Cyclotris(paraquat-p-phenylenes)

Article abstract of DOI:10.1002/anie.201907329

Reported here is the synthesis, solid-state characterization, and redox properties of new triangular, threefold symmetric, viologen-containing macrocycles. Cyclotris(paraquat-p-phenylene) (CTPQT⁶⁺) and cyclotris(paraquat-p-1,4-dimethoxyphenylen

Full text of DOI:10.1002/anie.201907329

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Accepted Article
Title: Cyclotris(paraquat-p-phenylenes)
Authors: Ommid Anamimoghadam, James A. Cooper, Minh T. Nguyen,
Qing-Hui Guo, Lorenzo Mosca, Indranil Roy, Junling Sun,
Charlotte L. Stern, Louis Redfern, Omar K. Farha, and Fraser
Stoddart
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Cyclotris(paraquat-p-phenylenes)
Ommid Anamimoghadam,* James A. Cooper, Minh T. Nguyen, Qing-Hui Guo, Lorenzo Mosca,
Indranil Roy, Junling Sun, Charlotte L. Stern, Louis Redfern, Omar K. Farha, J. Fraser
Stoddart*
Abstract: We report on the synthesis, solid-state characterization
and redox properties of a new class of triangular, three-fold
symmetric viologen-containing macrocycles. The preparation of
radicals, it is important to maintain the structural integrity[15] of
the bipyridinium units so as to allow the convenient generation
and isolation of the respective radical cations by chemical
means.
6+
cyclotris(paraquat-p-phenylene) (CTPQT ) and cyclotris(paraquat-p-
6
+
1
,4-dimethoxyphenylene) (MCTPQT ), can be executed by
following three-step protocol. Their X-ray single-crystal
super)structures reveal intricate three-dimensional packing. The
Herein, we describe the synthesis of (i) a new ‘triangular’
a
cyclophane,
cyclotris(paraquat-p-phenylene)
hexakis(hexa-
(
fluorophosphate) (CTPQT•6PF
6
), which incorporates three
6+
crystallization of MCTPQT results in nanometer-sized channels in
paraquat units, separated from each other by para-phenylene
groups, and (ii) its methoxy-substituted analog, cyclotris-
(paraquat-p-1,4-dimethoxyphenylene) hexakis(hexafluorophos-
6+
contrast with its parent counterpart CTPQT which crystallizes as a
couple of polymorphs in the form of intercalated assemblies. In the
3(•+)
solid-state, MCTPQT —the three-electron reduced form of
6
phate) (MCTPQT•6PF ), in which the phenylene groups are
6+
MCTPQT —exhibits stacks between the 1,4-dimethoxyphenylene
and bipyridinium radical cations, providing new opportunities for the
manipulation and control of the recognition motif associated with
viologen radical cations. These redox-active cyclophanes
demonstrate that geometry-matching and weak intermolecular
interactions are of paramount importance in dictating the formation
of their intricate solid-state superstructures.
replaced by electron-rich 1,4-dimethoxyphenylene (DMP) units,
resulting in a redox-active pillararene-like macrocycle. We have
investigated their optical properties, electrochemistry and solid-
state properties, in both their hexacationic and triply reduced
forms. We have found that the incorporation of DMP units has
important
MCTPQT•6PF
trication (MCTPQT•3PF
repercussions
and its corresponding triply reduced trisradical
) in the solid state that enabled us to
for
the
self-organization
of
6
6
obtain single crystals of sufficient quality for X-ray crystal
structure analysis.
The synthesis (Scheme 1) of CTPQT•6PF and MCTPQT•6PF
6 6
was carried out by heating their corresponding bis(bromomethyl-
aryl)dipyridinium and (aryl-bismethylene)dipyridinium salts under
Interest in research on macrocycles in the context of
[1]
supramolecular chemistry has spawned numerous examples
derived from common building blocks, including cyclodextrins,[
crown ethers,[ cucurbiturils,[4] calixarenes,[5] pillararenes,[6]
asararenes[ and several other cyclophanes,[8] along with
2]
3]
7]
reflux in the presence of TBAI in MeCN, affording CTPQT•6PF
and MCTPQT•6PF
6
[
9]
structures incorporating redox-active bipyridinium units. Since
6
in yields of 23% and 19%, respectively. See
the original report[10] of the synthesis of cyclobis(paraquat-p-
Scheme S1 for details.
4
+
phenylene) (CBPQT ), the range of bipyridinium-containing
cyclophanes has expanded enormously to produce an alluring
series of
superstructures,[
macrocycles that
can form well-defined
11]
as well as mechanically interlocked
molecules,[12] such as rotaxanes and catenanes. Notably, the
electrochemical properties of these macrocycles allow access to
their corresponding open-shell species following chemical or
electrochemical reduction. These additional redox states have
broadened their scope considerably, on account of the radical-
radical pimerization (π-dimerization) that occurs[13] between the
viologen radical cations. Their redox chemistry enriches their
properties with respect to both the self-organization and host–
[
14]
guest chemistry of these macrocyclic entities.
In order to
6 6
Scheme 1. Synthesis of CTPQT•6PF and MCTPQT•6PF .
4
+
explore extended derivatives of CBPQT , whilst retaining
access to the rich and varied chemistry of viologen-based
−
salt[10] of
CTPQT•6PF
CBPQT . In contrast, MCTPQT•6PF
6
is a white solid analogous to the PF
6
4
+
6
is yellow in the solid state
and in solution, exhibiting a weak absorption (Figure S7)
centered on 366 nm (log ε = 3.17) in MeCN, indicative of an
intramolecular charge-transfer interaction presumably between
[
*]
O. Anamimoghadam, J. A. Cooper, M. T. Nguyen, Q.-H. Guo, L.
Mosca, I. Roy, J. Sun, C. L. Stern, L. Redfern, O. K. Farha, J. F.
Stoddart
Department of Chemistry, Northwestern University, 2145 Sheridan
Road, Evanston, IL 60208 (USA)
[16]
the methoxy groups and the adjacent bipyridinium units.
Colorless crystals of CTPQT•6PF
6
were grown by vapor
in MeCN,
E-mail: stoddart@northwestern.edu
i
diffusion of Pr
2
O into a solution of CTPQT•6PF
6
J. F. Stoddart
resulting[17] in two polymorphs. The molecular dimensions of the
triangular cyclophane (Figure 1a) are determined by the lengths
of its constituent parts―bipyridinium (~10.0 Å) and xylylene
Institute for Molecular Design and Synthesis, Tianjin University, 92
Weijin Road, Tianjin, 300072 (China)
(
~5.8 Å)― and the cyclophane exhibits a cavity size of ~10 Å.
J.F. Stoddart
School of Chemistry, University of New South Wales, Sydney, NSW
The solid-state structures, as determined by X-ray
crystallography, of the two polymorphs demonstrate a lack of
2052 (Australia)
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COMMUNICATION
i
shape-persistence in CTPQT•6PF
6
. In both polymorphs, torsions
2 2
diffusion of Pr O into a solution of the cyclophane in Me CO.
of the three bipyridinium units of the cyclophane about their 4,4′-
C−C bonds range widely from 2° to 62°. In one of the two
polymorphs, the unusually low torsion angle of 2°, observed
After examining different crystalline samples, we found that
does not form polymorphs[
19]
under these
MCTPQT•6PF
6
conditions. The macrocycle shows (Figure 2a) similar side
lengths (~10.0 and ~5.8 Å) and cavity size (~10 Å) to its parent
counterpart CTPQT•6PF .
6
(
Figure 1c) in one of the three bipyridinium units, the one in
−
which the bipyridinium unit participates in interactions with PF
6
counterions, is presumably a result of packing constraints. The
angle between adjacent bipyridinium and xylylene units, linked
by bridging methylene groups, was observed to be 107−118°
and deviates moderately from the ideal angle for planar
truncated triangles (120°) and the characteristic angle (109.5°)
between hybrid orbitals with sp3 configuration. As
consequence, X-ray crystal structure analysis revealed four
Figure 1c) and two (Figure 1d) crystallographically equivalent
a
(
6+
CTPQT molecules in the unit cell of the two respective
polymorphs. Their extended superstructures (Figure 1b and
Figure S11 and S12 in the SI) show a clear offset assembly of
6+
CTPQT hexacationic cyclophanes, which are intercalated by
−
PF
6
counterions and solvent molecules.
Figure 2. X-Ray crystal (super)structures of MCTPQT•6PF
6
.
a) Single
MCTPQT6 molecule depicting side lengths, cavity size and angles. b) Angled
+
top view of MCTPQT6+ molecule illustrating C
3
-symmetry and close distances
between the methoxyl oxygen atoms and the nitrogen atom and α-carbons of
the bipyridinium units. c) Side-on view of tubular assembly of five MCTPQT6+
molecules illustrating van der Waals separations between methoxyl groups. d)
Top view of four MCTPQT6+ molecules illustrating noncovalent bonding
interactions. Counterions, hydrogen atoms, and Me
for the sake of clarity.
2
CO molecules are omitted
The torsion angles of all the bipyridinium units about their 4,4′-
C−C bond range from 27° to 43° and the angles between the
bipyridinium and the xylylene units that are linked by the
methylene groups were found to range between 108 and 112°. It
is worth noting that the oxygen atoms of the methoxy groups are
positioned (Figure 2b) in close proximity to the nitrogens
(2.9−3.3 Å) and α-carbons (2.8−3.3 Å) of the bipyridinium units,
which may contribute to charge-transfer interactions indicated by
the UV/Vis absorption spectrum (Figure S8 in the SI) of
Figure 1. X-Ray crystal (super)structure of CTPQT•6PF
6
obtained from MeCN
by vapor diffusion of iPr
O. a) Single CTPQT6+ hexacation depicting side
2
lengths and cavity size. b) Space-filling model of the CTPQT•6PF
6
superstructure of one of the two polymorphs showing offset-packing. c) Four
separate crystallographically equivalent CTPQT6+ molecules (colored in blue,
medium blue, cornflower blue and seafoam green) in the crystal lattice of one
polymorph. Top: Side-on angled view. Bottom-left: Top view. Bottom-right:
Side-on view. d) Two different crystallographically equivalent CTPQT6+
6
MCTPQT•6PF .
6
+
molecules (colored in blue and seafoam green) in the crystal lattice of the
Three crystallographically equivalent MCTPQT hexacationic
−
other polymorph. In a), c) and d) the PF
6
counterions and MeCN molecules
macrocycles were observed (Figure S13 in the SI) by X-ray
crystallography, all of which exhibited local C -symmetry and
3
are omitted for the sake of clarity.
revealed (Figure 2c) van der Waals separations (3.4‒3.6 Å)
between the methoxyl groups on the DMP pillars of adjacent
triangles, leading to the formation of channel-like architectures.
These channels are arranged in a tessellated manner, with
interactions (Figure 2d) between each tubular assembly
characterized by (i) offset π–π interactions (3.3–3.5 Å) between
DMP units and (ii) d(CH•••O) interactions (3.3–3.4 Å) between
the bipyridinium units and methoxyl groups of the DMP units.
We have taken note of investigations[18] on
pyromellitic diimide (PMDI) macrocycle, which is reminiscent
structurally of CTPQT•6PF —containing DMP and PMDI units
instead of phenylene and bipyridinium units, respectively—but is
more rigid and lacks counterions. In contrast to CTPQT•6PF
a triangular
6
6
,
the self-organization of these PMDI-based macrocycles to form
nanotubular superstructures is apparent when crystallized in the
6
The modification of CTPQT•6PF with DMP pillars creates
3
presence of toluene or [2 ]paracyclophane.
spacings between the rims of the cyclophanes in which all the
We have investigated the crystal growth of a derivative of the
CTPQT6+ hexacation which incorporates three DMP units into its
triangular scaffold, yielding MCTPQT•6PF , with the result that
6
an intricate tubular organization in the solid state is achieved.
Yellow needle-like crystals of sufficient quality for X-ray crystal
−
PF
6
counterions are situated giving rise for a porous
architecture (Figure S13 in the SI) with an accessible volume of
3.6 %. The pores are filled with solvent molecules coming from
3
the crystallization procedures. After removing the solvent under
vacuum overnight, powder X-ray diffraction analysis was carried
(
super)structure analysis were obtained from slow vapor
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out to assess whether the structural integrity of the pores is still
maintained when devoid of solvent molecules. While the PXRD
indicated that the material was still crystalline, it was not
consistent with the diffraction pattern for the channel-like
superstructure (Figure S15 in the SI) simulated from the single
X-ray crystal structure analysis. This observation suggests that
(Figure 3c, inset) with a bipyridinium unit that is stacked with the
DMP unit of another neighboring cyclophane, resulting in a
quadruple π-stack in which an unprecedented π-stack of a
viologen radical pimer, sandwiched between two DMP units. The
pimer is characterized by a marginal orbital overlap between two
α-carbon and two β-carbon atoms. The remaining bypyridinium
units of the respective cyclophane show radical–radical
interactions (3.1 Å) with near-planar viologen units with a high
interaxial angle of 76° and d(HC•••π) interactions (3.4 Å) with
DMP units. Furthermore, one DMP unit is paired (3.4 Å) with
another DMP unit of an adjacent cyclophane.
2
the Me CO molecules promote the stabilization of this highly
ordered superstructure. The dried microcrystalline sample could
not be investigated further by single crystal X-ray structure
analysis as a result of the poor diffraction of the sample. Indeed,
N
2
isotherm measurements (see SI Figure S16) of the
desolvated material confirmed a lack of porosity. We suggest
that MCTPQT•6PF undergoes polymorphism upon removal of
Me CO molecules.
Cyclic voltammetry of CTPQT•6PF
Figure S10 in SI) two reversible three-electron transfer
6
2
6
6
and MCTPQT•6PF reveal
(
6
+
processes. In the case of CTPQT , the redox waves at −0.74
and −1.17 V (vs. Fc /Fc couple) correspond to the reversible
+
formation of the (i) trisradical trication and (ii) the fully reduced
neutral state, respectively. MCTPQT6+ shows similar redox
behavior, with small cathodic shifts of the redox waves to −0.79
and −1.22 V, which can be ascribed to the increased electronic
density provided by the electron-rich DMP units.
Taking into account the potentials of the redox waves, we
employed Zn dust to generate their corresponding trisradical
tricationic species in MeCN. After removing the excess of Zn
6
Figure 3. X-Ray crystal (super)structures of trisradical trication MCTPQT•3PF .
a) Single MCTPQT3(●+) molecule depicting side lengths, cavity size and angles.
b) Angled top view of MCTPQT3(●+) molecule illustrating close distances
dust by filtration, the filtrate containing CTPQT•3PF
6
was
i
subjected to vapor diffusion with Pr
2
O, resulting in a dark purple
between the methoxyl oxygen atoms and the nitrogen atom and α-carbons of
the bipyridinium units. c) Top view of eight MCTPQT3(●+) molecules illustrating
predominant noncovalent bonding interactions and torsion angles between
pairs of bipyridinium units. Counterions, hydrogen atoms, and MeCN
molecules are omitted for the sake of clarity.
polycrystalline or film-like material of insufficient quality to
conduct a single crystal structure analysis. In contrast, purple-
colored single crystals of MCTPQT•3PF
the same conditions and were investigated[20] by X-ray
crystallography. In the solid state, MCTPQT•3PF (Figure 3a)
has similar side-lengths and cavity size to those exhibited by
CTPQT•6PF and MCTPQT•6PF . The torsion angles present in
6
could be grown under
6
6
6
In comparison with previously reported[14] bipyridinium-
containing cyclophanes in their radical states, the dramatic
differences in the recognition motif associated with the
bipyridinium radical cations presented by this system—as
determined by X-ray crystallography—demonstrate the
tunability[ of these radical–radical interactions, that can prove
effective when incorporated in macrocyclic scaffolds involving
DMP units. These findings may have implications for, and may
open up new opportunities in, the design of bipyridinium-based
materials,[ particularly from the point of view of their conductive
and magnetic properties.
the bipyridinium radical cations, however, are dramatically
decreased to below 5° when compared with those present in
bipyridinium dications in the fully cationic state: This observation
[
13]
is not uncommon
for open-shell viologens. The distances
13]
measured between the methoxyl oxygen atoms and the
pyridinium nitrogen and α-carbon atoms (Figure 3b) suggests
the presence of ion-dipole intramolecular interactions of 2.9−3.4
Å and 2.8−3.5 Å, respectively, between these two groups. The
21]
spatial orientation of the DMP pillars does not give rise to C
symmetry as was observed (Figure 2b) in the solid-state
structure of its fully oxidized form MCTPQT•6PF
The packing of the MCTPQT•3PF trisradical trications in the
3
6
.
In summary, we have synthesized a new class of paraquat-
containing cyclophanes―incorporating three bipyridinium units
separated by three xylylene groups― that adopt a triangular
arrangement. In contrast to the polymorphic and less-intricate
self-organization of the parent cyclotris(paraquat-p-phenylene)
6
extended solid-state superstructure (Figure 3c) was found to be
predominantly a result of (i) radical–radical interactions, (ii)
DMP-viologen interactions and (iii) offset π–π stacks between
the DMP units. The accessible volume of this superstructure is
decreased by more than half to 14.1% when compared with that
of its fully oxidized form.
The cyclophanes are paired (Figure 3c) by the inclusion of one
of the DMP units of one cyclophane inside the cavity of an
adjacent cyclophane, resulting in π–π interactions (3.4 Å)
between DMP and viologen radical cation units. These radical
cationic units are involved in distinctly offset radical–radical
interactions (3.3 Å), exhibiting a negligible interaxial angle
6
+
(
CTPQT ), the assembly of a porous superstructure in the case
6
+
of cyclotris(paraquat-p-1,4-dimethoxyphenylene) (MCTPQT )
was achieved by replacing the phenylene units with electron-rich
1,4-dimethoxyphenylene (DMP) units, highlighting the effective
tunability of their solid-state superstructures. The triangular
shape and orientation of the DMP pillars represent an increase
in structural complexity when compared with cyclic paraquat
dimeric analogs reported previously.[11] The intermolecular
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COMMUNICATION
interactions involved in the solid-state superstructure are not
characterized by interactions between π-electron rich and π-
electron poor units, but rather established by offset π–π
interactions between DMP units, d(CH•••O) interactions and van
der Waals interactions involving the methoxyl groups of the
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and obtained as high-quality single crystals for X-ray crystal
super)structural analysis. Notably, a quadruple stack composed
6
, has been synthesized
(
of a radical cationic pimer, sandwiched between two DMP units,
demonstrates the disruptive impact of the electron-rich pillars on
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This research is part of the Joint Center of Excellence in
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Energy Sciences Program (grant DE-FG02-08ER155967).
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Keywords: macrocycles • nanoporous material • radical-radical
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Angewandte Chemie International Edition
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COMMUNICATION
[17] (i) Polymorph
126 123 72 21 12
1 Crystal Parameters. C H F N P , Monoclinic,
space group P2
1
/n (no. 14), a = 31.3397(9), b = 13.9653(5), c =
3
3
5.0975(10) Å, β = 91.751(2)°, V = 15353.9(8) Å , Z = 4, T = 100 K,
μ(CuKα) = 2.54 mm–1, Dcalc = 1.59 g/mm , 83372 reflections measured
3.84 ≤ 2Θ ≤ 130.31), 25551 unique (Rint = 0.05, Rsigma = 0.05) which
were used in all calculations. The final R value was 0.06 (I > 2σ(I)) and
wR was 0.18 (all data). CCDC 1914248.
3
(
1
2
Refinement Details. Rigid bond restraints were imposed on the
displacement parameters as well as restraints on similar amplitudes
separated by less than 1.7 Å on the disordered fluorine atoms. Distance
−
restraints were imposed on the disordered PF
ii) Polymorph Crystal Parameters.
space group P2
6
counterions.
(
2
57 52.5 36 7.5 6
C H F N P , Monoclinic,
1
(no. 4), a = 18.0524(16), b = 29.672(2), c = 27.797(3)
3
Å, β = 101.125(5)°, V = 14609(2) Å , Z = 8, T = 100.0 K, μ(CuKα) =
.61 mm–1, Dcalc = 1.56 g/mm , 54005 reflections measured (3.24 ≤ 2Θ
3
2
≤
101.12), 29733 unique (Rint = 0.21, Rsigma = 0.22) which were used in
all calculations. The final R
1 2
value was 0.12 (I > 2σ(I)) and wR was
0.36 (all data). CCDC 1914249.
Refinement Details. The enhanced rigid-bond restraint (SHELX
keyword RIGU) was applied globally. The solvent masking procedure,
as implemented in Olex2 was used to remove the electronic
contribution of solvent molecules from the refinement. Since the exact
solvent content is not known, only the atoms used in the refinement
model are reported in the formula here. Total solvent accessible volume
3
/
cell = 1211.4 Å [8.3%]. Total electron count / cell = 297.8.
[18] T. Iwanaga, R. Nakamoto, M. Yasuitake, H. Takemura, K. Sato, T.
Shinmyozu, Angew. Chem. Int. Ed. 2006, 45, 3643−3647.
[
19] Crystal Parameters. C60
H
60 36
F N
6 6
O P
6
, Monoclinic, space group P2
1
/c
(
no. 14), a = 17.1935(6), b = 30.8524(10), c = 19.8570(6) Å, β =
08.675(2)°, V = 9978.8(6) Å , Z = 4, T = 100 K, μ(CuKα) = 1.98 mm–1,
3
1
3
Dcalc = 1.22 g/mm , 55688 reflections measured (5.73 ≤ 2Θ ≤ 120.43),
1
4759 unique (Rint = 0.12, Rsigma = 0.13) which were used in all
calculations. The final R value was 0.11 (I > 2σ(I)) and wR was 0.32
all data). CCDC 1914255.
1
2
(
Refinement Details. Distance restraints were imposed on the
−
disordered PF
6
counterion as well as the enhanced rigid-bond restraint
(SHELX keyword RIGU). The solvent masking procedure, as
implemented in Olex2 was used to remove the electronic contribution of
solvent molecules from the refinement. As the exact solvent content is
not known, only the atoms used in the refinement model are reported in
the formula here. Total solvent accessible volume / cell = 3354.5 Å3
[
33.6%]. Total electron count / cell = 719.0.
20] Crystal Parameters. C60 , Monoclinic, space group P2
no. 14), a = 18.4214(12), b = 10.9307(8), c = 32.7544(16) Å, β =
[
H
60 18
F N
6 6
O P
3
1
/n
(
9
3.482(4)°, V = 6583.2(7) Å , Z = 4, T = 100.0 K, μ(CuKα) = 1.76 mm–1,
3
3
Dcalc = 1.41 g/mm , 22423 reflections measured (5.37 ≤ 2Θ ≤ 124.77),
9924 unique (Rint
=
0.08,
R
sigma
=
0.10) which were used in all
calculations. The final R
1
was 0.09 (I > 2σ(I)) and wR was 0.28 (all
2
data). CCDC 1914256.
Refinement Details. The solvent masking procedure as implemented in
Olex2 was used to remove the electronic contribution of solvent
molecules from the refinement. As the exact solvent content is not
known, only the atoms used in the refinement model are reported in the
formula here. Total solvent accessible volume / cell = 927.8 Å3 [14.1%].
Total electron count / cell = 171.6.
[21] C. Gourlaouen, S. Vela, S. Choua, M. Berville, J. A. Wytko, J. Weiss, V.
Robert, Phys. Chem. Chem. Phys. 2018, 20, 27878−27884.
[
22] W. M. Jacobs, D. Frenkel, J. Am. Chem. Soc. 2016, 138, 2457−2467.
23] Y. Wu, M. D. Krzyaniak, J. F. Stoddart, M. R. Wasielewski, J. Am.
Chem. Soc. 2017, 139, 2948−2951.
[
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
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COMMUNICATION
COMMUNICATION
Ommid Anamimoghadam,* James A.
Cooper, Minh T. Nguyen, Qing-Hui Guo,
Lorenzo Mosca, Indranil Roy, Junling
Sun, Charlotte L. Stern, Louis Redfern,
Omar K. Farha, J. Fraser Stoddart*
Page No. – Page No.
Text for Table of Contents
Cyclotris(paraquat-p-phenylenes)
[*]
O. Anamimoghadam, J. A. Cooper, M. T.
Nguyen, Q.-H. Guo, L. Mosca, I. Roy, J.
Sun, C. L. Stern, L. Redfern, O. K. Farha,
J. F. Stoddart
Department of Chemistry, Northwestern
University, 2145 Sheridan Road, Evanston,
IL 60208 (USA)
Email: stoddart@northwestern.edu
J. F. Stoddart
Institute of Molecular Design and Synthesis,
Tianjin University, 92 Weijin Road, Tianjin,
300072 (China)
J. F. Stoddart
School of Chemistry, University of New
South Wales, Sidney, NSW 2052 (Australia)
This article is protected by copyright. All rights reserved.