Anilido-aldimine aluminum complexes: Synthesis, characterization and lactide polymerization

Article abstract of DOI:10.1016/j.jorganchem.2012.02.003

The synthesis of six bimetallic dimethyl aluminum anilido-aldimine complexes, active for the ring opening polymerization of rac-lactide, is reported. An efficient synthetic route to these ligands utilizing isolated lithium amide salts allows for the synthesis of novel cyclohexylamide- substituted ligands, as well as improving the yields of diisopropylphenylamino- substituted ligands, with both ethyl and propyl backbones. Less sterically encumbered methylamide-substituted ligands were prepared through the condensation of ortho-methylaminobenzaldehyde with the appropriate diamine. A monometallic intermediate species, L(AlMe₂), was isolated and crystallographically characterized, illustrating the preference these ligands display for bis(bidentate) coordination. L(AlMe₂)₂ complexes 8-13 are efficient mediators of the ring opening polymerization of rac-lactide, with solution polymerizations displaying first-order rate constants, molecular weights very close to the theoretical values and polydispersity indexes as low as 1.07.

Full text of DOI:10.1016/j.jorganchem.2012.02.003

Journal of Organometallic Chemistry 706-707 (2012) 106e112
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Anilido-aldimine aluminum complexes: Synthesis, characterization and lactide
polymerization
Laura E.N. Allan a, Justin A. Bélanger a, Laura M. Callaghan a, Donald J.A. Cameron a, Andreas Decken b,
,
Michael P. Shavera
*
a Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE, C1A 4P3, Canada
b Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, NB, E3B 5A3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of six bimetallic dimethyl aluminum anilido-aldimine complexes, active for the ring
opening polymerization of rac-lactide, is reported. An efficient synthetic route to these ligands utilizing
isolated lithium amide salts allows for the synthesis of novel cyclohexylamide-substituted ligands, as
well as improving the yields of diisopropylphenylamino-substituted ligands, with both ethyl and propyl
backbones. Less sterically encumbered methylamide-substituted ligands were prepared through the
Received 17 November 2011
Received in revised form
30 January 2012
Accepted 1 February 2012
condensation of ortho-methylaminobenzaldehyde with the appropriate diamine.
A monometallic
Keywords:
Aluminum
Lactide
Ligand design
Ring opening polymerization
intermediate species, L(AlMe2), was isolated and crystallographically characterized, illustrating the
preference these ligands display for bis(bidentate) coordination. L(AlMe2)2 complexes 8e13 are efficient
mediators of the ring opening polymerization of rac-lactide, with solution polymerizations displaying
first-order rate constants, molecular weights very close to the theoretical values and polydispersity
indexes as low as 1.07.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
l-lactide with polydispersities (PDIs) of 1.1e1.2 [6]. Bis(bidentate)
complexes of zinc, mimicking the
b-diketiminato complexes
A recent adaptation of the classic phenoxyimine ligand frame-
work has been the N-only donor ligands coined the anilido-
aldimines. Replacing the anionic alkoxide donor with an amido
functionality has offered different steric and electronic tunability
in these systems, however their application and scope remain
largely underdeveloped. Ligand frameworks include bidentate
[1e3], tridentate [4e7] and bis(bidentate) [8e11] systems, as
shown in Fig. 1.
These ligands have been applied to some speciality applica-
tions. Dialkylaluminum complexes supported by bidentate
ligands exhibit tunable fluorescence in solution and solid state [2],
while rare-earth yttrium, scandium and lutetium complexes
supported by tridentate ligands were tested as catalysts for the
ring-opening polymerization of ε-caprolactone and l-lactide,
exhibiting high activity but only moderate control [4,5]. Magne-
sium and zinc complexes incorporating tridentate ligands
were more effective, polymerizing both ε-caprolactone and
developed by Coates et al. [12], have been used successfully to
control the copolymerization of epoxides and carbon dioxide
[8,9]. Similar bis(bidentate) aluminum and zinc complexes are
active catalysts for the living ring opening polymerization of
ε-caprolactone [11].
Our interest in this ligand set arises from their close relationship
to the salen ligand. Aluminum salen complexes were first reported
by Spassky et al. and promoted a moderate isotactic bias for the
polymerization of lactide (Pm ¼ 0.68) [13]. Increased steric bulk
improved the isospecificity of the polymerization, with Pm values as
high as 0.88 when a 2,4-di(tert-butyl)phenol was used [14]. The
steric bulk proximal to the active site has been suggested to be the
determining factor in promoting chain-end control of polymer
tacticity [14,15]. Continued efforts to improve the design of lactide
polymerization catalysts is an essential area for growth in polymer
science as systems with improved activity or stereocontrol are
crucial in developing commercially relevant, sustainable and
degradable alternatives to petroleum-derived polymeric materials.
In this vein, it was envisaged that moving the bulky R0 substituents
from the ortho-substituted aromatic rings of the salen ligand to the
amido donor of an anilido-aldimine framework would improve
isospecificity (Fig. 2).
* Corresponding author. Tel.: þ1 902 566 9375; fax: þ1 902 566 0632.
E-mail address: mshaver@upei.ca (M.P. Shaver).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.02.003

L.E.N. Allan et al. / Journal of Organometallic Chemistry 706-707 (2012) 106e112
107
crystals were coated in Paratone-N oil, mounted using a polyimide
MicroMount and frozen in the cold nitrogen stream of the goni-
ometer. A hemisphere of data was collected on a Bruker AXS P4/
SMART 1000 diffractometer using
u and q scans with a scan width
of 0.3ꢁ and 30 s exposure times. The detector distance was 5 cm.
The data were reduced (SAINT) [18] and corrected for absorption
(SADABS) [19]. The structure was solved by direct methods and
refined by full-matrix least squares on F2(SHELXTL) [20] with
graphics processed using ORTEP [21]. For 7, the crystal was
a multiple twin and the major component determined. One of the
atom positions was disordered and the site occupancies deter-
mined using an isotropic model at 0.5 (N(3), N(30)) and fixed in
subsequent refinement cycles.
2.2. Synthesis
2.2.1. Ligand synthesis
2.2.1.1. Synthesis of N,N0ebis[(2-(2,6-diisopropylphenylamino)ben-
zylidene)]-1,2-ethanediamine (1). 3.50
g (19.7 mmol) of 2,6-
diisopropylaniline (177.29 g molꢀ1) was dissolved in 12 mL of
tetrahydrofuran. To this stirred solution was added 12.4 mL
(19.7 mmol) of n-butyllithium (1.6 M solution in hexanes) drop-
wise. Evolution of heat and gas was observed. Reaction contents
were allowed to stir at room temperature for 12 h, at which point
an off-white solid was observed. This precipitate was collected by
vacuum filtration and utilized without further purification. Yield:
3.25 g (90%). 3.25 g (17.7 mmol) of lithium 2,6-diisopropylanilide
(183.22 g molꢀ1) was added to 40 mL of THF to create a reaction
slurry. This mixture was placed in an addition funnel and stored
prior to use. Concurrently, 2.19 g (8.1 mmol) of N,N0-bis[(2-
fluorophenyl)methylene]-1,2-ethanediamine was dissolved in
5 mL of THF in a 125 mL Erlenmeyer flask and cooled to ꢀ35 ꢁC. The
LiC12H18N slurry was then added dropwise to this stirred and
chilled solution resulting in the formation of a deep red solution.
The reaction was sealed under nitrogen and allowed to stir at room
temperature for 12 h. Under an open atmosphere, 20 mL of N2-
sparged deionized water was added dropwise to the reaction
mixture forming an orange suspension. The mixture was poured
into a separatory funnel and extracted 3 times with hexanes
(100 mL). The organic phase was collected and dried in vacuo to
yield a yellow oil which was recrystallized from acetonitrile to
afford a white product. C40H50N4 (586.40 g molꢀ1) Yield: 3.56 g
(75%). Anal. Calcd for C40H50N4: C, 81.87 H, 8.59; N, 9.55. Found: C,
Fig. 1. Anilido-aldimine ligands and complexes.
2. Materials and methods
2.1. General experimental procedures
All chemicals and solvents were obtained from Sigma Aldrich in
the highest available purity unless otherwise stated. Purasorb d,l-
lactide and l-lactide (PURAC Biomaterials) were sublimed under
vacuum three times prior to use. 2,6-diisopropylaniline (90%),
benzyl alcohol (99%) and cyclohexylamine were dried over calcium
hydride and distilled under a nitrogen atmosphere prior to use.
N,N0-bis[(2-fluorophenyl)methylene]-1,2-ethanediamine
N,N0-bis[(2-fluorophenyl)methylene]-1,3-propanediamine
[11],
[10]
and ortho-methylaminobenzaldehyde [16] were prepared via
previously published procedures. Dry, reagent grade pentane,
tetrahydrofuran and toluene were collected from an Innovative
Technologies solvent purification system, consisting of columns
containing alumina and a copper catalyst. Acetonitrile was dried
over calcium hydride and freshly distilled as needed. All solvents
were degassed by three consecutive freeze-pump-thaw cycles
prior to use. Air-sensitive syntheses were performed in an MBraun
Labmaster glovebox equipped with a ꢀ35 ꢁC freezer and [O2] and
[H2O] analyzers or under a nitrogen atmosphere on a dual mani-
81.74; H, 8.99; N, 9.35. 1H NMR (300 MHz, CDCl3, 25 ꢁC)
d: 10.47 (s,
fold Schlenk line utilizing standard Schlenk techniques. 1H and 13
C
2H, NH), 8.39 (s, 2H, HC]N), 7.33e7.20 (m, 4H AreH), 7.10 (m, 2H,
AreH), 7.03 (m, 2H, AreH), 6.55 (m, 4H, AreH), 6.16 (m, 2H, AreH),
3.90 (s, 4H, CH2CH2), 3.07 (s, 4H, CH3CHCH3), 1.27 (dd, 24H, CH3,
NMR spectra were collected on a 300 MHz Bruker Avance Spec-
trometer. GPC analyses were performed on a PolymerLabs GPC50
system equipped with two Jordi Gel DVB mixed bed columns
(300 mm ꢂ 7.8 mm) and a refractive index detector. Samples were
dissolved and eluted in HPLC grade THF at a flow rate of 1 mL minꢀ1
at 50 ꢁC. Molecular weights were measured and corrected relative
to styrene standards [17]. Elemental analyses were conducted by
Guelph Analytical Laboratories. Crystals of 7 and 9 were grown by
precipitation from an acetonitrile solution at ꢀ35 ꢁC. Single
5.8 Hz, 3.6 Hz). 13C NMR (75 MHz, CDCl3, 25 ꢁC)
d: 161.0, 142.3,
138.8, 132.1, 131.3, 127.7, 125.8, 124.4, 123.1, 119.1, 115.7, 62.3, 28.5,
23.5, 23.3.
2.2.1.2. Synthesis of N,N0ebis[(2-(2,6-diisopropylphenylamino)ben-
zylidene)]-1,3-propanediamine (2). Ligand precursor 2 was prepared
analogously to 1 with 3.50 g (19.7 mmol) of 2,6-diisopropylaniline
(177.29 g molꢀ1), 12.4 mL (19.7 mmol) of n-butyllithium (1.6 M solu-
tion in hexanes) and 2.32 g (8.1 mmol) of N,N0-bis[(2-fluorophenyl)
methylene]-1,3-propanediamine. C41H52N4 (600.88 g molꢀ1) Yield:
3.94 g (81%). Anal. Calcd for C41H52N4: C, 81.95 H, 8.72; N, 9.32. Found:
C, 81.75; H, 8.90; N, 9.35. 1H NMR (300 MHz, CDCl3, 25 ꢁC)
d: 10.42 (s,
2H, NH), 8.42 (s, 2H, HC]N), 7.33 (m, 2H AreH), 7.17 (m, 2H, AreH),
7.00 (m, 2H, AreH), 6.58 (m, 4H, AreH), 6.24 (m, 2H, AreH), 3.90
(m, 4H, CH2CH2CH2), 3.07 (m, 4H, CH3CHCH3), 2.22, (p, 2H,
CH2CH2CH2, 6.8 Hz, 13.7 Hz), 1.27 (dd, 24H, CH3, 5.9 Hz, 3.7 Hz). 13
C
NMR (75 MHz, CDCl3, 25 ꢁC)
125.4, 124.6, 123.6, 119.4, 116.1, 62.5, 32.6, 28.9, 23.8, 23.5.
d: 162.3, 144.7, 138.6, 132.3, 131.3, 128.3,
Fig. 2. Proximity of substituent R0 to the metal centre in salen and anilido-aldimine
type ligand frameworks.

108
L.E.N. Allan et al. / Journal of Organometallic Chemistry 706-707 (2012) 106e112
2.2.1.3. Synthesis of N,N0ebis[(2-(cyclohexylamino)benzylidene)]-
1,2-ethanediamine (3). Ligand precursor 3 was prepared analo-
in 20 mL of toluene at ꢀ78 ꢁC. The reaction mixture was held at
this temperature for 1 h and then allowed to warm to room
temperature and transferred via cannula into an ampoule. The
ampoule was sealed under nitrogen and stirred at 80 ꢁC for 12 h,
forming a clear yellow solution. Removal of solvent in vacuo
produced a yellow paste that was recrystallized from acetonitrile
to afford 5 in moderate yield. C42H55AlN4 (642.89 g molꢀ1) Yield:
gously to
1 with 5.50 g (55.5 mmol) of cyclohexylamine
(99.17 g molꢀ1), 34.7 mL (55.5 mmol) of n-butyllithium (1.6 M
solution in hexanes) and 3.21 g (11.8 mmol) of N,N0-bis[(2-
fluorophenyl)methylene]-1,2-ethanediamine. The crude red oil
(3.50 g, 80% purity) was loaded on a packed alumina column (25 g)
pretreated with 50 mL of 1% (v,v) solution of triethylamine (NEt3)
and hexane. The product was eluted with hexanes (300 mL) to
1.20 g, 62%. 1H NMR (300 MHz, CDCl3, 25 ꢁC)
d: 10.29 (br, 1H, NH),
8.47 (s, 1H, CH]NAr), 8.10 (s, 1H, CH]NAr), 7.30e6.01 (m, 14H,
AreH), 3.96 (s, 2H, NCH2), 3.65 (s, 2H, NCH2), 3.45 (m, 2H, CHCH3),
3.32 (m, 2H, CHCH3), 1.54 (d, 12H, CH3CH), 1.25 (d, 12H,
afford an analytically pure yellow oil. C28H38N4 (430.31 g molꢀ1
)
Yield: 2.04 g (40%). Anal. Calcd for C28H38N4: C, 78.10 H, 8.89; N,
13.01. Found: C, 78.04; H, 9.01; N, 12.95. 1H NMR (300 MHz, CDCl3,
CH3CH), ꢀ0.84 (s, 6H, AlCH3). 13C NMR (75 MHz, CHCl3, 25 ꢁC)
d:
25 ꢁC)
d: 9.44 (br s, 2H, NH), 8.37 (s, 2H HC]N), 8.00 (m, 2H, AreH),
165.7, 157.0, 142.5, 137.0, 136.1, 133.9, 131.8, 129.1, 128.9, 128.7,
128.5, 126.3, 126.1, 116.0, 115.5, 115.0, 114.7, 69.4, 67.2, 28.7, 28.5,
23.6, 23.3, ꢀ8.3.
7.39 (m, 2H, AreH) 6.70 (m, 2H, AreH) 6.57 (m, 2H, AreH) 3.48 (s,
4H, CH2CH2) 2.56 (m, 2H, NHCHC5H10), 2.00e1.05 (m, 20H,
2(C5H10)). 13C NMR (75 MHz, CDCl3, 25 ꢁC)
d: 162.0, 149.6, 133.8,
133.4, 117.3, 114.6, 113.8, 61.9, 61.2, 33.4, 26.7, 25.1.
2.2.2.2. Synthesis of [1](AlMe2)2 (8). 1.00 g (1.71 mmol) of 1 was
dissolved in 3 mL of toluene, in a glass ampoule equipped with
magnetic stir bar. To this stirred solution was added 1.71 mL of
trimethylaluminum (TMA, 2.0 M solution in heptane), dropwise
with stirring. Heat and evolution of gas was observed. The glass
ampoule was sealed, removed from glovebox and allowed to stir at
110 ꢁC for 24 h. The volatiles were removed under reduced pressure
to reveal an impure orange solid. Washing with pentane gave the
2.2.1.4. Synthesis of N,N0ebis[(2-(cyclohexylamino)benzylidene)]-
1,3-propanediamine (4). Ligand precursor 4 was prepared analo-
gously to
1 with 5.50 g (55.5 mmol) of cyclohexylamine
(99.17 g molꢀ1), 34.7 mL (55.5 mmol) of n-butyllithium (1.6 M
solution in hexanes) and 3.38 g (11.8 mmol) of N,N0-bis[(2-
fluorophenyl)methylene]-1,3-propanediamine. The crude red oil
(3.00 g, 70% purity) was loaded on a packed alumina column (25 g)
pretreated with 50 mL of 1% (v,v) solution of triethylamine (NEt3)
and hexane. The product was eluted with hexanes (300 mL) to
aluminum complex in high yield. C44H60NAl2N4 (698.44 g molꢀ1
)
Yield: 0.89 g (75%). Anal. Calcd for C44H60Al2N4: C, 75.61; H, 8.65; N,
8.02. Found: C, 75.60; H, 8.47; N, 8.04. 1H NMR (300 MHz, C6D6,
afford an analytically pure yellow oil. C29H40N4 (444.33 g molꢀ1
)
25 ꢁC)
d: 7.67 (s, 2H, HC]N), 7.37e7.06 (m, 8H, AreH), 6.82 (d, 2H,
Yield: 1.57 g (31%). Anal. Calcd for C29H40N4: C, 78.33 H, 9.07; N,
AreH, 9.0 Hz), 6.34 (t, 2H, AreH, 7.0 Hz) 3.90 (s, 4H, CH2CH2), 3.48
(m, 4H, CH3CHCH3), 1.48 (d, 12H, CH3, 7.0 Hz), 1.18 (d, 12H, CH3,
12.60. Found: C, 78.41; H, 9.10; N, 12.49. 1H NMR (300 MHz, CDCl3,
25 ꢁC)
d
: 9.41 (s, 2H, NH) 8.34 (s, 2H, HC]N) 7.98 (m, 2H, AreH) 7.18
7.0 Hz), ꢀ0.33 (s, 12H, AleCH3). 13C NMR (75 MHz, C6D6, 25 ꢁC)
d:
(m, 2H, AreH) 6.67 (m, 2H, AreH) 6.56 (m, 2H, AreH) 3.11 (m, 4H,
CH2CH2CH2) 2.55 (m, 2H, NHCHC5H10), 2.07e1.14 (m, 22H,
161.0, 142.3, 138.8, 132.1, 131.3, 127.7, 125.8, 124.4, 123.1, 119.1, 115.7,
62.3, 28.5, 23.6, 23.3, ꢀ9.1.
CH2CH2CH2 and (C5H10)2). 13C NMR (75 MHz, CDCl3, 25 ꢁC)
d: 161.7,
149.8,134.5, 133.2, 117.7,114.2, 113.6, 62.3, 61.6, 33.1, 33.0, 26.5, 25.4.
2.2.2.3. Synthesis of [2](AlMe2)2 (9). Prepared as for 8, using 1.00 g
(1.66 mmol) of 2 and 1.66 mL of TMA in 3 mL of toluene.
C45H62Al2N4 (712.46 g molꢀ1) Yield: 0.97 g (82%). Anal. Calcd for
C45H62Al2N4: C, 75.81; H, 8.77; N, 7.86. Found: C, 75.61; H, 8.51; N,
2.2.1.5. Synthesis of N,N0ebis[(2-(methylamino)benzylidene)]-1,2-
ethanediamine (5). A solution of o-aminobenzaldehyde (3.22 g,
24 mmol) in MeOH (50 mL) was stirred at ambient temperature as
1,2-diaminoethane (0.72 g, 12 mmol) was added dropwise. The
yellow solution was heated to reflux for 1.5 h and then stirred at
room temperature overnight. The off-white precipitate was
collected and recrystallized from MeOH/CHCl3 to yield a white
solid, 2.04 g, 58%. Anal. Calcd for C18H22N4: C, 73.44; H, 7.53; N,
19.03. Found: C, 73.55; H, 7.64; N, 18.96. 1H NMR (300 MHz, CDCl3):
7.75. 1H NMR (300 MHz, C6D6, 25 ꢁC)
d: 7.57 (s, 2H, HC]N),
7.35e7.10 (m, 8H, AreH), 6.94 (d, 2H, AreH, 9 Hz), 6.43 (t, 2H, AreH,
7.0 Hz), 3.90 (m, 4H, CH2CH2CH2), 3.50 (m, 2H, CH2CH2CH2), 3.17 (m,
4H, CH3CHCH3), 1.48 (d, 12H, CH3, 7.0 Hz), 1.18(d, 12H, CH3,
7.0 Hz), ꢀ0.34 (s, 12H, AleCH3). 13C NMR (75 MHz, C6D6, 25 ꢁC)
d:
162.3, 144.7, 138.6, 132.3, 131.3, 128.3, 125.4, 124.6, 123.6, 119.4,
116.1, 62.5, 32.6, 28.9, 23.8, 23.5, ꢀ9.1.
d
8.91 (br s, 2H, NH), 8.37 (s, 2H HC]N), 7.28 (m, 2H, AreH), 7.21 (m,
2H, AreH), 6.63 (m, 4H, AreH), 3.86 (s, 4H, CH2CH2) 2.84 (d, 6H,
2.2.2.4. Synthesis of [3](AlMe2)2 (10). Prepared as for 8 using 1.00 g
(2.32 mmol) of 3 and 2.32 mL of TMA in 3 mL of toluene.
C32H48Al2N4 (542.71 g molꢀ1) Yield: 0.82 g (65%). Anal. Calcd for
C32H48Al2N4: C, 70.82; H, 8.91; N, 10.32. Found: C, 71.00; H, 8.69; N,
2JNH ¼ 5, CH3) ppm. 13C NMR (75 MHz, CDCl3):
d 165.4 (HC]N),
150.3, 117.1 (Ar Cq), 133.9, 131.5, 114.2, 109.5 (Ar CeH), 62.4 (CH2),
29.2 (CH3) ppm.
10.50. 1H NMR (300 MHz, C6D6, 25 ꢁC)
d: 7.57 (s, 2H, HC]N),
2.2.1.6. Synthesis of N,N0ebis[(2-(methylamino)benzylidene)]-1,3-
propanediamine (6). Prepared as for 5 using o-aminobenzaldehyde
(2.00 g, 15 mmol) and 1,3-diaminopropane (0.55 g, 7 mmol) to yield
1.51 g, 66%. Anal. Calcd for C19H24N4: C, 73.99; H, 7.84; N, 18.17.
7.22e7.14 (m, 4H, AreH), 6.84 (d, 2H, AreH, 8 Hz), 6.42 (t, 2H, AreH,
7 Hz), 3.48 (s, 4H, CH2CH2) 2.56 (m, 2H, NHCHC5H10) 2.00e1.05 (m,
20H, (C5H10)2), ꢀ0.17 (s, 12H, AleCH3). 13C NMR (75 MHz, C6D6,
25 ꢁC)
d: 162.0, 149.6, 133.8, 133.4, 117.3, 114.6, 113.8, 61.9, 61.2, 33.4,
Found: C, 74.05; H, 8.00; N, 18.25. 1H NMR (300 MHz, CDCl3):
d
8.97
26.7, 25.1, ꢀ5.1.
(br s, 2H, NH), 8.36 (s, 2H HC]N), 7.28 (m, 2H, AreH), 7.20 (m, 2H,
AreH), 6.66 (m, 4H, AreH), 3.67 (t, 4H, 3JHH ¼ 7, CH2CH2CH2) 2.94 (d,
2.2.2.5. Synthesis of [4](AlMe2)2 (11). Prepared as for 8 using 1.00 g
(2.25 mmol) of 4 and 2.25 mL of TMA in 3 mL of toluene.
C33H50Al2N4 (556.74 g molꢀ1) Yield: 0.85 g (68%). Anal. Calcd for
C33H50Al2N4: C, 71.19; H, 9.05; N, 10.06. Found: C, 71.25; H, 9.20; N,
6H, 2JNH ¼ 5, CH3), 2.08 (pentet, 2H, 3JHH ¼ 7, CH2CH2CH2) ppm. 13
C
NMR (75 MHz, CDCl3):
d 164.8 (HC]N), 150.4, 117.3 (Ar Cq), 133.9,
131.5, 114.4, 109.7 (Ar CeH), 59.5, 33.1 (CH2), 29.5 (CH3) ppm.
9.96. 1H NMR (300 MHz, C6D6, 25 ꢁC)
d: 7.57 (s, 2H, HC]N),
2.2.2. Aluminum complex synthesis
2.2.2.1. Synthesis of [1]AlMe2 (7). AlMe3 (3.0 mL, 1.0 M in toluene,
2.1 mmol) was added dropwise to a solution of 1 (1.76 g, 3.0 mmol)
7.22e7.14 (m, 4H, AreH), 6.84 (d, 2H, AreH, 8 Hz), 6.42 (t, 2H, AreH,
7 Hz), 3.48 (m, 4H, CH2CH2CH2), 3.22 (m, 2H, CH2CH2CH2), 2.56 (m,
2H, NHCHC5H10) 2.00e1.05 (m, 20H, 2(C5H10)), ꢀ0.17 (s, 12H,

L.E.N. Allan et al. / Journal of Organometallic Chemistry 706-707 (2012) 106e112
109
AleCH3). 13C NMR (75 MHz, C6D6, 25 ꢁC)
117.7, 114.2, 113.6, 62.3, 61.6, 33.1, 33.0, 26.5, 25.4, ꢀ10.4.
d
: 161.7, 149.8, 134.5, 133.2,
precursors with lithium methylamide yielded a complex mixture
of products which could not be separated. Proligands 5 and 6
were successfully prepared through modification of a literature
procedure [16], through the condensation of ortho-methyl-
aminobenzaldehyde with the appropriate diamine. The general
reaction scheme for ligand synthesis is outlined in Scheme 1.
Purification of 1e4 was challenging, as the solubility of both the
ligand and fluorinated precursor precluded a rapid filtration and
washing methodology. Slow recrystallization of 1 and 2 from
acetonitrile afforded these desired ligands in good yields, while
cyclohexyl derivatives 3 and 4 were purified on alumina columns
pretreated with a 1% v/v solution of triethylamine (NEt3) in hexanes
to limit ligand binding to the alumina. Ligands 5 and 6 were more
straightforward to purify and were recrystallized from a methanol/
chloroform mixture. 1H and 13C NMR spectroscopy confirmed
product formation and purity, particularly noted by the presence of
2.2.2.6. Synthesis of [5](AlMe2)2 (12). 0.50 g (2 mmol) of 5 was
dissolved in 15 mL of toluene, in a glass ampoule equipped with
magnetic stir bar. 1.18 g of trimethylaluminum (TMA, 2.0 M solution
in heptane) was added dropwise, with stirring. Heat and evolution
of gas was observed. The glass ampoule was sealed, removed from
glovebox and allowed to stir at 110 ꢁC for 24 h. The volatiles were
removed under reduced pressure to yield an orange residue which
was washed with pentane, filtered and dried under vacuum to give
a yellow precipitate. Yield: 0.41 g, 69%. Anal. Calcd for C22H32Al2N4:
C, 65.01; H, 7.93; N, 13.78. Found: C, 65.30; H, 8.04; N, 13.95.1H NMR
(300 MHz, CDCl3) d: 7.86 (s, 2H, HC]N), 7.33e7.28 (m, 2H, AreH),
3
4
3
6.95 (dd, 2H, JHH ¼ 8, JHH ¼ 2, AreH), 6.72 (d, 2H, JHH ¼ 8,
AreH), 6.39 (m, 2H, AreH), 3.81 (s, 4H, CH2CH2), 2.91 (s, 6H,
NeCH3), ꢀ0.70 (s, 12H, AleCH3) ppm. 13C NMR (75 MHz, CDCl3)
d:
the NH peak at
6). In the case of 1e4, an upfield shift of the imine signals from
8.6 ppm in the fluorinated precursors to w 8.4 ppm in the
anilido-aldimine proligands was also observed, whereas the
d 10.4 ppm (1, 2), d 9.4 ppm (3, 4) and d 8.9 ppm (5,
171.7 (HC]N), 157.7, 115.2 (Ar Cq), 137.4, 137.0, 113.4, 113.2 (Ar CeH),
56.7 (CH2), 33.1 (NCH3), ꢀ9.0 (AleCH3) ppm.
d
d
2.2.2.7. Synthesis of [6](AlMe2)2 (13). Prepared as for 12 using
0.30 g (1 mmol) of 6 and 0.67 g (2 mmol) of TMA in 10 mL of
toluene. Yield 0.19 g, 41%. Anal. Calcd for C23H34Al2N4: C, 65.69; H,
8.15; N, 13.32. Found: C, 65.73; H, 8.24; N, 13.46. 1H NMR (300 MHz,
appearance of the imine resonances at d 8.4 ppm confirmed the
formation of 5 and 6.
Aluminum complexes supported by these ligands were
prepared via protonolysis of the proligand and trimethylaluminum
(TMA). Initial experiments focused on a 1:1 ratio of 1:TMA. This
reaction gave a mixture of products at high (110 ꢁC) and ambient
temperatures. Careful interpretation of the 1H NMR spectrum of
crude reaction products suggested that the major product was
a bimetallic complex, (L)(AlMe2)2. Other products included free
ligand and an unknown aluminum-containing species. Lowering
the reaction temperature to ꢀ78 ꢁC allowed for isolation of the
monometallic compound after recrystallization from acetonitrile.
NMR and X-ray crystallographic characterization showed that [1]
AlMe2, 7, had formed: the ligand prefers a bidentate coordination
mode, with a free pendant arm. A similar structure was prepared
purposefully as an intermediate in the preparation of hetero-
bimetallic complexes [22]. The molecular structure for 7 is shown
in Fig. 3 while relevant bond lengths, angles and crystallographic
parameters are given in Tables 1 and 2. Repeated attempts to grow
crystals which would yield a higher quality molecular structure
were unsuccessful.
CDCl3) d: 7.96 (s, 2H, HC]N), 7.37e7.31 (m, 2H, AreH), 7.08 (dd, 2H,
3JHH ¼ 8, 4JHH ¼ 2, AreH), 6.72 (d, 2H, 3JHH ¼ 9, AreH), 6.47 (m, 2H,
AreH), 3.60 (t, 4H, 3JHH ¼ 8, CH2CH2CH2), 2.88 (s, 6H, NeCH3), 2.20
(pentet, 2H, 3JHH ¼ 8, CH2CH2CH2), ꢀ0.75 (s, 12H, AleCH3) ppm. 13
C
NMR (75 MHz, CDCl3) d: 170.4 (HC]N), 157.6, 120.5 (Ar Cq), 137.2,
136.8, 115.3, 113.2 (Ar CeH), 55.2, 31.1 (CH2), 33.1 (NCH3), ꢀ9.2
(AleCH3) ppm.
2.3. Lactide polymerization
0.020 g of the desired precatalyst (8e13), 2.00 M equivalents of
benzyl alcohol and 200 M equivalents of rac-lactide were added to
an oven-dried ampoule charged with a magnetic stir bar and 3 mL
of toluene when appropriate. The ampoule was sealed and heated
to 120 ꢁC (bulk polymerization) or 70 ꢁC (solution polymerization)
with stirring for the desired period of time. The ampoule was
cooled to room temperature and the resulting viscous mixture was
dissolved in a 10:1 v:v dichloromethane:methanol solution. Once
fully dissolved, the solution was left to stir at ambient temperature
for 30 min and then precipitated by dropwise addition of the
solution into 100 mL of cold methanol. The resulting white
precipitate was filtered and dried in vacuo to constant weight.
Samples were analyzed by 1H{1H} NMR spectroscopy and gel-
permeation chromatography.
As expected, the Al(1)eN(2) bond is shorter than the Al(1)eN(1)
bond, confirming the stronger amido-aluminum bond. The imine
bond length is short, although significantly elongated by w0.15 Å
3. Results and discussion
Six potentially tetradentate anilido-aldimine ligands were
targeted in this study, containing 2,6-diisopropylphenyl, cyclo-
hexyl or methyl substituents on the amine donors and ethyl or
propyl chains bridging the imine functionalities. Proligands 1e4
were synthesized by modifying previously reported proce-
dures [10,11], with the fluorinated precursors first prepared via
the acid-catalyzed condensation of 2-fluorobenzaldehyde and
the appropriate diamine. Controlled addition of lithium 2,6-
diisopropylphenylamide or lithium cyclohexylamide to the
fluorinated precursor at ꢀ35 ꢁC promoted a metathesis reaction,
forming the desired products in good yield. Of note, isolation of
the lithium amido intermediates was important to maximize
ligand yields. This route was unsuccessful for the synthesis of N-
methyl substituted ligands, as the reaction of the fluorinated
Scheme 1. Synthesis of substituted anilido-aldimine ligands 1e6.

110
L.E.N. Allan et al. / Journal of Organometallic Chemistry 706-707 (2012) 106e112
Table 2
Crystallographic data and details for 7 ([1]AlMe2).
Empirical Formula
Crystal system
a, b, c (Å)
C42H55AlN4
Monoclinic
13.496(2), 13.913(3), 20.820(5)
90, 99.431(2), 90
3856.7(13)
4
1.107
0.71073
173(1)
1.53e27.50ꢁ
Formula weight
Space group
642.88
P21/c
a, b, g
(ꢁ)
V, Å3
Z
Total reflections
Unique reflections
25 597
8584
437
0.1010
0.1701
1.052
Dcalc, mg mꢀ3
Wavelength, Å
T, K
Parameters
a
R1
b
wR2
q
range (ꢁ)
Goodness-of-fit
a
R1 ¼ S (Fo ꢀ Fc)/S Fo.
P
P
wR2 ¼ ð wðF2o ꢀ F2c Þ2= ðwF4oÞ1=2Þ.
b
Fig. 3. ORTEP drawing (spheroids at 50% probability) of [1]AlMe2, 7. Hydrogen atoms
omitted for clarity.
when compared to the unbound imine fragment. Bond angles
reveal that chelation of the ligand to the aluminum centre causes
a pinching of the ligand framework and a narrowing of the bond
angles. This results in a distorted tetrahedral geometry around the
aluminum, exemplified by the small N(2)eAl(1)eN(1) bite angle of
94.38(8)ꢁ. 1H NMR spectroscopy confirms both the monometallic
nature of the complex and the pendant arm in solution. Further
attempts with 1 and 2e6 to isolate a tetradentate aluminum
complex [L]AlMe by controlling reaction temperatures, times and
concentrations were unsuccessful.
Exclusive formation of the bimetallic complexes [L](AlMe2)2
(8e13) is readily achieved by altering the reagent ratio to 1:2
ligand:TMA, greatly improving reaction yields. The complexes are
shown in Fig. 4. Complex 8 has been previously reported and
crystallographically characterized [11]. For complex 9, crystals were
grown from a saturated acetonitrile solution and analyzed by single
crystal X-ray crystallography. The molecular structure for 9 is
shown in Fig. 5 while relevant bond lengths, angles and crystallo-
graphic parameters are given in Tables 3 and 4. The symmetry
observed in the crystal structure ensures the two aluminum
centres, and thus catalytically active sites, are equivalent. The
bidentate coordination of the anilido-aldimine ligand is main-
tained, linked by the flexible aliphatic chain to give the bimetallic
product. The bond lengths and angles around the metal centres in 9
are nearly unchanged from the monometallic structure 7 and are
also similar to related and previously published complexes [10,11].
Fig. 4. Bimetallic aluminum complexes 8e13.
Synthesis of the cyclohexylamido substituted complexes 10 and
11 suggest an even higher propensity to favour the bimetallic
complex, despite their reduced steric demands. Reactions of 3 and 4
with one equivalent of TMA showed mixtures of products,
including the bimetallic complexes, even at low temperatures.
Similar results were obtained when using the least sterically
encumbered ligands 5 and 6, with bimetallic complexes 12 and 13
the major product under all attempted reaction conditions.
Synthesis of 10e13 was improved through the use of 2:1 TMA:li-
gand ratios and high temperatures, affording the desired bimetallic
complexes in moderate yields. Diagnostic resonances, such as the
AlCH3 signal at
this assignment. Reaction progress was monitored over time by
observing the loss of the NH resonance at
9.4 ppm in the 1H NMR
d
ꢀ0.17 ppm, and accurate integration supported
d
spectra. While the desired monometallic complexes could not be
isolated, six bimetallic aluminum alkyl complexes were prepared
and tested for the controlled ring-opening polymerization of rac-
lactide. Interestingly, bimetallic catalysts have been shown to offer
improved activity and control over lactide polymerization [23],
although reduced stereocontrol would be expected from the more
open active site.
Table 1
Selected bond lengths and angles for 7 ([1]AlMe2).
Atoms
Length (Å)
Atoms
Angle (ꢁ)
Al(1)eN(1)
Al(1)eN(2)
A1(1)eC(1)
A1(1)eC(2)
N(1)eC(21)
N(1)eC(22)
N(2)eC(3)
N(2)eC(15)
N(3)eC(23)
N(3)eC(24)
N(4)eC(25)
N(4)eC(37)
C(16)eC(21)
C(22)eC(23)
1.9292(18)
1.8881(18)
1.958(3)
1.967(3)
1.295(3)
1.468(3)
1.449(2)
1.369(3)
1.52(4)
N(1)eAl(1)eC(1)
N(1)eAl(1)eC(2)
N(2)eAl(1)eC(1)
N(2)eAl(1)eC(2)
N(2)eAl(1)eN(1)
C(1)eAl(1)eC(2)
C(3)eN(2)eAl(1)
C(15)eN(2)eAl(1)
C(15)eN(2)eC(3)
C(21)eN(1)eAl(1)
C(21)eN(1)eC(22)
C(22)eN(1)eAl(1)
C(24)eN(3)eC(23)
C(24)e(N30)eC(23)
C(37)eN(4)eC(25)
110.23(8)
104.12(10)
115.16(10)
117.58(10)
94.38(8)
112.82(13)
115.25(12)
127.07(13)
117.65(16)
123.84(15)
118.50(18)
117.25(14)
123.0(3)
1.15(3)
1.430(3)
1.378(3)
1.429(3)
1.506(4)
113.0(2)
122.54(17)
Fig. 5. ORTEP drawing (spheroids at 50% probability) of [2](AlMe2)2, 9. Hydrogen
atoms omitted for clarity.

L.E.N. Allan et al. / Journal of Organometallic Chemistry 706-707 (2012) 106e112
111
Table 3
Bond lengths and angles for 9 ([2](AlMe2)2).
Bond
Length (Å)
Bond
Length (Å)
N(1)eAl(1)
N(2)eAl(1)
Al(l)eC(1)
Al(1)eC(2)
N(2)eC(21)
N(1)eC(15)
N(1)eC(3)
N(2)eC(22)
C(22)eC(23)
C(16)eC(21)
1.8918(14)
1.9482(15)
1.963(2)
1.965(2)
1.293(2)
1.365(2)
1.445(2)
1.481(2)
1.512(3)
1.436(2)
N(1)eAl(1)eN(2)
N(1)eAl(1)eC(1)
N(1)eAl(1)eC(2)
N(2)eAl(1)eC(1)
N(2)eAl(1)eC(2)
C(1)eAl(1)eC(2)
C(15)eN(1)eAl(1)
C(3)eN(1)eAl(1)
C(21)eN(2)eAl(1)
C(22)eN(2)eAl(1)
95.15(2)
114.35(8)
115.45(8)
112.55(9)
104.90(8)
112.70(11)
127.65(11)
113.89(11)
123.78(12)
117.76(13)
Table 4
Crystallographic data for 9, [2](AlMe2)2.
Fig. 6. Plot of Mn versus conversion for the solution polymerization of rac-lactide at
70 ꢁC by [3](AlMe2)2, 10. Solid line represents theoretical molecular weights.
Empirical Formula
Crystal system
a, b, c (Å)
C45H62Al2N4
Orthorhombic
15.7648(16), 17.2749(17), 15.2425(15)
90, 90, 90
4151.1(7)
4
1.141
0.71073
173(1)
Formula weight
Space group
712.95
Pbcn
a, b, g
(ꢁ)
V, Å3
Z
Total reflections
Unique reflections
Parameters
27 336
4742
355
0.0676
0.1339
1.123
suggesting the initial protonolysis reaction between the aluminum
alkyl species and benzyl alcohol to form the desired alkoxide
initiator is rapid relative to propagation in each case. Cyclohexyl-
substituted ethyl-bridged catalyst 10 achieved the highest conver-
sions under these conditions. All catalysts displayed living charac-
teristics for these polymerizations, as evidenced by linear plots of
Mn vs. conversion with R2 values higher than 0.98 and low poly-
dispersities. A typical plot is shown in Fig. 6.
Dcalc, mg mꢀ3
Wavelength, Å
T, K
a
R1
b
Rw
q
range (ꢁ)
1.75 to 27.50
Goodness-of-fit
a
R1 ¼ S (Fo ꢀ Fc)/S Fo.
P
P
wR2 ¼ ð wðF2o ꢀ F2c Þ2= ðwF4oÞ1=2Þ.
b
The rates of polymerization for these reactions, kobs, were also
determined. Plotting ln(Mo/Mt) versus time for each kinetic run
further supported the living nature of these catalysts, but also
revealed that little productive polymerization occurs after 16 h of
reaction time. Rates ranged from 1.64 ꢂ 10ꢀ5 sꢀ1 for 9 to
2.80 ꢂ 10ꢀ5 sꢀ1 for 10. Interestingly, the rates for the ethyl-bridged
catalysts were significantly higher than their propyl-bridged
counterparts, a trend opposite to that observed in monometallic
aluminum salen systems [14]. A representative plot is given in
Fig. 7. The rates of these catalysts are significantly lower than for the
fastest aluminum salen initiators which possess first-order rate
constants in the range of 101 sꢀ1, a six-order of magnitude decrease
in our system [24].
Polymerizations were initiated by the addition of benzyl
alcohol, forming the aluminum alkoxide in situ, and were carried
out in toluene at 70 ꢁC and in molten lactide at 120 ꢁC. Lacti-
de:initiator:catalyst ratios of 200:2:1 were used under the
presumption that one active, growing chain per metal centre was
the steric limit for these systems. Results are summarized in
Table 5. Polymerizations with 7 were uncontrolled and exhibited
extremely broad, multimodal PDIs, which can be attributed to
the presence of an alternate initiating site on the pendant
ligand arm.
Neat polymerizations of lactide mediated by 8e13 are rapid but
lack control. High polydispersities and molecular weights are
indicative of poor catalyst stability and activity at these high
temperatures. However, the same catalysts offer good control over
the ROP of rac-lactide in toluene at 70 ꢁC, reaching moderate to high
conversions in 16 h. Attempts to extend these reactions to higher
conversions led to eventual catalyst degradation and trans-
esterification of the polymer. No induction period was noted,
In all cases these bimetallic aluminum catalysts (8e13) were
found to possess no capacity for tacticity control in the polymeri-
zation of rac-lactide. The selectively decoupled 1H NMR spectra of
the resultant PLA revealed the presence of the five possible tetrads
arising from rac-lactide, characteristic of an atactic chain. The steric
Table 5
Polymerization data for bimetallic anilido-aldimine aluminum catalysts (8e13) with
rac-lactide.a
Catalyst
Temp (ꢁC)
Time (hr)
Conv. (%)
Mn,GPC
Mn,th
9594
8592
11 742
9308
9322
8701
9447
9792
9908
8986
11 416
13 590
PDI
8
9
70
70
70
70
70
16
16
16
16
16
16
3
3
3
3
3
67
60
82
65
66
58
82
85
86
78
81
91
9010
9123
11 244
9034
7324
6780
18 626
30 539
16 132
26 507
11 834
10 175
1.17
1.25
1.23
1.37
1.07
1.07
1.59
1.64
1.47
1.49
1.58
1.56
10
11
12
13
8
70
120
120
120
120
120
120
9
10
11
12
13
3
a
Lactide:benzyl alcohol:catalyst ratio of 200:2:1 employed. GPC molecular
weights measured and corrected relative to styrene standards [17].
Fig. 7. Plot of ln(Mo/Mt) versus time for the solution polymerization of rac-lactide at
70 ꢁC by [3](AlMe2)2, 10.

112
L.E.N. Allan et al. / Journal of Organometallic Chemistry 706-707 (2012) 106e112
influence of the diisopropylphenyl, cyclohexyl and methyl
substituents proximal to the metal centre, coupled with the change
in ligand electronics, hinders the formation of monometallic
aluminum complexes. This promotes the formation of bimetallic
aluminum complexes that, while active for polymerization, do not
possess the sterically inhibited active site required to promote
tacticity control. The open coordination sphere around the metal
centre also means that the growing polymer chain is not affected by
changes to the ligand framework, resulting in very similar ROP data
for complexes 8e13.
Appendix. Supplementary Data
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2012.02.003.
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9 characterized
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