Full Papers
doi.org/10.1002/ejoc.202001392
assembled lamellar arrays, containing near-rectangular features
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
with dimensions close to that of a (ꢀ)-Zr(1) molecule were
2
observed (Figure 4A). The slightly oblique unit cell appears to
contain two molecules of (ꢀ)-Zr(1) and is defined by vectors
2
with dimensions a=3.5ꢀ0.2 nm and b=2.4ꢀ0.15 nm, under
an angle of 93ꢀ2°. Although the near-rectangular features
contain sub-molecular features, the structure of (ꢀ)-Zr(1)
2
cannot be directly identified in them. This is not surprising,
given the fact that the adsorbed double cages are large 3D
architectures of which the aromatic and aliphatic parts are
superimposed from the point of view of the STM probe.
Nevertheless, two types of roughly equally sized near-rectan-
gular features can be discerned, differing in brightness, which
alternate along each of the lamellar arrays. The observed
difference in brightness may be caused by (i) the alternating
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Figure 4. (A) STM image of a self-assembled monolayer of (ꢀ)-Zr(1)
interface of HOPG and 1-phenyloctane; the unit cell is indicated in white;
bias =À 570 mV, Iset =3 pA. (B) Molecular model of a possible composition of
the self-assembled monolayer, in which the red and blue structures
represent the two different enantiomers of Zr(1) , (+)-Zr(1)
and (À )-Zr(1)
respectively. The unit cell is indicated by the black arrows.
2
at the
V
2
2
2
,
adsorption of the (M)-isomer and the (P)-isomer of (ꢀ)-Zr(1) , (ii)
2
two different adsorption geometries of the double cage
molecules (i.e., independent of the enantiomorph), or (iii) a
combination of these two possibilities. Unfortunately, the
complex appearance of the rectangular features prohibited
identification of either the isomer identity or the cage
orientation. A molecular model was constructed assuming the
possibility that both enantiomers of Zr(1)2 adsorb on the
surface, i.e. as a 2D racemate (Figure 4B). Within a lamellar
unit cell, and 58° in the other molecule, which is somewhat
[
52,54]
larger than the angle of 37° reported for Zr(TPP) .
The
2
average distance between the mean planes through the
porphyrin nitrogen atoms is 2.56 Å, which is the same distance
[52,54]
as was found in the X-ray structure of Zr(TPP)2.
The average
distance between the mean porphyrin planes (plane through
the 24 atoms of the porphyrin ring) is 3.33 Å, which is close to
array, the 2D assembly of (ꢀ)-Zr(1) (the two enantiomers are
2
[52,54]
the reported value of 3.28 Å for Zr(TPP)2.
The dihedral angle
indicated in red and blue, respectively) is stabilized by
intermolecular π-π stacking interactions between the meso-
phenyl rings of adjacent porphyrin planes, while between the
lamellae the phenyl rings of the glycoluril frameworks of the
double cages are within Van der Waals contact distance. As
mentioned above, a stable monolayer of the racemate formed
readily at the solid/liquid interface, while multiple attempts to
construct and image stable monolayers of the pure enantiomers
failed. This might imply that the presence of both enantiomers
of the double cage compounded is required for the formation
of a stable layer. However, since layer formation at a solid/liquid
interface can be quite unpredictable and is dependent on a
variety of conditions, such a conclusion may be premature and
requires additional research.
between the pyrrole rings and the mean porphyrin planes is an
indication of the distortion of the porphyrin. For Zr(TPP) , these
2
[52,54]
angles ranged from 9.9° to 24.2°,
while for (À )-Zr(1) they
2
vary from 11.2° to 26.2°, indicating that the porphyrins in (À )-Zr
(
1) are slightly more distorted than those in Zr(TPP) . The
2
2
average distance between the protons H-30 (see Figure 5 for
proton numbering) on opposite xylylene sidewalls at the same
portal of the two cavities is 6.40 Å, which is similar to the
distance in the X-ray structure of these protons in the parent
[55]
cage compound H 1, i.e., 6.34 Å.
The distance from the
2
carbonyl groups at the bottom of the cavity to the mean
porphyrin plane is 8.67 Å in H 1 and on average 9.03 Å in (À )-Zr
2
(
1) , indicating a slight elongation of the cavities in the latter
2
complex, presumably due to the distortion of the porphyrin
planes.
NMR characterization
Self-assembly at a surface
The structure of (ꢀ)-Zr(1) in solution was investigated by
2
means of 1D and 2D NMR spectroscopy, which allowed the
assignment of all proton and carbon signals. Compared to the
The solubility of (ꢀ)-Zr(1)2 in most common solvents and
solvent mixtures turned out to be quite low, i.e. <1 mg/mL,
except in dichloromethane, in which the compound showed a
relatively high solubility of �5 mg/mL. We rationalized that in
1
H NMR spectrum of H 1, the spectrum of (ꢀ)-Zr(1) (Figure 5) is
2
2
more complex, since asymmetry is induced into this compound
upon coordination of the two cages to the zirconium(IV) center.
This induced asymmetry causes a doubling of nearly all proton
signals of the cages, making the protons located at opposite
cage ‘quadrants’ (I and III, and II and IV, see Figure 5) chemically
equivalent. In addition to this doubling of signals, the majority
of them shifted upfield compared to the signals in H21
(Table S1). The porphyrin β-pyrrole protons were assigned
based on the observed shifts of their signals upon the inclusion
of viologen guests G1 inside the cavities (vide infra): the signals
the poor solvents (ꢀ)-Zr(1) was prone to aggregation. To
2
further investigate the ability of (ꢀ)-Zr(1) to self-assemble,
2
droplets of solutions of racemic and enantiopure Zr(1) in 1-
2
À 4
phenyloctane (c�10 M) were brought onto a freshly cleaved
surface of highly oriented pyrolytic graphite (HOPG) in a
scanning tunneling microscope (STM) operating at a solid/liquid
interface. The pure enantiomers failed to form stable mono-
layers, but for the racemate extended domains of self-
Eur. J. Org. Chem. 2021, 607–617
609
© 2020 The Authors. European Journal of Organic Chemistry published
by Wiley-VCH GmbH