Self-Assembly of Extended Head-to-Tail Triangular Pt3 Macrocycles into Nanotubes

Article abstract of DOI:10.1021/acs.inorgchem.7b00475

New pyridylsalicylaldimine-based ligands with extended conjugation have been synthesized. The reaction of these ligands with K₂PtCl₄ yielded triangular Pt₃ macrocycles rather than the anticipated Pt₄ macrocycles. The macrocycles have been analyzed in solution by variable-temperature and variable-concentration ¹H NMR, NOESY, and DOSY spectroscopy studies. These investigations show that the macrocycles aggregate at room temperature in solution by an entropy-driven process. In the solid state, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and transmission electron microscopy studies show that the macrocycles aggregate into nanotubular structures. Triangular platinum-containing macrocycles with the expanded pyridylsalicylaldimine ligands are promising for constructing nanotubes and discotic liquid crystals.

Full text of DOI:10.1021/acs.inorgchem.7b00475

Article
pubs.acs.org/IC
Self-Assembly of Extended Head-to-Tail Triangular Pt Macrocycles
3
into Nanotubes
Zhengyu Chen, Brian J. Sahli, and Mark J. MacLachlan*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada
*
S Supporting Information
ABSTRACT: New pyridylsalicylaldimine-based ligands with ex-
tended conjugation have been synthesized. The reaction of these
ligands with K PtCl yielded triangular Pt macrocycles rather than
2
4
3
the anticipated Pt macrocycles. The macrocycles have been analyzed
4
1
in solution by variable-temperature and variable-concentration H
NMR, NOESY, and DOSY spectroscopy studies. These inves-
tigations show that the macrocycles aggregate at room temperature
in solution by an entropy-driven process. In the solid state, matrix-
assisted laser desorption/ionization time-of-flight mass spectrometry
and transmission electron microscopy studies show that the
macrocycles aggregate into nanotubular structures. Triangular
platinum-containing macrocycles with the expanded pyridylsalicy-
laldimine ligands are promising for constructing nanotubes and discotic liquid crystals.
INTRODUCTION
artificial membrane ion channels from self-assembling peptide
19
■
nanotubes. Moreover, Gong’s group developed a family of
oligoamide macrocycles for spontaneous self-assembly into
nanotubes that can serve as highly conductive transmenbrane
Supramolecular chemistry, where intermolecular interactions
are used to organize structures, is a powerful paradigm in the
1
synthesis of complex substances such as metal−organic
20
21
2
3
4
channels. Other examples abound in the literature.
frameworks, catenanes, and gels. Macrocycles have had a
prominent role in the development of the field of supra-
molecular chemistry, perhaps most notably Pederson’s develop-
In 2010, we reported new platinum-containing macrocycles
that are constructed from a head-to-tail assembly (Scheme
22
5
6
S1). In each case, only the Pt macrocycle was observed by
4
ment of crown ethers. Shape-persistent macrocycles have
been investigated for several decades and have already attracted
significant attention for developing new functional materials,
mass spectrometry, with no evidence of forming either smaller
or larger rings. These macrocycles can self-assemble into
columnar structures by π−π stacking. Interestingly, by using
7
8
9
such as liquid crystals, nanofibers, and ion channels. They are
also of interest for exploring new concepts in host−guest
large substituents, it was possible to isolate discrete hexamers
23
and tetramers of the Pt macrocycles in the solid state. We set
1
0
11
4
chemistry and self-assembly.
out to extend the organic linker in order to make larger Pt4
macrocycles, so that they would have better guest-accessible
channels for exploring their supramolecular chemistry. We
believed that the geometry of the ligand was robust and that
extending the distance between the platinum salicylaldehyde
groups while maintaining the geometry of the structure would
Among those properties, the self-assembly of shape-
persistent macrocycles into nanoscale structures has been
12
extensively pursued. Many organic macrocycles have been
investigated as building blocks to form tubular assemblies upon
stacking with the goal of developing new families of molecule-
1
3
based nanotubes. Forming tubular structures by stacking
cyclic building blocks has some inherent advantages over other
methods of constructing nanotubes. For example, the size and
shape of the macrocycles can be readily modified, leading to
tunable and predictable cavity sizes for the resulting nano-
lead to larger Pt macrocycles.
4
In this paper, we report our investigations of platinum-
containing macrocycles with extended conjugated ligands based
on the pyridylsalicylaldehyde system. We describe their
aggregation behavior in the solid state and solution as probed
by many techniques.
1
4
tubes. Furthermore, the properties of the inside or outside of
the nanotubes may be modified by changing the internal or
side-chain functionalities, respectively, during the synthesis of
macrocycles. This could allow for control of the hydro-
EXPERIMENTAL SECTION
■
1
5
16
Materials and Equipment. All reactions were carried out under
air unless otherwise stated. Tetrahydrofuran was distilled from
phobicity, metal binding ability, and reactivity, for instance.
The stacking of shape-persistent macrocycles into nanotubes
17
may be facilitated by weak π−π interactions (which are
largely driven by desolvation of the rings upon stacking) or
Received: February 21, 2017
18
hydrogen bonding. Ghadiri and co-workers constructed
©
XXXX American Chemical Society
A
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Article
sodium/benzophenone under nitrogen. Diisopropylamine was distilled
from NaOH under nitrogen. Acetonitrile, methanol (MeOH), and
triethylamine were purged with nitrogen gas and dried over molecular
sieves before use. All reagents were used as received unless otherwise
stated. H and C{ H} NMR spectra were recorded on either a
Bruker AV-300 or a Bruker AV-400 spectrometer. Mass spectrometry
0.25 mmol), and two cycles of evacuation/N purging were conducted.
2
The K PtCl4 solution was transferred via a syringe to the flask
2
containing imine and K CO , and the mixture was heated at 150 °C
2
3
for 4 h. The yellow-brown suspension was cooled to room
temperature, followed by centrifugation. After the supernatant solution
was decanted, three cycles of washing, centrifuging, and decanting
were performed, once with water and twice with MeOH. Upon drying
1
13
1
(
MS) data and elemental analyses were obtained at the Microanalytical
Services Laboratory, University of British Columbia. Matrix-assisted
laser desorption/ionization (MALDI) MS spectra were obtained on a
Bruker Biflex IV time-of-flight (TOF) mass spectrometer equipped
with a MALDI ion source. Electrospray ionization (ESI-)MS spectra
were obtained on a Bruker Esquire-LC ion-trap mass spectrometer
equipped with an electrospray ion source. High-resolution electrospray
ionization (HR-ESI) MS spectra were obtained on a Micromass LCT
TOF mass spectrometer equipped with an electrospray ion source.
Gramicidin S, Rifampicin, and Erythromycin were used as the
references for HR-ESI. Elemental analyses were obtained on a Carlo
Erba EA 1108 elemental analyzer. Melting points were obtained on a
Fisher-John’s melting point apparatus. IR spectra were obtained using
a Thermo Scientific Nicolet 6700 FT-IR spectrometer equipped with a
diamond attenuated-total-reflectance accessory. UV−vis spectra were
obtained in chloroform on a Varian Cary 5000 UV−vis−near-IR
spectrophotometer using a 1 cm quartz cuvette. For transmission
electron microscopy (TEM) studies, macrocycle 1a was suspended in
ethanol, and the suspension was then transferred onto a copper TEM
grid and placed in the oven at 333 K to dry. TEM was performed on a
Hitachi H7600 transmission electron microscope.
of the precipitate, the macrocycle was isolated as a yellow powder (28
1
mg, 0.028 mmol, 23%). H NMR (400 MHz, CDCl
, 25 °C): δ 9.19
3
(m, 2H), 8.11 (s, 1H), 7.95 (tt, J = 8.0(2) and 1.8(2) Hz, 1H), 7.63 (d,
J = 2.0 Hz, 1H), 7.55 (dd, J = 8.8 and 2.0 Hz, 1H), 7.46−7.53 (m,
5H), 7.31−7.36 (m, 3H), 7.28−7.31 (m, 6H), 7.18−7.21 (m, 6H),
13
1
7.15−7.18 (m, 1H), 6.96−7.01 (m, 2H), 1.35 (s, 27H). C{ H} NMR
(400 MHz, CDCl ): δ 165.7, 162.0, 149.9, 148.5, 144.1, 142.4, 138.8,
3
138.4, 137.5, 135.5, 135.1, 132.8, 131.3, 130.8, 128.3, 127.7, 125.2,
124.1, 123.8, 122.2, 121.8, 117.2, 116.3, 111.0, 89.2, 87.8, 63.1, 34.4,
31.4 ppm. IR: ν 2963, 2902, 2867, 2212, 1671, 1606, 1595, 1494, 1484,
−1
1375, 1314, 1271, 1177, 1018, 922, 840, 822, 757, 687 cm . ESI-MS
+
(MeOH). Calcd: m/z 996.4 ([M + H] ). Found: m/z 996.5. Mp:
>360 °C. Anal. Calcd for C57
H N O Pt: C, 68.73; H, 5.67; N, 2.81.
Found: C, 69.03; H, 5.71; N, 2.83.
56 2 2
RESULTS AND DISCUSSION
■
In an effort to prepare Pt macrocycles 1a′ and 1b′ (Figure 1)
4
with expanded diameters, we first constructed compounds 3a
Synthesis of Macrocycle 1a. A suspension of K PtCl (58 mg, 0.14
2
4
mmol) in dimethyl sulfoxide (DMSO; 8 mL) was sparged with
nitrogen and then heated to 100 °C until all of the salt dissolved. A
separate Schlenk flask was charged with imine 2a (0.10 g, 0.14 mmol)
and K CO (48 mg, 0.35 mmol), and two cycles of evacuation/N
2
2
3
purging were conducted. The K PtCl solution was transferred via a
2
4
syringe to the flask containing imine and K CO , and the mixture was
2
3
then heated at 120 °C for 6 h. The yellow-brown suspension was
cooled to room temperature, followed by centrifugation. After the
supernatant solution was decanted, three cycles of washing,
centrifuging, and decanting were performed, once with water and
twice with MeOH. Upon drying of the precipitate, the macrocycle was
1
isolated as a yellow powder (49 mg, 0.018 mmol, 39%). H NMR (400
MHz, CDCl , 25 °C): δ 8.74 (br s, 1H), 8.57 (br s, 1H), 7.64 (br s,
3
1
6
H), 7.15−7.42 (m, 16H), 7.13 (br s, 1H), 6.89 (d, J = 8.0 Hz, 1H),
.61 (d, J = 8.0 Hz, 1H), 6.23 (br s, 1H), 1.39 (s, 27H). 13C{ H} NMR
1
(
400 MHz, CDCl ): δ 166.2, 162.0, 148.8, 148.6, 146.3, 144.7, 139.7,
3
1
1
39.0, 136.4, 136.1, 135.4, 134.2, 132.1, 131.0, 124.2, 123.9, 122.8,
22.0, 117.6, 116.8, 109.9, 95.8, 84.4, 63.2, 34.4, 31.5 (missing one
peak). IR: ν 3083, 3031, 2958, 2903, 2869, 2206, 1596, 1571, 1492,
1
7
415, 1380, 1362, 1316, 1269, 1195, 1135, 1109, 1064, 1017, 916, 818,
08, 688 cm . MALDI-TOF MS: m/z 2755.7 ([M + H] ). Mp: >360
−1
+
°
C.
Synthesis of Macrocycle 1b. A suspension of K PtCl (146 mg,
2
4
0
.351 mmol) in DMSO (20 mL) was sparged with nitrogen and then
heated to 100 °C until all of the salt dissolved. A separate Schlenk flask
was charged with imine 2b (290 mg, 0.351 mmol) and K CO (97 mg,
2
3
0
.70 mmol), and two cycles of evacuation/N purging were conducted.
2
The K PtCl4 solution was transferred via a syringe to the flask
Figure 1. (a and b) Structures of the target extended macrocycles 1a′
and 1b′ anticipated in this study (not obtained). (c and d) Structures
2
containing imine and K CO , and the mixture was heated at 120 °C
2
3
for 6 h. The yellow-brown suspension was cooled to room
temperature, followed by centrifugation. After decanting the super-
natant solution, three cycles of washing, centrifuging, and decanting
were performed, once with water and twice with MeOH. Upon drying
of the precipitate, the macrocycle was isolated as a yellow powder (136
mg, 0.0445 mmol, 38%). IR: ν 3083, 3031, 2960, 2904, 2869, 2201,
of the Pt macrocycles 1a and 1b obtained in this study.
3
and 3b (Scheme 1) by Sonogashira coupling of acetylene
derivatives to aryl halides. Ligands 2a and 2b were prepared by
condensing aminophenol 6 with 3a and 3b, respectively. The
bulky substituents were selected to inhibit aggregation of the
macrocycles and improve their solubility so that they could be
studied in solution. Importantly, these new ligands have the
same overall geometry as the ligands used previously to prepare
1
9
596, 1508, 1489, 1409, 1363, 1314, 1269, 1197, 1175, 1135, 1018,
18, 820, 709, 685 cm . MALDI-TOF MS: m/z 3055.88 ([M + H] ).
−1
+
Mp: >360 °C.
Synthesis of Model Compound 1c′. A suspension of K PtCl (52
2
4
mg, 0.12 mmol) in DMSO (6 mL) was sparged with nitrogen and then
heated to 100 °C until all of the salt dissolved. A separate Schlenk flask
was charged with imine 2c (90 mg, 0.12 mmol) and K CO (34 mg,
the Pt macrocycles in Scheme S1 in terms of the orientation of
4
the pyridyl nitrogen with respect to the salicylaldehyde.
2
3
B
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Scheme 1. Syntheses of (a) Aminophenol 6, (b) Ligand 2a
and Macrocycle 1a, and (c) Ligand 2b and Macrocycle 1b
tendency to stack (as previously observed with the Pt4
macrocycles).
Figure 2. MALDI-TOF MS of macrocycle 1a. The peak at m/z 2755.7
+
corresponds to that of the protonated Pt macrocycle, [1a+H] .
3
Surprisingly, Pt rings were exclusively obtained with the
3
expanded ligand 2a. The change of the geometry can be
explained by two factors. First, the insertion of acetylene groups
25
introduced flexibility to the organic linkers. The additional
flexibility could contribute to the formation of macrocycles with
26
unexpected geometry. Second, in our previous organic linker
containing pyridyl-substituted salicylaldehyde, the ground-state
conformation of the ligand has a torsion between the two
aromatic rings. This nonplanarity may favor the square-shaped
macrocycle, whereas in 1a and 1b, the ligands can adopt a
conformation where the pyridyl and salicylaldimine compo-
nents are coplanar. Thus, the flat conformation of the
macrocycle may favor the triangular macrocycles. The
enhanced planarity of macrocycle 1a may lead to enhanced
stacking. These results suggest that macrocycle 1a has excellent
potential for supramolecular assembly into nanotubes. As noted
in Figure 2, MALDI-TOF MS shows a strong stacking pattern
for macrocycle 1a. In fact, we could observe up to 13
macrocycles stacked together. Qualitatively, macrocycle 1a
appears to be more prone to stacking than the Pt macrocycles
4
with the same substituents.
A computational study was conducted to investigate the
macrocycle conformation. To simplify the system, a Pt3
macrocycle with R = H was used as a model compound
(
Figure 3). The geometry was optimized using the density
functional theory (DFT) PBE0 exchange-correlation functional,
with the LANL2DZ core potential and basis set for platinum
and the 6-31g(d) basis set for all other atoms. This level of
theory is commonly chosen for complexes of iridium,
palladium, and platinum based on favorable comparisons with
gas-phase and high-level ab initio structural data. In the
optimized conformation, all three repeating units have similar
bond and twisting angles. Overall, the optimized conformation
of the macrocycle is almost planar, but it bends very slightly to
one side to give a shallow bowl-shaped conformation.
Consequently, we expected that the reaction of these new
ligands with Pt2 salts would afford Pt macrocycles 1a′ and
+
4
1
b′.
The reaction of 2a with K PtCl gave a yellow powder.
2
4
27
MALDI-TOF MS of the product revealed that the product was
not the intended Pt macrocycle 1a′ (MW = 3670.4 g/mol) but
4
24
rather Pt macrocycle 1a (MW = 2754.1 g/mol) (Figure 2).
3
Also, MALDI-TOF MS showed aggregates of the Pt3
macrocycle, suggesting that the macrocycles have a strong
C
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Figure 3. (a) Chemical structure of Pt macrocycle 1a with R = H. (b)
3
Top and (c) side views of the DFT-optimized geometry of the
macrocycle with R = H, computed at the PBE0/LANL2DZ(Pt) and 6-
3
1g(d) (other atoms) levels of theory.
All CC triple bonds in the organic linkers showed a
CCC bond angle of about 6°. The extra flexibility
introduced by acetylene groups contributed to the formation
of the 3-fold-symmetric macrocycle. In addition, in the
calculated structure, two aromatic rings belonging to the
same organic linker almost stay in the same plane, where they
can maximize conjugation. In fact, all of the aromatic rings from
the same Pt macrocycle are nearly coplanar. The flattened
3
conformation of the organic linkers with flexible acetylene
groups allowed the triangular macrocycle to form.
Another Pt3 macrocycle 1b was prepared by a similar
approach from 2b, and MALDI-TOF MS verified that it was
also the Pt macrocycle that predominated in the product.
3
However, macrocycles with other geometries could also be
observed in the MALDI-TOF MS spectrum as minor products.
Moreover, strong stacking was not observed in this case.
Notably, macrocycle 1b showed much weaker signals in the
MALDI-TOF MS spectrum compared to macrocycle 1a.
Because macrocycle 1b demonstrated very poor solubility in
organic solvents, macrocycle 1a was chosen for the following
self-assembly studies.
The MALDI-TOF MS data for macrocycle 1a suggested that
it aggregates strongly in the solid state. More experiments were
performed to investigate the self-assembly. Unfortunately,
attempts to grow a single crystal of macrocycle 1a were
unsuccessful. Powder X-ray diffraction (PXRD) of macrocycle
Figure 4. TEM images of macrocycle 1a. The repeating distance
between adjacent stripes is approximately 2.6 nm, and the inset shows
the proposed organization of 1a in the solid state.
columns in the solid state. The proposed model of the solid-
state organization of 1a is shown in the inset of Figure 4.
The actual stacking structure of the macrocycles is likely
disordered. When a head-to-tail macrocycle stacks on top of
another one, the second macrocycle could stack with either the
same or opposite orientation as the first molecule (Figure S24).
Within a long stack, the possibility of different relative
orientations between adjacent macrocycles will lead to a large
number of possible stacks and, consequently, substantial
disorder. This stacking disorder might be the reason for the
lack of sharp peaks in the PXRD pattern of macrocycle 1a.
1
a showed no sharp diffraction peaks beyond 2θ = 5° (Figure
S23). However, a peak was evident at 2θ = 3.75° (23.5 Å d
spacing), suggesting a higher-order organization within the
substance. We felt that this could arise from columnar stacking
of the macrocycles in the solid state. Because the typical pattern
of columnar stacks of disklike molecules is usually a hexagonal
2
8
lattice, the diameter of the macrocycles can be calculated
from the 23.5 Å d spacing as 2.71 nm, which is consistent with
the estimated diameter of macrocycle 1a.
To corroborate this interpretation, TEM studies of macro-
cycle 1a were undertaken. Stripes with an average separation of
about 2.6 nm are clearly evident in the TEM images (Figure 4).
This value is close to the diameter estimated for a stacked
macrocycle nanotube and the PXRD result. Thus, we believe
that the TEM images show that the macrocycles have stacked
one on top of the other into a nanotubular structure, which is
organized into a hexagonal lattice. This organization is
Although it was shown that Pt macrocycle 1a self-assembles
3
in the solid state, its behavior in solution remained unknown.
Therefore, we conducted NMR experiments to study its
behavior in solution. Because of the macrocycle’s poor
solubility in most organic solvents, NMR experiments were
1
only conducted in CDCl . The H NMR spectrum of
3
macrocycle 1a shows a set of broad peaks (Figure 5). The
1
broadening of the proton peaks in H NMR studies of
reminiscent of the Pt macrocycles, which also stacked into
macrocycles is often a good indication of aggregation because
4
D
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1
study this aggregation, we performed a variable-temperature H
NMR study of macrocycle 1a (Figure 7). Upon cooling of the
1
Figure 5. H NMR spectrum of macrocycle 1a in CDCl at room
3
−3
temperature (400 MHz, c = 7.0 × 10 mol/L). Only the aromatic
region is shown. *: CHCl from the CDCl solvent.
3
3
large aggregates have shorter T2 relaxation times and
macrocycles located in different environments within an
aggregate are both effects that can lead to broadening.
To probe this further, we carried out a dilution experiment in
−3
CDCl (Figure 6). At high concentration (7.3 × 10 mol/L),
3
1
Figure 7. Variable-temperature H NMR spectra of macrocycle 1a in
−3
CDCl (400 MHz, c = 7.0 × 10 mol/L). Only the aromatic region is
3
shown.
sample, the peaks significantly sharpened and underwent large
changes in chemical shift. Notably, the peak observed at ∼6.2
ppm shifted upfield as the sample was cooled, to a value closer
to that of the monomer. Also, J coupling of this peak and others
became evident as the sample was cooled. Our interpretation of
these data is that the macrocycles are aggregated at room
temperature (dimers, trimers, maybe larger) but dissociate
upon cooling. This is characteristic of entropy-driven
aggregation, as was previously observed in the cases of the
29
Pt rings and Zn /Cd cluster complexes. Thus, aggregation
4
7
7
of the macrocycles results in desolvation and, consequently, an
increase in the entropy. Lowering the temperature therefore
favors disaggregation, resulting in sharper peaks.
1
Figure 6. H NMR spectra of macrocycle 1a in CDCl at different
3
To help understand the behavior of Pt macrocycles in
3
concentrations at room temperature (400 MHz). Only the aromatic
region is shown. Blue squares indicate the original broad signals at high
concentrations, and red circles indicate the new, relatively sharp signals
at low concentrations.
solution and to investigate these unusual NMR results, model
compound 1c′ was synthesized. Scheme S2 shows the synthetic
route to model compound 1c′. Compound 1c′ was designed to
have chemical shifts similar to those of macrocycle 1a. To
prevent cyclization, the pyridyl group was replaced by a phenyl
ring.
the macrocycle only shows one set of major peaks that
correspond reasonably well with those expected for macrocycle
a. After being diluted by 16 times (to 4.5 × 10 mol/L), a
−4
1
new set of sharper peaks are observed, with the original peaks
remaining. With further dilution, this new set of peaks grows at
the expense of the original set of broad peaks. At low
−4
concentration (1.1 × 10 mol/L), the original broad signals
disappeared and only the new, relatively sharp peaks were
present. This dilution experiment not only confirms the
aggregation of macrocycle 1a in CDCl at room temperature
3
but also suggests that the aggregation process is slow compared
to the NMR time scale because both aggregates and monomer
could be simultaneously resolved.
We attempted to investigate the aggregation behavior of
macrocycle 1a by dynamic light scattering and boiling-point-
elevation measurements. However, these measurements gave
unreliable results owing to the limited concentration ranges and
low sensitivity. UV−vis spectroscopy of the macrocycle showed
no deviation from Beer’s law at concentrations below 10−4
mol/L (Figure S30), indicating that the macrocycle does not
aggregate at these low concentrations. Therefore, to further
When model compound 1c′ was dissolved in CDCl , it
3
1
showed a set of sharp peaks in the H NMR spectrum. Dilution
experiments with 1c′ were conducted and are shown in Figure
S25. Different from 1a, which demonstrated a conversion
between two sets of peaks during dilution, model compound
1
1c′ only showed one set of peaks in the H NMR spectrum at
different concentrations. Also, chemical shift changes were
minimal, indicating no self-association in this concentration
E
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range. Thus, macrocycle 1a aggregates when concentrated, but
compound 1c′ does not.
In order to determine the size of the aggregates in solution at
different temperatures, diffusion ordered spectroscopy (DOSY)
was conducted at 298 and 250 K. Because the experiment
depends on the viscosity of the solution (see the Supporting
Information for details) and the viscosity depends on the
3
0
temperature, we incorporated compound 1c′ as a reference.
A
1
variable-temperature H NMR study of compound 1c′ between
2
98 and 250 K showed no significant changes (Figure S26).
This suggests that the size of compound 1c′ does not change
much over this temperature range.
DOSY showed that the relative size of macrocycle 1a (vs 1c′)
decreases as the temperature is lowered. Considering that the
size of 1c′ does not change much with the temperature and
when any interactions between 1a and the reference compound
are neglected, these data suggest that the size of the aggregates
of macrocycle 1a decrease as the temperature is lowered,
consistent with our interpretation of the variable-temperature
Figure 9. 2D NOESY NMR spectrum of macrocycle 1a in CDCl
40 K (400 MHz, mixing time 0.1 s). Only the aromatic region is
shown.
at
3
2
1
H NMR data for 1a.
A nuclear Overhauser effect NMR spectroscopy (NOESY)
Table 1. Relative Integral and Estimated Distance Data from
a NOESY NMR Study of Macrocycle 1a at 240 K
study of macrocycle 1a in CDCl was also conducted to probe
3
its aggregation state. To study the distances between certain
protons in macrocycle 1a, the H NMR peaks need to be
relative
integral
relative
distance
estimated closest
expected closest
a
1
H1 H2
b
distance /Å
distance /Å
assigned to individual protons. Because several peaks of 1a
overlapped at room temperature, which caused difficulties in
peak assignment, we studied the macrocycle’s behavior at lower
b
a
h
h
a
a
a
c
1.00
1.02
1.67
2.40
0.12
0.11
0.21
1.00
1.00
0.92
0.86
1.42
1.44
1.30
2.51
2.51
2.30
2.17
3.57
3.63
3.26
2.51
2.51
2.24
2.05
5.10
7.20
8.58
b
g
k
g
h
k
1
13
temperatures. With the aid of HSQC [ H and C] and HMBC
1
13
[
H and C], peaks at 240 K were assigned (Figure 8). The 2D
NOESY NMR spectrum of 1a at 240 K is shown in Figure 9.
a
The expected distance was measured from one single macrocycle
b
with optimized geometry (Figure 3b). The estimated distance was
calculated from the relative distance assuming rbc = 2.51 Å.
the expected intramolecular value (8.58 Å). Similar unusually
strong dipolar interactions such as a , a , and a should not
bg bh
bk
be observed within a single macrocycle.
Considering that macrocycle 1a has a very rigid framework
and is impossible to fold, these surprisingly strong coupling
signals likely arise from the stacking of macrocycle 1a. When
stacked, pairs of protons are in closer proximity, leading to
stronger dipolar coupling (Figure 10). Protons a and h from the
same macrocycle (labeled with the same color) are too far apart
to offer NOESY signals. The observed nuclear Overhauser
effects are due to dipolar interaction between protons from
different macrocycles (interaction between green and light-blue
protons).
1
Figure 8. Aromatic region of the H NMR spectrum of macrocycle 1a
−3
in CDCl with peak assignments (400 MHz, c = 7.0 × 10 mol/L, T =
3
2
40 K).
By using the previously calculated conformation of the
This stacking behavior also explains the coupling between
protons a/b and aromatic protons from substituents R, which
are far apart in a single macrocycle. In addition, because the
distances r , r , and r show similarly high values, it is likely
macrocycle (Figure 3b), the expected distances between some
protons could be calculated (Table 1). Although the distance
between two types of protons have three different values in one
macrocycle due to the 3-fold symmetry, only the closest value is
listed in the table.
With protons b and c as the reference protons, the distance
between the many protons could be estimated from their
integral intensities (see Table 1 and the Supporting
Information for details). For example, the estimated distance
between protons g and h is about 2.36 Å, which is consistent
with the expected value (2.24 Å). Notably, the distance
between protons a and h is about 3.34 Å, which is very close to
the distance between protons a and g (3.35 Å). However, the
distance between protons a and k (3.21 Å) is much smaller than
ag ah
ak
that one macrocycle stacks on top of another macrocycle with
proton a located close to protons g, h, and k from another
macrocycle. Moreover, the similar values of the distances rag, rah,
and rak are not consistent with a fixed conformation of
aggregates, which implies that the relative twisting angle
between two stacked adjacent macrocycles varies within a
31
stack. Notably, the unexpectedly short distances such as rag
and r are between 3.3 and 3.8 Å, consistent with typical π−π-
ah
32
stacking distances.
For the purpose of comparing the aggregation behavior at
room temperature with the behavior at low temperature,
F
DOI: 10.1021/acs.inorgchem.7b00475
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
analyzed by a combination of computation, UV−vis experi-
ments, variable-temperature NMR, and DOSY and NOESY
NMR. These results challenge our design principles for
platinum-containing macrocycles, and they may be useful for
constructing molecule-based nanotubes and discotic liquid
crystals.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00475.
Syntheses, characterization, and calculations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: mmaclach@chem.ubc.ca.
■
Figure 10. Top view of two stacked macrocycles 1a, where the
macrocycles have opposite orientations. Only protons a and h from
different macrocycles are shown in green and light-blue color; other
protons and R groups are removed for clarity.
ORCID
Mark J. MacLachlan: 0000-0002-3546-7132
Author Contributions
The manuscript was written through contributions of all
authors.
NOESY NMR of macrocycle 1a at room temperature was also
performed. Due to overlapping peaks at room temperature,
1
however, not all signals in its H NMR spectrum could be
Notes
assigned; peak overlap also caused trouble in the assignment of
the coupling signals between 7.2 and 7.5 ppm in the 2D
NOESY NMR spectrum. Hence, less data could be used for
distance estimation in this case. Figure S29 shows the NOESY
NMR spectrum of macrocycle 1a at room temperature. Despite
coupling of the signals between 7.2 and 7.5 ppm, calculation
was conducted to estimate the distances between some protons
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Natural Sciences and Engineering Research
Council of Canada for funding (Discovery Grant).
■
(
Table S3). The observation of cross-peaks for protons that are
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