Synthesis of aluminum complexes of triaza framework ligands and their catalytic activity toward polymerization of ε-caprolactone

Article abstract of DOI:10.1021/om3011432

The synthesis and characterization of 1,5-bis(2,6-diisopropylphenyl)-2,4- diphenyl-1,3,5-triazapenta-1,3-diene (L¹H), a triaza ligand, and Al complexes of its anion are reported. A neat condensation reaction between N-(Dipp)benzamidine (Dipp =

Full text of DOI:10.1021/om3011432

Article
pubs.acs.org/Organometallics
Synthesis of Aluminum Complexes of Triaza Framework Ligands and
Their Catalytic Activity toward Polymerization of ε‑Caprolactone
K. Bakthavachalam and N. Dastagiri Reddy*
Department of Chemistry, Pondicherry University, Pondicherry 605014, India
S
* Supporting Information
ABSTRACT: The synthesis and characterization of 1,5-
bis(2,6-diisopropylphenyl)-2,4-diphenyl-1,3,5-triazapenta-1,3-
diene (L1H), a triaza ligand, and Al complexes of its anion are
reported. A neat condensation reaction between N-(Dipp)-
benzamidine (Dipp = 2,6-diisopropylphenyl) and N-(Dipp)-
benzimidoyl chloride affords L1H in good yield. The Al
complexes [L1AlMe2] (1), [L1AlMe(Cl)] (2), and [L1AlCl2]
(3) are prepared by treating L1H with a slight excess of AlMe3,
AlMe2Cl, and AlMeCl2, respectively, in toluene. Further, the
aluminum complexes [L2AlMe2] (5), [L2AlMe(Cl)] (6), and
[L2AlCl2] (7) are obtained in good yields from 1,3-bis(2-
pyridylimino)isoindoline (L2H) in a similar fashion. The
ligand L1H and complexes 1, 2, and 4−6 have been structurally
characterized. All of the complexes have been explored for their catalytic activity toward the ring-opening polymerization (ROP)
of ε-caprolactone. It has been found that [L1AlMe2], upon the addition of cocatalyst (benzyl alcohol), gives a tetranuclear Al
alkoxide (8), which is highly efficient in catalyzing the ROP of ε-caprolactone. [L2AlMe2] has also been found to be a good
catalyst. The crystal structure of 8 and the catalytic activities of all the complexes in detail are reported.
bis(2-pyridyl)amine7 and phthalocyanin,8 reported in the
INTRODUCTION
■
literature. A report on cationic Al complexes of a quaternary
Poly(caprolactone) (PCL) is a synthetic biodegradable and
1,3,5-triazapenta-1,3-diene has also appeared.9 Consequently,
biocompatible polymer which has a wide range of applications
we have synthesized a tertiary 1,3,5-triazapenta-1,3-diene ligand
in agriculture and the biomedical and pharmaceutical
and its Al complexes. The catalytic activity of these complexes
industries.1 The most common route for the synthesis of
toward polymerization of caprolactone has also been explored
PCL is ring-opening polymerization (ROP) of caprolactone
and compared with that of Al complexes of another triaza
using a catalyst. Several catalysts based on main group as well as
framework ligand, 1,3-bis(2-pyridylimino)isoindolinate.10 The
transition metals have been reported to catalyze the ROP of
details of these reactions and observations are reported herein.
caprolactone.2 Among them, Al-based catalysts have been
promising because they are highly active and inexpensive. β-
RESULTS AND DISCUSSION
Diketiminate ligands have been widely employed in developing
efficient homogeneous catalysts for carrying out various
transformations,3 including the ROP of caprolactone.4 How-
ever, triazapentadienate ligands, which closely resemble β-
diketiminates (Figure 1), have been comparatively less
explored.5 In our continuing efforts in exploring Al chemistry,6
especially the structural diversity in triazaalane networks, we
have realized that there have been no reports on Al complexes
of triazapentadienate ligands, though there have been a good
number of Al complexes of triaza framework ligands, such as
■
Synthesis and Structural Characterization. Recently,
Wurthwein and his co-workers have synthesized tertiary 1,3,5-
̈
triazapenta-1,3-dienes by modifying a literature procedure,5c
which utilizes a condensation reaction between imidoyl
chlorides (RC(Cl)NR) and N-substituted amidines
(RC(NH)NHR) in diethyl ether. However, they have
observed that this method is not effective when aromatic
imidoyl chlorides are used. Subsequently, they have found an
alternate route, which involves the synthesis of imidoylimi-
doates (RC(OEt)NC(R)NR)) and their reaction with
primary amines. This two-step process suffers from low overall
yield. Therefore, we decided to revisit the condensation
reaction between imidoyl chlorides and N-substituted amidines,
which are easily accessible. In a neat reaction, an equimolar
Received: November 26, 2012
Figure 1.
© XXXX American Chemical Society
A
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mixture of N-(Dipp)benzamidine (Dipp = 2,6-diisopropyl-
phenyl) and N-(Dipp)benzimidoyl chloride was heated to 160
°C for 2.5 h to afford 1,5-bis(2,6-diisopropylphenyl)-2,4-
diphenyl-1,3,5-triazapenta-1,3-diene (L1H) (Scheme 1) in
Scheme 1. Synthesis of 1,5-Bis(2,6-diisopropylphenyl)-2,4-
diphenyl-1,3,5-triazapenta-1,3-diene (L1H)
Figure 3. Single-crystal X-ray structure of 1. All hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
level.
Figure 2. Single-crystal X-ray structure of L1H. All hydrogen atoms,
except that on N1, are omitted for clarity. Thermal ellipsoids are
drawn at the 40% probability level.
Figure 4. Single-crystal X-ray structure of 2. All hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
level.
Scheme 2. Synthesis of Aluminum and Lithium Complexes
of L1
1,3-dienes similar to L1H have been reported in the
literature,5a−e and interestingly, none of them show intra-
molecular hydrogen bonding. An ORTEP diagram along with
selected bond distances and bond angles is given in Figure 2.
The Al complexes [L1AlMe2] (1), [L1AlMe(Cl)] (2), and
[L1AlCl2] (3) were prepared by refluxing L1H with a slight
excess of AlMe3, AlMe2Cl, and AlMeCl2, respectively, in
toluene. Our initial efforts to synthesize the complex [L1AlCl2]
(3) by treating the lithiated ligand [(L1Li)2] or [L1Li(THF)3]
with AlCl3 were unsuccessful. These reactions yielded large
amounts of precipitate, which is presumably a polymeric
substance, insoluble in common organic solvents. Using 2 equiv
of AlMe3 and 1 equiv of L1H resulted in the formation of
[L1AlMe2(AlMe3)] (4), in which AlMe3 is coordinated by the
central N atom. An illustration of the synthesis of these
complexes is given in Scheme 2. 4 is found to react with
chloroform, and hence its NMR spectra have been recorded in
toluene-d8. The 1H NMR spectrum at room temperature is not
informative and contains very broad resonances (Supporting
Information, Figure S1). As the temperature of the sample
decreases the peaks resolve and at −70 °C, a well resolved
spectrum, though quite complicated, has been obtained
(Supporting Information, Figure S2). Interestingly, the peaks
also resolve as the temperature increases above room
temperature. A simple and well-resolved spectrum can be
obtained at 80 °C (Supporting Information, Figure S3). As can
be seen in the spectrum (80 °C), the AlMe3 group shows two
good yield (67%). L1H was characterized by NMR and
single-crystal X-ray techniques. The molecular structure showed
the triazapentadiene frame as a planar ring with terminal
nitrogen atoms held together by intramolecular hydrogen
bonding. Crystal structures of a few tertiary 1,3,5-triazapenta-
B
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Figure 7. Single-crystal X-ray structure of [L1Li(THF)3]. All hydrogen
atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths (Å): N1−Li1 = 2.052(6), N1−
C2 = 1.319(4), N2−C2 = 1.343(4), N2−C1 = 1.343(4), N3−C1 =
1.295(4), O1−Li1 2.011(7), O2−Li1 = 1.928(6), O3−Li1 2.004(6).
Selected bond angles (deg): C2−N1−Li1 = 133.8(3), C2−N2−C1 =
120.9(3), N3−C1−N2 = 125.7(3), O1−Li1−N1 = 122.1(3). O2−
Li1−N1 = 112.8(3), O3−Li1−N1 = 115.5(3), O2−Li1−O3 =
102.2(3), O3−Li1−O1 = 97.7(3).
Figure 5. Single-crystal X-ray structure of 4. All hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
level. Labels of symmetry equivalent atoms have been changed for easy
comparison of bond parameters with those of L1H, 1, and 2 (Table 1).
Figure 6. Single-crystal X-ray structure of [(L1Li)2]. The diethyl ether
molecule and all hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 50% probability level. Selected bond lengths
(Å): N6−C5 = 1.296(3), N6−Li1 = 1.991(5), N1−Li1 = 1.978(5),
N1−C2 = 1.297(3), N2−C2 = 1.371(3), N2−Li1 2.164(5), N2−C3 =
1.370(3), N3−C3 = 1.296(3), N2−Li2 2.125(5), N3−Li2 = 1.996(5),
N4−Li2 = 1.988(5), N4−C4 = 1.298(3), N5−Li2 2.150(5), N5−C5 =
1.368(3), N5−C4 = 1.376(3), N5−Li1 2.108(5). Selected bond angles
(deg): C5−N6−Li1 = 92.6(2), C3−N3−Li2 = 92.8(2), C4−N4−Li2
= 93.7(2), C2−N1−Li1 = 94.6(2), C5−N5−C4 = 122.3(2), C5−N5−
Li2 = 146.1(2), C5−N5−Li1 = 85.65(19), C4−N5−Li2 = 84.73(19),
C4−N5−Li1 = 147.0(2), Li1−N5−Li2 = 77.86(18), C2−N2−Li2 =
147.4(2), C2−N2−Li1 = 84.63(18), C3−N2−C2 = 122.1(2), C3−
N2−Li2 = 85.35(19), C3−N2−Li1 = 147.6(2), Li2−N2−Li1 =
77.20(18).
Figure 8. Single-crystal X-ray structure of 5. All hydrogens are omitted
for clarity. Thermal ellipsoids are drawn at the 50% probability level.
Selected bond lengths (Å): Al1−N1 = 2.1351(18), Al1−N5 =
2.1267(18), Al1−N3 = 1.9234(18), Al1−C1 = 1.987(2), Al1−C2 =
1.981(2), N1−C3 = 1.349(3), C3−N2 = 1.382(3), N2−C4 =
1.296(3), C4−N3 = 1.394(3), N3−C5 = 1.405(3), C5−N4 =
1.292(3), N4−C6 = 1.380(3), C6−N5 = 1.347(3). Selected bond
angles (deg): C1−Al1−C2 = 122.27(10), C1−Al1−N1 = 91.74(8),
C2−Al1−N1 = 90.81(8), N1−C3−N2 = 122.62(18), C3−N2−C4 =
123.64(18), N2−C4−N3 = 131.15(19), C4−N3−C5 = 106.81(16),
N3−C5−N4 = 131.4(2), C5−N4−C6 = 124.14(18), N4−C6−N5 =
122.51(19), C6−N5−Al1 = 128.05(15), N5−Al1−C2 = 90.12(9),
N5−Al1−C1 = 93.18(8), N5−Al1−N3 = 87.28(7), N3−Al1−N1 =
86.71(7), N3−Al1−C2 = 120.26(9), N3−Al1−C1 = 117.47(9), N5−
Al1−N1 = 173.52(8).
peaks in a 1:2 ratio. While one of the methyl groups appears in
the downfield region (δ 0.23 ppm, 3H), the other two appear in
the upfield region (δ −0.83 ppm, 6H) in comparison to AlMe2
protons (δ −0.38 ppm). This behavior can be attributed to the
orientation of methyl groups (AlMe3) with respect to the
phenyl substituents, coupled with hindered rotation of the
Me3Al−N bond.
Though the NMR spectra of these complexes are quite
conclusive of their formation, single-crystal X-ray structures of
1, 2, and 4 were obtained for further confirmation. Molecular
structures are depicted in Figures 3−5. Important bond
parameters are given in Table 1. The structures of 1, 2, and
4 contain a six-membered C2N3Al ring with similar bond
lengths. In 1 and 2, except for Al, all the other atoms in the ring
are almost in one plane. However, in 4, the central N atom also
deviates significantly from the plane, presumably due to the
steric repulsion between AlMe3, to which it is coordinated, and
the phenyl substituents on the adjacent C atoms. The structural
parameters of 1, 2, and 4 have been compared with those of
similar β-diketiminate Al complexes reported, and it is found
that Al in 1 has deviated from the mean plane of the rest of the
ring atoms (0.825 Å) slightly more than that of a similar β-
C
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Table 1. Selected Bond Lengths and Bond Angles for L1H, 1, 2, and 4
L1H
1
2
4
Bond Lengths (Å)
1.322(3)
N1−C1
N2−C1
N2−C2
N3−C2
Al1−N3
Al1−N1
Al1−C4
Al1−C3
Al1−Cl1
Al2−N2
Al2−C5
Al2−C6
1.324(2)
1.341(2)
1.359(2)
1.316(2)
1.333(4)
1.339(4)
1.341(4)
1.338(4)
1.899(3)
1.893(3)
1.319(2)
1.352(3)
1.3827(18)
1.3827(18)
1.319(2)
1.344(3)
1.329(3)
1.924(2)
1.9415(14)
1.9415(14)
1.971(3)
1.938(2)
1.950(3)
1.958(3)
2.002(3)
1.955(3)
2.126(13)
2.076(2)
1.975(2)
1.963(3)
Bond Angles (deg)
124.9(2)
N1−C1−N2
C1−N2−C2
N3−C2−N2
N3−Al1−N1
N3−Al1−C4
N3−Al1−C3
N1−Al1−C4
N1−Al1−C3
C4−Al1−C3
C1−N1−Al1
C2−N3−Al1
N1−Al1−Cl1
N3−Al1−Cl1
C5−Al2−N2
C6−Al2−N2
C5−Al2−C6
123.03(17)
122.68(16)
122.54(17)
124.3(3)
127.2(3)
124.3(3)
95.02(11)
123.04(15)
123.69(19)
123.04(15)
92.61(9)
126.1(2)
124.8(2)
93.05(8)
111.65(11)
110.92(11)
116.99(11)
106.78(11)
115.29(13)
118.13(16)
117.01(16)
107.00(7)
109.97(8)
107.00(7)
109.97(8)
125.34(13)
115.60(11)
115.60(11)
119.02(12)
113.46(12)
117.7(2)
118.4(2)
108.72(9)
105.57(9)
113.28(12)
103.93(7)
112.25(8)
diketiminate complex (0.721,11a 0.729 Å11b). It is also observed
that the N−Al−N bond angle is somewhat smaller in 1 and 4
(∼93°) than in β-diketiminate complexes (∼96°).11 However,
it is quite similar to those found in cationic Al complexes of a
quaternary 1,3,5-triazapenta-1,3-diene ligand (N−Al−N =
∼92°) reported in the literature.9 While the Al−N bond
distances in 1 and 4 are comparable with those in the
symmetric [((Dipp)N(C(Me)N(Dipp))2)AlI2]+ complex, Al−
C bond distances are like those found in [(Dipp)NC(
CH2)(N(Dipp))(C(Me)N(Dipp)AlMe2].
(THF)3], is a monomer. It is a linear molecule with only one of
the N atoms coordinated to the Li ion. The molecular structure
of [L1Li(THF)3], along with selected bond parameters, is given
in figure 7.
1,3-Bis(2-pyridylimino)isoindoline (L2H) was synthesized by
following a literature procedure.10 The aluminum complexes of
its anion, [L2AlMe2] (5), [L2AlMe(Cl)] (6), and [L2AlCl2] (7),
were obtained in good yields by refluxing L2H for 4 h in toluene
1
with an appropriate Al reagent as shown in Scheme 3. Its H
Scheme 3. Synthesis of Aluminum Complexes of L2
A dried sample of the lithium complex made in ether
medium showed no coordinated solvent, while that made in
1
THF medium showed three THF molecules in H NMR. This
observation generated curiosity about their structures, and
therefore we obtained single-crystal X-ray structures of both.
These structures reveal that the lithium complex obtained from
ether, [(L1Li)2], exists as a dimer, as depicted in Figure 6.
Though there are two ether molecules present in the
asymmetric unit, they do not coordinate to Li ions. Each Li
is coordinated by four N atoms, two from each ligand molecule
and found in a highly distorted tetrahedral geometry. Basically
the ligand molecules adopt a “W” shape and the Li ions are
sandwiched between two ligand molecules. There are two
articles on similar structures with triaza ligands having
guanidine moieties reported in the literature.12 However, it
has been reported that triaza ligands bearing perfluoroalkyl
groups in α positions form six-membered chelate Li complexes,
similar to the case for β-diketiminate ligands.13 In contrast, the
lithium complex that has been obtained from THF, [L1Li-
NMR spectrum suggests a C2-symmetric structure, which can
be envisaged by placing Al in a pentacoordinated geometry.
This structure was further confirmed by undertaking single-
crystal X-ray studies for 5 and 6. Molecular structures along
with important bond parameters of 5 and 6 are given in Figures
8 and 9, respectively. The crystal structures show Al in a slightly
D
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the equatorial Al−N bond (5, Al1−N3 = 1.9234(18) Å; 6,
Al1−N3 = 1.892(2) Å). The whole molecule, except Me and/
or Cl substituents on Al, is almost planar and hence a tight
packing of the molecules (intermolecular shortest C···C contact
is 3.358 Å in 5 and 3.380 Å in 6) has been observed. These
kinds of short contacts, which are due to π−π interactions, are
common in isoindoline complexes.14 Since these are the first
structural reports on Al complexes of isoindolinate type ligands,
the bond parameters were compared only with standard bonds
and found to be in the same range.
Polymerization Studies. Ring-opening polymerization
(ROP) of ε-caprolactone (CL) using Al complexes 1−7 was
carried out in the presence of benzyl alcohol as cocatalyst, and
the results are given in Table 2. The polymer was characterized
by NMR, and the Mn and Mw values were obtained from gel
permeation chromatography (GPC). In this paper, the
nomenclature for catalytic activities has been used as per the
classification given in Redshaw’s review.2a Complex 1 was
found to show high activity in initiating ROP of ε-caprolactone.
It showed catalytic activity within 2 min of the addition of the
monomer, and in 10 min, the conversion reached 97% for a
[CL]/[Al] ratio of 200/1 (entry 1). Even when the [CL]/[Al]
ratio was increased to 1000, a very good conversion (80%) was
observed in 30 min (entry 5). The linear relationship between
the [CL]/[Al] ratio and number-averaged molecular weight
(Mn) that is shown in Figure 10 demonstrates the living nature
of the polymerization. It is noteworthy that the neat reactions
(entries 7−10) produced polymer with relatively low Mn and
high Mw/Mn values, presumably because of the reaction mixture
becoming highly viscous as the reaction progressed. Com-
pounds 2 and 3 were found to be catalytically inactive toward
ROP of ε-caprolactone. They yielded only traces of polymer
even after several hours of stirring the reaction mixture at 80 °C
(entries 12 and 13). Complex 4, which is an AlMe3 adduct of 1,
showed a good activity (entries 14 and 15). The isoindolinate−
AlMe2 complex 5 was quite slow in initiating ROP of ε-
Figure 9. Single-crystal X-ray structure of 6. All hydrogens are omitted
for clarity. Thermal ellipsoids are drawn at the 50% probability level.
Selected bond lengths (Å): Al1−N1 = 2.074(2), Al1−N5 = 2.081(2),
Al1−N3 = 1.892(2), Al1−Cl1A= 2.141(2), Al1−Cl1B = 2.142(2),
Al1−C1A= 2.187(9), Al1−C1B= 2.192(10), N1−C2 = 1.348(3), C2−
N2 = 1.375(4), N2−C3 = 1.296(4), C3−N3 = 1.395(3), N3−C4 =
1.397(3), C4−N4 = 1.295(3), N4−C5 = 1.379(3), C5−N5 =
1.346(3). Selected bond angles (deg): Cl1A−Al1−ClA =
125.81(19), Cl1B−Al1−C1B = 125.0(2), N5−Al1−Cl1A =
90.54(7), N5−Al1−C1B = 90.78(8), N5−Al1−Cl1A = 90.78(8),
N5−Al1−Cl1B = 90.54(7), N1−Al1−C1A = 90.0(2), N1−Al1−C1B
= 89.5(2), N1−Al1−Cl1A = 90.16(8), N1−Al1−Cl1B = 91.35(7),
N3−Al1−Cl1A = 122.10(8), N3−Al1−Cl1B = 120.22(7), N3−Al1−
C1B = 114.8(2), Al1−N1−C2 = 126.97(18), N1−C2−N2 = 123.5(2),
C3−N2−C2 = 107(2), N2−C3−N3 = 130.4(2), C3−N3−C4 =
107.4(2), N3−C4−N4 = 130.0(2), C4−N4−C5 = 123.8(2), N4−
C5−N5 = 123.0(2), C5−N5−Al1 = 127.56(17), N5−Al1−N1 =
177.21(9), N5−Al1−N3 = 88.45(9), N3−Al1−N1 = 88.82(9).
distorted trigonal bipyramidal geometry with pyridine nitrogens
occupying axial positions. In both cases, axial Al−N bonds (5,
Al1−N1 = 2.1351(18) Å, Al1−N5 = 2.1267(18) Å; 6, Al1−N1
= 2.074(2) Å, Al1−N5 = 2.081(2) Å) are slightly longer than
a
Table 2. Ring-Opening Polymerization of ε-Caprolactone Initiated by 1−7/Benzyl Alcohol
b
c
d
e
entry
complex
[CL]/[Al]/ [BnOH]
temp/°C
t/min
conversn (%)
Mn(GPC)
Mn(calcd)
TOF
PDI
1
2
3
4
5
6
1
1
1
1
1
1
1
1
1
1
1
2
3
4
4
5
5
6
7
200/1/1
500/1/1
500/1/1
1000/1/1
1000/1/1
2000/1/1
200/1/1
500/1/1
1000/1/1
1000/1/1
100/1/1
200/1/1
200/1/1
200/1/1
200/1/1
200/1/1
200/1/1
200/1/1
200/1/1
80
80
80
80
80
80
80
80
80
80
90
80
80
80
80
80
80
80
80
10
10
97
75
29319
46685
49930
84544
92911
131800
45100
41478
49250
53127
15862
22224
42858
49698
63948
91308
118668
22680
50838
45708
90168
10710
1168
2259
870
1.61
1.44
1.52
1.58
1.51
1.56
2.09
2.53
1.32
1.49
1.73
30
87
10
56
3373
1600
6265
396
30
80
10
52
f
7
30
99
f
8
30
89
890
f
9
2
40
12121
1580
2818
f
10
30
79
11
12
13
14
15
16
17
18
19
2
93
3840
3840
10
trace
trace
91
21640
36996
20856
22224
1096
388
1.67
1.54
30
97
180
540
3840
3840
trace
81
32060
18576
18
1.63
trace
trace
a
b
c
Conditions: 0.02 mmol of catalyst, 2 mL of toluene (wherever used). Determined by 1H NMR spectroscopy. Obtained from GPC analysis and
d
calibrated by polystyrene standard (uncorrected). Theoretical Mn = (monomer/initiator) × (% conversion) × (Mw of ε-CL) + (Mw of BnOH).
e
f
Moles of CL consumed per mole of catalyst per hour. Solvent -free reaction.
E
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of the alkoxide 8 was confirmed by single-crystal X-ray studies.
The molecular structure and selected bond parameters are
given in Figure 11. There are four Al atoms in the structure,
Figure 11. Single-crystal X-ray structure of 8. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
level. Selected bond lengths (Å): Al1−O1 = 1.908(2), Al1−O2 =
1.900(2), Al2−O1 = 1.844(2), Al2−O2′ = 1.833(2), Al2−C1 =
1.954(3), Al2−C2 = 1.957(4), Selected bond angles (deg): O1−Al1−
O2′ = 76.19(8), O1−Al2−O2′ = 79.44(9), C1−Al2−C2 =
114.34(17), O1−Al2−C1 = 114.72(14), O1−Al2−C2 = 114.82(15),
O2−Al1−O2′ = 94.92(9), O1−Al1−O2″ = 166.97(8), Al1−O1−Al2
= 101.82(10), Al1−O2−Al2″ = 102.54(10), O1−Al1−O2 = 95.31(8),
O1−Al1−O1″ = 94.94(9).
Figure 10. (a) Plot of number-averaged molecular weights (Mn) and
polydispersity (PDI) vs monomer conversion for the polymerization
of ε-CL. Conditions: [CL]/[Al]/[BnOH] = 200/1/1 in 2 mL of
toluene using complex 1 at 80 °C. (b) Plot of number-averaged
molecular weights (Mn) and polydispersity (PDI) vs [CL]/[Al]/
[BnOH] for the polymerization of ε-CL using complex 1 at 80 °C
and all of them are found to be in one plane. Three Al atoms
form a triangle, and the fourth Al atom occupies the center of
the triangle. The terminal Al atoms are connected to the central
Al via μ2-O benzyloxido bridges. The central Al, which is
coordinated by six benzyloxido groups, is in a slightly distorted
octahedral geometry. The terminal Al atoms, which possess two
methyl groups each, exist in a tetrahedral geometry. Quite a few
crystal structures of similar compounds have been reported in
the literature, and the bond parameters of 8 are similar to those
of the reported structures.16
Once the alkoxide 8 was thoroughly characterized, it was
synthesized in high yield (85%) from AlMe3 and benzyl alcohol
in toluene and explored for its catalytic activity toward ROP of
ε-caprolactone. The results of these polymerization reactions
are provided in Table 3. The reactions were carried out at 80
°C in toluene, without the addition of a cocatalyst. 8 is found to
be highly active and polymerizes ε-caprolactone with >90%
conversion within 10 min, even when the [CL]/[Al] ratio is as
high as 2000/1 (entry 23). The controlled nature of the
polymerization can be seen in plots of conversion vs Mn and
[CL]/[Al] vs Mn (Figure 12). Kinetic studies performed on the
reactions catalyzed by 1 and 8 reveal that these polymerization
reactions proceed with first-order dependence on monomer
concentration (Figure 13).
These results suggest that, on treatment with benzyl alcohol,
complex 1 readily forms 8, which initiates the polymerization
reaction. Though 8 can be prepared from AlMe3 and benzyl
alcohol, the AlMe3/benzyl alcohol mixtures are far less active15a
in comparison to 1. The difference in the catalytic activities of
AlMe3 and 1 can be attributed to the rate of formation of the
active alkoxide, 8, when they are treated with benzyl alcohol.
While 1 produces 8 within a few seconds, AlMe3 takes more
than 1 h to form 8 in any significant quantities. Recently, the
catalytic activity of Al complexes of β-diketiminate ligands
■
(entries 1, 2, 4, and 6 in Table 2). In both plots, red squares ( )
▲
represent Mn (uncorrected) values and blue triangles ( ) represent
PDI values.
caprolactone, and it produced only traces of polymer even after
3 h of the addition of the monomer (entry 16). Nonetheless, by
the end of 9 h, a very good conversion of the monomer (81%)
was observed (entry 17). On the basis of TOF values, complex
5 can be considered as moderately active. Similar to the case for
2 and 3, complexes 6 and 7 also failed to initiate
polymerization. In all those entries found in Table 2, benzyl
alcohol was used as a cocatalyst, without which the reactions
did not proceed.
Recently, several articles have appeared in the literature on
ring-opening polymerization of ε-caprolactone by Al cata-
lysts.2a−i,15 Most of the catalysts contain alkyl or alkoxide
moieties on aluminum. Most of the catalysts that do not
possess Al−O bonds require a cocatalyst, which is mostly an
alcohol. This envisages the presence of an alkoxide intermediate
during the course of the reaction. In order to isolate the
intermediate in case of 1, we treated it with 1 equiv of benzyl
alcohol at 80 °C in toluene. To our surprise, we obtained only a
simple alkoxide, [Al{(μ-OCH2Ph)2AlMe2}3] (8), which does
not contain the ligand. The yield of crystals was good (71%
based on BnOH), and the mother liquor was also found to
contain a mixture of the free ligand and 8, along with some
impurities (NMR). However, an NMR tube reaction between 1
and benzyl alcohol in a 4/6 ratio showed a clear and complete
conversion of 1 into 8 even at room temperature. The structure
F
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a
Table 3. Ring-Opening Polymerization of ε-Caprolactone Initiated by 8 and 9
b
c
d
e
entry
complex
[CL]/[Al]
time/min
conversn (%)
Mn(GPC)
Mn(calcd)
TOF
PDI
20
21
22
23
24
25
26
27
8
8
8
8
9
9
9
9
200/1
500/1
1000/1
2000/1
50/1
10
10
10
10
30
30
30
30
99
99
97
92
99
99
95
94
15393
22072
31342
45817
6837
22680
56538
110688
209868
5751
1192
2981
5843
11084
99
1.79
1.64
1.83
1.94
1.31
1.16
1.36
1.03
100/1
200/1
500/1
8236
11394
21768
53688
198
20752
380
61080
940
a
b
c
Conditions: 0.02 mmol of catalyst, 2 mL of toluene, 80 °C. Determined by 1H NMR spectroscopy. Obtained from GPC analysis and calibrated
d
e
by polystyrene standard (uncorrected). Theoretical Mn = (monomer/initiator) × (% conversion) × (Mw of ε-CL) + (Mw of BnOH). Moles of CL
consumed per mole of catalyst per hour.
Figure 13. Plots of ln([CL]0/[CL]t) vs time for the polymerization of
■
▲
ε-CL catalyzed by 1 (red ) and 8 (blue ). Conditions: for complex
1, [CL]/[Al]/[BnOH] = 200/1/1 in 2 mL of toluene at 80 °C; for
complex 8, [CL]/[Al] = 200/1 in 2 mL of toluene at 80 °C.
do not decompose readily to form 8. This difference in the
reactivity can be explained by comparing the donor abilities of
both the types of ligands. β-Diketiminate ligands are strong σ
donors, and replacing a carbon atom in the ligand backbone by
nitrogen (as in triaza ligands) will decrease their donor ability.
Hence, the triaza ligand is too weak a donor to retain Al in
presence of BnOH. In order to find out the reason for the
inactivity of complexes [L1AlMe(Cl)] (2) and [L1AlCl2] (3),
we investigated their reactions with benzyl alcohol. Both 2 and
3 readily reacted with benzyl alcohol in toluene to give a white
precipitate, which is found to be a mixture of [L1H2]Cl and
some highly insoluble substance, presumably polymeric
alkoxides of the type [AlClx(OCH2Ph)3−x]n. Formation of
Figure 12. (a) Plot of number-averaged molecular weights (Mn) and
polydispersity (PDI) vs monomer conversion for the polymerization
of ε-CL. Conditions: [CL]/[Al] = 200/1 in 2 mL of toluene using
complex 8 at 80 °C. (b) Plot of number-averaged molecular weights
(Mn) and polydispersity (PDI) vs [CL]/[Al] for the polymerization of
ε-CL using complex 8 at 80 °C (entries 20−23 in Table 3). In both
1
[L1H2]Cl was confirmed by H NMR and single crystal X-ray
analysis (see the Supporting Information, Figures S8 and S9).
Evaporation of volatiles from the filtrates of these reactions
yielded no residue. These observations clearly explain why 2
and 3 cannot initiate polymerization.
■
plots, red squares ( ) represent Mn (uncorrected) values and blue
▲
triangles ( ) represent PDI values.
Unlike 1, complex 5 ([L2AlMe2]) does not react with benzyl
alcohol at room temperature and even at 80 °C only a very slow
reaction occurred, which explains the long hours of catalysis
reaction time. It has been established that the formation of
alkoxide involves nucleophilic attack by benzyl alcohol at the
aluminum center and the rate of the reaction depends upon the
acidity of aluminum. In complex 5, aluminum is five-
coordinated and moreover there are two pyridine donors. As
a result, the Al center becomes less acidic and hence the
reaction is sluggish. Complexes 6 and 7 did not react with
benzyl alcohol even when the reaction mixtures were heated at
toward ROP of ε-caprolactone has been reported.4 Since β-
diketiminates have striking resemblance to 1,3,5-triazapenta-
dienates, it would be interesting to compare the catalytic
activities of their Al complexes with those of 1. Most of the β-
diketiminate complexes reported took several hours (7−24 h)
for a good conversion of ε-caprolactone into poly(ε-
caprolactone), and the polymer produced has a relatively
high molecular weight distribution. Comparison of these results
with those of 1 or 8 suggests that the β-diketiminate complexes
G
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an atmosphere of dry nitrogen. Trimethylaluminum, methylaluminum
dichloride, dimethylaluminum chloride, n-butyllithium, benzoyl
chloride, pyridine, benzyl alcohol, and 2,6-diisopropylaniline were
procured from Aldrich and used as received. ε-Caprolactone,
purchased from Acros Organics, was stirred over CaH2 for 24 h and
distilled under vacuum. N-(Dipp)benzamidine6a was prepared by
following the literature procedure. Hexane, benzene, diethyl ether, and
toluene (from Na/benzophenone ketyl) were distilled fresh when
required. CDCl3 was distilled from calcium hydride. C6D6 and toluene-
d8 were dried over sodium metal. 1H and 13C spectra were recorded on
a Bruker 400 MHz instrument. Elemental analyses were performed
using an Elementar Vario EL III analyzer.
80 °C for 24 h. This may be due to the inert nature of the Al−
Cl bond compounded by the five-coordinate aluminum
center.2c
The dibenzyloxidoaluminum complex [L2Al(OCH2Ph)2] (9)
was synthesized in high yield by refluxing 5 with 2 equiv of
benzyl alcohol in toluene for 4 h. Compound 9 was
characterized by NMR. The results of the catalytic activity of
9 have been summarized in Table 3. As anticipated, 9 is found
to be more efficient than the parent complex 5. It takes only 30
min for 9 to convert >90% of ε-caprolactone into poly(ε-
caprolactone), whereas 5 takes more than 6 h. Narrow
molecular weight distributions and a linear relationship
between the [CL]/[Al] ratio and Mn (Figure 14) suggest that
the polymerization proceeds in a living fashion.
Structural Determinations for L1H, 1, 2, 4, [(L1Li)2]·2Et2O,
[L1Li(THF)3], 5, 6, and 8. Single crystals of 1, 2, 4, [(L1Li)2]·2Et2O,
[L1Li(THF)3], 5, 6, and 8 were mounted on glass fibers in paraffin oil
and then brought into the cold nitrogen stream of a low-temperature
device so that the oil solidified. Data collections were performed on an
OXFORD XCALIBUR diffractometer, equipped with a CCD area
detector, using graphite-monochromated Mo Kα (λ = 0.71073 Å)
radiation and a low-temperature device. Data collection for L1H was
done at room temperature. All calculations were performed using
SHELXS-97 and SHELXL-97.17 The structures were solved by direct
methods and successive interpretation of the difference Fourier maps,
followed by full-matrix least-squares refinement (against F2). All non-
hydrogen atoms, except the disordered phenyl carbons in 1, were
refined anisotropically. In 6, Me and Cl groups on Al exchange their
positions, leading to substitutional disorder. They were refined by
using the PART instruction together with free variables. Disorders in
the phenyl group in 1, indoline moiety in 6, and methylene group in 8
were also treated similarly. The contribution of the hydrogen atoms, in
their calculated positions, was included in the refinement using a riding
model. Upon convergence, the final Fourier difference map of the X-
ray structures showed no significant peaks.
All the data sets were collected to 2θ values >50°, but they were cut
off to 2θ = 50° during the refinement. Relevant data concerning
crystallographic data, data collection, and refinement details are
summarized in Table S4 (see Supporting Information). Crystallo-
graphic information files (CIF) for the structures reported in this
paper have also been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC 910941−
910949 and 933359. Copies of the data can be obtained free of charge
on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. (fax, (+44) 1223-336-033; e-mail, deposit@ccdc.cam.ac.uk).
Preparation of N-(Dipp)benzamide (PhC(O)NH(2,6-i-
Pr2C6H3)). A literature method was modified.18 Freshly distilled
benzoyl chloride (3.95 g, 28.24 mmol) was added dropwise to a
solution of 2,6-diisopropylaniline (5.00 g, 28.24 mmol) and pyridine
(6.70 g, 84.74 mmol) in 100 mL of toluene. A white precipitate was
formed, and the mixture was refluxed for 2.5 h. The mixture was
brought to room temperature and washed with NaHCO3 solution, and
the organic layer was dried over Na2SO4. A white solid was obtained
after removal of the volatiles using a rotary evaporator. Recrystalliza-
tion from a toluene/hexane mixture gave colorless crystals. Yield: 6.50
Figure 14. Plot of number-averaged molecular weights (Mn) and
polydispersity (PDI) vs [CL]/[Al] for the polymerization of ε-CL
■
using complex 9 at 80 °C (entries 24−27 in Table 3). Red squares ( )
▲
represent Mn (uncorrected) values, and blue triangles ( ) represent
PDI values.
In addition to the above studies, we also examined complexes
1, 5, and 8 for initiating ROP of rac-lactide. Reactions were
carried out with and without BnOH for 1 and 5 and without
BnOH for 8. Even after 36 h of stirring in toluene at 80 °C,
surprisingly, none of these complexes displayed activity and no
polymer was produced in these reactions.
CONCLUSION
■
Al complexes of a Dipp-substituted tertiary 1,3,5-triazapenta-
1,3-dienate ligand (L1) and 1,3-bis(2-pyridylimino)-
isoindolinate ligand (L2) have been synthesized, and some of
them have been characterized structurally. The catalytic activity
of all of the complexes toward ROP of ε-caprolactone has been
explored, and it was found that the complex [L1AlMe2], when
treated with the cocatalyst benzyl alcohol, readily forms a
tetranuclear alkoxide (8), which is highly active in initiating the
polymerization reaction. In the case of [L2AlMe2], the
dibenzyloxido intermediate [L2Al(OCH2C6H5)2] was isolated.
This complex showed a good catalytic activity toward ROP of
ε-caprolactone. Compound 8, which can be readily prepared
from AlMe3 and benzyl alcohol, is the first simple aluminum
alkoxide (without any supporting ligand) reported to initiate ε-
caprolactone polymerization.
1
g (82%). Mp: 135−136 °C. H NMR (CDCl3, 400 MHz): δ 1.11 (d,
12H, CH(CH3)2), 3.03 (m, 2H, CH(CH3)2), 7.13 (d, 2H, Ar H), 7.26
(t, 1H, Ar H), 7.37 (m, 2H, Ph H), 7.46 (m, 1H, Ph H), 7.51 (1H, s,
NH), 7.82 (m, 2H, Ph H). 13C NMR (CDCl3, 100 MHz): δ 23.66,
28.89, 123.55, 127.25, 128.48, 128.78, 131.75, 134.48, 146.48, 146.48,
167.04.
Synthesis of N-(Dipp)benzimidoyl Chloride (2,6-i-
Pr2C6H3NC(Ph)Cl). A literature method was modified.9,18 PCl5 (5.29
g, 25.44 mmol) was dissolved in benzene (30 mL) with heating under
N2, and the mixture was cooled before adding N-(Dipp)benzamide
(6.50 g, 23.13 mmol). The mixture was refluxed for 1 h, until gas
evolution ceased. Volatiles were removed under high vacuum, and the
residue was subjected to vacuum distillation to give the product as a
yellow oil, which solidified after 1 day. Yield: 95% (6.56 g). Mp: 104−
1
EXPERIMENTAL SECTION
General Methods and Instrumentation. All manipulations were
carried out using standard Schlenk line and glovebox techniques under
105 °C. H NMR (CDCl3, 400 MHz): δ 1.07 (d, 6H, CH(CH3)2),
■
1.13 (d, 6H, CH(CH3)2), 2.73 (m, 2H, CH(CH3)2), 7.41 (m, 3H, Ar
H), 7.46 (m, 3H, Ph H), 8.13 (m, 2H, Ph H). 13C NMR (CDCl3, 100
H
dx.doi.org/10.1021/om3011432 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
MHz): δ 22.95, 23.38, 28.73, 123.13, 124.94, 128.61, 129.47, 132.14,
135.07, 136.75, 143.59, 143.8.
to colorless. The volume of the reaction mixture was reduced to 10 mL
and kept at 0 °C to afford colorless crystals of [(L1Li)2] (0.32 g, 93%).
Mp: 266 °C dec. 1H NMR (C6D6, 400 MHz): δ 0.92, (d, 12H,
CH(CH3)2), 1.04 (d, 12H, CH(CH3)2), 1.41 (d, 12H,CH(CH3)2),
3.35 (m, 4H, CH(CH3)2), 3.68 (m, 4H, CH(CH3)2), 6.58 (m, 8H, Ar
H), 6.94 (m, 4H, Ar H), 7.01 (m, 5H, Ph H), 7.09 (m, 5H, Ph H),
7.158 (m, 10H, Ph H). 13C NMR (C6D6, 100 MHz): δ 22.01, 22.69,
23.97, 24.53, 28.61, 29.33, 122.73, 123.18, 124.14, 137.99, 138.70,
141.36, 144.21, 172.73. Anal. Calcd for C76H88Li2N6: C, 83.03; H,
8.07; N, 7.64. Found: C, 83.09; H, 8.02; N, 7.63.
Synthesis of 1,5-Bis(2,6-diisopropylphenyl)-2,4-diphenyl-
1,3,5-triazapenta-1,3-diene (L1H). A Schlenk flask was charged
with N-(Dipp)benzamidine (14.00 g, 50.00 mmol) and N-(Dipp)-
benzimidoyl chloride (14.97 g, 50.00 mmol) and heated to 160 °C for
2.5 h. The reaction mixture was cooled to room temperature, 150 mL
of CH2Cl2 and 100 mL of saturated NaHCO3 were added, and the
organic layer was separated. The aqueous layer was extracted two
times with CH2Cl2. All of the organic layers were combined and dried
over Na2SO4. After removal of volatiles using a rotary evaporator, a
viscous yellow oil was obtained. This was dissolved in 100 mL of
hexane and kept in the refrigerator to give a yellow crystalline material
Synthesis of [L1Li(THF)3]. A solution of L1H (0.50 g, 0.92 mmol)
in tetrahydrofuran (25 mL) was cooled to −78 °C in a 100 mL
Schlenk flask, and n-BuLi (0.6 mL, 1.6 M in hexane, 0.96 mmol) was
added. The reaction mixture was stirred for 1 h at this temperature and
warmed to room temperature. The volume of the reaction mixture was
reduced to 10 mL and kept at 0 °C to afford colorless crystals of
[L1Li(THF)3] (0.61 g, 87%). Mp: 235 °C dec. 1H NMR (CDCl3, 400
MHz): δ 0.71 (m, 12H, CH(CH3)2), 1.25 (d, 6H, CH(CH3)2), 1.31
(d, 6H, CH(CH3)2), 1.85 (m, 12H, THF), 3.03 (m, 2H, CH(CH3)2),
3.36 (m, 2H, CH(CH3)2), 3.74 (m, 12H, THF), 6.75 (m, 6H, Ar H),
6.87 (m, 10H, Ph H). 13C NMR (CDCl3, 100 MHz): δ 21.69, 22.38,
23.49, 24.13, 25.77, 28.23, 28.95, 68.13, 122.19, 122.57, 123.25,
127.04, 127.92, 128.21, 137.83, 138.40, 141.16, 143.87. Anal. Calcd for
C50H68LiN3O3: C, 78.40; H, 8.95; N, 5.49. Found: C, 78.37; H, 8.92;
N, 5.53.
1
(18.41 g, 67%). Mp: 159−160 °C. H NMR (CDCl3, 400 MHz): δ
0.98 (d, 12H, CH(CH3)2), 1.15 (d, 12H, CH(CH3)2), 3.27 (m, 4H,
CH(CH3)2), 7.11 (m, 6H, Ar H), 7.25 (m, 6H, Ph H), 7.64 (m, 4H,
Ph H), 14.61 (s, 1H, NH). 13C NMR (CDCl3, 100 MHz): δ 22.34,
24.76, 28.59, 123.51, 125.63, 127.62, 129.68, 129.74, 136.75,139.25,
140.94, 164.40. Anal. Calcd for C38H45N3: C, 83.93; H, 8.34; N, 7.73.
Found: C, 83.66; H, 8.40; N, 7.71.
General Procedure for the Synthesis of [L1AlRR′] and
[L1AlMe2(AlMe3)] (1−4). A solution of L1H in toluene (20 mL)
was cooled to −78 °C in a 100 mL Schlenk flask, and AlMeRR′ was
added. The reaction mixture was warmed to room temperature and
stirred for an additional 1 h. The mixture was refluxed for 2 h and
cooled to room temperature. The volume of the reaction mixture was
reduced to 10 mL and kept at ambient temperature to afford colorless
crystals of [L1AlRR′].
General Procedure for the Synthesis of [L2AlRR′] (5−7). To a
stirred solution of L2H in toluene (15 mL) was added MeAlRR′ at
−78 °C. The reaction mixture was warmed to room temperature, and
the mixture was refluxed for 4 h. Volatiles were removed under
vacuum to give yellow crystalline material. Recrystallization from
toluene afforded yellow solid/crystals.
[L1AlMe2], (1). L1H (0.45 g, 0.82 mmol), AlMe3 (0.5 mL, 2 M in
toluene, 1.00 mmol). Yield: 97% (0.48 g). Mp: 213−214 °C. 1H NMR
(CDCl3, 400 MHz): δ −0.80 (s, 6H, AlMe), 0.76 (d, 12H,
CH(CH3)2), 1.23 (d, 12H, CH(CH3)2), 3.29 (m, 4H, CH(CH3)2),
7.10 (m, 6H, Ar H), 7.21 (m, 6H, Ph H), 7.38 (m, 4H, Ph H). 13C
NMR (CDCl3, 100 MHz): δ −10.13, 23.79, 22.58, 28.61, 124.65,
127.31, 127.42, 129.77, 130.43, 138.20, 139.67, 143.58, 169.50. Anal.
Calcd for C40H50AlN3: C, 80.09; H, 8.40; N, 7.01. Found: C, 79.48; H,
8.55; N, 6.66.
[L2AlMe2] (5). L2H (0.45 g, 1.50 mmol), AlMe3 (0.8 mL, 2 M in
toluene, 1.60 mmol). Yield: 97% (0.52 g). Mp: 245 °C dec. 1H NMR
(CDCl3, 400 MHz): δ −0.66 (s, 6H, AlMe), 7.13 (m, 2H, Py H), 7.44
(m, 2H, Py H), 7.59 (m, 2H, Ar H), 7.77 (m, 2H, Py H), 8.00 (m, 2H,
Ar H), 8.43 (m, 2H, Py H). 13C NMR (CDCl3, 100 MHz): δ −0.80,
119.91, 122.51, 124.92, 131.59, 138.44, 139.92, 146.48, 156.77,
164.03.Anal. Calcd for C20H18AlN5: C, 67.60; H, 5.11; N, 19.71.
Found: C, 67.52; H, 5.07; N, 19.64.
[L1AlMe(Cl)] (2). L1H (0.50 g, 0.92 mmol), AlMe2Cl (1 mL, 1 M in
hexanes, 1.00 mmol). Yield: 84% (0.48 g). Mp: 175 °C dec. 1H NMR
(CDCl3, 400 MHz): δ −0.79 (s, 3H, AlMe), 0.33 (d, 6H, CH(CH3)2),
1.15 (m, 12H, CH(CH3)2), 1.38 (d, 6H, CH(CH3)2), 3.27 (m, 2H,
CH(CH3)2), 3.44 (m, 2H, CH(CH3)2), 7.10 (m, 6H, Ar H), 7.26 (m,
6H, Ph H), 7.39 (m, 4H, Ph H). 13C NMR (CDCl3, 100 MHz): δ
−7.12, 22.63, 24.10, 24.67, 26.98, 28.32, 29.72, 123.93, 125.88, 127.55,
128.03, 130.22, 130.48, 137.55, 138.23, 142.99, 144.60, 170.07. Anal.
Calcd for C39H47AlClN3: C, 75.52; H, 7.64; N, 6.77. Found: C, 75.47;
H, 7.69; N, 6.67.
[L2AlMe(Cl)] (6). L2H (0.33 g, 1.10 mmol), AlMe2Cl (1.2 mL, 1 M
1
in hexanes, 1.20 mmol). Yield: 97% (0.40 g). Mp: 282 °C dec. H
NMR (CDCl3, 400 MHz): δ −0.44 (s, 3H, AlMe), 7.20 (m, 2H, Py
H), 7.51 (m, 2H Py H), 7.63 (m, 2H, Ar H), 7.85 (m, 2H, Py H), 8.03
(m, 2H, Ar H), 8.77 (m, 2H, Py H). 13C NMR (CDCl3, 100 MHz): δ
−7.56, 120.27, 120.44, 122.87, 123.14, 125.50, 125.95, 132.11, 132.53,
137.47, 137.92, 140.76, 141.48, 147.62, 148.78, 156.14, 156.29,
163.16.. Anal. Calcd for C19H15AlClN5: C, 60.73; H, 4.02; N, 18.64.
Found: C, 60.66; H, 4.16; N, 18.56.
[L1AlCl2] (3). L1H (0.50 g, 0.92 mmol), AlMeCl2 (1 mL, 1 M in
hexanes, 1.00 mmol). Yield: 88% (0.52 g). Mp: 199 °C dec. 1H NMR
(CDCl3, 400 MHz): δ 0.73 (d, 12H, CH(CH3)2), 1.24 (d, 12H,
CH(CH3)2), 3.29 (m, 4H, CH(CH3)2), 7.08 (m, 6H, Ar H), 7.22 (m,
6H, Ph H), 7.37 (m, 4H, Ph H). 13C NMR (CDCl3, 100 MHz): δ
23.75, 25.57, 29.09, 125.08, 127.64, 128.51, 130.75, 130.83, 136.89,
137.04, 144.06, 171.25. Anal. Calcd for C38H44AlCl2N3: C, 71.24; H,
6.92; N, 6.56. Found: C, 71.17; H, 6.95; N, 6.51.
[L2AlCl2] (7). L2H (1.00 g, 3.34 mmol), AlMeCl2 (3.6 mL, 1 M in
hexanes, 3.60 mmol). Yield: 96% (1.27 g). Mp: 285 °C dec. 1H NMR
(CDCl3, 400 MHz): δ 7.13 (m, 2H, Py H), 7.44 (m, 2H, Py H), 7.59
(m, 2H, Ar H), 7.77 (m, 2H, Py H), 8.00 (m, 2H, Ar H), 8.43 (m, 2H,
Py H). 13C NMR (CDCl3, 100 MHz): δ 120.61, 123.35, 125.83,
132.69, 137.25, 141.58, 148.81, 155.81, 162.39.Anal. Calcd for
C18H12AlCl2N5: C, 54.57; H, 3.05; N, 17.68. Found: C, 54.50; H,
3.11; N, 17.62.
[L1AlMe2(AlMe3)] (4). L1H (0.45 g, 0.82 mmol), AlMe3 (0.9 mL, 2
1
M in toluene, 1.65 mmol). Yield 83% (0.46 g) Mp: 193 °C dec. H
Formation of [Al{(μ-OCH2Ph)2AlMe2}3] (8) in a Reaction
between 1 and Benzyl Alcohol. To a stirred solution of 1 (0.30 g,
0.50 mmol) in toluene (5 mL) was added benzyl alcohol (0.05 g, 0.50
mmol) at room temperature. The mixture was stirred for 15 min at
room temperature before volatiles were removed under vacuum. The
residue was extracted with hexane to give colorless crystals of 8 at
room temperature (0.05 g, 71%, based on BnOH). Mp: 176−177 °C.
1H NMR (C6D6, 400 MHz): δ −0.60 (s, 18H, AlMe), 5.30 (d, 6H,
CH2), 5.38 (d, 6H, CH2), 7.16 (t, 6H, Ph H), 7.22 (t, 12H, Ph H),
7.65 (d, 12H, Ph H). 13C NMR (C6D6, 100 MHz): δ −6.75, 67.77,
129.06, 129.14, 130.45, 138.09. Anal. Calcd for C48H60Al4O6: C, 68.56;
H, 7.19. Found: C, 68.49; H, 7.23.
NMR (toluene-d8, 400 MHz at 80 °C): δ −0.73 (s, 6H, AlMe3), −0.39
(s, 6H, AlMe2), 0.21 (s, 3H, AlMe3), 0.94 (d, 12H, CH(CH3)2), 1.21
(d, 12H, CH(CH3)2), 3.41 (m, 4H, CH(CH3)2), 6.58 (s, 1H, Ph H),
6.92 (m, 5H, Ph H), 6.99 (m, 6H, Ar H) 7.64 (m, 4H, Ph H). 13C
NMR (toluene-d8, 100 MHz at 80 °C): δ −8.97, −4.84, 1.42, 24.12,
26.82, 28.99, 127.59, 131.50, 132.86, 136.13, 137.37, 137.94, 139.62,
144.13. Anal. Calcd for C43H59Al2N3: C, 76.86; H, 8.85; N, 6.25.
Found: C, 76.76; H, 8.91; N, 6.20.
Synthesis of [(L1Li)2]. A solution of L1H (0.34 g, 0.62 mmol) in
diethyl ether (20 mL) was cooled to −78 °C in a 100 mL Schlenk
flask, and n-BuLi (0.4 mL, 1.6 M in hexane, 0.64 mmol) was added.
The reaction mixture was warmed to room temperature and stirred for
an additional 1 h, during which time the solution changed from yellow
Synthesis of 8 from AlMe3 and Benzyl Alcohol. To a stirred
solution of AlMe3 (3.0 mL, 2 M in toluene, 6.00 mmol) in hexane (40
I
dx.doi.org/10.1021/om3011432 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mL) was added benzyl alcohol (1.00 g, 9.25 mmol) dropwise at −78
°C. The resultant slurry was allowed to come to room temperature
slowly and stirred overnight. The precipitate was separated by
filtration, washed with hexane, and dried under high vacuum. Yield:
85% (1.10 g).
of a glovebox. We thank the Central Instrumentation Facility,
Pondicherry University, for NMR spectra and DST-FIST for
the single-crystal X-ray diffraction facility.
Synthesis of [L2Al(OCH2Ph)2] (9). A solution of L2H (0.55 g, 1.83
mmol) in toluene (20 mL) was cooled to −78 °C in a 100 mL Schlenk
flask, and AlMe3 (1 mL, 2 M in toluene, 2.00 mmol) was added. The
reaction mixture was warmed to room temperature, and the mixture
was refluxed for 4 h. The reaction mixture was cooled to room
temperature, and benzyl alcohol (0.40 g, 3.67 mmol) was added. The
mixture was refluxed again for 8 h. Volatiles were removed under high
vacuum, and the residue was washed with hexane and dried under
vacuum, to give a yellow powder. Yield: 80% (0.79 g). Mp: 214 °C
dec. 1H NMR (CDCl3, 400 MHz): δ 4.34 (s, 4H, CH2), 6.98 (m, 8H,
Ph H), 7.16 (m, 2H, Ph H), 7.52 (m, 2H, Py H), 7.58 (m, 2H, Ar H),
7.85 (m, 2H, Py H), 7.96 (m, 2H, Ar H), 8.95 (m, 2H, Py H). 13C
NMR (CDCl3, 100 MHz): δ 65.27, 120.03, 122.67, 125.36, 125.84,
127.81, 131.81, 138.06, 140.45, 145.22, 147.75, 157.14, 163.12. Anal.
Calcd for C32H26AlN5O2: C, 71.23; H, 4.86; N, 12.98. Found: C,
71.01; H, 4.90; N, 12.98.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for N.D.R.: ndreddy.che@pondiuni.edu.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Department of Science and
Technology, New Delhi, for financial support and the
Alexander von Humboldt Stiftung of Germany for donation
J
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