Synthesis and structure of neutral and cationic aluminum complexes supported by bidentate O,P-phosphinophenolate ligands and their reactivity with propylene oxide and ε-caprolactone

Article abstract of DOI:10.1021/om9003362

The O,P-type phosphinophenol ligands 1a-c were found to readily react with 1 equiv of AlMe₃ to afford in high yields the corresponding Al chelate complexes {η²-O,P-(2-PPh₂-4-R′-6-R- C₆H₂O)}-AlMe₂, 2a-c (R = Me, R′ = H, 2a; R = Ph, R′ = H, 2b; R = ^tBu, R′ = Me, 2c). The bis-adduct Al methyl complexes {η²-O,P-(2-PPh₂-4-R′-6-R- C₆H₂O)}₂AlMe (R = Ph, R′ = H, 3b; R = ^tBu, R′ = Me, 3c) also formed quantitatively upon reaction of phosphinophenols 1b,c with 0.5 equiv of AlMe₃. Both the mono- and bis-adduct Al methyl species 2a-c and 3b,c are stable monomeric species whether in solution or in the solid state and remain stable in coordinating solvents such as thf. In contrast, the bis-adduct Al methyl complex 3c undergoes a ligand exchange reaction in the presence of an alcohol source (^tPrOH, BnOH) to generate the homoleptic tris-adduct Al complex {η²-O,P-(2- PPh₂-4-Me-6-^tBu-C₆H₃O)} ₃Al (5c), as determined from X-ray crystallographic studies. Both the mono- and bis-adduct Al methyl species 2b,c and 3b,c react fast with B(C ₆F₅)₃ via a methide abstraction reaction to afford the stable and well-defined Al cationic species {η²-O,P- (2-PPh₂-6-Ph-C₆H₃O)}Al(Me) (THF)⁺ (6b,c⁺) and {η²-O,P-(2-PPh₂-4-R′-6-R- C₆H₃O)}₂Al⁺ (7b,c⁺), respectively, which were found to be highly active in propylene oxide polymerization to afford atactic poly(propylene oxide). These cations also readily initiate the ring-opening polymerization of e-caprolactone via successive ring-opening insertions of the monomer into the Al-O phenoxide bond of the phosphinophenolate chelating ligand to exclusively afford linear poly(e-caprolactone) capped, at the ester end, with a (phosphino oxide)phenolate group, as deduced from NMR and MALDI-TOF data. In these cationic systems, the PO^- chelating moiety may thus act as both a supporting ligand and an initiating group for the ROP of ε-CL.

Full text of DOI:10.1021/om9003362

4584 Organometallics 2009, 28, 4584–4592
DOI: 10.1021/om9003362
Synthesis and Structure of Neutral and Cationic Aluminum Complexes
Supported by Bidentate O,P-Phosphinophenolate Ligands and Their
Reactivity with Propylene Oxide and ε-Caprolactone
Mansour Haddad,‡ Mohamed Laghzaoui,§ Richard Welter,§ and Samuel Dagorne*,§
§
ꢀ
Laboratoire DECOMET, UMR CNRS 7177, Universite de Strasbourg, 1, Rue Blaise Pascal, 67000
ꢀ
‡
Strasbourg, France, and Laboratoire Charles Friedel, UMR CNRS 7223, Ecole Nationale Superieure de
Chimie de Paris, 11 Rue Pierre et Marie Curie, 75005 Paris, France
Received April 29, 2009
The O,P-type phosphinophenol ligands 1a-c were found to readily react with 1 equiv of AlMe3 to
afford in high yields the corresponding Al chelate complexes {η2-O,P-(2-PPh2-4-R0-6-R-C6H2O)}-
AlMe2, 2a-c (R=Me, R0=H, 2a; R=Ph, R0=H, 2b; R=tBu, R0=Me, 2c). The bis-adduct Al methyl
complexes {η2-O,P-(2-PPh2-4-R0-6-R-C6H2O)}2AlMe (R=Ph, R0=H, 3b; R=tBu, R0=Me, 3c) also
formed quantitatively upon reaction of phosphinophenols 1b,c with 0.5 equiv of AlMe3. Both the
mono- and bis-adduct Al methyl species 2a-c and 3b,c are stable monomeric species whether in
solution or in the solid state and remain stable in coordinating solvents such as thf. In contrast, the
bis-adduct Al methyl complex 3c undergoes a ligand exchange reaction in the presence of an alcohol
source (iPrOH, BnOH) to generate the homoleptic tris-adduct Al complex {η2-O,P-(2-PPh2-4-Me-
6-tBu-C6H3O)}3Al (5c), as determined from X-ray crystallographic studies. Both the mono- and
bis-adduct Al methyl species 2b,c and 3b,c react fast with B(C6F5)3 via a methide abstraction reaction
to afford the stable and well-defined Al cationic species {η2-O,P-(2-PPh2-6-Ph-C6H3O)}Al(Me)
(THF)þ (6b,cþ) and {η2-O,P-(2-PPh2-4-R0-6-R-C6H3O)}2Alþ (7b,cþ), respectively, which were
found to be highly active in propylene oxide polymerization to afford atactic poly(propylene oxide).
These cations also readily initiate the ring-opening polymerization of ε-caprolactone via successive
ring-opening insertions of the monomer into the Al-O phenoxide bond of the phosphinophenolate
chelating ligand to exclusively afford linear poly(ε-caprolactone) capped, at the ester end, with a
(phosphino oxide)phenolate group, as deduced from NMR and MALDI-TOF data. In these cationic
systems, the PO- chelating moiety may thus act as both a supporting ligand and an initiating group
for the ROP of ε-CL.
Introduction
of Al cations have been primarily used in polymerization
catalysis and were found to be effective initiators of oxiranes
and isobutene polymerization.2,3 Although less studied thus
far, the use of cationic aluminum species for the ROP of
cyclic esters such as rac-lactide and ε-caprolactone has also
been the subject of a few reports;2c,4 some of these cations
were found to exhibit high activity in ε-caprolactone poly-
merization.2c,4b
Aluminum cations are typically supported by bi-, tri-, and
tetradentate chelating and anionic ligands containing, in
various combination, nitrogen and oxygen hard donors
and whose steric properties greatly impact the structure
and stability of the derived cationic species.1b In contrast,
thus far, whether in neutral or in cationic aluminum chem-
istry, anionic chelating ligands incorporating L-type soft
donors such as phosphine donors have been much less
explored as supporting ligands for coordination to the hard
metal center Al(III); examples in this area include the use
of bidentate and tridentate amidophosphine-type anionic
Cationic aluminum species, which are of interest for their
enhanced Lewis acidity versus that of their neutral analo-
gues, have established themselves over the last 10 years as a
novel class of highly electrophilic species, able to efficiently
mediate and/or catalyze a variety of organic transforma-
tions.1 While some of these cations, namely, low-coordinate
Al alkyl cations, may be involved in stoichiometric reactions
such as the alkylation of benzene and as fast transalkylating
agents in the presence of a terminal olefin,2 the vast majority
*Corresponding author. E-mail: dagorne@chimie.u-strasbg.fr.
(1) (a) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407. (b) Dagorne,
S.; Atwood, D. A. Chem. Rev. 2008, 108, 4037.
(2) (a) Korolev, A. V.; Ihara, E.; Guzei, I. A.; Young, V. G. Jr.;
Jordan, R. F. J. Am. Chem. Soc. 2001, 123, 8291. (b) Kim, K.-C.; Reed,
C. A.; Long, G. S.; Sen, A. J. Am. Chem. Soc. 2002, 124, 7662. (c) Dagorne,
S.; Le Bideau, F.; Welter, R.; Bellemin-Laponnaz, S.; Maisse-Franc-ois, A.
Chem.-Eur. J. 2007, 13, 3202.
(3) For other representative examples, see: (a) Atwood, D. A.; Jegier,
J. A.; Rutherford, D. J. Am. Chem. Soc. 1995, 117, 6779. (b) Bochmann,
M.; Dawson, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2226.
(c) Munoz-Hernandez, M.-A.; McKee, M. L.; Keizer, T. S.; Yearwood,
B. C.; Atwood, D. A. J. Chem. Soc., Dalton Trans. 2002, 410. (d) Dagorne,
S.; Lavanant, L.; Welter, R.; Chassenieux, C.; Haquette, P.; Jaouen, G.
Organometallics 2003, 22, 3732.
~
ꢀ
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(4) (a) Emig, N.; Nguyen, H.; Krautschied, H.; Reau, R.; Cazaux, J.-B.;
ꢀ
Bertrand, G. Organometallics 1998, 17, 3599. (b) Lewinski; Horeglad, P.;
Dranka, M.; Justiniak, I. Inorg. Chem. 2004, 43, 5789. (c) Milione, S.; Grisi, F.;
Centore, R.; Tuzi, A. Organometallics 2006, 25, 266.
r
pubs.acs.org/Organometallics
Published on Web 06/18/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 15, 2009 4585
Chart 1
Scheme 1
ligands and of bidentate monoanionic phosphinomethanide
ligands.5 This lack of interest may arise from the expected
decreased stability that such complexes would exhibit com-
pared to more classical hard-donor-stabilized Al species.
Nevertheless, recent work on early transition-metal and
high-oxidation-state complexes such as group 4 metal com-
plexes strongly suggests that the use of ancillary ligands with
softer L donors, such as phosphorus and sulfur, may be
beneficial for olefin polymerization activity.6
Within the above context, we have become interested into
the synthesis of cationic aluminum compounds supported by
bidentate ligands with softer donor atoms to evaluate their
potential interest in polymerization catalysis. Bidentate
phosphinophenolates of type A (Chart 1) appeared suitable
for such studies, as they combine a hard phenoxide donor,
able to anchor the ligand onto the Al(III) hard metal center,
with a soft phosphine donor able to coordinate to the
aluminum center, to yield the formation of a stable five-
membered {PO}Al chelate. In addition, such phosphinophe-
nols may be synthesized in a straightforward manner and
their steric properties may easily be tuned via introduction of
various ortho substituents on the phenol ring.7 The latter
feature is of importance given the general tendency of
aluminum to readily form aggregates.
Here we report the synthesis and structural characteriza-
tion of cationic aluminum supported by one or two bidentate
phosphinophenolate ligands as well as those of the neutral
aluminum alkyl precursors prepared along the way. Both
neutral and cationic aluminum derivatives were tested as
propylene oxide, ε-caprolactone, and rac-lactide polymeri-
zation initiators, and the results of these studies are also
discussed herein.
analysis, establishing its monomeric nature. The molecular
structure of 2c and selected bond distances and angles are
shown in Figure 1, and crystallographic data are given in the
Supporting Information (Table S2). Compound 2c crystallizes
as a monomer in which the Al center adopts a distorted
tetrahedral geometry due to the rather acute η2-Al-bonded
phosphinophenolate bite angle (O-Al-P = 81.42(7)°). The
latter structural feature results in the opening of the C(1)-
Al-C(2) and O-Al-C bond angles (average 121.4(1)° and
112.9(1)°, respectively). The C-Al-P bond angles (110.6(1)°)
are very close to the ideal tetrahedral angle (109.49°). The
five-membered-ring Al metallacycle is significantly puckered,
with the AlMe2 moiety well above the nearly planar O-C(3)-
C(4)-P chelating ligand backbone, as shown by the P-Al-
O-C(3) and C(4)-P-Al-O torsion angles (40.4(1)° and
28.6(1)°, respectively). The Al-P and Al-C bond distances
Results and Discussion
Synthesis and Structure of Mono(phosphinophenolate)Al
Dimethyl Complexes (2a-c). The phosphinophenolate alumi-
num dimethyl complexes {η2-O,P-(2-PPh2-4-R0-6-R-C6H2O)}-
AlMe2, 2a-c (R = Me, R0=H, 2a; R=Ph, R0=H, 2b; R=tBu,
R0=Me, 2c; Scheme 1), were found to be readily accessible in
high yields via an alkane elimination route involving the
reaction of 1 equiv of the corresponding phosphinophenol
proligand 1a-c and AlMe3. X-ray-quality crystals of 2c were
grown from a saturated 1:1 Et2O/pentane solution, and its
molecular structure was determined by X-ray crystallographic
˚
(2.469(1) and 1.948(1) A, respectively) are shorter than those in
˚
the Al-phosphine adduct (PPh3)AlMe3 (2.535 and 1.981 A,
respectively), which may reflect a more electron-deficient Al
center in 2c versus (PPh3)AlMe3.8 While the Al-P bond
distance lies within the normal range for aluminum phosphine
5b
˚
˚
bonds (from 2.4 to 2.8 A), the Al-O bond distance (1.791(2) A)
is a bit above that found for aluminum phenolates (1.640(5)-
Interestingly, this comes along with a rela-
3d,9
˚
1.773(2) A).
tively more acute Al-O-C(3) bond angle (119.2(2)°) com-
pared to other aluminum phenoxides;9 the latter structural
feature is most likely imposed by the five-membered-ring
{PO}Al chelate. On the basis of structural and theoretical
studies reported on aluminum phenoxide derivatives,9,10 the
longer Al-O bond distance observed here might arise from
decreased π-symmetry interactions between the oxygen phen-
oxide and the Al center as a result of the smaller O-Al-C bond
angle.
€
(5) (a) Karsch, H. H.; Appelt, A.; Riede, J.; Muller, G. Organome-
tallics 1987, 6, 316. (b) Fryzuk, M. D.; Giesbrecht, G. R.; Olovsson, G.;
Rettig, S. J. Organometallics 1996, 15, 4832. (c) Fryzuk, M. D.; Giesbrecht,
G. R.; Rettig, S. J. Inorg. Chem. 1998, 37, 6928. (d) Liang, L.-C.; Huang,
M.-H.; Hung, C.-H. Inorg. Chem. 2004, 43, 2166. (e) Lee, W.-Y.; Liang,
L.-C. Dalton Trans. 2005, 1952.
(6) (a) Oakes, D. C. H.; Kimberley, B. S.; Gibson, V. C.; Jones, D. J.;
White, A. J. P.; Williams, D. J. Chem. Commun. 2004, 2174. (b) Lavanant,
L.; Silvestru, A.; Faucheux, A.; Toupet, L.; Jordan, R. F.; Carpentier, J.-F.
Organometallics 2005, 24, 5604. (c) Wang, C.; Ma, Z.; Sun, X.-L.; Gao, Y.;
Guo, Y.-H.; Tang, Y.; Shi, L.-P. Organometallics 2006, 25, 3259. (d) Long,
R. J.; Gibson, V. C.; White, A. J. P. Organometallics 2008, 27, 235.
(7) (a) Rauchfuss, T. B. Inorg. Chem. 1977, 16, 2966. (b) Willoughby,
C. A.; Duff, R. R. Jr.; Davis, W. M.; Buchwald, S. L. Organometallics 1996,
15, 472.
(8) Wierda, D. A.; Barron, A. R. Polyhedron 1989, 8, 831.
(9) Healy, M. D.; Power, M. B.; Barron, A. R. Coord. Chem. Rev.
1994, 130, 63.
(10) Barron, A. R.; Dobbs, K. D.; Franck, M. M. J. Am. Chem. Soc.
1991, 113, 39.

4586 Organometallics, Vol. 28, No. 15, 2009
Haddad et al.
Figure 1. Molecular structure (ORTEP drawing) of the Al com-
plex 2c. The hydrogen atoms are omitted for clarity. Selected bond
distances (A): Al-O = 1.791(2), Al-C(1)=1.948(3), Al-C(2)=
1.949(3), Al-P = 2.469(1). Selected bond angles (deg): O-Al-
C(1) = 113.7(1), O-Al-C(2) = 112.0(1), O-Al-P = 81.42(7),
Al-O-C(3) = 119.2(2).
Figure 2. Molecular structure (ORTEP drawing) of the Al com-
plex 3c. The hydrogen atoms are omitted for clarity. Selected bond
distances (A): Al-O(1)=1.774(2), Al-O(2)=1.768(2), Al-P(1)=
2.705(1), Al-P(2) = 2.607(1), Al-C(1) = 1.951(3). Selected
bond angles (deg): O(2)-Al-O(1)=122.06(9), O(2)-Al-C(1)=
118.2(1), O(1)-Al-C(1)=119.6(1), O(1)-Al-P(2)=79.48(6), O
(2)-Al-P(1)=75.30(6), P(2)-Al-P(1)=158.71(4), C(2)-O(1)-
Al=125.3(1), C(13)-O(2)-Al=123.6(1).
˚
˚
The NMR data for compounds 2a-c are overall similar to
one another and are consistent with the effective chelation of
one phosphinophenolate ligand to the aluminum center. For
1
As a comparison, the methane elimination reaction between
AlMe3 and 2 equiv of N,O-and N,N-based bidentate proligands
(LX-H-type proligands) generally affords the monochelate
{LX}AlMe2 when performed under the above conditions;
subsequent heating is usually required to promote the reaction
between the formed {LX}AlMe2 species and the second equiva-
lent of proligand. For instance, the reaction of AlMe3 with
2 equiv of aminophenol 2-CH2NMe2-6-tBu-C6H3OH at room
temperature quantitatively yields the mono(aminophenolate)
Al dimethyl complex {η2-O,N-(2-CH2NMe2-6-tBu-C6H3O)}-
AlMe2,3d which may then be slowly converted to the corre-
sponding bis(aminophenolate) Al species (toluene, 100 °C,
2 days, 20% conv by NMR).11 In the present case, the increased
reactivity of phosphinophenols 1b,c toward Al alkyls has to be
related to the decreased stabilizing properties of phosphines
versus amines as supporting ligands for Al, which, in the former
case, renders the Al center more electrophilic and thus more
susceptible to increase its coordination.
The molecular structures of compounds 3b and 3c were
determined by X-ray crystallographic analysis, establishing
their monomeric nature (see Figure 2 for 3c and Supporting
Information for 3b). Selected bond distances and angles for 3c
are given in Figure 2, and crystallographic data are summar-
ized in the Supporting Information (Table S2). Compounds
3b,c exhibit very similar structural features, and thus, only
those for 3c will be discussed. As illustrated in Figure 2, the
geometry of the Al center in 3c is best described as a slightly
distorted trigonal-bipyramidal structure, in which the oxygen
phenoxides and the Al-Me group occupy the equatorial posi-
tions while the phosphines are axially coordinated and thus
trans to one another (P(1)-Al-P(2) = 158.71(4)°). The bond
angles O(1)-Al-O(2), O(1)-Al-C(1), and O(2)-Al-C(1)
(122.06(9)°, 119.6(1)°, and 118.2(1)°, respectively) are similar
to one another, and their sum nearly perfectly equals 360°,
thus reflecting that the Al metal is centrally located in the
O(1)-C(1)-O(2) bipyramidal base. The Al-O and Al-C bond
instance, the H NMR spectra of 2a-c all contain a char-
acteristic doublet resonance (average δ 0.38, d, 3JHP = 4 Hz)
assigned to the AlMe2 moiety, which indicates the effective
coordination of phosphorus to aluminum. These data also
agree with an overall Cs-symmetry for 2a-c on the NMR
time scale in C6D6 or CD2Cl2 at room temperature, indicat-
ing a fast conformation change of the five-membered-ring
Al metallacycle under these conditions. No reaction was
observed between either 2b or 2c after 2 days at 65 °C in
thf-d8, indicating the unreactivity of the {PO}Al chelate with
regard to an external Lewis base such as thf. In contrast,
under these conditions, compound 2a was found to readily
and quantitatively decompose to unidentified species. Such a
difference of reactivity between 2a and 2b,c presumably
results from a better steric protection of the Al center in 2b,
c versus 2a.
Synthesis and Structure of Bis(phosphinophenolate)Al
Methyl and Chloro Complexes (3b,c and 4c) and of a Tris-
(phosphinophenolate)Al Complex (5c). Trimethylaluminum
reacts fast (toluene,-35 °C to room temperature, 3 h) with
2 equiv of the phosphinophenol proligand 1b,c to cleanly
afford the corresponding bis(phosphinophenolate)Al methyl
complexes {η2-O,P-(2-PPh2-4-R0-6-R-C6H2O)}2AlMe (R=
Ph, R0 =H, 3b; R=tBu, R0 =Me, 3c; Scheme 1) in good
yields as analytically pure colorless solids. The correspond-
ing Al chloro derivative {η2-O,P-(2-PPh2-4-Me-6-tBu-C6-
H3O)}2AlCl (4c, Scheme 1) could also be prepared and
isolated in good yield via a salt metathesis route involving
the reaction of the Li salt [2-PPh2-4-Me-6-tBu-C6H3O]Li,
generated in situ by reaction of 1c with nBuLi at -78 °C, with
0.5 equiv of AlCl3. In contrast, the Me-ortho-substituted-
phenol proligand 1a yields an intractable mixture upon reac-
tion with AlMe3 at low temperature (-35 and -78 °C),
which further highlights the importance of steric hindrance
around aluminum for the synthesis of well-defined species.
The salt metathesis reaction of the Li salt [2-PPh2-6-Me-
C6H3O]Li, generated in situ by reaction of 1a with nBuLi at
-78 °C, with 0.5 equiv of AlCl2Me also was unsuccessful.
(11) Lavanant, L.; Dagorne, S. Unpublished results.

Article
Organometallics, Vol. 28, No. 15, 2009 4587
Scheme 2
˚
distances (1.770(1) and 1.951(3) A average, respectively) in 3c
˚
arenormalwhiletheAl-P bond distances (2.656(1) A average)
are significantly longer than those in the monophosphinophe-
˚
nolate analogue 2c (2.469(1) A).
Figure 3. Molecular structure (ORTEP drawing) of the Al com-
plex 5c. The hydrogen atoms and the phenyl groups of the PPh2
˚
moieties are omitted for clarity. Selected bond distances (A):
Al-O(1) = 1.791(4), Al-O(2) = 1.783(4), Al-O(3) = 1.787(4),
Al-P(1) = 2.647(2), Al-P(2) = 2.714 (2), Al-P(3) = 2.659(3).
Selected bond angles (deg): O(2)-Al-O(1)=122.06(9), O(2)-Al-
C(1) = 118.2(1), O(1)-Al-C(1) = 119.6(1), O(1)-Al-P(2) =
79.48(6), O(2)-Al-P(1)=75.30(6), P(2)-Al-P(1)=158.71(4), C
(2)-O(1)-Al = 125.3(1), C(13)-O(2)-Al = 123.6(1).
The NMR data for compounds 3b,c are consistent with
their solid-state structure being retained in solution and with
effective C2-symmetric structures in C6D6 or CD2Cl2 at
1
room temperature. In particular, the H NMR spectra of
3b,c both contain a characteristic triplet resonance (average
3
δ -0.37, t, JHP = 4 Hz) assigned to the AlMe group,
indicating the effective coordination of both phosphorus
atoms to aluminum under the studied conditions. On the
basis of NMR data, the Al chloro analogue 4c most probably
adopts a structure similar to those of 3b,c.
angles (average 78.7(1)°) and the Al-P and Al-O bond
distances (2.673(4) and 1.786(5) A, respectively) compare to
˚
In attempts to prepare the bis(phosphinophenolate) alu-
minum alkoxide derivative {η2-O,P-(2-PPh2-4-Me-6-tBu-
C6H2O)}2AlOiPr, thought to be of potential interest as a
ROP initiator of cyclic esters,12 the corresponding Al tris-
adduct {PO}3Al was prepared instead. Thus, the reaction of
phosphinophenol 1c with 0.5 equiv of Al(OiPr)3 (toluene,
12 h, 80 °C) afforded the Al tris-adduct {η2-O,P-(2-PPh2-4-
Me-6-tBu-C6H2O)}3Al (5c) as the major product along with
other organoaluminum species (Scheme 2). Changing the
reaction conditions resulted either in no reaction, when the
reaction was carried out at room temperature, or in a lower
yield in 5c at shorter reaction time (toluene, 2 h, 80 °C). A 1H
NMR monitoring of the latter reaction in C6D6 at 75 °C
revealed the presence of the desired Al alkoxide {PO}2AlOR
all along the reaction process, albeit consistently as a minor
component: the considered {PO}2AlOR species is unstable
under the conditions required for its formation and under-
goes a ligand exchange reaction to yield the Al tris-adduct 5c,
presumably the thermodynamic product. It should also be
pointed out that both the reaction of bis(phosphinopheno-
late)Al methyl complexes 3c with 1 equiv of ROH (R=Bn,
iPr) and that of the Al chloro derivative 4c with 1 equiv of
LiOiPr also yielded the isolation of the trisphosphinopheno-
late Al species 5c, which illustrates the propensity of the bis
(phosphinophenolate) aluminum complexes of the type de-
scribed here to undergo ligand exchange reactions in the
presence of an alcohol or an alkoxide source.
those of the bis-adduct Al methyl analogue 3c. As for its
solution structure, the 1H and 13C NMR spectra (C6D6, RT)
exhibit only one set of resonances for the chelating ligand,
while the 31P NMR spectrum consists of a sharp singlet
resonance; thus, these data are consistent with compound 5c
retaining its solid-state structure in solution under the stu-
died conditions.
Synthesis of Mono(phosphinophenolate) and Bis(phosphi-
nophenolate) Al Methyl Cationic Complexes (6b,cþ, 7b,cþ,
and 8cþ). While the ionization chemistry of N,N- and N,
O-based Al neutral dialkyls has yielded novel families of
highly Lewis acidic Al species, the synthesis and use in
catalysis of cationic Al alkyls supported by “softer” chelating
ligands have not been investigated.1 Thus, the prepared
mono- and bis-phosphinophenolate Al methyl complexes
(2a-c and 3b,c) were reacted with methide abstracting
reagents such as B(C6F5)3 and [Ph3C][B(C6F5)4] so as to
generate the corresponding Al cations.
The reaction of complexes 2b,c with 1 equiv of B(C6F5)3
(CH2Cl2, RT, 10 min) in the presence of 1 equiv of THF
quantitatively affords the corresponding Al-THF cationiþc
adducts {η2-O,P-(2-PPh2-4-R0-6-R-C6H2O)}Al(Me)(THF)
(R = Ph, R0 = Ph, 6bþ; R = tBu, R0 = Me, 6cþ; Scheme 3)
as MeB(C6F5)-3 salts. The salt species [6b,c][MeB(C6F5)3],
which were isolated as analytically pure colorless solids in good
yields, are air-stable in the solid state and stable in solution
(CH2Cl2) under N2 for days. Compounds [6b,c][MeB(C6F5)3]
are fully dissociated in CD2Cl2 with no cation-anion interac-
tion at room temperature, as deduced from 1H and 19F NMR
data.13 Key characteristic 1H NMR resonances for cations 6b,
cþ include (i) the AlMeþ resonances (δ -0.21, -0.12 for 6b,cþ,
respectively) are downfield shifted as compared to those in the
neutral precursors 2b,c (δ -0.73, -0.68), a result of the cationic
As determined by X-ray crystallography, the molecular
structure of 5c features a central hexacoordinated Al atom
adopting a slightly distorted octahedral geometry, in which
all three oxygens of the phosphinophenolate bidentate li-
gands are coordinated in a mer-fashion (Figure 3). Its
principal structural features including the O-Al-P bite
(12) (a) O’Keefe, B. J.; Himmeyer, M. A.; Tolman, W. B. J. Chem.
Soc., Dalton Trans. 2001, 2215. (b) Dechy-Cabaret, O.; Martin-Vaca, B.;
Bourissou, D. Chem. Rev. 2004, 104, 6147.
(13) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015.

4588 Organometallics, Vol. 28, No. 15, 2009
Haddad et al.
Scheme 3
Figure 4. Molecular structure (ORTEP drawing) of the Al
cation 7cþ. The hydrogen atoms are omitted for clarity. Selected
˚
bond distances (A): Al-O(1)=1.732(1), Al-O(2)=1.725(1),
Al-P(1)=2.367(7), Al-P(2)=2.3890(8). Selected bond angles
(deg): O(2)-Al-P(2) = 88.10(5), O(2)-Al-P(1) = 88.92(5),
O(1)-Al-P(2)=120.18(6), O(2)-Al-P(1)=125.47(6), P(2)-
Al-P(1)=120.00(3).
charge on the Al metal center, and (ii) the THF resonances
(δ (average) 4.18, 1.96) also appear at lower field than those for
free THF (δ 3.67, 1.81), showing effective THF coordination to
the cationic Al center.
7cþ adopts a distorted tetrahedral structure with both
P-Al-O bite angles (average 88.5(1)°) being significantly
wider than those in the neutral Al precursors 2c (81.42(7)°)
and 3c (77.4(1)° average), which arises, at least in part, from
shorter Al-P and, to a lesser extent, Al-O bond distances in
Unlike compounds 2b,c, complex 2a reacts with B(C6F5)3
in the presence of 1 equiv or excess of THF to afford a
mixture of products. The ionization of 2a is evidenced by the
formation of the MeB(C6F5)-3 anion, as observed in NMR
spectroscopy; nevertheless, the lack of steric protection at the
Al center in the resulting cationic species probably favors the
formation of aggregates, which precluded, in the present
case, the isolation of a well-defined salt species. In addition,
two experimental observations should also be pointed out:
(i) all three neutral precursors 2a-c afforded decomposition
products when ionized by B(C6F5)3 in the absence of an
external Lewis base, such as THF, to stabilize the formed Al
cationic center and (ii) no reaction was observed between
complexes 2a-c and [Ph3C][B(C6F5)4] under the studied
conditions (CD2Cl2, RT, 2 h) whether or not in the presence
of THF. Such a lack of reactivity of [Ph3C][B(C6F5)4] versus
B(C6F5)3 toward {LX}AlMe2-type species is unusual.
The bis(phosphinophenolate) aluminum methyl com-
plexes 3b,c cleanly and quantitatively react with 1 equiv of
B(C6F5)3 (CH2Cl2, RT, 10 min) to afford the corresponding
Al cations {η2-O,P-(2-PPh2-4-R0-6-R-C6H2O)}2Alþ (R=Ph,
R0 = H, 7bþ; R = tBu, R0 = Me, 7cþ; Scheme 3) as MeB-
(C6F5)-3 salts in good yields. Both salt species are air-stable
whether in the solid state or in CH2Cl2 solution. The NMR
data for [7b,c][MeB(C6F5)3] (CD2Cl2, room temperature)
feature one set of resonances for the phosphinophenolate
ligand, which is consistent with a symmetrical environment
at Al in solution, and no cation-anion interactions are
observed under these conditions.13 The molecular structure
of [7c][MeB(C6F5)3], as determined by X-ray crystallo-
graphic analysis, unambiguously establishes cation 7cþ as a
four-coordinate Al cation effectively η2-chelated by two
bidentate phosphinophenolate ligands and with no interac-
tion with the MeB(C6F5)-3 anion in the solid state (Figure 4).
Selected bond distances and angles for 7cþ are given in
Figure 4, and crystallographic data are summarized in the
Supporting Information (Table S2). The Al center in cation
7cþ (average 2.378(5) and 1.72(1) A, respectively) versus
˚
˚
those in 2c (2.469(1) and 1.791(2) A) and 3c (average 2.656(1)
˚
and 1.770(1) A, respectively). The latter bonding parameters
reflect an increased electrophilicity of the cationic Al center
in 7cþ versus neutral analogues. In solution, despite its
excellent stability and the tetrahedral environment at Al,
the four-coordinate Al cation 7cþ remains quite Lewis acidic,
as it reacts fast with 1 equiv of NMe2Ph (CD2Cl2, RT,
10 min) to yield the corresponding pentacoordinate Al-
NMe2Ph adduct 8cþ, as deduced by 1H and 31P NMR
spectroscopy.
Reactivity of Phosphinophenolate Al Complexes with Pro-
pylene Oxide (PO). While all neutral phosphinophenolate
Al complexes synthesized in the present work are inactive in
PO polymerization at room temperature in CH2Cl2, the
more Lewis acidic Al cations 6b,cþ and 7b,cþ were all found
to initiate the polymerization of PO. Thus, the monopho-
sphinophenolate Al-THF adduct cations 6b,cþ readily poly-
merizes PO to afford atactic poly(propylene) oxide (PPO), as
deduced from 13C NMR data [RT, 15 min, CH2Cl2; 100
equiv of PO ([PO]0 = 1 M); yield in PPO: 6bþ, 52%, 6cþ,
61%]. As estimated from SEC data, both systems appear to
be similar to one another and feature a monomodal low-
molecular-weight distribution with a moderate polydisper-
sity (6bþ, Mn = 3120, Mw/Mn = 1.45; 6cþ, Mn = 3540, Mw/
Mn = 1.63). For both 6b,cþ cations, longer reaction time did
not improve conversion to PPO, suggesting a fast deactiva-
tion/decomposition of the catalytically active species under
the studied conditions. Nevertheless, in the case of 6cþ,
higher molecular weight PPO, along with a better conversion
of PO to PPO, was achieved at lower temperature [0 °C,
CH2Cl2, 1 h; 100 equiv of PO ([PO]0 = 1 M); 79% yield in
PPO; Mn = 7420, Mw/Mn = 1.68]. Analyses of the latter
PPOs by 1H, 13C, and 31P NMR spectroscopy are consistent

Article
Organometallics, Vol. 28, No. 15, 2009 4589
with the absence of aromatics and phosphorus in the synthe-
sized materials, which suggests that, unlike that of ε-CL (vide
infra), the polymerization process may not proceed via
insertion into the Al-phenolate bond. Presumably, the
present PO polymerization catalysis proceeds via a Lewis
acid-assisted cationic mechanism, as observed in related
N,N- and N,O-supported Al cationic systems.3c,3d Overall,
the catalytic activity of 6b,cþ at room temperature compares
with that of aminophenolate-supported Al methyl cations
and Schiff base-AlEt2Cl initiating systems as well as with
that of (Salen)Al(thf)þ2 and (Salpen)Al(thf)þ2 Al cations.3d,14
Although less reactive than cations 6b,cþ, the bis-phos-
phinophenolate Al cations 7b,cþ also initiate the polymeri-
zation of PO under mild conditions to afford atactic PPO in
high yield (RT, 12 h, CH2Cl2; 100 equiv of PO ([PO]0=1 M);
yield in PPO: 7bþ, 87%, 7cþ, 92%). The SEC data for 7b,cþ
exhibit a monomodal molecular weight distribution along
with significantly higher Mn values than those observed
with the 6b,cþ systems (7bþ, Mn = 7789, Mw/Mn = 1.74;
7cþ, Mn=8740, Mw/Mn=1.57). The higher Mn values and
the better conversion in PPO observed for 7b,cþ versus 6b,cþ
Al cations most likely reflect the better stability of the former
systems under the studied catalytic conditions. Overall, the
catalytic activity of 7b,cþ at room temperature compares
with that of (Salen)Al(thf)þ2 and (Salpen)Al(thf)þ2 Al cations,
although the molecular weight of the obtained PPO is much
higher with the Salen systems.3c
Reactivity of Phosphinophenolate Al Complexes with rac-
Lactide (rac-LA) and ε-Caprolactone (ε-CL). The reactivity of
cationic Al complexes toward cyclic esters such as rac-lactide
and ε-caprolactone has thus far been the subject of very few
studies despite the potential interest of such Lewis acidic and
electrophilic species as initiators of the ROP of cyclic esters.2c,4
In particular, while a couple of Al alkoxide cations of the type
{LX}Al(OR)(L)þ have been reported to be highly active in the
ROP of ε-CL,2c,4b the potential in such catalysis of Al cations
of the types {LX}Al(Me)(L)þ and {LX}2Alþ remains to be
explored. Thus, all phosphinophenolate Al cations were tested
in the ROP of rac-LA and ε-CL. While none of the synthesized
Al cations were found to initiate the ROP of rac-LA under the
studied conditions (100 equiv of rac-LA, toluene, 75 °C, 12 h),
they all, but cation 8cþ, readily and quantitatively polymerize
ε-CL to yield poly(ε-caprolactone) (100 equiv of ε-CL, toluene,
75 °C, 2 h), as summarized in Table 1. The SEC data for all
polymers exhibit monomodal molecular weight distributions
along with rather narrow polydispersities (Table 1), which
agrees with a relatively well-behaved polymerization process.
Nevertheless, in all cases, the Mn values are much higher than
those expected for a “living”-type polymerization proceeding in
ideal conditions and suggest that roughly only 30% of the initial
catalyst is actually involved in the polymerization process,
implying the presence of a rate-limiting initiation step prior to
chain propagation. To gain more insight into the nature of the
formed polymers, the identity of their end-groups was deter-
mined by NMR spectroscopy and MALDI-TOF spectrometry.
Thus, the reactions of cations 6b,cþ and 7b,cþ with 5 equiv of
ε-CL (toluene, 75 °C, 1 h) all yielded, after workup, the isolation
Table 1. Polymerization of ε-CL Initiated by the Al Cations 6b,cþ
and 7b,cþa
catalyst
conv (%)b
yield (%)
Mn(obsd)c
PDI
Mn(theor)d
6bþ
6cþ
7bþ
7cþ
>95
>95
>95
>95
84
78
91
95
37 700
36 500
32 100
37 400
1.33
1.24
1.31
1.35
11 400
11 400
11 400
11 400
a Polymerization conditions: toluene, 75 °C, 2 h, [ε-CL]0/Al=100,
[ε-CL]0=1 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)].17 d Calculated
for one growing chain per aluminum atom.
Scheme 4
of poly(ε-caprolactone) with a (phosphine oxide)phenolate
1
end-group (Scheme 4).15 In particular, the H NMR spectra
of all polymers contain one set of (phosphine oxide)phenolate
resonances, while the 31P NMR spectra exhibit only a single
sharp resonance (δ average 41.2) consistent with the presence of
a phosphine oxide moiety. In addition, the MALDI-TOF
spectrum recorded with the ε-PCL derived from the polymer-
ization of 100 equiv of ε-CL by cation 7bþ is also consistent with
a (phosphine oxide)phenolate end-group (presence of M-Naþ
and M-Kþ ions; see Supporting Information). Altogether, these
data agree with a ε-CL polymerization proceeding via an initial
insertion of ε-CL into the Al-O bond of the phosphinopheno-
late Al chelate, very likely to be the rate-limiting initiation step,
and subsequent faster insertions allowing the polymer chain to
grow. Notably, both cations 6b,cþ and 7b,cþ exhibit similar
catalytic activities, and the formed polymers are also closely
related, as deduced from SEC data and end-group analysis; this
suggests, in the case of the bis-phosphinolate Al cations 7b,cþ,
the presence of only one growing chain per metal center during
the polymerization process. In the present cationic systems, the
chelating phosphinophenolate moiety thus appears to act as
both the ancillary ligand and the initiating group, which is rather
uncommon. On that matter, these results may be related to
earlier observations on neutral bis-thiophenolate Al systems that
were found to be active in the ROP of rac-lactide and in which
the thiophenolate ancillary ligand is believed to also initiate the
polymerization, as deduced from end-group analysis.16
As a comparison, it is noteworthy that the neutral Al
complexes 2a-c and 3a,b were found to initiate the ROP of
both rac-LA and ε-CL (toluene, 75 °C, 12 h) to yield the
corresponding polyester materials albeit, in the case of ε-CL
polymerization, with an activity lower than that of the
cationic derivatives. Nevertheless, as deduced from SEC
data, in all cases, the molecular weight distribution is multi-
modal along with a wide polydispersity (ranging from 2.5
to 4), indicating an uncontrolled polymerization process.
(14) (a) Sugimoto, H.; Kawamura, C.; Kuroki, M.; Aida, T.; Inoue,
S. Macromolecules 1994, 27, 2013. (b) Vincens, V.; Le Borgne, A.; Spassky,
N. Makromol. Chem. Rapid Commun. 1989, 10, 623.
(15) The oxidation of the phosphine end-group presumably arises
from the workup conditions (MeOH with a few drops of CH3COOH).
Likewise, phosphinophenols 1a-c were found to readily and quantita-
tively oxidize when left a few hours in acidic MeOH medium.
(16) Huang, C.-H.; Wang, F.-C.; Ko, B.-T.; Yu, T.-L.; Lin, C.-C.
Macromolecules 2001, 34, 356.
(17) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci.,
ꢀ
ꢀ
Part A: Polym. Chem. 1993, 31, 505.

4590 Organometallics, Vol. 28, No. 15, 2009
Haddad et al.
Summary and Conclusion
concentration; 2,5-dihydroxybenzoic acid (DHB) was used as
the matrix in 5:1 volume ratio.
All the salt species were obtained as dissociated Al cations and
MeB(C6F5)-3 salts in solution. The NMR data for the MeB
(C6F5)-3 anion are listed below for all of the compounds.
The present work shows that bidentate O,P-phosphino-
phenolates, which combine a hard phenoxide donor with
a soft phosphine, are suitable ligands for coordination to
Al(III), as they readily form stable and monomeric mono-
and bis-adducts of the type {PO}AlMe2 and {PO}2AlX (X=
Me, Cl) provided a sufficient steric protection of the metal
center is achieved. While the formed {PO}AlMe2 and
{PO}2AlMe complexes are stable in the presence of Lewis
bases such THF, the bis-adduct Al species {PO}2AlX (X=
Me, Cl) disproportionate in the presence of an alcohol or an
alkoxide source, respectively, to yield the corresponding tris-
adduct {PO}3Al, thereby precluding access to Al alkoxide
species of the type {PO}2AlOR under the studied conditions.
Both {PO}AlMe2 and {PO}2AlMe react fast with B(C6F5)3
to yield the quantitative formation of stable and well-defined
cationic Al species of the type {PO}Al(Me)(THF)þ and
{PO}2Alþ, respectively, which are highly active in PO poly-
merization to yield atactic PPO via presumably a Lewis acid-
assisted cationic mechanism. These cations also readily
initiate the ROP of ε-CL via successive ring-opening inser-
tions of the monomer into the Al-OPhO bond of the phos-
phinophenolate chelating ligand to exclusively afford ε-PCL
capped, at the ester end, with a phosphinophenolate oxide
group. Thus, in these cationic systems, the {PO}- chelating
moiety may act as both a supporting ligand and an initiating
group for the ROP of ε-CL.
Data for MeB(C6F5)3-. H NMR (400 MHz, CD2Cl2): δ 0.48
1
(BMe). 11B{1H} NMR (128 MHz, CD2Cl2): δ -11.9 (br s, BMe).
13C{1H} NMR (100 MHz, CD2Cl2): δ 9.8 (br, BMe), 136.7
1
1
(d, JCF =233 Hz, m-C6F5), 137.9 (d, JCF = 238 Hz, p-C6F5),
148.6 (d, 1JCF=233 Hz, o-C6F5). 19F NMR (376 MHz, CD2Cl2):
δ -133.5 (d, 3JFF=19 Hz, 2F, o-C6F5), -165.7 (t, 3JFF = 20 Hz,
1F, p-C6F5), -168.2 (m, 3JFF = 19 Hz, 2F, m-C6F5).
Phosphinophenols 1a-c. The O,P pro-ligands 1a-c were all
synthesized according to known literature procedures, and the
NMR data for 1b matched those reported in the literature.6d,7
Data for 2-PPh2-6-Me-C6H3OH (1a):. 65% isolated yield.
Anal. Calcd for C19H17OP: C, 78.07; H, 5.87. Found: C, 77.93;
H, 5.76. 1H NMR (300 MHz, CDCl3): δ 2.17 (s, 3H, Me), 6.30
(m, 1H, PhO), 6.70 (m, 1H, PhO), 6.74 (m, 1H, PhO), 7.23 (m,
10H, PPh2). 13C{1H} NMR (75 MHz, CDCl3): δ 16.2 (Me),
120.1, 120.7, 124.5, 128.6, 128.9, 132.4, 132.9, 133.5, 135.2,
157.6. 31P{1H} NMR (121.5 MHz, C6D6): δ -31.1.
Data for 2-PPh2-4-Me-6-tBu-C6H2OH (1c):. 55% isolated
yield. Anal. Calcd for C23H25OP: C, 79.29; H, 7.23. Found: C,
78.86; H, 7.32. 1H NMR (300 MHz, C6D6): δ 1.51 (s, 9H, tBu),
1.96 (s, 3H, Me), 6.91 (m, 1H, PhO), 6.96-7.00 (m, 6H, Ph), 7.20
(m, 1H, Ph), 7.32-7.38 (m, 4H, Ph). 13C{1H} NMR (75 MHz,
C6D6): δ 20.7 (Me), 29.7 (tBu), 35.0 (tBu), 121.0, 128.8, 129.5,
130.5, 132.7, 133.6, 135.6, 136.3, 156.8 (one resonance is pre-
sumably obscured by the solvent peak). 31P{1H} NMR (121.5
MHz, C6D6): δ -32.0.
{η2-O,P-(2-PPh2-6-Me-C6H3O)}AlMe2 (2a), {η2-O,P-(2-PPh2-
6-Ph-C6H3O)}AlMe2 (2b), and {η2-O,P-(2-PPh2-4-Me-6-tBu-C6-
H2O)}AlMe2 (2c). The three phosphinophenolate aluminum di-
methyl complexes 2a-c were all synthesized using an identical
synthetic procedure and were isolated in good yields (2a, 78%;
2b, 73%; 2c, 82%). As an example, the experimental procedure for
2c is described here.
Experimental Section
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 going through drying columns (SPS appa-
˚
ratus, MBraun) and stored over activated molecular sieves (4 A)
for 24 h in a glovebox prior to use. CH2Cl2, CD2Cl2, and C6D6
were distilled from CaH2, degassed under a N2 flow, and stored
In a glovebox filled with N2, a pentane solution (5 mL) of
AlMe3 (52.6 mg, 0.729 mmol) and a 1:1 toluene/pentane solution
(5 mL) of 1c (254.0 mg, 0.729 mmol) are prepared in two separate
vials and are stored in a freezer at -35 °C for 1 h. After this time,
both vials were taken out and the phosphinophenol solution was
added via a pipet to the AlMe3 solution under vigorous stirring,
which provoked immediate methane bubbling. The resulting
colorless solution was allowed to warm to room temperature
and then stirred for a couple of hours, after which it was
evaporated to dryness to yield a colorless solid. Pure compound
2c was obtained as colorless crystals (242 mg, 82% yield) from a
1:1 Et2O/pentane solution stored overnight at -35 °C.
Data for 2a:. 78% yield (260 mg) from a Et2O solution stored
overnight at -35 °C. Anal. Calcd for C21H22AlOP: C, 72.40; H,
6.37. Found: C, 72.56; H, 6.19. 1H NMR (300 MHz, CD2Cl2):
δ -0.73 (d, 3JHP = 3.5 Hz, 6H, AlMe2), 2.13 (s, 3H, Me), 6.72-
6.86 (m, 2H, Ph), 7.20-7.23 (m, 1H, Ph), 7.35-7.45 (m, 10H,
Ph). 13C{1H} NMR (100 MHz, CD2Cl2): δ -9.2 (AlMe2), 16.5
(Me), 118.6 (o-PhO), 119.9 (p-PhO), 128.5 (Ph), 129.8 (Ph),
130.4 (Cquat), 130.6 (Cquat), 131.4 (Ph), 133.2 (Ph), 133.8 (Ph),
161.5 (Cipso-PhO). 31P{1H} NMR (121.5 MHz, C6D6): δ -27.8.
Data for 2b:. 73% yield (425 mg) from a Et2O solution stored
overnight at -35 °C. Anal. Calcd for C26H24AlOP: C, 76.09; H,
5.89. Found: C, 76.25; H, 5.86. 1H NMR (300 MHz, C6D6): δ -
0.23 (d, 3JHP=4.1 Hz, 6H, AlMe2), 6.76 (t, 3JHH=7.6 Hz, 1H,
˚
over activated molecular sieves (4 A) in a glovebox prior to use.
B(C6F5)3 was purchased from Strem and used as received.
[Ph3C][B(C6F5)4] was purchased from Asahi Glass Europe,
while 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
propylene oxide (PO) and ε-caprolactone (ε-CL), which were
distilled over CaH2 prior to use. NMR spectra were recorded on
Bruker AC 300 or 400 MHz NMR spectrometers, in Teflon-
valved J-Young NMR tubes at ambient temperature, unless
1
otherwise indicated. H and 13C chemical shifts are reported
versus SiMe4 and were determined by reference to the residual
1H and 13C solvent peaks. 11B, 19F, and 31P chemical shifts are
reported versus BF3-Et2O in CD2Cl2, neat CFCl3, and 85%
H3PO4 in aqueous solution, respectively. Elemental analyses for
all compounds were performed at the Services de Microanalyse
ꢀ
of the Universite Pierre et Marie Curie (Paris, France) and the
Universite de Strasbourg (Strasbourg, France). SEC analyses
ꢀ
were performed at the Institut 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 cal-
culated using polystyrene standards. In the case of molecular
weights, these were corrected with Mark-Houwink corrections
for Mn [Mn(obsd) = 0.56Mn(SEC)].16 MALDI-TOF mass
spectroscopic analyses were performed at the Service de Spectro-
Ph), 7.04-7.27 (m, 15H, Ph), 7.85 (d, 3JHH=7.8 Hz, 2H, Ph). 13
C
{1H} NMR (100 MHz, CD2Cl2): δ -9.2 (AlMe), 117.9, 126.4,
128.1, 128.8, 129.4, 130.0, 130.9, 131.6, 132.8, 133.3, 133.5,
134.2, 139.9, 160.3 (Cipso-PhO). 31P{1H} NMR (121.5 MHz,
CD2Cl2): δ -29.8.
ꢀ
metrie de Masse de l’Institut de Chimie de Strasbourg and run
in a positive mode, and samples were prepared by mixing a
solution of the polymers in CH2Cl2 with a 0.5 mg/100 mL
Data for 2c:. 82% yield (242 mg) from a 1:1 Et2O/pentane
solution stored overnight at -35 °C. Anal. Calcd for C24H28AlOP:

Article
Organometallics, Vol. 28, No. 15, 2009 4591
C, 73.83; H, 7.23. Found: C, 73.98; H, 7.19. 1H NMR (300 MHz,
C6D6): δ -0.18 (d, 3JHP=4.0 Hz, 6H, AlMe2), 1.61 (s, 9H, tBu),
2.01 (s, 3H, Me), 6.86-6.94 (m, 7H, Ph), 7.27-7.34 (m, 5H, Ph).
13C{1H} NMR (100 MHz, C6D6): δ -9.8 (AlMe2), 20.9 (Me), 29.8
(tBu), 35.6 (tBu), 115.5 (Cquat), 116.0 (Cquat), 127.3 (Cquat), 129.2
(Ph), 130.7 (Ph), 131.0 (Ph), 132.9 (Ph), 133.5 (Ph), 139.8 (Cquat),
165.0 (Cipso-PhO). 31P{1H} NMR (121.5 MHz, C6D6): δ -26.6.
{η2-O,P-(2-PPh2-6-Ph-C6H3O)}2AlMe (3b) and {η2-O,P-(2-
PPh2-4-Me-6-tBu-C6H2O)}2AlMe (3c). The bis-phosphinophe-
nolate aluminum dimethyl complexes 3b and 3c were both
synthesized using the same synthetic procedure and were iso-
lated in 65% and 61% yield, respectively. The experimental
procedure for 3c is described below.
In a glovebox filled with N2, a toluene solution (3 mL) of
AlMe3 (82.0 mg, 1.14 mmol) and a toluene solution (10 mL) of
phosphinophenol 1c (793.0 mg, 2.28 mmol) were prepared in
two separate vials and stored in a freezer at -35 °C for 1 h. After
this time, both vials were taken out, and the phosphinophenol
solution was added via a pipet to the AlMe3 solution under
vigorous stirring, which provoked immediate methane bub-
bling. The resulting colorless solution was allowed to warm to
room temperature and then stirred for a couple of hours, after
which it was evaporated to dryness to yield a colorless foam.
Addition of pentane and subsequent trituration provoked the
precipitation of a colorless solid, which was filtered through
a glass frit. Drying of the latter solid in vacuo afforded pure 3c
(510 mg, 61% yield).
Data for 3b:. 65% yield (452 mg) in a similar manner to that used
for compound 3c. Anal. Calcd for C49H39AlO2P2: C, 78.60; H, 5.25.
Found: C, 78.93; H, 5.02. 1H NMR (300 MHz, CD2Cl2): δ -0.60
(t, 3JHP=4.1 Hz, 3H, AlMe), 6.89 (t, 3JHH = 7.6 Hz, 2H, Ph), 7.14-
7.41 (m, 30H, Ph), 7.65 (d, 3JHH=7.6 Hz, 4H, Ph). 13C{1H} NMR
(100 MHz, CD2Cl2): δ -9.9 (AlMe), 118.9, 126.5, 127.5, 127.8,
128.4, 128.6, 129.4, 129.5, 132.8, 133.3, 133.5, 133.7, 139.8, 155.2
(Cipso-PhO). 31P{1H} NMR (121.5 MHz, CD2Cl2): δ -35.3.
Data for 3c:. 61% yield. Anal. Calcd for C47H51AlO2P2: C,
76.61; H, 6.98. Found: C, 76.95; H, 6.89. 1H NMR (300 MHz,
C6D6): δ -0.14 (t, 3JHP=3.8 Hz, 3H, AlMe), 1.64 (s, 18H, tBu),
2.05 (s, 6H, Me), 6.92-6.97 (m, 12H, Ph), 7.01 (d, 4JHH=1.5 Hz,
2H, PhO), 7.31 (d, 4JHH=1.5 Hz, 2H, PhO), 7.53-7.57 (m, 8H,
Ph). 13C{1H} NMR (100 MHz, CD2Cl2): δ -8.3 (AlMe), 21.0
(Me), 30.1 (tBu), 35.5 (tBu), 119.7 (Cquat), 119.9 (Cquat), 128.9
(Ph), 129.6 (Ph), 131.6 (Ph), 131.9 (Ph), 133.0 (Cquat), 133.8 (Ph),
139.6 (Cquat), 162.8 (Cipso-PhO). 31P{1H} NMR (121.5 MHz,
C6D6): δ -31.2.
13C{1H} NMR (100 MHz, CD2Cl2): δ 20.9 (Me), 30.0 (tBu), 35.2
(tBu), 118.9 (Cquat), 119.4 (Cquat), 128.9 (Ph), 129.3 (Cquat), 130.1
(Ph), 130.6 (Ph), 132.1 (Ph), 134.3 (Ph), 133.8 (Ph), 139.2 (Cquat),
161.9 (Cipso-PhO). 31P{1H} NMR (121.5 MHz, C6D6): δ -36.5.
{η2-O,P-(2-PPh2-4-Me-6-tBu-C6H2O)}3Al (5c). In an attempt
to prepare the Al-OiPr complex {η2-O,P-(2-PPh2-6-tBu-C6H2O)}2-
AlOiPr by reaction between the phosphinophenol 1c and Al(OiPr)3,
the tris-phosphinophenolate Al complex 5c was instead prepared,
as described below.
NMR-Scale Reaction. In a nitrogen-filled glovebox, 2 equiv of
compound 1c (30.0 mg, 0.0861 mmol) and 1 equiv of Al(OiPr)3
(8.8 mg, 0.043 mmol) were charged in a J-young NMR tube and
dissolved in 0.75 mL of C6D6 to yield a pale yellow solution.
While no reaction occurred at room temperature after 18 h, as
deduced from NMR monitoring, the concomitant formation of
the Al-OiPr complex {η2-O,P-(2-PPh2-6-tBu-C6H2O)}2AlOiPr
and the tris-adduct 5c was observed upon heating the reaction
mixture at 80 °C. Compound 5c remained the major product
all along the reaction, and complete conversion (along with
unidentified organoaluminum species) was observed after 18 h
at 80 °C.
Preparative-Scale Reaction. Two equivalents of compound 1c
(300.0 mg, 0.861 mmol) and 1 equiv of Al(OiPr)3 (87.8 mg,
0.430 mmol) were charged in a 10 mL vial sample equipped with
a Teflon-capped screw-cap and dissolved in 5 mL of toluene.
The resulting pale yellow solution was heated to 80 °C for 12 h,
after which it was stored at -35 °C, which allowed the crystal-
lization of compound 5c as a colorless solid in an analytically
pure form (255 mg, 83% yield). Anal. Calcd for C69H72AlO3P3:
C, 77.51; H, 6.79. Found: C, 77.32; H, 6.85. 1H NMR (300 MHz,
C6D6): δ 1.75 (s, 27H, tBu), 1.89 (s, 9H, Me), 6.49 (br m, 6H, Ph),
6.72-6.94 (m, 21H, Ph), 7.34 (d, 4JHH=1.5 Hz, 3H, PhO), 7.51
(br m, 6H, Ph). 13C{1H} NMR (100 MHz, CD2Cl2): δ 20.2 (Me),
29.6 (tBu), 34.9 (tBu), 118.0 (Cquat), 118.4 (Cquat), 128.9 (Ph),
129.6 (Ph), 131.6 (Ph), 131.9 (Ph), 132.9 (Cquat), 134.7 (Ph),
138.1 (Cquat), 162.7 (Cipso-PhO). 31P{1H} NMR (121.5 MHz,
C6D6): δ -30.4.
[{η2-O,P-(2-PPh2-6-Ph-C6H3O)}Al(Me)(THF)][MeB(C6F5)3]-
([6b][MeB(C6F5)3]) and [{η2-O,P-(2-PPh2-4-Me-6-tBu-C6H2O)}Al-
(Me)(THF)][MeB(C6F5)3] ([6c][MeB(C6F5)3]). In a nitrogen-filled
glovebox, an equimolar amount of the neutral dimethyl Al pre-
cursor 2b or 2c (2b, 80.2 mg; 2c, 79.0 mg, 0.195 mmol) and B(C6F5)3
(100.0 mg, 0.195 mmol) were charged in a small Schlenk flask
and dissolved in THF (3 mL) to yield a colorless solution, which
was stirred at room temperature for 1 h. The resulting color-
less solution was evaporated to dryness to afford a pale yellow
foam, to which cold pentane was added, causing the formation of a
colorless solid. Subsequent drying in vacuo of the latter solid and
NMR analysis identified it as the salt species [6b][MeB(C6F5)3]
(145 mg, 75% yield) or [6c][MeB(C6F5)3] (158 mg, 82% yield) in a
pure form.
Data for [6b][MeB(C6F5)3]. Anal. Calcd for C48H32AlB-
F15O2P: C, 57.97; H, 3.24. Found: C, 58.23; H, 3.56. 1H NMR
(300 MHz, CD2Cl2): δ -0.21 (d, 3JHP=5.0 Hz, 3H, AlMe), 1.99
(br, 4H, THF), 4.21 (br, 4H, THF), 6.75 (t, 3JHH=7.6 Hz, 1H,
Ph), 7.32-7.58 (m, 15H, Ph), 7.84 (d, 3JHH=7.6 Hz, 2H, Ph). 13C-
{1H} NMR (100 MHz, CD2Cl2): δ -6.2 (AlMe), 24.3 (THF),
70.5 (THF), 117.9, 126.1, 126.9, 127.4, 128.6, 129.0, 129.4, 129.9,
132.9, 133.6, 134.0, 134.7, 139.5, 156.2 (Cipso-PhO). 31P{1H}
NMR (121.5 MHz, CD2Cl2): δ -38.1.
Data for [6c][MeB(C6F5)3]. Anal. Calcd for C47H38AlB-
F15O2P: C, 57.10; H, 3.87. Found: C, 57.63; H, 4.12. 1H NMR
(300 MHz, CD2Cl2): δ -0.12 (d, 3JHP=5.4 Hz, 3H, AlMe), 1.47
(s, 9H, tBu), 1.94 (4H, THF), 2.24 (s, 3H, Me), 4.16 (4H, THF),
6.91 (m, 1H, Ph), 7.34 (br s, 1H, Ph), 7.33-7.56 (m, 10H, Ph).
13C{1H} NMR (100 MHz, CD2Cl2): δ -7.1 (AlMe), 22.0 (Me),
27.3 (THF), 30.1 (tBu), 36.2 (tBu), 72.4 (THF), 116.5 (Cquat),
116.9 (Cquat), 128.3 (Cquat), 129.9 (Ph), 131.7 (Ph), 132.0 (Ph),
132.9 (Ph), 133.8 (Ph), 142.3 (Cquat), 162.4 (Cipso-PhO). 31P{1H}
NMR (121.5 MHz, CD2Cl2): δ -34.8.
{η2-O,P-(2-PPh2-4-Me-6-tBu-C6H2O)}2AlCl (4c). In a nitro-
gen-filled glovebox, nBuLi (750.0 μL of a 1.6 M pentane solution,
1.20 mmol) was added dropwise via a syringe to a precooled
(-35 °C) pentane solution (5 mL) of phosphinophenol 1c. After
the addition was completed, the resulting pale yellow solution was
stirred for 1 h at room temperature, after which it was evaporated
to dryness to afford an off-white solid residue. The latter solid was
subsequently washed twice with cold pentane to yield the pre-
sumed phosphinophenolate Li salt [2-PPh2-4-Me-6-tBu-C6H3O]Li
(223 mg, 0.663 mmol) as a colorless solid, which was used as is.
Thus, the latter Li salt was dissolved in 5 mL of toluene, and the
resulting solution was added all at once (via a pipet) to a precooled
(-35 °C) 1:1 THF/toluene solution (3 mL) of 0.5 equiv of AlCl3
(42.0 mg, 0.330 mmol). The initially pale yellow solution became
cloudy upon warming to room temperature due to the formation
of LiCl. The reaction mixture was stirred at room temperature
overnight, after which the volatiles were removed in vacuo to yield a
colorless solid. Toluene was then added, and the resulting suspen-
sion was filtered through a glass frit; subsequent evaporation of the
mother liquor afforded crude 4c, which was isolated in pure form
from a Et2O solution stored at -35 °C overnight (249 mg, 69%
yield). Anal. Calcd for C46H58AlClO2P2: C, 72.96; H, 6.39. Found:
C, 72.55; H, 6.55. 1H NMR (300 MHz, C6D6): δ 1.51 (s, 18H, tBu),
2.03 (s, 6H, Me), 6.95-7.00 (m, 12H, Ph), 7.1 (d, 4JHH=1.5 Hz,
2H, PhO), 7.28 (d, 4JHH=1.5 Hz, 2H, PhO), 7.82 (br m, 8H, Ph).

4592 Organometallics, Vol. 28, No. 15, 2009
Haddad et al.
[{η2-O,P-(2-PPh2-6-Ph-C6H3O)}2Al][MeB(C6F5)3] ([7b][MeB-
(C6F5)3]) and [{η2-O,P-(2-PPh2-4-Me-6-tBu-C6H2O)}2Al][MeB-
(C6F5)3] ([7c][MeB(C6F5)3]). In a nitrogen-filled glovebox, an
equimolar amount of the neutral bis-phosphinophenolate methyl
Al precursor 3b or 3c (3b, 146.0 mg; 3c, 143.7 mg, 0.195 mmol) and
B(C6F5)3 (100.0 mg, 0.195 mmol) were charged in a small Schlenk
flask and dissolved in CH2Cl2 (3 mL) to yield a colorless solution,
which was stirred at room temperature for 1 h. The resulting
colorless solution was evaporated to dryness to afford a pale yellow
foam, to which cold pentane was added, causing the formation of a
colorless solid. Subsequent drying in vacuo of the latter solid
and NMR analysis identified it as the salt species [7b][MeB-
(C6F5)3] (175 mg, 71% yield) or [7c][MeB(C6F5)3] (146 mg, 60%
yield) in a pure form.
129.7 (Cquat), 129.9 (Ph), 130.1 (Ph), 131.1 (Ph), 132.9 (Ph),
133.8 (Ph), 138.2 (Cquat), 145.1 (Cipso-PhN), 160.3 (Cipso-PhO).
31P{1H} NMR (121.5 MHz, CD2Cl2): δ -37.3.
Typical Procedure for ε-Caprolactone Polymerization. In a
nitrogen-filled glovebox, the Al initiator (0.0150 mmol) was
charged in a 5 mL sample vial (equipped with a magnet stirring
bar) and dissolved in 1.50 mL of toluene. One hundred equiva-
lents of ε-caprolactone (172.6 mg, 1.50 mmol) was then added;
the sample was tightly closed with a Teflon-tight screw-cap, and
the mixture was heated to the appropriate temperature for the
desired time. After this time, an aliquot was then taken and an
1H NMR spectrum was then recorded to determine the mono-
mer conversion. The overall reaction mixture was quenched
with MeOH and a few drops of acetic acid, provoking the
precipitation of poly(ε-caprolactone) as a colorless solid. This
solid was washed several times and dried in vacuo until constant
weight, after which it was subjected to NMR and SEC analysis
as well as, in one case, to MALDI-TOF mass spectrometry.
Typical Procedure for Propylene Oxide Polymerization. In a
nitrogen-filled glovebox, the Al initiator (0.0150 mmol) was
charged in a 5 mL sample vial (equipped with a magnet stirring
bar) and dissolved in 1.50 mL of toluene. One hundred equiva-
lents of propylene oxide (88 mg, 1.50 mmol) was then added; the
sample was tightly closed with a Teflon-tight screw-cap, and the
mixture was vigorously stirred for the desired time, after which it
was quenched with MeOH and a few drops of an aqueous HCl
solution (0.1 M) and evaporated to dryness to yield a pale yellow
oily residue. CH2Cl2 (2 mL) was added, and the resulting cloudy
suspension was filtered to remove Al hydroxide residues. The
filtrate was evaporated to yield a colorless oil that revealed to be
Data for [7b][MeB(C6F5)3]. Anal. Calcd for C67H39AlB-
F15O2P2: C, 63.83; H, 3.12. Found: C, 64.25; H, 3.33. 1H
3
NMR (300 MHz, CD2Cl2): δ 6.96 (t, JHH=7.9 Hz, 2H, Ph),
7.35-7.69 (m, 30H, Ph), 8.12 (d, 3JHH = 7.8 Hz, 4H, Ph). 13C-
{1H} NMR (100 MHz, CD2Cl2): δ 117.0, 124.3, 125.7, 127.8,
128.2, 129.0, 129.5, 130.7, 133.9, 134.8, 135.0, 136.7, 139.5, 154.7
(Cipso-PhO). 31P{1H} NMR (121.5 MHz, CD2Cl2): δ -38.1.
NMR data for [7c][MeB(C6F5)3]. Anal. Calcd for C65H51AlB-
F15O2P2: C, 62.51; H, 4.12. Found: C, 62.73; H, 4.22. 1H NMR
(300 MHz, CD2Cl2): δ 1.45 (s, 18H, tBu), 2.28 (s, 6H, Me), 7.03
(m, 2H, Ph), 7.42-7.48 (m, 18H, Ph), 7.62 (m, 4H, Ph). 13C{1H}
NMR (100 MHz, CD2Cl2): δ 20.2 (Me), 28.8 (tBu), 34.9 (tBu),
111.7 (Cquat), 120.7 (Cquat), 129.7 (Ph), 130.0 (Ph), 130.1 (Ph),
131.5 (Cquat), 132.9 (Ph), 135.0 (Ph), 139.9 (Cquat), 160.4 (Cipso
-
PhO). 31P{1H} NMR (121.5 MHz, CD2Cl2): δ -36.3.
Generation of [{η2-O,P-(2-PPh2-4-Me-6-tBu-C6H2O)}2Al(N-
Me2Ph)][MeB(C6F5)3] ([8c][MeB(C6F5)3]). In a nitrogen-filled
glovebox, a J-young NMR tube was charged with the salt
species [7c][MeB(C6F5)3] (40 mg, 0.032 mmol) and dissolved in
0.75 mL of CD2Cl2. N,N-Dimethylaniline (4.1 μL, 0.032 mmol)
was then syringed in the NMR tube. The tube was tightly capped
and vigorously shaken, and a 1H NMR spectrum was immedi-
ately recorded showing the quantitative formation of the Al-
NMe2Ph cationic adduct {η2-O,P-(2-PPh2-4-Me-6-tBu-C6H3-
O)}Al(NMe2Ph)þ (8cþ).
1
atactic poly(propylene oxide) (PPO), as deduced from H and
13C{1H} NMR analysis. All PPO samples were analyzed by
SEC.
Acknowledgment. The authors thank the CNRS (Centre
ꢀ
National de la Recherche Scientifique) and the Universite de
Strasbourg for financial support.
NMR data for 8cþ. 1H NMR (300 MHz, CD2Cl2): δ 1.33 (s,
18H, tBu), 2.28 (s, 6H, Me), 2.90 (s, 6H, NMe), 6.92-6.99 (m,
6H, Ph), 7.11-7.18 (m, 10H, Ph), 7.27 (br, 6H, Ph), 7.38 (s, 2H,
Ph), 7.45-7.51 (m, 5H, Ph). 13C{1H} NMR (100 MHz,
CD2Cl2): δ 20.2 (Me), 29.6 (tBu), 34.7 (tBu), 49.6 (br, NMe),
113.7 (Cquat), 114.3 (Cquat), 120.0 (Ph), 128.0 (Ph), 129.1 (Ph),
Supporting Information Available: Crystallographic data for
compounds 2c, 3b, 3c, 5c, and 7c and an ORTEP drawing of the
Al complex 3b as well as the MALDI-TOF spectrum of the
ε-PCL obtained by ROP of ε-CL initiated by cation 7bþ. This
material is available free of charge via the Internet at http://
pubs.acs.org.