(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
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