Pseudo-5-Fold-Symmetrical Ligand Drives Geometric Frustration in Porous Metal-Organic and Hydrogen-Bonded Frameworks

Article abstract of DOI:10.1021/jacs.0c04450

Reticular framework materials thrive on designability, but unexpected reaction outcomes are crucial in exploring new structures and functionalities. By combining "incompatible"building blocks, we employed geometric frustration in reticular materials leadi

Full text of DOI:10.1021/jacs.0c04450

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Article
A pseudo five-fold symmetrical ligand drives geometric frustration
in porous metal-organic and hydrogen bonded frameworks
Frederik Haase, Gavin A. Craig, Mickaele Bonneau, Kunihisa Sugimoto, and Shuhei Furukawa
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04450 • Publication Date (Web): 15 Jul 2020
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Journal of the American Chemical Society
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A pseudo five-fold symmetrical ligand drives geometric
frustration in porous metal-organic and hydrogen bonded
frameworks
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,
a
a
a
a,b
Frederik Haase,* Gavin A. Craig, Mickaële Bonneau, Kunihisa Sugimoto, Shuhei
Furukawa*
,
a,c
a
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, iCeMS Research Bldg, Yoshida,
Sakyo-ku, Kyoto 606-8501, Japan
b
Japan Synchrotron Radiation Research Institute/SPring-8, Kouto, Sayo, Hyogo 679-5198, Japan
c
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
ABSTRACT: Reticular framework materials thrive on designability, but unexpected reaction outcomes are crucial in
exploring new structures and functionalities. By combining “incompatible” building blocks, we employed geometric
frustration in reticular materials leading to emergent structural features. The combination of a pseudo C
5
symmetrical
organic building unit based on a pyrrole core, with a C symmetrical copper paddlewheel synthon led to three distinct
4
frameworks by tuning the synthetic conditions. The frameworks show structural features typical for geometric
frustration: self-limiting assembly, internally stressed equilibrium structures and topological defects in the equilibrium
structure, which manifested in the formation of a hydrogen bonded framework, distorted and broken secondary building
units and dangling functional groups, respectively. The influence of geometric frustration on the CO sorption behavior
2
and the discovery of a new secondary building unit shows geometric frustration can serve as a strategy to obtain highly
complex porous frameworks.
use of competing interactions to obtain complex systems
is a widespread phenomenon. We therefore wanted to
INTRODUCTION
13
Reticular framework materials are a class of materials
that are constructed by the selection or design of
building blocks and linkage groups such that a desired
induce geometric frustration, in order to explore new
13,15
emergent
preconception of the reaction outcome.
In this work, we introduced geometric frustration
through a linker with pseudo-C symmetry, as the
structures
and
features
without
1
net is formed. This approach is successfully applied for
the introduction of functionality on the organic linkers and
5
2
on known metal based secondary building units (SBU),
symmetry cannot be propagated to a classical periodic
lattice based-on translation, while periodic lattices based
on rotation and translation such as in a quasicrystal
while structural complexity can be increased by the
rational construction of new organic nodes and new
3
nets or by the synthesis of controlled multivariate
12,16
could support this symmetry.
for the introduction of C
Several strategies exist
4–6
materials.
Recently, design strategies have emerged
5
symmetry in organic building
as these are used for the synthesis of large
that seek to deviate from this approach and attempt to
increase complexity by introducing well-defined defects
and mismatches into the backbone that lead to reduced
17–20
blocks,
cage coordination compounds,
for the synthesis of quasicrystal approximants. As
19,21–24
or as metal nodes
25
1
symmetry, new nets and open metal sites. This can be
introducing
challenging,
C
5
-symmetry
instead we designed
can
be
synthetically
pseudo-C
7
achieved by linker clip-off, dissolution and introduction
20,26
a
5
8
–11
of defects
or the installation of size and shape
symmetrical ligand based on a pyrrole core (see method
section, Figure S2). The pseudo-C symmetric linker
containing pendant carboxylic acid groups, 1,2,3,4,5-
penta (4-(benzoic acid))pyrrole (pLH ; Figure 1) was
reacted with copper(II) ions with the intention of creating
the C -symmetric paddlewheel motif. While metal-
organic frameworks based on paddlewheel have been
1
mismatched linkers. These approaches are all based on
the knowledge of a well-defined parent lattice. Our
strategy here is to use mismatched symmetry in order to
introduce geometric frustration, where the mismatch is
large enough to prohibit the formation of predetermined
lattices or nets. Geometric frustration arises when local
geometric constraints hinder the energetic minimization
5
5
4
27
28
29
30
reported with planar 2-c, 3-c, 4-c and 6-c nodes
12
of all of the interactions at the same time. This is a
31
before, a search of MOF databases revealed no nets
sought after phenomenon for spin ices in magnetic
with alternating 5-c and 4-c nodes. The combination of
materials and their application in data storage,14 but the
13
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C
4
and C
5
local symmetries was, therefore, expected to
S2).32 The pentaHOF-1 structure contains two types of
1D channels parallel to the c-axis. The first type is purely
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be difficult to reconcile while maintaining the symmetry of
the building blocks.
2 4 4
intermolecular, created between stacks of Cu (pLH )
2
molecules with pore apertures of 11.3×11.3 Å (indicated
as A in Figure S5). The second type of void is created by
connecting the inter-molecular voids within one stack of
2
molecules by small intramolecular apertures (4.6×3.8 Å )
(
indicated as B in Figure S5).
Changing the solvent system led to the
formation of a MOF with the composition Cu (pLH).
2
pentaMOF–2 single crystals were obtained with
monoclinic crystal system in the C2/c space group by
reaction in a mixture of dimethyl formamide (DMF) and
n-butanol heated at 40°C. The structure is composed of
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a fully 3D metal-organic network containing [pLH] units
and copper paddlewheels. Two out of four carboxylate
groups create a square lattice layer with regular copper
paddlewheels (Figure 2c), while the remaining two
carboxylate groups twist out of plane and connect these
layers with a distorted copper paddlewheel leading to a
33
sqc176 (lvt) topology (Figure 2d). The fifth, potential
carboxylate group remains protonated and is therefore
not involved in binding to copper. The carboxylic acid
group protrudes into the interstitial voids and has an
average dO---O of 6.19 Å to the nearest free carboxylic
acid. It is therefore not able to participate in hydrogen
bonding to the MOF backbone. Individual linker
molecules in the structure are not in contact with each
other and are separated by solvent molecules, which
could be resolved by SXRD (Figure S6). The close
proximity of the linkers without close contact leads to a
three-dimensional pore structure in pentaMOF-2 with
small diameter accessible voids (Figure S7).
Figure 1. Schematic illustration of the synthesis of materials
from the pentagonal linker pLH .
5
RESULTS AND DISCUSSION
Structural description of three frameworks based on
the pentagonal linker. Using pLH for the synthesis of
5
frameworks, three materials were crystallized:
hydrogen bonded framework (HOF), based on
molecular copper(II) complex containing four [pLH
a
a
-
4
]
units (Cu
organic frameworks (MOF) consisting of [pLH] units
Cu (pLH); pentaMOF-2 and pentaMOF-3). The
2 4 4
(pLH ) ; pentaHOF-1) and two different metal-
4-
(
2
materials differ in their connectivity, node-distortion,
porosity and linker-to-linker contacts, which point
towards the pathways in which the frustration is released
in this system.
The interdiffusion of an aqueous copper nitrate
solution with a solution of pLH
temperature leads to the formation of a metal containing
HOF (pentaHOF-1). This material consists of
molecular copper complex Cu (pLH that is composed
of a single copper paddlewheel surrounded by four
5
in ethanol at room
a
2
4 4
)
-
[
pLH
pentaHOF-1 crystallizes in the tetragonal space group
P4/nnc (Table S1) in which the Cu (pLH molecules
4
] moieties in a propeller shape (Figure 2a). The
2
4 4
)
arrange to form a square lattice layer. The layers are
stacked in an alternating fashion to form a distorted body
centered arrangement (Figure 2b). The structure is
stabilized by hydrogen bonding interactions where one
Cu (pLH ) molecule interacts with 10 of its neighbors (8
2 4 4
from the body centered arrangement and 2 from
interaction with the molecules above and below the
Cu
2
(pLH
4
)
4
molecules; Figure S3) through its 16
carboxylic acids. The structure contains a disordered
hydrogen bonding motif where four carboxylic acids
meet. The disorder can be disentangled to two binding
modes (Figure S4), where either four carboxylic acids
form three hydrogen bonds or two carboxylic acid pairs
can form one hydrogen bond each. The oxygen-oxygen
distances and the angles between the carboxylic acids
are in line with moderately strong hydrogen bonds (Table
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Journal of the American Chemical Society
coordinates to one copper in an equatorial fashion and
through pseudo-coordination to the axial position of a
neighboring copper pair, thus leading to one-
dimensional SBU (Figure 2f). The copper pairs are
bridged by three axial carboxylate anions with a dCu---Cu
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4
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a
2
.98 Å, which is considerably larger than in the regular
paddlewheels present in the structure with
Cu---Cu = 2.64 Å. The inter-pair distance, bridged by an
d
axial-equatorial carboxylate binding motif is even larger
with dCu---Cu = 4.67 Å. The pseudo-coordination of the rod
SBU and the hydrogen bonding connect the individual
layers with each other, leading to a 3D net based on the
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sqc54 topology (Figure S9). The structure contains
small undulating 1D channels with an aperture of
5.5×3.6Å (Figure S10).
2
Nets. The net of the pentaHOF-1 can be described by
simplifying the structure by different degrees. The copper
paddlewheel is a 4-c node the pL presents a 5-c node
and the hydrogen bonded node can be considered as a
4-c node on arrives at a 4,4,5-c net with a transitivity of
(3 3) (Figure S11). However, since the bonding situation
is ambiguous due to disorder, the hydrogen bonded
node can also be considered as a linear connection, thus
a 4,5-c net is obtained with a transitivity of (2 2) as two
kinds of edges are observed in this structure. In this
structure the nodes are not alternating, like in many
31
other observed 4,5-c nets. When considering the
hydrogen bonded node as a 4-c node, the question
arises, why no additional paddlewheel is formed in this
position, which would lead to the compound Cu
When constructing a model of Cu pL with the same
,4,5-c net, the paddlewheel placed instead of the
5 2
pL .
5
2
4
hydrogen bonded node is highly strained (Figure S12).
This node is much more distorted than pentaMOF-2 and
pentaaMOF-3 paddlewheels. The highly distorted
coordination environment might prohibit the formation of
a copper paddlewheel. This might explain why the
hydrogen bonded framework pentaHOF-1 is formed
instead of Cu
4,5-c net might not be possible, the 4,4,5-c net is a
plausible candidate for Cu pL . The proposed structure
is however only one possibility for the net and others are
unknown. Attempts to obtain Cu pL were unsuccessful,
5 2
pL . As the formation of an edge transitive
Figure 2. Detail illustrations of the crystal structures of (a,b)
pentaHOF-1, (c,d) pentaMOF-2 and (e,f) pentaMOF-3,
which emphasize the connectivity of the structure.
37
5
2
By increasing the reaction temperature up to
5
2
1
10°C and using an n-butanol/water mixture, we
obtained single crystals of pentaMOF-3 in an
orthorhombic Pna2 space group (Table S1) with the
same composition Cu (pLH) as pentaMOF-2, but with a
different connectivity. The structure is composed of
with increasing the reaction temperature or introducing
base only leading to pentaMOF-3. The assembly of the
hydrogen bonded molecules can also be considered as
just molecules that connect to neighboring molecules.
This leads to a 10-c unimodal net obtained with a
transitivity (1 2) bct.
1
2
4-
[
pLH] units where four of the five available carboxylates
bind to copper and lead to the formation of a two-
dimensional layer with a distorted square lattice net
(Figure 2e). The remaining fifth carboxylate is protonated
and is able to form moderately strong hydrogen bond
The change in connectivity from a 5-c node to a
4-c node as observed in the structures of penta-MOF-2
and pentaMOF-3 also have a profound effect on the nets
of the materials. While the pentaMOF-2 crystallized with
a lvt net which is based on only 4-c nodes, the
pentaMOF-3 crystallizes with 2-periodic sql net layers,
which are however connected through the rod SBU
leading to a 6-c node instead of a 4-c node in one half of
the available copper paddlewheels. This leads to a
three-dimensional periodic sqc54 net.
32
with a second linker. The carboxylic acid is disordered,
with one state resembling a typical carboxylic acid dimer
with two hydrogen bonds, and another state a carboxylic
acid dimer where only one hydrogen bond can be
formed (Table S2, Figure S8) The copper binds the
linker with two distinct secondary building units (SBU).
The first is a regular copper paddlewheel, albeit a
distorted one. The second SBU contains pairs of copper
ions that resemble a copper paddlewheel only in three of
four carboxylates, while the fourth carboxylate group
Sorption properties of framework materials. The
single crystal structures of the materials presented here
showed promising solvent accessible voids, and we
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therefore investigated gas sorption properties. Only the
pentaHOF-1 showed uptake of N at 77 K with a type‐I
isotherm, with a BET surface area of 849 m g (Figure
3a), which confirms its permanent porosity.
considering the total uptake of 108.8 and 124.2 ml/g for
pentaMOF-2 and -3, respectively. In pentaHOF-1, the
metal adsorption sites account for only 16.0 ml/g CO
which is small compared to the total uptake of 297.5 ml/g
~5% of the total uptake). In addition to the adsorption on
the metal sites, the pore sizes of the materials are
strongly influencing the adsorption properties of the
materials. In pentaMOF-2 and -3 only small pores are
present, while pentaHOF-1 has a much larger channel
with a ~1.1 nm diameter in addition to small pores close
to the metal centers. The small pores in the materials
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seem to reject N , which explains why pentaMOF-2 and -
3 show no uptake of nitrogen but a small uptake of CO
2
.
On the other hand, it is surprising, that a hydrogen
bonded structure of a metal containing molecular
complex possesses larger porosity than the
corresponding MOFs. Normally, the formation of metal-
ligand bonds is the driving force that leads to porous
MOF. However, in this case, the formation of pentaHOF-
1
with a high porosity and low number of copper-ligand
bonds is an unexpected outcome that can be linked to
geometric frustration. We therefore systematically
investigated structural constraints, to elucidate this
peculiar behavior.
Effect of geometric frustration on the structures.
12
Grason offers a theoretical context for interpreting the
effects of geometric frustration by defining anomalous
structural properties such as “internally stressed
equilibrium structures, self-limiting assembly, and
topological defects in the equilibrium assembly
12
structures”. All structures derived from the pL building
block show these signs of frustration. In self-limiting
equilibrium structures, the size of an assembly is limited,
since the local interactions lead to stresses that sum up
to limit the growth. In pentaHOF-1 this is observed in an
2 4 4
extreme fashion as the molecule Cu (pLH ) consists of
only the first sphere of coordination, as a single copper
paddlewheel is surrounded by four pL molecules that are
not connected to further metal SBU. This is surprising,
since under the applied synthetic conditions, a full
reaction could be expected, as it is possible to
synthesize copper paddlewheel MOFs such as, for
Figure 3. Sorption measurement of pentaHOF-1,
pentaMOF-2 and pentaMOF-3 for (a) N
CO at 195 K.
2
at 77 K and (b)
2
In contrast, all three materials showed CO
2
adsorption at 195K. The isotherm of pentaHOF-1
exhibits a steep rise at low partial pressures that
28
example, HKUST-1 at room temperature.
Internally stressed equilibrium structures arise,
where the self-limiting growth is overcome and extended
metal-ligand nets are formed. The high coordination
number of the nodes and the rigid nature of the building
blocks lead to few degrees of freedom for the system to
form a stable structure. This leads to suboptimal
coordination environments, as the angles and distances
of the carboxylic acid groups cannot be rearranged to
accommodate the energetic minimum for an unstrained
copper paddlewheel. In pentaHOF-1, the copper
paddlewheel (Figure 4) and the pL molecules are
unstrained as a result of the formation of a discrete
molecule. However, the structures of pentaMOF-2 and
pentaMOF-3 show a copper paddlewheel that is more
distorted than in any previously reported phenyl based
copper paddlewheel structure in the Cambridge
0
includes a step at around 0.055 P/P (Figure 3b, inset).
Such a stepped increase could be attributed to a
3
4
dynamic feature of the framework material
or a
35
preferential adsorption site. The single crystal structure
of pentaHOF-1 shows copper paddlewheels situated
directly above each other with a Cu---Cu separation of
7
.37 Å, which conforms well with a previously described
35
“optimum” distance for the binding of CO
2
.
In the case
of pentaHOF-1, the adsorption step occurs at much lower
partial pressures than described in the literature,
possibly indicating
a
stronger binding. The CO
2
isotherms of pentaMOF-2 and -3 described a type‐I
shape with a total uptake significantly smaller than
pentaHOF-1. The differences in total CO
2
uptake
between the three structures are in stark contrast to the
available metal binding sites per gram of material. In
pentaMOF-2 and -3 the available number of metal
binding sites are the same, corresponding to 56.6 ml/g
CO2. This value accounts nearly half of the adsorption,
38
structural database (CSD, 454 structures) (Figure 4,
see method section). In pentaMOF-2 one paddlewheel
remains largely unaffected by strain, while the other is
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markedly distorted. The pentaMOF-3 structure shows
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one paddlewheel that is distorted more strongly than in
pentaMOF-2 (Figure 4 blue circles), and additionally the
second SBU is strained so strongly that the paddlewheel
motif breaks down and a rod SBU is formed instead
(
Figure 2f, Figure 4 blue X). The copper SBU is
experiencing strain in the equatorial plane and also in
the axial direction (Figure 2f, Figure S13) as
consequence of the partially overlapping linkers (Figure
e) within one layer of pentaMOF-3. The equatorial
a
2
strain can be resolved by an in-plane change of the bite
angle (Figure S14), while the axial strain is released by
altering the binding motif from a paddlewheel to a rod
SBU. This effect can be quantified by calculating the out-
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of-plane distortion of the [pLH] carboxyphenyl groups in
relation to the central pyrrole core. The groups
connected to the rod SBU show a significantly reduced
out-of-plane bending compared to those connected to
the copper paddlewheels (Figure S15, Table S3). The
internal stress of the structures can also be observed in
the hydrogen bonding in pentaHOF-1 and pentaMOF-3,
where the disorder of the free carboxylic acids suggests
competition between two hydrogen bonding states. In
pentaHOF-1, one state with three hydrogen bonds
between four carboxylic acids (Figure S4b, d) competes
with a state where two pairs of carboxylic acids can form
one hydrogen bond each (Figure S4c). In pentaMOF-3,
the two binding modes are characterized by a carboxylic
acid dimer with two hydrogen bonds and a carboxylic
acid dimer with only one hydrogen bond (Figure S8). The
rigid backbone prohibits the complete relaxation of the
structure to maximize the number of hydrogen bonds,
leading to strained hydrogen bonding interactions.
Figure 4. Distortion of the copper paddlewheel structure of
pentaHOF-1, pentaMOF-2 and pentaMOF-3, in comparison
to structures reported in the Cambridge Structural
Database. As assessed by the carbon distances as
indicated in the top. The dashes line indicates values where
x=y.
Dangling carboxylic acid groups remain in the
structures of pentaMOF-2 and -3, even as the number of
ligand-to-copper bonds are increased compared to
pentaHOF-1. These topological defects in the equilibrium
structure lead to coordination nets based on a 4-c node
5
for the nominally 5-c pLH linker (Figure 2e). This can be
understood as a result of the difficulty translating the C
5
symmetry of the linker to a periodic pattern. Additionally,
the high but unequal number of points of extension leads
to an overload in points of extension already in the first
sphere of reticulation, which is then resolved by dangling
carboxylic acids.
Applying this analysis to other reported MOFs,
reveals geometric frustration leading to anomalous
structures such as in the formation of a hydrogen
bonded material consisting of metal organic polyhedra
39
instead of a MOF and the formation distorted or
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untypical metal SBU due to incompatibilities between the
linker and the SBU (see also supplemental discussion).
comparatively weak interactions between linkers play a
significant role in determining the structure of both
MOFs. The diagram also reveals that some structural
features can completely relax in some structures, such
as the completely undistorted paddlewheel in pentHOF-
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1. Other features such as the carboxylate binding to
copper is at a maximum 4/5 per pL linker, thus not
binding to metals.
CONCLUSION
We have reported three porous materials in a frustrated
linker - metal node system. The geometry of the linker
and the combination of a nominally 5-c with a 4-c node
induced geometric frustration that manifested in the
distorted linkers and nodes, sub-stoichiometry, and
porosity. The analysis of the linker distortion was used
as a qualitative basis to evaluate the frustration in the
obtained structures. The structures reported here point
towards using geometrically frustrated assembly as a
design principle in reticular materials to form metal
containing HOFs, unusual metal SBUs, dangling
functional groups and polymorphic structures.
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Figure 5. Qualitative assessment of the competing forces
that lead to frustration (details can be found in Figure S16).
Values further from the center indicate a stronger penalty.
ASSOCIATED CONTENT
The hallmark of geometric frustration is the
absence of a simple ground state, which leads to
structural solutions that show compromises of competing
Supporting Information The Supporting Information is
available free of charge on the ACS Publications website at
DOI:.
Materials and methods; Supplementary Figures S1−S32,
Supplementary Tables S1-S3 containing SRXD data,
PXRD, TGA and additional information about the nets.
driving
forces;
metal-to-ligand
bonding,
metal
coordination
environment,
hydrogen
bonding,
hydrophobic interactions, porosity and geometric
constraints of the linker. The described system follows
divergent pathways to trade-off several dimensions of
structural degrees of freedom to release some of the
frustration; five of the most prominent are shown in
Figure 5. Each structure occupies a different area in this
qualitative frustration diagram, illustrating the structural
trade-offs. The driving force in MOF formation is the
formation of the metal-ligand bond, yet protonated
carboxylic acids remain in the resulting structures to
different extent. In pentaHOF-1, only one of five
carboxylate groups bind to copper and in pentaMOF-2
and -3, four out of five carboxylic acids bind to copper. At
the same time, the distortion of the copper
paddlewheels, which also carries a large penalty, follows
(PDF)
Single Crystal X-ray Data for pentaHOF-1 (CCDC:
1991981)
Single Crystal X-ray Data for pentaMOF-2 (CCDC:
1991980)
Single Crystal X-ray Data for pentaMOF-3 (CCDC:
1991982)
AUTHOR INFORMATION
Corresponding Author
To whom correspondence should be addressed:
shuhei.furukawa@icems.kyoto-u.ac.jp
haase.frederik.35c@st.kyoto-u.ac.jp
the opposite trend: pentaHOF-1 has
paddlewheel without distortion, while in pentaMOF-2 and
3 some or all of the paddlewheel motifs are distorted.
a
copper
Notes
The authors declare no competing financial interests.
-
Comparing the pentaMOF-2 to -3, the number of free
carboxylic acids are the same, but a trade-off can be
observed between the paddlewheel distortion and the
remaining three factors: hydrogen bonding, linker
isolation, and accessible voids. The paddlewheel
distortion is the largest in pentaMOF-3, but the free
carboxylic acids are able to participate in hydrogen
bonding unlike in pentaMOF-2, where the carboxylic
acids are completely isolated. The linkers within the
MOF backbone are also much more isolated in
pentaMOF-2 than in pentaMOF-3, which also leads to an
increased solvent accessible surface in pentaMOF-2.
Consequently, the penalty from the isolation of the
backbone in pentaMOF-2, allows it to reduce the
distortion of the paddlewheels, and vice versa in
pentaMOF-3. These results indicate that the
ACKNOWLEDGMENT
F.H. is grateful to the Japan Society for the Promotion of
Science (JSPS) for Post-Doctoral Fellowships. The authors
acknowledge iCeMS Analysis Center for access to
analytical facilities.
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