Macrocyclic Metal Complexes Bearing Rigid Polyaromatic Ligands: Synthesis and Catalytic Activity

Article abstract of DOI:10.1002/asia.201901561

We synthesised palladium and platinum complexes possessing cyclic and acyclic pincer-type polyaromatic ligands and investigated their structural effect on the catalysis. The pincer-type bis(6-arylpyridin-2-yl)benzene skeleton was constructed via Kr?hnke pyridine synthesis under transition metal-free conditions on gram-scale quantity. Ligand structure significantly influenced catalytic activity toward the platinum-catalysed hydrosilylation of diphenyl acetylenes, despite the ligand-independence of the conformations and electronic properties of these complexes.

Full text of DOI:10.1002/asia.201901561

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Accepted Article
Title: Macrocyclic Metal Complexes Bearing Rigid Polyaromatic
Ligands: Synthesis and Catalytic Activity
Authors: Koji Yamamoto, Kazuki Higuchi, Masahiro Ogawa, Hiromitsu
Sogawa, Shigeki Kuwata, Yoshihiro Hayashi, Susumu
Kawauchi, and Toshikazu Takata
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COMMUNICATION
Macrocyclic Metal Complexes Bearing Rigid Polyaromatic
Ligands: Synthesis and Catalytic Activity
[
a]
[a]
[a]
[a]
[b]
Koji Yamamoto, Kazuki Higuchi, Masahiro Ogawa, Hiromitsu Sogawa, Shigeki Kuwata,
[
b]
[b]
[a]
Yoshihiro Hayashi, Susumu Kawauchi and Toshikazu Takata*
3
Abstract: We synthesised palladium and platinum complexes
possessing cyclic and acyclic pincer-type polyaromatic ligands and
investigated their structural effect on the catalysis. The pincer-type
bis(6-arylpyridin-2-yl)benzene skeleton was constructed via Kröhnke
pyridine synthesis under transition metal-free conditions at gram-
scale quantity. Ligand structure significantly influenced catalytic
activity toward the platinum-catalysed hydrosilylation of diphenyl
acetylenes, despite the ligand-independence of these complexes’
conformations and electronic properties.
sp -carbon-rich macrocycle frameworks (Figure 1a). The flexible
nature of the ligand would tend to place the coordination site for
substrate molecules outside the macrocycle, decreasing the
potential of the reaction space defined by the cyclic ligand.
To overcome this circumstance, macrocyclic metal
complexes bearing rigid polyaromatic pincer ligands should be
promising candidates. However, such example is limited only to
the report by Lusby et al. to our best knowledge. They reported
macrocyclic platinum complex possessing diphenylpyridine
[
7]
ligand. Yet, its catalytic activity was unexamined. In this work,
we focused on 2,6-di(pyridin-2-yl)phenyl (NCN) ligands because
they are known to form stable complexes with various transition
Macrocyclic metal complexes possessing a cavity for catalytic
reaction have attracted much attention in recent years from the
viewpoint of new catalyst design along with their complex
stability, because they can bring about a specific reaction field
that shows unique reactivity and selectivity. Cavity-containing
[8]
[9]
[8a,8b]
metals such as palladium,
platinum,
mercury,
[10]
[10a]
[11]
ruthenium, osmium and gold. In addition, some of these
have been applied to catalytic reactions.
[8a–8c]
Moreover,
introduction of an aryl moiety at the 6-position of the pyridine
arms should create a more rigid reaction space around the metal
centre. In this study, cyclic and acyclic bis(6-arylpyridin-2-
yl)phenyl ligands and their palladium and platinum complexes
were synthesised (Figure 1b). X-ray crystal structure analyses,
[
1,2]
macrocyclic transition metal complexes,
macrocyclic
[
3]
[4]
organocatalysts, and biohybrid-type catalysts have already
been reported. The development of macrocyclic transition metal
complexes might facilitate various reactions according to the
[
5]
combination of metals and ligands. However, differences in
catalytic activity due to ligand or complex topology, i.e. cyclic vs.
acyclic, remains to be systematically studied. Therefore, we
must clarify the relation between structure, topology and
catalytic activity in order to move toward the important goal of
developing robust macrocyclic transition metal catalysts with
high activity and stability.
1
H-NMR, UV/vis, DFT calculations of the obtained complexes
and platinum-catalysed hydrosilylation reactions were performed.
Even though the structure and electronic state in the vicinity of
the metal centre were unaffected by the ligand topology,
differences in catalytic activity in platinum-catalysed
hydrosilylation were observed.
Citing the generality and usefulness of pincer-type ligands in
[
6]
homogeneous metal complex catalysts, we previously reported
macrocyclic Pd(II) complexes composed of pyridine-2,6-
bisamide ligand, which could catalyse hydroamination and
[
2j,2o,2r]
chemoselective Suzuki coupling.
Meanwhile, Saito et al.
reported N-heterocyclic carbene (NHC)-type macrocyclic Pd(II)
complexes capable of catalysing the Heck reaction and oxidative
[
2i]
alkynyl-alkynyl homocoupling reaction. Chaplin et al. reported
a
terminal alkyne coupling reaction catalysed by Rh(I)
[
2t]
complexes of macrocyclic NHC-based ligands.
However,
these reported macrocyclic pincer catalysts showed bent or
twisted conformations around the metal centre because of their
[
a]
b]
Dr. K. Yamamoto, K. Higuchi, Dr. M. Ogawa, Dr. H. Sogawa, Prof.
Dr. T. Takata
Department of Chemical Science and Engineering
Tokyo Institute of Technology
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503 (Japan)
E-mail: takata.t.ab@m.titech.ac.jp
[
Prof. Dr. S. Kuwata, Dr. Y. Hayashi, Prof. Dr. S. Kawauchi
Department of Chemical Science and Engineering
Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
Figure 1. Reported macrocyclic pincer catalysts. Macrocyclic pincer catalysts
possessing (a) flexible structures and (b) NCN metal complexes synthesised
in this work.
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
The cyclic palladium and platinum complexes and acyclic
palladium and platinum complexes were synthesised through
construction of an NCN-type bis(6-arylpyridin-2-yl)benzene
skeleton (NCN), etherification and metallation (Scheme 1). Note
that the polyaromatic NCN was prepared under transition metal-
free conditions at gram-scale quantity via Kröhnke pyridine
metal centres in these complexes, summarised in Table 1, are
mostly unexceptional. One notable feature is the significant
deviation from typical square-planar coordination of the
[
8a,8d,9]
palladium(II) and platinum(II) centre,
as shown in the side
views (Figure S3) along with the small C6-M-Cl angles
(155.3(2)–157.4(3)°). The deformation may be ascribed to steric
[
12]
[12]
synthesis.
Treatment of 1,3-diacetylbenzene (1) with I
2
in
repulsion between the chlorine atom and the two phenyl rings.
1
pyridine afforded bis(pyridinium iodide) 2. One-step formation of
two pyridine rings occurred by refluxing of 2 and Mannich base 3
in acetic acid in the presence of excess ammonium acetate to
give the desired NCN at 42% yield at gram scale. NCN was
converted to an acyclic methyl ether ligand (NCN-Me) and a 27-
membered macrocyclic ligand (NCN-27) by etherification with
the corresponding electrophile. An X-ray single-crystal analysis
of NCN-Me confirmed the construction of a bis(6-arylpyridin-2-
yl)benzene skeleton. Heating of NCN-Me, potassium
tetrachloropalladate(II) and sodium hydrogen carbonate in a
mixture of acetic acid and water at reflux temperature for 2 h
gave palladium complex PdCl-NCN-Me at 42% yield. The
platinum complex PtCl-NCN-Me was synthesised by the
reaction of NCN-Me with potassium tetrachloroplatinate(II) under
similar conditions at 47% yield. In the same manner, macrocyclic
complexes PdCl-NCN-27 and PtCl-NCN-27 were obtained from
Consistent with the H-NMR spectra, the structures of all four
complexes are almost superimposable, except for the presence
of a cavity around the chlorido ligand in cyclic complexes PdCl-
NCN-27 and PtCl-NCN-27; Figures 2c and 2f highlight the
[
14]
differences between acyclic and cyclic platinum complexes.
These structural characteristics were reproduced well by
structural optimisation using DFT calculations (ωB97X-D/6-
311G(d,p) for C, H, N, O, Cl atoms and SDD+f for Pd, Pt atoms)
for both acyclic and cyclic complexes of palladium and platinum
(Figure S5 and Table S1), suggesting that crystal packing does
not affect the solid-state structures. Figure S6 displays the
HOMO and LUMO shapes of the four metal complexes. PdCl-
NCN-Me and PdCl-NCN-27 showed almost similar shapes of
the HOMO and LUMO. In addition, the shapes of these orbitals
of PtCl-NCN-Me were almost identical with those of PtCl-NCN-
27.
1
13
NCN-27 with 66% and 43% yields, respectively. The H and
C
NMR spectra and ESI-MS spectra were congruent with
1
formation of all of the complexes (See SI). The H-NMR
spectrum of PdCl-NCN-27 showed little difference in the
aromatic region compared with that of PdCl-NCN-Me (Figure
S1), indicating that their structures in solution were similar. PtCl-
NCN-27 also gave aromatic proton signals similar to those of
PtCl-NCN-Me (Figure S2).
Figure 2. Crystal structures of (a) PdCl-NCN-Me, (b) PtCl-NCN-Me, (d) PdCl-
NCN-27 and (e) PtCl-NCN-27. CPK model representations of (c) PtCl-NCN-
Me and (f) PtCl-NCN-27 are also shown. Hydrogen atoms and the co-
crystalised molecules are omitted for clarity (a, b, d and e). Ellipsoids are
drawn at the 50% probability level.
In order to investigate the effect of the cyclic skeleton on
photophysical properties, UV/vis analyses of the metal
complexes were conducted in DMSO (0.020 mM, 1 cm, 25 °C).
The UV/vis spectra of palladium and platinum were not affected
by the cyclic skeleton (Figure 3). The UV/vis spectrum of PdCl-
NCN-Me showed absorptions at 331, 354 and 374 nm. PdCl-
NCN-27 exhibited a similar spectrum to PdCl-NCN-Me (Figure
3a). PtCl-NCN-Me displayed absorption bands at 317, 360, 383
and 404 nm. The absorption spectrum of PtCl-NCN-27 was also
almost identical with that of PtCl-NCN-Me. These results show
Scheme 1. Synthesis of acyclic (left) and macrocyclic (right) palladium and
platinum complexes. Reaction conditions: (a) K
2
PdCl
O (10/1), reflux, 2 h. (b) K PtCl
1.1 equiv.), AcOH, reflux, 3 days.
4
(1.1 equiv.), NaHCO
4
(1.1 equiv.), NaHCO
3
(
(
1.1 equiv.), AcOH/H
2
2
3
The four metal complexes, PdCl-NCN-Me, PtCl-NCN-Me,
PdCl-NCN-27 and PtCl-NCN-27, were determined by X-ray
analysis (Figure 2). The geometrical parameters around the
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that the cyclic skeleton had little influence on photophysical
In addition, no difference on induction period of approximate ten
hours was observed between acyclic PtCl-NCN-Me and cyclic
PtCl-NCN-27 (Figure S8). For comparison of the catalytic
effectivity, Karstedt's complex, which is the well-known catalyst
for hydrosilylation, provided 7a in 29% under the same condition.
The reaction of bis(4-acetoxyphenyl)acetylene (5b) in the
presence of PtCl-NCN-Me gave the hydrosilylated product 7b in
1
properties, in good agreement with the results of H-NMR, X-ray
crystallographic analyses and DFT calculations. Unlike platinum
[
9]
complexes bearing NCN ligands reported previously, PdCl-
Table 1. Selected experimental bond lengths (Å) and angles (deg) in PdCl-
NCN-Me, PdCl-NCN-27, PtCl-NCN-Me and PtCl-NCN-27.
7
8% yield along with the recovery of 12% of the unreacted 5b
(
Figure S9). PtCl-NCN-27 provided 7b in 93% yields. When
bis(4-methoxyphenyl)acetylene (5c) was used as the substrate,
the catalysts PtCl-NCN-Me afforded the hydrosilylated 7c and
the unreacted 5c at 68% and 25% yield, respectively (Figure
S10). The use of a PtCl-NCN-27 as catalyst resulted in low yield
of 7c (39%) along with a large amount of unreacted 5c (55%).
The hydrosilylation of bis(4-methoxycarbonylphenyl)acetylene
PdCl-NCN
Me
PtCl-NCN
Me
27
27
[
[
[
a]
a]
a]
M-N2
M-C6
M-N1
2.104(2)
1.914(3)
2.109(2)
2.447(1)
156.5(1)
2.117(5)
1.900(6)
2.123(6)
2.426(3)
155.3(2)
2.081(7)
1.86(1)
2.068(8)
2.430(3)
157.4(3)
2.069(5)
1.903(7)
2.071(6)
2.414(2)
156.5(2)
(
5d) proceeded in the presence of PtCl-NCN-Me and PtCl-NCN-
27 to give the hydrosilylated product 7d at 51% and 34% yields,
and the unreacted 5d at 10% and 62% yields, respectively
[
a]
M-Cl
(
Figure S11). The yields of products 7c and 7d obtained with
[
a]
PtCl-NCN-27 were lower than those obtained with PtCl-NCN-
Me. Reactions proceeded with high stereoselectivity, and only
cis-adducts 7 were observed for all substrates, which implies
that this reaction likely proceeds through the standard Chalk-
C6-M-Cl
[
a] M = Pd (PdCl-NCN-Me, PdCl-NCN-27) or Pt (PtCl-NCN-Me, PtCl-NCN-
7)
2
[
17]
Harrod mechanism.
by correspondence with literature (for 7a) or NOE experiments
for 7b–d). The large difference in the yield of 7 between PtCl-
The stereochemistry of 7 was confirmed
(
NCN-Me and PtCl-NCN-27 shows that, despite little difference
in local structure around the metal or in electronic properties, the
cyclic skeleton had a considerable effect on the catalytic activity
alone. The relation between ligand topology and the electronic
nature of the substituents is currently under investigation.
Table
2.
Platinum-catalyzed
cis-selective
hydrosilylation
of
diphenylacetylenes 5 with triethylsilane (6).
Figure 3. UV/vis spectra (0.020 mM, 1 cm cell, DMSO, 25 °C) of (a) palladium
and (b) platinum complexes.
NCN-Me, PdCl-NCN-27, PtCl-NCN-Me and PtCl-NCN-27 were
[
15]
[a]
nonfluorescent in DMSO under air.
yield
To evaluate the impact of the cyclic skeleton in NCN
complexes, the catalytic activities of PtCl-NCN-Me and PtCl-
NCN-27 were compared in hydrosilylation of diarylacetylenes
R
PtCl-NCN-Me
PtCl-NCN-27
7
a
H
12
78
68
51
74
93
39
34
(
(
Table 2). Diphenylacetylene (5a) was reacted with triethylsilane
6; 1.2 equiv.) in the presence of each PtCl-NCN complex (0.5
7b
OAc
OMe
mol%) in toluene at 60 °C for 20 h (Figure S7). The reactions
proceeded in visually homogeneous solution for both catalysts.
PtCl-NCN-Me provided the cis-adduct 7a and the unreacted 5a
at 12% and 88% yield, respectively. On the contrary, in the
presence of macrocyclic catalyst PtCl-NCN-27, the reaction
proceeded much more efficiently to give 7a at 74% yield along
7
7
c
d
2
CO Me
[
a] NMR yields
[
16]
with the unreacted 5a at 20% yield under the same conditions.
These reactions were re-examined to yield 7a in 20 and 68%
with the use of PtCl-NCN-Me and PtCl-NCN-27, respectively. It
also should be noted 7a was not volatile because the weight of
isolated 7a did not change before and after decompression with
vacuum pump. Thus, variation of yields is considered to be small.
In conclusion, we synthesised acyclic and cyclic rigid NCN
ligands and their palladium and platinum complexes. Based on
1
H-NMR, X-ray crystallographic analyses, DFT calculations,
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COMMUNICATION
UV/vis spectroscopy and analyses of catalytic activity in
hydrosilylation, we conclude that the cyclic skeleton had a
considerable effect on catalytic activity despite no substantial
effect on the complexes’ local structure around the metal or
electronic properties. These findings open the door to the design
of novel macrocyclic pincer-type catalysts including CNC (2,6-
diphenylpyridine) and NNN (terpyridine) donor sets.
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Technology (CREST) project from the Japan Science and
Technology Agency (JST) grant number JPMJCR1522, a Grant-
in-Aid for Specially Promoted Research (JSPS KAKENHI Grant
Number JP17H06092 to S. K.) and a Grant-in-Aid for Young
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[
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Chemistry - An Asian Journal
10.1002/asia.201901561
COMMUNICATION
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COMMUNICATION
Rigid macrocyclic pincer catalysts
were synthesised. The ligand
structure significantly influenced the
catalytic activity alone.
Koji Yamamoto, Kazuki Higuchi,
Masahiro Ogawa, Hiromitsu Sogawa,
Shigeki Kuwata, Yoshihiro Hayashi,
Susumu Kawauchi and Toshikazu
Takata*
Page No. – Page No.
Macrocyclic Metal Complexes
Bearing Rigid Polyaromatic
Ligands: Synthesis and Catalytic
Activity
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