Well-Designed Unsymmetrical Salphen-Al Complexes: Synthesis, Characterization, and Ring-Opening Polymerization Catalysis

Article abstract of DOI:10.1021/acs.organomet.7b00106

The unsymmetrical salphen ligand (3,5-^tBu-1-OH-C₆H₂)CHN-C₆H₄-NCH(3-Ph-1-OH-C₆H₃) (^tBu-PhLH₂) was designed and synthesized to support aluminum complexes. Its symmetric analogues, (3,5-^tBu-1-OH-C₆H₂)CHN-C₆H₄-NCH(3,5-^tBu-1-OH-C₆H₂) (^tBuLH₂) and (3-Ph-1-OH-C₆H₃)CHN-C₆H₄-NCH(3-Ph-1-OH-C₆H₃) (^PhLH₂), were also explored and compared. The methane elimination reactions between ligands and AlMe₃ resulted in formation of ^tBu-PhLAlMe (1),^tBuLAlMe (2), and ^PhLAlMe (3) in high yields, which were characterized by elemental analysis, ¹H and ¹³C NMR. The coordination geometries of unsymmetrical and symmetric ligands in 1 and 2 were studied by X-ray diffractions, which revealed a five-coordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1-3 reacted with BnOH at 70 °C to give ^tBu-PhLAlOBn (4), ^tBuLAlOBn (5), and ^PhLAlOBn (6), respectively, which existed as monometallic species in solution as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reaction of ^tBu-PhLH₂ with 1 equiv of Al(OⁱPr)₃ contained three species, one monometallic ^tBu-PhLAlOⁱPr (7) and two bridged dimers μ-O₂-(cis-^tBu-PhLAlOⁱPr)₂ (8) and μ-O₂-(trans-^tBu-PhLAlOⁱPr)₂ (9). The catalytic performances of unsymmetrical 4 for the ring-opening polymerization of racemic lactide (rac-LA) were preliminarily studied and compared to those of symmetric 5 and 6.

Full text of DOI:10.1021/acs.organomet.7b00106

Article
pubs.acs.org/Organometallics
Well-Designed Unsymmetrical Salphen-Al Complexes: Synthesis,
Characterization, and Ring-Opening Polymerization Catalysis
†
†
,†,‡
‡
,†
Wenlong Luo, Tong Shi, Shaofeng Liu,*
Weiwei Zuo, and Zhibo Li*
†
School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China
‡
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, P. R.
China
*
S Supporting Information
t
ABSTRACT: The unsymmetrical salphen ligand (3,5- Bu-1-
OH-C H )CHN-C H -NCH(3-Ph-1-OH-C H )
6
2
6
4
6
3
tBu‑Ph
(
LH ) was designed and synthesized to support
aluminum complexes. Its symmetric analogues, (3,5- Bu-1-
2
t
t
OH-C H )CHN-C H -NCH(3,5- Bu-1-OH-C H )
6
2
6
4
6
2
tBu
(
LH ) and (3-Ph-1-OH-C H )CHN-C H -NCH(3-
2
6
3
6
4
Ph
Ph-1-OH-C H ) ( LH ), were also explored and compared.
6 3 2
The methane elimination reactions between ligands and AlMe
3
tBu‑Ph
tBu
resulted in formation of
LAlMe (1), LAlMe (2), and
Ph
LAlMe (3) in high yields, which were characterized by
1
13
elemental analysis, H and C NMR. The coordination
geometries of unsymmetrical and symmetric ligands in 1 and 2
were studied by X-ray diffractions, which revealed a five-
coordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1−3 reacted with BnOH
tBu‑Ph
tBu
Ph
at 70 °C to give
LAlOBn (4), LAlOBn (5), and LAlOBn (6), respectively, which existed as monometallic species in
tBu‑Ph
solution as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reaction of
LH with 1 equiv
2
i
tBu‑Ph
i
tBu‑Ph
i
of Al(O Pr) contained three species, one monometallic
LAlO Pr (7) and two bridged dimers μ-O -(cis-
LAlO Pr) (8)
2
3
2
tBu‑Ph
i
and μ-O -(trans-
LAlO Pr) (9). The catalytic performances of unsymmetrical 4 for the ring-opening polymerization of
2
2
racemic lactide (rac-LA) were preliminarily studied and compared to those of symmetric 5 and 6.
1
. INTRODUCTION
reported aluminum complexes ligating an achiral salen ligand,
exhibiting stereocontrol ROP of rac-LA by a chain-end control
The tetradentate Schiff bases, salen ligand and its derivatives,
have more than a century of history. In the last two decades,
they were used as one of the most important synthetic ligand
systems in the areas of coordination chemistry and asymmetric
18
mechanism. Gibson systematically studied the relationship
between the structures and the catalytic performances of this
19
system for ROP of rac-LA.
The current research mainly focuses on the symmetric chiral
1
−6
catalysis.
One major breakthrough is the discovery of
20−25
or achiral ligands having a C -symmetric feature.
In
2
Jacobsen’s catalyst in 1990s, which is enormously powerful and
universally applied for catalyzing asymmetric epoxidations of
unfunctionalized olefins. Since the first reports of manganese-
salen complexes by Jacobsen and Katsuki, other transition
and main group metals bearing the same ligand framework have
contrast, the unsymmetrical ligands might offer a much greater
diversity of metal-salen species with more flexible and tunable
electronic and steric properties, and thus lead to a possibility for
better catalytic performances. However, the study of metal
catalysts based on unsymmetrical salen-type ligands is far less
7
8
9
10−13
been widely developed.
The success of Jacobsen’s catalyst
26
investigated due to the challenge of synthesis. The most
efficient methodology for synthesis of unsymmetrical salen-type
ligands is a stepwise approach involving protection of one
is apparently attributed to the coordinated chiral salen ligands,
which create strongly stereogenic environments around the
metal centers and lead to the remarkable discrimination to
27−29
7
amino group by hydrochloric acid.
Using this method,
enantiomers. On the other hand, the controlled modifications
Carpentier synthesized an unsymmetrical salen-type ligand,
of the catalytic properties are readily achieved by tuning of the
substituents on two phenolate rings and the backbones.
Inspired by these studies, Spassky reported an aluminum
complex based on a chiral Schiff base and successfully
synthesized isotactic PLA from rac-LA by a site control
which combined a fluorous alcohol and a phenol, and prepared
30
corresponding Al complexes. Very recently, an unsymmetrical
2+
salphen-Cu complex was reported through the Cu -templating
approach synthesis and showed unique catalytic properties in
1
4
15
16,17
mechanism. Baker and Coates
extended this research
and synthesized a series of PLA with different tacticity
Received: February 9, 2017
depending on the nature of catalysts and feed stock. Nomura
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00106
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Organometallics
Article
Scheme 1. Synthesis of the Unsymmetrical Salphen Ligand tBu‑PhLH2
Scheme 2. Synthesis of the Unsymmetrical and Symmetric Al Complexes 1−6
3
1
methyl methacrylate polymerization. Unfortunately, this
convenient methodology could not be employed for the
preparation of Al complexes due to the high sensitivity of Al
precursor and led to an untraceable mixture. Herein, we
reported the straightforward synthesis of the unsymmetrical
salphen ligand with different substituents on the two phenolate
rings without protection of an amino group. Aluminum
complexes supported by this unsymmetrical ligand were
prepared and characterized by NMR and X-ray diffractions.
The Al complexes bearing symmetric salphen ligands were also
synthesized for comparison. Preliminary investigations for ROP
of rac-LA using unsymmetrical Al complexes were reported and
compared to the symmetric analogues.
protection (Scheme 1). Thus, the reaction of 3-phenyl-
salicylaldehyde and a in EtOH selectively formed unsym-
tBu‑Ph
metrical
LH as a precipitate in 85% yield. However, the
2
t
intermediate b reacted with 3,5- Bu-salicylaldehyde under a
wide range of conditions to give a mixture of unsymmetrical
tBu‑Ph
tBu
Ph
tBu‑Ph
LH and symmetric LH and LH2.
LH was
2
2
2
stable in the solid state and in H O-free solution. Its structure
2
was confirmed by elemental analysis and NMR spectroscopy. In
1
tBu‑Ph
the H NMR spectrum of
LH , the resonances at 13.66
2
pm and 13.52 ppm for OH and 8.72 and 8.64 ppm for CHN
clearly illustrated the unsymmetrical structure of the ligand. In
sharp contrast, there was only one set of signals for OH and
1
tBu
CHN in the H NMR spectra of symmetric LH and
2
Ph
LH , which were prepared according to the previous
2
2
. RESULTS AND DISCUSSION
.1. Synthesis of the Unsymmetrical Salphen Ligand.
19,32
reports.
We also tried to synthesize other similar unsym-
2
metrical ligands using this method. When 3,5-dibromosalicy-
The straightforward condensation of diamine with two different
laldehyde was employed as the second sequential salicylalde-
salicylaldehydes usually results in a statistical mixture of one
tBu‑Br
27−29
hyde, the unsymmetrical
LH (Scheme S1) was obtained
2
unsymmetrical and two symmetric salen-type ligands.
In
selectively as an orange solid. However, the synthesis of
order to prepare unsymmetrical salen-type ligands, the
methodology that protecting one amino group and isolating
the monoimine intermediate is generally employed. In this
context, we found that it was possible to synthesize unsym-
metrical salphen ligand via an intermediate a without the
tBu‑H
LH2 (Scheme S1) using this methodology was not
successful. The details of synthesis and characterization of
tBu‑Br
unsymmetrical
LH ligand and the corresponding Al
2
compound are shown in the Supporting Information.
B
DOI: 10.1021/acs.organomet.7b00106
Organometallics XXXX, XXX, XXX−XXX

Organometallics
.2. Synthesis of the Aluminum Complexes. The
Article
2
factor. These differences would lead to their different catalytic
properties in the ROP of rac-LA (vide infra).
ligands reacted with 1 equiv of AlMe in toluene at room
3
tBu‑Ph
temperature to give Al methyl complexes
LAlMe (2), and LAlMe (3) in high yields (Scheme 2).
Again, the unsymmetrical nature of
LAlMe (1),
Since aluminum alkoxides are better catalysts for ROP
tBu
Ph
35−39
regarding the efficiency and molecular weight control,
we
tBu‑Ph
LAlMe was
tried to convert 1−3 into the corresponding Al-alkoxides 4−6
suggested by nonequivalent two CHN and phenolate rings
using 1 equiv of BnOH in toluene. The reactions were slow, but
1
1
according to H NMR (see the Experimental Section). Crystals
completed in 12 h at 70 °C (Scheme 2). Comparing the H
tBu‑Ph
tBu
of unsymmetrical
LAlMe (1) and symmetric LAlMe
NMR spectra of 1 and 4, the resonance at high field for Al-Me
(
2) were obtained by slow diffusion of hexane to their CH Cl2
in 1 disappeared and a new resonance around 5.00 ppm for Ph-
2
1
solutions. The thermal ellipsoid plots for 1 and 2 are given in
Figures 1 and 2, both of which revealed a distorted square-
CH -O in 4 was observed. Note that the H NMR spectrum of
2
4 had only one set of resonances (see the Experimental
Section), indicating a monometallic isomer in solution.
Otherwise, there should be two bridged dimers due to the
unsymmetrical substituents on the phenolate rings (see the
i
27
discussion below for AlO Pr compounds). Moreover, the Al
tBu‑Ph
NMR of LOBn (4) had a resonance at 38 ppm and no
resonance around 0 ppm (Figure S1), which also indicated a
five-coordinated Al center and a monometallic species.
The aluminum isopropoxide complexes could be prepared by
34
tBu‑Ph
i
the alcohol elimination reaction of
LH and Al(O Pr) in
2 3
toluene (Scheme 3). The reaction was found to proceed slowly
and completed in 2 days at 70 °C. However, this reaction was
not selective, but formed a mixture of monometallic and
1
bridged dinuclear complexes in a ratio of 1:1 according to H
NMR (Figure 3A and C, blue). In the region of 1.65−1.30 ppm
Figure 1. ORTEP of the molecular structure of 1. Ellipsoids at 50%
t
(
Figure 3D), there were six singlets for Bu groups. The two
probability level. Hydrogen atoms and uncoordinated CH Cl2 are
2
bigger singlets (1.59 and 1.36 ppm) belonged to the
omitted for clarity.
monometallic 7, while the other four were attributed to two
tBu‑Ph
i
bridged dimers μ-O -(cis-
LAlO Pr) (8) and μ-O2-
2
2
tBu‑Ph
i
(
trans-
LAlO Pr) (9) as shown in Scheme 3. To the
above mixture was added 8 equiv of Al(O Pr) at room
2
i
3
temperature to selectively form monometallic compound 7
i
(
Figure 3B, red and E). After removing the excess Al(O Pr)
3
through washing with hexane, the mixture of 7, 8, and 9 was
recurrent in solution according to NMR experiments. It was
reported that the Al metal center could be stabilized by the
40
formation of Al···O interactions in solution. The excess
i
tBu‑Ph
i
Al(O Pr) in solution could compete with the other
LO Pr
3
molecule and interact with the Al center in 7. This would
Figure 2. ORTEP of the molecular structure of 2. Ellipsoids at 50%
stabilize the monometallic species and shift the equilibrium to
probability level. Hydrogen atoms are omitted for clarity.
form 7 selectively (Figure 3). However, this interaction was so
i
weak and once Al(O Pr) was removed from the solution, the
3
equilibrium of monomer and dimers was recurrent. To get a
better understanding of this reaction, we ran the reaction of
pyramidal geometry. The Me groups located in the axial
position, which was common for salen-Al alkyl complex. The
selected bond distances and angles for 1 and 2 are summarized
in Tables S1 and S2 in the Supporting Information,
respectively. The bond distances and angles in both of 1 and
tBu‑Ph
i
LH and Al(O Pr) in THF at 70 °C. It was found that the
2
3
reaction in THF was very slow, and after 48 h, there was over
0% of free ligand unreacted. Moreover, this reaction resulted
5
in a mixture of untraceable compounds (Figure S3 in the SI),
probably due to the partial coordination of THF. The reaction
2
were like those of previously reported salen-Al com-
33,34
pounds.
However, due to the unsymmetrical feature in 1
tBu‑Ph
i
of
LAlMe (1) and PrOH in toluene was also investigated,
as compared to 2, the geometry around the Al center in the
structure of 1 was more distorted than 2, which could be
quantified by the torsion angle between the two imino-
phenolate mean planes (42.25° in 1 and 22.50° in 2). This
value in 1 was even larger than that observed in unsymmetrical
and a mixture of monomer and dimers was obtained similar as
tBu‑Ph
i
the reaction of
LH and Al(O Pr) in toluene. With the
2 3
comparison of the cases of benzyloxide and isopropoxide Al
compounds, it was believed that the bulky OBn group led to
the formation of monomer due to steric hindrance, while the Al
compound with a relatively small isopropoxide group existed as
a mixture of monometallic and bridged dinuclear in solution.
To the best of our knowledge, this is the first time to
systematically study the relationship of monomer and dimer for
this Al system, and it is of great importance to better
understand the catalytic species of ROP for aluminum
alkoxides.
Ar
Cy
CF3
30
ON NO Al complex (36.47°) reported by Carpentier.
Thus, the upper side of the coordination plane in unsym-
metrical 1 (Figure 1) was more open comparing to that in
symmetric 2 (Figure 2). The mechanism of ROP by Al
complexes is usually believed as a coordination−insertion one.
In both of complexes 1 and 2, the coordination and insertion of
monomer would occur at the Me (or resulted alkoxide) located
side, namely, the upper side, which is the main considered
C
DOI: 10.1021/acs.organomet.7b00106
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 3. Synthesis of the Monometallic 7 and Bridged Dinuclear 8 and 9
1
tBu‑Ph
i
i
Figure 3. H NMR spectra: (A)
LH with 1 equiv of Al(O Pr) ; (B) mixture of 7, 8, and 9 with 8 equiv of Al(O Pr) ; (C) zoom in A from 3.5
2 3 3
to 4.5 ppm; (D) zoom in A from 0.9 to 1.7 ppm; (E) zoom in stacked A and B from 6.6 to 8.2 ppm.
2
.3. Preliminary Studies in the Ring-Opening Poly-
Table 1. Ring-Opening Polymerization of rac-LA using 1 and
4−6
a
merization of rac-LA. Salen-type aluminum complexes are
well-known as initiators for the ring-opening polymerization
ROP) of lactide in a living and stereoselective manner.
Therefore, we are interested in evaluating the performances of
these new complexes in the ROP of rac-LA. Complex 1 was
first tested with 1 equiv of BnOH (Table 1, entry 1). The
polymerization was slow, and the produced PLA had a much
higher M as compared to the calculated one, which was
consistent with the previous report using an Al-alkyl compound
as initiator. This was probably because the reaction of 1 and
BnOH was slow and only part of the Al compound was
activated (vide supra). Generally, the metal alkoxides catalyze
ROP of lactide in a better controlled way than the alkyl
b
c
d
c
e
entry
cat.
t/h
conv. (%)
Mn
Mn (cal.)
PDI
Pi
(
f
1
2
3
4
1
24
12
24
12
75
95
85
95
9200
6700
6800
6800
5500
6900
6200
6900
1.24
1.12
1.13
1.12
78
77
79
68
4
5
6
a
Conditions: 20 μmol of Al, LA/Al = 50, 2 mL of toluene solution,
b
1
c
n
110 °C. Determined by H NMR. GPC data in THF vs polystyrene
44 d
standards, using a correcting factor of 0.58. Mn (cal.) = MLA × ([LA]:
e
1
1
8
[Al]) × conversion + MBnOH. Determined by homodecoupled H
f
NMR. 20 μmol of BnOH was added.
D
DOI: 10.1021/acs.organomet.7b00106
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
6,41,42
t
) (tBuLH
19
analogues.
4
Thus, the benzyloxide aluminum complexes
(3,5- Bu-1-OH-C
6
6
H
2
)
2
and (3-Ph-1-OH-C
6
H
3
)CHN-
Ph
32
C H -NCH(3-Ph-1-OH-C H ) ( LH ), were synthesized ac-
−6 were next used for the evaluations of catalysis. With 4−6
as the initiators, the PLAs with M ’s matching the calculated
values and narrow PDIs were synthesized (Table 1, entries 2−
), suggesting a controlled polymerization. According to
Gibson’s research, the steric effect of ortho-phenoxy sub-
stituents dramatically influences the catalytic activity and
selectivity in a reversed manner, in which bulky substituents
accelerate the polymerization rate but reduce the isoselectiv-
ity. Fujita reported that, in the bis(phenoxy−imine) titanium
complexes, the ortho-phenyl group provided a more open active
4
6
3
2
cording to the methods reported in the literatures.
.2. Synthesis of the Unsymmetrical Ligand. 3,5- Bu-2-
OH)C H CHN-C H -NH . To an EtOH solution of o-phenylene-
n
t
4
(
6
2
6
4
2
4
t
diamine (2.16 g, 20 mmol) were added 3,5- Bu-salicylaldehyde (2.34 g,
0 mmol) and p-TsOH (20 mg) at room temperature. The mixture
1
was then refluxed for 12 h, cooled down to room temperature, and
filtered. The filter cake was washed with 20 mL of EtOH to give a
yellow solid (2.85 g, 8.8 mmol, 88%). H NMR (CDCl ): δ 13.40 (s, 1
H, OH), 8.64 (s, 1 H, CHN), 7.45 (s, 1 H), 7.23 (s, 1 H), 7.09 (t, 1
H, J = 7.5 Hz), 7.04 (d, 1 H, J = 7.5 Hz), 6.83−6.75 (m, 2 H), 4.41−
3.39 (br, 2 H, NH ), 1.50 (s, 9 H, CMe ), 1.32 (s, 9 H, CMe ). Anal.
1
3
1
9
t
43
2
3
3
site than did the ortho- Bu group due to its rotation.
t
Calcd for C21
.62; N, 8.55.
H N O: C, 77.74; H, 8.70; N, 8.63. Found: C, 77.75; H,
28 2
Therefore, introduction of phenyl groups instead of Bu groups
onto the phenolate rings would accelerate the polymerization
process but reduce the isotacticity of PLAs (Table 1, entry 3 vs
8
t
(
3,5- Bu-1-OH-C H )CHN-C H -NCH(3-Ph-1-OH-C H )
6
2
6
4
6
3
tBu‑Ph
t
(
LH ). To an EtOH solution of 3,5- Bu-2-(OH)C H CHN-
2
6
2
4
). Note that the unsymmetrical 4 was more active than
C H -NH (2.85 g, 8.8 mmol) were added 3-Ph-salicylaldehyde (1.74
6 4
2
symmetric 5 and comparable to 6. In regard to the
isoselectivity, the value of 4 was higher than that of 6 (77
versus 68), but similar as that of 5 (77 versus 79). This result
indicated the unsymmetrical 4 took the advantage of combining
Bu and Ph groups and showed interesting catalytic properties
in activity and selectivity as compared to its symmetric
analogues (Table 1, entry 2 vs entries 3 and 4).
g, 8.8 mmol) and p-TsOH (20 mg) at room temperature, and then the
mixture was stirred for 12 h. The mixture was concentrated under
vacuum to 10 mL and filtered. The yellow solid obtained was purified
1
by washing with 10 mL of EtOH (3.76 g, 7.5 mmol, 85%). H NMR
t
(CDCl ): δ 13.66 (s, 1 H, OH), 13.52 (s, 1 H, OH), 8.72 (s, 1 H,
3
CHN), 8.64 (s, 1 H, CHN), 7.66 (d, 2 H, J = 7.5 Hz), 7.49−7.43
(
m, 2 H), 7.41−7.38 (m, 3 H), 7.36−7.31 (m, 3 H), 7.26−7.18 (m, 3
H), 7.00 (t, 1 H, J = 7.8 Hz), 1.45 (s, 9 H, CMe ), 1.32 (s, 9 H,
3
CMe3). 13C NMR (CDCl ): δ 164.89, 164.16, 158.86, 158.71, 142.84,
3
3
. CONCLUSION
1
1
42.41, 140.58, 137.72, 137.30, 134.37, 132.00, 130.14, 129.58, 128.40,
28.15, 127.82, 127.47, 127.14, 126.95, 120.14, 119.89, 119.59, 118.98,
In summary, we designed and synthesized unsymmetrical
tBu‑Ph
118.49, 35.26, 34.30, 31.60, 29.52. Anal. Calcd for C H N O : C,
34 36 2 2
salphen Al complex
LAlMe (1) and its symmetric
tBu
Ph
80.92; H, 7.19; N, 5.55. Found: C, 80.81; H, 7.33; N, 5.55.
.3. Synthesis of the Unsymmetrical and Symmetric
analogues
complexes 1−3 slowly reacted with 1 equiv of BnOH at 70
C in toluene to form the Al benzyloxide complexes
LAlMe (2) and LAlMe (3). The methyl
4
tBu‑Ph
Aluminum Complexes. Synthesis of
LAlMe (1). To a stirred
°
tBu‑Ph
toluene solution (30 mL) of
LH (1.01 g, 2.00 mmol) was added
2
tBu‑Ph
tBu
Ph
LAlOBn (4), LAlOBn (5), and LAlOBn (6), which
Me Al (1 mL, 2 M in toluene). The mixture was allowed to stir at 25
3
were monometallic compounds in solution as indicated by
°C for 12 h and concentrated to 5 mL under reduced pressure. 10 mL
n-hexane was added and filtered to obtain a yellow powder (1.03 g,
NMR studies. In contrast, the Al isopropoxide complexes
1
tBu‑Ph
1.89 mmol, 95%). H NMR (C D ): δ 8.02 (s, 1 H, CHN), 7.98 (s,
prepared from the reaction of
LH2 with 1 equiv of
6
6
i
1 H, CHN), 7.92 (d, 2 H, J = 7.2 Hz), 7.76 (s, 1 H), 7.50 (d, 1 H, J
= 7.0 Hz), 7.41 (t, 2 H, J = 7.4 Hz), 7.28 (t, 1 H, J = 7.4 Hz), 6.99 (s, 1
H), 6.93 (d, 1 H, J = 7.6 Hz), 6.90−6.84 (m, 2 H), 6.75−6.68 (m, 3
H), 1.54 (s, 9 H, CMe ), 1.38 (s, 9 H, CMe ), −0.39 (s, 3 H, AlMe).
13
Al(O Pr) contained three species, one monometallic
3
tBu‑Ph
i
LAlO Pr (7) and two bridged dimers μ-O -
2
i
tBu‑Ph
i
(
cis-tBu‑PhLAlO Pr) (8) and μ-O -(trans-
LAlO Pr) (9).
2
2
2
3
3
According to the NMR experiments, the bridged dimers 8 and
would convert to monometallic 7 in the presence of excess
C NMR (C D ): δ 164.67, 162.35, 161.86, 141.80, 139.55, 139.11,
6
6
9
1
38.93, 138.38, 137.71, 134.74, 133.69, 132.03, 129.79, 128.75, 128.31,
i
Al(O Pr) , and this transformation was reversed by removing
the Al(O Pr) . The preliminary study for ROP of rac-LA
128.17, 127.41, 126.55, 119.62, 118.17, 116.82, 116.30, 116.20, 35.21,
3
i
34.17, 31.45, 29.10, −10.08. Anal. Calcd for C H AlN O : C, 77.18;
3
35 37
2
2
revealed that the unsymmetrical 4 took the advantage of
H, 6.85; N, 5.14. Found: C, 77.32; H, 6.80; N, 5.01.
tBu
t
Synthesis of LAlMe (2). Using the similar method for 1, 2 was
combining Bu and Ph groups and exhibited intriguing catalytic
1
obtained as a yellow solid (1.01 g, 1.74 mmol, 87%). H NMR (C D ):
properties in activity and selectivity as compared to its
symmetric analogues 5 and 6. Further studies including
copolymerization and using the unsymmetrical ligands to
support rare earth metals are in progress in our group.
6
6
δ 8.13 (s, 2 H, CHN), 7.85 (d, 2 H, J = 2.1 Hz), 7.03 (d, 2 H, J = 1.9
Hz), 6.88 (dd, 2 H, J = 5.9 Hz, J = 3.2 Hz), 6.75 (dd, 2 H, J = 5.8 Hz, J
=
3.3 Hz), 1.88 (s, 18 H, CMe ), 1.41 (s, 18 H, CMe ), −0.47 (s, 3 H,
13
3
3
AlMe). C NMR (C D ): δ 165.67, 162.14, 142.05, 139.01, 138.33,
6
6
1
−
32.17, 128.25, 127.87, 119.22, 115.91, 36.19, 34.24, 31.62, 30.35,
7.96. Anal. Calcd for C H AlN O : C, 76.52; H, 8.50; N, 4.82.
4
. EXPERIMENTAL SECTION
37
49
2
2
4
.1. General Considerations. All moisture/oxygen-sensitive
Found: C, 76.32; H, 8.39; N, 4.85.
Ph
reactions/compounds were performed using standard Schlenk
techniques or glovebox techniques in an atmosphere of high-purity
nitrogen. CH Cl was dried over CaH , distilled, and stored over 4A
MS. THF, toluene, and n-hexane were dried by refluxing over sodium
and benzophenone. CDCl dried over CaH and C D dried over Na/
Synthesis of LAlMe (3). Using the similar method for 1, 3 was
1
obtained as a yellow solid (0.934 g, 1.84 mmol, 92%). H NMR
(CDCl
2 H, J = 5.9 Hz, J = 3.4 Hz), 7.59 (d, 2 H, J = 7.3 Hz), 7.45 (dd, 2 H, J
= 6.0 Hz, J = 3.3 Hz), 7.33 (d, 2 H, J = 7.7 Hz), 7.20 (t, 2 H, J = 7.4
Hz), 7.06 (t, 4 H, J = 7.7 Hz), 6.85 (t, 2 H, J = 7.5 Hz), −1.00 (s, 3 H,
AlMe). C NMR (CDCl
): δ 8.78 (s, 2 H, CHN), 7.71 (d, 4 H, J = 7.5 Hz), 7.68 (dd,
2
2
2
3
3
2
6
6
K were vacuum transferred prior to use. rac-LA, anhydrous BnOH, and
Al(O Pr) were purchased from TCI and used as received. FT-IR,
i
13
): δ 164.42, 161.66, 138.89, 138.15, 137.43,
3
3
elemental analyses, and NMR were performed on a Bruker Tensor 27
133.30, 129.24, 128.48, 127.79, 126.64, 119.80, 117.07, 116.15, 114.81,
−6.67. Anal. Calcd for C33 : C, 77.94; H, 4.96; N, 5.51.
Found: C, 77.68; H, 4.72; N, 5.52.
1
instrument, PE2400II Series and Bruker DMX-500 (500 MHz for H,
H25AlN O
2 2
1
25 MHz for 13C) instrument, respectively. The GPC measurements
Synthesis of tBu‑PhLAlOBn (4). To a stirred toluene solution (30
mL) of 1 (0.544 g, 1.00 mmol) was added BnOH (0.108 g, 1.00
mmol) at room temperature. The mixture was allowed to stir at 70 °C
for 12 h. After cooling down to room temperature, the solvent was
were collected on a Wyatt OPTILAB rEX refractive index detector
using THF as the eluent (flow rate: 1 mL min , at 40 °C) and
polystyrenes as standard with a correcting factor of 0.58. The
symmetric ligands, (3,5- Bu-1-OH-C H )CHN-C H -NCH-
−1
4
4
t
6
2
6
4
E
DOI: 10.1021/acs.organomet.7b00106
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
removed under vacuum and the residue was washed with 20 mL of
desired time, the reaction was quenched by drops of glacial acetic acid.
A small part of the solution was taken and dried for H NMR
1
1
hexane (0.591 g, 0.93 mmol, 93%). H NMR (CDCl ): δ 8.11 (s, 1 H,
3
CHN), 8.08 (s, 1 H, CHN), 7.88 (s, 1 H), 7.87 (s, 1 H), 7.78 (d,
characterization to determine the conversion. The other solution was
poured into methanol (200 mL) to precipitate polymer.
1
H, J = 2.2 Hz), 7.52−7.47 (m, 1 H), 7.42 (d, 2 H, J = 7.6 Hz), 7.30
(
6
1
t, 1 H, J = 7.4 Hz), 7.24 (s, 1 H), 7.22 (s, 1 H), 7.04−6.87 (m, 7 H),
4.5. Crystal Structure Determinations. Single crystals of
tBu‑Ph
tBu
.79 (s, 1 H), 6.77 (s, 1 H), 6.71 (t, 1 H, J = 7.5 Hz), 5.03 (d, 1 H, J =
4.0 Hz, OCHH), 4.99 (d, 1 H, J = 14.0 Hz, OCHH), 1.56 (s, 9 H,
LAlMe (1) and
LAlMe (2) suitable for X-ray structural
analysis were obtained by diffusing hexane into their CH Cl solutions
2
2
13
CMe ), 1.37 (s, 9 H, CMe ). C NMR (CDCl ): δ 165.78, 163.63,
at room temperature. The data were collected on a Rigaku RAXIS
3
3
3
1
1
1
63.35, 146.59, 142.74, 140.28, 139.96, 139.71, 139.03, 134.74, 133.26,
30.87, 129.59, 129.37, 129.01, 128.47, 127.63, 127.18, 126.57, 120.49,
19.28, 118.30, 117.15, 117.01, 66.25, 36.25, 35.22, 32.45, 30.27. Anal.
Rapid IP diffractometer with graphite-monochromated Mo Kα
4
5
radiation (λ = 0.71073 Å) at 100(2) K. Using Olex2, the structures
4
6
47
were solved and refined by XS and SHELXL, respectively. The H
atoms were placed into the geometrically calculated positions and
refined riding on the atoms to which they should be attached. Crystal
data and processing parameters for 1 and 2 are given in Table S3.
Calcd for C H AlN O : C, 77.34; H, 6.49; N, 4.40. Found: C, 77.21;
H, 6.32; N, 4.39.
41
41
2
3
tBu
Synthesis of LAlOBn (5). Using the similar method for 4, 5 was
1
obtained as a yellow solid (0.646 g, 0.96 mmol, 96%). H NMR
(
C D ): δ 8.20 (s, 2 H, CHN), 7.87 (d, 2 H, J = 2.4 Hz), 7.06 (d, 2
6
6
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00106.
■
H, J = 6.8 Hz), 7.03 (d, 2 H, J = 2.3 Hz), 6.94−6.87 (m, 5 H), 6.77
*
S
(
dd, 2 H, J = 6.0 Hz, J = 3.4 Hz), 4.89 (s, 2 H, OCH ), 1.90 (s, 18 H,
2
13
CMe ), 1.41 (s, 18 H, CMe ). C NMR (C D ): δ 165.64, 162.19,
3
3
6
6
1
46.75, 141.88, 138.84, 138.72, 132.21, 129.62, 127.47, 126.90, 126.29,
25.53, 119.22, 115.61, 65.54, 36.25, 34.27, 31.60, 30.42. Anal. Calcd
1
for C H AlN O : C, 76.75; H, 7.94; N, 4.16. Found: C, 76.55; H,
Synthesis of unsymmetrical tBu‑BrLH and corresponding
Al complexes. Selected distances (Å), angles (deg),
crystal data, and structure refinement for 1 and 2 (PDF)
Crystallographic details for 1 and 2 (CIF)
43
53
2
3
2
7
.78; N, 4.30.
Synthesis of LAlOBn (6). Using the similar method for 4, 6 was
Ph
1
obtained as a yellow solid (0.582 g, 0.97 mmol, 97%). H NMR
(
7
7
6
CDCl ): δ 8.81 (s, 2 H, CHN), 7.69 (d, 4 H, J = 7.5 Hz), 7.64−
3
.60 (m, 4 H), 7.47−7.42 (m, 2 H), 7.32 (dd, 2 H, J = 7.7, 1.4 Hz),
.21 (t, 2 H, J = 7.4 Hz), 7.04 (t, 4 H, J = 7.7 Hz), 6.97−6.95 (m, 1 H),
.93 (d, 4 H, J = 4.2 Hz), 6.89 (t, 2 H, J = 7.6 Hz), 4.60 (s, 2 H,
AUTHOR INFORMATION
Corresponding Authors
*E-mail: shaofengliu@qust.edu.cn (S.L.).
*E-mail: zbli@qust.edu.cn (Z.L.).
ORCID
■
13
OCH Ph). C NMR (CDCl ): δ 164.19, 162.07, 145.11, 138.57,
2
3
1
1
7
37.77, 133.58, 129.41, 128.47, 127.95, 127.44, 126.78, 126.05, 125.51,
19.50, 117.51, 115.79, 64.96. Anal. Calcd for C H AlN O : C,
39
29
2
3
7.99; H, 4.87; N, 4.66. Found: C, 77.88; H, 4.59; N, 4.80.
tBu‑Ph
i
tBu‑Ph
i
Synthesis of
2
LAlO Pr (7), μ-O -(cis-
LAlO Pr) (8), and μ-
2
2
tBu‑Ph
i
Shaofeng Liu: 0000-0002-1230-0946
Zhibo Li: 0000-0001-9512-1507
O -(trans-
LAlO Pr) (9). To a stirred toluene solution (30 mL) of
LH (0.504 g, 1.00 mmol) was added Al(O Pr) (0.204 g, 1.00
2
tBu‑Ph
i
2
3
mmol) at room temperature. The mixture was allowed to stir at 70 °C
for 2 days. After cooling down to room temperature, the solvent was
removed under vacuum and the residue was washed with 20 mL of
Notes
The authors declare no competing financial interest.
1
hexane. H NMR of the product indicated three isomers: one
ACKNOWLEDGMENTS
monomer 7 and two dimers 8 and 9 in the ratio of 1.0:0.5:0.5. To the
■
i
above mixture was added 8 equiv of Al(O Pr) , and the mixture was
The project was supported by NSFC No. B040102, the State
Key Laboratory for Modification of Chemical Fibers and
Polymer Materials (Donghua University) No, LK1501, and the
Department of Science and Technology of Qingdao and
Shandong Province Nos. 159181jch and 2015GGX107015.
3
stirred overnight. All the solvent was removed under vacuum to give 7
i
1
+
Al(O Pr) : H NMR (C D ): δ 8.17 (s, 1 H, CHN), 8.14 (s, 1 H,
3
6
6
CHN), 7.98 (d, 2 H, J = 7.5 Hz), 7.78 (d, 1 H, J = 2.4 Hz), 7.51 (d,
1
7
H, J = 7.2 Hz), 7.47 (t, 2 H, J = 7.7 Hz), 7.32 (t, 1 H, J = 7.4 Hz),
.02 (d, 1 H, J = 2.2 Hz), 6.98 (d, 1 H, J = 7.8 Hz), 6.93−6.84 (m, 4
H), 6.71 (t, 1 H, J = 7.5 Hz), 4.70 (dt, 12 H, J = 12.4 Hz, J = 6.2 Hz,
i
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■
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DOI: 10.1021/acs.organomet.7b00106
Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00106
Organometallics XXXX, XXX, XXX−XXX