β-Diketiminate aluminium complexes: Synthesis, characterization and ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1039/b802638f

A series of aluminium alkyl complexes (BDI)AlEt₂ (3a-m) bearing symmetrical or unsymmetrical β-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding β-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize β-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex "(BDI-2a)AlMeCl" (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe]₂(μ-O) (7) as characterized by X-ray diffraction methods. Complexes 3a-m and [(2,6-ⁱPr₂C₆H₃NCMe) ₂HC]AlEt₂ (8) were found to catalyze the ring-opening polymerization (ROP) of ε-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (M_η) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of ε-caprolactone initiated by complexes 3a-m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66-3.74, M_w/M_n) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b802638f

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PAPER
www.rsc.org/dalton | Dalton Transactions
b-Diketiminate aluminium complexes: synthesis, characterization and
ring-opening polymerization of cyclic esters†
Shaogang Gong and Haiyan Ma*
Received 15th February 2008, Accepted 11th April 2008
First published as an Advance Article on the web 16th May 2008
DOI: 10.1039/b802638f
A series of aluminium alkyl complexes (BDI)AlEt2 (3a–m) bearing symmetrical or unsymmetrical
b-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the
corresponding b-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by
an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the
nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral
geometry. Attempt to synthesize b-diketiminate aluminium alkoxide complexes by the reactions of
monochloride complex “(BDI-2a)AlMeCl” (4) with alkali salts of 2-propanol gave unexpectedly an
aluminoxane [(BDI-2a)AlMe]2(l-O) (7) as characterized by X-ray diffraction methods. Complexes
3a–m and [(2,6-iPr2C6H3NCMe)2HC]AlEt2 (8) were found to catalyze the ring-opening polymerization
(ROP) of e-caprolactone with moderate activities. The steric and electronic characteristics of the
ancillary ligands have a significant influence on the polymerization performance of the corresponding
aluminium complexes. The introduction of electron-donating substituents at the para-positions of the
aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the
highest activity among the investigated aluminium complexes due to the high electrophilicity of the
metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric
hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a
decrease in the catalytic activity. Although the viscosity average molecular weights (Mg) of the obtained
poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of
e-caprolactone initiated by complexes 3a–m and 8 were not well-controlled, as judged from the broad
molecular weight distributions (PDI = 1.66–3.74, Mw/Mn) of the obtained polymers and the nonlinear
relationship of molecular weight versus monomer conversion.
initiating living ring-opening polymerization, producing polymers
with well-controlled molecular weight and narrow molecular
Introduction
Poly(e-caprolactone) (PCL) and poly(lactide) (PLA) as well as
weight distribution; (2) stereocontrolled polymerization of chiral
their copolymers are the most important synthetic biodegradable
monomers such as lactides, for example Salen or Salen supporting
polymers, and have attracted considerable attention mainly due to
their biomedical and pharmaceutical applications.1–6 In industry
PCL and PLA are synthesized by ring-opening polymerizations
aluminium complexes exhibit excellent stereoselectivity as well
as reasonable catalytic activities in the polymerization of rac-
lactide and meso-lactide.7–11 Besides the aluminium bis(phenolate)
(ROPs) using tin(II) bis(2-ethylhexanoate) (SnOct2) as catalyst.
systems, aluminium amide complexes have also proved to be
active in the ROP of cyclic esters. Chakraborty and Chen12
synthesized neutral three-coordinate chelating diamide aluminium
complexes, which produced telechelic PCLs with high molecular
weights up to 1.21 × 106 Da. Comparatively, aluminium complexes
bearing purely nitrogen-containing polydentate ligands and their
application in ROP of cyclic esters are still less investigated.
Nevertheless, nitrogen-containing ligands prove to benefit other
metals well. Coates et al. and Gibson et al. reported a series
of zinc and magnesium complexes supported by b-diketiminate
ligands, with zinc b-diketiminate complexes polymerizing rac-
lactide to heterotactic PLA (Pr = 0.94 at 0 ◦C),13 and meso-lactide
to syndiotactic PLA (Pr = 0.76 at 0 ◦C).13 On the other hand, the
magnesium b-diketiminate complexes were extremely active for the
polymerization of rac-lactide, polymerizing500equiv. of monomer
Although Sn(Oct)2 has been accepted as a food additive by the
U.S. FDA, the toxicity associated with most tin compounds is
a considerable drawback in the case of biomedical applications.5
So researchers all over the world endeavor to explore novel well-
defined catalysts which possess the characteristics of good biocom-
patibility, high catalytic activity and excellent stereoselectivity.
In recent years, aluminium bis(phenolate)-based complexes
have proved to be efficient for the ROP of cyclic esters in the
presence of alcohol, mainly due to their great success in: (1)
Laboratory of Organometallic Chemistry, East China University of Science
and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China.
E-mail: haiyanma@ecust.edu.cn; Fax: +86 21 64253519; Tel: +86 21
64253519
† Electronic supplementary information (ESI) available: Kinetics of e-
caprolactone polymerization initiated with complexes 3a and 3e–3g.
CCDC reference numbers 678287 and 628288 for 3k and 7. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b802638f
◦
up to 96% conversion in less than 5 min at 20 C.14 Based on
the success with zinc and magnesium metals, Chisholm’s group15
synthesized b-diketiminate calcium complexes, which showed high
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activity but hardly any stereoselectivity in the ROP of rac-
lactide.
Aluminium complexes with b-diketiminate ligands have been re-
ported extensively.16–36 To the best of our knowledge, the studies of
these complexes described in the literature are however limited to
rearrangement reactions,16–20 being employed as cocatalysts/active
species for olefin polymerizations,21–24 synthetic methodologies,25–36
and have not yet been extended to act as initiators for the
polymerization of cyclic esters; furthermore, nearly all of the b-
diketiminate ligands involved possess a symmetrical structure.37
b-diketimines as versatile ligands, allowing easy modulation of
steric and electronic factors by varying the amide moieties,14,38–44
would be ideal ligand sets for the design of novel aluminium-
based catalysts/initiators for the polymerization of cyclic esters.
It is conceivable that b-diketiminate aluminium complexes will
possess catalytic activity for the ROP of cyclic esters due to the
high Lewis acidity of the metal center that accounts for effective
monomer activation via r-bond coordination.12 As a monoanionic
bidentate ligand, b-diketiminate ligands tend to adopt a planar
configuration when bonded to a metal atom, and construct an
achiral metal center for most of the divalent or trivalent metals
if the b-diketiminate itself is symmetric. Therefore it is desired to
introduce some unsymmetrical features into the ligand framework
in order to construct metal complexes capable of initiating
stereoselective polymerization of chiral monomers. Herein we
report on the synthesis of a series of aluminium diethyl complexes
ligated by symmetrical or unsymmetrical b-diketiminate ligands,
and their catalytic behavior for ROPs of cyclic esters. The steric
and electronic effects of ligands on the polymerization of e-
caprolactone were explored. As far as we know, this is the first time
that b-diketiminate aluminium complexes have been reported as
initiators for the ring-opening polymerization of cyclic esters.
Scheme 1
ethane. Pale yellow crystals were obtained in each case in moderate
to good yields after recrystallization from aliphatic solvents.
Similar two-step condensation reactions were adopted to
synthesize the unsymmetrical b-diketimines. Successful synthesis
was limited to the reactions of 4-(2,6-diisopropylphenyl)amino-
3-penten-2-one (1i) with the corresponding aromatic amines
(Scheme 2). The condensation of enaminoketones having smaller
ortho-substituents than 2-propyl with the studied primary aro-
matic amines led to mixtures of all three possible b-diketiminates,
which however are too similar to be separated from one another.
Similar results have been reported by Park and Marshall.42
Possibly a steric bulky group such as 2-propyl is essential to create
sufficient space encumbrances to restrain the exchange reactions
between the two different amine moieties involved in the structure.
In contrast to symmetrical b-diketimines, the pretreatment of
aromatic amine with para-toluenesulfonic acid is not necessary
for the synthesis of unsymmetrical ligands 2i–m, most likely due
to the presence of bulky 2-propyl groups which block the reaction
of 1i with para-toluenesulfonic acid. Aluminium complexes 3i–m
Results and discussion
Synthesis of b-diketiminate aluminium complexes
With the aim of unravelling the effect of the catalyst/initiator
structure parameters on polymerization activity, various sub-
stituents such as alkyl, alkoxy, halide were adopted to obtain
b-diketiminate ligands containing N-aryl moieties with differing
electron-donating abilities. As depicted in Scheme 1, the symmet-
rical b-diketimines 2a–h were synthesized in two steps using classic
literature procedures:14 (1) condensation of 2,4-pentanedione with
one equiv. of primary aromatic amine in toluene, with para-
toluenesulfonic acid as catalyst, afforded the enaminoketone
intermediate 1; (2) another equiv. of the same aromatic amine
was pre-treated with para-toluenesulfonic acid in a 1 : 1 ratio for
3 h to afford para-toluenesulfonate, which was then reacted with
the corresponding enaminoketone 1 to give b-diketimines 2a–
h in reasonable yields. In the second step, the one-pot reaction
of enaminoketone, aromatic amine and para-toluenesulfonic acid
failed to give any target product; the reaction of enaminoketone
with the para-toluenesulfonic acid took place instead, preventing
further reaction with the aromatic amine. The reactions of ligands
2a–h with triethyl aluminium in a 1 : 1 molar ratio in n-hexane
readily generated the four-coordinated aluminium complexes 3a–
h (Scheme 1), proceeding along with the elimination of 1 equiv. of
Scheme 2
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with unsymmetrical b-diketimine were synthesized accordingly via
the reactions of ligands 2i–m with triethyl aluminium.
1
The H NMR spectra indicate that, for aluminium complexes
3a–d, 3g–h the chemical environments of both aromatic rings as
well as the two backbone methyl groups are identical. In contrast,
aluminium complexes 3i–m with unsymmetrical b-diketiminate
ligands exhibit different resonances for them in 1H NMR spectra.
The chemical shifts of the c-CH in complexes 3a–h are about
4.9 ppm, almost the same as those in the neutral b-diketimines.
For complexes 3i–m, the chemical shifts of the c-CH move to lower
field significantly. One signal is displayed for the methine protons
of 2-propyl groups in complexes 3i–m, but restricted rotation
about the N–aryl bonds gives rise to two separated doublets for
the –CH(CH3)2 methyl groups. In addition, 1H NMR spectra
of aluminium complexes 3i–m reveal two diastereotopic methene
protons in each ethyl group (Al-CH2CH3) which resonate at about
d −0.37 ppm as two separate multipeaks of dq mode,45 suggesting
that the bulky ortho-2-propyl substituents restrict the free rotation
of the phenyl moiety on the NMR time scale.45,46
Scheme 4
Alumoxanes as potential active catalysts and cocatalysts for
the polymerization of a wide range of organic monomers have
attracted much attention from chemists.50–53 In most of the cases,
the alumoxanes were prepared by stoichiometric hydrolysis of
aluminium alkyls or hydride with water or water contained in hy-
drated metal salts.34,36,54,55 Reaction of aluminium alkyls or hydride
with reactive oxygen-containing inorganic or organic substrates,
such as alkali metal oxides,56 anhydrous lithium hydroxide,57
siloxanes,58 or carboxylate acid,59 also provided effective synthetic
approaches to well-characterized alumoxanes.60,61 More recently,
Roesky and co-workers reported the stoichiometric hydrolysis of
the aluminium–halide bond in aluminium b-diketiminate com-
plexes LAlMeCl to afford aluminoxanes,22,24 however a compli-
cated system of KOH/H2O/KH in liquid ammonia and toluene,
or a strong nucleophilic reagent such as N-heterocyclic carbene
acting as hydrogen halide acceptor, are essentially required to
fulfill the transformation under mild conditions.34,62–64 Simple
treatment of LAlMeCl with water in THF only led to complete
hydrolysis yielding an insoluble aluminium oxide or hydroxide
under elimination of b-diketimine.64 In view of the selective
formation of complex 7 in good yield in this work, the possibility
that it was produced by the contamination of moisture was
therefore excluded. Although the mechanism is not clear at the
present stage, it is conceivable that the alkali salt may take part
in the reaction and play an important role. As we know, this
is the first example that an alkali salt of an alcohol may have
promoted the selective transformation of aluminium monohalide
to an alumoxane complex.
Further attempts to synthesize the corresponding alkoxide com-
plexes were carried out by reacting the b-diketiminate aluminium
diethyl complex 3e with 2-propanol in toluene. No reaction
occurred even under reflux conditions for long time, or using excess
2-propanol. The replacement of 2-propanol with the more acidic
benzyl alcohol did not work either. Therefore an alternative salt
metathesis route of treating a monochloride aluminium complex
with LiOiPr was designed. In the first step b-diketimine 2a was
reacted with Me2AlCl to synthesize the monochloride aluminium
complex (BDI-2a)AlMeCl (4). However, the reaction gave a
mixture of monochloride complex (4) and dimethyl complex (5)
in about a 1 : 2 molar ratio, which could hardly be separated
from each other (Scheme 3). The elimination of hydrogen chloride
seems even faster under the reaction conditions, possibly due to
the presence of b-diketimine acting as a base. In consequence
of this fact, a two-step salt metathesis route was adopted as
outlined in Scheme 4. Monochloride aluminium complex 447
could be obtained in moderate yield from the reaction of the
lithium salt of 2a and MeAlCl2; the sequential treatment of 4
with LiOiPr gave a white solid after workup, however it was
1
shown to be a mixture by H NMR spectroscopy. In addition
to one set of resonances assignable to “[(BDI-2a)AlMe(OiPr)]”
(6),48 another set of resonances accounting for an unknown
structure was also displayed, which was later characterized as
an aluminoxane complex [(BDI-2a)AlMe]2(l-O) (7) by an X-ray
diffraction study (vide infra). Due to the similar solubility of 6
and 7, further separation processes failed to give analytically pure
products. In consideration of the better reactivity of the sodium
salt than the lithium one, NaOiPr was reacted with complex 4. This
strategy however could not improve the reaction to give aluminium
isopropoxide 6; instead, unexpectedly, only complex 7 was isolated
as colorless crystals in high yield.49
Crystal structure of aluminium complexes 3k and 7
Single crystals of aluminium complexes 3k and 7 suitable for X-
ray diffraction measurement were obtained by slowly cooling a
saturated n-hexane solution to −20 ◦C and 0 ◦C respectively.
Scheme 3
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Table 1 Crystal data and structure refinement details for 3k and 7
3k
7
Empirical formula
Formula weight
T/K
C27H38AlClN2
453.02
293(2)
0.71073
C36H40Al2N4O
598.68
293(2)
˚
Wavelength/A
0.71073
Crystal size/mm
Crystal system
Space group
0.50 × 0.49 × 0.25
0.48 × 0.36 × 0.30
Orthorhombic
Pbca
15.5268(11)
19.5571(15)
22.6748(17)
90.00
90.00
90.00
6885.4(9)
8
1.155
Monoclinic
P2(1)/n
12.3283(13)
10.2992(10)
21.520(2)
90.00
93.270(2)
90.00
2727.9(5)
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
Volume/A
Z
4
Calcd density/Mg m−3
Absorp coeff/mm−1
F(000)
1.103
0.188
976
1.86 to 27.00
0.117
2544
1.80 to 27.00
h range for data collection/◦
Limiting indices
−15<= h <= 14, −6<= k <= 13, −27<= l <= 25
15695/5952 [R(int) = 0.1071]
1.00000 and 0.76883
5952/0/288
−19<= h <= 16, −23<= k <= 24, −28<= l<= 28
38829/7492 [R(int) = 0.0968]
1.00000 and 0.75196
7492/0/395
0.789
R1 = 0.0512, wR2 = 0.1159
R1 = 0.1337, wR2 = 0.1356
0.319 and −0.185
Reflns collected/unique
Max. and min. transmn
Data/restrains/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R Indices (all data)
0.882
R1 = 0.0550, wR2 = 0.1364
R1 = 0.0932, wR2 = 0.1505
0.266 and −0.204
−3
˚
Largest diff. peak and hole/e A
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for complex 3k
Al–N(1)
Al–C(24)
1.9062(16)
1.960(2)
Al–N(2)
Al–C(26)
1.9223(16)
1.964(2)
N(1)–C(1)
C(1)–C(2)
1.334(3)
1.386(3)
N(2)–C(3)
C(2)–C(3)
1.329(2)
1.390(3)
C(1)–C(4)
N(1)–C(6)
N(1)–Al–N(2)
N(1)–Al–C(24)
N(1)–Al–C(26)
1.506(3)
1.441(2)
94.50(7)
113.48(10)
105.58(9)
C(3)–C(5)
1.508(3)
1.454(2)
115.59(11)
112.68(9)
112.90(9)
N(2)–C(12)
C(24)–Al–C(26)
N(2)–Al–C(24)
N(2)–Al–C(26)
◦
˚
Table 3 Selected bond distances (A) and angles ( ) for complex 7
Al(1)–N(1)
Al(1)–N(2)
1.904(2)
1.912(2)
Al(2)–N(3)
Al(2)–N(4)
1.909(2)
1.902(2)
Fig. 1 ORTEP diagram of the molecular structure of (BDI-2k)AlEt2 (3k).
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity.
Al(1)–O(1)
1.6797(18)
1.942(3)
94.69(10)
168.90(12)
Al(2)–O(1)
Al(2)–C(36)
N(3)–Al(2)–N(4)
1.6807(18)
1.934(3)
95.46(11)
Al(1)–C(18)
N(1)–Al(1)–N(2)
Al(1)–O–Al(2)
C2–C3–N2 backbone of the ligand in complex 3k is essen-
tially planar with the aluminium center deviating from it by
˚
Crystallographic data, results of structure refinements, and se-
lected bond lengths and angles are summarized in Tables 1, 2 and 3.
As shown in Fig. 1, being surrounded by two nitrogen
donors of the chelating b-diketiminate ligand and two ethyl
groups, the aluminium center in complex 3k possesses a dis-
torted tetrahedral geometry with the bite angle of N(1)–Al–
N(2) (94.50(7)◦) significantly smaller than the regular tetrahedral
angle of 109.28◦, which however falls into the normal range
of 86.76–100.04◦ for the tetracoordinate b-diketiminate metal
complexes.24,27,45,46,65–68 The C(24)–Al–C(26) angle of 115.59(11)◦ is
comparable to those observed in symmetrical b-diketiminate alu-
minium complexes [(4-MeC6H4NCMe)2HC]AlMe2 (115.4(2)◦)25
and [(2,6-iPr2C6H3NCMe)2HC]AlMe2 (117.4(1)◦).16 The N1–C1–
0.5595 A. This value tends to be sensitive to the steric bulk-
iness of the ortho-substituents of the phenyl moieties in the
˚
˚
b-diketiminate ligand, since deviations of 0.33 A and 0.72 A
have been observed for [(4-MeC6H4NCMe)2HC]AlMe2 and [(2,6-
iPr2C6H3NCMe)2HC]AlMe2 respectively. Obviously the more
bulky the ortho-substituents, the larger the deviation of the
aluminium center from the backbone plane will be. The very close
˚
bond lengths of N(1)–C(1) (1.334(3) A) and N(2)–C(3) (1.329(2)
˚
˚
˚
A), as well as C(1)–C(2) (1.386(3) A) and C(2)–C(3) (1.390(3) A)
indicate the multiple bond character and significant delocalization
in these bonds. The two aryl rings have similar orientations
towards the ligand backbone, with the dihedral angle of 78.15◦
formed by the 2,6-diisopropylphenyl ring and that of 80.74◦ by
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the 4-chlorophenyl ring. The two Al–N bond lengths (Al–N(1) =
could be isolated after long reaction times of more than 10 d,
implying that neither the Al–N bonding nor the Al–alkyl bonding
in these complexes is active enough to initiate the ring opening
polymerization of lactide monomer. Lewin´ski et al.70 found that
the complex [Me2Al(l-OCH2CH2OMe)]2 was inactive towards
polymerization of lactide at 40 ◦C in CH2Cl2; after introducing the
bulky tert-butyl group, the complex [tBu2Al(l-OCH2CH2OMe)]2
was effective for the ROP of lactide at 40 ◦C. The authors suggested
that the different chelating extent of the lactide monomer with
the metal center in these two cases could be responsible for the
inverse catalytic behavior. It is reasonable that the inactivity of our
complexes for the ROP of lactide might also be due to the strong
chelation of lactide with aluminium, which suppressed the further
insertion of incoming monomer, although the chelate species was
not obtained.
˚
1.9062(16), Al–N(2) = 1.9223(16) A) in 3k are almost identical,
with the Al–N(2) (N-phenyl-2,6-diisopropyl) bond length slightly
˚
elongated by about 0.02 A. The rather long Al · · · C (C1, C2,
˚
C3) contacts (2.849–3.147 A) and the orientation of the chelate
ring N1–C1–C2–C3–N2 exclude any p interaction between the b-
diketiminate ligand and the metal center in 3k, just as has been
reported for b-diketiminate aluminium or zinc complexes.13,14,16,25
The X-ray structural analysis (Fig. 2) of compound 7 un-
ambiguously indicated the formation of an Al(1)–O–Al(2) unit
and its nearly linear structure, with this angle (168.90(12)◦)
being smaller than that in the pentafluoro-sub◦stituted complex
{HC[(CMe)(NC6F5)]2AlMe}2(l-O) (174.42(11) ),34 but signifi-
cantly larger than the one in {HC[(CMe)(2,6-iPrNC6H3)]2-
AlOH}2(l-O) (143.84(16)◦).62 In complex 7 each aluminium
center adopts a distorted tetrahedral geometry constructed by
two nitrogen atoms of the ligand, a methyl group and one (l-
Ring-opening polymerization of e-caprolactone
˚
O) unit. The Al–O bond lengths (1.6797(18), 1.6807(18) A) are
slightly shorter than those in {HC[(CMe)(NC6F5)]2AlMe}2(l-O)
b-Diketiminate aluminium complexes 3a–m as well as
the previously reported bulky aluminium complex [(2,6-
iPr2C6H3NCMe)2HC]AlEt2 (8) however do initiate the ring-
opening polymerization of e-caprolactone (e-CL) in toluene at
80 ◦C. The polymerization results are listed in Table 4, all
complexes displayed moderate activities for the polymerization
of e-caprolactone, and the structure of the ancillary ligands has
a significant influence on the polymerization behavior of the
corresponding aluminium complexes.
˚
(1.689(2), 1.685(2) A), possibly resulting from the smaller Al(1)–
O–Al(2) angle.34 The two Al–C bonds are in trans-position relative
to the Al(1)–O–Al(2) plane, with the bond lengths (avg. 1.938(3)
˚
A) slightly shorter than those in {HC[(CMe)(NC6F5)]2AlMe}2(l-
O) (avg 1.956(2) A).34 Besides, other structural features such as the
˚
N–Al–N bond angles (94.69(10)◦, 95.46(11)◦) are comparative to
those in 3k and other reported b-diketiminate dialkyl aluminium
complexes.16,25
Comparing the polymerization runs performed for 9 h in
Table 4, several structure–activity trends may be drawn. It is
found that for aluminium complexes 3a–h with symmetrical b-
diketiminate ligands, the electronic nature of the para- or meta-
substituents of the phenyl groups exerts great influence on the
ROP of e-CL. A clear decreasing tendency of catalytic activity
is found for complexes 3a–c in the order 3a (p-H) > 3b (p-
Me) > 3c (p-OMe) (runs 1, 3, 5), indicating that electron-donating
substituents at the para-position are a disadvantage to the catalytic
activity. The introduction of electron-donating groups on the
phenyl rings decreases the electrophilicity of the aluminium center
through the chelating p-system of the ancillary ligand, and is
thus unfavorable for the coordination/insertion of the cyclic ester
monomer and leads to a decrease in activity.71 Based on this point
of view, complexes 3d and 3e bearing electron-withdrawing para-
chloro or -fluoro substituents are expected to display increased
activities. However the polymerization experiments gave inverse
results. Complexes 3d (p-Cl) and 3e (p-F) show much lower activity
compared with the unsubstituted complex 3a, and complex 3d is
slightly more active than 3e (runs 1, 7, 9). Nevertheless, when a
fluorine atom was introduced at the meta-position of the phenyl
moieties, complex 3f does show higher activity than 3a (run 11 vs.
1). The influence of halogen substitution at the ancillary ligands
on the polymerization performance of the corresponding metal
catalysts/initiators for cyclic esters has been investigated in other
cases,36,72–75 but conflicting results are usually obtained. A decrease
in catalytic activity was observed for bis(phenolato)bis(amine) alu-
minium complexes with para-bromo substitution at the phenoxy
moieties;72 whereas a great improvement in activity was obtained
for salen–aluminium complexes with para-chloro substitution.36,73
When halide-substituted amine-phenolate ligands were employed
with Group IV metals, the opposite influence was found for
Fig. 2 ORTEP diagram of the molecular structure of [(BDI-2a)-
AlMe]2(l-O) (7). Thermal ellipsoids are drawn at the 30% probability
level. Hydrogen atoms are omitted for clarity.
Ring-opening polymerization of rac-lactide
As we have mentioned that the corresponding aluminium alkox-
ides could not be generated from the reaction of this series of b-
diketiminate aluminium complexes and alcohol, complex 3e was
added directly to a solution of rac-lactide (rac-LA) in toluene at
80 ◦C with an [Al]/[rac-LA] ratio of 1 : 100. However no polymer
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Table 4 Ring-opening polymerization of e-CL initiated by aluminium complexes 3a–m and 8a
c
d
ꢀf
f
Run
Cat.
3a
3b
3c
3d
3e
3f
Time/h
Conv.b (%)
MCald
Mg (104)
Mw (104)
Mw/Mn
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
9
17
9
17
9
24
9
17
9
17
9
17
7
9
7
9
9
56
9
48
9
48
9
48
240
240
73
89
62
92
50
85
57
89
49
92
86
96
74
91
85
93
30
74
11
82
23
91
18
90
72
6.9
8322
10 146
7068
10 488
5700
9690
6498
10 146
5586
10 488
9804
10 944
8436
10 374
9690
10 602
3420
8436
1254
9348
2622
10 374
2052
10 260
8208
786
11.2
12.5
6.20
7.54
4.13
6.58
4.02
5.32
5.65
9.50
11.3
10.2
7.66
8.39
6.98
7.85
4.41
3.48
2.06
3.18
5.52
5.35
3.74
2.48
1.66
3g
3h
3i
5.10
e
3j
3.73
e
3k
3l
3.44
2.72
5.04
3.22
0.23
4.27
2.42
3m
8
◦
1
c
a [e-CL]0/[Al]0 = 100, [e-CL]0 = 1.0 M, toluene, 80 C. b Determined by H NMR spectroscopy. MCald = ([e-CL]0/[Al]0) × 114 × conv.%. d Viscosity
measurements, in DMF, 30 ◦C. e Not enough sample available for the measurement. f Determined by gel permeation chromatography, calibrated with
polystyrene standards in THF, Mw = Mw, GPC × 0.56.69
ꢀ
titanium and zirconium complexes.74 Obviously, the effect of
halogen substitution on activity is considerably complicated.
Halogen atoms, especially fluorine, being strongly electronegative
are considered to display marked electron-withdrawing charac-
teristics, but the lone pair in the p-orbital of the halogen atom
can also lead to an electron-donating conjugated effect via p–
p bonding to its para- and ortho-positions when introduced to a
phenyl ring. From a comparison of catalytic activities of complexes
3d–f, it seems that the lower activity of complexes 3d and 3e
should be attributed to this electron-donating conjugated effect.
In order to exclude the conjugated effect of the para-halogen
substitution and verify the influence of an electron-withdrawing
group, the trifluoromethyl group was introduced to the para- or
meta-position of the phenyl rings. As expected, complexes 3g,
3h exhibit significantly higher activities for the ROP of e-CL;
and 3h shows the highest activity among these b-diketiminate
aluminium complexes; high conversion up to 93% was obtained
within 9 h. With the aim of having a detailed understanding of the
influence of these substituents on the polymerization processes,
such as initiation, chain propagation, kinetic experiments were
carried out for complexes 3a, 3e, 3f, and 3g. Semilogarithmic
plots (ln{([CL]0 − [CL]eq)/([CL]t − [CL]eq)} versus time), typical of
slow initiation, were observed in all cases (Fig. S1–S4, see ESI†).
However, due to the lack of a method to determine accurately the
concentration of active initiators at specified time intervals, this
study finally failed to give believable results.†
methoxy group to the para-position of the aromatic ring led to a
decrease in catalytic activity (run 19). Complexes 3k and 3l with
para-halogen substitution display lower polymerization activities
(runs 21 and 23) than the unsubstituted complex 3i. In general,
aluminium complexes 3i–l with unsymmetrical b-diketiminate
ligands show much lower catalytic activity when compared with
complexes 3a–h, the bulky ortho-2-propyl groups in one of the
aryl rings remarkably block the coordination sphere of the metal
center and restrain the coordination/insertion of the monomer.
The unfavorable steric influence of the ortho-substitution is also
incorporated into complexes 3m and 8 (runs 25 and 26), only
low to moderate monomer conversion was reached after long
polymerization times of 240 h. Due to the difficulty encountered
in the synthesis of the unsymmetrical b-diketiminate ligands with
less steric hindrance than 2-propyl at the ortho-positions, further
study concerning the steric and electronic cooperative effect on the
ROP of e-CL is restricted. It should also be pointed that with the
advancement of the polymerizations, the monomer conversions
were not able to reflect the real activities of the aluminium
complexes due to the quite high viscosity of the reaction solution
in some cases, which significantly restricted the effective monomer
diffusion and coordination/insertion.
Molecular weight information of all poly(e-caprolactone) sam-
ples was obtained by viscosity measurements and in selected cases
by gel permeation chromatography (GPC). As shown in Table 4, in
general, with the increase of monomer conversion, the viscosity-
average molecular weights (Mg) of the polymer samples increase.
The Mg of all the polymers deviate considerably from the theoret-
ical values (calculated with the assumption that each aluminium
A similar electronic influence of substituent on polymerization
was also found for aluminium complexes 3i–m with unsymmetrical
b-diketiminate ligands. In the case of complex 3j, introducing a
3350 | Dalton Trans., 2008, 3345–3357
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center initiates one polymer chain). The poorly controlled nature
of the ROP of e-CL by these aluminium complexes is also indicated
by the broad molecular weight distributions (MWD = 1.66–3.74)
of the polymer samples via GPC analysis. Similar results were
observed by Chakraborty and Chen when diamide aluminium
complexes [N∩N]AlR (N∩N = ArN(CH2)3NAr) were used to
catalyze the ROP of e-CL.12
complexes 3a–h bearing less hindered b-diketiminate ligands at
similar monomer conversions.
Experimental
General considerations
In order to understand the polymerization mechanism of e-
CL by aluminium complexes 3a–m, e-CL was polymerized with
complex 3c as initiator at the molar ratio of [e-CL]/[Al] = 20 : 1
under the same conditions. The 1H NMR spectrum of the purified
oligomer, obtained at 34% monomer conversion, indicates the
existence of a tiny peak at 3.65 ppm which is assignable to
the methene protons (–CH2–) adjacent to a terminal hydroxyl
group, suggesting a linear structure of the oligomer instead of
cyclic. The molecular weight of more than 11 000 estimated from
1H NMR spectroscopy however hampers a detailed end-group
analysis with ESI or MALDI-TOF spectrometry.70 Due to the
fact that complexes 3a–m are hardly sensitive to oxygen/moisture
and are inert to alcohol, the possibility of any impurity initiating
the polymerization is excluded.
All manipulations were carried out under a dry argon atmosphere
using standard Schlenk techniques unless otherwise indicated.
Toluene and n-hexane were refluxed over sodium benzophenone
prior to use. Chloroform-d was dried over calcium hydride.
e-Caprolactone (ACROS ORGANICS, 99%) was dried over
CaH2 for 1 d at 80 ◦C, then vacuum distilled and stored
under argon. n-BuLi (2.3 M in n-hexane) was purchased from
Chemetall. AlEt3 (Fluka, 99%), Me2AlCl (supplied by Nanjing
University, China) and MeAlCl2 (Aldrich, 1.0 M in n-hexane) were
used as received. Enaminoketones 1a–i,41,81 b-diketimines 2a,39
2b,39 2g,82 2i,41 2k,83 and [(2,6-iPr2C6H3NCMe)2HC]AlEt2 (8)16
were synthesized according to the published procedures. NMR
spectra were recorded on Bruker AVANCE-500 and AVANCE-
300 spectrometers with CDCl3 as solvent (1H: 500 MHz; 13C:
1
125 MHz and 75 MHz). Chemical shifts for H and 13C NMR
Based on an activity comparison between ketoiminate alu-
minium complexes (OCMeCHCMeNAr)2AlR (R = Me, Et) and
their corresponding chloride complexes, Huang et al.46 assumed
that Al–alkyl bonding is responsible for the initiation in the ROP of
e-CL. Nevertheless, no further evidence was observed to confirm
it. Lewin´ski and co-workers even reported that, in the presence of
dioxygen, methylaluminium(bisphenoxide) might be oxidized to
give Al–OMe species which finally led to poly(e-CL)s end-capped
with a methoxy group.76,77 The in situ 1H NMR polymerizations of
e-CL in the presence of [N∩N]AlMe conducted by Chakraborty
and Chen12 however evidenced retaining the methyl group at
the aluminium center and the initiation step involving monomer
insertion into an Al–N bond. Since no characteristic signals of
terminal ethyl (COCH2CH3) or ethoxy (COOCH2CH3) groups
could be observed in the 1H NMR spectrum of the oligomer
sample by complex 3c, we suggest that, similar to the results
of Chakraborty and Chen,12 the ROP of e-CL by complexes
3a–m and 8 were most likely initiated by one of the Al–N
bonds. After quenching with normal methanol, the terminal amide
was hydrolyzed78–80 and polymers with hydroxyl and carboxyl as
terminals were obtained.
Compared to an alkoxy group, the initiation by an amide group
is rather slow, which usually leads to a broad molecular weight
distribution of the obtained polymer.12 In complexes 3a–m, the
two Al–N bonds are averaged due to the delocalization of the
b-diketiminate framework, it is clear that these Al–N bonds are
even more inert than usual Al–amido bonds. After insertion of
the first monomer into an Al–N bond, an Al–O bond is formed
and sequentially induces a fast propagation of the polymer chain.
The rather slow initiation step combined with quite fast chain
propagation thus results in PCLs with significant deviation of
experimental molecular weights from the theoretical values and
broad molecular weight distributions as well. On the other hand,
the bulky ortho-2-propyl groups at one of the aromatic rings in
complexes 3i–m obviously hindered the coordination/insertion
of the monomer to the aluminium center, leading to a decrease
in propagation rate. As a result, the Mg of polymers produced
by complexes 3i–m are generally smaller than those obtained by
spectra were referenced internally using the residual solvent
resonances and reported relative to tetramethylsilane (TMS).
Elemental analyses were performed on an EA-1106 instrument.
The intrinsic viscosity of poly(e-caprolactone) was measured with
an Ubbelohde viscometer in N,N-dimethylformamide (DMF)
at 30 ◦C. The viscosity average molecular weight of PCL was
calculated according to the equation:84 [g] (dL/g) = 1.91 × 10−4
Mg0.73. Gel permeation chromatography (GPC) analyses were
carried out on a Wat◦ers instrument (M515 pump, Optilab Rex
injector) in THF at 25 C, at a flow rate of 1 mL min−1. Calibration
standards were commercially available narrowly distributed linear
polystyrene samples that cover a broad range of molar masses
(103 < M < 2 × 106 g mol−1).
Syntheses
2-(4-Methoxyphenyl)amino-4-(4-methoxyphenyl)imino-2-pen-
tene (2c). To a stirred solution of 4-methoxyaniline (6.150 g,
50.00 mmol) in 80 mL of toluene was added para-toluenesulfonic
acid monohydrate (9.510 g, 50.00 mmol), and the mix-
ture was stirred for 3 h at room temperature, then 4-(4-
methoxyphenyl)amino-3-penten-2-one (1c) (10.26 g, 50.00 mmol)
was added to it. A Dean–Stark apparatus was attached and the
mixture was heated to reflux for 24 h. The reaction mixture
was cooled and dried under reduced pressure to give a yellow
solid. The obtained solid was treated with diethyl ether (100 mL),
water (100 mL) and sodium carbonate (10.60 g, 100.0 mmol), and
kept stirring. After complete dissolution, the aqueous phase was
separated and extracted with diethyl ether. The combined organic
phases were dried over MgSO4 and rotary evaporated to dryness
under reduced pressure to afford a yellow solid. Yellow crystals
of 2c (11.95 g, 77%) were obtained after recrystallization from
methanol (Found: C, 73.55; H, 7.25; N, 9.06. Calc. for C19H22N2O2:
C, 73.52; H, 7.14; N, 9.03%); mp 93.5–94.5 ◦C; dH (500 MHz,
CDCl3): 1.96 (s, 6H, CH3), 3.79 (s, 6H, OCH3), 4.84 (s, 1H, c -
CH), 6.85 (d, 3J = 8.7 Hz, 4H, o-Ar-H), 6.91 (d, 3J = 8.7 Hz, 4H,
m-Ar-H), 12.57 (br s, 1H, NH); dC (75 MHz, CDCl3): 20.7 (CMe),
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55.5 (OMe), 96.4 (CH), 114.1 (Ar-C), 124.3 (Ar-C), 138.9 (Ar-C),
156.1 (Ar-C), 160.2 (NCMe).
4.96 (s, 1H, c -CH), 7.12 (d, 3J = 8.0 Hz, 2H, o-Ar-H), 7.21 (s, 2H,
o-Ar-H), 7.32 (d, 3J = 8.0 Hz, 2H, p-Ar-H), 7.41 (t, 3J = 8.0 Hz,
2H, m-Ar-H), 12.71 (br s, 1H, NH).
2-(4-Chlorophenyl)amino-4-(4-chlorophenyl)imino-2-pentene (2d).
b-Diketimine 2d was synthesized by the same procedure as 2c.
9.510 g (50.00 mmol) of para-toluenesulfonic acid monohydrate,
6.380 g (50.00 mmol) of 4-chloroaniline, and 10.48 g (50.00 mmol)
of 4-(4-chlorophenyl)amino-3-penten-2-one (1d) were used to give
2d (11.80 g, 74%) as yellow crystals (Found: C, 63.94; H, 5.09; N,
8.86. Calc. for C17H16Cl2N2: C, 63.96; H, 5.05; N, 8.78%); mp
2-(2,6-Diisopropylphenyl)amino-4-(4-methoxyphenyl)imino-2-
pentene (2j). 9.510 g (50.00 mmol) of para-toluenesulfonic acid
monohydrate, 6.160 g (50.00 mmol) of 4-methoxyaniline, 12.97 g
(50.00 mmol) of 4-(2,6-diisopropylphenyl)amino-3-penten-2-one
(1i), and 80 mL of toluene were combined in a round bottomed
flask. A Dean–Stark apparatus was attached and the mixture was
heated to reflux for 24 h. The reaction mixture was cooled and
dried under reduced pressure to give a yellow solid. The obtained
solid was treated with diethyl ether (100 mL), water (100 mL)
and sodium carbonate (10.60 g, 100.0 mmol), and kept stirring.
After complete dissolution, the aqueous phase was separated and
extracted with diethyl ether. The combined organic phases were
dried over MgSO4 and rotary evaporated to dryness under reduced
pressure to afford a yellow solid. Yellow crystals of 2j (13.85 g,
76%) were obtained after recrystallization from methanol (Found:
C, 79.21; H, 8.95; N, 7.69. Calc. for C24H32N2O: C, 79.08; H, 8.85;
N, 7.68%); mp 104–105 ◦C; dH (500 MHz, CDCl3): 1.13 (d, 3J =
6.9 Hz, 6H, -CH(CH3)2), 1.20 (d, 3J = 6.9 Hz, 6H, -CH(CH3)2),
◦
86–87 C; dH (500 MHz, CDCl3): 1.98 (s, 6H, CH3), 4.89 (s, 1H,
c -CH), 6.88 (d, 3J = 8.7 Hz, 4H, o-Ar-H), 7.25 (d, 3J = 8.7 Hz,
4H, m-Ar-H), 12.59 (br s, 1H, NH); dC (75 MHz, CDCl3): 20.9
(CMe), 97.9 (CH), 123.8 (Ar-C), 128.6 (Ar-C), 128.9 (Ar-C), 144.1
(Ar-C), 159.8 (NCMe).
2-(4-Fluorophenyl)amino-4-(4-fluorophenyl)imino-2-pentene (2e).
b-Diketimine 2e was synthesized by the same procedure as 2c.
9.510 g (50.00 mmol) of para-toluenesulfonic acid monohydrate,
5.561 g (50.00 mmol) of 4-fluoroaniline, and 9.650 g (50.00 mmol)
of 4-(4-fluorophenyl)amino-3-penten-2-one (1e) were used to give
2e (10.73 g, 75%) as yellow crystals (Found: C, 71.36; H, 5.63;
N, 9.80. Calc. for C17H16F2N2: C, 71.31; H, 5.63; N 9.78%); mp
3
◦
1.69 (s, 3H, CH3), 2.00 (s, 3H, CH3), 2.97 (sept, J = 6.9 Hz,
69–70 C; dH (500 MHz, CDCl3): 1.95 (s, 6H, CH3), 4.87 (s, 1H,
2H, -CH(CH3)2), 3.78 (s, 3H, OCH3), 4.84 (s, 1H, c -CH), 6.82 (d,
c -CH), 6.88–6.93 (m, 4H, o-Ar-H), 6.96–7.01 (m, 4H, m-Ar-H),
12.53 (br s, 1H, NH); dC (75 MHz, CDCl3): 20.7 (CMe), 97.1
(CH), 115.5 (Ar-C), 115.6 (Ar-C), 124.3 (Ar-C), 141.7 (Ar-C),
157.9 (Ar-C), 160.2 (Ar-C), 161.1 (NCMe).
3
3J = 8.8 Hz, 2H, o-Ar-H), 6.90 (d, J = 8.8 Hz, 2H, m-Ar-H),
7.06–7.13 (m, 3H, m-, p-Ar-H), 12.61 (br s, 1H, NH); dC (75 MHz,
CDCl3): 20.5 (CMe), 21.3 (CMe), 22.7 (CHMe2), 24.1 (CHMe2),
28.3 (CHMe2), 55.5 (OMe), 95.1 (CH), 114.1 (Ar-C), 122.9 (Ar-
C), 124.0 (Ar-C), 124.8 (Ar-C), 136.6 (Ar-C), 140.2 (Ar-C), 143.6
(Ar-C), 156.2 (Ar-C), 156.8 (NCMe), 163.6 (NCMe).
2-(3-Fluorophenyl)amino-4-(3-fluorophenyl)imino-2-pentene (2f).
b-Diketimine 2f was synthesized by the same procedure as 2c.
9.510 g (50.00 mmol) of para-toluenesulfonic acid monohydrate,
5.561 g (50.00 mmol) of 3-fluoroaniline and 9.650 g (50.00 mmol)
of 4-(3-fluorophenyl)amino-3-penten-2-one (1f) were used to give
2f (7.292 g, 51%) as light yellow crystals (Found: C, 71.27; H,
5.60; N, 9.84. Calc. for C17H16F2N2: C, 71.31; H, 5.63; N, 9.78%);
2-(2,6-Diisopropylphenyl)amino-4-(4-fluorophenyl)imino-2-pen-
tene (2l). b-Diketimine 2l was synthesized by the same proce-
dure as 2j. 9.510 g (50.00 mmol) of para-toluenesulfonic acid
monohydrate, 5.561 g (50.00 mmol) of 4-fluoroaniline and 12.97 g
(50.00 mmol) of 4-(2,6-diisopropylphenyl)amino-3-penten-2-one
(1i) were used to give 2l (12.86 g, 73%) as colorless crystals (Found:
C, 78.33; H, 8.30; N, 7.99. Calc. for C23H29FN2: C, 78.37; H, 8.29;
N, 7.95%); mp 100.5–101.5 ◦C; dH (500 MHz, CDCl3): 1.13 (d, 3J =
6.9 Hz, 6H, -CH(CH3)2), 1.21 (d, 3J = 6.9 Hz, 6H, -CH(CH3)2),
◦
mp 31–32 C; dH (500 MHz, CDCl3): 2.02 (s, 6H, CH3), 4.90 (s,
1H, c -CH), 6.71 (m, 6H, o-, m-Ar-H), 7.22 (d, 3J = 8.0 Hz, 1H,
p-Ar-H), 7.25 (d, 3J = 8.0 Hz, 1H, p-Ar-H), 12.64 (br s, 1H, NH);
dC (75 MHz, CDCl3): 21.0 (CMe), 98.4 (CH), 109.2 (Ar-C), 109.6
(Ar-C), 109.8 (Ar-C), 110.1 (Ar-C), 118.1 (Ar-C), 118.5 (Ar-C),
129.9 (Ar–C), 130.1 (Ar–C), 147.3 (Ar–C), 147.5 (Ar–C), 159.7
(Ar–C), 161.5 (Ar–C), 164.8 (NCMe).
3
1.70 (s, 3H, CH3), 1.99 (s, 3H, CH3), 3.00 (sept, J = 6.9 Hz,
2H, -CH(CH3)2), 4.87 (s, 1H, c -CH), 6.78–6.80 (m, 2H, o-Ar-H),
6.93–6.96 (m, 2H, m-Ar-H), 7.13 (s, 3H, m-, p-Ar-H), 12.57 (br s,
1H, NH); dC (75 MHz, CDCl3): 20.7 (CMe), 20.9 (CMe), 22.7
(CHMe2), 24.3 (CHMe2), 28.3 (CHMe2), 95.5 (CH), 115.3 (Ar-
C), 115.6 (Ar-C), 123.0 (Ar-C), 124.1 (Ar-C), 124.2 (Ar-C), 125.0
(Ar-C), 141.4 (Ar-C), 141.7 (Ar-C), 157.6 (Ar-C), 158.8 (Ar-C),
160.8 (NCMe), 162.1 (NCMe).
2-(3-Trifluoromethylphenyl)amino-4-(3-trifluoromethylphenyl)-
imino-2-pentene (2h). To a stirred solution of 3-trifluoro-
methylaniline (8.056 g, 50.00 mmol) in 80 mL of toluene was added
para-toluenesulfonic acid monohydrate (9.510 g, 50.00 mmol).
The mixture was stirred for 3 h at room temperature, then
4-(3-trifluoromethylphenyl)amino-3-penten-2-one (1h) (12.15 g,
50.00 mmol) was added to it. A Dean–Stark apparatus was
attached and the mixture was heated to reflux for 24 h. The reaction
mixture was cooled and all the volatiles were evacuated under
reduced pressure to give a yellow solid. The obtained solid was
treated with diethyl ether (100 mL), water (100 mL) and sodium
carbonate (10.60 g, 100.0 mmol), and kept stirring until dissolution
was complete. The aqueous phase was separated and extracted
with diethyl ether. The combined organic phase was dried over
MgSO4 and concentrated under reduced pressure. The obtained
residue was distilled to afford 2h as a yellow oil (12.56 g, 65%); bp
128–130 ◦C (0.3 Torr); dH (500 MHz, CDCl3,): 2.05 (s, 6H, CH3),
2-(2,6-Diisopropylphenyl)amino-4-(2,6-dimethylphenyl)imino-2-
pentene (2m). b-Diketimine 2m was synthesized by the same
procedure as 2j. 9.510 g (50.00 mmol) of para-toluenesulfonic
acid monohydrate, 6.090 g (50.00 mmol) of 2,6-dimethylaniline
and 12.97 g (50.00 mmol) of 4-(2,6-diisopropylphenyl)amino-3-
penten-2-one (1i) were used to give 2m (12.87 g, 71%) as colorless
crystals (Found: C, 82.91; H, 9.43; N, 7.71. Calc. for C25H34N2: C,
◦
82.82; H, 9.45; N, 7.73%); mp 93–94 C; dH (500 MHz, CDCl3):
1.12 (d, 3J = 6.9 Hz, 6H, -CH(CH3)2), 1.23 (d, 3J = 6.9 Hz, 6H,
-CH(CH3)2), 1.70 (s, 3H, CH3), 1.71 (s, 3H, CH3), 2.15 (s, 6H,
3
Ar-CH3), 3.09 (sept, J = 6.9 Hz, 2H, -CH(CH3)2), 4.88 (s, 1H,
3352 | Dalton Trans., 2008, 3345–3357
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3
3
c -CH), 6.94 (t, J = 7.4 Hz, 1H, p-Ar-H), 7.04 (d, J = 7.4 Hz,
2H, m-Ar-H), 7.13 (m, 3H, m-, p-Ar-H), 12.28 (br s, 1H, NH);
dC (75 MHz, CDCl3): 18.3 (Ar-Me), 20.6 (CMe), 22.9 (CHMe2),
24.5 (CHMe2), 28.5 (CHMe2), 93.3 (CH), 123.1 (Ar-C), 124.0
(Ar-C), 125.7 (Ar-C), 127.9 (Ar-C), 131.5 (Ar-C), 139.8 (Ar-C),
143.1 (Ar-C), 144.8 (Ar-C), 160.2 (NCMe), 162.0 (NCMe).
(BDI-2e)AlEt2 (3e). Complex 3e was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.144 g (4.000 mmol) of b-diketimine 2e were used to obtain
3e (1.182 g, 80%) as yellow crystals (Found: C, 68.10; H, 6.74; N,
7.51. Calc. for C21H25AlF2N2: C, 68.09; H, 6.80; N, 7.56%); mp 91–
93 ◦C; dH (500 MHz, CDCl3): −0.40 (q, 3J = 8.1, 4H, AlCH2CH3),
0.73 (t, 3J = 8.1 Hz, 6H, AlCH2CH3), 1.81 (s, 6H, CH3), 4.86 (s,
1H, c -CH), 6.96–7.06 (m, 8H, o-, m-Ar-H); dC (75 MHz, CDCl3):
0.1 (AlCH2CH3), 9.1 (AlCH2CH3), 23.0 (CMe), 96.9 (CH), 115.8
(Ar-C), 116.1 (Ar-C), 127.5 (Ar-C), 141.5 (Ar-C), 159.1 (Ar-C),
162.4 (Ar-C), 168.9 (NCMe).
(BDI-2a)AlEt2 (3a). A solution of AlEt3 (0.457 g, 4.000 mmol)
in 10 mL of n-hexane was added dropwise to a solution of b-
diketimine 2a (1.001 g, 4.000 mmol) in 20 mL of n-hexane at
0
◦C with rapid stirring. The reaction solution was then stirred
overnight at room temperature and filtered. The filtrate was
concentrated under vacuum to approximate 2 mL. 3a deposited
after 24 h at −40 ◦C as yellow crystals (1.003 g, 75%) (Found: C,
75.30; H, 8.18; N, 8.39. Calc. for C21H27AlN2: C, 75.42; H, 8.14; N,
8.38%); mp 63–64 ◦C; dH (500 MHz, CDCl3): −0.36 (q, 3J = 8.1 Hz,
(BDI-2f)AlEt2 (3f). Complex 3f was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.144 g (4.000 mmol) of b-diketimine 2f were used to obtain
3f (0.738 g, 50%) as yellow crystals (Found: C, 68.00; H, 6.72;
N, 7.63. Calc. for C21H25AlF2N2: C, 68.09; H, 6.80; N, 7.56%);
mp 45–46 ◦C; dH (500 MHz, CDCl3): −0.35 (q, 3J = 8.1 Hz, 4H,
3
4H, AlCH2CH3), 0.75 (t, J = 8.1 Hz, 6H, AlCH2CH3), 1.83 (s,
6H, CH3), 4.86 (s, 1H, c -CH), 7.06 (d, 3J = 7.4 Hz, 4H, o-Ar-H),
3
3
3
AlCH2CH3), 0.76 (t, J = 8.1 Hz, 6H, AlCH2CH3), 1.86 (s, 6H,
7.21 (t, J = 7.4 Hz, 2H, p-Ar-H), 7.35 (t, J = 7.4 Hz, 4H, m-
Ar-H); dC (75 MHz, CDCl3): 0.2 (AlCH2CH3), 9.1 (AlCH2CH3),
23.0 (CMe), 96.7 (CH), 125.7 (Ar-C), 126.1 (Ar-C), 129.1 (Ar-C),
145.7 (Ar-C), 168.3 (NCMe).
CH3), 4.90 (s, 1H, c -CH), 6.79 (d, 3J = 8.2 Hz, 2H, o-Ar-H), 6.85
3
3
(d, J = 7.8 Hz, 2H, o-Ar-H), 6.94 (t, J = 8.2 Hz, 2H, p-Ar-
H), 7.32 (q, 3J = 7.8 Hz, 2H, m-Ar-H); dC (75 MHz, CDCl3): 0.1
(AlCH2CH3), 9.1 (AlCH2CH3), 23.0 (CMe), 97.3 (CH), 112.7 (Ar-
C), 113.0 (Ar-C), 113.3 (Ar-C), 113.5 (Ar-C), 121.8 (Ar-C), 121.9
(Ar-C), 130.2 (Ar-C), 130.3 (Ar-C), 147.2 (Ar-C), 147.4 (Ar-C),
161.4 (Ar-C), 164.7 (Ar-C), 168.6 (NCMe).
(BDI-2b)AlEt2 (3b). Complex 3b was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.113 g (4.000 mmol) of b-diketimine 2b were used to obtain 3b
(1.015 g, 70%) as yellow crystals (Found: C, 76.12; H, 8.63; N, 7.67.
Calc. for C23H31AlN2: C, 76.21; H, 8.62; N 7.73%); mp 86–87 ◦C;
(BDI-2g)AlEt2 (3g). A solution of AlEt3 (0.514 g, 4.000 mmol)
in 10 mL of toluene was added dropwise to a solution of b-
diketimine 2g (1.545 g, 4.000 mmol) in 20 mL of toluene at 0 ◦C
with rapid stirring. The reacti◦on solution was stirred overnight at
room temperature and at 60 C for 48 h, then was concentrated
under vacuum to afford an orange-yellow sticky solid. 10 mL of n-
hexane was added and the slightly turbid solution was filtered. The
filtrate was concentrate under reduced pressure to approximate
3 mL. 3g deposited after 24 h at −40 ◦C as yellow crystals (1.072 g,
57%) (Found: C, 58.47; H, 5.41; N, 5.95. Calc. for C23H25AlF6N2:
C, 58.72; H, 5.36; N, 5.95%); mp 73–74 ◦C; dH (500 MHz, CDCl3):
3
dH (500 MHz, CDCl3): −0.37 (q, J = 8.1 Hz, 4H, AlCH2CH3),
0.77 (t, 3J = 8.1 Hz, 6H, AlCH2CH3), 1.82 (s, 6H, CH3), 2.35 (s,
6H, Ar-CH3), 4.82 (s, 1H, c -CH), 6.93 (d, 3J = 8.0 Hz, 4H, o-Ar-
H), 7.15 (d, 3J = 8.0 Hz, 4H, m-Ar-H); dC (75 MHz, CDCl3): 0.3
(AlCH2CH3), 9.2 (AlCH2CH3), 21.0 (Ar-Me), 23.0 (CMe), 96.4
(CH), 125.8 (Ar-C), 129.6 (Ar-C), 135.2 (Ar-C), 143.1 (Ar-C),
168.4 (NCMe).
(BDI-2c)AlEt2 (3c). Complex 3c was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.241 g (4.000 mmol) of b-diketimine 2c were used to obtain
3c (1.231 g, 78%) as yellow crystals (Found: C, 70.32; H, 7.84;
N, 7.07. Calc. for C23H31AlN2O2: C, 70.03; H, 7.92; N, 7.10%);
mp 82–83 ◦C; dH (500 MHz, CDCl3): −0.38 (q, 3J = 8.1 Hz, 4H,
3
3
−0.37 (q, J = 8.1 Hz, 4H, AlCH2CH3), 0.74 (t, J = 8.1 Hz,
6H, AlCH2CH3), 1.84 (s, 6H, CH3), 4.95 (s, 1H, c -CH), 7.17 (d,
3J = 8.3 Hz, 4H, o-Ar-H), 7.64 (d, J = 8.3 Hz, 4H, m-Ar-H);
3
dC (125 MHz, CDCl3): 0.1 (AlCH2CH3), 9.0 (AlCH2CH3), 23.1
(CMe), 97.8 (CH), 124.1 (q, 1JC–F = 270.0 Hz, CF3), 126.4 (Ar-C),
128.1 (q, 2JC–C–F = 32.4 Hz, Ar-C), 146.1 (Ar-C), 168.7 (NCMe).
3
AlCH2CH3), 0.76 (t, J = 8.1 Hz, 6H, AlCH2CH3), 1.80 (s, 6H,
CH3), 3.81 (s, 6H, Ar-OCH3), 4.81 (s, 1H, c -CH), 6.87 (dt, 3J =
4
3
4
8.9 Hz, J = 2.2 Hz, 4H, o-Ar-H), 6.95 (dt, J = 8.9 Hz, J =
2.2 Hz, 4H, m-Ar-H); dC (75 MHz, CDCl3): 0.2 (AlCH2CH3), 9.2
(AlCH2CH3), 22.9 (CMe), 55.4 (OMe), 96.3 (CH), 114.2 (Ar-C),
126.9 (Ar-C), 138.6 (Ar-C), 157.4 (Ar-C), 168.8 (NCMe).
(BDI-2h)AlEt2 (3h). Complex 3h was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.545 g (4.000 mmol) of b-diketimine 2h were used to obtain
3h (1.279 g, 68%) as yellow crystals (Found: C, 58.96; H, 5.22;
N, 5.78. Calc. for C23H25AlF6N2: C, 58.72; H, 5.36; N, 5.95%);
mp 79–80 ◦C; dH (500 MHz, CDCl3): −0.38 (q, 3J = 8.1 Hz, 4H,
(BDI-2d)AlEt2 (3d). Complex 3d was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.276 g (4.000 mmol) of b-diketimine 2d were used to obtain
3d (1.258 g, 78%) as yellow crystals (Found: C, 62.31; H, 6.29;
N, 6.99. C◦alc. for C21H25AlCl2N2: C, 62.54; H, 6.25; N, 6.95%);
mp 96–98 C; dH (500 MHz, CDCl3): −0.37 (q,3J = 8.0 Hz, 4H,
3
AlCH2CH3), 0.73 (t, J = 8.1 Hz, 6H, AlCH2CH3), 1.84 (s, 6H,
CH3), 4.95 (s, 1H, c -CH), 7.25 (m, 2H, o-Ar-H), 7.32 (s, 2H, o-
Ar-H), 7.47–7.50 (m, 4H, m-, p-Ar-H); dC (125 MHz, CDCl3): d
0.1 (AlCH2CH3), 8.8 (AlCH2CH3), 23.1 (CMe), 97.7 (CH), 122.6
(Ar-C), 122.9 (Ar-C), 123.7 (q, 1JC–F = 270.0 Hz, CF3), 129.4 (Ar-
C), 129.7 (Ar-C), 131.8 (q, 2JC–C–F = 32.4 Hz, Ar-C), 146.1 (Ar-C),
168.7 (NCMe).
3
AlCH2CH3), 0.75 (t, J = 8.0 Hz, 6H, AlCH2CH3), 1.82 (s, 6H,
3
CH3), 4.88 (s, 1H, c -CH), 6.98 (d, J = 8.2 Hz, 4H, o-Ar-H),
3
7.33 (d, J = 8.2 Hz, 4H, m-Ar-H); dC (75 MHz, CDCl3): d 0.2
(AlCH2CH3), 9.2 (AlCH2CH3), 23.1 (CMe), 97.3 (CH), 127.7 (Ar-
C), 129.6 (Ar-C), 129.7 (Ar-C), 131.5 (Ar-C), 168.6 (NCMe).
(BDI-2i)AlEt2 (3i). Complex 3i was synthesized using the same
procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3 and
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3345–3357 | 3353
©

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2
1.338 g (4.000 mmol) of b-diketimine 2i were used to obtain 3i
(1.155 g, 69%) as colorless crystals (Found: C, 77.57; H, 9.41;
N, 6.54. Calc. for C27H39AlN2: C, 77.47; H, 9.39; N, 6.69%); mp
67–68 ◦C; dH (500 MHz, CDCl3): −0.41 (dq, 2J = 14.3 Hz, 3J =
8.1 Hz, 2H, AlCH2CH3), −0.32 (dq, 2J = 14.3 Hz, 3J = 8.1 Hz, 2H,
AlCH2CH3), 0.73 (t, 3J = 8.1 Hz, 6H, AlCH2CH3), 1.17 (d, 3J =
6.8 Hz, 6H, -CH(CH3)2), 1.20 (d, 3J = 6.8 Hz, 6H, -CH(CH3)2),
8.1 Hz, 2H, AlCH2CH3), −0.33 (dq, J = 14.3 Hz,3J = 8.1 Hz,
3
2H, AlCH2CH3), 0.72 (t, J = 8.1 Hz, 6H, AlCH2CH3), 1.16
3
3
(d, J = 6.8 Hz, 6H, -CH(CH3)2), 1.19 (d, J = 6.8 Hz, 6H, -
CH(CH3)2), 1.77 (s, 3H, CH3), 1.89 (s, 3H, CH3), 3.06 (sept, 3J =
6.8 Hz, 2H, -CH(CH3)2), 5.12 (s, 1H, c -CH), 6.96 (dt, 3J = 8.5 Hz,
4J = 2.0 Hz, 2H, o-Ar-H), 7.13 (d, J = 8.2 Hz, 2H, m-Ar-H),
3
7.21 (t, J = 8.2 Hz, 1H, p-Ar-H), 7.30 (dt, J = 8.5 Hz,4J =
2.0 Hz, 2H, m-Ar-H); dC (75 MHz, CDCl3): −0.2 (AlCH2CH3),
9.6 (AlCH2CH3), 22.8 (CMe), 23.3 (CMe), 24.5 (CHMe2), 24.6
(CHMe2), 27.9 (CHMe2), 99.0 (CH), 115.7 (Ar-C), 116.0 (Ar-C),
123.9 (Ar-C), 126.4 (Ar-C), 127.0 (Ar-C), 127.1 (Ar-C), 140.9 (Ar-
C), 142.1 (Ar-C), 142.2 (Ar-C), 143.8 (Ar-C), 158.9 (Ar-C), 162.1
(Ar-C), 167.5 (NCMe), 169.9 (NCMe).
3
3
3
1.77 (s, 3H, CH3), 1.91 (s, 3H, CH3), 3.08 (sept, J = 6.8 Hz,
3
2H, -CH(CH3)2), 5.12 (s, 1H, c -CH), 7.00 (d, J = 7.3 Hz, 2H,
o-Ar-H), 7.14 (d, 3J = 7.3 Hz, 2H, m-Ar-H), 7.20 (m, 2H, p-Ar-
3
H), 7.34 (t, J = 7.7 Hz, 2H, m-Ar-H); dC (75 MHz, CDCl3):
d −0.1 (AlCH2CH3), 9.6 (AlCH2CH3), 22.8 (CMe), 23.3 (CMe),
24.6 (CHMe2), 24.9 (CHMe2), 27.8 (CHMe2), 99.0 (CH), 123.8
(Ar-C), 125.3 (Ar-C), 125.7 (Ar-C), 126.4 (Ar-C), 129.0 (Ar-C),
141.0 (Ar-C), 143.9 (Ar-C), 146.2 (Ar-C), 167.3 (NCMe), 169.5
(NCMe).
(BDI-2m)AlEt2 (3m). Complex 3m was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3
and 1.450 g (4.000 mmol) of b-diketimine 2m were used to obtain
3m (1.250 g yield 70%) as colorless crystals (Found: C, 77.40; H,
10.15; N, 6.37. Calc. for C29H43AlN2: C, 77.98; H, 9.70; N, 6.27%);
mp 88–90 ◦C; dH (500 MHz, CDCl3): −0.37 (m, 4H, AlCH2CH3),
(BDI-2j)AlEt2 (3j). Complex 3j was synthesized using the same
procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3 and
1.458 g (4.000 mmol) of b-diketimine 2j were used to obtain 3j
(1.310 g, 73%) as yellow crystals (Found: C, 74.89; H, 9.33; N,
6.11. Calc. for C28H41AlN2O: C, 74.96; H, 9.21; N, 6.24%); mp
90–91 ◦C; dH (500 MHz, CDCl3): −0.41 (dq, 2J = 14.1 Hz, 3J =
8.1 Hz, 2H, AlCH2CH3), −0.33 (dq, 2J = 14.1 Hz, 3J = 8.1 Hz, 2H,
AlCH2CH3), 0.73 (t, 3J = 8.1 Hz, 6H, AlCH2CH3), 1.17 (d, 3J =
6.8 Hz, 6H, -CH(CH3)2), 1.19 (d, 3J = 6.8 Hz, 6H, -CH(CH3)2),
1.76 (s, 3H, CH3), 1.90 (s, 3H, CH3), 3.09 (sept, 3J = 6.8 Hz, 2H,
-CH(CH3)2), 3.80 (s, 3H, OCH3), 5.09 (s, 1H, c -CH), 6.90 (m, 4H,
3
3
0.70 (t, J = 8.1 Hz, 6H, AlCH2CH3), 1.15 (d, J = 6.8 Hz, 6H,
3
-CH(CH3)2), 1.23 (d, J = 6.8 Hz, 6H, -CH(CH3)2), 1.73 (s, 3H,
3
CH3), 1.79 (s, 3H, CH3), 2.27 (s, 6H, Ar–CH3), 3.26 (sept, J =
6.8 Hz, 2H, -CH(CH3)2), 5.08 (s, 1H, c -CH), 7.02–7.08 (m, 3H,
3
3
m-, p-Ar-H), 7.16 (d, J = 7.8 Hz, 2H, m-Ar-H), 7.22 (t, J =
7.8 Hz, 1H, p-Ar-H); dC (75 MHz, CDCl3): 0.9 (AlCH2CH3),
9.2 (AlCH2CH3), 18.4 (Ar-Me), 22.7 (CMe), 23.5 (CMe), 24.8
(CHMe2), 24.9 (CHMe2), 27.6 (CHMe2), 97.7 (CH), 124.1 (Ar-C),
125.6 (Ar-C), 126.5 (Ar-C), 128.6 (Ar-C), 133.5 (Ar-C), 140.9 (Ar-
C), 143.6 (Ar-C), 144.2 (Ar-C), 168.7 (NCMe), 170.2 (NCMe).
3
3
o-, m-Ar-H), 7.13 (d, J = 7.3 Hz, 2H, m-Ar-H), 7.20 (t, J =
7.3 Hz, 1H, p-Ar-H); dC (75 MHz, CDCl3): −0.1 (AlCH2CH3),
9.7 (AlCH2CH3), 20.5 (CMe), 21.2 (CMe), 24.1 (CHMe2), 24.6
(CHMe2), 27.8 (CHMe2), 55.4 (OMe), 98.7 (CH), 114.2 (Ar-C),
123.8 (Ar-C), 126.3 (Ar-C), 126.5 (Ar-C), 139.0 (Ar-C), 141.1 (Ar-
C), 144.0 (Ar-C), 157.2 (Ar-C), 167.9 (NCMe), 169.3 (NCMe).
Generation of “(BDI-2a)AlMeCl” (4). To a solution of ligand
2a (1.000 g, 4.000 mmol) in 10 mL of toluene was added dropwise
n-BuLi (2.3 M, 1.74 mL, 4.000 mmol) at −78 ◦C. The mixture
was stirred and allowed to warm to ambient temperature. After
being stirred for an additional 12 h, the solution was cooled
to −78 ◦C and MeAlCl2 (1.0 M, 4.00 mL, 4.000 mmol) was
added. The resulting solution was allowed to warm to ambient
temperature. and stirred overnight. After workup, the insoluble
LiCl was removed by filtration, the filtrate was condensed to about
4 mL. 4 was deposited after 24 h at −20 ◦C as pale yellow crystals
(0.925 g, 71%). dH (500 MHz, CDCl3): −0.98 (s, 3H, Al-CH3), 1.87
(s, 6H, CH3), 5.12 (s, 1H, c -CH), 7.16–7.39 (m, 10H, Ar-H).
(BDI-2k)AlEt2 (3k). Complex 3k was synthesized using the
same procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3,
and 1.475 g (4.000 mmol) of b-diketimine 2k were used to obtain
3k (1.359 g, 75%) as yellow crystals (Found: C, 71.78; H, 8.49;
N, 6.12. Calc. for C27H38AlClN2: C, 71.58; H, 8.45; N, 6.18%);
mp 91–92 ◦C; dH (500 MHz, CDCl3): −0.41 (dq, J = 14.3 Hz,
2
2
3J = 8.2 Hz, 2H, AlCH2CH3), −0.33 (dq, J = 14.3 Hz,3J =
3
8.2 Hz, 2H, AlCH2CH3), 0.73 (t, J = 8.2 Hz, 6H, AlCH2CH3),
1.09 (d, 3J = 6.8 Hz, 6H, -CH(CH3)2), 1.12 (d, 3J = 6.8 Hz, 6H,
-CH(CH3)2), 1.77 (s, 3H, CH3), 1.91 (s, 3H, CH3), 3.03 (sept, 3J =
6.8 Hz, 2H, -CH(CH3)2), 5.13 (s, 1H, c -CH), 6.93 (dt, 3J = 8.6 Hz,
Reaction of complex 4 with LiOiPr. To a solution of 2-propanol
(0.240 g, 4.000 mmol) in 10 mL of toluene was added dropwise n-
BuLi (2.30 M, 1.74 mL, 4.000 mmol) at −78 ◦C. The mixture was
stirred and allowed to warm to ambient temperature. After being
stirred for an additional 12 h, the solution was added dropwise
to a solution of complex 4 (1.300 g, 4.000 mmol) in 10 mL of
toluene at 0 ◦C. The resulting solution was allowed to warm to
r.t. and was stirred overnight. After workup, the insoluble LiCl
was removed by filtration, the filtrate was condensed to about
5 mL. 0.670 g white solid was deposited after 24 h at 0 ◦C. The 1H
NMR spectrum indicated that the isolated solid was a mixture of
6 and 7.
3
4J = 2.0 Hz, 2H, o-Ar-H), 7.13 (d, J = 7.3 Hz, 2H, m-Ar-H),
3
3
7.21 (t, J = 7.3 Hz, 1H, p-Ar-H), 7.31 (dt, J = 8.6 Hz,4J =
2.0 Hz, 2H, m-Ar-H); dC (75 MHz, CDCl3): −0.3 (AlCH2CH3),
9.6 (AlCH2CH3), 22.9 (CMe), 23.3 (CMe), 24.5 (CHMe2), 24.6
(CHMe2), 27.9 (CHMe2), 99.3 (CH), 123.9 (Ar-C), 126.5 (Ar-C),
127.1 (Ar-C), 129.2 (Ar-C), 130.9 (Ar-C), 140.8 (Ar-C), 143.7
(Ar-C), 144.8 (Ar-C), 167.0 (NCMe), 170.1 (NCMe).
(BDI-2l)AlEt2 (3l). Complex 3l was synthesized using the same
procedure as for complex 3a. 0.457 g (4.000 mmol) of AlEt3 and
1.409 g (4.000 mmol) of b-diketimine 2l were used to obtain 3l
(1.327 g, 76%) as yellow crystals (Found: C, 74.47; H, 8.61; N,
6.54. Calc. for C27H38AlFN2: C, 74.28; H, 8.77; N, 6.42%); mp
Reaction of complex 4 with NaOiPr. To a suspension of sodium
(0.083 g, 3.600 mmol) in 10 mL of toluene was added dropwise
2-propanol (0.106 g, 1.800 mmol) at 0 ◦C. The mixture was stirred
and allowed to warm to ambient temperature. After being stirred
◦
2
82–83 C; dH (500 MHz, CDCl3): −0.41 (dq, J = 14.3 Hz,3J =
3354 | Dalton Trans., 2008, 3345–3357
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for an additional 12 h, the solution was filtered into a solution of
complex 4 (0.588 g, 1.800 mmol) in 10 mL of toluene at 0 ◦C. The
resulting solution was allowed to warm to room temperature and
stirred overnight. After workup, the solvent was removed under
vacuum, then 30 mL of n-hexane was added and the insoluble
residue was removed by filtration, the filtrate was condensed to
about 15 mL. Complex 7 was deposited at 0 ◦C after 24 h as
colorless crystals (0.296 g, 55%). dH (500 MHz, CDCl3): −1.34
(s, 6H, Al-CH3), 1.83 (s, 12H, CH3), 4.95 (s, 2H, c -CH), 6.92 (d,
complexes have been applied to catalyze the ROP of cyclic esters.
The effect of the ligand substituents on the ROP of e-caprolactone
is significant. In general, electron-donating substituents at the
para-position of the phenyl rings decrease the electrophilicity of
the aluminium center, and are unfavorable for the coordination
and insertion of e-caprolactone monomers. The introduction of
fluorine atoms to the meta-positions of the phenyl rings improved
the catalytic activity; whereas the para-fluoro substituent led
to an inverse result. CF3 substitution at either the para- or
meta-position of the phenyl rings improved the activity of the
corresponding aluminium complex, with the meta-CF3 substituted
complex 3h showing the highest catalytic activity among the
investigated b-diketiminate aluminium complexes. In addition,
increasing the steric hindrance of the ortho-substituents on the aryl
ring also resulted in a decrease in catalytic activity. The ROP of e-
caprolactone initiated by b-diketiminate aluminium complexes 3a–
m are not well-controlled, the measured molecular weights deviate
significantly from the theoretical values. End group analysis of
the obtained oligomer sample excludes the possibility that the
polymerization is initiated from the Al–alkyl bond; the Al–amido
moiety is most likely the active site for initiation.
3
3J = 7.4 Hz, 8H, o-Ar-H). 7.10 (t, J = 7.4 Hz, 4H, p-Ar-H),
3
7.20 (t, J = 7.4 Hz, 8H, m-Ar-H). EI-MS: m/z (%) 583 (100,
[M+ − Me]).
Typical polymerization procedure
◦
To a solution of e-caprolactone (e-CL) in toluene kept at 80 C,
a solution of b-diketiminate aluminium complex 3a in toluene
was injected. The concentration of e-CL was 1 M. 1 mL of poly-
merization aliquots were withdrawn at appropriate time intervals
under the protection of argon and quenched with methanol. After
1
removal of the volatiles, the residue was subjected to H NMR
analysis. Monomer conversion was determined by observing the
integration of monomer vs. polymer methylene resonance in the
1H NMR (CDCl3, 500 MHz) spectrum. The polymer was purified
by dissolving the crude samples in CH2Cl2 and precipitating into
methanol. The obtained polymers were further dried in a vacuum
oven at 60 ◦C for 24 h. The dry polymer samples were subjected to
viscosity measurements and in selected cases analyzed by GPC.
Acknowledgements
This work is subsidized by the National Basic Research Program
of China (2005CB623801), National Natural Science Foundation
of China (NNSFC, 20604009), the Program for New Century
Excellent Talents in University (for H. Ma, NCET-06-0413),
Shanghai Rising-Star Program (06QA14014), and the Scientific
Research Foundation for the Returned Overseas Chinese Scholars
(SRF for ROCS, SEM). All the financial support is gratefully
acknowledged.
X-Ray diffraction measurements
The crystallographic data for complexes 3k and 7 were collected on
a Bruker AXSD8 diffractometer with graphite-monochromated
◦
˚
Mo-Ka(k = 0.71073 A) radiation. All data were collected at 20 C
using omega-scan techniques. The structures of 3k and 7 were
solved by direct methods and refined using Fourier techniques.
An absorption correction based on SADABS was applied.85 All
non-hydrogen atoms were refined by full-matrix least-squares on
F2 using the SHELXTL program package.86 Hydrogen atoms
were located and refined by the geometry method. The cell
refinement, data collection, and reduction were done using Bruker
SAINT.87 The structure solution and refinement were performed
with SHELXS-9788 and SHELXL-9789 respectively. For further
crystal data and details of measurements see Tables 1, 2 and 3.
Molecule structures were generated using ORTEP.90
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1
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