Article
pubs.acs.org/accounts
Versatile and Resilient Hydrogen-Bonded Host Frameworks
Takuji Adachi and Michael D. Ward*
Department of Chemistry and Molecular Design Institute, New York University, 100 Washington Square East, New York, New York
10003-6688, United States
CONSPECTUS: Low-density molecular host frameworks,
whether equipped with persistent molecular-scale pores or
virtual pores that are sustainable only when occupied by guest
molecules, have emerged as a promising class of materials
owing to the ability to tailor the size, geometry, and chemical
character of their free space through the versatility of organic
synthesis. As such, molecular frameworks are promising
candidates for storage, separations of commodity and fine
chemicals, heterogeneous catalysis, and optical and electronic
materials. Frameworks assembled through hydrogen bonds,
though generally not stable toward collapse in the absence of guests, promise significant chemical and structural diversity, with
pores that can be tailored for a wide range of guest molecules. The utility of these frameworks, however, depends on the
resilience of n-dimensional hydrogen-bonded motifs that serve as reliable building blocks so that the molecular constituents can
be manipulated without disruption of the anticipated global solid-state architecture. Though many hydrogen-bonded frameworks
have been reported, few exist that are amenable to systematic modification, thus limiting the design of functional materials.
This Account reviews discoveries in our laboratory during the past decade related to a series of host frameworks based on
guanidinium cations and interchangeable organosulfonate anions, in which the 3-fold symmetry and hydrogen-bonding
complementarity of these ions prompt the formation of a two-dimensional (2-D) quasi-hexagonal hydrogen-bonding network
that has proven to be remarkably resilient toward the introduction of a wide range of organic pendant groups attached to the
sulfonate. Since an earlier report in this journal that focused primarily on organodisulfonate host frameworks with lamellar
architectures, this unusually persistent network has afforded an unparalleled range of framework architectures and hundreds of
new crystalline materials with predictable solid-state architectures. These range from the surprising discovery of inclusion
compounds in organomonosulfonates (GMS), as well as organodisulfonates (GDS), structural isomerism reminiscent of
microstructures observed in soft matter, a retrosynthetic approach to the synthesis of polar crystals, separation of molecular
isomers, storage of unstable molecules, formation of a zeolite-like hydrogen-bonded framework, and postsynthetic pathways to
inclusion compounds through reversible guest swapping in flexible GS frameworks.
Collectively, the examples described in this Account illustrate the potential for hydrogen-bonded frameworks in the design of
molecular materials, the prediction of solid-state architecture from simple empirical parameters, and the importance of design
principles based on robust high dimensional networks. These and other emerging hydrogen-bonded frameworks promise new
advanced materials that capitalize fully on the ability of materials chemists to manipulate solid-state structure through molecular
design.
■
INTRODUCTION
The formation of molecular crystals has been described as
“supramolecular chemistry par excellence,”1 owing to the
extraordinary fidelity that is characteristic of a growing crystal:
a typical 1 mm3 organic crystal is the product of approximately
1018 events that occur with near perfection! The potential
versatility and functions of these materials has motivated a
modular approach to their synthesis, as illustrated clearly by the
advances in metal−organic frameworks.2 Moreover, during the
past five decades organic solid-state chemistry has produced a
wide range of molecular crystals, with an equally wide range of
solid-state properties, capitalizing on the versatility of organic
synthesis. Solid-state properties are cooperative, however. As
such, they are inextricably linked to solid-state structure, the
control of which often is thwarted by the delicate, noncovalent
forces that govern molecular organization in the solid state, to
© 2016 American Chemical Society
the extent that even the slightest modification of a constituent
can lead to unpredictable changes in crystal architecture.
Although recent advances are encouraging, crystal structure
prediction through computational methods, including space
group, lattice parameters, and atomic positions, remains
elusive.3 Consequently, organic solid-state chemists often rely
on empirical strategies using structure-directing interactions
such as hydrogen bonds4 to override the cumulative effect of
weaker and less directional crystal packing forces. In these
cases, crystal structure sometimes can be anticipated from the
molecular symmetry of the building blocks, as exemplified by
the two-dimensional (2D) “chicken wire” network formed by
Received: July 12, 2016
Published: September 30, 2016
2669
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
Scheme 1. (A) Quasi-hexagonal GS Sheet, Illustrating the Major and Minor Hydrogen-Bonded Ribbons and (B) “Shifted
Ribbon” Motif Observed Occasionally in GS Compounds
trimesic acid5 and the three-dimensional (3D) diamond-like
network formed from adamantane-1,3,5,7-tetracarboxylic acid.6
In 2001, an Account from our laboratory reported lowdensity molecular frameworks based on a robust 2D network
assembled through charge-assisted (guanidinium)N−H···O(sulfonate) hydrogen bonds,7 wherein guanidinium organomonosulfonates (GMS) formed guest-free phases while
guanidinium organodisulfonates (GDS) readily formed frameworks with guests included. Since that Account, more than 500
crystalline compounds based on the guanidinium sulfonate
(GS) network with interchangeable sulfonates, exhibiting
various lamellar, cylindrical, and cubic architectures, have
been discovered, a benchmark in crystal engineering and one
that continues to surprise. In this Account, we describe
examples that illustrate the versatility and unique character of
these frameworks, including the discovery of GMS inclusion
compounds, new architectures in GMS and GDS frameworks,
structural isomerism reminiscent of soft matter, strategies for
the synthesis of polar crystals, selective encapsulation for
separation and chemical storage, a zeolite-like hydrogen bonded
framework assembled from molecular tiles, and postsynthetic
modification through guest swapping in flexible frameworks.
attributed to the strength of the charge-assisted hydrogen
bonds and the unique ability of the GS sheet to pucker like an
accordian, which provides a pathway to close packing with
retention of the hydrogen-bond connectivity.
■
ARCHITECTURAL DIVERSITY IN GS FRAMEWORKS
The organic groups appended to sulfonates can be viewed as
posts (for monosulfonates) or pillars (for disulfonates and
some polysulfonates) for constructing a GS framework in the
third dimension from the 2D GS sheets. The GS sheet can be
described as consisting of one “major” (M) and two “minor”
(m) ribbons, and the “up-down” projections of the organosulfonate groups from the opposing sides of a GS sheet can be
depicted as filled or open circles (Figure 1). The number of
possible “up-down” arrangements of organosulfonate groups is
actually indefinite. For example, guest-free guanidinium
organomonosulfonates (GMS) can assemble through interdigitation of organosulfonate posts that project from the same
side of each GS sheet to form a bilayer architecture (PT-I),
while equal numbers of rows of organic groups projecting from
opposite sides form a simple continuously layered (s-CL)
architecture (PT-II). The architecture formed depends on the
cross-sectional area of the organic group, with the s-CL more
accommodating of groups with larger footprints (Figure 2A,B).
Guanidinium organodisulfonates (GDS) assemble via organodisulfonate “pillars” that connect opposing GS sheets, thus
enforcing inclusion cavities in the gallery regions between
adjacent GS sheets (Figure 2C,D). Numerous low-density
frameworks have been realized, discrete bilayer, simple brick,
double brick, zigzag brick, V-brick, that can be attributed to
templating by differently sized and shaped guests during
framework assembly.8
Despite the facile formation of the guest-free GMS
compounds and the absence of predestined inclusion cavities
like those in related GDS host frameworks, GMS inclusion
compounds form readily.9,10 Using a combinatorial library of 24
GMS hosts and 26 guest molecules, a total of 304 inclusion
compounds out of a possible 624 host−guest combinations
were realized, demonstrating a remarkable capacity of the GMS
hosts to form inclusion compounds while revealing the role of
guest templating. Many GMS−guest combinations generated a
lamellar (simple continuous layered inclusion compounds: sCLIC) architecture with the PT-II projection topology,
identical to the guest-free s-CL and the GDS s-CLIC
compounds but with guests confined between the organic
■
THE GUANIDINIUM SULFONATE
HYDROGEN-BONDED NETWORK
The 3-fold symmetry and hydrogen bond complementarity of
the guanidinium cation (G = C(NH2)3+) and the sulfonate
moieties of organodisulfonate anions (DS; S = -O3S-R-SO3-)
typically affords a two-dimensional quasi-hexagonal hydrogenbonded GS sheet that can be described as 1-D GS “ribbons”
fused along the ribbon edges by lateral (G)N−H···O(S) Hbonds (Scheme 1, left), which serve as flexible “hinges” that
permit puckering of the GS sheet. The interribbon puckering
angle (θIR), determined from the centroid of two sulfur atoms
on a selected GS ribbon and the nearest sulfur atoms on the
two adjacent ribbons, dictates the repeat distance normal to the
ribbon direction (b1). The observed values of b1 range from
13.0 Å (twice the width of a GS ribbon) for a perfectly flat
sheet to as little as 7.0 Å for a highly puckered sheet, while
retaining the quasi-hexagonal motif. Occasionally, the GS sheet
adopts a “shifted-ribbon” motif (Scheme 1, right) in which
adjacent connected ribbons are shifted from the quasihexagonal arrangement, by as much as a1/2. The GS network
is remarkably resilient to a wide range of organosulfonates and
guests (in the case of inclusion compounds), which can be
2670
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
Figure 1. Projection topologies (PT) observed for GS sheets and their
corresponding architectures. Filled and open circles depict organic
groups projecting from the sulfonate nodes above and below the sheet,
respectively. The G ions sit on the undecorated nodes of the hexagonal
tiling. The major ribbon (M) and minor ribbons m(1) and m(2) are
depicted on PT-1. The parallelograms depict the translational repeat
unit of each sheet. The loops sketched on the cylindrical projection
topologies (CPT-I and CPT-II) denote hydrogen-bonded fusion of
the edges of the GS ribbons at the top and bottom of each diagram,
which results in formation of cylinders. s-CL = simple continuous
layered; s-CLIC = simple continuous layered inclusion compound; dCL = double continuous layered; d-CLIC = double continuous layered
inclusion compound; zz-CL = zigzag continuous layered; zz-CLIC =
zigzag continuous layered inclusion compound (two versions); TIC =
tubular inclusion compound. Adapted with permission from ref 10.
Copyright 2007 American Chemical Society.
groups protruding from opposing GS sheets (Figure 2E), akin
to “molecular jaws.” The ubiquity of the GMS inclusion
compounds can be attributed to fewer constraints on packing
compared with the GDS compounds due to the absence of a
covalent connection between the GS sheets, thereby removing
the requirement of registry between adjacent sheets that
otherwise may frustrate optimal packing. A double continuously
layered inclusion compound (d-CLIC) with projection topology PT-III and “double-wide” channels formed when guests
were too large for the s-CLIC architecture, as well as two
distinct architectures with “zigzag” channels (zz-CLICs) with
PT-IV and PT-V configurations directed by guest shape as well
as size. Although the assembly of so many constituents seems
counterintuitive, GMS inclusion compounds are likely favored
by the gain in entropy associated with the loss of structured
solvent (usually water or methanol) around the nonpolar guest
molecules during crystallization.
Figure 2. (A,B) Guest-free GMS bilayer and s-CL architecture (PT-I
topology). (C) GDS bilayer inclusion compound (PT-I topology).
(D) GDS s-CLIC architecture (PT-II topology). (E) GMS s-CLIC
architecture (PT-II topology). 4CBS = 4-chlorobenzenesulfonate;
4IBS = 4-iodobenzenesulfonate; NDS = naphthalenedisulfonate. The
metrics denote distance along the puckering direction, distance
between GS layers, and puckering angle.
Surprisingly, GMS inclusion compounds are not constrained
to layered architectures. Certain host−guest combinations
afforded crystalline hexagonal rods, dubbed tubular inclusion
compounds (TICs), in which the GS sheet curled into a
cylinder consisting of six GS ribbons while retaining the quasihexagonal motif (Figure 3). The organic groups attached to the
2671
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
spaced guests. One-half of the guests are contained within the
cylinders and the other half between the cylinders, resulting in
the overall stoichiometry GMS·2/3(guest).
The large number of GMS inclusion compounds permitted
the sorting of the architectural isomers in a phase diagram
based on simple molecular parameters, specifically the sulfonate
volume/guest volume ratio (Vsulf/Vguest) and the guest
eccentricity (εguest), both measured readily from molecular
models (Figure 4). Inspection of the phase diagram reveals that
the TIC and d-CLIC architectures reside primarily in separate
sectors, with the TIC architecture templated by guests that are
small and disk-shaped and the d-CLIC architecture, with its
wide channels, templated by larger guests. The ability to sort
architectures by relatively simple molecular parameters enables
a more informed design of new compounds, and it is a
significant step in crystal engineering. The isomerism displayed
by the various CLICs and TICs is somewhat reminiscent of
lamellar and hexagonal cylinder phases observed in soft matter
surfactant assemblies and block copolymers. The correspondence to soft matter is especially apparent for guanidinium
phenyl alkanesulfonates, biphenylalkanesulfonate, and alkanesulfonates, which are crystalline with lamellar architectures at
room temperature but form smectic liquid crystal phases upon
heating and lyotropic lamellar liquid crystals in certain organic
solvents.11,12 Like soft matter microstructures, the phase
diagram in Figure 4 is described by relatively simple parameters,
and the GMS inclusion compounds are equipped with a welldefined, elastic interface, the GS sheet, although the length scale
defining curvature and periodicity in the GMS compounds is
smaller.
The tubular inclusion compounds prompted the question as
to whether the 3-fold packing of the cylinders, driven by
dispersive interactions between the organosulfonate groups
could be replicated with rigid trisulfonates having 3-fold
symmetry. Indeed, the guanidinium salts of 1,3,5-benzenetrisulfonate (BTS), tri(4-sulfophenyl)methane (TSPM), and 1,3,5tri(4-sulfophenyl)benzene (TSPHB) crystallized in a cylindrical
architecture (G3BTS and G3TSPHB in space group P63/m;
G3TSPM in P63), but with neighboring GS cylinders connected
through covalent nodes provided by the trisulfonates rather
than dispersive interactions (Figure 5).13 This topological
equivalence demonstrates that symmetry can be preserved from
Figure 3. (A) Schematic of the GMS TIC architectures. (B) Spacefilled rendering of a single cylinder in G4BBS·2/3(o-xylene) (4BBS =
4-bromobenzenesulfonate). (C) Seven adjoining cylinders. (D) GS
tube, illustrating the commensurism between guest stacks and the
intersulfonate distance along the major ribbon. Hydrogen atoms and
all but the α-C atoms of 4BBS are omitted for clarity. Adapted with
permission from reference 10. Copyright 2007 American Chemical
Society.
sulfonate nodes project outward from the surface of each
cylinder (Figure 1, CPT-II), interdigitating to form hexagonal
arrays of cylinders with a pore diameter of approximately 9 Å.
Each cylinder contains a commensurate stack of uniformly
Figure 4. Structural phase diagram for 206 GMS inclusion compounds. Each data point represents a unique host−guest combination. Reproduced
with permission from ref 10. Copyright 2007 American Chemical Society.
2672
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
the guest molecules, once again demonstrating the templating
role of guest molecules.
Interestingly, the rigid tetrasulfonates afforded architectures
with differently sized and shaped cylinders in the same crystal.
Figure 5. (left) Schematic representation of the tubular architecture
generated by rigid trisulfonates (right) and space-filled model of
G3(1,3,5-trisulfonatobenzene). Adapted with permission from reference 13. Copyright 2015 American Chemical Society.
a well-defined blueprint even though significantly different
intermolecular forces direct assembly. This concept was limited
to rigid connectors, however, as trisulfonates with flexible arms
tend to form lamellar architectures with “molecular baskets”
because the sulfonate groups on each molecule can bend and
insert into the same sheet.13
■
EXPANDING VALENCY
The series of guanidinium organotetrasulfonates, G4T4SMB,
G4TSPB, and G4TSP, revealed a trend from lamellar to
cylindrical with increasing rigidity (Figure 6).13 Whereas the
flexible arms allow lamellar structures due to metric
compatibility of the sulfonate groups and their positions in
the GS sheet, rigid G4TSP formed cylindrical architectures with
a topology enforced by the disposition of the sulfonate groups.
The moderate conformational freedom in G4TSPB, however,
allowed both lamellar and cylindrical structures, depending on
Figure 7. (top) Reversible single crystal−single crystal transformation
accompanying guest exchange in the flexible framework between
G4TSPB·(dioxane)5 (left) and G4TSPB·(tetrahydrofuran)5 (right).
The tetrahydrofuran molecules are depicted as green to distinguish
them from the dioxane molecules in G4TSPB·(dioxane)5. (bottom)
Schematic illustration of reversible single crystal−single crystal
transformations based on G4TSPB framework. Adapted with
permission from ref 14. Copyright 2014 American Chemical Society.
The cylindrical architecture of G4TSPB exhibits three crystallographically unique one-dimensional channels (Figure 6). The
framework is sufficiently flexible to permit reversible release and
adsorption of various guest molecules with retention of single
crystallinity (Figure 7).14 The release and adsorption of dioxane
between G4TSPB·(dioxane)4 and G4TSPB·(dioxane)5 revealed
a reversible single crystal−single crystal transformation
accompanied by cyclic “breathing” of the framework. Reversible
guest exchange between G4TSPB·(dioxane)5 and tetrahydofuran, toluene, aniline, and nitrobenzene occurred with retention
of single crystallinity, signaling a postsynthetic approach to
inclusion compounds that could not be synthesized directly.
Moroever, the different channels discriminate between
exchanging guests (Figure 7). Whereas all five dioxane guests
in G4TSPB·(dioxane)5 could be replaced by five tetrahydrofuran guests, only partial exchange could be achieved with toluene
to generate G4TSPB·(toluene)3(dioxane). Partial exchange also
was observed in the conversion of G4TSPB·(tetrahydrofuran)5
to G4TSPB·(toluene)3(tetrahydrofuran)0.5. This surprising
Figure 6. Tendency to form cylindrical architectures increasing with
tetrasulfonate rigidity.
2673
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
Figure 8. (A, B) Complementary [G3NO3]2+ (yellow) and HSPB6− (green) tiles, and their corresponding edge lengths. (C) An unfolded quasitruncated octahedron (q-TO) based on the complementary [G3NO3]2+ (yellow) and HSPB6− (green) tiles. (D) The quasi-truncated octahedron.
The squares in panels C and D correspond to the openings that define the channels between adjacent q-TOs in the solid state. (E, F) The quasitruncated octahedron viewed perpendicular to a HSPB6− tile and a (G3NO3)2+ tile. The purple sphere represents the maximum isotropic volume
accommodated by the interior cavity, measuring approximately 12 Å in diameter. (Photo) Plastic model of the zeolite-like hydrogen bonded
framework. Adapted from ref 15. Reprinted with permission from AAAS.
Figure 9. (top) Protocol for an inclusion-based separation, wherein guest inclusion results in rapid crystallization of an inclusion compound.
(bottom) Selectivity profiles of pairwise competition for inclusion of xylene isomers by (left) G2NDS and (right) G4CBS, where X and Y are the
mole fractions for a particular isomer in solution and in the inclusion compound retrieved after crystallization, respectively. The offset of the triangle
from the center is a measure of the selectivity for competition experiments performed with an equimolar ratio of the three isomers. The curves
represent fits to the data, from which selectivity coefficients (K) can be calculated. The 45° line corresponds to K = 1 (no selectivity). Adapted with
permission from ref 16. Copyright 2001 American Chemical Society.
invoking some kind of cooperative peristaltic motion wherein
entry of a new guest on one side of the channel forces all the
molecules in the channel to move and expel an original guest
molecule from the other end. The other possible explanation
would invoke defects, but single crystallinity is preserved during
these exchange processes.
Rounding out the de novo framework design is a zeolite-like
hydrogen bonded framework formed with the hexa(sulfonatophenyl)benzene hexaanion (HSPB6−) by recognizing
that the assembly of two complementary hexagonal tiles of
discrimination can be attributed to the size constraints of the
different channels, but the mechanism of exchange is puzzling.
The diffusivities of molecules in the channels during exchange,
estimated from 1H NMR assays, were estimated to be ∼10−6
cm2 s−1, which is surprisingly large. Rapid diffusion in the two
channels having cross sections sufficient to accommodate two
guest molecules can be explained by two-way or ring diffusion,
most likely vacancy assisted. Guest exchange in the smaller
channel (in the center of each schematic in Figure 7) requires
single-file diffusion, which is difficult to explain without
2674
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
comparable size could fold into a quasi-truncated octahedron
(q-TO), similar to the Archimedean truncated octahedron. As
such, four HSPB6− tiles and four supramolecular [G3NO3]2+
tiles, hexagonal, complementary, and metrically matched along
the edges, formed a convex polyhedron through 72 hydrogen
bonds, with a [46.64.64] tiling and 4̅3m symmetry, assembling
further into a body-centered cubic framework with features
resembling those of sodalite and zeolite A (Figure 8).15 Single
crystal X-ray diffraction revealed I43̅ m space group symmetry
and a = 26.7 Å, with q-TOs interconnected by 4 Å channels
formed by sodium bridges between sulfonate oxygen atoms.
The q-TO can accommodate a sphere with a diameter of 12 Å,
corresponding to a sphere volume of 905 Å3. The total free
volume in the interior of each approaches 2200 Å3. The q-TO
and its framework exhibit a remarkable ability to encapsulate
and tolerate an assortment of molecular species with wideranging shapes, sizes, substituents, and charges, ranging from
transition metal complexes to “ship-in-a-bottle” metal-iodide
nanoclusters. The pervasiveness of the q-TO framework for
such a wide range of guests signifies an inherent thermodynamic stability of the framework alone, rather than assembly
directed by templating as observed for most other GS
compounds.
■
SELECTIVE ENCAPSULATION BY DESIGN
Traditional separation methods for molecular isomers often can
be unfeasible. Unlike covalent host frameworks such as zeolites
or metal−organic frameworks, which rely on selective sorption
and diffusion through pre-existing pores, the tailored inclusion
cavities in the GS frameworks are created during crystal
assembly. Trapped guests can be retrieved subsequently by
dissolution of the crystallized inclusion compound and
extraction under mild conditions, and the host material can
be recycled. This concept was demonstrated by our laboratory
for the GS frameworks,16 most notably the separation of 2,6dimethylnaphthalene from its nine other isomers in approximately 95% purity after crystallization of its inclusion
compound with the guanidinium biphenylsulfonate framework.
The guanidinium naphthalene-2,6-disulfonate (G2NDS) framework exhibited a selectivity for the isomers of xylene in the
order p-xylene ≫ o-xylene > m-xylene, considerably greater
than that typically exhibited by industry-standard zeolitic
materials. In contrast, the GMS host frameworks with 4methylbenzenesulfonate (4MBS), 4-chlorobenzenesulfonate
(4CBS), and 4-bromobenzenesulfonate (4BBS) preferentially
included m-xylene (Figure 9, bottom right for G4CBS).
Though these examples are limited to rather simple isomers,
they suggest protocols for separation of high value-added
compounds, for example, enantioselective separations using
chiral organosulfonates.
Selective encapsulation of a different kind was demonstrated
for a lamellar GS architecture built from a tetrasulfonated
calixarene, in which the footprint of the four sulfonate groups
on the upper rim of the calixarene match up with the sulfonate
sites on a GS sheet, creating an “endo-inclusion” cavity,
illustrated schematically in Figure 10.17 These capsules can
incorporate 3,4-dihydro-2H-pyrrole (Δ1-pyrroline),18 the active
pheromone of the Mediterranean fruit fly (Medfly), one of the
most destructive fruit pests. The Δ1-pyrroline monomer
coexists with its corresponding trimer in both solution and
neat liquid form (by 1H NMR), but the monomer is the active
form. Whereas the pure liquid pheromone gradually polymerizes after four months storage in neat liquid form, the monomer
Figure 10. (top) Schematic representation of the calixarene footprint
on the GS sheet and the appended “capsules”. (bottom) Selective
encapsulation of the monomer during crystallization of the endoinclusion compound G1.5SC⊃Δ1-pyrroline. Adapted with permission
from ref 18. Copyright 2013 American Chemical Society.
can be stored indefinitely without decomposition when
included in the endo-inclusion cavities of the GS calixarene
framework and released slowly by heating above ambient,
suggesting an agricultural adjuvant to trap female Medflies.
■
SEPARATING STRUCTURE FROM FUNCTION
The GS frameworks present an opportunity for synthesizing
functional materials, wherein the function can be introduced
through guest inclusion organized in a manner directed by a
reliable host framework scaffold, obviating the need to use
molecular components that direct structure and provide
function simultaneously, a daunting prospect that severely
limits crystal engineering. In this way, the GS frameworks
resemble a skyscraper: floors, ceilings, and walls articulate the
structure but the function is added separately, from lighting to
computers to office workers.
This can be exemplified by the synthesis of a polar
framework that produced a series of inclusion compounds
with predictable space group symmetry and lattice parameters,
while guiding the assembly of various acentric guest molecules
into polar arrays.19 Puckering of the GS sheet in a simple brick
GDS framework forces straight pillars in adjacent layers to tilt
in opposite directions. A retrosynthetic analysis suggested that
the same puckering could be achieved with banana-shaped
pillars, such that the pillars would point in the same direction
2675
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
Figure 11. A retrosynthetic approach to a 3D polar host frameworks (and, consequently, polar inclusion compounds): Linear organodisulfonate
pillars of a puckered simple brick GS framework (PT-II) can be cleaved virtually, rotated 180° with respect to each other, and reassembled to
produce banana-shaped pillars and a polar framework. This framework can be made directly with “bent” pillars, such as meta-substituted arene
disulfonates. From ref 19. Reprinted with permission from AAAS.
Figure 12. (top left) Schematic representation of GDS lamellar BL∥, simple brick, and BL architectures. The guest alignment in the bilayer structures
depends on cavity size. (top right) Libraries of eight GDS hosts and nine linear π-conjugated guests, characterized by (bottom left) metric variables
lS−S, the distance between sulfur atoms in the host pillar, and lg, the length of the long axis of the guest. (bottom right) Structural phase diagram for
GDS inclusion compounds sorted according to distinct sectors defined by the value of lg/lS−S. Blue circles (●) denote BL⊥ compounds, red
diamonds (◆,◇) denote simple brick compounds, and green triangles (▲,△) denote BL∥ compounds. Adapted with permission from ref 20.
Copyright 2010 American Chemical Society.
(Figure 11), generating a polar framework. Pillars having C2v
symmetry would be expected to afford a framework with
orthorhombic Imm2 space group symmetry. This was reduced
to practice using meta-substituted arenedisulfonates, which
generated polar frameworks that enforced polar alignment of
nitroarene guests, producing single crystals with SHG activity
that scaled with the guest hyperpolarizablities. These results
demonstrated the potential for separating structure from
function in the design of crystalline materials.
Polyconjugated molecules have substantial potential in
electronics, ranging from light-emitting diodes to nonlinear
optical devices to thin-film transistors. The optical and
electronic properties of these compounds are governed by
optical absorption, emission, charge generation, and carrier
transport, which typically are cooperative effects that depend on
2676
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
the arrangement of molecular constituents in the solid state.
GDS frameworks were found to direct the organization of
various oligothiophenes20 in bilayer and brick frameworks.
Large guests typically promote the formation of the more open
brick frameworks, but oligothiophene guests can be included in
the bilayer architecture as well, in either a “parallel” or
“perpendicular” orientation, depending on relative heights of
the pillar and guest (Figure 12). Various guest configurations,
edge-to-edge, face-to-edge, and end-to-end, were realized by the
systematic transition from the BL∥ to simple brick to BL⊥
architecture, achieved with increasing values of the guest length
relative to the pillar. By examining a total of 59 unique inclusion
compounds, a structural “phase diagram” was constructed based
on the parameters lg and lS−S (Figure 12), once again illustrating
structure prediction of a kind based on simple parameters.
Controlled guest arrangement enabled tuning of the optical
properties of the guests due to confinement in the host matrix,
manifested as bathochromic shifts in the absorption and
emission spectra of the guests compared with methanol
solutions.20 Similarly, a variety of coumarin dyes were included
in GDS frameworks, bilayer, simple brick, zigzag brick, and a
“chevron” brick, with larger coumarin dyes templating the more
open frameworks.21 This permitted control over the formation
of J- and H-aggregates, as well as their corresponding emission
behavior, which was distinct from that of the monomers. The
ability to regulate intermolecular association of laser dyes in
high concentrations in a robust host framework may lead to
opportunities for tunable solid-state lasing materials.
The effect of dye aggregation in GDS frameworks also has
been explored through the use of “tetris-shaped” pillars,22 such
as the stilbene disulfonates PV3DS2− and DSBDS2−, wherein
the only possible orientation of the pillar is with its long axis
parallel to the GS sheet (Figure 13). A discrete bilayer
architecture would require a Z-conformer in which the
sulfonate−sulfonate distance within each pillar was commensurate with the sulfonate nodes in a single GS sheet, an unlikely
possibility. Conversely, an E-conformer could assemble the
framework in a continuously layered architecture because this
distance criterion is relaxed, requiring only planar PV3 and
DSBDS moieties and registry of the opposing sheets in a
manner that avoids steric interference between neighboring
pillars. Molecular models based on the lengths of the PV3DS2−
and DSBDS2− pillars and the possible projection topologies of
the sulfonate nodes revealed that these pillars would pack in a
continuously layered architecture with their long axes diagonal
to the GS major ribbon axis, along a vector defined by two
sulfonate nodes in adjacent ribbons on a GS sheet. Dense
packing of the DSB and PV3 residues between the GS sheets
precluded the incorporation of guest molecules, allowing strong
intermolecular electronic couplings between the stilbene-like
fragments. G2DSBDS adopted a face-to-face brickwork packing
motif (Figure 13) and G2PV3DS adopted a face-to-face
herringbone motif. Strong intermolecular electronic couplings
between the pillars were confirmed by the spectral shifts in the
absorption and emission spectra. Collectively, these observations promise manipulation of aggregate structure for the
optimization of optoelectronic properties, even for GS
compounds in which the function is delivered by the host
framework alone.
Figure 13. (top) GS architecture with “Tetris-shaped pillars”. The
long axes of the pillars align parallel to the GS sheets, precluding guest
inclusion. The length of pillars, L, determines the azimuthal
orientation with respect to the sheet. (bottom) Packing of 11
DSBDS pillars (space filling) on a GS sheet (wireframe). G ions are
omitted for clarity. Adapted with permission from ref 22. Copyright
2015 American Chemical Society.
block, the guanidinium-sulfonate network, that reveal unprecedented control of solid-state architecture, spanning
lamellar to cylindrical to cubic, based on relatively simple
parameters such as size and shape of molecular constituents,
while demonstrating the value of separating architecture from
function when attempting the synthesis of functional materials.
The GS compounds demonstrate the utility of constraining
design to one dimension, in this case by locking in two
dimensions with a resilient hydrogen-bonded sheet equipped
with “builders”, the organic substituents on the sulfonates, for
engineering the third dimension. One definition of engineering
is “the art of skillful contrivance,” and the GS compounds
accumulated in our laboratory and others23,24 are consummate
examples of “crystal engineering.” Proton conduction recently
was described25 in GDS frameworks previously reported by our
group,26,27 further expanding on the suite of properties
resulting from their unique architecture. The GS concept has
been emulated in reports of analogous pillared networks
constructed from organosulfonates and transition metals
equipped with amine ligands,28−31 as well as hydrogen bonded
guanidinium borate networks in which the guanidinium ion acts
as a 3-fold connecting node to generate a cubic boracite
network.32 Nonetheless, universal design principles that are
applicable to molecular frameworks in general remain a
challenge. We hope that this Account encourages further
exploration of other classes of compounds based on common
motifs that illuminate the underlying principles governing
architectural forms. Moreover, hydrogen-bonding frameworks
with persistent porosity, including flexible frameworks, have
been reported recently with capabilities for carbon dioxide
capture,33 gas separations,34 and chiral separations,35 promising
further advances in functional host frameworks. Although the
utility of designer frameworks ultimately will rely on their
■
OUTLOOK
This Account encompasses hundreds of organic solid-state
compounds designed from a common supramolecular building
2677
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
(9) Horner, M. J.; Holman, K. T.; Ward, M. D. Lamellae-nanotube
isomerism in hydrogen-bonded host frameworks. Angew. Chem., Int.
Ed. 2001, 40, 4045−4048.
(10) Horner, M. J.; Holman, K. T.; Ward, M. D. Architectural
diversity and elastic networks in hydrogen-bonded host frameworks:
From molecular jaws to cylinders. J. Am. Chem. Soc. 2007, 129,
14640−14660.
(11) Martin, S. M.; Yonezawa, J.; Horner, M. J.; Macosko, C. W.;
Ward, M. D. Structure and Rheology of Hydrogen Bond Reinforced
Liquid Crystals. Chem. Mater. 2004, 16, 3045−3055.
(12) Martin, S. M.; Ward, M. D. Lyotropic Phases Reinforced by
Hydrogen Bonding. Langmuir 2005, 21, 5324−5331.
(13) Liu, Y.; Xiao, W.; Yi, J. J.; Hu, C.; Park, S.-J.; Ward, M. D.
Regulating the Architectures of Hydrogen-Bonded Frameworks
through Topological Enforcement. J. Am. Chem. Soc. 2015, 137,
3386−3392.
(14) Xiao, W. C.; Hu, C. H.; Ward, M. D. Guest Exchange through
Single Crystal-Single Crystal Transformations in a Flexible HydrogenBonded Framework. J. Am. Chem. Soc. 2014, 136, 14200−14206.
(15) Liu, Y. Z.; Hu, C. H.; Comotti, A.; Ward, M. D. Supramolecular
Archimedean Cages Assembled with 72 Hydrogen Bonds. Science
2011, 333, 436−440.
(16) Pivovar, A. M.; Holman, K. T.; Ward, M. D. Shape-selective
separation of molecular isomers with tunable hydrogen-bonded host
frameworks. Chem. Mater. 2001, 13, 3018−3031.
(17) Liu, Y. Z.; Ward, M. D. Molecular Capsules in Modular
Frameworks. Cryst. Growth Des. 2009, 9, 3859−3861.
(18) Xiao, W. C.; Hu, C. H.; Ward, M. D. Isolation and Stabilization
of a Pheromone in Crystalline Molecular Capsules. Cryst. Growth Des.
2013, 13, 3197−3200.
(19) Holman, K. T.; Pivovar, A. M.; Ward, M. D. Engineering crystal
symmetry and polar order in molecular host frameworks. Science 2001,
294, 1907−1911.
(20) Soegiarto, A. C.; Comotti, A.; Ward, M. D. Controlled
Orientation of Polyconjugated Guest Molecules in Tunable Host
Cavities. J. Am. Chem. Soc. 2010, 132, 14603−14616.
(21) Soegiarto, A. C.; Ward, M. D. Directed Organization of Dye
Aggregates in Hydrogen-Bonded Host Frameworks. Cryst. Growth Des.
2009, 9, 3803−3815.
(22) Adachi, T.; Connors, D. M.; Xiao, W. C.; Hu, C. H.; Ward, M.
D. Strong Intermolecular Electronic Coupling of Chromophores
Confined in Hydrogen-Bonded Frameworks. Cryst. Growth Des. 2015,
15, 3366−3373.
(23) Burke, N. J.; Burrows, A. D.; Mahon, M. F.; Teat, S. J.
Incorporation of sulfonate dyes into hydrogen-bonded networks.
CrystEngComm 2004, 6, 429−436.
(24) Bouchmella, K.; Dutremez, S. G.; Guérin, C.; Longato, J.-C.;
Dahan, F. Guanidinium Alkynesulfonates with Single-Layer Stacking
Motif: Interlayer Hydrogen Bonding Between Sulfonate Anions
Changes the Orientation of the Organosulfonate R Group from
“Alternate Side” to “Same Side”. Chem. - Eur. J. 2010, 16, 2528−2536.
(25) Karmakar, A.; Illathvalappil, R.; Anothumakkool, B.; Sen, A.;
Samanta, P.; Desai, A. V.; Kurungot, S.; Ghosh, S. K. HydrogenBonded Organic Frameworks (HOFs): A New Class of Porous
Crystalline Proton-Conducting Materials. Angew. Chem., Int. Ed. 2016,
55, 10667−10671.
(26) Russell, V. A.; Evans, C.; Li, W.; Ward, M. D. Nanoporous
Molecular Sandwiches: Pillared Two-Dimensional Hydrogen-Bonded
Networks with Adjustable Porosity. Science 1997, 276, 575−579.
(27) Swift, J. A.; Reynolds, A. M.; Ward, M. D. Cooperative HostGuest Recognition in Crystalline Clathrates: Steric Guest Ordering by
Molecular Gears. Chem. Mater. 1998, 10, 4159−4168.
(28) Reddy, D. S.; Duncan, S.; Shimizu, G. K. H. A Family of
Supramolecular Inclusion Solids Based Upon Second-Sphere Interactions. Angew. Chem., Int. Ed. 2003, 42, 1360−1364.
(29) Wang, X. Y.; Justice, R.; Sevov, S. C. Inorg. Chem. 2007, 46,
4626−4631.
competitive economic advantage, crystal engineering remains a
promising frontier for the discovery of functional molecular
materials.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Funding
This work was supported primarily by the National Science
Foundation under Award Number DMR-1308677. T.A. thanks
the JSPS Postdoctoral Fellowships for Research Abroad for the
financial support.
Notes
The authors declare no competing financial interest.
Biographies
Takuji Adachi received a B.Eng. at Osaka University and a Ph.D.
degree at the University of Texas at Austin under supervision of Paul
F. Barbara and David A. Vanden Bout. After a postdoctoral research
position at New York University, Adachi moved to the Institut de
Science et d’Ingénierie Supramoléculaires at Université de Strasbourg,
where he is a postdoctoral research associate.
Michael D. Ward received a B.S. in Chemistry from the William
Paterson College of New Jersey and a Ph.D. in Chemistry from
Princeton University. After a postdoctoral position at the University of
Texas, Austin, he held research positions at Standard Oil of Ohio and
Dupont Central Research, joining the faculty of the Department of
Chemical Engineering and Materials Science at the University of
Minnesota in 1990. Ward moved to New York University in 2006,
where he established the Molecular Design Institute and is a Silver
Professor in the Department of Chemistry.
■
ACKNOWLEDGMENTS
The authors take this opportunity to acknowledge the
contributions and support of Professor Paul F. Barbara, who
unfortunately passed away in October of 2010. Paul was a
renowned scientist, a former editor of this journal, the Ph.D.
advisor of T.A., and a longstanding colleague and friend of
M.D.W. His dedication to science was unparalleled, and he is
sorely missed.
■
REFERENCES
(1) Dunitz, J. D. Phase transitions in molecular crystals from a
chemical viewpoint. Pure Appl. Chem. 1991, 63, 177−185.
(2) Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Metal−Organic
Frameworks Special Issue. Chem. Rev. 2012, 12, 673−1268.
(3) Price, S. L. Predicting crystal structures of organic compounds.
Chem. Soc. Rev. 2014, 43, 2098−2111.
(4) Etter, M. C. Encoding and Decoding Hydrogen-Bond Patterns of
Organic-Compounds. Acc. Chem. Res. 1990, 23, 120−126.
(5) Duchamp, D. J.; Marsh, R. E. The Crystal Structure of Trimesic
Acid (Benzene-1,3,5-tricarboxylic acid). Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem. 1969, B25, 5−19.
(6) Ermer, O. Five-fold diamond structure of adamantane-1,3,5,7tetracarboxylic acid. J. Am. Chem. Soc. 1988, 110, 3747−3754.
(7) Holman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Metric
engineering of soft molecular host frameworks. Acc. Chem. Res. 2001,
34, 107−118.
(8) Holman, K. T.; Martin, S. M.; Parker, D. P.; Ward, M. D. The
generality of architectural isomerism in designer inclusion frameworks.
J. Am. Chem. Soc. 2001, 123, 4421−4431.
2678
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679
Article
Accounts of Chemical Research
(30) Dalrymple, S. A.; Shimizu, G. K. H. Crystal Engineering of a
Permanently Porous Network Sustained Exclusively by ChargeAssisted Hydrogen Bonds. J. Am. Chem. Soc. 2007, 129, 12114−12116.
(31) Wang, X.-Y.; Sevov, S. C. Hydrogen-Bonded Host Frameworks
of Cationic Metal Complexes and Anionic Disulfonate Linkers: Effects
of the Guest Molecules and the Charge of the Metal Complex. Chem.
Mater. 2007, 19, 4906−4912.
(32) Abrahams, B. F.; Haywood, M. G.; Robson, R. Guanidinium Ion
as a Symmetrical Template in the Formation of Cubic HydrogenBonded Borate Networks with the Boracite Topology. J. Am. Chem.
Soc. 2005, 127, 816−817.
(33) Wang, H.; Li, B.; Wu, H.; Hu, T.-L.; Yao, Z.; Zhou, W.; Xiang,
S.; Chen, B. A Flexible Microporous Hydrogen-Bonded Organic
Framework for Gas Sorption and Separation. J. Am. Chem. Soc. 2015,
137, 9963−9970.
(34) He, Y.; Xiang, S.; Chen, B. A Microporous Hydrogen-Bonded
Organic Framework for Highly Selective C2H2/C2H4 Separation at
Ambient Temperature. J. Am. Chem. Soc. 2011, 133, 14570−14573.
(35) Li, P.; He, Y.; Guang, J.; Weng, L.; Zhao, J. C.-G.; Xiang, S.;
Chen, B. A Homochiral Microporous Hydrogen-Bonded Organic
Framework for Highly Enantioselective Separation of Secondary
Alcohols. J. Am. Chem. Soc. 2014, 136, 547−549.
2679
DOI: 10.1021/acs.accounts.6b00360
Acc. Chem. Res. 2016, 49, 2669−2679