Synthesis of a trinuclear tropolone-palladium(II) macrocycle and its C60 inclusion properties

Article abstract of DOI:10.1246/cl.140638

4,5-Dialkyl-1,2-bis(5-tropolonylethynyl)benzenes 5a and 5b were synthesized, and dioctyl derivative 5a was converted into the corresponding trinuclear palladium(II) macrocycle 1a by react ing it w ith Pd(OAc)₂. Macrocyclic supramolecular coordi

Full text of DOI:10.1246/cl.140638

CL-140638
Received: July 4, 2014 | Accepted: July 19, 2014 | Web Released: August 1, 2014
Synthesis of a Trinuclear Tropolone-Palladium(II) Macrocycle and Its C60 Inclusion Properties
Hiroyuki Otani,*1 Chisaki Sumi,1 Hideyuki Shimizu,2 Masashi Hasegawa,3 and Masahiko Iyoda*2
1Graduate School of Environment and Information Sciences, Yokohama National University,
Hodogaya-ku, Yokohama, Kanagawa 240-8501
2Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University,
Hachioji, Tokyo 192-0397
3Department of Chemistry, School of Science, Kitasato University,
1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373
(E-mail: iyoda@tmu.ac.jp, otani-h@ynu.ac.jp)
4,5-Dialkyl-1,2-bis(5-tropolonylethynyl)benzenes 5a and 5b
were synthesized, and dioctyl derivative 5a was converted into
the corresponding trinuclear palladium(II) macrocycle 1a by
reacting it with Pd(OAc)2. Macrocyclic supramolecular coor-
dination complex 1a has an inner cavity just large enough
to include C60. Although trinuclear palladium complex 1a
incompletely incorporated C60 in solution owing to weak
interactions between the bis(tropolono)palladium(II) moieties
and C60, it formed an inclusion complex with C60 in the solid
state. The inclusion complex showed a moderate charge-transfer
absorption in the UV-vis-NIR spectra.
R
R
O
Pd
O
O
O
O
R
R
O
O
O
Pd
O
O
Pd
O
O
Study of self-assembled supramolecular coordination com-
plexes with well-defined inner cavities has become important for
gaining specific information concerning structural, electronic,
and optical properties, especially in relation to “inner” and
“outer” domains.1,2 Macrocyclic metal complexes with shape-
persistent, noncollapsible backbones are useful for building
porous surface networks and inclusion complexes.
1a: R = n-C8H17
1c: R = H
R
R
Figure 1. Molecular structure of trinuclear tropolone-Pd complexes
1a and 1c.
Tropolone and related 2-substituted tropones act as versatile
ligands in inorganic and organometallic chemistry.3 They form
chelate complexes with various metal ions via the carbonyl
oxygen and vicinal substituents. In this respect, the metal
complexes formed with tropolone and related 2-substituted
tropones have been employed as catalysts for polymerization,4
building blocks for supramolecular coordination complexes,5
stable liquid crystals,6 and magnetic units for molecular-
based magnetic materials.7 However, only a limited number of
tropolone-metal macrocycles have been reported, presumably
due to the difficulty in forming cyclic structures among common
octahedral tropolone-metal complexes.5 Recently, we found
that a square-planar bis(tropolono)palladium(II) complex can be
employed in constructing a macrocyclic framework, which has
an inner cavity just large enough to include C60 as a guest.
Herein, we report the synthesis of large triangular tropolone-
palladium complex 1a (Figure 1). Moreover, we also report the
complexation of 1a with C60.
O
1) HC≡CSiMe3,
[PdCl2(PPh3)2],
CuI, iPr2NH
I
NEt2
I
R
R
R
R
3
2) nBu3NF, THF
I
[PdCl2(PPh3)2],
CuI, iPr2NH
2a
: R = n-C8H17 (89%)
2b: R = n-C6H13 (68%)
O
O
NEt2
OH
Hc
Ha
R
R
R
R
LiOH
Hb
Hb
Ha
EtOH
H2O
O
O
NEt2
OH
Building blocks 5a and 5b for synthesizing tropolone-
palladium(II) macrocycle 1 were synthesized by modifying our
previously reported procedure (Scheme 1). The reaction of
dialkyldiethynylbenzenes 2a and 2b with 2-diethylamino-5-
iodotropone (3) under Sonogashira coupling conditions pro-
ceeded smoothly to afford bis(aminotropone) compounds 4a
(71%) and 4b (74%). 4a and 4b were hydrolyzed with LiOH
in aq. EtOH to afford bis(tropolone) compounds 5a and 5b,
respectively. Although 5a and 5b could not be purified by using
5a
: R = n-C8H17 (97%)
4a: R = n-C8H17 (71%)
4b: R = n-C6H13 (74%)
5b: R = n-C6H13 (94%)
Na2[PdCl4], EtOH, toluene
(81%)
C60
1a·C60
5a
1a
toluene,
pyridine,
MeOH
or Pd(OAc)2, EtOH, toluene
(96%)
Scheme 1. Synthesis of 5a, 5b, 1a, and 1a¢C60.
1710 | Chem. Lett. 2014, 43, 1710–1712 | doi:10.1246/cl.140638
© 2014 The Chemical Society of Japan

(a)
(b)
Py
25°
Py
(a)
(b)
22°
8°
13°
Py
Hc
Ha
Hb
C12
C10
C10*
4°
4°
C12*
13°
Py
34°
Py
c Py
H
Hb
Ha
Figure 2. X-ray crystal structures of (a) 4a and (b) 5b. The yellow
curved arrows show the dihedral angle between the central benzene
and terminal tropone rings. Wedges represent the bend angles between
the carbonyl groups and C3-C4-C5-C6 carbons in the tropone and
tropolone rings.
8.5
8.0
7.5
δ / ppm
column chromatography on silica gel due to their high polarity,
they could be purified by recrystallization from CH2Cl2-hexane
to give pure 5a (97%) and 5b (94%) as yellow prisms. Both 5a
and 5b are stable and can be stored in air at room temperature for
more than 1 year.
Figure 3. Aromatic region in the 1H NMR spectra (500 MHz) of
(a) 5a and (b) 1a in pyridine-d5 at 25 °C.
(a)
(b)
The crystal structures of 4a and 5b, single crystals of which
were obtained by recrystallization from CHCl3-hexane and
THF-hexane, respectively, were determined by using X-ray
analysis.8 As shown in Figure 2a, the two tropone rings in 4a
are twisted toward the central benzene ring presumably due to
packing effects of the bulky diethylamino groups. The dihedral
angles between the central benzene and terminal tropone rings in
4a are 8 and 22°. Furthermore, to decrease steric congestion, the
carbonyl groups in the tropone rings are bent by 34 and 25° to
form an envelope-like conformation of the tropone ring. The
molecular structure of 5b has crystallographic C2 symmetry with
a twofold axis passing through the midpoints of C(10)-C(10*)
and C(12)-C(12*) (Figure 2b). In contrast to the structure of 4a,
the dihedral angles between the central benzene and terminal
tropone rings in 5b are 13°, and the carbonyl groups in the
tropone ring in 5b are almost planar (bend angle: 4°) because
there is only a small amount of steric repulsion between the
carbonyl and hydroxy groups.
Figure 4. Calculated structures of 1c and 1c¢C60.8 (a) Planar 1c at
B3LYP/LAN2DZ level. (b) 1c with C60 in the inner cavity at HF/
STO-3G. The diameter of inner cavity is 12.4 ¡, which is large enough
to fit C60 (7.1 ¡), considering van der Waals radii.
except for pyridine, 1a could be characterized by using
spectroscopic analysis. MALDI-TOF MS showed [M + H]+ at
2085.51 with the correct isotopic ratio (Figure S2). In the
1H NMR spectra acquired in pyridine-d5 (Figure 3), the tropo-
lone protons were shifted downfield (¤ 7.96 (Ha), 7.46 (Hb))
compared to those of 5a (¤ 7.75 (Ha), 7.40 (Hb)), whereas the
benzene proton (Hc) was shifted slightly upfield (5a: ¤ 7.69; 1a:
¤ 7.66). In the UV-vis absorption spectra in pyridine, 1a showed
a small red shift (5a: 380 and 418(sh) nm; 1a: 397 and
430(sh) nm; (sh): shoulder).
DFT calculations (B3LYP/LAN2DZ) on unsubstituted
macrocyclic trinuclear palladium(II) complex 1c predicted an
almost planar structure with roughly C2 symmetry (Figure 4a).
Since there were two doublets for the tropolone units in the
1H NMR spectrum of 1a, the bis(tropolono)palladium(II) moiety
freely rotates on the NMR time scale. Although the planar
structure of 1c cannot incorporate C60 in the inner cavity,
the conformation with the bis(tropolono)palladium(II) moieties
nearly vertical to the molecular plane (Figure 4b) has an inner
cavity size large enough to fit C60. Therefore, we examined the
incorporation of C60 inside 1a.10
To construct a macrocyclic supramolecular coordination
complex composed of benzene rings, acetylene linkages, and
bis(tropolono)metal moieties, a square-planar metal-complex
unit is indispensable, although most bis(tropolono)metal com-
plexes adopt nonplanar octahedral structures.3c,3d,3f We chose a
diamagnetic bis(tropolono)palladium(II) complex as a metal unit
to prepare supramolecular coordination complex 1. The coordi-
nation sphere around the palladium(II) ions in bis(tropolono)-
palladium(II) moieties is almost square planar (O-Pd-O angles
are 81.83-99.28°), and there are no steric interactions among
the atoms in 1.9 The palladium complexes could be easily
1
characterized by using H NMR analysis.
Complex 1b with hexyl substituents is much less soluble
than 1a with octyl substituents. Therefore, further studies on the
synthesis of macrocyclic palladium complexes and their incor-
poration of C60 in the inner cavity were performed only using
1a. Trinuclear tropolone-palladium(II) complex 1a was prepared
by mixing a solution of 5a in toluene with a solution of
Na2[PdCl4] in ethanol and with a solution of Pd(OAc)2 in
ethanol in 81% and 96% yields, respectively, as a dark brown
solid. Although 1a is hardly soluble in common organic solvents
A solution of 1a in pyridine was mixed with a solution of
C
60 in toluene to afford a dark brown solution, from which a dark
brown solid was obtained after evaporation of the solvent.11 The
solubility of the dark brown solid in either toluene or pyridine
was greater than those of 1a and C60, indicating the formation
Chem. Lett. 2014, 43, 1710–1712 | doi:10.1246/cl.140638
© 2014 The Chemical Society of Japan | 1711

Py
Py
Hc'
Py
Supporting Information is available electronically on J-STAGE.
References and Notes
Hb'
Hb
Ha'
1
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9.0
8.5
8.0
7.5
δ / ppm
Figure 5. Aromatic region in the 1H NMR spectrum (500 MHz)
measured 1 h after dissolving 1.4 mg of 1a¢C60 in pyridine-d5 (0.5 mL)
at 25 °C [1a¢C60 (Ha, Hb, and Hc) and 1a (Ha¤, Hb¤, and Hc¤)].
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Crystallographic data reported in this manuscript have been deposited
with Cambridge Crystallographic Data Centre as supplementary pub-
lication No. CCDC 988875 for 4a and CCDC 988876 for 5b. Copies of
the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge, CB2 1EZ, U.K.; fax: +44 1223
336033; or deposit@ccdc.cam.ac.uk).
λ / nm
Figure 6. Normalized solid-state UV-vis-NIR spectra of 1a, C60,
and 1a¢C60 using integrating spheres.
of 1a¢C60, as shown in Figure 4b. However, C60 gradually
precipitated from the toluene solution, followed by precipitation
of 1a. In the 1H NMR spectra of 1a¢C60 in pyridine-d5
(Figure 5), the tropolone protons at ¤ 8.65 (Ha) and 7.49 (Hb)
and the benzene proton at 8.30 (Hc) were markedly downfield-
shifted from those of 1a (¤ 7.94 (Ha), 7.44 (Hb), and 7.66 (Hc)).
Because of the weak binding constant for 1a¢C60 (Ka = 580
at 25 °C) in pyridine-d5,12,14 1a¢C60 was rapidly dissociated in
pyridine-d5.15 The large downfield shift of the Hc proton in
1a¢C60 indicates that aggregation occurs in pyridine-d5.16 Thus,
we believe that C60 can be incorporated in the cavity of 1a.
The UV-vis-NIR absorption spectra of 1a, C60, and 1a¢C60
in the solid state are shown in Figure 6. Although the UV-vis-
NIR spectra of 1a and 1a¢C60 are similar, a charge-transfer (CT)
absorption was observed at 700 nm with tailing up to 1000 nm.17
In summary, we synthesized a new supramolecular tropo-
lone-palladium(II) complex, 1a, with a large inner cavity and
investigated its ability to incorporate C60. Although the low
solubility of 1a, C60, and 1a¢C60 in common organic solvents
made it difficult to characterize them in solution, a peak
indicating a CT interaction between 1a and C60 was observed in
the solid-state UV-vis-NIR spectrum of 1a¢C60. We believe that
our results are important for developing functional supramolec-
ular coordination complexes with new electronic and optical
properties.
9
X-ray structure of bis(tropolono)palladium(II): G. Steyl, Acta
Crystallogr., Sect. E: Struct. Rep. Online 2005, 61, m1860.
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11 XRD analysis of the dark brown solid of 1a¢C60 revealed an amorphous
pattern.
12 By assuming the formation of 1:1 complex 1a¢C60, the binding constant
was determined by the 1H NMR spectrum of 1a¢C60 (1.35 mg) in
pyridine-d5 (0.5 mL) at 25 °C (Figure S5). All 1a, C60, and 1a¢C60 were
soluble in pyridine-d5 under these conditions.13
13 R. S. Ruoff, D. S. Tse, R. Malhotra, D. C. Lorents, J. Phys. Chem. 1993,
97, 3379.
14 Mixing a solution of 1a in pyridine with C60 in the same solvent at room
temperature very slowly formed only a small amount of 1a¢C60
.
15 The ratio of 1a¢C60 and 1a was 0.5:1 5 min after dissolving 1a¢C60 in
pyridine-d5 at 25 °C. However, the ratio of 1a¢C60 and 1a changed to
0.4:1 1 h after dissolving 1a¢C60 under the same conditions (Figure S6).
16 Tropolone-metal complexes easily form a dimeric structure.3c-3f There-
fore, the formation of a 2:1 1a¢C60 complex or (1a)m¢(C60)n complexes
This work was supported by a Grant-in-Aid for Scientific
Research (A) (No. 50115995) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We thank
Dr. Fatema Sultana (Tokyo Metropolitan University) for her
experimental assistance.
can be expected. However, the low solubility of 1a, C60, and 1a¢C60
in common organic solvents prevented further studies on aggregation
behavior in solution.
17 HOMO and LUMO orbitals of 1a¢C60 are consistent with the CT
absorption (Figure S7).
1712 | Chem. Lett. 2014, 43, 1710–1712 | doi:10.1246/cl.140638
© 2014 The Chemical Society of Japan