Synthesis and characterization of sterically crowded aryloxides: ‘Mitsubishi’-class of tetrametallic aluminum complexes

Article abstract of DOI:10.1016/j.poly.2016.10.019

A new series of tetrameric aluminum molecules with a Mitsubishi structure, composed of two cage-shaped triphenolic ligands with a tripod-chelated structure, have been synthesized and characterized. Transmetalation from the cage-shaped borates was a proper

Full text of DOI:10.1016/j.poly.2016.10.019

Polyhedron xxx (2016) xxx–xxx
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Synthesis and characterization of sterically crowded aryloxides:
‘
Mitsubishi’-class of tetrametallic aluminum complexes
a,b
a
a
a
a
Akihito Konishia, , Hideto Nakajima , Hikaru Maruyama , Sachiko Yoshioka , Akio Baba ,
⇑
Makoto Yasuda
a
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of tetrameric aluminum molecules with a Mitsubishi structure, composed of two cage-
shaped triphenolic ligands with a tripod-chelated structure, have been synthesized and characterized.
Transmetalation from the cage-shaped borates was a proper method for the synthesis of these aluminum
complexes. All complexes were determined via X-ray crystallography and NMR measurements. These
Lewis acidic aluminum complexes catalyze the hetero Diels Alder reaction of the Danishefsky diene with
benzaldehyde.
Received 27 June 2016
Accepted 14 October 2016
Available online xxxx
Keywords:
Tripodal ligand
Aluminum complex
Mitsubishi-type molecules
Cage-shape
Ó 2016 Elsevier Ltd. All rights reserved.
Crystal structure
1
. Introduction
The elements in Group 13, such as boron (B), aluminum (Al),
to the weaker electronic communication between O and B atoms
induced by the geometric restrictions. As shown in Fig. 1(A), the
Lewis acidity of the cage-shaped borate 4 can be electronically
tuned by the introduction of substituents, R, such as halogens
[15], aryl groups [16], and hetero aromatic rings [17]. They also
can be sterically controlled by changing a tether atom, X, from car-
bon to silicon [18], which allows precise control of the LUMO levels
and dihedral angles of C(Aripso)AOABAO in the cage. These acces-
sible chemical modification of the tripod ligands is a promising
way to design the cage-shaped Lewis acids with a variety of char-
acteristics. Applying this strategy to an aluminum atom instead of
a boron atom is a fascinating proposition. It is well known that an
aluminum complex with aryloxide or alkoxide ligands can exhibit
many types of structures [19–22] ranging from monomeric [23–
27], dimeric [25–32], and trimeric [13,28–30,33] to tetrameric
[33–36] species depending on the degree of the steric encum-
brance incorporated into the ligands (Fig. 1(B)). These facts raised
the expectations that our triphenolic ligand with a tripod-chelated
structure could impart dramatic influence on the molecular geom-
etry of an aluminum complex. Here, we describe the tetrameric
aluminum complexes 6a–c based on cage-shaped triphenolic
ligands 5a–c, which contain a central six-coordinate aluminum
with three peripheral four-coordinate aluminum atoms (Fig. 1
gallium (Ga) and indium (In), possess derivatives that behave as
a Lewis acid and originate from a vacant p-orbital. These are versa-
tile metals that are used in precise organic syntheses [1–4]. The
properties of these metals are often controlled by the steric or elec-
tronic factors of ligands, which play important roles in developing
peculiar metal complexes with novel reactivities and properties.
The use of the multidentate ligands is one of the promising strate-
gies in ligand design, because the multidentate ligands can reduce
the freedom of the metal–ligand coordination and impart higher
steric requirements to the resultant metal complexes, compared
with monodentate ligands [5–13].
Scott and co-workers reported pioneering studies on the struc-
tural control of the coordination geometry of aluminum com-
plexes. They employed a tripodal ligand and synthesized trimeric
aluminum complex 1 as the basic platform of a reactive Lewis acid
catalyst (Fig. 1(A)) [13]. Recently, we elucidated the effectiveness
of a cage-shaped triphenolic ligand to control the Lewis acidity of
a boron atom (Fig. 1(A)) [14]. The cage-shaped borate esters with
a tripod-chelated structure such as 3 show a higher level of Lewis
acidity than that of the planar structure of a common borate 2 due
(
C)). The aluminum–oxygen framework in the obtained complexes
formed a tri-diamond skeleton, which is reminiscent of Mitsubishi
Ltd.’s logo [37]. These previously reported aluminum complexes
with a Mitsubishi structure may be viewed as metal alkoxides
⇑
Corresponding author.
E-mail address: yasuda@chem.eng.osaka-u.ac.jp (M. Yasuda).
http://dx.doi.org/10.1016/j.poly.2016.10.019
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0
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A. Konishi et al. / Polyhedron xxx (2016) xxx–xxx
Fig. 1. (A) Scott’s trimeric aluminum complex 1 and planar-/cage-shaped borates 2–4, (B) structures of aryloxide or alkoxide aluminum complexes, and (C) this work.
[
14,20,21,38–46], whereas those reported here are alkyl aluminum
100.0 ppm, which corresponds to the two types of aluminum
atoms in the tetrameric complex 6a (see below). Under dilute or
low-temperature conditions, the generation of 7a was observed
prior to that of 6a, which suggests that the trimeric complex 7a
is one of the kinetically-controlled products that is shifted to the
thermodynamically favored tetrameric complex 6a. After optimiz-
aryloxides based on tripodal ligands.
2
. Synthesis of Mitsubishi-type aluminum complexes
2.1. Direct treatment of the triphenolic ligand with AlMe
3
3
ing the reaction condition, a 2.0 equivalent of AlMe was suitable
for the synthesis of the complex 6a, giving the product quantita-
tively (Scheme 1(A)). Unfortunately, this method can only accom-
plish the synthesis of complex 6a; a change in the tether atom of
the ligand from silicon to carbon, such as 5b and 5c, yielded
unidentified products, presumably due to a decrease in the ligand
flexibility derived from the downsizing of the tether atom.
First, we attempted to synthesize a monomeric cage-shaped
aluminum complex similar to the cage-shaped borates 2 and 3
14]. However, the treatment of the triphenolic ligand 5a with
.0 equivalent AlMe in CH Cl /THF selectively afforded a tetra-
[
1
3
2
2
meric aluminum complex 6a with a Mitsubishi structure as an isol-
able product regardless of the stoichiometry. The observation of
the reaction mixture via 27Al NMR measurement indicated that a
broad signal at around 230 ppm emerged in the early reaction
stages, and is believed to have come from a trimeric aluminum
species such as 7a. The signal then was gradually converted into
a set of sharp signals at 52.0 ppm and a broad signal at around
2.2. Transmetalation via the cage-shaped borates
In order to establish versatility in the synthesis of the tetra-
meric aluminum complexes with a Mitsubishi structure, we devel-
oped a transmetalation method where the cage-shaped borate 4
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A. Konishi et al. / Polyhedron xxx (2016) xxx–xxx
3
Scheme 1. The synthetic scheme for 6a–c. (A) Direct treatment of the triphenolic ligand with trimethylaluminum and (B) transmetalation via cage-shaped borates.
was generated in situ and employed as a precursor. As shown in
Scheme 1(B), the treatment of the cage-shaped borate 4a with an
3.2. Molecular geometries
excess of AlMe
3
gave the desired tetrameric aluminum complex
The highly symmetric structure of complex 6 was successfully
identified by X-ray crystallography. The ORTEP diagrams of com-
plex 6 are shown in Fig. 2. Although the data for 6a remained the
unsolved alerts, the others were uneventfully solved. The geometry
around the aluminum atoms is shown in Fig. 3. Selected bond
lengths and bond angles are included in Table 1.
6
a, quantitatively. Moreover, the class of the tether atom of car-
bons 6b and 6c was obtained under the same conditions. This is
the first example of a transmetalation reaction of cage-shaped
borates to other metal complexes. It should be noted that the
cage-shaped borates play a template-like role in forming these tet-
rameric aluminum complexes with a Mitsubishi structure. The
moderate fixation of metal by the tripodal ligands may promote
reorganization from the monomeric cage-shaped borate to a tetra-
meric aluminum complex.
In Figs. 2(A) and 3(A), the geometry of the aluminum–oxygen
framework clearly exhibits a Mitsubishi structure with an almost
3
D symmetry, which is consistent with the high symmetry
observed via the NMR measurements. The Xtether–Y bonds at the
tether position of 6 have an anti-orientation against the coordi-
nated aluminum atoms (Fig. 2(B)), which reveals distinct differ-
3
. Results and discussion
ences with the trimeric aluminum complex
1 of a syn-
orientation, as reported by Scott and coworkers [13]. The molecu-
lar structure consists of a central six-coordinate aluminum atom
and three peripheral four-coordinate aluminum atoms. The periph-
eral aluminum atoms occupy an equilateral-triangular plane and
the six-coordinate aluminum atoms are located at the center of
the triangular plane. The two triphenolic ligands approach from
either side of the triangular plane of the peripheral aluminums
and each of the oxygen atoms of the ligand bidentately coordinates
both of the central and peripheral aluminum atoms. The central
3
.1. NMR studies
1
In the H NMR spectra of 6, the absence of the AOH resonance
1
of the triphenolic ligands 5 was clear. The d( H) of Xtether–Y at the
tether position showed typical signals for forming a cage-shaped
complex, as the downfield shift of the d( H) of SiAMe for 6a
1
1
(
0.95 ? 1.12 ppm) and the upfield shift of the d( H) of CAH for
6
b (6.07 ? 5.41 ppm) and 6c (6.08 ? 5.50 ppm) were observed
in comparison with those of the free ligands 5a–c. In addition,
the H NMR signals of the aromatic protons of 6 maintained a C
+
1
aluminum atom is designated a 3 cation surrounded by six oxygen
3
À
2
atoms from three monoanioic [Me Al(OArupper)(OArbottom)]
symmetry similar to those of free ligands, which implies the for-
mation of a product with high symmetry. The highly symmetric
structures of the obtained complexes was also consolidated by
the Al NMR measurement. The Al NMR spectra of 6 showed
two signals at around ꢀ100 ppm and ꢀ55 ppm for Al atoms, with
coordination numbers of four and six, respectively, by the integra-
tion values of these signals. Compared with the previously
(
Ar = triphenolic ligand) groups in a slightly distorted octahedral
configuration.
2
7
27
As shown in Fig. 2 and Table 1, the structures of the three com-
plexes are almost identical. For three complexes, no significant dif-
ference in the average bond distances of the AlperipheralAO (1.834
(
4)–1.861(3) Å), AlcentralAO (1.880(4)–1.895(4) Å) and the average
2
7
bond angles of OAAlperipheralAO (78.65(13)–77.8(18)°) is observed,
which also demonstrates the similarities in the previously reported
methyl aluminum alkoxides with a Mitsubishi structure [43,44]. In
contrast, the average distance between the central aluminum and
the tether atom of the triphenolic ligand (Alcentral–Xtether) varies
according to the size of the tether atom, 3.753 Å for 6a (Xtether = Si)
versus 3.553–3.571 Å for 6b and 6c (Xtether = C), which reflects the
flexibility of the cage-shaped ligand. This fine structural control of
reported Al NMR spectra of the methyl aluminum alkoxides with
27
a Mitsubishi structure [43,44], the upfield shift of the d( Al) four-
coordinate aluminum atom (ꢀ150 ppm) and the downfield shift of
2
7
the d( Al) six-coordinate aluminum atom (ꢀ11 ppm) were
observed. We believe the ring current effect from the aryl groups
in ligand 5 caused this discrepancy. Examination of the molecular
3
geometry suggests that the complex has an idealized D symmetry
in a solution state.
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4
A. Konishi et al. / Polyhedron xxx (2016) xxx–xxx
Fig. 2. ORTEP diagrams of the molecular structures of 6a–c showing 50% thermal ellipsoid probability. (A) Top view and (B) side view. Hydrogen atoms, except for those at the
tether positions, are omitted for clarity.
Table 2
Hetero Diels–Alder reaction catalyzed by 6a or 6b.
Entry
Catalyst
Yield of 10
Fig. 3. The coordination geometry around the aluminum atoms of 6a. (A) Side view
and (B) Top view. Selected aluminum and oxygen atoms are depicted as a ball and
stick model, and the other atoms are depicted as a wireframe model; hydrogen
atoms are omitted for clarity.
1
2
6a
6b
29%
53%
atom possess a distorted octahedral coordination (Fig. 3(B)); the
angles between O atoms occupying the axial positions of the cen-
tral aluminum atom vary from 180 ° for a perfect octahedron
Table 1
Selected bond lengths (mean values reported for 6b) and bond angles (mean values
reported for 6b) for 6a–c.
(
169.05(12)° for 6a, 166.1(11)° for 6b and 166.60(17)° for 6c).
6a
6b
6c
3
.3. Application to the Lewis acid catalyzed reaction
Alperipheral–O/Å
Alcentral–O/Å
Alcentral–Xtether/Å
1.853(4)
1.895(4)
3.753
78.4(2)
76.33(13)
169.05(12)
1.834(4)
1.880(4)
3.553
77.8(18)
75.7(16)
166.1(11)
1.861(3)
1.891(3)
3.571
78.65(13)
77.15(12)
166.60(17)
To estimate Lewis acid catalytic ability, a hetero Diels-Alder
O–Alperipheral–O/°
reaction of the Danishefsky diene 8 with benzaldehyde 9 was
examined in the presence of a tetrameric aluminum complex 6a
or 6b [15,47,48]. The results are shown in Table 2. Both complexes
catalyzed the reaction and complex 6b afforded a much higher
yield of the product 10 compared with that given by 6a. It seemed
strange that the tetrameric aluminum complexes 6 would possess
Lewis acidity because the aluminum atoms in complex 6 have no
vacant p-orbital, which is perfectly filled by the coordination of
O
O
ax–Alcentral–Oeq/°
ax-Alcentral-Oax/°
such a complicated Mitsubishi-type complex shows the effective-
ness of the cage-shaped ligand with the accessibility of chemical
modification. The geometries around the six-coordinate aluminum
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A. Konishi et al. / Polyhedron xxx (2016) xxx–xxx
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Tetrameric aluminum molecules with a Mitsubishi structure
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and characterized. The transmetalation from a cage-shaped borate
to the tetrameric aluminum complex enabled us to evaluate the
structural properties from among a series of cage-shaped Mit-
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CCDC 1486007, 1486008 and 1486009 contain the supplemen-
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