated at different monomer
conversions, exhibit a narrow polydispersity as deduced from SEC
data (Table 2); (ii) the monomer conversion linearly correlates
with the Mn values of PLAs (Fig. 4) and (iii) the polymerization
is first order in the monomer (Fig. 5). As estimated from SEC
data and shown in Table 2, the Mn values of the PLAs are lower
than those expected for a “living”-type ROP proceeding in ideal
conditions, which is most likely due to transesterification side-
reactions during the present polymerization process. This was
substantiated by MALDI-TOF mass-spectrometric analysis of
one PLA sample; its mass spectrum primarily exhibits metal ions
peaks (M + Na+ and M + K+) that are equally spaced by 72 a.u.
and with exact masses consistent with a linear PLA bearing a
benzyloxide group at the ester end (see ESI†). In addition, the
MALDI-TOF spectrum also contains ion peaks corresponding
Fig. 6 Plots of ln([M]0/[M]) versus time with rac-lactide and (S)-lactide
as the monomer M. Conditions: complex 5a as the initiator, 230 equiv. of
monomer, [M]0 = 1.6 M, CH2Cl2, room temperature.
Thus, while Al-based metal alkoxides classically require heating
to initiate the ROP of lactide, the well-defined lithium Al alkoxide
species 5a and 5b readily undergo such a polymerization at
room temperature. To rationalize this difference of activity, it
was conjectured that 5a and 5b, which are of the type {(RN-o-
C6H4)2O}Al(m -OCH2Ph)2Li(THF)2, may well dissociate, at least
partially under catalytic conditions, to yield the neutral Al alkoxide
complex {(RN-o-C6H4)2O}Al(OCH2Ph), likely to be stabilized by
the monomer, and LiOCH2Ph; the latter Li species may then, on
Fig.
5 Plot of ln([M]0/[M]) versus time with rac-lactide as the
monomer M. Conditions: complex 5a as the initiator, 230 equiv. of rac-LA,
[rac-LA]0 = 1.6 M, CH2Cl2, room temperature.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 533–540 | 537
©

its own, initiate the ROP of rac-lactide.15 To gain an insight into
this matter and taking the case of 5b, the synthesis of {(CyN-o-
C6H4)2O}Al(OCH2Ph)(THF) (6b, see Experimental section) and
of LiOCH2Ph, generated by deprotonation of PhCH2OH with
nBuLi, were carried out. Both compounds were found to be
inactive in the ROP of rac-LA under conditions found suitable
for complexes 5a and 5b (CH2Cl2, room temperature, 16 h). The
latter results are consistent with an active ROP species in which
both the Al and Li fragments may be involved and co-operate.
Charles Sadron (Strasbourg, France) on a system equipped with
a Shimadzu RID10A refractometer detector using dry THF (on
CaH2) as an eluant. Molecular weights and polydispersity indices
(PDIs) were calculated using polystyrene standards. MALDI-
TOF mass spectroscopic analysis were performed at the Service de
Spectrome´trie de Masse of the University of 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;
a-HCCA (in saturated acetone) was used as the matrix in 5/1
volume ratio.
Summary and conclusions
Proligands (RNH-o-C6H4)2O (1a, R = C5H9; 1b, R = Cy).
Compounds 1a and 1b were synthesized according to a literature
procedure involving the reaction of (NH2-o-C6H4)2O with two
equivalents of cyclopentanone or cyclohexanone in the presence
of a Zn/CH3COOH mixture.6 The analytical data obtained for
1b matched those reported in the literature while those for 1a are
listed below.
Tridentate diamido-ether dianionic ligands of the type NON2-
are suitable ligands for coordination to Al and the synthesis
of the derived lithium Al hydrido and alkoxide species 2a–b,
4a–b and 5a–b, which, formally, are of the type {LX2}AlX2 ,
-
Li+. All these salt species crystallize as associated Al and Li
fragments and may be structurally different depending on the
nature of the X ligand (hydrido or benzyloxide), as illustrated
by the observed structural variation when going from 2a–b to 4b
and then 5a. While the lithium Al mono-alkoxide species 4a–b are
inactive in the ROP of lactide, their bis-alkoxide Al analogues 5a–b
polymerize rac-lactide and (S)-lactide at room temperature, which
is rather uncommon for Al-based alkoxide systems. Kinetic studies
of the lactide ROP initiated by compound 5a suggest a strong
preference for racemic enchainment during the ROP chain growth;
the resulting PLAs are however only moderately heterotactic due
to detrimental transesterification processes occurring as the chain
grows. Future studies will focus on the use and screening of various
dianionic and trianionic chelating ligands for coordination to Al
so as to probe the potential of the derived anionic Al complexes
as lactide ROP initiators.
Data for [(C5H9)NH-o-C6H4]2O (1a). 1H NMR (300 MHz,
C6D6): d 1.21–1.37 (m, 12H, C5H9), 1.68–1.76 (m, 4H, C5H9), 3.58
3
3
(septet, JHH = 6.6 Hz, 2H, N–CH), 4.30 (br. d, JHH = 6.3 Hz,
2H, NH), 6.58 (dt, 3JHH = 7.2 Hz, 4JHH = 1.5 Hz, 2H), 6.70 (dd,
3JHH = 7.5 Hz, JHH = 1.5 Hz, 2H), 6.90 (dd, JHH = 7.5 Hz,
4
3
4JHH = 1.5 Hz, 2H), 7.01 (dt, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 2H).
1
13
{ H} C NMR (75 MHz, C6D6): d 24.1 (C5H9), 33.5 (C5H9), 54.4
(N-CH), 112.3 (CH), 116.7 (CH), 118.3 (CH), 124.4 (CH), 140.0
(Cipso), 144.4 (Cipso). Anal. Calcd for C22H28N2O: C, 78.53; H, 8.39.
Found: C, 78.36; H, 8.18.
[g2-N,N-{(C5H9)N-o-C6H4)2O}AlH(l-H)Li(THF)]2 (2a). In
a glovebox, LiAlH4 (113.5 mg, 2.99 mmol) was added in several
portions via a spatula to a precooled (-35 ◦C) THF solution
(15 mL) of the diamino proligand 1a (1.00 g, 2.98 mmol). The
resulting grey suspension was then allowed to warm to room
temperature; upon warming, bubbling occurred (H2 formation).
The reaction mixture was stirred at room temperature for 20 h,
filtered through a glass frit and the obtained colorless solution
evaporated to dryness in vacuo to afford a colorless solid as
the crude product. The latter residue was recrystallized from a
5/1 pentane/CH2Cl2 solvent mixture cooled at -35 ◦C to yield
Experimental
General procedures
All experiments were carried out under N2 using standard Schlenk
techniques or in a Mbraun Unilab glovebox. Toluene, pentane,
diethyl ether and tetrahydrofuran were collected after passage
through drying columns (SPS apparatus, MBraun) and stored
over activated molecular sieves (4 A) for 24 h in a glovebox
prior to use. CH2Cl2, CD2Cl2, C6D6 and THF-d8 were distilled
1
analytically pure 2a (660 mg, 50% yield). H NMR (300 MHz,
CD2Cl2): d 1.40–1.89 (m, 40H, C5H9 and Hb-THF), 3.53–3.60 (m,
˚
3
12H, N-CH and Ha-THF), 6.48 (t, JHH = 7.5 Hz, 4H), 6.75 (d,
3JHH = 8.1 Hz, 4H), 6.84 (d, 3JHH = 7.8 Hz, 4H), 7.00 (t, 3JHH
=
from CaH2, degassed under a N2 flow and stored over activated
1
13
7.6 Hz, 4H). { H} C NMR (75 MHz, CD2Cl2): d 24.2 (CH2),
25.2 (THF), 34.0 (CH2), 60.6 (N–CH), 68.4 (THF), 114.4 (CH),
116.4 (CH), 118.9 (CH), 125.6 (CH), 147.4 (Cipso), 148.5 (Cipso). IR
(cm-1): 1789 (br s, Al–H). Anal. Calcd for C52H72Al2Li2N4O4: C,
70.57; H, 8.20. Found: C, 70.23; H, 8.38.
˚
molecular sieves (4 A) 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 rac-lactide, which was twice
sublimed prior to use. NMR spectra were recorded on Bruker
AC 300 or 400 MHz NMR spectrometers, in TeflonTM-valved
J-Young NMR tubes at ambient temperature, unless otherwise
indicated. 1H and 13C chemical shifts are reported vs. SiMe4
and were determined by reference to the residual 1H and 13C
solvent peaks. 19F and 7Li chemical shifts are reported versus
BF3–Et2O in CD2Cl2, LiCl (1M) in D2O, respectively. Elemental
analysis for all compounds were performed at the Service de
Microanalyse of the Universite´ Pierre et Marie Curie (Paris,
France) and the Universite´ de Strasbourg (Strasbourg France).
SEC analysis of poly(lactic) acid were performed at the Institut
[g2-N,N-{(CyN-o-C6H4)2O}AlH(l-H)Li(THF)]2 (2b). The
diamido Al hydrido complex 2b was synthesized following an
identical procedure to that used for 2a and involved the reaction of
LiAlH4 (133.0 mg, 3.50 mmol) with ligand 1b (1.15 g, 3.20 mmol).
Complex 2b was isolated as an analytically colorless crystalline
powder after a toluene wash of the crude product (630 mg,
1
42% yield). H NMR (300 MHz, CD2Cl2): d 1.12–1.26 (br m,
12H, Cy), 1.68–1.76 (m, 36H, Cy and Hb-THF), 2.95 (br s, 4H,
3
N–CH), 3.51 (br s, 8H, Ha-THF), 6.45 (t, JHH = 7.5 Hz, 4H),
3
3
6.72 (d, JHH = 8.1 Hz, 4H), 6.83 (d, JHH = 7.8 Hz, 4H), 6.97
538 | Dalton Trans., 2010, 39, 533–540
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The Royal Society of Chemistry 2010
©

IR (cm-1): 1811 (br s, Al–H). Anal. Calcd for C39H54AlLiN2O4: C,
72.20; H, 8.39. Found: C, 72.56; H, 8.17.
1
13
(t, 3JHH = 7.6 Hz, 4H). { H} C NMR (75 MHz, CD2Cl2): d 25.2
(CH2), 26.1 (CH2), 26.5 (CH2), 34.2 (CH2), 58.9 (N–CH), 68.3
(THF), 113.8 (CH), 114.6 (CH), 119.2 (CH), 125.9 (CH), 146.5
(Cipso), 147.9 (Cipso). 7Li NMR (155.5 MHz, CD2Cl2): d -0.14 (s).
IR (cm-1): 1751 (br s, Al–H). Anal. Calcd for C28H40AlLiN2O2: C,
71.47; H, 8.57. Found: C, 71.15; H, 8.32.
g2-N, N -{((C5H9)N-o-C6H4)2O}Al(l-OCH2Ph)2Li(THF)2 (5a).
In a glovebox, two equivalents of benzaldehyde (245 mL,
2.42 mmol) were added via a syringe to a precooled (-35 ◦C)
THF solution (10 mL) of the Al hydrido species 2a. The initial
colorless solution turned immediately yellow upon addition of
PhCHO; it was then allowed to warm to room temperature and
stirred overnight at this temperature. The resulting yellow solution
was then evaporated to dryness to afford a yellow residue as the
crude product, which was recrystallized at -35 ◦C from a 1/5
THF/pentane solvent mixture to afford compound 5a as a pale
{(CyN-o-C6H4)2O}Al(H)(THF) (3b) NMR-scale reaction.
A
J-Young NMR tube containing a CD2Cl2 solution of pure 2b was
1
left for 24 h at RT. Subsequent H NMR analysis revealed the
complete conversion of the Al salt species 2b into the neutral
monohydrido aluminium complex 3b, along with formation of
CHD2Cl (d 2.99, q, 2JHD = 1.6 Hz). Preparative scale. In a glove
box, a CH2Cl2 of complex 2b (200 mg, 0.425 mmol) was stirred
at room temperature overnight to yield, after drying in vacuo,
a pale light suspension (formation of LiCl), which was filtered
through a glass frit to afford a colorless foam as the crude product.
Recrystallization of the latter residue from a 9/1 pentane/CH2Cl2
solvent mixture stored at -5 ◦C afforded pure 3b (277 mg, 63%
1
yellow powder in pure form (400.0 mg, 50% yield). H NMR
(300 MHz, CD2Cl2): d 1.49–1.53 (m, 4H, C5H9), 1.68–1.88 (m,
20H, C5H9 and Hb-THF), 3.50 (m, 8H, Ha-THF), 3.87 (m, 2H,
3
N–CH), 4.76 (s, 4H, PhCH2), 6.47 (t, JHH = 7.5 Hz, 2H), 6.88
3
(d,, JHH = 8.1 Hz, 2H), 6.97–7.03 (m, 4H), 7.20–7.30 (m, 10H).
1
13
{ H} C NMR (75 MHz, CD2Cl2): d 24.5 (CH2), 25.3 (THF),
31.0 (CH2), 59.7 (N–CH), 65.4 (PhCH2), 67.9 (THF), 112.9 (CH),
116.4 (CH), 118.0 (CH), 124.9 (CH), 126.2 (CH), 126.7 (CH),
127.9 (CH), 144.9 (Cipso), 147.5 (Cipso), 147.8 (Cipso). Anal. Calcd
for C44H56AlLiN2O5: C, 72.71; H, 7.77. Found: C, 73.05; H, 7.92.
1
yield). H NMR (300 MHz, CD2Cl2): d 1.14–1.33 (m, 8H, Cy),
1.59–1.82 (m, 16H, Cy and Hb-THF), 2.78 (br s, 1H, Al–H), 3.09
(br s, 2H, N–CH), 3.73 (br s, 4H, Ha-THF), 6.44 (t, 3JHH = 7.6 Hz,
3
3
2H), 6.70 (d, JHH = 8.1 Hz, 2H), 6.79 (d, JHH = 7.8 Hz, 2H),
6.96 (t, 3JHH = 7.7 Hz, 2H). { H} C NMR (75 MHz, CD2Cl2): d
24.8 (CH2), 25.9 (CH2), 26.3 (CH2), 36.3 (CH2), 55.9 (N–CH), 69.9
(THF), 110.7 (CH), 111.1 (CH), 116.2 (CH), 125.8 (CH), 144.1
(Cipso), 145.6 (Cipso). IR (cm-1): 1842 (br s, Al–). Anal. Calcd for
C28H39AlN2O2: C, 72.70; H, 8.50. Found: C, 72.93; H, 8.77.
1
13
g2-N,N-{(CyN-o-C6H4)2O}Al(l-OCH2Ph)2Li(THF)2
(5b).
The dialkoxide Al complex 4b was synthesized following an
identical procedure to that used for 5a and was isolated as
an analytically pale yellow powder from a 1/5 THF/pentane
solvent mixture cooled at -35 ◦C (630 mg, 42% yield). 1H NMR
(300 MHz, CD2Cl2): d 1.08–1.29 (m, 8H, Cy), 1.55–1.62 (m, 8H,
Hb-THF), 1.71–1.78 (m, 8H, Cy), 1.91–2.04 (m, 4H, Cy), 3.09 (m,
2H, N–CH), 3.25 (m, 8H, Ha-THF), 4.83 (s, 4H, PhCH2), 6.44
g2-N,
N -{(C5H9)N-o-C6H4)2O}Al(H)(l-OCH2Ph)Li(THF)2
(4a). The mono-alkoxide species 4a was synthesized using an
identical procedure to that for 5a (vide infra) but using one
equivalent of benzaldehyde. Compound 5a was isolated as a
colorless solid after recrystallization of the crude product at
3
(t, JHH = 7.5 Hz, 2H), 6.87–6.96 (m, 6H), 7.17–7.28 (m, 10H).
1
13C{ H} NMR (75 MHz, CD2Cl2): d 25.0 (CH2), 25.3 (CH2),
◦
-35 C from a 1/5 THF/pentane solvent mixture (352 mg, 47%
yield). H NMR (300 MHz, CD2Cl2): d 1.41–.55 (m, 4H, C5H9),
25.9 (CH2), 33.3 (CH2), 58.8 (N–CH), 65.4 (PhCH2), 67.9 (THF),
112.9 (CH), 115.6 (CH), 118.4 (CH), 124.2 (CH), 126.3 (CH),
126.8 (CH), 126.0 (CH), 144.9 (Cipso), 146.9 (Cipso), 148.5 (Cipso).
Anal. Calcd for C46H60AlLiN2O5: C, 73.19; H, 8.01. Found: C,
73.56; H, 8.21.
1
1.61–1.82 (m, 20H, C5H9 and Hb-THF), 3.50 (m, 8H, Ha-THF),
3.91 (m, 2H, N–CH), 4.65 (s, 2H, PhCH2), 6.37 (t, 3JHH = 7.5 Hz,
3
2H), 6.81 (d, JHH = 8.1 Hz, 2H), 7.01–7.09 (m, 4H), 7.27–7.35
1
13
(m, 5H). { H} C NMR (75 MHz, CD2Cl2): d 24.4 (CH2), 25.6
(CH2), 31.8 (CH2), 60.0 (N–CH), 65.9 (PhCH2), 68.5 (THF),
113.9 (CH), 116.7 (CH), 117.5 (CH), 125.1 (CH), 126.3 (CH),
126.9 (CH), 128.4 (CH), 144.4 (Cipso), 147.52 (Cipso), 148.1 (Cipso).
IR (cm-1): 1805 (br s, Al–H). Anal. Calcd for C37H50AlLiN2O4: C,
71.79; H, 8.12. Found: C, 72.21; H, 7.98.
{(CyN-o-C6H4)2O}Al(OCH2Ph)(THF) (6b). NMR-scale reac-
tion. In a J-Young NMR tube, the Al hydrido complex 3b
(30.0 mg, 0.053 mmol) was charged and dissolved in CD2Cl2
(0.75 mL) and one equivalent of benzaldehyde (5.4 mL,
0.053 mmol) was immediately added via a microsyringe to the
latter solution. The NMR tube was tightly capped, vigorously
shaken and a 1H NMR spectrum was immediately recorded show-
ing the quantitative formation of the Al alkoxide 6b. Preparative
scale. In a glove box, one equivalent of benzaldehyde (35.7 mL,
0.352 mmol) was added via a microsyringe to a precooled CH2Cl2
g2-N, N -{(CyN-o-C6H4)2O}Al(H)(l-OCH2Ph)Li(THF)2 (4b).
The mono-alkoxide species 4b was synthesized using an identical
procedure to that for 5b (vide infra) but using one equivalent
of benzaldehyde. Compound 5b was isolated as a colorless solid
after recrystallization of the crude product at -35 ◦C from a 1/5
◦
solution (-35 C, 10 mL) of complex 3b (200 mg, 0.352 mmol).
1
THF/pentane solvent mixture (274 mg, 45% yield). H NMR
The resulting mixture was left under stirring at room temperature
overnight and subsequently evaporated to dryness to afford a
colorless solid as the crude pro◦duct. Recrystallization of the latter
residue in Et2O stored at -35 C afforded pure 6b as a colorless
(300 MHz, CD2Cl2): d 1.10–1.33 (m, 8H, Cy), 1.66–1.73 (m, 8H,
Hb-THF), 1.71–1.82 (m, 8H, Cy), 1.95–2.09 (m, 4H, Cy), 3.06 (m,
2H, N–CH), 3.49 (m, 8H, Ha-THF), 4.70 (s, 2H, PhCH2), 6.38 (t,
1
13
1
3JHH = 7.5 Hz, 2H), 6.91–7.06 (m, 6H), 7.14–7.34 (m, 5H). { H} C
NMR (75 MHz, CD2Cl2): d 25.2 (CH2), 26.3 (CH2), 27.2 (CH2),
33.7 (CH2), 34.3 (CH2), 58.7 (N–CH), 64.2 (PhCH2), 67.9 (THF),
111.5 (CH), 113.3 (CH), 119.0 (CH), 125.1. (CH), 125.8 (CH),
126.8 (CH), 127.0 (CH), 145.1 (Cipso), 147.4 (Cipso), 148.8 (Cipso).
powder (112 mg, 56% yield). H NMR (300 MHz, CD2Cl2): d
1.16–1.34 (m, 4H, Cy), 1.64–1.89 (m, 20H, Cy and Hb-THF), 2.99
(br m, 2H, N–CH), 3.41 (m, 4H, Ha-THF), 4.52 (s, 2H, PhCH2),
3
3
6.45 (t, JHH = 7.6 Hz, 2H), 6.76 (d, JHH = 7.6 Hz, 2H), 6.92
3
(d, JHH = 7.6 Hz, 2H), 6.93–7.03 (m, 4H), 7.12–7.28 (m, 3H).
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 533–540 | 539
©

1
13
D. Bourissou, Chem. Rev., 2004, 104, 6147; (c) J. Wu, T.-L. Yu, C.-T.
Chen and C.-C. Lin, Coord. Chem. Rev., 2006, 250, 602; (d) A. P. Dove,
Chem. Commun., 2008, 6446.
{ H} C NMR (75 MHz, CD2Cl2): d 24.8 (CH2), 25.9 (CH2), 26.3
(CH2), 33.4 (CH2), 34.6 (CH2), 58.7 (N–CH), 63.9 (PhCH2), 67.9
(THF), 113.1 (CH), 114.5 (CH), 118.7 (CH), 125.5 (CH), 126.0
(CH), 126.5 (CH), 127.7 (CH), 143.8 (Cipso), 146.4 (Cipso), 147.9
(Cipso). Anal. Calcd for C33H45AlN2O3: C, 73.92; H, 7.98. Found:
C, 73.65; H, 8.11.
2 For representative examples, see: (a) N. Spassky, M. Wisniewski, C.
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5 H. Li, C. Wang, F. Bai, J. Yue and H.-G. Woo, Organometallics, 2004,
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6 R. Baumann, R. Stumpf, W. M. Davis, L.-C. Liang and R. R. Schrock,
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9 P. Deslongchamps and M. Caron, Can. J. Chem., 1980, 58, 2061.
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Typical procedure for lactide polymerisation by 5a–b. In a
glovebox, the desired Al alkoxide ROP initiator (0.012 mmol) was
charged in a vial equipped with a TeflonTM-tight screw-cap and a
CH2Cl2 solution (1.8 mL) of rac-lactide (230 equiv., 400.0 mg) was
added via a syringe all at once. The resulting colorless solution
was vigorously stirred at room temperature for the appropriate
time, during which several aliquots were taken and analyzed by
1H NMR spectroscopy to monitor the conversion. When the
appropriate conversion was reached, the vial was removed from
the glovebox and the reaction mixture was quenched with MeOH
along with a few drops of acetic acid provoking the precipitation
of PLA. The resulting polymer was washed several times with
MeOH, dried in vacuo until constant weight and subsequently
analyzed by 1H NMR, SEC and, in some cases, by MALDI-TOF
spectrometry. In the case of the ROP of (S)-lactide by the Al
complexes 5a–b, the isolated PLA was also analyzed by 13C NMR
spectroscopy which confirmed it to be (S)-PLA, thus showing that
no epimerization takes place during the ROP process.
Acknowledgements
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The authors thank the CNRS (Centre Nationale de la Recherche
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