Anion Exchange in Cationic Frameworks: Structures of Channel-Forming Triarylpyrylium Tetrafluoroborate Salts

Article abstract of DOI:10.1021/acs.cgd.6b00005

The crystal structure of a co-crystal of 2,4,6-tris(4-methylphenyl)pyridine and 2,4,6-tris(4-methylphenyl)pyrylium revealed a pseudohexagonal motif assembled from π-stacked helices that enclose 1-D channels containing disordered tetrafluoroborate counteri

Full text of DOI:10.1021/acs.cgd.6b00005

Article
pubs.acs.org/crystal
Anion Exchange in Cationic Frameworks: Structures of Channel-
Forming Triarylpyrylium Tetrafluoroborate Salts
,†
‡,§
†
Ren A. Wiscons,* Matthias Zeller, and Jesse L. C. Rowsell
†Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074, United States
‡Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States
§Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States
*
S Supporting Information
ABSTRACT: The crystal structure of a co-crystal of 2,4,6-tris(4-methylphenyl)pyridine
and 2,4,6-tris(4-methylphenyl)pyrylium revealed a pseudohexagonal motif assembled from
π-stacked helices that enclose 1-D channels containing disordered tetrafluoroborate
counterions and solvent molecules. Though convoluted by disorder, aperiodicity, and
complex twinning patterns, further investigation was motivated by the unique packing
arrangement of this material, which enables solid-state anion diffusion, while resisting
dissolution in water. Such properties are ideal for the development of anion exchange
materials that can be employed for the remediation of agricultural and nuclear waste. To
develop a more complete understanding of the intermolecular interactions that give rise to
this helical motif, we have compiled and analyzed the local interactions and extended
structural motifs of 19 pore- and channel-forming 2,4,6-triarylpyrylium crystal structures.
−
Three conserved local interactions were identified, including C−H···X interactions, the
rhombic binding pocket, and four-ring antiparallel dimerization. Development of this
library of interaction motifs rationalizes the packing arrangement of the co-crystal and
contributes to the informed design of anion exchange materials.
INTRODUCTION
materials also have drastically lowered performances after
6
■
,9−16
moving from organic to aqueous exchange environments.
Anionic species are central to many environmental and health-
related crises such as eutrophication in wetland environ-
3,4
Increased attention has been dedicated to the design of
metal−organic frameworks (MOFs) over the past decade with
the goal of developing porous, robust, and tunable materials
capable of selectively adsorbing gases for hydrogen storage,
carbon-dioxide capture, liquid-phase separations, catalysis, and
1
,2
ments, unintended dispersal of vitrified radioactive waste,
and introduction of organic acids and radioactive species (e.g.,
9
9m
−
131
3−5
TcO4 and I) into biogeochemical cycles.
Though
these concerns are pressing, selective noncovalent binding of
anionic guests in cationic hosts is a relatively nascent focus of
supramolecular chemistry. Challenges are, for example,
presented by the size, solubility, and nondirectional electron
density distribution of halides and spherical oxyanions (sulfates,
17−22
ion exchange.
Anion exchange has long been recognized as
a potential application of MOFs in which the organic ligands
that define the distance between metal vertices are neutral, and
anion entrapment arises as a result of balancing residual positive
5
5
,6
charge introduced by the metal coordination centers. Such
phosphates, perchlorates, etc.).
materials allow for exchange under less extreme conditions than
The development of solid-state anion exchange materials
remains a challenge, as the complexities intrinsic to
heterogeneous-phase anion exchange are far from being well
22
other materials may demand. MOF geometries are often
restricted by directional bonding of linkers to the coordination
23
1
−9
geometries of metal centers. Molecular crystals built from
noncovalently interacting discrete molecular units are capable
of accessing softer frameworks that can flex to accommodate
guests of a nonideal size and defy the traditional geometric
understood.
One approach that has been investigated quite
extensively is the use of molecular recognition for the selective
entrapment of anions based on shape and electrostatic surface
potential. Macrocyclic cages such as polyamide katapinands and
cryptands encapsulate anionic guests in solution, thus
preventing anions from interacting with other dissolved
24
skeletons achievable by MOFs.
Pyrylium, a cationic heterocycle (Figure 1), introduces a
unique opportunity to construct positively charged organic
frameworks. As one of the most electronegative nuclei yet
introduced into an aromatic system, the pyrylium’s oxygen
1
0−14
species.
Most of these cages suffer from pH sensitivity
and as a result are often not suitable for anion extraction from a
broad range of aqueous environments. More recently, research
groups have begun addressing these limitations by tuning weak
−
C−H···X interactions for anion binding and by designing
Received: January 2, 2016
Revised: February 20, 2016
macrocycles that aggregate upon anion capture, allowing
14,16
separation by filtration.
Unfortunately, many of these
©
XXXX American Chemical Society
A
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triarylpyridine compounds, due to increased cross-conjugative
demand.
Mean torsion angles between the central heterocycle and the
aryl substituents para and ortho to the aromatic oxygen/
nitrogen in triarylpyrylium cations (20.8° and 10.9°) and
triarylpyridine (26.1° and 21.0°) molecules were measured and
the average of all three torsion angles were calculated for
triarylbenzene molecules (35.3°). The decrease in mean torsion
angle between the triarylbenzene population and the
triarylpyridine and pyrylium populations suggests that
decreased hydrogen−hydrogen repulsion allows greater
planarity, while significant differences in the mean torsion
angles of the substituent arene para to the heteroatom
calculated between the triarylpyridine and pyrylium populations
suggests that triarylpyrylium cations gain thermodynamic
stability by delocalization of positive charge across multiple
rings by cross-conjugation.
Figure 1. 2,4,6-Triarylpyrylium heterocyclic core.
atom induces a dipole moment on the heterocycle that
polarizes the C−H bonds of carbon atoms C3 and C5 of
25
2
,4,6-triarylated pyrylium molecules, which promotes its
propensity toward anion binding in the solid state through
−
26
reinforced C−H···X interactions.
Of the 21 known 2,4,6-triarylpyrylium-containing crystal
structures (Cambridge Structural Database, CSD, surveyed in
July 2015), 15 exhibit solid-state arrangements with continuous
anion-containing channels or pores that are inherently
compatible with reversible anion exchange and ion capture.
In addition, three new crystal structures of solvated pyrylium
salts and the crystal structure of a uniquely helical pyrylium/
pyridine co-crystal are reported herein. These 19 triarylpyry-
lium-containing crystal structures (Figure 2) were analyzed to
develop a library of intermolecular motifs persistent across
pyrylium salts that display solid-state anion exchange potential.
For each of the aforementioned 19 pore- and channel-forming
triarylpyrylium crystal structures, we have modeled and
compared the molecular conformations against the analogous
triarylbenzenes and pyridines to understand the unique
planarity and exocyclic bond deformation measured from the
pyrylium cations.
The combination of hydrogen−hydrogen repulsion and
coplanarity of the substituent rings with the core pyrylium
heterocycle causes the mean exocyclic bond angles at position
+
C2 and C6 on the triarylpyrylium ring, ∠(C−C−O ), to
deviate from the idealized 120°, instead adopting angles
between 112.2 and 116.8°. This deformation of exocyclic
angles in triarylpyrylium salts was first observed in a 2,6-
diphenyl-4-(4-nitrophenyl)pyrylium perchlorate salt (8) and
was attributed to a combination of hydrogen−hydrogen
+
repulsion and increased polarization of C−O bonds. The
intermolecular bond distance between ortho hydrogen atoms
on substituent aryls and the aromatic oxygen was found to be
too long to be attractive, indicating that this deviation in angle
3
3
is not attributed to intramolecular hydrogen bonding.
40
Computational models (B3LYP//6-31G(d)) of triarylpyry-
lium molecules 1 and 4−15 suggest that deviation from ideal σ
C−C bond lengths between central and substituent rings,
The role of crystallization conditions and trapped solvent
molecules in the local and extended motifs of solvated pyrylium
frameworks was studied by characterizing the structural
differences between the three new 2,4,6-tris(4-methylphenyl)-
pyrylium tetrafluoroborate solvates crystallized from THF and
water, acetic acid, and ethanol and water. These materials were
found to decompose rapidly by hydrolysis in neutral and basic
aqueous environments, limiting their functionality to acidic
solutions. However, we have found that the solubility and
stability of pyrylium salts in aqueous environments may be
modified through co-crystallization with isosteric triarylpyridine
molecules, extending the lifetime of this material in anion
exchange settings and broadening the working pH range of
pyrylium-based exchange material.
+
d(C−C), and exocyclic bond angles, ∠(C−C−O ), are
positively correlated with the magnitude of the torsion angle
between the central aromatic ring and the aryl substituents.
Molecules 2 and 3 were excluded from analyses because
coplanarity is enabled by the smaller size of the thionyl and
methylfuranyl groups relative to the phenyl substituents. These
calculations support the prediction that π-character of the bond
connecting the central and substituent arenes (cross-
conjugation) increases with decreasing torsion angle and drives,
in combination with hydrogen−hydrogen repulsion, the
deformation of exocyclic bond angles. In lieu of these findings,
it is proposed that the tendency for triarylpyrylium cations to
+
adopt torsion angles, d(C−C), and ∠(C−C−O ) that are
RESULTS AND DISCUSSION
lower than those measured for the identical triarylpyridine
molecules is in part attributed to the electronic stability that can
be gained by the pyrylium cation from substituent arene
electron density donation.
■
Structural Organization in Triarylpyrylium Salts.
Coplanarity, though seen in triarylpyrylium molecules, is rarely
observed in 1,3,5- and 2,4,6-triaryl-substituted arenes due to the
barrier to planarization presented by steric clashing between
hydrogen atoms on the central aromatic ring and hydrogen
atoms on the substituent aryl rings. To investigate the origin of
this coplanarity, mean values were calculated from torsion
angles measured from populations of metal-free 1,3,5- and
Conserved Local Interactions in Solid-State Pyrylium
Salts. Pore- and channel-forming triarylpyrylium salts adopt
elaborate architectures that have a tendency to exhibit disorder,
twinning, and stacking faults in the solid state, often leading to
substantial non-Bragg behavior of their diffraction patterns and
limiting the resolution of the crystal structures that can be
obtained. The repeat sequence of the pyrylium framework and
of the anions and solvent molecules in the channels are not
necessarily commensurate with each other. More often than
not, the content of the channels is ill defined, with problems
ranging from simple disorder to completely diffuse electron
density within the channels. Despite these substantial
2
,4,6-triarylpyrylium, triarylpyridine, and triarylbenzene crystal
structures obtained from the CSD (137 total structures) to
determine whether the planarity observed from pyrylium
compounds could be strictly attributed to a decreased barrier
to rotation in heterocyclic systems. Alternatively, the low
resonance energy of the pyrylium moiety may encourage near-
planar molecular conformations, beyond what is observed in
B
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Figure 2. Crystal structures 7−39 of pore- and channel-forming triarylpyrylium salts sampled from the CSD (1−14) and reported in this article
2
(
15a−15d) that give rise to channelized solid-state materials. For structures 1−14, the codes given are the entry codes of the CSD, Cambridge
a
Structural Database. indicates the new triarylpyrylium crystal structures reported herein.
challenges, enough data can be obtained from the available
structures to elucidate some trends common to channel-
forming triarylpyrylium salts. The structures can be generally
understood as arising from a combination of three local
geometries that persist across the 15 triarylpyrylium crystal
structures sampled from the CSD and four triarylpyrylium
C
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structures reported here for the first time. Of these crystal
structures, 15 exhibit weak C−H···X interactions (Figure 3a),
29% of the crystal structures containing only one short d(C−
−
−
H···X ), 29% of the structures containing two short d(C−H···
−
−
X ), and 41% containing all three short d(C−H···X ).
−
Structure-specific d(C−H···X ) and radial overlap data are
presented in the Supporting Information, SI.
The coplanarity of substituent aryl rings with the pyrylium
core preorganizes a polydentate binding site for anions. When
paired with that of a second triarylpyrylium molecule, this
binding site mimics the geometry of macrocyclic anion
receptors with a greater degree of flexibility with respect to
the size of the target anion that can be accommodated. This
motif is referred to as a “rhombic binding pocket” and persists
in 63% of the pore- and channel-forming triarylpyrylium salt
crystal structures. Triarylpyrylium salts that do not adopt this
motif include those salts with notably large and/or soft anions
Figure 3. Conserved local motifs identified in triarylpyrylium salts
−
including (a) reinforced C−H···X hydrogen bonding, (b) the
rhombic anion binding pocket, and (c) four-ring antiparallel
dimerization.
−
−
−
(
I , FeF , ClO ) and pyrylium molecules with bulky aryl
3 4 4
1
(
0 display two-molecule in-plane coordination of anions
Figure 3b), and 13 have four-ring antiparallel dimerization of
triarylpyrylium molecules through cation-π aromatic stacking
substituents, 1. The predominance of the rhombic binding
pocket in crystal structures suggests that this motif may exist as
a precrystallization complex and potentially in solution as a
homogeneous-phase anion receptor in addition to solid-state
triarylpyrylium exchange materials (Figure 5).
(Figure 3c).
Nonclassical C−H hydrogen-bonds are not widely employed
in crystal design due to their weakness relative to classical
hydrogen-bond donor/acceptor pairs, though they often play
an underlying role in crystal packing.
15,41
However, in the case
of pyrylium salts, the presence of the electronegative oxygen
atom causes a substantial polarization of the triarylpyrylium
core’s C−H bonds, which reinforces the strength of its C−H···
−
26
X interactions and makes these interactions a viable crystal
engineering tool.
Previous groups studying the interactions between C−H
groups and O atoms have measured the hydrogen-bond-
donating carbon to hydrogen-bond-accepting oxygen distances,
d(C···O), as a proxy for C−H···O−C interactions, as the
4
1
hydrogen atoms cannot be accurately located. Additionally,
without experimentally determined hydrogen atom positions,
the approach linearity between C−H donors and acceptors can−
only be qualitatively described. For this reason, the C···X
Figure 5. Mimicry of macrocyclic anion binding by the rhombic
binding pocket.
−
distances, d(C···X ), between triarylpyrylium C−H bond
donors and anionic acceptors were measured for the 19
The final conserved local geometry does not involve
interactions with anions but is limited to the pyrylium
framework. This interaction arises as a result of cation-π-
facilitated face-to-face aromatic stacking (Figure 6) in which a
substituent aryl ring (A) participates in aromatic stacking
interactions with the pyrylium heterocycle (B) on the molecule
above/below and vice versa, heterocycle (C) to substituent
−
triarylpyrylium crystal structures that exhibit weak C−H···X
−
interactions. Three d(C···X ) were measured for each
triarylpyrylium anion pair, a, b, and c (Figure 4).
(
D). Lateral slipping lengthens the centroid-centroid distance
between rings (A) and (B) and (C) and (D) with the longest
centroid-centroid distance measured at 4.158 Å and the
shortest at 3.436 Å. The four-ring antiparallel dimerization
geometry is adopted by all pore- and channel-forming
triarylpyrylium salts with the exception of the triiodide salts
(
17 structures), in which π-stacking is disrupted by aromatic
−
−
Figure 4. Carbon atoms for which the closest C···X and C−H···X
sandwiching of the triiodide anion.
distances were measured.
For more meaningful interpretation, the H···X− contac2t
distances were approximated by subtracting the idealized sp
4
2
−
d(C−H) bond length, 1.2 Å, from the measured d(C···X )
−
values. These hydrogen-normalized d(C−H···X ) measure-
−
ments were analyzed for H···X nonbonded radial overlap. It
−
was found that all structures contained at least one C−H···X
−
interaction in which the d(C−H···X ) was shorter than the sum
Figure 6. Centroid-to-centroid (A···B and C···D) distance measure-
ments for the four-ring antiparallel dimerization.
−
42
of the predicted nonbonded H···X contact distances, with
D
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Role of Crystallization Solvent in Pyrylium Architec-
tures. Solvent choice is often critical in accessing specific
polymorphs in systems that express geometrical frustration and
in crystallizing soft supramolecular assemblies. Though the
individual interactions between solvent molecules and
precrystalline complexes are only loosely understood, trapped
solvent molecules and the inherent disorder in positions,
orientations, and conformations of these molecules in
triarylpyrylium solvates play a fundamental role in the control
of channel morphology in the solid state. Of the 19
triarylpyrylium salts, 8 structures are solvates, 7 of which
contain solvent molecules that are crystallographically well
enough defined to be modeled and refined.
To directly study the effect of solvent type on triarylpyrylium
salt frameworks, we built a library of three such patterns within
a series of 2,4,6-tris(4-methylphenyl)pyrylium tetrafluoroborate
(TMPPyry+ BF ) solvated salts. TMPPyry BF4 was
−
+
−
4
synthesized by an acid-catalyzed cyclocondensation adapted
4
3
from the literature (synthetic details available in the
Experimental Section). In all three solvates, the nonpolar
+
character of the TMPPyry methyl substituents gives rise to
various hydrophobic barriers to anion mobility diagrammed in
Figure 8a−c. These nonpolar barriers have been labeled
−
Solvents capable of accepting C−H···X hydrogen-bonds,
such as acetic acid, ethanol, nitromethane, and water, were
found noncovalently bound to the framework by accepting
hydrogen-bonds donated by C3−H and/or C5−H of the
pyrylium heterocycles. THF, a solvent molecule less inclined to
form hydrogen-bonds with the framework, is instead
surrounded by the triarylpyrylium substituents donated by six
separate molecules and is more severely disordered in the
crystal structure, indicating that the interaction between the
solvent molecule and framework is nonspecific compared to the
hydrogen-bond-accepting solvents. These solvent coordination
motifs give rise to the two general solvent pocket structures
that, when alternated with the rhombic anion binding pocket,
result in the diversity of channel types observed in
triarylpyrylium materials (Figure 7).
Figure 8. Diagram of inner channel wall nonpolar barriers to anion
diffusion with polar surfaces shown in blue and nonpolar surfaces in
gray: (a) elliptical, (b) parabolic, and (c) alternating parabolic.
elliptical, parabolic, and alternating parabolic. A detailed
understanding of these patterns enables precise control over
the steric and electrostatic barriers encountered by mobile
anions during exchange and can be exploited in order to design
and tune anion exchange materials that are selective for
particular guests. These electrostatic surfaces, in addition to
steric barriers, affect the mode of anion binding. The following
two sections discuss, in detail, how channel structures may be
combined to synthesize anion exchange materials within a
+
−
series of TMPPyry BF4 solvated salts and co-crystals.
−
Solvates of 2,4,6-Tris(4-methylphenyl)pyrylium BF4
+
Salts. The one-pot synthesis of TMPPyry takes place by
reacting two molar equivalents of 4-methylacetophenone with
one molar equivalent of 4-methylbenzaldehyde in the presence
of boron trifluoride to form the central pyrylium cation. The
TMPPyry BF4 solid was then recrystallized solvothermally
from various solvents.
43
+
−
+
−
A THF and water solvate of TMPPyry BF4 (15a) can be
crystallized from a biphasic THF/water solvent mixture. The
solvent molecules are located in channels and alternate with
tetrafluoroborate anions (Figure 9a). The two types of solvent
molecules give rise to two different channel morphologies as a
result of the variable hydrogen-bond-forming capabilities of the
two molecules. The first channel type arises from alternation of
tetrafluoroborate anions and THF molecules. The THF
molecules, incapable of forming strong hydrogen-bonds
(relative to protic solvent molecules), are surrounded by
Figure 7. Conserved local solvent pockets, including the nonpolar (a)
THF pocket and (b) the hydrogen-bond acceptor pocket with a water
molecule located within the weak hydrogen-bonding distance (2.787
Å) from the polarized pyrylium C−H, which is indicated by a dashed
line.
Persistent Extended Motifs in Solid-State Pyrylium
Salts. Poor diffraction quality arising from disorder, twinning,
and stacking faults suggests that the combinations of local
interactions that give rise to extended triarylpyrylium motifs are
nonspecific relative to conventional crystal engineering
synthons. Channels of variable morphologies arise from the
alignment and alternation of the two-dimensional solvent and
rhombic anion binding pockets. Trapped solvent stabilizes the
channel continuity by screening anion−anion Coulombic
repulsion. Additionally, the character of the solvent can
determine the patterning of the inner channel walls with the
electrostatic character of the triarylpyrylium molecule’s
substituent by promoting alternation of a particular solvent
pocket type to form the channel.
+
methyl substituents from six separate TMPPyry molecules,
giving rise to elliptical nonpolar barriers to anion diffusion
(Figure 8a). Because interactions between the THF molecules
and the framework are nonspecific, THF molecules are 2-fold
rotationally disordered in the solid state. The second channel
type contains tetrafluoroborate anions and water molecules and
exhibits a parabolic nonpolar electrostatic patterning of the
channel surface (Figure 8b). This patterning arises as a result of
the water molecules’ hydrogen-bonds to the framework
bringing three methyl groups to one side of the channel and
acidic C−H moieties on the other. Individual water molecules
are engaged in four hydrogen-bonds, accepting two C−H
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contain water molecules as well, consistent with the depend-
ence of this phase crystallizing out of a solvent solution doped
with water (see SI for more details). The two channel types are
differentiated by the orientation, disorder, and interactions of
the ethanol and water molecules relative to the surrounding
framework. In both channels, ethanol molecules and tetra-
fluoroborate anions are connected via strong O−H···F
hydrogen-bonds, forming solvate−anion pairs (Figure 9c).
Channels containing ordered ethanol molecules are nearly
trigonal cross-sectionally, accepting hydrogen-bonds donated
by framework molecules that bond to the well-ordered solvate-
anion pairs within the channel. The contents of the second
channel type refined to an 86% ethanol and 14% water
occupancy. The disordered solvent molecules are arranged
around a crystallographic 2-fold axis, imposing additional whole
ethanol molecule disorder. The symmetry imposed solvent
molecule disorder breaks the directionality of the O−H···F
hydrogen-bonds leading to anion disorder as well. This disorder
is likely correlated with the larger channel diameters of the
second channel type, offering greater conformational flexibility
of the ethanol molecules and greater mobility of the
tetrafluoroborate anions.
Co-crystals of 2,4,6-Tris(4-methylphenyl)pyrylium
−
BF4 and 2,4,6-Tris(4-methylphenyl)pyridine. Co-crystal-
lization has been identified as a nonsynthetic means by which
to attenuate physical properties of solids, such as the
bioavailability of pharmaceuticals, the sensitivity and detonation
velocity of energetic materials, and the solubilities of solids. Co-
crystals of TMPPyry+ BF4 and 2,4,6-tris(4-methylphenyl)-
pyridine, TMPPy, (15d) crystallized from a biphasic THF and
water solvent mixture, exhibit both a new extended structural
phase not observed in pure-phase triarylpyrylium salts and
decreased solubility in water, a property desirable for anion
Figure 9. Channel structures of (a) 15a, (b) 15b, and (c) 15c,
highlighting the channel cross-section, the solvent molecule and
tetrafluoroborate repeat sequence in the channel, and the hydrogen-
bonding interaction in each of the crystal structures between the
solvent and tetrafluoroborate.
−
groups from the framework (from two ortho phenylene groups)
and donating two O−H···F hydrogen-bonds to the tetrafluor-
oborate anions in the channel, forming a chain of hydrogen-
bonded guest molecules (Figure 9a).
exchange materials to be utilized in natural settings.
Co-crystallization of TMPPyry+ BF
−
and TMPPy was
4
+
−
attempted as a method by which to stabilize TMPPyry BF
4
+
−
The acetic acid solvate of the TMPPyry BF salt (15b)
solids in water. As a result of the oxygen atom’s electro-
negativity, pyrylium reactivity differs greatly from that of
benzene and pyridine, having weaker resonance energy and a
greater susceptibility toward nucleophilic attack. This phenom-
enon makes the pyrylium cation subject to rapid decomposition
when dissolved in water. Though this hydrolysis can be slowed
and even reversed in acidic solutions, co-crystallization with
isosteric pyridine compounds was predicted to slow dissolution,
protecting against pyrylium hydrolysis in aqueous environ-
ments of unknown pH.
4
forms a complicated series of channels containing the
tetrafluoroborate anions and two types of disordered acetic
2
2
acid dimers that exhibit the typical R (8) carboxylic acid dimer
H-bonding motif. Continuous aromatic stacking between the
cations is disrupted by π−π interactions between acetic acid
dimers and the tolyl substituent of a neighboring TMPPyry+
cation with a centroid-centroid stacking distance of 3.411 Å.
Intercalation of the acetic acid dimers between TMPPyry+
molecules limits aromatic stacking interactions to discrete
+
cation-π-facilitated stacked TMPPyry dimers. The acetic acid
The pyridine derivative, TMPPy, was prepared by reacting
two molar equivalents of 4-methylacetophenone with one
molar equivalent of 4-methylbenzaldehyde in the presence of
an acid catalyst, acetic acid, and a large excess of ammonium
dimers are the sole occupants of continuous channels parallel to
the c-axis. A second set of channels propagates parallel to the a-
axis and contains alternating tetrafluoroborate anions and acetic
acid molecules (Figure 9b) with an alternating parabolic
nonpolar channel surface pattern (Figure 8c). The acetic acid
molecules in the second channel type accept hydrogen-bonds
donated by the framework and donate C−H···O hydrogen-
bonds to the tetrafluoroborate anions within the channel,
forming a directional solvent-anion hydrogen-bonded chain
44
+
−
acetate to form the central pyridine ring. TMPPyry BF
and
4
TMPPy can be co-crystallized from a 1:1 molar ratio of the two
solids in a 1 THF: 1 water (v/v) solvent mixture. The
compounds are dissolved by heating the suspension in THF/
water to 70 °C, inducing phase separation of the system. Slow
cooling of the system to 4 °C causes collapse of the separated
phases, and bright red crystals are obtained (details provided in
the Experimental Section).
(Figure 9b).
Solvothermal crystallization of the TMPPyry BF4− salt from
+
an ethanol and water mixed solvent system yields crystals with
two channel types containing chains of tetrafluoroborate anions
and solvent molecules: alternating and continuous parabolic
channel surface patterning (15c). Both channel types contain
ethanol molecules, though one of the channel types is found to
The co-crystal structure has been solved and refined in the
orthorhombic space group Fddd, but is convoluted by
disordered anions and solvent molecules in the channels,
solid solution disorder of the pyrylium cations and pyridine
molecules, and twinning through pseudomerohedry emulating
F
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a hexagonal primitive packing; initial structure solutions
compositions, while a variation in lattice site occupancy can be
predicted from changes in relative peak intensity.
modeled in P2 , a subgroup of the actual space group Fddd
1
not obscuring the pseudohexagonal lattice dimensions, refined
with a β-angle of 119.759° and a- and c-axes of 38.5754 and
Variable temperature PXRD, VT-PXRD, and thermogravi-
metric analysis, TGA, (Figure 10c, d) reveal that mobilization
of trapped solvent is correlated with anisotropic distortion of
the helical channels. This can be observed by PXRD peak-
splitting and symmetry changes that result from heating bulk
samples of the co-crystal. The quality of the single crystals was
severely compromised, preventing more detailed structural
analysis of the desolvated co-crystalline phase. These results
suggest flexibility of the framework to accommodate guest
diffusion and exchange.
+
−
3
0
8.6371 Å, respectively and refined to a 2.125 TMPPyry BF :
4
.875 TMPPy molar ratio. Though solid solution of the
+
−
TMPPyry BF4 and TMPPy prevents these molecules from
being definitively discerned from one another in the crystal
structure, a UV−vis absorbance shift of the co-crystalline solid
relative to pure-phase triarylpyrylium solids suggests a strong
electronic interaction between the pyridine and pyrylium
moieties. Such an interaction is predicted to provide electron
density to the core pyrylium cation, stabilizing the heterocycle.
The 2-fold triple helical channels arise from a series of
twisted and interlocked rhombic binding pockets (Figure 11b),
+
−
The solid solution between TMPPyry BF and TMPPy
4
1
molecules in this system was explored by H NMR and powder
XRD, PXRD, (Figure 10a, b). Precrystallization molar ratios
Figure 11. Crystal structure of 15d in Fddd (a) parallel to the a-axis,
illustrating the two major channel types: (b) the 2-fold triple-helical
pseudohexagonal channel of rhombic binding pocket motifs and (c)
the alternating parabolic channel.
Figure 10. Solid solution, thermal, and structural properties of 15d.
1
(
a) H NMR spectra and (b) PXRD of bulk co-crystalline material of
+
variable mole percent TMPPyry : 81% (i), 71% (ii), 68% (iii), and 0%
iv) crystallized from starting mass ratios of 90%, 70%, 50%, and 10%
(
+
−
TMPPyry BF . (c) TGA and (d) VT-PXRD of co-crystal bulk
4
and as a direct result, the inner channel environment lacks
nonpolar patterning from the methyl substituents. Difference
density F − F maps of the single crystal data confirm that both
revealing structural deformation and retained crystallinity upon solvent
evacuation with the temperature (°C) at which each pattern was taken
indicated to the left of the pattern.
o
c
channel types enclose disordered anions and solvent molecules.
Atomic positions are not well resolved, though the helical
channel contains guests that are better defined than the
completely diffuse electron density enclosed by the second
channel type. The framework of the second channel type
(Figure 11c) is built from the alternation of the polar solvent
pocket, Figure 7b, and rhombic binding pockets, resulting in an
alternating parabolic channel surface, which provides diffusing
anions with a continuous helical path. The difference in
resolution of the channel guests is most likely attributable to a
difference in mean solvent occupancy between the two channel
types, one channel containing predominantly anions, which can
form more favorable interactions with the walls of the triple-
helical channel and the other containing disordered water
molecules.
between the two species were varied in 10% intervals and the
final stoichiometries of the resultant co-crystals determined by
1
integration ratios of H NMR peaks of redissolved crystals. It
was found that the co-crystals have to contain between 68 and
+
8
1 molar percent TMPPyry in order to form the co-crystalline
structural phase. Above and below of this range, crystals of pure
+
−
TMPPyry BF4 and colorless crystals of TMPPy are formed
beside the bright red solid solution co-crystals. Solid solution of
the framework molecules contributes to guest disorder, as a
variable number of counteranions must be entrained in the
channels in order to charge balance the framework. Minimal
peak shifts in the PXRD patterns (Figure 10b) support a
conserved co-crystalline structural phase across solid solution
G
DOI: 10.1021/acs.cgd.6b00005
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Two structural roles can be assigned to TMPPy that cause
the motifs unique to the co-crystal structure to deviate from
those adopted by the pure-phase pyrylium salts and solvates: as
an isosteric uncharged structural spacer and as a solubility
modifier.
neutral pentenedione/pentadieneone and recyclization with the
acid or aqueous inorganic salt of choice to reform the pyrylium
4
,44−46
cation.
Preliminary results suggest that 15d is capable of
undergoing anion exchange through either heterogeneous and
homogeneous exchange mechanisms depending on the nature
of the anion, as supported by PXRD and IR studies (see SI for
The triple-helical channel observed in the co-crystal is the
+
−
46
only extended motif observed in the TMPPyry BF4 series
that does not exhibit alternation between solvent and anionic
binding pocket types, but it instead exclusively contains twisted
interlocked anion binding geometries. The tetrafluoroborate
anion has a van der Waals radius of approximately 4.32 Å,
though the space available for a counteranion in an individual
rhombic pocket in the triple helix is only 3.62 Å, due to
interlocking of the rhombic anionic binding pockets. The
more details).
CONCLUSIONS
■
With great structural diversity in extended cationic lattice
motifs arising from a small set of conserved local interactions,
triarylpyrylium salts can be considered both tunable and
predictable solids for the design of anion-exchangeable
materials. The electronic and conformational features of
channel-forming triarylpyrylium cations revealed a competition
between electrostatic and geometric stabilities, which results in
a planar polyaryl building block with unique optical properties.
Though the planarity of triarylpyrylium cations distinguishes
these molecules from alternative arenes, the low resonance
energy of the core heterocycle makes the pyrylium ring more
susceptible to hydrolysis and rapid degradation, limiting the
utility of solution-phase pyrylium-based anion receptors.
+
−
helical morphology cannot arise in pure TMPPyry BF4 crystal
structures because there is not enough space available to
−
accommodate the number of BF4 anions required to charge-
balance the framework. The helical morphology is allowed in
the co-crystal because the TMPPy molecule serves as a spacer
to alleviate charge balance complications that arise from the
1
uninterrupted stacking of anions in the binding pockets. H
NMR spectra of redissolved co-crystals show a solid solution
between the two heterocyclic molecules, suggesting that this
intercalation is necessary to relieve frustration, but diffraction
reveals that this replacement does not occur in predictable
lattice positions in the crystal structure.
The conserved local motifs and unique extended structures
formed by triarylpyrylium salts in the solid state were
characterized and shown to display prerequisite features for
solid-state anion exchange and capture, such as continuous
cationic pore and channels with tunable electron density
The second major role of the TMPPy is its notable
modification of TMPPyry+ BF4 salt solubility in aqueous
media, which is an important feature in developing anion
exchange materials that are useful and efficient in natural
aquatic settings. The strong electronic attraction between the
pyrylium and pyridine moieties greatly extends the lifetime of
the material in water by decreasing the solubility of the material
thus preventing hydrolysis of dissolved triarylpyrylium cations.
The Mechanisms of Anion Exchange. Theory and
mechanistic details of anion separation as they apply to
potential exchange in triarylpyrylium cationic frameworks and
−
−
distributions about predicted C−H···X binding sites. These
binding sites may be engineered through chemical substitution
of the triarylpyrylium cation with retention of the local binding
environment and functionally inconsequential modification of
the extended structure. The specific effects of solvent-inclusion
were addressed by careful analysis of the crystal structures of
three solvated 2,4,6-tris(4-methylphenyl)pyrylium tetrafluoro-
borate salts. Additionally, co-crystallization of triarylpyrylium
compounds with isosteric pyridine compounds was found to
subtly modify common local motifs shared by triarylpyrylium
salts to give rise to extended structures not observed in pure-
phase triarylpyrylium salts. Co-crystallization also resulted in
cationic pyrylium-based frameworks with reduced solubilities in
aqueous media, making this material a candidate for
heterogeneous anion exchange in the remediation of
contaminated surface and groundwater systems. Modern
anion exchange materials suffer from pH sensitivity and
reduced performances in aqueous environments, which can
be reduced in triarylpyrylium materials by co-crystallization.
Though solid-state triarylpyrylium-based frameworks are a
relatively unexplored area of anion exchange solids, they have
already been shown to exhibit key properties that may side-step
these engineering difficulties.
5
,44
other solid-state materials have been presented elsewhere
but will be briefly summarized here. The three predominan5t
mechanisms include heterogeneous-phase anion diffusion,
5
homogeneous-phase competitive crystallization, and synthetic
4
4
anion exchange. Ideal heterogeneous anion exchange
materials facilitate anion diffusion through the salt lattice at a
5
rate much faster than that of framework dissolution, thus
leaving the framework functionally intact. Viability of
heterogeneous exchange can be limited by structural deviation
between the frameworks of the original and exchanged salts and
can be biased for anions less soluble in the exchange media due
to variable enthalpy changes during anion desolvation as the
5
dissolved and framework anions swap. Solvation bias, though
still present, may be lessened in homogeneous-phase exchange
settings by receptor complementarity. Anion separation by
selective crystallization is here defined as partial or complete
dissolution of the crystal followed by reprecipitation of a salt
encasing the target anionic guest. This separation mechanism is
driven by a significant difference in solubility between the initial
EXPERIMENTAL SECTION
■
All reagents were used as received from the manufacturer without
further purification, with the exception of tetrahydrofuran and ethanol,
which were dried over 3A molecular sieves. BHT-free biograde
tetrahydrofuran, boron trifluoride diethyl etherate, and 48% (w/w)
HBF4(aq) were received from Alfa Aesar. Ethanol (200 proof,
anhydrous), glacial acetic acid, and concentrated HCl were obtained
from Pharmaco-Aaper. BHT-stabilized diethyl ether and toluene were
purchased from Fischer Scientific. Ammonium acetate and 57% (w/w)
HI(aq) were purchased from Sigma-Aldrich, and 4-methylacetophenone
and 4-methylbenzaldehyde were purchased from Eastman.
5
and exchanged salts and, unlike heterogeneous exchange, the
lattice isoreticularity need not be retained. The third exchange
method, synthetic anion exchange, is not subject to solvation
bias and proceeds by taking advantage of the pH-reversible
charged and uncharged states of substituted pyrylium
compounds. Synthetic anion exchange is a brute-force method
of exchange by hydrolysis of the pyrylium ring to a substituted
A Varian 400 MHz NMR spectrometer was used. All NMR spectra
were taken at room temperature with spinning at 20 Hz. Shimming
H
DOI: 10.1021/acs.cgd.6b00005
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
and locking were performed using Varian VnmrJ software. A Rigaku
Ultima IV diffractometer (λ = 1.5418 Å) with Bragg−Brentano
geometry was used for room temperature PXRD experiments with a
scan speed of 2°/min. Variable-temperature PXRD experiments were
performed with parallel-beam geometry and kept under a flowing N2
atmosphere. A scan-speed of 4°/min was used for VT-PXRD
experiments.
Crystallization of 15b. Crystals were grown by dissolving 15 mg
+
−
of TMPPyry BF in 1 mL glacial acetic acid at 115 °C. Once the
4
+
−
TMPPyry BF
was completely dissolved, the solution was cooled to
4
17 °C, crystallizing gold rods.
Crystallization of 15c. Crystals were grown by dissolving 3 mg
+
−
TMPPyry BF
in 1 mL ethanol with 5 drops of diH
O at 80 °C.
4
2
+
−
Once the TMPPyry BF
was completely dissolved, the solution was
4
Single-Crystal X-ray Diffraction Methods. Data for compound
5a were collected on a Bruker AXS D8 Quest CMOS diffractometer
cooled to 17 °C, yielding golden rods. Crystals grown from a dry
ethanol solution had a blocky habit and exhibited non-Bragg behavior,
preventing structural determination by single-crystal XRD. PXRD
patterns of the needles and the block habits confirmed that the two
habits are not isostructural.
1
with a molybdenum I-μ-S microsource X-ray tube (λ = 0.71073 Å).
Single crystal data for all other compounds (15b−15d) were collected
using a Bruker AXS X8 Prospector CCD diffractometer with a copper
I-μ-S microsource X-ray tube (λ = 1.54178 Å). Both feature laterally
graded multilayer (Goebel) mirrors for monochromatization. Single
crystals were mounted on Mitegen micromesh mounts with the help
of a trace of mineral oil and flash cooled to 100 K.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.6b00005.
The Apex2 software package was used to determine unit cells and
for data collection, and data were integrated using SAINT. The data
were processed with SADABS and corrected for absorption using
multiscan techniques. The space groups were assigned using XPREP of
the Bruker SHELXTL package, solved with ShelXD, and refined with
SHELXL 2013 or 2014 and the graphical interface Shelxle. All non-
hydrogen atoms were refined anisotropically. H atoms attached to
carbon and hydroxyl oxygen atoms were positioned geometrically and
constrained to ride on their parent atoms, with carbon hydrogen bond
distances of 0.95 Å for aromatic C−H, 0.99 and 0.98 Å for aliphatic
CH and CH , respectively. Methyl H atoms were allowed to rotate
Details of the single-crystal structure determinations,
structure factor tables, and Ortep-style plots for
structures 15a−d are included. Additionally, experimen-
tal and calculated PXRD diffractograms, a table of
hydrogen-anion radial overlap values for 1−15c, photo-
graphs of 15c and 15d crystallization, and further
discussion of the anion exchange capabilities of 15d
(PDF)
2
3
but not to tip to best fit the experimental electron density. U (H)
iso
Accession Codes
values were set to a multiple of U (C/O) with 1.5 for CH , and 1.2
eq
3
CCDC 1444843−1444846 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
for C−H.
All structures feature solvated anions within channels of the rigid
frameworks. The residual electron density peaks corresponding to the
solvent molecules occupying channels in the frameworks are highly
disordered and often not arranged in an interpretable pattern.
Complete crystallographic data, in CIF format, have been deposited
with the Cambridge Crystallographic Data Centre. CCDC 1444843−
1444846 contain the supplementary crystallographic data for this
AUTHOR INFORMATION
Corresponding Author
Tel: (440) 775-8300. Fax: (440) 775-6682. E-mail: renwis@
umich.edu.
■
*
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
+
Synthesis of TMPPyry BF . Adapted from the published
4
4
3
literature. 4-Methylacetophenone (91.8 mL, 92.26 g, 687.58
mmol) and 4-methylbenzaldehyde (40.3 mL, 41.07 g, 341.79 mmol)
were refluxed in dry toluene (280 mL) and boron trifluoride diethyl
etherate (86.96 mL) for 2 h during which time the golden starting
solution rapidly converted to a deep red solution. A yellow solid
Present Address
Department of Chemistry, University of Michigan, Ann Arbor,
MI 48109, United States.
Notes
The authors declare no competing financial interest.
Dr. Jesse L. C. Rowsell passed away on January 30, 2015. Dr.
Rowsell’s curiosity toward questions that challenge present
paradigms resulted in this research. His relentless dedication
motivated the completion of this manuscript. The authors are
grateful for his guidance in and outside of the lab.
precipitated upon addition of diethyl ether (yield: 59.1042 g, 134.86
1
mmol, 39.4%). H NMR (D -Acetone, 400 MHz): δ (ppm) 2.538 (s,
6
6
H), 2.538 (s, 3H), 7.623 (d, 2H), 7.642 (d, 4H), 8.471 (d, 4H), 8.523
d, 4H), 9.066 (s, 2H).
Synthesis of TMPPy. Adapted from the published literature. 4-
Methylacetophenone (1.14 g, 8.50 mmol) and 4-methylbenzaldehyde
0.50 g, 4.16 mmol) were refluxed in 50 mL glacial acetic acid with
(
4
4
(
excess ammonium acetate (7.27 g) for 16 h. Water was added to the
cooled reaction solution, precipitating a cream-colored solid.
ACKNOWLEDGMENTS
■
Recrystallization from ethanol yielded white needles (yield: 0.2370
The X-ray diffractometers at Youngstown State University were
funded by NSF Grants DMR-1337296, CHE-0087210, by Ohio
Board of Regents Grant CAP-491, and by Youngstown State
University. The powder X-ray diffractometer at Oberlin College
was purchased with support of NSF grant DMR-0922588.
Though the crystal structure for the co-crystal was solved from
data collected at YSU, data were also collected at Beamline 15-
ID B ChemMatCARS at the Advanced Photon Source through
the Synchrotron Crystallography at the Advanced Photon
Source, SCrAPS program supported by Indiana University. The
authors would like to thank Drs. Maren Pink and Josh Chen for
data collection and continued mentorship in addition to Drs.
Adam J. Matzger and Antek G. Wong-Foy as well as Kyle A.
1
g, 0.678 mmol, 16.3%.) H NMR (CDCl , 400 MHz): δ (ppm) 2.441
3
(
s, 6H), 2.429 (s, 3H), 7.312 (d, 4H), 7.330 (d, 2H), 7.647 (d, 2H),
7.824 (s, 2H), 8.093 (d, 4H).
Crystallization of 15d. Crystals were grown by dissolving 5 mg of
+
−
TMPPyry BF4 and 5 mg TMPPy in 1 mL of a 1 THF: 1 diH O (v/
2
v) mixed solvent by heating to 70 °C. Once the compound was
dissolved and the solvent mixture phase separated, the system was
cooled to 4 °C, crystallizing red rods with anisotropic hexagonal faces.
Crystallization of 15a. Crystals were grown by dissolving 10 mg
+
−
of TMPPyry BF4 in 1 mL of a 1 THF: 1 diH O (v/v) mixed solvent
2
by heating to 70 °C. Once the compound was dissolved and solvent
phase separated, the mixture was cooled to 4 °C, yielding gold rods
with rhombic and orthorhombic faces.
I
DOI: 10.1021/acs.cgd.6b00005
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
McDonald for their help during the completion of this
manuscript.
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J
DOI: 10.1021/acs.cgd.6b00005
Cryst. Growth Des. XXXX, XXX, XXX−XXX